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

Integrating transcriptomic and proteomic data for a comprehensive molecular perspective on the association between sarcopenia and osteoporosis

BACKGROUND

Osteoporosis and sarcopenia are common age-related conditions characterized by the progressive loss of bone density and muscle mass, respectively. Their co-occurrence, often referred to as osteosarcopenia, presents significant challenges in elderly care due to increased fragility and functional impairment. Existing studies have identified shared pathological mechanisms between these conditions, including inflammation, hormonal imbalances, and metabolic dysregulation, but a comprehensive understanding of their molecular interplay remains incomplete.

OBJECTIVE

This study aims to deepen our understanding of the molecular interactions between sarcopenia and osteoporosis through an integrated omics approach, revealing potential therapeutic targets and biomarkers.

METHODS

Employing a combination of proteomics and transcriptomics analyses, this study analyzed bone and muscle tissue samples from patients diagnosed with osteoporosis and osteosarcopenia. Techniques included high-throughput sequencing and label-free proteomics, supported by advanced bioinformatics tools for data analysis and functional annotation of genes and proteins.

RESULTS

The study found marked differences in gene and protein expressions between osteoporosis and osteosarcopenia tissues. Specifically, genes like PDIA5, TUBB1, and CYFIP2 in bone, along with MYH7 and NCAM1 in muscle, exhibited differential expression at both mRNA and protein levels. Pathway analyses revealed the significance of oxidative-reduction balance, cellular metabolism, and immune response in the progression of these conditions. Importantly, the study pinpointed osteoclast differentiation and NF-kappa B signaling pathways as critical in the molecular dynamics of osteosarcopenia, suggesting potential targets for therapy.

CONCLUSIONS

This study utilized transcriptomics and proteomics to identify key genes and proteins impacting sarcopenia and osteoporosis, employing advanced network tools to delineate interaction networks and crucial signaling pathways. It highlighted genes like PDIA5 and TUBB1, consistently expressed in both analyses, involved in pathways such as osteoclast differentiation and cytokine interactions. These insights enhance understanding of the molecular interplay in bone and muscle degeneration with aging, suggesting directions for future research into therapeutic interventions and prevention strategies for age-related degenerative diseases.

Accelerated sarcopenia precedes learning and memory impairments in the P301S mouse model of tauopathies and Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) impairs cognitive functions and peripheral systems, including skeletal muscles. The PS19 mouse, expressing the human tau P301S mutation, shows cognitive and muscular pathologies, reflecting the central and peripheral atrophy seen in AD.

METHODS

We analysed skeletal muscle morphology and neuromuscular junction (NMJ) through immunohistochemistry and advanced image quantification. A factorial Analysis of Variance assessed muscle weight, NCAM expression, NMJ, myofibre type distribution, cross-sectional areas, expression of single or multiple myosin heavy-chain isoforms, and myofibre grouping in PS19 and wild type (WT) mice over their lifespan (1-12 months).

RESULTS

Significant weight differences in extensor digitorum longus (EDL) and soleus muscles between WT and PS19 mice were noted by 7-8 months. For EDL muscle in females, WT weighed 0.0113 ± 0.0005 compared with PS19's 0.0071 ± 0.0008 (P < 0.05), and in males, WT was 0.0137 ± 0.0001 versus PS19's 0.0069 ± 0.0006 (P < 0.005). Similarly, soleus muscle showed significant differences; females (WT: 0.0084 ± 0.0004; PS19: 0.0057 ± 0.0005, P < 0.005) and males (WT: 0.0088 ± 0.0003; PS19: 0.0047 ± 0.0004, P < 0.0001). Analysis of the NMJ in PS19 mice revealed a marked reduction in myofibre innervation at 5 months, with further decline by 10 months. NMJ pre-terminals in PS19 mice became shorter and simpler by 5 months, showing a steep decline by 10 months. Genotype and age strongly influenced muscle NCAM immunoreactivity, denoting denervation as early as 5-6 months in EDL muscle Type II fibres, with earlier effects in soleus muscle Type I and II fibres at 3-4 months. Muscle denervation and subsequent myofibre atrophy were linked to a reduction in Type IIB fibres in the EDL muscle and Type IIA fibres in the soleus muscle, accompanied by an increase in hybrid fibres. The EDL muscle showed Type IIB fibre atrophy with WT females at 1505 ± 110 μm2 versus PS19's 1208 ± 94 μm2, and WT males at 1731 ± 185 μm2 versus PS19's 1227 ± 116 μm2. Similarly, the soleus muscle demonstrated Type IIA fibre atrophy from 5 to 6 months, with WT females at 1194 ± 52 μm2 versus PS19's 858 ± 62 μm2, and WT males at 1257 ± 43 μm2 versus PS19's 1030 ± 55 μm2. Atrophy also affected Type IIX, I + IIA, and IIA + IIX fibres in both muscles. The timeline for both myofibre and overall muscle atrophy in PS19 mice was consistent, indicating a simultaneous decline.

CONCLUSIONS

Progressive and accelerated neurogenic sarcopenia may precede and potentially predict cognitive deficits observed in AD.

Muscle preservation in proximal nerve injuries: a current update

Optimal recovery of muscle function after proximal nerve injuries remains a complex and challenging problem. After a nerve injury, alterations in the affected muscles lead to atrophy, and later degeneration and replacement by fat-fibrous tissues. At present, several different strategies for the preservation of skeletal muscle have been reported, including various sets of physical exercises, muscle massage, physical methods (e.g. electrical stimulation, magnetic field and laser stimulation, low-intensity pulsed ultrasound), medicines (e.g. nutrients, natural and chemical agents, anti-inflammatory and antioxidants, hormones, enzymes and enzyme inhibitors), regenerative medicine (e.g. growth factors, stem cells and microbiota) and surgical procedures (e.g. supercharge end-to-side neurotization). The present review will focus on methods that aimed to minimize the damage to muscles after denervation based on our present knowledge.

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

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

The effects of resistance training on denervated myofibers, senescent cells, and associated protein markers in middle-aged adults

Denervated myofibers and senescent cells are hallmarks of skeletal muscle aging. However, sparse research has examined how resistance training affects these outcomes. We investigated the effects of unilateral leg extensor resistance training (2 days/week for 8 weeks) on denervated myofibers, senescent cells, and associated protein markers in apparently healthy middle-aged participants (MA, 55 ± 8 years old, 17 females, 9 males). We obtained dual-leg vastus lateralis (VL) muscle cross-sectional area (mCSA), VL biopsies, and strength assessments before and after training. Fiber cross-sectional area (fCSA), satellite cells (Pax7+), denervated myofibers (NCAM+), senescent cells (p16+ or p21+), proteins associated with denervation and senescence, and senescence-associated secretory phenotype (SASP) proteins were analyzed from biopsy specimens. Leg extensor peak torque increased after training (p < .001), while VL mCSA trended upward (interaction p = .082). No significant changes were observed for Type I/II fCSAs, NCAM+ myofibers, or senescent (p16+ or p21+) cells, albeit satellite cells increased after training (p = .037). While >90% satellite cells were not p16+ or p21+, most p16+ and p21+ cells were Pax7+ (>90% on average). Training altered 13 out of 46 proteins related to muscle-nerve communication (all upregulated, p < .05) and 10 out of 19 proteins related to cellular senescence (9 upregulated, p < .05). Only 1 out of 17 SASP protein increased with training (IGFBP-3, p = .031). In conclusion, resistance training upregulates proteins associated with muscle-nerve communication in MA participants but does not alter NCAM+ myofibers. Moreover, while training increased senescence-related proteins, this coincided with an increase in satellite cells but not alterations in senescent cell content or SASP proteins. These latter findings suggest shorter term resistance training is an unlikely inducer of cellular senescence in apparently healthy middle-aged participants. However, similar study designs are needed in older and diseased populations before definitive conclusions can be drawn.

Swimming alters some proteins of skeletal muscle tissue in rats with Alzheimer-like phenotype

OBJECTIVES

Exercise training plays a significant role in preventing the destruction of central nerve neurons and muscle atrophy. The purpose of the present study was to investigate the effect of a period of swimming training on the expression of Neural cell adhesion molecule (NCAM), Semaphorin 3A (SEMA3A), and Profilin-1 (PFN1) proteins in the gastrocnemius muscle of Alzheimer-like phenotype rats.

