Increased sphingosine-1-phosphate improves muscle regeneration in acutely injured mdx mice

The current landscape of antifibrotic therapy across different organs: A systematic approach

Fibrosis is a common pathological process that can affect virtually all the organs, but there are hardly any effective therapeutic options. This has led to an intense search for antifibrotic therapies over the last decades, with a great number of clinical assays currently underway. We have systematically reviewed all current and recently finished clinical trials involved in the development of new antifibrotic drugs, and the preclinical studies analyzing the relevance of each of these pharmacological strategies in fibrotic processes affecting tissues beyond those being clinically studied. We analyze and discuss this information with the aim of determining the most promising options and the feasibility of extending their therapeutic value as antifibrotic agents to other fibrotic conditions.

Optimization of Culture Media and Cell Ratios for 3D In Vitro Skeletal Muscle Tissues with Endothelial Cells

A major challenge of engineering larger macroscale tissues in vitro is the limited diffusion of nutrients and oxygen to the interior. For skeletal muscle, this limitation results in millimeter scale outcomes to avoid necrosis. One method to address this constraint may be to vascularize in vitro-grown muscle tissue, to support nutrient (culture media) flow into the interior of the structure. In this exploratory study, we examine culture conditions that enable myogenic development and endothelial cell survival within tissue engineered 3D muscles. Myoblasts (C2C12s), endothelial cells (HUVECs), and endothelial support cells (C3H 10T1/2s) were seeded into Matrigel-fibrin hydrogels and cast into 3D printed frames to form 3D in vitro skeletal muscle tissues. Our preliminary results suggest that the simultaneous optimization of culture media formulation and cell concentrations is necessary for 3D cultured muscles to exhibit robust myosin heavy chain expression and GFP expression from GFP-transfected endothelial cells. The ability to form differentiated 3D muscles containing endothelial cells is a key step toward achieving vascularized 3D muscle tissues, which have potential use as tissue for implantation in a medical setting, as well as for future foods such as cultivated meats.

Irisin Is Target of Sphingosine-1-Phosphate/Sphingosine-1-Phosphate Receptor-Mediated Signaling in Skeletal Muscle Cells

Irisin is a hormone-like myokine produced in abundance by skeletal muscle (SkM) in response to exercise. This myokine, identical in humans and mice, is involved in many signaling pathways related to metabolic processes. Despite much evidence on the regulators of irisin and the relevance of sphingolipids for SkM cell biology, the contribution of these latter bioactive lipids to the modulation of the myokine in SkM is missing. In particular, we have examined the potential involvement in irisin formation/release of sphingosine-1-phosphate (S1P), an interesting bioactive molecule able to act as an intracellular lipid mediator as well as a ligand of specific G-protein-coupled receptors (S1PR). We demonstrate the existence of distinct intracellular pools of S1P able to affect the expression of the irisin precursor FNDC. In addition, we establish the crucial role of the S1P/S1PR axis in irisin formation/release as well as the autocrine/paracrine effects of irisin on myoblast proliferation and myogenic differentiation. Altogether, these findings provide the first evidence for a functional crosstalk between the S1P/S1PR axis and irisin signaling, which may open new windows for potential therapeutic treatment of SkM dysfunctions.

Metabolic Pathways and Ion Channels Involved in Skeletal Muscle Atrophy: A Starting Point for Potential Therapeutic Strategies

Skeletal muscle tissue has the important function of supporting and defending the organism. It is the largest apparatus in the human body, and its function is important for contraction and movements. In addition, it is involved in the regulation of protein synthesis and degradation. In fact, inhibition of protein synthesis and/or activation of catabolism determines a pathological condition called muscle atrophy. Muscle atrophy is a reduction in muscle mass resulting in a partial or complete loss of function. It has been established that many physiopathological conditions can cause a reduction in muscle mass. Nevertheless, it is not well known that the molecular mechanisms and signaling processes caused this dramatic event. There are multiple concomitant processes involved in muscle atrophy. In fact, the gene transcription of some factors, oxidative stress mechanisms, and the alteration of ion transport through specific ion channels may contribute to muscle function impairment. In this review, we focused on the molecular mechanisms responsible for muscle damage and potential drugs to be used to alleviate this disabling condition.

Sphingosine Phosphate Lyase Is Upregulated in Duchenne Muscular Dystrophy, and Its Inhibition Early in Life Attenuates Inflammation and Dystrophy in Mdx Mice

Duchenne muscular dystrophy (DMD) is a congenital myopathy caused by mutations in the dystrophin gene. DMD pathology is marked by myositis, muscle fiber degeneration, and eventual muscle replacement by fibrosis and adipose tissue. Satellite cells (SC) are muscle stem cells critical for muscle regeneration. Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that promotes SC proliferation, regulates lymphocyte trafficking, and is irreversibly degraded by sphingosine phosphate lyase (SPL). Here, we show that SPL is virtually absent in normal human and murine skeletal muscle but highly expressed in inflammatory infiltrates and degenerating fibers of dystrophic DMD muscle. In mdx mice that model DMD, high SPL expression is correlated with dysregulated S1P metabolism. Perinatal delivery of the SPL inhibitor LX2931 to mdx mice augmented muscle S1P and SC numbers, reduced leukocytes in peripheral blood and skeletal muscle, and attenuated muscle inflammation and degeneration. The effect on SC was also observed in SCID/mdx mice that lack mature T and B lymphocytes. Transcriptional profiling in the skeletal muscles of LX2931-treated vs. control mdx mice demonstrated changes in innate and adaptive immune functions, plasma membrane interactions with the extracellular matrix (ECM), and axon guidance, a known function of SC. Our cumulative findings suggest that by raising muscle S1P and simultaneously disrupting the chemotactic gradient required for lymphocyte egress, SPL inhibition exerts a combination of muscle-intrinsic and systemic effects that are beneficial in the context of muscular dystrophy.

Skeletal Muscle and COVID-19: The Potential Involvement of Bioactive Sphingolipids

SARS-CoV-2 virus infection is the cause of the coronavirus disease 2019 (COVID-19), which is still spreading over the world. The manifestation of this disease can range from mild to severe and can be limited in time (weeks) or persist for months in about 30–50% of patients. COVID-19 is considered a multiple organ dysfunction syndrome and the musculoskeletal system manifestations are beginning to be considered of absolute importance in both COVID-19 patients and in patients recovering from the SARS-CoV-2 infection. Musculoskeletal manifestations of COVID-19 and other coronavirus infections include loss of muscle mass, muscle weakness, fatigue or myalgia, and muscle injury. The molecular mechanisms by which SARS-CoV-2 can cause damage to skeletal muscle (SkM) cells are not yet well understood. Sphingolipids (SLs) represent an important class of eukaryotic lipids with structural functions as well as bioactive molecules able to modulate crucial processes, including inflammation and viral infection. In the last two decades, several reports have highlighted the role of SLs in modulating SkM cell differentiation, regeneration, aging, response to insulin, and contraction. This review summarizes the consequences of SARS-CoV-2 infection on SkM and the potential involvement of SLs in the tissue responses to virus infection. In particular, we highlight the role of sphingosine 1-phosphate signaling in order to aid the prediction of novel targets for preventing and/or treating acute and long-term musculoskeletal manifestations of virus infection in COVID-19.

