(B) Western blotting of microsomal proteins shows increased Axl phosphorylation in DT-treated kidneys. evidence of tubular damage or increased apoptosis, resulting in an increase in total tubular cell numbers. The proliferative response coincided with detection of the growth factor Gas6 in the urine and phosphorylation of the Gas6 receptor Axl in the apical membrane of renal tubular cells. In contrast, ablation of >40% of podocytes led to CHM 1 progressive glomerulosclerosis, profound tubular injury, and renal failure. These data suggest that glomerular proteinuria in the absence of severe structural glomerular injury activates tubular proliferation, potentially as an adaptive response to minimize the loss of filtered proteins. Podocytes, mesangial cells, and glomerular endothelial cells together contribute to the formation and maintenance of the kidney ultrafiltration barrier that serves to retain the majority of plasma proteins within the vascular space.15This barrier can be compromised by injury to any of these cells, resulting in glomerular proteinuria that has in turn been associated with progressive renal dysfunction. Of these cells, podocyte damage is the most problematic because podocytes provide a critical component of the filtration barrier and seem to lack the ability to proliferate in response to cell loss. Multiple causes of podocyte dysfunction have been identified, including genetic defects in components of the slit diaphragm, toxins, deposition of abnormal proteins in the glomerular basement membrane (GBM), and immune-mediated injury.68 There have been numerous animal models developed to study the effect of glomerular injury on kidney function.9,10Typically, these models involve injection of general cellular toxins such as adriamycin or nephritic serum containing antibodies directed against either the GBM or podocyte-specific antigens. In most cases, there is widespread damage to multiple cell types in the glomerulus with a substantial inflammatory response, making it difficult to determine which components of the subsequent kidney injury are related specifically to podocyte injury and which are related to the ensuing response. One of the key responses to glomerular damage is albuminuria, and several studies have suggested that glomerular albuminuria can lead to subsequent tubule damage.11,12In vitrostudies in which albuminuria was modeled by culturing tubular cells in the presence of albumin have led to conflicting results. Whereas some groups demonstrated that albumin can cause oxidative stress13and programmed cell death,14,15other studies showed that albumin is one of the major serum survival factors for renal tubular cells and can serve to scavenge reactive oxygen species.16,17 To induce selective podocyte loss via a nonimmune mechanism, we utilized mice CHM 1 that contained the transgene for diphtheria toxin receptor (DTR) downstream DKFZp781B0869 of a polyadenylation signal flanked by loxP sites (iDTR mouse18). Because mice normally lack the DTR, expression of the Cre recombinase in a podocyte-specific manner (Pod-Cre19) results in DTR expression selectively in podocytes, allowing subsequent podocyte ablation induced by diphtheria toxin (DT) to be performed in a dose-dependent manner without concomitant injury to other glomerular components. A similar approach has been successfully used for selective podocyte ablation in the rat. 20In these studies, we demonstrate that structural tubular injury occurs only when podocyte loss is sufficient to cause glomerular damage, and is not seen in the setting of isolated glomerular albuminuria. In fact, modest podocyte loss results in marked albuminuria that is accompanied by a proliferative response of proximal tubule cells without evident tubular injury, leading to an increase in total proximal tubule cell numbers. We propose that the proteinuria that accompanies selective podocyte loss, in the absence of more generalized glomerular injury, leads to an increase in proximal tubule cell numbers that may provide an adaptive response to limit the loss of filtered proteins in the urine. == Results == == iDTR+/;Pod-Cre+/Mice Express DTR in Podocytes == iDTR+/;Pod-Cre+/mice were CHM 1 found to be viable and fertile. To examine the expression pattern of DTR, multiple organs were isolated from double transgenic (iDTR+/;Pod-Cre+/)18,19and control mice (iDTR+/, Pod-Cre+/, and wild type). These included heart, lung, liver, pancreas, small intestine, spleen, kidney, and skeletal muscle. Immunofluorescence microscopy demonstrated that DTR expression was found only in the glomeruli of theiDTR+/;Pod-Cre+/mice, with anti-Wt1 and antinephrin staining confirming co-localization restricted to podocytes (Figure 1, A and Band data not shown). No DTR expression was detected in any other organ ofiDTR+/;Pod-Cre+/mice or in any organ ofiDTR+/-orPod-Cre+/mice. Analysis of DTR immunostaining of multiple kidney sections fromiDTR+/;Pod-Cre+/mice demonstrated that 99.5% of Wt1+podocytes co-expressed the DTR. == Figure 1. == DTR expression in podocytes. Immunofluorescent staining with anti-DTR (green) and anti-Wt1 (pink) was performed to evaluate kidney sections. (A) Podocytes of the single transgeniciDTR+/mouse.