Elevated SOCS3 and altered IL-6 signaling is associated with age-related human muscle stem cell dysfunction

Advancing immunomodulatory functions in mesenchymal stem/stromal cells through targeting the GATA6-mediated pathway

BACKGROUND AIMS

The immunomodulatory capacity of mesenchymal stem/stromal cells (MSCs) is a key feature that makes them particularly valuable for regenerative medicine. However, this potential is affected by the chronological aging of the donors and the cell expansion procedures in culture. We have demonstrated that GATA binding protein 6 (GATA6) plays a pivotal role in the aging of MSCs and inhibiting GATA6 rejuvenates the characteristics of MSCs.

METHODS

In this study, we compared the immunomodulatory capabilities of young and old MSC models, using induced pluripotent stem cells-derived rejuvenated MSCs (rMSCs) and their parental MSCs (pMSCs), respectively, to identify a key mechanism involved in the differential regulation of these capabilities. Additionally, we explored the role of GATA6 in mediating the mechanism.

RESULTS

Our results demonstrated that rMSCs exhibited downregulated aging-associated regulators, including p53, p21 and GATA6, and showed enhanced suppression of T cell proliferation compared to pMSCs. Through analyzing our previous RNA-seq data and employing target gene knockdown, we determined both suppressors of cytokine signaling 3 (SOCS3) and interleukin 6 were involved in GATA6-induced regulation, collectively affecting the expression of programmed death ligand 1 (PDL1) in both pMSCs and rMSCs.

CONCLUSIONS

Our findings underline the significance of the GATA6/SOCS3/PDL1 pathway in regulating aging-associated changes in MSC immunomodulatory activity, providing valuable insights into the potential use of rMSCs in the treatment of immune diseases and regenerative medicine.

Approaching the Sarcopenic Patient with Nonalcoholic Steatohepatitis-related Cirrhosis

Sarcopenia is a well-known complication of chronic liver disease (CLD), and it is almost always observed in patients with cirrhosis, at least in those with decompensated disease. Since nonalcoholic fatty liver disease (NAFLD), recently renamed metabolic dysfunction-associated steatotic liver disease (MASLD), is becoming the leading cause of end-stage liver disease, a new scenario characterized by the frequent coexistence of NAFLD, obesity, and sarcopenia is emerging. Although it is not yet resolved whether the bidirectional relationship between sarcopenia and NAFLD subtends causal determinants, it is clear that the interaction of these two conditions is associated with an increased risk of poor outcomes. Notably, during the course of CLD, deregulation of the liver-muscle-adipose tissue axis has been described. Unfortunately, owing to the lack of properly designed studies, specific therapeutic guidelines for patients with sarcopenia in the context of NAFLD-related CLD have not yet been defined. Strategies aimed to induce the loss of fat mass together with the maintenance of lean body mass seem most appropriate. This can be achieved by properly designed diets integrated with specific nutritional supplementations and accompanied by adequate physical exercise. Future studies aiming to add to the knowledge of the correct assessment and approach to sarcopenia in the context of NAFLD-related CLD are eagerly awaited.

Age-Associated Differences in Recovery from Exercise-Induced Muscle Damage

Understanding the intricate mechanisms governing the cellular response to resistance exercise is paramount for promoting healthy aging. This narrative review explored the age-related alterations in recovery from resistance exercise, focusing on the nuanced aspects of exercise-induced muscle damage in older adults. Due to the limited number of studies in older adults that attempt to delineate age differences in muscle discovery, we delve into the multifaceted cellular influences of chronic low-grade inflammation, modifications in the extracellular matrix, and the role of lipid mediators in shaping the recovery landscape in aging skeletal muscle. From our literature search, it is evident that aged muscle displays delayed, prolonged, and inefficient recovery. These changes can be attributed to anabolic resistance, the stiffening of the extracellular matrix, mitochondrial dysfunction, and unresolved inflammation as well as alterations in satellite cell function. Collectively, these age-related impairments may impact subsequent adaptations to resistance exercise. Insights gleaned from this exploration may inform targeted interventions aimed at enhancing the efficacy of resistance training programs tailored to the specific needs of older adults, ultimately fostering healthy aging and preserving functional independence.

Dynamic model assuming mutually inhibitory biomarkers of frailty suggests bistability with contrasting mobility phenotypes

Bistability is a fundamental biological phenomenon associated with "switch-like" behavior reflecting the capacity of a system to exist in either of two stable states. It plays a role in gene regulation, cell fate switch, signal transduction and cell oscillation, with relevance for cognition, hearing, vision, sleep, gait and voiding. Here we consider a potential role for bistability in the existence of specific frailty states or phenotypes as part of disablement pathways. We use mathematical modeling with two frailty biomarkers (insulin growth factor-1, IGF-1 and interleukin-6, IL-6), which mutually inhibit each other. In our model, we demonstrate that small variations around critical IGF-1 or IL-6 blood levels lead to strikingly different mobility outcomes. We employ deterministic modeling of mobility outcomes, calculating the average trends in population health. Our model predicts the bistability of clinical outcomes: the deterministically-computed likelihood of an individual remaining mobile, becoming less mobile, or dying over time either increases to almost 100% or decreases to almost zero. Contrary to statistical models that attempt to estimate the likelihood of final outcomes based on probabilities and correlations, our model predicts functional outcomes over time based on specific hypothesized molecular mechanisms. Instead of estimating probabilities based on stochastic distributions and arbitrary priors, we deterministically simulate model outcomes over a wide range of physiological parameter values within experimentally derived boundaries. Our study is "a proof of principle" as it is based on a major assumption about mutual inhibition of pathways that is oversimplified. However, by making such an assumption, interesting effects can be described qualitatively. As our understanding of molecular mechanisms involved in aging deepens, we believe that such modeling will not only lead to more accurate predictions, but also help move the field from using mostly studies of associations to mechanistically guided approaches.

Distinct human skeletal muscle-derived CD90 progenitor subsets for myo-fibro-adipogenic disease modeling and treatment in multiplexed conditions

Chronic muscle injuries, such as massive rotator cuff tears, are associated with progressive muscle wasting, fibrotic scarring, and intramuscular fat accumulation. While progenitor cell subsets are usually studied in culture conditions that drive either myogenic, fibrogenic, or adipogenic differentiation, it is still unknown how combined myo-fibro-adipogenic signals, which are expected to occur in vivo, modulate progenitor differentiation. We therefore evaluated the differentiation potential of retrospectively generated subsets of primary human muscle mesenchymal progenitors in multiplexed conditions in the presence or absence of 423F drug, a modulator of gp130 signaling. We identified a novel CD90+CD56- non-adipogenic progenitor subset that maintained a lack of adipogenic potential in single and multiplexed myo-fibro-adipogenic culture conditions. CD90-CD56- demarcated fibro-adipogenic progenitors (FAP) and CD56+CD90+ progenitors were typified as myogenic. These human muscle subsets exhibited varying degrees of intrinsically regulated differentiation in single and mixed induction cultures. Modulation of gp130 signaling via 423F drug mediated muscle progenitor differentiation in a dose-, induction-, and cell subset-dependent manner and markedly decreased fibro-adipogenesis of CD90-CD56- FAP. Conversely, 423F promoted myogenesis of CD56+CD90+ myogenic subset, indicated by increased myotube diameter and number of nuclei per myotube. 423F treatment eliminated FAP-derived mature adipocytes from mixed adipocytes-FAP cultures but did not modify the growth of non-differentiated FAP in these cultures. Collectively, these data demonstrate that capability of myogenic, fibrogenic, or adipogenic differentiation is largely dependent on the intrinsic features of cultured subsets, and that the degree of lineage differentiation varies when signals are multiplexed. Moreover, our tests performed in primary human muscle cultures reveal and confirm the potential triple-therapeutic effects of 423F drug which simultaneously attenuates degenerative fibrosis, fat accumulation and promotes myo-regeneration.

Pax7+ Satellite Cells in Human Skeletal Muscle After Exercise: A Systematic Review and Meta-analysis

BACKGROUND

Skeletal muscle has extraordinary regenerative capabilities against challenge, mainly owing to its resident muscle stem cells, commonly identified by Pax7⁺, which expediently donate nuclei to the regenerating multinucleated myofibers. This local reserve of stem cells in damaged muscle tissues is replenished by undifferentiated bone marrow stem cells (CD34⁺) permeating into the surrounding vascular system.

OBJECTIVE

The purpose of the study was to provide a quantitative estimate for the changes in Pax7⁺ muscle stem cells (satellite cells) in humans following an acute bout of exercise until 96 h, in temporal relation to circulating CD34⁺ bone marrow stem cells. A subgroup analysis of age was also performed.

