Drp1 knockdown induces severe muscle atrophy and remodelling, mitochondrial dysfunction, autophagy impairment and denervation

Unlocking mitochondrial dysfunction-associated senescence (MiDAS) with NAD+ - A Boolean model of mitochondrial dynamics and cell cycle control

The steady accumulation of senescent cells with aging creates tissue environments that aid cancer evolution. Aging cell states are highly heterogeneous. 'Deep senescent' cells rely on healthy mitochondria to fuel a strong proinflammatory secretome, including cytokines, growth and transforming signals. Yet, the physiological triggers of senescence such as reactive oxygen species (ROS) can also trigger mitochondrial dysfunction, and sufficient energy deficit to alter their secretome and cause chronic oxidative stress - a state termed Mitochondrial Dysfunction-Associated Senescence (MiDAS). Here, we offer a mechanistic hypothesis for the molecular processes leading to MiDAS, along with testable predictions. To do this we have built a Boolean regulatory network model that qualitatively captures key aspects of mitochondrial dynamics during cell cycle progression (hyper-fusion at the G1/S boundary, fission in mitosis), apoptosis (fission and dysfunction) and glucose starvation (reversible hyper-fusion), as well as MiDAS in response to SIRT3 knockdown or oxidative stress. Our model reaffirms the protective role of NAD+ and external pyruvate. We offer testable predictions about the growth factor- and glucose-dependence of MiDAS and its reversibility at different stages of reactive oxygen species (ROS)-induced senescence. Our model provides mechanistic insights into the distinct stages of DNA-damage induced senescence, the relationship between senescence and epithelial-to-mesenchymal transition in cancer and offers a foundation for building multiscale models of tissue aging.

The interplay between mitochondrial dynamics and autophagy: From a key homeostatic mechanism to a driver of pathology

The complex relationship between mitochondrial dynamics and autophagy illustrates how two cellular housekeeping processes are intimately linked, illuminating fundamental principles of cellular homeostasis and shedding light on disparate pathological conditions including several neurodegenerative disorders. Here we review the basic tenets of mitochondrial dynamics i.e., the concerted balance between fusion and fission of the organelle, and its interplay with macroautophagy and selective mitochondrial autophagy, also dubbed mitophagy, in the maintenance of mitochondrial quality control and ultimately in cell viability. We illustrate how conditions of altered mitochondrial dynamics reverberate on autophagy and vice versa. Finally, we illustrate how altered interplay between these two key cellular processes participates in the pathogenesis of human disorders affecting multiple organs and systems.

Metabolism and bioenergetics in the pathophysiology of organ fibrosis

Fibrosis is the tissue scarring characterized by excess deposition of extracellular matrix (ECM) proteins, mainly collagens. A fibrotic response can take place in any tissue of the body and is the result of an imbalanced reaction to inflammation and wound healing. Metabolism has emerged as a major driver of fibrotic diseases. While glycolytic shifts appear to be a key metabolic switch in activated stromal ECM-producing cells, several other cell types such as immune cells, whose functions are intricately connected to their metabolic characteristics, form a complex network of pro-fibrotic cellular crosstalk. This review purports to clarify shared and particular cellular responses and mechanisms across organs and etiologies. We discuss the impact of the cell-type specific metabolic reprogramming in fibrotic diseases in both experimental and human pathology settings, providing a rationale for new therapeutic interventions based on metabolism-targeted antifibrotic agents.

Mitochondrial mechanisms in the pathogenesis of chronic inflammatory musculoskeletal disorders

Chronic inflammatory musculoskeletal disorders characterized by prolonged muscle inflammation, resulting in enduring pain and diminished functionality, pose significant challenges for the patients. Emerging scientific evidence points to mitochondrial malfunction as a pivotal factor contributing to these ailments. Mitochondria play a critical role in powering skeletal muscle activity, but in the context of persistent inflammation, disruptions in their quantity, configuration, and performance have been well-documented. Various disturbances, encompassing alterations in mitochondrial dynamics (such as fission and fusion), calcium regulation, oxidative stress, biogenesis, and the process of mitophagy, are believed to play a central role in the progression of these disorders. Additionally, unfolded protein responses and the accumulation of fatty acids within muscle cells may adversely affect the internal milieu, impairing the equilibrium of mitochondrial functioning. The structural discrepancies between different mitochondrial subsets namely, intramyofibrillar and subsarcolemmal mitochondria likely impact their metabolic capabilities and susceptibility to inflammatory influences. The release of signals from damaged mitochondria is known to incite inflammatory responses. Intriguingly, migrasomes and extracellular vesicles serve as vehicles for intercellular transfer of mitochondria, aiding in the removal of impaired mitochondria and regulation of inflammation. Viral infections have been implicated in inducing stress on mitochondria. Prolonged dysfunction of these vital organelles sustains oxidative harm, metabolic irregularities, and heightened cytokine release, impeding the body's ability to repair tissues. This review provides a comprehensive analysis of advancements in understanding changes in the intracellular environment, mitochondrial architecture and distribution, biogenesis, dynamics, autophagy, oxidative stress, cytokines associated with mitochondria, vesicular structures, and associated membranes in the context of chronic inflammatory musculoskeletal disorders. Strategies targeting key elements regulating mitochondrial quality exhibit promise in the restoration of mitochondrial function, alleviation of inflammation, and enhancement of overall outcomes.

Delineating the contribution of ageing and physical activity to changes in mitochondrial characteristics across the lifespan

Ageing is associated with widespread physiological changes prominent within all tissues, including skeletal muscle and the brain, which lead to a decline in physical function. To tackle the growing health and economic burdens associated with an ageing population, the concept of healthy ageing has become a major research priority. Changes in skeletal muscle mitochondrial characteristics have been suggested to make an important contribution to the reductions in skeletal muscle function with age, and age-related changes in mitochondrial content, respiratory function, morphology, and mitochondrial DNA have previously been reported. However, not all studies report changes in mitochondrial characteristics with ageing, and there is increasing evidence to suggest that physical activity (or inactivity) throughout life is a confounding factor when interpreting age-associated changes. Given that physical activity is a potent stimulus for inducing beneficial adaptations to mitochondrial characteristics, delineating the influence of physical activity on the changes in skeletal muscle that occur with age is complicated. This review aims to summarise our current understanding and knowledge gaps regarding age-related changes to mitochondrial characteristics within skeletal muscle, as well as to provide some novel insights into brain mitochondria, and to propose avenues of future research and targeted interventions. Furthermore, where possible, we incorporate discussions of the modifying effects of physical activity, exercise, and training status, to purported age-related changes in mitochondrial characteristics.

Autophagy gene expression in skeletal muscle of older individuals is associated with physical performance, muscle volume and mitochondrial function in the study of muscle, mobility and aging (SOMMA)

Autophagy is essential for proteostasis, energetic balance, and cell defense and is a key pathway in aging. Identifying associations between autophagy gene expression patterns in skeletal muscle and physical performance outcomes would further our knowledge of mechanisms related with proteostasis and healthy aging. Muscle biopsies were obtained from participants in the Study of Muscle, Mobility, and Aging (SOMMA). For 575 participants, RNA was sequenced and expression of 281 genes related to autophagy regulation, mitophagy, and mTOR/upstream pathways was determined. Associations between gene expression and outcomes including mitochondrial respiration in muscle fiber bundles (MAX OXPHOS), physical performance (VO2 peak, 400 m walking speed, and leg power), and thigh muscle volume, were determined using negative binomial regression models. For autophagy, key transcriptional regulators including TFE3 and NFKB-related genes (RELA, RELB, and NFKB1) were negatively associated with outcomes. On the contrary, regulators of oxidative metabolism that also promote overall autophagy, mitophagy, and pexophagy (PPARGC1A, PPARA, and EPAS1) were positively associated with multiple outcomes. In line with this, several mitophagy, fusion, and fission-related genes (NIPSNAP2, DNM1L, and OPA1) were also positively associated with outcomes. For mTOR pathway and related genes, expression of WDR59 and WDR24, both subunits of GATOR2 complex (an indirect inhibitor of mTORC1), and PRKAG3, which is a regulatory subunit of AMPK, were negatively correlated with multiple outcomes. Our study identifies autophagy and selective autophagy such as mitophagy gene expression patterns in human skeletal muscle related to physical performance, muscle volume, and mitochondrial function in older persons which may lead to target identification to preserve mobility and independence.

MFN2 overexpression in skeletal muscles of young and old mice causes a mild hypertrophy without altering mitochondrial respiration and H2O2 emission

AIM

Sarcopenia, the aging-related loss of muscle mass and function, is a debilitating process negatively impacting the quality of life of affected individuals. Although the mechanisms underlying sarcopenia are incompletely understood, impairments in mitochondrial dynamics, including mitochondrial fusion, have been proposed as a contributing factor. However, the potential of upregulating mitochondrial fusion proteins to alleviate the effects of aging on skeletal muscles remains unexplored. We therefore hypothesized that overexpressing Mitofusin 2 (MFN2) in skeletal muscle in vivo would mitigate the effects of aging on muscle mass and improve mitochondrial function.

METHODS

MFN2 was overexpressed in young (7 mo) and old (24 mo) male mice for 4 months through intramuscular injections of an adeno-associated viruses. The impacts of MFN2 overexpression on muscle mass and fiber size (histology), mitochondrial respiration, and H2O2 emission (Oroboros fluororespirometry), and various signaling pathways (qPCR and western blotting) were investigated.

