High mTORC1 signaling is maintained, while protein degradation pathways are perturbed in old murine skeletal muscles in the fasted state

The association between body roundness index and sarcopenia in older adults: a population-based study

Background

Sarcopenia, defined by the gradual decline in skeletal muscle mass and functionality, is a common disorder in the aging population and is linked to an elevated risk of falls and osteoporotic fractures. The contemporary diagnosis of sarcopenia depends on intricate and expensive techniques, such as computed tomography (CT) scans or dual-energy X-ray absorptiometry (DXA), which hinder the timely prevention of sarcopenia.

Objective

This study seeks to explore the association between the Body Roundness Index (BRI) and sarcopenia in the older adult cohort, utilizing data from the National Health and Nutrition Examination Survey (NHANES) in the United States.

Methods

Our study adopted a cross-sectional design, encompassing 9,411 older individuals, of which 1,147 were diagnosed with sarcopenia. After weighting, the number of individuals with sarcopenia was 23,985,011. The study employed multivariate logistic regression analysis to evaluate the association between BRI and sarcopenia, incorporating stepwise adjustments for potential confounders.

Results

The outcomes of the multivariate logistic regression analysis revealed that, in contrast to individuals without sarcopenia, those with sarcopenia exhibited significantly higher mean BRI values and a greater prevalence of comorbid conditions, including hypertension and diabetes. A significant positive correlation was observed between BRI and the likelihood of developing sarcopenia. Specifically, after controlling for all covariates, each one-unit increase in BRI was linked to a 64% elevation in the risk of sarcopenia (OR = 1.64, 95% CI = 1.58-1.71). Furthermore, the receiver operating characteristic (ROC) curve analysis indicated that BRI is a robust predictor for diagnosing sarcopenia, with an AUC of 0.744.

Conclusion

These findings suggest that, within the U.S. older adult population, an elevated BRI is associated with a heightened risk of sarcopenia. BRI can function as a practical and cost-effective anthropometric index for more precise prediction of sarcopenia risk in older adults.

Mikhail 'Misha' Blagosklonny's enduring legacy in geroscience: the hyperfunction theory and the therapeutic potential of rapamycin

The untimely passing of Dr. Mikhail "Misha" Blagosklonny has left a lasting void in geroscience and oncology. This review examines his profound contributions, focusing on his pioneering the Hyperfunction Theory and his advocacy for rapamycin, an mTOR inhibitor, as a therapeutic agent for lifespan extension. Contrary to traditional damage-centric models, the Hyperfunction Theory rejects damage accumulation as the primary driver of aging. Instead, it redefines aging as a quasi-programmed process driven by the persistent, excessive activity of growth-promoting pathways beyond their developmental roles, leading to age-related pathologies. We explore how Blagosklonny's insights predict rapamycin's ability to decelerate aging by modulating excessive mTOR signaling, supported by empirical evidence across multiple physiological systems, including immune, cardiovascular, cognitive, and oncologic health. His forward-thinking approach, advocating for the cautious clinical use of rapamycin and suggesting personalized, preventive, and combination therapy strategies, has catalyzed interest in translational geroscience. This review synthesizes Blagosklonny's legacy, presenting rapamycin as a foundational pharmacological intervention with potential in managing age-related decline and extending healthspan, and underlines his impact in shifting aging research from theoretical frameworks to actionable interventions. Blagosklonny's work remains an enduring inspiration, paving the way toward treating aging as a modifiable condition.

Muscle aging and sarcopenia: The pathology, etiology, and most promising therapeutic targets

Sarcopenia is a progressive muscle wasting disorder that severely impacts the quality of life of elderly individuals. Although the natural aging process primarily causes sarcopenia, it can develop in response to other conditions. Because muscle function is influenced by numerous changes that occur with age, the etiology of sarcopenia remains unclear. However, recent characterizations of the aging muscle transcriptional landscape, signaling pathway disruptions, fiber and extracellular matrix compositions, systemic metabolomic and inflammatory responses, mitochondrial function, and neurological inputs offer insights and hope for future treatments. This review will discuss age-related changes in healthy muscle and our current understanding of how this can deteriorate into sarcopenia. As our elderly population continues to grow, we must understand sarcopenia and find treatments that allow individuals to maintain independence and dignity throughout an extended lifespan.

Sex difference, proteostasis and mitochondrial function impact stroke-related sarcopenia-A systematic review and meta-analysis

BACKGROUND

The prevalence of stroke-related sarcopenia has been noted; however, epidemiological data and interventions that increase or reduce the incidence of stroke-related sarcopenia remain lacking.

METHODS

Studies on stroke-related sarcopenia were included in association or interventional analyses. All analyses were performed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Two evaluators independently extracted the data.

RESULTS

Female stroke patients had a higher preference for sarcopenia than male patients (pooled odds ratio [OR] = 0.670, 95 % CI 0.533-0.842, p = 0.001). Although stroke patients without drug use have improved skeletal muscle mass index (SMI) (MD = 0.272, 95 % CI 0.087-0.457, p = 0.004), handgrip strength (HGS) was not significantly altered (MD = -0.068, 95 % CI -0.221-0.076, p = 0.354). Stroke patients with nutrient interventions have improved SMI (MD = -0.354, 95 % CI -0.635- -0.073, p = 0.014) and HGS (MD = -0.394, 95 % CI -0.678- -0.111, p = 0.006); the synergistic effect of rehabilitation exercise has not been ruled out. Whether a sex difference exists in these interventions remains to be investigated. The underlying pathological mechanisms and potential therapeutic strategies for this disease are discussed.

CONCLUSION

Sex difference, proteostasis, and mitochondrial function may impact the incidence of stroke-related sarcopenia. Understanding the underlying pathological mechanisms and potential therapeutic targets for this disease will provide new insights into disease treatment, prevention, and drug development.

Lost in translation: challenges of current pharmacotherapy for sarcopenia

A healthy lifespan relies on independent living, in which active skeletal muscle is a critical element. The cost of not recognizing and acting earlier on unhealthy or aging muscle could be detrimental, since muscular weakness is inversely associated with all-cause mortality. Sarcopenia is characterized by a decline in skeletal muscle mass and strength and is associated with aging. Exercise is the only effective therapy to delay sarcopenia development and improve muscle health in older adults. Although numerous interventions have been proposed to reduce sarcopenia, none has yet succeeded in clinical trials. This review evaluates the biological gap between recent clinical trials targeting sarcopenia and the preclinical studies on which they are based, and suggests an alternative approach to bridge the discrepancy.

