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.

Mitigating disuse-induced skeletal muscle atrophy in ageing: Resistance exercise as a critical countermeasure.

The gradual deterioration of physiological systems with ageing makes it difficult to maintain skeletal muscle mass (sarcopenia), at least partly due to the presence of 'anabolic resistance', resulting in muscle loss. Sarcopenia can be transiently but markedly accelerated through periods of muscle disuse-induced (i.e., unloading) atrophy due to reduced physical activity, sickness, immobilisation or hospitalisation. Periods of disuse are detrimental to older adults' overall quality of life and substantially increase their risk of falls, physical and social dependence, and early mortality. Disuse events induce skeletal muscle atrophy through various mechanisms, including anabolic resistance, inflammation, disturbed proteostasis and mitochondrial dysfunction, all of which tip the scales in favour of a negative net protein balance and subsequent muscle loss. Concerningly, recovery from disuse atrophy is more difficult for older adults than their younger counterparts. Resistance training (RT) is a potent anabolic stimulus that can robustly stimulate muscle protein synthesis and mitigate muscle losses in older adults when implemented before, during and following unloading. RT may take the form of traditional weightlifting-focused RT, bodyweight training and lower- and higher-load RT. When combined with sufficient dietary protein, RT can accelerate older adults' recovery from a disuse event, mitigate frailty and improve mobility; however, few older adults regularly participate in RT. A feasible and practical approach to improving the accessibility and acceptability of RT is through the use of resistance bands. Moving forward, RT must be prescribed to older adults to mitigate the negative consequences of disuse atrophy.

Exercise Preconditioning Blunts Early Atrogenes Expression and Atrophy in Gastrocnemius Muscle of Hindlimb Unloaded Mice.

A large set of FoxOs-dependent genes play a primary role in controlling muscle mass during hindlimb unloading. Mitochondrial dysfunction can modulate such a process. We hypothesized that endurance exercise before disuse can protect against disuse-induced muscle atrophy by enhancing peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) expression and preventing mitochondrial dysfunction and energy-sensing AMP-activated protein kinase (AMPK) activation. We studied cross sectional area (CSA) of muscle fibers of gastrocnemius muscle by histochemistry following 1, 3, 7, and 14 days of hindlimb unloading (HU). We used Western blotting and qRT-PCR to study mitochondrial dynamics and FoxOs-dependent atrogenes' expression at 1 and 3 days after HU. Preconditioned animals were submitted to moderate treadmill exercise for 7 days before disuse. Exercise preconditioning protected the gastrocnemius from disuse atrophy until 7 days of HU. It blunted alterations in mitochondrial dynamics up to 3 days after HU and the expression of most atrogenes at 1 day after disuse. In preconditioned mice, the activation of atrogenes resumed 3 days after HU when mitochondrial dynamics, assessed by profusion and pro-fission markers (mitofusin 1, MFN1, mitofusin 2, MFN2, optic atrophy 1, OPA1, dynamin related protein 1, DRP1 and fission 1, FIS1), PGC1α levels, and AMPK activation were at a basal level. Therefore, the normalization of mitochondrial dynamics and function was not sufficient to prevent atrogenes activation just a few days after HU. The time course of sirtuin 1 (SIRT1) expression and content paralleled the time course of atrogenes' expression. In conclusion, seven days of endurance exercise counteracted alterations of mitochondrial dynamics and the activation of atrogenes early into disuse. Despite the normalization of mitochondrial dynamics, the effect on atrogenes' suppression died away within 3 days of HU. Interestingly, muscle protection lasted until 7 days of HU. A longer or more intense exercise preconditioning may prolong atrogenes suppression and muscle protection.

Impact of disuse muscular atrophy on the compound muscle action potential.

BACKGROUND: Disuse atrophy from immobilization is the result of decreased neural activity and muscle unloading. METHODS: We studied the impact of disuse on hand intrinsic compound muscle action potentials (CMAPs) in a cohort of 39 patients with unilateral 6-week immobilization of the hand in a cast, after distal radius fracture. We excluded patients with nerve injury. We compared side-to-side CMAP characteristics at the time of cast removal and at a subsequent follow-up visit, after a mean interval of 7.8 weeks. RESULTS: Statistically significant reductions in CMAP amplitude were noted for the abductor pollicis brevis (29.2%), abductor digiti minimi (19.0%), and first dorsal interosseus (24.9%). There was partial repair of the relative CMAP reduction at the follow-up visit (20.1%, 10.7%, and 8.7%, respectively). There was no significant change in CMAP duration. CONCLUSIONS: These results provide a framework for quantifying the degree of hand intrinsic CMAP amplitude reduction attributed to disuse.

