Impaired skeletal muscle regeneration in diabetes: From cellular and molecular mechanisms to novel treatments

Diabetes represents a major public health concern with a considerable impact on human life and healthcare expenditures. It is now well established that diabetes is characterized by a severe skeletal muscle pathology that limits functional capacity and quality of life. Increasing evidence indicates that diabetes is also one of the most prevalent disorders characterized by impaired skeletal muscle regeneration, yet underlying mechanisms and therapeutic treatments remain poorly established. In this review, we describe the cellular and molecular alterations currently known to occur during skeletal muscle regeneration in people with diabetes and animal models of diabetes, including its associated comorbidities, e.g., obesity, hyperinsulinemia, and insulin resistance. We describe the role of myogenic and non-myogenic cell types on muscle regeneration in conditions with or without diabetes. Therapies for skeletal muscle regeneration and gaps in our knowledge are also discussed, while proposing future directions for the field.

Regulation of adult stem cell function by ketone bodies

Adult stem cells play key roles in tissue homeostasis and regeneration. Recent evidence suggests that dietary interventions can significantly impact adult stem cell function. Some of these effects depend on ketone bodies. Adult stem cells could therefore potentially be manipulated through dietary regimens or exogenous ketone body supplementation, a possibility with significant implications for regenerative medicine. In this review we discuss recent findings of the mechanisms by which ketone bodies could influence adult stem cells, including ketogenesis in adult stem cells, uptake and transport of circulating ketone bodies, receptor-mediated signaling, and changes to cellular metabolism. We also discuss the potential effects of ketone bodies on intracellular processes such as protein acetylation and post-transcriptional control of gene expression. The exploration of mechanisms underlying the effects of ketone bodies on stem cell function reveals potential therapeutic targets for tissue regeneration and age-related diseases and suggests future research directions in the field of ketone bodies and stem cells.

Six weeks of high-load resistance and low-load blood flow restricted training increase Na/K-ATPase sub-units α2 and β1 equally, but does not alter ClC-1 abundance in untrained human skeletal muscle

Contractile function of skeletal muscle relies on the ability of muscle fibers to trigger and propagate action potentials (APs). These electrical signals are created by transmembrane ion transport through ion channels and membrane transporter systems. In this regard, the Cl^- ion channel 1 (ClC-1) and the Na⁺/K^--ATPase (NKA) are central for maintaining ion homeostasis across the sarcolemma during intense contractile activity. Therefore, this randomized controlled trial aimed to investigate the changes in ClC-1 and specific NKA subunit isoform expression in response to six weeks (18 training sessions) of high-load resistance exercise (HLRE) and low-load blood flow restricted resistance exercise (BFRRE), respectively. HLRE was conducted as 4 sets of 12 repetitions of knee extensions performed at 70% of 1 repetition maximum (RM), while BFRRE was conducted as 4 sets of knee extensions at 30% of 1RM performed to volitional fatigue. Furthermore, the potential associations between protein expression and contractile performance were investigated. We show that muscle ClC-1 abundance was not affected by either exercise modality, whereas NKA subunit isoforms [Formula: see text]² and [Formula: see text]¹ increased equally by appx. 80-90% with BFRRE (p < 0.05) and 70-80% with HLRE (p < 0.05). No differential impact between exercise modalities was observed. At baseline, ClC-1 protein expression correlated inversely with dynamic knee extensor strength (r=-0.365, p = 0.04), whereas no correlation was observed between NKA subunit content and contractile performance at baseline. However, training-induced changes in NKA [Formula: see text]² subunit (r = 0.603, p < 0.01) and [Formula: see text]¹ subunit (r = 0.453, p < 0.05) correlated with exercise-induced changes in maximal voluntary contraction. These results suggest that the initial adaptation to resistance-based exercise does not involve changes in ClC-1 abundance in untrained skeletal muscle, and that increased content of NKA subunits may facilitate increases in maximal force production.

Fluorescence-activated cell sorting and phenotypic characterization of human fibro-adipogenic progenitors

The ability of stem cells to activate and differentiate is critical for maintaining the regenerative capacity of skeletal muscle. Here, we detail steps for specific quantification and isolation of primary human fibro-adipogenic progenitors and skeletal muscle stem cells using fluorescence-activated cell sorting. We describe important phenotypic traits such as time to enter the cell cycle and assessment of cell differentiation for the isolated cell populations. The technique has been applied on tissue obtained from surgery and needle biopsies. For complete details on the use and execution of this protocol, please refer to Farup et al. (2021).1.

Low-load blood flow-restricted resistance exercise produce fiber type-independent hypertrophy and improves muscle functional capacity in older individuals

Low-load blood flow-restricted resistance exercise (BFRRE) constitute an effective means to produce skeletal muscle hypertrophy. Nonetheless, its applicability to counteract the age-related skeletal muscle decay at a cellular level, is not clear. Therefore, we investigated the effect of BFRRE on muscle fiber morphology, integrated muscle protein synthesis, muscle stem cells (MuSCs), myonuclear content and muscle functional capacity in healthy older individuals. Twenty-three participants with a mean age of 66 years (56-75 years) were randomized to six weeks of supervised BFRRE (3 sessions x week) or non-intervention control (CON). Biopsies were collected from vastus lateralis before and after the intervention. Immunofluorescent microscopy was utilized to assess muscle fiber type-specific cross-sectional area (CSA) as well as MuSC and myonuclear content. Deuterium oxide was orally administered throughout the intervention period, enabling assessment of integrated myofibrillar and connective tissue protein fractional synthesis rate (FSR). BFRRE produced uniform ~20% increases in the fiber CSA of both type I and type II fibers (p<0.05). This occurred concomitantly with improvements in both maximal strength and muscle strength-endurance, but in the absence of increased MuSC content and myonuclear addition. The observed muscle fiber hypertrophy was not mirrored by increases in either myofibrillar or connective tissue FSR. In conclusion, BFRRE proved effective in stimulating skeletal muscle growth and increased muscle function in older individuals, which advocates for the use of BFRRE as a countermeasure of age-related deterioration of skeletal muscle mass and function.

A randomized placebo-controlled trial of nicotinamide riboside+pterostilbene supplementation in experimental muscle injury in elderly subjects

BACKGROUND

During ageing there is a functional decline in the pool of muscle stem cells (MuSCs) which influences the functional and regenerative capacity of skeletal muscle. Preclinical evidence have suggested that Nicotinamide Riboside (NR) and Pterostilbene (PT) can improve muscle regeneration e.g. by increasing MuSC function. The objective of the present study was to investigate if NRPT-supplementation promotes skeletal muscle regeneration after muscle injury in elderly humans by improved recruitment of MuSCs.

