High-intensity interval training increases SIRT1 activity in human skeletal muscle

Rugby-specific small-sided games training is an effective alternative to stationary cycling at reducing clinical risk factors associated with the development of type 2 diabetes: a randomized, controlled trial

INTRODUCTION

The present study investigated whether rugby small-sided games (SSG) could be an effective alternative to continuous stationary cycling (CYC) training at reducing clinical risk factors associated with the development of type 2 diabetes mellitus (T2DM).

METHODS

Thirty-three middle-aged (48.6±6.6y), inactive men were randomized into a CYC (n=11), SSG (n=11), or control (CON, n=11) group. Participants trained 3d·wk(-1) for 8 weeks, while control participants maintained normal activity and dietary patterns. Exercise duration was matched between groups, which involved CYC or SSG (four quarters, interspersed with 2-min passive recovery). Both training programs were designed to induce similar internal loads of maximal heart rate (~80-85%HRmax) and rating of perceived exertion. Pre- and post-intervention testing included dual-energy x-ray absorptiometry scan, graded exercise test, fasting 2 h oral glucose tolerance test and resting muscle biopsy. Western blotting was used to assess the content of skeletal muscle proteins associated with mitochondrial biogenesis and glucose regulation.

RESULTS

Both CYC and SSG increased VO2 at 80%HRmax, and reduced glycated haemoglobin, glucose area under the curve (AUC; SSG, -2.3±2.4; CYC -2.2±1.6 mmol·L(1)(120 min)(1); p<0.05), and total body fat-mass (SSG -2.6±0.9%; CYC -2.9±1.1%), compared to no change in CON (p<0.05). SSG reduced insulin AUC (-30.4±40.7 µlU·mL(1)(120 min)(1); p<0.05) and increased total body fat-free mass (1.1±1.2 kg; p<0.05), with no change in CYC or CON (P>0.05). There were no differences within or between conditions for protein content of peroxisome proliferator-activated receptor gamma coactivator-1α, sirtuin-1, p53, glucose transporter-4, protein kinase AKT/PKB, myocyte enhancer factor 2A, mitochondrial transcription factor, nuclear respiratory factor (NRF)-1, NRF-2 or mitochondrial complexes I-V (p>0.05).

CONCLUSION

Rugby small-sided games is an effective alternative to continuous cycling for improving metabolic risk-factors associated with the prevention of T2DM. Despite such positive adaptations in clinical risk factors, there were no changes in the content of skeletal muscle proteins associated with glucose regulation and mitochondrial biogenesis.

TRIAL REGISTRATION

Australian New Zealand Clinical Trial Registry ACTRN12613000874718.

AMP-activated protein kinase controls exercise training- and AICAR-induced increases in SIRT3 and MnSOD

The mitochondrial protein deacetylase sirtuin (SIRT) 3 may mediate exercise training-induced increases in mitochondrial biogenesis and improvements in reactive oxygen species (ROS) handling. We determined the requirement of AMP-activated protein kinase (AMPK) for exercise training-induced increases in skeletal muscle abundance of SIRT3 and other mitochondrial proteins. Exercise training for 6.5 weeks increased SIRT3 (p < 0.01) and superoxide dismutase 2 (MnSOD; p < 0.05) protein abundance in quadriceps muscle of wild-type (WT; n = 13–15), but not AMPK α2 kinase dead (KD; n = 12–13) mice. We also observed a strong trend for increased MnSOD abundance in exercise-trained skeletal muscle of healthy humans (p = 0.051; n = 6). To further elucidate a role for AMPK in mediating these effects, we treated WT (n = 7–8) and AMPK α2 KD (n = 7–9) mice with 5-amino-1-β-D-ribofuranosyl-imidazole-4-carboxamide (AICAR). Four weeks of daily AICAR injections (500 mg/kg) resulted in AMPK-dependent increases in SIRT3 (p < 0.05) and MnSOD (p < 0.01) in WT, but not AMPK α2 KD mice. We also tested the effect of repeated AICAR treatment on mitochondrial protein levels in mice lacking the transcriptional coactivator peroxisome proliferator-activated receptor γ-coactivator 1α (PGC-1α KO; n = 9–10). Skeletal muscle SIRT3 and MnSOD protein abundance was reduced in sedentary PGC-1α KO mice (p < 0.01) and AICAR-induced increases in SIRT3 and MnSOD protein abundance was only observed in WT mice (p < 0.05). Finally, the acetylation status of SIRT3 target lysine residues on MnSOD (K122) or oligomycin-sensitivity conferring protein (OSCP; K139) was not altered in either mouse or human skeletal muscle in response to acute exercise. We propose an important role for AMPK in regulating mitochondrial function and ROS handling in skeletal muscle in response to exercise training.

