Striking denervation of neuromuscular junctions without lumbar motoneuron loss in geriatric mouse muscle

Age-related changes in the structure and function of mammalian neuromuscular junctions

As mammals age, their neuromuscular junctions (NMJs) change their form, with an increasingly complex system of axonal branches innervating increasingly fragmented regions of postsynaptic differentiation. It has been suggested that this remodeling is associated with impairment of neuromuscular transmission and that this contributes to age-related muscle weakness in mammals, including humans. Here, we review previous work on NMJ aging, most of which has focused on either structure or function, as well as a new study aimed at seeking correlation between the structure and function of individual NMJs. While it is clear that extensive structural changes occur as part of the aging process, it is much less certain how, if at all, these are correlated with an impairment of function. This leaves open the question of whether loss of NMJ function is a significant cause of age-related muscle weakness.

Aging Induces Changes in the Somatic Nerve and Postsynaptic Component without Any Alterations in Skeletal Muscles Morphology and Capacity to Carry Load of Wistar Rats

The present study aimed to analyze the morphology of the peripheral nerve, postsynaptic compartment, skeletal muscles and weight-bearing capacity of Wistar rats at specific ages. Twenty rats were divided into groups: 10 months-old (ADULT) and 24 months-old (OLD). After euthanasia, we prepared and analyzed the tibial nerve using transmission electron microscopy and the soleus and plantaris muscles for cytofluorescence and histochemistry. For the comparison of the results between groups we used dependent and independent Student's t-test with level of significance set at p ≤ 0.05. For the tibial nerve, the OLD group presented the following alterations compared to the ADULT group: larger area and diameter of both myelinated fibers and axons, smaller area occupied by myelinated and unmyelinated axons, lower numerical density of myelinated fibers, and fewer myelinated fibers with normal morphology. Both aged soleus and plantaris end-plate showed greater total perimeter, stained perimeter, total area and stained area compared to ADULT group (p < 0.05). Yet, aged soleus end-plate presented greater dispersion than ADULT samples (p < 0.05). For the morphology of soleus and plantaris muscles, density of the interstitial volume was greater in the OLD group (p < 0.05). No statistical difference was found between groups in the weight-bearing tests. The results of the present study demonstrated that the aging process induces changes in the peripheral nerve and postsynaptic compartment without any change in skeletal muscles and ability to carry load in Wistar rats.

Functional decline at the aging neuromuscular junction is associated with altered laminin-α4 expression

Laminin-α4 is involved in the alignment of active zones to postjunctional folds at the neuromuscular junction (NMJ). Prior study has implicated laminin-α4 in NMJ maintenance, with altered NMJ morphology observed in adult laminin-α4 deficient mice (lama4^−/−). The present study further investigated the role of laminin-α4 in NMJ maintenance by functional characterization of transmission properties, morphological investigation of synaptic proteins including synaptic laminin-α4, and neuromotor behavioral testing. Results showed maintained perturbed transmission properties at lama4^−/− NMJs from adult (3 months) through to aged (18-22 months). Hind-limb grip force demonstrated similar trends as transmission properties, with maintained weaker grip force across age groups in lama4^−/−. Interestingly, both transmission properties and hind-limb grip force in aged wild-types resembled those observed in adult lama4^−/−. Most significantly, altered expression of laminin-α4 was noted at the wild-type NMJs prior to the observed decline in transmission properties, suggesting that altered laminin-α4 expression precedes the decline of neurotransmission in aging wild-types. These findings significantly support the role of laminin-α4 in maintenance of the NMJ during aging.

Denervated muscle fibers induce mitochondrial peroxide generation in neighboring innervated fibers: Role in muscle aging

Disruption of neuromuscular junctions and denervation of some muscle fibers occurs in ageing skeletal muscle and contribute to loss of muscle mass and function. Aging is associated with mitochondrial dysfunction and loss of redox homeostasis potentially occurs through increased mitochondrial generation of reactive oxygen species (ROS). No specific link between increased mitochondrial ROS generation and denervation has been defined in muscle ageing. To address this, we have examined the effect of experimental denervation of all fibers, or only a proportion of the fibers, in the mouse tibialis anterior (TA) muscle on muscle mitochondrial peroxide generation. Transection of the peroneal nerve of mice caused loss of pre-synaptic axons within 1-3 days with no significant morphological changes in post-synaptic structures up to 10 days post-surgery when decreased TA mass and fiber size were apparent. Mitochondria in the denervated muscle showed increased peroxide generation by 3 days post-transection. Use of electron transport chain (ETC) substrates and inhibitors of specific pathways indicated that the ETC was unlikely to contribute to increased ROS generation, but monoamine oxidase B, NADPH oxidase and phospholipase enzymes were implicated. Transection of one of the 3 branches of the peroneal nerve caused denervation of some TA muscle fibers while others retained innervation, but increased mitochondrial peroxide generation occurred in both denervated and innervated fibers. Thus the presence of recently denervated fibers leads to increased ROS generation by mitochondria in neighboring innervated fibers providing a novel explanation for the increased mitochondrial oxidative stress and damage seen with aging in skeletal muscles.

Electrical impedance myography detects age-related muscle change in mice

Loss of muscle mass and strength represents one of the most significant contributors to impaired function in older adults. Convenient and non-invasive biomarkers are needed that can readily identify and track age-related muscle change. Previous data has suggested electrical impedance myography (EIM) has the potential to serve in this capacity. In this study we investigated how changes in EIM compared with other standard measures of muscle structure and function in aged compared with young mice. A total of 19 male mice aged approximately 25 months and 19 male mice aged 3 months underwent surface multifrequency EIM of the right gastrocnemius muscle using standard methods. Fore and hind limb grip strength, sciatic compound muscle action potential amplitude, and in-situ force of the gastrocnemius were also measured; after sacrifice, gastrocnemius myofiber size was assessed using standard histology. Spearman correlation coefficients were calculated to investigate the association between EIM and muscle characteristics. EIM in aged animals demonstrated significantly lower 50 kHz impedance phase (p<0.001) and reactance (p<0.01) values as well as reduced multifrequency parameters. In contrast, absolute gastrocnemius muscle mass was no different between young and aged mice (p = 0.58) but was reduced in aged mice after normalization to body mass (p<0.001). Median myofiber size in the aged mice was not different from that of young mice (p = 0.72). Aged mice showed reduced muscle function on the basis of normalized fore limb (p<0.001) and normalized hind limb (p<0.001) grip strength, as well as normalized gastrocnemius twitch (p<0.001) and normalized maximal isometric force (p<0.001). Sciatic compound muscle action potential amplitude was reduced in aged mice (p<0.05). EIM parameters showed good correlation with reduced standard physiological and electrophysiological measures of muscle health. Our study suggests that EIM is sensitive to aged-related muscle change and may represent a convenient and valuable method of quantifying loss of muscle health.