PET Study of Sphingosine-1-phosphate Receptor 1 Expression in Response to S. aureus Infection

Sphingosine-1-phosphate receptor 1 (S1PR1) plays a crucial role in infectious diseases. Targeting S1PR1 provides protection against pathogens, such as influenza viruses. This study is aimed at investigating S1PR1 in response to bacterial infection by assessing S1PR1 expression in S. aureus-infected mice. A rodent local muscle bacterial infection model was developed by injecting S. aureus to the lower hind limb of Balb/c mice. The changes of S1PR1 expression in response to bacterial infection and blocking treatment were assessed using ex vivo biodistribution and in vivo positron emission tomography (PET) after intravenous injection of an S1PR1-specific radiotracer [18F]TZ4877. The specificity of [18F]TZ4877 was assessed using S1PR1-specific antagonist, NIBR-0213, and S1PR1-specific DsiRNA pretreated the animals. Immunohistochemical studies were performed to confirm the increase of S1PR1 expression in response to infection. Ex vivo biodistribution data showed that the uptake of [18F]TZ4877 was increased 30.6%, 54.3%, 74.3%, and 115.3% in the liver, kidney, pancreas, and thymus of the infected mice, respectively, compared to that in normal control mice, indicating that S1PR1 is involved in the early immune response to bacterial infection. NIBR-0213 or S1PR1-specific DsiRNA pretreatment reduced the tissue uptake of [18F]TZ4877, suggesting that uptake of [18F]TZ4877 is specific. Our PET/CT study data also confirmed that infected mice have increased [18F]TZ4877 uptake in several organs comparing to that in normal control mice. Particularly, compared to control mice, a 39% increase of [18F]TZ4877 uptake was observed in the infected muscle of S. aureus mice, indicating that S1PR1 expression was directly involved in the inflammatory response to infection. Overall, our study suggested that S1PR1 plays an important role in the early immune response to bacterial infection. The uptake of [18F]TZ4877 is tightly correlated with the S1R1 expression in response to S. aureus infection. PET with S1PR1-specific radiotracer [18F]TZ4877 could provide a noninvasive tool for detecting the early S1PR1 immune response to infectious diseases.

Promoting endogenous repair of skeletal muscle using regenerative biomaterials

Skeletal muscles normally have a remarkable ability to repair themselves; however, large muscle injuries and several myopathies diminish this ability leading to permanent loss of function. No clinical therapy yet exists that reliably restores muscle integrity and function following severe injury. Consequently, numerous tissue engineering techniques, both acellular and with cells, are being investigated to enhance muscle regeneration. Biomaterials are an essential part of these techniques as they can present physical and biochemical signals that augment the repair process. Successful tissue engineering strategies require regenerative biomaterials that either actively promote endogenous muscle repair or create an environment supportive of regeneration. This review will discuss several acellular biomaterial strategies for skeletal muscle regeneration with a focus on those under investigation in vivo. This includes materials that release bioactive molecules, biomimetic materials and immunomodulatory materials.

The Role of Innate and Adaptive Immune Cells in Skeletal Muscle Regeneration

Skeletal muscle regeneration is highly dependent on the inflammatory response. A wide variety of innate and adaptive immune cells orchestrate the complex process of muscle repair. This review provides information about the various types of immune cells and biomolecules that have been shown to mediate muscle regeneration following injury and degenerative diseases. Recently developed cell and drug-based immunomodulatory strategies are highlighted. An improved understanding of the immune response to injured and diseased skeletal muscle will be essential for the development of therapeutic strategies.

Role of Sphingosine 1-Phosphate Signalling Axis in Muscle Atrophy Induced by TNFα in C2C12 Myotubes

Skeletal muscle atrophy is characterized by a decrease in muscle mass causing reduced agility, increased fatigability and higher risk of bone fractures. Inflammatory cytokines, such as tumor necrosis factor-alpha (TNFα), are strong inducers of skeletal muscle atrophy. The bioactive sphingolipid sphingosine 1-phoshate (S1P) plays an important role in skeletal muscle biology. S1P, generated by the phosphorylation of sphingosine catalyzed by sphingosine kinase (SK1/2), exerts most of its actions through its specific receptors, S1P_1-5. Here, we provide experimental evidence that TNFα induces atrophy and autophagy in skeletal muscle C2C12 myotubes, modulating the expression of specific markers and both active and passive membrane electrophysiological properties. NMR-metabolomics provided a clear picture of the deep remodelling of skeletal muscle fibre metabolism induced by TNFα challenge. The cytokine is responsible for the modulation of S1P signalling axis, upregulating mRNA levels of S1P₂ and S1P₃ and downregulating those of SK2. TNFα increases the phosphorylated form of SK1, readout of its activation. Interestingly, pharmacological inhibition of SK1 and specific antagonism of S1P₃ prevented the increase in autophagy markers and the changes in the electrophysiological properties of C2C12 myotubes without affecting metabolic remodelling induced by the cytokine, highlighting the involvement of S1P signalling axis on TNFα-induced atrophy in skeletal muscle.

Perimenopausal transdermal estradiol replacement reduces serum HDL cholesterol efflux capacity but improves cardiovascular risk factors

BACKGROUND

The cardiovascular (CV) safety of estrogen replacement therapy (ERT) in perimenopausal women remains uncertain. Although exogenous estrogens increase HDL cholesterol (HDL-C), estrogen-mediated effects on alternative metrics of HDL that may better predict CV risk are unknown.

OBJECTIVE

To determine the effects of transdermal ERT on HDL composition and cholesterol efflux capacity (CEC), as well as the relationships between these metrics and CV risk factors.

METHODS

Fasting plasma samples were analyzed from 101 healthy, perimenopausal women randomized to receive either transdermal placebo or transdermal estradiol (100 μg/24 h) with intermittent micronized progesterone. At baseline and after 6 months of treatment, serum HDL CEC, HDL particle concentration, HDL protein composition, insulin resistance and brachial artery flow-mediated dilatation (FMD) were measured.

RESULTS

No difference between groups was found for change in plasma HDL-C (p = 0.69). Between-group differences were found for changes in serum HDL total CEC [median change from baseline -5.4 (-17.3,+8.4)% ERT group versus +5.8 (-6.3,+16.9)% placebo group, p = 0.01] and ABCA1-specific CEC [median change from baseline -5.3 (-10.7,+6.7)% ERT group versus +7.4 (-1.5,+18.1)% placebo group, p = 0.0002]. Relative to placebo, transdermal ERT led to reductions in LDL-C (p < 0.0001) and insulin resistance (p = 0.0002). An inverse correlation was found between changes in serum HDL total CEC and FMD (β = -0.26, p = 0.004).

