The impact of sarcopenia and exercise training on skeletal muscle satellite cells

Strength and endurance training prescription in healthy and frail elderly

Aging is associated with declines in the neuromuscular and cardiovascular systems, resulting in an impaired capacity to perform daily activities. Frailty is an age-associated biological syndrome characterized by decreases in the biological functional reserve and resistance to stressors due to changes in several physiological systems, which puts older individuals at special risk of disability. To counteract the neuromuscular and cardiovascular declines associated with aging, as well as to prevent and treat the frailty syndrome, the strength and endurance training seems to be an effective strategy to improve muscle hypertrophy, strength and power output, as well as endurance performance. The first purpose of this review was discuss the neuromuscular adaptations to strength training, as well as the cardiovascular adaptations to endurance training in healthy and frail elderly subjects. In addition, the second purpose of this study was investigate the concurrent training adaptations in the elderly. Based on the results found, the combination of strength and endurance training (i.e., concurrent training) performed at moderate volume and moderate to high intensity in elderly populations is the most effective way to improve both neuromuscular and cardiorespiratory functions. Moreover, exercise interventions that include muscle power training should be prescribed to frail elderly in order to improve the overall physical status of this population and prevent disability.

Myogenic potential of canine craniofacial satellite cells

The skeletal fibers have different embryological origin; the extraocular and jaw-closer muscles develop from prechordal mesoderm while the limb and trunk muscles from somites. These different origins characterize also the adult muscle stem cells, known as satellite cells (SCs) and responsible for the fiber growth and regeneration. The physiological properties of presomitic SCs and their epigenetics are poorly studied despite their peculiar characteristics to preserve muscle integrity during chronic muscle degeneration. Here, we isolated SCs from canine somitic [somite-derived muscle (SDM): vastus lateralis, rectus abdominis, gluteus superficialis, biceps femoris, psoas] and presomitic [pre-somite-derived muscle (PSDM): lateral rectus, temporalis, and retractor bulbi] muscles as myogenic progenitor cells from young and old animals. In addition, SDM and PSDM-SCs were obtained also from golden retrievers affected by muscular dystrophy (GRMD). We characterized the lifespan, the myogenic potential and functions, and oxidative stress of both somitic and presomitic SCs with the aim to reveal differences with aging and between healthy and dystrophic animals. The different proliferation rate was consistent with higher telomerase activity in PSDM-SCs compared to SDM-SCs, although restricted at early passages. SDM-SCs express early (Pax7, MyoD) and late (myosin heavy chain, myogenin) myogenic markers differently from PSDM-SCs resulting in a more efficient and faster cell differentiation. Taken together, our results showed that PSDM-SCs elicit a stronger stem cell phenotype compared to SDM ones. Finally, myomiR expression profile reveals a unique epigenetic signature in GRMD SCs and miR-206, highly expressed in dystrophic SCs, seems to play a critical role in muscle degeneration. Thus, miR-206 could represent a potential target for novel therapeutic approaches.

Satellite cells in human skeletal muscle; from birth to old age

Changes in satellite cell content play a key role in regulating skeletal muscle growth and atrophy. Yet, there is little information on changes in satellite cell content from birth to old age in humans. The present study defines muscle fiber type-specific satellite cell content in human skeletal muscle tissue over the entire lifespan. Muscle biopsies were collected in 165 subjects, from different muscles of children undergoing surgery (<18 years; n = 13) and from the vastus lateralis muscle of young adult (18–49 years; n = 50), older (50–69 years; n = 53), and senescent subjects (70–86 years; n = 49). In a subgroup of 51 aged subjects (71 ± 6 years), additional biopsies were collected after 12 weeks of supervised resistance-type exercise training. Immunohistochemistry was applied to assess skeletal muscle fiber type-specific composition, size, and satellite cell content. From birth to adulthood, muscle fiber size increased tremendously with no major changes in muscle fiber satellite cell content, and no differences between type I and II muscle fibers. In contrast to type I muscle fibers, type II muscle fiber size was substantially smaller with increasing age in adults (r = −0.56; P < 0.001). This was accompanied by an age-related reduction in type II muscle fiber satellite cell content (r = −0.57; P < 0.001). Twelve weeks of resistance-type exercise training significantly increased type II muscle fiber size and satellite cell content. We conclude that type II muscle fiber atrophy with aging is accompanied by a specific decline in type II muscle fiber satellite cell content. Resistance-type exercise training represents an effective strategy to increase satellite cell content and reverse type II muscle fiber atrophy.

Neuromuscular electrical stimulation prevents muscle disuse atrophy during leg immobilization in humans

AIM

Short periods of muscle disuse, due to illness or injury, result in substantial skeletal muscle atrophy. Recently, we have shown that a single session of neuromuscular electrical stimulation (NMES) increases muscle protein synthesis rates. The aim was to investigate the capacity for daily NMES to attenuate muscle atrophy during short-term muscle disuse.

METHODS

Twenty-four healthy, young (23 ± 1 year) males participated in the present study. Volunteers were subjected to 5 days of one-legged knee immobilization with (NMES; n = 12) or without (CON; n = 12) supervised NMES sessions (40-min sessions, twice daily). Two days prior to and immediately after the immobilization period, CT scans and single-leg one-repetition maximum (1RM) strength tests were performed to assess quadriceps muscle cross-sectional area (CSA) and leg muscle strength respectively. Furthermore, muscle biopsies were taken to assess muscle fibre CSA, satellite cell content and mRNA and protein expression of selected genes.

RESULTS

In CON, immobilization reduced quadriceps CSA by 3.5 ± 0.5% (P < 0.0001) and muscle strength by 9 ± 2% (P < 0.05). In contrast, no significant muscle loss was detected following immobilization in NMES although strength declined by 7 ± 3% (P < 0.05). Muscle MAFbx and MuRF1 mRNA expression increased following immobilization in CON (P < 0.001 and P = 0.07 respectively), whereas levels either declined (P < 0.01) or did not change in NMES, respectively. Immobilization led to an increase in muscle myostatin mRNA expression in CON (P < 0.05), but remained unchanged in NMES.

CONCLUSION

During short-term disuse, NMES represents an effective interventional strategy to prevent the loss of muscle mass, but it does not allow preservation of muscle strength. NMES during disuse may be of important clinical relevance in both health and disease.

