The impact of exercise on mitochondrial biogenesis in skeletal muscle: A systematic review and meta-analysis of randomized trials.

Caffeine is one of the most widely used substances as recreational drug for performance-enhancement in sport, underpinned by a strong evidence base. Although the effects of caffeine are widely investigated within the scope of performance physiology, the molecular effects of caffeine within skeletal muscle remain unclear. Evidence from invitro and invivo models suggest that caffeine regulates the glucose metabolism in the skeletal muscle. Moreover, caffeine seems to stimulate CaMKII, PPARδ/β, AMPK and PGC1α, classical markers of exercise-adaptations, including mitochondrial biogenesis and mitochondrial content. This review summarizes evidence to suggest caffeine-effects within skeletal muscle fibers, focusing on the putative role of caffeine on mitochondrial biogenesis to explore whether caffeine supplementation might be a strategy to enhance mitochondrial biogenesis.

AMP-activated protein kinase (AMPK) is a heterotrimeric complex, composed of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma), that works as a cellular energy sensor. The existence of multiple heterotrimeric complexes provides a molecular basis for the multiple roles of this highly conserved signaling system. The AMPK gamma3 subunit is predominantly expressed in skeletal muscle, mostly in type II glycolytic fiber types. We determined whether the AMPK gamma3 subunit has a role in signaling pathways that mediate mitochondrial biogenesis in skeletal muscle. We provide evidence that overexpression or ablation of the AMPK gamma3 subunit does not appear to play a critical role in defining mitochondrial content in resting skeletal muscle. However, overexpression of a mutant form (R225Q) of the AMPK gamma3 subunit (Tg-AMPKgamma3(225Q)) increases mitochondrial biogenesis in glycolytic skeletal muscle. These adaptations are associated with an increase in expression of the co-activator PGC-1alpha and several transcription factors that regulate mitochondrial biogenesis, including NRF-1, NRF-2, and TFAM. Succinate dehydrogenase staining, a marker of the oxidative profile of individual fibers, was also increased in transversal skeletal muscle sections of white gastrocnemius muscle from Tg-AMPKgamma3(225Q) mice, independent of changes in fiber type composition. In conclusion, a single nucleotide mutation (R225Q) in the AMPK gamma3 subunit is associated with mitochondrial biogenesis in glycolytic skeletal muscle, concomitant with increased expression of the co-activator PGC-1alpha and several transcription factors that regulate mitochondrial proteins, without altering fiber type composition.

Adiponectin Increases Skeletal Muscle Mitochondrial Biogenesis by Suppressing Mitogen-Activated Protein Kinase Phosphatase-1

Nitric oxide is a potential regulator of mitochondrial biogenesis. Therefore, we investigated if mice deficient in endothelial nitric oxide synthase (eNOS-/-) or neuronal NOS (nNOS-/-) have attenuated activation of skeletal muscle mitochondrial biogenesis in response to exercise. eNOS-/-, nNOS-/- and C57Bl/6 (CON) mice (16.3 +/- 0.2 weeks old) either remained in their cages (basal) or ran on a treadmill (16 m min(-1), 5% grade) for 60 min (n = 8 per group) and were killed 6 h after exercise. Other eNOS-/-, nNOS-/- and CON mice exercise trained for 9 days (60 min per day) and were killed 24 h after the last bout of exercise training. eNOS-/- mice had significantly higher nNOS protein and nNOS-/- mice had significantly higher eNOS protein in the EDL, but not the soleus. The basal mitochondrial biogenesis markers NRF1, NRF2alpha and mtTFA mRNA were significantly (P< 0.05) higher in the soleus and EDL of nNOS-/- mice whilst basal citrate synthase activity was higher in the soleus and basal PGC-1alpha mRNA higher in the EDL. Also, eNOS-/- mice had significantly higher basal citrate synthase activity in the soleus but not the EDL. Acute exercise increased (P< 0.05) PGC-1alpha mRNA in soleus and EDL and NRF2alpha mRNA in the EDL to a similar extent in all genotypes. In addition, short-term exercise training significantly increased cytochrome c protein in all genotypes (P< 0.05) in the EDL. In conclusion, eNOS and nNOS are differentially involved in the basal regulation of mitochondrial biogenesis in skeletal muscle but are not critical for exercise-induced increases in mitochondrial biogenesis in skeletal muscle.

since the identification of the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α as a factor interacting with PPARγ in brown adipose tissue (BAT) ([12][1]), it has become clear that PGC-1α plays important roles in regulating several cellular processes

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.

