Canonical transient receptor potential-6 (TRPC6) channels drive certain familial forms of focal and segmental glomerulosclerosis (FSGS) and there is evidence that they contribute to much more common acquired forms of FSGS and to renal fibrosis. TRPC6 dysregulation in podocytes occurs in animal models of FSGS, and in podocytes exposed to serum or plasma from patients with recurrent FSGS, or in cells treated with the soluble urokinase receptor (suPAR). While it is known that TRPC6 activation is required for Ca2+ influx driven by e.g. angiotensin II, the Ca2+-permeability of TRPC6 is quite limited, and there are many conditions in which TRPC6 functions primarily as a monovalent cation channel. This fundamental property of TRPC6 suggests that while it is necessary, it may not be sufficient to drive Ca2+ overload in podocytes or other cells.
Specific Aim 1 of this proposal tests the hypothesis that TRPC6 is part of a multichannel complex that includes KCa1.1 channels, and that coordinated KCa1.1 activation is necessary for TRPC6 to drive significant Ca2+ influx into podocytes. KCa1.1 is a Ca2+-activated K+ channel that interacts with TRPC6 and other slit diaphragm proteins including nephrin, Neph1, and podocin. Preliminary data show that KCa1.1 activation in podocytes is coupled to TRPC6, and that KCa1.1 is dysregulated in an animal model of FSGS and in response to recurrent FSGS plasma samples or suPAR.
Specific Aim 1 will characterize changes in KCa1.1 gating and current density in glomerular disease models previously shown to alter TRPC6. Quantitative Ca2+ imaging will address if KCa1.1 or its auxiliary ?- and ?-subunits facilitate TRPC6-dependent Ca2+ influx into podocytes, which drives normal cell signaling as well as pathological Ca2+ overload. There is now extensive evidence that suPAR contributes to progression of multiple kidney diseases, including some cases of primary FSGS, but the mechanisms of its signaling pathways are not well understood. Preliminary data show that the receptor for advanced glycation endproducts (RAGE) functions as an essential co-receptor for suPAR on the pathway leading to Rac1, oxidative stress, c-Src, and dysregulation of podocyte TRPC6.
Specific Aim 2 will examine the role of RAGE in driving albuminuria and glomerulosclerosis in a mouse model (suPAR2-Tg) that overexpresses a suPAR variant in adipocytes and secretes it into the circulation. These mice exhibit albuminuria that later progresses to glomerulosclerosis in a manner similar to human FSGS.
In Specific Aim 2 we will cross suPAR2-Tg mice with mice homozygous for an inducible knockout of RAGE. We will induce RAGE knockout both prior to and after the onset of albuminuria. We will also determine if two different small molecule RAGE antagonists reduce albuminuria and glomerulosclerosis in suPAR2-Tg mice. One of these antagonists, azeliragon, is orally bioavailable in mice and humans, and is currently undergoing a Phase 2/3 clinical trial in a subset of patients with Alzheimer's disease. Therefore, a positive result in Specific Aim 2 could have considerable translational significance for primary and recurrent FSGS, which in many patients is refractory to current treatments.
Focal and segmental glomerulosclerosis (FSGS) is a leading cause of kidney failure, and primary FSGS is often refractory to treatment. This research will probe two novel potential therapeutic targets for FSGS. These targets are the ion channel, KCa1.1 which may be necessary Ca2+ overload in podocytes, and the receptor for advanced glycation endproducts (RAGE), which is necessary for pathogenic effects mediated by the soluble urokinase receptor (suPAR) in podocytes and other cell types.
