Therapeutic targeting of GDF11 in muscle atrophy: Insights and strategies

The exploration of novel therapeutic avenues for skeletal muscle atrophy is imperative due to its significant health impact. Recent studies have spotlighted growth differentiation factor 11 (GDF11), a TGFβ superfamily member, for its rejuvenating role in reversing age-related tissue dysfunction. This review synthesizes current findings on GDF11, elucidating its distinct biological functions and the ongoing debates regarding its efficacy in muscle homeostasis. By addressing discrepancies in current research outcomes and its ambiguous role due to its homological identity to myostatin, a negative regulator of muscle mass, this review aims to clarify the role of GDF11 in muscle homeostasis and its potential as a therapeutic target for muscle atrophy. Through a thorough examination of GDF11's mechanisms and effects, this review provides insights that could pave the way for innovative treatments for muscle atrophy, emphasizing the need and strategies to boost endogenous GDF11 levels for therapeutic potential.

Identification of key lncRNAs and mRNAs in muscle development pathways of Tan sheep

The study aimed to identify the long noncoding RNA (lncRNA) responsible for regulating muscle development in Tan sheep. RNA-seq analysis was conducted on longissimus dorsi samples from 1-day-old and 60-day-old Tan sheep to investigate the molecular processes involved in muscle development. A total of 5517 lncRNAs and 2885 mRNAs were found to be differentially expressed in the 60-day-old Tan sheep. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that these differentially expressed lncRNAs and mRNAs were linked to pathways crucial for muscle development, such as MAPK, cAMP, and calcium-mediated signaling pathways. Key genes like CDKN1A, MAPK14, TGFB1, MEF2C, MYOD1, and CD53 were identified as significant players in muscle development. The study validated the RNA-seq results through RT-qPCR, confirming the consistency of expression levels of differentially expressed lncRNAs and mRNAs. These findings indicate that lncRNA-mRNA networks produce a remarked effect on modulating muscle development in Tan sheep, such as lncRNAs (MSTRG.12808.1/MSTRG.22662.3/MSTRG.18310.1) and mRNAs (MSTRG.10027/MSTRG.10029/MSTRG.10258/MSTRG.11011/MSTRG.10354), laying the groundwork for future research in this area.

Network model of skeletal muscle cell signalling predicts differential responses to endurance and resistance exercise training

Exercise-induced muscle adaptations vary based on exercise modality and intensity. We constructed a signalling network model from 87 published studies of human or rodent skeletal muscle cell responses to endurance or resistance exercise in vivo or simulated exercise in vitro. The network comprises 259 signalling interactions between 120 nodes, representing eight membrane receptors and eight canonical signalling pathways regulating 14 transcriptional regulators, 28 target genes and 12 exercise-induced phenotypes. Using this network, we formulated a logic-based ordinary differential equation model predicting time-dependent molecular and phenotypic alterations following acute endurance and resistance exercises. Compared with nine independent studies, the model accurately predicted 18/21 (85%) acute responses to resistance exercise and 12/16 (75%) acute responses to endurance exercise. Detailed sensitivity analysis of differential phenotypic responses to resistance and endurance training showed that, in the model, exercise regulates cell growth and protein synthesis primarily by signalling via mechanistic target of rapamycin, which is activated by Akt and inhibited in endurance exercise by AMP-activated protein kinase. Endurance exercise preferentially activates inflammation via reactive oxygen species and nuclear factor κB signalling. Furthermore, the expected preferential activation of mitochondrial biogenesis by endurance exercise was counterbalanced in the model by protein kinase C in response to resistance training. This model provides a new tool for investigating cross-talk between skeletal muscle signalling pathways activated by endurance and resistance exercise, and the mechanisms of interactions such as the interference effects of endurance training on resistance exercise outcomes.

The Role of Muscle Biomarkers in Adolescent Idiopathic Scoliosis

Adolescent idiopathic scoliosis (AIS) is the predominant orthopedic disorder in children, affecting 1-3% of the global population. Research in this field has tried to delineate the genetic factors behind scoliosis and its association with heredity since AIS is considered a polygenic disease and has different genetic and epigenetic factors. The current study conducted a narrative review of the literature, focusing on biomarkers in the pathophysiology of muscle in AIS patients. Articles were collected from Scopus, Pubmed, and Web of Science. The key screening parameters were scoliosis classification, sampling, and the biomarkers evaluated. This review emphasizes potential key mechanisms and molecular regulators in muscle tissue. While there has been limited focus on the proteins contributing to muscle changes in AIS, significant attention has been given to genomic studies of single-nucleotide polymorphisms, particularly in LBX1. Despite these efforts, the exact causes of AIS remain elusive, with several theories suggesting genetic and hormonal factors. This review identified critical protein biomarkers such as Gi-protein alpha subunits, fibrillin-1 and -2, and various differentially expressed proteins, which may be linked to muscle alterations in AIS. This field of research is still limited due to a lack of homogeneity in the distinction of patients by groups and curve severity. Although the pathophysiology of AIS is still unclear, molecular research is important to guide the treatment of AIS before achieving skeletal maturity, thus avoiding serious problems associated with posture changes and low quality of life. In the future, a more comprehensive synergy between orthopedic and molecular research might ameliorate the diagnosis and treatment of AIS patients.

The identification and validation of target genes of IGFBP3 protein in sheep skeletal muscle cells

This study aimed to identify the target genes of IGFBP3（insulin growth factor binding protein）protein and to investigate its target genes effects on the proliferation and differentiation of Hu sheep skeletal muscle cells. IGFBP3 was an RNA-binding protein that regulates mRNA stability. Previous studies have reported that IGFBP3 promotes the proliferation of Hu sheep skeletal muscle cells and inhibits differentiation, but the downstream genes that bind to it have not been reported yet. We predicted the target genes of IGFBP3 through RNAct and sequencing data, and verified by qPCR and RIP（RNA Immunoprecipitation）experiments, and demonstrated GNAI2（G protein subunit alpha i2）as one of the target gene of IGFBP3. After interference with siRNA, we carried out qPCR, CCK8, EdU, and immunofluorescence experiments, and found that GNAI2 can promote the proliferation and inhibit differentiation of Hu sheep skeletal muscle cells. This study revealed the effects of GNAI2 and provided one of the regulatory mechanisms of IGFBP3 protein underlying sheep muscle development.

Non-canonical Ca2+- Akt signaling pathway mediates the antiproteolytic effects induced by oxytocin receptor stimulation in skeletal muscle

Although it has been previously demonstrated that oxytocin (OXT) receptor stimulation can control skeletal muscle mass in vivo, the intracellular mechanisms that mediate this effect are still poorly understood. Thus, rat oxidative skeletal muscles were isolated and incubated with OXT or WAY-267,464, a non-peptide selective OXT receptor (OXTR) agonist, in the presence or absence of atosiban (ATB), an OXTR antagonist, and overall proteolysis was evaluated. The results indicated that both OXT and WAY-267,464 suppressed muscle proteolysis, and this effect was blocked by the addition of ATB. Furthermore, the WAY-induced anti-catabolic action on protein metabolism did not involve the coupling between OXTR and Gαi since it was insensitive to pertussis toxin (PTX). The decrease in overall proteolysis induced by WAY was probably due to the inhibition of the autophagic/lysosomal system, as estimated by the decrease in LC3 (an autophagic/lysosomal marker), and was accompanied by an increase in the content of Ca2+-dependent protein kinase (PKC)-phosphorylated substrates, pSer473-Akt, and pSer256-FoxO1. Most of these effects were blocked by the inhibition of inositol triphosphate receptors (IP3R), which mediate Ca2+ release from the sarcoplasmic reticulum to the cytoplasm, and triciribine, an Akt inhibitor. Taken together, these findings indicate that the stimulation of OXTR directly induces skeletal muscle protein-sparing effects through a Gαq/IP3R/Ca2+-dependent pathway and crosstalk with Akt/FoxO1 signaling, which consequently decreases the expression of genes related to atrophy, such as LC3, as well as muscle proteolysis.

