Sarcopenia (Rosenberg, 1997) was originally coined by Evans and Campbell as a combination of two Greek words sarx (meat) and penia (loss). In the 1980s, Irwin Rosenberg defined sarcopenia as age-related muscle mass loss. In 2010, the European Working Group on Sarcopenia in Older People (EWGSOP) defined low muscle function (characterized by low muscle strength) and reduced muscle mass as two criteria for the clinical diagnosis of sarcopenia (Cruz-Jentoft et al., 2010). In 2016, sarcopenia was recognized as an independent disease (Cruz-Jentoft et al., 2019) and in 2018, EWGSOP updated EWGSOP2 to include low muscle strength as the primary index for diagnosis of sarcopenia, and define suspected sarcopenia as the presence of decreased muscle strength. Sarcopenia is characterized by decreased muscle strength and muscle mass, whereas severe sarcopenia is characterized by decreased muscle strength, muscle mass, and physical function (Cruz-Jentoft et al., 2019). In 2020, the Asian Working Group of Sarcopenia (AWGS) updated its Expert Consensus on the Diagnosis and Treatment of Sarcopenia Asia (AWGS2019), which defines sarcopenia as low muscle mass, low muscle strength, and/or low physical function. Currently, there is no internationally accepted diagnostic standard for the definition of sarcopenia.

Diagnosis of sarcopenia is based on the definition of sarcopenia, which requires a comprehensive assessment of the patient’s muscle mass, muscle function (strength), and body function. According to the stepwise diagnostic approach and defined cutoff points proposed by EWGSOP2 (Cruz-Jentoft et al., 2019), sarcopenia is usually evaluated from grip strength, which is easy to measure and highly predictive of adverse outcomes, to detect muscle strength. According to EWGSOP2, the grip strength of men with sarcopenia is less than 27 kg, and that of women is less than 16 kg (Cruz-Jentoft et al., 2019). The second step in diagnosis of sarcopenia is measurement of muscle mass. AWGS2019 recommends using DXA or DSM-BIA combined with the height-corrected appendicular skeletal muscle mass (ASM) index (ASM/height²) to evaluate muscle mass. According to EWGSOP2, this ratio is less than 7 in men and 5.5 in women with sarcopenia (Cruz-Jentoft and Sayer, 2019). The concept of body function includes muscle function as well as central and peripheral nerve function, which are used to evaluate the cooperation of individual tissues and organs. The normal walking pace of patients is often used in clinical evaluation of physical function because it can predict the occurrence of adverse health events related to sarcopenia, such as disability, cognitive impairment, weakness, and fall susceptibility. According to EWGSOP2, the walking pace of men and women with sarcopenia is less than 0.8 m/s (Cruz-Jentoft et al., 2019).

Like other syndromes, the causes of sarcopenia syndrome are varied. Even healthy individuals can gradually lose muscle mass and function with aging. During early growth and development, muscle weight gradually increases to a peak and is generally maintained subsequently. Each person loses about half a pound of muscle a year between the ages of 30 and 60 (Figure 1). But this loss of muscle mass is accompanied by a doubling of fat gain (Forbes, 1999; Barazzoni et al., 2018), so it is usually not noticeable (Gallagher et al., 2000). Changes in amounts of muscle and fat can also lead to changes in muscle composition, with fat infiltration into muscle to form “marbling” (Visser et al., 2002), which can also reduce muscle mass and performance.