Muscle disuse atrophy is not accompanied by changes in skeletal muscle satellite cell content

Two weeks of muscle disuse led to a loss in muscle mass and strength. The loss in muscle mass was attributed to both type I and type II muscle fibre atrophy, and was not accompanied by a decline in satellite cell content.

How to simultaneously optimize muscle strength, power, functional capacity, and cardiovascular gains in the elderly: an update

The purpose of the present study was to review the scientific literature that investigated concurrent training adaptations in elderly populations, with the aim of identifying the optimal combination of both training program variables (i.e., strength and endurance) to avoid or minimize the interference effect in the elderly. Scielo, Science Citation Index, MEDLINE, Scopus, SPORTDiscus, and ScienceDirect databases were searched. Concurrent training is the most effective strategy by which to improve neuromuscular and cardiorespiratory functions as well as functional capacity in the elderly. The volume and frequency of training appears to play a critical role in concurrent training-induced adaptations in elderly subjects. Furthermore, new evidence indicates that the intra-session exercise order may influence the magnitude of physiological adaptations. Despite the interference effect on strength gains that is caused by concurrent training, this type of training is advantageous in that the combination of strength and endurance training produces both neuromuscular and cardiovascular adaptations in the elderly. The interference phenomenon may be observed in elderly subjects when a moderate weekly volume of concurrent training (i.e., three times per week) is performed. However, even with the occurrence of this phenomenon, the performance of three concurrent training sessions per week appears to optimize the strength gains in relative brief periods of training (12 weeks). Moreover, performing strength prior to endurance exercise may optimize both neuromuscular and cardiovascular gains.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Nutritional influences on age-related skeletal muscle loss

Age-related muscle loss impacts on whole-body metabolism and leads to frailty and sarcopenia, which are risk factors for fractures and mortality. Although nutrients are integral to muscle metabolism the relationship between nutrition and muscle loss has only been extensively investigated for protein and amino acids. The objective of the present paper is to describe other aspects of nutrition and their association with skeletal muscle mass. Mechanisms for muscle loss relate to imbalance in protein turnover with a number of anabolic pathways of which the mechanistic TOR pathway and the IGF-1–Akt–FoxO pathways are the most characterised. In terms of catabolism the ubiquitin proteasome system, apoptosis, autophagy, inflammation, oxidation and insulin resistance are among the major mechanisms proposed. The limited research associating vitamin D, alcohol, dietary acid–base load, dietary fat and anti-oxidant nutrients with age-related muscle loss is described. Vitamin D may be protective for muscle loss; a more alkalinogenic diet and diets higher in the anti-oxidant nutrients vitamin C and vitamin E may also prevent muscle loss. Although present recommendations for prevention of sarcopenia focus on protein, and to some extent on vitamin D, other aspects of the diet including fruits and vegetables should be considered. Clearly, more research into other aspects of nutrition and their role in prevention of muscle loss is required.

Isolation, characterization, and molecular regulation of muscle stem cells

Skeletal muscle has great regenerative capacity which is dependent on muscle stem cells, also known as satellite cells. A loss of satellite cells and/or their function impairs skeletal muscle regeneration and leads to a loss of skeletal muscle power; therefore, the molecular mechanisms for maintaining satellite cells in a quiescent and undifferentiated state are of great interest in skeletal muscle biology. Many studies have demonstrated proteins expressed by satellite cells, including Pax7, M-cadherin, Cxcr4, syndecan3/4, and c-met. To further characterize satellite cells, we established a method to directly isolate satellite cells using a monoclonal antibody, SM/C-2.6. Using SM/C-2.6 and microarrays, we measured the genes expressed in quiescent satellite cells and demonstrated that Hesr3 may complement Hesr1 in generating quiescent satellite cells. Although Hesr1- or Hesr3-single knockout mice show a normal skeletal muscle phenotype, including satellite cells, Hesr1/Hesr3-double knockout mice show a gradual decrease in the number of satellite cells and increase in regenerative defects dependent on satellite cell numbers. We also observed that a mouse's genetic background affects the regenerative capacity of its skeletal muscle and have established a line of DBA/2-background mdx mice that has a much more severe phenotype than the frequently used C57BL/10-mdx mice. The phenotype of DBA/2-mdx mice also seems to depend on the function of satellite cells. In this review, we summarize the methodology of direct isolation, characterization, and molecular regulation of satellite cells based on our results. The relationship between the regenerative capacity of satellite cells and progression of muscular disorders is also summarized. In the last part, we discuss application of the accumulating scientific information on satellite cells to treatment of patients with muscular disorders.

Musculoskeletal ageing and primary prevention

Loss of musculoskeletal mass and function is a natural ageing trait, reinforced by an unhealthy life style. Loss of bone (osteoporosis) and muscle (sarcopaenia) are conditions whose prevalence are increasing because of the change in population distribution in the western world towards an older mean age. Improvements in lifestyle factors, such as diet, smoking and exercise, are the most powerful tools to combat this decline efficiently; however, public health interventions aimed at tackling these problems have shown abysmal success at the population level, mostly due to failure in compliance. With these issues in mind, we believe that the primary prevention modality in coming decades will be pharmacological. We review the basic biology of musculoskeletal ageing and what measures can be taken to prevent ageing-associated loss of musculoskeletal mass and function, with particular emphasis on pharmacological means.

Eccentric exercise increases satellite cell content in type II muscle fibers

INTRODUCTION

Satellite cells (SCs) are of key importance in skeletal muscle tissue growth, repair, and regeneration. A single bout of high-force eccentric exercise has been demonstrated to increase mixed muscle SC content after 1-7 d of postexercise recovery. However, little is known about fiber type-specific changes in SC content and their activation status within 24 h of postexercise recovery.

METHODS

Nine recreationally active young men (23 ± 1 yr) performed 300 eccentric actions of the knee extensors on an isokinetic dynamometer. Skeletal muscle biopsies from the vastus lateralis were collected preexercise and 24 h postexercise. Muscle fiber type-specific SC content and the number of activated SCs were determined by immunohistochemical analyses.

