Biochemical and functional characterization of skeletal muscle cells differentiated from tonsil-derived mesenchymal stem cells

INTRODUCTION/AIMS

Human tonsils are a readily accessible source of stem cells for the potential treatment of skeletal muscle disorders. We reported previously that tonsil-derived mesenchymal stem cells (TMSCs) can differentiate into skeletal muscle cells (SKMCs), which renders TMSCs promising candidates for cell therapy for skeletal muscle disorders. However, the functional properties of the myocytes differentiated from mesenchymal stem cells have not been clearly evaluated. In this study we investigated whether myocytes differentiated from TMSCs (skeletal muscle cells derived from tonsil mesenchymal stem cells [TMSC-SKMCs]) exhibit the functional characteristics of SKMCs.

METHODS

To test the insulin reactivity of TMSC-SKMCs, the expression of glucose transporter 4 (GLUT4) and phosphatidylinositol 3-kinase/Akt was analyzed after the cells were treated for 30 minutes with 100 nmol/L insulin in normal or high-glucose medium. We also examined whether these cells formed a neuromuscular junction (NMJ) when cocultured with motor neurons, and whether they were stimulated by electrical signals using whole-cell patch clamping.

RESULTS

Skeletal muscle cells derived from tonsil mesenchymal stem cells expressed SKMC markers, such as MYOD, MYH3, MYH8, TNNI1, and TTN, at high levels, and exhibited a multinucleated cell morphology and a myotube-like shape. The expression of the acetylcholine receptor and GLUT4 was confirmed in TMSC-SKMCs. In addition, these cells exhibited insulin-mediated glucose uptake, NMJ formation, and transient changes in cell membrane action potential, all of which are representative functions of human SKMCs.

DISCUSSION

Tonsil-derived mesenchymal stem cells can be functionally differentiated into SKMCs and may have potential for clinical application for the treatment of skeletal muscle disorders.

Fibroblast growth factor-21 improves insulin action in nonlactating ewes

During metabolically demanding physiological states, ruminants and other mammals coordinate nutrient use among tissues by varying the set point of insulin action. This set point is regulated in part by metabolic hormones with some antagonizing (e.g., growth hormone and TNFα) and others potentiating (e.g., adiponectin) insulin action. Fibroblast growth factor-21 (FGF21) was recently identified as a sensitizing hormone in rodent and primate models of defective insulin action. FGF21 administration, however, failed to improve insulin action in dairy cows during the naturally occurring insulin resistance of lactation, raising the possibility that ruminants as a class of animals or lactation as a physiological state are unresponsive to FGF21. To start addressing this question, we asked whether FGF21 could improve insulin action in nonlactating ewes. Gene expression studies showed that the ovine FGF21 system resembles that of other species, with liver as the major site of FGF21 expression and adipose tissue as a target tissue based on high expression of the FGF21 receptor complex and activation of p44/42 extracellular signal-regulated kinase (ERK1/2) following exogenous FGF21 administration. FGF21 treatment for 13 days reduced plasma glucose and insulin over the entire treatment period and improved glucose disposal during a glucose tolerance test. FGF21 increased plasma adiponectin by day 3 of treatment but had no effect on the plasma concentrations of total, C16:0-, or C18:0-ceramide. Overall, these data confirm that the insulin-sensitizing effects of FGF21 are conserved in ruminants and raise the possibility that lactation is an FGF21-resistant state.

In Vitro Effects of Emerging Bisphenols on Myocyte Differentiation and Insulin Responsiveness