METHODS

32 elderly men and women (55-80 yr) were randomized to daily supplementation with either NRPT (1000 mg NR + 200 mg PT) or matched placebo. Two weeks after initiation of supplementation, a skeletal muscle injury was induced by electrically-induced eccentric muscle work. Skeletal muscle biopsies were obtained pre, 2h, 2, 8, and 30 days post injury.

RESULTS

A substantial skeletal muscle injury was induced by the protocol and associated with release of myoglobin and creatine kinase, muscle soreness, tissue edema, and a decrease in muscle strength. MuSC content, proliferation and cell size revealed a large demand for recruitment post injury but was not affected by NRPT. Furthermore, histological analyses of muscle fiber area, internal nuclei and embryonic Myosin Heavy Chain showed no effect of NRPT supplementation.

CONCLUSION

Daily supplementation with 1000 mg NR+200 mg PT is safe but does not improve recruitment of the MuSC pool or other measures of muscle recovery in response to injury or subsequent regeneration in elderly subjects.

TRIAL REGISTRATION

NCT03754842.

FUNDING

Novo Nordisk Foundation (Ref. NNF17OC0027242) given to JTT and NJ. JTT, ED, SC, MVD, KT, and TM are supported by the Novo Nordisk Foundation Center for Basic Metabolic Research (CBMR). CBMR is an independent Research Center at the University of Copenhagen that is partially funded by an unrestricted donation from the Novo Nordisk Foundation (NNF18CC0034900).

Sex Hormones and Satellite Cell Regulation in Women

Recent years have seen growing scholarly interest in female physiology in general. Moreover, particular attention has been devoted to how concentrations of female sex hormones vary during the menstrual cycle and menopausal transition and how hormonal contraception and hormonal therapy influence skeletal muscle tissue. While much effort has been paid to macro outcomes, such as muscle function or mass, rather less attention has been paid to mechanistic work that may help explain the underlying mechanism through which sex hormones regulate skeletal muscle tissue. Evidence from animal studies shows a strong relationship between the female sex hormone estrogen and satellite cells (SCs), a population of muscle stem cells involved in skeletal muscle regulation. A few human studies investigating this relationship have been published only recently. Thus, the purpose of this study was to bring an updated review on female sex hormones and their role in SC regulation. First, we describe how SCs regulate skeletal muscle maintenance and repair and introduce sex hormone signaling within the muscle. Second, we present evidence from animal studies elucidating how estrogen deficiency and supplementation influence SCs. Third, we present results from investigations from human trials including women whose concentrations of female hormones differ due to menopause, hormone therapy, hormonal contraceptives, and the menstrual cycle. Finally, we discuss research and methodological recommendations for future studies aiming at elucidating the link between female sex hormones and SCs with respect to aging and training.

Human skeletal muscle CD90+ fibro-adipogenic progenitors are associated with muscle degeneration in type 2 diabetic patients

Type 2 diabetes mellitus (T2DM) is associated with impaired skeletal muscle function and degeneration of the skeletal muscles. However, the mechanisms underlying the degeneration are not well described in human skeletal muscle. Here we show that skeletal muscle of T2DM patients exhibit degenerative remodeling of the extracellular matrix that is associated with a selective increase of a subpopulation of fibro-adipogenic progenitors (FAPs) marked by expression of THY1 (CD90)-the FAP^CD90+. We identify platelet-derived growth factor (PDGF) as a key FAP regulator, as it promotes proliferation and collagen production at the expense of adipogenesis. FAPs^CD90+ display a PDGF-mimetic phenotype, with high proliferative activity, clonogenicity, and production of extracellular matrix. FAP^CD90+ proliferation was reduced by in vitro treatment with metformin. Furthermore, metformin treatment reduced FAP content in T2DM patients. These data identify a PDGF-driven conversion of a subpopulation of FAPs as a key event in the fibrosis development in T2DM muscle.

Nampt controls skeletal muscle development by maintaining Ca2+ homeostasis and mitochondrial integrity

Objective

NAD⁺ is a co-factor and substrate for enzymes maintaining energy homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT) controls NAD⁺ synthesis, and in skeletal muscle, NAD⁺ is essential for muscle integrity. However, the underlying molecular mechanisms by which NAD⁺ synthesis affects muscle health remain poorly understood. Thus, the objective of the current study was to delineate the role of NAMPT-mediated NAD⁺ biosynthesis in skeletal muscle development and function.

Methods

To determine the role of Nampt in muscle development and function, we generated skeletal muscle-specific Nampt KO (SMNKO) mice. We performed a comprehensive phenotypic characterization of the SMNKO mice, including metabolic measurements, histological examinations, and RNA sequencing analyses of skeletal muscle from SMNKO mice and WT littermates.

Results

SMNKO mice were smaller, with phenotypic changes in skeletal muscle, including reduced fiber area and increased number of centralized nuclei. The majority of SMNKO mice died prematurely. Transcriptomic analysis identified that the gene encoding the mitochondrial permeability transition pore (mPTP) regulator Cyclophilin D (Ppif) was upregulated in skeletal muscle of SMNKO mice from 2 weeks of age, with associated increased sensitivity of mitochondria to the Ca²⁺-stimulated mPTP opening. Treatment of SMNKO mice with the Cyclophilin D inhibitor, Cyclosporine A, increased membrane integrity, decreased the number of centralized nuclei, and increased survival.

Conclusions

Our study demonstrates that NAMPT is crucial for maintaining cellular Ca²⁺ homeostasis and skeletal muscle development, which is vital for juvenile survival.

•

NAD⁺ salvage capacity is important for skeletal muscle development and survival.

•

Skeletal muscle-specific Nampt knockout mice exhibit a dystrophy-like phenotype.

•

Nampt deletion alters Ca²⁺ homeostasis and impairs mitochondrial function.

•

Low NAD⁺ levels signals mitochondrial permeability transition pore opening.

•

Cyclosporin A treatment improves sarcolemma integrity and increases survival rate.

