Differential activation of mTOR signaling by contractile activity in skeletal muscle

Tenotomy-induced muscle atrophy is sex-specific and independent of NFκB

The nuclear factor-κB (NFκB) pathway is a major thoroughfare for skeletal muscle atrophy and is driven by diverse stimuli. Targeted inhibition of NFκB through its canonical mediator IKKβ effectively mitigates loss of muscle mass across many conditions, from denervation to unloading to cancer. In this study, we used gain- and loss-of-function mouse models to examine the role of NFκB in muscle atrophy following rotator cuff tenotomy - a model of chronic rotator cuff tear. IKKβ was knocked down or constitutively activated in muscle-specific inducible transgenic mice to elicit a twofold gain or loss of NFκB signaling. Surprisingly, neither knockdown of IKKβ nor overexpression of caIKKβ significantly altered the loss of muscle mass following tenotomy. This finding was consistent across measures of morphological adaptation (fiber cross-sectional area, fiber length, fiber number), tissue pathology (fibrosis and fatty infiltration), and intracellular signaling (ubiquitin-proteasome, autophagy). Intriguingly, late-stage tenotomy-induced atrophy was exacerbated in male mice compared with female mice. This sex specificity was driven by ongoing decreases in fiber cross-sectional area, which paralleled the accumulation of large autophagic vesicles in male, but not female muscle. These findings suggest that tenotomy-induced atrophy is not dependent on NFκB and instead may be regulated by autophagy in a sex-specific manner.

Strength Training in Swimming

This narrative review deals with the topic of strength training in swimming, which has been a controversial issue for decades. It is not only about the importance for the performance at start, turn and swim speed, but also about the question of how to design a strength training program. Different approaches are discussed in the literature, with two aspects in the foreground. On the one hand is the discussion about the optimal intensity in strength training and, on the other hand, is the question of how specific strength training should be designed. In addition to a summary of the current state of research regarding the importance of strength training for swimming, the article shows which physiological adaptations should be achieved in order to be able to increase performance in the long term. Furthermore, an attempt is made to explain why some training contents seem to be rather unsuitable when it comes to increasing strength as a basis for higher performance in the start, turn and clean swimming. Practical training consequences are then derived from this. Regardless of the athlete's performance development, preventive aspects should also be considered in the discussion. The article provides a critical overview of the abovementioned key issues. The most important points when designing a strength training program for swimming are a sufficiently high-load intensity to increase maximum strength, which in turn is the basis for power, year-round strength training, parallel to swim training and working on the transfer of acquired strength skills in swim training, and not through supposedly specific strength training exercises on land or in the water.

Insights into the Interaction of Lysosomal Amino Acid Transporters SLC38A9 and SLC36A1 Involved in mTORC1 Signaling in C2C12 Cells

Amino acids are critical for mammalian target of rapamycin complex 1 (mTORC1) activation on the lysosomal surface. Amino acid transporters SLC38A9 and SLC36A1 are the members of the lysosomal amino acid sensing machinery that activates mTORC1. The current study aims to clarify the interaction of SLC38A9 and SLC36A1. Here, we discovered that leucine increased expressions of SLC38A9 and SLC36A1, leading to mTORC1 activation. SLC38A9 interacted with SLC36A1 and they enhanced each other's expression levels and locations on the lysosomal surface. Additionally, the interacting proteins of SLC38A9 in C2C12 cells were identified to participate in amino acid sensing mechanism, mTORC1 signaling pathway, and protein synthesis, which provided a resource for future investigations of skeletal muscle mass.

The effects of acute aerobic and resistance exercise on mTOR signaling and autophagy markers in untrained human skeletal muscle

PURPOSE

Aerobic (AE) and resistance (RE) exercise elicit unique adaptations in skeletal muscle. The purpose here was to compare the post-exercise response of mTOR signaling and select autophagy markers in skeletal muscle to acute AE and RE.

METHODS

In a randomized, cross-over design, six untrained men (27 ± 3 years) completed acute AE (40 min cycling, 70% HRmax) and RE (8 sets, 10 repetitions, 65% 1RM). Muscle biopsies were taken at baseline, and at 1 h and 4 h following each exercise. Western blot analyses were performed to examine total and phosphorylated protein levels. Upstream regulator analyses of skeletal muscle transcriptomics were performed to discern the predicted activation states of mTOR and FOXO3.

RESULTS

Compared to AE, acute RE resulted in greater phosphorylation (P < 0.05) of mTOR^Ser2448 at 4 h, S6K1^Thr389 at 1 h, and 4E- BP1^Thr37/46 during the post-exercise period. However, both AE and RE increased mTOR^Ser2448 and S6K1^Thr389 phosphorylation at 4 h (P < 0.05). Upstream regulator analyses revealed the activation state of mTOR was increased for both AE (z score, 2.617) and RE (z score, 2.789). No changes in LC3BI protein were observed following AE or RE (P > 0.05), however, LC3BII protein was decreased after both AE and RE at 1 h and 4 h (P < 0.05). p62 protein content was also decreased at 4 h following AE and RE (P < 0.05).

CONCLUSION

Both acute AE and RE stimulate mTOR signaling and similarly impact select markers of autophagy. These findings indicate the early adaptive response of untrained human skeletal muscle to divergent exercise modes is not likely mediated through large differences in mTOR signaling or autophagy.

Running and Swimming Differently Adapt the BDNF/TrkB Pathway to a Slow Molecular Pattern at the NMJ

Physical exercise improves motor control and related cognitive abilities and reinforces neuroprotective mechanisms in the nervous system. As peripheral nerves interact with skeletal muscles at the neuromuscular junction, modifications of this bidirectional communication by physical activity are positive to preserve this synapse as it increases quantal content and resistance to fatigue, acetylcholine receptors expansion, and myocytes' fast-to-slow functional transition. Here, we provide the intermediate step between physical activity and functional and morphological changes by analyzing the molecular adaptations in the skeletal muscle of the full BDNF/TrkB downstream signaling pathway, directly involved in acetylcholine release and synapse maintenance. After 45 days of training at different intensities, the BDNF/TrkB molecular phenotype of trained muscles from male B6SJLF1/J mice undergo a fast-to-slow transition without affecting motor neuron size. We provide further knowledge to understand how exercise induces muscle molecular adaptations towards a slower phenotype, resistant to prolonged trains of stimulation or activity that can be useful as therapeutic tools.

Exercise-Stimulated ROS Sensitive Signaling Pathways in Skeletal Muscle

Physical exercise represents a major challenge to whole-body homeostasis, provoking acute and adaptative responses at the cellular and systemic levels. Different sources of reactive oxygen species (ROS) have been described in skeletal muscle (e.g., NADPH oxidases, xanthine oxidase, and mitochondria) and are closely related to the physiological changes induced by physical exercise through the modulation of several signaling pathways. Many signaling pathways that are regulated by exercise-induced ROS generation, such as adenosine monophosphate-activated protein kinase (AMPK), mitogen activated protein kinase (MAPK), nuclear respiratory factor2 (NRF2), and PGC-1α are involved in skeletal muscle responses to physical exercise, such as increased glucose uptake, mitochondriogenesis, and hypertrophy, among others. Most of these adaptations are blunted by antioxidants, revealing the crucial role played by ROS during and after physical exercise. When ROS generation is either insufficient or exacerbated, ROS-mediated signaling is disrupted, as well as physical exercise adaptations. Thus, an understanding the limit between "ROS that can promote beneficial effects" and "ROS that can promote harmful effects" is a challenging question in exercise biology. The identification of new mediators that cause reductive stress and thereby disrupt exercise-stimulated ROS signaling is a trending on this topic and are covered in this current review.