The effect of SIRT1 protein knock down on PGC-1α acetylation during skeletal muscle contraction

[Purpose]

The purpose of this study was to investigate the effect of Sirtuin 1 (SIRT1) and General control nonderepressible 5 (GCN5) knock down on peroxisome proliferator- activated receptor gamma coactivator 1-alpha (PGC-1α) deacetylation during electrical stimulated skeletal muscle contraction.

[Methods]

Skeletal muscle primary cell were isolated from C57BL/6 mice gastrocnemius and transfected lentiviral SIRT1 and GCN5 shRNA. Knock downed muscle cell were stimulated by electrical stimulation (1Hz, 3min) and collected for PGC-1α deceatylation assays. Immunoprecipitation performed for PGC-1α deacetylation, acetyl-lysine level was measured.

[Results]

Our resulted showed SIRT1 knock down not influenced to PGC-1α deacetylation during electrical stimulation induced muscle contraction while GCN5 knock down decreased PGC-1α deacetylation significantly (p<0.05).

[Conclusion]

This study can be concluded that GCN5 is a critical factor for muscle contraction induced PGC-1α deacetylation.

Two weeks of reduced-volume sprint interval or traditional exercise training does not improve metabolic functioning in sedentary obese men

AIMS

To investigate the effects of short-term, reduced-volume sprint interval training (SIT) compared to traditional exercise recommendations (TER) in sedentary obese men.

METHODS

Sixteen subjects [37.8 ± 5.8 years; body mass index (BMI) 32.8 ± 4.7 kg/m(2)] were randomly allocated to 2 weeks of either SIT (6 sessions of 8-12 × 10 s sprints) or TER [10 sessions of 30 min at 65% peak oxygen consumption (VO(2peak))] cycle exercise. Fasting plasma glucose, insulin, non-esterified fatty acids (NEFA), homeostasis model assessment of insulin sensitivity (HOMA-IR), body composition and VO(2peak) were assessed at baseline and approximately 72 h after the final training bout. Skeletal muscle biopsy samples were also obtained before and 72 h after training and analysed for AS160 phosphorylation and COX II, COX IV, GLUT-4, Nur77 and SIRT1 protein expression.

RESULTS

No changes in BMI, body composition, VO(2peak), glucose, insulin, NEFA and HOMA-IR were observed after training, either within or between groups. Skeletal muscle markers of glucose metabolism and mitochondrial function also remained unaltered after 2 weeks of exercise training.

CONCLUSIONS

Our findings show that 2 weeks of reduced-volume SIT or TER did not elicit any measurable metabolic adaptations in sedentary obese men. Further work is needed to determine the minimal amount of exercise required for short-term adaptations in this population.

VO2max trainability and high intensity interval training in humans: a meta-analysis

Endurance exercise training studies frequently show modest changes in VO₂max with training and very limited responses in some subjects. By contrast, studies using interval training (IT) or combined IT and continuous training (CT) have reported mean increases in VO₂max of up to ∼1.0 L · min⁻¹. This raises questions about the role of exercise intensity and the trainability of VO₂max. To address this topic we analyzed IT and IT/CT studies published in English from 1965–2012. Inclusion criteria were: 1)≥3 healthy sedentary/recreationally active humans <45 yrs old, 2) training duration 6–13 weeks, 3) ≥3 days/week, 4) ≥10 minutes of high intensity work, 5) ≥1∶1 work/rest ratio, and 6) results reported as mean ± SD or SE, ranges of change, or individual data. Due to heterogeneity (I² value of 70), statistical synthesis of the data used a random effects model. The summary statistic of interest was the change in VO₂max. A total of 334 subjects (120 women) from 37 studies were identified. Participants were grouped into 40 distinct training groups, so the unit of analysis was 40 rather than 37. An increase in VO₂max of 0.51 L ·min⁻¹ (95% CI: 0.43 to 0.60 L · min⁻¹) was observed. A subset of 9 studies, with 72 subjects, that featured longer intervals showed even larger (∼0.8–0.9 L · min⁻¹) changes in VO₂max with evidence of a marked response in all subjects. These results suggest that ideas about trainability and VO₂max should be further evaluated with standardized IT or IT/CT training programs.