Isolation and characterization of muscle stem cells, fibro-adipogenic progenitors, and macrophages from human skeletal muscle biopsies

Animal models clearly illustrate that the maintenance of skeletal muscle mass depends on the function and interaction of a heterogeneous population of resident and infiltrating mononuclear cells. Several lines of evidence suggest that mononuclear cells also play a role in muscle wasting in humans, and targeting these cells may open new treatment options for intervention or prevention in sarcopenia. Methodological and ethical constraints have perturbed exploration of the cellular characteristics and function of mononuclear cells in human skeletal muscle. Thus, investigations of cellular phenotypes often depend on immunohistochemical analysis of small tissue samples obtained by needle biopsies, which do not match the deep phenotyping of mononuclear cells obtained from animal models. Here, we have developed a protocol for fluorescence-activated cell sorting (FACS), based on single-cell RNA-sequencing data, for quantifying and characterizing mononuclear cell populations in human skeletal muscle. Muscle stem cells, fibro-adipogenic progenitors, and two subsets of macrophages (CD11c^+/-) are present in needle biopsies in comparable quantities per milligram tissue to open surgical biopsies. We find that direct cell isolation is preferable due to a substantial shift in transcriptome when using preculture before the FACS procedure. Finally, in vitro validation of the cellular phenotype of muscle stem cells, fibro-adipogenic progenitors, and macrophages confirms population-specific traits. This study demonstrates that mononuclear cell populations can be quantified and subsequently analyzed from needle biopsy material and opens the perspective for future clinical studies of cellular mechanisms in muscle wasting.

Efficient correction of Duchenne muscular dystrophy mutations by SpCas9 and dual gRNAs

CRISPR gene therapy is one promising approach for treatment of Duchenne muscular dystrophy (DMD), which is caused by a large spectrum of mutations in the dystrophin gene. To broaden CRISPR gene editing strategies for DMD treatment, we report the efficient restoration of dystrophin expression in induced myotubes by SpCas9 and dual guide RNAs (gRNAs). We first sequenced 32 deletion junctions generated by this editing method and revealed that non-homologous blunt-end joining represents the major indel type. Based on this predictive repair outcome, efficient in-frame deletion of a part of DMD exon 51 was achieved in HEK293T cells with plasmids expressing SpCas9 and dual gRNAs. More importantly, we further corrected a frameshift mutation in human DMD (exon45del) fibroblasts with SpCas9-dual gRNA ribonucleoproteins. The edited DMD fibroblasts were transdifferentiated into myotubes by lentiviral-mediated overexpression of a human MYOD transcription factor. Restoration of DMD expression at both the mRNA and protein levels was confirmed in the induced myotubes. With further development, the combination of SpCas9-dual gRNA-corrected DMD patient fibroblasts and transdifferentiation may provide a valuable therapeutic strategy for DMD.

Exercise-dependent increases in protein synthesis are accompanied by chromatin modifications and increased MRTF-SRF signalling

AIM

Resistance exercise increases muscle mass over time. However, the early signalling events leading to muscle growth are not yet well-defined. Here, we aim to identify new signalling pathways important for muscle remodelling after exercise.

METHODS

We performed a phosphoproteomics screen after a single bout of exercise in mice. As an exercise model we used unilateral electrical stimulation in vivo and treadmill running. We analysed muscle biopsies from human subjects to verify if our findings in murine muscle also translate to exercise in humans.

RESULTS

We identified a new phosphorylation site on Myocardin-Related Transcription Factor B (MRTF-B), a co-activator of serum response factor (SRF). Phosphorylation of MRTF-B is required for its nuclear translocation after exercise and is accompanied by the transcription of the SRF target gene Fos. In addition, high-intensity exercise also remodels chromatin at specific SRF target gene loci through the phosphorylation of histone 3 on serine 10 in myonuclei of both mice and humans. Ablation of the MAP kinase member MSK1/2 is sufficient to prevent this histone phosphorylation, reduce induction of SRF-target genes, and prevent increases in protein synthesis after exercise.

CONCLUSION

Our results identify a new exercise signalling fingerprint in vivo, instrumental for exercise-induced protein synthesis and potentially muscle growth.

Alternative polyadenylation of Pax3 controls muscle stem cell fate and muscle function

Adult stem cells are essential for tissue homeostasis. In skeletal muscle, muscle stem cells (MuSCs) reside in a quiescent state, but little is known about the mechanisms that control homeostatic turnover. Here we show that, in mice, the variation in MuSC activation rate among different muscles (for example, limb versus diaphragm muscles) is determined by the levels of the transcription factor Pax3. We further show that Pax3 levels are controlled by alternative polyadenylation of its transcript, which is regulated by the small nucleolar RNA U1. Isoforms of the Pax3 messenger RNA that differ in their 3' untranslated regions are differentially susceptible to regulation by microRNA miR206, which results in varying levels of the Pax3 protein in vivo. These findings highlight a previously unrecognized mechanism of the homeostatic regulation of stem cell fate by multiple RNA species.

Blood flow-restricted resistance exercise alters the surface profile, miRNA cargo and functional impact of circulating extracellular vesicles

Ischemic exercise conducted as low-load blood flow restricted resistance exercise (BFRE) can lead to muscle remodelling and promote muscle growth, possibly through activation of muscle precursor cells. Cell activation can be triggered by blood borne extracellular vesicles (EVs) as these nano-sized particles are involved in long distance signalling. In this study, EVs isolated from plasma of healthy human subjects performing a single bout of BFRE were investigated for their change in EV surface profiles and miRNA cargos as well as their impact on skeletal muscle precursor cell proliferation. We found that after BFRE, five EV surface markers and 12 miRNAs were significantly altered. Furthermore, target prediction and functional enrichment analysis of the miRNAs revealed several target genes that are associated to biological pathways involved in skeletal muscle protein turnover. Interestingly, EVs from BFRE plasma increased the proliferation of muscle precursor cells. In addition, alterations in surface markers and miRNAs indicated that the combination of exercise and ischemic conditioning during BFRE can stimulate blood cells to release EVs. These results support that BFRE promotes EV release to engage in muscle remodelling and/or growth processes.