RESULTS

MFN2 overexpression increased muscle mass and fiber size in both young and old mice. No sign of fibrosis, necrosis, or inflammation was found upon MFN2 overexpression, indicating that the hypertrophy triggered by MFN2 overexpression was not pathological. MFN2 overexpression even reduced the proportion of fibers with central nuclei in old muscles. Importantly, MFN2 overexpression had no impact on muscle mitochondrial respiration and H2O2 emission in both young and old mice. MFN2 overexpression attenuated the increase in markers of impaired autophagy in old muscles.

CONCLUSION

MFN2 overexpression may be a viable approach to mitigate aging-related muscle atrophy and may have applications for other muscle disorders.

The role of mitochondrial dynamics and mitophagy in skeletal muscle atrophy: from molecular mechanisms to therapeutic insights

Skeletal muscle is the largest metabolic organ of the human body. Maintaining the best quality control and functional integrity of mitochondria is essential for the health of skeletal muscle. However, mitochondrial dysfunction characterized by mitochondrial dynamic imbalance and mitophagy disruption can lead to varying degrees of muscle atrophy, but the underlying mechanism of action is still unclear. Although mitochondrial dynamics and mitophagy are two different mitochondrial quality control mechanisms, a large amount of evidence has indicated that they are interrelated and mutually regulated. The former maintains the balance of the mitochondrial network, eliminates damaged or aged mitochondria, and enables cells to survive normally. The latter degrades damaged or aged mitochondria through the lysosomal pathway, ensuring cellular functional health and metabolic homeostasis. Skeletal muscle atrophy is considered an urgent global health issue. Understanding and gaining knowledge about muscle atrophy caused by mitochondrial dysfunction, particularly focusing on mitochondrial dynamics and mitochondrial autophagy, can greatly contribute to the prevention and treatment of muscle atrophy. In this review, we critically summarize the recent research progress on mitochondrial dynamics and mitophagy in skeletal muscle atrophy, and expound on the intrinsic molecular mechanism of skeletal muscle atrophy caused by mitochondrial dynamics and mitophagy. Importantly, we emphasize the potential of targeting mitochondrial dynamics and mitophagy as therapeutic strategies for the prevention and treatment of muscle atrophy, including pharmacological treatment and exercise therapy, and summarize effective methods for the treatment of skeletal muscle atrophy.

Mitochondrial Transplantation's Role in Rodent Skeletal Muscle Bioenergetics: Recharging the Engine of Aging

BACKGROUND

Mitochondria are the 'powerhouses of cells' and progressive mitochondrial dysfunction is a hallmark of aging in skeletal muscle. Although different forms of exercise modality appear to be beneficial to attenuate aging-induced mitochondrial dysfunction, it presupposes that the individual has a requisite level of mobility. Moreover, non-exercise alternatives (i.e., nutraceuticals or pharmacological agents) to improve skeletal muscle bioenergetics require time to be effective in the target tissue and have another limitation in that they act systemically and not locally where needed. Mitochondrial transplantation represents a novel directed therapy designed to enhance energy production of tissues impacted by defective mitochondria. To date, no studies have used mitochondrial transplantation as an intervention to attenuate aging-induced skeletal muscle mitochondrial dysfunction. The purpose of this investigation, therefore, was to determine whether mitochondrial transplantation can enhance skeletal muscle bioenergetics in an aging rodent model. We hypothesized that mitochondrial transplantation would result in sustained skeletal muscle bioenergetics leading to improved functional capacity.

METHODS

Fifteen female mice (24 months old) were randomized into two groups (placebo or mitochondrial transplantation). Isolated mitochondria from a donor mouse of the same sex and age were transplanted into the hindlimb muscles of recipient mice (quadriceps femoris, tibialis anterior, and gastrocnemius complex).

RESULTS

The results indicated significant increases (ranging between ~36% and ~65%) in basal cytochrome c oxidase and citrate synthase activity as well as ATP levels in mice receiving mitochondrial transplantation relative to the placebo. Moreover, there were significant increases (approx. two-fold) in protein expression of mitochondrial markers in both glycolytic and oxidative muscles. These enhancements in the muscle translated to significant improvements in exercise tolerance.

CONCLUSIONS

This study provides initial evidence showing how mitochondrial transplantation can promote skeletal muscle bioenergetics in an aging rodent model.

Assessment of the effects of transforming growth factor beta1 (TGF-β1)-Smad2/3 on fibrosis in rat myofascial trigger points using point shear wave elastography

Background & Aims

Myofascial trigger points (MTrPs) are highly sensitive irritated points within a tense belt of skeletal muscle, and are the main cause of muscle pain and dysfunction. MTrPs can also cause paraesthesia and autonomic nervous dysfunction. Furthermore, long-term and chronic MTrPs can cause muscle atrophy and even disability, seriously affecting the quality of life and mental health of patients, and increasing the social and economic burden. However, to date, there have been few studies on fibrogenesis and changes in MTrPs. Therefore, this study investigated whether transforming growth factor beta1 (TGF-β1)-Smad2/3 participates in the formation of MTrPs and how it affects fibrosis using point shear wave elastography.

Methods

Forty Sprague‒Dawley rats were randomly divided into the MTrPs group and the control group. Blunt injury combined with eccentric exercise was used to establish an MTrPs model. Electromyography (EMG), haematoxylin and eosin (H&E) staining and transmission electron microscopy (TEM) were used to verify the model. The collagen volume fraction was measured by Masson staining, the protein expression of TGF-β1 and p-Smad2/3 was measured by Western blotting (WB) and immunohistochemistry (IHC), and the shear wave velocity (SWV) was measured by point shear wave elastography.

Results

EMG, H&E and TEM examination indicated that the modelling was successful. The collagen volume fraction and the protein expression of TGF-β1 and p-Smad2/3 were higher in the MTrPs group than in the control group. The SWV of the MTrPs group was also higher than that of the control group. These differences suggest that MTrPs may exhibit fibrosis. The correlations between the collagen volume fraction and SWV and between the collagen volume fraction and TGF-β1 were positive.

Conclusion

Fibrotic conditions may be involved in the formation of MTrPs. Ultrasound point shear wave elastography and assessment of TGF-β1 and p-Smad2/3 expression can reflect the degree of MTrPs fibrosis to some extent. Further exploration of the important role of TGF-β1 and Smad2/3 in the pathogenesis of MTrPs will be of great significance for clinical treatment.

The impact of 60 days of -6° head down tilt bed rest on mitochondrial content, respiration and regulators of mitochondrial dynamics

It is unclear how skeletal muscle metabolism and mitochondrial function adapt to long duration bed rest and whether changes can be prevented by nutritional intervention. The present study aimed (1) to assess the effect of prolonged bed rest on skeletal muscle mitochondrial function and dynamics and (2) to determine whether micronutrient supplementation would mitigate the adverse metabolic effect of bed rest. Participants were maintained in energy balance throughout 60 days of bed rest with micronutrient supplementation (INT) (body mass index: 23.747 ± 1.877 kg m-2 ; 34.80 ± 7.451 years; n = 10) or without (control) (body mass index: 24.087 ± 2.088 kg m-2 ; 33.50 ± 8.541 years; n = 10). Indirect calorimetry and dual-energy x-ray absorptiometry were used for measures of energy expenditure, exercise capacity and body composition. Mitochondrial respiration was determined by high-resolution respirometry in permeabilized muscle fibre bundles from vastus lateralis biopsies. Protein and mRNA analysis further examined the metabolic changes relating to regulators of mitochondrial dynamics induced by bed rest. INT was not sufficient in preserving whole body metabolic changes conducive of a decrease in body mass, fat-free mass and exercise capacity within both groups. Mitochondrial respiration, OPA1 and Drp1 protein expression decreased with bed rest, with an increase pDrp1s616 . This reduction in mitochondrial respiration was explained through an observed decrease in mitochondrial content (mtDNA:nDNA). Changes in regulators of mitochondrial dynamics indicate an increase in mitochondrial fission driven by a decrease in inner mitochondrial membrane fusion (OPA1) and increased pDrp1s616 . KEY POINTS: Sixty days of -6° head down tilt bed rest leads to significant changes in body composition, exercise capacity and whole-body substrate metabolism. Micronutrient supplementation throughout bed rest did not preserve whole body metabolic changes. Bed rest results in a decrease in skeletal muscle mitochondrial respiratory capacity, mainly as a result of an observed decrease in mitochondrial content. Prolonged bed rest ensues changes in key regulators of mitochondrial dynamics. OPA1 and Drp1 are significantly reduced, with an increase in pDrp1s616 following bed rest indicative of an increase in mitochondrial fission. Given the reduction in mitochondrial content following 60 days of bed rest, the maintenance of regulators of mitophagy in line with the increase in regulators of mitochondrial fission may act to maintain mitochondrial respiration to meet energy demands.

The role of mitochondrial dynamics in disease

Mitochondria are multifaceted and dynamic organelles regulating various important cellular processes from signal transduction to determining cell fate. As dynamic properties of mitochondria, fusion and fission accompanied with mitophagy, undergo constant changes in number and morphology to sustain mitochondrial homeostasis in response to cell context changes. Thus, the dysregulation of mitochondrial dynamics and mitophagy is unsurprisingly related with various diseases, but the unclear underlying mechanism hinders their clinical application. In this review, we summarize the recent developments in the molecular mechanism of mitochondrial dynamics and mitophagy, particularly the different roles of key components in mitochondrial dynamics in different context. We also summarize the roles of mitochondrial dynamics and target treatment in diseases related to the cardiovascular system, nervous system, respiratory system, and tumor cell metabolism demanding high-energy. In these diseases, it is common that excessive mitochondrial fission is dominant and accompanied by impaired fusion and mitophagy. But there have been many conflicting findings about them recently, which are specifically highlighted in this view. We look forward that these findings will help broaden our understanding of the roles of the mitochondrial dynamics in diseases and will be beneficial to the discovery of novel selective therapeutic targets.

