Neuregulin-dependent protein synthesis in C2C12 myotubes and rat diaphragm muscle

Recombinant human neuregulin-1 alleviates immobilization-induced neuromuscular dysfunction via neuregulin-1/ErbB signaling pathway in rat

Immobilization-induced Neuromuscular Dysfunction (NMD) increases morbidity and mortality of patients in Intensive Care Units. However, the underlying mechanism of NMD remain poorly elucidated which limited the development of therapeutic method for NMD. Here we developed an immobilization rat model and tested the hypothesis that decreased expression of NRG-1, abnormal expression and distribution of nicotinic acetylcholine receptors (nAChRs) in skeletal muscle caused by immobilization can lead to NMD. To investigate the role of NRG-1/ErbB pathway on immobilization-induced NMD, exogenous recombinant human neuregulin-1 (rhNRG-1) was used to increase the expression of NRG-1 in skeletal muscle during immobilization. It was observed rhNRG-1 significantly alleviated the muscle loss and enhanced the expression of ε-nAChR, while diminished the expression of γ- and α7-nAChR and NMD. Interestingly, ErbB inhibitor PD158780 blocked the protective effects of rhNRG-1. Collectively, the results of present study suggested that rhNRG-1 attenuated immobilization-induced muscle loss and NMD, suppressed γ- and α7-nAChR production, enhanced ε-nAChR synthesis via activating NRG-1/ErbB pathway. Taken together, our findings provide novel insights into NMD contribution, suggesting that the rhNRG-1 is a promising therapy to protect against immobilization-induced myopathy.

Neuregulin-1β Protects the Rat Diaphragm during Sepsis against Oxidative Stress and Inflammation by Activating the PI3K/Akt Pathway

Sepsis-induced diaphragm dysfunction (SIDD) which is mainly characterized by decrease in diaphragmatic contractility has been identified to cause great harms to patients. Therefore, there is an important and pressing need to find effective treatments for improving SIDD. In addition, acetylcholinesterase (AChE) activity is a vital property of the diaphragm, so we evaluated both diaphragmatic contractility and AChE activity. Though neuregulin-1β (NRG-1β) is known to exert organ-protective effects in some inflammatory diseases, little is known about the potential of NRG-1β therapy in the diaphragm during sepsis. Our study was aimed at exploring the effects of NRG-1β application on diaphragmatic contractility and AChE activity during sepsis. Proinflammatory cytokines, muscle injury biomarkers in serum, contractile force, AChE activity, proinflammatory cytokines, oxidative parameters, histological condition, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining, and expression of phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB/Akt) signaling proteins in the diaphragm were measured and compared between nonseptic and septic groups with or without NRG-1β treatment. In vitro, the effects of NRG-1β on reactive oxygen species (ROS) production in the lipopolysaccharide- (LPS-) stimulated L6 rat muscle skeletal cells with or without the Akt inhibitor MK-2206 were detected. NRG-1β inhibited proinflammatory cytokine release and muscle injury biomarkers soaring in serum and improved the sepsis-induced diaphragm dysfunction and AChE activity decrease significantly during sepsis. Meanwhile, the inflammatory response, oxidative stress, pathological impairment, and cell apoptosis in the diaphragm were mitigated by NRG-1β. And NRG-1β activated the PI3K/Akt signaling in the diaphragm of septic rats. Elevated ROS production in the LPS-stimulated L6 rat skeletal muscle cells was reduced after treatment with NRG-1β, while MK-2206 blocked these effects of NRG-1β. In conclusion, our findings underlined that NRG-1β could reduce circulating levels of proinflammatory cytokines in rats with sepsis, adjust diaphragmatic proinflammatory cytokine level, mitigate diaphragmatic oxidative injury, and lessen diaphragm cell apoptosis, thereby improving diaphragmatic function, and play a role in diaphragmatic protection by activating PI3K/Akt signaling.

