Single muscle fibre contractile characteristics with lifelong endurance exercise

Aging Skeletal Muscles: What Are the Mechanisms of Age-Related Loss of Strength and Muscle Mass, and Can We Impede Its Development and Progression?

As we age, we lose muscle strength and power, a condition commonly referred to as sarcopenia (ICD-10-CM code (M62.84)). The prevalence of sarcopenia is about 5-10% of the elderly population, resulting in varying degrees of disability. In this review we emphasise that sarcopenia does not occur suddenly. It is an aging-induced deterioration that occurs over time and is only recognised as a disease when it manifests clinically in the 6th-7th decade of life. Evidence from animal studies, elite athletes and longitudinal population studies all confirms that the underlying process has been ongoing for decades once sarcopenia has manifested. We present hypotheses about the mechanism(s) underlying this process and their supporting evidence. We briefly review various proposals to impede sarcopenia, including cell therapy, reducing senescent cells and their secretome, utilising targets revealed by the skeletal muscle secretome, and muscle innervation. We conclude that although there are potential candidates and ongoing preclinical and clinical trials with drug treatments, the only evidence-based intervention today for humans is exercise. We present different exercise programmes and discuss to what extent the interindividual susceptibility to developing sarcopenia is due to our genetic predisposition or lifestyle factors.

Fast and slow myofiber nuclei, satellite cells, and size distribution with lifelong endurance exercise in men and women

We previously observed lifelong endurance exercise (LLE) influenced quadriceps whole-muscle and myofiber size in a fiber-type and sex-specific manner. The current follow-up exploratory investigation examined myofiber size regulators and myofiber size distribution in vastus lateralis biopsies from these same LLE men (n = 21, 74 ± 1 years) and women (n = 7, 72 ± 2 years) as well as old, healthy nonexercisers (OH; men: n = 10, 75 ± 1 years; women: n = 10, 75 ± 1 years) and young exercisers (YE; men: n = 10, 25 ± 1 years; women: n = 10, 25 ± 1 years). LLE exercised ~5 days/week, ~7 h/week for the previous 52 ± 1 years. Slow (myosin heavy chain (MHC) I) and fast (MHC IIa) myofiber nuclei/fiber, myonuclear domain, satellite cells/fiber, and satellite cell density were not influenced (p > 0.05) by LLE in men and women. The aging groups had ~50%-60% higher proportion of large (>7000 μm2) and small (<3000 μm2) myofibers (OH; men: 44%, women: 48%, LLE; men: 42%, women: 42%, YE; men: 27%, women: 29%). LLE men had triple the proportion of large slow fibers (LLE: 21%, YE: 7%, OH: 7%), while LLE women had more small slow fibers (LLE: 15%, YE: 8%, OH: 9%). LLE reduced by ~50% the proportion of small fast (MHC II containing) fibers in the aging men (OH: 14%, LLE: 7%) and women (OH: 35%, LLE: 18%). These data, coupled with previous findings, suggest that myonuclei and satellite cell content are uninfluenced by lifelong endurance exercise in men ~60-90 years, and this now also extends to septuagenarian lifelong endurance exercise women. Additionally, lifelong endurance exercise appears to influence the relative abundance of small and large myofibers (fast and slow) differently between men and women.

