Myeloid cell-derived tumor necrosis factor-alpha promotes sarcopenia and regulates muscle cell fusion with aging muscle fibers

The Causal Relationship between PCSK9 Inhibitors and Osteoporosis Based on Drug-Targeted Mendelian Combined Mediation Analysis

PCSK9 inhibitors have been shown to lower serum low density lipoprotein cholesterol (LDL-C) levels and are considered integral in the treatment of cardiovascular diseases. However, the potential association between PCSK9 inhibitors and osteoporosis is unclear now. In this study, drug-targeted mendelian randomization (MR) was utilized in conjunction with mediation analysis including bone mineral density (BMD), total 25-hydroxyvitamin D (T25(OH)D) levels and calcium supplementation to investigate the causal relationship between PCSK9 inhibitors and osteoporosis. The LDL-C level was chosen as the exposure variable in a sample size of 173,082 individuals. We conducted a MR analysis on the relationship between PCSK9 inhibitors and osteoporosis, elucidating the mediators involved. Utilizing the inverse variance weighted (IVW) method, we found the risk of osteoporosis was reduced by 0.6% in those who used PCSK9 inhibitors compared with non-users (OR: 0.994, 95%CI: 0.991-0.998, P < 0.001). In people aged 30-45 years, the risk of low BMD was 1.176 times higher among PCSK9 inhibitor users compared to non-users (OR: 1.176, 95%CI: 1.017-1.336, P = 0.045). Conversely, people aged 45-60 years who used PCSK9 inhibitors had a 14.9% lower risk of low BMD compared to non-users (OR: 0.851, 95%CI: 0.732-0.968, P = 0.007). Mediation analysis revealed that 43.33% of the impact of PCSK9 inhibitors on osteoporosis was mediated through BMD levels, with the remaining 56.67% being a direct effect. Effects of PCSK9 inhibitors on BMD levels varied in different ages. In addition, the risk of high serum T25(OH)D levels were 1.091 times among PCSK9 inhibitor users compared to non-users (OR: 1.091, 95%CI: 1.065-1.112, P < 0.001), providing valuable insights for clinicians.

Dynamic equilibrium of skeletal muscle macrophage ontogeny in the diaphragm during homeostasis, injury, and recovery

The diaphragm is a unique skeletal muscle due to its continuous activation pattern during the act of breathing. The ontogeny of macrophages, pivotal cells for skeletal muscle maintenance and regeneration, is primarily based on two distinct origins: postnatal bone marrow-derived monocytes and prenatal embryonic progenitors. Here we employed chimeric mice to study the dynamics of these two macrophage populations under different conditions. Traditional chimeric mice generated through whole body irradiation showed virtually complete elimination of the original tissue-resident macrophage pool. We then developed a novel method which employs lead shielding to protect the diaphragm tissue niche from irradiation. This allowed us to determine that up to almost half of tissue-resident macrophages in the diaphragm can be maintained independently from bone marrow-derived monocytes under steady-state conditions. These findings were confirmed by long-term (5 months) parabiosis experiments. Acute diaphragm injury shifted the macrophage balance toward an overwhelming predominance of bone marrow (monocyte)-derived macrophages. However, there was a remarkable reversion to the pre-injury ontological landscape after diaphragm muscle recovery. This diaphragm shielding method permits analysis of the dynamics of macrophage origin and corresponding function under different physiological and pathological conditions. It may be especially useful for studying diseases which are characterized by acute or chronic injury of the diaphragm and accompanying inflammation.

Sarcopenia is associated with hypomethylation of TWEAK and increased plasma levels of TWEAK and its downstream inflammatory factor TNF-α in older adults: A case-control study

BACKGROUND

Sarcopenia is a harmful condition common among older adults for which no treatment is available. Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and its receptor fibroblast growth factor inducible 14 (FN14) are known to play important roles in the pathogenesis of sarcopenia. This study investigated alterations in methylation in TWEAK and Fn14 to identify potential targets for the managing sarcopenia.

MATERIALS AND METHODS

Through an epidemiological investigation, we detected methylation of CpG islands (CpGs) in TWEAK and Fn14 in community-dwelling older adult of Xinjiang by bisulfite sequencing. Significant CpGs associated with sarcopenia were selected for detection in 152 older individuals by pyrosequencing. Associations between CpG methylation, plasma inflammatory marker levels, and sarcopenia were analyzed.

RESULTS

Of 38 CpGs in TWEAK and 30 CpGs in Fn14 detected in 60 individuals, 6 CpGs showed lower methylation in sarcopenia patients compared with control individuals. In 152 older adults, covariance analysis with adjustment for age, gender, triglyceride level, obesity, diabetes, and hypertension showed that the methylation levels of 6 CpGs (CpG8, CpG12, CpG13, CpG20 and CpG21of TWEAK, and CpG24 of Fn14) were significantly lower in sarcopenia patients than in control individuals. With adjustment for additional confounding factors, covariate variance analysis showed that plasma TWEAK, TNF-α and IL-10 levels in the sarcopenia group were significant higher than those in the control group (P = 0.007, P < 0.001, P = 0.003). Multivariate logistic regression analysis showed that CpG8, CpG13, CpG21, and total methylation of TWEAK (OR = 0.767, 95 % CI = 0.622-0.947; OR = 0.740, 95 % CI = 0.583-0.941; OR = 0.734, 95 % CI = 0.561-0.958; OR = 0.883, 95 % CI = 0.795-0.980) as well as CpG22 and total methylation of Fn14 were significantly associated with sarcopenia (OR = 826, 95 % CI = 0.704-0.968; OR = 0.918, 95 % CI = 0.852-0.989). From partial correlation analysis, plasma TWEAK was correlated with plasma TNF-α (r = 0.172, P = 0.042).

CONCLUSION

Sarcopenia is associated with hypomethylation of TWEAK and increased plasma levels of TWEAK and its downstream inflammatory factor TNF-α in a community-dwelling population of older adults in Xinjiang.