Cardiac troponin T and fast skeletal muscle denervation in ageing

BACKGROUND

Ageing skeletal muscle undergoes chronic denervation, and the neuromuscular junction (NMJ), the key structure that connects motor neuron nerves with muscle cells, shows increased defects with ageing. Previous studies in various species have shown that with ageing, type II fast-twitch skeletal muscle fibres show more atrophy and NMJ deterioration than type I slow-twitch fibres. However, how this process is regulated is largely unknown. A better understanding of the mechanisms regulating skeletal muscle fibre-type specific denervation at the NMJ could be critical to identifying novel treatments for sarcopenia. Cardiac troponin T (cTnT), the heart muscle-specific isoform of TnT, is a key component of the mechanisms of muscle contraction. It is expressed in skeletal muscle during early development, after acute sciatic nerve denervation, in various neuromuscular diseases and possibly in ageing muscle. Yet the subcellular localization and function of cTnT in skeletal muscle is largely unknown.

METHODS

Studies were carried out on isolated skeletal muscles from mice, vervet monkeys, and humans. Immunoblotting, immunoprecipitation, and mass spectrometry were used to analyse protein expression, real-time reverse transcription polymerase chain reaction was used to measure gene expression, immunofluorescence staining was performed for subcellular distribution assay of proteins, and electromyographic recording was used to analyse neurotransmission at the NMJ.

RESULTS

Levels of cTnT expression in skeletal muscle increased with ageing in mice. In addition, cTnT was highly enriched at the NMJ region-but mainly in the fast-twitch, not the slow-twitch, muscle of old mice. We further found that the protein kinase A (PKA) RIα subunit was largely removed from, while PKA RIIα and RIIβ are enriched at, the NMJ-again, preferentially in fast-twitch but not slow-twitch muscle in old mice. Knocking down cTnT in fast skeletal muscle of old mice: (i) increased PKA RIα and reduced PKA RIIα at the NMJ; (ii) decreased the levels of gene expression of muscle denervation markers; and (iii) enhanced neurotransmission efficiency at NMJ.

CONCLUSIONS

Cardiac troponin T at the NMJ region contributes to NMJ functional decline with ageing mainly in the fast-twitch skeletal muscle through interfering with PKA signalling. This knowledge could inform useful targets for prevention and therapy of age-related decline in muscle function.

Role of nerve-muscle interactions and reactive oxygen species in regulation of muscle proteostasis with ageing

Skeletal muscle ageing is characterised by atrophy, a deficit in specific force generation, increased susceptibility to injury, and incomplete recovery after severe damage. The hypothesis that increased generation of reactive oxygen species (ROS) in vivo plays a key role in the ageing process has been extensively studied, but remains controversial. Skeletal muscle generates ROS at rest and during exercise. ROS can cause oxidative damage particularly to proteins. Indeed, products of oxidative damage accumulate in skeletal muscle during ageing and the ability of muscle cells to respond to increased ROS becomes defective. The aim of this review is to examine the evidence that ROS manipulation in peripheral nerves and/or muscle modifies mechanisms of proteostasis in skeletal muscle and plays a key role in initiating sarcopenia.

Milk fat globule membrane supplementation with voluntary running exercise attenuates age-related motor dysfunction by suppressing neuromuscular junction abnormalities in mice

Age-related loss of skeletal muscle mass and function attenuates physical performance, and maintaining fine muscle innervation is known to play an important role in its prevention. We had previously shown that consumption of milk fat globule membrane (MFGM) with habitual exercise improves the muscle mass and motor function in humans and mice. Improvement of neuromuscular junction (NMJ) was suggested as one of the mechanisms underlying these effects. In this study, we evaluated the effect of MFGM intake combined with voluntary running (MFGM-VR) on morphological changes of NMJ and motor function in aging mice. Seven months following the intervention, the MFGM-VR group showed a significantly improved motor coordination in the rotarod test and muscle force in the grip strength test compared with the control group at 13 and 14months of age, respectively. In 14-month old control mice, the extensor digitorum longus muscle showed increased abnormal NMJs, such as fragmentation and denervation, compared with 6-month old young mice. However, such age-related deteriorations of NMJs were significantly suppressed in the MFGM-VR group. Increase in the expression of NMJ formation-related genes, such as agrin and LDL Receptor Related Protein 4 (LRP4), might contribute to this beneficial effect. Rotarod performance and grip strength showed significant negative correlation with the status of denervation and fragmentation of NMJs. These results suggest that MFGM intake with voluntary running exercise effectively suppresses age-related morphological deterioration of NMJ, thus contributing to improvement of motor function.

The role of attenuated redox and heat shock protein responses in the age-related decline in skeletal muscle mass and function

The loss of muscle mass and weakness that accompanies ageing is a major contributor to physical frailty and loss of independence in older people. A failure of muscle to adapt to physiological stresses such as exercise is seen with ageing and disruption of redox regulated processes and stress responses are recognized to play important roles in theses deficits. The role of redox regulation in control of specific stress responses, including the generation of heat shock proteins (HSPs) by muscle appears to be particularly important and affected by ageing. Transgenic and knockout studies in experimental models in which redox and HSP responses were modified have demonstrated the importance of these processes in maintenance of muscle mass and function during ageing. New data also indicate the potential of these processes to interact with and influence ageing in other tissues. In particular the roles of redox signalling and HSPs in regulation of inflammatory pathways appears important in their impact on organismal ageing. This review will briefly indicate the importance of this area and demonstrate how an understanding of the manner in which redox and stress responses interact and how they may be controlled offers considerable promise as an approach to ameliorate the major functional consequences of ageing of skeletal muscle (and potentially other tissues) in man.

Ablation of S1P3 receptor protects mouse soleus from age-related drop in muscle mass, force, and regenerative capacity

We investigated the effects of S1P3 deficiency on the age-related atrophy, decline in force, and regenerative capacity of soleus muscle from 23-mo-old male (old) mice. Compared with muscle from 5-mo-old (adult) mice, soleus mass and muscle fiber cross-sectional area (CSA) in old wild-type mice were reduced by ~26% and 24%, respectively. By contrast, the mass and fiber CSA of soleus muscle in old S1P3-null mice were comparable to those of adult muscle. Moreover, in soleus muscle of wild-type mice, twitch and tetanic tensions diminished from adulthood to old age. A slowing of contractile properties was also observed in soleus from old wild-type mice. In S1P3-null mice, neither force nor the contractile properties of soleus changed during aging. We also evaluated the regenerative capacity of soleus in old S1P3-null mice by stimulating muscle regeneration through myotoxic injury. After 10 days of regeneration, the mean fiber CSA of soleus in old wild-type mice was significantly smaller (−28%) compared with that of regenerated muscle in adult mice. On the contrary, the mean fiber CSA of regenerated soleus in old S1P3-null mice was similar to that of muscle in adult mice. We conclude that in the absence of S1P3, soleus muscle is protected from the decrease in muscle mass and force, and the attenuation of regenerative capacity, all of which are typical characteristics of aging.

