Role of adenosine 5'-monophosphate-activated protein kinase subunits in skeletal muscle mammalian target of rapamycin signaling

Regulating eEF2 and eEF2K in skeletal muscle by exercise

Skeletal muscle is a flexible and adaptable tissue that strongly responds to exercise training. The skeletal muscle responds to exercise by increasing muscle protein synthesis (MPS) when energy is available. One of protein synthesis's major rate-limiting and critical regulatory steps is the translation elongation pathway. The process of translation elongation in skeletal muscle is highly regulated. It requires elongation factors that are intensely affected by various physiological stimuli such as exercise and the total available energy of cells. Studies have shown that exercise involves the elongation pathway by numerous signalling pathways. Since the elongation pathway, has been far less studied than the other translation steps, its comprehensive prospect and quantitative understanding remain in the dark. This study highlights the current understanding of the effect of exercise training on the translation elongation pathway focussing on the molecular factors affecting the pathway, including Ca2+, AMPK, PKA, mTORC1/P70S6K, MAPKs, and myostatin. We further discussed the mode and volume of exercise training intervention on the translation elongation pathway.What is the topic of this review? This review summarises the impacts of exercise training on the translation elongation pathway in skeletal muscle focussing on eEF2 and eEF2K.What advances does it highlight? This review highlights mechanisms and factors that profoundly influence the translation elongation pathway and argues that exercise might modulate the response. This review also combines the experimental observations focussing on the regulation of translation elongation during and after exercise. The findings widen our horizon to the notion of mechanisms involved in muscle protein synthesis (MPS) through translation elongation response to exercise training.

AMP-activated Protein Kinase Activation Suppresses Protein Synthesis and mTORC1 Signaling in Chick Myotube Cultures

Protein synthesis in skeletal muscle is considered one of the most energy-consuming cellular processes. AMP-activated protein kinase (AMPK) is a metabolic master switch that regulates glucose and lipid metabolism, and it is implicated in protein synthesis control in skeletal muscles. The mechanistic target of rapamycin complex 1 (mTORC1) is a central regulator of protein metabolism in cells. However, the effect of AMPK activation on protein synthesis and mTORC1 signaling in chicken skeletal muscle remains unclear. Therefore, in this study, we aimed to investigate the effect of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), an AMPK activator, on protein synthesis and mTORC1 signaling in chick myotube cultures. The incubation of chick myotubes with AICAR (1 mM) for 3 h led to a significant increase in AMPK (Thr172) phosphorylation. Nonetheless, protein synthesis, measured using the surface sensing of translation method, was significantly decreased by AICAR. In addition, the phosphorylation of p70 ribosomal S6 kinase 1 (S6K1, Thr389), S6 ribosomal protein (Ser240/244), and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1, Thr37/46) was significantly reduced by AICAR. These results suggest that AMPK activation suppresses protein synthesis and mTORC1 signaling (through the phosphorylation of S6K1, S6 ribosomal protein, and 4E-BP1) in chick myotubes.

Amino Acid Trafficking and Skeletal Muscle Protein Synthesis: A Case of Supply and Demand

Skeletal muscle protein synthesis is a highly complex process, influenced by nutritional status, mechanical stimuli, repair programs, hormones, and growth factors. The molecular aspects of protein synthesis are centered around the mTORC1 complex. However, the intricacies of mTORC1 regulation, both up and downstream, have expanded overtime. Moreover, the plastic nature of skeletal muscle makes it a unique tissue, having to coordinate between temporal changes in myofiber metabolism and hypertrophy/atrophy stimuli within a tissue with considerable protein content. Skeletal muscle manages the push and pull between anabolic and catabolic pathways through key regulatory proteins to promote energy production in times of nutrient deprivation or activate anabolic pathways in times of nutrient availability and anabolic stimuli. Branched-chain amino acids (BCAAs) can be used for both energy production and signaling to induce protein synthesis. The metabolism of BCAAs occur in tandem with energetic and anabolic processes, converging at several points along their respective pathways. The fate of intramuscular BCAAs adds another layer of regulation, which has consequences to promote or inhibit muscle fiber protein anabolism. This review will outline the general mechanisms of muscle protein synthesis and describe how metabolic pathways can regulate this process. Lastly, we will discuss how BCAA availability and demand coordinate with synthesis mechanisms and identify key factors involved in intramuscular BCAA trafficking.

DW14006 as a Direct AMPKα Activator Ameliorates Diabetic Peripheral Neuropathy in Mice

Diabetic peripheral neuropathy (DPN) is a long-term complication of diabetes with a complicated pathogenesis. AMP-activated protein kinase (AMPK) senses oxidative stress, and mitochondrial function plays a central role in the regulation of DPN. Here, we reported that DW14006 (2-[3-(7-chloro-6-[2'-hydroxy-(1,1'-biphenyl)-4-yl]-2-oxo-1,2-dihydroquinolin-3-yl)phenyl]acetic acid) as a direct AMPKα activator efficiently ameliorated DPN in both streptozotocin (STZ)-induced type 1 and BKS db/db type 2 diabetic mice. DW14006 administration highly enhanced neurite outgrowth of dorsal root ganglion neurons and improved neurological function in diabetic mice. The underlying mechanisms have been intensively investigated. DW14006 treatment improved mitochondrial bioenergetics profiles and restrained oxidative stress and inflammation in diabetic mice by targeting AMPKα, which has been verified by assay against the STZ-induced diabetic mice injected with adeno-associated virus 8-AMPKα-RNAi. To our knowledge, our work might be the first report on the amelioration of the direct AMPKα activator on DPN by counteracting multiple risk factors including mitochondrial dysfunction, oxidative stress, and inflammation, and DW14006 has been highlighted as a potential leading compound in the treatment of DPN.

