PGC-1 coactivators and skeletal muscle adaptations in health and disease

Dietary exercise as a novel strategy for the prevention and treatment of metabolic syndrome: effects on skeletal muscle function

A sedentary lifestyle can cause metabolic syndrome to develop. Metabolic syndrome is associated with metabolic function in the skeletal muscle, a major consumer of nutrients. Dietary exercise, along with an adequate diet, is reported to be one of the major preventive therapies for metabolic syndrome; exercise improves the metabolic capacity of muscles and prevents the loss of muscle mass. Epidemiological studies have shown that physical activity reduces the risk of various common diseases such as cardiovascular disease, diabetes, and cancer; it also helps in reducing visceral adipose tissue. In addition, laboratory studies have demonstrated the mechanisms underlying the benefits of single-bout and regular exercise. Exercise regulates the expression/activity of proteins associated with metabolic and anabolic signaling in muscle, leading to a change in phenotype. The extent of these changes depends on the intensity, the duration, and the frequency of the exercise. The effect of exercise is also partly due to a decrease in inflammation, which has been shown to be closely related to the development of various diseases. Furthermore, it has been suggested that several phytochemicals contained in natural foods can improve nutrient metabolism and prevent protein degradation in the muscle.

Amelioration of lipid-induced insulin resistance in rat skeletal muscle by overexpression of Pgc-1β involves reductions in long-chain acyl-CoA levels and oxidative stress

AIMS/HYPOTHESIS

To determine if acute overexpression of peroxisome proliferator-activated receptor, gamma, coactivator 1 beta (Pgc-1β [also known as Ppargc1b]) in skeletal muscle improves insulin action in a rodent model of diet-induced insulin resistance.

METHODS

Rats were fed either a low-fat or high-fat diet (HFD) for 4 weeks. In vivo electroporation was used to overexpress Pgc-1β in the tibialis cranialis (TC) and extensor digitorum longus (EDL) muscles. Downstream effects of Pgc-1β on markers of mitochondrial oxidative capacity, oxidative stress and muscle lipid levels were characterised. Insulin action was examined ex vivo using intact muscle strips and in vivo via a hyperinsulinaemic-euglycaemic clamp.

RESULTS

Pgc-1β gene expression was increased >100% over basal levels. The levels of proteins involved in mitochondrial function, lipid metabolism and antioxidant defences, the activity of oxidative enzymes, and substrate oxidative capacity were all increased in muscles overexpressing Pgc-1β. In rats fed a HFD, increasing the levels of Pgc-1β partially ameliorated muscle insulin resistance, in association with decreased levels of long-chain acyl-CoAs (LCACoAs) and increased antioxidant defences.

CONCLUSIONS

Our data show that an increase in Pgc-1β expression in vivo activates a coordinated subset of genes that increase mitochondrial substrate oxidation, defend against oxidative stress and improve lipid-induced insulin resistance in skeletal muscle.

Module-based multiscale simulation of angiogenesis in skeletal muscle

Background

Mathematical modeling of angiogenesis has been gaining momentum as a means to shed new light on the biological complexity underlying blood vessel growth. A variety of computational models have been developed, each focusing on different aspects of the angiogenesis process and occurring at different biological scales, ranging from the molecular to the tissue levels. Integration of models at different scales is a challenging and currently unsolved problem.

Results

We present an object-oriented module-based computational integration strategy to build a multiscale model of angiogenesis that links currently available models. As an example case, we use this approach to integrate modules representing microvascular blood flow, oxygen transport, vascular endothelial growth factor transport and endothelial cell behavior (sensing, migration and proliferation). Modeling methodologies in these modules include algebraic equations, partial differential equations and agent-based models with complex logical rules. We apply this integrated model to simulate exercise-induced angiogenesis in skeletal muscle. The simulation results compare capillary growth patterns between different exercise conditions for a single bout of exercise. Results demonstrate how the computational infrastructure can effectively integrate multiple modules by coordinating their connectivity and data exchange. Model parameterization offers simulation flexibility and a platform for performing sensitivity analysis.

Conclusions

This systems biology strategy can be applied to larger scale integration of computational models of angiogenesis in skeletal muscle, or other complex processes in other tissues under physiological and pathological conditions.

