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

Skeletal Muscle Injury in Chronic Kidney Disease-From Histologic Changes to Molecular Mechanisms and to Novel Therapies

Chronic kidney disease (CKD) is associated with significant reductions in lean body mass and in the mass of various tissues, including skeletal muscle, which causes fatigue and contributes to high mortality rates. In CKD, the cellular protein turnover is imbalanced, with protein degradation outweighing protein synthesis, leading to a loss of protein and cell mass, which impairs tissue function. As CKD itself, skeletal muscle wasting, or sarcopenia, can have various origins and causes, and both CKD and sarcopenia share common risk factors, such as diabetes, obesity, and age. While these pathologies together with reduced physical performance and malnutrition contribute to muscle loss, they cannot explain all features of CKD-associated sarcopenia. Metabolic acidosis, systemic inflammation, insulin resistance and the accumulation of uremic toxins have been identified as additional factors that occur in CKD and that can contribute to sarcopenia. Here, we discuss the elevation of systemic phosphate levels, also called hyperphosphatemia, and the imbalance in the endocrine regulators of phosphate metabolism as another CKD-associated pathology that can directly and indirectly harm skeletal muscle tissue. To identify causes, affected cell types, and the mechanisms of sarcopenia and thereby novel targets for therapeutic interventions, it is important to first characterize the precise pathologic changes on molecular, cellular, and histologic levels, and to do so in CKD patients as well as in animal models of CKD, which we describe here in detail. We also discuss the currently known pathomechanisms and therapeutic approaches of CKD-associated sarcopenia, as well as the effects of hyperphosphatemia and the novel drug targets it could provide to protect skeletal muscle in CKD.

The effects of resistance training on denervated myofibers, senescent cells, and associated protein markers in middle-aged adults

Denervated myofibers and senescent cells are hallmarks of skeletal muscle aging. However, sparse research has examined how resistance training affects these outcomes. We investigated the effects of unilateral leg extensor resistance training (2 days/week for 8 weeks) on denervated myofibers, senescent cells, and associated protein markers in apparently healthy middle-aged participants (MA, 55 ± 8 years old, 17 females, 9 males). We obtained dual-leg vastus lateralis (VL) muscle cross-sectional area (mCSA), VL biopsies, and strength assessments before and after training. Fiber cross-sectional area (fCSA), satellite cells (Pax7+), denervated myofibers (NCAM+), senescent cells (p16+ or p21+), proteins associated with denervation and senescence, and senescence-associated secretory phenotype (SASP) proteins were analyzed from biopsy specimens. Leg extensor peak torque increased after training (p < .001), while VL mCSA trended upward (interaction p = .082). No significant changes were observed for Type I/II fCSAs, NCAM+ myofibers, or senescent (p16+ or p21+) cells, albeit satellite cells increased after training (p = .037). While >90% satellite cells were not p16+ or p21+, most p16+ and p21+ cells were Pax7+ (>90% on average). Training altered 13 out of 46 proteins related to muscle-nerve communication (all upregulated, p < .05) and 10 out of 19 proteins related to cellular senescence (9 upregulated, p < .05). Only 1 out of 17 SASP protein increased with training (IGFBP-3, p = .031). In conclusion, resistance training upregulates proteins associated with muscle-nerve communication in MA participants but does not alter NCAM+ myofibers. Moreover, while training increased senescence-related proteins, this coincided with an increase in satellite cells but not alterations in senescent cell content or SASP proteins. These latter findings suggest shorter term resistance training is an unlikely inducer of cellular senescence in apparently healthy middle-aged participants. However, similar study designs are needed in older and diseased populations before definitive conclusions can be drawn.

Coordinated Regulation of Myonuclear DNA Methylation, mRNA, and miRNA Levels Associates With the Metabolic Response to Rapid Synergist Ablation-Induced Skeletal Muscle Hypertrophy in Female Mice

The central dogma of molecular biology dictates the general flow of molecular information from DNA that leads to a functional cellular outcome. In skeletal muscle fibers, the extent to which global myonuclear transcriptional alterations, accounting for epigenetic and post-transcriptional influences, contribute to an adaptive stress response is not clearly defined. In this investigation, we leveraged an integrated analysis of the myonucleus-specific DNA methylome and transcriptome, as well as myonuclear small RNA profiling to molecularly define the early phase of skeletal muscle fiber hypertrophy. The analysis of myonucleus-specific mature microRNA and other small RNA species provides new directions for exploring muscle adaptation and complemented the methylation and transcriptional information. Our integrated multi-omics interrogation revealed a coordinated myonuclear molecular landscape during muscle loading that coincides with an acute and rapid reduction of oxidative metabolism. This response may favor a biosynthesis-oriented metabolic program that supports rapid hypertrophic growth.

Genome-wide epigenetic dynamics during postnatal skeletal muscle growth in Hu sheep

Hypertrophy and fiber transformation are two prominent features of postnatal skeletal muscle development. However, the role of epigenetic modifications is less understood. ATAC-seq, whole genome bisulfite sequencing, and RNA-seq were applied to investigate the epigenetic dynamics of muscle in Hu sheep at 3 days, 3 months, 6 months, and 12 months after birth. All 6865 differentially expressed genes were assigned into three distinct tendencies, highlighting the balanced protein synthesis, accumulated immune activities, and restrained cell division in postnatal development. We identified 3742 differentially accessible regions and 11799 differentially methylated regions that were associated with muscle-development-related pathways in certain stages, like D3-M6. Transcription factor network analysis, based on genomic loci with high chromatin accessibility and low methylation, showed that ARID5B, MYOG, and ENO1 were associated with muscle hypertrophy, while NR1D1, FADS1, ZFP36L2, and SLC25A1 were associated with muscle fiber transformation. Taken together, these results suggest that DNA methylation and chromatin accessibility contributed toward regulating the growth and fiber transformation of postnatal skeletal muscle in Hu sheep.

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

EV-mediated promotion of myogenic differentiation is dependent on dose, collection medium, and isolation method

Extracellular vesicles (EVs) have been implicated in the regulation of myogenic differentiation. C2C12 murine myoblast differentiation was reduced following treatment with GW4869 or heparin (to inhibit exosome biogenesis and EV uptake, respectively). Conversely, treatment with C2C12 myotube-conditioned medium enhanced myogenic differentiation. Ultrafiltration-size exclusion liquid chromatography (UF-SEC) was used to isolate EVs and non-EV extracellular protein in parallel from C2C12 myoblast- and myotube-conditioned medium. UF-SEC-purified EVs promoted myogenic differentiation at low doses (≤2 × 108 particles/mL) and were inhibitory at the highest dose tested (2 × 1011 particles/mL). Conversely, extracellular protein fractions had no effect on myogenic differentiation. While the transfer of muscle-enriched miRNAs (myomiRs) has been proposed to mediate the pro-myogenic effects of EVs, we observed that they are scarce in EVs (e.g., 1 copy of miR-133a-3p per 195 EVs). Furthermore, we observed pro-myogenic effects with undifferentiated myoblast-derived EVs, in which myomiR concentrations are even lower, suggestive of a myomiR-independent mechanism underlying the observed pro-myogenic effects. During these investigations we identified technical factors with profound confounding effects on myogenic differentiation. Specifically, co-purification of insulin (a component of Opti-MEM) in non-EV LC fractions and polymer precipitated EV preparations. These findings provide further evidence that polymer-based precipitation techniques should be avoided in EV research.

The Potential Modulatory Effects of Exercise on Skeletal Muscle Redox Status in Chronic Kidney Disease

Chronic Kidney Disease (CKD) is a global health burden with high mortality and health costs. CKD patients exhibit lower cardiorespiratory and muscular fitness, strongly associated with morbidity/mortality, which is exacerbated when they reach the need for renal replacement therapies (RRT). Muscle wasting in CKD has been associated with an inflammatory/oxidative status affecting the resident cells' microenvironment, decreasing repair capacity and leading to atrophy. Exercise may help counteracting such effects; however, the molecular mechanisms remain uncertain. Thus, trying to pinpoint and understand these mechanisms is of particular interest. This review will start with a general background about myogenesis, followed by an overview of the impact of redox imbalance as a mechanism of muscle wasting in CKD, with focus on the modulatory effect of exercise on the skeletal muscle microenvironment.

