Satellite Cell Pool Research Articles

Cell Transplantation and “Stem Cell Therapy” in the Treatment of Myopathies: Many Promises in Mice, Few Realities in Humans

Myopathies produce deficits in skeletal muscle function and, in some cases, progressive and irreversible loss of skeletal muscles. The transplantation of myogenic cells, that is, cells able to differentiate into myofibers, is an experimental strategy for the potential treatment of some of these diseases. The objectives pursued by the transplantation of these cells are essentially three: (a) the fusion with the patient’s myofibers to obtain the expression of therapeutic proteins into them, (b) the neoformation of new functional myofibers in skeletal muscles that were too degenerated by the progressive degeneration, and (c) the formation of a new pool of healthy donor-derived satellite cells. Although the repertoire of myogenic cells appears to have expanded in recent years, myoblasts are the only cells that have been demonstrated to engraft in humans. The present work aims to make a comprehensive review of the subject, from its beginnings to recent advances, including the preclinical experience in different animal models and recent clinical findings.

O.19 Specification of muscle stem cells : A genome-wide screen to identify new regulators of skeletal muscle satellite cell renewal

Stem cells play a central role in development and maintenance of tissues and organs in the body of animals and humans. During embryonic development, growth or regeneration, skeletal muscle cells are unable to divide but replenish from a population of progenitor stem cells, which have the unique ability to divide, to produce copies of themselves as well as differentiating muscle cells. Around birth, the fetal muscle progenitor cells adopt a satellite cell position, becoming embedded within the basal lamina in close contact to the muscle fibers. Importantly, in addition to this morphological change, the emerging satellite cell enter quiescence, a molecular state poorly characterized in vivo. During post-natal growth or in response to injury or disruption of the basal lamina, a subset of the satellite cells become activated, proliferate and either fuse to form multinucleated myotubes or re-establish a residual pool of quiescent satellite cells that have the capability of supporting additional rounds of growth/regeneration. Little is known regarding the molecular pathways involved in the emergence of post-natal muscle progenitors in vertebrates, as well as the genetic changes associated with the cellular and stemness capacity of these cells. I will present our screening strategy to identify candidate genes potentially involved in the formation of adult satellite cells, that led to the identification of new factors involved in satellite cells biology.

P38α MAPK Regulates Adult Muscle Stem Cell Fate by Restricting Progenitor Proliferation During Postnatal Growth and Repair

Stem cell function is essential for the maintenance of adult tissue homeostasis. Controlling the balance between self-renewal and differentiation is crucial to maintain a receptive satellite cell pool capable of responding to growth and regeneration cues. The mitogen-activated protein kinase p38α has been implicated in the regulation of these processes but its influence in adult muscle remains unknown. Using conditional satellite cell p38α knockout mice we have demonstrated that p38α restricts excess proliferation in the postnatal growth phase while promoting timely myoblast differentiation. Differentiation was still able to occur in the p38α-null satellite cells, however, but was delayed. An absence of p38α resulted in a postnatal growth defect along with the persistence of an increased reservoir of satellite cells into adulthood. This population was still capable of responding to cardiotoxin-induced injury, resulting in complete, albeit delayed, regeneration, with further enhancement of the satellite cell population. Increased p38γ phosphorylation accompanied the absence of p38α, and inhibition of p38γ ex vivo substantially decreased the myogenic defect. We have used genome-wide transcriptome analysis to characterize the changes in expression that occur between resting and regenerating muscle, and the influence p38α has on these expression profiles. This study provides novel evidence for the fundamental role of p38α in adult muscle homeostasis in vivo.

