Mitochondrial Mechanisms of Neuromuscular Junction Degeneration with Aging

Astragaloside IV protects against autoimmune myasthenia gravis in rats via regulation of mitophagy and apoptosis

Astragaloside IV (AS‑IV) has various pharmacological effects, including antioxidant and immunoregulatory properties, which can improve myasthenia gravis (MG) symptoms. However, the potential mechanism underlying the effects of AS‑IV on MG remains to be elucidated. The present study aimed to investigate whether AS‑IV has a therapeutic effect on MG and its potential mechanism of action. By subcutaneously immunizing rats with R97‑116 peptide, an experimental autoimmune (EA) MG rat model was established. AS‑IV (40 or 80 mg/kg/day) treatment was then applied for 28 days after modeling. The results demonstrated that AS‑IV significantly ameliorated the weight loss, Lennon score and pathological changes in the gastrocnemius muscle of EAMG rats compared with the model group. Additionally, the levels of acetylcholine receptor antibody (AChR‑Ab) were significantly decreased, whereas mitochondrial function [ATPase and cytochrome c (Cyt‑C) oxidase activities] and ultrastructure were improved in the AS‑IV treated rats. Moreover, the mRNA and protein expression levels of phosphatase and tensin homolog‑induced putative kinase 1, Parkin, LC3II and Bcl‑2, key signaling molecules for mitophagy and apoptosis, were upregulated, whereas the mRNA and protein expression levels of p62, Cyt‑C, Bax, caspase 3 and caspase 9 were downregulated following AS‑IV intervention. In conclusion, AS‑IV may protect against EAMG in a rat model by modulating mitophagy and apoptosis. These findings indicated the potential mechanism underlying the effects of AS‑IV on MG and provided novel insights into treatment strategies for MG.

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.

Skeletal muscle immobilisation-induced atrophy: mechanistic insights from human studies

Periods of skeletal muscle disuse lead to rapid declines in muscle mass (atrophy), which is fundamentally underpinned by an imbalance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). The complex interplay of molecular mechanisms contributing to the altered regulation of muscle protein balance during disuse have been investigated but rarely synthesised in the context of humans. This narrative review discusses human models of muscle disuse and the ensuing inversely exponential rate of muscle atrophy. The molecular processes contributing to altered protein balance are explored, with a particular focus on growth and breakdown signalling pathways, mitochondrial adaptations and neuromuscular dysfunction. Finally, key research gaps within the disuse atrophy literature are highlighted providing future avenues to enhance our mechanistic understanding of human disuse atrophy.

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.

Neuroprotective treatment with the nitrone compound OKN-007 mitigates age-related muscle weakness in aging mice

Despite the universal impact of sarcopenia on compromised health and quality of life in the elderly, promising pharmaceutical approaches that can effectively mitigate loss of muscle and function during aging have been limited. Our group and others have reported impairments in peripheral motor neurons and loss of muscle innervation as initiating factors in sarcopenia, contributing to mitochondrial dysfunction and elevated oxidative stress in muscle. We recently reported a reduction in α motor neuron loss in aging mice in response to the compound OKN-007, a proposed antioxidant and anti-inflammatory agent. In the current study, we asked whether OKN-007 treatment in wildtype male mice for 8-9 months beginning at 16 months of age can also protect muscle mass and function. At 25 months of age, we observed a reduction in the loss of whole-body lean mass, a reduced loss of innervation at the neuromuscular junction and well-preserved neuromuscular junction morphology in OKN-007 treated mice versus age matched wildtype untreated mice. The loss in muscle force generation in aging mice (~ 25%) is significantly improved with OKN-007 treatment. In contrast, OKN-007 treatment provided no protection in loss of muscle mass in aging mice. Mitochondrial function was improved by OKN-007 treatment, consistent with its potential antioxidative properties. Together, these exciting findings are the first to demonstrate that interventions through neuroprotection can be an effective therapy to counter aging-related muscle dysfunction.

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.

Downregulation of mitochondrial metabolism is a driver for fast skeletal muscle loss during mouse aging

Skeletal muscle aging is characterized by the loss of muscle mass, strength and function, mainly attributed to the atrophy of glycolytic fibers. Underlying mechanisms driving the skeletal muscle functional impairment are yet to be elucidated. To unbiasedly uncover its molecular mechanisms, we recurred to gene expression and metabolite profiling in a glycolytic muscle, Extensor digitorum longus (EDL), from young and aged C57BL/6JRj mice. Employing multi-omics approaches we found that the main age-related changes are connected to mitochondria, exhibiting a downregulation in mitochondrial processes. Consistent is the altered mitochondrial morphology. We further compared our mouse EDL aging signature with human data from the GTEx database, reinforcing the idea that our model may recapitulate muscle loss in humans. We are able to show that age-related mitochondrial downregulation is likely to be detrimental, as gene expression signatures from commonly used lifespan extending interventions displayed the opposite direction compared to our EDL aging signature.