Molecular predictors of resistance training outcomes in young untrained female adults

We sought to determine if the myofibrillar protein synthetic (MyoPS) response to a naïve resistance exercise (RE) bout, or chronic changes in satellite cell number and muscle ribosome content, were associated with hypertrophic outcomes in females or differed in those who classified as higher (HR) or lower (LR) responders to resistance training (RT). Thirty-four untrained college-aged females (23.4 ± 3.4 kg/m2) completed a 10-wk RT protocol (twice weekly). Body composition and leg imaging assessments, a right leg vastus lateralis biopsy, and strength testing occurred before and following the intervention. A composite score, which included changes in whole body lean/soft tissue mass (LSTM), vastus lateralis (VL) muscle cross-sectional area (mCSA), midthigh mCSA, and deadlift strength, was used to delineate upper and lower HR (n = 8) and LR (n = 8) quartiles. In all participants, training significantly (P < 0.05) increased LSTM, VL mCSA, midthigh mCSA, deadlift strength, mean muscle fiber cross-sectional area, satellite cell abundance, and myonuclear number. Increases in LSTM (P < 0.001), VL mCSA (P < 0.001), midthigh mCSA (P < 0.001), and deadlift strength (P = 0.001) were greater in HR vs. LR. The first-bout 24-hour MyoPS response was similar between HR and LR (P = 0.367). While no significant responder × time interaction existed for muscle total RNA concentrations (i.e., ribosome content) (P = 0.888), satellite cell abundance increased in HR (P = 0.026) but not LR (P = 0.628). Pretraining LSTM (P = 0.010), VL mCSA (P = 0.028), and midthigh mCSA (P < 0.001) were also greater in HR vs. LR. Female participants with an enhanced satellite cell response to RT, and more muscle mass before RT, exhibited favorable resistance training adaptations.NEW & NOTEWORTHY This study continues to delineate muscle biology differences between lower and higher responders to resistance training and is unique in that a female population was interrogated. As has been reported in prior studies, increases in satellite cell numbers are related to positive responses to resistance training. Satellite cell responsivity, rather than changes in muscle ribosome content per milligrams of tissue, may be a more important factor in delineating resistance-training responses in women.

Resident muscle stem cell myogenic characteristics in postnatal muscle growth impairments in children with cerebral palsy

Children with cerebral palsy (CP), a perinatal brain alteration, have impaired postnatal muscle growth, with some muscles developing contractures. Functionally, children are either able to walk or primarily use wheelchairs. Satellite cells are muscle stem cells (MuSCs) required for postnatal development and source of myonuclei. Only MuSC abundance has been previously reported in contractured muscles, with myogenic characteristics assessed only in vitro. We investigated whether MuSC myogenic, myonuclear, and myofiber characteristics in situ differ between contractured and noncontractured muscles, across functional levels, and compared with typically developing (TD) children with musculoskeletal injury. Open muscle biopsies were obtained from 36 children (30 CP, 6 TD) during surgery; contracture correction for adductors or gastrocnemius, or from vastus lateralis [bony surgery in CP, anterior cruciate ligament (ACL) repair in TD]. Muscle cross sections were immunohistochemically labeled for MuSC abundance, activation, proliferation, nuclei, myofiber borders, type-1 fibers, and collagen content in serial sections. Although MuSC abundance was greater in contractured muscles, primarily in type-1 fibers, their myogenic characteristics (activation, proliferation) were lower compared with noncontractured muscles. Overall, MuSC abundance, activation, and proliferation appear to be associated with collagen content. Myonuclear number was similar between all muscles, but only in contractured muscles were there associations between myonuclear number, MuSC abundance, and fiber cross-sectional area. Puzzlingly, MuSC characteristics were similar between ambulatory and nonambulatory children. Noncontractured muscles in children with CP had a lower MuSC abundance compared with TD-ACL injured children, but similar myogenic characteristics. Contractured muscles may have an intrinsic deficiency in developmental progression for postnatal MuSC pool establishment, needed for lifelong efficient growth and repair.

Extracellular matrix content and remodeling markers do not differ in college-aged men classified as higher and lower responders to resistance training

We determined if skeletal muscle extracellular matrix (ECM) content and remodeling markers adapted with resistance training or were associated with hypertrophic outcomes. Thirty-eight untrained males (21 ± 3 yr) participated in whole body resistance training (10 wk, 2 × weekly). Participants completed testing [ultrasound, peripheral quantitative computed tomography (pQCT)] and donated a vastus lateralis (VL) biopsy 1 wk before training and 72 h following the last training bout. Higher responders (HR, n = 10) and lower responders (LR, n = 10) were stratified based on a composite score considering changes in pQCT-derived mid-thigh cross-sectional area (mCSA), ultrasound-derived VL thickness, and mean fiber cross-sectional area (fCSA). In all participants, training reduced matrix metalloprotease (MMP)-14 protein (P < 0.001) and increased satellite cell abundance (P < 0.001); however, VL fascial thickness, ECM protein content per myofiber, MMP-2/-9 protein content, tissue inhibitor of metalloproteinase (TIMP)-1/-2 protein content, collagen-1/-4 protein content, macrophage abundance, or fibroadipogenic progenitor cell abundance were not altered. Regarding responder analysis, MMP-14 exhibited an interaction (P = 0.007), and post hoc analysis revealed higher protein content in HR versus LR before training (P = 0.026) and a significant decrease from pre to posttraining in HR only (P = 0.002). In summary, basal skeletal muscle ECM markers are minimally affected with 10 wk of resistance training, and these findings could be related to not capturing more dynamic alterations in the assayed markers earlier in training. However, the downregulation in MMP-14 in college-aged men classified as HR is a novel finding and warrants continued investigation, and further research is needed to delineate muscle connective tissue strength attributes between HR and LR.NEW & NOTEWORTHY Although past studies have examined aspects of extracellular matrix remodeling in relation to mechanical overload or resistance training, this study serves to expand our knowledge on a multitude of extracellular matrix markers and whether these markers adapt to resistance training or are associated with differential hypertrophic responses.

Effects of transdermal estrogen therapy on satellite cell number and molecular markers for muscle hypertrophy in response to resistance training in early postmenopausal women

PURPOSE

To investigate the effects of resistance training with or without transdermal estrogen therapy (ET) on satellite cell (SC) number and molecular markers for muscle hypertrophy in early postmenopausal women.

METHODS

Using a double-blinded randomized controlled design, we allocated healthy, untrained postmenopausal women to perform 12 weeks of resistance training with placebo (PLC, n = 16) or ET (n = 15). Muscle biopsies obtained before and after the intervention, and two hours after the last training session were analyzed for fiber type, SC number and molecular markers for muscle hypertrophy and degradation (real-time PCR, western blotting).

RESULTS

The analysis of SCs per Type I fiber showed a time x treatment interaction caused by a 47% decrease in PLC, and a 26% increase after ET after the training period. Also, SCs per Type II fiber area was lower after the intervention driven by a 57% decrease in PLC. Most molecular markers changed similarly in the two groups.

CONCLUSION

A decline in SC per muscle fiber was observed after the 12-week training period in postmenopausal women, which was counteracted when combined with use of transdermal ET.

CLINICAL TRIAL REGISTRATION NUMBER

nct03020953.

Cyclic Tensile Stress Induces Skeletal Muscle Hypertrophy and Myonuclear Accretion in a 3D Model

Skeletal muscle is highly adaptive to mechanical stress due to its resident stem cells and the pronounced level of myotube plasticity. Herein, we study the adaptation to mechanical stress and its underlying molecular mechanisms in a tissue-engineered skeletal muscle model. We subjected differentiated 3D skeletal muscle-like constructs to cyclic tensile stress using a custom-made bioreactor system, which resulted in immediate activation of stress-related signal transducers (Erk1/2, p38). Cell cycle re-entry, increased proliferation, and onset of myogenesis indicated subsequent myoblast activation. Furthermore, elevated focal adhesion kinase and β-catenin activity in mechanically stressed constructs suggested increased cell adhesion and migration. After 3 days of mechanical stress, gene expression of the fusogenic markers MyoMaker and MyoMixer, myotube diameter, myonuclear accretion, as well as S6 activation, were significantly increased. Our results highlight that we established a promising tool to study sustained adaptation to mechanical stress in healthy, hypertrophic, or regenerating skeletal muscle.