Multiple factors, not just aging, can contribute to the onset and progression sarcopenia, including poor nutrition, lack of exercise (Glover et al., 2008; Tanner et al., 2015), disease, inflammation, and iatrogenic factors (Cruz-Jentoft et al., 2019). Protein intake is closely related to skeletal muscle health (Cruz-Jentoft et al., 2010; Cruz-Jentoft et al., 2014; Lorenzo-Lopez et al., 2017). In sarcopenia, muscle protein synthesis and muscle protein breakdown (MPB) are unbalanced (Paddon-Jones et al., 2008; Breen and Phillips, 2011), and the response of older people to muscle myofibrillar protein synthesis supplied by protein/amino acid is also weakened (Cuthbertson et al., 2004; Katsanos et al., 2005; Moore et al., 2015; Brook et al., 2016). Long-term bed rest or an otherwise sedentary or low-exercise lifestyle negatively impact the volume, structure, chemical composition, and muscle nerve excitability of muscle fibers (Zampieri et al., 2015), which accelerates the process of sarcopenia. Chronic heart and lung diseases (such as chronic heart failure, chronic obstructive pulmonary disease) (Buchman et al., 2009; Bouillon et al., 2013; Elbaz et al., 2014; Navarro-Cruz et al., 2019), diabetes (Sinclair et al., 2017), metabolic diseases (Cleasby et al., 2016), endocrine diseases (such as androgen deficiency or deprivation), cancer, liver, and kidney diseases, bone and joint diseases, and even nervous system disorders can aggravate sarcopenia by restricting the absorption of nutrients and increasing inflammation.

Mechanisms of sarcopenia are complex and varied. Reduction of insulin-like growth factor-1 (IGF-1), a major anabolic signal of muscle protein synthesis, is associated with skeletal muscle mass (Ari et al., 2004; Bian et al., 2020), lower grip strength (van Nieuwpoort et al., 2018), and reduced physical performance. Insulin is also an anabolic hormone acting on myocyte (Bouchi et al., 2017; Tokarz et al., 2018). The sensitivity of myocyte to insulin decreases during aging, resulting in a decrease of muscle mass. In addition, elevation of glucocorticoids under stress, cachexia, and other conditions can increase MPB (Hoppeler, 2016). Decreased numbers of mitochondria and mitochondrial dysfunction occurring with aging can lead to muscle reduction and functional decline (Chabi et al., 2008; Wawrzyniak et al., 2016; Picca et al., 2018). Simultaneously, accumulated ROS leads to production of faulty proteins and overall dysfunction of mitochondria (Chabi et al., 2008; Chocron et al., 2019), which reduce muscle function. One of characteristic of aging is the increase of circulating proinflammatory cytokines [such as interleukin 1 (IL-1), IL-6, tumor necrosis factor (TNF-α), and C-reactive protein] that can promote MPB (Argiles, 2017; Dalle et al., 2017) and downregulate IGF-1 (Piotrowicz and Gasowski, 2020), thereby affecting muscle strength and activity. In addition, decreases of autophagy associated with aging can lead to accumulation of damaged cell components and misfolded proteins (Mizushima et al., 2008), as well as decreased numbers and function of MuSCs (Garcia-Prat et al., 2016), which affect the function and regeneration of myocytes (Garcia-Prat et al., 2016; Jiao and Demontis, 2017). The negative effects of MuSC quiescence and impaired self-renewal capacity on sarcopenia will be discussed in the next section.

Several approaches, including exercise, dietary supplements, and hormonal interventions are presently the main interventions for managing sarcopenia. Exercise reduces age-related oxidative damage and chronic inflammation, increases autophagy, improves mitochondrial function, and enhances IGF-1 signaling and insulin sensitivity. Resistance exercise (RE) is generally considered to alleviate sarcopenia by increasing skeletal muscle mass and strength (Cruz-Jentoft and Landi, 2014; Landi et al., 2014; Karlsen et al., 2020). Mechanistic studies found that RE can reduce age-induced intracellular lipid (IMCL) accumulation and enhance expression of glucose homeostasis regulatory components (e.g., glucose transporter type 4, glucose-6-phosphate dehydrogenase, hexokinase 2, and glycogen synthase 1) to resist sarcopenia (Ribeiro et al., 2017). RE can also induce autophagy, which is closely related to sarcopenia (Masiero et al., 2009; Sandri, 2010), by regulating Akt/mechanistic target of rapamycin (mTOR) and Akt/FOXO3a signaling pathways, as well as AMPK-mediated mitochondrial quality control (Zeng et al., 2020). Most exercise interventions currently used to treat sarcopenia involve a combination of aerobic, balance, and resistance exercises, which are more effective than RE alone (Moore et al., 2020). However, even in elderly individuals who are physically active, the loss of muscle weight and increase in body fat percentage are still significant (Hawkins et al., 2001), so exercise is not a radical cure.

