Mighty is a novel promyogenic factor in skeletal myogenesis

An Overview of Sarcopenia: Focusing on Nutritional Treatment Approaches

Sarcopenia is a syndrome characterized by the progressive and generalized loss of skeletal muscle mass and strength. This condition is associated with physical disability, decreased quality of life, and increased mortality. Therefore, reducing the prevalence of sarcopenia could significantly lower healthcare costs. Sarcopenia can be classified into primary and secondary sarcopenia. The former is related to aging and begins after the fourth decade of life; after that, there is a muscle loss of around 8% per decade until age 70 years, which subsequently increases to 15% per decade. On the other hand, secondary sarcopenia can affect all individuals and may result from various factors including physical inactivity, malnutrition, endocrine disorders, neurodegenerative diseases, inflammation, and cachexia. Understanding the multiple mechanisms involved in the onset and progression of sarcopenia allows for us to develop strategies that can prevent, treat, or at least mitigate muscle loss caused by increased protein breakdown. One potential treatment of sarcopenia is based on nutritional interventions, including adequate caloric and protein intake and specific nutrients that support muscle health. Such nutrients include natural food rich in whey protein and omega-3 fatty acids as well as nutritional supplements like branched-chain amino acids, β-hydroxy-β-methylbutyrate, and vitamin D along with food for special medical purposes. It is important to emphasize that physical exercises, especially resistance training, not only promote muscle protein synthesis on their own but also work synergistically with nutritional strategies to enhance their effectiveness.

miR-224 activates cancer-associated fibroblasts to enhance lung cancer cell migration and invasion by targeting Akirin1

Cancer-associated fibroblasts (CAFs) actively contribute to the formation of tumor-supportive microenvironments, thereby promoting cancer progression and impacting therapeutic outcomes. This study utilized global microRNA (miRNA) expression profiling to identify specific miRNAs responsible for reprogramming normal lung fibroblasts (LFs) into CAFs. miR-224 demonstrates increased expression in CAFs, and its levels are elevated in lung tumors compared to those in normal tissues, according to data from public databases. Overexpression of miR-224 in LFs increases the overall expression of CAF activation markers. Furthermore, LFs overexpressing miR-224 enhanced the migration and invasion of lung cancer cells via direct cell-to-cell contact in a co-culture system. In a mouse orthotopic injection model, miR-224 overexpression in LFs increased lung cancer metastasis. Using target prediction tools and subsequent 3'-UTR luciferase assay, Akirin1 was validated as a direct target gene of miR-224. In addition, LFs depleted of Akirin1 by siRNAs stimulated the migration and invasion of lung cancer cells compared to control LFs. Overall, these findings indicate that miR-224 induces CAF activation and promotes the migration and invasion of lung cancer cells by targeting Akirin1 in co-culture systems.

Myokines and Microbiota: New Perspectives in the Endocrine Muscle-Gut Axis

This review explores the dual role of skeletal muscle as both a mechanical and endocrine organ, highlighting its contributions to overall health and its adaptability to various inputs such as nutrition, hormones, exercise, and injuries. In addition to its role in metabolism and energy conversion, skeletal muscle secretes signalling molecules called myokines (at rest) and exerkines (during/after physical exercise), which communicate with other organs like the brain, the cardiovascular system, and the immune system. Key molecules such as interleukins, irisin, and myostatin are discussed for their roles in mediating muscle health and inter-organ communication. This work also focuses on the muscle-gut axis, emphasising the bidirectional interaction between skeletal muscle and the gut microbiota, a complex ecosystem influencing immune defence, digestion, and metabolism. Muscle activity, particularly exercise, alters the gut microbial composition, promoting beneficial species, while gut-derived metabolites like short-chain fatty acids (SCFAs) impact muscle metabolism, mitochondrial function, and insulin sensitivity. Dysbiosis, or an imbalanced microbiota, can lead to muscle atrophy, inflammation, and metabolic dysfunction. This evidence highlights emerging research into myokines and exerkines as potential therapeutic targets for managing conditions like muscle decline, ageing, and metabolic diseases through muscle-gut interactions.

SMN Deficiency Induces an Early Non-Atrophic Myopathy with Alterations in the Contractile and Excitatory Coupling Machinery of Skeletal Myofibers in the SMN∆7 Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is caused by a deficiency of the ubiquitously expressed survival motor neuron (SMN) protein. The main pathological hallmark of SMA is the degeneration of lower motor neurons (MNs) with subsequent denervation and atrophy of skeletal muscle. However, increasing evidence indicates that low SMN levels not only are detrimental to the central nervous system (CNS) but also directly affect other peripheral tissues and organs, including skeletal muscle. To better understand the potential primary impact of SMN deficiency in muscle, we explored the cellular, ultrastructural, and molecular basis of SMA myopathy in the SMNΔ7 mouse model of severe SMA at an early postnatal period (P0-7) prior to muscle denervation and MN loss (preneurodegenerative [PND] stage). This period contrasts with the neurodegenerative (ND) stage (P8-14), in which MN loss and muscle atrophy occur. At the PND stage, we found that SMN∆7 mice displayed early signs of motor dysfunction with overt myofiber alterations in the absence of atrophy. We provide essential new ultrastructural data on focal and segmental lesions in the myofibrillar contractile apparatus. These lesions were observed in association with specific myonuclear domains and included abnormal accumulations of actin-thin myofilaments, sarcomere disruption, and the formation of minisarcomeres. The sarcoplasmic reticulum and triads also exhibited ultrastructural alterations, suggesting decoupling during the excitation-contraction process. Finally, changes in intermyofibrillar mitochondrial organization and dynamics, indicative of mitochondrial biogenesis overactivation, were also found. Overall, our results demonstrated that SMN deficiency induces early and MN loss-independent alterations in myofibers that essentially contribute to SMA myopathy. This strongly supports the growing body of evidence indicating the existence of intrinsic alterations in the skeletal muscle in SMA and further reinforces the relevance of this peripheral tissue as a key therapeutic target for the disease.

Identification, Expression, Characteristic Analysis, and Immune Function of Two Akirin Genes in Grass Carp (Ctenopharyngodon idella)

Intensive aquaculture of grass carp often leads to decreased immunity and increased disease prevalence, resulting in economic losses. Improving grass carp immunity is therefore a critical strategy for addressing these challenges. Akirin reportedly participates in myogenesis, growth, and immune responses. However, its role in grass carp remains unclear. Herein, we isolated akirins from the spleen of grass carp and analyzed their tissue-specific expression. Akirin expression was detected following treatment with poly (I:C), LPS, and Aeromonas hydrophila (A. hydrophila). The immunological function of the akirin protein was evaluated in head kidney leukocytes (HKLs). The results revealed that the coding sequence (CDS) of akirin1 is 570 bp, encoding 189 amino acids. There was one predicted nuclear localization signal (NLS) and two predicted α- helix domains. The CDS of akirin2 is 558 bp, encoding 185 amino acids. There were two predicted NLSs and two predicted α-helix domains. Tissue-specific expression analysis showed that akirins are widely detected in grass carp tissues. akirin1 was highly detected in the brain, kidneys, heart, spleen, and gonads, while akirin2 was highly detected in the brain, liver, gonads, kidneys, spleen, and heart. The mRNA levels of akirins were promoted after treatment with poly (I:C), LPS, and A. hydrophila. Recombinant akirin proteins were produced in Escherichia coli (E. coli). il-1β, ifnγ, il-6, tnfα, il-4, iκbα, and nfκb were markedly increased in grass carp HKLs by treatment with the akirin protein. These results suggest that akirins play a role in the immunological regulation of grass carp.

Transcriptomics and gut microbiome analysis of the edible herb Bidens pilosa as a functional feed additive to promote growth and metabolism in tilapia (Oreochromis spp.)

