Radiation-Induced Damage to Prepubertal Pax7+ Skeletal Muscle Stem Cells Drives Lifelong Deficits in Myofiber Size and Nuclear Number

Unraveling radiation-induced skeletal muscle damage: Insights from a 3D human skeletal muscle organoid model

BACKGROUND

Three-dimensional (3D) organoids derived from human pluripotent stem cells (hPSCs) have revolutionized in vitro tissue modeling, offering a unique opportunity to replicate physiological tissue organization and functionality. This study investigates the impact of radiation on skeletal muscle response using an innovative in vitro human 3D skeletal muscle organoids (hSMOs) model derived from hPSCs.

METHODS

The hSMOs model was established through a differentiation protocol faithfully recapitulating embryonic myogenesis and maturation via paraxial mesodermal differentiation of hPSCs. Key skeletal muscle characteristics were confirmed using immunofluorescent staining and RT-qPCR. Subsequently, the hSMOs were exposed to a clinically relevant dose of 2 Gy of radiation, and their response was analyzed using immunofluorescent staining and RNA-seq.

RESULTS

The hSMO model faithfully recapitulated embryonic myogenesis and maturation, maintaining key skeletal muscle characteristics. Following exposure to 2 Gy of radiation, histopathological analysis revealed deficits in hSMOs expansion, differentiation, and repair response across various cell types at early (30 min) and intermediate (18 h) time points post-radiation. Immunofluorescent staining targeting γH2AX and 53BP1 demonstrated elevated levels of foci per cell, particularly in PAX7+ cells, during early and intermediate time points, with a distinct kinetic pattern showing a decrease at 72 h. RNA-seq data provided comprehensive insights into the DNA damage response within the hSMOs.

CONCLUSIONS

Our findings highlight deficits in expansion, differentiation, and repair response in hSMOs following radiation exposure, enhancing our understanding of radiation effects on skeletal muscle and contributing to strategies for mitigating radiation-induced damage in this context.

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.

Dynamic equilibrium of skeletal muscle macrophage ontogeny in the diaphragm during homeostasis, injury, and recovery

The diaphragm is a unique skeletal muscle due to its continuous activation pattern during the act of breathing. The ontogeny of macrophages, pivotal cells for skeletal muscle maintenance and regeneration, is primarily based on two distinct origins: postnatal bone marrow-derived monocytes and prenatal embryonic progenitors. Here we employed chimeric mice to study the dynamics of these two macrophage populations under different conditions. Traditional chimeric mice generated through whole body irradiation showed virtually complete elimination of the original tissue-resident macrophage pool. We then developed a novel method which employs lead shielding to protect the diaphragm tissue niche from irradiation. This allowed us to determine that up to almost half of tissue-resident macrophages in the diaphragm can be maintained independently from bone marrow-derived monocytes under steady-state conditions. These findings were confirmed by long-term (5 months) parabiosis experiments. Acute diaphragm injury shifted the macrophage balance toward an overwhelming predominance of bone marrow (monocyte)-derived macrophages. However, there was a remarkable reversion to the pre-injury ontological landscape after diaphragm muscle recovery. This diaphragm shielding method permits analysis of the dynamics of macrophage origin and corresponding function under different physiological and pathological conditions. It may be especially useful for studying diseases which are characterized by acute or chronic injury of the diaphragm and accompanying inflammation.

