The Role of mTOR in Cardiac Function.

Abstract

Despite many therapeutic advances, the rate of heart failure after myocardial infarction remains very high. Therefore, there is still significant need to develop better therapeutics to treat heart failure. One potential mark is the protein, mechanistic target of rapamycin (mTOR), a key signaling kinase for most cell types. Overexpression of mTOR has been previously shown by our lab as sufficient to protect the heart against different stressors including ischemia-reperfusion (I/R) injury, TAC-induced hypertrophy, and metabolic syndrome and obesity. As our laboratory is most concerned with the role of mTOR in I/R injury, in this dissertation I wanted to determine if mTOR was necessary for cardioprotection in I/R against various pathological settings, including diabetes mellitus (DM). To do this, I generated a tamoxifen-inducible, cardiac muscle specific mTOR knockout (CKO) mouse model to evaluate the loss of mTOR in functional studies, especially I/R injury. I initially characterized heart physiology in the CKO mouse at baseline using echocardiography. Preliminary in vivo I/R injury and ex vivo Langendorff I/R using acute inhibition of mTOR by Torin1 administration suggested mTOR was necessary to protect the heart as the recovery of the CKO group was significantly worse than that of littermate controls in both settings. However, when CKO hearts were subjected to I/R using the ex vivo system, CKO hearts surprisingly exhibited had better cardiac function following I/R than control hearts. This was also the case in a mouse model of obesity and hyperglycemia. CKO hearts also had irregular contractility, a finding that led to the investigation of Ca2+ handling in these CKO mice. Cardiomyocytes (CM) isolated from CKO mice displayed weaker contractions and smaller calcium (Ca2+) transients as well as a reduction in relative SR Ca2+ content. Insulin also blunted the recovery of these mice, showing mTOR is at least partially necessary for cardioprotection against I/R injury. The findings in this study may be caused by decreased expression of the IP3R, which plays an important role in regulating Ca2+ transfer from the ER to the mitochondria, since that was found to be lower in the SR/mitochondria fraction of CKO hearts. However, a clear mechanism for explaining these results still needs to be identified. A novel role for mTOR in Ca2+ handling and contraction could bring new insights into potential therapeutics for treating and managing heart failure.