METHODS

Four databases (Web of Science, PubMed, Scopus, and BASE) were used for the literature search until February 2022. Pax7⁺ cells in human skeletal muscle were the primary outcome. Circulating CD34⁺ cells were the secondary outcome. The standardized mean difference (SMD) was calculated using a random-effects meta-analysis. Subgroup analyses were conducted to examine the influence of age, training status, type of exercise, and follow-up time after exercise.

RESULTS

The final search identified 20 studies for Pax7⁺ cells comprising a total of 370 participants between the average age of 21 and 74 years and 26 studies for circulating CD34⁺ bone marrow stem cells comprising 494 participants between the average age of 21 and 67 years. Only one study assessed Pax7⁺ cells immediately after aerobic exercise and showed a 32% reduction in exercising muscle followed by a fast repletion to pre-exercise level within 3 h. A large effect on increasing Pax7⁺ cell content in skeletal muscles was observed 24 h after resistance exercise (SMD = 0.89, p < 0.001). Pax7⁺ cells increased to ~ 50% above pre-exercise level 24-72 h after resistance exercise. For a subgroup analysis of age, a large effect (SMD = 0.81, p < 0.001) was observed on increasing Pax7⁺ cells in exercised muscle among adults aged > 50 years, whereas adults at younger age presented a medium effect (SMD = 0.64, p < 0.001). Both resistance exercise and aerobic exercise showed a medium overall effect in increasing circulating CD34⁺ cells (SMD = 0.53, p < 0.001), which declined quickly to the pre-exercise baseline level after exercise within 6 h.

CONCLUSIONS

An immediate depletion of Pax7⁺ cells in exercising skeletal muscle concurrent with a transient release of CD34⁺ cells suggest a replenishment of the local stem cell reserve from bone marrow. A protracted Pax7⁺ cell expansion in the muscle can be observed during 24-72 h after resistance exercise. This result provides a scientific basis for exercise recommendations on weekly cycles allowing for adequate recovery time. Exercise-induced Pax7⁺ cell expansion in muscle remains significant at higher age, despite a lower stem cell reserve after age 50 years. More studies are required to confirm whether Pax7⁺ cell increment can occur after aerobic exercise.

CLINICAL TRIAL REGISTRATION

Registered at the International Prospective Register of Systematic Reviews (PROSPERO) [identification code CRD42021265457].

Rehabilitation of Older Adults with Sarcopenia: From Cell to Functioning

The 20th and 21st centuries have witnessed a substantial increase in human life expectancy and in the number of men and women aged 60 years and older. Aging is associated with a large number of health conditions, including sarcopenia, which has been the subject of important research in the past 30 years. Sarcopenia is characterized by an age-related loss of muscle mass, weakness, and impaired physical performance. The condition can be diagnosed with a combination of measurements of these three elements. The precise definition of sarcopenia and the selection of optimal assessment methods have changed significantly in the past 20 years; nonetheless, the prevalence of sarcopenia in the general older population is in the range of 5-15%. Molecular and cellular events at the muscle cell level impact the size and quality of muscles (force adjusted for size). The active and passive mechanical properties of single muscle fibers are altered by changes in the structure and function of various cellular elements. Systemic factors such as inflammation, loss of hormonal influence, and deleterious lifestyle choices also contribute to sarcopenia. The consequences of sarcopenia include many adverse effects such as impairments in activities of daily living, falls, loss of independence, and increased mortality. Several rehabilitative interventions have been tested, and the safest and most effective is the use of progressive resistance exercise. An increase in dietary protein intake has synergistic effects. Future research should focus on a consensus definition of sarcopenia, identification of the best assessment methods, understanding of biological mechanisms, and testing of innovative interventions.

Inflammation and Wasting of Skeletal Muscles in Kras-p53-Mutant Mice with Intraepithelial Neoplasia and Pancreatic Cancer-When Does Cachexia Start?

Skeletal muscle wasting critically impairs the survival and quality of life in patients with pancreatic ductal adenocarcinoma (PDAC). To identify the local factors initiating muscle wasting, we studied inflammation, fiber cross-sectional area (CSA), composition, amino acid metabolism and capillarization, as well as the integrity of neuromuscular junctions (NMJ, pre-/postsynaptic co-staining) and mitochondria (electron microscopy) in the hindlimb muscle of LSL-KrasG12D/+; LSL-TrP53R172H/+; Pdx1-Cre mice with intraepithelial-neoplasia (PanIN) 1-3 and PDAC, compared to wild-type mice (WT). Significant decreases in fiber CSA occurred with PDAC but not with PanIN 1-3, compared to WT: These were found in the gastrocnemius (type 2x: −20.0%) and soleus (type 2a: −21.0%, type 1: −14.2%) muscle with accentuation in the male soleus (type 2a: −24.8%, type 1: −17.4%) and female gastrocnemius muscle (−29.6%). Significantly higher densities of endomysial CD68+ and cyclooxygenase-2+ (COX2+) cells were detected in mice with PDAC, compared to WT mice. Surprisingly, CD68+ and COX2+ cell densities were also higher in mice with PanIN 1-3 in both muscles. Significant positive correlations existed between muscular and hepatic CD68+ or COX2+ cell densities. Moreover, in the gastrocnemius muscle, suppressor-of-cytokine-3 (SOCS3) expressions was upregulated >2.7-fold with PanIN 1A-3 and PDAC. The intracellular pools of proteinogenic amino acids and glutathione significantly increased with PanIN 1A-3 compared to WT. Capillarization, NMJ, and mitochondrial ultrastructure remained unchanged with PanIN or PDAC. In conclusion, the onset of fiber atrophy coincides with the manifestation of PDAC and high-grade local (and hepatic) inflammatory infiltration without compromised microcirculation, innervation or mitochondria. Surprisingly, muscular and hepatic inflammation, SOCS3 upregulation and (proteolytic) increases in free amino acids and glutathione were already detectable in mice with precancerous PanINs. Studies of initial local triggers and defense mechanisms regarding cachexia are warranted for targeted anti-inflammatory prevention.

Effect of chronic intermittent hypoxia (CIH) on neuromuscular junctions and mitochondria in slow- and fast-twitch skeletal muscles of mice—the role of iNOS

Background

Obstructive sleep apnea (OSA) imposes vascular and metabolic risks through chronic intermittent hypoxia (CIH) and impairs skeletal muscle performance. As studies addressing limb muscles are rare, the reasons for the lower exercise capacity are unknown. We hypothesize that CIH-related morphological alterations in neuromuscular junctions (NMJ) and mitochondrial integrity might be the cause of functional disorders in skeletal muscles.

Methods

Mice were kept under 6 weeks of CIH (alternating 7% and 21% O₂ fractions every 30 s, 8 h/day, 5 days/week) compared to normoxia (NOX). Analyses included neuromuscular junctions (NMJ) postsynaptic morphology and integrity, fiber cross-sectional area (CSA) and composition (ATPase), mitochondrial ultrastructure (transmission-electron-microscopy), and relevant transcripts (RT-qPCR). Besides wildtype (WT), we included inducible nitric oxide synthase knockout mice (iNOS^−/−) to evaluate whether iNOS is protective or risk-mediating.

Results

In WT soleus muscle, CIH vs. NOX reduced NMJ size (− 37.0%, p < 0.001) and length (− 25.0%, p < 0.05) together with fiber CSA of type IIa fibers (− 14%, p < 0.05) and increased centronucleated fiber fraction (p < 0.001). Moreover, CIH vs. NOX increased the fraction of damaged mitochondria (1.8-fold, p < 0.001). Compared to WT, iNOS^−/− similarly decreased NMJ area and length with NOX (− 55%, p < 0.001 and − 33%, p < 0.05, respectively) or with CIH (− 37%, p < 0.05 and − 29%, p < 0.05), however, prompted no fiber atrophy. Moreover, increased fractions of damaged (2.1-fold, p < 0.001) or swollen (> 6-fold, p < 0.001) mitochondria were observed with iNOS^−/− vs. WT under NOX and similarly under CIH. Both, CIH- and iNOS^−/− massively upregulated suppressor-of-cytokine-signaling-3 (SOCS3) > 10-fold without changes in IL6 mRNA expression. Furthermore, inflammatory markers like CD68 (macrophages) and IL1β were significantly lower in CIH vs. NOX. None of these morphological alterations with CIH- or iNOS^−/− were detected in the gastrocnemius muscle. Notably, iNOS expression was undetectable in WT muscle, unlike the liver, where it was massively decreased with CIH.

