Deciphering Cardiovascular Differentiation using Whole Genome miRNA Screening

Unmasking the essential genes and pathways that control cardiac lineage decision has paramount importance not only for cardiac development but also for regenerative medicine, including the directed differentiation of adult progenitors or resident cardiac fibroblasts and stem cells to cardiac fates. Self repair is unquestionably insufficient to offset cell death in heart disease and this has triggered considerable efforts to restore myocardium with cardiac cells from either exogenous sources via transplantation or by stimulating endogenous regeneration. Although some reports cite improvement in cardiac performance other show equivocal results clearly showing that more research is needed to achieve  adequate cardiac regeneration.

microRNAs (miRs) exert cell-autonomous control over many biological processes, including heart formation and physiology, and they represent a powerful means to identify key genes and pathways regulating cardiovascular differentiation. Here we present the results of two studies using whole genome miR screens that led us to uncover two groups of miRs regulating cardiovascular differentiation. First, a systematic screen of a whole genome miR library (875 miRs) revealed that let-7 and miR-18 families increase mesoderm at the expense of endoderm in mouse embryonic stem cells. siRNA screening of computationally predicted targets revealed that the miRs exert their action by attenuating Nodal signaling, consistent with their expression overlapping Acvr1b in vertebrate embryos. Blocking let-7 function in vivo dramatically affected cell fate, converting presumptive mesoderm and ectoderm into endoderm. This is the first demonstration that miRs apportion germ-layer fate, and reveals a remarkable conservation of function from amphibians to mammals. Next, a second screen revealed that 47 families of miRs are able to modulate the fate of a cardiovascular progenitor population co-expressing globally Gata4, Mef2c and Tbx5, to differentiate into either cardiac or endothelial cells. Here, we will emphasize on the use of a powerful combinatorial method aiming at increasing the outcome of miR functional screening.