This month’s featured article is from the labs of Robert Schwartz and Yu Liu of the University of Houston in Houston, Texas. The article was published 23 August 2016 in the journal PNAS. This study investigates the mechanisms of early cardiac fate determination. Understanding the molecular events early in cardiac progenitor cell (CPC) development is important because these pathways can be targeted by therapeutics aiming to promote cardiac regeneration. Regeneration of heart tissue is important for treating cardiac and skeletal muscle injuries.
The authors used Mesp1 genetic tracing and next generation sequencing to identify miRNAs enriched during CPC development. This approach identified miRNAs that were already known to be essential for cardiac development (e.g. miR-17 and miR-92), but the most enriched miRNAs were miR-322 and miR-503. Mesp1 is a known high-level regulator of cardiac development, and the authors further found that MESP1 directly transactivates the miRNAs enriched in MESP1-positive CPCs. The authors also found that miR-322 and miR-503 have strong cardiomyocyte-promoting activity by measuring calcium transients following electric pulses.
The authors then focused on the regulatory activities of miR-322 and miR-503. They found that this miRNA cluster specifically drives a cardiomyocyte differentiation program, as indicated by the presence of sarcomeric structions, high cardiac troponin expression, and increased expression of cardiomyocyte markers Tbx5, Mef2c, Nkx2-5, and a-MHC. Next, the authors used Phalanx Biotech’s Mouse OneArray Whole Genome Microarray to determine global transcriptome changes following induced expression of miR-322 and miR-502. Some of the results are shown above. Gene Ontology terms enriched in the up-regulated genes include “heart development” and “skeletal system development”, while down-regulated GO terms are “ectoderm development” and “neural tube development”. The heatmaps above show up-regulation of Cardiac and Endoderm genes and down-regulation of Neural and Celf genes following induction of miR-502 and miR-322.
Lastly, the authors determined that miR-322 and miR-502 directly target the gene Celf1. They also found that Celf1 drives neuroectoderm and inhibits cardiac development; thus, targeting of Celf1 by miR-322 and miR-502 is the key event in driving cardiac cell development.
In summary, the authors identified miRNAs downstream of Mesp1 that represent a significant addition to the network of early cardiac-fate regulators. They found that the miR-322/-503 cluster specifically regulates the cardiomyocyte program, and thus further study of these miRNAs may lead to new drug candidates for treating cardiac and skeletal muscle injuries.
Shen X et al. miR-322/-503 cluster is expressed in the earliest cardiac progenitor cells and drives cardiomyocyte specification (2016). Proc Natl Acad Sci USA 113(34):9551-6.