Effects of one bout of endurance exercise on the expression of myogenin in human quadriceps muscle

Skeletal Muscle MicroRNA Patterns in Response to a Single Bout of Exercise in Females: Biomarkers for Subsequent Training Adaptation?

microRNAs (miRs) have been proposed as a promising new class of biomarkers in the context of training adaptation. Using microarray analysis, we studied skeletal muscle miR patterns in sedentary young healthy females (n = 6) before and after a single submaximal bout of endurance exercise ('reference training'). Subsequently, participants were subjected to a structured training program, consisting of six weeks of moderate-intensity continuous endurance training (MICT) and six weeks of high-intensity interval training (HIIT) in randomized order. In vastus lateralis muscle, we found significant downregulation of myomiRs, specifically miR-1, 133a-3p, and -5p, -133b, and -499a-5p. Similarly, exercise-associated miRs-23a-3p, -378a-5p, -128-3p, -21-5p, -107, -27a-3p, -126-3p, and -152-3p were significantly downregulated, whereas miR-23a-5p was upregulated. Furthermore, in an untargeted approach for differential expression in response to acute exercise, we identified n = 35 miRs that were downregulated and n = 20 miRs that were upregulated by factor 4.5 or more. Remarkably, KEGG pathway analysis indicated central involvement of this set of miRs in fatty acid metabolism. To reproduce these data in a larger cohort of all-female subjects (n = 29), qPCR analysis was carried out on n = 15 miRs selected from the microarray, which confirmed their differential expression. Furthermore, the acute response, i.e., the difference between miR concentrations before and after the reference training, was correlated with changes in maximum oxygen uptake (V̇O₂max) in response to the training program. Here, we found that miRs-199a-3p and -19b-3p might be suitable acute-response candidates that correlate with individual degrees of training adaptation in females.

Endurance training changes the expression of miR-1 and miR-133 and predicted genes in slow and fast twitch muscles

PURPOSE OF THE RESEARCH

Endurance training can modify signaling and gene expression pathways that play a pivotal role in determining the phenotype of the fibers. The present study aimed to investigate the effects of endurance training on the expression of some myomiRs and related genes in slow and fast twitch muscles.

METHODS

Twenty healthy male adult Wistar rats (281 ± 14 g) were randomized to either control (n = 10) or treated (n = 10). The treated group performed an endurance program for eight weeks (running on a treadmill for eight weeks, 50 min, 23 m/min). After the end of the training protocol, the slow (soleus) and fast (EDL) twitch muscles were removed to assess the miR-1, miR-133 expression, and hdac4, mef2c genes, and protein by real-time PCR and western blot, respectively.

RESULTS

The soleus muscle miR-1 expression and mef2c gene in the treated group were significantly lower compared control (p = 0.0001). In contrast, miR-133 and hdac4 gene expression of the soleus muscle of the treated group increased significantly (p = 0001), and the EDL miR-133 and mef2c expression of the treated group increased in the compared control group (p = 0.0001). The EDL MEF2c protein expression in the treated group significantly decreased compared to the control group, although the expression of EDL HDAC4 protein significantly increased (p = 0.0001).

CONCLUSIONS

Endurance training changes the expression of the miR-1, miR-133, and their predicted genes in slow and fast twitch muscles. Also, the rate of HDAC4 and MEF2c protein synthesis, which are upstream and downstream of these myomiRs, was affected by endurance training.

Astragalus polysaccharide promotes sheep satellite cell differentiation by regulating miR-133a through the MAPK/ERK signaling pathway

Astragalus polysaccharide (APS) possesses extensive biological activities, pharmacological effects, and anti-fatigue function. MiR-133a is a specifically expressed miRNA in skeletal muscle that participates in the regulation of myoblast proliferation and differentiation. However, little is known about the role of APS in the development of sheep skeletal muscle. In this study, we aimed to investigate the underlying mechanism of APS and miR-133a on the differentiation of sheep skeletal muscle satellite cells (SMSCs) and the regulatory relationship between APS and miR-133a. The results suggested that APS plays a positive regulatory role in the proliferation and differentiation of sheep SMSCs. Moreover, miR-133a significantly promotes SMSC differentiation and the activity of the MAPK/ERK signaling pathway. Importantly, we found that APS function requires the mediation of miR-133a in the differentiation of sheep SMSCs. Taken together, our results indicate that APS accelerates SMSC differentiation by regulating miR-133a via the MAPK/ERK signaling pathway in sheep.

No dynamic changes in the expression of genes related to the epigenetic mechanism during acute exercise

Physical exercise results in structural remodeling in tissues and modifies cellular metabolism. Changes in gene expression lie at the root of these adaptations. Epigenetic changes are one of the factors responsible for such exercise-related alterations. One-hour acute exercise will change DNMT1, HDAC1, and JHDM1D transcriptions in PBMC. This study examined changes in the expression of genes responsible for epigenetic modifications (HDAC1, DNMT1, and JHDM1D) during and after an incremental exercise test on a treadmill and a 30-min recovery. Blood samples from 9 highly trained triathletes were tested. Examination of the transcripts showed no significant changes. Correlations between transcript results and biochemical indices revealed a significant (p = 0.007) relationship between JHDM1D mRNA and the number of monocytes at peak exercise intensity (exhaustion), while there was no significant (p = 0.053) correlation at rest. There are no rapid changes in the mRNA levels of the genes studied in blood cells in competitive athletes during acute exercise and recovery. Due to the small group of subjects studied, more extensive research is needed to verify correlations between transcription and biochemical variables.

Non-Coding RNAs as Regulators of Myogenesis and Postexercise Muscle Regeneration

miRNAs and lncRNAs do not encode proteins, but they play an important role in the regulation of gene expression. They differ in length, biogenesis, and mode of action. In this work, we focus on the selected miRNAs and lncRNAs involved in the regulation of myogenesis and muscle regeneration. We present selected miRNAs and lncRNAs that have been shown to control myogenic differentiation and show that manipulation of their levels could be used to improve myogenic differentiation of various types of stem and progenitor cells. Finally, we discuss how physical activity affects miRNA and lncRNA expression and how it affects muscle well-being.

