Akirin2 regulates proliferation and differentiation of porcine skeletal muscle satellite cells via ERK1/2 and NFATc1 signaling pathways

Comparative Transcriptome Analysis of mRNA and miRNA during the Development of Longissimus Dorsi Muscle of Gannan Yak and Tianzhu White Yak

The Gannan yak, a superior livestock breed found on the Tibetan Plateau, exhibits significantly enhanced body size, weight, and growth performance in comparison to the Tianzhu white yak. MiRNAs play a pivotal role in regulating muscle growth by negatively modulating target genes. In this study, we found the average diameter, area, and length of myofibers in Gannan yaks were significantly higher than those of Tianzhu white yaks. Further, we focused on analyzing the longissimus dorsi muscle from both Gannan yaks and Tianzhu white yaks through transcriptome sequencing to identify differentially expressed (DE)miRNAs that influence skeletal muscle development. A total of 254 DE miRNAs were identified, of which 126 miRNAs were up-regulated and 128 miRNAs were down-regulated. GO and KEGG enrichment analysis showed that the target genes of these DE miRNAs were significantly enriched in signaling pathways associated with muscle growth and development. By constructing a DE miRNA- DE mRNA interaction network, we screened 18 key miRNAs, and notably, four of the candidates (novel-m0143-3p, novel-m0024-3p, novel-m0128-5p, and novel-m0026-3p) targeted six genes associated with muscle growth and development (DDIT4, ADAMTS1, CRY2, AKIRIN2, SIX1, and FOXO1). These findings may provide theoretical references for further studies on the role of miRNAs in muscle growth and development in Gannan yaks.

Deletion of TECRL promotes skeletal muscle repair by up-regulating EGR2

Myogenic regeneration relies on the proliferation and differentiation of satellite cells. TECRL (trans-2,3-enoyl-CoA reductase like) is an endoplasmic reticulum protein only expressed in cardiac and skeletal muscle. However, its role in myogenesis remains unknown. We show that TECRL expression is increased in response to injury. Satellite cell-specific deletion of TECRL enhances muscle repair by increasing the expression of EGR2 through the activation of the ERK1/2 signaling pathway, which in turn promotes the expression of PAX7. We further show that TECRL deletion led to the upregulation of the histone acetyltransferase general control nonderepressible 5, which enhances the transcription of EGR2 through acetylation. Importantly, we showed that AAV9-mediated TECRL silencing improved muscle repair in mice. These findings shed light on myogenic regeneration and muscle repair.

MAPK family genes' influences on myogenesis in cattle: Genome-wide analysis and identification

The mitogen-activated protein kinase (MAPK) family is highly conserved in mammals, and is involved in a variety of physiological phenomena like regeneration, development, cell proliferation, and differentiation. In this study, 13 MAPK genes were identified in cattle and their corresponding protein properties were characterized using genome-wide identification and analysis. Phylogenetic analysis showed that the 13 BtMAPKs were cluster grouped into eight major evolutionary branches, which were segmented into three large subfamilies: ERK, p38 and JNK MAPK. BtMAPKs from the same subfamily had similar protein motif compositions, but considerably different exon-intron patterns. The heatmap analysis of transcriptome sequencing data showed that the expression of BtMAPKs was tissue-specific, with BtMAPK6 and BtMAPK12 highly expressed in muscle tissues. Furthermore, knockdown of BtMAPK6 and BtMAPK12 revealed that BtMAPK6 had no effect on myogenic cell proliferation, but negatively affected the differentiation of myogenic cells. In contrast, BtMAPK12 improved both the cell proliferation and differentiation. Taken together, these results provide novel insights into the functions of MAPK families in cattle, which could serve as a basis for further studies on the specific mechanisms of the genes in myogenesis.