Dissociation of increases in PGC-1α and its regulators from exercise intensity and muscle activation following acute exercise

Muscle activation as well as changes in peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) following high-intensity interval exercise (HIIE) were examined in young healthy men (n  = 8; age, 21.9±2.2 yrs; VO2peak, 53.1±6.4 ml/min/kg; peak work rate, 317±23.5 watts). On each of 3 visits HIIE was performed on a cycle ergometer at a target intensity of 73, 100, or 133% of peak work rate. Muscle biopsies were taken at rest and three hours after each exercise condition. Total work was not different between conditions (∼730 kJ) while average power output (73%, 237±21; 100%, 323±26; 133%, 384±35 watts) and EMG derived muscle activation (73%, 1262±605; 100%, 2089±737; 133%, 3029±1206 total integrated EMG per interval) increased in an intensity dependent fashion. PGC-1α mRNA was elevated after all three conditions (p<0.05), with a greater increase observed following the 100% condition (∼9 fold, p<0.05) compared to both the 73 and 133% conditions (∼4 fold). When expressed relative to muscle activation, the increase in PGC-1α mRNA for the 133% condition was less than that for the 73 and 100% conditions (p<0.05). SIRT1 mRNA was also elevated after all three conditions (∼1.4 fold, p<0.05), with no difference between conditions. These findings suggest that intensity-dependent increases in PGC-1α mRNA following submaximal exercise are largely due to increases in muscle recruitment. As well, the blunted response of PGC-1α mRNA expression following supramaximal exercise may indicate that signalling mediated activation of PGC-1α may also be blunted. We also indentify that increases in PDK4, SIRT1, and RIP140 mRNA following acute exercise are dissociated from exercise intensity and muscle activation, while increases in EGR1 are augmented with supramaximal HIIE (p<0.05).

Exercise training increases the expression and nuclear localization of mRNA destabilizing proteins in skeletal muscle

While a paucity of information exists regarding posttranscriptional mechanisms influencing mitochondrial biogenesis, in resting muscle the stability of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) mRNA has been linked to mitochondrial content. Therefore, in the current study we have examined whether exercise promotes mRNA accumulation through the induction of proteins affiliated with mRNA stabilization (human antigen R, HuR) or conversely by decreasing the expression of mRNA destabilizing proteins [AU-rich binding factor (AUF1) and CUG binding protein (CUG-BP1)]. A single bout of exercise increased (P < 0.05) the mRNA content of the transcriptional coactivator PGC-1α ∼3.5-fold without affecting mRNA content for HuR, CUG-BP1, or AUF1. One week of treadmill exercise training did not alter markers of mitochondrial content, the mRNA stabilizing protein HuR, or the mRNA destabilizing protein AUF1. In contrast, the mRNA destabilizing protein CUG-BP1 increased ∼40%. Four weeks of treadmill training increased the content of subunits of the electron transport chain ∼50%, suggesting induction of mitochondrial biogenesis. Expression levels for HuR and CUG-BP1 were not altered with chronic training; however, AUF1 expression was increased posttraining. Specifically, training increased (P < 0.05) total muscle expression of two of four AUF1 isoforms ∼50% (AUF1(p37), AUF1(p40)). Interestingly, these two isoforms were not detected in isolated nuclei; however, a large band representing the other two isoforms (AUF1(p42), AUF1(p45)) was present in nuclei and increased ∼35% following chronic training. Altogether the current data provides evidence that mitochondrial biogenesis occurs in the presence of increased CUG-BP1 and AUF1, suggesting that reductions in known mRNA destabilizing proteins likely does not contribute to exercise-induced mitochondrial biogenesis.