Exercise Partially Rejuvenates Muscle Stem Cells

Exercise has long been known to extend health and lifespan in humans and other mammals. However, typically exercise is thought to slow the loss of function that accompanies aging. Brett et al. have now shown that exercise restores functional competency to regenerate muscle stem cells (MuSCs) in mice as well as restore a significant portion of the transcriptional signature associated with young MuSCs. The mechanism involves the likely induction of plasma-borne factors that upregulate cell cycle regulator cyclin D1, which otherwise decreases with increasing age. Cyclin D1, in turn, through its noncanonical attenuation of TGF-beta/Smad3 signaling, helps maintain the regenerative capacity of MuSCs, which is lost as TGF-beta signaling increases with age. Interestingly, elevated levels of some proinflammatory regulators including NF-κB, TNF-alpha, and interleukin 6 (IL-6) are also reduced by exercise or ectopic expression of cyclin D1. Importantly, the rejuvenation is not complete, as Notch signaling, which also decreases with age, remains at old levels and the rejuvenative effect is not permanent: wearing off in ∼2 weeks after cessation of exercise. Understanding the limitations of the rejuvenative effect of exercise on MuSCs at the molecular level, including changes in the epigenome such as altered DNA methylation age, will be critical in developing more significant rejuvenative therapies including some for aged people wherein morbidities limit exercise.

The protective effect of rosmarinic acid on myotube formation during myoblast differentiation under heat stress

High ambient temperature is one of the most important environmental factors that caused the reduction of livestock productivity and the increase of mortality. It has been shown that heat stress could affect the meat quality characteristics by physiological and metabolic perturbations in live livestock. Rosmarinic acid (RA) is a natural polyphenolic phytochemical compound that has many important biological activities, such as antioxidant, antimutagenic, and antitumor. The purpose of this study was to investigate the possible function and mechanism of RA on myoblast proliferation and differentiation under heat stress condition. The results showed that heat stress reduced the viability of myoblast and increased the percentage of apoptotic cells, and it also disrupted myotube formation by altering the expression of myogenic regulatory factors MyoD, myogenin, and MyHC. However, pretreatment of RA can protect C2C12 cells from heat stress-induced apoptosis, and it also increased the expression level of MyoD, myogenin, and MyHC under heat stress, which indicated that RA have protective effect on heat stress-caused failure of myotube formation during myoblast differentiation. Above all, our finding demonstrated that RA can promote the differentiation of C2C12 myoblast and maintain the formation of myotubes even under heat stress condition.

Exercise rejuvenates quiescent skeletal muscle stem cells in old mice through restoration of Cyclin D1

Aging impairs tissue repair. This is pronounced in skeletal muscle, whose regeneration by muscle stem cells (MuSCs) is robust in young adult animals but inefficient in older organisms. Despite this functional decline, old MuSCs are amenable to rejuvenation through strategies that improve the systemic milieu, such as heterochronic parabiosis. One such strategy, exercise, has long been appreciated for its benefits on healthspan, but its effects on aged stem cell function in the context of tissue regeneration are incompletely understood. Here we show that exercise in the form of voluntary wheel running accelerates muscle repair in old animals and improves old MuSC function. Through transcriptional profiling and genetic studies, we discovered that the restoration of old MuSC activation ability hinges on restoration of Cyclin D1, whose expression declines with age in MuSCs. Pharmacologic studies revealed that Cyclin D1 maintains MuSC activation capacity by repressing TGFβ signaling. Taken together, these studies demonstrate that voluntary exercise is a practicable intervention for old MuSC rejuvenation. Furthermore, this work highlights the distinct role of Cyclin D1 in stem cell quiescence.

Facilitating Exercise Engagement among Community Dwelling Stroke Survivors: Is a once Per Week Group Session Sufficient?

Although exercise is proven as an effective strategy to combat post-stroke complications and the risk of stroke recurrence, many stroke survivors fail to engage in this activity following rehabilitation. In this study, we assessed the feasibility and usefulness of a low-frequency group exercise to determine its suitability as an approach to facilitate exercise engagement among stroke survivors. Forty-one stroke survivors, mean (SD) age 59.34 (10.02) years, mean time post-stroke 17.13 (17.58) months, completed a 90 minute, once per week, group exercise supervised by therapists for 12 weeks. The exercise outcomes were measured using standardized clinical tests. We observed improvement in the group's physical performance; balance score by 3 units (Z = -3.88, p < 0.001), speed of repetitive sit to stand by 3.4 s (Z = -4.69, p < 0.001), and walking speed by 8.22 m/min (Z = -3.25, p < 0.001). Scores of seven out of 14 Berg's balance scale items increased significantly, indicating better balance ability among the survivors. In conclusion, a 12-week, once per week group exercise session seems feasible and sufficient to improve the physical performance of community dwelling stroke survivors. This exercise arrangement may be offered to stroke survivors to facilitate exercise practice following rehabilitation.

The redox-dependent regulation of satellite cells following aseptic muscle trauma (SpEED): study protocol for a randomized controlled trial

Background

Muscle satellite cells (SCs) are crucial for muscle regeneration following muscle trauma. Acute skeletal muscle damage results in inflammation and the production of reactive oxygen species (ROS) which may be implicated in SCs activation. Protection of these cells from oxidative damage is essential to ensure sufficient muscle regeneration. The aim of this study is to determine whether SCs activity under conditions of aseptic skeletal muscle trauma induced by exercise is redox-dependent.

Methods/design

Based on the SCs content in their vastus lateralis skeletal muscle, participants will be classified as either high or low respondents. In a randomized, double-blind, crossover, repeated-measures design, participants will then receive either placebo or N-acetylcysteine (alters redox potential in muscle) during a preliminary 7-day loading phase, and for eight consecutive days following a single bout of intense muscle-damaging exercise. In both trials, blood samples and muscle biopsies will be collected, and muscle performance and soreness will be measured at baseline, pre-exercise, 2 and 8 days post exercise. Biological samples will be analyzed for redox status and SCs activity. Between trials, a 4-week washout period will be implemented.

Discussion

This study is designed to investigate the impact of redox status on SCs mobilization and thus skeletal muscle potential for regeneration under conditions of aseptic inflammation induced by exercise. Findings of this trial should provide insight into (1) molecular pathways involved in SCs recruitment and muscle healing under conditions of aseptic skeletal muscle trauma present in numerous catabolic conditions and (2) whether skeletal muscle’s potential for regeneration depends on its basal SCs content.

