Putative Myokine Research Articles

Integrated procedures for accelerating, deepening, and leading genetic inquiry: A first application on human muscle secretome

PurposeBeyond classical procedures, bioinformatic-assisted approaches and computational biology offer unprecedented opportunities for scholars. However, these amazing possibilities still need epistemological criticism, as well as standardized procedures. Especially those topics with a huge body of data may benefit from data science (DS)-assisted methods. Therefore, the current study dealt with the combined expert-assisted and DS-assisted approaches to address the broad field of muscle secretome. We aimed to apply DS tools to fix the literature research, suggest investigation targets with a data-driven approach, predict possible scenarios, and define a workflow. MethodsRecognized scholars with expertise on myokines were invited to provide a list of the most important myokines. GeneRecommender, GeneMANIA, HumanNet, and STRING were selected as DS tools. Networks were built on STRING and GeneMANIA. The outcomes of DS tools included the top 5 recommendations. Each expert-led discussion has been then integrated with an DS-led approach to provide further perspectives. ResultsAmong the results, 11 molecules had already been described as bona-fide myokines in literature, and 11 molecules were putative myokines. Most of the myokines and the putative myokines recommended by the DS tools were described as present in the cargo of extracellular vesicles. ConclusionsIncluding both supervised and unsupervised learning methods, as well as encompassing algorithms focused on both protein interaction and gene represent a comprehensive approach to tackle complex biomedical topics. DS-assisted methods for reviewing existent evidence, recommending targets of interest, and predicting original scenarios are worth exploring as in silico recommendations to be integrated with experts' ideas for optimizing molecular studies.

Identification of myokines susceptible to improve glucose homeostasis after bariatric surgery.

The identification of myokines susceptible to improve glucose homeostasis following bariatric surgery could lead to new therapeutic approaches for type 2 diabetes. Changes in the Homeostasis Model Assessment (HOMA) test were assessed in patients before and 3 months after bariatric surgery. Changes in myokines expression and circulating levels were assessed using RT-qPCR and ELISA. Myokines known to regulate glucose homeostasis were identified using literature (Targeted study) and putative myokines using RNA-Sequencing (Untargeted study). A linear regression analysis adjusted for age and sex was used to search for associations between changes in the HOMA test and changes in myokines. In the targeted study, BDNF expression was upregulated (+30%, p=0.006) while BDNF circulating levels were decreased (-12%, p=0.001). Upregulated BDNF expression was associated with decreased HOMA-IR (adjusted estimate [95%CI]: -0.51 [-0.88; -0.13], p=0.010). Decreased BDNF serum levels were associated with decreased HOMA-B (adjusted estimate [95%CI] = 0.002 [0.00002-0.0031], p=0.046). In the untargeted study, upregulated putative myokines included XYLT1 (+64%, p<0.001), LGR5 (+57, p<0.001), and SPINK5 (+46%, p<0.001). Upregulated LGR5 was associated with decreased HOMA-IR (adjusted estimate -0.50 [-0.86; -0.13], p=0.009). Upregulated XYLT1 and SPINK5 were associated with increased HOMA-S (respectively, adjusted estimate [95%CI]: 109.1 [28.5; 189.8], p=0.009 and 16.5 [0.87; 32.19], p=0.039). Improved glucose homeostasis following bariatric surgery is associated with changes in myokines expression and circulating levels. In particular, upregulation of BDNF, XYLT1, SPINK5, and LGR5 is associated with improved insulin sensitivity. These results suggest that these myokines could contribute to improved glucose homeostasis following bariatric surgery.

Aortic carboxypeptidase-like protein, a putative myokine, stimulates the differentiation and survival of bone-forming osteoblasts.