Apol9a regulates myogenic differentiation via the ERK1/2 pathway in C2C12 cells

Background: The rising prevalence of obesity and its complications is a big challenge for the global public health. Obesity is accompanied by biological dysfunction of skeletal muscle and the development of muscle atrophy. The deep knowledge of key molecular mechanisms underlying myogenic differentiation is crucial for discovering novel targets for the treatment of obesity and obesity-related muscle atrophy. However, no effective target is currently known for obesity-induced skeletal muscle atrophy. Methods: Transcriptomic analyses were performed to identify genes associated with the regulation of myogenic differentiation and their potential mechanisms of action. C2C12 cells were used to assess the myogenic effect of Apol9a through immunocytochemistry, western blotting, quantitative polymerase chain reaction, RNA interference or overexpression, and lipidomics. Results: RNA-seq of differentiated and undifferentiated C2C12 cells revealed that Apol9a expression significantly increased following myogenic differentiation and decreased during obesity-induced muscle atrophy. Apol9a silencing in these C2C12 cells suppressed the expression of myogenesis-related genes and reduced the accumulation of intracellular triglycerides. Furthermore, RNA-seq and western blot results suggest that Apol9a regulates myogenic differentiation through the activation of extracellular signal-regulated kinase 1/2 (ERK1/2). This assumption was subsequently confirmed by intervention with PD98059. Conclusion: In this study, we found that Apol9a regulates myogenic differentiation via the ERK1/2 pathway. These results broaden the putative function of Apol9a during myogenic differentiation and provide a promising therapeutic target for intervention in obesity and obesity-induced muscle atrophy.

Satellite cell-specific deletion of Cipc alleviates myopathy in mdx mice

Skeletal muscle regeneration relies on satellite cells that can proliferate, differentiate, and form new myofibers upon injury. Emerging evidence suggests that misregulation of satellite cell fate and function influences the severity of Duchenne muscular dystrophy (DMD). The transcription factor Pax7 determines the myogenic identity and maintenance of the pool of satellite cells. The circadian clock regulates satellite cell proliferation and self-renewal. Here, we show that the CLOCK-interacting protein Circadian (CIPC) a negative-feedback regulator of the circadian clock, is up-regulated during myoblast differentiation. Specific deletion of Cipc in satellite cells alleviates myopathy, improves muscle function, and reduces fibrosis in mdx mice. Cipc deficiency leads to activation of the ERK1/2 and JNK1/2 signaling pathways, which activates the transcription factor SP1 to trigger the transcription of Pax7 and MyoD. Therefore, CIPC is a negative regulator of satellite cell function, and loss of Cipc in satellite cells promotes muscle regeneration.

Decoding the role of inflammation-related microRNAs in cancer cachexia: a study using HPV16-transgenic mice and in silico approaches

Cachexia is associated with poor prognosis in cancer patients, and inflammation is one of its main drive factors. MicroRNAs have recently emerged as important players in cancer cachexia and are involved in reciprocal regulation networks with pro-inflammatory signaling pathways. We hypothesize that inflammation-driven cancer cachexia is regulated by specific microRNAs. The aim of this study is to explore the expression and role of inflammation-related microRNAs in muscle wasting. HPV16-transgenic mice develop systemic inflammation and muscle wasting and are a model for cancer cachexia. We employed gastrocnemius muscle samples from these mice to study the expression of microRNAs. Bioinformatic tools were then used to explore their potential role in muscle wasting. Among the microRNAs studied, miR-223-3p (p = 0.004), let-7b-5p (p = 0.034), miR-21a-5p (p = 0.034), miR-150-5p (p = 0.027), and miR-155-5p (p = 0.011) were significantly upregulated in muscles from cachectic mice. In silico analysis showed that these microRNAs participate in several processes related to muscle wasting, including muscle structure development and regulation of the MAPK pathway. When analyzing protein-protein interactions (PPI)-networks, two major clusters and the top 10 hubs were obtained. From the top 10, Kras (p = 0.050) and Ccdn1 (p = 0.009) were downregulated in cachectic muscles, as well as Map2k3 (p = 0.007). These results show that miR-223-3p, let-7b-5p, miR-21a-5p, miR-150-5p, and miR-155-5p, play a role in muscle wasting in HPV16 transgenic mice, possible through regulating the MAPK cascades. Future experimental studies are required to validate our in silico analysis, and to explore the usefulness of these microRNAs and MAPK signaling as new potential biomarkers or therapy targets for cancer cachexia.

TLR4 activation inhibits the proliferation and osteogenic differentiation of skeletal muscle stem cells by downregulating LGI1

