Bioactive Glass and Silica Particles for Skeletal and Cardiac Muscle Tissue Regeneration

When skeletal and cardiac tissues are damaged, surgical approaches are not always successful and tissue regeneration approaches are investigated. Reports in the literature indicate that silica nanoparticles and bioactive glasses (BGs), including silicate bioactive glasses (e.g., 45S5 BG), phosphate glass fibers, boron-doped mesoporous BGs, borosilicate glasses, and aluminoborates, are promising for repairing skeletal muscle tissue. Silica nanoparticles and BGs have been combined with polymers to obtain aligned nanofibers and to maintain controlled delivery of nanoparticles for skeletal muscle repair. The literature indicates that cardiac muscle regeneration can be also triggered by the ionic products of BGs. This was observed to be due to the release of vascular endothelial growth factor and other growth factors from cardiomyocytes, which regulate endothelial cells to form capillary structures (angiogenesis). Specific studies, including both in vitro and in vivo approaches, are reviewed in this article. The analysis of the literature indicates that although the research field is still very limited, BGs are showing great promise for muscle tissue engineering and further research in the field should be carried out to expand our basic knowledge on the application of BGs in muscle (skeletal and cardiac) tissue regeneration. Impact statement This review highlights the potential of silica particles and bioactive glasses (BGs) for skeletal and cardiac tissue regeneration. These biomaterials create scaffolds triggering muscle cell differentiation. Ionic products from BGs stimulate growth factors, supporting angiogenesis in cardiac tissue repair. Further research is required to expand our know-how on silica particles and BGs in muscle tissue engineering.

Integrating analysis of mRNA expression profiles indicates Sgk1 as a key mediator in muscle-brain crosstalk during resistance exercise

Abundant evidence has shown the protective effect of aerobic exercise on central neuronal system, however, research about resistance exercise remains limited. To evaluate the effect and potential molecular mechanisms of resistance exercise in improving cognition and mental health, three-month-old male C57BL/6J mice underwent resistance training for five weeks. Body parameters, cognitive performance and synaptic plasticity were then assessed. In both groups, total RNA from the frontal cortex, hippocampus and gastrocnemius was isolated and sequenced, GO term and KEGG analysis were performed to identify molecular mechanisms. The results from RNA sequencing were then verified by RT-PCR. Our data found that mice in training group showed reduced anxiety-like behavior and better spatial memory. Accordingly, resistance exercise specifically increased the number of thin spines without affecting the number of other kind of spines. mRNA sequence analysis showed that resistance exercise induced differential expression of hundreds of genes in the above three tissues. KEGG analysis indicated the FoxO signaling pathway the most significant changed pathway throughout the brain and muscle. GO terms analysis showed that Sgk1 was enriched in the three key cognition related BP, including long-term memory, learning or memory and memory, and the expression level of Sgk1 was positive related with cognitive performance in the water maze. In conclusion, resistance exercise improved the mental health, cognition and synaptic plasticity of mice. Integrating analysis of mRNA expression profiles in frontal cortex, hippocampus and muscle reveals Sgk1 as the key mediator in brain-muscle crosstalk.

Drosophila neuronal Glucose-6-Phosphatase is a modulator of neuropeptide release that regulates muscle glycogen stores via FMRFamide signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a noncanonical function of the Drosophila Glucose-6-Phosphatase (G6P) gene in a subset of neurosecretory cells in the central nervous system that governs systemic glucose homeostasis in food-deprived flies. Here, we show that G6P-expressing neurons define six groups of NP-secreting cells, four in the brain and two in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide NPs (FMRFaG6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFaG6P neurons for attaining a prominent Golgi apparatus and secreting NPs efficiently. Finally, we establish that G6P-dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of NPs and enhancing signaling to respective target tissues expressing cognate receptors.

The "Sunshine Vitamin" and Its Antioxidant Benefits for Enhancing Muscle Function

Pathological states marked by oxidative stress and systemic inflammation frequently compromise the functional capacity of muscular cells. This progressive decline in muscle mass and tone can significantly hamper the patient's motor abilities, impeding even the most basic physical tasks. Muscle dysfunction can lead to metabolic disorders and severe muscle wasting, which, in turn, can potentially progress to sarcopenia. The functionality of skeletal muscle is profoundly influenced by factors such as environmental, nutritional, physical, and genetic components. A well-balanced diet, rich in proteins and vitamins, alongside an active lifestyle, plays a crucial role in fortifying tissues and mitigating general weakness and pathological conditions. Vitamin D, exerting antioxidant effects, is essential for skeletal muscle. Epidemiological evidence underscores a global prevalence of vitamin D deficiency, which induces oxidative harm, mitochondrial dysfunction, reduced adenosine triphosphate production, and impaired muscle function. This review explores the intricate molecular mechanisms through which vitamin D modulates oxidative stress and its consequent effects on muscle function. The aim is to evaluate if vitamin D supplementation in conditions involving oxidative stress and inflammation could prevent decline and promote or maintain muscle function effectively.

Current Aspects of Selected Factors to Modulate Brain Health and Sports Performance in Athletes

This review offers a comprehensive evaluation of current aspects related to nutritional strategies, brain modulation, and muscle recovery, focusing on their applications and the underlying mechanisms of physiological adaptation for promoting a healthy brain, not only in athletes but also for recreationally active and inactive individuals. We propose that applying the rule, among others, of good sleep, regular exercise, and a properly balanced diet, defined as "SPARKS", will have a beneficial effect on the function and regeneration processes of the gut-brain-muscle axis. However, adopting the formula, among others, of poor sleep, stress, overtraining, and dysbiosis, defined as "SMOULDER", will have a detrimental impact on the function of this axis and consequently on human health as well as on athletes. Understanding these dynamics is crucial for optimizing brain health and cognitive function. This review highlights the significance of these factors for overall well-being, suggesting that adopting the "SPARKS" approach may benefit not only athletes but also older adults and individuals with health conditions.

Muscle-bone cross-talk through the FNIP1-TFEB-IGF2 axis is associated with bone metabolism in human and mouse

Clinical evidence indicates a close association between muscle dysfunction and bone loss; however, the underlying mechanisms remain unclear. Here, we report that muscle dysfunction-related bone loss in humans with limb-girdle muscular dystrophy is associated with decreased expression of folliculin-interacting protein 1 (FNIP1) in muscle tissue. Supporting this finding, murine gain- and loss-of-function genetic models demonstrated that muscle-specific ablation of FNIP1 caused decreased bone mass, increased osteoclastic activity, and mechanical impairment that could be rescued by myofiber-specific expression of FNIP1. Myofiber-specific FNIP1 deficiency stimulated expression of nuclear translocation of transcription factor EB, thereby activating transcription of insulin-like growth factor 2 (Igf2) at a conserved promoter-binding site and subsequent IGF2 secretion. Muscle-derived IGF2 stimulated osteoclastogenesis through IGF2 receptor signaling. AAV9-mediated overexpression of IGF2 was sufficient to decrease bone volume and impair bone mechanical properties in mice. Further, we found that serum IGF2 concentration was negatively correlated with bone health in humans in the context of osteoporosis. Our findings elucidate a muscle-bone cross-talk mechanism bridging the gap between muscle dysfunction and bone loss. This cross-talk represents a potential target to treat musculoskeletal diseases and osteoporosis.

