Researchers here report on a life span study of mice genetically engineered to lack the Akt2 gene. The outcome is a greater resistance to the effects of aging on cardiac tissue, and a modest extension of life span. The researchers offer some thoughts on the likely mechanisms, suggesting that this is an example of the class of results that can be obtained via improved autophagy – though in this case, it is interesting that effects appear limited to cardiovascular tissues. The processes of autophagy act to remove damaged cellular components, particularly mitochondria, as well as many forms of metabolic waste. Thus I tend to read evidence for improved autophagy to slow aging as generally supportive of the SENS view of what should be done about aging, which is to say repair the molecular damage that causes aging. In principle the research community can build therapies that achieve a far greater and more effective level of repair than is possible through evolved mechanisms such as autophagy.
A number of hypotheses have been postulated for cardiac aging including oxidative stress, mitochondrial injury, autophagy dysregulation, and intracellular Ca2+ mishandling. Nonetheless, the precise machineries behind cardiac aging still remain somewhat elusive. Recent evidence from our laboratory and others has depicted a unique role for phosphoinositide 3-kinase (PI3K) and its downstream-signaling target protein kinase B (Akt) in aging-induced pathological changes in the heart. It was shown that the on-and-off switching of the PI3K/Akt pathway, particularly by insulin and insulin-like growth factor-1 (IGF-1), serves as a powerful physiological integrator rudimentary to life span and aging.
Our data have revealed an essential role for diminished autophagy, an evolutionarily conserved lysosome-dependent process for turnover of proteins and organelles, in Akt overactivation-induced accentuation of cardiac aging process. Autophagy plays a key role for biological aging process and cardiac homeostasis. Diminished autophagy has been shown to accelerate mammalian aging, in association with accumulation of damaged intracellular components including protein aggregate. Moreover, defective autophagy facilitates ventricular remodeling, contractile defects, and heart failure. Given the critical role for Akt in the regulation of cardiac survival and life span, this study was designed to examine the role of Akt2 ablation on aging-induced geometric, functional, and intracellular Ca2+ homeostatic changes in the heart, with a focus on autophagy and mitochondrial integrity.
Our findings indicated that Akt2 ablation prolongs life span and improves myocardial contractile function with a possible adaptive cardiac remodeling through the Sirt1-mediated autophagy regulation. In addition, Akt2 ablation alleviated aging-associated mitochondrial injury. Cardiac aging is characterized by unfavorable cardiac remodeling and function including cardiac hypertrophy, interstitial fibrosis, compromised contractility, and prolonged diastolic duration. To our surprise, Akt2 ablation negated aging-induced cardiac contractile dysfunction with a more pronounced remodeling. More prominent changes in heart mass/size, and cardiomyocyte cross-sectional area (but not fibrosis) were noted in aged Akt2-/- mice, favoring an important role for Akt2 in aging as opposed to young hearts. With the improved cardiac function in aging, the more pronounced cardiac hypertrophy in the face of Akt2 ablation seemed to suggest a state of adaptive cardiac hypertrophy in aged Akt2-/- hearts. Akt2 knockout did not elicit any notable cardiac effect at young age, suggesting that ablation of Akt2 may take time to impose cardiac remodeling and contractile effects.
Perhaps the most intriguing finding from our study is that Akt2 ablation prolonged life span and rescued against aging-induced cardiac dysfunction despite more pronounced cardiac hypertrophy. Several theories may be proposed for Akt2 ablation-elicited responses in aging. Earlier findings from our group depicted dampened phosphorylation of the Akt-negative regulator PTEN with aging, consistent with present observation of Akt activation in aging and the rationale of beneficial Akt2 ablation. Second, restored autophagy and mitophagy seem to play an important role for Akt2 ablation-induced cardioprotection. Both Akt activation, a key molecule governing cardiac survival, autophagy, and mitochondrial function, and aging have been shown to suppress autophagy. Our results revealed that Akt2 ablation restored autophagy and mitophagy in aging hearts. Our in vitro findings further revealed that autophagy induction with rapamycin improved mitophagy and contractile function. It is likely that restored autophagy and mitophagy may be responsible for prolonged survival in Akt2 knockout mice, in line with the prolonged life span with autophagy induction. Improved autophagy may improve diastolic function in senescent myocardium via preserved intracellular Ca2+ handling.
In summary, our findings suggest that Akt2 seems play an essential role in the regulation of longevity, cardiac geometry, and function in aging. Our data favor the notion that increased Akt signaling and downregulated Sirt1 with advanced aging may underscore reduced autophagy and mitophagy in aging, indicating the therapeutic potentials for Akt and autophagy/mitophagy in the management of cardiac aging. Although our study sheds some light on the interaction of Akt-Sirt1 signaling cascades on autophagy and cardiac homeostasis, the pathogenesis of cardiac dysfunction in aging, particularly in association with autophagy and mitochondria still deserves further investigation.