Control of muscle satellite cell function by specific exercise-induced cytokines and their applications in muscle maintenance

Exercise is recognized to play an observable role in improving human health, especially in promoting muscle hypertrophy and intervening in muscle mass loss-related diseases, including sarcopenia. Recent rapid advances have demonstrated that exercise induces the release of abundant cytokines from several tissues (e.g., liver, muscle, and adipose tissue), and multiple cytokines improve the functions or expand the numbers of adult stem cells, providing candidate cytokines for alleviating a wide range of diseases. Muscle satellite cells (SCs) are a population of muscle stem cells that are mitotically quiescent but exit from the dormancy state to become activated in response to physical stimuli, after which SCs undergo asymmetric divisions to generate new SCs (stem cell pool maintenance) and commit to later differentiation into myocytes (skeletal muscle replenishment). SCs are essential for the postnatal growth, maintenance, and regeneration of skeletal muscle. Emerging evidence reveals that exercise regulates muscle function largely via the exercise-induced cytokines that govern SC potential, but this phenomenon is complicated and confusing. This review provides a comprehensive integrative overview of the identified exercise-induced cytokines and the roles of these cytokines in SC function, providing a more complete picture regarding the mechanism of SC homeostasis and rejuvenation therapies for skeletal muscle.

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.

Isolation, identification, and induced differentiation of satellite cells from skeletal muscle of adult tree shrews

A method for the in vitro isolation, purification, identification, and induced differentiation of satellite cells from adult tree shrew skeletal muscle was established. The mixed enzyme digestion method and differential adhesion method were used to obtain skeletal muscle satellite cells, which were identified and induced to differentiate to verify their pluripotency. The use of a mixture of collagenase II, hyaluronidase IV, and DNase I is an efficient method for isolating adult tree shrew skeletal muscle satellite cells. The P3 generation of cells had good morphology, rapid proliferation, high viability, and an "S"-shaped growth curve. Reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence staining indicated that marker genes or proteins were expressed in skeletal muscle satellite cells. After myogenic differentiation was induced, multiple-nucleated myotubes were observed, and the MyHC protein was expressed. The expression of myogenic marker genes changed with the differentiation process. After the induction of adipogenic differentiation, orange-red lipid droplets were observed, and the expression of adipogenic marker genes increased gradually with the differentiation process. In summary, satellite cells from adult tree shrew skeletal muscle were successfully isolated using a mixed enzyme digestion method, and their potential for differentiation into myogenic and adipogenic cells was confirmed, laying a foundation for further in vitro study of tree shrew muscle damage.

Radiation induces long-term muscle fibrosis and promotes a fibrotic phenotype in fibro-adipogenic progenitors

BACKGROUND

Radiation-induced muscle pathology, characterized by muscle atrophy and fibrotic tissue accumulation, is the most common debilitating late effect of therapeutic radiation exposure particularly in juvenile cancer survivors. In healthy muscle, fibro/adipogenic progenitors (FAPs) are required for muscle maintenance and regeneration, while in muscle pathology FAPs are precursors for exacerbated extracellular matrix deposition. However, the role of FAPs in radiation-induced muscle pathology has not previously been explored.

METHODS

Four-week-old Male CBA or C57Bl/6J mice received a single dose (16 Gy) of irradiation (IR) to a single hindlimb with the shielded contralateral limb (CLTR) serving as a non-IR control. Mice were sacrificed 3, 7, 14 (acute IR response), and 56 days post-IR (long-term IR response). Changes in skeletal muscle morphology, myofibre composition, muscle niche cellular dynamics, DNA damage, proliferation, mitochondrial respiration, and metabolism and changes in progenitor cell fate where assessed.

RESULTS

Juvenile radiation exposure resulted in smaller myofibre cross-sectional area, particularly in type I and IIA myofibres (P < 0.05) and reduced the proportion of type I myofibres (P < 0.05). Skeletal muscle fibrosis (P < 0.05) was evident at 56 days post-IR. The IR-limb had fewer endothelial cells (P < 0.05) and fibro-adipogenic progenitors (FAPs) (P < 0.05) at 56 days post-IR. Fewer muscle satellite (stem) cells were detected at 3 and 56 days in the IR-limb (P < 0.05). IR induced FAP senescence (P < 0.05), increased their fibrogenic differentiation (P < 0.01), and promoted their glycolytic metabolism. Further, IR altered the FAP secretome in a manner that impaired muscle satellite (stem) cell differentiation (P < 0.05) and fusion (P < 0.05).