METHODS & MATERIALS

32 Wistar males were (6 weeks of age) divided into four groups: Healthy Control (HC), Alzheimer-like phenotype's Control (AC), Healthy Training (HT), and Alzheimer-like phenotype's Training (AT). Alzheimer-like phenotypes were induced by beta-amyloid injection in the hippocampus. The training program consisted of 20 swimming sessions. Gastrocnemius muscle was removed after the intervention, and NCAM, SEMA3A, and PFN1 proteins were measured by the immunohistoflorescent method.

RESULTS

The results showed that SEMA3A was increased (p = 0.001), and NCAM (p = 0.001), and PFN1 (p = 0.001) were decreased in AC compared to the HC group. Also, the results showed that NCAM (p = 0.001) and Pfn1 (p = 0.002) increased in the HT group compared to HC, and the NCAM (p = 0.001) and Pfn1 (p = 0.002) in AT group compared to AC (p = 0.001) increased significantly, while SEMA3A was reduced in the HT group compared to HC (p = 0.001) and AT group compared to AC (p = 0.001) CONCLUSION: Swimming effectively improves axon regeneration and neuronal formation in motor neurons and, therefore, can be an effective intervention to prevent and control the complications of Alzheimer-like phenotype.

Serum Proteomic Signatures of Common Health Outcomes among Older Adults

INTRODUCTION

In aging populations, the coexistence of multiple health comorbidities represents a significant challenge for clinicians and researchers. Leveraging advances in omics techniques to characterize these health conditions may provide insight into disease pathogenesis as well as reveal biomarkers for monitoring, prognostication, and diagnosis. Researchers have previously established the utility of big data approaches with respect to comprehensive health outcome measurements in younger populations, identifying protein markers that may provide significant health information with a single blood sample.

METHODS

Here, we employed a similar approach in two cohorts of older adults, the Baltimore Longitudinal Study of Aging (mean age = 76.12 years) and InCHIANTI Study (mean age = 66.05 years), examining the relationship between levels of serum proteins and 5 key health outcomes: kidney function, fasting glucose, physical activity, lean body mass, and percent body fat.

RESULTS

Correlations between proteins and health outcomes were primarily shared across both older adult cohorts. We further identified that most proteins associated with health outcomes in the older adult cohorts were not associated with the same outcomes in a prior study of a younger population. A subset of proteins, adiponectin, MIC-1, and NCAM-120, were associated with at least three health outcomes in both older adult cohorts but not in the previously published younger cohort, suggesting that they may represent plausible markers of general health in older adult populations.

CONCLUSION

Taken together, these findings suggest that comprehensive protein health markers have utility in aging populations and are distinct from those identified in younger adults, indicating unique mechanisms of disease with aging.

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

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

Vitamin D Supplementation and Muscle Power, Strength and Physical Performance in Older Adults: A Randomized Controlled Trial

BACKGROUND

Low 25-hydroxyvitamin D (25[OH]D) concentrations (<30 ng/mL [<50 nmol/L]) have been associated with muscle weakness and impaired physical performance in observational studies. However, the effect of vitamin D supplementation on changes in muscle strength and physical performance in randomized controlled trials has been mixed.

OBJECTIVES

To determine the effect of daily vitamin D supplementation on leg power, strength, and physical performance in low-functioning older adults with 25(OH)D concentrations of 18 to <30 ng/mL.

METHODS

In this double-blind, randomized controlled trial, 136 low-functioning [Short Physical Performance Battery (SPPB) scores ≤10] adults aged 65-89 y with 25(OH)D concentrations of 18 to <30 ng/mL were randomly assigned to 2000 IU/d vitamin D3 or placebo for 12 mo. Lower-extremity leg power (primary outcome), leg and grip strength, SPPB, timed up and go (TUG), postural sway, and gait velocity and spatiotemporal parameters (secondary outcomes) were assessed at baseline, 4 and 12 mo. A subset (n = 37) also underwent a muscle biopsy at baseline and 4 mo and muscle fiber composition and contractile properties were assessed.

RESULTS

Participants' mean ± SD age and SPPB scores at baseline were 73.4 ± 6.3 y and 7.8 ± 1.8, respectively. Mean ± SD 25(OH)D concentrations at baseline and 12 mo were 19.4 ± 4.2 ng/mL and 28.6 ± 6.7 ng/mL in the vitamin D group and 19.9 ± 4.9 ng/mL and 20.2 ± 5.0 ng/mL in the placebo group for a mean ± SE difference of 9.1 ± 1.1 ng/mL (P < 0.0001). However, there were no differences in change in leg power, leg or grip strength, SPPB score, TUG, postural sway, or gait velocity and spatiotemporal parameters by intervention group over 12 mo or muscle fiber composition and contractile properties over 4 mo.

CONCLUSIONS

In low-functioning older adults with 25(OH)D concentrations of 18 to <30 ng/mL, randomization to 2000 IU/d vitamin D3 did not result in improvements in leg power, strength, or physical performance or muscle fiber composition and contractile properties. This trial was registered at clinicaltrials.gov as NCT02015611.

Linking cortex and contraction-Integrating models along the corticomuscular pathway

Computational models of the neuromusculoskeletal system provide a deterministic approach to investigate input-output relationships in the human motor system. Neuromusculoskeletal models are typically used to estimate muscle activations and forces that are consistent with observed motion under healthy and pathological conditions. However, many movement pathologies originate in the brain, including stroke, cerebral palsy, and Parkinson's disease, while most neuromusculoskeletal models deal exclusively with the peripheral nervous system and do not incorporate models of the motor cortex, cerebellum, or spinal cord. An integrated understanding of motor control is necessary to reveal underlying neural-input and motor-output relationships. To facilitate the development of integrated corticomuscular motor pathway models, we provide an overview of the neuromusculoskeletal modelling landscape with a focus on integrating computational models of the motor cortex, spinal cord circuitry, α-motoneurons  and skeletal muscle in regard to their role in generating voluntary muscle contraction. Further, we highlight the challenges and opportunities associated with an integrated corticomuscular pathway model, such as challenges in defining neuron connectivities, modelling standardisation, and opportunities in applying models to study emergent behaviour. Integrated corticomuscular pathway models have applications in brain-machine-interaction, education, and our understanding of neurological disease.

Epigenome-wide association studies of meat traits in Chinese Yorkshire pigs highlights several DNA methylation loci and genes

In this study, we aimed to identified CpG sites at which DNA methylation levels are associated with meat quality traits in 140 Yorkshire pigs, including pH at 45 min (pH_45min), pH at 24 h (pH_24h), drip loss (DL), meat redness value (a*), yellowness (b*) and lightness (L*). Genome-wide methylation levels were measured in muscular tissue using reduced representation bisulfite sequencing (RRBS). Associations between DNA methylation levels and meat quality traits were examined using linear mixed-effect models that were adjusted for gender, year, month and body weight. A Bonferroni-corrected p-value lower than 7.79 × 10 - 8 was considered statistically significant threshold. Eight CpG sites were associated with DL, including CpG sites annotated to RBM4 gene (cpg301054, cpg301055, cpg301058, cpg301059, cpg301066, cpg301072 and cpg301073) and NCAM1 gene (cpg1802985). Two CpG sites were associated with b*, including RNFT1 and MED13 (cpg2272837) and TRIM37 gene (cpg2270611). Five CpG sites were associated with L*, including GSDMA and LRRC3C gene (cpg2252750) and ENSSSCG00000043539 and IRX1 gene (cpg2820178, cpg2820179, cpg2820181 and cpg2820182). No significant associations were observed with pH_45min, pH_24h or a*. We reported associations of meat quality traits with DNA methylation and identified some candidate genes associated with these traits, such as NCAM1, MED13 and TRIM37 gene. These results provide new insight into the epigenetic molecular mechanisms of meat quality traits in pigs.