RESULTS

There was no difference between Type I and Type II muscle fiber SC content before exercise. SC content significantly increased 24 h postexercise in Type II muscle fibers (from 0.085 ± 0.012 to 0.133 ± 0.016 SCs per fiber, respectively; P < 0.05), whereas there was no change in Type I fibers. In accordance, activation status increased from preexercise to 24 h postexercise as demonstrated by the increase in the number of DLK1+ SCs in Type II muscle fibers (from 0.027 ± 0.008 to 0.070 ± 0.017 SCs per muscle fiber P < 0.05). Although no significant changes were observed in the number of Ki-67+ SCs, we did observe an increase in the number of proliferating cell nuclear antigen-positive SCs after 24 h of postexercise recovery.

CONCLUSION

A single bout of high-force eccentric exercise increases muscle fiber SC content and activation status in Type II but not Type I muscle fibers.

Age-dependent alteration in muscle regeneration: the critical role of tissue niche

Although adult skeletal muscle is composed of fully differentiated fibers, it retains the capacity to regenerate in response to injury and to modify its contractile and metabolic properties in response to changing demands. The major role in the growth, remodeling and regeneration is played by satellite cells, a quiescent population of myogenic precursor cells that reside between the basal lamina and plasmalemma and that are rapidly activated in response to appropriate stimuli. However, in pathologic conditions or during aging, the complete regenerative program can be precluded by fibrotic tissue formation and resulting in functional impairment of the skeletal muscle. Our study, along with other studies, demonstrated that although the regenerative program can also be impaired by the limited proliferative capacity of satellite cells, this limit is not reached during normal aging, and it is more likely that the restricted muscle repair program in aging is presumably due to missing signals that usually render the damaged muscle a permissive environment for regenerative activity.

Expression and modification proteomics during skeletal muscle ageing

Skeletal muscle ageing is characterized by a progressive and dramatic loss of muscle mass and strength leading to decreased muscular function resulting in muscle weakness which is often referred to as sarcopenia. Following the standardisation of "omics" approaches to study the genome (genomics) and the transcriptome (transcriptomics), the study of the proteins encoded by the genome, referred to as proteomics, is a tremendous challenge. Unlike the genome, the proteome varies in response to many physiological or pathological factors. In addition, the proteome is orders of magnitude more complex than the transcriptome due to post-translational modifications, protein oxidation and limited protein degradation. Proteomic studies, including the analysis of protein abundance as well as post-translational modified proteins have been shown to provide valuable information to unravel the key molecular pathways implicated in complex biological processes, such as tissue and organ ageing. In this article, we will describe proteomic approaches for the analysis of protein abundance as well as the specific protein targets for oxidative damage upon oxidative stress and/or during skeletal muscle ageing.

Troponin T nuclear localization and its role in aging skeletal muscle

Troponin T (TnT) is known to mediate the interaction between Tn complex and tropomyosin (Tm), which is essential for calcium-activated striated muscle contraction. This regulatory function takes place in the myoplasm, where TnT binds Tm. However, recent findings of troponin I and Tm nuclear translocation in Drosophila and mammalian cells imply other roles for the Tn-Tm complex. We hypothesized that TnT plays a nonclassical role through nuclear translocation. Immunoblotting with different antibodies targeting the NH2- or COOH-terminal region uncovered a pool of fast skeletal muscle TnT3 localized in the nuclear fraction of mouse skeletal muscle as either an intact or fragmented protein. Construction of TnT3-DsRed fusion proteins led to the further observation that TnT3 fragments are closely related to nucleolus and RNA polymerase activity, suggesting a role for TnT3 in regulating transcription. Functionally, overexpression of TnT3 fragments produced significant defects in nuclear shape and caused high levels of apoptosis. Interestingly, nuclear TnT3 and its fragments were highly regulated by aging, thus creating a possible link between the deleterious effects of TnT3 and sarcopenia. We propose that changes in nuclear TnT3 and its fragments cause the number of myonuclei to decrease with age, contributing to muscle damage and wasting.

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

Aging is associated with increased circulating interleukin-6 (IL-6) and a reduced myogenic capacity, marked by reduced muscle stem cell [satellite cell (SC)] activity. Although IL-6 is important for normal SC function, it is unclear whether elevated IL-6 associated with aging alters SC function. We hypothesized that mild chronically elevated IL-6 would be associated with a blunted SC response through altered IL-6 signaling and elevated suppressor of cytokine signaling-3 (SOCS3) in the elderly. Nine healthy older adult men (OA; 69.6 ± 3.9 yr) and 9 young male controls (YC; 21. 3 ± 3.1 yr) completed 4 sets of 10 repetitions of unilateral leg press and knee extension (75% of 1-RM). Muscle biopsies and blood were obtained before and 3, 24, and 48 h after exercise. Basal SC number was 33% lower in OA vs. YC, and the response was blunted in OA. IL-6(+)/Pax7(+) cells demonstrated a divergent response in OA, with YC increasing to 69% at 3 h and peaking at 24 h (72%), while IL-6(+)/Pax7(+) cells were not increased until 48 h in OA (61%). Type II fiber-associated phosphorylated signal transducer and activator of transcription (pSTAT3)(+)/Pax7(+) cells demonstrated a similar delay in OA, not increasing until 48 h (vs. 3 h in YC). SOCS3 protein was 86% higher in OA. These data demonstrate an age-related impairment in normal SC function that appears to be influenced by SOCS3 protein and delayed induction of IL-6 and pSTAT3 in the SCs of OA. Collectively, these data suggest dysregulated IL-6 signaling as a consequence of aging contributes to the blunted muscle stem cell response.

Skeletal muscle degenerative diseases and strategies for therapeutic muscle repair

Skeletal muscle is a highly specialized, postmitotic tissue that must withstand chronic mechanical and physiological stress throughout life to maintain proper contractile function. Muscle damage or disease leads to progressive weakness and disability, and manifests in more than 100 different human disorders. Current therapies to treat muscle degenerative diseases are limited mostly to the amelioration of symptoms, although promising new therapeutic directions are emerging. In this review, we discuss the pathological basis for the most common muscle degenerative diseases and highlight new and encouraging experimental and clinical opportunities to prevent or reverse these afflictions.