Skeletal muscle stem cells (SMSCs) are vital to the growth, maintenance, and repair of the muscles; emerging evidence has indicated that Toll-like receptor 4 (TLR4) can potentially regulate muscle regeneration. In present study, in vitro and in vivo experiments were performed to explore the correlation of TLR4 with leucine-rich glioma-inactivated 1 (LGI1) as well as their effects on the proliferation and osteogenesis potential of SMSCs. In order to examine the regulatory mechanisms of TLR4 and LGI1 in SMSCs, the obtained cells were treated with lipopolysaccharide (LPS, used as an activator of TLR4) of different concentration at different time points as well as the siRNA against LGI1. Subsequently, a series of detection was undertaken in order to measure the proliferation and differentiation potential of SMSCs, which involved detection of the related factors, cell activity, and the sphere-forming capability. Following LPS treatment, the increased TLR4 expression and reduced LGI1 expression were observed. Consequently, we also discovered that Erk signaling pathway was inactivated and cell proliferation and osteogenesis capabilities declined, presented by the downregulation of related factors such as cyclin B1 and runt-related transcription factor 2. Moreover, the cell activity and sphere-formation performance of SMSCs were also declined. These results were also validated in rats with cecal ligation and perforation-induced rat models with sepsis. In conclusion, the present study reveals a regulatory mechanism in SMSCs whereby LGI1 expression is reduced by TLR4, thus impeding cell proliferation and osteogenesis, highlighting TLR4 as a potential therapeutic target against many diseases related to SMSCs.

Ellagic acid enhances muscle endurance by affecting the muscle fiber type, mitochondrial biogenesis and function

Ellagic acid (EA) is a natural polyphenolic compound, which shows various effects, such as anti-inflammatory and antioxidant effects and inhibition of platelet aggregation. In this study, we investigated the effect of EA on muscle endurance and explored its possible underlying mechanism. Our data showed that EA significantly improved muscle endurance in mice. EA increased the protein level of slow myosin heavy chain (MyHC) I and decreased the protein level of fast MyHC. We also found that the AMP-activated protein kinase (AMPK) signaling pathway was activated by EA. Finally, our data indicated that EA could increase mitochondrial biogenesis and function by increasing the content of mitochondrial DNA (mtDNA), the concentration of ATP, the activities of succinodehydrogenase (SDH) and malate dehydrogenase (MDH), and the mRNA levels of ATP synthase (ATP5G), mtDNA transcription factor A (TFAM), mitochondrial transcription factor b1 (Tfb1m) and citrate synthase (Cs) in mice and C2C12 myotubes. These results proved that EA could enhance muscle endurance via transforming the muscle fiber type and improving mitochondrial biogenesis and function.

Contingent intramuscular boosting of P2XR7 axis improves motor function in transgenic ALS mice

Amyotrophic lateral sclerosis is a fatal neurodegenerative disorder that leads to progressive degeneration of motor neurons and severe muscle atrophy without effective treatment. Most research on the disease has been focused on studying motor neurons and supporting cells of the central nervous system. Strikingly, the recent observations have suggested that morpho-functional alterations in skeletal muscle precede motor neuron degeneration, bolstering the interest in studying muscle tissue as a potential target for the delivery of therapies. We previously showed that the systemic administration of the P2XR7 agonist, 2′(3′)-O‐(4-benzoylbenzoyl) adenosine 5-triphosphate (BzATP), enhanced the metabolism and promoted the myogenesis of new fibres in the skeletal muscles of SOD1G93A mice. Here we further corroborated this evidence showing that intramuscular administration of BzATP improved the motor performance of ALS mice by enhancing satellite cells and the muscle pro-regenerative activity of infiltrating macrophages. The preservation of the skeletal muscle retrogradely propagated along with the motor unit, suggesting that backward signalling from the muscle could impinge on motor neuron death. In addition to providing the basis for a suitable adjunct multisystem therapeutic approach in ALS, these data point out that the muscle should be at the centre of ALS research as a target tissue to address novel therapies in combination with those oriented to the CNS.

2-D08 treatment regulates C2C12 myoblast proliferation and differentiation via the Erk1/2 and proteasome signaling pathways

SUMOylation is one of the post-translational modifications that involves the covalent attachment of the small ubiquitin-like modifier (SUMO) to the substrate. SUMOylation regulates multiple biological processes, including myoblast proliferation, differentiation, and apoptosis. 2-D08 is a synthetically available flavone, which acts as a potent cell-permeable SUMOylation inhibitor. Its mechanism of action involves preventing the transfer of SUMO from the E2 thioester to the substrate without influencing SUMO-activating enzyme E1 (SAE-1/2) or E2 Ubc9-SUMO thioester formation. However, both the effects and mechanisms of 2-D08 on C2C12 myoblast cells remain unclear. In the present study, we found that treatment with 2-D08 inhibits C2C12 cell proliferation and differentiation. We confirmed that 2-D08 significantly hampers the viability of C2C12 cells. Additionally, it inhibited myogenic differentiation, decreasing myosin heavy chain (MHC), MyoD, and myogenin expression. Furthermore, we confirmed that 2-D08-mediated anti-myogenic effects impair myoblast differentiation and myotube formation, reducing the number of MHC-positive C2C12 cells. In addition, we found that 2-D08 induces the activation of ErK1/2 and the degradation of MyoD and myogenin in C2C12 cells. Taken together, these results indicated that 2-D08 treatment results in the deregulated proliferation and differentiation of myoblasts. However, further research is needed to investigate the long-term effects of 2-D08 on skeletal muscles.