The myonuclear domain in adult skeletal muscle fibres: past, present and future

Most cells in the body are mononuclear whereas skeletal muscle fibres are uniquely multinuclear. The nuclei of muscle fibres (myonuclei) are usually situated peripherally which complicates the equitable distribution of gene products. Myonuclear abundance can also change under conditions such as hypertrophy and atrophy. Specialised zones in muscle fibres have different functions and thus distinct synthetic demands from myonuclei. The complex structure and regulatory requirements of multinuclear muscle cells understandably led to the hypothesis that myonuclei govern defined 'domains' to maintain homeostasis and facilitate adaptation. The purpose of this review is to provide historical context for the myonuclear domain and evaluate its veracity with respect to mRNA and protein distribution resulting from myonuclear transcription. We synthesise insights from past and current in vitro and in vivo genetically modified models for studying the myonuclear domain under dynamic conditions. We also cover the most contemporary knowledge on mRNA and protein transport in muscle cells. Insights from emerging technologies such as single myonuclear RNA-sequencing further inform our discussion of the myonuclear domain. We broadly conclude: (1) the myonuclear domain can be flexible during muscle fibre growth and atrophy, (2) the mechanisms and role of myonuclear loss and motility deserve further consideration, (3) mRNA in muscle is actively transported via microtubules and locally restricted, but proteins may travel far from a myonucleus of origin and (4) myonuclear transcriptional specialisation extends beyond the classic neuromuscular and myotendinous populations. A deeper understanding of the myonuclear domain in muscle may promote effective therapies for ageing and disease.

Decrease in the expression of muscle-specific miRNAs, miR-133a and miR-1, in myoblasts with replicative senescence

Muscles that are injured or atrophied by aging undergo myogenic regeneration. Although myoblasts play a pivotal role in myogenic regeneration, their function is impaired with aging. MicroRNAs (miRNAs) are also involved in myogenic regeneration. MiRNA (miR)-1 and miR-133a are muscle-specific miRNAs that control the proliferation and differentiation of myoblasts. In this study, we determined whether miR-1 and miR-133a expression in myoblasts is altered with cellular senescence and involved in senescence-impaired myogenic differentiation. C2C12 murine skeletal myoblasts were converted to a replicative senescent state by culturing to a high passage number. Although miR-1 and miR-133a expression was largely induced during myogenic differentiation, expression was suppressed in cells at high passage numbers (passage 10 and/or passage 20). Although the senescent myoblasts exhibited a deterioration of myogenic differentiation, transfection of miR-1 or miR-133a into myoblasts ameliorated cell fusion. Treatment with the glutaminase 1 inhibitor, BPTES, removed senescent cells from C2C12 myoblasts with a high passage number, whereas myotube formation and miR-133a expression was increased. In addition, primary cultured myoblasts prepared from aged C57BL/6J male mice (20 months old) exhibited a decrease in miR-1 and miR-133a levels compared with younger mice (3 months old). The results suggest that replicative senescence suppresses muscle-specific miRNA expression in myoblasts, which contributes to the senescence-related dysfunction of myogenic regeneration.

Cutting edge concepts: Does bilirubin enhance exercise performance?

Exercise performance is dependent on many factors, such as muscular strength and endurance, cardiovascular capacity, liver health, and metabolic flexibility. Recent studies show that plasma levels of bilirubin, which has classically been viewed as a liver dysfunction biomarker, are elevated by exercise training and that elite athletes may have significantly higher levels. Other studies have shown higher plasma bilirubin levels in athletes and active individuals compared to general, sedentary populations. The reason for these adaptions is unclear, but it could be related to bilirubin's antioxidant properties in response to a large number of reactive oxygen species (ROS) that originates from mitochondria during exercise. However, the mechanisms of these are unknown. Current research has re-defined bilirubin as a metabolic hormone that interacts with nuclear receptors to drive gene transcription, which reduces body weight. Bilirubin has been shown to reduce adiposity and improve the cardiovascular system, which might be related to the adaption of bilirubin increasing during exercise. No studies have directly tested if elevating bilirubin levels can influence athletic performance. However, based on the mechanisms proposed in the present review, this seems plausible and an area to consider for future studies. Here, we discuss the importance of bilirubin and exercise and how the combination might improve metabolic health outcomes and possibly athletic performance.

Murine Models of Tenotomy-Induced Mechanical Overloading and Tail-Suspension-Induced Mechanical Unloading

Skeletal muscle is a highly plastic tissue that can alter its mass and strength in response to mechanical stimulation, such as overloading and unloading, which lead to muscle hypertrophy and atrophy, respectively. Mechanical loading in the muscle influences muscle stem cell dynamics, including activation, proliferation, and differentiation. Although experimental models of mechanical overloading and unloading have been widely used for the investigation of the molecular mechanisms regulating muscle plasticity and stem cell function, few studies have described the methods in detail. Here, we describe the appropriate procedures for tenotomy-induced mechanical overloading and tail-suspension-induced mechanical unloading, which are the most common and simple methods to induce muscle hypertrophy and atrophy in mouse models.

Skeletal Muscle Nuclei in Mice are not Post-mitotic

The skeletal muscle research field generally accepts that nuclei in skeletal muscle fibers (ie, myonuclei) are post-mitotic and unable to proliferate. Because our deuterium oxide (D2O) labeling studies showed DNA synthesis in skeletal muscle tissue, we hypothesized that resident myonuclei can replicate in vivo. To test this hypothesis, we used a mouse model that temporally labeled myonuclei with GFP followed by D2O labeling during normal cage activity, functional overload, and with satellite cell ablation. During normal cage activity, we observed deuterium enrichment into myonuclear DNA in 7 out of 7 plantaris (PLA), 6 out of 6 tibialis anterior (TA), 5 out of 7 gastrocnemius (GAST), and 7 out of 7 quadriceps (QUAD). The average fractional synthesis rates (FSR) of DNA in myonuclei were: 0.0202 ± 0.0093 in PLA, 0.0239 ± 0.0040 in TA, 0.0076 ± 0. 0058 in GAST, and 0.0138 ± 0.0039 in QUAD, while there was no replication in myonuclei from EDL. These FSR values were largely reproduced in the overload and satellite cell ablation conditions, although there were higher synthesis rates in the overloaded PLA muscle. We further provided evidence that myonuclear replication is through endoreplication, which results in polyploidy. These novel findings contradict the dogma that skeletal muscle nuclei are post-mitotic and open potential avenues to harness the intrinsic replicative ability of myonuclei for muscle maintenance and growth.

Multi-transcriptome analysis following an acute skeletal muscle growth stimulus yields tools for discerning global and MYC regulatory networks

Myc is a powerful transcription factor implicated in epigenetic reprogramming, cellular plasticity, and rapid growth as well as tumorigenesis. Cancer in skeletal muscle is extremely rare despite marked and sustained Myc induction during loading-induced hypertrophy. Here, we investigated global, actively transcribed, stable, and myonucleus-specific transcriptomes following an acute hypertrophic stimulus in mouse plantaris. With these datasets, we define global and Myc-specific dynamics at the onset of mechanical overload-induced muscle fiber growth. Data collation across analyses reveals an under-appreciated role for the muscle fiber in extracellular matrix remodeling during adaptation, along with the contribution of mRNA stability to epigenetic-related transcript levels in muscle. We also identify Runx1 and Ankrd1 (Marp1) as abundant myonucleus-enriched loading-induced genes. We observed that a strong induction of cell cycle regulators including Myc occurs with mechanical overload in myonuclei. Additionally, in vivo Myc-controlled gene expression in the plantaris was defined using a genetic muscle fiber-specific doxycycline-inducible Myc-overexpression model. We determined Myc is implicated in numerous aspects of gene expression during early-phase muscle fiber growth. Specifically, brief induction of Myc protein in muscle represses Reverbα, Reverbβ, and Myh2 while increasing Rpl3, recapitulating gene expression in myonuclei during acute overload. Experimental, comparative, and in silico analyses place Myc at the center of a stable and actively transcribed, loading-responsive, muscle fiber-localized regulatory hub. Collectively, our experiments are a roadmap for understanding global and Myc-mediated transcriptional networks that regulate rapid remodeling in postmitotic cells. We provide open webtools for exploring the five RNA-seq datasets as a resource to the field.

