Lack of Musashi-2 induces type IIa fiber-dominated muscle atrophy.

Skeletal muscle is a highly plastic tissue, adapting its structure and metabolism in response to diverse conditions such as contractile activity, nutrients, and diseases. Finding a novel master regulator of muscle mass and quality will provide new therapeutic targets for the prevention and treatment of muscle weakness. Musashi is an RNA-binding protein that dynamically regulates protein expression; it was originally discovered as a cell fate determination factor in neural cells. Here, we report that Musashi-2 (Msi2) is dominantly expressed in slow-type muscle fibers, fibers characterized by high metabolism and endurance. Msi2 knockout (KO) mice exhibited a decrease in both soleus myofiber size and number compared to control mice. Biochemical and histological analyses revealed that type IIa fibers, which are of the fast type but have high metabolic capacity, were decreased in Msi2 KO mice. The contraction force of isolated soleus muscle was lower in KO mice, and the expression of the metabolic proteins, cytochrome c oxidase and myoglobin, was also decreased in KO muscle. Our data demonstrate the critical role of Msi2 in the maintenance of normal fiber-type composition and metabolism.

PDGF-B secreted from skeletal muscle enhances myoblast proliferation and myotube maturation via activation of the PDGFR signaling cascade.

Myokines, secreted factors from skeletal muscle, act locally on muscle cells or satellite cells, which is important in regulating muscle mass and function. Here, we found platelet-derived growth factor subunit B (PDGF-B) is constitutively secreted from muscle cells without muscle contraction. Furthermore, PDGF-B secretion increased with myoblast to myotube differentiation. To examine the role of PDGF-B as a paracrine or autocrine myokine, myoblasts or myotubes were treated with PDGF-B. As a result, myoblast proliferation was significantly enhanced via several signaling pathways. Intriguingly, myotubes treated with PDGF-B showed enhanced maturation as indicated by their increased myotube diameter, myosin heavy chain expression, and strengthened contractile force. These findings suggest that PDGF-B is constitutively secreted by myokines to enhance myoblast proliferation and myotube maturation, which may contribute to skeletal muscle regeneration.

Mass spectrometry imaging reveals local metabolic changes in skeletal muscle due to chronic training.

Muscle atrophy is a major health problem that needs effective prevention and treatment approaches. Chronic exercise, an effective treatment strategy for atrophy, promotes muscle hypertrophy, which leads to dynamic metabolic changes; however, the metabolic changes vary among myofiber types. To investigate local metabolic changes due to chronic exercise, we utilized comprehensive proteome and mass spectrometry (MS) imaging analyses. Our training model exhibited hypertrophic features only in glycolytic myofibers. The proteome analyses demonstrated that exercise promoted anabolic pathways, such as protein synthesis, and significant changes in lipid metabolism, but not in glucose metabolism. Furthermore, the fundamental energy sources, glycogen, neutral lipids, and ATP, were sensitive to exercise, and the changes in these sources differed between glycolytic and oxidative myofibers. MS imaging revealed that the lipid composition differs among myofibers; arachidonic acid might be an effective target for promoting lipid metabolism during muscle hypertrophy in oxidative myofibers.

R3hdml regulates satellite cell proliferation and differentiation.

In this study, we identified a previously uncharacterized skeletal satellite cell-secreted protein, R3h domain containing-like (R3hdml). Expression of R3hdml increases during skeletal muscle development and differentiation in mice. Body weight and skeletal muscle mass of R3hdml knockout (KO) mice are lower compared to control mice. Expression levels of cell cycle-related markers, phosphorylation of Akt, and expression of insulin-like growth factor within the skeletal muscle are reduced in R3hdml KO mice compared to control mice. Expression of R3hdml increases during muscle regeneration in response to cardiotoxin (CTX)-induced muscle injury. Recovery of handgrip strength after CTX injection was significantly impaired in R3hdml KO mice, which is rescued by R3hdml. Our results indicate that R3hdml is required for skeletal muscle development, regeneration, and, in particular, satellite cell proliferation and differentiation.

A new in vitro muscle contraction model and its application for analysis of mTORC1 signaling in combination with contraction and beta-hydroxy-beta-methylbutyrate administration.