The immunology of sickness metabolism

Everyone knows that an infection can make you feel sick. Although we perceive infection-induced changes in metabolism as a pathology, they are a part of a carefully regulated process that depends on tissue-specific interactions between the immune system and organs involved in the regulation of systemic homeostasis. Immune-mediated changes in homeostatic parameters lead to altered production and uptake of nutrients in circulation, which modifies the metabolic rate of key organs. This is what we experience as being sick. The purpose of sickness metabolism is to generate a metabolic environment in which the body is optimally able to fight infection while denying vital nutrients for the replication of pathogens. Sickness metabolism depends on tissue-specific immune cells, which mediate responses tailored to the nature and magnitude of the threat. As an infection increases in severity, so do the number and type of immune cells involved and the level to which organs are affected, which dictates the degree to which we feel sick. Interestingly, many alterations associated with metabolic disease appear to overlap with immune-mediated changes observed following infection. Targeting processes involving tissue-specific interactions between activated immune cells and metabolic organs therefore holds great potential for treating both people with severe infection and those with metabolic disease. In this review, we will discuss how the immune system communicates in situ with organs involved in the regulation of homeostasis and how this communication is impacted by infection.

Biological basis and treatment of frailty and sarcopenia

In an ageing society, the importance of maintaining healthy life expectancy has been emphasized. As a result of age-related decline in functional reserve, frailty is a state of increased vulnerability and susceptibility to adverse health outcomes with a serious impact on healthy life expectancy. The decline in skeletal muscle mass and function, also known as sarcopenia, is key in the development of physical frailty. Both frailty and sarcopenia are highly prevalent in patients not only with advanced age but also in patients with illnesses that exacerbate their progression like heart failure (HF), cancer, or dementia, with the prevalence of frailty and sarcopenia in HF patients reaching up to 50-75% and 19.5-47.3%, respectively, resulting in 1.5-3 times higher 1-year mortality. The biological mechanisms of frailty and sarcopenia are multifactorial, complex, and not yet fully elucidated, ranging from DNA damage, proteostasis impairment, and epigenetic changes to mitochondrial dysfunction, cellular senescence, and environmental factors, many of which are further linked to cardiac disease. Currently, there is no gold standard for the treatment of frailty and sarcopenia, however, growing evidence supports that a combination of exercise training and nutritional supplement improves skeletal muscle function and frailty, with a variety of other therapies being devised based on the underlying pathophysiology. In this review, we address the involvement of frailty and sarcopenia in cardiac disease and describe the latest insights into their biological mechanisms as well as the potential for intervention through exercise, diet, and specific therapies.

Lysosomes as coordinators of cellular catabolism, metabolic signalling and organ physiology

Every cell must satisfy basic requirements for nutrient sensing, utilization and recycling through macromolecular breakdown to coordinate programmes for growth, repair and stress adaptation. The lysosome orchestrates these key functions through the synchronised interplay between hydrolytic enzymes, nutrient transporters and signalling factors, which together enable metabolic coordination with other organelles and regulation of specific gene expression programmes. In this Review, we discuss recent findings on lysosome-dependent signalling pathways, focusing on how the lysosome senses nutrient availability through its physical and functional association with mechanistic target of rapamycin complex 1 (mTORC1) and how, in response, the microphthalmia/transcription factor E (MiT/TFE) transcription factors exert feedback regulation on lysosome biogenesis. We also highlight the emerging interactions of lysosomes with other organelles, which contribute to cellular homeostasis. Lastly, we discuss how lysosome dysfunction contributes to diverse disease pathologies and how inherited mutations that compromise lysosomal hydrolysis, transport or signalling components lead to multi-organ disorders with severe metabolic and neurological impact. A deeper comprehension of lysosomal composition and function, at both the cellular and organismal level, may uncover fundamental insights into human physiology and disease.

Geroprotector drugs and exercise: friends or foes on healthy longevity?

Physical activity and several pharmacological approaches individually combat age-associated conditions and extend healthy longevity in model systems. It is tantalizing to extrapolate that combining geroprotector drugs with exercise could extend healthy longevity beyond any individual treatment. However, the current dogma suggests that taking leading geroprotector drugs on the same day as exercise may limit several health benefits. Here, we review leading candidate geroprotector drugs and their interactions with exercise and highlight salient gaps in knowledge that need to be addressed to identify if geroprotector drugs can have a harmonious relationship with exercise.

Hallmarks of ageing in human skeletal muscle and implications for understanding the pathophysiology of sarcopenia in women and men

Ageing is a complex biological process associated with increased morbidity and mortality. Nine classic, interdependent hallmarks of ageing have been proposed involving genetic and biochemical pathways that collectively influence ageing trajectories and susceptibility to pathology in humans. Ageing skeletal muscle undergoes profound morphological and physiological changes associated with loss of strength, mass, and function, a condition known as sarcopenia. The aetiology of sarcopenia is complex and whilst research in this area is growing rapidly, there is a relative paucity of human studies, particularly in older women. Here, we evaluate how the nine classic hallmarks of ageing: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication contribute to skeletal muscle ageing and the pathophysiology of sarcopenia. We also highlight five novel hallmarks of particular significance to skeletal muscle ageing: inflammation, neural dysfunction, extracellular matrix dysfunction, reduced vascular perfusion, and ionic dyshomeostasis, and discuss how the classic and novel hallmarks are interconnected. Their clinical relevance and translational potential are also considered.

The proteasome regulates body weight and systemic nutrient metabolism during fasting

The ubiquitin-proteasome system (UPS) and the autophagy-lysosome pathway are the primary means of degradation in mammalian tissues. We sought to determine the individual contribution of the UPS and autophagy to tissue catabolism during fasting. Mice were overnight fasted for 15 h before regaining food access ("Fed" group, n = 6) or continuing to fast ("Fast" group, n = 7) for 3 h. In addition, to investigate the effects of autophagy on systemic metabolism and tissue degradation, one group of mice was fasted for 18 h and treated with chloroquine ("Fast + CLQ" group, n = 7) and a fourth group of mice was treated with bortezomib ("Fast + Bort" group, n = 7) to assess the contribution of the UPS. Body weight, tissue weight, circulating hormones and metabolites, intracellular signaling pathways, and protein synthesis were investigated. Fasting induced the loss of body weight, liver mass, and white adipose tissue in the Fast and the Fast + CLQ group, whereas the Fast + Bort group maintained tissue and body weight. Fasting reduced glucose and increased β hydroxybutyrate in the circulation of all mice. Both changes were most profound in the Fast + Bort group compared with the other fasting conditions. Molecular signaling indicated a successful inhibition of hepatic UPS with bortezomib and an upregulation of the PI3K/AKT/mTOR pathway. The latter was further supported by an increase in hepatic protein synthesis with bortezomib. Inhibition of the UPS through bortezomib blocks body weight loss and tissue catabolism during an acute overnight fast in mice. The effects were likely mediated through a combined effect of the drug on biomolecule degradation and synthesis.NEW & NOTEWORTHY Bortezomib treatment prevents tissue and body weight loss during fasting. The loss of proteasome activity with bortezomib exacerbates fasting-induced ketogenesis. During fasting, bortezomib increases AMPK and PI3K/AKT signaling in the liver, which promotes protein synthesis.

