Dynamics of the skeletal muscle secretome during myoblast differentiation

Synergistic short-term and long-term effects of TGF-β1 and 3 on collagen production in differentiating myoblasts

Skeletal muscle fibrosis and regeneration are modulated by transforming growth factor β (TGF-β) superfamily. Amongst them, TGF-β1 is a highly potent pro-fibrotic factor, while TGF-β3 has been implicated to reduce scar formation and collagen production in skin and vocal mucosa. However, little is known about the individual and combined short- and long-term effects of TGF-β1 and TGF-β3 on collagen expression in myoblasts and myotubes. Here we show that in C2C12 myoblasts TGF-β1 and/or TGF-β3 increased mRNA expression of Ctgf and Fgf-2 persistently after 3 h and of Col1A1 after 24 h, while TGF-β1+TGF-β3 mitigated these effects after 48 h incubation. Gene expression of Tgf-β1 was enhanced by TGF-β1 and/or TGF-β3 after 24 h and 48 h. However, Tgfbr1 mRNA expression was reduced at 48 h. After 48 h incubation with TGF-β1 and/or TGF-β3, Col3A1 and Col4A1 mRNA expression levels were decreased. Myoblasts produced collagen after three days incubation with TGF-β1 and/or TGF-β3 in a dose independent manner. Collagen deposition was doubled when myoblasts differentiated into myotubes and TGF-β1 and/or TGF-β3 did not stimulate collagen production any further. TGF-β type I receptor (TGFBR1) inhibitor, LY364947, suppressed TGF-βs-induced collagen production. Collagen I expression was higher in myotubes than in myoblasts. TGF-β1 and/or TGF-β3 inhibited myotube differentiation which was antagonized by LY364947. These results indicate that both C2C12 myoblasts and myotubes produce collagen. Whereas TGF-β1 and TGF-β3 individually and simultaneously stimulate collagen production in C2C12 differentiating myoblasts, in myotubes these effects are less prominent. In muscle cells, TGF-β3 is ineffective to antagonize TGF-β1-induced collagen production.

Sarcopenic Obesity in Non-Alcoholic Fatty Liver Disease-The Union of Two Culprits

Non-alcoholic fatty liver disease (NAFLD) continues to rise and has become the most common cause of chronic liver disease among all ages and ethnicities. Metabolic disorders, such as obesity and insulin resistance, are closely associated with sarcopenia and NAFLD. Sarcopenic obesity is a clinical disorder characterized by the simultaneous loss of skeletal muscle and gain of adipose tissue. It is associated with worse outcomes in individuals with NAFLD. It is projected that NAFLD and sarcopenia will rise as the prevalence of obesity continues to increase at an unparallel rate. Recently, sarcopenia and sarcopenic obesity have gained considerable interest, but we still lack a well-defined definition and a management approach. Therefore, it is imperative to continue shining the light on this topic and better understand the underlying mechanism as well as treatment options. In this review article, we aimed to address the pathophysiology, impact, and outcomes of sarcopenic obesity on NAFLD.

The role of Irisin in multiorgan protection

Physical exercise is an effective strategy for improving human health. Various organs, including the heart, lung and kidney, can benefit from exercise. However, the underlying molecular mechanisms by which exercise protects organs remain unknown. Irisin, a myokine secreted from muscle in response to exercise, has attracted increased attention from researchers. The role of irisin in multiorgan protection has been gradually revealed, and this muscle-derived circulating factor is regarded as an essential bridge linking exercise and organ health. The mechanisms by which irisin protects diverse organs are different. Here, we review the research progress on the multiorgan protective effects of irisin and discuss the underlying molecular mechanisms.

Early Post-Stroke Electroacupuncture Promotes Motor Function Recovery in Post-Ischemic Rats by Increasing the Blood and Brain Irisin

OBJECTIVE

Recent studies have shown that irisin, a novel peptide hormone derived from muscles, could be used as a potential therapeutic drug against ischemic stroke. Moreover, electroacupuncture (EA) is widely used in the treatment of ischemic stroke. Yet, whether irisin is involved in the EA neuroprotection remains unclear. The following study investigated the association between serum and peri-lesional cortex irisin and EA-induced post-stroke motor recovery in rats.

METHODS

The middle cerebral artery occlusion (MCAO) method was used to induce ischemic stroke in rats. Rats were randomly divided into two groups: a middle cerebral artery occlusion (MCAO) group (MCAO rats without treatment) and an electroacupuncture (EA) group (MCAO rats treated with EA). On the 3rd day post-stroke, infarct volume, behavioral deficits, surviving neurons, irisin protein expression in peri-infarction cortex, muscle tissue, and serum were evaluated to identify the neuroprotective of EA in acute ischemic stroke.

RESULTS

Compared with the MCAO group, the EA group showed better behavioral performance, a smaller cerebral infarct volume, more surviving neurons, and a significant increase in irisin expression in the peri-infarction cortex and serum (p<0.05). However, no difference in irisin expression in the muscle tissue was found between the MCAO group and the EA group (p>0.05).

CONCLUSION

EA promotes motor function recovery, reduces the volume of cerebral infarction, and alleviates neuronal death following ischemic stroke by enhancing the expression of irisin in both the blood and peri-lesional cortex.

Impact of testosterone treatment on circulating irisin in men with late-onset hypogonadism and metabolic syndrome

OBJECTIVES

The beneficial effects of testosterone replacement therapy (TRT) in men with late-onset hypogonadism (LOH) on the body composition and metabolic outcomes are well-established. A potential explanation might lie in the hormones, secreted from skeletal muscles, named "myokines". The aim of this study was to evaluate the effects of TRT on the levels of serum irisin in subjects with LOH.

STUDY DESIGN

A total 40 men with metabolic syndrome (MS) and LOH (measured serum testosterone concentration < 12 nmol/l). TRT with Testosterone Undecanoate (Nebido™) was performed at baseline and at week 6. Irisin serum concentration was determined at baseline and at week 18 by means of ELISA.

RESULTS

Circulating irisin was positively associated with serum testosterone (r = 0.283, p < 0.05). TRT has led to a statistically significant rise in circulating serum irisin levels (7.12 ± 0.76 mcg/ml versus 7.76 ± 0.75 mcg/ml; paired-samples t-test p < 0.001). ROC-analyses determined irisin to be predictive of treatment response (AUC = 0.741, p = 0.014).

CONCLUSIONS

Irisin is positively associated with serum testosterone in a population of men with MS and LOH. TRT in these subjects has led to a significant improvement in associated clinical symptoms as well as to a significant rise in serum irisin levels.

