New insights in the regulation of skeletal muscle PGC-1α by exercise and metabolic diseases

Molecular aspects of the exercise response and training adaptation in skeletal muscle

Skeletal muscle plasticity enables an enormous potential to adapt to various internal and external stimuli and perturbations. Most notably, changes in contractile activity evoke a massive remodeling of biochemical, metabolic and force-generating properties. In recent years, a large number of signals, sensors, regulators and effectors have been implicated in these adaptive processes. Nevertheless, our understanding of the molecular underpinnings of training adaptation remains rudimentary. Specifically, the mechanisms that underlie signal integration, output coordination, functional redundancy and other complex traits of muscle adaptation are unknown. In fact, it is even unclear how stimulus-dependent specification is brought about in endurance or resistance exercise. In this review, we will provide an overview on the events that describe the acute perturbations in single endurance and resistance exercise bouts. Furthermore, we will provide insights into the molecular principles of long-term training adaptation. Finally, current gaps in knowledge will be identified, and strategies for a multi-omic and -cellular analyses of the molecular mechanisms of skeletal muscle plasticity that are engaged in individual, acute exercise bouts and chronic training adaptation discussed.

The Genomic Context and Corecruitment of SP1 Affect ERRα Coactivation by PGC-1α in Muscle Cells

The peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) coordinates the transcriptional network response to promote an improved endurance capacity in skeletal muscle, eg, by coactivating the estrogen-related receptor-α (ERRα) in the regulation of oxidative substrate metabolism. Despite a close functional relationship, the interaction between these 2 proteins has not been studied on a genomic level. We now mapped the genome-wide binding of ERRα to DNA in a skeletal muscle cell line with elevated PGC-1α and linked the DNA recruitment to global PGC-1α target gene regulation. We found that, surprisingly, ERRα coactivation by PGC-1α is only observed in the minority of all PGC-1α recruitment sites. Nevertheless, a majority of PGC-1α target gene expression is dependent on ERRα. Intriguingly, the interaction between these 2 proteins is controlled by the genomic context of response elements, in particular the relative GC and CpG content, monomeric and dimeric repeat-binding site configuration for ERRα, and adjacent recruitment of the transcription factor specificity protein 1. These findings thus not only reveal a novel insight into the regulatory network underlying muscle cell plasticity but also strongly link the genomic context of DNA-response elements to control transcription factor-coregulator interactions.

Complex Coordination of Cell Plasticity by a PGC-1α-controlled Transcriptional Network in Skeletal Muscle

Skeletal muscle cells exhibit an enormous plastic capacity in order to adapt to external stimuli. Even though our overall understanding of the molecular mechanisms that underlie phenotypic changes in skeletal muscle cells remains poor, several factors involved in the regulation and coordination of relevant transcriptional programs have been identified in recent years. For example, the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) is a central regulatory nexus in the adaptation of muscle to endurance training. Intriguingly, PGC-1α integrates numerous signaling pathways and translates their activity into various transcriptional programs. This selectivity is in part controlled by differential expression of PGC-1α variants and post-translational modifications of the PGC-1α protein. PGC-1α-controlled activation of transcriptional networks subsequently enables a spatio-temporal specification and hence allows a complex coordination of changes in metabolic and contractile properties, protein synthesis and degradation rates and other features of trained muscle. In this review, we discuss recent advances in our understanding of PGC-1α-regulated skeletal muscle cell plasticity in health and disease.

