p38gamma mitogen-activated protein kinase is a key regulator in skeletal muscle metabolic adaptation in mice

The interplay of p38 MAPK signaling and mitochondrial metabolism, a dynamic target in cancer and pathological contexts

Mitochondria play a crucial role in cellular metabolism and bioenergetics, orchestrating various cellular processes, including energy production, metabolism, adaptation to stress, and redox balance. Besides, mitochondria regulate cellular metabolic homeostasis through coordination with multiple signaling pathways. Importantly, the p38 mitogen-activated protein kinase (MAPK) signaling pathway is a key player in the intricate communication with mitochondria, influencing various functions. This review explores the multifaced interaction between the mitochondria and p38 MAPK signaling and the consequent impact on metabolic alterations. Overall, the p38 MAPK pathway governs the activities of key mitochondrial proteins, which are involved in mitochondrial biogenesis, oxidative phosphorylation, thermogenesis, and iron homeostasis. Additionally, p38 MAPK contributes to the regulation of mitochondrial responses to oxidative stress and apoptosis induced by cancer therapies or natural substances by coordinating with other pathways responsible for energy homeostasis. Therefore, dysregulation of these interconnected pathways can lead to various pathologies characterized by aberrant metabolism. Consequently, gaining a deeper understanding of the interaction between mitochondria and the p38 MAPK pathway and their implications presents exciting forecasts for novel therapeutic interventions in cancer and other disorders characterized by metabolic dysregulation.

Redox signaling and skeletal muscle adaptation during aerobic exercise

Redox regulation is a fundamental physiological phenomenon related to oxygen-dependent metabolism, and skeletal muscle is mainly regarded as a primary site for oxidative phosphorylation. Several studies have revealed the importance of reactive oxygen and nitrogen species (RONS) in the signaling process relating to muscle adaptation during exercise. To date, improving knowledge of redox signaling in modulating exercise adaptation has been the subject of comprehensive work and scientific inquiry. The primary aim of this review is to elucidate the molecular and biochemical pathways aligned to RONS as activators of skeletal muscle adaptation and to further identify the interconnecting mechanisms controlling redox balance. We also discuss the RONS-mediated pathways during the muscle adaptive process, including mitochondrial biogenesis, muscle remodeling, vascular angiogenesis, neuron regeneration, and the role of exogenous antioxidants.

CHCHD4-TRIAP1 regulation of innate immune signaling mediates skeletal muscle adaptation to exercise

Exercise training can stimulate the formation of fatty-acid-oxidizing slow-twitch skeletal muscle fibers, which are inversely correlated with obesity, but the molecular mechanism underlying this transformation requires further elucidation. Here, we report that the downregulation of the mitochondrial disulfide relay carrier CHCHD4 by exercise training decreases the import of TP53-regulated inhibitor of apoptosis 1 (TRIAP1) into mitochondria, which can reduce cardiolipin levels and promote VDAC oligomerization in skeletal muscle. VDAC oligomerization, known to facilitate mtDNA release, can activate cGAS-STING/NFKB innate immune signaling and downregulate MyoD in skeletal muscle, thereby promoting the formation of oxidative slow-twitch fibers. In mice, CHCHD4 haploinsufficiency is sufficient to activate this pathway, leading to increased oxidative muscle fibers and decreased fat accumulation with aging. The identification of a specific mediator regulating muscle fiber transformation provides an opportunity to understand further the molecular underpinnings of complex metabolic conditions such as obesity and could have therapeutic implications.

Role of Mitogen-Activated Protein (MAP) Kinase Pathways in Metabolic Diseases

Physiological processes that govern the normal functioning of mammalian cells are regulated by a myriad of signalling pathways. Mammalian mitogen-activated protein (MAP) kinases constitute one of the major signalling arms and have been broadly classified into four groups that include extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK), p38, and ERK5. Each signalling cascade is governed by a wide array of external and cellular stimuli, which play a critical part in mammalian cells in the regulation of various key responses, such as mitogenic growth, differentiation, stress responses, as well as inflammation. This evolutionarily conserved MAP kinase signalling arm is also important for metabolic maintenance, which is tightly coordinated via complicated mechanisms that include the intricate interaction of scaffold proteins, recognition through cognate motifs, action of phosphatases, distinct subcellular localisation, and even post-translational modifications. Aberration in the signalling pathway itself or their regulation has been implicated in the disruption of metabolic homeostasis, which provides a pathophysiological foundation in the development of metabolic syndrome. Metabolic syndrome is an umbrella term that usually includes a group of closely associated metabolic diseases such as hyperglycaemia, hyperlipidaemia, and hypertension. These risk factors exacerbate the development of obesity, diabetes, atherosclerosis, cardiovascular diseases, and hepatic diseases, which have accounted for an increase in the worldwide morbidity and mortality rate. This review aims to summarise recent findings that have implicated MAP kinase signalling in the development of metabolic diseases, highlighting the potential therapeutic targets of this pathway to be investigated further for the attenuation of these diseases.

GADD45A is a mediator of mitochondrial loss, atrophy, and weakness in skeletal muscle

Aging and many illnesses and injuries impair skeletal muscle mass and function, but the molecular mechanisms are not well understood. To better understand the mechanisms, we generated and studied transgenic mice with skeletal muscle-specific expression of growth arrest and DNA damage inducible α (GADD45A), a signaling protein whose expression in skeletal muscle rises during aging and a wide range of illnesses and injuries. We found that GADD45A induced several cellular changes that are characteristic of skeletal muscle atrophy, including a reduction in skeletal muscle mitochondria and oxidative capacity, selective atrophy of glycolytic muscle fibers, and paradoxical expression of oxidative myosin heavy chains despite mitochondrial loss. These cellular changes were at least partly mediated by MAP kinase kinase kinase 4, a protein kinase that is directly activated by GADD45A. By inducing these changes, GADD45A decreased the mass of muscles that are enriched in glycolytic fibers, and it impaired strength, specific force, and endurance exercise capacity. Furthermore, as predicted by data from mouse models, we found that GADD45A expression in skeletal muscle was associated with muscle weakness in humans. Collectively, these findings identify GADD45A as a mediator of mitochondrial loss, atrophy, and weakness in mouse skeletal muscle and a potential target for muscle weakness in humans.

Expression profile and bioinformatics analysis of circRNA and its associated ceRNA networks in longissimus dorsi from Lufeng cattle and Leiqiong cattle

This paper aims to explore the role of circRNA expression profiles and circRNA-associated ceRNA networks in the regulation of myogenesis in the longissimus dorsi of cattle breeds surviving under subtropical conditions in southern China by RNA sequencing and bioinformatics analysis. It also aims to provide comprehensive understanding of the differences in muscle fibers in subtropical cattle breeds and to expand the knowledge of the molecular networks that regulate myogenesis. With regard to meat quality indicators, results showed that the longissimus dorsi of LQC had lower pH (P < 0.0001), lower redness (P < 0.01), lower shear force (P < 0.05), and higher brightness (P < 0.05) than the longissimus dorsi of LFC. With regard to muscle fiber characteristics, the longissimus dorsi of LQC had a smaller diameter (P < 0.0001) and higher density of muscle fibers (P < 0.05). The analysis results show that the function of many circRNA-targeted mRNAs was related to myogenesis and metabolic regulation. Furthermore, in the analysis of the function of circRNA source genes, we hypothesized that btacirc_00497 and btacirc_034497 may regulate the function and type of myofibrils by affecting the expression of MYH6, MYH7, and NEB through competitive linear splicing.