Sarcopenia in older adults

PURPOSE OF REVIEW

Sarcopenia, or the decline of skeletal muscle tissue with age, is one of the most important causes of functional decline and loss of independence in older adults. The purpose of this article is to review the current definitions of sarcopenia, its potential causes and clinical consequences, and the potential for intervention.

RECENT FINDINGS

Although no consensus diagnosis has been reached, sarcopenia is increasingly defined by both loss of muscle mass and loss of muscle function or strength. Its cause is widely regarded as multifactorial, with neurological decline, hormonal changes, inflammatory pathway activation, declines in activity, chronic illness, fatty infiltration, and poor nutrition, all shown to be contributing factors. Recent molecular findings related to apoptosis, mitochondrial decline, and the angiotensin system in skeletal muscle have highlighted biological mechanisms that may be contributory. Interventions in general continue to target nutrition and exercise.

SUMMARY

Efforts to develop a consensus definition are ongoing and will greatly facilitate the development and testing of novel interventions for sarcopenia. Although pharmaceutical agents targeting multiple biological pathways are being developed, adequate nutrition and targeted exercise remain the gold standard for therapy.

Effect of nutritional interventions and resistance exercise on aging muscle mass and strength

Sarcopenia, defined as the age-related loss of muscle mass, has a negative effect on strength, functional independence and overall quality of life. Sarcopenia is a multifactorial phenomenon characterized by changes in muscle morphology, protein and hormonal kinetics, oxidative stress, inflammation, physical activity and nutrition. It is well known that resistance exercise increases aging muscle mass and strength and these physiological adaptations from exercise may be further enhanced with certain nutritional interventions. Research indicates that essential amino acids and milk-based proteins, creatine monohydrate, essential fatty acids, and vitamin D may all have beneficial effects on aging muscle biology.

The effect of physiological stimuli on sarcopenia; impact of Notch and Wnt signaling on impaired aged skeletal muscle repair

The age-related loss of skeletal muscle mass and function that is associated with sarcopenia can result in ultimate consequences such as decreased quality of life. The causes of sarcopenia are multifactorial and include environmental and biological factors. The purpose of this review is to synthesize what the literature reveals in regards to the cellular regulation of sarcopenia, including impaired muscle regenerative capacity in the aged, and to discuss if physiological stimuli have the potential to slow the loss of myogenic potential that is associated with sarcopenia. In addition, this review article will discuss the effect of aging on Notch and Wnt signaling, and whether physiological stimuli have the ability to restore Notch and Wnt signaling resulting in rejuvenated aged muscle repair. The intention of this summary is to bring awareness to the benefits of consistent physiological stimulus (exercise) to combating sarcopenia as well as proclaiming the usefulness of contraction-induced injury models to studying the effects of local and systemic influences on aged myogenic capability.

Satellite cell senescence underlies myopathy in a mouse model of limb-girdle muscular dystrophy 2H

Mutations in the E3 ubiquitin ligase tripartite motif-containing 32 (TRIM32) are responsible for the disease limb-girdle muscular dystrophy 2H (LGMD2H). Previously, we generated Trim32 knockout mice (Trim32-/- mice) and showed that they display a myopathic phenotype accompanied by neurogenic features. Here, we used these mice to investigate the muscle-specific defects arising from the absence of TRIM32, which underlie the myopathic phenotype. Using 2 models of induced atrophy, we showed that TRIM32 is dispensable for muscle atrophy. Conversely, TRIM32 was necessary for muscle regrowth after atrophy. Furthermore, TRIM32-deficient primary myoblasts underwent premature senescence and impaired myogenesis due to accumulation of PIAS4, an E3 SUMO ligase and TRIM32 substrate that was previously shown to be associated with senescence. Premature senescence of myoblasts was also observed in vivo in an atrophy/regrowth model. Trim32-/- muscles had substantially fewer activated satellite cells, increased PIAS4 levels, and growth failure compared with wild-type muscles. Moreover, Trim32-/- muscles exhibited features of premature sarcopenia, such as selective type II fast fiber atrophy. These results imply that premature senescence of muscle satellite cells is an underlying pathogenic feature of LGMD2H and reveal what we believe to be a new mechanism of muscular dystrophy associated with reductions in available satellite cells and premature sarcopenia.

Genetic variation in human muscle strength--opportunities for therapeutic interventions?

Inter-individual variation in muscle mass and muscular fitness is broad; being at the upper tail of the distribution not only contributes to improve elite sport performance, but is also associated with longer independent living and higher quality-of-life in the aging population. Heritability estimates of muscle phenotypes are substantial and warrant the search for genetic components underlying this individual variability. The 'kinesiogenomics' field is young, but genetic associations with muscle strength-related phenotypes have been reported already for more than 40 candidate genes, and genome-wide scans revealed several additional regions of interest in the genome. Although genetic findings may reveal attractive targets for novel muscle atrophy therapy, the benefit of exercise as a major stimulus for natural muscle mass enhancement or maintenance cannot be underestimated.

Inactivity-induced oxidative stress: a central role in age-related sarcopenia?

Ageing causes a progressive decline in skeletal muscle mass that may lead to decreased strength and functionality. The term sarcopenia is especially used to characterise this geriatric syndrome. Numerous conditions and behaviours are considered to accelerate the progression of sarcopenia such as chronic diseases, malnutrition and physical inactivity. As people in modern countries are more and more sedentary, the impact of physical inactivity on the prevalence of sarcopenia might be more and more important in the future. In this review, we discuss how reactive oxygen species (ROS) could mediate the effects of lifelong inactivity in the onset and progression of age-related sarcopenia. Although the cellular mechanisms responsible for muscle ROS production are not necessarily the same, both inactivity and ageing are indeed known to increase basal ROS concentrations in skeletal muscle. New data and literature review are provided showing that chronic ROS overproduction induced by physical inactivity may exacerbate the activation of some redox-sensitive signalling pathways involved in age-related sarcopenia. We also address the scientific evidences implicating the role of ROS overproduction in the precocious failure of aged muscles to activate intracellular signalling responses to contractions.

Voluntary resistance wheel exercise during post-natal growth in rats enhances skeletal muscle satellite cell and myonuclear content at adulthood

AIM

To determine whether voluntary free wheel (FW) or resistance wheel (RW) exercise or reduced muscle activity would influence maturational increases in muscle mass and the number of satellite cells (SCs) and myonuclei (MN) accrued by adulthood.