Nutritional supplementation combined with exercise intervention is a very common treatment for sarcopenia (Abizanda et al., 2015; Naclerio and Larumbe-Zabala, 2016; McKendry et al., 2020). In young individuals, RE makes the body more sensitive to protein supplemented in the diet (Symons et al., 2009). During this time, a combined approach of RE with intake of more protein can be directly used for skeletal muscle synthesis. However, owing to problems related to anabolic resistance (Burd et al., 2013), older people need to eat more protein to maintain muscle mass and function. Protein intake of 1.0–1.3 g/kg/day reduced muscle mass loss in older people by 40% (Houston et al., 2008; Valenzuela et al., 2013) compared with the previously recommended (Manore and Meacham, 2003; Wolfe et al., 2017) intake of 0.66–0.80 g/kg/day. Balanced meal frequency and total protein intake at each meal are more beneficial to MSP in older people (Loenneke et al., 2016; Smeuninx et al., 2020). Notably, animal-derived proteins are more widely recommended than plant-based proteins for sarcopenia owing to their comprehensive amino acids (Gorissen et al., 2018; Churchward-Venne, 2019), high digestibility, and low anti-nutritional factors (Gilani et al., 2012). In particular, branched-chain amino acids, including leucine, valine, and isoleucine, can increase muscle mass by directly promoting protein synthesis through activation of the mTOR signaling pathway (Katsanos et al., 2006; Le Couteur et al., 2020). In addition, the leucine metabolite β-hydroxy-β-methylbutyrate has a positive effect on protein synthesis and mitochondrial dynamics in muscle (Sanz-Paris et al., 2018; Standley et al., 2020), and its oral administration of together with other protein supplements is beneficial for maintaining muscle mass in older people (Sanz-Paris et al., 2018). Supplementation with omega-3 polyunsaturated fatty acids, polyphenols, and vitamin D can also suppress inflammation and improve metabolism, thereby supporting skeletal muscle function (Domingues-Faria et al., 2016; Dupont et al., 2019; Du et al., 2020).

Hormonal modulation, a classic strategy to alleviate sarcopenia (Kwak and Kwon, 2019), offers new options for elderly individuals suffering from exercise intolerance. One of the key options is to promote myogenic cells and protein synthesis by activating androgen receptor (AR) expression in satellite cells and myoblasts. Testosterone is age-dependently reduced as a ligand for ARs. Supplementation with testosterone promotes satellite cell entry into the cell cycle, thereby increasing the number of satellite cells (Hikim et al., 2002). Moreover, testosterone increases the response of fusion-impaired myoblasts, enhances AR levels, and increases the abundance of AKT expression (Hughes et al., 2016; Pal et al., 2019). In addition, testosterone improves the efficiency of amino acid utilization after proteolysis and promotes muscle synthesis (Ferrando et al., 1998). Experiments have proven that testosterone can increase muscle protein synthesis in frail and elderly patients with heart failure, thereby improving muscle strength and increase walking distance. However, there are risks associated with testosterone therapy, including sleep apnea, thrombotic complications, and increased risk of prostate cancer. Accordingly, the potential risks of testosterone use in older people population may outweigh its benefits, so it is not widely used. Selective androgen receptor modulators (SARMs), a new class of AR ligands, have the same effect as androgens with higher selectivity for bone and muscle tissues with fewer side effects. Embosarm, for example, successfully increased muscle mass and body functions in patients without cancer and patients with advanced lung, colorectal, breast cancer, or lymphoma. The Embosarm Phase II study in 120 healthy older adults showed both lean body mass gain and dose-dependent improvement in stair climbing (Dalton et al., 2011). However, Embosarm has not been approved by the United States Food and Drug Administration owing to hepatotoxicity and unpredictable post-administration prognosis.

Melatonin, an effective substitute for testosterone, was shown to prevent muscle atrophy induced by the IGF-1 axis in rats. In addition, melatonin can reduce adipose formation in obese mice by regulating autocrine and paracrine responses in muscle and adipose tissue (Jin et al., 2021).