Transient changes to metabolic homeostasis initiate mitochondrial adaptation to endurance exercise

Endurance exercise is well established to increase mitochondrial content and function in skeletal muscle, a process termed mitochondrial biogenesis. Current understanding is that exercise initiates skeletal muscle mitochondrial remodeling via modulation of cellular nutrient, energetic and contractile stress pathways. These subtle changes in the cellular milieu are sensed by numerous transduction pathways that serve to initiate and coordinate an increase in mitochondrial gene transcription and translation. The result of these acute signaling events is the promotion of growth and assembly of mitochondria, coupled to a greater capacity for aerobic ATP provision in skeletal muscle. The aim of this review is to highlight the acute metabolic events induced by endurance exercise and the subsequent molecular pathways that sense this transient change in cellular homeostasis to drive mitochondrial adaptation and remodeling.

p38γ MAPK Inflammatory and Metabolic Signaling in Physiology and Disease

p38γ MAPK (also called ERK6 or SAPK3) is a family member of stress-activated MAPKs and has common and specific roles as compared to other p38 proteins in signal transduction. Recent studies showed that, in addition to inflammation, p38γ metabolic signaling is involved in physiological exercise and in pathogenesis of cancer, diabetes, and Alzheimer's disease, indicating its potential as a therapeutic target. p38γphosphorylates at least 19 substrates through which p38γ activity is further modified to regulate life-important cellular processes such as proliferation, differentiation, cell death, and transformation, thereby impacting biological outcomes of p38γ-driven pathogenesis. P38γ signaling is characterized by its unique reciprocal regulation with its specific phosphatase PTPH1 and by its direct binding to promoter DNAs, leading to transcriptional activation of targets including cancer-like stem cell drivers. This paper will review recent findings about p38γ inflammation and metabolic signaling in physiology and diseases. Moreover, we will discuss the progress in the development of p38γ-specific pharmacological inhibitors for therapeutic intervention in disease prevention and treatment by targeting the p38γ signaling network.

P38 MAPK and Radiotherapy: Foes or Friends?

Over the last 30 years, the study of the cellular response to ionizing radiation (IR) has increased exponentially. Among the various signaling pathways affected by IR, p38 MAPK has been shown to be activated both in vitro and in vivo, with involvement in key processes triggered by IR-mediated genotoxic insult, such as the cell cycle, apoptosis or senescence. However, we do not yet have a definitive clue about the role of p38 MAPK in terms of radioresistance/sensitivity and its potential use to improve current radiotherapy. In this review, we summarize the current knowledge on this family of MAPKs in response to IR as well as in different aspects related to radiotherapy, such as their role in the control of REDOX, fibrosis, and in the radiosensitizing effect of several compounds.

Duodenal Metabolic Profile Changes in Heat-Stressed Broilers

Simple Summary

Heat stress (HS) represents an environmental and socio-economic burden to the poultry industry worldwide. However, the underpinning mechanisms for HS responses are still not well defined. Here, we used a high-throughput analysis to determine the metabolite profiles in acute and chronic heat-stressed broilers in comparison with thermoneutral and pair-fed birds. The results showed that HS altered several duodenal metabolites in a duration-dependent manner and identified potential metabolite signatures.

Abstract

Heat stress (HS) is devastating to poultry production sustainability worldwide. In addition to its adverse effects on growth, welfare, meat quality, and mortality, HS alters the gut integrity, leading to dysbiosis and leaky gut syndrome; however, the underlying mechanisms are not fully defined. Here, we used a high-throughput mass spectrometric metabolomics approach to probe the metabolite profile in the duodenum of modern broilers exposed to acute (AHS, 2 h) or chronic cyclic (CHS, 8 h/day for 2 weeks) HS in comparison with thermoneutral (TN) and pair-fed birds. Ultra high performance liquid chromatography coupled with high resolution mass spectrometry (UHPLC–HRMS) identified a total of 178 known metabolites. The trajectory analysis of the principal component analysis (PCA) score plots (both 2D and 3D maps) showed clear separation between TN and each treated group, indicating a unique duodenal metabolite profile in HS birds. Within the HS groups, partial least squares discriminant analysis (PLS-DA) displayed different clusters when comparing metabolite profiles from AHS and CHS birds, suggesting that the metabolite signatures were also dependent on HS duration. To gain biologically related molecule networks, the above identified duodenal metabolites were mapped into the Ingenuity Pathway Analysis (IPA) knowledge-base and analyzed to outline the most enriched biological functions. Several common and specific top canonical pathways were generated. Specifically, the adenosine nucleotide degradation and dopamine degradation pathways were specific for the AHS group; however, the UDP-D-xylose and UDP-D-glucuronate biosynthesis pathways were generated only for the CHS group. The top diseases enriched by the IPA core analysis for the DA metabolites, including cancer, organismal (GI) injury, hematological, cardiovascular, developmental, hereditary, and neurological disorders, were group-specific. The top altered molecular and cellular functions were amino acid metabolism, molecular transport, small molecule biochemistry, protein synthesis, cell death and survival, and DNA damage and repair. The IPA-causal network predicted that the upstream regulators (carnitine palmitoyltransferase 1B, CPT1B; histone deacetylase 11, HDAC11; carbonic anhydrase 9, CA9; interleukin 37, IL37; glycine N-methyl transferase, GNMT; GATA4) and the downstream mediators (mitogen-activated protein kinases, MAPKs; superoxide dismutase, SOD) were altered in the HS groups. Taken together, these data showed that, independently of feed intake depression, HS induced significant changes in the duodenal metabolite profile in a duration-dependent manner and identified a potential duodenal signature for HS.

Mitochondrial Plasticity Promotes Resistance to Sorafenib and Vulnerability to STAT3 Inhibition in Human Hepatocellular Carcinoma

Simple Summary

Enhanced expression of mitochondrial ribosomal proteins and marked reprogramming of the mitochondrial network are associated with sorafenib resistance in human cell lines and hepatocarcinoma patients, providing novel actionable targets for increasing therapeutic efficacy.

Abstract

The multi-kinase inhibitor sorafenib is a primary treatment modality for advanced-stage hepatocellular carcinoma (HCC). However, the therapeutic benefits are short-lived due to innate and acquired resistance. Here, we examined how HCC cells respond to sorafenib and adapt to continuous and prolonged exposure to the drug. Sorafenib-adapted HCC cells show a profound reprogramming of mitochondria function and marked activation of genes required for mitochondrial protein translation and biogenesis. Mitochondrial ribosomal proteins and components of translation and import machinery are increased in sorafenib-resistant cells and sorafenib-refractory HCC patients show similar alterations. Sorafenib-adapted cells also exhibited increased serine 727 phosphorylated (pSer727) STAT3, the prevalent form in mitochondria, suggesting that STAT3 might be an actionable target to counteract resistance. Consistently, a small-molecule STAT3 inhibitor reduces pSer727, reverts mitochondrial alterations, and enhances the response to sorafenib in resistant cells. These results sustain the importance of mitochondria plasticity in response to sorafenib and identify a clinically actionable strategy for improving the treatment efficacy in HCC patients.

