Abstract
The performance of adult stem cells is crucial for tissue homeostasis but their regenerative capacity declines with age, leading to failure of multiple organs. In skeletal muscle this failure is manifested by the loss of functional tissue, the accumulation of fibrosis, and reduced satellite cell-mediated myogenesis in response to injury. While recent studies have shown that changes in the composition of the satellite cell niche are at least in part responsible for the impaired function observed with aging, little is known about the effects of aging on the intrinsic properties of satellite cells. For instance, their ability to repair DNA damage and the effects of a potential accumulation of DNA double strand breaks (DSBs) on their regenerative performance remain unclear. This work demonstrates that old muscle stem cells display no significant accumulation of DNA DSBs when compared to those of young, as assayed after cell isolation and in tissue sections, either in uninjured muscle or at multiple time points after injury. Additionally, there is no significant difference in the expression of DNA DSB repair proteins or globally assayed DNA damage response genes, suggesting that not only DNA DSBs, but also other types of DNA damage, do not significantly mark aged muscle stem cells. Satellite cells from DNA DSB-repair-deficient SCID mice do have an unsurprisingly higher level of innate DNA DSBs and a weakened recovery from gamma-radiation-induced DNA damage. Interestingly, they are as myogenic in vitro and in vivo as satellite cells from young wild type mice, suggesting that the inefficiency in DNA DSB repair does not directly correlate with the ability to regenerate muscle after injury. Overall, our findings suggest that a DNA DSB-repair deficiency is unlikely to be a key factor in the decline in muscle regeneration observed upon aging.
Highlights
Adult organisms are subject to various physical and biochemical injuries throughout their lifespan and regenerative capacities differ greatly across organs
In order to test whether or not DNA double strand breaks (DSBs) contribute to impaired satellite cell function with age, in vivo activated satellite cells were isolated from cardiotoxin (CTX)-injured muscle of 2 to 4 monthold C57BL/6 mice and of 20 to 24 month-old C57BL/6 mice 72 hours post injury, and were quantified for the number of c-H2AX nuclear foci, which is a commonly used method to monitor DNA DSB induction and repair [12,13,33,34]
Activated satellite cells from young severe combined immunodeficiency (SCID) B6.CB17Prkdcscid/SzJ mice, which have a defect in DNA DSB repair [14,15,28], showed a higher percentage of c-H2AX positive nuclei (81%64) than did satellite cells isolated from both young and old mice (61%613 and 53%615, respectively)
Summary
Adult organisms are subject to various physical and biochemical injuries throughout their lifespan and regenerative capacities differ greatly across organs. Skeletal muscle robustly regenerates during youth and adulthood owing to muscle stem cells, known as satellite cells. Muscle injury provides a synchronizing signal for the activation/proliferation of satellite cells and satellite cell isolation from uninjured muscle mimics an injury causing their simultaneous activation ex vivo [3,4]. The majority of these activated satellite cells differentiate into myoblasts, migrate to the injury site, and either differentiate into new, multinucleated myotubes or fuse with existing ones to repair the injury [5]. A smaller portion of these cells return to quiescence to maintain the stem cell pool [2]
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