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.

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.

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.

The molecular basis of glomerular permselectivity remains largely unknown. The congenital nephrotic syndrome of the Finnish type (CNF) characterized by massive proteinuria already present but without extrarenal symptoms is a unique human disease model of pure proteinuria. In search of genes and pathophysiologic mechanisms associated with proteinuria, we used differential display-PCR to identify differences in gene expression between glomeruli from CNF and control kidneys. A distinctly underexpressed PCR product of the CNF kidneys showed over 98% identity with a mitochondrially encoded cytochrome c oxidase (COX I). Using a full-length COX I cDNA probe, we verified down-regulation of COX I mRNA to 1/4 of normal kidney values on Northern blots. In addition, transcripts of other mitochondrially encoded respiratory chain complexes showed a similar down-regulation whereas the respective nuclearly encoded complexes were expressed at comparable levels. Additional studies using histochemical, immunohistochemical, in situ hybridization, RT-PCR, and biochemical and electron microscopic methods all showed a mitochondrial involvement in the diseased kidneys but not in extrarenal blood vessels. As a secondary sign of mitochondrial dysfunction, excess lipid peroxidation products were found in glomerular structures in CNF samples. Our data suggest that mitochondrial dysfunction occurs in the kidneys of patients with CNF, with subsequent lipid peroxidation at the glomerular basement membrane. Our additional studies have revealed similar down-regulation of mitochondrial functions in experimental models of proteinuria. Thus, mitochondrial dysfunction may be a crucial pathophysiologic factor in this symptom.

The novel gene NPHS1 is defective in the patients with congenital nephrotic syndrome of the Finnish type (CNF) leading to abnormal expression of the respective protein product nephrin in glomerular cells. CNF patients are treated with early nephrectomy and renal transplantation, but about 20% show recurrence of nephrotic syndrome (NS). We used indirect immunofluorescence microscopy and immunoblotting and an ELISA assay to search for circulating autoantibodies to nephrin, the protein defect in CNF patient kidneys. In serial serum samples gathered before and after recurrence of NS, we show an increased antibody titer to nephrin prior to the NS episode and a subsequent drop in antibody level after its successful treatment and reactivity of the high titer sera with glomeruli in indirect immunofluorescence microscopy as well. The results show that the transplantation treatment introduces a neoantigen inducing production of autoantibodies, which may be pathogenic for perturbation of the function of the glomerular filtration barrier.