mTOR inactivation in myocardium from infant mice rapidly leads to dilated cardiomyopathy due to translation defects and p53/JNK-mediated apoptosis

Mechanistic target of rapamycin (mTOR) is a central regulator of cell growth, proliferation, survival and metabolism, as part of mTOR complex 1 (mTORC1) and mTORC2. While partial inhibition of mTORC1 using rapamycin was shown to be cardioprotective, genetic studies in mouse models revealed that mTOR is essential for embryonic heart development and cardiac function in adults. However, the physiological role of mTOR during postnatal cardiac maturation is not fully elucidated. We have therefore generated a mouse model in which cardiac mTOR was inactivated at an early postnatal stage. Mutant mTORcmKO mice rapidly developed a dilated cardiomyopathy associated with cardiomyocyte growth defects, apoptosis and fibrosis, and died during their third week. Here, we show that reduced cardiomyocyte growth results from impaired protein translation efficiency through both 4E-BP1-dependent and -independent mechanisms. In addition, infant mTORcmKO hearts displayed markedly increased apoptosis linked to stretch-induced ANKRD1 (Anlcyrin repeat-domain containing protein 1) up-regulation, JNK kinase activation and p53 accumulation. Pharmacological inhibition of p53 with pifithrin-a attenuated caspase-3 activation. Cardiomyocyte death did not result from activation of the MST1/Hippo proapoptotic pathway as reported in adult rictor/mTORC2 KO hearts. As well, mTORcmKO hearts showed a strong downregulation of myoglobin content, thereby leading to a hypoxic environment. Nevertheless, they lacked a HIFI alpha-mediated adaptive response, as mTOR is required for hypoxia-induced HIF-1 alpha activation. Altogether, our results demonstrate that mTOR is critically required for cardiomyocyte growth, viability and oxygen supply in early postnatal myocardium and provide insight into the molecular mechanisms involved in apoptosis of mTOR-depleted cardiomyocytes. (C) 2016 Elsevier Ltd. All rights reserved.