CONCLUSIONS

Our study suggests that following juvenile radiation exposure, FAPs contribute to long-term skeletal muscle atrophy and fibrosis. These findings provide rationale for investigating FAP-targeted therapies to ameliorate the negative late effects of radiation exposure in skeletal muscle.

Impacts of radiation exposure, hindlimb unloading, and recovery on murine skeletal muscle cell telomere length

Astronauts are exposed to harsh conditions, including cosmic radiation and microgravity. Spaceflight elongates human telomeres in peripheral blood, which shorten upon return to Earth and approach baseline levels during postflight recovery. Astronauts also encounter muscle atrophy, losing up to 20% loss of muscle mass on spaceflights. Telomere length changes in muscle cells of astronauts remain unexplored. This study investigates telomere alterations in grounded mice experiencing radiation exposure and muscle atrophy, via a hindlimb unloading spaceflight mimicking model. We find telomere lengthening is present in muscle stem cells and in myofiber nuclei, but not in muscle-resident endothelial cells. We further assessed telomere length in the model following hindlimb unloading recovery. We find that telomere length failed to return to baseline values. Our results suggest a role for telomeres in muscle acclimatization, which is relevant for the well-being of astronauts in space, and upon their return to Earth.

Ex vivo adult stem cell characterization from multiple muscles in ambulatory children with cerebral palsy during early development of contractures

Cerebral palsy (CP) is one of the most common conditions leading to lifelong childhood physical disability. Literature reported previously altered muscle properties such as lower number of satellite cells (SCs), with altered fusion capacity. However, these observations highly vary among studies, possibly due to heterogeneity in patient population, lack of appropriate control data, methodology and different assessed muscle. In this study we aimed to strengthen previous observations and to understand the heterogeneity of CP muscle pathology. Myogenic differentiation of SCs from the Medial Gastrocnemius (MG) muscle of patients with CP (n = 16, 3-9 years old) showed higher fusion capacity compared to age-matched typically developing children (TD, n = 13). Furthermore, we uniquely assessed cells of two different lower limb muscles and showed a decreased myogenic potency in cells from the Semitendinosus (ST) compared to the MG (TD: n = 3, CP: n = 6). Longitudinal assessments, one year after the first botulinum toxin treatment, showed slightly reduced SC representations and lower fusion capacity (n = 4). Finally, we proved the robustness of our data, by assessing in parallel the myogenic capacity of two samples from the same TD muscle. In conclusion, these data confirmed previous findings of increased SC fusion capacity from MG muscle of young patients with CP compared to age-matched TD. Further elaboration is reported on potential factors contributing to heterogeneity, such as assessed muscle, CP progression and reliability of primary outcome parameters.

Roadmap for precision preclinical x-ray radiation studies

This Roadmap paper covers the field of precision preclinical x-ray radiation studies in animal models. It is mostly focused on models for cancer and normal tissue response to radiation, but also discusses other disease models. The recent technological evolution in imaging, irradiation, dosimetry and monitoring that have empowered these kinds of studies is discussed, and many developments in the near future are outlined. Finally, clinical translation and reverse translation are discussed.

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

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

Resistance and endurance exercise training improves muscle mass and the inflammatory/fibrotic transcriptome in a rhabdomyosarcoma model

BACKGROUND

Rhabdomyosarcoma (RMS) is an aggressive soft tissue sarcoma that most often develops in children. Chemoradiation therapy is a standard treatment modality; however, the detrimental long-term skeletal muscle consequences of this therapy in juvenile cancer survivors include muscle atrophy and fibrosis resulting in decreased physical performance. Using a novel model of murine resistance and endurance exercise training, we investigate its role in preventing the long-term effects of juvenile RMS plus therapy.