Mitochondrial Structure and Function in the Metabolic Myopathy Accompanying Patients with Critical Limb Ischemia

Mitochondrial dysfunction has been implicated as a central mechanism in the metabolic myopathy accompanying critical limb ischemia (CLI). However, whether mitochondrial dysfunction is directly related to lower extremity ischemia and the structural and molecular mechanisms underpinning mitochondrial dysfunction in CLI patients is not understood. Here, we aimed to study whether mitochondrial dysfunction is a distinctive characteristic of CLI myopathy by assessing mitochondrial respiration in gastrocnemius muscle from 14 CLI patients (65.3 ± 7.8 y) and 15 matched control patients (CON) with a similar comorbidity risk profile and medication regimen but without peripheral ischemia (67.4 ± 7.4 y). Furthermore, we studied potential structural and molecular mechanisms of mitochondrial dysfunction by measuring total, sub-population, and fiber-type-specific mitochondrial volumetric content and cristae density with transmission electron microscopy and by assessing mitophagy and fission/fusion-related protein expression. Finally, we asked whether commonly used biomarkers of mitochondrial content are valid in patients with cardiovascular disease. CLI patients exhibited inferior mitochondrial respiration compared to CON. This respiratory deficit was not related to lower whole-muscle mitochondrial content or cristae density. However, stratification for fiber types revealed ultrastructural mitochondrial alterations in CLI patients compared to CON. CLI patients exhibited an altered expression of mitophagy-related proteins but not fission/fusion-related proteins compared to CON. Citrate synthase, cytochrome c oxidase subunit IV (COXIV), and 3-hydroxyacyl-CoA dehydrogenase (β-HAD) could not predict mitochondrial content. Mitochondrial dysfunction is a distinctive characteristic of CLI myopathy and is not related to altered organelle content or cristae density. Our results link this intrinsic mitochondrial deficit to dysregulation of the mitochondrial quality control system, which has implications for the development of therapeutic strategies.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

Molecular and cellular adaptations to exercise training in skeletal muscle from cancer patients treated with chemotherapy

BACKGROUND

A growing body of evidence suggests that exercise training has beneficial effects in cancer patients. The aim of the present study was to investigate the molecular basis underlying these beneficial effects in skeletal muscle from cancer patients.

METHODS

We investigated expression of selected proteins involved in cellular processes known to orchestrate adaptation to exercise training by western blot. Skeletal muscle biopsies were sampled from ten cancer patients before and after 4-7 weeks of ongoing chemotherapy, and subsequently after 10 weeks of continued chemotherapy in combination with exercise training. Biopsies from ten healthy matched subjects served as reference.

RESULTS

The expression of the insulin-regulated glucose transporter, GLUT4, increased during chemotherapy and continued to increase during exercise training. A similar trend was observed for ACC, a key enzyme in the biosynthesis and oxidation of fatty acids, but we did not observe any changes in other regulators of substrate metabolism (AMPK and PDH) or mitochondrial proteins (Cyt-C, COX-IV, SDHA, and VDAC). Markers of proteasomal proteolysis (MURF1 and ATROGIN-1) decreased during chemotherapy, but did not change further during chemotherapy combined with exercise training. A similar pattern was observed for autophagy-related proteins such as ATG5, p62, and pULK1 Ser⁷⁵⁷, but not ULK1 and LC3BII/LC3BI. Phosphorylation of FOXO3a at Ser^318/321 did not change during chemotherapy, but decreased during exercise training. This could suggest that FOXO3a-mediated transcriptional regulation of MURF1 and ATROGIN-1 serves as a mechanism by which exercise training maintains proteolytic systems in skeletal muscle in cancer patients. Phosphorylation of proteins that regulate protein synthesis (mTOR at Ser²⁴⁴⁸ and 4EBP1 at Thr^37/46) increased during chemotherapy and leveled off during exercise training. Finally, chemotherapy tended to increase the number of satellite cells in type 1 fibers, without any further change during chemotherapy and exercise training. Conversely, the number of satellite cells in type 2 fibers did not change during chemotherapy, but increased during chemotherapy combined with exercise training.

CONCLUSIONS

Molecular signaling cascades involved in exercise training are disturbed during cancer and chemotherapy, and exercise training may prevent further disruption of these pathways.

TRIAL REGISTRATION

The study was approved by the local Scientific Ethics Committee of the Central Denmark Region (Project ID: M-2014-15-14; date of approval: 01/27/2014) and the Danish Data Protection Agency (case number 2007-58-0010; date of approval: 01/28/2015). The trial was registered at http//www.clinicaltrials.gov (registration number: NCT02192216; date of registration 07/17-2014).

Skeletal muscle stem cell characteristics and myonuclei content in patients with rheumatoid arthritis: a cross-sectional study

To investigate satellite cells (SCs) and myonuclei characteristics in patients with rheumatoid arthritis (RA). Resting biopsies from m. vastus lateralis were obtained from thirteen RA patients and thirteen matched healthy controls (CON). Muscle biopsies were immunohistochemically stained and analyzed for fiber type specific content of SCs (Pax7⁺), proliferating SCs (Pax7⁺/MyoD⁺) and differentiating SCs (myogenin⁺). Furthermore, we quantified fiber type specific content of myonuclei and myofiber cross-sectional area (CSA). Finally, newly formed/regenerating fibers expressing neonatal MHC (nMHC⁺) were determined. The fiber type specific number of SCs did not differ between RA patients and CON, nor did the content of proliferating or differentiating SCs. In contrast, the content of myonuclei per fiber was higher in RA patients than CON for both type I (2.01 ± 0.41 vs. 1.42 ± 0.40 myonuclei/fiber, p < 0.01) and type II fibers (2.01 ± 0.41 vs. 1.37 ± 0.32 myonuclei/fiber, p < 0.01). No differences were observed in fiber composition, fiber type specific CSA or content of nMHC⁺ fibers. Our results indicate an increased propensity for myogenic differentiation of SC leading to an elevated myonuclear content in the skeletal muscle of RA patients. It is hypothesized that this could be a compensatory regulatory response related to the chronic inflammation in these patients.

Lean body mass, muscle fibre size and muscle function in cancer patients during chemotherapy and 10 weeks exercise

Background: Chemotherapy can reduce muscle mass in cancer patients but the potential of exercise to ameliorate this are understudied, particularly at the myocellular level. The primary purpose was to investigate changes in lean body mass (LBM) and secondly single fibre cross-sectional area (CSA) in cancer patients during chemotherapy and in combination with 10 weeks of exercise. Methods: In a single-arm trial, patients adhered to chemotherapy for at least 4 weeks (control period) before 10 weeks of exercise adjunct to chemotherapy (exercise period). LBM (Dual Energy X-ray Absorptiometry) and single fibre CSA (muscle biopsies) were assessed at baseline, pre- and post-exercise.  Muscle strength, functional performance and aerobic capacity were also assessed. Results: Ten patients were included, however only six patients completed the protocol. LBM changed over time (p=0.013), but no significant changes were observed between specific time points. Numerically, LBM decreased by 0.3 kg (p=0.41, 95% CI: -1.1;0.5) from 41.3-41.0 kg, during the control period and increased by 0.7 kg (p=0.16, 95% CI: -0.6;2.0) from 40.4-41.1 kg during exercise. Muscle fibre CSA did not change significantly over time (p=0.13), but decreased numerically in the control period by 703 μm2 (p=0.20, 95% CI: -1877; 470) and increased by 846 μm2 (trend, p=0.08, 95% CI: -162; 1854) following exercise. Muscle strength and functional performance were unchanged during the control period but improved post-exercise. Conclusions:  Despite non-significant changes in muscle mass (due to small sample size), this study adds novel information on LBM and myocellular changes in cancer patients during chemotherapy and concurrent exercise.

