| Summary: | Background Translocation of miR‐181c into cardiac mitochondria downregulates the mitochondrial gene, mt‐COX1. miR‐181c/d−/− hearts experience less oxidative stress during ischemia/reperfusion (I/R) and are protected against I/R injury. Additionally, miR‐181c overexpression can increase mitochondrial matrix Ca2+ ([Ca2+]m), but the mechanism by which miR‐181c regulates [Ca2+]m is unknown. Methods and Results By RNA sequencing and analysis, here we show that hearts from miR‐181c/d−/− mice overexpress nuclear‐encoded Ca2+ regulatory and metabolic pathway genes, suggesting that alterations in miR‐181c and mt‐COX1 perturb mitochondria‐to‐nucleus retrograde signaling and [Ca2+]m regulation. Quantitative polymerase chain reaction validation of transcription factors that are known to initiate retrograde signaling revealed significantly higher Sp1 (specificity protein) expression in the miR‐181c/d−/− hearts. Furthermore, an association of Sp1 with the promoter region of MICU1 was confirmed by chromatin immunoprecipitation‐quantitative polymerase chain reaction and higher expression of MICU1 was found in the miR‐181c/d−/− hearts. Conversely, downregulation of Sp1 by small interfering RNA decreased MICU1 expression in neonatal mouse ventricular myocytes. Changes in PDH activity provided evidence for a change in [Ca2+]m via the miR‐181c/MICU1 axis. Moreover, this mechanism was implicated in the pathology of I/R injury. When MICU1 was knocked down in the miR‐181c/d−/− heart by lentiviral expression of a short‐hairpin RNA against MICU1, cardioprotective effects against I/R injury were abrogated. Furthermore, using an in vitro I/R model in miR‐181c/d−/− neonatal mouse ventricular myocytes, we confirmed the contribution of both Sp1 and MICU1 in ischemic injury. Conclusions miR‐181c regulates mt‐COX1, which in turn regulates MICU1 expression through the Sp1‐mediated mitochondria‐to‐nucleus retrograde pathway. Loss of miR‐181c can protect the heart from I/R injury by modulating [Ca2+]m through the upregulation of MICU1.
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