Blazing a trail for the clinical use of rapamycin as a geroprotecTOR

Treatment with rapamycin, an inhibitor of the mechanistic Target Of Rapamycin Complex One (mTORC1) protein kinase, has been repeatedly demonstrated to extend lifespan and prevent or delay age-related diseases in diverse model systems. Concerns over the risk of potentially serious side effects in humans, including immunosuppression and metabolic disruptions, have cautiously limited the translation of rapamycin and its analogs as a treatment for aging associated conditions. During the last decade, we and others have developed a working model that suggests that while inhibition of mTORC1 promotes healthy aging, many of the negative side effects of rapamycin are associated with "off-target" inhibition of a second mTOR complex, mTORC2. Differences in the kinetics and molecular mechanisms by which rapamycin inhibits mTORC1 and mTORC2 suggest that a therapeutic window for rapamycin could be exploited using intermittent dosing schedules or alternative rapalogs that may enable more selective inhibition of mTORC1. However, the optimal dosing schedules and the long-term efficacy of such interventions in humans are unknown. Here, we highlight ongoing or upcoming clinical trials that will address outstanding questions regarding the safety, pharmacokinetics, pharmacodynamics, and efficacy of rapamycin and rapalogs on several clinically oriented outcomes. Results from these early phase studies will help guide the design of phase 3 clinical trials to determine whether rapamycin can be used safely to inhibit mTORC1 for the treatment and prevention of age-related diseases in humans.

Targeting the biology of aging with mTOR inhibitors

Inhibition of the protein kinase mechanistic target of rapamycin (mTOR) with the Food and Drug Administration (FDA)-approved therapeutic rapamycin promotes health and longevity in diverse model organisms. More recently, specific inhibition of mTORC1 to treat aging-related conditions has become the goal of basic and translational scientists, clinicians and biotechnology companies. Here, we review the effects of rapamycin on the longevity and survival of both wild-type mice and mouse models of human diseases. We discuss recent clinical trials that have explored whether existing mTOR inhibitors can safely prevent, delay or treat multiple diseases of aging. Finally, we discuss how new molecules may provide routes to the safer and more selective inhibition of mTOR complex 1 (mTORC1) in the decade ahead. We conclude by discussing what work remains to be done and the questions that will need to be addressed to make mTOR inhibitors part of the standard of care for diseases of aging.

Age-Related Dysfunction in Proteostasis and Cellular Quality Control in the Development of Sarcopenia

Sarcopenia is a debilitating skeletal muscle disease that accelerates in the last decades of life and is characterized by marked deficits in muscle strength, mass, quality, and metabolic health. The multifactorial causes of sarcopenia have proven difficult to treat and involve a complex interplay between environmental factors and intrinsic age-associated changes. It is generally accepted that sarcopenia results in a progressive loss of skeletal muscle function that exceeds the loss of mass, indicating that while loss of muscle mass is important, loss of muscle quality is the primary defect with advanced age. Furthermore, preclinical models have suggested that aged skeletal muscle exhibits defects in cellular quality control such as the degradation of damaged mitochondria. Recent evidence suggests that a dysregulation of proteostasis, an important regulator of cellular quality control, is a significant contributor to the aging-associated declines in muscle quality, function, and mass. Although skeletal muscle mammalian target of rapamycin complex 1 (mTORC1) plays a critical role in cellular control, including skeletal muscle hypertrophy, paradoxically, sustained activation of mTORC1 recapitulates several characteristics of sarcopenia. Pharmaceutical inhibition of mTORC1 as well as caloric restriction significantly improves muscle quality in aged animals, however, the mechanisms controlling cellular proteostasis are not fully known. This information is important for developing effective therapeutic strategies that mitigate or prevent sarcopenia and associated disability. This review identifies recent and historical understanding of the molecular mechanisms of proteostasis driving age-associated muscle loss and suggests potential therapeutic interventions to slow or prevent sarcopenia.

Limb-girdle muscular dystrophy type 2B causes HDL-C abnormalities in patients and statin-resistant muscle wasting in dysferlin-deficient mice

Limb-girdle muscular dystrophy (MD) type 2B (LGMD2B) and Duchenne MD (DMD) are caused by mutations to the Dysferlin and Dystrophin genes, respectively. We have recently demonstrated in typically mild dysferlin- and dystrophin-deficient mouse models that increased plasma cholesterol levels severely exacerbate muscle wasting, and that DMD patients display primary dyslipidemia characterized by elevated plasma cholesterol and triglycerides. Herein, we investigate lipoprotein abnormalities in LGMD2B and if statin therapy protects dysferlin-deficient mice (Dysf) from muscle damage. Herein, lipoproteins and liver enzymes from LGMD2B patients and dysferlin-null (Dysf) mice were analyzed. Simvastatin, which exhibits anti-muscle wasting effects in mouse models of DMD and corrects aberrant expression of key markers of lipid metabolism and endogenous cholesterol synthesis, was tested in Dysf mice. Muscle damage and fibrosis were assessed by immunohistochemistry and cholesterol signalling pathways via Western blot. LGMD2B patients show reduced serum high-density lipoprotein cholesterol (HDL-C) levels compared to healthy controls and exhibit a greater prevalence of abnormal total cholesterol (CHOL)/HDL-C ratios despite an absence of liver dysfunction. While Dysf mice presented with reduced CHOL and associated HDL-C and LDL-C-associated fractions, simvastatin treatment did not prevent muscle wasting in quadriceps and triceps muscle groups or correct aberrant low-density lipoprotein receptor (LDLR) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) protein expression. LGMD2B patients present with reduced serum concentrations of HDL-C, a major metabolic comorbidity, and as a result, statin therapy is unlikely to prevent muscle wasting in this population. We propose that like DMD, LGMD2B should be considered as a new type of genetic dyslipidemia.

Mammalian Target of Rapamycin (mTOR) Signaling at the Crossroad of Muscle Fiber Fate in Sarcopenia

The mammalian target of rapamycin (mTOR) is a major regulator of skeletal myocyte viability. The signaling pathways triggered by mTOR vary according to the type of endogenous and exogenous factors (e.g., redox balance, nutrient availability, physical activity) as well as organismal age. Here, we provide an overview of mTOR signaling in skeletal muscle, with a special focus on the role played by mTOR in the development of sarcopenia. Intervention strategies targeting mTOR in sarcopenia (e.g., supplementation of plant extracts, hormones, inorganic ions, calorie restriction, and exercise) have also been discussed.

