Senescent cells accumulate with age. Researchers here provide evidence for yet another age-related decline to be added to the long list of issues in which this accumulation of senescent cells is an important contributing cause. In this case, the problem is the loss of regenerative capacity in muscle tissue that occurs with age. Can this be due to loss of stem cell activity? Past research has indicated that muscle stem cell populations are largely intact in old individuals, but increasingly quiescent and inactive. Removing senescent cells removes some portion of the inflammatory signaling characteristic of old age, and this signaling may be influential in the loss of stem cell function in many tissue types.
Equally, the loss of regenerative capacity may have more to do with changes in the timing of clearance of senescent cells during the process of tissue regrowth following injury. The immune system becomes less able to rapidly clear senescent cells in later life. There has been some concern that intermittent removal of senescent cells via senolytic treatment would impair wound healing, given that the short-term presence of senescent cells is involved in the intricate coordination of different cell types that is needed for regeneration. As this study shows, the benefits of removing senescent cells in this way outweigh the downsides, at least in the old mice. In young mice, removal of senescent cells is disruptive to wound healing.
The evidence here suggests that issues in aged muscle regeneration are caused in part by (a) the inability of the immune system to rapidly clear the senescent cells created during the healing process, and (b) senescent immune cells that enter the injured area. The aged environment may be forcing immune cells into senescence rather than allowing them to perform the usual pro-regenerative activities.
Systemic deletion of senescent cells leads to robust improvements in cognitive, cardiovascular, and whole-body metabolism, but their role in tissue reparative processes is incompletely understood. We hypothesized that senolytic drugs would enhance regeneration in aged skeletal muscle. Young (3 months) and old (20 months) male C57Bl/6J mice were administered the senolytics dasatinib (5 mg/kg) and quercetin (50 mg/kg) or vehicle bi-weekly for 4 months. Tibialis anterior (TA) was then injected with 1.2% BaCl2 or PBS 7 days or 28 days prior to euthanization. Senescence-associated β-Galactosidase positive (SA β-Gal+) cell abundance was low in muscle from both young and old mice and increased similarly 7 days following injury in both age groups, with no effect of D+Q. Most SA β-Gal+ cells were also CD11b+ in young and old mice 7 days and 14 days following injury, suggesting they are infiltrating immune cells.
By 14 days, SA β-Gal+/CD11b+ cells from old mice expressed senescence genes, whereas those from young mice expressed higher levels of genes characteristic of anti-inflammatory macrophages. SA β-Gal+ cells remained elevated in old compared to young mice 28 days following injury, which were reduced by D+Q only in the old mice. In D+Q-treated old mice, muscle regenerated following injury to a greater extent compared to vehicle-treated old mice, having larger fiber cross-sectional area after 28 days. Conversely, D+Q blunted regeneration in young mice. In vitro experiments suggested D+Q directly improve myogenic progenitor cell proliferation. Enhanced physical function and improved muscle regeneration demonstrate that senolytics have beneficial effects only in old mice.