Lying low but ready for action: the quiescent muscle satellite cell

The muscle satellite cell is essential for skeletal muscle regeneration. It is located on the muscle fibre, under the basal lamina as a quiescent cell, which becomes activated after injury, when it leaves the fibre, proliferates, and either undergoes myogenesis to form new fibres or reconstitutes the satellite cell pool. In this review, we discuss the cellular environment of the quiescent cell, including the extracellular matrix, which constitutes its niche. Cell adhesion molecules and some signalling pathways reinforce its quiescent state, whereas other signals lead to activation. We discuss how the satellite cell is ready to respond with the appropriate receptors, but protects its quiescence by mechanisms that include immobilization of ligands by extracellular matrix components and synthesis of inhibitors for intracellular signalling pathways and for metalloproteinases that break down the matrix and promote ligand processing and receptor activation. The quiescent satellite cell is also well protected against toxins and oxidative stress. It has a low metabolic rate, as shown by few active mitochondria and anaerobic glycolysis. Different subpopulations of quiescent satellite cells can be distinguished on the basis of cell surface markers and stem cell-like properties. We discuss the latter in the context of the small proportion of satellite cells that express high levels of Pax7, or that are derived from cells that have never activated the Myf5 myogenic determination gene. However, many quiescent satellite cells transcribe Myf5, but do not enter myogenesis because of post-transcriptional regulation, which prevents Myf5 protein accumulation. Post-transcriptional regulation, through microRNA repression of a potential cell cycle activator, further illustrates how these cells are ready for action.

Transient systemic mtDNA damage leads to muscle wasting by reducing the satellite cell pool

With age, muscle mass and integrity are progressively lost leaving the elderly frail, weak and unable to independently care for themselves. Defined as sarcopenia, this age-related muscle atrophy appears to be multifactorial but its definite cause is still unknown. Mitochondrial dysfunction has been implicated in this process. Using a novel transgenic mouse model of mitochondrial DNA (mtDNA) double-strand breaks (DSBs) that presents a premature aging-like phenotype, we studied the role of mtDNA damage in muscle wasting. We caused DSBs in mtDNA of adult mice using a ubiquitously expressed mitochondrial-targeted endonuclease, mito-PstI. We found that a short, transient systemic mtDNA damage led to muscle wasting and a decline in locomotor activity later in life. We found a significant decline in muscle satellite cells, which decreases the muscle's capacity to regenerate and repair during aging. This phenotype was associated with impairment in acetylcholinesterase (AChE) activity and assembly at the neuromuscular junction (NMJ), also associated with muscle aging. Our data suggests that systemic mitochondrial dysfunction plays important roles in age-related muscle wasting by preferentially affecting the myosatellite cell pool.

Myf5 expression during fetal myogenesis defines the developmental progenitors of adult satellite cells

Myf5 is a member of the muscle-specific determination genes and plays a critical role in skeletal muscle development. Whereas the expression of Myf5 during embryonic and fetal myogenesis has been extensively studied, its expression in progenitors that will ultimately give rise to adult satellite cells, the stem cells responsible for muscle repair, is still largely unexplored. To investigate this aspect, we have generated a mouse strain carrying a CreER coding sequence in the Myf5 locus. In this strain, Tamoxifen-inducible Cre activity parallels endogenous Myf5 expression. Combining Myf5CreER and Cre reporter alleles, we were able to evaluate the contribution of cells expressing Myf5 at distinct developmental stages to the pool of satellite cells in adult hindlimb muscles. Although it was possible to trace back the origin of some rare satellite cells to a subpopulation of Myf5+ve progenitors in the limb buds at the late embryonic stage (∼E12), a significant number of satellite cells arise from cells which expressed Myf5 for the first time at the fetal stage (∼E15). These studies provide direct evidence that adult satellite cells derive from progenitors that first express the myogenic determination gene Myf5 during fetal stages of myogenesis.