Sarcopenia in a type 2 diabetic state: Reviewing literature on the pathological consequences of oxidative stress and inflammation beyond the neutralizing effect of intracellular antioxidants

Sarcopenia remains one of the major pathological features of type 2 diabetes (T2D), especially in older individuals. This condition describes gradual loss of muscle mass, strength, and function that reduces the overall vitality and fitness, leading to increased hospitalizations and even fatalities to those affected. Preclinical evidence indicates that dysregulated mitochondrial dynamics, together with impaired activity of the NADPH oxidase system, are the major sources of oxidative stress that drive skeletal muscle damage in T2D. While patients with T2D also display relatively higher levels of circulating inflammatory markers in the serum, including high sensitivity-C-reactive protein, interleukin-6, and tumor necrosis factor-α that are independently linked with the deterioration of muscle function and sarcopenia in T2D. In fact, beyond reporting on the pathological consequences of both oxidative stress and inflammation, the current review highlights the importance of strengthening intracellular antioxidant systems to preserve muscle mass, strength, and function in individuals with T2D.

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.

The relationship between sarcopenia and serum irisin and TNF-α levels in newly diagnosed cancer patients

OBJECTIVE

Sarcopenia in patients with cancer makes patients physically weak and adversely affects their compliance with treatment. In this study, we investigated the relationship between sarcopenia in patients with cancer and circulating irisin and tumor necrosis factor-alpha (TNF-α) levels.

MATERIAL AND METHOD

A total of 141 patients with different types of newly diagnosed cancer were divided into two groups, sarcopenia (n = 72) and non-sarcopenia (n = 69) groups. The body compositions of the patients were measured using bioelectrical impedance (BIA) and muscle strength using hand grip strength (HGS) tests. Serum irisin and TNF-α levels were measured using an enzyme-linked immunosorbent assay.

RESULTS

In our study, serum irisin levels were found to be significantly lower (p < 0.01) and TNF-α levels were found to be significantly higher (p = 0.014) in the sarcopenia group. Skeletal muscle index (SMI) and HGS values and serum irisin levels were positively correlated [(r: 0.451, p < 0.001), (r: 0.469, p < 0.001)], and SMI and HGS values and serum TNF-α levels were negatively correlated [(r: -0.181, p = 0.032) and (r: -0.143, p = 0.090), respectively]. In addition, multiple linear regression analysis showed that serum irisin and TNF-α levels were independent predictors of sarcopenia.

CONCLUSION

Serum irisin levels were found to be significantly lower in patients with cancer with sarcopenia, and TNF-α levels were found to be significantly higher. These two markers can be used as potential biomarkers for the diagnosis of sarcopenia in patients with cancer. The efficacy and possible mechanisms of action of irisin and TNF-α in the diagnosis of sarcopenia should be investigated with larger patient groups.

Are inflammatory markers associated with sarcopenia-related traits in older adults with sarcopenia? - A cross-sectional analysis of the ENHANce study

AIMS

To explore the relationship between inflammatory markers and sarcopenia-related traits in sarcopenic older adults.

METHODS

Baseline data of the ongoing Exercise and Nutrition for Healthy AgeiNg (ENHANce) study were used for a secondary, exploratory, cross-sectional analysis. ENHANce is a 5-armed triple blinded randomized controlled trial, in older adults (>65y) with sarcopenia defined according to the revised criteria of the European Working Group of Sarcopenia in Older People (EWGSOP2) aiming to assess the effect of combined anabolic interventions (protein supplement, omega-3 supplement and physical exercise) on physical performance, compared to single/placebo interventions. Inflammatory markers C-reactive protein (hs-CRP), albumin, interleukin-1β (IL-1β), IL-6, IL-8, and tumour necrosis factor-α (TNF-α) were assessed at baseline. Spearman's rho (ρ) correlation coefficients were calculated to associate these inflammatory markers with baseline sarcopenia-defining parameters (handgrip strength, chair stand test, appendicular lean mass [aLM], gait speed, Short Physical Performance Battery), physical activity (step count) and quality of life (SF-36, SarQoL).

RESULTS

We included 40 sarcopenic subjects (15 men/25 women, age 77.1 ± 6.8 years). Contrary to expectations, the pro-inflammatory IL-1β correlated positively with handgrip strength (ρ: 0.376; p = 0.024) and IL-6 with aLM (ρ: 0.334; p = 0.0433). IL-6 inversely correlated with step count (ρ:-0.358; p = 0.048). Subgroup analysis revealed important gender differences. IL-8 inversely correlated with handgrip strength in women (ρ: -0.425; p = 0.034) but not in men. In contrast, pro-inflammatory cytokines CRP (ρ: -0.615; p = 0.019), IL-6 (ρ: -0.604; p = 0.029) and TNF-α (ρ: -0.615; p = 0.025) inversely correlated with the SF-36 physical component score in men but not in women.

CONCLUSION

Although Inflammageing might play a role in sarcopenia-related traits, this exploratory study highlights an important role of gender. Future research should take this into account when elucidating the Inflammageing-sarcopenia interplay.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

m6A epitranscriptomic regulation of tissue homeostasis during primate aging

How N⁶-methyladenosine (m⁶A), the most abundant mRNA modification, contributes to primate tissue homeostasis and physiological aging remains elusive. Here, we characterize the m⁶A epitranscriptome across the liver, heart and skeletal muscle in young and old nonhuman primates. Our data reveal a positive correlation between m⁶A modifications and gene expression homeostasis across tissues as well as tissue-type-specific aging-associated m⁶A dynamics. Among these tissues, skeletal muscle is the most susceptible to m⁶A loss in aging and shows a reduction in the m⁶A methyltransferase METTL3. We further show that METTL3 deficiency in human pluripotent stem cell-derived myotubes leads to senescence and apoptosis, and identify NPNT as a key element downstream of METTL3 involved in myotube homeostasis, whose expression and m⁶A levels are both decreased in senescent myotubes. Our study provides a resource for elucidating m⁶A-mediated mechanisms of tissue aging and reveals a METTL3-m⁶A-NPNT axis counteracting aging-associated skeletal muscle degeneration.