Inflammation o'clock: interactions of circadian rhythms with inflammation-induced skeletal muscle atrophy

Circadian rhythms are ∼24 h cycles evident in behaviour, physiology and metabolism. The molecular mechanism directing circadian rhythms is the circadian clock, which is composed of an interactive network of transcription-translation feedback loops. The core clock genes include Bmal1, Clock, Rev-erbα/β, Per and Cry. In addition to keeping time, the core clock regulates a daily programme of gene expression that is important for overall cell homeostasis. The circadian clock mechanism is present in all cells, including skeletal muscle fibres, and disruption of the muscle clock is associated with changes in muscle phenotype and function. Skeletal muscle atrophy is largely associated with a lower quality of life, frailty and reduced lifespan. Physiological and genetic modification of the core clock mechanism yields immune dysfunction, alters inflammatory factor expression and secretion and is associated with skeletal muscle atrophy in multiple conditions, such as ageing and cancer cachexia. Here, we summarize the possible interplay between the circadian clock modulation of immune cells, systemic inflammatory status and skeletal muscle atrophy in chronic inflammatory conditions. Although there is a clear disruption of circadian clocks in various models of atrophy, the mechanism behind such alterations remains unknown. Understanding the modulatory potential of muscle and immune circadian clocks in inflammation and skeletal muscle health is essential for the development of therapeutic strategies to protect skeletal muscle mass and function of patients with chronic inflammation.

Gαi2 regulates the adult myogenesis of masticatory muscle satellite cells

Although similar to trunk and limb skeletal muscles, masticatory muscles are believed as unique in both developmental origins and myogenesis. Gαi2 has been demonstrated to promote muscle hypertrophy and muscle satellite cell differentiation in limb muscles. However, the effect of Gαi2 on masticatory muscles is still unexplored. This study aimed to identify the role of Gαi2 in the proliferation and differentiation of masticatory muscle satellite cells, further exploring the metabolic mechanism of masticatory muscles. The proliferation rate, myotube size, fusion index of masticatory muscle satellite cells and Pax7, Myf5, MyoD, Tcf21 and Musculin expressions were significantly decreased by Gαi2 knockdown, while in cells infected with AdV4-Gαi2, the proliferation rate, myotube size, fusion index and Tbx1 expression were significantly increased. Masticatory muscle satellite cells also displayed phenotype transformation as Gαi2 changed. In addition, Gαi2 altered myosin heavy chain (MyHC) isoforms of myotubes with less MyHC-2A expression in siGαi2 group and more MyHC-slow expression in AdV4-Gαi2 group. In conclusion, Gαi2 could positively affect the adult myogenesis of masticatory muscle satellite cells and maintain the superiority of MyHC-slow. Masticatory muscle satellite cells may have their unique Gαi2-regulated myogenic transcriptional networks, although they may share some common characteristics with trunk and limb muscles.

Organokines, Sarcopenia, and Metabolic Repercussions: The Vicious Cycle and the Interplay with Exercise

Sarcopenia is a disease that becomes more prevalent as the population ages, since it is directly linked to the process of senility, which courses with muscle atrophy and loss of muscle strength. Over time, sarcopenia is linked to obesity, being known as sarcopenic obesity, and leads to other metabolic changes. At the molecular level, organokines act on different tissues and can improve or harm sarcopenia. It all depends on their production process, which is associated with factors such as physical exercise, the aging process, and metabolic diseases. Because of the seriousness of these repercussions, the aim of this literature review is to conduct a review on the relationship between organokines, sarcopenia, diabetes, and other metabolic repercussions, as well the role of physical exercise. To build this review, PubMed-Medline, Embase, and COCHRANE databases were searched, and only studies written in English were included. It was observed that myokines, adipokines, hepatokines, and osteokines had direct impacts on the pathophysiology of sarcopenia and its metabolic repercussions. Therefore, knowing how organokines act is very important to know their impacts on age, disease prevention, and how they can be related to the prevention of muscle loss.

Exploring the Muscle Metabolomics in the Mouse Model of Sepsis-Induced Acquired Weakness

Background/Aim

We aimed to identify the differentially expressing metabolites (DEMs) in the muscles of the mouse model of sepsis-induced acquired weakness (sepsis-AW) using liquid chromatography-mass spectrometry (LC-MS).

Materials and Methods

Sepsis by cecal ligation puncture (CLP) with lower limb immobilization was used to produce a sepsis-AW model. After this, the grip strength of the C57BL/6 male mice was investigated. The transmission electron microscopy was utilized to determine the pathological model. LC-MS was used to detect the metabolic profiles within the mouse muscles. Additionally, a statistically diversified analysis was carried out.

Results

Compared to the sepsis group, 30 DEMs, including 17 upregulated and 13 down-regulated metabolites, were found in the sepsis-AW group. The enriched metabolic pathways including purine metabolism, valine/leucine/isoleucine biosynthesis, cGMP-PKG pathway, mTOR pathway, FoxO pathway, and PI3K-Akt pathway were found to differ between the two groups. The targeted metabolomics analysis explored significant differences between four amino acid metabolites (leucine, cysteine, tyrosine, and serine) and two energy metabolites (AMP and cAMP) in the muscles of the sepsis-AW experimental model group, which was comparable to the sepsis group.

Conclusion

The present work identified DEMs and metabolism-related pathways within the muscles of the sepsis-AW mice, which offered valuable experimental data for diagnosis and identification of the pathogenic mechanism underlying sepsis-AW.

Molecular networks underlying cannabinoid signaling in skeletal muscle plasticity

The cannabinoid system is ubiquitously present and is classically considered to engage in neural and immunity processes. Yet, the role of the cannabinoid system in the whole body and tissue metabolism via central and peripheral mechanisms is increasingly recognized. The present review provides insights in (i) how cannabinoid signaling is regulated via receptor-independent and -dependent mechanisms and (ii) how these signaling cascades (might) affect skeletal muscle plasticity and physiology. Receptor-independent mechanisms include endocannabinoid metabolism to eicosanoids and the regulation of ion channels. Alternatively, endocannabinoids can act as ligands for different classic (cannabinoid receptor 1 [CB₁ ], CB₂ ) and/or alternative (e.g., TRPV1, GPR55) cannabinoid receptors with a unique affinity, specificity, and intracellular signaling cascade (often tissue-specific). Antagonism of CB₁ might hold clues to improve oxidative (mitochondrial) metabolism, insulin sensitivity, satellite cell growth, and muscle anabolism, whereas CB₂ agonism might be a promising way to stimulate muscle metabolism and muscle cell growth. Besides, CB₂ ameliorates muscle regeneration via macrophage polarization toward an anti-inflammatory phenotype, induction of MyoD and myogenin expression and antifibrotic mechanisms. Also TRPV1 and GPR55 contribute to the regulation of muscle growth and metabolism. Future studies should reveal how the cannabinoid system can be targeted to improve muscle quantity and/or quality in conditions such as ageing, disease, disuse, and metabolic dysregulation, taking into account challenges that are inherent to modulation of the cannabinoid system, such as central and peripheral side effects.

