Adult expression of PGC-1α and -1β in skeletal muscle is not required for endurance exercise-induced enhancement of exercise capacity

Temporal expression of mitochondrial life cycle markers during acute and chronic overload of rat plantaris muscles

Skeletal muscle hypertrophy is generally associated with a fast-to-slow phenotypic adaptation in both human and rodent models. Paradoxically, this phenotypic shift is not paralleled by a concomitant increase in mitochondrial content and aerobic markers that would be expected to accompany a slow muscle phenotype. To understand the temporal response of the mitochondrial life cycle (i.e., biogenesis, oxidative phosphorylation, fission/fusion, and mitophagy/autophagy) to hypertrophic stimuli, in this study, we used the functional overload (FO) model in adult female rats and examined the plantaris muscle responses at 1 and 10 weeks. As expected, the absolute plantaris muscle mass increased by ∼12 and 26% at 1 and 10 weeks following the FO procedure, respectively. Myosin heavy-chain isoform types I and IIa significantly increased by 116% and 17%, respectively, in 10-week FO plantaris muscles. Although there was a general increase in protein markers associated with mitochondrial biogenesis in acute FO muscles, this response was unexpectedly sustained under 10-week FO conditions after muscle hypertrophy begins to plateau. Furthermore, the early increase in mito/autophagy markers observed under acute FO conditions was normalized by 10 weeks, suggesting a cellular environment favoring mitochondrial biogenesis to accommodate the aerobic demands of the plantaris muscle. We also observed a significant increase in the expression of mitochondrial-, but not nuclear-, encoded oxidative phosphorylation (OXPHOS) proteins and peptides (i.e., humanin and MOTS-c) under chronic, but not acute, FO conditions. Taken together, the temporal response of markers related to the mitochondrial life cycle indicates a pattern of promoting biogenesis and mitochondrial protein expression to support the energy demands and/or enhanced neural recruitment of chronically overloaded skeletal muscle.

Retention of stress susceptibility in the mdx mouse model of Duchenne muscular dystrophy after PGC-1α overexpression or ablation of IDO1 or CD38

Duchenne muscular dystrophy (DMD) is a lethal degenerative muscle wasting disease caused by the loss of the structural protein dystrophin with secondary pathological manifestations including metabolic dysfunction, mood and behavioral disorders. In the mildly affected mdx mouse model of DMD, brief scruff stress causes inactivity, while more severe subordination stress results in lethality. Here, we investigated the kynurenine pathway of tryptophan degradation and the nicotinamide adenine dinucleotide (NAD+) metabolic pathway in mdx mice and their involvement as possible mediators of mdx stress-related pathology. We identified downregulation of the kynurenic acid shunt, a neuroprotective branch of the kynurenine pathway, in mdx skeletal muscle associated with attenuated peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) transcriptional regulatory activity. Restoring the kynurenic acid shunt by skeletal muscle-specific PGC-1α overexpression in mdx mice did not prevent scruff -induced inactivity, nor did abrogating extrahepatic kynurenine pathway activity by genetic deletion of the pathway rate-limiting enzyme, indoleamine oxygenase 1. We further show that reduced NAD+ production in mdx skeletal muscle after subordination stress exposure corresponded with elevated levels of NAD+ catabolites produced by ectoenzyme cluster of differentiation 38 (CD38) that have been implicated in lethal mdx response to pharmacological β-adrenergic receptor agonism. However, genetic CD38 ablation did not prevent mdx scruff-induced inactivity. Our data do not support a direct contribution by the kynurenine pathway or CD38 metabolic dysfunction to the exaggerated stress response of mdx mice.

Exercise induces tissue-specific adaptations to enhance cardiometabolic health

The risk associated with multiple cancers, cardiovascular disease, diabetes, and all-cause mortality is decreased in individuals who meet the current recommendations for physical activity. Therefore, regular exercise remains a cornerstone in the prevention and treatment of non-communicable diseases. An acute bout of exercise results in the coordinated interaction between multiple tissues to meet the increased energy demand of exercise. Over time, the associated metabolic stress of each individual exercise bout provides the basis for long-term adaptations across tissues, including the cardiovascular system, skeletal muscle, adipose tissue, liver, pancreas, gut, and brain. Therefore, regular exercise is associated with a plethora of benefits throughout the whole body, including improved cardiorespiratory fitness, physical function, and glycemic control. Overall, we summarize the exercise-induced adaptations that occur within multiple tissues and how they converge to ultimately improve cardiometabolic health.

