Making Mice Mighty: recent advances in translational models of load-induced muscle hypertrophy

Stuart has got the PoWeR! Skeletal muscle adaptations to a novel heavy progressive weighted wheel running exercise model in C57BL/6 mice

Murine exercise models are developed to study the molecular and cellular mechanisms regulating muscle mass. A progressive weighted wheel running model, named 'PoWeR', was previously developed to serve as a more translatable alternative to involuntary resistance-type exercise models in rodents, such as synergist ablation. However, mice still run great distances despite the added resistance as evidenced by a large glycolytic-to-oxidative shift in muscle fibre type. Thus, PoWeR reflects a blended resistance/endurance model. In an attempt to bias PoWeR further towards resistance-type exercise, we developed a novel heavy PoWeR model (hPoWeR) utilizing higher wheel loads (max of 12.5 g vs 6 g). Adult male C57BL/6 mice voluntarily performed an 8-week progressive loading protocol (PoWeR or hPoWeR). Running distance peaked at ∼5-6 km day-1 in both treatments and was maintained by PoWeR mice, but declined in the hPoWeR mice as load increased beyond 7.5 g. Peak isometric force of the gastrocnemius-soleus-plantaris complex tended to increase in wheel running treatments. Soleus mass increased by 19% and 24% in PoWeR and hPoWeR treatments, respectively, and plantaris fibre cross-sectional area was greater in hPoWeR, compared to PoWeR. There were fewer glycolytic and more oxidative fibres in the soleus and plantaris muscles in the PoWeR treatment, but not hPoWeR. Collectively, these data suggest hPoWeR may modestly alter skeletal muscle supporting the aim of better reflecting typical resistance training adaptations, in line with decreased running volume and exposure to higher resistance. Regardless, PoWeR remains an effective hypertrophic concurrent training model in mice.

Conserved and species-specific transcriptional responses to daily programmed resistance exercise in rat and mouse

Mice are often used in gain or loss of function studies to understand how genes regulate metabolism and adaptation to exercise in skeletal muscle. Once-daily resistance training with electrical nerve stimulation produces hypertrophy of the dorsiflexors in rat, but not in mouse. Using implantable pulse generators, we assessed the acute transcriptional response (1-h post-exercise) after 2, 10, and 20 days of training in free-living mice and rats using identical nerve stimulation paradigms. RNA sequencing revealed strong concordance in the timecourse of many transcriptional responses in the tibialis anterior muscles of both species including responses related to "stress responses/immediate-early genes, and "collagen homeostasis," "ribosomal subunits," "autophagy," and "focal adhesion." However, pathways associated with energy metabolism including "carbon metabolism," "oxidative phosphorylation," "mitochondrial translation," "propanoate metabolism," and "valine, leucine, and isoleucine degradation" were oppositely regulated between species. These pathways were suppressed in the rat but upregulated in the mouse. Our transcriptional analysis suggests that although many pathways associated with growth show remarkable similarities between species, the absence of an actual growth response in the mouse may be because the mouse prioritizes energy metabolism, specifically the replenishment of fuel stores and intermediate metabolites.

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.

Intrinsic contractile dysfunction due to impaired sarcoplasmic reticulum Ca2+ release in compensatory hypertrophied muscle fibers following synergist ablation

