Neuron specific reduction in CuZnSOD is not sufficient to initiate a full sarcopenia phenotype

Reactive oxygen species in the pathogenesis of sarcopenia

One of the most critical factors impacting healthspan in the elderly is the loss of muscle mass and function, clinically referred to as sarcopenia. Muscle atrophy and weakness lead to loss of mobility, increased risk of injury, metabolic changes and loss of independence. Thus, defining the underlying mechanisms of sarcopenia is imperative to enable the development of effective interventions to preserve muscle function and quality in the elderly and improve healthspan. Over the past few decades, understanding the roles of mitochondrial dysfunction and oxidative stress has been a major focus of studies seeking to reveal critical molecular pathways impacted during aging. In this review, we will highlight how oxidative stress might contribute to sarcopenia by discussing the impact of oxidative stress on the loss of innervation and alteration in the neuromuscular junction (NMJ), on muscle mitochondrial function and atrophy pathways, and finally on muscle contractile function.

Mouse models used to test the role of reactive oxygen species in aging and age-related chronic diseases

With the development of the technology to generate transgenic and knockout mice in the 1990s, investigators had a powerful tool to directly test the impact of altering a specific gene on a biological process or disease. Over the past three decades, investigators have used transgenic and knockout mouse models, which have altered expression of antioxidant genes, to test the role of oxidative stress/damage in aging and age-related diseases. In this comprehensive review, we describe the studies using transgenic and knockout mouse models to test the role of oxidative stress/damage in aging (longevity) and three age-related diseases, e.g., sarcopenia, cardiac aging, and Alzheimer's Disease. While longevity was consistently altered only by one transgenic and one knockout mouse model as predicted by the Oxidative Stress Theory of Aging, the incidence/progression of the three age-related diseases (especially Alzheimer's disease) were robustly impacted when the expression of various antioxidant genes was altered using transgenic and knockout mouse models.

Exercise-induced adaptations to homeostasis of reactive oxygen species in skeletal muscle

Reactive oxygen species are generated by multiple mechanisms during contractile activity in exercising skeletal muscle and are recognised to play a role in signaling adaptations to the contractions. The sources of the superoxide and hydrogen peroxide generated are now relatively well understood but how the resulting low concentrations of hydrogen peroxide induce activation of multiple signaling pathways remains obscure. Several theories are presented together with accumulating evidence that 2-Cys peroxiredoxins may play a role of "effector" proteins in mediating the signaling actions of hydrogen peroxide. Identification of the mechanisms underlying these pathways offers the potential in the longer term for development of novel interventions to maintain exercise responses in the elderly with the potential to maintain muscle mass and function and consequent quality of life.

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

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

Tracking the Plasma C-Terminal Agrin Fragment as a Biomarker of Neuromuscular Decline in 18- to 87-Year-Old Men

OBJECTIVES

Plasma C-terminal agrin-fragment-22 (CAF22), a breakdown product of neuromuscular junction, is a potential biomarker of muscle loss. However, its levels from adolescence to octogenarians are unknown.

METHODS

We evaluated young (18-34 years, n = 203), middle-aged (35-59 years, n = 163), and old men (60-87 years, n = 143) for CAF22, handgrip strength (HGS), appendicular skeletal-mass index (ASMI), and gait speed.

RESULTS

We found an age-associated increase in CAF22 from young (100.9 ± 29 pmol) to middle-aged (128.3 ± 38.7 pmol) and older men (171.5 ± 35.5 pmol) (all p<0.05). This was accompanied by a gradual reduction in HGS (37.7 ± 6.1 kg, 30.2 ± 5.2 kg, and 26.6 ± 4.7 kg, for young, middle-aged, and old men, respectively), ASMI (8.02 ± 1.02 kg/m2, 7.65 ± 0.92 kg/m2, 6.87 ± 0.93 kg/m2, for young, middle-aged, and old men, respectively), and gait speed (1.29 ± 0.24 m/s, 1.05 ± 0.16 m/s, and 0.81 ± 0.13 m/s, for young, middle-aged, and old men, respectively). After adjustment for age, we found negative regressions of CAF22 with HGS (- 0.0574, p < 0.001) and gait speed (- 0.0162, p < 0.001) in the cumulative cohort. The receiver operating characteristics analysis revealed significant efficacy of plasma CAF22 in diagnosing muscle weakness (HGS < 27 kg) (middle-aged men; AUC = 0.731, 95% CI = 0.629-0.831, p < 0.001, Older men; AUC = 0.816, 95% CI = 0.761-0.833, p < 0.001), and low gait speed (0.8 m/s) (middle-aged men; AUC = 0.737, 95% CI = 0.602-0.871, p < 0.001, older men; AUC = 0.829, 95% CI = 0.772-0.886, p < 0.001), and a modest efficacy in diagnosing sarcopenia (middle-aged men; AUC = 0.701, 95% CI = 0.536-0.865, p = 0.032, older men; AUC = 0.822, 95% CI = 0.759-0.884, p < 0.001) in middle-aged and older men.

CONCLUSION

Altogether, CAF22 increases with advancing age and may be a reliable marker of muscle weakness and low gait speed.

Plasma levels of Neurofilament light chain correlate with handgrip strength and sarcopenia in patients with chronic obstructive pulmonary disease

BACKGROUND

Age-associated muscle decline, termed sarcopenia, is a common systemic effect of chronic obstructive pulmonary disease (COPD). Circulating Neurofilament light chain (NfL) levels reflect neuronal degradation and may be relevant to sarcopenia phenotype. However, such an association in COPD patients remains elusive.

METHODS

We investigated male, 60-76 years old controls (n = 50) and COPD patients (n = 139) for plasma NfL levels in relation to sarcopenia and physical capacity markers. We measured handgrip strength (HGS), body composition, and short physical performance battery (SPPB) to evaluate sarcopenia and physical capacity.

RESULTS

COPD patients had higher plasma NfL and lower HGS and SPPB performance than controls. Plasma NfL levels demonstrated negative associations with HGS and gait speed in COPD patients (all p < 0.05). Further, NfL levels were negatively associated with total SPPB scores in controls and patients with advanced COPD (p < 0.05). Plasma NfL also demonstrated an acceptable accuracy in diagnosing sarcopenia in controls (AUC = 0.757, p < 0.05) and COPD (AUC = 0.806, p < 0.05) patients.

CONCLUSION

Collectively, plasma NfL may be helpful in evaluating sarcopenia phenotype and physical capacity in geriatric patients with COPD.

Frontiers in sarcopenia: Advancements in diagnostics, molecular mechanisms, and therapeutic strategies

The onset of sarcopenia is intimately linked with aging, posing significant implications not only for individual patient quality of life but also for the broader societal healthcare framework. Early and accurate identification of sarcopenia and a comprehensive understanding of its mechanistic underpinnings and therapeutic targets paramount to addressing this condition effectively. This review endeavors to present a cohesive overview of recent advancements in sarcopenia research and diagnosis. We initially delve into the contemporary diagnostic criteria, specifically referencing the European Working Group on Sarcopenia in Older People (EWGSOP) 2 and Asian Working Group on Sarcopenia (AWGS) 2019 benchmarks. Additionally, we elucidate comprehensive assessment techniques for muscle strength, quantity, and physical performance, highlighting tools such as grip strength, chair stand test, dual-energy X-ray Absorptiometry (DEXA), bioelectrical impedance analysis (BIA), gait speed, and short physical performance battery (SPPB), while also discussing their inherent advantages and limitations. Such diagnostic advancements pave the way for early identification and unequivocal diagnosis of sarcopenia. Proceeding further, we provide a deep-dive into sarcopenia's pathogenesis, offering a thorough examination of associated signaling pathways like the Myostatin, AMP-activated protein kinase (AMPK), insulin/IGF-1 Signaling (IIS), and the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathways. Each pathway's role in sarcopenia mediation is detailed, underscoring potential therapeutic target avenues. From a mechanistic perspective, the review also underscores the pivotal role of mitochondrial dysfunction in sarcopenia, emphasizing elements such as mitochondrial oxidative overload, mitochondrial biogenesis, and mitophagy, and highlighting their therapeutic significance. At last, we capture recent strides made in sarcopenia treatment, ranging from nutritional and exercise interventions to potential pharmacological and supplementation strategies. In sum, this review meticulously synthesizes the latest scientific developments in sarcopenia, aiming to enhance diagnostic precision in clinical practice and provide comprehensive insights into refined mechanistic targets and innovative therapeutic interventions, ultimately contributing to optimized patient care and advancements in the field.