Exercise-induced modulation of myokine irisin on muscle-bone unit in the rat model of post-traumatic osteoarthritis

BACKGROUND AND AIM

Post-traumatic osteoarthritis (PTOA) is a subtype of osteoarthritis (OA). Exercise may produce and release the myokine irisin through muscle fiber contraction. However, the effect of exercise-promoted irisin production on the internal interactions of the muscle-bone unit in PTOA studies remains unclear.

METHODS

Eighteen 8-week-old Sprague-Dawley (SD) rats were randomly divided into three groups: Sham/sedentary (Sham/Sed), PTOA/sedentary (PTOA/Sed), and PTOA/treadmill-walking (PTOA/TW). The PTOA model was established by transection of anterior cruciate ligament (ACLT) and destabilization of medial meniscus (DMM). After 4 weeks of modeling, the PTOA/TW group underwent treadmill exercise (15 m/min, 30 min/d, 5 d/ week, 8 weeks), and the other two groups were free to move in the cage. Evaluation and correlation analysis of muscle, cartilage, subchondral bone and serological indexes were performed after euthanasia.

RESULTS

Eight weeks of treadmill exercise effectively alleviated the trauma-induced OA phenotype, thereby maintaining cartilage and subchondral bone integrity in PTOA, and reducing quadriceps atrophy and myofibril degradation. Exercise reversed the down-regulated expression of peroxisome proliferator-activated receptor-gamma coactivator-1α (PGC-1α) and fibronectin type III structural domain protein 5 (FNDC5) in muscle tissue of PTOA rats, and increased the blood irisin level, and the irisin level was positively correlated with the expression of PGC-1α and FNDC5. In addition, correlation analysis showed that irisin metabolism level was strongly negatively correlated with Osteoarthritis Research Society International (OARSI) and subchondral bone loss, indicating that irisin may be involved in cartilage biology and PTOA-related changes in cartilage and subchondral bone. Moreover, the metabolic level of irisin was strongly negatively correlated with muscle fiber cross-sectional area (CSA), Atrogin-1 and muscle ring-finger protein-1(MuRF-1) expression, suggesting that irisin may alleviate muscle atrophy through autocrine action.

CONCLUSION

Treadmill exercise can alleviate the atrophy and degeneration of muscle fibers in PTOA rats, reduce the degradation of muscle fibrin, promote the expression of serum irisin, and alleviate the degeneration of articular cartilage and subchondral bone loss in PTOA rats. These results indicate that treadmill exercise can affect the process of PTOA by promoting the expression of myokine irisin in rat muscle-bone unit.

Exercise metabolism and adaptation in skeletal muscle

Viewing metabolism through the lens of exercise biology has proven an accessible and practical strategy to gain new insights into local and systemic metabolic regulation. Recent methodological developments have advanced understanding of the central role of skeletal muscle in many exercise-associated health benefits and have uncovered the molecular underpinnings driving adaptive responses to training regimens. In this Review, we provide a contemporary view of the metabolic flexibility and functional plasticity of skeletal muscle in response to exercise. First, we provide background on the macrostructure and ultrastructure of skeletal muscle fibres, highlighting the current understanding of sarcomeric networks and mitochondrial subpopulations. Next, we discuss acute exercise skeletal muscle metabolism and the signalling, transcriptional and epigenetic regulation of adaptations to exercise training. We address knowledge gaps throughout and propose future directions for the field. This Review contextualizes recent research of skeletal muscle exercise metabolism, framing further advances and translation into practice.

Adult skeletal muscle peroxisome proliferator-activated receptor γ -related coactivator 1 is involved in maintaining mitochondrial content