Muscle fibre size and myonuclear positioning in trained and aged humans

Changes in myonuclear architecture and positioning are associated with exercise adaptations and ageing. However, data on the positioning and number of myonuclei following exercise are inconsistent. Additionally, whether myonuclear domains (MNDs; i.e., the theoretical volume of cytoplasm within which a myonucleus is responsible for transcribing DNA) and myonuclear positioning are altered with age remains unclear. The aim of this investigation was to investigate relationships between age and activity status and myonuclear domains and positioning. Vastus lateralis muscle biopsies from younger endurance-trained (YT) and older endurance-trained (OT) individuals were compared with age-matched untrained counterparts (YU and OU; OU samples were acquired during surgical operation). Serial, optical z-slices were acquired throughout isolated muscle fibres and analysed to give three-dimensional coordinates for myonuclei and muscle fibre dimensions. The mean cross-sectional area (CSA) of muscle fibres from OU individuals was 33%-53% smaller compared with the other groups. The number of nuclei relative to fibre CSA was 90% greater in OU compared with YU muscle fibres. Additionally, scaling of MND volume with fibre size was altered in older untrained individuals. The myonuclear arrangement, in contrast, was similar across groups. Fibre CSA and most myonuclear parameters were significantly associated with age in untrained individuals, but not in trained individuals. These data indicate that regular endurance exercise throughout the lifespan might better preserve the size of muscle fibres in older age and maintain the relationship between fibre size and MND volumes. Inactivity, however, might result in reduced muscle fibre size and altered myonuclear parameters.

Effects of aging and lifelong aerobic exercise on expression of innate immune components in skeletal muscle of women

This study examined the effects of aging and lifelong aerobic exercise on innate immune system components in the skeletal muscle of healthy women in the basal state and after an unaccustomed resistance exercise (RE) challenge. We also made exploratory between-sex comparisons with our previous report on men. Three groups of women were studied: young exercisers (YE, n = 10, 25 ± 1 yr, V̇o2max: 44 ± 2 mL/kg/min), lifelong aerobic exercisers with a 48 ± 2 yr training history (LLE, n = 7, 72 ± 2 yr, V̇o2max: 26 ± 2 mL/kg/min), and old healthy nonexercisers (OH, n = 10, 75 ± 1 yr, V̇o2max: 18 ± 1 mL/kg/min). Ten Toll-like receptors (TLRs)1-10, TLR adaptors (Myd88, TRIF), and NF-κB pathway components (IκBα, IKKβ) were assessed at the mRNA level in vastus lateralis biopsies before and 4 h after RE [3×10 repetitions, 70% 1-repetition maximum (1RM)]. Basal TLR1-10 expression was minimally influenced by age or LLE in women (TLR9 only; OH > YE, +43%, P < 0.05; OH > LLE, +30%, P < 0.10) and was on average 24% higher in women versus men. Similarly, basal adaptor expression was not influenced (P > 0.05) by age or LLE in women but was on average 26% higher (myeloid differentiation primary response 88, Myd88) and 23% lower [Toll interleukin (IL)-1 receptor-containing adaptor-inducing interferon-γ, TRIF] in women versus men. RE-induced changes in women, independent of the group, in TLR3, TLR4, TLR6 (∼2.1-fold, P < 0.05), Myd88 (∼1.2-fold, P < 0.10), and IκBα (∼0.3-fold, P < 0.05). Although there were some similar RE responses in men (TLR4: 2.1-fold, Myd88: 1.2-fold, IκBα: 0.4-fold), several components responded only in men to RE (TLR1, TLR8, TRIF, and IKKβ). Our findings support the sexual dimorphism of immunity, with women having greater basal skeletal muscle TLR expression and differential response to unaccustomed exercise than men.NEW & NOTEWORTHY We recently reported that aging increases basal expression of many Toll-like receptors (TLRs) in men and lifelong aerobic exercise does not prevent this effect. In addition, a resistance exercise (RE) challenge increased the expression of many TLRs. Here we show that basal TLR expression is minimally influenced by aging in women and findings support the sexual dimorphism of immunity, with women having greater basal skeletal muscle TLR expression and a differential response to unaccustomed exercise than men.

Fast and slow muscle fiber transcriptome dynamics with lifelong endurance exercise