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

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

Human skeletal muscle acetylcholine receptor gene expression in elderly males performing heavy resistance exercise

Muscle fiber denervation is a major contributor to the decline in muscle mass and function during aging. Heavy resistance exercise is an effective tool for increasing muscle mass and strength, but whether it can rescue denervated muscle fibers remains unclear. Therefore, the purpose of this study was to investigate the potential of heavy resistance exercise to modify indices of denervation in healthy elderly individuals. 38 healthy elderly men (72±5 years) underwent 16 weeks of heavy resistance exercise while 20 healthy elderly men (72±6 years) served as non-exercising sedentary controls. Muscle biopsies were obtained pre and post training, and midway at eight weeks. Biopsies were analysed by immunofluorescence for the prevalence of myofibers expressing embryonic myosin (MyHCe), neonatal myosin (MyHCn), nestin, and neural cell adhesion molecule (NCAM), and by RT-qPCR for gene expression levels of acetylcholine receptor (AChR) subunits, MyHCn, MyHCe, p16 and Ki67. In addition to increases in strength and type II fiber hypertrophy, heavy resistance exercise training led to a decrease in AChR α1 and ε subunit mRNA (at eight weeks). Changes in gene expression levels of the α1 and ε AChR subunits with eight weeks of heavy resistance exercise supports the role of this type of exercise in targeting stability of the neuromuscular junction. The number of fibers positive for NCAM, nestin, and MyHCn was not affected, suggesting that a longer timeframe is needed for adaptations to manifest at the protein level.

Skeletal muscle sympathetic denervation disrupts the neuromuscular junction postterminal organization: A single-cell quantitative approach

The sympathetic nervous system (SNS) regulates skeletal muscle motor innervation and stabilizes the NMJ in health, disease and aging. Previous studies using both chemical (6-hydroxydopamine, 6-OHDA) and microsurgically-induced sympathetic denervation examined the NMJ organization and transmission in the mouse; however, a detailed quantification of the postterminal on larger hindlimb muscles involved in gait mechanics and posture is lacking. The purpose of this study was to determine whether targets of the sympathetic neuron (SN) exhibiting different intrinsic composition such as the fast-twitch extensor digitorum longus (EDL) and the slow-twitch soleus muscles differ in their response to SN deprivation, and to develop a strategy to accurately quantify the impact of sympathectomy on the NMJ postterminal including those fibers located deeper in the muscle. This approach included muscle fixed ex vivo or through transcardial perfusion in mice treated with 6-OHDA or control ascorbic acid. We measured NMJ postterminal mean terminal total area, number of postterminal fragments, mean fragment area, and mean distance between fragments in free-floating alpha-bungarotoxin-stained in 1038 isolated muscle fibers. We found that muscle fiber sympathetic innervation plays a crucial role in the structural organization of the motorneuron-myofiber synapse postterminal and its deprivation leads to AChR cluster dispersion or shrinking as described in various neuromuscular diseases and aging.

Ageing and exercise-induced motor unit remodelling

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

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

Exercise-mediated reinnervation of skeletal muscle in elderly people: An update

Sarcopenia is defined by the loss of muscle mass and function. In aging sarcopenia is due to mild chronic inflammation but also to fiber-intrinsic defects, such as mitochondrial dysfunction. Age-related sarcopenia is associated with physical disability and lowered quality of life. In addition to skeletal muscle, the nervous tissue is also affected in elderly people. With aging, type 2 fast fibers preferentially undergo denervation and are reinnervated by slow-twitch motor neurons. They spread forming new neuro-muscular junctions with the denervated fibers: the result is an increased proportion of slow fibers that group together since they are associated in the same motor unit. Grouping and fiber type shifting are indeed major histological features of aging skeletal muscle. Exercise has been proposed as an intervention for age-related sarcopenia due to its numerous beneficial effects on muscle mechanical and biochemical features. In 2013, a precursor study in humans was published in the European Journal of Translation Myology (formerly known as Basic and Applied Myology), highlighting the occurrence of reinnervation in the musculature of aged, exercise-trained individuals as compared to the matching control. This paper, entitled «Reinnervation of Vastus lateralis is increased significantly in seniors (70-years old) with a lifelong history of high-level exercise», is now being reprinted for the second issue of the «Ejtm Seminal Paper Series». In this short review we discuss those results in the light of the most recent advances confirming the occurrence of exercise-mediated reinnervation, ultimately preserving muscle structure and function in elderly people who exercise.

Central Suppression of the GH/IGF Axis and Abrogation of Exercise-Related mTORC1/2 Activation in the Muscle of Phenotype-Selected Male Marathon Mice (DUhTP)

The somatotropic axis is required for a number of biological processes, including growth, metabolism, and aging. Due to its central effects on growth and metabolism and with respect to its positive effects on muscle mass, regulation of the GH/IGF-system during endurance exercise is of particular interest. In order to study the control of gene expression and adaptation related to physical performance, we used a non-inbred mouse model, phenotype-selected for high running performance (DUhTP). Gene expression of the GH/IGF-system and related signaling cascades were studied in the pituitary gland and muscle of sedentary males of marathon and unselected control mice. In addition, the effects of three weeks of endurance exercise were assessed in both genetic groups. In pituitary glands from DUhTP mice, reduced expression of Pou1f1 (p = 0.002) was accompanied by non-significant reductions of Gh mRNA (p = 0.066). In addition, mRNA expression of Ghsr and Sstr2 were significantly reduced in the pituitary glands from DUhTP mice (p ≤ 0.05). Central downregulation of Pou1f1 expression was accompanied by reduced serum concentrations of IGF1 and coordinated downregulation of multiple GH/IGF-signaling compounds in muscle (e.g., Ghr, Igf1, Igf1r, Igf2r, Irs1, Irs2, Akt3, Gskb, Pik3ca/b/a2, Pten, Rictor, Rptor, Tsc1, Mtor; p ≤ 0.05). In response to exercise, the expression of Igfbp3, Igfbp 4, and Igfbp 6 and Stc2 mRNA was increased in the muscle of DUhTP mice (p ≤ 0.05). Training-induced specific activation of AKT, S6K, and p38 MAPK was found in muscles from control mice but not in DUhTP mice (p ≤ 0.05), indicating a lack of mTORC1 and mTORC2 activation in marathon mice in response to physical exercise. While hormone-dependent mTORC1 and mTORC2 pathways in marathon mice were repressed, robust increases of Ragulator complex compounds (p ≤ 0.001) and elevated sirtuin 2 to 6 mRNA expression were observed in the DUhTP marathon mouse model (p ≤ 0.05). Activation of AMPK was not observed under the experimental conditions of the present study. Our results describe coordinated downregulation of the somatotropic pathway in long-term selected marathon mice (DUhTP), possibly via the pituitary gland and muscle interaction. Our results, for the first time, demonstrate that GH/IGF effects are repressed in a context of superior running performance in mice.

Long-term, induced expression of Hand2 in peripheral sympathetic neurons ameliorates sarcopenia in geriatric mice

BACKGROUND

The discovery of adrenoceptors, which mediate the effects of the sympathetic nervous system neurotransmitter norepinephrine on specific tissues, sparked the development of sympathomimetics that have profound influence on skeletal muscle mass. However, chronic administration has serious side effects that preclude their use for muscle-wasting conditions such as sarcopenia, the age-dependent decline in muscle mass, force, and power. Devising interventions that can adjust neurotransmitter release to changing physiological demands will require understanding how the sympathetic nervous system affects muscle motor innervation and muscle mass, which will prevent sarcopenia-associated impaired mobility, falls, institutionalization, co-morbidity, and premature death. Here, we tested the hypothesis that prolonged heart and neural crest derivative 2 (Hand2) expression in peripheral sympathetic neurons (SNs) ameliorates sympathetic muscle denervation, motor denervation, and sarcopenia in geriatric mice.

METHODS

We delivered either a viral vector encoding the transcription factor Hand2 or an empty vector (EV) driven to SNs by the PRSx8 promoter by injecting the saphenous vein in 16-month-old C57BL/6 mice that were sacrificed 10-11 months later. Studies relied on sympathetic and muscle immunohistochemistry analysed by confocal microscopy, nerve and muscle protein expression assessed by immunoblots, nerve-evoked and muscle-evoked maximal muscle contraction force, extensor digitorum longus (EDL) muscle RNA sequencing, SN real-time PCR, and tests of physical performance using an inverted-cling grip test and in an open-arena setting.

RESULTS

Examining the mice 10-11 months later, we found that inducing Hand2 expression in peripheral SNs preserved (i) the number of neurons (EV: 0.32 ± 0.03/μm² , n = 6; Hand2: 0.92 ± 0.08/μm² , n = 7; P < 0.0001) and size (EV: 279 ± 18 μm² , n = 6; Hand2: 396 ± 18 μm² , n = 7; P < 0.0001); (ii) lumbricalis muscle sympathetic innervation (EV: 1.4 ± 1.5 μm/μm² , n = 5; Hand2: 12 ± 1.8 μm/μm² , n = 5; P < 0.001); (iii) tibialis anterior, gastrocnemius, EDL, and soleus muscles weight and whole-body strength (EV: 48 ± 6.4 s, n = 6; Hand2: 102 ± 6.8 s, n = 6; P < 0.001); (iv) EDL type IIb, IIx, and II/IIx and soleus type I, IIa, IIx, IIa/IIx, and IIb/IIx myofibre cross-sectional area; (v) nerve-evoked (EV: 16 ± 2.7 mN; Hand2: 30 ± 4.4 mN; P < 0.001) and muscle-evoked (EV: 24 ± 3.8 mN, n = 5; Hand2: 38 ± 3.0 mN, n = 8; P < 0.001) muscle force by 150 Hz-3 s pulses; and (vi) motor innervation assessed by measuring presynaptic/postsynaptic neuromuscular junction area overlay.