Quantitative proteomic analysis of dystrophic dog muscle

Duchenne muscular dystrophy (DMD) is caused by null mutations in the dystrophin gene, leading to progressive and unrelenting muscle loss. Although the genetic basis of DMD is well resolved, the cellular mechanisms associated with the physiopathology remain largely unknown. Increasing evidence suggests that secondary mechanisms, as the alteration of key signaling pathways, may play an important role. In order to identify reliable biomarkers and potential therapeutic targets, and taking advantage of the clinically relevant Golden Retriever Muscular Dystrophy (GRMD) dog model, a proteomic study was performed. Isotope-coded affinity tag (ICAT) profiling was used to compile quantitative changes in protein expression profiles of the vastus lateralis muscles of 4-month old GRMD vs healthy dogs. Interestingly, the set of under-expressed proteins detected appeared primarily composed of metabolic proteins, many of which have been shown to be regulated by the transcriptional peroxisome proliferator-activated receptor-gamma co-activator 1 alpha (PGC-1α). Subsequently, we were able to showed that PGC1-α expression is dramatically reduced in GRMD compared to healthy muscle. Collectively, these results provide novel insights into the molecular pathology of the clinically relevant animal model of DMD, and indicate that defective energy metabolism is a central hallmark of the disease in the canine model.

Prolonged Nrf1 overexpression triggers adipocyte inflammation and insulin resistance

Adipose tissue is currently being recognized as an important endocrine organ, carrying defects in a number of metabolic diseases. Mitochondria play a key role in normal adipose tissue function and mitochondrial alterations can result in pathology, like lipodystrophy or type 2 diabetes. Although Pgc1α is regarded as the main regulator of mitochondrial function, downstream Nrf1 is the key regulator of mitochondrial biogenesis. Nrf1 is also involved in a wide range of other processes, including proliferation, innate immune response, and apoptosis. To determine transcriptional targets of Nrf1, 3T3-L1 preadipocytes were transfected with either pNrf1 or a control vector. Two days post-confluence, 3T3-L1 preadipocytes were allowed to differentiate. At day 8 of differentiation, Nrf1 overexpressing cells had an increased mtDNA copy number and reduced lipid content. This was not associated with an increased ATP production rate per cell. Using global gene expression analysis, we observed that Nrf1 overexpression stimulated cell proliferation, apoptosis, and cytokine expression. In addition, prolonged Nrf1 induced an adipokine expression profile of insulin resistant adipocytes. Nrf1 has a wide range of transcriptional targets, stimulators as well as inhibitors of adipose tissue functioning. Therefore, post-transcriptional regulation of Nrf1, or stimulating specific Nrf1 targets may be a more suitable approach for stimulating mitochondrial biogenesis and treating adipose tissue defects, instead of directly stimulating Nrf1 expression. In addition, our results show that short-term effects can drastically differ from long-term effects.

Increased fatigue resistance linked to Ca2+-stimulated mitochondrial biogenesis in muscle fibres of cold-acclimated mice

Mammals exposed to a cold environment initially generate heat by repetitive muscle activity (shivering). Shivering is successively replaced by the recruitment of uncoupling protein-1 (UCP1)-dependent heat production in brown adipose tissue. Interestingly, adaptations observed in skeletal muscles of cold-exposed animals are similar to those observed with endurance training. We hypothesized that increased myoplasmic free [Ca2+] ([Ca2+]i) is important for these adaptations. To test this hypothesis, experiments were performed on flexor digitorum brevis (FDB) muscles, which do not participate in the shivering response, of adult wild-type (WT) and UCP1-ablated (UCP1-KO) mice kept either at room temperature (24°C) or cold-acclimated (4°C) for 4-5 weeks. [Ca2+]i (measured with indo-1) and force were measured under control conditions and during fatigue induced by repeated tetanic stimulation in intact single fibres. The results show no differences between fibres from WT and UCP1-KO mice. However, muscle fibres from cold-acclimated mice showed significant increases in basal [Ca2+]i (∼50%), tetanic [Ca2+]i (∼40%), and sarcoplasmic reticulum (SR) Ca2+ leak (∼fourfold) as compared to fibres from room-temperature mice. Muscles of cold-acclimated mice showed increased expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and increased citrate synthase activity (reflecting increased mitochondrial content). Fibres of cold-acclimated mice were more fatigue resistant with higher tetanic [Ca2+]i and less force loss during fatiguing stimulation. In conclusion, cold exposure induces changes in FDB muscles similar to those observed with endurance training and we propose that increased [Ca2+]i is a key factor underlying these adaptations.

