Cloning of rat homologue of podocin: expression in proteinuric states and in developing glomeruli.

Podocin is identified as a product of the gene mutated in a patient with autosomal recessive steroid-resistant nephrotic syndrome. Although podocin is reported to be located at the slit diaphragm area, the precise role of podocin for maintaining the barrier function of the slit diaphragm has not been clearly elucidated. A rat homologue of podocin was cloned, and the expression of podocin was investigated and then compared with the nephrin and the ZO-1 expressions in rat experimental proteinuric models and in developing glomeruli. Amino acid sequences of rat and human podocin are highly homologous (84.3% identity). The domain structure of podocin is also highly conserved between rat and human. The mRNA expression for podocin was detected in glomeruli and the nerve tissues. The localization of podocin has close proximity to that of nephrin in normal adult rat glomeruli. Podocin staining was restricted to the basal side of the podocyte of the early developing stage, whereas nephrin staining was detected on the basolateral surface of podocyte. The redistribution of podocin was observed in the anti-nephrin antibody (ANA)-induced nephropathy and puromycin aminonucleoside (PAN) nephropathy. The redistribution of podocin paralleled with nephrin in ANA nephropathy but not in PAN nephropathy. Podocin is observed at the site of tight junction newly formed in proteinuric state in PAN nephropathy. It is postulated that podocin is one of the critical components of a slit diaphragm for maintaining the barrier function of the glomerular capillary wall.