Satellite cell pool expansion is affected by skeletal muscle characteristics

We investigated changes in satellite cell (SC) pool size after an acute bout of strenuous exercise and evaluated the influence of baseline SC count and fiber type. Participants completed a downhill running (DHR) intervention (5 × 8 min, 2-min rest; 80% VO2max ; -10% gradient). Muscle biopsies were taken 7 days before VO₂max and 7-9 days after the DHR intervention. Delayed-onset muscle soreness (DOMS) and creatine kinase activity (CK) were measured on days 1, 2, 7, and 9 post-DHR. SCs were identified by Pax7 and laminin staining. Relative distribution of MHC isoforms was determined by electrophoresis. DOMS and CK peaked on day 1 post-DHR (P < 0.01). The SC pool increased (26%) after DHR (P = 0.005). SCs/total myonuclei after recovery correlated with baseline SCs (r = 0.979, P = 0.003) and VO₂max (r = 0.956, P = 0.011), whereas change in SC pool (Pax7(+) cells/total myonuclei: recovery minus baseline) tended to correlate with percent MHC II (r = 0.848; P = 0.06). Interindividual physiological characteristics affect SC pool expansion after a single bout of DHR and are influenced by VO₂max .

Moderate‐intensity treadmill running promotes expansion of the satellite cell pool in young and old mice

Satellite cells, the myogenic progenitors located at the myofibre surface, are essential for the repair of adult skeletal muscle. There is ample evidence for an age-linked decline in the number of satellite cells and performance in limb muscles. Hence, an effective means of activating and expanding the satellite cell pool may enhance muscle maintenance and reduce the impact of age-associated muscle deterioration (sarcopaenia). Accordingly, in the present study, we explored the beneficial effects of endurance exercise on satellite cells in young and old mice. Animals were subjected to an 8-week moderate-intensity treadmill-running approach that does not inflict apparent muscle damage (0° inclination, 11.5 m·min(-1) for 30 min·day(-1) , 6 days·week(-1) ). Myofibres of extensor digitorum longus muscles were then isolated from exercised and sedentary mice and used for monitoring the number of satellite cells, as well as for harvesting individual satellite cells for clonal growth assays. We specifically focused on satellite cell pools of single myofibres, with the view that daily wear of muscles probably affects individual myofibres rather than causing overall muscle damage. We found an expansion of the satellite cell pool in the exercised groups compared to the sedentary groups, with the same increase (~ 1.6-fold) in both ages. The results of the present study are in agreement with our findings obtained using rat gastrocnemius, indicating the consistent effect of exercise on satellite cell expansion in limb muscles. The experimental paradigm established in the present study is useful for investigating satellite cell dynamics at the myofibre niche, as well as for broader investigations of the impact of physiologically and pathologically relevant factors on adult myogenesis.

The influence of satellite cell‐depletion on glycosaminoglycan accumulation in aged skeletal muscle

We utilized the Pax7‐DTA mouse model to directly test the role of satellite cells in aged muscle fibrosis. The Pax7‐DTA mouse allows for the conditional depletion of satellite cells in adult muscle with tamoxifen treatment. Four month old mice were treated with tamoxifen or vehicle and sacrificed at 24 months of age. Pax7 immunohistochemistry of the gastrocnemius muscle demonstrated a reduction of 78% of the satellite cell pool in tamoxifen‐compared to vehicle‐treated aged mice. Thus, early‐adulthood tamoxifen treatment successfully reduces the satellite cell pool, which does not effectively recover during the course of the lifespan. Alpha‐wheat germ agglutinin immunostaining demonstrated a 40% increase in glycosaminoglycan expression from muscle depleted of satellite cells, with no differences in muscle cross‐sectional area compared to muscle from vehicle‐treated muscle. The robust increase in glycosaminoglycan expression in satellite cell depleted muscle provides strong evidence that satellite cells participate in the regulation of muscle extracellular matrix remodeling with age, however do not influence overall muscle size.Funding: NIH R01AR060701 and R21AG34453 (CAP); Ellison Foundation/American Federation of Aging Research EPD12102 (JDL); American Physiological Society undergraduate research fellowship (ZJH); University of Kentucky Center for Muscle Biology.