Bisphenols are endocrine disrupting chemicals to which humans are ubiquitously exposed to. Prenatal bisphenol A exposure can lead to insulin resistance. However, the metabolic effects of other emerging bisphenols, such as bisphenol S (BPS) and bisphenol F (BPF), are less understood. Because the skeletal muscle is the largest of the insulin target tissues, the goal of this study was to evaluate the effects of 2 emerging bisphenols (BPS and BPF) on cytotoxicity, proliferation, myogenic differentiation, and insulin responsiveness in skeletal muscle cells. We tested this using a dose-response approach in C2C12 mouse and L6 rat myoblast cell lines. The results showed that C2C12 mouse myoblasts were more susceptible to bisphenols compared with L6 rat myoblasts. In both cell lines, bisphenol A was more cytotoxic, followed by BPF and BPS. C2C12 myoblast proliferation was higher upon BPF exposure at the 10-4 M dose and the fusion index was increased after exposure to either BPF or BPS at doses over 10-10 M. Exposure to BPS and BPF also reduced baseline expression of p-AKT (Thr) and p-GSK-3β, but not downstream effectors such as mTOR and glucose transporter-4. In conclusion, at noncytotoxic doses, BPS and BPF can alter myoblast cell proliferation, differentiation, and partially modulate early effectors of the insulin receptor signaling pathway. However, BPS or BPF short-term exposure evaluated here does not result in impaired insulin responsiveness.

Insulin-dependent glucose metabolism in dairy cows with variable fat mobilization around calving

Dairy cows undergo significant metabolic and endocrine changes during the transition from pregnancy to lactation, and impaired insulin action influences nutrient partitioning toward the fetus and the mammary gland. Because impaired insulin action during transition is thought to be related to elevated body condition and body fat mobilization, we hypothesized that over-conditioned cows with excessive body fat mobilization around calving may have impaired insulin metabolism compared with cows with low fat mobilization. Nineteen dairy cows were grouped according to their average concentration of total liver fat (LFC) after calving in low [LLFC; LFC <24% total fat/dry matter (DM); n=9] and high (HLFC; LFC >24.4% total fat/DM; n=10) fat-mobilizing cows. Blood samples were taken from wk 7 antepartum (ap) to wk 5 postpartum (pp) to determine plasma concentrations of glucose, insulin, glucagon, and adiponectin. We applied euglycemic-hyperinsulinemic (EGHIC) and hyperglycemic clamps (HGC) in wk 5 ap and wk 3 pp to measure insulin responsiveness in peripheral tissue and pancreatic insulin secretion during the transition period. Before and during the pp EGHIC, [(13)C6] glucose was infused to determine the rate of glucose appearance (GlucRa) and glucose oxidation (GOx). Body condition, back fat thickness, and energy-corrected milk were greater, but energy balance was lower in HLFC than in LLFC. Plasma concentrations of glucose, insulin, glucagon, and adiponectin decreased at calving, and this was followed by an immediate increase of glucagon and adiponectin after calving. Insulin concentrations ap were higher in HLFC than in LLFC cows, but the EGHIC indicated no differences in peripheral insulin responsiveness among cows ap and pp. However, GlucRa and GOx:GlucRa during the pp EGHIC were greater in HLFC than in LLFC cows. During HGC, pancreatic insulin secretion was lower, but the glucose infusion rate was higher pp than ap in both groups. Plasma concentrations of nonesterified fatty acids decreased during HGC and EGHIC, but in both clamps, pp nonesterified fatty acid concentrations did not reach the ap levels. The study demonstrated a minor influence of different degrees of body fat mobilization on insulin metabolism in cows during the transition period. The distinct decrease in the glucose-dependent release of insulin pp is the most striking finding that explains the impaired insulin action after calving, but does not explain differences in body fat mobilization between HLFC and LLFC cows.

Milk-soluble formula increases food intake and reduces Il6 expression in elderly rat hypothalami