A new target that stimulates bone formation is needed to overcome limitations of current anti-osteoporotic drugs. Myokines, factors secreted from muscles, may modulate it. In this study, we investigated the role of aortic carboxypeptidase-like protein (ACLP), which is highly expressed in skeletal muscles, on bone formation. MC3T3-E1 cells and/or calvaria osteoblasts were treated with recombinant N-terminal mouse ACLP containing a signal peptide [rmACLP (N)]. The expression and secretion of ACLP were higher in skeletal muscle and differentiated myotube than in other tissues and undifferentiated myoblasts, respectively. rmACLP (N) increased bone formation, ALP activity, and phosphorylated p38 mitogen-activated protein (MAP) kinase in osteoblasts; reversal was achieved by pre-treatment with a TGF-β receptor inhibitor. Under H2 O2 treatment, rmACLP (N) increased osteoblast survival, phosphorylated p38 MAP kinase, and the nuclear translocation of FoxO3a in osteoblasts. H2 O2 treatment caused rmACLP (N) to suppress its apoptotic, oxidative, and caspase-9 activities. rmACLP (N)-stimulated osteoblast survival was reversed by pre-treatment with a p38 inhibitor, a TGF-β-receptor II blocking antibody, and a FoxO3a shRNA. Conditioned media (CM) from muscle cells stimulated osteoblast survival under H2 O2 treatment, in contrast to CM from ACLP knockdown muscle cells. rmACLP (N) increased the expressions of FoxO3a target anti-oxidant genes such as Sod2, Trx2, and Prx5. In conclusion, ACLP stimulated the differentiation and survival of osteoblasts. This led to the stimulation of bone formation by the activation of p38 MAP kinase and/or FoxO3a via TGF-β receptors. These findings suggest a novel role for ACLP in bone metabolism as a putative myokine.

Insight Into the Metabolic Adaptations of Electrically Pulse-Stimulated Human Myotubes Using Global Analysis of the Transcriptome and Proteome.

Electrical pulse stimulation (EPS) has proven to be a useful tool to interrogate cell-specific responses to muscle contraction. In the present study, we aimed to uncover networks of signaling pathways and regulatory molecules responsible for the metabolic effects of exercise in human skeletal muscle cells exposed to chronic EPS. Differentiated myotubes from young male subjects were exposed to EPS protocol 1 (i.e. 2 ms, 10 V, and 0.1 Hz for 24 h), whereas myotubes from middle-aged women and men were exposed to protocol 2 (i.e. 2 ms, 30 V, and 1 Hz for 48 h). Fuel handling as well as the transcriptome, cellular proteome, and secreted proteins of EPS-treated myotubes from young male subjects were analyzed using a combination of high-throughput RNA sequencing, high-resolution liquid chromatography-tandem mass spectrometry, oxidation assay, and immunoblotting. The data showed that oxidative metabolism was enhanced in EPS-exposed myotubes from young male subjects. Moreover, a total of 81 differentially regulated proteins and 952 differentially expressed genes (DEGs) were observed in these cells after EPS protocol 1. We also found 61 overlapping genes while comparing the DEGs to mRNA expression in myotubes from the middle-aged group exposed to protocol 2, assessed by microarray. Gene ontology (GO) analysis indicated that significantly regulated proteins and genes were enriched in biological processes related to glycolytic pathways, positive regulation of fatty acid oxidation, and oxidative phosphorylation, as well as muscle contraction, autophagy/mitophagy, and oxidative stress. Additionally, proteomic identification of secreted proteins revealed extracellular levels of 137 proteins were changed in myotubes from young male subjects exposed to EPS protocol 1. Selected putative myokines were measured using ELISA or multiplex assay to validate the results. Collectively, our data provides new insight into the transcriptome, proteome and secreted proteins alterations following in vitro exercise and is a valuable resource for understanding the molecular mechanisms and regulatory molecules mediating the beneficial metabolic effects of exercise.

Genetic variation of putative myokine signaling is dominated by biological sex and sex hormones.