DNA methylation may affect beef tenderness through signal transduction in Bos indicus

BACKGROUND

Beef tenderness is a complex trait of economic importance for the beef industry. Understanding the epigenetic mechanisms underlying this trait may help improve the accuracy of breeding programs. However, little is known about epigenetic effects on Bos taurus muscle and their implications in tenderness, and no studies have been conducted in Bos indicus.

RESULTS

Comparing methylation profile of Bos indicus skeletal muscle with contrasting beef tenderness at 14 days after slaughter, we identified differentially methylated cytosines and regions associated with this trait. Interestingly, muscle that became tender beef had higher levels of hypermethylation compared to the tough group. Enrichment analysis of predicted target genes suggested that differences in methylation between tender and tough beef may affect signal transduction pathways, among which G protein signaling was a key pathway. In addition, different methylation levels were found associated with expression levels of GNAS, PDE4B, EPCAM and EBF3 genes. The differentially methylated elements correlated with EBF3 and GNAS genes overlapped CpG islands and regulatory elements. GNAS, a complex imprinted gene, has a key role on G protein signaling pathways. Moreover, both G protein signaling pathway and the EBF3 gene regulate muscle homeostasis, relaxation, and muscle cell-specificity.

CONCLUSIONS

We present differentially methylated loci that may be of interest to decipher the epigenetic mechanisms affecting tenderness. Supported by the previous knowledge about regulatory elements and gene function, the methylation data suggests EBF3 and GNAS as potential candidate genes and G protein signaling as potential candidate pathway associated with beef tenderness via methylation.

Apelin Resistance Contributes to Muscle Loss during Cancer Cachexia in Mice

Simple Summary

Cancer cachexia is a highly debilitating syndrome involving severe body weight loss. Worldwide around 9–14.5 million cancer patients suffer from cachexia every year and many of them die because of cachexia. Our study aimed to assess the possible role of apelin against muscle loss during cancer growth given its beneficial effects against muscle atrophy during aging. We found apelin exhibiting advantageous effects against atrophy in in vitro models, but not in in vivo models, where we unraveled undesirable apelin resistance that may nullify apelin-based therapy for cancer cachexia.

Abstract

Cancer cachexia consists of dramatic body weight loss with rapid muscle depletion due to imbalanced protein homeostasis. We found that the mRNA levels of apelin decrease in muscles from cachectic hepatoma-bearing rats and three mouse models of cachexia. Furthermore, apelin expression inversely correlates with MuRF1 in muscle biopsies from cancer patients. To shed light on the possible role of apelin in cachexia in vivo, we generated apelin 13 carrying all the last 13 amino acids of apelin in D isomers, ultimately extending plasma stability. Notably, apelin D-peptides alter cAMP-based signaling in vitro as the L-peptides, supporting receptor binding. In vitro apelin 13 protects myotube diameter from dexamethasone-induced atrophy, restrains rates of degradation of long-lived proteins and MuRF1 expression, but fails to protect mice from atrophy. D-apelin 13 given intraperitoneally for 13 days in colon adenocarcinoma C26-bearing mice does not reduce catabolic pathways in muscles, as it does in vitro. Puzzlingly, the levels of circulating apelin seemingly deriving from cachexia-inducing tumors, increase in murine plasma during cachexia. Muscle electroporation of a plasmid expressing its receptor APJ, unlike apelin, preserves myofiber area from C26-induced atrophy, supporting apelin resistance in vivo. Altogether, we believe that during cachexia apelin resistance occurs, contributing to muscle wasting and nullifying any possible peptide-based treatment.

Amelioration of muscle wasting by gintonin in cancer cachexia

Cancer cachexia is characterized by systemic inflammation, protein degradation, and loss of skeletal muscle. Despite extensive efforts to develop therapeutics, only few effective treatments are available to protect against cancer cachexia. Here, we found that gintonin (GT), a ginseng-derived lysophosphatidic acid receptor (LPAR) ligand, protected C2C12 myotubes from tumor necrosis factor α (TNFα)/interferon γ (IFNγ)- induced muscle wasting condition. The activity of GT was found to be dependent on LPAR/Gαi2, as the LPAR antagonist Ki16425 and Gαi2 siRNA abolished the anti-atrophic effects of GT on myotubes. GT suppressed TNFα-induced oxidative stress by reducing reactive oxygen species and suppressing inflammation-related genes, such as interleukin 6 (IL-6) and NADPH oxidase 2 (NOX-2). In addition, GT exhibited anti-atrophy effects in primary normal human skeletal myoblasts. Further, GT protected against Lewis lung carcinoma cell line (LLC1)-induced cancer cachexia in a mouse model. Specifically, GT rescued the lower levels of grip strength, hanging, and cross-sectional area caused by LLC1. Collectively, our findings suggest that GT may be a good therapeutic candidate for protecting against cancer cachexia.

Translational control by DHX36 binding to 5'UTR G-quadruplex is essential for muscle stem-cell regenerative functions

Skeletal muscle has a remarkable ability to regenerate owing to its resident stem cells (also called satellite cells, SCs). SCs are normally quiescent; when stimulated by damage, they activate and expand to form new fibers. The mechanisms underlying SC proliferative progression remain poorly understood. Here we show that DHX36, a helicase that unwinds RNA G-quadruplex (rG4) structures, is essential for muscle regeneration by regulating SC expansion. DHX36 (initially named RHAU) is barely expressed at quiescence but is highly induced during SC activation and proliferation. Inducible deletion of Dhx36 in adult SCs causes defective proliferation and muscle regeneration after damage. System-wide mapping in proliferating SCs reveals DHX36 binding predominantly to rG4 structures at various regions of mRNAs, while integrated polysome profiling shows that DHX36 promotes mRNA translation via 5′-untranslated region (UTR) rG4 binding. Furthermore, we demonstrate that DHX36 specifically regulates the translation of Gnai2 mRNA by unwinding its 5′ UTR rG4 structures and identify GNAI2 as a downstream effector of DHX36 for SC expansion. Altogether, our findings uncover DHX36 as an indispensable post-transcriptional regulator of SC function and muscle regeneration acting through binding and unwinding rG4 structures at 5′ UTR of target mRNAs.

Skeletal muscle stem cells (or satellite cells, SCs) are normally quiescent but activate and expand in response to injury. Here the authors show that induction of DHX36 helicase during SC activation promotes mRNA translation by binding to 5′UTR mRNA G-quadruplexes (rG4) in targets including Gnai2 and unwinding them.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

Transcriptome analysis reveals the long intergenic noncoding RNAs contributed to skeletal muscle differences between Yorkshire and Tibetan pig

The difference between the skeletal muscle growth rates of Western and domestic breeds is remarkable, but the potential regulatory mechanism involved is still unclear. Numerous studies have pointed out that long intergenic noncoding RNA (lincRNA) plays a key role in skeletal muscle development. This study used published Yorkshire (LW) and Tibetan pig (TP) transcriptome data to explore the possible role of lincRNA in the difference in skeletal muscle development between the two breeds. 138 differentially expressed lincRNAs (DELs) were obtained between the two breeds, and their potential target genes (PTGs) were predicted. The results of GO and KEGG analysis revealed that PTGs are involved in multiple biological processes and pathways related to muscle development. The quantitative trait loci (QTLs) of DELs were predicted, and the results showed that most QTLs are related to muscle development. Finally, we constructed a co-expression network between muscle development related PTGs (MDRPTGs) and their corresponding DELs on the basis of their expression levels. The expression of DELs was significantly correlated with the corresponding MDRPTGs. Also, multiple MDRPTGs are involved in the key regulatory pathway of muscle fiber hypertrophy, which is the IGF-1-AKT-mTOR pathway. In summary, multiple lincRNAs that may cause differences in skeletal muscle development between the two breeds were identified, and their possible regulatory roles were explored. The findings of this study may provide a valuable reference for further research on the role of lincRNA in skeletal muscle development.