Key Genes Regulating Skeletal Muscle Development and Growth in Farm Animals

Simple Summary

Skeletal muscle mass is an important economic trait, and muscle development and growth is a crucial factor to supply enough meat for human consumption. Thus, understanding (candidate) genes regulating skeletal muscle development is crucial for understanding molecular genetic regulation of muscle growth and can be benefit the meat industry toward the goal of increasing meat yields. During the past years, significant progress has been made for understanding these mechanisms, and thus, we decided to write a comprehensive review covering regulators and (candidate) genes crucial for muscle development and growth in farm animals. Detection of these genes and factors increases our understanding of muscle growth and development and is a great help for breeders to satisfy demands for meat production on a global scale.

Abstract

Farm-animal species play crucial roles in satisfying demands for meat on a global scale, and they are genetically being developed to enhance the efficiency of meat production. In particular, one of the important breeders’ aims is to increase skeletal muscle growth in farm animals. The enhancement of muscle development and growth is crucial to meet consumers’ demands regarding meat quality. Fetal skeletal muscle development involves myogenesis (with myoblast proliferation, differentiation, and fusion), fibrogenesis, and adipogenesis. Typically, myogenesis is regulated by a convoluted network of intrinsic and extrinsic factors monitored by myogenic regulatory factor genes in two or three phases, as well as genes that code for kinases. Marker-assisted selection relies on candidate genes related positively or negatively to muscle development and can be a strong supplement to classical selection strategies in farm animals. This comprehensive review covers important (candidate) genes that regulate muscle development and growth in farm animals (cattle, sheep, chicken, and pig). The identification of these genes is an important step toward the goal of increasing meat yields and improves meat quality.

Leucine regulates porcine muscle fiber type transformation via adiponectin signaling pathway

Leucine can promote slow-twitch muscle fibers formation, and this effect may be mediated by AMPK signaling pathway. In addition, adiponectin (AdipoQ) plays an important role in regulation of muscle fiber type transformation. AdipoQ is located in the upstream of AMPK and its secretion can be regulated by leucine. Therefore, the aim of this study was to explore whether leucine affects muscle fiber type transformation through AdipoQ signaling pathway. Our data showed that 4 mM leucine significantly increased protein expression levels of slow MyHC, Myoglobin, Troponin I-SS, AdipoQ, AdipoR1, phospho-AMPK (p-AMPK) and PGC-1α and mRNA expression levels of AMPKα2, PGC-1α, AdipoQ and AdipoR1, and significantly decreased fast MyHC protein expression. In addition, 4 mM leucine significantly increased the SDH activity while significantly decreased the LDH activity. However, knockdown of AdipoR1 expression by AdipoR1-siRNA abolished leucine-induced upregulation of protein expressions of slow MyHC, AdipoR1, p-AMPK, PGC-1α and NRF1, mRNA expressions of MyHC I, MyHC IIa, AdipoR1, AMPKα2 and PGC-1α, ATP5G, TFAM and NRF1, and mtDNA level, as well as downregulation of protein expression of fast MyHC and mRNA expression of MyHC IIb. Together, our data revealed that leucine promotes muscle fiber type transformation from fast-twitch to slow-twitch through AdipoQ signaling pathway.

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.

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.

