Podocyte-specific RAP1GAP expression contributes to focal segmental glomerulosclerosis–associated glomerular injury
Uma Potla, … , Paul E. Klotman, Lewis Kaufman
Uma Potla, … , Paul E. Klotman, Lewis Kaufman
Published March 18, 2014
Citation Information: J Clin Invest. 2014;124(4):1757-1769. https://doi.org/10.1172/JCI67846.
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Research Article

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.

Authors

Uma Potla, Jie Ni, Justin Vadaparampil, Guozhe Yang, Jeremy S. Leventhal, Kirk N. Campbell, Peter Y. Chuang, Alexei Morozov, John C. He, Vivette D. D’Agati, Paul E. Klotman, Lewis Kaufman

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Figure 6

Podocyte-specific Rap1 haploinsufficiency induces FSGS.

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Podocyte-specific Rap1 haploinsufficiency induces FSGS. (A) Mice with ...
(A) Mice with 1 functional copy of Rap1 developed proteinuria and early death. All animals that developed proteinuria (60% overall) died early. HI, haploinsufficient. (B) Calculated urinary albumin/creatinine ratios showed heavy proteinuria. (C) Representative pathology for 6-week-old Rap1-haploinsufficient mice. Top: Whereas age-matched controls (left) exhibited normal histology, in haploinsufficient mice (right), 2 of 3 glomeruli showed severe segmental lesions. Middle: Control glomeruli (left) showed delicate mesangium, compared with moderate mesangial expansion affecting most glomeruli in haploinsufficient mice (right). Bottom: Many of these lesions progressed to FSGS (left), in which collapsing glomerulopathy was a prominent feature (right). (D) On electron microscopy, podocytes demonstrated complete foot process effacement with microvillous transformation and loss of normal primary and secondary process architecture. Scale bars: 5 μm (top), 2 μm (bottom). (E) Synaptopodin expression was completely lost in DKO mice and severely diminished in haploinsufficient mice, even in nonsclerotic glomeruli. (F) Severe podocyte loss was evident in DKO mice. Podocyte loss was present, but to a lesser extent, in haploinsufficient mice. A representative WT1-stained glomerulus for each group is shown. The number of WT1-positive cells per glomerular tuft area was determined for approximately 20–30 glomeruli per animal. n = 2–4 mice per group. *P < 0.001, **P < 0.02 versus age-matched WT control. Original magnification, ×200 (C, top); ×600 (C, middle and bottom); ×400 (E); ×630 (F).