Cessation of electrically-induced muscle contraction activates autophagy in cultured myotubes

Exercise is known to improve skeletal muscle function. The mechanism involves muscle contraction-induced activation of the mTOR pathway, which plays a central role in protein synthesis. However, mTOR activation blocks autophagy, a recycling mechanism with a critical role in cellular maintenance/homeostasis. These two responses to muscle contraction look contradictory to the functional improvement of exercise. Herein, we investigate these paradoxical muscle responses in a series of active-inactive phases in a cultured myotube model receiving electrical stimulation to induce intermittent muscle contraction. Our model shows that (1) contractile activity induces mTOR activation and muscle hypertrophy but blocks autophagy, resulting in the accumulation of damaged proteins, while (2) cessation of muscle contraction rapidly activates autophagy, removing damaged protein, yet a prolonged inactive state results in muscle atrophy. Our findings provide new insights into muscle biology and suggest that not only muscle contraction, but also the subsequent cessation of contraction plays a substantial role for the improvement of skeletal muscle function.

Moving human muscle physiology research forward: an evaluation of fiber type-specific protein research methodologies

Human skeletal muscle is a heterogeneous tissue composed of multiple fiber types that express unique contractile and metabolic properties. While analysis of mixed fiber samples predominates and holds value, increasing attention has been directed toward studying proteins segregated by fiber type, a methodological distinction termed “fiber type-specific.” Fiber type-specific protein studies have the advantage of uncovering key molecular effects that are often missed in mixed fiber homogenate studies but also require greater time and resource-intensive methods, particularly when applied to human muscle. This review summarizes and compares current methods used for fiber type-specific protein analysis, highlighting their advantages and disadvantages for human muscle studies, in addition to recent advances in these techniques. These methods can be grouped into three categories based on the initial processing of the tissue: 1) muscle-specific fiber homogenates, 2) cross sections of fiber bundles, and 3) isolated single fibers, with various subtechniques for performing fiber type identification and protein quantification. The relative implementation for each unique methodological approach is analyzed from 83 fiber type-specific studies of proteins in live human muscle found in the literature to date. These studies have investigated several proteins involved in a wide range of cellular functions that are important to muscle tissue. The second half of this review summarizes key findings from this ensemble of fiber type-specific human protein studies. We highlight examples of where this analytical approach has helped to improve understanding of important physiological topics such as insulin sensitivity, muscle hypertrophy, muscle fatigue, and adaptation to different exercise programs.

High-intensity muscle contraction-mediated increases in Akt1 and Akt2 phosphorylation do not contribute to mTORC1 activation and muscle protein synthesis

High-intensity muscle contraction (HiMC) is known to induce muscle protein synthesis, a process in which mechanistic target of rapamycin (mTOR) is reported to play a critical role. However, the mechanistic details have not been completely elucidated. Here, we investigated whether Akt plays a role in regulating HiMC-induced mTORC1 activation and muscle protein synthesis using a rodent model of resistance exercise and MK2206 (an Akt kinase inhibitor). The right gastrocnemius muscle of male C57BL/6J mice aged 10 wk was isometrically contracted via percutaneous electrical stimulation (100 Hz, 5 sets of 10 3-s contractions, 7-s rest between contractions, and 3-min rest between sets), while the left gastrocnemius muscle served as a control. Vehicle or MK2206 was injected intraperitoneally 6 h before contraction. MK2206 inhibited both resting and HiMC-induced phosphorylation of Akt1 Ser-473 and Akt2 Ser-474. MK2206 also inhibited the resting phosphorylation of p70S6K and 4E-BP1, which are downstream targets of mTORC1; however, it did not inhibit the HiMC-induced increase in phosphorylation of these targets. Similarly, MK2206 inhibited the resting muscle protein synthesis, but not the resistance exercise-induced muscle protein synthesis. On the basis of these observations, we conclude that although Akt2 regulates resting mTORC1 activity and muscle protein synthesis, HiMC-induced increases in mTORC1 activity and muscle protein synthesis are Akt-independent processes.

NEW & NOTEWORTHY Akt is well known to be an upstream regulator of mechanistic target of rapamycin (mTOR) and has three isoforms in mammals, namely, Akt1, Akt2, and Akt3. We found that high-intensity muscle contraction (HiMC) increases Akt1 and Akt2 phosphorylation; however, HiMC-induced increases in mTORC1 activity and muscle protein synthesis are Akt-independent processes.

Resistance Isn't Futile: The Physiological Basis of the Health Effects of Resistance Exercise in Individuals With Type 1 Diabetes

The importance of regular exercise for glucose management in individuals with type 1 diabetes is magnified by its acknowledgment as a key adjunct to insulin therapy by several governmental, charitable, and healthcare organisations. However, although actively encouraged, exercise participation rates remain low, with glycaemic disturbances and poor cardiorespiratory fitness cited as barriers to long-term involvement. These fears are perhaps exacerbated by uncertainty in how different forms of exercise can considerably alter several acute and chronic physiological outcomes in those with type 1 diabetes. Thus, understanding the bodily responses to specific forms of exercise is important for the provision of practical guidelines that aim to overcome these exercise barriers. Currently, the majority of existing exercise research in type 1 diabetes has focused on moderate intensity continuous protocols with less work exploring predominately non-oxidative exercise modalities like resistance exercise. This is surprising, considering the known neuro-muscular, osteopathic, metabolic, and vascular benefits associated with resistance exercise in the wider population. Considering that individuals with type 1 diabetes have an elevated susceptibility for complications within these physiological systems, the wider health benefits associated with resistance exercise may help alleviate the prevalence and/or magnitude of pathological manifestation in this population group. This review outlines the health benefits of resistance exercise with reference to evidence in aiding some of the common complications associated with individuals with type 1 diabetes.

Licorice flavonoid oil enhances muscle mass in KK-Ay mice

AIMS

Muscle mass is regulated by the balance between the synthesis and degradation of muscle proteins. Loss of skeletal muscle mass is associated with an increased risk of developing metabolic diseases such as obesity and type 2 diabetes mellitus. The aim of this study was to clarify the effects of licorice flavonoid oil on muscle mass in KK-A^y/Ta mice.

MAIN METHODS

Male genetically type II diabetic KK-A^y/Ta mice received 0, 1, or 1.5 g/kg BW of licorice flavonoid oil by mouth once daily for 4 weeks. After 4 weeks, the femoral and soleus muscles were collected for western blotting for evaluation of the mTOR/p70 S6K, p38/FoxO3a, and Akt/FoxO3a signaling pathways.

KEY FINDINGS

Ingestion of licorice flavonoid oil significantly enhanced femoral muscle mass without affecting body weight in KK-A^y/Ta mice. Licorice flavonoid oil also decreased expression of MuRF1 and atrogin-1, which are both markers of muscle atrophy. The mechanisms by which licorice flavonoid oil enhances muscle mass include activation of mTOR and p70 S6K, and regulation of phosphorylation of FoxO3a.

SIGNIFICANCE

Ingestion of licorice flavonoids may help to prevent muscle atrophy.