The peroxisome proliferator-activated receptor γ coactivator-1 (PGC-1) family of transcriptional coactivators are regulators of mitochondrial oxidative capacity and content in skeletal muscle. Many of these conclusions are based primarily on gain-of-function studies using muscle-specific overexpression of PGC1s. We have previously reported that genetic deletion of both PGC-1α and PGC-1β in adult skeletal muscle resulted in a significant reduction in oxidative capacity with no effect on mitochondrial content. However, the contribution of PGC-1-related coactivator (PRC), the third PGC-1 family member, in regulating skeletal muscle mitochondria is unknown. Therefore, we generated an inducible skeletal muscle-specific PRC knockout mouse (iMS-PRC-KO) to assess the contribution of PRC in skeletal muscle mitochondrial function. We measured mRNA expression of electron transport chain (ETC) subunits as well as markers of mitochondrial content in the iMS-PRC-KO animals and observed an increase in ETC gene expression and mitochondrial content. Furthermore, the increase in ETC gene expression and mitochondrial content was associated with increased expression of PGC-1α and PGC-1β. We therefore generated an adult-inducible PGC-1 knockout mouse in which all PGC-1 family members are deleted (iMS-PGC-1TKO). The iMS-PGC-1TKO animals exhibited a reduction in ETC mRNA expression and mitochondrial content. These data suggest that in the absence of PRC alone, compensation occurs by increasing PGC-1α and PGC-1β to maintain mitochondrial content. Moreover, the removal of all three PGC-1s in skeletal muscle results in a reduction in both ETC mRNA expression and mitochondrial content. Taken together, these results suggest that PRC plays a role in maintaining baseline mitochondrial content in skeletal muscle.

Aerobic exercise-mediated changes in the expression of glucocorticoid responsive genes in skeletal muscle differ across the day

Glucocorticoids are released in response to acute aerobic exercise. The objective was to define changes in the expression of glucocorticoid target genes in skeletal muscle in response to acute aerobic exercise at different times of day. We identified glucocorticoid target genes altered in skeletal muscle by acute exercise by comparing data sets from rodents subjected to acute aerobic exercise in the light or dark cycles to data sets from C2C12 myotubes treated with glucocorticoids. The role of glucocorticoid receptor signaling and REDD1 protein in mediating gene expression was assessed in exercised mice. Changes to expression of glucocorticoid genes were greater when exercise occurred in the dark cycle. REDD1 was required for the induction of genes induced at both times of day. In all, the time of day at which aerobic exercise is conducted dictates changes to the expression of glucocorticoid target genes in skeletal muscle with REDD1 contributing to those changes.

Larger improvements in fatigue resistance and mitochondrial function with high- than with low-intensity contractions during interval training of mouse skeletal muscle

Interval training (IT) results in improved fatigue resistance in skeletal muscle mainly due to an increased aerobic capacity, which involves increased muscle mitochondrial content and/or improved mitochondrial function. We hypothesized that IT with high-intensity contractions is more effective in increasing mitochondrial function, and hence fatigue resistance, than low-intensity contractions. To study this hypothesis without interference from differences in muscle fiber recruitment obliged to occur during voluntary contractions, IT was performed with in situ supramaximal electrical stimulation where all muscle fibers are recruited. We compared the effect of IT with repeated low-intensity (20 Hz stimulation, IT20) and high-intensity (100 Hz stimulation, IT100) contractions on fatigue resistance and mitochondrial content and function in mouse plantar flexor muscles. Muscles were stimulated every other day for 4 weeks. The averaged peak torque during IT bouts was 4.2-fold higher with IT100 than with IT20. Both stimulation protocols markedly improved in situ fatigue resistance, although the improvement was larger with IT100. The citrate synthase activity, a biomarker of mitochondrial content, was similarly increased with IT20 and IT100. Conversely, increased expression of mitochondrial respiratory chain (MRC) complexes I, III, and IV was only observed with IT100 and this was accompanied by increases in MRC supercomplex formation and pyruvate-malate-driven state 3 respiration in isolated mitochondria. In conclusion, the IT-induced increase in fatigue resistance is larger with high-intensity than with low-intensity contractions and this is linked to improved mitochondrial function due to increased expression of MRC complexes and assembly of MRC supercomplexes.

Reduced mitochondrial DNA and OXPHOS protein content in skeletal muscle of children with cerebral palsy

AIM

To provide a detailed gene and protein expression analysis related to mitochondrial biogenesis and assess mitochondrial content in skeletal muscle of children with cerebral palsy (CP).

METHOD

Biceps brachii muscle samples were collected from 19 children with CP (mean [SD] age 15y 4mo [2y 6mo], range 9-18y, 16 males, three females) and 10 typically developing comparison children (mean [SD] age 15y [4y], range 7-21y, eight males, two females). Gene expression (quantitative reverse transcription polymerase chain reaction [PCR]), mitochondrial DNA (mtDNA) to genomic DNA ratio (quantitative PCR), and protein abundance (western blotting) were analyzed. Microarray data sets (CP/aging/bed rest) were analyzed with a focused query investigating metabolism- and mitochondria-related gene networks.