L-carnitine exerts a nutrigenomic effect via direct modulation of nuclear receptor signaling in adipocytes, hepatocytes and SKMC, demonstrating its nutritional impact

L-carnitine is an indispensable metabolite facilitating the transport of fatty acids into the mitochondrial matrix and has been previously postulated to exert a nutrigenomic effect. However, the underlying molecular mechanisms remain mostly unclear. We hypothesized that L-carnitine interacts with nuclear receptors involved in metabolic regulation, thereby modulating downstream targets of cellular metabolism. Therefore, we investigated the effect of L-carnitine supplementation on protein activity, mRNA expression, and binding affinities of nuclear receptors as well as mRNA expression of downstream targets in skeletal muscle cells, hepatocytes, and differentiated adipocytes. L-carnitine supplementation to hepatocytes increased the protein activity of multiple nuclear receptors (RAR, RXR, VDR, PPAR, HNF4, ER, LXR). Diverging effects on the mRNA expression of PPAR-α, PPAR-δ, PPAR-γ, RAR-β, LXR-α, and RXR-α were observed in adipocytes, hepatocytes, and skeletal muscle cells. mRNA levels of PPAR-α, a key regulator of lipolysis and β-oxidation, were significantly upregulated, emphasizing a role of L-carnitine as a promoter of lipid catabolism. L-carnitine administration to hepatocytes modulated the transcription of key nuclear receptor target genes, including ALDH1A1, a promoter of adipogenesis, and OGT, a contributor to insulin resistance. Electrophoretic mobility shift assays proved L-carnitine to increase binding affinities of nuclear receptors to their promoter target sequences, suggesting a molecular mechanism for the observed transcriptional modulation. Overall, these findings indicate that L-carnitine modulates the activity and expression of nuclear receptors, thereby promoting lipolytic gene expression and decreasing transcription of target genes linked to adipogenesis and insulin resistance.

Drosophila PDGF/VEGF signaling from muscles to hepatocyte-like cells protects against obesity

PDGF/VEGF ligands regulate a plethora of biological processes in multicellular organisms via autocrine, paracrine, and endocrine mechanisms. We investigated organ-specific metabolic roles of Drosophila PDGF/VEGF-like factors (Pvfs). We combine genetic approaches and single-nuclei sequencing to demonstrate that muscle-derived Pvf1 signals to the Drosophila hepatocyte-like cells/oenocytes to suppress lipid synthesis by activating the Pi3K/Akt1/TOR signaling cascade in the oenocytes. Functionally, this signaling axis regulates expansion of adipose tissue lipid stores in newly eclosed flies. Flies emerge after pupation with limited adipose tissue lipid stores and lipid level is progressively accumulated via lipid synthesis. We find that adult muscle-specific expression of pvf1 increases rapidly during this stage and that muscle-to-oenocyte Pvf1 signaling inhibits expansion of adipose tissue lipid stores as the process reaches completion. Our findings provide the first evidence in a metazoan of a PDGF/VEGF ligand acting as a myokine that regulates systemic lipid homeostasis by activating TOR in hepatocyte-like cells.

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.

Role of Myokines in Myositis Pathogenesis and Their Potential to be New Therapeutic Targets in Idiopathic Inflammatory Myopathies

Idiopathic inflammatory myopathies (IIM) represent a heterogeneous group of autoimmune diseases whose treatment is often a challenge. Many patients, even after immunosuppressive therapy, do not respond to treatment, so new alternatives have been sought for this. Therefore, other signaling pathways that could contribute to the pathogenesis of myositis have been investigated, such as the expression of myokines in skeletal muscle in response to the inflammatory process. In this review, we will refer to these muscle cytokines that are overexpressed or downregulated in skeletal muscle in patients with various forms of IIM, thus being able to contribute to the maintenance of the autoimmune process. Some muscle cytokines, through their antagonistic action, may be a helpful contributor to the disease modulation, and thus, they could represent personalized treatment targets. Here, we consider the main myokines involved in the pathogenesis of myositis, expressing our view on the possibility of using them as potential therapeutic targets: interleukins IL-6, IL-15, and IL-18; chemokines CXCL10, CCL2, CCL3, CCL4, CCL5, and CCL20; myostatin; follistatin; decorin; osteonectin; and insulin-like 6. An interesting topic regarding the complex connection between myokines and noninflammatory pathways implied in IIM has also been briefly described, because it is an important scientific approach to the pathogenesis of IIM and can be a therapeutic alternative to be considered, especially for the patients who do not respond to immunosuppressive treatment.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

Optimized method for extraction of exosomes from human primary muscle cells

Skeletal muscle is increasingly considered an endocrine organ secreting myokines and extracellular vesicles (exosomes and microvesicles), which can affect physiological changes with an impact on different pathological conditions, including regenerative processes, aging, and myopathies. Primary human myoblasts are an essential tool to study the muscle vesicle secretome. Since their differentiation in conditioned media does not induce any signs of cell death or cell stress, artefactual effects from those processes are unlikely. However, adult human primary myoblasts senesce in long-term tissue culture, so a major technical challenge is posed by the need to avoid artefactual effects resulting from pre-senescent changes. Since these cells should be studied within a strictly controlled pre-senescent division count (<21 divisions), and yields of myoblasts per muscle biopsy are low, it is difficult or impossible to amplify sufficiently large cell numbers (some 250 × 10⁶ myoblasts) to obtain sufficient conditioned medium for the standard ultracentrifugation approach to exosome isolation.

Thus, an optimized strategy to extract and study secretory muscle vesicles is needed. In this study, conditions are optimized for the in vitro cultivation of human myoblasts, and the quality and yield of exosomes extracted using an ultracentrifugation protocol are compared with a modified polymer-based precipitation strategy combined with extra washing steps. Both vesicle extraction methods successfully enriched exosomes, as vesicles were positive for CD63, CD82, CD81, floated at identical density (1.15-1.27 g.ml⁻¹), and exhibited similar size and cup-shape using electron microscopy and NanoSight tracking. However, the modified polymer-based precipitation was a more efficient strategy to extract exosomes, allowing their extraction in sufficient quantities to explore their content or to isolate a specific subpopulation, while requiring >30 times fewer differentiated myoblasts than what is required for the ultracentrifugation method. In addition, exosomes could still be integrated into recipient cells such as human myotubes or iPSC-derived motor neurons.

Modified polymer-based precipitation combined with extra washing steps optimizes exosome yield from a lower number of differentiated myoblasts and less conditioned medium, avoiding senescence and allowing the execution of multiple experiments without exhausting the proliferative capacity of the myoblasts.

Skeletal muscle antagonizes antiviral CD8+ T cell exhaustion

CD8⁺ T cells become functionally impaired or "exhausted" in chronic infections, accompanied by unwanted body weight reduction and muscle mass loss. Whether muscle regulates T cell exhaustion remains incompletely understood. We report that mouse skeletal muscle increased interleukin (IL)-15 production during LCMV clone 13 chronic infection. Muscle-specific ablation of Il15 enhanced the CD8⁺ T cell exhaustion phenotype. Muscle-derived IL-15 was required to maintain a population of CD8⁺CD103⁺ muscle-infiltrating lymphocytes (MILs). MILs resided in a less inflamed microenvironment, expressed more T cell factor 1 (Tcf1), and had higher proliferative potential than splenic T cells. MILs differentiated into functional effector T cells after reentering lymphoid tissues. Increasing muscle mass via muscle-specific inhibition of TGFβ signaling enhanced IL-15 production and antiviral CD8⁺ T cell responses. We conclude that skeletal muscle antagonizes T cell exhaustion by protecting T cell proliferative potential from inflammation and replenishing the effector T cell progeny pool in lymphoid organs.

