MicroRNA regulation and role in stem cell maintenance, cardiac differentiation and hypertrophy.

There are currently 1527 known microRNAs (miRNAs) in human, each of which may regulate hundreds or thousands of target genes. miRNA expression levels vary between cell types; for example, miR- 302 and miR-290 families are highly enriched in embryonic stem cells, while miR-1 is a muscle specific miRNA. miRNA biosynthesis and function are highly regulated and this regulation may be cell type specific. The processing enzymes and factors that recognize features in sequence and secondary structure of the miRNA play key roles in regulating the production of mature miRNA. Mature miRNA enriched in stem cells control stem cell self-renewal as well as their differentiation. Though specific miRNAs have been shown to control differentiation towards various lineages such as neural or skin cells, some of the most well characterized miRNAs have been found in promoting the formation of cardiac cells. In addition, miRNAs also play a critical role in cardiomyocyte hypertrophy, especially in a pathological context. Such miRNAs are predicted to be therapeutic targets for treating cardiovascular diseases. In this review we will discuss how miRNAs act to maintain the stem cell state and also explore the current knowledge of the mechanisms that regulate miRNAs. Furthermore, we will discuss the emerging roles of miRNAs using cardiomyocyte differentiation and maturation as a paradigm. Emphasis will also be given on some of the less ventured areas such as the role of miRNAs in the physiological maturation of cardiomyocytes. These potentially beneficial miRNAs are likely to improve cardiac function in both in vivo and in vitro settings and thus provide additional strategy to treat heart diseases and more importantly serve as a good model for understanding cardiomyocyte maturation in vitro.