Trial registration

ClinicalTrials.gov, ID: NCT03711838. Registered on 19 Oct 2018.

Electronic supplementary material

The online version of this article (10.1186/s13063-019-3557-3) contains supplementary material, which is available to authorized users.

Satellite cell activation and mTOR signaling pathway response to resistance and combined exercise in elite weight lifters

PURPOSE

Our aim was to compare the effects of a single exercise training mode (resistance exercise) with a combined exercise training (resistance and plyometric exercise) mode on satellite cell activity and anabolic signaling at the molecular level.

METHODS

Eighteen male weight lifters (20 ± 4 years, BMI 27 ± 6 kg/m²) were randomly assigned to either a series of resistance exercise or a series of combined exercise group. The intensity of the exercise was set at 60% of their 1 RM weight and subjects completed three sets each of six repetitions. The combined exercise group performed three different types of resistance exercise alternating with three different types of plyometric exercise, whereas the resistance exercise group performed only the three different types of resistance exercise which was repeated twice. Muscle biopsies were obtained the vastus lateralis muscle immediately before and 3 h after one bout of exercise.

RESULTS

Exercise induced increases in satellite cell activation and myofibrillar protein synthesis following both exercise modes, but the resistance exercise group was superior compared to the combined exercise group in satellite cell activity expressed by Ki67/CD56 (165 vs 232%) and PI3K/Akt protein expression (121 vs 157%), mTOR protein expression (117 vs 288%), p70S6K protein expression (253 vs 809%), and 4E-BP1 protein expression (70 vs 139%) of anabolic signaling pathway.

CONCLUSIONS

These results suggest that the previous findings showing a greater effect of combined as opposed to a single exercise mode could be the effect of a greater training volume rather than a true-training effect of a combined exercise program.

Neuromuscular electrical stimulation improves skeletal muscle regeneration through satellite cell fusion with myofibers in healthy elderly subjects

The aim of this study was to determine whether neuromuscular electrical stimulation (NMES) affects skeletal muscle regeneration through a reduction of oxidative status in satellite cells of healthy elderly subjects. Satellite cells from the vastus lateralis skeletal muscle of 12 healthy elderly subjects before and after 8 wk of NMES were allowed to proliferate to provide myogenic populations of adult stem cells [myogenic precursor cells (MPCs)]. These MPCs were then investigated in terms of their proliferation, their basal cytoplasmic free Ca2+concentrations, and their expression of myogenic regulatory factors ( PAX3, PAX7, MYF5, MYOD, and MYOG) and micro-RNAs (miR-1, miR-133a/b, and miR-206). The oxidative status of these MPCs was evaluated through superoxide anion production and superoxide dismutase and glutathione peroxidase activities. On dissected single skeletal myofibers, the nuclei were counted to determine the myonuclear density, the fiber phenotype, cross-sectional area, and tension developed. The MPCs obtained after NMES showed increased proliferation rates along with increased cytoplasmic free Ca2+concentrations and gene expression of MYOD and MYOG on MPCs. Muscle-specific miR-1, miR-133a/b, and miR-206 were upregulated. This NMES significantly reduced superoxide anion production, along with a trend to reduction of superoxide dismutase activity. The NMES-dependent stimulation of muscle regeneration enhanced satellite cell fusion with mature skeletal fibers. NMES improved the regenerative capacity of skeletal muscle in elderly subjects. Accordingly, the skeletal muscle strength and mobility of NMES-stimulated elderly subjects significantly improved. NMES may thus be further considered for clinical or ageing populations.

NEW & NOTEWORTHY The neuromuscular electrical stimulation (NMES) effect on skeletal muscle regeneration was assessed in healthy elderly subjects for the first time. NMES improved the regenerative capacity of skeletal muscle through increased myogenic precursor cell proliferation and fusion with mature myofibers. The increased cytoplasmic free Ca2+concentration along with MYOD, MYOG, and micro-RNA upregulation could be related to reduced O2·−production, which, in turn, favors myogenic regeneration. Accordingly, the skeletal muscle strength of NMES-stimulated lower limbs of healthy elderly subjects improved along with their mobility.

Low Intensity Exercise Training Improves Skeletal Muscle Regeneration Potential

Purpose: The aim of this study was to determine whether 12 days of low-to-moderate exercise training at low altitude (598 m a.s.l.) improves skeletal muscle regeneration in sedentary adult women.

Methods: Satellite cells were obtained from the vastus lateralis skeletal muscle of seven women before and after this exercise training at low altitude. They were investigated for differentiation aspects, superoxide anion production, antioxidant enzymes, mitochondrial potential variation after a depolarizing insult, intracellular Ca²⁺ concentrations, and micro (mi)RNA expression (miR-1, miR-133, miR-206).

Results: In these myogenic populations of adult stem cells, those obtained after exercise training, showed increased Fusion Index and intracellular Ca²⁺ concentrations. This exercise training also generally reduced superoxide anion production in cells (by 12–67%), although not in two women, where there was an increase of ~15% along with a reduced superoxide dismutase activity. miRNA expression showed an exercise-induced epigenetic transcription profile that was specific according to the reduced or increased superoxide anion production of the cells.

Conclusions: The present study shows that low-to-moderate exercise training at low altitude improves the regenerative capacity of skeletal muscle in adult women. The differentiation of cells was favored by increased intracellular calcium concentration and increased the fusion index. This low-to-moderate training at low altitude also depicted the epigenetic signature of cells.

Satellite cells in human skeletal muscle plasticity

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

Mega roles of microRNAs in regulation of skeletal muscle health and disease

Skeletal muscle is a dynamic tissue with remarkable plasticity. Skeletal muscle growth and regeneration are highly organized processes thus it is not surprising that a high degree of complexity exists in the regulation of these processes. Recent discovery of non-coding microRNAs (miRNAs) has prompted extensive research in understanding the roles of these molecules in skeletal muscle. Research so far shows that miRNAs play a very significant role at every aspect of muscle biology. Besides muscle growth, development, and regeneration miRNAs are also involved in the pathology of muscle diseases and metabolism. In this review, recent advancements in miRNA function during myogenesis, exercise, atrophy, aging, and dystrophy are discussed.