The quiescence of MuSCs is jointly regulated by extracellular cues and intracellular molecules. Abnormal activation of QSCs leads to the accumulation of aging-related damage and depletion of the stem cell pool (Chakkalakal et al., 2012; Kimmel et al., 2020). Therefore, ensuring accurate expression of these regulatory factors is very important for muscle regeneration.

Just as MuSCs are regulated to remain quiescent, their activation is also directly influenced by cell cycle regulators. Perturbation of the signaling pathways and regulators that maintain MuSC quiescence activates QSCs to enter the cell cycle. As the regulatory center of MuSC activation, PI3K-AKT mediates the activation of mTOR signaling and inhibits high expression of FOXO in QSCs, which is closely related to their activation.

In 2014, Rodgers et al. reported the existence of an intermediate state (G_alert) between quiescence and activation of MuSCs (Rodgers et al., 2014). Compared with QSCs, G_alert MuSCs were in a state of mobilization, with larger cell sizes and higher mitochondrial activity (Rodgers et al., 2014). Later studies confirmed that hepatocyte growth factor activator generated from tissue injury binds to C-Met on the surface of MuSCs, whereby it activates mTORC1 through PI3K to trigger the G_alert state of MuSCs (Rodgers et al., 2017).

MuSCs undergo two cell division patterns: symmetrical division to produce two stem cells and asymmetric division to produce one differentiated cell and one stem cell. MuSCs produced by symmetric division and the offspring MuSC produced by asymmetric division need to return to quiescence to supplement the MuSC pool. In contrast, myogenically committed cells produced by asymmetric division need to differentiate and fuse to form muscle fibers (Figure 2). Regardless of the division mode and ultimate cell fate, both processes are accompanied by withdrawal from the cell cycle.

p38α stimulates cell cycle exit by inducing expression of cell cycle inhibitors. In addition, p38α induces MuSC myogenic differentiation by activating MyoD and MEF2 transcriptional activities (Aziz et al., 2010) and promoting the action of chromatin remodeling complex on myogenic promoters (Wu et al., 2000; Forcales et al., 2012). MyoD can also induce expression of muscle-specific miRNAs (miR-206 and miR-133), downregulate Pax7, and activate the main regulator of differentiated myogenin through transcriptional (Dey et al., 2011; Dong et al., 2020) and post-transcriptional mechanisms. Finally, MyoD and myogenin work together to activate MRF4 and other muscle-derived terminal differentiation genes, causing the cell to exit the cell cycle (Bi et al., 2017).

Induction of myogenic differentiation should be avoided in the selection of a self-renewal fate during MuSC proliferation. p38-mitogen-activated kinase (MAPK) (Bernet et al., 2014), fibroblast growth factor (FGF)-extracellular-regulated kinase (ERK)/MAPK (Pawlikowski et al., 2017) and JAK-STAT3 (Pietrosemoli et al., 2017) pathways are involved in determination of the self-renewal fate. During asymmetric division and in contrast to the myogenically committed cell, the self-renewing MuSC daughter cell exhibits Notch3 receptor expression, Notch pathway activation, increased Pax7 expression, and inhibited MyoD expression, which induce the renewed MuSC to resume quiescence.

At present, how MuSCs choose a differentiation or self-renewal fate is not fully understood. In vivo and in vitro studies found that the division pattern and number of MuSCs depend on their internal counting mechanism (Evano et al., 2020), indicating the complexity of their proliferative regulation. Activation of QSCs and subsequent determination of their cell fate are precisely controlled by a variety of signals and factors.

Not all stem cells within a tissue are homogeneous, and this is true of MuSCs. Indeed, heterogeneity is considered a beneficial feature of stem cells, enabling them to improve their ability to adapt to the environment and respond to signals. The complex quiescent and active regulatory networks of MuSCs are also inseparable from their heterogeneity. Accordingly, the heterogeneity of MuSCs can be described from multiple perspectives.

