Dynamics of the skeletal muscle secretome during myoblast differentiation

Analysis of secreted proteins

Most biological processes including growth, proliferation, differentiation, and apoptosis are coordinated by tightly regulated signaling pathways, which also involve secreted proteins acting in an autocrine and/or paracrine manner. In addition, extracellular signaling molecules affect local niche biology and influence the cross-talking with the surrounding tissues. The understanding of this molecular language may provide an integrated and broader view of cellular regulatory networks under physiological and pathological conditions. In this context, the profiling at a global level of cell secretomes (i.e., the subpopulations of a proteome secreted from the cell) has become an active area of research. The current interest in secretome research also deals with its high potential for the biomarker discovery and the identification of new targets for therapeutic strategies. Several proteomic and mass spectrometry platforms and methodologies have been applied to secretome profiling of conditioned media of cultured cell lines and primary cells. Nevertheless, the analysis of secreted proteins is still a very challenging task, because of the technical difficulties that may hamper the subsequent mass spectrometry analysis. This chapter describes a typical workflow for the analysis of proteins secreted by cultured cells. Crucial issues related to cell culture conditions for the collection of conditioned media, secretome preparation, and mass spectrometry analysis are discussed. Furthermore, an overview of quantitative LC-MS-based approaches, computational tools for data analysis, and strategies for validation of potential secretome biomarkers is also presented.

Satellite cells produce neural chemorepellent semaphorin 3A upon muscle injury

Regenerative mechanisms that regulate intramuscular motor innervation. including configuration of the neuromuscular connections are thought to reside in the spatiotemporal expression of axon-guidance molecules. Our previous studies proposed a heretofore unexplored role of satellite cells as a key source of a secreted neural chemorepellent semaphorin 3A (Sema3A) expression. In order to verify this concept, there is still a critical need to provide direct evidence to show the up-regulation of Sema3A protein in satellite cells in vivo upon muscle injury. The present study employed a Sema3A/MyoD double-immunohistochemical staining for cryo-sections prepared from cardiotoxin injected gastrocnemius muscle of adult mouse lower hind-limb. Results clearly demonstrated that Sema3A expression was up-regulated in myogenic differentiation-positive satellite cells at 4-12 days post-injury period, the time that corresponds to the cell differentiation phase characterized by increasing myogenin messenger RNA expression. This direct proof encourages a possible implication of satellite cells in the spatiotemporal regulation of extracellular Sema3A concentrations, which potentially ensures coordinating a delay in neurite sprouting and re-attachment of motoneuron terminals onto damaged muscle fibers early in muscle regeneration in synchrony with recovery of muscle-fiber integrity.

Myostatin is a novel tumoral factor that induces cancer cachexia

Humoral and tumoral factors collectively promote cancer-induced skeletal muscle wasting by increasing protein degradation. Although several humoral proteins, namely TNFα (tumour necrosis factor α) and IL (interleukin)-6, have been shown to induce skeletal muscle wasting, there is a lack of information regarding the tumoral factors that contribute to the atrophy of muscle during cancer cachexia. Therefore, in the present study, we have characterized the secretome of C26 colon cancer cells to identify the tumoral factors involved in cancer-induced skeletal muscle wasting. In the present study, we show that myostatin, a procachectic TGFβ (transforming growth factor β) superfamily member, is abundantly secreted by C26 cells. Consistent with myostatin signalling during cachexia, treating differentiated C2C12 myotubes with C26 CM (conditioned medium) resulted in myotubular atrophy due to the up-regulation of muscle-specific E3 ligases, atrogin-1 and MuRF1 (muscle RING-finger protein 1), and enhanced activity of the ubiquitin–proteasome pathway. Furthermore, the C26 CM also activated ActRIIB (activin receptor type II B)/Smad and NF-κB (nuclear factor κB) signalling, and reduced the activity of the IGF-I (insulin-like growth factor 1)/PI3K (phosphoinositide 3-kinase)/Akt pathway, three salient molecular features of myostatin action in skeletal muscles. Antagonists to myostatin prevented C26 CM-induced wasting in muscle cell cultures, further confirming that tumoral myostatin may be a key contributor in the pathogenesis of cancer cachexia. Finally, we show that treatment with C26 CM induced the autophagy–lysosome pathway and reduced the number of mitochondria in myotubes. These two previously unreported observations were recapitulated in skeletal muscles collected from C26 tumour-bearing mice.