CONCLUSIONS

Preserving Hand2 expression in SNs from middle-aged to very old mice attenuates decreases in muscle mass and force by (i) maintaining skeletal muscle sympathetic and motor innervation, (ii) improving membrane and total acetylcholine receptor stability and nerve-evoked and muscle-evoked muscle contraction, (iii) preventing the elevation of inflammation and myofibrillar protein degradation markers, and (iv) increasing muscle autophagy.

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

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

Cannabinoid receptor 1 expression is higher in muscle of old vs. young males, and increases upon resistance exercise in older adults

Aged skeletal muscle undergoes metabolic and structural alterations eventually resulting in a loss of muscle strength and mass, i.e. age-related sarcopenia. Therefore, novel targets for muscle growth purposes in elderly are needed. Here, we explored the role of the cannabinoid system in muscle plasticity through the expression of muscle cannabinoid receptors (CBs) in young and old humans. The CB1 expression was higher (+ 25%; p = 0.04) in muscle of old (≥ 65 years) vs. young adults (20-27 years), whereas CB2 was not differently expressed. Furthermore, resistance exercise tended to increase the CB1 (+ 11%; p = 0.055) and CB2 (+ 37%; p = 0.066) expression in muscle of older adults. Interestingly, increases in the expression of CB2 following resistance exercise positively correlated with changes in key mechanisms of muscle homeostasis, such as catabolism (FOXO3a) and regenerative capacity (Pax7, MyoD). This study for the first time shows that CB1 is differentially expressed with aging and that changes in CB2 expression upon resistance exercise training correlate with changes in mediators that play a central role in muscle plasticity. These data confirm earlier work in cells and mice showing that the cannabinoid system might orchestrate muscle growth, which is an incentive to further explore CB-based strategies that might counteract sarcopenia.

Neuromuscular Junction Aging: A Role for Biomarkers and Exercise

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

Muscle-nerve communication and the molecular assessment of human skeletal muscle denervation with aging

Muscle fiber denervation is a major contributor to the decline in physical function observed with aging. Denervation can occur through breakdown of the neuromuscular junctions (NMJ) itself, affecting only that particular fiber, or through the death of a motor neuron, which can lead to a loss of all the muscle fibers in that motor unit. In this review, we discuss the muscle-nerve relationship, where signaling from both the motor neuron and the muscle fiber is required for maximal preservation of neuromuscular function in old age. Physical activity is likely to be the most important single factor that can contribute to this preservation. Furthermore, we propose that inactivity is not an innocent bystander, but plays an active role in denervation through the production of signals hostile to neuron survival. Investigating denervation in human muscle tissue samples is challenging due to the shared protein profile of regenerating and denervated muscle fibers. In this review, we provide a detailed overview of the key traits observed in immunohistochemical preparations of muscle biopsies from healthy, young, and elderly individuals. Overall, a combination of assessing tissue samples, circulating biomarkers, and electrophysiological assessments in humans will prove fruitful in the quest to gain more understanding of denervation of skeletal muscle. In addition, cell culture models represent a valuable tool in the search for key signaling factors exchanged between muscle and nerve, and which exercise has the capacity to alter.

Walking Exercise Therapy Effects on Lower Extremity Skeletal Muscle in Peripheral Artery Disease

Walking exercise is the most effective noninvasive therapy that improves walking ability in peripheral artery disease (PAD). Biologic mechanisms by which exercise improves walking in PAD are unclear. This review summarizes evidence regarding effects of walking exercise on lower extremity skeletal muscle in PAD. In older people without PAD, aerobic exercise improves mitochondrial activity, muscle mass, capillary density, and insulin sensitivity in skeletal muscle. However, walking exercise increases lower extremity ischemia in people with PAD, and therefore, mechanisms by which this exercise improves walking may differ between people with and without PAD. Compared with people without PAD, gastrocnemius muscle in people with PAD has greater mitochondrial impairment, increased reactive oxygen species, and increased fibrosis. In multiple small trials, walking exercise therapy did not consistently improve mitochondrial activity in people with PAD. In one 12-week randomized trial of people with PAD randomized to supervised exercise or control, supervised treadmill exercise increased treadmill walking time from 9.3 to 15.1 minutes, but simultaneously increased the proportion of angular muscle fibers, consistent with muscle denervation (from 7.6% to 15.6%), while angular myofibers did not change in the control group (from 9.1% to 9.1%). These findings suggest an adaptive response to exercise in PAD that includes denervation and reinnervation, an adaptive process observed in skeletal muscle of people without PAD during aging. Small studies have not shown significant effects of exercise on increased capillary density in lower extremity skeletal muscle of participants with PAD, and there are no data showing that exercise improves microcirculatory delivery of oxygen and nutrients in patients with PAD. However, the effects of supervised exercise on increased plasma nitrite abundance after a treadmill walking test in people with PAD may be associated with improved lower extremity skeletal muscle perfusion and may contribute to improved walking performance in response to exercise in people with PAD. Randomized trials with serial, comprehensive measures of muscle biology, and physiology are needed to clarify mechanisms by which walking exercise interventions improve mobility in PAD.

The emerging role of the sympathetic nervous system in skeletal muscle motor innervation and sarcopenia

Examining neural etiologic factors'role in the decline of neuromuscular function with aging is essential to our understanding of the mechanisms underlying sarcopenia, the age-dependent decline in muscle mass, force and power. Innervation of the skeletal muscle by both motor and sympathetic axons has been established, igniting interest in determining how the sympathetic nervous system (SNS) affect skeletal muscle composition and function throughout the lifetime. Selective expression of the heart and neural crest derivative 2 gene in peripheral SNs increases muscle mass and force regulating skeletal muscle sympathetic and motor innervation; improving acetylcholine receptor stability and NMJ transmission; preventing inflammation and myofibrillar protein degradation; increasing autophagy; and probably enhancing protein synthesis. Elucidating the role of central SNs will help to define the coordinated response of the visceral and neuromuscular system to physiological and pathological challenges across ages. This review discusses the following questions: (1) Does the SNS regulate skeletal muscle motor innervation? (2) Does the SNS regulate presynaptic and postsynaptic neuromuscular junction (NMJ) structure and function? (3) Does sympathetic neuron (SN) regulation of NMJ transmission decline with aging? (4) Does maintenance of SNs attenuate aging sarcopenia? and (5) Do central SN group relays influence sympathetic and motor muscle innervation?

Nonimmune mechanisms in idiopathic inflammatory myopathies

PURPOSE OF REVIEW

This review encompasses the main novelties regarding nonimmune mechanisms implicated in the pathogenesis of idiopathic inflammatory myopathies (IIM).

RECENT FINDINGS

In recent years, growing data support a role for endoplasmic-reticulum (ER) stress as a propagator of muscular damage, together with the release of interferon type I and reactive oxygen species in hypoxemic muscle fibers. Other studies evaluating the relationship between autophagy and Toll-like receptors (TLRs) in IIM subtypes have shown increased TLR3 and TLR4 expression in fibers of IIM patients and colocalization with LC3, an autophagy marker, submitting autophagy as a likely player in IIM pathogenesis. Most novel evidences concern the potential role of denervation of the neuromuscular junction in IIM, possibly connected to hyperexpression of MHC-I, and trafficking of extracellular vesicles, which may represent a connection between nonimmune and immune-mediated mechanisms of muscle inflammation and damage.

SUMMARY

Nonimmune mechanisms contribute to the pathogenesis of IIM, likely cooperating with immune-mediated inflammation. Consistent data were released for ER stress, autophagy, mitochondrial dysfunction and hypoxia; in addition to, neuromuscular denervation and extracellular vesicles have been proposed as thoughtful links between muscle inflammation, damage and atrophy. Further understanding of nonimmune abnormalities and potential reversible pathways is needed to improve the management of IIM.

