Human Skeletal Muscle Cell Cultures Research Articles

Small extracellular vesicle microRNAs in pediatric myasthenia gravis plasma and skeletal muscle.

The diagnosis of myasthenia gravis (MG) in children remains difficult. Circulating small extracellular vesicle (sEV)-derived miRNAs (sEV-miRNAs) have been recognized as biomarkers of various diseases and can be excreted by different cell types. These biomarker candidates also play a vital role in autoimmune diseases via intercellular communication. In the present study, we used sEV isolation and purification methods to extract the plasma-derived sEV-miRNAs from children with MG and healthy controls. A small RNA sequencing analysis confirmed the miRNA expression features in plasma-derived sEVs from MG patients. The miRNA expression analysis in vitro was determined using microarray analysis. The enrichment and network analyses of altered sEV-miRNAs were performed using miRNA databases and Database for Annotation, Visualization, and Integrated Discovery website. Quantitative real-time polymerase chain reaction was performed for validation of sEV-miRNA. The diagnostic power of altered sEV-miRNAs was evaluated using receiver operating characteristic curve analyses. Twenty-four sEV-miRNAs with altered expression level were identified between groups by DESeq2 method. The miRNAs were extracted from the sEVs, which were isolated from human primary skeletal muscle cell culture treated with mAb198. The target genes and enriched pathways of sEV-miRNAs partially overlapped between cell supernatant and plasma samples. The significantly downregulated miR-143-3p was validated in quantitative real-time polymerase chain reaction analysis. For the first time, we report that plasma-derived sEV-miRNAs may act as novel circulating biomarkers and therapeutic targets in pediatric MG.

The effect of metformin on autophagy by LC3 expression in Type 2 Diabetes Mellitus (T2DM) human skeletal muscle cell culture

Background: Skeletal muscle is an essential tissue in glucose metabolism. Reduced autophagic capacity to remove damaged contractile proteins in skeletal muscle cells will contribute to the loss of muscle mass. Metformin is the first-line agent for treating type 2 diabetes mellitus (T2DM) patients. This study aims to investigate the effects of metformin on autophagy through LC3 expression in human skeletal muscle cell culture (SkMC). Methods: The T2DM human SkMC was obtained from T2DM treatment naïve patients, purchased from AcceGen Biotech®. Fully differentiated myotubes were randomized into the control and treatment groups. The treatment group was given Metformin in three doses (1 mM, 2 mM, and 3 mM). An immunoblotting assay of AMPKα and LC3 was performed using electrochemiluminescence (ECL). The quantitative expression of AMPKα and LC3 were measured at baseline, after 24-hour, 48-hour, and 72-hour. Data were analyzed using SPSS version 22 for Windows.Results: AMPKα and LC3 expression were higher in the treatment group compared to the control group. The levels of AMPKα and LC3 expression in the treatment group increased dose-dependent. Linear regression analysis demonstrated a significant correlation between metformin administration and LC3 expression levels (p<0.0001). Conclusion: Metformin administration on T2DM human SkMC resulted in increased autophagic activity, marked by increased LC3 expression.

β-Hydroxybutyrate is reduced in humans with obesity-related NAFLD and displays a dose-dependent effect on skeletal muscle mitochondrial respiration in vitro.

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic fat accumulation and impaired insulin sensitivity. Reduced hepatic ketogenesis may promote these pathologies, but data are inconclusive in humans and the link between NAFLD and reduced insulin sensitivity remains obscure. We investigated individuals with obesity-related NAFLD and hypothesized that β-hydroxybutyrate (βOHB; the predominant ketone species) would be reduced and related to hepatic fat accumulation and insulin sensitivity. Furthermore, we hypothesized that ketones would impact skeletal muscle mitochondrial respiration in vitro. Hepatic fat was assessed by 1H-MRS in 22 participants in a parallel design, case control study [Control: n = 7, age 50 ± 6 yr, body mass index (BMI) 30 ± 1 kg/m2; NAFLD: n = 15, age 57 ± 3 yr, BMI 35 ± 1 kg/m2]. Plasma assessments were conducted in the fasted state. Whole body insulin sensitivity was determined by the gold-standard hyperinsulinemic-euglycemic clamp. The effect of ketone dose (0.5-5.0 mM) on mitochondrial respiration was conducted in human skeletal muscle cell culture. Fasting βOHB, a surrogate measure of hepatic ketogenesis, was reduced in NAFLD (-15.6%, P < 0.01) and correlated negatively with liver fat (r2 = 0.21, P = 0.03) and positively with insulin sensitivity (r2 = 0.30, P = 0.01). Skeletal muscle mitochondrial oxygen consumption increased with low-dose ketones, attributable to increases in basal respiration (135%, P < 0.05) and ATP-linked oxygen consumption (136%, P < 0.05). NAFLD pathophysiology includes impaired hepatic ketogenesis, which is associated with hepatic fat accumulation and impaired insulin sensitivity. This reduced capacity to produce ketones may be a potential link between NAFLD and NAFLD-associated reductions in whole body insulin sensitivity, whereby ketone concentrations impact skeletal muscle mitochondrial respiration.

