Aging In Human Skeletal Muscle Research Articles

Human skeletal muscle aging atlas

Skeletal muscle aging is a key contributor to age-related frailty and sarcopenia with substantial implications for global health. Here we profiled 90,902 single cells and 92,259 single nuclei from 17 donors to map the aging process in the adult human intercostal muscle, identifying cellular changes in each muscle compartment. We found that distinct subsets of muscle stem cells exhibit decreased ribosome biogenesis genes and increased CCL2 expression, causing different aging phenotypes. Our atlas also highlights an expansion of nuclei associated with the neuromuscular junction, which may reflect re-innervation, and outlines how the loss of fast-twitch myofibers is mitigated through regeneration and upregulation of fast-type markers in slow-twitch myofibers with age. Furthermore, we document the function of aging muscle microenvironment in immune cell attraction. Overall, we present a comprehensive human skeletal muscle aging resource (https://www.muscleageingcellatlas.org/) together with an in-house mouse muscle atlas to study common features of muscle aging across species.

Aging-affiliated post-translational modifications of skeletal muscle myosin affect biochemical properties, myofibril structure, muscle function, and proteostasis.

The molecular motor myosin is post-translationally modified in its globular head, its S2 hinge, and its thick filament domain during human skeletal muscle aging. To determine the importance of such modifications, we performed an integrative analysis of transgenic Drosophila melanogaster expressing myosin containing post-translational modification mimic mutations. We determined effects on muscle function, myofibril structure, and myosin biochemistry. Modifications in the homozygous state decreased jump muscle function by a third at 3 weeks of age and reduced indirect flight muscle function to negligible levels in young flies, with severe effects on flight muscle myofibril assembly and/or maintenance. Expression of mimic mutations in the heterozygous state or in a wild-type background yielded significant, but less severe, age-dependent effects upon flight muscle structure and function. Modification of the residue in the globular head disabled ATPase activity and invitro actin filament motility, whereas the S2 hinge mutation reduced actin-activated ATPase activity by 30%. The rod modification diminished filament formation invitro. The latter mutation also reduced proteostasis, as demonstrated by enhanced accumulation of polyubiquitinated proteins. Overall, we find that mutation of amino acids at sites that are chemically modified during human skeletal muscle aging can disrupt myosin ATPase, myosin filament formation, and/or proteostasis, providing a mechanistic basis for the observed muscle defects. We conclude that age-specific post-translational modifications present in human skeletal muscle are likely to act in a dominant fashion to affect muscle structure and function and may therefore be implicated in degeneration and dysfunction associated with sarcopenia.

MUSCLE GROUP SPECIFIC SKELETAL MUSCLE AGING: A FIVE YEAR LONGITUDINAL STUDY IN SEPTUAGENARIANS

Abstract Most of the scientific literature on human skeletal muscle aging has focused on the quadriceps femoris, with the remaining few hundred skeletal muscles receiving limited specific investigation. This longitudinal investigation compared changes in skeletal muscle size via computed tomography of the quadriceps (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), hamstrings (biceps femoris short and long heads, semitendinosus, semimembranosus), psoas, rectus abdominis, lateral abdominals (obliques and transversus abdominus), and paraspinal muscles (erector spinae and multifidi) of older individuals from the Health ABC study at baseline and 5.0±0.1 years later (n=469, 73±3y & 78±3y, 49% women, 33% black). Skeletal muscle size decreased (p < 0.05) in quadriceps (-3.3%), hamstrings (-5.9%), psoas (-0.4%), and rectus abdominus (-7.0%). The hamstrings and rectus abdominus atrophied approximately twice as much as the quadriceps (p < 0.05), while the quadriceps atrophied substantially more than the psoas (p < 0.05). The paraspinals (+4.3%) and lateral abdominals (+5.9%) hypertrophied (p < 0.05) to a similar degree (p>0.05) over the 5 years. These data suggest that skeletal muscle mass in older individuals changes in a muscle group specific fashion in the eighth decade, a critical time period in the aging process. A broader understanding of muscle group specific skeletal muscle aging is needed to better guide exercise programs and other interventions that mitigate decrements in physical function with aging.

Single-nucleus profiling unveils a geroprotective role of the FOXO3 in primate skeletal muscle aging.

