Cellular Biology Nuclear miRNA Regulates the Mitochondrial Genome in the Heart

Rationale: Mitochondria are semiautonomous cellular organelles with their own genome, which not only supply energy but also participate in cell death pathways. MicroRNAs (miRNAs) are usually 19 to 25 nt long, noncoding RNAs, involved in posttranscriptional gene regulation by binding to the 3؅-untranslated regions of target mRNA, which impact on diverse cellular processes. Objective: To determine if nuclear miRNAs translocate into the mitochondria and regulate mitochondrial function with possible pathophysiological implications in cardiac myocytes. Methods and Results: We find that miR-181c is encoded in the nucleus, assembled in the cytoplasm, and finally translocated into the mitochondria of cardiac myocytes. Immunoprecipitation of Argonaute 2 from the mitochondrial fraction indicates binding of cytochrome c oxidase subunit 1 (mt-COX1) mRNA from the mitochondrial genome with miR-181c. Also, a luciferase reporter construct shows that mi-181c binds to the 3؅UTR of mt-COX1. To study whether miR-181c regulates mt-COX1, we overexpressed precursor miR-181c (or a scrambled sequence) in primary cultures of neonatal rat ventricular myocytes. Overexpression of miR-181c did not change mt-COX1 mRNA but significantly decreased mt-COX1 protein, suggesting that miR-181c is primarily a translational regulator of mt-COX1. In addition to altering mt-COX1, overexpression of miR-181c results in increased mt-COX2 mRNA and protein content, with an increase in both mitochondrial respiration and reactive oxygen species generation in neonatal rat ventricular myocytes. Thus, our data show for the first time that miR-181c can enter and target the mitochondrial genome, ultimately causing electron transport chain complex IV remodeling and mitochondrial dysfunction. Conclusions: Nuclear miR-181c translocates into the mitochondria and regulates mitochondrial genome expression. This unique observation may open a new dimension to our understanding of mitochondrial dynamics and the role of miRNA in mitochondrial dysfunction. Key Words: microRNA Ⅲ mitochondria Ⅲ cytochrome c oxidase Ⅲ reactive oxygen species Ⅲ miR M itochondria, in addition to supplying energy, are also involved in many other essential cellular processes, such as signaling, cellular differentiation, growth, and cell death. 1,2 More than 2000 proteins are found in the mitochon-dria, but the mitochondrial genome codes for only 13 proteins. 3 The majority of mitochondrial proteins are products of nuclear genes and are synthesized in the cytosol, then translocated into the mitochondria. 3,4 Editorial, see p 1540 In This Issue, see p 1539 MicroRNAs (miRNAs) are small noncoding RNA molecules that regulate gene expression by inhibiting mRNA translation and/or inducing mRNA degradation. 5,6 Several groups have proposed that miRNAs play critical roles in cardiovascular …