Skeletal muscle plays an integral role in coordinating physiological homeostasis, where signaling to other tissues via myokines allows for coordination of complex processes. Here, we aimed to leverage natural genetic correlation structure of gene expression both within and across tissues to understand how muscle interacts with metabolic tissues. Specifically, we performed a survey of genetic correlations focused on myokine gene regulation, muscle cell composition, cross-tissue signaling, and interactions with genetic sex in humans. While expression levels of a majority of myokines and cell proportions within skeletal muscle showed little relative differences between males and females, nearly all significant cross-tissue enrichments operated in a sex-specific or hormone-dependent fashion; in particular, with estradiol. These sex- and hormone-specific effects were consistent across key metabolic tissues: liver, pancreas, hypothalamus, intestine, heart, visceral, and subcutaneous adipose tissue. To characterize the role of estradiol receptor signaling on myokine expression, we generated male and female mice which lack estrogen receptor α specifically in skeletal muscle (MERKO) and integrated with human data. These analyses highlighted potential mechanisms of sex-dependent myokine signaling conserved between species, such as myostatin enriched for divergent substrate utilization pathways between sexes. Several other putative sex-dependent mechanisms of myokine signaling were uncovered, such as muscle-derived tumor necrosis factor alpha (TNFA) enriched for stronger inflammatory signaling in females compared to males and GPX3 as a male-specific link between glycolytic fiber abundance and hepatic inflammation. Collectively, we provide a population genetics framework for inferring muscle signaling to metabolic tissues in humans. We further highlight sex and estradiol receptor signaling as critical variables when assaying myokine functions and how changes in cell composition are predicted to impact other metabolic organs.

Altered irisin/BDNF axis parallels excessive daytime sleepiness in obstructive sleep apnea patients

Study objectivesObstructive sleep apnea hypopnea syndrome (OSAHS) is a sleep-related breathing disorder, characterized by excessive daytime sleepiness (EDS), paralleled by intermittent collapse of the upper airway. EDS may be the symptom of OSAHS per se but may also be due to the alteration of central circadian regulation. Irisin is a putative myokine and has been shown to induce BDNF expression in several sites of the brain. BDNF is a key factor regulating photic entrainment and consequent circadian alignment and adaptation to the environment. Therefore, we hypothesized that EDS accompanying OSAHS is reflected by alteration of irisin/BDNF axis.MethodsCase history, routine laboratory parameters, serum irisin and BDNF levels, polysomnographic measures and Epworth Sleepiness Scale questionnaire (ESS) were performed in a cohort of OSAHS patients (n = 69). Simple and then multiple linear regression was used to evaluate data.ResultsWe found that EDS reflected by the ESS is associated with higher serum irisin and BDNF levels; β: 1.53; CI: 0.35, 6.15; p = 0.012 and β: 0.014; CI: 0.0.005, 0.023; p = 0.02, respectively. Furthermore, influence of irisin and BDNF was significant even if the model accounted for their interaction (p = 0.006 for the terms serum irisin, serum BDNF and their interaction). Furthermore, a concentration-dependent effect of both serum irisin and BDNF was evidenced with respect to their influence on the ESS.ConclusionsThese results suggest that the irisin-BDNF axis influences subjective daytime sleepiness in OSAS patients reflected by the ESS. These results further imply the possible disruption of the circadian regulation in OSAHS. Future interventional studies are needed to confirm this observation.

Effects of sprint interval or combined aerobic and resistance training on myokines in overweight women with type 2 diabetes: A randomized controlled trial

Our primary aim was to assess the effects of two different training modalities: sprint interval training (SIT) or combined aerobic and resistance training (A + R) on circulating myokines related to metabolic profile and adiposity in type 2 diabetes (T2D). Fifty-two overweight women with T2D [55 ± 6 yrs., BMI 28.9 ± 4.1 kg/m2, HbA1c 9.4 ± 0.82% (79 mmol/mol)] were randomized to SIT (n = 17), A + R training (n = 17) or control (n = 18) for 10 weeks. Myokines, metabolic outcomes, body composition and cardiorespiratory fitness were assessed at baseline and 48 hours after the last training session/control period. Relationships between myokines and other variables were investigated via linear regression models. Completion rate was 81%. There was no effect of either exercise modality on any myokine. Interlukin-15 decreased over time irrespective of group assignment (p = 0.02). Aerobic capacity (p = 0.01), fasting glucose (p = 0.03) and HbA1c (p = 0.006) improved significantly and similarly in both exercise groups compared to controls. Insulin (p = 0.02), weight (p = 0.020, body max index (BMI) (p = 0.01) decreased significantly over time irrespective of group. Changes in myokines were unrelated to changes in body composition or metabolic profile. Neither SIT or A + R training altered myokines measured 48 h after exercise in T2D, despite improving aerobic capacity and glucose homeostasis relative to controls. Future studies are needed to elucidate the time course and clinical relevance of putative myokine responses to exercise in this and other cohorts.