Synergist ablation (SA) is an experimental procedure for the induction of hypertrophy. However, SA causes a decrease in specific force (i.e., force per cross-sectional area), likely due to excessive muscle use. Here, we investigated the mechanisms behind the SA-induced intrinsic contractile dysfunction, especially focusing on the excitation-contraction (EC) coupling. Male Wistar rats had unilateral surgical ablation of gastrocnemius and soleus muscles to induce compensatory hypertrophy in the plantaris muscles. Two weeks after SA, plantaris muscle was dissected from each animal and used for later analyses. SA significantly increased the mean fiber cross-sectional area (+18%). On the other hand, the ratio of depolarization-induced force to the maximum Ca2+-activated specific force, an indicator of sarcoplasmic reticulum (SR) Ca2+ release, was markedly reduced in mechanically skinned fibers from the SA group (-51%). These functional defects were accompanied by an extensive fragmentation of the SR Ca2+ release channel, the ryanodine receptor 1 (RyR1), and a decrease in the amount of other triad proteins (i.e., DHPR, STAC3, and junctophilin1). SA treatment also caused activation of calpain-1 and increased the amount of NADPH oxidase 2, endoplasmic reticulum (ER) stress proteins (i.e., Grp78, Grp94, PDI, and Ero1), and lipid peroxidation [i.e., 4-hydroxynonenal (4-HNE)] in SA-treated muscles. Our findings show that SA causes skeletal muscle weakness due to impaired EC coupling. This is likely to be induced by Ca2+-dependent degradation of triad proteins, which may result from Ca2+ leak from fragmented RyR1 triggered by increased oxidative stress.NEW & NOTEWORTHY Synergist ablation (SA) has widely been used to understand the mechanisms behind skeletal muscle hypertrophy. However, compensatory hypertrophied muscles display intrinsic contractile dysfunction, i.e., a hallmark of overuse. Here, we demonstrate that SA-induced compensatory hypertrophy is accompanied by muscle weakness due to impaired sarcoplasmic reticulum Ca2+ release. This dysfunction may be caused by the degradation of triad proteins due to the reciprocal amplification of reactive oxygen species and Ca2+ signaling at the junctional space microdomain.

MicroRNA control of the myogenic cell transcriptome and proteome: the role of miR-16

MicroRNAs (miRs) control stem cell biology and fate. Ubiquitously expressed and conserved miR-16 was the first miR implicated in tumorigenesis. miR-16 is low in muscle during developmental hypertrophy and regeneration. It is enriched in proliferating myogenic progenitor cells but is repressed during differentiation. The induction of miR-16 blocks myoblast differentiation and myotube formation, whereas knockdown enhances these processes. Despite a central role for miR-16 in myogenic cell biology, how it mediates its potent effects is incompletely defined. In this investigation, global transcriptomic and proteomic analyses after miR-16 knockdown in proliferating C2C12 myoblasts revealed how miR-16 influences myogenic cell fate. Eighteen hours after miR-16 inhibition, ribosomal protein gene expression levels were higher relative to control myoblasts and p53 pathway-related gene abundance was lower. At the protein level at this same time point, miR-16 knockdown globally upregulated tricarboxylic acid (TCA) cycle proteins while downregulating RNA metabolism-related proteins. miR-16 inhibition induced specific proteins associated with myogenic differentiation such as ACTA2, EEF1A2, and OPA1. We extend prior work in hypertrophic muscle tissue and show that miR-16 is lower in mechanically overloaded muscle in vivo. Our data collectively point to how miR-16 is implicated in aspects of myogenic cell differentiation. A deeper understanding of the role of miR-16 in myogenic cells has consequences for muscle developmental growth, exercise-induced hypertrophy, and regenerative repair after injury, all of which involve myogenic progenitors.

Different Resistance Exercise Loading Paradigms Similarly Affect Skeletal Muscle Gene Expression Patterns of Myostatin-Related Targets and mTORC1 Signaling Markers

