Specific attenuation of protein kinase phosphorylation in muscle with a high mitochondrial content

Better maintenance of enzymatic capacity and higher levels of substrate transporter proteins in skeletal muscle of aging female mice

This study investigated sex-specific differences in high-energy phosphate, glycolytic, and mitochondrial enzyme activities and also metabolite transporter protein levels in the skeletal muscles of adult (5 months old), middle-aged (12 months old), and advanced-aged (24 months old) mice. While gastrocnemius glycogen content increased with age regardless of sex, gastrocnemius triglyceride levels increased only in advanced-aged female mice. Aging decreased creatine kinase and adenylate kinase activities in the plantaris muscle of both sexes and in the soleus muscle of male mice but not in female mice. Irrespective of sex, phosphofructokinase and lactate dehydrogenase (LDH) activities decreased in the plantaris and soleus muscles. Additionally, hexokinase activity in the plantaris muscle and LDH activity in the soleus muscle decreased to a greater extent in aged male mice compared with those in aged female mice. Mitochondrial enzyme activities increased in the plantaris muscle of aged female mice but did not change in male mice. The protein content of the glucose transporter 4 in the aged plantaris muscle and fatty acid translocase/cluster of differentiation 36 increased in the aged plantaris and soleus muscles of both sexes, with a significantly higher content in female mice. These findings suggest that females possess a better ability to maintain metabolic enzyme activity and higher levels of metabolite transport proteins in skeletal muscle during aging, despite alterations in lipid metabolism. Our data provide a basis for studying muscle metabolism in the context of age-dependent metabolic perturbations and diseases that affect females and males differently.

Differential Mitochondrial Adaptation of the Slow and Fast Skeletal Muscles by Endurance Running Exercise in Streptozotocin-Induced Diabetic Mice

The skeletal muscle is the main organ responsible for insulin action, and glucose disposal and metabolism. Endurance and/or resistance training raises the number of mitochondria in diabetic muscles. The details of these adaptations, including mitochondrial adaptations of the slow and fast muscles in diabetes, are unclear. This study aimed to determine whether exercise training in streptozotocin (STZ)-induced mice leads to differential adaptations in the slow and fast muscles, and improving glucose clearance. Eight-week-old mice were randomly distributed into normal control (CON), diabetes (DM), and diabetes and exercise (DM+Ex) groups. In the DM and DM+Ex groups, mice received a freshly prepared STZ (100 mg/kg) intraperitoneal injection on two consecutive days. Two weeks after the injection, the mice in the groups ran on a treadmill for 60 min at 20 m/min for a week and subsequently at 25 m/min for 5 weeks (5 days/week). The analyses indicated that running training at low speed (25 m/min) enhanced mitochondrial enzyme activity and expression of lactate and glucose transporters in the plantaris (low-oxidative) muscle that improved whole-body glucose metabolism in STZ-induced diabetic mice. There were no differences in glucose transporter expression levels in the soleus (high-oxidative) muscle. The endurance running exercise at 20-25 m/min was sufficient to induce mitochondrial adaptation in the low-oxidative muscles, but not in the high-oxidative muscles, of diabetic mice. In conclusion, the present study indicated that running training at 25 m/min improved glucose metabolism by increasing the mitochondrial enzyme activity and glucose transporter 4 and monocarboxylate transporter 4 protein contents in the low-oxidative muscles in STZ-induced diabetic mice.

Insights into the development of insulin resistance: Unraveling the interaction of physical inactivity, lipid metabolism and mitochondrial biology

While impairments in peripheral tissue insulin signalling have a well-characterized role in the development of insulin resistance and type 2 diabetes (T2D), the specific mechanisms that contribute to these impairments remain debatable. Nonetheless, a prominent hypothesis implicates the presence of a high-lipid environment, resulting in both reactive lipid accumulation and increased mitochondrial reactive oxygen species (ROS) production in the induction of peripheral tissue insulin resistance. While the etiology of insulin resistance in a high lipid environment is rapid and well documented, physical inactivity promotes insulin resistance in the absence of redox stress/lipid-mediated mechanisms, suggesting alternative mechanisms-of-action. One possible mechanism is a reduction in protein synthesis and the resultant decrease in key metabolic proteins, including canonical insulin signaling and mitochondrial proteins. While reductions in mitochondrial content associated with physical inactivity are not required for the induction of insulin resistance, this could predispose individuals to the detrimental effects of a high-lipid environment. Conversely, exercise-training induced mitochondrial biogenesis has been implicated in the protective effects of exercise. Given mitochondrial biology may represent a point of convergence linking impaired insulin sensitivity in both scenarios of chronic overfeeding and physical inactivity, this review aims to describe the interaction between mitochondrial biology, physical (in)activity and lipid metabolism within the context of insulin signalling.

Protein signaling in response to ex vivo dynamic contractions is independent of training status in rat skeletal muscle

New Findings

What is the central question of this study?

Are myofibre protein signalling responses to ex vivo dynamic contractions altered by accustomization to voluntary endurance training in rats?

What is the main finding and its importance?

In response to ex vivo dynamic muscle contractions, canonical myofibre protein signalling pertaining to metabolic transcriptional regulation, as well as translation initiation and elongation, was not influenced by prior accustomization to voluntary endurance training in rats. Accordingly, intrinsic myofibre protein signalling responses to standardized contractile activity may be independent of prior exercise training in rat skeletal muscle.

Abstract

Skeletal muscle training status may influence myofibre regulatory protein signalling in response to contractile activity. The current study employed a purpose‐designed ex vivo dynamic contractile protocol to evaluate the effect of exercise‐accustomization on canonical myofibre protein signalling for metabolic gene expression and for translation initiation and elongation. To this end, rats completed 8 weeks of in vivo voluntary running training versus no running control intervention, whereupon an ex vivo endurance‐type dynamic contraction stimulus was conducted in isolated soleus muscle preparations from both intervention groups. Protein signalling response by phosphorylation was evaluated by immunoblotting at 0 and 3 h following ex vivo stimulation. Phosphorylation of AMP‐activated protein kinase α‐isoforms and its downstream target, acetyl‐CoA carboxylase, as well as phosphorylation of eukaryotic elongation factor 2 (eEF2) was increased immediately following the dynamic contraction protocol (at 0 h). Signalling for translation initiation and elongation was evident at 3 h after dynamic contractile activity, as evidenced by increased phosphorylation of p70 S6 kinase and eukaryotic translation initiation factor 4E‐binding protein 1, as well as a decrease in phosphorylation of eEF2 back to resting control levels. However, prior exercise training did not alter phosphorylation responses of the investigated signalling proteins. Accordingly, protein signalling responses to standardized endurance‐type contractions may be independent of training status in rat muscle during ex vivo conditions. The present findings add to our current understanding of molecular regulatory events responsible for skeletal muscle plasticity.