Transcriptomic Profile of Primary Culture of Skeletal Muscle Cells Isolated from Semitendinosus Muscle of Beef and Dairy Bulls

The aim of the study was to identify differences in the transcriptomic profiles of primary muscle cell cultures derived from the semitendinosus muscle of bulls of beef breeds (Limousin (LIM) and Hereford (HER)) and a dairy breed (Holstein-Friesian (HF)) (n = 4 for each breed). Finding a common expression pattern for proliferating cells may point to such an early orientation of the cattle beef phenotype at the transcriptome level of unfused myogenic cells. To check this hypothesis, microarray analyses were performed. The analysis revealed 825 upregulated and 1300 downregulated transcripts similar in both beef breeds (LIM and HER) and significantly different when compared with the dairy breed (HF) used as a reference. Ontological analyses showed that the largest group of genes were involved in muscle organ development. Muscle cells of beef breeds showed higher expression of genes involved in myogenesis (including erbb-3, myf5, myog, des, igf-1, tgfb2) and those encoding proteins comprising the contractile apparatus (acta1, actc1, myh3, myh11, myl1, myl2, myl4, tpm1, tnnt2, tnnc1). The obtained results confirmed our hypothesis that the expression profile of several groups of genes is common in beef breeds at the level of proliferating satellite cells but differs from that observed in typical dairy breeds.

Sarcopenia in Cirrhosis: Looking Beyond the Skeletal Muscle Loss to See the Systemic Disease

Sarcopenia is a common complication of cirrhosis and is defined as a progressive and generalized loss of skeletal muscle mass, strength, and function. Sarcopenia is associated with poor prognosis and increased mortality. How sarcopenia and muscle wasting relate to such poor outcomes requires looking beyond the overt muscle loss and at this entity as a systemic disease that affects muscles of vital organs including cardiac and respiratory muscles. This review explores the pathophysiological pathways and mechanisms that culminate in poor outcomes associated with sarcopenia. This provides a launching pad to identify potential targets for therapeutic intervention and optimization to improve patient outcomes.

Phenethyl isothiocyanate stimulates glucose uptake through the Akt pathway in C2C12 myotubes

Phenethyl isothiocyanate (PEITC) is an aromatic isothiocyanate present in cruciferous vegetables. Several studies have shown that isothiocyanates regulate various intracellular signaling pathways, and thereby show anti-inflammatory and detoxifying activities. However, little is known about the effects of PEITC on glucose metabolism. In this study, we examined whether PEITC promotes glucose utilization in mouse skeletal muscle cells, C2C12 myotubes. PEITC induced glucose uptake, glucose transporter 4 (Glut4) translocation to the plasma membrane, and activation of Akt and ERK in C2C12 cells. Inhibition of Akt suppressed PEITC-induced Glut4 translocation and glucose uptake, whereas ERK inhibition did not. Furthermore, PEITC increased phosphorylation of ErbB2 and ErbB3. Treatment with a pan-ErbB inhibitor reduced Akt activation and the subsequent glucose uptake induced by PEITC. These results indicate that PEITC promotes glucose utilization through the ErbB/Akt pathway in C2C12 myotubes. PEITC may therefore serve as a dietary constituent with beneficial effects on the carbohydrate metabolism.

Abbreviations: PEITC: phenethyl isothiocyanate; Glut4: glucose transporter 4; PI3K: phosphatidylinositide 3-kinase; Nrf2: erythroid−2-related factor; ARE: antioxidant response element; HO−1: heme oxygenase−1; NRG: neuregulin

Neuregulin-1β attenuates sepsis-induced diaphragm atrophy by activating the PI3K/Akt signaling pathway

