Nuclear numbers in syncytial muscle fibers promote size but limit the development of larger myonuclear domains

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Epigenetic control of skeletal muscle atrophy

Skeletal muscular atrophy is a complex disease involving a large number of gene expression regulatory networks and various biological processes. Despite extensive research on this topic, its underlying mechanisms remain elusive, and effective therapeutic approaches are yet to be established. Recent studies have shown that epigenetics play an important role in regulating skeletal muscle atrophy, influencing the expression of numerous genes associated with this condition through the addition or removal of certain chemical modifications at the molecular level. This review article comprehensively summarizes the different types of modifications to DNA, histones, RNA, and their known regulators. We also discuss how epigenetic modifications change during the process of skeletal muscle atrophy, the molecular mechanisms by which epigenetic regulatory proteins control skeletal muscle atrophy, and assess their translational potential. The role of epigenetics on muscle stem cells is also highlighted. In addition, we propose that alternative splicing interacts with epigenetic mechanisms to regulate skeletal muscle mass, offering a novel perspective that enhances our understanding of epigenetic inheritance's role and the regulatory network governing skeletal muscle atrophy. Collectively, advancements in the understanding of epigenetic mechanisms provide invaluable insights into the study of skeletal muscle atrophy. Moreover, this knowledge paves the way for identifying new avenues for the development of more effective therapeutic strategies and pharmaceutical interventions.

Skeletal muscle: molecular structure, myogenesis, biological functions, and diseases

Skeletal muscle is an important motor organ with multinucleated myofibers as its smallest cellular units. Myofibers are formed after undergoing cell differentiation, cell-cell fusion, myonuclei migration, and myofibril crosslinking among other processes and undergo morphological and functional changes or lesions after being stimulated by internal or external factors. The above processes are collectively referred to as myogenesis. After myofibers mature, the function and behavior of skeletal muscle are closely related to the voluntary movement of the body. In this review, we systematically and comprehensively discuss the physiological and pathological processes associated with skeletal muscles from five perspectives: molecule basis, myogenesis, biological function, adaptive changes, and myopathy. In the molecular structure and myogenesis sections, we gave a brief overview, focusing on skeletal muscle-specific fusogens and nuclei-related behaviors including cell-cell fusion and myonuclei localization. Subsequently, we discussed the three biological functions of skeletal muscle (muscle contraction, thermogenesis, and myokines secretion) and its response to stimulation (atrophy, hypertrophy, and regeneration), and finally settled on myopathy. In general, the integration of these contents provides a holistic perspective, which helps to further elucidate the structure, characteristics, and functions of skeletal muscle.

Whole-body replacement of larval myofibers generates permanent adult myofibers in zebrafish

Drastic increases in myofiber number and size are essential to support vertebrate post-embryonic growth. However, the collective cellular behaviors that enable these increases have remained elusive. Here, we created the palmuscle myofiber tagging and tracking system for in toto monitoring of the growth and fates of ~5000 fast myofibers in developing zebrafish larvae. Through live tracking of individual myofibers within the same individuals over extended periods, we found that many larval myofibers readily dissolved during development, enabling the on-site addition of new and more myofibers. Remarkably, whole-body surveillance of multicolor-barcoded myofibers further unveiled a gradual yet extensive elimination of larval myofiber populations, resulting in near-total replacement by late juvenile stages. The subsequently emerging adult myofibers are not only long-lasting, but also morphologically and functionally distinct from the larval populations. Furthermore, we determined that the elimination-replacement process is dependent on and driven by the autophagy pathway. Altogether, we propose that the whole-body replacement of larval myofibers is an inherent yet previously unnoticed process driving organismic muscle growth during vertebrate post-embryonic development.

Valosin-Containing Protein (VCP): A Review of Its Diverse Molecular Functions and Clinical Phenotypes

In this review we examine the functionally diverse ATPase associated with various cellular activities (AAA-ATPase), valosin-containing protein (VCP/p97), its molecular functions, the mutational landscape of VCP and the phenotypic manifestation of VCP disease. VCP is crucial to a multitude of cellular functions including protein quality control, endoplasmic reticulum-associated degradation (ERAD), autophagy, mitophagy, lysophagy, stress granule formation and clearance, DNA replication and mitosis, DNA damage response including nucleotide excision repair, ATM- and ATR-mediated damage response, homologous repair and non-homologous end joining. VCP variants cause multisystem proteinopathy, and pathology can arise in several tissue types such as skeletal muscle, bone, brain, motor neurons, sensory neurons and possibly cardiac muscle, with the disease course being challenging to predict.

Neurofibromin 1 controls metabolic balance and Notch-dependent quiescence of murine juvenile myogenic progenitors

Patients affected by neurofibromatosis type 1 (NF1) frequently show muscle weakness with unknown etiology. Here we show that, in mice, Neurofibromin 1 (Nf1) is not required in muscle fibers, but specifically in early postnatal myogenic progenitors (MPs), where Nf1 loss led to cell cycle exit and differentiation blockade, depleting the MP pool resulting in reduced myonuclear accretion as well as reduced muscle stem cell numbers. This was caused by precocious induction of stem cell quiescence coupled to metabolic reprogramming of MPs impinging on glycolytic shutdown, which was conserved in muscle fibers. We show that a Mek/Erk/NOS pathway hypersensitizes Nf1-deficient MPs to Notch signaling, consequently, early postnatal Notch pathway inhibition ameliorated premature quiescence, metabolic reprogramming and muscle growth. This reveals an unexpected role of Ras/Mek/Erk signaling supporting postnatal MP quiescence in concert with Notch signaling, which is controlled by Nf1 safeguarding coordinated muscle growth and muscle stem cell pool establishment. Furthermore, our data suggest transmission of metabolic reprogramming across cellular differentiation, affecting fiber metabolism and function in NF1.