Nitrate uptake and metabolism in human skeletal muscle cell cultures

Several studies show that dietary nitrate enhances exercise performance, presumably by increasing muscle blood flow and improving oxygen utilization. These effects are likely mediated by nitrate metabolites, including nitrite and nitric oxide (NO). However, the mechanisms of nitrate production, storage, and metabolism to nitrite and NO in skeletal muscle cells are still unclear. We hypothesized that exogenous nitrate can be taken up and metabolized to nitrite/NO inside the skeletal muscle. We found rapid uptake of exogeneous nitrate in both myoblasts and myotubes, increasing nitrite levels in myotubes, but not myoblasts. During differentiation we found increased expression of molybdenum containing proteins, such as xanthine oxidoreductase (XOR) and the mitochondrial amidoxime-reducing component (MARC); nitrate and nitrite reductases. Sialin, a known nitrate transporter, was detected in myoblasts; nitrate uptake decreased after sialin knockdown. Inhibition of chloride channel 1 (CLC1) also led to significantly decreased uptake of nitrate. Addition of exogenous nitrite, which resulted in higher intracellular nitrite levels, increased intracellular cGMP levels in myotubes. In summary, our results demonstrate for the first time the presence of the nitrate/nitrite/NO pathway in skeletal muscle cells, namely the existence of strong uptake of exogenous nitrate into cells and conversion of intracellular nitrate to nitrite and NO. Our results further support our previously formulated hypothesis about the importance of the nitrate to nitrite to NO intrinsic reduction pathways in skeletal muscles, which likely contributes to improved exercise tolerance after nitrate ingestion.

Pharmacological activation of AMPK and glucose uptake in cultured human skeletal muscle cells from patients with ME/CFS.

Skeletal muscle fatigue and post-exertional malaise are key symptoms of myalgic encephalomyelitis (ME)/chronic fatigue syndrome (ME/CFS). We have previously shown that AMP-activated protein kinase (AMPK) activation and glucose uptake are impaired in primary human skeletal muscle cell cultures derived from patients with ME/CFS in response to electrical pulse stimulation (EPS), a method which induces contraction of muscle cells in vitro. The aim of the present study was to assess if AMPK could be activated pharmacologically in ME/CFS. Primary skeletal muscle cell cultures from patients with ME/CFS and healthy controls were treated with either metformin or compound 991. AMPK activation was assessed by Western blot and glucose uptake measured. Both metformin and 991 treatment significantly increased AMPK activation and glucose uptake in muscle cell cultures from both controls and ME/CFS. Cellular ATP content was unaffected by treatment although ATP content was significantly decreased in ME/CFS compared with controls. Pharmacological activation of AMPK can improve glucose uptake in muscle cell cultures from patients with ME/CFS. This suggests that the failure of EPS to activate AMPK in these muscle cultures is due to a defect proximal to AMPK. Further work is required to delineate the defect and determine whether pharmacological activation of AMPK improves muscle function in patients with ME/CFS.

Effects of Roux‐en‐Y Gastric Bypass Surgery on Mitochondrial Quality Control Proteins in Human Myotubes Derived from Severely Obese Humans

