Abstract Body (Do not enter title and authors here): Regular physical activity is critical to maintain cardiovascular health. Endurance exercise reduces ischemic injury in the heart. Both ischemic preconditioning (IPC), and acute exercise preconditioning, in which exercise is performed prior to infarction can reduce myocardial damage post-infarction. The precise mechanisms governing preconditioning protection are incompletely understood. An internally translated isoform of Connexin-43 (Cx43), GJA1-20k, has been established as a critical mediator of myocardial protection from ischemia. GJA1-20k is the most abundant internally translated isoform of the GJA1 gene. GJA1-20k translation is regulated by mTOR and MNK/ERK signaling. GJA1-20k is a stress-response protein that increases in the setting of ischemic heart failure. Exogenous GJA1-20k delivery via AAV9-gene therapy recapitulates the protective IPC phenotype in mice. The goal of this study was to determine if acute exercise regulates translation of GJA1-20k and cardiac preconditioning protection.
Six-to-ten-week-old wild-type mice were subjected to a 90-minute episode of acute swim exercise (AE) or 90 minutes of rest (UE). A third cohort of mice were injected with insulin (AE-I) prior to exercise. Mice were sacrificed immediately following the exercise. Whole heart immunoblotting revealed increased GJA1-20k expression in AE mice relative to UE mice by two-fold, a phenomenon which was blunted by insulin pretreatment. Furthermore, we measured a reduction in mTOR activity as measured by phosphorylated 4EBP1 and S6K by half following exercise, consistent with the increase in GJA1-20k expression. Insulin pretreatment prevented mTOR inhibition via analysis of phosphorylated 4EBP1 relative to GAPDH. In addition to mTOR, ERK1/2 inhibition has been shown to increase GJA1-20k expression. We measured phosphorylated ERK1/2 in AE and UE hearts and found a trending decrease in ERK1/2 activity with exercise intervention. In parallel studies, AE hearts subjected to ex vivo ischemia/reperfusion demonstrated reduced infarct size by two-fold relative to UE. Insulin pretreatment was sufficient to prevent cardioprotection.
In summary, acute exercise regulates GJA1-20k translation via mTOR signaling and GJA1-20k is a critical driver of exercise-mediated cardiac preconditioning. Future directions will focus on leveraging these cellular mechanisms for augmenting GJA1-20k expression for cardioprotection in human disease.