Prior acetaminophen consumption impacts the early adaptive cellular response of human skeletal muscle to resistance exercise

Resistance exercise (RE) is a powerful stimulus for skeletal muscle adaptation. Previous data demonstrate that cyclooxygenase (COX)-inhibiting drugs alter the cellular mechanisms regulating the adaptive response of skeletal muscle. The purpose of this study was to determine whether prior consumption of the COX inhibitor acetaminophen (APAP) alters the immediate adaptive cellular response in human skeletal muscle after RE. In a double-blinded, randomized, crossover design, healthy young men ( n = 8, 25 ± 1 yr) performed two trials of unilateral knee extension RE (8 sets, 10 reps, 65% max strength). Subjects ingested either APAP (1,000 mg/6 h) or placebo (PLA) for 24 h before RE (final dose consumed immediately after RE). Muscle biopsies (vastus lateralis) were collected at rest and 1 h and 3 h after exercise. Mammalian target of rapamycin (mTOR) complex 1 signaling was assessed through immunoblot and immunohistochemistry, and mRNA expression of myogenic genes was examined via RT-qPCR. At 1 h p-rpS6Ser240/244 was increased in both groups but to a greater extent in PLA. At 3 h p-S6K1Thr389 was elevated only in PLA. Furthermore, localization of mTOR to the lysosome (LAMP2) in myosin heavy chain (MHC) II fibers increased 3 h after exercise only in PLA. mTOR-LAMP2 colocalization in MHC I fibers was greater in PLA vs. APAP 1 h after exercise. Myostatin mRNA expression was reduced 1 h after exercise only in PLA. MYF6 mRNA expression was increased 1 h and 3 h after exercise only in APAP. APAP consumption appears to alter the early adaptive cellular response of skeletal muscle to RE. These findings further highlight the mechanisms through which COX-inhibiting drugs impact the adaptive response of skeletal muscle to exercise.

NEW & NOTEWORTHY The extent to which the cellular reaction to acetaminophen impacts the mechanisms regulating the adaptive response of human skeletal muscle to resistance exercise is not well understood. Consumption of acetaminophen before resistance exercise appears to suppress the early response of mTORC1 activity to acute resistance exercise. These data also demonstrate, for the first time, that resistance exercise elicits fiber type-specific changes in the intracellular colocalization of mTOR with the lysosome in human skeletal muscle.

Effect of Resistance Exercise on Muscle Metabolism and Autophagy in sIBM

PURPOSE

Sporadic inclusion body myositis (sIBM), a muscular degenerative disease in the elderly, is an inflammatory myopathy characterized by muscle weakness in the wrist flexor, quadriceps, and tibialis anterior muscles. We aimed to identify the therapeutic effect of resistance exercise (RE) in improving sIBM symptoms in an sIBM animal model.

METHODS

Six-week-old male Wistar rats were divided into a sham group (sham, n = 12), chloroquine-control group (CQ-con, n = 12), and chloroquine-RE group (CQ-RE, n = 12). The rats were subjected to 1 wk of exercise adaptation and 8 wk of exercise (three sessions per week). Protein expression was measured by Western blotting. Rimmed vacuoles (RV) were identified by hematoxylin and eosin staining and modified Gömöri trichrome staining, and amyloid deposition was examined by Congo red staining.

RESULTS

The effects of CQ and RE differed depending on myofiber characteristics. Soleus muscles showed abnormal autophagy in response to CQ, which increased RV generation and amyloid-β accumulation, resulting in atrophy. RE generated RV and decreased amyloid deposition in soleus muscles and also improved autophagy without generating hypertrophy. This reduced the atrophy signal transduction, resulting in decreased atrophy compared with the CQ-con group. Despite no direct effect of CQ, flexor hallucis longus muscles showed loss of mass because of reduced activity or increased inflammatory response; however, RE increased the hypertrophy signal, resulting in reduced autophagy and atrophy.

CONCLUSIONS

These results demonstrate that RE had a preventive effect on sIBM induced by CQ treatment in an animal model. However, because the results were from an animal experiment, a more detailed study should be conducted over a longer period, and the effectiveness of different RE programs should also be investigated.

Lean and Obese Zucker Rat Extensor Digitorum Longus Muscle high-frequency electrical stimulation (HFES) Data: Regulation of p70S6kinase Associated Proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the AKT, GSK3beta, mTor, p70s6K, Pten, and Shp2 in the lean and obese (fa/fa) Zucker rat Extensor Digitorum Longus (EDL) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009a, 2009b; Tullgren et al., 1991) [1–3] and concurrent Data in Brief articles (Ginjupalli et al., 2017a, 2017b; Rice et al., 2017a, 2017b) [4–7].

Lean and Obese Zucker Rat Extensor Digitorum Longus Muscle high-frequency electrical stimulation (HFES) Data: Regulation of MAPKs Associated Proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the mitogen-activated protein kinases (MAPKs) ERK 1/2, p38, and JNK in the lean and obese (fa/fa) Zucker rat extensor digitorum longus (EDL) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our (Katta et al., 2009a, 2009b, 2008) [1-3] and concurrent Data in Brief articles (Ginjupalli et al., 2017a, 2017b; Rice et al., 2017a, 2017b) [4-7].

Diabetic Zucker rat Tibialis anterior muscle high-frequency electrical stimulation (HFES) data: Regulation of MAPKs associated proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the mitogen-activated protein kinases (MAPKs) ERK 1/2, p38, and JNK in the lean and obese (fa/fa) Zucker rat tibialus anterior (TA) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009, Katta et al., 2009, Tullgren et al., 1991) [1–3] and concurrent Data in Brief articles (Ginjupalli et al., 2017, Rice et al., 2017, Rice et al., 2017, Rice et al., 2017) [4–7].

High-frequency electrical stimulation (HFES) data lean and obese Zucker rat tibialis anterior muscle: Regulation of glycogen synthase kinase 3 beta (GSK3B) associated proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the AMPK, GSK3beta, and Shp2 in the lean and obese (fa/fa) Zucker rat tibialis anterior (TA) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009; Katta et al., 2009; Tullgren et al., 1991) [1–3] and concurrent Data in Brief articles (Ginjupalli et al., 2017; Rice et al., 2017; Rice et al., 2017; Rice et al., 2017) [4–7].

High-frequency electrical stimulation (HFES) Data Lean and Obese Zucker Rat Soleus Muscle: Regulation of p70S6kinase Associated Proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the AKT, GSK3beta, mTor, p70s6K, Pten, and Shp2 proteins in the lean and obese (fa/fa) Zucker rat soleus muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009a, 2009b; Tullgren et al., 1991) [1-3] and concurrent Data in Brief articles (Ginjupalli et al., 2017a, 2017b; Rice et al., 2017a, 2017b) [4-7].

Considerations on mTOR regulation at serine 2448: implications for muscle metabolism studies

The mammalian target of rapamycin (mTOR) complex exerts a pivotal role in protein anabolism and cell growth. Despite its importance, few studies adequately address the complexity of phosphorylation of the mTOR protein itself to enable conclusions to be drawn on the extent of kinase activation following this event. In particular, a large number of studies in the skeletal muscle biology field have measured Serine 2448 (Ser2448) phosphorylation as a proxy of mTOR kinase activity. However, the evidence to be described is that Ser2448 is not a measure of mTOR kinase activity nor is a target of AKT activity and instead has inhibitory effects on the kinase that is targeted by the downstream effector p70S6K in a negative feedback loop mechanism, which is evident when revisiting muscle research studies. It is proposed that this residue modification acts as a fine-tuning mechanism that has been gained during vertebrate evolution. In conclusion, it is recommended that Ser2448 is an inadequate measure and that preferential analysis of mTORC1 activation should focus on the downstream and effector proteins, including p70S6K and 4E-BP1, along mTOR protein partners that bind to mTOR protein to form the active complexes 1 and 2.

