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From the Department of Dermatology,*
Kyoto
Prefectural University of Medicine, Kamigyo-ku, Kyoto; and the
Department of Dermatology,
Kyoto University
Graduate School of Medicine, Sakyo-ku, Kyoto, Japan
| Abstract |
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-(
-glutamyl)lysine cross-linking of substrate
proteins such as involucrin and loricrin to generate the cornified
envelope at the cell periphery of the stratum corneum. We have shown
that disruption of the TGase 1 gene in mice
results in neonatal lethality, absence of the cornified
envelope, and impaired skin barrier function. Based on the
importance of TGase 1 in epidermal morphogenesis, we have now
assessed its role in wound healing. In neonatal mouse skin,
TGase 1 mRNA as well as keratin 6
was induced in the epidermis at
the wound edges as early as 2 hours after injury and that expression
continued in the migrating epidermis until completion of
re-epithelialization. The TGase 1 enzyme co-localized on the plasma
membrane of migrating keratinocytes with involucrin, but not
with loricrin, which suggests the premature assembly of the
cornified envelope. Similar injuries to TGase 1 knockout mouse skins
grafted on athymic nude mice showed substantial delays in wound healing
concomitant with sustained K6
mRNA induction. From these
results, we suggest that activation of the TGase 1gene is essential for facilitated repair of skin
injury.
| Introduction |
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-(
-glutamyl)lysine cross-links between peptide-bound
glutamine residues and the primary amine group of various
amines.1
Such cross-links are generally formed in or
between proteins by reaction with the
-amino group of lysine
residues and they act to stabilize proteins via polymerization against
chemicals, proteolytic enzymes, and physical disruption. TGases have
been proposed to participate in a wide variety of biological events
such as fertilization, development, differentiation, apoptosis,
coagulation, and wound
healing.2
Cutaneous wound healing is a complex process consisting of three
overlapping phases, inflammation, proliferation, and
remodeling.3
Skin injury elicits a temporary repair by
formation of a clot that plugs the defect, followed by invasion of
inflammatory cells and then fibroblasts and capillaries into the clot
to form a contractile granulation tissue. Meanwhile, the cut epidermal
edges migrate forward to cover the denuded wound surface. During those
wound healing processes, TGases are activated in all layers of the skin
corresponding to the epidermis, the dermis, and the panniculus
carnosis.4
Coagulation factor XIIIa (plasma TGase) is
activated by thrombin and contributes to the formation of a fibrin clot
at the site of injury.5
Tissue TGase (TGase 2) is
expressed in endothelial cells, macrophages, and skeletal muscle cells
throughout the healing process.6
TGase 2 might be induced
in response to acute-phase injury cytokines, such as transforming
growth factor-ß,7
interleukin-6,8
and/or
tumor necrosis factor-
,9
because the TGase
2 gene has response elements to these cytokines. TGase 2 that
binds to a cell surface complex composed of plasminogen, urokinase
receptor, and mannose-6-phosphate receptor, converts latent
transforming growth factor-ß to its active form, providing a positive
amplification loop for TGase 2 expression in wound
sites.10
TGase 2 and factor XIIIa may participate in the
cross-linking of extracellular matrix molecules, such as
fibrin/fibrinogen,11,12
fibronectin,13
collagen,14
laminin/nidogen,15
osteopontin,16
and vitronectin.17
Thus,
TGases effectively construct a scaffold for migration of inflammatory
and endothelial cells by stabilizing the extracellular
matrix,6
and the end product of the process is the
replacement of the fibrin matrix with granulation tissue. Even after
the wound has healed, those TGase isozymes possibly promote contraction
throughout the wound by cross-linking extracellular matrix molecules.