Satellite cells in the growth and maintenance of muscle

Embryonic skeletal muscle growth is contingent upon a population of somite derived satellite cells, however, the contribution of these cells to early postnatal skeletal muscle growth remains relatively high. As prepubertal postnatal development proceeds, the activity and contribution of satellite cells to skeletal muscle growth diminishes. Eventually, at around puberty, a population of satellite cells escapes terminal commitment, continues to express the paired box transcription factor Pax7, and reside in a quiescent state orbiting the myofiber periphery adjacent to the basal lamina. After adolescence, some satellite cell contributions to muscle maintenance and adaptation occur, however, their necessity is reduced relative to embryonic, early postnatal, and prepubertal growth.

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

The mitochondrial ATP-dependent potassium channel (mitoKATP) controls skeletal muscle structure and function

MitoKATP is a channel of the inner mitochondrial membrane that controls mitochondrial K+ influx according to ATP availability. Recently, the genes encoding the pore-forming (MITOK) and the regulatory ATP-sensitive (MITOSUR) subunits of mitoKATP were identified, allowing the genetic manipulation of the channel. Here, we analyzed the role of mitoKATP in determining skeletal muscle structure and activity. Mitok-/- muscles were characterized by mitochondrial cristae remodeling and defective oxidative metabolism, with consequent impairment of exercise performance and altered response to damaging muscle contractions. On the other hand, constitutive mitochondrial K+ influx by MITOK overexpression in the skeletal muscle triggered overt mitochondrial dysfunction and energy default, increased protein polyubiquitination, aberrant autophagy flux, and induction of a stress response program. MITOK overexpressing muscles were therefore severely atrophic. Thus, the proper modulation of mitoKATP activity is required for the maintenance of skeletal muscle homeostasis and function.

Advances in the discovery of genetic elements underlying longissimus dorsi muscle growth and development in the pig

As a major source of protein in human diets, pig meat plays a crucial role in ensuring global food security. Key determinants of meat production refer to the chemical and physical compositions or characteristics of muscle fibers, such as the number, hypertrophy potential, fiber-type conversion and intramuscular fat deposition. However, the growth and formation of muscle fibers comprises a complex process under spatio-temporal regulation, that is, the intermingled and concomitant proliferation, differentiation, migration and fusion of myoblasts. Recently, with the fast and continuous development of next-generation sequencing technology, the integration of quantitative trait loci mapping with genome-wide association studies (GWAS) has greatly helped animal geneticists to discover and explore thousands of functional or causal genetic elements underlying muscle growth and development. However, owing to the underlying complex molecular mechanisms, challenges to in-depth understanding and utilization remain, and the cost of large-scale sequencing, which requires integrated analyses of high-throughput omics data, is high. In this review, we mainly elaborate on research advances in integrative analyses (e.g. GWAS, omics) for identifying functional genes or genomic elements for longissimus dorsi muscle growth and development for different pig breeds, describing several successful transcriptome analyses and functional genomics cases, in an attempt to provide some perspective on the future functional annotation of genetic elements for muscle growth and development in pigs.

White epaxial muscle aerobic and anaerobic potential and muscle fiber structure in surface and cave morphotypes of the Mexican cavefish (Astyanax mexicanus)

Proper muscle function and muscle fiber structures that match the environmental demands of organisms are imperative to their success in any ecosystem. The Mexican cavefish, Astyanax mexicanus, has two morphotypes: an obligate cave-dwelling form that lives in thermally insulated caves and an O2 poor environment, and a surface form that lives in a more thermally variable, but O2 rich river environment. As environment can determine physiological adaptations, it is of interest to compare the aerobic and anaerobic metabolic profiles of white muscle metabolism in both morphotypes of this species, as well as their muscle structures. Here, we used white muscle of both morphotypes of the Mexican cavefish to determine citrate synthase (CS) activity as a measure of aerobic potential, and lactate concentration as a measure of anaerobic potential at three different chronic acclimation temperatures (14°C, 25°C, and 31°C). By examining aerobic and anaerobic potential in both morphs, we sought to link environmental thermal flexibility to muscle metabolism. We found that the surface morphotype had higher CS activity and lower lactate concentration, suggesting an overall more efficient usage of aerobic metabolism; whereas the cave morphotype showed lower CS activity and higher lactate concentration, suggesting a stronger reliance on anaerobic pathways. We also measured white muscle histological variables that have been previously linked to whole-animal metabolism: fiber diameter, number of nuclei per mm of fiber and myonuclear domain (MND) of both morphotypes at 25°C to examine cell-level differences in muscle morphology. However, we found no differences in fiber diameter, number of nuclei per mm of fiber or MND between the two morphotypes. Thus, although the cellular morphology is similar in these species, the environmental differences in the evolution of the two morphs has led to differences in their metabolic profiles.

Lineage tracing of newly accrued nuclei in skeletal myofibers uncovers distinct transcripts and interplay between nuclear populations

Multinucleated skeletal muscle cells have an obligatory need to acquire additional nuclei through fusion with activated skeletal muscle stem cells when responding to both developmental and adaptive growth stimuli. A fundamental question in skeletal muscle biology has been the reason underlying this need for new nuclei in syncytial cells that already harbor hundreds of nuclei. To begin to answer this long-standing question, we utilized nuclear RNA-sequencing approaches and developed a lineage tracing strategy capable of defining the transcriptional state of recently fused nuclei and distinguishing this state from that of pre-existing nuclei. Our findings reveal the presence of conserved markers of newly fused nuclei both during development and after a hypertrophic stimulus in the adult. However, newly fused nuclei also exhibit divergent gene expression that is determined by the myogenic environment to which they fuse. Moreover, accrual of new nuclei through fusion is required for nuclei already resident in adult myofibers to mount a normal transcriptional response to a load-inducing stimulus. We propose a model of mutual regulation in the control of skeletal muscle development and adaptations, where newly fused and pre-existing myonuclear populations influence each other to maintain optimal functional growth.

