Changes in podocyte number or density have been suggested to play an important role in renal disease progression. Here , we investigated the temporal relationship between glomerular podocyte number and development of proteinuria and glomerulosclerosis in the male Munich Wistar Fromter (MWF) rat. We also assessed whether changes in podocyte number affect podocyte function and focused specifically on the slit diaphragm-associated protein nephrin. Age-matched Wistar rats were used as controls. Estimation of podocyte number per glomerulus was determined by digital morphometry of WT1-positive cells. MWF rats developed moderate hypertension , massive proteinuria , and glomerulosclerosis with age. Glomerular hypertrophy was already observed at 10 weeks of age and progressively increased thereafter. By contrast , mean podocyte number per glomerulus was lower than normal in young animals and further decreased with time. As a consequence , the capillary tuft volume per podocyte was more than threefold increased in older rats. Electron microscopy showed important changes in podocyte structure of MWF rats , with expansion of podocyte bodies surrounding glomerular filtration membrane. Glomerular nephrin expression was markedly altered in MWF rats and inversely correlated with both podocyte loss and proteinuria. Our findings suggest that reduction in podocyte number is an important determinant of podocyte dysfunction and progressive impairment of the glomerular permselectivity that lead to the development of massive proteinuria and ultimately to renal scarring. Proteinuric nephropathies progress toward end-stage renal failure independently of the primary insult. Proteinuria is the leakage of plasma proteins into the urine due to dysfunction of the glomerular barrier, which loses its permselective properties. Increasing evidence suggests that the visceral glomerular epithelial cell is a key determinant in the maintenance of the permselective function of the glomerular capillary.1-5 Podocytes are highly differentiated and specialized epithelial cells anchored to the glomerular basement membrane (GBM). Foot processes of neighboring podocytes interdigitate each other over the capillary wall and are bridged by the slit diaphragm forming the filtration barrier. The most characteristic structural change of damaged podocytes, concomitant with proteinuria, consists of foot process effacement, reorganization of actin cytoskeleton, and apical dislocation of the slit diaphragm. [5][6][7][8] We have recently demonstrated that in a genetic model of spontaneous glomerulosclerosis, the male Munich Wistar Fromter (MWF) rat, 9 proteinuria paralleled redistribution of the slit diaphragm protein zonula occludens-1 in the absence of changes in the ultrastructure of the podocyte foot processes as measured by mean foot process width.

Today angiotensin II inhibition is primarily used to slow the rate of progression of kidney diseases. There is evidence that these therapies can induce a partial regression of glomerular lesions. However, we do not know yet the extent of sclerotic lesion regression and whether new glomerular tissue is formed to help support the renal function. We used male Munich Wistar Fromter (MWF) rats, an experimental model for progressive kidney disease, to quantify kidney structural lesions upon angiotensin-converting enzyme (ACE) inhibition therapy. Animals were studied at 50 weeks of age, when renal function and structure are severely altered, and after a 10-week observation period, without or with treatment with lisinopril (80 mg/l in drinking water). A group of untreated Wistar rats was used as controls. With age, proteinuria, and serum creatinine worsen, but lisinopril almost normalized proteinuria and stabilized serum creatinine. Serial section analysis of whole glomerular tufts showed that at baseline, glomerulosclerosis affected the entire glomerular population, and that these changes further increased with age. Lisinopril significantly reduced incidence and extent of glomerulosclerosis, with the presence of glomerular tufts not affected by sclerosis (23% of glomeruli). Glomerular volume was not significantly affected by treatment, and glomerular mass spared from sclerosis increased from 46.9 to 65.5% upon treatment, indicating consistent regeneration of glomerular tissue. Lisinopril normalized baseline glomerular transforming growth factor-beta and alpha-smooth muscle actin overexpression, and prevented worsening of interstitial changes. Hence, ACE inhibition, which is widely used in human kidney disease, may not only halt the progression of renal failure, but also actually induce the regeneration of new renal tissue.

Angiotensin-converting enzyme (ACE) inhibition induces glomerular repair in the Munich WistarFrömter (MWF) rat, a model of spontaneous glomerular injury. In this study, we investigated whether this effect is related to changes in glomerular cell number, particularly of podocytes, which are progressively lost with age. MWF rats with advanced nephropathy were studied at both 40 weeks and after 20 weeks of observation either with or without treatment with the ACE inhibitor lisinopril. Forty-weekold Wistar rats were used as controls. In untreated MWF rats, proteinuria, hypertension, glomerulosclerosis, and renal function worsened, while lisinopril induced regression of both functional and structural changes. Despite glomerular hypercellularity in untreated MWF rats, the number of endothelial cells per glomerulus did not change, and podocyte number even decreased. ACE inhibition halted the progressive increase in glomerular cell number and enhanced endothelial cell volume density. Surprisingly, lisinopril not only halted age-related podocyte loss but also increased the number of glomerular podocytes above baseline, which was associated with an increased number of proliferating Wilms tumor 1-positive cells, loss of cyclin-dependent kinase inhibitor p27 expression, and increased number of parietal podocytes. These data indicate that ACE inhibition restructures glomerular capillary , primarily by restoring the podocyte population in this model of glomerular injury. Increased parietal podocyte number in lisinopril-treated MWF rats suggests that the remodeling of Bowman's capsule epithelial cells contributes to this effect.

Ameliorating the function of the glomerular barrier to circulating proteins by blocking angiotensin II (Ang II) translates into less risk of progression toward endstage renal failure in diabetic and nondiabetic nephropathies. However, the mechanisms underlying this barrier protection are not clear. Specialized contacts between adjacent podocytes are major candidate targets, and the actin cytoskeleton is emerging as a regulatory element. Here, we present data demonstrating that Ang II induced reorganization of F-actin fibers and redistribution of zonula occludens-1 (ZO-1) that is physically associated with actin in murine podocytes. These effects were paralleled by increased albumin permeability across podocyte monolayers. The F-actin stabilizer jasplakinolide prevented both ZO-1 redistribution and albumin leakage, suggesting that actin cytoskeleton rearrangement is instrumental to podocyte permselective dysfunction induced by Ang II. Changes in both F-actin and ZO-1 patterns were confirmed in glomeruli of rat isolated perfused kidneys on short infusion of Ang II, leading to increased protein excretion. Podocyte dysfunction was mediated by Ang II type 1 receptor and was partly dependent on Src kinase-phospholipase C activation. These data demonstrate that strategies aimed at stabilizing podocyte-podocyte contacts and targeting the relevant intracellular signal transduction are crucial to renoprotection.