Regulation of the muscle fiber microenvironment by activated satellite cells during hypertrophy

Muscle contracture and passive mechanics in cerebral palsy

Skeletal muscle contractures represent the permanent shortening of a muscle-tendon unit, resulting in loss of elasticity and, in extreme cases, joint deformation. They may result from cerebral palsy, spinal cord injury, stroke, muscular dystrophy, and other neuromuscular disorders. Contractures are the prototypic and most severe clinical presentation of increased passive mechanical muscle force in humans, often requiring surgical correction. Intraoperative experiments demonstrate that high muscle passive force is associated with sarcomeres that are abnormally stretched, although otherwise normal, with fewer sarcomeres in series. Furthermore, changes in the amount and arrangement of collagen in the extracellular matrix also increase muscle stiffness. Structural light and electron microscopy studies demonstrate that large bundles of collagen, referred to as perimysial cables, may be responsible for this increased stiffness and are regulated by interaction of a number of cell types within the extracellular matrix. Loss of muscle satellite cells may be related to changes in both sarcomeres and extracellular matrix. Future studies are required to determine the underlying mechanism for changes in muscle satellite cells and their relationship (if any) to contracture. A more complete understanding of this mechanism may lead to effective nonsurgical treatments to relieve and even prevent muscle contractures.

The Influence of Post-Exercise Cold-Water Immersion on Adaptive Responses to Exercise: A Review of the Literature

Post-exercise cold-water immersion (CWI) is used extensively in exercise training as a means to minimise fatigue and expedite recovery between sessions. However, debate exists around its merit in long-term training regimens. While an improvement in recovery following a single session of exercise may improve subsequent training quality and stimulus, reports have emerged suggesting CWI may attenuate long-term adaptations to exercise training. Recent developments in the understanding of the molecular mechanisms governing the adaptive response to exercise in human skeletal muscle have provided potential mechanistic insight into the effects of CWI on training adaptations. Preliminary evidence suggests that CWI may blunt resistance signalling pathways following a single exercise session, as well as attenuate key long-term resistance training adaptations such as strength and muscle mass. Conversely, CWI may augment endurance signalling pathways and the expression of genes key to mitochondrial biogenesis following a single endurance exercise session, but have little to no effect on the content of proteins key to mitochondrial biogenesis following long-term endurance training. This review explores current evidence regarding the underlying molecular mechanisms by which CWI may alter cellular signalling and the long-term adaptive response to exercise in human skeletal muscle.

Prepubertal skeletal muscle growth requires Pax7-expressing satellite cell-derived myonuclear contribution

The functional role of Pax7-expressing satellite cells (SCs) in postnatal skeletal muscle development beyond weaning remains obscure. Therefore, the relevance of SCs during prepubertal growth, a period after weaning but prior to the onset of puberty, has not been examined. Here, we have characterized mouse skeletal muscle growth during prepuberty and found significant increases in myofiber cross-sectional area that correlated with SC-derived myonuclear number. Remarkably, genome-wide RNA-sequencing analysis established that post-weaning juvenile and early adolescent skeletal muscle have markedly different gene expression signatures. These distinctions are consistent with extensive skeletal muscle maturation during this essential, albeit brief, developmental phase. Indelible labeling of SCs with Pax7^CreERT2/+; Rosa26^nTnG/+ mice demonstrated SC-derived myonuclear contribution during prepuberty, with a substantial reduction at puberty onset. Prepubertal depletion of SCs in Pax7^CreERT2/+; Rosa26^DTA/+ mice reduced myofiber size and myonuclear number, and caused force generation deficits to a similar extent in both fast and slow-contracting muscles. Collectively, these data demonstrate SC-derived myonuclear accretion as a cellular mechanism that contributes to prepubertal hypertrophic skeletal muscle growth.

Summary: Examination of gene expression and morphological changes in mouse skeletal muscle during prepuberty demonstrates that satellite cell-derived myonuclear accretion contributes to prepubertal hypertrophic skeletal muscle growth.

Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise

One of the most striking adaptations to exercise is the skeletal muscle hypertrophy that occurs in response to resistance exercise. A large body of work shows that a mammalian target of rapamycin complex 1 (mTORC1)-mediated increase of muscle protein synthesis is the key, but not sole, mechanism by which resistance exercise causes muscle hypertrophy. While much of the hypertrophy signaling cascade has been identified, the initiating, resistance exercise-induced and hypertrophy-stimulating stimuli have remained elusive. For the purpose of this review, we define an initiating, resistance exercise-induced and hypertrophy-stimulating signal as "hypertrophy stimulus," and the sensor of such a signal as "hypertrophy sensor." In this review we discuss our current knowledge of specific mechanical stimuli, damage/injury-associated and metabolic stress-associated triggers, as potential hypertrophy stimuli. Mechanical signals are the prime hypertrophy stimuli candidates, and a filamin-C-BAG3-dependent regulation of mTORC1, Hippo, and autophagy signaling is a plausible albeit still incompletely characterized hypertrophy sensor. Other candidate mechanosensing mechanisms are nuclear deformation-initiated signaling or several mechanisms related to costameres, which are the functional equivalents of focal adhesions in other cells. While exercise-induced muscle damage is probably not essential for hypertrophy, it is still unclear whether and how such muscle damage could augment a hypertrophic response. Interventions that combine blood flow restriction and especially low load resistance exercise suggest that resistance exercise-regulated metabolites could be hypertrophy stimuli, but this is based on indirect evidence and metabolite candidates are poorly characterized.

Potential Roles of Vascular Endothelial Growth Factor During Skeletal Muscle Hypertrophy

Vascular endothelial growth factor (VEGF) deletion in adult mouse muscle fibers contributes to impaired contractile and muscular adaptations to a hypertrophic stimulus suggesting a critical role in adult muscle growth. This review explores the hypothesis that VEGF is essential for adult muscle growth by impacting inflammatory processes, satellite-endothelial cell interactions, and contractile protein accumulation by functioning within known hypertrophic signaling pathways including insulin-like growth factor-1 (IGF-1-Akt) and Wnt-ß-catenin.

Skeletal muscle fibrosis is associated with decreased muscle inflammation and weakness in patients with chronic kidney disease

Muscle dysfunction is an important cause of morbidity among patients with chronic kidney disease (CKD). Although muscle fibrosis is present in a CKD rodent model, its existence in humans and its impact on physical function are currently unknown. We examined isometric leg extension strength and measures of skeletal muscle fibrosis and inflammation in vastus lateralis muscle from CKD patients ( n = 10) and healthy, sedentary controls ( n = 10). Histochemistry and immunohistochemistry were used to assess muscle collagen and macrophage and fibro/adipogenic progenitor (FAP) cell populations, and RT-qPCR was used to assess muscle-specific inflammatory marker expression. Muscle collagen content was significantly greater in CKD compared with control (18.8 ± 2.1 vs. 11.7 ± 0.7% collagen area, P = 0.008), as was staining for collagen I, pro-collagen I, and a novel collagen-hybridizing peptide that binds remodeling collagen. Muscle collagen was inversely associated with leg extension strength in CKD ( r = -0.74, P = 0.01). FAP abundance was increased in CKD, was highly correlated with muscle collagen ( r = 0.84, P < 0.001), and was inversely associated with TNF-α expression ( r = -0.65, P = 0.003). TNF-α, CD68, CCL2, and CCL5 mRNA were significantly lower in CKD than control, despite higher serum TNF-α and IL-6. Immunohistochemistry confirmed fewer CD68+ and CD11b+ macrophages in CKD muscle. In conclusion, skeletal muscle collagen content is increased in humans with CKD and is associated with functional parameters. Muscle fibrosis correlated with increased FAP abundance, which may be due to insufficient macrophage-mediated TNF-α secretion. These data provide a foundation for future research elucidating the mechanisms responsible for this newly identified human muscle pathology.