Estimated Cardiorespiratory Fitness Attenuates the Impacts of Sarcopenia and Obesity on Non-Alcoholic Fatty Liver in Korean Adults

This population-based, cross-sectional study examined the preventive role of non-exercise-based estimation of cardiorespiratory fitness (eCRF) against the impacts of sarcopenia and obesity on the non-alcoholic fatty liver (NAFL) in Korean adults. Data were obtained from the 2008-2011 Korea National Health and Nutrition Examination Surveys IV and V (n = 14,015 Koreans aged ≥ 18 years, 64% women). eCRF was calculated with the age- and sex-specific algorithms, and classified as lower (lowest 25%), middle (middle 50%) and upper (highest 25%). Individuals were classified as optimal (i.e., the absence of both sarcopenia and obesity), sarcopenia (i.e., the presence of sarcopenia), obesity (i.e., the presence of obesity) or sarcopenic obesity (i.e., the coexistence of sarcopenia and obesity). Limited to the sarcopenia phenotype, the adjusted odds ratio (OR) of NAFL was 2.2 (95% confidence interval, CI, 1.5-3.1) for the lower eCRF, 1.6 (95% CI, 1.3-2.1) for the middle eCRF and 2.1 (95% CI, 1.4-3.1) for the upper eCRF, compared to the optimal phenotype. Limited to the obesity phenotype, the adjusted OR of NAFL was 2.9 (95% CI, 2.0-4.2) for the lower eCRF, 3.5 (95% CI, 2.7-4.6) for the middle eCRF and 1.8 (95% CI, 1.2-2.8) for the upper eCRF, compared to the optimal phenotype. Limited to the sarcopenic obesity phenotype, the adjusted OR of NAFL was 5.9 (95% CI, 4.3-8.2) for the lower eCRF, 4.2 (95% CI, 3.2-5.5) for the middle eCRF and 2.5 (95% CI, 1.5-4.1) for the upper eCRF, compared to the optimal phenotype. The current findings suggest that high eCRF attenuates the individual and synergistic impacts of sarcopenia and obesity on NAFL in Korean adults.

The role of extracellular vesicles in skeletal muscle and systematic adaptation to exercise

Regular exercise has a central role in human health by reducing the risk of type 2 diabetes, obesity, stroke and cancer. How exercise is able to promote such systemic benefits has remained somewhat of a mystery but has been thought to be in part mediated by the release of myokines, skeletal muscle-specific cytokines, in response to exercise. Recent studies have revealed skeletal muscle can also release extracellular vesicles (EVs) into circulation following a bout of exercise. EVs are small membrane-bound vesicles capable of delivering biomolecules to recipient cells and subsequently altering their metabolism. The notion that EVs may have a role in both skeletal muscle and systemic adaptation to exercise has generated a great deal of excitement within a number of different fields including exercise physiology, neuroscience and metabolism. The purpose of this review is to provide an introduction to EV biology and what is currently known about skeletal muscle EVs and their potential role in the response of muscle and other tissues to exercise.

Extracellular transglutaminase 2 induces myotube hypertrophy through G protein-coupled receptor 56

Skeletal muscle secretes biologically active proteins that contribute to muscle hypertrophy in response to either exercise or dietary intake. The identification of skeletal muscle-secreted proteins that induces hypertrophy can provide critical information regarding skeletal muscle health. Dietary provitamin A, β-carotene, induces hypertrophy of the soleus muscle in mice. Here, we hypothesized that skeletal muscle produces hypertrophy-inducible secretory proteins via dietary β-carotene. Knockdown of retinoic acid receptor (RAR) γ inhibited the β-carotene-induced increase soleus muscle mass in mice. Using RNA sequencing, bioinformatic analyses, and literature searching, we predicted transglutaminase 2 (TG2) to be an all-trans retinoic acid (ATRA)-induced secretory protein in cultured C2C12 myotubes. Tg2 mRNA expression increased in ATRA- or β-carotene-stimulated myotubes and in the soleus muscle of β-carotene-treated mice. Knockdown of RARγ inhibited β-carotene-increased mRNA expression of Tg2 in the soleus muscle. ATRA increased endogenous TG2 levels in conditioned medium from myotubes. Extracellular TG2 promoted the phosphorylation of Akt, mechanistic target of rapamycin (mTOR), and ribosomal p70 S6 kinase (p70S6K), and inhibitors of mTOR, phosphatidylinositol 3-kinase, and Src (rapamycin, LY294002, and Src I1, respectively) inhibited TG2-increased phosphorylation of mTOR and p70S6K. Furthermore, extracellular TG2 promoted protein synthesis and hypertrophy in myotubes. TG2 mutant lacking transglutaminase activity exerted the same effects as wild-type TG2. Knockdown of G protein-coupled receptor 56 (GPR56) inhibited the effects of TG2 on mTOR signaling, protein synthesis, and hypertrophy. These results indicated that TG2 expression was upregulated through ATRA-mediated RARγ and that extracellular TG2 induced myotube hypertrophy by activating mTOR signaling-mediated protein synthesis through GPR56, independent of transglutaminase activity.

Multi-omics characterization of a diet-induced obese model of non-alcoholic steatohepatitis

To improve the understanding of the complex biological processes underlying the development of non-alcoholic steatohepatitis (NASH), a multi-omics approach combining bulk RNA-sequencing based transcriptomics, quantitative proteomics and single-cell RNA-sequencing was used to characterize tissue biopsies from histologically validated diet-induced obese (DIO) NASH mice compared to chow-fed controls. Bulk RNA-sequencing and proteomics showed a clear distinction between phenotypes and a good correspondence between mRNA and protein level regulations, apart from specific regulatory events discovered by each technology. Transcriptomics-based gene set enrichment analysis revealed changes associated with key clinical manifestations of NASH, including impaired lipid metabolism, increased extracellular matrix formation/remodeling and pro-inflammatory responses, whereas proteomics-based gene set enrichment analysis pinpointed metabolic pathway perturbations. Integration with single-cell RNA-sequencing data identified key regulated cell types involved in development of NASH demonstrating the cellular heterogeneity and complexity of NASH pathogenesis.

Phosphoproteomic profiling reveals a defined genetic program for osteoblastic lineage commitment of human bone marrow-derived stromal stem cells

Bone marrow-derived mesenchymal stem cells (MSCs) differentiate into osteoblasts upon stimulation by signals present in their niche. Because the global signaling cascades involved in the early phases of MSCs osteoblast (OB) differentiation are not well-defined, we used quantitative mass spectrometry to delineate changes in human MSCs proteome and phosphoproteome during the first 24 h of their OB lineage commitment. The temporal profiles of 6252 proteins and 15,059 phosphorylation sites suggested at least two distinct signaling waves: one peaking within 30 to 60 min after stimulation and a second upsurge after 24 h. In addition to providing a comprehensive view of the proteome and phosphoproteome dynamics during early MSCs differentiation, our analyses identified a key role of serine/threonine protein kinase D1 (PRKD1) in OB commitment. At the onset of OB differentiation, PRKD1 initiates activation of the pro-osteogenic transcription factor RUNX2 by triggering phosphorylation and nuclear exclusion of the histone deacetylase HDAC7.

