SRGAP1 Controls Small Rho GTPases To Regulate Podocyte Foot Process Maintenance
Authors: Rogg Manuel , Maier Jasmin I. , Dotzauer Robert , Artelt Nadine , Kretz Oliver , Helmstädter Martin , Abed Ahmed , Sammarco Alena , Sigle August , Sellung Dominik , Dinse Patrick , Reiche Karoline , Yasuda-Yamahara Mako , Biniossek Martin L. , Walz Gerd , Werner Martin , Endlich Nicole , Schilling Oliver , Huber Tobias B. , Schell Christoph 
Published: January 2021

Significance Statement
Although the role of the podocyte cytoskeleton in the integrity of glomerular filtration is well established, the specific contribution of an upstream regulatory network comprising the Rho family of small GTPases—modulators of the actin cytoskeleton—is poorly characterized. The authors provide a comprehensive map of the podocyte Rho GTPase affinity interactome and identify a small GTPase-activating protein, SRGAP1, as a podocyte-specific RhoGAP. Through in vivo models of experimental FSGS, they demonstrate that SRGAP1 prevents podocyte foot-process effacement. They also show that SRGAP1 mediates spatial restriction of the activity of the Rho GTPase RAC1, thereby maintaining morphologic plasticity in disease conditions. These findings indicate that a comprehensive understanding of the regulatory networks of small Rho GTPases is needed for precisely targeted therapeutic interventions in glomerular diseases.

Background
Previous research demonstrated that small Rho GTPases, modulators of the actin cytoskeleton, are drivers of podocyte foot-process effacement in glomerular diseases, such as FSGS. However, a comprehensive understanding of the regulatory networks of small Rho GTPases in podocytes is lacking.

Methods
We conducted an analysis of podocyte transcriptome and proteome datasets for Rho GTPases; mapped in vivo, podocyte-specific Rho GTPase affinity networks; and examined conditional knockout mice and murine disease models targeting Srgap1. To evaluate podocyte foot-process morphology, we used super-resolution microscopy and electron microscopy; in situ proximity ligation assays were used to determine the subcellular localization of the small GTPase-activating protein SRGAP1. We performed functional analysis of CRISPR/Cas9-generated SRGAP1 knockout podocytes in two-dimensional and three-dimensional cultures and quantitative interaction proteomics.

Results
We demonstrated SRGAP1 localization to podocyte foot processes in vivo and to cellular protrusions in vitro. Srgap1fl/fl*Six2Cre but not Srgap1fl/fl*hNPHS2Cre knockout mice developed an FSGS-like phenotype at adulthood. Podocyte-specific deletion of Srgap1 by hNPHS2Cre resulted in increased susceptibility to doxorubicin-induced nephropathy. Detailed analysis demonstrated significant effacement of podocyte foot processes. Furthermore, SRGAP1-knockout podocytes showed excessive protrusion formation and disinhibition of the small Rho GTPase machinery in vitro. Evaluation of a SRGAP1-dependent interactome revealed the involvement of SRGAP1 with protrusive and contractile actin networks. Analysis of glomerular biopsy specimens translated these findings toward human disease by displaying a pronounced redistribution of SRGAP1 in FSGS.

Conclusions
SRGAP1, a podocyte-specific RhoGAP, controls podocyte foot-process architecture by limiting the activity of protrusive, branched actin networks. Therefore, elucidating the complex regulatory small Rho GTPase affinity network points to novel targets for potentially precise intervention in glomerular diseases.