Distinct and additive effects of calorie restriction and rapamycin in aging skeletal muscle

Preserving skeletal muscle function is essential to maintain life quality at high age. Calorie restriction (CR) potently extends health and lifespan, but is largely unachievable in humans, making “CR mimetics” of great interest. CR targets nutrient-sensing pathways centering on mTORC1. The mTORC1 inhibitor, rapamycin, is considered a potential CR mimetic and is proven to counteract age-related muscle loss. Therefore, we tested whether rapamycin acts via similar mechanisms as CR to slow muscle aging. Here we show that long-term CR and rapamycin unexpectedly display distinct gene expression profiles in geriatric mouse skeletal muscle, despite both benefiting aging muscles. Furthermore, CR improves muscle integrity in mice with nutrient-insensitive, sustained muscle mTORC1 activity and rapamycin provides additive benefits to CR in naturally aging mouse muscles. We conclude that rapamycin and CR exert distinct, compounding effects in aging skeletal muscle, thus opening the possibility of parallel interventions to counteract muscle aging.

The anti-aging intervention calorie restriction (CR) is thought to act via the nutrient-sensing multiprotein complex mTORC1. Here the authors show that the mTORC1-inhibitor rapamycin and CR use largely distinct mechanisms to slow mouse muscle aging.

Role of autophagy in muscle disease

Beside inherited muscle diseases many catabolic conditions such as insulin resistance, malnutrition, cancer growth, aging, infections, chronic inflammatory status, inactivity, obesity are characterized by loss of muscle mass, strength and function. The decrease of muscle quality and quantity increases morbidity, mortality and has a major impact on the quality of life. One of the pathogenetic mechanisms of muscle wasting is the dysregulation of the main protein and organelles quality control system of the cell: the autophagy-lysosome. This review will focus on the role of the autophagy-lysosome system in the different conditions of muscle loss. We will also dissect the signalling pathways that are involved in excessive or defective autophagy regulation. Finally, the state of the art of autophagy modulators that have been used in preclinical or clinical studies to ameliorate muscle mass will be also described.

Analysis of proteostasis during aging with western blot of detergent-soluble and insoluble protein fractions

Defects in protein quality control are the underlying cause of age-related diseases. The western blot analysis of detergent-soluble and insoluble protein fractions has proven useful in identifying interventions that regulate proteostasis. Here, we describe the protocol for such analyses in Drosophila tissues, mouse skeletal muscle, human organoids, and HEK293 cells. We describe key adaptations of this protocol and provide key information that will help modify this protocol for future studies in other tissues and disease models. For complete details on the use and execution of this protocol, please refer to Rai et al. (2021) and Hunt el al. (2021).

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?

Manifestations of Age on Autophagy, Mitophagy and Lysosomes in Skeletal Muscle

Sarcopenia is the loss of both muscle mass and function with age. Although the molecular underpinnings of sarcopenia are not fully understood, numerous pathways are implicated, including autophagy, in which defective cargo is selectively identified and degraded at the lysosome. The specific tagging and degradation of mitochondria is termed mitophagy, a process important for the maintenance of an organelle pool that functions efficiently in energy production and with relatively low reactive oxygen species production. Emerging data, yet insufficient, have implicated various steps in this pathway as potential contributors to the aging muscle atrophy phenotype. Included in this is the lysosome, the end-stage organelle possessing a host of proteolytic and degradative enzymes, and a function devoted to the hydrolysis and breakdown of defective molecular complexes and organelles. This review provides a summary of our current understanding of how the autophagy-lysosome system is regulated in aging muscle, highlighting specific areas where knowledge gaps exist. Characterization of the autophagy pathway with a particular focus on the lysosome will undoubtedly pave the way for the development of novel therapeutic strategies to combat age-related muscle loss.

Molecular routes to sarcopenia and biomarker development: per aspera ad astra

Sarcopenia, the age-related decline in muscle mass and strength/function, is a prototypical geroscience condition. The dissection of muscle-specific molecular pathways through analyses of tissue biopsies has provided valuable insights into the pathophysiology of sarcopenia. However, such an approach is unsuitable for capturing the dynamic nature of the condition. Furthermore, the muscle sampling procedure may be perceived as burdensome especially by multimorbid, frail older adults. To overcome these limitations, sophisticated statistical methods have been devised for the simultaneous analysis of circulating factors related to the multiple domains of sarcopenia. This approach has shown potential for achieving a more comprehensive appraisal of the condition, unveiling new therapeutic targets, and identifying meaningful biomarkers. Here, we discuss the main pathogenetic pathways of sarcopenia, with a focus on mediators that are currently in the spotlight as biomarkers and potential treatment targets.

Cancer-Associated Muscle Wasting-Candidate Mechanisms and Molecular Pathways

Excessive muscle loss is commonly observed in cancer patients and its association with poor prognosis has been well-established. Cancer-associated sarcopenia differs from age-related wasting in that it is not responsive to nutritional intervention and exercise. This is related to its unique pathogenesis, a result of diverse and interconnected mechanisms including inflammation, disordered metabolism, proteolysis and autophagy. There is a growing body of evidence that suggests that the tumor is the driver of muscle wasting by its elaboration of mediators that influence each of these pro-sarcopenic pathways. In this review, evidence for these tumor-derived factors and putative mechanisms for inducing muscle wasting will be reviewed. Potential targets for future research and therapeutic interventions will also be reviewed.

The neuromuscular junction is a focal point of mTORC1 signaling in sarcopenia

With human median lifespan extending into the 80s in many developed countries, the societal burden of age-related muscle loss (sarcopenia) is increasing. mTORC1 promotes skeletal muscle hypertrophy, but also drives organismal aging. Here, we address the question of whether mTORC1 activation or suppression is beneficial for skeletal muscle aging. We demonstrate that chronic mTORC1 inhibition with rapamycin is overwhelmingly, but not entirely, positive for aging mouse skeletal muscle, while genetic, muscle fiber-specific activation of mTORC1 is sufficient to induce molecular signatures of sarcopenia. Through integration of comprehensive physiological and extensive gene expression profiling in young and old mice, and following genetic activation or pharmacological inhibition of mTORC1, we establish the phenotypically-backed, mTORC1-focused, multi-muscle gene expression atlas, SarcoAtlas (https://sarcoatlas.scicore.unibas.ch/), as a user-friendly gene discovery tool. We uncover inter-muscle divergence in the primary drivers of sarcopenia and identify the neuromuscular junction as a focal point of mTORC1-driven muscle aging.

The TOR Pathway at the Neuromuscular Junction: More Than a Metabolic Player?