Conclusion

CIH leads to NMJ and mitochondrial damage associated with fiber atrophy/centronucleation selectively in slow-twitch muscle of WT. This effect is largely mimicked by iNOS^−/− at NOX (except for atrophy). Both conditions involve massive SOCS3 upregulation likely through denervation without Il6 upregulation but accompanied by a decrease of macrophage density especially next to denervated endplates. In the absence of muscular iNOS expression in WT, this damage may arise from extramuscular, e.g., motoneuronal iNOS deficiency (through CIH or knockout) awaiting functional evaluation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00288-7.

Increased IL-6 receptor expression and signaling in ageing cartilage can be explained by loss of TGF-β-mediated IL-6 receptor suppression

OBJECTIVE

Osteoarthritis (OA) development is strongly associated with ageing, possibly due to age-related changes in transforming growth factor-β (TGF-β) signaling in cartilage. Recently, we showed that TGF-β suppresses interleukin (IL)-6 receptor (IL-6R) expression in chondrocytes. As IL-6 is involved in cartilage degeneration, we hypothesized that age-related loss of TGF-β signaling results in increased IL-6R expression and signaling in ageing cartilage.

DESIGN

Bovine articular cartilage was collected and immediately processed to study age-related changes in IL-6R expression using qPCR and IHC (age-range: 0.5-14 years). Moreover, cartilage from young and aged cows was stimulated with rhIL-6 and/or rhTGF-β1 to measure IL-6-induced p-STAT3 using Western blot. Expression of STAT3-responsive genes was analyzed using qPCR.

RESULTS

Expression of IL-6 receptor (bIL-6R) significantly increased in cartilage upon ageing (slope: 0.32, 95%CI: 0.20-0.45), while expression of glycoprotein 130 (bGP130) was unaffected. Cartilage stimulation with IL-6 showed increased induction of p-STAT3 upon ageing (slope: 0.14, 95%CI: 0.08-0.20). Furthermore, IL-6-mediated induction of STAT3-responsive genes like bSOCS3 and bMMP3 was increased in aged compared to young cartilage. Interestingly, the ability of TGF-β to suppress bIL6R expression in young cartilage was lost upon ageing (slope: 0.21, 95%CI: 0.13-0.30). Concurrently, an age-related loss in TGF-β-mediated suppression of IL-6-induced p-STAT3 and bSOCS3 expression was observed.

CONCLUSIONS

Ageing results in enhanced IL-6R expression and subsequent IL-6-induced p-STAT3 signaling in articular cartilage. This is likely caused by age-related loss of protective TGF-β signaling, resulting in loss of TGF-β-mediated IL-6R suppression. Because of the detrimental role of IL-6 in cartilage, this mechanism may be involved in age-related OA development.

Senescence in Post-Mitotic Cells: A Driver of Aging?

Significance: Cell senescence was originally defined by an acute loss of replicative capacity and thus believed to be restricted to proliferation-competent cells. More recently, senescence has been recognized as a cellular stress and damage response encompassing multiple pathways or senescence domains, namely DNA damage response, cell cycle arrest, senescence-associated secretory phenotype, senescence-associated mitochondrial dysfunction, autophagy/mitophagy dysfunction, nutrient and stress signaling, and epigenetic reprogramming. Each of these domains is activated during senescence, and all appear to interact with each other. Cell senescence has been identified as an important driver of mammalian aging. Recent Advances: Activation of all these senescence domains has now also been observed in a wide range of post-mitotic cells, suggesting that senescence as a stress response can occur in nondividing cells temporally uncoupled from cell cycle arrest. Here, we review recent evidence for post-mitotic cell senescence and speculate about its possible relevance for mammalian aging. Critical Issues: Although a majority of senescence domains has been found to be activated in a range of post-mitotic cells during aging, independent confirmation of these results is still lacking for most of them. Future Directions: To define whether post-mitotic senescence plays a significant role as a driver of aging phenotypes in tissues such as brain, muscle, heart, and others. Antioxid. Redox Signal. 34, 308-323.

Harnessing Muscle-Liver Crosstalk to Treat Nonalcoholic Steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) has reached epidemic proportions, affecting an estimated one-quarter of the world's adult population. Multiple organ systems have been implicated in the pathophysiology of NAFLD; however, the role of skeletal muscle has until recently been largely overlooked. A growing body of evidence places skeletal muscle-via its impact on insulin resistance and systemic inflammation-and the muscle-liver axis at the center of the NAFLD pathogenic cascade. Population-based studies suggest that sarcopenia is an effect-modifier across the NAFLD spectrum in that it is tightly linked to an increased risk of non-alcoholic fatty liver, non-alcoholic steatohepatitis (NASH), and advanced liver fibrosis, all independent of obesity and insulin resistance. Longitudinal studies suggest that increases in skeletal muscle mass over time may both reduce the incidence of NAFLD and improve preexisting NAFLD. Adverse muscle composition, comprising both low muscle volume and high muscle fat infiltration (myosteatosis), is highly prevalent in patients with NAFLD. The risk of functional disability conferred by low muscle volume in NAFLD is further exacerbated by the presence of myosteatosis, which is twice as common in NAFLD as in other chronic liver diseases. Crosstalk between muscle and liver is influenced by several factors, including obesity, physical inactivity, ectopic fat deposition, oxidative stress, and proinflammatory mediators. In this perspective review, we discuss key pathophysiological processes driving sarcopenia in NAFLD: anabolic resistance, insulin resistance, metabolic inflexibility and systemic inflammation. Interventions that modify muscle quantity (mass), muscle quality (fat), and physical function by simultaneously engaging multiple targets and pathways implicated in muscle-liver crosstalk may be required to address the multifactorial pathogenesis of NAFLD/NASH and provide effective and durable therapies.

Effects of aging and lifelong aerobic exercise on basal and exercise-induced inflammation in women

Low muscle mass and frailty are especially prevalent in older women and may be accelerated by age-related inflammation. Habitual physical activity throughout the life span (lifelong exercise) may prevent muscle inflammation and associated pathologies, but this is unexplored in women. This investigation assessed basal and acute exercise-induced inflammation in three cohorts of women: young exercisers (YE, n = 10, 25 ± 1 yr, [Formula: see text]: 44 ± 2 mL/kg/min, quadriceps size: 59 ± 2 cm²), old healthy nonexercisers (OH, n = 10, 75 ± 1 yr, [Formula: see text]: 18 ± 1 mL/kg/min, quadriceps size: 40 ± 1 cm²), and lifelong aerobic exercisers with a 48 ± 2 yr aerobic training history (LLE, n = 7, 72 ± 2 yr, [Formula: see text]: 26 ± 2 mL/kg/min, quadriceps size: 42 ± 2 cm²). Resting serum IL-6, TNF-α, C-reactive protein (CRP), and IGF-1 were measured. Vastus lateralis muscle biopsies were obtained at rest (basal) and 4 h after an acute exercise challenge (3 × 10 reps, 70% 1-repetition maximum) to assess gene expression of cytokines (IL-6, TNF-α, IL-1β, IL-10, IL-4, IL-1Ra, TGF-β), chemokines (IL-8, MCP-1), cyclooxygenase enzymes (COX-1, COX-2), prostaglandin E₂ synthases (mPGES-1, cPGES) and receptors (EP3-4), and macrophage markers (CD16b, CD163), as well as basal macrophage abundance (CD68⁺ cells). The older cohorts (LLE + OH combined) demonstrated higher muscle IL-6 and COX-1 (P ≤ 0.05) than YE, whereas LLE expressed lower muscle IL-1β (P ≤ 0.05 vs. OH). Acute exercise increased muscle IL-6 expression in YE only, whereas the older cohorts combined had the higher postexercise expression of IL-8 and TNF-α (P ≤ 0.05 vs. YE). Only LLE had increased postexercise expression of muscle IL-1β and MCP-1 (P ≤ 0.05 vs. preexercise). Thus, aging in women led to mild basal and exercise-induced inflammation that was unaffected by lifelong aerobic exercise, which may have implications for long-term function and adaptability.NEW & NOTEWORTHY We previously reported a positive effect of lifelong exercise on skeletal muscle inflammation in aging men. This parallel investigation in women revealed that lifelong exercise did not protect against age-related increases in circulating or muscle inflammation and that preparedness to handle loading stress was not preserved by lifelong exercise. Further investigation is necessary to understand why lifelong aerobic exercise may not confer the same anti-inflammatory benefits in women as it does in men.

