Age‐related changes to the satellite cell niche are associated with reduced activation following exercise

Skeletal muscle satellite cell (SC) function and responsiveness is regulated, in part, through interactions within the niche, in which they reside. Evidence suggests that structural changes occur in the SC niche as a function of aging. In the present study, we investigated the impact of aging on SC niche properties. Muscle biopsies were obtained from the vastus lateralis of healthy young (YM; 21 ± 1 yr; n = 10) and older men (OM; 68 ± 1 yr; n = 16) at rest. A separate group of OM performed a single bout of resistance exercise and additional muscle biopsies were taken 24 and 48 hours post‐exercise; this was performed before and following 12 wks of combined exercise training (OM‐Ex; 73 ± 1; n = 24). Muscle SC niche measurements were assessed using high resolution immunofluorescent confocal microscopy. Type II SC niche laminin thickness was greater in OM (1.86 ± 0.06 µm) as compared to YM (1.55 ± 0.09 µm, P < .05). The percentage of type II‐associated SC that were completely surrounded by laminin was greater in OM (13.6%±4.2%) as compared to YM (3.5%±1.5%; P < .05). In non‐surrounded SC, the proportion of active MyoD+/Pax7+ SC were higher compared to surrounded SC (P < .05) following a single bout of exercise. This “incarceration” of the SC niche by laminin appears with aging and may inhibit SC activation in response to exercise

Sarcopenia; Aging-related loss of muscle mass and function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of -motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems

Locomotor function shapes the passive mechanical properties and operating lengths of muscle

Locomotor muscles often perform diverse roles, functioning as motors that produce mechanical energy, struts that produce force and brakes that dissipate mechanical energy. In many vertebrate muscles, these functions are not mutually exclusive and a single muscle often performs a range of mechanically diverse tasks. This functional diversity has obscured the relationship between a muscle's locomotor function and its mechanical properties. I use hopping in toads as a model system for comparing muscles that primarily produce mechanical energy with muscles that primarily dissipate mechanical energy. During hopping, hindlimb muscles undergo active shortening to produce mechanical energy and propel the animal into the air, whereas the forelimb muscles undergo active lengthening to dissipate mechanical energy during landing. Muscles performing distinct mechanical functions operate on different regions of the force-length curve. These findings suggest that a muscle's operating length may be shaped by potential trade-offs between force production and sarcomere stability. In addition, the passive force-length properties of hindlimb and forelimb muscles vary, suggesting that passive stiffness functions to restrict the muscle's operating length in vivo. These results inform our understanding of vertebrate muscle variation by providing a clear link between a muscle's locomotor function and its mechanical properties