Time-dependent reduction in oxidative capacity among cultured myotubes from spinal cord injured individuals

BACKGROUND

Skeletal muscle adapts in reaction to contractile activity to efficiently utilize energy substrates, primarily glucose and free fatty acids (FA). Inactivity leads to atrophy and a change in energy utilization in individuals with spinal cord injury (SCI). The present study aimed to characterize possible inactivity-related differences in the energy metabolism between skeletal muscle cells cultured from satellite cells isolated 1- and 12-months post-SCI.

METHODS

To characterize inactivity-related disturbances in spinal cord injury, we studied skeletal muscle cells isolated from SCI subjects. Cell cultures were established from biopsy samples from musculus vastus lateralis from subjects with SCI 1 and 12 months after the injury. The myoblasts were proliferated and differentiated into myotubes before fatty acid and glucose metabolism were assessed and gene and protein expressions were measured.

RESULTS

The results showed that glucose uptake was increased, while oleic acid oxidation was reduced at 12 months compared to 1 month. mRNA expressions of PPARGC1α, the master regulator of mitochondrial biogenesis, and MYH2, a determinant of muscle fiber type, were significantly reduced at 12 months. Proteomic analysis showed reduced expression of several mitochondrial proteins.

CONCLUSION

In conclusion, skeletal muscle cells isolated from immobilized subjects 12 months compared to 1 month after SCI showed reduced fatty acid metabolism and reduced expression of mitochondrial proteins, indicating an increased loss of oxidative capacity with time after injury.

Microarchitected Compliant Scaffolds of Pyrolytic Carbon for 3D Muscle Cell Growth

The integration of additive manufacturing technologies with the pyrolysis of polymeric precursors enables the design-controlled fabrication of architected 3D pyrolytic carbon (PyC) structures with complex architectural details. Despite great promise, their use in cellular interaction remains unexplored. This study pioneers the utilization of microarchitected 3D PyC structures as biocompatible scaffolds for the colonization of muscle cells in a 3D environment. PyC scaffolds are fabricated using micro-stereolithography, followed by pyrolysis. Furthermore, an innovative design strategy using revolute joints is employed to obtain novel, compliant structures of architected PyC. The pyrolysis process results in a pyrolysis temperature- and design-geometry-dependent shrinkage of up to 73%, enabling the geometrical features of microarchitected compatible with skeletal muscle cells. The stiffness of architected PyC varies with the pyrolysis temperature, with the highest value of 29.57 ± 0.78 GPa for 900 °C. The PyC scaffolds exhibit excellent biocompatibility and yield 3D cell colonization while culturing skeletal muscle C2C12 cells. They further induce good actin fiber alignment along the compliant PyC construction. However, no conclusive myogenic differentiation is observed here. Nevertheless, these results are highly promising for architected PyC scaffolds as multifunctional tissue implants and encourage more investigations in employing compliant architected PyC structures for high-performance tissue engineering applications.

Research progress of intramuscular fat formation based on co-culture

Intramuscular fat (IMF) is closely related to the meat quality of livestock and poultry. As a new cell culture technique in vitro, cell co-culture has been gradually applied to the related research of IMF formation because it can simulate the changes of microenvironment in vivo during the process of IMF cell formation. In the co-culture model, in addition to studying the effects of skeletal muscle cells on the proliferation and differentiation of IMF, we can also consider the role of many secretion factors in the formation of IMF, thus making the cell research in vitro closer to the real level in vivo. This paper reviewed the generation and origin of IMF, summarized the existing co-culture methods and systems, and discussed the advantages and disadvantages of each method as well as the challenges faced in the establishment of the system, with emphasis on the current status of research on the formation of IMF for human and animal based on co-culture technology.

Pulsed Electromagnetic Fields Induce Skeletal Muscle Cell Repair by Sustaining the Expression of Proteins Involved in the Response to Cellular Damage and Oxidative Stress

Pulsed electromagnetic fields (PEMF) are employed as a non-invasive medicinal therapy, especially in the orthopedic field to stimulate bone regeneration. However, the effect of PEMF on skeletal muscle cells (SkMC) has been understudied. Here, we studied the potentiality of 1.5 mT PEMF to stimulate early regeneration of human SkMC. We showed that human SkMC stimulated with 1.5 mT PEMF for four hours repeated for two days can stimulate cell proliferation without inducing cell apoptosis or significant impairment of the metabolic activity. Interestingly, when we simulated physical damage of the muscle tissue by a scratch, we found that the same PEMF treatment can speed up the regenerative process, inducing a more complete cell migration to close the scratch and wound healing. Moreover, we investigated the molecular pattern induced by PEMF among 26 stress-related cell proteins. We found that the expression of 10 proteins increased after two consecutive days of PEMF stimulation for 4 h, and most of them were involved in response processes to oxidative stress. Among these proteins, we found that heat shock protein 70 (HSP70), which can promote muscle recovery, inhibits apoptosis and decreases inflammation in skeletal muscle, together with thioredoxin, paraoxonase, and superoxide dismutase (SOD2), which can also promote skeletal muscle regeneration following injury. Altogether, these data support the possibility of using PEMF to increase SkMC regeneration and, for the first time, suggest a possible molecular mechanism, which consists of sustaining the expression of antioxidant enzymes to control the important inflammatory and oxidative process occurring following muscle damage.

Bioprocessing Considerations towards the Manufacturing of Therapeutic Skeletal and Smooth Muscle Cells

Tissue engineering approaches within the muscle context represent a promising emerging field to address the current therapeutic challenges related with multiple pathological conditions affecting the muscle compartments, either skeletal muscle or smooth muscle, responsible for involuntary and voluntary contraction, respectively. In this review, several features and parameters involved in the bioprocessing of muscle cells are addressed. The cell isolation process is depicted, depending on the type of tissue (smooth or skeletal muscle), followed by the description of the challenges involving the use of adult donor tissue and the strategies to overcome the hurdles of reaching relevant cell numbers towards a clinical application. Specifically, the use of stem/progenitor cells is highlighted as a source for smooth and skeletal muscle cells towards the development of a cellular product able to maintain the target cell's identity and functionality. Moreover, taking into account the need for a robust and cost-effective bioprocess for cell manufacturing, the combination of muscle cells with biomaterials and the need for scale-up envisioning clinical applications are also approached.

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.

RNF7 Induces Skeletal Muscle Cell Apoptosis and Arrests Cell Autophagy via Upregulation of THBS1 and Inactivation of the PI3K/Akt Signaling Pathway in a Rat Sepsis Model

Recently, long noncoding RNAs (lncRNAs) have been highlighted for extensive functionality in sepsis. In this study, we aimed to explore the role of RNF7 in the progression of sepsis. We initially established a rat model of sepsis through cecal ligation and puncture induction, whereupon RNF7 expression was determined by RT-qPCR. Following adenovirus infection, the role of RNF7 in muscle injury, skeletal muscle protein metabolism, oxidative stress, and inflammation in sepsis rats was analyzed. Then, downstream mechanisms of RNF7 were identified and validated. Further, lipopolysaccharide was applied to treat myoblast to further demonstrate the in vitro role of RNF7. Our results showed that RNF7 expression was upregulated during sepsis. Overexpression of RNF7 worsened the sepsis-induced skeletal muscle injury, induced skeletal muscle protein metabolism, oxidative stress, and inflammation in sepsis rats. Meanwhile, overexpression of RNF7 elevated thrombospondin-1 (THBS1) expression. Silencing of RNF7 inhibited THBS1 and activated the PI3K/Akt signaling pathway, arresting the release of inflammatory factors and oxidative stress levels in skeletal muscle cells. Altogether, RNF7 may promote skeletal muscle cell apoptosis while simultaneously inhibiting cell autophagy through the promotion of THBS1 and inactivation of the PI3K/Akt signaling pathway.

Phosphoproteomics Profiling Defines a Target Landscape of the Basophilic Protein Kinases AKT, S6K, and RSK in Skeletal Myotubes

Phosphorylation-dependent signal transduction plays an important role in regulating the functions and fate of skeletal muscle cells. Central players in the phospho-signaling network are the protein kinases AKT, S6K, and RSK as part of the PI3K-AKT-mTOR-S6K and RAF-MEK-ERK-RSK pathways. However, despite their functional importance, knowledge about their specific targets is incomplete because these kinases share the same basophilic substrate motif RxRxxp[ST]. To address this, we performed a multifaceted quantitative phosphoproteomics study of skeletal myotubes following kinase inhibition. Our data corroborate a cross talk between AKT and RAF, a negative feedback loop of RSK on ERK, and a putative connection between RSK and PI3K signaling. Altogether, we report a kinase target landscape containing 49 so far unknown target sites. AKT, S6K, and RSK phosphorylate numerous proteins involved in muscle development, integrity, and functions, and signaling converges on factors that are central for the skeletal muscle cytoskeleton. Whereas AKT controls insulin signaling and impinges on GTPase signaling, nuclear signaling is characteristic for RSK. Our data further support a role of RSK in glucose metabolism. Shared targets have functions in RNA maturation, stability, and translation, which suggests that these basophilic kinases establish an intricate signaling network to orchestrate and regulate processes involved in translation.