METHODS

Hind limb muscles of male rats housed with, or without, FWs from 4 to 5, 7 or 10 weeks of age, and rats housed with RWs from 4 to 10 week of age, were evaluated. To assess the effect of reduced muscle activity, gastrocnemius muscles of 4-week-old rats were injected with botulinum toxin (Btx) and collected at 7 weeks of age. Muscle fibre size and the frequency of Pax7-positive SCs and MN were determined in 7- and 10-week-old muscles via immunohistochemical methods.

RESULTS

Free wheel exercise enhanced muscle growth and the frequency of SCs in the medial gastrocnemius (MG) (threefold) and vastus lateralis (VL) (twofold) of rats at 10 week of age. Resistance wheel exercise increased the number of SCs and MN (22-30%), with more muscle fibre nuclei being associated with larger fibre size, in the soleus, MG and VL muscles. Btx impaired the normal increases in muscle fibre size and the accrual of MN but not SCs.

CONCLUSION

A greater volume of exercise during maturational growth was important for enhancing SC numbers, whereas their conversion to MN required higher-intensity exercise. The enhanced muscle fibre nuclear populations may influence the capacity of the muscle to adapt to exercise, injury or disuse in later adulthood.

Leucine as a pharmaconutrient to prevent and treat sarcopenia and type 2 diabetes

Amino acids function as precursors for de novo protein synthesis. In addition, however, they play a key role as nutritional signals that regulate multiple cellular processes. There is ample in vitro and in vivo evidence showing that muscle tissue responds to increases in amino acid availability via signal transduction pathways that are also regulated by insulin, glucagon, growth hormone, and insulin growth factor 1. The increased amino acid availibility results in the upregulation of mRNA translation, thereby increasing muscle protein synthesis, which, in turn, leads to greater net muscle protein accretion. These findings have been particularly pronounced for the amino acid leucine. Furthermore, leucine has the ability to act as a strong insulin secretagogue. Consequently, it has been suggested that leucine represents an effective pharmaconutrient for the prevention and treatment of sarcopenia and type 2 diabetes. In accordance, recent in vivo studies in humans show that free leucine ingestion can reverse the blunted response of muscle protein synthesis to amino acid/protein intake in the elderly. Although short-term studies suggest that leucine supplementation can stimulate muscle mass accretion in the elderly, there are no long-term nutritional intervention studies to confirm this or the other proposed benefits of leucine as a pharmaconutrient.

Eccentric exercise facilitates mesenchymal stem cell appearance in skeletal muscle

Eccentric, or lengthening, contractions result in injury and subsequently stimulate the activation and proliferation of satellite stem cells which are important for skeletal muscle regeneration. The discovery of alternative myogenic progenitors in skeletal muscle raises the question as to whether stem cells other than satellite cells accumulate in muscle in response to exercise and contribute to post-exercise repair and/or growth. In this study, stem cell antigen-1 (Sca-1) positive, non-hematopoetic (CD45^-) cells were evaluated in wild type (WT) and α7 integrin transgenic (α7Tg) mouse muscle, which is resistant to injury yet liable to strain, 24 hr following a single bout of eccentric exercise. Sca-1⁺CD45⁻ stem cells were increased 2-fold in WT muscle post-exercise. The α7 integrin regulated the presence of Sca-1⁺ cells, with expansion occurring in α7Tg muscle and minimal cells present in muscle lacking the α7 integrin. Sca-1⁺CD45⁻ cells isolated from α7Tg muscle following exercise were characterized as mesenchymal-like stem cells (mMSCs), predominantly pericytes. In vitro multiaxial strain upregulated mMSC stem cells markers in the presence of laminin, but not gelatin, identifying a potential mechanistic basis for the accumulation of these cells in muscle following exercise. Transplantation of DiI-labeled mMSCs into WT muscle increased Pax7⁺ cells and facilitated formation of eMHC⁺DiI⁻ fibers. This study provides the first demonstration that mMSCs rapidly appear in skeletal muscle in an α7 integrin dependent manner post-exercise, revealing an early event that may be necessary for effective repair and/or growth following exercise. The results from this study also support a role for the α7 integrin and/or mMSCs in molecular- and cellular-based therapeutic strategies that can effectively combat disuse muscle atrophy.

Beta-alanine (Carnosyn™) supplementation in elderly subjects (60-80 years): effects on muscle carnosine content and physical capacity

The aim of this study was to investigate the effects of beta-alanine supplementation on exercise capacity and the muscle carnosine content in elderly subjects. Eighteen healthy elderly subjects (60-80 years, 10 female and 4 male) were randomly assigned to receive either beta-alanine (BA, n=12) or placebo (PL, n=6) for 12 weeks. The BA group received 3.2 g of beta-alanine per day (2×800 mg sustained-release Carnosyn™ tablets, given 2 times per day). The PL group received 2× (2×800 mg) of a matched placebo. At baseline (PRE) and after 12 weeks (POST-12) of supplementation, assessments were made of the muscle carnosine content, anaerobic exercise capacity, muscle function, quality of life, physical activity and food intake. A significant increase in the muscle carnosine content of the gastrocnemius muscle was shown in the BA group (+85.4%) when compared with the PL group (+7.2%) (p=0.004; ES: 1.21). The time-to-exhaustion in the constant-load submaximal test (i.e., TLIM) was significantly improved (p=0.05; ES: 1.71) in the BA group (+36.5%) versus the PL group (+8.6%). Similarly, time-to-exhaustion in the incremental test was also significantly increased (p=0.04; ES 1.03) following beta-alanine supplementation (+12.2%) when compared with placebo (+0.1%). Significant positive correlations were also shown between the relative change in the muscle carnosine content and the relative change in the time-to-exhaustion in the TLIM test (r=0.62; p=0.01) and in the incremental test (r=0.48; p=0.02). In summary, the current data indicate for the first time, that beta-alanine supplementation is effective in increasing the muscle carnosine content in healthy elderly subjects, with subsequent improvement in their exercise capacity.

