Aging of the immune system causes reductions in muscle stem cell populations, promotes their shift to a fibrogenic phenotype, and modulates sarcopenia

Aging-related hyperphosphatemia triggers the release of TNF-α from macrophages, promoting indicators of sarcopenia through the reduction of IL-15 expression in skeletal muscle

AIMS

The association between aging-related hyperphosphatemia and sarcopenia has been documented, and evidence suggests that inflammaging is involved in the manifestation of sarcopenia. The present study investigates whether hyperphosphatemia triggers inflammation, thereby inducing the appearance of sarcopenia along with the cytokines involved in these processes.

MATERIALS AND METHODS

RAW 264.7 macrophages were incubated with β-glycerophosphate (BGP), as a phosphate donor, at different time intervals, to assess the production of proinflammatory markers. Conditioned medium from macrophages was collected and added to cultured C2C12 myoblasts to analyse whether proinflammatory molecules, released by macrophages, modified myogenic differentiation, cell senescence or myokine IL-15 expression. A neutralising antibody anti-TNF-α and recombinant IL-15 were added to evaluate the role of these cytokines in the observed effects. Additionally, TNF-α, IL-15, serum phosphate, and sarcopenia signs were evaluated in 5-month-old mice, 24-month-old mice and 24-month-old mice fed with a hypophosphatemic diet.

KEY FINDINGS

BGP increased TNF-α expression in macrophages through NFkB activation. Conditioned medium from BGP-treated macrophages impaired myogenic differentiation in differentiating myoblasts and promoted cellular senescence and reduced IL-15 expression in undifferentiated myoblasts. These effects were mediated by TNF-α. Old mice displayed reduced expression of muscle IL-15 and elevated circulating TNF-α, along with increased serum phosphate levels, which correlated with the appearance of sarcopenia indicators. The hypophosphatemic diet prevented these changes in old mice.

SIGNIFICANCE

Hyperphosphatemia induces TNF-α production in macrophages, which contributes to the reduced expression of muscular IL-15. This mechanism may play a role in inducing sarcopenia in elderly mice.

Sarcopenia and cachexia: molecular mechanisms and therapeutic interventions

Sarcopenia is defined as a muscle-wasting syndrome that occurs with accelerated aging, while cachexia is a severe wasting syndrome associated with conditions such as cancer and immunodeficiency disorders, which cannot be fully addressed through conventional nutritional supplementation. Sarcopenia can be considered a component of cachexia, with the bidirectional interplay between adipose tissue and skeletal muscle potentially serving as a molecular mechanism for both conditions. However, the underlying mechanisms differ. Recognizing the interplay and distinctions between these disorders is essential for advancing both basic and translational research in this area, enhancing diagnostic accuracy and ultimately achieving effective therapeutic solutions for affected patients. This review discusses the muscle microenvironment's changes contributing to these conditions, recent therapeutic approaches like lifestyle modifications, small molecules, and nutritional interventions, and emerging strategies such as gene editing, stem cell therapy, and gut microbiome modulation. We also address the challenges and opportunities of multimodal interventions, aiming to provide insights into the pathogenesis and molecular mechanisms of sarcopenia and cachexia, ultimately aiding in innovative strategy development and improved treatments.

Mechanisms of muscle cells alterations and regeneration decline during aging

Skeletal muscles are essential for locomotion and body metabolism regulation. As muscles age, they lose strength, elasticity, and metabolic capability, leading to ineffective motion and metabolic derangement. Both cellular and extracellular alterations significantly influence muscle aging. Satellite cells (SCs), the primary muscle stem cells responsible for muscle regeneration, become exhausted, resulting in diminished population and functionality during aging. This decline in SC function impairs intercellular interactions as well as extracellular matrix production, further hindering muscle regeneration. Other muscle-resident cells, such as fibro-adipogenic progenitors (FAPs), pericytes, and immune cells, also deteriorate with age, reducing local growth factor activities and responsiveness to stress or injury. Systemic signaling, including hormonal changes, contributes to muscle cellular catabolism and disrupts muscle homeostasis. Collectively, these cellular and environmental components interact, disrupting muscle homeostasis and regeneration in advancing age. Understanding these complex interactions offers insights into potential regenerative strategies to mitigate age-related muscle degeneration.

The immune landscape of murine skeletal muscle regeneration and aging

Age-related alterations in the immune system are starting to emerge as key contributors to impairments found in aged organs. A decline in regenerative capacity is a hallmark of tissue aging; however, the contribution of immune aging to regenerative failure is just starting to be explored. Here, we apply a strategy combining single-cell RNA sequencing with flow cytometry, histological analysis, and functional assays to perform a complete analysis of the immune environment of the aged regenerating skeletal muscle on a time course following injury with single-cell resolution. Our results reveal an unanticipated complexity and functional heterogeneity in immune populations within the skeletal muscle that have been regarded as homogeneous. Furthermore, we uncover a profound remodeling of both myeloid and lymphoid compartments in aging. These discoveries challenge established notions on immune regulation of skeletal muscle regeneration, providing a set of potential targets to improve skeletal muscle health and regenerative capacity in aging.

Scientific landscape of oxidative stress in sarcopenia: from bibliometric analysis to hotspots review

Objective

Sarcopenia is a significant healthcare challenge in the aging population. Oxidative stress (OS) is acknowledged to play a pivotal role in the pathological progression of sarcopenia. Numerous studies have demonstrated that mitigating or eliminating OS can ameliorate the pathological manifestations associated with sarcopenia. However, current clinical antioxidant therapies often fall short of anticipated outcomes. This bibliometric analysis aims to delineate prevailing research trends, thematic emphases, focal points, and developmental trajectories within the domain of OS in sarcopenia, while also endeavoring to explore prospective anti-oxidative stress strategies for future clinical interventions.

Methods

Relevant publications were retrieved from the Web of Science (WOS) Core Collection database for the period 2000-2024. Citespace was employed for retrieving and analyzing trends and emerging topics.

Results

In the field of OS in sarcopenia, the number of publications has significantly increased from 2000 to 2024. The United States and China are the primary contributors to global publication output. The most productive research institution is INRAE. The most prolific author is Holly Van Remmen from the United States, while the most frequently cited author is Cruz-Jentoft AJ from Spain. Experimental Gerontology is the journal with the highest volume of published articles, whereas the Journal of Gerontology Series A: Biological Sciences and Medical Sciences holds the record for the highest number of citations. The research keywords in this field can be categorized into eight domains: "Physiology and anatomy", "Physiological mechanisms", "Pathology associations", "Experimental studies", "Nutrition and metabolism", "Sports and physical activities", "Age" and "Oxidation and antioxidation". Moreover, recent years have seen the emergence of "TNF-α," "insulin resistance", "mitochondrial autophagy", "signal pathways", and "mechanisms" as focal points in the realm of OS in sarcopenia, encompassing related fundamental research and clinical translation.

Conclusion

This bibliometric and visualization provides a comprehensive analysis of the global research landscape in the field of OS in sarcopenia, identifies priorities, summarizes the current research status and suggests possible future research priorities. In addition, in order to benefit more sarcopenia patients, strengthening cooperation and communication between institutions and research teams is the key to the future development of this field. Given the expectation that research on OS in sarcopenia will remain a prominent area of interest in the future, this article could serve as a valuable resource for scholars seeking to shape future studies through an understanding of influential scholarly contributions and key research findings.

Systematic review registration

https://www.crd.york.ac.uk, identifier CRD42024528628.