Mammalian Target of Rapamycin Signaling Pathway Regulates Mitochondrial Quality Control of Brown Adipocytes in Mice

The mammalian target of rapamycin (mTOR) is an important protein kinase that senses changes in extracellular and intracellular energy levels and plays a key role in regulating energy metabolism. Brown adipose tissue, which can be converted to white adipose tissue, contains a large number of mitochondria and regulates energy expenditure through thermogenesis. Because obesity is a process of fat accumulation due to chronic excessive energy intake, we attempted to determine whether the mTOR signaling pathway can affect the mitochondrial quality control of brown adipocytes through sensing energy status, thereby regulating brown/white adipocyte transformation. In the present study, through activation or inhibition of mTOR signaling, we detected mitochondrial biogenesis, dynamics, and autophagy-related markers in brown adipocytes. We found that activation of mTOR signaling downregulated the expression of mitochondrial biogenesis, dynamics, and autophagy-relevant markers and inhibited the mitochondrial quality control of brown adipocytes, indicating a phenotypic transformation of brown to white adipocytes. In contrast, inhibition of mTOR signaling upregulated the expression of mitochondrial biogenesis, dynamics, and mitophagy-relevant markers and strengthened mitochondrial quality control, suggesting an inhibition of the phenotypic transformation of brown to white adipocytes. In conclusion, the mTOR signaling pathway plays an important role in modulating the transformation of adipocytes by regulating mitochondrial quality control.

Exercise Is Muscle Mitochondrial Medicine

Exercise stimulates the biogenesis of mitochondria in muscle. Some literature supports the use of pharmaceuticals to enhance mitochondria as a substitute for exercise. We provide evidence that exercise rejuvenates mitochondrial function, thereby augmenting muscle health with age, in disease, and in the absence of cellular regulators. This illustrates the power of exercise to act as mitochondrial medicine in muscle.

The role of stress kinases in metabolic disease

Obesity is a health condition that has reached pandemic levels and is implicated in the development and progression of type 2 diabetes mellitus, cancer and heart failure. A key characteristic of obesity is the activation of stress-activated protein kinases (SAPKs), such as the p38 and JNK stress kinases, in several organs, including adipose tissue, liver, skeletal muscle, immune organs and the central nervous system. The correct timing, intensity and duration of SAPK activation contributes to cellular metabolic adaptation. By contrast, uncontrolled SAPK activation has been proposed to contribute to the complications of obesity. The stress kinase signalling pathways have therefore been identified as potential targets for the development of novel therapeutic approaches for metabolic syndrome. The past few decades have seen intense research efforts to determine how these kinases are regulated in a cell-specific manner and to define their contribution to the development of obesity and insulin resistance. Several studies have uncovered new and unexpected functions of the non-classical members of both pathways. Here, we provide an overview of the role of SAPKs in metabolic control and highlight important discoveries in the field.

Ribosomal stress-surveillance: three pathways is a magic number

Cells rely on stress response pathways to uphold cellular homeostasis and limit the negative effects of harmful environmental stimuli. The stress- and mitogen-activated protein (MAP) kinases, p38 and JNK, are at the nexus of numerous stress responses, among these the ribotoxic stress response (RSR). Ribosomal impairment is detrimental to cell function as it disrupts protein synthesis, increase inflammatory signaling and, if unresolved, lead to cell death. In this review, we offer a general overview of the three main translation surveillance pathways; the RSR, Ribosome-associated Quality Control (RQC) and the Integrated Stress Response (ISR). We highlight recent advances made in defining activation mechanisms for these pathways and discuss their commonalities and differences. Finally, we reflect on the physiological role of the RSR and consider the therapeutic potential of targeting the sensing kinase ZAKα for treatment of ribotoxin exposure.

Ethanolic Extract of Vaccinium corymbosum Alleviates Muscle Fatigue in Mice

The aim of this study was to evaluate the effects of ethanol extracts of Vaccinium corymbosum (VCE) on exercise-induced fatigue in mice. Mice were randomly divided into three groups; nonexercise control group (CON), exercise control group (Ex-CON), and exercise and VCE supplementation group (Ex-VCE). Compared with Ex-CON, Ex-VCE showed increased endurance exercise capacity on day 21. In Ex-VCE mice, the accumulation of lactate was inhibited and the consumption of fatty acids was enhanced, indicating the delay of muscle fatigue. In addition, VCE supplementation elevated mRNA expression levels of mitochondrial biogenesis-associated genes such as peroxisome proliferator-activated receptor-1γ coactivator 1α (PGC-1α), nuclear respiratory factor (NRF), and mitochondrial transcription factor A (Tfam) and fatty acid β-oxidation-associated genes such as carnitine palmitoyltransferase-1 (CPT-1), β-hydroxyacyl coenzyme A dehydrogenase (β-HAD), and peroxisome proliferator-activated receptor-δ (PPAR-δ). These results suggest that VCE can potentially prevent muscle fatigue by enhancing mitochondrial biogenesis and fatty acid β-oxidation.

Functions of p38 MAP Kinases in the Central Nervous System

Mitogen-activated protein (MAP) kinases are a central component in signaling networks in a multitude of mammalian cell types. This review covers recent advances on specific functions of p38 MAP kinases in cells of the central nervous system. Unique and specific functions of the four mammalian p38 kinases are found in all major cell types in the brain. Mechanisms of p38 activation and downstream phosphorylation substrates in these different contexts are outlined and how they contribute to functions of p38 in physiological and under disease conditions. Results in different model organisms demonstrated that p38 kinases are involved in cognitive functions, including functions related to anxiety, addiction behavior, neurotoxicity, neurodegeneration, and decision making. Finally, the role of p38 kinases in psychiatric and neurological conditions and the current progress on therapeutic inhibitors targeting p38 kinases are covered and implicate p38 kinases in a multitude of CNS-related physiological and disease states.

p38 MAPK in Glucose Metabolism of Skeletal Muscle: Beneficial or Harmful?

Skeletal muscles respond to environmental and physiological changes by varying their size, fiber type, and metabolic properties. P38 mitogen-activated protein kinase (MAPK) is one of several signaling pathways that drive the metabolic adaptation of skeletal muscle to exercise. p38 MAPK also participates in the development of pathological traits resulting from excessive caloric intake and obesity that cause metabolic syndrome and type 2 diabetes (T2D). Whereas p38 MAPK increases insulin-independent glucose uptake and oxidative metabolism in muscles during exercise, it contrastingly mediates insulin resistance and glucose intolerance during metabolic syndrome development. This article provides an overview of the apparent contradicting roles of p38 MAPK in the adaptation of skeletal muscles to exercise and to pathological conditions leading to glucose intolerance and T2D. Here, we focus on the involvement of p38 MAPK in glucose metabolism of skeletal muscle, and discuss the possibility of targeting this pathway to prevent the development of T2D.

Reduction of advanced tau-mediated memory deficits by the MAP kinase p38γ

Hyperphosphorylation of the neuronal tau protein contributes to Alzheimer's disease (AD) by promoting tau pathology and neuronal and cognitive deficits. In contrast, we have previously shown that site-specific tau phosphorylation can inhibit toxic signals induced by amyloid-β (Aβ) in mouse models. The post-synaptic mitogen-activated protein (MAP) kinase p38γ mediates this site-specific phosphorylation on tau at Threonine-205 (T205). Using a gene therapeutic approach, we draw on this neuroprotective mechanism to improve memory in two Aβ-dependent mouse models of AD at stages when advanced memory deficits are present. Increasing activity of post-synaptic kinase p38γ that targets T205 in tau reduced memory deficits in symptomatic Aβ-induced AD models. Reconstitution experiments with wildtype human tau or phosphorylation-deficient tauT205A showed that T205 modification is critical for downstream effects of p38γ that prevent memory impairment in APP-transgenic mice. Furthermore, genome editing of the T205 codon in the murine Mapt gene showed that this single side chain in endogenous tau critically modulates memory deficits in APP-transgenic Alzheimer's mice. Ablating the protective effect of p38γ activity by genetic p38γ deletion in a tau transgenic mouse model that expresses non-pathogenic tau rendered tau toxic and resulted in impaired memory function in the absence of human Aβ. Thus, we propose that modulating neuronal p38γ activity serves as an intrinsic tau-dependent therapeutic approach to augment compromised cognition in advanced dementia.