Phosphorylated ERK1/2 protein levels are closely associated with the fast fiber phenotypes in rat hindlimb skeletal muscles

The relationship between the extracellular signal-regulated kinase 1 and 2 (ERK1/2), one of the mitogen-activated protein kinases (MAPKs), and mammalian skeletal muscle fiber phenotype is unclear. We looked at this relationship in three in vivo conditions in male Wistar rats. First, the levels of phosphorylated (active) ERK1/2 protein were closely associated with the fiber type composition of sedentary rat hindlimb muscles: highest in the superficial portion of the gastrocnemius (100% fast fibers), lower in the plantaris (~ 80% fast fibers), and lowest in the soleus (~ 15% fast fibers). Second, during growth, there was a gradual decrease in the percentage of fast fibers from 40% at 3 weeks to 1.5% at 65 weeks and a concomitant gradual decrease in the levels of phosphorylated ERK1/2 in the soleus muscle. Third, sciatic nerve denervation induced a significant decrease in the weight of both the soleus and plantaris, but a slow-to-fast fiber type shift and increase in phosphorylated ERK1/2 protein were observed only in the soleus. Although only a few fast and fast + slow hybrid fibers of the denervated soleus muscle reacted positively to the anti-phosphorylated ERK1/2 antibody by immuno-histochemical analysis, our results suggest that the phosphorylated form of ERK1/2 seems to be closely related to the fast fiber phenotype program. Further evidence for this relationship was provided by the observation that several slow fiber phenotype-specific proteins, i.e., Hsp72, Hsp60, and PGC-1, changed in the opposite direction of the levels of phosphorylated ERK1/2 protein.

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.

Fluid shear stress promotes osteoblast proliferation through the NFATc1-ERK5 pathway

PURPOSE

Extracellular-regulated kinase 5 (ERK5) is thought to regulate osteoblast proliferation. To further understand how ERK5 signaling regulates osteoblast proliferation induced by fluid shear stress (FSS), we examined some potential signaling targets associated with ERK5 in MC3T3-E1 cells.

METHODS

MC3T3-E1 cells were treated with XMD8-92 (an ERK5 inhibitor) or Cyclosporin A (CsA, a nuclear factor of activated T cells (NFAT) c1 inhibitor) and/or exposed to 12 dyn/cm² FSS. Phosphorylated-ERK5 (p-ERK5) and expression levels of NFATc1, ERK5, E2F2, and cyclin E1 were analyzed by western blot. The mRNA levels of genes associated with cell proliferation were analyzed by Polymerase Chain Reaction (PCR) array. Subcellular localization of p-ERK5 and NFATc1 were determined by immunofluorescence. Cell proliferation was evaluated by MTT assay.

RESULTS

NFATc1 expression was up-regulated by FSS. XMD8-92 only blocked ERK5 activation; however, CsA decreased NFATc1 and p-ERK5 levels, including after FSS stimulation. Exposure to NFATc1 inhibitor or ERK5 inhibitor resulted in decreased E2F2 and cyclin E1 expression and proliferation by proliferative MC3T3-E1 cells. Furthermore, immunofluorescence results illustrated that NFATc1 induced ERK5 phosphorylation, resulting in p-ERK5 translocation to the nucleus.

CONCLUSIONS

Our results reveal that NFATc1 acts as an intermediate to promote the phosphorylation of ERK5 induced by FSS. Moreover, activated NFATc1-ERK5 signaling up-regulates the expression of E2F2 and cyclin E1, which promote osteoblast proliferation.

Ferulic acid regulates muscle fiber type formation through the Sirt1/AMPK signaling pathway

Ferulic acid promotes slow-twitch and inhibits fast-twitch myofiber formation via Sirt1/AMPK.

Leucine regulates slow-twitch muscle fibers expression and mitochondrial function by Sirt1/AMPK signaling in porcine skeletal muscle satellite cells

A previous study demonstrated that leucine upregulates the slow myosin heavy chain mRNA expression in C2C12 cells. However, the role of leucine in slow-twitch muscle fibers expression and mitochondrial function of porcine skeletal muscle satellite cells as well as its mechanism remain unclear. In this study, porcine skeletal muscle satellite cells cultured in differentiation medium were treated with 2 mM leucine for 3 days. Sirt1 inhibitor EX527, AMPK inhibitor compound C, and AMPKα1 siRNA were used to examine its underlying mechanism. Here we showed that leucine increased slow-twitch muscle fibers and mitochondrial function-related gene expression, as well as increased succinic dehydrogenase (SDH) and malate dehydrogenase (MDH) activities. Moreover, leucine increased the protein levels of Sirt1 and phospho-AMPK. We also found that AMPKα1 siRNA, AMPK inhibitor compound C, or Sirt1 inhibitor EX527 attenuated the positive effect of leucine on slow-twitch muscle fibers and mitochondrial function-related gene expression. Finally, we showed that Sirt1 was required for leucine-induced AMPK activation. Our results provide, for the first time, evidence that leucine induces slow-twitch muscle fibers expression and improves mitochondrial function through Sirt1/AMPK signaling pathway in porcine skeletal muscle satellite cells.

