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F U L L T E X T S O U R C E : FEBSPRESS
Abstract
Aging is characterized by the progressive dysfunction of most tissues and organs, which has been linked to the regenerative decline of their resident stem cells over time. Skeletal muscle provides a stark example of this decline. Its stem cells, also called satellite cells, sustain muscle regeneration throughout life, but at advanced age they fail for largely undefined reasons. Here, we discuss current understanding of the molecular processes regulating satellite cell maintenance throughout life and how age‐related failure of these processes contributes to muscle aging. We also highlight the emerging field of rejuvenating biology to restore features of youthfulness in satellite cells, with the ultimate goal of slowing down or reversing the age‐related decline in muscle regeneration.
Introduction
Stem cells are rare and specialized cells within a fully differentiated tissue that are essential for host‐organ repair and renewal. In an adult organism, stem cell populations have the unique capacity both to self‐renew and to generate progeny that differentiates to replace lost or damaged cells [1, 2]. In most instances, stem cell properties and behavior are determined by the regenerative requirements of the host tissue: High‐turnover tissues like the intestine or the hematopoietic system maintain active populations of stem‐ or progenitor‐cell populations, whereas tissues like the skeletal muscle maintain most of their stem cell pool in a quiescent state, activating it only upon injury [3]. The central role played by stem cells in tissue replacement throughout the human lifespan means that their functional decline often results in compromised organ maintenance and regeneration. At the same time, stem cell‐based therapies hold immense potential for regenerative medicine to restore or rejuvenate tissues.
Aging is accompanied by a progressive decline in tissue function and increased vulnerability to disease. Current understanding of the aging process centers on the interplay of cell‐intrinsic, intercellular communication and systemic dysregulations. In this context, the adult stem cell is simultaneously an important integrator of multiple age‐related alterations and a key contributor to the progression of age‐related tissue dysfunction [4, 5].
Skeletal muscle harbors a population of quiescent stem cells, called satellite cells, that are essential for its extraordinary regenerative capacity. Upon injury, satellite cells exit quiescence and proliferate, giving rise to a population of committed progenitors capable of engaging the myogenic program to generate new myofibers and replace damaged tissue [6, 7]. This remarkable regenerative capacity is greatly affected by the aging process and is associated with an age‐related decline in satellite cell numbers and functionality. Moreover, skeletal muscle aging is characterized by a loss of mass and decline in muscle strength, a phenotype that broadly defines sarcopenia. Although the involvement of stem cells in the etiology of sarcopenia is debated [8], the maintenance of a healthy population of satellite cells or the exogenous delivery of a rejuvenated progenitor population to the aging muscle has the potential to correct acquired defects that give rise to age‐related muscle wasting. Skeletal muscle is thus an attractive model for the study of stem cell and tissue aging, as well as a potential future target tissue for stem cell‐based rejuvenating interventions.
In this review, we examine and discuss our current understanding of the determinants of satellite cell aging and their contribution to age‐related loss of regenerative capacity in skeletal muscle. We also highlight recent advances in the field pointing to promising rejuvenating interventions that could help restore skeletal muscle regenerative capacity in the elderly. Finally, we discuss the potential of stem cell‐based interventions as rejuvenating strategies to prevent or delay age‐related loss of muscle mass and force.
