Morphological and functional evidence suggests that the glomerular visceral epithelial cells, podocytes, are crucial for maintaining the glomerular permeability barrier. 1-3Podocytes attach to the underlying basement membrane by mechanisms involving matrix receptors capable of inside-out and outside-in signaling that is important for rapid cell shape changes. 4,5 The attachment via integrins also appears crucial for the filtration function as evidenced by recent data for mice with genetic disruption especially of ␣3␤1 integrin. 6 The distinct structure of podocytes with primary and secondary foot processes interlinked by filtration slits appears to provide additional means for dynamic shape changes and regulation of functions. The composition and exact role of the filtration slits, however, remain to be determined. The structural complexity of podocytes is the characteristic feature of an intact glomerular filtration barrier, as contrasted with the flattening, retraction, and fusion of foot processes in human and experimental diseases with proteinuria.

SUMMARY:Diabetic nephropathy is a major complication of diabetes leading to thickening of the glomerular basement membrane, glomerular hypertrophy, mesangial expansion, and overt renal disease. The pathophysiologic mechanisms of diabetic nephropathy remain poorly understood. Nephrin is a recently found podocyte protein crucial for the interpodocyte slit membrane structure and maintenance of an intact filtration barrier. Here we have assessed the role of nephrin in two widely used animal models of diabetes, the streptozotocin model of the rat and the nonobese diabetic mouse. In both models, the expression levels of nephrin-specific mRNA as determined by real-time quantitative polymerase chain reaction increased up to two-fold during several weeks of follow-up. Immunohistochemical stainings revealed nephrin also more centrally within the glomerular tuft along with its preferential site in podocytes. Interestingly, as detected by immunoblotting, nephrin protein was also found in the urine of streptozotocin-induced rats. We conclude that nephrin is connected to the early changes of diabetic nephropathy and thus may contribute to the loss of glomerular filtration function. (Lab Invest 2001, 81:1185-1190.

Nephrin appears to be an important causative molecule of proteinuria and shows a remarkable redistribution from the filtration slits to the podocyte plasma membrane, especially in PAN.

Abstract. The molecular mechanisms maintaining glomerular filtration barrier are under intensive study. This study describes a mutant Nphs1 mouse line generated by gene-trapping. Nephrin, encoded by Nphs1, is a structural protein of interpodocyte filtration slits crucial for formation of primary urine. Nephrin trap/trap mutants show characteristic features of proteinuric disease and die soon after birth. Morphologically, fibrotic glomeruli with distorted structures and cystic tubular lesions were observed, but no prominent changes in the branching morphogenesis of the developing collecting ducts could be found. Western blotting and immunohistochemical analyses confirmed the absence of nephrin in nephrin trap/trap glomeruli. The immunohistochemical staining showed also that the interaction partner of nephrin, CD2-associated protein (CD2AP), and the slit-diaphragm-associated protein, ZO-1␣ Ϫ , appeared unchanged, whereas the major anionic apical membrane protein of podocytes, podocalyxin, somewhat punctate as compared with the wild-type (wt) and nephrin wt/trap stainings. Electron microscopy revealed that Ͼ90% of the podocyte foot processes were fused. The remaining interpodocyte junctions lacked slit diaphragms and, instead, showed tight adhering areas. In the heterozygote glomeruli, approximately one third of the foot processes were fused and real-time RT-PCR showed Ͼ60% decrease of nephrin-specific transcripts. These results show an effective nephrin gene elimination, resulting in a phenotype that resembles human congenital nephrotic syndrome. Although the nephrin trap/trap mice can be used to study the pathophysiology of the disease, the heterozygous mice may provide a useful model to study the gene dose effect of this crucial protein of the glomerular filtration barrier.

Despite of the increased availability of genetically modified mouse strains , the experimental models in the rat have provided the most widely employed and versatile models for the study of renal pathophysiology and functional genetics. The identification of the human gene mutated in the congenital nephrotic syndrome of the Finnish type (NPHS1) has recently been reported , and its protein product has been termed nephrin. Here we report the molecular cloning and characterization of rat nephrin cDNA. Rat nephrin cDNA has an open reading frame of 3705 bp , shows 82% sequence identity with human nephrin cDNA, and shows characteristic rat-specific splicing variants. The translated nucleotide sequence has 89% sequence identity at the amino acid level. The signal sequence , glycosylation , and cysteine localization patterns are nearly identical to those of human nephrin. As in the human , the rat nephrin transcript is expressed in a tissue-restricted pattern. Antipeptide antibodies raised to the intracellular nephrin-specific domain identified immunoreactivity exclusively within the rat kidney glomerulus by indirect immunofluorescence. Initial results with semiquantitative reverse transcriptase-polymerase chain reaction analysis showed a remarkable down-regulation of nephrin-specific mRNA in the puromycin nephrosis of the rat. (Am J Pathol 1999, 155:907-913)The molecular pathogenesis of diseases that result in abnormalities of glomerular filtration has remained poorly understood. Recent results indicate that podocytes play an important role in regulating the passage of macromolecules and have several structural properties suitable for allowing the rapid modulation of the permeability barrier.