Prospect of early vascular tone and satellite cell modulations on white striping muscle myopathy

Polyphasic myodegeneration potentially causes severe physiological and metabolic disorders in the breast muscle of fast-growing broiler chickens. To date, the etiology of recent muscle myopathies, such as the white striping (WS) phenotype, is still unknown. White striping–affected breast meats compromise the water holding capacity and predispose muscle to poor vascular tone, leading to the deterioration of meat qualities. Herein, this review article provides insight on the complexities around chicken breast myopathies: (i) the etiologies of WS occurrence in chicken; (ii) the metabolic changes that occur in WS defect in pectoralis major; and (iii) the interactions between breast muscle physiology and vascular tone. It also addressed the effects of nutritional supplements on muscle myopathies on chicken breast meats. Moreover, the review explored breast muscle biology focusing on the early preparation of satellite and vascular cells in fast-growth chicken breeds. Transcriptomics and histological analyses revealed poor vascularity in breast muscle of fast growth chickens. Thus, we suggest in ovo feeding of nutrients promoting vascularization and satellite cells replenishment as a potential strategy to enhance endothelium-derived nitric oxide availability to promote vascularization in the pectoralis major muscle region.

Targeting RAGE prevents muscle wasting and prolongs survival in cancer cachexia

BACKGROUND

Cachexia, a multifactorial syndrome affecting more than 50% of patients with advanced cancer and responsible for ~20% of cancer-associated deaths, is still a poorly understood process without a standard cure available. Skeletal muscle atrophy caused by systemic inflammation is a major clinical feature of cachexia, leading to weight loss, dampening patients' quality of life, and reducing patients' response to anticancer therapy. RAGE (receptor for advanced glycation end-products) is a multiligand receptor of the immunoglobulin superfamily and a mediator of muscle regeneration, inflammation, and cancer.

METHODS

By using murine models consisting in the injection of colon 26 murine adenocarcinoma (C26-ADK) or Lewis lung carcinoma (LLC) cells in BALB/c and C57BL/6 or Ager^-/- (RAGE-null) mice, respectively, we investigated the involvement of RAGE signalling in the main features of cancer cachexia, including the inflammatory state. In vitro experiments were performed using myotubes derived from C2C12 myoblasts or primary myoblasts isolated from C57BL/6 wild type and Ager^-/- mice treated with the RAGE ligand, S100B (S100 calcium-binding protein B), TNF (tumor necrosis factor)α±IFN (interferon) γ, and tumour cell- or masses-conditioned media to analyse hallmarks of muscle atrophy. Finally, muscles of wild type and Ager^-/- mice were injected with TNFα/IFNγ or S100B in a tumour-free environment.

RESULTS

We demonstrate that RAGE is determinant to activate signalling pathways leading to muscle protein degradation in the presence of proinflammatory cytokines and/or tumour-derived cachexia-inducing factors. We identify the RAGE ligand, S100B, as a novel factor able to induce muscle atrophy per se via a p38 MAPK (p38 mitogen-activated protein kinase)/myogenin axis and STAT3 (signal transducer and activator of transcription 3)-dependent MyoD (myoblast determination protein 1) degradation. Lastly, we found that in cancer conditions, an increase in serum levels of tumour-derived S100B and HMGB1 (high mobility group box 1) occurs leading to chronic activation/overexpression of RAGE, which induces hallmarks of cancer cachexia (i.e. muscle wasting, systemic inflammation, and release of tumour-derived pro-cachectic factors). Absence of RAGE in mice translates into reduced serum levels of cachexia-inducing factors, delayed loss of muscle mass and strength, reduced tumour progression, and increased survival.

CONCLUSIONS

RAGE is a molecular determinant in inducing the hallmarks of cancer cachexia, and molecular targeting of RAGE might represent a therapeutic strategy to prevent or counteract the cachectic syndrome.

Pancreatic Cancer and Cachexia-Metabolic Mechanisms and Novel Insights

Cachexia is a major characteristic of multiple non-malignant diseases, advanced and metastatic cancers and it is highly prevalent in pancreatic cancer, affecting almost 70-80% of the patients. Cancer cachexia is a multifactorial condition accompanied by compromised appetite and changes in body composition, i.e., loss of fat. It is associated with lower effectiveness of treatment, compromised quality of life, and higher mortality. Understanding the complex pathways underlying the pathophysiology of cancer cachexia, new therapeutic targets will be unraveled. The interplay between tumor and host factors, such as cytokines, holds a central role in cachexia pathophysiology. Cytokines are possibly responsible for anorexia, hypermetabolism, muscle proteolysis, and apoptosis. In particular, cachexia in pancreatic cancer might be the result of the surgical removal of pancreas parts. In recent years, many studies have been carried out to identify an effective treatment algorithm for cachexia. Choosing the most appropriate treatment, the clinical effect and the risk of adverse effects should be taken under consideration. The purpose of this review is to highlight the pathophysiological mechanisms as well as the current ways of cachexia treatment in the pharmaceutical and the nutrition field.

Depletion of resident muscle stem cells negatively impacts running volume, physical function, and muscle fiber hypertrophy in response to lifelong physical activity

To date, studies that have aimed to investigate the role of satellite cells during adult skeletal muscle adaptation and hypertrophy have utilized a nontranslational stimulus and/or have been performed over a relatively short time frame. Although it has been shown that satellite cell depletion throughout adulthood does not drive skeletal muscle loss in sedentary mice, it remains unknown how satellite cells participate in skeletal muscle adaptation to long-term physical activity. The current study was designed to determine whether reduced satellite cell content throughout adulthood would influence the transcriptome-wide response to physical activity and diminish the adaptive response of skeletal muscle. We administered vehicle or tamoxifen to adult Pax7-diphtheria toxin A (DTA) mice to deplete satellite cells and assigned them to sedentary or wheel-running conditions for 13 mo. Satellite cell depletion throughout adulthood reduced balance and coordination, overall running volume, and the size of muscle proprioceptors (spindle fibers). Furthermore, satellite cell participation was necessary for optimal muscle fiber hypertrophy but not adaptations in fiber type distribution in response to lifelong physical activity. Transcriptome-wide analysis of the plantaris and soleus revealed that satellite cell function is muscle type specific; satellite cell-dependent myonuclear accretion was apparent in oxidative muscles, whereas initiation of G protein-coupled receptor (GPCR) signaling in the glycolytic plantaris may require satellite cells to induce optimal adaptations to long-term physical activity. These findings suggest that satellite cells play a role in preserving physical function during aging and influence muscle adaptation during sustained periods of physical activity.