First, MuSC subsets can be distinguished according to different cell surface markers. CD34 is the first candidate, according to which QSCs can be divided into two subgroups: CD34^hi and CD 34^low. The former exhibits more stem-like properties, but represents no more than 15% of total cells, whereas the latter is more committed to myogenic differentiation (Garcia-Prat et al., 2020). In addition, expression of CD9 alone or in combination with CD104 can be used to distinguish between quiescent and activated myogenic progenitor cell populations (Porpiglia et al., 2017)—the CD9⁺/CD104⁻ subgroup of MuSCs is more likely to self-renew (Porpiglia et al., 2017).

MuSC activation and subsequent myogenic differentiation or self-renewal are continuously regulated by transcription factors, which are regarded as myogenic markers. Thus, QSC heterogeneity can also be defined by the expression patterns of transcription factors. Previous studies linked the loss of Myf5 expression to the maintenance of MuSC quiescence. However, with more widespread use of lineage-tracing techniques, a subset of Pax7⁺ was identified in mice that never express Myf5. Compared with Myf5-expressing subpopulations, Pax7⁺/Myf5- MuSCs performed better at reconstructing the quiescent MuSC pool after transplantation, suggesting that Pax7⁺/Myf5⁻ MuSCs retained more stemness (Kuang et al., 2007). In addition, Pax7^hi and Pax7^low MuSCs were identified by their relative expression of Pax7, with Pax7^hi MuSCs exhibiting a more quiescent state with lower metabolic activity (Rocheteau et al., 2012).

MuSC heterogeneity can also be distinguished according to differences in gene expression, which can be confirmed by single-cell sequencing (scRNA-seq). MuSC subsets encoding growth arrest-specific protein (Gas1), Notch-targeted Hes1, Pax7, and CALCR genes are more quiescent. MuSC subsets expressing ribosomal genes Myf5 and MyoD were more inclined to exhibit early activation (Dell'Orso et al., 2019).

To determine whether labeling was retained during proliferation, MuSCs were artificially labeled with a histone H₂B pulse-chase system or the fluorescent dye PKH26. MuSCs that retained the label (H₂B-GFP⁺) exhibited a low degree of differentiation, preference for self-renewal to maintain the stem cell pool, and high expression of cell cycle inhibitors p21 and p27. Unlabeled MuSCs (H₂B-GFP-) retained fast division speeds and tended to undergo myogenic differentiation. Similarly, MuSCs exhibiting high retention of the fluorescent dye PKH26 exhibited higher stemness than MuSCs exhibiting low retention.

Aging impairs both autonomous and niche-dependent regulation of MuSCs, eliciting changes in their ability to maintain quiescence and functional loss of self-renewal and regeneration—the main factors driving age-related muscle changes (Carlson et al., 2008; Sousa-Victor et al., 2014). External cues, such as changes in inflammatory factors and extracellular components [e.g., Wnt ligands (Brack et al., 2007), transforming growth factor β (TGF-β) (Carlson et al., 2008), and FGF2 (Chakkalakal et al., 2012)], may influence MuSC function. In addition, intrinsic factors including oxidative stress, DNA damage, altered signaling pathways, protein damage, and metabolic changes, can impair MuSC function (Sousa-Victor et al., 2014).

The mainstream view is that the number of MuSCs decreases in an age-dependent manner (Day et al., 2010; Verdijk et al., 2012; Verdijk et al., 2014). The data showed a significant decrease in the number of MuSC in type II muscle fibers of the vastus lateralis muscle in older (50–59 years) and senescent (70–86 years) populations compared with younger adults (18–49 years) (Verdijk et al., 2014). However, other studies show no significant difference in the number of MuSCs between young and aged mice (van der Meer et al., 2011). Therefore, this review is skeptical about the correlated changes between MuSCs and aging. In any case, there are various mechanisms for aging to affect the number of MuSCs, as suggested by the mainstream view. Although QSCs are in a dormant state of low metabolism, they still exhibit continuous autophagy to remove damaged proteins and organelles. With age, this protective mechanism is gradually lost (Sandri, 2010). Aging-dependent autophagy impairments or genetic damage of MuSCs will lead to the loss of protein homeostasis and increased oxidative stress, which further promotes aging, resulting in declining numbers and function of MuSCs (Revuelta and Matheu, 2017). These negative effects can be reversed by genetic and pharmacological manipulation. MuSCs isolated from elderly mice exhibited increased numbers of lesions containing the DNA damage marker γH2AX (Sinha et al., 2014), suggesting a potential influence of genomic instability on muscle aging. Owing to the non-mitogenic nature of QSCs, repair of DNA damage is achieved by non-homologous end joining, a repair mechanism different from cell cycle-dependent double-strand break repair (Ferdousi et al., 2014). However, because non-homologous end joining does not require template strands for DNA repair, its error rate is increased, thus increasing the possibility of cell mutagenesis (Burkhalter et al., 2015).