Immunosenescence, Inflammaging, and Frailty: Role of Myeloid Cells in Age-Related Diseases

The immune system is the central regulator of tissue homeostasis, ensuring tissue regeneration and protection against both pathogens and the neoformation of cancer cells. Its proper functioning requires homeostatic properties, which are maintained by an adequate balance of myeloid and lymphoid responses. Aging progressively undermines this ability and compromises the correct activation of immune responses, as well as the resolution of the inflammatory response. A subclinical syndrome of "homeostatic frailty" appears as a distinctive trait of the elderly, which predisposes to immune debilitation and chronic low-grade inflammation (inflammaging), causing the uncontrolled development of chronic and degenerative diseases. The innate immune compartment, in particular, undergoes to a sequela of age-dependent functional alterations, encompassing steps of myeloid progenitor differentiation and altered responses to endogenous and exogenous threats. Here, we will review the age-dependent evolution of myeloid populations, as well as their impact on frailty and diseases of the elderly.

Main Pathogenic Mechanisms and Recent Advances in COPD Peripheral Skeletal Muscle Wasting

Chronic obstructive pulmonary disease (COPD) is a worldwide prevalent respiratory disease mainly caused by tobacco smoke exposure. COPD is now considered as a systemic disease with several comorbidities. Among them, skeletal muscle dysfunction affects around 20% of COPD patients and is associated with higher morbidity and mortality. Although the histological alterations are well characterized, including myofiber atrophy, a decreased proportion of slow-twitch myofibers, and a decreased capillarization and oxidative phosphorylation capacity, the molecular basis for muscle atrophy is complex and remains partly unknown. Major difficulties lie in patient heterogeneity, accessing patients' samples, and complex multifactorial process including extrinsic mechanisms, such as tobacco smoke or disuse, and intrinsic mechanisms, such as oxidative stress, hypoxia, or systemic inflammation. Muscle wasting is also a highly dynamic process whose investigation is hampered by the differential protein regulation according to the stage of atrophy. In this review, we report and discuss recent data regarding the molecular alterations in COPD leading to impaired muscle mass, including inflammation, hypoxia and hypercapnia, mitochondrial dysfunction, diverse metabolic changes such as oxidative and nitrosative stress and genetic and epigenetic modifications, all leading to an impaired anabolic/catabolic balance in the myocyte. We recapitulate data concerning skeletal muscle dysfunction obtained in the different rodent models of COPD. Finally, we propose several pathways that should be investigated in COPD skeletal muscle dysfunction in the future.

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.

Mechanisms of skeletal muscle-tendon development and regeneration/healing as potential therapeutic targets

Skeletal muscle contraction is essential for the movement of our musculoskeletal system. Tendons and ligaments that connect the skeletal muscles to bones in the correct position at the appropriate time during development are also required for movement to occur. Since the musculoskeletal system is essential for maintaining basic bodily functions as well as enabling interactions with the environment, dysfunctions of these tissues due to disease can significantly reduce quality of life. Unfortunately, as people live longer, skeletal muscle and tendon/ligament diseases are becoming more common. Sarcopenia, a disease in which skeletal muscle function declines, and tendinopathy, which involves chronic tendon dysfunction, are particularly troublesome because there have been no significant advances in their treatment. In this review, we will summarize previous reports on the development and regeneration/healing of skeletal muscle and tendon tissues, including a discussion of the molecular and cellular mechanisms involved that may be used as potential therapeutic targets.

TNF-α contributes to sarcopenia through caspase-8/caspase-3/GSDME-mediated pyroptosis

Sarcopenia has become a leading cause of disability and mortality in the elderly. It has been reported that programmed cell death (PCD) is associated with the development of sarcopenia that is characterized by reduction of muscle fiber size and number. TNF-α is also validated to play a prominent role in sarcopenia through its complex signaling pathways including cell death signaling. However, it is still unclear whether TNF-α contributes to sarcopenia by mediating pyroptosis, one type of PCD. Here, we first established naturally aged mice with sarcopenia model and confirmed an inflammatory state represented by TNF-α in aged mice. Evidence of GSDME-mediated pyroptosis and activation of apoptotic caspase-8/-3 were also found in skeletal muscle cells of aged mice with sarcopenia. We demonstrated that TNF-α triggered GSDME-mediated pyroptosis in myotubes through activating caspase-8 and caspase-3 by using caspase-8 and caspase-3 inhibitors. Comparing the activation of caspase-8 and GSDME expression between TNF Complex IIa and TNF Complex IIb, TNF-α was found to be more inclined to assemble TNF Complex IIb in activating caspase-8 and triggering pyroptosis. Moreover, pyroptotic myotubes were validated to result in decreased expression of MHC1 and finally loss of myotubes by knockdown of GSDME. Our work reveals a novel mechanism that TNF-ɑ/caspase-8/caspase-3/GSDME signaling-mediated pyroptosis contributes to the development of sarcopenia. Caspase-3/GSDME signaling-mediated pyroptosis may be a promising therapeutic target for sarcopenia.

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.

Contribution of skeletal muscle to cancer immunotherapy: A focus on muscle function, inflammation, and microbiota

Sarcopenia, characterized by degenerative and systemic loss of skeletal muscle mass and function, is a multifactorial syndrome commonly observed in individuals with cancer. Additionally, it represents a poor nutritional status and indicates possible presence of cancer cachexia. Recently, with the extensive application of cancer immunotherapy, the effects of sarcopenia/cachexia on cancer immunotherapy, have gained attention. The aim of this review was to summarize the influence of low muscle mass (sarcopenia/cachexia) on the response and immune-related adverse events to immunotherapy from the latest literature. It was revealed that low muscle mass (sarcopenia/cachexia) has detrimental effects on cancer immunotherapy in most cases, although there were results that were not consistent with this finding. This review also discussed potential causes of the paradox, such as different measure methods, research types, muscle indicators, time point, and cancer type. Mechanically, chronic inflammation, immune cells, and microbiota may be critically involved in regulating the efficacy of immunotherapy under the condition of low muscle mass (sarcopenia/cachexia). Thus, nutritional interventions will likely be promising ways for individuals with cancer to increase the efficacy of immunotherapy in the future, for low muscle mass (sarcopenia/cachexia) is an important prognostic factor for cancer immunotherapy.

