Advertisement

The Pathological Relevance of Increased Endothelial Glycocalyx Permeability

Open ArchivePublished:February 06, 2020DOI:https://doi.org/10.1016/j.ajpath.2019.11.015
      The endothelial glycocalyx is a vital regulator of vascular permeability. Damage to this delicate layer can result in increased protein and water transit. The clinical importance of albuminuria as a predictor of kidney disease progression and vascular disease has driven research in this area. This review outlines how research to date has attempted to measure the contribution of the endothelial glycocalyx to vessel wall permeability. We discuss the evidence for the role of the endothelial glycocalyx in regulating permeability in discrete areas of the vasculature and highlight the inherent limitations of the data that have been produced to date. In particular, this review emphasizes the difficulties in interpreting urinary albumin levels in early disease models. In addition, the research that supports the view that glycocalyx damage is a key pathologic step in a diverse array of clinical conditions, including diabetic complications, sepsis, preeclampsia, and atherosclerosis, is summarized. Finally, novel methods are discussed, including an ex vivo glomerular permeability assay that enhances the understanding of permeability changes in disease.
      Water and solute exchange across the walls of the microcirculation are dynamic processes that are fundamental to tissue homeostasis. The net rates of exchange are regulated by alterations in systemic blood pressure (hydrostatic pressure), vessel density and size (surface area), flow rate, concentration gradients, and the intrinsic permeability properties of the vessel wall. The basic structure of the capillary wall is conserved throughout the body, consisting of an endothelial cell monolayer, basement membrane, and supporting cells. However, a high level of specialization occurs within discrete areas of the vasculature, optimizing the structure for the individual demands placed on it. The endothelial glycocalyx layer found on the luminal surface of all endothelial cells contributes to the permeability barrier formed by the vessel wall.
      • Betteridge K.B.
      • Arkill K.P.
      • Neal C.R.
      • Harper S.J.
      • Foster R.R.
      • Satchell S.C.
      • Bates D.O.
      • Salmon A.H.J.
      Sialic acids regulate microvessel permeability, revealed by novel in vivo studies of endothelial glycocalyx structure and function.
      ,
      • Butler M.J.
      • Ramnath R.
      • Kadoya H.
      • Desposito D.
      • Riquier-Brison A.
      • Ferguson J.K.
      • Onions K.L.
      • Ogier A.S.
      • ElHegni H.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Peti-Peterdi J.
      • Satchell S.C.
      Aldosterone induces albuminuria via matrix metalloproteinase-dependent damage of the endothelial glycocalyx.
      Glycocalyx literally translates from the Greek for sugar coat (glykys means sweet and kalyx means husk). This adherent structure includes proteoglycans, glycoproteins, and glycolipids (Figure 1).
      • Betteridge K.B.
      • Arkill K.P.
      • Neal C.R.
      • Harper S.J.
      • Foster R.R.
      • Satchell S.C.
      • Bates D.O.
      • Salmon A.H.J.
      Sialic acids regulate microvessel permeability, revealed by novel in vivo studies of endothelial glycocalyx structure and function.
      ,
      • Dane M.J.
      • van den Berg B.M.
      • Lee D.H.
      • Boels M.G.
      • Tiemeier G.L.
      • Avramut M.C.
      • van Zonneveld A.J.
      • van der Vlag J.
      • Vink H.
      • Rabelink T.J.
      A microscopic view on the renal endothelial glycocalyx.
      • Singh A.
      • Satchell S.C.
      • Neal C.R.
      • McKenzie E.A.
      • Tooke J.E.
      • Mathieson P.W.
      Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.
      • Curry F.R.
      Microvascular solute and water transport.
      Proteoglycans consist of core proteins (eg, syndecans and glypicans) with covalently bound glycosaminoglycan side chains [eg, heparan sulfate (HS) and chondroitin sulfate]. The glycocalyx is not uniform across its depth.
      • Curry F.E.
      Layer upon layer: the functional consequences of disrupting the glycocalyx-endothelial barrier in vivo and in vitro.
      The 2-layer fiber matrix model suggests a dense 200- to 300-nm meshlike inner layer rich in proteoglycans covalently bound to the endothelial cell membrane and adherent glycosaminoglycans, including long chains of hyaluronan (HA), and an outer, more porous, gellike layer up to 1-μm thick, including adsorbed plasma proteins.
      • Curry F.E.
      Layer upon layer: the functional consequences of disrupting the glycocalyx-endothelial barrier in vivo and in vitro.
      ,
      • Curry F.E.
      • Michel C.C.
      The endothelial glycocalyx: barrier functions versus red cell hemodynamics: a model of steady state ultrafiltration through a bi-layer formed by a porous outer layer and more selective membrane-associated inner layer.
      This review discusses the evidence for the importance of the endothelial glycocalyx as a regulator of endothelial and vascular permeability, while noting the inherent difficulties in studying it. The human diseases in which glycocalyx damage and associated permeability alterations appear to be key pathogenic steps are also reviewed.
      Figure thumbnail gr1
      Figure 1The glomerular filtration barrier consists of glomerular endothelial cells (GEnCs), the glomerular basement membrane, and podocytes. GEnCs possess numerous transcellular fenestrations, which permit the high hydraulic permeability necessary for filtration, and a glycocalyx, which covers the luminal surface, extending over the fenestrations. Podocytes form a second cellular layer by interdigitating their foot processes, which connect at the slit diaphragms. The glycocalyx is a complex structure that contains core proteoglycans, such as syndecans and glypicans, holding glycosaminoglycans, heparan sulfate, chondroitin sulfate, and hyaluronan to the cell surface. The glycocalyx contributes to the filtration barrier by depleting the filtrated protein concentration before it reaches the fenestrated glomerular endothelial cell surface. The generation of a zone of protein-depleted filtrate adjacent to the luminal membrane of endothelial cells (shown in pink) limits loss of macromolecules from the plasma and reduces the effective oncotic pressure across the endothelial cell body.

      The Complexities of Studying the Endothelial Glycocalyx as a Permeability Barrier

