Human Skeletal Muscle Cells Research Articles

Caspase 3 exhibits a yeast metacaspase proteostasis function that protects mitochondria from toxic TDP43 aggregates.

Caspase 3 activation is a hallmark of cell death and there is a strong correlation between elevated protease activity and evolving pathology in neurodegenerative disease, such as amyotrophic lateral sclerosis (ALS). At the cellular level, ALS is characterized by protein aggregates and inclusions, comprising the RNA binding protein TDP-43, which are hypothesized to trigger pathogenic activation of caspase 3. However, a growing body of evidence indicates this protease is essential for ensuring cell viability during growth, differentiation and adaptation to stress. Here, we explored whether caspase 3 acts to disperse toxic protein aggregates, a proteostasis activity first ascribed to the distantly related yeast metacaspase ScMCA1. We demonstrate that human caspase 3 can functionally substitute for the ScMCA1 and limit protein aggregation in yeast, including TDP-43 inclusions. Proteomic analysis revealed that disrupting caspase 3 in the same yeast substitution model resulted in detrimental TDP-43/mitochondrial protein associations. Similarly, suppression of caspase 3, in either murine or human skeletal muscle cells, led to accumulation of TDP-43 aggregates and impaired mitochondrial function. These results suggest that caspase 3 is not inherently pathogenic, but may act as a compensatory proteostasis factor, to limit TDP-43 protein inclusions and protect organelle function in aggregation related degenerative disease.

Ascorbic acid reduces Ropivacaine-induced myotoxicity in cultured human osteoporotic skeletal muscle cells

BackgroundOsteoporosis is a worldwide health issue. Loss of bone mass is a potential risk factor for fragility fractures, and osteoporotic fractures place a considerable burden on society. Bone and muscle represent a functional unit in which the two tissues are intimately interconnected. Ropivacaine is a potent local anesthetic used in clinical practice for intraoperative anesthesia and postoperative pain management, in particular for hip surgery. When injected, Ropivacaine can diffuse locally through, in particular in surrounding skeletal muscle tissue, causing dose-dependent cytotoxicity, oxidative stress and myogenesis impairment. Based on those evidences, we focused our attention on Ropivacaine-induced cytotoxicity on cultured human myoblasts.MethodsPrimary human myoblasts and myotubes from healthy subjects, osteoarthritic and osteoporotic patients (OP) were cultured in the presence of Ropivacaine. In some experiments, ascorbic acid (AsA) was added as a potent antioxidant agent. Cell viability and ROS levels were evaluated to investigate the myotoxic activity and Real-Time PCR and Western blot analysis carried out to investigate the expression of proliferation and myogenic markers.ResultsA dose-dependent decrease of cell viability was observed after Ropivacaine exposure in both OP myoblasts and myotubes cultures, whereas those effects were not observed in the presence of Propofol, a general anesthetic. The adding of AsA reduced Ropivacaine negative effects in OP myoblast cultures. In addition, Ropivacaine exposure also increased ROS levels and upregulated Nox4 expression, an enzyme primarily implicated in skeletal muscle ROS generation. AsA treatment counteracted the oxidant activity of Ropivacaine and partially restored the basal condition in cultures. Positive myogenic markers, such as MyoD and Myf5, were downregulated by Ropivacaine exposure, whereas myostatin, a negative regulator of muscle growth and differentiation, was upregulated. The phenotypic deregulation of myogenic controllers in the presence of Ropivacaine was counteracted by AsA treatment.ConclusionsOur findings highlight the oxidative stress-mediated myotoxic effect of Ropivacaine on human skeletal muscle tissue cell cultures, and suggest treatment with AsA as valid strategy to mitigate its negative effects and allowing an ameliorated functional skeletal muscle recovery in patients undergoing hip replacement surgery for osteoporotic bone fracture.

Secretion of Interleukin 6 in Human Skeletal Muscle Cultures Depends on Ca2+ Signalling.