Effect of protein quality on recovery after intense resistance training

PURPOSE

The present study investigated the effects of high- versus low-quality protein supplementation on the regain of exercise performance during recovery from a period of high-intensity resistance training.

METHODS

In a diet-controlled crossover study, 12 resistance-trained participants performed two identical training periods, with each training period including four sessions of high-intensity resistance exercise during 5 days, while receiving either high- or low-quality protein. Prior to and at 3, 24 and 48 h after the training periods, performance was evaluated in knee extensor and flexor isometric maximal voluntary contraction (MVC), counter-movement jumping height (CMJ), and peak and mean anaerobic power. In addition, prior to and at 48 h after the training periods, performance in time-to-exhaustion at 70 % of VO_2max (TTE) was evaluated.

RESULTS

After the intense training periods, decrements in the order of 4-24 % were observed for MVC_ext, CMJ, mean anaerobic power, and TTE. In particular for TTE, this decrement in exercise performance did not attain full recovery at 48 h post-exercise. The regain of exercise performance was not dictated by type of protein supplement.

CONCLUSION

The regain of muscle strength as well as anaerobic or aerobic performances were not markedly influenced by the type of protein supplement.

High Intensity Training May Reverse the Fiber Type Specific Decline in Myogenic Stem Cells in Multiple Sclerosis Patients

Multiple sclerosis (MS) is associated with loss of skeletal muscle mass and function. The myogenic stem cells (satellite cells—SCs) are instrumental to accretion of myonuclei, but remain to be investigated in MS. The present study aimed to compare the SC and myonuclei content between MS patients (n = 23) and age matched healthy controls (HC, n = 18). Furthermore, the effects of 12 weeks of high intensity training on SC and myonuclei content were explored in MS. Muscle biopsies were obtained from m. Vastus Lateralis at baseline (MS and HC) and following 12 weeks of training (MS only). Frozen biopsies were sectioned followed by immunohistochemical analysis for fiber type specific SCs (Pax7⁺), myonuclei (MN) and central nuclei content and fiber cross-sectional area (fCSA) was quantified using ATPase histochemistry. At baseline the SCs per fiber was lower in type II compared to type I fibers in both MS (119%, p < 0.01) and HC (69%, p < 0.05), whereas the SCs per fCSA was lower in type II fibers compared to type I only in MS (72%, p < 0.05). No differences were observed in MN or central nuclei between MS and HC. Following training the type II fiber SCs per fiber and per fCSA in MS patients increased by 165% (p < 0.05) and 135% (p < 0.05), respectively. Furthermore, the type II fiber MN content tended (p = 0.06) to be increased by 35% following training. In conclusion, the SC content is lower in type II compared to type I fibers in both MS and HC. Furthermore, high intensity training was observed to selectively increase the SC and myonuclei content in type II fibers in MS patients.

Effect of degree of hydrolysis of whey protein on in vivo plasma amino acid appearance in humans

Whey protein is generally found to be faster digested and to promote faster and higher increases in plasma amino acid concentrations during the immediate ~60 min following protein ingestion compared to casein. The aim of the present study was to compare three different whey protein hydrolysates with varying degrees of hydrolysis (DH, % cleaved peptide bonds) to evaluate if the degree of whey protein hydrolysis influences the rate of amino acid plasma appearance in humans. A casein protein was included as reference. The three differentially hydrolysed whey proteins investigated were: High degree of hydrolysis (DH, DH = 48 %), Medium DH (DH = 27 %), and Low DH (DH = 23 %). The casein protein was intact. Additionally, since manufacturing of protein products may render some amino acids unavailable for utilisation in the body the digestibility and the biological value of all four protein fractions were evaluated in a rat study. A two-compartment model for the description of the postprandial plasma amino acid kinetics was applied to investigate the rate of postprandial total amino acid plasma appearance of the four protein products. The plasma amino acid appearance rates of the three whey protein hydrolysates (WPH) were all significantly higher than for the casein protein, however, the degree of hydrolysis of the WPH products did not influence plasma total amino acid appearance rate (estimates of DH and 95 % confidence intervals [CI] (mol L⁻¹ min⁻¹): High DH 0.0585 [0.0454, 0.0754], Medium DH 0.0594 [0.0495, 0.0768], Low DH 0.0560 [0.0429, 0.0732], Casein 0.0194 [0.0129, 0.0291]). The four protein products were all highly digestible, while the biological value decreased with increasing degree of hydrolysis. In conclusion, the current study does not provide evidence that the degree of whey protein hydrolysis is a strong determinant for plasma amino acid appearance rate within the studied range of hydrolysis and protein dose.

Satellite cell response to erythropoietin treatment and endurance training in healthy young men