The influence of age, sex, and exercise on autophagy, mitophagy, and lysosome biogenesis in skeletal muscle

Background

Aging decreases skeletal muscle mass and quality. Maintenance of healthy muscle is regulated by a balance between protein and organellar synthesis and their degradation. The autophagy-lysosome system is responsible for the selective degradation of protein aggregates and organelles, such as mitochondria (i.e., mitophagy). Little data exist on the independent and combined influence of age, biological sex, and exercise on the autophagy system and lysosome biogenesis. The purpose of this study was to characterize sex differences in autophagy and lysosome biogenesis in young and aged muscle and to determine if acute exercise influences these processes.

Methods

Young (4–6 months) and aged (22–24 months) male and female mice were assigned to a sedentary or an acute exercise group. Mitochondrial content, the autophagy-lysosome system, and mitophagy were measured via protein analysis. A TFEB-promoter-construct was utilized to examine Tfeb transcription, and nuclear-cytosolic fractions allowed us to examine TFEB localization in sedentary and exercised muscle with age and sex.

Results

Our results indicate that female mice, both young and old, had more mitochondrial protein than age-matched males. However, mitochondria in the muscle of females had a reduced respiratory capacity. Mitochondrial content was only reduced with age in the male cohort. Young female mice had a greater abundance of autophagy, mitophagy, and lysosome proteins than young males; however, increases were evident with age irrespective of sex. Young sedentary female mice had indices of greater autophagosomal turnover than male counterparts. Exhaustive exercise was able to stimulate autophagic clearance solely in young male mice. Similarly, nuclear TFEB protein was enhanced to a greater extent in young male, compared to young female mice following exercise, but no changes were observed in aged mice. Finally, TFEB-promoter activity was upregulated following exercise in both young and aged muscle.

Conclusions

The present study demonstrates that biological sex influences mitochondrial homeostasis, the autophagy-lysosome system, and mitophagy in skeletal muscle with age. Furthermore, our data suggest that young male mice have a more profound ability to activate these processes with exercise than in the other groups. Ultimately, this may contribute to a greater remodeling of muscle in response to exercise training in males.

Regulatory networks controlling mitochondrial quality control in skeletal muscle

The adaptive plasticity of mitochondria within skeletal muscle is regulated by signals converging on a myriad of regulatory networks that operate during conditions of increased (i.e. exercise) and decreased (inactivity, disuse) energy requirements. Notably, some of the initial signals that induce adaptive responses are common to both conditions, differing in their magnitude and temporal pattern, to produce vastly opposing mitochondrial phenotypes. In response to exercise, signaling to PGC-1α and other regulators ultimately produces an abundance of high quality mitochondria, leading to reduced mitophagy and a higher mitochondrial content. This is accompanied by the presence of an enhanced protein quality control system that consists of the protein import machinery as well chaperones and proteases termed the UPR^mt. The UPR^mt monitors intra-organelle proteostasis, and strives to maintain a mito-nuclear balance between nuclear- and mtDNA-derived gene products via retrograde signaling from the organelle to the nucleus. In addition, antioxidant capacity is improved, affording greater protection against oxidative stress. In contrast, chronic disuse conditions produce similar signaling but result in decrements in mitochondrial quality and content. Thus, the interactive cross-talk of the regulatory networks that control organelle turnover during wide variations in muscle use and disuse remain incompletely understood, despite our improving knowledge of the traditional regulators of organelle content and function. This brief review acknowledges existing regulatory networks and summarizes recent discoveries of novel biological pathways involved in determining organelle biogenesis, dynamics, mitophagy, protein quality control and antioxidant capacity, identifying ample protein targets for therapeutic intervention that determine muscle and mitochondrial health.

Promoting a pro-oxidant state in skeletal muscle: Potential dietary, environmental, and exercise interventions for enhancing endurance-training adaptations

Accumulating evidence now shows that supplemental antioxidants including vitamin C, vitamin E and N-Acetylcysteine consumption can suppress adaptations to endurance-type exercise by attenuating reactive oxygen and nitrogen species (RONS) formation within skeletal muscle. This emerging evidence points to the importance of pro-oxidation as an important stimulus for endurance-training adaptations, including mitochondrial biogenesis, endogenous antioxidant production, insulin signalling, angiogenesis and growth factor signaling. Although sustained oxidative distress is associated with many chronic diseases, athletes have, on average, elevated levels of certain endogenous antioxidants to maintain redox homeostasis. As a result, trained athletes may have a better capacity to buffer oxidants during and after exercise, resulting in a reduced oxidative eustress stimulus for adaptations. Thus, higher levels of RONS input and exercise-induced oxidative stress may benefit athletes in the pursuit of continuous endurance training redox adaptations. This review addresses why athletes should be looking to enhance exercise-induced oxidative stress and how it can be accomplished. Methods covered include high-intensity interval training, hyperthermia and heat stress, dietary antioxidant restriction and modified antioxidant timing, dietary antioxidants and polyphenols as adjuncts to exercise, and vitamin C as a pro-oxidant.