We investigated fast and slow muscle fiber transcriptome exercise dynamics among three groups of men: lifelong exercisers (LLE, n = 8, 74 ± 1 yr), old healthy nonexercisers (OH, n = 9, 75 ± 1 yr), and young exercisers (YE, n = 8, 25 ± 1 yr). On average, LLE had exercised ∼4 day·wk-1 for ∼8 h·wk-1 over 53 ± 2 years. Muscle biopsies were obtained pre- and 4 h postresistance exercise (3 × 10 knee extensions at 70% 1-RM). Fast and slow fiber size and function were assessed preexercise with fast and slow RNA-seq profiles examined pre- and postexercise. LLE fast fiber size was similar to OH, which was ∼30% smaller than YE (P < 0.05) with contractile function variables among groups, resulting in lower power in LLE (P < 0.05). LLE slow fibers were ∼30% larger and more powerful compared with YE and OH (P < 0.05). At the transcriptome level, fast fibers were more responsive to resistance exercise compared with slow fibers among all three cohorts (P < 0.05). Exercise induced a comprehensive biological response in fast fibers (P < 0.05) including transcription, signaling, skeletal muscle cell differentiation, and metabolism with vast differences among the groups. Fast fibers from YE exhibited a growth and metabolic signature, with LLE being primarily metabolic, and OH showing a strong stress-related response. In slow fibers, only LLE exhibited a biological response to exercise (P < 0.05), which was related to ketone and lipid metabolism. The divergent exercise transcriptome signatures provide novel insight into the molecular regulation in fast and slow fibers with age and exercise and suggest that the ∼5% weekly exercise time commitment of the lifelong exercisers provided a powerful investment for fast and slow muscle fiber metabolic health at the molecular level.NEW & NOTEWORTHY This study provides the first insights into fast and slow muscle fiber transcriptome dynamics with lifelong endurance exercise. The fast fibers were more responsive to exercise with divergent transcriptome signatures among young exercisers (growth and metabolic), lifelong exercisers (metabolic), and old healthy nonexercisers (stress). Only lifelong exercisers had a biological response in slow fibers (metabolic). These data provide novel insights into fast and slow muscle fiber health at the molecular level with age and exercise.

Bmi-1 Overexpression Improves Sarcopenia Induced by 1,25(OH)2 D3 Deficiency and Downregulates GATA4-Dependent Rela Transcription

Sarcopenia increases with age, and an underlying mechanism needs to be determined to help with designing more effective treatments. This study aimed to determine whether 1,25(OH)₂ D₃ deficiency could cause cellular senescence and a senescence-associated secretory phenotype (SASP) in skeletal muscle cells to induce sarcopenia, whether GATA4 could be upregulated by 1,25(OH)₂ D₃ deficiency to promote SASP, and whether Bmi-1 reduces the expression of GATA4 and GATA4-dependent SASP induced by 1,25(OH)₂ D₃ deficiency in skeletal muscle cells. Bioinformatics analyses with RNA sequencing data in skeletal muscle from physiologically aged and young mice were conducted. Skeletal muscles from 2-month-old young and 2-year-old physiologically aged wild-type (WT) mice and 8-week-old WT, Bmi-1 mesenchymal transgene (Bmi-1^Tg ), Cyp27b1 homozygous (Cyp27b1^-/- ), and Bmi-1^Tg Cyp27b1^-/- mice were observed for grip strength, cell senescence, DNA damage, and NF-κB-mediated SASP signaling of skeletal muscle. We found that muscle-derived Bmi-1 and vitamin D receptor (VDR) decreased with physiological aging, and DNA damage and GATA4-dependent SASP activation led to sarcopenia. Furthermore, 1,25(OH)₂ D₃ deficiency promoted DNA damage-induced GATA4 accumulation in muscles. GATA4 upregulated Rela at the region from -1448 to -1412 bp at the transcriptional level to cause NF-κB-dependent SASP for aggravating cell senescence and muscular dysfunction and sarcopenia. Bmi-1 overexpression promoted the ubiquitination and degradation of GATA4 by binding RING1B, which prevented cell senescence, SASP, and dysfunctional muscle, and improved sarcopenia induced by 1,25(OH)₂ D₃ deficiency. Thus, Bmi-1 overexpression improves sarcopenia induced by 1,25(OH)₂ D₃ deficiency, downregulates GATA4-dependent Rela transcription, and sequentially inhibits GATA4-dependent SASP in muscle cells. Therefore, Bmi-1 overexpression could be used for translational gene therapy for the ubiquitination of GATA4 and prevention of sarcopenia. © 2023 American Society for Bone and Mineral Research (ASBMR).