Heart and neural crest derivative 2-induced preservation of sympathetic neurons attenuates sarcopenia with aging

BACKGROUND

Sarcopenia, or age-dependent decline in muscle force and power, impairs mobility, increasing the risk of falls, institutionalization, co-morbidity, and premature death. The discovery of adrenoceptors, which mediate the effects of the sympathetic nervous system (SNS) neurotransmitter norepinephrine on specific tissues, sparked the development of sympathomimetics that have profound influence on skeletal muscle mass. However, chronic administration has serious side effects that preclude their use for muscle-wasting conditions. Interventions that can adjust neurotransmitter release to changing physiological demands depend on understanding how the SNS affects neuromuscular transmission, muscle motor innervation, and muscle mass.

METHODS

We examined age-dependent expression of the heart and neural crest derivative 2 (Hand2), a critical transcription factor for SN maintenance, and we tested the possibility that inducing its expression exclusively in sympathetic neurons (SN) will prevent (i) motor denervation, (ii) impaired neuromuscular junction (NMJ) transmission, and (iii) loss of muscle mass and function in old mice. To test this hypothesis, we delivered a viral vector carrying Hand2 expression or an empty vector exclusively in SNs by vein injection in 16-month-old C57BL/6 mice that were sacrificed 6 months later. Techniques include RNA-sequencing, real-time PCR, genomic DNA methylation, viral vector construct, tissue immunohistochemistry, immunoblot, confocal microscopy, electrophysiology, and in vivo mouse physical performance.

RESULTS

Hand2 expression declines throughout life, but inducing its expression increased (i) the number and size of SNs, (ii) muscle sympathetic innervation, (iii) muscle weight and force and whole-body strength, (iv) myofiber size but not muscle fibre-type composition, (v) NMJ transmission and nerve-evoked muscle force, and (vi) motor innervation in old mice. Additionally, the SN controls a set of genes to reduce inflammation and to promote transcription factor activity, cell signalling, and synapse in the skeletal muscle. Hand2 DNA methylation may contribute, at least partially, to gene silencing.

CONCLUSIONS

Selective expression of Hand2 in the mouse SNs from middle age through old age increases muscle mass and force by (i) regulating skeletal muscle sympathetic and motor innervation; (ii) improving acetylcholine receptor stability and NMJ transmission; (iii) preventing inflammation and myofibrillar protein degradation; (iv) increasing autophagy; and (v) probably enhancing protein synthesis.

Synergistic effect of high-intensity interval training and stem cell transplantation with amniotic membrane scaffold on repair and rehabilitation after volumetric muscle loss injury

Despite the high regenerative capacity of skeletal muscle, volumetric muscle loss (VML) is an irrecoverable injury. One therapeutic approach is the implantation of engineered biologic scaffolds enriched with stem cells. The objective of this study is to investigate the synergistic effect of high-intensity interval training (HIIT) and stem cell transplantation with an amniotic membrane scaffold on innervation, vascularization and muscle function after VML injury. A VML injury was surgically created in the tibialis anterior (TA) muscle in rats. The animals were randomly assigned to three groups: untreated negative control group (untreated), decellularized human amniotic membrane bio-scaffold group (dHAM) and dHAM seeded with adipose-derived stem cells, which differentiate into skeletal muscle cells (dHAM-ADSCs). Then, each group was divided into sedentary and HIIT subgroups. The exercise training protocol consisted of treadmill running for 8 weeks. The animals underwent in vivo functional muscle tests to evaluate maximal isometric contractile force. Regenerated TA muscles were harvested for molecular analyses and explanted tissues were analyzed with histological methods. The main finding was that HIIT promoted muscle regeneration, innervation and vascularization in regenerated areas in HIIT treatment subgroups, especially in the dHAM-ADSC subgroup. In parallel with innervation, maximal isometric force also increased in vivo. HIIT upregulated neurotrophic factor gene expression in skeletal muscle. The amniotic membrane bio-scaffold seeded with differentiated ADSC, in conjunction with exercise training, improved vascular perfusion and innervation and enhanced the functional and morphological healing process after VML injury. The implications of these findings are of potential importance for future efforts to develop engineered biological scaffolds and for the use of interval training programs in rehabilitation after VML injury.

Semiautomatic morphometric analysis of skeletal muscle obtained by needle biopsy in older adults

Analysis of skeletal muscle mass and composition is essential for studying the biology of age-related sarcopenia, loss of muscle mass, and function. Muscle immunohistochemistry (IHC) allows for simultaneous visualization of morphological characteristics and determination of fiber type composition. The information gleaned from myosin heavy chain (MHC) isoform, and morphological measurements offer a more complete assessment of muscle health and properties than classical techniques such as SDS-PAGE and ATPase immunostaining; however, IHC quantification is a time-consuming and tedious method. We developed a semiautomatic method to account for issues frequently encountered in aging tissue. We analyzed needle-biopsied vastus lateralis (VL) of the quadriceps from a cohort of 14 volunteers aged 74.9 ± 2.2 years. We found a high correlation between manual quantification and semiautomatic analyses for the total number of fibers detected (r² = 0.989) and total fiber cross-sectional area (r² = 0.836). The analysis of the VL fiber subtype composition and the cross-sectional area also did not show statistically significant differences. The semiautomatic approach was completed in 10-15% of the time required for manual quantification. The results from these analyses highlight some of the specific issues which commonly occur in aged muscle. Our methods which address these issues underscore the importance of developing efficient, accurate, and reliable methods for quantitatively analyzing the skeletal muscle and the standardization of collection protocols to maximize the likelihood of preserving tissue quality in older adults. Utilizing IHC as a means of exploring the progression of disease, aging, and injury in the skeletal muscle allows for the practical study of muscle tissue down to the fiber level. By adding editing modules to our semiautomatic approach, we accurately quantified the aging muscle and addressed common technical issues.

Long Non-coding RNAs Are Differentially Expressed After Different Exercise Training Programs

Background

Molecular regulation related to the health benefits of different exercise modes remains unclear. Long non-coding RNAs (lncRNAs) have emerged as an RNA class with regulatory functions in health and diseases. Here, we analyzed the expression of lncRNAs after different exercise training programs and their possible modes of action related to physical exercise adaptations.

Methods

Public high-throughput RNA-seq data (skeletal muscle biopsies) were downloaded, and bioinformatics analysis was performed. We primarily analyzed data reports of 12 weeks of resistance training (RT), high-intensity interval training (HIIT), and combined (CT) exercise training. In addition, we analyzed data from 8 weeks of endurance training (ET). Differential expression analysis of lncRNAs was performed, and an adjusted P-value < 0.1 and log2 (fold change) ≥0.5 or ≤-0.5 were set as the cutoff values to identify differentially expressed lncRNAs (DELs).

Results

We identified 204 DELs after 12 weeks of HIIT, 43 DELs after RT, and 15 DELs after CT. Moreover, 52 lncRNAs were differentially expressed after 8 weeks of ET. The lncRNA expression pattern after physical exercise was very specific, with distinct expression profiles for the different training programs, where few lncRNAs were common among the exercise types. LncRNAs may regulate molecular responses to exercise, such as collagen fibril organization, extracellular matrix organization, myoblast and plasma membrane fusion, skeletal muscle contraction, synaptic transmission, PI3K and TORC regulation, autophagy, and angiogenesis.

Conclusion

For the first time, we show that lncRNAs are differentially expressed in skeletal muscle after different physical exercise programs, and these lncRNAs may act in various biological processes related to physical activity adaptations.

Optimizing Skeletal Muscle Anabolic Response to Resistance Training in Aging

Loss of muscle mass and strength with aging, also termed sarcopenia, results in a loss of mobility and independence. Exercise, particularly resistance training, has proven to be beneficial in counteracting the aging-associated loss of skeletal muscle mass and function. However, the anabolic response to exercise in old age is not as robust, with blunted improvements in muscle size, strength, and function in comparison to younger individuals. This review provides an overview of several physiological changes which may contribute to age-related loss of muscle mass and decreased anabolism in response to resistance training in the elderly. Additionally, the following supplemental therapies with potential to synergize with resistance training to increase muscle mass are discussed: nutrition, creatine, anti-inflammatory drugs, testosterone, and growth hormone (GH). Although these interventions hold some promise, further research is necessary to optimize the response to exercise in elderly patients.