Muscle Involvement in Amyotrophic Lateral Sclerosis: Understanding the Pathogenesis and Advancing Therapeutics

Amyotrophic lateral sclerosis (ALS) is a fatal condition characterized by the selective loss of motor neurons in the motor cortex, brainstem, and spinal cord. Muscle involvement, muscle atrophy, and subsequent paralysis are among the main features of this disease, which is defined as a neuromuscular disorder. ALS is a persistently progressive disease, and as motor neurons continue to degenerate, individuals with ALS experience a gradual decline in their ability to perform daily activities. Ultimately, muscle function loss may result in paralysis, presenting significant challenges in mobility, communication, and self-care. While the majority of ALS research has traditionally focused on pathogenic pathways in the central nervous system, there has been a great interest in muscle research. These studies were carried out on patients and animal models in order to better understand the molecular mechanisms involved and to develop therapies aimed at improving muscle function. This review summarizes the features of ALS and discusses the role of muscle, as well as examines recent studies in the development of treatments.

ACE Inhibitors Improve Skeletal Muscle by Preserving Neuromuscular Junctions in Patients with Alzheimer's Disease

BACKGROUND

Hypertension and skeletal muscle decline are common findings in patients with Alzheimer's disease (AD). Angiotensin-converting enzyme (ACE) inhibitors preserve skeletal muscle and physical capacity; however, the driving mechanisms are poorly understood.

OBJECTIVE

We investigated the effects of ACE inhibitors on the neuromuscular junction (NMJ) with relevance to skeletal muscle and physical capacity in AD patients and age-matched controls.

METHODS

We evaluated controls (n = 59) and three groups of AD patients, including normotensive (n = 51) and patients with hypertension taking ACE inhibitors (n = 53) or other anti-hypertensive medications (n = 49) at baseline and one year later. We measure plasma c-terminal agrin fragment-22 (CAF22) as a marker of NMJ degradation, handgrip strength (HGS), and Short Physical Performance Battery (SPPB) as markers of physical capacity.

RESULTS

At baseline AD patients demonstrated lower HGS and SPPB scores and higher CAF22 levels than controls, irrespective of the hypertension status (all p < 0.05). The use of ACE inhibitors was associated with higher HGS and relative maintenance of SPPB scores, gait speed, and plasma CAF22 levels. Conversely, other anti-hypertensive medications were associated with an unaltered HGS, reduced SPPB scores and elevated plasma CAF22 levels (both p < 0.05). We also found dynamic associations of CAF22 with HGS, gait speed, and SPPB in AD patients taking ACE inhibitors (all p < 0.05). These changes were associated with reduced oxidative stress in AD patients taking ACE inhibitors (p < 0.05).

CONCLUSION

Altogether, ACE inhibitors are associated with higher HGS, preserved physical capacity, and the prevention of NMJ degradation in hypertensive AD patients.

Degradation of neuromuscular junction contributes to muscle weakness but not physical compromise in chronic obstructive pulmonary disease patients taking lipids-lowering medications

OBJECTIVES

The relevant data about the effects and the associated mechanisms of statins on muscle strength and physical capacity is inconsistent. We investigated the potential contribution of neuromuscular junction (NMJ) degradation to muscle weakness and physical compromise in patients with chronic obstructive pulmonary disease (COPD) on statins.

METHOD

We recruited male COPD patients (age range = 63-75 years, n = 150) as nonusers (n = 71) and users of statin medications (n = 79) along with age-matched controls (n = 76). The COPD patients were evaluated at baseline and one year later. The data about handgrip strength (HGS), body composition, short physical performance battery (SPPB), and plasma c-terminal agrin fragment-22 (CAF22) as a marker of NMJ disintegration was collected at two time points.

RESULTS

We observed lower HGS, SPPB scores, and higher CAF22 levels in all COPD patients than controls, irrespective of the treatment status (all p < 0.05). Statins further reduced HGS and elevated CAF22 in COPD patients (both p < 0.05). The decline in SPPB was relatively modest in statin users (≈3.7%, p = 0.032) than in nonusers (≈8.7%, p = 0.002). The elevated plasma CAF22 exhibited robust negative correlations with a reduction in HGS but not with SPPB in COPD patients taking statins. We also found a reduction in markers of inflammation and no increase in oxidative stress markers following statin use in COPD patients.

CONCLUSION

Altogether, statin-induced NMJ degradation exacerbates muscle decline but does not contribute to physical compromise in COPD patients.

Deletion of Sod1 in Motor Neurons Exacerbates Age-Related Changes in Axons and Neuromuscular Junctions in Mice

Whole-body knock-out of Cu,Zn superoxide dismutase (Sod1KO) results in accelerated, age-related loss of muscle mass and function associated with neuromuscular junction (NMJ) breakdown similar to sarcopenia. In order to determine whether altered redox in motor neurons underlies this phenotype, an inducible neuron-specific deletion of Sod1 (i-mnSod1KO) was compared with wild-type (WT) mice of different ages (adult, mid-age, and old) and whole-body Sod1KO mice. Nerve oxidative damage, motor neuron numbers and structural changes to neurons and NMJ were examined. Tamoxifen-induced deletion of neuronal Sod1 from two months of age. No specific effect of a lack of neuronal Sod1 was seen on markers of nerve oxidation (electron paramagnetic resonance of an in vivo spin probe, protein carbonyl, or protein 3-nitrotyrosine contents). i-mnSod1KO mice showed increased denervated NMJ, reduced numbers of large axons and increased number of small axons compared with old WT mice. A large proportion of the innervated NMJs in old i-mnSod1KO mice displayed a simpler structure than that seen in adult or old WT mice. Thus, previous work showed that neuronal deletion of Sod1 induced exaggerated loss of muscle in old mice, and we report that this deletion leads to a specific nerve phenotype including reduced axonal area, increased proportion of denervated NMJ, and reduced acetyl choline receptor complexity. Other changes in nerve and NMJ structure seen in the old i-mnSod1KO mice reflect aging of the mice.

Neuronal deletion of MnSOD in mice leads to demyelination, inflammation and progressive paralysis that mimics phenotypes associated with progressive multiple sclerosis

Neuronal oxidative stress has been implicated in aging and neurodegenerative disease. Here we investigated the impact of elevated oxidative stress induced in mouse spinal cord by deletion of Mn-Superoxide dismutase (MnSOD) using a neuron specific Cre recombinase in Sod2 floxed mice (i-mn-Sod2 KO). Sod2 deletion in spinal cord neurons was associated with mitochondrial alterations and peroxide generation. Phenotypically, i-mn-Sod2 KO mice experienced hindlimb paralysis and clasping behavior associated with extensive demyelination and reduced nerve conduction velocity, axonal degeneration, enhanced blood brain barrier permeability, elevated inflammatory cytokines, microglia activation, infiltration of neutrophils and necroptosis in spinal cord. In contrast, spinal cord motor neuron number, innervation of neuromuscular junctions, muscle mass, and contractile function were not altered. Overall, our findings show that loss of MnSOD in spinal cord promotes a phenotype of demyelination, inflammation and progressive paralysis that mimics phenotypes associated with progressive multiple sclerosis.