Thyroarytenoid Oxidative Metabolism and Synaptic Signaling Dysregulation in the Female Pink1-/- Rat

OBJECTIVES AND HYPOTHESIS

Vocal dysfunction, including hypophonia, in Parkinson disease (PD) manifests in the prodromal period and significantly impacts an individual's quality of life. Data from human studies suggest that pathology leading to vocal deficits may be structurally related to the larynx and its function. The Pink1-/- rat is a translational model used to study pathogenesis in the context of early-stage mitochondrial dysfunction. The primary objective of this work was to identify differentially expressed genes in the thyroarytenoid muscle and examine the dysregulated biological pathways in the female rat.

METHODS

RNA sequencing was used to determine thyroarytenoid (TA) muscle gene expression in adult female Pink1-/- rats compared with controls. A bioinformatic approach and the ENRICHR gene analysis tool were used to compare the sequencing dataset with biological pathways and processes, disease relationships, and drug-repurposing compounds. Weighted Gene Co-expression Network Analysis was used to construct biological network modules. The data were compared with a previously published dataset in male rats.

RESULTS

Significant upregulated pathways in female Pink1-/- rats included fatty acid oxidation and muscle contraction, synaptic transmission, and neuromuscular processes. Downregulated pathways included anterograde transsynaptic signaling, chemical synaptic transmission, and ion release. Several drug treatment options including cetuximab, fluoxetine, and resveratrol are hypothesized to reverse observed genetic dysregulation.

CONCLUSIONS

Data presented here are useful for identifying biological pathways that may underlie the mechanisms of peripheral dysfunction including neuromuscular synaptic transmission to the TA muscle. These experimental biomarkers have the potential to be targeted as sites for improving the treatment for hypophonia in early-stage PD.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:3412-3421, 2023.

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.

Neuronal Plasticity and Age-Related Functional Decline in the Motor Cortex

Physiological aging causes a decline of motor function due to impairment of motor cortex function, losses of motor neurons and neuromuscular junctions, sarcopenia, and frailty. There is increasing evidence suggesting that the changes in motor function start earlier in the middle-aged stage. The mechanism underlining the middle-aged decline in motor function seems to relate to the central nervous system rather than the peripheral neuromuscular system. The motor cortex is one of the responsible central nervous systems for coordinating and learning motor functions. The neuronal circuits in the motor cortex show plasticity in response to motor learning, including LTP. This motor cortex plasticity seems important for the intervention method mechanisms that revert the age-related decline of motor function. This review will focus on recent findings on the role of plasticity in the motor cortex for motor function and age-related changes. The review will also introduce our recent identification of an age-related decline of neuronal activity in the primary motor cortex of middle-aged mice using electrophysiological recordings of brain slices.

Neuromuscular junction transmission failure in aging and sarcopenia: The nexus of the neurological and muscular systems

Sarcopenia, or age-related decline in muscle form and function, exerts high personal, societal, and economic burdens when untreated. Integrity and function of the neuromuscular junction (NMJ), as the nexus between the nervous and muscular systems, is critical for input and dependable neural control of muscle force generation. As such, the NMJ has long been a site of keen interest in the context of skeletal muscle function deficits during aging and in the context of sarcopenia. Historically, changes of NMJ morphology during aging have been investigated extensively but primarily in aged rodent models. Aged rodents have consistently shown features of NMJ endplate fragmentation and denervation. Yet, the presence of NMJ changes in older humans remains controversial, and conflicting findings have been reported. This review article describes the physiological processes involved in NMJ transmission, discusses the evidence that supports NMJ transmission failure as a possible contributor to sarcopenia, and speculates on the potential of targeting these defects for therapeutic development. The technical approaches that are available for assessment of NMJ transmission, whether each approach has been applied in the context of aging and sarcopenia, and the associated findings are summarized. Like morphological studies, age-related NMJ transmission deficits have primarily been studied in rodents. In preclinical studies, isolated synaptic electrophysiology recordings of endplate currents or potentials have been mostly used, and paradoxically, have shown enhancement, rather than failure, with aging. Yet, in vivo assessment of single muscle fiber action potential generation using single fiber electromyography and nerve-stimulated muscle force measurements show evidence of NMJ failure in aged mice and rats. Together these findings suggest that endplate response enhancement may be a compensatory response to post-synaptic mechanisms of NMJ transmission failure in aged rodents. Possible, but underexplored, mechanisms of this failure are discussed including the simplification of post-synaptic folding and altered voltage-gated sodium channel clustering or function. In humans, there is limited clinical data that has selectively investigated single synaptic function in the context of aging. If sarcopenic older adults turn out to exhibit notable impairments in NMJ transmission (this has yet to be examined but based on available evidence appears to be plausible) then these NMJ transmission defects present a well-defined biological mechanism and offer a well-defined pathway for clinical implementation. Investigation of small molecules that are currently available clinically or being testing clinically in other disorders may provide a rapid route for development of interventions for older adults impacted by sarcopenia.