Aging and aging-related diseases: from molecular mechanisms to interventions and treatments

Aging is a gradual and irreversible pathophysiological process. It presents with declines in tissue and cell functions and significant increases in the risks of various aging-related diseases, including neurodegenerative diseases, cardiovascular diseases, metabolic diseases, musculoskeletal diseases, and immune system diseases. Although the development of modern medicine has promoted human health and greatly extended life expectancy, with the aging of society, a variety of chronic diseases have gradually become the most important causes of disability and death in elderly individuals. Current research on aging focuses on elucidating how various endogenous and exogenous stresses (such as genomic instability, telomere dysfunction, epigenetic alterations, loss of proteostasis, compromise of autophagy, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing) participate in the regulation of aging. Furthermore, thorough research on the pathogenesis of aging to identify interventions that promote health and longevity (such as caloric restriction, microbiota transplantation, and nutritional intervention) and clinical treatment methods for aging-related diseases (depletion of senescent cells, stem cell therapy, antioxidative and anti-inflammatory treatments, and hormone replacement therapy) could decrease the incidence and development of aging-related diseases and in turn promote healthy aging and longevity.

The landscape of aging

Aging is characterized by a progressive deterioration of physiological integrity, leading to impaired functional ability and ultimately increased susceptibility to death. It is a major risk factor for chronic human diseases, including cardiovascular disease, diabetes, neurological degeneration, and cancer. Therefore, the growing emphasis on "healthy aging" raises a series of important questions in life and social sciences. In recent years, there has been unprecedented progress in aging research, particularly the discovery that the rate of aging is at least partly controlled by evolutionarily conserved genetic pathways and biological processes. In an attempt to bring full-fledged understanding to both the aging process and age-associated diseases, we review the descriptive, conceptual, and interventive aspects of the landscape of aging composed of a number of layers at the cellular, tissue, organ, organ system, and organismal levels.

Immunometabolism of macrophages regulates skeletal muscle regeneration

Sarcopenia is an age-related progressive loss of skeletal muscle mass, quality, and strength disease. In addition, sarcopenia is tightly correlated with age-associated pathologies, such as sarcopenic obesity and osteoporosis. Further understanding of disease mechanisms and the therapeutic strategies in muscle regeneration requires a deeper knowledge of the interaction of skeletal muscle and other cells in the muscle tissue. Skeletal muscle regeneration is a complex process that requires a series of highly coordinated events involving communication between muscle stem cells and niche cells, such as muscle fibro/adipogenic progenitors and macrophages. Macrophages play a critical role in tissue regeneration and the maintenance of muscle homeostasis by producing growth factors and cytokines that regulate muscle stem cells and myofibroblast activation. Furthermore, the aging-related immune dysregulation associated with the release of trophic factors and the polarization in macrophages transiently affect the inflammatory phase and impair muscle regeneration. In this review, we focus on the role and regulation of macrophages in skeletal muscle regeneration and homeostasis. The aim of this review is to highlight the important roles of macrophages as a therapeutic target in age-related sarcopenia and the increasing understanding of how macrophages are regulated will help to advance skeletal muscle regeneration.

Extra Virgin Olive Oil (EVOO), a Mediterranean Diet Component, in the Management of Muscle Mass and Function Preservation

Aging results in a progressive decline in skeletal muscle mass, strength and function, a condition known as sarcopenia. This pathological condition is due to multifactorial processes including physical inactivity, inflammation, oxidative stress, hormonal changes, and nutritional intake. Physical therapy remains the standard approach to treat sarcopenia, although some interventions based on dietary supplementation are in clinical development. In this context, thanks to its known anti-inflammatory and antioxidative properties, there is great interest in using extra virgin olive oil (EVOO) supplementation to promote muscle mass and health in sarcopenic patients. To date, the molecular mechanisms responsible for the pathological changes associated with sarcopenia remain undefined; however, a complete understanding of the signaling pathways that regulate skeletal muscle protein synthesis and their behavior during sarcopenia appears vital for defining how EVOO might attenuate muscle wasting during aging. This review highlights the main molecular players that control skeletal muscle mass, with particular regard to sarcopenia, and discusses, based on the more recent findings, the potential of EVOO in delaying/preventing loss of muscle mass and function, with the aim of stimulating further research to assess dietary supplementation with EVOO as an approach to prevent or delay sarcopenia in aging individuals.

Inflammaging: The ground for sarcopenia?

Sarcopenia is a progressive skeletal muscle disease that occurs most commonly in the elderly population, contributing to increased costs and hospitalization. Exercise and nutritional therapy have been proven to be effective for sarcopenia, and some drugs can also alleviate declines in muscle mass and function due to sarcopenia. However, there is no specific pharmacological treatment for sarcopenia at present. This review will mainly discuss the relationship between inflammaging and sarcopenia. The increased secretion of proinflammatory cytokines with aging may be because of cellular senescence, immunosenescence, alterations in adipose tissue, damage-associated molecular patterns (DAMPs), and gut microbes due to aging. These sources of inflammaging can impact the sarcopenia process through direct or indirect pathways. Conversely, sarcopenia can also aggravate the process of inflammaging, creating a vicious cycle. Targeting sources of inflammaging can influence muscle function, which could be considered a therapeutic target for sarcopenia. Moreover, not only proinflammatory cytokines but also anti-inflammatory cytokines can influence muscle and inflammation and participate in the progression of sarcopenia. This review focuses on the effects of TNF-α, IL-6, and IL-10, which can be detected in plasma. Therefore, clearing chronic inflammation by targeting proinflammatory cytokines (TNF-α, IL-1, IL-6) and the inflammatory pathway (JAK/STAT, autophagy, NF-κB) may be effective in treating sarcopenia.

Resistin as a Systemic Inflammation-Related Biomarker for Sarcopenia in Patients With Chronic Obstructive Pulmonary Disease

Background

Sarcopenia is common in patients with chronic obstructive pulmonary disease (COPD) and is mainly caused by systemic inflammation. Resistin acts as a proinflammatory cytokine and is involved in the activation of multiple inflammatory signaling pathways. The aim of this study was to determine the relationship between resistin levels and systemic inflammation and to assess the clinical value of circulating resistin for sarcopenia in patients with COPD.