Baicalin increases VEGF expression and angiogenesis by activating the ERR{alpha}/PGC-1{alpha} pathway

AIMS

Baicalin is the major component found in Scutellaria baicalensis root, a widely used herb in traditional Chinese medicine. Although it has been used for thousands of years to treat stroke, the mechanisms of action of S. baicalensis have not been clearly elucidated. In this report, we studied the modulation of angiogenesis as one possible mechanism by investigating the effects of these agents on expression of vascular endothelial growth factor (VEGF), a critical factor for angiogenesis.

METHODS AND RESULTS

The effects of baicalin and an extract of S. baicalensis on VEGF expression were tested in several cell lines. Both agents induced VEGF expression in all cells without increasing expression of hypoxia-inducible factor-1α (HIF-1α). The expression of reporter genes was also activated under the control of the VEGF promoter containing either a functional or a defective HIF response element (HRE). Only minimal effects were observed on reporter activation under the HRE promoter. Instead, both agents significantly induced oestrogen-related receptor (ERRα) expression as well as the activity of reporter genes under the control of ERRα-binding element. Their ability to induce VEGF expression was suppressed once ERRα expression was knocked down by siRNA or ERRα-binding sites were deleted in the VEGF promoter. We also found that both agents stimulated cell migration and vessel sprout formation from the aorta.

CONCLUSION

Our results implicate baicalin and S. baicalensis in angiogenesis by inducing VEGF expression through the activation of the ERRα pathway. These data may facilitate a better understanding of the potential health benefits of these agents in the treatment of cardiovascular diseases.

Exercise increases mitochondrial PGC-1alpha content and promotes nuclear-mitochondrial cross-talk to coordinate mitochondrial biogenesis

Endurance exercise is known to induce metabolic adaptations in skeletal muscle via activation of the transcriptional co-activator peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α). PGC-1α regulates mitochondrial biogenesis via regulating transcription of nuclear-encoded mitochondrial genes. Recently, PGC-1α has been shown to reside in mitochondria; however, the physiological consequences of mitochondrial PGC-1α remain unknown. We sought to delineate if an acute bout of endurance exercise can mediate an increase in mitochondrial PGC-1α content where it may co-activate mitochondrial transcription factor A to promote mtDNA transcription. C57Bl/6J mice (n = 12/group; ♀ = ♂) were randomly assigned to sedentary (SED), forced-endurance (END) exercise (15 m/min for 90 min), or forced endurance +3 h of recovery (END+3h) group. The END group was sacrificed immediately after exercise, whereas the SED and END+3h groups were euthanized 3 h after acute exercise. Acute exercise coordinately increased the mRNA expression of nuclear and mitochondrial DNA-encoded mitochondrial transcripts. Nuclear and mitochondrial abundance of PGC-1α in END and END+3h groups was significantly higher versus SED mice. In mitochondria, PGC-1α is in a complex with mitochondrial transcription factor A at mtDNA D-loop, and this interaction was positively modulated by exercise, similar to the increased binding of PGC-1α at the NRF-1 promoter. We conclude that in response to acute altered energy demands, PGC-1α re-localizes into nuclear and mitochondrial compartments where it functions as a transcriptional co-activator for both nuclear and mitochondrial DNA transcription factors. These results suggest that PGC-1α may dynamically facilitate nuclear-mitochondrial DNA cross-talk to promote net mitochondrial biogenesis.

Metabolic benefits of resistance training and fast glycolytic skeletal muscle

Skeletal muscle exhibits remarkable plasticity with respect to its metabolic properties. Recent work has shown that interventions such as resistance training, genetic alterations and pharmacological strategies that increase muscle mass and glycolytic capacity, and not necessarily oxidative competence, can improve body composition and systemic metabolism. We review here recent advances in our understanding of the signaling and transcriptional regulatory pathways of this strategy and review new evidence obtained from mice and humans that supports the notion that increasing muscle mass and glycolytic capacity may effectively counter insulin resistance and type 2 diabetes mellitus.