Insulin and IGF provided by aging muscle activate the PI3K-Akt signal pathway (classical pathway regulating the transformation of QSCs into ASCs), which subsequently activates expression of mTORC1 and inhibits expression of FOXO. Chronic activation of mTORC1 associated with aging or repeated stimulation of mTORC1 to maintain ASC proliferation and differentiation leads to depletion of the MuSC pool (Haller et al., 2017). Moreover, reduced inhibition of FOXO activity by Akt promotes expression of myogenic regulatory factors (e.g., MyoG), leading to the withdrawal of the MuSC from its quiescent state. Measures to inhibit mTORC1 activation are conducive to protection of the MuSC pool. FGF2, another cytokine whose production is increased in aging muscle (Bernet et al., 2014), binds FGF receptors on MuSCs to cause their continuous activation, which also causes MuSC depletion (Bernet et al., 2014). Overexpression of SPRY1 can counteract increases of FGF2 and protect the quiescence of aging MuSCs (Chakkalakal et al., 2012). Other studies evaluating signaling pathways found that Notch-dependent activation of p53 was decreased during regeneration of aging MuSCs, which led to impaired MuSC self-renewal and increased mitosis-related mortality of MuSCs (Liu et al., 2018). Restoring the activity of p53 in aging MuSCs can promote their survival and regeneration. Notably, a reduction of AMPK/p27 pathway activation was observed in aging MuSCs, which exhibited impaired autophagy and an increased propensity for apoptosis, thus consuming the MuSC pool (White et al., 2018).

During aging, the loss of MuSC stemness and their withdrawal from the quiescent state caused by abnormal activation can occur simultaneously. TGF-β promotes expression of CDK inhibitors and inhibits MuSC proliferation through the pSmad3 intracellular pathway (Carlson et al., 2008). Notch signaling can antagonize this function by removing pSmad3 from the promoter of CDK inhibitor genes. Therefore, continuous enhancement of TGF-β signaling and continuous consumption of Notch signaling in aging muscle may lead to increased expression of CDK inhibitors, which are related to defects in proliferation and differentiation (Carlson et al., 2008). Accordingly, blocking TGF-β signal transduction is beneficial to reducing aging and promoting MuSC function. The key reason for the loss of regenerative potential of MuSCs with aging is upregulated expression of the CDK inhibitor p16^INK4a, which helps MuSCs permanently withdraw from the cell cycle, resulting in impaired self-renewal ability (Sousa-Victor et al., 2014). Factors associated with high expression of p16^INK4a include ROS accumulation in mitochondria, constitutive activation of p38/MAPK signaling (Cosgrove et al., 2014), and low expression of the p16^INK4a repressor Slug (Li et al., 2019). Silencing of p16^INK4a in MuSCs by gene knockdown or overexpression of Slug restores MuSC quiescence and rescues their ability to transplant and return to quiescence. During mitosis occurring with muscle regeneration, p53 can inhibit and defect the spindle assembly, causing apoptosis of MuSCs in the process of regeneration (Liu et al., 2018).

