Integrins are extracellular matrix receptors, which regulate vital cell functions. The ?8 subunit partners with alpha-v, and expression is restricted to kidney, brain, and placenta. ?8 expression is localized to mesangial cells (MCs) in vivo and in vitro. Itg?8-/- mice develop mild renal disease, but the phenotype is incompletely defined due to premature lethality. Kidney ?8 expression is decreased in mouse models of glomerular disease, and MCs derived from Itg?8-/- mice develop myofibroblast features. Of known ?8 ligands, only latency-associated peptide (LAP), which combines with TGF? to form the small latent complex (SLC), to maintain TGF? in an inactive (latent) state, is detected in kidney. SLC binding to ?8 can lead to TGF? activation through MT1-MMP cleavage of LAP, though MT1-MMP is not expressed in glomeruli, but can be induced in disease states. We show that LAP is expressed in MCs, and stimulates ?8-dependent lamellopodia and migration, consistent with Rac1 activation. As a screen for MC ?8 ligands, we show that matrix secreted by podocytes is superior to mesangial matrix for ?8-dependent cell adhesion - the first demonstration of any ?8 ligand supporting stable adhesions. We have shown that ?8 interacts with Rho GDI. The major function of GDI is to sequester Rho family G-proteins in cytosol, which prevents G-protein activation or association with plasma membrane effectors. We show that ?8 clustering enhances ?8-GDI interaction and Rac1 activation in MCs, whereas Itg?8-/- and Gdi-/- MCs exhibit myofibroblast features, such as alpha-smooth muscle actin (SMA) assembly and RhoA activation, as well as Rac1 suppression. We hypothesize that latent TGF? ligation with MC ?8 recruits GDI-bound Rac1 to the ?8 cytoplasmic tail. The ?8-GDI interaction dissociates Rac1 from GDI, which facilitates Rac1 activation to suppress MC myofibroblast differentiation. In pathologic states, MC ?8 expression is decreased, resulting in loss of LAP stimulation, cytoplasmic retention of Rac1 by GDI, and impaired MC binding to GBM. The net effect is that MCs transform into myofibroblasts with altered surveillance of the glomerular capillary endothelium.
Specific aims : (1) In vivo characterization of ?8 function using mice with MC-targeted ?8 deletion and a rat model of acquired glomerulosclerosis. (2) Characterize the MC ?8 ligand. (3) Test whether ?8 functions as a RacGDI displacement factor to suppress MC myofibroblast differentiation.
Over 20 million people in the U.S. suffer from chronic kidney disease, which usually starts with injury to the glomerulus, the filtering unit of the kidney. Our proposal aims to define the role of a specific receptor, the beta-8 integrin, which is expressed on the glomerular mesangial cell, in regulating kidney disease. This integrin is found in the brain, kidney and placenta and has been well-characterized only in brain. We are currently the only group working on kidney beta-8 integrin, and we provide preliminary data that beta-8 loss of function leads to kidney disease. To achieve the goals of this proposal, we are using mouse models, in which the beta-8 integrin has been selectively deleted from mesangial cells, to avoid confounding effects from brain. In addition, we are employing mesangial cell culture models, which can be more easily manipulated, upon completion of this project;we hope to provide new information about the kidney beta-8 integrin, which could lead to new therapies for chronic kidney diseases.
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