Protein Arginine Methyltransferase 1 Ablation in Motor Neurons Causes Mitochondrial Dysfunction Leading to Age-related Motor Neuron Degeneration with Muscle Loss

Neuromuscular dysfunction is tightly associated with muscle wasting that occurs with age or due to degenerative diseases. However, the molecular mechanisms underlying neuromuscular dysfunction are currently unclear. Recent studies have proposed important roles of Protein arginine methyltransferase 1 (Prmt1) in muscle stem cell function and muscle maintenance. In the current study, we set out to determine the role of Prmt1 in neuromuscular function by generating mice with motor neuron-specific ablation of Prmt1 (mnKO) using Hb9-Cre. mnKO exhibited age-related motor neuron degeneration and neuromuscular dysfunction leading to premature muscle loss and lethality. Prmt1 deficiency also impaired motor function recovery and muscle reinnervation after sciatic nerve injury. The transcriptome analysis of aged mnKO lumbar spinal cords revealed alterations in genes related to inflammation, cell death, oxidative stress, and mitochondria. Consistently, mnKO lumbar spinal cords of sciatic nerve injury model or aged mice exhibited elevated cellular stress response in motor neurons. Furthermore, Prmt1 inhibition in motor neurons elicited mitochondrial dysfunction. Our findings demonstrate that Prmt1 ablation in motor neurons causes age-related motor neuron degeneration attributing to muscle loss. Thus, Prmt1 is a potential target for the prevention or intervention of sarcopenia and neuromuscular dysfunction related to aging.

Mitochondria Have Made a Long Evolutionary Path from Ancient Bacteria Immigrants within Eukaryotic Cells to Essential Cellular Hosts and Key Players in Human Health and Disease

Mitochondria have made a long evolutionary path from ancient bacteria immigrants within the eukaryotic cell to become key players for the cell, assuming crucial multitasking skills critical for human health and disease. Traditionally identified as the powerhouses of eukaryotic cells due to their central role in energy metabolism, these chemiosmotic machines that synthesize ATP are known as the only maternally inherited organelles with their own genome, where mutations can cause diseases, opening up the field of mitochondrial medicine. More recently, the omics era has highlighted mitochondria as biosynthetic and signaling organelles influencing the behaviors of cells and organisms, making mitochondria the most studied organelles in the biomedical sciences. In this review, we will especially focus on certain 'novelties' in mitochondrial biology "left in the shadows" because, although they have been discovered for some time, they are still not taken with due consideration. We will focus on certain particularities of these organelles, for example, those relating to their metabolism and energy efficiency. In particular, some of their functions that reflect the type of cell in which they reside will be critically discussed, for example, the role of some carriers that are strictly functional to the typical metabolism of the cell or to the tissue specialization. Furthermore, some diseases in whose pathogenesis, surprisingly, mitochondria are involved will be mentioned.

Effects of Monoamino-Oxidase-A (MAO-A) Inhibition on Skeletal Muscle Inflammation and Wasting through Pancreatic Ductal Adenocarcinoma in Triple Transgenic Mice

Cancer cachexia describes a syndrome of muscle wasting and lipolysis that is still largely untreatable and negatively impacts prognosis, mobility, and healthcare costs. Since upregulation of skeletal muscle monoamine-oxidase-A (MAO-A), a source of reactive oxygen species, may contribute to cachexia, we investigated the effects of the MAO-inhibitor harmine-hydrochloride (HH, intraperitoneal, 8 weeks) on muscle wasting in a triple-transgenic mouse model of pancreatic ductal adenocarcinoma (PDAC) and wild type (WT) mice. Gastrocnemius and soleus muscle cryo-cross-sections were analyzed for fiber type-specific cross-sectional area (CSA), fraction and capillarization using ATPase- and lectin-stainings. Transcripts of pro-apoptotic, -atrophic, and -inflammatory signals were determined by RT-qPCR. Furthermore, we evaluated the integrity of neuromuscular junction (NMJ, pre-/post-synaptic co-staining) and mitochondrial ultrastructure (transmission electron microscopy). MAO-A expression in gastrocnemius muscle was increased with PDAC vs. WT (immunohistochemistry: p < 0.05; Western blot: by trend). PDAC expectedly reduced fiber CSA and upregulated IL-1β in both calf muscles, while MuRF1 expression increased in soleus muscle only. Although IL-1β decreased, HH caused an additional 38.65% (p < 0.001) decrease in gastrocnemius muscle (IIBX) fiber CSA. Moreover, soleus muscle CSA remained unchanged despite the downregulation of E3-ligases FBXO32 (p < 0.05) and MuRF1 (p < 0.01) through HH. Notably, HH significantly decreased the post-synaptic NMJ area (quadriceps muscle) and glutathione levels (gastrocnemius muscle), thereby increasing mitochondrial damage and centronucleation in soleus and gastrocnemius type IIBX fibers. Moreover, although pro-atrophic/-inflammatory signals are reversed, HH unfortunately fails to stop and rather promotes PDAC-related muscle wasting, possibly via denervation or mitochondrial damage. These differential adverse vs. therapeutic effects warrant studies regarding dose-dependent benefits and risks with consideration of other targets of HH, such as the dual-specificity tyrosine phosphorylation regulated kinases 1A and B (DYRK1A/B).