RUNX1: an emerging therapeutic target for cardiovascular disease

Runt-related transcription factor-1 (RUNX1), also known as acute myeloid leukaemia 1 protein (AML1), is a member of the core-binding factor family of transcription factors which modulate cell proliferation, differentiation, and survival in multiple systems. It is a master-regulator transcription factor, which has been implicated in diverse signalling pathways and cellular mechanisms during normal development and disease. RUNX1 is best characterized for its indispensable role for definitive haematopoiesis and its involvement in haematological malignancies. However, more recently RUNX1 has been identified as a key regulator of adverse cardiac remodelling following myocardial infarction. This review discusses the role RUNX1 plays in the heart and highlights its therapeutic potential as a target to limit the progression of adverse cardiac remodelling and heart failure.

Resistance exercise training promotes fiber type-specific myonuclear adaptations in older adults

Aging induces physiological decline in human skeletal muscle function and morphology, including type II fiber atrophy and an increase in type I fiber frequency. Resistance exercise training (RET) is an effective strategy to overcome muscle mass loss and improve strength, with a stronger effect on type II fibers. In the present study, we sought to determine the effect of a 12-wk progressive RET program on the fiber type-specific skeletal muscle hypertrophic response in older adults. Nineteen subjects [10 men and 9 women (71.1 ± 4.3 yr)] were studied before and after the 12-wk program. Immunohistochemical analysis was used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area (CSA), satellite cell abundance, myonuclear content, and lipid droplet density. RET induced an increase in MyHC type II fiber frequency and a concomitant decrease in MyHC type I fiber frequency. Mean CSA increased significantly only in MyHC type II fibers (+23.3%, P < 0.05), but myonuclear content increased only in MyHC type I fibers (P < 0.05), with no change in MyHC type II fibers. Satellite cell content increased ~40% in both fiber types (P > 0.05). RET induced adaptations to the capillary supply to satellite cells, with the distance between satellite cells and the nearest capillary increasing in type I fibers and decreasing in type II fibers. Both fiber types showed similar decrements in intramuscular lipid density with training (P < 0.05). Our data provide intriguing evidence for a fiber type-specific response to RET in older adults and suggest flexibility in the myonuclear domain of type II fibers during a hypertrophic stimulus.NEW & NOTEWORTHY In older adults, progressive resistance exercise training (RET) increased skeletal muscle fiber volume and cross-sectional area independently of myonuclear accretion, leading to an expansion of the myonuclear domain. Fiber type-specific analyses illuminated differential adaptation; type II fibers underwent hypertrophy and exhibited myonuclear domain plasticity, whereas myonuclear accretion occurred in type I fibers in the absence of a robust hypertrophic response. RET also augmented satellite cell-capillary interaction and reduced intramyocellular lipid density to improve muscle quality.

Relationship of Physical Function to Single Muscle Fiber Contractility in Older Adults: Effects of Resistance Training With and Without Caloric Restriction

BACKGROUND

Previous studies support beneficial effects of both resistance exercise training (RT) and caloric restriction (CR) on skeletal muscle strength and physical performance. The goal of this study was to determine the effects of adding CR to RT on single-muscle fiber contractility responses to RT in older overweight and obese adults.

METHODS

We analyzed contractile properties in 1,253 single myofiber from muscle biopsies of the vastus lateralis, as well as physical performance and thigh muscle volume, in 31 older (65-80 years), overweight or obese (body mass index = 27-35 kg/m2) men (n = 19) and women (n = 12) who were randomly assigned to a standardized, progressive RT intervention with CR (RT+CR; n = 15) or without CR (RT; n = 16) for 5 months.

RESULTS

Both interventions evoked an increase in force normalized to cross-sectional area (CSA), in type-I and type-II fibers and knee extensor quality. However, these improvements were not different between intervention groups. In the RT group, changes in total thigh fat volume inversely correlated with changes in type-II fiber force (r = -.691; p = .019). Within the RT+CR group, changes in gait speed correlated positively with changes in type-I fiber CSA (r = .561; p = .030). In addition, increases in type-I normalized fiber force were related to decreases in thigh intermuscular fat volume (r = -0.539; p = .038).

CONCLUSION

Single muscle fiber force and knee extensor quality improve with RT and RT+CR; however, CR does not enhance improvements in single muscle fiber contractility or whole muscle in response to RT in older overweight and obese men and women.

Discovery proteomics in aging human skeletal muscle finds change in spliceosome, immunity, proteostasis and mitochondria

A decline of skeletal muscle strength with aging is a primary cause of mobility loss and frailty in older persons, but the molecular mechanisms of such decline are not understood. Here, we performed quantitative proteomic analysis from skeletal muscle collected from 58 healthy persons aged 20 to 87 years. In muscle from older persons, ribosomal proteins and proteins related to energetic metabolism, including those related to the TCA cycle, mitochondria respiration, and glycolysis, were underrepresented, while proteins implicated in innate and adaptive immunity, proteostasis, and alternative splicing were overrepresented. Consistent with reports in animal models, older human muscle was characterized by deranged energetic metabolism, a pro-inflammatory environment and increased proteolysis. Changes in alternative splicing with aging were confirmed by RNA-seq analysis. We propose that changes in the splicing machinery enables muscle cells to respond to a rise in damage with aging.

As humans age, their muscles become weaker, making it increasingly harder for them to move, a condition known as sarcopenia. Analyzing old muscles in other animals revealed that they produce energy inefficiently, they destroy more proteins than younger muscles, and they have high levels of molecules that cause inflammation. These characteristics may be involved in causing muscle weakness.

Proteomics is the study of proteins, the molecules that play many roles in keeping the body working: for example, they accelerate chemical reactions, participate in copying DNA and help cells respond to stimuli. Using proteomics, it is possible to examine a large number of the different proteins in a tissue, which can provide information about the state of that tissue. Ubaida-Mohien et al. used this approach to answer the question of why muscles become weaker with age.

First, they analyzed the levels of all the proteins found in skeletal muscle collected from 58 healthy volunteers between 20 and 87 years of age. This revealed that the muscles of older people have fewer copies of the proteins that make up ribosomes – the cellular machines that produce new proteins – and fewer proteins involved in providing the cell with chemical energy. In contrast, proteins implicated in the immune system, in the maintenance of existing proteins, and in processing other molecules called RNAs were more abundant in older muscles.

Ubaida-Mohien et al. then looked more closely at changes involving RNA processing. Cells make proteins by copying DNA sequences into an RNA template and using this template to instruct the ribosomes on how to make the specific protein. Before the RNA can be ‘read’ by a ribosome, however, some parts must be cut out and others added, which can lead to different versions of the final RNA, also known as alternative transcripts.

In order to check whether the difference in the levels of proteins that process RNAs was affecting the RNAs being produced, Ubaida-Mohien et al. extracted the RNAs from older and younger muscles and compared them. This showed that the RNA in older people had more alternative transcripts, confirming that the change in protein levels was having downstream effects.

Currently, it is not possible to prevent or delay the loss of muscle strength associated with aging. Understanding how the protein make-up of muscles changes as humans grow older may help find new ways to prevent and perhaps even reverse this decline.

Resistance training-induced gains in knee extensor strength are related to increased neural cell adhesion molecule expression in older adults with knee osteoarthritis

Objective

Resistance training (RT) can improve whole muscle strength without increasing muscle fiber size or contractility. Neural adaptations, which lead to greater neural activation of muscle, may mediate some of these improvements, particularly in older adults, where motor neuron denervation is common. The purpose of this study was to explore the relationship of neural adaptations, as reflected by neural cell adhesion molecule (NCAM) expression, to improvements in (1) whole muscle strength and (2) muscle fiber size following RT in older adults with knee osteoarthritis. We performed whole muscle strength measurements and immunohistochemical analysis of fiber size, type, and NCAM expression before and after a 14-week RT program.

Results

RT increased whole-muscle strength as measured by 1-repetition maximum (1-RM) leg press (P = 0.01), leg extension (P = 0.03), and knee extensor peak torque (P = 0.050), but did not alter NCAM expression. Greater NCAM expression in myosin heavy chain (MHC) II fibers was associated with greater whole muscle strength gains (knee extensor peak torque r = 0.93; P < 0.01) and greater MHC II fiber size (r = 0.79; P < 0.01). Our results suggest that training-induced NCAM expression, and neural adaptations more generally, may be important for RT-induced morphological and functional improvements in older adults.