Sarcopenic Obesity, Insulin Resistance, and Their Implications in Cardiovascular and Metabolic Consequences

The prevalence of sarcopenic obesity is increasing worldwide, particularly amongst aging populations. Insulin resistance is the core mechanism of sarcopenic obesity and is also associated with variable cardiometabolic diseases such as cardiovascular disease, type 2 diabetes mellitus, and non-alcoholic fatty liver disease. Fat accumulation in muscle tissue promotes a proinflammatory cascade and oxidative stress, leading to mitochondrial dysfunction, impaired insulin signaling, and muscle atrophy. To compound the problem, decreased muscle mass aggravates insulin resistance. In addition, the crosstalk between myokines and adipokines leads to negative feedback, which in turn aggravates sarcopenic obesity and insulin resistance. In this review, we focus on the molecular mechanisms linking sarcopenic obesity and insulin resistance with various biological pathways. We also discuss the impact and mechanism of sarcopenic obesity and insulin resistance on cardiometabolic disease.

Interleukin-6 Induces Myogenic Differentiation via JAK2-STAT3 Signaling in Mouse C2C12 Myoblast Cell Line and Primary Human Myoblasts

Postnatal muscle growth and exercise- or injury-induced regeneration are facilitated by myoblasts. Myoblasts respond to a variety of proteins such as cytokines that activate various signaling cascades. Cytokines belonging to the interleukin 6 superfamily (IL-6) influence myoblasts' proliferation but their effect on differentiation is still being researched. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway is one of the key signaling pathways identified to be activated by IL-6. The aim of this study was to investigate myoblast fate as well as activation of JAK-STAT pathway at different physiologically relevant IL-6 concentrations (10 pg/mL; 100 pg/mL; 10 ng/mL) in the C2C12 mouse myoblast cell line and primary human myoblasts, isolated from eight young healthy male volunteers. Myoblasts' cell cycle progression, proliferation and differentiation in vitro were assessed. Low IL-6 concentrations facilitated cell cycle transition from the quiescence/Gap1 (G0/G1) to the synthesis (S-) phases. Low and medium IL-6 concentrations decreased the expression of myoblast determination protein 1 (MyoD) and myogenin and increased proliferating cell nuclear antigen (PCNA) expression. In contrast, high IL-6 concentration shifted a larger proportion of cells to the pro-differentiation G0/G1 phase of the cell cycle, substantiated by significant increases of both MyoD and myogenin expression and decreased PCNA expression. Low IL-6 concentration was responsible for prolonged JAK1 activation and increased suppressor of cytokine signaling 1 (SOCS1) protein expression. JAK-STAT inhibition abrogated IL-6-mediated C2C12 cell proliferation. In contrast, high IL-6 initially increased JAK1 activation but resulted in prolonged JAK2 activation and elevated SOCS3 protein expression. High IL-6 concentration decreased interleukin-6 receptor (IL-6R) expression 24 h after treatment whilst low IL-6 concentration increased IL-6 receptor (IL-6R) expression at the same time point. In conclusion, this study demonstrated that IL-6 has concentration- and time-dependent effects on both C2C12 mouse myoblasts and primary human myoblasts. Low IL-6 concentration induces proliferation whilst high IL-6 concentration induces differentiation. These effects are mediated by specific components of the JAK/STAT/SOCS pathway.

The influence of capillarization on satellite cell pool expansion and activation following exercise-induced muscle damage in healthy young men

KEY POINTS

Skeletal muscle stem cells (satellite cells) play a crucial role in repair and remodelling of muscle in response to exercise. Satellite cells are in close spatial proximity to muscle capillaries and therefore may be influenced by them. In this study, we describe the activation and expansion of the satellite cell pool in response to eccentric contraction-induced muscle damage in individuals with significantly different levels of muscle capillarization. Individuals with greater capillarization and capacity for muscle perfusion demonstrated enhanced activation and/or expansion of the satellite cell pool allowing for an accelerated recovery of muscle function. These results provide insight into the critical relationship between muscle capillarization and satellite cells during skeletal muscle repair.

ABSTRACT

Factors that determine the skeletal muscle satellite cell (SC) response remain incompletely understood. It is known, however, that SC activation status is closely related to the anatomical relationship between SCs and muscle capillaries. We investigated the impact of muscle fibre capillarization on the expansion and activation status of SCs following a muscle-damaging exercise protocol in healthy young men. Twenty-nine young men (21 ± 0.5 years) performed 300 unilateral eccentric contractions (180 deg s^-1 ) of the knee extensors. Percutaneous muscle biopsies from the vastus lateralis and blood samples from the antecubital vein were taken prior to (Pre) exercise and at 6, 24, 72 and 96 h of post-exercise recovery. A comparison was made between subjects who had a relative low mixed muscle capillary-to-fibre perimeter exchange index (CFPE; Low group) and high mixed muscle CFPE index (High group) at baseline. Type I and type II muscle fibre size, myonuclear content, capillarization, and SC response were determined via immunohistochemistry. Overall, there was a significant correlation (r = 0.39; P < 0.05) between the expansion of SC content (change in total Pax7⁺ cells/100 myofibres) 24 h following eccentric exercise and mixed muscle CFPE index. There was a greater increase in activated SCs (MyoD⁺ /Pax7⁺ cells) in the High as compared to the Low CFPE group 72 h following eccentric exercise (P < 0.05). The current study provides further evidence that muscle fibre capillarization may play an important role in the activation and expansion of the SC pool during the process of skeletal muscle repair.

Deletion of suppressor of cytokine signaling 3 (SOCS3) in muscle stem cells does not alter muscle regeneration in mice after injury

Muscles of older animals are more susceptible to injury and regenerate poorly, in part due to a persistent inflammatory response. The janus kinase (Jak)/signal transducer and activator of transcription (Stat) pathway mediates inflammatory signaling and is tightly regulated by the suppressor of cytokine signaling (SOCS) proteins, especially SOCS3. SOCS3 expression is altered in the muscle of aged animals and may contribute to the persistent inflammation and impaired regeneration. To test this hypothesis, we performed myotoxic injuries on mice with a tamoxifen-inducible deletion of SOCS3 specifically within the muscle stem cell compartment. Muscle stem cell-specific SOCS3 deletion reduced muscle mass at 14 days post-injury (-14%, P < 0.01), altered the myogenic transcriptional program, and reduced myogenic fusion based on the number of centrally-located nuclei per muscle fiber. Despite the delay in myogenesis, muscles with a muscle stem cell-specific deletion of SOCS3 were still able to regenerate after a single bout or multiple bouts of myotoxic injury. A reduction in SOCS3 expression in muscle stem cells is unlikely to be responsible for the incomplete muscle repair in aged animals.

An Overview about the Biology of Skeletal Muscle Satellite Cells

The peculiar ability of skeletal muscle tissue to operate adaptive changes during post-natal de-velopment and adulthood has been associated with the existence of adult somatic stem cells. Satellite cells, occupying an exclusive niche within the adult muscle tissue, are considered bona fide stem cells with both stem-like properties and myogenic activities. Indeed, satellite cells retain the capability to both maintain the quiescence in uninjured muscles and to be promptly activated in response to growth or re-generative signals, re-engaging the cell cycle. Activated cells can undergo myogenic differentiation or self-renewal moving back to the quiescent state. Satellite cells behavior and their fate decision are finely controlled by mechanisms involving both cell-autonomous and external stimuli. Alterations in these regu-latory networks profoundly affect muscle homeostasis and the dynamic response to tissue damage, con-tributing to the decline of skeletal muscle that occurs under physio-pathologic conditions. Although the clear myogenic activity of satellite cells has been described and their pivotal role in muscle growth and regeneration has been reported, a comprehensive picture of inter-related mechanisms guiding muscle stem cell activity has still to be defined. Here, we reviewed the main regulatory networks determining satellite cell behavior. In particular, we focused on genetic and epigenetic mechanisms underlining satel-lite cell maintenance and commitment. Besides intrinsic regulations, we reported current evidences about the influence of environmental stimuli, derived from other cell populations within muscle tissue, on satel-lite cell biology.

Prolonged exercise training improves the acute type II muscle fibre satellite cell response in healthy older men

KEY POINTS

Skeletal muscle stem cells, termed satellite cells, play a crucial role in repair and remodelling of muscle in response to exercise An age-related decline in satellite cell number and/or function has been hypothesized to be a key factor in the development of sarcopenia and/or the blunted muscle fibre adaptive response to prolonged exercise training in older persons We report that performing prolonged exercise training improves the acute type II muscle fibre satellite cell response following a single bout of resistance exercise in older men. The observed improvement in muscle satellite function is associated with an increase in muscle fibre capillarization following exercise training suggesting a possible functional link between capillarization and satellite cell function.