Age-dependent loss of skeletal muscle mass and function is a feature of sarcopenia, and increases the risk of many aging-related metabolic diseases. Here, we report phenotypic and single-nucleus transcriptomic analyses of non-human primate skeletal muscle aging. A higher transcriptional fluctuation was observed in myonuclei relative to other interstitial cell types, indicating a higher susceptibility of skeletal muscle fiber to aging. We found a downregulation of FOXO3 in aged primate skeletal muscle, and identified FOXO3 as a hub transcription factor maintaining skeletal muscle homeostasis. Through the establishment of a complementary experimental pipeline based on a human pluripotent stem cell-derived myotube model, we revealed that silence of FOXO3 accelerates human myotube senescence, whereas genetic activation of endogenous FOXO3 alleviates human myotube aging. Altogether, based on a combination of monkey skeletal muscle and human myotube aging research models, we unraveled the pivotal role of the FOXO3 in safeguarding primate skeletal muscle from aging, providing a comprehensive resource for the development of clinical diagnosis and targeted therapeutic interventions against human skeletal muscle aging and the onset of sarcopenia along with aging-related disorders.

Macrophage Subpopulations and the Acute Inflammatory Response of Elderly Human Skeletal Muscle to Physiological Resistance Exercise.

The current model for repair of damaged tissue includes immune cells, mediating the progression from a pro-inflammatory to an anti-inflammatory environment. How this process changes with aging in human skeletal muscle under conditions of physiological exercise loading remains unclear. To investigate this, 25 elderly males (mean age 70 ± SD 7 years), as well as 12 young (23 ± 3 years) and 12 elderly (74 ± 3 years) females, performed a unilateral bout of heavy resistance leg extension exercise. Biopsies were collected from the vastus lateralis muscle of the rested (control) leg, and post exercise from the exercised leg at 4.5 h, and on days 1, 4, and 7 for the male participants, or on day 5 for the female participants. Total macrophages (CD68+) as well as pro- (CD11b+) and anti-inflammatory (CD163+, CD206+) subpopulations were identified on sections by immunohistochemistry. Gene expression levels of COL1A1, TNF-a, CD68, myostatin, TCF7L2, IL-1B, IL-1R, IL-10, and Ki67 were determined by real-time RT-PCR. At rest, the muscle tissue from the elderly vs. young females was characterized by higher gene expression levels of CD68, IL-10, lower myostatin mRNA, and trends for a greater number of macrophages, while COL1A1 mRNA post exercise values were greater in the elderly vs young. For the male participants, mRNA levels of the inflammatory cytokines IL-1B, IL-1R were elevated in the early phase following exercise, followed by increases in COL1A1 and Ki67 on days 4 and 7. In general, exercise induced increases in all types of macrophages counted in the elderly, but not in young, individuals. Cells expressing CD68, CD11b, and CD206 simultaneously were the most frequently observed cell type, which raises the possibility that pure pro- and anti-inflammatory macrophages populations do not exist in healthy human skeletal muscle within the spectrum of tissue remodeling induced by physiological exercise designed to induce hypertrophy. Together these data provide insight into the time course of macrophage activity and associated molecular targets in human skeletal muscle in the context of aging and exercise.

An Evaluation Of Skeletal Muscle Aging Using A Novel Guinea Pig Model

Maintenance of the musculoskeletal system is critical to prevent falls and loss of mobility with aging. Sarcopenia, the age-related loss of muscle mass and function, affects 10% of those over 65 years and as much as 50% of people over 80 years of age. Aging is a major risk factor for osteoarthritis (OA) which is characterized by a concomitant loss of skeletal muscle, further contributing to decreased mobility. The “inflammaging” phenotype, (i.e., age-related increases in low-grade inflammation and oxidative stress) is common to both OA and sarcopenia. While progress has been made in understanding the mechanisms of sarcopenia a preclinical model that capitulates human conditions is lacking. Dunkin Hartley (DH) guinea pigs rapidly and spontaneously develop primary knee OA beginning at about 4 months of age. Thus, we speculate that DH guinea pigs may also be a valuable model of sarcopenia. PURPOSE: To determine if DH guinea pigs can serve as a model to understand human skeletal muscle aging. METHODS: We compared skeletal muscle age-related changes in the gastrocnemius (GAS) and soleus (SOL) from 5, 9, and 15-mo DH guinea pigs. We also compared these changes to a strain of guinea pig, strain 13, that does not develop knee OA at an early age. Magnetic resonance imaging was used to examine volume and then used to calculate muscle density. Immunofluorescent histochemistry was used to assess myofiber size distribution. Formalin fixed muscles were stained in India ink to measure pennation angle. Fibrosis was assessed using muscles paraffin embedded and stained with Masson’s Trichrome to quantify % collagen. Analyses are ongoing to identify if DH pigs are characterized by an inflammaging phenotype similar to aging human muscle. RESULTS: DH guinea pigs had a significant decrease in GAS density between 5 and 15-mo that was not present in the SOL (p<0.05). Both in the GAS and SOL, DH guinea pigs also demonstrated a shift towards a smaller average myofiber size with age. However, there were no age-related changes in pennation angle or fibrosis. CONCLUSION: Based on these analyses, the DH guinea pig appears to be a potentially valuable model of musculoskeletal aging. Identifying a model to study muscular aging that mimics human conditions but in a shortened time frame, will potentially allow for effective screening and treatment interventions.