RESULTS

The mtDNA to genomic DNA ratio was lower in the children with CP compared to the typically developing group (-23%, p=0.002). Out of five investigated complexes in the mitochondrial respiratory chain, we observed lower protein levels of all complexes (I, III, IV, V, -20% to -37%; p<0.05) except complex II. Total peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) messenger RNA (p<0.004), isoforms PGC1α1 (p=0.05), and PGC1α4 (p<0.001) were reduced in CP. Transcriptional similarities were observed between CP, aging, and 90 days' bed rest.

INTERPRETATION

Mitochondrial biogenesis, mtDNA, and oxidative phosphorylation protein content are reduced in CP muscle compared with typically developing muscle. Transcriptional pathways shared between aging and long-term unloading suggests metabolic dysregulation in CP, which may guide therapeutic strategies for combatting CP muscle pathology. What this paper adds Cerebral palsy (CP) muscle contains fewer energy-generating organelles than typically developing muscle. Gene expression in CP muscle is similar to aging and long-term bed rest.

Effects of hyperbaric environment on endurance and metabolism are exposure time-dependent in well-trained mice

Hyperbaric exposure (1.3 atmospheres absolute with 20.9% O2 ) for 1 h a day was shown to improve exercise capacity. The present study was designed to reveal whether the daily exposure time affects exercise performance and metabolism in skeletal and cardiac muscles. Male mice in the training group were housed in a cage with a wheel activity device for 7 weeks from 5 weeks old. Trained mice were then subjected to hybrid training (HT, endurance exercise for 30 min followed by sprint interval exercise for 30 min). Hyperbaric exposure was applied following daily HT for 15 min (15HT), 30 min (30HT), or 60 min (60HT) for 4 weeks. In the endurance capacity test, maximal work values were significantly increased by 30HT and 60HT. In the left ventricle (LV), activity levels of 3-hydroxyacyl-CoA-dehydrogenase, citrate synthase, and carnitine palmitoyl transferase (CPT) 2 were significantly increased by 60HT. CPT2 activity levels were markedly increased by hyperbaric exposure in red gastrocnemius (Gr) and plantaris muscle (PL). Pyruvate dehydrogenase complex activity values in PL were enhanced more by 30HT and 60HT than by HT. Protein levels of N-terminal isoform of PGC1α (NT-PGC1α) protein were significantly enhanced in three hyperbaric exposed groups in Gr, but not in LV. These results indicate that hyperbaric exposure for 30 min or longer has beneficial effects on endurance, and 60-min exposure has the potential to further increase performance by facilitating fatty acid metabolism in skeletal and cardiac muscles in highly trained mice. NT-PGC1α may have important roles for these adaptations in skeletal muscle.

Molecular Basis for the Therapeutic Effects of Exercise on Mitochondrial Defects

Mitochondrial dysfunction is common to many organ system disorders, including skeletal muscle. Aging muscle and diseases of muscle are often accompanied by defective mitochondrial ATP production. This manuscript will focus on the pre-clinical evidence supporting the use of regular exercise to improve defective mitochondrial metabolism and function in skeletal muscle, through the stimulation of mitochondrial turnover. Examples from aging muscle, muscle-specific mutations and cancer cachexia will be discussed. We will also examine the effects of exercise on the important mitochondrial regulators PGC-1α, and Parkin, and summarize the effects of exercise to reverse mitochondrial dysfunction (e.g., ROS production, apoptotic susceptibility, cardiolipin synthesis) in muscle pathology. This paper will illustrate the breadth and benefits of exercise to serve as "mitochondrial medicine" with age and disease.

PGC-1α-mediated regulation of mitochondrial function and physiological implications

The majority of human energy metabolism occurs in skeletal muscle mitochondria emphasizing the importance of understanding the regulation of myocellular mitochondrial function. The transcriptional co-activator peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) has been characterized as a major factor in the transcriptional control of several mitochondrial components. Thus, PGC-1α is often described as a master regulator of mitochondrial biogenesis as well as a central player in regulating the antioxidant defense. However, accumulating evidence suggests that PGC-1α is also involved in the complex regulation of mitochondrial quality beyond biogenesis, which includes mitochondrial network dynamics and autophagic removal of damaged mitochondria. In addition, mitochondrial reactive oxygen species production has been suggested to regulate skeletal muscle insulin sensitivity, which may also be influenced by PGC-1α. This review aims to highlight the current evidence for PGC-1α-mediated regulation of skeletal muscle mitochondrial function beyond the effects on mitochondrial biogenesis as well as the potential PGC-1α-related impact on insulin-stimulated glucose uptake in skeletal muscle. Novelty PGC-1α regulates mitochondrial biogenesis but also has effects on mitochondrial functions beyond biogenesis. Mitochondrial quality control mechanisms, including fission, fusion, and mitophagy, are regulated by PGC-1α. PGC-1α-mediated regulation of mitochondrial quality may affect age-related mitochondrial dysfunction and insulin sensitivity.

