Remodeling of the neuromuscular junction precedes sarcopenia related alterations in myofibers

Exercise-induced CLCF1 attenuates age-related muscle and bone decline in mice

Skeletal muscle undergoes many alterations with aging. However, the impact of aging on muscle's ability to secrete myokines and its subsequent effects on the body remain largely unexplored. Here, we identify myokines that have the potential to ameliorate age-related muscle and bone decline. Notably, circulating levels of cardiotrophin-like cytokine factor 1 (CLCF1) decrease with age, while exercise significantly upregulates CLCF1 levels in both humans and rodents. Restoring CLCF1 levels in aged male mice improves their physical performance, glucose tolerance, and mitochondrial activity. Furthermore, CLCF1 protects against age-induced bone loss by inhibiting osteoclastogenesis and promoting osteoblast differentiation in aged male mice. These improvements mirror some of the effects of exercise training. Conversely, blocking CLCF1 activity significantly abolishes these beneficial effects, confirming the crucial role of CLCF1 in mediating the positive effects of exercise on muscle and bone health in male mice. These findings collectively suggest that CLCF1 may contribute to the regulation of age-associated musculoskeletal deterioration, and warrant further investigation into its potential role as a modulator of musculoskeletal health during aging.

Pharmacological reduction of lipid hydroperoxides as a potential modulator of sarcopenia

We previously reported that elevated expression of phospholipid hydroperoxide glutathione peroxidase 4, an enzyme that regulates membrane lipid hydroperoxides, can mitigate sarcopenia in mice. However, it is still unknown whether a pharmacological intervention designed to modulate lipid hydroperoxides might be an effective strategy to reduce sarcopenia in aged mice. Here we asked whether a newly developed compound, CMD-35647 (CMD), can reduce muscle atrophy induced by sciatic nerve transection. We treated mice daily with vehicle or CMD (15 mg/kg, i.p. injection) starting 1 day prior to denervation. CMD treatment reduced hydroperoxide generation and blunted muscle atrophy by over 17% in denervated muscle. To test whether CMD can reduce ageing-induced muscle atrophy and weakness, we treated mice with either vehicle or CMD (15 mg/kg, i.p. injection) 3 days per week for 8 months, starting at 18 months of age until 26 months of age. We measured muscle mass, functional status of neuromuscular junctions, muscle contractile function and mitochondrial function in control and CMD-treated 26-month-old female mice. Treatment with CMD conferred protection against muscle atrophy in both tibialis anterior and extensor digitorum longus that was associated with maintenance of fibre size of MHC 2b and 2x fibres. Mitochondrial respiration was also protected in CMD-treated mice. We also found that muscle force generation was protected with CMD treatment despite denervation in ∼25% of the muscle fibres. Overall, this study shows that pharmacological interventions designed to reduce lipid hydroperoxides might be effective for preventing sarcopenia. KEY POINTS: Sarcopenia in aged mice is associated with muscle loss, contractile dysfunction, denervation, and reduced mitochondrial respiration. CMD-35647 is a pharmocological compound that can neutralize lipid hydroperoxides. 8 month treatment of CMD-35647 mitigated muscle atrophy in tibialis anterior and extensor digitorum longus. 8 month treatment of CMD-35647 improved muscle function in aged mice independent of the neuromuscular junction. Aged mice treated with CMD-35647 had greater respiration in red gastrocnemius muscle when compared to vehicle treated mice.

Heat exposure promotes sarcopenia via gut microbiota-derived metabolites

The unprecedented rise in global ambient temperatures in the last decade has significantly impacted human health, yet how heat exposure affects the development of sarcopenia remains enigmatic. Here, we demonstrate that chronic heat exposure induces skeletal muscle volume loss, leading to muscle strength and functional decline in mice. The microbiota composition of heat-exposed mice was analyzed using 16S ribosomal DNA analysis. Liquid chromatography-mass spectrometry (LC-MS) was used to explore the effects of heat exposure on the blood metabolome and to further analyze the correlation between blood metabolism and gut microbiota. Transplantation of microbiota from heat-exposed mice to germ-free mice was sufficient to increase adverse effects on skeletal muscle function in the host. Mechanistically, using an untargeted metabolomics strategy, we reveal that altered gut microbiota due to high temperatures is associated with elevated serum levels of homocitrulline. Homocitrulline causes mitochondrial dysfunction in myocytes by exacerbating ferroptosis levels. And Nrf2 activator (Oltipraz) supplementation alleviates muscle atrophy and dysfunction induced by heat exposure. Our findings reveal the detrimental effects of heat exposure on muscle function and provide new strategies for treating sarcopenia.

Effects of Aging on Orbicularis Oculi Muscle Strength and Ultrastructure in Dermatochalasis: A Pilot Study

Background: Age-related changes to the orbicularis oculi muscle include impaired eyelid function, such as lagophthalmos, alterations in tear film dynamics, and aesthetic changes like wrinkles, festoons, and the descent of soft tissue. To date, the structural and functional changes that would comprehensively increase our understanding of orbicularis aging have not been analyzed. This study aims to investigate functional outcomes using surface electromyography and correlate them with ultrastructural changes in orbicularis during aging. Methods: This study enrolled 26 patients aged 37 to 78 years with a clinical diagnosis of dermatochalasis. Patients were divided into two age groups (<60 years; ≥60 years). Ultrastructural and electromyographical examinations were performed, and the electromyographical signals were correlated with the ultrastructural damage in the orbicularis. Results: This study revealed significantly lower values of average voluntary contraction and RMS of the surface electromyography signals in the older age group compared to the younger age group (p = 0.029 and p = 0.045, respectively). There was no statistically significant association between age and muscle damage (χ2(2) = 2.86, p > 0.05). There was no correlation between average voluntary contraction and the degree of ultrastructural damage in both groups (Spearman's coefficient equaled 0.06923 and 0.64366, respectively). Conclusions: sEMG measurements are valuable for monitoring age-related functional changes in the orbicularis. Aging diminishes the functional capacity of the orbicularis, as evidenced by reduced contraction strength. This study, the first to compare ultrastructural and electromyographical changes in the orbicularis among dermatochalasis patients of different ages, finds that ultrastructural damage to muscle fibers is not directly responsible for the contraction strength decline.

Factors, mechanisms and improvement methods of muscle strength loss

Muscle strength is a crucial aspect of muscle function, essential for maintaining normal physical activity and quality of life. The global aging population coupled with the increasing prevalence of muscle disorders and strength loss, poses a remarkable public health challenge. Understanding the mechanisms behind muscle strength decline is vital for improving public health outcomes. This review discusses recent research advancements on muscle strength loss from various perspectives, including factors contributing to muscle strength decline, the signaling pathways involved in the deterioration of muscle function, and the methods for assessing muscle strength. The final section explores the influence of exercise stimulation and nutrition on muscle strength.

Impact of ageing and disuse on neuromuscular junction and mitochondrial function and morphology: Current evidence and controversies

Inactivity and ageing can have a detrimental impact on skeletal muscle and the neuromuscular junction (NMJ). Decreased physical activity results in muscle atrophy, impaired mitochondrial function, and NMJ instability. Ageing is associated with a progressive decrease in muscle mass, deterioration of mitochondrial function in the motor axon terminals and in myofibres, NMJ instability and loss of motor units. Focusing on the impact of inactivity and ageing, this review examines the consequences on NMJ stability and the role of mitochondrial dysfunction, delving into their complex relationship with ageing and disuse. Evidence suggests that mitochondrial dysfunction can be a pathogenic driver for NMJ alterations, with studies revealing the role of mitochondrial defects in motor neuron degeneration and NMJ instability. Two perspectives behind NMJ instability are discussed: one is that mitochondrial dysfunction in skeletal muscle triggers NMJ deterioration, the other envisages dysfunction of motor terminal mitochondria as a primary contributor to NMJ instability. While evidence from these studies supports both perspectives on the relationship between NMJ dysfunction and mitochondrial impairment, gaps persist in the understanding of how mitochondrial dysfunction can cause NMJ deterioration. Further research, both in humans and in animal models, is essential for unravelling the mechanisms and potential interventions for age- and inactivity-related neuromuscular and mitochondrial alterations.

Lifelong dietary protein restriction induces denervation and skeletal muscle atrophy in mice

As a widespread global issue, protein deficiency hinders development and optimal growth in offspring. Maternal low-protein diet influences the development of age-related diseases, including sarcopenia, by altering the epigenome and organ structure through potential increase in oxidative stress. However, the long-term effects of lactational protein restriction or postnatal lifelong protein restriction on the neuromuscular system have yet to be elucidated. Our results demonstrated that feeding a normal protein diet after lactational protein restriction did not have significant impacts on the neuromuscular system in later life. In contrast, a lifelong low-protein diet induced a denervation phenotype and led to demyelination in the sciatic nerve, along with an increase in the number of centralised nuclei and in the gene expression of atrogenes at 18 months of age, indicating an induced skeletal muscle atrophy. These changes were accompanied by an increase in proteasome activity in skeletal muscle, with no significant alterations in oxidative stress or mitochondrial dynamics markers in skeletal muscle later in life. Thus, lifelong protein restriction may induce skeletal muscle atrophy through changes in peripheral nerves and neuromuscular junctions, potentially contributing to the early onset or exaggeration of sarcopenia.