TAK1 preserves skeletal muscle mass and mitochondrial function through redox homeostasis

Skeletal muscle atrophy is debilitating consequence of a large number of chronic disease states, aging, and disuse conditions. Skeletal muscle mass is regulated through coordinated activation of a number of signaling cascades. Transforming growth factor-β activated kinase 1 (TAK1) is a central kinase that mediates the activation of multiple signaling pathways in response to various growth factors, cytokines, and microbial products. Accumulating evidence suggests that TAK1 promotes skeletal muscle growth and essential for the maintenance of muscle mass in adults. Targeted inactivation of TAK1 leads to severe muscle wasting and kyphosis in mice. However, the mechanisms by which TAK1 prevents loss of muscle mass remain poorly understood. Through generation of inducible skeletal muscle-specific Tak1-knockout mice, we demonstrate that targeted ablation of TAK1 disrupts redox signaling leading to the accumulation of reactive oxygen species and loss of skeletal muscle mass and contractile function. Suppression of oxidative stress using Trolox improves muscle contractile function and inhibits the activation of catabolic signaling pathways in Tak1-deficient muscle. Moreover, Trolox inhibits the activation of ubiquitin-proteasome system and autophagy markers in skeletal muscle of Tak1-deficient mice. Furthermore, inhibition of oxidative stress using Trolox prevents the slow-to-fast type fiber transition and improves mitochondrial respiration in skeletal muscle of Tak1-deficient mice. Overall, our results demonstrate that TAK1 maintains skeletal muscle mass and health through redox homeostasis.

Impact of Conventional and Atypical MAPKs on the Development of Metabolic Diseases

The family of mitogen-activated protein kinases (MAPKs) consists of fourteen members and has been implicated in regulation of virtually all cellular processes. MAPKs are divided into two groups, conventional and atypical MAPKs. Conventional MAPKs are further classified into four sub-families: extracellular signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK1, 2 and 3), p38 (α, β, γ, δ), and extracellular signal-regulated kinase 5 (ERK5). Four kinases, extracellular signal-regulated kinase 3, 4, and 7 (ERK3, 4 and 7) as well as Nemo-like kinase (NLK) build a group of atypical MAPKs, which are activated by different upstream mechanisms than conventional MAPKs. Early studies identified JNK1/2 and ERK1/2 as well as p38α as a central mediators of inflammation-evoked insulin resistance. These kinases have been also implicated in the development of obesity and diabetes. Recently, other members of conventional MAPKs emerged as important mediators of liver, skeletal muscle, adipose tissue, and pancreatic β-cell metabolism. Moreover, latest studies indicate that atypical members of MAPK family play a central role in the regulation of adipose tissue function. In this review, we summarize early studies on conventional MAPKs as well as recent findings implicating previously ignored members of the MAPK family. Finally, we discuss the therapeutic potential of drugs targeting specific members of the MAPK family.

Activin A induces tumorigenesis of leiomyoma via regulation of p38β MAPK-mediated signal cascade

OBJECTIVE

The objective of this study was to investigate the role of p38-C/EBPβ signaling in leiomyoma cells and myometrial cells challenged with Activin A, and to identify specifically the isoform of p38 MAPK that mediates the effects of Activin A.

METHODS

The immortalization human leiomyoma cells (HuLM) and human myometrial cells (HM), and mouse myometrial tissues were treated with Activin A (4 nM) in response to p38α/β inhibition (10 μM SB202190) or depletion (p38 α/β-targeting siRNA or p38β muscle specific-knock out mice). p38 MAPK signaling molecules (p-p38 and p-C/EBPβ) and ECM components (COL1A1 and/or FN) were analyzed by Western blotting.

RESULTS

Activin A induced ECM accumulation in leiomyoma cells and myofibroblastic transformation in myometrical cells specifically by p38β MAPK.

CONCLUSION

This study is the first to demonstrate that activation of C/EBPβ by p38β MAPK may contribute to tumorigenesis and progression of Activin A-induced leiomyoma. Specific p38β inhibition may represent a novel and promising intervention for leiomyoma.

p38γ MAPK Is Essential for Aerobic Glycolysis and Pancreatic Tumorigenesis

KRAS is mutated in most pancreatic ductal adenocarcinomas (PDAC) and yet remains undruggable. Here, we report that p38γ MAPK, which promotes PDAC tumorigenesis by linking KRAS signaling and aerobic glycolysis (also called the Warburg effect), is a novel therapeutic target. p38γ interacted with a glycolytic activator PFKFB3 that was dependent on mutated KRAS. KRAS transformation and overexpression of p38γ increased expression of PFKFB3 and glucose transporter GLUT2, conversely, silencing mutant KRAS, and p38γ decreased PFKFB3 and GLUT2 expression. p38γ phosphorylated PFKFB3 at S467, stabilized PFKFB3, and promoted their interaction with GLUT2. Pancreatic knockout of p38γ decreased p-PFKFB3/PFKFB3/GLUT2 protein levels, reduced aerobic glycolysis, and inhibited PDAC tumorigenesis in KPC mice. PFKFB3 and GLUT2 depended on p38γ to stimulate glycolysis and PDAC growth and p38γ required PFKFB3/S467 to promote these activities. A p38γ inhibitor cooperated with a PFKFB3 inhibitor to blunt aerobic glycolysis and PDAC growth, which was dependent on p38γ. Moreover, overexpression of p38γ, p-PFKFB3, PFKFB3, and GLUT2 in PDAC predicted poor clinical prognosis. These results indicate that p38γ links KRAS oncogene signaling and aerobic glycolysis to promote pancreatic tumorigenesis through PFKFB3 and GLUT2, and that p38γ and PFKFB3 may be targeted for therapeutic intervention in PDAC. SIGNIFICANCE: These findings show that p38γ links KRAS oncogene signaling and the Warburg effect through PFKBF3 and Glut2 to promote pancreatic tumorigenesis, which can be disrupted via inhibition of p38γ and PFKFB3.