The neuromuscular junction (NMJ) is the chemical synapse connecting motor neurons and skeletal muscle fibers. NMJs allow all voluntary movements, and ensure vital functions like breathing. Changes in the structure and function of NMJs are hallmarks of numerous pathological conditions that affect muscle function including sarcopenia, the age-related loss of muscle mass and function. However, the molecular mechanisms leading to the morphological and functional perturbations in the pre- and post-synaptic compartments of the NMJ remain poorly understood. Here, we discuss the role of the metabolic pathway associated to the kinase TOR (Target of Rapamycin) in the development, maintenance and alterations of the NMJ. This is of particular interest as the TOR pathway has been implicated in aging, but its role at the NMJ is still ill-defined. We highlight the respective functions of the two TOR-associated complexes, TORC1 and TORC2, and discuss the role of localized protein synthesis and autophagy regulation in motor neuron terminals and sub-synaptic regions of muscle fibers and their possible effects on NMJ maintenance.

β-Cryptoxanthin Improves p62 Accumulation and Muscle Atrophy in the Soleus Muscle of Senescence-Accelerated Mouse-Prone 1 Mice

We investigated the effects of β-cryptoxanthin on skeletal muscle atrophy in senescence-accelerated mouse-prone 1 (SAMP1) mice. For 15 weeks, SAMP1 mice were intragastrically administered vehicle or β-cryptoxanthin. At 35 weeks of age, the skeletal muscle mass in SAMP1 mice was reduced compared with that in control senescence-accelerated mouse-resistant 1 (SAMR1) mice. β-cryptoxanthin increased muscle mass with an increase in the size of muscle fibers in the soleus muscle of SAMP1 mice. The expressions of autophagy-related factors such as beclin-1, p62, LC3-I, and LC3-II were increased in the soleus muscle of SAMP1 mice; however, β-cryptoxanthin administration inhibited this increase. Unlike in SAMR1 mice, p62 was punctately distributed throughout the cytosol in the soleus muscle fibers of SAMP1 mice; however, β-cryptoxanthin inhibited this punctate distribution. The cross-sectional area of p62-positive fiber was smaller than that of p62-negative fiber, and the ratio of p62-positive fibers to p62-negative fibers was increased in SAMP1 mice. β-cryptoxanthin decreased this ratio in SAMP1 mice. Furthermore, β-cryptoxanthin decreased the autophagy-related factor expression in murine C2C12 myotube. The autophagy inhibitor bafilomycin A1, but not the proteasome inhibitor MG132, inhibited the β-cryptoxanthin-induced decrease in p62 and LC3-II expressions. These results indicate that β-cryptoxanthin inhibits the p62 accumulation in fibers and improves muscle atrophy in the soleus muscle of SAMP1 mice.

Experimental Models of Sarcopenia: Bridging Molecular Mechanism and Therapeutic Strategy

Sarcopenia has been defined as a progressive decline of skeletal muscle mass, strength, and functions in elderly people. It is accompanied by physical frailty, functional disability, falls, hospitalization, and mortality, and is becoming a major geriatric disorder owing to the increasing life expectancy and growing older population worldwide. Experimental models are critical to understand the pathophysiology of sarcopenia and develop therapeutic strategies. Although its etiologies remain to be further elucidated, several mechanisms of sarcopenia have been identified, including cellular senescence, proteostasis imbalance, oxidative stress, and "inflammaging." In this article, we address three main aspects. First, we describe the fundamental aging mechanisms. Next, we discuss both in vitro and in vivo experimental models based on molecular mechanisms that have the potential to elucidate the biochemical processes integral to sarcopenia. The use of appropriate models to reflect sarcopenia and/or its underlying pathways will enable researchers to understand sarcopenia and develop novel therapeutic strategies for sarcopenia. Lastly, we discuss the possible molecular targets and the current status of drug candidates for sarcopenia treatment. In conclusion, the development of experimental models for sarcopenia is essential to discover molecular targets that are valuable as biochemical biomarkers and/or therapeutic targets for sarcopenia.

Characterization of a novel compound that promotes myogenesis via Akt and transcriptional co-activator with PDZ-binding motif (TAZ) in mouse C2C12 cells

Transcriptional co-activator with PDZ-binding motif (TAZ) plays versatile roles in the regulation of cell proliferation and differentiation. TAZ activity changes in response to the cellular environment such as mechanic and nutritional stimuli, osmolarity, and hypoxia. To understand the physiological roles of TAZ, chemical compounds that activate TAZ in cells are useful as experimental reagents. Kaempferol, TM-25659, and ethacridine are reported as TAZ activators. However, as each TAZ activator has a distinct property in cellular functions, additional TAZ activators are awaiting. We screened for TAZ activators and previously reported IB008738 as a TAZ activator that promotes myogenesis in C2C12 cells. In this study, we have characterized IBS004735 that was obtained in the same screening. IBS004735 also promotes myogenesis in C2C12 cells, but is not similar to IBS008738 in the structure. IBS004735 activates TAZ via Akt and has no effect on TAZ phosphorylation, which is the well-described key modification to regulate TAZ activity. Thus, we introduce IBS004735 as a novel TAZ activator that regulates TAZ in a yet unidentified mechanism.

Pharmacological targeting of age-related changes in skeletal muscle tissue

Sarcopenia, the age-related loss of skeletal muscle mass and function, increases the risk of developing chronic diseases in older individuals and is a strong predictor of disability and death. Because of the ongoing demographic transition, age-related muscle weakness is responsible for an alarming and increasing contribution to health care costs in Western countries. Exercise-based interventions are most successful in preventing the decline in skeletal muscle mass and in preserving or ameliorating functional capacities with increasing age. However, other treatment options are still scarce. In this review, we explore currently applied nutritional and pharmacological approaches to mitigate age-related muscle wasting, and discuss potential future therapeutic avenues.

Sarcopenia: Tilting the Balance of Protein Homeostasis

Sarcopenia, defined as age-associated decline of muscle mass and function, is a risk factor for mortality and disability, and comorbid with several chronic diseases such as type II diabetes and cardiovascular diseases. Clinical trials showed that nutritional supplements had positive effects on muscle mass, but not on muscle function and strength, demonstrating our limited understanding of the molecular events involved in the ageing muscle. Protein homeostasis, the equilibrium between protein synthesis and degradation, is proposed as the major mechanism underlying the development of sarcopenia. As the key central regulator of protein homeostasis, the mammalian target of rapamycin (mTOR) is proposed to be essential for muscle hypertrophy. Paradoxically, sustained activation of mTOR complex 1 (mTORC1) is associated with a loss of sensitivity to extracellular signaling in the elderly. It is not understood why sustained mTORC1 activity, which should induce muscle hypertrophy, instead results in muscle atrophy. Here, recent findings on the implications of disrupting protein homeostasis on muscle physiology and sarcopenia development in the context of mTOR/protein kinase B (AKT) signaling are reviewed. Understanding the role of these molecular mechanisms during the ageing process will contribute towards the development of targeted therapies that will improve protein metabolism and reduce sarcopenia.