The loss of muscle mass in sarcopenia is mainly characterized by type II muscle fiber atrophy, which is accompanied by a decrease in the amount of type II muscle fiber type-specific MuSCs in aging muscle (Leenders et al., 2013; Mackey et al., 2014; Verdijk et al., 2014). Based on studies showing that the onset of MuSC reduction precedes age-related muscle fiber atrophy (Brack et al., 2005) and MuSC content is a strong predictor of muscle fiber size in aged individuals (Verdijk et al., 2010), it has been hypothesized that age-related decreases in type II muscle fiber MuSC content may lead to decreased muscle fiber maintenance capacity, resulting in the specific type II muscle fiber atrophy observed in aged muscle (Snijders et al., 2009). Although it has been reported that muscle mass in humans is relatively stable between developmental maturity and 40 years of age, and MuSC function is often well preserved thereafter, signs of sarcopenia and a slow and steady decline in muscle mass can also occur (Mitchell et al., 2012). Compared with younger mice, old mice (20–24 months of age) showed a significant decrease in MuSCs along with fiber atrophy, but geriatric mice (28–32 months of age) showed no further loss of MuSCs when multiple sarcopenic symptoms increased (Sousa-Victor et al., 2014). Considering the conflicting evidence, it remains controversial whether the loss of MuSCs directly causes sarcopenia, although MuSCs are thought to be the sole source of new muscle nuclei and there is an inextricable link between the decay of MuSCs and poor muscle progression.

Fry et al. (2015) observed significant muscle mass loss only in flounder muscle after induced depletion of MuSCs from adult mouse skeletal muscle. In aged mice (24 months old), muscle atrophy consistent with criteria for human sarcopenia was observed in both control and MuSC-depleted groups, indicating that MuSC-dependent loss of regenerative capacity in adulthood does not accelerate sarcopenia in aging mice. However, there was a low frequency of MuSC depletion in this experiment (range 64%–87%) and normally aging control mice may still lack the muscle fiber regeneration that inhibits sarcopenia. Keefe et al. (2015) used a similar strategy to induce MuSC depletion in adult mouse skeletal muscle and found that MuSCs continued to contribute to muscle fibers even in the absence of injury, but were unnecessary to maintain muscle fiber CSA in uninjured adult muscle. The reduced number of muscle fibers in sarcopenia (Lushaj et al., 2008) is not mentioned in the MuSC-depleted mouse model.

The changes that muscles undergo over the lifetime of model mice cannot be fully analogous to humans, and a recent study based on human muscle samples is exciting. Although it is generally accepted that the decreased regenerative capacity of MuSCs correlates with aging (Forcina et al., 2019), Novak et al. (2021), who xenografted human muscle from different age sources into an immunodeficient mouse model, found that the muscle-forming capacity of MuSCs was not impaired with age and loss of muscle mass during aging was not attributable to any intrinsic myogenic loss of MuSCs. These findings confirmed that muscle regeneration can still occur even in MuSCs from elderly individuals in a friendly environment, suggesting the possibility of improving the environment for MuSC survival to protect muscle health.

Neuromuscular junctions (NMJs) are synapses located between motor neurons and skeletal muscle cells that are important for maintaining muscle mass and function (Liu et al., 2018). Degeneration of NMJs is associated with aging and one of the manifestations of sarcopenia. Depletion of MuSCs accelerates age-related degeneration of NMJs and significantly induces early onset of reduced cross-sectional area of muscle fibers, a sarcopenic phenotype (Liu et al., 2017). Activation of MuSCs in the vicinity of NMJs has been shown to be essential for promoting myofiber reinnervation and restoring muscle atrophy and fibrosis (Liu et al., 2015). Previous experiments demonstrated that overexpression of Spry1 in SCs attenuates age-related SC decline (Chakkalakal et al., 2012; Bigot et al., 2015). To verify that functional MuSCs are the key factor for maintaining muscle mass and strength, and slowing the progression of sarcopenia, Liu et al. (Liu et al., 2017) compared SC-specific Spry1-overexpressing Pax7 ^CreER/+; CAG^Spry1/+ (Spry1OX) mice with Pax7^+/+; CAG^Spry1/+ (control) mice. Flow cytometric analysis confirmed that aging muscles of Spry1OX mice had significantly more SCs at 12 months of age, and that Spry1OX muscles had diminished age-related neuromuscular junction degeneration and a robust increase in postsynaptic myonuclear cluster size. In behavioral tests, older Spry1OX mice had significantly longer run times and distances, and exhibited a significant increase in the peak absolute force produced by the muscle. This experiment demonstrates that retention of functional MuSCs in aging skeletal muscle is essential to maintain muscle mass and strength, and to slow the development of sarcopenia. Collectively, these findings lay the theoretical foundation for subsequent treatment of sarcopenia by improving MuSC function.