Autophagy gene expression in skeletal muscle of older individuals is associated with physical performance, muscle volume and mitochondrial function in the Study of Muscle, Mobility and Aging (SOMMA)

Autophagy is an essential component of proteostasis and a key pathway in aging. Identifying associations between autophagy gene expression patterns in skeletal muscle and physical performance outcomes would further our knowledge of mechanisms related with proteostasis and healthy aging. Muscle biopsies were obtained from participants in the Study of Muscle, Mobility and Aging (SOMMA). For 575 participants, RNA was sequenced and expression of 281 genes related to autophagy regulation, mitophagy and mTOR/upstream pathways were determined. Associations between gene expression and outcomes including mitochondrial respiration in muscle fiber bundles (MAX OXPHOS), physical performance (VO2 peak, 400m walking speed, and leg power), and thigh muscle volume were determined using negative binomial regression models. For autophagy, key transcriptional regulators including TFE3 and NFKB-related genes (RELA, RELB, NFKB1) were negatively associated with outcomes. On the contrary, regulators of oxidative metabolism that also promote overall autophagy, mitophagy and pexophagy (PPARGC1A, PPARA, EPAS1) were positively associated with multiple outcomes. In line with this, several mitophagy, fusion and fission related genes (NIPSNAP2, DNM1L, OPA1) were also positively associated with outcomes. For mTOR pathway and related genes, expression of WDR59 and WDR24, both subunits of GATOR2 complex (an indirect inhibitor of mTORC1) and PRKAG3, which is a regulatory subunit of AMPK, were negatively correlated with multiple outcomes. Our study identifies autophagy and selective autophagy such as mitophagy gene expression patterns in human skeletal muscle related to physical performance, muscle volume and mitochondrial function in older persons which may lead to target identification to preserve mobility and independence.

Mitochondrial dysfunction and skeletal muscle atrophy: Causes, mechanisms, and treatment strategies

Skeletal muscle, which accounts for approximately 40% of total body weight, is one of the most dynamic and plastic tissues in the human body and plays a vital role in movement, posture and force production. More than just a component of the locomotor system, skeletal muscle functions as an endocrine organ capable of producing and secreting hundreds of bioactive molecules. Therefore, maintaining healthy skeletal muscles is crucial for supporting overall body health. Various pathological conditions, such as prolonged immobilization, cachexia, aging, drug-induced toxicity, and cardiovascular diseases (CVDs), can disrupt the balance between muscle protein synthesis and degradation, leading to skeletal muscle atrophy. Mitochondrial dysfunction is a major contributing mechanism to skeletal muscle atrophy, as it plays crucial roles in various biological processes, including energy production, metabolic flexibility, maintenance of redox homeostasis, and regulation of apoptosis. In this review, we critically examine recent knowledge regarding the causes of muscle atrophy (disuse, cachexia, aging, etc.) and its contribution to CVDs. Additionally, we highlight the mitochondrial signaling pathways involvement to skeletal muscle atrophy, such as the ubiquitin-proteasome system, autophagy and mitophagy, mitochondrial fission-fusion, and mitochondrial biogenesis. Furthermore, we discuss current strategies, including exercise, mitochondria-targeted antioxidants, in vivo transfection of PGC-1α, and the potential use of mitochondrial transplantation as a possible therapeutic approach.

Mitochondrial Properties in Skeletal Muscle Fiber

Mitochondria are the primary source of energy production and are implicated in a wide range of biological processes in most eukaryotic cells. Skeletal muscle heavily relies on mitochondria for energy supplements. In addition to being a powerhouse, mitochondria evoke many functions in skeletal muscle, including regulating calcium and reactive oxygen species levels. A healthy mitochondria population is necessary for the preservation of skeletal muscle homeostasis, while mitochondria dysregulation is linked to numerous myopathies. In this review, we summarize the recent studies on mitochondria function and quality control in skeletal muscle, focusing mainly on in vivo studies of rodents and human subjects. With an emphasis on the interplay between mitochondrial functions concerning the muscle fiber type-specific phenotypes, we also discuss the effect of aging and exercise on the remodeling of skeletal muscle and mitochondria properties.

Mitochondrial Dynamics in Neurodegenerative Diseases: Unraveling the Role of Fusion and Fission Processes

Neurodegenerative diseases (NDs) are a diverse group of disorders characterized by the progressive degeneration and death of neurons, leading to a range of neurological symptoms. Despite the heterogeneity of these conditions, a common denominator is the implication of mitochondrial dysfunction in their pathogenesis. Mitochondria play a crucial role in creating biomolecules, providing energy through adenosine triphosphate (ATP) generated by oxidative phosphorylation (OXPHOS), and producing reactive oxygen species (ROS). When they're not functioning correctly, becoming fragmented and losing their membrane potential, they contribute to these diseases. In this review, we explore how mitochondria fuse and undergo fission, especially in the context of NDs. We discuss the genetic and protein mutations linked to these diseases and how they impact mitochondrial dynamics. We also look at the key regulatory proteins in fusion (MFN1, MFN2, and OPA1) and fission (DRP1 and FIS1), including their post-translational modifications. Furthermore, we highlight potential drugs that can influence mitochondrial dynamics. By unpacking these complex processes, we aim to direct research towards treatments that can improve life quality for people with these challenging conditions.

Autophagy ablation in skeletal muscles worsens sepsis-induced muscle wasting, impairs whole-body metabolism, and decreases survival

Septic patients frequently develop skeletal muscle wasting and weakness, resulting in severe clinical consequences and adverse outcomes. Sepsis triggers sustained induction of autophagy, a key cellular degradative pathway, in skeletal muscles. However, the impact of enhanced autophagy on sepsis-induced muscle dysfunction remains unclear. Using an inducible and muscle-specific Atg7 knockout mouse model (Atg7iSkM-KO), we investigated the functional importance of skeletal muscle autophagy in sepsis using the cecal ligation and puncture model. Atg7iSkM-KO mice exhibited a more severe phenotype in response to sepsis, marked by severe muscle wasting, hypoglycemia, higher ketone levels, and a decreased in survival as compared to mice with intact Atg7. Sepsis and Atg7 deletion resulted in the accumulation of mitochondrial dysfunction, although sepsis did not further worsen mitochondrial dysfunction in Atg7iSkM-KO mice. Overall, our study demonstrates that autophagy inactivation in skeletal muscles triggers significant worsening of sepsis-induced muscle and metabolic dysfunctions and negatively impacts survival.

Implications of mitochondrial fusion and fission in skeletal muscle mass and health

The continuous dynamic reshaping of mitochondria by fusion and fission events is critical to keep mitochondrial quality and function under control in response to changes in energy and stress. Maintaining a functional, highly interconnected mitochondrial reticulum ensures rapid energy production and distribution. Moreover, mitochondrial networks act as dynamic signaling hub to adapt to the metabolic demands imposed by contraction, energy expenditure, and general metabolism. However, excessive mitochondrial fusion or fission results in the disruption of the skeletal muscle mitochondrial network integrity and activates a retrograde response from mitochondria to the nucleus, leading to muscle atrophy, weakness and influencing whole-body homeostasis. These actions are mediated via the secretion of mitochondrial-stress myokines such as FGF21 and GDF15. Here we will summarize recent discoveries in the role of mitochondrial fusion and fission in the control of muscle mass and in regulating physiological homeostasis and disease progression.

Genetic deletion of skeletal muscle iPLA2γ results in mitochondrial dysfunction, muscle atrophy and alterations in whole-body energy metabolism

Skeletal muscle is the major site of glucose utilization in mammals integrating serum glucose clearance with mitochondrial respiration. To mechanistically elucidate the roles of iPLA2γ in skeletal muscle mitochondria, we generated a skeletal muscle-specific calcium-independent phospholipase A2γ knockout (SKMiPLA2γKO) mouse. Genetic ablation of skeletal muscle iPLA2γ resulted in pronounced muscle weakness, muscle atrophy, and increased blood lactate resulting from defects in mitochondrial function impairing metabolic processing of pyruvate and resultant bioenergetic inefficiency. Mitochondria from SKMiPLA2γKO mice were dysmorphic displaying marked changes in size, shape, and interfibrillar juxtaposition. Mitochondrial respirometry demonstrated a marked impairment in respiratory efficiency with decreases in the mass and function of oxidative phosphorylation complexes and cytochrome c. Further, a pronounced decrease in mitochondrial membrane potential and remodeling of cardiolipin molecular species were prominent. Collectively, these alterations prevented body weight gain during high-fat feeding through enhanced glucose disposal without efficient capture of chemical energy thereby altering whole-body bioenergetics.