Skeletal muscle: A review of molecular structure and function, in health and disease

Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The "omics" revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross-talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems-level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Developmental Biology > Developmental Processes in Health and Disease Models of Systems Properties and Processes > Cellular Models.

mTORC2 affects the maintenance of the muscle stem cell pool

Background

The mammalian target of rapamycin complex 2 (mTORC2), containing the essential protein rictor, regulates cellular metabolism and cytoskeletal organization by phosphorylating protein kinases, such as PKB/Akt, PKC, and SGK. Inactivation of mTORC2 signaling in adult skeletal muscle affects its metabolism, but not muscle morphology and function. However, the role of mTORC2 in adult muscle stem cells (MuSCs) has not been investigated.

Method

Using histological, biochemical, and molecular biological methods, we characterized the muscle phenotype of mice depleted for rictor in the Myf5-lineage (RImyfKO) and of mice depleted for rictor in skeletal muscle fibers (RImKO). The proliferative and myogenic potential of MuSCs was analyzed upon cardiotoxin-induced injury in vivo and in isolated myofibers in vitro.

Results

Skeletal muscle of young and 14-month-old RImyfKO mice appeared normal in composition and function. MuSCs from young RImyfKO mice exhibited a similar capacity to proliferate, differentiate, and fuse as controls. In contrast, the number of MuSCs was lower in young RImyfKO mice than in controls after two consecutive rounds of cardiotoxin-induced muscle regeneration. Similarly, the number of MuSCs in RImyfKO mice decreased with age, which correlated with a decline in the regenerative capacity of mutant muscle. Interestingly, reduction in the number of MuSCs was also observed in 14-month-old RImKO muscle.

Conclusions

Our study shows that mTORC2 signaling is dispensable for myofiber formation, but contributes to the homeostasis of MuSCs. Loss of mTORC2 does not affect their myogenic function, but impairs the replenishment of MuSCs after repeated injuries and their maintenance during aging. These results point to an important role of mTORC2 signaling in MuSC for muscle homeostasis.

Musclin, A Myokine Induced by Aerobic Exercise, Retards Muscle Atrophy During Cancer Cachexia in Mice

Physical activity improves the prognosis of cancer patients, partly by contrasting the associated muscle wasting (cachexia), through still unknown mechanisms. We asked whether aerobic exercise causes secretion by skeletal muscles of proteins (myokines) that may contrast cachexia. Media conditioned by peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α)-expressing myotubes, reproducing some metabolic adaptations of aerobic exercise, as increased mitochondrial biogenesis and oxidative phosphorylation, restrained constitutively active Forkhead box-containing subfamily O3 (caFoxO3)-induced proteolysis. Microarray analysis identified amphiregulin (AREG), natriuretic peptide precursor B (NppB), musclin and fibroblast growth factor 18 (FGF18) as myokines highly induced by PGC1α. Notably, only musclin tended to be low in muscle of mice with a rare human renal carcinoma; it was reduced in plasma and in muscles of C26-bearing mice and in atrophying myotubes, where PGC1α expression is impaired. Therefore, we electroporated the Tibialis Anterior (TA) of C26-bearing mice with musclin or (its receptor) natriuretic peptide receptor 3 (Npr3)-encoding plasmids and found a preserved fiber area, as a result of restrained proteolysis. Musclin knockout (KO) mice lose more muscle tissue during growth of two distinct cachexia-causing tumors. Running protected C26-bearing mice from cachexia, not changing tumor growth, and rescued the C26-induced downregulation of musclin in muscles and plasma. Musclin expression did not change in overloaded plantaris of mice, recapitulating partially muscle adaptations to anaerobic exercise. Musclin might, therefore, be beneficial to cancer patients who cannot exercise and are at risk of cachexia and may help to explain how aerobic exercise alleviates cancer-induced muscle wasting.

The sympathetic nervous system regulates skeletal muscle motor innervation and acetylcholine receptor stability

AIM

Symptoms of autonomic failure are frequently the presentation of advanced age and neurodegenerative diseases that impair adaptation to common physiologic stressors. The aim of this work was to examine the interaction between the sympathetic and motor nervous system, the involvement of the sympathetic nervous system (SNS) in neuromuscular junction (NMJ) presynaptic motor function, the stability of postsynaptic molecular organization, and the skeletal muscle composition and function.

METHODS

Since muscle weakness is a symptom of diseases characterized by autonomic dysfunction, we studied the impact of regional sympathetic ablation on muscle motor innervation by using transcriptome analysis, retrograde tracing of the sympathetic outflow to the skeletal muscle, confocal and electron microscopy, NMJ transmission by electrophysiological methods, protein analysis, and state of the art microsurgical techniques, in C57BL6, MuRF1KO and Thy-1 mice.

RESULTS

We found that the SNS regulates motor nerve synaptic vesicle release, skeletal muscle transcriptome, muscle force generated by motor nerve activity, axonal neurofilament phosphorylation, myelin thickness, and myofibre subtype composition and CSA. The SNS also modulates the levels of postsynaptic membrane acetylcholine receptor by regulating the Gαi2 -Hdac4-Myogenin-MuRF1pathway, which is prevented by the overexpression of the guanine nucleotide-binding protein Gαi2 (Q205L), a constitutively active mutant G protein subunit.

CONCLUSION

The SNS regulates NMJ transmission, maintains optimal Gαi2 expression, and prevents any increase in Hdac4, myogenin, MuRF1, and miR-206. SNS ablation leads to upregulation of MuRF1, muscle atrophy, and downregulation of postsynaptic AChR. Our findings are relevant to clinical conditions characterized by progressive decline of sympathetic innervation, such as neurodegenerative diseases and aging.

Insulin-Like Growth Factor I Regulation and Its Actions in Skeletal Muscle

The insulin-like growth factor (IGF) pathway is essential for promoting growth and survival of virtually all tissues. It bears high homology to its related protein insulin, and as such, there is an interplay between these molecules with regard to their anabolic and metabolic functions. Skeletal muscle produces a significant proportion of IGF-1, and is highly responsive to its actions, including increased muscle mass and improved regenerative capacity. In this overview, the regulation of IGF-1 production, stability, and activity in skeletal muscle will be described. Second, the physiological significance of the forms of IGF-1 produced will be discussed. Last, the interaction of IGF-1 with other pathways will be addressed. © 2019 American Physiological Society. Compr Physiol 9:413-438, 2019.

Cellular and molecular mechanisms of sarcopenia: the S100B perspective

Primary sarcopenia is a condition of reduced skeletal muscle mass and strength, reduced agility, and increased fatigability and risk of bone fractures characteristic of aged, otherwise healthy people. The pathogenesis of primary sarcopenia is not completely understood. Herein, we review the essentials of the cellular and molecular mechanisms of skeletal mass maintenance; the alterations of myofiber metabolism and deranged properties of muscle satellite cells (the adult stem cells of skeletal muscles) that underpin the pathophysiology of primary sarcopenia; the role of the Ca²⁺ -sensor protein, S100B, as an intracellular factor and an extracellular signal regulating cell functions; and the functional role of S100B in muscle tissue. Lastly, building on recent results pointing to S100B as to a molecular determinant of myoblast-brown adipocyte transition, we propose S100B as a transducer of the deleterious effects of accumulation of reactive oxygen species in myoblasts and, potentially, myofibers concurring to the pathophysiology of sarcopenia.