Human skeletal muscle fibre contractile properties and proteomic profile: adaptations to 3 weeks of unilateral lower limb suspension and active recovery

KEY POINTS

It is generally assumed that muscle fibres go through atrophy following disuse with a loss of specific force and an increase in unloaded shortening velocity. However, the underlying mechanisms remain to be clarified. Most studies have focused on events taking place during the development of disuse, whereas the subsequent recovery phase, which is equally important, has received little attention. Our findings support the hypotheses that the specific force of muscle fibres decreased following unilateral lower limb suspension (ULLS) and returned to normal after 3 weeks of active recovery as a result of a loss and recovery of myosin and actin content. Furthermore, muscle fibres went through extensive qualitative changes in muscle protein pattern following ULLS, and these were reversed by active recovery. Resistance training was very effective in restoring both muscle mass and qualitative muscle changes, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

ABSTRACT

Following disuse, muscle fibre function goes through adaptations such as a loss of specific force (PO /CSA) and an increase in unloaded shortening velocity, which could be a result of both quantitative changes (i.e. atrophy) and qualitative changes in protein pattern. The underlying mechanisms remain to be clarified. In addition, little is known about the recovery of muscle mass and strength following disuse. In the present study, we report an extensive dataset describing, in detail,the functional and protein content adaptations of skeletal muscle in response to both disuse and re-training. Eight young healthy subjects were subjected to 3 weeks of unilateral lower limb suspension (ULLS), a widely used human model of disuse skeletal muscle atrophy. Needle biopsies samples were taken from the vastus lateralis muscle Pre-ULLS, Post-ULLS and after 3 weeks of recovery during which heavy resistance training was performed. After disuse, cross-sectional area (CSA), PO /CSA and myosin concentration (MC) decreased in both type 1 and 2A skinned muscle fibres. After recovery, CSA and MC returned to levels comparable to those observed before disuse, whereas Po/CSA and unloaded shortening velocity reached a higher level. Myosin heavy chain isoform composition of muscle samples did not differ among the experimental groups. To study the mechanisms underlying such adaptations, a two-dimensional proteomic analysis was performed. ULLS induced a reduction of myofibrillar, metabolic (glycolytic and oxidative) and anti-oxidant defence system protein content. Resistance training was very effective in counteracting ULLS-induced alterations, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

Muscle fiber type diversification during exercise and regeneration

The plasticity of skeletal muscle can be traced down to extensive metabolic, structural and molecular remodeling at the single fiber level. Skeletal muscle is comprised of different fiber types that are the basis of muscle plasticity in response to various functional demands. Resistance and endurance exercises are two external stimuli that differ in their duration and intensity of contraction and elicit markedly different responses in muscles adaptation. Further, eccentric contractions that are associated with exercise-induced injuries, elicit varied muscle adaptation and regenerative responses. Most adaptive changes are fiber type-specific and are highly influenced by diverse structural, metabolic and functional characteristics of individual fiber types. Regulation of signaling pathways by reactive oxygen species (ROS) and oxidative stress also plays an important role in muscle fiber adaptation during exercise. This review focuses on cellular and molecular responses that regulate the adaptation of skeletal muscle to exercise and exercise-related injuries.

Recovery of strength is dependent on mTORC1 signaling after eccentric muscle injury

INTRODUCTION

Eccentric contractions may cause immediate and long-term reductions in muscle strength that can be recovered through increased protein synthesis rates. The purpose of this study was to determine whether the mechanistic target-of-rapamycin complex 1 (mTORC1), a vital controller of protein synthesis rates, is required for return of muscle strength after injury.

METHODS

Isometric muscle strength was assessed before, immediately after, and then 3, 7, and 14 days after a single bout of 150 eccentric contractions in mice that received daily injections of saline or rapamycin.

RESULTS

The bout of eccentric contractions increased the phosphorylation of mTORC1 (1.8-fold) and p70s6k1 (13.8-fold), mTORC1's downstream effector, 3 days post-injury. Rapamycin blocked mTORC1 and p70s6k1 phosphorylation and attenuated recovery of muscle strength (∼20%) at 7 and 14 days.

CONCLUSION

mTORC1 signaling is instrumental in the return of muscle strength after a single bout of eccentric contractions in mice. Muscle Nerve 54: 914-924, 2016.

The Nuclear Receptor, Nor-1, Induces the Physiological Responses Associated With Exercise

Skeletal muscle remodels metabolic capacity, contractile and exercise phenotype in response to physiological demands. This adaptive remodeling response to physical activity can ameliorate/prevent diseases associated with poor diet and lifestyle. Our previous work demonstrated that skeletal muscle-specific transgenic expression of the neuron-derived orphan nuclear receptor, Nor-1 drives muscle reprogramming, improves exercise endurance, and oxidative metabolism. The current manuscript investigates the association between exercise, Nor-1 expression and the role of Nor-1 in adaptive remodeling. We demonstrate that Nor-1 expression is induced by exercise and is dependent on calcium/calcineurin signaling (in vitro and in vivo). Analysis of fatigue-resistant transgenic mice that express Nor-1 in skeletal muscle revealed increased hypertrophy and vascularization of muscle tissue. Moreover, we demonstrate that transgenic Nor-1 expression is associated with increased intracellular recycling, ie, autophagy, involving 1) increased expression of light chain 3A or LC3A-II, autophagy protein 5, and autophagy protein 12 in quadriceps femoris muscle extracts from Tg-Nor-1 (relative to Wild-type (WT) littermates); 2) decreased p62 expression indicative of increased autophagolysosome assembly; and 3) decreased mammalian target of rapamycin complex 1 activity. Transfection of LC3A-GFP-RFP chimeric plasmid demonstrated that autophagolysosome formation was significantly increased by Nor-1 expression. Furthermore, we demonstrated a single bout of exercise induced LC3A-II expression in skeletal muscle from C57BL/6 WT mice. This study, when combined with our previous studies, demonstrates that Nor-1 expression drives multiple physiological changes/pathways that are critical to the beneficial responses of muscle to exercise and provides insights into potential pharmacological manipulation of muscle reprogramming for the treatment of lifestyle induced chronic diseases.

Carbohydrate supplementation attenuates decrement in performance in overtrained rats

Carbohydrate ingestion at the end of a single exercise is recognized as delaying fatigue and accelerating recovery, but whether chronic ingestion can prevent overtraining during periods of intense training has not yet been elucidated. This study aimed to determine whether carbohydrate supplementation minimizes overtraining in Wistar rats. The animals underwent 11 weeks of training (running) on a treadmill, and the last 3 weeks were designed to induce overtraining. One group was supplemented with carbohydrates (EX-CHO) (n = 13), 1 group had no supplementation (EX) (n = 10), and a third group remained inactive (C) (n = 9). Performance tests were given before training (Pr1) and at the 8th (Pr2) and 11th (Pr3) training week. Food intake, body weight, testosterone, cortisol, malondialdehyde, creatine kinase, and activities of the PI3-K, Akt-1, mTOR, and GSK-3 enzymes were measured. In the EX group, there was a significant 32.6% performance decrease at Pr3 when compared with Pr2. In addition, at protocol completion, the EX-CHO group had a greater gastrocnemius weight than did the C group (p = 0.02), which the EX group did not. Training caused anorexia, decreased testosterone (p = 0.001), and increased malondialdehyde (p = 0.009) in both exercise groups compared with the C group, with no influence of carbohydrate supplementation on these variables (p > 0.05). Compared with in the C group, the activity of Akt-1 was higher in the EX-CHO group but not in the EX group (p = 0.013). Carbohydrate supplementation promoted an attenuation in the performance decrement and maintained gastrocnemius muscle mass in animals that had undergone overtraining protocols, which was accompanied by increased activity of the Akt-1 molecular indicator.