Conserved chamber-specific polyploidy maintains heart function in Drosophila

Developmentally programmed polyploidy (whole-genome duplication) of cardiomyocytes is common across evolution. Functions of such polyploidy are essentially unknown. Here, in both Drosophila larvae and human organ donors, we reveal distinct polyploidy levels in cardiac organ chambers. In Drosophila, differential growth and cell cycle signal sensitivity leads the heart chamber to reach a higher ploidy/cell size relative to the aorta chamber. Cardiac ploidy-reduced animals exhibit reduced heart chamber size, stroke volume and cardiac output, and acceleration of circulating hemocytes. These Drosophila phenotypes mimic human cardiomyopathies. Our results identify productive and likely conserved roles for polyploidy in cardiac chambers and suggest that precise ploidy levels sculpt many developing tissues. These findings of productive cardiomyocyte polyploidy impact efforts to block developmental polyploidy to improve heart injury recovery.

Regenerating Myofibers after an Acute Muscle Injury: What Do We Really Know about Them?

Injury to skeletal muscle through trauma, physical activity, or disease initiates a process called muscle regeneration. When injured myofibers undergo necrosis, muscle regeneration gives rise to myofibers that have myonuclei in a central position, which contrasts the normal, peripheral position of myonuclei. Myofibers with central myonuclei are called regenerating myofibers and are the hallmark feature of muscle regeneration. An important and underappreciated aspect of muscle regeneration is the maturation of regenerating myofibers into a normal sized myofiber with peripheral myonuclei. Strikingly, very little is known about processes that govern regenerating myofiber maturation after muscle injury. As knowledge of myofiber formation and maturation during embryonic, fetal, and postnatal development has served as a foundation for understanding muscle regeneration, this narrative review discusses similarities and differences in myofiber maturation during muscle development and regeneration. Specifically, we compare and contrast myonuclear positioning, myonuclear accretion, myofiber hypertrophy, and myofiber morphology during muscle development and regeneration. We also discuss regenerating myofibers in the context of different types of myofiber necrosis (complete and segmental) after muscle trauma and injurious contractions. The overall goal of the review is to provide a framework for identifying cellular and molecular processes of myofiber maturation that are unique to muscle regeneration.

Scaling of nuclear numbers and their spatial arrangement in skeletal muscle cell size regulation

Many cells display considerable functional plasticity and depend on the regulation of numerous organelles and macromolecules for their maintenance. In large cells, organelles also need to be carefully distributed to supply the cell with essential resources and regulate intracellular activities. Having multiple copies of the largest eukaryotic organelle, the nucleus, epitomizes the importance of scaling gene products to large cytoplasmic volumes in skeletal muscle fibers. Scaling of intracellular constituents within mammalian muscle fibers is, however, poorly understood, but according to the myonuclear domain hypothesis, a single nucleus supports a finite amount of cytoplasm and is thus postulated to act autonomously, causing the nuclear number to be commensurate with fiber volume. In addition, the orderly peripheral distribution of myonuclei is a hallmark of normal cell physiology, as nuclear mispositioning is associated with impaired muscle function. Because underlying structures of complex cell behaviors are commonly formalized by scaling laws and thus emphasize emerging principles of size regulation, the work presented herein offers more of a unified conceptual platform based on principles from physics, chemistry, geometry, and biology to explore cell size-dependent correlations of the largest mammalian cell by means of scaling.

Overlapping functions of SIX homeoproteins during embryonic myogenesis

Four SIX homeoproteins display a combinatorial expression throughout embryonic developmental myogenesis and they modulate the expression of the myogenic regulatory factors. Here, we provide a deep characterization of their role in distinct mouse developmental territories. We showed, at the hypaxial level, that the Six1:Six4 double knockout (dKO) somitic precursor cells adopt a smooth muscle fate and lose their myogenic identity. At the epaxial level, we demonstrated by the analysis of Six quadruple KO (qKO) embryos, that SIX are required for fetal myogenesis, and for the maintenance of PAX7+ progenitor cells, which differentiated prematurely and are lost by the end of fetal development in qKO embryos. Finally, we showed that Six1 and Six2 are required to establish craniofacial myogenesis by controlling the expression of Myf5. We have thus described an unknown role for SIX proteins in the control of myogenesis at different embryonic levels and refined their involvement in the genetic cascades operating at the head level and in the genesis of myogenic stem cells.

MuSK-BMP signaling in adult muscle stem cells maintains quiescence and regulates myofiber size

A central question in the biology of adult stem cells is elucidating the signaling pathways regulating their dynamics and function in diverse physiological and age-related contexts. Adult muscle stem cells (Satellite Cells; SCs) are generally quiescent but can activate and contribute to muscle homeostasis and repair. Here we tested the role of the MuSK-BMP pathway in regulating adult SC quiescence and myofiber size. We attenuated MuSK-BMP signaling by deletion of the BMP-binding MuSK Ig3 domain ('ΔIg3-MuSK') and studied the fast TA and EDL muscles. In germ line mutants at 3 months of age SC and myonuclei numbers as well as myofiber size were comparable in ΔIg3-MuSK and WT animals. However, in 5-month-old ΔIg3-MuSK animals SC density was decreased while myofiber size, myonuclear number and grip strength were increased - indicating that SCs had activated and productively fused into the myofibers over this interval. Notably, myonuclear domain size was conserved. Following injury, the mutant muscle fully regenerated with restoration of myofiber size and SC pool to WT levels, indicating that ΔIg3-MuSK SCs maintain full stem cell function. Conditional expression of ΔIg3-MuSK in adult SCs showed that the MuSK-BMP pathway regulates quiescence and myofiber size in a cell autonomous fashion. Transcriptomic analysis revealed that SCs from uninjured ΔIg3-MuSK mice exhibit signatures of activation, including elevated Notch and epigenetic signaling. We conclude that the MuSK-BMP pathway regulates SC quiescence and myofiber size in a cell autonomous, age-dependent manner. Targeting MuSK-BMP signaling in muscle stem cells thus emerges a therapeutic strategy for promoting muscle growth and function in the settings of injury, disease, and aging.