Impaired skeletal muscle glucose oxidation in response to insulin is a hallmark characteristic of humans with severe obesity and Type 2 Diabetes. Roux‐en‐Y gastric bypass (RYGB) surgery is an effective treatment of these metabolic diseases by markedly improving metabolic health, including enhanced glucose oxidation and insulin action in skeletal muscle. However, the underlying mechanisms responsible for these improvements are not fully understood. Skeletal muscle mitochondria are critical in preserving cellular health, but exhibit defective functions in severely obese humans. Recent studies have suggested that dysregulated mitochondrial dynamics are associated with mitochondrial dysfunction and insulin resistance. This study aims to determine the effects of severe obesity and RYGB surgery on mitochondrial dynamic proteins and its association with insulin sensitivity and glucose oxidation in human skeletal muscle. Human skeletal muscle cell culture model was utilized by isolating human skeletal muscle cells from muscle biopsies obtained from lean subjects (n=6, BMI = 22.7 ± 1.2 kg/m2, HOMA‐IR = 1.9 ± 0.4) and RYGB patients prior to, 1‐month and 7‐months following surgery (n=6, BMI = 47.3 ± 2.8 kg/m2, 41.0 ± 2.9 kg/m2, 33.7 ± 2.2 kg/m2, HOMA‐IR = 3.4 ± 0.4, 2.9 ± 0.9, 1.7 ± 0.2, respectively). Human skeletal muscle cells were differentiated to myotubes. On day 7 of differentiation, myotubes were harvested for immunoblot analysis in order to assess the expressions of mitochondria dynamics proteins. Myotubes derived from severely obese patients had a 35% increase in mitochondrial fusion protein Optic Atrophy 1 (Opa1) expression before surgery when compared to lean controls (p &lt;0.05). In addition, there was a trend towards a decrease of fusion proteins Opa1 and Mitofusin 1 (Mfn1) expressions in myotubes derived from the same patients at 7‐months following surgery (26% and 18%, p=0.065, respectively) in comparison to pre‐surgery state, and virtually no difference from lean controls. Furthermore, Opa1 expression negatively correlated with insulin‐stimulated glucose oxidation in myotubes (r = −0.614, p &lt;0.05), while Mfn1 is positively correlated with HOMA‐IR (r = 0.495, p &lt;0.05). Mitochondrial fission and mitophagy proteins observed no differences between lean, pre‐surgery, 1‐month and 7‐months following RYGB surgery. In conclusion, these data suggest that RYGB surgery corrected severe obesity‐induced dysregulation in mitochondrial fusion process in human myotubes, which may contribute to improved insulin sensitivity and skeletal muscle glucose oxidation in RYGB patients.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Metabolic Inflexibility with Obesity and the Effects of Fenofibrate on Skeletal Muscle Fatty Acid Oxidation.

This study was designed to investigate mechanisms of lipid metabolic inflexibility in human obesity and the ability of fenofibrate (FENO) to increase skeletal muscle fatty acid oxidation (FAO) in primary human skeletal muscle cell cultures (HSkMC) exhibiting metabolic inflexibility. HSkMC from 10 lean and 10 obese, insulin resistant subjects were treated with excess fatty acid for 24 h (24hFA) to gauge lipid-related metabolic flexibility. Metabolically inflexible HSkMC from obese individuals were then treated with 24hFA in combination with FENO to determine effectiveness for increasing FAO. Mitochondrial enzyme activity and FAO were measured in skeletal muscle from subjects with prediabetes (n=11) before and after 10 weeks of fenofibrate in vivo. 24hFA increased FAO to a greater extent in HSkMC from lean versus obese subjects (+49% vs. +9%, for lean vs. obese, respectively; p<0.05) indicating metabolic inflexibility with obesity. Metabolic inflexibility was not observed for measures of cellular respiration in permeabilized cells using carbohydrate substrate. Fenofibrate co-incubation with 24hFA, increased FAO in a subset of HSkMC from metabolically inflexible, obese subjects (p<0.05), which was eliminated by PPARα antagonist. In vivo, fenofibrate treatment increased skeletal muscle FAO in a subset of subjects with prediabetes but did not affect gene transcription or mitochondrial enzyme activity. Lipid metabolic inflexibility observed in HSkMC from obese subjects is not due to differences in electron transport flux, but rather upstream decrements in lipid metabolism. Fenofibrate increases the capacity for FAO in human skeletal muscle cells, though its role in skeletal muscle metabolism in vivo remains unclear.