Metabolic and endocrine response to exercise: sympathoadrenal integration with skeletal muscle

Skeletal muscle has the capacity to increase energy turnover by ∼1000 times its resting rate when contracting at the maximum force/power output. Since ATP is not stored in any appreciable quantity, the muscle requires a coordinated metabolic response to maintain an adequate supply of ATP to sustain contractile activity. The integration of intracellular metabolic pathways is dependent upon the cross-bridge cycling rate of myosin and actin, substrate availability and the accumulation of metabolic byproducts, all of which can influence the maintenance of contractile activity or result in the onset of fatigue. In addition, the mobilisation of extracellular substrates is dependent upon the integration of both the autonomic nervous system and endocrine systems to coordinate an increase in both carbohydrate and fat availability. The current review examines the evidence for skeletal muscle to generate power over short and long durations and discusses the metabolic response to sustain these processes. The review also considers the endocrine response from the perspective of the sympathoadrenal system to integrate extracellular substrate availability with the increased energy demands made by contracting skeletal muscle. Finally, the review briefly discusses the evidence that muscle acts in an endocrine manner during exercise and what role this might play in mobilising extracellular substrates to augment the effects of the sympathoadrenal system.

Association between myosin heavy chain protein isoforms and intramuscular anabolic signaling following resistance exercise in trained men

Resistance exercise stimulates an increase in muscle protein synthesis regulated by intracellular anabolic signaling molecules in a mammalian/mechanistic target of rapamycin (mTOR)‐dependent pathway. The purpose of this study was to investigate acute anabolic signaling responses in experienced, resistance‐trained men, and to examine the association between myosin heavy chain (MHC) isoform composition and the magnitude of anabolic signaling. Eight resistance‐trained men (24.9 ± 4.3 years; 91.2 ± 12.4 kg; 176.7 ± 8.0 cm; 13.3 ± 3.9 body fat %) performed a whole body, high‐volume resistance exercise protocol (REX) and a control protocol (CTL) in a balanced, randomized order. Participants were provided a standardized breakfast, recovery drink, and meal during each protocol. Fine needle muscle biopsies were completed at baseline (BL), 2 h (2H) and 6 h post‐exercise (6H). BL biopsies were analyzed for MHC isoform composition. Phosphorylation of proteins specific to the Akt/mTOR signaling pathway and MHC mRNA expression was quantified. Phosphorylation of p70S6k was significantly greater in REX compared to CTL at 2H (P = 0.04). MHC mRNA expression and other targets in the Akt/mTOR pathway were not significantly influenced by REX. The percentage of type IIX isoform was inversely correlated (P < 0.05) with type I and type IIA MHC mRNA expression (r = −0.69 to −0.93). Maximal strength was also observed to be inversely correlated (P < 0.05) with Type I and Type IIA MHC mRNA expression (r = −0.75 to −0.77) and p70S6k phosphorylation (r = −0.75). Results indicate that activation of p70S6k occurs within 2‐h following REX in experienced, resistance‐trained men. Further, results also suggest that highly trained, stronger individuals have an attenuated acute anabolic response.

Resistance exercise stimulates an increase in muscle protein synthesis regulated by intracellular anabolic signaling molecules in a mammalian/mechanistic target of rapamycin (mTOR)‐dependent pathway. Results indicate that activation of p70S6k occurs within 2‐h following REX in experienced, resistance‐trained men. Further, results also suggest that highly trained, stronger individuals have an attenuated acute anabolic response.

Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables

Concurrent training is defined as simultaneously incorporating both resistance and endurance exercise within a periodized training regime. Despite the potential additive benefits of combining these divergent exercise modes with regards to disease prevention and athletic performance, current evidence suggests that this approach may attenuate gains in muscle mass, strength, and power compared with undertaking resistance training alone. This has been variously described as the interference effect or concurrent training effect. In recent years, understanding of the molecular mechanisms mediating training adaptation in skeletal muscle has emerged and provided potential mechanistic insight into the concurrent training effect. Although it appears that various molecular signaling responses induced in skeletal muscle by endurance exercise can inhibit pathways regulating protein synthesis and stimulate protein breakdown, human studies to date have not observed such molecular 'interference' following acute concurrent exercise that might explain compromised muscle hypertrophy following concurrent training. However, given the multitude of potential concurrent training variables and the limitations of existing evidence, the potential roles of individual training variables in acute and chronic interference are not fully elucidated. The present review explores current evidence for the molecular basis of the specificity of training adaptation and the concurrent interference phenomenon. Additionally, insights provided by molecular and performance-based concurrent training studies regarding the role of individual training variables (i.e., within-session exercise order, between-mode recovery, endurance training volume, intensity, and modality) in the concurrent interference effect are discussed, along with the limitations of our current understanding of this complex paradigm.

Effect of gluteus medius muscle sample collection depth on postprandial mammalian target of rapamycin signaling in mature Thoroughbred mares

OBJECTIVE

To determine the effect of biopsy collection depth on the postprandial activation of mammalian target of rapamycin (mTOR) signaling factors, particularly protein kinase B, ribosomal protein S6 kinase, ribosomal protein S6, and eukaryotic initiation factor 4E binding protein 1 in middle-aged horses.

ANIMALS

6 healthy Thoroughbred mares (mean ± SD age, 13.4 ± 3.4 years).

PROCEDURES

Horses were fed a high-protein feed at 3 g/kg. Sixty minutes after horses were fed, the percutaneous needle biopsy technique was used to collect biopsy specimens from the gluteus medius muscle at 6, 8, and 10 cm below the surface of the skin. Muscle specimens were analyzed for the activation of upstream and downstream mTOR signaling factors, myosin heavy chain (MHC) isoform composition, and amino acid concentrations.

RESULTS

A 21% increase in MHC IIA isoform expression and a 21% decrease in MHC IIX isoform expression were identified as biopsy depth increased from 8 to 10 cm below the surface of the skin; however, no significant change was evident in the degree of MHC I expression with muscle depth. Biopsy depth had no significant effect on the phosphorylation of any of the mTOR signaling factors evaluated.

CONCLUSIONS AND CLINICAL RELEVANCE

Postprandial mTOR signaling could be compared between middle-aged horses when biopsy specimens were collected between 6 and 10 cm below the surface of the skin. Optimization of muscle biopsy techniques for evaluating mTOR signaling in horses will facilitate the design of future investigations into the factors that regulate muscle mass in horses.

The role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

Key signalling factors and pathways in the molecular determination of skeletal muscle phenotype

The molecular basis and control of the biochemical and biophysical properties of skeletal muscle, regarded as muscle phenotype, are examined in terms of fibre number, fibre size and fibre types. A host of external factors or stimuli, such as ligand binding and contractile activity, are transduced in muscle into signalling pathways that lead to protein modifications and changes in gene expression which ultimately result in the establishment of the specified phenotype. In skeletal muscle, the key signalling cascades include the Ras-extracellular signal regulated kinase-mitogen activated protein kinase (Erk-MAPK), the phosphatidylinositol 3'-kinase (PI3K)-Akt1, p38 MAPK, and calcineurin pathways. The molecular effects of external factors on these pathways revealed complex interactions and functional overlap. A major challenge in the manipulation of muscle of farm animals lies in the identification of regulatory and target genes that could effect defined and desirable changes in muscle quality and quantity. To this end, recent advances in functional genomics that involve the use of micro-array technology and proteomics are increasingly breaking new ground in furthering our understanding of the molecular determinants of muscle phenotype.