CONCLUSIONS

Natural menopause leads to an increase in serum HDL CEC, an effect that is abrogated by transdermal ERT. However, transdermal ERT leads to favorable changes in major CV risk factors.

Sphingosine-1-phosphate in acute exercise and training

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid found in all eukaryotic cells. Although it may function as an intracellular second messenger, most of its effects are induced extracellularly via activation of a family of five specific membrane receptors. Sphingosine-1-phosphate is enriched in plasma, where it is transported by high-density lipoprotein and albumin, as well as in erythrocytes and platelets which store and release large amounts of this sphingolipid. Sphingosine-1-phosphate regulates a host of cellular processes such as growth, proliferation, differentiation, migration, and apoptosis suppression. It was also shown to play an important role in skeletal muscle physiology and pathophysiology. In recent years, S1P metabolism in both muscle and blood was found to be modulated by exercise. In this review, we summarize the current knowledge on the effect of acute exercise and training on S1P metabolism, highlighting the role of this sphingolipid in skeletal muscle adaptation to physical effort.

Role of sphingosine 1-phosphate signalling in tissue fibrosis

Fibrosis is characterized by the excessive accumulation of extracellular matrix components, leading to loss of tissue function in affected organs. Although the majority of fibrotic diseases have different origins, they have in common a persistent inflammatory stimulus and lymphocyte-monocyte interactions that determine the production of numerous fibrogenic cytokines. Treatment to contrast fibrosis is urgently needed, since some fibrotic diseases lead to systemic fibrosis and represent a major cause of death. In this article, the role of the bioactive sphingolipid sphingosine 1-phosphate (S1P) and its signalling pathway in the fibrosis of different tissue contexts is extensively reviewed, highlighting that it may represent an innovative and promising pharmacological therapeutic target for treating this devastating multifaceted disease. In multiple tissues S1P influences different aspects of fibrosis modulating the recruitment of inflammatory cells, as well as cell proliferation, migration and transdifferentiation into myofibroblasts, the cell type mainly involved in fibrosis development. Moreover, at the level of fibrotic lesions, S1P metabolism is profoundly influenced by multiple cross-talk with profibrotic mediators, such as transforming growth factor β, thus finely regulating the development of fibrosis. This article is part of a Special Issue entitled "Physiological and pathological roles of bioactive sphingolipids".

Reduction of circulating sphingosine-1-phosphate worsens mdx soleus muscle dystrophic phenotype

NEW FINDINGS

What is the central question of the study? What are the consequences of reducing circulating sphingosine-1-phosphate (S1P) for muscle physiology in the murine model of Duchenne muscular dystrophy (DMD)? What is the main result and its importance? Reduction of the circulating S1P level in mdx mice aggravates the dystrophic phenotype, as seen by an increase in fibre atrophy, fibrosis and loss of specific force, suggesting that S1P signalling is a potential therapeutic target in DMD. Although further studies are needed, plasma S1P levels have the intriguing possibility of being used as a biomarker for disease severity, an important issue in DMD.

ABSTRACT

Sphingosine-1-phosphate (S1P) is an important regulator of skeletal muscle properties. The dystrophin-deficient mdx mouse possesses low levels of S1P (∼50%) compared with wild type. Increased S1P availability was demonstrated to ameliorate the dystrophic phenotype in Drosophila and in mdx mice. Here, we analysed the effects produced by further reduction of S1P availability on the mass, force and regenerative capacity of dystrophic mdx soleus. Circulating S1P was neutralized by a specific anti-S1P antibody (S1P-Ab) known to lower the extracellular concentration of this signalling lipid. The S1P-Ab was administered intraperitoneally in adult mdx mice every 2 days for the duration of experiments. Soleus muscle properties were analysed 7 or 14 days after the first injection. The decreased availability of circulating S1P after the 14 day treatment reduced mdx soleus fibre cross-sectional area (-16%, P < 0.05), an effect that was associated with an increase in markers of proteolytic (MuRF1 and atrogin-1) and autophagic (p62 and LC3-II/LC3-I ratio) pathways. Moreover, an increase of fibrosis was also observed (+26%, P < 0.05). Notably, the treatment also caused a reduction of specific tetanic tension (-29%, P < 0.05). The mdx soleus regenerative capacity was only slightly influenced by reduced S1P. In conclusion, neutralization of circulating S1P reduces the mass and specific force and increases fibrosis of mdx soleus muscle, thus worsening the dystrophic phenotype. The results confirm that active, functional S1P signalling might counteract the progression of soleus mdx pathology and validate the pathway as a potential therapeutic target for muscular dystrophies.

An adiponectin-S1P autocrine axis protects skeletal muscle cells from palmitate-induced cell death

Background

The prevalence of type 2 diabetes, obesity and their various comorbidities have continued to rise. In skeletal muscle lipotoxicity is well known to be a contributor to the development of insulin resistance. Here it was examined if the small molecule adiponectin receptor agonist AdipoRon mimicked the effect of adiponectin to attenuate palmitate induced reactive oxygen species (ROS) production and cell death in L6 skeletal muscle cells.

Methods

L6 cells were treated ±0.1 mM PA, and ± AdipoRon, then assays analyzing reactive oxygen species (ROS) production and cell death, and intracellular and extracellular levels of sphingosine-1 phosphate (S1P) were conducted. To determine the mechanistic role of S1P gain (using exogenous S1P or using THI) or loss of function (using the SKI-II) were conducted.

Results

Using both CellROX and DCFDA assays it was found that AdipoRon reduced palmitate-induced ROS production. Image-IT DEAD, MTT and LDH assays all indicated that AdipoRon reduced palmitate-induced cell death. Palmitate significantly increased intracellular accumulation of S1P, whereas in the presence of AdipoRon there was increased release of S1P from cells to extracellular medium. It was also observed that direct addition of extracellular S1P prevented palmitate-induced ROS production and cell death, indicating that S1P is acting in an autocrine manner. Pharmacological approaches to enhance or decrease S1P levels indicated that accumulation of intracellular S1P correlated with enhanced cell death.

Conclusion

This data indicates that increased extracellular levels of S1P in response to adiponectin receptor activation can activate S1P receptor-mediated signaling to attenuate lipotoxic cell death. Taken together these findings represent a possible novel mechanism for the protective action of adiponectin.

Duchenne muscular dystrophy animal models for high-throughput drug discovery and precision medicine

Introduction: Duchenne muscular dystrophy (DMD) is an X-linked handicapping disease due to the loss of an essential muscle protein dystrophin. Dystrophin-null animals have been extensively used to study disease mechanisms and to develop experimental therapeutics. Despite decades of research, however, treatment options for DMD remain very limited.Areas covered: High-throughput high-content screening and precision medicine offer exciting new opportunities. Here, the authors review animal models that are suitable for these studies.Expert opinion: Nonmammalian models (worm, fruit fly, and zebrafish) are particularly attractive for cost-effective large-scale drug screening. Several promising lead compounds have been discovered using these models. Precision medicine for DMD aims at developing mutation-specific therapies such as exon-skipping and genome editing. To meet these needs, models with patient-like mutations have been established in different species. Models that harbor hotspot mutations are very attractive because the drugs developed in these models can bring mutation-specific therapies to a large population of patients. Humanized hDMD mice carry the entire human dystrophin gene in the mouse genome. Reagents developed in the hDMD mouse-based models are directly translatable to human patients.