A novel treatment strategy targeting cellular pathways with natural products to alleviate sarcopenia

Sarcopenia is a condition marked by a significant reduction in muscle mass and strength, primarily due to the aging process, which critically impacts muscle protein dynamics, metabolic functions, and overall physical functionality. This condition leads to increased body fat and reduced daily activity, contributing to severe health issues and a lower quality of life among the elderly. Recognized in the ICD-10-CM only in 2016, sarcopenia lacks definitive treatment options despite its growing prevalence and substantial social and economic implications. Given the aging global population, addressing sarcopenia has become increasingly relevant and necessary. The primary causes include aging, cachexia, diabetes, and nutritional deficiencies, leading to imbalances in protein synthesis and degradation, mitochondrial dysfunction, and hormonal changes. Exercise remains the most effective intervention, but it is often impractical for individuals with limited mobility, and pharmacological options such as anabolic steroids and myostatin inhibitors are not FDA-approved and are still under investigation. This review is crucial as it examines the potential of natural products as a novel treatment strategy for sarcopenia, targeting multiple mechanisms involved in its pathogenesis. By exploring natural products' multi-targeted effects, this study aims to provide innovative and practical solutions for sarcopenia management. Therefore, this review indicates significant improvements in muscle mass and function with the use of specific natural compounds, suggesting promising alternatives for those unable to engage in regular physical activity.

Characterizing the skeletal muscle immune microenvironment for sarcopenia: insights from transcriptome analysis and histological validation

Background

Sarcopenia is a condition characterized by the age-related loss of skeletal muscle mass and function. The pathogenesis of the disease is influenced by chronic low-grade inflammation. However, the specific changes in the immune landscape changes of sarcopenic muscle are not yet fully understood.

Methods

To gain insights into the immune cell composition and interactions, we combined single-nucleus RNA sequencing data, bulk RNA sequencing dataset, and comprehensive bioinformatic analyses on the skeletal muscle samples from young, aged, and sarcopenic individuals. Histological staining was then performed on skeletal muscles to validate the distribution of immune cells in clinical samples.

Results

We analyzed the transcriptomes of 101,862 single nuclei, revealing a total of 10 major cell types and 6 subclusters of immune cell types within the human skeletal muscle tissues. Notable variations were identified in the immune microenvironment between young and aged skeletal muscle. Among the immune cells from skeletal muscle microenvironment, macrophages constituted the largest fraction. A specific marker gene LYVE1 for skeletal muscle resident macrophages was further identified. Cellular subclasses included four distinct groups of resident macrophages, which play different roles in physiological or non-physiological conditions. Utilizing bulk RNA sequencing data, we observed a significant enrichment of macrophage-rich inflammation in sarcopenia.

Conclusions

Our findings demonstrate age-related changes in the composition and cross-talk of immune cells in human skeletal muscle microenvironment, which contribute to chronic inflammation in aged or sarcopenia muscle. Furthermore, macrophages emerge as a potential therapeutic target, thus advancing our understanding of the pathogenesis of sarcopenia.

Bioinformatics and system biology approach to identify potential common pathogenesis for COVID-19 infection and sarcopenia

Sarcopenia is a condition characterized by age-related loss of muscle mass and strength. Increasing evidence suggests that patients with sarcopenia have higher rates of coronavirus 2019 (COVID-19) infection and poorer post-infection outcomes. However, the exact mechanism and connections between the two is unknown. In this study, we used high-throughput data from the GEO database for sarcopenia (GSE111016) and COVID-19 (GSE171110) to identify common differentially expressed genes (DEGs). We conducted GO and KEGG pathway analyses, as well as PPI network analysis on these DEGs. Using seven algorithms from the Cytoscape plug-in cytoHubba, we identified 15 common hub genes. Further analyses included enrichment, PPI interaction, TF-gene and miRNA-gene regulatory networks, gene-disease associations, and drug prediction. Additionally, we evaluated immune cell infiltration with CIBERSORT and assessed the diagnostic accuracy of hub genes for sarcopenia and COVID-19 using ROC curves. In total, we identified 66 DEGs (34 up-regulated and 32 down-regulated) and 15 hub genes associated with sarcopenia and COVID-19. GO and KEGG analyses revealed functions and pathways between the two diseases. TF-genes and TF-miRNA regulatory network suggest that FOXOC1 and hsa-mir-155-5p may be identified as key regulators, while gene-disease analysis showed strong correlations with hub genes in schizophrenia and bipolar disorder. Immune infiltration showed a correlation between the degree of immune infiltration and the level of infiltration of different immune cell subpopulations of hub genes in different datasets. The ROC curves for ALDH1L2 and KLF5 genes demonstrated their potential as diagnostic markers for both sarcopenia and COVID-19. This study suggests that sarcopenia and COVID-19 may share pathogenic pathways, and these pathways and hub genes offer new targets and strategies for early diagnosis, effective treatment, and tailored therapies for sarcopenia patients with COVID-19.

The role of magnesium in the pathogenesis of osteoporosis

Magnesium (Mg), a nutritional element which is essential for bone development and mineralization, has a role in the progression of osteoporosis. Osteoporosis is a multifactorial disease characterized by significant deterioration of bone microstructure and bone loss. Mg deficiency can affect bone structure in an indirect way through the two main regulators of calcium homeostasis (parathyroid hormone and vitamin D). In human osteoblasts (OBs), parathyroid hormone regulates the expression of receptor activator of nuclear factor-κ B ligand (RANKL) and osteoprotegerin (OPG) to affect osteoclast (OC) formation. In addition, Mg may also affect the vitamin D3 -mediated bone remodeling activity. vitamin D3 usually coordinates the activation of the OB and OC. The unbalanced activation OC leads to bone resorption. The RANK/RANKL/OPG axis is considered to be a key factor in the molecular mechanism of osteoporosis. Mg participates in the pathogenesis of osteoporosis by affecting the regulation of parathyroid hormone and vitamin D levels to affect the RANK/RANKL/OPG axis. Different factors affecting the axis and enhancing OC function led to bone loss and bone tissue microstructure damage, which leads to the occurrence of osteoporosis. Clinical research has shown that Mg supplementation can alleviate the symptoms of osteoporosis to some extent.

The effect of T cell aging on the change of human tissue structure

The trend of aging of the global population is becoming more and more significant, and the incidence of age-related diseases continues to rise.This phenomenon makes the problem of aging gradually attracted wide attention of the society, and gradually developed into an independent research field.As a vital defense mechanism of the human body, the immune system changes significantly during the aging process.Age-induced changes in the body's immune system are considered harmful and are commonly referred to as immune aging, which may represent the beginning of systemic aging.Immune cells, especially T cells, are the biggest influencers and participants in age-related deterioration of immune function, making older people more susceptible to different age-related diseases.More and more evidence shows that T cells play an important role in the change of human tissue structure after aging, which fundamentally affects the health and survival of the elderly.In this review, we discuss the general characteristics of age-related T cell immune alterations and the possible effects of aging T cells in various tissue structures in the human body.

Restoring Mitochondrial Function and Muscle Satellite Cell Signaling: Remedies against Age-Related Sarcopenia

Sarcopenia has a complex pathophysiology that encompasses metabolic dysregulation and muscle ultrastructural changes. Among the drivers of intracellular and ultrastructural changes of muscle fibers in sarcopenia, mitochondria and their quality control pathways play relevant roles. Mononucleated muscle stem cells/satellite cells (MSCs) have been attributed a critical role in muscle repair after an injury. The involvement of mitochondria in supporting MSC-directed muscle repair is unclear. There is evidence that a reduction in mitochondrial biogenesis blunts muscle repair, thus indicating that the delivery of functional mitochondria to injured muscles can be harnessed to limit muscle fibrosis and enhance restoration of muscle function. Injection of autologous respiration-competent mitochondria from uninjured sites to damaged tissue has been shown to reduce infarct size and enhance cell survival in preclinical models of ischemia-reperfusion. Furthermore, the incorporation of donor mitochondria into MSCs enhances lung and cardiac tissue repair. This strategy has also been tested for regeneration purposes in traumatic muscle injuries. Indeed, the systemic delivery of mitochondria promotes muscle regeneration and restores muscle mass and function while reducing fibrosis during recovery after an injury. In this review, we discuss the contribution of altered MSC function to sarcopenia and illustrate the prospect of harnessing mitochondrial delivery and restoration of MSCs as a therapeutic strategy against age-related sarcopenia.