Changes in muscle quality and biomarkers of neuromuscular junctions and muscle protein turnover following 12 weeks of resistance training in older men

The present study aimed to investigate the effects of 12 weeks of resistance training (RT) on body composition [fat mass (FM), lean body mass (LBM)], muscle quality upper and lower (MQU, MQL), muscle size [cross sectional area (CSA), quadriceps cross-sectional area (QCSA)], biomarkers of neuromuscular junctions [C-terminal agrin fragment (CAF)], and muscle protein turnover [N-terminal peptide (P3NP), 3-methylhistidine (3MH), skeletal muscle-specific isoform of troponin T (sTnT)] in older men. Thirty elderly men (age 66.23 ± 0.57 years) were randomly divided into two groups: resistance training group (RT, n=15) and control group (CON, n=15). Participants in RT performed resistance training protocols with the intensity of 60% one-repetition maximum (3 × /week, 4 sets of the six exercise circuits). Blood samples were assessed before (pre-test) and after (post-test) a 12-week intervention. The ANCOVA (2 × 2; group × time; with the baseline variable as the covariate) revealed significant interaction effects; the greater increases for LBM (P < 0.001), CSA (P < 0.001), QCSA (P < 0.001), MQU (P < 0.05), MQL (P < 0.01), 3MH (P < 0.05) were noted in RT than CON, and greater decreases for FM (P < 0.001) and CAF (P < 0.001) in RT than CON. No interaction effect was found for P3NP and sTnT. The time effect was found for all variables besides P3NP in RT, but no time effect was revealed in CON. The 12-week RT was an effective strategy for improving the concentrations of neuromuscular junction biomarkers such as 3-MH and CAF in older adults, and may lead to favourable changes in body composition, muscle quality, and muscle size.

Neuromuscular impairment at different stages of human sarcopenia

BACKGROUND

Degeneration of the motoneuron and neuromuscular junction (NMJ) and loss of motor units (MUs) contribute to age-related muscle wasting and weakness associated with sarcopenia. However, these features have not been comprehensively investigated in humans. This study aimed to compare neuromuscular system integrity and function at different stages of sarcopenia, with a particular focus on NMJ stability and MU properties.

METHODS

We recruited 42 young individuals (Y) (aged 25.98 ± 4.6 years; 57% females) and 88 older individuals (aged 75.9 ± 4.7 years; 55% females). The older group underwent a sarcopenia screening according to the revised guidelines of the European Working Group on Sarcopenia in Older People 2. In all groups, knee extensor muscle force was evaluated by isometric dynamometry, muscle morphology by ultrasound and MU potential properties by intramuscular electromyography (iEMG). MU number estimate (iMUNE) and blood samples were obtained. Muscle biopsies were collected in a subgroup of 16 Y and 52 older participants.

RESULTS

Thirty-nine older individuals were non-sarcopenic (NS), 31 pre-sarcopenic (PS) and 18 sarcopenic (S). A gradual decrease in quadriceps force, cross-sectional area and appendicular lean mass was observed across the different stages of sarcopenia (for all P < 0.0001). Handgrip force and the Short Physical Performance Battery score also showed a diminishing trend. iEMG analyses revealed elevated near fibre segment jitter in NS, PS and S compared with Y (Y vs. NS and S: P < 0.0001; Y vs. PS: P = 0.0169), suggestive of age-related impaired NMJ transmission. Increased C-terminal agrin fragment (P < 0.0001) and altered caveolin 3 protein expression were consistent with age-related NMJ instability in all the older groups. The iMUNE was lower in all older groups (P < 0.0001), confirming age-related loss of MUs. An age-related increase in MU potential complexity was also observed. These observations were accompanied by increased muscle denervation and axonal damage, evinced by the increase in neural cell adhesion molecule-positive fibres (Y vs. NS: P < 0.0001; Y vs. S: P = 0.02) and the increase in serum concentration of neurofilament light chain (P < 0.0001), respectively. Notably, most of these MU and NMJ parameters did not differ when comparing older individuals with or without sarcopenia.

CONCLUSIONS

Alterations in MU properties, axonal damage, an altered innervation profile and NMJ instability are prominent features of the ageing of the neuromuscular system. These neuromuscular alterations are accompanied by muscle wasting and weakness; however, they appear to precede clinically diagnosed sarcopenia, as they are already detectable in older NS individuals.

Effects of exercise and doxorubicin on acute diaphragm neuromuscular transmission failure

Doxorubicin (DOX) is a highly effective anthracycline antibiotic used to treat a wide variety of cancers including breast cancer, leukemia and lymphoma. Unfortunately, clinical use of DOX is limited due to adverse off-target effects resulting in fatigue, respiratory muscle weakness and dyspnea. The diaphragm is the primary muscle of inspiration and respiratory insufficiency is likely the result of both muscle weakness and neural impairment. However, the contribution of neuropathology to DOX-induced respiratory muscle dysfunction is unclear. We hypothesized that diaphragm weakness following acute DOX exposure is associated with neurotoxicity and that exercise preconditioning is sufficient to improve diaphragm muscle contractility by maintaining neuromuscular integrity. Adult female Sprague-Dawley rats were randomized into four experimental groups: 1) sedentary-saline, 2) sedentary-DOX, 3) exercise-saline or 4) exercise-DOX. Endurance exercise preconditioning consisted of treadmill running for 1 h/day at 30 m/min for 10 days. Twenty-four hours after the last bout of exercise, animals were treated with DOX (20 mg/kg, I.P.) or saline (equal volume). Our results demonstrate that 48-h following DOX administration diaphragm muscle specific force is reduced in sedentary-DOX rats in response to both phrenic nerve and direct diaphragm stimulation. Importantly, endurance exercise preconditioning in DOX-treated rats attenuated the decrease in diaphragm contractile function, reduced neuromuscular transmission failure and altered phrenic nerve morphology. These changes were associated with an exercise-induced reduction in circulating biomarkers of inflammation, nerve injury and reformation. Therefore, the results are consistent with exercise preconditioning as an effective way of reducing respiratory impairment via preservation of phrenic-diaphragm neuromuscular conduction.

Skeletal muscle dysfunction with advancing age

As a result of advances in medical treatments and associated policy over the last century, life expectancy has risen substantially and continues to increase globally. However, the disconnect between lifespan and 'health span' (the length of time spent in a healthy, disease-free state) has also increased, with skeletal muscle being a substantial contributor to this. Biological ageing is accompanied by declines in both skeletal muscle mass and function, termed sarcopenia. The mechanisms underpinning sarcopenia are multifactorial and are known to include marked alterations in muscle protein turnover and adaptations to the neural input to muscle. However, to date, the relative contribution of each factor remains largely unexplored. Specifically, muscle protein synthetic responses to key anabolic stimuli are blunted with advancing age, whilst alterations to neural components, spanning from the motor cortex and motoneuron excitability to the neuromuscular junction, may explain the greater magnitude of function losses when compared with mass. The consequences of these losses can be devastating for individuals, their support networks, and healthcare services; with clear detrimental impacts on both clinical (e.g., mortality, frailty, and post-treatment complications) and societal (e.g., independence maintenance) outcomes. Whether declines in muscle quantity and quality are an inevitable component of ageing remains to be completely understood. Nevertheless, strategies to mitigate these declines are of vital importance to improve the health span of older adults. This review aims to provide an overview of the declines in skeletal muscle mass and function with advancing age, describes the wide-ranging implications of these declines, and finally suggests strategies to mitigate them, including the merits of emerging pharmaceutical agents.

