The Role of FXR1 in Vascular Smooth Muscle Cytoskeletal Dynamics

Abstract

Appropriate cytoskeletal organization and protein-protein interactions are essential for vascular smooth muscle cell (VSMC) physiological processes relevant to hypertensive conditions. Proteins that link actin dynamics and post-transcriptional mRNA metabolism potentially have a high impact on VSMC pathophysiological processes. We recently described Fragile-X-Related protein (FXR1) as a muscle-enhanced, cytokine-inducible RNA-binding protein that regulates mRNA transcript stability in hVSMC. This study identified FXR1 as a key protein linking cytoskeletal dynamics with post-transcriptional mRNA modification with impact on VSMC pathophysiological processes. RNA immunoprecipitation sequencing (RIPseq) analysis in human VSMC identified that FXR1 binds to mRNA that participates in VSMC contractility and cytoskeletal reorganization, and FXR1 depletion decreases mRNA abundance and stability of these transcripts. FXR1 has signatures of an actin-binding protein, and depletion of FXR1 impairs actin polymerization. Mass-spectrometry identified that FXR1 predominantly interacts with cytoskeletal proteins, particularly Arp2, a protein crucial for VSMC contraction, and CYFIP1, a member of the WAVE Regulatory complex (WRC) known to link mRNA processing with actin polymerization. Depletion of FXR1 also decreased the expression of these proteins, altered VSMC morphology, and significantly decreased cytoskeletal processes, including VSMC adhesion, migration, contraction, and Rac1 and CDC42 activation. Using telemetry under baseline conditions, conditional FXR1SMC/SMC mice have decreased mean arterial and diastolic blood pressure and decreased abundance of cytoskeletal-associated transcripts. Taken together, this indicates that FXR1 is a muscle-enhanced WRC modulatory protein that regulates VSMC cytoskeletal dynamics by linking two processes: post-translational stability of cytoskeletal transcripts, and actin polymerization and cytoskeletal protein-protein interactions which can regulate blood pressure.