How muscle stem cells age
Skeletal muscle aging is accompanied by a considerable reduction in the size of the satellite cell pool. The decline in satellite cell numbers occurs in the early stages of muscle aging and is likely due to changes affecting the niche and to cell‐autonomous alterations that together disturb the proper balance between cell quiescence, proliferation, and apoptosis. Increased expression of myofiber‐derived Fibroblast growth factor (FGF)2 in old muscles has been shown to bring satellite cells out of quiescence, causing spontaneous mitogenic activity that can contribute to the exhaustion of the satellite cell pool [9]. Additionally, FGF2 inhibits the expression of sprouty1, an intracellular inhibitor of the Extracellular signal‐regulated kinase (ERK)/Mitogen‐activated protein kinase (MAPK) signaling pathway, and mouse models with satellite cell‐specific deletion of Sprouty1 have defects in the muscle stem cell pool. Sprouty1‐null cycling satellite cells are impaired in their capacity to return to quiescence, succumbing instead to apoptosis [10]. Depletion of resident satellite cells after regenerative events in aged muscles also involves deficiencies in the aged niche microenvironment. A recent study revealed that an age‐related decline in Notch activators in the niche provokes satellite cell death by mitotic catastrophe, impairing proliferative expansion of muscle stem cell in aged mice [11]. The idea that stem cell survival is an important factor in the maintenance of the satellite cell pool is further supported by the observations that old human satellite cells are susceptible to nuclear apoptosis [12] and that the promotion of anti‐apoptotic pathways in aged satellite cells improves muscle regenerative capacity [13].
Although the satellite cell pool diminishes with age, a fraction of the satellite cells survives within the skeletal muscle until very old age. However, even when they are present, these satellite cells have functional defects that undermine their ability to sustain muscle regenerative capacity. The defects in satellite cell proliferative activity in response to regenerative pressure seen in the early stages of the aging process are exacerbated in very old (geriatric) mice. Whereas the early defects are driven by alterations in the environment, at advanced ages quiescent satellite cells are intrinsically changed and become pre‐senescent, a state that leads to full senescence in response to regenerative pressure. As a consequence of these defects, old satellite cells show a reduced capacity for activation and expansion after injury and produce insufficient progeny to sustain muscle regeneration. Moreover, the progeny of muscle stem cells that overcome these limitations have a limited differentiation capacity, with poor myogenic potential and a tendency to commit to alternative lineages. The consequence of these defects in old skeletal muscle is an enhanced fibrotic response to injury. These functional alterations have multiple underlying molecular causes, including altered signaling cues from the aging environment and intrinsic alterations in genomic integrity and metabolic regulation within the satellite cell (Fig. 1 [14-16]).
Figure 1.
Extrinsic and intrinsic drivers of satellite cell aging. Changes in niche‐derived and systemic signaling molecules, along with intrinsic changes in the satellite cell, contribute to the functional impairments of aged muscle stem cells and the consequent defects in regenerative capacity of the aged skeletal muscle. Changes in the niche and systemic environment include alterations in the inflammatory signaling and changes in secreted and local growth factors that synergize with alterations in ECM signaling to impair satellite cell function. Intrinsic changes to the satellite cell include epigenetic changes and defects in autophagy, leading to increased senescence and apoptosis. These changes manifest in impaired satellite cell function characterized by loss of lineage commitment and low myogenic potential, defects in activation/proliferation that impair self‐renewal capacity.
The aging environment
In a young organism, satellite cell function is tightly regulated by signals originating from the niche and the systemic environment. Alterations to the surrounding milieu have profound effects on satellite cell quiescence, differentiation, and self‐renewal, and deterioration of the extracellular environment is one of the main factors determining age‐related stem cell dysfunction [17].
The contribution of the aging environment to the decline in satellite cell function was clearly demonstrated by heterochronic tissue transplant studies, which revealed that the age of the host animal was a key determinant factor of the regenerative success of the transplant [18-20]. Subsequent studies employed heterochronic parabiosis, an experimental model involving the surgically pairing and fusion of the circulatory systems of two mice; this approach demonstrated that the regenerative capacity of the muscle is modulated by exposure to blood from an animal of a different age [21, 22]. A recent study obtained similar results through direct blood exchange between a young mouse and an aged mouse, supporting the established idea that circulatory factors directly affect satellite cell aging [23].