An essential role for the nuclear protein Akirin2 in mouse limb interdigital tissue regression

The regulation of interdigital tissue regression requires the interplay of multiple spatiotemporally-controlled morphogen gradients to ensure proper limb formation and release of individual digits. Disruption to this process can lead to a number of limb abnormalities, including syndactyly. Akirins are highly conserved nuclear proteins that are known to interact with chromatin remodelling machinery at gene enhancers. In mammals, the analogue Akirin2 is essential for embryonic development and critical for a wide variety of roles in immune function, meiosis, myogenesis and brain development. Here we report a critical role for Akirin2 in the regulation of interdigital tissue regression in the mouse limb. Knockout of Akirin2 in limb epithelium leads to a loss of interdigital cell death and an increase in cell proliferation, resulting in retention of the interdigital web and soft-tissue syndactyly. This is associated with perdurance of Fgf8 expression in the ectoderm overlying the interdigital space. Our study supports a mechanism whereby Akirin2 is required for the downregulation of Fgf8 from the apical ectodermal ridge (AER) during limb development, and implies its requirement in signalling between interdigital mesenchymal cells and the AER.

Effects of Cyclic Mechanical Stretch on the Proliferation of L6 Myoblasts and Its Mechanisms: PI3K/Akt and MAPK Signal Pathways Regulated by IGF-1 Receptor

Myoblast proliferation is crucial to skeletal muscle hypertrophy and regeneration. Our previous study indicated that mechanical stretch altered the proliferation of C2C12 myoblasts, associated with insulin growth factor 1 (IGF-1)-mediated phosphoinositide 3-kinase (PI3K)/Akt (also known as protein kinase B) and mitogen-activated protein kinase (MAPK) pathways through IGF-1 receptor (IGF-1R). The purpose of this study was to explore the same stretches on the proliferation of L6 myoblasts and its association with IGF-1-regulated PI3K/Akt and MAPK activations. L6 myoblasts were divided into three groups: control, 15% stretch, and 20% stretch. Stretches were achieved using FlexCell Strain Unit. Cell proliferation and IGF-1 concentration were detected by CCK8 and ELISA, respectively. IGF-1R expression, and expressions and activities of PI3K, Akt, and MAPKs (including extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38) were determined by Western blot. We found that 15% stretch promoted, while 20% stretch inhibited L6 myoblast proliferation. A 15% stretch increased IGF-1R level, although had no effect on IGF-1 secretion of L6 myoblasts, and PI3K/Akt and ERK1/2 (not p38) inhibitors attenuated 15% stretch-induced pro-proliferation. Exogenous IGF-1 reversed 20% stretch-induced anti-proliferation, accompanied with increases in IGF-1R level as well as PI3K/Akt and MAPK (ERK1/2 and p38) activations. In conclusion, stretch regulated L6 myoblasts proliferation, which may be mediated by the changes in PI3K/Akt and MAPK activations regulated by IGF-1R, despite no detectable IGF-1 from stretched L6 myoblasts.

Arginine Promotes Slow Myosin Heavy Chain Expression via Akirin2 and the AMP-Activated Protein Kinase Signaling Pathway in Porcine Skeletal Muscle Satellite Cells

This study aimed to investigate the effect of arginine on the expression of slow myosin heavy chain (MyHC) I and its underlying mechanism in porcine skeletal muscle satellite cells. Our results showed that arginine upregulated the mRNA (1.54 ± 0.08; p < 0.01) and protein (2.01 ± 0.01; p < 0.001) levels of MyHC I. We also showed that arginine upregulated the expression of Akirin2 (1.35 ± 0.1; p < 0.05) and increased the NO content (1.56 ± 0.04; p < 0.001). Akirin2 siRNA abolished arginine-induced upregulation of MyHC I and the increase of the NO content. In addition, arginine significantly increased the phospho-AMP-activated protein kinase (AMPK)/AMPK level (1.33 ± 0.06; p < 0.05), the AMPK content (79.55 ± 0.13; p < 0.001), and the AMPKα2 mRNA level (2.03 ± 0.20; p < 0.01). AMPKα2 silencing or AMPK inhibitor Compound C abolished arginine-induced upregulation of MyHC I. Our results provide, for the first time, evidence for the involvement of Akirin2 and the AMPK signaling pathway in arginine-induced MyHC I expression in porcine skeletal muscle satellite cells.