Drosophila Neuronal Glucose 6 Phosphatase is a Modulator of Neuropeptide Release that Regulates Muscle Glycogen Stores via FMRFamide Signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a non-canonical function of the Drosophila Glucose-6-Phosphatase ( G6P ) gene in a subset of neurosecretory cells in the CNS that governs systemic glucose homeostasis in food deprived flies. Here, we show that G6P expressing neurons define 6 groups of neuropeptide secreting cells, 4 in the brain and 2 in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide neuropeptides ( FMRFa G6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFa G6P neurons for attaining a prominent Golgi apparatus and secreting neuropeptides efficiently. Finally, we establish that G6P dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of neuropeptides and enhancing signaling to respective target tissues expressing cognate receptors.

SIGNIFICANCE STATEMENT

Glucose-6-phosphtase (G6P) is a critical enzyme in sugar synthesis and catalyzes the final step in glucose production. In Drosophila - and insects in general - where trehalose is the circulating sugar and Trehalose phosphate synthase, and not G6P, is used for sugar production, G6P has adopted a novel and unique role in peptidergic neurons in the CNS. Interestingly, flies lacking G6P show diminished Neuropeptide secretions and have a smaller Golgi apparatus in peptidergic neurons. It is hypothesized that the role of G6P is to counteract glycolysis, thereby creating a cellular environment that is more amenable to efficient neuropeptide secretion.

The ubiquitin-conjugating enzyme UBE2D/eff maintains a youthful proteome and ensures protein quality control during aging

Ubiquitin-conjugating enzymes (E2s) are key for regulating protein function and turnover via ubiquitination but it remains undetermined which E2s maintain proteostasis during aging. Here, we find that E2s have diverse roles in handling a model aggregation-prone protein (huntingtin-polyQ) in the Drosophila retina: while some E2s mediate aggregate assembly, UBE2D/effete (eff) and other E2s are required for huntingtin-polyQ degradation. UBE2D/eff is key for proteostasis also in skeletal muscle: eff protein levels decline with aging, and muscle-specific eff knockdown causes an accelerated buildup in insoluble poly-ubiquitinated proteins (which progressively accumulate with aging) and shortens lifespan. Transgenic expression of human UBE2D2, homologous to eff, partially rescues the lifespan and proteostasis deficits caused by muscle-specific effRNAi by re-establishing the physiological levels of effRNAi-regulated proteins, which include several regulators of proteostasis. Interestingly, UBE2D/eff knockdown in young age reproduces part of the proteomic changes that normally occur in old muscles, suggesting that the decrease in UBE2D/eff protein levels that occurs with aging contributes to reshaping the composition of the muscle proteome. Altogether, these findings indicate that UBE2D/eff is a key E2 ubiquitin-conjugating enzyme that ensures protein quality control and helps maintain a youthful proteome composition during aging.

Skeletal muscle TFEB signaling promotes central nervous system function and reduces neuroinflammation during aging and neurodegenerative disease

Skeletal muscle has recently arisen as a regulator of central nervous system (CNS) function and aging, secreting bioactive molecules known as myokines with metabolism-modifying functions in targeted tissues, including the CNS. Here, we report the generation of a transgenic mouse with enhanced skeletal muscle lysosomal and mitochondrial function via targeted overexpression of transcription factor E-B (TFEB). We discovered that the resulting geroprotective effects in skeletal muscle reduce neuroinflammation and the accumulation of tau-associated pathological hallmarks in a mouse model of tauopathy. Muscle-specific TFEB overexpression significantly ameliorates proteotoxicity, reduces neuroinflammation, and promotes transcriptional remodeling of the aged CNS, preserving cognition and memory in aged mice. Our results implicate the maintenance of skeletal muscle function throughout aging in direct regulation of CNS health and disease and suggest that skeletal muscle originating factors may act as therapeutic targets against age-associated neurodegenerative disorders.

Age Is Just a Number: Progress and Obstacles in the Discovery of New Candidate Drugs for Sarcopenia

Sarcopenia is a disease characterized by the progressive loss of skeletal muscle mass and function that occurs with aging. The progression of sarcopenia is correlated with the onset of physical disability, the inability to live independently, and increased mortality. Due to global increases in lifespan and demographic aging in developed countries, sarcopenia has become a major socioeconomic burden. Clinical therapies for sarcopenia are based on physical therapy and nutritional support, although these may suffer from low adherence and variable outcomes. There are currently no clinically approved drugs for sarcopenia. Consequently, there is a large amount of pre-clinical research focusing on discovering new candidate drugs and novel targets. In this review, recent progress in this research will be discussed, along with the challenges that may preclude successful translational research in the clinic. The types of drugs examined include mitochondria-targeting compounds, anti-diabetes agents, small molecules that target non-coding RNAs, protein therapeutics, natural products, and repositioning candidates. In light of the large number of drugs and targets being reported, it can be envisioned that clinically approved pharmaceuticals to prevent the progression or even mitigate sarcopenia may be within reach.

Muscle-bone crosstalk via endocrine signals and potential targets for osteosarcopenia-related fracture

Background

Osteosarcopenia is a syndrome coexisting sarcopenia and osteopenia/osteoporosis, with a high fracture risk. Recently, skeletal muscle and bone have been recognized as endocrine organs capable of communication through secreting myokines and osteokines, respectively. With a deeper understanding of the muscle-bone crosstalk, these endocrine signals exhibit an important role in osteosarcopenia development and fracture healing.

Methods

This review summarizes the role of myokines and osteokines in the development and treatment of osteosarcopenia and fracture, and discusses their potential for osteosarcopenia-related fracture treatment.

Results

Several well-defined myokines (myostatin and irisin) and osteokines (RANKL and SOST) are found to not only regulate skeletal muscle and bone metabolism but also influence fracture healing processes. Systemic interventions targeting these biochemical signals has shown promising results in improving the mass and functions of skeletal muscle and bone, as well as accelerating fracture healing processes.

Conclusion

The regulation of muscle-bone crosstalk via biochemical signals presents a novel and promising strategy for treating osteosarcopenia and fracture by simultaneously enhancing bone and muscle anabolism. We propose that myostatin, irisin, RANKL, and SOST may serve as potential targets to treat fracture patients with osteosarcopenia.

The translational potential of this article

Osteosarcopenia is an emerging geriatric syndrome where sarcopenia and osteoporosis coexist, with high fracture risk, delayed fracture healing, and increased mortality. However, no pharmacological agent is available to treat fracture patients with osteosarcopenia. This review summarizes the role of several myokines and osteokines in the development and treatment of osteosacropenia and fracture, as well as discusses their potential as intervention targets for osteosarcopenia-related fracture, which provides a novel and promising strategy for future osteosarcopenia-related fracture treatment.

Nutritional regulation of skeletal muscle energy metabolism, lipid accumulation and meat quality in pigs

The quality of pork determines consumers' purchase intention, which directly affects the economic value of pork. Minimizing the proportion of inferior pork and producing high quality pork are the ultimate goals of the pig industry. Muscle energy metabolism, serving as a regulative hub in organism energy expenditure and storage as a fat deposit, is compatible with myofiber type composition, affecting meat color, intramuscular fat content, tenderness, pH values and drip loss. Increasing data illustrate that dietary nutrients and bioactive ingredients affect muscle energy metabolism, white adipose browning and fat distribution, and myofiber type composition in humans, and rodents. Recently, some studies have shown that modulating muscle energy metabolism and lipid accumulation through nutritional approaches could effectively improve meat quality. This article reviews the progress and development in this field, and specifically discusses the impacts of dietary supply of amino acids, lipids, and gut microbiota as well as maternal nutrition on skeletal muscle energy metabolism, lipid accumulation and meat quality of pigs, so as to provide comprehensive overview with respect to effective avenues for improving meat quality.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

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.