Coordination of Satellite Cell Activation and Self-Renewal by Par-Complex-Dependent Asymmetric Activation of p38α/β MAPK

In response to muscle injury, satellite cells activate the p38α/β MAPK pathway to exit quiescence, thenproliferate, repair skeletal muscle, and self-renew, replenishing the quiescent satellite cell pool. Although satellite cells are capable of asymmetric division, the mechanisms regulating satellite cell self-renewal are not understood. We found that satellite cells, once activated, enter the cell cycle and a subset undergoes asymmetric division, renewing the satellite cell pool. Asymmetric localization of the Par complex activates p38α/β MAPK in only one daughter cell, inducing MyoD, which permits cell cycle entry and generates a proliferating myoblast. The absence of p38α/β MAPK signaling in the other daughter cell prevents MyoD induction, renewing the quiescent satellite cell. Thus, satellite cells employ a mechanism to generate distinct daughter cells, coupling the Par complex and p38α/β MAPK signaling to link the response to muscle injury with satellite cell self-renewal.

Muscle Fiber Atrophy and Regeneration Coexist in Collagen VI-Deficient Human Muscle: Role of Calpain-3 and Nuclear Factor-κB Signaling

Ullrich congenital muscular dystrophy (UCMD) is a common form of muscular dystrophy associated with defects in collagen VI. It is characterized by loss of individual muscle fibers and muscle mass and proliferation of connective and adipose tissues. We sought to investigate the mechanisms by which collagen VI regulates muscle cell survival, size, and regeneration and, in particular, the potential role of the ubiquitin-proteasome and calpain-proteolytic systems. We studied muscle biopsies of UCMD (n = 6), other myopathy (n = 12), and control patients (n = 10) and found reduced expression of atrogin-1, MURF1, and calpain-3 mRNAs in UCMD cases. Downregulation of calpain-3 was associated with changes in the nuclear immunolocalization of nuclear factor-κB. We also observed increased expression versus controls of regeneration markers at the protein and RNA levels. Satellite cell numbers did not differ in collagen VI-deficient muscle versus normal nonregenerating muscle, indicating that collagen VI does not play a key role in the maintenance of the satellite cell pool. Our results indicate that alterations in calpain-3 and nuclear factor-κB signaling pathways may contribute to muscle mass loss in UCMD muscle, whereas atrogin-1 and MURF1 are not likely to play a major role.

Satellite cell heterogeneity revealed by G-Tool, an open algorithm to quantify myogenesis through colony-forming assays

BackgroundMuscle growth and repair is accomplished by the satellite cell pool, a self-renewing population of myogenic progenitors. Functional heterogeneity within the satellite cell compartment and changes in potential with experimental intervention can be revealed by in vitro colony-forming cell (CFC) assays, however large numbers of colonies need to be assayed to give meaningful data, and manually quantifying nuclei and scoring markers of differentiation is experimentally limiting.MethodsWe present G-Tool, a multiplatform (Java) open-source algorithm that analyzes an ensemble of fluorescent micrographs of satellite cell-derived colonies to provide quantitative and statistically meaningful metrics of myogenic potential, including proliferation capacity and propensity to differentiate.ResultsWe demonstrate the utility of G-Tool in two applications: first, we quantify the response of satellite cells to oxygen concentration. Compared to 3% oxygen which approximates tissue levels, we find that 21% oxygen, the ambient level, markedly limits the proliferative potential of transit amplifying progeny but at the same time inhibits the rate of terminal myogenic differentiation. We also test whether satellite cells from different muscles have intrinsic differences that can be read out in vitro. Compared to masseter, dorsi, forelimb and hindlimb muscles, we find that the diaphragm satellite cells have significantly increased proliferative potential and a reduced propensity to spontaneously differentiate. These features may be related to the unique always-active status of the diaphragm.ConclusionsG-Tool facilitates consistent and reproducible CFC analysis between experiments and individuals. It is released under an open-source license that enables further development by interested members of the community.