From mitochondria to sarcopenia: role of 17β-estradiol and testosterone

Sarcopenia, characterized by a loss of muscle mass and strength with aging, is prevalent in older adults. Although the exact mechanisms underlying sarcopenia are not fully understood, evidence suggests that the loss of mitochondrial integrity in skeletal myocytes has emerged as a pivotal contributor to the complex etiology of sarcopenia. Mitochondria are the primary source of ATP production and are also involved in generating reactive oxygen species (ROS), regulating ion signals, and initiating apoptosis signals in muscle cells. The accumulation of damaged mitochondria due to age-related impairments in any of the mitochondrial quality control (MQC) processes, such as proteostasis, biogenesis, dynamics, and mitophagy, can contribute to the decline in muscle mass and strength associated with aging. Interestingly, a decrease in sex hormones (e.g., 17β-estradiol and testosterone), which occurs with aging, has also been linked to sarcopenia. Indeed, 17β-estradiol and testosterone targeted mitochondria and exhibited activities in regulating mitochondrial functions. Here, we overview the current literature on the key mechanisms by which mitochondrial dysfunction contribute to the development and progression of sarcopenia and the potential modulatory effects of 17β-estradiol and testosterone on mitochondrial function in this context. The advance in its understanding will facilitate the development of potential therapeutic agents to mitigate and manage sarcopenia.

Impact of aging and oxidative stress on specific components of excitation contraction coupling in regulating force generation

Muscle weakness associated with sarcopenia is a major contributor to reduced health span and quality of life in the elderly. However, the underlying mechanisms of muscle weakness in aging are not fully defined. We investigated the effect of oxidative stress and aging on specific molecular mechanisms involved in muscle force production in mice and skinned permeabilized single fibers in mice lacking the antioxidant enzyme CuZnSod (Sod1KO) and in aging (24-month-old) wild-type mice. Loss of muscle strength occurs in both models, potentially because of reduced membrane excitability with altered NKA signaling and RyR stability, decreased fiber Ca2+ sensitivity and suppressed SERCA activity via modification of the Cys674 residue, dysregulated SR and cytosolic Ca2+ homeostasis, and impaired mitochondrial Ca2+ buffering and respiration. Our results provide a better understanding of the specific impacts of aging and oxidative stress on mechanisms related to muscle weakness that may point to future interventions for countering muscle weakness.

Analysis of the Effects of Ninjin’yoeito on Physical Frailty in Mice

Physical frailty is an aging-related clinical syndrome involving decreases in body weight, mobility, activity, and walking speed that occurs in individuals with sarcopenia and is accelerated by increased oxidative stress. Ninjin’yoeito, a traditional Japanese Kampo medicine, is used for treating conditions, including anemia and physical weakness. Here, we investigated whether ninjin’yoeito could improve physical frailty by controlling oxidative stress in the senescence-accelerated mouse prone 8 (SAMP8) model. First, SAMP8 mice were divided into two groups, ninjin’yoeito treated and untreated, with the former consuming a diet containing 3% ninjin’yoeito from 3 months of age. At 7 months of age, body weight, motor function, locomotor activity, and mean walking speed were measured. Subsequently, mice were euthanized and measured for muscle weight, 8-hydroxy-2′-deoxyguanosine levels in muscle and brain, and cleaved caspase-3 expression in brain. The results showed reductions in weight, locomotor function, locomotion, and average walking speed in the untreated group, which were significantly improved by ninjin’yoeito. Furthermore, 8-hydroxy-2′-deoxyguanosine levels were reduced in muscle and brain from ninjin’yoeito-treated mice, compared with the levels in untreated mice; cleaved caspase-3 expression was similarly reduced in brain from the treated mice, indicating reduced apoptosis. Our findings suggest that ninjin’yoeito inhibits sarcopenia-based physical frailty through its antioxidant effects.

Scavenging mitochondrial hydrogen peroxide by peroxiredoxin 3 overexpression attenuates contractile dysfunction and muscle atrophy in a murine model of accelerated sarcopenia

Age-related muscle atrophy and weakness, or sarcopenia, are significant contributors to compromised health and quality of life in the elderly. While the mechanisms driving this pathology are not fully defined, reactive oxygen species, neuromuscular junction (NMJ) disruption, and loss of innervation are important risk factors. The goal of this study is to determine the impact of mitochondrial hydrogen peroxide on neurogenic atrophy and contractile dysfunction. Mice with muscle-specific overexpression of the mitochondrial H₂ O₂  scavenger peroxiredoxin3 (mPRDX3) were crossed to Sod1KO mice, an established mouse model of sarcopenia, to determine whether reduced mitochondrial H₂ O₂ can prevent or delay the redox-dependent sarcopenia. Basal rates of H₂ O₂  generation were elevated in isolated muscle mitochondria from Sod1KO, but normalized by mPRDX3 overexpression. The mPRDX3 overexpression prevented the declines in maximum mitochondrial oxygen consumption rate and calcium retention capacity in Sod1KO. Muscle atrophy in Sod1KO was mitigated by ~20% by mPRDX3 overexpression, which was associated with an increase in myofiber cross-sectional area. With direct muscle stimulation, maximum isometric specific force was reduced by ~20% in Sod1KO mice, and mPRDX3 overexpression preserved specific force at wild-type levels. The force deficit with nerve stimulation was exacerbated in Sod1KO compared to direct muscle stimulation, suggesting NMJ disruption in Sod1KO. Notably, this defect was not resolved by overexpression of mPRDX3. Our findings demonstrate that muscle-specific PRDX3 overexpression reduces mitochondrial H₂ O₂  generation, improves mitochondrial function, and mitigates loss of muscle quantity and quality, despite persisting NMJ impairment in a murine model of redox-dependent sarcopenia.

Skeletal muscle transcriptomics identifies common pathways in nerve crush injury and ageing

Motor unit remodelling involving repeated denervation and re-innervation occurs throughout life. The efficiency of this process declines with age contributing to neuromuscular deficits. This study investigated differentially expressed genes (DEG) in muscle following peroneal nerve crush to model motor unit remodelling in C57BL/6 J mice. Muscle RNA was isolated at 3 days post-crush, RNA libraries were generated using poly-A selection, sequenced and analysed using gene ontology and pathway tools. Three hundred thirty-four DEG were found in quiescent muscle from (26mnth) old compared with (4-6mnth) adult mice and these same DEG were present in muscle from adult mice following nerve crush. Peroneal crush induced 7133 DEG in muscles of adult and 699 DEG in muscles from old mice, although only one DEG (ZCCHC17) was found when directly comparing nerve-crushed muscles from old and adult mice. This analysis revealed key differences in muscle responses which may underlie the diminished ability of old mice to repair following nerve injury.

Effects of prenatal testosterone on cumulative markers of oxidative damage to organs of young adult zebra finches (Taeniopygia guttata)

We tested the hypothesis that exposure of avian embryos to androgens in ovo entails long-term costs in the form of oxidative damage to vital cells and organs in adulthood. We injected zebra finch eggs with testosterone (T), monitored postnatal growth, and analyzed markers of oxidative damage in heart and liver in mature birds. We measured 8-oxo-2'-deoxyguanosine and isoprostanes, markers of oxidative damage to DNA and membrane lipids, respectively. T treatment (1) reduced growth rates of female but not male nestlings vs. controls; (2) resulted in less accumulation of 8-oxo-dG, but not IsoPs, in liver tissue of 60-day-old females, but not males; and (3) a trend toward elevated 8-oxo-dG levels in heart tissue of males and females at 60 and 180 days old combined. These results generally support the testosterone oxidative damage hypothesis, in that embryonic exposure to higher T resulted in damage to DNA of heart tissue in both sexes. They also suggest that sex-specific effects of androgens on early growth rates may carry over as differences in some forms of oxidative damage in adults. This supports a basic tenet of evolutionary aging theory that developmental influences early in life can be linked to costs later on.

Evaluation of Sarcopenia Using Biomarkers of the Neuromuscular Junction in Parkinson's Disease

Patients with Parkinson's disease (PD) present with an advanced form of age-related muscle loss or sarcopenia. However, the search for a biomarker to accurately predict muscle loss in PD remains elusive. We evaluated the biomarkers of neuromuscular junction (NMJ) stability, including c-terminal agrin fragment-22 (CAF22), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF) as predictors of muscle wasting and physical capacity in PD. Male, 63-78 years patients of PD, were investigated for physical capacity, handgrip strength (HGS), and circulating biomarkers at the diagnosis and follow-up during rehabilitation 6 months apart. Patients with PD presented with elevated CAF22 and reduced BDNF and GDNF levels, which were partially restored to normal levels with rehabilitation. All three biomarkers showed significant dynamic associations with HGS and indexes of sarcopenia. Logistic regression revealed that the combination of biomarkers levels into a cumulative risk score enhanced the diagnostic accuracy of sarcopenia. In brief, measurements of plasma BDNF, GDNF, and CAF22 may be helpful in timely diagnosis and/or evaluation of sarcopenia.

