THO Complex–Dependent Posttranscriptional Control Contributes to Vascular Smooth Muscle Cell Fate Decision [Original Research]

Rationale: Modulation of vascular smooth muscle cell (VSMC) phenotype plays a fundamental role in vascular development and diseases. While extensive studies uncovered the roles of transcriptional regulation in VSMC-specific gene expression, how posttranscriptional regulation contributes to VSMC fate decisions remains to be determined. Objective: To establish THO complex-dependent VSMC gene expression as a novel regulatory basis controlling VSMC phenotypes. Methods and Results: Immunohistochemical staining against THOC2 and THOC5, two components of the THO complex, revealed a dramatic reduction in their expression in human arteries undergoing carotid endarterectomy (CEA) compared to normal internal mammary arteries (IMA). Silencing of THOC2 or THOC5 led to dedifferentiation of VSMCs in vitro, characterized by decreased VSMC marker gene expression, and increased migration and proliferation. Furthermore, RNA-Seq revealed that THOC5 silencing closely resembled the gene expression changes induced upon PDGF-BB/PDGF-DD treatments in cultured VSMCs. Mechanistically, THOC2 and THOC5 physically interacted with and functionally relied upon each other to bind to specific motifs on VSMC marker gene mRNAs. Interestingly, mRNAs that lost THOC2 or THOC5 binding during VSMC dedifferentiation were enriched for genes important for the differentiated VSMC phenotype. Lastly, THOC5 overexpression in injured rat carotid arteries significantly repressed loss of VSMC marker gene expression and neointima formation. Conclusions: Our data introduce dynamic binding of THO to VSMC marker gene mRNAs as a novel mechanism contributing to VSMC phenotypic switching, and imply THOC5 as a potential intervention node for vascular diseases.