Hydroxytyrosol modifies skeletal muscle GLUT4/AKT/Rac1 axis in trained rats

Training induces a number of healthy effects including a rise in skeletal muscle (SKM) glucose uptake. These adaptations are at least in part due to the reactive oxygen species produced within SKM, which is in agreement with the notion that antioxidant supplementation blunts some training-induced adaptations. Here, we tested whether hydroxytyrosol (HT), the main polyphenol of olive oil, would modify the molecular regulators of glucose uptake when HT is supplemented during exercise. Rats were included into sedentary and exercised (EXE) groups. EXE group was further divided into a group consuming a low HT dose (0.31 mg·kg·d; EXElow), a moderate HT dose (4.61 mg·kg·d; EXEmid), and a control group (EXE). EXE raised glucose transporter type 4 (GLUT4) protein content, Ras-related C3 botulinum toxin substrate 1 (Rac1) activity, and protein kinase b (AKT) phosphorylation in SKM. Furthermore, EXElow blunted GLUT4 protein content and AKT phosphorylation while EXEmid showed a downregulation of the GLUT4/AKT/Rac1 axis. Hence, a low-to-moderate dose of HT, when it is supplemented as an isolated compound, might alter the beneficial effect of training on basal AKT phosphorylation and Rac1 activity in rats.

Moderate, but Not Excessive, Training Attenuates Autophagy Machinery in Metabolic Tissues

The protective effects of chronic moderate exercise-mediated autophagy include the prevention and treatment of several diseases and the extension of lifespan. In addition, physical exercise may impair cellular structures, requiring the action of the autophagy mechanism for clearance and renovation of damaged cellular components. For the first time, we investigated the adaptations on basal autophagy flux in vivo in mice's liver, heart, and skeletal muscle tissues submitted to four different chronic exercise models: endurance, resistance, concurrent, and overtraining. Measuring the autophagy flux in vivo is crucial to access the functionality of the autophagy pathway since changes in this pathway can occur in more than five steps. Moreover, the responses of metabolic, performance, and functional parameters, as well as genes and proteins related to the autophagy pathway, were addressed. In summary, the regular exercise models exhibited normal/enhanced adaptations with reduced autophagy-related proteins in all tissues. On the other hand, the overtrained group presented higher expression of Sqstm1 and Bnip3 with negative morphological and physical performance adaptations for the liver and heart, respectively. The groups showed different adaptions in autophagy flux in skeletal muscle, suggesting the activation or inhibition of basal autophagy may not always be related to improvement or impairment of performance.

Effects of lactate administration on mitochondrial enzyme activity and monocarboxylate transporters in mouse skeletal muscle

Growing evidence shows that lactate is not merely an intermediate metabolite, but also a potential signaling molecule. However, whether daily lactate administration induces physiological adaptations in skeletal muscle remains to be elucidated. In this study, we first investigated the effects of daily lactate administration (equivalent to 1 g/kg of body weight) for 3 weeks on mitochondrial adaptations in skeletal muscle. We demonstrated that 3-week lactate administration increased mitochondrial enzyme activity (citrate synthase, 3-hydroxyacyl CoA dehydrogenase, and cytochrome c oxidase) in the plantaris muscle, but not in the soleus muscle. MCT1 and MCT4 protein contents were not different after 3-week lactate administration. Next, we examined whether lactate administration enhances training-induced adaptations in skeletal muscle. Lactate administration prior to endurance exercise training (treadmill running, 20 m/min, 60 min/day), which increased blood lactate concentration during exercise, enhanced training-induced mitochondrial enzyme activity in the skeletal muscle after 3 weeks. MCT protein content and blood lactate removal were not different after 3-week lactate administration with exercise training compared to exercise training alone. In a single bout experiment, lactate administration did not change the phosphorylation state of AMPK, ACC, p38 MAPK, and CaMKII in skeletal muscle. Our results suggest that lactate can be a key factor for exercise-induced mitochondrial adaptations, and that the efficacy of high-intensity training is, at least partly, attributed to elevated blood lactate concentration.

Low pre-exercise muscle glycogen availability offsets the effect of post-exercise cold water immersion in augmenting PGC-1α gene expression

We assessed the effects of post-exercise cold-water immersion (CWI) in modulating PGC-1α mRNA expression in response to exercise commenced with low muscle glycogen availability. In a randomized repeated-measures design, nine recreationally active males completed an acute two-legged high-intensity cycling protocol (8 × 5 min at 82.5% peak power output) followed by 10 min of two-legged post-exercise CWI (8°C) or control conditions (CON). During each trial, one limb commenced exercise with low (LOW: <300 mmol·kg-1 dw) or very low (VLOW: <150 mmol·kg-1 dw) pre-exercise glycogen concentration, achieved via completion of a one-legged glycogen depletion protocol undertaken the evening prior. Exercise increased (P < 0.05) PGC-1α mRNA at 3 h post-exercise. Very low muscle glycogen attenuated the increase in PGC-1α mRNA expression compared with the LOW limbs in both the control (CON VLOW ~3.6-fold vs. CON LOW ~5.6-fold: P = 0.023, ES 1.22 Large) and CWI conditions (CWI VLOW ~2.4-fold vs. CWI LOW ~8.0 fold: P = 0.019, ES 1.43 Large). Furthermore, PGC-1α mRNA expression in the CWI-LOW trial was not significantly different to the CON LOW limb (P = 0.281, ES 0.67 Moderate). Data demonstrate that the previously reported effects of post-exercise CWI on PGC-1α mRNA expression (as regulated systemically via β-adrenergic mediated cell signaling) are offset in those conditions in which local stressors (i.e., high-intensity exercise and low muscle glycogen availability) have already sufficiently activated the AMPK-PGC-1α signaling axis. Additionally, data suggest that commencing exercise with very low muscle glycogen availability attenuates PGC-1α signaling.

Effects of Royal Jelly Administration on Endurance Training-Induced Mitochondrial Adaptations in Skeletal Muscle

We investigated the effect of royal jelly (RJ), a natural secretion from worker bees, on the endurance training-induced mitochondrial adaptations in skeletal muscles of ICR mice. Mice received either RJ (1.0 mg/g body weight) or distilled water for three weeks. The mice in the training group were subjected to endurance training (20 m/min; 60 min; 5 times/week). There was a main effect of endurance training on the maximal activities of the mitochondrial enzymes, citrate synthase (CS), and β-hydroxyacyl coenzyme Adehydrogenase (β-HAD), in the plantaris and tibialis anterior (TA) muscles, while no effect of RJ treatment was observed. In the soleus muscle, CS and β-HAD maximal activities were significantly increased by endurance training in the RJ-treated group, while there was no effect of training in the control group. Furthermore, we investigated the effects of acute RJ treatment on the signaling cascade involved in mitochondrial biogenesis. In the soleus, phosphorylation of 5′-AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) were additively increased by a single RJ treatment and endurance exercise, while only an exercise effect was found in the plantaris and TA muscles. These results indicate that the RJ treatment induced mitochondrial adaptation with endurance training by AMPK activation in the soleus muscles of ICR mice.