Vitamin D as a Shield against Aging

Aging can be seen as a physiological progression of biomolecular damage and the accumulation of defective cellular components, which trigger and amplify the process, toward whole-body function weakening. Senescence initiates at the cellular level and consists in an inability to maintain homeostasis, characterized by the overexpression/aberrant expression of inflammatory/immune/stress responses. Aging is associated with significant modifications in immune system cells, toward a decline in immunosurveillance, which, in turn, leads to chronic elevation of inflammation/oxidative stress, increasing the risk of (co)morbidities. Albeit aging is a natural and unavoidable process, it can be regulated by some factors, like lifestyle and diet. Nutrition, indeed, tackles the mechanisms underlying molecular/cellular aging. Many micronutrients, i.e., vitamins and elements, can impact cell function. This review focuses on the role exerted by vitamin D in geroprotection, based on its ability to shape cellular/intracellular processes and drive the immune response toward immune protection against infections and age-related diseases. To this aim, the main biomolecular paths underlying immunosenescence and inflammaging are identified as biotargets of vitamin D. Topics such as heart and skeletal muscle cell function/dysfunction, depending on vitamin D status, are addressed, with comments on hypovitaminosis D correction by food and supplementation. Albeit research has progressed, still limitations exist in translating knowledge into clinical practice, making it necessary to focus attention on the role of vitamin D in aging, especially considering the growing number of older individuals.

Triterpenoid CDDO-EA inhibits lipopolysaccharide-induced inflammatory responses in skeletal muscle cells through suppression of NF-κB

Chronic inflammation is a major contributor to the development of obesity-induced insulin resistance, which then can lead to the development of type 2 diabetes (T2D). Skeletal muscle plays a pivotal role in insulin-stimulated whole-body glucose disposal. Therefore, dysregulation of glucose metabolism by inflammation in skeletal muscle can adversely affect skeletal muscle insulin sensitivity and contribute to the pathogenesis of T2D. The mechanism underlying insulin resistance is not well known; however, macrophages are important initiators in the development of the chronic inflammatory state leading to insulin resistance. Skeletal muscle consists of resident macrophages which can be activated by lipopolysaccharide (LPS). These activated macrophages affect myocytes via a paracrine action of pro-inflammatory mediators resulting in secretion of myokines that contribute to inflammation and ultimately skeletal muscle insulin resistance. Therefore, knowing that synthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acids (CDDOs) can attenuate macrophage pro-inflammatory responses in chronic disorders, such as cancer and obesity, and that macrophage pro-inflammatory responses can modulate skeletal muscle inflammation, we first examined whether CDDO-ethyl amide (CDDO-EA) inhibited chemokine and cytokine production in macrophages since this had not been reported for CDDO-EA. CDDO-EA blocked LPS-induced tumor necrosis factor-alpha (TNF-α), monocyte chemotactic protein-1 (MCP-1), interleukine-1beta (IL-1β), and interleukine-6 (IL-6) production in RAW 264.7 mouse and THP-1 human macrophages. Although many studies show that CDDOs have anti-inflammatory properties in several tissues and cells, little is known about the anti-inflammatory effects of CDDOs on skeletal muscle. We hypothesized that CDDO-EA protects skeletal muscle from LPS-induced inflammation by blocking nuclear factor kappa B (NF-κB) signaling. Our studies demonstrate that CDDO-EA prevented LPS-induced TNF-α and MCP-1 gene expression by inhibiting the NF-κB signaling pathway in L6-GLUT4myc rat myotubes. Our findings suggest that CDDO-EA suppresses LPS-induced inflammation in macrophages and myocytes and that CDDO-EA is a promising compound as a therapeutic agent for protecting skeletal muscle from inflammation.

L-Carnitine Functionalization to Increase Skeletal Muscle Tropism of PLGA Nanoparticles

Muscular dystrophies are a group of rare genetic pathologies, encompassing a variety of clinical phenotypes and mechanisms of disease. Several compounds have been proposed to treat compromised muscles, but it is known that pharmacokinetics and pharmacodynamics problems could occur. To solve these issues, it has been suggested that nanocarriers could be used to allow controlled and targeted drug release. Therefore, the aim of this study was to prepare actively targeted poly(lactide-co-glycolide) (PLGA) nanoparticles (NPs) for the treatment of muscular pathologies. By taking advantage of the high affinity for carnitine of skeletal muscle cells due to the expression of Na⁺-coupled carnitine transporter (OCTN), NPs have been actively targeted via association to an amphiphilic derivative of L-carnitine. Furthermore, pentamidine, an old drug repurposed for its positive effects on myotonic dystrophy type I, was incorporated into NPs. We obtained monodispersed targeted NPs, with a mean diameter of about 100 nm and a negative zeta potential. To assess the targeting ability of the NPs, cell uptake studies were performed on C2C12 myoblasts and myotubes using confocal and transmission electron microscopy. The results showed an increased uptake of carnitine-functionalized NPs compared to nontargeted carriers in myotubes, which was probably due to the interaction with OCTN receptors occurring in large amounts in these differentiated muscle cells.

Effect of sera from elite athletes on cytokine secretion and insulin signaling in preadipocytes and skeletal muscle cells

Introduction: The immunomodulatory effect of physical activity can impact insulin signaling differentially in adipose tissues and skeletal muscle cells, depending on sport intensity. In this study, the effect of serum from elite athletes with varying endurance levels and playing different power sports on cytokine secretion and insulin signaling in preadipocyte and skeletal muscle cell lines was investigated. Methods: Preadipocytes (3T3-L1) and skeletal muscle cells (C2C12) were cultured in media containing pooled sera from elite athletes who play high-endurance (HE), high-power (HP), or low-endurance/low-power (LE/LP) sports for 72 h. Secreted cytokines (IL-6 and TNF-alpha) were assessed in the supernatant, and insulin signaling phosphoproteins levels were measured in lysates following treatment using cells multiplex immunoassays. Results: Sera from LE/LP and HP induced TNF-α secretion in C2C12, while serum from HE reduced IL-6 secretion compared to non-athlete serum control. All elite athlete sera groups caused decreased insulin sensitivity in 3T3-L1 cells, whereas in C2C12 cells, only HE athlete serum reduced insulin signaling, while LE/LP and HP caused increased insulin sensitivity. Conclusion: Sera from elite athletes of different sport disciplines can affect the inflammatory status and insulin signaling of preadipocytes and myoblasts differently, with risk of developing insulin resistance. Furthermore, investigation of the functional relevance of these effects on exercise physiology and pathophysiology is warranted.

Geometric basis of action potential of skeletal muscle cells and neurons

Although we know something about single-cell neuromuscular junctions, it is still unclear how multiple skeletal muscle cells coordinate to complete intricate spatial curve movement. Here, we hypothesize that skeletal muscle cell populations with action potentials are aligned according to curved manifolds in space (a curved shape in space). When a specific motor nerve impulse is transmitted, the skeletal muscle also moves according to the corresponding shape (manifolds). The action potential of motor nerve fibers has the characteristics of a time curve manifold, and this time-manifold curve of motor nerve fibers comes from the visual cortex in which spatial geometric manifolds are formed within the synaptic connection of neurons. This spatial geometric manifold of the synaptic connection of neurons originates from spatial geometric manifolds outside nature that are transmitted to the brain through the cone cells and ganglion cells of the retina. The essence of life is that life is an object that can move autonomously, and the essence of life's autonomous movement is the movement of proteins. Theoretically, because of the infinite diversity of geometric manifold shapes in nature, the arrangement and combination of 20 amino acids should have infinite diversity, and the geometric manifold formed by the protein three-dimensional spatial structure should also have infinite diversity.