Effect of resveratrol on adipokines and myokines involved in fat browning: Perspectives in healthy weight against obesity

Obesity is a globally widespread metabolic disorder, characterized by immoderate fat accumulation in the body. There are different types of body fats such as white adipose tissue (WAT), which stores surplus energy in the body, and brown adipose tissue (BAT) which utilize energy to produce heat during metabolism. BAT acts many beneficial functions in metabolic disorders including type 2 diabetes and obesity. Recent studies have investigated methods for promoting the fat browning process of WAT in obesity because of various reasons such as the improvement of insulin resistance, and weight loss. Among natural polyphenolic compounds, resveratrol has been highlighted due to its anti-oxidant and anti-obesity as well as anti-inflammation and anti-cancer properties. Recent studies have paid a lot of attention to that resveratrol may act as a fat browning activator, involved in the secretion of many myokines and adipokines. Here, we reviewed the role of resveratrol in fat browning and also the association between resveratrol and adipokines/myokines in the fat browning process. Our review may provide novel insight into the role of resveratrol in fat browning, leading to the maintenance of a healthy weight against obesity.

Skeletal Muscle-Released Extracellular Vesicles: State of the Art

All cells export part of their intracellular content into the extracellular space through the release of various types of extracellular vesicles (EVs). They are synthetized either from the budding of the plasma membrane [i.e., microparticles (MPs, 150–300 nm size)] or from the late endosomes in which intraluminal vesicles progressively (ILVs) accumulate during their maturation into multivesicular bodies (MVBs). ILVs are then released into the extracellular space through MVB fusion with the plasma membrane [i.e., exosomes (50–100 nm size)]. In the context of metabolic diseases, recent data have highlighted the role of EVs in inflammation associated with pancreas dysfunction, adipose tissue homeostasis, liver steatosis, inflammation, and skeletal muscle (SkM) insulin resistance (IR). Among these insulin-sensitive tissues, SkM is the largest organ in human and is responsible for whole-body glucose disposal and locomotion. Therefore, understanding the contribution of SkM-EVs in the development of diabetes/obesity/dystrophy/,-related diseases is a hot topic. In this review, we have summarized the role of SkM-EVs in muscle physiology and in the development of metabolic diseases and identify important gaps that have to be filled in order to have more precise information on SkM-EVs biological actions and to understand the functions of the different subpopulations of SkM-EVs on the whole-body homeostasis.

Adipose Tissue-Derived Stromal Cells in Matrigel Impacts the Regeneration of Severely Damaged Skeletal Muscles

In case of large injuries of skeletal muscles the pool of endogenous stem cells, i.e., satellite cells, might be not sufficient to secure proper regeneration. Such failure in reconstruction is often associated with loss of muscle mass and excessive formation of connective tissue. Therapies aiming to improve skeletal muscle regeneration and prevent fibrosis may rely on the transplantation of different types of stem cell. Among such cells are adipose tissue-derived stromal cells (ADSCs) which are relatively easy to isolate, culture, and manipulate. Our study aimed to verify applicability of ADSCs in the therapies of severely injured skeletal muscles. We tested whether 3D structures obtained from Matrigel populated with ADSCs and transplanted to regenerating mouse gastrocnemius muscles could improve the regeneration. In addition, ADSCs used in this study were pretreated with myoblasts-conditioned medium or anti-TGFβ antibody, i.e., the factors modifying their ability to proliferate, migrate, or differentiate. Analyses performed one week after injury allowed us to show the impact of 3D cultured control and pretreated ADSCs at muscle mass and structure, as well as fibrosis development immune response of the injured muscle.

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.

Muscle endocrinology and its relation with nutrition

Recent years have demonstrated clear evidence that skeletal muscle is an active endocrine organ. During contraction of muscle fibers, the skeletal muscle produces and releases, into the blood stream, cytokines and other peptides, called myokines, thanks to which it can both communicate with cells locally within the muscle, in an autocrine and paracrine fashion, or with other distant tissues, exerting its endocrine effects. With the progress of sophisticated technologies, the interest towards the skeletal muscle secretome is rapidly grown and the discovery of new myokines represents a prolific field for the identification of new pharmacological approaches for the management and treatment of many clinical diseases. Considering the importance of the muscle proteome and the cross-talk with other organs, the preservation of a skeletal muscle in good health represents a fundamental aspect in life, especially in ageing. Sarcopenia is the age-dependent loss of skeletal muscle mass and strength, bringing to increases of the risk of adverse outcomes, such as physical disability and poor quality of life, as well as alteration of several hormonal networks. For that reasons, the scientific community has risen its interest to find new interventions to prevent and manage the sarcopenia. Adequate nutrition during ages plays a fundamental role in the health and function of the skeletal muscle and it can represents, alone or in combination with physical exercise, a possible preventive measure against sarcopenia. This review will overview the endocrinology of the skeletal muscle, making a focus on food intake as a strategy for preventing skeletal muscle decay.

Impact of Lipotoxicity on Tissue "Cross Talk" and Metabolic Regulation

Obesity-associated comorbidities include non-alcoholic fatty liver disease, Type 2 diabetes, and cardiovascular disease. These diseases are associated with accumulation of lipids in non-adipose tissues, which can impact many intracellular cellular signaling pathways and functions that have been broadly defined as "lipotoxic." This review moves beyond understanding intracellular lipotoxic outcomes and outlines the consequences of lipotoxicity on protein secretion and inter-tissue "cross talk," and the impact this exerts on systemic metabolism.

Sarcopenic obesity in fatty liver

PURPOSE OF REVIEW

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steato hepatitis have an increasing prevalence among liver diseases. Overweight and obesity are frequently associated conditions in patients with fatty liver. Skeletal muscle mass depletion may also coexist with chronic liver disease even in obese patients. This review will focus on the relationship between sarcopenic obesity and fatty liver.

RECENT FINDINGS

Obesity and sarcopenia are frequently encountered in patients with NAFLD. Adipose tissue is able to release molecules (adipokines) that regulate lipid metabolism, interact with insulin sensitivity and may contribute to induce fibrogenesis in the liver. Skeletal muscle tissue is able to secrete myokines regulating muscle metabolism and insulin sensitivity. Myokines perturbation has been reported to influence adipose tissue mass and fat deposition in the liver. Sarcopenia has been reported as independent risk factor for the development of NAFLD, and for a more severe liver fibrosis in patients with NAFLD.

SUMMARY

The interaction between skeletal muscle, adipose tissue and the liver may play a role in the development of NAFLD. Sarcopenia and sarcopenic obesity are risk factors for the development of fatty liver and associated with more severe liver fibrosis. Management is not standardized, but dietary counseling and physical training have been proposed as promising strategies. Bariatric surgery may be considered in patients with severe 'resistant' obesity.

Myonectin deletion promotes adipose fat storage and reduces liver steatosis

We recently described myonectin (also known as erythroferrone) as a novel skeletal muscle-derived myokine with metabolic functions. Here, we use a genetic mouse model to determine myonectin's requirement for metabolic homeostasis. Female myonectin-deficient mice had larger gonadal fat pads and developed mild insulin resistance when fed a high-fat diet (HFD) and had reduced food intake during refeeding after an unfed period but were otherwise indistinguishable from wild-type littermates. Male mice lacking myonectin, however, had reduced physical activity when fed ad libitum and in the postprandial state but not during the unfed period. When stressed with an HFD, myonectin-knockout male mice had significantly elevated VLDL-triglyceride (TG) and strikingly impaired lipid clearance from circulation following an oral lipid load. Fat distribution between adipose and liver was also altered in myonectin-deficient male mice fed an HFD. Greater fat storage resulted in significantly enlarged adipocytes and was associated with increased postprandial lipoprotein lipase activity in adipose tissue. Parallel to this was a striking reduction in liver steatosis due to significantly reduced TG accumulation. Liver metabolite profiling revealed additional significant changes in bile acids and 1-carbon metabolism pathways. Combined, our data affirm the physiologic importance of myonectin in regulating local and systemic lipid metabolism.-Little, H. C., Rodriguez, S., Lei, X., Tan, S. Y., Stewart, A. N., Sahagun, A., Sarver, D. C., Wong, G. W. Myonectin deletion promotes adipose fat storage and reduces liver steatosis.