The systemic effects of physical activity are mediated by the release of IL-6 and other myokines from contracting muscle. Although the release of IL-6 from muscle has been extensively studied, the information on the cellular mechanisms is fragmentary and scarce, especially regarding the role of Ca2+ signals. The aim of this study was to characterize the role of the main components of Ca2+ signals in human skeletal muscle cells during IL-6 secretion stimulated by the Ca2+ mobilizing agonist ATP. Primary cultures were prepared from surgical samples, fluorescence microscopy was used to evaluate the Ca2+ signals and the stimulated release of IL-6 into the medium was determined using ELISA. Intracellular calcium chelator Bapta, low extracellular calcium and the Ca2+ channels blocker La3+ reduced the ATP-stimulated, but not the basal secretion. Secretion was inhibited by blockers of L-type (nifedipine, verapamil), T-type (NNC55-0396) and Orai1 (Synta66) Ca2+ channels and by silencing Orai1 expression. The same effect was achieved with inhibitors of ryanodine receptors (ryanodine, dantrolene) and IP3 receptors (xestospongin C, 2-APB, caffeine). Inhibitors of calmodulin (calmidazolium) and calcineurin (FK506) also decreased secretion. IL-6 transcription in response to ATP was not affected by Bapta or by the T channel blocker. Our results prove that ATP-stimulated IL-6 secretion is mediated at the post-transcriptional level by Ca2+ signals, including the mobilization of calcium stores, the activation of store-operated Ca2+ entry, and the subsequent activation of voltage-operated Ca2+ channels and calmodulin/calcineurin pathways.

1643-P: Unique Signature of Circular RNA in Skeletal Muscle of Different Insulin Sensitivity

Diabetes pathologies are complex, affecting numerous organs such as liver, pancreas, and skeletal muscle. Circular RNAs (circRNA) are a class of non-coding RNAs that are characterized by a covalently closed loop structure. Functionally, they can act on cell physiology by sponging microRNAs and thus regulating gene and protein expression. The emerging function of these circRNA is not fully understood, but initial studies have recently shown that circRNA are involved in insulin secretion regulation. Therefore, deregulation of this class of RNAs may lead to metabolic disorders in pancreatic β-cells and thus be involved in the pathogenesis of type 1 (T1DM) and type 2 diabetes (T2DM). Our research is focused on the unique signature by circular RNA and the impact on skeletal muscle metabolism. We demonstrated that human skeletal muscle cells in culture maintain their gene expression phenotype. A different gene signature of the cells according to their different muscular fiber type sources (oxidative versus glycolytic versus mixt) was confirmed. Moreover, an independent microRNA composition has also been highlighted. As CircRNA can regulate microRNAs and then gene expression and proteins, we wondered whether these observed signatures could be explained by circRNA. We therefore focused on key circRNAs involved in myogenesis (such as circZNF609, circHIPK3) and we were able to see that there is a difference in expression of circRNA according to the muscle type. Moreover, we analyzed correlations between circRNA levels and insulin sensitivity (measured by PKB phosphorylation). Finally, we are currently completing our analysis with in-depth sequencing techniques to identify all expressed circRNA in muscle fiber types to potentially identify new targets in insulin metabolic pathways. Disclosure M.Yepmo: None. E.Meugnier: None. M.Pinget: Consultant; Novo Nordisk, Stock/Shareholder; ILONOV. K.Bouzakri: Consultant; Adocia, Stock/Shareholder; ILONOV.

Functional expression of the thermally activated transient receptor potential channels TRPA1 and TRPM8 in human myotubes