ABSTRACT

Age-related type II muscle fibre atrophy is accompanied by a fibre type-specific decline in satellite cell number and function. Exercise training restores satellite cell quantity in older adults; however, whether it can restore the impaired satellite cell response to exercise in older adults remains unknown. Therefore we assessed the acute satellite cell response to a single exercise session before and after prolonged exercise training in older men. Fourteen older men (74 ± 8 years) participated in a 12-week exercise training programme (resistance exercise performed twice per week, high intensity interval training once per week). Before and after training, percutaneous biopsies from the vastus lateralis muscle were taken prior to and following 24 and 48 h of post-exercise recovery. Muscle fibre characteristics were evaluated by immunohistochemistry and mRNA expression by RT-PCR. Whereas no changes were observed in type II muscle fibres, type I muscle fibre satellite cell content increased significantly at 24 and 48 h after a single bout of resistance exercise before the exercise training programme (P < 0.01). Following the exercise training programme, both type I and type II muscle fibre satellite cell content increased significantly at 24 and 48 h after a single bout of resistance exercise (P < 0.05). The greater acute increase in type II muscle fibre satellite cell content at 24 h post-exercise recovery after training was correlated with an increase in type II muscle fibre capillarization (r = 0.671, P = 0.012). We show that the acute muscle satellite cell response following exercise can be improved by prolonged exercise training in older men.

Blunted satellite cell response is associated with dysregulated IGF-1 expression after exercise with age

PURPOSE

Insulin-like growth factor-1 (IGF-1) regulates protein synthesis and cell cycle kinetics. Given that aging is associated with anabolic resistance, we sought to determine if the attenuated exercise-induced satellite cell (SC) expression in older muscle is associated with a blunted IGF-1 response.

METHODS

SC expression (Pax7⁺ cells) and protein (Western blot) and mRNA (RT-PCR) expression of IGF-1 splice variants and ubiquitous (IGFBP4) and muscle-specific (IGFBP3 and -5) IGF-1 binding proteins were measured in skeletal muscle of young (Y: 22 ± 2, n = 7) and older (O: 70 ± 2, n = 7) adults up to 48 h after an acute bout of resistance exercise.

RESULTS

SC expression was greater in Y compared to O (age; P < 0.01) and increased (interaction; P < 0.05) by 24 h after exercise in Y only. IGF-1Ea and IGF-1Eb mRNA tended to be greater in O (age; P < 0.06-0.09). IGF-1Eb mRNA increased at 48 h (time; P < 0.05), whereas IGF-1Ec mRNA increased (interaction; P < 0.05) at 24 and 48 h in O only. IGF binding protein (IGFBP)4 mRNA was greater (age; P < 0.01) in O with the increase at 24 h and 48 h (time; P < 0.01) primarily driven by changes in O (interaction; P < 0.01). Despite IGFBP3 mRNA being greater in O (age; P < 0.01) and increasing at 48 h (time; P < 0.01), there was no effect of age or exercise on IGFBP3 protein expression. In contrast, IGFBP5 mRNA was greater (age; P < 0.01) despite IGFBP5 protein expression being lower (age; P < 0.01) in O compared to Y.

CONCLUSIONS

The greater muscle-specific expression of IGF-1 family members with a blunted post-exercise SC expression may be a compensatory attempt to rescue age-related anabolic resistance.

Systemic IL-6 and Myoglobin Response to Three Different Resistance Exercise Intensities in Older Men

The purpose of this research was to identify if three different intensities of resistance exercise would acutely and differentially effect the systemic release of interleukin-6 (IL-6) and myoglobin in older men (≥65 years). A total of 11 older men performed isovolume resistance exercise on six different apparatuses at three different intensities (144 reps at 60%, 120 reps at 72%, and 108 reps at 80% of 1-repetition maximum), with the intensity order randomly allocated, to determine the systemic release of IL-6 and myoglobin in the blood. Blood samples were collected at six time points, including preexercise, immediately postexercise, and 3, 6, 24, and 48 hr postexercise. There were no differences between intensity levels; therefore, data for all conditions were pooled. IL-6 did not show any change from baseline values throughout all time points (p > .05), whereas myoglobin was elevated at 3, 6, and 24 hr postexercise and returned to baseline after 48 hr (p < .05).

Neural and musculotendinous mechanisms underpinning age-related force reductions

Ageing leads to substantial force production capacity reductions, which is an indicator of frailty and disability, and a mortality predictor in elders. Understanding the age-dependent neuromuscular mechanisms underlying force reductions can optimize healthcare professionals' exercise protocol choices for patients and allows researchers to investigate new interventions to mitigate these reductions. Our primary goal was to provide an updated review about the main neural and musculotendinous mechanisms underpinning age-related reductions in force capacity. Our secondary goal was to summarize how aerobic and strength training can lessen these age-related reductions. This review suggests that several steps in the force production pathway, from cortical to muscular mechanisms, are negatively affected by ageing. However, combining aerobic and strength training can attenuate these effects. Strength training (i.e. moderate to high- intensity, progressive volume, accentuated eccentric loading and fast concentric contraction velocity) can increase overall force production capacity by producing beneficial neural and musculotendinous adaptations. Additionally, aerobic training (i.e. moderate and high intensities) plays an essential role in preserving the structure and function of the neuromuscular system.

The physiopathologic interplay between stem cells and tissue niche in muscle regeneration and the role of IL-6 on muscle homeostasis and diseases

Skeletal muscle is a complex, dynamic tissue characterized by an elevated plasticity. Although the adult muscle is mainly composed of multinucleated fibers with post mitotic nuclei, it retains a remarkable ability to regenerate in response to traumatic events. The regenerative potential of the adult skeletal muscle relies in the activity of satellite cells, mononucleated cells residing within the muscle in intimate association with myofibers. Satellite cells normally remain quiescent in their sublaminar position, sporadically entering the cell cycle to guarantee an efficient cellular turnover, by fusing with pre-existing myofibers, and to maintain the stem cell pool. However, after muscle injury satellite cells undergo an extensive increase of their activity in response to environmental stimuli, thereby participating to the regeneration of a functional muscle tissue. Nevertheless, regeneration is affected in several pathologic conditions and by a wide range of environmental signals that are highly variable, not only through time, but also depending on the physiological or pathological conditions of the musculature. Among these factors, the interleukin-6 (IL-6) plays a critical physiopathologic role on muscle homeostasis and diseases. The basis of muscle regeneration and the impact of IL-6 on the physiopathology of skeletal muscle will be discussed.

Genetic Variant of SOCS3 Gene is Functionally Associated With Lumbar Adolescent Idiopathic Scoliosis

STUDY DESIGN

This is a genetic association study.

OBJECTIVE

To investigate association between suppressor of cytokine signaling-3 (SOCS3) gene polymorphisms and the onset and progression of lumbar adolescent idiopathic scoliosis (AIS) and to further clarify its role in the regulation of SOCS3 expression in AIS patients.

SUMMARY OF BACKGROUND DATA

Some studies showed that muscle development imbalance may be responsible for onset and progression of lumbar AIS. SOCS3 is one of the significant regulators of skeletal muscle development, and in vitro study showed that SOCS3 influences myoblast differentiation.

MATERIALS AND METHODS

Rs4969198 was genotyped in 476 lumbar AIS patients and 672 controls. The differences of genotype and allele distributions between patients and controls were calculated using the χ test. Paravertebral muscles were collected from 53 AIS, 23 congenital scoliosis, and 18 lumbar disk herniation patients. AIS patients were classified into 3 groups according to the genotypes of each single nucleotide polymorphisms, and 1-way analysis of variance test was used to compare SOCS3 expression among different groups and genotypes.

RESULTS

Patients were found to have a significantly higher frequency of GG than the controls (40.8% vs. 29.9%, odds ratio=1.36; P=0.000), and the frequency of allele G was found to be remarkably higher in the patients than the controls (65.3% vs. 56.7%, odds ratio=1.15; P=0.000). AIS patients had significantly less muscle expression of the SOCS3 than the congenital scoliosis patients (2.73±2.17 vs. 4.62±2.41; P=0.006) and the lumbar disk herniation patients (2.73±2.17 vs. 4.12±2.93; P=0.009). The SOCS3 expression was significantly correlated with the curve severity (r=0.472; P=0.014).

CONCLUSIONS

The SOCS3 gene is significantly associated with the development of lumbar AIS in Chinese population. Decreased expression of SOCS3 is associated with larger severity of lumbar AIS.

LEVEL OF EVIDENCE

Level III.