Several food constituents augment exercise-induced muscle strength improvement; however, the detailed mechanism underlying these combined effects is unknown because of the lack of a cultured cell model for evaluating the contraction-induced muscle protein synthesis level. Here, we aimed to establish a new in vitro muscle contraction model for analyzing the activation of mammalian target of rapamycin complex 1 (mTORC1) signaling. We adopted the tetanic electric stimulation of 50 V at 100 Hz for 10 min in L6.C11 myotubes. Akt, ERK1/2, and p70S6K phosphorylation increased significantly after electrical pulse stimulation (EPS), compared to untreated cells. Next, we used this model to analyze mTORC1 signaling in combination with exercise and beta-hydroxy-beta-methylbutyrate (HMB), an l-leucine metabolite. p70S6K phosphorylation increased significantly in the EPS+HMB group compared to that in the EPS-alone group. These findings show that our model could be used to analyze mTORC1 signaling and that HMB enhances muscle contraction-activated mTORC1 signaling.

Dammarane-type triterpene extracts of Panax notoginseng root ameliorates hyperglycemia and insulin sensitivity by enhancing glucose uptake in skeletal muscle.

Skeletal muscle is an important organ for controlling the development of type 2 diabetes. We discovered Panax notoginseng roots as a candidate to improve hyperglycemia through in vitro muscle cells screening test. Saponins are considered as the active ingredients of ginseng. However, in the body, saponins are converted to dammarane-type triterpenes, which may account for the anti-hyperglycemic activity. We developed a method for producing a dammarane-type triterpene extract (DTE) from Panax notoginseng roots and investigated the extract's potential anti-hyperglycemic activity. We found that DTE had stronger suppressive activity on blood glucose levels than the saponin extract (SE) did in KK-Ay mice. Additionally, DTE improved oral glucose tolerance, insulin sensitivity, glucose uptake, and Akt phosphorylation in skeletal muscle. These results suggest that DTE is a promising agent for controlling hyperglycemia by enhancing glucose uptake in skeletal muscle.

Evaluation of an in vitro muscle contraction model in mouse primary cultured myotubes.

To construct an in vitro contraction model with the primary cultured myotubes, we isolated satellite cells from the mouse extensor digitorum longus. Differentiated myotubes possessed a greater number of sarcomere assemblies and higher expression levels of myosin heavy chain, cytochrome c oxidase IV, and myoglobin than in C2C12 myotubes. In agreement with these results regarding the sarcomere assemblies and protein expressions, the primary myotubes showed higher contractile activity stimulated by the electric pulses than that in the C2C12 myotubes. These data suggest that mouse primary myotubes will be a valuable research tool as an in vitro muscle contraction model.

A fragmented form of annexin A1 is secreted from C2C12 myotubes by electric pulse-induced contraction.

The main function of annexin A1 (ANXA1), a member of the annexin superfamily, is to bind to cellular membranes in a Ca(2+)-dependent manner. In skeletal muscle, ANXA1 is thought to be involved in the repair of damaged membrane tissue and in the migration of muscle cells. We hypothesized that ANXA1 is one of the myokines secreted during muscle contractions to accelerate the repair of cell damage after contraction. Here we performed cell contractions by electric pulse stimulation; the results revealed that a fragmented form of ANXA1 was cleaved by calpain and selectively secreted from skeletal muscle cells by contraction. We therefore realized that muscle-contraction-induced calpain-dependent ANXA1 fragmentation has a wound-healing effect on damaged cells. This suggested that not the intact form but rather fragmented ANXA1 is a contraction-induced myokine.

An improved glucose transport assay system for isolated mouse skeletal muscle tissues.

There is a growing demand for a system in the field of sarcopenia and diabetes research that could be used to evaluate the effects of functional food ingredients that enhance muscle mass/contractile force or muscle glucose uptake. In this study, we developed a new type of in vitro muscle incubation system that systemizes an apparatus for muscle incubation, using an electrode, a transducer, an incubator, and a pulse generator in a compact design. The new system enables us to analyze the muscle force stimulated by the electric pulses and glucose uptake during contraction and it may thus be a useful tool for analyzing the metabolic changes that occur during muscle contraction. The system may also contribute to the assessments of new food ingredients that act directly on skeletal muscle in the treatment of sarcopenia and diabetes.

Imaging mass spectrometry reveals fiber-specific distribution of acetylcarnitine and contraction-induced carnitine dynamics in rat skeletal muscles.