METHODS

Four-week-old male (n = 10) and female (n = 10) C57Bl/6J mice were injected with M3-9-M RMS cell into the left gastrocnemius with the right limb serving as an internal control (CON). Mice received a systemic vincristine injection and then five doses of 4.8 Gy of gamma radiation localized to the left hindlimb (RMS + Tx). Mice were then randomly divided into either sedentary (SED) or resistance and endurance exercise training (RET) groups. Changes in exercise performance, body composition, myocellular adaptations and the inflammatory/fibrotic transcriptome were assessed.

RESULTS

RET improved endurance performance (P < 0.0001) and body composition (P = 0.0004) compared to SED. RMS + Tx resulted in significantly lower muscle weight (P = 0.015) and significantly smaller myofibre cross-sectional area (CSA) (P = 0.014). Conversely, RET resulted in significantly higher muscle weight (P = 0.030) and significantly larger Type IIA (P = 0.014) and IIB (P = 0.015) fibre CSA. RMS + Tx resulted in significantly more muscle fibrosis (P = 0.028), which was not prevented by RET. RMS + Tx resulted in significantly fewer mononuclear cells (P < 0.05) and muscle satellite (stem) cells (MuSCs) (P < 0.05) and significantly more immune cells (P < 0.05) than CON. RET resulted in significantly more fibro-adipogenic progenitors (P < 0.05), a trend for more MuSCs (P = 0.076) than SED and significantly more endothelial cells specifically in the RMS + Tx limb. Transcriptomic changes revealed significantly higher expression of inflammatory and fibrotic genes in RMS + Tx, which was prevented by RET. In the RMS + Tx model, RET also significantly altered expression of genes involved in extracellular matrix turnover.

CONCLUSIONS

Our study suggests that RET preserves muscle mass and performance in a model of juvenile RMS survivorship while partially restoring cellular dynamics and the inflammatory and fibrotic transcriptome.

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.

Physical Activity as the Best Supportive Care in Cancer: The Clinician's and the Researcher's Perspectives

Multidisciplinary supportive care, integrating the dimensions of exercise alongside oncological treatments, is now regarded as a new paradigm to improve patient survival and quality of life. Its impact is important on the factors that control tumor development, such as the immune system, inflammation, tissue perfusion, hypoxia, insulin resistance, metabolism, glucocorticoid levels, and cachexia. An increasing amount of research has been published in the last years on the effects of physical activity within the framework of oncology, marking the appearance of a new medical field, commonly known as "exercise oncology". This emerging research field is trying to determine the biological mechanisms by which, aerobic exercise affects the incidence of cancer, the progression and/or the appearance of metastases. We propose an overview of the current state of the art physical exercise interventions in the management of cancer patients, including a pragmatic perspective with tips for routine practice. We then develop the emerging mechanistic views about physical exercise and their potential clinical applications. Moving toward a more personalized, integrated, patient-centered, and multidisciplinary management, by trying to understand the different interactions between the cancer and the host, as well as the impact of the disease and the treatments on the different organs, this seems to be the most promising method to improve the care of cancer patients.

Short-term and bystander effects of radiation on murine submandibular glands

Many patients treated for head and neck cancers experience salivary gland hypofunction due to radiation damage. Understanding the mechanisms of cellular damage induced by radiation treatment is important in order to design methods of radioprotection. In addition, it is crucial to recognize the indirect effects of irradiation and the systemic responses that may alter saliva secretion. In this study, radiation was delivered to murine submandibular glands (SMGs) bilaterally, using a 137Cs gamma ray irradiator, or unilaterally, using a small-animal radiation research platform (SARRP). Analysis at 3, 24 and 48 h showed dynamic changes in mRNA and protein expression in SMGs irradiated bilaterally. Unilateral irradiation using the SARRP caused similar changes in the irradiated SMGs, as well as significant off-target, bystander effects in the non-irradiated contralateral SMGs.