Pharmacological Mitigation of Fibrosis in a Porcine Model of Volumetric Muscle Loss Injury

Volumetric muscle loss (VML) resulting from extremity trauma presents functional deficits and fibrosis, ultimately manifesting disability. The extensive fibrotic accumulation is expected to interfere with neural, trophic, vascular, and mechanical connectivity of any possible regenerative medicine approaches. Our objective was to quantify the muscle properties and stiffness following injury and investigate if the fibrotic deposition could be mitigated using an antifibrotic agent; we hypothesized that antifibrotic treatment would prevent the overwhelming fibrotic response. Yorkshire Cross pigs (n = 10) were randomized to sham or a nontreated ∼20% VML injury. Immediately following surgery, injured animals were further randomized to nintedanib (Ofev; 300 mg/day) or no treatment for 30 days. Longitudinal analysis of muscle function via peroneal nerve stimulation, compartment volume, and quantitative muscle stiffness using shearwave elastography were conducted. Terminally comprehensive histopathologic, biochemical, and genetic investigations were conducted on the skeletal muscle and fibrosis. Through 4 weeks post-VML, nontreated muscles presented a significant deficit (23%) in maximal torque compared to the sham operated (p < 0.01). The stiffness in the VML defect area increased significantly (7-fold) in the VML-nontreated leg than the VML antifibrotic-treated legs by 4 weeks postinjury, which was coupled with the nontreated muscle having ∼40% more hydroxyproline per mg of tissue than those receiving antifibrotic treatment (p = 0.01). This work indicates that VML injury progressively induces fibrosis and muscle stiffness. Antifibrotic treatment can mitigate the pathologic development of fibrosis. Future work should evaluate optimal timing and duration of treatments combined with regenerative medicine approaches in efforts to improve function. Impact statement This work primarily evaluated the effect of a clinically available antifibrotic therapy (nintedanib) on the development of fibrosis after volumetric muscle loss (VML) injury in a large animal model. As a primary outcome measure of fibrosis, skeletal muscle stiffness was repeatedly measured in vivo and noninvasively using a quantitative ultrasound device with shearwave elastography capability. The most salient finding of the study is that the antifibrotic nintedanib significantly reduced the development of VML injury-induced fibrous tissue deposition and stiffness.

Redox Signaling from Mitochondria: Signal Propagation and Its Targets

Progress in mass spectroscopy of posttranslational oxidative modifications has enabled researchers to experimentally verify the concept of redox signaling. We focus here on redox signaling originating from mitochondria under physiological situations, discussing mechanisms of transient redox burst in mitochondria, as well as the possible ways to transfer such redox signals to specific extramitochondrial targets. A role of peroxiredoxins is described which enables redox relay to other targets. Examples of mitochondrial redox signaling are discussed: initiation of hypoxia-inducible factor (HIF) responses; retrograde redox signaling to PGC1α during exercise in skeletal muscle; redox signaling in innate immune cells; redox stimulation of insulin secretion, and other physiological situations.

Cancer Takes a Toll on Skeletal Muscle by Releasing Heat Shock Proteins-An Emerging Mechanism of Cancer-Induced Cachexia

Cancer-associated cachexia (cancer cachexia) is a major contributor to the modality and mortality of a wide variety of solid tumors. It is estimated that cachexia inflicts approximately ~60% of all cancer patients and is the immediate cause of ~30% of all cancer-related death. However, there is no established treatment of this disorder due to the poor understanding of its underlying etiology. The key manifestations of cancer cachexia are systemic inflammation and progressive loss of skeletal muscle mass and function (muscle wasting). A number of inflammatory cytokines and members of the TGFβ superfamily that promote muscle protein degradation have been implicated as mediators of muscle wasting. However, clinical trials targeting some of the identified mediators have not yielded satisfactory results. Thus, the root cause of the muscle wasting associated with cancer cachexia remains to be identified. This review focuses on recent progress of laboratory studies in the understanding of the molecular mechanisms of cancer cachexia that centers on the role of systemic activation of Toll-like receptor 4 (TLR4) by cancer-released Hsp70 and Hsp90 in the development and progression of muscle wasting, and the downstream signaling pathways that activate muscle protein degradation through the ubiquitin-proteasome and the autophagy-lysosome pathways in response to TLR4 activation. Verification of these findings in humans could lead to etiology-based therapies of cancer cachexia by targeting multiple steps in this signaling cascade.

Protective transcriptional mechanisms in cardiomyocytes and cardiac fibroblasts

Heart failure is the leading cause of morbidity and mortality worldwide. Several lines of evidence suggest that physical activity and exercise can pre-condition the heart to improve the response to acute cardiac injury such as myocardial infarction or ischemia/reperfusion injury, preventing the progression to heart failure. It is becoming more apparent that cardioprotection is a concerted effort between multiple cell types and converging signaling pathways. However, the molecular mechanisms of cardioprotection are not completely understood. What is clear is that the mechanisms underlying this protection involve acute activation of transcriptional activators and their corresponding gene expression programs. Here, we review the known stress-dependent transcriptional programs that are activated in cardiomyocytes and cardiac fibroblasts to preserve function in the adult heart after injury. Focus is given to prominent transcriptional pathways such as mechanical stress or reactive oxygen species (ROS)-dependent activation of myocardin-related transcription factors (MRTFs) and transforming growth factor beta (TGFβ), and gene expression that positively regulates protective PI3K/Akt signaling. Together, these pathways modulate both beneficial and pathological responses to cardiac injury in a cell-specific manner.

ERK1/2 signaling induces skeletal muscle slow fiber-type switching and reduces muscular dystrophy disease severity

Mitogen-activated protein kinase (MAPK) signaling consists of an array of successively acting kinases. The extracellular signal-regulated kinases 1/2 (ERK1/2) are major components of the greater MAPK cascade that transduce growth factor signaling at the cell membrane. Here we investigated ERK1/2 signaling in skeletal muscle homeostasis and disease. Using mouse genetics, we observed that the muscle-specific expression of a constitutively active MEK1 mutant promotes greater ERK1/2 signaling that mediates fiber-type switching to a slow, oxidative phenotype with type I myosin heavy chain expression. Using a conditional and temporally regulated Cre strategy as well as Mapk1 (ERK2) and Mapk3 (ERK1) genetically targeted mice, MEK1-ERK2 signaling was shown to underlie this fast-to-slow fiber type switching in adult skeletal muscle as well as during development. Physiologic assessment of these activated MEK1-ERK1/2 mice showed enhanced metabolic activity and oxygen consumption with greater muscle fatigue resistance. Moreover, induction of MEK1-ERK1/2 signaling increased dystrophin and utrophin protein expression in a mouse model of limb-girdle muscle dystrophy and protected myofibers from damage. In summary, sustained MEK1-ERK1/2 activity in skeletal muscle produces a fast-to-slow fiber-type switch that protects from muscular dystrophy, suggesting a therapeutic approach to enhance the metabolic effectiveness of muscle and protect from dystrophic disease.

Perm1 regulates CaMKII activation and shapes skeletal muscle responses to endurance exercise training

Objective

Endurance exercise training remodels skeletal muscle, leading to increased mitochondrial content and oxidative capacity. How exercise entrains skeletal muscle signaling pathways to induce adaptive responses remains unclear. In past studies, we identified Perm1 (PGC-1 and ERR induced regulator, muscle 1) as an exercise-induced gene and showed that Perm1 overexpression elicits similar muscle adaptations as endurance exercise training. The mechanism of action and the role of Perm1 in exercise-induced responses are not known. In this study, we aimed to determine the pathway by which Perm1 acts as well as the importance of Perm1 for acute and long-term responses to exercise.

Methods

We performed immunoprecipitation and mass spectrometry to identify Perm1 associated proteins, and validated Perm1 interactions with the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). We also knocked down Perm1 expression in gastrocnemius muscles of mice via AAV-mediated delivery of shRNA and assessed the impact of reduced Perm1 expression on both acute molecular responses to a single treadmill exercise bout and long-term adaptive responses to four weeks of voluntary wheel running training. Finally, we asked whether Perm1 levels are modulated by diet or diseases affecting skeletal muscle function.