Evidence for dying-back axonal degeneration in age-associated skeletal muscle decline

INTRODUCTION

Age-associated muscle strength decline is a major contributing factor to increased late-life functional decline and comorbidity, and is strongly associated with early mortality. Although all parts of the neuromuscular system seem to be affected by aging, dying-back of motor axons likely plays a major role.

METHODS

We compared the degeneration in ventral roots and neuromuscular junction denervation in young and aged mice and correlated the findings with strength and electrophysiological measures.

RESULTS

With normal aging, there is little decline in motor axon numbers in the ventral roots, but the neuromuscular junctions show marked partial denervation that is associated with increased jitter on stimulated single fiber electromyography and a decrease in muscle strength.

CONCLUSIONS

These findings suggest that dying-back axonal degeneration may be partially responsible for the electrophysiological and strength changes observed with aging. Muscle Nerve 55: 894-901, 2017.

Role of reactive oxygen species in age-related neuromuscular deficits

Although it is now clear that reactive oxygen species (ROS) are not the key determinants of longevity, a number of studies have highlighted the key role that these species play in age-related diseases and more generally in determining individual health span. Age-related loss of skeletal muscle mass and function is a key contributor to physical frailty in older individuals and our current understanding of the key areas in which ROS contribute to age-related deficits in muscle is through defective redox signalling and key roles in maintenance of neuromuscular integrity. This topical review will describe how ROS stimulate adaptations to contractile activity in muscle that include up-regulation of short-term stress responses, an increase in mitochondrial biogenesis and an increase in some catabolic processes. These adaptations occur through stimulation of redox-regulated processes that lead to the activation of transcription factors such as NF-κB, AP-1 and HSF1 which mediate changes in gene expression. They are attenuated during ageing and this appears to occur through an age-related increase in mitochondrial hydrogen peroxide production. The potential for redox-mediated cross-talk between motor neurons and muscle is also described to illustrate how ROS released from muscle fibres during exercise may help maintain the integrity of axons and how the degenerative changes in neuromuscular structure that occur with ageing may contribute to mitochondrial ROS generation in skeletal muscle fibres.

Voluntary resistance wheel exercise from mid-life prevents sarcopenia and increases markers of mitochondrial function and autophagy in muscles of old male and female C57BL/6J mice

Background

There is much interest in the capacity of resistance exercise to prevent the age-related loss of skeletal muscle mass and function, known as sarcopenia. This study investigates the molecular basis underlying the benefits of resistance exercise in aging C57BL/6J mice of both sexes.

Results

This study is the first to demonstrate that long-term (34 weeks) voluntary resistance wheel exercise (RWE) initiated at middle age, from 15 months, prevents sarcopenia in selected hindlimb muscles and causes hypertrophy in soleus, by 23 months of age in both male and female C57BL/6J mice. Compared with 23-month-old sedentary (SED) controls, RWE (0–6 g of resistance) increased intramuscular mitochondrial density and oxidative capacity (measured by citrate synthase and NADH-TR) and increased LC3II/I ratios (a marker of autophagy) in exercised mice of both sexes. RWE also reduced mRNA expression of Gadd45α (males only) and Runx1 (females only) but had no effect on other markers of denervation including Chrng, Chrnd, Musk, and Myog. RWE increased heart mass in all mice, with a more pronounced increase in females. Significant sex differences were also noted among SED mice, with Murf1 mRNA levels increasing in male, but decreasing in old female mice between 15 and 23 months.

Conclusions

Overall, long-term RWE initiated from 15 month of age significantly improved some markers of the mitochondrial and autophagosomal pathways and prevented age-related muscle wasting.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0117-3) contains supplementary material, which is available to authorized users.

Emerging roles for histone deacetylases in age-related muscle atrophy

BACKGROUND: Skeletal muscle atrophy during aging, a process known as sarcopenia, is associated with muscle weakness, frailty, and the loss of independence in older adults. The mechanisms contributing to sarcopenia are not totally understood, but muscle fiber loss due to apoptosis, reduced stimulation of anabolic pathways, activation of catabolic pathways, denervation, and altered metabolism have been observed in muscle from old rodents and humans.

OBJECTIVE: Recently, histone deacetylases (HDACs) have been implicated in muscle atrophy and dysfunction due to denervation, muscular dystrophy, and disuse, and HDACs play key roles in regulating metabolism in skeletal muscle. In this review, we will discuss the role of HDACs in muscle atrophy and the potential of HDAC inhibitors for the treatment of sarcopenia.

CONCLUSIONS: Several HDAC isoforms are potential targets for intervention in sarcopenia. Inhibition of HDAC1 prevents muscle atrophy due to nutrient deprivation. HDAC3 regulates metabolism in skeletal muscle and may inhibit oxidative metabolism during aging. HDAC4 and HDAC5 have been implicated in muscle atrophy due to denervation, a process implicated in sarcopenia. HDAC inhibitors are already in use in the clinic, and there is promise in targeting HDACs for the treatment of sarcopenia.

Vitamin D and/or calcium deficient diets may differentially affect muscle fiber neuromuscular junction innervation

INTRODUCTION

There is evidence that supports a role for Vitamin D (Vit. D) in muscle. The exact mechanism by which Vit. D deficiency impairs muscle strength and function is not clear.

METHODS

Three-week-old mice were fed diets with varied combinations of Vit. D and Ca²⁺ deficiency. Behavioral testing, genomic and protein analysis, and muscle histology were performed with a focus on neuromuscular junction (NMJ) -related genes.

RESULTS

Vit. D and Ca²⁺ deficient mice performed more poorly on given behavioral tasks than animals with Vit. D deficiency alone. Genomic and protein analysis of the soleus and tibialis anterior muscles revealed changes in several Vit. D metabolic, NMJ-related, and protein chaperoning and refolding genes.

CONCLUSIONS

These data suggest that detrimental effects of a Vit. D deficient or a Vit. D and Ca²⁺ deficient diet may be a result of differential alterations in the structure and function of the NMJ and a lack of a sustained stress response in muscles. Muscle Nerve 54: 1120-1132, 2016.