Skeletal muscle wasting with disuse atrophy is multi-dimensional: the response and interaction of myonuclei, satellite cells and signaling pathways

Maintenance of skeletal muscle is essential for health and survival. There are marked losses of skeletal muscle mass as well as strength and physiological function under conditions of low mechanical load, such as space flight, as well as ground based models such as bed rest, immobilization, disuse, and various animal models. Disuse atrophy is caused by mechanical unloading of muscle and this leads to reduced muscle mass without fiber attrition. Skeletal muscle stem cells (satellite cells) and myonuclei are integrally involved in skeletal muscle responses to environmental changes that induce atrophy. Myonuclear domain size is influenced differently in fast and slow twitch muscle, but also by different models of muscle wasting, a factor that is not yet understood. Although the myonuclear domain is 3-dimensional this is rarely considered. Apoptosis as a mechanism for myonuclear loss with atrophy is controversial, whereas cell death of satellite cells has not been considered. Molecular signals such as myostatin/SMAD pathway, MAFbx, and MuRF1 E3 ligases of the ubiquitin proteasome pathway and IGF1-AKT-mTOR pathway are 3 distinctly different contributors to skeletal muscle protein adaptation to disuse. Molecular signaling pathways activated in muscle fibers by disuse are rarely considered within satellite cells themselves despite similar exposure to unloading or low mechanical load. These molecular pathways interact with each other during atrophy and also when various interventions are applied that could alleviate atrophy. Re-applying mechanical load is an obvious method to restore muscle mass, however how nutrient supplementation (e.g., amino acids) may further enhance recovery (or reduce atrophy despite unloading or ageing) is currently of great interest. Satellite cells are particularly responsive to myostatin and to growth factors. Recently, the hibernating squirrel has been identified as an innovative model to study resistance to atrophy.

Effects of myogenin on muscle fiber types and key metabolic enzymes in gene transfer mice and C2C12 myoblasts

Skeletal muscle fiber type composition is one of the important factors influencing muscle growth and meat quality. As a member of the myogenic transcription factors, myogenin (MyoG) is required for embryonic myoblast differentiation, but the expression of MyoG continues in mature muscle tissue of adult animals, especially in oxidative metabolic muscle, which suggests that MyoG may play a more extended role. Therefore, using MyoG gene transfer mice and C2C12 myoblasts as in vivo and in vitro models, respectively, we elected to study the role of MyoG in muscle fiber types and oxidative metabolism by using overexpression and siRNA suppression strategies. The overexpression of MyoG by DNA electroporation in mouse gastrocnemius muscle had no significant effect on fiber type composition but upregulated the mRNA expression (P<0.01) and enzyme activity (P<0.05) of oxidative succinic dehydrogenase (SDH). In addition, downregulation of the activity of the glycolytic enzymes lactate dehydrogenase (LDH, P<0.05) and pyruvate kinase (PK, P<0.05) was observed in MyoG gene transfer mice. In vitro experiments verified the results obtained in mice. Stable MyoG-transfected differentiating C2C12 cells showed higher mRNA expression levels of myosin heavy chain (MyHC) isoform IIX (P<0.01) and SDH (P<0.05), while the LDH mRNA was attenuated. The enzyme activities of SDH (P<0.01) and LDH (P<0.05) were similarly altered at the mRNA level. When MyoG was knocked down in C2C12 cells, MyHC IIX expression (P<0.05) was decreased, but the mRNA level (P<0.05) and the enzyme activity (P<0.05) of SDH were increased. Downregulating MyoG also increased the activity of the glycolytic enzymes PK (P<0.05) and hexokinase (HK, P<0.05). Based on those results, we concluded that MyoG barely changes the MyHC isoforms, except MyHC IIX, in differentiating myoblasts but probably influences the shift from glycolytic metabolism towards oxidative metabolism both in vivo and in vitro. These results contribute to further understand the role of MyoG in skeletal muscle energy metabolism and also help to explore the key genes that regulate meat quality.

Role of nitric oxide in muscle regeneration following eccentric muscle contractions in rat skeletal muscle

We examined the role of nitric oxide (NO) in muscle repair and regeneration following repetitive eccentric contractions (ECC). A standardized exercise protocol was used to create eccentric contraction-induced injury to the left tibialis anterior muscle of 48 male Wistar rats (body wt 250-350 g), using a customized isokinetic test device and a bout of 40 ECCs under electrical stimulation. A nitric oxide synthase inhibitor, N(G)-nitro-L-arginine-methyl ester (L-NAME; 35 mg kg(-1) day(-1)), was included in the diet for half the animals (n = 24) beginning 3 days prior to the ECC and continuing throughout the experiment, whereas the other half (n = 24) received a control diet. ECC/+L-NAME and ECC/-L-NAME were killed after the ECC protocol at 0, 1, 3 and 7 days (n = 6 on each day). An unexercised contralateral limb with and without L-NAME infusion served as a respective control muscle at each time point. Muscle NO content, skeletal muscle damage, leukocyte infiltration, calpain activity, and MyoD and myogenin expression were assessed. NO has both pro-inflammatory and anti-inflammatory properties, and several possible roles for NO in skeletal muscle damage have been postulated. NO content was greater in the ECC/-L-NAME group at all time points (p < 0.05) compared to ECC/+L-NAME. Additionally, significant differences in NO content were observed on day 0 (p < 0.05), and day 3 (p < 0.05), ECC/+L-NAME versus ECC/-L-NAME. One day following the bout of ECC, and NO levels were increased in the ECC/-L-NAME group. Three days following ECC, there was greater myofiber damage (measured by β-glucuronidase activity) and leukocyte invasion in the ECC/-L-NAME group as compared to the ECC/+L-NAME group. One day after ECC, calpain activity was significantly increased in ECC/-L-NAME compared with control muscles (p < 0.05). On days 3 and 7, Myo-D and myogenin gene expression was increased in both groups; however, the degree of regeneration was less in the ECC/+L-NAME-treated animals. These data suggest that NO dynamics have important implications in the regulation of various factors during skeletal muscle regeneration following damaging eccentric muscle contractions.

