Differential gene expression profiling of short and long term denervated muscle

Establishing a Murine Model of Muscle Changes in Chronic Nerve Compression

PURPOSE

We aimed to build upon a previously validated model of early chronic nerve compression (CNC) by evaluating changes in gross muscle weight, muscle gene expression, and muscle function, and correlating the mechanism and timing of muscle- and nerve related changes.

METHODS

Chronic nerve compression was induced by placing a Silastic tube around the sciatic nerve with the contralateral limb as control. At 6, 8, and 12 weeks of compression, gait analysis, muscle force measurements, and electrodiagnostics (EDX) were performed, and the sciatic nerve, tibialis anterior (TA), extensor digitorum longus (EDL), and gastrocnemius were harvested. Muscle weight (MW), cross-sectional area (CSA), g-ratio, axon area, and axon density were measured. Reverse transcripton polymerase chain reaction of TA+EDL muscle was performed. Genes assayed included atrogenes (Foxo-3, Atrogin-1, and MuRF1), markers of myogenesis (MyoD and MyoG), fatty acid synthase, type-I collagen (Col1a1), and inflammatory markers (tumor necrosis factor-α and interleukin-1β).

RESULTS

At 6 weeks, we observed a maximum 30.8% decrease in nerve conduction speed. G-ratio was increased 14.4% at 8 weeks, and at all time points, we observed a 25%-26% decrease in axon area. At 12 weeks, we observed a 10.4% decrease in TA+EDL MW, and at 8 weeks, CSA was reduced 13.9%. At 8 weeks, expression of atrogenes was increased 2-3-fold implying ongoing atrophy. MyoD/MyoG expression was reduced 0.3 times, and fatty acid synthase, type-1 collagen, and inflammatory marker expression was increased 1.3-, 1.4-, and >2-fold, respectively. There were no clinically important differences in gait analysis or muscle force measurement between compressed and control limbs at any time-point.

CONCLUSIONS

The murine model of muscle changes in CNC demonstrates reduced nerve conduction speed, demyelination, and a shift in axon size consistent with early CNC. Changes in MW, CSA, and gene expression occur in the absence of significant differences in muscle function.

CLINICAL RELEVANCE

These findings establish a mouse model of early muscle changes in CNC that can be used to investigate interventions to reduce or delay muscle changes in compression neuropathies.

Unravelling the complexities of non-alcoholic steatohepatitis: The role of metabolism, transporters, and herb-drug interactions

Nonalcoholic fatty liver disease (NAFLD) is a mainstream halting liver disease with high prevalence in North America, Europe, and other world regions. It is an advanced form of NAFLD caused by the amassing of fat in the liver and can progress to the more severe form known as non-alcoholic steatohepatitis (NASH). Until recently, there was no authorized pharmacotherapy reported for NASH, and to improve the patient's metabolic syndrome, the focus is mainly on lifestyle modification, weight loss, ensuring a healthy diet, and increased physical activity; however, the recent approval of Rezdiffra (Resmetirom) by the US FDA may change this narrative. As per the reported studies, there is an increased articulation of uptake and efflux transporters of the liver, including OATP and MRP, in NASH, leading to changes in the drug's pharmacokinetic properties. This increase leads to alterations in the pharmacokinetic properties of drugs. Furthermore, modifications in Cytochrome P450 (CYP) enzymes can have a significant impact on these properties. Xenobiotics are metabolized primarily in the liver and constitute liver enzymes and transporters. This review aims to delve into the role of metabolism, transport, and potential herb-drug interactions in the context of NASH.

MiR-21 regulates skeletal muscle atrophy and fibrosis by targeting TGF-beta/SMAD7-SMAD2/3 signaling pathway

Long-term denervation-induced atrophy and fibrosis of skeletal muscle due to denervation leads to poor recovery of muscle function. Studies have shown that the transforming growth factor-β1 (TGF-β1)-Smad signaling pathway plays a central role in muscle atrophy and fibrosis. Recent studies demonstrate the role of microRNAs (miRs) in various pathological conditions, including muscle regeneration. miR-21 has been shown to play a dynamic role in inflammatory responses and in accelerating injury responses to fibrosis. We used both RNA sequencing and quantitative RT-PCR strategies to examine the alternations of miRNAs during denervation-induced gastrocnemius muscle atrophy and fibrosis. Our data showed that MiR-21 was upregulated in denervated gastrocnemius muscle tissue, and TGF-β1treatment increased miR-21 expression. Inhibition of miR-21 reduced gastrocnemius muscle fibrosis and significantly downregulated the expression of p-SMAD2/3 and the fibrosis-associated markers TGF-β1, connective tissue growth factor, alpha smooth muscle actin. Masson's trichrome staining revealed that atrophy and fibrosis in gastrocnemius muscle tissue were reduced in the miR-21 inhibition group compared to the control group. We confirmed that SMAD7 is a direct target of miR-21 using a dual luciferase assay. Furthermore, Immunofluorescence and Western blot analyses revealed that miR-21 inhibition reduced SMAD2/3 phosphorylation and nuclear translocation. While SMAD7-siRNA abolished the effect. Consequently, the discovery that miR-21 regulates the atrophy and fibrosis of the gastrocnemius muscle offers a possible therapeutic approach for their management.