Pax7-expressing satellite cells are indispensable for adult skeletal muscle regeneration

Distinct cell populations with regenerative capacity have been reported to contribute to myofibres after skeletal muscle injury, including non-satellite cells as well as myogenic satellite cells. However, the relative contribution of these distinct cell types to skeletal muscle repair and homeostasis and the identity of adult muscle stem cells remain unknown. We generated a model for the conditional depletion of satellite cells by expressing a human diphtheria toxin receptor under control of the murine Pax7 locus. Intramuscular injection of diphtheria toxin during muscle homeostasis, or combined with muscle injury caused by myotoxins or exercise, led to a marked loss of muscle tissue and failure to regenerate skeletal muscle. Moreover, the muscle tissue became infiltrated by inflammatory cells and adipocytes. This localised loss of satellite cells was not compensated for endogenously by other cell types, but muscle regeneration was rescued after transplantation of adult Pax7(+) satellite cells alone. These findings indicate that other cell types with regenerative potential depend on the presence of the satellite cell population, and these observations have important implications for myopathic conditions and stem cell-based therapeutic approaches.

Physical training modulates structural and functional features of cell nuclei in type II myofibers of old mice

Aging is associated with a progressive loss of muscle mass, strength, and function, a condition known as sarcopenia, which represents an important risk factor for physical disability in elderly. The mechanisms leading to sarcopenia are still largely unknown, and no specific therapy is presently available to counteract its onset or progress. Many studies have stressed the importance of physical exercise as an effective approach to prevent/limit the age-related muscle mass loss. This study investigated the effects of physical training on pre-mRNA pathways in quadriceps and gastrocnemius muscles of old mice by ultrastructural cytochemistry: Structural and in situ molecular features of myonuclei and satellite cell nuclei of type II fibers were compared in exercised versus sedentary old mice, using adult individuals as control. Our results demonstrated that in myonuclei of old mice physical exercise stimulates pre-mRNA transcription, splicing, and export to the cytoplasm, likely increasing muscle protein turnover. In satellite cells, the effect of physical exercise seems to be limited to the reactivation of some factors involved in the transcriptional and splicing apparatus without increasing RNA production, probably making these quiescent cells more responsive to activating stimuli.

Sarcopenia and obesity

Four body composition phenotypes exist in older adults: normal, sarcopenic, obese, and a combination of sarcopenic and obese. There is no consensus, however, on the definitions and classifications of these phenotypes and their etiology and consequences continue to be debated. The lack of standard definitions, particularly for sarcopenia and sarcopenic obesity, creates challenges for determining prevalence across different populations. The etiology of these phenotypes is multifactorial with complex covariate relationships. This review focuses on the current literature addressing the classification, prevalence, etiology, and correlates of sarcopenia, obesity, and the combination of sarcopenia and obesity, referred to as sarcopenic obesity.

Cellular and molecular mechanisms regulating fibrosis in skeletal muscle repair and disease

The repair of an injured tissue is a complex biological process involving the coordinated activities of tissue-resident and infiltrating cells in response to local and systemic signals. Following acute tissue injury, inflammatory cell infiltration and activation/proliferation of resident stem cells is the first line of defense to restore tissue homeostasis. However, in the setting of chronic tissue damage, such as in Duchenne Muscular Dystrophy, inflammatory infiltrates persist, the ability of stem cells (satellite cells) is blocked and fibrogenic cells are continuously activated, eventually leading to the conversion of muscle into nonfunctional fibrotic tissue. This review explores our current understanding of the cellular and molecular mechanisms underlying efficient muscle repair that are dysregulated in muscular dystrophy-associated fibrosis and in aging-related muscle dysfunction.

The α7β1-integrin accelerates fiber hypertrophy and myogenesis following a single bout of eccentric exercise

The α(7)β(1)-integrin is a heterodimeric transmembrane protein that adheres to laminin in the extracellular matrix, representing a critical link that maintains structure in skeletal muscle. In addition to preventing exercise-induced skeletal muscle injury, the α(7)-integrin has been proposed to act as an intrinsic mechanosensor, initiating cellular growth in response to mechanical strain. The purpose of this study was to determine the extent to which the α(7)-integrin regulates muscle hypertrophy following eccentric exercise. Wild-type (WT) and α(7)-integrin transgenic (α(7)Tg) mice completed a single bout of downhill running exercise (-20°, 17 m/min, 60 min), and gastrocnemius-soleus complexes were collected 1, 2, 4, and 7 days (D) postexercise (PE). Maximal isometric force was maintained and macrophage accumulation was suppressed in α(7)Tg muscle 1D PE. Mean fiber cross-sectional area was unaltered in WT mice but increased 40% in α(7)Tg mice 7D PE. In addition, a rapid and striking fivefold increase in embryonic myosin heavy chain-positive fibers appeared in α(7)Tg mice 2D PE. Although Pax7-positive satellite cells were increased in α(7)Tg muscle 1D PE, the number of nuclei per myofiber was not altered 7D PE. Phosphorylation of mammalian target of rapamycin (mTOR) was significantly elevated in α(7)Tg 1D PE. This study provides the first demonstration that the presence of the α(7)β(1)-integrin in skeletal muscle increases fiber hypertrophy and new fiber synthesis in the early time course following a single bout of eccentric exercise. Further studies are necessary to elucidate the precise mechanism by which the α(7)-integrin can enhance muscle hypertrophy following exercise.

Acute loading and aging effects on myostatin pathway biomarkers in human skeletal muscle after three sequential bouts of resistance exercise

To determine the influence of age and resistance exercise on myostatin pathway-related genes, younger (n = 10; 28 ± 5 years) and older (n = 10; 68 ± 6 years) men underwent four testing conditions (T1-T4). A baseline (T1) muscle sample was obtained, whereas the second and third biopsies were obtained 48 hours following the first and second training sessions (T2, T3), and a final biopsy was taken 24 hours following T3. The training sessions consisted of 3 sets of 10 repetitions (80% of one repetition maximum) on leg press, hack squat, and leg extension exercises. Follistatin (FST) messenger RNA was greater in older compared with younger men at T1 and T2 (p < .05). Follistatin-like 3 (FSTL3) messenger RNA was greater in older compared with younger men at T1 and T4 (p < .05). In older men, there was a significant decrease in myostatin (MSTN) messenger RNA at T4 (p < .05). Older men contained less active (Ser-425 phosphorylated) SMAD3 (p-SMAD3) protein than younger men at T3 and T4 (p < .05).Although it is well known that younger individuals possess a greater hypertrophic potential to resistance exercise, it appears that older individuals may paradoxically possess a more favorable resistance exercise response regarding myostatin pathway-related genes and a protein marker of pathway activity. Future research is warranted to examine the physiological significance of this age-dependent mechanism.