Metabolomic profiling reveals muscle metabolic changes following iliac arteriovenous fistula creation in mice

End-stage kidney disease, the most advanced stage of chronic kidney disease (CKD), requires renal replacement therapy or kidney transplant to sustain life. To accomplish durable dialysis access, the creation of an arteriovenous fistula (AVF) has emerged as a preferred approach. Unfortunately, a significant proportion of patients that receive an AVF experience some form of hand dysfunction; however, the mechanisms underlying these side effects are not understood. In this study, we used nuclear magnetic resonance spectroscopy to investigate the muscle metabolome following iliac AVF placement in mice with CKD. To induce CKD, C57BL6J mice were fed an adenine-supplemented diet for 3 wk and then randomized to receive AVF or sham surgery. Two weeks following surgery, the quadriceps muscles were rapidly dissected and snap frozen for metabolite extraction and subsequent nuclear magnetic resonance analysis. Principal component analysis demonstrated clear separation between groups, confirming a unique metabolome in mice that received an AVF. AVF creation resulted in reduced levels of creatine, ATP, and AMP as well as increased levels of IMP and several tricarboxylic acid cycle metabolites suggesting profound energetic stress. Pearson correlation and multiple linear regression analyses identified several metabolites that were strongly linked to measures of limb function (grip strength, gait speed, and mitochondrial respiration). In summary, AVF creation generates a unique metabolome profile in the distal skeletal muscle indicative of an energetic crisis and myosteatosis.NEW & NOTEWORTHY Creation of an arteriovenous fistula (AVF) is the preferred approach for dialysis access, but some patients experience hand dysfunction after AVF creation. In this study, we provide a detailed metabolomic analysis of the limb muscle in a murine model of AVF. AVF creation resulted in metabolite changes associated with an energetic crisis and myosteatosis that associated with limb function.

Changes in neuromuscular function in elders: Novel techniques for assessment of motor unit loss and motor unit remodeling with aging

Functional muscle fiber denervation is a major contributor to the decline in physical function observed with aging and is now a recognized cause of sarcopenia, a muscle disorder characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength. There is an interrelationship between muscle strength, motor unit (MU) number, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functional MUs. During normal aging, there is a time-related progression of MU loss, an adaptive sprouting followed by a maladaptive sprouting, and continuing recession of terminal Schwann cells leading to a reduced capacity for compensatory reinnervation in elders. In amyotrophic lateral sclerosis, increasing age at onset predicts worse survival ALS and it is possible that age-related depletion of the motor neuron pool may worsen motor neuron disease. MUNE methods are used to estimate the number of functional MU, data from MUNIX arguing for motor neuron loss with aging will be reviewed. Recently, a new MRI technique MU-MRI could be used to assess the MU recruitment or explore the activity of a single MU. This review presents published studies on the changes of neuromuscular function with aging, then focusing on these two novel techniques for assessment of MU loss and MU remodeling.

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.

Integrating Mechanisms of Exacerbated Atrophy and Other Adverse Skeletal Muscle Impact in COPD

The normal decline in skeletal muscle mass that occurs with aging is exacerbated in patients with chronic obstructive pulmonary disease (COPD) and contributes to poor health outcomes, including a greater risk of death. There has been controversy about the causes of this exacerbated muscle atrophy, with considerable debate about the degree to which it reflects the very sedentary nature of COPD patients vs. being precipitated by various aspects of the COPD pathophysiology and its most frequent proximate cause, long-term smoking. Consistent with the latter view, recent evidence suggests that exacerbated aging muscle loss with COPD is likely initiated by decades of smoking-induced stress on the neuromuscular junction that predisposes patients to premature failure of muscle reinnervation capacity, accompanied by various alterations in mitochondrial function. Superimposed upon this are various aspects of COPD pathophysiology, such as hypercapnia, hypoxia, and inflammation, that can also contribute to muscle atrophy. This review will summarize the available knowledge concerning the mechanisms contributing to exacerbated aging muscle affect in COPD, consider the potential role of comorbidities using the specific example of chronic kidney disease, and identify emerging molecular mechanisms of muscle impairment, including mitochondrial permeability transition as a mechanism of muscle atrophy, and chronic activation of the aryl hydrocarbon receptor in driving COPD muscle pathophysiology.