Transient potential (TRP) ion channels expressed in primary sensory neurons act as the initial detectors of environmental cold and heat, information which controls muscle energy expenditure. We hypothesize that non-neuronal TRPs have direct cellular responses to thermal exposure, also affecting cellular metabolism. In the present study we show expression of TRPA1, TRPM8 and TRPV1 in rat skeletal muscle and human primary myotubes by qPCR. Effects of TRP activity on metabolism in human myotubes were studied using radiolabeled glucose. FURA-2 was used for Ca2+ imaging.TRPA1, TRPM8 and TRPV1 were expressed at low levels in primary human myotubes and in m. gastrocnemius, m. soleus, and m. trapezius from rat. Activation of TRPA1 by ligustilide resulted in an increased glucose uptake and oxidation in human myotubes, whereas activation of TRPM8 by menthol and icilin significantly decreased glucose uptake and oxidation. Activation of heat sensing TRPV1 by capsaicin had no effect on glucose metabolism. Agonist-induced increases in intracellular Ca2+ levels by ligustilide and icilin in human myotubes confirmed a direct activation of TRPA1 and TRPM8, respectively. The mRNA expression of some genes involved in thermogenesis, i.e. peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), uncoupling protein (UCP) 1 and UCP3, were downregulated in human myotubes following TRPA1 activation, while the mRNA expression of TRPM8 and TRPA1 were downregulated following TRPM8 activation by menthol and icilin, respectively. Cold exposure (18 °C) of cultured myotubes followed by a short recovery period had no effect on glucose uptake and oxidation in the basal situation, however when TRPA1 and TRPM8 channels were chemically inhibited a temperature-induced difference in glucose metabolism was found.In conclusion, mRNA of TRPA1, TRPM8 and TRPV1 are expressed in rat skeletal muscle and human skeletal muscle cells. Modulation of TRPA1 and TRPM8 by chemical agents induced changes in Ca2+ levels and glucose metabolism in human skeletal muscle cells, indicating functional receptors.

L-Anserine Increases Muscle Differentiation and Muscle Contractility in Human Skeletal Muscle Cells.

l-Anserine, an imidazole peptide, has a variety of physiological activities, but its effects on skeletal muscle differentiation and muscle contractile force remain unknown. Thus, in this study, we investigated the effect of l-anserine on muscle differentiation and muscle contractile force in human skeletal muscle cells. In two-dimensional culture, 1 μM l-anserine significantly increased the myotube diameters (26.5 ± 1.71, 27.7 ± 1.08, and 28.8 ± 0.85 μm with 0, 0.1, and 1 μM l-anserine, respectively) and the expression levels of genes involved in muscle differentiation and the sarcomere structure. In three-dimensional culture, 1 μM l-anserine significantly increased the contractile force of engineered human skeletal muscle tissues cultured on a microdevice (1.99 ± 0.30, 2.17 ± 0.62, 2.66 ± 0.39, and 3.28 ± 0.85 μN with 0, 0.1, 0.5, and 1 μM l-anserine, respectively). l-Anserine also increased the myotube diameters and the proportion of myotubes with sarcomere structures in the cultured tissues. Furthermore, the histamine receptor 1 (H1R) antagonist attenuated the l-anserine-induced increase in the contractile force, suggesting the involvement of H1R in the mechanism of action of l-anserine. This study showed for the first time that l-anserine enhances muscle differentiation and muscle contractility via H1R.

DEHP exposure impairs human skeletal muscle cell proliferation in primary culture conditions: preliminary study.

DEHP exposure impairs human skeletal muscle cell proliferation in primary culture conditions: preliminary study.

Role of mitochondria in the myopathy of juvenile dermatomyositis and implications for skeletal muscle calcinosis