Potential of Food Protein-Derived Bioactive Peptides against Sarcopenia: A Comprehensive Review

Sarcopenia is an age-related progressive muscle disorder characterized by accelerated loss of muscle mass, strength, and function, which are important causes of physiological dysfunctions in the elderly. At present, the main alleviating method includes protein supplements to stimulate synthesis of muscle proteins. Food protein-derived peptides containing abundant branched-chain amino acids have a remarkable effect on the improvement of sarcopenia. Understanding the underlying molecular mechanism and clarifying the structure-activity relationship is essential for the mitigation of sarcopenia. This present review recaps the epidemiology, pathogenesis, diagnosis, and treatment of sarcopenia, which facilitates a comprehensive understanding of sarcopenia. Moreover, the latest research progress on food-derived antisarcopenic peptides is reviewed, including their antisarcopenic activity, molecular mechanism as well as structural characteristics. Food-derived bioactive peptides can indeed alleviate/mitigate sarcopenia. These antisarcopenic peptides play a pivotal role mainly by activating the PI3K/Akt/mTOR and MAPK pathways and inhibiting the ubiquitin-proteasome system and AMPK pathway, thus promoting the synthesis of muscle proteins and inhibiting their degradation. Antisarcopenic peptides alleviate sarcopenia via specific peptides, which may be absorbed into the circulation and exhibit their bioactivity in intact forms. The present review provides a theoretical reference for mitigation and prevention of sarcopenia by food protein-derived bioactive peptides.

Modulation of protease expression by the transcription factor Ptx1/PITX regulates protein quality control during aging

Protein quality control is important for healthy aging and is dysregulated in age-related diseases. The autophagy-lysosome and ubiquitin-proteasome are key for proteostasis, but it remains largely unknown whether other proteolytic systems also contribute to maintain proteostasis during aging. Here, we find that expression of proteolytic enzymes (proteases/peptidases) distinct from the autophagy-lysosome and ubiquitin-proteasome systems declines during skeletal muscle aging in Drosophila. Age-dependent protease downregulation undermines proteostasis, as demonstrated by the increase in detergent-insoluble poly-ubiquitinated proteins and pathogenic huntingtin-polyQ levels in response to protease knockdown. Computational analyses identify the transcription factor Ptx1 (homologous to human PITX1/2/3) as a regulator of protease expression. Consistent with this model, Ptx1 protein levels increase with aging, and Ptx1 RNAi counteracts the age-associated downregulation of protease expression. Moreover, Ptx1 RNAi improves muscle protein quality control in a protease-dependent manner and extends lifespan. These findings indicate that proteases and their transcriptional modulator Ptx1 ensure proteostasis during aging.

Sarcopenia is an independent risk factor for all-cause mortality rate in patients with diabetic foot ulcers

Objective

This study aimed to determine whether sarcopenia affects the all-cause mortality rate of patients with diabetic foot ulcers (DFUs).

Research design and methods

The clinic-based observational study included 217 patients treated at the Department of Endocrinology, the First Affiliated Hospital of Chongqing Medical University during a 4-year period. All subjects underwent dual-energy X-ray absorptiometry to determine their body composition during hospitalization. Diagnosis of sarcopenia was based on the Baumgartner diagnostic criteria. Patients were followed up regularly by phone calls until April 1, 2019, and their survival status was recorded.Univariate and multivariate Cox risk ratio regression models were used to analyze factors influencing the all-cause mortality rate of patients with DFUs.

Results

Of the 217 patients, 158 people survived (82.7%), 33 died (17.3%), and 26 were lost to follow-up. The median follow-up time was 23 (Range 11-34) months. The majority of patients were male (68.6%), with a mean age of 67.29 ± 11.14 years. The 5-year survival rate was 68.3% and 45.9% for all study patients (n = 217) and sarcopenia patients (n = 81), respectively. Multivariate Cox risk regression model showed that age (HR 1.042[95%CI:1.006, 1.078], P = 0.021), sarcopenia (HR 5.051[95%CI:1.968, 12.961], P = 0.001), and serum creatinine (HR 1.007[95%CI: 1.003, 1.010], P < 0.001) were independent risk factors for all-cause mortality rate of patients with DFUs. Kaplan-Meier survival curve indicated that the survival rate of patients with sarcopenia was significantly lower than non-sarcopenia patients (P < 0.001).

Conclusions

Sarcopenia is an independent risk factor for all-cause mortality of patients with DFUs and hence an important prognostic factor for patients with DFUs. Active prevention and improvement of sarcopenia can potentially improve the survival outcomes of this patient population.

The myokine Fibcd1 is an endogenous determinant of myofiber size and mitigates cancer-induced myofiber atrophy

Decline in skeletal muscle cell size (myofiber atrophy) is a key feature of cancer-induced wasting (cachexia). In particular, atrophy of the diaphragm, the major muscle responsible for breathing, is an important determinant of cancer-associated mortality. However, therapeutic options are limited. Here, we have used Drosophila transgenic screening to identify muscle-secreted factors (myokines) that act as paracrine regulators of myofiber growth. Subsequent testing in mouse myotubes revealed that mouse Fibcd1 is an evolutionary-conserved myokine that preserves myofiber size via ERK signaling. Local administration of recombinant Fibcd1 (rFibcd1) ameliorates cachexia-induced myofiber atrophy in the diaphragm of mice bearing patient-derived melanoma xenografts and LLC carcinomas. Moreover, rFibcd1 impedes cachexia-associated transcriptional changes in the diaphragm. Fibcd1-induced signaling appears to be muscle selective because rFibcd1 increases ERK activity in myotubes but not in several cancer cell lines tested. We propose that rFibcd1 may help reinstate myofiber size in the diaphragm of patients with cancer cachexia.

Long noncoding RNA ZFP36L2-AS functions as a metabolic modulator to regulate muscle development

Skeletal muscle is the largest metabolic organ in the body, and its metabolic flexibility is essential for maintaining systemic energy homeostasis. Metabolic inflexibility in muscles is a dominant cause of various metabolic disorders, impeding muscle development. In our previous study, we found lncRNA ZFP36L2-AS (for “ZFP36L2-antisense transcript”) is specifically enriched in skeletal muscle. Here, we report that ZFP36L2-AS is upregulated during myogenic differentiation, and highly expressed in breast and leg muscle. In vitro, ZFP36L2-AS inhibits myoblast proliferation but promotes myoblast differentiation. In vivo, ZFP36L2-AS facilitates intramuscular fat deposition, as well as activates fast-twitch muscle phenotype and induces muscle atrophy. Mechanistically, ZFP36L2-AS interacts with acetyl-CoA carboxylase alpha (ACACA) and pyruvate carboxylase (PC) to induce ACACA dephosphorylation and damaged PC protein stability, thus modulating muscle metabolism. Meanwhile, ZFP36L2-AS can activate ACACA to reduce acetyl-CoA content, which enhances the inhibition of PC activity. Our findings present a novel model about the regulation of lncRNA on muscle metabolism.