Mitochondrial breakdown in skeletal muscle and the emerging role of the lysosomes

Skeletal muscle mitochondria are essential in providing the energy required for locomotion. In response to contractile activity, the production of mitochondria is upregulated to meet the energy demands placed upon muscle cells. In a coordinated fashion, exercise also promotes the breakdown of dysfunctional mitochondria via mitophagy. Mitophagy is characterized by the selection of poorly functioning organelles, engulfment in an autophagosome and transport to lysosomes for degradation. In addition to the activation of mitophagy, exercise also elevates lysosome biogenesis. This coordinated increase in mitophagy targeting and lysosomal biogenesis serves to enhance the capacity for autophagosomal degradation, thereby aiding in the maintenance of mitochondrial quality. Lysosome dysfunction, as observed in lysosomal storage disorders (LSDs), negatively impacts mitochondrial function likely through the suppression of mitophagy. Since exercise is capable of activating mitophagy and lysosome biogenesis, researchers have begun to investigate physical activity as an effective therapy for LSDs. This review summarizes the current understanding of how mitophagy and lysosomal biogenesis are regulated in exercising skeletal, with potential therapeutic implications.

Regulation of autophagic and mitophagic flux during chronic contractile activity-induced muscle adaptations

Autophagy and mitophagy are important for training-inducible muscle adaptations, yet it remains unclear how these systems are regulated throughout the adaptation process. Here, we studied autophagic and mitophagic flux in the skeletal muscles of Sprague-Dawley rats (300-500 g) exposed to chronic contractile activity (CCA; 3 h/day, 9 V, 10 Hz continuous, 0.1 ms pulse duration) for 1, 2, 5, and 7 days (N = 6-8/group). In order to determine the flux rates, colchicine (COL; 0.4 mg/ml/kg) was injected 48 h before tissue collection, and we evaluated differences of autophagosomal protein abundances (LC3-II and p62) between colchicine- and saline-injected animals. We confirmed that CCA resulted in mitochondrial adaptations, including improved state 3 respiration as early as day 1 in permeabilized muscle fibers, as well significant increases in mitochondrial respiratory capacity and marker proteins in IMF mitochondria by day 7. Mitophagic and autophagic flux (LC3-II and p62) were significantly decreased in skeletal muscle following 7 days of CCA. Notably, the mitophagic system seemed to be downregulated prior (day 3-5) to changes in autophagic flux (day 7), suggesting enhanced sensitivity of mitophagy compared to autophagy with chronic muscle contraction. Although we detected no significant change in the nuclear translocation of TFEB, a regulator of lysosomal biogenesis, CCA increased total TFEB protein, as well as LAMP1, in skeletal muscle. Thus, chronic muscle activity reduces mitophagy in parallel with improved mitochondrial function, and this is supported by enhanced lysosomal degradation capacity.

The Role of p53 in Determining Mitochondrial Adaptations to Endurance Training in Skeletal Muscle

p53 plays an important role in regulating mitochondrial homeostasis. However, it is unknown whether p53 is required for the physiological and mitochondrial adaptations with exercise training. Furthermore, it is also unknown whether impairments in the absence of p53 are a result of its loss in skeletal muscle, or a secondary effect due to its deletion in alternative tissues. Thus, we investigated the role of p53 in regulating mitochondria both basally, and under the influence of exercise, by subjecting C57Bl/6J whole-body (WB) and muscle-specific p53 knockout (mKO) mice to a 6-week training program. Our results confirm that p53 is important for regulating mitochondrial content and function, as well as proteins within the autophagy and apoptosis pathways. Despite an increased proportion of phosphorylated p53 (Ser¹⁵) in the mitochondria, p53 is not required for training-induced adaptations in exercise capacity or mitochondrial content and function. In comparing mouse models, similar directional alterations were observed in basal and exercise-induced signaling modifications in WB and mKO mice, however the magnitude of change was less pronounced in the mKO mice. Our data suggest that p53 is required for basal mitochondrial maintenance in skeletal muscle, but is not required for the adaptive responses to exercise training.

Maximizing Cellular Adaptation to Endurance Exercise in Skeletal Muscle

The application of molecular techniques to exercise biology has provided novel insight into the complexity and breadth of intracellular signaling networks involved in response to endurance-based exercise. Here we discuss several strategies that have high uptake by athletes and, on mechanistic grounds, have the potential to promote cellular adaptation to endurance training in skeletal muscle. Such approaches are based on the underlying premise that imposing a greater metabolic load and provoking extreme perturbations in cellular homeostasis will augment acute exercise responses that, when repeated over months and years, will amplify training adaptation.

Role of Parkin and endurance training on mitochondrial turnover in skeletal muscle

Background

Parkin is a ubiquitin ligase that is involved in the selective removal of dysfunctional mitochondria. This process is termed mitophagy and can assist in mitochondrial quality control. Endurance training can produce adaptations in skeletal muscle toward a more oxidative phenotype, an outcome of enhanced mitochondrial biogenesis. It remains unknown whether Parkin-mediated mitophagy is involved in training-induced increases in mitochondrial content and function. Our purpose was to determine a role for Parkin in maintaining mitochondrial turnover in muscle, and its requirement in mediating mitochondrial biogenesis following endurance exercise training.

Methods

Wild-type and Parkin knockout (KO) mice were trained for 6 weeks and then treated with colchicine or vehicle to evaluate the role of Parkin in mediating changes in mitochondrial content, function and acute exercise-induced mitophagy flux.