Physical activity and sarcopenia

Physical activity can be a valuable countermeasure to sarcopenia in its treatment and prevention. In considering physical training strategies for sarcopenic subjects, it is critical to consider personal and environmental obstacles to access opportunities for physical activity for any patient with chronic disease. This article presents an overview of current knowledge of the effects of physical training on muscle function and the physical activity recommended for sarcopenic patients. So that this countermeasure strategy can be applied in practice, the authors propose a standardized protocol for prescribing physical activity in chronic diseases such as sarcopenia.

Aberrant repair and fibrosis development in skeletal muscle

The repair process of damaged tissue involves the coordinated activities of several cell types in response to local and systemic signals. Following acute tissue injury, infiltrating inflammatory cells and resident stem cells orchestrate their activities to restore tissue homeostasis. However, during chronic tissue damage, such as in muscular dystrophies, the inflammatory-cell infiltration and fibroblast activation persists, while the reparative capacity of stem cells (satellite cells) is attenuated. Abnormal dystrophic muscle repair and its end stage, fibrosis, represent the final common pathway of virtually all chronic neurodegenerative muscular diseases. As our understanding of the pathogenesis of muscle fibrosis has progressed, it has become evident that the muscle provides a useful model for the regulation of tissue repair by the local microenvironment, showing interplay among muscle-specific stem cells, inflammatory cells, fibroblasts and extracellular matrix components of the mammalian wound-healing response. This article reviews the emerging findings of the mechanisms that underlie normal versus aberrant muscle-tissue repair.

Effects of preexercise feeding on markers of satellite cell activation

PURPOSE

The purpose of this study was to determine if consuming isoenergetic (25 g) doses of carbohydrate or protein versus a noncaloric placebo before conventional resistance training affected the myogenic expression of cell cycle-regulating genes as well as the muscle [DNA] acutely after exercise.

METHODS

Ten untrained men (mean +/- SD: age = 22 +/- 4 yr, body mass = 77.8 +/- 8.3 kg, percent body fat = 17.8 +/- 4.0) participated in three resistance exercise sessions (three sets of 10 repetitions at 80% one-repetition maximum for the bilateral hack squat, leg press, and leg extension exercises) in a crossover fashion, which were preceded by carbohydrate, protein, or placebo ingestion 30 min before training. Presupplement/preexercise and 2- and 6-h postexercise muscle biopsies were obtained during each session and analyzed for fold changes in CDK4, CYCLIN D1, MGF, MYOD, P21(CIP1), and P27(KIP1) messenger RNA expression using real-time reverse transcriptase-polymerase chain reaction as well as muscle [DNA] using cuvette-based fluorometric methods.

RESULTS

Nonparametric statistics were completed, and no conditions x time interaction effects were revealed. Several exercise-mediated responses were found to occur independent of condition: 1) muscle [DNA] increased at 6 h (+40%, P < 0.05), 2) CDK4 expression increased at 6 h (+86%, P < 0.05), 3) MYOD expression increased at 6 h (+98%, P < 0.05), 4) P27(KIP1) expression decreased at 2 h (j35%, P < 0.05) and 6 h (-59%, P < 0.001), and 5) P21(CIP1) expression substantially increased 2 and 6 h postexercise (+1.250% and +4.670%, respectively, P < 0.001).

CONCLUSIONS

The tandem DNA and cell cycle regulator gene expression analyses provide preliminary evidence to suggest that satellite cell activation and proliferation may be occurring at early post-exercise time points after a conventional resistance exercise bout, a phenomenon that may seemingly be independent of preexercise macronutrient ingestion.

Sarcopenic obesity: satellite cells in the aging muscle

PURPOSE OF REVIEW

Current knowledge on satellite cells in relation to suggested mechanisms of loss of muscle mass and strength, induction of fat infiltration, and countermeasures is highlighted.

RECENT FINDINGS

Consensus on the definition of sarcopenia and sarcopenic obesity is proposed. Human satellite cell heterogeneity has now unequivocally been verified in situ as well as an adipogenic potential, though in mice other muscle stem cells are the hot topic to induce adipogenesis upon muscle damage. Inflammation, oxidative stress, proteolytic degradation, and nuclear apoptosis are discussed as pathogenetic mechanisms of sarcopenia, although little evidence exists that they are important in human muscle. In rodents, exercise-induced muscle injury is a hallmark for sequential events leading to muscle fiber necrosis and sarcopenia. Exercise in humans, on the contrary, is the key event to countermeasure sarcopenia. Cautions to extrapolate observation in rodents to explain human conditions have been presented.

SUMMARY

Human satellite cells are indispensable for maintenance of human muscle mass, but their implications in the pathogenesis of sarcopenia and sarcopenic obesity are still under debate. Nevertheless, satellite cell activation upon exercise seems unequivocally together with adequate nutrition to be the most effective countermeasure for sarcopenia and sarcopenic obesity.

Dietary protein and exercise training in ageing

Ageing is accompanied by a progressive loss of skeletal muscle mass and strength, leading to the loss of functional capacity and an increased risk for developing chronic metabolic diseases such as diabetes. The age-related loss of skeletal muscle mass results from a chronic disruption in the balance between muscle protein synthesis and degradation. As basal muscle protein synthesis rates are likely not different between healthy young and elderly human subjects, it was proposed that muscles from older adults lack the ability to regulate the protein synthetic response to anabolic stimuli, such as food intake and physical activity. Indeed, the dose-response relationship between myofibrillar protein synthesis and the availability of essential amino acids and/or resistance exercise intensity is shifted down and to the right in elderly human subjects. This so-called 'anabolic resistance' represents a key factor responsible for the age-related decline in skeletal muscle mass. Interestingly, long-term resistance exercise training is effective as a therapeutic intervention to augment skeletal muscle mass, and improves functional performance in the elderly. The consumption of different types of proteins, i.e. protein hydrolysates, can have different stimulatory effects on muscle protein synthesis in the elderly, which may be due to their higher rate of digestion and absorption. Current research aims to elucidate the interactions between nutrition, exercise and the skeletal muscle adaptive response that will define more effective strategies to maximise the therapeutic benefits of lifestyle interventions in the elderly.