Reducing the intensity and volume of interval training diminishes cardiovascular adaptation but not mitochondrial biogenesis in overweight/obese men

OBJECTIVE

The purpose of this research was to determine if the adaptations to high intensity interval training (HIT) are mitigated when both intensity and training volume (i.e. exercise energy expenditure) are reduced.

METHODS

19 overweight/obese, sedentary males (Age: 22.7±3.9 yrs, Body Mass Index: 31.4±2.6 kg/m(2), Waist Circumference: 106.5±6.6 cm) performed 9 sessions of interval training using a 1-min on, 1-min off protocol on a cycle ergometer over three weeks at either 70% (LO) or 100% (HI) peak work rate.

RESULTS

Cytochrome oxidase I protein content, cytochrome oxidase IV protein content, and citrate synthase maximal activity all demonstrated similar increases between groups with a significant effect of training for each. β-hydroxyacyl-CoA dehydrogenase maximal activity tended to increase with training but did not reach statistical significance (p = 0.07). Peroxisome proliferator-activated receptor gamma coactivator-1α and silent mating type information regulator 2 homolog 1 protein contents also increased significantly (p = 0.047), while AMP-activated protein kinase protein content decreased following the intervention (p = 0.019). VO2peak increased by 11.0±7.4% and 27.7±4.4% in the LO and HI groups respectively with significant effects of both training (p<0.001) and interaction (p = 0.027). Exercise performance improved by 8.6±7.6% in the LO group and 14.1±4.3% in the HI group with a significant effect of training and a significant difference in the improvement between groups. There were no differences in perceived enjoyment or self-efficacy between groups despite significantly lower affect scores during training in the HI group.

CONCLUSIONS

While improvements in aerobic capacity and exercise performance were different between groups, changes in oxidative capacity were similar despite reductions in both training intensity and volume.

Sirtuins: the molecular basis of beneficial effects of physical activity

The research of the last decade highlighted the existence of a family of genes activated by cellular stresses that allow the cells to reactivate defense and repair activities regardless of age. The prolonged activation of these genes enhances the organism health and lifespan. Members of this gene family are called sirtuins (SIRT). The founding member of the SIRT protein family, Sir2 is a limiting component of yeast longevity. Many members of this family have been also identified as key longevity regulators in species ranging from yeast to fly. On the other hand, the role of SIRTs in the regulation of mammalian ageing has been questioned. While SIRTs' effects on lifespan are still a matter of scientific debate, the beneficial effects of SIRTs in terms of physical health and quality of aging are widely accepted. Increasing evidence suggests a pivotal role for SIRTs in mediating the adaptive response to physical exercise. The following review summarizes the knowledge so far acquired on sirtuins' role in mediating beneficial effects of physical exercise. In particular, the first paragraph gives an overture on mammalian sirtuins defining their localization, function when possible, and substrates. In the second paragraph, we discuss recent data regarding alteration of sirtuins expression and activity after physical exercise collected by our laboratory and others'.

Six weeks of a polarized training-intensity distribution leads to greater physiological and performance adaptations than a threshold model in trained cyclists