Trial registration NCT01190046

Fidelity of muscle fibre reinnervation modulates ageing muscle impact in elderly women

KEY POINTS

Susceptibility to age-related muscle atrophy relates to the degree of muscle denervation and the capacity of successful reinnervation. However, the specific role of denervation as a determinant of the severity of muscle aging between populations with low versus high physical function has not been addressed. We show that prefrail/frail elderly women exhibited marked features of muscle denervation, whereas world class octogenarian female master athletes showed attenuated indices of denervation and greater reinnervation capacity. These findings suggest that the difference in age-related muscle impact between low- and high-functioning elderly women is the robustness of the response to denervation of myofibers.

ABSTRACT

Ageing muscle degeneration is a key contributor to physical frailty; however, the factors responsible for exacerbated vs. muted ageing muscle impact are largely unknown. Based upon evidence that susceptibility to neurogenic impact is an important determinant of the severity of ageing muscle degeneration, we aimed to determine the presence and extent of denervation in pre-frail/frail elderly (FE, 77.9 ± 6.2 years) women compared to young physically inactive (YI, 24.0 ± 3.5 years) females, and contrast these findings to high-functioning world class octogenarian female masters athletes (MA, 80.9 ± 6.6 years). Muscle biopsies from vastus lateralis muscle were obtained from all three groups to assess denervation-related morphological and transcriptional markers. The FE group displayed marked grouping of slow fibres, accumulation of very small myofibres, a severe reduction in type IIa/I size ratio, highly variable inter-subject accumulation of neural cell adhesion molecule (NCAM)-positive myofibres, and an accumulation of pyknotic nuclei, indicative of recurring cycles of denervation/reinnervation and persistent denervation. The MA group exhibited a smaller decline in type IIa/I size ratio and fewer pyknotic nuclei, accompanied by a higher degree of type I fibre grouping and larger fibre group size, suggesting a greater reinnervation of denervated fibres. Consistent with this interpretation, MA had higher mRNA levels of the reinnervation-promoting cytokine fibroblast growth factor binding protein 1 (FGFBP1) than FE. Our results indicate that the muscle of FE women has significant neurogenic atrophy, whereas MA muscle exhibit superior reinnervation capacity, suggesting that the difference in age-related muscle impact between low- and high-functioning elderly women is the robustness of the response to denervation of myofibres.

Combating osteoporosis and obesity with exercise: leveraging cell mechanosensitivity

Osteoporosis, a condition of skeletal decline that undermines quality of life, is treated with pharmacological interventions that are associated with poor adherence and adverse effects. Complicating efforts to improve clinical outcomes, the incidence of obesity is increasing, predisposing the population to a range of musculoskeletal complications and metabolic disorders. Pharmacological management of obesity has yet to deliver notable reductions in weight and debilitating complications are rarely avoided. By contrast, exercise shows promise as a non-invasive and non-pharmacological method of regulating both osteoporosis and obesity. The principal components of exercise - mechanical signals - promote bone and muscle anabolism while limiting formation and expansion of fat mass. Mechanical regulation of bone and marrow fat might be achieved by regulating functions of differentiated cells in the skeletal tissue while biasing lineage selection of their common progenitors - mesenchymal stem cells. An inverse relationship between adipocyte versus osteoblast fate selection from stem cells is implicated in clinical conditions such as childhood obesity and increased marrow adiposity in type 2 diabetes mellitus, as well as contributing to skeletal frailty. Understanding how exercise-induced mechanical signals can be used to improve bone quality while decreasing fat mass and metabolic dysfunction should lead to new strategies to treat chronic diseases such as osteoporosis and obesity.

Moderate-intensity aerobic exercise improves skeletal muscle quality in older adults

Sarcopenia, age-associated involuntary loss of muscle and strength, can progress to clinically relevant functional decline. Resistance exercise attenuates muscle and strength loss but may not be feasible for some older adults. Aerobic exercise training (AET) improves cardiopulmonary health; however, effects on protein turnover, muscle mass, and strength are less clear. We aimed to determine whether AET improves basal myofibrillar protein synthesis (MPS) and capillarization, promoting hypertrophy and strength. We hypothesized that AET improves strength with increased MPS and capillarization. Older adults were randomized to non-exercise (NON; n = 11, 71.4 ± 4.18 years) or exercise (EX; n = 12, 73.7 ± 4.05 years). EX completed 24 weeks of AET (walking 3×/week, 45 minutes, 70% heart rate reserve); NON remained sedentary. A stable isotope tracer was infused. MPS and capillarization were analyzed from vastus lateralis muscle biopsies. Strength was measured via isokinetic dynamometry. Lean mass was determined with dual-energy X-ray absorptiometry. Basal MPS increased in EX (+50.7%, P = 0.01) along with capillary density (+66.4%, P = 0.03), peak oxygen consumption (+15.8%, P = 0.01), quadriceps strength (+15.1%, P = 0.01), and muscle quality (peak torque divided by leg lean mass, +15.5%, P = 0.01). Lean mass did not change (P > 0.05). AET increases muscle protein turnover and capillarization in older adults, improving muscle quality.

Building strength, endurance, and mobility using an astaxanthin formulation with functional training in elderly

BACKGROUND

Building both strength and endurance has been a challenge in exercise training in the elderly, but dietary supplements hold promise as agents for improving muscle adaptation. Here, we test a formulation of natural products (AX: astaxanthin, 12 mg and tocotrienol, 10 mg and zinc, 6 mg) with both anti-inflammatory and antioxidant properties in combination with exercise. We conducted a randomized, double-blind, placebo-controlled study of elderly subjects (65-82 years) on a daily oral dose with interval walking exercise on an incline treadmill.

METHODS

Forty-two subjects were fed AX or placebo for 4 months and trained 3 months (3×/week for 40-60 min) with increasing intervals of incline walking. Strength was measured as maximal voluntary force (MVC) in ankle dorsiflexion exercise, and tibialis anterior muscle size (cross-sectional area, CSA) was determined from magnetic resonance imaging.

RESULTS

Greater endurance (exercise time in incline walking, >50%) and distance in 6 min walk (>8%) accompanied training in both treatments. Increases in MVC by 14.4% (±6.2%, mean ± SEM, P < 0.02, paired t-test), CSA by 2.7% (±1.0%, P < 0.01), and specific force by 11.6% (MVC/CSA, ±6.0%, P = 0.05) were found with AX treatment, but no change was evident in these properties with placebo treatment (MVC, 2.9% ± 5.6%; CSA, 0.6% ± 1.2%; MVC/CSA, 2.4 ± 5.7%; P > 0.6 for all).

CONCLUSIONS

The AX formulation improved muscle strength and CSA in healthy elderly in addition to the elevation in endurance and walking distance found with exercise training alone. Thus, the AX formulation in combination with a functional training programme uniquely improved muscle strength, endurance, and mobility in the elderly.

Neural and musculotendinous mechanisms underpinning age-related force reductions

Ageing leads to substantial force production capacity reductions, which is an indicator of frailty and disability, and a mortality predictor in elders. Understanding the age-dependent neuromuscular mechanisms underlying force reductions can optimize healthcare professionals' exercise protocol choices for patients and allows researchers to investigate new interventions to mitigate these reductions. Our primary goal was to provide an updated review about the main neural and musculotendinous mechanisms underpinning age-related reductions in force capacity. Our secondary goal was to summarize how aerobic and strength training can lessen these age-related reductions. This review suggests that several steps in the force production pathway, from cortical to muscular mechanisms, are negatively affected by ageing. However, combining aerobic and strength training can attenuate these effects. Strength training (i.e. moderate to high- intensity, progressive volume, accentuated eccentric loading and fast concentric contraction velocity) can increase overall force production capacity by producing beneficial neural and musculotendinous adaptations. Additionally, aerobic training (i.e. moderate and high intensities) plays an essential role in preserving the structure and function of the neuromuscular system.

NMJ maintenance and repair in aging

As the final output of the somatic nervous system, the neuromuscular junction (NMJ) is essential for all voluntary movements. The NMJ is also necessary for connected cells to function and survive. Because of this central role, much effort has been devoted to understanding the effects of aging, diseases, and injuries on the NMJ. These efforts have revealed a close relationship between aberrant changes at NMJs and its three cellular components - the presynaptic site on motor axons, the postsynaptic region on muscle fibers and perisynaptic Schwann cells. Here, we review the morphological and molecular changes associated with aging NMJs in rodents and humans. We also provide an overview of factors with potential roles in maintaining and repairing adult and aged NMJs.

