Defective trafficking of nephrin missense mutants rescued by a chemical chaperone.

The nephrin gene (NPHS1) is mutated in congenital nephrotic syndrome of the Finnish type. Most mutations found in non-Finnish patients are missense mutations. The most common consequence of missense mutations in congenital nephrotic syndrome is a defect in intracellular transport and retention of the mutant proteins in the endoplasmic reticulum (ER), possibly as a result of misfolding and unfavored conformation. Because sodium 4-phenylbutyrate has been shown to function as a chemical chaperone and to correct the cellular trafficking of several mislocalized or misfolded mutant plasma membrane proteins, the effects of this compound on the missense mutants identified in patients with congenital nephrotic syndrome of the Finnish type were investigated. This study was performed using human embryonic kidney 293 cells stably expressing wild-type or missense nephrin mutants trapped in the ER. Immunofluorescence microscopy and cell surface biotinylation showed that treatment with sodium 4-phenylbutyrate rescued several of the missense mutants from the ER to the cell surface. All of the rescued mutants were found to be able to interact with Neph1. Furthermore, their tyrosine phosphorylation was rapidly induced by clustering with anti-nephrin antibodies, suggesting that the rescued mutants may be functionally intact.