To reduce the use of antibiotics and chemicals in aquaculture, an edible herb, Bidens pilosa, has been selected as a multifunctional feed additive. Although there has been considerable research into the effects of B. pilosa on poultry, the wider effects of B. pilosa, particularly on the growth and gut microbiota of fish, remain largely unexplored. We aimed to investigate the interactive effects between the host on growth and the gut microbiota using transcriptomics and the gut microbiota in B. pilosa-fed tilapia. In this study, we added 0.5% and 1% B. pilosa to the diet and observed that the growth performance of tilapia significantly increased over 8 weeks of feeding. Comparative transcriptome analysis was performed on RNA sequence profiles obtained from liver and muscle tissues. Functional enrichment analysis revealed that B. pilosa regulates several pathways and genes involved in amino acid metabolism, lipid metabolism, carbohydrate metabolism, endocrine system, signal transduction, and metabolism of other amino acids. The expression of the selected growth-associated genes was validated by qRT-PCR. The qRT-PCR results indicated that B. pilosa may enhance growth performance by activating the expression of the liver igf1 and muscle igf1rb genes and inhibiting the expression of the muscle negative regulator mstnb. Both the enhancement of liver endocrine IGF1/IGF1Rb signaling and the suppression of muscle autocrine/paracrine MSTN signaling induced the expression of myogenic regulatory factors (MRFs), myod1, myog and mrf4 in muscle to promote muscle growth in tilapia. The predicted function of the gut microbiota showed several significantly different pathways that overlapped with the KEGG enrichment results of differentially expressed genes in the liver transcriptomes. This finding suggested that the gut microbiota may influence liver metabolism through the gut-liver axis in B. pilosa-fed tilapia. In conclusion, dietary B. pilosa can regulate endocrine IGF1 signaling and autocrine/paracrine MSTN signaling to activate the expression of MRFs to promote muscle growth and alter the composition of gut bacteria, which can then affect liver amino acid metabolism, carbohydrate metabolism, endocrine system, lipid metabolism, metabolism of other amino acids, and signal transduction in the host, ultimately enhancing growth performance. Our results suggest that B. pilosa has the potential to be a functional additive that can be used as an alternative to reduce antibiotic use as a growth promoter in aquaculture.

Defining and identifying satellite cell-opathies within muscular dystrophies and myopathies

Muscular dystrophies and congenital myopathies arise from specific genetic mutations causing skeletal muscle weakness that reduces quality of life. Muscle health relies on resident muscle stem cells called satellite cells, which enable life-course muscle growth, maintenance, repair and regeneration. Such tuned plasticity gradually diminishes in muscle diseases, suggesting compromised satellite cell function. A central issue however, is whether the pathogenic mutation perturbs satellite cell function directly and/or indirectly via an increasingly hostile microenvironment as disease progresses. Here, we explore the effects on satellite cell function of pathogenic mutations in genes (myopathogenes) that associate with muscle disorders, to evaluate clinical and muscle pathological hallmarks that define dysfunctional satellite cells. We deploy transcriptomic analysis and comparison between muscular dystrophies and myopathies to determine the contribution of satellite cell dysfunction using literature, expression dynamics of myopathogenes and their response to the satellite cell regulator PAX7. Our multimodal approach extends current pathological classifications to define Satellite Cell-opathies: muscle disorders in which satellite cell dysfunction contributes to pathology. Primary Satellite Cell-opathies are conditions where mutations in a myopathogene directly affect satellite cell function, such as in Progressive Congenital Myopathy with Scoliosis (MYOSCO) and Carey-Fineman-Ziter Syndrome (CFZS). Primary satellite cell-opathies are generally characterised as being congenital with general hypotonia, and specific involvement of respiratory, trunk and facial muscles, although serum CK levels are usually within the normal range. Secondary Satellite Cell-opathies have mutations in myopathogenes that affect both satellite cells and muscle fibres. Such classification aids diagnosis and predicting probable disease course, as well as informing on treatment and therapeutic development.

Effect of eccentric and concentric contraction mode on myogenic regulatory factors expression in human vastus lateralis muscle

Skeletal muscle contractions are caused to release myokines by muscle fiber. This study investigated the myogenic regulatory factors, as MHC I, IIA, IIX, Myo-D, MRF4, Murf, Atrogin-1, Decorin, Myonection, and IL-15 mRNA expression in the response of eccentric vs concentric contraction. Eighteen healthy men were randomly divided into two eccentric and concentric groups, each of 9 persons. Isokinetic contraction protocols included maximal single-leg eccentric or concentric knee extension tasks at 60°/s with the dominant leg. Contractions consisted of a maximum of 12 sets of 10 reps, and the rest time between each set was 30 s. The baseline biopsy was performed 4 weeks before the study, and post-test biopsies were taken immediately after exercise protocols from the vastus lateralis muscle. The gene expression levels were evaluated using Real-Time PCR methods. The eccentric group showed a significantly lower RPE score than the concentric group (P ≤ 0.05). A significant difference in MyoD, MRF4, Myonection, and Decorin mRNA, were observed following eccentric or concentric contractions (P ≤ 0.05). The MHC I, MHC IIA, IL-15 mRNA has been changed significantly compared to the pre-exercise in the concentric group (P ≤ 0.05). While only MHC IIX and Atrogin-1 mRNA changed significantly in the eccentric group (P ≤ 0.05). Additionally, the results showed a significant difference in MyoD, MRF4, IL-15, and Decorin at the follow-up values between eccentric or concentric groups (P ≤ 0.05). Our findings highlight the growing importance of elucidating the different responses of muscle growth factors associated with a myogenic activity such as MHC IIA, Decorin, IL-15, Myonectin, Decorin, MuRF1, and MHC IIX mRNA in following various types of exercise.

Genome-Wide Analysis of Smad7-Mediated Transcription in Mouse Embryonic Stem Cells

Smad7 has been identified as a negative regulator of the transforming growth factor TGF-β pathway by direct interaction with the TGF-β type I receptor (TβR-I). Although Smad7 has also been shown to play TGF-β unrelated functions in the cytoplasm and in the nucleus, a comprehensive analysis of its nuclear function has not yet been performed. Here, we show that in ESCs Smad7 is mainly nuclear and acts as a general transcription factor regulating several genes unrelated to the TGF-β pathway. Loss of Smad7 results in the downregulation of several key stemness master regulators, including Pou5f1 and Zfp42, and in the upregulation of developmental genes, with consequent loss of the stem phenotype. Integrative analysis of genome-wide mapping data for Smad7 and ESC self-renewal and pluripotency transcriptional regulators revealed that Smad7 co-occupies promoters of highly expressed key stemness regulators genes, by binding to a specific consensus response element NCGGAAMM. Altogether, our data establishes Smad7 as a new, integral component of the regulatory circuitry that controls ESC identity.

Exercise-induced myokines and their effect on prostate cancer

Exercise is recognized by clinicians in the field of clinical oncology for its potential role in reducing the risk of certain cancers and in reducing the risk of disease recurrence and progression; yet, the underlying mechanisms behind this reduction in risk are not fully understood. Studies applying post-exercise blood serum directly to various types of cancer cell lines provide insight that exercise might have a role in inhibiting cancer growth via altered soluble and cell-free blood contents. Myokines, which are cytokines produced by muscle and secreted into the bloodstream, might offer multiple benefits to cellular metabolism (such as a reduction in insulin resistance, improved glucose uptake and reduced adiposity), and blood myokine levels can be altered with exercise. Alterations in the levels of myokines such as IL-6, IL-15, IL-10, irisin, secreted protein acidic risk in cysteine (SPARC), myostatin, oncostatin M and decorin might exert a direct inhibitory effect on cancer growth via inhibiting proliferation, promoting apoptosis, inducing cell-cycle arrest and inhibiting the epithermal transition to mesenchymal cells. The association of insulin resistance, hyperinsulinaemia and hyperlipidaemia with obesity can create a tumour-favourable environment; exercise-induced myokines can manipulate this environment by regulating adipose tissue and adipocytes. Exercise-induced myokines also have a critical role in increasing cytotoxicity and the infiltration of immune cells into the tumour.