Satellite cell depletion does not affect diaphragm adaptations to hypoxia

The diaphragm is the main skeletal muscle responsible for inspiration and is susceptible to age-associated decline in function and morphology. Satellite cells in diaphragm fuse into unperturbed muscle fibers throughout life, yet their role in adaptations to hypoxia in diaphragm is unknown. Given their continual fusion, we hypothesize that satellite cell depletion will negatively impact adaptations to hypoxia in the diaphragm, particularly with aging. We used the Pax7CreER/CreER:R26RDTA/DTA genetic mouse model of inducible satellite cell depletion to investigate diaphragm responses to hypoxia in adult (6 mo) and aged (22 mo) male mice. The mice were subjected to normobaric hypoxia at 10% [Formula: see text] or normoxia for 4 wk. We showed that satellite cell depletion had no effect on diaphragm muscle fiber cross-sectional area, fiber-type distribution, myonuclear density, or regulation of extracellular matrix in either adult or aged mice. Furthermore, we showed lower muscle fiber cross-sectional area with hypoxia and age (main effects), while extracellular matrix content was higher and satellite cell abundance was lower with age (main effect) in diaphragm. Lastly, a greater number of Pax3-mRNA+ cells was observed in diaphragm muscle of satellite cell-depleted mice independent of hypoxia (main effect), potentially as a compensatory mechanism for the loss of satellite cells. We conclude that satellite cells are not required for diaphragm muscle adaptations to hypoxia in either adult or aged mice.NEW & NOTEWORTHY Satellite cells show consistent fusion into diaphragm muscle fibers throughout life, suggesting a critical role in maintaining homeostasis. Here, we report identical diaphragm adaptations to hypoxia with and without satellite cells in adult and aged mice. In addition, we propose that the higher number of Pax3-positive cells in satellite cell-depleted diaphragm muscle acts as a compensatory mechanism.

A glitch in the matrix: the pivotal role for extracellular matrix remodeling during muscle hypertrophy

Multinuclear muscle fibers are the most voluminous cells in skeletal muscle and the primary drivers of growth in response to loading. Outside the muscle fiber, however, is a diversity of mononuclear cell types that reside in the extracellular matrix (ECM). These muscle-resident cells are exercise-responsive and produce the scaffolding for successful myofibrillar growth. Without proper remodeling and maintenance of this ECM scaffolding, the ability to mount an appropriate response to resistance training in adult muscles is severely hindered. Complex cellular choreography takes place in muscles following a loading stimulus. These interactions have been recently revealed by single-cell explorations into muscle adaptation with loading. The intricate ballet of ECM remodeling involves collagen production from fibrogenic cells and ECM modifying signals initiated by satellite cells, immune cells, and the muscle fibers themselves. The acellular collagen-rich ECM is also a mechanical signal-transducer and rich repository of growth factors that may directly influence muscle fiber hypertrophy once liberated. Collectively, high levels of collagen expression, deposition, and turnover characterize a well-trained muscle phenotype. The purpose of this review is to highlight the most recent evidence for how the ECM and its cellular components affect loading-induced muscle hypertrophy. We also address how the muscle fiber may directly take part in ECM remodeling, and whether ECM dynamics are rate limiting for muscle fiber growth.

Contribution of muscle satellite cells to sarcopenia

Sarcopenia, a disorder characterized by age-related muscle loss and reduced muscle strength, is associated with decreased individual independence and quality of life, as well as a high risk of death. Skeletal muscle houses a normally mitotically quiescent population of adult stem cells called muscle satellite cells (MuSCs) that are responsible for muscle maintenance, growth, repair, and regeneration throughout the life cycle. Patients with sarcopenia are often exhibit dysregulation of MuSCs homeostasis. In this review, we focus on the etiology, assessment, and treatment of sarcopenia. We also discuss phenotypic and regulatory mechanisms of MuSC quiescence, activation, and aging states, as well as the controversy between MuSC depletion and sarcopenia. Finally, we give a multi-dimensional treatment strategy for sarcopenia based on improving MuSC function.

Myonuclear addition is associated with sex-specific fiber hypertrophy and occurs in relation to fiber perimeter not cross-sectional area

It is unclear whether resistance training-induced myofiber hypertrophy is affected by sex, and whether myonuclear addition occurs in relation to the myonuclear domain and can contribute to explaining a potential sex-specific hypertrophic response. This study investigated the effect of 8 weeks of resistance training on myofiber hypertrophy and myonuclear addition in 12 males (28±7 years; mean±SD) and 12 females (27±7 years). Muscle biopsies were collected from m. vastus lateralis before and after the training intervention and analyzed by immunohistochemistry for fiber type and size, satellite cells, and myonuclei. Hypertrophy of type I fibers was greater in males than females (P<0.05), whereas hypertrophy of type II fibers was similar between sexes (P=0.158‒0.419). Expansion of the satellite cell pool (P=0.132‒0.667) and myonuclear addition (P=0.064‒0.228) did not differ significantly between sexes, irrespective of myofiber type. However, when individual responses to resistance training were assessed, myonuclear addition was strongly correlated with fiber hypertrophy (r=0.68‒0.85, P<0.001). While myofiber hypertrophy was accompanied by an increase in myonuclear domain (P<0.05), fiber perimeter per myonucleus remained constant throughout the study (P=0.096‒0.666). These findings indicate that myonuclear addition occurs in relation to the fiber perimeter per myonucleus, not the myonuclear domain, and has a substantial role in muscle hypertrophy, but does not fully explain greater hypertrophy of type I fibers in males than females.

Engineered meatballs via scalable skeletal muscle cell expansion and modular micro-tissue assembly using porous gelatin micro-carriers

The emerging field of cultured meat faces several technical hurdles, including the scale-up production of quality muscle and adipose progenitor cells, and the differentiation and bioengineering of these cellular materials into large, meat-like tissue. Here, we present edible, 3D porous gelatin micro-carriers (PoGelat-MCs), as efficient cell expansion scaffolds, as well as modular tissue-engineering building blocks for lab-grown meat. PoGelat-MC culture in spinner flasks, not only facilitated the scalable expansion of porcine skeletal muscle satellite cells and murine myoblasts, but also triggered their spontaneous myogenesis, in the absence of myogenic reagents. Using 3D-printed mold and transglutaminase, we bio-assembled pork muscle micro-tissues into centimeter-scale meatballs, which exhibited similar mechanical property and higher protein content compared to conventional ground pork meatballs. PoGelat-MCs also supported the expansion and differentiation of 3T3L1 murine pre-adipocytes into mature adipose micro-tissues, which could be used as modular assembly unit for engineered fat-containing meat products. Together, our results highlight PoGelat-MCs, in combination with dynamic bioreactors, as a scalable culture system to produce large quantity of highly-viable muscle and fat micro-tissues, which could be further bio-assembled into ground meat analogues.

Differences in muscle satellite cell dynamics during muscle hypertrophy and regeneration

Skeletal muscle homeostasis and function are ensured by orchestrated cellular interactions among several types of cells. A noticeable aspect of skeletal muscle biology is the drastic cell–cell communication changes that occur in multiple scenarios. The process of recovering from an injury, which is known as regeneration, has been relatively well investigated. However, the cellular interplay that occurs in response to mechanical loading, such as during resistance training, is poorly understood compared to regeneration. During muscle regeneration, muscle satellite cells (MuSCs) rebuild multinuclear myofibers through a stepwise process of proliferation, differentiation, fusion, and maturation, whereas during mechanical loading-dependent muscle hypertrophy, MuSCs do not undergo such stepwise processes (except in rare injuries) because the nuclei of MuSCs become directly incorporated into the mature myonuclei. In this review, six specific examples of such differences in MuSC dynamics between regeneration and hypertrophy processes are discussed.