Augmentation of aerobic respiration and mitochondrial biogenesis in skeletal muscle by hypoxia preconditioning with cobalt chloride

Exercise confers numerous health benefits, many of which are thought to stem from exercise-induced mitochondrial biogenesis (EIMB) in skeletal muscle. The transcriptional coactivator PGC-1α, a potent regulator of metabolism in numerous tissues, is widely believed to be required for EIMB. We show here that this is not the case. Mice engineered to lack PGC-1α specifically in skeletal muscle (Myo-PGC-1αKO mice) retained intact EIMB. The exercise capacity of these mice was comparable to littermate controls. Induction of metabolic genes after 2 weeks of in-cage voluntary wheel running was intact. Electron microscopy revealed no gross abnormalities in mitochondria, and the mitochondrial biogenic response to endurance exercise was as robust in Myo-PGC-1αKO mice as in wildtype mice. The induction of enzymatic activity of the electron transport chain by exercise was likewise unperturbed in Myo-PGC-1αKO mice. These data demonstrate that PGC-1α is dispensable for exercise-induced mitochondrial biogenesis in skeletal muscle, in sharp contrast to the prevalent assumption in the field.

MicroRNA-761 regulates mitochondrial biogenesis in mouse skeletal muscle in response to exercise

Transient receptor potential vanilloid 1 (TRPV1) is a protein that is susceptible to cell environment temperature. High temperatures of 40–45°C can activate the TRPV1 channel. TRPV1 is highly expressed in skeletal muscle and located on the sarcoplasmic reticulum (SR). Therefore, TRPV1 activated by high-temperature stress releases Ca2+ from the SR to the cytoplasm. Cellular Ca2+ accumulation is a key event that enhances TRPV1 activity by directly binding to the N-terminus and C-terminus. Moreover, Ca2+ is the key messenger involved in regulating mitochondrial biogenesis in skeletal muscle. Long-term activation of TRPV1 may promote mitochondrial biogenesis in skeletal muscle through the Ca2+-CaMKII-p38 MAPK-PGC-1α signaling axis. The discovery of the TRPV1 channel highlights the potential mechanism for high-temperature stress improving muscle mitochondrial biogenesis. The appropriate hot stimulus in thermal environments might be beneficial to the muscular mitochondrial adaptation for aerobic capacity. However, the investigation of TRPV1 on mitochondrial biogenesis is at an early stage. Further investigations need to examine the role of TRPV1 in response to mitochondrial biogenesis in skeletal muscle induced by different thermal environments.

The purpose of this study was to evaluate the effect of chronic quercetin treatment on mitochondrial biogenesis, endurance exercise performance and activation levels of AMP-activated protein kinase (AMPK) in rat skeletal muscle. Rats were assigned to a control or quercetin group and were fed for 7 days. Rats treated with quercetin showed no changes in the protein levels of citrate synthase or cytochrome C oxidase IV or those of sirtuin 1, peroxisome proliferator-activated receptor gamma coactivator-1α or phosphorylated AMPK. After endurance swimming exercise, quercetin-treated rats demonstrated no differences in blood and muscle lactate levels or glycogen utilization speed compared to control rats. These results indicate that quercetin treatment does not stimulate mitochondrial biogenesis in skeletal muscle and does not influence metabolism in a way that might enhance endurance exercise capacity. On the other hand, the AMPK phosphorylation level immediately after exercise was significantly lower in quercetin-treated muscles, suggesting that quercetin treatment might provide a disadvantage to muscle adaptation when administered with exercise training. The molecular results of this study indicate that quercetin treatment may not be advantageous for improving endurance exercise performance, at least after high-dose and short-term therapy.

Exercise is a recognized non-pharmacological treatment for improving glucose homeostasis in type 2 diabetes (T2DM), with resistance exercise (RE) showing promising results. However, the mechanism of RE improving T2DM has not been clarified. This study aims to investigate the effects of RE on glucose and lipid metabolism, insulin signaling, and mitochondrial function in T2DM mice, with a focus on the regulatory role of miR-30d-5p. Our results confirmed that RE significantly improved fasting blood glucose, IPGTT, and ITT in T2DM mice. Enhanced expression of IRS-1, p-PI3K, and p-Akt indicated improved insulin signaling. RE improved glycolipid metabolism, as well as mitochondrial biogenesis and dynamics in skeletal muscle of T2DM mice. We also found that miR-30d-5p was upregulated in T2DM, and was downregulated after RE. Additionally, in vitro, over-expression of miR-30d-5p significantly increased lipid deposition, and reduced glucose uptake and mitochondrial biogenesis. These observations were reversed after transfection with the miR-30d-5p inhibitor. Mechanistically, miR-30d-5p regulates glycolipid metabolism in skeletal muscle by directly targeting SIRT1, which affects the expression of PGC-1α, thereby influencing mitochondrial function and glycolipid metabolism. Taken together, RE effectively improves glucose and lipid metabolism and mitochondrial function in T2DM mice, partly through regulating the miR-30d-5p/SIRT1/PGC-1α axis. miR-30d-5p could serve as a potential therapeutic target for T2DM management.