ObjectivesTo elucidate mechanisms contributing to skeletal muscle calcinosis in patients with juvenile dermatomyositis. MethodsA well-characterized cohorts of JDM (n = 68), disease controls (polymyositis, n = 7; juvenile SLE, n = 10, and RNP + overlap syndrome, n = 12), and age-matched health controls (n = 17) were analyzed for circulating levels of mitochondrial (mt) markers including mtDNA, mt-nd6, and anti-mitochondrial antibodies (AMAs) using standard qPCR, ELISA, and novel-in-house assays, respectively. Mitochondrial calcification of affected tissue biopsies was confirmed using electron microscopy and energy dispersive X-ray analysis. A human skeletal muscle cell line, RH30, was used to generate an in vitro calcification model. Intracellular calcification is measured by flow cytometry and microscopy. Mitochondria were assessed for mtROS production and membrane potential by flow cytometry and real-time oxygen consumption rate by Seahorse bioanalyzer. Inflammation (interferon-stimulated genes) was measured by qPCR. ResultsIn the current study, patients with JDM exhibited elevated levels of mitochondrial markers associated with muscle damage and calcinosis. Of particular interest are AMAs predictive of calcinosis. Human skeletal muscle cells undergo time- and dose-dependent accumulation of calcium phosphate salts with preferential localization to mitochondria. Calcification renders skeletal muscle cells mitochondria stressed, dysfunctional, destabilized, and interferogenic. Further, we report that inflammation induced by interferon-alpha amplifies mitochondrial calcification of human skeletal muscle cells via the generation of mitochondrial reactive oxygen species (mtROS). ConclusionsOverall, our study demonstrates the mitochondrial involvement in the skeletal muscle pathology and calcinosis of JDM and mtROS as a central player in the calcification of human skeletal muscle cells. Therapeutic targeting of mtROS and/or upstream inducers, such as inflammation, may alleviate mitochondrial dysfunction, leading to calcinosis. AMAs can potentially identify patients with JDM at risk for developing calcinosis.

Non-canonical telomere protection role of FOXO3a of human skeletal muscle cells regulated by the TRF2-redox axis

Telomeric repeat binding factor 2 (TRF2) binds to telomeres and protects chromosome ends against the DNA damage response and senescence. Although the expression of TRF2 is downregulated upon cellular senescence and in various aging tissues, including skeletal muscle tissues, very little is known about the contribution of this decline to aging. We previously showed that TRF2 loss in myofibers does not trigger telomere deprotection but mitochondrial dysfunction leading to an increased level of reactive oxygen species. We show here that this oxidative stress triggers the binding of FOXO3a to telomeres where it protects against ATM activation, revealing a previously unrecognized telomere protective function of FOXO3a, to the best of our knowledge. We further showed in transformed fibroblasts and myotubes that the telomere properties of FOXO3a are dependent on the C-terminal segment of its CR2 domain (CR2C) but independent of its Forkhead DNA binding domain and of its CR3 transactivation domain. We propose that these non-canonical properties of FOXO3a at telomeres play a role downstream of the mitochondrial signaling induced by TRF2 downregulation to regulate skeletal muscle homeostasis and aging.

PARP1 mediated PARylation contributes to myogenic progression and glucocorticoid transcriptional response

The ADP-ribosyltransferase, PARP1 enzymatically generates and applies the post-translational modification, ADP-Ribose (ADPR). PARP1 roles in genome maintenance are well described, but recent work highlights roles in many fundamental processes including cellular identity and energy homeostasis. Herein, we show in both mouse and human skeletal muscle cells that PARP1-mediated PARylation is a regulator of the myogenic program and the muscle transcriptional response to steroid hormones. Chemical PARP1 modulation impacts the expression of major myocellular proteins, including troponins, key in dictating muscle contractile force. Whilst PARP1 in absence of DNA damage is often assumed to be basally inactive, we show PARylation to be acutely sensitive to extracellular glucose concentrations and the steroid hormone class, glucocorticoids which exert considerable authority over muscle tissue mass. Specifically, we find during myogenesis, a transient and significant rise in PAR. This early-stage differentiation event, if blocked with PARP1 inhibition, reduced the abundance of important muscle proteins in the fully differentiated myotubes. This suggests that PAR targets during early-stage differentiation are central to the proper development of the muscle contractile unit. We also show that reduced PARP1 in myoblasts impacts a variety of metabolic pathways in line with the recorded actions of glucocorticoids. Currently, as both regulators of myogenesis and muscle mass loss, glucocorticoids represent a clinical conundrum. Our work goes on to identify that PARP1 influences transcriptional activation by glucocorticoids of a subset of genes critical to human skeletal muscle pathology. These genes may therefore signify a regulatory battery of targets through which selective glucocorticoid modulation could be achieved. Collectively, our data provide clear links between PARP1-mediated PARylation and skeletal muscle homeostatic mechanisms crucial to tissue mass maintenance and endocrine response.