Although transcriptome profiling has been used in several resistance training studies, the associated analytical approaches seldom provide in-depth information on individual genes linked to skeletal muscle hypertrophy. Therefore, a secondary analysis was performed herein on a muscle transcriptomic dataset we previously published involving trained college-aged men (n = 11) performing two resistance exercise bouts in a randomized and crossover fashion. The lower-load bout (30 Fail) consisted of 8 sets of lower body exercises to volitional fatigue using 30% one-repetition maximum (1 RM) loads, whereas the higher-load bout (80 Fail) consisted of the same exercises using 80% 1 RM loads. Vastus lateralis muscle biopsies were collected prior to (PRE), 3 h, and 6 h after each exercise bout, and 58 genes associated with skeletal muscle hypertrophy were manually interrogated from our prior microarray data. Select targets were further interrogated for associated protein expression and phosphorylation induced-signaling events. Although none of the 58 gene targets demonstrated significant bout x time interactions, ~57% (32 genes) showed a significant main effect of time from PRE to 3 h (15↑ and 17↓, p < 0.01), and ~26% (17 genes) showed a significant main effect of time from PRE to 6 h (8↑ and 9↓, p < 0.01). Notably, genes associated with the myostatin (9 genes) and mammalian target of rapamycin complex 1 (mTORC1) (9 genes) signaling pathways were most represented. Compared to mTORC1 signaling mRNAs, more MSTN signaling-related mRNAs (7 of 9) were altered post-exercise, regardless of the bout, and RHEB was the only mTORC1-associated mRNA that was upregulated following exercise. Phosphorylated (phospho-) p70S6K (Thr389) (p = 0.001; PRE to 3 h) and follistatin protein levels (p = 0.021; PRE to 6 h) increased post-exercise, regardless of the bout, whereas phospho-AKT (Thr389), phospho-mTOR (Ser2448), and myostatin protein levels remained unaltered. These data continue to suggest that performing resistance exercise to volitional fatigue, regardless of load selection, elicits similar transient mRNA and signaling responses in skeletal muscle. Moreover, these data provide further evidence that the transcriptional regulation of myostatin signaling is an involved mechanism in response to resistance exercise.

Going nuclear: Molecular adaptations to exercise mediated by myonuclei

Muscle fibers are multinucleated, and muscle fiber nuclei (myonuclei) are believed to be post-mitotic and are typically situated near the periphery of the myofiber. Due to the unique organization of muscle fibers and their nuclei, the cellular and molecular mechanisms regulating myofiber homeostasis in unstressed and stressed conditions (e.g., exercise) are unique. A key role myonuclei play in regulating muscle during exercise is gene transcription. Only recently have investigators had the capability to identify molecular changes at high resolution exclusively in myonuclei in response to perturbations in vivo. The purpose of this review is to describe how myonuclei modulate their transcriptome, epigenetic status, mobility and shape, and microRNA expression in response to exercise in vivo. Given the relative paucity of high-fidelity information on myonucleus-specific contributions to exercise adaptation, we identify specific gaps in knowledge and provide perspectives on future directions of research.

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.

Improved Electrical Stimulation-Based Exercise Model to Induce Mice Tibialis Anterior Muscle Hypertrophy and Function

Efficient and suitable animal models directed to skeletal muscle hypertrophy are highly needed; nevertheless, the currently available models have limitations, such as restricted hypertrophy outcome and prolonged protocols; thus, additional research is required. In this study, we developed an improved muscle training protocol for mice by directly stimulating the tibialis anterior (TA) muscle motor point using electrical stimulation. C57BL/6 adult male mice were separated into four groups: CTR (control groups for one and two weeks), ES1 (electrical stimulation for one week), and ES2 (electrical stimulation for two weeks). Following muscle training, TA was taken for further examination. The results demonstrated a steady increase in the fiber cross-sectional area as a result of muscle training (ES1, 14.6% and ES2, 28.9%, p < 0.0001). Two weeks of muscle training enhanced muscle mass and maximal tetanic force by 18 (p = 0.0205) and 30%, respectively (p = 0.0260). To assess the tissue remodeling response in this model, we evaluated satellite cell activity and observed an increase in the number of Pax-7-positive nuclei after one and two weeks of muscle training (both >2-fold, p < 0.0001). In addition, we observed an increase in the number of positive nuclei for MyoD after two weeks (2.6-fold, p = 0.0057) without fiber damage. Accordingly, phosphorylation of mTOR and p70 increased following two weeks of muscle training (17%, p = 0.0215 and 66%, p = 0.0364, respectively). The results indicate that this muscle training strategy is appropriate for promoting quick and intense hypertrophy.