KEY POINT

Erythropoietin (Epo) treatment may induce myogenic differentiation factor (MyoD) expression and prevent apoptosis in satellite cells (SCs) in murine and in vitro models. Endurance training stimulates SC proliferation in vivo in murine and human skeletal muscle. In the present study, we show, in human skeletal muscle, that treatment with an Epo-stimulating agent (darbepoetin-α) in vivo increases the content of MyoD(+) SCs in healthy young men. Moreover, we report that Epo receptor mRNA is expressed in adult human SCs, suggesting that Epo may directly target SCs through ligand-receptor interaction. Moreover, endurance training, but not Epo treatment, increases the SC content in type II myofibres, as well as the content of MyoD(+) SCs. Collectively, our results suggest that Epo treatment can regulate human SCs in vivo, supported by Epo receptor mRNA expression in human SCs. In effect, long-term Epo treatment during disease conditions involving anaemia may impact SCs and warrants further investigation. Satellite cell (SC) proliferation is observed following erythropoitin treatment in vitro in murine myoblasts and endurance training in vivo in human skeletal muscle. The present study aimed to investigate the effects of prolonged erythropoiesis-stimulating agent (ESA; darbepoetin-α) treatment and endurance training, separately and combined, on SC quantity and commitment in human skeletal muscle. Thirty-five healthy, untrained men were randomized into four groups: sedentary-placebo (SP, n = 9), sedentary-ESA (SE, n = 9), training-placebo (TP, n = 9) or training-ESA (TE, n = 8). ESA/placebo was injected once weekly and training consisted of ergometer cycling three times a week for 10 weeks. Prior to and following the intervention period, blood samples and muscle biopsies were obtained and maximal oxygen uptake (V̇O2, max) was measured. Immunohistochemical analyses were used to quantify fibre type specific SCs (Pax7(+)), myonuclei and active SCs (Pax7(+)/MyoD(+)). ESA treatment led to elevated haematocrit, whereas endurance training increased V̇O2, max. Endurance training led to an increase in SCs associated with type II fibres (P < 0.05), whereas type I fibres showed no changes. Both ESA treatment and endurance training increased Pax7(+)/MyoD(+) cells, whereas only ESA treatment increased the total content of MyoD(+) cells. Epo-R mRNA presence in adult SC was tested with real-time RT-PCR using fluorescence-activated cell sorting (CD56(+)/CD45(-)/CD31(-)) to isolate cells from a human rectus abdominis muscle and was found to be considerably higher than in whole muscle. In conclusion, endurance training and ESA treatment may separately stimulate SC commitment to the myogenic program. Furthermore, ESA-treatment may alter SC activity by direct interaction with the Epo-R expressed on SCs.

Pericyte response to contraction mode-specific resistance exercise training in human skeletal muscle

Skeletal muscle satellite cells (SCs) are important for muscle repair and hypertrophy in response mechanical stimuli. Neuron-glial antigen 2-positive (NG2(+)) and alkaline phosphatase-positive (ALP(+)) pericytes may provide an alternative source of myogenic progenitors and/or secrete paracrine factors to induce Pax7(+) SC proliferation and differentiation. The purpose of this study was to investigate NG2(+) and ALP(+) cell quantity, as well as SC content and activation, in human skeletal muscle following prolonged concentric (Conc) or eccentric (Ecc) resistance training. Male subjects engaged in unilateral resistance training utilizing isolated Ecc or Conc contractions. After 12 wk, muscle biopsies were analyzed for NG2(+) and ALP(+) pericytes, total Pax7(+) SCs, activated SCs (Pax7(+)MyoD(+)), and differentiating myogenic cells (Pax7(-) MyoD(+)). NG2(+) cells localized to CD31(+) vessels and the majority coexpressed ALP. NG2(+) pericyte quantity decreased following both Conc and Ecc training (P < 0.05). ALP(+) pericyte quantity declined following Conc (P < 0.05) but not Ecc training. Conversely, total Pax7(+) SC content was elevated following Conc only (P < 0.001), while Pax7(+)MyoD(+) SC content was increased following Conc and Ecc (P < 0.001). Follow up analyses demonstrated that CD90(+) and platelet-derived growth factor receptor-α (PDGFRα)(+) mononuclear cell proliferation was also increased in response to both Conc and Ecc training (P < 0.01). In summary, resistance training results in a decline in pericyte quantity and an increase in mesenchymal progenitor cell proliferation, and these events likely influence SC pool expansion and increased activation observed posttraining.

Interactions between muscle stem cells, mesenchymal-derived cells and immune cells in muscle homeostasis, regeneration and disease

Recent evidence has revealed the importance of reciprocal functional interactions between different types of mononuclear cells in coordinating the repair of injured muscles. In particular, signals released from the inflammatory infiltrate and from mesenchymal interstitial cells (also known as fibro-adipogenic progenitors (FAPs)) appear to instruct muscle stem cells (satellite cells) to break quiescence, proliferate and differentiate. Interestingly, conditions that compromise the functional integrity of this network can bias muscle repair toward pathological outcomes that are typically observed in chronic muscular disorders, that is, fibrotic and fatty muscle degeneration as well as myofiber atrophy. In this review, we will summarize the current knowledge on the regulation of this network in physiological and pathological conditions, and anticipate the potential contribution of its cellular components to relatively unexplored conditions, such as aging and physical exercise.

The acute response of pericytes to muscle-damaging eccentric contraction and protein supplementation in human skeletal muscle

Skeletal muscle pericytes increase in quantity following eccentric exercise (ECC) and contribute to myofiber repair and adaptation in mice. The purpose of the present investigation was to examine pericyte quantity in response to muscle-damaging ECC and protein supplementation in human skeletal muscle. Male subjects were divided into protein supplement (WHY; n = 12) or isocaloric placebo (CHO; n = 12) groups and completed ECC using an isokinetic dynamometer. Supplements were consumed 3 times/day throughout the experimental time course. Biopsies were collected prior to (PRE) and 3, 24, 48, and 168 h following ECC. Reflective of the damaging protocol, integrin subunits, including α7, β1A, and β1D, increased (3.8-fold, 3.6-fold and 3.9-fold, respectively, P < 0.01) 24 h post-ECC with no difference between supplements. Pericyte quantity did not change post-ECC. WHY resulted in a small, but significant, decrease in ALP(+) pericytes when expressed as a percentage of myonuclei (CHO 6.8 ± 0.3% vs. WHY 5.8 ± 0.3%, P < 0.05) or per myofiber (CHO 0.119 ± 0.01 vs. WHY 0.098 ± 0.01, P < 0.05). The quantity of myonuclei expressing serum response factor and the number of pericytes expressing serum response factor, did not differ as a function of time post-ECC or supplement. These data demonstrate that acute muscle-damaging ECC increases α7β1 integrin content in human muscle, yet pericyte quantity is largely unaltered. Future studies should focus on the capacity for ECC to influence pericyte function, specifically paracrine factor release as a mechanism toward pericyte contribution to repair and adaptation postexercise.