Postexercise essential amino acid supplementation amplifies skeletal muscle satellite cell proliferation in older men 24 hours postexercise

Aged skeletal muscle has an attenuated and delayed ability to proliferate satellite cells in response to resistance exercise. The mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway is a focal point for cell growth, however, the effect of postexercise mTORC1 activation on human skeletal muscle satellite cell (SC) proliferation is unknown. To test the proliferative capacity of skeletal muscle SC in aging muscle to a potent mTORC1 activator (i.e., EAA; essential amino acids) we recruited older (~72y) men to conduct leg resistance exercise (8setsx10reps) without (−EAA; n = 8) and with (+EAA: n = 11) ingestion of 10 g of EAA 1 h postexercise. Muscle biopsies were taken before exercise (Pre) and 24 h postexercise (Post) for assessment of expression and fiber type‐specific Pax7⁺SC, Ki67⁺Pax7⁺SC and MyoD⁺SC. −EAA did not show an increase in Pax7⁺ satellite cells at Post(P > 0.82). Although statistical significance for an increase in Pax7 +  SC at 24 h post‐RE was not observed in +EAA versus −EAA, we observed trends for a treatment difference (P < 0.1). When examining the change from Pre to Post trends were demonstrated (#/myofiber: P = 0.076; and %/myonuclei: P = 0.065) for a greater increase in +EAA versus −EAA. Notably, we found an increase SC proliferation in +EAA, but not −EAA with increase in Ki67⁺SC and MyoD⁺ cells (P < 0.05). Ki67⁺SC also exhibited a significant group difference Post (P < 0.010). Pax7⁺SC in fast twitch myofibers did not change and were not different between groups (P > 0.10). CDK2, MEF2C, RB1 mRNA only increased in +EAA (P < 0.05). Acute muscle satellite cell proliferative capacity may be partially rescued with postexercise EAA ingestion in older men.

Potential Roles of n-3 PUFAs during Skeletal Muscle Growth and Regeneration

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), which are commonly found in fish oil supplements, are known to possess anti-inflammatory properties and more recently alter skeletal muscle function. In this review, we discuss novel findings related to how n-3 PUFAs modulate molecular signaling responsible for growth and hypertrophy as well as the activity of muscle stem cells. Muscle stem cells commonly known as satellite cells, are primarily responsible for driving the skeletal muscle repair process to potentially damaging stimuli, such as mechanical stress elicited by exercise contraction. To date, there is a paucity of human investigations related to the effects of n-3 PUFAs on satellite cell content and activity. Based on current in vitro investigations, this review focuses on novel mechanisms linking n-3 PUFA’s to satellite cell activity and how they may improve muscle repair. Understanding the role of n-3 PUFAs during muscle growth and regeneration in association with exercise could lead to the development of novel supplementation strategies that increase muscle mass and strength, therefore possibly reducing the burden of muscle wasting with age.

"Known Unknowns": Current Questions in Muscle Satellite Cell Biology

Our understanding of satellite cells, now known to be the obligate stem cells of skeletal muscle, has increased dramatically in recent years due to the introduction of new molecular, genetic, and technical resources. In addition to their role in acute repair of damaged muscle, satellite cells are of interest in the fields of aging, exercise, neuromuscular disease, and stem cell therapy, and all of these applications have driven a dramatic increase in our understanding of the activity and potential of satellite cells. However, many fundamental questions of satellite cell biology remain to be answered, including their emergence as a specific lineage, the degree and significance of heterogeneity within the satellite cell population, the roles of their interactions with other resident and infiltrating cell types during homeostasis and regeneration, and the relative roles of intrinsic vs extrinsic factors that may contribute to satellite cell dysfunction in the context of aging or disease. This review will address the current state of these open questions in satellite cell biology.

The Muscle Stem Cell Niche in Health and Disease

The regulation of stem cells that maintain and regenerate postnatal tissues depends on extrinsic signals originating from their microenvironment, commonly referred to as the stem cell niche. Complex higher-order regulatory interrelationships with the tissue and factors in the systemic circulation are integrated and propagated to the stem cells through the niche. The stem cell niche in skeletal muscle tissue is both a paradigm for a structurally and functionally relatively static niche that maintains stem cell quiescence during tissue homeostasis, and a highly dynamic regenerative niche that is subject to extensive structural remodeling and a flux of different support cell populations. Conditions ranging from aging to chronically degenerative skeletal muscle diseases affect the composition of the niche and thereby impair the regenerative potential of muscle stem cells. A holistic and integrative understanding of the extrinsic mechanisms regulating muscle stem cells in health and disease in a broad systemic context will be imperative for the identification of regulatory hubs in the niche interactome that can be targeted to maintain, restore, or enhance the regenerative capacity of muscle tissue. Here, we review the microenvironmental regulation of muscle stem cells, summarize how niche dysfunction can contribute to disease, and discuss emerging therapeutic implications.

Muscle Stem Cells and Aging

Satellite cells (SCs) are a population of muscle-resident stem cells that are essential for efficient tissue repair. SCs reside in a relatively quiescent state during normal tissue turnover, but are activated in response to injury through the microenvironment and cell-intrinsic signals. During aging, SC dysfunction is a major contributor to the decline in regenerative potential of muscle tissue. Recent studies have demonstrated that both cell-intrinsic and cell-extrinsic factors are deregulated during aging. Interventions that reverse age-associated changes in SCs or the niche have shown to partially rejuvenate the regenerative capacity of aged muscle SCs. In this review, we discuss recent advances in SC biology as it pertains to the deleterious effects of aging. A better understanding of how age-dependent changes in the SC and its environment niche impact muscle regeneration could lead to interventions to ameliorate the effects of aging in humans.

Acute and chronic effects of exercise on mRNA expression in the skeletal muscle of two mouse models of peripheral artery disease

Endurance exercise improves walking performance in patients with peripheral artery disease (PAD), which is characterized by skeletal muscle dysfunction caused by lower extremity ischemia. Although transcriptional analyses of exercise-induced changes in normal animals and healthy volunteers have been reported, no detailed study has explored exercise-induced alterations in gene expression in PAD animal models. Here, we determined the acute and chronic effects of exercise on mRNA expression in the skeletal muscles of two mouse models of PAD. Three particular gene categories were investigated: known exercise-responsive genes (Pgc1a, Il6, Nr4a1, Nr4a2, and Nr4a3); myogenic and muscle regeneration-related genes (Myf5, Myogenin, Myomaker, and Myh3); and Gpr56 and its ligand Col3a1. PAD was induced by bilateral femoral artery ligation in normal C57BL/6 and diabetic KK-A^y mice. From 1 week after surgery, repetitive twice-weekly 30-min treadmill endurance exercise sessions were applied. Altered mRNA expression in the soleus muscles was measured in both the acute and chronic phases. In the acute phase, transcript levels of exercise-inducible genes showed significant increases in both C57BL/6 and diabetic KK-A^y PAD mice; levels of regeneration-related genes showed little alteration, and those of Gpr56 increased immediately and significantly after exercise in both models. In the chronic phase, transcript levels of Pgc1a, Myf5, Myogenin, Myomaker, Myh3, Gpr56, and Col3a1 were upregulated significantly in sedentary C57BL/6 PAD mice compared with that in sham-operated mice. Exercise training inhibited the upregulation of Col3a1, Myf5, and Myogenin significantly. In KK-A^y PAD mice, only Gpr56 mRNA levels increased significantly compared with those in sham-operated mice. RNA sequence analysis revealed 33 and 166 differentially upregulated, and 363 and 99 downregulated, genes after exercise training in C57BL/6 PAD and KK-A^y PAD mice, respectively. In summary, we detected significant alterations of skeletal muscle genes after exercise in PAD mouse models and characterized their expression patterns.

Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men

BACKGROUND

Adequate muscle fibre perfusion is critical for the maintenance of muscle mass; it is essential in the rapid delivery of oxygen, nutrients and growth factors to the muscle, stimulating muscle fibre growth. Muscle fibre capillarization is known to decrease substantially with advancing age. However, whether (relative) low muscle fibre capillarization negatively impacts the muscle hypertrophic response following resistance exercise training in older adults is unknown.

METHODS

Twenty-two healthy older men (71 ± 1 years) performed 24 weeks of progressive resistance type exercise training. To assess the change in muscle fibre characteristics, percutaneous biopsies from the vastus lateralis muscle were taken before and following 12 and 24 weeks of the intervention programme. A comparison was made between participants who had a relatively low type II muscle fibre capillary-to-fibre perimeter exchange index (CFPE; LOW group) and high type II muscle fibre CFPE (HIGH group) at baseline. Type I and type II muscle fibre size, satellite cell, capillary content and distance between satellite cells to the nearest capillary were determined by immunohistochemistry.