CORP: Using transgenic mice to study skeletal muscle physiology

The development of tissue-specific inducible transgenic mice has provided a powerful tool to study gene function and cell biology in almost any tissue of interest at any given time within the animal's life. The purpose of this review is to describe how to use two different inducible transgenic systems, the Cre-loxP system and the Tet-ON/OFF system, that can be used to study skeletal muscle physiology. Myofiber- and satellite cell-specific Cre-loxP transgenic mice are described as is how these mice can be used to knockout a gene of interest or to deplete satellite cells in adult skeletal muscle, respectively. A myofiber-specific Tet-ON system is described as is how such mice can be used to overexpress a gene of interest or to label myonuclei. How to effectively breed and genotype the transgenic mice are also described in detail. The hope is this review will provide the basic information necessary to facilitate the incorporation of tissue-specific inducible transgenic mice into a skeletal muscle research program.

Synergic effects of exercise training and octopamine on peroxisome proliferator-activated receptor-gamma coactivator -1a and uncoupling protein 1 mRNA in heart tissue of rat treated with deep frying oil

Octopamine (OCT) have an adverse effect on heart function. One of the positive effects of exercise training is improving cardiac function and cardiomyocytes signaling, which along with herbal supplements can have better effects on the heart tissue. Therefore, the aim of this study was to evaluate the effects of exercise training and OCT on changes of PGC1α and UCP1 expression in heart tissue of rat treated with deep frying oil (DFO). In this study, 45 male wistar rats were divided into 5 groups (n = 9 in each): I) control (Co), II) DFO, III) DFO + exercise, IV) DFO + OCT, and V) DFO + OCT + exercise. The quantification of apoptotic effects of DFO in heart tissue was assessed by TUNEL assay. Masson's trichrome stain applied to study cardiomyocytic fibers. Moreover, PGC1α and UCP1 genes and proteins expression in all groups were investigated using quantitative real-time PCR and immunohistochemical method. A significant increase in apoptotic cells was observed in the DFO-treated group (p < 0.05). In Masson's Trichrome stain study, more cardiomyocytic fibers were observed and some lymphocytic cells were present in some fibers. Also, the expression of PGC1α and UCP1 was significantly increase in DFO + exercise group, DFO + OCT group, and DFO + OCT + exercise group compare to DFO group (p < 0.05). Based on these findings, exercise and octopamine can be considered as factors affecting the expression of PGC1α genes and UCP1 as well as drug poisoning.

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Exercise training with octopamine can be upregulate expression of PGC1α.

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Exercise training with octopamine can be upregulate expression of UCP1.

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Increase the PGC1α and UCP1 can improve the mitochondria disfunction induced by DFO.

The impact of acute and chronic exercise on Nrf2 expression in relation to markers of mitochondrial biogenesis in human skeletal muscle

PURPOSE

To examine the relationship between changes in nuclear factor erythroid 2-related factor 2 (Nrf2) expression and markers of mitochondrial biogenesis in acutely and chronically exercised human skeletal muscle.

METHODS

The impact of acute submaximal endurance (END) and supramaximal interval (Tabata) cycling on the upregulation of Nrf2 (and its downstream targets), nuclear respiratory factor-1 (NRF-1) and mitochondrial transcription factor A (TFAM) mRNA expression was examined in healthy young males (n = 10). The relationship between changes in citrate synthase (CS) maximal activity and the protein content of Nrf2, heme oxygenase 1 (HO-1), NRF-1, and TFAM was also investigated following 4 weeks of Tabata in a separate group of males (n = 21).

RESULTS

Nrf2, NRF-1, and HO-1 mRNA expression increased after acute exercise (p < 0.05), whereas the increase in superoxide dismutase 2 (SOD2) mRNA expression approached significance (p = 0.08). Four weeks of Tabata increased CS activity and Nrf2, NRF-1, and TFAM protein content (p < 0.05), but decreased HO-1 protein content (p < 0.05). Training-induced changes in Nrf2 protein were strongly correlated with NRF-1 (r = 0.63, p < 0.01). When comparing protein content changes between individuals with the largest (HI: + 23%) and smallest (LO: - 1%) observed changes in CS activity (n = 8 each), increases in Nrf2 and TFAM protein content were apparent in the HI group only (p < 0.02) with medium-to-large effect sizes for between-group differences in changes in Nrf2 (η_p²₌0.15) and TFAM (η_p² = 0.12) protein content.