Human Skeletal Muscle Glyoxalase‐1 Protein Expression is Attenuated with Type 2 Diabetes

INTRODUCTIONIndividuals with type 2 diabetes mellitus (T2DM) present with increased levels of advanced glycation endproducts (AGEs), which have been associated with progression and complications of the disease. AGEs are known to contribute to the disruption of insulin signaling in skeletal muscle and are primarily produced in vivo by the damaging glycolytic byproduct, methylglyoxal (MG). MG is effectively detoxified by the enzyme, glyoxalase‐1 (GLO1), whose protein expression has been previously shown to be inversely related to MG‐mediated protein‐damage and AGE formation. Despite the role of skeletal muscle insulin resistance in the pathogenesis of T2DM, GLO1 protein expression has not been fully elucidated in skeletal muscle. Additionally, the metabolic effects of insulin stimulation on GLO1 protein in human skeletal muscle has not been described.PURPOSETherefore, the purpose of this study was to examine skeletal muscle GLO1 protein content in individuals with T2DM compared to lean, healthy controls (LHC). Further, we sought to examine the effects of insulin stimulation on GLO1 protein expression through in vivo and in vitro approaches that included the hyperinsulinemic‐euglycemic clamp and primary human skeletal muscle cell culture experiments, respectively.METHODSSkeletal muscle biopsies from the vastus lateralis were obtained from 7 individuals with T2DM (age: 55 ± 4 yrs, BMI: 35.6 ± 2.4 kg/m2) and 10 LHCs (age: 27 ± 1 yrs, BMI: 22.3 ± 0.8 kg/m2) during basal and insulin stimulated states of the hyperinsulinemic (40mU/m2/min)‐euglycemic (5 mM) clamp. To further investigate the effect of insulin, primary human skeletal muscle cells were incubated with 300 pM insulin (a concentration similar to that experienced during the hyperinsulinemic‐euglycemic clamp) for 0, 30, and 180 mins. GLO1 protein content in human tissue and cell culture was assessed via western blot.RESULTSIndividuals with T2DM displayed a 5.6 fold reduction (p&lt;0.0001) in GLO1 compared to LHC at baseline. Clamp‐induced hyperinsulinemia had no significant effect on GLO1 protein expression in either group (p&gt;0.05). In cell culture, 30 minutes of insulin stimulation increased GLO1 protein expression by 37 % (p=0.0093). Interestingly, continued insulin stimulation for 180 mins returned GLO1 protein expression to levels comparable to baseline (0 min vs 180 min, p&gt;0.05).CONCLUSIONThese data suggest that the skeletal muscle of T2DM may be susceptible to MG‐mediated protein damage and accumulation of AGEs. Further, prolonged exposure to hyperinsulinemia may attenuate early insulin mediated increases in GLO1 expression profiles. Further study is need to elucidate the exact mechanisms of these observations.

Skeletal muscle myotubes in severe obesity exhibit altered ubiquitin‐proteasome and autophagic/lysosomal proteolytic flux

ObjectiveWhole-body protein metabolism is dysregulated with obesity. Our goal was to determine if activity and expression of major protein degradation pathways are compromised specifically in human skeletal muscle with obesity.MethodsWe utilized primary Human Skeletal Muscle cell (HSkM) cultures since cellular mechanisms can be studied absent of hormones and contractile activity that could independently influence metabolism. HSkM from 10 lean (BMI ≤ 26.0 kg/m2) and 8 severely obese (BMI ≥ 39.0) women were examined basally and when stimulated to atrophy (serum and amino acid starvation).ResultsHSkM from obese donors had a lower proportion of type I myosin heavy chain and slower flux through the autophagic/lysosomal pathway. During starvation, flux through the ubiquitin-proteasome system diverged according to obesity status, with a decrease in the lean and an increase in HSkM from obese subjects. HSkMC from the obese also displayed elevated proteasome activity despite no difference in proteasome content. Atrophy-related gene expression and myotube area were similar in myotubes derived from lean and obese individuals under basal and starved conditions.ConclusionsOur data indicate that muscle cells of the lean and severely obese have innate differences in management of protein degradation, which may explain their metabolic differences.

A High‐Fat Diet Activates Inflammatory Signaling in Human Skeletal Muscle Cultures from Severely Obese Women

The incidence of metabolic disease and severe obesity continues to rise; likely associated with consumption of a Western, high-fat diet. Several mechanisms have been postulated to account for comorbidities associated with severe obesity; one being inflammation. The objective of this study was to determine whether metabolic and/or inflammatory gene networks were differentially regulated in human skeletal muscle cell cultures (HSkMC) from lean and severely obese women as a response to lipid oversupply. RNA in HSkMC from lean and severely obese women were isolated after a 48hr lipid incubation (250µM oleate:palmitate), designed to imitate a high-fat diet. Genome-wide expression data was acquired using Illumina HumanHT-12 v4.0 Expression BeadChip. Network and pathway analysis was done using MetaCore software. The number of inflammatory response pathways associated with genes differentially regulated by the lipid treatment was 66, including the top responses, HMGB1/RAGE, TNF-R2, TSLP, and IL-17 signaling pathways, all of which are associated with pro-inflammatory cytokine production. In conclusion, the activation of inflammatory signaling due to the up-regulation of cytokine regulators in response to a high fat diet in the severely obese could be the link between obesity and metabolic dysfunction.

