Epigenetics and miRNA emerge as key regulators of smooth muscle cell phenotype and function

Abstract

Regulation of phenotypic plasticity in smooth muscle requires an understanding of the mechanisms regulating phenotype-specific genes and the processes dysregulated during pathogenesis. Decades of study in airway smooth muscle has provided extensive knowledge of the gene expression patterns and signaling pathways necessary to maintain and alter smooth muscle cell phenotype. With this solid foundation, the importance and complexity of inheritable epigenetic modifications and mechanisms silencing gene expression have now emerged as fundamental components regulating aspects of inflammation, proliferation and remodeling.

Fig 1. Epigenetic regulation of gene expression

A) Histone modifications occur on the N-terminal tails of the four core histones. Histone acetylation (Ac) by histone acetyltransferases (HATs) stabilizes the unmethylated DNA in an open chromatin conformation to allow for transcription factor (TF) binding, recruitment of RNA polymerase II (RNAPol II), mRNA expression and the synthesis of proteins coded by the coding DNA sequence (CDS). B) DNA methylation occurs when DNA nucleotide methyltransferase (Dnmt) adds methyl groups (CH₃) to cytosine residues at CpG sites. Association of methylated DNA with histone deacetylases (HDAC) and histones methylated (Me) by histone methyltransferases (HMT) closes the chromatin structure to inhibit TF binding and silence gene expression.

Fig. 2. miRNA biogenesis and function

miRNA are transcribed by RNA polymerase II (RNAPol II) as primary miRNA (pri-miRNA). The transcripts are cleaved by the ribonuclease Drosha to form precursor miRNA (pre-miRNA). Pre-miRNA are exported out of the nucleus and further cleaved by Dicer to yield a mature miRNA duplex bound by a miRNA-associated RNA-induced silencing complex (miRISC). Binding of complementary mRNA sequences by one strand of the miRNA leads to Argonaute (AGO2 in mammals) mediated cleavage, resulting in decreased mRNA stability and transcript cleavage or translational repression and reduced synthesis of proteins coded by the coding DNA sequence (CDS).

Fig. 3. Epigenetic regulation of genes associated with remodeling

Histone lysine methylation regulates differential VEGF expression from ASM cells isolated from non-asthmatic donors compared to non-asthmatic donors. Histone acetylation is involved in the transcriptional regulation of Endothelin-1 (ET-1) and MMPs but its involvement in dysregulation of these genes expression in disease has yet to be investigated. Increased CpG methylation of the 5′ flanking region of the extracellular superoxide dismutase (EC-SOD) gene represses EC-SOD expression and may be implicated in pulmonary arterial hypertension pathogenesis.