Introduction: A hallmark of aging is poor muscle recovery following disuse atrophy. Efficacious strategies to enhance muscle recovery following disuse atrophy in aging are non-existent. Prior exercise training could result in favorable muscle morphological and cellular adaptations that may promote muscle recovery in aging. Here, we characterized the impact of exercise training on skeletal muscle inflammatory and metabolic profiles and cellular remodeling and function, together with femoral artery reactivity prior to and following recovery from disuse atrophy in aged male mice. We hypothesized that 12 weeks of treadmill training in aged male mice would improve skeletal muscle cellular remodeling at baseline and during recovery from disuse atrophy, resulting in improved muscle regrowth. Methods: Physical performance, ex vivo muscle and vascular function, tissue and organ mass, hindlimb muscle cellular remodeling (macrophage, satellite cell, capillary, myofiber size, and fibrosis), and proteolytic, inflammatory, and metabolic muscle transcripts were evaluated in aged exercise-trained and sedentary mice. Results: We found that at baseline following exercise training (vs. sedentary mice), exercise capacity and physical function increased, fat mass decreased, and endothelial function improved. However, exercise training did not alter tibialis anterior or gastrocnemius muscle transcriptional profile, macrophage, satellite cell, capillarity or collagen content, or myofiber size and only tended to increase tibialis mass during recovery from disuse atrophy. Conclusion: While exercise training in old male mice improved endothelial function, physical performance, and whole-body tissue composition as anticipated, 12 weeks of treadmill training had limited impact on skeletal muscle remodeling at baseline or in response to recovery following disuse atrophy. Here, we characterized the impact of exercise training on skeletal muscle inflammatory and metabolic profiles and cellular remodeling and function, together with femoral artery reactivity prior to and following recovery from disuse atrophy in aged male mice. We hypothesized that 12 weeks of treadmill training in aged male mice would improve skeletal muscle cellular remodeling at baseline and during recovery from disuse atrophy, resulting in improved muscle regrowth. However, exercise training did not alter tibialis anterior or gastrocnemius muscle transcriptional profile, macrophage, satellite cell, capillarity or collagen content, or myofiber size and only tended to increase tibialis mass during recovery from disuse atrophy. Conclusion: While exercise training in old male mice improved endothelial function, physical performance, and whole-body tissue composition as anticipated, 12 weeks of treadmill training had limited impact on skeletal muscle remodeling at baseline or in response to recovery following disuse atrophy. Therefore, dysfunctional macrophage, satellite cell, and endothelial cell/angiogenesis responses to cellular stressors (e.g., reloading) likely contribute to impaired muscle remodeling during recovery in aging. Despite age-related muscle cellular dysfunction, human and rodent skeletal muscle retains the ability to undergo adaptation to exercise training including muscle fiber hypertrophy and increased muscle capillarity and satellite cell content ( Moro et al., 2019 ; Dungan et al., 2022 ; Jones et al., 2023 ). In older adults, aerobic exercise training increased muscle macrophage content which corresponded with muscle size ( Walton et al., 2019 ). Moreover, we have shown that treadmill exercise training in aged mice improves vascular function ( Cho et al., 2021 ; Cho et al., 2022 ). Similarly, voluntary wheel running when administered after muscle disuse (hindlimb unloading) promotes physical function, satellite cell abundance, and muscle recovery following disuse atrophy in young mice ( Hanson et al., 2010 ; Brooks et al., 2018 ), while treadmill exercise pre-conditioning in aged mice was capable of enhancing muscle regeneration ( Joanisse et al., 2016 ). However, it is unknown whether prior exercise training in aged mice improve muscle size and cellular remodeling during recovery following disuse atrophy. We used a progressive resistance treadmill training protocol previously demonstrated to affect cardiovascular, anthropometric, and skeletal muscle oxidative enzyme activity improvements in aged male mice ( Cho et al., 2021 ). Therefore, we hypothesized that 12 weeks of treadmill exercise training in aged mice would improve muscle cellular content and remodeling (e.g., macrophages, satellite cells, capillary, collagen content, and fiber size), muscle transcriptional responses, and whole body and muscle function, resulting in enhanced muscle recovery following muscle disuse. Fewer mice were assigned to the HU group since we were primarily interested in the recovery from hindlimb unloading. Therefore, 11 sedentary and nine exercise-trained mice completed 4 days of reloading after HU. Ex vivo skeletal muscle force Force production in soleus and EDL muscles were measured, as previously described ( Petrocelli et al., 2021 ; Ferrara et al., 2022 ). Soleus muscles were sutured at each tendon and placed in a tissue bath (Aurora Scientific, Model 801C). Briefly, optimal length (L0) was reached Discussion This study aimed to determine if exercise training in aged mice would remodel skeletal muscle cellular and transcriptional responses that translate into improved muscle size and function at baseline and in response to recovery following disuse atrophy. As anticipated, exercise training in aged male mice (vs. sedentary age-matched controls) improved exercise capacity, whole-body function and tissue composition, and arterial vasodilation. Contrary to our hypothesis, skeletal muscle cellular remodeling, transcriptional changes, and muscle fiber size during recovery from hindlimb unloading were refractory to chronic exercise training.