In addition to the important role of TGase 2 in dermal wound healing,
the enzyme is also involved in the cross-linking of anchoring fibrils
at the dermo-epidermal junction via ligation of collagen VII as a
potential substrate.18
In contrast to those TGase isoforms, little attention has been
given to the role of TGase 1 (keratinocyte TGase) in wound healing
since the report by Mansbridge and Knapp19
that TGase
immunoreactivity is detectable in the epidermis 15 hours after
tape-stripping and 3 days after generation of a suction blister. TGase
1 is essential for the development and maturation of the stratum
corneum, especially for production of the cornified envelope at the
cell periphery of the stratum corneum,20
by assembly of
precursor proteins such as involucrin21
and
loricrin.22
Furthermore, TGase 1 may be necessary to
maintain skin barrier function by covalent cross-linking of
-OH
ceramides with involucrin.23
The TGase 1
gene is one of the genes responsible for autosomal recessive lamellar
ichthyosis and a variety of mutations in that gene have been identified
in some families with the disorder.24,25
The TGase
1 gene is expressed mainly in differentiating layers of normal
stratified squamous epithelia and in inner root sheath
cells,26
although TGase 1 localizes not only in the
stratified squamous epithelia but also concentrates at cadherin-based
adherens junctions of simple epithelial cells.27
In
addition to these roles of TGase 1 in the skin, the unusual expression
of the enzyme in the brain has been implicated in the pathogenesis of
Alzheimers disease.28
Recently, we have generated transgenic mice bearing a lacZ gene directed by a 2.5-kb 5' upstream promoter of the human TGase 1 gene and we have analyzed ß-galactosidase expression in these mice.26 In a series of studies using those TGM1-lacZ transgenic mice, we have noticed that ß-galactosidase staining is strongly induced in the epidermis close to the skin injury. Furthermore, based on the phenotype of our TGase 1 knockout mice, which have defective epidermal development and keratinization, lack the cornified envelope, and have impaired barrier function,20 we have speculated on the importance of TGase 1 during skin regeneration. In this study, we characterize TGase 1 gene expression during wound healing of the skin and the results suggest the essential role of TGase 1 in that process.
| Materials and Methods |
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Rat monoclonal TGase 1 antibody was kindly supplied by Dr. Hiiragi, Department of Cell Biology, Kyoto University Graduate School of Medicine, Kyoto, Japan.27 Rabbit polyclonal mouse loricrin and mouse involucrin antibodies were from Covance Research Products Inc. (Richmond, CA); Cy3-conjugated anti-rat IgG and fluorescein isothiocyanate-conjugated anti-mouse IgG were from Jackson ImmunoResearch Laboratories, Inc. (West Grove, PA); fluorescein isothiocyanate-conjugated and Texas Red-conjugated anti-rabbit IgGs were from Vector Laboratories, Inc. (Burlingame, CA).
Experimental Wounding, Harvesting, and Sectioning
All studies involving animals were reviewed by the Animal Use and Care Committee of the Kyoto Prefectural University of Medicine. BDF1 neonatal mice were anesthetized and a 10-mm linear full-thickness incision was made on the dorsal skin. Wounds were harvested from three mice at 1, 2, 3, 4, 8, 12, 16, 20, 24 hours and 2, 3, 4, 5, 6 days after wounding. For skin grafting, nine normal controls and eight TGase 1-/- neonatal mice were killed by decapitation and their dorsal skins were excised and transplanted onto athymic nude mice. Two weeks after the skin grafting, a round wound 3 mm in diameter was made at the center of each graft with a biopsy punch. Wounds made on the grafted skins were harvested for histological examinations at 48 hours and 5, 10, and 11 days after wounding.
Wound tissues were fixed in 10% formaldehyde in phosphate-buffered saline (PBS), then embedded in paraffin. Those tissues were sectioned at 4-µm thickness and were used for hematoxylin and eosin staining and for in situ hybridization. For immunofluorescence, tissues were embedded in OCT compound (Miles Inc., Elkhart, IN) and were frozen at -80°C, then sectioned at 5-µm thickness at -20°C.
In Situ Hybridization
pMTG11.1 containing a 1.1-kb mouse TGase 1 cDNA26
was linearized by SalI digestion and antisense TGase 1 cRNA
was synthesized with T7 RNA polymerase, using a digoxigenin RNA
Labeling Kit (Roche Diagnostics Corp., Basel, Switzerland). Mouse
keratin 6
(K6
) and keratin 6ß (K6ß) cRNA probes were prepared
as described previously.29
In situ hybridization was performed using a method described elsewhere.26 Tissue sections were deparaffinized in xylene and dehydrated through a graded ethanol series. After proteinase K digestion (18 µg/ml), the sections were postfixed with 4% (w/v) paraformaldehyde in PBS for 10 minutes and were then treated with 0.1 mol/L triethanolamine-HCl (pH 8.0) for 1 minute. After acetylation for 10 minutes, the sections were dehydrated, air-dried, and then incubated at 50°C in a hybridization buffer composed of 50% formamide, 10 mmol/L Tris-HCl (pH 7.5), 1 mg/ml yeast tRNA (Sigma Chemical Co., St. Louis, MO), 1x Denhardts solution (Sigma Chemical Co.), 10% polyethylene glycol 6,000, 600 mmol/L NaCl, 0.25% sodium dodecyl sulfate, 1 mmol/L ethylenediaminetetraacetic acid (pH 8.0), and 0.2 µg/ml of probe. After hybridization, the sections were washed at 45°C for 1 hour in 50% formamide and 2x standard saline citrate, and were then digested with 20 µg/ml RNase (Sigma Chemical Co.) in 10 mmol/L Tris-HCl (pH 8.0) and 500 mmol/L NaCl at 37°C for 30 minutes. Hybridized digoxigenin-labeled probes were visualized with a Nucleic Acid Detection Kit (Roche Diagnostics Corp.).
