ARHGEF7 (?-PIX) Is Required for the Maintenance of Podocyte Architecture and Glomerular Function

Significance StatementDysregulation of Cdc42 and other members of the Rho family of small GTPases in podocytes contributes to the pathogenesis of proteinuria. However, the upstream regulatory mechanisms for Cdc42 activity in podocytes are largely unknown. The authors identified ARHGEF7 (commonly known as ?-PIX) as a predominant guanine nucleotide exchange factor and activator of Cdc42 in podocytes. They also demonstrated that ?-PIX is required for the maintenance of podocyte architecture and glomerular function via Cdc42 and its downstream effects on Yes-associated protein (YAP) activity. Elucidating the precise details of how numerous regulatory proteins maintain the delicate balance of Rho GTPases in podocytes will be essential in understanding the pathogenesis of proteinuric glomerular diseases and identifying therapeutic targets. BackgroundPrevious studies showed that Cdc42, a member of the prototypical Rho family of small GTPases and a regulator of the actin cytoskeleton, is critical for the normal development and health of podocytes. However, upstream regulatory mechanisms for Cdc42 activity in podocytes are largely unknown.MethodsWe used a proximity-based ligation assay, BioID, to identify guanine nucleotide exchange factors that activate Cdc42 in immortalized human podocytes. We generated podocyte-specific ARHGEF7 (commonly known as ?-PIX) knockout mice by crossing ?-PIX floxed mice with Podocin-Cre mice. Using shRNA, we established cultured mouse podocytes with ?-PIX knockdown and their controls.ResultsWe identified ?-PIX as a predominant guanine nucleotide exchange factor that interacts with Cdc42 in human podocytes. Podocyte-specific ?-PIX knockout mice developed progressive proteinuria and kidney failure with global or segmental glomerulosclerosis in adulthood. Glomerular podocyte density gradually decreased in podocyte-specific ?-PIX knockout mice, indicating podocyte loss. Compared with controls, glomeruli from podocyte-specific ?-PIX knockout mice and cultured mouse podocytes with ?-PIX knockdown exhibited significant reduction in Cdc42 activity. Loss of ?-PIX promoted podocyte apoptosis, which was mediated by the reduced activity of the prosurvival transcriptional regulator Yes-associated protein.ConclusionsThese findings indicate that ?-PIX is required for the maintenance of podocyte architecture and glomerular function via Cdc42 and its downstream Yes-associated protein activities. This appears to be the first evidence that a Rho?guanine nucleotide exchange factor plays a critical role in podocytes.