Enhanced Glycogen Storage of a Subcellular Hot Spot in Human Skeletal Muscle during Early Recovery from Eccentric Contractions

Unaccustomed eccentric exercise is accompanied by muscle damage and impaired glucose uptake and glycogen synthesis during subsequent recovery. Recently, it was shown that the role and regulation of glycogen in skeletal muscle are dependent on its subcellular localization, and that glycogen synthesis, as described by the product of glycogen particle size and number, is dependent on the time course of recovery after exercise and carbohydrate availability. In the present study, we investigated the subcellular distribution of glycogen in fibers with high (type I) and low (type II) mitochondrial content during post-exercise recovery from eccentric contractions. Analysis was completed on five male subjects performing an exercise bout consisting of 15 x 10 maximal eccentric contractions. Carbohydrate-rich drinks were subsequently ingested throughout a 48 h recovery period and muscle biopsies for analysis included time points 3, 24 and 48 h post exercise from the exercising leg, whereas biopsies corresponding to prior to and at 48 h after the exercise bout were collected from the non-exercising, control leg. Quantitative imaging by transmission electron microscopy revealed an early (post 3 and 24 h) enhanced storage of intramyofibrillar glycogen (defined as glycogen particles located within the myofibrils) of type I fibers, which was associated with an increase in the number of particles. In contrast, late in recovery (post 48 h), intermyofibrillar, intramyofibrillar and subsarcolemmal glycogen in both type I and II fibers were lower in the exercise leg compared with the control leg, and this was associated with a smaller size of the glycogen particles. We conclude that in the carbohydrate-supplemented state, the effect of eccentric contractions on glycogen metabolism depends on the subcellular localization, muscle fiber’s oxidative capacity, and the time course of recovery. The early enhanced storage of intramyofibrillar glycogen after the eccentric contractions may entail important implications for muscle function and fatigue resistance.

Blood flow restricted and traditional resistance training performed to fatigue produce equal muscle hypertrophy

This study investigated the hypertrophic potential of load-matched blood-flow restricted resistance training (BFR) vs free-flow traditional resistance training (low-load TRT) performed to fatigue. Ten healthy young subjects performed unilateral BFR and contralateral low-load TRT elbow flexor dumbbell curl with 40% of one repetition maximum until volitional concentric failure 3 days per week for 6 weeks. Prior to and at 3 (post-3) and 10 (post-10) days post-training, magnetic resonance imaging (MRI) was used to estimate elbow flexor muscle volume and muscle water content accumulation through training. Acute changes in muscle thickness following an early vs a late exercise bout were measured with ultrasound to determine muscle swelling during the immediate 0-48 h post-exercise. Total work was threefold lower for BFR compared with low-load TRT (P < 0.001). Both BRF and low-load TRT increased muscle volume by approximately 12% at post-3 and post-10 (P < 0.01) with no changes in MRI-determined water content. Training increased muscle thickness during the immediate 48 h post-exercise (P < 0.001) and to greater extent with BRF (P < 0.05) in the early training phase. In conclusion, BFR and low-load TRT, when performed to fatigue, produce equal muscle hypertrophy, which may partly rely on transient exercise-induced increases in muscle water content.

Muscle strength and functional performance is markedly impaired at the recommended time point for sport return after anterior cruciate ligament reconstruction in recreational athletes

PURPOSE

To examine potential deficits in muscle strength or functional capacity when comparing (1) an ACL reconstructed group to matched healthy controls, (2) the ACL reconstructed leg to the non-injured leg and (3) the non-injured leg to matched healthy controls, at the time-point of recommended sport return 9-12months post-surgery.

METHODS

Sixteen patients (male-female ratio: 9:7) 9-12months post ACL reconstruction and sixteen age and sex matched healthy controls were included. Outcome measures included maximal knee extensor (KE) and knee flexor (KF) dynamometry, including measurement of rate of force development, functional capacity (counter movement jump (CMJ) and single distance hop (SDH)) and the Lysholm score.

RESULTS

Compared to the control group, maximal KE and KF muscle strength were impaired in the ACL reconstructed leg by 27-39% and 16-35%, respectively (p<.001). Also, impairments of both CMJ (38%) and SDH (33%) were observed (p<.001). Rate of force development for KE were reduced in the ACL group compared to the control group (p<.001). Similarly, the KE and KF muscle strength, CMJ and SDH of the ACL reconstructed leg were impaired, when compared to the non-injured leg by 15-23%, 8-20%, 23% and 20%, respectively (p<.05).

CONCLUSION

Muscle strength and functional capacity are markedly impaired in the ACL reconstructed leg of recreationally active people 9-12months post-surgery when compared to healthy matched controls and to their non-injured leg. This suggests that objective criteria rather than "time-since-surgery" criteria should guide return to sport.

Influence of exercise contraction mode and protein supplementation on human skeletal muscle satellite cell content and muscle fiber growth

Skeletal muscle satellite cells (SCs) are involved in remodeling and hypertrophy processes of skeletal muscle. However, little knowledge exists on extrinsic factors that influence the content of SCs in skeletal muscle. In a comparative human study, we investigated the muscle fiber type-specific association between emergence of satellite cells (SCs), muscle growth, and remodeling in response to 12 wk unilateral resistance training performed as eccentric (Ecc) or concentric (Conc) resistance training ± whey protein (Whey, 19.5 g protein + 19.5 g glucose) or placebo (Placebo, 39 g glucose) supplementation. Muscle biopsies (vastus lateralis) were analyzed for fiber type-specific SCs, myonuclei, and fiber cross-sectional area (CSA). Following training, SCs increased with Conc in both type I and type II fibers (P < 0.01) and exhibited a group difference from Ecc (P < 0.05), which did not increase. Myonuclei content in type I fibers increased in all groups (P < 0.01), while a specific accretion of myonuclei in type II fibers was observed in the Whey-Conc (P < 0.01) and Placebo-Ecc (P < 0.01) groups. Similarly, whereas type I fiber CSA increased independently of intervention (P < 0.001), type II fiber CSA increased exclusively with Whey-Conc (P < 0.01) and type II fiber hypertrophy correlated with whole muscle hypertrophy exclusively following Conc training (P < 0.01). In conclusion, isolated concentric knee extensor resistance training appears to constitute a stronger driver of SC content than eccentric resistance training while type II fiber hypertrophy was accentuated when combining concentric resistance training with whey protein supplementation.