Prevention of age-related neuromuscular junction degeneration in sarcopenia by low-magnitude high-frequency vibration

Neuromuscular junction (NMJ) degeneration is one of pathological factors of sarcopenia. Low-magnitude high-frequency vibration (LMHFV) was reported effective in alleviating the sarcopenia progress. However, no previous study has investigated treatment effects of LMHFV targeting NMJ degeneration in sarcopenia. We first compared morphological differences of NMJ between sarcopenic and non-sarcopenic subjects, as well as young and old C57BL/6 mice. We then systematically characterized the age-related degeneration of NMJ in SAMP8 against its control strain, SAMR1 mice, from 3 to 12 months old. We also investigated effects of LMHFV in SAMP8 on the maintenance of NMJ during the onset of sarcopenia with respect to the Agrin-LRP4-MuSK-Dok7 pathway and investigated the mechanism related to ERK1/2 signaling. We observed sarcopenic/old NMJ presented increased acetylcholine receptors (AChRs) cluster fragmentation and discontinuity than non-sarcopenic/young NMJ. In SAMP8, NMJ degeneration (morphologically at 6 months and functionally at 8 months) was observed associated with the sarcopenia onset (10 months). SAMR1 showed improved NMJ morphology and function compared with SAMP8 at 10 months. Skeletal muscle performance was improved at Month 4 post-LMHFV treatment. Vibration group presented improved NMJ function at Months 2 and 6 posttreatment, accompanied with alleviated morphological degeneration at Month 4 posttreatment. LMHFV increased Dok7 expression at Month 4 posttreatment. In vitro, LMHFV could promote AChRs clustering in myotubes by increasing Dok7 expression through suppressing ERK1/2 phosphorylation. In conclusion, NMJ degeneration was observed associated with the sarcopenia onset in SAMP8. LMHFV may attenuate NMJ degeneration and sarcopenia progression by increasing Dok7 expression through suppressing ERK1/2 phosphorylation.

Higher expression of denervation-responsive genes is negatively associated with muscle volume and performance traits in the study of muscle, mobility, and aging (SOMMA)

With aging skeletal muscle fibers undergo repeating cycles of denervation and reinnervation. In approximately the 8th decade of life reinnervation no longer keeps pace, resulting in the accumulation of persistently denervated muscle fibers that in turn cause an acceleration of muscle dysfunction. The significance of denervation in important clinical outcomes with aging is poorly studied. The Study of Muscle, Mobility, and Aging (SOMMA) is a large cohort study with the primary objective to assess how aging muscle biology impacts clinically important traits. Using transcriptomics data from vastus lateralis muscle biopsies in 575 participants we have selected 49 denervation-responsive genes to provide insights to the burden of denervation in SOMMA, to test the hypothesis that greater expression of denervation-responsive genes negatively associates with SOMMA participant traits that included time to walk 400 meters, fitness (VO2peak), maximal mitochondrial respiration, muscle mass and volume, and leg muscle strength and power. Consistent with our hypothesis, increased transcript levels of: a calciumdependent intercellular adhesion glycoprotein (CDH15), acetylcholine receptor subunits (CHRNA1, CHRND, CHRNE), a glycoprotein promoting reinnervation (NCAM1), a transcription factor regulating aspects of muscle organization (RUNX1), and a sodium channel (SCN5A) were each negatively associated with at least 3 of these traits. VO2peak and maximal respiration had the strongest negative associations with 15 and 19 denervation-responsive genes, respectively. In conclusion, the abundance of denervationresponsive gene transcripts is a significant determinant of muscle and mobility outcomes in aging humans, supporting the imperative to identify new treatment strategies to restore innervation in advanced age.

Effects of physical exercise on neuromuscular junction degeneration during ageing: A systematic review

The neuromuscular junction (NMJ) is a specialized chemical synapse that converts neural impulses into muscle action. Age-associated NMJ degeneration, which involves nerve terminal and postsynaptic decline, denervation, and loss of motor units, significantly contributes to muscle weakness and dysfunction. Although physical training has been shown to make substantial modifications in NMJ of both young and aged animals, the results are often influenced by methodological variables in existing studies. Moreover, there is still lack of strong consensus on the specific effects of exercise on improving the morphology and function of the ageing NMJ. Consequently, the purpose of this study was to conduct a systematic review to elucidate the effects of exercise training on NMJ compartments in the elderly. We conducted a systematic review using PubMed, Embase, and Web of Science databases, employing relevant keywords. Two independent reviewers selected studies that detailed NMJ changes during exercise in ageing, written in English, and available in full text. In total, 20 papers were included. We examined the altered adaptation of the NMJ to exercise, focusing on presynaptic and postsynaptic structures and myofibers in older animals or humans. Our findings indicated that aged NMJs exhibited different adaptive responses to physical exercise compared to younger counterparts. Endurance training, compared with resistance and voluntary exercise regimens, was found to have a more pronounced effect on NMJ structural remodeling, particularly in fast twitch muscle fibers. Physical exercise was observed to promote the formation and maintenance of acetylcholine receptor (AChR) clusters by increasing the recombinant docking protein 7 (Dok7) expression and stabilizing Agrin and lipoprotein receptor-related protein 4 (LRP4). These insights suggest that research on exercise-related therapies could potentially attenuate the progression of neuromuscular degeneration. Translational potential of this article: This systematic review provides a detailed overview of the effects of different types of physical exercise on improving NMJ in the elderly, providing scientific support for the timely intervention of muscle degeneration in the elderly by physical exercise, and providing help for the development of new therapeutic interventions in the future.

Synergistically Acting on Myostatin and Agrin Pathways Increases Neuromuscular Junction Stability and Endurance in Old Mice

Sarcopenia is the primary cause of impaired motor performance in the elderly. The current prevailing approach to counteract such condition is increasing the muscle mass through inhibition of the myostatin system: however, this strategy only moderately improves muscular strength, not being able to sustain the innervation of the hypertrophic muscle per se, leading to a progressive worsening of motor performances. Thus, we proposed the administration of ActR-Fc-nLG3, a protein that combines the soluble activin receptor, a strong myostatin inhibitor, with the C-terminal agrin nLG3 domain. This compound has the potential of reinforcing neuro-muscular stability to the hypertrophic muscle. We previously demonstrated an enhancement of motor endurance and ACh receptor aggregation in young mice after ActR-Fc-nLG3 administration. Now we extended these observations by demonstrating that also in aged (2 years-old) mice, long-term administration of ActR-Fc-nLG3 increases in a sustained way both motor endurance and muscle strength, compared with ActR-Fc, a myostatin inhibitor, alone. Histological data demonstrate that the administration of this biological improves neuromuscular stability and fiber innervation maintenance, preventing muscle fiber atrophy and inducing only moderate hypertrophy. Moreover, at the postsynaptic site we observe an increased folding in the soleplate, a likely anatomical substrate for improved neurotransmission efficiency in the NMJ, that may lead to enhanced motor endurance. We suggest that ActR-Fc-nLG3 may become a valid option for treating sarcopenia and possibly other disorders of striatal muscles.

Respiratory Muscle Strength Is Related to Handgrip Performance in Community-Dwelling Persons Aged 80+ from the BUTTERFLY Study

INTRODUCTION

This study aimed to evaluate the association of respiratory muscle strength with sarcopenia and its indicators in the oldest old.

METHODS

Maximum inspiratory pressure (MIP), maximum expiratory pressure (MEP), and sarcopenia-related factors (handgrip strength and appendicular lean mass) were evaluated in a cohort of n = 286 (45.5% female) non-frail, community-dwelling persons aged 83.6 ± 3.0 years (age range 80-97 years).

RESULTS

The sample presented a sarcopenia prevalence of 32.2%. Sarcopenic subjects showed comparable MIP and MEP as non-sarcopenic ones (female: MIP 43.9 ± 18.9 vs. 50.3 ± 19.5, p = 0.053; MEP 63.0 ± 23.0 vs. 69.2 ± 19.1, p = 0.067; male: MIP, 65.1 ± 24.4 vs. 64.4 ± 23.9, p = 0.433; MEP 87.7 ± 33.3 vs. 93.8 ± 30.9, p = 0.124). Statistically significant but very low associations were found between grip strength and MIP (r = 0.193 for male, p < 0.05 and r = 0.257 for female participants, p < 0.01) and MEP (r = 0.200 for male, p < 0.01 and r = 0.191 for female participants, p < 0.05). Lean mass was significantly correlated to MIP and MEP in female (r = 0.253, p < 0.01 and r = 0.343, p < 0.01, respectively), whereas this association was not found in male participants. Grip strength was the only statistically significant predictor of MEP (r2 = 0.212, p < 0.001), while MIP was independently predicted by age, male sex, and grip strength (r2 = 0.177, p < 0.001).

CONCLUSIONS

Peripheral muscle strength is a statistically significant, albeit weak predictor for respiratory muscle strength in well-functioning, community-dwelling persons aged 80+. When confronted to a low grip strength, one should be aware of concomitant respiratory muscle weakness, as this is a known risk factor for atelectasis and pneumonia. Given the relatively low association with handgrip strength, respiratory muscle strength testing might be indicated.