A Multifactorial Approach for Sarcopenia Assessment: A Literature Review

Simple Summary

Sarcopenia is characterized by an accelerated decline in skeletal muscle mass and strength, which results in poor quality of life, disability, and death. In the literature, sarcopenia is defined as the progressive breakdown of muscle tissue. The prevalence ranges from 5% to 13% in people 60–70 years old and from 11% to 50% in people older than 80 years. The comparison of risk factors associated with sarcopenia based on the European Working Group on Sarcopenia (1 and 2) in Older People, the Asian Working Group for Sarcopenia (1 and 2), the International Working Group on Sarcopenia, and the Foundation for the National Institutes of Health revealed no consistent patterns. Accordingly, the identification of a single risk factor for sarcopenia is unpredictable. Due to its “multifactorial” pathogenesis related to the involvement of a multitude of factors. In this review, we summarize 13 relevant risk factors associated with this disease that are important to consider prior to embarking on any related sarcopenia research. We suggest that researchers should concentrate on the biology of sarcopenia to develop a uniform consensus for screening this condition. In this review, we identify 50 biochemical markers across six pathways that have previously been investigated in subjects with sarcopenia. We suggest that these summarized biomarkers can be considered in future diagnosis to determine the biology of this disorder, thereby contributing to further research findings. As a result, a uniform consensus may also need to be established for screening and defining the disease. Sarcopenia is associated with a number of adverse economic and social outcomes, including disability, hospitalization, and death. In relation to this, we propose that we need to develop strategies including exercise interventions in the COVID-19 era to delay the onset and effects of sarcopenia. This suggestion should impact on sarcopenia’s primary and secondary outcomes, including physical, medical, social, and financial interactions.

Abstract

Sarcopenia refers to a progressive and generalized weakness of skeletal muscle as individuals age. Sarcopenia usually occurs after the age of 60 years and is associated with a persistent decline in muscle strength, function, and quality. A comparison of the risk factors associated with sarcopenia based on the European Working Group on Sarcopenia (1 and 2) in Older People, the Asian Working Group for Sarcopenia (1 and 2), the International Working Group on Sarcopenia, and the Foundation for the National Institutes of Health revealed no consistent patterns. Accordingly, the identification of a single risk factor for sarcopenia is unpredictable due to its “multifactorial” pathogenesis, with the involvement of a multitude of factors. Therefore, the first aim of this review was to outline and propose that the multiple factors associated with sarcopenia need to be considered in combination in the design of new experimentation in this area. A secondary aim was to highlight the biochemical risk factors that are already identified in subjects with sarcopenia to assist scientists in understanding the biology of the pathophysiological mechanisms affecting the old people with sarcopenia. We also briefly discuss primary outcomes (physical) and secondary outcomes (social and financial) of sarcopenia. For future investigative purposes, this comprehensive review may be useful in considering important risk factors in the utilization of a panel of biomarkers emanating from all pathways involved in the pathogenesis of this disease. This may help to establish a uniform consensus for screening and defining this disease. Considering the COVID-19 pandemic, its impact may be exacerbated in older populations, which requires immediate attention. Here, we briefly suggest strategies for advancing the development of smart technologies to deliver exercise in the COVID-19 era in an attempt regress the onset of sarcopenia. These strategies may also have an impact on sarcopenia’s primary and secondary outcomes.

miR-23a suppression accelerates functional decline in the rNLS8 mouse model of TDP-43 proteinopathy

Skeletal muscle dysfunction may contribute to the progression and severity of amyotrophic lateral sclerosis (ALS). In the present study, we characterized the skeletal muscle pathophysiology in an inducible transgenic mouse model (rNLS8) that develops a TAR-DNA binding protein (TDP-43) proteinopathy and ALS-like neuropathology and disease progression; representative of >90% of all familial and sporadic ALS cases. As we previously observed elevated levels of miR-23a in skeletal muscle of patients with familial and sporadic ALS, we also investigated the effect of miR-23a suppression on skeletal muscle pathophysiology and disease severity in rNLS8 mice. Five weeks after disease onset TDP-43 protein accumulation was observed in tibialis anterior (TA), quadriceps (QUAD) and diaphragm muscle lysates and associated with skeletal muscle atrophy. In the TA muscle TDP-43 was detected in muscle fibres that appeared atrophied and angular in appearance and that also contained β-amyloid aggregates. These fibres were also positive for neural cell adhesion molecule (NCAM), but not embryonic myosin heavy chain (eMHC), indicating TDP-43/ β-amyloid localization in denervated muscle fibres. There was an upregulation of genes associated with myogenesis and NMJ degeneration and a decrease in the MURF1 atrophy-related protein in skeletal muscle. Suppression of miR-23a impaired rotarod performance and grip strength and accelerated body weight loss during early stages of disease progression. This was associated with increased AchRα mRNA expression and decreased protein levels of PGC-1α. The TDP-43 proteinopathy-induced impairment of whole body and skeletal muscle functional performance is associated with muscle wasting and elevated myogenic and NMJ stress markers. Suppressing miR-23a in the rNLS8 mouse model of ALS contributes to an early acceleration of disease progression as measured by decline in motor function.

Sarcopenia in pulmonary diseases is associated with elevated sarcoplasmic reticulum stress and myonuclear disorganization

Chronic obstructive pulmonary disease (COPD) is frequently associated with age-related muscle loss or sarcopenia. However, the exact molecular mechanism of muscle loss in COPD remains elusive. We investigated the association of chronic dysregulation of sarcoplasmic reticulum (SR) protein homeostasis (a condition called SR stress) and myonuclear disorganization with sarcopenia in patients with COPD. Markers of SR stress and their downstream consequences, including apoptosis and inflammation, were upregulated in patients with COPD. The maximal SR Ca²⁺ ATPase (SERCA) activity was significantly reduced in advanced COPD as compared to healthy controls. Single muscle fiber diameter and cytoplasmic domain per myonucleus were significantly smaller in patients with advanced COPD than in healthy controls. Increased disruption of myonuclear organization was found in the COPD patients as compared to healthy controls. These changes in SR dysfunction were accompanied by elevated global levels of oxidative stress, including lipid peroxidation and mitochondrial reactive oxygen species (ROS) production. Altogether, our data suggest that muscle weakness in advanced COPD is in part associated with the disruption of SR protein and calcium homeostasis and their pathological consequences.

Prediction of Sarcopenia Using Multiple Biomarkers of Neuromuscular Junction Degeneration in Chronic Obstructive Pulmonary Disease

Patients with chronic obstructive pulmonary disease (COPD) present with an advanced form of age-related muscle loss or sarcopenia. Among multiple pathomechanisms of sarcopenia, neuromuscular junction (NMJ) degradation may be of primary relevance. We evaluated the circulating biomarkers of NMJ degradation, including c-terminal agrin fragment -22 (CAF22), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF) as predictors of sarcopenia in COPD during pulmonary rehabilitation (PR). Male, 61-77-year-old healthy controls and patients of COPD (n = 77-84/group) were recruited for measurements of circulating CAF22, BDNF, and GDNF levels. Functional assessment and measurements of plasma biomarkers were performed at diagnosis and following six months of PR. CAF22 levels were elevated while BDNF and GDNF levels were reduced in COPD patients at diagnosis, which were incompletely restored to normal levels following PR. These biomarkers showed varying degrees of associations with indexes of sarcopenia and functional recovery during PR. Logistic regression revealed that the combined use of three biomarkers enhanced the diagnostic accuracy of sarcopenia better than single biomarkers. Altogether, measurements of plasma CAF22, BDNF, and GDNF may be helpful for the accurate diagnosis of sarcopenia and functional capacity in COPD during PR.