Fiber type conversion by PGC-1α activates lysosomal and autophagosomal biogenesis in both unaffected and Pompe skeletal muscle

PGC-1α is a transcriptional co-activator that plays a central role in the regulation of energy metabolism. Our interest in this protein was driven by its ability to promote muscle remodeling. Conversion from fast glycolytic to slow oxidative fibers seemed a promising therapeutic approach in Pompe disease, a severe myopathy caused by deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA) which is responsible for the degradation of glycogen. The recently approved enzyme replacement therapy (ERT) has only a partial effect in skeletal muscle. In our Pompe mouse model (KO), the poor muscle response is seen in fast but not in slow muscle and is associated with massive accumulation of autophagic debris and ineffective autophagy. In an attempt to turn the therapy-resistant fibers into fibers amenable to therapy, we made transgenic KO mice expressing PGC-1α in muscle (tgKO). The successful switch from fast to slow fibers prevented the formation of autophagic buildup in the converted fibers, but PGC-1α failed to improve the clearance of glycogen by ERT. This outcome is likely explained by an unexpected dramatic increase in muscle glycogen load to levels much closer to those observed in patients, in particular infants, with the disease. We have also found a remarkable rise in the number of lysosomes and autophagosomes in the tgKO compared to the KO. These data point to the role of PGC-1α in muscle glucose metabolism and its possible role as a master regulator for organelle biogenesis - not only for mitochondria but also for lysosomes and autophagosomes. These findings may have implications for therapy of lysosomal diseases and other disorders with altered autophagy.

Alterations of glucose-dependent insulinotropic polypeptide (GIP) during cold acclimation

Cold acclimation is initially associated with shivering thermogenesis in skeletal muscle followed by adaptive non-shivering thermogenesis, particularly in brown adipose tissue (BAT). In response, hyperphagia occurs to meet increased metabolic demand and thermoregulation. The present study investigates the effects of cold (4 ± 1 °C) acclimation and hyperphagia on circulating and intestinal levels of gastric inhibitory polypeptide (GIP) in rats. Pair fed animals were used as additional controls in some experiments. Cold acclimation for 42 days significantly (p<0.01) increased daily food intake. There was no corresponding change in body weight. However, body weights of pair fed cold exposed rats were significantly (p<0.01) reduced compared to controls and ad libitum fed cold exposed rats. By day 42, non-fasting plasma glucose was increased (p<0.05) by chronic cold exposure regardless of food intake. Corresponding plasma insulin concentrations were significantly (p<0.01) lower in pair fed cold exposed rats. Circulating GIP levels were elevated (p<0.05) in ad libitum fed cold acclimated rats on days 18 and 24, but returned to normal levels by the end of the study. The glycaemic response to oral glucose was improved (p<0.01) in all cold exposed rats, with significantly (p<0.05) elevated GIP responses in ad libitum fed rats and significantly (p<0.05) reduced insulin responses in pair fed rats. In keeping with this, insulin sensitivity was enhanced (p<0.05) in cold exposed rats compared to controls. By the end of the study, cold acclimated rats had significantly (p<0.01) increased BAT mass and intestinal concentrations of GIP and GLP-1 compared to controls, independent of food intake. These data indicate that changes in the secretion and actions of GIP may be involved in the metabolic adaptations to cold acclimation in rats.

The transcriptional coregulators TIF2 and SRC-1 regulate energy homeostasis by modulating mitochondrial respiration in skeletal muscles

The two p160 transcriptional coregulator family members SRC-1 and TIF2 have important metabolic functions in white and brown adipose tissues as well as in the liver. To analyze TIF2 cell-autonomous functions in skeletal muscles, we generated TIF2((i)skm)⁻(/)⁻ mice in which TIF2 was selectively ablated in skeletal muscle myofibers at adulthood. We found that increased mitochondrial uncoupling in skeletal muscle myocytes protected these mice from decreased muscle oxidative capacities induced by sedentariness, delayed the development of type 2 diabetes, and attenuated high-caloric-diet-induced obesity. Moreover, our results demonstrate that SRC-1 and TIF2 can modulate the expression of the uncoupling protein 3 (UCP3) in an antagonistic manner and that enhanced SRC-1 levels in TIF2-deficient myofibers are critically involved in the metabolic changes of TIF2((i)skm)⁻(/)⁻ mice. Thus, modulation of the expression and/or activity of these coregulators represents an attractive way to prevent or treat metabolic disorders.

PGC-1 coactivators in cardiac development and disease

The beating heart requires a constant flux of ATP to maintain contractile function, and there is increasing evidence that energetic defects contribute to the development of heart failure. The last 10 years have seen a resurgent interest in cardiac intermediary metabolism and a dramatic increase in our understanding of transcriptional networks that regulate cardiac energetics. The PPAR-γ coactivator (PGC)-1 family of proteins plays a central role in these pathways. The mechanisms by which PGC-1 proteins regulate transcriptional networks and are regulated by physiological cues, as well as the roles they play in cardiac development and disease, are reviewed here.