Unraveling the causes of sarcopenia: Roles of neuromuscular junction impairment and mitochondrial dysfunction

Sarcopenia is a systemic skeletal muscle disease characterized by a decline in skeletal muscle mass and function. Originally defined as an age-associated condition, sarcopenia presently also encompasses muscular atrophy due to various pathological factors, such as intensive care unit-acquired weakness, inactivity, and malnutrition. The exact pathogenesis of sarcopenia is still unknown; herein, we review the pathological roles of the neuromuscular junction and mitochondria in this condition. Sarcopenia is caused by complex and interdependent pathophysiological mechanisms, including aging, neuromuscular junction impairment, mitochondrial dysfunction, insulin resistance, lipotoxicity, endocrine factors, oxidative stress, and inflammation. Among these, neuromuscular junction instability and mitochondrial dysfunction are particularly significant. Dysfunction in neuromuscular junction can lead to muscle weakness or paralysis. Mitochondria, which are plentiful in neurons and muscle fibers, play an important role in neuromuscular junction transmission. Therefore, impairments in both mitochondria and neuromuscular junction may be one of the key pathophysiological mechanisms leading to sarcopenia. Moreover, this article explores the structural and functional alterations in the neuromuscular junction and mitochondria in sarcopenia, suggesting that a deeper understanding of these changes could provide valuable insights for the prevention or treatment of sarcopenia.

Higher Expression of Denervation-responsive Genes is Negatively Associated with Muscle Volume and Performance Traits in the Study of Muscle, Mobility and Aging (SOMMA)

With aging skeletal muscle fibers undergo repeating cycles of denervation and reinnervation. In approximately the 8 th decade of life reinnervation no longer keeps pace, resulting in the accumulation of persistently denervated muscle fibers that in turn cause an acceleration of muscle dysfunction. The significance of denervation in important clinical outcomes with aging is poorly studied. The Study of Muscle, Mobility and Aging (SOMMA) is a large cohort study with the primary objective to assess how aging muscle biology impacts clinically important traits. Using transcriptomics data from vastus lateralis muscle biopsies in 575 participants we have selected 49 denervation-responsive genes to provide insights to the burden of denervation in SOMMA, to test the hypothesis that greater expression of denervation-responsive genes negatively associates with SOMMA participant traits that included time to walk 400 meters, fitness (VO 2peak ), maximal mitochondrial respiration, muscle mass and volume, and leg muscle strength and power. Consistent with our hypothesis, increased transcript levels of: a calcium-dependent intercellular adhesion glycoprotein (CDH15), acetylcholine receptor subunits (Chrna1, Chrnd, Chrne), a glycoprotein promoting reinnervation (NCAM1), a transcription factor regulating aspects of muscle organization (RUNX1), and a sodium channel (SCN5A) were each negatively associated with at least 3 of these traits. VO 2peak and maximal respiration had the strongest negative associations with 15 and 19 denervation-responsive genes, respectively. In conclusion, the abundance of denervation-responsive gene transcripts is a significant determinant of muscle and mobility outcomes in aging humans, supporting the imperative to identify new treatment strategies to restore innervation in advanced age.

Cellular and molecular evidence that synaptic Schwann cells contribute to aging of mouse neuromuscular junctions

Age-induced degeneration of the neuromuscular junction (NMJ) is associated with motor dysfunction and muscle atrophy. While the impact of aging on the NMJ presynapse and postsynapse is well-documented, little is known about the changes perisynaptic Schwann cells (PSCs), the synaptic glia of the NMJ, undergo during aging. Here, we examined PSCs in young, middle-aged, and old mice in three muscles with different susceptibility to aging. Using light and electron microscopy, we found that PSCs acquire age-associated cellular features either prior to or at the same time as the onset of NMJ degeneration. Notably, we found that aged PSCs fail to completely cap the NMJ even though they are more abundant in old compared with young mice. We also found that aging PSCs form processes that either intrude into the synaptic cleft or guide axonal sprouts to innervate other NMJs. We next profiled the transcriptome of PSCs and other Schwann cells (SCs) to identify mechanisms altered in aged PSCs. This analysis revealed that aged PSCs acquire a transcriptional pattern previously shown to promote phagocytosis that is absent in other SCs. It also showed that aged PSCs upregulate unique pro-inflammatory molecules compared to other aged SCs. Interestingly, neither synaptogenesis genes nor genes that are typically upregulated by repair SCs were induced in aged PSCs or other SCs. These findings provide insights into cellular and molecular mechanisms that could be targeted in PSCs to stave off the deleterious effects of aging on NMJs.

Association between the 110-kDa C-terminal agrin fragment and skeletal muscle decline among community-dwelling older women

BACKGROUND

C-terminal agrin fragment (CAF) is a biomarker for neuromuscular junction degradation. This study aimed to investigate whether 110-kDa CAF (CAF110) was associated with the presence and incidence of low muscle mass and strength.

METHODS

This cross-sectional retrospective cohort study comprised women aged ≥65 years. We measured muscle mass using a dual-energy X-ray absorptiometry scanner, hand-grip strength, and blood sampling between 2011 and 2012. A follow-up study with the same measurements was conducted between 2015 and 2017. Low muscle mass and strength were defined as an appendicular skeletal muscle mass index <5.4 kg/m2 and hand-grip strength <18 kg, respectively. The CAF110 level was measured using enzyme-linked immunosorbent assay kits.

RESULTS

In total, 515 women (74.3 ± 6.3 years) were included in this cross-sectional analysis. Of these, 101 (19.6%) and 128 (24.9%) women presented with low muscle mass and strength, respectively. For low muscle mass, the odds ratios (ORs) of the middle and highest CAF110 tertile groups, compared with the lowest group, were 1.93 (95% confidence interval: 1.09-3.43; P = 0.024) and 2.15 (1.22-3.80; P = 0.008), respectively. After adjusting for age, the ORs remained significant: 1.98 (1.11-3.52; P = 0.020) and 2.27 (1.28-4.03; P = 0.005), respectively. Low muscle strength ORs of all the CAF110 tertile groups were not significant. In the longitudinal analysis, 292 and 289 women were assessed for incidents of low muscle mass and strength, respectively. Of those, 34 (11.6%) and 20 (6.9%) women exhibited low muscle mass and strength, respectively. For incident low muscle mass, the crude OR of the CAF110 ≥ the median value group was marginally higher than that of the CAF110 < median value group (median [interquartile range]: 1.98 [0.94-4.17] (P = 0.072). After adjusting for age and baseline muscle mass, the OR was 2.22 [0.97-5.06] (P = 0.058). All low muscle strength ORs of the median categories of CAF110 were not significant.

CONCLUSIONS

CAF110 was not associated with low muscle strength. However, CAF110 may be a potential marker for the incidence of low muscle mass.

A combination of metformin and galantamine exhibits synergistic benefits in the treatment of sarcopenia

Age-associated sarcopenia, characterized by a progressive loss in muscle mass and strength, is the largest cause of frailty and disability in the elderly worldwide. Current treatments involve nonpharmacological guidelines that few subjects can abide by, highlighting the need for effective drugs. Preclinical models were employed to test the benefits of RJx-01, a combination drug composed of metformin and galantamine, on sarcopenia. In worms, RJx-01 treatment improved lifespan, locomotion, pharyngeal pumping, and muscle fiber organization. The synergistic effects of RJx-01 were recapitulated in a transgenic mouse model that displays an exacerbated aging phenotype (Opa1-/-). In these mice, RJx-01 ameliorated physical performance, muscle mass and force, neuromuscular junction stability, and systemic inflammation. RJx-01 also improved physical performance and muscle strength in 22-month-old WT mice and also improved skeletal muscle ultrastructure, mitochondrial morphology, autophagy, lysosomal function, and satellite cell content. Denervation and myofiber damage were decreased in RJx-01-treated animals compared with controls. RJx-01 improved muscle quality rather than quantity, indicating that the improvement in quality underlies the beneficial effects of the combination drug. The studies herein indicate synergistic beneficial effects of RJx-01 in the treatment of sarcopenia and support the pursuit of RJx-01 in a human clinical trial as a therapeutic intervention for sarcopenia.

Discovery and pharmacological characterization of novel positive allosteric modulators acting on skeletal muscle-type nicotinic acetylcholine receptors

Skeletal muscle-type nicotinic acetylcholine receptors (m-nAChRs) are ligand-gated ion channels that open after activation by ACh and whose signals cause muscle contraction. Defects in neurotransmission are reported in disorders such as myasthenia gravis (MG) and congenital myasthenia syndromes (CMS). Although treatments for these disorders exist, therapies which significantly increase muscle strength have yet to be reported. Positive allosteric modulators (PAMs), which promote ACh signaling through AChRs, are expected to be promising therapeutic agents. In this study, we identified an m-nAChR PAM called AS3513678 by high-throughput screening using human myotube cells and modified it to obtain novel compounds (AS3566987 and AS3580239) that showed even stronger PAM activity. AS3580239 caused a leftward shift in the ACh concentration-response curve and was 14.0-fold potent at 10 μM compared with vehicle. Next, we examined the effect of AS3580239 on electrically-induced isometric contraction of the extensor digitorum longus (EDL) muscle in wild-type (WT) and MG model rats. AS3580239 enhanced EDL muscle contraction in both WT and MG model rats at 30 μM. These data suggest that AS3580239 improved neurotransmission and enhanced muscle strength. Thus, m-nAChR PAMs may be a useful treatment for neuromuscular diseases.