Alterations in quadriceps muscle cellular and molecular properties in adults with moderate knee osteoarthritis

OBJECTIVE

Quadriceps muscle weakness is common in knee osteoarthritis (OA). While pain, disuse, and atrophy are commonly cited causes for muscle weakness in OA, emerging evidence suggests changes in muscle quality also occur. Alterations in muscle quality are not well understood, but likely include both cellular and morphologic adaptions. The purpose of this study was to conduct the first cellular-level analysis of the vastus lateralis in adults with moderate knee OA.

METHODS

Vastus lateralis biopsies were obtained from 24 subjects with moderate knee OA and 15 healthy controls. Quadriceps strength, muscle fiber cross sectional area (CSA), fiber type distribution, extracellular matrix (ECM) content, satellite cell abundance, and profibrotic gene expression were assessed.

RESULTS

Relative to controls, quadriceps strength was significantly lower in OA subjects (OA 62.23, 50.67-73.8 Nm vs 91.46, 75.91-107.0 Nm, P = 0.003) despite no difference in fiber CSA. OA subjects had significantly fewer Type I fibers (OA 41.51, 35.56-47.47% vs 53.07, 44.86-61.29%, P = 0.022) and more hybrid IIa/x fibers (OA 24.61, 20.61-28.61% vs 16.4, 11.60-21.20%, P = 0.009). Significantly greater ECM content, lower satellite cell density, and higher profibrotic gene expression was observed with OA, and muscle collagen content was inversely correlated to strength and satellite cell (SC) density.

CONCLUSION

Lower quadriceps function with moderate OA may not result from fiber size impairments, but is associated with ECM expansion. Impaired satellite cell density, high profibrotic gene expression, and a slow-to-fast fiber type transition may contribute to reduced muscle quality in OA. These findings can help guide therapeutic interventions to enhance muscle function with OA.

Laminin-111 functionalized polyethylene glycol hydrogels support myogenic activity in vitro

Skeletal muscle has a remarkable regenerative capability following mild physical or chemical insult. However, following a critical loss of muscle tissue, the regeneration process is impaired due to the inadequate myogenic activity of muscle resident stem cells (i.e., satellite cells). Laminin (LM) is a heterotrimeric structural protein in the satellite cell niche that is crucial for maintaining its function. In this study, we created hydrogels composed of poly (ethylene glycol) (PEG) and LM-111 to provide an elastic substrate for satellite cell proliferation at the site of injury. The PEG-LM111 conjugates were mixed with 5% and 10% (w/v) pure PEG-diacrylate (PEGDA) and photopolymerized to form 5% and 10% PEGLM gels. Pure 5% and 10% PEGDA gels were used as controls. The modulus of both hydrogels containing 10% (w/v) PEGDA was significantly higher than the hydrogels containing 5% (w/v) PEGDA. The 5% PEGLM hydrogels showed significantly higher swelling in aqueous medium suggesting a more porous structure. C₂C₁₂ myoblasts cultured on the softer 5% PEGLM hydrogels showed a flat and spread-out morphology when compared to the rounded, multicell clusters formed on the 5% PEGDA, 10% PEGDA, and 10% PEGLM hydrogels. The 5% PEGLM hydrogels also promoted a significant increase in both vascular endothelial growth factor and interleukin-6 (IL-6) production from the myoblasts. Additionally, the expression of MyoD was significantly higher while that of myogenin and α-actinin trended higher on the 5% PEGLM hydrogels compared to 5% PEGDA on day 5. Our data suggests that the introduction of LM-111 into compliant PEG hydrogels promoted myoblast adhesion, survival, pro-regenerative growth factor production, and myogenic activity.

Losartan has no additive effect on the response to heavy-resistance exercise in human elderly skeletal muscle

Our purpose here was to investigate the potential of blocking the angiotensin II type I receptor (AT1R) on the hypertrophy response of elderly human skeletal muscle to 4 mo of heavy-resistance exercise training. Fifty-eight healthy elderly men (+65 yr) were randomized into three groups, consuming either AT1R blocker (losartan, 100 mg/day) or placebo for 4 mo. Two groups performed resistance training (RT) and were treated with either losartan or placebo, and one group did not train but was treated with losartan. Quadriceps muscle biopsies, MR scans, and strength tests were performed at baseline and after 8 and 16 wk. Biopsies were sectioned for immunohistochemistry to determine the number of satellite cells, capillaries, fiber type distribution, and fiber area. Gene expression levels of myostatin, connective tissue, and myogenic signaling pathways were determined by real-time RT-PCR. Four months of heavy-resistance training led in both training groups to expected improvements in quadriceps (∼3-4%) and vastus lateralis (∼5-6%), cross-sectional area, and type II fiber area (∼10-18%), as well as dynamic (∼13%) and isometric (∼19%) quadriceps peak force, but with absolutely no effect of losartan on these outcomes. Furthermore, no changes were seen in satellite cell number with training, and most gene targets failed to show any changes induced by training or losartan treatment. We conclude that there does not appear to be any effect of AT1R blocking in elderly men during 4 mo of resistance training. Therefore, we do not find any support for using AT1R blockers for promoting muscle adaptation to training in humans. NEW & NOTEWORTHY Animal studies have suggested that blocking angiotensin II type I receptor (AT1R) enhances muscle regeneration and prevents disuse atrophy, but studies in humans are limited. Focusing on hypertrophy, satellite cells, and gene expression, we found that AT1R blocking did not result in any greater responses with 4 mo of resistance training. These results do not support previous findings and question the value of blocking AT1R in the context of preserving aging human muscle.

Transcriptome changes in muscle of Nellore cows submitted to recovery weight gain under grazing condition

The aim of this study was to evaluate transcriptome changes in the muscle tissue of Bos taurus indicus cull cows subjected to recovery weight gain under grazing conditions. In all, 38 Nellore cull cows were divided randomly into two different management groups: (1) Maintenance (MA) and (2) Recovery gain (RG) from weight loss by moderate growth under high forage availability. After slaughter, RNA analysis was performed on the Longissimus thoracis muscle. Semaphorin 4A, solute carrier family 11 member 1, and Ficolin-2 were expressed in the RG, which may indicate an inflammatory response during tissue regrowth. Signaling factors, such as Myostatin, related to fibroblast activation, negative control of satellite cell proliferation in adults and muscle protein synthesis were less abundant in the RG group. The only gene related to anabolic processes that were more abundant in the MA group was related to fat deposition. The genes that were differentially expressed in the experiment showed muscle repair-related changes during RG based on the greater expression of genes involved in inflammatory responses and the lower expression of negative regulators of muscle cell proliferation and hypertrophy.