      Endothelial cells in vitro produce a surface glycocalyx that provides an accessible model to study glycocalyx functions, including shear stress sensing
      • Butler M.J.
      • Ramnath R.
      • Kadoya H.
      • Desposito D.
      • Riquier-Brison A.
      • Ferguson J.K.
      • Onions K.L.
      • Ogier A.S.
      • ElHegni H.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Peti-Peterdi J.
      • Satchell S.C.
      Aldosterone induces albuminuria via matrix metalloproteinase-dependent damage of the endothelial glycocalyx.
      and permeability regulation.
      • Singh A.
      • Satchell S.C.
      • Neal C.R.
      • McKenzie E.A.
      • Tooke J.E.
      • Mathieson P.W.
      Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.
      However, it is much thinner than the glycocalyx seen in vivo, limiting the applicability of in vitro research.
      • Singh A.
      • Satchell S.C.
      • Neal C.R.
      • McKenzie E.A.
      • Tooke J.E.
      • Mathieson P.W.
      Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.
      ,
      • Potter D.R.
      • Damiano E.R.
      The hydrodynamically relevant endothelial cell glycocalyx observed in vivo is absent in vitro.
      ,
      • Chappell D.
      • Jacob M.
      • Paul O.
      • Rehm M.
      • Welsch U.
      • Stoeckelhuber M.
      • Conzen P.
      • Becker B.F.
      The glycocalyx of the human umbilical vein endothelial cell: an impressive structure ex vivo but not in culture.
      As a result, most studies on the permeability of the glycocalyx have been conducted in vivo.
      Much research to date has focused on the glomerular endothelial glycocalyx, which may be driven by the physiologic importance of permeability in glomerular function and the clinical importance of albuminuria. Furthermore, urinary albumin creatinine ratios (uACRs) can be easily quantified to provide a measure of albumin permeability across the glomerular filtration barrier (GFB) (Figure 1). Early models of the GFB overestimated the contribution of the slit diaphragm to the restriction of albumin filtration.
      • Edwards A.
      • Daniels B.S.
      • Deen W.M.
      Ultrastructural model for size selectivity in glomerular filtration.
      Recent models of the GFB, however, suggest that the glycocalyx represents a significant protein barrier.
      • Punyaratabandhu N.
      • Kongoup P.
      • Dechadilok P.
      • Katavetin P.
      • Triampo W.
      Transport of spherical particles through fibrous media and a row of parallel cylinders: applications to glomerular filtration.
      This finding concurs with accumulating experimental evidence,
      • Satchell S.
      The role of the glomerular endothelium in albumin handling.
      indicating that the remaining components of the GFB, including the endothelial glycocalyx, provide the major barrier to protein permeability (Figure 1).
      • Singh A.
      • Satchell S.C.
      • Neal C.R.
      • McKenzie E.A.
      • Tooke J.E.
      • Mathieson P.W.
      Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.
      ,
      • Weinbaum S.
      • Tarbell J.M.
      • Damiano E.R.
      The structure and function of the endothelial glycocalyx layer.
      • Salmon A.H.
      • Satchell S.C.
      Endothelial glycocalyx dysfunction in disease: albuminuria and increased microvascular permeability.
      • Oltean S.
      • Qiu Y.
      • Ferguson J.K.
      • Stevens M.
      • Neal C.
      • Russell A.
      • Kaura A.
      • Arkill K.P.
      • Harris K.
      • Symonds C.
      • Lacey K.
      • Wijeyaratne L.
      • Gammons M.
      • Wylie E.
      • Hulse R.P.
      • Alsop C.
      • Cope G.
      • Damodaran G.
      • Betteridge K.B.
      • Ramnath R.
      • Satchell S.C.
      • Foster R.R.
      • Ballmer-Hofer K.
      • Donaldson L.F.
      • Barratt J.
      • Baelde H.J.
      • Harper S.J.
      • Bates D.O.
      • Salmon A.H.
      Vascular endothelial growth factor-A165b is protective and restores endothelial glycocalyx in diabetic nephropathy.
      These estimates, combined with the increased understanding of cellular crosstalk within the GFB, should make us question assumptions about the pathogenesis of albuminuria in multiple historical models.
      • Eremina V.
      • Jefferson J.A.
      • Kowalewska J.
      • Hochster H.
      • Haas M.
      • Weisstuch J.
      • Richardson C.
      • Kopp J.B.
      • Kabir M.G.
      • Backx P.H.
      • Gerber H.P.
      • Ferrara N.
      • Barisoni L.
      • Alpers C.E.
      • Quaggin S.E.
      VEGF inhibition and renal thrombotic microangiopathy.
      ,
      • Dimke H.
      • Maezawa Y.
      • Quaggin S.E.
      Crosstalk in glomerular injury and repair.
      Albuminuria is the net result of albumin passage across the GFB (influenced by permeability) and albumin reabsorption and metabolism within the renal tubules. However, despite the widespread use of uACR, there is increasing evidence in rodent models that uACR is not a sensitive test for changes in GFB permeability and hence not an ideal index of glycocalyx integrity. Using in vivo multiphoton microscopy in mice (Figure 2) (image provided by M.J.B.), we found that glomerular albumin leakage can be significantly increased before detectable levels of albumin appear in the urine.
      • Butler M.J.
      • Ramnath R.
      • Kadoya H.
      • Desposito D.
      • Riquier-Brison A.
      • Ferguson J.K.
      • Onions K.L.
      • Ogier A.S.
      • ElHegni H.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Peti-Peterdi J.
      • Satchell S.C.
      Aldosterone induces albuminuria via matrix metalloproteinase-dependent damage of the endothelial glycocalyx.
      Other groups have reported similar data, demonstrating increased glomerular albumin leakage but no significant change in uACR.
      • Dane M.J.
      • van den Berg B.M.
      • Avramut M.C.
      • Faas F.G.
      • van der Vlag J.
      • Rops A.L.
      • Ravelli R.B.
      • Koster B.J.
      • van Zonneveld A.J.
      • Vink H.
      • Rabelink T.J.
      Glomerular endothelial surface layer acts as a barrier against albumin filtration.
      Evidence suggests that that this discrepancy is explained by tubular reabsorption of filtered albumin, resulting in a threshold effect whereby increased glomerular albumin permeability will not result in uACR increases until mechanisms of uptake and metabolism are overwhelmed.
      • Lazzara M.J.
      • Deen W.M.
      Model of albumin reabsorption in the proximal tubule.
      The role of tubular albumin uptake in humans is debated, but diseases resulting in tubular dysfunction, such as Fanconi syndrome and Dent disease result in significant albuminuria.
      • Norden A.G.
      • Scheinman S.J.
      • Deschodt-Lanckman M.M.
      • Lapsley M.
      • Nortier J.L.
      • Thakker R.V.
      • Unwin R.J.
      • Wrong O.
      Tubular proteinuria defined by a study of Dent's (CLCN5 mutation) and other tubular diseases.
      Historical studies in patients with diabetes using lysine to inhibit tubular albumin reabsorption similarly indicate that tubular albumin uptake may influence uACR results.
      • Gambaro G.
      • Baggio B.
      • Cicerello E.
      • Mastrosimone S.
      • Marzaro G.
      • Borsatti A.
      • Crepaldi G.
      Abnormal erythrocyte charge in diabetes mellitus: link with microalbuminuria.
      In summary, the clinical importance of albuminuria is now well established, but the absence of albuminuria in early rodent models of disease does not exclude increased albumin passage across the GFB.
      Figure thumbnail gr2
      Figure 2A live perfused mouse glomerulus under anesthesia was imaged using multiphoton microscopy. The image was taken 10 minutes after an intravenous bolus of fluorescein isothiocyanate–wheat germ agglutinin. This lectin binds to sialic acid residues within the glycocalyx (labeled green). The plasma within the capillary loops was labeled red with Alexa Flur–conjugated albumin. The dark areas within the capillary loops represent circulating blood cells. The glomerular endothelial glycocalyx is a continuous layer within the glomerular capillaries that contribute to the restriction of macromolecule and water leakage from the glomerulus into the Bowman space. Scale bar = 50 μm.
      Relatively little research has been conducted studying glycocalyx-dependent permeability changes in the systemic vasculature. In part, this lack of research is attributable to the complex methods needed to study glycocalyx-specific changes. Previous studies in mesenteric microvessels by intravital confocal microscopy have demonstrated that neuraminidase, which disrupts sialic acid residues and reduces glycocalyx depth, increases microvessel permeability.
      • Betteridge K.B.
      • Arkill K.P.
      • Neal C.R.
      • Harper S.J.
      • Foster R.R.
      • Satchell S.C.
      • Bates D.O.
      • Salmon A.H.J.
      Sialic acids regulate microvessel permeability, revealed by novel in vivo studies of endothelial glycocalyx structure and function.
      Work that compares the depth of the endothelial glycocalyx within the continuous capillaries of the systemic and pulmonary vasculature highlighted significant variability in glycocalyx depth.
      • Ando Y.
      • Okada H.
      • Takemura G.
      • Suzuki K.
      • Takada C.
      • Tomita H.
      • Zaikokuji R.
      • Hotta Y.
      • Miyazaki N.
      • Yano H.
      • Muraki I.
      • Kuroda A.
      • Fukuda H.
      • Kawasaki Y.
      • Okamoto H.
      • Kawaguchi T.
      • Watanabe T.
      • Doi T.
      • Yoshida T.
      • Ushikoshi H.
      • Yoshida S.
      • Ogura S.
      Brain-specific ultrastructure of capillary endothelial glycocalyx and its possible contribution for blood brain barrier.
      To date, little is known about whether the composition of the glycocalyx varies among these sites. It seems likely that the glycocalyx within discrete areas of the vasculature is specialized, adapting to perform the combination of tasks needed at each tissue site optimally. Variability in the glycocalyx structure means that pathologic insults may not affect all areas of the glycocalyx equally. Specialization is also seen in the endothelial monolayer itself. The double barrier concept was first introduced by Rehm et al
      • Rehm M.
      • Zahler S.
      • Lotsch M.
      • Welsch U.
      • Conzen P.
      • Jacob M.
      • Becker B.F.
      Endothelial glycocalyx as an additional barrier determining extravasation of 6% hydroxyethyl starch or 5% albumin solutions in the coronary vascular bed.
      and is illustrated in Figure 3. They found in guinea pig hearts that simultaneous disruption of both the cellular barrier (using ischemia or histamine) and glycocalyx damage (using heparinase) was needed to increase coronary vessel leakage.
      • Rehm M.
      • Zahler S.
      • Lotsch M.
      • Welsch U.
      • Conzen P.
      • Jacob M.
      • Becker B.F.
      Endothelial glycocalyx as an additional barrier determining extravasation of 6% hydroxyethyl starch or 5% albumin solutions in the coronary vascular bed.
      This work led to the hypothesis that glycocalyx damage overlying a tight cellular barrier will have minimal (direct) influence on monolayer or vessel wall permeability. In contrast, identical glycocalyx damage overlying a leaky cellular monolayer will result in rapid measurable increases in vessel permeability. When glycocalyx-dependent permeability changes within the systemic vasculature are studied, it is therefore important to consider how both glycocalyx structural adaptations and the underlying endothelial cell phenotype will influence detectable permeability changes.
      Figure thumbnail gr3
      Figure 3The double barrier concept. A: In health, both the intact glycocalyx and a tight endothelial monolayer can limit vascular wall permeability to macromolecules (including albumin). B: When damage to the endothelial barrier is limited to the glycocalyx, in areas of the vasculature where a tight cellular monolayer exists, an intact second barrier remains. This tight endothelial monolayer continues to limit macromolecule transit. In such areas, it is currently not possible to directly measure the contribution of the endothelial glycocalyx to the vessel wall permeability. C: When damage affects both the glycocalyx and the permeability of the underlying monolayer, marked increases in the vascular wall permeability will result, but again calculating the relative contribution of the glycocalyx to the vessel wall permeability is not possible. In summary, to directly measure the contribution of the glycocalyx to vessel permeability, vessels that lack a tight endothelial monolayer must be selected until new techniques are developed that can measure macromolecule concentrations in the subglycocalyx space.
      To measure the contribution of the endothelial glycocalyx to vessel permeability, comparisons have generally been made after a glycocalyx insult. Enzymatic removal or genetic knockdown of a specific glycocalyx component is commonly used for this purpose. However, the method used can significantly alter the results of such studies. Rapid removal of HS, using human heparanase or bacterial heparinase III, increased albumin passage across endothelial monolayers.
      • Singh A.
      • Satchell S.C.
      • Neal C.R.
      • McKenzie E.A.
      • Tooke J.E.
      • Mathieson P.W.
      Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.
      However, knockout of the HS proteoglycan syndecan 1 did not result in albuminuria in mice.
      • Rops A.L.
      • Gotte M.
      • Baselmans M.H.
      • van den Hoven M.J.
      • Steenbergen E.J.
      • Lensen J.F.
      • Wijnhoven T.J.
      • Cevikbas F.
      • van den Heuvel L.P.
      • van Kuppevelt T.H.
      • Berden J.H.
      • van der Vlag J.
      Syndecan-1 deficiency aggravates anti-glomerular basement membrane nephritis.
      In addition, mice that lacked endothelial N-deacetylase and N-sulfotransferase, a key enzyme in modifying HS chains, did not become albuminuric.
      • Garsen M.
      • Rops A.L.
      • Rabelink T.J.
      • Berden J.H.
      • van der Vlag J.
      The role of heparanase and the endothelial glycocalyx in the development of proteinuria.
      A similar pattern of results was seen in experiments that target HA. Hyaluronidase, an enzyme that degrades HA, has a plasma half-life of approximately 3 minutes before being taken up in the liver via a mannose-dependent mechanism.
      • Earnshaw J.S.
      • Curtis C.G.
      • Powell G.M.
      • Dodgson K.S.
      • Olavesen A.H.
      • Gacesa P.
      The fate of intravenously administered highly purified bovine testicular hyaluronidase (Hyalosidase) in the rat.
      The short plasma half-life and the relatively high molecular weight (55 to 61 kDa) of hyaluronidase result in a high level of glycocalyx-degrading activity with limited off-target effects. Several studies found that removing HA from the endothelial glycocalyx, using hyaluronidase, increased glomerular albumin transit 5.6-fold, a figure consistent with findings.
      • Landsverk S.A.
      • Tsai A.G.
      • Cabrales P.
      • Intaglietta M.
      Impact of enzymatic degradation of the endothelial glycocalyx on vascular permeability in an awake hamster model.
      • Jeansson M.
      • Haraldsson B.
      Glomerular size and charge selectivity in the mouse after exposure to glucosaminoglycan-degrading enzymes.
      • Onions K.L.
      • Gamez M.
      • Buckner N.R.
      • Baker S.L.
      • Betteridge K.B.
      • Desideri S.
      • Dallyn B.P.
      • Ramnath R.D.
      • Neal C.R.
      • Farmer L.K.
      • Mathieson P.W.
      • Gnudi L.
      • Alitalo K.
      • Bates D.O.
      • Salmon A.H.J.
      • Welsh G.I.
      • Satchell S.C.
      • Foster R.R.
      VEGFC reduces glomerular albumin permeability and protects against alterations in VEGF receptor expression in diabetic nephropathy.
      Tamoxifen-induced endothelial-specific knockdown of HA synthase 2 (a membrane-bound HA synthesis enzyme) also resulted in significant albuminuria from 4 weeks after induction, which persisted to 12 weeks (experimental end point).
      • van den Berg B.M.
      • Wang G.
      • Boels M.G.S.
      • Avramut M.C.
      • Jansen E.
      • Sol W.
      • Lebrin F.
      • Jan van Zonneveld A.
      • de Koning E.J.P.
      • Vink H.
      • Grone H.J.
      • Carmeliet P.
      • van der Vlag J.
      • Rabelink T.J.
      Glomerular function and structural integrity depend on hyaluronan synthesis by glomerular endothelium.
      In contrast, Dane et al
      • Dane M.J.
      • van den Berg B.M.
      • Avramut M.C.
      • Faas F.G.
      • van der Vlag J.
      • Rops A.L.
      • Ravelli R.B.
      • Koster B.J.
      • van Zonneveld A.J.
      • Vink H.
      • Rabelink T.J.
      Glomerular endothelial surface layer acts as a barrier against albumin filtration.
      found that 4 weeks of hyaluronidase infusion did not result in measurable albuminuria, although they found increased glomerular albumin leakage in 90% of glomeruli.
      Although this work, again, highlights the fact that albuminuria is a poor measure of low-level glomerular albumin leakage, it also suggests that the differences in time scale, as well as the precise component targeted, may influence glycocalyx permeability changes. Knockdown in genetic models may have resulted in compensatory adaptations (eg, up-regulation of other glycocalyx components). In contrast, the rapid enzymatic removal of glycocalyx components may prevent adaptations from occurring before functional assessments are made.
      • Garsen M.
      • Rops A.L.
      • Rabelink T.J.
      • Berden J.H.
      • van der Vlag J.
      The role of heparanase and the endothelial glycocalyx in the development of proteinuria.
      ,
      • Landsverk S.A.
      • Tsai A.G.
      • Cabrales P.
      • Intaglietta M.
      Impact of enzymatic degradation of the endothelial glycocalyx on vascular permeability in an awake hamster model.
      • Jeansson M.
      • Haraldsson B.
      Glomerular size and charge selectivity in the mouse after exposure to glucosaminoglycan-degrading enzymes.
      • Onions K.L.
      • Gamez M.
      • Buckner N.R.
      • Baker S.L.
      • Betteridge K.B.
      • Desideri S.
      • Dallyn B.P.
      • Ramnath R.D.
      • Neal C.R.
      • Farmer L.K.
      • Mathieson P.W.
      • Gnudi L.
      • Alitalo K.
      • Bates D.O.
      • Salmon A.H.J.
      • Welsh G.I.
      • Satchell S.C.
      • Foster R.R.
      VEGFC reduces glomerular albumin permeability and protects against alterations in VEGF receptor expression in diabetic nephropathy.
      ,
      • Gil N.
      • Goldberg R.
      • Neuman T.
      • Garsen M.
      • Zcharia E.
      • Rubinstein A.M.
      • van Kuppevelt T.
      • Meirovitz A.
      • Pisano C.
      • Li J.P.
      • van der Vlag J.
      • Vlodavsky I.
      • Elkin M.
      Heparanase is essential for the development of diabetic nephropathy in mice.
      In addition, the rapid removal of a single structural component of the glycocalyx may leave the structure as a whole vulnerable to further nonspecific destruction by the shear forces applied by the circulation. Another contrast between enzymatic removal and genetic knockdown of a glycocalyx component is the generation of circulating glycocalyx fragments. The enzymatic release of fragments is also nonspecific. Heparanase increased syndecan 1 and 4 loss from the glycocalyx structure, possibly by exposing cleavage sites for the actions of other circulating enzymes.
      • Jung O.
      • Trapp-Stamborski V.
      • Purushothaman A.
      • Jin H.
      • Wang H.
      • Sanderson R.D.
      • Rapraeger A.C.
      Heparanase-induced shedding of syndecan-1/CD138 in myeloma and endothelial cells activates VEGFR2 and an invasive phenotype: prevention by novel synstatins.
      The influence of active signaling fragments released after glycocalyx enzymatic degradation on vessel permeability has not been directly investigated, but they represent a potentially important pharmacologic target.
      • Kim E.Y.
      • Roshanravan H.
      • Dryer S.E.
      Syndecan-4 ectodomain evokes mobilization of podocyte TRPC6 channels and their associated pathways: an essential role for integrin signaling.
      ,
      • Zhang X.
      • Han X.
      • Xia K.
      • Xu Y.
      • Yang Y.
      • Oshima K.
      • Haeger S.M.
      • Perez M.J.
      • McMurtry S.A.
      • Hippensteel J.A.
      • Ford J.A.
      • Herson P.S.
      • Liu J.
      • Schmidt E.P.
      • Linhardt R.J.
      Circulating heparin oligosaccharides rapidly target the hippocampus in sepsis, potentially impacting cognitive functions.
      In summary, when the glycocalyx is studied, careful consideration needs to be given to the chosen method and tissue. Although the glomerular endothelial glycocalyx lends itself to studying permeability, the uACR should be interpreted with caution. Measures of systemic permeability need to be considered in context. A double barrier can exist, and in the absence of specific manipulations, the relative contributions of the components are difficult to establish. With the development of more sensitive and specific methods to assess permeability, the role of individual glycocalyx components are expected to be better understood in the next decade.
      • Desideri S.
      • Onions K.L.
      • Qiu Y.
      • Ramnath R.D.
      • Butler M.J.
      • Neal C.R.
      • King M.L.R.
      • Salmon A.E.
      • Saleem M.A.
      • Welsh G.I.
      • Michel C.C.
      • Satchell S.C.
      • Salmon A.H.J.
      • Foster R.R.
      A novel assay provides sensitive measurement of physiologically relevant changes in albumin permeability in isolated human and rodent glomeruli.

