Podocyte-specific RAP1GAP expression contributes to focal segmental glomerulosclerosis-associated glomerular injury

Injury to the specialized epithelial cells of the glomerulus (podocytes) underlies the pathogenesis of all forms of proteinuric kidney disease; however, the specific genetic changes that mediate podocyte dysfunction after injury are not fully understood. Here, we performed a large-scale insertional mutagenic screen of injury-resistant podocytes isolated from mice and found that increased expression of the gene Rap1gap, encoding a RAP1 activation inhibitor, ameliorated podocyte injury resistance. Furthermore, injured podocytes in murine models of disease and kidney biopsies from glomerulosclerosis patients exhibited increased RAP1GAP, resulting in diminished glomerular RAP1 activation. In mouse models, podocyte-specific inactivation of Rap1a and Rap1b induced massive glomerulosclerosis and premature death. Podocyte-specific Rap1a and Rap1b haploinsufficiency also resulted in severe podocyte damage, including features of podocyte detachment. Over-expression of RAP1GAP in cultured podocytes induced loss of activated β1 integrin, which was similarly observed in kidney biopsies from patients. Furthermore, preventing elevation of RAP1GAP levels in injured podocytes maintained β1 integrin-mediated adhesion and prevented cellular detachment. Taken together, our findings suggest that increased podocyte expression of RAP1GAP contributes directly to podocyte dysfunction by a mechanism that involves loss of RAP1-mediated activation of β1 integrin.