OBJECTIVE:Metabolic flexibility refers to the ability of tissues to adjust cellular fuel choice in response to conditional changes in metabolic demand and activity. A loss of metabolic flexibility is a defining feature of various diseases and cellular dysfunction. This study investigated the role of microRNA-1 (miR-1), the most abundant microRNA in skeletal muscle, in maintaining whole-body metabolic flexibility. METHODS:We used an inducible, skeletal muscle-specific knockout (KO) mouse model to examine miR-1 function. Argonaute 2 enhanced crosslinking and immunoprecipitation sequencing (AGO2 eCLIP-seq) and RNA-seq analyses identified miR-1 target genes. Metabolism was investigated using metabolomics, proteomics, and comprehensive bioenergetic and activity phenotyping. Corroborating information was provided from cell culture, C. elegans, and exercised human muscle tissue. RESULTS:miR-1 KO mice demonstrated loss of diurnal oscillations in whole-body respiratory exchange ratio and higher fasting blood glucose. For the first time, we identified bona fide miR-1 target genes in adult skeletal muscle that regulated pyruvate metabolism through mechanisms including the alternative splicing of pyruvate kinase (Pkm). The maintenance of metabolic flexibility by miR-1 was necessary for sustained endurance activity in mice and in C. elegans. Loss of metabolic flexibility in the miR-1 KO mouse was rescued by pharmacological inhibition of the miR-1 target, monocarboxylate transporter 4 (MCT4), which redirects glycolytic carbon flux toward oxidation. The physiological down-regulation of miR-1 in response to hypertrophic stimuli caused a similar metabolic reprogramming necessary for muscle cell growth. CONCLUSIONS:These data identify a novel post-transcriptional mechanism of whole-body metabolism regulation mediated by a tissue-specific miRNA.

In cell biology, ribosomal RNA (rRNA) 2′ O-methyl (2'- O-Me) is the most prevalent post-transcriptional chemical modification contributing to ribosome heterogeneity. The modification involves a family of small nucleolar RNAs (snoRNAs) and is specified by box C/D snoRNAs (SNORDs). Given the importance of ribosome biogenesis for skeletal muscle growth, we asked if rRNA 2'- O-Me in nascent ribosomes synthesized in response to a growth stimulus is an unrecognized mode of ribosome heterogeneity in muscle. To determine the pattern and dynamics of 2'- O-Me rRNA, we used a sequencing-based profiling method called RiboMeth-seq. We applied this method to tissue-derived rRNA of skeletal muscle and rRNA specifically from the muscle fiber using an inducible myofiber-specific RiboTag mouse in sedentary and mechanically overloaded conditions. These analyses were complemented by myonuclear-specific small RNA sequencing to profile SNORDs and link the rRNA epitranscriptome to known regulatory elements generated within the muscle fiber. We demonstrate for the first time that mechanical overload of skeletal muscle 1) induces decreased 2'- O-Me at a subset of skeletal muscle rRNA and 2) alters the SNORD profile in isolated myonuclei. These findings point to a transient diversification of the ribosome pool via 2'- O-Me during growth and adaptation in skeletal muscle. These findings suggest changes in ribosome heterogeneity at the 2'- O-Me level during muscle hypertrophy and lay the foundation for studies investigating the functional implications of these newly identified "growth-induced" ribosomes.

Adult muscle stem cells (MuSCs) are known to replicate upon activation before differentiating and fusing to regenerate myofibers. It is unclear whether MuSC differentiation is intrinsically linked to cell division, which has implications for stem cell population maintenance. We use single-cell RNA-sequencing to identify transcriptionally diverse subpopulations of MuSCs after 5 days of a growth stimulus in adult muscle. Trajectory inference in combination with a novel mouse model for tracking MuSC-derived myonuclei and in vivo labeling of DNA replication revealed an MuSC population that exhibited division-independent differentiation and fusion. These findings demonstrate that in response to a growth stimulus in the presence of intact myofibers, MuSC division is not obligatory.