CoQ10Phytosomes Improve Cellular Ubiquinone Uptake in Skeletal Muscle Cells: An Ex Vivo Study Using CoQ10-Enriched Low-Density Lipoproteins Obtained in a Randomized Crossover Study.

Coenzyme Q10 (CoQ10) bioavailability in vivo is limited due to its lipophilic nature. Moreover, a large body of evidence in the literature shows that muscle CoQ10 uptake is limited. In order to address cell specific differences in CoQ uptake, we compared cellular CoQ10 content in cultured human dermal fibroblasts and murine skeletal muscle cells that were incubated with lipoproteins from healthy volunteers and enriched with different formulations of CoQ10 following oral supplementation. Using a crossover design, eight volunteers were randomized to supplement 100 mg/daily CoQ10 for two weeks, delivered both in phytosome form (UBQ) as a lecithin formulation and in CoQ10 crystalline form. After supplementation, plasma was collected for CoQ10 determination. In the same samples, low density lipoproteins (LDL) were extracted and normalized for CoQ10 content, and 0.5 µg/mL in the medium were incubated with the two cell lines for 24 h. The results show that while both formulations were substantially equivalent in terms of plasma bioavailability in vivo, UBQ-enriched lipoproteins showed a higher bioavailability compared with crystalline CoQ10-enriched ones both in human dermal fibroblasts (+103%) and in murine skeletal myoblasts (+48%). Our data suggest that phytosome carriers might provide a specific advantage in delivering CoQ10 to skin and muscle tissues.

Altered Cathepsin B Expression as a Diagnostic Marker of Skeletal Muscle Insulin Resistance in Type 2 Diabetes.

Insulin resistance (IR) of skeletal muscle is critical for type 2 diabetes mellitus (T2DM). Herein, we tried to identify genes critical for the IR of skeletal muscle in T2DM based on the Gene Expression Omnibus (GEO) database and in vitro cell experiments. Data sets related to skeletal muscle samples of T2DM patients were downloaded from the GEO database, and clinical information on T2DM patients in the GSE18732 data set was extracted, followed by determination of the module most related to T2DM. Then, the key genes were found after intersection analysis, followed by the analysis of the diagnostic markers of IR of skeletal muscle in T2DM. Subsequently, the mechanistic role of the key gene was illustrated by in vitro experiments in palmitate-stimulated human skeletal muscle cells (SkMCs). The black module was most associated with T2DM. Following intersection analysis with differential genes, eight key genes were obtained, including CTSB, ESR2, OAT, MSTN, PVALB, MAPK6, PHKB, and ATP2B2. Among them, CTSB had the highest diagnostic value, and its expression adversely correlated to the homeostasis assessment model for IR. Furthermore, in vitro experiments indicated that overexpression of CTSB inhibited the protein degradation of IRS-1 and GLUT4, thus attenuating the IR in palmitate-induced human SkMCs. The current study demonstrated that CTSB could act as a diagnostic marker of skeletal muscle IR in T2DM, and its overexpression inhibited palmitate-induced IR in human skeletal muscle cells.

Kaempferol improves glucose uptake in skeletal muscle via an AMPK-dependent mechanism