Mitochondrial transplantation as a possible therapeutic option for sarcopenia

With advancing age, the skeletal muscle phenotype is characterized by a progressive loss of mass, strength, and quality. This phenomenon, known as sarcopenia, has a negative impact on quality of life and increases the risk of morbidity and mortality in older adults. Accumulating evidence suggests that damaged and dysfunctional mitochondria play a critical role in the pathogenesis of sarcopenia. Lifestyle modifications, such as physical activity, exercise, and nutrition, as well as medical interventions with therapeutic agents, are effective in the management of sarcopenia and offer solutions to maintain and improve skeletal muscle health. Although a great deal of effort has been devoted to the identification of the best treatment option, these strategies are not sufficient to overcome sarcopenia. Recently, it has been reported that mitochondrial transplantation may be a possible therapeutic approach for the treatment of mitochondria-related pathological conditions such as ischemia, liver toxicity, kidney injury, cancer, and non-alcoholic fatty liver disease. Given the role of mitochondria in the function and metabolism of skeletal muscle, mitochondrial transplantation may be a possible option for the treatment of sarcopenia. In this review, we summarize the definition and characteristics of sarcopenia and molecular mechanisms associated with mitochondria that are known to contribute to sarcopenia. We also discuss mitochondrial transplantation as a possible option. Despite the progress made in the field of mitochondrial transplantation, further studies are needed to elucidate the role of mitochondrial transplantation in sarcopenia. KEY MESSAGES: Sarcopenia is the progressive loss of skeletal muscle mass, strength, and quality. Although the specific mechanisms that lead to sarcopenia are not fully understood, mitochondria have been identified as a key factor in the development of sarcopenia. Damaged and dysfunctional mitochondria initiate various cellular mediators and signaling pathways, which largely contribute to the age-related loss of skeletal muscle mass and strength. Mitochondrial transplantation has been reported to be a possible option for the treatment/prevention of several diseases. Mitochondrial transplantation may be a possible therapeutic option for improving skeletal muscle health and treating sarcopenia. Mitochondrial transplantation as a possible treatment option for sarcopenia.

MYTHO is a novel regulator of skeletal muscle autophagy and integrity

Autophagy is a critical process in the regulation of muscle mass, function and integrity. The molecular mechanisms regulating autophagy are complex and still partly understood. Here, we identify and characterize a novel FoxO-dependent gene, d230025d16rik which we named Mytho (Macroautophagy and YouTH Optimizer), as a regulator of autophagy and skeletal muscle integrity in vivo. Mytho is significantly up-regulated in various mouse models of skeletal muscle atrophy. Short term depletion of MYTHO in mice attenuates muscle atrophy caused by fasting, denervation, cancer cachexia and sepsis. While MYTHO overexpression is sufficient to trigger muscle atrophy, MYTHO knockdown results in a progressive increase in muscle mass associated with a sustained activation of the mTORC1 signaling pathway. Prolonged MYTHO knockdown is associated with severe myopathic features, including impaired autophagy, muscle weakness, myofiber degeneration, and extensive ultrastructural defects, such as accumulation of autophagic vacuoles and tubular aggregates. Inhibition of the mTORC1 signaling pathway in mice using rapamycin treatment attenuates the myopathic phenotype triggered by MYTHO knockdown. Skeletal muscles from human patients diagnosed with myotonic dystrophy type 1 (DM1) display reduced Mytho expression, activation of the mTORC1 signaling pathway and impaired autophagy, raising the possibility that low Mytho expression might contribute to the progression of the disease. We conclude that MYTHO is a key regulator of muscle autophagy and integrity.

The role of the mechanistic target of rapamycin complex 1 in the regulation of mitochondrial adaptation during skeletal muscle atrophy under denervation or calorie restriction in mice

Mechanistic target of rapamycin complex 1 (mTORC1) is a protein complex that regulates skeletal muscle protein synthesis and hypertrophy. mTORC1-mediated signaling activities are activated during denervation-induced skeletal muscle atrophy and suppressed during calorie restriction-induced atrophy. Mitochondria control the qualitative plasticity of skeletal muscles primarily through biogenesis, fusion, and fission. We recently showed that mTORC1 activation contributes toward mitochondrial homeostasis. In this study, we examined the role of mTORC1 in mitochondrial adaptation during denervation- or calorie restriction-induced skeletal muscle atrophy. Seven-week-old Institute of Cancer Research mice were subjected to 14 days of denervation or calorie restriction combined with the administration of the mTORC1 inhibitor-"rapamycin". Our results showed that although mTORC1 inhibition did not alter mitochondrial biogenesis, content and enzyme activity, it suppressed the activation of dynamin-related protein 1 (DRP1), a mitochondrial fission-related protein in denervated muscle, and reduced DRP1 expression in calorie-restricted muscle. Furthermore, calorie restriction-induced mitochondrial fragmentation was partially suppressed by mTORC1 inhibition. Taken together, our results indicate that mTORC1 activation upon denervation and inhibition upon calorie restriction contributes to qualitative changes in muscle plasticity by at least partially regulating the mitochondrial fission response.

The Role of Mitophagy in Skeletal Muscle Damage and Regeneration

Mitochondria are cellular organelles that play an essential role in generating the chemical energy needed for the biochemical reactions in cells. Mitochondrial biogenesis, i.e., de novo mitochondria formation, results in enhanced cellular respiration, metabolic processes, and ATP generation, while autophagic clearance of mitochondria (mitophagy) is required to remove damaged or useless mitochondria. The balance between the opposing processes of mitochondrial biogenesis and mitophagy is highly regulated and crucial for the maintenance of the number and function of mitochondria as well as for the cellular homeostasis and adaptations to metabolic demands and extracellular stimuli. In skeletal muscle, mitochondria are essential for maintaining energy homeostasis, and the mitochondrial network exhibits complex behaviors and undergoes dynamic remodeling in response to various conditions and pathologies characterized by changes in muscle cell structure and metabolism, such as exercise, muscle damage, and myopathies. In particular, the involvement of mitochondrial remodeling in mediating skeletal muscle regeneration following damage has received increased attention, as modifications in mitophagy-related signals arise from exercise, while variations in mitochondrial restructuring pathways can lead to partial regeneration and impaired muscle function. Muscle regeneration (through myogenesis) following exercise-induced damage is characterized by a highly regulated, rapid turnover of poor-functioning mitochondria, permitting the synthesis of better-functioning mitochondria to occur. Nevertheless, essential aspects of mitochondrial remodeling during muscle regeneration remain poorly understood and warrant further characterization. In this review, we focus on the critical role of mitophagy for proper muscle cell regeneration following damage, highlighting the molecular mechanisms of the mitophagy-associated mitochondrial dynamics and network reformation.

The impact of high-fat feeding and parkin overexpression on skeletal muscle mass, mitochondrial respiration, and H2O2 emission

Obesity is a major risk factor for developing various health problems, including insulin resistance and type 2 diabetes. Although controversial, accumulation of mitochondrial dysfunction, and notably an increase in mitochondrial reactive oxygen species (ROS) production, was proposed as a key contributor leading to obesity-induced insulin resistance. Here, our goal was to investigate whether Parkin overexpression, a key regulator of the removal of dysfunctional mitochondria through mitophagy, could confer protection against obesity-induced mitochondrial dysfunction. To this end, intramuscular injections of adeno-associated viruses (AAVs) were performed to overexpress Parkin in limb muscle of 6-mo-old mice fed a control diet (CD) or a high-fat diet (HFD) for 12 wk. An AAV-expressing the green fluorescent protein (GFP) was used as control. HFD increased fat mass, altered glycemia, and resulted in insulin resistance. Parkin overexpression resulted in an increase in muscle mass in both CD and HFD mice. In CD mice, Parkin overexpression increased maximal mitochondrial respiration and lowered H₂O₂ emission. HFD increased mitochondrial respiration and, surprisingly, also lowered H₂O₂ emission. Parkin overexpression did not significantly impact mitochondrial function in HFD mice. Taken altogether, our results indicate that Parkin overexpression positively impacts muscle and mitochondrial health under basal conditions and challenges the notion that intrinsic mitochondrial dysfunction is involved in the development of insulin resistance caused by high-fat feeding.

The protective effect of inhibiting mitochondrial fission on the juvenile rat brain following PTZ kindling through inhibiting the BCL2L13/LC3 mitophagy pathway

Maintaining the balance of mitochondrial fission and mitochondrial autophagy on seizures is helpful to find a solution to control seizures and reduce brain injuries. The present study is to investigate the protective effect of inhibiting mitochondrial fission on brain injury in juvenile rat epilepsy induced by pentatetrazol (PTZ) by inhibiting the BCL2L13/LC3-mediated mitophagy pathway. PTZ was injected (40 mg/kg) to induce kindling once every other day, for a total of 15 times. In the PTZ + DMSO (DMSO), PTZ + Mdivi-1 (Mdivi-1), and PTZ + WY14643 (WY14643) groups, rats were pretreated with DMSO, Mdivi-1 and WY14643 for half an hour prior to PTZ injection. The seizure attacks of young rats were observed for 30 min after model establishment. The Morris water maze (MWM) was used to test the cognition of experimental rats. After the test, the numbers of NeuN(+) neurons and GFAP(+) astrocytes were observed and counted by immunofluorescence (IF). The protein expression levels of Drp1, BCL2L13, LC3 and caspase 3 in the hippocampus of young rats were detected by immunohistochemistry (IHC) and Western blotting (WB). Compared with the PTZ and DMSO groups, the seizure latency in the Mdivi-1 group was longer (P < 0.01), and the severity degree and frequency of seizures were lower (P < 0.01). The MWM test showed that the incubation periods of crossing the platform in the Mdivi-1 group was significantly shorter. The number of platform crossings, the platform stay time, and the ratio of residence time/total stay time were significantly increased in the Mdivi-1 group (P < 0.01). The IF results showed that the number of NeuN(+) neurons in the Mdivi-1 group was greater, while the number of GFAP(+) astrocytes was lower. IHC and WB showed that the average optical density (AOD) and relative protein expression levels of Drp1, BCL2L13, LC3 and caspase 3 in the hippocampi of rats in the Mdivi-1 group were higher (P < 0.05). The above results in the WY14643 group were opposite to those in the Mdivi-1 group. Inhibition of mitochondrial fission could reduce seizure attacks, protect injured neurons, and improve cognition following PTZ-induced epilepsy by inhibiting mitochondrial autophagy mediated by the BCL2L13/LC3 mitophagy pathway.