This study was undertaken to investigate physiological adaptation with two endurance-training periods differing in intensity distribution. In a randomized crossover fashion, separated by 4 wk of detraining, 12 male cyclists completed two 6-wk training periods: 1) a polarized model [6.4 (±1.4 SD) h/wk; 80%, 0%, and 20% of training time in low-, moderate-, and high-intensity zones, respectively]; and 2) a threshold model [7.5 (±2.0 SD) h/wk; 57%, 43%, and 0% training-intensity distribution]. Before and after each training period, following 2 days of diet and exercise control, fasted skeletal muscle biopsies were obtained for mitochondrial enzyme activity and monocarboxylate transporter (MCT) 1 and 4 expression, and morning first-void urine samples were collected for NMR spectroscopy-based metabolomics analysis. Endurance performance (40-km time trial), incremental exercise, peak power output (PPO), and high-intensity exercise capacity (95% maximal work rate to exhaustion) were also assessed. Endurance performance, PPOs, lactate threshold (LT), MCT4, and high-intensity exercise capacity all increased over both training periods. Improvements were greater following polarized rather than threshold for PPO [mean (±SE) change of 8 (±2)% vs. 3 (±1)%, P < 0.05], LT [9 (±3)% vs. 2 (±4)%, P < 0.05], and high-intensity exercise capacity [85 (±14)% vs. 37 (±14)%, P < 0.05]. No changes in mitochondrial enzyme activities or MCT1 were observed following training. A significant multilevel, partial least squares-discriminant analysis model was obtained for the threshold model but not the polarized model in the metabolomics analysis. A polarized training distribution results in greater systemic adaptation over 6 wk in already well-trained cyclists. Markers of muscle metabolic adaptation are largely unchanged, but metabolomics markers suggest different cellular metabolic stress that requires further investigation.

Hibernating squirrel muscle activates the endurance exercise pathway despite prolonged immobilization

Skeletal muscle atrophy is a very common clinical challenge in many disuse conditions. Maintenance of muscle mass is crucial to combat debilitating functional consequences evoked from these clinical conditions. In contrast, hibernation represents a physiological state in which there is natural protection against disuse atrophy despite prolonged periods of immobilization and lack of nutrient intake. Even though peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1-α (PGC-1α) is a central mediator in muscle remodeling pathways, its role in the preservation of skeletal muscle mass during hibernation remains unclear. Since PGC-1α regulates muscle fiber type formation and mitochondrial biogenesis, we analyzed muscles of 13-lined ground squirrels. We find that animals in torpor exhibit a shift to slow-twitch Type I muscle fibers. This switch is accompanied by activation of the PGC-1α-mediated endurance exercise pathway. In addition, we observe increased antioxidant capacity without evidence of oxidative stress, a marked decline in apoptotic susceptibility, and enhanced mitochondrial abundance and metabolism. These results show that activation of the endurance exercise pathway can be achieved in vivo despite prolonged periods of immobilization, and therefore might be an important mechanism for skeletal muscle preservation during hibernation. This PGC-1α regulated pathway may be a potential therapeutic target promoting skeletal muscle homeostasis and oxidative balance to prevent muscle loss in a variety of inherited and acquired neuromuscular disease conditions.

Extremely low volume, whole-body aerobic-resistance training improves aerobic fitness and muscular endurance in females

The current study evaluated changes in aerobic fitness and muscular endurance following endurance training and very low volume, whole-body, high-intensity, interval-style aerobic-resistance training. Subjects' enjoyment and implementation intentions were also examined prior to and following training. Subjects (22 recreationally active females (20.3 ± 1.4 years)) completed 4 weeks of exercise training 4 days per week consisting of either 30 min of endurance treadmill training (~85% maximal heart rate; n = 7) or whole-body aerobic-resistance training involving one set of 8 × 20 s of a single exercise (burpees, jumping jacks, mountain climbers, or squat thrusts) separated by 10 s of rest per session (n = 7). A third group was assigned to a nontraining control group (n = 8). Following training, [Formula: see text]O(2peak) was increased in both the endurance (~7%) and interval (~8%) groups (p < 0.05), whereas muscle endurance was improved (p < 0.05) in the interval group (leg extensions, +40%; chest presses, +207%; sit-ups, +64%; push-ups, +135%; and back extensions, +75%). Perceived enjoyment of, and intentions to engage in, very low volume, high-intensity, whole-body interval exercise were both increased following training (p < 0.05). No significant changes were observed for any variable in the control (nontraining) group. These data demonstrate that although improvements in cardiovascular fitness are induced by both endurance and extremely low volume interval-style training, whole-body aerobic-resistance training imparted addition benefit in the form of improved skeletal muscle endurance.