Thermogenesis in CD-1 mice after combined chronic hypoxia and cold acclimation

Many small mammals thermoregulate through shivering in muscle and/or non-shivering thermogenesis (NST) via brown adipose tissue (BAT) by the actions of mitochondrial uncoupling proteins (UCPs). An up-regulation of these mechanisms would be advantageous in a cold environment but not in conditions of low oxygen as it leads to needless increases in energy expenditure. We examined the chronic effect of 4 weeks of exposure to hypobaric hypoxia (H, 480 mm Hg), cold (C, 5 degrees C) and the combination of the two stressors (HC) compared to normoxic thermoneutral controls (N, 28 degrees C) in male CD-1 mice. We found that hypoxic/cold acclimated mice had significantly lower body temperatures (T(b)) after acclimation along with complete abolishment of diurnal T(b) fluctuations. Capacity for NST was assessed by changes in intrascapular BAT mass, mitochondrial content and UCP1 content per milligram mitochondria. Acclimation caused distinct remodeling of BAT that was reflected in differences in NE-induced increases in oxygen consumption (VO(2)) used to assess NST capacity. Reduction of T(b) in HC acclimated mice was not due to a decreased heat-generating capacity of BAT. VO(2) during an acute temperature challenge (32 to 4 degrees C) in normoxia was similar in all treatment groups compared to controls but thermal conductance was greater in C acclimated mice and T(b) higher in HC acclimated mice. We propose that an overriding inhibition by hypoxia on neural feedback pathways persists even after weeks of acclimation when combined with chronic cold.

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.

The microRNA miR-696 regulates PGC-1{alpha} in mouse skeletal muscle in response to physical activity

MicroRNAs (miRNAs) are small noncoding RNAs involved in posttranscriptional gene regulation that have been shown to be involved in growth, development, function, and stress responses of various organs. The purpose of this study was to identify the miRNA response to physical activity, which was related to functions such as nutrient metabolism, although the miRNAs involved are currently unknown. C57BL/6 mice were divided into exercise and control groups. The exercise group performed running exercise, with a gradual increase of the load over 4 wk. On the other hand, to examine the effect of muscle inactivity, the unilateral hindlimbs of other mice were fixed in a cast for 5 days. Microarray analysis for miRNA in gastrocnemius revealed that miR-696 was markedly affected by both exercise and immobilization, showing opposite responses to these two interventions. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), which was increased by exercise and decreased by immobilization in the protein level, was predicted as a target regulated by miR-696. In cultured myocytes, intracellular miR-696 variation led to negative regulation of PGC-1alpha protein along with the expression of mRNAs for downstream genes. In addition, we found decreases in the biogenesis of mitochondria and fatty acid oxidation in miR-696-overexpressing myocytes compared with normal control myocytes. These observations demonstrate that miR-696 is a physical activity-dependent miRNA involved in the translational regulation of PGC-1alpha and skeletal muscle metabolism in mice.

p38 mitogen-activated protein kinase and calcium channels mediate signaling in depolarization-induced activation of peroxisome proliferator-activated receptor gamma coactivator-1alpha in neurons

Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) coactivates a number of transcription factors critical for mitochondrial biogenesis. Previously, we found that the expression of PGC-1alpha is governed by neuronal activity, but the signaling mechanism is poorly understood. The present study aimed at testing our hypothesis that depolarizing activation of PGC-1alpha in neurons is mediated by p38 mitogen-activated protein kinase (MAPK) and calcium channels. Cultured primary neurons and N2a cells were depolarized with 20 mM KCl for varying times, and increases in PGC-1alpha mRNA and protein levels were found after 0.5 and 1 hr of stimulation, respectively. These levels returned to those of controls after the withdrawal of KCl. Significantly, 15 min of KCl stimulation induced an up-regulation of both p38 MAPK and phosphorylated p38 MAPK that were suppressed by 30 min of pretreatment with SB203580, a blocker of p38 MAPK that also blocked the up-regulation of PGC-1alpha by KCl. Likewise, 30 min of pretreatment with nifedipine, a calcium channel blocker, also prevented the up-regulation of PGC-1alpha mRNA and proteins by KCl. Furthermore, a knockdown of p38 MAPK with small interference hairpin RNA significantly suppressed PGC-1alpha mRNA and protein levels. Our results indicate that both p38 MAPK and calcium play important roles in mediating signaling in depolarization-induced activation of PGC-1alpha at the protein and message levels in neurons.