Quantitative Evaluation of the Reduced Capacity of Skeletal Muscle Hypertrophy after Total Body Irradiation in Relation to Stem/Progenitor Cells

The effects of total body irradiation (TBI) to the capacity of skeletal muscle hypertrophy were quantified using the compensatory muscle hypertrophy model. We additionally assessed the responses of stem and/or progenitor cells in the muscles. A single TBI of 9.0, 5.0 and 2.5 Gy was delivered to C57BL/6 mice. Bone marrow stromal cells were obtained from GFP-Tg mice, and were injected into the tail vein of the recipient mice (1 × 106 cells/mouse), for bone marrow transplantation (BMT). Five weeks after TBI, the mean GFP-chimerism in the blood was 96 ± 0.8% in the 9 Gy, 83 ± 3.9% in the 5 Gy, and 8.4 ± 3.4% in the 2.5 Gy groups. This implied that the impact of 2.5 Gy is quite low and unavailable as the BMT treatment. Six weeks after the TBI/BMT procedure, muscle hypertrophy was induced in the right plantaris muscle by surgical ablation (SA) of the synergist muscles (gastrocnemius and soleus), and the contralateral left side was preserved as a control. The muscle hypertrophy capacity significantly decreased by 95% in the 9 Gy, 48% in the 5 Gy, and 36% in the 2.5 Gy groups. Furthermore, stem/progenitor cells in the muscle were enzymatically isolated and fractionated into non-sorted bulk cells, CD45-/34-/29+ (Sk-DN), and CD45-/34+ (Sk-34) cells, and myogenic capacity was confirmed by the presence of Pax7+ and MyoD+ cells in culture. Myogenic capacity also declined significantly in the Bulk and Sk-DN cell groups in all three TBI conditions, possibly implying that skeletal muscles are more susceptible to TBI than bone marrow. However, interstitial Sk-34 cells were insusceptible to TBI, retaining their myogenic/proliferative capacity.

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.

Endurance exercise attenuates juvenile irradiation-induced skeletal muscle functional decline and mitochondrial stress

BACKGROUND

Radiotherapy is commonly used to treat childhood cancers and can have adverse effects on muscle function, but the underlying mechanisms have yet to be fully elucidated. We hypothesized that endurance exercise following radiation treatment would improve skeletal muscle function.

METHODS

We utilized the Small Animal Radiation Research Platform (SARRP) to irradiate juvenile male mice with a clinically relevant fractionated dose of 3× (every other day over 5 days) 8.2 Gy X-ray irradiation locally from the knee to footpad region of the right hindlimb. Mice were then singly housed for 1 month in cages equipped with either locked or free-spinning voluntary running wheels. Ex vivo muscle contractile function, RT-qPCR analyses, resting cytosolic and sarcoplasmic reticulum (SR) store Ca2+ levels, mitochondrial reactive oxygen species levels (MitoSOX), and immunohistochemical and biochemical analyses of muscle samples were conducted to assess the muscle pathology and the relative therapeutic impact of voluntary wheel running (VWR).

RESULTS

Irradiation reduced fast-twitch extensor digitorum longus (EDL) muscle-specific force by 27% compared to that of non-irradiated mice, while VWR post-irradiation improved muscle-specific force by 37%. Radiation treatment similarly reduced slow-twitch soleus muscle-specific force by 14% compared to that of non-irradiated mice, while VWR post-irradiation improved specific force by 18%. We assessed intracellular Ca2+ regulation, oxidative stress, and mitochondrial homeostasis as potential mechanisms of radiation-induced pathology and exercise-mediated rescue. We found a significant reduction in resting cytosolic Ca2+ concentration following irradiation in sedentary mice. Intriguingly, however, SR Ca2+ store content was increased in myofibers from irradiated mice post-VWR compared to mice that remained sedentary. We observed a 73% elevation in the overall protein oxidization in muscle post-irradiation, while VWR reduced protein nitrosylation by 35% and mitochondrial reactive oxygen species (ROS) production by 50%. Finally, we found that VWR significantly increased the expression of PGC1α at both the transcript and protein levels, consistent with an exercise-dependent increase in mitochondrial biogenesis.