Redox Signaling and Sarcopenia: Searching for the Primary Suspect

Sarcopenia, the age-related decline in muscle mass and function, derives from multiple etiological mechanisms. Accumulative research suggests that reactive oxygen species (ROS) generation plays a critical role in the development of this pathophysiological disorder. In this communication, we review the various signaling pathways that control muscle metabolic and functional integrity such as protein turnover, cell death and regeneration, inflammation, organismic damage, and metabolic functions. Although no single pathway can be identified as the most crucial factor that causes sarcopenia, age-associated dysregulation of redox signaling appears to underlie many deteriorations at physiological, subcellular, and molecular levels. Furthermore, discord of mitochondrial homeostasis with aging affects most observed problems and requires our attention. The search for the primary suspect of the fundamental mechanism for sarcopenia will likely take more intense research for the secret of this health hazard to the elderly to be unlocked.

The Skeletal Muscle Emerges as a New Disease Target in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that leads to progressive degeneration of motor neurons (MNs) and severe muscle atrophy without effective treatment. Most research on ALS has been focused on the study of MNs and supporting cells of the central nervous system. Strikingly, the recent observations of pathological changes in muscle occurring before disease onset and independent from MN degeneration have bolstered the interest for the study of muscle tissue as a potential target for delivery of therapies for ALS. Skeletal muscle has just been described as a tissue with an important secretory function that is toxic to MNs in the context of ALS. Moreover, a fine-tuning balance between biosynthetic and atrophic pathways is necessary to induce myogenesis for muscle tissue repair. Compromising this response due to primary metabolic abnormalities in the muscle could trigger defective muscle regeneration and neuromuscular junction restoration, with deleterious consequences for MNs and thereby hastening the development of ALS. However, it remains puzzling how backward signaling from the muscle could impinge on MN death. This review provides a comprehensive analysis on the current state-of-the-art of the role of the skeletal muscle in ALS, highlighting its contribution to the neurodegeneration in ALS through backward-signaling processes as a newly uncovered mechanism for a peripheral etiopathogenesis of the disease.

Prediction of sarcopenia using a battery of circulating biomarkers

Loss of muscle mass and strength with aging, termed sarcopenia is accelerated in several comorbidities including chronic heart failure (CHF) and chronic obstructive pulmonary diseases (COPD). However, the effective circulating biomarkers to accurately diagnose and assess sarcopenia are not known. We recruited male healthy controls and patients with CHF and COPD (n = 81–87/group), aged 55–74 years. Sarcopenia was clinically identified based on hand-grip strength, appendicular skeletal muscle index and physical capacity as recommended by the European working group for sarcopenia. The serum levels of amino-terminal pro-peptide of type-III procollagen, c-terminal agrin fragment-22, osteonectin, irisin, fatty acid-binding protein-3 and macrophage migration inhibitory factor were significantly different between healthy controls and patients with CHF and COPD. Risk scores for individual biomarkers were calculated by logistic regressions and combined into a cumulative risk score. The median cutoff value of 3.86 was used to divide subjects into high- and low-risk groups for sarcopenia with the area under the curve of 0.793 (95% CI = 0.738–0.845, p < 0.001). A significantly higher incidence of clinical sarcopenia was found in high-risk group. Taken together, the battery of biomarkers can be an effective tool in the early diagnosis and assessment of sarcopenia.

Reduced sarcoplasmic reticulum Ca2+ ATPase activity underlies skeletal muscle wasting in asthma

AIMS

Skeletal muscle mass and strength are reduced in asthma and contribute to compromised functional capacity in asthmatic patients. However, an effective pharmacological intervention remains elusive, partly because molecular mechanisms dictating muscle decline in asthma are not known.

MATERIALS

We investigated the potential contribution(s) of skeletal muscle sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) to muscle atrophy and weakness in asthmatic patients. Quadriceps muscle biopsies were taken from 58 to 72 years old male patients with mild and advanced asthma and the SERCA activity was analyzed in association with cellular redox environment and myonuclear domain (MND) size.

KEY FINDINGS

Maximal SERCA activity was reduced in skeletal muscles of mild and advanced asthmatics and was associated with reduced expression of SERCA2 protein and upregulation of sarcolipin, a SERCA inhibitory lipoprotein. We also found downregulation of Ca²⁺ release protein calstabin and upregulation of Ca²⁺ buffer, calsequestrin in skeletal muscles of asthmatic patients. The atrophic single muscle fibers had smaller cytoplasmic domains per myonucleus possibly indicating the reduced transcriptional reserves of individual myonuclei. Plasma periostin and CAF22 levels were significantly elevated in asthmatic patients and showed a strong correlation with hand-grip strength. These changes were accompanied by substantially elevated markers of global oxidative stress including lipid peroxidation and mitochondrial ROS production.

CONCLUSION

Taken together, our data suggest that muscle weakness and atrophy in asthma is in part driven by SERCA dysfunction and oxidative stress. The data propose SERCA dysfunction as a therapeutic intervention to address muscle decline in asthma.

Transgenic expression of SOD1 specifically in neurons of Sod1 deficient mice prevents defects in muscle mitochondrial function and calcium handling

Aging is accompanied by loss of muscle mass and force, known as sarcopenia. Muscle atrophy, weakness, and neuromuscular junction (NMJ) degeneration reminiscent of normal muscle aging are observed early in adulthood for mice deficient in Cu, Zn-superoxide dismutase (SOD, Sod1^-/-). Muscles of Sod1^-/- mice also display impaired mitochondrial ATP production and increased mitochondrial reactive oxygen species (ROS) generation implicating oxidative stress in sarcopenia. Restoration of CuZnSOD specifically in neurons of Sod1^-/- mice (SynTgSod1^-/-) prevents muscle atrophy and loss of force, but whether muscle mitochondrial function is preserved is not known. To establish links among CuZnSOD expression, mitochondrial function, and sarcopenia, we examined contractile properties, mitochondrial function and ROS production, intracellular calcium transients (ICT), and NMJ morphology in lumbrical muscles of 7-9 month wild type (WT), Sod1^-/-, and SynTgSod1^-/- mice. Compared with WT values, mitochondrial ROS production was increased 2.9-fold under basal conditions and 2.2-fold with addition of glutamate and malate in Sod1^-/- muscle fibers while oxygen consumption was not significantly altered. In addition, NADH recovery was blunted following contraction and the peak of the ICT was decreased by 25%. Mitochondrial function, ROS generation and calcium handling were restored to WT values in SynTgSod1^-/- mice, despite continued lack of CuZnSOD in muscle. NMJ denervation and fragmentation were also fully rescued in SynTgSod1^-/- mice suggesting that muscle mitochondrial and calcium handling defects in Sod1^-/- mice are secondary to neuronal oxidative stress and its effects on the NMJ rather than the lack of muscle CuZnSOD. We conclude that intact neuronal function and innervation are key to maintaining excitation-contraction coupling and muscle mitochondrial function.

Impairment Mechanisms and Intervention Approaches for Aged Human Neuromuscular Junctions

The neuromuscular junction (NMJ) is a chemical synapse formed between a presynaptic motor neuron and a postsynaptic muscle cell. NMJs in most vertebrate species share many essential features; however, some differences distinguish human NMJs from others. This review will describe the pre- and postsynaptic structures of human NMJs and compare them to NMJs of laboratory animals. We will focus on age-dependent declines in function and changes in the structure of human NMJs. Furthermore, we will describe insights into the aging process revealed from mouse models of accelerated aging. In addition, we will compare aging phenotypes to other human pathologies that cause impairments of pre- and postsynaptic structures at NMJs. Finally, we will discuss potential intervention approaches for attenuating age-related NMJ dysfunction and sarcopenia in humans.