The role of laminins in the organization and function of neuromuscular junctions

The synapse between motor neurons and skeletal muscle is known as the neuromuscular junction (NMJ). Proper alignment of presynaptic and post-synaptic structures of motor neurons and muscle fibers, respectively, is essential for efficient motor control of skeletal muscles. The synaptic cleft between these two cells is filled with basal lamina. Laminins are heterotrimer extracellular matrix molecules that are key members of the basal lamina. Laminin α4, α5, and β2 chains specifically localize to NMJs, and these laminin isoforms play a critical role in maintenance of NMJs and organization of synaptic vesicle release sites known as active zones. These individual laminin chains exert their role in organizing NMJs by binding to their receptors including integrins, dystroglycan, and voltage-gated calcium channels (VGCCs). Disruption of these laminins or the laminin-receptor interaction occurs in neuromuscular diseases including Pierson syndrome and Lambert-Eaton myasthenic syndrome (LEMS). Interventions to maintain proper level of laminins and their receptor interactions may be insightful in treating neuromuscular diseases and aging related degeneration of NMJs.

The effect of lengthening contractions on neuromuscular junction structure in adult and old mice

Skeletal muscles of old mice demonstrate a profound inability to regenerate fully following damage. Such a failure could be catastrophic to older individuals where muscle loss is already evident. Degeneration and regeneration of muscle fibres following contraction-induced injury in adult and old mice are well characterised, but little is known about the accompanying changes in motor neurons and neuromuscular junctions (NMJs) following this form of injury although defective re-innervation of muscle following contraction-induced damage has been proposed to play a role in sarcopenia. This study visualised and quantified structural changes to motor neurons and NMJs in Extensor digitorum longus (EDL) muscles of adult and old Thy1-YFP transgenic mice during regeneration following contraction-induced muscle damage. Data demonstrated that the damaging contraction protocol resulted in substantial initial disruption to NMJs in muscles of adult mice, which was reversed entirely within 28 days following damage. In contrast, in quiescent muscles of old mice, ∼15 % of muscle fibres were denervated and ∼80 % of NMJs showed disruption. This proportion of denervated and partially denervated fibres remained unchanged following recovery from contraction-induced damage in muscles of old mice although ∼25 % of muscle fibres were completely lost by 28 days post-contractions. Thus, in old mice, the failure to restore full muscle force generation that occurs following damage does not appear to be due to any further deficit in the percentage of disrupted NMJs, but appears to be due, at least in part, to the complete loss of muscle fibres following damage.

MHCI promotes developmental synapse elimination and aging-related synapse loss at the vertebrate neuromuscular junction

Synapse elimination at the developing neuromuscular junction (NMJ) sculpts motor circuits, and synapse loss at the aging NMJ drives motor impairments that are a major cause of loss of independence in the elderly. Here we provide evidence that at the NMJ, both developmental synapse elimination and aging-related synapse loss are promoted by specific immune proteins, members of the major histocompatibility complex class I (MHCI). MHCI is expressed at the developing NMJ, and three different methods of reducing MHCI function all disrupt synapse elimination during the second postnatal week, leaving some muscle fibers multiply-innervated, despite otherwise outwardly normal synapse formation and maturation. Conversely, overexpressing MHCI modestly accelerates developmental synapse elimination. MHCI levels at the NMJ rise with aging, and reducing MHCI levels ameliorates muscle denervation in aged mice. These findings identify an unexpected role for MHCI in the elimination of neuromuscular synapses during development, and indicate that reducing MHCI levels can preserve youthful innervation of aging muscle.

Perturbed cholesterol homeostasis in aging spinal cord

The spinal cord is vital for the processing of sensorimotor information and for its propagation to and from both the brain and the periphery. Spinal cord function is affected by aging, however, the mechanisms involved are not well-understood. To characterize molecular mechanisms of spinal cord aging, microarray analyses of gene expression were performed on cervical spinal cords of aging rats. Of the metabolic and signaling pathways affected, cholesterol-associated pathways were the most comprehensively altered, including significant downregulation of cholesterol synthesis-related genes and upregulation of cholesterol transport and metabolism genes. Paradoxically, a significant increase in total cholesterol content was observed-likely associated with cholesterol ester accumulation. To investigate potential mechanisms for the perturbed cholesterol homeostasis, we quantified the expression of myelin and neuroinflammation-associated genes and proteins. Although there was minimal change in myelin-related expression, there was an increase in phagocytic microglial and astrogliosis markers, particularly in the white matter. Together, these results suggest that perturbed cholesterol homeostasis, possibly as a result of increased inflammatory activation in spinal cord white matter, may contribute to impaired spinal cord function with aging.

Reactive oxygen species in sarcopenia: Should we focus on excess oxidative damage or defective redox signalling?

Physical frailty in the elderly is driven by loss of muscle mass and function and hence preventing this is the key to reduction in age-related physical frailty. Our current understanding of the key areas in which ROS contribute to age-related deficits in muscle is through increased oxidative damage to cell constituents and/or through induction of defective redox signalling. Recent data have argued against a primary role for ROS as a regulator of longevity, but studies have persistently indicated that aspects of the aging phenotype and age-related disorders may be mediated by ROS. There is increasing interest in the effects of defective redox signalling in aging and some studies now indicate that this process may be important in reducing the integrity of the aging neuromuscular system. Understanding how redox-signalling pathways are altered by aging and the causes of the defective redox homeostasis seen in aging muscle provides opportunities to identify targeted interventions with the potential to slow or prevent age-related neuromuscular decline with a consequent improvement in quality of life for older people.

Ageing-induced changes in the redox status of peripheral motor nerves imply an effect on redox signalling rather than oxidative damage

Ageing is associated with loss of skeletal muscle fibres, atrophy of the remaining fibres and weakness. These changes in muscle are accompanied by disruption of motor neurons and neuromuscular junctions although the direct relationship between the nerve and muscle degeneration is not understood. Oxidative changes have been implicated in the mechanisms leading to age-related loss of muscle mass and in degeneration of the central nervous system, but little is known about age-related changes in oxidation in specific peripheral nerves that supply muscles that are affected by ageing. We have therefore examined the sciatic nerve of old mice at an age when loss of tibialis anterior muscle mass and function is apparent. Sciatic nerve from old mice did not show a gross increase in oxidative damage, but electron paramagnetic resonance (EPR) studies indicated an increase in the activity of superoxide and/or peroxynitrite in the nerves of old mice at rest that was further exacerbated by electrical stimulation of the nerve to activate muscle contractions. Proteomic analyses indicated that specific redox-sensitive proteins are increased in content in the nerves of old mice that may reflect an adaptation to regulate the increased superoxide/peroxynitrite and maintain redox homoeostasis. Analysis of redox active cysteines showed some increase in reversible oxidation in specific proteins in nerves of old mice, but this was not universally seen across all redox-active cysteines. Detailed analysis of the redox-active cysteine in one protein in the nerve of old mice that is key to redox signalling (Peroxiredoxin 6, Cys 47) showed a minor increase in reversible oxidation that would be compatible with a change in its redox signalling function. In conclusion, the data presented indicate that sciatic nerve from old mice does not show a gross increase in oxidative damage similar to that seen in the TA and other muscles that it innervates. Our results indicate an adaptation to increased oxidation with minor changes in the oxidation of key cysteines that may contribute to defective redox signalling in the nerve.