LncEDCH1 improves mitochondrial function to reduce muscle atrophy by interacting with SERCA2

Skeletal muscle is a regulator of the body's energy expenditure and metabolism. Abnormal regulation of skeletal muscle-specific genes leads to various muscle diseases. Long non-coding RNAs (lncRNAs) have been demonstrated to play important roles in muscle growth and muscle atrophy. To explore the potential function of muscle-associated lncRNA, we analyzed our previous RNA-sequencing data and selected the lncRNA (LncEDCH1) as the research object. In this study, we report that LncEDCH1 is specifically enriched in skeletal muscle, and its transcriptional activity is positively regulated by transcription factor SP1. LncEDCH1 regulates myoblast proliferation and differentiation in vitro. In vivo, LncEDCH1 reduces intramuscular fat deposition, activates slow-twitch muscle phenotype, and inhibits muscle atrophy. Mechanistically, LncEDCH1 binds to sarcoplasmic/ER calcium ATPase 2 (SERCA2) protein to enhance SERCA2 protein stability and increase SERCA2 activity. Meanwhile, LncEDCH1 improves mitochondrial efficiency possibly through a SERCA2-mediated activation of the AMPK pathway. Our findings provide a strategy for using LncEDCH1 as an effective regulator for the treatment of muscle atrophy and energy metabolism.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

Conserved and convergent mechanisms underlying performance-life-history trade-offs

Phenotypic trade-offs are inevitable in nature, but the mechanisms driving them are poorly understood. Movement and oxygen are essential to all animals, and as such, the common ancestor to all living animals passed on mechanisms to acquire oxygen and contract muscle, sometimes at the expense of other activities or expression of traits. Nevertheless, convergent pathways have also evolved to deal with critical trade-offs that are necessary to survive ubiquitous environmental challenges. We discuss how whole-animal performance traits, such as locomotion, are important to fitness, yet costly, resulting in trade-offs with other aspects of the phenotype via specific conserved and convergent mechanistic pathways across all animals. Specifically, we discuss conserved pathways involved in muscle structure and signaling, insulin/insulin-like signaling, sirtuins, mitochondria and hypoxia-inducible factors, as well as convergent pathways involved in energy regulation, development, reproductive investment and energy storage. The details of these mechanisms are only known from a few model systems, and more comparative studies are needed. We make two main recommendations as a framework for future studies of animal form and function. First, studies of performance should consider the broader life-history context of the organism, and vice versa, as performance expression can require a large portion of acquired resources. Second, studies of life histories or mechanistic pathways that measure performance should do so in meaningful and standardized ways. Understanding proximate mechanisms of phenotypic trade-offs will not only better explain the phenotypes of the organisms we study, but also allow predictions about phenotypic variation at the evolutionary scale.

Muscle-to-Brain Signaling Via Myokines and Myometabolites

Skeletal muscle health and function are important determinants of systemic metabolic homeostasis and organism-wide responses, including disease outcome. While it is well known that exercise protects the central nervous system (CNS) from aging and disease, only recently this has been found to depend on the endocrine capacity of skeletal muscle. Here, we review muscle-secreted growth factors and cytokines (myokines), metabolites (myometabolites), and other unconventional signals (e.g. bioactive lipid species, enzymes, and exosomes) that mediate muscle-brain and muscle-retina communication and neuroprotection in response to exercise and associated processes, such as the muscle unfolded protein response and metabolic stress. In addition to impacting proteostasis, neurogenesis, and cognitive functions, muscle-brain signaling influences complex brain-dependent behaviors, such as depression, sleeping patterns, and biosynthesis of neurotransmitters. Moreover, myokine signaling adapts feeding behavior to meet the energy demands of skeletal muscle. Contrary to protective myokines induced by exercise and associated signaling pathways, inactivity and muscle wasting may derange myokine expression and secretion and in turn compromise CNS function. We propose that tailoring muscle-to-CNS signaling by modulating myokines and myometabolites may combat age-related neurodegeneration and brain diseases that are influenced by systemic signals.

Defend or reproduce? Muscle-derived glutamate determines an immune-reproductive energetic tradeoff

There are dramatic disparities in infection susceptibility within populations. In this issue of Cell Metabolism, Zhao and Karpac uncover a muscle-adipose-gut axis in Drosophila that explains variability in pathogen susceptibility. They show that the degree of intramuscular NF-κB activation accounts for differences in circulating glutamate, which enhances infection resistance at the expense of reproduction.

Cholinergic signaling at the body wall neuromuscular junction distally inhibits feeding behavior in Caenorhabditis elegans

Complex biological functions within organisms are frequently orchestrated by systemic communication between tissues. In the model organism Caenorhabditis elegans, the pharyngeal and body wall neuromuscular junctions are two discrete structures that control feeding and locomotion, respectively. Separate, the well-defined neuromuscular circuits control these distinct tissues. Nonetheless, the emergent behaviors, feeding and locomotion, are coordinated to guarantee the efficiency of food intake. Here, we show that pharmacological hyperactivation of cholinergic transmission at the body wall muscle reduces the rate of pumping behavior. This was evidenced by a systematic screening of the effect of the cholinesterase inhibitor aldicarb on the rate of pharyngeal pumping on food in mutant worms. The screening revealed that the key determinants of the inhibitory effect of aldicarb on pharyngeal pumping are located at the body wall neuromuscular junction. In fact, the selective stimulation of the body wall muscle receptors with the agonist levamisole inhibited pumping in a lev-1-dependent fashion. Interestingly, this response was independent of unc-38, an alpha subunit of the nicotinic receptor classically expressed with lev-1 at the body wall muscle. This implies an uncharacterized lev-1-containing receptor underpins this effect. Overall, our results reveal that body wall cholinergic transmission not only controls locomotion but simultaneously inhibits feeding behavior.

Sarcopenia is associated with decreased gray matter volume in the parietal lobe: a longitudinal cohort study

Background

Substantial evidence supports an association between physical activity and cognitive function. However, the role of muscle mass and function in brain structural changes is not well known. This study investigated whether sarcopenia, defined as low muscle mass and strength, accelerates brain volume atrophy.

Methods

A total of 1284 participants with sarcopenic measurements and baseline and 4-year follow-up brain magnetic resonance images were recruited from the Korean Genome and Epidemiology Study. Muscle mass was represented as appendicular skeletal muscle mass divided by the body mass index. Muscle function was measured by handgrip strength. The low mass and strength groups were defined as being in the lowest quintile of each variable for one’s sex. Sarcopenia was defined as being in the lowest quintile for both muscle mass and handgrip strength.

Results

Of the 1284 participants, 12·6%, 10·8%, and 5·4% were classified as the low mass, low strength, and sarcopenia groups, respectively. The adjusted mean changes of gray matter (GM) volume during 4-year follow-up period were − 9·6 mL in the control group, whereas − 11·6 mL in the other three groups (P < 0·001). The significantly greater atrophy in parietal GM was observed in the sarcopenia group compared with the control group. In a joint regression model, low muscle mass, but not muscle strength, was an independent factor associated with a decrease of GM volume.

Conclusions

Sarcopenia is associated with parietal GM volume atrophy, in a middle-aged population. Maintaining good levels of muscle mass could be important for brain health in later adulthood.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12877-021-02581-4.

Exercise mimetics: harnessing the therapeutic effects of physical activity

Exercise mimetics are a proposed class of therapeutics that specifically mimic or enhance the therapeutic effects of exercise. Increased physical activity has demonstrated positive effects in preventing and ameliorating a wide range of diseases, including brain disorders such as Alzheimer disease and dementia, cancer, diabetes and cardiovascular disease. This article discusses the molecular mechanisms and signalling pathways associated with the beneficial effects of physical activity, focusing on effects on brain function and cognitive enhancement. Emerging therapeutic targets and strategies for the development of exercise mimetics, particularly in the field of central nervous system disorders, as well as the associated opportunities and challenges, are discussed.