Primary cilia in satellite cells are the mechanical sensors for muscle hypertrophy

Skeletal muscle atrophy is commonly associated with aging, immobilization, muscle unloading, and congenital myopathies. Generation of mature muscle cells from skeletal muscle satellite cells (SCs) is pivotal in repairing muscle tissue. Exercise therapy promotes muscle hypertrophy and strength. Primary cilium is implicated as the mechanical sensor in some mammalian cells, but its role in skeletal muscle cells remains vague. To determine mechanical sensors for exercise-induced muscle hypertrophy, we established three SC-specific cilium dysfunctional mouse models-Myogenic factor 5 (Myf5)-Arf-like Protein 3 (Arl3)^-/-, Paired box protein Pax-7 (Pax7)-Intraflagellar transport protein 88 homolog (Ift88)^-/-, and Pax7-Arl3^-/--by specifically deleting a ciliary protein ARL3 in MYF5-expressing SCs, or IFT88 in PAX7-expressing SCs, or ARL3 in PAX7-expressing SCs, respectively. We show that the Myf5-Arl3^-/- mice develop grossly the same as WT mice. Intriguingly, mechanical stimulation-induced muscle hypertrophy or myoblast differentiation is abrogated in Myf5-Arl3^-/- and Pax7-Arl3^-/- mice or primary isolated Myf5-Arl3^-/- and Pax7-Ift88^-/- myoblasts, likely due to defective cilia-mediated Hedgehog (Hh) signaling. Collectively, we demonstrate SC cilia serve as mechanical sensors and promote exercise-induced muscle hypertrophy via Hh signaling pathway.

Muscle-Specific Cellular and Molecular Adaptations to Late-Life Voluntary Concurrent Exercise

Murine exercise models can provide information on factors that influence muscle adaptability with aging, but few translatable solutions exist. Progressive weighted wheel running (PoWeR) is a simple, voluntary, low-cost, high-volume endurance/resistance exercise approach for training young mice. In the current investigation, aged mice (22-mo-old) underwent a modified version of PoWeR for 8 wk. Muscle functional, cellular, biochemical, transcriptional, and myonuclear DNA methylation analyses provide an encompassing picture of how muscle from aged mice responds to high-volume combined training. Mice run 6-8 km/d, and relative to sedentary mice, PoWeR increases plantarflexor muscle strength. The oxidative soleus of aged mice responds to PoWeR similarly to young mice in every parameter measured in previous work; this includes muscle mass, glycolytic-to-oxidative fiber type transitioning, fiber size, satellite cell frequency, and myonuclear number. The oxidative/glycolytic plantaris adapts according to fiber type, but with modest overall changes in muscle mass. Capillarity increases markedly with PoWeR in both muscles, which may be permissive for adaptability in advanced age. Comparison to published PoWeR RNA-sequencing data in young mice identified conserved regulators of adaptability across age and muscles; this includes Aldh1l1 which associates with muscle vasculature. Agrn and Samd1 gene expression is upregulated after PoWeR simultaneous with a hypomethylated promoter CpG in myonuclear DNA, which could have implications for innervation and capillarization. A promoter CpG in Rbm10 is hypomethylated by late-life exercise in myonuclei, consistent with findings in muscle tissue. PoWeR and the data herein are a resource for uncovering cellular and molecular regulators of muscle adaptation with aging.

Skeletal muscle properties show collagen organization and immune cell content are associated with resistance exercise response heterogeneity in older persons

In older individuals, hypertrophy from progressive resistance training (PRT) is compromised in approximately one- third of participants in exercise trials. The objective of this study was to establish novel relationships between baseline muscle features and/or their PRT-induced change in vastus lateralis muscle biopsies with hypertrophy outcomes. Multiple linear regression analyses adjusted for sex were performed on phenotypic data from older adults (n=48, 70.8±4.5 years) completing 14 weeks of PRT. Results show that baseline muscle size associates with growth regardless of hypertrophy outcome measure (fiber cross-sectional area (fCSA), β=-0.76, Adj. p<0.01; thigh muscle area by CT, β=-0.75, Adj. p<0.01; DXA thigh lean mass, β=-0.47, Adj. p<0.05). Furthermore, loosely packed collagen organization (β=-0.44, Adj. p<0.05) and abundance of CD11b+/CD206- immune cells (β=-0.36, Adj. p=0.10) were negatively associated with whole muscle hypertrophy, with a significant sex interaction on the latter. Additionally, a composite hypertrophy score generated using all three measures reinforces significant fiber level findings that changes in myonuclei (β=0.67, Adj. p<0.01), changes in immune cells (β=0.48, Adj. p<0.05; both CD11b+/CD206+ and CD11b+/CD206- cells), and capillary density (β=0.56, Adj. p<0.01) are significantly associated with growth. Exploratory single cell RNA-sequencing of CD11b+ cells in muscle in response to resistance exercise showed that macrophages have a mixed phenotype. Collagen associations with macrophages may be an important aspect in muscle response heterogeneity. Detailed histological phenotyping of muscle combined with multiple measures of growth response to resistance training in older persons identify potential new mechanisms underlying response heterogeneity and possible sex differences.

Sex Hormones and Satellite Cell Regulation in Women

Recent years have seen growing scholarly interest in female physiology in general. Moreover, particular attention has been devoted to how concentrations of female sex hormones vary during the menstrual cycle and menopausal transition and how hormonal contraception and hormonal therapy influence skeletal muscle tissue. While much effort has been paid to macro outcomes, such as muscle function or mass, rather less attention has been paid to mechanistic work that may help explain the underlying mechanism through which sex hormones regulate skeletal muscle tissue. Evidence from animal studies shows a strong relationship between the female sex hormone estrogen and satellite cells (SCs), a population of muscle stem cells involved in skeletal muscle regulation. A few human studies investigating this relationship have been published only recently. Thus, the purpose of this study was to bring an updated review on female sex hormones and their role in SC regulation. First, we describe how SCs regulate skeletal muscle maintenance and repair and introduce sex hormone signaling within the muscle. Second, we present evidence from animal studies elucidating how estrogen deficiency and supplementation influence SCs. Third, we present results from investigations from human trials including women whose concentrations of female hormones differ due to menopause, hormone therapy, hormonal contraceptives, and the menstrual cycle. Finally, we discuss research and methodological recommendations for future studies aiming at elucidating the link between female sex hormones and SCs with respect to aging and training.

The functional and molecular effects of problematic alcohol consumption on skeletal muscle: a focus on athletic performance

Background: Chronic alcohol misuse is associated with alcoholic myopathy, characterized by skeletal muscle weakness and atrophy. Moreover, there is evidence that sports-related people seem to exhibit a greater prevalence of problematic alcohol consumption, especially binge drinking (BD), which might not cause alcoholic myopathy but can negatively impact muscle function and amateur and professional athletic performance.Objective: To review the literature concerning the effects of alcohol consumption on skeletal muscle function and structure that can affect muscle performance.Methodology: We examined the currently available literature (PubMed, Google Scholars) to develop a narrative review summarizing the knowledge about the effects of alcohol on skeletal muscle function and exercise performance, obtained from studies in human beings and animal models for problematic alcohol consumption.Results: Exercise- and sport-based studies indicate that alcohol consumption can negatively affect muscle recovery after vigorous exercise, especially in men, while women seem less affected. Clinical studies and pre-clinical laboratory research have led to the knowledge of some of the mechanisms involved in alcohol-related muscle dysfunction, including an imbalance between anabolic and catabolic pathways, reduced regeneration, increased inflammation and fibrosis, and deficiencies in energetic balance and mitochondrial function. These pathological features can appear not only under chronic alcohol misuse but also in other alcohol consumption patterns.Conclusions: Most laboratory-based studies use chronic or acute alcohol exposure, while episodic BD, the most common drinking pattern in amateur and professional athletes, is underrepresented. Nevertheless, alcohol consumption negatively affects skeletal muscle health through different mechanisms, which collectively might contribute to reduced sports performance.