Physiological Differences Between Low Versus High Skeletal Muscle Hypertrophic Responders to Resistance Exercise Training: Current Perspectives and Future Research Directions

Numerous reports suggest there are low and high skeletal muscle hypertrophic responders following weeks to months of structured resistance exercise training (referred to as low and high responders herein). Specifically, divergent alterations in muscle fiber cross sectional area (fCSA), vastus lateralis thickness, and whole body lean tissue mass have been shown to occur in high versus low responders. Differential responses in ribosome biogenesis and subsequent protein synthetic rates during training seemingly explain some of this individual variation in humans, and mechanistic in vitro and rodent studies provide further evidence that ribosome biogenesis is critical for muscle hypertrophy. High responders may experience a greater increase in satellite cell proliferation during training versus low responders. This phenomenon could serve to maintain an adequate myonuclear domain size or assist in extracellular remodeling to support myofiber growth. High responders may also express a muscle microRNA profile during training that enhances insulin-like growth factor-1 (IGF-1) mRNA expression, although more studies are needed to better validate this mechanism. Higher intramuscular androgen receptor protein content has been reported in high versus low responders following training, and this mechanism may enhance the hypertrophic effects of testosterone during training. While high responders likely possess “good genetics,” such evidence has been confined to single gene candidates which typically share marginal variance with hypertrophic outcomes following training (e.g., different myostatin and IGF-1 alleles). Limited evidence also suggests pre-training muscle fiber type composition and self-reported dietary habits (e.g., calorie and protein intake) do not differ between high versus low responders. Only a handful of studies have examined muscle biomarkers that are differentially expressed between low versus high responders. Thus, other molecular and physiological variables which could potentially affect the skeletal muscle hypertrophic response to resistance exercise training are also discussed including rDNA copy number, extracellular matrix and connective tissue properties, the inflammatory response to training, and mitochondrial as well as vascular characteristics.

Molecular Regulation of Exercise-Induced Muscle Fiber Hypertrophy

Skeletal muscle hypertrophy is a widely sought exercise adaptation to counteract the muscle atrophy of aging and disease, or to improve athletic performance. While this desired muscle enlargement is a well-known adaptation to resistance exercise training (RT), the mechanistic underpinnings are not fully understood. The purpose of this review is thus to provide the reader with a summary of recent advances in molecular mechanisms-based on the most current literature-that are thought to promote RT-induced muscle hypertrophy. We have therefore focused this discussion on the following areas of fertile investigation: ribosomal function and biogenesis, muscle stem (satellite) cell activity, transcriptional regulation, mechanotransduction, and myokine signaling.

Myonuclear Domain Flexibility Challenges Rigid Assumptions on Satellite Cell Contribution to Skeletal Muscle Fiber Hypertrophy

Satellite cell-mediated myonuclear accretion is thought to be required for skeletal muscle fiber hypertrophy, and even drive hypertrophy by preceding growth. Recent studies in humans and rodents provide evidence that challenge this axiom. Specifically, Type 2 muscle fibers reliably demonstrate a substantial capacity to hypertrophy in the absence of myonuclear accretion, challenging the notion of a tightly regulated myonuclear domain (i.e., area that each myonucleus transcriptionally governs). In fact, a “myonuclear domain ceiling”, or upper limit of transcriptional output per nucleus to support hypertrophy, has yet to be identified. Satellite cells respond to muscle damage, and also play an important role in extracellular matrix remodeling during loading-induced hypertrophy. We postulate that robust satellite cell activation and proliferation in response to mechanical loading is largely for these purposes. Future work will aim to elucidate the mechanisms by which Type 2 fibers can hypertrophy without additional myonuclei, the extent to which Type 1 fibers can grow without myonuclear accretion, and whether a true myonuclear domain ceiling exists.

The roles of muscle stem cells in muscle injury, atrophy and hypertrophy

Skeletal muscle is composed of multinuclear cells called myofibers. Muscular dystrophy (a genetic muscle disorder) induces instability in the cell membrane of myofibers and eventually causes myofibre damage. Non-genetic muscle disorders, including sarcopenia, diabetes, bedridden immobility and cancer cachexia, lead to atrophy of myofibres. In contrast, resistance training induces myofibre hypertrophy. Thus, myofibres exhibit a plasticity that is strongly affected by both intrinsic and extrinsic factors. There is no doubt that muscle stem cells (MuSCs, also known as muscle satellite cells) are indispensable for muscle repair/regeneration, but their contributions to atrophy and hypertrophy are still controversial. The present review focuses on the relevance of MuSCs to (i) muscle diseases and (ii) hypertrophy. Further, this review addresses fundamental questions about MuSCs to clarify the onset or progression of these diseases and which might lead to development of a MuSC-based therapy.

Postexercise essential amino acid supplementation amplifies skeletal muscle satellite cell proliferation in older men 24 hours postexercise

Aged skeletal muscle has an attenuated and delayed ability to proliferate satellite cells in response to resistance exercise. The mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway is a focal point for cell growth, however, the effect of postexercise mTORC1 activation on human skeletal muscle satellite cell (SC) proliferation is unknown. To test the proliferative capacity of skeletal muscle SC in aging muscle to a potent mTORC1 activator (i.e., EAA; essential amino acids) we recruited older (~72y) men to conduct leg resistance exercise (8setsx10reps) without (−EAA; n = 8) and with (+EAA: n = 11) ingestion of 10 g of EAA 1 h postexercise. Muscle biopsies were taken before exercise (Pre) and 24 h postexercise (Post) for assessment of expression and fiber type‐specific Pax7⁺SC, Ki67⁺Pax7⁺SC and MyoD⁺SC. −EAA did not show an increase in Pax7⁺ satellite cells at Post(P > 0.82). Although statistical significance for an increase in Pax7 +  SC at 24 h post‐RE was not observed in +EAA versus −EAA, we observed trends for a treatment difference (P < 0.1). When examining the change from Pre to Post trends were demonstrated (#/myofiber: P = 0.076; and %/myonuclei: P = 0.065) for a greater increase in +EAA versus −EAA. Notably, we found an increase SC proliferation in +EAA, but not −EAA with increase in Ki67⁺SC and MyoD⁺ cells (P < 0.05). Ki67⁺SC also exhibited a significant group difference Post (P < 0.010). Pax7⁺SC in fast twitch myofibers did not change and were not different between groups (P > 0.10). CDK2, MEF2C, RB1 mRNA only increased in +EAA (P < 0.05). Acute muscle satellite cell proliferative capacity may be partially rescued with postexercise EAA ingestion in older men.

MicroRNAs, heart failure, and aging: potential interactions with skeletal muscle

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression by targeting mRNAs for degradation or translational repression. MiRNAs can be expressed tissue specifically and are altered in response to various physiological conditions. It has recently been shown that miRNAs are released into the circulation, potentially for the purpose of communicating with distant tissues. This manuscript discusses miRNA alterations in cardiac muscle and the circulation during heart failure, a prevalent and costly public health issue. A potential mechanism for how skeletal muscle maladaptations during heart failure could be mediated by myocardium-derived miRNAs released to the circulation is presented. An overview of miRNA alterations in skeletal muscle during the ubiquitous process of aging and perspectives on miRNA interactions during heart failure are also provided.

Potential Roles of n-3 PUFAs during Skeletal Muscle Growth and Regeneration

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), which are commonly found in fish oil supplements, are known to possess anti-inflammatory properties and more recently alter skeletal muscle function. In this review, we discuss novel findings related to how n-3 PUFAs modulate molecular signaling responsible for growth and hypertrophy as well as the activity of muscle stem cells. Muscle stem cells commonly known as satellite cells, are primarily responsible for driving the skeletal muscle repair process to potentially damaging stimuli, such as mechanical stress elicited by exercise contraction. To date, there is a paucity of human investigations related to the effects of n-3 PUFAs on satellite cell content and activity. Based on current in vitro investigations, this review focuses on novel mechanisms linking n-3 PUFA’s to satellite cell activity and how they may improve muscle repair. Understanding the role of n-3 PUFAs during muscle growth and regeneration in association with exercise could lead to the development of novel supplementation strategies that increase muscle mass and strength, therefore possibly reducing the burden of muscle wasting with age.