Hematopoietic Microenvironment in the Fetal Liver: Roles of Different Cell Populations

Hematopoiesis is the main function of the liver during a considerable period of mammalian prenatal development. Hematopoietic cells of the fetal liver exist in a specific microenvironment that controls their proliferation and differentiation. This microenvironment is created by different cell populations, including epitheliocytes, macrophages, various stromal elements (hepatic stellate cells, fibroblasts, myofibroblasts, vascular smooth muscle and endothelial cells, mesenchymal stromal cells), and also cells undergoing epithelial-to-mesenchymal transition. This paper considers the involvement of these cell types in the regulation of fetal liver hematopoiesis.

Comparative analysis of semaphorin 3A in soleus and EDL muscle satellite cells in vitro toward understanding its role in modulating myogenin expression

Resident myogenic stem cells, satellite cells, up-regulate a secreted multi-functional modulator, semaphorin 3A (Sema3A), exclusively at the early-differentiation phase in response to muscle-crush injury and treatment with hepatocyte growth factor (HGF) or basic fibroblast growth factor (FGF2). Here, we add evidence that the Sema3A expression and secretion induced by the growth factors is significantly higher in primary cultures from adult rat soleus than from the fast-twitch extensor digitorum longus (EDL) muscle. The higher Sema3A response, revealed by quantitative PCR and Western blotting of cell lysates and conditioned media, may account for the higher myogenin expression of soleus muscle satellite cells early in differentiation since addition of recombinant Sema3A stimulates myogenin expression in cultures. These experiments also showed that mRNA expression of plexin A2, which together with neuropilins, constitutes Sema3A composite-receptors, was higher in satellite cells from soleus than EDL with no difference in plexin A1 and A3 and neuropilin-1 and 2 levels. These comparative studies, therefore, highlight a possible Sema3A-plexin A2-myogenin signaling axis that may ensure promoting early differentiation by soleus muscle satellite cells.

Regulation of tissue crosstalk by skeletal muscle-derived myonectin and other myokines

The integrated control of animal physiology requires intimate tissue crosstalk, a vital task mediated by circulating humoral factors. As one type of these factors, adipose tissue-derived adipokines have recently garnered attention as important regulators of systemic insulin sensitivity and metabolic homeostasis. However, the realization that skeletal muscle also secretes a variety of biologically and metabolically active polypeptide factors (collectively called myokines) has provided a new conceptual framework to understand the critical role skeletal muscle plays in coordinating whole-body energy balance. Here, we highlight recent progress made in the myokine field and discuss possible roles of myonectin, which we have recently identified as a potential postprandial signal derived from skeletal muscle to integrate metabolic processes in other tissues, such as adipose and liver; one of its roles is to promote fatty acid uptake into cells. Myonectin is also likely an important mediator in inter-tissue crosstalk.

Time-coordinated prevalence of extracellular HGF, FGF2 and TGF-β3 in crush-injured skeletal muscle

Successful regeneration and remodeling of neuromuscular junctions are critical for restoring functional capacities and properties of skeletal muscle after damage, and axon-guidance molecules may be involved in the signaling that regulates such restoration. Recently, we found that early-differentiated satellite cells up-regulate a secreted neural chemorepellent Sema3A upon in vivo muscle-crush injury. The study also revealed that Sema3A expression is up-regulated in primary satellite-cell cultures in response to hepatocyte growth factor (HGF) and basic fibroblast growth factor (FGF2) and is prevented by transforming growth factor (TGF)-β2, 3. In order to verify the physiological significance of this regulation in vitro, the present study was designed to estimate the time-course of extracellular HGF, FGF2 and TGF-β3 concentrations after crush-injury of Gastrocnemius muscle in the rat lower hind-limb, using a combination of a non-homogenization/non-spin extraction of extracellular wound fluids and enhanced chemiluminescence-Western blotting analyses. Results clearly demonstrated that active HGF and FGF2 are prevalent in 2-8 days post-crush, whereas active TGF-β3 increases after 12 days, providing a better understanding of the time-coordinated levels of HGF, FGF2 and TGF-β3 that drive regulation of Sema3A expression during regenerative intramuscular moto-neuritogenesis.

