Systematic Control of Aged Skeletal Muscle Following High-Intensity Stretch-Shortening Contraction Exercise Training: Epigenomic Regulation and Signaling Factors Underpinning Adaptation

Sarcopenia, the age-related decline in skeletal muscle mass, results in a loss of strength and functional capacity, which subsequently increases the risk of disease, disability, frailty, and all-cause mortality. Exercise is known to be an efficacious paradigm for improving health and attenuating or preventing many chronic diseases. For the previous two decades, our laboratory has established an in vivo rodent dynamometer model to explore the effects of various skeletal muscle training paradigms following stretch-shortening contractions (SSCs).The responses tohigh-intensity resistance-type exercise training (RTET) using this physiological model ranges from adaptation, characterized by enhanced skeletal muscle performance along with increased muscle mass, to maladaptation, defined as an absence or diminishment of skeletal muscle performance and no improvements in muscle mass. Utilizing a non-injurious SSC protocol, training-induced adaptation occurs in young rodents; this response is altered with age in which old rodents undergo maladaptation when exposed to this same chronic loading protocol. Additionally, with respect to chronologically advancing age, our previous work indicates an altered adaptive phenotype following SSC RTET occurs prior to complete biological development of the rodent – six months of age in the adult rat, which we believe may indicate the onset of a loss in homeostatic control leading to an age-specific biological departure from the adaptive response. However, recently we have shown that modifying the frequency of RTET from three to two days per week in older rodents (e.g. 30-31 months of age) attenuates age-dependent maladaptation and restores muscle quality to a younger phenotype. Despite the therapeutic potential of RTET, a fundamental basis for evidence-based exercise prescription is still largely undetermined because the molecular, cellular and integrated physiological pathways involved in exercise-induced muscle adaptation are not fully understood. Aging in-and-of-itself is a biological process associated with an altered phenotype, and emerging evidence suggests these changes are possibly linked to epigenomic processes. Excitingly, recent research has shown that exercise can influence changes in DNA methylation in skeletal muscle. However, it is currently unknown how exactly DNA methylation may be influencing the adaptation of skeletal muscle to high-intensity SSC RTET, which could be an important mechanism underlying the responsivity of the muscle to training in the context of aging. Traditionally, the term muscle memory has been defined as describing the capability of skeletal muscle to respond more quickly to an applied stimulus that has been encountered previously in spite of periods of inactivity. Recently, emerging evidence has pointed to the existence of a cellular foundation of skeletal muscle memory. Because environmental stimuli and stressors lead to modifications in gene expression, epigenetics/epigenomics are highly likely to form the underlying basis for this cellular memory. However, despite this collective knowledge, to date no studies have determined whether or not changes via DNA methylation that occur as a result of exposure to an adaptive exercise stimulus has a lasting influence on the adaptability of skeletal muscle upon reintroduction to the same stimulus at a later life. In order to examine these unresolved issues, the purpose of this dissertation followed three specific aims: 1) To determine the effects of aging and a reduced training frequency on the activation of molecular signaling pathways associated with the adaptation of skeletal muscle following one month of high-intensity SSC RTET in old rats; 2) to investigate whether DNA methylation influences the molecular signaling activity and adaptability of skeletal muscle following one month of high-intensity SSC RTET, and whether reducing the training frequency modifies the methylation profile of skeletal muscle in response to the training stimulus; 3) to examine if introducing high-intensity SSC RTET at an earlier relative age promotes changes in molecular signaling and DNA methylation that positively influences the ability of skeletal muscle to adapt upon re-exposure to the same paradigm at a later agepreviously shown to have the inability to go through the full adaptive response . The hypotheses for this research were that the ability of aged muscle to adapt to high-intensity SSC RTET would be compromised when exposed to an inappropriate stimulus (i.e., maladaptive) as a consequence of a dysregulated molecular signaling response which would be observable in distinct pathways crucial in muscle homeostasis and remodeling; furthermore, these potential age-related dysregulated events in gene activity would occur as a consequence to altered DNA methylation. Moreover, older animals exposed to a reduced frequency of high-intensity SSC RTET would respond favorably to the training stimulus and in an appropriate manner (i.e., adaptation) and would have a resemblance more like young rats in terms of the molecular signaling pathway and DNA methylation responses compared to age-matched counterparts exposed to a higher frequency that induces maladaptation. Additionally, training rodents at a younger relative age compared to where age-dependent maladaptation occurs would attenuate DNA methylation and therefore positively augment the adaptability of muscle to respond favorably to chronic SSC RTET at a later age following detraining. The results from this study could be vital in understanding the underlying performance, physiological, molecular, and environmental factors influencing the capability of aged skeletal muscle to undergo adaptation in response to RTET; and, thus have important ramifications in the attenuation and/or reversal of sarcopenia. Additionally, we sought to determine the therapeutic efficacy of a training-retraining paradigm using our in vivo high-intensity RTET paradigm by investigating whether or not training at an earlier age is able to prevent functional and physiological decrements of skeletal muscle during the later stages of life