Integrated expression analysis of muscle hypertrophy identifies Asb2 as a negative regulator of muscle mass

The transforming growth factor-β (TGF-β) signaling network is a critical regulator of skeletal muscle mass and function and, thus, is an attractive therapeutic target for combating muscle disease, but the underlying mechanisms of action remain undetermined. We report that follistatin-based interventions (which modulate TGF-β network activity) can promote muscle hypertrophy that ameliorates aging-associated muscle wasting. However, the muscles of old sarcopenic mice demonstrate reduced response to follistatin compared with healthy young-adult musculature. Quantitative proteomic and transcriptomic analyses of young-adult muscles identified a transcription/translation signature elicited by follistatin exposure, which included repression of ankyrin repeat and SOCS box protein 2 (Asb2). Increasing expression of ASB2 reduced muscle mass, thereby demonstrating that Asb2 is a TGF-β network-responsive negative regulator of muscle mass. In contrast to young-adult muscles, sarcopenic muscles do not exhibit reduced ASB2 abundance with follistatin exposure. Moreover, preventing repression of ASB2 in young-adult muscles diminished follistatin-induced muscle hypertrophy. These findings provide insight into the program of transcription and translation events governing follistatin-mediated adaptation of skeletal muscle attributes and identify Asb2 as a regulator of muscle mass implicated in the potential mechanistic dysfunction between follistatin-mediated muscle growth in young and old muscles.

A robust neuromuscular system protects rat and human skeletal muscle from sarcopenia

Declining muscle mass and function is one of the main drivers of loss of independence in the elderly. Sarcopenia is associated with numerous cellular and endocrine perturbations, and it remains challenging to identify those changes that play a causal role and could serve as targets for therapeutic intervention. In this study, we uncovered a remarkable differential susceptibility of certain muscles to age-related decline. Aging rats specifically lose muscle mass and function in the hindlimbs, but not in the forelimbs. By performing a comprehensive comparative analysis of these muscles, we demonstrate that regional susceptibility to sarcopenia is dependent on neuromuscular junction fragmentation, loss of motoneuron innervation, and reduced excitability. Remarkably, muscle loss in elderly humans also differs in vastus lateralis and tibialis anterior muscles in direct relation to neuromuscular dysfunction. By comparing gene expression in susceptible and non-susceptible muscles, we identified a specific transcriptomic signature of neuromuscular impairment. Importantly, differential molecular profiling of the associated peripheral nerves revealed fundamental changes in cholesterol biosynthetic pathways. Altogether our results provide compelling evidence that susceptibility to sarcopenia is tightly linked to neuromuscular decline in rats and humans, and identify dysregulation of sterol metabolism in the peripheral nervous system as an early event in this process.

Innervation and neuromuscular control in ageing skeletal muscle

Changes in the neuromuscular system affecting the ageing motor unit manifest structurally as a reduction in motor unit number secondary to motor neuron loss; fibre type grouping due to repeating cycles of denervation-reinnervation; and instability of the neuromuscular junction that may be due to either or both of a gradual perturbation in postsynaptic signalling mechanisms necessary for maintenance of the endplate acetylcholine receptor clusters or a sudden process involving motor neuron death or traumatic injury to the muscle fibre. Functionally, these changes manifest as a reduction in strength and coordination that precedes a loss in muscle mass and contributes to impairments in fatigue. Regular muscle activation in postural muscles or through habitual physical activity can attenuate some of these structural and functional changes up to a point along the ageing continuum. On the other hand, regular muscle activation in advanced age (>75 years) loses its efficacy, and at least in rodents may exacerbate age-related motor neuron death. Transgenic mouse studies aimed at identifying potential mechanisms of motor unit disruptions in ageing muscle are not conclusive due to many different mechanisms converging on similar motor unit alterations, many of which phenocopy ageing muscle. Longitudinal studies of ageing models and humans will help clarify the cause and effect relationships and thus, identify relevant therapeutic targets to better preserve muscle function across the lifespan.

Evaluation of follistatin as a therapeutic in models of skeletal muscle atrophy associated with denervation and tenotomy

Follistatin is an inhibitor of TGF-β superfamily ligands that repress skeletal muscle growth and promote muscle wasting. Accordingly, follistatin has emerged as a potential therapeutic to ameliorate the deleterious effects of muscle atrophy. However, it remains unclear whether the anabolic effects of follistatin are conserved across different modes of non-degenerative muscle wasting. In this study, the delivery of a recombinant adeno-associated viral vector expressing follistatin (rAAV:Fst) to the hind-limb musculature of mice two weeks prior to denervation or tenotomy promoted muscle hypertrophy that was sufficient to preserve muscle mass comparable to that of untreated sham-operated muscles. However, administration of rAAV:Fst to muscles at the time of denervation or tenotomy did not prevent subsequent muscle wasting. Administration of rAAV:Fst to innervated or denervated muscles increased protein synthesis, but markedly reduced protein degradation only in innervated muscles. Phosphorylation of the signalling proteins mTOR and S6RP, which are associated with protein synthesis, was increased in innervated muscles administered rAAV:Fst, but not in treated denervated muscles. These results demonstrate that the anabolic effects of follistatin are influenced by the interaction between muscle fibres and motor nerves. These findings have important implications for understanding the potential efficacy of follistatin-based therapies for non-degenerative muscle wasting.

Resistance Training Enhances Skeletal Muscle Innervation Without Modifying the Number of Satellite Cells or their Myofiber Association in Obese Older Adults

Studies in humans and animal models provide compelling evidence for age-related skeletal muscle denervation, which may contribute to muscle fiber atrophy and loss. Skeletal muscle denervation seems relentless; however, long-term, high-intensity physical activity appears to promote muscle reinnervation. Whether 5-month resistance training (RT) enhances skeletal muscle innervation in obese older adults is unknown. This study found that neural cell-adhesion molecule, NCAM+ muscle area decreased with RT and was inversely correlated with muscle strength. NCAM1 and RUNX1 gene transcripts significantly decreased with the intervention. Type I and type II fiber grouping in the vastus lateralis did not change significantly but increases in leg press and knee extensor strength inversely correlated with type I, but not with type II, fiber grouping. RT did not modify the total number of satellite cells, their number per area, or the number associated with specific fiber subtypes or innervated/denervated fibers. Our results suggest that RT has a beneficial impact on skeletal innervation, even when started late in life by sedentary obese older adults.