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.

Mini-review: Local and downstream responses to the neuromuscular junction: Potential roles for integrins, connexins and ephrins in altering muscle characteristics and function

Skeletal muscle develops in a manner directly related to its innervating motor neuron. The formation of the neuromuscular junction (NMJ) is a well-described process that is coordinated to allow for efficient communication between the central nervous system and muscle for muscle contraction and movement. Some of the major mediators of NMJ formation, like muscle-specific kinase, agrin and laminin, have been thoroughly described but there are other important proteins that have an integral role in muscle health that have also been associated with proper NMJ integrity and fiber health and function. This mini-review focuses on integrins, connexin hemichannels and ephrins and their relationship with the NMJin regulating muscle health.

The effect of long chain omega-3 polyunsaturated fatty acids on muscle mass and function in sarcopenia: A scoping systematic review and meta-analysis

BACKGROUND & AIMS

Sarcopenia is characterized by the progressive loss of skeletal muscle mass and function, which reduces mobility and quality of life. Risk factors for sarcopenia include advanced age, physical inactivity, obesity, and chronic diseases such as cancer or rheumatoid arthritis. Omega-3 long chain polyunsaturated fatty acids (LC PUFAs) might be associated with a reduction in risk of sarcopenia due to their anti-inflammatory effects.

METHODS

We conducted a systematic review and meta-analysis to quantify the effects of omega-3 LC PUFAs on muscle mass, volume and function parameters. The National Library of Medicine's MEDLINE/PubMed database was searched on 9th October 2020 for randomized controlled trials that used omega-3 LC PUFAs as an intervention with muscle-related endpoints. A snowballing search to identify additional studies was completed on 23rd April 2021. The meta-analysis was conducted using meta-essentials worksheet 3. Bias was assessed using the Jadad scale.

RESULTS

123 studies were identified with the systematic searches. Most studies were performed in disease populations, such as cancer or chronic obstructive pulmonary disease (COPD), or in healthy individuals after a fatiguing exercise bout. The endpoints lean body mass, skeletal muscle mass, mid-arm muscle circumference, handgrip strength, quadriceps maximal voluntary capacity (MVC), and 1-repetition maximum chest press were selected for meta-analysis based on the number of available studies; thus 66 studies were included in the quantitative synthesis. Using a random effects model and 2-tailed p-value, there was a significant relationship in favor of omega-3 LC PUFA supplementation for lean body mass (effect size 0.27, 95%CI 0.04 to 0.51), skeletal muscle mass (effect size 0.31, 95%CI 0.01 to 0.60) and quadriceps MVC (effect size 0.47, 95%CI 0.02 to 0.93).

CONCLUSION

The results indicate that there is a positive effect of omega-3 LC PUFA supplementation on overall body muscle mass and strength. Small study size and heterogeneity limit the applicability of these findings for sarcopenia prevention. Larger trials in populations at risk of sarcopenia would strengthen the evidence base.

An Overview of Mitochondrial Protein Defects in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are dysfunctions that involve skeletal muscle and cause incorrect communication between the nerves and muscles. The specific causes of NMDs are not well known, but most of them are caused by genetic mutations. NMDs are generally progressive and entail muscle weakness and fatigue. Muscular impairments can differ in onset, severity, prognosis, and phenotype. A multitude of possible injury sites can make diagnosis of NMDs difficult. Mitochondria are crucial for cellular homeostasis and are involved in various metabolic pathways; for this reason, their dysfunction can lead to the development of different pathologies, including NMDs. Most NMDs due to mitochondrial dysfunction have been associated with mutations of genes involved in mitochondrial biogenesis and metabolism. This review is focused on some mitochondrial routes such as the TCA cycle, OXPHOS, and β-oxidation, recently found to be altered in NMDs. Particular attention is given to the alterations found in some genes encoding mitochondrial carriers, proteins of the inner mitochondrial membrane able to exchange metabolites between mitochondria and the cytosol. Briefly, we discuss possible strategies used to diagnose NMDs and therapies able to promote patient outcome.

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.

Matrisome, innervation and oxidative metabolism affected in older compared with younger males with similar physical activity

BACKGROUND

Due to the interaction between skeletal muscle ageing and lifestyle factors, it is often challenging to attribute the decline in muscle mass and quality to either changes in lifestyle or to advancing age itself. Because many of the physiological factors affecting muscle mass and quality are modulated by physical activity and physical activity declines with age, the aim of this study is to better understand the effects of early ageing on muscle function by comparing a population of healthy older and young males with similar physical activity patterns.