Loss of mitochondrial energetics is associated with poor recovery of muscle function but not mass following disuse atrophy.

Skeletal muscle atrophy is a clinically important outcome of disuse because of injury, immobilization, or bed rest. Disuse atrophy is accompanied by mitochondrial dysfunction, which likely contributes to activation of the muscle atrophy program. However, the linkage of muscle mass and mitochondrial energetics during disuse atrophy and its recovery is incompletely understood. Transcriptomic analysis of muscle biopsies from healthy older adults subject to complete bed rest revealed marked inhibition of mitochondrial energy metabolic pathways. To determine the temporal sequence of muscle atrophy and changes in intramyocellular lipid and mitochondrial energetics, we conducted a time course of hind limb unloading-induced atrophy in adult mice. Mitochondrial respiration and calcium retention capacity were diminished, whereas H2O2 emission was increased within 3 days of unloading before significant muscle atrophy. These changes were associated with a decrease in total cardiolipin and profound changes in remodeled cardiolipin species. Hind limb unloading performed in muscle-specific peroxisome proliferator-activated receptor-γ coactivator-1α/ß knockout mice, a model of mitochondrial dysfunction, did not affect muscle atrophy but impacted muscle function. These data suggest early mitochondrial remodeling affects muscle function but not mass during disuse atrophy. Early alterations in mitochondrial energetics and lipid remodeling may represent novel targets to prevent muscle functional impairment caused by disuse and to enhance recovery from periods of muscle atrophy.

Increased mitophagy in the skeletal muscle of spinal and bulbar muscular atrophy patients.

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disorder caused by polyglutamine expansion in the androgen receptor (AR) and characterized by the loss of lower motor neurons. Here we investigated pathological processes occurring in muscle biopsy specimens derived from SBMA patients and, as controls, age-matched healthy subjects and patients suffering from amyotrophic lateral sclerosis (ALS) and neurogenic atrophy. We detected atrophic fibers in the muscle of SBMA, ALS and neurogenic atrophy patients. In addition, SBMA muscle was characterized by the presence of a large number of hypertrophic fibers, with oxidative fibers having a larger size compared with glycolytic fibers. Polyglutamine-expanded AR expression was decreased in whole muscle, yet enriched in the nucleus, and localized to mitochondria. Ultrastructural analysis revealed myofibrillar disorganization and streaming in zones lacking mitochondria and degenerating mitochondria. Using molecular (mtDNA copy number), biochemical (citrate synthase and respiratory chain enzymes) and morphological (dark blue area in nicotinamide adenine dinucleotide-stained muscle cross-sections) analyses, we found a depletion of the mitochondria associated with enhanced mitophagy. Mass spectrometry analysis revealed an increase of phosphatidylethanolamines and phosphatidylserines in mitochondria isolated from SBMA muscles, as well as a 50% depletion of cardiolipin associated with decreased expression of the cardiolipin synthase gene. These observations suggest a causative link between nuclear polyglutamine-expanded AR accumulation, depletion of mitochondrial mass, increased mitophagy and altered mitochondrial membrane composition in SBMA muscle patients. Given the central role of mitochondria in cell bioenergetics, therapeutic approaches toward improving the mitochondrial network are worth considering to support SBMA patients.

Low-Frequency Electrical Stimulation of Denervated Skeletal Muscle Retards Muscle and Trabecular Bone Loss in Aged Rats.

Electrical stimulation (ES)-induced muscle contraction has multiple effects; however, mechano-responsiveness of bone tissue declines with age. Here, we investigated whether daily low-frequency ES-induced muscle contraction treatment reduces muscle and bone loss and ameliorates bone fragility in early-stage disuse musculoskeletal atrophy in aged rats. Twenty-seven-month-old male rats were assigned to age-matched groups comprising the control (CON), sciatic nerve denervation (DN), or DN with direct low-frequency ES (DN+ES) groups. The structural and mechanical properties of the trabecular and cortical bone of the tibiae, and the morphological and functional properties of the tibialis anterior (TA) muscles were assessed one week after DN. ES-induced muscle contraction force mitigated denervation-induced muscle and trabecular bone loss and deterioration of the mechanical properties of the tibia mid-diaphysis, such as the stiffness, but not the maximal load, in aged rats. The TA muscle in the DN+ES group showed significant improvement in the myofiber cross-sectional area and muscle force relative to the DN group. These results suggest that low-frequency ES-induced muscle contraction treatment retards trabecular bone and muscle loss in aged rats in early-stage disuse musculoskeletal atrophy, and has beneficial effects on the functional properties of denervated skeletal muscle.