Carnitine is well recognized as a key regulator of long-chain fatty acyl group translocation into the mitochondria. In addition, carnitine, as acetylcarnitine, acts as an acceptor of excess acetyl-CoA, a potent inhibitor of pyruvate dehydrogenase. Here, we provide a new methodology for accurate quantification of acetylcarnitine content and determination of its localization in skeletal muscles. We used matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) to visualize acetylcarnitine distribution in rat skeletal muscles. MALDI-IMS and immunohistochemistry of serial cross-sections showed that acetylcarnitine was enriched in the slow-type muscle fibers. The concentration of ATP was lower in muscle regions with abundant acetylcarnitine, suggesting a relationship between acetylcarnitine and metabolic activity. Using our novel method, we detected an increase in acetylcarnitine content after muscle contraction. Importantly, this increase was not detected using traditional biochemical assays of homogenized muscles. We also demonstrated that acetylation of carnitine during muscle contraction was concomitant with glycogen depletion. Our methodology would be useful for the quantification of acetylcarnitine and its contraction-induced kinetics in skeletal muscles.

Macrophage migration inhibitory factor diminishes muscle glucose transport induced by insulin and AICAR in a muscle type-dependent manner.

Skeletal muscle is a primary organ that uses blood glucose. Insulin- and 5'AMP-activated protein kinase (AMPK)-regulated intracellular signaling pathways are known as major mechanisms that regulate muscle glucose transport. It has been reported that macrophage migration inhibitory factor (MIF) is secreted from adipose tissue and heart, and affects these two pathways. In this study, we examined whether MIF is a myokine that is secreted from skeletal muscles and affects muscle glucose transport induced by these two pathways. We found that MIF is expressed in several different types of skeletal muscle. Its secretion was also confirmed in C2C12 myotubes, a skeletal muscle cell line. Next, the extensor digitorum longus (EDL) and soleus muscles were isolated from mice and treated with recombinant MIF in an in vitro muscle incubation system. MIF itself did not have any effect on glucose transport in both types of muscles. However, glucose transport induced by a submaximal dose of insulin was diminished by co-incubation with MIF in the soleus muscle. MIF also diminished glucose transport induced by a maximal dose of 5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR), an AMPK activator, in the EDL muscle. These results suggest that MIF is a negative regulator of insulin- and AICAR-induced glucose transport in skeletal muscle. Since MIF secretion from C2C12 myotubes to the culture medium decreased during contraction evoked by electrical stimulations, MIF may be involved in the mechanisms underlying exercise-induced sensitization of glucose transport in skeletal muscle.

Chronic exercise enhances insulin secretion ability of pancreatic islets without change in insulin content in non-diabetic rats.

We evaluated the effect of chronic exercise on insulin secretion in response to high-glucose by using a perifusion method with isolated pancreatic islets from normal rats. Male Wistar rats were assigned to one of two groups: a sedentary group and a trained group. Running exercise was carried out on a treadmill for one hour per day, five days per week, for six, nine, or 12 weeks. The chronic exercise significantly enhanced the insulin secretion ability of pancreatic islets in response to the high-glucose stimulation upon nine and 12 weeks of exercise. The insulin content in the pancreas and the weight of the pancreas did not change upon nine weeks of exercise. Potassium-stimulated insulin secretion was also increased in the islets isolated from rats that trained for nine weeks compared with that in sedentary rats, suggesting that insulin secretion events downstream of membrane depolarization are involved in targets of the exercise effect. These findings suggest that chronic exercise could be a useful strategy not only for the maintenance of peripheral insulin sensitivity but also for the promotion of islet function to secrete insulin in non-diabetics.

Stable isotope-labeled carnitine reveals its rapid transport into muscle cells and acetylation during contraction.

Carnitine plays multiple roles in skeletal muscle metabolism, including fatty acid transport and buffering of excess acetyl-CoA in the mitochondria. The skeletal muscle cannot synthesize carnitine; therefore, carnitine must be taken up from the blood into the cytoplasm. Carnitine metabolism, its uptake into cells, and the subsequent reactions of carnitine are accelerated by muscle contraction. Isotope tracing enables the marking of target molecules and monitoring of tissue distribution. In this study, stable isotope-labeled carnitine tracing was combined with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging to determine carnitine distribution in mouse skeletal muscle tissues. Deuterium-labeled carnitine (d3-carnitine) was intravenously injected into the mice and diffused to the skeletal muscles for 30 and 60 min. To examine whether muscle contraction changes the distribution of carnitine and its derivatives, unilateral in situ muscle contraction was performed; 60 min muscle contraction showed increased d3-carnitine and its derivative d3-acetylcarnitine in the muscle, indicating that carnitine uptake in cells is promptly converted to acetylcarnitine, consequently, buffering accumulated acetyl-CoA. While the endogenous carnitine was localized in the slow type fibers rather than fast type, the contraction-induced distributions of d3-carnitine and acetylcarnitine were not necessarily associated with muscle fiber type. In conclusion, the combination of isotope tracing and MALDI-MS imaging can reveal carnitine flux during muscle contraction and show the significance of carnitine in skeletal muscles.

