microRNA-1 (miR-1) is an evolutionarily conserved, striated muscle-enriched miRNA. Most mammalian genomes contain two copies of miR-1, and in mice, deletion of a single locus, miR-1-2, causes incompletely penetrant lethality and subtle cardiac defects. Here, we report that deletion of miR-1-1 resulted in a phenotype similar to that of the miR-1-2 mutant. Compound miR-1 knockout mice died uniformly before weaning due to severe cardiac dysfunction. miR-1-null cardiomyocytes had abnormal sarcomere organization and decreased phosphorylation of the regulatory myosin light chain-2 (MLC2), a critical cytoskeletal regulator. The smooth muscle-restricted inhibitor of MLC2 phosphorylation, Telokin, was ectopically expressed in the myocardium, along with other smooth muscle genes. miR-1 repressed Telokin expression through direct targeting and by repressing its transcriptional regulator, Myocardin. Our results reveal that miR-1 is required for postnatal cardiac function and reinforces the striated muscle phenotype by regulating both transcriptional and effector nodes of the smooth muscle gene expression network.
MicroRNAs are tiny RNAs that do not encode proteins. Instead, they regulate the expression of genes by preventing protein-encoding messenger RNAs from being translated into protein. MicroRNAs are expressed throughout the body, including the heart, where the most abundant microRNA is called miR-1. This is encoded by two nearly identical genes: miR-1-1 and miR-1-2.
Mice that lack the miR-1-2 gene have various heart abnormalities, but generally survive because they still produce some miR-1 from their remaining miR-1-1 gene. Now, Heidersbach et al. have generated the first mice that specifically lack both miR-1 genes, and shown that these animals die before weaning.
When viewed under the electron microscope, heart muscle from miR-1 double knockout mice lacks the characteristic ‘striped’, or striated, appearance of normal heart muscle. Additionally, miR-1 double knockout hearts have some gene expression characteristics more similar to the smooth muscle found in the gut and in the walls of blood vessels. Smooth muscle differs from striated muscle in that it lacks sarcomeres: these are bands of fibrous proteins, such as myosin, that are essential for muscle contraction.
In normal mice, an enzyme called MLCK contributes to the formation and function of sarcomeres by adding phosphate groups to myosin molecules. By contrast, in smooth muscle an enzyme called Telokin promotes phosphate group removal, and thus affects the function of sarcomeres. Heidersbach et al. showed that miR-1 interacts directly with Telokin mRNA to prevent its expression in the heart, and simultaneously represses a protein called Myocardin, which directly activates transcription of Telokin. However, when miR-1 is absent, as in the miR-1 double knockout mice, Telokin is expressed in heart muscle, along with many other genes characteristic of smooth muscle.
As well as improving our understanding of the development and functioning of the heart, these findings should shed new light on the role of microRNAs in maintaining the patterns of gene expression that characterize unique cell fates.