Contractile activity-induced mitochondrial biogenesis and mTORC1

In response to exercise training, or chronic contractile activity, mitochondrial content is known to be enriched within skeletal muscle. However, the molecular mechanisms that mediate this adaptation are incompletely defined. Recently, the protein complex, mammalian target of rapamycin complex 1 (mTORC1), has been identified to facilitate the expression of nuclear genes encoding mitochondrial proteins (NUGEMPs) in resting muscle cells via the interaction of the mTORC1 components, mTOR and raptor, the transcription factor Yin Yang 1, and peroxisome proliferator-activated receptor-γ coactivator-1α. It is currently unknown if this mechanism is operative during the increase in mitochondrial content that occurs within skeletal muscle with chronic contractile activity (CCA). Thus we employed a cell culture model of murine skeletal muscle and subjected the myotubes to CCA for 3 h per day for 4 consecutive days in the presence or absence of the mTORC1 inhibitor rapamycin. CCA produced increases in the mitochondrial markers cytochrome oxidase (COX) IV (2.5-fold), Tfam (1.5-fold), and COX activity (1.6-fold). Rapamycin-mediated inhibition of mTORC1 did not suppress these CCA-induced increases in mitochondrial proteins and organelle content. mTORC1 inhibition alone produced a selective upregulation of mitochondrial proteins (COX IV, Tfam), but diminished organelle state 3 respiration. CCA restored this impairment to normal. Our results suggest that mTORC1 activity is not integral for the increase in mitochondrial content elicited by CCA, but is required to maintain mitochondrial function and homeostasis in resting muscle.

Deficiency of inducible nitric oxide synthase attenuates immobilization-induced skeletal muscle atrophy in mice

The present study examined the effects of inducible nitric oxide synthase (iNOS) deficiency on skeletal muscle atrophy in single leg-immobilized iNOS knockout (KO) and wild-type (WT) mice. The left leg was immobilized for 1 wk, and the right leg was used as the control. Muscle weight and contraction-stimulated glucose uptake were reduced by immobilization in WT mice, which was accompanied with increased iNOS expression in skeletal muscle. Deficiency of iNOS attenuated muscle weight loss and the reduction in contraction-stimulated glucose uptake by immobilization. Phosphorylation of Akt, mTOR, and p70S6K was reduced to a similar extent by immobilization in both WT and iNOS KO mice. Immobilization decreased FoxO1 phosphorylation and increased mRNA and protein levels of MuRF1 and atrogin-1 in WT mice, which were attenuated in iNOS KO mice. Aconitase and superoxide dismutase activities were reduced by immobilization in WT mice, and deficiency of iNOS normalized these enzyme activities. Increased nitrotyrosine and carbonylated protein levels by immobilization in WT mice were reversed in iNOS KO mice. Phosphorylation of ERK and p38 was increased by immobilization in WT mice, which was reduced in iNOS KO mice. Immobilization-induced muscle atrophy was also attenuated by an iNOS-specific inhibitor N6-(1-iminoethyl)-l-lysine, and this finding was accompanied by increased FoxO1 phosphorylation and reduced MuRF1 and atrogin-1 levels. These results suggest that deficiency of iNOS attenuates immobilization-induced skeletal muscle atrophy through reduced oxidative stress, and iNOS-induced oxidative stress may be required for immobilization-induced skeletal muscle atrophy.

mTORC1 and the regulation of skeletal muscle anabolism and mass

The mass and integrity of skeletal muscle is vital to whole-body substrate metabolism and health. Indeed, defects in muscle metabolism and functions underlie or exacerbate diseases like diabetes, rheumatoid arthritis, and cancer. Physical activity and nutrition are the 2 most important environmental factors that can affect muscle health. At the molecular level, the mammalian target of rapamycin complex 1 (mTORC1) is a critical signalling complex that regulates muscle mass. In response to nutrition and resistance exercise, increased muscle mass and activation of mTORC1 occur in parallel. In this review, we summarize recent findings on mTORC1 and its regulation in skeletal muscle in response to resistance exercise, alone or in combination with intake of protein or amino acids. Because increased activity of the complex is implicated in the development of muscle insulin resistance, obesity, and some cancers (e.g., ovarian, breast), drugs that target mTORC1 are being developed or are in clinical trials. However, various cancers are associated with extensive muscle wasting, due in part to tumour burden and malnutrition. This muscle wasting may also be a side effect of anticancer drugs. Because loss of muscle mass is associated not only with metabolic abnormalities but also dose limiting toxicity, we review the possible implications for skeletal muscle of long-term inhibition of mTORC1, especially in muscle wasting conditions.

Sex-based comparisons of myofibrillar protein synthesis after resistance exercise in the fed state

We made sex-based comparisons of rates of myofibrillar protein synthesis (MPS) and anabolic signaling after a single bout of high-intensity resistance exercise. Eight men (20 ± 10 yr, BMI = 24.3 ± 2.4) and eight women (22 ± 1.8 yr, BMI = 23.0 ± 1.9) underwent primed constant infusions of l-[ ring-13C6]phenylalanine on consecutive days with serial muscle biopsies. Biopsies were taken from the vastus lateralis at rest and 1, 3, 5, 24, 26, and 28 h after exercise. Twenty-five grams of whey protein was ingested immediately and 26 h after exercise. We also measured exercise-induced serum testosterone because it is purported to contribute to increases in myofibrillar protein synthesis (MPS) postexercise and its absence has been hypothesized to attenuate adaptative responses to resistance exercise in women. The exercise-induced area under the testosterone curve was 45-fold greater in men than women in the early (1 h) recovery period following exercise ( P < 0.001). MPS was elevated similarly in men and women (2.3- and 2.7-fold, respectively) 1–5 h postexercise and after protein ingestion following 24 h recovery. Phosphorylation of mTORSer2448 was elevated to a greater extent in men than women acutely after exercise ( P = 0.003), whereas increased phosphorylation of p70S6K1Thr389 was not different between sexes. Androgen receptor content was greater in men (main effect for sex, P = 0.049). Atrogin-1 mRNA abundance was decreased after 5 h recovery in both men and women ( P < 0.001), and MuRF-1 expression was elevated in men after protein ingestion following 24 h recovery ( P = 0.003). These results demonstrate minor sex-based differences in signaling responses and no difference in the MPS response to resistance exercise in the fed state. Interestingly, our data demonstrate that exercise-induced increases in MPS are dissociated from postexercise testosteronemia and that stimulation of MPS occurs effectively with low systemic testosterone concentrations in women.

LIF is a contraction-induced myokine stimulating human myocyte proliferation

The cytokine leukemia inhibitory factor (LIF) is expressed by skeletal muscle and induces proliferation of myoblasts. We hypothesized that LIF is a contraction-induced myokine functioning in an autocrine fashion to activate gene regulation of human muscle satellite cell proliferation. Skeletal muscle LIF expression, regulation, and action were examined in two models: 1) young men performing a bout of heavy resistance exercise of the quadriceps muscle and 2) cultured primary human satellite cells. Resistance exercise induced a ninefold increase in LIF mRNA content in skeletal muscle, but LIF was not detectable in plasma of the subjects. However, electrically stimulated cultured human myotubes produced and secreted LIF, suggesting that LIF is a myokine with local effects. The well established exercise-induced signaling molecules PI3K, Akt, and mTor contributed to the regulation of LIF in cultured human myotubes as chemical inhibition of PI3K and mTor and siRNA knockdown of Akt1 were independently sufficient to downregulate LIF. Human myoblast proliferation was increased by recombinant exogenous LIF and decreased by siRNA knockdown of the endogenous LIF receptor. Finally, the transcription factors JunB and c-Myc, which promote myoblast proliferation, were induced by LIF in cultured human myotubes. Indeed, both JunB and c-Myc were also increased in skeletal muscle following resistance exercise. Our data suggest that LIF is a contraction-induced myokine, potentially acting in an autocrine or paracrine fashion to promote satellite cell proliferation.