Patient-identified impact of symptoms in spinal and bulbar muscular atrophy.

INTRODUCTION: The effects of spinal bulbar muscular atrophy (SBMA) on quality of life (QoL) are not well understood. This study describes symptoms from the patient's perspective and the impact these symptoms have on QoL. METHODS: We conducted open-ended interviews with 21 adult men with genetically confirmed SBMA. Using a qualitative framework technique, we coded and analyzed interviews to identify symptoms and resulting themes. RESULTS: From these interviews, 729 quotations were extracted. We identified 200 SBMA-specific symptoms and 20 symptomatic themes. Weakness was mentioned by all interviewees. Symptoms within the domain of mental health and the specific themes of emotional issues and psychological impact were also frequently mentioned. DISCUSSION: Numerous symptoms affect QoL for patients with SBMA. We identified previously unrecognized symptoms that are important to address in enhancing clinical care for patients with SBMA and in developing tools to evaluate efficacy in future clinical trials. Muscle Nerve 57: 40-44, 2018.

Defects in Neuromuscular Transmission May Underlie Motor Dysfunction in Spinal and Bulbar Muscular Atrophy.

UNLABELLED: Spinal and bulbar muscular atrophy (SBMA) in men is an androgen-dependent neuromuscular disease caused by expanded CAG repeats in the androgen receptor (AR). Whether muscle or motor neuron dysfunction or both underlies motor impairment in SBMA is unknown. Muscles of SBMA mice show significant contractile dysfunction, implicating them as a likely source of motor dysfunction, but whether disease also impairs neuromuscular transmission is an open question. Thus, we examined synaptic function in three well-studied SBMA mouse models-the AR97Q, knock-in (KI), and myogenic141 models-by recording in vitro miniature and evoked end-plate potentials (MEPPs and EPPs, respectively) intracellularly from adult muscle fibers. We found striking defects in neuromuscular transmission suggesting that toxic AR in SBMA impairs both presynaptic and postsynaptic mechanisms. Notably, SBMA causes neuromuscular synapses to become weak and muscles to become hyperexcitable in all three models. Presynaptic defects included deficits in quantal content, reduced size of the readily releasable pool, and impaired short-term facilitation. Postsynaptic defects included prolonged decay times for both MEPPs and EPPs, marked resistance to µ-conotoxin (a sodium channel blocker), and enhanced membrane excitability. Quantitative PCR revealed robust upregulation of mRNAs encoding neonatal isoforms of the AChR (γ-subunit) and the voltage-gated sodium channel (NaV1.5) in diseased adult muscles of all three models, consistent with the observed slowing of synaptic potentials and resistance to µ-conotoxin. These findings suggest that muscles of SBMA patients regress to an immature state that impairs neuromuscular function. SIGNIFICANCE STATEMENT: We have discovered that SBMA is accompanied by marked defects in neuromuscular synaptic transmission involving both presynaptic and postsynaptic mechanisms. For three different mouse models, we find that diseased synapses are weak, having reduced quantal content due to reductions in the size of the readily releasable pool and/or probability of release. Synaptic potentials in diseased adult fibers are slowed, explained by an aberrant upregulation of the neonatal isoform of the acetylcholine receptor. Diseased fibers also show marked resistance to µ-conotoxin, explained by an aberrant upregulation in the neonatal isoform of the sodium channel, and are hyperexcitable, reminiscent of myotonic dystrophy, showing anode-break action potentials. This work identifies several new molecular targets for recovering function in SBMA.

Membrane lipid rafts are disturbed in the response of rat skeletal muscle to short-term disuse.