NAD+: An Old but Promising Therapeutic Agent for Skeletal Muscle Ageing

More than a century after the discovery of nicotinamide adenine dinucleotide (NAD+), our understanding of the molecule's role in the biology of ageing continues to evolve. As a coenzyme or substrate for many enzymes, NAD+ governs a wide range of biological processes, including energy metabolism, genomic stability, signal transduction, and cell fate. NAD+ deficiency has been recognised as a bona fide hallmark of tissue degeneration, and restoring NAD+ homeostasis helps to rejuvenate multiple mechanisms associated with tissue ageing. The progressive loss of skeletal muscle homeostasis with age is directly associated with high morbidity, disability and mortality. The aetiology of skeletal muscle ageing is complex, involving mitochondrial dysfunction, senescence and stem cell depletion, autophagy defects, chronic cellular stress, intracellular ion overload, immune cell dysfunction, circadian clock disruption, microcirculation disorders, persistent denervation, and gut microbiota dysbiosis. This review focuses on the therapeutic potential of NAD+ restoration to alleviate the above pathological factors and discusses the effects of in vivo administration of different NAD+ boosting strategies on skeletal muscle homeostasis, aiming to provide a reference for combating skeletal muscle ageing.

The multifaceted role of macrophages in homeostatic and injured skeletal muscle

Skeletal muscle is essential for body physical activity, energy metabolism, and temperature maintenance. It has excellent capabilities to maintain homeostasis and to regenerate after injury, which indispensably relies on muscle stem cells, satellite cells (MuSCs). The quiescence, activation, and differentiation of MuSCs are tightly regulated in homeostatic and regenerating muscles. Among the important regulators are intramuscular macrophages, which are functionally heterogeneous with different subtypes present in a spatiotemporal manner to regulate the balance of different MuSC statuses. During chronic injury and aging, intramuscular macrophages often undergo aberrant activation, which in turn disrupts muscle homeostasis and regenerative repair. Growing evidence suggests that the aberrant activation is mainly triggered by altered muscle microenvironment. The trained immunity that affects myeloid progenitors during hematopoiesis may also contribute. Aged immune system may contribute, in part, to the aging-related sarcopenia and compromised skeletal muscle injury repair. As macrophages are actively involved in the progression of many muscle diseases, manipulating their functional activation has become a promising therapeutic approach, which requires comprehensive knowledge of the cellular and molecular mechanisms underlying the diverse activation. To this end, we discuss here the current knowledge of multifaceted role of macrophages in skeletal muscle homeostasis, injury, and repair.

Treadmill training does not enhance skeletal muscle recovery following disuse atrophy in older male mice

Introduction: A hallmark of aging is poor muscle recovery following disuse atrophy. Efficacious strategies to enhance muscle recovery following disuse atrophy in aging are non-existent. Prior exercise training could result in favorable muscle morphological and cellular adaptations that may promote muscle recovery in aging. Here, we characterized the impact of exercise training on skeletal muscle inflammatory and metabolic profiles and cellular remodeling and function, together with femoral artery reactivity prior to and following recovery from disuse atrophy in aged male mice. We hypothesized that 12 weeks of treadmill training in aged male mice would improve skeletal muscle cellular remodeling at baseline and during recovery from disuse atrophy, resulting in improved muscle regrowth. Methods: Physical performance, ex vivo muscle and vascular function, tissue and organ mass, hindlimb muscle cellular remodeling (macrophage, satellite cell, capillary, myofiber size, and fibrosis), and proteolytic, inflammatory, and metabolic muscle transcripts were evaluated in aged exercise-trained and sedentary mice. Results: We found that at baseline following exercise training (vs. sedentary mice), exercise capacity and physical function increased, fat mass decreased, and endothelial function improved. However, exercise training did not alter tibialis anterior or gastrocnemius muscle transcriptional profile, macrophage, satellite cell, capillarity or collagen content, or myofiber size and only tended to increase tibialis mass during recovery from disuse atrophy. Conclusion: While exercise training in old male mice improved endothelial function, physical performance, and whole-body tissue composition as anticipated, 12 weeks of treadmill training had limited impact on skeletal muscle remodeling at baseline or in response to recovery following disuse atrophy.

Palmitate-Induced Inflammation and Myotube Atrophy in C2C12 Cells Are Prevented by the Whey Bioactive Peptide, Glycomacropeptide

BACKGROUND

Metabolic diseases are often associated with muscle atrophy and heightened inflammation. The whey bioactive compound, glycomacropeptide (GMP), has been shown to exhibit anti-inflammatory properties and therefore may have potential therapeutic efficacy in conditions of skeletal muscle inflammation and atrophy.

OBJECTIVES

The purpose of this study was to determine the role of GMP in preventing lipotoxicity-induced myotube atrophy and inflammation.

METHODS

C2C12 myoblasts were differentiated to determine the effect of GMP on atrophy and inflammation and to explore its mechanism of action in evaluating various anabolic and catabolic cellular signaling nodes. We also used a lipidomic analysis to evaluate muscle sphingolipid accumulation with the various treatments. Palmitate (0.75 mM) in the presence and absence of GMP (5 μg/mL) was used to induce myotube atrophy and inflammation and cells were collected over a time course of 6-24 h.

RESULTS

After 24 h of treatment, GMP prevented the palmitate-induced decrease in the myotube area and myogenic index and the increase in the TLR4-mediated inflammatory genes tumor necrosis factor-α and interleukin 1β. Moreover, phosphorylation of Erk1/2, and gene expression of myostatin, and the E3 ubiquitin ligases, FBXO32, and MuRF1 were decreased with GMP treatment. GMP did not alter palmitate-induced ceramide or diacylglycerol accumulation, muscle insulin resistance, or protein synthesis.

CONCLUSIONS

In summary, GMP prevented palmitate-induced inflammation and atrophy in C2C12 myotubes. The GMP protective mechanism of action in muscle cells during lipotoxic stress may be related to targeting catabolic signaling associated with cellular stress and proteolysis but not protein synthesis.

Autophagy in sarcopenia: Possible mechanisms and novel therapies

With global population aging, age-related diseases, especially sarcopenia, have attracted much attention in recent years. Characterized by low muscle strength, low muscle quantity or quality and low physical performance, sarcopenia is one of the major factors associated with an increased risk of falls and disability. Much effort has been made to understand the cellular biological and physiological mechanisms underlying sarcopenia. Autophagy is an important cellular self-protection mechanism that relies on lysosomes to degrade misfolded proteins and damaged organelles. Research designed to obtain new insight into human diseases from the autophagic aspect has been carried out and has made new progress, which encourages relevant studies on the relationship between autophagy and sarcopenia. Autophagy plays a protective role in sarcopenia by modulating the regenerative capability of satellite cells, relieving oxidative stress and suppressing the inflammatory response. This review aims to reveal the specific interaction between sarcopenia and autophagy and explore possible therapies in hopes of encouraging more specific research in need and unlocking novel promising therapies to ameliorate sarcopenia.

Exploring cellular senescence in the musculoskeletal system: Any insights for biomarkers discovery?

The locomotor system comprises skeletal muscles and bones with active metabolism and cellular turnover. Chronic locomotor system disorders gradually arising with aging are inversely associated with the correct function of bone and muscles. Senescent cells appear more frequently in advanced ages or pathological conditions, and the accumulation of senescent cells in muscle tissue negatively correlates with muscle regeneration, which is crucial for maintaining strength and preventing frailty. Senescence in the bone microenvironment, osteoblasts, and osteocytes affects bone turnover favoring osteoporosis. It is likely that in response to injury and age-related damage over the lifetime, a subset of niche cells accumulates oxidative stress and DNA damage beyond the threshold that primes the onset of cellular senescence. These senescent cells may acquire resistance to apoptosis that, combined with the weakened immune system, results in impaired clearance of senescent cells and their accumulation. The secretory profile of senescent cells causes local inflammation, further spreading senescence in neighboring niche cells and impairing tissue homeostasis. The resulting impairment of turnover/tissue repair in the musculoskeletal system reduces the efficiency of the organ in response to environmental needs that finally lead to functional decline. Management of the musculoskeletal system at the cellular level can benefit the quality of life and reduce early aging. This work discusses current knowledge of cellular senescence of musculoskeletal tissues to conclude with biologically active biomarkers effective enough to reveal the underlying mechanisms of tissue flaws at the earliest possible.