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.

Conserved Chamber-Specific Polyploidy Maintains Heart Function in Drosophila

Developmentally programmed polyploidy (whole-genome-duplication) of cardiomyocytes is common across evolution. Functions of such polyploidy are essentially unknown. Here, we reveal roles for precise polyploidy levels in cardiac tissue. We highlight a conserved asymmetry in polyploidy level between cardiac chambers in Drosophila larvae and humans. In Drosophila , differential Insulin Receptor (InR) sensitivity leads the heart chamber to reach a higher ploidy/cell size relative to the aorta chamber. Cardiac ploidy-reduced animals exhibit reduced heart chamber size, stroke volume, cardiac output, and acceleration of circulating hemocytes. These Drosophila phenotypes mimic systemic human heart failure. Using human donor hearts, we reveal asymmetry in nuclear volume (ploidy) and insulin signaling between the left ventricle and atrium. Our results identify productive and likely conserved roles for polyploidy in cardiac chambers and suggest precise ploidy levels sculpt many developing tissues. These findings of productive cardiomyocyte polyploidy impact efforts to block developmental polyploidy to improve heart injury recovery.

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.

A juvenile climbing exercise establishes a muscle memory boosting the effects of exercise in adult rats

AIM

Investigate whether juvenile exercise could induce a long-term muscle memory, boosting the effects of exercise in adults.

METHODS

We devised a 5-week climbing exercise scheme with food reward administered to male juvenile rats (post-natal week 4-9). Subsequently, the animals were subjected to 10 weeks of detraining (week 9-19) without climbing and finally retraining during week 19-21.

RESULTS

The juvenile exercise increased fiber cross-sectional area (fCSA) by 21% (p = 0.0035), boosted nuclear accretion by 13% (p = 0.057), and reduced intraperitoneal fat content by 28% (p = 0.007) and body weight by 9% (p = 0.001). During detraining, the fCSA became similar in the animals that had been climbing compared to naive controls, but the elevated number of myonuclei induced by the climbing were maintained (15%, p = 0.033). When the naive rats were subjected to 2 weeks of adult exercise there was little effect on fCSA, while the previously trained rats displayed an increase of 19% (p = 0.0007). Similarly, when the rats were subjected to unilateral surgical overload in lieu of the adult climbing exercise, the increase in fCSA was 20% (p = 0.0039) in the climbing group, while there was no significant increase in naive rats when comparing to the contralateral leg.

CONCLUSION

This demonstrates that juvenile exercise can establish a muscle memory boosting the effects of adult exercise. The juvenile climbing exercise with food reward also led to leaner animals with lower body weight. These differences were to some extent maintained throughout the adult detraining period in spite of all animals being fed ad libitum, indicating a form of body weight memory.

All for One and One for All: Regenerating Skeletal Muscle

Despite the evolutionary loss of tissue regenerative potential, robust skeletal muscle repair processes are largely retained even in higher vertebrates. In mammals, the skeletal muscle regeneration program is driven by resident stem cells termed satellite cells, guided by the coordinated activity of multiple intrinsic and extrinsic factors and other cell types. A thorough understanding of muscle repair mechanisms is crucial not only for combating skeletal myopathies, but for its prospective aid in devising therapeutic strategies to endow regenerative potential on otherwise regeneration-deficient organs. In this review, we discuss skeletal muscle regeneration from an evolutionary perspective, summarize the current knowledge of cellular and molecular mechanisms, and highlight novel paradigms of muscle repair revealed by explorations of the recent decade.

Skeletal muscle and metabolic flexibility in response to changing energy demands in wild birds

Phenotypically plastic responses of animals to adjust to environmental variation are pervasive. Reversible plasticity (i.e., phenotypic flexibility), where adult phenotypes can be reversibly altered according to prevailing environmental conditions, allow for better matching of phenotypes to the environment and can generate fitness benefits but may also be associated with costs that trade-off with capacity for flexibility. Here, we review the literature on avian metabolic and muscle plasticity in response to season, temperature, migration and experimental manipulation of flight costs, and employ an integrative approach to explore the phenotypic flexibility of metabolic rates and skeletal muscle in wild birds. Basal (minimum maintenance metabolic rate) and summit (maximum cold-induced metabolic rate) metabolic rates are flexible traits in birds, typically increasing with increasing energy demands. Because skeletal muscles are important for energy use at the organismal level, especially to maximum rates of energy use during exercise or shivering thermogenesis, we consider flexibility of skeletal muscle at the tissue and ultrastructural levels in response to variations in the thermal environment and in workloads due to flight exercise. We also examine two major muscle remodeling regulatory pathways: myostatin and insulin-like growth factor -1 (IGF-1). Changes in myostatin and IGF-1 pathways are sometimes, but not always, regulated in a manner consistent with metabolic rate and muscle mass flexibility in response to changing energy demands in wild birds, but few studies have examined such variation so additional study is needed to fully understand roles for these pathways in regulating metabolic flexibility in birds. Muscle ultrastrutural variation in terms of muscle fiber diameter and associated myonuclear domain (MND) in birds is plastic and highly responsive to thermal variation and increases in workload, however, only a few studies have examined ultrastructural flexibility in avian muscle. Additionally, the relationship between myostatin, IGF-1, and satellite cell (SC) proliferation as it relates to avian muscle flexibility has not been addressed in birds and represents a promising avenue for future study.