Age-Related Increase in Lactate Dehydrogenase Activity in Skeletal Muscle Reduces Lifespan in Drosophila

Metabolic adaptations occur with aging but the significance and causal roles of such changes are only partially known. In Drosophila, we find that skeletal muscle aging is paradoxically characterized by increased readouts of glycolysis (lactate, NADH/NAD+) but reduced expression of most glycolytic enzymes. This conundrum is explained by lactate dehydrogenase (LDH), an enzyme necessary for anaerobic glycolysis and whose expression increases with aging. Experimental Ldh overexpression in skeletal muscle of young flies increases glycolysis and shortens lifespan, suggesting that age-related increases in muscle LDH contribute to mortality. Similar results are also found with overexpression of other glycolytic enzymes (Pfrx/PFKFB, Pgi/GPI). Conversely, hypomorphic mutations in Ldh extend lifespan whereas reduction in PFK, Pglym78/PGAM, Pgi/GPI, and Ald/ALDO levels shorten lifespan to various degrees, indicating that glycolysis needs to be tightly controlled for optimal aging. Altogether, these findings indicate a role for muscle LDH and glycolysis in aging.

Systemic manifestation and contribution of peripheral tissues to Huntington's disease pathogenesis

Huntington disease (HD) is an autosomal dominant neurodegenerative disease that is caused by expansion of cytosine/adenosine/guanine repeats in the huntingtin (HTT) gene, which leads to a toxic, aggregation-prone, mutant HTT-polyQ protein. Beyond the well-established mechanisms of HD progression in the central nervous system, growing evidence indicates that also peripheral tissues are affected in HD and that systemic signaling originating from peripheral tissues can influence the progression of HD in the brain. Herein, we review the systemic manifestation of HD in peripheral tissues, and the impact of systemic signaling on HD pathogenesis. Mutant HTT induces a body wasting syndrome (cachexia) primarily via its activity in skeletal muscle, bone, adipose tissue, and heart. Additional whole-organism effects induced by mutant HTT include decline in systemic metabolic homeostasis, which stems from derangement of pancreas, liver, gut, hypothalamic-pituitary-adrenal axis, and circadian functions. In addition to spreading via the bloodstream and a leaky blood brain barrier, HTT-polyQ may travel long distance via its uptake by neurons and its axonal transport from the peripheral to the central nervous system. Lastly, signaling factors that are produced and/or secreted in response to therapeutic interventions such as exercise or in response to mutant HTT activity in peripheral tissues may impact HD. In summary, these studies indicate that HD is a systemic disease that is influenced by intertissue signaling and by the action of pathogenic HTT in peripheral tissues. We propose that treatment strategies for HD should include the amelioration of HD symptoms in peripheral tissues. Moreover, harnessing signaling between peripheral tissues and the brain may provide a means for reducing HD progression in the central nervous system.

Mini review: Biomaterials in repair and regeneration of nerve in a volumetric muscle loss

Volumetric muscle loss (VML) following a severe trauma or injury is beyond the intrinsic regenerative capacity of muscle tissues, and hence interventional therapy is required. Extensive muscle loss concomitant with damage to neuromuscular components overwhelms the muscles' remarkable regenerative capacity. The loss of nervous and vascular tissue leads to further damage and atrophy, so a combined treatment for neuromuscular junction (NMJ) along with the volumetric muscle regeneration is important. There have been immense advances in the field of tissue engineering for skeletal muscle tissue and peripheral nerve regeneration, but very few address the interdependence of the tissues and the need for combined therapies to repair and regenerate fully functional muscle tissue. This review addresses the problem and presents an overview of the biomaterials that have been studied for tissue engineering of neuromuscular tissues associated with skeletal muscles.

Skeletal muscle-specific calpastatin overexpression mitigates muscle weakness in aging and extends life span

Calpain activation has been postulated as a potential contributor to the loss of muscle mass and function associated with both aging and disease, but limitations of previous experimental approaches have failed to completely examine this issue. We hypothesized that mice overexpressing calpastatin (CalpOX), an endogenous inhibitor of calpain, solely in skeletal muscle would show an amelioration of the aging muscle phenotype. We assessed four groups of mice (age in months): 1) young wild type (WT; 5.71 ± 0.43), 2) young CalpOX (5.6 ± 0.5), 3) old WT (25.81 ± 0.56), and 4) old CalpOX (25.91 ± 0.60) for diaphragm and limb muscle (extensor digitorum longus, EDL) force frequency relations. Aging significantly reduced diaphragm and EDL peak force in old WT mice, and decreased the force-time integral during a fatiguing protocol by 48% and 23% in aged WT diaphragm and EDL, respectively. In contrast, we found that CalpOX mice had significantly increased diaphragm and EDL peak force in old mice, similar to that observed in young mice. The impact of aging on the force-time integral during a fatiguing protocol was abolished in the diaphragm and EDL of old CalpOX animals. Surprisingly, we found that CalpOX had a significant impact on longevity, increasing median survival from 20.55 mo in WT mice to 24 mo in CalpOX mice (P = 0.0006).NEW & NOTEWORTHY This is the first study to investigate the role of calpastatin overexpression on skeletal muscle specific force in aging rodents. Muscle-specific overexpression of calpastatin, the endogenous calpain inhibitor, prevented aging-induced reductions in both EDL and diaphragm specific force and, remarkably, increased life span. These data suggest that diaphragm dysfunction in aging may be a major factor in determining longevity. Targeting the calpain/calpastatin pathway may elucidate novel therapies to combat skeletal muscle weakness in aging.

Forgot to Exercise? Exercise Derived Circulating Myokines in Alzheimer's Disease: A Perspective

Regular exercise plays an essential role in maintaining healthy neurocognitive function and central nervous system (CNS) immuno-metabolism in the aging CNS. Physical activity decreases the risk of developing Alzheimer's Disease (AD), is associated with better AD prognosis, and positively affects cognitive function in AD patients. Skeletal muscle is an important secretory organ, communicating proteotoxic and metabolic stress to distant tissues, including the CNS, through the secretion of bioactive molecules collectively known as myokines. Skeletal muscle undergoes significant physical and metabolic remodeling during exercise, including alterations in myokine expression profiles. This suggests that changes in myokine and myometabolite secretion may underlie the well-documented benefits of exercise in AD. However, to date, very few studies have focused on specific alterations in skeletal muscle-originating secreted factors and their potential neuroprotective effects in AD. In this review, we discuss exercise therapy for AD prevention and intervention, and propose the use of circulating myokines as novel therapeutic tools for modifying AD progression.

Proteasome stress in skeletal muscle mounts a long-range protective response that delays retinal and brain aging

Neurodegeneration in the central nervous system (CNS) is a defining feature of organismal aging that is influenced by peripheral tissues. Clinical observations indicate that skeletal muscle influences CNS aging, but the underlying muscle-to-brain signaling remains unexplored. In Drosophila, we find that moderate perturbation of the proteasome in skeletal muscle induces compensatory preservation of CNS proteostasis during aging. Such long-range stress signaling depends on muscle-secreted Amyrel amylase. Mimicking stress-induced Amyrel upregulation in muscle reduces age-related accumulation of poly-ubiquitinated proteins in the brain and retina via chaperones. Preservation of proteostasis stems from the disaccharide maltose, which is produced via Amyrel amylase activity. Correspondingly, RNAi for SLC45 maltose transporters reduces expression of Amyrel-induced chaperones and worsens brain proteostasis during aging. Moreover, maltose preserves proteostasis and neuronal activity in human brain organoids challenged by thermal stress. Thus, proteasome stress in skeletal muscle hinders retinal and brain aging by mounting an adaptive response via amylase/maltose.