Satellite cell pool enhancement in rat plantaris muscle by endurance training depends on intensity rather than duration

Increases in the number of satellite cells are necessary for the maintenance of normal muscle function. Endurance training enhances the satellite cell pool. However, it remains unclear whether exercise intensity or exercise duration is more important to enhance the satellite cell pool. This study examined the effects of different intensity and duration of endurance training on the satellite cell pool in rat skeletal muscle. Forty-one 17-week-old female Sprague-Dawley rats were assigned to control (n = 8), high intensity and high duration (n = 7), high intensity and low duration (n = 8), low intensity and high duration (n = 9) and low intensity and low duration (n = 9) groups. Training groups exercised 5 days per week on a motor driven treadmill for 10 weeks. After the training period, animals were anaesthetized and the plantaris muscles were removed, weighed and analysed for immunohistochemical and histochemical properties. Although no significant differences were found in muscle mass, mean fibre area and myonuclei per muscle fibre between all groups, the percentage of satellite cells was significantly higher in the high-intensity groups than in the other groups (P < 0.05). Increases in the satellite cell pool of skeletal muscle following endurance training depend on the intensity rather than duration of exercise.

Satellite cell pool size expansion is affected by skeletal muscle characteristics

The purpose of this study was to determine the change of satellite cells (SC) pool size after an acute bout of strenuous exercise and to determine whether this is affected by baseline SC count and fiber type.Participants completed 40min (5 × 8 bouts, 2 min rest) downhill running (DHR) at 80% VO2max at −10% gradient. Muscle biopsy was taken a week before VO2max test and 7–9 days after DHR. Delayed onset muscle soreness (DOMS) and creatine kinase activities (CK) were measured on d1, d2 d7, and d9 after the DHR bout. Satellite cells were analyzed with Pax7 and laminin immunohistochemistry staining.DOMS and CK increased 1 day after DHR (P &lt; 0.01) and returned to baseline. SC pool size was increased by 26% 7–9 days after the exercise intervention (P = 0.005). SC count after the acute exercise intervention was significantly correlated with VO2max (P = 0.03). A trend was seen in correlation between change in SC and percentage MHC type II (P = 0.10).This study reports that 7–9 days after a single bout of DHR, the SC pool expands and the expansion extent is associated with inter‐individual physiological characteristics: VO2max and with a tendency, skeletal muscle fiber type.Grant Funding Source : Claude Leon Foundation, NRF (National Research Foundation)

Nitric Oxide Sustains Long-Term Skeletal Muscle Regeneration by Regulating Fate of Satellite Cells Via Signaling Pathways Requiring Vangl2 and Cyclic GMP

Satellite cells are myogenic precursors that proliferate, activate, and differentiate on muscle injury to sustain the regenerative capacity of adult skeletal muscle; in this process, they self-renew through the return to quiescence of the cycling progeny. This mechanism, while efficient in physiological conditions does not prevent exhaustion of satellite cells in pathologies such as muscular dystrophy where numerous rounds of damage occur. Here, we describe a key role of nitric oxide, an important signaling molecule in adult skeletal muscle, on satellite cells maintenance, studied ex vivo on isolated myofibers and in vivo using the α-sarcoglycan null mouse model of dystrophy and a cardiotoxin-induced model of repetitive damage. Nitric oxide stimulated satellite cells proliferation in a pathway dependent on cGMP generation. Furthermore, it increased the number of Pax7+/Myf5− cells in a cGMP-independent pathway requiring enhanced expression of Vangl2, a member of the planar cell polarity pathway involved in the Wnt noncanonical pathway. The enhanced self-renewal ability of satellite cells induced by nitric oxide is sufficient to delay the reduction of the satellite cell pool during repetitive acute and chronic damages, favoring muscle regeneration; in the α-sarcoglycan null dystrophic mouse, it also slowed disease progression persistently. These results identify nitric oxide as a key messenger in satellite cells maintenance, expand the significance of the Vangl2-dependent Wnt noncanonical pathway in myogenesis, and indicate novel strategies to optimize nitric oxide-based therapies for muscular dystrophy. Stem Cells 2012; 30:197–209.