Semaphorin 7a links nerve regeneration and inflammation in the cornea

PURPOSE

We determined Semaphorin 7a (Sema7a) localization and abundance in naive corneas and in corneas after nerve-transecting lamellar flap surgery, and determined the effect of Sema7a supplementation on corneal nerve regeneration and inflammation.

METHODS

Immunolocalization and Western blot analyses were performed to evaluate the abundance of Sema7a in naive corneas and corneas undergoing nerve regeneration after lamellar corneal surgery in thy1-YFP+ neurofluorescent mice. We used compartmental cultures of dissociated trigeminal ganglion cells to determine the effect of Sema7a exposure on neurite outgrowth in vitro. Finally, a Sema7a pellet was implanted under the corneal flap after lamellar transection surgery to determine the neuronal and inflammatory effects of Sema7a supplementation in vivo.

RESULTS

Sema7a was expressed in the corneal epithelium and stromal keratocytes, but was more abundant in the epithelium (74.3%) compared to the stroma (25.7%, P = 0.02). Sema7a expression was increased significantly in the cornea after lamellar corneal surgery and was localized to stromal cells near the regenerating nerve fronds. Exposure of trigeminal neurites to Sema7a (20 nM) in the side compartment increased neurite length significantly. The implanted Sema7a pellet increased significantly YFP+ inflammatory cell influx into the cornea as well as increased corneal nerve length.

CONCLUSIONS

Sema7a is expressed constitutively in the cornea, and potently stimulates nerve regeneration and inflammatory cell influx. Therefore, this immune semaphorin links nerve regeneration and inflammatory processes in the cornea.

Muscle-to-organ cross talk mediated by myokines

Cytokines and other peptides are secreted from skeletal muscles in response to exercise and function as hormones either locally within the muscle or by targeting distant organs. Such proteins are recognized as myokines, with the prototype myokine being IL-6. Several studies have established a role of these muscle-derived factors as important contributors of the beneficial effects of exercise, and the myokines are central to our understanding of the cross talk during and after exercise between skeletal muscles and other organs. In a study into the mechanisms of a newly defined myokine, CXCL-1, we found that CXCL-1 overexpression increases muscular fatty acid oxidation with concomitant attenuation of diet-induced fat accumulation in the adipose tissue. Clearly this study adds to the concept of myokines playing an important role in mediating the whole-body adaptive effects of exercise through the regulation of skeletal muscle metabolism. Yet, myokines also contribute to whole-body metabolism by directly signaling to distant organs, regulating metabolic processes in liver and adipose tissue. Thus accumulating data shows that myokines play an important role in restoring a healthy cellular environment, reducing low-grade inflammation and thereby preventing metabolic related diseases like insulin resistance and cancer.

Secretomics for skeletal muscle cells: a discovery of novel regulators?

Metabolic tissues, including skeletal muscle, adipose tissue and the digestive system, dynamically secrete various factors depending on the metabolic state, communicate with each other and orchestrate functions to maintain body homeostasis. Skeletal muscle secretes cytokines such as interleukin-6 (IL-6), IL-15, fibroblast growth factor-21 (FGF21) and IL-8. These compounds, myokines, play important roles in biological homeostasis such as energy metabolism, angiogenesis and myogenesis. New technological advances have allowed secretomics - analysis of the secretome - to be performed. The application of highly sensitive mass spectrometry makes qualitative and quantitative analysis of the secretome of skeletal muscle possible. Secretory proteins derived from skeletal muscle cells under various conditions were analyzed, and many important factors were suggested. In-depth studies of the secretome from metabolic cells in various conditions are strongly recommended. This study will provide information on methods of novel communication between metabolic tissues.