Marked loss of skeletal muscle mass occurs under various conditions of disuse, but the molecular and cellular mechanisms leading to atrophy are not completely understood. We investigate early molecular events that might play a role in skeletal muscle remodeling during mechanical unloading (disuse). The effects of acute (6-12 h) hindlimb suspension on the soleus muscles from adult rats were examined. The integrity of plasma membrane lipid rafts was tested utilizing cholera toxin B subunit or fluorescent sterols. In addition, resting intracellular Ca2+ level was analyzed. Acute disuse disturbed the plasma membrane lipid-ordered phase throughout the sarcolemma and was more pronounced in junctional membrane regions. Ouabain (1 µM), which specifically inhibits the Na-K-ATPase α2 isozyme in rodent skeletal muscles, produced similar lipid raft changes in control muscles but was ineffective in suspended muscles, which showed an initial loss of α2 Na-K-ATPase activity. Lipid rafts were able to recover with cholesterol supplementation, suggesting that disturbance results from cholesterol loss. Repetitive nerve stimulation also restores lipid rafts, specifically in the junctional sarcolemma region. Disuse locally lowered the resting intracellular Ca2+ concentration only near the neuromuscular junction of muscle fibers. Our results provide evidence to suggest that the ordering of lipid rafts strongly depends on motor nerve input and may involve interactions with the α2 Na-K-ATPase. Lipid raft disturbance, accompanied by intracellular Ca2+ dysregulation, is among the earliest remodeling events induced by skeletal muscle disuse.

Skeletal muscle PGC-1ß signaling is sufficient to drive an endurance exercise phenotype and to counteract components of detraining in mice.

Peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α and -1ß serve as master transcriptional regulators of muscle mitochondrial functional capacity and are capable of enhancing muscle endurance when overexpressed in mice. We sought to determine whether muscle-specific transgenic overexpression of PGC-1ß affects the detraining response following endurance training. First, we established and validated a mouse exercise-training-detraining protocol. Second, using multiple physiological and gene expression end points, we found that PGC-1ß overexpression in skeletal muscle of sedentary mice fully recapitulated the training response. Lastly, PGC-1ß overexpression during the detraining period resulted in partial prevention of the detraining response. Specifically, an increase in the plateau at which O2 uptake (V̇o2) did not change from baseline with increasing treadmill speed [peak V̇o2 (ΔV̇o2max)] was maintained in trained mice with PGC-1ß overexpression in muscle 6 wk after cessation of training. However, other detraining responses, including changes in running performance and in situ half relaxation time (a measure of contractility), were not affected by PGC-1ß overexpression. We conclude that while activation of muscle PGC-1ß is sufficient to drive the complete endurance phenotype in sedentary mice, it only partially prevents the detraining response following exercise training, suggesting that the process of endurance detraining involves mechanisms beyond the reversal of muscle autonomous mechanisms involved in endurance fitness. In addition, the protocol described here should be useful for assessing early-stage proof-of-concept interventions in preclinical models of muscle disuse atrophy.

Prevention of muscle wasting and osteoporosis: the value of examining novel animal models.

Bone mass and skeletal muscle mass are controlled by factors such as genetics, diet and nutrition, growth factors and mechanical stimuli. Whereas increased mechanical loading of the musculoskeletal system stimulates an increase in the mass and strength of skeletal muscle and bone, reduced mechanical loading and disuse rapidly promote a decrease in musculoskeletal mass, strength and ultimately performance (i.e. muscle atrophy and osteoporosis). In stark contrast to artificially immobilised laboratory mammals, animals that experience natural, prolonged bouts of disuse and reduced mechanical loading, such as hibernating mammals and aestivating frogs, consistently exhibit limited or no change in musculoskeletal performance. What factors modulate skeletal muscle and bone mass, and what physiological and molecular mechanisms protect against losses of muscle and bone during dormancy and following arousal? Understanding the events that occur in different organisms that undergo natural periods of prolonged disuse and suffer negligible musculoskeletal deterioration could not only reveal novel regulatory factors but also might lead to new therapeutic options. Here, we review recent work from a diverse array of species that has revealed novel information regarding physiological and molecular mechanisms that dormant animals may use to conserve musculoskeletal mass despite prolonged inactivity. By highlighting some of the differences and similarities in musculoskeletal biology between vertebrates that experience disparate modes of dormancy, it is hoped that this Review will stimulate new insights and ideas for future studies regarding the regulation of atrophy and osteoporosis in both natural and clinical models of muscle and bone disuse.