Muscular contraction's therapeutic potential for cancer-induced wasting

Skeletal muscle atrophy and dysfunction contribute to cancer patient morbidity and mortality. Cachexia pathophysiology is highly complex, given that perturbations to the systemic cancer environment and the interaction with diverse tissues can contribute to wasting processes. Systemic interleukin 6 (IL-6) and glycoprotein 130 (gp130) receptor signaling have established roles in some types of cancer-induced muscle wasting through disruptions to protein turnover and oxidative capacity. While exercise has documented benefits for cancer prevention and patient survival, there are significant gaps in our understanding of muscle adaptation and plasticity during severe cachexia. Preclinical models have provided valuable insight into the adaptive potential of muscle to contraction within the cancer environment. We summarize the current understanding of how resistance-type exercise impacts mechanisms involved in cancer-induced muscle atrophy and dysfunction. Specifically, the role of IL-6 and gp130 receptor in the pathophysiology of muscle wasting and the adaptive response to exercise is explained. The discussion includes current knowledge gaps and future research directions needed to improve preclinical research and accelerate clinical translation in human cancer patients.

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.

Exercise Counteracts the Deleterious Effects of Cancer Cachexia

Simple Summary

This review provides an overview of the effects of exercise training on the major mechanisms related to cancer cachexia (CC). The review also discusses how cancer comorbidities can influence the ability of patients/animals with cancer to perform exercise training and what precautions should be taken when they exercise. The contribution of other factors, such as exercise modality and biological sex, to exercise effectiveness in ameliorating CC are also elaborated in the final sections. We provide meticulous evidence for how advantageous exercise training can be in patients/animals with CC at molecular and cellular levels. Finally, we emphasise what factors should be considered to optimise and personalise an exercise training program in CC.

Abstract

Cancer cachexia (CC) is a multifactorial syndrome characterised by unintentional loss of body weight and muscle mass in patients with cancer. The major hallmarks associated with CC development and progression include imbalanced protein turnover, inflammatory signalling, mitochondrial dysfunction and satellite cell dysregulation. So far, there is no effective treatment to counteract muscle wasting in patients with CC. Exercise training has been proposed as a potential therapeutic approach for CC. This review provides an overview of the effects of exercise training in CC-related mechanisms as well as how factors such as cancer comorbidities, exercise modality and biological sex can influence exercise effectiveness in CC. Evidence in mice and humans suggests exercise training combats all of the hallmarks of CC. Several exercise modalities induce beneficial adaptations in patients/animals with CC, but concurrent resistance and endurance training is considered the optimal type of exercise. In the case of cancer patients presenting comorbidities, exercise training should be performed only under specific guidelines and precautions to avoid adverse effects. Observational comparison of studies in CC using different biological sex shows exercise-induced adaptations are similar between male and female patients/animals with cancer, but further studies are needed to confirm this.

Dysbiosis of the gut microbiome impairs mouse skeletal muscle adaptation to exercise