Insulin resistance is a hallmark of type-2 diabetes (T2D) pathogenesis. Because skeletal muscle (SkM) is the major tissue for insulin-mediated glucose disposal, insulin resistance in SkM is considered a major risk factor for developing T2D. Thus, the identification of compounds that enhance the ability of SkM to take up glucose is a promising strategy for preventing T2D. Our previous work showed that kaempferol, a flavonol present in many foods, improves insulin sensitivity in obese mice, however, the mechanism underlying this beneficial action remains unclear. Here, we show that kaempferol directly stimulates glucose uptake and prevents lipotoxicity-impaired glucose uptake in primary human SkM. Kaempferol stimulates Akt phosphorylation in a time-dependent manner in human SkM cells. The effect of kaempferol on glucose uptake was blunted by inhibition of glucose transporter 4, phosphoinositide 3-kinase (PI3K), or AMPK. In addition, kaempferol induced AMPK phosphorylation, and inhibition of AMPK prevented kaempferol-stimulated Akt phosphorylation. In vivo, kaempferol administration induced rapid glucose disposal accompanied with increased Akt and AMPK phosphorylation in SkM tissue of the mice. Taken together, these findings suggest that kaempferol stimulates glucose uptake in SkM via an AMPK/Akt dependent mechanism, and it may be a viable therapeutic agent for insulin resistance.

In Vitro Effects of PTH (1-84) on Human Skeletal Muscle-Derived Satellite Cells.

Parathyroid hormone (PTH) is a hormone secreted by the parathyroid glands. Despite its well-known characterized anabolic and catabolic actions on the skeleton, the in vitro effects of PTH on skeletal muscle cells are limited and generally performed on animal models. The aim of this study was to evaluate the effects of a short impulse of PTH (1-84) on the proliferation and the differentiation of skeletal muscle satellite cells isolated from human biopsies. The cells were exposed for 30 min to different concentrations of PTH (1-84), from 10-6 mol/L to 10-12 mol/L. ELISA was used to assay cAMP and the myosin heavy-chain (MHC) protein. The proliferation was assayed by BrdU and the differentiation by RealTime-qPCR. A statistical analysis was performed by ANOVA followed by Bonferroni's test. No significant variations in cAMP and the proliferation were detected in the isolated cells treated with PTH. On the other hand, 10-7 mol/L PTH on differentiated myotubes has shown significant increases in cAMP (p ≤ 0.05), in the expression of myogenic differentiation genes (p ≤ 0.001), and in the MHC protein (p ≤ 0.01) vs. untreated controls. This work demonstrates for the first time the in vitro effects of PTH (1-84) on human skeletal muscle cells and it opens new fields of investigation in muscle pathophysiology.

Insulin, dibutyryl-cAMP, and glucose modulate expression of patatin-like domain containing protein 7 in cultured human myotubes.

Expression of patatin-like phospholipase domain containing protein 7 (PNPLA7), also known as neuropathy target esterase-related esterase (NRE), a lysophospholipase, increases with fasting and decreases with feeding in mouse skeletal muscle, indicating it is regulated by insulin, counterregulatory hormones, such as glucocorticoids and catecholamines, and/or nutrients. In cultured mouse adipocytes insulin reduces Pnpla7 expression, underscoring the possibility that insulin regulates PNPLA7 in skeletal muscle. The first aim of this study was to establish whether PNPLA7 is functionally expressed in cultured human skeletal muscle cells. The second aim was to determine whether PNPLA7 is regulated by insulin, glucocorticoids, cAMP/protein kinase A pathway, and/or glucose. Cultured human skeletal muscle cells expressed PNPLA7 mRNA and protein. Gene silencing of PNPLA7 in myoblasts reduced the phosphorylation of 70 kDa ribosomal protein S6 kinase and ribosomal protein S6 as well as the abundance of α1-subunit of Na+,K+-ATPase and acetyl-CoA carboxylase, indirectly suggesting that PNPLA7 is functionally important. In myotubes, insulin suppressed PNPLA7 mRNA at 1 g/L glucose, but not at low (0.5 g/L) or high (4.5 g/L) concentrations. Treatment with synthetic glucocorticoid dexamethasone and activator of adenylyl cyclase forskolin had no effect on PNPLA7 regardless of glucose concentration, while dibutyryl-cAMP, a cell-permeable cAMP analogue, suppressed PNPLA7 mRNA at 4.5 g/L glucose. The abundance of PNPLA7 protein correlated inversely with the glucose concentrations. Collectively, our results highlight that PNPLA7 in human myotubes is regulated by metabolic signals, implicating a role for PNPLA7 in skeletal muscle energy metabolism.