Mitochondria transplant therapy improves regeneration and restoration of injured skeletal muscle

BACKGROUND

Injection of exogenous mitochondria has been shown to improve the ischaemia-damaged myocardium, but the effect of mitochondrial transplant therapy (MTT) to restore skeletal muscle mass and function has not been tested following neuromuscular injury. Therefore, we tested the hypothesis that MTT would improve the restoration of muscle function after injury.

METHODS

BaCl₂ was injected into the gastrocnemius muscle of one limb of 8-12-week-old C57BL/6 mice to induce damage without injury to the resident stem cells. The contralateral gastrocnemius muscle was injected with phosphate-buffered saline (PBS) and served as the non-injured intra-animal control. Mitochondria were isolated from donor mice. Donor mitochondria were suspended in PBS or PBS without mitochondria (sham treatment) and injected into the tail vein of BaCl₂ injured mice 24 h after the initial injury. Muscle repair was examined 7, 14 and 21 days after injury.

RESULTS

MTT did not increase systemic inflammation in mice. Muscle mass 7 days following injury was 21.9 ± 2.1% and 17.4 ± 1.9% lower (P < 0.05) in injured as compared with non-injured intra-animal control muscles in phosphate-buffered saline (PBS)- and MTT-treated animals, respectively. Maximal plantar flexor muscle force was significantly lower in injured as compared with uninjured muscles of PBS-treated (-43.4 ± 4.2%, P < 0.05) and MTT-treated mice (-47.7 ± 7.3%, P < 0.05), but the reduction in force was not different between the experimental groups. The percentage of collagen and other non-contractile tissue in histological muscle cross sections, was significantly greater in injured muscles of PBS-treated mice (33.2 ± 0.2%) compared with MTT-treated mice (26.5 ± 0.2%) 7 days after injury. Muscle wet weight and maximal muscle force from injured MTT-treated mice had recovered to control levels by 14 days after the injury. However, muscle mass and force had not improved in PBS-treated animals by 14 days after injury. The non-contractile composition of the gastrocnemius muscle tissue cross sections was not different between control, repaired PBS-treated and repaired MTT-treated mice 14 days after injury. By 21 days following injury, PBS-treated mice had fully restored gastrocnemius muscle mass of the injured muscle to that of the uninjured muscle, although maximal plantar flexion force was still 19.4 ± 3.7% (P < 0.05) lower in injured/repaired gastrocnemius as compared with uninjured intra-animal control muscles.

CONCLUSIONS

Our results suggest that systemic mitochondria delivery can enhance the rate of muscle regeneration and restoration of muscle function following injury.

Acute resistance exercise training does not augment mitochondrial remodelling in master athletes or untrained older adults

Background: Ageing is associated with alterations to skeletal muscle oxidative metabolism that may be influenced by physical activity status, although the mechanisms underlying these changes have not been unraveled. Similarly, the effect of resistance exercise training (RET) on skeletal muscle mitochondrial regulation is unclear. Methods: Seven endurance-trained masters athletes ([MA], 74 ± 3 years) and seven untrained older adults ([OC]. 69 ± 6 years) completed a single session of knee extension RET (6 x 12 repetitions, 75% 1-RM, 120-s intra-set recovery). Vastus lateralis muscle biopsies were collected pre-RET, 1 h post-RET, and 48h post-RET. Skeletal muscle biopsies were analyzed for citrate synthase (CS) enzyme activity, mitochondrial content, and markers of mitochondrial quality control via immunoblotting. Results: Pre-RET CS activity and protein content were ∼45% (p < .001) and ∼74% greater in MA compared with OC (p = .006). There was a significant reduction (∼18%) in CS activity 48 h post-RET (p < .05) in OC, but not MA. Pre-RET abundance of individual and combined mitochondrial electron transport chain (ETC) complexes I-V were significantly greater in MA compared with OC, as were markers of mitochondrial fission and fusion dynamics (p-DRP-1Ser616, p-MFFSer146, OPA-1 & FIS-1, p < .05 for all). Moreover, MA displayed greater expression of p-AMPKThr172, PGC1α, TFAM, and SIRT-3 (p < .05 for all). Notably, RET did not alter the expression of any marker of mitochondrial content, biogenesis, or quality control in both OC and MA. Conclusion: The present data suggest that long-term aerobic exercise training supports superior skeletal muscle mitochondrial density and protein content into later life, which may be regulated by greater mitochondrial quality control mechanisms and supported via superior fission-fusion dynamics. However, a single session of RET is unable to induce mitochondrial remodelling in the acute (1h post-RET) and delayed (48 h post-RET) recovery period in OC and MA.

Bile Acids Alter the Autophagy and Mitogenesis in Skeletal Muscle Cells

Muscle atrophy decreases muscle mass with the subsequent loss of muscle function. Among the mechanisms that trigger sarcopenia is mitochondrial dysfunction. Mitochondria, whose primary function is to produce ATP, are dynamic organelles that present the process of formation (mitogenesis) and elimination (mitophagy). Failure of any of these processes contributes to mitochondrial malfunction. Mitogenesis is mainly controlled by Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1α), a transcriptional coactivator that regulates the expression of TFAM, which participates in mitogenesis. Mitophagy is a process of selective autophagy. Autophagy corresponds to a degradative pathway of protein complexes and organelles. Liver disease caused sarcopenia and increased bile acids in the blood. We demonstrated that the treatment with cholic (CA) or deoxycholic (DCA) bile acids generates mitochondrial dysfunction and loss of biomass. This work assessed whether CA and DCA alter autophagy and mitogenesis. For this, western blot evaluated the autophagy process by determining the protein levels of the LC3II/LC3I ratio. In addition, we assessed mitogenesis using a luciferase-coupled plasmid reporter for the PGC-1α promoter and the protein levels of TFAM by western blot. Our results indicate that treatment with CA or DCA induces autophagy, represented by an increase in the LC3II/LC3I ratio. In addition, a decreased autophagic flux was observed. On the other hand, when treated with CA or DCA, a decrease in the activity of the PGC-1α promoter was observed. However, the levels of TFAM increased in myotubes incubated with CA and DCA. Our results demonstrate that CA and DCA modulate autophagy ad mitogenesis in C₂C₁₂ myotubes.

Ischemic preconditioning upregulates Mitofusin2 and preserves muscle strength in tourniquet-induced ischemia/reperfusion

Background

Tourniquet-induced ischemia and reperfusion (I/R) has been related to postoperative muscle atrophy through mechanisms involving protein synthesis/breakdown, cellular metabolism, mitochondrial dysfunction, and apoptosis. Ischemic preconditioning (IPC) could protect skeletal muscle against I/R injury. This study aims to determine the underlying mechanisms of IPC and its effect on muscle strength after total knee arthroplasty (TKA).

Methods

Twenty-four TKA patients were randomized to receive either sham IPC or IPC (3 cycles of 5-min ischemia followed by 5-min reperfusion). Vastus medialis muscle biopsies were collected at 30 ​min after tourniquet (TQ) inflation and the onset of reperfusion. Western blot analysis was performed in muscle protein for 4-HNE, SOD2, TNF-ɑ, IL-6, p-Drp1^ser616, Drp1, Mfn1, Mfn2, Opa1, PGC-1ɑ, ETC complex I-V, cytochrome c, cleaved caspase-3, and caspase-3. Clinical outcomes including isokinetic muscle strength and quality of life were evaluated pre- and postoperatively.

Results

IPC significantly increased Mfn2 (2.0 ​± ​0.2 vs 1.2 ​± ​0.1, p ​= ​0.001) and Opa1 (2.9 ​± ​0.3 vs 1.9 ​± ​0.2, p ​= ​0.005) proteins expression at the onset of reperfusion, compared to the ischemic phase. There were no differences in 4-HNE, SOD2, TNF-ɑ, IL-6, p-Drp1^ser616/Drp1, Mfn1, PGC-1ɑ, ETC complex I-V, cytochrome c, and cleaved caspase-3/caspase-3 expression between the ischemic and reperfusion periods, or between the groups. Clinically, postoperative peak torque for knee extension significantly reduced in the sham IPC group (-16.6 [-29.5, -3.6] N.m, p ​= ​0.020), while that in the IPC group was preserved (-4.7 [-25.3, 16.0] N.m, p ​= ​0.617).

Conclusion

In TKA with TQ application, IPC preserved postoperative quadriceps strength and prevented TQ-induced I/R injury partly by enhancing mitochondrial fusion proteins in the skeletal muscle.

The translational potential of this article

Mitochondrial fusion is a potential underlying mechanism of IPC in preventing skeletal muscle I/R injury. IPC applied before TQ-induced I/R preserved postoperative quadriceps muscle strength after TKA.