The skeletal muscle secretome: an emerging player in muscle-bone crosstalk

In vitro and in vivo studies provide evidence that a variety of growth factors and cytokines are actively secreted by muscle tissue. Muscle can therefore function as an endocrine and paracrine organ. These peptides characterize the muscle secretome, and many muscle-derived factors such as insulin-like growth factor-1, basic fibroblast growth factor, interleukin-15, myostatin and secreted protein acidic and rich in cysteine (osteonectin) are also known to have significant effects on bone metabolism. The factors secreted by muscle may vary according to muscle activity, in that muscle contraction, muscle atrophy or traumatic muscle injury can alter the type and relative abundance of particular factors released from muscle cells. The molecular and cellular pathways by which muscle-derived factors affect different types of bone cells (for example, osteoblasts, osteoclasts and osteocytes) are, however, poorly understood. Nevertheless, these findings further underscore the complex nature of muscle-bone interactions, and highlight the importance of integrating muscle biology and physiology into our understanding of bone growth, development and aging.

Myonectin (CTRP15), a novel myokine that links skeletal muscle to systemic lipid homeostasis

Skeletal muscle plays important roles in whole-body glucose and fatty acid metabolism. However, muscle also secretes cytokines and growth factors (collectively termed myokines) that can potentially act in an autocrine, a paracrine, and/or an endocrine manner to modulate metabolic, inflammatory, and other processes. Here, we report the identification and characterization of myonectin, a novel myokine belonging to the C1q/TNF-related protein (CTRP) family. Myonectin transcript was highly induced in differentiated myotubes and predominantly expressed by skeletal muscle. Circulating levels of myonectin were tightly regulated by the metabolic state; fasting suppressed, but refeeding dramatically increased, its mRNA and serum levels. Although mRNA and circulating levels of myonectin were reduced in a diet-induced obese state, voluntary exercise increased its expression and circulating levels. Accordingly, myonectin transcript was up-regulated by compounds (forskolin, epinephrine, ionomycin) that raise cellular cAMP or calcium levels. In vitro, secreted myonectin forms disulfide-linked oligomers, and when co-expressed, forms heteromeric complexes with other members of the C1q/TNF-related protein family. In mice, recombinant myonectin administration reduced circulating levels of free fatty acids without altering adipose tissue lipolysis. Consistent with this, myonectin promoted fatty acid uptake in cultured adipocytes and hepatocytes, in part by up-regulating the expression of genes (CD36, FATP1, Fabp1, and Fabp4) that promote lipid uptake. Collectively, these results suggest that myonectin links skeletal muscle to lipid homeostasis in liver and adipose tissue in response to alterations in energy state, revealing a novel myonectin-mediated metabolic circuit.

Muscles, exercise and obesity: skeletal muscle as a secretory organ

During the past decade, skeletal muscle has been identified as a secretory organ. Accordingly, we have suggested that cytokines and other peptides that are produced, expressed and released by muscle fibres and exert either autocrine, paracrine or endocrine effects should be classified as myokines. The finding that the muscle secretome consists of several hundred secreted peptides provides a conceptual basis and a whole new paradigm for understanding how muscles communicate with other organs, such as adipose tissue, liver, pancreas, bones and brain. However, some myokines exert their effects within the muscle itself. Thus, myostatin, LIF, IL-6 and IL-7 are involved in muscle hypertrophy and myogenesis, whereas BDNF and IL-6 are involved in AMPK-mediated fat oxidation. IL-6 also appears to have systemic effects on the liver, adipose tissue and the immune system, and mediates crosstalk between intestinal L cells and pancreatic islets. Other myokines include the osteogenic factors IGF-1 and FGF-2; FSTL-1, which improves the endothelial function of the vascular system; and the PGC-1α-dependent myokine irisin, which drives brown-fat-like development. Studies in the past few years suggest the existence of yet unidentified factors, secreted from muscle cells, which may influence cancer cell growth and pancreas function. Many proteins produced by skeletal muscle are dependent upon contraction; therefore, physical inactivity probably leads to an altered myokine response, which could provide a potential mechanism for the association between sedentary behaviour and many chronic diseases.

