Muscle stem cell function and the aging environment
The influence of the extrinsic environment on the efficiency of the regeneration process arouse from whole muscle transplantation experiments and parabiosis. Transplantation of whole muscle grafts of young and old mice showed that the regenerative ability of young and old muscles depended mainly on the age of the host, as young hosts allowed successful engraftment of both young and old muscles while both graft types failed to regenerate in old rats [24]. However, further studies showed delayed regeneration in old muscle grafts even in a youthful environment [7, 8]. This was ascribed to the altered inflammatory cell function at old age [25]. In agreement with these results, studies by Lee et al. [26], using different paradigms of muscle regeneration, support a delay rather than impairment in muscle regeneration at old age. More recent studies have shown, however, that whole muscles from geriatric mice (28Â months of age or older) grafted onto young recipient muscles maintain a regenerative defect [14]. In support, Chakkalakal et al. compared muscle repair 30Â days after BaCl2-induced muscle injury and observed that muscle fiber size did not return back to pre-injured levels in aged mice [13]. Possibly, the differences in the extent of regeneration among these studies might be due to the distinct ages, gender of mice, and models of muscle injury employed. It is noteworthy that even if repair of muscle is delayed rather than impaired, the rate at which an aged human tissue recovers after injury or surgery will have significant ramifications for their health and general well-being.
Parabiosis has been used to test the effects of circulatory or systemic factors of one animal on the other for over 100 years [5]. Anastomosis of the circulatory system between adult and old mice provided definite evidence that the effects of aging on muscle regeneration can be modulated by the age of the systemic environment [4]. Based on short-term injury-repair models, youthful factors accelerated muscle repair, and aging factors delayed repair [4]. In vitro cell culture experiments supported this observation at the level of the satellite cell and their progeny [4, 27]. This seminal work paved the path to the discovery of circulating factors as regulators of satellite cell aging. Numerous studies have now shown that aged muscle repair can be augmented by modulation of many signaling pathways including RTK/ERK, Notch, Wnt, TGF?, and hormones, such as oxytocin, in vivo [23, 27–30]. Importantly, neutralization of Wnt and TGF? signaling in old mice restored efficient muscle repair. It is worth noting that both signaling pathways antagonize Notch signaling, which is critical for satellite cell activation, proliferation, and self-renewal [23, 27, 31]. In addition, treatment of aged mice with the reproductive hormone oxytocin promotes muscle regeneration [28]. In sum, targeting cell-extrinsic regulators in old mice, via parabiosis (or exposure to factors in young serum in cell culture), via inhibition of Wnt and TGF? signaling, or oxytocin administration, significantly rescues defective satellite cell-mediated muscle regeneration.
Recently, the assessment of the expression and role of a circulating factor, growth differentiation factor 11 (GDF11), has provided conflicting results [32, 33]. The results from the first study implied that circulating GDF11 levels decline with aging and showed that the treatment of old mice with recombinant GDF11 improved muscle regeneration after injury [33]. A subsequent study showed that GDF11 levels do not decline with aging and that administration of recombinant GDF11 protein does not affect repair of old muscle [32]. The reasons for the discrepancies are discussed elsewhere [34].
Overall, these findings demonstrate reversibility of satellite cell dysfunction in old animals through restoration by, or mimicking of, a young extrinsic environment. Due to the nature of the muscle injury assay, and the influence of many cell types to its outcome, it is not possible to determine if these stimulatory effects are acting at the level of the stem cell, myogenic progenitors, or different cell types. Indeed, direct evidence for non-stem cell-mediated control of muscle repair comes from studies investigating inflammation. A transient blood-mediated inflammatory response to muscle injury is key for successful regeneration [35, 36]. Paradoxically, organismal inflammation increases during aging [37]. Yet, the initial inflammatory response to muscle injury in old mice is delayed [7]. This, together with the increased production of osteopontin by old macrophages infiltrating injured muscle, could contribute to the poor regenerative outcome of aged muscle [30]. Indeed, neutralization of osteopontin rejuvenated the behavior of old satellite cells in vivo and in vitro, reinforcing the idea that age-related inflammatory responses become counterproductive for muscle regeneration. Furthermore, recent studies have shown a detrimental role for IL-6-Jak-Stat3 signaling [17, 18]. Thus, while transient inflammation in response to muscle injury is required for efficient regeneration, chronic elevation may be deleterious [17, 18]. Finally, a recent report showed that the increased levels of TGF? in aged mice promoted inflammation, rather than exerting its canonical role in attenuating immune responses, and hence inhibition of TGF? signaling improved regeneration [31]. Interestingly, increased levels of TGF? have been recently shown to promote survival of fibro-adipogenic progenitors (FAPs), a muscle-resident mesenchymal stomal cell population known to crosstalk with satellite cells during acute muscle regeneration in young mice [38]. Therefore, increased TGF? in aged environment may have deleterious functions on satellite cells through an increase in pro-fibrotic muscle-resident stem cell types.