Inflammatory, mitochondrial, and senescence-related markers: Underlying biological pathways of muscle aging and new therapeutic targets

The maintenance of functional health is pivotal for achieving independent life in older age. The aged muscle is characterized by ultrastructural changes, including loss of type I and type II myofibers and a greater proportion of cytochrome c oxidase deficient and succinate dehydrogenase positive fibers. Both intrinsic (e.g., altered proteostasis, DNA damage, and mitochondrial dysfunction) and extrinsic factors (e.g., denervation, altered metabolic regulation, declines in satellite cells, and inflammation) contribute to muscle aging. Being a hub for several cellular activities, mitochondria are key to myocyte viability and mitochondrial dysfunction has been implicated in age-associated physical decline. The maintenance of functional organelles via mitochondrial quality control (MQC) processes is, therefore, crucial to skeletal myofiber viability and organismal health. The autophagy-lysosome pathway has emerged as a critical step of MQC in muscle by disposing organelles and proteins via their tagging for autophagosome incorporation and delivery to the lysosome for clearance. This pathway was found to be altered in muscle of physically inactive older adults. A relationship between this pathway and muscle tissue composition of the lower extremities as well as physical performance was also identified. Therefore, integrating muscle structure and myocyte quality control measures in the evaluation of muscle health may be a promising strategy for devising interventions fostering muscle health.

Polarizing Macrophage Functional Phenotype to Foster Cardiac Regeneration

There is an increasing interest in understanding the connection between the immune and cardiovascular systems, which are highly integrated and communicate through finely regulated cross-talking mechanisms. Recent evidence has demonstrated that the immune system does indeed have a key role in the response to cardiac injury and in cardiac regeneration. Among the immune cells, macrophages appear to have a prominent role in this context, with different subtypes described so far that each have a specific influence on cardiac remodeling and repair. Similarly, there are significant differences in how the innate and adaptive immune systems affect the response to cardiac damage. Understanding all these mechanisms may have relevant clinical implications. Several studies have already demonstrated that stem cell-based therapies support myocardial repair. However, the exact role that cardiac macrophages and their modulation may have in this setting is still unclear. The current need to decipher the dual role of immunity in boosting both heart injury and repair is due, at least for a significant part, to unresolved questions related to the complexity of cardiac macrophage phenotypes. The aim of this review is to provide an overview on the role of the immune system, and of macrophages in particular, in the response to cardiac injury and to outline, through the modulation of the immune response, potential novel therapeutic strategies for cardiac regeneration.

Changes in macrophage immunometabolism as a marker of skeletal muscle dysfunction across the lifespan

One of the most pronounced changes in the elderly is loss of strength and mobility due to the decline of skeletal muscle function, resulting in a multifactorial condition termed sarcopenia. Although significant clinical changes begin to manifest at advanced ages, recent studies have shown that changes at the cellular and molecular level precede the symptomatology of sarcopenia. By utilizing a single-cell transcriptomic atlas of mouse skeletal muscle across the lifespan, we identified a clear sign of immune senescence that presents during middle age. More importantly, the change in macrophage phenotype in middle age may explain the changes in extracellular matrix composition, especially collagen synthesis, that contributes to fibrosis and overall muscle weakness with advanced age. Our results show a novel paradigm whereby skeletal muscle dysfunction is driven by alterations in tissue-resident macrophages before the appearance of clinical symptoms in middle-aged mice, providing a new therapeutic approach via regulation of immunometabolism.

Loss of the matrix metalloproteinase-10 causes premature features of aging in satellite cells

Aged muscles accumulate satellite cells with a striking decline response to damage. Although intrinsic defects in satellite cells themselves are the major contributors to aging-associated stem cell dysfunction, increasing evidence suggests that changes in the muscle-stem cell local microenvironment also contribute to aging. Here, we demonstrate that loss of the matrix metalloproteinase-10 (MMP-10) in young mice alters the composition of the muscle extracellular matrix (ECM), and specifically disrupts the extracellular matrix of the satellite cell niche. This situation causes premature features of aging in the satellite cells, contributing to their functional decline and a predisposition to enter senescence under proliferative pressure. Similarly, reduction of MMP-10 levels in young satellite cells from wild type animals induces a senescence response, while addition of the protease delays this program. Significantly, the effect of MMP-10 on satellite cell aging can be extended to another context of muscle wasting, muscular dystrophy. Systemic treatment of mdx dystrophic mice with MMP-10 prevents the muscle deterioration phenotype and reduces cellular damage in the satellite cells, which are normally under replicative pressure. Most importantly, MMP-10 conserves its protective effect in the satellite cell-derived myoblasts isolated from a Duchenne muscular dystrophy patient by decreasing the accumulation of damaged DNA. Hence, MMP-10 provides a previously unrecognized therapeutic opportunity to delay satellite cell aging and overcome satellite cell dysfunction in dystrophic muscles.

Aging disrupts MANF-mediated immune modulation during skeletal muscle regeneration

Age-related decline in skeletal muscle regenerative capacity is multifactorial, yet the contribution of immune dysfunction to regenerative failure is unknown. Macrophages are essential for effective debris clearance and muscle stem cell activity during muscle regeneration, but the regulatory mechanisms governing macrophage function during muscle repair are largely unexplored. Here, we uncover a new mechanism of immune modulation operating during skeletal muscle regeneration that is disrupted in aged animals and relies on the regulation of macrophage function. The immune modulator mesencephalic astrocyte-derived neurotrophic factor (MANF) is induced following muscle injury in young mice but not in aged animals, and its expression is essential for regenerative success. Regenerative impairments in aged muscle are associated with defects in the repair-associated myeloid response similar to those found in MANF-deficient models and could be improved through MANF delivery. We propose that restoring MANF levels is a viable strategy to improve myeloid response and regenerative capacity in aged muscle.

Train your T cells: How skeletal muscles and T cells keep each other fit during aging

Frailty and a failing immune system lead to significant morbidities in the final years of life and bring along a significant burden on healthcare systems. The good news is that regular exercise provides an effective countermeasure for losing muscle tissue when we age while supporting proper immune system functioning. For a long time, it was assumed that exercise-induced immune responses are predominantly mediated by myeloid cells, but it has become evident that they receive important help from T lymphocytes. Skeletal muscles and T cells interact, not only in muscle pathology but also during exercise. In this review article, we provide an overview of the most important aspects of T cell senescence and discuss how these are modulated by exercise. In addition, we describe how T cells are involved in muscle regeneration and growth. A better understanding of the complex interactions between myocytes and T cells throughout all stages of life provides important insights needed to design strategies that effectively combat the wave of age-related diseases the world is currently faced with.

Nano-Immunomodulation: A New Strategy for Skeletal Muscle Diseases and Aging?

The skeletal muscle has a very remarkable ability to regenerate upon injury under physiological conditions; however, this regenerative capacity is strongly diminished in physio-pathological conditions, such as those present in diseased or aged muscles. Many muscular dystrophies (MDs) are characterized by aberrant inflammation due to the deregulation of both the lymphoid and myeloid cell populations and the production of pro-inflammatory cytokines. Pathological inflammation is also observed in old muscles due to a systemic change in the immune system, known as "inflammaging". Immunomodulation represents, therefore, a promising therapeutic opportunity for different skeletal muscle conditions. However, the use of immunomodulatory drugs in the clinics presents several caveats, including their low stability in vivo, the need for high doses to obtain therapeutically relevant effects, and the presence of strong side effects. Within this context, the emerging field of nanomedicine provides the powerful tools needed to control the immune response. Nano-scale materials are currently being explored as biocarriers to release immunomodulatory agents in the damaged tissues, allowing therapeutic doses with limited off-target effects. In addition, the intrinsic immunomodulatory properties of some nanomaterials offer further opportunities for intervention that still need to be systematically explored. Here we exhaustively review the state-of-the-art regarding the use of nano-sized materials to modulate the aberrant immune response that characterizes some physio-pathological muscle conditions, such as MDs or sarcopenia (the age-dependent loss of muscle mass). Based on our learnings from cancer and immune tolerance induction, we also discuss further opportunities, challenges, and limitations of the emerging field of nano-immunomodulation.