The aim of this study was to investigate the protective effects of neuregulin-1β (NRG-1β) on sepsis-induced diaphragm atrophy and the possible underlying mechanisms. Sprague-Dawley rats were randomly divided into sham, sepsis and NRG groups. Sepsis was induced by cecal ligation and puncture (CLP). In the NRG group, rats received tail vein injections of NRG-1β (10 μg/kg) every 12 h for 72 h after CLP. At 3 days after surgery, diaphragm contractile forces were measured by determining the force-frequency curve and muscle fiber areas by hematoxylin-eosin staining. Moreover, the NRG-1 expression level in the diaphragm was detected by Western blotting. Furthermore, the proteins in the PI3K/Akt signaling pathway and its downstream Akt-mTOR and Akt-FOXO axes were detected by Western blotting analysis. In L6 myotubes treated with lipopolysaccharide (LPS) and NRG-1β, PI3K/Akt signaling pathway-related protein expression was further determined using the PI3K inhibitor LY294002. Exogenous NRG-1β could compensate for sepsis-induced diminished NRG-1 in the diaphragm and attenuate the reduction in diaphragm contractile forces and muscle fiber areas during sepsis. Moreover, NRG-1β treatment could activate the PI3K/Akt signaling pathway in the diaphragm during sepsis. The inhibition of p70S6K and 4E-BP1 on the Akt-mTOR axis and the increased expression of Murf1 on the Akt-FOXO axis were reversed after NRG-1 treatment. In addition, NRG-1β could activate the PI3K/Akt signaling pathway in L6 myotubes treated with LPS, while the PI3K inhibitor LY294002 blocked the effects of NRG-1β. NRG-1 expression in the diaphragm was reduced during sepsis, and exogenously administered recombinant human NRG-1β could attenuate sepsis-induced diaphragm atrophy by activating the PI3K/Akt signaling pathway.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Effects of different types of exercise on skeletal muscle atrophy, antioxidant capacity and growth factors expression following myocardial infarction

AIMS

Myocardial infarction (MI) is accompanied with skeletal muscle abnormalities. The aims are to explore an optimal exercise mode to improve cardiac function and prevent skeletal muscle atrophy, and detect the possible mechanisms of exercise-induced inhibition of muscle atrophy.

MAIN METHODS

Rats were subjected to four weeks of different types of exercise after MI surgery (resistance training, RT; moderated-intensity continuous aerobic exercise, MCE and high-intensity intermittent aerobic exercise, HIA). Cardiac function, histological changes of heart and skeletal muscle, oxidative stress, antioxidant capacity and the expression of muscle atrophy-related factors were detected in skeletal muscle.

KEY FINDINGS

The three types of exercise improved heart function, reduced cardiac fibrosis and increased muscle weight and cross-section area (CSA) of muscle fibers in different degrees. The survival rates of MI rats intervened by RT and MCE were higher than HIA. Exercise down-regulated the mRNA levels of murf1 and atrogin-1, decreased reactive oxygen species level, increased antioxidant capacity, regulated the expression of insulin-like growth factor 1 (IGF1), mechano growth factor (MGF), Neuregulin1 (NRG1) and Myostatin (MSTN), and activated Akt and Erk1/2 signalings in soleus muscle. Furthermore, CSA of muscle fibers and the expression of IGF1, MGF, NRG1 in skeletal muscle had correlations with cardiac function.

SIGNIFICANCE

RT and MCE are the first two choices for the early exercise rehabilitation following MI. All types of exercise can effectively inhibit skeletal muscle atrophy through increasing the antioxidant capacity, reducing oxidative stress and protein degradation, and regulating the growth factors expression in skeletal muscle.

Neuregulin 1 improves complex 2-mediated mitochondrial respiration in skeletal muscle of healthy and diabetic mice

It has been reported that neuregulin1 (NRG1) improves glucose tolerance in healthy and diabetic rodents. In vitro studies also suggest that NRG1 regulates myocyte oxidative capacity. To confirm this observation in vivo, we evaluated the effect on mitochondrial function of an 8-week treatment with NRG1 in db/db diabetic mice and C57BL/6JRJ healthy controls. NRG1 treatment improved complex 2-mediated mitochondrial respiration in the gastrocnemius of both control and diabetic mice and increased mitochondrial complex 2 subunit content by 2-fold. This effect was not associated with an increase in mitochondrial biogenesis markers. Enhanced ERBB4 phosphorylation could mediate NRG1 effects on mitochondrial function through signalling pathways, independently of ERK1/2, AKT or AMPK.

Activin A more prominently regulates muscle mass in primates than does GDF8

Growth and differentiation factor 8 (GDF8) is a TGF-β superfamily member, and negative regulator of skeletal muscle mass. GDF8 inhibition results in prominent muscle growth in mice, but less impressive hypertrophy in primates, including man. Broad TGF-β inhibition suggests another family member negatively regulates muscle mass, and its blockade enhances muscle growth seen with GDF8-specific inhibition. Here we show that activin A is the long-sought second negative muscle regulator. Activin A specific inhibition, on top of GDF8 inhibition, leads to pronounced muscle hypertrophy and force production in mice and monkeys. Inhibition of these two ligands mimics the hypertrophy seen with broad TGF-β blockers, while avoiding the adverse effects due to inhibition of multiple family members. Altogether, we identify activin A as a second negative regulator of muscle mass, and suggest that inhibition of both ligands provides a preferred therapeutic approach, which maximizes the benefit:risk ratio for muscle diseases in man.