GROWTH AND DEVELOPMENT SYMPOSIUM: Adenosine monophosphate-activated protein kinase and mitochondria in Rendement Napole pig growth

The Rendement Napole mutation (RN-), which is well known to influence pork quality, also has a profound impact on metabolic characteristics of muscle. Pigs with RN- possess a SNP in the γ3 subunit of adenosine monophosphate (AMP)-activated protein kinase (AMPK); AMPK, a key energy sensor in skeletal muscle, modulates energy producing and energy consuming pathways to maintain cellular homeostasis. Importantly, AMPK regulates not only acute response to energy stress but also facilitates long-term adaptation via changes in gene and protein expression. The RN- allele increases AMPK activity, which alters the metabolic phenotype of skeletal muscle by increasing mitochondrial content and oxidative capacity. Fibers with greater oxidative capacity typically exhibit increased protein turnover and smaller fiber size, which indicates that RN- pigs may exhibit decreased efficiency and growth potential. However, whole body and muscle growth of RN- pigs appear similar to that of wild-type pigs and despite increased oxidative capacity, fibers maintain the capacity for hypertrophic growth. This indicates that compensatory mechanisms may allow RN- pigs to achieve rates of muscle growth similar to those of wild-type pigs. Intriguingly, lipid oxidation and mitochondria function are enhanced in RN- pig muscle. Thus far, characteristics of RN- muscle are largely based on animals near market weight. To better understand interaction between energy signaling and protein accretion in muscle, further work is needed to define age-dependent relationships between AMPK signaling, metabolism, and muscle growth.

Intensive training and reduced volume increases muscle FXYD1 expression and phosphorylation at rest and during exercise in athletes

The present study examined the effect of intensive training in combination with marked reduction in training volume on phospholemman (FXYD1) expression and phosphorylation at rest and during exercise. Eight well-trained cyclists replaced their regular training with speed-endurance training (10-12 × ∼30-s sprints) two or three times per week and aerobic high-intensity training (4-5 × 3-4 min at 90-95% of peak aerobic power output) 1-2 times per week for 7 wk and reduced the training volume by 70%. Muscle biopsies were obtained before and during a repeated high-intensity exercise protocol, and protein expression and phosphorylation were determined by Western blot analysis. Expression of FXYD1 (30%), actin (40%), mammalian target of rapamycin (mTOR) (12%), phospholamban (PLN) (16%), and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) γ/δ (25%) was higher (P < 0.05) than before the training intervention. In addition, after the intervention, nonspecific FXYD1 phosphorylation was higher (P < 0.05) at rest and during exercise, mainly achieved by an increased FXYD1 Ser-68 phosphorylation, compared with before the intervention. CaMKII, Thr-287, and eukaryotic elongation factor 2 Thr-56 phosphorylation at rest and during exercise, overall PKCα/β, Thr-638/641, and mTOR Ser-2448 phosphorylation during repeated intense exercise as well as resting PLN Thr-17 phosphorylation were also higher (P < 0.05) compared with before the intervention period. Thus, a period of high-intensity training with reduced training volume increases expression and phosphorylation levels of FXYD1, which may affect Na(+)/K(+) pump activity and muscle K(+) homeostasis during intense exercise. Furthermore, higher expression of CaMKII and PLN, as well as increased phosphorylation of CaMKII Thr-287 may have improved intracellular Ca(2+) handling.

Effect of N-acetylcysteine infusion on exercise-induced modulation of insulin sensitivity and signaling pathways in human skeletal muscle

-Reactive oxygen species (ROS) produced in skeletal muscle may play a role in potentiating the beneficial responses to exercise; however, the effects of exercise-induced ROS on insulin action and protein signaling in humans has not been fully elucidated. Seven healthy, recreationally active participants volunteered for this double-blind, randomized, repeated-measures crossover study. Exercise was undertaken with infusion of saline (CON) or the antioxidant N-acetylcysteine (NAC) to attenuate ROS. Participants performed two 1-h cycling exercise sessions 7-14 days apart, 55 min at 65% V̇o2peak plus 5 min at 85%V̇o2peak, followed 3 h later by a 2-h hyperinsulinemic euglycemic clamp (40 mIU·min(-1)·m(2)) to determine insulin sensitivity. Four muscle biopsies were taken on each trial day, at baseline before NAC infusion (BASE), after exercise (EX), after 3-h recovery (REC), and post-insulin clamp (PI). Exercise, ROS, and insulin action on protein phosphorylation were evaluated with immunoblotting. NAC tended to decrease postexercise markers of the ROS/protein carbonylation ratio by -13.5% (P = 0.08) and increase the GSH/GSSG ratio twofold vs. CON (P < 0.05). Insulin sensitivity was reduced (-5.9%, P < 0.05) by NAC compared with CON without decreased phosphorylation of Akt or AS160. Whereas p-mTOR was not significantly decreased by NAC after EX or REC, phosphorylation of the downstream protein synthesis target kinase p70S6K was blunted by 48% at PI with NAC compared with CON (P < 0.05). We conclude that NAC infusion attenuated muscle ROS and postexercise insulin sensitivity independent of Akt signaling. ROS also played a role in normal p70S6K phosphorylation in response to insulin stimulation in human skeletal muscle.