METHODS

Eighteen older (69 ± 2.0 years) and 20 young (22 ± 2.0 years) males were recruited based on similar self-reported physical activity, which was verified using accelerometry measurements. Gene expression profiles of vastus lateralis biopsies obtained by RNA sequencing were compared, and key results were validated using quantitative polymerase chain reaction and western blot.

RESULTS

Total physical activity energy expenditure was similar between the young and old group (404 ± 215 vs. 411 ± 189 kcal/day, P = 0.11). Three thousand seven hundred ninety-seven differentially expressed coding genes (DEGs) were identified (adjusted P-value cut-off of <0.05), of which 1891 were higher and 1906 were lower expressed in the older muscle. The matrisome, innervation and inflammation were the main upregulated processes, and oxidative metabolism was the main downregulated process in old compared with young muscle. Lower protein levels of mitochondrial transcription factor A (TFAM, P = 0.030) and mitochondrial respiratory Complexes IV and II (P = 0.011 and P = 0.0009, respectively) were observed, whereas a trend was observed for Complex I (P = 0.062), in older compared with young muscle. Protein expression of Complexes I and IV was significantly correlated to mitochondrial capacity in the vastus lateralis as measured in vivo (P = 0.017, R2  = 0.42 and P = 0.030, R2  = 0.36). A trend for higher muscle-specific receptor kinase (MUSK) protein levels in the older group was observed (P = 0.08).

CONCLUSIONS

There are clear differences in the transcriptome signatures of the vastus lateralis muscle of healthy older and young males with similar physical activity levels, including significant differences at the protein level. By disentangling physical activity and ageing, we appoint early skeletal muscle ageing processes that occur despite similar physical activity. Improved understanding of these processes will be key to design targeted anti-ageing therapies.

Beneficial effects of dietary supplementation with green tea catechins and cocoa flavanols on aging-related regressive changes in the mouse neuromuscular system

Besides skeletal muscle wasting, sarcopenia entails morphological and molecular changes in distinct components of the neuromuscular system, including spinal cord motoneurons (MNs) and neuromuscular junctions (NMJs); moreover, noticeable microgliosis has also been observed around aged MNs. Here we examined the impact of two flavonoid-enriched diets containing either green tea extract (GTE) catechins or cocoa flavanols on age-associated regressive changes in the neuromuscular system of C57BL/6J mice. Compared to control mice, GTE- and cocoa-supplementation significantly improved the survival rate of mice, reduced the proportion of fibers with lipofuscin aggregates and central nuclei, and increased the density of satellite cells in skeletal muscles. Additionally, both supplements significantly augmented the number of innervated NMJs and their degree of maturity compared to controls. GTE, but not cocoa, prominently increased the density of VAChT and VGluT2 afferent synapses on MNs, which were lost in control aged spinal cords; conversely, cocoa, but not GTE, significantly augmented the proportion of VGluT1 afferent synapses on aged MNs. Moreover, GTE, but not cocoa, reduced aging-associated microgliosis and increased the proportion of neuroprotective microglial phenotypes. Our data indicate that certain plant flavonoids may be beneficial in the nutritional management of age-related deterioration of the neuromuscular system.

Circulating Mediators of Apoptosis and Inflammation in Aging; Physical Exercise Intervention

Sarcopenia is an age-related loss of skeletal muscle mass caused by many cellular mechanisms and also by lifestyle factors such as low daily physical activity. In addition, it has been shown that sarcopenia may be associated with inflammation and cognitive impairment in old age. Regular exercise is key in reducing inflammation and preventing sarcopenia and diseases related to cognitive impairment. The study was designed to assess the impact of exercise training on circulating apoptotic and inflammatory markers of sarcopenia in older adults. Eighty older adults aged 70.5 ± 5.8 years were randomized to the physically active group who participated in a 10-month Tai-Chi training session (TC, n = 40) and the control group who participated in health education sessions (HE, n = 40). Tai-Chi training caused a significant decrease in fat mass (FM) by 3.02 ± 3.99%, but an increase in appendicular skeletal muscle mass index (ASMI) by 1.76 ± 3.17% and gait speed by 9.07 ± 11.45%. Tai-Chi training elevated the plasma levels of C-reactive protein (CRP), tumor necrosis factor (TNFα), and tumor necrosis receptor factor II (TNFRII), and decreased caspases 8 and 9. Despite the increase in TNFα, apoptosis was not initiated, i.e., the cell-free DNA level did not change in the TC group. The study demonstrated that Tai-Chi training significantly reduced the symptoms of sarcopenia through the changes in body composition and physical performance, and improvements in cytokine-related mechanisms of apoptosis.