The objectives of this study were to measure to what extent exercise stimulates partial molecular reprogramming in skeletal muscle. We hypothesized that exercise training, beyond its known functional improvements in aged tissues, could recapitulate some of the same effects that are seen with in vivo partial reprogramming by Yamanaka factors. Using transcriptome profiling from 1) a skeletal muscle-specific in vivo Oct3/4, Klf4, Sox2, and Myc (OKSM) reprogramming-factor expression murine model, 2) an in vivo inducible muscle-specific Myc induction murine model, 3) a translatable high-volume hypertrophic exercise training approach in aged mice, and 4) human exercise muscle biopsies, we collectively defined exercise-induced genes common to partial reprogramming. Late-life exercise training lowered murine DNA methylation age according to several contemporary muscle-specific clocks. A comparison of the murine soleus transcriptome after late-life exercise training to the soleus transcriptome after OKSM induction revealed an overlapping signature. Within this signature, downregulation of specific mitochondrial and muscle-enriched genes was conserved in skeletal muscle of long-term exercise-trained humans; among these was muscle-specific Abra/Stars. Myc is the OKSM factor most induced by exercise in muscle and was elevated following exercise training in aged mice. A pulse of MYC rewired the global soleus muscle methylome, and the transcriptome after a MYC pulse partially recapitulated OKSM induction. A common signature also emerged in the murine MYC-controlled and exercise adaptation transcriptomes, including lower muscle-specific Melusin and reactive oxygen species-associated Romo1. With Myc, OKSM, and exercise training in mice as well habitual exercise in humans, the complex I accessory subunit Ndufb11 was lower; low Ndufb11 is linked to longevity in rodents. Our data collectively suggest: 1) a biological age-mitigating effect on the epigenetic landscape by late-life exercise-training in murine skeletal muscle, 2) a common gene expression signature of partial reprogramming by OKSM and exercise training in muscle of humans and aged mice, and 3) that Myc is an exercise-responsive factor that contributes to a rewired molecular profile at the transcriptome and methylome levels. National Institutes of Health: Kevin A. Murach, AG063994; American Federation for Aging Research (AFAR): Kevin A. Murach, Junior Investigator Grant This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

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.

Recently, the gut microbiome has emerged as a potent modulator of exercise-induced systemic adaptation and appears to be crucial for mediating some of the benefits of exercise. This study builds upon previous evidence establishing a gut microbiome-skeletal muscle axis, identifying exercise-induced changes in microbiome composition. Metagenomics sequencing of fecal samples from non-exercise-trained controls or exercise-trained mice was conducted. Biodiversity indices indicated exercise training did not change alpha diversity. However, there were notable differences in beta-diversity between trained and untrained microbiomes. Exercise significantly increased the level of the bacterial species Muribaculaceae bacterium DSM 103720. Computation simulation of bacterial growth was used to predict metabolites that accumulate under in silico culture of exercise-responsive bacteria. We identified acetate and succinate as potential gut microbial metabolites that are produced by Muribaculaceae bacterium, which were then administered to mice during a period of mechanical overload-induced muscle hypertrophy. Although no differences were observed for the overall muscle growth response to succinate or acetate administration during the first 5 days of mechanical overload-induced hypertrophy, acetate and succinate increased skeletal muscle mitochondrial respiration. When given as post-biotics, succinate or acetate treatment may improve oxidative metabolism during muscle hypertrophy.

Introduction: Apolipoprotein E (ApoE) has been shown to be necessary for proper skeletal muscle regeneration. Consistent with this finding, single-cell RNA-sequencing analyses of skeletal muscle stem cells (MuSCs) revealed that Apoe is a top marker of quiescent MuSCs that is downregulated upon activation. The purpose of this study was to determine if muscle regeneration is altered in mice which harbor one of the three common human ApoE isoforms, referred to as ApoE2, E3 and E4.Methods: Histomorphometric analyses were employed to assess muscle regeneration in ApoE2, E3, and E4 mice after 14 days of recovery from barium chloride-induced muscle damage in vivo, and primary MuSCs were isolated to assess proliferation and differentiation of ApoE2, E3, and E4 MuSCs in vitro.Results: There was no difference in the basal skeletal muscle phenotype of ApoE isoforms as evaluated by section area, myofiber cross-sectional area (CSA), and myonuclear and MuSC abundance per fiber. Although there were no differences in fiber-type frequency in the soleus, Type IIa relative frequency was significantly lower in plantaris muscles of ApoE4 mice compared to ApoE3. Moreover, ApoE isoform did not influence muscle regeneration as assessed by fiber frequency, fiber CSA, and myonuclear and MuSC abundance. Finally, there were no differences in the proliferative capacity or myogenic differentiation potential of MuSCs between any ApoE isoform.Discussion: Collectively, these data indicate nominal effects of ApoE isoform on the ability of skeletal muscle to regenerate following injury or the in vitro MuSC phenotype.

Abstract 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.