Mechanisms modulating skeletal muscle phenotype

Mammalian skeletal muscles are composed of a variety of highly specialized fibers whose selective recruitment allows muscles to fulfill their diverse functional tasks. In addition, skeletal muscle fibers can change their structural and functional properties to perform new tasks or respond to new conditions. The adaptive changes of muscle fibers can occur in response to variations in the pattern of neural stimulation, loading conditions, availability of substrates, and hormonal signals. The new conditions can be detected by multiple sensors, from membrane receptors for hormones and cytokines, to metabolic sensors, which detect high-energy phosphate concentration, oxygen and oxygen free radicals, to calcium binding proteins, which sense variations in intracellular calcium induced by nerve activity, to load sensors located in the sarcomeric and sarcolemmal cytoskeleton. These sensors trigger cascades of signaling pathways which may ultimately lead to changes in fiber size and fiber type. Changes in fiber size reflect an imbalance in protein turnover with either protein accumulation, leading to muscle hypertrophy, or protein loss, with consequent muscle atrophy. Changes in fiber type reflect a reprogramming of gene transcription leading to a remodeling of fiber contractile properties (slow-fast transitions) or metabolic profile (glycolytic-oxidative transitions). While myonuclei are in postmitotic state, satellite cells represent a reserve of new nuclei and can be involved in the adaptive response.

The biochemistry and cell biology of aging: metabolic regulation through mitochondrial signaling

Cellular and organ metabolism affects organismal lifespan. Aging is characterized by increased risks for metabolic disorders, with age-associated degenerative diseases exhibiting varying degrees of mitochondrial dysfunction. The traditional view of the role of mitochondria generated reactive oxygen species (ROS) in cellular aging, assumed to be causative and simply detrimental for a long time now, is in need of reassessment. While there is little doubt that high levels of ROS are detrimental, mounting evidence points toward a lifespan extension effect exerted by mild to moderate ROS elevation. Dietary caloric restriction, inhibition of insulin-like growth factor-I signaling, and inhibition of the nutrient-sensing mechanistic target of rapamycin are robust longevity-promoting interventions. All of these appear to elicit mitochondrial retrograde signaling processes (defined as signaling from the mitochondria to the rest of the cell, for example, the mitochondrial unfolded protein response, or UPR(mt)). The effects of mitochondrial retrograde signaling may even spread to other cells/tissues in a noncell autonomous manner by yet unidentified signaling mediators. Multiple recent publications support the notion that an evolutionarily conserved, mitochondria-initiated signaling is central to the genetic and epigenetic regulation of cellular aging and organismal lifespan.

Regulation of muscle protein synthesis and the effects of catabolic states

Protein synthesis and degradation are dynamically regulated processes that act in concert to control the accretion or loss of muscle mass. The present article focuses on the mechanisms involved in the impairment of protein synthesis that are associated with skeletal muscle atrophy. The vast majority of mechanisms known to regulate protein synthesis involve modulation of the initiation phase of mRNA translation, which comprises a series of reactions that result in the binding of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. The function of the proteins involved in both events has been shown to be repressed under atrophic conditions such as sepsis, cachexia, chronic kidney disease, sarcopenia, and disuse atrophy. The basis for the inhibition of protein synthesis under such conditions is likely to be multifactorial and includes insulin/insulin-like growth factor 1 resistance, pro-inflammatory cytokine expression, malnutrition, corticosteroids, and/or physical inactivity. The present article provides an overview of the existing literature regarding mechanisms and signaling pathways involved in the regulation of mRNA translation as they apply to skeletal muscle wasting, as well as the efficacy of potential clinical interventions such as nutrition and exercise in the maintenance of skeletal muscle protein synthesis under atrophic conditions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

High-frequency electrical stimulation reveals a p38-mTOR signaling module correlated with force-time integral

High-frequency electrical stimulation (HFES) leads to muscle hypertrophy, and attention has been drawn to the high forces involved. However, both mechanical and metabolic stresses occur simultaneously, and both stimuli influence signaling cascades related to protein synthesis. This study aimed to identify the immediate signaling correlates of contraction-induced force and metabolic stresses under the hypothesis that HFES induces growth-related signaling through mechanical stimulation. Force-time integral (FTI) signaling in mouse tibialis anterior muscle was examined by separately manipulating the time of contraction to emphasize the metabolic aspect or the force of contraction to emphasize the mechanical aspect. When FTI was manipulated by changing the total time of activation, phosphorylation of p54 JNK, ERK and p70S6k(T421/S424) was independent of FTI, while phosphorylation of acetyl-CoA carboxylase and p38 correlated with FTI. When FTI was manipulated by changing the force of contraction, p54 JNK, ERK and p70S6k(T421/S424) were again independent of FTI, while phosphorylation of p38 and FAK correlated with FTI. Factor analysis identified a p38-mTOR signaling module that correlated with FTI in both experiments. The consistent link among p38, mTOR and FTI suggests that they form a connected signaling module sensitive to the mechanical aspects of FTI, separate from markers of metabolic load.

AMPK and Exercise: Glucose Uptake and Insulin Sensitivity

AMPK is an evolutionary conserved sensor of cellular energy status that is activated during exercise. Pharmacological activation of AMPK promotes glucose uptake, fatty acid oxidation, mitochondrial biogenesis, and insulin sensitivity; processes that are reduced in obesity and contribute to the development of insulin resistance. AMPK deficient mouse models have been used to provide direct genetic evidence either supporting or refuting a role for AMPK in regulating these processes. Exercise promotes glucose uptake by an insulin dependent mechanism involving AMPK. Exercise is important for improving insulin sensitivity; however, it is not known if AMPK is required for these improvements. Understanding how these metabolic processes are regulated is important for the development of new strategies that target obesity-induced insulin resistance. This review will discuss the involvement of AMPK in regulating skeletal muscle metabolism (glucose uptake, glycogen synthesis, and insulin sensitivity).