Rapamycin for longevity: opinion article

From the dawn of civilization, humanity has dreamed of immortality. So why didn't the discovery of the anti-aging properties of mTOR inhibitors change the world forever? I will discuss several reasons, including fear of the actual and fictional side effects of rapamycin, everolimus and other clinically-approved drugs, arguing that no real side effects preclude their use as anti-aging drugs today. Furthermore, the alternative to the reversible (and avoidable) side effects of rapamycin/everolimus are the irreversible (and inevitable) effects of aging: cancer, stroke, infarction, blindness and premature death. I will also discuss why it is more dangerous not to use anti-aging drugs than to use them and how rapamycin-based drug combinations have already been implemented for potential life extension in humans. If you read this article from the very beginning to its end, you may realize that the time is now.

The clinical impact and biological mechanisms of skeletal muscle aging

Skeletal muscle is a highly plastic tissue that remarkably adapts to diverse stimuli including exercise, injury, disuse, and, as discussed here, aging. Humans achieve peak skeletal muscle mass and strength in mid-life and then experience a progressive decline of up to 50% by the ninth decade. The loss of muscle mass and function with aging is a phenomenon termed sarcopenia. It is evidenced by the loss and atrophy of muscle fibers and the concomitant accretion of fat and fibrous tissue. Sarcopenia has been recognized as a key driver of limitations in physical function and mobility, but is perhaps less appreciated for its role in age-related metabolic dysfunction and loss of organismal resilience. Similar to other tissues, muscle is prone to multiple forms of age-related molecular and cellular damage, including disrupted protein turnover, impaired regenerative capacity, cellular senescence, and mitochondrial dysfunction. The objective of this review is to highlight the clinical consequences of skeletal muscle aging, and provide insights into potential biological mechanisms. In light of population aging, strategies to improve muscle health in older adults promise to have a profound public health impact.

Partial Inhibition of mTORC1 in Aged Rats Counteracts the Decline in Muscle Mass and Reverses Molecular Signaling Associated with Sarcopenia

There is a lack of pharmacological interventions available for sarcopenia, a progressive age-associated loss of muscle mass, leading to a decline in mobility and quality of life. We found mTORC1 (mammalian target of rapamycin complex 1), a well-established positive modulator of muscle mass, to be surprisingly hyperactivated in sarcopenic muscle.

There is a lack of pharmacological interventions available for sarcopenia, a progressive age-associated loss of muscle mass, leading to a decline in mobility and quality of life. We found mTORC1 (mammalian target of rapamycin complex 1), a well-established positive modulator of muscle mass, to be surprisingly hyperactivated in sarcopenic muscle. Furthermore, partial inhibition of the mTORC1 pathway counteracted sarcopenia, as determined by observing an increase in muscle mass and fiber type cross-sectional area in select muscle groups, again surprising because mTORC1 signaling has been shown to be required for skeletal muscle mass gains in some models of hypertrophy. Additionally, several genes related to senescence were downregulated and gene expression indicators of neuromuscular junction denervation were diminished using a low dose of a “rapalog” (a pharmacological agent related to rapamycin). Therefore, partial mTORC1 inhibition may delay the progression of sarcopenia by directly and indirectly modulating multiple age-associated pathways, implicating mTORC1 as a therapeutic target to treat sarcopenia.

Angiotensin II receptor blocker losartan exacerbates muscle damage and exhibits weak blood pressure-lowering activity in a dysferlin-null model of Limb-Girdle muscular dystrophy type 2B

There is no cure or beneficial management option for Limb-Girdle muscular dystrophy (MD) type 2B (LGMD2B). Losartan, a blood pressure (BP) lowering angiotensin II (AngII) receptor type 1 (ATR1) blocker (ARB) with unique anti-transforming growth factor-β (TGF-β) properties, can protect muscles in various types of MD such as Duchenne MD, suggesting a potential benefit for LGMD2B patients. Herein, we show in a mild, dysferlin-null mouse model of LGMD2B that losartan increased quadriceps muscle fibrosis (142%; P<0.0001). In a severe, atherogenic diet-fed model of LGMD2B recently described by our group, losartan further exacerbated dysferlin-null mouse muscle wasting in quadriceps and triceps brachii, two muscles typically affected by LGMD2B, by 40% and 51%, respectively (P<0.05). Lower TGF-β signalling was not observed with losartan, therefore plasma levels of atherogenic lipids known to aggravate LGMD2B severity were investigated. We report that losartan increased both plasma triglycerides and cholesterol concentrations in dysferlin-null mice. Other protective properties of losartan, such as increased nitric oxide release and BP lowering, were also reduced in the absence of dysferlin expression. Our data suggest that LGMD2B patients may show some resistance to the primary BP-lowering effects of losartan along with accelerated muscle wasting and dyslipidemia. Hence, we urge caution on the use of ARBs in this population as their ATR1 pathway may be dysfunctional.

Regulation of murine skeletal muscle growth by STAT5B is age- and sex-specific

Background

Sexually dimorphic growth has been attributed to the growth hormone (GH)/insulin-like growth factor 1 (IGF1) axis, particularly GH-induced activation of the intracellular signal transducer and activator of transcription 5B (STAT5B), because deletion of STAT5B reduces body mass and the mass of skeletal muscles in male mice to that in female mice. However, it remains unclear why these effects are sex- and species-specific, because the loss of STAT5B retards growth in girls, but not in male mice. Our objectives were to determine whether sexually dimorphic growth of skeletal muscle persisted in STAT5B^−/− mice and investigate the mechanisms by which STAT5B regulates sexually dimorphic growth.

Methods

Blood and skeletal muscle were harvested from male and female STAT5B^−/− mice and their wild-type littermates from the onset of puberty to adulthood.

Results

Growth of the skeleton and skeletal muscles was retarded in both sexes of STAT5B^−/− mice, but more so in males. Although reduced, sexually dimorphic growth of skeletal muscle persisted in STAT5B^−/− mice with an oxidative shift in the composition of myofibres in both sexes. Concentrations of IGF1 in blood and skeletal muscle were reduced in male STAT5B^−/− mice at all ages, but only in female STAT5B^−/− mice at the onset of puberty. Expression of androgen receptor (AR) and oestrogen receptor alpha (ERα) mRNA and protein was reduced in skeletal muscles of male and female STAT5B^−/− mice, respectively. Loss of STAT5B abolished the sexually dimorphic expression of myostatin protein and Igf1, Ar, Erα, suppressor of cytokine signalling 2 (Socs2), and cytokine-inducible SH2-containing protein (Cis) mRNA in skeletal muscle.

Conclusions

STAT5B appears to mediate GH signalling in skeletal muscles of male mice at all ages, but only until puberty in female mice. STAT5B also appears to mediate the actions of androgens and oestrogens in both male and female mice, but sexually dimorphic growth persists in STAT5B^−/− mice.