Electrical Stimulation Influences Satellite Cell Proliferation and Apoptosis in Unloading-Induced Muscle Atrophy in Mice

Muscle atrophy caused by disuse is accompanied by adverse physiological and functional consequences. Satellite cells are the primary source of skeletal muscle regeneration. Satellite cell dysfunction, as a result of impaired proliferative potential and/or increased apoptosis, is thought to be one of the causes contributing to the decreased muscle regeneration capacity in atrophy. We have previously shown that electrical stimulation improved satellite cell dysfunction. Here we test whether electrical stimulation can also enhance satellite cell proliferative potential as well as suppress apoptotic cell death in disuse-induced muscle atrophy. Eight-week-old male BALB/c mice were subjected to a 14-day hindlimb unloading procedure. During that period, one limb (HU-ES) received electrical stimulation (frequency: 20 Hz; duration: 3 h, twice daily) while the contralateral limb served as control (HU). Immunohistochemistry and western blotting techniques were used to characterize specific proteins in cell proliferation and apoptosis. The HU-ES soleus muscles showed significant improvement in muscle mass, cross-sectional area, and peak tetanic force relative to the HU limb (p<0.05). The satellite cell proliferative activity as detected within the BrdU+/Pax7+ population was significantly higher (p<0.05). The apoptotic myonuclei (detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) and the apoptotic satellite cells (detected by cleaved Poly [ADP-ribose] polymerase co-labeled with Pax7) were reduced (p<0.05) in the HU-ES limb. Furthermore the apoptosis-inducing factor and cleaved caspase-3 were down-regulated while the anti-apoptotic Bcl-2 protein was up-regulated (p<0.05), in the HU-ES limb. These findings suggest that the electrical stimulation paradigm provides an effective stimulus to rescue the loss of myonuclei and satellite cells in disuse muscle atrophy, thus maintaining a viable satellite cell pool for subsequent muscle regeneration. Optimization of stimulation parameters may enhance the outcome of the intervention.

Extrinsic Regulation of Satellite Cell Function and Muscle Regeneration Capacity during Aging

Optimal regeneration of skeletal muscle in response to injury requires the contribution of tissue resident stem cells termed satellite cells. Normally residing at the interface between the muscle fiber and overlying basal lamina it is generally understood with age the satellite cell pool exhibits decline both in number and function. Over the past decade mechanisms that contribute to these declines have begun to emerge. Implicit in aged-related satellite cell dysfunction and decline is the involvement of signals from the environment. Many of the signals that become deregulated with age have conserved functions during distinct stages of muscle fiber formation both in early development and regeneration. In particular, modulations in Wnt, TGFβ, Notch and FGF emanating from aged skeletal muscle fibers or the systemic milieu have emerged as age-related alterations that significantly impact both the maintenance of the satellite cell pool and skeletal muscle regenerative efficacy. In this review we will summarize how the aforementioned pathways contribute to skeletal muscle development and regeneration.We will then discuss deregulation of these cascades with age and how they contribute to satellite cell depletion and dysfunction. The review will also summarize some of the challenges we face in trying to draw parallels between murine and human satellite cell aging. Finally, we will highlight the few examples whereby FDA approved drugs may be exploited to modulate specific signaling cascades in effort to preserve skeletal muscle regenerative function with age.

Apoptosis-Inducing Factor Regulates Skeletal Muscle Progenitor Cell Number and Muscle Phenotype