The skeletal muscle tissue in the adult is relatively stable under normal conditions but retains a striking ability to regenerate by its resident stem cells (satellite cells). Satellite cells exist in a quiescent (G0) state; however, in response to an injury, they reenter the cell cycle and start proliferating to provide sufficient progeny to form new myofibers or undergo self-renewal and returning to quiescence. Maintenance of satellite cell quiescence and entry of satellite cells into the activation state requires autophagy, a fundamental degradative and recycling process that preserves cellular proteostasis. With aging, satellite cell regenerative capacity declines, correlating with loss of autophagy. Enhancing autophagy in aged satellite cells restores their regenerative functions, underscoring this proteostatic activity’s relevance for tissue regeneration. Here we describe two strategies for assessing autophagic activity in satellite cells from GFP-LC3 reporter mice, which allows direct autophagosome labeling, or from non-transgenic (wild-type) mice, where autophagosomes can be immunostained. Treatment of GFP-LC3 or WT satellite cells with compounds that interfere with autophagosome-lysosome fusion enables measurement of autophagic activity by flow cytometry and immunofluorescence. Thus, the methods presented permit a relatively rapid assessment of autophagy in stem cells from skeletal muscle in homeostasis and in different pathological scenarios such as regeneration, ag

Aging is associated with impaired recovery of muscle mass and function after a period of disuse, often leading to an increased risk of falls and fractures. The exact mechanisms behind impaired recovery due to aging after disuse are not fully understood. Membrane-derived lipids mediators (LM) in muscle orchestrate inflammatory actions that are critical for restoration of muscle following injury. We tested the hypothesis that muscle LM associated with inflammation would be dysregulated in adult vs older mice during a time course of recovery following disuse atrophy. Therefore, we designed a study in which adult (3-6m/o) and old (23-26m/o) male mice underwent 14 days of hindlimb unloading followed by 2, 4, or 7 days of reloading. The gastrocnemius muscle was collected at each timepoint and between age groups and processed for lipidomic analysis of the eicosanoids. Gastrocnemius muscle mass showed impaired recovery after HU in the old group when compared to adult, at 4 (P=0.0420) and 7 days reload (P=0.0092). We also observed that muscle LM’s associated with acute inflammation (arachidonic acid, prostaglandins, leukotrienes) were significantly overexpressed in old mice during recovery after disuse. Importantly, several of these lipids corresponded with impaired muscle recovery. PGE2 and PGF2a levels showed a correlation (P=0.03 and P=0.02, respectively) with gastrocnemius mass at the 4 days reload. We conclude that recovery from disuse in aged mice are characterized with the upregulation of pro-inflammatory lipids in muscle and this may be related to poor muscle regrowth. Future studies will be considered to determine the role and cell source of these pro-inflammatory LM during muscle recovery in aged mice. Funding was provided by NIH: R01AG076075 awarded to M.J.D. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.