Hypoxanthine Induces Muscular ATP Depletion and Fatigue via UCP2

Hypoxanthine (Hx), an intermediate metabolite of the purine metabolism pathway which is dramatically increased in blood and skeletal muscle during muscle contraction and metabolism, is characterized as a marker of exercise exhaustion. However, the physiological effects of Hx on skeletal muscle remain unknown. Herein, we demonstrate that chronic treatment with Hx through dietary supplementation resulted in skeletal muscle fatigue and impaired the exercise performance of mice without affecting their growth and skeletal muscle development. Hx increased the uncoupling protein 2 (UCP2) expression in the skeletal muscle, which led to decreased energy substrate storage and enhanced glycolysis. These effects could also be verified in acute treatment with Hx through intraperitoneal injection. In addition, muscular specifically knockout of UCP2 through intra-muscle tissue injection of adenovirus-associated virus reversed the effects of Hx. In conclusion, we identified a novel role of Hx in the skeletal muscular fatigue mediated by UCP2-dependent mitochondrial uncoupling. This finding may shed light on the pathological mechanism of clinical muscle dysfunctions due to abnormal metabolism, such as muscle fatigue and weakness.

Impaired capillary tube formation induced by elevated secretion of IL8 involves altered signaling via the CXCR1/PI3K/MMP2 pathway

Angiogenesis is a multistep process requiring endothelial cell activation, migration, proliferation and tube formation. We recently reported that elevated secretion of interlukin 8 (IL8) by myotubes (MT) from subjects with Type-2 Diabetes (T2D) reduced angiogenesis by human umbilical vein endothelial cells (HUVEC) and human skeletal muscle explants. This lower vascularization was mediated through impaired activation of the phosphatidylinositol 3-kinase (PI3K)-pathway. We sought to investigate additional signaling elements that might mediate reduced angiogenesis. HUVEC were exposed to levels of IL8 equal to those secreted by MT from non-diabetic (ND) and T2D subjects and the involvement of components in the angiogenic response pathway examined. Cellular content of reactive oxygen species and Nitrate secretion were similar after treatment with [ND-IL8] and [T2D-IL8]. CXCR1 protein was down-regulated after treatment with [T2D-IL8] (p < 0.01 vs [ND-IL8] treatment); CXCR2 expression was unaltered. Addition of neutralizing antibodies against CXCR1 and CXCR2 to HUVEC treated with IL8 confirmed that CXCR1 alone mediated the angiogenic response to IL8. A key modulator of angiogenesis is matrix metalloproteinase-2 (MMP2). MMP2 secretion was higher after treatment with [ND-IL8] vs [T2D-IL8] (p < 0.01). MMP2 inhibition reduced tube formation to greater extent with [ND-IL8] than with [T2D-IL8] (p < 0.005). The PI3K-pathway inhibitor LY294002 reduced IL8-induced MMP2 release. IL8 regulation of MMP2 release was CXCR1 dependent, as anti-CXCR1 significantly reduced MMP2 release (p < 0.05). These results suggest that high levels of IL8 secreted by T2D MT trigger reduced capillarization via lower activation of a CXCR1-PI3K pathway, followed by impaired release and activity of MMP2.

Ionic Silicon Protects Oxidative Damage and Promotes Skeletal Muscle Cell Regeneration

Volumetric muscle loss injuries overwhelm the endogenous regenerative capacity of skeletal muscle, and the associated oxidative damage can delay regeneration and prolong recovery. This study aimed to investigate the effect of silicon-ions on C2C12 skeletal muscle cells under normal and excessive oxidative stress conditions to gain insights into its role on myogenesis during the early stages of muscle regeneration. In vitro studies indicated that 0.1 mM Si-ions into cell culture media significantly increased cell viability, proliferation, migration, and myotube formation compared to control. Additionally, MyoG, MyoD, Neurturin, and GABA expression were significantly increased with addition of 0.1, 0.5, and 1.0 mM of Si-ion for 1 and 5 days of C2C12 myoblast differentiation. Furthermore, 0.1-2.0 mM Si-ions attenuated the toxic effects of H2O2 within 24 h resulting in increased cell viability and differentiation. Addition of 1.0 mM of Si-ions significantly aid cell recovery and protected from the toxic effect of 0.4 mM H2O2 on cell migration. These results suggest that ionic silicon may have a potential effect in unfavorable situations where reactive oxygen species is predominant affecting cell viability, proliferation, migration, and differentiation. Furthermore, this study provides a guide for designing Si-containing biomaterials with desirable Si-ion release for skeletal muscle regeneration.

Potential Roles of Exercise-Induced Plasma Metabolites Linking Exercise to Health Benefits

Regular exercise has a myriad of health benefits. An increase in circulating exercise factors following exercise is a critical physiological response. Numerous studies have shown that exercise factors released from tissues during physical activity may contribute to health benefits via autocrine, paracrine, and endocrine mechanisms. Myokines, classified as proteins secreted from skeletal muscle, are representative exercise factors. The roles of myokines have been demonstrated in a variety of exercise-related functions linked to health benefits. In addition to myokines, metabolites are also exercise factors. Exercise changes the levels of various metabolites via metabolic reactions. Several studies have identified exercise-induced metabolites that positively influence organ functions. Here, we provide an overview of selected metabolites secreted into the circulation upon exercise.

Muscle-Organ Crosstalk: The Emerging Roles of Myokines

Physical activity decreases the risk of a network of diseases, and exercise may be prescribed as medicine for lifestyle-related disorders such as type 2 diabetes, dementia, cardiovascular diseases, and cancer. During the past couple of decades, it has been apparent that skeletal muscle works as an endocrine organ, which can produce and secrete hundreds of myokines that exert their effects in either autocrine, paracrine, or endocrine manners. Recent advances show that skeletal muscle produces myokines in response to exercise, which allow for crosstalk between the muscle and other organs, including brain, adipose tissue, bone, liver, gut, pancreas, vascular bed, and skin, as well as communication within the muscle itself. Although only few myokines have been allocated to a specific function in humans, it has been identified that the biological roles of myokines include effects on, for example, cognition, lipid and glucose metabolism, browning of white fat, bone formation, endothelial cell function, hypertrophy, skin structure, and tumor growth. This suggests that myokines may be useful biomarkers for monitoring exercise prescription for people with, for example, cancer, diabetes, or neurodegenerative diseases.