Unraveling age-related impairment of the neuromuscular system: exploring biomechanical and neurophysiological perspectives

With extended life expectancy, the quality of life of elders is a priority. Loss of mobility, increased morbidity and risks of falls have dramatic individual and societal impacts. Here we consider the age-related modifications of gait, from a biomechanical and neurophysiological perspective. Among the many factors of frailty involved (e.g., metabolic, hormonal, immunological), loss of muscle strength and neurodegenerative changes inducing slower muscle contraction may play a key role. We highlight that the impact of the multifactorial age-related changes in the neuromuscular systems results in common features of gait in the immature gait of infants and older adults. Besides, we also consider the reversibility of age-related neuromuscular deterioration by, on the one hand, exercise training, and the other hand, novel techniques such as direct spinal stimulation (tsDCS).

Effect of targeted intervention on C-terminal agrin fragment and its association with the components of sarcopenia: a scoping review

BACKGROUND

C-terminal Agrin Fragment (CAF) has emerged as a potent biomarker for identifying sarcopenia. However, the effect of interventions on CAF concentration and the association of CAF with sarcopenia components are unclear.

OBJECTIVE

To review the association between CAF concentration and muscle mass, muscle strength, and physical performance among individuals with primary and secondary sarcopenia and to synthesize the effect of interventions on the change in the level of CAF concentration.

METHODS

A systematic literature search was conducted in six electronic databases, and studies were included if they met the selection criteria decided a priori. The data extraction sheet was prepared, validated, and extracted relevant data.

RESULTS

A total of 5,158 records were found, of which 16 were included. Among studies conducted on individuals with primary sarcopenia, muscle mass was significantly associated with CAF levels, followed by hand grip strength (HGS) and physical performance, with more consistent findings in males. While in secondary sarcopenics, the strongest association was found for HGS and CAF levels, followed by physical performance and muscle mass. CAF concentration was reduced in trials that used functional, dual task, and power training, whereas resistance training and physical activity raised CAF levels. Hormonal therapy did not affect serum CAF concentration.

CONCLUSION(S)

The association between CAF and sarcopenic assessment parameters varies in primary and secondary sarcopenics. The findings would help practitioners and researchers choose the best training mode/parameters/exercises to reduce CAF levels and, eventually, manage sarcopenia.

Decline of regenerative potential of old muscle stem cells: contribution to muscle aging

Muscle stem cells (MuSCs) are required for life-long muscle regeneration. In general, aging has been linked to a decline in the numbers and the regenerative potential of MuSCs. Muscle regeneration depends on the proper functioning of MuSCs, which is itself dependent on intricate interactions with its niche components. Aging is associated with both cell-intrinsic and niche-mediated changes, which can be the result of transcriptional, posttranscriptional, or posttranslational alterations in MuSCs or in the components of their niche. The interplay between cell intrinsic alterations in MuSCs and changes in the stem cell niche environment during aging and its impact on the number and the function of MuSCs is an important emerging area of research. In this review, we discuss whether the decline in the regenerative potential of MuSCs with age is the cause or the consequence of aging skeletal muscle. Understanding the effect of aging on MuSCs and the individual components of their niche is critical to develop effective therapeutic approaches to diminish or reverse the age-related defects in muscle regeneration.

Age-Related Dysfunction in Proteostasis and Cellular Quality Control in the Development of Sarcopenia

Sarcopenia is a debilitating skeletal muscle disease that accelerates in the last decades of life and is characterized by marked deficits in muscle strength, mass, quality, and metabolic health. The multifactorial causes of sarcopenia have proven difficult to treat and involve a complex interplay between environmental factors and intrinsic age-associated changes. It is generally accepted that sarcopenia results in a progressive loss of skeletal muscle function that exceeds the loss of mass, indicating that while loss of muscle mass is important, loss of muscle quality is the primary defect with advanced age. Furthermore, preclinical models have suggested that aged skeletal muscle exhibits defects in cellular quality control such as the degradation of damaged mitochondria. Recent evidence suggests that a dysregulation of proteostasis, an important regulator of cellular quality control, is a significant contributor to the aging-associated declines in muscle quality, function, and mass. Although skeletal muscle mammalian target of rapamycin complex 1 (mTORC1) plays a critical role in cellular control, including skeletal muscle hypertrophy, paradoxically, sustained activation of mTORC1 recapitulates several characteristics of sarcopenia. Pharmaceutical inhibition of mTORC1 as well as caloric restriction significantly improves muscle quality in aged animals, however, the mechanisms controlling cellular proteostasis are not fully known. This information is important for developing effective therapeutic strategies that mitigate or prevent sarcopenia and associated disability. This review identifies recent and historical understanding of the molecular mechanisms of proteostasis driving age-associated muscle loss and suggests potential therapeutic interventions to slow or prevent sarcopenia.

Demographic and Genome Wide Association Analyses According to Muscle Mass Using Data of the Korean Genome and Epidemiology Study

BACKGROUND

Sarcopenia is commonly found in the elderly due to a decline in muscle mass. Many researchers have performed genome-wide association studies (GWAS) to find genetic risk factors of sarcopenia. Although many studies have discovered sarcopenia associated single nucleotide polymorphisms (SNPs), most of them are studies targeting Caucasians. The purpose of this study was to evaluate genetic correlation according to muscle mass in middle aged Koreans using data of the Korean Genome and Epidemiology Study (KOGES), a large population-based genomic cohort study.

METHODS

Baseline participants were 10,030 subjects aged 40 to 69 years who were from Ansan or Anseong in Gyeonggi-do, South Korea. Among them, 9,351 subjects with laboratory data available were included in this study. To identify sarcopenia associated variants, those in the top 30% and bottom 30% of muscle mass index (MMI) were compared. A total of 7,452 people with an MMI of 30-70% were excluded. A total of 1,004 people were also excluded due to missing data. Finally, 895 people were selected for this study. The Korea Biobank Array generated 500,568 SNPs for this dataset.

RESULTS

When subjects were divided into top 30% and bottom 30% of MMI, the top 30% had 169 men and 308 women and the bottom 30% had 220 men and 198 women. In men, age, body mass index (BMI), waist and hip were significantly (P < 0.005) different between top 30% and bottom 30% MMI groups. In women, age, BMI, waist, hip, and hypertension history were significantly different between the two MMI groups. There were 13 significant SNPs in men and 14 significant SNPs in women. Genes associated with variants in men based on the single-nucleotide polymorphism database (dbSNP) were LRP1B containing rs11679458 and RGS6 containing rs11848300. A gene associated with variants in women was Pi4K2A, which contained rs1189312 as a variant. In addition, rs11189312 was associated with expression quantitative trait loci (eQTL) of ZFYVE27 in skeletal muscles and other SNPs of ZFYVE27 (rs10882883, rs17108378, rs35077384) known to be associated with spastic paraplegia. The eQTL analysis revealed that rs11189312 was a variant associated with SNPs of ZFYVE27.

CONCLUSIONS

In the demographic study, significant results were found in BMI, waist, hip, history of hyperlipidemia, and sedentary life status in male group, and significant results were found in BMI, waist, hip, and hypertension history in female group. Variant rs11189312 was found to be a novel variant affecting ZFYVE27 expressed in skeletal muscles, suggesting that rs11189312 might be related to sarcopenia as a novel discovery of this study. Further study is needed to determine the association between sarcopenia and ZFYVE27 known to be associated with spastic paraplegia.

Lipid hydroperoxides and oxylipins are mediators of denervation induced muscle atrophy

Loss of innervation is a key driver of age associated muscle atrophy and weakness (sarcopenia). Our laboratory has previously shown that denervation induced atrophy is associated with the generation of mitochondrial hydroperoxides and lipid mediators produced downstream of cPLA2 and 12/15 lipoxygenase (12/15-LOX). To define the pathological impact of lipid hydroperoxides generated in denervation-induced atrophy in vivo, we treated mice with liproxstatin-1, a lipid hydroperoxide scavenger. We treated adult male mice with 5 mg/kg liproxstain-1 or vehicle one day prior to sciatic nerve transection and daily for 7 days post-denervation before tissue analysis. Liproxstatin-1 treatment protected gastrocnemius mass and fiber cross sectional area (∼40% less atrophy post-denervation in treated versus untreated mice). Mitochondrial hydroperoxide generation was reduced 80% in vitro and by over 65% in vivo by liproxstatin-1 treatment in denervated permeabilized muscle fibers and decreased the content of 4-HNE by ∼25% post-denervation. Lipidomic analysis revealed detectable levels of 25 oxylipins in denervated gastrocnemius muscle and significantly increased levels for eight oxylipins that are generated by metabolism of fatty acids through 12/15-LOX. Liproxstatin-1 treatment reduced the level of three of the eight denervation-induced oxylipins, specifically 15-HEPE, 13-HOTrE and 17-HDOHE. Denervation elevated protein degradation rates in muscle and treatment with liproxstatin-1 reduced rates of protein breakdown in denervated muscle. In contrast, protein synthesis rates were unchanged by denervation. Targeted proteomics revealed a number of proteins with altered expression after denervation but no effect of liproxstain-1. Transcriptomic analysis revealed 203 differentially expressed genes in denervated muscle from vehicle or liproxstatin-1 treated mice, including ER stress, nitric oxide signaling, Gαi signaling, glucocorticoid receptor signaling, and other pathways. Overall, these data suggest lipid hydroperoxides and oxylipins are key drivers of increased protein breakdown and muscle loss associated with denervation induced atrophy and a potential target for sarcopenia intervention.