There is emerging evidence of a gut microbiome-skeletal muscle axis. The purpose of this study was to determine if an intact gut microbiome was necessary for skeletal muscle adaptation to exercise. Forty-two 4-month-old female C57BL/6J mice were randomly assigned to untreated (U) or antibiotic-treated (T) non-running controls (CU or CT, respectively) or progressive weighted wheel running (PoWeR, P) untreated (PU) or antibiotic-treated (PT) groups. Antibiotic treatment resulted in disruption of the gut microbiome as indicated by a significant depletion of gut microbiome bacterial species in both CT and PT groups. The training stimulus was the same between PU and PT groups as assessed by weekly (12.35 ± 2.06 vs. 11.09 ± 1.76 km/week, respectively) and total (778.9 ± 130.5 vs. 703.8 ± 112.9 km, respectively) running activity. In response to PoWeR, PT showed less hypertrophy of soleus type 1 and 2a fibres and plantaris type 2b/x fibres compared to PU. The higher satellite cell and myonuclei abundance of PU plantaris muscle after PoWeR was not observed in PT. The fibre-type shift of PU plantaris muscle to a more oxidative type 2a fibre composition following PoWeR was blunted in PT. There was no difference in serum cytokine levels among all groups suggesting disruption of the gut microbiome did not induce systemic inflammation. The results of this study provide the first evidence that an intact gut microbiome is necessary for skeletal muscle adaptation to exercise. KEY POINTS: Dysbiosis of the gut microbiome caused by continuous antibiotic treatment did not affect running activity. Continuous treatment with antibiotics did not result in systemic inflammation as indicated by serum cytokine levels. Gut microbiome dysbiosis was associated with blunted fibre type-specific hypertrophy in the soleus and plantaris muscles in response to progressive weighted wheel running (PoWeR). Gut microbiome dysbiosis was associated with impaired PoWeR-induced fibre-type shift in the plantaris muscle. Gut microbiome dysbiosis was associated with a loss of PoWeR-induced myonuclei accretion in the plantaris muscle.

Weight Pulling: A Novel Mouse Model of Human Progressive Resistance Exercise

This study describes a mouse model of progressive resistance exercise that utilizes a full-body/multi-joint exercise (weight pulling) along with a training protocol that mimics a traditional human paradigm (three training sessions per week, ~8–12 repetitions per set, 2 min of rest between sets, approximately two maximal-intensity sets per session, last set taken to failure, and a progressive increase in loading that is based on the individual’s performance). We demonstrate that weight pulling can induce an increase in the mass of numerous muscles throughout the body. The relative increase in muscle mass is similar to what has been observed in human studies, and is associated with the same type of long-term adaptations that occur in humans (e.g., fiber hypertrophy, myonuclear accretion, and, in some instances, a fast-to-slow transition in Type II fiber composition). Moreover, we demonstrate that weight pulling can induce the same type of acute responses that are thought to drive these long-term adaptations (e.g., the activation of signaling through mTORC1 and the induction of protein synthesis at 1 h post-exercise). Collectively, the results of this study indicate that weight pulling can serve as a highly translatable mouse model of progressive resistance exercise.

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.

Nucleus Type-Specific DNA Methylomics Reveals Epigenetic "Memory" of Prior Adaptation in Skeletal Muscle

Using a mouse model of conditional and inducible in vivo fluorescent myonuclear labeling (HSA-GFP), sorting purification of nuclei, low-input reduced representation bisulfite sequencing (RRBS), and a translatable and reversible model of exercise (progressive weighted wheel running, PoWeR), we provide the first nucleus type-specific epigenetic information on skeletal muscle adaptation and detraining. Adult (>4 mo) HSA-GFP mice performed PoWeR for 8 wk then detrained for 12 wk; age-matched untrained mice were used to control for the long duration of the study. Myonuclei and interstitial nuclei from plantaris muscles were isolated for RRBS. Relative to untrained, PoWeR caused similar myonuclear CpG hypo- and hyper-methylation of promoter regions and substantial hypomethylation in interstitial nuclear promoters. Over-representation analysis of promoters revealed a larger number of hyper- versus hypo-methylated pathways in both nuclear populations after training and evidence for reciprocal regulation of methylation between nucleus types, with hypomethylation of promoter regions in Wnt signaling-related genes in myonuclei and hypermethylation in interstitial nuclei. After 12 wk of detraining, promoter CpGs in documented muscle remodeling-associated genes and pathways that were differentially methylated immediately after PoWeR were persistently differentially methylated in myonuclei, along with long-term promoter hypomethylation in interstitial nuclei. No enduring gene expression changes in muscle tissue were observed using RNA-sequencing. Upon 4 wk of retraining, mice that trained previously grew more at the whole muscle and fiber type-specific cellular level than training naïve mice, with no difference in myonuclear number. Muscle nuclei have a methylation epi-memory of prior training that may augment muscle adaptability to retraining.