The effect of exercise hypertrophy and disuse atrophy on muscle contractile properties: a mechanomyographic analysis.

PURPOSE: To determine whether mechanomyographic (MMG) determined contractile properties of the biceps brachii change during exercise-induced hypertrophy and subsequent disuse atrophy. METHODS: Healthy subjects (mean ± SD, 23.7 ± 2.6 years, BMI 21.8 ± 2.4, n = 19) performed unilateral biceps curls (9 sets × 12 repetitions, 5 sessions per week) for 8 weeks (hypertrophic phase) before ceasing exercise (atrophic phase) for the following 8 weeks (non-dominant limb; treatment, dominant limb; control). MMG measures of muscle contractile properties (contraction time; T c, maximum displacement; D max, contraction velocity; V c), electromyographic (EMG) measures of muscle fatigue (median power frequency; MPF), strength measures (maximum voluntary contraction; MVC) and measures of muscle thickness (ultrasound) were obtained. RESULTS: Two-way repeated measures ANOVA showed significant differences (P < 0.05) between treatment and control limbs. During the hypertrophic phase treatment MVC initially declined (weeks 1-3), due to fatigue (decline in MPF), followed by improvement against control during weeks 6-8. Between weeks 5 and 8 treatment, muscle thickness was greater than control, reflecting gross hypertrophy. MMG variables Dmax (weeks 2, 7) and Vc (weeks 7, 8) declined. During the atrophic phase, MVC (weeks 9-12) and muscle thickness (weeks 9, 10) initially remained high before declining to control levels, reflecting gross atrophy. MMG variables D max (weeks 9, 14) and V c (weeks 9, 14, 15) also declined during the atrophic phase. No change in T c was found throughout the hypertrophic or atrophic phases. CONCLUSIONS: MMG detects changes in contractile properties during stages of exercise-induced hypertrophy and disuse atrophy suggesting its applicability as a clinical tool in musculoskeletal rehabilitation.

Endoplasmic reticulum stress in spinal and bulbar muscular atrophy: a potential target for therapy.

Spinal and bulbar muscular atrophy is an X-linked degenerative motor neuron disease caused by an abnormal expansion in the polyglutamine encoding CAG repeat of the androgen receptor gene. There is evidence implicating endoplasmic reticulum stress in the development and progression of neurodegenerative disease, including polyglutamine disorders such as Huntington's disease and in motor neuron disease, where cellular stress disrupts functioning of the endoplasmic reticulum, leading to induction of the unfolded protein response. We examined whether endoplasmic reticulum stress is also involved in the pathogenesis of spinal and bulbar muscular atrophy. Spinal and bulbar muscular atrophy mice that carry 100 pathogenic polyglutamine repeats in the androgen receptor, and develop a late-onset neuromuscular phenotype with motor neuron degeneration, were studied. We observed a disturbance in endoplasmic reticulum-associated calcium homeostasis in cultured embryonic motor neurons from spinal and bulbar muscular atrophy mice, which was accompanied by increased endoplasmic reticulum stress. Furthermore, pharmacological inhibition of endoplasmic reticulum stress reduced the endoplasmic reticulum-associated cell death pathway. Examination of spinal cord motor neurons of pathogenic mice at different disease stages revealed elevated expression of markers for endoplasmic reticulum stress, confirming an increase in this stress response in vivo. Importantly, the most significant increase was detected presymptomatically, suggesting that endoplasmic reticulum stress may play an early and possibly causal role in disease pathogenesis. Our results therefore indicate that the endoplasmic reticulum stress pathway could potentially be a therapeutic target for spinal and bulbar muscular atrophy and related polyglutamine diseases.

p62/SQSTM1 differentially removes the toxic mutant androgen receptor via autophagy and inclusion formation in a spinal and bulbar muscular atrophy mouse model.

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders that are caused by the expansion of trinucleotide CAG repeats in the causative genes. Spinal and bulbar muscular atrophy (SBMA) is an inherited motor neuron disease that is caused by the expansion of a polyQ tract within the androgen receptor (AR). p62 is a ubiquitin- and light-chain 3-binding protein that is known to regulate the degradation of targeted proteins via autophagy and inclusion formation. In this study, we examined the effects of p62 depletion and overexpression on cultured cells and in a transgenic mouse model that overexpressed the mutant AR. Here, we demonstrate that depletion of p62 significantly exacerbated motor phenotypes and the neuropathological outcome, whereas overexpression of p62 protected against mutant AR toxicity in SBMA mice. Depletion of p62 significantly increased the levels of monomeric mutant AR and mutant AR protein complexes in an SBMA mouse model via the impairment of autophagic degradation. In addition, p62 overexpression improved SBMA mouse phenotypes by inducing cytoprotective inclusion formation. Our results demonstrate that p62 provides two different therapeutic targets in SBMA pathogenesis: (1) autophagy-dependent degradation and (2) benevolent inclusion formation of the mutant AR.