CONCLUSION

Altogether, our findings support a potential role for Nrf2 in exercise-induced mitochondrial biogenesis in human skeletal muscle.

PGC-1α regulates mitochondrial properties beyond biogenesis with aging and exercise training

Impaired mitochondrial function has been implicated in the pathogenesis of age-associated metabolic diseases through regulation of cellular redox balance. Exercise training is known to promote mitochondrial biogenesis in part through induction of the transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α). Recently, mitochondrial ADP sensitivity has been linked to reactive oxygen species (ROS) emission with potential impact on age-associated physiological outcomes, but the underlying molecular mechanisms remain unclear. Therefore, the present study investigated the effects of aging and exercise training on mitochondrial properties beyond biogenesis, including respiratory capacity, ADP sensitivity, ROS emission, and mitochondrial network structure, in myofibers from inducible muscle-specific PGC-1α-knockout mice and control mice. Aged mice displayed lower running endurance and mitochondrial respiratory capacity than young mice. This was associated with intermyofibrillar mitochondrial network fragmentation, diminished submaximal ADP-stimulated respiration, increased mitochondrial ROS emission, and oxidative stress. Exercise training reversed the decline in maximal respiratory capacity independent of PGC-1α, whereas exercise training rescued the age-related mitochondrial network fragmentation and the impaired submaximal ADP-stimulated respiration in a PGC-1α-dependent manner. Furthermore, lack of PGC-1α was associated with altered phosphorylation and carbonylation of the inner mitochondrial membrane ADP/ATP exchanger adenine nucleotide translocase 1. In conclusion, the present study provides evidence that PGC-1α regulates submaximal ADP-stimulated respiration, ROS emission, and mitochondrial network structure in mouse skeletal muscle during aging and exercise training.

Concepts About V˙O2max and Trainability Are Context Dependent

Some individuals show little or no increase in maximal oxygen consumption (V˙O2max) in response to training programs consistent with public health guidelines. However, results from studies using more intense programs challenge the concept that some humans have limited trainability. We explore the implications of these divergent observations on the biology of trainability and propose a new set of twin studies to explore them.

Adult skeletal muscle deletion of Mitofusin 1 and 2 impedes exercise performance and training capacity

Endurance exercise has been shown to be a positive regulator of skeletal muscle metabolic function. Changes in mitochondrial dynamics (fusion and fission) have been shown to influence mitochondrial oxidative capacity. We therefore tested whether genetic disruption of mitofusins (Mfns) affected exercise performance in adult skeletal muscle. We generated adult-inducible skeletal muscle-specific Mfn1 (iMS-Mfn1KO), Mfn2 (iMS-Mfn2KO), and Mfn1/2 (iMS-MfnDKO) knockout mice. We assessed exercise capacity by performing a treadmill time to exhaustion stress test before deletion and up to 8 wk after deletion. Analysis of either the iMS-Mfn1KO or the iMS-Mfn2KO did not reveal an effect on exercise capacity. However, analysis of iMS-MfnDKO animals revealed a progressive reduction in exercise performance. We measured individual electron transport chain (ETC) complex activity and observed a reduction in ETC activity in both the subsarcolemmal and intermyofibrillar mitochondrial fractions specifically for NADH dehydrogenase (complex I) and cytochrome- c oxidase (complex IV), which was associated with a decrease in ETC subunit expression for these complexes. We also tested whether voluntary exercise training would prevent the decrease in exercise capacity observed in iMS-MfnDKO animals ( n = 10/group). However, after 8 wk of training we did not observe any improvement in exercise capacity or ETC subunit parameters in iMS-MfnDKO animals. These data suggest that the decrease in exercise capacity observed in the iMS-MfnDKO animals is in part the result of impaired ETC subunit expression and ETC complex activity. Taken together, these results provide strong evidence that mitochondrial fusion in adult skeletal muscle is important for exercise performance. NEW & NOTEWORTHY This study is the first to utilize an adult-inducible skeletal muscle-specific knockout model for Mitofusin (Mfn)1 and Mfn2 to assess exercise capacity. Our findings reveal a progressive decrease in exercise performance with Mfn1 and Mfn2 deletion. The decrease in exercise capacity was accompanied by impaired oxidative phosphorylation specifically for complex I and complex IV. Furthermore, voluntary exercise training was unable to rescue the impairment, suggesting that normal fusion is essential for exercise-induced mitochondrial adaptations.