Results

Our results indicate that Parkin is required for the basal maintenance of mitochondrial function. The absence of Parkin did not significantly alter mitophagy basally; however, acute exercise produced an elevation in mitophagy flux, a response that was Parkin-dependent. Mitochondrial content was increased following training in both genotypes, but this occurred without an induction of PGC-1α signaling in KO animals. Interestingly, the increased muscle mitochondrial content in response to training did not influence basal mitophagy flux, despite an enhanced expression and localization of Parkin to mitochondria in WT animals. Furthermore, exercise-induced mitophagy flux was attenuated with training in WT animals, suggesting a lower rate of mitochondrial degradation resulting from improved organelle quality with training. In contrast, training led to a higher mitochondrial content, but with persistent dysfunction, in KO animals. Thus, the lack of a rescue of mitochondrial dysfunction with training in the absence of Parkin is the likely reason for the impaired training-induced attenuation of mitophagy flux compared to WT animals.

Conclusions

Our study demonstrates that Parkin is required for exercise-induced mitophagy flux. Exercise-induced mitophagy is reduced with training in muscle, likely due to attenuated signaling consequent to increased mitochondrial content and quality. Our data suggest that Parkin is essential for the maintenance of basal mitochondrial function, as well as for the accumulation of normally functioning mitochondria as a result of training adaptations in muscle.

Effect of contractile activity on PGC-1α transcription in young and aged skeletal muscle

Mitochondrial impairments are often noted in aged skeletal muscle. The transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) is integral to maintaining mitochondria, and its expression declines in aged muscle. It remains unknown whether this is due to a transcriptional deficit during aging. Our study examined PGC-1α transcription in muscle from young and old F344BN rats. Using a rat PGC-1α promoter-reporter construct, we found that PGC-1α transcription was reduced by ∼65% in aged TA muscle, accompanied by decreases in PGC-1α mRNA and transcript stability. Altered expression patterns in PGC-1α transcription regulatory factors, including nuclear respiratory factor 2, upstream transcription factor 1, activating transcription factor 2, and yin yang 1, were noted in aged muscle. Acute contractile activity (CA) followed by recovery was employed to examine whether PGC-1α transcription could be activated in aged muscle similar to that observed in young muscle. AMPK and p38 signaling was attenuated in aged muscle. CA evoked an upregulation of PGC-1α transcription in both young and aged groups, whereas mRNAs encoding PGC-1α and cytochrome oxidase subunit IV were induced during the recovery period. Global DNA methylation, an inhibitory event for transcription, was enhanced in aged muscle, likely a result of elevated methyltransferase enzyme Dnmt3b in aged muscle. Successive bouts of CA for 7 days to evaluate longer-term consequences resulted in a rescue of PGC-1α and downstream mRNAs in aged muscle. Our data indicate that diminished mitochondria in aged muscle is due partly to a deficit in PGC-1α transcription, a result of attenuated upstream signaling. Contractile activity is an appropriate countermeasure to restore PGC-1α expression and mitochondrial content in aged muscle. NEW & NOTEWORTHY PGC-1α is a regulator of mitochondrial biogenesis in muscle. We demonstrate that PGC-1α expression is reduced in aging muscle due to decreases in transcriptional and posttranscriptional mechanisms. The transcriptional deficit is due to alterations in transcription factor expression, reduced signaling, and DNA methylation. Acute exercise can initiate signaling to reverse the transcriptional defect, restoring PGC-1α expression toward young values, suggesting a mechanism whereby aged muscle can respond to exercise for the promotion of mitochondrial biogenesis.

Exercise-induced skeletal muscle signaling pathways and human athletic performance

Skeletal muscle is a highly malleable tissue capable of altering its phenotype in response to external stimuli including exercise. This response is determined by the mode, (endurance- versus resistance-based), volume, intensity and frequency of exercise performed with the magnitude of this response-adaptation the basis for enhanced physical work capacity. However, training-induced adaptations in skeletal muscle are variable and unpredictable between individuals. With the recent application of molecular techniques to exercise biology, there has been a greater understanding of the multiplicity and complexity of cellular networks involved in exercise responses. This review summarizes the molecular and cellular events mediating adaptation processes in skeletal muscle in response to exercise. We discuss established and novel cell signaling proteins mediating key physiological responses associated with enhanced exercise performance and the capacity for reactive oxygen and nitrogen species to modulate training adaptation responses. We also examine the molecular bases underpinning heterogeneous responses to resistance and endurance exercise and the dissociation between molecular 'markers' of training adaptation and subsequent exercise performance.

Exercise Promotes Healthy Aging of Skeletal Muscle

Primary aging is the progressive and inevitable process of bodily deterioration during adulthood. In skeletal muscle, primary aging causes defective mitochondrial energetics and reduced muscle mass. Secondary aging refers to additional deleterious structural and functional age-related changes caused by diseases and lifestyle factors. Secondary aging can exacerbate deficits in mitochondrial function and muscle mass, concomitant with the development of skeletal muscle insulin resistance. Exercise opposes deleterious effects of secondary aging by preventing the decline in mitochondrial respiration, mitigating aging-related loss of muscle mass and enhancing insulin sensitivity. This review focuses on mechanisms by which exercise promotes "healthy aging" by inducing modifications in skeletal muscle.

Mitochondria, muscle health, and exercise with advancing age

Skeletal muscle health is dependent on the optimal function of its mitochondria. With advancing age, decrements in numerous mitochondrial variables are evident in muscle. Part of this decline is due to reduced physical activity, whereas the remainder appears to be attributed to age-related alterations in mitochondrial synthesis and degradation. Exercise is an important strategy to stimulate mitochondrial adaptations in older individuals to foster improvements in muscle function and quality of life.

Ramping up the signal: promoting endurance training adaptation in skeletal muscle by nutritional manipulation

Mitochondrial biogenesis in skeletal muscle results from the cumulative effect of transient increases in mRNA transcripts encoding mitochondrial proteins in response to repeated exercise sessions. This process requires the coordinated expression of both nuclear and mitochondrial (mt) DNA genomes and is regulated, for the most part, by the peroxisome proliferator-activated receptor γ coactivator 1α. Several other exercise-inducible proteins also play important roles in promoting an endurance phenotype, including AMP-activated protein kinase, p38 mitogen-activated protein kinase and tumour suppressor protein p53. Commencing endurance-based exercise with low muscle glycogen availability results in greater activation of many of these signalling proteins compared with when the same exercise is undertaken with normal glycogen concentration, suggesting that nutrient availability is a potent signal that can modulate the acute cellular responses to a single bout of exercise. When exercise sessions are repeated in the face of low glycogen availability (i.e. chronic training), the phenotypic adaptations resulting from such interventions are also augmented.

