Project Summary: Vascular smooth muscle cell (SMC) phenotypic modulation plays critical roles in the pathogenesis of vascular diseases. SRF (Serum Response Factor) is a critical transcription factor that plays a central role in regulating gene transcription in SMC phenotypic modulation by competitively binding with Myocardin (the key differentiation regulator) and other key transcription regulators such as Elk and NF-kB. Extensive studies have characterized the mechanisms of actin-Myocardin interaction in SRF-mediated transcription, yet surprisingly the question of whether actin directly targets SRF to modulate SMC differentiation and phenotypic modulation has not been addressed. Downregulation of actin cytoskeleton proteins including actin and SM22 (an actin binding protein) has long been recognized as a marker of SMC phenotypic modulation and was until now regarded as a consequence of SMC dedifferentiation. However, we have now accumulated compelling evidence suggesting that SM22 but not actin can target SRF to regulate its transcriptional activities. The goal of this project is to characterize the molecular mechanisms of SM22 in coordinatively regulating the transcription of a variety of genes involved in SMC modulation from contractile phenotype to pathogenic phenotypes. Based on our published work and exciting preliminary results, we hypothesize that SM22 regulates SMC phenotypes as a transcription cofactor to modulate the interplay of SRF and other key transcription regulators for SMC differentiation and dedifferentiation in the vessel wall. We will take the system biology approach using integrated molecular, cellular, genetic, and bioinformatics methods to test this hypothesis. The Specific Aims are (i) to determine the molecular mechanisms whereby SM22 regulates the function of SRF in SMC phenotypic modulation in cultured SMCs. (ii) to determine the roles of SM22 in the pathogenesis of vascular wall remodeling in response to vascular injury using knockout and transgenic mice generated in our lab. Successful completion of this project will likely validate a new paradigm whereby actin cytoskeleton proteins actively participate in regulating smooth muscle phenotypic modulation during the pathogenesis of vascular diseases. We expect that the proposed studies will have the positive impact of identifying cytoskeleton proteins as a new class of targets for future pharmaceutical intervention.