Podocyte depletion and glomerulosclerosis have a direct relationship in the PAN-treated rat

Background. Podocytes are highly differentiated glomerular epithelial cells with limited potential to divide. They are responsible for maintaining and supporting the glomerular basement membrane so as to facilitate efficient filtration. The hypothesis tested was whether the development of glomerulos clerosis in the puromycin aminonucleoside (PAN)-treated rat could be attrib uted to podocyte depletion. Methods. PAN was injected in Sprague-Dawley rats once, twice, or three time s at 30-day intervals. Podocytes were counted in glomeruli using immunopero xidase histochemistry and antibodies to both GLEPP1 (PTPRO) and WT-1. Podoc ytes were assayed in urine using reverse transcription-quantitative polymer ase chain reaction (RT-QPCR). Glomerular areas were measured by computerize d morphometry. Results. In a preliminary experiment, a single injection of PAN caused a re duction in the glomerular podocyte count by 25%. Additional independent con firmation that podocytes were lost from glomeruli after PAN injection was o btained identifying detached podocytes in Bowman's space, measurement of ne phrin and GLEPP1 mRNAs in urine, ultrastructural analysis of glomeruli, and identification of TUNEL-positive apoptotic podocytes in glomeruli. In a se cond experiment, sequential podocyte depletion by 15, 31, and 53% was achie ved by the administration of one, two, or three injections of PAN at 30-day intervals. The region of the glomerulus devoid of podocytes developed glom erulosclerosis, and this area progressively increased as podocytes were pro gressively depleted. The correlation coefficient (r(2)) value for the relat ionship between percent podocyte depletion and glomerulosclerotic area was 0.99. The Y intercept of this plot showed that glomerulosclerosis was initi ated when only 10 to 20% of podocytes were lost. Conclusion. This report supports the growing body of data linking glomerulo sclerosis directly to a reduction in relative podocyte number [increased gl omerular area per podocyte (GAPP)]. It raises important questions related t o the mechanisms of podocyte loss, strategies for prevention of podocyte de pletion, and the prevention of progression of glomerular diseases.