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.

ATAC-Seq of a Single Myofiber from Mus musculus

Chromatin accessibility is a key determinant of gene expression that can be altered under different physiological and disease conditions. Skeletal muscle is made up of myofibers that are highly plastic and adaptive. Therefore, assessing the genome-wide chromatin state of myofibers under various conditions is very important to gain insight into the epigenetic state of myonuclei. The rigid nature of myofibers, as well as the low number of myonuclei that they contain, have rendered genome-wide studies with myofibers challenging. In recent years, ATAC-Seq from whole muscle and single nucleus ATAC-Seq have been performed. However, these techniques cannot distinguish between different fiber and cell types present in the muscle. In addition, due to the limited depth capacity obtained from single nucleus ATAC-Seq, an extensive comparative analysis cannot be performed. Here, we introduce a protocol where we combine the isolation of a single myofiber with OMNI ATAC-Seq. This protocol allows for genome-wide analysis of accessible chromatin regions of a selected single myofiber at a sufficient depth for comparative analysis under various physiological and disease conditions. This protocol can also allow for a specific myofiber to be selected, such as a regenerating myofiber. In the future, this protocol can help identify global changes in chromatin state under various conditions, as well as between different types of myofibers. Graphical abstract.

Impaired activity of the fusogenic micropeptide Myomixer causes myopathy resembling Carey-Fineman-Ziter syndrome

Skeletal muscle fibers contain hundreds of nuclei, which increase the overall transcriptional activity of the tissue and perform specialized functions. Multinucleation occurs through myoblast fusion, mediated by the muscle fusogens Myomaker (MYMK) and Myomixer (MYMX). We describe a human pedigree harboring a recessive truncating variant of the MYMX gene that eliminates an evolutionarily conserved extracellular hydrophobic domain of MYMX, thereby impairing fusogenic activity. Homozygosity of this human variant resulted in a spectrum of abnormalities that mimicked the clinical presentation of Carey-Fineman-Ziter syndrome (CFZS), caused by hypomorphic MYMK variants. Myoblasts generated from patient-derived induced pluripotent stem cells displayed defective fusion, and mice bearing the human MYMX variant died perinatally due to muscle abnormalities. In vitro assays showed that the human MYMX variant conferred minimal cell-cell fusogenicity, which could be restored with CRISPR/Cas9-mediated base editing, thus providing therapeutic potential for this disorder. Our findings identify MYMX as a recessive, monogenic human disease gene involved in CFZS, and provide new insights into the contribution of myoblast fusion to neuromuscular diseases.

Positional Context of Myonuclear Transcription During Injury-Induced Muscle Regeneration

Fundamental aspects underlying downstream processes of skeletal muscle regeneration, such as myonuclear positioning and transcription are poorly understood. This investigation begins to address deficiencies in knowledge by examining the kinetics of myonuclear accretion, positioning, and global transcription during injury-induced muscle regeneration in mice. We demonstrate that myonuclear accretion plateaus within 7 days of an injury and that the majority (∼70%) of myonuclei are centrally aligned in linear arrays (nuclear chains) throughout the course of regeneration. Relatively few myonuclei were found in a peripheral position (∼20%) or clustered (∼10%) together during regeneration. Importantly, transcriptional activity of individual myonuclei in nuclear chains was high, and greater than that of peripheral or clustered myonuclei. Transcription occurring primarily in nuclear chains elevated the collective transcriptional activity of regenerating myofibers during the later stage of regeneration. Importantly, the number of myonuclei in chains and their transcriptional activity were statistically correlated with an increase in myofiber size during regeneration. Our findings demonstrate the positional context of transcription during regeneration and highlight the importance of centralized nuclear chains in facilitating hypertrophy of regenerating myofibers after injury.

Insights into muscle stem cell dynamics during postnatal development

During development, resident stem cell populations contribute to the growth and maturation of tissue and organs. In skeletal muscle, muscle stem cells, or satellite cells (SCs), are responsible for the maturation of postnatal myofibers. However, the role SCs play in later stages of postnatal growth, and thus, when they enter a mature quiescent state is controversial. Here, we discuss the current literature regarding the role SCs play in all stages of postnatal growth, from birth to puberty onset to young adulthood. We additionally highlight the implications of SC loss or dysfunction during developmental stages, both in the context of experimental paradigms and disease settings.

Regulation of the myoblast fusion reaction for muscle development, regeneration, and adaptations

Fusion of plasma membranes is essential for skeletal muscle development, regeneration, exercise-induced adaptations, and results in a cell that contains hundreds to thousands of nuclei within a shared cytoplasm. The differentiation process in myocytes culminates in their fusion to form a new myofiber or fusion to an existing myofiber thereby contributing more synthetic material to the syncytium. The choice for two cells to fuse and become one could be a dangerous event if the two cells are not committed to an allied function. Thus, fusion events are highly regulated with positive and negative factors to fine-tune the process, and requires muscle-specific fusogens (Myomaker and Myomerger) as well as general cellular machinery to achieve the union of membranes. While a unified vertebrate myoblast fusion pathway is not yet established, recent discoveries should make this pursuit attainable. Not only does myocyte fusion impact the normal biology of skeletal muscle, but new evidence indicates dysregulation of the process impacts pathologies of skeletal muscle. Here, I will highlight the molecular players and biochemical mechanisms that drive fusion events in muscle, and discuss how this key myogenic process impacts skeletal muscle diseases.

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.