Identification of a Novel Function of Resveratrol and Genistein as a Regulator of β2 -Adrenergic Receptor Expression in Skeletal Muscle Cells and Characterization of Promoter Elements Required for Promoter Activation

SCOPE

Modulating β₂ -adrenergic receptor (β₂ -AR) expression and activation is important for maintaining skeletal muscle function. In this study, two food factors, resveratrol (RSV) and genistein (GEN), that are able to regulate β₂ -AR promoter activity and may improve skeletal muscle function are identified.

METHODS AND RESULTS

Using luciferase reporter assay, 357 functional food factors as candidates for β₂ -AR promoter activity have been screened and subsequently RSV and GEN increase β₂ -AR promoter activity and β₂ -AR mRNA expression. Using promoter sequence analysis, it is shown that the CCAAT box and the GC box on the β₂ -AR promoter are required for the regulation of β₂ -AR expression by RSV or GEN. It is also ascertained that transcription factor NF-YA binds to the CCAAT box on the β₂ -AR promoter and that the amount of NF-YA bound to the CCAAT box is unchanged by RSV or GEN treatment. Finally, it is confirmed that a GEN-containing diet increases β₂ -AR expression in mouse skeletal muscle and increased skeletal muscle mass.

CONCLUSIONS

The findings show that food-derived molecules have the potential to influence skeletal muscle mass and function by regulating G protein-coupled receptor expression.

Effects of ursolic acid on sub-lesional muscle pathology in a contusion model of spinal cord injury

Spinal Cord Injury (SCI) results in severe sub-lesional muscle atrophy and fiber type transformation from slow oxidative to fast glycolytic, both contributing to functional deficits and maladaptive metabolic profiles. Therapeutic countermeasures have had limited success and muscle-related pathology remains a clinical priority. mTOR signaling is known to play a critical role in skeletal muscle growth and metabolism, and signal integration of anabolic and catabolic pathways. Recent studies show that the natural compound ursolic acid (UA) enhances mTOR signaling intermediates, independently inhibiting atrophy and inducing hypertrophy. Here, we examine the effects of UA treatment on sub-lesional muscle mTOR signaling, catabolic genes, and functional deficits following severe SCI in mice. We observe that UA treatment significantly attenuates SCI induced decreases in activated forms of mTOR, and signaling intermediates PI3K, AKT, and S6K, and the upregulation of catabolic genes including FOXO1, MAFbx, MURF-1, and PSMD11. In addition, UA treatment improves SCI induced deficits in body and sub-lesional muscle mass, as well as functional outcomes related to muscle function, motor coordination, and strength. These findings provide evidence that UA treatment may be a potential therapeutic strategy to improve muscle-specific pathological consequences of SCI.

ATP Citrate Lyase Regulates Myofiber Differentiation and Increases Regeneration by Altering Histone Acetylation

ATP citrate lyase (ACL) plays a key role in regulating mitochondrial function, as well as glucose and lipid metabolism in skeletal muscle. We report here that ACL silencing impairs myoblast and satellite cell (SC) differentiation, and it is accompanied by a decrease in fast myosin heavy chain isoforms and MYOD. Conversely, overexpression of ACL enhances MYOD levels and promotes myogenesis. Myogenesis is dependent on transcriptional but also other mechanisms. We show that ACL regulates the net amount of acetyl groups available, leading to alterations in acetylation of H3(K9/14) and H3(K27) at the MYOD locus, thus increasing MYOD expression. ACL overexpression in murine skeletal muscle leads to improved regeneration after cardiotoxin-mediated damage. Thus, our findings suggest a mechanism for regulating SC differentiation and enhancing regeneration, which might be exploited for devising therapeutic approaches for treating skeletal muscle disease.

α-Ketoglutarate prevents skeletal muscle protein degradation and muscle atrophy through PHD3/ADRB2 pathway

Skeletal muscle atrophy due to excessive protein degradation is the main cause for muscle dysfunction, fatigue, and weakening of athletic ability. Endurance exercise is effective to attenuate muscle atrophy, but the underlying mechanism has not been fully investigated. α-Ketoglutarate (AKG) is a key intermediate of tricarboxylic acid cycle, which is generated during endurance exercise. Here, we demonstrated that AKG effectively attenuated corticosterone-induced protein degradation and rescued the muscle atrophy and dysfunction in a Duchenne muscular dystrophy mouse model. Interestingly, AKG also inhibited the expression of proline hydroxylase 3 (PHD3), one of the important oxidoreductases expressed under hypoxic conditions. Subsequently, we identified the β2 adrenergic receptor (ADRB2) as a downstream target for PHD3. We found AKG inhibited PHD3/ADRB2 interaction and therefore increased the stability of ADRB2. In addition, combining pharmacologic and genetic approaches, we showed that AKG rescues skeletal muscle atrophy and protein degradation through a PHD3/ADRB2 mediated mechanism. Taken together, these data reveal a mechanism for inhibitory effects of AKG on muscle atrophy and protein degradation. These findings not only provide a molecular basis for the potential use of exercise-generated metabolite AKG in muscle atrophy treatment, but also identify PHD3 as a potential target for the development of therapies for muscle wasting.-Cai, X., Yuan, Y., Liao, Z., Xing, K., Zhu, C., Xu, Y., Yu, L., Wang, L., Wang, S., Zhu, X., Gao, P., Zhang, Y., Jiang, Q., Xu, P., Shu, G. α-Ketoglutarate prevents skeletal muscle protein degradation and muscle atrophy through PHD3/ADRB2 pathway.

Flavan 3-ol delays the progression of disuse atrophy induced by hindlimb suspension in mice

Periods of skeletal muscle disuse, for example due to a sedentary lifestyle or bed rest, are associated with aging and can lead to muscle atrophy. We previously found that the flavan 3-ol fraction derived from cocoa (FL) enhanced energy expenditure with metabolic changes in skeletal muscle. In the present study, we examined the effect of FL on disuse muscle atrophy induced by hindlimb suspension in mice. Male C57BL/6J mice were assigned to four groups as follows: unsuspended-vehicle, unsuspended-FL, suspended-vehicle, and suspended-FL. Mice in the vehicle treatment groups were administered distilled water and those in the FL treatment groups were dosed with FL (50mg/kg/day) for 2weeks. The weights of the gastrocnemius (GC), tibialis anterior (TA), and soleus (SOL), but not the extensor digitorum longus (EDL), decreased significantly in mice with hindlimb suspension (-11.8%, -16.5%, and -41.0%, respectively). This reduction in GC, TA, and SOL mass was inhibited by FL (-5.3%, +2.0%, and -16.6%, respectively). The FL increased the EDL weight >20% with or without hindlimb suspension. The protein level of the ubiquitin ligase, muscle ring finger-1, in the SOL was significantly increased by hindlimb suspension, but inhibited by treatment with FL. Protein expression of p70S6 kinase in the SOL was significantly decreased by hindlimb suspension, and FL treatment inhibited this change. These results suggested that FL delayed disuse muscle atrophy by metabolic alteration.

Harnessing cAMP signaling in musculoskeletal regenerative engineering

This paper reviews the most recent findings in the search for small molecule cyclic AMP analogues regarding their potential use in musculoskeletal regenerative engineering.

Transcriptome profiling of Arabian horse blood during training regimens

BACKGROUND

Arabian horses are believed to be one of the oldest and most influential horse breeds in the world. Blood is the main tissue involved in maintaining body homeostasis, and it is considered a marker of the processes taking place in the other tissues. Thus, the aim of our study was to identify the genetic basis of changes occurring in the blood of Arabian horses subjected to a training regimen and to compare the global gene expression profiles between different training periods (T₁: after a slow canter phase that is considered a conditioning phase, T₂: after an intense gallop phase, and T₃: at the end of the racing season) and between trained and untrained horses (T₀). RNA sequencing was performed on 37 samples with a 75-bp single-end run on a HiScanSQ platform (Illumina), and differentially expressed genes (DEGs) were identified based on DESeq2 (v1.11.25) software.