Calcineurin signaling and PGC-1alpha expression are suppressed during muscle atrophy due to diabetes

PGC-1alpha is a transcriptional coactivator that controls energy homeostasis through regulation of glucose and oxidative metabolism. Both PGC-1alpha expression and oxidative capacity are decreased in skeletal muscle of patients and animals undergoing atrophy, suggesting that PGC-1alpha participates in the regulation of muscle mass. PGC-1alpha gene expression is controlled by calcium- and cAMP-sensitive pathways. However, the mechanism regulating PGC-1alpha in skeletal muscle during atrophy remains unclear. Therefore, we examined the mechanism responsible for decreased PGC-1alpha expression using a rodent streptozotocin (STZ) model of chronic diabetes and atrophy. After 21days, the levels of PGC-1alpha protein and mRNA were decreased. We examined the activation state of CREB, a potent activator of PGC-1alpha transcription, and found that phospho-CREB was paradoxically high in muscle of STZ-rats, suggesting that the cAMP pathway was not involved in PGC-1alpha regulation. In contrast, expression of calcineurin (Cn), a calcium-dependent phosphatase, was suppressed in the same muscles. PGC-1alpha expression is regulated by two Cn substrates, MEF2 and NFATc. Therefore, we examined MEF2 and NFATc activity in muscles from STZ-rats. Target genes MRF4 and MCIP1.4 mRNAs were both significantly reduced, consistent with reduced Cn signaling. Moreover, levels of MRF4, MCIP1.4, and PGC-1alpha were also decreased in muscles of CnAalpha-/- and CnAbeta-/- mice without diabetes indicating that decreased Cn signaling, rather than changes in other calcium- or cAMP-sensitive pathways, were responsible for decreased PGC-1alpha expression. These findings demonstrate that Cn activity is a major determinant of PGC-1alpha expression in skeletal muscle during diabetes and possibly other conditions associated with loss of muscle mass.

Deficiency in APOBEC2 leads to a shift in muscle fiber type, diminished body mass, and myopathy

The apoB RNA-editing enzyme, catalytic polypeptide-like (APOBEC) family of proteins includes APOBEC1, APOBEC3, and activation-induced deaminase, all of which are zinc-dependent cytidine deaminases active on polynucleotides and involved in RNA editing or DNA mutation. In contrast, the biochemical and physiological functions of APOBEC2, a muscle-specific member of the family, are unknown, although it has been speculated, like APOBEC1, to be an RNA-editing enzyme. Here, we show that, although expressed widely in striated muscle (with levels peaking late during myoblast differentiation), APOBEC2 is preferentially associated with slow-twitch muscle, with its abundance being considerably greater in soleus compared with gastrocnemius muscle and, within soleus muscle, in slow as opposed to fast muscle fibers. Its abundance also decreases following muscle denervation. We further show that APOBEC2-deficient mice harbor a markedly increased ratio of slow to fast fibers in soleus muscle and exhibit an approximately 15-20% reduction in body mass from birth onwards, with elderly mutant animals revealing clear histological evidence of a mild myopathy. Thus, APOBEC2 is essential for normal muscle development and maintenance of fiber-type ratios; although its molecular function remains to be identified, biochemical analyses do not especially argue for any role in RNA editing.

The transcriptional coactivator PGC-1alpha mediates exercise-induced angiogenesis in skeletal muscle

Peripheral arterial disease (PAD) affects 5 million people in the US and is the primary cause of limb amputations. Exercise remains the single best intervention for PAD, in part thought to be mediated by increases in capillary density. How exercise triggers angiogenesis is not known. PPARgamma coactivator (PGC)-1alpha is a potent transcriptional co-activator that regulates oxidative metabolism in a variety of tissues. We show here that PGC-1alpha mediates exercise-induced angiogenesis. Voluntary exercise induced robust angiogenesis in mouse skeletal muscle. Mice lacking PGC-1alpha in skeletal muscle failed to increase capillary density in response to exercise. Exercise strongly induced expression of PGC-1alpha from an alternate promoter. The induction of PGC-1alpha depended on beta-adrenergic signaling. beta-adrenergic stimulation also induced a broad program of angiogenic factors, including vascular endothelial growth factor (VEGF). This induction required PGC-1alpha. The orphan nuclear receptor ERRalpha mediated the induction of VEGF by PGC-1alpha, and mice lacking ERRalpha also failed to increase vascular density after exercise. These data demonstrate that beta-adrenergic stimulation of a PGC-1alpha/ERRalpha/VEGF axis mediates exercise-induced angiogenesis in skeletal muscle.