Cellular and molecular pathways controlling muscle size in response to exercise

From the discovery of ATP and motor proteins to synaptic neurotransmitters and growth factor control of cell differentiation, skeletal muscle has provided an extreme model system in which to understand aspects of tissue function. Muscle is one of the few tissues that can undergo both increase and decrease in size during everyday life. Muscle size depends on its contractile activity, but the precise cellular and molecular pathway(s) by which the activity stimulus influences muscle size and strength remain unclear. Four correlates of muscle contraction could, in theory, regulate muscle growth: nerve-derived signals, cytoplasmic calcium dynamics, the rate of ATP consumption and physical force. Here, we summarise the evidence for and against each stimulus and what is known or remains unclear concerning their molecular signal transduction pathways and cellular effects. Skeletal muscle can grow in three ways, by generation of new syncytial fibres, addition of nuclei from muscle stem cells to existing fibres or increase in cytoplasmic volume/nucleus. Evidence suggests the latter two processes contribute to exercise-induced growth. Fibre growth requires increase in sarcolemmal surface area and cytoplasmic volume at different rates. It has long been known that high-force exercise is a particularly effective growth stimulus, but how this stimulus is sensed and drives coordinated growth that is appropriately scaled across organelles remains a mystery.

Preserved stem cell content and innervation profile of elderly human skeletal muscle with lifelong recreational exercise

Abstract

Muscle fibre denervation and declining numbers of muscle stem (satellite) cells are defining characteristics of ageing skeletal muscle. The aim of this study was to investigate the potential for lifelong recreational exercise to offset muscle fibre denervation and compromised satellite cell content and function, both at rest and under challenged conditions. Sixteen elderly lifelong recreational exercisers (LLEX) were studied alongside groups of age‐matched sedentary (SED) and young subjects. Lean body mass and maximal voluntary contraction were assessed, and a strength training bout was performed. From muscle biopsies, tissue and primary myogenic cell cultures were analysed by immunofluorescence and RT‐qPCR to assess myofibre denervation and satellite cell quantity and function. LLEX demonstrated superior muscle function under challenged conditions. When compared with SED, the muscle of LLEX was found to contain a greater content of satellite cells associated with type II myofibres specifically, along with higher mRNA levels of the beta and gamma acetylcholine receptors (AChR). No difference was observed between LLEX and SED for the proportion of denervated fibres or satellite cell function, as assessed in vitro by myogenic cell differentiation and fusion index assays. When compared with inactive counterparts, the skeletal muscle of lifelong exercisers is characterised by greater fatigue resistance under challenged conditions in vivo, together with a more youthful tissue satellite cell and AChR profile. Our data suggest a little recreational level exercise goes a long way in protecting against the emergence of classic phenotypic traits associated with the aged muscle.

Key points

The detrimental effects of ageing can be partially offset by lifelong self‐organized recreational exercise, as evidence by preserved type II myofibre‐associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions.

Satellite cell function (in vitro), muscle fibre size and muscle fibre denervation determined by immunofluorescence were not affected by recreational exercise.

Individuals that are recreationally active are far more abundant than master athletes, which sharply increases the translational perspective of the present study. Future studies should further investigate recreational activity in relation to muscle health, while also including female participants.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Implication of satellite cell behaviors in capillary growth via VEGF expression-independent mechanism in response to mechanical loading in HeyL-null mice

Angiogenesis and muscle satellite cell (SC)-mediated myonuclear accretion are considered essential for the robust response of contraction-induced muscle hypertrophy. Moreover, both myonucleus and SCs are physically adjacent to capillaries and are the major sites for the expression of proangiogenic factors, such as VEGF, in the skeletal muscle. Thus, events involving the addition of new myonuclei via activation of SCs may play an important role in angiogenesis during muscle hypertrophy. However, the relevance among myonuclei number, capillary supply, and angiogenesis factor is not demonstrated. The Notch effector HeyL is specifically expressed in SCs in the skeletal muscle and is crucial for SC proliferation by inhibiting MyoD in overload-induced muscle hypertrophy. Here, we tested whether the addition of new myonuclei by SC in overloaded muscle is associated with angiogenic adaptation by reanalyzing skeletal muscle from HeyL-knockout (KO) mice, which show blunted responses of SC proliferation, myonucleus addition, and overload-induced muscle hypertrophy. Reanalysis confirmed blunted SC proliferation and myonuclear accretion in the plantaris muscle of HeyL-KO mice 9 wk after synergist ablation. Interestingly, the increase in capillary-to-fiber ratio observed in wild-type (WT) mice was impaired in HeyL-KO mice. In both WT and HeyL-KO mice, the expression of VEGFA and VEGFB was similarly increased in response to overload. In addition, the expression pattern of TSP-1, a negative regulator of angiogenesis, was also not changed between WT and HeyL-KO mice. Collectively, these results suggest that SCs activation-myonuclear accretion plays a crucial role in angiogenesis during overload-induced muscle hypertrophy via independent of angiogenesis regulators.

Epigenetic evidence for distinct contributions of resident and acquired myonuclei during long-term exercise adaptation using timed in vivo myonuclear labeling

Muscle fibers are syncytial postmitotic cells that can acquire exogenous nuclei from resident muscle stem cells, called satellite cells. Myonuclei are added to muscle fibers by satellite cells during conditions such as load-induced hypertrophy. It is difficult to dissect the molecular contributions of resident versus satellite cell-derived myonuclei during adaptation due to the complexity of labeling distinct nuclear populations in multinuclear cells without label transference between nuclei. To sidestep this barrier, we used a genetic mouse model where myonuclear DNA can be specifically and stably labeled via nonconstitutive H2B-GFP at any point in the lifespan. Resident myonuclei (Mn) were GFP-tagged in vivo before 8 wk of progressive weighted wheel running (PoWeR) in adult mice (>4-mo-old). Resident + satellite cell-derived myonuclei (Mn+SC Mn) were labeled at the end of PoWeR in a separate cohort. Following myonuclear isolation, promoter DNA methylation profiles acquired with low-input reduced representation bisulfite sequencing (RRBS) were compared to deduce epigenetic contributions of satellite cell-derived myonuclei during adaptation. Resident myonuclear DNA has hypomethylated promoters in genes related to protein turnover, whereas the addition of satellite cell-derived myonuclei shifts myonuclear methylation profiles to favor transcription factor regulation and cell-cell signaling. By comparing myonucleus-specific methylation profiling to previously published single-nucleus transcriptional analysis in the absence (Mn) versus the presence of satellite cells (Mn+SC Mn) with PoWeR, we provide evidence that satellite cell-derived myonuclei may preferentially supply specific ribosomal proteins to growing myofibers and retain an epigenetic "memory" of prior stem cell identity. These data offer insights on distinct epigenetic myonuclear characteristics and contributions during adult muscle growth.

Evidence of myomiR regulation of the pentose phosphate pathway during mechanical load-induced hypertrophy

Many of the molecular and cellular mechanisms discovered to regulate skeletal muscle hypertrophy were first identified using the rodent synergist ablation model. This model reveals the intrinsic capability and necessary pathways of skeletal muscle growth in response to mechanical overload (MOV). Reminiscent of the rapid cellular growth observed with cancer, we hypothesized that in response to MOV, skeletal muscle would undergo metabolic programming to sustain increased demands to support hypertrophy. To test this hypothesis, we analyzed the gene expression of specific metabolic pathways taken from transcriptomic microarray data of a MOV time course. We found an upregulation of genes involved in the oxidative branch of the pentose phosphate pathways (PPP) and mitochondrial branch of the folate cycle suggesting an increase in the production of NADPH. In addition, we sought to determine the potential role of skeletal muscle-enriched microRNA (myomiRs) and satellite cells in the regulation of the metabolic pathways that changed during MOV. We observed an inverse pattern in gene expression between muscle-enriched myomiR-1 and its known target gene glucose-6-phosphate dehydrogenase, G6pdx, suggesting myomiR regulation of PPP activation in response to MOV. Satellite cell fusion had a significant but modest impact on PPP gene expression. These transcriptomic findings suggest the robust muscle hypertrophy induced by MOV requires enhanced redox metabolism via PPP production of NADPH which is potentially regulated by a myomiR network.

Muscle fibrosis and maladaptation occur progressively in CKD and are rescued by dialysis

BACKGROUND

Skeletal muscle maladaptation accompanies chronic kidney disease (CKD) and negatively impacts physical function. Emphasis in CKD has historically been placed on muscle fiber intrinsic deficits, such as altered protein metabolism and atrophy. However, targeted treatment of fiber intrinsic dysfunction has produced limited improvement, whereas alterations within the fiber extrinsic environment have scarcely been examined.