Iron Metabolism of the Skeletal Muscle and Neurodegeneration

Recent studies clearly indicate that the endocrine function of the skeletal muscle is essential for a long and healthy life. Regular exercise, which has been shown to stimulate the release of myokines, lowers the risk of many diseases, including Alzheimer’s and Parkinson’s disease, emphasizing the role of skeletal muscle in proper functioning of other tissues. In addition, exercise increases insulin sensitivity, which may also impact iron metabolism. Even though the role of iron in neurodegeneration is well established, the exact mechanisms of iron toxicity are not known. Interestingly, exercise has been shown to modulate iron metabolism, mainly by reducing body iron stores. Insulin signaling and iron metabolism are interconnected, as high tissue iron stores are associated with insulin resistance, and conversely, impaired insulin signaling may lead to iron accumulation in an affected tissue. Excess iron accumulation in tissue triggers iron-dependent oxidative stress. Further, iron overload in the skeletal muscle not only negatively affects muscle contractility but also might impact its endocrine function, thus possibly affecting the clinical outcome of diseases, including neurodegenerative diseases. In this review, we discuss possible mechanisms of iron dependent oxidative stress in skeletal muscle, its impact on muscle mass and endocrine function, as well as on neurodegeneration processes.

Sliding window haplotype approaches overcome single SNP analysis limitations in identifying genes for meat tenderness in Nelore cattle

Background

Traditional single nucleotide polymorphism (SNP) genome-wide association analysis (GWAA) can be inefficient because single SNPs provide limited genetic information about genomic regions. On the other hand, using haplotypes in the statistical analysis may increase the extent of linkage disequilibrium (LD) between haplotypes and causal variants and may also potentially capture epistastic interactions between variants within a haplotyped locus, providing an increase in the power and robustness of the association studies. We performed GWAA (413,355 SNP markers) using haplotypes based on variable-sized sliding windows and compared the results to a single-SNP GWAA using Warner-Bratzler shear force measured in the longissimus thorasis muscle of 3161 Nelore bulls to ascertain the optimal window size for identifying the genomic regions that influence meat tenderness.

Results

The GWAA using single SNPs identified eight variants influencing meat tenderness on BTA 3, 4, 9, 10 and 11. However, thirty-three putative meat tenderness QTL were detected on BTA 1, 3, 4, 5, 8, 9, 10, 11, 15, 17, 18, 24, 25, 26 and 29 using variable-sized sliding haplotype windows. Analyses using sliding window haplotypes of 3, 5, 7, 9 and 11 SNPs identified 57, 61, 42, 39, and 21% of all thirty-three putative QTL regions, respectively; however, the analyses using the 3 and 5 SNP haplotypes, cumulatively detected 88% of the putative QTL. The genes associated with variation in meat tenderness participate in myogenesis, neurogenesis, lipid and fatty acid metabolism and skeletal muscle structure or composition processes.

Conclusions

GWAA using haplotypes based on variable-sized sliding windows allowed the detection of more QTL than traditional single-SNP GWAA. Analyses using smaller haplotypes (3 and 5 SNPs) detected a higher proportion of the putative QTL.

Electronic supplementary material

The online version of this article (10.1186/s12863-019-0713-4) contains supplementary material, which is available to authorized users.

Mammalian Target of Rapamycin at the Crossroad Between Alzheimer's Disease and Diabetes

Accumulating evidence suggests that Alzheimer's disease may manifest as a metabolic disorder with pathology and/or dysfunction in numerous tissues. Adults with Alzheimer's disease suffer with significantly more comorbidities than demographically matched Medicare beneficiaries (Zhao et al, BMC Health Serv Res 8:108, 2008b). Reciprocally, comorbid health conditions increase the risk of developing Alzheimer's disease (Haaksma et al, PLoS One 12(5):e0177044, 2017). Type 2 diabetes mellitus is especially notable as the disease shares many overlapping pathologies observed in patients with Alzheimer's disease, including hyperglycemia, hyperinsulinemia, insulin resistance, glucose intolerance, dyslipidemia, inflammation, and cognitive dysfunction, as described in Chap. 8 of this book (Yoshitake et al, Neurology 45(6):1161-1168, 1995; Leibson et al, Am J Epidemiol 145(4):301-308, 1997; Ott et al, Neurology 53(9):1937-1942, 1999; Voisin et al, Rev Med Interne 24(Suppl 3):288s-291s, 2003; Janson et al. Diabetes 53(2):474-481, 2004; Ristow M, J Mol Med (Berl) 82(8):510-529, 2004; Whitmer et al, BMJ 330(7504):1360, 2005, Curr Alzheimer Res 4(2):103-109, 2007; Ohara et al, Neurology 77(12):1126-1134, 2011). Although nondiabetic older adults also experience age-related cognitive decline, diabetes is uniquely associated with a twofold increased risk of Alzheimer's disease, as described in Chap. 2 of this book (Yoshitake et al, Neurology 45(6):1161-1168, 1995; Leibson et al, Am J Epidemiol 145(4):301-308, 1997; Ott et al. Neurology 53(9):1937-1942, 1999; Ohara et al, Neurology 77(12):1126-1134, 2011). Good glycemic control has been shown to improve cognitive status (Cukierman-et al, Diabetes Care 32(2):221-226, 2009), and the use of insulin sensitizers is correlated with a lower rate of cognitive decline in older adults (Morris JK, Burns JM, Curr Neurol Neurosci Rep 12(5):520-527, 2012). At the molecular level, the mechanistic/mammalian target of rapamycin (mTOR) plays a key role in maintaining energy homeostasis. Nutrient availability and cellular stress information, both extracellular and intracellular, are integrated and transduced through mTOR signaling pathways. Aberrant regulation of mTOR occurs in the brains of patients with Alzheimer's disease and in numerous tissues of individuals with type 2 diabetes (Mannaa et al, J Mol Med (Berl) 91(10):1167-1175, 2013). Moreover, modulating mTOR activity with a pharmacological inhibitor, rapamycin, provides wide-ranging health benefits, including healthy life span extension in numerous model organisms (Vellai et al, Nature 426(6967):620, 2003; Jia et al, Development 131(16):3897-3906, 2004; Kapahi et al, Curr Biol 14(10):885-890, 2004; Kaeberlein et al, Science 310(5751):1193-1196, 2005; Powers et al, Genes Dev 20(2):174-184, 2006; Harrison et al, Nature 460(7253):392-395, 2009; Selman et al, Science 326(5949):140-144, 2009; Sharp ZD, Strong R, J Gerontol A Biol Sci Med Sci 65(6):580-589, 2010), which underscores its importance to overall organismal health and longevity. In this chapter, we discuss the physiological role of mTOR signaling and the consequences of mTOR dysregulation in the brain and peripheral tissues, with emphasis on its relevance to the development of Alzheimer's disease and link to type 2 diabetes.