Malnutrition in the elderly is accompanied by several metabolic dysfunctions, especially alterations in energy homeostasis regulation and a loss of insulin responsiveness. Nutritional recommendations aim to enrich food with high protein and energy supplements, and protein composition and lipid quality have been widely studied. Despite the numerous studies that have examined attempts to overcome malnutrition in the elderly through such nutritional supplementation, it is still necessary to study the effects of a combination of protein, lipids, and vitamin D (VitD). This can be done in animal models of elderly malnutrition. In the present study, we investigated the effects of several diet formulae on insulin responsiveness, inflammation, and the hypothalamic expression of key genes that are involved in energy homeostasis control. To mimic elderly malnutrition in humans, elderly Wistar rats were food restricted (R, -50%) for 12 weeks and then refed for 4 weeks with one of four different isocaloric diets: a control diet; a diet where milk soluble protein (MSP) replaced casein; a blend of milk fat, rapeseed, and DHA (MRD); or a full formula (FF) diet that combined MSP and a blend of MRD (FF). All of the refeeding diets contained VitD. We concluded that: (i) food restriction led to the upregulation of insulin receptor in liver and adipose tissue accompanied by increased Tnfα in the hypothalamus; (ii) in all of the refed groups, refeeding led to similar body weight gain during the refeeding period; and (iii) refeeding with MSP and MRD diets induced higher food intake on the fourth week of refeeding, and this increase was associated with reduced hypothalamic interleukin 6 expression.

Sphingosine 1-phosphate axis: a new leader actor in skeletal muscle biology

Sphingosine 1-phosphate (S1P) is a bioactive lipid involved in the regulation of biological processes such as proliferation, differentiation, motility, and survival. Here we review the role of S1P in the biology and homeostasis of skeletal muscle. S1P derives from the catabolism of sphingomyelin and is produced by sphingosine phosphorylation catalyzed by sphingosine kinase (SK). S1P can act either intracellularly or extracellularly through specific ligation to its five G protein-coupled receptors (GPCR) named S1P receptors (S1PR). Many experimental findings obtained in the last 20 years demonstrate that S1P and its metabolism play a multifaceted role in the regulation of skeletal muscle regeneration. Indeed, this lipid is known to activate muscle-resident satellite cells, regulating their proliferation and differentiation, as well as mesenchymal progenitors such as mesoangioblasts that originate outside skeletal muscle, both involved in tissue repair following an injury or disease. The molecular mechanism of action of S1P in skeletal muscle cell precursors is highly complex, especially because S1P axis is under the control of a number of growth factors and cytokines, canonical regulators of skeletal muscle biology. Moreover, this lipid is crucially involved in the regulation of skeletal muscle contractile properties, responsiveness to insulin, fatigue resistance and tropism. Overall, on the basis of these findings S1P signaling appears to be an appealing pharmacological target for improving skeletal muscle repair. Nevertheless, further understanding is required on the regulation of S1P downstream signaling pathways and the expression of S1PR. This article will resume our current knowledge on S1P signaling in skeletal muscle, hopefully stimulating further investigation in the field, aimed at individuating novel molecular targets for ameliorating skeletal muscle regeneration and reducing fibrosis of the tissue after a trauma or due to skeletal muscle diseases.

Ginsenoside Re rapidly reverses insulin resistance in muscles of high-fat diet fed rats

OBJECTIVE

In a previous study, it was found that a ginseng berry extract with a high content of the ginsenoside Re normalized blood glucose in ob/ob mice. The objective of this study was to evaluate the effect of the ginsenoside Re on insulin resistance of glucose transport in muscles of rats made insulin resistant with a high-fat diet.

MATERIAL/METHOD

Rats were fed either rat chow or a high-fat diet for 5 weeks. The rats were then euthanized, and insulin stimulated glucose transport activity was measured in epitrochlearis and soleus muscle strips in vitro.

RESULTS

Treatment of muscles with Re alone had no effect on glucose transport. The high-fat diet resulted in ~50% decreases in insulin responsiveness of GLUT4 translocation to the cell surface and glucose transport in epitrochlearis and soleus muscles. Treatment of muscles with Re in vitro for 90 min completely reversed the high-fat diet-induced insulin resistance of glucose transport and GLUT4 translocation. This effect of Re is specific for insulin stimulated glucose transport, as Re treatment did not reverse the high-fat diet-induced resistance of skeletal muscle glucose transport to stimulation by contractions or hypoxia.

CONCLUSIONS

Our results show that the ginsenoside Re induces a remarkably rapid reversal of high-fat diet-induced insulin resistance of muscle glucose transport by reversing the impairment of insulin-stimulated GLUT4 translocation to the cell surface.