A novel in vitro model for the assessment of postnatal myonuclear accretion

BACKGROUND

Due to the post-mitotic nature of myonuclei, postnatal myogenesis is essential for skeletal muscle growth, repair, and regeneration. This process is facilitated by satellite cells through proliferation, differentiation, and subsequent fusion with a pre-existing muscle fiber (i.e., myonuclear accretion). Current knowledge of myogenesis is primarily based on the in vitro formation of syncytia from myoblasts, which represents aspects of developmental myogenesis, but may incompletely portray postnatal myogenesis. Therefore, we aimed to develop an in vitro model that better reflects postnatal myogenesis, to study the cell intrinsic and extrinsic processes and signaling involved in the regulation of postnatal myogenesis.

METHODS

Proliferating C2C12 myoblasts were trypsinized and co-cultured for 3 days with 5 days differentiated C2C12 myotubes. Postnatal myonuclear accretion was visually assessed by live cell time-lapse imaging and cell tracing by cell labeling with Vybrant® DiD and DiO. Furthermore, a Cre/LoxP-based cell system was developed to semi-quantitatively assess in vitro postnatal myonuclear accretion by the conditional expression of luciferase upon myoblast-myotube fusion. Luciferase activity was assessed luminometrically and corrected for total protein content.

RESULTS

Live cell time-lapse imaging, staining-based cell tracing, and recombination-dependent luciferase activity, showed the occurrence of postnatal myonuclear accretion in vitro. Treatment of co-cultures with the myogenic factor IGF-I (p < 0.001) and the cytokines IL-13 (p < 0.05) and IL-4 (p < 0.001) increased postnatal myonuclear accretion, while the myogenic inhibitors cytochalasin D (p < 0.001), myostatin (p < 0.05), and TNFα (p < 0.001) decreased postnatal myonuclear accretion. Furthermore, postnatal myonuclear accretion was increased upon recovery from electrical pulse stimulation-induced fiber damage (p < 0.001) and LY29004-induced atrophy (p < 0.001). Moreover, cell type-specific siRNA-mediated knockdown of myomaker in myoblasts (p < 0.001), but not in myotubes, decreased postnatal myonuclear accretion.

CONCLUSIONS

We developed a physiologically relevant, sensitive, high-throughput cell system for semi-quantitative assessment of in vitro postnatal myonuclear accretion, which can be used to mimic physiological myogenesis triggers, and can distinguish the cell type-specific roles of signals and responses in the regulation of postnatal myogenesis. As such, this method is suitable for both basal and translational research on the regulation of postnatal myogenesis, and will improve our understanding of muscle pathologies that result from impaired satellite cell number or function.

Alterations in the in vitro and in vivo regulation of muscle regeneration in healthy ageing and the influence of sarcopenia

BACKGROUND

Sarcopenia is defined as the age-related loss of skeletal muscle mass and function. While all humans lose muscle with age, 2-5% of elderly adults develop functional consequences (disabilities). The aim of this study was to investigate muscle myogenesis in healthy elderly adults, with or without sarcopenia, compared with middle-aged controls using both in vivo and in vitro approaches to explore potential biomarker or causative molecular pathways associated with sarcopenic versus non-sarcopenic skeletal muscle phenotypes during ageing.

METHODS

Biomarkers of multiple molecular pathways associated with muscle regeneration were analysed using quantitative polymerase chain reaction in quadriceps muscle samples obtained from healthy elderly sarcopenic (HSE, n = 7) or non-sarcopenic (HENS, n = 21) and healthy middle-aged control (HMC, n = 22) groups. An in vitro system of myogenesis (using myoblasts from human donors aged 17-83 years) was used to mimic the environmental challenges of muscle regeneration over time.

RESULTS

The muscle biopsies showed evidence of satellite cell activation in HENS (Pax3, P < 0.01, Pax7, P < 0.0001) compared with HMC. Early myogenesis markers Myogenic Differentiation 1 (MyoD1) and Myogenic factor 5 (Myf5) (P < 0.0001) and the late myogenesis marker myogenin (MyoG) (P < 0.01) were increased in HENS. In addition, there was a 30-fold upregulation of TNF-α in HENS compared with HMC (P < 0.0001). The in vitro system demonstrated age-related upregulation of pro-inflammatory cytokines (2-fold upregulation of interleukin (IL)-6, IL-8 mRNA, increased secretion of tumor necrosis factor-α (TNF-α) and IL-6, all P < 0.05) associated with impaired kinetics of myotube differentiation. The HSE biopsy samples showed satellite cell activation (Pax7, P < 0.05) compared with HMC. However, no significant upregulation of the early myogenesis (MyoD and Myf5) markers was evident; only the late myogenesis marker myogenin was upregulated (P < 0.05). Higher activation of the oxidative stress pathway was found in HENS compared with the HSE group. In contrast, there was 10-fold higher upregulation of HSPA1A a stress-induced chaperone acting upon misfolded proteins in HSE compared with the HENS group.

CONCLUSIONS

Both pathological and adaptive processes are active in skeletal muscle during healthy ageing. Muscle regeneration pathways are activated during healthy ageing, but there is evidence of dysregulation in sarcopenia. In addition, increased cellular stress, with an impaired oxidative-stress and mis-folded protein response (HSPA1A), may be associated with the development of sarcopenia. The in vitro system of young and old myoblasts replicated some of the differences between young and old muscle.

Enhanced skeletal muscle regrowth and remodelling in massaged and contralateral non-massaged hindlimb

KEY POINTS

Muscle fibre cross sectional area is enhanced with massage in the form of cyclic compressive loading during regrowth after atrophy. Massage enhances protein synthesis of the myofibrillar and cytosolic, but not the mitochondrial fraction, in muscle during regrowth. Focal adhesion kinase activation and satellite cell number are elevated in muscles undergoing massage during regrowth. Muscle fibre cross sectional area and protein synthesis of the myofibrillar fraction, but not DNA synthesis, are elevated in muscle of the contralateral non-massaged limb. Massage in the form of cyclic compressive loading is a potential anabolic intervention during muscle regrowth after atrophy.

ABSTRACT

Massage, in the form of cyclic compressive loading (CCL), is associated with multiple health benefits, but its potential anabolic effect on atrophied muscle has not been investigated. We hypothesized that the mechanical activity associated with CCL induces an anabolic effect in skeletal muscle undergoing regrowth after a period of atrophy. Fischer-Brown Norway rats at 10 months of age were hindlimb unloaded for a period of 2 weeks. The rats were then allowed reambulation with CCL applied at a 4.5 N load at 0.5 Hz frequency for 30 min every other day for four bouts during a regrowth period of 8 days. Muscle fibre cross sectional area was enhanced by 18% with massage during regrowth compared to reloading alone, and this was accompanied by elevated myofibrillar and cytosolic protein as well as DNA synthesis. Focal adhesion kinase phosphorylation indicated that CCL increased mechanical stimulation, while a higher number of Pax7+ cells likely explains the elevated DNA synthesis. Surprisingly, the contralateral non-massaged limb exhibited a comparable 17% higher muscle fibre size compared to reloading alone, and myofibrillar protein synthesis, but not DNA synthesis, was also elevated. We conclude that massage in the form of CCL induces an anabolic response in muscles regrowing after an atrophy-inducing event. We suggest that massage can be used as an intervention to aid in the regrowth of muscle lost during immobilization.

Starring or Supporting Role? Satellite Cells and Skeletal Muscle Fiber Size Regulation

Recent loss-of-function studies show that satellite cell depletion does not promote sarcopenia or unloading-induced atrophy, and does not prevent regrowth. Although overload-induced muscle fiber hypertrophy is normally associated with satellite cell-mediated myonuclear accretion, hypertrophic adaptation proceeds in the absence of satellite cells in fully grown adult mice, but not in young growing mice. Emerging evidence also indicates that satellite cells play an important role in remodeling the extracellular matrix during hypertrophy.