Major determinants of circulating myostatin in polycystic ovary syndrome

The present study was designed to investigate the possible impact of hormonal and demographic parameters of patients with polycystic ovary syndrome (PCOS) on the circulating levels of myostatin. The study cohort comprised 46 patients with PCOS and 42 healthy female controls, and all subjects were of normal weight. Multiple regression analysis was applied to investigate the possible associations between serum myostatin levels and other laboratory parameters. Evaluation of the levels of myostatin revealed no significant differences between the PCOS and control groups (P>0.05). In the control group, no significant correlations were identified between the myostatin levels and any other laboratory parameters. Only low-density-lipoprotein cholesterol (LDL-C) levels in the PCOS group were revealed to be significantly, although negatively, associated with myostatin levels (P=0.018). In the regression model of the PCOS group, an increase in LDL-C and prolactin (PRL) were associated with a decrease in myostatin (P=0.001 and P=0.013, respectively). Furthermore, a decrease in sex hormone-binding globulin (SHBG), fasting blood glucose (FBG) and monocytes were associated with an increase in myostatin (P=0.028, P<0.001 and P=0.026, respectively). An increase in triglycerides was also associated with an increase in myostatin (P=0.001). In the regression model of the control group, a decrease in LDL-C was associated with an increase in myostatin (P=0.003) and a decrease in thyroid-stimulating hormone was associated with a decrease in myostatin (P=0.028). These results indicated that the normal range of myostatin levels in patients with PCOS is regulated by changes in the circulating levels of PRL, LDL-C, SHBG, triglycerides, monocytes and FBG.

The extracellular SEMA domain attenuates intracellular apoptotic signaling of semaphorin 6A in lung cancer cells

Semaphorin 6A (SEMA6A), a membrane-bound protein, is downregulated in lung cancer tissue compared to its adjacent normal tissue. However, the functions of SEMA6A in lung cancer cells are still unclear. In the present study, full length SEMA6A and various truncations were transfected into lung cancer cells to investigate the role of the different domains of SEMA6A in cell proliferation and survival, apoptosis, and in vivo tumor growth. SEMA6A-induced cell signaling was explored using gene silencing, co-immunoprecipitation, and co-culture assays. Our results showed that overexpression of SEMA6A reduced the growth of lung cancer cells in vitro and in vivo, and silencing SEMA6A increased the proliferation of normal lung fibroblasts. Truncated SEMA6A lacking the SEMA domain or the extracellular region induced more apoptosis than full length SEMA6A, and reintroducing the SEMA domain attenuated the apoptosis. Fas-associated protein with death domain (FADD) bound to the cytosolic region of truncated SEMA6A and was involved in SEMA6A-associated cytosol-induced apoptosis. This study suggests a novel function of SEMA6A in inducing apoptosis via FADD binding in lung cancer cells.

Redefining Tissue Crosstalk via Shotgun Proteomic Analyses of Plasma Extracellular Vesicles

Protein signaling between tissues, or tissue cross-talk is becoming recognized as a fundamental biological process that is incompletely understood. Shotgun proteomic analyses of tissues and plasma to explore this concept are regularly challenged by high dynamic range of protein abundance, which limits the identification of lower abundance proteins. In this viewpoint article, it is highlighted how a focus on proteins contained within extracellular vesicles (EVs) not only partially addresses this issue, but can also reveal an underappreciated complexity of the circulating proteome in various physiological and pathological contexts. Furthermore, how quantitative proteomics can inform EV mediated crosstalk is highlighted and the importance of high coverage, sensitive proteomic analyses of EVs to identify both the optimal methods to isolate EV subtypes of interest and proteins that characterize them is stressed.

Effect of poly (l-lactic acid) scaffolds seeded with aligned diaphragmatic myoblasts overexpressing connexin-43 on infarct size and ventricular function in sheep with acute coronary occlusion

Diaphragmatic myoblasts (DM) are stem cells of the diaphragm, a muscle displaying high resistance to stress and exhaustion. We hypothesized that DM modified to overexpress connexin-43 (cx43), seeded on aligned poly (l-lactic acid) (PLLA) sheets would decrease infarct size and improve ventricular function in sheep with acute myocardial infarction (AMI). Sheep with AMI received PLLA sheets without DM (PLLA group), sheets with DM (PLLA-DM group), sheets with DM overexpressing cx43 (PLLA-DMcx43) or no treatment (control group, n = 6 per group). Infarct size (cardiac magnetic resonance) decreased ∼25% in PLLA-DMcx43 [from 8.2 ± 0.6 ml (day 2) to 6.5 ± 0.7 ml (day 45), p < .01, ANOVA-Bonferroni] but not in the other groups. Ejection fraction (EF%) (echocardiography) at 3 days post-AMI fell significantly in all groups. At 45 days, PLLA-DM y PLLA-DMcx43 recovered their EF% to pre-AMI values (PLLA-DM: 61.1 ± 0.5% vs. 58.9 ± 3.3%, p = NS; PLLA-DMcx43: 64.6 ± 2.9% vs. 56.9 ± 2.4%, p = NS), but not in control (56.8 ± 2.0% vs. 43.8 ± 1.1%, p < .01) and PLLA (65.7 ± 2.1% vs. 56.6 ± 4.8%, p < .01). Capillary density was higher (p < .05) in PLLA-DMcx43 group than in the remaining groups. In conclusion, PLLA-DMcx43 reduces infarct size in sheep with AMI. PLLA-DMcx43 and PLLA-DM improve ventricular function similarly. Given its safety and feasibility, this novel approach may prove beneficial in the clinic.

Increased Serpina3n release into circulation during glucocorticoid-mediated muscle atrophy

BACKGROUND

Glucocorticoids (GC) play a major role in muscle atrophy. As skeletal muscle is a secretory organ, characterization of the muscle secretome elicited by muscle atrophy should allow to better understand the cellular mechanisms and to identify circulating biomarkers of this condition. Our project aimed to identify the changes in the muscle secretome associated with GC-induced muscle atrophy and susceptible to translate into circulation.

METHODS

We have identified the GC-induced changes in the secretome of C₂ C₁₂ muscle cells by proteomic analysis, and then, we have determined how these changes translate into the circulation of mice or human subjects exposed to high concentrations of GC.

RESULTS

This approach led us to identify Serpina3n as one of the most markedly secreted protein in response to GC. Our original in vitro results were confirmed in vivo by an increased expression of Serpina3n in skeletal muscle (3.9-fold; P < 0.01) and in the serum (two-fold; P < 0.01) of mice treated with GC. We also observed increased levels of the human orthologue Serpina3 in the serum of Cushing's syndrome patients compared with healthy controls matched for age and sex (n = 9/group, 2.5-fold; P < 0.01). An increase of Serpina3n was also demonstrated in muscle atrophy models mediated by GC such as cancer cachexia (four-fold; P < 0.01), sepsis (12.5-fold; P < 0.001), or diabetes (two-fold; P < 0.01). In contrast, levels of Serpina3n both in skeletal muscle and in the circulation were reduced in several models of muscle hypertrophy induced by myostatin inhibition (P < 0.01). Furthermore, a cluster of data suggests that the regulation of muscle Serpina3n involves mTOR, an essential determinant of the muscle cell size.