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Single muscle fibre contractile function with ageing

Ageing is accompanied by decrements in the size and function of skeletal muscle that compromise independence and quality of life in older adults. Developing therapeutic strategies to ameliorate these changes is critical but requires an in-depth mechanistic understanding of the underlying physiology. Over the past 25 years, studies on the contractile mechanics of isolated human muscle fibres have been instrumental in facilitating our understanding of the cellular mechanisms contributing to age-related skeletal muscle dysfunction. The purpose of this review is to characterize the changes that occur in single muscle fibre size and contractile function with ageing and identify key areas for future research. Surprisingly, most studies observe that the size and contractile function of fibres expressing slow myosin heavy chain (MHC) I are well-preserved with ageing. In contrast, there are profound age-related decrements in the size and contractile function of the fibres expressing the MHC II isoforms. Notably, lifelong aerobic exercise training is unable to prevent most of the decrements in fast fibre contractile function, which have been implicated as a primary mechanism for the age-related loss in whole-muscle power output. These findings reveal a critical need to investigate the effectiveness of other nutritional, pharmaceutical or exercise strategies, such as lifelong resistance training, to preserve fast fibre size and function with ageing. Moreover, integrating single fibre contractile mechanics with the molecular profile and other parameters important to contractile function (e.g. phosphorylation of regulatory proteins, innervation status, mitochondrial function, fibre economy) is necessary to comprehensively understand the ageing skeletal muscle phenotype.

The Aging Athlete: Paradigm of Healthy Aging

The Exercise Boom of the 1970's resulted in the adoption of habitual exercise in a significant portion of the population. Many of these individuals are defying the cultural norms by remaining physically active and competing at a high level in their later years. The juxtaposition between masters athletes and non-exercisers demonstrate the importance of remaining physically active throughout the lifespan on physiological systems related to healthspan (years of healthy living). This includes ~50% improved maximal aerobic capacity (VO₂max) and enhanced skeletal muscle health (size, function, as well as metabolic and communicative properties) compared to non-exercisers at a similar age. By taking a reductionist approach to VO₂max and skeletal muscle health, we can gain insight into how aging and habitual exercise affects the aging process. Collectively, this review provides a physiological basis for the elite performances seen in masters athletes, as well as the health implications of lifelong exercise with a focus on VO₂max, skeletal muscle metabolic fitness, whole muscle size and function, single muscle fiber physiology, and communicative properties of skeletal muscle. This review has significant public health implications due to the potent health benefits of habitual exercise across the lifespan.

Transcriptome features of striated muscle aging and predictability of protein level changes

We performed total RNA sequencing and multi-omics analysis comparing skeletal muscle and cardiac muscle in young adult (4 months) vs. early aging (20 months) mice to examine the molecular mechanisms of striated muscle aging. We observed that aging cardiac and skeletal muscles both invoke transcriptomic changes in innate immune system and mitochondria pathways but diverge in extracellular matrix processes. On an individual gene level, we identified 611 age-associated signatures in skeletal and cardiac muscles, including a number of myokine and cardiokine encoding genes. Because RNA and protein levels correlate only partially, we reason that differentially expressed transcripts that accurately reflect their protein counterparts will be more valuable proxies for proteomic changes and by extension physiological states. We applied a computational data analysis workflow to estimate which transcriptomic changes are more likely relevant to protein-level regulation using large proteogenomics data sets. We estimate about 48% of the aging-associated transcripts predict protein levels well (r ≥ 0.5). In parallel, a comparison of the identified aging-regulated genes with public human transcriptomics data showed that only 35-45% of the identified genes show an age-dependent expression in corresponding human tissues. Thus, integrating both RNA-protein correlation and human conservation across data sources, we nominate 134 prioritized aging striated muscle signatures that are predicted to correlate strongly with protein levels and that show age-dependent expression in humans. The results here reveal new details into how aging reshapes gene expression in striated muscles at the transcript and protein levels.