Eukaryotic initiation factor 2B epsilon induces cap-dependent translation and skeletal muscle hypertrophy

The purpose of this study was to identify signalling components known to control mRNA translation initiation in skeletal muscle that are responsive to mechanical load and may be partly responsible for myofibre hypertrophy. To accomplish this, we first utilized a human cluster model in which skeletalmuscle samples fromsubjects with widely divergent hypertrophic responses to resistance training were used for the identification of signalling proteins associated with the degree myofibre hypertrophy. We found that of 11 translational signalling molecules examined, the response of p(T421/S424)-p70S6K phosphorylation and total eukaryotic initiation factor 2Bε (eIF2Bε) protein abundance after a single bout of unaccustomed resistance exercise was associated with myofibre hypertrophy following 16 weeks of training. Follow up studies revealed that overexpression of eIF2Bε alone was sufficient to induce an 87% increase in cap-dependent translation in L6 myoblasts in vitro and 21% hypertrophy of myofibres in mouse skeletal muscle in vivo (P<0.05).However, genetically altering p70S6K activity had no impact on eIF2Bε protein abundance in mouse skeletal muscle in vivo or multiple cell lines in vitro (P >0.05), suggesting that the two phenomena were not directly related. These are the first data that mechanistically link eIF2Bε abundance to skeletal myofibre hypertrophy, and indicate that eIF2Bε abundance may at least partially underlie the widely divergent hypertrophic phenotypes in human skeletal muscle exposed to mechanical stimuli.

Heat stress enhances mTOR signaling after resistance exercise in human skeletal muscle

This study investigated the effect of heat stress (HS) on mammalian target of rapamycin (mTOR) signaling involved in translation initiation after resistance exercise in human skeletal muscle. Eight young male subjects performed four sets of six maximal repetitions of knee extension exercises, with or without HS, in a randomized crossover design. HS was applied to the belly of the vastus lateralis by using a microwave therapy unit prior to and during exercise. Muscle biopsies were taken from the vastus lateralis before, immediately after, and 1 h after exercise. HS significantly increased the phosphorylation of Akt/PKB, mTOR, and ribosomal protein S6 at 1 h after exercise (P < 0.05), and the 4E-BP1 phosphorylation level, which had initially decreased with exercise, had recovered by 1 h after exercise with HS. In addition, the phosphorylation of ribosomal S6 kinase 1 was significantly increased immediately after exercise with HS (P < 0.05). These results indicate that HS enhances mTOR signaling after resistance exercise in human skeletal muscle.

Post-exercise protein synthesis rates are only marginally higher in type I compared with type II muscle fibres following resistance-type exercise

We examined the effect of an acute bout of resistance exercise on fractional muscle protein synthesis rates in human type I and type II muscle fibres. After a standardised breakfast (31 ± 1 kJ kg⁻¹ body weight, consisting of 52 Energy% (En%) carbohydrate, 34 En% protein and 14 En% fat), 9 untrained men completed a lower-limb resistance exercise bout (8 sets of 10 repetitions leg press and leg extension at 70% 1RM). A primed, continuous infusion of l-[ring-¹³C₆]phenylalanine was combined with muscle biopsies collected from both legs immediately after exercise and after 6 h of post-exercise recovery. Single muscle fibres were dissected from freeze-dried biopsies and stained for ATPase activity with pre-incubation at a pH of 4.3. Type I and II fibres were separated under a light microscope and analysed for protein-bound l-[ring-¹³C₆]phenylalanine labelling. Baseline (post-exercise) l-[ring-¹³C₆]phenylalanine muscle tissue labelling, expressed as (∂¹³C/¹²C), averaged −32.09 ± 0.28, −32.53 ± 0.10 and −32.02 ± 0.16 in the type I and II muscle fibres and mixed muscle, respectively (P = 0.14). During post-exercise recovery, muscle protein synthesis rates were marginally (8 ± 2%) higher in the type I than type II muscle fibres, at 0.100 ± 0.005 versus 0.094 ± 0.005%/h, respectively (P < 0.05), whereby rates of mixed muscle protein were 0.091 ± 0.005%/h. Muscle protein synthesis rates following resistance-type exercise are only marginally higher in type I compared with type II muscle fibres.

Electrostimulation during hindlimb unloading modulates PI3K-AKT downstream targets without preventing soleus atrophy and restores slow phenotype through ERK

Our aim was to analyze the role of phosphatidylinositol 3-kinase (PI3K)-AKT and MAPK signaling pathways in the regulation of muscle mass and slow-to-fast phenotype transition during hindlimb unloading (HU). For that purpose, we studied, in rat slow soleus and fast extensor digitorum longus muscles, the time course of anabolic PI3K-AKT-mammalian target of rapamycin, catabolic PI3K-AKT-forkhead box O (FOXO), and MAPK signaling pathway activation after 7, 14, and 28 days of HU. Moreover, we performed chronic low-frequency soleus electrostimulation during HU to maintain exclusively contractile phenotype and so to determine more precisely the role of these signaling pathways in the modulation of muscle mass. HU induced a downregulation of the anabolic AKT, mammalian target of rapamycin, 70-kDa ribosomal protein S6 kinase, 4E-binding protein 1, and glycogen synthase kinase-3β targets, and an upregulation of the catabolic FOXO1 and muscle-specific RING finger protein-1 targets correlated with soleus muscle atrophy. Unexpectedly, soleus electrostimulation maintained 70-kDa ribosomal protein S6 kinase, 4E-binding protein 1, FOXO1, and muscle-specific RING finger protein-1 to control levels, but failed to reduce muscle atrophy. HU decreased ERK phosphorylation, while electrostimulation enabled the maintenance of ERK phosphorylation similar to control level. Moreover, slow-to-fast myosin heavy chain phenotype transition and upregulated glycolytic metabolism were prevented by soleus electrostimulation during HU. Taken together, our data demonstrated that the processes responsible for gradual disuse muscle plasticity in HU conditions involved both PI3-AKT and MAPK pathways. Moreover, electrostimulation during HU restored PI3K-AKT activation without counteracting soleus atrophy, suggesting the involvement of other signaling pathways. Finally, electrostimulation maintained initial contractile and metabolism properties in parallel to ERK activation, reinforcing the idea of a predominant role of ERK in the regulation of muscle slow phenotype.

Influence of supplementation with branched-chain amino acids in combination with resistance exercise on p70S6 kinase phosphorylation in resting and exercising human skeletal muscle

AIM

Skeletal muscle growth is thought to be regulated by the mammalian target of rapamycin (mTOR) pathway, which can be activated by resistance exercise and branched-chain amino acids (BCAA). The major aim of the present study was to distinguish between the influence of resistance exercise and BCAA on key enzymes considered to be involved in the regulation of protein synthesis, including p70(S6) kinase (p70(S6k)).

METHODS

Nine healthy subjects (four men and five women) performed unilateral resistance exercise on two occasions separated by 1 month. Subjects were randomly supplied either a mixture of BCAA or flavoured water. Muscle biopsies were taken from both resting and exercising muscle before, after and 1 h after exercise.