Skeletal muscle as an endocrine organ: PGC-1α, myokines and exercise

An active lifestyle is crucial to maintain health into old age; inversely, sedentariness has been linked to an elevated risk for many chronic diseases. The discovery of myokines, hormones produced by skeletal muscle tissue, suggests the possibility that these might be molecular mediators of the whole body effects of exercise originating from contracting muscle fibers. Even though less is known about the sedentary state, the lack of contraction-induced myokines or the production of a distinct set of hormones in the inactive muscle could likewise contribute to pathological consequences in this context. In this review, we try to summarize the most recent developments in the study of muscle as an endocrine organ and speculate about the potential impact on our understanding of exercise and sedentary physiology, respectively. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Regulation of skeletal muscle mitochondrial function by nuclear receptors: implications for health and disease

Skeletal muscle metabolism is highly dependent on mitochondrial function, with impaired mitochondrial biogenesis associated with the development of metabolic diseases such as insulin resistance and type 2 diabetes. Mitochondria display substantial plasticity in skeletal muscle, and are highly sensitive to levels of physical activity. It is thought that physical activity promotes mitochondrial biogenesis in skeletal muscle through increased expression of genes encoded in both the nuclear and the mitochondrial genome; however, how this process is co-ordinated at the cellular level is poorly understood. Nuclear receptors (NRs) are key signalling proteins capable of integrating environmental factors and mitochondrial function, thereby providing a potential link between exercise and mitochondrial biogenesis. The aim of this review is to highlight the function of NRs in skeletal muscle mitochondrial biogenesis and discuss the therapeutic potential of NRs for the management and treatment of chronic metabolic disease.

The coactivator PGC-1α regulates skeletal muscle oxidative metabolism independently of the nuclear receptor PPARβ/δ in sedentary mice fed a regular chow diet

AIMS/HYPOTHESIS

Physical activity improves oxidative capacity and exerts therapeutic beneficial effects, particularly in the context of metabolic diseases. The peroxisome proliferator-activated receptor (PPAR) γ coactivator-1α (PGC-1α) and the nuclear receptor PPARβ/δ have both been independently discovered to play a pivotal role in the regulation of oxidative metabolism in skeletal muscle, though their interdependence remains unclear. Hence, our aim was to determine the functional interaction between these two factors in mouse skeletal muscle in vivo.

METHODS

Adult male control mice, PGC-1α muscle-specific transgenic (mTg) mice, PPARβ/δ muscle-specific knockout (mKO) mice and the combination PPARβ/δ mKO + PGC-1α mTg mice were studied under basal conditions and following PPARβ/δ agonist administration and acute exercise. Whole-body metabolism was assessed by indirect calorimetry and blood analysis, while magnetic resonance was used to measure body composition. Quantitative PCR and western blot were used to determine gene expression and intracellular signalling. The proportion of oxidative muscle fibre was determined by NADH staining.

RESULTS

Agonist-induced PPARβ/δ activation was only disrupted by PPARβ/δ knockout. We also found that the disruption of the PGC-1α-PPARβ/δ axis did not affect whole-body metabolism under basal conditions. As expected, PGC-1α mTg mice exhibited higher exercise performance, peak oxygen consumption and lower blood lactate levels following exercise, though PPARβ/δ mKO + PGC-1α mTg mice showed a similar phenotype. Similarly, we found that PPARβ/δ was dispensable for PGC-1α-mediated enhancement of an oxidative phenotype in skeletal muscle.

CONCLUSIONS/INTERPRETATION

Collectively, these results indicate that PPARβ/δ is not an essential partner of PGC-1α in the control of skeletal muscle energy metabolism.

The transcriptional coactivator PGC-1α is dispensable for chronic overload-induced skeletal muscle hypertrophy and metabolic remodeling

Skeletal muscle mass loss and dysfunction have been linked to many diseases. Conversely, resistance exercise, mainly by activating mammalian target of rapamycin complex 1 (mTORC1), promotes skeletal muscle hypertrophy and exerts several therapeutic effects. Moreover, mTORC1, along with peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), regulates skeletal muscle metabolism. However, it is unclear whether PGC-1α is required for skeletal muscle adaptations after overload. Here we show that although chronic overload of skeletal muscle via synergist ablation (SA) strongly induces hypertrophy and a switch toward a slow-contractile phenotype, these effects were independent of PGC-1α. In fact, SA down-regulated PGC-1α expression and led to a repression of energy metabolism. Interestingly, however, PGC-1α deletion preserved peak force after SA. Taken together, our data suggest that PGC-1α is not involved in skeletal muscle remodeling induced by SA.