Intramuscular injection of sotagliflozin promotes neovascularization in diabetic mice through enhancing skeletal muscle cells paracrine function

Diabetes mellitus is associated with series of macrovascular and microvascular pathological changes that cause a wide range of complications. Diabetic patients are highly susceptible to hindlimb ischemia (HLI), which remains incurable. Evidence shows that skeletal muscle cells secrete a number of angiogenic factors to promote neovascularization and restore blood perfusion, this paracrine function is crucial for therapeutic angiogenesis in diabetic HLI. In this study we investigated whether sotagliflozin, an anti-hyperglycemia SGLT2 inhibitor, exerted therapeutic angiogenesis effects in diabetic HLI in vitro and in vivo. In C2C12 skeletal muscle cells, we showed that high glucose (HG, 25 mM) under hypoxia markedly inhibited cell viability, proliferation and migration potentials, which were dose-dependently reversed by pretreatment with sotagliflozin (5-20 μM). Sotagliflozin pretreatment enhanced expression levels of angiogenic factors HIF-1α, VEGF-A and PDGF-BB in HG-treated C2C12 cells under hypoxia as well as secreted amounts of VEGF-A and PDGF-BB in the medium; pretreatment with the HIF-1α inhibitor 2-methoxyestradiol (2-ME2, 10 μM) or HIF-1α knockdown abrogated sotagliflozin-induced increases in VEGF-A and PDGF-BB expression, as well as sotagliflozin-stimulated cell proliferation and migration potentials. Furthermore, the conditioned media from sotagliflozin-treated C2C12 cells in HG medium enhanced the migration and proliferation capabilities of vascular endothelial and smooth muscle cells, two types of cells necessary for forming functional blood vessels. In vivo study was conducted in diabetic mice subjected to excising the femoral artery of the left limb. After the surgery, sotagliflozin (10 mg/kg) was directly injected into gastrocnemius muscle of the left hindlimb once every 3 days for 3 weeks. We showed that intramuscular injection of sotagliflozin effectively promoted the formation of functional blood vessels, leading to significant recovery of blood perfusion in diabetic HLI mice. Together, our results highlight a new indication of SGLT2 inhibitor sotagliflozin as a potential therapeutic angiogenesis agent for diabetic HLI.

Regulation of Myogenesis by a Na/K-ATPase α1 Caveolin-Binding Motif

The N-terminal caveolin-binding motif (CBM) in Na/K-ATPase (NKA) α1 subunit is essential for cell signaling and somitogenesis in animals. To further investigate the molecular mechanism, we have generated CBM mutant human-induced pluripotent stem cells (iPSCs) through CRISPR/Cas9 genome editing and examined their ability to differentiate into skeletal muscle (Skm) cells. Compared with the parental wild-type human iPSCs, the CBM mutant cells lost their ability of Skm differentiation, which was evidenced by the absence of spontaneous cell contraction, marker gene expression, and subcellular myofiber banding structures in the final differentiated induced Skm cells. Another NKA functional mutant, A420P, which lacks NKA/Src signaling function, did not produce a similar defect. Indeed, A420P mutant iPSCs retained intact pluripotency and ability of Skm differentiation. Mechanistically, the myogenic transcription factor MYOD was greatly suppressed by the CBM mutation. Overexpression of a mouse Myod cDNA through lentiviral delivery restored the CBM mutant cells' ability to differentiate into Skm. Upstream of MYOD, Wnt signaling was demonstrated from the TOPFlash assay to have a similar inhibition. This effect on Wnt activity was further confirmed functionally by defective induction of the presomitic mesoderm marker genes BRACHYURY (T) and MESOGENIN1 (MSGN1) by Wnt3a ligand or the GSK3 inhibitor/Wnt pathway activator CHIR. Further investigation through immunofluorescence imaging and cell fractionation revealed a shifted membrane localization of β-catenin in CBM mutant iPSCs, revealing a novel molecular component of NKA-Wnt regulation. This study sheds light on a genetic regulation of myogenesis through the CBM of NKA and control of Wnt/β-catenin signaling.

Cerium Oxide Nanoparticle Administration to Skeletal Muscle Cells under Different Gravity and Radiation Conditions

For their remarkable biomimetic properties implying strong modulation of the intracellular and extracellular redox state, cerium oxide nanoparticles (also termed "nanoceria") were hypothesized to exert a protective role against oxidative stress associated with the harsh environmental conditions of spaceflight, characterized by microgravity and highly energetic radiations. Nanoparticles were supplied to proliferating C2C12 mouse skeletal muscle cells under different gravity and radiation levels. Biological responses were thus investigated at a transcriptional level by RNA next-generation sequencing. Lists of differentially expressed genes (DEGs) were generated and intersected by taking into consideration relevant comparisons, which led to the observation of prevailing effects of the space environment over those induced by nanoceria. In space, upregulation of transcription was slightly preponderant over downregulation, implying involvement of intracellular compartments, with the majority of DEGs consistently over- or under-expressed whenever present. Cosmic radiations regulated a higher number of DEGs than microgravity and seemed to promote increased cellular catabolism. By taking into consideration space physical stressors alone, microgravity and cosmic radiations appeared to have opposite effects at transcriptional levels despite partial sharing of molecular pathways. Interestingly, gene ontology denoted some enrichment in terms related to vision, when only effects of radiations were assessed. The transcriptional regulation of mitochondrial uncoupling protein 2 in space-relevant samples suggests perturbation of the intracellular redox homeostasis, and leaves open opportunities for antioxidant treatment for oxidative stress reduction in harsh environments.

Fabrication of hybrid scaffolds obtained from combinations of PCL with gelatin or collagen via electrospinning for skeletal muscle tissue engineering

The creation of skeletal muscle tissue in vitro is a major topic of interest today in the field of biomedical research, due to the lack of treatments for muscle loss due to traumatic accidents or disease. For this reason, the intrinsic properties of nanofibrillar structures to promote cell adhesion, proliferation, and cell alignment presents an attractive tool for regenerative medicine to recreate organized tissues such as muscle. Electrospinning is one of the processing techniques often used for the fabrication of these nanofibrous structures and the combination of synthetic and natural polymers is often required to achieve optimal mechanical and physiochemical properties. Here, polycaprolactone (PCL) is selected as a synthetic polymer used for the fabrication of scaffolds, and the effect of protein addition on the final scaffolds' properties is studied. Collagen and gelatin were the proteins selected and two different concentrations were analyzed (2 and 4 wt/vol%). Different PCL/protein systems were prepared, and a structural, mechanical and functional characterization was performed. The influence of fiber alignment on the properties of the final scaffolds was assessed through morphological, mechanical and biological evaluations. A bioreactor was used to promote cell proliferation and differentiation within the scaffolds. The results revealed that protein addition produced a decrease in the fiber size of the membranes, an increase in their hydrophilicity, and a softening of their mechanical properties. The biological study showed the ability of the selected systems to harbor cells, allow their growth and, potentially, develop musculoskeletal tissues.

New Functions of Vav Family Proteins in Cardiovascular Biology, Skeletal Muscle, and the Nervous System

Simple Summary

In this review, we provide information on the role of Vav proteins, a group of signaling molecules that act as both Rho GTPase activators and adaptor molecules, in the cardiovascular system, skeletal muscle, and the nervous system. We also describe how these functions impact in other physiological and pathological processes such as sympathoregulation, blood pressure regulation, systemic metabolism, and metabolic syndrome.

Abstract

Vav proteins act as tyrosine phosphorylation-regulated guanosine nucleotide exchange factors for Rho GTPases and as molecular scaffolds. In mammals, this family of signaling proteins is composed of three members (Vav1, Vav2, Vav3) that work downstream of protein tyrosine kinases in a wide variety of cellular processes. Recent work with genetically modified mouse models has revealed that these proteins play key signaling roles in vascular smooth and skeletal muscle cells, specific neuronal subtypes, and glia cells. These functions, in turn, ensure the proper regulation of blood pressure levels, skeletal muscle mass, axonal wiring, and fiber myelination events as well as systemic metabolic balance. The study of these mice has also led to the discovery of new physiological interconnection among tissues that contribute to the ontogeny and progression of different pathologies such as, for example, hypertension, cardiovascular disease, and metabolic syndrome. Here, we provide an integrated view of all these new Vav family-dependent signaling and physiological functions.

RNA-Seq-Based Analysis of Cortisol-Induced Differential Gene Expression Associated with Piscirickettsia salmonis Infection in Rainbow Trout (Oncorhynchus mykiss) Myotubes

Salmonid rickettsial septicemia (SRS) is the major infectious disease of the Chilean salmonid aquaculture industry caused by Piscirickettsia salmonis. Intensive farming conditions generate stress and increased susceptibility to diseases, being skeletal muscle mainly affected. However, the interplay between pathogen infection and stress in muscle is poorly understood. In this study, we perform an RNA-seq analysis on rainbow trout myotubes that are pretreated for 3 h with cortisol (100 ng/mL) and then infected with P. salmonis strain LF-89 for 8 h (MOI 50). Twelve libraries are constructed from RNA samples (n = 3 per group) and sequenced on Illumina HiSeq 4000. A total of 704,979,454 high-quality reads are obtained, with 70.25% mapped against the reference genome. In silico DETs include 175 total genes-124 are upregulated and 51 are downregulated. GO enrichment analysis reveals highly impacted biological processes related to apoptosis, negative regulation of cell proliferation, and innate immune response. These results are validated by RT-qPCR of nine candidate transcripts. Furthermore, cortisol pretreatment significantly stimulated bacterial gene expression of ahpC and 23s compared to infection. In conclusion, for the first time, we describe a transcriptomic response of trout myotubes infected with P. salmonis by inducing apoptosis, downregulating cell proliferation, and intrinsic immune-like response that is differentially regulated by cortisol.