Electronic supplementary material

The online version of this article (10.1186/s13395-019-0204-3) contains supplementary material, which is available to authorized users.

Regulation of Amino Acid Transporters and Sensors in Response to a High protein Diet: A Randomized Controlled Trial in Elderly Men

BACKGROUND

The mammalian target of rapamycin complex 1 (mTORC1) is fundamental for many cellular processes, yet it is often dysregulated with aging. Increased amino acid (AA) availability is correlated with the expression of AA transporters (AAT) and mTORC1 activity. Although many AA sensors and mediators have been proposed to relay the AA signal to mTORC1, it has not yet been determined if chronic dietary intervention affects the expression of AAT, sensors and mediators and their relationships with mTORC1 activity.

OBJECTIVE AND DESIGN

This study investigated whether the consumption of a diet containing either the current recommended daily allowance (RDA) of protein intake (0.8 g/kg/d) or twice the RDA (2RDA) for ten weeks affected the expression of targets associated with AA transport, sensing and mTORC1 regulation in 26 older men (70-81 years).

METHOD

Muscle biopsies were collected before and after the intervention under fasting conditions. Diets were controlled by providing fully prepared meals and snacks. Western blot and quantitative polymerase chain reaction were used to measure protein and gene expression respectively.

RESULTS

Consumption of 2RDA reduced the protein expression of L-type amino acid transporter 1 (LAT1). However, plasma leucine concentration and basal mTORC1 activity were unaltered. The downregulation of LAT1 did not affect the expression of AA sensors and mediators, including leucyl tRNA synthetase (LRS), cytosolic arginine sensor for mTORC1 (CASTOR1), Sestrin2 and Rag proteins. Instead, total ribosomal protein S6 (RPS6) was upregulated with 2RDA.

CONCLUSION

Ten weeks of 2RDA diet did not affect the fasting mTORC1 signaling, but increased total RPS6 might suggest improved muscular translational capacity to maintain muscular mass.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

MicroRNA and Long Non-coding RNA Regulation in Skeletal Muscle From Growth to Old Age Shows Striking Dysregulation of the Callipyge Locus

MicroRNAs (miRNAs) undergo high levels of regulation in skeletal muscle development and control skeletal muscle mass, function and metabolism over the lifespan. More recently, the role of long non-coding RNAs (lncRNAs) in skeletal muscle regulation has started to emerge. Following up on our recent study describing the expression pattern and putative roles of 768 miRNAs in the quadriceps muscle of mice at early life stages, we used a high-throughput miRNA qPCR-based array to assess the expression of the same miRNAs in 28-month old male mouse quadriceps muscle. In addition, we report the expression patterns of lncRNAs playing a putative role in muscle development and adaptation from growth to old age. Twelve miRNAs were significantly downregulated in 28-month old muscle when compared with 12-week old muscle. Ten of them clustered at the Dlk1-Dio3 locus, known as ‘Callipyge,’ which is associated with muscle development and hypertrophy. This collective downregulation was paralleled by decreases in the expression levels of the maternally expressed imprinted LncRNA coding genes Meg3 and Rian stemming from the same chromosomal region. In contrast, the paternally expressed imprinted Dlk1-Dio3 locus members Rtl1, Dio3, and Dlk1 and the muscle related lncRNAs lncMyoD1, Neat_v1, Neat_v2, and Malat1 underwent significant changes during growth, but their expression levels were not altered past the age of 12 weeks, suggesting roles limited to hyperplasia and early hypertrophy. In conclusion, collective muscle miRNA expression gradually decreases over the lifespan and a cluster of miRNAs and maternally expressed lncRNAs stemming from the Callipyge locus is significantly dysregulated in aging muscle. The Dlk1-Dio3 locus therefore represents a potential new mechanism for age-related muscle decline.

Renal cold storage followed by transplantation impairs proteasome function and mitochondrial protein homeostasis

Identifying pathways related to renal cold storage (CS) that lead to renal damage after transplantation (Tx) will help us design novel pathway-specific therapies to improve graft outcome. Our recent report showed that mitochondrial function was compromised after CS alone, and this was exacerbated when CS was combined with Tx (CS/Tx). The goal of this study was to determine whether the proteasome exacerbates mitochondrial dysfunction after CS/Tx. We exposed the kidneys of male Lewis rats (in vivo) and rat renal proximal tubular (NRK) cells (in vitro) to CS/Tx or rewarming (CS/RW), respectively. To compare CS-induced effects, in vivo kidney Tx without CS exposure (autotransplantation; ATx) was also used. Our study provides the first evidence that the chymotrypsin-like (ChT-L) peptidase activity of the proteasome declined only after CS/Tx or CS/RW, but not after CS or ATx. Interestingly, key mitochondrial proteins involved with respiration [succinate dehydrogenase complex, subunit A (SDHA), a complex II subunit, and ATP5B, an ATP synthase/complex V subunit] were detected in the detergent-insoluble fraction after CS/Tx or CS/RW, with compromised complex V activity. Pharmacological inhibition of ChT-L activity in NRK cells decreased the activity of mitochondrial complexes I, II, and V and also increased the levels of SDHA and ATP5B in the insoluble fraction. On the other hand, inhibiting mitochondrial respiration in NRK cells with antimycin A compromised ChT-L function and increased the amounts of SDHA and ATP5B in the insoluble fraction. Our results suggest that mitochondrial respiratory dysfunction during CS precedes compromised ChT-L function after CS/Tx and proteasome dysfunction further alters mitochondrial protein homeostasis and decreases respiration in the kidneys after CS/Tx. Therefore, therapeutics that preserve mitochondrial and proteasome function during CS may provide beneficial outcomes following transplantation.

Muscle strength and size are associated with motor unit connectivity in aged mice

In older adults, the loss of muscle strength (dynapenia) and the loss of muscle mass (sarcopenia) are important contributors to the loss of physical function. We sought to investigate dynapenia, sarcopenia, and the loss of motor unit function in aging mice. C57BL/6J mice were analyzed with cross-sectional (males: 3 vs. 27 months; males and females: 8 vs. 12 vs. 20 months) and longitudinal studies (males: 10-25 months) using in vivo electrophysiological measures of motor unit connectivity (triceps surae compound muscle action potential and motor unit number estimation), in vivo measures of plantar flexion torque, magnetic resonance imaging of hind limb muscle volume, and grip strength. Compound muscle action potential amplitude, motor unit number estimation, and plantar flexion torque were decreased at 20 months. In contrast, grip strength was reduced at 24 months. Motor unit number estimates correlated with muscle torque and hind limb muscle volume. Our results demonstrate that the loss of motor unit connectivity is an early finding in aging male and female mice and that muscle size and contractility are both associated with motor unit number.

