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

Abstract

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. IntroductionPodocytes, the terminally differentiated visceral epithelial cells of the glomerulus, are responsible for forming and regulating the kidney filtration barrier. These cells have a remarkably complex cellular morphology, extending numerous interdigitating foot processes that surround the glomerular capillary walls and form unique specialized intercellular junctions known as slit diaphragms. The importance of slit diaphragms is enforced by the abundance of glomerular disorders that are caused by mutations in genes that encode components of this complex. A podocyte's intricate shape is maintained by a well-organized and dynamic actin cytoskeleton that is tightly regulated. In all forms of human proteinuric kidney disease, the podocyte undergoes cytoskeletal remodeling that results in foot process effacement and loss of normal filtration barrier selectivity, a process that is common to nearly all forms of podocyte injury, regardless of the underlying cause (1). The molecular mechanisms driving foot process effacement versus recovery are only beginning to be understood and are paramount to the identification of novel therapeutic strategies for proteinuria.The original goals of our studies were not only to identify pathways that are dysregulated in podocytes in response to injury, but also to select for those pathways that have the largest functional impact when dysregulated. To accomplish this, we designed and

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The FERM protein EPB41L5 regulates actomyosin contractility and focal adhesion formation to maintain the kidney filtration barrier

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