RESULTS

An increase in the number of DEGs between subsequent training periods was observed, and the highest amount of DEGs (440) was detected between untrained horses (T₀) and horses at the end of the racing season (T₃). The comparisons of the T₂ vs. T₃ transcriptomes and the T₀ vs. T₃ transcriptomes showed a significant gain of up-regulated genes during long-term exercise (up-regulation of 266 and 389 DEGs in the T₃ period compared to T₂ and T₀, respectively). Forty differentially expressed genes were detected between the T₁ and T₂ periods, and 296 between T₂ and T₃. Functional annotation showed that the most abundant genes up-regulated in exercise were involved in pathways regulating cell cycle (PI3K-Akt signalling pathway), cell communication (cAMP-dependent pathway), proliferation, differentiation and apoptosis, as well as immunity processes (Jak-STAT signalling pathway).

CONCLUSIONS

We investigated whether training causes permanent transcriptome changes in horse blood as a reflection of adaptation to conditioning and the maintenance of fitness to compete in flat races. The present study identified the overrepresented molecular pathways and genes that are essential for maintaining body homeostasis during long-term exercise in Arabian horses. Selected DEGs should be further investigated as markers that are potentially associated with racing performance in Arabian horses.

Schisandrae fructus enhances myogenic differentiation and inhibits atrophy through protein synthesis in human myotubes

Schisandrae fructus (SF) has recently been reported to increase skeletal muscle mass and inhibit atrophy in mice. We investigated the effect of SF extract on human myotube differentiation and its acting pathway. Various concentrations (0.1–10 μg/mL) of SF extract were applied on human skeletal muscle cells in vitro. Myotube area and fusion index were measured to quantify myotube differentiation. The maximum effect was observed at 0.5 μg/mL of SF extract, enhancing differentiation up to 1.4-fold in fusion index and 1.6-fold in myotube area at 8 days after induction of differentiation compared to control. Phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 and 70 kDa ribosomal protein S6 kinase, which initiate translation as downstream of mammalian target of rapamycin pathway, was upregulated in early phases of differentiation after SF treatment. SF also attenuated dexamethasone-induced atrophy. In conclusion, we show that SF augments myogenic differentiation and attenuates atrophy by increasing protein synthesis through mammalian target of rapamycin/70 kDa ribosomal protein S6 kinase and eukaryotic translation initiation factor 4E-binding protein 1 signaling pathway in human myotubes. SF can be a useful natural dietary supplement in increasing skeletal muscle mass, especially in the aged with sarcopenia and the patients with disuse atrophy.

Using AAV vectors expressing the β2-adrenoceptor or associated Gα proteins to modulate skeletal muscle mass and muscle fibre size

Anabolic β₂-adrenoceptor (β₂-AR) agonists have been proposed as therapeutics for treating muscle wasting but concerns regarding possible off-target effects have hampered their use. We investigated whether β₂-AR-mediated signalling could be modulated in skeletal muscle via gene delivery to the target tissue, thereby avoiding the risks of β₂-AR agonists. In mice, intramuscular administration of a recombinant adeno-associated virus-based vector (rAAV vector) expressing the β₂-AR increased muscle mass by >20% within 4 weeks. This hypertrophic response was comparable to that of 4 weeks’ treatment with the β₂-AR agonist formoterol, and was not ablated by mTOR inhibition. Increasing expression of inhibitory (Gαi2) and stimulatory (GαsL) G-protein subunits produced minor atrophic and hypertrophic changes in muscle mass, respectively. Furthermore, Gαi2 over-expression prevented AAV:β₂-AR mediated hypertrophy. Introduction of the non-muscle Gαs isoform, GαsXL elicited hypertrophy comparable to that achieved by AAV:β₂-AR. Moreover, GαsXL gene delivery was found to be capable of inducing hypertrophy in the muscles of mice lacking functional β₁- and β₂-ARs. These findings demonstrate that gene therapy-based interventions targeting the β₂-AR pathway can promote skeletal muscle hypertrophy independent of ligand administration, and highlight novel methods for potentially modulating muscle mass in settings of disease.

Control of Skeletal Muscle Cell Growth and Size Through Adhesion GPCRs

Skeletal muscle homeostasis is regulated by a constant influx of chemicals and exposure to mechanical stimuli. A number of key signaling pathways that translate these stimuli into changes in muscle physiology have been established. The GPCR family known as adhesion GPCRs (aGPCRs) has largely elusive roles in skeletal muscle biology; however, their unique capacity to activate adhesion and G protein signaling pathways makes them an attractive point of investigation. The skeletal muscle myofiber contains a highly organized cytoarchitecture to ensure contractile function. This requires intricate interactions with components of the extracellular matrix (ECM) surrounding each fiber. aGPCRs possess extended N-termini known to interact with ECM proteins and complexes suggesting a compatible role in skeletal muscle biology. Furthermore, recent work demonstrated the involvement of certain aGPCRs in whole muscle hypertrophy and differentiation of muscle progenitor cells. Signaling pathways downstream of aGPCRs are still incompletely understood; however, initial findings show involvement of the Gα12/13 subunit signaling to the pro-anabolic Akt/mTOR pathway. Together, this chapter will review the emerging role of aGPCRs in skeletal muscle biology and putative mechanism(s) employed to regulate skeletal muscle growth.

Acetoacetate Accelerates Muscle Regeneration and Ameliorates Muscular Dystrophy in Mice

Acetoacetate (AA) is a ketone body and acts as a fuel to supply energy for cellular activity of various tissues. Here, we uncovered a novel function of AA in promoting muscle cell proliferation. Notably, the functional role of AA in regulating muscle cell function is further evidenced by its capability to accelerate muscle regeneration in normal mice, and it ameliorates muscular dystrophy in mdx mice. Mechanistically, our data from multiparameter analyses consistently support the notion that AA plays a non-metabolic role in regulating muscle cell function. Finally, we show that AA exerts its function through activation of the MEK1-ERK1/2-cyclin D1 pathway, revealing a novel mechanism in which AA serves as a signaling metabolite in mediating muscle cell function. Our findings highlight the profound functions of a small metabolite as signaling molecule in mammalian cells.

Skeletal Muscle Abnormalities in Heart Failure

Exercise capacity is lowered in patients with heart failure, which limits their daily activities and also reduces their quality of life. Furthermore, lowered exercise capacity has been well demonstrated to be closely related to the severity and prognosis of heart failure. Skeletal muscle abnormalities including abnormal energy metabolism, transition of myofibers from type I to type II, mitochondrial dysfunction, reduction in muscular strength, and muscle atrophy have been shown to play a central role in lowered exercise capacity. The skeletal muscle abnormalities can be classified into the following main types: 1) low endurance due to mitochondrial dysfunction; and 2) low muscle mass and muscle strength due to imbalance of protein synthesis and degradation. The molecular mechanisms of these skeletal muscle abnormalities have been studied mainly using animal models. The current review including our recent study will focus upon the skeletal muscle abnormalities in heart failure.