RESULTS

Overall, type II muscle fibre size (from 5150 ± 234 to 6719 ± 446 µm² , P < 0.05) and satellite cell content (from 0.058 ± 0.006 to 0.090 ± 0.010 satellite cells per muscle fibre, P < 0.05) had increased significantly in response to 24 weeks of resistance exercise training. However, these improvements where mainly driven by differences in baseline type II muscle fibre capillarization, whereas muscle fibre size (from 5170 ± 390 to 7133 ± 314 µm² , P < 0.05) and satellite cell content (from 0.059 ± 0.009 to 0.102 ± 0.017 satellite cells per muscle fibre, P < 0.05) increased significantly in the HIGH group, no significant changes were observed in LOW group following exercise training. No significant changes in type I and type II muscle fibre capillarization were observed in response to 12 and 24 weeks of resistance exercise training in both the LOW and HIGH group.

CONCLUSIONS

Type II muscle fibre capillarization at baseline may be a critical factor for allowing muscle fibre hypertrophy to occur during prolonged resistance exercise training in older men.

Skeletal muscle satellite cells are located at a closer proximity to capillaries in healthy young compared with older men

BACKGROUND

Skeletal muscle satellite cells (SC) are instrumental in maintenance of muscle fibres, the adaptive responses to exercise, and there is an age-related decline in SC. A spatial relationship exists between SC and muscle fibre capillaries. In the present study, we aimed to investigate whether chronologic age has an impact on the spatial relationship between SC and muscle fibre capillaries. Secondly, we determined whether this spatial relationship changes in response to a single session of resistance exercise.

METHODS

Muscle biopsies were obtained from the vastus lateralis of previously untrained young men (YM, 24 ± 3 years; n = 23) and older men (OM, 67 ± 4 years; n = 22) at rest. A subset of YM (n = 9) performed a single bout of resistance exercise, where additional muscle biopsies taken at 24 and 72 h post-exercise recovery. Skeletal muscle fibre capillarization, SC content, and activation status were assessed using immunofluorescent microscopy of muscle cross sections.

RESULTS

Type II muscle fibre SC and capillary content was significantly lower in the YM compared with OM (P < 0.05). Furthermore, type II muscle fibre SC were located at a greater distance from the nearest capillary in OM compared with YM (21.6 ± 1.3 vs. 17.0 ± 0.8 µm, respectively; P < 0.05). In response to a single bout of exercise, we observed a significant increase in SC number and activation status (P < 0.05). In addition, activated vs. quiescent SC were situated closer (P < 0.05) to capillaries.

CONCLUSIONS

We demonstrate that there is a greater distance between capillaries and type II fibre-associated SC in OM as compared with YM. Furthermore, quiescent SC are located significantly further away from capillaries than active SC after single bout of exercise. Our data have implications for how muscle adapts to exercise and how aging may affect such adaptations.

Muscle-specific deletion of SOCS3 increases the early inflammatory response but does not affect regeneration after myotoxic injury

BACKGROUND

Muscles of old animals are injured more easily and regenerate poorly, attributed in part to increased levels of circulating pro-inflammatory cytokines. The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade is a key mediator of inflammatory cytokine action, and signaling via this pathway is increased in muscles with aging. As a negative regulator of JAK/STAT signaling, a key mediator of myogenic proliferation and differentiation, altered expression of suppressor of cytokine signaling (SOCS3) is likely to have important consequences for muscle regeneration. To model this scenario, we investigated the effect of SOCS3 deletion within mature muscle fibers on injury and repair. We tested the hypothesis that reduced SOCS3 function would alter the inflammatory response and impair muscle regeneration after myotoxic injury.

METHODS

Mice with a specific deletion of SOCS3 within mature skeletal muscle fibers were used to assess the effect of SOCS3 deletion on muscle injury and repair. Twelve-week-old or 24-month-old SOCS3 muscle-specific knockout (SOCS3 MKO) mice and littermate controls were either left uninjured or injured with a single injection of notexin (10 μg/ml) into the right tibialis anterior (TA) muscle. At 1, 2, 3, 5, 7, or 14 days post-injury, the right TA muscle was excised and subjected to histological, western immunoblotting, and gene expression analyses. Force production and fatigue were assessed in uninjured muscles and at 7 days post-notexin injury.

RESULTS

In uninjured muscles, SOCS3 deletion decreased force production during fatigue but had no effect on the gross or histological appearance of the TA muscles. After notexin injury, deletion of SOCS3 increased STAT3 phosphorylation at day 1 and increased the mRNA expression of the inflammatory cytokine TNF-α, and the inflammatory cell markers F4/80 and CD68 at day 2. Gene expression analysis of the regeneration markers Pax7, MyoD, and Myogenin indicated SOCS3 deletion had no effect on the progression of muscle repair after notexin injury. Inflammation and regeneration were also unchanged in the muscles of 24-month-old SOCS3 MKO mice compared with control.

CONCLUSIONS

Loss of SOCS3 expression in mature muscle fibers increased the inflammatory response to myotoxic injury but did not impair muscle regeneration in either adult or old mice. Therefore, reduced SOCS3 expression in muscle fibers is unlikely to underlie impaired muscle regeneration. Further investigation into the role of SOCS3 in other cell types involved in muscle repair is warranted.

Frailty and sarcopenia as the basis for the phenotypic manifestation of chronic diseases in older adults

Frailty is a functional status that precedes disability and is characterized by decreased functional reserve and increased vulnerability. In addition to disability, the frailty phenotype predicts falls, institutionalization, hospitalization and mortality. Frailty is the consequence of the interaction between the aging process and some chronic diseases and conditions that compromise functional systems and finally produce sarcopenia. Many of the clinical manifestations of frailty are explained by sarcopenia which is closely related to poor physical performance. Reduced regenerative capacity, malperfusion, oxidative stress, mitochondrial dysfunction and inflammation compose the sarcopenic skeletal muscle alterations associated to the frailty phenotype. Inflammation appears as a common determinant for chronic diseases, sarcopenia and frailty. The strategies to prevent the frailty phenotype include an adequate amount of physical activity and exercise as well as pharmacological interventions such as myostatin inhibitors and specific androgen receptor modulators. Cell response to stress pathways such as Nrf2, sirtuins and klotho could be considered as future therapeutic interventions for the management of frailty phenotype and aging-related chronic diseases.

Computed Biological Relations among Five Select Treatment-Related Organ/Tissue Toxicities

Drug toxicity presents a major challenge in drug development and patient care. We set to build upon previous works regarding select drug-induced toxicities to find common patterns in the mode of action of the drugs associated with these toxicities. In particular, we focused on five disparate organ toxicities, peripheral neuropathy (PN), rhabdomyolysis (RM), Stevens-Johnson syndrome/toxic epidermal necrosis (SJS/TEN), lung injury (LI), and heart contraction-related cardiotoxicity (CT), and identified biological commonalities between and among the toxicities in terms of pharmacological targets and nearest neighbors (indirect effects) using the hyper-geometric test and a distance metric of Spearman correlation. There were 20 significant protein targets associated with two toxicities and 0 protein targets associated with three or more toxicities. Per Spearman distance, PN was closest to SJS/TEN compared to other pairs, whereas the pairs involving RM were more different than others excluding RM. The significant targets associated with RM outnumbered those associated with every one of the other four toxicities. Enrichment analysis of drug targets that are expressed in corresponding organ/tissues determined proteins that should be avoided in drug discovery. The identified biological patterns emerging from the mode of action of these drugs are statistically associated with these serious toxicities and could potentially be used as predictors for new drug candidates. The predictive power and usefulness of these biological patterns will increase with the database of these five toxicities. Furthermore, extension of our approach to all severe adverse reactions will produce useful biological commonalities for reference in drug discovery and development.

Expansion of myeloid-derived suppressor cells with arginase activity lasts longer in aged than in young mice after CpG-ODN plus IFA treatment

As we age, the homeostatic function of many systems in the body, such as the immune function declines, which in turn contributes to augment susceptibility to disease. Here we describe that challenging aged mice with synthetic oligodeoxynucleotides containing unmethylated cytosine guanine motifs (CpG-ODN) emulsified in incomplete Freund's adjuvant (IFA), (CpG-ODN+IFA) an inflammatory stimulus, led to the expansion of CD11b⁺Gr1⁺ myeloid cells with augmented expression of CD124 and CD31. These myeloid cells lasted longer in the spleen of aged mice than in their younger counterparts after CpG-ODN+IFA treatment and were capable of suppressing T cell proliferative response by arginase induction. Myeloid cells from aged CpG-ODN+IFA-treated mice presented increased arginase-1 expression and enzyme activity. In addition, we found a different requirement of cytokines for arginase induction according to mice age. In myeloid cells from young treated mice, arginase-1 expression and activity is induced by the presence of each IL-4 or IL-6 in their extracellular medium, unlike myeloid cells from aged treated mice which need the presence of both IL-4 and IL-6 together for arginase induction and suppressor function.