Circulating Biomarkers of Accelerated Sarcopenia in Respiratory Diseases

Skeletal muscle dysfunction is a critical finding in many respiratory diseases. However, a definitive biomarker to assess muscle decline in respiratory diseases is not known. We analyzed the association of plasma levels of glycoprotein Dickkopf-3 (Dkk-3), c-terminal agrin fragment-22 (CAF22) and microRNAs miR-21, miR-134a, miR-133 and miR-206 with hand-grip strength (HGS) and appendicular skeletal mass index (ASMI) in male, 54-73-year-old patients with chronic obstructive pulmonary diseases (COPD), asthma or pulmonary TB (n = 83-101/group). Patients with respiratory diseases showed a reduction in HGS and gait speed, while a reduction in ASMI was only found in patients with pulmonary TB. Among the sarcopenia indexes, HGS showed the strongest correlation with plasma CAF22, miR-21 and miR-206 levels while ASMI showed the strongest correlation with Dkk-3 and miR-133 in respiratory diseases. We found a modest-to-significant increase in the plasma markers of inflammation, oxidative stress and muscle damage, which had varying degrees of correlations with Dkk-3, CAF22 and selected micro RNAs (miRs) in respiratory diseases. Taken together, our data show that plasma levels of Dkk-3, CAF22 and selected miRs can be useful tools to assess accelerated sarcopenia phenotype in the elderly with respiratory diseases.

Macpherson P, Ahn B, Piekarz KM, Staunton CA, Brown JL, Qaisar R, Vasilaki A, Richardson A, McArdle A, Jackson MJ, Brooks SV, Van Remmen H. Neuron-specific deletion of CuZnSOD leads to an advanced sarcopenic phenotype in older mice

Age-associated loss of muscle mass and function (sarcopenia) has a profound effect on the quality of life in the elderly. Our previous studies show that CuZnSOD deletion in mice (Sod1-/- mice) recapitulates sarcopenia phenotypes, including elevated oxidative stress and accelerated muscle atrophy, weakness, and disruption of neuromuscular junctions (NMJs). To determine whether deletion of Sod1 initiated in neurons in adult mice is sufficient to induce muscle atrophy, we treated young (2- to 4-month-old) Sod1flox/SlickHCre mice with tamoxifen to generate i-mn-Sod1KO mice. CuZnSOD protein was 40-50% lower in neuronal tissue in i-mn-Sod1KO mice. Motor neuron number in ventral spinal cord was reduced 28% at 10 months and more than 50% in 18- to 22-month-old i-mn-Sod1KO mice. By 24 months, 22% of NMJs in i-mn-Sod1KO mice displayed a complete lack of innervation and deficits in specific force that are partially reversed by direct muscle stimulation, supporting the loss of NMJ structure and function. Muscle mass was significantly reduced by 16 months of age and further decreased at 24 months of age. Overall, our findings show that neuronal-specific deletion of CuZnSOD is sufficient to cause motor neuron loss in young mice, but that NMJ disruption, muscle atrophy, and weakness are not evident until past middle age. These results suggest that loss of innervation is critical but may not be sufficient until the muscle reaches a threshold beyond which it cannot compensate for neuronal loss or rescue additional fibers past the maximum size of the motor unit.

Targeting cPLA2 derived lipid hydroperoxides as a potential intervention for sarcopenia

Defects in neuromuscular innervation contribute significantly to the age-related decline in muscle mass and function (sarcopenia). Our previous studies demonstrated that denervation induces muscle mitochondrial hydroperoxide production (H2O2 and lipid hydroperoxides (LOOHs)). Here we define the relative contribution of mitochondrial electron transport chain (ETC) derived H2O2 versus cytosolic phospholipase A2 (cPLA2) derived LOOHs in neurogenic muscle atrophy. We show that denervation increases muscle cPLA2 protein content, activity, and metabolites downstream of cPLA2 including LOOHs. Increased scavenging of mitochondrial H2O2 does not protect against denervation atrophy, suggesting ETC generated H2O2 is not a critical player. In contrast, inhibition of cPLA2 in vivo mitigates LOOH production and muscle atrophy and maintains individual muscle fiber size while decreasing oxidative damage. Overall, we show that loss of innervation in several muscle atrophy models including aging induces generation of LOOHs produced by arachidonic acid metabolism in the cPLA2 pathway contributing to loss of muscle mass.

Plasma CAF22 Levels as a Useful Predictor of Muscle Health in Patients with Chronic Obstructive Pulmonary Disease

Skeletal muscle dysfunction and reduced physical capacity are characteristic features of chronic obstructive pulmonary disease (COPD). However, the search for a reliable biomarker to assess muscle health in CODP remains elusive. We analyzed the course of hand-grip strength (HGS) and appendicular skeletal mass index (ASMI) in COPD in relation to spirometry decline and plasma extracellular heat shock protein-72 (eHSP72) and c-terminal fragment of agrin-22 (CAF22) levels. We evaluated male, 62-73 years old patients of COPD (N = 265) and healthy controls (N = 252) at baseline and after 12 and 24 months for plasma biomarkers, spirometry and HGS measurements. HGS declined significantly over time and plasma CAF22, but not eHSP72 levels, had a significant negative association with HGS and ASMI in COPD. Plasma CAF22 also had an association with walking speed and daily steps count in advanced COPD. Lower ASMI was associated with reduced HGS at all time-point. Narrow age-span of the study cohort and exclusion of lower-limb muscles from the analysis are limitations of this study. Taken together, we report that the plasma CAF22 may be a useful tool to assess muscle weakness and atrophy in COPD patients.

Molecular changes in transcription and metabolic pathways underlying muscle atrophy in the CuZnSOD null mouse model of sarcopenia

Mice lacking the superoxide anion scavenger CuZn superoxide dismutase (Sod1^-/- mice) develop a number of age-related phenotypes, including an early progression of muscle atrophy and weakness (sarcopenia) associated with loss of innervation. The purpose of this study was to delineate the early development of sarcopenia in the Sod1^-/- mice and to measure changes in the muscle transcriptome, proteome, and eicosanoid profile at the stage when sarcopenia is markedly induced in this model (7-9 months of age). We found a strong correlation between muscle atrophy and mitochondrial state 1 hydroperoxide production, which was 40% higher in isolated mitochondria from Sod1^-/- mouse gastrocnemius muscle by 2 months of age. The primary pathways showing altered gene expression in Sod1^-/- mice identified by RNA-seq transcriptomic analysis are protein ubiquitination, synaptic long-term potentiation, calcium signaling, phospholipase C signaling, AMPK, and TWEAK signaling. Targeted proteomics shows elevated expression of mitochondrial proteins, fatty acid metabolism enzymes, tricarboxylic acid (TCA) cycle enzymes, and antioxidants, while enzymes involved in carbohydrate metabolism are downregulated in Sod1^-/- mice. LC-MS analysis of lipids in gastrocnemius muscle detected 78 eicosanoids, of which 31 are significantly elevated in muscle from Sod1^-/- mice. These data suggest that mitochondrial hydroperoxide generation is elevated prior to muscle atrophy and may be a potential driving factor of changes in the transcriptome, proteome, and eicosanoid profile of the Sod1^-/- mice. Together, these analyses revealed important molecular events that occur during muscle atrophy, which will pave the way for future studies using new approaches to treat sarcopenia.

Muscle unloading: A comparison between spaceflight and ground-based models

Prolonged unloading of skeletal muscle, a common outcome of events such as spaceflight, bed rest and hindlimb unloading, can result in extensive metabolic, structural and functional changes in muscle fibres. With advancement in investigations of cellular and molecular mechanisms, understanding of disuse muscle atrophy has significantly increased. However, substantial gaps exist in our understanding of the processes dictating muscle plasticity during unloading, which prevent us from developing effective interventions to combat muscle loss. This review aims to update the status of knowledge and underlying mechanisms leading to cellular and molecular changes in skeletal muscle during unloading. We have also discussed advances in the understanding of contractile dysfunction during spaceflights and in ground-based models of muscle unloading. Additionally, we have elaborated on potential therapeutic interventions that show promising results in boosting muscle mass and strength during mechanical unloading. Finally, we have identified key gaps in our knowledge as well as possible research direction for the future.