Sphingolipid Metabolism and Signaling in Skeletal Muscle: From Physiology to Physiopathology

Sphingolipids represent one of the major classes of eukaryotic lipids. They play an essential structural role, especially in cell membranes where they also possess signaling properties and are capable of modulating multiple cell functions, such as apoptosis, cell proliferation, differentiation, and inflammation. Many sphingolipid derivatives, such as ceramide, sphingosine-1-phosphate, and ganglioside, have been shown to play many crucial roles in muscle under physiological and pathological conditions. This review will summarize our knowledge of sphingolipids and their effects on muscle fate, highlighting the role of this class of lipids in modulating muscle cell differentiation, regeneration, aging, response to insulin, and contraction. We show that modulating sphingolipid metabolism may be a novel and interesting way for preventing and/or treating several muscle-related diseases.

S1P/S1P Receptor Signaling in Neuromuscolar Disorders

The bioactive sphingolipid metabolite, sphingosine 1-phosphate (S1P), and the signaling pathways triggered by its binding to specific G protein-coupled receptors play a critical regulatory role in many pathophysiological processes, including skeletal muscle and nervous system degeneration. The signaling transduced by S1P binding appears to be much more complex than previously thought, with important implications for clinical applications and for personalized medicine. In particular, the understanding of S1P/S1P receptor signaling functions in specific compartmentalized locations of the cell is worthy of being better investigated, because in various circumstances it might be crucial for the development or/and the progression of neuromuscular diseases, such as Charcot-Marie-Tooth disease, myasthenia gravis, and Duchenne muscular dystrophy.

Sphingosine 1-Phosphate (S1P)/ S1P Receptor Signaling and Mechanotransduction: Implications for Intrinsic Tissue Repair/Regeneration

Tissue damage, irrespective from the underlying etiology, destroys tissue structure and, eventually, function. In attempt to achieve a morpho-functional recover of the damaged tissue, reparative/regenerative processes start in those tissues endowed with regenerative potential, mainly mediated by activated resident stem cells. These cells reside in a specialized niche that includes different components, cells and surrounding extracellular matrix (ECM), which, reciprocally interacting with stem cells, direct their cell behavior. Evidence suggests that ECM stiffness represents an instructive signal for the activation of stem cells sensing it by various mechanosensors, able to transduce mechanical cues into gene/protein expression responses. The actin cytoskeleton network dynamic acts as key mechanotransducer of ECM signal. The identification of signaling pathways influencing stem cell mechanobiology may offer therapeutic perspectives in the regenerative medicine field. Sphingosine 1-phosphate (S1P)/S1P receptor (S1PR) signaling, acting as modulator of ECM, ECM-cytoskeleton linking proteins and cytoskeleton dynamics appears a promising candidate. This review focuses on the current knowledge on the contribution of S1P/S1PR signaling in the control of mechanotransduction in stem/progenitor cells. The potential contribution of S1P/S1PR signaling in the mechanobiology of skeletal muscle stem cells will be argued based on the intriguing findings on S1P/S1PR action in this mechanically dynamic tissue.

Acute Hypobaric Hypoxia-Mediated Biochemical/Metabolic Shuffling and Differential Modulation of S1PR-SphK in Cardiac and Skeletal Muscles

AIM

High altitude exposure alters biochemical, metabolic, and physiological features of heart and skeletal muscles, and hence has pathological consequences in these tissues. Central to these hypoxia-associated biochemical/metabolic shuffling are energy deficit accumulation of free radicals and ensuing oxidative damage in the tissue. Recent preclinical/clinical studies indicate sphingosine-1-phosphate (S1P) axis, comprising S1P G protein coupled receptors (S1PR_1-5) and its synthesizing enzyme-sphingosine kinase (SphK) to have key regulatory roles in homeostatic cardiac and skeletal muscle biology. In view of this, the aim of the present study was to chart the initiation and progression of biochemical/metabolic shuffling and assess the coincident differential modulation of S1PR_(1-5) expression and total SphK activity in cardiac and skeletal muscles from rats exposed to progressive hypobaric hypoxia (HH; 21,000 feet for 12, 24, and 48 hours).

RESULTS

HH-associated responses were evident as raised damage markers in plasma, oxidative stress, decreased total tissue protein, imbalance of intermediate metabolites, and aerobic/anaerobic enzyme activities in cardiac and skeletal muscles (gastrocnemius and soleus) culminating as energy deficit.

CONCLUSION

Cardiac and gastrocnemius muscles were more susceptible to hypoxic environment than soleus muscle. These differential responses were directly and indirectly coincident with temporal expression of S1PR_(1-5) and SphK activity.

Contribution of Extracellular Vesicles in Rebuilding Injured Muscles

Skeletal myofibers are injured due to mechanical stresses experienced during physical activity, or due to myofiber fragility caused by genetic diseases. The injured myofiber needs to be repaired or regenerated to restore the loss in muscle tissue function. Myofiber repair and regeneration requires coordinated action of various intercellular signaling factors-including proteins, inflammatory cytokines, miRNAs, and membrane lipids. It is increasingly being recognized release and transmission of these signaling factors involves extracellular vesicle (EV) released by myofibers and other cells in the injured muscle. Intercellular signaling by these EVs alters the phenotype of their target cells either by directly delivering the functional proteins and lipids or by modifying longer-term gene expression. These changes in the target cells activate downstream pathways involved in tissue homeostasis and repair. The EVs are heterogeneous with regards to their size, composition, cargo, location, as well as time-course of genesis and release. These differences impact on the subsequent repair and regeneration of injured skeletal muscles. This review focuses on how intracellular vesicle production, cargo packaging, and secretion by injured muscle, modulates specific reparative, and regenerative processes. Insights into the formation of these vesicles and their signaling properties offer new understandings of the orchestrated response necessary for optimal muscle repair and regeneration.