Regulation of hypoxia-inducible genes by PGC-1 alpha

Atmospheric oxygen appeared approximately 2.3 billion years ago and sustains most complex life on earth. As mitochondria evolved to harness the energy in oxygen, systems developed to sense and respond to local oxygen concentrations and metabolic conditions. For more than a decade, research has focused on hypoxia-inducible factor 1 (HIF1), a key component of the eukaryotic oxygen-response system. Recently, evidence for other systems has also surfaced. One of these systems involves the PGC-1 alpha coactivator, a powerful transcriptional regulator of mitochondria and oxidative metabolic programs. This brief review will focus on this burgeoning role for PGC-1 alpha and will highlight the many questions that remain unanswered.

Alterations in oxidative gene expression in equine skeletal muscle following exercise and training

Intense selection for elite racing performance in the Thoroughbred horse (Equus caballus) has resulted in a number of adaptive physiological phenotypes relevant to exercise; however, the underlying molecular mechanisms responsible for these characteristics are not well understood. Adaptive changes in mRNA expression in equine skeletal muscle were investigated by real-time qRT-PCR for a panel of candidate exercise-response genes following a standardized incremental-step treadmill exercise test in eight untrained Thoroughbred horses. Biopsy samples were obtained from the gluteus medius before, immediately after, and 4 h after exercise. Significant (P < 0.05) differences in gene expression were detected for six genes (CKM, COX4I1, COX4I2, PDK4, PPARGC1A, and SLC2A4) 4 h after exercise. Investigation of relationships between mRNA and velocity at maximum heart rate (VHR(max)) and peak postexercise plasma lactate concentration ([La]T(1)) revealed significant (P < 0.05) associations with postexercise COX4I1 and PPARCG1A expression and between [La]T(1) and basal COX4I1 expression. Gene expression changes were investigated in a second cohort of horses after a 10 mo period of training. In resting samples, COX4I1 gene expression had significantly increased following training, and, after exercise, significant differences were identified for COX4I2, PDK4, and PPARGC1A. Significant relationships with VHR(max) and [La]T(1) were detected for PPARGC1A and COX4I1. These data highlight the roles of genes responsible for the regulation of oxygen-dependent metabolism, glucose metabolism, and fatty acid utilization in equine skeletal muscle adaptation to exercise.

BAT: a new target for human obesity?

Two types of adipose tissue can be distinguished histologically and functionally: white (WAT) and brown adipose tissue (BAT). Whereas BAT is specialized in the production of heat, WAT stores excess energy as triacylglycerols. BAT is present throughout life in rodents, whereas in humans it was thought to involute rapidly postnatally, having essentially disappeared within the first years after birth. However, positron emission tomography has provided evidence that adults retain metabolically active BAT depots that can be induced in response to cold and sympathetic nervous system activation. These findings together with the recent identification of specific molecular determinants (PRDM16 and BMP7) activating brown adipogenesis highlights BAT as a potential relevant target for pharmacological and gene expression manipulation to combat human obesity.

Alterations in mitochondrial function as a harbinger of cardiomyopathy: lessons from the dystrophic heart

While compelling evidence supports the central role of mitochondrial dysfunction in the pathogenesis of heart failure, there is comparatively less information available on mitochondrial alterations that occur prior to failure. Building on our recent work with the dystrophin-deficient mdx mouse heart, this review focuses on how early changes in mitochondrial functional phenotype occur prior to overt cardiomyopathy and may be a determinant for the development of adverse cardiac remodelling leading to failure. These include alterations in energy substrate utilization and signalling of cell death through increased permeability of mitochondrial membranes, which may result from abnormal calcium handling, and production of reactive oxygen species. Furthermore, we will discuss evidence supporting the notion that these alterations in the dystrophin-deficient heart may represent an early "subclinical" signature of a defective nitric oxide/cGMP signalling pathway, as well as the potential benefit of mitochondria-targeted therapies. While the mdx mouse is an animal model of Duchenne muscular dystrophy (DMD), changes in the structural integrity of dystrophin, the mutated cytoskeletal protein responsible for DMD, have also recently been implicated as a common mechanism for contractile dysfunction in heart failure. In fact, altogether our findings support a critical role for dystrophin in maintaining optimal coupling between metabolism and contraction in the heart.