Regulation of miRNAs in human skeletal muscle following acute endurance exercise and short-term endurance training

  The identification of microRNAs (miRNAs) has established new mechanisms that control skeletal muscle adaptation to exercise. The present study investigated the mRNA regulation of components of the miRNA biogenesis pathway (Drosha, Dicer and Exportin-5), muscle enriched miRNAs, (miR-1, -133a, -133b and -206), and several miRNAs dysregulated in muscle myopathies (miR-9, -23, -29, -31 and -181). Measurements were made in muscle biopsies from nine healthy untrained males at rest, 3 h following an acute bout of moderate-intensity endurance cycling and following 10 days of endurance training. Bioinformatics analysis was used to predict potential miRNA targets. In the 3 h period following the acute exercise bout, Drosha, Dicer and Exportin-5, as well as miR-1, -133a, -133-b and -181a were all increased. In contrast miR-9, -23a, -23b and -31 were decreased. Short-term training increased miR-1 and -29b, while miR-31 remained decreased. Negative correlations were observed between miR-9 and HDAC4 protein (r=-0.71; P=0.04), miR-31 and HDAC4 protein (r=-0.87; P=0.026) and miR-31 and NRF1 protein (r=-0.77; P=0.01) 3 h following exercise. miR-31 binding to the HDAC4 and NRF1 3 untranslated region (UTR) reduced luciferase reporter activity. Exercise rapidly and transiently regulates several miRNA species in muscle. Several of these miRNAs may be involved in the regulation of skeletal muscle regeneration, gene transcription and mitochondrial biogenesis. Identifying endurance exercise-mediated stress signals regulating skeletal muscle miRNAs, as well as validating their targets and regulatory pathways post exercise, will advance our understanding of their potential role/s in human health.

Muscle specific microRNAs are regulated by endurance exercise in human skeletal muscle

Muscle specific miRNAs, myomiRs, have been shown to control muscle development in vitro and are differentially expressed at rest in diabetic skeletal muscle. Therefore, we investigated the expression of these myomiRs, including miR-1, miR-133a, miR-133b and miR-206 in muscle biopsies from vastus lateralis of healthy young males (n = 10) in relation to a hyperinsulinaemic–euglycaemic clamp as well as acute endurance exercise before and after 12 weeks of endurance training. The subjects increased their endurance capacity, VO2max (l min−1) by 17.4% (P < 0.001), and improved insulin sensitivity by 19% (P < 0.01). While myomiR expression remained stable during a hyperinsulinaemic–euglycaemic clamp, an acute bout of exercise increased mir-1 (P < 0.05) and mir-133a (P < 0.05) expression before, but not after, training. In resting biopsies, endurance training for 12 weeks decreased basal expression of all four myomiRs (P < 0.05). Interestingly, all myomiRs reverted to their pre-training expression levels 14 days after ceasing the training programme. Components of major pathways involved in endurance adaptation such as MAPK and TGF-β were predicted to be targeted by the myomiRs examined. Tested predicted target proteins included Cdc42 and ERK 1/2. Although these proteins were downregulated between post-training period and 2 weeks of cessation, an inverse correlation between myomiR and target proteins was not found. In conclusion, our data suggest myomiRs respond to physiological stimuli, but their role in regulating human skeletal muscle adaptation remains unknown.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

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.

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

It has been well-established that the age-related loss of muscle mass and strength, or sarcopenia, impairs skeletal muscle function and reduces functional performance at a more advanced age. Skeletal muscle satellite cells (SC), as precursors of new myonuclei, have been suggested to be involved in the development of sarcopenia. In accordance with the type II muscle fiber atrophy observed in the elderly, recent studies report a concomitant fiber type specific reduction in SC content. Resistance type exercise interventions have proven effective to augment skeletal muscle mass and improve muscle function in the elderly. In accordance, recent work shows that resistance type exercise training can augment type II muscle fiber size and reverse the age-related decline in SC content. The latter is supported by an increase in SC activation and proliferation factors that generally appear following exercise training. Present findings strongly suggest that the skeletal muscle SC control myogenesis and have an important, but yet unresolved, function in the loss of muscle mass with aging. This review discusses the contribution of skeletal muscle SC in the age-related loss of muscle mass and the efficacy of exercise training as a means to attenuate and/or reverse this process.

Structural protein alterations to resistance and endurance cycling exercise training

The muscle cytoskeleton is necessary for the effective transmission of forces generated by actin-myosin interactions. We have examined the impact of muscle force and exercise volume on the cytoskeleton by measuring desmin and dystrophin content in human skeletal muscle after 12 weeks of progressive resistance or endurance cycle training. Muscle biopsies of the vastus lateralis were obtained before and after training. Desmin and dystrophin content was determined using immunoblotting techniques. After resistance training, desmin content increased 82 +/- 18% (p < 0.05), whereas there was no change in desmin content with endurance cycling. Dystrophin content did not change in either group. One-repetition maximum and VO2max increased (p < 0.05) in the resistance and endurance groups, respectively. These data demonstrate that a high-tension stimulus impacts the cytoskeleton in contrast to high-volume concentric contractions. The tensile loading and eccentric components of resistance training are implicated in desmin alterations. Indeed, the functional improvements resulting from resistance training may be related in part to the mechanical integration provided by the desmin protein.

Role of MyoD in denervated, disused, and exercised muscle

The myogenic regulatory factor MyoD plays an important role in embryonic and adult skeletal muscle growth. Even though it is best known as a marker for activated satellite cells, it is also expressed in myonuclei, and its expression can be induced by a variety of different conditions. Several model systems have been used to study the mechanisms behind MyoD regulation, such as exercise, stretch, disuse, and denervation. Since MyoD reacts in a highly muscle-specific manner, and its expression varies over time and between species, universally valid predictions and explanations for changes in MyoD expression are not possible. This review explores the complex role of MyoD in muscle plasticity by evaluating the induction of MyoD expression in the context of muscle composition and electrical and mechanical stimulation.