Sphingosine-1-phosphate analog FTY720 reverses obesity but not age-induced anabolic resistance to muscle contraction

Sarcopenia, the age-associated loss of skeletal muscle mass and function, is coupled with declines in physical functioning leading to subsequent higher rates of disability, frailty, morbidity, and mortality. Aging and obesity independently contribute to muscle atrophy that is assumed to be a result of the activation of mutual physiological pathways. Understanding mechanisms contributing to the induction of skeletal muscle atrophy with aging and obesity is important for determining targets that may have pivotal roles in muscle loss in these conditions. We find that aging and obesity equally induce an anabolic resistance to acute skeletal muscle contraction as observed with decreases in anabolic signaling activation after contraction. Furthermore, treatment with the sphingosine-1-phosphate analog FTY720 for 4 wk increased lean mass and strength, and the anabolic signaling response to contraction was improved in obese but not older animals. To determine the role of chronic inflammation and different fatty acids on anabolic resistance in skeletal muscle cells, we overexpressed IKKβ with and without exposure to saturated fatty acid (SFA; palmitic acid), polyunsaturated fatty acid (eicosapentaenoic acid), and monounsaturated fatty acid (oleic acid). We found that IKKβ overexpression increased inflammation markers in muscle cells, and this chronic inflammation exacerbated anabolic resistance in response to SFA. Pretreatment with FTY720 reversed the inflammatory effects of palmitic acid in the muscle cells. Taken together, these data demonstrate chronic inflammation can induce anabolic resistance, SFA aggravates these effects, and FTY720 can reverse this by decreasing ceramide accumulation in skeletal muscle.

Engineering Biomimetic Materials for Skeletal Muscle Repair and Regeneration

Although skeletal muscle is highly regenerative following injury or disease, endogenous self-regeneration is severely impaired in conditions of volume traumatic muscle loss. Consequently, tissue engineering approaches are a promising means to regenerate skeletal muscle. Biological scaffolds serve as not only structural support for the promotion of cellular ingrowth but also impart potent modulatory signaling cues that may be beneficial for tissue regeneration. In this work, the progress of tissue engineering approaches for skeletal muscle engineering and regeneration is overviewed, with a focus on the techniques to create biomimetic engineered tissue using extracellular cues. These factors include mechanical and electrical stimulation, geometric patterning, and delivery of growth factors or other bioactive molecules. The progress of evaluating the therapeutic efficacy of these approaches in preclinical models of muscle injury is further discussed.

The role of sphingosine-1-phosphate in skeletal muscle: Physiology, mechanisms, and clinical perspectives

Sphingolipids were discovered more than a century ago and were simply considered as a class of cell membrane lipids for a long time. However, after the discovery of several intracellular functions and their role in the control of many physiological and pathophysiological conditions, these molecules have gained much attention. For instance, the sphingosine-1-phosphate (S1P) is a circulating bioactive sphingolipid capable of triggering strong intracellular reactions through the family of S1P receptors (S1PRs) spread in several cell types and tissues. Recently, the role of S1P in the control of skeletal muscle metabolism, atrophy, regeneration, and metabolic disorders has been widely investigated. In this review, we summarized the knowledge of S1P and its effects in skeletal muscle metabolism, highlighting the role of S1P/S1PRs axis in skeletal muscle regeneration, fatigue, ceramide accumulation, and insulin resistance. Finally, we discussed the physical exercise role in S1P/S1PRs signaling in skeletal muscle cells, and how this nonpharmacological strategy may be prospective for future investigations due to its ability to increase S1P levels.

Involvement of released sphingosine 1-phosphate/sphingosine 1-phosphate receptor axis in skeletal muscle atrophy

Skeletal muscle (SkM) atrophy is caused by several and heterogeneous conditions, such as cancer, neuromuscular disorders and aging. In most types of SkM atrophy overall rates of protein synthesis are suppressed, protein degradation is consistently elevated and atrogenes, such as the ubiquitin ligase Atrogin-1/MAFbx, are up-regulated. The molecular regulators of SkM waste are multiple and only in part known. Sphingolipids represent a class of bioactive molecules capable of modulating the destiny of many cell types, including SkM cells. In particular, we and others have shown that sphingosine 1phosphate (S1P), formed by sphingosine kinase (SphK), is able to act as trophic and morphogenic factor in myoblasts. Here, we report the first evidence that the atrophic phenotype observed in both muscle obtained from mice bearing the C26 adenocarcinoma and C2C12 myotubes treated with dexamethasone was characterized by reduced levels of active phospho-SphK1. The importance of SphK1 activity is also confirmed by the specific pharmacological inhibition of SphK1 able to increase Atrogin-1/MAFbx expression and reduce myotube size and myonuclei number. Furthermore, we found that SkM atrophy was accomplished by significant increase of S1P transporter Spns2 and in changes in the pattern of S1P receptor (S1PRs) subtype expression paralleled by increased Atrogin-1/MAFbx expression, suggesting a role for the released S1P and of specific S1PR-mediated signaling pathways in the control of the ubiquitin ligase. Altogether, these findings provide the first evidence that SphK1/released S1P/S1PR axis acts as a molecular regulator of SkM atrophy, thereby representing a new possible target for therapy in many patho-physiological conditions.

Chemical modulation of cell fates: in situ regeneration

Chemical modulation of cell fates has been widely used to promote tissue and organ regeneration. Small molecules can target the self-renewal, expansion, differentiation, and survival of endogenous stem cells for enhancing their regenerative power or induce dedifferentiation or transdifferentiation of mature cells into proliferative progenitors or specialized cell types needed for regeneration. Here, we discuss current progress and potential using small molecules to promote in vivo regenerative processes by regulating the cell fate. Current studies of small molecules in regeneration will provide insights into developing safe and efficient chemical approaches for in situ tissue repair and regeneration.

Type 2 diabetes is associated with loss of HDL endothelium protective functions

Aims/Hypothesis

One of the hallmarks of diabetes is impaired endothelial function. Previous studies showed that HDL can exert protective effects on endothelium stimulating NO production and protecting from inflammation and suggested that HDL in obese people with diabetes and dyslipidemia may have lower endothelial protective function. We aimed to investigate whether type 2 diabetes impairs HDL endothelium protective functions in people with otherwise normal lipid profile.

Methods

In a case-control study (n = 41 per group) nested in the Cooper Center Longitudinal Study we tested the ability of HDL to protect endothelium by stimulating endothelial nitric oxide synthase activity and suppressing NFκB-mediated inflammatory response in endothelial cells. In parallel we measured HDL protein composition, sphinogosine-1-phosphate and P-selectin.

Results

Despite similar levels of plasma HDL-C the HDL in individuals with type 2 diabetes lost almost 40% of its ability to stimulate eNOS activity (P<0.001) and 20% of its ability to suppress TNFα-dependent NFκB-mediated inflammatory response in endothelial cells (P<0.001) compared to non-T2D controls despite similar BMI and lipid profile (HDL-C, LDL-C, TC, TG). Significantly, the ability of HDL to stimulate eNOS activity was negatively associated with plasma levels of P-selectin, an established marker of endothelial dysfunction (r = −0.32, P<0.001). Furthermore, sphingosine-1-phosphate (S1P) levels were decreased in diabetic plasma (P = 0.017) and correlated with HDL-mediated eNOS activation.

Conclusions/Interpretations

Collectively, our data suggest that HDL in individuals with type 2 diabetes loses its ability to maintain proper endothelial function independent of HDL-C, perhaps due to loss of S1P, and may contribute to development of diabetic complications.