Why Are Masters Sprinters Slower Than Their Younger Counterparts? Physiological, Biomechanical, and Motor Control Related Implications for Training Program Design

Elite sprint performances typically peak during an athlete's 20s and decline thereafter with age. The mechanisms underpinning this sprint performance decline are often reported to be strength-based in nature with reductions in strength capacities driving increases in ground contact time and decreases in stride lengths and frequency. However, an as-of-yet underexplored aspect of Masters sprint performance is that of age-related degradation in neuromuscular infrastructure, which manifests as a decline in both strength and movement coordination. Here, the authors explore reductions in sprint performance in Masters athletes in a holistic fashion, blending discussion of strength and power changes with neuromuscular alterations along with mechanical and technical age-related alterations. In doing so, the authors provide recommendations to Masters sprinters-and the aging population, in general-as to how best to support sprint ability and general function with age, identifying nutritional interventions that support performance and function and suggesting useful programming strategies and injury-reduction techniques.

Impaired muscle mitochondrial energetics is associated with uremic metabolite accumulation in chronic kidney disease

Chronic kidney disease (CKD) causes progressive skeletal myopathy involving atrophy, weakness, and fatigue. Mitochondria have been thought to contribute to skeletal myopathy; however, the molecular mechanisms underlying muscle metabolism changes in CKD are unknown. We employed a comprehensive mitochondrial phenotyping platform to elucidate the mechanisms of skeletal muscle mitochondrial impairment in mice with adenine-induced CKD. CKD mice displayed significant reductions in mitochondrial oxidative phosphorylation (OXPHOS), which was strongly correlated with glomerular filtration rate, suggesting a link between kidney function and muscle mitochondrial health. Biochemical assays uncovered that OXPHOS dysfunction was driven by reduced activity of matrix dehydrogenases. Untargeted metabolomics analyses in skeletal muscle revealed a distinct metabolite profile in CKD muscle including accumulation of uremic toxins that strongly associated with the degree of mitochondrial impairment. Additional muscle phenotyping found CKD mice experienced muscle atrophy and increased muscle protein degradation, but only male CKD mice had lower maximal contractile force. CKD mice had morphological changes indicative of destabilization in the neuromuscular junction. This study provides the first comprehensive evaluation of mitochondrial health in murine CKD muscle to our knowledge and uncovers several unknown uremic metabolites that strongly associate with the degree of mitochondrial impairment.

Innovations in Geroscience to enhance mobility in older adults

Aging is the primary risk factor for functional decline; thus, understanding and preventing disability among older adults has emerged as an important public health challenge of the 21st century. The science of gerontology - or geroscience - has the practical purpose of "adding life to the years." The overall goal of geroscience is to increase healthspan, which refers to extending the portion of the lifespan in which the individual experiences enjoyment, satisfaction, and wellness. An important facet of this goal is preserving mobility, defined as the ability to move independently. Despite this clear purpose, this has proven to be a challenging endeavor as mobility and function in later life are influenced by a complex interaction of factors across multiple domains. Moreover, findings over the past decade have highlighted the complexity of walking and how targeting multiple systems, including the brain and sensory organs, as well as the environment in which a person lives, can have a dramatic effect on an older person's mobility and function. For these reasons, behavioral interventions that incorporate complex walking tasks and other activities of daily living appear to be especially helpful for improving mobility function. Other pharmaceutical interventions, such as oxytocin, and complementary and alternative interventions, such as massage therapy, may enhance physical function both through direct effects on biological mechanisms related to mobility, as well as indirectly through modulation of cognitive and socioemotional processes. Thus, the purpose of the present review is to describe evolving interventional approaches to enhance mobility and maintain healthspan in the growing population of older adults in the United States and countries throughout the world. Such interventions are likely to be greatly assisted by technological advances and the widespread adoption of virtual communications during and after the COVID-19 era.

Apitherapy for Age-Related Skeletal Muscle Dysfunction (Sarcopenia): A Review on the Effects of Royal Jelly, Propolis, and Bee Pollen