Cardiac troponin T and autoimmunity in skeletal muscle aging

Age-related muscle mass and strength decline (sarcopenia) impairs the performance of daily living activities and can lead to mobility disability/limitation in older adults. Biological pathways in muscle that lead to mobility problems have not been fully elucidated. Immunoglobulin G (IgG) infiltration in muscle is a known marker of increased fiber membrane permeability and damage vulnerability, but whether this translates to impaired function is unknown. Here, we report that IgG1 and IgG4 are abundantly present in the skeletal muscle (vastus lateralis) of ~ 50% (11 out of 23) of older adults (&gt; 65 years) examined. Skeletal muscle IgG1 was inversely correlated with physical performance (400 m walk time: r = 0.74, p = 0.005; SPPB score: r =  - 0.73, p = 0.006) and muscle strength (r =  - 0.6, p = 0.05). In a murine model, IgG was found to be higher in both muscle and blood of older, versus younger, C57BL/6 mice. Older mice with a higher level of muscle IgG had lower motor activity. IgG in mouse muscle co-localized with cardiac troponin T (cTnT) and markers of complement activation and apoptosis/necroptosis. Skeletal muscle-inducible cTnT knockin mice also showed elevated IgG in muscle and an accelerated muscle degeneration and motor activity decline with age. Most importantly, anti-cTnT autoantibodies were detected in the blood of cTnT knockin mice, old mice, and older humans. Our findings suggest a novel cTnT-mediated autoimmune response may be an indicator of sarcopenia.

Ranolazine Counteracts Strength Impairment and Oxidative Stress in Aged Sarcopenic Mice

Sarcopenia is defined as the loss of muscle mass associated with reduced strength leading to poor quality of life in elderly people. The decline of skeletal muscle performance is characterized by bioenergetic impairment and severe oxidative stress, and does not always strictly correlate with muscle mass loss. We chose to investigate the ability of the metabolic modulator Ranolazine to counteract skeletal muscle dysfunctions that occur with aging. For this purpose, we treated aged C57BL/6 mice with Ranolazine/vehicle for 14 days and collected the tibialis anterior and gastrocnemius muscles for histological and gene expression analyses, respectively. We found that Ranolazine treatment significantly increased the muscle strength of aged mice. At the histological level, we found an increase in centrally nucleated fibers associated with an up-regulation of genes encoding MyoD, Periostin and Osteopontin, thus suggesting a remodeling of the muscle even in the absence of physical exercise. Notably, these beneficial effects of Ranolazine were also accompanied by an up-regulation of antioxidant and mitochondrial genes as well as of NADH-dehydrogenase activity, together with a more efficient protection from oxidative damage in the skeletal muscle. These data indicate that the protection of muscle from oxidative stress by Ranolazine might represent a valuable approach to increase skeletal muscle strength in elderly populations.

Ubiquitin Ligases in Longevity and Aging Skeletal Muscle

The development and prevalence of diseases associated with aging presents a global health burden on society. One hallmark of aging is the loss of proteostasis which is caused in part by alterations to the ubiquitin-proteasome system (UPS) and lysosome-autophagy system leading to impaired function and maintenance of mass in tissues such as skeletal muscle. In the instance of skeletal muscle, the impairment of function occurs early in the aging process and is dependent on proteostatic mechanisms. The UPS plays a pivotal role in degradation of misfolded and aggregated proteins. For the purpose of this review, we will discuss the role of the UPS system in the context of age-related loss of muscle mass and function. We highlight the significant role that E3 ubiquitin ligases play in the turnover of key components (e.g., mitochondria and neuromuscular junction) essential to skeletal muscle function and the influence of aging. In addition, we will briefly discuss the contribution of the UPS system to lifespan. By understanding the UPS system as part of the proteostasis network in age-related diseases and disorders such as sarcopenia, new discoveries can be made and new interventions can be developed which will preserve muscle function and maintain quality of life with advancing age.

The Effects of the 'Catabolic Crisis' on Patients' Prolonged Immobility after COVID-19 Infection

Background and Objectives: Quarantine, isolation and bed rest associated with COVID-19 infection favored the loss of muscle and bone mass, especially in elderly patients. The current study aims to compare the presence of sarcopenia and osteoporosis in patients with a recent (one month) history of SARS-CoV-2 infection versus the general population. Materials and Methods: A cross-sectional study was conducted in which 157 patients were enrolled, divided into two groups, comparable in structure. The COVID-19 group (group C) consisted of 86 patients who were diagnosed with SARS-CoV-2 respiratory infection within the last 30 days. The non-COVID-19 group (group NC) consists of 71 patients who had no clinical signs of respiratory infection and were not quarantined/hospitalized in the last 3 months. Muscle strength, incidence of sarcopenia (using SARC-F score) and osteoporosis (DEXA determination) and physical performance (SPPB score) in the two groups were assessed and compared. Results: No statistically significant differences were found between the SPPB scores of the C group versus the NC group. Statistically significant differences were found in the evaluation of three parameters included in the SARC-F score. Patients in the C group had difficulties in standing up from a chair (p = 0.009) and climbing stairs (p = 0.030) due to lower muscle strength (p = 0.002) compared with patients in the NC group. No correlation of the SARC F and SPPB scores with the T score values obtained by osteo-densitometry was found. Conclusions: The sudden and significant reduction in physical activity, through various measures taken in the general population during the pandemic, led to an increased incidence of sarcopenia, both in patients who did not have COVID-19 infection and among those quarantined/hospitalized for this condition.

Ageing and exercise-induced motor unit remodelling

A motor unit (MU) comprises the neuron cell body, its corresponding axon and each of the muscle fibres it innervates. Many studies highlight age-related reductions in the number of MUs, yet the ability of a MU to undergo remodelling and to expand to rescue denervated muscle fibres is also a defining feature of MU plasticity. Remodelling of MUs involves two coordinated processes: (i) axonal sprouting and new branching growth from adjacent surviving neurons, and (ii) the formation of key structures around the neuromuscular junction to resume muscle-nerve communication. These processes rely on neurotrophins and coordinated signalling in muscle-nerve interactions. To date, several neurotrophins have attracted focus in animal models, including brain-derived neurotrophic factor and insulin-like growth factors I and II. Exercise in older age has demonstrated benefits in multiple physiological systems including skeletal muscle, yet evidence suggests this may also extend to peripheral MU remodelling. There is, however, a lack of research in humans due to methodological limitations which are easily surmountable in animal models. To improve mechanistic insight of the effects of exercise on MU remodelling with advancing age, future research should focus on combining methodological approaches to explore the in vivo physiological function of the MU alongside alterations of the localised molecular environment.

Adaptive Remodeling of the Neuromuscular Junction with Aging

Aging is associated with gradual degeneration, in mass and function, of the neuromuscular system. This process, referred to as “sarcopenia”, is considered a disease by itself, and it has been linked to a number of other serious maladies such as type II diabetes, osteoporosis, arthritis, cardiovascular disease, and even dementia. While the molecular causes of sarcopenia remain to be fully elucidated, recent findings have implicated the neuromuscular junction (NMJ) as being an important locus in the development and progression of that malady. This synapse, which connects motor neurons to the muscle fibers that they innervate, has been found to degenerate with age, contributing both to senescent-related declines in muscle mass and function. The NMJ also shows plasticity in response to a number of neuromuscular diseases such as amyotrophic lateral sclerosis (ALS) and Lambert-Eaton myasthenic syndrome (LEMS). Here, the structural and functional degradation of the NMJ associated with aging and disease is described, along with the measures that might be taken to effectively mitigate, if not fully prevent, that degeneration.

Microbiopsy Sampling for Examining Age-Related Differences in Skeletal Muscle Fiber Morphology and Composition

Introduction: The increasingly popular microbiopsy is an appealing alternative to the more invasive Bergström biopsy given the challenges associated with harvesting skeletal muscle in older populations. Parameters of muscle fiber morphology and composition derived from the microbiopsy have not been compared between young and older adults.

Purpose: The purpose of this study was to examine muscle fiber morphology and composition in young (YM) and older (OM) males using the microbiopsy sampling technique. A secondary aim was to determine if specific strength is associated with serum levels of C-terminal agrin fragment [CAF; an indicator of neuromuscular junction (NMJ) degradation].

