BACKGROUND: Males and females exhibit distinct anatomic and functional characteristics of the heart, predisposing them to specific disease states. METHODS: We identified microRNAs (miRNAs/miR) with sex-differential expression in mouse hearts. RESULTS: Four conserved miRNAs are present in a single locus on the X-chromosome and are expressed at higher levels in females than males. We show miRNA, miR-871, is responsible for decreased expression of the protein SRL (sarcalumenin) in females. SRL is involved in calcium signaling, and we show it contributes to differences in electrophysiology between males and females. miR-871 overexpression mimics the effects of the cardiac physiology of conditional cardiomyocyte-specific Srl-null mice. Inhibiting miR-871 with an antagomir in females shortened ventricular repolarization. The human orthologue of miR-871, miR-888, coevolved with the SRL 3′ untranslated region and regulates human SRL. CONCLUSIONS: These data highlight the importance of sex-differential miRNA mechanisms in mediating sex-specific functions and their potential relevance to human cardiac diseases.

Males and females differ in the basic anatomy and physiology of the heart. Sex differences are evident in cardiac repolarization in humans; women have longer corrected QT and JT intervals. However, the molecular mechanisms that lead to these differences are incompletely understood. Here, we present that, like in humans, sex differences in QT and JT intervals exist in mouse models; female mice had longer corrected QT and JT intervals compared with age-matched males. To further understand the molecular underpinning of these sex differences, we developed a novel technology using fluorescent confocal microscopy that allows the simultaneous visualization of action potential, Ca²⁺ transients, and contractions in isolated cardiomyocytes at a high temporal resolution. From this approach, we uncovered that females at baseline have increased action potential duration, decreased Ca²⁺ release and reuptake rates, and decreased contraction and relaxation velocities compared with males. Additionally, males had a shorter overall time from action potential onset to peak contraction. In aggregate, our studies uncovered male and female differences in excitation-contraction coupling that account for differences observed in the EKG. Overall, a better understanding of sex differences in electrophysiology is essential for equitably treating cardiac disease.

Sex differences influence congenital heart disease (CHD) development, yet underlying molecular mechanisms remain largely unclear. We demonstrate that the X-linked RNA helicase DDX3X associates in the heart with ribosomal subunit proteins, and eCLIP mapping reveals its preferential binding to cardiac mRNAs with long, structured 5′ untranslated regions (UTRs) that can hinder translation. Using a cardiomyocyte-specific mouse Ddx3x knockout model, we show that female embryos lacking Ddx3x die at midgestation from heart failure due to impaired translation of key cardiac regulators, whereas male littermates survive. Ribosome profiling and proteomics demonstrate that DDX3X is required for efficient translation of female differential cardiac mRNAs. Reporter assays confirm that translation of essential cardiac genes such as Srf and Rcor2 depends on their 5′ UTRs and requires DDX3X. These findings uncover a sex-specific posttranscriptional mechanism by which DDX3X safeguards female heart development through selective mRNA translation, providing insight into how X-linked dosage-sensitive regulators contribute to CHD.