G-CSF does not influence C2C12 myogenesis despite receptor expression in healthy and dystrophic skeletal muscle.

Granulocyte-colony stimulating factor (G-CSF) increases recovery of rodent skeletal muscles after injury, and increases muscle function in rodent models of neuromuscular disease. However, the mechanisms by which G-CSF mediates these effects are poorly understood. G-CSF acts by binding to the membrane spanning G-CSFR and activating multiple intracellular signaling pathways. Expression of the G-CSFR within the haematopoietic system is well known, but more recently it has been demonstrated to be expressed in other tissues. However, comprehensive characterization of G-CSFR expression in healthy and diseased skeletal muscle, imperative before implementing G-CSF as a therapeutic agent for skeletal muscle conditions, has been lacking. Here we show that the G-CSFR is expressed in proliferating C2C12 myoblasts, differentiated C2C12 myotubes, human primary skeletal muscle cell cultures and in mouse and human skeletal muscle. In mdx mice, a model of human Duchenne muscular dystrophy (DMD), G-CSF mRNA and protein was down-regulated in limb and diaphragm muscle, but circulating G-CSF ligand levels were elevated. G-CSFR mRNA in the muscles of mdx mice was up-regulated however steady-state levels of the protein were down-regulated. We show that G-CSF does not influence C2C12 myoblast proliferation, differentiation or phosphorylation of Akt, STAT3, and Erk1/2. Media change alone was sufficient to elicit increases in Akt, STAT3, and Erk1/2 phosphorylation in C2C12 muscle cells and suggest previous observations showing a G-CSF increase in phosphoprotein signaling be viewed with caution. These results suggest that the actions of G-CSF may require the interaction with other cytokines and growth factors in vivo, however these data provides preliminary evidence supporting the investigation of G-CSF for the management of muscular dystrophy.

Leucine modulates dynamic phosphorylation events in insulin signaling pathway and enhances insulin-dependent glycogen synthesis in human skeletal muscle cells

BackgroundBranched-chain amino acids, especially leucine, are known to interact with insulin signaling pathway and glucose metabolism. However, the mechanism by which this is exerted, remain to be clearly defined. In order to examine the effect of leucine on muscle insulin signaling, a set of experiments was carried out to quantitate phosphorylation events along the insulin signaling pathway in human skeletal muscle cell cultures. Cells were exposed to insulin, leucine or both, and phosphorylation events of key insulin signaling molecules were tracked over time so as to monitor time-related responses that characterize the signaling events and could be missed by a single sampling strategy limited to pre/post stimulus events.ResultsLeucine is shown to increase the magnitude of insulin-dependent phosphorylation of protein kinase B (AKT) at Ser473 and glycogen synthase kinase (GSK3β) at Ser21-9. Glycogen synthesis follows the same pattern of GSK3β, with a significant increase at 100 μM leucine plus insulin stimulus. Moreover, data do not show any statistically significant increase of pGSK3β and glycogen synthesis at higher leucine concentrations. Leucine is also shown to increase the magnitude of insulin-mediated extracellularly regulated kinase (ERK) phosphorylation; however, differently from AKT and GSK3β, ERK shows a transient behavior, with an early peak response, followed by a return to the baseline condition.ConclusionsThese experiments demonstrate a complementary effect of leucine on insulin signaling in a human skeletal muscle cell culture, promoting insulin-activated GSK3β phosphorylation and glycogen synthesis.