Effect of Physical Activity/Exercise on Oxidative Stress and Inflammation in Muscle and Vascular Aging

Functional status is considered the main determinant of healthy aging. Impairment in skeletal muscle and the cardiovascular system, two interrelated systems, results in compromised functional status in aging. Increased oxidative stress and inflammation in older subjects constitute the background for skeletal muscle and cardiovascular system alterations. Aged skeletal muscle mass and strength impairment is related to anabolic resistance, mitochondrial dysfunction, increased oxidative stress and inflammation as well as a reduced antioxidant response and myokine profile. Arterial stiffness and endothelial function stand out as the main cardiovascular alterations related to aging, where increased systemic and vascular oxidative stress and inflammation play a key role. Physical activity and exercise training arise as modifiable determinants of functional outcomes in older persons. Exercise enhances antioxidant response, decreases age-related oxidative stress and pro-inflammatory signals, and promotes the activation of anabolic and mitochondrial biogenesis pathways in skeletal muscle. Additionally, exercise improves endothelial function and arterial stiffness by reducing inflammatory and oxidative damage signaling in vascular tissue together with an increase in antioxidant enzymes and nitric oxide availability, globally promoting functional performance and healthy aging. This review focuses on the role of oxidative stress and inflammation in aged musculoskeletal and vascular systems and how physical activity/exercise influences functional status in the elderly.

Lysyl oxidase-like 2 inhibitor rescues D-galactose-induced skeletal muscle fibrosis

Aging-related sarcopenia is currently the most common sarcopenia. The main manifestations are skeletal muscle atrophy, replacement of muscle fibers with fat and fibrous tissue. Excessive fibrosis can impair muscle regeneration and function. Lysyl oxidase-like 2 (LOXL2) has previously been reported to be involved in the development of various tissue fibrosis. Here, we investigated the effects of LOXL2 inhibitor on D-galactose (D-gal)-induced skeletal muscle fibroblast cells and mice. Our molecular and physiological studies show that treatment with LOXL2 inhibitor can alleviate senescence, fibrosis, and increased production of reactive oxygen species in fibroblasts caused by D-gal. These effects are related to the inhibition of the TGF-β1/p38 MAPK pathway. Furthermore, in vivo, mice treatment with LOXL2 inhibitor reduced D-gal-induced skeletal muscle fibrosis, partially enhanced skeletal muscle mass and strength and reduced redox balance disorder. Taken together, these data indicate the possibility of using LOXL2 inhibitors to prevent aging-related sarcopenia, especially with significant fibrosis.

Sex-specific mitochondrial dynamics and mitophagy response to muscle damage

Muscle damage imposes stress on mitochondria resulting in mitochondrial fusion, fission, and mitophagy. Testosterone is a regulator of these processes. However, no study has examined the effect of sex-specific resistance exercise (RE)-induced hormonal response on mitochondrial dynamics and mitophagy after muscle damage in untrained men and women. Untrained men and women performed two sessions of 80 unilateral maximal eccentric knee extensions (ECC) followed by upper-body RE (ECC+RE) aimed to induce hormonal changes and maintain a similar lower body demands between conditions or 20 min seated rest (ECC+REST). Vastus lateralis samples were analyzed for gene and protein expression of OPA1, MFN1, DRP1, PINK1, and Parkin at baseline (BL), 12 and 24 h. Testosterone area under the curve was greater for ECC+RE than ECC+REST in men and was greater in men than women for both conditions. A significant time × sex × condition effect was found for Parkin protein expression. At 12 and 24 h, Parkin was lower for ECC + REST than ECC + RE for men; whereas, Parkin was increased at 24 h for women regardless of condition. A significant time effect was found for OPA1 protein expression increasing at 12 and 24 h. A significant time × sex × condition effects were found for MFN1, DRP1, and PINK1 gene expression with increases at 12 h in men for ECC + RE. A significant time × sex effect was found for OPA1 gene expression with a decrease at 12 h in men, and 12 h expression in men was lower than women. RE-induced hormonal changes promoted expression of fission, fusion, and mitophagy markers in men. With muscle damage, regardless of condition, expression of inner mitochondrial membrane fusion markers are promoted in both sexes; whereas, those for mitophagy were promoted in women but reduced in men.

Unbiased proteomics, histochemistry, and mitochondrial DNA copy number reveal better mitochondrial health in muscle of high-functioning octogenarians

Background

Master athletes (MAs) prove that preserving a high level of physical function up to very late in life is possible, but the mechanisms responsible for their high function remain unclear.

Methods

We performed muscle biopsies in 15 octogenarian world-class track and field MAs and 14 non-athlete age/sex-matched controls (NA) to provide insights into mechanisms for preserving function in advanced age. Muscle samples were assessed for respiratory compromised fibers, mitochondrial DNA (mtDNA) copy number, and proteomics by liquid-chromatography mass spectrometry.

Results

MA exhibited markedly better performance on clinical function tests and greater cross-sectional area of the vastus lateralis muscle. Proteomics analysis revealed marked differences, where most of the ~800 differentially represented proteins in MA versus NA pertained to mitochondria structure/function such as electron transport capacity (ETC), cristae formation, mitochondrial biogenesis, and mtDNA-encoded proteins. In contrast, proteins from the spliceosome complex and nuclear pore were downregulated in MA. Consistent with proteomics data, MA had fewer respiratory compromised fibers, higher mtDNA copy number, and an increased protein ratio of the cristae-bound ETC subunits relative to the outer mitochondrial membrane protein voltage-dependent anion channel. There was a substantial overlap of proteins overrepresented in MA versus NA with proteins that decline with aging and that are higher in physically active than sedentary individuals. However, we also found 176 proteins related to mitochondria that are uniquely differentially expressed in MA.

Conclusions

We conclude that high function in advanced age is associated with preserving mitochondrial structure/function proteins, with underrepresentation of proteins involved in the spliceosome and nuclear pore complex. Whereas many of these differences in MA appear related to their physical activity habits, others may reflect unique biological (e.g., gene, environment) mechanisms that preserve muscle integrity and function with aging.

Funding

Funding for this study was provided by operating grants from the Canadian Institutes of Health Research (MOP 84408 to TT and MOP 125986 to RTH). This work was supported in part by the Intramural Research Program of the National Institute on Aging, NIH, Baltimore, MD, USA.

Mitochondrial fusion- and fission-related protein expression in the regulation of skeletal muscle mass

Mitochondria in the skeletal muscle are essential for maintaining metabolic plasticity and function. Mitochondrial quality control encompasses the dynamics of the biogenesis and remodeling of mitochondria, characterized by the constant fission and fusion of mitochondria in response to metabolic stressors. However, the roles of mitochondrial fission or fusion in muscle hypertrophy and atrophy remain unclear. The aim of this study was to determine whether mitochondrial fusion and fission events are influenced by muscle hypertrophy or atrophy stimulation. Twenty-six male F344 rats were randomly assigned to a control group or were subjected to up to 14 days of either plantaris overload (via tenotomy of the gastrocnemius and soleus muscles; hypertrophy group) or hindlimb cast immobilization (atrophy group). After 14 days of treatment, plantaris muscle samples were collected to determine the expression levels of mitochondrial fusion- and fission-related proteins. Muscle weight and total muscle protein content increased following plantaris overload in the hypertrophy group, but decreased following immobilization for 14 days in the atrophy group. In the hypertrophied muscle, the level of activated dynamin-related protein 1 (Drp1), phosphorylated at Ser616, significantly increased by 25.8% (p = 0.014). Moreover, the protein expression level of mitochondrial fission factor significantly decreased by 36.5% in the hypertrophy group compared with that of the control group (p = 0.017). In contrast, total Drp1 level significantly decreased in the atrophied plantaris muscle (p = 0.011). Our data suggest that mitochondrial fission events may be influenced by both muscle hypertrophy and atrophy stimulation, and that mitochondrial fission- related protein Drp1 plays an important role in the regulation of skeletal muscle in response to mechanical stimulation.

Effects of Excessive Activation of N-methyl-D-aspartic Acid Receptors in Neonatal Cardiac Mitochondrial Dysfunction Induced by Intrauterine Hypoxia

Intrauterine hypoxia is a common complication during pregnancy and could increase the risk of cardiovascular disease in offspring. However, the underlying mechanism is controversial. Memantine, an NMDA receptor antagonist, is reported to be a potential cardio-protective agent. We hypothesized that antenatal memantine treatment could prevent heart injury in neonatal offspring exposed to intrauterine hypoxia. Pregnant rats were exposed to gestational hypoxia or antenatal memantine treatment during late pregnancy. Newborns were then sacrificed to assess multiple parameters. The results revealed that Intrauterine hypoxia resulted in declining birth weight, heart weight, and an abnormally high heart weight/birth weight ratio. Furthermore, intrauterine hypoxia caused mitochondrial structural, functional abnormalities and decreased expression of DRP1, and upregulation of NMDAR1 in vivo. Antenatal memantine treatment,an NMDARs antagonist, improved these changes. In vitro, hypoxia increased the glutamate concentration and expression of NMDAR1. NMDAR activation may lead to similar changes in mitochondrial function, structure, and downregulation of DRP1 in vitro. Pharmacological blockade of NMDARs by the non-competitive NMDA antagonist MK-801 or knockdown of the glutamate receptor NR1 significantly attenuated the increased mitochondrial reactive oxygen species and calcium overload-induced by hypoxia exposure. These facts suggest that memantine could provide a novel and promising treatment for clinical use in intrauterine hypoxia during pregnancy to protect the cardiac mitochondrial function in the offspring. To our best knowledge, our research is the first study that shows intrauterine hypoxia can excessively activate cardiac NMDARs and thus cause mitochondrial dysfunction.