Effects of divergent resistance exercise contraction mode and dietary supplementation type on anabolic signalling, muscle protein synthesis and muscle hypertrophy

Greater force produced with eccentric (ECC) compared to concentric (CONC) contractions, may comprise a stronger driver of muscle growth, which may be further augmented by protein supplementation. We investigated the effect of differentiated contraction mode with either whey protein hydrolysate and carbohydrate (WPH + CHO) or isocaloric carbohydrate (CHO) supplementation on regulation of anabolic signalling, muscle protein synthesis (MPS) and muscle hypertrophy. Twenty-four human participants performed unilateral isolated maximal ECC versus CONC contractions during exercise habituation, single-bout exercise and 12 weeks of training combined with WPH + CHO or CHO supplements. In the exercise-habituated state, p-mTOR, p-p70S6K, p-rpS6 increased by approximately 42, 206 and 213 %, respectively, at 1 h post-exercise, with resistance exercise per se; whereas, the phosphorylation was exclusively maintained with ECC at 3 and 5 h post-exercise. This acute anabolic signalling response did not differ between the isocaloric supplement types, neither did protein fractional synthesis rate differ between interventions. Twelve weeks of ECC as well as CONC resistance training augmented hypertrophy with WPH + CHO group compared to the CHO group (7.3 ± 1.0 versus 3.4 ± 0.8 %), independently of exercise contraction type. Training did not produce major changes in basal levels of Akt-mTOR pathway components. In conclusion, maximal ECC contraction mode may constitute a superior driver of acute anabolic signalling that may not be mirrored in the muscle protein synthesis rate. Furthermore, with prolonged high-volume resistance training, contraction mode seems less influential on the magnitude of muscle hypertrophy, whereas protein and carbohydrate supplementation augments muscle hypertrophy as compared to isocaloric carbohydrate supplementation .

Differentiated mTOR but not AMPK signaling after strength vs endurance exercise in training-accustomed individuals

The influence of adenosine mono phosphate (AMP)-activated protein kinase (AMPK) vs Akt-mammalian target of rapamycin C1 (mTORC1) protein signaling mechanisms on converting differentiated exercise into training specific adaptations is not well-established. To investigate this, human subjects were divided into endurance, strength, and non-exercise control groups. Data were obtained before and during post-exercise recovery from single-bout exercise, conducted with an exercise mode to which the exercise subjects were accustomed through 10 weeks of prior training. Blood and muscle samples were analyzed for plasma substrates and hormones and for muscle markers of AMPK and Akt-mTORC1 protein signaling. Increases in plasma glucose, insulin, growth hormone (GH), and insulin-like growth factor (IGF)-1, and in phosphorylated muscle phospho-Akt substrate (PAS) of 160 kDa, mTOR, 70 kDa ribosomal protein S6 kinase, eukaryotic initiation factor 4E, and glycogen synthase kinase 3a were observed after strength exercise. Increased phosphorylation of AMPK, histone deacetylase5 (HDAC5), cAMP response element-binding protein, and acetyl-CoA carboxylase (ACC) was observed after endurance exercise, but not differently from after strength exercise. No changes in protein phosphorylation were observed in non-exercise controls. Endurance training produced an increase in maximal oxygen uptake and a decrease in submaximal exercise heart rate, while strength training produced increases in muscle cross-sectional area and strength. No changes in basal levels of signaling proteins were observed in response to training. The results support that in training-accustomed individuals, mTORC1 signaling is preferentially activated after hypertrophy-inducing exercise, while AMPK signaling is less specific for differentiated exercise.

Influence of divergent exercise contraction mode and whey protein supplementation on atrogin-1, MuRF1, and FOXO1/3A in human skeletal muscle

Knowledge from human exercise studies on regulators of muscle atrophy is lacking, but it is important to understand the underlying mechanisms influencing skeletal muscle protein turnover and net protein gain. This study examined the regulation of muscle atrophy-related factors, including atrogin-1 and MuRF1, their upstream transcription factors FOXO1 and FOXO3A and the atrogin-1 substrate eIF3-f, in response to unilateral isolated eccentric (ECC) vs. concentric (CONC) exercise and training. Exercise was performed with whey protein hydrolysate (WPH) or isocaloric carbohydrate (CHO) supplementation. Twenty-four subjects were divided into WPH and CHO groups and completed both single-bout exercise and 12 wk of training. Single-bout ECC exercise decreased atrogin-1 and FOXO3A mRNA compared with basal and CONC exercise, while MuRF1 mRNA was upregulated compared with basal. ECC exercise downregulated FOXO1 and phospho-FOXO1 protein compared with basal, and phospho-FOXO3A was downregulated compared with CONC. CONC single-bout exercise mediated a greater increase in MuRF1 mRNA and increased FOXO1 mRNA compared with basal and ECC. CONC exercise downregulated FOXO1, FOXO3A, and eIF3-f protein compared with basal. Following training, an increase in basal phospho-FOXO1 was observed. While WPH supplementation with ECC and CONC training further increased muscle hypertrophy, it did not have an additional effect on mRNA or protein levels of the targets measured. In conclusion, atrogin-1, MuRF1, FOXO1/3A, and eIF3-f mRNA, and protein levels, are differentially regulated by exercise contraction mode but not WPH supplementation combined with hypertrophy-inducing training. This highlights the complexity in understanding the differing roles these factors play in healthy muscle adaptation to exercise.

Effect of resistance exercise contraction mode and protein supplementation on members of the STARS signalling pathway

The striated muscle activator of Rho signalling (STARS) pathway is suggested to provide a link between external stress responses and transcriptional regulation in muscle. However, the sensitivity of STARS signalling to different mechanical stresses has not been investigated. In a comparative study, we examined the regulation of the STARS signalling pathway in response to unilateral resistance exercise performed as either eccentric (ECC) or concentric (CONC) contractions as well as prolonged training; with and without whey protein supplementation. Skeletal muscle STARS, myocardian-related transcription factor-A (MRTF-A) and serum response factor (SRF) mRNA and protein, as well as muscle cross-sectional area and maximal voluntary contraction, were measured. A single-bout of exercise produced increases in STARS and SRF mRNA and decreases in MRTF-A mRNA with both ECC and CONC exercise, but with an enhanced response occurring following ECC exercise. A 31% increase in STARS protein was observed exclusively after CONC exercise (P < 0.001), while pSRF protein levels increased similarly by 48% with both CONC and ECC exercise (P < 0.001). Prolonged ECC and CONC training equally stimulated muscle hypertrophy and produced increases in MRTF-A protein of 125% and 99%, respectively (P < 0.001). No changes occurred for total SRF protein. There was no effect of whey protein supplementation. These results show that resistance exercise provides an acute stimulation of the STARS pathway that is contraction mode dependent. The responses to acute exercise were more pronounced than responses to accumulated training, suggesting that STARS signalling is primarily involved in the initial phase of exercise-induced muscle adaptations.