Effects of aging and Parkinson's disease on motor unit remodeling: influence of resistance exercise training

Aging muscle atrophy is in part a neurodegenerative process revealed by denervation/reinnervation events leading to motor unit remodeling (i.e., myofiber type grouping). However, this process and its physiological relevance are poorly understood, as is the wide-ranging heterogeneity among aging humans. Here, we attempted to address 1) the relation between myofiber type grouping and molecular regulators of neuromuscular junction (NMJ) stability; 2) the impact of motor unit remodeling on recruitment during submaximal contractions; 3) the prevalence and impact of motor unit remodeling in Parkinson's disease (PD), an age-related neurodegenerative disease; and 4) the influence of resistance exercise training (RT) on regulators of motor unit remodeling. We compared type I myofiber grouping, molecular regulators of NMJ stability, and the relative motor unit activation (MUA) requirement during a submaximal sit-to-stand task among untrained but otherwise healthy young (YA; 26 yr, n = 27) and older (OA; 66 yr, n = 91) adults and OA with PD (PD; 67 yr, n = 19). We tested the effects of RT on these outcomes in OA and PD. PD displayed more motor unit remodeling, alterations in NMJ stability regulation, and a higher relative MUA requirement than OA, suggesting PD-specific effects. The molecular and physiological outcomes tracked with the severity of type I myofiber grouping. Together these findings suggest that age-related motor unit remodeling, manifested by type I myofiber grouping, 1) reduces MUA efficiency to meet submaximal contraction demand, 2) is associated with disruptions in NMJ stability, 3) is further impacted by PD, and 4) may be improved by RT in severe cases. NEW & NOTEWORTHY Because the physiological consequences of varying amounts of myofiber type grouping are unknown, the current study aims to characterize the molecular and physiological correlates of motor unit remodeling. Furthermore, because exercise training has demonstrated neuromuscular benefits in aged humans and improved innervation status and neuromuscular junction integrity in animals, we provide an exploratory analysis of the effects of high-intensity resistance training on markers of neuromuscular degeneration in both Parkinson's disease (PD) and age-matched older adults.

Effects of Exercise and Aging on Skeletal Muscle

A substantial loss of muscle mass and strength (sarcopenia), a decreased regenerative capacity, and a compromised physical performance are hallmarks of aging skeletal muscle. These changes are typically accompanied by impaired muscle metabolism, including mitochondrial dysfunction and insulin resistance. A challenge in the field of muscle aging is to dissociate the effects of chronological aging per se on muscle characteristics from the secondary influence of lifestyle and disease processes. Remarkably, physical activity and exercise are well-established countermeasures against muscle aging, and have been shown to attenuate age-related decreases in muscle mass, strength, and regenerative capacity, and slow or prevent impairments in muscle metabolism. We posit that exercise and physical activity can influence many of the changes in muscle during aging, and thus should be emphasized as part of a lifestyle essential to healthy aging.

ACL injury reduces satellite cell abundance and promotes fibrogenic cell expansion within skeletal muscle

Anterior cruciate ligament (ACL) injuries are associated with significant loss of strength in knee extensor muscles that persists despite physical therapy. The underlying mechanisms responsible for this protracted muscle weakness are poorly understood; however, we recently showed significant myofiber atrophy and altered muscle phenotype following ACL injury. We sought to further explore perturbations in skeletal muscle morphology and progenitor cell activity following an ACL injury. Muscle biopsies were obtained from the injured and non-injured vastus lateralis of young adults (n = 10) following ACL injury, and histochemical/immunohistochemical analyses were undertaken to determine collagen content, abundance of connective tissue fibroblasts, fibrogenic/adipogenic progenitor (FAP) cells, satellite cells, in addition to indices of muscle fiber denervation and myonuclear apoptosis. The injured limb showed elevated collagen content (p < 0.05), in addition to a greater abundance of fibroblasts and FAPs (p < 0.05) in the injured limb. Fibroblast content was correlated with increased accumulation of extracellular matrix in the injured limb as well. A higher frequency of interstitial nuclei were positive for phospho-SMAD3 in the injured limb (p < 0.05), providing some evidence for activation of a fibrogenic program through transforming growth factor β following an ACL injury. The injured limb also displayed reduced satellite cell abundance, increased fiber denervation and DNA damage associated with apoptosis (p < 0.05), indicating alterations within the muscle itself after the ligament injury. Injury of the ACL induces a myriad of negative outcomes within knee extensor muscles, which likely compromise the restorative capacity and plasticity of skeletal muscle, impeding rehabilitative efforts. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1876-1885, 2017.

Lipid modulation of skeletal muscle mass and function

Loss of skeletal muscle mass is a characteristic feature of various pathologies including cancer, diabetes, and obesity, as well as being a general feature of ageing. However, the processes underlying its pathogenesis are not fully understood and may involve multiple factors. Importantly, there is growing evidence which supports a role for fatty acids and their derived lipid intermediates in the regulation of skeletal muscle mass and function. In this review, we discuss evidence pertaining to those pathways which are involved in the reduction, increase and/or preservation of skeletal muscle mass by such lipids under various pathological conditions, and highlight studies investigating how these processes may be influenced by dietary supplementation as well as genetic and/or pharmacological intervention.

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

Background

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

Results

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

Conclusions

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

Electronic supplementary material

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

High Fat Diet-Induced Skeletal Muscle Wasting Is Decreased by Mesenchymal Stem Cells Administration: Implications on Oxidative Stress, Ubiquitin Proteasome Pathway Activation, and Myonuclear Apoptosis

Obesity can lead to skeletal muscle atrophy, a pathological condition characterized by the loss of strength and muscle mass. A feature of muscle atrophy is a decrease of myofibrillar proteins as a result of ubiquitin proteasome pathway overactivation, as evidenced by increased expression of the muscle-specific ubiquitin ligases atrogin-1 and MuRF-1. Additionally, other mechanisms are related to muscle wasting, including oxidative stress, myonuclear apoptosis, and autophagy. Stem cells are an emerging therapy in the treatment of chronic diseases such as high fat diet-induced obesity. Mesenchymal stem cells (MSCs) are a population of self-renewable and undifferentiated cells present in the bone marrow and other mesenchymal tissues of adult individuals. The present study is the first to analyze the effects of systemic MSC administration on high fat diet-induced skeletal muscle atrophy in the tibialis anterior of mice. Treatment with MSCs reduced losses of muscle strength and mass, decreases of fiber diameter and myosin heavy chain protein levels, and fiber type transitions. Underlying these antiatrophic effects, MSC administration also decreased ubiquitin proteasome pathway activation, oxidative stress, and myonuclear apoptosis. These results are the first to indicate that systemically administered MSCs could prevent muscle wasting associated with high fat diet-induced obesity and diabetes.

Fourteen days of bed rest induces a decline in satellite cell content and robust atrophy of skeletal muscle fibers in middle-aged adults

Bed rest, a ground-based spaceflight analog, induces robust atrophy of skeletal muscle, an effect that is exacerbated with increasing age. We examined the effect of 14 days of bed rest on skeletal muscle satellite cell content and fiber type atrophy in middle-aged adults, an understudied age demographic with few overt signs of muscle aging that is representative of astronauts who perform long-duration spaceflight. Muscle biopsies were obtained from the vastus lateralis of healthy middle-aged adults [n= 7 (4 male, 3 female); age: 51 ± 1 yr] before (Pre-BR) and after (Post-BR) 14 days of bed rest. Immunohistochemical analyses were used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area (CSA), satellite cell and myonuclear content, and capillary density. Peak oxygen consumption, knee extensor strength, and body composition were also measured Pre-BR and Post-BR. Post-BR MyHC type 2a fiber percentage was reduced, and mean CSA decreased in all fiber types (-24 ± 5%;P< 0.05). Satellite cell content was also reduced Post-BR (-39 ± 9%;P< 0.05), and the change in satellite cell content was significantly correlated with the change in mean fiber CSA (r(2)= 0.60;P< 0.05). A decline in capillary density was observed Post-BR (-23 ± 6%;P< 0.05), and Post-BR capillary content was significantly associated with Post-BR peak aerobic capacity (r(2)= 0.59;P< 0.05). A subtle decline in myonuclear content occurred during bed rest (-5 ± 1%;P< 0.05). The rapid maladaptation of skeletal muscle to 14 days of mechanical unloading in middle-aged adults emphasizes the need for robust countermeasures to preserve muscle function in astronauts.