Coapplication of Magnesium Supplementation and Vibration Modulate Macrophage Polarization to Attenuate Sarcopenic Muscle Atrophy through PI3K/Akt/mTOR Signaling Pathway

Sarcopenia is an age-related geriatric syndrome characterized by the gradual loss of muscle mass and function. Low-magnitude high-frequency vibration (LMHFV) was shown to be beneficial to structural and functional outcomes of skeletal muscles, while magnesium (Mg) is a cofactor associated with better indices of skeletal muscle mass and strength. We hypothesized that LMHFV, Mg and their combinations could suppress inflammation and sarcopenic atrophy, promote myogenesis via PI3k/Akt/mTOR pathway in senescence-accelerated mouse P8 (SAMP8) mice and C2C12 myoblasts. Results showed that Mg treatment and LMHFV could significantly decrease inflammatory expression (C/EBPα and LYVE1) and modulate a CD206-positive M2 macrophage population at month four. Mg treatment also showed significant inhibitory effects on FOXO3, MuRF1 and MAFbx mRNA expression. Coapplication showed a synergistic effect on suppression of type I fiber atrophy, with significantly higher IGF-1, MyoD, MyoG mRNA (p < 0.05) and pAkt protein expression (p < 0.0001) during sarcopenia. In vitro inhibition of PI3K/Akt and mTOR abolished the enhancement effects on myotube formation and inhibited MRF mRNA and p85, Akt, pAkt and mTOR protein expressions. The present study demonstrated that the PI3K/Akt/mTOR pathway is the predominant regulatory mechanism through which LMHFV and Mg enhanced muscle regeneration and suppressed atrogene upregulation.

Macrophage immunomodulation accelerates skeletal muscle functional recovery in aged mice following disuse atrophy

Poor recovery of muscle size and strength with aging coincides with a dysregulated macrophage response during the early stages of regrowth. Immunomodulation in the form of ex vivo cytokine (macrophage-colony stimulating factor) or polarized macrophage delivery has been demonstrated to improve skeletal muscle regeneration. However, it is unclear if these macrophage-promoting approaches would be effective to improve skeletal muscle recovery following disuse in aged animals. Here, we isolated bone marrow-derived macrophages from donor mice of different ages under various experimental conditions and polarized them into proinflammatory macrophages. Macrophages were delivered intramuscularly into young adult or aged recipient mice during the early recovery period following a period of hindlimb unloading (HU). Delivery of proinflammatory macrophages from donor young adults or aged mice was sufficient to increase muscle function of aged mice during the recovery period. Moreover, proinflammatory macrophages derived from aged donor mice collected during recovery were similarly able to increase muscle function of aged mice following disuse. In addition to the delivery of macrophages, we showed that the intramuscular injection of the cytokine, macrophage-colony stimulating factor, to the muscle of aged mice following HU was able to increase muscle macrophage content and muscle force production during recovery. Together, these results suggest that macrophage immunomodulation approaches in the form of ex vivo proinflammatory macrophage or macrophage-colony stimulating factor delivery during the early recovery phase following disuse atrophy were sufficient to restore the loss of aged skeletal muscle function.NEW & NOTEWORTHY A single intramuscular administration of polarized macrophages into muscles of aged mice following a bout of disuse atrophy was sufficient to improve functional recover similarly to young adults after disuse atrophy regardless of the age or experimental condition of the donor mice. Additionally, intramuscular delivery of macrophage-colony stimulating factor into aged mice was similarly effective. Targeting macrophage function early during the regrowth phase may be a novel tool to bolster muscle recovery in aging.

Myeloid cell-specific mutation of Spi1 selectively reduces M2-biased macrophage numbers in skeletal muscle, reduces age-related muscle fibrosis and prevents sarcopenia

Intramuscular macrophages play key regulatory roles in determining the response of skeletal muscle to injury and disease. Recent investigations showed that the numbers and phenotype of intramuscular macrophages change during aging, suggesting that those changes could influence the aging process. We tested that hypothesis by generating a mouse model that harbors a myeloid cell-specific mutation of Spi1, which is a transcription factor that is essential for myeloid cell development. The mutation reduced the numbers of macrophages biased to the CD163+/CD206+ M2 phenotype in muscles of aging mice without affecting the numbers of CD68-expressing macrophages and reduced the expression of transcripts associated with the M2-biased phenotype. The mutation did not affect the colony-forming ability or the frequency of specific subpopulations of bone marrow hematopoietic cells and did not affect myeloid/lymphoid cell ratios in peripheral blood leukocyte populations. Cellularity of most myeloid lineage cells was not influenced by the mutation. The Spi1 mutation in bone marrow-derived macrophages in vitro also did not affect expression of transcripts that indicate the M2-biased phenotype. Thus, myeloid cell-targeted mutation of Spi1 influences macrophage phenotype in muscle but did not affect earlier stages of differentiation of cells in the macrophage lineage. The mutation reduced age-related muscle fibrosis, which is consistent with the reduction of M2-biased macrophages, and reduced expression of the pro-fibrotic enzyme arginase. Most importantly, the mutation prevented sarcopenia. Together, our observations indicate that intramuscular, M2-biased macrophages play significant roles in promoting detrimental, age-related changes in muscle.

Bibliometric Analysis of the Knowledge Base and Future Trends on Sarcopenia from 1999–2021

Sarcopenia is characterized by progressive loss of muscle mass and function, and it is becoming a serious public health problem with the aging population. However, a comprehensive overview of the knowledge base and future trends is still lacking. The articles and reviews with “sarcopenia” in their title published from 1999 to 2021 in the SCIE database were retrieved. We used Microsoft Excel, VOSviewer, and CiteSpace to conduct a descriptive and bibliometric analysis. A total of 3582 publications were collected, from 4 published in 2000 increasing dramatically to 850 documents in 2021. The USA was the most productive country, with the most citations. The Catholic University of the Sacred Heart and Landi F were the most influential organization and author in this field, respectively. The core journal in this field was the Journal of Cachexia Sarcopenia and Muscle. According to the analysis of keywords and references, we roughly categorized the main research areas into four domains as follows: 1. Definition and diagnosis; 2. Epidemiology; 3. Etiology and pathogenesis; 4. Treatments. Comparing different diagnostic tools and the epidemiology of sarcopenia in different populations are recent hotspots, while more efforts are needed in the underlying mechanism and developing safe and effective treatments. In conclusion, this study provides comprehensive insights into developments and trends in sarcopenia research that can help researchers and clinicians better manage and implement their work.

Inhibiting uptake of extracellular vesicles derived from senescent bone marrow mesenchymal stem cells by muscle satellite cells attenuates sarcopenia

Objective

Osteoporosis is associated with senescence of bone marrow mesenchymal stem cells (BMSCs). Extracellular vesicles derived from senescent BMSCs (BMSC-EVs) could be uptaken by muscle satellite cells (SCs). We hypothesized that inhibiting the uptake of harmful BMSC-EVs by SCs could prevent patients with osteoporosis complicated with sarcopenia.

Methods

Bioinformatics analysis was used to analyze senescent SCs. Myogenic potential of SCs was measured using myogenesis assay and immunofluorescence while muscle atrophy was measured using histological evaluation. And the interaction of cluster of differentiation (CD) 81 and the membrane proteins of SCs was verified using biotin pulldown assay.. CD81-specific siRNA (si-CD81) was used to knockdown CD81 and anti-CD81 antibody (anti-CD81 Ab) was used to block CD81.