Since these early discoveries, several signaling molecules, including niche‐derived signaling cues and systemic factors, have been identified as mediators of the extrinsic effects of the aging environment on satellite cell function [17]. Changes in the myofiber, the most abundant source of niche signaling, have a significant impact on satellite cell activity: an age‐related increase in Transforming growth factor (TGF)β and FGF signaling from the myofiber synergize with a decline in Dl‐driven Notch signaling and a decreased deposition of the Extracellular matrix (ECM) protein fibronectin to disrupt satellite cell activity, contributing to impaired regenerative capacity at old age [9, 24-28]. Changes in TGFβ and Notch activity contribute to an imbalance in satellite cell activation and differentiation cues [11, 24, 25], while increased FGF singling breaks satellite cell quiescence leading to stem cell loss [9]. The remaining satellite cells become unresponsive to FGF under regenerative pressure and fail to expand or self‐renew [26], a defect that is exacerbated by defective fibronectin deposition in old muscles undergoing regeneration, and the consequent impairment in integrin signaling [27, 28]. Aging also impairs the supportive function of fibro‐adipogenic progenitors (FAPs), which fail to induce the matricellular protein Wnt1‐inducible signaling pathway protein 1 (WISP1), required for satellite cell expansion and commitment [29].
Heterochronic parabiosis has identified systemic factors as key regulators of muscle stem cell function. These include the unconventional Wnt‐activating ligand complement component 1q, the TGFβ family member growth differentiation factor 11 (GDF11), and the hormone oxytocin [21, 30-32]. Increased Wnt signaling driven by systemic factors promotes aberrant fibrogenic commitment in aged satellite cells, leading to a fibrotic response upon regenerative pressure [21]. Conversely, oxytocin and GDF11 have been identified as rejuvenating factors, decreased in the circulation of old animals, and offer a possible route to improving satellite cell function and muscle regenerative capacity. However, the effects of GDF11 on the muscle and its age‐related changes are still debated, with contradictory reports highlighting the need for further investigation [33-37]. Another important circulatory factor affecting satellite cell function in sarcopenic muscles is the exercise‐induced myokine apelin [38]. Apelin levels decrease with age, and studies in mice show that this has important consequences for healthspan [39]. A recent study revealed an association between apelin signaling and the beneficial effects of exercise, and also identified a positive effect on the regenerative capacity of aged muscle stem cells [38].
Age‐related alterations in the immune environment have also emerged in recent years as important contributors to satellite cell impairments in old muscles. Regenerative success depends on a regulated immune response to muscle injury, involving multiple immune cell types and the coordination of pro‐inflammatory and anti‐inflammatory signaling [40, 41]. Muscle regeneration is critically regulated by macrophages [42, 43] and bone marrow transplant from old donors is sufficient to impair satellite cell function in young mice, reducing the number of Pax7+ cells and promoting fibrogenic conversion [44]. A potential mediator of these effects is an age‐related increase in myeloid‐derived TNFα signaling: TNFα‐expressing macrophages are present in aged skeletal muscle, and old TNFα‐null mice show improved satellite cell activation and myogenic commitment in response to injury [45]. Consistently with these observations, there is evidence that over‐activation of the Nuclear factor kappa‐light‐chain enhancer of activated B cells (NFκB) pathway (an important TNFα target) in muscle of old mice is sufficient to disrupt satellite cell function and limit regenerative success [46]. In addition to TNFα‐NFκB signaling, age‐related impairments of satellite cell function are also linked to Janus kinases (JAK)/Signal transducer and activator of transcription proteins (STAT) signaling. Increased STAT3 activation in old satellite cells compromises symmetric expansion by promoting direct myogenic commitment and limits regenerative capacity [47]; however, the source of STAT‐activating factors in the old muscles is still unknown. The age‐related impairment of satellite cell function is thus likely to be the result of the over‐activation of multiple inflammatory pathways acting synergistically. Muscle regenerative capacity is also thought to be limited by age‐related impairments to Regulatory T cell (Treg) signaling: Interleukin 33 (IL‐33)‐dependent recruitment of Tregs during muscle regeneration is decreased in old mice, due to ineffective production of IL‐33 by FAP‐like cells, and these defects compromise muscle regenerative capacity [48]. Since one of the functions of Tregs during muscle regeneration is to coordinate the transition between pro‐inflammatory and anti‐inflammatory macrophage states, this age‐related impairment may be an additional way in which chronic activation of inflammatory signaling compromises muscle regeneration in old animals.