Satellite cell heterogeneity with respect to expression of MyoD, myogenin, Dlk1 and c-Met in human skeletal muscle: application to a cohort of power lifters and sedentary men

Human satellite cells (SCs) are heterogeneous with respect to markers for their identification in the niche between the muscle fibre plasma membrane and its basal lamina. We have previously shown that, in biopsies from highly competitive power lifters, power lifters with long-term use of anabolic steroids and a population of healthy sedentary men, antibodies against the neuronal cell adhesion molecule (NCAM) and the paired box transcription factor Pax7 together label 94% of the SCs, NCAM alone labels 4% and Pax7 alone labels 1%. In the present study, we have further studied these biopsies with four markers related to SC activation and differentiation. Our study unequivocally shows that staining for MyoD and myogenin are present in nuclei of SCs and of myoblasts and myotubes in areas of muscle fibre regeneration. Staining for c-Met was observed in a proportion of Pax7⁺ SCs. However, widespread labelling of the sarcolemma precluded the quantification of c-Met⁺/Pax7⁺ SCs and the use of c-Met as a reliable SC marker. Pax7⁺ SCs labelled by anti-Delta like1 (Dlk1) were present in all samples but in variable proportions, whereas muscle progenitor cells related to repair were Dlk1⁻. Staining for Dlk1 was also observed in Pax7⁻ interstitial cells and in the cytoplasm of some small muscle fibres. Interestingly, the proportion of Dlk1⁺/Pax7⁺ SCs was significantly different between the groups of power lifters. Thus, our study confirms that human SCs show marked heterogeneity and this is discussed in terms of SC activation, myonuclei turnover, muscle fibre growth and muscle fibre damage and repair.

Molecular attributes of human skeletal muscle at rest and after unaccustomed exercise: an age comparison

The current study examined muscle DNA and protein concentrations ([ ]) and the [RNA] (assumed to represent translational capacity), [RNA]:[DNA] (assumed to represent transcriptional efficiency) and [protein]:[RNA] (assumed to represent translational efficiency) in younger vs. older participants during a resting state. Further, changes in muscle [DNA], translational capacity, and transcriptional efficiency were analyzed 24 hours after an unaccustomed resistance exercise bout. Younger (20.9 +/- 0.5 years, 84.0 +/- 5.2 kg, 26.6 +/- 1.8 kg x m(-2); n = 13) and older men (67.6 +/- 1.3 years, 88.7 +/- 4.8 kg, 28.6 +/- 1.4 kg x m(-2); n = 13) reported to the laboratory and completed an unaccustomed bout of lower-body resistance training (i.e., 3 sets of 10 repetitions at 80% 1 repetition maximum for Smith squat, leg press, and leg extensions). Muscle biopsies from the vastus lateralis were obtained before and 24 hours after exercise. Baseline [RNA], [DNA], [protein], and [RNA]:[DNA] were not different between age groups (p > 0.05). Baseline [protein]:[RNA] was greater in younger vs. older men (p = 0.045), whereas 24-hour postexercise [RNA]:[DNA] tended to be greater in older men (p = 0.087). These findings suggest that a decrease in the efficiency of translational processes occurs in older human skeletal muscle, whereas global transcriptional processes appear to be unaltered when compared with those in younger men. In lieu of these data, it remains apparent that muscle-protein synthesis is impaired in aging skeletal muscle and effective countermeasures such as resistance exercise and nutritional adequacy must be undertaken by older populations to offset this phenomenon.

Are human and mouse satellite cells really the same?

Satellite cells are quiescent cells located under the basal lamina of skeletal muscle fibers that contribute to muscle growth, maintenance, repair, and regeneration. Mouse satellite cells have been shown to be muscle stem cells that are able to regenerate muscle fibers and self-renew. As human skeletal muscle is also able to regenerate following injury, we assume that the human satellite cell is, like its murine equivalent, a muscle stem cell. In this review, we compare human and mouse satellite cells and highlight their similarities and differences. We discuss gaps in our knowledge of human satellite cells, compared with that of mouse satellite cells, and suggest ways in which we may advance studies on human satellite cells, particularly by finding new markers and attempting to re-create the human satellite cell niche in vitro.

Nutritional recommendations for the management of sarcopenia

The Society for Sarcopenia, Cachexia, and Wasting Disease convened an expert panel to develop nutritional recommendations for prevention and management of sarcopenia. Exercise (both resistance and aerobic) in combination with adequate protein and energy intake is the key component of the prevention and management of sarcopenia. Adequate protein supplementation alone only slows loss of muscle mass. Adequate protein intake (leucine-enriched balanced amino acids and possibly creatine) may enhance muscle strength. Low 25(OH) vitamin D levels require vitamin D replacement.

Modulation of muscle atrophy, fatigue and MLC phosphorylation by MuRF1 as indicated by hindlimb suspension studies on MuRF1-KO mice

MuRF1 is a member of the TRIM/RBCC superfamily, a gene family that encompasses a large variety of proteins, all sharing the conserved TRIM (Tripartite Motive) sequential array of RING, B-box, and coiled-coil domains. Within this family, MuRF1(also named TRIM63) is a specialized member that contributes to the development of muscle atrophy and sarcopenia. Here we studied MuRF1's role in muscle atrophy during muscle unloading induced by hindlimb suspension. Consistent with previous studies, we found that MuRF1 inactivation leads to an attenuated muscle atrophy response. The amount of protection was higher as compared to the denervation model, and within the 10 day-suspension period the soleus muscle was spared from atrophy in MuRF1-KO mice. Contractility studies on hindlimb suspended muscle tissues suggested that MuRF1's functions extend beyond muscle trophicity and implicate MuRF1 in muscle fatigue and MLC phosphorylation control: soleus muscle from MuRF1-KO mice fatigued significantly faster and in addition showed a reduced posttetanic twitch potentiation. Thus the present work further established the role of MuRF1 in muscle atrophy and for the first time shows that MuRF1 plays a role in muscle fatigue and twitch potentiation.