Co-expression of IGF-1 family members with myogenic regulatory factors following acute damaging muscle-lengthening contractions in humans

Muscle regeneration following injury is dependent on the ability of muscle satellite cells to activate, proliferate and fuse with damaged fibres. This process is controlled by the myogenic regulatory factors (MRF). Little is known about the temporal relation of the MRF with the expression of known myogenic growth factors (i.e. IGF-1) in humans following muscle damage. Eight subjects (20.6 +/- 2.1 years; 81.4 +/- 9.8 kg) performed 300 lengthening contractions (180 deg s(-1)) of their knee extensors in one leg on a dynamometer. Blood and muscle samples were collected before and at 4 (T4), 24 (T24), 72 (T72) and 120 h (T120) post-exercise. Mechano growth factor (MGF), IGF-1Ea and IGF-1Eb mRNA were quantified. Serum IGF-1 did not change over the post-exercise time course. IGF-1Ea and IGF-1Eb mRNA increased approximately 4- to 6-fold by T72 (P < 0.01) and MGF mRNA expression peaked at T24 (P = 0.005). MyoD mRNA expression increased approximately 2-fold at T4 (P < 0.05). Myf5 expression peaked at T24 (P < 0.05), while MRF4 and myogenin mRNA expression peaked at T72 (P < 0.05). Myf5 expression strongly correlated with the increase in MGF mRNA (r(2) = 0.83; P = 0.03), while MRF4 was correlated with both IGF-1Ea and -Eb (r(2) = 0.90; r(2) = 0.81, respectively; P < 0.05). Immunofluorescence analysis showed IGF-1 protein expression localized to satellite cells at T24, and to satellite cells and the myofibre at T72 and T120; IGF-1 was not detected at T0 or T4. These results suggest that the temporal response of MGF is probably related to the activation/proliferation phase of the myogenic programme as marked by an increase in both Myf5 and MyoD, while IGF-1Ea and -Eb may be temporally related to differentiation as marked by an increase in MRF4 and myogenin expression following acute muscle damage.

Skeletal muscle changes after hemiparetic stroke and potential beneficial effects of exercise intervention strategies

Stroke is the leading cause of disability in the United States. New evidence reveals significant structural and metabolic changes in skeletal muscle after stroke. Muscle alterations include gross atrophy and shift to fast myosin heavy chain in the hemiparetic (contralateral) leg muscle; both are related to gait deficit severity. The underlying molecular mechanisms of this atrophy and muscle phenotype shift are not known. Inflammatory markers are also present in contralateral leg muscle after stroke. Individuals with stroke have a high prevalence of insulin resistance and diabetes. Skeletal muscle is a major site for insulin-glucose metabolism. Increasing evidence suggests that inflammatory pathway activation and oxidative injury could lead to wasting, altered function, and impaired insulin action in skeletal muscle. The health benefits of exercise in disabled populations have now been recognized. Aerobic exercise improves fitness, strength, and ambulatory performance in subjects with chronic stroke. Therapeutic exercise may modify or reverse skeletal muscle abnormalities.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Skeletal Muscle Morphology in Patients with Restless Legs Syndrome

AIM

The aim of the study was to assess the cellular and structural properties of skeletal muscle in restless legs syndrome (RLS).

METHOD

Twenty patients and 16 controls were included. Aerobic performance was assessed using a submaximal test. On muscle biopsies taken from the tibialis anterior, fiber distribution and fiber area were analyzed together with parameters surveying the microvascularization, especially the tortuosity, which is expressed as a percent of muscle fiber perimeter in contact with the wall of the microvessel, length of capillary/perimeter of fiber (LC/PF) index.

RESULTS

The RLS group had significantly lower predicted maximal oxygen uptake (p = 0.01) and significantly higher LC/PF index (p = 0.01) compared to the controls.

CONCLUSION

The higher capillary tortuosity in RLS patients indicates the occurrence of significant remodeling in capillary geometry in RLS.

Influence of preexercise muscle glycogen content on transcriptional activity of metabolic and myogenic genes in well-trained humans

To determine whether preexercise muscle glycogen content influences the transcription of several early-response genes involved in the regulation of muscle growth, seven male strength-trained subjects performed one-legged cycling exercise to exhaustion to lower muscle glycogen levels (Low) in one leg compared with the leg with normal muscle glycogen (Norm) and then the following day completed a unilateral bout of resistance training (RT). Muscle biopsies from both legs were taken at rest, immediately after RT, and after 3 h of recovery. Resting glycogen content was higher in the control leg (Norm leg) than in the Low leg (435 +/- 87 vs. 193 +/- 29 mmol/kg dry wt; P < 0.01). RT decreased glycogen content in both legs (P < 0.05), but postexercise values remained significantly higher in the Norm than the Low leg (312 +/- 129 vs. 102 +/- 34 mmol/kg dry wt; P < 0.01). GLUT4 (3-fold; P < 0.01) and glycogenin mRNA abundance (2.5-fold; not significant) were elevated at rest in the Norm leg, but such differences were abolished after exercise. Preexercise mRNA abundance of atrogenes was also higher in the Norm compared with the Low leg [atrogin: approximately 14-fold, P < 0.01; RING (really interesting novel gene) finger: approximately 3-fold, P < 0.05] but decreased for atrogin in Norm following RT (P < 0.05). There were no differences in the mRNA abundance of myogenic regulatory factors and IGF-I in the Norm compared with the Low leg. Our results demonstrate that 1) low muscle glycogen content has variable effects on the basal transcription of select metabolic and myogenic genes at rest, and 2) any differences in basal transcription are completely abolished after a single bout of heavy resistance training. We conclude that commencing resistance exercise with low muscle glycogen does not enhance the activity of genes implicated in promoting hypertrophy.

Predicting tissue-specific enhancers in the human genome

Determining how transcriptional regulatory signals are encoded in vertebrate genomes is essential for understanding the origins of multicellular complexity; yet the genetic code of vertebrate gene regulation remains poorly understood. In an attempt to elucidate this code, we synergistically combined genome-wide gene-expression profiling, vertebrate genome comparisons, and transcription factor binding-site analysis to define sequence signatures characteristic of candidate tissue-specific enhancers in the human genome. We applied this strategy to microarray-based gene expression profiles from 79 human tissues and identified 7187 candidate enhancers that defined their flanking gene expression, the majority of which were located outside of known promoters. We cross-validated this method for its ability to de novo predict tissue-specific gene expression and confirmed its reliability in 57 of the 79 available human tissues, with an average precision in enhancer recognition ranging from 32% to 63% and a sensitivity of 47%. We used the sequence signatures identified by this approach to successfully assign tissue-specific predictions to approximately 328,000 human-mouse conserved noncoding elements in the human genome. By overlapping these genome-wide predictions with a data set of enhancers validated in vivo, in transgenic mice, we were able to confirm our results with a 28% sensitivity and 50% precision. These results indicate the power of combining complementary genomic data sets as an initial computational foray into a global view of tissue-specific gene regulation in vertebrates.