      The Relevance of Glycocalyx Permeability Changes in Disease

      Diabetes

      There is evidence that the diabetic milieu affects the vasculature globally. Nieuwdorp et al
      • Nieuwdorp M.
      • van Haeften T.W.
      • Gouverneur M.C.
      • Mooij H.L.
      • van Lieshout M.H.
      • Levi M.
      • Meijers J.C.
      • Holleman F.
      • Hoekstra J.B.
      • Vink H.
      • Kastelein J.J.
      • Stroes E.S.
      Loss of endothelial glycocalyx during acute hyperglycemia coincides with endothelial dysfunction and coagulation activation in vivo.
      measured the total glycocalyx volume by comparing the volume of distribution of erythrocytes and dextran 40 in healthy volunteers. They found that 6 hours of hyperglycemia reduced the glycocalyx volume to 50% of the baseline value. More recently, an intervention trial found that intensification of glycemic control in poorly controlled type 2 diabetic patients during 12 months (glycated hemoglobin reduction from 74 to 54 mol/mol) was associated with a significant increase in glycocalyx depth compared with baseline vaulues.
      • Lambadiari V.
      • Pavlidis G.
      • Kousathana F.
      • Maratou E.
      • Georgiou D.
      • Andreadou I.
      • Kountouri A.
      • Varoudi M.
      • Balampanis K.
      • Parissis J.
      • Triantafyllidi H.
      • Katogiannis K.
      • Birba D.
      • Lekakis J.
      • Dimitriadis G.
      • Ikonomidis I.
      Effects of different antidiabetic medications on endothelial glycocalyx, myocardial function, and vascular function in type 2 diabetic patients: one year follow-up study.
      Glycocalyx depth in this study was measured using side-stream dark field imaging to assess the perfused boundary region depth (a measure inversely proportional to glycocalyx thickness) on sublingual vessels, which suggests that systemic glycocalyx damage in early diabetes is at least partially reversible.
      • Lambadiari V.
      • Pavlidis G.
      • Kousathana F.
      • Maratou E.
      • Georgiou D.
      • Andreadou I.
      • Kountouri A.
      • Varoudi M.
      • Balampanis K.
      • Parissis J.
      • Triantafyllidi H.
      • Katogiannis K.
      • Birba D.
      • Lekakis J.
      • Dimitriadis G.
      • Ikonomidis I.
      Effects of different antidiabetic medications on endothelial glycocalyx, myocardial function, and vascular function in type 2 diabetic patients: one year follow-up study.
      Changes to the endothelial glycocalyx occur early in the disease course, suggesting that they could represent a valuable direct therapeutic target.
      • Desideri S.
      • Onions K.L.
      • Qiu Y.
      • Ramnath R.D.
      • Butler M.J.
      • Neal C.R.
      • King M.L.R.
      • Salmon A.E.
      • Saleem M.A.
      • Welsh G.I.
      • Michel C.C.
      • Satchell S.C.
      • Salmon A.H.J.
      • Foster R.R.
      A novel assay provides sensitive measurement of physiologically relevant changes in albumin permeability in isolated human and rodent glomeruli.
      Electron microscopy on glomerular capillaries showed that the percentage surface covered by the glycocalyx is one of the earliest detectable vascular changes in diabetes.
      • Desideri S.
      • Onions K.L.
      • Qiu Y.
      • Ramnath R.D.
      • Butler M.J.
      • Neal C.R.
      • King M.L.R.
      • Salmon A.E.
      • Saleem M.A.
      • Welsh G.I.
      • Michel C.C.
      • Satchell S.C.
      • Salmon A.H.J.
      • Foster R.R.
      A novel assay provides sensitive measurement of physiologically relevant changes in albumin permeability in isolated human and rodent glomeruli.
      Consistent with this, Targosz-Korecka et al
      • Targosz-Korecka M.
      • Jaglarz M.
      • Malek-Zietek K.E.
      • Gregorius A.
      • Zakrzewska A.
      • Sitek B.
      • Rajfur Z.
      • Chlopicki S.
      • Szymonski M.
      AFM-based detection of glycocalyx degradation and endothelial stiffening in the db/db mouse model of diabetes.
      used atomic force microscopy to map the glycocalyx on diabetic db/db mouse aortas. They found that endothelial glycocalyx coverage was significantly reduced by week 11.
      • Targosz-Korecka M.
      • Jaglarz M.
      • Malek-Zietek K.E.
      • Gregorius A.
      • Zakrzewska A.
      • Sitek B.
      • Rajfur Z.
      • Chlopicki S.
      • Szymonski M.
      AFM-based detection of glycocalyx degradation and endothelial stiffening in the db/db mouse model of diabetes.
      At this time point, significant reductions in the glycocalyx depth were not detected.
      • Targosz-Korecka M.
      • Jaglarz M.
      • Malek-Zietek K.E.
      • Gregorius A.
      • Zakrzewska A.
      • Sitek B.
      • Rajfur Z.
      • Chlopicki S.
      • Szymonski M.
      AFM-based detection of glycocalyx degradation and endothelial stiffening in the db/db mouse model of diabetes.
      Although glycocalyx damage occurs early in diabetic disease, the clinical manifestations of this damage may be significantly delayed and remain dependent on the function of the vascular bed studied.

      Diabetic Nephropathy

      The hallmark of diabetic nephropathy (DN) is microalbuminuria. In diabetes, uACR is a key screening test, predicting patients at the highest risk of progressive renal and vascular disease.
      • Satchell S.C.
      • Tooke J.E.
      What is the mechanism of microalbuminuria in diabetes: a role for the glomerular endothelium?.
      Electron microscopy to study glomerular structural changes in patients with diabetes found that loss of endothelial fenestration area correlated with the uACR more strongly than podocyte detachment.
      • Toyoda M.
      • Najafian B.
      • Kim Y.
      • Caramori M.L.
      • Mauer M.
      Podocyte detachment and reduced glomerular capillary endothelial fenestration in human type 1 diabetic nephropathy.
      These data suggest that endothelial damage is likely to be one of the key steps that results in albuminuria in diabetes.
      • Toyoda M.
      • Najafian B.
      • Kim Y.
      • Caramori M.L.
      • Mauer M.
      Podocyte detachment and reduced glomerular capillary endothelial fenestration in human type 1 diabetic nephropathy.
      The authors of this study did not examine the glycocalyx, but it seems likely that the endothelial cell damage, resulting in fenestration loss, was the result of altered vascular endothelial growth factor (VEGF) signaling.
      • Tschulakow A.
      • Christner S.
      • Julien S.
      • Ludinsky M.
      • van der Giet M.
      • Schraermeyer U.
      Effects of a single intravitreal injection of aflibercept and ranibizumab on glomeruli of monkeys.
      ,
      • Eleftheriadis T.
      • Antoniadi G.
      • Pissas G.
      • Liakopoulos V.
      • Stefanidis I.
      The renal endothelium in diabetic nephropathy.
      Altered VEGF signaling results in glycocalyx loss.
      • Oltean S.
      • Qiu Y.
      • Ferguson J.K.
      • Stevens M.
      • Neal C.
      • Russell A.
      • Kaura A.
      • Arkill K.P.
      • Harris K.
      • Symonds C.
      • Lacey K.
      • Wijeyaratne L.
      • Gammons M.
      • Wylie E.
      • Hulse R.P.
      • Alsop C.
      • Cope G.
      • Damodaran G.
      • Betteridge K.B.
      • Ramnath R.
      • Satchell S.C.
      • Foster R.R.
      • Ballmer-Hofer K.
      • Donaldson L.F.
      • Barratt J.
      • Baelde H.J.
      • Harper S.J.
      • Bates D.O.
      • Salmon A.H.
      Vascular endothelial growth factor-A165b is protective and restores endothelial glycocalyx in diabetic nephropathy.
      ,
      • Onions K.L.
      • Gamez M.
      • Buckner N.R.
      • Baker S.L.
      • Betteridge K.B.
      • Desideri S.
      • Dallyn B.P.
      • Ramnath R.D.
      • Neal C.R.
      • Farmer L.K.
      • Mathieson P.W.
      • Gnudi L.
      • Alitalo K.
      • Bates D.O.
      • Salmon A.H.J.
      • Welsh G.I.
      • Satchell S.C.
      • Foster R.R.
      VEGFC reduces glomerular albumin permeability and protects against alterations in VEGF receptor expression in diabetic nephropathy.
      ,
      • Foster R.R.
      • Armstrong L.
      • Baker S.
      • Wong D.W.
      • Wylie E.C.
      • Ramnath R.
      • Jenkins R.
      • Singh A.
      • Steadman R.
      • Welsh G.I.
      • Mathieson P.W.
      • Satchell S.C.
      Glycosaminoglycan regulation by VEGFA and VEGFC of the glomerular microvascular endothelial cell glycocalyx in vitro.
      Given the evidence that supports the importance of the glycocalyx in glomerular filtration, it seems likely that glycocalyx damage contributed to the microalbuminuria seen in these patients.
      Experimental evidence supports this view. On cultured monolayers of conditionally immortalized glomerular endothelial cells, high concentrations of glucose result in a marked reduction in the biosynthesis of the glycocalyx component HS, with an associated increase in albumin permeability.
      • Singh A.
      • Friden V.
      • Dasgupta I.
      • Foster R.R.
      • Welsh G.I.
      • Tooke J.E.
      • Haraldsson B.
      • Mathieson P.W.
      • Satchell S.C.
      High glucose causes dysfunction of the human glomerular endothelial glycocalyx.
      Jeansson et al
      • Jeansson M.
      • Granqvist A.B.
      • Nystrom J.S.
      • Haraldsson B.
      Functional and molecular alterations of the glomerular barrier in long-term diabetes in mice.
      confirmed that increased glomerular albumin leakage contributed to the albuminuria detected in animal models of diabetes by cooling isolated kidneys to 8°C to limit tubular effects. Loss of HA and HS from the glomerular capillary wall has been confirmed in streptozocin-induced diabetic rats and Zucker fatty rats, suggesting that glycocalyx damage is likely to contribute to the increase in albumin filtration seen in these disease models.
      • Satoh M.
      • Kobayashi S.
      • Kuwabara A.
      • Tomita N.
      • Sasaki T.
      • Kashihara N.
      In vivo visualization of glomerular microcirculation and hyperfiltration in streptozotocin-induced diabetic rats.
      ,
      • Kuwabara A.
      • Satoh M.
      • Tomita N.
      • Sasaki T.
      • Kashihara N.
      Deterioration of glomerular endothelial surface layer induced by oxidative stress is implicated in altered permeability of macromolecules in Zucker fatty rats.
      More recently, it is reported that diabetic glomerular glycocalyx damage can be prevented in the mouse streptozocin model of diabetes by administering a matrix metalloprotease (MMP) 2/9 inhibitor.
      • Ramnath R.D.
      • Butler M.J.
      • Newman G.
      • Desideri S.
      • Russell A.
      • Lay A.C.
      • Neal C.R.
      • Qiu Y.
      • Fawaz S.
      • Onions K.
      • Gamez M.
      • Crompton M.
      • Michie C.
      • Finch N.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Satchell S.C.
      Blocking matrix metalloproteinase-mediated syndecan-4 shedding restores the endothelial glycocalyx and glomerular filtration barrier function in early diabetic kidney disease.
      A daily injection of MMP2/9 inhibitor reduced syndecan 4 loss from the glycocalyx and significantly reduced albuminuria by reducing glomerular albumin permeability.
      • Ramnath R.D.
      • Butler M.J.
      • Newman G.
      • Desideri S.
      • Russell A.
      • Lay A.C.
      • Neal C.R.
      • Qiu Y.
      • Fawaz S.
      • Onions K.
      • Gamez M.
      • Crompton M.
      • Michie C.
      • Finch N.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Satchell S.C.
      Blocking matrix metalloproteinase-mediated syndecan-4 shedding restores the endothelial glycocalyx and glomerular filtration barrier function in early diabetic kidney disease.
      As previously reported, high-power electron microscopy images of the GFB and glomerular endothelial glycocalyx suggested that glycocalyx changes had occurred before other significant changes to the GFB.
      • Onions K.L.
      • Gamez M.
      • Buckner N.R.
      • Baker S.L.
      • Betteridge K.B.
      • Desideri S.
      • Dallyn B.P.
      • Ramnath R.D.
      • Neal C.R.
      • Farmer L.K.
      • Mathieson P.W.
      • Gnudi L.
      • Alitalo K.
      • Bates D.O.
      • Salmon A.H.J.
      • Welsh G.I.
      • Satchell S.C.
      • Foster R.R.
      VEGFC reduces glomerular albumin permeability and protects against alterations in VEGF receptor expression in diabetic nephropathy.
      ,
      • Desideri S.
      • Onions K.L.
      • Qiu Y.
      • Ramnath R.D.
      • Butler M.J.
      • Neal C.R.
      • King M.L.R.
      • Salmon A.E.
      • Saleem M.A.
      • Welsh G.I.
      • Michel C.C.
      • Satchell S.C.
      • Salmon A.H.J.
      • Foster R.R.
      A novel assay provides sensitive measurement of physiologically relevant changes in albumin permeability in isolated human and rodent glomeruli.
      ,
      • Ramnath R.D.
      • Butler M.J.
      • Newman G.
      • Desideri S.
      • Russell A.
      • Lay A.C.
      • Neal C.R.
      • Qiu Y.
      • Fawaz S.
      • Onions K.
      • Gamez M.
      • Crompton M.
      • Michie C.
      • Finch N.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Satchell S.C.
      Blocking matrix metalloproteinase-mediated syndecan-4 shedding restores the endothelial glycocalyx and glomerular filtration barrier function in early diabetic kidney disease.
      An isolated glomerular albumin permeability assay used early in the diabetic disease course confirmed that glomerular glycocalyx loss is associated with increased glomerular albumin permeability in both the mouse and rat streptozocin diabetic models.
      • Onions K.L.
      • Gamez M.
      • Buckner N.R.
      • Baker S.L.
      • Betteridge K.B.
      • Desideri S.
      • Dallyn B.P.
      • Ramnath R.D.
      • Neal C.R.
      • Farmer L.K.
      • Mathieson P.W.
      • Gnudi L.
      • Alitalo K.
      • Bates D.O.
      • Salmon A.H.J.
      • Welsh G.I.
      • Satchell S.C.
      • Foster R.R.
      VEGFC reduces glomerular albumin permeability and protects against alterations in VEGF receptor expression in diabetic nephropathy.
      ,
      • Ramnath R.D.
      • Butler M.J.
      • Newman G.
      • Desideri S.
      • Russell A.
      • Lay A.C.
      • Neal C.R.
      • Qiu Y.
      • Fawaz S.
      • Onions K.
      • Gamez M.
      • Crompton M.
      • Michie C.
      • Finch N.
      • Coward R.J.
      • Welsh G.I.
      • Foster R.R.
      • Satchell S.C.
      Blocking matrix metalloproteinase-mediated syndecan-4 shedding restores the endothelial glycocalyx and glomerular filtration barrier function in early diabetic kidney disease.
      This unique assay has the advantage of studying glomerular permeability in the absence of hemodynamic influence, ensuring that alterations in the glomerular perfusion pressure will not influence permeability measurements. In addition, this assay allows us to study changes in glomerular permeability in isolation from the renal tubules and hence confounding tubular albumin reuptake. This assay found that glomerular albumin leakage may be significantly increased before an increase in uACR is detectable. In summary, the current evidence suggests that glycocalyx injury is likely to contribute to the early pathogenesis of DN.