As humans age, numbers of MuSCs decrease dramatically. Sarcopenia also elicits age-related loss of skeletal muscle, encompassing declines in both muscle fiber size and number. Experiments have validated the relationship between MuSCs and muscle hypertrophy, which is related to their potential to differentiate into muscle fibers.

In the 1990s, Rosenblatt and Parry used γ-irradiation to ablate MuSCs and observed virtually no muscle fiber growth in mice (Rosenblatt and Parry, 1992; Rosenblatt and Parry, 1993). However, due to controversy surrounding the low specificity of γ-irradiation on MuSCs, this test cannot fully prove a relationship between MuSCs and muscle hypertrophy. Therefore, in 2011, McCarthy et al. (2011) used the Pax7^CreER-diphtheria toxin A (Pax7^CreER-DTA) strain of mice that allows for conditional ablation of Pax7⁺ cells (MuSCs) to verify the hypothesis that MuSCs supports skeletal muscle hypertrophy. The results showed that muscle fiber hypertrophy induced by 2 weeks of overload was accompanied by the production of new myonuclei in muscle with MuSCs. However, similar hypertrophy of muscle fibers was observed in MuSC-ablated muscle. Therefore, the authors believe that MuSCs are not necessary for mouse skeletal muscle hypertrophy. In 2014, Fry et al. (2014) used the same mouse model and found that muscle fiber hypertrophy without satellite cells was impaired after 8 weeks of overload, providing a role for satellite cells in long-term muscle fiber hypertrophy. To explain the above controversy, Egner et al. (2016) used the exact same experimental model and found that increases of muscle fiber cross-sectional area and myonuclei contents were observed after 2-weeks overload, but not in MuSC-deficient muscles. The authors suggest that the reason for this discrepancy results from the use of different criteria for evaluating muscle fiber cross-sectional area in McCarthy’s experiment. McCarthy’s assessment included fibers showing signs of damage, whereas (Egner et al., 2016) observed that damaged muscle fibers were generally smaller than normal muscle fibers. If a large number of damaged muscle fibers are included in the statistics, it will mask the growth of muscle fibers in response to overload. Notably, (Egner et al., 2016) did observe a difference in muscle mass gain between normal and MuSC-ablated mice (112% and 55%, respectively), but they considered muscle mass to be an unreliable indicator of hypertrophy in the plantaris overload model due to complications after synergistic ablation. Hall et al. (2010) showed that transplantation of MuSCs (along with some muscle fibers) into injured muscles of young mice contributed to an increase in muscle mass and strength, and this positive effect persisted as the mice aged. In this transplantation model, the lifetime increases in muscle mass arose from an increase in muscle fiber number and progressive muscle fiber hypertrophy, providing new evidence that MuSCs can cause muscle hypertrophy.

Studies on the relationship between MuSCs and muscle hypertrophy in humans are mostly descriptive, however numerous studies have confirmed that the hypertrophy of muscle fibers during long-term resistance exercise is accompanied by increases in MuSCs and muscle nuclei content, as observed in both healthy young and old adults (Petrella et al., 2006; Leenders et al., 2013; Bellamy et al., 2014; Farup et al., 2014; Abou Sawan et al., 2021). Thus, MuSCs do indeed play an important role in muscle fiber hypertrophy under stimulation with human muscle growth factors, regardless of age.

These results demonstrate that MuSCs have a positive role in long-term muscle fiber hypertrophy, which is of great significance in resisting muscle loss associated with sarcopenia. Moreover, these findings suggest that optimizing the function of MuSCs with aging may be a feasible strategy to combat skeletal sarcopenia.

The quantity and function of MuSCs are subject to a combination of intra- and extracellular signals. Therefore, restoration of MuSC function should be addressed from multiple perspectives (Figure 4).