Myogenin (Myf4) upregulation in trans-differentiating fibroblasts from a congenital myopathy with arrest of myogenesis and defects of myotube formation

Congenital myopathies often have an unclear aetiology. Here, we studied a novel case of a severe congenital myopathy with a failure of myotube formation. Polymerase chain reaction-based analysis was performed to characterize the expression patterns of the Desmin, p21, p57, and muscle regulatory factors (MRFs) MyoD, Myf4, Myf5 and Myf6 in differentiating skeletal muscle cells (SkMCs), normal human fibroblasts and patient-derived fibroblasts during trans-differentiation. The temporal and spatial pattern of MRFs was further characterized by immunocyto- and immunohistochemical stainings. In differentiating SkMCs, each MRF showed a characteristic expression pattern. Normal trans-differentiating fibroblasts formed myotubes and expressed all of the MRFs, which were detected. Interestingly, the patient's fibroblasts also showed some fusion events during trans-differentiation with a comparable expression profile for the MRFs, particularly, with increased expression of Myf4 and p21. Immunohistochemical analysis of normal and patient-derived skeletal musculature revealed that Myf4, which is downregulated during normal fetal development, was still present in patient-derived skeletal head muscle, which was also positive for Desmin and sarcomeric actin. The abnormal upregulation of Myf4 and p21 in the patient who suffered from a severe congenital myopathy suggests that the regulation of Myf4 and p21 gene expression during myogenesis might be of interest for further studies.

Interaction of contractile activity and training history on mRNA abundance in skeletal muscle from trained athletes

Skeletal muscle displays enormous plasticity to respond to contractile activity with muscle from strength- (ST) and endurance-trained (ET) athletes representing diverse states of the adaptation continuum. Training adaptation can be viewed as the accumulation of specific proteins. Hence, the altered gene expression that allows for changes in protein concentration is of major importance for any training adaptation. Accordingly, the aim of the present study was to quantify acute subcellular responses in muscle to habitual and unfamiliar exercise. After 24-h diet/exercise control, 13 male subjects (7 ST and 6 ET) performed a random order of either resistance (8 x 5 maximal leg extensions) or endurance exercise (1 h of cycling at 70% peak O2 uptake). Muscle biopsies were taken from vastus lateralis at rest and 3 h after exercise. Gene expression was analyzed using real-time PCR with changes normalized relative to preexercise values. After cycling exercise, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (ET approximately 8.5-fold, ST approximately 10-fold, P < 0.001), pyruvate dehydrogenase kinase-4 (PDK-4; ET approximately 26-fold, ST approximately 39-fold), vascular endothelial growth factor (VEGF; ET approximately 4.5-fold, ST approximately 4-fold), and muscle atrophy F-box protein (MAFbx) (ET approximately 2-fold, ST approximately 0.4-fold) mRNA increased in both groups, whereas MyoD (approximately 3-fold), myogenin (approximately 0.9-fold), and myostatin (approximately 2-fold) mRNA increased in ET but not in ST (P < 0.05). After resistance exercise PDK-4 (approximately 7-fold, P < 0.01) and MyoD (approximately 0.7-fold) increased, whereas MAFbx (approximately 0.7-fold) and myostatin (approximately 0.6-fold) decreased in ET but not in ST. We conclude that prior training history can modify the acute gene responses in skeletal muscle to subsequent exercise.

Efficacy of 3 days/wk resistance training on myofiber hypertrophy and myogenic mechanisms in young vs. older adults

Resistance training (RT) has shown the most promise in reducing/reversing effects of sarcopenia, although the optimum regime specific for older adults remains unclear. We hypothesized myofiber hypertrophy resulting from frequent (3 days/wk, 16 wk) RT would be impaired in older (O; 60-75 yr; 12 women, 13 men), sarcopenic adults compared with young (Y; 20-35 yr; 11 women, 13 men) due to slowed repair/regeneration processes. Myofiber-type distribution and cross-sectional area (CSA) were determined at 0 and 16 wk. Transcript and protein levels of myogenic regulatory factors (MRFs) were assessed as markers of regeneration at 0 and 24 h postexercise, and after 16 wk. Only Y increased type I CSA 18% (P < 0.001). O showed smaller type IIa (-16%) and type IIx (-24%) myofibers before training (P < 0.05), with differences most notable in women. Both age groups increased type IIa (O, 16%; Y, 25%) and mean type II (O, 23%; Y, 32%) size (P < 0.05). Growth was generally most favorable in young men. Percent change scores on fiber size revealed an age x gender interaction for type I fibers (P < 0.05) as growth among Y (25%) exceeded that of O (4%) men. Myogenin and myogenic differentiation factor D (MyoD) mRNAs increased (P < 0.05) in Y and O, whereas myogenic factor (myf)-5 mRNA increased in Y only (P < 0.05). Myf-6 protein increased (P < 0.05) in both Y and O. The results generally support our hypothesis as 3 days/wk training led to more robust hypertrophy in Y vs. O, particularly among men. However, this differential hypertrophy adaptation was not explained by age variation in MRF expression.

Recent progress in histochemistry and cell biology: the state of the art 2005

Advances in the field of histochemistry, a multidisciplinary area including the detection, localization and functional characterization of molecules in single cells and complex tissues, often drives the attainment of new knowledge in the broadly defined discipline of cell biology. These two disciplines, histochemistry and cell biology, have been joined in this journal to facilitate the flow of information with celerity from technical advancement in histochemical procedures, to their utilization in experimental models. This review summarizes advancements in these fields during the past year.

The behaviour of satellite cells in response to exercise: what have we learned from human studies?