Inhibition of GDF8 increases muscle mass in mice, but is less effective in monkeys and humans. Here the authors show that activin A also inhibits muscle hypertrophy and that concomitant inhibition of activin A and GDF8 synergistically increases muscle mass in mice and non-human primates.

Hormonal Treatment Effects on the Cross-sectional Area of Pubococcygeus Muscle Fibers After Denervation and Castration in Male Rats

We explore the interaction of muscle innervation and gonadal hormone action in the pubococcygeus muscle (Pcm) after castration and hormone replacement. Male Wistar rats were castrated and the Pcm was unilaterally denervated; after 2 or 6 weeks, the cross-sectional area (CSA) of Pcm fibers was assessed. Additional groups of castrated rats were used to examine the effects of hormone replacement. At 2 weeks post surgeries, rats were implanted with Silastic capsules containing either dihydrotestosterone (DHT), estradiol benzoate (EB) or both hormones, and the CSA of Pcm fibers was assessed after 4 weeks of hormone treatment. At 2 weeks post surgeries, gonadectomy without hormone replacement resulted in reductions in the CSA of Pcm fibers, and denervation combined with castration increased the magnitude of this effect; further reductions in CSA were present at 6 weeks post surgeries, but again denervation combined with castration increased the magnitude of this effect. Hormone replacement with DHT resulted in hypertrophy in the CSA of nondenervated muscles compared to those of intact normal males, but this effect was attenuated in denervated muscles. Hormone replacement with EB treatment prevented further castration-induced reductions in CSA of nondenervated muscles, but denervation prevented this effect. Similar to that seen with treatment with EB alone, combined treatment with both DHT and EB prevented further reductions in CSA of Pcm fibers in nondenervated muscles, but again denervation attenuated this effect. Thus, while hormone replacement can reverse or prevent further castration-induced atrophy of Pcm fibers, these effects are dependent on muscle innervation. Anat Rec, 300:1327-1335, 2017. © 2017 Wiley Periodicals, Inc.

Connexin hemichannels explain the ionic imbalance and lead to atrophy in denervated skeletal muscles

Denervated fast skeletal muscles undergo atrophy, which is associated with an increase in sarcolemma permeability and protein imbalance. However, the mechanisms responsible for these alterations remain largely unknown. Recently, a close association between de novo expression of hemichannels formed by connexins 43 and 45 and increase in sarcolemma permeability of denervated fast skeletal myofibers was demonstrated. However, it remains unknown whether these connexins cause the ionic imbalance of denervates fast myofibers. To elucidate the latter and the role of hemichannels formed by connexins (Cx HCs) in denervation-induced atrophy, skeletal myofibers deficient in Cx43 and Cx45 expression (Cx43^fl/flCx45^fl/fl:Myo-Cre mice) and control (Cx43^fl/flCx45^fl/fl mice) were denervated and several muscle features were systematically analyzed at different post-denervation (PD) times (1, 3, 5, 7 and 14days). The following sequence of events was found in denervated myofibers of Cx43^fl/flCx45^fl/fl mice: 1) from day 3 PD, increase in sarcolemmal permeability, 2) from day 5 PD, increases of intracellular Ca²⁺ and Na⁺ signals as well as a significant increase in protein synthesis and degradation, yielding a negative protein balance and 3) from day 7 PD, a fall in myofibers cross-section area. All the above alterations were either absent or drastically reduced in denervated myofibers of Cx43^fl/flCx45^fl/fl:Myo-Cre mice. Thus, the denervation-induced Cx HCs expression is an early event that precedes the electrochemical gradient dysregulation across the sarcolemma and critically contributes to the progression of skeletal muscle atrophy. Consequently, Cx HCs could be a therapeutic target to drastically prevent the denervation-induced atrophy of fast skeletal muscles.

