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Technical Advance |



From the Department of Medical Biochemistry* and the Electron Microscopy Unit,
Institute of Anatomy and Cell Biology, Göteborg University, Gothenburg, Sweden; AngioGenetics AB,
Gothenburg, Sweden; and Clinical Cell Biology,
Graduate School of Medicine, Chiba University, Chiba, Japan
| Abstract |
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10% of whole kidney tissues and are unique structures of microvasculature mainly made up of three highly specialized cell types; fenestrated endothelial cells, mesangial cells, and podocytes. These cell types together with the glomerular basement membrane form the permeable barrier across which blood is filtered to produce primary urine. During the past decade several gene products have been shown to play essential roles in glomerulus development,3-5
function, and pathology.6
However, our knowledge of the molecular mechanisms governing glomerulus morphogenesis and development of the specialized features of its individual cells is still very limited. An obvious difficulty in addressing these issues stems from the low abundance of the glomerulus cells and the inability of the glomerulus cell types to retain their differentiated features in cell culture. Podocytes, for example, make up less than 2% of kidney tissues. Although endothelial cells and pericytes exist outside the glomerulus, their phenotype within the glomerulus is quite distinct from related cells elsewhere.7 We describe a new protocol for the isolation of glomeruli from mice. The protocol is fast and allows for the isolation of virtually all glomeruli present in a mouse kidney at 97% purity. The method thus allows for transcript profiling and proteomic analysis of the glomerulus using standard procedures.
| Materials and Methods |
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Collagenase A was purchased from Roche (Roche Diagnostics GmbH, Mannheim, Germany). Deoxyribonuclease I and Hanks balanced salt solution (HBSS) were from Invitrogen (Invitrogen AB, Lidingö, Sweden). Dynabeads M-450 tosylactivated (
4.5 µm) and magnetic particle concentrator (MPC) were from Dynal (Dynal A.S., Oslo, Norway). The surface of the Dynabeads was inactivated according to manufacturers instructions before use. Cell strainers were from Falcon (BD Biosciences, Stockholm, Sweden). The mouse nephrin cDNA was kindly provided by Dr. Heli Putaala of Karolinska Institute, Stockholm, Sweden. The mouse Tie 2 cDNA was kindly provided by Dr. Tom Sato of University of Texas Southwestern Medical Center, Dallas, TX.
Mice
Mice used were C57BL6 and 129/sv or hybrids between the two. Mice were housed at the Department of Experimental Biomedicine at Göteborg University according to Swedish animal research regulations. All experiments were approved by a local committee for ethics in animal research.
Isolation of Glomeruli
Mice were anesthetized by an intraperitoneal injection of Avertin (2,2,2-tribromoethyl and tertiary amyl alcohol; 17 µl/g mice) and perfused with 8 x 107 Dynabeads diluted in 40 ml of phosphate-buffered saline through the heart. The kidneys were removed, minced into 1-mm3 pieces, and digested in collagenase (1 mg/ml collagenase A, 100 U/ml deoxyribonuclease I in HBSS) at 37°C for 15 minutes (for newborn mice) or 30 minutes (for adult mice) with gentle agitation. The collagenase-digested tissue was gently pressed through a 100-µm cell strainer using a flattened pestle and the cell strainer was then washed with 5 ml of HBSS. The filtered cells were passed through a new cell strainer without pressing and the cell strainer washed with 5 ml of HBSS. The cell suspension was then centrifuged at 200 x g for 5 minutes. The supernatant was discarded and the cell pellet was resuspended in 2 ml of HBSS. Finally, glomeruli containing Dynabeads were gathered by a magnetic particle concentrator and washed for at least three times with HBSS. During the procedure, kidney tissues were kept at 4°C except for the collagenase digestion at 37°C.