Nonsurgically induced disuse muscle atrophy and neuromuscular dysfunction upregulates alpha7 acetylcholine receptors.

Previous models of muscle disuse have invariably used surgical methods that require the repetitive application of plaster casts. A method of disuse atrophy that does not require such repetitive applications is described herein. Modified plastic pipette tubing was applied to a single hindlimb (mouse), from thigh to foot, resulting in immobilization of the knee in the extension position, and the ankle in the plantar flexion position. This method resulted in the loss of soleus muscle to 11%, 22%, 39%, and 45% of its original mass at 3, 7, 14, and 21 days, respectively, in association with a significant decrease of tibialis twitch (25%) and tetanic tensions (26%) at 21 days, compared with the contralateral side and (or) sham-immobilized controls. Immunohistochemical analysis of the soleus using fluorescent α-bungarotoxin revealed a significant increase in the number of synapses per unit area (818 + 31 compared with 433 + 16/mm(2)) and an increase in muscle fibers per unit area (117 compared with 83/mm(2)), most likely related to the atrophy of muscle fibers bringing synapses closer. A 3-fold increase in alpha7 acetylcholine receptor (α7AChR) protein expression, along with increased expression of α1AChR subunit in the immobilized side compared with the contralateral side was observed. The physiology and pharmacology of the novel finding of upregulation of α7AChRs with disuse requires further study.

Electrical stimulation of denervated rat skeletal muscle retards trabecular bone loss in early stages of disuse musculoskeletal atrophy.

OBJECTIVES: We aimed to determine the intensity of muscle stimulation required to prevent structural failure as well as bone and skeletal muscle loss after denervation-induced disuse. METHODS: Seven-week-old rats (weight, 198-225 g) were randomly assigned to age-matched groups comprising control (CON), sciatic nerve denervation (DN) or direct electrical stimulation (ES) one day later [after denervation] with 4, 8 and 16 mA at 10 Hz for 30 min/day, six days/week, for one or three weeks. Bone architecture and mean osteoid thickness in histologically stained tibial sections and tension in tibialis anterior muscles were assessed at one and three weeks after denervation. RESULTS: Direct ES with 16 mA generated 23-30% maximal contraction force. Denervation significantly decreased trabecular bone volume fraction, thickness and number, connectivity density and increased trabecular separation in the DN group at weeks one and three. Osteoid thickness was significantly greater in the ES16 group at week one than in the DN and other ES groups. Trabecular bone volume significantly correlated with muscle weight. CONCLUSIONS: Relatively low-level muscle contraction induced by low-frequency, high-intensity electrical muscle stimulation delayed trabecular bone loss during the early stages (one week after DN) of musculoskeletal atrophy due to disuse.

Age-related differences in the loss and recovery of serial sarcomere number following disuse atrophy in rats.

