CARMN is an Evolutionarily Conserved Smooth Muscle Cell-specific LncRNA that Maintains Contractile Phenotype by Binding Myocardin

Circulation. 2021 Oct 25. doi: 10.1161/CIRCULATIONAHA.121.055949. Online ahead of print.

Abstract

Background: Vascular homeostasis is maintained by the differentiated phenotype of vascular smooth muscle cells (VSMCs). The landscape of protein coding genes comprising the transcriptome of differentiated VSMCs has been intensively investigated but many gaps remain including the emerging roles of non-coding genes. Methods: We re-analyzed large-scale, publicly available bulk and scRNA-seq datasets from multiple tissues and cell types to identify VSMC-enriched lncRNAs. The in vivo expression pattern of a novel SMC expressed lncRNA, Carmn (CARdiac Mesoderm Enhancer-associated Non-coding RNA) was investigated using a novel Carmn GFP knock-in reporter mouse model. Bioinformatics and qRT-PCR analysis were employed to assess CARMN expression changes during VSMC phenotypic modulation in human and murine vascular disease models. In vitro, functional assays were performed by knocking down CARMN with antisense oligonucleotides and over-expressing Carmn by adenovirus in human coronary artery SMCs. Carotid artery injury was performed in SMC-specific Carmn knockout mice to assess neointima formation and the therapeutic potential of reversing CARMN loss was tested in a rat carotid artery balloon injury model. The molecular mechanisms underlying CARMN function were investigated using RNA pull-down, RNA immunoprecipitation and luciferase reporter assays. Results: We identified CARMN, which was initially annotated as the host gene of the MIR143/145 cluster and recently reported to play a role in cardiac differentiation, as a highly abundant and conserved, SMC-specific lncRNA. Analysis of the Carmn GFP knock-in mouse model confirmed that Carmn is transiently expressed in embryonic cardiomyocytes and thereafter becomes restricted to SMCs. We also found that Carmn is transcribed independently of Mir143/145. CARMN expression is dramatically decreased by vascular disease in humans and murine models and regulates the contractile phenotype of VSMCs in vitro. In vivo, SMC-specific deletion of Carmn significantly exacerbated, while overexpression of Carmn markedly attenuated, injury-induced neointima formation in mouse and rat, respectively. Mechanistically, we found that Carmn physically binds to the key transcriptional cofactor myocardin, facilitating its activity and thereby maintaining the contractile phenotype of VSMCs Conclusions: CARMN is an evolutionarily conserved SMC-specific lncRNA with a previously unappreciated role in maintaining the contractile phenotype of VSMCs and is the first non-coding RNA discovered to interact with myocardin.