Muscle-tendon Crosstalk During Muscle Wasting

In organisms from flies to mammals, the initial formation of a functional tendon is completely dependent on chemical signals from muscle (myokines). However, how myokines affect the maturation, maintenance, and regeneration of tendons as a function of age is completely unstudied. Here we discuss the role of four myokines - fibroblast growth factors (FGF), myostatin, the secreted protein acidic and rich in cysteine (SPARC), and miR-29 - in tendon development and hypothesize a role for these factors in the progressive changes in tendon structure and function as a result of muscle wasting (disuse, aging and disease). Because of the close relationship between mechanical loading and muscle and tendon regulation, disentangling muscle-tendon crosstalk from simple mechanical loading is experimentally quite difficult. Therefore, we propose an experimental framework that hopefully will be useful in demonstrating muscle-tendon crosstalk in vivo. Though understudied, the promise of a better understanding of muscle-tendon crosstalk is the development of new interventions that will improve tendon development, regeneration, and function throughout the lifespan.

The identification of a nuclear factor Akirin with regulating the expression of antimicrobial peptides in red swamp crayfish (Procambarus clarkii)

Akirin is a highly conserved nuclear factor among different species. It is closely related to skeletal muscle development, innate immune response, and tumorigenesis in a variety of animals. In invertebrates, Akirin is mainly involved in gene transcription and NF-κB dependent natural immune response. In the present study, a nuclear factor Akirin was identified from Procambarus clarkii. The Akirin protein of crayfish consists of 204 amino acids and is conserved among its family members, especially the nuclear localization signal peptide motif (KRRR). PcAkirin was highly expressed in stomach, intestines, and hepatopancreas. After A. hydrophila challenge, the transcription level of Akirin significantly increased in hemocyte and hepatopancreas. In addition, the recombinant Akirin protein was produced successfully and helpful to resist WSSV infection by increasing the expression level of some immune related genes. On the contrary, after interfering with Akirin gene by dsRNA, the crayfish increased the sensitivity to A. hydrophila and WSSV infections. The results are more obvious in the accumulated mortality of P. clarkii infected with A. hydrophila and WSSV. All these results suggested that Akirin played a significant role in innate immune responses and protected it from WSSV and bacterial infection in crayfish.

Pathophysiology, Biomarkers, and Therapeutic Modalities Associated with Skeletal Muscle Loss Following Spinal Cord Injury

A spinal cord injury (SCI) may lead to loss of strength, sensation, locomotion and other body functions distal to the lesion site. Individuals with SCI also develop secondary conditions due to the lack of skeletal muscle activity. As SCI case numbers increase, recent studies have attempted to determine the best options to salvage affected musculature before it is lost. These approaches include pharmacotherapeutic options, immunosuppressants, physical activity or a combination thereof. Associated biomarkers are increasingly used to determine if these treatments aid in the protection and reconstruction of affected musculature.

Akirin proteins in development and disease: critical roles and mechanisms of action

The Akirin genes, which encode small, nuclear proteins, were first characterized in 2008 in Drosophila and rodents. Early studies demonstrated important roles in immune responses and tumorigenesis, which subsequent work found to be highly conserved. More recently, a multiplicity of Akirin functions, and the associated molecular mechanisms involved, have been uncovered. Here, we comprehensively review what is known about invertebrate Akirin and its two vertebrate homologues Akirin1 and Akirin2, highlighting their role in regulating gene expression changes across a number of biological systems. We detail essential roles for Akirin family proteins in the development of the brain, limb, and muscle, in meiosis, and in tumorigenesis, emphasizing associated signaling pathways. We describe data supporting the hypothesis that Akirins act as a "bridge" between a variety of transcription factors and major chromatin remodeling complexes, and discuss several important questions remaining to be addressed. In little more than a decade, Akirin proteins have gone from being completely unknown to being increasingly recognized as evolutionarily conserved mediators of gene expression programs essential for the formation and function of animals.

Role of Myokines in Myositis Pathogenesis and Their Potential to be New Therapeutic Targets in Idiopathic Inflammatory Myopathies

Idiopathic inflammatory myopathies (IIM) represent a heterogeneous group of autoimmune diseases whose treatment is often a challenge. Many patients, even after immunosuppressive therapy, do not respond to treatment, so new alternatives have been sought for this. Therefore, other signaling pathways that could contribute to the pathogenesis of myositis have been investigated, such as the expression of myokines in skeletal muscle in response to the inflammatory process. In this review, we will refer to these muscle cytokines that are overexpressed or downregulated in skeletal muscle in patients with various forms of IIM, thus being able to contribute to the maintenance of the autoimmune process. Some muscle cytokines, through their antagonistic action, may be a helpful contributor to the disease modulation, and thus, they could represent personalized treatment targets. Here, we consider the main myokines involved in the pathogenesis of myositis, expressing our view on the possibility of using them as potential therapeutic targets: interleukins IL-6, IL-15, and IL-18; chemokines CXCL10, CCL2, CCL3, CCL4, CCL5, and CCL20; myostatin; follistatin; decorin; osteonectin; and insulin-like 6. An interesting topic regarding the complex connection between myokines and noninflammatory pathways implied in IIM has also been briefly described, because it is an important scientific approach to the pathogenesis of IIM and can be a therapeutic alternative to be considered, especially for the patients who do not respond to immunosuppressive treatment.

Akirin Is Required for Muscle Function and Acts Through the TGF-β Sma/Mab Signaling Pathway in Caenorhabditis elegans Development

Akirin, a conserved metazoan protein, functions in muscle development in flies and mice. However, this was only tested in the rodent and fly model systems. Akirin was shown to act with chromatin remodeling complexes in transcription and was established as a downstream target of the NFκB pathway. Here we show a role for Caenorhabditis elegans Akirin/AKIR-1 in the muscle and body length regulation through a different pathway. Akirin localizes to somatic tissues throughout the body of C. elegans, including muscle nuclei. In agreement with its role in other model systems, Akirin loss of function mutants exhibit defects in muscle development in the embryo, as well as defects in movement and maintenance of muscle integrity in the C. elegans adult. We also have determined that Akirin acts downstream of the TGF-β Sma/Mab signaling pathway in controlling body size. Moreover, we found that the loss of Akirin resulted in an increase in autophagy markers, similar to mutants in the TGF-β Sma/Mab signaling pathway. In contrast to what is known in rodent and fly models, C. elegans Akirin does not act with the SWI/SNF chromatin-remodeling complex, and is instead involved with the NuRD chromatin remodeling complex in both movement and regulation of body size. Our studies define a novel developmental role (body size) and a new pathway (TGF-β Sma/Mab) for Akirin function, and confirmed its evolutionarily conserved function in muscle development in a new organism.

A novel nuclear factor Akirin regulating the expression of antimicrobial peptides in Chinese mitten crab Eriocheir sinensis

Akirin, a recently discovered nuclear factor, participates in regulating various processes, including cell proliferation and differentiation, embryonic development, and immunity. In the present study, a novel Akirin was identified from Chinese mitten crab Eriocheir sinensis (designated as EsAkirin), and its primary functions in regulating antimicrobial peptides were explored. The open reading frame of EsAkirin was of 615 bp, encoding a polypeptide of 204 amino acid residues. The deduced amino acid sequence of EsAkirin shared high similarities ranging from 44.1% to 89.2% with other Akirins. In the phylogenetic tree, EsAkirin was firstly clustered with Akirins from shrimp and then assigned into the invertebrate branch. The mRNA transcripts of EsAkirin were constitutively expressed in all the tested tissues, with the highest expression level (5.07-fold compared to the stomach, p < 0.01) in hepatopancreas. The mRNA expression of EsAkirin in hemocytes was significantly increased at 6 h, and reached the maximum level at 24 h post stimulations with either lipopolysaccharide (LPS) (5.04-fold, p < 0.01) or Aeromonas hydrophila (3.10-fold, p < 0.01). After the injection of EsAkirin-dsRNA, the mRNA expressions of EsALF2, EsLYZ, EsCrus2 and EsDWD1 were significantly decreased (p < 0.01) upon LPS stimulation. EsAkirin protein was prominently distributed in the nucleus of E. sinensis hemocytes after LPS and A. hydrophila stimulations. The relative luciferase reporter system analysis revealed that the activity of nuclear factor-κB was significantly up-regulated (2.64-fold, p < 0.01) in human embryonic kidney (HEK293T) cells after the over-expression of EsAkirin. Collectively, these results suggested that EsAkirin might play an important role in the immune responses of E. sinensis by regulating the expression of antimicrobial peptides.