Skeletal muscle adaptation to external stimuli, such as regeneration following injury and hypertrophy in response to resistance exercise, are blunted with advanced age. The accumulation of senescent cells, along with defects in myogenic progenitor cell (MPC) proliferation, have been strongly linked as contributing factors to age-associated impairment in muscle adaptation. p53 plays an integral role in all these processes, as upregulation of p53 causes apoptosis in senescent cells and prevents mitotic catastrophe in MPCs from old mice. The goal of this study was to determine if a novel pharmaceutical agent (BI01), which functions by upregulating p53 through inhibition of binding to MDM2, the primary p53 regulatory protein, improves muscle regeneration and hypertrophy in old mice. BI01 effectively reduced the number of senescent cells in vitro but had no effect on MPC survival or proliferation at a comparable dose. Following repeated oral gavage with 2 mg/kg of BI01 (OS) or vehicle (OV), old mice (24 months) underwent unilateral BaCl 2 injury in the tibialis anterior (TA) muscle, with PBS injections serving as controls. After 7 days, satellite cell number was higher in the TA of OS compared to OV mice, as was the expression of genes involved in ATP production. By 35 days, old mice treated with BI01 displayed reduced senescent cell burden, enhanced regeneration (higher muscle mass and fiber cross-sectional area) and restoration of muscle function relative to OV mice. To examine the impact of 2 mg/kg BI01 on muscle hypertrophy, the plantaris muscle was subjected to 28 days of mechanical overload (MOV) in OS and OV mice. In response to MOV, OS mice had larger plantaris muscles and muscle fibers than OV mice, particularly type 2b + x fibers, associated with reduced senescent cells. Together our data show that BI01 is an effective senolytic agent that may also augment muscle metabolism to enhance muscle regeneration and hypertrophy in old mice.

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.

Exosomes are small extracellular vesicles (EVs) that can function as effective delivery vehicles through systemic circulation due to the protective effects of the phospholipid bilayer. Our previous work in mice indicated that resistance exercise induces the release of skeletal muscle EVs to promote adipose lipolysis through the action of microRNA-1. To better understand the effect of resistance exercise on exosomes in humans, we collected blood samples as well as skeletal muscle and adipose tissue biopsies before and after a single bout of resistance exercise. Using the ExoView platform, we were able to analyze subpopulations of exosomes based on their surface tetraspanin markers (CD63, CD81, and CD9). Our preliminary findings show that serum CD81/CD9 exosome abundance is significantly lower in high BMI (>30) subjects, and serum CD63/CD81 exosome abundance may be differentially affected by exercise in subjects with high BMI. Further studies are necessary to define the nucleic acid and protein contents of the exercise-induced exosomes. To this end, we have generated a transgenic mouse with an inducible skeletal muscle specific exosome surface protein reporter, which will allow definitive tracking of exosomes released from skeletal muscle that are delivered to other organ systems.

With aging, skeletal muscle plasticity is attenuated in response to exercise. Here, we report that senescent cells, identified using senescence-associated β-galactosidase (SA β-Gal) activity and p21 immunohistochemistry, are very infrequent in resting muscle, but emerge approximately 2 weeks after a bout of resistance exercise in humans. We hypothesized that these cells contribute to blunted hypertrophic potential in old age. Using synergist ablation-induced mechanical overload (MOV) of the plantaris muscle to model resistance training in adult (5–6-month) and old (23–24-month) male C57BL/6 J mice, we found increased senescent cells in both age groups during hypertrophy. Consistent with the human data, there were negligible senescent cells in plantaris muscle from adult and old sham controls, but old mice had significantly more senescent cells 7 and 14 days following MOV relative to young. Old mice had blunted whole-muscle hypertrophy when compared to adult mice, along with smaller muscle fibers, specifically glycolytic type 2x + 2b fibers. To ablate senescent cells using a hit-and-run approach, old mice were treated with vehicle or a senolytic cocktail consisting of 5 mg/kg dasatinib and 50 mg/kg quercetin (D + Q) on days 7 and 10 during 14 days of MOV; control mice underwent sham surgery with or without senolytic treatment. Old mice given D + Q had larger muscles and muscle fibers after 14 days of MOV, fewer senescent cells when compared to vehicle-treated old mice, and changes in the expression of genes (i.e., Igf1 , Ddit4 , Mmp14 ) that are associated with hypertrophic growth. Our data collectively show that senescent cells emerge in human and mouse skeletal muscle following a hypertrophic stimulus and that D + Q improves muscle growth in old mice.

Abstract 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.