CONCLUSIONS

Taken together, these data suggest that Serpina3n may represent a circulating biomarker of muscle atrophy associated to GC and, broadly, a reflection of dynamic changes in muscle mass.

Physical Exercise-Induced Myokines and Muscle-Adipose Tissue Crosstalk: A Review of Current Knowledge and the Implications for Health and Metabolic Diseases

Physical exercise has beneficial effects on metabolic diseases, and a combined therapeutic regimen of regular exercise and pharmaceutical treatment is often recommended for their clinical management. However, the mechanisms by which exercise produces these beneficial effects are not fully understood. Myokines, a group of skeletal muscle (SkM) derived peptides may play an important part in this process. Myokines are produced, expressed and released by muscle fibers under contraction and exert both local and pleiotropic effects. Myokines such as IL-6, IL-10, and IL-1ra released during physical exercise mediate its health benefits. Just as exercise seems to promote the myokine response, physical inactivity seems to impair it, and could be a mechanism to explain the association between sedentary behavior and many chronic diseases. Myokines help configure the immune-metabolic factor interface and the health promoting effects of physical exercise through the release of humoral factors capable of interacting with other tissues, mainly adipose tissue (AT). AT itself secretes proinflammatory cytokines (adipokines) as a result of physical inactivity and it is well recognized that AT inflammation can lead to the development of metabolic diseases, such as type 2 diabetes mellitus (T2DM) and atherosclerosis. On the other hand, the browning phenotype of AT has been suggested to be one of the mechanisms through which physical exercise improves body composition in overweight/obese individuals. Although, many cytokines are involved in the crosstalk between SkM and AT, in respect of these effects, it is IL-6, IL-15, irisin, and myostatin which seem to have the decisive role in this “conversation” between AT and SkM. This review article proposes to bring together the latest “state of the art” knowledge regarding Myokines and muscle-adipose tissue crosstalk. Furthermore, it is intended to particularly focus on the immune-metabolic changes from AT directly mediated by myokines.

Skeletal muscle secretion of IL-6 is muscle type specific: Ex vivo evidence

Emerging evidence indicates that skeletal muscle possesses endocrine function to secret myokines. Interleukin 6 (IL-6) is a well-characterized myokine that is involved in regulation of metabolism and muscle function. Metabolism type and contractile dynamics vary in different muscle types. It is not clear, however, if IL-6 secretion differs in different muscle types. In this study, we first established an ex vivo approach to test the inducible muscle secretion. Freshly isolated muscles were incubated in Krebs solution at 37 °C with oxygen supply. Secreted IL-6 in the incubation media was measure using Western blot and ELISA assay. We first confirmed that the IL-6 release was inducible by treating the incubated muscle with a cytokine stimulant. We demonstrated that physiological temperature (37 °C) and O₂ supply were essential for the induction of IL-6 release from the incubated muscle, suggesting it is a controlled secretion rather than a spontaneous leak. Using this approach, we found that IL-6 release was only inducible from soleus muscle but not EDL muscle. We further showed that IL-6 protein level was higher in slow oxidative muscle fibers. Moreover, we showed that EDL, although lacks of IL-6 release, surely has inducible secretory function that had different secretory pattern from soleus.

Effects of knee loading on obesity-related non-alcoholic fatty liver disease in an ovariectomized mouse model with high-fat diet

AIM

Hormonal and nutritional disorders are the main causes of obesity and non-alcoholic fatty liver disease, especially in the elderly and in postmenopausal women. Although physical activity might alleviate these disorders, the elderly may often have difficulty in carrying out physical exercise. The purpose of this study was to investigate the therapeutic effect of knee loading, a new form of physical stimulation, on the symptoms of obesity and fatty liver.

METHODS

Using ovariectomized mice fed a high-fat diet, we evaluated the effect of knee loading that applies gentle cyclic loads to the knee. Female C57BL/6 mice were divided into five groups: control (SCD), high-fat diet (HF), HF with loading (HF + L), HF with ovariectomy (HF + OVX), and HF + OVX with loading (HF + OVX + L). Except for SCD, mice underwent sham operation or ovariectomy and were maintained on HF diet. After 6 weeks, the mice in the HF + L and HF + OVX + L groups were treated with knee loading for 6 weeks.

RESULTS

Compared to the obesity groups (HF and HF + OVX), knee loading significantly decreased a gain in body weight, liver weight, and white adipose tissue (all P < 0.01). It also reduced the lipid level in the serum (P < 0.01) and histological severity of hepatic steatosis (P < 0.01). Furthermore, knee loading downregulated biomarkers related to endoplasmic reticulum (ER) stress (GRP78, p-eIF2α, and ATF4) and altered biomarkers in autophagy (LC3 and p62).

CONCLUSIONS

Knee loading suppressed obesity-associated metabolic alterations and hepatic steatosis. These effects with knee loading might be associated with suppression of ER stress and promotion of autophagy.

Fractal dimension analysis reveals skeletal muscle disorganization in mdx mice

Duchenne Muscular Dystrophy (DMD) is characterized by muscle extracellular matrix disorganization due to the increased collagen deposition leading to fibrosis that significantly exacerbates disease progression. Fractal dimension analysis is a method that quantifies tissue/cellular disorganization and characterizes complex structures. The first objective of the present study was use fractal analysis to evaluate extracellular matrix disorganization in mdx mice soleus muscle. Next, we mimic a hyper-proliferation of fibrogenic cells by co-culturing NIH3T3 fibroblasts and C2C12 myoblasts to test whether fibroblasts induce disorganization in myoblast arrangement. Here, we show mdx presented high skeletal muscle disorganization as revealed by fractal analysis. Similarly, this method revealed that myoblasts co-cultured with fibroblast also presented cellular arrangement disorganization. We also reanalyzed skeletal muscle microarrays transcriptomic data from mdx and DMD patients that revealed transcripts related to extracellular matrix organization. This analysis also identified Osteoglycin, which was validated as a potential regulator of ECM organization in mdx dystrophic muscles. Our results demonstrate that fractal dimension is useful tool for the analysis of skeletal muscle disorganization in DMD and also reveal a fibroblast-myoblast cross-talk that contributes to "in vitro" myoblast disarrangement.

Lactate increases myotube diameter via activation of MEK/ERK pathway in C2C12 cells

AIM

Lactate is produced in and released from skeletal muscle cells. Lactate receptor, G-protein-coupled receptor 81 (GPR81), is expressed in skeletal muscle cells. However, a physiological role of extracellular lactate on skeletal muscle is not fully clarified. The purpose of this study was to investigate extracellular lactate-associated morphological changes and intracellular signals in C2C12 skeletal muscle cells.

METHODS

Mouse myoblast C2C12 cells were differentiated for 5 days to form myotubes. Sodium lactate (lactate) or GPR81 agonist, 3,5-dihydroxybenzoic acid (3,5-DHBA), was administered to the differentiation medium.