Methods

In this prospective observational study, we enrolled 235 patients with COPD who were divided into development and validation sets. The definition of sarcopenia followed the guidelines from the Asian Working Group for Sarcopenia. Serum concentrations of resistin and TNF-α were measured using an enzyme-linked immunosorbent assay (ELISA).

Results

In this study, higher serum resistin levels were significantly associated with lower skeletal muscle mass and muscular strength. The serum resistin levels in patients with sarcopenia were significantly higher than those in patients without sarcopenia. The serum resistin level had positive correlations with the serum TNF-α level (r = 0.250, p = 0.007). The predictive efficacy of the serum resistin level (AUC: 0.828) for sarcopenia was superior to that of the serum TNF-α level (AUC: 0.621). The cutoff point (7.138 ng/ml) for the serum resistin level was validated in the validation set (AUC: 0.818).

Conclusions

Serum resistin levels were associated with systemic inflammation and can be used accurately and easily to predict sarcopenia in patients with COPD.

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.

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.

Cellular and molecular modulation of rotator cuff muscle pathophysiology

Rotator cuff (RC) tendon tears are common shoulder injuries that result in irreversible and persistent degeneration of the associated muscles, which is characterized by severe inflammation, atrophy, fibrosis, and fatty infiltration. Although RC muscle degeneration strongly dictates the overall clinical outcomes, strategies to stimulate RC muscle regeneration have largely been overlooked to date. In this review, we highlight the current understanding of the cellular processes that coordinate muscle regeneration, and the roles of muscle resident cells, including immune cells, fibroadipogenic progenitors, and muscle satellite cells in the pathophysiologic regulation of RC muscles following injury. This review also provides perspectives for potential therapies to alleviate the hallmarks of RC muscle degeneration to address current limitations in postsurgical recovery.

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.

Inflammation and sarcopenia: A focus on circulating inflammatory cytokines

Sarcopenia is an aged-related syndrome that is progressive and can be accelerated by other concomitant disease states. Sarcopenia, characterized by loss of skeletal muscle mass, reduced muscle strength, and/or reduced physical performance, is one of the main reasons for limitation of daily activities in the elderly. It is associated with an increased incidence of many adverse events, such as dysfunction, falls, weakness, hospitalization, disability and even death. Sarcopenia justifies one of the most widely accepted theories that low-grade chronic inflammation associated with aging, known as inflammatory aging, is important to the pathogenesis of many age-related diseases. Currently, the diagnosis of sarcopenia is based on a comprehensive assessment of three aspects: muscle mass, muscle strength and physical performance. The measurement of muscle mass is complicated, as the measurement of muscle strength and gait speed is easily affected by the physical conditions of the subjects. This makes the measurements inaccurate and prospective, and it is difficult to achieve continuous, purposeful monitoring. In addition, serum levels of inflammatory cytokines change as inflammatory states develop in the elderly population. This manuscript focuses on the correlation between serum inflammatory cytokines and sarcopenia in recent years, plus the possible underlying mechanisms.

Interactive effect of sarcopenia and falls on vertebral osteoporotic fracture in patients with rheumatoid arthritis

Patients with rheumatoid arthritis (RA) had higher incidences of sarcopenia, falls, osteoporosis, and vertebral osteoporotic fractures (VOPF). Sarcopenia was associated with longer disease duration, higher disease activity, and more severe RA. The interactive effect of sarcopenia and falls was associated with a higher risk of VOPF in patients with RA.

PURPOSE

Whether sarcopenia and falls are a risk factor for vertebral fracture in RA patients has not been demonstrated. This study aimed to explore the incidence of vertebral osteoporotic fracture (VOPF) and its relationship with sarcopenia and falls in RA patients.

METHODS

A total of 474 RA patients and 156 controls were enrolled in this study. Anteroposterior and lateral X-ray examinations of the vertebral column (T4-L4) were used for the semiquantitative assessment of VOPF. Bone mineral density was measured by dual-energy X-ray absorptiometry. Skeletal muscle mass was measured by direct segmental multifrequency bioelectrical impedance analysis (DSM-BIA method).

RESULTS

RA patients had an increased risk of sarcopenia (62.4% vs 9.0%, x² = 47.478, P < 0.001), falls (30.2% vs 3.2%), osteoporosis (OP) (33.5% vs 12.8%, x² = 134.276, P < 0.001), and VOPF (20.3% vs 3.8%, x² = 47.478, P < 0.001) than controls. Patients with sarcopenia were more likely to have VOPF than RA without sarcopenia (24.0% vs 14.0%, x² = 6.802, P = 0.009). RA with sarcopenia and prior falls had the highest incidences of VOPF (36.7%). Older age (OR = 1.056, P < 0.001, 95% CI 1.030-1.083), falls (OR = 2.043, P = 0.003, 95% CI 1.238-3.371), OP (OR = 1.819, P = 0.034, 95% CI 1.046-3.163), and usage of glucocorticoids (GCs) (OR = 1.862, P = 0.022, 95% CI 1.093-3.172) were risk factors for VOPF in RA patients, while a higher skeletal muscle index (SMI) was a protective factor (OR = 0.754, P = 0.038, 95% CI 0.578-0.984) for VOPF in RA patients.

CONCLUSIONS

The interactive effect of sarcopenia and falls is associated with a higher risk of VOPF in patients with RA.

Role of Protein Arginine Methyltransferases and Inflammation in Muscle Pathophysiology

Arginine methylation mediated by protein arginine methyltransferases (PRMTs) is a post-translational modification of both histone and non-histone substrates related to diverse biological processes. PRMTs appear to be critical regulators in skeletal muscle physiology, including regeneration, metabolic homeostasis, and plasticity. Chronic inflammation is commonly associated with the decline of skeletal muscle mass and strength related to aging or chronic diseases, defined as sarcopenia. In turn, declined skeletal muscle mass and strength can exacerbate chronic inflammation. Thus, understanding the molecular regulatory pathway underlying the crosstalk between skeletal muscle function and inflammation might be essential for the intervention of muscle pathophysiology. In this review, we will address the current knowledge on the role of PRMTs in skeletal muscle physiology and pathophysiology with a specific emphasis on its relationship with inflammation.

