Rac1 Suppression by the Focal Adhesion Protein GIT ArfGAP2 and Podocyte Protection.

[KEY POINTS] Focal adhesion protein GIT2 protected podocytes from injury in rodent proteinuric disease models. GIT2 facilitated translocation of tyrosine phosphatase PTP1B to focal adhesions where it dephosphorylates p130Cas, thereby suppressing Rac1 activity. Stabilizing GIT2 or facilitating GIT2 localization to focal adhesions in podocytes could be a therapeutic strategy in proteinuric kidney diseases.

[BACKGROUND] Podocytes have an intricate structure featured by numerous actin-based projections called foot processes. Rho family of small GTPases, including Ras-related C3 botulinum toxin substrate 1 (Rac1), play important roles in actin cytoskeletal remodeling required for cell morphology and adhesion. We previously showed that Rac1 activation in podocytes causes foot process effacement and proteinuria, but the upstream and spatiotemporal regulatory mechanism directing Rac1 is largely unknown. Recently, we identified the focal adhesion protein GIT ArfGAP2 (GIT2) as one of the Rac1 interactors in human podocytes by proximity-dependent biotin identification and proteomics.

[METHODS] Systemic and podocyte-specific GIT2 knockout mice were generated and assessed for kidney phenotypes. Human podocytes with GIT2 knockdown (KD) and overexpression were established using lentiviral transduction and characterized.

[RESULTS] GIT2 was enriched in glomeruli, including podocytes, in the mouse kidney. Gene deletion of in podocytes caused exacerbated proteinuria and foot process effacement when subjected to the minimal change disease model and salt-sensitive hypertension model, which were improved by pharmacological inhibition of Rac1. In cultured podocytes, GIT2 KD resulted in Rac1-dependent cell spreading with marked lamellipodial protrusions, accelerated focal adhesion disassembly, and shorter focal adhesion lifetime. In GIT2 KD podocytes, tyrosine phosphorylation of the focal adhesion protein p130 Crk-associated substrate (Cas) was significantly increased, accompanied by impaired localization of the tyrosine phosphatase PTP1B to focal adhesions. These phenotypes observed in GIT2 KD podocytes were reversed by GIT2 overexpression.

[CONCLUSIONS] The results indicate that GIT2 facilitates translocation of PTP1B to focal adhesions where it dephosphorylates p130Cas, thereby suppressing local Rac1 activity and protecting against podocyte injury and proteinuria.