Visualization of dynamic change in contraction-induced lipid composition in mouse skeletal muscle by matrix-assisted laser desorption/ionization imaging mass spectrometry.

Lipids in skeletal muscle play a fundamental role both in normal muscle metabolism and in disease states. Skeletal muscle lipid accumulation is associated with several chronic metabolic disorders, including obesity, insulin resistance, and type 2 diabetes. However, it is poorly understood whether the lipid composition of skeletal muscle changes by contraction, due to the complexity of lipid molecular species. In this study, we used matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) to investigate changes in skeletal muscle lipid composition induced by contraction. We successfully observed the reduction of diacylglycerol and triacylglycerol, which are generally associated with muscle contraction. Interestingly, we found the accumulation of some saturated and mono-unsaturated fatty acids and poly-unsaturated fatty acids containing phosphatidylcholine in contracted muscles. Moreover, the distributions of several types of lipid were changed by contraction. Our results show that changes in the lipid amount, lipid composition, and energy metabolic activity can be evaluated in each local spot of cells and tissues at the same time using MALDI-IMS. In conclusion, MALDI-IMS is a powerful tool for studying lipid changes associated with contractions.

Establishment of a system evaluating the contractile force of electrically stimulated myotubes from wrinkles formed on elastic substrate.

Muscle weakness is detrimental not only to quality of life but also life expectancy. However, effective drugs have still not been developed to improve and prevent muscle weakness associated with aging or diseases. One reason for the delay in drug discovery is that no suitable in vitro screening system has been established to test whether drugs improve muscle strength. Here, we used a specific deformable silicone gel substrate to effectively and sensitively evaluate the contractile force generated by myotubes from wrinkles formed on the substrate. Using this system, it was found that the contractile force generated by an atrophic phenotype of myotubes induced by dexamethasone or cancer cell-conditioned medium treatment significantly decreased while that generated by hypertrophic myotubes induced by insulin-like growth factor-1 significantly increased. Notably, it was found that changes in the index related to contractile force can detect atrophic or hypertrophic phenotypes more sensitively than changes in myotube diameter or myosin heavy chain expression, both commonly used to evaluate myotube function. These results suggest that our proposed system will be an effective tool for assessing the contractile force-related state of myotubes, which are available for the development of drugs to prevent and/or treat muscle weakness.

R-spondin3 is a myokine that differentiates myoblasts to type I fibres.

Muscle fibres are broadly categorised into types I and II; the fibre-type ratio determines the contractile and metabolic properties of skeletal muscle tissue. The maintenance of type I fibres is essential for the prevention of obesity and the treatment of muscle atrophy caused by type 2 diabetes or unloading. Some reports suggest that myokines are related to muscle fibre type determination. We thus explored whether a myokine determines whether satellite cells differentiate to type I fibres. By examining the fibre types separately, we identified R-spondin 3 (Rspo3) as a myokine of interest, a secreted protein known as an activator of Wnt signalling pathways. To examine whether Rspo3 induces type I fibres, primary myoblasts prepared from mouse soleus muscles were exposed to a differentiation medium containing the mouse recombinant Rspo3 protein. Expression of myosin heavy chain (MyHC) I, a marker of type I fibre, significantly increased in the differentiated myotubes compared with a control. The Wnt/ß-catenin pathway was shown to be the dominant signalling pathway which induces Rspo3-induced MyHC I expression. These results revealed Rspo3 as a myokine that determines whether satellite cells differentiate to type I fibres.

Excess Glucose Impedes the Proliferation of Skeletal Muscle Satellite Cells Under Adherent Culture Conditions.