LINCing Nuclear Mechanobiology With Skeletal Muscle Mass and Function

Skeletal muscle demonstrates a high degree of adaptability in response to changes in mechanical input. The phenotypic transformation in response to mechanical cues includes changes in muscle mass and force generating capabilities, yet the molecular pathways that govern skeletal muscle adaptation are still incompletely understood. While there is strong evidence that mechanotransduction pathways that stimulate protein synthesis play a key role in regulation of muscle mass, there are likely additional mechano-sensitive mechanisms important for controlling functional muscle adaptation. There is emerging evidence that the cell nucleus can directly respond to mechanical signals (i.e., nuclear mechanotransduction), providing a potential additional level of cellular regulation for controlling skeletal muscle mass. The importance of nuclear mechanotransduction in cellular function is evident by the various genetic diseases that arise from mutations in proteins crucial to the transmission of force between the cytoskeleton and the nucleus. Intriguingly, these diseases preferentially affect cardiac and skeletal muscle, suggesting that nuclear mechanotransduction is critically important for striated muscle homeostasis. Here we discuss our current understanding for how the nucleus acts as a mechanosensor, describe the main cytoskeletal and nuclear proteins involved in the process, and propose how similar mechanoresponsive mechanisms could occur in the unique cellular environment of a myofiber. In addition, we examine how nuclear mechanotransduction fits into our current framework for how mechanical stimuli regulates skeletal muscle mass.

Chemotherapy-Induced Myopathy: The Dark Side of the Cachexia Sphere

Simple Summary

In addition to cancer-related factors, anti-cancer chemotherapy treatment can drive life-threatening body wasting in a syndrome known as cachexia. Emerging evidence has described the impact of several key chemotherapeutic agents on skeletal muscle in particular, and the mechanisms are gradually being unravelled. Despite this evidence, there remains very little research regarding therapeutic strategies to protect muscle during anti-cancer treatment and current global grand challenges focused on deciphering the cachexia conundrum fail to consider this aspect—chemotherapy-induced myopathy remains very much on the dark side of the cachexia sphere. This review explores the impact and mechanisms of, and current investigative strategies to protect against, chemotherapy-induced myopathy to illuminate this serious issue.

Abstract

Cancer cachexia is a debilitating multi-factorial wasting syndrome characterised by severe skeletal muscle wasting and dysfunction (i.e., myopathy). In the oncology setting, cachexia arises from synergistic insults from both cancer–host interactions and chemotherapy-related toxicity. The majority of studies have surrounded the cancer–host interaction side of cancer cachexia, often overlooking the capability of chemotherapy to induce cachectic myopathy. Accumulating evidence in experimental models of cachexia suggests that some chemotherapeutic agents rapidly induce cachectic myopathy, although the underlying mechanisms responsible vary between agents. Importantly, we highlight the capacity of specific chemotherapeutic agents to induce cachectic myopathy, as not all chemotherapies have been evaluated for cachexia-inducing properties—alone or in clinically compatible regimens. Furthermore, we discuss the experimental evidence surrounding therapeutic strategies that have been evaluated in chemotherapy-induced cachexia models, with particular focus on exercise interventions and adjuvant therapeutic candidates targeted at the mitochondria.

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.

Targeting the Activin Receptor Signaling to Counteract the Multi-Systemic Complications of Cancer and Its Treatments

Muscle wasting, i.e., cachexia, frequently occurs in cancer and associates with poor prognosis and increased morbidity and mortality. Anticancer treatments have also been shown to contribute to sustainment or exacerbation of cachexia, thus affecting quality of life and overall survival in cancer patients. Pre-clinical studies have shown that blocking activin receptor type 2 (ACVR2) or its ligands and their downstream signaling can preserve muscle mass in rodents bearing experimental cancers, as well as in chemotherapy-treated animals. In tumor-bearing mice, the prevention of skeletal and respiratory muscle wasting was also associated with improved survival. However, the definitive proof that improved survival directly results from muscle preservation following blockade of ACVR2 signaling is still lacking, especially considering that concurrent beneficial effects in organs other than skeletal muscle have also been described in the presence of cancer or following chemotherapy treatments paired with counteraction of ACVR2 signaling. Hence, here, we aim to provide an up-to-date literature review on the multifaceted anti-cachectic effects of ACVR2 blockade in preclinical models of cancer, as well as in combination with anticancer treatments.