Methods: Thirty healthy, YM (n = 15, age = 20.7 ± 2.2 years) and OM (n = 15, age = 71.6 ± 3.9 years) underwent ultrasound imaging to determine whole-muscle cross-sectional area (CSA) of the vastus lateralis and rectus femoris as well as isometric and isokinetic (60°⋅s^–1 and 180°⋅s^–1) peak torque testing of the knee extensors. Microbiopsy samples of the vastus lateralis were collected from 13 YM and 11 OM, and immunofluorescence was used to calculate CSA and proportion of type I and type II fibers.

Results: Peak torque was lower in OM at all velocities (p ≤ 0.001; d = 1.39–1.86) but only lower at 180°⋅s^–1 (p = 0.003; d = 1.23) when normalized to whole-muscle CSA. Whole-muscle CSA was smaller in OM (p = 0.001; d = 1.34), but atrophy was not present at the single fiber level (p > 0.05). Per individual, ∼900 fibers were analyzed, and type I fiber CSA was larger (p = 0.05; d = 0.94) in OM which resulted in a smaller type II/I fiber CSA ratio (p = 0.015; d = 0.95). CAF levels were not sensitive to age (p = 0.159; d = 0.53) nor associated with specific strength or whole-muscle CSA in OM.

Conclusion: The microbiopsy appears to be a viable alternative to the Bergström biopsy for histological analyses of skeletal muscle in older adults. NMJ integrity was not influential for age-related differences in specific strength in our healthy, non-sarcopenic older sample.

Increased MFG-E8 at neuromuscular junctions is an exacerbating factor for sarcopenia-associated denervation

Sarcopenia is an important health problem associated with adverse outcomes. Although the etiology of sarcopenia remains poorly understood, factors apart from muscle fibers, including humoral factors, might be involved. Here, we used cytokine antibody arrays to identify humoral factors involved in sarcopenia and found a significant increase in levels of milk fat globule epidermal growth factor 8 (MFG‐E8) in skeletal muscle of aged mice, compared with young mice. We found that the increase in MFG‐E8 protein at arterial walls and neuromuscular junctions (NMJs) in muscles of aged mice. High levels of MFG‐E8 at NMJs and an age‐related increase in arterial MFG‐E8 have also been identified in human skeletal muscle. In NMJs, MFG‐E8 is localized on the surface of terminal Schwann cells, which are important accessory cells for the maintenance of NMJs. We found that increased MFG‐E8 at NMJs precedes age‐related denervation and is more prominent in sarcopenia‐susceptible fast‐twitch than in sarcopenia‐resistant slow‐twitch muscle. Comparison between fast and slow muscles further revealed that arterial MFG‐E8 can be uncoupled from sarcopenic phenotype. A genetic deficiency in MFG‐E8 attenuated age‐related denervation of NMJs and muscle weakness, providing evidence of a pathogenic role of increased MFG‐E8. Thus, our study revealed a mechanism by which increased MFG‐E8 at NMJs leads to age‐related NMJ degeneration and suggests that targeting MFG‐E8 could be a promising therapeutic approach to prevent sarcopenia.

Current Thoughts of Notch's Role in Myoblast Regulation and Muscle-Associated Disease

The evolutionarily conserved signaling pathway Notch is unequivocally essential for embryogenesis. Notch’s contribution to the muscle repair process in adult tissue is complex and obscure but necessary. Notch integrates with other signals in a functional antagonist manner to direct myoblast activity and ultimately complete muscle repair. There is profound recent evidence describing plausible mechanisms of Notch in muscle repair. However, the story is not definitive as evidence is slowly emerging that negates Notch’s importance in myoblast proliferation. The purpose of this review article is to examine the prominent evidence and associated mechanisms of Notch’s contribution to the myogenic repair phases. In addition, we discuss the emerging roles of Notch in diseases associated with muscle atrophy. Understanding the mechanisms of Notch’s orchestration is useful for developing therapeutic targets for disease.

Variation in muscle and neuromuscular junction morphology between atrophy-resistant and atrophy-prone muscles supports failed re-innervation in aging muscle atrophy

In advanced age, there is an accelerated decline in skeletal muscle mass that appears to be secondary to repeated cycles of denervation-reinnervation and eventually, failed reinnervation. However, whether variation in reinnervation capacity explains why some muscles are less vulnerable to age-related atrophy has not been addressed. In this study we examined changes in neuromuscular junction (NMJ) morphology, fiber cross-sectional area (CSA) and fiber type, accumulation of severely atrophied myofibers, and expression of a marker of denervation in four muscles that exhibit differences in the degree of age-related atrophy and which span the extremes of fiber type composition in 8 mo old (8 M) and 34 mo old (34 M) male Fischer 344 Brown Norway F1 hybrid rats. Aging muscle atrophy was most pronounced in the fast twitch gastrocnemius (Gas; 25%) and similar between extensor digitorum longus (EDL) and slow-twitch soleus (Sol) muscle (14-15%), whereas the slow-twitch adductor longus (AL) increased in mass by 21% between 8 M and 34 M (P < 0.05 for all). Only the Sol exhibited significant alterations in fiber type with aging, and there was a decrease in fiber CSA in the Gas, EDL, and Sol (P < 0.05) with aging that was not seen in the AL. Muscles that atrophied had an increased fraction of severely atrophic myofibers (P < 0.05), but this was not observed in the AL. The Gas and EDL both demonstrated a similar degree of age-related remodeling of pre- and post-synaptic NMJ components. On the other hand, pre- and post-synaptic morphology underwent greater changes with aging in the AL, and many of these same morphological variables were already greater in the Sol vs AL at 8 M, suggesting the Sol had already undergone substantial remodeling and may be nearing its adaptive limits. Consistent with this idea, analysis of NMJ morphology in Sol from 3 M rats exhibited similar values as 8 M AL, and the Sol demonstrated greater expression of the denervation marker neural cell adhesion molecule (NCAM) compared to the AL at 34 M. Collectively, our results are consistent with NMJ remodeling capacity being finite with aging and that maintained remodeling potential confers atrophy protection in aging skeletal muscle by reducing the degree of persistent denervation.

Assessment of Sarcopenia and Obesity in Patients with Myasthenia Gravis Using Dual-Energy X-ray Absorptiometry: A Cross-Sectional Study

Sarcopenia and obesity can negatively impact quality of life and cause chronic fragility, and are associated with neuromuscular diseases, including myasthenia gravis (MG). The long-term consequences of body composition changes in chronic MG remain unknown; we therefore evaluated changes in body composition, including sarcopenia, obesity, lean body mass, and the prevalence of sarcopenic obesity in patients. In this cross-sectional study, 35 patients with MG (mean age: 56.1 years) and 175 matched controls were enrolled. Body fat mass and skeletal muscle mass were measured using whole body dual-energy X-ray absorptiometry. Patients with MG exhibited a higher prevalence of obesity and higher android adiposity and total body fat percentage than those of controls. Although the prevalence of sarcopenia and sarcopenic obesity did not increase with age, there was a decrease in arm and android muscle mass in patients with MG compared with controls. Lower muscle mass percentages were correlated with increased age and MG severity, but not with corticosteroid use. Thus, MG is associated with increased risk for obesity and decreased muscle mass with aging, regardless of corticosteroid use. Therefore, accurate diagnosis of body composition changes in MG could facilitate the application of appropriate therapies to promote health, improve quality of life, and prevent fragility.

Neuromuscular Junction Aging: A Role for Biomarkers and Exercise

Age-related skeletal muscle degradation known as "sarcopenia" exerts considerable strain on public health systems globally. While the pathogenesis of such atrophy is undoubtedly multifactorial, disruption at the neuromuscular junction (NMJ) has recently gained traction as a key explanatory factor. The NMJ, an essential communicatory link between nerve and muscle, undergoes profound changes with advancing age. Ascertaining whether such changes potentiate the onset of sarcopenia would be paramount in facilitating a timely implementation of targeted therapeutic strategies. Hence, there is a growing level of importance to further substantiate the effects of age on NMJs, in parallel with developing measures to attenuate such changes. As such, this review aimed to establish the current standpoint on age-related NMJ deterioration and consequences for skeletal muscle, while illuminating a role for biomarkers and exercise in ameliorating these alterations. Recent insights into the importance of key biomarkers for NMJ stability are provided, while the stimulative benefits of exercise in preserving NMJ function are demonstrated. Further elucidation of the diagnostic and prognostic relevance of biomarkers, coupled with the therapeutic benefits of regular exercise may be crucial in combating age-related NMJ and skeletal muscle degradation.