BACKGROUND: Older adults exhibit a slower recovery of muscle mass following disuse atrophy than young adults. At a smaller scale, muscle fibre cross-sectional area (i.e., sarcomeres in parallel) exhibits this same pattern. Less is known, however, about age-related differences in the recovery of muscle fibre length, driven by increases in serial sarcomere number (SSN), following disuse. The purpose of this study was to investigate age-related differences in SSN adaptations and muscle mechanical function during and following muscle immobilization. We hypothesized that older adult rats would experience a similar magnitude of SSN loss during immobilization, however, take longer to recover SSN than young following cast removal, which would limit the recovery of muscle mechanical function. METHODS: We casted the plantar flexors of young (8 months) and old (32 months) male rats in a shortened position for 2 weeks, and assessed recovery during 4 weeks of voluntary ambulation. Following sacrifice, legs were fixed in formalin for measurement of soleus SSN and physiological cross-sectional area (PCSA) with the un-casted soleus acting as a control. Ultrasonographic measurements of pennation angle (PA) and muscle thickness (MT) were conducted weekly. In-vivo active and passive torque-angle relationships were constructed pre-cast, post-cast, and following 4 weeks of recovery. RESULTS: From pre- to post-cast, young and older adult rats experienced similar decreases in SSN (-20%, P < 0.001), muscle wet weight (-25%, P < 0.001), MT (-30%), PA (-15%, P < 0.001), and maximum isometric torque (-40%, P < 0.001), but there was a greater increase in passive torque in older (+ 180%, P < 0.001) compared to young adult rats (+ 68%, P = 0.006). Following cast removal, young exhibited quicker recovery of SSN and MT than old, but SSN recovered sooner than PA and MT in both young and old. PCSA nearly recovered and active torque fully recovered in young adult rats, whereas in older adult rats these remained unrecovered at ∼ 75%. CONCLUSIONS: This study showed that older adult rats retain a better ability to recover longitudinal compared to parallel muscle morphology following cast removal, making SSN a highly adaptable target for improving muscle function in elderly populations early on during rehabilitation.

Mitochondrial pathways in sarcopenia of aging and disuse muscle atrophy.

Muscle loss during aging and disuse is a highly prevalent and disabling condition, but knowledge about cellular pathways mediating muscle atrophy is still limited. Given the postmitotic nature of skeletal myocytes, the maintenance of cellular homeostasis relies on the efficiency of cellular quality control mechanisms. In this scenario, alterations in mitochondrial function are considered a major factor underlying sarcopenia and muscle atrophy. Damaged mitochondria are not only less bioenergetically efficient, but also generate increased amounts of reactive oxygen species, interfere with cellular quality control mechanisms, and display a greater propensity to trigger apoptosis. Thus, mitochondria stand at the crossroad of signaling pathways that regulate skeletal myocyte function and viability. Studies on these pathways have sometimes provided unexpected and counterintuitive results, which suggests that they are organized into a complex, heterarchical network that is currently insufficiently understood. Untangling the complexity of such a network will likely provide clinicians with novel and highly effective therapeutics to counter the muscle loss associated with aging and disuse. In this review, we summarize the current knowledge on the mechanisms whereby mitochondrial dysfunction intervenes in the pathogenesis of sarcopenia and disuse atrophy, and highlight the prospect of targeting specific processes to treat these conditions.

Longitudinal changes of outcome measures in spinal and bulbar muscular atrophy.

Spinal and bulbar muscular atrophy is an adult-onset, hereditary motor neuron disease caused by the expansion of a trinucleotide CAG repeat within the gene encoding the androgen receptor. To date, several agents have been shown to prevent or slow disease progression in animal models of this disease. For the translational research of these agents, it is necessary to perform the detailed analysis of natural history with quantitative outcome measures and to establish sensitive and validated disease-specific endpoints in the clinical trials. To this end, we performed a prospective observation of disease progression over 3 years in 34 genetically confirmed Japanese patients with spinal and bulbar muscular atrophy by using quantitative outcome measures, including functional and blood parameters. The baseline evaluation revealed that CAG repeat length in the androgen receptor gene correlated not only with the age of onset but also with the timing of substantial changes in activity of daily living. Multiple regression analyses indicated that the serum level of creatinine is the most useful blood parameter that reflects the severity of motor dysfunction in spinal and bulbar muscular atrophy. In 3-year prospective analyses, a slow but steady progression was affirmed in most of the outcome measures we examined. In the analyses using random coefficient models that summarize the individual data into a representative line, disease progression was not affected by CAG repeat length or onset age. These models showed large interindividual variation, which was also independent of the differences of CAG repeat size. Analyses using these models also demonstrated that the subtle neurological deficits at an early or preclinical stage were more likely to be detected by objective motor functional tests such as the 6-min walk test and grip power or serum creatinine levels than by functional rating scales, such as the revised amyotrophic lateral sclerosis functional rating scale or modified Norris scale. Categorization of the clinical phenotypes using factor analysis showed that upper limb function is closely related to bulbar function, but not to lower limb function at baseline, whereas the site of onset had no substantial effects on disease progression. These results suggest that patients with spinal and bulbar muscular atrophy show a slow but steady progression of motor dysfunction over time that is independent of CAG repeat length or clinical phenotype, and that objective outcome measures may be used to evaluate disease severity at an early stage of this disease.