Cross-talk between AMPK and mTOR in regulating energy balance

Energy balance is maintained by a complex homeostatic system involving some signaling pathways and "nutrient sensors" in multiple tissues and organs. Any defect associated with the pathways can lead to metabolic disorders including obesity, type 2 diabetes, and the metabolic syndrome. The 5'-adenosine monophosphate-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) appear to play a significant role in the intermediary metabolism of these diseases. AMPK is involved in the fundamental regulation of energy balance at the whole body level by responding to hormonal and nutrient signals in the central nervous system and peripheral tissues that modulate food intake and energy expenditure. Mammalian target of rapamycin (mTOR),is one of the downstream targets of AMPK functions as an intracellular nutrient sensor to control protein synthesis, cell growth, and metabolism. Recent research demonstrated the possible interplay between mTOR and AMPK signaling pathways. In this review, we will present current knowledge of AMPK and mTOR pathways in regulating energy balance and demonstrate the convergence between these two pathways.

The role of AMP-activated protein kinase in the coordination of skeletal muscle turnover and energy homeostasis

The AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that acts as a sensor of cellular energy status switch regulating several systems including glucose and lipid metabolism. Recently, AMPK has been implicated in the control of skeletal muscle mass by decreasing mTORC1 activity and increasing protein degradation through regulation of ubiquitin-proteasome and autophagy pathways. In this review, we give an overview of the central role of AMPK in the control of skeletal muscle plasticity. We detail particularly its implication in the control of the hypertrophic and atrophic signaling pathways. In the light of these cumulative and attractive results, AMPK appears as a key player in regulating muscle homeostasis and the modulation of its activity may constitute a therapeutic potential in treating muscle wasting syndromes in humans.

AMPK promotes skeletal muscle autophagy through activation of forkhead FoxO3a and interaction with Ulk1

In skeletal muscle, protein levels are determined by relative rates of protein synthesis and breakdown. The balance between synthesis and degradation of intracellular components determines the overall muscle fiber size. AMP-activated protein kinase (AMPK), a sensor of cellular energy status, was recently shown to increase myofibrillar protein degradation through the expression of MAFbx and MuRF1. In the present study, the effect of AMPK activation by AICAR on autophagy was investigated in muscle cells. Our results show that FoxO3a transcription factor activation by AMPK induces the expression of the autophagy-related proteins LC3B-II, Gabarapl1, and Beclin1 in primary mouse skeletal muscle myotubes and in the Tibialis anterior (TA) muscle. Time course studies reveal that AMPK activation by AICAR leads to a transient nuclear relocalization of FoxO3a followed by an increase of its cytosolic level. Moreover, AMPK activation leads to the inhibition of mTORC1 and its subsequent dissociation of Ulk1, Atg13, and FIP200 complex. Interestingly, we identify Ulk1 as a new interacting partner of AMPK in muscle cells and we show that Ulk1 is associated with AMPK under normal conditions and dissociates from AMPK during autophagy process. Moreover, we find that AMPK phosphorylates FoxO3a and Ulk1. In conclusion, our data show that AMPK activation stimulates autophagy in skeletal muscle cells through its effects on the transcriptional function of FoxO3a and takes part in the initiation of autophagosome formation by interacting with Ulk1. Here, we present new evidences that AMPK plays a crucial role in the fine tuning of protein expression programs that control skeletal muscle mass.

Crosstalk between mitochondrial (dys)function and mitochondrial abundance

A controlled regulation of mitochondrial mass through either the production (biogenesis) or the degradation (mitochondrial quality control) of the organelle represents a crucial step for proper mitochondrial and cell function. Key steps of mitochondrial biogenesis and quality control are overviewed, with an emphasis on the role of mitochondrial chaperones and proteases that keep mitochondria fully functional, provided the mitochondrial activity impairment is not excessive. In this case, the whole organelle is degraded by mitochondrial autophagy or "mitophagy." Beside the maintenance of adequate mitochondrial abundance and functions for cell homeostasis, mitochondrial biogenesis might be enhanced, through discussed signaling pathways, in response to various physiological stimuli, like contractile activity, exposure to low temperatures, caloric restriction, and stem cells differentiation. In addition, mitochondrial dysfunction might also initiate a retrograde response, enabling cell adaptation through increased mitochondrial biogenesis.

Mitochondrial therapeutics for cardioprotection

Mitochondria represent approximately one-third of the mass of the heart and play a critical role in maintaining cellular function-however, they are also a potent source of free radicals and pro-apoptotic factors. As such, maintaining mitochondrial homeostasis is essential to cell survival. As the dominant source of ATP, continuous quality control is mandatory to ensure their ongoing optimal function. Mitochondrial quality control is accomplished by the dynamic interplay of fusion, fission, autophagy, and mitochondrial biogenesis. This review examines these processes in the heart and considers their role in the context of ischemia-reperfusion injury. Interventions that modulate mitochondrial turnover, including pharmacologic agents, exercise, and caloric restriction are discussed as a means to improve mitochondrial quality control, ameliorate cardiovascular dysfunction, and enhance longevity.