Suppression of Pyruvate Dehydrogenase Kinase by Dichloroacetate in Cancer and Skeletal Muscle Cells Is Isoform Specific and Partially Independent of HIF-1α

Inhibition of pyruvate dehydrogenase kinase (PDK) emerged as a potential strategy for treatment of cancer and metabolic disorders. Dichloroacetate (DCA), a prototypical PDK inhibitor, reduces the abundance of some PDK isoenzymes. However, the underlying mechanisms are not fully characterized and may differ across cell types. We determined that DCA reduced the abundance of PDK1 in breast (MDA-MB-231) and prostate (PC-3) cancer cells, while it suppressed both PDK1 and PDK2 in skeletal muscle cells (L6 myotubes). The DCA-induced PDK1 suppression was partially dependent on hypoxia-inducible factor-1α (HIF-1α), a transcriptional regulator of PDK1, in cancer cells but not in L6 myotubes. However, the DCA-induced alterations in the mRNA and the protein levels of PDK1 and/or PDK2 did not always occur in parallel, implicating a role for post-transcriptional mechanisms. DCA did not inhibit the mTOR signaling, while inhibitors of the proteasome or gene silencing of mitochondrial proteases CLPP and AFG3L2 did not prevent the DCA-induced reduction of the PDK1 protein levels. Collectively, our results suggest that DCA reduces the abundance of PDK in an isoform-dependent manner via transcriptional and post-transcriptional mechanisms. Differential response of PDK isoenzymes to DCA might be important for its pharmacological effects in different types of cells.

A muscle fatigue-like contractile decline was recapitulated using skeletal myotubes from Duchenne muscular dystrophy patient-derived iPSCs

Duchenne muscular dystrophy (DMD) is a muscle degenerating disease caused by dystrophin deficiency, for which therapeutic options are limited. To facilitate drug development, it is desirable to develop in vitro disease models that enable the evaluation of DMD declines in contractile performance. Here, we show MYOD1-induced differentiation of hiPSCs into functional skeletal myotubes in vitro with collagen gel and electrical field stimulation (EFS). Long-term EFS training (0.5 Hz, 20 V, 2 ms, continuous for 2 weeks) mimicking muscle overuse recapitulates declines in contractile performance in dystrophic myotubes. A screening of clinically relevant drugs using this model detects three compounds that ameliorate this decline. Furthermore, we validate the feasibility of adapting the model to a 96-well culture system using optogenetic technology for large-scale screening. Our results support a disease model using patient-derived iPSCs that allows for the recapitulation of the contractile pathogenesis of DMD and a screening strategy for drug development.

Current Strategies for the Regeneration of Skeletal Muscle Tissue

Traumatic injuries, tumor resections, and degenerative diseases can damage skeletal muscle and lead to functional impairment and severe disability. Skeletal muscle regeneration is a complex process that depends on various cell types, signaling molecules, architectural cues, and physicochemical properties to be successful. To promote muscle repair and regeneration, various strategies for skeletal muscle tissue engineering have been developed in the last decades. However, there is still a high demand for the development of new methods and materials that promote skeletal muscle repair and functional regeneration to bring approaches closer to therapies in the clinic that structurally and functionally repair muscle. The combination of stem cells, biomaterials, and biomolecules is used to induce skeletal muscle regeneration. In this review, we provide an overview of different cell types used to treat skeletal muscle injury, highlight current strategies in biomaterial-based approaches, the importance of topography for the successful creation of functional striated muscle fibers, and discuss novel methods for muscle regeneration and challenges for their future clinical implementation.

[Cloning and expression of the growth hormone gene and its effect on the proliferation of skeletal muscle cells of Guizhou cattle]

The aim of this study was to investigate the expression of growth hormone (GH) gene on skeletal muscle cell proliferation of Guizhou cattle. The coding sequence of cattle GH gene was amplified by reverse transcription PCR, cloned into the pUCM-T vector and then used to construct the GH gene overexpression vector pEGFP-N3-GH. The expression of the GH gene in skeletal muscle-related tissues (psoas major and longissimus dorsi) of Guizhou cattle was determined by real-time fluorescent quantitative PCR (RT-qPCR). This was followed by culturing and identification of the bovine primary skeletal muscle cells. Subsequently, we introduced the GH gene overexpression vector into the cells to investigate its effect on the proliferation of bovine skeletal muscle cells and the expression of insulin like growth factor 1 and 2 genes related to skeletal muscle growth and development. RT-qPCR results showed that the expression level of GH gene was higher in the psoas major than in the longissimus dorsi of Guizhou cattle, and the expression level in the psoas major of Guanling cattle and Weining cattle was significantly higher than in the longissimus dorsi (P<0.05). The transfection and proliferation results showed that pEGFP-N3-GH significantly increased the expression of GH, IGF-1, and IGF-2 genes in skeletal muscle cells compared to pEGFP-N3 (PP<0.05), and that overexpression of the GH gene also significantly increased the proliferation rate of skeletal muscle cells at the four periods examined (PP<0.01). Our results suggest that GH gene can promote the proliferation of skeletal muscle cells of Guizhou cattle and exerts a positive regulatory effect. This lays the foundation for further exploring the mechanism by which the GH gene affects the growth and development of Guizhou cattle.

Incorporation of Elastin to Improve Polycaprolactone-Based Scaffolds for Skeletal Muscle via Electrospinning

Skeletal muscle regeneration is increasingly necessary, which is reflected in the increasing number of studies that are focused on improving the scaffolds used for such regeneration, as well as the incubation protocol. The main objective of this work was to improve the characteristics of polycaprolactone (PCL) scaffolds by incorporating elastin to achieve better cell proliferation and biocompatibility. In addition, two cell incubation protocols (with and without dynamic mechanical stimulation) were evaluated to improve the activity and functionality yields of the regenerated cells. The results indicate that the incorporation of elastin generates aligned and more hydrophilic scaffolds with smaller fiber size. In addition, the mechanical properties of the resulting scaffolds make them adequate for use in both bioreactors and patients. All these characteristics increase the biocompatibility of these systems, generating a better interconnection with the tissue. However, due to the low maturation achieved in biological tests, no differences could be found between the incubation with and without dynamic mechanical stimulation.

The effect of young and old ex vivo human serum on cellular protein synthesis and growth in an in vitro model of aging

In vitro models of muscle aging are useful for understanding mechanisms of age-related muscle loss and aiding the development of targeted therapies. To investigate mechanisms of age-related muscle loss in vitro utilizing ex vivo human serum, fasted blood samples were obtained from four old (72 ± 1 yr) and four young (26 ± 3 yr) men. Older individuals had elevated levels of plasma CRP, IL-6, HOMA-IR, and lower concentric peak torque and work-per-repetition compared with young participants (P < 0.05). C2C12 myotubes were serum and amino acid starved for 1 h and conditioned with human serum (10%) for 4 h or 24 h. After 4 h, C2C12 cells were treated with 5 mM leucine for 30 min. Muscle protein synthesis (MPS) was determined through the surface sensing of translation (SUnSET) technique and regulatory signaling pathways were measured via Western blot. Myotube diameter was significantly reduced in myotubes treated with serum from old, in comparison to young donors (84%, P < 0.001). MPS was reduced in myotubes treated with old donor serum, compared with young serum before leucine treatment (32%, P < 0.01). MPS and the phosphorylation of Akt, p70S6K, and eEF2 were increased in myotubes treated with young serum in response to leucine treatment, with a blunted response identified in cells treated with old serum (P < 0.05). Muscle protein breakdown signaling pathways did not differ between groups. In summary, we show that myotubes conditioned with serum from older individuals had decreased myotube diameter and MPS compared with younger individuals, potentially driven by low-grade systemic inflammation.