Machine Learning on Human Muscle Transcriptomic Data for Biomarker Discovery and Tissue-Specific Drug Target Identification.

For the past several decades, research in understanding the molecular basis of human muscle aging has progressed significantly. However, the development of accessible tissue-specific biomarkers of human muscle aging that may be measured to evaluate the effectiveness of therapeutic interventions is still a major challenge. Here we present a method for tracking age-related changes of human skeletal muscle. We analyzed publicly available gene expression profiles of young and old tissue from healthy donors. Differential gene expression and pathway analysis were performed to compare signatures of young and old muscle tissue and to preprocess the resulting data for a set of machine learning algorithms. Our study confirms the established mechanisms of human skeletal muscle aging, including dysregulation of cytosolic Ca2+ homeostasis, PPAR signaling and neurotransmitter recycling along with IGFR and PI3K-Akt-mTOR signaling. Applying several supervised machine learning techniques, including neural networks, we built a panel of tissue-specific biomarkers of aging. Our predictive model achieved 0.91 Pearson correlation with respect to the actual age values of the muscle tissue samples, and a mean absolute error of 6.19 years on the test set. The performance of models was also evaluated on gene expression samples of the skeletal muscles from the Gene expression Genotype-Tissue Expression (GTEx) project. The best model achieved the accuracy of 0.80 with respect to the actual age bin prediction on the external validation set. Furthermore, we demonstrated that aging biomarkers can be used to identify new molecular targets for tissue-specific anti-aging therapies.

MicroRNA Regulated Transcriptomic Changes during Aging in Human Skeletal Muscle Are Reversible with Exercise Training

Muscle mass declines with age (sarcopenia) in both men and women resulting in impaired physical function and altered glucose metabolism. Endurance exercise training can mitigate and even reverse some of the changes in skeletal muscle structure and function, which are observed at advanced age. To gain insight into the potential mechanisms driving sarcopenia and the adaptive response to training, we conducted deep sequencing on vastus lateralis muscle obtained from healthy, younger (n=14; age=27±0.89 y; 8 women, 6 men) and older (n=14, age=71±1.39 y; 7 women, 7 men) subjects before and after a 4 month endurance training program. We analyzed the sequencing data with bioinformatics tools to determine whether miRNAs that change with age can be reverted back to youthful levels with training. We found that skeletal muscle from women had 7 miRNAs and men had 8 miRNAs that changed with age and were reverted back to youthful levels with training, respectively. In older trained females these 7 miRNAs targeted 94 unique transcripts in the top 1000 differentially expressed genes between post-training compared to pre-training and these transcripts were involved in lipid metabolism, glycolysis, and growth. In older men the 8 miRNAs with youthful signatures after training had 146 unique targets involved in phosphoprotein and PI3K-AKT signaling, as well as cholinergic synapses. Notably, miR-29b-3p was primarily responsible for the regulation of target transcripts within the PI3K-AKT pathway in older trained men; similar family members (miR-29a-3p and miR-29c-3p) have been directly linked to insulin sensitivity in preclinical models. All miRNAs with youthful signatures in older trained subjects had opposing expression patterns to their target mRNAs. We conclude that some of the aging related changes to the skeletal muscle transcriptome are under the regulation of miRNAs and that exercise training can reverse these changes. Disclosure J. Valentine: None. S. Ghosh: None. B. Milholland: None. J.A. Gelfond: None. N. Musi: None.

Somatic mutagenesis in satellite cells associates with human skeletal muscle aging

Human aging is associated with a decline in skeletal muscle (SkM) function and a reduction in the number and activity of satellite cells (SCs), the resident stem cells. To study the connection between SC aging and muscle impairment, we analyze the whole genome of single SC clones of the leg muscle vastus lateralis from healthy individuals of different ages (21–78 years). We find an accumulation rate of 13 somatic mutations per genome per year, consistent with proliferation of SCs in the healthy adult muscle. SkM-expressed genes are protected from mutations, but aging results in an increase in mutations in exons and promoters, targeting genes involved in SC activity and muscle function. In agreement with SC mutations affecting the whole tissue, we detect a missense mutation in a SC propagating to the muscle. Our results suggest somatic mutagenesis in SCs as a driving force in the age-related decline of SkM function.