Mechanistic models to guide redox investigations and interventions in musculoskeletal ageing

Age is the greatest risk factor for the major chronic musculoskeletal disorders, osteoarthritis, osteoporosis and age-related loss of skeletal muscle mass and function (sarcopenia). Dramatic advances in understanding of the fundamental mechanisms underlying the ageing process are being exploited to understand the causes of these age-related disorders and identify approaches to prevent or treat these disorders. This review will focus on one of these fundamental mechanisms, redox regulation, and the role of redox changes in age-related loss of skeletal muscle mass and function (sarcopenia). Key to understanding the role of such pathways has been the development and study of experimental models of musculoskeletal ageing that are designed to examine the effect of modification of ROS regulatory enzymes. These have primarily involved genetic deletion of regulatory enzymes for ROS in mice. Many of the models studied show increased oxidative damage in tissues, but no clear relationship with skeletal muscle aging has been seen The exception to this has been mice with disruption of the superoxide dismutases and, in particular, deletion of Cu,ZnSOD (SOD1) localised in the cytosol and mitochondrial intermembrane space. Studies of tissue specific models lacking SOD1 have highlighted the potential role that disrupted redox pathways can play in muscle loss and weakness and have demonstrated the need to study both motor neurons and muscle to understand age-related loss of skeletal muscle. The complex interplay that has been identified between changes in redox homeostasis in the motor neuron and skeletal muscle and their role in premature loss of muscle mass and function illustrates the utility of modifiable models to establish key pathways that may contribute to age-related changes and identify potential logical approaches to intervention.

Using MRI to measure in vivo free radical production and perfusion dynamics in a mouse model of elevated oxidative stress and neurogenic atrophy

Mitochondrial dysfunction, reactive oxygen species (ROS) and oxidative damage have been implicated to play a causative role in age-related skeletal muscle atrophy and weakness (i.e. sarcopenia). Mice lacking the superoxide scavenger CuZnSOD (Sod1-/-) exhibit high levels of oxygen-derived radicals and oxidative damage, associated with neuronal and muscular phenotypes consistent with sarcopenia. We used magnetic resonance imaging (MRI) technology combined with immunospin-trapping (IST) to measure in vivo free radical levels in skeletal muscle from wildtype, Sod1-/- and SynTgSod1-/- mice, a mouse model generated using targeted expression of the human Sod1 transgene specifically in neuronal tissues to determine the impact of motor neuron degeneration in muscle atrophy. By combining the spin trap DMPO (5,5-dimethyl-1-pyrroline N-oxide) and molecular MRI (mMRI), we monitored the level of free radicals in mouse hindlimb muscle. The level of membrane-bound macromolecular radicals in the quadriceps muscle was elevated by ~3-fold in Sod1-/- mice, but normalized to wildtype levels in SynTgSod1-/- rescue mice. Skeletal muscle mass was reduced by ~25-30% in Sod1-/- mice, but fully reversed in muscle from SynTgSod1-/- mice. Using perfusion MRI we also measured the dynamics of blood flow within mouse hindlimb. Relative muscle blood flow in Sod1-/- is decreased to ~50% of wildtype and remained low in the SynTgSod1-/- mice. Our findings are significant in that we have shown for the first time that in vivo free radical production in skeletal muscle is directly correlated to muscle atrophy in an experimental model of oxidative stress. Neuron-specific expression of CuZnSOD reverses the in vivo free radical production in skeletal muscle in the Sod1-/- mouse model and prevents muscle atrophy. These results further support the feasibility of using in vivo assessments of redox status in the progression of a pathological process such as sarcopenia. This approach can also be valuable for evaluating responses to pharmacologic interventions.

Metabolic and Stress Response Changes Precede Disease Onset in the Spinal Cord of Mutant SOD1 ALS Mice

Many Amyotrophic Lateral Sclerosis (ALS) patients experience hypermetabolism, or an increase in measured vs. calculated metabolic rate. The cause of hypermetabolism and the effects on neuronal metabolism in ALS are currently unknown, but the efficacy of dietary interventions shows promise for metabolism as an ALS therapeutic target. The goal of this study is to measure changes in metabolic pathways as a function of disease progression in spinal cords of the SOD1^G93A mouse model of ALS. We conducted a comprehensive assessment of protein expression for metabolic pathways, antioxidants, chaperones, and proteases in lumbar spinal cord from male SOD1^G93A mice at pre-onset, onset, and end-stages of the disease using targeted proteomic analysis. These results reveal that protein content of metabolic proteins including proteins involved in glycolysis, β-oxidation, and mitochondrial metabolism is altered in SOD1^G93A mouse spinal cord well before disease onset. The changes in mitochondrial metabolism proteins are associated with decreased maximal respiration and glycolytic flux in SOD1^G93A dermal fibroblasts and increased hydrogen peroxide and lipid hydroperoxide production in mitochondria from sciatic nerve and gastrocnemius muscle fibers at end stage of disease. Consistent with redox dysregulation, expression of the glutathione antioxidant system is decreased, and peroxiredoxins and catalase expression are increased. In addition, stress response proteases and chaperones, including those involved in the mitochondrial unfolded protein response (UPR^mt), are induced before disease onset. In summary, we report that metabolic and stress response changes occur in SOD1^G93A lumbar spinal cord before motor symptom onset, and are primarily caused by SOD1^G93A expression and do not vary greatly as a function of disease course.

Accelerated sarcopenia in Cu/Zn superoxide dismutase knockout mice

Mice lacking Cu/Zn-superoxide dismutase (Sod1^-/- or Sod1KO mice) show high levels of oxidative stress/damage and a 30% decrease in lifespan. The Sod1KO mice also show many phenotypes of accelerated aging with the loss of muscle mass and function being one of the most prominent aging phenotypes. Using various genetic models targeting the expression of Cu/Zn-superoxide dismutase to specific tissues, we evaluated the role of motor neurons and skeletal muscle in the accelerated loss of muscle mass and function in Sod1KO mice. Our data are consistent with the sarcopenia in Sod1KO mice arising through a two-hit mechanism involving both motor neurons and skeletal muscle. Sarcopenia is initiated in motor neurons leading to a disruption of neuromuscular junctions that results in mitochondrial dysfunction and increased generation of reactive oxygen species (ROS) in skeletal muscle. The mitochondrial ROS generated in muscle feedback on the neuromuscular junctions propagating more disruption of neuromuscular junctions and more ROS production by muscle resulting in a vicious cycle that eventually leads to disaggregation of neuromuscular junctions, denervation, and loss of muscle fibers.

Smoke-induced neuromuscular junction degeneration precedes the fibre type shift and atrophy in chronic obstructive pulmonary disease

KEY POINTS

Chronic obstructive pulmonary disease (COPD) is largely caused by smoking, and patient limb muscle exhibits a fast fibre shift and atrophy. We show that this fast fibre shift is associated with type grouping, suggesting recurring cycles of denervation-reinnervation underlie the type shift. Compared to patients with normal fat-free mass index (FFMI), patients with low FFMI exhibited an exacerbated fibre type shift, marked accumulation of very small persistently denervated muscle fibres, and a blunted denervation-responsive transcript profile, suggesting failed denervation precipitates muscle atrophy in patients with low FFMI. Sixteen weeks of passive tobacco smoke exposure in mice caused neuromuscular junction degeneration, consistent with a key role for smoke exposure in initiating denervation in COPD.

ABSTRACT

A neurological basis for the fast fibre shift and atrophy seen in limb muscle of patients with chronic obstructive pulmonary disease (COPD) has not been considered previously. The objective of our study was: (1) to determine if denervation contributes to fast fibre shift and muscle atrophy in COPD; and (2) to assess using a preclinical smoking mouse model whether chronic tobacco smoke (TS) exposure could initiate denervation by causing neuromuscular junction (NMJ) degeneration. Vastus lateralis muscle biopsies were obtained from severe COPD patients [n = 10 with low fat-free mass index (FFMI), 65 years; n = 15 normal FFMI, 65 years) and healthy age- and activity-matched non-smoker control subjects (CON; n = 11, 67 years), to evaluate morphological and transcriptional markers of denervation. To evaluate the potential for chronic TS exposure to initiate these changes, we examined NMJ morphology in male adult mice following 16 weeks of passive TS exposure. We observed a high proportion of grouped fast fibres and a denervation transcript profile in COPD patients, suggesting that motor unit remodelling drives the fast fibre type shift in COPD patient limb muscle. A further exacerbation of fast fibre grouping in patients with low FFMI, coupled with blunted reinnervation signals, accumulation of very small non-specific esterase hyperactive fibres and neural cell adhesion molecule-positive type I and type II fibres, suggests denervation-induced exhaustion of reinnervation contributes to muscle atrophy in COPD. Evidence from a smoking mouse model showed significant NMJ degeneration, suggesting that recurring denervation in COPD is probably caused by decades of chronic TS exposure.