Functional impact of sarcopenia in respiratory muscles

The risk for respiratory complications and infections is substantially increased in old age, which may be due, in part, to sarcopenia (aging-related weakness and atrophy) of the diaphragm muscle (DIAm), reducing its force generating capacity and impairing the ability to perform expulsive non-ventilatory motor behaviors critical for airway clearance. The aging-related reduction in DIAm force generating capacity is due to selective atrophy of higher force generating type IIx and/or IIb muscle fibers, whereas lower force generating type I and IIa muscle fiber sizes are preserved. Fiber type specific DIAm atrophy is also seen following unilateral phrenic nerve denervation and in other neurodegenerative disorders. Accordingly, the effect of aging on DIAm function resembles that of neurodegeneration and suggests possible common mechanisms, such as the involvement of several neurotrophic factors in mediating DIAm sarcopenia. This review will focus on changes in two neurotrophic signaling pathways that represent potential mechanisms underlying the aging-related fiber type specific DIAm atrophy.

ErbB3 binding protein-1 (Ebp1) controls proliferation and myogenic differentiation of muscle stem cells

Satellite cells are resident stem cells of skeletal muscle, supplying myoblasts for post-natal muscle growth, hypertrophy and repair. Many regulatory networks control satellite cell function, which includes EGF signalling via the ErbB family of receptors. Here we investigated the role of ErbB3 binding protein-1 (Ebp1) in regulation of myogenic stem cell proliferation and differentiation. Ebp1 is a well-conserved DNA/RNA binding protein that is implicated in cell growth, apoptosis and differentiation in many cell types. Of the two main Ebp1 isoforms, only p48 was expressed in satellite cells and C2C12 myoblasts. Although not present in quiescent satellite cells, p48 was strongly induced during activation, remaining at high levels during proliferation and differentiation. While retroviral-mediated over-expression of Ebp1 had only minor effects, siRNA-mediated Ebp1 knockdown inhibited both proliferation and differentiation of satellite cells and C2C12 myoblasts, with a clear failure of myotube formation. Ebp1-knockdown significantly reduced ErbB3 receptor levels, yet over-expression of ErbB3 in Ebp1 knockdown cells did not rescue differentiation. Ebp1 was also expressed by muscle cells during developmental myogenesis in mouse. Since Ebp1 is well-conserved between mouse and chick, we switched to chick to examine its role in muscle formation. In chick embryo, Ebp1 was expressed in the dermomyotome, and myogenic differentiation of muscle progenitors was inhibited by specific Ebp1 down-regulation using shRNA electroporation. These observations demonstrate a conserved function of Ebp1 in the regulation of embryonic muscle progenitors and adult muscle stem cells, which likely operates independently of ErbB3 signaling.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Systems biology of skeletal muscle: fiber type as an organizing principle

Skeletal muscle force generation and contraction are fundamental to countless aspects of human life. The complexity of skeletal muscle physiology is simplified by fiber type classification where differences are observed from neuromuscular transmission to release of intracellular Ca(2+) from the sarcoplasmic reticulum and the resulting recruitment and cycling of cross-bridges. This review uses fiber type classification as an organizing and simplifying principle to explore the complex interactions between the major proteins involved in muscle force generation and contraction.

Respiratory muscle plasticity

Muscle plasticity is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it. As such, respiratory muscle is in a constant state of remodeling, and the basis of muscle's plasticity is its ability to change protein expression and resultant protein balance in response to varying environmental conditions. Here, we will describe the changes of respiratory muscle imposed by extrinsic changes in mechanical load, activity, and innervation. Although there is a large body of literature on the structural and functional plasticity of respiratory muscles, we are only beginning to understand the molecular-scale protein changes that contribute to protein balance. We will give an overview of key mechanisms regulating protein synthesis and protein degradation, as well as the complex interactions between them. We suggest future application of a systems biology approach that would develop a mathematical model of protein balance and greatly improve treatments in a variety of clinical settings related to maintaining both muscle mass and optimal contractile function of respiratory muscles.