Intact regulation of muscle expression and circulating levels of myokines in response to exercise in patients withtype 2 diabetes.

Regular exercise plays an important role in the prevention and treatment of type 2 diabetes (T2D). The synthesis and secretion of myokines in response to contraction may contribute to the beneficial metabolic effects of exercise. However, some exercise‐induced responses may be attenuated in T2D. Here, we investigated whether the effect of acute exercise on selected myokines are impaired in T2D. Skeletal muscle biopsies and blood samples were obtained from 13 men with T2D and 14 weight‐matched, glucose‐tolerant men before, immediately after and 3‐h after acute exercise (60 min cycling) to examine muscle expression and plasma/serum levels of selected myokines. One‐hour of exercise increased muscle expression of IL6, FGF21, ANGPTL4, CHI3L1, CTGF and CYR61, of which FGF21, ANGPTL4 and CHI3L1 increased further 3‐h into recovery, whereas expression of IL6, CYR61, and CTGF returned to baseline levels. There was no immediate effect of exercise on IL15 expression, but it decreased 3‐h into recovery. Plasma IL‐6 increased robustly, whereas circulating levels of FGF21, ANGPTL4, IL‐15, and CHI3L1 increased only modestly in response to exercise. All returned toward baseline levels 3‐h into recovery except for plasma ANGPTL4, which increased further. No significant differences in these responses to exercise were observed between the groups. Our results demonstrate that muscle expression and circulating levels of selected known and putative myokines were equally regulated by acute exercise in patients with T2D and weight‐matched controls. This suggests that the potential beneficial metabolic effects of these myokines are not impaired in patients with T2D.

Transcriptional profiling reveals extraordinary diversity among skeletal muscle tissues.

Skeletal muscle comprises a family of diverse tissues with highly specialized functions. Many acquired diseases, including HIV and COPD, affect specific muscles while sparing others. Even monogenic muscular dystrophies selectively affect certain muscle groups. These observations suggest that factors intrinsic to muscle tissues influence their resistance to disease. Nevertheless, most studies have not addressed transcriptional diversity among skeletal muscles. Here we use RNAseq to profile mRNA expression in skeletal, smooth, and cardiac muscle tissues from mice and rats. Our data set, MuscleDB, reveals extensive transcriptional diversity, with greater than 50% of transcripts differentially expressed among skeletal muscle tissues. We detect mRNA expression of hundreds of putative myokines that may underlie the endocrine functions of skeletal muscle. We identify candidate genes that may drive tissue specialization, including Smarca4, Vegfa, and Myostatin. By demonstrating the intrinsic diversity of skeletal muscles, these data provide a resource for studying the mechanisms of tissue specialization.

Transcriptional Profiling Reveals Extraordinary Diversity Among Skeletal Muscle Tissues