Exercise improves mitochondrial and redox-regulated stress responses in the elderly: better late than never!

Ageing is associated with several physiological declines to both the cardiovascular (e.g. reduced aerobic capacity) and musculoskeletal system (muscle function and mass). Ageing may also impair the adaptive response of skeletal muscle mitochondria and redox-regulated stress responses to an acute exercise bout, at least in mice and rodents. This is a functionally important phenomenon, since (1) aberrant mitochondrial and redox homeostasis are implicated in the pathophysiology of musculoskeletal ageing and (2) the response to repeated exercise bouts promotes exercise adaptations and some of these adaptations (e.g. improved aerobic capacity and exercise-induced mitochondrial remodelling) offset age-related physiological decline. Exercise-induced mitochondrial remodelling is mediated by upstream signalling events that converge on downstream transcriptional co-factors and factors that orchestrate a co-ordinated nuclear and mitochondrial transcriptional response associated with mitochondrial remodelling. Recent translational human investigations have demonstrated similar exercise-induced mitochondrial signalling responses in older compared with younger skeletal muscle, regardless of training status. This is consistent with data indicating normative mitochondrial remodelling responses to long-term exercise training in the elderly. Thus, human ageing is not accompanied by diminished mitochondrial plasticity to acute and chronic exercise stimuli, at least for the signalling pathways measured to date. Exercise-induced increases in reactive oxygen and nitrogen species promote an acute redox-regulated stress response that manifests as increased heat shock protein and antioxidant enzyme content. In accordance with previous reports in rodents and mice, it appears that sedentary ageing is associated with a severely attenuated exercise-induced redox stress response that might be related to an absent redox signal. In this regard, regular exercise training affords some protection but does not completely override age-related defects. Despite some failed redox-regulated stress responses, it seems mitochondrial responses to exercise training are intact in skeletal muscle with age and this might underpin the protective effect of exercise training on age-related musculoskeletal decline. Whilst further investigation is required, recent data suggest that it is never too late to begin exercise training and that lifelong training provides protection against several age-related declines at both the molecular (e.g. reduced mitochondrial function) and whole-body level (e.g. aerobic capacity).

The order of concurrent endurance and resistance exercise modifies mTOR signaling and protein synthesis in rat skeletal muscle

Concurrent training, a combination of endurance (EE) and resistance exercise (RE) performed in succession, may compromise the muscle hypertrophic adaptations induced by RE alone. However, little is known about the molecular signaling interactions underlying the changes in skeletal muscle adaptation during concurrent training. Here, we used an animal model to investigate whether EE before or after RE affects the molecular signaling associated with muscle protein synthesis, specifically the interaction between RE-induced mammalian target of rapamycin complex 1 (mTORC1) signaling and EE-induced AMP-activated protein kinase (AMPK) signaling. Male Sprague-Dawley rats were divided into five groups: an EE group (treadmill, 25 m/min, 60 min), an RE group (maximum isometric contraction via percutaneous electrical stimulation for 3 × 10 s, 5 sets), an EE before RE group, an EE after RE group, and a nonexercise control group. Phosphorylation of p70S6K, a marker of mTORC1 activity, was significantly increased 3 h after RE in both the EE before RE and EE after RE groups, but the increase was smaller in latter. Furthermore, protein synthesis was greatly increased 6 h after RE in the EE before RE group. Increases in the phosphorylation of AMPK and Raptor were observed only in the EE after RE group. Akt and mTOR phosphorylation were increased in both groups, with no between-group differences. Our results suggest that the last bout of exercise dictates the molecular responses and that mTORC1 signaling induced by any prior bout of RE may be downregulated by a subsequent bout of EE.

Exercise training enhances white adipose tissue metabolism in rats selectively bred for low- or high-endurance running capacity

Impaired visceral white adipose tissue (WAT) metabolism has been implicated in the pathogenesis of several lifestyle-related disease states, with diminished expression of several WAT mitochondrial genes reported in both insulin-resistant humans and rodents. We have used rat models selectively bred for low- (LCR) or high-intrinsic running capacity (HCR) that present simultaneously with divergent metabolic phenotypes to test the hypothesis that oxidative enzyme expression is reduced in epididymal WAT from LCR animals. Based on this assumption, we further hypothesized that short-term exercise training (6 wk of treadmill running) would ameliorate this deficit. Approximately 22-wk-old rats (generation 22) were studied. In untrained rats, the abundance of mitochondrial respiratory complexes I-V, citrate synthase (CS), and PGC-1 was similar for both phenotypes, although CS activity was greater than 50% in HCR (P = 0.09). Exercise training increased CS activity in both phenotypes but did not alter mitochondrial protein content. Training increased the expression and phosphorylation of proteins with roles in β-adrenergic signaling, including β3-adrenergic receptor (16% increase in LCR; P < 0.05), NOR1 (24% decrease in LCR, 21% decrease in HCR; P < 0.05), phospho-ATGL (25% increase in HCR; P < 0.05), perilipin (25% increase in HCR; P < 0.05), CGI-58 (15% increase in LCR; P < 0.05), and GLUT4 (16% increase in HCR; P < 0.0001). A training effect was also observed for phospho-p38 MAPK (12% decrease in LCR, 20% decrease in HCR; P < 0.05) and phospho-JNK (29% increase in LCR, 20% increase in HCR; P < 0.05). We conclude that in the LCR-HCR model system, mitochondrial protein expression in WAT is not affected by intrinsic running capacity or exercise training. However, training does induce alterations in the activity and expression of several proteins that are essential to the intracellular regulation of WAT metabolism.

