miRNAs are small RNAs directing many developmental processes by posttranscriptional regulation of protein-coding genes. We uncovered a new role for miR-1-1/133a-2 and miR-1-2/133a-1 clusters in the specification of embryonic cardiomyocytes allowing transition from an immature state characterized by expression of smooth muscle (SM) genes to a more mature fetal phenotype. Concomitant knockout of miR-1-1/133a-2 and miR-1-2/133a-1 released suppression of the transcriptional co-activator myocardin, a major regulator of SM gene expression, but not of its binding partner SRF. Overexpression of myocardin in the embryonic heart essentially recapitulated the miR-1/133a mutant phenotype at the molecular level, arresting embryonic cardiomyocytes in an immature state. Interestingly, the majority of postulated miR-1/133a targets was not altered in double mutant mice, indicating that the ability of miR-1/133a to suppress target molecules strongly depends on the cellular context. Finally, we show that myocardin positively regulates expression of miR-1/133a, thus constituting a negative feedback loop that is essential for early cardiac development.

The electrical properties of the heart are primarily determined by the activity of ion channels and the activity of these molecules is permanently modulated and adjusted to the physiological needs by adrenergic signaling. miRNAs are known to control the expression of many proteins and to fulfill distinct functions in the mammalian heart, though the in vivo effects of miRNAs on the electrical activity of the heart are poorly characterized. The miRNAs miR-1 and miR-133a are the most abundant miRNAs of the heart and are expressed from two miR-1/133a genomic clusters. Genetic modulation of miR-1/133a cluster expression without concomitant severe disturbance of general cardiomyocyte physiology revealed that these miRNA clusters govern cardiac muscle repolarization. Reduction of miR-1/133a dosage induced a longQT phenotype in mice especially at low heart rates. Longer action potentials in cardiomyocytes are caused by modulation of the impact of β-adrenergic signaling on the activity of the depolarizing L-type calcium channel. Pharmacological intervention to attenuate β-adrenergic signaling or L-type calcium channel activity in vivo abrogated the longQT phenotype that is caused by modulation of miR-1/133a activity. Thus, we identify the miR-1/133a miRNA clusters to be important to prevent a longQT-phenotype in the mammalian heart.

Dedifferentiation of cardiomyocytes is part of the survival program in the remodeling myocardium and may be essential for enabling cardiomyocyte proliferation. In addition to transcriptional processes, non-coding RNAs play important functions for the control of cell cycle regulation in cardiomyocytes and cardiac regeneration. Here, we demonstrate that suppression of FGFR1 and OSMR by miR-1/133a is instrumental to prevent cardiomyocyte dedifferentiation and cell cycle entry in the adult heart. Concomitant inactivation of both miR-1/133a clusters in adult cardiomyocytes activates expression of cell cycle regulators, induces a switch from fatty acid to glycolytic metabolism, and changes expression of extracellular matrix genes. Inhibition of FGFR and OSMR pathways prevents most effects of miR-1/133a inactivation. Short-term miR-1/133a depletion protects cardiomyocytes against ischemia, while extended loss of miR-1/133a causes heart failure. Our results demonstrate a crucial role of miR-1/133a-mediated suppression of Osmr and Ffgfr1 in maintaining the postmitotic differentiated state of cardiomyocytes.

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