3D-bioengineered model of human skeletal muscle tissue with phenotypic features of aging for drug testing purposes

Three-dimensional engineering of skeletal muscle is becoming increasingly relevant for tissue engineering, disease modeling and bio-hybrid robotics, where flexible, versatile and multidisciplinary approaches for the evaluation of tissue differentiation, functionality and force measurement are required. This works presents a 3D-printed platform of bioengineered human skeletal muscle which can efficiently model the three-dimensional structure of native tissue, while providing information about force generation and contraction profiles. Proper differentiation and maturation of myocytes is demonstrated by the expression of key myo-proteins using immunocytochemistry and analyzed by confocal microscopy, and the functionality assessed via electrical stimulation and analysis of contraction kinetics. To validate the flexibility of this platform for complex tissue modeling, the bioengineered muscle is treated with tumor necrosis factorαto mimic the conditions of aging, which is supported by morphological and functional changes. Moreover, as a proof of concept, the effects of Argireline® Amplified peptide, a cosmetic ingredient that causes muscle relaxation, are evaluated in both healthy and aged tissue models. Therefore, the results demonstrate that this 3D-bioengineered human muscle platform could be used to assess morphological and functional changes in the aging process of muscular tissue with potential applications in biomedicine, cosmetics and bio-hybrid robotics.

Ageing of the gut microbiome: Potential influences on immune senescence and inflammageing

Advancing age is accompanied by changes in the gut microbiota characterised by a loss of beneficial commensal microbes that is driven by intrinsic and extrinsic factors such as diet, medications, sedentary behaviour and chronic health conditions. Concurrently, ageing is accompanied by an impaired ability to mount a robust immune response, termed immunesenescence, and age-associated inflammation, termed inflammaging. The microbiome has been proposed to impact the immune system and is a potential determinant of healthy aging. In this review we summarise the knowledge on the impact of ageing on microbial dysbiosis, intestinal permeability, inflammaging, and the immune system and investigate whether dysbiosis of the gut microbiota could be a potential mechanism underlying the decline in immune function, overall health and longevity with advancing age. Furthermore, we examine the potential of altering the gut microbiome composition as a novel intervention strategy to reverse the immune ageing clock and possibly support overall good health during old age.

Aging-related genes are potential prognostic biomarkers for patients with gliomas

Aging has a significant role in the proliferation and development of cancers. This study explored the expression profiles, prognostic value, and potential roles of aging-related genes in gliomas. We designed risk score and cluster models based on aging-related genes and glioma cases using LASSO Cox regression analysis, consensus clustering analysis and univariate cox regression analyses. High risk score was related to malignant clinical features and poor prognosis based on 10 datasets, 2953 cases altogether. Genetic alterations analysis revealed that high risk scores were associated with genomic aberrations of aging-related oncogenes. GSVA analysis exhibited the potential function of the aging-related genes. More immune cell infiltration was found in high-risk group cases, and glioma patients in high-risk group may be more responsive to immunotherapy. Knock-down of CTSC, an aging-related gene, can inhibit cell cycle progression, colony formation, cell proliferation and increase cell senescence in glioma cell lines in vitro. Indeed, high expression of CTSC was associated with poor prognosis in glioma cases. In conclusion, this study revealed that aging-related genes have prognostic potential for glioma patients and further identified potential mechanisms for aging-related genes in tumorigenesis and progression in gliomas.

Magnoliae Cortex Alleviates Muscle Wasting by Modulating M2 Macrophages in a Cisplatin-Induced Sarcopenia Mouse Model

Cachexia causes high mortality, low quality of life, and rapid weight loss in cancer patients. Sarcopenia, a condition characterized by the loss of muscle, is generally present in cachexia and is associated with inflammation. M2 macrophages, also known as an anti-inflammatory or alternatively activated macrophages, have been shown to play a role in muscle repair. Magnoliae Cortex (M.C) is a widely used medicinal herb in East Asia reported to have a broad range of anti-inflammatory activities; however, the effects of M.C on sarcopenia and on M2 macrophage polarization have to date not been studied. This study was designed to investigate whether the oral administration of M.C could decrease cisplatin-induced sarcopenia by modulating M2 macrophage polarization in mice. C57BL/6 mice were injected intraperitoneally with cisplatin (2.5 mg/kg) to mimic chemotherapy-induced sarcopenia. M.C extract (50, 100, and 200 mg/kg) was administered orally every 3 days (for a total of 12 times). M.C (100 and 200 mg/kg) significantly alleviated the cisplatin-induced loss of body mass, skeletal muscle weight, and grip strength. In addition, M.C increased the expression of M2 macrophage markers, such as MRC1, CD163, TGF-β, and Arg-1, and decreased the expression of M1-specific markers, including NOS2 and TNF-α, in skeletal muscle. Furthermore, the levels of like growth factor-1(IGF-1), as well as the number of M2a and M2c macrophages, significantly increased in skeletal muscle after M.C administration. M.C did not interfere with the anticancer effect of cisplatin in colon cancer. Our results demonstrated that M.C can alleviate cisplatin-induced sarcopenia by increasing the number of M2 macrophages. Therefore, our findings suggest that M.C could be used as an effective therapeutic agent to reverse or prevent cisplatin-induced sarcopenia.

Invalidation of the Transcriptional Modulator of Lipid Metabolism PPARβ/δ in T Cells Prevents Age-Related Alteration of Body Composition and Loss of Endurance Capacity

Anti-inflammatory regulatory T cells (Tregs) are the most metabolically flexible CD4⁺ T cells by using both glycolysis and fatty acid oxidation (FAO) which allow them to migrate in tissues. With aging, Tregs accumulate in secondary lymphoid organs and are involved in impairment of skeletal muscle (SKM) regeneration and mass maintenance. In this study, we showed that a deletion of a FAO modulator, peroxisome proliferator-activated receptor beta/delta (PPARβ/δ), specifically in T cells (KO-T PPARβ/δ), increased the number of CD4⁺ T cells at day 2 following a cardiotoxin-induced SKM regeneration. Older KO-T PPARβ/δ mice maintained a Tregs prevalence in lymph nodes similar to young mice. Surprisingly, KO-T PPARβ/δ mice were protected from the effects of age on lean and fat mass and endurance capacity. Our results lead us to propose an original potential role of T cell metabolism in the effects of aging on the maintenance of body composition and endurance capacity.