Skeletal muscle comprises a family of diverse tissues with highly specialized morphology, function, and metabolism. Many acquired diseases – including HIV, COPD, cancer cachexia, critical illness myopathy, and sepsis – affect specific muscles while sparing others. Even monogenic muscular dystrophies tend to selectively affect certain muscle groups, despite their causative genetic mutations being present in all tissues. These observations suggest that factors intrinsic to muscle tissues influence their susceptibility to various disease mechanisms. Nevertheless, most studies have not addressed transcriptional diversity among skeletal muscles. Here we use unbiased RNA sequencing (RNAseq) to profile global mRNA expression in a wide array of skeletal, smooth, and cardiac muscle tissues from mice and rats. Our data set, MuscleDB, reveals extensive transcriptional diversity, with greater than 50% of transcripts differentially expressed among skeletal muscle tissues. This diversity is only partly explained by fiber type composition and developmental history, suggesting that specialized transcriptional profiles establish the functional identity of muscle tissues. We find conservation in the transcriptional profiles across species as well as between males and females, indicating that these data may be useful in predicting gene expression in related species, such as humans. Notably, thousands of differentially expressed genes in skeletal muscle are associated with human disease, and hundreds of these genes encode targets of drugs on the market today. Related to this observation, we suggest a mechanistic explanation for how myotonic dystrophy induces weakness in the extensor digitorum longus (EDL) while sparing nearby muscles. These data may therefore provide the means by which muscle-specific sensitivity to disease may be unraveled. In addition, we detect hundreds of putative myokines that may underlie the endocrine functions of skeletal muscle. We anticipate that in conjunction with transcriptional modeling, this resource will catalyze more sophisticated tissue engineering of skeletal muscle to improve the efficacy of regenerative medicine.

Skeletal muscle as a gene regulatory endocrine organ

Skeletal muscle is gaining increased attention as an endocrine organ. Recently, novel myokines and new effects of already established myokines have been identified. The objective of this review is to give an update on the recent advances in the field. Several hundred putative myokines have been described, some of which are induced by contraction and differentially regulated between healthy and metabolically diseased individuals. Interleukin-6 (IL-6) is the prototype myokine, which was identified as a muscle-derived cytokine 15 years ago. Recently, IL-6 has been linked to β-cell survival and inhibition of cancer-cell growth. Moreover, trans-signaling appears to determine whether IL-6 acts as a proinflammatory or an anti-inflammatory cytokine. Irisin has been shown to be a secreted myokine, which contribute to circulating concentrations dependent on training status. IL-15 has been established as a cytokine mediating cross-talk between skeletal muscle and skin tissue, and decorin has been characterized as a contraction-induced myokine which apparently is differentially regulated between healthy and dysglycemic individuals. Skeletal muscle is an endocrine organ which, by the release of myokines, may influence metabolism in virtually all organs in the body. This knowledge may potentially open up for the possibility of designing new drugs that mimic the effects of myokine signaling.

Muscle-specific deletion of exons 2 and 3 of the IL15RA gene in mice: effects on contractile properties of fast and slow muscles.

Interleukin-15 (IL-15) is a putative myokine hypothesized to induce an oxidative skeletal muscle phenotype. The specific IL-15 receptor alpha subunit (IL-15Rα) has also been implicated in specifying this contractile phenotype. The purposes of this study were to determine the muscle-specific effects of IL-15Rα functional deficiency on skeletal muscle isometric contractile properties, fatigue characteristics, spontaneous cage activity, and circulating IL-15 levels in male and female mice. Muscle creatine kinase (MCK)-driven IL-15Rα knockout mice (mIl15ra(fl/fl)/Cre(+)) were generated using the Cre-loxP system. We tested the hypothesis that IL-15Rα functional deficiency in skeletal muscle would increase resistance to contraction-induced fatigue, cage activity, and circulating IL-15 levels. There was a significant effect of genotype on the fatigue curves obtained in extensor digitorum longus (EDL) muscles from female mIl15ra(fl/fl)/Cre(+) mice, such that force output was greater during the repeated contraction protocol compared with mIl15ra(fl/fl)/Cre(-) control mice. Muscles from female mIl15ra(fl/fl)/Cre(+) mice also had a twofold greater amount of the mitochondrial genome-specific COXII gene compared with muscles from mIl15ra(fl/fl)/Cre(-) control mice, indicating a greater mitochondrial density in these skeletal muscles. There was a significant effect of genotype on the twitch:tetanus ratio in EDL and soleus muscles from mIl15ra(fl/fl)/Cre(+) mice, such that the ratio was lower in these muscles compared with mIl15ra(fl/fl)/Cre(-) control mice, indicating a pro-oxidative shift in muscle phenotype. However, spontaneous cage activity was not different and IL-15 protein levels were lower in male and female mIl15ra(fl/fl)/Cre(+) mice compared with control. Collectively, these data support a direct effect of muscle IL-15Rα deficiency in altering contractile properties and fatigue characteristics in skeletal muscles.