Targeted functional analysis of p300 coactivator in Wnt/β-catenin signaling pathway using phosphoproteomic and biochemical approaches

Both p300 and β-catenin are transcriptional activators and phosphoproteins, and play a central role in Wnt/β-catenin-dependent transcriptional regulation. The minimum β-catenin binding domain of p300 has been mapped to the N-terminus 1-111 amino acids. Here, we performed phosphoproteomic analysis for the critical binding region using LC-MS/MS approach to investigate potential phosphosites that may affect the binding affinity. By implementing TiO(2)-based phosphopeptide affinity purification followed by LC-MS/MS analysis with both collision-induced dissociation (CID) and electron transfer dissociation (ETD) methods, two unique phosphosites Ser12 and Ser89 were identified, of which, phosphorylation at Ser12 is novel. Functional studies aided by site-directed mutagenesis, co-immunoprecipitation and mammalian two-hybrid assay have concluded that phosphorylation at Ser12 critically mediates the binding ability of p300 with β-catenin. Further studies utilizing specific MAPK inhibitors suggest that the p38 MAPK activation is the upstream signal required for Ser12 phosphorylation. The transcriptional roles of p300/β-catenin complex in myoblast differentiation are discussed.

Investigating the secretome: lessons about the cells that comprise the heart

The cell/environment interface is composed of the proteins of plasma membrane which face the extracellular space and by the proteins secreted directly by the cell of origin or by neighboring cells. The secreted proteins can act as extracellular matrix proteins and/or autocrine/paracrine proteins. This report discusses the technical aspects involved in the identification and characterization of the secreted proteins of specific cell types that comprise the heart. These aspects include the culturing of the cells, cell co-culturing and quantitative labeling, conditioned media collection and dealing with high abundant serum proteins, post-translational modification enrichment, the use of protein separation methods and mass spectrometry, protein identification and validation and the incorporation of pathway analysis to better understand the novel discovery on the background of already known experimental biological systems. The proteomic methods have the solid emplacement in cardiovascular research and the identification of proteins secreted by cardiac cells has been used in various applications such as determination the specificity between secretomes of different cell types, e.g. cardiac stem cells and cardiac myocytes, for the global secretome screening of e.g. human arterial smooth muscle cells, for the mapping of the beneficial effect of conditioned medium of one cell type on the other cell type, e.g. conditioned medium of human mesenchymal stem cells on cardiac myocytes, and for the searching the candidate paracrine factors and potential biomarkers.

Increased FAT/CD36 cycling and lipid accumulation in myotubes derived from obese type 2 diabetic patients

BACKGROUND

Permanent fatty acid translocase (FAT/)CD36 relocation has previously been shown to be related to abnormal lipid accumulation in the skeletal muscle of type 2 diabetic patients, however mechanisms responsible for the regulation of FAT/CD36 expression and localization are not well characterized in human skeletal muscle.

METHODOLOGY/PRINCIPAL FINDINGS

Primary muscle cells derived from obese type 2 diabetic patients (OBT2D) and from healthy subjects (Control) were used to examine the regulation of FAT/CD36. We showed that compared to Control myotubes, FAT/CD36 was continuously cycling between intracellular compartments and the cell surface in OBT2D myotubes, independently of lipid raft association, leading to increased cell surface FAT/CD36 localization and lipid accumulation. Moreover, we showed that FAT/CD36 cycling and lipid accumulation were specific to myotubes and were not observed in reserve cells. However, in Control myotubes, the induction of FAT/CD36 membrane translocation by the activation of (AMP)-activated protein kinase (AMPK) pathway did not increase lipid accumulation. This result can be explained by the fact that pharmacological activation of AMPK leads to increased mitochondrial beta-oxidation in Control cells.

CONCLUSION/SIGNIFICANCE

Lipid accumulation in myotubes derived from obese type 2 diabetic patients arises from abnormal FAT/CD36 cycling while lipid accumulation in Control cells results from an equilibrium between lipid uptake and oxidation. As such, inhibiting FAT/CD36 cycling in the skeletal muscle of obese type 2 diabetic patients should be sufficient to diminish lipid accumulation.