METHODS

We investigated alterations to the skeletal muscle interstitial environment with deep cellular phenotyping of biopsies from patients with CKD compared to age-matched control participants and performed transcriptome profiling to define the molecular underpinnings of CKD-associated muscle impairments. We further examined changes in the observed muscle maladaptation following initiation of dialysis therapy for kidney failure.

RESULTS

Patients with CKD exhibited a progressive fibrotic muscle phenotype, which was associated with impaired regenerative capacity and lower vascular density. The severity of these deficits was strongly associated with the degree of kidney dysfunction. Consistent with these profound deficits, CKD was associated with broad alterations to the muscle transcriptome, including altered extracellular matrix organization, downregulated angiogenesis, and altered expression of pathways related to stem cell self-renewal. Remarkably, despite the seemingly advanced nature of this fibrotic transformation, dialysis treatment rescued these deficits, restoring a healthier muscle phenotype. Furthermore, after accounting for muscle atrophy, strength and endurance improved after dialysis initiation.

CONCLUSION

These data identify a dialysis-responsive muscle fibrotic phenotype in CKD and suggest that the early dialysis window presents a unique opportunity of improved muscle regenerative capacity during which targeted interventions may achieve maximal impact.

TRIAL REGISTRATION

NCT01452412FUNDING. NIH.

Regulation of muscle hypertrophy: Involvement of the Akt-independent pathway and satellite cells in muscle hypertrophy

Skeletal muscles are composed of multinuclear cells called myofibers and have unique abilities, one of which is plasticity. In response to the mechanical load induced by physical activity, skeletal muscle exerts several local adaptations, including an increase in myofiber size and myonuclear number, known as muscle hypertrophy. Protein synthesis and muscle satellite cells (MuSCs) are mainly responsible for these adaptations. However, the upstream signaling pathways that promote protein synthesis remain controversial. Further, the necessity of MuSCs in muscle hypertrophy is also a highly debated issue. In this review, we summarized the insulin-like growth factor 1 (IGF-1)/Akt-independent activation of mammalian target of rapamycin (mTOR) signaling in muscle hypertrophy and the involvement of mTOR signaling in age-related loss of skeletal muscle function and mass and in sarcopenia. The roles and behaviors of MuSCs, characteristics of new myonuclei in muscle hypertrophy, and their relevance to sarcopenia have also been updated in this review.

Fusion and beyond: Satellite cell contributions to loading-induced skeletal muscle adaptation

Satellite cells support adult skeletal muscle fiber adaptations to loading in numerous ways. The fusion of satellite cells, driven by cell-autonomous and/or extrinsic factors, contributes new myonuclei to muscle fibers, associates with load-induced hypertrophy, and may support focal membrane damage repair and long-term myonuclear transcriptional output. Recent studies have also revealed that satellite cells communicate within their niche to mediate muscle remodeling in response to resistance exercise, regulating the activity of numerous cell types through various mechanisms such as secretory signaling and cell-cell contact. Muscular adaptation to resistance and endurance activity can be initiated and sustained for a period of time in the absence of satellite cells, but satellite cell participation is ultimately required to achieve full adaptive potential, be it growth, function, or proprioceptive coordination. While significant progress has been made in understanding the roles of satellite cells in adult muscle over the last few decades, many conclusions have been extrapolated from regeneration studies. This review highlights our current understanding of satellite cell behavior and contributions to adaptation outside of regeneration in adult muscle, as well as the roles of satellite cells beyond fusion and myonuclear accretion, which are gaining broader recognition.

Importance of Nutrient Availability and Metabolism for Skeletal Muscle Regeneration

Skeletal muscle is fundamentally important for quality of life. Deterioration of skeletal muscle, such as that observed with advancing age, chronic disease, and dystrophies, is associated with metabolic and functional decline. Muscle stem/progenitor cells promote the maintenance of skeletal muscle composition (balance of muscle mass, fat, and fibrotic tissues) and are essential for the regenerative response to skeletal muscle damage. It is increasing recognized that nutrient and metabolic determinants of stem/progenitor cell function exist and are potential therapeutic targets to improve regenerative outcomes and muscle health. This review will focus on current understanding as well as key gaps in knowledge and challenges around identifying and understanding nutrient and metabolic determinants of skeletal muscle regeneration.

Satellite cell and myonuclear accretion is related to training-induced skeletal muscle fiber hypertrophy in young males and females

Satellite cells (SC) play an integral role in the recovery from skeletal muscle damage and supporting muscle hypertrophy. Acute resistance exercise typically elevates type I and type II SC content 24-96 h post exercise in healthy young males, although comparable research in females is lacking. We aimed to elucidate whether sex-based differences exist in fiber type-specific SC content after resistance exercise in the untrained (UT) and trained (T) states. Ten young males (23.0 ± 4.0 yr) and females (23.0 ± 4.8 yr) completed an acute bout of resistance exercise before and after 8 wk of whole body resistance training. Muscle biopsies were taken from the vastus lateralis immediately before and 24 and 48 h after each bout to determine SC and myonuclear content by immunohistochemistry. Males had greater SC associated with type II fibers (P ≤ 0.03). There was no effect of acute resistance exercise on SC content in either fiber type (P ≥ 0.58) for either sex; however, training increased SC in type II fibers (P < 0.01) irrespective of sex. The change in mean 0-48 h type II SC was positively correlated with muscle fiber hypertrophy in type II fibers (r = 0.47; P = 0.035). Furthermore, the change in myonuclei per fiber was positively correlated with type I and type II fiber hypertrophy (both r = 0.68; P < 0.01). Our results suggest that SC responses to acute and chronic resistance exercise are similar in males and females and that SC and myonuclear accretion is related to training-induced muscle fiber hypertrophy.NEW & NOTEWORTHY We demonstrate that training-induced increase in SC content in type II fibers and myonuclear content in type I and II fibers is similar between males and females. Furthermore, these changes are related to the extent of muscle fiber hypertrophy. Thus, SC and myonuclear accretion appear to contribute to muscle hypertrophy irrespective of sex, highlighting the importance of these muscle stem cells in human skeletal muscle growth.

Myonuclear transcriptional dynamics in response to exercise following satellite cell depletion

Skeletal muscle is composed of post-mitotic myofibers that form a syncytium containing hundreds of myonuclei. Using a progressive exercise training model in the mouse and single nucleus RNA-sequencing (snRNA-seq) for high-resolution characterization of myonuclear transcription, we show myonuclear functional specialization in muscle. After 4 weeks of exercise training, snRNA-seq reveals that resident muscle stem cells, or satellite cells, are activated with acute exercise but demonstrate limited lineage progression while contributing to muscle adaptation. In the absence of satellite cells, a portion of nuclei demonstrates divergent transcriptional dynamics associated with mixed-fate identities compared with satellite cell replete muscles. These data provide a compendium of information about how satellite cells influence myonuclear transcription in response to exercise.

Resistance Exercise, Aging, Disuse, and Muscle Protein Metabolism

Skeletal muscle is the organ of locomotion, its optimal function is critical for athletic performance, and is also important for health due to its contribution to resting metabolic rate and as a site for glucose uptake and storage. Numerous endogenous and exogenous factors influence muscle mass. Much of what is currently known regarding muscle protein turnover is owed to the development and use of stable isotope tracers. Skeletal muscle mass is determined by the meal- and contraction-induced alterations of muscle protein synthesis and muscle protein breakdown. Increased loading as resistance training is the most potent nonpharmacological strategy by which skeletal muscle mass can be increased. Conversely, aging (sarcopenia) and muscle disuse lead to the development of anabolic resistance and contribute to the loss of skeletal muscle mass. Nascent omics-based technologies have significantly improved our understanding surrounding the regulation of skeletal muscle mass at the gene, transcript, and protein levels. Despite significant advances surrounding the mechanistic intricacies that underpin changes in skeletal muscle mass, these processes are complex, and more work is certainly needed. In this article, we provide an overview of the importance of skeletal muscle, describe the influence that resistance training, aging, and disuse exert on muscle protein turnover and the molecular regulatory processes that contribute to changes in muscle protein abundance. © 2021 American Physiological Society. Compr Physiol 11:2249-2278, 2021.