Results

Differentially expressed genes in senescent SCs were enriched in muscle cell differentiation. The myogenic potential of senescent SCs was significantly decreased. Senescent BMSC-EVs impaired myogenesis of SCs. CD81 on the surface of BMSC-EVs could bind to membrane proteins of SCs. Both knockdown of CD81 and blocking CD81 prevented the uptake of senescent BMSC-EVs by SCs, thus relieving harmful effects of senescent BMSC-EVs on muscle atrophy.

Conclusion

Blocking CD81 on the surface of senescent BMSC-EVs attenuates sarcopenia in aged mice, which could be useful for prevention of sarcopenia in patients with osteoporosis in clinical practice.

Translational potential of this article

Inhibiting uptake of extracellular vesicles derived from senescent bone marrow mesenchymal stem cells by muscle satellite cells can prevent muscle atrophy in aged mice and has potential for application in treating sarcopenia.

Remodeling of the Aged and Emphysematous Lungs: Roles of Microenvironmental Cues

Aging is a slow process that affects all organs, and the lung is no exception. At the alveolar level, aging increases the airspace size with thicker and stiffer septal walls and straighter and thickened collagen and elastic fibers. This creates a microenvironment that interferes with the ability of cells in the parenchyma to maintain normal homeostasis and respond to injury. These changes also make the lung more susceptible to disease such as emphysema. Emphysema is characterized by slow but progressive remodeling of the deep alveolar regions that leads to airspace enlargement and increased but disorganized elastin and collagen deposition. This remodeling has been attributed to ongoing inflammation that involves inflammatory cells and the cytokines they produce. Cellular senescence, another consequence of aging, weakens the ability of cells to properly respond to injury, something that also occurs in emphysema. These factors conspire to make alveolar walls more prone to mechanical failure, which can set emphysema in motion by driving inflammation through immune stimulation by protein fragments. Both aging and emphysema are influenced by microenvironmental conditions such as local inflammation, chemical makeup, tissue stiffness, and mechanical stresses. Although aging and emphysema are not equivalent, they have the potential to influence each other in synergistic ways; aging sets up the conditions for emphysema to develop, while emphysema may accelerate cellular senescence and thus aging itself. This article focuses on the similarities and differences between the remodeled microenvironment of the aging and emphysematous lung, with special emphasis on the alveolar septal wall. © 2022 American Physiological Society. Compr Physiol 12:3559-3574, 2022.

Lysyl oxidase-like 2 inhibitor rescues D-galactose-induced skeletal muscle fibrosis

Aging-related sarcopenia is currently the most common sarcopenia. The main manifestations are skeletal muscle atrophy, replacement of muscle fibers with fat and fibrous tissue. Excessive fibrosis can impair muscle regeneration and function. Lysyl oxidase-like 2 (LOXL2) has previously been reported to be involved in the development of various tissue fibrosis. Here, we investigated the effects of LOXL2 inhibitor on D-galactose (D-gal)-induced skeletal muscle fibroblast cells and mice. Our molecular and physiological studies show that treatment with LOXL2 inhibitor can alleviate senescence, fibrosis, and increased production of reactive oxygen species in fibroblasts caused by D-gal. These effects are related to the inhibition of the TGF-β1/p38 MAPK pathway. Furthermore, in vivo, mice treatment with LOXL2 inhibitor reduced D-gal-induced skeletal muscle fibrosis, partially enhanced skeletal muscle mass and strength and reduced redox balance disorder. Taken together, these data indicate the possibility of using LOXL2 inhibitors to prevent aging-related sarcopenia, especially with significant fibrosis.

Blood Flow Restriction Training for the Intervention of Sarcopenia: Current Stage and Future Perspective

Sarcopenia is a geriatric syndrome that is characterized by a progressive and generalized skeletal muscle disorder and can be associated with many comorbidities, including obesity, diabetes, and fracture. Its definitions, given by the AWGS and EWGSOP, are widely used. Sarcopenia is measured by muscle strength, muscle quantity or mass and physical performance. Currently, the importance and urgency of sarcopenia have grown. The application of blood flow restriction (BFR) training has received increased attention in managing sarcopenia. BFR is accomplished using a pneumatic cuff on the proximal aspect of the exercising limb. Two main methods of exercise, aerobic exercise and resistance exercise, have been applied with BFR in treating sarcopenia. Both methods can increase muscle mass and muscle strength to a certain extent. Intricate mechanisms are involved during BFRT. Currently, the presented mechanisms mainly include responses in the blood vessels and related hormones, such as growth factors, tissue hypoxia-related factors and recruitment of muscle fiber as well as muscle satellite cells. These mechanisms contribute to the positive balance of skeletal muscle synthesis, which in turn mitigates sarcopenia. As a more suited and more effective way of treating sarcopenia and its comorbidities, BFRT can serve as an alternative to traditional exercise for people who have marked physical limitations or even show superior outcomes under low loads. However, the possibility of causing stress or muscle damage must be considered. Cuff size, pressure, training load and other variables can affect the outcome of sarcopenia, which must also be considered. Thoroughly studying these factors can help to better determine an ideal BFRT scheme and better manage sarcopenia and its associated comorbidities. As a well-tolerated and novel form of exercise, BFRT offers more potential in treating sarcopenia and involves deeper insights into the function and regulation of skeletal muscle.

The role and therapeutic potential of stem cells in skeletal muscle in sarcopenia

Sarcopenia is a common age-related skeletal muscle disorder featuring the loss of muscle mass and function. In regard to tissue repair in the human body, scientists always consider the use of stem cells. In skeletal muscle, satellite cells (SCs) are adult stem cells that maintain tissue homeostasis and repair damaged regions after injury to preserve skeletal muscle integrity. Muscle-derived stem cells (MDSCs) and SCs are the two most commonly studied stem cell populations from skeletal muscle. To date, considerable progress has been achieved in understanding the complex associations between stem cells in muscle and the occurrence and treatment of sarcopenia. In this review, we first give brief introductions to sarcopenia, SCs and MDSCs. Then, we attempt to untangle the differences and connections between these two types of stem cells and further elaborate on the interactions between sarcopenia and stem cells. Finally, our perspectives on the possible application of stem cells for the treatment of sarcopenia in future are presented. Several studies emerging in recent years have shown that changes in the number and function of stem cells can trigger sarcopenia, which in turn leads to adverse influences on stem cells because of the altered internal environment in muscle. A better understanding of the role of stem cells in muscle, especially SCs and MDSCs, in sarcopenia will facilitate the realization of novel therapy approaches based on stem cells to combat sarcopenia.

Stem cell aging in the skeletal muscle: The importance of communication

Adult stem cells sustain tissue homeostasis and regeneration; their functional decline is often linked to aging, which is characterized by the progressive loss of physiological functions across multiple tissues and organs. The resident stem cells in skeletal muscle, termed satellite cells, are normally quiescent but activate upon injury to reconstitute the damaged tissue. In this review, we discuss the current understanding of the molecular processes that contribute to the functional failure of satellite cells during aging. This failure is due not only to intrinsic changes but also to extrinsic factors, most of which are still undefined but originate from the muscle tissue microenvironment of the satellite cells (the niche), or from the systemic environment. We also highlight the emerging applications of the powerful single-cell sequencing technologies in the study of skeletal muscle aging, particularly in the heterogeneity of the satellite cell population and the molecular interaction of satellite cells and other cell types in the niche. An improved understanding of how satellite cells communicate with their environment, and how this communication is perturbed with aging, will be helpful for defining countermeasures against loss of muscle regenerative capacity in sarcopenia.

