Intracellular mislocalization of mutant podocin and correction by chemical chaperones

The NPHS2 gene encoding the podocin protein was causally linked to the autosomal recessive type of steroid-resistant nephrotic syndrome. In this study, we investigated the consequence of the R138Q mutation of podocin, one of the most common missense mutations in the NPHS2 gene, by examining the expression of the wild-type and R138Q mutant podocins in mammalian cells. Either myc- or FLAG-tagged wild-type podocin was strongly stained in plasma membrane, particularly in the fine processes wherein the protein was colocalized with actin stress fibers. On the other hand, the R138Q mutant podocin was completely retained intracellularly and colocalized with the endoplasmic reticulum (ER) marker, calnexin. These results suggest that the R138Q mutation affected podocin protein folding, thereby interfering with the mutant protein's departure from the ER. To determine if the ER retention of R138Q mutant is correctable, cells were incubated with the chemical chaperones glycerol, trimethylamine-N-oxide, and DMSO. Using these two methods, namely, cell surface labeling with sulfo-NHS-S-S-biotin and Alexa 488-streptavidin, and immunostaining to detect the podocin protein close to the plasma membrane, we confirmed that these chemical chaperone treatments elicit a cellular redistribution of R138Q podocin. Our results reveal defective cellular processing of the mutant podocin, and provide evidence for pharmacological correction of the processing defect.