Upregulation of the CaV 1.1-ryanodine receptor complex in a rat model of critical illness myopathy

The processes that trigger severe muscle atrophy and loss of myosin in critical illness myopathy (CIM) are poorly understood. It has been reported that muscle disuse alters Ca(2+) handling by the sarcoplasmic reticulum. Since inactivity is an important contributor to CIM, this finding raises the possibility that elevated levels of the proteins involved in Ca(2+) handling might contribute to development of CIM. CIM was induced in 3- to 5-mo-old rats by sciatic nerve lesion and infusion of dexamethasone for 1 wk. Western blot analysis revealed increased levels of ryanodine receptor (RYR) isoforms-1 and -2 as well as the dihydropyridine receptor/voltage-gated calcium channel type 1.1 (DHPR/Ca(V) 1.1). Immunostaining revealed a subset of fibers with elevation of RYR1 and Ca(V) 1.1 that had severe atrophy and disorganization of sarcomeres. These findings suggest increased Ca(2+) release from the sarcoplasmic reticulum may be an important contributor to development of CIM. To assess the endogenous functional effects of increased intracellular Ca(2+) in CIM, proteolysis of α-fodrin, a well-known target substrate of Ca(2+)-activated proteases, was measured and found to be 50% greater in CIM. There was also selective degradation of myosin heavy chain relative to actin in CIM muscle. Taken together, our findings suggest that increased Ca(2+) release from the sarcoplasmic reticulum may contribute to pathology in CIM.

Intracellular signaling pathways regulating net protein balance following diaphragm muscle denervation

Unilateral denervation (DNV) of rat diaphragm muscle increases protein synthesis at 3 days after DNV (DNV-3D) and degradation at DNV-5D, such that net protein breakdown is evident by DNV-5D. On the basis of existing models of protein balance, we examined DNV-induced changes in Akt, AMP-activated protein kinase (AMPK), and ERK&frac12; activation, which can lead to increased protein synthesis via mammalian target of rapamycin (mTOR)/p70S6 kinase (p70S6K), glycogen synthase kinase-3β (GSK3β), or eukaryotic initiation factor 4E (eIF4E), and increased protein degradation via forkhead box protein O (FoxO). Protein phosphorylation was measured using Western analyses through DNV-5D. Akt phosphorylation decreased at 1 h and 6 h after DNV compared with sham despite decreased AMPK phosphorylation. Both Akt and AMPK phosphorylation returned to sham levels by DNV-1D. Phosphorylation of their downstream effector mTOR (Ser2481) did not change at any time point after DNV, and phosphorylated p70S6K and eIF4E-binding protein 1 (4EBP1) increased only by DNV-5D. In contrast, ERK&frac12; phosphorylation and its downstream effector eIF4E increased 1.7-fold at DNV-1D and phosphorylated GSK3β increased 1.5-fold at DNV-3D (P < 0.05 for both comparisons). Thus, following DNV there are differential effects on protein synthetic pathways with preferential activation of GSK3β and eIF4E over p70S6K. FoxO1 nuclear translocation occurred by DNV-1D, consistent with its role in increasing expression of atrogenes necessary for subsequent ubiquitin-proteasome activation evident by DNV-5D. On the basis of our results, increased protein synthesis following DNV is associated with changes in ERK&frac12;-dependent pathways, but protein degradation results from downregulation of Akt and nuclear translocation of FoxO1. No single trigger is responsible for protein balance following DNV. Protein balance in skeletal muscle depends on multiple synthetic/degradation pathways that should be studied in concert.

Emerging role of neuregulin as a modulator of muscle metabolism

Neuregulin was described initially as a neurotrophic factor involved in the formation of the neuromuscular junction in skeletal muscle. However, in recent years, neuregulin has been reported to be a myokine that exerts relevant effects on myogenesis and the regulation of muscle metabolism. In this new context, the rapid and chronic metabolic effects of neuregulin appear to be related to muscle contraction. Indeed, the effects of neuregulin resemble those of exercise, which are accompanied by an improvement in insulin sensitivity. In this review, we challenge the classical role assigned to neuregulin in muscle and propound the emerging concept of its involvement in the regulation of energetic metabolism and insulin responsiveness.