      Diabetic Retinopathy

      Diabetic retinopathy (DR), like DN, is a manifestation of diabetic microvascular damage. The retina is a continuation of the central nervous system with a blood retina barrier. The pathogenesis of DR has been reviewed in detail elsewhere,
      • Kowluru R.A.
      • Mishra M.
      Regulation of matrix metalloproteinase in the pathogenesis of diabetic retinopathy.
      ,
      • Wong T.Y.
      • Cheung C.M.
      • Larsen M.
      • Sharma S.
      • Simo R.
      Diabetic retinopathy.
      but the role of the retinal endothelial glycocalyx and its impairment in diabetes is an area of ongoing research. DR is characterized by microaneurysms, leukocyte-endothelial adhesion, hemorrhage, capillary occlusion, neovascularization, and increased permeability.
      • Leskova W.
      • Pickett H.
      • Eshaq R.S.
      • Shrestha B.
      • Pattillo C.B.
      • Harris N.R.
      Effect of diabetes and hyaluronidase on the retinal endothelial glycocalyx in mice.
      Diabetic rats have a significantly thinner retinal endothelial glycocalyx compared with control rats (28.3 versus 60.2 nmol/L, P < 0.01), measured using transmission electron microscopy.
      • Kumase F.
      • Morizane Y.
      • Mohri S.
      • Takasu I.
      • Ohtsuka A.
      • Ohtsuki H.
      Glycocalyx degradation in retinal and choroidal capillary endothelium in rats with diabetes and hypertension.
      In addition, in the Akita mouse model of type 1 diabetes, anesthetized diabetic mice have a significantly thinner glycocalyx in retinal arterioles.
      • Leskova W.
      • Pickett H.
      • Eshaq R.S.
      • Shrestha B.
      • Pattillo C.B.
      • Harris N.R.
      Effect of diabetes and hyaluronidase on the retinal endothelial glycocalyx in mice.
      In human volunteers, combined fluorescein and indocyanine green angiography also demonstrated a significantly thinner glycocalyx in patients with type 2 diabetes compared with healthy controls.
      • Broekhuizen L.N.
      • Lemkes B.A.
      • Mooij H.L.
      • Meuwese M.C.
      • Verberne H.
      • Holleman F.
      • Schlingemann R.O.
      • Nieuwdorp M.
      • Stroes E.S.
      • Vink H.
      Effect of sulodexide on endothelial glycocalyx and vascular permeability in patients with type 2 diabetes mellitus.
      In the same study, the authors measured the rate at which 125I-labeled albumin left the plasma, confirming an increased rate of loss in patients with diabetes, which is consistent with an increase in systemic albumin permeability.
      • Broekhuizen L.N.
      • Lemkes B.A.
      • Mooij H.L.
      • Meuwese M.C.
      • Verberne H.
      • Holleman F.
      • Schlingemann R.O.
      • Nieuwdorp M.
      • Stroes E.S.
      • Vink H.
      Effect of sulodexide on endothelial glycocalyx and vascular permeability in patients with type 2 diabetes mellitus.
      In streptozocin-induced diabetic rats, retinal permeability was also increased (measured by fluorescein isothiocyanate–dextran accumulation in retinal tissue).
      • Niu T.
      • Zhao M.
      • Jiang Y.
      • Xing X.
      • Shi X.
      • Cheng L.
      • Jin H.
      • Liu K.
      Endomucin restores depleted endothelial glycocalyx in the retinas of streptozotocin-induced diabetic rats.
      In the same study, overexpression of endomucin prevented damage to the endothelial glycocalyx, reduced leukocyte adhesion, and reduced retinal vessel permeability.
      • Niu T.
      • Zhao M.
      • Jiang Y.
      • Xing X.
      • Shi X.
      • Cheng L.
      • Jin H.
      • Liu K.
      Endomucin restores depleted endothelial glycocalyx in the retinas of streptozotocin-induced diabetic rats.
      A reduction in tight junctions within the interendothelial cleft of retinal vessels has also been reported early in diabetic disease, making the direct contribution of glycocalyx injury impossible to assess.
      • Niu T.
      • Zhao M.
      • Jiang Y.
      • Xing X.
      • Shi X.
      • Cheng L.
      • Jin H.
      • Liu K.
      Endomucin restores depleted endothelial glycocalyx in the retinas of streptozotocin-induced diabetic rats.
      ,
      • Wallow I.H.
      • Engerman R.L.
      Permeability and patency of retinal blood vessels in experimental diabetes.
      However, it seems likely that glycocalyx injury is contributing to the pathogenesis of DR through direct increases in permeability and by facilitating leukocyte adhesion; glycocalyx preservation therefore remains a promising therapeutic option.
      • Niu T.
      • Zhao M.
      • Jiang Y.
      • Xing X.
      • Shi X.
      • Cheng L.
      • Jin H.
      • Liu K.
      Endomucin restores depleted endothelial glycocalyx in the retinas of streptozotocin-induced diabetic rats.

