The transcriptional regulation of cardiovascular development requires precise spatiotemporal control of gene expression, and heterozygous mutations of transcription factors have frequently been implicated in human cardiovascular malformations. in the human genome, and each is usually assumed to target? 100?mRNAs, resulting in mRNA degradation or translational inhibition. Interactions between miRNAs and mRNAs are thought to require sequence homology in the 5 end of the miRNA; however, significant variance in the degree of complementation in the remaining sequence allows a single miRNA to target a wide range of mRNAs, often regulating multiple genes within a common pathway. As a result, more than one third of mRNAs in the mammalian genome are believed to be regulated by one or more miRNAs [8]. Despite advances in miRNA discovery, the role of miRNAs 175481-36-4 in physiologic and pathophysiologic processes is just emerging. It has become clear that miRNAs play diverse roles in fundamental biologic processes, such as cell proliferation, differentiation, apoptosis, stress response, and tumorigenesis. Identification of miRNAs expressed in specific cardiac cell types has led to the discovery of important regulatory roles for these small RNAs during cardiomyocyte differentiation, cell cycle, and conduction, as well as during stages of cardiac hypertrophy in adults, indicating that miRNAs may be as important as transcription factors in controlling cardiac gene expression. Here, we review the basic mechanisms by which miRNAs function, with a focus on the role of miRNAs during development of the heart and vessels. It appears that a network of miRNAs can be superimposed on well-described signaling and transcriptional networks with considerable intersection between the two. Ultimately, knowledge of the function and regulation of specific miRNAs and their mRNA targets in the heart will lead to a deeper understanding of cardiac cell-fate decisions and morphogenesis and ultimately could result in the development of novel therapeutic or preventive approaches for heart disease. miRNA Organization, Biogenesis, and Target Recognition miRNAs regulate gene expression at the post-transcriptional level through mRNA degradation, translational repression, or miRNA-mediated mRNA decay. Mature miRNAs are formed in a multistep biologic process involving critical endonucleases (Fig.?1). miRNAs are initially transcribed from the genome into long (several kilobases) 5 capped, polyadenylated (poly(A)) primary transcripts (primiRNAs) by RNA polymerase II [7]. Some miRNAs interspersed among repetitive DNA elements, such as Alu repeats (5 AG/CT 3), can also be transcribed by RNA polymerase III [5]. The miRNA-encoding portion of the pri-miRNA forms a hairpin structure that is recognized and cleaved in the nucleus by a microprocessing complex. This complex consists of the double-stranded RNA-specific nuclease DROSHA and its cofactor, DiGeorge syndrome critical region 8 (DGCR8) [25]. The resulting approximately 70-nt hairpin precursor miRNA (pre-miRNA) is exported to the cytoplasm by the RAN-GTPCdependent nuclear transport receptor, exportin-5, which acts by recognizing a 2- to 3-base pair overhang of the pre-miRNA stem-loop structure [4, 56]. A complex of the RNAse III-like ribonuclease, Dicer, and the transactivator RNA-binding protein then cleaves the pre-miRNA to release the mature miRNA duplex. Open in a separate window Fig.?1 Schematic representation of miRNA biogenesis and function. Transcription of miRNA genes is typically mediated by RNA polymerase II (pol II) and can be controlled by various transcription factors (TF). The initial transcripts, termed primiRNAs, can range from a few hundred nucleotides to several kilobases long. The primiRNA has a characteristic stem-loop structure that can be recognized and cleaved by the RNase III endonuclease Drosha, along with its partner DGCR8 (DiGeorge syndrome critical region 8 gene; also known as Pasha). The cleavage product, an approximately 70-nt stem-loop pre-miRNA, is exported from the nucleus by Exportin 5. In the 175481-36-4 cytoplasm, another RNase III enzyme, Dicer, further cleaves the pre-miRNA into a double-stranded mature miRNA (approximately 21?nt), which is incorporated into the RISC, thus allowing preferential strand KLF11 antibody separation of the mature miRNA to 175481-36-4 repress mRNA translation or destabilize mRNA transcripts through cleavage or deadenylation (adapted from Zhao and Srivastava [57]) An asymmetry in the relative thermodynamic stability of the 5 ends of the miRNA duplex results in preferential loading of the less stable approximately 22-nt strand into the RNA-induced silencing complex (RISC); the other strand is degraded, although in some cases both strands are incorporated into the RISC [22, 40, 43]. The RISC helps mediate miRNACmRNA interactions and subsequent mRNA repression or destabilization [19]. miRNAs typically bind to the 3 UTRs of their mRNA targets with imprecise complementarity. Typically, the degree of.