MicroRNA regulation of cardiovascular development

Abstract

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. A novel mechanism involving posttranscriptional regulation by small, noncoding microRNAs (miRNAs) has emerged as a central regulator of many cardiogenic processes. We are beginning to understand the functions that miRNAs play during essential biological processes, such as cell proliferation, differentiation, apoptosis, stress response, and tumorigenesis. The identification of miRNAs expressed in specific cardiac and vascular cell types has led to the discovery of important regulatory roles for these small RNAs during cardiomyocyte differentiation, cell cycle, conduction, vessel formation, and during stages of cardiac hypertrophy in the adult. Here, we overview the recent findings on miRNA regulation in cardiovascular development and report the latest advances in understanding their function by unveiling their mRNA targets. Further analysis of miRNA function during cardiovascular development will allow us to determine the potential for novel miRNA-based therapeutic strategies.

Figure 1

Schematic representation of miRNA biogenesis and function. Transcription of miRNA genes is typically mediated by RNA polymerase II (pol II). The initial miRNA-containing transcript, termed primary miRNAs (pri-miRNAs), can range from a few hundred nucleotides (nt) to several kilobases long. Inside the nucleus, the pri-miRNA has a characteristic stem-loop structure that can be recognized and cleaved by the ribonuclease III (RNase III) endonuclease Drosha along with its partner DGCR8 (DiGeorge syndrome critical region 8 gene; also known as Pasha). The cleavage product, a ~70-nt stem-loop pre-miRNA, is exported from the nucleus by Exportin 5. In the cytoplasm, another RNase III enzyme, Dicer, further cleaves the pre-miRNA into a double-stranded mature miRNA (~21-nt), which is incorporated into the RNA-induced silencing complex (RISC) allowing preferential strand-separation of the mature miRNA to repress mRNA translation or destabilize mRNA transcripts through cleavage or deadenylation. Abbreviations: SRF, serum response factor; TF, transcription factor (adapted from 4).

Figure 2

Summary of miR-1 and miR-133 genomic organization, regulation, and expression during mouse cardiogenesis. A, Chromosome locations of miR-1 and miR-133a orthologs and miR-206/133b. The miR-1-2/miR-133a-1 cluster is intragenic, and the miR-1-1/miR-133a-2 and miR-206/133b clusters are intergenic. The miR-1/ 133a and miR-206/133b clusters are transcribed as a bicistronic transcript. B, Cardiac (red) and muscle (green) -specific expression of miR-1 and miR-133 clusters is regulated by SRF and myogenic transcription factors, Mef2 and Myod. Targets of miR-1 and miR-133 that regulate cardiac or skeletal muscle are shown. C, LacZ directed by an upstream enhancer of the miR-1-2/ miR-133a-2 cluster and the miR-1-1/miR-133a-1 cluster, respectively, shows expression in the heart (ht) and somites (arrowhead) at mouse embryonic day 11.5.

Figure 3

Cardiac defects in the miR-1-2 mutants. A, Transverse sections of wild-type (wt) or miR-1-2^−/− hearts at E15.5 showing ventricular septal defect (arrowhead). B, Representative diagrams of electrocardiograms indicate the location of PR and QRS intervals. The second peak in the QRS complex (R’) was observed in the majority of mutant mice representing delay of electrical conduction. RV, right ventricle; LV, left ventricle; bpm, beats per minute; msec, milliseconds (adapted from 1)

Figure 4

Putative model of miR-126 function in endothelial cells. miR-126 represses SPRED1 and PIK3R2, which negatively regulate VEGF signaling (and possibly other growth factor signaling pathways) via the MAP kinase and PI3 kinase pathways, respectively. Thus, miR-126 promotes VEGF signaling, angiogenesis and vascular integrity by inhibiting protein production of endogenous VEGF repressors within endothelial cells (adapted from61).