Castration induces satellite cell activation that contributes to skeletal muscle maintenance

BACKGROUND

Sarcopenia, the age-related loss of skeletal muscle, is a side effect of androgen deprivation therapy (ADT) for prostate cancer patients. Resident stem cells of skeletal muscle, satellite cells (SCs), are an essential source of progenitors for the growth and regeneration of skeletal muscle. Decreased androgen signaling and deficits in the number and function of SCs are features of aging. Although androgen signaling is known to regulate skeletal muscle, the cellular basis for ADT-induced exacerbation of sarcopenia is unknown. Furthermore, the consequences of androgen deprivation on SC fate in adult skeletal muscle remain largely unexplored.

METHODS

We examined SC fate in an androgen-deprived environment using immunofluorescence and fluorescence-activated cell sorting (FACS) with SC-specific markers in young castrated mice. To study the effects of androgen deprivation on SC function and skeletal muscle regenerative capacity, young castrated mice were subjected to experimental regenerative paradigms. SC-derived-cell contributions to skeletal muscle maintenance were examined in castrated Pax7CreER/+; ROSA26mTmG/+ mice. SCs were depleted in Pax7CreER/+; ROSA26DTA/+ mice to ascertain the consequences of SC ablation in sham and castrated skeletal muscles. Confocal immunofluorescence analysis of neuromuscular junctions (NMJs), and assessment of skeletal muscle physiology, contractile properties, and integrity were conducted.

RESULTS

Castration led to SC activation, however this did not result in a decline in SC function or skeletal muscle regenerative capacity. Surprisingly, castration induced SC-dependent maintenance of young skeletal muscle. The functional dependence of skeletal muscles on SCs in young castrated mice was demonstrated by an increase in SC-derived-cell fusion within skeletal muscle fibers. SC depletion was associated with further atrophy and functional decline, as well as the induction of partial innervation and the loss of NMJ-associated myonuclei in skeletal muscles from castrated mice.

CONCLUSION

The maintenance of skeletal muscles in young castrated mice relies on the cellular contributions of SCs. Considering the well-described age-related decline in SCs, the results in this study highlight the need to devise strategies that promote SC maintenance and activity to attenuate or reverse the progression of sarcopenia in elderly androgen-deprived individuals.

Protein Supplementation Does Not Affect Myogenic Adaptations to Resistance Training

It has been proposed that protein supplementation during resistance exercise training enhances muscle hypertrophy. The degree of hypertrophy during training is controlled in part through the activation of satellite cells and myonuclear accretion.

PURPOSE

This study aimed to determine the efficacy of protein supplementation (and the type of protein) during traditional resistance training on myofiber cross-sectional area, satellite cell content, and myonuclear addition.

METHODS

Healthy young men participated in supervised whole-body progressive resistance training 3 d·wk for 12 wk. Participants were randomized to one of three groups ingesting a daily 22-g macronutrient dose of soy-dairy protein blend (PB, n = 22), whey protein isolate (WP, n = 15), or an isocaloric maltodextrin placebo (MDP, n = 17). Lean mass, vastus lateralis myofiber-type-specific cross-sectional area, satellite cell content, and myonuclear addition were assessed before and after resistance training.

RESULTS

PB and the pooled protein treatments (PB + WP = PRO) exhibited a greater whole-body lean mass %change compared with MDP (P = 0.057 for PB) and (P = 0.050 for PRO), respectively. All treatments demonstrated similar leg muscle hypertrophy and vastus lateralis myofiber-type-specific cross-sectional area (P < 0.05). Increases in myosin heavy chain I and II myofiber satellite cell content and myonuclei content were also detected after exercise training (P < 0.05).

CONCLUSION

Protein supplementation during resistance training has a modest effect on whole-body lean mass as compared with exercise training without protein supplementation, and there was no effect on any outcome between protein supplement types (blend vs whey). However, protein supplementation did not enhance resistance exercise-induced increases in myofiber hypertrophy, satellite cell content, or myonuclear addition in young healthy men. We propose that as long as protein intake is adequate during muscle overload, the adaptations in muscle growth and function will not be influenced by protein supplementation.

Inducible satellite cell depletion attenuates skeletal muscle regrowth following a scald-burn injury

KEY POINTS

Severe burns result in significant skeletal muscle cachexia that impedes recovery. Activity of satellite cells, skeletal muscle stem cells, is altered following a burn injury and likely hinders regrowth of muscle. Severe burn injury induces satellite cell proliferation and fusion into myofibres with greater activity in muscles proximal to the injury site. Conditional depletion of satellite cells attenuates recovery of myofibre area and volume following a scald burn injury in mice. Skeletal muscle regrowth following a burn injury requires satellite cell activity, underscoring the therapeutic potential of satellite cells in the prevention of prolonged frailty in burn survivors.

ABSTRACT

Severe burns result in profound skeletal muscle atrophy; persistent muscle atrophy and weakness are major complications that hamper recovery from burn injury. Many factors contribute to the erosion of muscle mass following burn trauma, and we have previously shown concurrent activation and apoptosis of muscle satellite cells following a burn injury in paediatric patients. To determine the necessity of satellite cells during muscle recovery following a burn injury, we utilized a genetically modified mouse model (Pax7CreER -DTA) that allows for the conditional depletion of satellite cells in skeletal muscle. Additionally, mice were provided 5-ethynyl-2'-deoxyuridine to determine satellite cell proliferation, activation and fusion. Juvenile satellite cell-wild-type (SC-WT) and satellite cell-depleted (SC-Dep) mice (8 weeks of age) were randomized to sham or burn injury consisting of a dorsal scald burn injury covering 30% of total body surface area. Both hindlimb and dorsal muscles were studied at 7, 14 and 21 days post-burn. SC-Dep mice had >93% depletion of satellite cells compared to SC-WT (P < 0.05). Burn injury induced robust atrophy in muscles located both proximal and distal to the injury site (∼30% decrease in fibre cross-sectional area, P < 0.05). Additionally, burn injury induced skeletal muscle regeneration, satellite cell proliferation and fusion. Depletion of satellite cells impaired post-burn recovery of both muscle fibre cross-sectional area and volume (P < 0.05). These findings support an integral role for satellite cells in the aetiology of lean tissue recovery following a severe burn injury.

Contraction mode itself does not determine the level of mTORC1 activity in rat skeletal muscle

Resistance training with eccentric contraction has been shown to augment muscle hypertrophy more than other contraction modes do (i.e., concentric and isometric contraction). However, the molecular mechanisms involved remain unclear. The purpose of this study was to investigate the effect of muscle contraction mode on mammalian target of rapamycin complex 1 (mTORC1) signaling using a standardized force‐time integral (load (weight) × contraction time). Male Sprague–Dawley rats were randomly assigned to three groups: eccentric contraction, concentric contraction, and isometric contraction. The right gastrocnemius muscle was exercised via percutaneous electrical stimulation‐induced maximal contraction. In experiment 1, different modes of muscle contraction were exerted using the same number of reps in all groups, while in experiment 2, muscle contractions were exerted using a standardized force‐time integral. Muscle samples were obtained immediately and 3 h after exercise. Phosphorylation of molecules associated with mTORC1 activity was assessed using western blot analysis. In experiment 1, the force‐time integral was significantly different among contraction modes with a higher force‐time integral for eccentric contraction compared to that for other contraction modes (P < 0.05). In addition, the force‐time integral was higher for concentric contraction compared to that for isometric contraction (P < 0.05). Similarly, p70S6K phosphorylation level was higher for eccentric contraction than for other modes of contraction (P < 0.05), and concentric contraction was higher than isometric contraction (P < 0.05) 3 h after exercise. In experiment 2, under the same force‐time integral, p70S6K (Thr389) and 4E‐BP1 phosphorylation levels were similar among contraction modes 3 h after exercise. Our results suggest that mTORC1 activity is not determined by differences in muscle contraction mode itself. Instead, mTORC1 activity is determined by differences in the force‐time integral during muscle contraction.