Immunofluorescence and Confocal Fluorescence Imaging
Cryosections mounted on glass slides were air-dried and rinsed with PBS containing 1% bovine serum albumin for 15 minutes, and were then incubated for 1 hour with rat TGase 1 (1:2) and rabbit involucrin antibodies (1:600) or with rabbit loricrin antibodies (1:500). Sections were then rinsed with PBS containing 1% bovine serum albumin and 0.15% Triton X-100, followed by incubation for 1 hour with Cy3-conjugated anti-rat IgG and fluorescein isothiocyanate-conjugated anti-rabbit IgG or Texas Red-conjugated anti-rabbit IgG. A confocal laser-scanning microscope, Olympus Fluoview (Olympus Optical Co., Ltd., Tokyo, Japan) was used for immunofluorescence imaging.
| Results |
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A linear 10-mm full-thickness incision was made with a scalpel on
the dorsal skin of BDF1 neonatal mice and the expression of the
TGase 1 gene in the epidermis was compared with that of
the K6
and K6ß genes. Histological
changes at the wound edges were stained with hematoxylin and eosin and
are shown in Figure 1
(left panels A, D,
G, J, M, and P). The morphological changes noted throughout the time
course were basically similar to those in adult mice reported by Croft
and Tarin.30
Before wounding, TGase 1 mRNA was evident in
the upper spinous and granular layers of the epidermis (Figure 1B)
, but
K6
mRNA was undetectable there (Figure 1C)
. One hour after wounding,
no obvious induction of TGase 1, K6
, or K6ß mRNA was observed.
However, 2 hours after injury, TGase 1 mRNA was focally increased in a
few layers of suprabasal keratinocytes near the wound edge (Figure 1E)
.
At that time, K6
mRNA was induced in a few suprabasal spinous layers
(Figure 1F)
. The TGase 1 mRNA signals were accentuated in keratinocytes
near the wound edge from 4 to 8 hours after wounding (Figure 1H)
and at
later times TGase 1 mRNA was distributed over the epidermis around the
wound. This event occurred earlier than did massive leukocyte
infiltration at the injured sites, which was found from 8 hours after
the injury. The strongest induction of TGase 1 mRNA was observed in
migrating keratinocytes 20 hours after incision (Figure 1K)
, at which
time the epidermis was thickening and re-epithelialization was becoming
obvious under the scab. At 48 hours, when intercellular spaces between
keratinocytes were widened (Figure 1M)
, intense signals were still
evident there, but the expression of TGase 1 mRNA was decreasing in the
epidermis away from the migrating edge (Figure 1N)
. Ninety-six hours
after wounding, TGase 1 mRNA expression was back to normal in the
thickened re-epithelializing epidermis (Figure 1Q)
. At 144 hours, the
re-epithelialized epidermis was arranged in regular layers and the
cutaneous surface became flattened (data not shown). The induction of
K6
mRNA was found at 2 hours (Figure 1I)
and the distribution of
K6
mRNA seemed to be optimal at 20 hours after wounding (Figure 1L)
.
At 48 hours, K6
mRNA expression was detected in the initial wound
site and in the regenerating epidermis (Figure 1O)
and was still
evident 96 hours after the injury (Figure 1R)
, by which time the
induction of TGase 1 mRNA had normalized. The expression of K6ß mRNA
began 2 hours after injury as did K6
, and strong signals were
observed 48 hours after the injury (data not shown). K6
mRNA often
extended down to the basal layer, whereas K6ß mRNA was primarily
restricted to the suprabasal layers of the epidermis.