Molecular characterization and functional analysis of Akirin from black tiger shrimp (Penaeus monodon)

Akirin, which are members of the NF-κB signaling pathway, play critical roles in regulating the expression of antimicrobial peptides. In the present study, the Akirin gene from Penaeus monodon was identified from a transcriptome database and designated as PmAkirin. The complete sequence of the PmAkirin cDNA was 1508 bp, encoding a protein of 213 amino acids, and it showed 99% amino acid identity to the Litopenaeus vannamei Akirin. Two predicted nuclear localization signals (NLSs) were found, and the amino acid sequence alignments showed that PmAkirin was highly conserved at the N-terminus and C-terminus. PmAkirin expression was found to be the highest in the hemolymph, followed by the heart, gill, stomach, hepatopancreas, intestine, and muscle. When challenged with Vibrio parahaemolyticus infection, the PmAkirin mRNA and three antimicrobial peptides (AMPs: PmALF2, PmALF3, and PmCrus4) were upregulated. However, another five AMPs (PmALF6, PmCrus1, PmPEN3a, PmPEN3b, and PmPEN5) were downregulated by V. parahaemolyticus infection. Silencing PmAkirin by dsRNA significantly decreased the expression of the eight AMPs, which lead to an increase in the blood concentration of V. parahaemolyticus and higher mortality in the shrimp. In contrast, the overexpression of PmAkirin significantly increased the expression of the eight AMPs, which led to a reduction in the blood concentration of V. parahaemolyticus and promoted the survival of the shrimp. Taken together, we concluded that PmAkirin plays an important role in regulating the expression of AMPs in black tiger shrimp to defend against V. parahaemolyticus infection.

Role of Akirin1 in the regulation of skeletal muscle fiber-type switch

Akirin1 is a highly conserved ubiquitously expressed nuclear protein. Owing to its strong nuclear localization signal and protein-protein interaction properties, Akirin1 has been speculated to regulate transcription of target genes as a cofactor. Previous studies have reported Akirin1 as a downstream target of myostatin, a potent negative regulator of myogenesis. Mice lacking myostatin displayed enhanced Akirin1 gene expression. Further, in vitro evidence has shown Akirin1 overexpression leads to hypertrophy in C₂ C ₁₂ myotubes. In this study, we used Akirin1 knockout mice as a model system to further elucidate the function of Akirin1 in fully differentiated skeletal muscle. Akirin1 knockout mice did not show any obvious phenotypic difference when compared with wild type. However, promoter-reporter assay suggested that Akirin1 regulated the transcription of muscle-specific RING finger 1 (MuRF-1), an important E3 ubiquitin ligase in skeletal muscle. Furthermore, ablation of Akirin1 resulted in increased type IIa and decreased type I muscle fibers, which was further supported by an increase in Myh2 and decrease in Myh7 gene expression. Also, histochemical studies for succinate dehydrogenase activity revealed a less oxidative muscle in the absence of Akirin1. Together, our study suggests a novel role of Akirin1 in maintaining the muscle fiber type and regulation of the metabolic activity of the skeletal muscle.

A critical role for the nuclear protein Akirin2 in the formation of mammalian muscle in vivo

Evolutionarily conserved Akirin nuclear proteins interact with chromatin remodeling complexes at gene enhancers and promoters, and have been reported to regulate cell proliferation and differentiation. Of the two mouse Akirin genes, Akirin2 is essential during embryonic development, with known in vivo roles in immune system function and the formation of the cerebral cortex. Here we demonstrate that Akirin2 is critical for mouse myogenesis, a tightly regulated developmental process through which myoblast precursors fuse to form mature skeletal muscle fibers. Loss of Akirin2 in somitic muscle precursor cells via Sim1-Cre-mediated excision of a conditional Akirin2 allele results in neonatal lethality. Mutant embryos exhibit a complete lack of forelimb, intercostal, and diaphragm muscles due to extensive apoptosis and loss of Pax3-positive myoblasts. Severe skeletal defects, including craniofacial abnormalities, disrupted ossification, and rib fusions are also observed, attributable to lack of skeletal muscles as well as patchy Sim1-Cre activity in the embryonic sclerotome. We further show that Akirin2 levels are tightly regulated during muscle cell differentiation in vitro, and that Akirin2 is required for the proper expression of muscle differentiation factors myogenin and myosin heavy chain. Our results implicate Akirin2 as a major regulator of mammalian muscle formation in vivo.

Akirin1 promotes myoblast differentiation by modulating multiple myoblast differentiation factors

Akirin1 is found to be involved in myoblast differentiation. However, the mechanism by which the Akirin1 gene regulates myoblast differentiation still remains unclear. In the present study, we found that ectopic expression of Akirin1 promoted myoblast differentiation by increasing the expression of myogenic regulatory factor (MRF) 4 (MRF4) and myocyte enhancer factor 2B (MEF2B) mRNA. Additionally, we showed that ectopic Akirin1 induced cell cycle arrest by up-regulating p21 mRNA. To further uncover the mechanism by which Akirin1 promotes myoblast differentiation, we showed that the enhanced Akirin1 increased the mRNA expression of P38α. Importantly, the enhanced MRF4 expression by Akirin1 can be abrogated by treatment of SB203580, a p38 inhibitor. Similarly, we found that enhanced MEF2B expression by Akirin1 can be abrogated by treatment with LY294002, a PI3K inhibitor. Together, our results indicate that Akirin1 promotes myoblast differentiation by acting on the p38 and PI3K pathways and subsequently inducing the expression of myoblast differentiation factors.

Role of Myokines in Regulating Skeletal Muscle Mass and Function

Loss of skeletal muscle mass and strength has recently become a hot research topic with the extension of life span and an increasingly sedentary lifestyle in modern society. Maintenance of skeletal muscle mass is considered an essential determinant of muscle strength and function. Myokines are cytokines synthesized and released by myocytes during muscular contractions. They are implicated in autocrine regulation of metabolism in the muscle as well as in the paracrine/endocrine regulation of other tissues and organs including adipose tissue, the liver, and the brain through their receptors. Till date, secretome analysis of human myocyte culture medium has revealed over 600 myokines. In this review article, we summarize our current knowledge of major identified and characterized myokines focusing on their biological activity and function, particularly in muscle mass and function.

A Novel Role for α-Importins and Akirin in Establishment of Meiotic Sister Chromatid Cohesion in Caenorhabditis elegans

During meiotic prophase I, sister chromatid cohesion is established in a way that supports the assembly of the synaptonemal complex (SC). The SC connects homologous chromosomes, directing meiotic recombination to create crossovers. In this paper, we identify two proteins that cooperate to import and load meiotic cohesins, thus indirectly promoting SC assembly. AKIR-1 is a protein with a previously identified meiotic role in SC disassembly. akir-1 mutants have no obvious defects in sister chromatid cohesion. We identified ima-2, a gene encoding for an α-importin nuclear transport protein, as a gene interacting with akir-1 Analysis of akir-1;ima-2 double mutants reveals a decrease in the number of germline nuclei and the formation of polycomplexes (PCs) (an SC protein aggregate). These PCs contain proteins that are part of the two main substructures of the SC: the central region and the lateral element. Unlike typical PCs, they also contain sister chromatid cohesion proteins. In akir-1;ima-2 double mutants, PCs are located in both the nucleus and the cytoplasm. This suggests that the defects observed in the double mutants are both in nuclear import and in the assembly of sister chromatid cohesion. PC formation is also associated with recombination defects leading to reduced numbers of crossovers. Similarly to cohesion mutants, the pairing center protein HIM-8 is mislocalized in akir-1;ima-2 double mutants, forming multiple foci. We propose that AKIR-1 and IMA-2 operate in parallel pathways to import and load chromosomally associated cohesin complex proteins in meiotic nuclei, a novel finding for both of these conserved proteins.