Muscle memory: myonuclear accretion, maintenance, morphology, and miRNA levels with training and detraining in adult mice

BACKGROUND

In the context of mass regulation, 'muscle memory' can be defined as long-lasting cellular adaptations to hypertrophic exercise training that persist during detraining-induced atrophy and may facilitate future adaptation. The cellular basis of muscle memory is not clearly defined but may be related to myonuclear number and/or epigenetic changes within muscle fibres.

METHODS

Utilizing progressive weighted wheel running (PoWeR), a novel murine exercise training model, we explored myonuclear dynamics and skeletal muscle miRNA levels with training and detraining utilizing immunohistochemistry, single fibre myonuclear analysis, and quantitative analysis of miRNAs. We also used a genetically inducible mouse model of fluorescent myonuclear labelling to study myonuclear adaptations early during exercise.

RESULTS

In the soleus, oxidative type 2a fibres were larger after 2 months of PoWeR (P = 0.02), but muscle fibre size and myonuclear number did not return to untrained levels after 6 months of detraining. Soleus type 1 fibres were not larger after PoWeR but had significantly more myonuclei, as well as central nuclei (P < 0.0001), the latter from satellite cell-derived or resident myonuclei, appearing early during training and remaining with detraining. In the gastrocnemius muscle, oxidative type 2a fibres of the deep region were larger and contained more myonuclei after PoWeR (P < 0.003), both of which returned to untrained levels after detraining. In the gastrocnemius and plantaris, two muscles where myonuclear number was comparable with untrained levels after 6 months of detraining, myonuclei were significantly elongated with detraining (P < 0.0001). In the gastrocnemius, miR-1 was lower with training and remained lower after detraining (P < 0.002).

CONCLUSIONS

This study found that (i) myonuclei gained during hypertrophy are lost with detraining across muscles, even in oxidative fibres; (ii) complete reversal of muscle adaptations, including myonuclear number, to untrained levels occurs within 6 months in the plantaris and gastrocnemius; (iii) the murine soleus is resistant to detraining; (iv) myonuclear accretion occurs early with wheel running and can be uncoupled from muscle fibre hypertrophy; (v) resident (non-satellite cell-derived) myonuclei can adopt a central location; (vi) myonuclei change shape with training and detraining; and (vii) miR-1 levels may reflect a memory of previous adaptation that facilitates future growth.

Satellite Cell Depletion Disrupts Transcriptional Coordination and Muscle Adaptation to Exercise

Satellite cells are required for postnatal development, skeletal muscle regeneration across the lifespan, and skeletal muscle hypertrophy prior to maturity. Our group has aimed to address whether satellite cells are required for hypertrophic growth in mature skeletal muscle. Here, we generated a comprehensive characterization and transcriptome-wide profiling of skeletal muscle during adaptation to exercise in the presence or absence of satellite cells in order to identify distinct phenotypes and gene networks influenced by satellite cell content. We administered vehicle or tamoxifen to adult Pax7-DTA mice and subjected them to progressive weighted wheel running (PoWeR). We then performed immunohistochemical analysis and whole-muscle RNA-seq of vehicle (SC+) and tamoxifen-treated (SC-) mice. Further, we performed single myonuclear RNA-seq to provide detailed information on how satellite cell fusion affects myonuclear transcription. We show that while skeletal muscle can mount a robust hypertrophic response to PoWeR in the absence of satellite cells, growth, and adaptation are ultimately blunted. Transcriptional profiling reveals several gene networks key to muscle adaptation are altered in the absence of satellite cells.