Neuromuscular adaptations to respiratory muscle inactivity

Cervical spinal cord injury results in significant functional impairment. It is important to understand the neuroplasticity in response to inactivity of respiratory muscles in order to prevent any associated effects that limit functional recovery. Recent studies have examined the mechanisms involved in inactivity-induced neuroplasticity of diaphragm motor units. Both spinal hemisection at C2 (C2HS) and tetrodotoxin (TTX)-induced phrenic nerve blockade result in diaphragm paralysis and inactivity of axon terminals. However, phrenic motoneurons are inactive with C2HS but remain active after TTX. Diaphragm muscle fibers ipsilateral to C2HS display minimal changes post-injury. Neuromuscular transmission is enhanced following C2HS but impaired following TTX. Synaptic vesicle pool size at diaphragm neuromuscular junctions increases after C2HS, but decreases after TTX. Thus, inactivity-induced neuromuscular plasticity reflects specific adaptations that depend on inactivity at the motoneuron rather than at axon terminals or muscle fibers. These results indicate that neuromuscular transmission and functional properties of diaphragm fibers can be maintained after spinal cord injury, providing a substrate for functional recovery and/or specific therapeutic approaches such as phrenic pacing.

The effect of denervation on protein synthesis and degradation in adult rat diaphragm muscle

Previous studies showed that unilateral denervation (DNV) of the rat diaphragm muscle (DIAm) results in loss of myosin heavy chain protein by 1 day after DNV. We hypothesize that DNV decreases net protein balance as a result of activation of the ubiquitin-proteasome pathway. In DIAm strips, protein synthesis was measured by incorporation of 3H-Tyr, and protein degradation was measured by Tyr release at 1, 3, 5, 7, and 14 days after DNV. Total protein ubiquitination, caspase-3 expression/activity, and actin fragmentation were analyzed by Western analysis. We found that, at 3 days after DNV, protein synthesis increased by 77% relative to sham controls. Protein synthesis remained elevated at 5 (85%), 7 (53%), and 14 days (123%) after DNV. At 5 days after DNV, protein degradation increased by 43% relative to sham controls and remained elevated at 7 (49%) and 14 days (74%) after DNV. Thus, by 5 days after DNV, net protein balance decreased by 43% compared with sham controls and was decreased compared with sham at 7 (49%) and 14 days (72%) after DNV. Protein ubiquitination increased at 5 days after DNV and remained elevated. DNV had no effect on caspase-3 activity or actin fragmentation, suggesting that the ubiquitin-proteasome pathway rather than caspase-3 activation is important in the DIAm response to DNV. Early loss of contractile proteins, such as myosin heavy chain, is likely the result of selective protein degradation rather than generalized protein breakdown. Future studies should evaluate this selective effect of DNV.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

Phytoecdysteroids increase protein synthesis in skeletal muscle cells

Phytoecdysteroids, which are structurally similar or identical to insect molting hormones, produce a range of effects in mammals, including increasing growth and physical performance. To study the mechanism of action of phytoecdysteroids in mammalian tissue, an in vitro cellular assay of protein synthesis was developed. In C2C12 murine myotubes and human primary myotubes, phytoecdysteroids increased protein synthesis by up to 20%. In vivo, ecdysteroids increased rat grip strength. Ecdysteroid-containing plant extracts produced similar results. The effect was inhibited by a phosphoinositide kinase-3 inhibitor, which suggests a PI3K-mediated mechanism.

Key aspects of phrenic motoneuron and diaphragm muscle development during the perinatal period

At the time of birth, respiratory muscles must be activated to sustain ventilation. The perinatal development of respiratory motor units (comprising an individual motoneuron and the muscle fibers it innervates) shows remarkable features that enable mammals to transition from in utero conditions to the air environment in which the remainder of their life will occur. In addition, significant postnatal maturation is necessary to provide for the range of motor behaviors necessary during breathing, swallowing, and speech. As the main inspiratory muscle, the diaphragm muscle (and the phrenic motoneurons that innervate it) plays a key role in accomplishing these behaviors. Considerable diversity exists across diaphragm motor units, but the determinant factors for this diversity are unknown. In recent years, the mechanisms underlying the development of respiratory motor units have received great attention, and this knowledge may provide the opportunity to design appropriate interventions for the treatment of respiratory disease not only in the perinatal period but likely also in the adult.