The Neuromuscular Junction: Roles in Aging and Neuromuscular Disease

The neuromuscular junction (NMJ) is a specialized synapse that bridges the motor neuron and the skeletal muscle fiber and is crucial for conversion of electrical impulses originating in the motor neuron to action potentials in the muscle fiber. The consideration of contributing factors to skeletal muscle injury, muscular dystrophy and sarcopenia cannot be restricted only to processes intrinsic to the muscle, as data show that these conditions incur denervation-like findings, such as fragmented NMJ morphology and corresponding functional changes in neuromuscular transmission. Primary defects in the NMJ also influence functional loss in motor neuron disease, congenital myasthenic syndromes and myasthenia gravis, resulting in skeletal muscle weakness and heightened fatigue. Such findings underscore the role that the NMJ plays in neuromuscular performance. Regardless of cause or effect, functional denervation is now an accepted consequence of sarcopenia and muscle disease. In this short review, we provide an overview of the pathologic etiology, symptoms, and therapeutic strategies related to the NMJ. In particular, we examine the role of the NMJ as a disease modifier and a potential therapeutic target in neuromuscular injury and disease.

Neuroprotective effects of exercise on the aging human neuromuscular system

Human biological aging from maturity to senescence is associated with a gradual loss of muscle mass and neuromuscular function. It is not until very old age (>80 years) however, that these changes often manifest into functional impairments. A driving factor underlying the age-related loss of muscle mass and function is the reduction in the number and quality of motor units (MUs). A MU consists of a single motoneuron, located either in the spinal cord or the brain stem, and all of the muscle fibres it innervates via its peripheral axon. Throughout the adult lifespan, MUs are slowly, but progressively lost. The compensatory process of collateral reinnervation attempts to recapture orphaned muscle fibres following the death of a motoneuron. Whereas this process helps mitigate loss of muscle mass during the latter decades of adult aging, the neuromuscular system has fewer and larger MUs, which have lower quality connections between the axon terminal and innervated muscle fibres. Whether this process of MU death and degradation can be attenuated with habitual physical activity has been a challenging question of great interest. This review focuses on age-related alterations of the human neuromuscular system, with an emphasis on the MU, and presents findings on the potential protective effects of lifelong physical activity. Although there is some discrepancy across studies of masters athletes, if one considers all experimental limitations as well as the available literature in animals, there is compelling evidence of a protective effect of chronic physical training on human MUs. Our tenet is that high-levels of physical activity can mitigate the natural trajectory of loss of quantity and quality of MUs in old age.

Effects of dual-task training with blood flow restriction on cognitive functions, muscle quality, and circulatory biomarkers in elderly women

This study investigated the effects of an eight-week dual-task training with and without blood flow restriction (BFR) on muscle quality (MQ) biomarkers, physical and cognitive functions in older women. Twenty-four healthy volunteers (62.9 ± 3.1years) were randomly assigned into either dual-task (DT), DT with blood flow restriction (DTBFR), or control (C) group. The experimental groups performed cognitive tasks while walked on a treadmill at 45% of heart rate reserve (HRR), 20 min per session, three sessions per week for eight weeks. The cuff pressure for the DTBFR group was 50% of the calculated arterial blood occlusion and was increased by 10% every 2 weeks. Pre and post-training intervention, biomarkers of MQ and cognitive functions, body composition, maximal strength, and psychological status were measured. There was a significant interaction effect on BDNF, CAF, P3NP, body mass, BMI, fat percentage, visceral fat mass, and total fat mass (p<0.05). Furthermore, a significant interaction effect on knee extension, 30-s stand-up test, 6-min walk test, timed up&go test, and QOL was observed. There was a significant effect of time on MMSE, sleep quality, and mood score. These findings suggested that including BFR to DT training result in greater physiological and psychological responses and should be considered a training method to combat undesired changes associated with increasing age.

NMJ-Analyser identifies subtle early changes in mouse models of neuromuscular disease

The neuromuscular junction (NMJ) is the peripheral synapse formed between a motor neuron axon terminal and a muscle fibre. NMJs are thought to be the primary site of peripheral pathology in many neuromuscular diseases, but innervation/denervation status is often assessed qualitatively with poor systematic criteria across studies, and separately from 3D morphological structure. Here, we describe the development of ‘NMJ-Analyser’, to comprehensively screen the morphology of NMJs and their corresponding innervation status automatically. NMJ-Analyser generates 29 biologically relevant features to quantitatively define healthy and aberrant neuromuscular synapses and applies machine learning to diagnose NMJ degeneration. We validated this framework in longitudinal analyses of wildtype mice, as well as in four different neuromuscular disease models: three for amyotrophic lateral sclerosis (ALS) and one for peripheral neuropathy. We showed that structural changes at the NMJ initially occur in the nerve terminal of mutant TDP43 and FUS ALS models. Using a machine learning algorithm, healthy and aberrant neuromuscular synapses are identified with 95% accuracy, with 88% sensitivity and 97% specificity. Our results validate NMJ-Analyser as a robust platform for systematic and structural screening of NMJs, and pave the way for transferrable, and cross-comparison and high-throughput studies in neuromuscular diseases.

Skeletal Muscle and Childhood Cancer: Where are we now and where we go from here

Skeletal muscle (muscle) is essential for physical health and for metabolic integrity, with sarcopenia (progressive muscle mass loss and weakness), a pre-curser of aging and chronic disease. Loss of lean mass and muscle quality (force generation per unit of muscle) in the general population are associated with fatigue, weakness, and slowed walking speed, eventually interfering with the ability to maintain physical independence, and impacting participation in social roles and quality of life. Muscle mass and strength impairments are also documented during childhood cancer treatment, which often persist into adult survivorship, and contribute to an aging phenotype in this vulnerable population. Although several treatment exposures appear to confer increased risk for loss of mass and strength that persists after therapy, the pathophysiology responsible for poor muscle quantity and quality is not well understood in the childhood cancer survivor population. This is partly due to limited access to both pediatric and adult survivor muscle tissue samples, and to difficulties surrounding non-invasive investigative approaches for muscle assessment. Because muscle accounts for just under half of the body's mass, and is essential for movement, metabolism and metabolic health, understanding mechanisms of injury responsible for both initial and persistent dysfunction is important, and will provide a foundation for intervention. The purpose of this review is to provide an overview of the available evidence describing associations between childhood cancer, its treatment, and muscle outcomes, identifying gaps in current knowledge.

Secondary denervation is a chronic pathophysiologic sequela of volumetric muscle loss

Volumetric muscle loss (VML) is the traumatic loss of muscle tissue that results in long-term functional impairments. Despite the loss of myofibers, there remains an unexplained significant decline in muscle function. VML injury likely extends beyond the defect area, causing negative secondary outcomes to the neuromuscular system, including the neuromuscular junctions (NMJs), yet the extent to which VML induces denervation is unclear. This study systematically examined NMJs surrounding the VML injury, hypothesizing that the sequela of VML includes denervation. The VML injury removed ∼20% of the tibialis anterior (TA) muscle in adult male inbred Lewis rats (n = 43), the noninjured leg served as an intra-animal control. Muscles were harvested up to 48 days post-VML. Synaptic terminals were identified immunohistochemically, and quantitative confocal microscopy evaluated 2,613 individual NMJ. Significant denervation was apparent by 21 and 48 days post-VML. Initially, denervation increased ∼10% within 3 days of injury; with time, denervation further increased to ∼22% and 32% by 21 and 48 days post-VML, respectively, suggesting significant secondary denervation. The appearance of terminal axon sprouting and polyinnervation were observed as early as 7 days post-VML, increasing in number and complexity throughout 48 days. There was no evidence of VML-induced NMJ size alteration, which may be beneficial for interventions aimed at restoring muscle function. This work recognizes VML-induced secondary denervation and poor remodeling of the NMJ as part of the sequela of VML injury; moreover, secondary denervation is a possible contributing factor to the chronic functional impairments and potentially an overlooked treatment target.NEW & NOTEWORTHY This work advances our understanding of the pathophysiologic complexity of volumetric muscle loss injury. Specifically, we identified secondary denervation in the muscle remaining after volumetric muscle loss injuries as a novel aspect of the injury sequela. Denervation increased chronically, in parallel with the appearance of irregular morphological characteristics and destabilization of the neuromuscular junction, which is expected to further confound chronic functional impairments.

Terminal Schwann Cell Aging: Implications for Age-Associated Neuromuscular Dysfunction

Action potential is transmitted to muscle fibers through specialized synaptic interfaces called neuromuscular junctions (NMJs). These structures are capped by terminal Schwann cells (tSCs), which play essential roles during formation and maintenance of the NMJ. tSCs are implicated in the correct communication between nerves and muscles, and in reinnervation upon injury. During aging, loss of muscle mass and strength (sarcopenia and dynapenia) are due, at least in part, to the progressive loss of contacts between muscle fibers and nerves. Despite the important role of tSCs in NMJ function, very little is known on their implication in the NMJ-aging process and in age-associated denervation. This review summarizes the current knowledge about the implication of tSCs in the age-associated degeneration of NMJs. We also speculate on the possible mechanisms underlying the observed phenotypes.