Skeletal Muscle Remodeling: Interconnections Between Stem Cells and Protein Turnover

Nutrition and exercise are important components of a healthy lifestyle to improve rates of hypertrophic and nonhypertrophic skeletal muscle remodeling. We provide evidence to support the hypothesis that muscle stem cells and protein turnover are collaborative, not separate, mechanisms supporting muscle remodeling by facilitating protein, nuclear, and cellular turnover in response to the ingestion of protein dense foods and exercise.

The response of muscle progenitor cells to cutaneous thermal injury

BACKGROUND

Severe burn results in a systemic response that leads to significant muscle wasting. It is believed that this rapid loss in muscle mass occurs due to increased protein degradation combined with reduced protein synthesis. Alterations in the microenvironment of muscle progenitor cells may partially account for this pathology. The aim of this study was to ascertain the response of muscle progenitor cells following thermal injury in mice and to enlighten the cellular cascades that contribute to the muscle wasting.

METHODS

C57BL/6 mice received a 20% total body surface area (TBSA) thermal injury. Gastrocnemius muscle was harvested at days 2, 7, and 14 following injury for protein and histological analysis.

RESULTS

We observed a decrease in myofiber cross-sectional area at 2 days post-burn. This muscle atrophy was compensated for by an increase in myofiber cross-sectional area at 7 and 14 days post-burn. Myeloperoxidase (MPO)-positive cells (neutrophils) increased significantly at 2 days. Moreover, through Western blot analysis of two key mediators of the proteolytic pathway, we show there is an increase in Murf1 and NF-κB 2 days post-burn. MPO-positive cells were also positive for NF-κB, suggesting that neutrophils attain NF-κB activity in the muscle. Unlike inflammatory and proteolytic pathways, the number of Pax7-positive muscle progenitor cells decreased significantly 2 days post-burn. This was followed by a recovery in the number of Pax7-positive cells at 7 and 14 days, suggesting proliferation of muscle progenitors that accompanied regrowth.

CONCLUSION

Our data show a biphasic response in the muscles of mice exposed to burn injury, with phenotypic characteristics of muscle atrophy at 2 days while compensation was observed later with a change in Pax7-positive muscle progenitor cells. Targeting muscle progenitors may be of therapeutic benefit in muscle wasting observed after burn injury.

MyoVision: software for automated high-content analysis of skeletal muscle immunohistochemistry

Analysis of skeletal muscle cross sections is an important experimental technique in muscle biology. Many aspects of immunohistochemistry and fluorescence microscopy can now be automated, but most image quantification techniques still require extensive human input, slowing progress and introducing the possibility of user bias. MyoVision is a new software package that was developed to overcome these limitations. The software improves upon previously reported automatic techniques and analyzes images without requiring significant human input and correction. When compared with data derived by manual quantification, MyoVision achieves an accuracy of ≥94% for basic measurements such as fiber number, fiber type distribution, fiber cross-sectional area, and myonuclear number. Scientists can download the software free from www.MyoVision.org and use it to automate the analysis of their own experimental data. This will improve the efficiency and consistency of the analysis of muscle cross sections and help to reduce the burden of routine image quantification in muscle biology. NEW & NOTEWORTHY Scientists currently analyze images of immunofluorescently labeled skeletal muscle using time-consuming techniques that require sustained human supervision. As well as being inefficient, these techniques can increase variability in studies that quantify morphological adaptations of skeletal muscle at the cellular level. MyoVision is new software that overcomes these limitations by performing high-content analysis of muscle cross sections with minimal manual input. It is open source and freely available.

Satellite cell depletion prevents fiber hypertrophy in skeletal muscle

The largest mammalian cells are the muscle fibers, and they have multiple nuclei to support their large cytoplasmic volumes. During hypertrophic growth, new myonuclei are recruited from satellite stem cells into the fiber syncytia, but it was recently suggested that such recruitment is not obligatory: overload hypertrophy after synergist ablation of the plantaris muscle appeared normal in transgenic mice in which most of the satellite cells were abolished. When we essentially repeated these experiments analyzing the muscles by immunohistochemistry and in vivo and ex vivo imaging, we found that overload hypertrophy was prevented in the satellite cell-deficient mice, in both the plantaris and the extensor digitorum longus muscles. We attribute the previous findings to a reliance on muscle mass as a proxy for fiber hypertrophy, and to the inclusion of a significant number of regenerating fibers in the analysis. We discuss that there is currently no model in which functional, sustainable hypertrophy has been unequivocally demonstrated in the absence of satellite cells; an exception is re-growth, which can occur using previously recruited myonuclei without addition of new myonuclei.

ACL injury reduces satellite cell abundance and promotes fibrogenic cell expansion within skeletal muscle

Anterior cruciate ligament (ACL) injuries are associated with significant loss of strength in knee extensor muscles that persists despite physical therapy. The underlying mechanisms responsible for this protracted muscle weakness are poorly understood; however, we recently showed significant myofiber atrophy and altered muscle phenotype following ACL injury. We sought to further explore perturbations in skeletal muscle morphology and progenitor cell activity following an ACL injury. Muscle biopsies were obtained from the injured and non-injured vastus lateralis of young adults (n = 10) following ACL injury, and histochemical/immunohistochemical analyses were undertaken to determine collagen content, abundance of connective tissue fibroblasts, fibrogenic/adipogenic progenitor (FAP) cells, satellite cells, in addition to indices of muscle fiber denervation and myonuclear apoptosis. The injured limb showed elevated collagen content (p < 0.05), in addition to a greater abundance of fibroblasts and FAPs (p < 0.05) in the injured limb. Fibroblast content was correlated with increased accumulation of extracellular matrix in the injured limb as well. A higher frequency of interstitial nuclei were positive for phospho-SMAD3 in the injured limb (p < 0.05), providing some evidence for activation of a fibrogenic program through transforming growth factor β following an ACL injury. The injured limb also displayed reduced satellite cell abundance, increased fiber denervation and DNA damage associated with apoptosis (p < 0.05), indicating alterations within the muscle itself after the ligament injury. Injury of the ACL induces a myriad of negative outcomes within knee extensor muscles, which likely compromise the restorative capacity and plasticity of skeletal muscle, impeding rehabilitative efforts. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1876-1885, 2017.

Depletion of Pax7+ satellite cells does not affect diaphragm adaptations to running in young or aged mice

KEY POINTS

Satellite cell depletion does not affect diaphragm adaptations to voluntary wheel running in young or aged mice. Satellite cell depletion early in life (4 months of age) has minimal effect on diaphragm phenotype by old age (24 months). Prolonged satellite cell depletion in the diaphragm does not result in excessive extracellular matrix accumulation, in contrast to what has been reported in hind limb muscles. Up-regulation of Pax3 mRNA+ cells after satellite cell depletion in young and aged mice suggests that Pax3+ cells may compensate for a loss of Pax7+ satellite cells in the diaphragm. Future investigations should focus on the role of Pax3+ cells in the diaphragm during adaptation to exercise and ageing.