Molecular Basis of Exercise-Induced Skeletal Muscle Mitochondrial Biogenesis: Historical Advances, Current Knowledge, and Future Challenges

We provide an overview of groundbreaking studies that laid the foundation for our current understanding of exercise-induced mitochondrial biogenesis and its contribution to human skeletal muscle fitness. We highlight the mechanisms by which skeletal muscle responds to the acute perturbations in cellular energy homeostasis evoked by a single bout of endurance-based exercise and the adaptations resulting from the repeated demands of exercise training that ultimately promote mitochondrial biogenesis through hormetic feedback loops. Despite intense research efforts to elucidate the cellular mechanisms underpinning mitochondrial biogenesis in skeletal muscle, translating this basic knowledge into improved metabolic health at the population level remains a future challenge.

Skeletal muscle mitochondrial remodeling in exercise and diseases

Skeletal muscle fitness and plasticity is an important determinant of human health and disease. Mitochondria are essential for maintaining skeletal muscle energy homeostasis by adaptive re-programming to meet the demands imposed by a myriad of physiologic or pathophysiological stresses. Skeletal muscle mitochondrial dysfunction has been implicated in the pathogenesis of many diseases, including muscular dystrophy, atrophy, type 2 diabetes, and aging-related sarcopenia. Notably, exercise counteracts the effects of many chronic diseases on skeletal muscle mitochondrial function. Recent studies have revealed a finely tuned regulatory network that orchestrates skeletal muscle mitochondrial biogenesis and function in response to exercise and in disease states. In addition, increasing evidence suggests that mitochondria also serve to "communicate" with the nucleus and mediate adaptive genomic re-programming. Here we review the current state of knowledge relevant to the dynamic remodeling of skeletal muscle mitochondria in response to exercise and in disease states.

ERRγ Promotes Angiogenesis, Mitochondrial Biogenesis, and Oxidative Remodeling in PGC1α/β-Deficient Muscle

PGC1α is a pleiotropic co-factor that affects angiogenesis, mitochondrial biogenesis, and oxidative muscle remodeling via its association with multiple transcription factors, including the master oxidative nuclear receptor ERRγ. To decipher their epistatic relationship, we explored ERRγ gain of function in muscle-specific PGC1α/β double-knockout (PKO) mice. ERRγ-driven transcriptional reprogramming largely rescues muscle damage and improves muscle function in PKO mice, inducing mitochondrial biogenesis, antioxidant defense, angiogenesis, and a glycolytic-to-oxidative fiber-type transformation independent of PGC1α/β. Furthermore, in combination with voluntary exercise, ERRγ gain of function largely restores mitochondrial energetic deficits in PKO muscle, resulting in a 5-fold increase in running performance. Thus, while PGC1s can interact with multiple transcription factors, these findings implicate ERRs as the major molecular target through which PGC1α/β regulates both innate and adaptive energy metabolism.

Expression of striated activator of rho-signaling in human skeletal muscle following acute exercise and long-term training

The striated activator of rho-signaling (STARS) protein acts as a link between external stimuli and exercise adaptation such as muscle hypertrophy. However, the acute and long-term adaptational response of STARS is still unclear. This study aimed at investigating the acute and long-term endurance training response on the mRNA and protein expression of STARS and its related upstream and downstream factors in human skeletal muscle. mRNA and protein levels of STARS and related factors were assessed in skeletal muscle of healthy young men and women following an acute bout of endurance exercise (n = 15) or 12 weeks of one-legged training (n = 23). Muscle biopsies were obtained before (acute and long-term), at 30 min, 2, and 6 h following acute exercise, and at 24 h following both acute exercise and long-term training. Following acute exercise, STARS mRNA was significantly elevated 3.9-fold at 30 min returning back to baseline 24 h after exercise. STARS protein levels were numerically but nonsignificantly increased 7.2-fold at 24 h. No changes in STARS or ERRα mRNA or STARS protein expression were seen following long-term training. PGC-1α mRNA increased 1.7-fold following long-term training. MRTF-A mRNA was increased both following acute exercise and long-term training, in contrast to SRF mRNA and protein which did not change. STARS mRNA is acutely upregulated with exercise, but there is no cumulative effect to long-term training as seen in PGC-1α mRNA expression. Exercise intensity might play a role in manifestation of protein expression, suggesting a more complex regulation of STARS.

