Uncontrolled accumulation of collagen within glomeruli leads to glomerulosclerosis, the final common pathway of end stage renal disease. The control of collagen homeostasis is complex and is mediated, at least in part by integrins, transmembrane receptors for extracellular matrix components. Mesangial cells, one of the major cell types in the glomerulus, express the two major collagen binding receptors, namely integrins alpha1beta1 and alpha2beta1 which are structurally highly conserved, yet have diverse affinities for ligands, and mediate opposite signals upon ligand stimulation. Integrin a1p1 binds collagen IV with high affinity, while integrin a2p1 has lower affinity for this ligand. In vitro studies suggest that integrin a2p1 enhances collagen I synthesis, whereas integrin a1(31 downregulates both collagen I and IV production. In agreement with these observations we demonstrated that deletion of the integrin a1 subunit results in enhanced collagen IV production, whereas loss of the a2 subunit decreases collagen IV deposition in models of glomerular injury. The molecular mechanisms whereby integrins a1p1 and a2p1 differentially regulate homeostasis of collagen IV networks is poorly understood, especially when both integrins are co-expressed in the same cell type as seen in MCs. Furthermore, it is not known how affinity of binding of these two integrins to collagen IV regulates cell function and signaling. The overall goal of this project is to determine how integrins a1p1 and oc2p1 regulate mesangial collagen IV synthesis. The central hypothesis of this grant is: the interaction of collagen IV with integrins a1/31 and/or a2/!1 differentially regulates mesangial collagen IV homeostasis. To test this hypothesis we will:
Aim 1) Identify the molecular mechanisms whereby integrins oc1p1 and <x2p1 differentially regulate collagen IV homeostasis in MCs.
Aim 2) Determine whether the degree of affinity of integrins a1p1 and a2p1 to collagen IV controls collagen IV homeostasis in MCs. This will be tested by expressing mutants of the ligand binding domains of the integrin a1 and a2 subunits with altered affinity for collagen IV into integrin a1- or a2-null MCs and determining their effects on collagen IV homeostasis.
Aim 3) Determine the atomic structures of integrin a1 and a2 ligand binding domain mutants with altered affinity for collagen IV. This will be done by solving the 3D structures of known mutants of ligand binding domains with or without ligand by nuclear magnetic resonance and X-ray crystallography. We anticipate that this study will generate novel insights into the molecular basis whereby integrin a1 p1 and a2p1 regulate collagen IV synthesis. This critical event is fundamental for the regulation of mesangial collagen homeostasis and the maintenance of glomerular function.
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