Sphingosine-1-phosphate pretreatment amends hypoxia-induced metabolic dysfunction and impairment of myogenic potential in differentiating C2C12 myoblasts by stimulating viability, calcium homeostasis and energy generation

Sphingosine-1-phosphate (S1P) has a role in transpiration in patho-physiological signaling in skeletal muscles. The present study evaluated the pre-conditioning efficacy of S1P in facilitating differentiation of C2C12 myoblasts under a normoxic/hypoxic cell culture environment. Under normoxia, exogenous S1P significantly promoted C2C12 differentiation as evident from morphometric descriptors and differentiation markers of the mature myotubes, but it could facilitate only partial recovery from hypoxia-induced compromised differentiation. Pretreatment of S1P optimized the myokine secretion, intracellular calcium release and energy generation by boosting the aerobic/anaerobic metabolism and mitochondrial mass. In the hypoxia-exposed cells, there was derangement of the S1PR1-3 expression patterns, while the same could be largely restored with S1P pretreatment. This is being proposed as a plausible underlying mechanism for the observed pro-myogenic efficacy of exogenous S1P preconditioning. The present findings are an invaluable addition to the existing knowledge on the pro-myogenic potential of S1P and may prove beneficial in the field of hypoxia-related myo-pathologies.

Harnessing Sphingosine-1-Phosphate Signaling and Nanotopographical Cues To Regulate Skeletal Muscle Maturation and Vascularization

Despite possessing substantial regenerative capacity, skeletal muscle can suffer from loss of function due to catastrophic traumatic injury or degenerative disease. In such cases, engineered tissue grafts hold the potential to restore function and improve patient quality of life. Requirements for successful integration of engineered tissue grafts with the host musculature include cell alignment that mimics host tissue architecture and directional functionality, as well as vascularization to ensure tissue survival. Here, we have developed biomimetic nanopatterned poly(lactic-co-glycolic acid) substrates conjugated with sphingosine-1-phosphate (S1P), a potent angiogenic and myogenic factor, to enhance myoblast and endothelial maturation. Primary muscle cells cultured on these functionalized S1P nanopatterned substrates developed a highly aligned and elongated morphology and exhibited higher expression levels of myosin heavy chain, in addition to genes characteristic of mature skeletal muscle. We also found that S1P enhanced angiogenic potential in these cultures, as evidenced by elevated expression of endothelial-related genes. Computational analyses of live-cell videos showed a significantly improved functionality of tissues cultured on S1P-functionalized nanopatterns as indicated by greater myotube contraction displacements and velocities. In summary, our study demonstrates that biomimetic nanotopography and S1P can be combined to synergistically regulate the maturation and vascularization of engineered skeletal muscles.

The low down on sphingosine-1-phosphate lyase as a regulator of thymic egress

After undergoing positive and negative selection in the thymus, surviving mature T cells egress from the thymic parenchyma and enter the bloodstream to participate in adaptive immunity. Thymic egress requires signals mediated by sphingosine-1-phosphate (S1P), a bioactive lipid that serves as the ligand for a family of G protein-coupled receptors (S1P1-5) expressed on many cell types, including T cells. In the final stage of their development, T cells upregulate S1P1 expression on the cell surface, which enables them to recognize and respond to a chemotactic S1P gradient that lures them into the bloodstream. The gradient is generated by an S1P source close to the site of egress combined with an S1P sink generated by the actions of S1P catabolic enzymes including S1P lyase (SPL), the only enzyme that irreversibly degrades S1P. The requisite contribution of SPL to thymic egress is demonstrated by the profound lymphopenia observed in SPL knockout (KO) mice and wild type mice treated with SPL inhibitors. SPL is robustly expressed in thymic epithelial cells (TECs), which make up the stromal reticular network of the thymus. However, TEC SPL was recently found to be dispensable for thymic egress. In contrast, deletion of SPL in dendritic cells (DCs) - which represent only a small percent of thymic stroma - disrupts the S1P gradient and blocks thymic egress. These recent observations identify DCs as homeostatic regulators of thymic export through the actions of SPL, thereby adding one more piece to the complex puzzle of how S1P signaling contributes to the regulation of T cell trafficking.

Sphingosine-1-Phosphate Immobilized on Nanotopographical Scaffolds Improve Myogenic Differentiation

The skeletal muscle consists of highly aligned dense cables of collagen fibers with nanometer feature size to support muscle fibers. The skeletal myocyte can be greatly affected to differentiate by their surrounding topographical structure. To improve myogenic differentiation, we fabricated cell culture platform that sphingosine-1-phosphate (S1P) which regulated myocyte behavior is immobilized on a biomimetic nanopatterned polyurethaneacrylate (PUA) substrate using 3,4-dihydroxyphenylalanine (L-DOPA) for providing topographical and biological cues synergistically. In the present study, we hypothesized that cultured C2C12 cells can be induced to synergistically promote myogenic differenntiation on nanopatterned PUA-L-DOPA-S1P. We confirmed that nanopatterned PUA-L-DOPA-S1P has high hydrophilicity with a suitable range of water contact angle and small intensity of phosphate peak (P2p) by analyses of water contact analyzer and X-ray photoelectron spectroscopy. In addition, C2C12 cells culured on nanopatterned PUA-L-DOPA-S1P has well-oriented and organized myodubes formed with greater expression of myogenic regulatory factors such as MyoD and MyoG comapred to flat PUA groups. This functional platform which is not only provided topographical and biological cues has a suitable potential function to apply muscle cell niche as similar structure of muscle fiber but also utilized cell behavior within tissue engineered scaffolds and cellular microenvironment.

Ablation of S1P3 receptor protects mouse soleus from age-related drop in muscle mass, force, and regenerative capacity

We investigated the effects of S1P3 deficiency on the age-related atrophy, decline in force, and regenerative capacity of soleus muscle from 23-mo-old male (old) mice. Compared with muscle from 5-mo-old (adult) mice, soleus mass and muscle fiber cross-sectional area (CSA) in old wild-type mice were reduced by ~26% and 24%, respectively. By contrast, the mass and fiber CSA of soleus muscle in old S1P3-null mice were comparable to those of adult muscle. Moreover, in soleus muscle of wild-type mice, twitch and tetanic tensions diminished from adulthood to old age. A slowing of contractile properties was also observed in soleus from old wild-type mice. In S1P3-null mice, neither force nor the contractile properties of soleus changed during aging. We also evaluated the regenerative capacity of soleus in old S1P3-null mice by stimulating muscle regeneration through myotoxic injury. After 10 days of regeneration, the mean fiber CSA of soleus in old wild-type mice was significantly smaller (−28%) compared with that of regenerated muscle in adult mice. On the contrary, the mean fiber CSA of regenerated soleus in old S1P3-null mice was similar to that of muscle in adult mice. We conclude that in the absence of S1P3, soleus muscle is protected from the decrease in muscle mass and force, and the attenuation of regenerative capacity, all of which are typical characteristics of aging.