Application of ATAC-Seq for genome-wide analysis of the chromatin state at single myofiber resolution

Myofibers are the main components of skeletal muscle, which is the largest tissue in the body. Myofibers are highly adaptive and can be altered under different biological and disease conditions. Therefore, transcriptional and epigenetic studies on myofibers are crucial to discover how chromatin alterations occur in the skeletal muscle under different conditions. However, due to the heterogenous nature of skeletal muscle, studying myofibers in isolation proves to be a challenging task. Single-cell sequencing has permitted the study of the epigenome of isolated myonuclei. While this provides sequencing with high dimensionality, the sequencing depth is lacking, which makes comparisons between different biological conditions difficult. Here, we report the first implementation of single myofiber ATAC-Seq, which allows for the sequencing of an individual myofiber at a depth sufficient for peak calling and for comparative analysis of chromatin accessibility under various physiological and disease conditions. Application of this technique revealed significant differences in chromatin accessibility between resting and regenerating myofibers, as well as between myofibers from a mouse model of Duchenne Muscular Dystrophy (mdx) and wild-type (WT) counterparts. This technique can lead to a wide application in the identification of chromatin regulatory elements and epigenetic mechanisms in muscle fibers during development and in muscle-wasting diseases.

Automated cross-sectional analysis of trained, severely atrophied and recovering rat skeletal muscles using MyoVision 2.0

The number of myonuclei within a muscle fiber is an important factor in muscle growth, but its regulation during muscle adaptation is not well understood. We aimed to elucidate the timecourse of myonuclear dynamics during endurance training, loaded and concentric resistance training, and nerve silencing-induced disuse atrophy with subsequent recovery. We modified tibialis anterior muscle activity in free-living rats with electrical stimulation from implantable pulse generators, or with implantable osmotic pumps delivering tetrodotoxin (TTX) to silence the motor nerve without transection. We used the updated, automated software MyoVision to measure fiber type-specific responses in whole tibialis anterior cross-sections (~8000 fibers each). Seven days of continuous low frequency stimulation (CLFS) reduced muscle mass (-12%), increased slower myosin isoforms and reduced IIX/IIB fibers (-32%) and substantially increased myonuclei especially in IIX/IIB fibers (55.5%). High load resistance training (Spillover), produced greater hypertrophy (~16%) in muscle mass and fiber cross-sectional area (CSA) than low load resistance training (concentric, ~6%) and was associated with myonuclear addition in all fiber types (35-46%). TTX-induced nerve silencing resulted in progressive loss in muscle mass, fiber CSA, and myonuclei per fiber cross-section (-50.7%, -53.7%, -40.7%, respectively at 14 days). Myonuclear loss occurred in a fiber type-independent manner, but subsequent recovery during voluntary habitual activity suggested that type IIX/IIB fibers contained more new myonuclei during recovery from severe atrophy. This study demonstrates the power and accuracy provided by the updated MyoVision software and introduces new models for studying myonuclear dynamics in training, detraining, retraining, repeated disuse, and recovery.

Lack of variation in nuclear DNA content in avian muscle

The avian pectoralis muscle demonstrates plasticity with regard to size, so that temperate birds facing winter conditions or birds enduring a migration bout tend to have significant increases in the size and mass of this tissue due to muscular hypertrophy. Myonuclear domain (MND), the volume of cytoplasm a myonuclei services, in the pectoralis muscle of birds seems to be altered during thermal stress or changing seasons. However, there is no information available regarding muscle DNA content or ploidy level within the avian pectoralis. Changes in muscle DNA content can be used in this tissue to aid in size and mass changes. Here, we hypothesized that long-distance migrants or temperate residents would use the process of endoreduplication to aid in altering muscle size. Mostly contradictory to our hypotheses, we found no differences in the mean muscle DNA content in any of the 62 species of birds examined in this study. We also found no correlations between mean muscle DNA content and other muscle structural measurements, such as the number of nuclei per millimeter of fiber, myonuclear domain, and fiber cross-sectional area. Thus, while avian muscle seems more phenotypically plastic than mammalian muscle, the biological processes surrounding myonuclear function may be more closely related to those seen in mammals.

Master regulators of skeletal muscle lineage development and pluripotent stem cells differentiation

In vertebrates, the skeletal muscles of the body and their associated stem cells originate from muscle progenitor cells, during development. The specification of the muscles of the trunk, head and limbs, relies on the activity of distinct genetic hierarchies. The major regulators of trunk and limb muscle specification are the paired-homeobox transcription factors PAX3 and PAX7. Distinct gene regulatory networks drive the formation of the different muscles of the head. Despite the redeployment of diverse upstream regulators of muscle progenitor differentiation, the commitment towards the myogenic fate requires the expression of the early myogenic regulatory factors MYF5, MRF4, MYOD and the late differentiation marker MYOG. The expression of these genes is activated by muscle progenitors throughout development, in several waves of myogenic differentiation, constituting the embryonic, fetal and postnatal phases of muscle growth. In order to achieve myogenic cell commitment while maintaining an undifferentiated pool of muscle progenitors, several signaling pathways regulate the switch between proliferation and differentiation of myoblasts. The identification of the gene regulatory networks operating during myogenesis is crucial for the development of in vitro protocols to differentiate pluripotent stem cells into myoblasts required for regenerative medicine.

Effects of acute temperature increases on House sparrow (Passer domesticus) pectoralis muscle myonuclear domain

With rapid climate change, heat wave episodes have become more intense and more frequent. This poses a significant threat to animals, and forces them to manage these physiologically challenging conditions by adapting and/or moving. As an invasive species with a large niche breadth, House sparrows (Passer domesticus) exhibit high phenotypic flexibility that caters to seasonal changes in function and metabolism. For example, their pectoral muscle complex exhibits size and mass plasticity with winter and summer acclimation. Here, we investigated the effects of acute whole-organism heat stress to 43°C on cellular-level changes in House sparrow pectoralis muscle myonuclear domain (MND), the volumetric portion each nucleus is responsible for, that have gone overlooked in the current literature. House sparrows were separated into a control group, a heat-shocked group subjected to thermal stress at 43°C for 24 h, and a recovery group that was returned to room temperature for 24 h after experiencing the same temperature treatment. Here, we found that heat-shocked and recovery groups demonstrated a decrease in number of nuclei per millimeter of fiber and increase in MND, when compared with the control. We also found a significant positive correlation between fiber diameter and MND in the recovery group, suggesting the possibility that nuclei number constrains the extent of muscle fiber size. Together, these results show that acute heat shock alters House sparrow pectoralis muscle cellular physiology in a rigid way that could prove detrimental to long-term muscle integrity and performance.