Glucose is a major energy source consumed by proliferating mammalian cells. Therefore, in general, proliferating cells have the preference of high glucose contents in extracellular environment. Here, we showed that high glucose concentrations impede the proliferation of satellite cells, which are muscle-specific stem cells, under adherent culture conditions. We found that the proliferation activity of satellite cells was higher in glucose-free DMEM growth medium (low-glucose medium with a glucose concentration of 2 mM) than in standard glucose DMEM (high-glucose medium with a glucose concentration of 19 mM). Satellite cells cultured in the high-glucose medium showed a decreased population of reserve cells, identified by staining for Pax7 expression, suggesting that glucose concentration affects cell fate determination. In conclusion, glucose is a factor that decides the cell fate of skeletal muscle-specific stem cells. Due to this unique feature of satellite cells, hyperglycemia may negatively affect the regenerative capability of skeletal muscle myofibers and thus facilitate sarcopenia.

Effect of antioxidant supplementation on skeletal muscle and metabolic profile in aging mice.

Aging induces drastic changes in muscle mass and function (sarcopenia); however, the detailed mechanisms underlying sarcopenia remain poorly understood. Recent studies suggested that age-related increases in oxidative stress induce muscle atrophy. In this study, we investigated the effect of 6-month supplementation of antioxidants, specifically piceatannol (PIC) and enzymatically modified isoquercitrin (EMIQ), on age-related physiological changes, including skeletal muscle weight and quality, in 25-month-old (OLD) mice, compared to in 4-month-old (young, YNG) C57BL/6J mice. Muscle weight corrected by body weight significantly declined in OLD mice, compared to in YNG mice. The control OLD mice also showed changes in the expression of genes related to muscle fiber type, reduced locomotor activity, and increased oxidative stress markers in blood. Consistent with the muscle weight and quality changes, whole-body fat oxidation during sedentary conditions and exercise periods in control OLD mice was significantly lower than that in YNG mice. Interestingly, compared to the control OLD mice, the PIC- or EMIQ-fed OLD mice showed higher fat oxidation. Furthermore, EMIQ, but not PIC, increased locomotor activity, the expression of genes encoding antioxidant enzymes, and suppressed the carbonylated protein in the skeletal muscle of OLD mice. These results suggested that chronic antioxidant intake could alleviate aging-related muscle function changes.

Effect of treatment with conditioned media derived from C2C12 myotube on adipogenesis and lipolysis in 3T3-L1 adipocytes.

Regular exercise is an effective strategy that is used to prevent and treat obesity as well as type 2 diabetes. Exercise-induced myokine secretion is considered a mechanism that coordinates communication between muscles and other organs. In order to examine the possibility of novel communications from muscle to adipose tissue mediated by myokines, we treated 3T3-L1 adipocytes with C2C12 myotube electrical pulse stimulation-conditioned media (EPS-CM), using a C2C12 myotube contraction system stimulated by an electrical pulse. Continuous treatment with myotube EPS-CM promoted adipogenesis of 3T3-L1 pre-adipocytes via the upregulation of the peroxisome proliferator-activated receptor-gamma (PPARγ) 2 and PPARγ-regulated gene expression. Furthermore, our results revealed that myotube EPS-CM induces lipolysis and secretion of adiponectin in mature adipocytes. EPS-CM obtained from a C2C12 myoblast culture did not induce such changes in these genes, suggesting that contraction-induced myokine(s) secretion occurs particularly in differentiated myotubes. Thus, contraction-induced secretion of myokine(s) promotes adipogenesis and lipid metabolism in 3T3-L1 adipocytes. These findings suggest the possibility that skeletal muscle communicates to adipose tissues during exercise, probably by the intermediary of unidentified myokines.

Trans-omic Analysis Reveals ROS-Dependent Pentose Phosphate Pathway Activation after High-Frequency Electrical Stimulation in C2C12 Myotubes.

Skeletal muscle adaptation is mediated by cooperative regulation of metabolism, signal transduction, and gene expression. However, the global regulatory mechanism remains unclear. To address this issue, we performed electrical pulse stimulation (EPS) in differentiated C2C12 myotubes at low and high frequency, carried out metabolome and transcriptome analyses, and investigated phosphorylation status of signaling molecules. EPS triggered extensive and specific changes in metabolites, signaling phosphorylation, and gene expression during and after EPS in a frequency-dependent manner. We constructed trans-omic network by integrating these data and found selective activation of the pentose phosphate pathway including metabolites, upstream signaling molecules, and gene expression of metabolic enzymes after high-frequency EPS. We experimentally validated that activation of these molecules after high-frequency EPS was dependent on reactive oxygen species (ROS). Thus, the trans-omic analysis revealed ROS-dependent activation in signal transduction, metabolome, and transcriptome after high-frequency EPS in C2C12 myotubes, shedding light on possible mechanisms of muscle adaptation.