Exercise as a Peripheral Circadian Clock Resynchronizer in Vascular and Skeletal Muscle Aging

Aging is characterized by several progressive physiological changes, including changes in the circadian rhythm. Circadian rhythms influence behavior, physiology, and metabolic processes in order to maintain homeostasis; they also influence the function of endothelial cells, smooth muscle cells, and immune cells in the vessel wall. A clock misalignment could favor vascular damage and indirectly also affect skeletal muscle function. In this review, we focus on the dysregulation of circadian rhythm due to aging and its relationship with skeletal muscle changes and vascular health as possible risk factors for the development of sarcopenia, as well as the role of physical exercise as a potential modulator of these processes.

Reversal of deficits in aged skeletal muscle during disuse and recovery in response to treatment with a secrotome product derived from partially differentiated human pluripotent stem cells

Aged individuals are at risk to experience slow and incomplete muscle recovery following periods of disuse atrophy. While several therapies have been employed to mitigate muscle mass loss during disuse and improve recovery, few have proven effective at both. Therefore, the purpose of this study was to examine the effectiveness of a uniquely developed secretome product (STEM) on aged skeletal muscle mass and function during disuse and recovery. Aged (22 months) male C57BL/6 were divided into PBS or STEM treatment (n = 30). Mice within each treatment were assigned to either ambulatory control (CON; 14 days of normal cage ambulation), 14 days of hindlimb unloading (HU), or 14 days of hindlimb unloading followed by 7 days of recovery (recovery). Mice were given an intramuscular delivery into the hindlimb muscle of either PBS or STEM every other day for the duration of their respective treatment group. We found that STEM-treated mice compared to PBS had greater soleus muscle mass, fiber cross-sectional area (CSA), and grip strength during CON and recovery experimental conditions and less muscle atrophy and weakness during HU. Muscle CD68 +, CD11b + and CD163 + macrophages were more abundant in STEM-treated CON mice compared to PBS, while only CD68 + and CD11b + macrophages were more abundant during HU and recovery conditions with STEM treatment. Moreover, STEM-treated mice had lower collagen IV and higher Pax7 + cell content compared to PBS across all experimental conditions. As a follow-up to examine the cell autonomous role of STEM on muscle, C2C12 myotubes were given STEM or horse serum media to examine myotube fusion/size and effects on muscle transcriptional networks. STEM-treated C2C12 myotubes were larger and had a higher fusion index and were related to elevated expression of transcripts associated with extracellular matrix remodeling. Our results demonstrate that STEM is a unique cocktail that possesses potent immunomodulatory and cytoskeletal remodeling properties that may have translational potential to improve skeletal muscle across a variety of conditions that adversely effect aging muscle.

Control of satellite cell function in muscle regeneration and its disruption in ageing

Skeletal muscle contains a designated population of adult stem cells, called satellite cells, which are generally quiescent. In homeostasis, satellite cells proliferate only sporadically and usually by asymmetric cell division to replace myofibres damaged by daily activity and maintain the stem cell pool. However, satellite cells can also be robustly activated upon tissue injury, after which they undergo symmetric divisions to generate new stem cells and numerous proliferating myoblasts that later differentiate to muscle cells (myocytes) to rebuild the muscle fibre, thereby supporting skeletal muscle regeneration. Recent discoveries show that satellite cells have a great degree of population heterogeneity, and that their cell fate choices during the regeneration process are dictated by both intrinsic and extrinsic mechanisms. Extrinsic cues come largely from communication with the numerous distinct stromal cell types in their niche, creating a dynamically interactive microenvironment. This Review discusses the role and regulation of satellite cells in skeletal muscle homeostasis and regeneration. In particular, we highlight the cell-intrinsic control of quiescence versus activation, the importance of satellite cell-niche communication, and deregulation of these mechanisms associated with ageing. The increasing understanding of how satellite cells are regulated will help to advance muscle regeneration and rejuvenation therapies.

Cellular senescence in musculoskeletal homeostasis, diseases, and regeneration

Emerging insights into cellular senescence highlight the relevance of senescence in musculoskeletal disorders, which represent the leading global cause of disability. Cellular senescence was initially described by Hayflick et al. in 1961 as an irreversible nondividing state in in vitro cell culture studies. We now know that cellular senescence can occur in vivo in response to various stressors as a heterogeneous and tissue-specific cell state with a secretome phenotype acquired after the initial growth arrest. In the past two decades, compelling evidence from preclinical models and human data show an accumulation of senescent cells in many components of the musculoskeletal system. Cellular senescence is therefore a defining feature of age-related musculoskeletal disorders, and targeted elimination of these cells has emerged recently as a promising therapeutic approach to ameliorate tissue damage and promote repair and regeneration of the skeleton and skeletal muscles. In this review, we summarize evidence of the role of senescent cells in the maintenance of bone homeostasis during childhood and their contribution to the pathogenesis of chronic musculoskeletal disorders, including osteoporosis, osteoarthritis, and sarcopenia. We highlight the diversity of the senescent cells in the microenvironment of bone, joint, and skeletal muscle tissue, as well as the mechanisms by which these senescent cells are involved in musculoskeletal diseases. In addition, we discuss how identifying and targeting senescent cells might positively affect pathologic progression and musculoskeletal system regeneration.

Disrupted macrophage metabolic reprogramming in aged soleus muscle during early recovery following disuse atrophy

Aged skeletal muscle is characterized by poor muscle recovery following disuse coinciding with an impaired muscle pro-inflammatory macrophage response. Macrophage inflammatory status is regulated by its metabolic state, but little is understood of macrophage metabolism and its relation to macrophage inflammation in the context of muscle recovery and aging. Therefore, the purpose of this study was to thoroughly characterize macrophage metabolism and inflammation in aged muscle during early recovery following disuse atrophy using single cell transcriptomics and functional assays. Young (4-5 months) and old (20-22 months) male C57BL/6 mice underwent 14 days of hindlimb unloading followed by 4 days of ambulatory recovery. CD45+ cells were isolated from solei muscles and analyzed using 10x Genomics single cell RNA sequencing. We found that aged pro-inflammatory macrophage clusters were characterized with an impaired inflammatory and glycolytic transcriptome, and this dysregulation was accompanied by a suppression of HIF-1α and its immediate downstream target, Glut1. As a follow-up, bone marrow-derived macrophages were isolated from a separate cohort of young and old mice at 4-d recovery and were polarized to a pro-inflammatory phenotype and used for glycolysis stress test, phagocytosis activity assay, and targeted GC-MS metabolomics. Aged bone marrow-derived pro-inflammatory macrophages were characterized with impaired glycolysis and phagocytosis function, decreased succinate and an accumulation of glycolytic metabolic intermediates overall supporting reduced glycolytic flux and macrophage function. Our results indicate that the metabolic reprograming and function of aged skeletal muscle pro-inflammatory macrophages are dysfunctional during early recovery from disuse atrophy possibly attributing to attenuated regrowth.

The Effects of Lifestyle and Diet on Gut Microbiota Composition, Inflammation and Muscle Performance in Our Aging Society

Living longer is associated with an increased risk of chronic diseases, including impairments of the musculoskeletal and immune system as well as metabolic disorders and certain cancers, each of which can negatively affect the relationship between host and microbiota up to the occurrence of dysbiosis. On the other hand, lifestyle factors, including regular physical exercise and a healthy diet, can affect skeletal muscle and immune aging positively at all ages. Accordingly, health benefits could partly depend on the effect of such interventions that influence the biodiversity and functionality of intestinal microbiota. In the present review, we first discuss the physiological effects of aging on the gut microbiota, immune system, and skeletal muscle. Secondly, we describe human epidemiological evidence about the associations between physical activity and fitness and the gut microbiota composition in older adults. The third part highlights the relevance and restorative mechanisms of immune protection through physical activity and specific exercise interventions during aging. Fourth, we present important research findings on the effects of exercise and protein as well as other nutrients on skeletal muscle performance in older adults. Finally, we provide nutritional recommendations to prevent malnutrition and support healthy active aging with a focus on gut microbiota. Key nutrition-related concerns include the need for adequate energy and protein intake for preventing low muscle mass and a higher demand for specific nutrients (e.g., dietary fiber, polyphenols and polyunsaturated fatty acids) that can modify the composition, diversity, and metabolic capacity of the gut microbiota, and may thus provide a practical means of enhancing gut and systemic immune function.