Epigenetic changes and genomic stability
Satellite cell aging is accompanied by genome‐wide epigenetic changes that translate abnormal signaling from the aged environment and a lifelong accumulation of molecular damage into altered gene expression programs. Comparative epigenomic analysis of young and old satellite cells has revealed complex changes with distinct consequences for quiescent and activated stem cells: While in quiescent satellite cells, an age‐related increase in repressive chromatin marks can contribute to changes in the expression of genes involved in stem cell self‐renewal and lineage commitment [49], in old activated satellite cells permissive chromatin states cause the aberrant induction of developmental pathways that impair stem cell function [50]. Understanding of the impact of these global changes at the level of individual gene expression is still limited; however, epigenetic changes are known to play an important role in the conversion of quiescent satellite cells to a pre‐senescent state in geriatric mice [51]. Specific analysis of the p16INK4a locus in satellite cells isolated from geriatric muscles revealed the loss of ubiquitinated H2A, a chromatin repressive mark associated with the polycomb repressor complex 1 [51]. More recently, the transcriptional repressor Slug was also found to be downregulated in aged satellite cells, contributing to the derepression of the p16INK4a gene and the conversion of aged muscle stem cells to a pre‐senescent state [52]. Future studies of global chromatin accessibility will likely reveal other changes to specific genomic loci that trigger age‐related perturbations in satellite cell function. It should also be remembered that epigenetic alterations can impact the health of neighboring cells in the satellite cell niche, indirectly contributing to muscle stem cell loss of function. Analysis of histone modifications in whole human skeletal muscle tissues found an age‐associated increase in the active enhancer marker H3K27ac. In mouse models, this enhancer activation is associated with the upregulation of ECM genes during aging, contributing to a decline in myogenic capacity and increased fibrogenic conversion of aged satellite cells [53].
The accumulation of DNA damage is a potential source of genomic instability and an important contributor to global changes in the epigenome with aging [54, 55]. The quiescent satellite cell combines a low risk of replication‐induced DNA damage with highly efficient mechanisms of DNA repair [56]. However, satellite cells isolated from aged mice still have an elevated number of foci containing the DNA damage marker γH2AX [31]. Furthermore, whole‐genome sequencing of human satellite cells isolated from individuals of different ages revealed an age‐related increase in the somatic mutation burden [57], reinforcing the notion that loss of genomic integrity is an important factor in satellite cell aging. Knowledge remains limited about how age‐related environmental changes converge to drive alterations in the satellite cell epigenetic landscape. A better understanding of these processes in aged satellite cells could provide important insights into how to expand the muscle stem cell healthspan.
Autophagic and metabolic defects
The lifelong accumulation of altered or damaged proteins is an important factor in age‐related stem cell dysfunction, affecting multiple intracellular signaling pathways and ultimately driving genomic instability, senescence, and apoptosis [58]. Satellite cells, mostly quiescent throughout life, cannot eliminate toxic organelles and protein aggregates through cell division and are therefore particularly susceptible to proteostatic stress [59]. As a consequence, mechanisms of intracellular macromolecular clearance, such as autophagy, play a fundamental role in the maintenance of satellite cell quiescence [58]. Impairment of autophagy causes the accumulation of damaged mitochondria and the generation of high reactive oxygen species (ROS) levels, further propagating protein and DNA damage. During aging, a decline in autophagic flux and the consequent increase in ROS levels are directly linked to the epigenetic derepression of the p16INK4a locus in old satellite cells and their entry into senescence [60]. The observed decline in defective organelle clearance in aged quiescent satellite cells may be related to an oscillatory rewiring that affects the expression of autophagy genes [61].
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