      Sepsis

      During sepsis, the endothelial glycocalyx becomes thinner and the cover sparser, contributing to tissue edema.
      • Uchimido R.
      • Schmidt E.P.
      • Shapiro N.I.
      The glycocalyx: a novel diagnostic and therapeutic target in sepsis.
      ,
      • Chelazzi C.
      • Villa G.
      • Mancinelli P.
      • De Gaudio A.R.
      • Adembri C.
      Glycocalyx and sepsis-induced alterations in vascular permeability.
      In mice, intravenous lipopolysaccharide injection results in activation of the innate immune system, shock, lactic acidosis, myocardial impairment, and increased levels of circulating tumor necrosis factor-α and IL-6.
      • Fink M.P.
      Animal models of sepsis.
      After lipopolysaccharide administration, mice have significantly thinner aortic glycocalyx compared with controls.
      • Wiesinger A.
      • Peters W.
      • Chappell D.
      • Kentrup D.
      • Reuter S.
      • Pavenstadt H.
      • Oberleithner H.
      • Kumpers P.
      Nanomechanics of the endothelial glycocalyx in experimental sepsis.
      Elevated levels of tumor necrosis factor-α are likely to contribute to glycocalyx damage in this model via increased MMP activity and syndecan loss.
      • Ramnath R.
      • Foster R.R.
      • Qiu Y.
      • Cope G.
      • Butler M.J.
      • Salmon A.H.
      • Mathieson P.W.
      • Coward R.J.
      • Welsh G.I.
      • Satchell S.C.
      Matrix metalloproteinase 9-mediated shedding of syndecan 4 in response to tumor necrosis factor alpha: a contributor to endothelial cell glycocalyx dysfunction.
      In human volunteers, a low-dose endotoxin model resulted in a significant reduction in the depth of the sublingual vessel glycocalyx (measured using side stream imaging) and a concurrent elevation in plasma HA, suggesting glycocalyx shedding.
      • Nieuwdorp M.
      • Meuwese M.C.
      • Mooij H.L.
      • van Lieshout M.H.
      • Hayden A.
      • Levi M.
      • Meijers J.C.
      • Ince C.
      • Kastelein J.J.
      • Vink H.
      • Stroes E.S.
      Tumor necrosis factor-alpha inhibition protects against endotoxin-induced endothelial glycocalyx perturbation.
      Multiple circulating biomarkers of glycocalyx shedding have been studied in humans as markers of sepsis, including syndecan 1, HS, and HA.
      • Uchimido R.
      • Schmidt E.P.
      • Shapiro N.I.
      The glycocalyx: a novel diagnostic and therapeutic target in sepsis.
      ,
      • Steppan J.
      • Hofer S.
      • Funke B.
      • Brenner T.
      • Henrich M.
      • Martin E.
      • Weitz J.
      • Hofmann U.
      • Weigand M.A.
      Sepsis and major abdominal surgery lead to flaking of the endothelial glycocalix.
      ,
      • Becker B.F.
      • Jacob M.
      • Leipert S.
      • Salmon A.H.
      • Chappell D.
      Degradation of the endothelial glycocalyx in clinical settings: searching for the sheddases.
      It is hoped that in the future these targets may provide practitioners with additional diagnostic and prognostic information. In sepsis, circulating sheddases, including A disintegrin and metalloproteinase 15, heparanase, and MMP2/9, have been implicated in the degradation of the glycocalyx.
      • Becker B.F.
      • Jacob M.
      • Leipert S.
      • Salmon A.H.
      • Chappell D.
      Degradation of the endothelial glycocalyx in clinical settings: searching for the sheddases.
      • Yang X.
      • Meegan J.E.
      • Jannaway M.
      • Coleman D.C.
      • Yuan S.Y.
      A disintegrin and metalloproteinase 15-mediated glycocalyx shedding contributes to vascular leakage during inflammation.
      • Lygizos M.I.
      • Yang Y.
      • Altmann C.J.
      • Okamura K.
      • Hernando A.A.
      • Perez M.J.
      • Smith L.P.
      • Koyanagi D.E.
      • Gandjeva A.
      • Bhargava R.
      • Tuder R.M.
      • Faubel S.
      • Schmidt E.P.
      Heparanase mediates renal dysfunction during early sepsis in mice.
      • Cui N.
      • Wang H.
      • Long Y.
      • Su L.
      • Liu D.
      Dexamethasone suppressed LPS-induced matrix metalloproteinase and its effect on endothelial glycocalyx shedding.
      It seems likely therefore that the albuminuria observed in sepsis, at least in part, results from glycocalyx injury and increased glomerular albumin permeability.
      • Salmon A.H.
      • Satchell S.C.
      Endothelial glycocalyx dysfunction in disease: albuminuria and increased microvascular permeability.
      ,
      • Adembri C.
      • Sgambati E.
      • Vitali L.
      • Selmi V.
      • Margheri M.
      • Tani A.
      • Bonaccini L.
      • Nosi D.
      • Caldini A.L.
      • Formigli L.
      • De Gaudio A.R.
      Sepsis induces albuminuria and alterations in the glomerular filtration barrier: a morphofunctional study in the rat.
      • Koike K.
      • Aiboshi J.
      • Shinozawa Y.
      • Sekine K.
      • Endo T.
      • Yamamoto Y.
      Correlation of glomerular permeability, endothelial injury, and postoperative multiple organ dysfunction.
      • De Gaudio A.R.
      • Adembri C.
      • Grechi S.
      • Novelli G.P.
      Microalbuminuria as an early index of impairment of glomerular permeability in postoperative septic patients.
      Increased pulmonary vascular permeability in sepsis manifests as acute lung injury and acute respiratory distress syndrome, which can occur early in sepsis.
      • Matthay M.A.
      • Zemans R.L.
      • Zimmerman G.A.
      • Arabi Y.M.
      • Beitler J.R.
      • Mercat A.
      • Herridge M.
      • Randolph A.G.
      • Calfee C.S.
      Acute respiratory distress syndrome.
      However, the evidence to date suggests that evidence should not be extrapolated, supporting peripheral glycocalyx damage mediating increased vasculature permeability, to the pulmonary vasculature in sepsis. Mouse pulmonary endothelial glycocalyx appears to be substantially thicker than that seen in the mouse systemic vessels (cremaster muscle).
      • Schmidt E.P.
      • Yang Y.
      • Janssen W.J.
      • Gandjeva A.
      • Perez M.J.
      • Barthel L.
      • Zemans R.L.
      • Bowman J.C.
      • Koyanagi D.E.
      • Yunt Z.X.
      • Smith L.P.
      • Cheng S.S.
      • Overdier K.H.
      • Thompson K.R.
      • Geraci M.W.
      • Douglas I.S.
      • Pearse D.B.
      • Tuder R.M.
      The pulmonary endothelial glycocalyx regulates neutrophil adhesion and lung injury during experimental sepsis.
      Conflicting evidence exists supporting the role of the pulmonary vessel glycocalyx as a permeability barrier. Although a study using bovine lung microvasculature endothelial cells in vitro suggested that HS within the glycocalyx contributed to cell barrier function, a study using ex vivo rat lungs did not.
      • Dull R.O.
      • Mecham I.
      • McJames S.
      Heparan sulfates mediate pressure-induced increase in lung endothelial hydraulic conductivity via nitric oxide/reactive oxygen species.
      ,
      • Dull R.O.
      • Cluff M.
      • Kingston J.
      • Hill D.
      • Chen H.
      • Hoehne S.
      • Malleske D.T.
      • Kaur R.
      Lung heparan sulfates modulate K(fc) during increased vascular pressure: evidence for glycocalyx-mediated mechanotransduction.
      In addition, in isolated mouse lungs perfused with 4% Evans blue–labeled albumin, neither water nor albumin permeability was measurably altered by glycocalyx degradation. In vivo heparinase III enzyme infusion did not result in detectable pulmonary edema in mice.
      • Yang Y.
      • Schmidt E.P.
      The endothelial glycocalyx: an important regulator of the pulmonary vascular barrier.
      These findings could be explained by the double barrier hypothesis (Figure 3). Glycocalyx injury in isolation may not result in measurable increases in pulmonary permeability to macromolecules in the presence of an intact second barrier (the endothelial monolayer). However, by altering the level of immune cell extravazation and inflammation, the pulmonary glycocalyx may still have a key role in maintaining pulmonary homeostasis.
      • Yang Y.
      • Schmidt E.P.
      The endothelial glycocalyx: an important regulator of the pulmonary vascular barrier.
      The endothelial cells within the cerebral circulation form part of the blood brain barrier, limiting transcytosis and solute diffusion.
      • Galea I.
      • Perry V.H.
      The blood-brain interface: a culture change.
      The glycocalyx forms the most luminal layer of the blood brain barrier. Although cytokines smaller than 40 kDa are likely to freely cross the glycocalyx, the passage of larger molecules and interactions between circulating cells and the endothelium are restricted by an intact glycocalyx.
      • Galea I.
      • Perry V.H.
      The blood-brain interface: a culture change.
      ,
      • Yoon J.H.
      • Lee E.S.
      • Jeong Y.
      In vivo imaging of the cerebral endothelial glycocalyx in mice.
      Groups are currently working to investigate the function and structure of the cerebral glycocalyx.
      • Yoon J.H.
      • Lee E.S.
      • Jeong Y.
      In vivo imaging of the cerebral endothelial glycocalyx in mice.
      ,
      • Haeren R.H.L.
      • Rijkers K.
      • Schijns O.
      • Dings J.
      • Hoogland G.
      • van Zandvoort M.
      • Vink H.
      • van Overbeeke J.J.
      In vivo assessment of the human cerebral microcirculation and its glycocalyx: a technical report.
      In vivo imaging in mice suggests that the cerebral arteries and capillaries have an intact glycocalyx, whereas veins and venules do not.
      • Yoon J.H.
      • Lee E.S.
      • Jeong Y.
      In vivo imaging of the cerebral endothelial glycocalyx in mice.
      As yet, it is not known if the same distribution will be seen in the human cerebral circulation. During sepsis, patients commonly develop cognitive impairment (acute delirium). The pathogenesis of this condition is complex, with evidence to date suggesting a role for cytokines (IL-1, IL-6, and tumor necrosis factor-α) penetrating the blood brain barrier and activating astrocytes.
      • Piva S.
      • McCreadie V.A.
      • Latronico N.
      Neuroinflammation in sepsis: sepsis associated delirium.
      Studies now suggest that during sepsis HS fragments penetrate the hippocampal blood brain barrier to inhibit long-term potentiation—the process responsible for memory formation.
      • Zhang X.
      • Han X.
      • Xia K.
      • Xu Y.
      • Yang Y.
      • Oshima K.
      • Haeger S.M.
      • Perez M.J.
      • McMurtry S.A.
      • Hippensteel J.A.
      • Ford J.A.
      • Herson P.S.
      • Liu J.
      • Schmidt E.P.
      • Linhardt R.J.
      Circulating heparin oligosaccharides rapidly target the hippocampus in sepsis, potentially impacting cognitive functions.
      ,
      • Hippensteel J.A.
      • Anderson B.J.
      • Orfila J.E.
      • McMurtry S.A.
      • Dietz R.M.
      • Su G.
      • Ford J.A.
      • Oshima K.
      • Yang Y.
      • Zhang F.
      • Han X.
      • Yu Y.
      • Liu J.
      • Linhardt R.J.
      • Meyer N.J.
      • Herson P.S.
      • Schmidt E.P.
      Circulating heparan sulfate fragments mediate septic cognitive dysfunction.
      The blood brain barrier of the hippocampal region appears to be susceptible to damage, with an apparent predisposition to age-related vascular dysfunction.
      • Montagne A.
      • Barnes S.R.
      • Sweeney M.D.
      • Halliday M.R.
      • Sagare A.P.
      • Zhao Z.
      • Toga A.W.
      • Jacobs R.E.
      • Liu C.Y.
      • Amezcua L.
      • Harrington M.G.
      • Chui H.C.
      • Law M.
      • Zlokovic B.V.
      Blood-brain barrier breakdown in the aging human hippocampus.
      It seems possible in sepsis, therefore, that an increase in the permeability of the blood brain barrier of the hippocampus to HS fragments, possibly as a direct result of sepsis-induced glycocalyx shedding, contributes to the hippocampal HS fragment accumilation.
      • Zhang X.
      • Han X.
      • Xia K.
      • Xu Y.
      • Yang Y.
      • Oshima K.
      • Haeger S.M.
      • Perez M.J.
      • McMurtry S.A.
      • Hippensteel J.A.
      • Ford J.A.
      • Herson P.S.
      • Liu J.
      • Schmidt E.P.
      • Linhardt R.J.
      Circulating heparin oligosaccharides rapidly target the hippocampus in sepsis, potentially impacting cognitive functions.
      Interestingly, a limited clinical study using side-stream dark field imaging found that the glycocalyx depth varies between cortical and hippocampal microvessels.
      • Haeren R.H.L.
      • Rijkers K.
      • Schijns O.
      • Dings J.
      • Hoogland G.
      • van Zandvoort M.
      • Vink H.
      • van Overbeeke J.J.
      In vivo assessment of the human cerebral microcirculation and its glycocalyx: a technical report.
      The hippocampal endothelial glycocalyx was thicker than the cortical endothelial glycocalyx, suggesting that structural differences in the glycocalyx at the two sites may exit.
      • Haeren R.H.L.
      • Rijkers K.
      • Schijns O.
      • Dings J.
      • Hoogland G.
      • van Zandvoort M.
      • Vink H.
      • van Overbeeke J.J.
      In vivo assessment of the human cerebral microcirculation and its glycocalyx: a technical report.
      Further work is needed to determine whether the hippocampal microvascular glycocalyx is predisposed to damage during sepsis because this could represent an exciting therapeutic target in the future to limit the comorbidity associated with sepsis.

      Preeclampsia

      Preeclampsia is a complication that affects 3% to 5% of pregnancies. The hallmark of the condition is endothelial cell damage, leading to altered microvascular permeability.
      • Boeldt D.S.
      • Bird I.M.
      Vascular adaptation in pregnancy and endothelial dysfunction in preeclampsia.
      ,
      • Tranquilli A.L.
      • Dekker G.
      • Magee L.
      • Roberts J.
      • Sibai B.M.
      • Steyn W.
      • Zeeman G.G.
      • Brown M.A.
      The classification, diagnosis and management of the hypertensive disorders of pregnancy: a revised statement from the ISSHP.
      Endothelial glycocalyx dysfunction may contribute to this. Activation and dysfunction of the maternal endothelium in preeclampsia is mediated (in part) through the release of placentally derived factors.
      • O'Brien M.
      • Baczyk D.
      • Kingdom J.C.
      Endothelial dysfunction in severe preeclampsia is mediated by soluble factors, rather than extracellular vesicles.
      Hypoxic trophoblasts release antiangiogenic cytokines, including soluble fms-like tyrosine kinase 1 into the maternal circulation. In addition, there is a reduction in the proangiogenic placental growth factor, a member of the VEGF family.
      • Levine R.J.
      • Maynard S.E.
      • Qian C.
      • Lim K.H.
      • England L.J.
      • Yu K.F.
      • Schisterman E.F.
      • Thadhani R.
      • Sachs B.P.
      • Epstein F.H.
      • Sibai B.M.
      • Sukhatme V.P.
      • Karumanchi S.A.
      Circulating angiogenic factors and the risk of preeclampsia.
      The resulting imbalance contributes to the altered vascular permeability seen in preeclampsia.
      • Maynard S.E.
      • Min J.-Y.
      • Merchan J.
      • Lim K.-H.
      • Li J.
      • Mondal S.
      • Libermann T.A.
      • Morgan J.P.
      • Sellke F.W.
      • Stillman I.E.
      • Epstein F.H.
      • Sukhatme V.P.
      • Karumanchi S.A.
      Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsia.
      Historically, VEGF-A, VEGF-C, and VEGF-A165b have altered the endothelial glycocalyx.
      • Oltean S.
      • Qiu Y.
      • Ferguson J.K.
      • Stevens M.
      • Neal C.
      • Russell A.
      • Kaura A.
      • Arkill K.P.
      • Harris K.
      • Symonds C.
      • Lacey K.
      • Wijeyaratne L.
      • Gammons M.
      • Wylie E.
      • Hulse R.P.
      • Alsop C.
      • Cope G.
      • Damodaran G.
      • Betteridge K.B.
      • Ramnath R.
      • Satchell S.C.
      • Foster R.R.
      • Ballmer-Hofer K.
      • Donaldson L.F.
      • Barratt J.
      • Baelde H.J.
      • Harper S.J.
      • Bates D.O.
      • Salmon A.H.
      Vascular endothelial growth factor-A165b is protective and restores endothelial glycocalyx in diabetic nephropathy.
      ,
      • Foster R.R.
      • Armstrong L.
      • Baker S.
      • Wong D.W.
      • Wylie E.C.
      • Ramnath R.
      • Jenkins R.
      • Singh A.
      • Steadman R.
      • Welsh G.I.
      • Mathieson P.W.
      • Satchell S.C.
      Glycosaminoglycan regulation by VEGFA and VEGFC of the glomerular microvascular endothelial cell glycocalyx in vitro.
      Interestingly, a failure in first trimester up-regulation of VEGF-A165b is predictive of the development of preeclampsia.
      • Bills V.L.
      • Varet J.
      • Millar A.
      • Harper S.J.
      • Soothill P.W.
      • Bates D.O.
      Failure to up-regulate VEGF(165)b in maternal plasma is a first trimester predictive marker for pre-eclampsia.
      The important link among VEGF, permeability, and the endothelial glycocalyx is an increasing focus in PE research.
      The net effect of PE is glycocalyx degradation, illustrated by an increased perfused boundary region in sublingual capillaries, assessed by side stream imaging in patients with early-onset preeclampsia.
      • Weissgerber T.L.
      • Garcia-Valencia O.
      • Milic N.M.
      • Codsi E.
      • Cubro H.
      • Nath M.C.
      • White W.M.
      • Nath K.A.
      • Garovic V.D.
      Early onset preeclampsia is associated with glycocalyx degradation and reduced microvascular perfusion.
      However the mechanism of damage remains unclear because of conflicting data to date. Both HS and HA are elevated in early-onset preeclampsia (onset before 34 weeks), severe preeclampsia, and the related syndrome of hemolysis, elevated liver enzyme levels, and low platelet counts.
      • Weissgerber T.L.
      • Garcia-Valencia O.
      • Milic N.M.
      • Codsi E.
      • Cubro H.
      • Nath M.C.
      • White W.M.
      • Nath K.A.
      • Garovic V.D.
      Early onset preeclampsia is associated with glycocalyx degradation and reduced microvascular perfusion.
      • Osmers R.G.
      • Schutz E.
      • Diedrich F.
      • Wehry B.
      • Krauss T.
      • Oellerich M.
      • Kuhn W.
      Increased serum levels of hyaluronic acid in pregnancies complicated by preeclampsia or hemolysis, elevated liver enzymes, and low platelets syndrome.
      • Berg S.
      • Engman A.
      • Holmgren S.
      • Lundahl T.
      • Laurent T.C.
      Increased plasma hyaluronan in severe pre-eclampsia and eclampsia.
      However, although soluble syndecan-1 increases with advancing gestation in normal pregnancy and preeclampsia, its utility as a marker of preeclampsia remains unclear.
      • Weissgerber T.L.
      • Garcia-Valencia O.
      • Milic N.M.
      • Codsi E.
      • Cubro H.
      • Nath M.C.
      • White W.M.
      • Nath K.A.
      • Garovic V.D.
      Early onset preeclampsia is associated with glycocalyx degradation and reduced microvascular perfusion.
      ,
      • Hofmann-Kiefer K.F.
      • Knabl J.
      • Martinoff N.
      • Schiessl B.
      • Conzen P.
      • Rehm M.
      • Becker B.F.
      • Chappell D.
      Increased serum concentrations of circulating glycocalyx components in HELLP syndrome compared to healthy pregnancy: an observational study.
      ,
      • Gandley R.E.
      • Althouse A.
      • Jeyabalan A.
      • Bregand-White J.M.
      • McGonigal S.
      • Myerski A.C.
      • Gallaher M.
      • Powers R.W.
      • Hubel C.A.
      Low soluble syndecan-1 precedes preeclampsia.
      The sample size, diagnostic criteria, and different preeclampsia subtypes may explain the discrepancies observed. Recently developed electron microscopy–based methods have reliably measured the glycocalyx depth in the fetal and maternal circulation of human placental tissue to aid future work in this area.
      • Fabre-Gray A.C.M.
      • Down C.J.
      • Neal C.R.
      • Foster R.R.
      • Satchell S.C.
      • Bills V.L.
      Imaging the placental glycocalyx with transmission electron microscopy.