Exercise-induced α-ketoglutaric acid stimulates muscle hypertrophy and fat loss through OXGR1-dependent adrenal activation

Beneficial effects of resistance exercise on metabolic health and particularly muscle hypertrophy and fat loss are well established, but the underlying chemical and physiological mechanisms are not fully understood. Here, we identified a myometabolite‐mediated metabolic pathway that is essential for the beneficial metabolic effects of resistance exercise in mice. We showed that substantial accumulation of the tricarboxylic acid cycle intermediate α‐ketoglutaric acid (AKG) is a metabolic signature of resistance exercise performance. Interestingly, human plasma AKG level is also negatively correlated with BMI. Pharmacological elevation of circulating AKG induces muscle hypertrophy, brown adipose tissue (BAT) thermogenesis, and white adipose tissue (WAT) lipolysis in vivo. We further found that AKG stimulates the adrenal release of adrenaline through 2‐oxoglutarate receptor 1 (OXGR1) expressed in adrenal glands. Finally, by using both loss‐of‐function and gain‐of‐function mouse models, we showed that OXGR1 is essential for AKG‐mediated exercise‐induced beneficial metabolic effects. These findings reveal an unappreciated mechanism for the salutary effects of resistance exercise, using AKG as a systemically derived molecule for adrenal stimulation of muscle hypertrophy and fat loss.

Myokine Regulation of Insulin Secretion: Impact of Inflammation and Type 2 Diabetes

Skeletal muscle (SkM) secretes protein factors (myokines) that can exert multiple actions. To study the control of myokine regulation of β-cell function, SkM biopsies were taken from non-diabetic (ND) and Type 2 diabetic (T2D) subjects and satellite cells cultured to myotubes (MT). MT were also treated with lipopolysaccharide (infectious inflammation – II) or a combination of glucose (10 mM), insulin (120 pM), and palmitate (0.4 mM) (metabolic inflammation – MI) to model the inflammatory and metabolic conditions seen in vivo with T2D. Conditioned media (CM) was collected from MT after 24 h and used to treat INS-1 cells for 24 h. Cell viability, total insulin content, glucose-stimulated insulin secretion (GSIS) and maximal (IBMX-stimulated) IS (IS_max) were monitored. Under baseline conditions, CM from ND and T2D MT had no effects on INS-1 cell viability, insulin content, GSIS, or IS_max. After exposure to II, CM from ND-MT augmented GSIS in INS-1 cells by 100 ± 25% over control (p < 0.05); T2D-CM had no effect. After exposure to MI, T2D-CM suppressed GSIS by 35 ± 5% (p < 0.05); ND-CM was without effect. Under either of these conditions cell viability, total insulin content and IS_max were unaffected. Effects of CM on GSIS were lost after CM was boiled. Both augmentation of GSIS by ND-CM from II-treated MT, and suppression by T2D-CM from MI-treated MT, were inhibited by wortmannin, Ro 31-8220, and SB203580. In summary: (1) ND-MT are able to augment GSIS when stressed, (2) T2D-MT responding to a diabetic-like environment secrete myokines that suppress GSIS, (3) Unknown protein factors exert effects specifically on GSIS, possibly through PI-3K, PKC, and/or p38 MAPK. In T2D, both insulin resistance and a suppression of adaptive increased insulin secretion are intrinsic properties of SkM that can contribute to the full T2D phenotype.

Muscle-derived Dpp regulates feeding initiation via endocrine modulation of brain dopamine biosynthesis

In this study, Robles-Murguia et al. set out to examine whether muscle-secreted hormones regulate feeding. Using Drosophila as a model system combined with several in vivo and in vitro experiments, the authors identify Decapentaplegic (Dpp) as a myokine that can signal from the muscle to the brain to control feeding by altering dopamine synthesis through transcriptional regulation of tyrosine hydroxylase (TH).

In animals, the brain regulates feeding behavior in response to local energy demands of peripheral tissues, which secrete orexigenic and anorexigenic hormones. Although skeletal muscle is a key peripheral tissue, it remains unknown whether muscle-secreted hormones regulate feeding. In Drosophila, we found that decapentaplegic (dpp), the homolog of human bone morphogenetic proteins BMP2 and BMP4, is a muscle-secreted factor (a myokine) that is induced by nutrient sensing and that circulates and signals to the brain. Muscle-restricted dpp RNAi promotes foraging and feeding initiation, whereas dpp overexpression reduces it. This regulation of feeding by muscle-derived Dpp stems from modulation of brain tyrosine hydroxylase (TH) expression and dopamine biosynthesis. Consistently, Dpp receptor signaling in dopaminergic neurons regulates TH expression and feeding initiation via the downstream transcriptional repressor Schnurri. Moreover, pharmacologic modulation of TH activity rescues the changes in feeding initiation due to modulation of dpp expression in muscle. These findings indicate that muscle-to-brain endocrine signaling mediated by the myokine Dpp regulates feeding behavior.

Rbfox-Splicing Factors Maintain Skeletal Muscle Mass by Regulating Calpain3 and Proteostasis

Maintenance of skeletal muscle mass requires a dynamic balance between protein synthesis and tightly controlled protein degradation by the calpain, autophagy-lysosome, and ubiquitin-proteasome systems (proteostasis). Several sensing and gene-regulatory mechanisms act together to maintain this balance in response to changing conditions. Here, we show that deletion of the highly conserved Rbfox1 and Rbfox2 alternative splicing regulators in adult mouse skeletal muscle causes rapid, severe loss of muscle mass. Rbfox deletion did not cause a reduction in global protein synthesis, but it led to altered splicing of hundreds of gene transcripts, including capn3, which produced an active form of calpain3 protease. Rbfox knockout also led to a reduction in autophagy flux, likely producing a compensatory increase in general protein degradation by the proteasome. Our results indicate that the Rbfox-splicing factors are essential for the maintenance of skeletal muscle mass and proteostasis.

Examination of the Myokine Response in Pregnant and Non-pregnant Women Following an Acute Bout of Moderate-Intensity Walking

Background

It is recommended that women accumulate 150-min of weekly moderate-intensity physical activity (MPA) when pregnant. Engaging in regular physical activity (PA) confers many health benefits to both the mother and the fetus. However, the molecular mechanisms by which these health benefits are bestowed are not well understood. One potential factor that may be contributing to the observed benefits is myokines, which are small peptides secreted by skeletal muscles. In the non-pregnant population, myokines are believed to be involved in the molecular mechanisms resulting from PA. The objective of this study was to characterize and compare the myokine profile of pregnant and non-pregnant women, after an acute bout of MPA.

Methods

Pregnant (n = 13) and non-pregnant (n = 17) women were recruited from the Ottawa region to undergo a treadmill walking session at moderate-intensity (40–60% heart rate reserve). Pre- and post-exercise serum samples were taken, and a set of 15 myokines were analyzed although only 10 were detected. IL-6 was analyzed using a high-sensitivity assay, while FGF21, EPO, BDNF, Fractalkine, IL-15, SPARC, FABP-3, FSTL-1, and oncostatin were analyzed using various multiplex assays.

Results

The pregnant and non-pregnant groups did not differ in terms of age, height, non/pre-pregnancy weight, BMI, and resting heart rate. Baseline levels of EPO and oncostatin were higher in the pregnant group while FGF21 was higher in the non-pregnant group. Circulating levels of three myokines, FGF21, EPO, and IL-15 significantly increased in response to the acute exercise in the pregnant group. Non-pregnant women exhibited an increase in three myokines, FABP-3, FSTL-1, and oncostatin, while one myokine, EPO, decreased post-exercise. SPARC, fractalkine and BDNF were shown to increase post-exercise regardless of pregnancy status while the response for BDNF was more pronounced in the non-pregnant group.

Conclusion

This is the first study examining myokine response following an acute bout of PA in pregnancy. Moderate intensity PA, which is recommended during pregnancy, elicited an increase in four myokines post-compared to pre-exercise in the pregnant group. Further research is warranted to understand the role of myokines in pregnancy.