Optimization of a proteomic approach for evidencing and identifying oxidized proteins during human skeletal muscle aging

Optimization of a proteomic approach for evidencing and identifying oxidized proteins during human skeletal muscle aging

Human Skeletal Muscle Aging and the Oxidative System: Cellular Events

As we age, the aerobic and functional capacities of our major physiological systems progressively decline. In the case of the neuromuscular system, reductions in strength and mobility cause a deterioration in motor performance and in turn a greater tendency to fall (with increased risk of fractures), impaired mobility, disability and loss of independence in the elderly. Given the increase in our life expectancy and the consequent growth in the elderly population, these conditions will have an increasing impact on modern healthcare systems, and their prevention and attenuation needs to be addressed. Several intervention strategies have been used to improve motor performance among the aging. At the cellular level, aging is caused by a progressive decline in mitochondrial function that results in the accumulation of reactive oxygen species (ROS) generated by the addition of a single electron to the oxygen molecule As the level of oxidative stress in skeletal muscle increases with age, the production of some antioxidant enzymes increases adaptively to compensate in part. The aging process is characterized by an imbalance between an increase in the production of reactive oxygen species in the organism and the antioxidant defences as a whole. The goal of this review is to examine the results of existing studies on oxidative stress in aging human skeletal muscles, taking into account different physiological factors (sex, fiber composition, muscle type and function).

Resistance Exercise Reverses Aging in Human Skeletal Muscle

Human aging is associated with skeletal muscle atrophy and functional impairment (sarcopenia). Multiple lines of evidence suggest that mitochondrial dysfunction is a major contributor to sarcopenia. We evaluated whether healthy aging was associated with a transcriptional profile reflecting mitochondrial impairment and whether resistance exercise could reverse this signature to that approximating a younger physiological age. Skeletal muscle biopsies from healthy older (N = 25) and younger (N = 26) adult men and women were compared using gene expression profiling, and a subset of these were related to measurements of muscle strength. 14 of the older adults had muscle samples taken before and after a six-month resistance exercise-training program. Before exercise training, older adults were 59% weaker than younger, but after six months of training in older adults, strength improved significantly (P<0.001) such that they were only 38% lower than young adults. As a consequence of age, we found 596 genes differentially expressed using a false discovery rate cut-off of 5%. Prior to the exercise training, the transcriptome profile showed a dramatic enrichment of genes associated with mitochondrial function with age. However, following exercise training the transcriptional signature of aging was markedly reversed back to that of younger levels for most genes that were affected by both age and exercise. We conclude that healthy older adults show evidence of mitochondrial impairment and muscle weakness, but that this can be partially reversed at the phenotypic level, and substantially reversed at the transcriptome level, following six months of resistance exercise training.

Oxidative stress and the mitochondrial theory of aging in human skeletal muscle

According to the mitochondrial theory of aging, an age-related increase in oxidative stress is responsible for cellular damage and ultimately cell death. Despite compelling evidence that supports the mitochondrial theory of aging in some tissues, data regarding aging skeletal muscle are inconsistent. We collected resting muscle biopsies from the vastus lateralis, and 24 h urine samples from, young ( N=12, ∼22 yr), and older ( N=12, ∼72 yr) men. Urinary 8-OHdG was significantly higher in older as compared to younger men (Old: 7714±1402, Young: 5333±1191 ng g −1 creatinine: p=0.005), as were levels of protein carbonyls (Old: 0.72±0.42, Young: 0.26±0.14 nmol mg −1 protein: p=0.007). MnSOD activity (Old: 7.1±0.8, Young: 5.2±1.8 U mg −1 protein: p=0.04) and catalase activity (Old: 8.5±2.0, Young: 6.2±2.4 μmol min −1 mg −1 protein: p=0.02) were significantly higher in old as compared to young men, respectively, with no differences observed for total or CuZnSOD. Full-length mtDNA appeared lower in old as compared to young men, and mtDNA deletions were present in 6/8 old and 0/6 young men ( p=0.003). The maximal activities of citrate synthase, and complex II+III, and IV were not different between young and old men, however, complex I+III activity was marginally higher in older as compared to younger men (Old: 2.5±0.5, Young: 1.9±0.5 μmol min −1 g −1 w.w: p=0.03) respectively. In conclusion, healthy aging is associated with oxidative damage to proteins and DNA, a compensatory up-regulation of antioxidant enzymes, and aberrations of mtDNA, with no reduction in electron transport chain maximal enzyme activity.