CONCLUSIONS

Juvenile irradiation stunted muscle development, disrupted proper Ca2+ handling, damaged mitochondria, and increased oxidative and nitrosative stress, paralleling significant deficits in muscle force production. Exercise mitigated aberrant Ca2+ handling, mitochondrial homeostasis, and increased oxidative and nitrosative stress in a manner that correlated with improved skeletal muscle function after radiation.

The anti-aging protein Klotho affects early postnatal myogenesis by downregulating Jmjd3 and the canonical Wnt pathway

Modulating the number of muscle stems cells, called satellite cells, during early postnatal development produces long-term effects on muscle growth. We tested the hypothesis that high expression levels of the anti-aging protein Klotho in early postnatal myogenesis increase satellite cell numbers by influencing the epigenetic regulation of genes that regulate myogenesis. Our findings show that elevated klotho expression caused a transient increase in satellite cell numbers and slowed muscle fiber growth, followed by a period of accelerated muscle growth that leads to larger fibers. Klotho also transcriptionally downregulated the H3K27 demethylase Jmjd3, leading to increased H3K27 methylation and decreased expression of genes in the canonical Wnt pathway, which was associated with a delay in muscle differentiation. In addition, Klotho stimulation and Jmjd3 downregulation produced similar but not additive reductions in the expression of Wnt4, Wnt9a, and Wnt10a in myogenic cells, indicating that inhibition occurred through a common pathway. Together, our results identify a novel pathway through which Klotho influences myogenesis by reducing the expression of Jmjd3, leading to reductions in the expression of Wnt genes and inhibition of canonical Wnt signaling.

Cryopreserved Stem Cells Incur Damages Due To Terrestrial Cosmic Rays Impairing Their Integrity Upon Long-Term Storage

Stem cells have the capacity to ensure the renewal of tissues and organs. They could be used in the future for a wide range of therapeutic purposes and are preserved at liquid nitrogen temperature to prevent any chemical or biological activity up to several decades before their use. We show that the cryogenized cells accumulate damages coming from natural radiations, potentially inducing DNA double-strand breaks (DSBs). Such DNA damage in stem cells could lead to either mortality of the cells upon thawing or a mutation diminishing the therapeutic potential of the treatment. Many studies show how stem cells react to different levels of radiation; the effect of terrestrial cosmic rays being key, it is thus also important to investigate the effect of the natural radiation on the cryopreserved stem cell behavior over time. Our study showed that the cryostored stem cells totally shielded from cosmic rays had less DSBs upon long-term storage. This could have important implications on the long-term cryostorage strategy and quality control of different cell banks.

Muscle-specific functional deficits and lifelong fibrosis in response to paediatric radiotherapy and tumour elimination

BACKGROUND

As paediatric cancer survivors are living into adulthood, they suffer from the age-related, accelerated decline of functional skeletal muscle tissue, termed sarcopenia. With ionizing radiation (radiotherapy) at the core of paediatric cancer therapies, its direct and indirect effects can have lifelong negative impacts on paediatric growth and maintenance of skeletal muscle. Utilizing our recently developed preclinical rhabdomyosarcoma mouse model, we investigated the late effects of paediatric radiation treatment on skeletal muscles from late adolescent (8 weeks old) and middle-aged (16 months old) mice.

METHODS

Paediatric C57BL/6J male mice (3 weeks old) were injected with rhabdomyosarcoma cells into their right hindlimbs, and then fractionated irradiation (3 × 8.2 Gy) was administered to those limbs at 4 weeks old to eliminate the tumours. Radiation-alone and tumour-irradiated mice were assessed at either 8 weeks (3 weeks post-irradiation) or 16 months (14 months post-irradiation) of age for muscle physiology, myofibre characteristics, cell loss, histopathology, fibrosis, inflammatory gene expression, and fibrotic gene expression.