Growth factor regulation of neural chemorepellent Sema3A expression in satellite cell cultures

Successful regeneration and remodeling of the intramuscular motoneuron network and neuromuscular connections are critical for restoring skeletal muscle function and physiological properties. The regulatory signals of such coordination remain unclear, although axon-guidance molecules may be involved. Recently, satellite cells, resident myogenic stem cells positioned beneath the basal lamina and at high density at the myoneural junction regions of mature fibers, were shown to upregulate a secreted neural chemorepellent semaphorin 3A (Sema3A) in response to in vivo muscle-crush injury. The initial report on that expression centered on the observation that hepatocyte growth factor (HGF), an essential cue in muscle fiber growth and regeneration, remarkably upregulates Sema3A expression in early differentiated satellite cells in vitro [Tatsumi et al., Am J Physiol Cell Physiol 297: C238–C252, 2009]. Here, we address regulatory effects of basic fibroblast growth factor (FGF2) and transforming growth factor (TGF)-βs on Sema3A expression in satellite cell cultures. When treated with FGF2, Sema3A message and protein were upregulated as revealed by reverse transcription-polymerase chain reaction and immunochemical studies. Sema3A upregulation by FGF2 was dose dependent with a maximum (8- to 1-fold relative to the control) at 2.5 ng/ml (150 pM) and occurred exclusively at the early differentiation stage. The response was highly comparable in dose response and timing to effects of HGF treatment, without any additive or synergistic effect from treatment with a combination of both potent upregulators. In contrast, TGF-β2 and -β3 potently decreased basal Sema3A expression; the maximum effect was at very low concentrations (40 and 8 pM, respectively) and completely cancelled the activities of FGF2 and HGF to upregulate Sema3A. These results therefore encourage the prospect that a time-coordinated increase in HGF, FGF2, and TGF-β ligands and their receptors promotes a programmed strategy for Sema3A expression that guarantees successful intramuscular motor reinnervation by delaying sprouting and reattachment of motoneuron terminals onto damaged muscle fibers early in regeneration pending restoration of muscle fiber contractile integrity.

Proteomic identification of secreted proteins from human skeletal muscle cells and expression in response to strength training

Regular physical activity protects against several types of diseases. This may involve altered secretion of signaling proteins from skeletal muscle. Our aim was to identify the most abundantly secreted proteins in cultures of human skeletal muscle cells and to monitor their expression in muscles of strength-training individuals. A total of 236 proteins were detected by proteome analysis in medium conditioned by cultured human myotubes, which was narrowed down to identification of 18 classically secreted proteins expressed in skeletal muscle, using the SignalP 3.0 and Human Genome Expression Profile databases together with a published mRNA-based reconstruction of the human skeletal muscle secretome. For 17 of the secreted proteins, expression was confirmed at the mRNA level in cultured human myotubes as well as in biopsies of human skeletal muscles. RT-PCR analyses showed that 15 of the secreted muscle proteins had significantly enhanced mRNA expression in m. vastus lateralis and/or m. trapezius after 11 wk of strength training among healthy volunteers. For example, secreted protein acidic and rich in cysteine, a secretory protein in the membrane fraction of skeletal muscle fibers, was increased 3- and 10-fold in m. vastus lateralis and m. trapezius, respectively. Identification of proteins secreted by skeletal muscle cells in vitro facilitated the discovery of novel responses in skeletal muscles of strength-training individuals.

Goodpasture antigen-binding protein (GPBP) directs myofibril formation: identification of intracellular downstream effector 130-kDa GPBP-interacting protein (GIP130)

Goodpasture antigen-binding protein-1 (GPBP-1) is an exportable non-conventional Ser/Thr kinase that regulates glomerular basement membrane collagen organization. Here we provide evidence that GPBP-1 accumulates in the cytoplasm of differentiating mouse myoblasts prior to myosin synthesis. Myoblasts deficient in GPBP-1 display defective myofibril formation, whereas myofibrils assemble with enhanced efficiency in those overexpressing GPBP-1. We also show that GPBP-1 targets the previously unidentified GIP130 (GPBP-interacting protein of 130 kDa), which binds to myosin and promotes its myofibrillar assembly. This report reveals that GPBP-1 directs myofibril formation, an observation that expands its reported role in supramolecular organization of structural proteins to the intracellular compartment.