Functional Evolution of Subolesin/Akirin

The Subolesin/Akirin constitutes a good model for the study of functional evolution because these proteins have been conserved throughout the metazoan and play a role in the regulation of different biological processes. Here, we investigated the evolutionary history of Subolesin/Akirin with recent results on their structure, protein-protein interactions and function in different species to provide insights into the functional evolution of these regulatory proteins, and their potential as vaccine antigens for the control of ectoparasite infestations and pathogen infection. The results suggest that Subolesin/Akirin evolved conserving not only its sequence and structure, but also its function and role in cell interactome and regulome in response to pathogen infection and other biological processes. This functional conservation provides a platform for further characterization of the function of these regulatory proteins, and how their evolution can meet species-specific demands. Furthermore, the conserved functional evolution of Subolesin/Akirin correlates with the protective capacity shown by these proteins in vaccine formulations for the control of different arthropod and pathogen species. These results encourage further research to characterize the structure and function of these proteins, and to develop new vaccine formulations by combining Subolesin/Akirin with interacting proteins for the control of multiple ectoparasite infestations and pathogen infection.

A comparative study of three akirin genes from big belly seahorse Hippocampus abdominalis: Molecular, transcriptional and functional characterization

Akirins, members of the NF-κB signaling pathway, are highly conserved nuclear proteins, which regulate gene expression in many physiological processes, including immunity, myogenesis, carcinogenesis, and embryogenesis. The akirin family in teleost fish consists of two to three genes. In the present study, three akirin genes from Hippocampus abdominalis were identified from a transcriptome database and designated as HaAkirin1, HaAkirin2(1), and HaAkirin2(2). The nuclear localization of HaAkirin1 and HaAkirin2(1) was confirmed by subcellular localization analysis. In contrast, diffused localization of HaAkirin2(2) was identified in the nucleus and cytoplasm that confirmed the aberrant nature of the nuclear localization signal. Phylogenetic analysis revealed a closer relationship of HaAkirins with other known teleost akirins. All three HaAkirin transcripts were ubiquitously expressed in all examined tissues with higher expression in ovary tissue. Immune challenge with LPS, poly I:C, and Streptococcus iniae exhibited a significant increase in the expression of all three HaAkirins in kidney and liver tissues. NF-κB luciferase assays revealed that relative luciferase activity was significantly higher for all three HaAkirin genes than mock controls. These results suggest that HaAkirin genes might play a role in regulating NF-κB dependent immune gene expression and their expression could be induced by bacterial and viral pathogen recognition molecular patterns.

Effects of Active Immunization Against Akirin2 on Muscle Fiber-type Composition in Pigs

The objective of this study was to investigate effects of active immunization against Akirin2 on muscle fiber-type composition in pigs. Here we showed that the titer of Akirin2 antibody in pigs immunized with porcine Akirin2 (pAkirin2) was significantly increased. Active immunization against pAkirin2 decreased succinic dehydrogenase and malate dehydrogenase activities and increased lactate dehydrogenase activity in the longissimus dorsi muscle of pigs. Active immunization against pAkirin2 significantly decreased MyHC I and MyHC IIa mRNA expressions and MyHC I protein expression and increased mRNA expressions of MyHC IIb as well as protein expressions of MyHC IIb and fast-MyHC. mRNA expressions of nuclear factors of activated T cells c1 (NFATc1), transcriptional coactivator PPARγ coactivator-1α, myocyte enhancer factor 2C, and modulatory calcineurin interacting protein 1 exon 4 isform were also notably decreased by active immunization against pAkirin2. Together, our data imply that active immunization against pAkirin2 may result in a slow to fast fiber-type shift in pigs, and which may be mediated by suppression of the calcineurin/NFATc1 signaling pathway.

Impaired fetal muscle development and JAK-STAT activation mark disease onset and progression in a mouse model for merosin-deficient congenital muscular dystrophy

Merosin-deficient congenital muscular dystrophy type 1A (MDC1A) is a dramatic neuromuscular disease in which crippling muscle weakness is evident from birth. Here, we use the dyW mouse model for human MDC1A to trace the onset of the disease during development in utero. We find that myotomal and primary myogenesis proceed normally in homozygous dyW-/- embryos. Fetal dyW-/- muscles display the same number of myofibers as wildtype (WT) muscles, but by E18.5 dyW-/- muscles are significantly smaller and muscle size is not recovered post-natally. These results suggest that fetal dyW-/- myofibers fail to grow at the same rate as WT myofibers. Consistent with this hypothesis between E17.5 and E18.5 dyW-/- muscles display a dramatic drop in the number of Pax7- and myogenin-positive cells relative to WT muscles, suggesting that dyW-/- muscles fail to generate enough muscle cells to sustain fetal myofiber growth. Gene expression analysis of dyW-/- E17.5 muscles identified a significant increase in the expression of the JAK-STAT target gene Pim1 and muscles from 2-day and 3-week old dyW-/- mice demonstrate a dramatic increase in pSTAT3 relative to WT muscles. Interestingly, myotubes lacking integrin α7β1, a laminin-receptor, also show a significant increase in pSTAT3 levels compared with WT myotubes, indicating that α7β1 can act as a negative regulator of STAT3 activity. Our data reveal for the first time that dyW-/- mice exhibit a myogenesis defect already in utero. We propose that overactivation of JAK-STAT signaling is part of the mechanism underlying disease onset and progression in dyW-/- mice.

Short-hairpin Mediated Myostatin Knockdown Resulted in Altered Expression of Myogenic Regulatory Factors with Enhanced Myoblast Proliferation in Fetal Myoblast Cells of Goats

Myostatin (MSTN) is a well-known negative regulator of skeletal muscle development. Reduced expression due to natural mutations in the coding region and knockout as well as knockdown of MSTN results in an increase in the muscle mass. In the present study, we demonstrated as high as 60 and 52% downregulation (p < 0.01) of MSTN mRNA and protein in the primary fetal myoblast cells of goats using synthetic shRNAs (n = 3), without any interferon response. We, for the first time, evaluated the effect of MSTN knockdown on the expression of MRFs (namely, MyoD, Myf5), follistatin (FST), and IGFs (IGF-1 & IGF-2) in goat myoblast cells. MSTN knockdown caused an upregulation (p < 0.05) of MyoD and downregulation (p < 0.01) of MYf5 and FST expression. Moreover, we report up to ∼four fold (p < 0.001) enhanced proliferation in myoblasts after four days of culture. The anti-MSTN shRNA demonstrated in the present study could be used for the production of transgenic goats to increase the muscle mass.

Akirin2 is essential for the formation of the cerebral cortex

Background

The proper spatial and temporal regulation of dorsal telencephalic progenitor behavior is a prerequisite for the formation of the highly-organized, six-layered cerebral cortex. Premature differentiation of cells, disruption of cell cycle timing, excessive apoptosis, and/or incorrect neuronal migration signals can have devastating effects, resulting in a number of neurodevelopmental disorders involving microcephaly and/or lissencephaly. Though genes encoding many key players in cortical development have been identified, our understanding remains incomplete. We show that the gene encoding Akirin2, a small nuclear protein, is expressed in the embryonic telencephalon. Converging evidence indicates that Akirin2 acts as a bridge between transcription factors (including Twist and NF-κB proteins) and the BAF (SWI/SNF) chromatin remodeling machinery to regulate patterns of gene expression. Constitutive knockout of Akirin2 is early embryonic lethal in mice, while restricted loss in B cells led to disrupted proliferation and cell survival.

Methods

We generated cortex-restricted Akirin2 knockouts by crossing mice harboring a floxed Akirin2 allele with the Emx1-Cre transgenic line and assessed the resulting embryos using in situ hybridization, EdU labeling, and immunohistochemistry.