RESULTS

Mice that received only paediatric radiation demonstrated reduced muscle mass (-17%, P < 0.001), muscle physiological function (-25%, P < 0.01), muscle contractile kinetics (-25%, P < 0.05), satellite cell number (-45%, P < 0.05), myofibre cross-sectional area (-30%, P < 0.0001), and myonuclear number (-17%, P < 0.001). Paediatric radiation increased inflammatory gene expression, increased fibrotic gene expression, and induced extracellular matrix protein deposition (fibrosis) with tumour elimination exacerbating some phenotypes. Paediatric tumour-eliminated mice demonstrated exacerbated deficits to function (-20%, P < 0.05) and myofibre size (-17%, P < 0.001) in some muscles as well as further increases to inflammatory and fibrotic gene expression. Examining the age-related effects of paediatric radiotherapy in middle-aged mice, we found persistent myofibre atrophy (-20%, P < 0.01), myonuclear loss (-18%, P < 0.001), up-regulated inflammatory and fibrotic signalling, and lifelong fibrosis.

CONCLUSIONS

The results from this paediatric radiotherapy model are consistent and recapitulate the clinical and molecular features of accelerated sarcopenia, musculoskeletal frailty, and radiation-induced fibrosis experienced by paediatric cancer survivors. We believe that this preclinical mouse model is well poised for future mechanistic insights and therapeutic interventions that improve the quality of life for paediatric cancer survivors.

Muscle wasting in cancer: opportunities and challenges for exercise in clinical cancer trials

BACKGROUND

Low muscle in cancer is associated with an increase in treatment-related toxicities and is a predictor of cancer-related and all-cause mortality. The mechanisms of cancer-related muscle loss are multifactorial, including anorexia, hypogonadism, anaemia, inflammation, malnutrition, and aberrations in skeletal muscle protein turnover and metabolism.

METHODS

In this narrative review, we summarise relevant literature to (i) review the factors influencing skeletal muscle mass regulation, (ii) provide an overview of how cancer/treatments negatively impact these, (iii) review factors beyond muscle signalling that can impact the ability to participate in and respond to an exercise intervention to counteract muscle loss in cancer, and (iv) provide perspectives on critical areas of future research.

RESULTS

Despite the well-known benefits of exercise, there remains a paucity of clinical evidence supporting the impact of exercise in cancer-related muscle loss. There are numerous challenges to reversing muscle loss with exercise in clinical cancer settings, ranging from the impact of cancer/treatments on the molecular regulation of muscle mass, to clinical challenges in responsiveness to an exercise intervention. For example, tumour-related/treatment-related factors (e.g. nausea, pain, anaemia, and neutropenia), presence of comorbidities (e.g. diabetes, arthritis, and chronic obstructive pulmonary disease), injuries, disease progression and bone metastases, concomitant medications (e.g., metformin), can negatively affect an individual's ability to exercise safely and limit subsequent adaptation.

CONCLUSIONS

This review identifies numerous gaps and oppportunities in the area of low muscle and muscle loss in cancer. Collaborative efforts between preclinical and clinical researchers are imperative to both understanding the mechanisms of atrophy, and develop appropriate therapeutic interventions.

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.

Cardiorespiratory fitness and physical performance after childhood hematopoietic stem cell transplantation: a systematic review and meta-analysis

The effects of childhood hematopoietic stem cell transplantation (HSCT) on key organs can impair cardiorespiratory fitness, muscle strength, and physical performance. We aimed to provide an overview of childhood HSCT survivors' status on these parameters compared with healthy controls and discuss current insights into clinical risk factors. We performed a systematic search in six scientific databases, including studies published before April 2019 and performed a meta-analysis on cardiorespiratory fitness. Muscle strength and physical performance status were presented narratively. We included ten studies embodying 517 childhood HSCT survivors (mean 17.8 years at follow-up). The meta-analysis (n = 4 studies) showed that childhood HSCT survivors have lower cardiorespiratory fitness compared with healthy controls (Standard mean difference (SMD) -1.32 [95% CI -1-58 to -1.07]; I² 2%, p < 0.00001). Collectively, the studies indicated that childhood HSCT survivors have lower muscle strength (n = 4 studies) and physical performance (n = 3 studies) compared with healthy controls. Childhood HSCT survivors have impaired cardiorespiratory fitness years after ended treatment. Muscle strength and physical performance seem to be impaired, although these measures are insufficiently investigated. Associations between HSCT-specific clinical risk factors and cardiorespiratory fitness, muscle strength, and physical performance are required.