Understanding the complex role played by satellite cells in the adaptive response to exercise in human skeletal muscle has just begun. The development of reliable markers for the identification of satellite cell status (quiescence/activation/proliferation) is an important step towards the understanding of satellite cell behaviour in exercised human muscles. It is hypothesised currently that exercise in humans can induce (1) the activation of satellite cells without proliferation, (2) proliferation and withdrawal from differentiation, (3) proliferation and differentiation to provide myonuclei and (4) proliferation and differentiation to generate new muscle fibres or to repair segmental fibre injuries. In humans, the satellite cell pool can increase as early as 4 days following a single bout of exercise and is maintained at higher level following several weeks of training. Cessation of training is associated with a gradual reduction of the previously enhanced satellite cell pool. In the elderly, training counteracts the normal decline in satellite cell number seen with ageing. When the transcriptional activity of existing myonuclei reaches its maximum, daughter cells generated by satellite cell proliferation are involved in protein synthesis by enhancing the number of nuclear domains. Clearly, delineating the events and the mechanisms behind the activation of satellite cells both under physiological and pathological conditions in human skeletal muscles remains an important challenge.

Transcriptional profiling of tissue plasticity: role of shifts in gene expression and technical limitations

Reprogramming of gene expression has been recognized as a main instructive modality for the adjustments of tissues to various kinds of stress. The recent application of gene expression profiling has provided a powerful tool to elucidate the molecular pathways underlying such tissue remodeling. However, the biological interpretations of expression profiling results critically depend on normalization of transcript signals to mRNA standards before statistical evaluation. A hypothesis is proposed whereby the “fluctuating nature” of gene expression represents an inherent limitation of the test system used to quantify RNA levels. Misinterpretation of gene expression data occurs when RNA quantities are normalized to a subset of mRNAs that are subject to strong regulation. The contention of contradictory biological outcomes using different RNA-normalization schemes is demonstrated in two models of skeletal muscle plasticity with data from custom-designed microarrays and biochemical and ultrastructural evidence for correspondingly altered RNA content and nucleolar activity. The prevalence of these biological constraints is underlined by a literature survey in different models of tissue plasticity with emphasis on the unique malleability of skeletal muscle. Finally, recommendations on the optimal experimental layout are given to control biological and technical variability in microarray and RT-PCR studies. It is proposed to approach normalization of transcript signals by measuring total RNA and DNA content per sample weight and by correcting for concurrently estimated endogenous standards such as major ribosomal RNAs and spiked RNA and DNA species. This allows for later conversion to diverse tissue-relevant references and should improve the physiological interpretations of phenotypic plasticity.

Time course of myogenic and metabolic gene expression in response to acute exercise in human skeletal muscle

The aim of this study was to examine the time course activation of select myogenic (MRF4, Myf5, MyoD, myogenin) and metabolic (CD36, CPT1, HKII, and PDK4) genes after an acute bout of resistance (RE) or run (Run) exercise. Six RE subjects [25 ± 4 yr (mean ± SD), 74 ± 14 kg, 1.71 ± 0.11 m] and six Run subjects (25 ± 4 yr, 72 ± 5 kg, 1.81 ± 0.07 m, 63 ± 8 ml·kg−1·min−1) were studied. Eight muscle biopsies were taken from the vastus lateralis (RE) and gastrocnemius (Run) before, immediately after, and 1, 2, 4, 8, 12 and 24 h after exercise. RE increased mRNA of MRF4 (3.7- to 4.5-fold 2–4 h post), MyoD (5.8-fold 8 h post), myogenin (2.6- and 3.5-fold 8–12 h post), HKII (3.6- to 10.5-fold 2–12 h post), and PDK4 (14- to 26-fold 2–8 h post). There were no differences in Myf5, CD36, and CPT1 mRNA levels 0–24 h post-RE. Run increased mRNA of MyoD (5.0- to 8.0-fold), HKII (12- to 16-fold), and PDK4 (32- to 52-fold) at 8–12 h postexercise. There were no differences in MRF4, Myf5, myogenin, CD36 and CPT1 mRNA levels 0–24 h post-Run. These data indicate a myogenic and metabolic gene induction with RE and Run exercise. The timing of the gene induction is variable and generally peaks 4–8 h postexercise with all gene expression not significantly different from the preexercise levels by 24 h postexercise. These data provide basic information for the timing of human muscle biopsy samples for gene-expression studies involving exercise.

News and views in Histochemistry and Cell Biology

Advances in histochemical methodology and ingenious applications of novel and improved methods continue to confirm the standing of morphological means and approaches in research efforts, and contribute significantly to increasing our knowledge about structures and functions in all areas of the life sciences from cell biology to pathology. Reports published during recent months documenting this progress are summarized in the present review.

Time course of molecular responses of human skeletal muscle to acute bouts of resistance exercise

Resistance exercise (RE) training, designed to induce hypertrophy, strives for optimal activation of anabolic and myogenic mechanisms to increase myofiber size. Clearly, activation of these mechanisms must precede skeletal muscle growth. Most mechanistic studies of RE have involved analysis of outcome variables after many training sessions. This study measured molecular level responses to RE on a scale of hours to establish a time course for the activation of myogenic mechanisms. Muscle biopsy samples were collected from nine subjects before and after acute bouts of RE. The response to a single bout was assessed at 12 and 24 h postexercise. Further samples were obtained 24 and 72 h after a second exercise bout. RE was induced by neuromuscular electrical stimulation to generate maximal isometric contractions in the muscle of interest. A single RE bout resulted in increased levels of mRNA for IGF binding protein-4 (84%), MyoD (83%), myogenin (approximately 3-fold), cyclin D1 (50%), and p21-Waf1 (16-fold), and a transient decrease in IGF-I mRNA (46%). A temporally conserved, significant correlation between myogenin and p21 mRNA was observed (r = 0.70, P < or = 0.02). The mRNAs for mechano-growth factor, IGF binding protein-5, and the IGF-I receptor were unchanged by RE. Total skeletal muscle RNA was increased 72 h after the second serial bout of RE. These results indicate that molecular adaptations of skeletal muscle to loading respond in a very short time. This approach should provide insights on the mechanisms that modulate adaptation to RE and may be useful in evaluating RE training protocol variables with high temporal resolution.