4D Biofabrication Using a Combination of 3D Printing and Melt-Electrowriting of Shape-Morphing Polymers

We report the fabrication of scroll-like scaffolds with anisotropic topography using 4D printing based on a combination of 3D extrusion printing of methacrylated alginate, melt-electrowriting of polycaprolactone fibers, and shape-morphing of the fabricated object. A combination of 3D extrusion printing and melt-electrowriting allows programmed deposition of different materials and fabrication of structures with high resolution. Shape-morphing allows the transformation of a patterned surface of a printed structure in a pattern on inner surface of a folded object that is used to align cells. We demonstrate that the concentration of calcium ions, the environment media, and the geometrical shape of the scaffold influences shape-morphing that allows it to be efficiently programmed. Myoblasts cultured inside a scrolled bilayer scaffold demonstrate excellent viability and proliferation. Moreover, the patterned surface generated by PCL fibers allow a very high degree of orientation of cells, which cannot be achieved on the alginate layer without fibers.

Vitamin D Restores Skeletal Muscle Cell Remodeling and Myogenic Program: Potential Impact on Human Health

Skeletal muscle cells, albeit classified as vitamin D receptor (VDR)-poor cells, are finely controlled by vitamin D through genomic and non-genomic mechanisms. Skeletal muscle constantly undergoes cell remodeling, a complex system under multilevel regulation, mainly orchestrated by the satellite niche in response to a variety of stimuli. Cell remodeling is not limited to satisfy reparative and hypertrophic needs, but, through myocyte transcriptome/proteome renewal, it warrants the adaptations necessary to maintain tissue integrity. While vitamin D insufficiency promotes cell maladaptation, restoring vitamin D levels can correct/enhance the myogenic program. Hence, vitamin D fortified foods or supplementation potentially represents the desired approach to limit or avoid muscle wasting and ameliorate health. Nevertheless, consensus on protocols for vitamin D measurement and supplementation is still lacking, due to the high variability of lab tests and of the levels required in different contexts (i.e., age, sex, heath status, lifestyle). This review aims to describe how vitamin D can orchestrate skeletal muscle cell remodeling and myogenic programming, after reviewing the main processes and cell populations involved in this important process, whose correct progress highly impacts on human health. Topics on vitamin D optimal levels, supplementation and blood determination, which are still under debate, will be addressed.

Role of miRNAs and lncRNAs in dexamethasone-induced myotube atrophy in vitro

Skeletal muscle atrophy is a well-known adverse effect of long-term glucocorticoid (GC) therapy. MicroRNAs (miRNAs or miRs) and long non-coding RNAs (lncRNAs) are important regulators in a number of physiological and pathological processes. However, the role of miRNAs and lncRNAs in the regulation of GC-treated muscle atrophy remains poorly understood. In the current study, muscular atrophy was induced and the results indicated that C2C12 myotubes were thinner than normal, while the expression of muscle ring finger protein 1 and Atrogin-1 was increased. The expression of nine miRNAs and seven lncRNAs associated with proliferation and differentiation were analyzed in a dexamethasone (DEX)-induced muscle atrophy cell model. In addition, the mRNA expression of the downstream targets of lncRNAs that were differentially expressed between DEX-treated and control cells were determined. The results indicated that the expression of miR-133a, miR-133b, miR-206 and five lncRNAs (increased Atrolnc-1, Dum, MAR1, linc-MD1 and decreased Myolinc) were significantly different between the DEX and the control group. Furthermore, the relative mRNA expression of Wnt5a and MyoD was significantly different between the two groups. The results of the current study indicated that some important miRNAs and lncRNAs are associated with DEX-induced muscle atrophy and have the potential to be further developed as a diagnostic tool for this condition.

Energetic Interactions Between Subcellular Organelles in Striated Muscles

Adult striated muscle cells present highly organized structure with densely packed intracellular organelles and a very sparse cytosol accounting for only few percent of cell volume. These cells have a high and fluctuating energy demand that, in continuously working oxidative muscles, is fulfilled mainly by oxidative metabolism. ATP produced by mitochondria should be directed to the main energy consumers, ATPases of the excitation-contraction system; at the same time, ADP near ATPases should rapidly be eliminated. This is achieved by phosphotransfer kinases, the most important being creatine kinase (CK). Specific CK isoenzymes are located in mitochondria and in close proximity to ATPases, forming efficient energy shuttle between these structures. In addition to phosphotransfer kinases, ATP/ADP can be directly channeled between mitochondria co-localized with ATPases in a process called “direct adenine nucleotide channeling, DANC.” This process is highly plastic so that inactivation of the CK system increases the participation of DANC to energy supply owing to the rearrangement of cell structure. The machinery for DANC is built during postnatal development in parallel with the increase in mitochondrial mass, organization, and complexification of the cell structure. Disorganization of cell architecture remodels the mitochondrial network and decreases the efficacy of DANC, showing that this process is intimately linked to cardiomyocyte structure. Accordingly, in heart failure, disorganization of the cell structure along with decrease in mitochondrial mass reduces the efficacy of DANC and together with alteration of the CK shuttle participates in energetic deficiency contributing to contractile failure.

Dapagliflozin Promotes Neovascularization by Improving Paracrine Function of Skeletal Muscle Cells in Diabetic Hindlimb Ischemia Mice Through PHD2/HIF-1α Axis

Diabetes mellitus is associated with a high risk of hindlimb ischemia (HLI) progression and an inevitably poor prognosis, including worse limb salvage and mortality. Skeletal muscle cells can secrete angiogenic factors, which could promote neovascularization and blood perfusion recovery. Thus, paracrine function of skeletal muscle cells, which is aberrant in diabetic conditions, is crucial for therapeutic angiogenesis in diabetic HLI. Dapagliflozin is a well-known anti-hyperglycemia and anti-obesity drug; however, its role in therapeutic angiogenesis is unknown. Herein, we found that dapagliflozin could act as an angiogenesis stimulator in diabetic HLI. We showed that dapagliflozin enhances the viability, proliferation, and migration potentials of skeletal muscle cells and promotes the secretion of multiple angiogenic factors from skeletal muscle cells, most plausibly through PHD2/HIF-1α axis. Furthermore, we demonstrated that conditioned medium from dapagliflozin-treated skeletal muscle cells enhances the proliferation and migration potentials of vascular endothelial and smooth muscle cells, which are two fundamental cells of functional mature vessels. Finally, an in vivo study demonstrated that intramuscular administration of dapagliflozin effectively enhances the formation of mature blood vessels and, subsequently, blood perfusion recovery in diabetic HLI mice. Hence, our results suggest a novel function of dapagliflozin as a potential therapeutic angiogenesis agent for diabetic HLI.

Exercise Ameliorates Insulin Resistance of Type 2 Diabetes through Motivating Short-Chain Fatty Acid-Mediated Skeletal Muscle Cell Autophagy

Background: Exercise can ameliorate type II diabetes mellitus (T2DM) by regulating intestinal flora metabolites. However, the detailed mechanism needs to be further explored. Methods: A T2DM model using mice was established by feeding them a high-fat diet and giving them subsequent streptozocin injections. Fasting blood glucose and serum insulin were determined by blood glucose meter and radioimmunoassay, respectively. Intestinal flora was measured by 16sRNA sequencing. SCFA content was measured by gas chromatography (GC) or enzyme-linked immunosorbent assay (ELISA). A fluorescently labeled 2-deoxyglucose (2-NBDG) kit was employed to detect glucose uptake capacity, and western blot was utilized to explore the signaling pathway of insulin resistance and cell autophagy. Results: In the T2DM model, along with a reduction in insulin resistance (IR), exercise reversed the decline of intestinal Bacteroidetes and the increase of Firmicutes. For metabolites of Bacteroides, exercise restored the decline in total intestinal and plasma short-chain fatty acids (SCFAs) in T2DM mice. However, the administration of GLPG0974—the inhibitor of G protein-coupled receptor 43 (GPR43), which is the receptor of SCFAs—abolished exercise-mediated alleviation in IR in vivo and acetate-mediated reduction of skeletal muscle IR (SMIR) in vitro. Mechanistically, exercise induced skeletal muscle cell autophagy, thereby ameliorating SMIR, which was neutralized by GLPG0974 exposure. Conclusions: Exercise-mediated SCFAs-upregulation may ameliorate insulin resistance (IR) through increasing autophagy of skeletal muscle cells by binding to GPR43. This study provides a theoretical basis for targeting gut bacterial metabolites to prevent T2DM.