Do thermal acclimation and an acute heat challenge alter myonuclear domain of control- and fast-growing quail?

Efforts to determine physiological traits that may render species resilient or susceptible to changing global temperatures have accelerated in recent years. Temperature is of critical importance to biological function; thus, climate change has the potential to severely affect all levels of biological organization in many species. For example, increases in environmental temperatures may alter muscle structure and function in birds. Myonuclear domain (MND), an under-studied aspect of avian muscle physiology that changes in response to thermal stress, is defined as the amount of cytoplasm within a muscle fiber that each nucleus is responsible for servicing. Here, we used two random bred lines of Japanese quail (Coturnix japonica) representing examples of control and fast growth rates. We used a factorial design to administer four treatment combinations to each line - an initial period of either heat-stress acclimation (Acclimation) or no acclimation (Not acclimated) followed by either a heat-stress challenge (HS) or no challenge (NC) after week 8 of age - to determine the effects of thermal acclimation and acute thermal stress on quail MND. We found a significant interaction between line * final treatment with fast-growing, HS birds demonstrating the lowest numbers of nuclei per mm of fiber, and Acclimated control-growing birds showing the highest numbers of nuclei per mm of fiber. There was a significant effect of line on MND with the fast-growing line having larger MNDs. Initial treatment with Not Acclimated birds showed larger MNDs. Additionally, control growing quail demonstrated positive correlations with fiber size, whereas fast growing quail did not. This may mean that nuclei in larger fibers of fast-growing quail may be functioning maximally, and that increases in temperature may also demonstrate similar effects.

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.

Regulative Mechanism of Guanidinoacetic Acid on Skeletal Muscle Development and Its Application Prospects in Animal Husbandry: A Review

Guanidinoacetic acid is the direct precursor of creatine and its phosphorylated derivative phosphocreatine in the body. It is a safe nutritional supplement that can be used to promote muscle growth and development. Improving the growth performance of livestock and poultry and meat quality is the eternal goal of the animal husbandry, and it is also the common demand of today's society and consumers. A large number of experimental studies have shown that guanidinoacetic acid could improve the growth performance of animals, promote muscle development and improve the health of animals. However, the mechanism of how it affects muscle development needs to be further elucidated. This article discusses the physical and chemical properties of guanidinoacetic acid and its synthesis pathway, explores its mechanism of how it promotes muscle development and growth, and also classifies and summarizes the impact of its application in animal husbandry, providing a scientific basis for this application. In addition, this article also proposes future directions for the development of this substance.

JNK activation in TA and EDL muscle is load-dependent in rats receiving identical excitation patterns

As the excitation-contraction coupling is inseparable during voluntary exercise, the relative contribution of the mechanical and neural input on hypertrophy-related molecular signalling is still poorly understood. Herein, we use a rat in-vivo strength exercise model with an electrically-induced standardized excitation pattern, previously shown to induce a load-dependent increase in myonuclear number and hypertrophy, to study acute effects of load on molecular signalling. We assessed protein abundance and specific phosphorylation of the four protein kinases FAK, mTOR, p70S6K and JNK after 2, 10 and 28 min of a low- or high-load contraction, in order to assess the effects of load, exercise duration and muscle-type on their response to exercise. Specific phosphorylation of mTOR, p70S6K and JNK was increased after 28 min of exercise under the low- and high-load protocol. Elevated phosphorylation of mTOR and JNK was detectable already after 2 and 10 min of exercise, respectively, but greatest after 28 min of exercise, and JNK phosphorylation was highly load-dependent. The abundance of all four kinases was higher in TA compared to EDL muscle, p70S6K abundance was increased after exercise in a load-independent manner, and FAK and JNK abundance was reduced after 28 min of exercise in both the exercised and control muscles. In conclusion, the current study shows that JNK activation after a single resistance exercise is load-specific, resembling the previously reported degree of myonuclear accrual and muscle hypertrophy with repetition of the exercise stimulus.

Extraordinarily rapid proliferation of cultured muscle satellite cells from migratory birds

Migratory birds experience bouts of muscle growth and depletion as they prepare for, and undertake prolonged flight. Our studies of migratory bird muscle physiology in vitro led to the discovery that sanderling (Calidris alba) muscle satellite cells proliferate more rapidly than other normal cell lines. Here we determined the proliferation rate of muscle satellite cells isolated from five migratory species (sanderling; ruff, Calidris pugnax; western sandpiper, Calidris mauri; yellow-rumped warbler, Setophaga coronata; Swainson's thrush, Catharus ustulatus) from two families (shorebirds and songbirds) and with different migratory strategies. Ruff and sanderling satellite cells exhibited rapid proliferation, with population doubling times of 9.3 ± 1.3 and 11.4 ± 2 h, whereas the remaining species' cell doubling times were greater than or equal to 24 h. The results indicate that the rapid proliferation of satellite cells is not associated with total migration distance but may be related to flight bout duration and interact with lifespan.