Update on mitochondria and muscle aging: all wrong roads lead to sarcopenia

Sarcopenia is a well-known geriatric syndrome that has been endorsed over the years as a biomarker allowing for the discrimination, at a clinical level, of biological from chronological age. Multiple candidate mechanisms have been linked to muscle degeneration during sarcopenia. Among them, there is wide consensus on the central role played by the loss of mitochondrial integrity in myocytes, secondary to dysfunctional quality control mechanisms. Indeed, mitochondria establish direct or indirect contacts with other cellular components (e.g. endoplasmic reticulum, peroxisomes, lysosomes/vacuoles) as well as the extracellular environment through the release of several biomolecules. The functional implications of these interactions in the context of muscle physiology and sarcopenia are not yet fully appreciated and represent a promising area of investigation. Here, we present an overview of recent findings concerning the interrelation between mitochondrial quality control processes, inflammation and the metabolic regulation of muscle mass in the pathogenesis of sarcopenia highlighting those pathways that may be exploited for developing preventive and therapeutic interventions against muscle aging.

Ablation of S1P3 receptor protects mouse soleus from age-related drop in muscle mass, force, and regenerative capacity

We investigated the effects of S1P3 deficiency on the age-related atrophy, decline in force, and regenerative capacity of soleus muscle from 23-mo-old male (old) mice. Compared with muscle from 5-mo-old (adult) mice, soleus mass and muscle fiber cross-sectional area (CSA) in old wild-type mice were reduced by ~26% and 24%, respectively. By contrast, the mass and fiber CSA of soleus muscle in old S1P3-null mice were comparable to those of adult muscle. Moreover, in soleus muscle of wild-type mice, twitch and tetanic tensions diminished from adulthood to old age. A slowing of contractile properties was also observed in soleus from old wild-type mice. In S1P3-null mice, neither force nor the contractile properties of soleus changed during aging. We also evaluated the regenerative capacity of soleus in old S1P3-null mice by stimulating muscle regeneration through myotoxic injury. After 10 days of regeneration, the mean fiber CSA of soleus in old wild-type mice was significantly smaller (−28%) compared with that of regenerated muscle in adult mice. On the contrary, the mean fiber CSA of regenerated soleus in old S1P3-null mice was similar to that of muscle in adult mice. We conclude that in the absence of S1P3, soleus muscle is protected from the decrease in muscle mass and force, and the attenuation of regenerative capacity, all of which are typical characteristics of aging.

Resistance wheel exercise from mid-life has minimal effect on sciatic nerves from old mice in which sarcopenia was prevented

The ability of resistance exercise, initiated from mid-life, to prevent age-related changes in old sciatic nerves, was investigated in male and female C57BL/6J mice. Aging is associated with cellular changes in old sciatic nerves and also loss of skeletal muscle mass and function (sarcopenia). Mature adult mice aged 15 months (M) were subjected to increasing voluntary resistance wheel exercise (RWE) over a period of 8 M until 23 M of age. This prevented sarcopenia in the old 23 M aged male and female mice. Nerves of control sedentary (SED) males at 3, 15 and 23 M of age, showed a decrease in the myelinated axon numbers at 15 and 23 M, a decreased g-ratio and a significantly increased proportion of myelinated nerves containing electron-dense aggregates at 23 M. Myelinated axon and nerve diameter, and axonal area, were increased at 15 M compared with 3 and 23 M. Exercise increased myelinated nerve profiles containing aggregates at 23 M. S100 protein, detected with immunoblotting was increased in sciatic nerves of 23 M old SED females, but not males, compared with 15 M, with no effect of exercise. Other neuronal proteins showed no significant alterations with age, gender or exercise. Overall the RWE had no cellular impact on the aging nerves, apart from an increased number of old nerves containing aggregates. Thus the relationship between cellular changes in aging nerves, and their sustained capacity for stimulation of old skeletal muscles to help maintain healthy muscle mass in response to exercise remains unclear.

Evidence that a mitochondrial death spiral underlies antagonistic pleiotropy

The antagonistic pleiotropy (AP) theory posits that aging occurs because alleles that are detrimental in older organisms are beneficial to growth early in life and thus are maintained in populations. Although genes of the insulin signaling pathway likely participate in AP, the insulin‐regulated cellular correlates of AP have not been identified. The mitochondrial quality control process called mitochondrial autophagy (mitophagy), which is inhibited by insulin signaling, might represent a cellular correlate of AP. In this view, rapidly growing cells are limited by ATP production; these cells thus actively inhibit mitophagy to maximize mitochondrial ATP production and compete successfully for scarce nutrients. This process maximizes early growth and reproduction, but by permitting the persistence of damaged mitochondria with mitochondrial DNA mutations, becomes detrimental in the longer term. I suggest that as mitochondrial ATP output drops, cells respond by further inhibiting mitophagy, leading to a further decrease in ATP output in a classic death spiral. I suggest that this increasing ATP deficit is communicated by progressive increases in mitochondrial ROS generation, which signals inhibition of mitophagy via ROS‐dependent activation of insulin signaling. This hypothesis clarifies a role for ROS in aging, explains why insulin signaling inhibits autophagy, and why cells become progressively more oxidized during aging with increased levels of insulin signaling and decreased levels of autophagy. I suggest that the mitochondrial death spiral is not an error in cell physiology but rather a rational approach to the problem of enabling successful growth and reproduction in a competitive world of scarce nutrients.

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.

L-Lysine suppresses myofibrillar protein degradation and autophagy in skeletal muscles of senescence-accelerated mouse prone 8

Sarcopenia is a condition of the loss of muscle mass that is associated with aging and that increases the risk for bedridden state, thereby warranting studies of interventions that attenuate sarcopenia. Here the effects of 2-month dietary L-lysine (Lys) supplementation (1.5-3.0 %) on myofibrillar protein degradation and major proteolytic systems were investigated in senescence-accelerated mouse prone 8 (SAMP8). At 36 weeks of age, skeletal muscle and lean body mass was reduced in SAMP8 when compared with control senescence-accelerated mouse resistant 1 (SAMR1). The myofibrillar protein degradation, which was evaluated by the release of 3-methylhistidine, was stimulated in SAMP8, and the autophagy activity, which was evaluated by light chain 3-II, was stimulated in the skeletal muscle of SAMP8. The activation of ubiquitin-proteasome system was not observed in the muscles of SAMP8. However, myofibrillar protein degradation and autophagic activity in skeletal muscles of SAMP8 were suppressed by dietary intake of 3.0 % Lys. The present data indicate that myofibrillar protein degradation by bulk autophagy is stimulated in the skeletal muscles of SAMP8 and that dietary Lys supplementation attenuates sarcopenia in SAMP8 by suppressing autophagic proteolysis.