Stretch-induced satellite cell deformation incontracturedmuscles in children with cerebral palsy

Satellite cells (SCs) are quiescent, adult skeletal muscle stem cells responsible for postnatal muscle growth and repair. Children with cerebral palsy (CP) have muscle contractures with reduced SC abundance, extracellular matrix abnormalities and reduced serial sarcomere number resulting in greatly increased in vivo sarcomere length, perhaps due to impaired sarcomere addition, compared to children with typical development (TD). Stretch is a strong activator of SCs that leads to addition of sarcomeres during bone-muscle growth. Mechanical loading and subsequent deformation of intracellular structures can lead to activation and proliferation, perhaps by cytoskeletal transmissions of extracellular mechanical signals to the nuclei. The primary aim of this study was to determine the effect of ex vivo stretch-induced sarcomere length change on SC deformation in children with CP and TD. Muscle biopsies were obtained from twelve children (7 CP, 5 TD) during surgery. Fiber bundles were labeled with fluorescent antibodies for Pax7 (SC), DRAQ5 (nuclei), and alpha-actinin (sarcomere protein). Fibers were stretched using a custom jig and imaged using confocal microscopy. SC nuclear length, height and aspect ratio underwent increased deformation with increasing sarcomere length (p < 0.05) in both groups. Slopes of association for SC nuclear length, aspect ratio and sarcomere lengths were similar between CP and TD. Our results indicate that SC in children with CP undergo similar deformation as TD across sarcomere lengths.

Mouse models of sarcopenia: classification and evaluation

Sarcopenia is a progressive and widespread skeletal muscle disease that is related to an increased possibility of adverse consequences such as falls, fractures, physical disabilities and death, and its risk increases with age. With the deepening of the understanding of sarcopenia, the disease has become a major clinical disease of the elderly and a key challenge of healthy ageing. However, the exact molecular mechanism of this disease is still unclear, and the selection of treatment strategies and the evaluation of its effect are not the same. Most importantly, the early symptoms of this disease are not obvious and are easy to ignore. In addition, the clinical manifestations of each patient are not exactly the same, which makes it difficult to effectively study the progression of sarcopenia. Therefore, it is necessary to develop and use animal models to understand the pathophysiology of sarcopenia and develop therapeutic strategies. This paper reviews the mouse models that can be used in the study of sarcopenia, including ageing models, genetically engineered models, hindlimb suspension models, chemical induction models, denervation models, and immobilization models; analyses their advantages and disadvantages and application scope; and finally summarizes the evaluation of sarcopenia in mouse models.

Muscle Stem Cell Quiescence: Controlling Stemness by Staying Asleep

Muscle stem cells (MuSCs) are tissue-resident stem cells required for growth and repair of skeletal muscle, that are otherwise maintained in a cell-cycle-arrested state called quiescence. While quiescence was originally believed to be a state of cellular inactivity, increasing evidence suggests that quiescence is dynamically regulated and contributes to stemness, the long-term capacity to maintain regenerative functions. Here, we review the current understanding of MuSC quiescence and highlight recently discovered molecular markers, which differentiate depth of quiescence and influence self-renewal capacity. We also discuss how quiescent MuSCs integrate paracrine factors from their niche and dynamically regulate cell signaling, metabolism and proteostasis as they anticipate physiological needs, and how perturbing these cues during aging impairs muscle regeneration.