NAD(+)/NADH and skeletal muscle mitochondrial adaptations to exercise

The pyridine nucleotides, NAD(+) and NADH, are coenzymes that provide oxidoreductive power for the generation of ATP by mitochondria. In skeletal muscle, exercise perturbs the levels of NAD(+), NADH, and consequently, the NAD(+)/NADH ratio, and initial research in this area focused on the contribution of redox control to ATP production. More recently, numerous signaling pathways that are sensitive to perturbations in NAD(+)(H) have come to the fore, as has an appreciation for the potential importance of compartmentation of NAD(+)(H) metabolism and its subsequent effects on various signaling pathways. These pathways, which include the sirtuin (SIRT) proteins SIRT1 and SIRT3, the poly(ADP-ribose) polymerase (PARP) proteins PARP1 and PARP2, and COOH-terminal binding protein (CtBP), are of particular interest because they potentially link changes in cellular redox state to both immediate, metabolic-related changes and transcriptional adaptations to exercise. In this review, we discuss what is known, and not known, about the contribution of NAD(+)(H) metabolism and these aforementioned proteins to mitochondrial adaptations to acute and chronic endurance exercise.

NAD(+)/NADH and skeletal muscle mitochondrial adaptations to exercise

The pyridine nucleotides, NAD(+) and NADH, are coenzymes that provide oxidoreductive power for the generation of ATP by mitochondria. In skeletal muscle, exercise perturbs the levels of NAD(+), NADH, and consequently, the NAD(+)/NADH ratio, and initial research in this area focused on the contribution of redox control to ATP production. More recently, numerous signaling pathways that are sensitive to perturbations in NAD(+)(H) have come to the fore, as has an appreciation for the potential importance of compartmentation of NAD(+)(H) metabolism and its subsequent effects on various signaling pathways. These pathways, which include the sirtuin (SIRT) proteins SIRT1 and SIRT3, the poly(ADP-ribose) polymerase (PARP) proteins PARP1 and PARP2, and COOH-terminal binding protein (CtBP), are of particular interest because they potentially link changes in cellular redox state to both immediate, metabolic-related changes and transcriptional adaptations to exercise. In this review, we discuss what is known, and not known, about the contribution of NAD(+)(H) metabolism and these aforementioned proteins to mitochondrial adaptations to acute and chronic endurance exercise.

Clenbuterol, a β2-adrenergic agonist, reciprocally alters PGC-1 alpha and RIP140 and reduces fatty acid and pyruvate oxidation in rat skeletal muscle

Clenbuterol, a β2-adrenergic agonist, reduces mitochondrial content and enzyme activities in skeletal muscle, but the mechanism involved has yet to be identified. We examined whether clenbuterol-induced changes in the muscles' metabolic profile and the intrinsic capacity of mitochondria to oxidize substrates are associated with reductions in the nuclear receptor coactivator PGC-1 alpha and/or an increase in the nuclear corepressor RIP140. In rats, clenbuterol was provided in the drinking water (30 mg/l). In 3 wk, this increased body (8%) and muscle weights (12–17%). In red (R) and white (W) muscles, clenbuterol induced reductions in mitochondrial content (citrate synthase: R, 27%; W, 52%; cytochrome- c oxidase: R, 24%; W, 34%), proteins involved in fatty acid transport (fatty acid translocase/CD36: R, 36%; W, 35%) and oxidation [β-hydroxyacyl CoA dehydrogenase (β-HAD): R, 33%; W, 62%], glucose transport (GLUT4: R, 8%; W, 13%), lactate transport monocarboxylate transporter (MCT1: R, 61%; W, 37%), and pyruvate oxidation (PDHE1α, R, 18%; W, 12%). Concurrently, only red muscle lactate dehydrogenase activity (25%) and MCT4 (31%) were increased. Palmitate oxidation was reduced in subsarcolemmal (SS) (R, 30%; W, 52%) and intermyofibrillar (IMF) mitochondria (R, 17%; W, 44%) along with reductions in β-HAD activity (SS: R, 17%; W, 51%; IMF: R, 20%; W, 57%). Pyruvate oxidation was only reduced in SS mitochondria (R, 20%; W, 28%), but this was not attributable solely to PDHE1α, which was reduced in both SS (R, 21%; W, 20%) and IMF mitochondria (R, 15%; W, 43%). These extensive metabolic changes induced by clenbuterol were associated with reductions in PGC-1α (R, 37%; W, 32%) and increases in RIP140 (R, 23%; W, 21%). This is the first evidence that clenbuterol appears to exert its metabolic effects via simultaneous and reciprocal changes in the nuclear receptor coactivator PGC-1α and the nuclear corepressor RIP140.