TNF-α Triggers RIP1/FADD/Caspase-8-Mediated Apoptosis of Astrocytes and RIP3/MLKL-Mediated Necroptosis of Neurons Induced by Angiostrongylus cantonensis Infection

Angiostrongylus cantonensis (AC) can cause severe eosinophilic meningitis or encephalitis in non-permissive hosts accompanied by apoptosis and necroptosis of brain cells. However, the explicit underlying molecular basis of apoptosis and necroptosis upon AC infection has not yet been elucidated. To determine the specific pathways of apoptosis and necroptosis upon AC infection, gene set enrichment analysis (GSEA) and protein-protein interaction (PPI) analysis for gene expression microarray (accession number: GSE159486) of mouse brain infected by AC revealed that TNF-α likely played a central role in the apoptosis and necroptosis in the context of AC infection, which was further confirmed via an in vivo rescue assay after treating with TNF-α inhibitor. The signalling axes involved in apoptosis and necroptosis were investigated via immunoprecipitation and immunoblotting. Immunofluorescence was used to identify the specific cells that underwent apoptosis or necroptosis. The results showed that TNF-α induced apoptosis of astrocytes through the RIP1/FADD/Caspase-8 axis and induced necroptosis of neurons by the RIP3/MLKL signalling pathway. In addition, in vitro assay revealed that TNF-α secretion by microglia increased upon LSA stimulation and caused necroptosis of neurons. The present study provided the first evidence that TNF-α was secreted by microglia stimulated by AC infection, which caused cell death via parallel pathways of astrocyte apoptosis (mediated by the RIP1/FADD/caspase-8 axis) and neuron necroptosis (driven by the RIP3/MLKL complex). Our research comprehensively elucidated the mechanism of cell death after AC infection and provided new insight into targeting TNF-α signalling as a therapeutic strategy for CNS injury.

NMR-based clinical metabolomics revealed distinctive serum metabolic profiles in patients with spondyloarthritis

Spondyloarthritis (SpA) is a common rheumatic disorder of the young, marred by delay in diagnosis, and paucity of biomarkers of disease activity. The present study aimed to explore the potential of serum metabolic profiling of patients with SpA to identify biomarker for the diagnosis and assessment of disease activity. The serum metabolic profiles of 81 patients with SpA were compared with that of 86 healthy controls (HCs) using nuclear magnetic resonance (NMR)-based metabolomics approach. Seventeen patients were followed up after 3 months of standard treatment, and paired sera were analyzed for effects of therapy. Comparisons were done using the multivariate partial least squares discriminant analysis (PLS-DA), and the discriminatory metabolic entities were identified based on variable importance in projection (VIP) statistics and further evaluated for statistical significance (p value < 0.05). We found that the serum metabolic profiles differed significantly in SpA as compared with HCs. Compared with HC, the SpA patients were characterized by increased serum levels of amino acids, acetate, choline, N-acetyl glycoproteins, Nα-acetyl lysine, creatine/creatinine, and so forth and decreased levels of low-/very low-density lipoproteins and polyunsaturated lipids. PLS-DA analysis also revealed metabolic differences between axial and peripheral SpA patients. Further metabolite profiles were found to differ with disease activity and treatment in responding patients. The results presented in this study demonstrate the potential of serum metabolic profiling of axial SpA as a useful tool for diagnosis, prediction of peripheral disease, assessment of disease activity, and treatment response.

Macrophage function in the elderly and impact on injury repair and cancer

Older age is associated with deteriorating health, including escalating risk of diseases such as cancer, and a diminished ability to repair following injury. This rise in age-related diseases/co-morbidities is associated with changes to immune function, including in myeloid cells, and is related to immunosenescence. Immunosenescence reflects age-related changes associated with immune dysfunction and is accompanied by low-grade chronic inflammation or inflammageing. This is characterised by increased levels of circulating pro-inflammatory cytokines such as tumor necrosis factor (TNF), interleukin (IL)-1β and IL-6. However, in healthy ageing, there is a concomitant age-related escalation in anti-inflammatory cytokines such as transforming growth factor-β1 (TGF-β1) and IL-10, which may overcompensate to regulate the pro-inflammatory state. Key inflammatory cells, macrophages, play a role in cancer development and injury repair in young hosts, and we propose that their role in ageing in these scenarios may be more profound. Imbalanced pro- and anti-inflammatory factors during ageing may also have a significant influence on macrophage function and further impact the severity of age-related diseases in which macrophages are known to play a key role. In this brief review we summarise studies describing changes to inflammatory function of macrophages (from various tissues and across sexes) during healthy ageing. We also describe age-related diseases/co-morbidities where macrophages are known to play a key role, focussed on injury repair processes and cancer, plus comment briefly on strategies to correct for these age-related changes.

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.

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.

Pharmacological blockade of TNFα prevents sarcopenia and prolongs survival in aging mice

Sarcopenia is a hallmark of aging. Inflammation due to increased generation of cytokines such as TNFα, IL-1β and IL-6 has been implicated in the pathogenesis of sarcopenia. In skeletal muscle of C57BL/6 mice from 12 until 28 months of age, we observed a progressive reduction of myofiber cross sectional area, loss of type II fibers and infiltration by inflammatory cells. Muscle strength decreased in parallel. Pharmacological TNFα blockade by weekly subcutaneous injection of Etanercept from 16 to 28 months of age prevented atrophy and loss of type II fibers, with significant improvements in muscle function and mice lifespan. The effects on leukocyte recruitment were limited. These results provide a proof of principle that endogenous TNFα is sufficient to cause sarcopenia and to reduce animal survival, and open a novel perspective on novel potential pharmacological treatment strategies based on TNFα blockade to prevent the noxious events associated with aging.