Recent progress toward understanding the molecular mechanisms that regulate skeletal muscle mass

The maintenance of muscle mass is critical for health and issues associated with the quality of life. Over the last decade, extensive progress has been made with regard to our understanding of the molecules that regulate skeletal muscle mass. Not surprisingly, many of these molecules are intimately involved in the regulation of protein synthesis and protein degradation [e.g. the mammalian target of rapamycin (mTOR), eukaryotic initiation factor 2B (eIF2B), eukaryotic initiation factor 3f (eIF3f) and the forkhead box O (FoxO) transcription factors]. It is also becoming apparent that molecules which sense, or control, the energetic status of the cell play a key role in the regulation of muscle mass [e.g. AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator-1 α (PGC1α)]. In this review we will attempt to summarize the current knowledge of how these molecules regulate skeletal muscle mass.

Effect of exercise and training on phospholemman phosphorylation in human skeletal muscle

Phospholemman (PLM, FXYD1) is a partner protein and regulator of the Na(+)-K(+)-ATPase (Na(+)-K(+) pump). We explored the impact of acute and short-term training exercise on PLM physiology in human skeletal muscle. A group of moderately trained males (n = 8) performed a 1-h acute bout of exercise by utilizing a one-legged cycling protocol. Muscle biopsies were taken from vastus lateralis at 0 and 63 min (non-exercised leg) and 30 and 60 min (exercised leg). In a group of sedentary males (n = 9), we determined the effect of a 10-day intense aerobic cycle training on Na(+)-K(+)-ATPase subunit expression, PLM phosphorylation, and total PLM expression as well as PLM phosphorylation in response to acute exercise (1 h at ∼72% Vo(2peak)). Biopsies were taken at rest, immediately following, and 3 h after an acute exercise bout before and at the conclusion of the 10-day training study. PLM phosphorylation was increased both at Ser(63) and Ser(68) immediately after acute exercise (75%, P < 0.05, and 30%, P < 0.05, respectively). Short-term training had no adaptive effect on PLM phosphorylation at Ser(63) and Ser(68), nor was the total amount of PLM altered posttraining. The protein expressions of α(1)-, α(2)-,and β(1)-subunits of Na(+)-K(+)-ATPase were increased after training (113%, P < 0.05, 49%, P < 0.05, and 27%, P < 0.05, respectively). Whereas an acute bout of exercise increased the phosphorylation of PKCα/βII on Thr(638/641) pre- and posttraining, phosphorylation of PKCζ/λ on Thr(403/410) was increased in response to acute exercise only after the 10-day training. In conclusion, we show that only acute exercise, and not short-term training, increases phosphorylation of PLM on Ser(63) and Ser(68), and data from one-legged cycling indicate that this effect of exercise on PLM phosphorylation is not due to systemic factors. Our results provide evidence that phosphorylation of PLM may play a role in the acute regulation of the Na(+)-K(+)-ATPase response to exercise.

Altered metabolism and persistent starvation behaviors caused by reduced AMPK function in Drosophila

Organisms must utilize multiple mechanisms to maintain energetic homeostasis in the face of limited nutrient availability. One mechanism involves activation of the heterotrimeric AMP-activated protein kinase (AMPK), a cell-autonomous sensor to energetic changes regulated by ATP to AMP ratios. We examined the phenotypic consequences of reduced AMPK function, both through RNAi knockdown of the gamma subunit (AMPKγ) and through expression of a dominant negative alpha (AMPKα) variant in Drosophila melanogaster. Reduced AMPK signaling leads to hypersensitivity to starvation conditions as measured by lifespan and locomotor activity. Locomotor levels in flies with reduced AMPK function were lower during unstressed conditions, but starvation-induced hyperactivity, an adaptive response to encourage foraging, was significantly higher than in wild type. Unexpectedly, total dietary intake was greater in animals with reduced AMPK function yet total triglyceride levels were lower. AMPK mutant animals displayed starvation-like lipid accumulation patterns in metabolically key liver-like cells, oenocytes, even under fed conditions, consistent with a persistent starved state. Measurements of O₂ consumption reveal that metabolic rates are greater in animals with reduced AMPK function. Lastly, rapamycin treatment tempers the starvation sensitivity and lethality associated with reduced AMPK function. Collectively, these results are consistent with models that AMPK shifts energy usage away from expenditures into a conservation mode during nutrient-limited conditions at a cellular level. The highly conserved AMPK subunits throughout the Metazoa, suggest such findings may provide significant insight for pharmaceutical strategies to manipulate AMPK function in humans.

Autophagy in health and disease. 5. Mitophagy as a way of life

Our understanding of autophagy has expanded greatly in recent years, largely due to the identification of the many genes involved in the process and to the development of better methods to monitor the process, such as green fluorescent protein-LC3 to visualize autophagosomes in vivo. A number of groups have demonstrated a tight connection between autophagy and mitochondrial turnover. Mitochondrial quality control is the process whereby mitochondria undergo successive rounds of fusion and fission with a dynamic exchange of components to segregate functional and damaged elements. Removal of the mitochondrion that contains damaged components is accomplished via autophagy (mitophagy). Mitophagy also serves to eliminate the subset of mitochondria producing the most reactive oxygen species, and episodic removal of mitochondria will reduce the oxidative burden, thus linking the mitochondrial free radical theory of aging with longevity achieved through caloric restriction. Mitophagy must be balanced by biogenesis to meet tissue energy needs, but the system is tunable and highly dynamic. This process is of greatest importance in long-lived cells such as cardiomyocytes, neurons, and memory T cells. Autophagy is known to decrease with age, and the failure to maintain mitochondrial quality control through mitophagy may explain why the heart, brain, and components of the immune system are most vulnerable to dysfunction as organisms age.