Deacetylation of PGC-1α by SIRT1: importance for skeletal muscle function and exercise-induced mitochondrial biogenesis

Activation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α)-mediated transcription is important for both the determination of mitochondrial content and the induction of mitochondrial biogenesis in skeletal muscle. SIRT1 (silent mating type information regulator 2 homolog 1) deactetylation is proposed as a potential activator of PGC-1α transcriptional activity. The current review examines the importance of SIRT1 deacetylation of PGC-1α in skeletal muscle. Models of SIRT1 overexpression and pharmacological activation are examined, but changes in SIRT1 expression and deacetylase activity following acute and chronic contractile activity will be emphasized. In addition, potential mechanisms of SIRT1 activation in skeletal muscle will be examined. The importance of the PGC-1α acetyltransferase GCN5 will also be briefly discussed. The current evidence supports the contribution of SIRT1 deacetylation of PGC-1α to exercise-induced mitochondrial biogenesis. Further research examining exercise-mediated activation of SIRT1 and the role of GCN5 in regulating PGC-1α transcriptional activity in skeletal muscle is required.

Neurophysiological and epigenetic effects of physical exercise on the aging process

Aging is a gradual process during which molecular and cellular processes deteriorate progressively, often leading to such pathological conditions as vascular and metabolic disorders and cognitive decline. Although the mechanisms of aging are not yet fully understood, inflammation, oxidative damage, mitochondrial dysfunction, functional alterations in specific neuronal circuits and a restricted degree of apoptosis are involved. Physical exercise improves the efficiency of the capillary system and increases the oxygen supply to the brain, thus enhancing metabolic activity and oxygen intake in neurons, and increases neurotrophin levels and resistance to stress. Regular exercise and an active lifestyle during adulthood have been associated with reduced risk and protective effects for mild cognitive impairment and Alzheimer's disease. Similarly, studies in animal models show that physical activity has positive physiological and cognitive effects that correlate with changes in transcriptional profiles. According to numerous studies, epigenetic events that include changes in DNA methylation patterns, histone modification and alterations in microRNA profiles seem to be a signature of aging. Hence, insight into the epigenetic mechanisms involved in the aging process and their modulation through lifestyle interventions such as physical exercise might open new avenues for the development of preventive and therapeutic strategies to treat aging-related diseases.

Sirtuin 1 (SIRT1) deacetylase activity is not required for mitochondrial biogenesis or peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) deacetylation following endurance exercise

The protein deacetylase, sirtuin 1 (SIRT1), is a proposed master regulator of exercise-induced mitochondrial biogenesis in skeletal muscle, primarily via its ability to deacetylate and activate peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). To investigate regulation of mitochondrial biogenesis by SIRT1 in vivo, we generated mice lacking SIRT1 deacetylase activity in skeletal muscle (mKO). We hypothesized that deacetylation of PGC-1α and mitochondrial biogenesis in sedentary mice and after endurance exercise would be impaired in mKO mice. Skeletal muscle contractile characteristics were determined in extensor digitorum longus muscle ex vivo. Mitochondrial biogenesis was assessed after 20 days of voluntary wheel running by measuring electron transport chain protein content, enzyme activity, and mitochondrial DNA expression. PGC-1α expression, nuclear localization, acetylation, and interacting protein association were determined following an acute bout of treadmill exercise (AEX) using co-immunoprecipitation and immunoblotting. Contrary to our hypothesis, skeletal muscle endurance, electron transport chain activity, and voluntary wheel running-induced mitochondrial biogenesis were not impaired in mKO versus wild-type (WT) mice. Moreover, PGC-1α expression, nuclear translocation, activity, and deacetylation after AEX were similar in mKO versus WT mice. Alternatively, we made the novel observation that deacetylation of PGC-1α after AEX occurs in parallel with reduced nuclear abundance of the acetyltransferase, general control of amino-acid synthesis 5 (GCN5), as well as reduced association between GCN5 and nuclear PGC-1α. These findings demonstrate that SIRT1 deacetylase activity is not required for exercise-induced deacetylation of PGC-1α or mitochondrial biogenesis in skeletal muscle and suggest that changes in GCN5 acetyltransferase activity may be an important regulator of PGC-1α activity after exercise.