We would firstly like to confirm that our eNOS−/− mice (Wadley et al. 2007) were indeed from a background generated by Shesely et al. (1996) and we apologise for any confusion this may have caused. We agree there seems to be distinct differences in strain between eNOS−/− mice generated from Huang et al. (1995) and Shesely et al. (1996) backgrounds. Nisoli et al. have shown quite comprehensively that mitochondrial biogenesis is reduced under basal conditions in a number of tissues in eNOS−/− mice generated from the Huang et al. (1995) background (Nisoli et al. 2003, 2004). We also feel that accurate consideration of methodological aspects between studies will contribute to a better understanding and discussion of the relevance of eNOS on basal (and exercise-induced) mitochondrial biogenesis. Indeed, the work by Ojaimi et al. (2005), showing decreased PGC-1α in the heart of very old (21 months) eNOS−/− mice from the Shesely et al. (1996) background, also showed a greater decline in exercise capacity with ageing compared with wildtype mice. So it is possible that mitochondrial biogenesis is impaired with ageing in eNOS−/− mice more than wildtype mice. Thus, caution should be taken when making comparisons between our findings in 16-week-old eNOS−/− mice and the 21-month-old mice used by Ojaimi et al. (2005). Very recently, Le Gouill et al. (2007) observed reduced mitochondrial protein and DNA content and lower beta oxidation in skeletal muscle of eNOS−/− mice generated from the Shesely et al. (1996) background, which would also suggest reduced basal skeletal muscle mitochondrial biogenesis, although unfortunately we do not know the age of these mice, so again direct comparison between our findings and those of Le Gouill et al. is difficult. However, it is highly unlikely that our observation of no impairments in basal mitochondrial biogenesis in the skeletal muscle of our eNOS−/− mice could be due to measurement error. This is because we analysed all strains of mice (contol (CON), eNOS−/− and nNOS−/−) together and we were able to detect altered basal mitochondrial biogenesis in the nNOS−/− mice and exercise-induced mitochondrial biogenesis in all strains of mice (Wadley et al. 2007). We do not agree that it has previously been demonstrated that mitochondrial biogenesis is markedly reduced in eNOS−/− mice under exercise conditions. Previous research is this area is quite limited and unclear. Prior to our study no previous study had examined the effect of an acute bout of exercise on mitochondrial biogenesis markers. In addition, the only study to examine the effect of exercise training on skeletal muscle of eNOS−/− was difficult to interpret (Momken et al. 2004). For instance, Momken et al. found that not only did citrate synthase (CS) activity not increase with exercise training (voluntary treadmill running) in either the eNOS−/− or the wildtype mice, but cytochrome c oxidase (COX) activity actually decreased with voluntary wheel running (Momken et al. 2004). Furthermore, basal COX protein expression and CS activity were not impaired in the eNOS−/− mice. They did not measure markers of mitochondrial biogenesis (i.e. PGC-1α, NRF1, NRF2, mtTFA). Also, voluntary wheel running may not be the ideal exercise protocol for these experiments, since eNOS−/− mice ran significantly less (about half) than their wildtype controls and thus the exercise was not matched between groups. To our knowledge, ours is the first study to measure markers of mitochondrial biogenesis in skeletal muscle of eNOS−/− (and nNOS−/−) mice following exercise and we clearly observe normal increases in skeletal muscle PGC-1α mRNA following acute exercise in mice that have no detectable eNOS protein (Wadley et al. 2007). Furthermore, we have previously shown that pharmacologically blocking all NOS isoforms with the non-specific NOS inhibitor, l-NAME, does not prevent increased skeletal muscle PGC-1α mRNA following acute exercise in rats (Wadley & McConell, 2007). Thus, we feel it is unlikely that NO is involved in exercise-induced mitochondrial biogenesis in skeletal muscle. In conclusion, we agree further work is required to investigate the differences in strain between eNOS−/− mice generated from the Huang et al. (1995) and Shesely et al. (1996) backgrounds on basal mitochondrial biogenesis. Careful consideration needs to be taken regarding methodological differences (i.e. age, body fatness, etc.), since this could impact on the interpretation of results. However, given our recent findings of normal exercise-induced mitochondrial biogenesis in both eNOS−/− mice (Wadley et al. 2007) and in rats treated with l-NAME (Wadley & McConell, 2007) we feel it is unlikely that eNOS has a direct role in exercise-induced mitochondrial biogenesis.

Curcumin treatment enhances the effect of exercise on mitochondrial biogenesis in skeletal muscle by increasing cAMP levels

dysregulation of energy balance is a primary constituent in the etiology of obesity and type 2 diabetes mellitus, which is manifested by altered metabolic homeostasis and insulin resistance in a variety of tissues, including brain, liver, and skeletal muscle. The discovery of the AMP-activated