Both Acute and Consecutive Days of Formoterol Stimulation Influence Myogenic, Mitochondrial, and myomiR Gene Expression in Human Skeletal Muscle Cells

Skeletal muscle physiology is regulated by microRNA that are localized within skeletal muscle (myomiRs). This study investigated how the expression of myomiRs and genes regulating skeletal muscle mass and myogenesis are influenced in response to acute and consecutive days of exercise-related signaling using the exercise mimetic, formoterol, in vitro. Human skeletal muscle cells were proliferated and differentiated for 6 days. Experimental conditions included: (a) control, (b) acute formoterol stimulation (AFS), and (c) consecutive days of formoterol stimulation (CFS). For AFS, myotubes were treated with 30 nM of formoterol for three hours on day 6 of differentiation, and this was immediately followed by RNA extraction. For CFS, myotubes were treated with 30 nM of formoterol for three hours on two or three consecutive days, with RNA extracted immediately following the final three-hour formoterol treatment. We observed increased myomiR expression for both AFS and CFS. AFS appeared to promote myogenesis, but this effect was lost with CFS. Additionally, we observed increased expression of genes involved in metabolism, mitochondrial biogenesis, and muscle protein degradation in response to AFS. myomiR and gene expression appear to be sensitive to acute and long-term exercise-related stimuli, and this likely contributes to the regulation of skeletal muscle mass.

SENP2 knockdown in human adipocytes reduces glucose metabolism and lipid accumulation, while increases lipid oxidation

Adipose tissue is one of the main regulative sites for energy metabolism. Excess lipid storage and expansion of white adipose tissue (WAT) is the primary contributor to obesity, a strong predisposing factor for development of insulin resistance. Sentrin-specific protease (SENP) 2 has been shown to play a role in metabolism in murine fat and skeletal muscle cells, and we have previously demonstrated its role in energy metabolism of human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism in primary human fat cells by using cultured primary human adipocytes to knock down the SENP2 gene. Glucose uptake and oxidation, as well as accumulation and distribution of oleic acid into complex lipids were decreased, while oleic acid oxidation was increased in SENP2-knockdown cells compared to control adipocytes. Furthermore, lipogenesis was reduced by SENP2-knockdown in adipocytes. Although TAG accumulation relative to total uptake was unchanged, there was increased mRNA expression of metabolically relevant genes such as UCP1 and PPARGC1A and mRNA and proteomic data revealed increased levels of mRNA and proteins related to mitochondrial function by SENP2-knockdown. In conclusion, SENP2 is an important regulator of energy metabolism in primary human adipocytes and its knockdown reduce glucose metabolism and lipid accumulation, while increasing lipid oxidation in human adipocytes.

Silencing MYOT Expression May Inhibit Autophagy in Human Skeletal Muscle Cells.

Muscle diseases are closely related to autophagy disorders. Studies of autophagy inhibition indicated the importance of autophagy in muscle regeneration, while activation of autophagy can restore muscle function in some myopathies. Previous studies have revealed that mutations in the MYOT gene may lead to several kinds of hereditary myopathies. However, whether the autophagy played a crucial role in hereditary myopathy caused by MYOT mutations was still not clear. In this study, we established the MYOT knockdown human skeletal muscle cell models (HSkMCs) by small interfering RNA. Real-time PCR and Western blot studies found that the expression of p62 and LC3B-II was decreased dramatically, which suggested that silencing MYOT expression may regulate the autophagy in HSkMCs. Further immunofluorescence study on Ad-mCherry-GFP-LC3B adenovirus transfection and monodansylcadaverine (MDC) staining revealed that knocking down the expression of MYOT may inhibit the autophagy. Next, we used the autophagy inducer Earle's balanced salt solution (EBSS) and late-autophagy inhibitor bafilomycin A1 (BAF A1) to treat the HSkMCs, respectively, and found that silencing MYOT expression can inhibit the activation of autophagy by EBSS and aggravate the inhibition of autophagy by BAF A1. Finally, we also found that silencing MYOT expression can downregulate the expression of ATG7 and ATG5, two important autophagy regulatory molecules. Hence, our study may first reveal that knocking down the expression of MYOT may inhibit the autophagy. Hereditary myopathies caused by MYOT mutations may partly result from the inhibition of autophagy in HSkMCs.

