Late Life Rapamycin Treatment Reverses Diastolic Dysfunction in Mice

Inhibitors of mTOR such as rapamycin are increasingly well studied. This class of drug stimulates cellular stress responses, principally autophagy, and thus produces outcomes that are broadly similar to the long-term improvement of health resulting from calorie restriction, exercise, or other demonstrated means of upregulating autophagy. This results in benefits to health, such as those noted in today's open access paper.

It is one thing to demonstrate that a drug improves measures of autophagy known to decline with age, and note that many of the interventions shown to modestly slow aging in laboratory species are characterized by improved autophagy. It is quite another to determine the links between low-level change in cell biochemistry and high level tissue properties. Cellular metabolism is enormously complex, and comparatively little headway has been made towards building broad bridges between (a) specific causative mechanisms of aging, (b) downstream issues with cellular biochemistry such as faltering autophagy, and (c) mechanical, structural, and other properties of tissue and organ function. It remains the case that knowing that a particular intervention works to improve health does not imply knowing how it works to improve health in detail.

Late-life Rapamycin Treatment Enhances Cardiomyocyte Relaxation Kinetics and Reduces Myocardial Stiffness

Diastolic function is controlled by active relaxation of cardiomyocytes and passive stiffness of the myocardium. Cardiomyocyte relaxation is controlled by the interplay of two macromolecular systems: membrane bound Ca2+ handling proteins to send the signal to start and stop contraction, and sarcomeric proteins for force generation and contraction regulation by Ca2+. Passive stiffness of the myocardium is controlled by mechanisms such as extracellular matrix remodeling, titin isoform shift and titin phosphorylation. It has been shown that rapamycin reduces the age-related increase in passive stiffness of the myocardium. The effects of rapamycin on active cardiomyocyte relaxation and the precise molecular mechanisms of rapamycin mediated reduction in passive myocardial stiffness remain unknown. Identifying the mechanisms by which rapamycin improves diastolic function in the aging heart will advance our understanding on its therapeutic potentials in cardiac aging and heart failure with preserved ejection fraction (HFpEF).

To dissect the mechanisms by which rapamycin improves diastolic function in old mice, we examined the effects of rapamycin treatment at the levels of single cardiomyocyte, myofibril, and multicellular cardiac muscle. Compared to young cardiomyocytes, isolated cardiomyocytes from old control mice exhibited prolonged time to 90% relaxation (RT90) and time to 90% Ca2+ transient decay (DT90), indicating slower relaxation kinetics and calcium reuptake with age. Late-life rapamycin treatment for 10 weeks completely normalized RT90 and partially normalized DT90, suggesting improved Ca2+ handling contributes partially to the rapamycin-induced improved cardiomyocyte relaxation.

In addition, rapamycin treatment in old mice enhanced the kinetics of sarcomere shortening and Ca2+ transient increase in old control cardiomyocytes. Myofibrils from old rapamycin-treated mice displayed increased rate of the fast, exponential decay phase of relaxation compared to old controls. The improved myofibrillar kinetics were accompanied by an increase in MyBP-C phosphorylation following rapamycin treatment. We also showed that late-life rapamycin treatment normalized the age-related increase in passive stiffness of demembranated cardiac trabeculae through a mechanism independent of titin isoform shift. In summary, our results showed that rapamycin treatment normalizes the age-related impairments in cardiomyocyte relaxation, which works conjointly with reduced myocardial stiffness to reverse age-related diastolic dysfunction.