ABSTRACT

Satellite cell contribution to unstressed diaphragm is higher compared to hind limb muscles, which is probably attributable to constant activation of this muscle to drive ventilation. Whether satellite cell depletion negatively impacts diaphragm quantitative and qualitative characteristics under stressed conditions in young and aged mice is unknown. We therefore challenged the diaphragm with prolonged running activity in the presence and absence of Pax7+ satellite cells in young and aged mice using an inducible Pax7^CreER -R26R^DTA model. Mice were vehicle (Veh, satellite cell-replete) or tamoxifen (Tam, satellite cell-depleted) treated at 4 months of age and were then allowed to run voluntarily at 6 months (young) and 22 months (aged). Age-matched, cage-dwelling, Veh- and Tam-treated mice without wheel access served as activity controls. Diaphragm muscles were analysed from young (8 months) and aged (24 months) mice. Satellite cell depletion did not alter diaphragm mean fibre cross-sectional area, fibre type distribution or extracellular matrix content in young or aged mice, regardless of running activity. Resting in vivo diaphragm function was also unaffected by satellite cell depletion. Myonuclear density was maintained in young satellite cell-depleted mice regardless of running, although it was modestly reduced in aged sedentary (-7%) and running (-19%) mice without satellite cells (P < 0.05). Using fluorescence in situ hybridization, we detected higher Pax3 mRNA+ cell density in both young and aged satellite cell-depleted diaphragm muscle (P < 0.05), which may compensate for the loss of Pax7+ satellite cells.

Exosomal Small Talk Carries Strong Messages from Muscle Stem Cells

Exosome-dependent intercellular communication is an emerging signaling mechanism involved in tissue repair and regeneration. Now in this issue of Cell Stem Cell, Fry et al. (2016) show that muscle stem cells communicate with fibrogenic cells by exosomal trafficking of microRNA-206 to regulate extracellular matrix deposition and muscle tissue remodeling.

Impact of Inflammation and Anti-inflammatory Modalities on Skeletal Muscle Healing: From Fundamental Research to the Clinic

Anti-inflammatory modalities are commonly used for the treatment of various musculoskeletal injuries. Although inflammation was originally believed to interfere with skeletal muscle regeneration, several recent studies have highlighted the beneficial effects of inflammatory cells on muscle healing. This discrepancy is attributable to an evolving understanding of the complex inflammatory process. To better appreciate the paradoxical roles of inflammation, clinicians must have a better comprehension of the fundamental mechanisms regulating the inflammatory response. In this perspective article, cellular, animal, and human studies were analyzed to summarize recent knowledge regarding the impact of inflammation on muscle regeneration in acute or chronic conditions. The effect of anti-inflammatory drugs on the treatment of various muscle injuries was also considered. Overall, this work aims to summarize the current state of the literature on the inflammatory process associated with muscle healing in order to give clinicians the necessary tools to have a more efficient and evidence-based approach to the treatment of muscle injuries and disorders.

The effect of calorie restriction on mouse skeletal muscle is sex, strain and time-dependent

Loss of skeletal muscle mass and function occurs with increasing age. Calorie restriction (CR) increases the lifespan of C57Bl/6 mice, but not in the shorter-lived DBA/2 strain. There is some evidence that calorie restriction reduces or delays many of the age-related defects that occur in rodent skeletal muscle. We therefore investigated the effect of short (2.5 month) and longer term (8.5 and 18.5 months) CR on skeletal muscle in male and female C57Bl/6 and DBA/2 mice. We found that short-term CR increased the satellite cell number and collagen VI content of muscle, but resulted in a delayed regenerative response to injury.Consistent with this, the in vitro proliferation of satellite cells derived from these muscles was reduced by CR. The percentage of stromal cells, macrophages, hematopoietic stem cells and fibroadipogenic cells in the mononucleated cell population derived from skeletal muscle was reduced by CR at various stages. But overall, these changes are neither consistent over time, nor between strain and sex. The fact that changes induced by CR do not persist with time and the dissimilarities between the two mouse strains, combined with sex differences, urge caution in applying CR to improve skeletal muscle function across the lifespan in humans.

Differential requirement for satellite cells during overload-induced muscle hypertrophy in growing versus mature mice

Background

Pax7+ satellite cells are required for skeletal muscle fiber growth during post-natal development in mice. Satellite cell-mediated myonuclear accretion also appears to persist into early adulthood. Given the important role of satellite cells during muscle development, we hypothesized that the necessity of satellite cells for adaptation to an imposed hypertrophic stimulus depends on maturational age.

Methods

Pax7^CreER-R26R^DTA mice were treated for 5 days with vehicle (satellite cell-replete, SC+) or tamoxifen (satellite cell-depleted, SC-) at 2 months (young) and 4 months (mature) of age. Following a 2-week washout, mice were subjected to sham surgery or 10 day synergist ablation overload of the plantaris (n = 6–9 per group). The surgical approach minimized regeneration, de novo fiber formation, and fiber splitting while promoting muscle fiber growth. Satellite cell density (Pax7+ cells/fiber), embryonic myosin heavy chain expression (eMyHC), and muscle fiber cross sectional area (CSA) were evaluated via immunohistochemistry. Myonuclei (myonuclei/100 mm) were counted on isolated single muscle fibers.

Results

Tamoxifen treatment depleted satellite cells by ≥90% and prevented myonuclear accretion with overload in young and mature mice (p < 0.05). Satellite cells did not recover in SC- mice after overload. Average muscle fiber CSA increased ~20% in young SC+ (p = 0.07), mature SC+ (p < 0.05), and mature SC- mice (p < 0.05). In contrast, muscle fiber hypertrophy was prevented in young SC- mice. Muscle fiber number increased only in mature mice after overload (p < 0.05), and eMyHC expression was variable, specifically in mature SC+ mice.

Conclusions

Reliance on satellite cells for overload-induced hypertrophy is dependent on maturational age, and global responses to overload differ in young versus mature mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-017-0132-z) contains supplementary material, which is available to authorized users.

Laminin-111 enriched fibrin hydrogels for skeletal muscle regeneration

Laminin (LM)-111 supplementation has improved muscle regeneration in several models of disease and injury. This study investigated a novel hydrogel composed of fibrinogen and LM-111. Increasing LM-111 concentration (50-450 μg/mL) in fibrin hydrogels resulted in highly fibrous scaffolds with progressively thinner interlaced fibers. Rheological testing showed that all hydrogels had viscoelastic behavior and the Young's modulus ranged from 2-6KPa. C₂C₁₂ myobalsts showed a significant increase in VEGF production and decrease in IL-6 production on LM-111 enriched fibrin hydrogels as compared to pure fibrin hydrogels on day 4. Western blotting results showed a significant increase in MyoD and desmin protein quantity but a significant decrease in myogenin protein quantity in myoblasts cultured on the LM-111 (450 μg/mL) enriched fibrin hydrogel. Combined application of electromechanical stimulation significantly enhanced the production of VEGF and IGF-1 from myoblast seeded fibrin-LM-111 hydrogels. Taken together, these observations offer an important first step toward optimizing a tissue engineered constructs for skeletal muscle regeneration.

Low-Intensity Pulsed Ultrasound Promotes Exercise-Induced Muscle Hypertrophy

The purpose of this study was to investigate whether low-intensity pulsed ultrasound (LIPUS) promotes exercise-induced muscle hypertrophy. Twenty-four adult Sprague-Dawley (SD) rats were randomly assigned to three groups (n = 8 per group): normal control group (NC), treadmill exercise group (TE) and treadmill exercise + LIPUS group (TE + LIPUS). The TE + LIPUS group received a LIPUS treatment (1 MHz, 30 mW/cm²) at the gastrocnemius for 20 min/d after treadmill exercise. The TE group was sham-treated. Eight weeks of treadmill training successfully established the exercise-induced muscle hypertrophy model. Muscle strength, muscle mass and muscle fiber cross-sectional area were significantly increased in the TE + LIPUS group compared with the TE group. Moreover, LIPUS treatment significantly upregulated the expression of Akt, mTOR, p-Akt and p-mTOR and significantly downregulated the expression of MSTN, ActRIIB, FoxO1 and its phosphorylation. The results indicated that LIPUS promotes exercise-induced muscle hypertrophy by facilitating protein synthesis and inhibiting the protein catabolism pathway.