RESULTS

Lactate administration increased the diameter of C2C12 myotubes in a dose-dependent manner. Administration of 3,5-DHBA also increased myotube diameter. Not only lactate but also 3,5-DHBA upregulated the phosphorylation level of mitogen-activated protein kinase kinase 1/2 (MEK1/2), p42/44 extracellular signal-regulated kinase-1/2 (ERK1/2) and p90 ribosomal S6 kinase (p90RSK). MEK inhibitor U0126 depressed the phosphorylation of ERK-p90RSK and increase in myotube diameter induced by lactate. On the other hand, both lactate and 3,5-DHBA failed to induce significant responses in the phosphorylation level of Akt, mammalian target of rapamycin, p70 S6 kinase and protein degradation-related signals.

CONCLUSION

These observations suggest that lactate-associated increase in the diameter of C2C12 myotubes is induced via activation of GRP81-mediated MEK/ERK pathway. Extracellular lactate might have a positive effect on skeletal muscle size.

Multiplex Quantification Identifies Novel Exercise-regulated Myokines/Cytokines in Plasma and in Glycolytic and Oxidative Skeletal Muscle

Exercise is known to confer major health benefits, but the underlying mechanisms are not well understood. The systemic effects of exercise on multi-organ systems are thought to be partly because of myokines/cytokines secreted by skeletal muscle. The extent to which exercise alters cytokine expression and secretion in different muscle fiber types has not been systematically examined. Here, we assessed changes in 66 mouse cytokines in serum, and in glycolytic (plantaris) and oxidative (soleus) muscles, in response to sprint, endurance, or chronic wheel running. Both acute and short-term exercise significantly altered a large fraction of cytokines in both serum and muscle, twenty-three of which are considered novel exercise-regulated myokines. Most of the secreted cytokine receptors profiled were also altered by physical activity, suggesting an exercise-regulated mechanism that modulates the generation of soluble receptors found in circulation. A greater overlap in cytokine profile was seen between endurance and chronic wheel running. Between fiber types, both acute and chronic exercise induced significantly more cytokine changes in oxidative compared with glycolytic muscle. Further, changes in a subset of circulating cytokines were not matched by their changes in muscle, but instead reflected altered expression in liver and adipose tissues. Last, exercise-induced changes in cytokine mRNA and protein were only minimally correlated in soleus and plantaris. In sum, our results indicate that exercise regulates many cytokines whose pleiotropic actions may be linked to positive health outcomes. These data provide a framework to further understand potential crosstalk between skeletal muscle and other organ compartments.

Gut Microbiota Contribute to Age-Related Changes in Skeletal Muscle Size, Composition, and Function: Biological Basis for a Gut-Muscle Axis

Skeletal muscle is a highly plastic tissue that plays a central role in human health and disease. Aging is associated with a decrease in muscle mass and function (sarcopenia) that is associated with a loss of independence and reduced quality of life. Gut microbiota, the bacteria, archaea, viruses, and eukaryotic microbes residing in the gastrointestinal tract are emerging as a potential contributor to age-associated muscle decline. Specifically, advancing age is characterized by a dysbiosis of gut microbiota that is associated with increased intestinal permeability, facilitating the passage of endotoxin and other microbial products (e.g., indoxyl sulfate) into the circulation. Upon entering the circulation, LPS and other microbial factors promote inflammatory signaling and skeletal muscle changes that are hallmarks of the aging muscle phenotype. This review will summarize existing literature suggesting cross-talk between gut microbiota and skeletal muscle health, with emphasis on the significance of this axis for mediating changes in aging skeletal muscle size, composition, and function.

Decorin, a growth hormone-regulated protein in humans

CONTEXT

Growth hormone (GH) stimulates connective tissue and muscle growth, an effect that is potentiated by testosterone. Decorin, a myokine and a connective tissue protein, stimulates connective tissue accretion and muscle hypertrophy. Whether GH and testosterone regulate decorin in humans is not known.

OBJECTIVE

To determine whether decorin is stimulated by GH and testosterone.

DESIGN

Randomized, placebo-controlled, double-blind study.

PARTICIPANTS AND INTERVENTION

96 recreationally trained athletes (63 men, 33 women) received 8 weeks of treatment followed by a 6-week washout period. Men received placebo, GH (2 mg/day), testosterone (250 mg/week) or combination. Women received either placebo or GH (2 mg/day).

MAIN OUTCOME MEASURE

Serum decorin concentration.

RESULTS

GH treatment significantly increased mean serum decorin concentration by 12.7 ± 4.2%; P < 0.01. There was a gender difference in the decorin response to GH, with greater increase in men than in women (∆ 16.5 ± 5.3%; P < 0.05 compared to ∆ 9.4 ± 6.5%; P = 0.16). Testosterone did not significantly change serum decorin. Combined GH and testosterone treatment increased mean decorin concentration by 19.5 ± 3.7% (P < 0.05), a change not significantly different from GH alone.

CONCLUSION

GH significantly increases circulating decorin, an effect greater in men than in women. Decorin is not affected by testosterone. We conclude that GH positively regulates decorin in humans in a gender-dimorphic manner.

Molecular Mechanisms Linking Exercise to Cancer Prevention and Treatment

The benefits of exercise training for cancer patients are becoming increasingly evident. Physical exercise has been shown to reduce cancer incidence and inhibit tumor growth. Here we provide the status of the current molecular understanding of the effect of exercise on cancer. We propose that exercise has a role in controlling cancer progression through a direct effect on tumor-intrinsic factors, interplay with whole-body exercise effects, alleviation of cancer-related adverse events, and improvement of anti-cancer treatment efficacy. These findings have wide-ranging societal implications, as this understanding may lead to changes in cancer treatment strategies.

Skeletal Muscle as an Endocrine Organ: The Role of Myokines in Exercise Adaptations

Exercise stimulates the release of proteins with autocrine, paracrine, or endocrine functions produced in skeletal muscle, termed myokines. Based on the current state of knowledge, the major physiological function of myokines is to protect the functionality and to enhance the exercise capacity of skeletal muscle. Myokines control adaptive processes in skeletal muscle by acting as paracrine regulators of fuel oxidation, hypertrophy, angiogenesis, inflammatory processes, and regulation of the extracellular matrix. Endocrine functions attributed to myokines are involved in body weight regulation, low-grade inflammation, insulin sensitivity, suppression of tumor growth, and improvement of cognitive function. Muscle-derived regulatory RNAs and metabolites, as well as the design of modified myokines, are promising novel directions for treatment of chronic diseases.

Mapping the human skeletal muscle proteome: progress and potential

INTRODUCTION

Human skeletal muscle represents 40% of our body mass and deciphering its proteome composition to further understand mechanisms regulating muscle function under physiological and pathological conditions has proved a challenge. The inter-individual variability, the presence of structurally and functionally different muscle types and the high protein dynamic range require carefully selected methodologies for the assessment of the muscle proteome. Furthermore, physiological studies are understandingly hampered by ethical issues related to biopsies on healthy subjects, making it difficult to recruit matched controls essential for comparative studies. Areas covered: This review critically analyses studies performed on muscle to date and identifies what still remains unknown or poorly investigated in physiological and pathological states, such as training, aging, metabolic disorders and muscular dystrophies. Expert commentary: Efforts should be made on biological fluid analyses targeting low abundant/low molecular weight fragments generated from muscle cell disruption to improve diagnosis and clinical monitoring. From a methodological point of view, particular attention should be paid to improve the characterization of intact proteins and unknown post translational modifications to better understand the molecular mechanisms of muscle disorders.