Effect of Lipopolysaccharide on Inflammation and Insulin Action in Human Muscle

Accumulating evidence from animal studies suggest that chronic elevation of circulating intestinal-generated lipopolysaccharide (LPS) (i.e., metabolic endotoxemia) could play a role in the pathogenesis of insulin resistance. However, the effect of LPS in human muscle is unclear. Moreover, it is unknown whether blockade/down regulation of toll-like receptor (TLR)4 can prevent the effect of LPS on insulin action and glucose metabolism in human muscle cells. In the present study we compared plasma LPS concentration in insulin resistant [obese non-diabetic and obese type 2 diabetic (T2DM)] subjects versus lean individuals. In addition, we employed a primary human skeletal muscle cell culture system to investigate the effect of LPS on glucose metabolism and whether these effects are mediated via TLR4. Obese non-diabetic and T2DM subjects had significantly elevated plasma LPS and LPS binding protein (LBP) concentrations. Plasma LPS (r = −0.46, P = 0.005) and LBP (r = −0.49, P = 0.005) concentrations negatively correlated with muscle insulin sensitivity (M). In human myotubes, LPS increased JNK phosphorylation and MCP-1 and IL-6 gene expression. This inflammatory response led to reduced insulin-stimulated IRS-1, Akt and AS160 phosphorylation and impaired glucose transport. Both pharmacologic blockade of TLR4 with TAK-242, and TLR4 gene silencing, suppressed the inflammatory response and insulin resistance caused by LPS in human muscle cells. Taken together, these findings suggest that elevations in plasma LPS concentration found in obese and T2DM subjects could play a role in the pathogenesis of insulin resistance and that antagonists of TLR4 may improve insulin action in these individuals.

Isolation of Myogenic Stem Cells from Cultures of Cryopreserved Human Skeletal Muscle

We demonstrate that subpopulations of adult human skeletal muscle-derived stem cells, myogenic endothelial cells (MECs), and perivascular stem cells (PSCs) can be simultaneously purified by fluorescence-activated cell sorting (FACS) from cryopreserved human primary skeletal muscle cell cultures (cryo-hPSMCs). For FACS isolation, we utilized a combination of cell lineage markers: the myogenic cell marker CD56, the endothelial cell marker UEA-1 receptor (UEA-1R), and the perivascular cell marker CD146. MECs expressing all three cell lineage markers (CD56(+)UEA-1R(+)CD146(+)/CD45(-)) and PSCs expressing only CD146 (CD146(+)/CD45(-)CD56(-)UEA-1R(-)) were isolated by FACS. To evaluate their myogenic capacities, the sorted cells, with and without expansion in culture, were transplanted into the cardiotoxin-injured skeletal muscles of immunodeficient mice. The purified MECs exhibited the highest regenerative capacity in the injured mouse muscles among all cell fractions tested, while PSCs remained superior to myoblasts and the unpurified primary skeletal muscle cells. Our findings show that both MECs and PSCs retain their high myogenic potentials after in vitro expansion, cryopreservation, and FACS sorting. The current study demonstrates that myogenic stem cells are prospectively isolatable from long-term cryopreserved primary skeletal muscle cell cultures. We emphasize the potential application of this new approach to extract therapeutic stem cells from human muscle cells cryogenically banked for clinical purposes.

Contractile activity of human skeletal muscle cells prevents insulin resistance by inhibiting pro-inflammatory signalling pathways

Obesity is closely associated with muscle insulin resistance and is a major risk factor for the pathogenesis of type 2 diabetes. Regular physical activity not only prevents obesity, but also considerably improves insulin sensitivity and skeletal muscle metabolism. We sought to establish and characterise an in vitro model of human skeletal muscle contraction, with a view to directly studying the signalling pathways and mechanisms that are involved in the beneficial effects of muscle activity. Contracting human skeletal muscle cell cultures were established by applying electrical pulse stimulation. To induce insulin resistance, skeletal muscle cells were incubated with human adipocyte-derived conditioned medium, monocyte chemotactic protein (MCP)-1 and chemerin. Similarly to in exercising skeletal muscle in vivo, electrical pulse stimulation induced contractile activity in human skeletal muscle cells, combined with the formation of sarcomeres, activation of AMP-activated protein kinase (AMPK) and increased IL-6 secretion. Insulin-stimulated glucose uptake was substantially elevated in contracting cells compared with control. The incubation of skeletal muscle cells with adipocyte-conditioned media, chemerin and MCP-1 significantly reduced the insulin-stimulated phosphorylation of Akt. This effect was abrogated by concomitant pulse stimulation of the cells. Additionally, pro-inflammatory signalling by adipocyte-derived factors was completely prevented by electrical pulse stimulation of the myotubes. We showed that the effects of electrical pulse stimulation on skeletal muscle cells were similar to the effect of exercise on skeletal muscle in vivo in terms of enhanced AMPK activation and IL-6 secretion. In our model, muscle contractile activity eliminates insulin resistance by blocking pro-inflammatory signalling pathways. This novel model therefore provides a unique tool for investigating the molecular mechanisms that mediate the beneficial effects of muscle contraction.