Lack of adequate appreciation of physical exercise's complexities can pre-empt appropriate design and interpretation in scientific discovery

Two major issues are presented. First, a challenge is made by us that a misunderstanding of physiology has led to incomplete or wrong functional designations of genes in some cases. Normal physiological processes are dynamic, integrated and periodic, and, therefore, it is difficult to define normal physiological function by looking at a single time point or single process in a non-stressed subject. The ability of the organism to successfully respond to homeostatic disruptions defines normal physiology. Genes were selected for survival and to appropriately respond to stresses, such as physical activity. Omitting gene functions by restricting them to non-stressful conditions could lead to less than optimal primary preventions, treatments and cures for diseases. Physical exercise, as a stressor, should be used to better demonstrate the complete functional classifications of some genes. Second, the challenge from others of an 'exercise pill' as a mimetic of natural physical activity will be shown to be lacking a scientific basis. The concept of an 'exercise pill'/'exercise mimetic' demonstrates an inadequate appreciation of the complexities in integrating cell, tissue, organ and systems during both acute disruptions in homeostasis by a single bout of exercise, and longer-term chronic adaptations to different types of exercise such as resistance and endurance. It is our opinion that those promoting drugs targeting a single or few molecules should not redefine the term 'exercise' and exercise concepts in an attempt to sensationalize findings. Additionally, the scientific criteria that the authors demand to be met to legitimately use the terms 'exercise pill' and 'exercise mimetic' are presented.

Impaired skeletal muscle beta-adrenergic activation and lipolysis are associated with whole-body insulin resistance in rats bred for low intrinsic exercise capacity

Rats selectively bred for high endurance running capacity (HCR) have higher insulin sensitivity and improved metabolic health compared with those bred for low endurance capacity (LCR). We investigated several skeletal muscle characteristics, in vitro and in vivo, that could contribute to the metabolic phenotypes observed in sedentary LCR and HCR rats. After 16 generations of selective breeding, HCR had approximately 400% higher running capacity (P < 0.001), improved insulin sensitivity (P < 0.001), and lower fasting plasma glucose and triglycerides (P < 0.05) compared with LCR. Skeletal muscle ceramide and diacylglycerol content, basal AMP-activated protein kinase (AMPK) activity, and basal lipolysis were similar between LCR and HCR. However, the stimulation of lipolysis in response to 10 mum isoproterenol was 70% higher in HCR (P = 0.004). Impaired isoproterenol sensitivity in LCR was associated with lower basal triacylglycerol lipase activity, Ser660 phosphorylation of HSL, and beta2-adrenergic receptor protein content in skeletal muscle. Expression of the orphan nuclear receptor Nur77, which is induced by beta-adrenergic signaling and is associated with insulin sensitivity, was lower in LCR (P < 0.05). Muscle protein content of Nur77 target genes, including uncoupling protein 3, fatty acid translocase/CD36, and the AMPK gamma3 subunit were also lower in LCR (P < 0.05). Our investigation associates whole-body insulin resistance with impaired beta-adrenergic response and reduced expression of genes that are critical regulators of glucose and lipid metabolism in skeletal muscle. We identify impaired beta-adrenergic signal transduction as a potential mechanism for impaired metabolic health after artificial selection for low intrinsic exercise capacity.

Lipid-induced mTOR activation in rat skeletal muscle reversed by exercise and 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside

The serine/threonine protein kinase, mammalian target of rapamycin (mTOR) is regulated by insulin and nutrient availability and has been proposed to play a central role as a nutrient sensor in skeletal muscle. mTOR associates with its binding partners, raptor and rictor, to form two structurally and functionally distinct complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) respectively. We have investigated the assembly of mTORC1/2 and the activation of their downstream substrates (i.e. Akt, S6K1) in response to known effectors of mTOR, excess lipid availability and AMP-activated protein kinase (AMPK) activation/exercise training in rat skeletal muscle. The in vivo formation of mTORC1 and 2 and the activation of their respective downstream substrates were increased in response to chronic (8 weeks) consumption of a high-fat diet. Diet-induced mTORC activation and skeletal muscle insulin resistance were reversed by 4 weeks of exercise training, which was associated with enhanced muscle AMPK activation. In order to determine whether AMPK activation reverses lipid-induced mTOR activation, L6 myotubes were exposed to 0.4 mM palmitate to activate mTORC1/2 in the absence or presence of 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). Palmitate exposure (4 h) increased insulin-stimulated S6K1 Thr389 phosphorylation by 60%, indicating activation of mTORC1. AMPK activation with 1 mM AICAR abolished lipid-induced mTOR activation in vitro. Our data implicates reductions in mTOR complex activation with the reversal of lipid-induced skeletal muscle insulin resistance in response to exercise training or AICAR and identifies mTOR as a potential target for the treatment of insulin resistance.

AMP-activated protein kinase in contraction regulation of skeletal muscle metabolism: necessary and/or sufficient?