Muscle-generated BDNF is a sexually dimorphic myokine that controls metabolic flexibility

The ability of skeletal muscle to switch between lipid and glucose oxidation for ATP production during metabolic stress is pivotal for maintaining systemic energy homeostasis, and dysregulation of this metabolic flexibility is a dominant cause of several metabolic disorders. However, the molecular mechanism that governs fuel selection in muscle is not well understood. Here, we report that brain-derived neurotrophic factor (BDNF) is a fasting-induced myokine that controls metabolic reprograming through the AMPK/CREB/PGC-1α pathway in female mice. Female mice with a muscle-specific deficiency in BDNF (MBKO mice) were unable to switch the predominant fuel source from carbohydrates to fatty acids during fasting, which reduced ATP production in muscle. Fasting-induced muscle atrophy was also compromised in female MBKO mice, likely a result of autophagy inhibition. These mutant mice displayed myofiber necrosis, weaker muscle strength, reduced locomotion, and muscle-specific insulin resistance. Together, our results show that muscle-derived BDNF facilitates metabolic adaption during nutrient scarcity in a gender-specific manner and that insufficient BDNF production in skeletal muscle promotes the development of metabolic myopathies and insulin resistance.

Physical activity and muscle-brain crosstalk

Neurological and mental illnesses account for a considerable proportion of the global burden of disease. Exercise has many beneficial effects on brain health, contributing to decreased risks of dementia, depression and stress, and it has a role in restoring and maintaining cognitive function and metabolic control. The fact that exercise is sensed by the brain suggests that muscle-induced peripheral factors enable direct crosstalk between muscle and brain function. Muscle secretes myokines that contribute to the regulation of hippocampal function. Evidence is accumulating that the myokine cathepsin B passes through the blood-brain barrier to enhance brain-derived neurotrophic factor production and hence neurogenesis, memory and learning. Exercise increases neuronal gene expression of FNDC5 (which encodes the PGC1α-dependent myokine FNDC5), which can likewise contribute to increased brain-derived neurotrophic factor levels. Serum levels of the prototype myokine, IL-6, increase with exercise and might contribute to the suppression of central mechanisms of feeding. Exercise also increases the PGC1α-dependent muscular expression of kynurenine aminotransferase enzymes, which induces a beneficial shift in the balance between the neurotoxic kynurenine and the neuroprotective kynurenic acid, thereby reducing depression-like symptoms. Myokine signalling, other muscular factors and exercise-induced hepatokines and adipokines are implicated in mediating the exercise-induced beneficial impact on neurogenesis, cognitive function, appetite and metabolism, thus supporting the existence of a muscle-brain endocrine loop.

Circadian gene variants and the skeletal muscle circadian clock contribute to the evolutionary divergence in longevity across Drosophila populations

Organisms use endogenous clocks to adapt to the rhythmicity of the environment and to synchronize social activities. Although the circadian cycle is implicated in aging, it is unknown whether natural variation in its function contributes to differences in lifespan between populations and whether the circadian clock of specific tissues is key for longevity. We have sequenced the genomes of Drosophila melanogaster strains with exceptional longevity that were obtained via multiple rounds of selection from a parental strain. Comparison of genomic, transcriptomic, and proteomic data revealed that changes in gene expression due to intergenic polymorphisms are associated with longevity and preservation of skeletal muscle function with aging in these strains. Analysis of transcription factors differentially modulated in long-lived versus parental strains indicates a possible role of circadian clock core components. Specifically, there is higher period and timeless and lower cycle expression in the muscle of strains with delayed aging compared to the parental strain. These changes in the levels of circadian clock transcription factors lead to changes in the muscle circadian transcriptome, which includes genes involved in metabolism, proteolysis, and xenobiotic detoxification. Moreover, a skeletal muscle-specific increase in timeless expression extends lifespan and recapitulates some of the transcriptional and circadian changes that differentiate the long-lived from the parental strains. Altogether, these findings indicate that the muscle circadian clock is important for longevity and that circadian gene variants contribute to the evolutionary divergence in longevity across populations.

Lipotoxicity, aging, and muscle contractility: does fiber type matter?

Sarcopenia is a universal characteristic of the aging process and is often accompanied by increases in whole-body adiposity. These changes in body composition have important clinical implications, given that loss of muscle and gain of fat mass are both significantly and independently associated with declining physical performance as well as an increased risk for disability, hospitalizations, and mortality in older individuals. This increased fat mass is not exclusively stored in adipose depots but may become deposited in non-adipose tissues, such as skeletal muscle, when the oxidative capacity of the adipose tissue itself is exceeded. The redistributed adipose tissue is thought to exert detrimental local effects on the muscle environment given the close proximity. Thus, sarcopenia observed with aging may be better defined in the context of loss of muscle quality rather than loss of muscle quantity per se. In this perspective, we briefly review the age-related physiological changes in cellularity, secretory profiles, and inflammatory status of adipose tissue which drive lipotoxicity (spillover) of skeletal muscle and then provide evidence of how this may affect specific fiber type contractility. We focus on biological contributors (cellular machinery) to contractility for which there is some evidence of vulnerability to lipid stress distinguishing between fiber types.

Fibroblast growth factor 21 controls mitophagy and muscle mass

BACKGROUND

Skeletal muscle is a plastic tissue that adapts to changes in exercise, nutrition, and stress by secreting myokines and myometabolites. These muscle-secreted factors have autocrine, paracrine, and endocrine effects, contributing to whole body homeostasis. Muscle dysfunction in aging sarcopenia, cancer cachexia, and diabetes is tightly correlated with the disruption of the physiological homeostasis at the whole body level. The expression levels of the myokine fibroblast growth factor 21 (FGF21) are very low in normal healthy muscles. However, fasting, ER stress, mitochondrial myopathies, and metabolic disorders induce its release from muscles. Although our understanding of the systemic effects of muscle-derived FGF21 is exponentially increasing, the direct contribution of FGF21 to muscle function has not been investigated yet.

METHODS

Muscle-specific FGF21 knockout mice were generated to investigate the consequences of FGF21 deletion concerning skeletal muscle mass and force. To identify the mechanisms underlying FGF21-dependent adaptations in skeletal muscle during starvation, the study was performed on muscles collected from both fed and fasted adult mice. In vivo overexpression of FGF21 was performed in skeletal muscle to assess whether FGF21 is sufficient per se to induce muscle atrophy.

RESULTS

We show that FGF21 does not contribute to muscle homeostasis in basal conditions in terms of fibre type distribution, fibre size, and muscle force. In contrast, FGF21 is required for fasting-induced muscle atrophy and weakness. The mass of isolated muscles from control-fasted mice was reduced by 15-25% (P < 0.05) compared with fed control mice. FGF21-null muscles, however, were significantly protected from muscle loss and weakness during fasting. Such important protection is due to the maintenance of protein synthesis rate in knockout muscles during fasting compared with a 70% reduction in control-fasted muscles (P < 0.01), together with a significant reduction of the mitophagy flux via the regulation of the mitochondrial protein Bnip3. The contribution of FGF21 to the atrophy programme was supported by in vivo FGF21 overexpression in muscles, which was sufficient to induce autophagy and muscle loss by 15% (P < 0.05). Bnip3 inhibition protected against FGF21-dependent muscle wasting in adult animals (P < 0.05).

CONCLUSIONS

FGF21 is a novel player in the regulation of muscle mass that requires the mitophagy protein Bnip3.