Effects of a Botanical Extract from Artemisia dracunculus L. on Insulin Sensitivity of Human Skeletal Muscle Cell Cultures

Insulin resistance is a key factor of metabolic syndrome and can lead to pre-diabetes, diabetes and cardiovascular disease. PMI-5011, an ethanolic extract from Russian Tarragon (Artemisia dracunculus L.) has been shown to increase insulin signaling in skeletal muscle of humans and rodents, resulting in improved insulin sensitivity. Despite extensive research, the exact mode of action of PMI-5011 has not been established. Here we present results from proteomic analyses of human skeletal muscle cell cultures (HSMC) derived from obese, insulin-resistant individuals. Using isobaric tagging for relative and absolute quantification (iTRAQ), we analyzed four treatments (baseline control; control with insulin stimulation; PMI-5011 treated; PMI-5011 treated with insulin stimulation) simultaneously using strong cation exchange chromatography and reverse-phase liquid chromatography-tandem mass spectrometry. Our results show that HSMCs retained the intrinsic properties of skeletal muscle cells; thus, HSMC from obese insulin resistant individuals showed a blunted response to insulin stimulation in vitro. Treatment of cells with PMI-5011 in the presence of insulin resulted in an increase of proteins involved in glycolysis/gluconeogenesis pathway, actin cytoskeleton signaling and interleukin signaling. Furthermore, we detected reduction in levels of proteins from inflammatory networks that involve tumor necrosis factor receptor that reduces activity of nuclear factor kappa B (NFκB). These results show that PMI-5011 increases protein abundance in key metabolic pathways of glucose metabolism and modulates proteins that lead to reduced inflammatory cytokine signaling, likely resulting in observed increase in insulin sensitivity.

Myostatin up-regulation is associated with the skeletal muscle response to hypoxic stimuli

Myostatin and hypoxia signalling pathways are able to induce skeletal muscle atrophy, but whether a relationship between these two pathways exists is currently unknown. Here, we tested the hypothesis that a potential mechanism for hypoxia effect on skeletal muscle may be through regulation of myostatin. We reported an induction of myostatin expression in muscles of rats exposed to chronic hypoxia. Interestingly, we also demonstrated increased skeletal muscle myostatin protein expression in skeletal muscle of hypoxemic patients with severe chronic obstructive pulmonary disease (COPD). Parallel studies in human skeletal muscle cell cultures showed that induction of myostatin expression in myotubes treated with hypoxia-mimicking agent such as cobalt chloride (CoCl2) is associated with myotube atrophy. Furthermore, we demonstrated that inhibition of myostatin by means of genetic deletion of myostatin or treatment with blocking antimyostatin antibodies inhibits the CoCl2-induced atrophy in muscle cells. Finally, addition of recombinant myostatin restored the CoCl2-induced atrophy in myostatin deficient myotubes. These results strongly suggest that myostatin can play an essential role in the adaptation of skeletal muscle to hypoxic environment.

Interferon‐stimulated gene 15 (ISG15) conjugates proteins in dermatomyositis muscle with perifascicular atrophy

We investigated interferon-stimulated gene 15 (ISG15), a poorly understood ubiquitin-like modifier, and its enzymatic pathway in dermatomyositis (DM), an autoimmune disease primarily involving muscle and skin. We generated microarray data measuring transcript abundance for approximately 18,000 genes in each of 113 human muscle biopsy specimens, and studied biopsy specimens and cultured skeletal muscle using immunohistochemistry, immunoblotting proteomics, real-time quantitative polymerase chain reaction, and laser-capture microdissection. Transcripts encoding ISG15-conjugation pathway proteins were markedly upregulated in DM with perifascicular atrophy (DM-PFA) muscle (ISG15 339-fold, HERC5 62-fold, and USP18 68-fold) compared with 99 non-DM samples. Combined analysis with publicly available microarray datasets showed that >50-fold ISG15 transcript elevation had 100% sensitivity and specificity for 28 biopsies from adult DM-PFA and juvenile DM patients compared with 199 muscle samples from other muscle diseases. Free ISG15 and ISG15-conjugated proteins were only found on immunoblots from DM-PFA muscle. Cultured human skeletal muscle exposed to type 1 interferons produced similar transcripts and ISG15 protein and conjugates. Laser-capture microdissection followed by proteomic analysis showed deficiency of titin in DM perifascicular atrophic myofibers. A large-scale microarray study of muscle samples demonstrated that among a diverse group of muscle diseases DM was uniquely associated with upregulation of the ISG15 conjugation pathway. Exposure of human skeletal muscle cell culture to type 1 interferons produced a molecular picture highly similar to that seen in human DM muscle. Perifascicular atrophic myofibers in DM were deficient in a number of skeletal muscle proteins including titin.