Skeletal muscle is a heterogenous tissue composed of multinucleated muscle fibers and accompanying supporting cells like satellite cells (SCs). Myonuclei are believed to be post‐mitotic, arrested in G0 phase of the cell cycle and thus unable to proliferate. Consequently, it is hypothesized that growth of muscle is dependent on the donation of nuclei from SCs. Although some studies suggest that under certain conditions myonuclei can replicate, it has been difficult to address whether this replicative potential exists in vivo. We hypothesized that skeletal muscle resident myonuclei possess the ability to replicate in vivo. To test this hypothesis, we used the HSArtTA;Tet‐O‐H2B‐GFP (HSA‐H2B) mouse model, which uses doxycycline (DOX) treatment for temporal labeling (Tet‐ON) of residential myonuclei. After removing DOX, and thus turning off GFP‐labeling, we provided the stable isotope deuterium oxide (D2O) for 8 weeks to measure DNA synthesis. At the end of D2O labeling, muscles were harvested and GFP‐positive myonuclei were sorted using an antibody‐independent approach and a restrictive gating strategy. Both GFP+ (myonuclei) and GFP‐ (non‐myonuclei) fractions were then analyzed for D2O incorporation into DNA by GC‐QQQ. By our approach, DNA from GFP‐positive myonuclei was deuterium‐enriched and therefore deemed to be replicating. DNA replication occurred in at least one muscle of the 7 mice studied. We observed deuterium enrichment into DNA in 6 out of 7 plantaris (PLA), 4 out of 7 extensor digitorum longus (EDL), 4 out of 7 tibialis anterior (TA), 6 out of 7 gastrocnemius (GAS) and 7 out of 7 quadriceps (QUAD). The average fractional synthesis rates (FSR, %/day) of replicating myonuclei were: 0.0351 ± 0.017 in PLA, 0.0156 ± 0.015 in EDL, 0.0220 ± 0.018 in TA, 0.0223 ± 0.018 in GAS, and 0.02692 ± 0.043 in QUAD. These data are the first to unambiguously demonstrate that myonuclei replicate DNA in vivo. The proposed project challenges current dogma and might significantly impact skeletal muscle treatment to maintain muscle mass with age and disease.

One aspect of skeletal muscle memory is the ability of a previously trained muscle to hypertrophy more rapidly following a period of detraining. Although the molecular basis of muscle memory remains to be fully elucidated, one potential mechanism thought to mediate muscle memory is the permanent retention of myonuclei acquired during the initial phase of hypertrophic growth. However, myonuclear permanence is debated and would benefit from a meta‐analysis to clarify the current state of the field for this important aspect of skeletal muscle plasticity. The objective of this study was to perform a meta‐analysis to assess the permanence of myonuclei associated with changes in physical activity and ageing. When available, the abundance of satellite cells (SCs) was also considered given their potential influence on changes in myonuclear abundance. One hundred forty‐seven peer‐reviewed articles were identified for inclusion across five separate meta‐analyses; (1–2) human and rodent studies assessed muscle response to hypertrophy; (3–4) human and rodent studies assessed muscle response to atrophy; and (5) human studies assessed muscle response with ageing. Skeletal muscle hypertrophy was associated with higher myonuclear content that was retained in rodents, but not humans, with atrophy (SMD = −0.60, 95% CI −1.71 to 0.51, P = 0.29, and MD = 83.46, 95% CI −649.41 to 816.32, P = 0.82; respectively). Myonuclear and SC content were both lower following atrophy in humans (MD = −11, 95% CI −0.19 to −0.03, P = 0.005, and SMD = −0.49, 95% CI −0.77 to −0.22, P = 0.0005; respectively), although the response in rodents was affected by the type of muscle under consideration and the mode of atrophy. Whereas rodent myonuclei were found to be more permanent regardless of the mode of atrophy, atrophy of ≥30% was associated with a reduction in myonuclear content (SMD = −1.02, 95% CI −1.53 to −0.51, P = 0.0001). In humans, sarcopenia was accompanied by a lower myonuclear and SC content (MD = 0.47, 95% CI 0.09 to 0.85, P = 0.02, and SMD = 0.78, 95% CI 0.37–1.19, P = 0.0002; respectively). The major finding from the present meta‐analysis is that myonuclei are not permanent but are lost during periods of atrophy and with ageing. These findings do not support the concept of skeletal muscle memory based on the permanence of myonuclei and suggest other mechanisms, such as epigenetics, may have a more important role in mediating this aspect of skeletal muscle plasticity.

Ribosomal protein L3-like (RPL3L) is a poorly characterized ribosomal protein that is exclusively expressed in skeletal and cardiac muscle. RPL3L is also downregulated in Duchenne muscular dystrophy (DMD), suggesting that it may play an important role in muscle biology. In this study, we investigated the role of RPL3L in skeletal muscle of healthy C57 and dystrophic mdx mice. We show that RPL3L is developmentally regulated and that intramuscular adeno-associated virus (AAV)-mediated RPL3L knockdown in the tibialis anterior of C57 and mdx mice results in increased specific force with improved resistance to eccentric contraction induced muscle damage in dystrophic muscles. The mechanism by which RPL3L knockdown improves muscle function remains unclear. Histological observations showed a significant increase in muscle length and decrease in muscle cross-sectional area after RPL3L inhibition suggesting that this ribosomal protein may play a role in myofiber morphology. The endogenous downregulation of RPL3L in DMD may be a protective mechanism that attempts to improve skeletal muscle function and counteract the dystrophic phenotype.

Summary: 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.