A Point Mutation in p190A RhoGAP Affects Ciliogenesis and Leads to Glomerulocystic Kidney Defects

Rho family GTPases act as molecular switches regulating actin cytoskeleton dynamics. Attenuation of their signaling capacity is provided by GTPase-activating proteins (GAPs), including p190A, that promote the intrinsic GTPase activity of Rho proteins. In the current study we have performed a small-scale ENU mutagenesis screen and identified a novel loss of function allele of the p190A gene Arhgap35, which introduces a Leu1396 to Gln substitution in the GAP domain. This results in decreased GAP activity for the prototypical Rho-family members, RhoA and Rac1, likely due to disrupted ordering of the Rho binding surface. Consequently, Arhgap35-deficient animals exhibit hypoplastic and glomerulocystic kidneys. Investigation into the cystic phenotype shows that p190A is required for appropriate primary cilium formation in renal nephrons. P190A specifically localizes to the base of the cilia to permit axoneme elongation, which requires a functional GAP domain. Pharmacological manipulations further reveal that inhibition of either Rho kinase (ROCK) or F-actin polymerization is able to rescue the ciliogenesis defects observed upon loss of p190A activity. We propose a model in which p190A acts as a modulator of Rho GTPases in a localized area around the cilia to permit the dynamic actin rearrangement required for cilia elongation. Together, our results establish an unexpected link between Rho GTPase regulation, ciliogenesis and glomerulocystic kidney disease.
Author Summary Glomerulocystic kidney disease occurs either in isolation or in combination with other cystic diseases. To date, the paucity of mouse models have impeded our progress in understanding the molecular mechanisms leading to glomerular cyst development. Using an ENU mutagenesis approach, we present here a novel mouse model of glomerular cyst formation caused by a point mutation in p190A RhoGAP (Arhgap35), which is associated with aberrant primary ciliogenesis. The primary cilium is a microtubule-based signalling center involved in cell differentiation and homeostasis. A role for the actin cytoskeleton in ciliogenesis has recently emerged, but the underlying regulatory mechanisms remain poorly understood. In this study we identify a requirement for the local modulation of Rho GTPase by p190A RhoGAP during primary cilium formation, which constitutes the first demonstration of ciliogenic regulation by RhoGAP proteins. Together, this work identifies deficiencies in actin cytoskeletal dynamics as an underlying cause for ciliary and glomerulocystic malformations.