The molecular basis for load-induced skeletal muscle hypertrophy

In a mature (weight neutral) animal, an increase in muscle mass only occurs when the muscle is loaded sufficiently to cause an increase in myofibrillar protein balance. A tight relationship between muscle hypertrophy, acute increases in protein balance, and the activity of the mechanistic target of rapamycin complex 1 (mTORC1) was demonstrated 15 years ago. Since then, our understanding of the signals that regulate load-induced hypertrophy has evolved considerably. For example, we now know that mechanical load activates mTORC1 in the same way as growth factors, by moving TSC2 (a primary inhibitor of mTORC1) away from its target (the mTORC activator) Rheb. However, the kinase that phosphorylates and moves TSC2 is different in the two processes. Similarly, we have learned that a distinct pathway exists whereby amino acids activate mTORC1 by moving it to Rheb. While mTORC1 remains at the forefront of load-induced hypertrophy, the importance of other pathways that regulate muscle mass are becoming clearer. Myostatin, is best known for its control of developmental muscle size. However, new mechanisms to explain how loading regulates this process are suggesting that it could play an important role in hypertrophic muscle growth as well. Last, new mechanisms are highlighted for how β2 receptor agonists could be involved in load-induced muscle growth and why these agents are being developed as non-exercise-based therapies for muscle atrophy. Overall, the results highlight how studying the mechanism of load-induced skeletal muscle mass is leading the development of pharmaceutical interventions to promote muscle growth in those unwilling or unable to perform resistance exercise.

Pharmacology of manipulating lean body mass

Dysfunction and wasting of skeletal muscle as a consequence of illness decreases the length and quality of life. Currently, there are few, if any, effective treatments available to address these conditions. Hence, the existence of this unmet medical need has fuelled large scientific efforts. Fortunately, these efforts have shown many of the underlying mechanisms adversely affecting skeletal muscle health. With increased understanding have come breakthrough disease-specific and broad spectrum interventions, some progressing through clinical development. The present review focuses its attention on the role of the antagonistic process regulating skeletal muscle mass before branching into prospective promising therapeutic targets and interventions. Special attention is given to therapies in development against cancer cachexia and Duchenne muscular dystrophy before closing remarks on design and conceptualization of future therapies are presented to the reader.

G protein-coupled receptor 56 regulates mechanical overload-induced muscle hypertrophy

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha 4 (PGC-1α4) is a protein isoform derived by alternative splicing of the PGC1α mRNA and has been shown to promote muscle hypertrophy. We show here that G protein-coupled receptor 56 (GPR56) is a transcriptional target of PGC-1α4 and is induced in humans by resistance exercise. Furthermore, the anabolic effects of PGC-1α4 in cultured murine muscle cells are dependent on GPR56 signaling, because knockdown of GPR56 attenuates PGC-1α4-induced muscle hypertrophy in vitro. Forced expression of GPR56 results in myotube hypertrophy through the expression of insulin-like growth factor 1, which is dependent on Gα12/13 signaling. A murine model of overload-induced muscle hypertrophy is associated with increased expression of both GPR56 and its ligand collagen type III, whereas genetic ablation of GPR56 expression attenuates overload-induced muscle hypertrophy and associated anabolic signaling. These data illustrate a signaling pathway through GPR56 which regulates muscle hypertrophy associated with resistance/loading-type exercise.

Muscle wasting: an overview of recent developments in basic research

The syndrome of cachexia, i.e., involuntary weight loss in patients with underlying diseases, sarcopenia, i.e., loss of muscle mass due to aging, and general muscle atrophy from disuse and/or prolonged bed rest have received more attention over the last decades. All lead to a higher morbidity and mortality in patients, and therefore, they represent a major socio-economic burden for the society today. This mini-review looks at recent developments in basic research that are relevant to the loss of skeletal muscle. It aims to cover the most significant publication of last 3 years on the causes and effects of muscle wasting, new targets for therapy development, and potential biomarkers for assessing skeletal muscle mass. The targets include the following: (1) E-3 ligases TRIM32, SOCS1, and SOCS3 by involving the elongin BC ubiquitin-ligase, Cbl-b, culling 7, Fbxo40, MG53 (TRIM72), and the mitochondrial Mul1; (2) the kinase MST1; and (3) the G-protein Gαi2. D(3)-creatine has the potential to be used as a novel biomarker that allows to monitor actual change in skeletal muscle mass over time. In conclusion, significant development efforts are being made by academic groups as well as numerous pharmaceutical companies to identify new target and biomarker muscles, as muscle wasting represents a great medical need, but no therapies have been approved in the last decades.

Muscle wasting: an overview of recent developments in basic research

The syndrome of cachexia, i.e. involuntary weight loss in patients with underlying diseases, sarcopenia, i.e. loss of muscle mass due to ageing, and general muscle atrophy from disuse and/or prolonged bed rest have received more attention over the last decades. All lead to a higher morbidity and mortality in patients and therefore, they represent a major socio-economic burden for the society today. This mini-review looks at recent developments in basic research that are relevant to the loss of skeletal muscle. It aims to cover the most significant publication of last three years on the causes and effects of muscle wasting, new targets for therapy development and potential biomarkers for assessing skeletal muscle mass. The targets include 1) E-3 ligases: TRIM32, SOCS1 and SOCS3 by involving the elongin BC ubiquitin-ligase, Cbl-b, culling 7, Fbxo40, MG53 (TRIM72) and the mitochondrial Mul1, 2) the kinase MST1 and 3) the G-protein Gαi2. D(3)-creatine has the potential to be used as a novel biomarker that allows to monitor actual change in skeletal muscle mass over time. In conclusion, significant development efforts are being made by academic groups as well as numerous pharmaceutical companies to identify new targets and biomarkers muscle, as muscle wasting represents a great medical need, but no therapies have been approved in the last decades.

Functional β-adrenoceptors are important for early muscle regeneration in mice through effects on myoblast proliferation and differentiation

Muscles can be injured in different ways and the trauma and subsequent loss of function and physical capacity can impact significantly on the lives of patients through physical impairments and compromised quality of life. The relative success of muscle repair after injury will largely determine the extent of functional recovery. Unfortunately, regenerative processes are often slow and incomplete, and so developing novel strategies to enhance muscle regeneration is important. While the capacity to enhance muscle repair by stimulating β₂-adrenoceptors (β-ARs) using β₂-AR agonists (β₂-agonists) has been demonstrated previously, the exact role β-ARs play in regulating the regenerative process remains unclear. To investigate β-AR-mediated signaling in muscle regeneration after myotoxic damage, we examined the regenerative capacity of tibialis anterior and extensor digitorum longus muscles from mice lacking either β₁-AR (β₁-KO) and/or β₂-ARs (β₂-KO), testing the hypothesis that muscles from mice lacking the β₂-AR would exhibit impaired functional regeneration after damage compared with muscles from β₁-KO or β₁/β₂-AR null (β₁/β₂-KO) KO mice. At 7 days post-injury, regenerating muscles from β₁/β₂-KO mice produced less force than those of controls but muscles from β₁-KO or β₂-KO mice did not exhibit any delay in functional restoration. Compared with controls, β₁/β₂-KO mice exhibited an enhanced inflammatory response to injury, which delayed early muscle regeneration, but an enhanced myoblast proliferation later during regeneration ensured a similar functional recovery (to controls) by 14 days post-injury. This apparent redundancy in the β-AR signaling pathway was unexpected and may have important implications for manipulating β-AR signaling to improve the rate, extent and efficacy of muscle regeneration to enhance functional recovery after injury.