Antioxidant pathways in human aged skeletal muscle: relationship with the distribution of type II fibers

Type II fiber loss and reactive oxygen species (ROS)-induced damage are hallmarks of muscle aging. The aim of this study was to analyze whether there exists a relationship between age-dependent changes in cellular antioxidant capacity and type II fiber loss in aged human skeletal muscles. Forty-five male and female subjects ranging in age from 65 to 90 year-old were divided into +40 and −40% type II fiber groups. We measured both total and Mn superoxide dismutase (total and MnSOD), glutathione peroxidase (GSHPx) and catalase (CAT) activities. We also measured the reduced and oxidized forms of glutathione (GSH and GSSG) and lipid peroxide (LPO) levels. Total SOD activity was lower in the −40% type II fiber group than in the +40% group; MnSOD tended to be lower but data are not statistically consistent. Both GSHPx and CAT activities remained unchanged; as did GSH, GSSG and GSH/GSSG ratio. Finally, muscle samples with −40% type II fibers had a significantly higher LPO content compared to those with +40% type II fibers. In summary, a relationship between human skeletal muscle aging, type II fiber loss and ROS reactions seems to exist.

Age and sex differences in human skeletal muscle: Role of reactive oxygen species

Previous studies, conducted on experimental animals, have indicated that reactive oxygen species (ROS) are involved in the aging process. The objective of this work was to study the relationship between oxidative damage and human skeletal muscle aging, measuring the activity of the main antioxidant enzymes superoxide dismutase (total and MnSOD), glutathione peroxidase (GPx) and catalase in the skeletal muscle of men and women in the age groups: young (17–40 years), adult (41–65 years) and aged (66–91 years). We also measured glutathione and glutathione disulfide (GSH and GSSG) levels and the redox index; lipid peroxidation and protein carbonyl content. Total SOD activity was lower in the 66–91 year-old vs. the 17–40 year-old men; MnSOD activity was significantly greater in 66–91 year-old vs. 17–40 year-old women. GPx activity remained unchanged. The activity of catalase was lower in adults than in young men but higher in the aged. We observed no changes in GSH levels and significantly higher GSSG levels only in aged men vs. adult men, and a significant decrease in aged women vs. aged men. The protein carbonyl content increased significantly in the 41–65 and 66–91 year-old vs. the 17–40 year-old men. Finally, young women have lower lipid peroxidation levels than young men. Significantly higher lipid peroxidation levels were observed in aged men vs. both young and adult men, and the same trend was noticed for women. We conclude that oxidative damage may play a crucial role in the decline of functional activity in human skeletal muscle with normal aging in both sexes; and that men appear to be more subject to oxidative stress than women.

Age-dependent changes of antioxidant activities and markers of free radical damage in human skeletal muscle

This study was conducted in order to provide evidence for the role of reactive oxygen species (ROS) in human skeletal muscle aging. We used human muscle samples obtained from hospitalized patients in an open study with matched pairs of individuals of different ages. The subjects, ranging in age from 17 to 91 years, were grouped as follows: 17–25-, 26–35-, 36–45-, 46–55-, 56–65-, 66–75-, 76–85-, and 86–91-year-old groups. To investigate the relationship between muscle aging and oxidative damage we measured total and Mn-dependent superoxide dismutase (total SOD, MnSOD), glutathione peroxidase (GSHPx), and catalase (CAT) activities; total reduced and oxidized glutathione (GSHtot, GSH, and GSSG) levels; lipid peroxidation (LPO), and protein carbonyl content (PrC). Total SOD activity decreases significantly with age in the 66–75-year-old group, although MnSOD activity increases significantly in the 76–85-year-old group. The activity of the two H 2O 2 detoxifying enzymes (GSHPx and CAT) did not change with age, as do GSHtot and GSH levels. GSSG levels increased significantly (76–85- and 86–91-year-old groups) with age. We observed a significant increase in LPO levels (66–75- and 76–85-year-old groups), although the PrC content shows a trend of increase without gaining the statistical significance. These results support the idea that ROS play an important role in the human muscle aging process.

Absence of Relationship between the Level of Electron Transport Chain Activities and Aging in Human Skeletal Muscle

We investigated the relationship between age and respiratory chain function of skeletal muscle mitochondria in 132 control individuals (15 to 95 years old). Muscle mitochondria were studied polarographically and spectrophotometrically. By regression analysis, we found a significant inverse correlation between age and oxygen uptake linked to substrate oxidation (succinate, glutamate-malate or ascorbate-TMPD). However, this significance disappeared after including physical activity and tobacco consumption as confounding variables in a multivariate statistical analysis. Similarly, the activity of respiratory chain complexes individually measured did not decline with age. It therefore appears that respiratory chain activity in human skeletal muscle mitochondria is substantially undamaged during the aging process.