RESULTS

Phosphorylation of Akt was unaltered by either resistance exercise and/or BCAA supplementation whereas mTOR phosphorylation was enhanced (P<0.05) to a similar extent in both exercising and resting muscle following exercise in the absence (70-90%) and presence of BCAA supplementation (80-130%). Phosphorylation of p70(S6k) was unaffected by resistance exercise alone; however, BCAA intake increased (P<0.05) this phosphorylation in both legs following exercise. In resting muscle, a 5- and 16-fold increase in p70(S6k) was observed immediately after and 1 h after exercise, respectively, as compared to 11- and 30-fold increases in the exercising muscle. Phosphorylation of eukaryotic elongation factor 2 was attenuated 1 h after exercise (P<0.05) in both resting (10-40%) and exercising muscle (30-50%) under both conditions.

CONCLUSION

The present findings indicate that resistance exercise and BCAA exert both separate and combined effects on the p70(S6k) phosphorylation in an Akt-independent manner.

Electromechanical modulation of catabolic and anabolic pathways in chronically inactive, but neurally intact, muscles

The extent and mechanisms by which neural input regulates skeletal muscle mass remain largely unknown. Adult spinal cord isolated (SI) rats were implanted unilaterally with a microstimulator, whereas the contralateral limb served as SI control (SI-C). A 100-HZ, 1-s stimulus was delivered every 30 s for 5 min, followed by a 5-min rest. This was repeated six times consecutively (SI-Stim1) or with a 9-h interval after the third bout (SI-Stim2) for 30 days (1 min of daily activity). SI-Stim1 and SI-Stim2 paradigms attenuated plantaris atrophy by 20% and 38%, respectively, whereas only SI-Stim2 blunted soleus atrophy (24%) relative to SI-C. Muscle mass changes occurred independent of the IGF-1/PI3K/Akt pathway. No relationships between SI or electromechanical stimulation and expression of several atrophy markers were observed. These data suggest that regulatory mechanisms for maintaining muscle mass previously shown in acute states of atrophy differ substantially from those observed in chronic states.

Altered regulation of contraction-induced Akt/mTOR/p70S6k pathway signaling in skeletal muscle of the obese Zucker rat

Increased muscle loading results in the phosphorylation of the 70 kDa ribosomal S6 kinase (p70S6k), and this event is strongly correlated with the degree of muscle adaptation following resistance exercise. Whether insulin resistance or the comorbidities associated with this disorder may affect the ability of skeletal muscle to activate p70S6k signaling following an exercise stimulus remains unclear. Here, we compare the contraction-induced activation of p70S6k signaling in the plantaris muscles of lean and insulin resistant obese Zucker rats following a single bout of increased contractile loading. Compared to lean animals, the basal phosphorylation of p70S6k (Thr389; 37.2% and Thr421/Ser424; 101.4%), Akt (Thr308; 25.1%), and mTOR (Ser2448; 63.0%) was higher in obese animals. Contraction increased the phosphorylation of p70S6k (Thr389), Akt (Ser473), and mTOR (Ser2448) in both models however the magnitude and kinetics of activation differed between models. These results suggest that contraction-induced activation of p70S6k signaling is altered in the muscle of the insulin resistant obese Zucker rat.

Contraction intensity and feeding affect collagen and myofibrillar protein synthesis rates differently in human skeletal muscle

Exercise stimulates muscle protein fractional synthesis rate (FSR), but the importance of contractile intensity and whether it interplays with feeding is not understood. This was investigated following two distinct resistance exercise (RE) contraction intensities using an intrasubject design in the fasted (n = 10) and fed (n = 10) states. RE consisted of 10 sets of knee extensions. One leg worked against light load (LL) at 16% of one-repetition maximum (1RM), the other leg against heavy load (HL) at 70% 1RM, with intensities equalized for total lifted load. Males were infused with [(13)C]leucine, and vastus lateralis biopsies were obtained bilaterally at rest as well as 0.5, 3, and 5.5 h after RE. Western blots were run on muscle lysates and phosphospecific antibodies used to detect phosphorylation status of targets involved in regulation of FSR. The intramuscular collagen FSR was evenly increased following LL- and HL-RE and was not affected by feeding. Myofibrillar FSR was unaffected by LL-RE, whereas HL-RE resulted in a delayed improvement (0.14 +/- 0.02%/h, P < 0.05). Myofibrillar FSR was increased at rest by feeding (P < 0.05) and remained elevated late in the postexercise period compared with the fasting condition. The Rp-s6k-4E-binding protein-1 (BP1) and the mitogen-activated protein kinase (MAPk) pathways were activated by the HL intensity and were suggested to be responsible for regulating myofibrillar FSR in response to adequate contractile activity. Feeding predominantly affected Rp-s6k and eukaryotic elongation factor 2 phosphorylations in correspondence with the observed changes in myofibrillar FSR, whereas 4E-BP1 remained to respond only to the HL contraction intensity. Thus the study design allows us to conclude that the MAPk- and mammalian target of rapamycin-dependent signaling responds to contractile activity, whereas elongation mainly was found to respond to feeding. Furthermore, although functionally linked, the contractile and the supportive matrix structures upregulate their protein synthesis rate quite differently in response to feeding and contractile activity and intensity.

Effects of chronic overload on muscle hypertrophy and mTOR signaling in young adult and aged rats

We examined the effect of 28 days of overload on mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase (ERK) signaling in young adult (Y; 6-month old) and aged (O; 30-month old) Fischer 344 x Brown Norway rats subjected to bilateral synergist ablation (SA) of two thirds of the gastrocnemius muscle or sham surgery (CON). Although plantaris (PLA) muscle hypertrophy was attenuated by aging, mTOR phosphorylation was 44% and 35% greater in Y SA and O SA compared with CON (p = .038). Ribosomal protein S6 phosphorylation was 114% and 24% higher in Y SA and O SA compared with CON (p = .009). Eukaryotic initiation factor 2Bepsilon phosphorylation was 33% and 9% higher in Y SA and O SA compared with CON (p = .04). Translational signaling in young adult and aged plantaris muscle is equally responsive to chronic overload.

Forskolin attenuates the action of insulin on the Akt–mTOR pathway in human skeletal muscle

Forskolin (FSK) is capable of both stimulating and inhibiting the intracellular signaling pathways of protein synthesis tissues other than skeletal muscle. The purpose of this investigation was to determine if FSK administration affects various elements of the protein kinase B (Akt)–mammalian target of rapamycin (mTOR) pathway in human skeletal muscle. Ten (n = 10) healthy, young (21.6 ± 1.3 years), nonobese (body mass index = 25.5 ± 3.5 kg·m–2), recreationally active males were selected for participation. Following an 8 h fast, 2 muscle biopsies of the vastus lateralis were performed. The samples were sectioned and exposed to 4 in vitro treatment conditions: basal, FSK, insulin (INS), and FSK+INS. The samples were then analyzed for total and phosphorylated levels of Akt, mTOR, S6 kinase (S6K1), and 4E binding protein (4EBP1). Akt phosphorylation was significantly greater in the INS-treated samples compared with the basal and FSK conditions (p = 0.007). Furthermore, the ratio of phosphorylated Akt to total Akt (P/T) was higher in the INS samples compared with the basal and FSK samples (p = 0.001). There were no differences in mTOR phosphorylation among the 4 groups; however, total mTOR was significantly greater in the FSK+INS group (p = 0.006). There were also no differences in phosphorylated or total levels of S6K1 among the 4 groups. However, 4EBP1 phosphorylation was significantly greater in the INS-treated samples compared with the basal (p = 0.003) and FSK (p = 0.004) treatments. There were no differences in the ratio of phosphorylated 4EBP1 to total 4EBP1 (P/T) among the 4 groups. These results indicate that FSK does not activate the Akt–mTOR pathway in human skeletal muscle; however, these results suggest that FSK may inhibit the actions of INS on this pathway.