      Atherosclerosis

      The major focus of this review has been on the function of the endothelial glycocalyx within the microvasculature. However, there is now an emerging field of research on the role of the glycocalyx in the prevention of atherosclerosis in larger blood vessels. Atherosclerosis is a chronic arterial vascular disease that results from lipid-filled plaque accumulation.
      • Mitra R.
      • O'Neil G.L.
      • Harding I.C.
      • Cheng M.J.
      • Mensah S.A.
      • Ebong E.E.
      Glycocalyx in atherosclerosis-relevant endothelium function and as a therapeutic target.
      The subsequent erosion or rupture of plaques can result in acute arterial occlusion, myocardial infarction, or cerebrovascular accident. Damage to the endothelial glycocalyx has been linked to the pathogenesis of atheroma through multiple pathways, which have recently been reviewed by Mitra et al,
      • Mitra R.
      • O'Neil G.L.
      • Harding I.C.
      • Cheng M.J.
      • Mensah S.A.
      • Ebong E.E.
      Glycocalyx in atherosclerosis-relevant endothelium function and as a therapeutic target.
      but alterations in transendothelial permeability appear to be a key step. Atheroma tends to form in areas of nonlaminar blood flow, adjacent to bends or branch points.
      • Mitra R.
      • O'Neil G.L.
      • Harding I.C.
      • Cheng M.J.
      • Mensah S.A.
      • Ebong E.E.
      Glycocalyx in atherosclerosis-relevant endothelium function and as a therapeutic target.
      ,
      • Zand T.
      • Majno G.
      • Nunnari J.J.
      • Hoffman A.H.
      • Savilonis B.J.
      • MacWilliams B.
      • Joris I.
      Lipid deposition and intimal stress and strain: a study in rats with aortic stenosis.
      In vitro, nonuniform fluid flow rapidly results in reduced glycocalyx expression of HS and sialic acids, with an associated reduction in glycocalyx thickness and coverage.
      • Mitra R.
      • O'Neil G.L.
      • Harding I.C.
      • Cheng M.J.
      • Mensah S.A.
      • Ebong E.E.
      Glycocalyx in atherosclerosis-relevant endothelium function and as a therapeutic target.
      Areas of glycocalyx damage correlated with oxidized low-density lipoprotein cholesterol uptake.
      • Mitra R.
      • O'Neil G.L.
      • Harding I.C.
      • Cheng M.J.
      • Mensah S.A.
      • Ebong E.E.
      Glycocalyx in atherosclerosis-relevant endothelium function and as a therapeutic target.
      In vivo, lipid accumulation in apolipoprotein E–deficient (ApoE−/−) mice increases in areas of endothelium with incomplete glycocalyx coverage (71% glycocalyx coverage in regions of plaque versus 97% coverage in plaque-free regions).
      • Cancel L.M.
      • Ebong E.E.
      • Mensah S.
      • Hirschberg C.
      • Tarbell J.M.
      Endothelial glycocalyx, apoptosis and inflammation in an atherosclerotic mouse model.
      Low-density lipoprotein cholesterol uptake is considered important because it triggers CD40/CD40 ligand signaling pathways, and subsequent macrophage uptake and degradation of oxidized low-density lipoprotein results in their transformation into foam cells, a key finding on histologic examination of plaques.
      • Parthasarathy S.
      • Raghavamenon A.
      • Garelnabi M.O.
      • Santanam N.
      Oxidized low-density lipoprotein.

      Conclusions

      Further research into the structural variability of the glycocalyx and the contribution of the endothelial glycocalyx to vessel wall permeability is needed. This research needs to be targeted to both the organ and vessel of interest. The glycocalyx is a highly specialized structure, and pathologic states should not be expected to affect remote vascular beds or the different vessels within them in the same way. Highly sensitive tools, such as the ex vivo glomerular permeability assay, are providing new evidence that glycocalyx injury in early disease models has functional consequences and should not be ignored. Glycocalyx damage is associated with increased vessel wall permeability in multiple clinical conditions and has massive potential medical relevance. Restoring or maintaining the glycocalyx represents an inviting therapeutic strategy, but for the full potential of this strategy to be realized, an increased understanding of how the glycocalyx structure responds to disease and the functional consequences of glycocalyx damage is needed.