In skeletal muscle, the contraction-activated heterotrimeric 5'-AMP-activated protein kinase (AMPK) protein is proposed to regulate the balance between anabolic and catabolic processes by increasing substrate uptake and turnover in addition to regulating the transcription of proteins involved in mitochondrial biogenesis and other aspects of promoting an oxidative muscle phenotype. Here, the current knowledge on the expression of AMPK subunits in human quadriceps muscle and evidence from rodent studies suggesting distinct AMPK subunit expression pattern in different muscle types is reviewed. Then, the intensity and time dependence of AMPK activation in human quadriceps and rodent muscle are evaluated. Subsequently, a major part of this review critically examines the evidence supporting a necessary and/or sufficient role of AMPK in a broad spectrum of skeletal muscle contraction-relevant processes. These include glucose uptake, glycogen synthesis, post-exercise insulin sensitivity, fatty acid (FA) uptake, intramuscular triacylglyceride hydrolysis, FA oxidation, suppression of protein synthesis, proteolysis, autophagy and transcriptional regulation of genes relevant to promoting an oxidative phenotype.

Regulatory mechanisms of skeletal muscle protein turnover during exercise

Skeletal muscle protein turnover is a relatively slow metabolic process that is altered by various physiological stimuli such as feeding, fasting, and exercise. During exercise, catabolism of amino acids contributes very little to ATP turnover in working muscle. With regard to protein turnover, there are now consistent data from tracer studies in rodents and humans showing that global protein synthesis is blunted in working skeletal muscle. Whether there is altered skeletal muscle protein breakdown during exercise remains unclear. The blunting of protein synthesis is believed to be mediated by suppressed mRNA translation initiation and elongation steps involving, but not limited to, changes in eukaryotic initiation factor 4E binding protein 1 and eukaryotic elongation factor 2 phosphorylation (eEF2), respectively. Evidence is provided that upstream signaling to translation factors is mediated by signaling downstream of changes in intracellular Ca2+ and energy turnover. In particular, a signaling cascade involving Ca2+/calmodulin-eEF2 kinase-eEF2 is implicated. The possible functional significance of altered protein turnover in working skeletal muscle during exercise is discussed. Further work with available and new techniques will undoubtedly reveal the functional significance and signaling mechanisms behind changes in skeletal muscle protein turnover during exercise.

LKB1 and AMP-activated protein kinase control of mTOR signalling and growth

The AMP-activated serine/threonine protein kinase (AMPK) is a sensor of cellular energy status found in all eukaryotes that is activated under conditions of low intracellular ATP following stresses such as nutrient deprivation or hypoxia. In the past 5 years, work from a large number of laboratories has revealed that one of the major downstream signalling pathways regulated by AMPK is the mammalian target-of-rapamycin [mammalian target of rapamycin (mTOR) pathway]. Interestingly, like AMPK, the mTOR serine/threonine kinase plays key roles not only in growth control and cell proliferation but also in metabolism. Recent work has revealed that across eukaryotes mTOR orthologues are found in two biochemically distinct complexes and only one of those complexes (mTORC1 in mammals) is acutely sensitive to rapamycin and regulated by nutrients and AMPK. Many details of the molecular mechanism by which AMPK inhibits mTORC1 signalling have also been decoded in the past 5 years. AMPK directly phosphorylates at least two proteins to induce rapid suppression of mTORC1 activity, the TSC2 tumour suppressor and the critical mTORC1 binding subunit raptor. Here we explore the molecular connections between AMPK and mTOR signalling pathways and examine the physiological processes in which AMPK regulation of mTOR is critical for growth or metabolic control. The functional conservation of AMPK and TOR in all eukaryotes, and the sequence conservation around the AMPK phosphorylation sites in raptor across all eukaryotes examined suggest that this represents a fundamental cell growth module connecting nutrient status to the cell growth machinery. These findings have broad implications for the control of cell growth by nutrients in a number of cellular and organismal contexts.

A Ca(2+)-calmodulin-eEF2K-eEF2 signalling cascade, but not AMPK, contributes to the suppression of skeletal muscle protein synthesis during contractions

Skeletal muscle protein synthesis rate decreases during contractions but the underlying regulatory mechanisms are poorly understood. It was hypothesized that there would be a coordinated regulation of eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E-binding protein 1 (4EBP1) phosphorylation by signalling cascades downstream of rises in intracellular [Ca(2+)] and decreased energy charge via AMP-activated protein kinase (AMPK) in contracting skeletal muscle. When fast-twitch skeletal muscles were contracted ex vivo using different protocols, the suppression of protein synthesis correlated more closely with changes in eEF2 than 4EBP1 phosphorylation. Using a combination of Ca(2+) release agents and ATPase inhibitors it was shown that the 60-70% suppression of fast-twitch skeletal muscle protein synthesis during contraction was equally distributed between Ca(2+) and energy turnover-related mechanisms. Furthermore, eEF2 kinase (eEF2K) inhibition completely blunted increases in eEF2 phosphorylation and partially blunted (i.e. 30-40%) the suppression of protein synthesis during contractions. The 3- to 5-fold increase in skeletal muscle eEF2 phosphorylation during contractions in situ was rapid and sustained and restricted to working muscle. The increase in eEF2 phosphorylation and eEF2K activation were downstream of Ca(2+)-calmodulin (CaM) but not other putative activating factors such as a fall in intracellular pH or phosphorylation by protein kinases. Furthermore, blunted protein synthesis and 4EBP1 dephosphorylation were unrelated to AMPK activity during contractions, which was exemplified by normal blunting of protein synthesis during contractions in muscles overexpressing kinase-dead AMPK. In summary, in fast-twitch skeletal muscle, the inhibition of eEF2 activity by phosphorylation downstream of Ca(2+)-CaM-eEF2K signalling partially contributes to the suppression of protein synthesis during exercise/contractions.