LIF is a contraction-induced myokine stimulating human myocyte proliferation

The cytokine leukemia inhibitory factor (LIF) is expressed by skeletal muscle and induces proliferation of myoblasts. We hypothesized that LIF is a contraction-induced myokine functioning in an autocrine fashion to activate gene regulation of human muscle satellite cell proliferation. Skeletal muscle LIF expression, regulation, and action were examined in two models: 1) young men performing a bout of heavy resistance exercise of the quadriceps muscle and 2) cultured primary human satellite cells. Resistance exercise induced a ninefold increase in LIF mRNA content in skeletal muscle, but LIF was not detectable in plasma of the subjects. However, electrically stimulated cultured human myotubes produced and secreted LIF, suggesting that LIF is a myokine with local effects. The well established exercise-induced signaling molecules PI3K, Akt, and mTor contributed to the regulation of LIF in cultured human myotubes as chemical inhibition of PI3K and mTor and siRNA knockdown of Akt1 were independently sufficient to downregulate LIF. Human myoblast proliferation was increased by recombinant exogenous LIF and decreased by siRNA knockdown of the endogenous LIF receptor. Finally, the transcription factors JunB and c-Myc, which promote myoblast proliferation, were induced by LIF in cultured human myotubes. Indeed, both JunB and c-Myc were also increased in skeletal muscle following resistance exercise. Our data suggest that LIF is a contraction-induced myokine, potentially acting in an autocrine or paracrine fashion to promote satellite cell proliferation.

Secreted proteins from adipose tissue and skeletal muscle - adipokines, myokines and adipose/muscle cross-talk

White adipose tissue and skeletal muscle are the largest organs in the body and both are composed of distinct cell types. The signature cell of adipose tissue is the adipocyte while myocytes are the defining cell of skeletal muscle. White adipocytes are major secretory cells and this is increasingly apparent also for myocytes. Both cells secrete a range of bioactive proteins, generally termed adipokines in the case of adipocytes and myokines for muscle cells. There has, however, been some confusion over nomenclature and we suggest that the name myokine is restricted to a protein that is secreted from myocytes, while the term adipokine should be used to describe all proteins secreted from any type of adipocyte (white, brown or brite). These definitions specifically exclude proteins secreted from other cells within adipose tissue and muscle, including macrophages. There is some commonality between the myokines and adipokines in that both groups include inflammation-related proteins - for example, IL-6, Il-8 and MCP-1. Adipokines and myokines appear to be involved in local autocrine/paracrine interactions within adipose tissue and muscle, respectively. They are also involved in an endocrine cross-talk with other tissues, including between adipose tissue and skeletal muscle, and this may be bi-directional. For example, IL-6, secreted from myocytes may stimulate lipolysis in adipose tissue, while adipocyte-derived IL-6 may induce insulin resistance in muscle.

Muscles and their myokines

In the past, the role of physical activity as a life-style modulating factor has been considered as that of a tool to balance energy intake. Although it is important to avoid obesity, physical inactivity should be discussed in a much broader context. There is accumulating epidemiological evidence that a physically active life plays an independent role in the protection against type 2 diabetes, cardiovascular diseases, cancer, dementia and even depression. For most of the last century, researchers sought a link between muscle contraction and humoral changes in the form of an 'exercise factor', which could be released from skeletal muscle during contraction and mediate some of the exercise-induced metabolic changes in other organs such as the liver and the adipose tissue. We have suggested that cytokines or other peptides that are produced, expressed and released by muscle fibres and exert autocrine, paracrine or endocrine effects should be classified as 'myokines'. Given that skeletal muscle is the largest organ in the human body, our discovery that contracting skeletal muscle secretes proteins sets a novel paradigm: skeletal muscle is an endocrine organ producing and releasing myokines, which work in a hormone-like fashion, exerting specific endocrine effects on other organs. Other myokines work via paracrine mechanisms, exerting local effects on signalling pathways involved in muscle metabolism. It has been suggested that myokines may contribute to exercise-induced protection against several chronic diseases.