Results

We show that Perm1 associates with skeletal muscle CaMKII and promotes CaMKII activation. In response to an acute exercise bout, muscles with a knock down of Perm1 showed defects in the activation of CaMKII and p38 MAPK and blunted induction of regulators of oxidative metabolism. Following four weeks of voluntary training, Perm1 knockdown muscles had attenuated mitochondrial biogenesis. Finally, we found that Perm1 expression is reduced in diet-induced obese mice and in muscular dystrophy patients and mouse models.

Conclusions

Our findings identify Perm1 as a muscle-specific regulator of exercise-induced signaling and Perm1 levels as tuners of the skeletal muscle response to exercise. The decreased Perm1 levels in states of obesity or muscle disease suggest that Perm1 may link pathological states to inefficient exercise responses.

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Perm1 interacts with CaMKII and activates the CaMKII-MEF2 pathway.

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Perm1 is important for CaMKII activation and PGC-1α induction by an exercise bout.

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In endurance training, Perm1 impacts muscle oxidative metabolism pathway responses.

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Skeletal muscle levels of Perm1 are reduced in obesity and muscular dystrophy.

p38β MAPK mediates ULK1-dependent induction of autophagy in skeletal muscle of tumor-bearing mice

Muscle wasting is the key manifestation of cancer-associated cachexia, a lethal metabolic disorder seen in over 50% of cancer patients. Autophagy is activated in cachectic muscle of cancer hosts along with the ubiquitin-proteasome pathway (UPP), contributing to accelerated protein degradation and muscle wasting. However, established signaling mechanism that activates autophagy in response to fasting or denervation does not seem to mediate cancer-provoked autophagy in skeletal myocytes. Here, we show that p38β MAPK mediates autophagy activation in cachectic muscle of tumor-bearing mice via novel mechanisms. Complementary genetic and pharmacological manipulations reveal that activation of p38β MAPK, but not p38α MAPK, is necessary and sufficient for Lewis lung carcinoma (LLC)-induced autophagy activation in skeletal muscle cells. Particularly, muscle-specific knockout of p38β MAPK abrogates LLC tumor-induced activation of autophagy and UPP, sparing tumor-bearing mice from muscle wasting. Mechanistically, p38β MAPK-mediated activation of transcription factor C/EBPβ is required for LLC-induced autophagy activation, and upregulation of autophagy-related genes LC3b and Gabarapl1. Surprisingly, ULK1 activation (phosphorylation at S555) by cancer requires p38β MAPK, rather than AMPK. Activated ULK1 forms a complex with p38β MAPK in myocytes, which is markedly increased by a tumor burden. Overexpression of a constitutively active p38Tbeta; MAPK in HEK293 cells increases phosphorylation at S555 and other amino acid residues of ULK1, but not several of AMPK-mediated sites. Finally, ULK1 activation is abrogated in tumor-bearing mice with muscle-specific knockout of p38β MAPK. Thus, p38β MAPK appears a key mediator of cancer-provoked autophagy activation, and a therapeutic target of cancer-induced muscle wasting.

Maintenance of Skeletal Muscle Mitochondria in Health, Exercise, and Aging

Mitochondria are critical organelles responsible for regulating the metabolic status of skeletal muscle. These organelles exhibit remarkable plasticity by adapting their volume, structure, and function in response to chronic exercise, disuse, aging, and disease. A single bout of exercise initiates signaling to provoke increases in mitochondrial biogenesis, balanced by the onset of organelle turnover carried out by the mitophagy pathway. This accelerated turnover ensures the presence of a high functioning network of mitochondria designed for optimal ATP supply, with the consequence of favoring lipid metabolism, maintaining muscle mass, and reducing apoptotic susceptibility over the longer term. Conversely, aging and disuse are associated with reductions in muscle mass that are in part attributable to dysregulation of the mitochondrial network and impaired mitochondrial function. Therefore, exercise represents a viable, nonpharmaceutical therapy with the potential to reverse and enhance the impaired mitochondrial function observed with aging and chronic muscle disuse.

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

TAK1 regulates skeletal muscle mass and mitochondrial function

Skeletal muscle mass is regulated by a complex array of signaling pathways. TGF-β-activated kinase 1 (TAK1) is an important signaling protein, which regulates context-dependent activation of multiple intracellular pathways. However, the role of TAK1 in the regulation of skeletal muscle mass remains unknown. Here, we report that inducible inactivation of TAK1 causes severe muscle wasting, leading to kyphosis, in both young and adult mice.. Inactivation of TAK1 inhibits protein synthesis and induces proteolysis, potentially through upregulating the activity of the ubiquitin-proteasome system and autophagy. Phosphorylation and enzymatic activity of AMPK are increased, whereas levels of phosphorylated mTOR and p38 MAPK are diminished upon inducible inactivation of TAK1 in skeletal muscle. In addition, targeted inactivation of TAK1 leads to the accumulation of dysfunctional mitochondria and oxidative stress in skeletal muscle of adult mice. Inhibition of TAK1 does not attenuate denervation-induced muscle wasting in adult mice. Finally, TAK1 activity is highly upregulated during overload-induced skeletal muscle growth, and inactivation of TAK1 prevents myofiber hypertrophy in response to functional overload. Overall, our study demonstrates that TAK1 is a key regulator of skeletal muscle mass and oxidative metabolism.

AMPK in skeletal muscle function and metabolism

Skeletal muscle possesses a remarkable ability to adapt to various physiologic conditions. AMPK is a sensor of intracellular energy status that maintains energy stores by fine-tuning anabolic and catabolic pathways. AMPK’s role as an energy sensor is particularly critical in tissues displaying highly changeable energy turnover. Due to the drastic changes in energy demand that occur between the resting and exercising state, skeletal muscle is one such tissue. Here, we review the complex regulation of AMPK in skeletal muscle and its consequences on metabolism (e.g., substrate uptake, oxidation, and storage as well as mitochondrial function of skeletal muscle fibers). We focus on the role of AMPK in skeletal muscle during exercise and in exercise recovery. We also address adaptations to exercise training, including skeletal muscle plasticity, highlighting novel concepts and future perspectives that need to be investigated. Furthermore, we discuss the possible role of AMPK as a therapeutic target as well as different AMPK activators and their potential for future drug development.—Kjøbsted, R., Hingst, J. R., Fentz, J., Foretz, M., Sanz, M.-N., Pehmøller, C., Shum, M., Marette, A., Mounier, R., Treebak, J. T., Wojtaszewski, J. F. P., Viollet, B., Lantier, L. AMPK in skeletal muscle function and metabolism.

Activin A induces skeletal muscle catabolism via p38β mitogen-activated protein kinase

BACKGROUND

Activation of type IIB activin receptor (ActRIIB) in skeletal muscle leads to muscle atrophy because of increased muscle protein degradation. However, the intracellular signalling mechanism that mediates ActRIIB-activated muscle catabolism is poorly defined.

METHODS

We investigated the role of p38β mitogen-activated protein kinases (MAPK) in mediating ActRIIB ligand activin A-activated muscle catabolic pathways in C2C12 myotubes and in mice with perturbation of this kinase pharmacologically and genetically.