Exercise/Resistance Training and Muscle Stem Cells

Skeletal muscle has attracted attention as endocrine organ, because exercise-dependent cytokines called myokines/exerkines are released from skeletal muscle and are involved in systemic functions. While, local mechanical loading to skeletal muscle by exercise or resistance training alters myofiber type and size and myonuclear number. Skeletal muscle-resident stem cells, known as muscle satellite cells (MuSCs), are responsible for the increased number of myonuclei. Under steady conditions, MuSCs are maintained in a mitotically quiescent state but exit from that state and start to proliferate in response to high physical activity. Alterations in MuSC behavior occur when myofibers are damaged, but the lethal damage to myofibers does not seem to evoke mechanical loading-dependent MuSC activation and proliferation. Given that MuSCs proliferate without damage, it is unclear how the different behaviors of MuSCs are controlled by different physical activities. Recent studies demonstrated that myonuclear number reflects the size of myofibers; hence, it is crucial to know the properties of MuSCs and the mechanism of myonuclear accretion by MuSCs. In addition, the elucidation of mechanical load-dependent changes in muscle resident cells, including MuSCs, will be necessary for the discovery of new myokines/exerkines and understating skeletal muscle diseases.

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.

Variability in vastus lateralis fiber type distribution, fiber size, and myonuclear content along and between the legs

Human skeletal muscle characteristics such as fiber type composition, fiber size, and myonuclear content are widely studied in clinical and sports-related contexts. Being aware of the methodological and biological variability of the characteristics is a critical aspect in study design and outcome interpretation, but comprehensive data on the variability of morphological features in human skeletal muscle are currently limited. Accordingly, in the present study, m. vastus lateralis biopsies (10 per subject) from young and healthy individuals, collected in a systematic manner, were analyzed for various characteristics using immunohistochemistry (n = 7) and SDS-PAGE (n = 25). None of the analyzed parameters, fiber type % (FT%), type I and II fiber cross-sectional area (fCSA), percentage fiber type area (fCSA%), myosin heavy chain composition (MyHC%), type IIX content, myonuclear content, or myonuclear domain, varied in a systematic manner longitudinally along the muscle or between the two legs. The average within-subject coefficient of variation for FT%, fCSA, fCSA%, and MyHC% ranged between 13% and 18% but was only 5% for fiber-specific myonuclear content, which reduced the variability for myonuclear domain size to 11%-12%. Pure type IIX fibers and type IIX MyHC were randomly distributed and present in <24% of the analyzed samples, with the average content being 0.1% and 1.1%, respectively. In conclusion, leg or longitudinal orientation does not seem to be an important aspect to consider when investigating human vastus lateralis characteristics. However, single muscle biopsies should preferably not be used when studying fiber type- and fiber size-related aspects, given the notable sample-to-sample variability.NEW & NOTEWORTHY This study provides a comprehensive analysis of the variability of key human skeletal muscle fiber characteristics in multiple sites along and between the m. vastus lateralis of healthy and active individuals. We found a notable but nonsystematic variability in fiber type and size, whereas myonuclear content was distinctively less variable, and the prevalence of type IIX fibers was random and very low. These data are important to consider when designing and interpreting studies including m. vastus lateralis biopsies.

The relationship between myonuclear number and protein synthesis in individual rat skeletal muscle fibres

Skeletal muscle has numerous nuclei within a cell. The nucleus is considered as the central organelle for muscle protein synthesis (MPS). However, it is unclear whether myonuclear number is associated with MPS capacity within the individual muscle fibres. Therefore, the purpose of the present study was to reveal the relationship between myonuclear number per unit muscle fibre length and MPS under basal and conditions of elevated MPS by high-intensity muscle contraction (HiMC) using an in vivo nascent protein labelling technique (SUnSET) in rodents. We found that myonuclear number was positively correlated with MPS in individual muscle fibres in the basal condition. Similarly, ribosomal protein S6 (rpS6) content, which is a rough estimate of ribosome content, was positively correlated with MPS. However, myonuclear number was not associated with rpS6 content. In contrast to the basal condition, when MPS was increased by acute HiMC, no correlation was observed between myonuclear number and MPS, but the association between rpS6 and MPS was maintained. Importantly, these observations indicate that the number of nuclei in individual myofibers is related only to MPS at rest. However, the ribosome content in individual fibres is related to MPS of individual myofibers both at rest and following HiMC.

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.

Communal living: the role of polyploidy and syncytia in tissue biology

Multicellular organisms are composed of tissues with diverse cell sizes. Whether a tissue primarily consists of numerous, small cells as opposed to fewer, large cells can impact tissue development and function. The addition of nuclear genome copies within a common cytoplasm is a recurring strategy to manipulate cellular size within a tissue. Cells with more than two genomes can exist transiently, such as in developing germlines or embryos, or can be part of mature somatic tissues. Such nuclear collectives span multiple levels of organization, from mononuclear or binuclear polyploid cells to highly multinucleate structures known as syncytia. Here, we review the diversity of polyploid and syncytial tissues found throughout nature. We summarize current literature concerning tissue construction through syncytia and/or polyploidy and speculate why one or both strategies are advantageous.

Myonuclear content regulates cell size with similar scaling properties in mice and humans

Muscle fibers are the largest cells in the body, and one of its few syncytia. Individual cell sizes are variable and adaptable, but what governs cell size has been unclear. We find that muscle fibers are DNA scarce compared to other cells, and that the nuclear number (N) adheres to the relationship N = aVb where V is the cytoplasmic volume. N invariably scales sublinearly to V (b < 1), making larger cells even more DNA scarce. N scales linearly to cell surface in adult humans, in adult and developing mice, and in mice with genetically reduced N, but in the latter the relationship eventually fails when they reach adulthood with extremely large myonuclear domains. Another exception is denervation-atrophy where nuclei are not eliminated. In conclusion, scaling exponents are remarkably similar across species, developmental stages and experimental conditions, suggesting an underlying scaling law where DNA-content functions as a limiter of muscle cell size.