Results

The vast majority of Akirin2 mutants do not survive past birth, and exhibit extreme microcephaly, with little dorsal telencephalic tissue and no recognizable cortex. This is primarily due to massive cell death of early cortical progenitors, which begins at embryonic day (E)10, shortly after Emx1-Cre is active. Immunostaining and cell cycle analysis using EdU labeling indicate that Akirin2-null progenitors fail to proliferate normally, produce fewer neurons, and undergo extensive apoptosis. All of the neurons that are generated in Akirin2 mutants also undergo apoptosis by E12. In situ hybridization for Wnt3a and Wnt-responsive genes suggest defective formation and/or function of the cortical hem in Akirin2 null mice. Furthermore, the apical ventricular surface becomes disrupted, and Sox2-positive progenitors are found to “spill” into the lateral ventricle.

Conclusions

Our data demonstrate a previously-unsuspected role for Akirin2 in early cortical development and, given its known nuclear roles, suggest that it may act to regulate gene expression patterns critical for early progenitor cell behavior and cortical neuron production.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-016-0076-8) contains supplementary material, which is available to authorized users.

Testosterone and trenbolone enanthate increase mature myostatin protein expression despite increasing skeletal muscle hypertrophy and satellite cell number in rodent muscle

The androgen-induced alterations in adult rodent skeletal muscle fibre cross-sectional area (fCSA), satellite cell content and myostatin (Mstn) were examined in 10-month-old Fisher 344 rats (n = 41) assigned to Sham surgery, orchiectomy (ORX), ORX + testosterone (TEST; 7.0 mg week^-1 ) or ORX + trenbolone (TREN; 1.0 mg week^-1 ). After 29 days, animals were euthanised and the levator ani/bulbocavernosus (LABC) muscle complex was harvested for analyses. LABC muscle fCSA was 102% and 94% higher in ORX + TEST and ORX + TREN compared to ORX (p < .001). ORX + TEST and ORX + TREN increased satellite cell numbers by 181% and 178% compared to ORX, respectively (p < .01), with no differences between conditions for myonuclear number per muscle fibre (p = .948). Mstn protein was increased 159% and 169% in the ORX + TEST and ORX + TREN compared to ORX (p < .01). pan-SMAD2/3 protein was ~30-50% greater in ORX compared to SHAM (p = .006), ORX + TEST (p = .037) and ORX + TREN (p = .043), although there were no between-treatment effects regarding phosphorylated SMAD2/3. Mstn, ActrIIb and Mighty mRNAs were lower in ORX, ORX + TEST and ORX + TREN compared to SHAM (p < .05). Testosterone and trenbolone administration increased muscle fCSA and satellite cell number without increasing myonuclei number, and increased Mstn protein levels. Several genes and signalling proteins related to myostatin signalling were differentially regulated by ORX or androgen therapy.

Acute resistance exercise activates rapamycin-sensitive and -insensitive mechanisms that control translational activity and capacity in skeletal muscle

KEY POINTS

Ribosome biogenesis is the primary determinant of translational capacity, but its regulation in skeletal muscle following acute resistance exercise is poorly understood. Resistance exercise increases muscle protein synthesis acutely, and muscle mass with training, but the role of translational capacity in these processes is unclear. Here, we show that acute resistance exercise activated pathways controlling translational activity and capacity through both rapamycin-sensitive and -insensitive mechanisms. Transcription factor c-Myc and its downstream targets, which are known to regulate ribosome biogenesis in other cell types, were upregulated after resistance exercise in a rapamycin-independent manner and may play a role in determining translational capacity in skeletal muscle. Local inhibition of myostatin was also not affected by rapamycin and may contribute to the rapamycin-independent effects of resistance exercise.

ABSTRACT

This study aimed to determine (1) the effect of acute resistance exercise on mechanisms of ribosome biogenesis, and (2) the impact of mammalian target of rapamycin on ribosome biogenesis, and muscle protein synthesis (MPS) and degradation. Female F344BN rats underwent unilateral electrical stimulation of the sciatic nerve to mimic resistance exercise in the tibialis anterior (TA) muscle. TA muscles were collected at intervals over the 36 h of exercise recovery (REx); separate groups of animals were administered rapamycin pre-exercise (REx+Rapamycin). Resistance exercise led to a prolonged (6-36 h) elevation (30-50%) of MPS that was fully blocked by rapamycin at 6 h but only partially at 18 h. REx also altered pathways that regulate protein homeostasis and mRNA translation in a manner that was both rapamycin-sensitive (proteasome activity; phosphorylation of S6K1 and rpS6) and rapamycin-insensitive (phosphorylation of eEF2, ERK1/2 and UBF; gene expression of the myostatin target Mighty as well as c-Myc and its targets involved in ribosome biogenesis). The role of c-Myc was tested in vitro using the inhibitor 10058-F4, which, over time, decreased basal RNA and MPS in a dose-dependent manner (correlation of RNA and MPS, r(2) = 0.98), even though it had no effect on the acute stimulation of protein synthesis. In conclusion, acute resistance exercise stimulated rapamycin-sensitive and -insensitive mechanisms that regulate translation activity and capacity.

Evolution of akirin family in gene and genome levels and coexpressed patterns among family members and rel gene in croaker

Akirins, which are highly conserved nuclear proteins, are present throughout the metazoan and regulate innate immunity, embryogenesis, myogenesis, and carcinogenesis. This study reports all akirin genes from miiuy croaker and analyzes comprehensively the akirin gene family combined with akirin genes from other species. A second nuclear localization signal (NLS) is observed in akirin2 homologues, which is not in akirin1 homologues in all teleosts and most other vertebrates. Thus, we deduced that the loss of second NLS in akirin1 homologues in teleosts likely occurred in an ancestor to all Osteichthyes after splitting with cartilaginous fish. Significantly, the akirin2(2) gene included six exons interrupted by five introns in the miiuy croaker, which may be caused by the intron insertion event as a novel evidence for the variation of akirin gene structure in some species. In addition, comparison of the genomic neighborhood genes of akirin1, akirin2(1), and akirin2(2) demonstrates a strong level of conserved synteny across the teleost classes, which further proved the deduction of Macqueen and Johnston 2009 that the produce of akirin paralogues can be attributed to whole-genome duplications and the loss of some akirin paralogues after genome duplications. Furthermore, akirin gene family members and relish gene are ubiquitously expressed across all tissues, and their expression levels are increased in three immune tissues after infection with Vibrio anguillarum. Combined with the expression patterns of LEAP-1 and LEAP-2 from miiuy croaker, an intricate network of co-regulation among family members is established. Thus, it is further proved that akirins acted in concert with the relish protein to induce the expression of a subset of downstream pathway elements in the NF-kB dependent signaling pathway.

Exploring evidence of positive selection reveals genetic basis of meat quality traits in Berkshire pigs through whole genome sequencing

BACKGROUND

Natural and artificial selection following domestication has led to the existence of more than a hundred pig breeds, as well as incredible variation in phenotypic traits. Berkshire pigs are regarded as having superior meat quality compared to other breeds. As the meat production industry seeks selective breeding approaches to improve profitable traits such as meat quality, information about genetic determinants of these traits is in high demand. However, most of the studies have been performed using trained sensory panel analysis without investigating the underlying genetic factors. Here we investigate the relationship between genomic composition and this phenotypic trait by scanning for signatures of positive selection in whole-genome sequencing data.

RESULTS

We generated genomes of 10 Berkshire pigs at a total of 100.6 coverage depth, using the Illumina Hiseq2000 platform. Along with the genomes of 11 Landrace and 13 Yorkshire pigs, we identified genomic variants of 18.9 million SNVs and 3.4 million Indels in the mapped regions. We identified several associated genes related to lipid metabolism, intramuscular fatty acid deposition, and muscle fiber type which attribute to pork quality (TG, FABP1, AKIRIN2, GLP2R, TGFBR3, JPH3, ICAM2, and ERN1) by applying between population statistical tests (XP-EHH and XP-CLR). A statistical enrichment test was also conducted to detect breed specific genetic variation. In addition, de novo short sequence read assembly strategy identified several candidate genes (SLC25A14, IGF1, PI4KA, CACNA1A) as also contributing to lipid metabolism.