Severe murine limb-girdle muscular dystrophy type 2C pathology is diminished by FTY720 treatment

INTRODUCTION

Limb-girdle muscular dystrophy type 2C (LGMD-2C) is caused by mutations in γ-sarcoglycan and is a devastating, progressive, and fully lethal human muscle-wasting disease that has no effective treatment. This study examined the efficacy of the sphingosine-1-phosphate receptor modulator FTY720 in treating Sgcg^-/- DBA2/J, a severe mouse model of LGMD-2C. FTY720 treatment was expected to target LGMD-2C disease progression at 2 key positions by reducing chronic inflammation and fibrosis.

METHODS

The treatment protocol was initiated at age 3 weeks and was continued with alternate-day injections for 3 weeks.

RESULTS

The treatment produced significant functional benefit by plethysmography and significant reductions of membrane permeability and fibrosis. Furthermore, the protocol elevated protein levels of δ-sarcoglycan, a dystrophin-glycoprotein family member.

CONCLUSION

This study showed that FTY720 is an effective muscular dystrophy treatment when therapy is initiated early in the disease progression. Muscle Nerve 56: 486-494, 2017.

PDGFRα signalling promotes fibrogenic responses in collagen-producing cells in Duchenne muscular dystrophy

Fibrosis is a characteristic of Duchenne muscular dystrophy (DMD), yet the cellular and molecular mechanisms responsible for DMD fibrosis are poorly understood. Utilizing the Collagen1a1-GFP transgene to identify cells producing Collagen-I matrix in wild-type mice exposed to toxic injury or those mutated at the dystrophin gene locus (mdx) as a model of DMD, we studied mechanisms of skeletal muscle injury/repair and fibrosis. PDGFRα is restricted to Sca1+, CD45- mesenchymal progenitors. Fate-mapping experiments using inducible CreER/LoxP somatic recombination indicate that these progenitors expand in injury or DMD to become PDGFRα+, Col1a1-GFP+ matrix-forming fibroblasts, whereas muscle fibres do not become fibroblasts but are an important source of the PDGFRα ligand, PDGF-AA. While in toxin injury/repair of muscle PDGFRα, signalling is transiently up-regulated during the regenerative phase in the DMD model and in human DMD it is chronically overactivated. Conditional expression of the constitutively active PDGFRα D842V mutation in Collagen-I+ fibroblasts, during injury/repair, hindered the repair phase and instead promoted fibrosis. In DMD, treatment of mdx mice with crenolanib, a highly selective PDGFRα/β tyrosine kinase inhibitor, reduced fibrosis, improved muscle strength, and was associated with decreased activity of Src, a downstream effector of PDGFRα signalling. These observations are consistent with a model in which PDGFRα activation of mesenchymal progenitors normally regulates repair of the injured muscle, but in DMD persistent and excessive activation of this pathway directly drives fibrosis and hinders repair. The PDGFRα pathway is a potential new target for treatment of progressive DMD. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

TLR4-HMGB1 signaling pathway affects the inflammatory reaction of autoimmune myositis by regulating MHC-I

OBJECTIVES

To analyze the effects of TLR4 on the expression of the HMGB1, MHC-I and downstream cytokines IL-6 and TNF-α, and to investigate the biological role of the TLR4-HMGB1 signaling pathway in the development of the autoimmune myositis.

METHODS

We built mice models with experimental autoimmune myositis (EAM) and used the inverted screen experiment to measure their muscle endurance; we also examined inflammatory infiltration of muscle tissues after HE staining; and we assessed the expression of MHC-I using immunohistochemistry. In addition, peripheral blood mononuclear cells (PBMC) were extracted and flow cytometry was utilized to detect the effect of IFN-γ on the expression of MHC-I. Furthermore, PBMCs were treated with IFN-γ, anti-TLR4, anti-HMGB1 and anti-MHC-I. Real-time PCR and western blotting were employed to examine the expressions of TLR4, HMGB1 and MHC-I in different groups. The ELISA method was also utilized to detect the expression of the downstream cytokines TNF-α and IL-6.

RESULTS

The expressions of TLR4, HMGB1 and MHC-I in muscle tissues from mice with EAM were significantly higher than those in the control group (all P<0.05). After IFN-γ treatment, the expressions of TLR4, HMGB1, MHC-I, TNF-α and IL-6 in PBMCs significantly increased (all P<0.05). The treatment of anti-TLR4, anti-HMGB1 and anti-MHC-I could significantly downregulate the expression of MHC-I (all P<0.05). In addition, anti-TLR4 and anti-HMGB1 significantly reduced the expression of TNF-α and IL-6 (all P<0.05).

CONCLUSIONS

The TLR4-HMGB1 signaling pathway affects the process of autoimmune myositis inflammation by regulating the expression of MHC-I and other pro-inflammatory cytokines.

S1P3 receptor influences key physiological properties of fast-twitch extensor digitorum longus muscle

To examine the role of sphingosine 1-phosphate (S1P) receptor 3 (S1P3) in modulating muscle properties, we utilized transgenic mice depleted of the receptor. Morphological analyses of extensor digitorum longus (EDL) muscle did not show evident differences between wild-type and S1P3-null mice. The body weight of 3-mo-old S1P3-null mice and the mean cross-sectional area of transgenic EDL muscle fibers were similar to those of wild-type. S1P3 deficiency enhanced the expression level of S1P1 and S1P2 receptors mRNA in S1P3-null EDL muscle. The contractile properties of S1P3-null EDL diverge from those of wild-type, largely more fatigable and less able to recover. The absence of S1P3 appears responsible for a lower availability of calcium during fatigue. S1P supplementation, expected to stimulate residual S1P receptors and signaling, reduced fatigue development of S1P3-null muscle. Moreover, in the absence of S1P3, denervated EDL atrophies less than wild-type. The analysis of atrophy-related proteins in S1P3-null EDL evidences high levels of the endogenous regulator of mitochondria biogenesis peroxisome proliferative-activated receptor-γ coactivator 1α (PGC-1α); preserving mitochondria could protect the muscle from disuse atrophy. In conclusion, the absence of S1P3 makes the muscle more sensitive to fatigue and slows down atrophy development after denervation, indicating that S1P3 is involved in the modulation of key physiological properties of the fast-twitch EDL muscle.

Long-Term Therapy With Omega-3 Ameliorates Myonecrosis and Benefits Skeletal Muscle Regeneration in Mdx Mice

In Duchenne muscle dystrophy (DMD) and in the mdx mouse model of DMD, a lack of dystrophin leads to myonecrosis and cardiorespiratory failure. Several lines of evidence suggest a detrimental role of the inflammatory process in the dystrophic process. Previously, we demonstrated that short-term therapy with eicosapentaenoic acid (EPA), at early stages of disease, ameliorated dystrophy progression in the mdx mouse. In the present study, we evaluated the effects of a long-term therapy with omega-3 later in dystrophy progression. Three-month-old mdx mice received omega-3 (300 mg/kg) or vehicle by gavage for 5 months. The quadriceps and diaphragm muscles were removed and processed for histopathology and Western blot. Long-term therapy with omega-3 increased the regulatory protein MyoD and muscle regeneration and reduced markers of inflammation (TNF-α and NF-kB) in both muscles studied. The present study supports the long-term use of omega-3 at later stages of dystrophy as a promising option to be investigated in DMD clinical trials.