High-Density Lipoprotein Modulates Glucose Metabolism in Patients With Type 2 Diabetes Mellitus

Low plasma high-density lipoprotein (HDL) is associated with elevated cardiovascular risk and aspects of the metabolic syndrome. We hypothesized that HDL modulates glucose metabolism via elevation of plasma insulin and through activation of the key metabolic regulatory enzyme, AMP-activated protein kinase, in skeletal muscle. Thirteen patients with type 2 diabetes mellitus received both intravenous reconstituted HDL (rHDL: 80 mg/kg over 4 hours) and placebo on separate days in a double-blind, placebo-controlled crossover study. A greater fall in plasma glucose from baseline occurred during rHDL than during placebo (at 4 hours rHDL=-2.6+/-0.4; placebo=-2.1+/-0.3 mmol/L; P=0.018). rHDL increased plasma insulin (at 4 hours rHDL=3.4+/-10.0; placebo= -19.2+/-7.4 pmol/L; P=0.034) and also the homeostasis model assessment beta-cell function index (at 4 hours rHDL=18.9+/-5.9; placebo=8.6+/-4.4%; P=0.025). Acetyl-CoA carboxylase beta phosphorylation in skeletal muscle biopsies was increased by 1.7+/-0.3-fold after rHDL, indicating activation of the AMP-activated protein kinase pathway. Both HDL and apolipoprotein AI increased glucose uptake (by 177+/-12% and 144+/-18%, respectively; P<0.05 for both) in primary human skeletal muscle cell cultures established from patients with type 2 diabetes mellitus (n=5). The mechanism is demonstrated to include stimulation of the ATP-binding cassette transporter A1 with subsequent activation of the calcium/calmodulin-dependent protein kinase kinase and the AMP-activated protein kinase pathway. rHDL reduced plasma glucose in patients with type 2 diabetes mellitus by increasing plasma insulin and activating AMP-activated protein kinase in skeletal muscle. These findings suggest a role for HDL-raising therapies beyond atherosclerosis to address type 2 diabetes mellitus.

Characterization of human myoblast cultures for tissue engineering

Skeletal muscle tissue engineering, a promising specialty, aims at the reconstruction of skeletal muscle loss. In vitro tissue engineering attempts to achieve this goal by creating differentiated, functional muscle tissue through a process in which stem cells are extracted from the patient, e.g. by muscle biopsies, expanded and differentiated in a controlled environment, and subsequently re-implanted. A prerequisite for this undertaking is the ability to cultivate and differentiate human skeletal muscle cell cultures. Evidently, optimal culture conditions must be investigated for later clinical utilization. We therefore analysed the proliferation of human cells in different environments and evaluated the differentiation potential of different culture media. It was shown that human myoblasts have a higher rate of proliferation in the alamarBlue assay when cultured on gelatin-coated culture flasks rather than polystyrene-coated flasks. We also demonstrated that myoblasts treated with a culture medium with a high concentration of growth factors [growth medium (GM)] showed a higher proliferation compared to cultures treated with a culture medium with lower amounts of growth factors [differentiation medium (DM)]. Differentiation of human myoblast cell cultures treated with GM and DM was analysed until day 16 and myogenesis was verified by expression of MyoD, myogenin, alpha-sarcomeric actin and myosin heavy chain by semi-quantitative RT-PCR. Immunohistochemical staining for desmin, Myf-5 and alpha-sarcomeric actin was performed to verify the myogenic phenotype of extracted satellite cells and to prove the maturation of cells. Cultures treated with DM showed positive staining for alpha-sarcomeric actin. Notably, markers of differentiation were also detected in cultures treated with GM, but there was no formation of myotubes. In the enzymatic assay of creatine phosphokinase, cultures treated with DM showed a higher activity, evidencing a higher degree of differentiation. In this study, we obtained detailed information regarding the cultivation and differentiation of human myoblast cultures in different environments. By exploring optimal culture conditions for skeletal muscle tissue engineering, we acquired culture data for comparison with other sources of stem cells in order to find the most applicable stem cell for focussed clinical usage.