CONCLUSIONS

Results revealed several candidate genes involved in Berkshire meat quality; most of these genes are involved in lipid metabolism and intramuscular fat deposition. These results can provide a basis for future research on the genomic characteristics of Berkshire pigs.

Chromatin Remodeling and Transcriptional Control in Innate Immunity: Emergence of Akirin2 as a Novel Player

Transcriptional regulation of inflammatory gene expression has been at the forefront of studies of innate immunity and is coordinately regulated by transcription factors, including NF-κB, and chromatin modifiers. The growing evidence for involvement of chromatin in the regulation of gene expression in innate immune cells, has uncovered an evolutionarily conserved role of microbial sensing and chromatin remodeling. Toll-like receptors and RIG-I-like receptors trigger these signaling pathways leading to transcriptional expression of a set of genes involved in inflammation. Tightly regulated control of this gene expression is a paramount, and often foremost, goal of most biological endeavors. In this review, we will discuss the recent progress about the molecular mechanisms governing control of pro-inflammatory gene expression by an evolutionarily conserved novel nuclear protein Akirin2 in macrophages and its emergence as an essential link between NF-κB and chromatin remodelers for transcriptional regulation.

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.

The identification of the first molluscan Akirin2 with immune defense function in the Hong Kong oyster Crassostrea hongkongensis

The Akirin protein is a nuclear factor in the innate immune system that is highly conserved from insects to mammals and plays key roles in diverse biological processes, including immunity, myogenesis, development and the cellular stress response. However, the function of Akirins in mollusk, the second most diverse group of animals, is still poorly understood. In this study, we report the discovery of an Akirin2 gene homolog (ChAkirin2) and its biological functions in the Hong Kong oyster Crassostrea hongkongensis. ChAkirin2 is 189 amino acids in length and shares significant homology with invertebrate homologs. Phylogenetic analysis results revealed that ChAkirin2 is clustered with invertebrate Akirin2s. A sequence analysis of the 5' flanking regions of ChAkirin2 indicated that it harbors several potential PAMP-activated transcription factor binding sites (TFB), including sites for NF-κB, C/EBPα, AP-1, SRF, Oct-1 and GATA-1. An RT-PCR analysis showed that ChAkirin2 mRNA was ubiquitously expressed in various tissues and at different embryonic and larval stages. Additionally, upon infection by pathogens (Vibrio alginolyticus, Staphylococcus haemolyticus and Saccharomyces cerevisiae) and pathogen-associated molecular patterns (PAMPs: LPS, PGN and polyI:C), the expression of ChAkirin2 was significantly up-regulated. Moreover, fluorescence microscopy observations show that ChAkirin2 is located in the nuclei of HeLa cells, and the overexpression of ChAkirin2 activated the transcriptional activities of the NF-κB reporter gene in HEK293T cells. Altogether, this report provided the first experimental demonstration that mollusks possess a functional Akirin2 that is involved in the innate defense and embryogenesis processes of the oyster.

Urocortin 3 activates AMPK and AKT pathways and enhances glucose disposal in rat skeletal muscle

Insulin resistance (IR) in skeletal muscle is an important component of both type 2 diabetes and the syndrome of sarcopaenic obesity, for which there are no effective therapies. Urocortins (UCNs) are not only well established as neuropeptides but also have their roles in metabolism in peripheral tissues. We have shown recently that global overexpression of UCN3 resulted in muscular hypertrophy and resistance to the adverse metabolic effects of a high-fat diet. Herein, we aimed to establish whether short-term local UCN3 expression could enhance glucose disposal and insulin signalling in skeletal muscle. UCN3 was found to be expressed in right tibialis cranialis and extensor digitorum longus muscles of rats by in vivo electrotransfer and the effects studied vs the contralateral muscles after 1 week. No increase in muscle mass was detected, but test muscles showed 19% larger muscle fibre diameter (P=0.030), associated with increased IGF1 and IGF1 receptor mRNA and increased SER256 phosphorylation of forkhead transcription factor. Glucose clearance into the test muscles after an intraperitoneal glucose load was increased by 23% (P=0.018) per unit mass, associated with increased GLUT1 (34% increase; P=0.026) and GLUT4 (48% increase; P=0.0009) proteins, and significantly increased phosphorylation of insulin receptor substrate-1, AKT, AKT substrate of 160 kDa, glycogen synthase kinase-3β, AMP-activated protein kinase and its substrate acetyl coA carboxylase. Thus, UCN3 expression enhances glucose disposal and signalling in muscle by an autocrine/paracrine mechanism that is separate from its pro-hypertrophic effects, implying that such a manipulation may have promised for the treatment of IR syndromes including sarcopaenic obesity.

L-leucine, beta-hydroxy-beta-methylbutyric acid (HMB) and creatine monohydrate prevent myostatin-induced Akirin-1/Mighty mRNA down-regulation and myotube atrophy

Background

The purpose of this study was to examine if L-leucine (Leu), β-hydroxy-β-methylbutyrate (HMB), or creatine monohydrate (Crea) prevented potential atrophic effects of myostatin (MSTN) on differentiated C2C12 myotubes.

Methods

After four days of differentiation, myotubes were treated with MSTN (10 ng/ml) for two additional days and four treatment groups were studied: 1) 3x per day 10 mM Leu, 2) 3x per day 10 mM HMB, 3) 3x per day 10 mM Crea, 4) DM only. Myotubes treated with DM without MSTN were analyzed as the control condition (DM/CTL). Following treatment, cells were analyzed for total protein, DNA content, RNA content, muscle protein synthesis (MPS, SUnSET method), and fiber diameter. Separate batch treatments were analyzed for mRNA expression patterns of myostatin-related genes (Akirin-1/Mighty, Notch-1, Ski, MyoD) as well as atrogenes (MuRF-1, and MAFbx/Atrogin-1).

Results

MSTN decreased fiber diameter approximately 30% compared to DM/CTL myotubes (p < 0.001). Leu, HMB and Crea prevented MSTN-induced atrophy. MSTN did not decrease MPS levels compared to DM/CTL myotubes, but MSTN treatment decreased the mRNA expression of Akirin-1/Mighty by 27% (p < 0.001) and MyoD by 26% (p < 0.01) compared to DM/CTL myotubes. shRNA experiments confirmed that Mighty mRNA knockdown reduced myotube size, linking MSTN treatment to atrophy independent of MPS. Remarkably, MSTN + Leu and MSTN + HMB myotubes had similar Akirin-1/Mighty and MyoD mRNA levels compared to DM/CTL myotubes. Furthermore, MSTN + Crea myotubes exhibited a 36% (p < 0.05) and 86% (p < 0.001) increase in Akirin-1/Mighty mRNA compared to DM/CTL and MSTN-only treated myotubes, respectively.

Conclusions

Leu, HMB and Crea may reduce MSTN-induced muscle fiber atrophy by influencing Akirin-1/Mighty mRNA expression patterns. Future studies are needed to examine if Leu, HMB and Crea independently or synergistically affect Akirin-1/Mighty expression, and how Akirin-1/Mighty expression mechanistically relates to skeletal muscle hypertrophy in vivo.

Akirin2 is critical for inducing inflammatory genes by bridging IκB-ζ and the SWI/SNF complex

Transcription of inflammatory genes in innate immune cells is coordinately regulated by transcription factors, including NF-κB, and chromatin modifiers. However, it remains unclear how microbial sensing initiates chromatin remodeling. Here, we show that Akirin2, an evolutionarily conserved nuclear protein, bridges NF-κB and the chromatin remodeling SWI/SNF complex by interacting with BRG1-Associated Factor 60 (BAF60) proteins as well as IκB-ζ, which forms a complex with the NF-κB p50 subunit. These interactions are essential for Toll-like receptor-, RIG-I-, and Listeria-mediated expression of proinflammatory genes including Il6 and Il12b in macrophages. Consistently, effective clearance of Listeria infection required Akirin2. Furthermore, Akirin2 and IκB-ζ recruitment to the Il6 promoter depend upon the presence of IκB-ζ and Akirin2, respectively, for regulation of chromatin remodeling. BAF60 proteins were also essential for the induction of Il6 in response to LPS stimulation. Collectively, the IκB-ζ-Akirin2-BAF60 complex physically links the NF-κB and SWI/SNF complexes in innate immune cell activation. By recruiting SWI/SNF chromatin remodellers to IκB-ζ, transcriptional coactivator for NF-κB, the conserved nuclear protein Akirin2 stimulates pro-inflammatory gene promoters in mouse macrophages during innate immune responses to viral or bacterial infection.