IL-1β and TNF-α Modulation of Proliferated and Committed Myoblasts: IL-6 and COX-2-Derived Prostaglandins as Key Actors in the Mechanisms Involved

In this study, we investigated the effects and mechanisms of the pro-inflammatory cytokines IL-1β and TNF-α on the proliferation and commitment phases of myoblast differentiation. C2C12 mouse myoblast cells were cultured to reach a proliferated or committed status and were incubated with these cytokines for the evaluation of cell proliferation, cyclooxygenase 2 (COX-2) expression, release of prostaglandins (PGs) and myokines, and activation of myogenic regulatory factors (MRFs). We found that inhibition of the IL-6 receptor reduced IL-1β- and TNF-α-induced cell proliferation, and that the IL-1β effect also involved COX-2-derived PGs. Both cytokines modulated the release of the myokines myostatin, irisin, osteonectin, and IL-15. TNF-α and IL-6 reduced the activity of Pax7 in proliferated cells and reduced MyoD and myogenin activity at both proliferative and commitment stages. Otherwise, IL-1β increased myogenin activity only in committed cells. Our data reveal a key role of IL-6 and COX-2-derived PGs in IL-1β and TNF-α-induced myoblast proliferation and support the link between TNF-α and IL-6 and the activation of MRFs. We concluded that IL-1β and TNF-α induce similar effects at the initial stages of muscle regeneration but found critical differences between their effects with the progression of the process, bringing new insights into inflammatory signalling in skeletal muscle regeneration.

Muscle stem cell aging: identifying ways to induce tissue rejuvenation

Aging is characterized by the functional and regenerative decline of tissues and organs. This regenerative decline is a consequence of the numerical and functional loss of adult stem cells, which are the corner stone of tissue homeostasis and repair. A palpable example of this decline is provided by skeletal muscle, a specialized tissue composed of postmitotic myofibers that contract to generate force. Skeletal muscle stem cells (satellite cells) are long-lived and support muscle regeneration throughout life, but at advanced age they fail for largely undefined reasons. Here, we discuss recent advances in the understanding of how satellite cells integrate diverse intrinsic and extrinsic processes to ensure optimal homeostatic function and how this integration is perturbed during aging, causing regenerative failure. With this increased understanding, it is now feasible to design and test interventions that delay satellite cell aging. We discuss the exciting new therapeutic potential of integrating and combining distinct anti-aging strategies for regenerative medicine.

Divergent Roles of Inflammation in Skeletal Muscle Recovery From Injury

A transient increase in local pro-inflammatory cytokine expression following skeletal muscle injury mediates the repair and regeneration of damaged myofibers through myogenesis. Regenerative capacity is diminished and muscle wasting occurs, however, when intramuscular inflammatory signaling is exceedingly high or persists chronically. An excessive and persistent inflammatory response to muscle injury may therefore impair recovery by limiting the repair of damaged tissue and triggering muscle atrophy. The concentration-dependent activation of different downstream signaling pathways by several pro-inflammatory cytokines in cell and animal models support these opposing roles of post-injury inflammation. Understanding these molecular pathways is essential in developing therapeutic strategies to attenuate excessive inflammation and accelerate functional recovery and muscle mass accretion following muscle damage. This is especially relevant given the observation that basal levels of intramuscular inflammation and the inflammatory response to muscle damage are not uniform across all populations, suggesting certain individuals may be more susceptible to an excessive inflammatory response to injury that limits recovery. This narrative review explores the opposing roles of intramuscular inflammation in muscle regeneration and muscle protein turnover. Factors contributing to an exceedingly high inflammatory response to damage and age-related impairments in regenerative capacity are also considered.

Skeletal muscle as potential central link between sarcopenia and immune senescence

As our population grows older, age-related pathologies are becoming more prevalent. Deterioration of skeletal muscle and the immune system manifests as sarcopenia and immune senescence respectively. The disease burden of these pathologies emphasizes the need for a better understanding of the underlying mechanisms. Skeletal muscle has emerged as a potent regulator of immune system function. As such, skeletal muscle might be the central integrator between sarcopenia and immune senescence in an aging biological system. Therapeutic approaches targeting skeletal muscle might be able to restore both muscle and immune system function. In this review, we therefore outline the current - however still fragmentary - knowledge about the potential communication pathways of muscle and immune system, how they are affected by aging of skeletal muscle and discuss possible treatment strategies. The review intends to be hypothesis-generating and should thereby stimulate further research in this important scientific field.

Myeloid cell-derived tumor necrosis factor-alpha promotes sarcopenia and regulates muscle cell fusion with aging muscle fibers

Sarcopenia is age‐related muscle wasting that lacks effective therapeutic interventions. We found that systemic ablation of tumor necrosis factor‐α (TNF‐α) prevented sarcopenia and prevented age‐related change in muscle fiber phenotype. Furthermore, TNF‐α ablation reduced the number of satellite cells in aging muscle and promoted muscle cell fusion in vivo and in vitro. Because CD68+ macrophages are important sources of TNF‐α and the number of CD68+ macrophages increases in aging muscle, we tested whether macrophage‐derived TNF‐α affects myogenesis. Media conditioned by TNF‐α‐null macrophages increased muscle cell fusion in vitro, compared to media conditioned by wild‐type macrophages. In addition, transplantation of bone marrow cells from wild‐type mice into TNF‐α‐null recipients increased satellite cell numbers and reduced numbers of centrally nucleated myofibers, indicating that myeloid cell‐secreted TNF‐α reduces muscle cell fusion. Transplanting bone marrow cells from wild‐type mice into TNF‐α‐null recipients also increased sarcopenia, although transplantation did not restore the age‐related change in muscle fiber phenotype. Collectively, we show that myeloid cell‐derived TNF‐α contributes to muscle aging by affecting sarcopenia and muscle cell fusion with aging muscle fibers. Our findings also show that TNF‐α that is intrinsic to muscle and TNF‐α secreted by immune cells work together to influence muscle aging.