Mitochondrial Function and Reactive Oxygen/Nitrogen Species in Skeletal Muscle

Skeletal muscle fibers contain a large number of mitochondria, which produce ATP through oxidative phosphorylation (OXPHOS) and provide energy for muscle contraction. In this process, mitochondria also produce several types of "reactive species" as side product, such as reactive oxygen species and reactive nitrogen species which have attracted interest. Mitochondria have been proven to have an essential role in the production of skeletal muscle reactive oxygen/nitrogen species (RONS). Traditionally, the elevation in RONS production is related to oxidative stress, leading to impaired skeletal muscle contractility and muscle atrophy. However, recent studies have shown that the optimal RONS level under the action of antioxidants is a critical physiological signal in skeletal muscle. Here, we will review the origin and physiological functions of RONS, mitochondrial structure and function, mitochondrial dynamics, and the coupling between RONS and mitochondrial oxidative stress. The crosstalk mechanism between mitochondrial function and RONS in skeletal muscle and its regulation of muscle stem cell fate and myogenesis will also be discussed. In all, this review aims to describe a comprehensive and systematic network for the interaction between skeletal muscle mitochondrial function and RONS.

From the Bench to the Bedside: Branched Amino Acid and Micronutrient Strategies to Improve Mitochondrial Dysfunction Leading to Sarcopenia

With extended life expectancy, the older population is constantly increasing, and consequently, so too is the prevalence of age-related disorders. Sarcopenia, the pathological age-related loss of muscle mass and function; and malnutrition, the imbalance in nutrient intake and resultant energy production, are both commonly occurring conditions in old adults. Altered nutrition plays a crucial role in the onset of sarcopenia, and both these disorders are associated with detrimental consequences for patients (e.g., frailty, morbidity, and mortality) and society (e.g., healthcare costs). Importantly, sarcopenia and malnutrition also share critical molecular alterations, such as mitochondrial dysfunction, increased oxidative stress, and a chronic state of low grade and sterile inflammation, defined as inflammageing. Given the connection between malnutrition and sarcopenia, nutritional interventions capable of affecting mitochondrial health and correcting inflammageing are emerging as possible strategies to target sarcopenia. Here, we discuss mitochondrial dysfunction, oxidative stress, and inflammageing as key features leading to sarcopenia. Moreover, we examine the effects of some branched amino acids, omega-3 PUFA, and selected micronutrients on these pathways, and their potential role in modulating sarcopenia, warranting further clinical investigation.

Mitochondrial dysfunction and mitochondrial therapies in heart failure

Cardiovascular diseases remain the leading cause of death worldwide in the last decade, accompanied by immense health and economic burdens. Heart failure (HF), as the terminal stage of many cardiovascular diseases, is a common, intractable, and costly medical condition. Despite significant improvements in pharmacologic and device therapies over the years, life expectancy for this disease remains poor. Current therapies have not reversed the trends in morbidity and mortality as expected. Thus, there is an urgent need for novel potential therapeutic agents. Although the pathophysiology of the failing heart is extraordinarily complex, targeting mitochondrial dysfunction can be an effective approach for potential treatment. Increasing evidence has shown that mitochondrial abnormalities, including altered metabolic substrate utilization, impaired mitochondrial oxidative phosphorylation (OXPHOS), increased reactive oxygen species (ROS) formation, and aberrant mitochondrial dynamics, are closely related to HF. Here, we reviewed the findings on the role of mitochondrial dysfunction in HF, along with novel mitochondrial therapeutics and their pharmacological effects.

Mitochondrial Quality Control in Sarcopenia: Updated Overview of Mechanisms and Interventions

Sarcopenia is a common geriatric disorder characterized by decreased muscle strength, low muscle mass and poor physical performance. This aging-related skeletal muscle deterioration leads to adverse outcomes and severely impairs the quality of life of patients. The accumulation of dysfunctional mitochondria with aging is an important factor in the occurrence and progression of sarcopenia. Mitochondrial quality control (MQC) fundamentally ensures the normal mitochondrial functions and is comprised of four main parts: proteostasis, biogenesis, dynamics and autophagy. Therefore, any pathophysiologic factors compromising the quality control of homeostasis in the skeletal muscle may lead to sarcopenia. However, the specific theoretical aspects of these processes have not been fully elucidated. Current therapeutic interventions using nutritional and pharmaceutical treatments show a modest therapeutic efficacy; however, only physical exercise is recommended as the first-line therapy for sarcopenia, which can ameliorate skeletal muscle deficiency by maintaining the homeostatic MQC. In this review, we summarized the known mechanisms that contribute to the pathogenesis of sarcopenia by impairing normal mitochondrial functions and described potential interventions that mitigate sarcopenia through improving MQC.

Short-term step reduction reduces CS activity without altering skeletal muscle markers of oxidative metabolism or insulin-mediated signalling in young males

Mitochondria are critical to skeletal muscle contractile function and metabolic health. Short-term periods of step reduction (SR) are associated with alterations in muscle protein turnover and mass. However, the effects of SR on mitochondrial metabolism/muscle oxidative metabolism and insulin-mediated signaling are unclear. We tested the hypothesis that the total and/or phosphorylated protein content of key skeletal muscle markers of mitochondrial/oxidative metabolism, and insulin-mediated signaling would be altered over 7 days of SR in young healthy males. Eleven, healthy, recreationally active males (means ± SE, age: 22 ± 1 yr, BMI: 23.4 ± 0.7 kg·m²) underwent a 7-day period of SR. Immediately before and following SR, fasted-state muscle biopsy samples were acquired and analyzed for the assessment of total and phosphorylated protein content of key markers of mitochondrial/oxidative metabolism and insulin-mediated signaling. Daily step count was significantly reduced during the SR intervention (13,054 ± 833 to 1,192 ± 99 steps·day⁻¹, P < 0.001). Following SR, there was a significant decline in maximal citrate synthase activity (fold change: 0.94 ± 0.08, P < 0.05) and a significant increase in the protein content of p-glycogen synthase (P-GS^S641; fold change: 1.47 ± 0.14, P < 0.05). No significant differences were observed in the total or phosphorylated protein content of other key markers of insulin-mediated signaling, oxidative metabolism, mitochondrial function, or mitochondrial dynamics (all P > 0.05). These results suggest that short-term SR reduces the maximal activity of citrate synthase, a marker of mitochondrial content, without altering the total or phosphorylated protein content of key markers of skeletal muscle mitochondrial metabolism and insulin signaling in young healthy males.

NEW & NOTEWORTHY Short-term (7 day) step reduction reduces the activity of citrate synthase without altering the total or phosphorylated protein content of key markers of skeletal muscle mitochondrial metabolism and insulin signaling in young healthy males.

Elephant seal muscle cells adapt to sustained glucocorticoid exposure by shifting their metabolic phenotype

Elephant seals experience natural periods of prolonged food deprivation while breeding, molting, and undergoing postnatal development. Prolonged food deprivation in elephant seals increases circulating glucocorticoids without inducing muscle atrophy, but the cellular mechanisms that allow elephant seals to cope with such conditions remain elusive. We generated a cellular model and conducted transcriptomic, metabolic, and morphological analyses to study how seal cells adapt to sustained glucocorticoid exposure. Seal muscle progenitor cells differentiate into contractile myotubes with a distinctive morphology, gene expression profile, and metabolic phenotype. Exposure to dexamethasone at three ascending concentrations for 48 h modulated the expression of six clusters of genes related to structural constituents of muscle and pathways associated with energy metabolism and cell survival. Knockdown of the glucocorticoid receptor (GR) and downstream expression analyses corroborated that GR mediates the observed effects. Dexamethasone also decreased cellular respiration, shifted the metabolic phenotype toward glycolysis, and induced mitochondrial fission and dissociation of mitochondria-endoplasmic reticulum (ER) interactions without decreasing cell viability. Knockdown of DNA damage-inducible transcript 4 (DDIT4), a GR target involved in the dissociation of mitochondria-ER membranes, recovered respiration and modulated antioxidant gene expression in myotubes treated with dexamethasone. These results show that adaptation to sustained glucocorticoid exposure in elephant seal myotubes involves a metabolic shift toward glycolysis, which is supported by alterations in mitochondrial morphology and a reduction in mitochondria-ER interactions, resulting in decreased respiration without compromising cell survival.

Voluntary exercise prevents abnormal muscle mitochondrial morphology in cancer cachexia mice

This study aimed to examine the effects of voluntary wheel running on cancer cachexia-induced mitochondrial alterations in mouse skeletal muscle. Mice bearing colon 26 adenocarcinoma (C26) were used as a model of cancer cachexia. C26 mice showed a lower gastrocnemius and plantaris muscle weight, but 4 weeks of voluntary exercise rescued these changes. Further, voluntary exercise attenuated observed declines in the levels of oxidative phosphorylation proteins and activities of citrate synthase and cytochrome c oxidase in the skeletal muscle of C26 mice. Among mitochondrial morphology regulatory proteins, mitofusin 2 (Mfn2) and dynamin-related protein 1 (Drp1) were decreased in the skeletal muscle of C26 mice, but exercise resulted in similar improvements as seen in markers of mitochondrial content. In isolated mitochondria, 4-hydroxynonenal and protein carbonyls were elevated in C26 mice, but exercise blunted the increases in these markers of oxidative stress. In addition, electron microscopy revealed that exercise alleviated the observed increase in the percentage of damaged mitochondria in C26 mice. These results suggest that voluntary exercise effectively counteracts mitochondrial dysfunction to mitigate muscle loss in cachexia.