Acute exercise induces tumour suppressor protein p53 translocation to the mitochondria and promotes a p53-Tfam-mitochondrial DNA complex in skeletal muscle

The major tumour suppressor protein p53 plays an important role in maintaining mitochondrial content and function in skeletal muscle. p53 has been shown to reside in the mitochondria complexed with mitochondrial DNA (mtDNA); however, the physiological repercussions of mitochondrial p53 remain unknown. We endeavoured to elucidate whether an acute bout of endurance exercise could mediate an increase in mitochondrial p53 levels. C57Bl6 mice (n = 6 per group) were randomly assigned to sedentary, acute exercise (AE, 15 m min(-1) for 90 min) or acute exercise + 3 h recovery (AER) groups. Exercise concomitantly increased the mRNA content of nuclear-encoded (PGC-1α, Tfam, NRF-1, COX-IV, citrate synthase) and mtDNA-encoded (COX-I) genes in the AE group, and further by ∼5-fold in the AER group. Nuclear p53 protein levels were reduced in the AE and AER groups, while in contrast, the abundance of p53 was drastically enhanced by ∼2.4-fold and ∼3.9-fold in subsarcolemmal and intermyofibrillar mitochondria, respectively, in the AER conditions. Within the mitochondria, the interaction of p53 with mtDNA at the D-loop and with Tfam was elevated by ∼4.6-fold and ∼3.6-fold, respectively, in the AER group. In the absence of p53, the enhanced COX-I mRNA content observed with AE and AER was abrogated. This study is the first to indicate that endurance exercise can signal to localize p53 to the mitochondria where it may serve to positively modulate the activity of the mitochondrial transcription factor Tfam. Our findings help us understand the mechanisms underlying the effects of exercise as a therapeutic intervention designed to trigger the pro-metabolic functions of p53.

mTOR signaling response to resistance exercise is altered by chronic resistance training and detraining in skeletal muscle

Resistance training-induced muscle anabolism and subsequent hypertrophy occur most rapidly during the early phase of training and become progressively slower over time. Currently, little is known about the intracellular signaling mechanisms underlying changes in the sensitivity of muscles to training stimuli. We investigated the changes in the exercise-induced phosphorylation of hypertrophic signaling proteins during chronic resistance training and subsequent detraining. Male rats were divided into four groups: 1 bout (1B), 12 bouts (12B), 18 bouts (18B), and detraining (DT). In the DT group, rats were subjected to 12 exercise sessions, detrained for 12 days, and then were subjected to 1 exercise session before being killed. Isometric training consisted of maximum isometric contraction, which was produced by percutaneous electrical stimulation of the gastrocnemius muscle every other day. Muscles were removed 24 h after the final exercise session. Levels of total and phosphorylated p70S6K, 4E-BP1, rpS6, and p90RSK levels were measured, and phosphorylation of p70S6K, rpS6, and p90RSK was elevated in the 1B group compared with control muscle (CON) after acute resistance exercise, whereas repeated bouts of exercise suppressed those phosphorylation in both 12B and 18B groups. Interestingly, these phosphorylation levels were restored after 12 days of detraining in the DT group. On the contrary, phosphorylation of 4E-BP1 was not altered with chronic training and detraining, indicating that, with chronic resistance training, anabolic signaling becomes less sensitive to resistance exercise stimuli but is restored after a short detraining period.

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.

One size may not fit all: anti-aging therapies and sarcopenia

Sarcopenia refers to age-related loss of muscle mass and function. Several age-related changes occur in skeletal muscle including a decrease in myofiber size and number and a diminished ability of satellite cells to activate and proliferate upon injury leading to impaired muscle remodeling. Although the molecular mechanisms underlying sarcopenia are unknown, it is tempting to hypothesize that interplay between biological and environmental factors cooperate in a positive feedback cycle contributing to the progression of sarcopenia. Indeed many essential biological mechanisms such as apoptosis and autophagy and critical signaling pathways involved in skeletal muscle homeostasis are altered during aging and have been linked to loss of muscle mass. Moreover, the environmental effects of the sedentary lifestyle of older people further promote and contribute the loss of muscle mass. There are currently no widely accepted therapeutic strategies to halt or reverse the progression of sarcopenia. Caloric restriction has been shown to be beneficial as a sarcopenia and aging antagonist. Such results have made the search for caloric restriction mimetics (CRM) a priority. However given the mechanisms of action, some of the currently investigated CRMs may not combat sarcopenia. Thus, sarcopenia may represent a unique phenotypic feature of aging that requires specific and individually tailored therapeutic strategies.

EMG-normalised kinase activation during exercise is higher in human gastrocnemius compared to soleus muscle

In mice, certain proteins show a highly confined expression in specific muscle groups. Also, resting and exercise/contraction-induced phosphorylation responses are higher in rat skeletal muscle with low mitochondrial content compared to muscles with high mitochondrial content, possibly related to differential reactive oxygen species (ROS)-scavenging ability or resting glycogen content. To evaluate these parameters in humans, biopsies from soleus, gastrocnemius and vastus lateralis muscles were taken before and after a 45 min inclined (15%) walking exercise bout at 69% VO2_max aimed at simultaneously activating soleus and gastrocnemius in a comparable dynamic work-pattern. Hexokinase II and GLUT4 were 46–59% and 26–38% higher (p<0.05) in soleus compared to the two other muscles. The type I muscle fiber percentage was highest in soleus and lowest in vastus lateralis. No differences were found in protein expression of signalling proteins (AMPK subunits, eEF2, ERK1/2, TBC1D1 and 4), mitochondrial markers (F1 ATPase and COX1) or ROS-handling enzymes (SOD2 and catalase). Gastrocnemius was less active than soleus measured as EMG signal and glycogen use yet gastrocnemius displayed larger increases than soleus in phosphorylation of AMPK Thr172, eEF2 Thr56 and ERK 1/2 Thr202/Tyr204 when normalised to the mean relative EMG-signal. In conclusion, proteins with muscle-group restricted expression in mice do not show this pattern in human lower extremity muscle groups. Nonetheless the phosphorylation-response is greater for a number of kinase signalling pathways in human gastrocnemius than soleus at a given activation-intensity. This may be due to the combined subtle effects of a higher type I muscle fiber content and higher training status in soleus compared to gastrocnemius muscle.

