Ataxia-Telangiectasia Mutated Kinase Deficiency Alters the Autophagic Response During Chronic Myocardial Infarction

Background: Environmental and endogenous stresses induce genomic DNA damage. In order to combat cellular assaults and maintain genomic integrity, reparative processes including DNA damage repair (DDR) and autophagy are activated. A key protein involved in DDR is ataxia telangiectasia mutated kinase (ATM). Mutations in ATM gene cause a multi-systemic disease called ataxia telangiectasia. Approximately 1.4-2.0% of the population has heterozygous mutation in ATM gene, which associates with enhanced susceptibility to cancer and ischemic heart disease. Autophagy, a conserved catabolic process, functions to maintain genomic stability by the sequestration and removal of misfolded proteins and damaged organelles. Dysregulation of autophagy contributes to the pathogenesis of many diseases including heart disease. Previous work from our lab has demonstrated autophagic impairment in the myocardium of ATM deficient mice during an acute phase (4 hr) of myocardial infarction (MI). The objective of this study was to examine the role of ATM deficiency in autophagic impairment during a chronic phase (28 days) post-MI. Methods: Wildtype (WT) and ATM heterozygous knockout (hKO) mice underwent MI by the ligation of the left anterior descending artery. Expression and activity of proteins associated with autophagy were examined in the infarct left ventricular tissue 28 days post-MI using western blot analyses. The data were analyzed using ANOVA followed by Student-Newman-Keuls test. A p-value of Results: The ratio of microtubule-associated protein light chain 3 (LC3-II-to-LC3-I; an indicator of autophagic turnover) lower in hKO-sham vs WT-sham. MI led to significant decrease in this ratio in WT-MI vs WT-sham. Protein levels of p62 (an autophagic transport protein) remained unchanged among the four groups. Expression levels of beclin-1 (aids in the formation of the autophagophore) were similarly increased in both MI groups vs their sham controls. Levels of mature cathepsin D (a lysosomal protease involved in lysosomal degradation of misfolded proteins) were significantly higher in WT-MI vs WT-sham group. Interestingly, cathepsin D levels were significantly lower in hKO-MI vs WT-MI group. Activation of mTOR (a coordinator of autophagy, cell growth and metabolism) was significantly higher in hKO-MI, not in WT-MI, vs hKO-sham group. Activation of AMPK (a sensor and regulator of cellular energy homeostasis) was higher in WT-MI, not in hKO-MI, vs WT-sham. Conclusion: Thus, ATM deficiency alters autophagic response in the heart chronic post-MI