Apoptosis Inducing Factor (AIF) is a highly conserved, ubiquitous flavoprotein localized in the mitochondrial intermembrane space. In vivo, AIF provides protection against neuronal and cardiomyocyte apoptosis induced by oxidative stress. Conversely in vitro, AIF has been demonstrated to have a pro-apoptotic role upon induction of the mitochondrial death pathway, once AIF translocates to the nucleus where it facilitates chromatin condensation and large scale DNA fragmentation. Given that the aif hypomorphic harlequin (Hq) mutant mouse model displays severe sarcopenia, we examined skeletal muscle from the aif hypomorphic mice in more detail. Adult AIF-deficient skeletal myofibers display oxidative stress and a severe form of atrophy, associated with a loss of myonuclei and a fast to slow fiber type switch, both in “slow” muscles such as soleus, as well as in “fast” muscles such as extensor digitorum longus, most likely resulting from an increase of MEF2 activity. This fiber type switch was conserved in regenerated soleus and EDL muscles of Hq mice subjected to cardiotoxin injection. In addition, muscle regeneration in soleus and EDL muscles of Hq mice was severely delayed. Freshly cultured myofibers, soleus and EDL muscle sections from Hq mice displayed a decreased satellite cell pool, which could be rescued by pretreating aif hypomorphic mice with the manganese-salen free radical scavenger EUK-8. Satellite cell activation seems to be abnormally long in Hq primary culture compared to controls. However, AIF deficiency did not affect myoblast cell proliferation and differentiation. Thus, AIF protects skeletal muscles against oxidative stress-induced damage probably by protecting satellite cells against oxidative stress and maintaining skeletal muscle stem cell number and activation.

Strength training increases the size of the satellite cell pool in type I and II fibres of chronically painful trapezius muscle in females

While strength training has been shown to be effective in mediating hypertrophy and reducing pain in trapezius myalgia, responses at the cellular level have not previously been studied. This study investigated the potential of strength training targeting the affected muscles (SST, n = 18) and general fitness training (GFT, n = 16) to augment the satellite cell (SC) and macrophage pools in the trapezius muscles of women diagnosed with trapezius myalgia. A group receiving general health information (REF, n = 8) served as a control. Muscle biopsies were collected from the trapezius muscles of the 42 women (age 44 ± 8 years; mean ± SD) before and after the 10 week intervention period and were analysed by immunohistochemistry for SCs, macrophages and myonuclei. The SC content of type I and II fibres was observed to increase significantly from baseline by 65% and 164%, respectively, with SST (P < 0.0001), together with a significant correlation between the baseline number of SCs and the extent of hypertrophy (r = -0.669, P = 0.005). SST also resulted in a 74% enhancement of the trapezius macrophage content (P < 0.01), accompanied by evidence for the presence of an increased number of actively dividing cells (Ki67(+)) post-SST (P < 0.001). GFT resulted in a significant 23% increase in the SC content of type II fibres, when expressed relative to myonuclear number only (P < 0.05). No changes in the number of myonuclei per fibre or myonuclear domain were detected in any group. These findings provide strong support at the cellular level for the potential of SST to induce a strong myogenic response in this population.

Satellite cell count, VO2max, and p38 MAPK in inactive to moderately active young men

Satellite cells (SCs) are responsible for muscle repair following strenuous exercise or injury. SC responses to intervention have been studied, but most studies do not discuss or take into account the substantial variability in SC number among young individuals. We hypothesized that an active lifestyle reflected in higher VO(2max) may be associated with greater SC number. As training alters basal p38-mitogen-activated protein kinase (MAPK) activity, which is associated with SC proliferation, SC count may also correlate with this stress signaling kinase. Muscle biopsies from vastus lateralis of eight male participants were analyzed for fiber type, myogenin, and p38/phospho-p38 MAPK using SDS-PAGE and Western blotting. Immunofluorescence was used to detect Pax7(+) SCs. Two weeks following the biopsy, subjects underwent an incremental treadmill test to determine VO(2max) . A strong positive correlation (P = 0.0087) was found between the number of Pax7(+) nuclei and VO(2max) . Pax7(+) cell number correlated negatively with phospho-p38/p38 MAPK (P = 0.0006), but had no correlation with fiber type or myogenin. SC number is proportional to VO(2max) , and hence it can be postulated that higher levels of physical activity activate SC proliferation but not fusion, underlining the relevance of exercise in stimulating SC pool size even without injury.