MicroRNA-22 regulates smooth muscle cell differentiation from stem cells by targeting methyl CpG binding protein 2.

Abstract

In recent years, microRNAs have emerged as important regulators in various biological processes, as a new class of biomarkers, and as novel therapeutic drugs for many diseases, including cardiovascular diseases. Tumour suppressor microRNA-22 (miRNA-22 or miR-22) has been reported to regulate cardiac aging and to play a role in hematopoietic cell differentiation and maturation. Moreover, DNA methylation, a major modification of eukaryotic genomes, plays an essential role in mammalian development. Methyl CpG-binding protein 2 (MECP2) is capable of binding specifically to methylated DNA and is involved in gene silencing. The main objectives of this PhD project were to determine the functional impact of miRNA-22 and its target gene, MECP2, in smooth muscle cell (SMC) differentiation and to delineate the molecular mechanism involved. Mouse embryonic stem (ES) cells were seeded on collagen-coated flasks in differentiation medium to promote SMC differentiation. MiRNA-22 was significantly upregulated during SMC differentiation from ES cells. Enforced expression of miRNA-22 by its mimic or knockdown of miR-22 by its antagomiR promoted or inhibited SMC differentiation from ES cells, respectively. As expected, miRNA-22 overexpression in stem cells promoted SMC differentiation in vivo. Consistently, a similar change in miR-22 expression and a similar functional role for miRNA-22 were observed during SMC differentiation from adventitia stem/progenitor cells isolated from murine blood vessels. MECP2, the founding member of the family of methyl CpG binding domain proteins, was identified by several computational miRNA target prediction tools as one of the top targets of miR-22. Methyl CpG binding domain proteins bind specifically to methylated and unmethylated DNA and recruit distinct interacting protein partners to establish a repressive or active chromatin - 9 - environment. Interestingly, expression of the gene encoding MECP2 decreased significantly during SMC differentiation. MECP2 decreased dramatically in miRNA-22–overexpressing cells, but significantly increased after miRNA-22 knockdown in differentiating stem cells. Moreover, luciferase assays showed that miR-22 substantially inhibited wild-type, but not mutant, MECP2-3′-UTR luciferase activity. In addition, modulation of MECP2 expression levels affected the expression of multiple SMC-specific marker genes in differentiated ES cells. At the mechanistic level, our data showed that MECP2 transcriptionally repressed SMC gene expression by modulating various SMC transcription factors as well as several established SMC differentiation regulators. Additionally, enrichment of H3K9 trimethylation around the promoter regions of SMC transcription factors and other known differentiation regulator genes increased after MECP2 overexpression, suggesting that modulation of DNA methylation is another mechanism underlying MECP2-mediated gene expression during SMC differentiation from stem cells. Finally, miR-22 was upregulated by platelet-derived growth factor-BB and transforming growth factor-β through a transcriptional mechanism during SMC differentiation. Taken together, the findings obtained from my PhD project strongly suggest that miR-22 plays an important role in SMC differentiation from both embryonic and adventitial stem cells and that epigenetic regulation through MECP2 is required for miR-22–mediated SMC differentiation.