Finnish-specific AKT2 gene variant leads to impaired insulin signalling in myotubes.

Finnish-specific gene variant p.P50T/AKT2 (MAF=1.1%), is associated with insulin resistance and increased predisposition to type 2 diabetes. Here, we have investigated in vitro the impact of the gene variant on glucose metabolism and intracellular signalling in human primary skeletal muscle cells, that were established from 14 male p.P50T/AKT2 variant carriers and 14 controls. Insulin-stimulated glucose uptake and glucose incorporation into glycogen were detected with 2-[1,2-3H]-deoxy-D-glucose and D-[14C]-glucose, respectively, and the rate of glycolysis was measured with a Seahorse XFe96 analyzer. Insulin signalling was investigated with western blotting. Binding of variant and control AKT2-PH domains to phosphatidylinositol (3,4,5)-trisphosphate ((PI(3,4,5)P3)) was assayed using PIP Strips™ Membranes. Tyrosine (PTK) and serine-threonine (STK) kinase assay was performed using the PamGene® kinome profiling system. Insulin-stimulated glucose uptake and glycogen synthesis in myotubes in vitro were not significantly affected by the genotype. However, insulin-stimulated glycolytic rate was impaired in variant myotubes. Western blot analysis showed that insulin-stimulated phosphorylation of Akt-Thr308, AS160-Thr642 and GSK3β-Ser9 was reduced in variant myotubes compared to controls. Binding of variant AKT2-PH domain to PI(3,4,5)P3 was reduced as compared to control protein. PamGene® kinome profiling revealed multiple differentially phosphorylated kinase substrates, e.g. calmodulin, between the genotypes. Further in silico upstream kinase analysis predicted a large-scale impairment in activities of kinases participating for example in intracellular signal transduction, protein translation and cell cycle events. In conclusion, myotubes from p.P50T/AKT2 variant carriers show multiple signalling alterations which may contribute to predisposition to insulin resistance and T2D in the carriers of this signalling variant.

Bioengineering approaches to promote maturity in human skeletal muscle constructs.

During ageing, cellular changes impairing normal responses to contractile activity causes our skeletal muscles to atrophy and weaken. Additionally, reduced innervation quality in muscle of older individuals results in denervation atrophy. This declining muscle mass and strength, termed sarcopenia, contributes to frailty and loss of independence in older populations. In order to study sarcopenia pathophysiology, there is a requirement for physiologically relevant in vitro models of mature human skeletal muscle which display neuromuscular junction (NMJ) formation. Recently, the MicroAge project, a UK Space Agency funded mission to the International Space Station, used microgravity to model accelerated musculoskeletal ageing of muscle constructs consisting of human skeletal muscle cells grown in 3D within a hydrogel. This current project aims to explore several bioengineering approaches to promote maturity of these constructs, developing a model of mature human skeletal muscle. The MicroAge constructs will first be characterised against a panel of maturation indices for skeletal muscle, including fibre typing and genetic markers of maturity. Development of a system to simultaneously measure force generation and impedance spectroscopy will evaluate how well impedance spectroscopy monitors tissue maturity as well as being a proxy for force generation during contractions. Subsequently, we aim to implement bioprinting to streamline construct formation, and investigate effects of varying patterns of electrical stimulation on maturity throughout construct differentiation. The longer-term aim is to develop an in vitro NMJ model through innervation of constructs by co-culture with human induced pluripotent stem cells differentiated into neuronal stem cells. Effects of these approaches on construct maturity will be measured by changes to the panel of maturation indices and contractile properties, determining the extent of maturity that can be achieved by human muscle constructs in vitro and producing a platform for screening therapeutic interventions targeting sarcopenia.