Mitochondrial Dynamics and Mitophagy in Skeletal Muscle Health and Aging

The maintenance of mitochondrial integrity is critical for muscle health. Mitochondria, indeed, play vital roles in a wide range of cellular processes, including energy supply, Ca²⁺ homeostasis, retrograde signaling, cell death, and many others. All mitochondria-containing cells, including skeletal muscle cells, dispose of several pathways to maintain mitochondrial health, including mitochondrial biogenesis, mitochondrial-derived vesicles, mitochondrial dynamics (fusion and fission process shaping mitochondrial morphology), and mitophagy—the process in charge of the removal of mitochondria though autophagy. The loss of skeletal muscle mass (atrophy) is a major health problem worldwide, especially in older people. Currently, there is no treatment to counteract the progressive decline in skeletal muscle mass and strength that occurs with aging, a process termed sarcopenia. There is increasing data, including our own, suggesting that accumulation of dysfunctional mitochondria contributes to the development of sarcopenia. Impairments in mitochondrial dynamics and mitophagy were recently proposed to contribute to sarcopenia. This review summarizes the current state of knowledge on the role played by mitochondrial dynamics and mitophagy in skeletal muscle health and in the development of sarcopenia. We also highlight recent studies showing that enhancing mitophagy in skeletal muscle is a promising therapeutic target to prevent or even treat skeletal muscle dysfunction in the elderly.

Regulation of muscle and mitochondrial health by the mitochondrial fission protein Drp1 in aged mice

KEY POINTS

The maintenance of mitochondrial integrity is critical for skeletal muscle health. Mitochondrial dynamics play key roles in mitochondrial quality control; however, the exact role that mitochondrial fission plays in the muscle ageing process remains unclear. Here we report that both Drp1 knockdown and Drp1 overexpression late in life in mice is detrimental to skeletal muscle function and mitochondrial health. Drp1 knockdown in 18-month-old mice resulted in severe skeletal muscle atrophy, mitochondrial dysfunction, muscle degeneration/regeneration, oxidative stress and impaired autophagy. Overexpressing Drp1 in 18-month-old mice resulted in mild skeletal muscle atrophy and decreased mitochondrial quality. Our data indicate that silencing or overexpressing Drp1 late in life is detrimental to skeletal muscle integrity. We conclude that modulating Drp1 expression is unlikely to be a viable approach to counter the muscle ageing process.

ABSTRACT

Sarcopenia, the ageing-related loss of skeletal muscle mass and function, is a debilitating process negatively impacting the quality of life of afflicted individuals. Although the mechanisms underlying sarcopenia are still only partly understood, impairments in mitochondrial dynamics, and specifically mitochondrial fission, have been proposed as an underlying mechanism. Importantly, conflicting data exist in the field and both excessive and insufficient mitochondrial fission were proposed to contribute to sarcopenia. In Drosophila melanogaster, enhancing mitochondrial fission in midlife through overexpression of dynamin-1-like protein (Drp1) extended lifespan and attenuated several key hallmarks of muscle ageing. Whether a similar outcome of Drp1 overexpression is observed in mammalian muscles remains unknown. In this study, we investigated the impact of knocking down and overexpressing Drp1 protein for 4 months in skeletal muscles of late middle-aged (18 months) mice using intra-muscular injections of adeno-associated viruses expressing shRNA targeting Drp1 or full Drp1 cDNA. We report that knocking down Drp1 expression late in life triggers severe muscle atrophy, mitochondrial dysfunctions, degeneration/regeneration, oxidative stress and impaired autophagy. Drp1 overexpression late in life triggered mild muscle atrophy and decreased mitochondrial quality. Taken altogether, our results indicate that both overexpression and silencing of Drp1 in late middle-aged mice negatively impact skeletal muscle mass and mitochondrial health. These data suggest that Drp1 content must remain within a narrow physiological range to preserve muscle and mitochondrial integrity during ageing. Altering Drp1 expression is therefore unlikely to be a viable target to counter sarcopenia.

TMEM135 is a Novel Regulator of Mitochondrial Dynamics and Physiology with Implications for Human Health Conditions

Transmembrane proteins (TMEMs) are integral proteins that span biological membranes. TMEMs function as cellular membrane gates by modifying their conformation to control the influx and efflux of signals and molecules. TMEMs also reside in and interact with the membranes of various intracellular organelles. Despite much knowledge about the biological importance of TMEMs, their role in metabolic regulation is poorly understood. This review highlights the role of a single TMEM, transmembrane protein 135 (TMEM135). TMEM135 is thought to regulate the balance between mitochondrial fusion and fission and plays a role in regulating lipid droplet formation/tethering, fatty acid metabolism, and peroxisomal function. This review highlights our current understanding of the various roles of TMEM135 in cellular processes, organelle function, calcium dynamics, and metabolism.

Loss of Fis1 impairs proteostasis during skeletal muscle aging in Drosophila

Increased levels of dysfunctional mitochondria within skeletal muscle are correlated with numerous age‐related physiopathological conditions. Improving our understanding of the links between mitochondrial function and muscle proteostasis, and the role played by individual genes and regulatory networks, is essential to develop treatments for these conditions. One potential player is the mitochondrial outer membrane protein Fis1, a crucial fission factor heavily involved in mitochondrial dynamics in yeast but with an unknown role in higher‐order organisms. By using Drosophila melanogaster as a model, we explored the effect of Fis1 mutations generated by transposon Minos‐mediated integration. Mutants exhibited a higher ratio of damaged mitochondria with age as well as elevated reactive oxygen species levels compared with controls. This caused an increase in oxidative stress, resulting in large accumulations of ubiquitinated proteins, accelerated muscle function decline, and mitochondrial myopathies in young mutant flies. Ectopic expression of Fis1 isoforms was sufficient to suppress this phenotype. Loss of Fis1 led to unbalanced mitochondrial proteostasis within fly muscle, decreasing both flight capabilities and lifespan. Fis1 thus clearly plays a role in fly mitochondrial dynamics. Further investigations into the detailed function of Fis1 are necessary for exploring how mitochondrial function correlates with muscle health during aging.

Manifestations of Age on Autophagy, Mitophagy and Lysosomes in Skeletal Muscle

Sarcopenia is the loss of both muscle mass and function with age. Although the molecular underpinnings of sarcopenia are not fully understood, numerous pathways are implicated, including autophagy, in which defective cargo is selectively identified and degraded at the lysosome. The specific tagging and degradation of mitochondria is termed mitophagy, a process important for the maintenance of an organelle pool that functions efficiently in energy production and with relatively low reactive oxygen species production. Emerging data, yet insufficient, have implicated various steps in this pathway as potential contributors to the aging muscle atrophy phenotype. Included in this is the lysosome, the end-stage organelle possessing a host of proteolytic and degradative enzymes, and a function devoted to the hydrolysis and breakdown of defective molecular complexes and organelles. This review provides a summary of our current understanding of how the autophagy-lysosome system is regulated in aging muscle, highlighting specific areas where knowledge gaps exist. Characterization of the autophagy pathway with a particular focus on the lysosome will undoubtedly pave the way for the development of novel therapeutic strategies to combat age-related muscle loss.

Pharmacological inhibition of dynamin-related protein 1 attenuates skeletal muscle insulin resistance in obesity

Dynamin-related protein-1 (Drp1) is a key regulator in mitochondrial fission. Excessive Drp1-mediated mitochondrial fission in skeletal muscle under the obese condition is associated with impaired insulin action. However, it remains unknown whether pharmacological inhibition of Drp1, using the Drp1-specific inhibitor Mitochondrial Division Inhibitor 1 (Mdivi-1), is effective in alleviating skeletal muscle insulin resistance and improving whole-body metabolic health under the obese and insulin-resistant condition. We subjected C57BL/6J mice to a high-fat diet (HFD) or low-fat diet (LFD) for 5-weeks. HFD-fed mice received Mdivi-1 or saline injections for the last week of the diet intervention. Additionally, myotubes derived from obese insulin-resistant humans were treated with Mdivi-1 or saline for 12 h. We measured glucose area under the curve (AUC) from a glucose tolerance test (GTT), skeletal muscle insulin action, mitochondrial dynamics, respiration, and H2 O2 content. We found that Mdivi-1 attenuated impairments in skeletal muscle insulin signaling and blood glucose AUC from a GTT induced by HFD feeding (p < 0.05). H2 O2 content was elevated in skeletal muscle from the HFD group (vs. LFD, p < 0.05), but was reduced with Mdivi-1 treatment, which may partially explain the improvement in skeletal muscle insulin action. Similarly, Mdivi-1 enhanced the mitochondrial network structure, reduced reactive oxygen species, and improved insulin action in myotubes from obese humans (vs. saline, p < 0.05). In conclusion, inhibiting Drp1 with short-term Mdivi-1 administration attenuates the impairment in skeletal muscle insulin signaling and improves whole-body glucose tolerance in the setting of obesity-induced insulin resistance. Targeting Drp1 may be a viable approach to treat obesity-induced insulin resistance.