      References

        • Betteridge K.B.
        • Arkill K.P.
        • Neal C.R.
        • Harper S.J.
        • Foster R.R.
        • Satchell S.C.
        • Bates D.O.
        • Salmon A.H.J.
        Sialic acids regulate microvessel permeability, revealed by novel in vivo studies of endothelial glycocalyx structure and function.
        J Physiol. 2017; 595: 5015-5035
        • Butler M.J.
        • Ramnath R.
        • Kadoya H.
        • Desposito D.
        • Riquier-Brison A.
        • Ferguson J.K.
        • Onions K.L.
        • Ogier A.S.
        • ElHegni H.
        • Coward R.J.
        • Welsh G.I.
        • Foster R.R.
        • Peti-Peterdi J.
        • Satchell S.C.
        Aldosterone induces albuminuria via matrix metalloproteinase-dependent damage of the endothelial glycocalyx.
        Kidney Int. 2019; 95: 94-107
        • Dane M.J.
        • van den Berg B.M.
        • Lee D.H.
        • Boels M.G.
        • Tiemeier G.L.
        • Avramut M.C.
        • van Zonneveld A.J.
        • van der Vlag J.
        • Vink H.
        • Rabelink T.J.
        A microscopic view on the renal endothelial glycocalyx.
        Am J Physiol Renal Physiol. 2015; 308: F956-F966
        • Singh A.
        • Satchell S.C.
        • Neal C.R.
        • McKenzie E.A.
        • Tooke J.E.
        • Mathieson P.W.
        Glomerular endothelial glycocalyx constitutes a barrier to protein permeability.
        J Am Soc Nephrol. 2007; 18: 2885-2893
        • Curry F.R.
        Microvascular solute and water transport.
        Microcirculation. 2005; 12: 17-31
        • Curry F.E.
        Layer upon layer: the functional consequences of disrupting the glycocalyx-endothelial barrier in vivo and in vitro.
        Cardiovasc Res. 2017; 113: 559-561
        • Curry F.E.
        • Michel C.C.
        The endothelial glycocalyx: barrier functions versus red cell hemodynamics: a model of steady state ultrafiltration through a bi-layer formed by a porous outer layer and more selective membrane-associated inner layer.
        Biorheology. 2019; 56: 113-130
        • Potter D.R.
        • Damiano E.R.
        The hydrodynamically relevant endothelial cell glycocalyx observed in vivo is absent in vitro.
        Circ Res. 2008; 102: 770-776
        • Chappell D.
        • Jacob M.
        • Paul O.
        • Rehm M.
        • Welsch U.
        • Stoeckelhuber M.
        • Conzen P.
        • Becker B.F.
        The glycocalyx of the human umbilical vein endothelial cell: an impressive structure ex vivo but not in culture.
        Circ Res. 2009; 104: 1313-1317
        • Edwards A.
        • Daniels B.S.
        • Deen W.M.
        Ultrastructural model for size selectivity in glomerular filtration.
        Am J Physiol. 1999; 276: F892-F902
        • Punyaratabandhu N.
        • Kongoup P.
        • Dechadilok P.
        • Katavetin P.
        • Triampo W.
        Transport of spherical particles through fibrous media and a row of parallel cylinders: applications to glomerular filtration.
        J Biomech Eng. 2017; 139: 121005
        • Satchell S.
        The role of the glomerular endothelium in albumin handling.
        Nat Rev Nephrol. 2013; 9: 717-725
        • Weinbaum S.
        • Tarbell J.M.
        • Damiano E.R.
        The structure and function of the endothelial glycocalyx layer.
        Annu Rev Biomed Eng. 2007; 9: 121-167
        • Salmon A.H.
        • Satchell S.C.
        Endothelial glycocalyx dysfunction in disease: albuminuria and increased microvascular permeability.
        J Pathol. 2012; 226: 562-574
        • Oltean S.
        • Qiu Y.
        • Ferguson J.K.
        • Stevens M.
        • Neal C.
        • Russell A.
        • Kaura A.
        • Arkill K.P.
        • Harris K.
        • Symonds C.
        • Lacey K.
        • Wijeyaratne L.
        • Gammons M.
        • Wylie E.
        • Hulse R.P.
        • Alsop C.
        • Cope G.
        • Damodaran G.
        • Betteridge K.B.
        • Ramnath R.
        • Satchell S.C.
        • Foster R.R.
        • Ballmer-Hofer K.
        • Donaldson L.F.
        • Barratt J.
        • Baelde H.J.
        • Harper S.J.
        • Bates D.O.
        • Salmon A.H.
        Vascular endothelial growth factor-A165b is protective and restores endothelial glycocalyx in diabetic nephropathy.
        J Am Soc Nephrol. 2015; 26: 1889-1904
        • Eremina V.
        • Jefferson J.A.
        • Kowalewska J.
        • Hochster H.
        • Haas M.
        • Weisstuch J.
        • Richardson C.
        • Kopp J.B.
        • Kabir M.G.
        • Backx P.H.
        • Gerber H.P.
        • Ferrara N.
        • Barisoni L.
        • Alpers C.E.
        • Quaggin S.E.
        VEGF inhibition and renal thrombotic microangiopathy.
        N Engl J Med. 2008; 358: 1129-1136
        • Dimke H.
        • Maezawa Y.
        • Quaggin S.E.
        Crosstalk in glomerular injury and repair.
        Curr Opin Nephrol Hypertens. 2015; 24: 231-238
        • Dane M.J.
        • van den Berg B.M.
        • Avramut M.C.
        • Faas F.G.
        • van der Vlag J.
        • Rops A.L.
        • Ravelli R.B.
        • Koster B.J.
        • van Zonneveld A.J.
        • Vink H.
        • Rabelink T.J.
        Glomerular endothelial surface layer acts as a barrier against albumin filtration.
        Am J Pathol. 2013; 182: 1532-1540
        • Lazzara M.J.
        • Deen W.M.
        Model of albumin reabsorption in the proximal tubule.
        Am J Physiol Renal Physiol. 2007; 292: F430-F439
        • Norden A.G.
        • Scheinman S.J.
        • Deschodt-Lanckman M.M.
        • Lapsley M.
        • Nortier J.L.
        • Thakker R.V.
        • Unwin R.J.
        • Wrong O.
        Tubular proteinuria defined by a study of Dent's (CLCN5 mutation) and other tubular diseases.
        Kidney Int. 2000; 57: 240-249
        • Gambaro G.
        • Baggio B.
        • Cicerello E.
        • Mastrosimone S.
        • Marzaro G.
        • Borsatti A.
        • Crepaldi G.
        Abnormal erythrocyte charge in diabetes mellitus: link with microalbuminuria.
        Diabetes. 1988; 37: 745-748
        • Ando Y.
        • Okada H.
        • Takemura G.
        • Suzuki K.
        • Takada C.
        • Tomita H.
        • Zaikokuji R.
        • Hotta Y.
        • Miyazaki N.
        • Yano H.
        • Muraki I.
        • Kuroda A.
        • Fukuda H.
        • Kawasaki Y.
        • Okamoto H.
        • Kawaguchi T.
        • Watanabe T.
        • Doi T.
        • Yoshida T.
        • Ushikoshi H.
        • Yoshida S.
        • Ogura S.
        Brain-specific ultrastructure of capillary endothelial glycocalyx and its possible contribution for blood brain barrier.
        Sci Rep. 2018; 8: 17523
        • Rehm M.
        • Zahler S.
        • Lotsch M.
        • Welsch U.
        • Conzen P.
        • Jacob M.
        • Becker B.F.
        Endothelial glycocalyx as an additional barrier determining extravasation of 6% hydroxyethyl starch or 5% albumin solutions in the coronary vascular bed.
        Anesthesiology. 2004; 100: 1211-1223
        • Rops A.L.
        • Gotte M.
        • Baselmans M.H.
        • van den Hoven M.J.
        • Steenbergen E.J.
        • Lensen J.F.
        • Wijnhoven T.J.
        • Cevikbas F.
        • van den Heuvel L.P.
        • van Kuppevelt T.H.
        • Berden J.H.
        • van der Vlag J.
        Syndecan-1 deficiency aggravates anti-glomerular basement membrane nephritis.
        Kidney Int. 2007; 72: 1204-1215
        • Garsen M.
        • Rops A.L.
        • Rabelink T.J.
        • Berden J.H.
        • van der Vlag J.
        The role of heparanase and the endothelial glycocalyx in the development of proteinuria.
        Nephrol Dial Transplant. 2014; 29: 49-55
        • Earnshaw J.S.
        • Curtis C.G.
        • Powell G.M.
        • Dodgson K.S.
        • Olavesen A.H.
        • Gacesa P.
        The fate of intravenously administered highly purified bovine testicular hyaluronidase (Hyalosidase) in the rat.
        Biochem Pharmacol. 1985; 34: 2199-2203
        • Landsverk S.A.
        • Tsai A.G.
        • Cabrales P.
        • Intaglietta M.
        Impact of enzymatic degradation of the endothelial glycocalyx on vascular permeability in an awake hamster model.
        Crit Care Res Pract. 2012; 2012: 842545
        • Jeansson M.
        • Haraldsson B.
        Glomerular size and charge selectivity in the mouse after exposure to glucosaminoglycan-degrading enzymes.
        J Am Soc Nephrol. 2003; 14: 1756-1765
        • Onions K.L.
        • Gamez M.
        • Buckner N.R.
        • Baker S.L.
        • Betteridge K.B.
        • Desideri S.
        • Dallyn B.P.
        • Ramnath R.D.
        • Neal C.R.
        • Farmer L.K.
        • Mathieson P.W.
        • Gnudi L.
        • Alitalo K.
        • Bates D.O.
        • Salmon A.H.J.
        • Welsh G.I.
        • Satchell S.C.
        • Foster R.R.
        VEGFC reduces glomerular albumin permeability and protects against alterations in VEGF receptor expression in diabetic nephropathy.
        Diabetes. 2019; 68: 172-187
        • van den Berg B.M.
        • Wang G.
        • Boels M.G.S.
        • Avramut M.C.
        • Jansen E.
        • Sol W.
        • Lebrin F.
        • Jan van Zonneveld A.
        • de Koning E.J.P.
        • Vink H.
        • Grone H.J.
        • Carmeliet P.
        • van der Vlag J.
        • Rabelink T.J.
        Glomerular function and structural integrity depend on hyaluronan synthesis by glomerular endothelium.
        J Am Soc Nephrol. 2019; 30: 1886-1897
        • Gil N.
        • Goldberg R.
        • Neuman T.
        • Garsen M.
        • Zcharia E.
        • Rubinstein A.M.
        • van Kuppevelt T.
        • Meirovitz A.
        • Pisano C.
        • Li J.P.
        • van der Vlag J.
        • Vlodavsky I.
        • Elkin M.
        Heparanase is essential for the development of diabetic nephropathy in mice.
        Diabetes. 2012; 61: 208-216
        • Jung O.
        • Trapp-Stamborski V.
        • Purushothaman A.
        • Jin H.
        • Wang H.
        • Sanderson R.D.
        • Rapraeger A.C.
        Heparanase-induced shedding of syndecan-1/CD138 in myeloma and endothelial cells activates VEGFR2 and an invasive phenotype: prevention by novel synstatins.
        Oncogenesis. 2016; 5: e202
        • Kim E.Y.
        • Roshanravan H.
        • Dryer S.E.
        Syndecan-4 ectodomain evokes mobilization of podocyte TRPC6 channels and their associated pathways: an essential role for integrin signaling.
        Biochim Biophys Acta. 2015; 1853: 2610-2620
        • Zhang X.
        • Han X.
        • Xia K.
        • Xu Y.
        • Yang Y.
        • Oshima K.
        • Haeger S.M.
        • Perez M.J.
        • McMurtry S.A.
        • Hippensteel J.A.
        • Ford J.A.
        • Herson P.S.
        • Liu J.
        • Schmidt E.P.
        • Linhardt R.J.
        Circulating heparin oligosaccharides rapidly target the hippocampus in sepsis, potentially impacting cognitive functions.
        Proc Natl Acad Sci U S A. 2019; 116: 9208-9213
        • Desideri S.
        • Onions K.L.
        • Qiu Y.
        • Ramnath R.D.
        • Butler M.J.
        • Neal C.R.
        • King M.L.R.
        • Salmon A.E.
        • Saleem M.A.
        • Welsh G.I.
        • Michel C.C.
        • Satchell S.C.
        • Salmon A.H.J.
        • Foster R.R.
        A novel assay provides sensitive measurement of physiologically relevant changes in albumin permeability in isolated human and rodent glomeruli.
        Kidney Int. 2018; 93: 1086-1097
        • Nieuwdorp M.
        • van Haeften T.W.
        • Gouverneur M.C.
        • Mooij H.L.
        • van Lieshout M.H.
        • Levi M.
        • Meijers J.C.
        • Holleman F.
        • Hoekstra J.B.
        • Vink H.
        • Kastelein J.J.
        • Stroes E.S.
        Loss of endothelial glycocalyx during acute hyperglycemia coincides with endothelial dysfunction and coagulation activation in vivo.
        Diabetes. 2006; 55: 480-486
        • Lambadiari V.
        • Pavlidis G.
        • Kousathana F.
        • Maratou E.
        • Georgiou D.
        • Andreadou I.
        • Kountouri A.
        • Varoudi M.
        • Balampanis K.
        • Parissis J.
        • Triantafyllidi H.
        • Katogiannis K.
        • Birba D.
        • Lekakis J.
        • Dimitriadis G.
        • Ikonomidis I.
        Effects of different antidiabetic medications on endothelial glycocalyx, myocardial function, and vascular function in type 2 diabetic patients: one year follow-up study.
        J Clin Med. 2019; 8
        • Targosz-Korecka M.
        • Jaglarz M.
        • Malek-Zietek K.E.
        • Gregorius A.
        • Zakrzewska A.
        • Sitek B.
        • Rajfur Z.
        • Chlopicki S.
        • Szymonski M.
        AFM-based detection of glycocalyx degradation and endothelial stiffening in the db/db mouse model of diabetes.
        Sci Rep. 2017; 7: 15951
        • Satchell S.C.
        • Tooke J.E.
        What is the mechanism of microalbuminuria in diabetes: a role for the glomerular endothelium?.
        Diabetologia. 2008; 51: 714-725
        • Toyoda M.
        • Najafian B.
        • Kim Y.
        • Caramori M.L.
        • Mauer M.
        Podocyte detachment and reduced glomerular capillary endothelial fenestration in human type 1 diabetic nephropathy.
        Diabetes. 2007; 56: 2155-2160
        • Tschulakow A.
        • Christner S.
        • Julien S.
        • Ludinsky M.
        • van der Giet M.
        • Schraermeyer U.
        Effects of a single intravitreal injection of aflibercept and ranibizumab on glomeruli of monkeys.
        PLoS One. 2014; 9: e113701
        • Eleftheriadis T.
        • Antoniadi G.
        • Pissas G.
        • Liakopoulos V.
        • Stefanidis I.
        The renal endothelium in diabetic nephropathy.
        Ren Fail. 2013; 35: 592-599
        • Foster R.R.
        • Armstrong L.
        • Baker S.
        • Wong D.W.
        • Wylie E.C.
        • Ramnath R.
        • Jenkins R.
        • Singh A.
        • Steadman R.
        • Welsh G.I.
        • Mathieson P.W.
        • Satchell S.C.
        Glycosaminoglycan regulation by VEGFA and VEGFC of the glomerular microvascular endothelial cell glycocalyx in vitro.
        Am J Pathol. 2013; 183: 604-616
        • Singh A.
        • Friden V.
        • Dasgupta I.
        • Foster R.R.
        • Welsh G.I.
        • Tooke J.E.
        • Haraldsson B.
        • Mathieson P.W.
        • Satchell S.C.
        High glucose causes dysfunction of the human glomerular endothelial glycocalyx.
        Am J Physiol Renal Physiol. 2011; 300: F40-F48
        • Jeansson M.
        • Granqvist A.B.
        • Nystrom J.S.
        • Haraldsson B.
        Functional and molecular alterations of the glomerular barrier in long-term diabetes in mice.
        Diabetologia. 2006; 49: 2200-2209
        • Satoh M.
        • Kobayashi S.
        • Kuwabara A.