Myofibre Hypertrophy in the Absence of Changes to Satellite Cell Content Following Concurrent Exercise Training in Young Healthy Men

Concurrent exercise training has been suggested to create an ‘interference effect,’ attenuating resistance training-based skeletal muscle adaptations, including myofibre hypertrophy. Satellite cells support myofibre hypertrophy and are influenced by exercise mode. To determine whether satellite cells contribute to the ‘interference effect’ changes in satellite cell and myonuclear content were assessed following a period of training in 32 recreationally active males (age: 25 ± 5 year; body mass index: 24 ± 3 kg⋅m^–2; mean ± SD) who undertook 12-week of either isolated (3 d⋅w^–1) resistance (RES; n = 10), endurance (END; n = 10), or alternate day (6 d⋅w^–1) concurrent (CET, n = 12) training. Skeletal muscle biopsies were obtained pre-intervention and after 2, 8, and 12 weeks of training to determine fibre type-specific cross-sectional area (CSA), satellite cell content (Pax7⁺DAPI⁺), and myonuclei (DAPI⁺) using immunofluorescence microscopy. After 12 weeks, myofibre CSA increased in all training conditions in type II (P = 0.0149) and mixed fibres (P = 0.0102), with no difference between conditions. Satellite cell content remained unchanged after training in both type I and type II fibres. Significant correlations were observed between increases in fibre type-specific myonuclear content and CSA of Type I (r = 0.63, P < 0.0001), Type II (r = 0.69, P < 0.0001), and mixed fibres (r = 0.72, P < 0.0001). Resistance, endurance, and concurrent training induce similar myofibre hypertrophy in the absence of satellite cell and myonuclear pool expansion. These findings suggest that myonuclear accretion via satellite cell fusion is positively correlated with hypertrophy after 12 weeks of concurrent training, and that individuals with more myonuclear content displayed greater myofibre hypertrophy.

Skeletal muscle fibers count on nuclear numbers for growth

Skeletal muscle cells are noteworthy for their syncytial nature, with each myofiber accumulating hundreds or thousands of nuclei derived from resident muscle stem cells (MuSCs). These nuclei are accrued through cell fusion, which is controlled by the two essential fusogens Myomaker and Myomerger that are transiently expressed within the myogenic lineage. While the absolute requirement of fusion for muscle development has been known for decades, the underlying need for the magnitude of multinucleation in muscle remains mysterious. Possible advantages of multinucleation include the potential it affords for transcriptional diversity within these massive cells, and as a means of increasing DNA content to support optimal cell size and function. In this article, we review recent advances that elucidate the relationship between myonuclear numbers and establishment of myofiber size, and discuss how this new information refines our understanding of the concept of myonuclear domains (MND), the cytoplasmic volumes that each resident myonucleus can support. Finally, we explore the potential consequences and costs of multinucleation and its impacts on myonuclear transcriptional reserve capacity, growth potential, myofiber size regulation, and muscle adaptability. We anticipate this report will not only serve to highlight the latest advances in the basic biology of syncytial muscle cells but also provide information to help design the next generation of therapeutic strategies to maintain muscle mass and function.

Early satellite cell communication creates a permissive environment for long-term muscle growth

Using in vivo muscle stem cell (satellite cell)-specific extracellular vesicle (EV) tracking, satellite cell depletion, in vitro cell culture, and single-cell RNA sequencing, we show satellite cells communicate with other cells in skeletal muscle during mechanical overload. Early satellite cell EV communication primes the muscle milieu for proper long-term extracellular matrix (ECM) deposition and is sufficient to support sustained hypertrophy in adult mice, even in the absence of fusion to muscle fibers. Satellite cells modulate chemokine gene expression across cell types within the first few days of loading, and EV delivery of miR-206 to fibrogenic cells represses Wisp1 expression required for appropriate ECM remodeling. Late-stage communication from myogenic cells during loading is widespread but may be targeted toward endothelial cells. Satellite cells coordinate adaptation by influencing the phenotype of recipient cells, which extends our understanding of their role in muscle adaptation beyond regeneration and myonuclear donation.

Modeling muscle regeneration in RNA toxicity mice

RNA toxicity underlies the pathogenesis of disorders such as myotonic dystrophy type 1 (DM1). Muscular dystrophy is a key element of the pathology of DM1. The means by which RNA toxicity causes muscular dystrophy in DM1 is unclear. Here, we have used the DM200 mouse model of RNA toxicity due to the expression of a mutant DMPK 3′UTR mRNA to model the effects of RNA toxicity on muscle regeneration. Using a BaCl₂-induced damage model, we find that RNA toxicity leads to decreased expression of PAX7, and decreased numbers of satellite cells, the stem cells of adult skeletal muscle (also known as MuSCs). This is associated with a delay in regenerative response, a lack of muscle fiber maturation and an inability to maintain a normal number of satellite cells. Repeated muscle damage also elicited key aspects of muscular dystrophy, including fat droplet deposition and increased fibrosis, and the results represent one of the first times to model these classic markers of dystrophic changes in the skeletal muscles of a mouse model of RNA toxicity. Using a ligand-conjugated antisense (LICA) oligonucleotide ASO targeting DMPK sequences for the first time in a mouse model of RNA toxicity in DM1, we find that treatment with IONIS 877864, which targets the DMPK 3′UTR mRNA, is efficacious in correcting the defects in regenerative response and the reductions in satellite cell numbers caused by RNA toxicity. These results demonstrate the possibilities for therapeutic interventions to mitigate the muscular dystrophy associated with RNA toxicity in DM1.

Post-exercise Cold Water Immersion Effects on Physiological Adaptations to Resistance Training and the Underlying Mechanisms in Skeletal Muscle: A Narrative Review

Post-exercise cold-water immersion (CWI) is a popular recovery modality aimed at minimizing fatigue and hastening recovery following exercise. In this regard, CWI has been shown to be beneficial for accelerating post-exercise recovery of various parameters including muscle strength, muscle soreness, inflammation, muscle damage, and perceptions of fatigue. Improved recovery following an exercise session facilitated by CWI is thought to enhance the quality and training load of subsequent training sessions, thereby providing a greater training stimulus for long-term physiological adaptations. However, studies investigating the long-term effects of repeated post-exercise CWI instead suggest CWI may attenuate physiological adaptations to exercise training in a mode-specific manner. Specifically, there is evidence post-exercise CWI can attenuate improvements in physiological adaptations to resistance training, including aspects of maximal strength, power, and skeletal muscle hypertrophy, without negatively influencing endurance training adaptations. Several studies have investigated the effects of CWI on the molecular responses to resistance exercise in an attempt to identify the mechanisms by which CWI attenuates physiological adaptations to resistance training. Although evidence is limited, it appears that CWI attenuates the activation of anabolic signaling pathways and the increase in muscle protein synthesis following acute and chronic resistance exercise, which may mediate the negative effects of CWI on long-term resistance training adaptations. There are, however, a number of methodological factors that must be considered when interpreting evidence for the effects of post-exercise CWI on physiological adaptations to resistance training and the potential underlying mechanisms. This review outlines and critiques the available evidence on the effects of CWI on long-term resistance training adaptations and the underlying molecular mechanisms in skeletal muscle, and suggests potential directions for future research to further elucidate the effects of CWI on resistance training adaptations.

Neuromuscular fatigue and recovery after strenuous exercise depends on skeletal muscle size and stem cell characteristics

Hamstring muscle injury is highly prevalent in sports involving repeated maximal sprinting. Although neuromuscular fatigue is thought to be a risk factor, the mechanisms underlying the fatigue response to repeated maximal sprints are unclear. Here, we show that repeated maximal sprints induce neuromuscular fatigue accompanied with a prolonged strength loss in hamstring muscles. The immediate hamstring strength loss was linked to both central and peripheral fatigue, while prolonged strength loss was associated with indicators of muscle damage. The kinematic changes immediately after sprinting likely protected fatigued hamstrings from excess elongation stress, while larger hamstring muscle physiological cross-sectional area and lower myoblast:fibroblast ratio appeared to protect against fatigue/damage and improve muscle recovery within the first 48 h after sprinting. We have therefore identified novel mechanisms that likely regulate the fatigue/damage response and initial recovery following repeated maximal sprinting in humans.