Leptin administration favors muscle mass accretion by decreasing FoxO3a and increasing PGC-1alpha in ob/ob mice

Absence of leptin has been associated with reduced skeletal muscle mass in leptin-deficient ob/ob mice. The aim of our study was to examine the effect of leptin on the catabolic and anabolic pathways regulating muscle mass. Gastrocnemius, extensor digitorum longus and soleus muscle mass as well as fiber size were significantly lower in ob/ob mice compared to wild type littermates, being significantly increased by leptin administration (P<0.001). This effect was associated with an inactivation of the muscle atrophy-related transcription factor forkhead box class O3 (FoxO3a) (P<0.05), and with a decrease in the protein expression levels of the E3 ubiquitin-ligases muscle atrophy F-box (MAFbx) (P<0.05) and muscle RING finger 1 (MuRF1) (P<0.05). Moreover, leptin increased (P<0.01) protein expression levels of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), a regulator of muscle fiber type, and decreased (P<0.05) myostatin protein, a negative regulator of muscle growth. Leptin administration also activated (P<0.01) the regulators of cell cycle progression proliferating cell nuclear antigen (PCNA) and cyclin D1, and increased (P<0.01) myofibrillar protein troponin T. The present study provides evidence that leptin treatment may increase muscle mass of ob/ob mice by inhibiting myofibrillar protein degradation as well as enhancing muscle cell proliferation.

PGC-1alpha mediates exercise-induced skeletal muscle VEGF expression in mice

The aim of the present study was to test the hypothesis that PGC-1alpha is required for exercise-induced VEGF expression in both young and old mice and that AMPK activation leads to increased VEGF expression through a PGC-1alpha-dependent mechanism. Whole body PGC-1alpha knockout (KO) and littermate wild-type (WT) mice were submitted to either 1) 5 wk of exercise training, 2) lifelong (from 2 to 13 mo of age) exercise training in activity wheel, 3) a single exercise bout, or 4) 4 wk of daily subcutaneous AICAR or saline injections. In skeletal muscle of PGC-1alpha KO mice, VEGF protein expression was approximately 60-80% lower and the capillary-to-fiber ratio approximately 20% lower than in WT. Basal VEGF mRNA expression was similar in WT and PGC-1alpha KO mice, but acute exercise and AICAR treatment increased the VEGF mRNA content in WT mice only. Exercise training of young mice increased skeletal muscle VEGF protein expression approximately 50% in WT mice but with no effect in PGC-1alpha KO mice. Furthermore, a training-induced prevention of an age-associated decline in VEGF protein content was observed in WT but not in PGC-1alpha KO muscles. In addition, repeated AICAR treatments increased skeletal muscle VEGF protein expression approximately 15% in WT but not in PGC-1alpha KO mice. This study shows that PGC-1alpha is essential for exercise-induced upregulation of skeletal muscle VEGF expression and for a training-induced prevention of an age-associated decline in VEGF protein content. Furthermore, the findings suggest an AMPK-mediated regulation of VEGF expression through PGC-1alpha.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscleThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Acute exercise initiates rapid cellular signals, leading to the subsequent activation of proteins that increase gene transcription. The result is a higher level of mRNA expression, often observed during the recovery period following exercise. These molecules are translated into precursor proteins for import into preexisting mitochondria. Once inside the organelle, the protein is processed to its mature form and either activates mitochondrial DNA gene expression, serves as a single subunit enzyme, or is incorporated into multi-subunit complexes of the respiratory chain devoted to electron transport and substrate oxidation. The result of this exercise-induced sequence of events is the expansion of the mitochondrial network within muscle cells and the capacity for aerobic ATP provision. An understanding of the molecular processes involved in this complex pathway of organelle synthesis is important for therapeutic purposes, and is a primary research undertaking in laboratories involved in the study of mitochondrial biogenesis. This pathway in muscle becomes impaired with chronic inactivity and aging, which leads to a reduced muscle aerobic capacity and an increased tendency for mitochondrially mediated apoptosis, a situation that can contribute to muscle atrophy. The resumption, or adoption, of an active lifestyle can ameliorate this metabolic dysfunction, improve endurance, and help maintain muscle mass.

PRDM16: the interconvertible adipo-myocyte switch

Both brown and white adipocytes were previously considered to be derived from the same precursor cell, despite being histologically and functionally different. However, a recent study shows that overexpression of the transcriptional regulator positive regulatory domain containing 16 (PRDM16) determines the development of brown adipocytes from a progenitor that expresses myoblast markers. Surprisingly, loss of PRDM16 from these precursors does not lead to white adipocyte differentiation. Thus, PRDM16 controls a bidirectional cell fate switch between skeletal myoblasts and brown adipocytes.