Multiple miRNAs implicated in different aspects of cardiac development To date, a wide range of miRNAs has been specifically implicated in different aspects of cardiovascular development. For example, miR-1, -133a, -133b, comprise a subset of skeletal- and cardiac-muscle specific

miRNAs that are induced during order Gambogic acid and regulate muscle differentiation (reviewed in 28,45,47 ). MiR-1 and miR-133 are two highly conserved miRNAs derived from a common precursor transcript, that exhibit cardiac- and skeletal- muscle specific expression during development and adult life. 46–47 According to studies, miR-1 (miR-1-1, mir-1-2) targets, amongst others, 46,54 the transcription factor (TF) Hand2, a promoter of ventricular cardiomyocyte expansion, whose levels are critical for normal cardiomyocyte morphogenesis and development. 46,48–52 Studies utilizing knockout mice of mir-1-2 have reported dysregulation of cardiac conduction, cell cycle and defective heart development in these animals, a subset of which suffered from early lethality, 53,54 thereby proposing a distinct role of miR-1-1 and mir-1-2 in cardiac development. MiR133a is also critical for cardiac development.

Interestingly, miR-133a-1 and miR-133a-2 present with at least partly overlapping roles, since the deletion of either one at a time results in phenotypically normal the mice. However, the double-mutant miR-133a mouse embryos and neonates present with ventricular-septal defects often leading to early lethality, whilst the surviving animals are prone to dilated cardiomyopathy and

heart failure. MiR-133a gene targets include Cyclin D2 and Serum Response Factor, the upregulation of which possibly underlies the dysregulation of cell cycle control and the aberrant activation of the smooth muscle gene program, as observed in miR-133a-1/ miR-133a-2 double mutant mice. 55 Cyclin D2 is also targeted by miR-29a, and this process has been shown to suppress cardiomyocyte proliferation during postnatal development in rats. 56 A recent global microRNA profiling study reported another miRNA, namely miR-27b, displaying a greatly elevated myocardial expression during heart development in mice. Interestingly, the TF Mef2c, which is involved in cardiac morphogenesis, was shown to be a target of miR-27b. 57 A series of studies in zebrafish has also provided valuable data for miRNAs implicated Cilengitide in heart development. For example, miR-23 has been shown to inhibit Hyaluronan synthase 2 (Has2) expression and extracellular hyaluronic acid production. 40 Has2 is an extracellular remodeling enzyme which is required for endocardial cushion and valve formation, and when inhibited by miR-23 the number of endocardial cells that differentiate into endocardial cushion cells during development in zebrafish embryos was restricted. 40 Endocardial cushions develop on the atrio-ventricular canal and play a role in proper heart septation during development.