|
The expression of TGase 1 enzyme and its substrate proteins,
involucrin and loricrin, during wound healing were assessed in five
control and four injured neonatal mice by confocal immunofluorescence
imaging. In control mice, TGase 1 enzyme was evident in a few layers
under the stratum corneum and in the inner root sheath of hair
follicles (Figure 2A)
. Involucrin and
loricrin were also located in the subcorneal layers, but loricrin was
not evident in hair follicles (Figure 2, B and C)
. The expression of
those substrate proteins was virtually unchanged 8 hours after the
injury (data not shown). At 48 hours, TGase 1 enzyme was abundantly
expressed suprabasally in the thickened epidermis near the wound edge
and in migrating keratinocytes (Figure 2D)
. The fluorescence of TGase 1
enzyme was clearly localized at the cell periphery, suggesting membrane
translocation of the enzyme (Figure 2G)
. At that time, the expression
of involucrin was detectable in the upper layers of the thickened
epidermis and in migrating keratinocytes (Figure 2, E and H)
.
Involucrin was also observed in differentiated layers of the epidermis
(Figure 2H)
, and almost completely co-localized with TGase 1 enzyme at
the cell periphery (Figure 2I)
. Some fluorescein
isothiocyanate-staining of cell debris and scabs at the wound sites and
the stratum corneum was nonspecific. In contrast, loricrin expression
was not evident at the wound edge and migrating epidermis (Figure 2F)
.
Even in the thickened epidermis, where the abundant expression of TGase
1 and involucrin was observed, loricrin expression was limited to a few
layers under the stratum corneum. The expression patterns of TGase 1
enzyme, involucrin, and loricrin were unchanged on the opposite side of
the wound edge examined in those samples (data not shown).
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Gene Expression in the TGase 1-/- Grafted
Skins
To elucidate the contribution of TGase 1 in wound healing, we
examined the effect of TGase 1 deficiency on epidermal regeneration.
Because of the neonatal lethality of TGase
1-/- mice,20
we grafted
their skins onto athymic nude mice and examined histological changes
and K6
mRNA expression during wound healing of those TGase
1-/- skin grafts. Two weeks after
transplantation, skin grafts from normal neonates resembled normal
adult mouse skin (Figure 3A)
, where the
expression of K6
mRNA was barely detectable in the epidermis (Figure 3B)
. In contrast, grafted skins from TGase
1-/- neonates showed thickened
epidermis with massive stratum corneum and immature hair follicles
(Figure 3G)
. K6
mRNA expression was evident in the epidermis (Figure 3H)
, suggesting hyperproliferation of the epidermis. An injury was then
made in the center of each graft, and those tissues were harvested up
to 11 days after wounding. At 48 hours after the injury, the wounds
were covered with bloody crusts but no apparent difference was observed
between wounds in control and in TGase
1-/- skin. In the control grafts,
epithelial spurs were elongated from the wound edges and had invaded
under the scab, and finally closed 5 days after the injury (Figure 3C)
.
At that time, K6
mRNA remained around the wound site, but its
expression was decreasing in the re-epithelialized epidermis (Figure 3D)
. On the other hand, at that same time, wounded TGase
1-/- skin grafts still had large clots
in the center, under which epithelial migration was apparently
interrupted (Figure 3I)
. A strong induction of K6
mRNA was evident
in the migrating epidermis (Figure 3J)
. At 11 days after wounding, the
control grafts had matured with hair follicles (Figure 3E)
and the
K6
mRNA expression had subsided. However, in TGase
1-/- skin grafts, epithelial spurs had
just fused in each center of the wound (Figure 3K)
and a diffuse
expression pattern of K6
mRNA remained. Thus, the regeneration of
the epidermis was markedly delayed and the expression of K6
was
uncontrolled in TGase 1-/-
grafted skins.
|
| Discussion |
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,31,32
interferon-
,33,34
interleukin-1,34
and/or activin A,35,36
might
participate in the maintenance of TGase 1 gene
expression during wound healing, because those cytokines are able to
activate the TGase 1 gene.
The regulation of TGase 1 and K6 genes
differ from each other in normal interfollicular epidermis because the
TGase 1 gene is primarily expressed at late stages of
keratinization, whereas K6 gene activation is not
evident except for limited epithelia.26,37
The machinery
for K6
and ß gene activation may not be identical
to each other because the induction of those mRNAs do not necessarily
co-localize in the wounded epidermis. However, the following evidence
suggests that regulatory systems common to TGase 1 and
those K6 genes are activated in the wound healing
process: 1) up-regulation of the TGase 1 gene occurs as
early as the K6 genes in injured neonatal mouse skin; 2)
the time course of TGase 1 gene expression overlaps that
of the K6 genes during wound healing; and 4) the 5'
upstream promoter of those genes are responsive to treatment with the
phorbol ester 12-O-tetradecanoylphorbol-13-acetate in the
epidermis of TGM1-LacZ and K6-LacZ transgenic
mice.26,37,38
The Jun transcription factor might mediate
signals for induction of those genes,39,40
but further
study of the complex mechanisms involved in gene regulation is
necessary to explore systems orchestrating epidermal regeneration.