The global pandemic of sarcopenia, skeletal muscle loss and weakness, which prevails in up to 50% of older adults is increasing worldwide due to the expansion of aging populations. It is now striking young and midlife adults as well because of sedentary lifestyle and increased intake of unhealthy food (e.g., western diet). The lockdown measures and economic turndown associated with the current outbreak of Coronavirus Disease 2019 (COVID-19) are likely to increase the prevalence of sarcopenia by promoting sedentarism and unhealthy patterns of eating. Sarcopenia has multiple detrimental effects including falls, hospitalization, disability, and institutionalization. Although a few pharmacological agents (e.g., bimagrumab, sarconeos, and exercise mimetics) are being explored in different stages of trials, not a single drug has been approved for sarcopenia treatment. Hence, research has focused on testing the effect of nutraceuticals, such as bee products, as safe treatments to prevent and/or treat sarcopenia. Royal jelly, propolis, and bee pollen are common bee products that are rich in highly potent antioxidants such as flavonoids, phenols, and amino acids. These products, in order, stimulate larval development into queen bees, promote defenses of the bee hive against microbial and environmental threats, and increase royal jelly production by nurse bees. Thanks to their versatile pharmacological activities (e.g., anti-aging, anti-inflammatory, anticarcinogenic, antimicrobial, etc.), these products have been used to treat multiple chronic conditions that predispose to muscle wasting such as hypertension, diabetes mellitus, cardiovascular disorder, and cancer, to name a few. They were also used in some evolving studies to treat sarcopenia in laboratory animals and, to a limited degree, in humans. However, a collective understanding of the effect and mechanism of action of these products in skeletal muscle is not well-developed. Therefore, this review examines the literature for possible effects of royal jelly, bee pollen, and propolis on skeletal muscle in aged experimental models, muscle cell cultures, and humans. Collectively, data from reviewed studies denote varying levels of positive effects of bee products on muscle mass, strength, and function. The likely underlying mechanisms include amelioration of inflammation and oxidative damages, promotion of metabolic regulation, enhancement of satellite stem cell responsiveness, improvement of muscular blood supply, inhibition of catabolic genes, and promotion of peripheral neuronal regeneration. This review offers suggestions for other mechanisms to be explored and provides guidance for future trials investigating the effects of bee products among people with sarcopenia.

Effects of resistance exercise training on redox homeostasis in older adults. A systematic review and meta-analysis

BACKGROUND

Resistance exercise training (RET) has proven effective at reducing the risk of chronic disease in older populations, and it appears to regulate redox homeostasis.

AIMS

To determine the effects of RET on redox homeostasis in older people.

STUDY DESIGN

A systematic review and meta-analysis of randomized clinical trials identified by searching MEDLINE, Web of Science, EMBASE, Sportdiscus, LILACS, CENTRAL and CINAHL. We included studies of subjects aged 65 years or older, with or without pathologies, and including RET metrics with quantified molecular oxidation and antioxidant capacity outcomes.

RESULTS

Fifteen studies were included in this review. Agreement between reviewers reached a kappa value of 0.725. There were a total of 614 participants, with an average age of 68.1 years. Five (for molecular oxidation markers) and three (for antioxidant capacity markers) studies included data that quantified the effects of RET on homeostasis redox. The results of the meta-analysis showed that there were no differences in the molecular oxidation markers (SMD = -0.26; 95% CI = -0.57 to 0.05; P = 0.10; I2 = 0%) and antioxidant capacity markers (SMD = 0.53; 95% CI = -0.20 to 1.26; P = 0.16; I2 = 71.5%) in healthy older people after a RET of 8-24 weeks compared to non-intervention.

CONCLUSIONS

Based on a small number of studies of low methodological quality, this systematic review with meta-analysis suggests that RET is not effective at reducing molecular oxidation markers in healthy older people. More research is needed on the effects of RET on redox homeostasis in older people.

PROSPERO REGISTRATION NUMBER

CRD42019121529.

Modulation of HOXA9 after skeletal muscle denervation and reinnervation

Homeobox A9 (HOXA9), the expression of which is promoted by mixed lineage leukemia 1 (MLL1) and WD-40 repeat protein 5 (WDR5), is a homeodomain-containing transcription factor that plays an essential role in regulating stem cell activity. HOXA9 has been found to inhibit skeletal muscle regeneration and delay recovery after muscle wounding in aged mice, but little is known about its role in denervated/reinnervated muscles. We performed detailed time-dependent expression analyses of HOXA9 and its promoters, MLL1 and WDR5, in rat gastrocnemius muscles after the following three types of sciatic nerve surgeries: nerve transection (denervation), end-to-end repair (repair), and sham operation (sham). Then, the specific mechanisms of HOXA9 were detected in vitro by transfecting primary satellite cells with empty pIRES2-DsRed2, pIRES2-DsRed2-HOXA9, empty pPLK/GFP-Puro, and pPLK/GFP-Puro-HOXA9 small hairpin RNA (shRNA) plasmids. We found, for the first time, that HOXA9 protein expression simultaneously increased with increasing denervated muscle atrophy severity and that upregulated MLL1 and WDR5 expression was partly associated with denervation. Indeed, in vitro experiments revealed that HOXA9 inhibited myogenic differentiation, affected the best known atrophic signaling pathways, and promoted apoptosis but did not eliminate the differentiation potential of primary satellite cells. HOXA9 may promote denervated muscle atrophy by regulating the activity of satellite cells.