Matched work high-intensity interval and continuous running induce similar increases in PGC-1α mRNA, AMPK, p38, and p53 phosphorylation in human skeletal muscle

The aim of the present study was to test the hypothesis that acute high-intensity interval (HIT) running induces greater activation of signaling pathways associated with mitochondrial biogenesis compared with moderate-intensity continuous (CONT) running matched for work done. In a repeated-measures design, 10 active men performed two running protocols consisting of HIT [6 × 3-min at 90% maximal oxygen consumption (Vo(2max)) interspersed with 3-min recovery periods at 50% Vo(2max) with a 7-min warm-up and cool-down period at 70% Vo(2max)] or CONT (50-min continuous running at 70% Vo(2max)). Both protocols were matched, therefore, for average intensity, duration, and distance run. Muscle biopsies (vastus lateralis) were obtained preexercise, postexercise, and 3 h postexercise. Muscle glycogen decreased (P < 0.05) similarly in HIT and CONT (116 ± 11 vs. 111 ± 17 mmol/kg dry wt, respectively). Phosphorylation (P-) of p38MAPK(Thr180/Tyr182) (1.9 ± 0.1- vs. 1.5 ± 0.2-fold) and AMPK(Thr172) (1.5 ± 0.3- vs. 1.5 ± 0.1-fold) increased immediately postexercise (P < 0.05) in HIT and CONT, respectively, and returned to basal levels at 3 h postexercise. P-p53(Ser15) (HIT, 2.7 ± 0.8-fold; CONT, 2.1 ± 0.8-fold), PGC-1α mRNA (HIT, 4.2 ± 1.7-fold; CONT, 4.5 ± 0.9-fold) and HSP72 mRNA (HIT, 4.4 ± 2-fold; CONT, 3.5 ± 1-fold) all increased 3 h postexercise (P < 0.05) although neither parameter increased (P > 0.05) immediately postexercise. There was no difference between trials for any of the above signaling or gene expression responses (P > 0.05). We provide novel data by demonstrating that acute HIT and CONT running (when matched for average intensity, duration, and work done) induces similar activation of molecular signaling pathways associated with regulation of mitochondrial biogenesis. Furthermore, this is the first report of contraction-induced p53 phosphorylation in human skeletal muscle, thus highlighting an additional pathway by which exercise may initiate mitochondrial biogenesis.

Chronic AMPK stimulation attenuates adaptive signaling in dystrophic skeletal muscle

In the present study, we evaluated how a pharmacologically induced phenotype shift in dystrophic skeletal muscle would affect subsequent intracellular signaling in response to a complementary, adaptive physiological stimulus. mdx mice were treated with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR; 500 mg·kg(-1)·day(-1)) for 30 days, and then one-half of the animals were subjected to a bout of treadmill running to induce acute AMPK and p38 MAPK signaling. The mRNA levels of phenotypic modifiers, including peroxisome proliferator-activated receptor-δ (PPARδ), PPARγ coactivator-1α (PGC-1α), receptor interacting protein 140 (RIP 140), and silent information regulator two ortholog 1 (SIRT1) were assessed in skeletal muscle, as well as the expression of the protein arginine methyltransferase genes PRMT1 and CARM1. We found unique AMPK and p38 phosphorylation and expression signatures between dystrophic and healthy muscle. In dystrophic skeletal muscle, treadmill running induced PPARδ, PGC-1α, and SIRT1 mRNAs, three molecules that promote the slow, oxidative myogenic program. In the mdx animals that received the chronic AICAR treatment, running-elicited AMPK and p38 phosphorylation was attenuated compared with vehicle-treated mice. Similarly, acute stress-evoked expression of PPARδ, PGC-1α, and SIRT1 was also blunted by chronic pharmacological AMPK stimulation. Skeletal muscle PRMT1 and CARM1 protein contents were higher in mdx mice compared with wild-type littermates. The acute running-evoked induction of PRMT1 and CARM1 mRNAs was also attenuated by the AICAR treatment. Our data demonstrate that prior pharmacological conditioning is a salient determinant in how dystrophic muscle adapts to subsequent complementary, acute physiological stress stimuli. These results provide insight into possible therapeutic applications of synthetic agonists in neuromuscular diseases, such as during chronic administration to Duchenne muscular dystrophy patients.

Carbohydrate availability and training adaptation: effects on cell metabolism

Several markers of endurance training adaptation are enhanced to a greater extent when individuals undertake selected training sessions with low compared with normal muscle glycogen content or with low exogenous carbohydrate availability. The potential mechanisms underlying the cellular responses arising from such nutrient-exercise interactions are discussed in the context of promoting training adaptation.

Effect of chronic contractile activity on mRNA stability in skeletal muscle

Repeated bouts of exercise promote the biogenesis of mitochondria by multiple steps in the gene expression patterning. The role of mRNA stability in controlling the expression of mitochondrial proteins is relatively unexplored. To induce mitochondrial biogenesis, we chronically stimulated (10 Hz; 3 or 6 h/day) rat muscle for 7 days. Chronic contractile activity (CCA) increased the protein expression of PGC-1alpha, c-myc, and mitochondrial transcription factor A (Tfam) by 1.6-, 1.7- and 2.0-fold, respectively. To determine mRNA stability, we incubated total RNA with cytosolic extracts using an in vitro cell-free system. We found that the intrinsic mRNA half-lives (t(1/2)) were variable within control muscle. Peroxisome proliferator-activated receptor-gamma, coactivator-1alpha (PGC-1alpha) and Tfam mRNAs decayed more rapidly (t(1/2) = 22.7 and 31.4 min) than c-myc mRNA (t(1/2) = 99.7 min). Furthermore, CCA resulted in a differential response in degradation kinetics. After CCA, PGC-1alpha and Tfam mRNA half-lives decreased by 48% and 44%, respectively, whereas c-myc mRNA half-life was unchanged. CCA induced an elevation of both the cytosolic RNA-stabilizing human antigen R (HuR) and destabilizing AUF1 (total) by 2.4- and 1.8-fold, respectively. Increases in the p37(AUF1), p40(AUF1), and p45(AUF1) isoforms were most evident. Thus these data indicate that CCA results in accelerated turnover rates of mRNAs encoding important mitochondrial biogenesis regulators in skeletal muscle. This adaptation is likely beneficial in permitting more rapid phenotypic plasticity in response to subsequent contractile activity.