Mechanisms of Muscle Denervation in Aging: Insights from a Mouse Model of Amyotrophic Lateral Sclerosis

Muscle denervation at the neuromuscular junction (NMJ) is thought to be a contributing factor in age-related muscle weakness. Therefore, understanding the mechanisms that modulate NMJ innervation is a key to developing therapies to combat age-related muscle weakness affecting the elderly. Two mouse models, one lacking the Cu/Zn superoxide dismutase (SOD1) gene and another harboring the transgenic mutant human SOD1 gene, display progressive changes at the NMJ, including muscle endplate fragmentation, nerve terminal sprouting, and denervation. These changes at the NMJ share many of the common features observed in the NMJs of aged mice. In this review, research findings demonstrating the effects of PGC-1α, IGF-1, GDNF, MyoD, myogenin, and miR-206 on NMJ innervation patterns in the G93A SOD1 mice will be highlighted in the context of age-related muscle denervation.

Elderly mouse skeletal muscle fibres have a diminished capacity to upregulate NCAM production in response to denervation

Sarcopenia is a major contributor to the loss of independence and deteriorating quality of life in elderly individuals, it manifests as a decline in skeletal muscle mass and strength beyond the age of 65. Muscle fibre atrophy is a major contributor to sarcopenia and the most severely atrophic fibres are commonly found in elderly muscles to have permanently lost their motor nerve input. By contrast with elderly fibres, when fibres in young animals lose their motor input they normally mount a response to induce restoration of nerve contact, and this is mediated in part by upregulated expression of the nerve cell adhesion molecule (NCAM). Therefore, skeletal muscles appear to progressively lose their ability to become reinnervated, and here we have investigated whether this decline occurs via loss of the muscle's ability to upregulate NCAM in response to denervation. We performed partial denervation (by peripheral nerve crush) of the extensor digitorum longus muscle of the lower limb in both young and elderly mice. We used immunohistochemistry to compare relative NCAM levels at denervated and control innervated muscle fibres, focused on measurements at neuromuscular junctional, extra-junctional and cytoplasmic locations. Muscle fibres in young animals responded to denervation with significant (32.9%) increases in unpolysialylated NCAM at extra-junctional locations, but with no change in polysialylated NCAM. The same partial denervation protocol applied to elderly animals resulted in no significant change in either polysialylated or unpolysialylated NCAM at junctional, extra-junctional or cytoplasmic locations, therefore muscle fibres in young mice upregulated NCAM in response to denervation but fibres in elderly mice failed to do so. Elevation of NCAM levels is likely to be an important component of the muscle fibre's ability to attract or reattract a neural input, so we conclude that the presence of increasing numbers of long-term denervated fibres in elderly muscles is due, at least in part, to the fibre's declining ability to mount a normal response to loss of motor input.

Redox regulation of muscle adaptations to contractile activity and aging

Superoxide and nitric oxide are generated by skeletal muscle, and these species are increased by contractile activity. Mitochondria have long been assumed to play the primary role in generation of superoxide in muscle, but recent studies indicate that, during contractile activity, membrane-localized NADPH oxidase(s) rapidly generate(s) superoxide that plays a role in redox signaling. This process is important in upregulation of rapid and specific cytoprotective responses that aid maintenance of cell viability following contractile activity, but the overall extent to which redox signaling contributes to regulation of muscle metabolism and homeostasis following contractile activity is currently unclear, as is identification of key redox-sensitive protein targets involved in these processes. Reactive oxygen and nitrogen species have also been implicated in the loss of muscle mass and function that occurs with aging, although recent work has questioned whether oxidative damage plays a key role in these processes. A failure of redox signaling occurs in muscle during aging and may contribute to the age-related loss of muscle fibers. Whether such changes in redox signaling reflect primary age-related changes or are secondary to the fundamental mechanisms is unclear. For instance, denervated muscle fibers within muscles from aged rodents or humans appear to generate large amounts of mitochondrial hydrogen peroxide that could influence adjacent innervated fibers. Thus, in this instance, a "secondary" source of reactive oxygen species may be potentially generated as a result of a primary age-related pathology (loss of neurons), but, nevertheless, may contribute to loss of muscle mass and function during aging.

Analysis of muscle fiber clustering in the diaphragm muscle of sarcopenic mice

INTRODUCTION

Sarcopenia likely comprises muscle fiber denervation and re-innervation, resulting in clustering of muscle fibers of the same type (classified by myosin heavy chain isoform composition). Development of methodology to quantitatively evaluate clustering of muscle fibers according to fiber type is necessary.

METHODS

Fiber type specific immunofluorescence histology was used to quantify fiber clustering in murine diaphragm muscle (n = 15) at ages 6 and 24 months.

RESULTS

With age, fiber type clustering is evidenced by fiber type specific changes in distances between fibers, specifically a 14% decrease to the closest fiber for type I and 24% increase for type IIx and/or IIb fibers (P < 0.001). Additionally, a 34% increase to the 3 closest type IIx and/or IIb fibers was found (P < 0.001).

CONCLUSIONS

This novel method of analyzing fiber type clustering may be useful in examining pathophysiological conditions of motor unit loss in neuromuscular disorders, myopathies, dystrophies, injuries, or amyotrophic lateral sclerosis.

Functional impact of diaphragm muscle sarcopenia in both male and female mice

To perform a range of ventilatory and nonventilatory behaviors, the diaphragm muscle (DIAm) must be able to generate sufficient forces throughout the lifespan. We hypothesized that sarcopenia impacts DIAm force generation and thus limits performance of expulsive, higher force, nonventilatory behaviors. Male and female mice (n = 79) at 6 and 24 mo of age (100 vs. 70-75% survival, respectively) were used to examine transdiaphragmatic pressure (Pdi) generation across motor behaviors in vivo and in vitro DIAm specific force. We found a significant effect of age on maximum Pdi (20-41% decline during tracheal occlusion and bilateral phrenic nerve stimulation), maximum DIAm specific force (30% decline), and DIAm fatigue resistance (15% increase). There were no differences between sexes in these age effects on DIAm performance. These results support our hypothesis that sarcopenia primarily impacts higher force, nonventilatory motor behaviors of the DIAm. Such functional limitations may have negative implications in the ability of the DIAm to generate forces needed for airway clearance in old age and thereby contribute to age-related respiratory complications.