Identification of human exercise-induced myokines using secretome analysis

Endurance exercise is associated with significant improvements in cardio-metabolic risk parameters. A role for myokines has been hypothesized, yet limited information is available about myokines induced by acute endurance exercise in humans. Therefore, the aim of the study was to identify novel exercise-induced myokines in humans. To this end, we carried out a 1 h one-legged acute endurance exercise intervention in 12 male subjects and a 12 wk exercise training intervention in 18 male subjects. Muscle biopsies were taken before and after acute exercise or exercise training and were subjected to microarray-based analysis of secreted proteins (secretome). For acute exercise, secretome analysis resulted in a list of 86 putative myokines, which was reduced to 29 by applying a fold-change cut-off of 1.5. Based on that shortlist, a selection of putative myokines was measured in the plasma by ELISA or multiplex assay. From that selection, CX3CL1 (fractalkine) and CCL2 (MCP-1) increased at both mRNA and plasma levels. From the known myokines, only IL-6 and FGF 21 changed at the mRNA level, whereas none of the known myokines changed at the plasma level. Secretome analysis of exercise training intervention resulted in a list of 69 putative myokines. Comparing putative myokines altered by acute exercise and exercise training revealed a limited overlap of only 13 genes. In conclusion, this study identified CX3CL1 and CCL2 as myokines that were induced by acute exercise at the gene expression and plasma level and that may be involved in communication between skeletal muscle and other organs.

The Structure of Irisin Reveals a Novel Intersubunit β-Sheet Fibronectin Type III (FNIII) Dimer: IMPLICATIONS FOR RECEPTOR ACTIVATION

Irisin was recently identified as a putative myokine that is induced by exercise. Studies suggest that it is produced by cleavage of the FNDC5 (fibronectin domain-containing protein 5) receptor; irisin corresponds to the extracellular receptor ectodomain. Data suggesting that irisin stimulates white-to-brown fat conversion have led to the hypothesis that it does so by binding an unknown receptor, thus functioning as a myokine. As brown fat promotes energy dissipation, myokines that elicit the transformation of white to brown fat have potentially profound benefits in the treatment of obesity and metabolic disorders. Understanding the molecular basis for such exercise-induced phenomena is thus of considerable interest. Moreover, FNDC5-like receptors are highly conserved and have been shown to be critical for neuronal development. However, the structural and molecular mechanisms utilized by these proteins are currently unknown. Here, we describe the crystal structure and biochemical characterization of the FNDC5 ectodomain, corresponding to the irisin myokine. The 2.28 Å structure shows that irisin consists of an N-terminal fibronectin III (FNIII)-like domain attached to a flexible C-terminal tail. Strikingly, the FNIII-like domain forms a continuous intersubunit β-sheet dimer, previously unobserved for any FNIII protein. Biochemical data confirm that irisin is a dimer and that dimerization is unaffected by glycosylation. This finding suggests a possible mechanism for receptor activation by the irisin domain as a preformed myokine dimer ligand or as a paracrine or autocrine dimerization module on FNDC5-like receptors.

Contraction and AICAR Stimulate IL-6 Vesicle Depletion From Skeletal Muscle Fibers In Vivo

Recent studies suggest that interleukin 6 (IL-6) is released from contracting skeletal muscles; however, the cellular origin, secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with enhanced green fluorescent protein (EGFP)-tagged IL-6 (IL-6-EGFP), and 5 days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP–containing vesicles and protein by 62% (P < 0.05), occurring rapidly and progressively over 25 min of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMP-activated protein kinase (AMPK) α2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6–positive vesicles that are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine and can be used to characterize the secretion kinetics of other putative myokines.