Coordination of mitochondrial biogenesis by PGC-1α in human skeletal muscle: A re-evaluation

The transcriptional co-activator peroxisome proliferator-activated receptor gamma co-activator-1 alpha (PGC-1α) is proposed to coordinate skeletal muscle mitochondrial biogenesis through the integrated induction of nuclear- and mitochondrial-encoded gene transcription. This paradigm is based largely on experiments demonstrating PGC-1α's ability to co-activate various nuclear transcription factors that increase the expression of mitochondrial genes, as well as PGC-1α's direct interaction with mitochondrial transcription factor A within mitochondria to increase the transcription of mitochondrial DNA. While this paradigm is supported by evidence from cellular and transgenic animal models, as well as acute exercise studies involving animals, the up-regulation of nuclear- and mitochondrial-encoded genes in response to exercise does not appear to occur in a coordinated fashion in human skeletal muscle. This review re-evaluates our current understanding of this phenomenon by highlighting evidence from recent studies examining the exercise-induced expression of nuclear- and mitochondrial-encoded genes targeted by PGC-1α. We also highlight several possible theories that may explain the apparent inability of PGC-1α to coordinately up-regulate the expression of genes required for mitochondrial biogenesis in human skeletal muscle, and provide directions for future work exploring mitochondrial biogenic gene expression following exercise.

REDD1 induction regulates the skeletal muscle gene expression signature following acute aerobic exercise

The metabolic stress placed on skeletal muscle by aerobic exercise promotes acute and long-term health benefits in part through changes in gene expression. However, the transducers that mediate altered gene expression signatures have not been completely elucidated. Regulated in development and DNA damage 1 (REDD1) is a stress-induced protein whose expression is transiently increased in skeletal muscle following acute aerobic exercise. However, the role of this induction remains unclear. Because REDD1 altered gene expression in other model systems, we sought to determine whether REDD1 induction following acute exercise altered the gene expression signature in muscle. To do this, wild-type and REDD1-null mice were randomized to remain sedentary or undergo a bout of acute treadmill exercise. Exercised mice recovered for 1, 3, or 6 h before euthanization. Acute exercise induced a transient increase in REDD1 protein expression within the plantaris only at 1 h postexercise, and the induction occurred in both cytosolic and nuclear fractions. At this time point, global changes in gene expression were surveyed using microarray. REDD1 induction was required for the exercise-induced change in expression of 24 genes. Validation by RT-PCR confirmed that the exercise-mediated changes in genes related to exercise capacity, muscle protein metabolism, neuromuscular junction remodeling, and Metformin action were negated in REDD1-null mice. Finally, the exercise-mediated induction of REDD1 was partially dependent upon glucocorticoid receptor activation. In all, these data show that REDD1 induction regulates the exercise-mediated change in a distinct set of genes within skeletal muscle.

Development of dilated cardiomyopathy and impaired calcium homeostasis with cardiac-specific deletion of ESRRβ

Mechanisms underlying the development of idiopathic dilated cardiomyopathy (DCM) remain poorly understood. Using transcription factor expression profiling, we identified estrogen-related receptor-β (ESRRβ), a member of the nuclear receptor family of transcription factors, as highly expressed in murine hearts and other highly oxidative striated muscle beds. Mice bearing cardiac-specific deletion of ESRRβ (MHC-ERRB KO) develop DCM and sudden death at ~10 mo of age. Isolated adult cardiomyocytes from the MHC-ERRB KO mice showed an increase in calcium sensitivity and impaired cardiomyocyte contractility, which preceded echocardiographic cardiac remodeling and dysfunction by several months. Histological analyses of myocardial biopsies from patients with various cardiomyopathies revealed that ESRRβ protein is absent from the nucleus of cardiomyocytes from patients with DCM but not other forms of cardiomyopathy (ischemic, hypertrophic, and arrhythmogenic right ventricular cardiomyopathy). Taken together these observations suggest that ESRRβ is a critical component in the onset of DCM by affecting contractility and calcium balance.NEW & NOTEWORTHY Estrogen-related receptor-β (ESRRβ) is highly expressed in the heart and cardiac-specific deletion results in the development of a dilated cardiomyopathy (DCM). ESRRβ is mislocalized in human myocardium samples with DCM, suggesting a possible role for ESRRβ in the pathogenesis of DCM in humans.