System-wide temporal characterization of the proteome and phosphoproteome of human embryonic stem cell differentiation

To elucidate cellular events underlying the pluripotency of human embryonic stem cells (hESCs), we performed parallel quantitative proteomic and phosphoproteomic analyses of hESCs during differentiation initiated by a diacylglycerol analog or transfer to media that had not been conditioned by feeder cells. We profiled 6521 proteins and 23,522 phosphorylation sites, of which almost 50% displayed dynamic changes in phosphorylation status during 24 hours of differentiation. These data are a resource for studies of the events associated with the maintenance of hESC pluripotency and those accompanying their differentiation. From these data, we identified a core hESC phosphoproteome of sites with similar robust changes in response to the two distinct treatments. These sites exhibited distinct dynamic phosphorylation patterns, which were linked to known or predicted kinases on the basis of the matching sequence motif. In addition to identifying previously unknown phosphorylation sites on factors associated with differentiation, such as kinases and transcription factors, we observed dynamic phosphorylation of DNA methyltransferases (DNMTs). We found a specific interaction of DNMTs during early differentiation with the PAF1 (polymerase-associated factor 1) transcriptional elongation complex, which binds to promoters of the pluripotency and known DNMT target genes encoding OCT4 and NANOG, thereby providing a possible molecular link for the silencing of these genes during differentiation.

Identification of differentially regulated secretome components during skeletal myogenesis

Myogenesis is a well-characterized program of cellular differentiation that is exquisitely sensitive to the extracellular milieu. Systematic characterization of the myogenic secretome (i.e. the ensemble of secreted proteins) is, therefore, warranted for the identification of novel secretome components that regulate both the pluripotency of these progenitor mesenchymal cells, and also their commitment and passage through the differentiation program. Previously, we have successfully identified 26 secreted proteins in the mouse skeletal muscle cell line C2C12 (1). In an effort to attain a more comprehensive picture of the regulation of myogenesis by its extracellular milieu, quantitative profiling employing stable isotope labeling by amino acids in cell culture was implemented in conjunction with two parallel high throughput online reverse phase liquid chromatography-tandem mass spectrometry systems. In summary, 34 secreted proteins were quantified, 30 of which were shown to be differentially expressed during muscle development. Intriguingly, our analysis has revealed several novel up- and down-regulated secretome components that may have critical biological relevance for both the maintenance of pluripotency and the passage of cells through the differentiation program. In particular, the altered regulation of secretome components, including follistatin-like protein-1, osteoglycin, spondin-2, and cytokine-induced apoptosis inhibitor-1, along with constitutively expressed factors, such as fibulin-2, illustrate dynamic changes in the secretome that take place when differentiation to a specific lineage occurs.

Exercise-induced liver chemokine CXCL-1 expression is linked to muscle-derived interleukin-6 expression

The chemokine CXC ligand-1 (CXCL-1) is a small cytokine that elicits effects by signalling through the chemokine receptor CXCR2. CXCL-1 has neutrophil chemoattractant activity, is involved in the processes of angiogenesis, inflammation and wound healing, and may possess neuroprotective effects. The aim of this study was to unravel the mechanisms whereby CXCL-1 is regulated by exercise inmice. After a single bout of exercise, CXCL-1 protein increased in serum(2.4-fold), and CXCL-1 mRNA in muscle (6.5-fold) and liver (41-fold). These increases in CXCL-1 were preceded by increases in serum interleukin-6 (IL-6) and muscle IL-6 mRNA. In contrast, exercise-induced regulation of liver CXCL-1 mRNA expression was completely blunted in IL-6 knockout mice. Based on these findings, we examined the possible existence of a muscle-to-liver axis by overexpressing IL-6 in muscles. This resulted in increases in serum CXCL-1 (5-fold) and liver CXCL-1 mRNA expression (24-fold) compared with control. Because IL-6 expression and release are known to be augmented during exercise in glycogen-depleted animals, CXCL-1 and IL-6 expression were examined after exercise in overnight-fasted mice.We found that fasting significantly augmented serum CXCL-1, and CXCL-1 expression in liver and muscle. Taken together, these data indicate that liver is the main source of serum CXCL-1 during exercise in mice, and that the CXCL-1 expression in the liver is regulated by muscle-derived IL-6.