RESULTS

Treatment of C2C12 myotubes with activin A or myostatin rapidly activated p38 MAPK and its effector C/EBPβ within 1 h. Paradoxically, Akt was activated at the same time through a p38 MAPK-independent mechanism. These events were followed by up-regulation of ubiquitin ligases atrogin1 (MAFbx) and UBR2 (E3α-II), as well as increase in LC3-II, a marker of autophagosome formation, leading to myofibrillar protein loss and myotube atrophy. The catabolic effects of activin A were abolished by p38α/β MAPK inhibitor SB202190. Using small interfering RNA-mediated gene knockdown, we found that the catabolic activity of activin A was dependent on p38β MAPK specifically. Importantly, systemic administration of activin A to mice similarly activated the catabolic pathways in vivo, and this effect was blocked by SB202190. Further, activin A failed to activate the catabolic pathways in mice with muscle-specific knockout of p38β MAPK. Interestingly, activin A up-regulated MuRF1 in a p38 MAPK-independent manner, and MuRF1 did not appear responsible for activin A-induced myosin heavy chain loss and muscle atrophy.

CONCLUSIONS

ActRIIB-mediated activation of muscle catabolism is dependent on p38β MAPK-activated signalling.

Physical activity prevents alterations in mitochondrial ultrastructure and glucometabolic parameters in a high-sugar diet model

Endurance exercise is a remarkable intervention for the treatment of many diseases. Mitochondrial changes on skeletal muscle are likely important for many of the benefits provided by exercise. In this study, we aimed to evaluate the effects that a regular physical activity (swimming without workload) has on mitochondrial morphological alterations and glucometabolic parameters induced by a high-sugar diet (HSD). Weaned male Wistar rats fed with a standard diet or a HSD (68% carbohydrate) were subjected to 60 minutes of regular physical activity by swimming (without workload) for four- (20 sessions) or eight-week (40 sessions) periods. After training, animals were euthanized and the sera, adipose tissues, and skeletal muscles were collected for further analysis. The HSD increased body weight after an 8-week period; it also increased the fat pads and the adipose index, resulting in glucose intolerance and insulin resistance (IR). Transmission electron microscopy showed an increase in alterations of mitochondrial ultrastructure in the gastrocnemius muscle, as well as a decrease in superoxide dismutase (SOD) activity, and an increase in protein carbonylation. Regular physical activity partially reverted these alterations in rats fed a HSD, preventing mitochondrial morphological alterations and IR. Moreover, we observed a decrease in Pgc1α expression (qPCR analysis) in STD-EXE group and a less pronounced reduction in HSD-EXE group after an 8-week period. Thus, regular physical activity (swimming without workload) in rats fed a HSD can prevent mitochondrial dysfunction and IR, highlighting the crucial role for physical activity on metabolic homeostasis.

Muscle injury, impaired muscle function and insulin resistance in Chromogranin A-knockout mice

Chromogranin A (CgA) is widely expressed in endocrine and neuroendocrine tissues as well as in the central nervous system. We observed CgA expression (mRNA and protein) in the gastrocnemius (GAS) muscle and found that performance of CgA-deficient Chga-KO mice in treadmill exercise was impaired. Supplementation with CgA in Chga-KO mice restored exercise ability suggesting a novel role for endogenous CgA in skeletal muscle function. Chga-KO mice display (i) lack of exercise-induced stimulation of pAKT, pTBC1D1 and phospho-p38 kinase signaling, (ii) loss of GAS muscle mass, (iii) extensive formation of tubular aggregates (TA), (iv) disorganized cristae architecture in mitochondria, (v) increased expression of the inflammatory cytokines Tnfα, Il6 and Ifnγ, and fibrosis. The impaired maximum running speed and endurance in the treadmill exercise in Chga-KO mice correlated with decreased glucose uptake and glycolysis, defects in glucose oxidation and decreased mitochondrial cytochrome C oxidase activity. The lack of adaptation to endurance training correlated with the lack of stimulation of p38MAPK that is known to mediate the response to tissue damage. As CgA sorts proteins to the regulated secretory pathway, we speculate that lack of CgA could cause misfolding of membrane proteins inducing aggregation of sarcoplasmic reticulum (SR) membranes and formation of tubular aggregates that is observed in Chga-KO mice. In conclusion, CgA deficiency renders the muscle energy deficient, impairs performance in treadmill exercise and prevents regeneration after exercise-induced tissue damage.

ADAPTAÇÃO DO MÚSCULO ESQUELÉTICO AO EXERCÍCIO FÍSICO: CONSIDERAÇÕES MOLECULARES E ENERGÉTICAS

RESUMO Os benefícios para a saúde e as adaptações fisiológicas ao exercício regular são amplamente conhecidos e, com o advento das ciências ômicas e moleculares, revelou-se uma complexa rede de vias de sinalização e moléculas reguladoras que coordenam a resposta adaptativa do músculo esquelético ao exercício. As mudanças orgânicas transientes, porém, são cumulativas no pós-exercício. Elas incluem, de forma principal, a transcrição de genes relacionados aos fatores regulatórios da miogênese, ao metabolismo de carboidratos, à mobilização de gorduras, ao transporte e oxidação de substratos, ao metabolismo mitocondrial através da fosforilação oxidativa e, por fim, à regulação transcricional de genes envolvidos na biogênese mitocondrial. Tendo em vista o grande impacto científico, resumiram-se neste trabalho, além de algumas das principais respostas moleculares sofridas pelo músculo esquelético com o exercício físico, fatores que coordenam a plasticidade muscular para o ganho de desempenho. Foram citadas dezenas de biomarcadores ligados a alguns aspectos moleculares das adaptações do músculo esquelético ao exercício físico, algumas principais vias sinalizadoras e o papel mitocondrial, revelando alguns novos paradigmas para o entendimento desta área científica.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

Skeletal muscle is a tissue with a low mitochondrial content under basal conditions, but it is responsive to acute increases in contractile activity patterns (i.e. exercise) which initiate the signalling of a compensatory response, leading to the biogenesis of mitochondria and improved organelle function. Exercise also promotes the degradation of poorly functioning mitochondria (i.e. mitophagy), thereby accelerating mitochondrial turnover, and preserving a pool of healthy organelles. In contrast, muscle disuse, as well as the aging process, are associated with reduced mitochondrial quality and quantity in muscle. This has strong negative implications for whole-body metabolic health and the preservation of muscle mass. A number of traditional, as well as novel regulatory pathways exist in muscle that control both biogenesis and mitophagy. Interestingly, although the ablation of single regulatory transcription factors within these pathways often leads to a reduction in the basal mitochondrial content of muscle, this can invariably be overcome with exercise, signifying that exercise activates a multitude of pathways which can respond to restore mitochondrial health. This knowledge, along with growing realization that pharmacological agents can also promote mitochondrial health independently of exercise, leads to an optimistic outlook in which the maintenance of mitochondrial and whole-body metabolic health can be achieved by taking advantage of the broad benefits of exercise, along with the potential specificity of drug action.

Oral quercetin administration transiently protects respiratory function in dystrophin-deficient mice

KEY POINT

PGC-1α pathway activation has been shown to decrease disease severity and can be driven by quercetin. Oral quercetin supplementation protected respiratory function for 4-6 months during a 12 month dosing regimen. This transient protection was probably due to a failure to sustain elevated SIRT1 activity and downstream PGC-1α signalling. Quercetin supplementation may be a beneficial treatment as part of a cocktail provided continued SIRT1 activity elevation is achieved.