        • Tomita N.
        • Sasaki T.
        • Kashihara N.
        In vivo visualization of glomerular microcirculation and hyperfiltration in streptozotocin-induced diabetic rats.
        Microcirculation. 2010; 17: 103-112
        • Kuwabara A.
        • Satoh M.
        • Tomita N.
        • Sasaki T.
        • Kashihara N.
        Deterioration of glomerular endothelial surface layer induced by oxidative stress is implicated in altered permeability of macromolecules in Zucker fatty rats.
        Diabetologia. 2010; 53: 2056-2065
        • Ramnath R.D.
        • Butler M.J.
        • Newman G.
        • Desideri S.
        • Russell A.
        • Lay A.C.
        • Neal C.R.
        • Qiu Y.
        • Fawaz S.
        • Onions K.
        • Gamez M.
        • Crompton M.
        • Michie C.
        • Finch N.
        • Coward R.J.
        • Welsh G.I.
        • Foster R.R.
        • Satchell S.C.
        Blocking matrix metalloproteinase-mediated syndecan-4 shedding restores the endothelial glycocalyx and glomerular filtration barrier function in early diabetic kidney disease.
        Kidney Int. 2019; ([Epub ahead of print] doi:10.1016/j.kint.2019.09.035)
        • Kowluru R.A.
        • Mishra M.
        Regulation of matrix metalloproteinase in the pathogenesis of diabetic retinopathy.
        Prog Mol Biol Transl Sci. 2017; 148: 67-85
        • Wong T.Y.
        • Cheung C.M.
        • Larsen M.
        • Sharma S.
        • Simo R.
        Diabetic retinopathy.
        Nat Rev Dis Primers. 2016; 2: 16012
        • Leskova W.
        • Pickett H.
        • Eshaq R.S.
        • Shrestha B.
        • Pattillo C.B.
        • Harris N.R.
        Effect of diabetes and hyaluronidase on the retinal endothelial glycocalyx in mice.
        Exp Eye Res. 2019; 179: 125-131
        • Kumase F.
        • Morizane Y.
        • Mohri S.
        • Takasu I.
        • Ohtsuka A.
        • Ohtsuki H.
        Glycocalyx degradation in retinal and choroidal capillary endothelium in rats with diabetes and hypertension.
        Acta Med Okayama. 2010; 64: 277-283
        • Broekhuizen L.N.
        • Lemkes B.A.
        • Mooij H.L.
        • Meuwese M.C.
        • Verberne H.
        • Holleman F.
        • Schlingemann R.O.
        • Nieuwdorp M.
        • Stroes E.S.
        • Vink H.
        Effect of sulodexide on endothelial glycocalyx and vascular permeability in patients with type 2 diabetes mellitus.
        Diabetologia. 2010; 53: 2646-2655
        • Niu T.
        • Zhao M.
        • Jiang Y.
        • Xing X.
        • Shi X.
        • Cheng L.
        • Jin H.
        • Liu K.
        Endomucin restores depleted endothelial glycocalyx in the retinas of streptozotocin-induced diabetic rats.
        FASEB J. 2019; 33: 13346-13357
        • Wallow I.H.
        • Engerman R.L.
        Permeability and patency of retinal blood vessels in experimental diabetes.
        Invest Ophthalmol Vis Sci. 1977; 16: 447-461
        • Uchimido R.
        • Schmidt E.P.
        • Shapiro N.I.
        The glycocalyx: a novel diagnostic and therapeutic target in sepsis.
        Crit Care. 2019; 23: 16
        • Chelazzi C.
        • Villa G.
        • Mancinelli P.
        • De Gaudio A.R.
        • Adembri C.
        Glycocalyx and sepsis-induced alterations in vascular permeability.
        Crit Care. 2015; 19: 26
        • Fink M.P.
        Animal models of sepsis.
        Virulence. 2014; 5: 143-153
        • Wiesinger A.
        • Peters W.
        • Chappell D.
        • Kentrup D.
        • Reuter S.
        • Pavenstadt H.
        • Oberleithner H.
        • Kumpers P.
        Nanomechanics of the endothelial glycocalyx in experimental sepsis.
        PLoS One. 2013; 8: e80905
        • Ramnath R.
        • Foster R.R.
        • Qiu Y.
        • Cope G.
        • Butler M.J.
        • Salmon A.H.
        • Mathieson P.W.
        • Coward R.J.
        • Welsh G.I.
        • Satchell S.C.
        Matrix metalloproteinase 9-mediated shedding of syndecan 4 in response to tumor necrosis factor alpha: a contributor to endothelial cell glycocalyx dysfunction.
        FASEB J. 2014; 28: 4686-4699
        • Nieuwdorp M.
        • Meuwese M.C.
        • Mooij H.L.
        • van Lieshout M.H.
        • Hayden A.
        • Levi M.
        • Meijers J.C.
        • Ince C.
        • Kastelein J.J.
        • Vink H.
        • Stroes E.S.
        Tumor necrosis factor-alpha inhibition protects against endotoxin-induced endothelial glycocalyx perturbation.
        Atherosclerosis. 2009; 202: 296-303
        • Steppan J.
        • Hofer S.
        • Funke B.
        • Brenner T.
        • Henrich M.
        • Martin E.
        • Weitz J.
        • Hofmann U.
        • Weigand M.A.
        Sepsis and major abdominal surgery lead to flaking of the endothelial glycocalix.
        J Surg Res. 2011; 165: 136-141
        • Becker B.F.
        • Jacob M.
        • Leipert S.
        • Salmon A.H.
        • Chappell D.
        Degradation of the endothelial glycocalyx in clinical settings: searching for the sheddases.
        Br J Clin Pharmacol. 2015; 80: 389-402
        • Yang X.
        • Meegan J.E.
        • Jannaway M.
        • Coleman D.C.
        • Yuan S.Y.
        A disintegrin and metalloproteinase 15-mediated glycocalyx shedding contributes to vascular leakage during inflammation.
        Cardiovasc Res. 2018; 114: 1752-1763
        • Lygizos M.I.
        • Yang Y.
        • Altmann C.J.
        • Okamura K.
        • Hernando A.A.
        • Perez M.J.
        • Smith L.P.
        • Koyanagi D.E.
        • Gandjeva A.
        • Bhargava R.
        • Tuder R.M.
        • Faubel S.
        • Schmidt E.P.
        Heparanase mediates renal dysfunction during early sepsis in mice.
        Physiol Rep. 2013; 1: e00153
        • Cui N.
        • Wang H.
        • Long Y.
        • Su L.
        • Liu D.
        Dexamethasone suppressed LPS-induced matrix metalloproteinase and its effect on endothelial glycocalyx shedding.
        Mediators Inflamm. 2015; 2015: 912726
        • Adembri C.
        • Sgambati E.
        • Vitali L.
        • Selmi V.
        • Margheri M.
        • Tani A.
        • Bonaccini L.
        • Nosi D.
        • Caldini A.L.
        • Formigli L.
        • De Gaudio A.R.
        Sepsis induces albuminuria and alterations in the glomerular filtration barrier: a morphofunctional study in the rat.
        Crit Care. 2011; 15: R277
        • Koike K.
        • Aiboshi J.
        • Shinozawa Y.
        • Sekine K.
        • Endo T.
        • Yamamoto Y.
        Correlation of glomerular permeability, endothelial injury, and postoperative multiple organ dysfunction.
        Surg Today. 2004; 34: 811-816
        • De Gaudio A.R.
        • Adembri C.
        • Grechi S.
        • Novelli G.P.
        Microalbuminuria as an early index of impairment of glomerular permeability in postoperative septic patients.
        Intensive Care Med. 2000; 26: 1364-1368
        • Matthay M.A.
        • Zemans R.L.
        • Zimmerman G.A.
        • Arabi Y.M.
        • Beitler J.R.
        • Mercat A.
        • Herridge M.
        • Randolph A.G.
        • Calfee C.S.
        Acute respiratory distress syndrome.
        Nat Rev Dis Primers. 2019; 5: 18
        • Schmidt E.P.
        • Yang Y.
        • Janssen W.J.
        • Gandjeva A.
        • Perez M.J.
        • Barthel L.
        • Zemans R.L.
        • Bowman J.C.
        • Koyanagi D.E.
        • Yunt Z.X.
        • Smith L.P.
        • Cheng S.S.
        • Overdier K.H.
        • Thompson K.R.
        • Geraci M.W.
        • Douglas I.S.
        • Pearse D.B.
        • Tuder R.M.
        The pulmonary endothelial glycocalyx regulates neutrophil adhesion and lung injury during experimental sepsis.
        Nat Med. 2012; 18: 1217-1223
        • Dull R.O.
        • Mecham I.
        • McJames S.
        Heparan sulfates mediate pressure-induced increase in lung endothelial hydraulic conductivity via nitric oxide/reactive oxygen species.
        Am J Physiol Lung Cell Mol Physiol. 2007; 292: L1452-L1458
        • Dull R.O.
        • Cluff M.
        • Kingston J.
        • Hill D.
        • Chen H.
        • Hoehne S.
        • Malleske D.T.
        • Kaur R.
        Lung heparan sulfates modulate K(fc) during increased vascular pressure: evidence for glycocalyx-mediated mechanotransduction.
        Am J Physiol Lung Cell Mol Physiol. 2012; 302: L816-L828
        • Yang Y.
        • Schmidt E.P.
        The endothelial glycocalyx: an important regulator of the pulmonary vascular barrier.
        Tissue Barriers. 2013; 1
        • Galea I.
        • Perry V.H.
        The blood-brain interface: a culture change.
        Brain Behav Immun. 2018; 68: 11-16
        • Yoon J.H.
        • Lee E.S.
        • Jeong Y.
        In vivo imaging of the cerebral endothelial glycocalyx in mice.
        J Vasc Res. 2017; 54: 59-67
        • Haeren R.H.L.
        • Rijkers K.
        • Schijns O.
        • Dings J.
        • Hoogland G.
        • van Zandvoort M.
        • Vink H.
        • van Overbeeke J.J.
        In vivo assessment of the human cerebral microcirculation and its glycocalyx: a technical report.
        J Neurosci Methods. 2018; 303: 114-125
        • Piva S.
        • McCreadie V.A.
        • Latronico N.
        Neuroinflammation in sepsis: sepsis associated delirium.
        Cardiovasc Hematol Disord Drug Targets. 2015; 15: 10-18
        • Hippensteel J.A.
        • Anderson B.J.
        • Orfila J.E.
        • McMurtry S.A.
        • Dietz R.M.
        • Su G.
        • Ford J.A.
        • Oshima K.
        • Yang Y.
        • Zhang F.
        • Han X.
        • Yu Y.
        • Liu J.
        • Linhardt R.J.
        • Meyer N.J.
        • Herson P.S.
        • Schmidt E.P.
        Circulating heparan sulfate fragments mediate septic cognitive dysfunction.
        J Clin Invest. 2019; 129: 1779-1784
        • Montagne A.
        • Barnes S.R.
        • Sweeney M.D.
        • Halliday M.R.
        • Sagare A.P.
        • Zhao Z.
        • Toga A.W.
        • Jacobs R.E.
        • Liu C.Y.
        • Amezcua L.
        • Harrington M.G.
        • Chui H.C.
        • Law M.
        • Zlokovic B.V.
        Blood-brain barrier breakdown in the aging human hippocampus.
        Neuron. 2015; 85: 296-302
        • Boeldt D.S.
        • Bird I.M.
        Vascular adaptation in pregnancy and endothelial dysfunction in preeclampsia.
        J Endocrinol. 2017; 232: R27-R44
        • Tranquilli A.L.
        • Dekker G.
        • Magee L.
        • Roberts J.
        • Sibai B.M.
        • Steyn W.
        • Zeeman G.G.
        • Brown M.A.
        The classification, diagnosis and management of the hypertensive disorders of pregnancy: a revised statement from the ISSHP.
        Pregnancy Hypertens. 2014; 4: 97-104
        • O'Brien M.
        • Baczyk D.
        • Kingdom J.C.
        Endothelial dysfunction in severe preeclampsia is mediated by soluble factors, rather than extracellular vesicles.
        Sci Rep. 2017; 7: 5887
        • Levine R.J.
        • Maynard S.E.
        • Qian C.
        • Lim K.H.
        • England L.J.
        • Yu K.F.
        • Schisterman E.F.
        • Thadhani R.
        • Sachs B.P.
        • Epstein F.H.
        • Sibai B.M.
        • Sukhatme V.P.
        • Karumanchi S.A.
        Circulating angiogenic factors and the risk of preeclampsia.
        N Engl J Med. 2004; 350: 672-683
        • Maynard S.E.
        • Min J.-Y.
        • Merchan J.
        • Lim K.-H.
        • Li J.
        • Mondal S.
        • Libermann T.A.
        • Morgan J.P.
        • Sellke F.W.
        • Stillman I.E.
        • Epstein F.H.
        • Sukhatme V.P.
        • Karumanchi S.A.
        Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsia.
        J Clin Invest. 2003; 111: 649-658
        • Bills V.L.
        • Varet J.
        • Millar A.
        • Harper S.J.
        • Soothill P.W.
        • Bates D.O.
        Failure to up-regulate VEGF(165)b in maternal plasma is a first trimester predictive marker for pre-eclampsia.
        Clin Sci (Lond). 2009; 116: 265-272
        • Weissgerber T.L.
        • Garcia-Valencia O.
        • Milic N.M.
        • Codsi E.
        • Cubro H.
        • Nath M.C.
        • White W.M.
        • Nath K.A.
        • Garovic V.D.
        Early onset preeclampsia is associated with glycocalyx degradation and reduced microvascular perfusion.
        J Am Heart Assoc. 2019; 8: e010647
        • Osmers R.G.
        • Schutz E.
        • Diedrich F.
        • Wehry B.
        • Krauss T.
        • Oellerich M.
        • Kuhn W.
        Increased serum levels of hyaluronic acid in pregnancies complicated by preeclampsia or hemolysis, elevated liver enzymes, and low platelets syndrome.
        Am J Obstet Gynecol. 1998; 178: 341-345
        • Berg S.
        • Engman A.
        • Holmgren S.
        • Lundahl T.
        • Laurent T.C.
        Increased plasma hyaluronan in severe pre-eclampsia and eclampsia.
        Scand J Clin Lab Invest. 2001; 61: 131-137
        • Hofmann-Kiefer K.F.
        • Knabl J.
        • Martinoff N.
        • Schiessl B.
        • Conzen P.
        • Rehm M.
        • Becker B.F.
        • Chappell D.
        Increased serum concentrations of circulating glycocalyx components in HELLP syndrome compared to healthy pregnancy: an observational study.
        Reprod Sci. 2013; 20: 318-325
        • Gandley R.E.
        • Althouse A.
        • Jeyabalan A.
        • Bregand-White J.M.
        • McGonigal S.
        • Myerski A.C.
        • Gallaher M.
        • Powers R.W.
        • Hubel C.A.
        Low soluble syndecan-1 precedes preeclampsia.
        PLoS One. 2016; 11: e0157608
        • Fabre-Gray A.C.M.
        • Down C.J.
        • Neal C.R.
        • Foster R.R.
        • Satchell S.C.
        • Bills V.L.
        Imaging the placental glycocalyx with transmission electron microscopy.
        Placenta. 2018; 74: 59-61
        • Mitra R.
        • O'Neil G.L.
        • Harding I.C.
        • Cheng M.J.
        • Mensah S.A.
        • Ebong E.E.
        Glycocalyx in atherosclerosis-relevant endothelium function and as a therapeutic target.
        Curr Atheroscler Rep. 2017; 19: 63
        • Zand T.
        • Majno G.
        • Nunnari J.J.
        • Hoffman A.H.
        • Savilonis B.J.
        • MacWilliams B.
        • Joris I.
        Lipid deposition and intimal stress and strain: a study in rats with aortic stenosis.
        Am J Pathol. 1991; 139: 101-113
        • Cancel L.M.
        • Ebong E.E.
        • Mensah S.
        • Hirschberg C.
        • Tarbell J.M.
        Endothelial glycocalyx, apoptosis and inflammation in an atherosclerotic mouse model.
        Atherosclerosis. 2016; 252: 136-146
        • Parthasarathy S.
        • Raghavamenon A.
        • Garelnabi M.O.
        • Santanam N.
        Oxidized low-density lipoprotein.
        Methods Mol Biol. 2010; 610: 403-417

      Linked Article