Naturally derived and synthetic scaffolds for skeletal muscle reconstruction

Skeletal muscle tissue has an inherent capacity for regeneration following injury. However, severe trauma, such as volumetric muscle loss, overwhelms these natural muscle repair mechanisms prompting the search for a tissue engineering/regenerative medicine approach to promote functional skeletal muscle restoration. A desirable approach involves a bioscaffold that simultaneously acts as an inductive microenvironment and as a cell/drug delivery vehicle to encourage muscle ingrowth. Both biologically active, naturally derived materials (such as extracellular matrix) and carefully engineered synthetic polymers have been developed to provide such a muscle regenerative environment. Next generation naturally derived/synthetic "hybrid materials" would combine the advantageous properties of these materials to create an optimal platform for cell/drug delivery and possess inherent bioactive properties. Advances in scaffolds using muscle tissue engineering are reviewed herein.

Silencing of drpr leads to muscle and brain degeneration in adult Drosophila

Mutations in the gene encoding the single transmembrane receptor multiple epidermal growth factor-like domain 10 (MEGF10) cause an autosomal recessive congenital muscle disease in humans. Although mammalian MEGF10 is expressed in the central nervous system as well as in skeletal muscle, patients carrying mutations in MEGF10 do not show symptoms of central nervous system dysfunction. drpr is the sole Drosophila homolog of the human genes MEGF10, MEGF11, and MEGF12 (JEDI, PEAR). The functional domains of MEGF10 and drpr bear striking similarities, and residues affected by MEGF10 mutations in humans are conserved in drpr. Our analysis of drpr mutant flies revealed muscle degeneration with fiber size variability and vacuolization, as well as reduced motor performance, features that have been observed in human MEGF10 myopathy. Vacuolization was also seen in the brain. Tissue-specific RNAi experiments demonstrated that drpr deficiency in muscle, but not in the brain, leads to locomotor defects. The histological and behavioral abnormalities seen in the affected flies set the stage for further studies examining the signaling pathway modulated by MEGF10/Drpr in muscle, as well as assessing the effects of genetic and/or pharmacological manipulations on the observed muscle defects. In addition, the absence of functional redundancy for Drpr in Drosophila may help elucidate whether paralogs of MEGF10 in humans (eg, MEGF11) contribute to maintaining wild-type function in the human brain.

Small-molecule based musculoskeletal regenerative engineering

Clinicians and scientists working in the field of regenerative engineering are actively investigating a wide range of methods to promote musculoskeletal tissue regeneration. Small-molecule-mediated tissue regeneration is emerging as a promising strategy for regenerating various musculoskeletal tissues and a large number of small-molecule compounds have been recently discovered as potential bioactive molecules for musculoskeletal tissue repair and regeneration. In this review, we summarize the recent literature encompassing the past 4 years in the area of small bioactive molecules for promoting repair and regeneration of various musculoskeletal tissues including bone, muscle, cartilage, tendon, and nerve.

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

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

The role of sphingosine-1 phosphate and ceramide-1 phosphate in trafficking of normal stem cells and cancer cells

INTRODUCTION

A common feature of many types of cells is their responsiveness to chemotactic gradients of factors for which they express the corresponding receptors. The most studied chemoattractants so far are peptide-based growth factors and a family of cytokines endowed with strong chemotactic properties, called chemokines. However, additional evidence has accumulated that, in addition to these peptide-based chemoattractants, an important role in cell migration is played by bioactive lipids.

AREAS COVERED

Solid evidence has accumulated that two bioactive phosphorylated sphingolipids that are derivatives of sphingolipid metabolism, namely sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P), are potent chemoattractants for a variety of cells. In this review, we will discuss the effect of these two phosphorylated sphingolipids on the trafficking of normal and malignant cells, and, in particular, we will focus on their role in trafficking of normal hematopoietic stem/progenitor cells. Unlike other mediators, S1P under steady-state conditions maintain a steep gradient between interstitial fluid and peripheral blood and lymph across the endothelial barrier, which is important in the egress of cells from bone marrow. Both S1P and C1P may be upregulated in damaged tissues, which may result in reversal of this gradient.

EXPERT OPINION

S1P and C1P are important regulators of the trafficking of normal and malignant cells, and modification of their biological effects will have important applications in optimizing stem cell mobilization and homing, tissue organ/regeneration, and preventing cancer metastasis.

Molecular mechanism of sphingosine-1-phosphate action in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle-wasting disease. Studies in Drosophila showed that genetic increase of the levels of the bioactive sphingolipid sphingosine-1-phosphate (S1P) or delivery of 2-acetyl-5-tetrahydroxybutyl imidazole (THI), an S1P lyase inhibitor, suppresses dystrophic muscle degeneration. In the dystrophic mouse (mdx), upregulation of S1P by THI increases regeneration and muscle force. S1P can act as a ligand for S1P receptors and as a histone deacetylase (HDAC) inhibitor. Because Drosophila has no identified S1P receptors and DMD correlates with increased HDAC2 levels, we tested whether S1P action in muscle involves HDAC inhibition. Here we show that beneficial effects of THI treatment in mdx mice correlate with significantly increased nuclear S1P, decreased HDAC activity and increased acetylation of specific histone residues. Importantly, the HDAC2 target microRNA genes miR-29 and miR-1 are significantly upregulated, correlating with the downregulation of the miR-29 target Col1a1 in the diaphragm of THI-treated mdx mice. Further gene expression analysis revealed a significant THI-dependent decrease in inflammatory genes and increase in metabolic genes. Accordingly, S1P levels and functional mitochondrial activity are increased after THI treatment of differentiating C2C12 cells. S1P increases the capacity of the muscle cell to use fatty acids as an energy source, suggesting that THI treatment could be beneficial for the maintenance of energy metabolism in mdx muscles.

Drosophila as a starting point for developing therapeutics for the rare disease Duchenne Muscular Dystrophy

Progress into developing therapeutics for rare diseases can be accelerated for those diseases that can be modeled in genetically tractable organisms. Here we comment on one disease, Duchenne Muscular Dystrophy (DMD), modeled in Drosophila that brought together disparate lines of research toward the goal of developing a therapeutic. Though the bioactive lipid sphingosine 1-phosphate (S1P) has been implicated in many anabolic processes in many cell types and tissues, including muscle, this work confirmed the therapeutic potential of assessing this pathway for DMD. Genetic dissection of sphingolipid metabolism showed the suppression of muscle structural and functional defects in flies. Moreover, improvement of muscle defects using known pharmacological agents that raise S1P levels in vivo highlight the potential of Drosophila as a drug-screening tool for DMD. We and others have extended S1P studies into the mouse model of DMD and have shown a partial amelioration of symptoms associated with DMD. Translation of this work to mammals makes the sphingolipid metabolism pathway a promising target for further drug development that may benefit the human condition.