Impaired structural and functional regeneration of skeletal muscles from β2-adrenoceptor knockout mice

Aims

β2-adrenergic stimulation causes beneficial effects on structure and function of regenerating muscles; thus, the β2-adrenoceptor may play an important role in the muscle regenerative process. Here, we investigated the role of the β₂-adrenoceptor in skeletal muscle regeneration.

Methods

Tibialis anterior (TA) muscles from β2-adrenoceptor knockout (β₂KO) mice were cryolesioned and analysed after 1, 3, 10 and 21 days. The role of β2-adrenoceptor on regenerating muscles was assessed through the analysis of morphological and contractile aspects, M1 and M2 macrophage profile, cAMP content, and activation of TGF-β signalling elements.

Results

Regenerating muscles from β₂KO mice showed decreased calibre of regenerating myofibres and reduced muscle contractile function at 10 days when compared with those from wild type. The increase in cAMP content in muscles at 10 days post-cryolesion was attenuated in the absence of the β₂-adrenoceptor. Furthermore, there was an increase in inflammation and in the number of macrophages in regenerating muscles lacking the β2-adrenoceptor at 3 and 10 days, a predominance of M1 macrophage phenotype, a decrease in TβR-I/Smad2/3 activation, and in the Smad4 expression at 3 days, while akirin1 expression increased at 10 days in muscles from β₂KO mice when compared to those from wild type.

Conclusions

Our results suggest that the β2-adrenoceptor contributes to the regulation of the initial phases of muscle regeneration, especially in the control of macrophage recruitment in regenerating muscle through activation of TβR-I/Smad2/3 and reduction in akirin1 expression. These findings have implications for the future development of better therapeutic approaches to prevent or treat muscle injuries.

The myokine decorin is regulated by contraction and involved in muscle hypertrophy

The health-promoting effects of regular exercise are well known, and myokines may mediate some of these effects. The small leucine-rich proteoglycan decorin has been described as a myokine for some time. However, its regulation and impact on skeletal muscle has not been investigated in detail. In this study, we report decorin to be differentially expressed and released in response to muscle contraction using different approaches. Decorin is released from contracting human myotubes, and circulating decorin levels are increased in response to acute resistance exercise in humans. Moreover, decorin expression in skeletal muscle is increased in humans and mice after chronic training. Because decorin directly binds myostatin, a potent inhibitor of muscle growth, we investigated a potential function of decorin in the regulation of skeletal muscle growth. In vivo overexpression of decorin in murine skeletal muscle promoted expression of the pro-myogenic factor Mighty, which is negatively regulated by myostatin. We also found Myod1 and follistatin to be increased in response to decorin overexpression. Moreover, muscle-specific ubiquitin ligases atrogin1 and MuRF1, which are involved in atrophic pathways, were reduced by decorin overexpression. In summary, our findings suggest that decorin secreted from myotubes in response to exercise is involved in the regulation of muscle hypertrophy and hence could play a role in exercise-related restructuring processes of skeletal muscle.

Vitamin D signaling in myogenesis: potential for treatment of sarcopenia

Muscle mass and strength progressively decrease with age, which results in a condition known as sarcopenia. Sarcopenia would lead to physical disability, poor quality of life, and death. Therefore, much is expected of an effective intervention for sarcopenia. Epidemiologic, clinical, and laboratory evidence suggest an effect of vitamin D on muscle function. However, the precise molecular and cellular mechanisms remain to be elucidated. Recent studies suggest that vitamin D receptor (VDR) might be expressed in muscle fibers and vitamin D signaling via VDR plays a role in the regulation of myoblast proliferation and differentiation. Understanding how vitamin D signaling contributes to myogenesis will provide a valuable insight into an effective nutritional strategy to moderate sarcopenia. Here we will summarize the current knowledge about the effect of vitamin D on skeletal muscle and myogenic cells and discuss the potential for treatment of sarcopenia.

Local overexpression of the myostatin propeptide increases glucose transporter expression and enhances skeletal muscle glucose disposal

Insulin resistance (IR) in skeletal muscle is a prerequisite for type 2 diabetes and is often associated with obesity. IR also develops alongside muscle atrophy in older individuals in sarcopenic obesity. The molecular defects that underpin this syndrome are not well characterized, and there is no licensed treatment. Deletion of the transforming growth factor-β family member myostatin, or sequestration of the active peptide by overexpression of the myostatin propeptide/latency-associated peptide (ProMyo) results in both muscle hypertrophy and reduced obesity and IR. We aimed to establish whether local myostatin inhibition would have a paracrine/autocrine effect to enhance glucose disposal beyond that simply generated by increased muscle mass, and the mechanisms involved. We directly injected adeno-associated virus expressing ProMyo in right tibialis cranialis/extensor digitorum longus muscles of rats and saline in left muscles and compared the effects after 17 days. Both test muscles were increased in size (by 7 and 11%) and showed increased radiolabeled 2-deoxyglucose uptake (26 and 47%) and glycogen storage (28 and 41%) per unit mass during an intraperitoneal glucose tolerance test. This was likely mediated through increased membrane protein levels of GLUT1 (19% higher) and GLUT4 (63% higher). Interestingly, phosphorylation of phosphoinositol 3-kinase signaling intermediates and AMP-activated kinase was slightly decreased, possibly because of reduced expression of insulin-like growth factor-I in these muscles. Thus, myostatin inhibition has direct effects to enhance glucose disposal in muscle beyond that expected of hypertrophy alone, and this approach may offer potential for the therapy of IR syndromes.

Identification and molecular characterization of an Akirin2 homolog in Chinese loach (Paramisgurnus dabryanus)

Akirin is a nuclear factor involved in innate immune responses of arthropods and mammals. In this study we have cloned an Akirin2 gene, pdakirin2, from freshwater Chinese loach (Paramisgurnus dabryanus) and characterized its biological functions. Phylogenetic analysis revealed deduced PdAkirin2 had high sequence identities to Akirin2 homologs from fish and mammals (70-91%), it contained two conserved nuclear localization signals (NLSs) with verified sub-cellular localization. Quantitative real-time (qRT)-PCR analysis indicated that PdAkirin2 was present in a wide range of loach tissues and showed up-regulation with challenges of Aeromonas hydrophila NJ-1, LPS and poly I:C. PdAkirin2 as an immune factor had significant effects on the expression of cytokines (TNFα, IFN-α, IFN-γ, IL-4 and IL-1β) and transcription factor NF-κB. This study provides insights into the potential role of PdAkirin2 in the innate immune system.

Akirin: a context-dependent link between transcription and chromatin remodeling

Embryonic patterning relies upon an exquisitely timed program of gene regulation. While the regulation of this process via the action of transcription factor networks is well understood, new lines of study have highlighted the importance of a concurrently regulated program of chromatin remodeling during development. Chromatin remodeling refers to the manipulation of the chromatin architecture through rearrangement, repositioning, or restructuring of nucleosomes to either favor or hinder the expression of associated genes. While the role of chromatin remodeling pathways during tumor development and cancer progression are beginning to be clarified, the roles of these pathways in the course of tissue specification, morphogenesis and patterning remains relatively unknown. Further, relatively little is understood as to the mechanism whereby developmentally critical transcription factors coordinate with chromatin remodeling factors to optimize target gene loci for gene expression. Such a mechanism might involve direct transcription factor/chromatin remodeling factor interactions, or could likely be mediated via an unknown intermediary. Our group has identified the relatively unknown protein Akirin as a putative member of this latter group: a secondary cofactor that serves as an interface between a developmentally critical transcription factor and the chromatin remodeling machinery. This role for the Akirin protein suggests a novel regulatory mode for regulating gene expression during development.