Regulation of Lin28a-miRNA let-7b-5p pathway in skeletal muscle cells by peroxisome proliferator-activated receptor delta

Lin28a/miRNA let-7b-5p pathway has emerged as a key regulators of energy homeostasis in the skeletal muscle. However, the mechanism through which this pathway is regulated in the skeletal muscle has remained unclear. We have found that 8 wk of aerobic training (Tr) markedly decreased let-7b-5p expression in murine skeletal muscle, whereas high-fat diet (Hfd) increased its expression. Conversely, Lin28a expression, a well-known inhibitor of let-7b-5p, was induced by Tr and decreased by Hfd. Similarly, in human muscle biopsies, Tr increased LIN28 expression and decreased let-7b-5p expression. Bioinformatics analysis of LIN28a DNA sequence revealed that its enrichment in peroxisome proliferator-activated receptor delta (PPARδ) binding sites, which is a well-known metabolic regulator of exercise. Treatment of primary mouse skeletal muscle cells or C2C12 cells with PPARδ activators GW501516 and AICAR increased Lin28a expression. Lin28a and let-7b-5p expression was also regulated by PPARδ coregulators. While PPARγ coactivator-1α (PGC1α) increased Lin28a expression, corepressor NCoR1 decreased its expression. Furthermore, PGC1α markedly reduced the let-7b-5p expression. PGC1α-mediated induction of Lin28a expression was blocked by the PPARδ inhibitor GSK0660. In agreement, Lin28a expression was downregulated in PPARδ knocked-down cells leading to increased let-7b-5p expression. Finally, we show that modulation of the Lin28a-let-7b-5p pathway in muscle cells leads to changes in mitochondrial metabolism in PGC1α dependent fashion. In summary, we demonstrate that Lin28a-let-7b-5p is a direct target of PPARδ in the skeletal muscle, where it impacts mitochondrial respiration.

lncRNA MALAT1 Accelerates Skeletal Muscle Cell Apoptosis and Inflammatory Response in Sepsis by Decreasing BRCA1 Expression by Recruiting EZH2

Sepsis is a serious and elusive syndrome caused by infection, which is accompanied by a high mortality worldwide. Recent evidence has documented the regulatory role of long non-coding RNA (lncRNA) metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) during the inflammatory process, the effects of which in the development of sepsis have become the focus of the current study. An in vivo mouse model and in vitro cell model of sepsis induced by lipopolysaccharide (LPS) were developed. High expression of lncRNA MALAT1 along with low expression of breast cancer susceptibility gene 1 (BRCA1) were identified in septic mice and human skeletal muscle cells of sepsis. Then, lncRNA MALAT1 expression was altered in vivo and in vitro to examine serum levels of inflammatory factors, as well as skeletal muscle cell apoptosis. lncRNA MALAT1 was noted to regulate the expression and export from the nucleus of BRCA1 by recruiting zeste homolog 2 (EZH2) in skeletal muscle cells of sepsis. Silencing lncRNA MALAT1 resulted in reduced serum levels of interleukin (IL)-6, IL-8, and tumor necrosis factor alpha (TNF-α), neutrophil migration, skeletal muscle cell apoptosis, and AKT-1 phosphorylation. Taken together, lncRNA MALAT1 interacting with EZH2 stimulated AKT-1 phosphorylation and decreased BRCA1 expression, consequently aggravating the progression of sepsis, highlighting a promising therapeutic option for sepsis.

Diosgenin Supplementation Prevents Lipid Accumulation and Induces Skeletal Muscle-Fiber Hypertrophy in Rats

Diosgenin (Dio) is a steroid sapogenin found in plants such as Dioscorea species, and is recognized as a phytochemical against various disorders as well as a natural precursor of steroidal drugs. The present study used rats fed high-cholesterol (Chol) diets supplemented with or without 0.5% Dio for 6 wk to investigate the effects of dietary Dio on lipid metabolism. Dio supplementation significantly increased serum high-density lipoprotein Chol concentrations and fecal Chol content, and significantly decreased fecal bile acid content compared rats fed a high-Chol diet alone, showing that dietary Dio may facilitate excretion of Chol rather than bile acids. A reduction in the liver triglyceride content and intra-abdominal visceral fat was observed in Dio-supplemented rats. Interestingly, dietary Dio also significantly increased the skeletal muscle-fiber diameter and area in the thigh muscles of the rats. Mouse myoblast-derived C2C12 cells were used to examine whether Dio directly affected skeletal muscle. Dio promoted fusion of myoblasts into multinucleated cells or myotubes. Furthermore, in myotube C2C12 cells, protein levels of phosphorylated AMP-activated protein kinase (AMPK) increased with Dio treatment in a dose-dependent manner. These results indicate that Dio may not only induce myoblast fusion and enhance skeletal muscle as an energy expenditure organ, but may also activate the catabolic pathway via AMPK in skeletal muscle cells. Thus, these effects of Dio on skeletal muscles may contribute to inhibition of visceral fat accumulation.

Tyrosol Facilitates Neovascularization by Enhancing Skeletal Muscle Cells Viability and Paracrine Function in Diabetic Hindlimb Ischemia Mice

As one of the most severe manifestations of diabetes, vascular complications are the main causes of diabetes-related morbidity and mortality. Hyperglycemia induces systemic abnormalities, including impaired angiogenesis, causing diabetic patients to be highly susceptible in suffering hindlimb ischemia (HLI). Despite its severe prognosis, there is currently no effective treatment for diabetic HLI. Skeletal muscle cells secrete multiple angiogenic factors, hence, recently are reported to be critical for angiogenesis; however, hyperglycemia disrupted the paracrine function in skeletal muscle cells, leading to the impaired angiogenesis potential observed in diabetic patients. The present study showed that tyrosol, a phenylethanoid compound, suppresses accumulation of intracellular reactive oxygen species (ROS) caused by hyperglycemia, most plausibly by promoting heme oxygenase-1 (HO-1) expression in skeletal muscle cells. Consequently, tyrosol exerts cytoprotective function against hyperglycemia-induced oxidative stress in skeletal muscle cells, increases their proliferation vigorously, and simultaneously suppresses apoptosis. Furthermore, tyrosol grossly increases the secretion of vascular endothelial growth factor-A (VEGF-A) and platelet-derived growth factor-BB (PDGF-BB) from skeletal muscle cells. This leads to enhanced proliferation and migration capabilities of vascular endothelial and smooth muscle cells, two types of cells that are responsible in forming blood vessels, through cell-cell communication. Finally, in vivo experiment using the diabetic HLI mouse model showed that tyrosol injection into the gastrocnemius muscle of the ischemic hindlimb significantly enhances the formation of functional blood vessels and subsequently leads to significant recovery of blood perfusion. Overall, our findings highlight the potential of the pharmacological application of tyrosol as a small molecule drug for therapeutic angiogenesis in diabetic HLI patients.

Development of Microfluidic Stretch System for Studying Recovery of Damaged Skeletal Muscle Cells

The skeletal muscle occupies about 40% mass of the human body and plays a significant role in the skeletal movement control. Skeletal muscle injury also occurs often and causes pain, discomfort, and functional impairment in daily living. Clinically, most studies observed the recovery phenomenon of muscle by massage or electrical stimulation, but there are limitations on quantitatively analyzing the effects on recovery. Although additional efforts have been made within in vitro biochemical research, some questions still remain for effects of the different cell microenvironment for recovery. To overcome these limitations, we have developed a microfluidic system to investigate appropriate conditions for repairing skeletal muscle injury. First, the muscle cells were cultured in the microfluidic chip and differentiated to muscle fibers. After differentiation, we treated hydrogen peroxide and 18% axial stretch to cause chemical and physical damage to the muscle fibers. Then the damaged muscle fibers were placed under the cyclic stretch condition to allow recovery. Finally, we analyzed the damage and recovery by quantifying morphological change as well as the intensity change of intracellular fluorescent signals and showed the skeletal muscle fibers recovered better in the cyclic stretched condition. In total, our in situ generation of muscle damage and induction recovery platform may be a key system for investigating muscle recovery and rehabilitation.

Icariin Ameliorates Palmitate-Induced Insulin Resistance Through Reducing Thioredoxin-Interacting Protein (TXNIP) and Suppressing ER Stress in C2C12 Myotubes

Both thioredoxin-interacting protein (TXNIP) and endoplasmic reticulum (ER) stress are implicated in skeletal muscle insulin resistance. Icariin has been found to mimic insulin action in normal skeletal muscle C2C12 cells and display anti-diabetic properties in diet-induced obese mice. However, the underlying molecular mechanism remains to be well-established. Herein, we tested the hypothesis that the protective effects of icariin on free fatty acid-induced insulin resistance were attributed to its regulation on TXNIP protein levels and ER stress in skeletal muscle cells. We found that TXNIP mediated the saturated fatty acid palmitate (PA)-induced insulin resistance in C2C12 myotubes. Icariin treatment significantly restored PA-reduced proteasome activity resulting in reduction of TXNIP protein and suppression of ER stress, as well as improvement of insulin sensitivity. Proteasome inhibition by its specific inhibitor MG132 obviously abolished the inhibitory effect of icariin on PA-induced insulin resistance. In addition, MG132 supplementation markedly abrogated the impacts of icariin on ER stress and TXNIP-mediated downstream events such as inflammation and STAT3 phosphorylation. These results clearly indicate that icariin improves PA-induced skeletal muscle insulin resistance through a proteasome-dependent mechanism, by which icariin downregulats TXNIP levels and inhibits ER stress.