Exercise attenuates the major hallmarks of aging

Regular exercise has multi-system anti-aging effects. Here we summarize how exercise impacts the major hallmarks of aging. We propose that, besides searching for novel pharmaceutical targets of the aging process, more research efforts should be devoted to gaining insights into the molecular mediators of the benefits of exercise and to implement effective exercise interventions for elderly people.

The effect of exercise mode on the acute response of satellite cells in old men

AIM

A dysregulation of satellite cells may contribute to the progressive loss of muscle mass that occurs with age; however, older adults retain the ability to activate and expand their satellite cell pool in response to exercise. The modality of exercise capable of inducing the greatest acute response is unknown. We sought to characterize the acute satellite cell response following different modes of exercise in older adults.

METHODS

Sedentary older men (n = 22; 67 ± 4 years; 27 ± 2.6 kg*m(-2) ) were randomly assigned to complete an acute bout of either resistance exercise, high-intensity interval exercise on a cycle ergometer or moderate-intensity aerobic exercise. Muscle biopsies were obtained before, 24 and 48 h following each exercise bout. The satellite cell response was analysed using immunofluorescent microscopy of muscle cross sections.

RESULTS

Satellite cell expansion associated with type I fibres was observed 24 and 48 h following resistance exercise only (P ˂ 0.05), while no expansion of type II-associated satellite cells was observed in any group. There was a greater number of activated satellite cells 24 h following resistance exercise (pre: 1.3 ± 0.1, 24 h: 4.8 ± 0.5 Pax7 + /MyoD+cells/100 fibres) and high-intensity interval exercise (pre: 0.7 ± 0.3, 24 h: 3.1 ± 0.3 Pax7 + /MyoD+cells/100 fibres) (P ˂ 0.05). The percentage of type I-associated SC co-expressing MSTN was reduced only in the RE group 24 h following exercise (pre: 87 ± 4, 24 h: 57 ± 5%MSTN+ type I SC) (P < 0.001).

CONCLUSION

Although resistance exercise is the most potent exercise type to induce satellite cell pool expansion, high-intensity interval exercise was also more potent than moderate-intensity aerobic exercise in inducing satellite cell activity.

Satellite cells in human skeletal muscle plasticity

Skeletal muscle satellite cells are considered to play a crucial role in muscle fiber maintenance, repair and remodeling. Our knowledge of the role of satellite cells in muscle fiber adaptation has traditionally relied on in vitro cell and in vivo animal models. Over the past decade, a genuine effort has been made to translate these results to humans under physiological conditions. Findings from in vivo human studies suggest that satellite cells play a key role in skeletal muscle fiber repair/remodeling in response to exercise. Mounting evidence indicates that aging has a profound impact on the regulation of satellite cells in human skeletal muscle. Yet, the precise role of satellite cells in the development of muscle fiber atrophy with age remains unresolved. This review seeks to integrate recent results from in vivo human studies on satellite cell function in muscle fiber repair/remodeling in the wider context of satellite cell biology whose literature is largely based on animal and cell models.

Preserved skeletal muscle protein anabolic response to acute exercise and protein intake in well-treated rheumatoid arthritis patients

Introduction

Rheumatoid arthritis (RA) is often associated with diminished muscle mass, reflecting an imbalance between protein synthesis and protein breakdown. To investigate the anabolic potential of both exercise and nutritional protein intake we investigated the muscle protein synthesis rate and anabolic signaling response in patients with RA compared to healthy controls.

Methods

Thirteen RA patients (age range 34–84 years; diagnosed for 1–32 years, median 8 years) were individually matched with 13 healthy controls for gender, age, BMI and activity level (CON). Plasma levels of C-reactive protein (CRP), interleukin (IL)-6 and tumor necrosis factor (TNF)-α were measured using enzyme-linked immunosorbent assay (ELISA) in resting blood samples obtained on two separate days. Skeletal muscle myofibrillar and connective tissue protein fractional synthesis rate (FSR) was measured by incorporation of the amino acid ¹³C₆-phenylalanine tracer in the overnight fasted state for 3 hours (BASAL) and 3 hours after intake of whey protein (0.5 g/kg lean body mass) alone (PROT, 3 hrs) and in combination with knee-extensor exercise (EX) with one leg (8 × 10 reps at 70 % of 1RM; PROT + EX, 3 hrs). Expression of genes related to inflammatory signaling, myogenesis and muscle growth/atrophy were analyzed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR).

Results

CRP was significantly higher in the RA patients (2.25 (0.50) mg/l) than in controls (1.07 (0.25) mg/l; p = 0.038) and so was TNF-α (RA 1.18 (0.30) pg/ml vs. CON 0.64 (0.07) pg/ml; p = 0.008). Muscle myofibrillar protein synthesis in both RA patients and CON increased in response to PROT and PROT + EX, and even more with PROT + EX (p < 0.001), with no difference between groups (p > 0.05). The gene expression response was largely similar in RA vs. CON, however, expression of the genes coding for TNF-α, myogenin and HGF1 were more responsive to exercise in RA patients than in CON.

Conclusions

The study demonstrates that muscle protein synthesis rate and muscle gene expression can be stimulated by protein intake alone and in combination with physical exercise in patients with well-treated RA to a similar extent as in healthy individuals. This indicates that moderately inflamed RA patients have maintained their muscle anabolic responsiveness to physical activity and protein intake.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0758-3) contains supplementary material, which is available to authorized users.

Satellite cell activity, without expansion, after nonhypertrophic stimuli

The purpose of the present studies was to determine the effect of various nonhypertrophic exercise stimuli on satellite cell (SC) pool activity in human skeletal muscle. Previously untrained men and women (men: 29 ± 9 yr and women: 29 ± 2 yr, n = 7 each) completed 6 wk of very low-volume high-intensity sprint interval training. In a separate study, recreationally active men (n = 16) and women (n = 3) completed 6 wk of either traditional moderate-intensity continuous exercise (n = 9, 21 ± 4 yr) or low-volume sprint interval training (n = 10, 21 ± 2 yr). Muscle biopsies were obtained from the vastus lateralis before and after training. The fiber type-specific SC response to training was determined, as was the activity of the SC pool using immunofluorescent microscopy of muscle cross sections. Training did not induce hypertrophy, as assessed by muscle cross-sectional area, nor did the SC pool expand in any group. However, there was an increase in the number of active SCs after each intervention. Specifically, the number of activated (Pax7(+)/MyoD(+), P ≤ 0.05) and differentiating (Pax7(-)/MyoD(+), P ≤ 0.05) SCs increased after each training intervention. Here, we report evidence of activated and cycling SCs that may or may not contribute to exercise-induced adaptations while the SC pool remains constant after three nonhypertrophic exercise training protocols.

Tyrosine 705 Phosphorylation of STAT3 Is Associated with Phenotype Severity in TGFβ1 Transgenic Mice

Transforming growth factor beta 1 (TGFβ1) is a key player in skeletal muscle degenerative and regenerative processes. We previously showed that conditionally overexpressing TGFβ1 in skeletal muscles caused myofiber atrophy and endomysial fibrosis in mice. However, the disease severity varied significantly among individual mice. While 40% of mice developed severe muscle pathology and lost body weight within 2 weeks of TGFβ1 transgene induction in muscles, the rest showed milder or no phenotype. This study aims at determining whether signal transducer and activator of transcription 3 (STAT3) plays a role in the phenotypic difference and whether it can be activated by TGFβ1 directly in muscle cells. Our results show that while total STAT3 was not differentially expressed between the two groups of mice, there was significantly higher pSTAT3 (Tyr705) in the muscles of the mice with severe phenotype. Immunohistochemistry showed that pSTAT3 (Tyr705) was localized in approximately 50% of the nuclei of the muscles. We further showed that TGFβ1 induced Tyr705 phosphorylation of STAT3 in C2C12 cells within 30 minutes of treatment while total STAT3 was not affected. Our findings suggest that TGFβ1 alone can induce Tyr705 phosphorylation of STAT3 in skeletal muscle cells and contribute to disease severity in transgenic TGFβ1 mice.