Nuclear SIRT1 activity, but not protein content, regulates mitochondrial biogenesis in rat and human skeletal muscle

Silent mating type information regulator 2 homolog 1 (SIRT1)-mediated peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) deacetylation is potentially key for activating mitochondrial biogenesis. Yet, at the whole muscle level, SIRT1 is not associated with mitochondrial biogenesis (Gurd, BJ, Yoshida Y, Lally J, Holloway GP, Bonen A. J Physiol 587: 1817-1828, 2009). Therefore, we examined nuclear SIRT1 protein and activity in muscle with varied mitochondrial content and in response to acute exercise. We also measured these parameters after stimulating mitochondrial biogenesis with chronic muscle contraction and 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) administration in rodents and exercise training in humans. In skeletal and heart muscles, nuclear SIRT1 protein was negatively correlated with indices of mitochondrial density (citrate synthase activity, CS; cytochrome oxidase IV, COX IV), but SIRT1 activity was positively correlated with these parameters (r > 0.98). Acute exercise did not alter nuclear SIRT1 protein but did induce a time-dependent increase in nuclear SIRT1 activity. This increase in SIRT1 activity was temporally related to increases in mRNA expression of genes activated by PGC-1α. Both chronic muscle stimulation and AICAR increased mitochondrial biogenesis and muscle PGC-1α, but not nuclear PGC-1α. Concomitantly, muscle and nuclear SIRT1 protein contents were reduced, but nuclear SIRT1 activity was increased. In human muscle, training-induced mitochondrial biogenesis did not alter muscle or nuclear SIRT1 protein content, but it did increase muscle and nuclear PGC-1α and SIRT1 activity. Thus, nuclear SIRT1 activity, but not muscle or nuclear SIRT1 protein content, is associated with contraction-stimulated mitochondrial biogenesis in rat and human muscle, possibly via AMPK activation.

Single-leg cycle training is superior to double-leg cycling in improving the oxidative potential and metabolic profile of trained skeletal muscle

Single-leg cycling may enhance the peripheral adaptations of skeletal muscle to a greater extent than double-leg cycling. The purpose of the current study was to determine the influence of 3 wk of high-intensity single- and double-leg cycle training on markers of oxidative potential and muscle metabolism and exercise performance. In a crossover design, nine trained cyclists (78 ± 7 kg body wt, 59 ± 5 ml·kg(-1)·min(-1) maximal O(2) consumption) performed an incremental cycling test and a 16-km cycling time trial before and after 3 wk of double-leg and counterweighted single-leg cycle training (2 training sessions per week). Training involved three (double) or six (single) maximal 4-min intervals with 6 min of recovery. Mean power output during the single-leg intervals was more than half that during the double-leg intervals (198 ± 29 vs. 344 ± 38 W, P < 0.05). Skeletal muscle biopsy samples from the vastus lateralis revealed a training-induced increase in Thr(172)-phosphorylated 5'-AMP-activated protein kinase α-subunit for both groups (P < 0.05). However, the increase in cytochrome c oxidase subunits II and IV and GLUT-4 protein concentration was greater following single- than double-leg cycling (P < 0.05). Training-induced improvements in maximal O(2) consumption (3.9 ± 6.2% vs. 0.6 ± 3.6%) and time-trial performance (1.3 ± 0.5% vs. 2.3 ± 4.2%) were similar following both interventions. We conclude that short-term high-intensity single-leg cycle training can elicit greater enhancement in the metabolic and oxidative potential of skeletal muscle than traditional double-leg cycling. Single-leg cycling may therefore provide a valuable training stimulus for trained and clinical populations.