Muscle wasting in disease: molecular mechanisms and promising therapies

Atrophy occurs in specific muscles with inactivity (for example, during plaster cast immobilization) or denervation (for example, in patients with spinal cord injuries). Muscle wasting occurs systemically in older people (a condition known as sarcopenia); as a physiological response to fasting or malnutrition; and in many diseases, including chronic obstructive pulmonary disorder, cancer-associated cachexia, diabetes, renal failure, cardiac failure, Cushing syndrome, sepsis, burns and trauma. The rapid loss of muscle mass and strength primarily results from excessive protein breakdown, which is often accompanied by reduced protein synthesis. This loss of muscle function can lead to reduced quality of life, increased morbidity and mortality. Exercise is the only accepted approach to prevent or slow atrophy. However, several promising therapeutic agents are in development, and major advances in our understanding of the cellular mechanisms that regulate the protein balance in muscle include the identification of several cytokines, particularly myostatin, and a common transcriptional programme that promotes muscle wasting. Here, we discuss these new insights and the rationally designed therapies that are emerging to combat muscle wasting.

Effects of loaded voluntary wheel exercise on performance and muscle hypertrophy in young and old male C57Bl/6J mice

This study compared the capacity of young and old male C57Bl/6J mice to exercise with increasing resistance over 10 weeks, and its impact on muscle mass. Young mice (aged 15-25 weeks) were subjected to low (LR) and high (HR) resistance exercise, whereas only LR was used for old mice (107-117 weeks). Weekly patterns of voluntary wheel activity, food consumption and body weights were measured. Running patterns changed over time and with age, with two peaks of activity detected for young, but only one for old mice: speed and distance run was also less for old mice. The mass for six limb muscles was measured at the end of the experiment. The most pronounced increase in mass in response to exercise was for the soleus in young and old mice, and also quadriceps and gastrocnemius in young mice. Soleus and quadriceps muscles were analyzed histologically for myofiber number and size. A striking feature was the many small myofibers in response to exercise in young (but not old) soleus, whereas these were not present after exercise in young or old quadriceps. Overall, there was a striking difference in response to exercise between muscles and this was influenced by age.

Muscle-derived extracellular signal-regulated kinases 1 and 2 are required for the maintenance of adult myofibers and their neuromuscular junctions

The Ras-extracellular signal-regulated kinase 1 and 2 (ERK1/2) pathway appears to be important for the development, maintenance, aging, and pathology of mammalian skeletal muscle. Yet no gene targeting of Erk1/2 in muscle fibers in vivo has been reported to date. We combined a germ line Erk1 mutation with Cre-loxP Erk2 inactivation in skeletal muscle to produce, for the first time, mice lacking ERK1/2 selectively in skeletal myofibers. Animals lacking muscle ERK1/2 displayed stunted postnatal growth, muscle weakness, and a shorter life span. Their muscles examined in this study, sternomastoid and tibialis anterior, displayed fragmented neuromuscular synapses and a mixture of modest fiber atrophy and loss but failed to show major changes in fiber type composition or absence of cell surface dystrophin. Whereas the lack of only ERK1 had no effects on the phenotypes studied, the lack of myofiber ERK2 explained synaptic fragmentation in the sternomastoid but not the tibialis anterior and a decrease in the expression of the acetylcholine receptor (AChR) epsilon subunit gene mRNA in both muscles. A reduction in AChR protein was documented in line with the above mRNA results. Evidence of partial denervation was found in the sternomastoid but not the tibialis anterior. Thus, myofiber ERK1/2 are differentially required for the maintenance of myofibers and neuromuscular synapses in adult mice.

Effects of aging and maternal protein restriction on the muscle fibers morphology and neuromuscular junctions of rats after nutritional recovery

Changes in the nutritional status of mothers may predispose their offspring to neuromuscular disorders in the long term. This study evaluated the effects of maternal protein restriction during pregnancy and lactation on the muscle fibers and neuromuscular junctions (NMJs) of the soleus muscle in the offspring of rats at 365 days of age that had undergone nutritional recovery. Wistar rats were divided into two groups: control (CG)--the offspring of mothers fed a normal protein diet (17%) and restricted (RG)--offspring of mothers fed a low protein diet (6%). After lactation, the male pups received standard chow ad libitum. At 365 days, samples of soleus muscle were collected for muscle fiber analysis (HE staining, NADH-TR reaction and ultrastructure), intramuscular collagen quantification (picrosirius red staining) and NMJs analysis (non-specific esterase technique). The cross-sectional area of type I fibers was reduced by 20% and type IIa fibers by 5% while type IIb fibers increased by 5% in the RG compared to the CG. The percentage of intramuscular collagen was 19% lower in the RG. Disorganization of the myofibrils and Z line was observed, with the presence of clusters of mitochondria in both groups. Regarding the NMJs, in the RG there was a reduction of 10% in the area and 17% in the small diameter and an increase of 7% in the large diameter. The results indicate that the effects of maternal protein restriction on muscle fibers and NMJs seem to be long-lasting and irreversible.

The need to more precisely define aspects of skeletal muscle regeneration

A more precise definition of the term 'skeletal muscle regeneration' is required to reduce confusion and misconceptions. In this paper the term is used only for events that follow myofibre necrosis, to result in myogenesis and new muscle formation: other key events include early inflammation and revascularisation, and later fibrosis and re-innervation. The term 'muscle regeneration' is sometimes used casually for situations that do not involve myonecrosis; such as restoration of muscle mass by hypertrophy after atrophy, and other forms of damage to muscle tissue components. These situations are excluded from the definition in this paper which is focussed on mammalian muscles with the long-term aim of clinical translation to enhance new muscle formation after acute or chronic injury or during surgery to replace whole muscles. The paper briefly outlines the cellular events involved in myogenesis during development and post-natal muscle growth, discusses the role of satellite cells in mature normal muscles, and the likely incidence of myofibre necrosis/regeneration in healthy ageing mammals (even when subjected to exercise). The importance of the various components of regeneration is outlined to emphasise that problems in each of these aspects can influence overall new muscle formation; thus care is needed for correct interpretation of altered kinetics. Various markers used to identify regenerating myofibres are critically discussed and, since these can all occur in other conditions, caution is required for accurate interpretation of these cellular events. Finally, clinical situations are outlined where there is a need to enhance skeletal muscle regeneration: these include acute and chronic injuries or transplantation with bioengineering to form new muscles, therapeutic approaches to muscular dystrophies, and comment on proposed stem cell therapies to reduce age-related loss of muscle mass and function. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Autophagy impairment in muscle induces neuromuscular junction degeneration and precocious aging

The cellular basis of age-related tissue deterioration remains largely obscure. The ability to activate compensatory mechanisms in response to environmental stress is an important factor for survival and maintenance of cellular functions. Autophagy is activated both under short and prolonged stress and is required to clear the cell of dysfunctional organelles and altered proteins. We report that specific autophagy inhibition in muscle has a major impact on neuromuscular synaptic function and, consequently, on muscle strength, ultimately affecting the lifespan of animals. Inhibition of autophagy also exacerbates aging phenotypes in muscle, such as mitochondrial dysfunction, oxidative stress, and profound weakness. Mitochondrial dysfunction and oxidative stress directly affect acto-myosin interaction and force generation but show a limited effect on stability of neuromuscular synapses. These results demonstrate that age-related deterioration of synaptic structure and function is exacerbated by defective autophagy.