ABSTRACT

Duchenne muscular dystrophy (DMD) impacts 1 : 3500 boys and leads to muscle dysfunction culminating in death due to respiratory or cardiac failure. There is an urgent need for effective therapies with the potential for immediate application for this patient population. Quercetin, a flavonoid with an outstanding safety profile, may provide therapeutic relief to DMD patients as the wait for additional therapies continues. This study evaluated the capacity of orally administered quercetin (0.2%) in 2 month old mdx mice to improve respiratory function and end-point functional and histological outcomes in the diaphragm following 12 months of treatment. Respiratory function was protected for the first 4-6 months of treatment but appeared to become insensitive to quercetin thereafter. Consistent with this, end-point functional measures were decreased and histopathological measures were more severe in dystrophic muscle compared to C57 and similar between control-fed and quercetin-fed mdx mice. To better understand the transient nature of improved respiratory function, we measured PGC-1α pathway activity, which is suggested to be up-regulated by quercetin supplementation. This pathway was largely suppressed in dystrophic muscle compared to healthy muscle, and at the 14 month time point dietary quercetin enrichment did not increase expression of downstream effectors. These data support the efficacy of quercetin as an intervention for DMD in skeletal muscle, and also indicate the development of age-dependent quercetin insensitivity when continued supplementation fails to drive the PGC-1α pathway. Continued study is needed to determine if this is related to disease severity, age or other factors.

The ageing neuromuscular system and sarcopenia: a mitochondrial perspective

Skeletal muscles undergo structural and functional decline with ageing, culminating in sarcopenia. The underlying neuromuscular mechanisms have been the subject of intense investigation, revealing mitochondrial abnormalities as potential culprits within both nerve and muscle cells. Implicated mechanisms involve impaired mitochondrial dynamics, reduced organelle biogenesis and quality control via mitophagy, accumulation of mitochondrial DNA (mtDNA) damage and respiratory chain defect, metabolic disturbance, pro-apoptotic signalling, and oxidative stress. This article provides an overview of the cellular mechanisms whereby mitochondria may promote maladaptive changes within motor neurons, the neuromuscular junction (NMJ) and muscle fibres. Lifelong physical activity, which promotes mitochondrial health across tissues, is emerging as an effective countermeasure for sarcopenia.

p38γ and p38δ Mitogen Activated Protein Kinases (MAPKs), New Stars in the MAPK Galaxy

The protein kinases p38γ and p38δ belong to the p38 mitogen-activated protein kinase (MAPK) family. p38MAPK signaling controls many cellular processes and is one of the most conserved mechanisms in eukaryotes for the cellular response to environmental stress and inflammation. Although p38γ and p38δ are widely expressed, it is likely that they perform specific functions in different tissues. Their involvement in human pathologies such as inflammation-related diseases or cancer is starting to be uncovered. In this article we give a general overview and highlight recent advances made in defining the functions of p38γ and p38δ, focusing in innate immunity and inflammation. We consider the potential of the pharmacological targeting of MAPK pathways to treat autoimmune and inflammatory diseases and cancer.

Gene profiling of embryonic skeletal muscle lacking type I ryanodine receptor Ca(2+) release channel

In mature skeletal muscle, the intracellular Ca²⁺ concentration rises dramatically upon membrane depolarization, constituting the link between excitation and contraction. This process requires Ca²⁺ release from the sarcoplasmic reticulum via the type 1 ryanodine receptor (RYR1). However, RYR1’s potential roles in muscle development remain obscure. We used an established RyR1- null mouse model, dyspedic, to investigate the effects of the absence of a functional RYR1 and, consequently, the lack of RyR1-mediated Ca²⁺ signaling, during embryogenesis. Homozygous dyspedic mice die after birth and display small limbs and abnormal skeletal muscle organization. Skeletal muscles from front and hind limbs of dyspedic fetuses (day E18.5) were subjected to microarray analyses, revealing 318 differentially expressed genes. We observed altered expression of multiple transcription factors and members of key signaling pathways. Differential regulation was also observed for genes encoding contractile as well as muscle-specific structural proteins. Additional qRT-PCR analysis revealed altered mRNA levels of the canonical muscle regulatory factors Six1, Six4, Pax7, MyoD, MyoG and MRF4 in mutant muscle, which is in line with the severe developmental retardation seen in dyspedic muscle histology analyses. Taken together, these findings suggest an important non-contractile role of RyR1 or RYR1-mediated Ca²⁺ signaling during muscle organ development.

The regulation of autophagy during exercise in skeletal muscle

The merits of exercise on muscle health and well-being are numerous and well documented. However, the mechanisms underlying the robust adaptations induced by exercise, particularly on mitochondria, are less clear and much sought after. Recently, an evolutionary conserved cellular recycling mechanism known as autophagy has been implicated in the adaptations to acute and chronic exercise. A basal level of autophagy is constantly ongoing in cells and tissues, ensuring cellular clearance and energy homeostasis. This pathway can be further induced, as a survival mechanism, by cellular perturbations, such as energetic imbalance and oxidative stress. During exercise, a biphasic autophagy response is mobilized, leading to both an acute induction and a long-term potentiation of the process. Posttranslational modifications arising from upstream signaling cascades induce an acute autophagic response during a single bout of exercise by mobilizing core autophagy machinery. A transcriptional program involving the regulators Forkhead box O, transcription factor EB, p53, and peroxisome proliferator coactivator-1α is also induced to fuel sustained increases in autophagic capacity. Autophagy has also been documented to mediate chronic exercise-induced metabolic benefits, and animal models in which autophagy is perturbed do not adapt to exercise to the same extent. In this review, we discuss recent developments in the field of autophagy and exercise. We specifically highlight the molecular mechanisms activated during acute exercise that lead to a prolonged adaptive response.

Sodium bicarbonate ingestion augments the increase in PGC-1α mRNA expression during recovery from intense interval exercise in human skeletal muscle

We tested the hypothesis that ingestion of sodium bicarbonate (NaHCO3) prior to an acute session of high-intensity interval training (HIIT) would augment signaling cascades and gene expression linked to mitochondrial biogenesis in human skeletal muscle. On two occasions separated by ∼1 wk, nine men (mean ± SD: age 22 ± 2 yr, weight 78 ± 13 kg, V̇O(2 peak) 48 ± 8 ml·kg(-1)·min(-1)) performed 10 × 60-s cycling efforts at an intensity eliciting ∼90% of maximal heart rate (263 ± 40 W), interspersed with 60 s of recovery. In a double-blind, crossover manner, subjects ingested a total of 0.4 g/kg body weight NaHCO3 before exercise (BICARB) or an equimolar amount of a placebo, sodium chloride (PLAC). Venous blood bicarbonate and pH were elevated at all time points after ingestion (P < 0.05) in BICARB vs. PLAC. During exercise, muscle glycogen utilization (126 ± 47 vs. 53 ± 38 mmol/kg dry weight, P < 0.05) and blood lactate accumulation (12.8 ± 2.6 vs. 10.5 ± 2.8 mmol/liter, P < 0.05) were greater in BICARB vs. PLAC. The acute exercise-induced increase in the phosphorylation of acetyl-CoA carboxylase, a downstream marker of AMP-activated protein kinase activity, and p38 mitogen-activated protein kinase were similar between treatments (P > 0.05). However, the increase in PGC-1α mRNA expression after 3 h of recovery was higher in BICARB vs. PLAC (approximately sevenfold vs. fivefold compared with rest, P < 0.05). We conclude that NaHCO3 before HIIT alters the mRNA expression of this key regulatory protein associated with mitochondrial biogenesis. The elevated PGC-1α mRNA response provides a putative mechanism to explain the enhanced mitochondrial adaptation observed after chronic HIIT supplemented with NaHCO3 in rats.