Long term rapamycin treatment improves mitochondrial DNA quality in aging mice

Age-induced mitochondrial DNA deletion mutations may underlie cell loss and tissue aging. Rapamycin extends mouse lifespan and modulates mitochondrial quality control. We hypothesized that reduced deletion mutation abundance may contribute to rapamycin's life extension effects. To test this hypothesis, genetically heterogeneous male and female mice were treated with rapamycin, compounded in chow at 14 or 42 ppm, from 9 months to 22 months of age. Mice under a 40% dietary restriction were included as a control known to protect mtDNA quality. To determine if chronic rapamycin treatment affects mitochondrial DNA quality, we assayed mtDNA deletion frequency and electron transport chain deficient fiber abundances in mouse quadriceps muscle. At 42 ppm rapamycin, we observed a 57% decrease in deletion frequency, a 2.8-fold decrease in ETC deficient fibers, and a 3.4-fold increase in the number of mice without electron transport chain deficient fibers. We observed a similar trend with the 14 ppm dose. DR significantly decreased ETC deficient fiber abundances with a trend toward lower mtDNA deletion frequency. The effects of rapamycin treatment on mitochondrial DNA quality were greatest in females at the highest dose. Rapamycin treatment at 14 ppm did not affect muscle mass or function. Dietary restriction also reduced deletion frequency and ETC deficient fibers. These data support the concept that the lifespan extending effects of rapamycin treatment result from enhanced mitochondrial DNA quality.

Comparison of Whole Body SOD1 Knockout with Muscle-Specific SOD1 Knockout Mice Reveals a Role for Nerve Redox Signaling in Regulation of Degenerative Pathways in Skeletal Muscle

AIMS

Lack of Cu,Zn-superoxide dismutase (CuZnSOD) in homozygous knockout mice (Sod1^-/-) leads to accelerated age-related muscle loss and weakness, but specific deletion of CuZnSOD in skeletal muscle (mSod1KO mice) or neurons (nSod1KO mice) resulted in only mild muscle functional deficits and failed to recapitulate the loss of mass and function observed in Sod1^-/- mice. To dissect any underlying cross-talk between motor neurons and skeletal muscle in the degeneration in Sod1^-/- mice, we characterized neuromuscular changes in the Sod1^-/- model compared with mSod1KO mice and examined degenerative molecular mechanisms and pathways in peripheral nerve and skeletal muscle.

RESULTS

In contrast to mSod1KO mice, myofiber atrophy in Sod1^-/- mice was associated with increased muscle oxidative damage, neuromuscular junction degeneration, denervation, nerve demyelination, and upregulation of proteins involved in maintenance of myelin sheaths. Proteomic analyses confirmed increased proteasomal activity and adaptive stress responses in muscle of Sod1^-/- mice that were absent in mSod1KO mice. Peripheral nerve from neither Sod1^-/- nor mSod1KO mice showed increased oxidative damage or molecular responses to increased oxidation compared with wild type mice. Differential cysteine (Cys) labeling revealed a specific redox shift in the catalytic Cys residue of peroxiredoxin 6 (Cys47) in the peripheral nerve from Sod1^-/- mice. Innovation and Conclusion: These findings demonstrate that neuromuscular integrity, redox mechanisms, and pathways are differentially altered in nerve and muscle of Sod1^-/- and mSod1KO mice. Results support the concept that impaired redox signaling, rather than oxidative damage, in peripheral nerve plays a key role in muscle loss in Sod1^-/- mice and potentially sarcopenia during aging. Antioxid. Redox Signal. 28, 275-295.

Redox Homeostasis in Age-Related Muscle Atrophy

Muscle atrophy and weakness, characterized by loss of lean muscle mass and function, has a significant effect on the independence and quality of life of older people. The cellular mechanisms that drive the age-related decline in neuromuscular integrity and function are multifactorial. Quiescent and contracting skeletal muscle can endogenously generate reactive oxygen and nitrogen species (RONS) from various cellular sites. Excessive RONS can potentially cause oxidative damage and disruption of cellular signaling pathways contributing to the initiation and progression of age-related muscle atrophy. Altered redox homeostasis and modulation of intracellular signal transduction processes have been proposed as an underlying mechanism of sarcopenia. This chapter summarizes the current evidence that has associated disrupted redox homeostasis and muscle atrophy as a result of skeletal muscle inactivity and aging.

Redox homeostasis and age-related deficits in neuromuscular integrity and function

Skeletal muscle is a major site of metabolic activity and is the most abundant tissue in the human body. Age-related muscle atrophy (sarcopenia) and weakness, characterized by progressive loss of lean muscle mass and function, is a major contributor to morbidity and has a profound effect on the quality of life of older people. With a continuously growing older population (estimated 2 billion of people aged >60 by 2050), demand for medical and social care due to functional deficits, associated with neuromuscular ageing, will inevitably increase. Despite the importance of this 'epidemic' problem, the primary biochemical and molecular mechanisms underlying age-related deficits in neuromuscular integrity and function have not been fully determined. Skeletal muscle generates reactive oxygen and nitrogen species (RONS) from a variety of subcellular sources, and age-associated oxidative damage has been suggested to be a major factor contributing to the initiation and progression of muscle atrophy inherent with ageing. RONS can modulate a variety of intracellular signal transduction processes, and disruption of these events over time due to altered redox control has been proposed as an underlying mechanism of ageing. The role of oxidants in ageing has been extensively examined in different model organisms that have undergone genetic manipulations with inconsistent findings. Transgenic and knockout rodent studies have provided insight into the function of RONS regulatory systems in neuromuscular ageing. This review summarizes almost 30 years of research in the field of redox homeostasis and muscle ageing, providing a detailed discussion of the experimental approaches that have been undertaken in murine models to examine the role of redox regulation in age-related muscle atrophy and weakness.

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.

Mitochondria Initiate and Regulate Sarcopenia

We present the hypothesis that an accumulation of dysfunctional mitochondria initiates a signaling cascade leading to motor neuron and muscle fiber death and culminating in sarcopenia. Interactions between neural and muscle cells that contain dysfunctional mitochondria exacerbate sarcopenia. Preventing sarcopenia will require identifying mitochondrial sources of dysfunction that are reversible.

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.

A new mouse model of frailty: the Cu/Zn superoxide dismutase knockout mouse

Frailty is a geriatric syndrome that is an important public health problem for the older adults living in the USA. Although several methods have been developed to measure frailty in humans, we have very little understanding of its etiology. Because the molecular basis of frailty is poorly understood, mouse models would be of great value in determining which pathways contribute to the development of frailty. More importantly, mouse models would be critical in testing potential therapies to treat and possibly prevent frailty. In this article, we present data showing that Sod1KO mice, which lack the antioxidant enzyme, Cu/Zn superoxide dismutase, are an excellent model of frailty, and we compare the Sod1KO mice to the only other mouse model of frailty, mice with the deletion of the IL-10 gene. Sod1KO mice exhibit four characteristics that have been used to define human frailty: weight loss, weakness, low physical activity, and exhaustion. In addition, Sod1KO mice show increased inflammation and sarcopenia, which are strongly associated with human frailty. The Sod1KO mice also show alterations in pathways that have been proposed to play a role in the etiology of frailty: oxidative stress, mitochondrial dysfunction, and cell senescence. Using Sod1KO mice, we show that dietary restriction can delay/prevent characteristics of frailty in mice.