Morphological Studies
Dynabead-perfused kidneys were snap-frozen for cryostat sectioning. Sections were stained with hematoxylin and eosin (H&E) and were examined by light microscopy. Isolated glomeruli were examined by both light and electron microscopy (EM). Specimens for EM were fixed with 2% paraformaldehyde and 2.5% glutaraldehyde. Glomeruli intended for transmission EM were subjected to ferrocyanide-reduced OsO4, dehydration, and plastic infiltration before ultrathin sectioning. For scanning EM glomeruli were osmicated according to the OTOTO protocol8 and dried using hexamethyldisilazane evaporation.9
Analysis of RNA Integrity
Total RNA was isolated using the RNase mini kit (Qiagen Inc., Valencia, CA) according to the manufacturers instruction. Northern blot analysis was performed as described previously10 using 32P-labeled nephrin cDNA, Tie 2 cDNA, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and ß-actin cDNA probes.
| Results |
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| Discussion |
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Methods to isolate glomeruli from rat15 and rabbit16 using sieving techniques have already been described. However, it has been difficult to isolate pure glomeruli from mouse by sieving because the diameter of mouse glomeruli is relatively similar to their tubules. As mice are widely used as experimental models of development and disease, it is important to be able to isolate glomeruli from mice. Isolation of mouse glomeruli after Fe3O4 perfusion has been reported,17 however the isolation efficiency was rather limited.
Here we describe a highly efficient method for the isolation of mice glomeruli using spherical Dynabeads containing iron. Dynabeads are made of a monodisperse polymer and exhibit magnetic properties within a magnetic field.18 Their surface is smooth with a coated polymer shell that reduces the direct damage to the tissues when they are perfused and protect from toxic exposure to iron. For our purpose, the Dynabeads were perfectly fitted not only to specifically embolizing the glomerular capillaries but their use also simplified and shortened the time of the isolation procedure which reasonably reduced cell damage to a minimum. The collagenase digestion and gentle filtration steps detached effectively the morphologically intact glomeruli from surrounding tissues. The method has good advantages not only for morphological studies but also for keeping intact the in vivo mRNA and protein profiles. The isolated glomeruli were well covered by podocytes and we could readily observe the fine structures of the podocyte slit diaphragms and fenestrated endothelial cells by electron microscopic analysis. The method can be applied to both developing glomeruli and to models of adult glomerular disease. In the Northern blotting analysis, we used nephrin cDNA and Tie 2 cDNA to verify the enrichment of glomeruli and their integrity in our preparations. Nephrin is a recently identified novel protein expressed only at the glomerular podocyte slit diaphragm. Nephrin has been reported to be mutated in congenital nephrotic syndrome of the Finnish type.19 Tie-2 is an endothelium-specific receptor tyrosine kinase, which binds to angiopoietin-1 and -2.20 Its strong expression in the kidney glomerulus compared with whole kidney probably reflects the fact that 50% of the glomerulus consists of endothelial cells, whereas such cells are far less abundant in the rest of the kidney. Thus, it seems clear that mRNAs with expression in a low proportion of cells are hard to detect when analyzing whole organs because of their dilution.
The yield in our isolation method was
20,000 glomeruli per adult mouse, which is consistent with the amount of glomeruli found in an adult mouse.11
This was also confirmed by the lack of glomeruli in the rest of the tissues not collected by the magnet (data not shown). Thus, the method described here is an efficient large-scale method for isolation of intact glomeruli from mice. The method enables the application of techniques for systematic analysis of gene and protein expression, such as Expressed Sequence Tag (EST) sequencing,21
serial analysis of gene expression,22
DNA microarray hybridization,23
and proteomics.24
The speed of the method is important for preserving the in vivo transcript and protein profile. The use of mice for these types of analyses opens up the possibility of exploring numerous transgenic models of developmental and pathophysiological disturbance of glomerular function at the molecular level. Because our method is based on perfusion, it will not work in cases in which there is a complete fallout of glomerular capillaries. However, it seems to work in cases of incomplete capillary development, as shown by the efficient isolation of immature stage glomeruli. The method also works well for diabetic mice and transgenic models with increased accumulation of mesangial matrix (our own unpublished observations). Thus, the method should prove useful when analyzing changes in gene and protein expression profiles during the later stages of glomerular development and during early stages of glomerular disease.
| Acknowledgements |
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| Footnotes |
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Supported by grants from the Swedish Cancer Foundation, Vetenskapsrådet, the Inga-Britt and Arne Lundberg Foundation, the Novo Nordisk Foundation, and an European Molecular Biology Organization (EMBO) postdoctoral fellowship (to H. G.).
Accepted for publication May 28, 2002.
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