Upregulation of circulating myomiR following short-term energy restriction is inversely associated with whole body protein synthesis

The objective of the present investigation was to determine whether energy restriction (ER) influences expression of skeletal muscle-specific microRNA (miRNA) in circulation (c-myomiR) and whether changes in c-myomiR are associated with rates of whole body protein synthesis. Sixteen older (64 ± 2 yr) overweight (28.5 ± 1.2 kg/m²) men enrolled in this 35-day controlled feeding trial. A 7-day weight maintenance (WM) period was followed by 28 days of 30% ER. Whole body protein turnover was determined from [¹⁵N]glycine enrichments in 24-h urine collections, and c-myomiR (miR-1-3p, miR-133a-3p, miR-133b, and miR-206) expression was assessed from serum samples by RT-quantitative PCR upon completion of the WM and ER periods. Participants lost 4.4 ± 0.3 kg body mass during ER (P < 0.05). After 28 days of ER, miR-133a and miR-133b expression was upregulated (P < 0.05) compared with WM. When all four c-myomiR were grouped as c-myomiR score (sum of the median fold change of all myomiR), overall expression of c-myomiR was higher (P < 0.05) at ER than WM. Backward linear regression analysis of whole body protein synthesis and breakdown and carbohydrate, fat, and protein oxidation determined protein synthesis to be the strongest predictor of c-myomiR score. An inverse association (P < 0.05) was observed with ER c-myomiR score and whole body protein synthesis (r = -0.729, r² = -0.530). Findings from the present investigation provide evidence that upregulation of c-myomiR expression profiles in response to short-term ER is associated with lower rates of whole body protein synthesis.

Human skeletal muscle fibroblasts stimulate in vitro myogenesis and in vivo muscle regeneration

KEY POINTS

Accumulation of skeletal muscle extracellular matrix is an unfavourable characteristic of many muscle diseases, muscle injury and sarcopenia. The extent of cross-talk between fibroblasts, as the source of matrix protein, and satellite cells in humans is unknown. We studied this in human muscle biopsies and cell-culture studies. We observed a strong stimulation of myogenesis by human fibroblasts in cell culture. In biopsies collected 30 days after a muscle injury protocol, fibroblast number increased to four times control levels, where fibroblasts were found to be preferentially located immediately surrounding regenerating muscle fibres. These novel findings indicate an important role for fibroblasts in supporting the regeneration of muscle fibres, potentially through direct stimulation of satellite cell differentiation and fusion, and contribute to understanding of cell-cell cross-talk during physiological and pathological muscle remodelling.

ABSTRACT

Accumulation of skeletal muscle extracellular matrix is an unfavourable characteristic of many muscle diseases, muscle injury and sarcopenia. In addition to the indispensable role satellite cells play in muscle regeneration, there is emerging evidence in rodents for a regulatory influence on fibroblast activity. However, the influence of fibroblasts on satellite cells and muscle regeneration in humans is unknown. The purpose of this study was to investigate this in vitro and during in vivo regeneration in humans. Following a muscle injury protocol in young healthy men (n = 7), the number of fibroblasts (TCF7L2+), satellite cells (Pax7+), differentiating myogenic cells (myogenin+) and regenerating fibres (neonatal/embryonic myosin+) was determined from biopsy cross-sections. Fibroblasts and myogenic precursor cells (MPCs) were also isolated from human skeletal muscle (n = 4) and co-cultured using different cell ratios, with the two cell populations either in direct contact with each other or separated by a permeable membrane. MPC proliferation, differentiation and fusion were assessed from cells stained for BrdU, desmin and myogenin. On biopsy cross-sections, fibroblast number was seen to increase, along with myogenic cell number, by d7 and increase further by d30, where fibroblasts were observed to be preferentially located immediately surrounding regenerating muscle fibres. In vitro, the presence of fibroblasts in direct contact with MPCs was found to moderately stimulate MPC proliferation and strongly stimulate both MPC differentiation and MPC fusion. It thus appears, in humans, that fibroblasts exert a strong positive regulatory influence on MPC activity, in line with observations during in vivo skeletal muscle regeneration.

Role of muscle stem cells in sarcopenia

PURPOSE OF REVIEW

The review is an update on recent research investigating the role of skeletal muscle stem cells (also known as satellite cells) during muscle fiber regeneration and growth in animal and human skeletal muscle, with an emphasis on their role in age-related sarcopenia.

RECENT FINDINGS

Studies indicate clear impairments in satellite cell function with aging, resulting in an impaired muscle fiber regenerative response. The autophagy-mediated switch to an irreversible presenescent state of geriatric satellite cells appears to play a key role in age-related impaired satellite cell function. In addition, inadequate muscle fiber vascularization may be a crucial factor underlying impaired regulation of satellite cells in older adults. Controversy remains on the actual contribution of satellite cells to the development of sarcopenia in later life, this clearly requires further research. Nevertheless, exercise training remains to be a potent intervention strategy, mediated through satellite cells or not, to counteract the ill effects of sarcopenia.

SUMMARY

Although important strides are made investigating the importance of satellite cells in the development and/or treatment of sarcopenia, the idea that satellite cell function is a therapeutic target to treat sarcopenia remains controversial.

Requirement of myomaker-mediated stem cell fusion for skeletal muscle hypertrophy

Fusion of skeletal muscle stem/progenitor cells is required for proper development and regeneration, however the significance of this process during adult muscle hypertrophy has not been explored. In response to muscle overload after synergist ablation in mice, we show that myomaker, a muscle specific membrane protein essential for myoblast fusion, is activated mainly in muscle progenitors and not myofibers. We rendered muscle progenitors fusion-incompetent through genetic deletion of myomaker in muscle stem cells and observed a complete reduction of overload-induced hypertrophy. This blunted hypertrophic response was associated with a reduction in Akt and p70s6k signaling and protein synthesis, suggesting a link between myonuclear accretion and activation of pro-hypertrophic pathways. Furthermore, fusion-incompetent muscle exhibited increased fibrosis after muscle overload, indicating a protective role for normal stem cell activity in reducing myofiber strain associated with hypertrophy. These findings reveal an essential contribution of myomaker-mediated stem cell fusion during physiological adult muscle hypertrophy.

Myogenic Satellite Cells: Biological Milieu and Possible Clinical Applications

Adult skeletal muscle is a post-mitotic terminally differentiated tissue that possesses an immense potential for regeneration after injury. This regeneration can be achieved by adult stem cells named satellite cells that inhabit the muscular tissue. These cells were first identified in 1961 and were described as being wedged between the plasma membrane of the muscle fiber and the surrounding basement membrane. Since their discovery, many researchers investigated their embryological origin and the exact role they play in muscle regeneration and repair. Under normal conditions, satellite cells are retained in a quiescent state and when required, these cells are activated to proliferate and differentiate to repair pre-existing muscle fibers or to a lesser extent fuse with each other to form new myofibers. During skeletal muscle regeneration, satellite cell actions are regulated through a cascade of complex signaling pathways that are influenced by multiple extrinsic factors within the satellite cell micro-environment. Here, the basic concepts were studied about satellite cells, their development, function, distribution and the different cellular and molecular mechanisms that regulate these cells. The recent findings about some of their clinical applications and potential therapeutic use were also discussed.