T lymphocyte depletion ameliorates age-related metabolic impairments in mice

Both glucose tolerance and adaptive immune function exhibit significant age-related alterations. The influence of the immune system on obesity-associated glucose intolerance is well characterized; however, whether the immune system contributes to age-related glucose intolerance is not as well understood. Here, we report that advancing age results in an increase in T cell infiltration in the epididymal white adipose tissue (eWAT), liver, and skeletal muscle. Subtype analyses show that both CD4+, CD8+ T cells are greater with advancing age in each of these tissues and that aging results in a blunted CD4 to CD8 ratio. Anti-CD3 F(ab')2 fragments depleted CD4+ and CD8+ cells in eWAT, CD4+ cells only in the liver, and did not deplete quadriceps T cells. In old mice, T cells producing both interferon-γ and tumor necrosis factor-α are accumulated in the eWAT and liver, and a greater proportion of skeletal muscle T cells produced interferon-γ. Aging resulted in increased proportion and numbers of T regulatory cells in eWAT, but not in the liver or muscle. Aging also resulted in greater numbers of eWAT and quadriceps CD206- macrophages and eWAT, liver and quadriceps B cells; neither cell type was altered by anti-CD3 treatment. Anti-CD3 treatment improved glucose tolerance in old mice and was accompanied by improved signaling related to liver and skeletal muscle insulin utilization and decreased gluconeogenesis-related gene expression in the liver. Our findings indicate a critical role of the adaptive immune system in the age-related metabolic dysfunction.

The biphasic and age-dependent impact of klotho on hallmarks of aging and skeletal muscle function

Aging is accompanied by disrupted information flow, resulting from accumulation of molecular mistakes. These mistakes ultimately give rise to debilitating disorders including skeletal muscle wasting, or sarcopenia. To derive a global metric of growing 'disorderliness' of aging muscle, we employed a statistical physics approach to estimate the state parameter, entropy, as a function of genes associated with hallmarks of aging. Escalating network entropy reached an inflection point at old age, while structural and functional alterations progressed into oldest-old age. To probe the potential for restoration of molecular 'order' and reversal of the sarcopenic phenotype, we systemically overexpressed the longevity protein, Klotho, via AAV. Klotho overexpression modulated genes representing all hallmarks of aging in old and oldest-old mice, but pathway enrichment revealed directions of changes were, for many genes, age-dependent. Functional improvements were also age-dependent. Klotho improved strength in old mice, but failed to induce benefits beyond the entropic tipping point.

Alterations of macrophage and neutrophil content in skeletal muscle of aged versus young mice

BACKGROUND

Skeletal muscle inflammation and oxidative stress are associated with aging-related loss of muscle mass and may be attributable to alterations in the number and types of leukocytes in skeletal muscle. Here, we tested the hypothesis that aging changes the number and composition of leukocyte subsets in skeletal muscle tissue.

METHODS

Skeletal muscle was sampled from 4-mo-old (young) and 27-mo-old (old) C57BL/6J mice. Mononuclear cells of the gastrocnemius muscle were isolated, and flow cytometry was used to characterize the number and types of immune cells.

RESULTS

The number of neutrophils and Ly-6C+ inflammatory macrophages in the skeletal muscle was significantly higher in old mice than in young mice. Inflammation and oxidative stress (measured using the markers phosphorylated JNK and nitrotyrosine) were also higher in the skeletal muscle of old mice than in that of young mice.

CONCLUSIONS

Increasing age promotes skeletal muscle inflammation and oxidative stress, as well as infiltration of inflammatory macrophages and neutrophils.

The Role of Innate and Adaptive Immune Cells in Skeletal Muscle Regeneration

Skeletal muscle regeneration is highly dependent on the inflammatory response. A wide variety of innate and adaptive immune cells orchestrate the complex process of muscle repair. This review provides information about the various types of immune cells and biomolecules that have been shown to mediate muscle regeneration following injury and degenerative diseases. Recently developed cell and drug-based immunomodulatory strategies are highlighted. An improved understanding of the immune response to injured and diseased skeletal muscle will be essential for the development of therapeutic strategies.

Delineating the relationship between immune system aging and myogenesis in muscle repair

Recruited immune cells play a critical role in muscle repair, in part by interacting with local stem cell populations to regulate muscle regeneration. How aging affects their communication during myogenesis is unclear. Here, we investigate how aging impacts the cellular function of these two cell types after muscle injury during normal aging or after immune rejuvenation using a young to old (Y-O) or old to old (O-O) bone marrow (BM) transplant model. We found that skeletal muscle from old mice (20 months) exhibited elevated basal inflammation and possessed fewer satellite cells compared with young mice (3 months). After cardiotoxin muscle injury (CTX), old mice exhibited a blunted inflammatory response compared with young mice and enhanced M2 macrophage recruitment and IL-10 expression. Temporal immune and cytokine responses of old mice were partially restored to a young phenotype following reconstitution with young cells (Y-O chimeras). Improved immune responses in Y-O chimeras were associated with greater satellite cell proliferation compared with O-O chimeras. To identify how immune cell aging affects myoblast function, conditioned media (CM) from activated young or old macrophages was applied to cultured C2C12 myoblasts. CM from young macrophages inhibited myogenesis while CM from old macrophages reduced proliferation. These functional differences coincided with age-related differences in macrophage cytokine expression. Together, this study examines the infiltration and proliferation of immune cells and satellite cells after injury in the context of aging and, using BM chimeras, demonstrates that young immune cells retain cell autonomy in an old host to increase satellite cell proliferation.

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20 years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration.

Physiopathology of Lifestyle Interventions in Non-Alcoholic Fatty Liver Disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is a major health problem, and its prevalence has increased in recent years. Diet and exercise interventions are the first-line treatment options, with weight loss via a hypocaloric diet being the most important therapeutic target in NAFLD. However, most NAFLD patients are not able to achieve such weight loss. Therefore, the requisite is the investigation of other effective therapeutic approaches. This review summarizes research on understanding complex pathophysiology underlying dietary approaches and exercise interventions with the potential to prevent and treat NAFLD.

Neuromuscular electrical stimulation and protein during bed rest increases CD11b+ skeletal muscle macrophages but does not correspond to muscle size or insulin sensitivity

With this cohort, we previously demonstrated preservation of thigh lean tissue with neuromuscular electrical stimulation combined with protein supplementation (NMES+PRO) treatment during bed rest in healthy older adults. Because macrophage polarization plays a significant role in the repair and maintenance of muscle size and insulin sensitivity, we hypothesized that muscle macrophages would be induced by NMES+PRO and would correspond to an increase in lean mass and an attenuated insulin resistance response altered by bed rest. Older adults (60-80 years old; body mass index < 30 kg/m2) underwent 5 days of bed rest and were randomized to either thrice daily treatment of NMES+PRO (n = 8) or CON (n = 8). Lean mass, insulin sensitivity, and markers of muscle macrophages, inflammation, and connective tissue were determined before and after bed rest. Glucose intolerance and insulin resistance occurred after bed rest but there was not a treatment effect (p > 0.10). Proinflammatory-like macrophages (CD11b+, CD206-) increased (p < 0.05) with NMES+PRO treatment and was different than CON. Minor changes in noncontractile tissue were observed. However, changes in muscle macrophages or extracellular matrix were not related to the preservation of thigh lean mass or insulin resistance. Daily NMES+PRO treatment during bed rest induced a muscle proinflammatory-like macrophage response and was unrelated to muscle size or metabolic function. This study is listed as clinical trial NCT02566590. Novelty Neuromuscular electrical stimulation combined with protein supplementation (NMES+PRO) increased proinflammatory-like macrophages and extracellular matrix content in older adults after bed rest. NMES+PRO changes in macrophages and noncontractile tissue macrophages were not related to muscle size preservation or insulin sensitivity.