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Autophagy in muscle decreases with age, and rescue prevents muscle loss

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Autophagy inhibition in muscles shortens animal lifespan

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Autophagy is required to maintain a normal muscle-nerve interaction

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Autophagy impairment induces oxidation of contractile proteins, causing weakness

The Neuromuscular Junction: Aging at the Crossroad between Nerves and Muscle

Aging is associated with a progressive loss of muscle mass and strength and a decline in neurophysiological functions. Age-related neuromuscular junction (NMJ) plays a key role in musculoskeletal impairment that occurs with aging. However, whether changes in the NMJ precede or follow the decline of muscle mass and strength remains unresolved. Many factors such as mitochondrial dysfunction, oxidative stress, inflammation, changes in the innervation of muscle fibers, and mechanical properties of the motor units probably perform an important role in NMJ degeneration and muscle mass and strength decline in late life. This review addresses the primary events that might lead to NMJ dysfunction with aging, including studies on biomarkers, signaling pathways, and animal models. Interventions such as caloric restriction and exercise may positively affect the NMJ through this mechanism and attenuate the age-related progressive impairment in motor function.

Therapies for sarcopenia and regeneration of old skeletal muscles: more a case of old tissue architecture than old stem cells

Age related loss of skeletal muscle mass and function (sarcopenia) reduces independence and the quality of life for individuals, and leads to falls and fractures with escalating health costs for the rapidly aging human population. Thus there is much interest in developing interventions to reduce sarcopenia. One area that has attracted recent attention is the proposed use of myogenic stem cells to improve regeneration of old muscles. This mini-review challenges the fundamental need for myogenic stem cell therapy for sarcopenia. It presents evidence that demonstrates the excellent capacity of myogenic stem cells from very old rodent and human muscles to form new muscles after experimental myofiber necrosis. The many factors required for successful muscle regeneration are considered with a strong focus on integration of components of old muscle bioarchitecture. The fundamental role of satellite cells in homeostasis of normal aging muscles and the incidence of endogenous regeneration in old muscles is questioned. These issues, combined with problems for clinical myogenic stem cell therapies for severe muscle diseases, raise fundamental concerns about the justification for myogenic stem cell therapy for sarcopenia.

Habitual exercise plus dietary supplementation with milk fat globule membrane improves muscle function deficits via neuromuscular development in senescence-accelerated mice

We examined the effects of habitual exercise plus nutritional intervention through consumption of milk fat globule membrane (MFGM), a milk component, on aging-related deficits in muscle mass and function in senescence-accelerated P1 mice. Combining wheel-running and MFGM (MFGMEx) intake significantly attenuated age-related declines in quadriceps muscle mass (control: 318 ± 6 mg; MFGMEx: 356 ± 9 mg; P < 0.05) and in contractile force (1.4-fold and 1.5-fold higher in the soleus and extensor digitorum longus muscles, respectively). Microarray analysis of genes in the quadriceps muscle revealed that MFGMEx stimulated neuromuscular development; this was supported by significantly increased docking protein-7 (Dok-7) and myogenin mRNA expression. Treatment of differentiating myoblasts with MFGM-derived phospholipid or sphingolipid fractions plus mechanical stretching also significantly increased Dok-7 mRNA expression. These findings suggest that habitual exercise plus dietary MFGM improves muscle function deficits through neuromuscular development, and that phospholipid and sphingolipid in MFGM contribute to its physiological actions.

Degeneration of neuromuscular junction in age and dystrophy

Functional denervation is a hallmark of aging sarcopenia as well as of muscular dystrophy. It is thought to be a major factor reducing skeletal muscle mass, particularly in the case of sarcopenia. Neuromuscular junctions (NMJs) serve as the interface between the nervous and skeletal muscular systems, and thus they may receive pathophysiological input of both pre- and post-synaptic origin. Consequently, NMJs are good indicators of motor health on a systemic level. Indeed, upon sarcopenia and dystrophy, NMJs morphologically deteriorate and exhibit altered characteristics of primary signaling molecules, such as nicotinic acetylcholine receptor and agrin. Since a remarkable reversibility of these changes can be observed by exercise, there is significant interest in understanding the molecular mechanisms underlying synaptic deterioration upon aging and dystrophy and how synapses are reset by the aforementioned treatments. Here, we review the literature that describes the phenomena observed at the NMJ in sarcopenic and dystrophic muscle as well as to how these alterations can be reversed and to what extent. In a second part, the current information about molecular machineries underlying these processes is reported.

Genome-wide DNA methylation changes with age in disease-free human skeletal muscle

A decline in skeletal muscle mass and function with aging is well recognized, but remains poorly characterized at the molecular level. Here, we report for the first time a genome-wide study of DNA methylation dynamics in skeletal muscle of healthy male individuals during normal human aging. We predominantly observed hypermethylation throughout the genome within the aged group as compared to the young subjects. Differentially methylated CpG (dmCpG) nucleotides tend to arise intragenically and are underrepresented in promoters and are overrepresented in the middle and 3′ end of genes. The intragenic methylation changes are overrepresented in genes that guide the formation of the junction of the motor neuron and myofibers. We report a low level of correlation of gene expression from previous studies of aged muscle with our current analysis of DNA methylation status. For those genes that had both changes in methylation and gene expression with age, we observed a reverse correlation, with the exception of intragenic hypermethylated genes that were correlated with an increased gene expression. We suggest that a minimal number of dmCpG sites or select sites are required to be altered in order to correlate with gene expression changes. Finally, we identified 500 dmCpG sites that perform well in discriminating young from old samples. Our findings highlight epigenetic links between aging postmitotic skeletal muscle and DNA methylation.

Respiratory chain deficiency in aged spinal motor neurons

Sarcopenia, muscle wasting, and strength decline with age, is an important cause of loss of mobility in the elderly individuals. The underlying mechanisms are uncertain but likely to involve defects of motor nerve, neuromuscular junction, and muscle. Loss of motor neurons with age and subsequent denervation of skeletal muscle has been recognized as one of the contributing factors. This study investigated aspects of mitochondrial biology in spinal motor neurons from elderly subjects. We found that protein components of complex I of mitochondrial respiratory chain were reduced or absent in a proportion of aged motor neurons–a phenomenon not observed in fetal tissue. Further investigation showed that complex I-deficient cells had reduced mitochondrial DNA content and smaller soma size. We propose that mitochondrial dysfunction in these motor neurons could lead to the cell loss and ultimately denervation of muscle fibers.