Expression and activity of the 5'-adenosine monophosphate-activated protein kinase pathway in selected tissues during chicken embryonic development

The 5'-adenosine monophosphate-activated protein kinase (AMPK) is a highly conserved serine-threonine protein kinase and a key part of a kinase-signaling cascade that senses cellular energy status (adenosine monophosphate:adenosine triphosphate ratio) and acts to maintain energy homeostasis by coordinately regulating energy-consuming and energy-generating metabolic pathways. The objective of this study was to investigate aspects of the AMPK pathway in the liver, brain, breast muscle, and heart from d 12 of incubation through hatch in chickens. We first determined mRNA and protein expression profiles for a major upstream AMPK kinase, LKB1, which is known to activate (phosphorylate) AMPK in response to increases in the adenosine monophosphate:adenosine triphosphate ratio. Expression of LKB1 protein was greatest in the brain, which demonstrated tissue-specific patterns for phosphorylation. Next, AMPK subunit mRNA and protein expression profiles were determined. Significant changes in AMPK subunit mRNA expression occurred in all tissues from d 12 of incubation to hatch. Differences in the levels of active (phosphorylated) AMPK as well as alpha and beta subunit proteins were observed in all 4 tissues during embryonic development. Finally, we determined the protein level and phosphorylation status of an important downstream target for AMPK, acetyl-coenzyme A carboxylase. The expression of acetyl-co-enzyme A carboxylase and phosphorylated acetyl-coenzyme A was greater in the brain than the liver, but was undetectable by Western blotting in the breast muscle and heart throughout the period of study. Together, our results are the first to demonstrate the expression and activity of the AMPK pathway in key tissues during the transition from embryonic to posthatch development in chickens.

Divergent cell signaling after short-term intensified endurance training in human skeletal muscle

Endurance training represents one extreme in the continuum of skeletal muscle plasticity. The molecular signals elicited in response to acute and chronic exercise and the integration of multiple intracellular pathways are incompletely understood. We determined the effect of 10 days of intensified cycle training on signal transduction in nine inactive males in response to a 1-h acute bout of cycling at the same absolute workload (164 +/- 9 W). Muscle biopsies were taken at rest and immediately and 3 h after the acute exercise. The metabolic signaling pathways, including AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR), demonstrated divergent regulation by exercise after training. AMPK phosphorylation increased in response to exercise ( approximately 16-fold; P < 0.05), which was abrogated posttraining (P < 0.01). In contrast, mTOR phosphorylation increased in response to exercise ( approximately 2-fold; P < 0.01), which was augmented posttraining (P < 0.01) in the presence of increased mTOR expression (P < 0.05). Exercise elicited divergent effects on mitogen-activated protein kinase (MAPK) pathways after training, with exercise-induced extracellular signal-regulated kinase (ERK) 1/2 phosphorylation being abolished (P < 0.01) and p38 MAPK maintained. Finally, calmodulin kinase II (CaMKII) exercise-induced phosphorylation and activity were maintained (P < 0.01), despite increased expression ( approximately 2-fold; P < 0.05). In conclusion, 10 days of intensified endurance training attenuated AMPK, ERK1/2, and mTOR, but not CaMKII and p38 MAPK signaling, highlighting molecular pathways important for rapid functional adaptations and maintenance in response to intensified endurance exercise and training.

Skeletal muscle eEF2 and 4EBP1 phosphorylation during endurance exercise is dependent on intensity and muscle fiber type

Protein synthesis in skeletal muscle is known to decrease during exercise, and it has been suggested that this may depend on the magnitude of the relative metabolic stress within the contracting muscle. To examine the mechanisms behind this, the effect of exercise intensity on skeletal muscle eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E binding protein 1 (4EBP1) phosphorylation, key components in the mRNA translation machinery, were examined together with AMP-activated protein kinase (AMPK) in healthy young men. Skeletal muscle eEF2 phosphorylation at Thr56 increased during exercise but was not influenced by exercise intensity, and was lower than rest 30 min after exercise. On the other hand, 4EBP1 phosphorylation at Thr37/46 decreased during exercise, and this decrease was greater at higher exercise intensities and was similar to rest 30 min after exercise. AMPK activity, as indexed by AMPK alpha-subunit phosphorylation at Thr172 and phosphorylation of the AMPK substrate ACCbeta at Ser221, was higher with higher exercise intensities, and these indices were higher than rest after high-intensity exercise only. Using immunohistochemistry, it was shown that the increase in skeletal muscle eEF2 Thr56 phosphorylation was restricted to type I myofibers. Taken together, these data suggest that the depression of skeletal muscle protein synthesis with endurance-type exercise may be regulated at both initiation (i.e., 4EBP1) and elongation (i.e., eEF2) steps, with eEF2 phosphorylation contributing at all exercise intensities but 4EBP1 dephosphorylation contributing to a greater extent at high vs. low exercise intensities.

Cellular mechanisms regulating protein synthesis and skeletal muscle hypertrophy in animals

Growth and maintenance of skeletal muscle mass is critical for long-term health and quality of life. Skeletal muscle is a highly adaptable tissue with well-known sensitivities to environmental cues such as growth factors, cytokines, nutrients, and mechanical loading. All of these factors act at the level of the cell and signal through pathways that lead to changes in phenotype through multiple mechanisms. In this review, we discuss the animal and cell culture models used and the signaling mechanisms identified in understanding regulation of protein synthesis in response to mechanical loading/resistance exercise. Particular emphasis has been placed on 1) alterations in mechanical loading and regulation of protein synthesis in both in vivo animal studies and in vitro cell culture studies and 2) upstream mediators regulating mammalian target of rapamycin signaling and protein synthesis during skeletal muscle hypertrophy.