Novel ultrashort self-assembling peptide bioinks for 3D culture of muscle myoblast cells

The ability of skeletal muscle to self-repair after a traumatic injury, tumor ablation, or muscular disease is slow and limited, and the capacity of skeletal muscle to self-regenerate declines steeply with age. Tissue engineering of functional skeletal muscle using 3D bioprinting technology is promising for creating tissue constructs that repair and promote regeneration of damaged tissue. Hydrogel scaffolds used as biomaterials for skeletal muscle tissue engineering can provide chemical, physical and mechanical cues to the cells in three dimensions thus promoting regeneration. Herein, we have developed two synthetically designed novel tetramer peptide biomaterials. These peptides are self-assembling into a nanofibrous 3D network, entrapping 99.9% water and mimicking the native collagen of an extracellular matrix. Different biocompatibility assays including MTT, 3D cell viability assay, cytotoxicity assay and live-dead assay confirm the biocompatibility of these peptide hydrogels for mouse myoblast cells (C2C12). Immunofluorescence analysis of cell-laden hydrogels revealed that the proliferation of C2C12 cells was well-aligned in the peptide hydrogels compared to the alginategelatin control. These results indicate that these peptide hydrogels are suitable for skeletal muscle tissue engineering. Finally, we tested the printability of the peptide bioinks using a commercially available 3D bioprinter. The ability to print these hydrogels will enable future development of 3D bioprinted scaffolds containing skeletal muscle myoblasts for tissue engineering applications.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

AMPK/AS160 mediates tiliroside derivatives-stimulated GLUT4 translocation in muscle cells

INTRODUCTION

The Chinese herb Potentilla chinensis can reduce blood glucose level of diabetic mice. Tiliroside is the main effective component, but the detailed mechanism is not clear. Skeletal muscles play an important role in whole body glucose homeostasis. Insulin and exercise/contraction stimulate glucose uptake by muscle cells via redistribution of glucose transporter GLUT4 to the cell surface.

MATERIALS AND METHODS

We explored the effects of tiliroside derivatives on cell surface GLUT4 level (GLUT4 translocation) and the underlying mechanism in L6-GLUT4myc muscle cells.

RESULTS

We showed that tiliroside derivatives D1-22 stimulated GLUT4myc translocation in L6-GLUT4myc skeletal muscle cells. Derivatives D1, D8 and D18 regulated GLUT4myc translocation in a time- and dose-dependent manner. Their effects on GLUT4 were additive with that of acute insulin stimulation. Moreover, they increased phosphorylated adenosine monophosphate-activated protein kinase (AMPK), but not protein kinase B (PKB, also called Akt). Their effects on GLUT4 were inhibited by Compound C. In addition, derivative D8 significantly stimulated AMPK and Akt substrate of 160 kDa (AS160) phosphorylation and GLUT4myc translocation in L6-GLUT4myc cells, but not in L6-AS160 4A-GLUT4myc cells.

CONCLUSION

Tiliroside derivatives D1, D8 and D18 stimulated GLUT4myc translocation by a mechanism different to that of insulin in skeletal muscle cells. The effect of derivative D8 on GLUT4myc translocation is mediated by AMPK/AS160 signaling pathway.

Egg White Ovotransferrin-Derived ACE Inhibitory Peptide Ameliorates Angiotensin II-Stimulated Insulin Resistance in Skeletal Muscle Cells

SCOPE

The renin-angiotensin system (RAS) is a major contributor to the development of insulin resistance and its related complications. Egg white ovotransferrin-derived tripeptides, IRW (Ile-Arg-Trp), IQW (Ile-Gln-Trp), or LKP (Leu-Lys-Pro) are previously identified as the inhibitors of angiotensin-converting enzyme (ACE), a key enzyme in the RAS. This study aims at determining whether these peptides are effective in improving insulin resistance, and their mechanisms of action, in a rat derived skeletal muscle cell line (L6 cells).

METHODS AND RESULTS

Insulin resistance is induced by treating L6 cells with 1 μm angiotensin II (Ang II) for 24 h. Effects of peptides on glucose uptake are determined using glucose uptake assay, glucose transporter 4 (GLUT4) translocation by immunofluorescence, reactive oxygen species (ROS) by dihydroethidium (DHE) staining, while insulin signaling pathway, Ang II receptor (AT1R or AT2R) levels, and NADPH oxidase activation are measured using Western Blot. Only IRW treatment significantly improves insulin resistance in L6 cells via stimulation of insulin signaling. IRW decreases Ang II-stimulated AT1R expression, ROS formation, and NADPH oxidase activation.

CONCLUSIONS

Of three ACE inhibitory peptides studied, only IRW improves insulin resistance in L6 cells, at least partially via reduced AT1R expression and its anti-oxidative activity.

Elevating VEGF-A and PDGF-BB secretion by salidroside enhances neoangiogenesis in diabetic hind-limb ischemia

Hind-limb ischemia (HLI) is one of the major complication of diabetic patients. Angiogenesis potential in diabetic patients is severely disrupted, and the mechanism underlying it has not been fully elucidated, making it an obstacle for developing an efficient therapeutic angiogenesis strategy. Skeletal muscle cells, through their paracrine function, had been known to be critical for neoangiogenesis. Here we found that hyperglycemia upregulates the expression of skeletal muscle cells prolyl hydroxylase domain 3 (PHD3), which resulted in the decrease of the secretion of angiogenic factors, especially VEGF-A and PDGF-BB. We showed that treatment with salidroside, a small molecule drug, significantly suppresses PHD3 expression and increases VEGF-A and PDGF-BB secretion from skeletal muscle cells, which in turn enhances the proliferation and migration potentials of endothelial and smooth muscle cells. Finally, we demonstrated that intramuscular injection of salidroside into the ischemic hind limbs of diabetic HLI model mice could efficiently induce neoangiogenesis and blood perfusion recovery. Thus, our novel findings not only reveal the effects of hyperglycemia on the angiogenesis potential of skeletal muscle cells and the mechanism underlying it, but also provides a novel finding suggesting that salidroside might be a potential small molecule drug for diabetic HLI.

Electrical pulse stimulation of cultured skeletal muscle cells as a model for in vitro exercise - possibilities and limitations

The beneficial health-related effects of exercise are well recognized, and numerous studies have investigated underlying mechanism using various in vivo and in vitro models. Although electrical pulse stimulation (EPS) for the induction of muscle contraction has been used for quite some time, its application on cultured skeletal muscle cells of animal or human origin as a model of in vitro exercise is a more recent development. In this review, we compare in vivo exercise and in vitro EPS with regard to effects on signalling, expression level and metabolism. We provide a comprehensive overview of different EPS protocols and their applications, discuss technical aspects of this model including critical controls and the importance of a proper maintenance procedure and finally discuss the limitations of the EPS model.

* Synthesis and Characterization of Electroactive Gellan Gum Spongy-Like Hydrogels for Skeletal Muscle Tissue Engineering Applications

Advances on materials' research for tissue engineering (TE) applications have shown that animal cells respond directly to the material physical, chemical, mechanical, and electrical stimuli altering a variety of cell signaling cascades, which consequently result in phenotypic and genotypic alterations. Gellan gum (GG) spongy-like hydrogels (SLH) with open microstructure, mechanical properties, and cell performance have shown promising results for soft TE applications. Taking advantage of intrinsic properties of GG-SLH and polypyrrole (PPy) electroactivity, we developed electroactive PPy-GG-SLH envisaging their potential use for skeletal muscle TE. Three different methods of in situ chemical oxidative polymerization were developed based on the availability of pyrrole: freely dissolved in solution (method I and III) or immobilized into GG hydrogels (method II). PPy was homogeneously distributed within (method I and III) and on the surface (method II) of GG-SLH, as also confirmed by Fourier Transform infrared spectra. PPy-GG-SLH showed higher conductivity than GG-SLH (p < 0.05) whereas PPy-GG-SLH (method I and II) showed the best conductivity among the 3 methods (∼1 to 2 × 10^-4 S/cm). The microarchitecture of PPy-GG-SLH (method I) was similar to GG-SLH but PPy-GG-SLH (method II and III) presented smaller pore sizes and lower porosity. PPy-GG-SLH (method I and II) compressive modulus (∼450-500 KPa) and recovering capacity (∼75-90%) was higher than GG-SLH, nevertheless the mechanical properties of PPy-GG-SLH (method III) were lower. The water uptake of PPy-GG-SLH was rapidly up to 2500% and were stable along 60 days of degradation being the maximum weight loss 20%. Mechanically stable and electroactive PPy-GG-SLH (method I and II) were analyzed regarding cellular performance. PPy-GG-SLH were not cytotoxic for L929 cells. In addition, L929 and C2C12 myoblast cells were able to adhere and spread within PPy-GG-SLH, showing improved spreading in comparison to GG-SLH performance. Overall, PPy-GG-SLH show promising features as an alternative electroactive platform to analyze the influence of electrical stimulation on skeletal muscle cells.