The expression of K16, a type I partner for the K6 protein, leads to retraction of tonofilaments in keratinocytes and allows cell movement for re-epithelialization during wound healing.41 There is no doubt that the abundant generation of TGase 1 enzyme is required to prepare for the rapid remodeling of the stratum corneum, but there are no obvious reasons for its expression in the leading edges of migrating keratinocytes. The expression of TGase 1 enzyme occurs with involucrin expression in migrating keratinocytes, and eventually they are co-localized on the plasma membrane to form premature cornified envelopes. In contrast, loricrin is not expressed in the leading edges and its expression is restricted to the granular layers behind the migrating epidermis. Those premature cornified envelopes might provide mechanical strength to migrating epidermal cells dissecting the wound between the collagenous dermis and the fibrin eschar. During the process, leading-edge keratinocytes release tissue plasminogen activator,42 urokinase-type PA,43 and various matrix metalloproteinases44 that degrade the extracellular matrix for the dissection. Stabilized membrane structures reinforced by protein cross-linking might protect keratinocytes themselves from proteolytic damage. Similar premature expression of the TGase 1 gene45,46 and involucrin47 have been found in hyperplastic psoriatic epidermis, where it is evident that thinner premature cornified envelopes are generated.48 Analysis of the physical and biochemical nature of the prematurely cross-linked cornified envelope will be necessary to elucidate the role of TGase 1 in wound healing as well as in psoriasis.
We also demonstrate the substantial delay in epidermal migration with
sustained expression of the K6
gene during wound
repair of TGase 1-/- grafted
skins. The evidence suggests that TGase 1 is essential to early wound
healing of the injured skin. Neonatal TGase
1-/- mice show severely impaired skin
barrier function,20
but it is unlikely that the aberrant
skin barrier function directly affects epidermal regeneration, because
skin barrier function has been restored to normal levels in grafted
TGase 1-/- skins, possibly by
the massive thick covering of scales even though the cornified envelope
is lacking (unpublished data). Hiiragi and colleagues27
have shown that TGase 1 is concentrated in adherence junctions in
simple epithelial cells. The minor expression of TGase 1 in simple
epithelia, including endothelial cells other than keratinocytes, might
play a role in wound healing. However, TGase 1 expression is not
evident in any components of neonatal mouse skin other than the
epidermis and inner root sheaths.
In conclusion, TGase 1 is important not only for development and maturation of the epidermis, but also for the progression of cutaneous regeneration. The TGase 1 gene is an early responder gene to acute cutaneous injury, and is possibly involved in major changes in keratinocyte cytoarchitecture for migration into the wound site.
| Footnotes |
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Supported in part by a grant-in-aid from the Ministry of Education, Science, Sports, and Culture of Japan and by the Nakatomi Foundation.
Accepted for publication August 30, 2000.
| References |
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modulates expression of the tissue transglutaminase gene in liver cells. Am J Physiol 1998, 274:G240-G245
-hydroxyceramides to involucrin by ester bond formation. Proc Natl Acad Sci USA 1999, 96:8402-8407
: antagonism by retinoic acid and transforming growth factor ß1. J Biol Chem 1994, 269:2016-2022
induce the regenerative epidermal phenotype of psoriasis in the transwell skin organ culture system. IFN-
up-regulates the expression of keratin 17 and keratinocyte transglutaminase via endogenous IL-1 production. J Pathol 1999, 187:358-364[Medline]
specifically induce the activation- and hyperproliferation-associated keratins 6 and 16. Proc Natl Acad Sci USA 1993, 90:6786-6790This article has been cited by other articles:
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S. B. Ting, J. Caddy, N. Hislop, T. Wilanowski, A. Auden, L.-l. Zhao, S. Ellis, P. Kaur, Y. Uchida, W. M. Holleran, et al. A Homolog of Drosophila grainy head Is Essential for Epidermal Integrity in Mice Science, April 15, 2005; 308(5720): 411 - 413. [Abstract] [Full Text] [PDF] |
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