Abstract 200: Novel Interaction of Spinophilin with Alpha1a-Adrenergic Receptor and its Genetic Variant in Cardiovascular Cells

Activation of α 1 -Adrenergic Receptors (α 1 ARs), members of the G protein-coupled receptor (GPCR) superfamily, in response to stimulation of the sympathetic nervous system by catecholamines plays a major role in regulating cardiovascular (CV) function. Among three α 1 AR subtypes (α 1a ,α 1b ,α 1d ), α 1a ARs predominate in human resistant vessels and in heart. Recently, we discovered that naturally occurring human α 1a AR-G247R (247R) genetic variant, identified in the 3 rd intracellular loop (3iL) of the receptor in highly hypertensive patient, triggers constitutive hyperproliferation in CV cells (cardiomyoblasts, smooth muscle cells (SMC) and fibroblasts), which may lead to myocardial fibrosis and remodeling. In fibroblasts and cardiomyoblasts 247R triggered hyperproliferation is due to constitutive active coupling to Gq-independent βarrestin1/MMP/EGFR/ERK dependent pathway, while in SMC it is Gq- and MMP/EGFR/ERK-dependent. Here we report that α 1a AR-WT (WT) and 247R differentially interact with ubiquitous multi-domain scaffold protein spinophilin (SPL) that binds to 3iL of several GPCRs competing with arrestin thereby prolonging their signaling. The role of SPL in CV regulation is poorly studied. We hypothesized that SPL mediates constitutive signaling of 247R and examined whether SPL directly interacts with α 1a AR-WT or 247R. Our preliminary results reveal a direct interaction of SPL with WT and 247R: the SPL-WT interaction appears to be stronger as determined by co-immunoprecipitation. Different domains of SPL differentially interact with WT or 247R. SPL 1-480aa fragment interacts stronger with WT indicating interaction with 3iL, while SPL 480-817 fragment interacts stronger with 247R. Our preliminary results also demonstrate that 247R expression in all three cell types elevates endogenous SPL protein levels. Importantly, inhibition of SPL expression with specific siRNA reduces 247R-triggered hyperproliferation in SMC and cardiomyoblasts to near normal levels, while SPL knockdown has no effect in WT cells. Thus, we identified SPL as a novel protein involved in interacting and signaling of α 1a AR and its genetic variant in CV cells and that SPL could be considered as a potentially novel target in α 1a AR-mediated cardiovascular disorders.