Development of Flexible Cell-Loaded Ultrathin Ribbons for Minimally Invasive Delivery of Skeletal Muscle Cells

Cell transplantation therapy provides a potential solution for treating skeletal muscle disorders, but cell survival after transplantation is poor. This limitation could be addressed by grafting donor cells onto biomaterials to protect them against harsh environments and processing, consequently improving cell viability in situ. Thus, we present here the fabrication of poly(lactic-co-glycolic acid) (PLGA) ultrathin ribbons with "canal-like" structures using a microfabrication technique to generate ribbons of aligned murine skeletal myoblasts (C2C12). We found that the ribbons functionalized with a solution of 3,4-dihydroxy-l-phenylalanine (DOPA) and then coated with poly-l-lysine (PLL) and fibronectin (FN) improve cell attachment and support the growth of C2C12. The viability of cells on the ribbons is evaluated following the syringe-handling steps of injection with different needle sizes. C2C12 cells readily adhere to the ribbon surface, proliferate over time, align (over 74%), maintain high viability (over 80%), and differentiate to myotubes longer than 400 μm. DNA content quantification carried out before and after injection and myogenesis evaluation confirm that cell-loaded ribbons can safely retain cells with high functionality after injection and are suitable for minimally invasive cell transplantation.

Platelet rich plasma releasate promotes proliferation of skeletal muscle cells in association with upregulation of PCNA, cyclins and cyclin dependent kinases

Platelet rich plasma (PRP) contains various cytokines and growth factors which may be beneficial to the healing process of injured muscle. The purpose of this study is to investigate the effect and molecular mechanism of PRP releasate on proliferation of skeletal muscle cells. Skeletal muscle cells intrinsic to Sprague-Dawley rats were treated with PRP releasate. Cell proliferation was evaluated by 3-[4,5-Dimethylthiazol- 2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay and immunocytochemistry with Ki-67 stain. Flow cytometric analysis was used to evaluate the cell cycle progression. Western blot analysis was used to evaluate the protein expressions of PCNA, cyclin E1, cyclin A2, cyclin B1, cyclin dependent kinase (cdk)1 and cdk2. The results revealed that PRP releasate enhanced proliferation of skeletal muscle cells by shifting cells from G1 phase to S phase and G2/M phases. Ki-67 stain revealed the increase of proliferative capability after PRP releasate treatment. Protein expressions including cyclin A2, cyclin B1, cdk1, cdk2 and PCNA were up-regulated by PRP releasate in a dose-dependent manner. It was concluded that PRP releasate promoted proliferation of skeletal muscle cells in association with the up-regulated protein expressions of PCNA, cyclin A2, cyclin B1, cdk1 and cdk2.

AMPK activation by prolonged stimulation with interleukin-1β contributes to the promotion of GLUT4 translocation in skeletal muscle cells

Impaired insulin signaling in skeletal muscle cells causes insulin resistance associated with the onset of type 2 diabetes. Although interleukin (IL)-1β has been considered to be implicated in the pathogenesis of type 2 diabetes, the action of prolonged stimulation with IL-1β on the insulin signaling pathway in skeletal muscle cells remains poorly understood. In the current study, we investigated the effect of IL-1β stimulation on insulin signal transduction from the insulin receptor (IR), resulting in glucose transporter 4 (GLUT4) translocation in skeletal muscle cells. In L6-GLUT4myc cells, stimulation with IL-1β for 24 h promoted GLUT4 translocation to the plasma membrane and increased glucose uptake in a concentration-dependent manner, whereas short-term stimulation with IL-1 for up to 6 h did not affect that. In addition, stimulation with IL-1β for 24 h further increased insulin-stimulated GLUT4 translocation. Interestingly, stimulation with IL-1β for 24 h did not cause any change in the phosphorylation of insulin signal molecules IR, insulin receptor substrate (IRS)-1, Akt, and p21-activated kinase (PAK1). Stimulation with IL-1β for 24 h significantly increased AMP-activated protein kinase (AMPK) phosphorylation and GLUT4 protein expression. Small interfering RNA (siRNA) targeting AMPK1/2 significantly inhibited IL-1β-stimulated GLUT4 translocation. These results suggest that prolonged stimulation with IL-1β positively regulates GLUT4 translocation in skeletal muscle cells. IL-1β may have a beneficial effect on maintaining glucose homeostasis in skeletal muscle cells in patients with type 2 diabetes. .

Generation of skeletal muscle cells from pluripotent stem cells: advances and challenges

Human pluripotent stem cells (hPSCs) possess unlimited proliferative potential while maintaining the ability to differentiate into any cell type including skeletal muscle cells (SMCs). hPSCs are amenable to genetic editing and can be derived from patient somatic cells, and thus represent a promising option for cell therapies for the treatment of degenerative diseases such as muscular dystrophies. There are unresolved challenges however associated with the derivation and scale-up of hPSCs and generation of differentiated cells in large quantity and high purity. Reported myogenic differentiation protocols are long, require cell sorting and/or rely on ectopic expression of myogenic master regulators. More recent advances have been made with the application of small molecules to enhance the myogenic differentiation efficiency and the identification of more selective markers for the enrichment of myogenic progenitors with enhanced regenerative potential. Here we review the field of myogenic differentiation and highlight areas requiring further research.

SIRT1 attenuates high glucose-induced insulin resistance via reducing mitochondrial dysfunction in skeletal muscle cells

Insulin resistance is often characterized as the most critical factor contributing to the development of type 2 diabetes mellitus (T2DM). Sustained high glucose is an important extracellular environment that induces insulin resistance. Acquired insulin resistance is associated with reduced insulin-stimulated mitochondrial activity as a result of increased mitochondrial dysfunction. Silent information regulator 1 (SIRT1) is one member of the SIRT2 (Sir2)-like family of proteins involved in glucose homeostasis and insulin secretion in mammals. Although SIRT1 has a therapeutic effect on metabolic deterioration in insulin resistance, it is still not clear how SIRT1 is involved in the development of insulin resistance. Here, we demonstrate that pcDNA3.1 vector-mediated overexpression of SIRT1 attenuates insulin resistance in the high glucose-induced insulin-resistant skeleton muscle cells. These beneficial effects were associated with ameliorated mitochondrial dysfunction. Further studies have demonstrated that SIRT1 restores mitochondrial complex I activity leading to decreased oxidative stress and mitochondrial dysfunction. Furthermore, SIRT1 significantly elevated the level of another SIRT which is named SIRT3, and SIRT3 siRNA-suppressed SIRT1-induced mitochondria complex activity increments. Taken together, these results showed that SIRT1 improves insulin sensitivity via the amelioration of mitochondrial dysfunction, and this is achieved through the SIRT1-SIRT3-mitochondrial complex I pathway.

TGFβ1 evokes myoblast apoptotic response via a novel signaling pathway involving S1P4 transactivation upstream of Rho-kinase-2 activation

In view of its multiple detrimental effects, transforming growth factor β1 (TGFβ1) is recognized as critical negative regulator of skeletal muscle repair. Apoptosis of skeletal muscle precursor cells driven by TGFβ1 contributes to the negative role exerted by the cytokine in tissue repair, although the underlying molecular mechanisms are still elusive. Herein we report the identification of a new signaling pathway, relying on Rho kinase-2 stimulation, subsequent to SMAD-dependent S1P4 up-regulation and transactivation via sphingosine kinase (SK)-2, that accounts for TGFβ1-induced apoptosis in cultured myoblasts. S1P4-specific gene silencing reduced by almost 50% activation of caspase-3 and poly-ADP ribosyl transferase cleavage elicited by TGFβ1. Moreover, the selective S1P4 antagonist CYM50358 also reduced the TGFβ1 proapoptotic effects. By employing pharmacological and molecular biological approaches, the involvement of SK2 and ROCK2 in the transmission of the TGFβ1 apoptotic action was also demonstrated. These results reinforce the notion that the SK/S1P axis plays a fundamental role in TGFβ1 mode of action in skeletal muscle cells and, by disclosing a novel mechanism by which TGFβ1 exerts its harmful action, pinpoint new molecular targets that in principle could be beneficial in the treatment of several skeletal muscle disorders or aging-dependent muscle atrophy.