MicroRNA-142a-3p regulates neurogenic skeletal muscle atrophy by targeting Mef2a

Peripheral nerve injury can lead to progressive muscle atrophy and poor motor function recovery, which is a difficult point of treatment, and the mechanism needs to be further explored. In previous studies, we found that miR-142a-3p was significantly upregulated and persistently highly expressed in denervated mouse skeletal muscle. Here, we show that overexpression of miR-142a-3p inhibited the growth and differentiation of C2C12 myoblast, while knockdown of miR-142a-3p had a promoting effect. In vitro, knockdown of miR-142a-3p in denervated mouse skeletal muscle effectively increased proliferating muscle satellite cells and ameliorated muscle atrophy. Mechanistically, the myoregulator Mef2a was proved to be an important downstream target of miR-142a-3p, and miR-142a-3p regulates skeletal muscle differentiation and regeneration by inhibiting the expression of Mef2a. The co-knockdown of Mef2a and miR-142a-3p effectively alleviated or offset the biological effects of miR-142a-3p knockdown. In conclusion, our data revealed that miR-142a-3p regulates neurogenic skeletal muscle atrophy by targeting Mef2a.

Different Resistance Exercise Loading Paradigms Similarly Affect Skeletal Muscle Gene Expression Patterns of Myostatin-Related Targets and mTORC1 Signaling Markers

Although transcriptome profiling has been used in several resistance training studies, the associated analytical approaches seldom provide in-depth information on individual genes linked to skeletal muscle hypertrophy. Therefore, a secondary analysis was performed herein on a muscle transcriptomic dataset we previously published involving trained college-aged men (n = 11) performing two resistance exercise bouts in a randomized and crossover fashion. The lower-load bout (30 Fail) consisted of 8 sets of lower body exercises to volitional fatigue using 30% one-repetition maximum (1 RM) loads, whereas the higher-load bout (80 Fail) consisted of the same exercises using 80% 1 RM loads. Vastus lateralis muscle biopsies were collected prior to (PRE), 3 h, and 6 h after each exercise bout, and 58 genes associated with skeletal muscle hypertrophy were manually interrogated from our prior microarray data. Select targets were further interrogated for associated protein expression and phosphorylation induced-signaling events. Although none of the 58 gene targets demonstrated significant bout x time interactions, ~57% (32 genes) showed a significant main effect of time from PRE to 3 h (15↑ and 17↓, p < 0.01), and ~26% (17 genes) showed a significant main effect of time from PRE to 6 h (8↑ and 9↓, p < 0.01). Notably, genes associated with the myostatin (9 genes) and mammalian target of rapamycin complex 1 (mTORC1) (9 genes) signaling pathways were most represented. Compared to mTORC1 signaling mRNAs, more MSTN signaling-related mRNAs (7 of 9) were altered post-exercise, regardless of the bout, and RHEB was the only mTORC1-associated mRNA that was upregulated following exercise. Phosphorylated (phospho-) p70S6K (Thr389) (p = 0.001; PRE to 3 h) and follistatin protein levels (p = 0.021; PRE to 6 h) increased post-exercise, regardless of the bout, whereas phospho-AKT (Thr389), phospho-mTOR (Ser2448), and myostatin protein levels remained unaltered. These data continue to suggest that performing resistance exercise to volitional fatigue, regardless of load selection, elicits similar transient mRNA and signaling responses in skeletal muscle. Moreover, these data provide further evidence that the transcriptional regulation of myostatin signaling is an involved mechanism in response to resistance exercise.

The Effect of Electrical Stimulation on Nerve Regeneration Following Peripheral Nerve Injury

Peripheral nerve injuries (PNI) are common and often result in lifelong disability. The peripheral nervous system has an inherent ability to regenerate following injury, yet complete functional recovery is rare. Despite advances in the diagnosis and repair of PNIs, many patients suffer from chronic pain, and sensory and motor dysfunction. One promising surgical adjunct is the application of intraoperative electrical stimulation (ES) to peripheral nerves. ES acts through second messenger cyclic AMP to augment the intrinsic molecular pathways of regeneration. Decades of animal studies have demonstrated that 20 Hz ES delivered post-surgically accelerates axonal outgrowth and end organ reinnervation. This work has been translated clinically in a series of randomized clinical trials, which suggest that ES can be used as an efficacious therapy to improve patient outcomes following PNIs. The aim of this review is to discuss the cellular physiology and the limitations of regeneration after peripheral nerve injuries. The proposed mechanisms of ES protocols and how they facilitate nerve regeneration depending on timing of administration are outlined. Finally, future directions of research that may provide new perspectives on the optimal delivery of ES following PNI are discussed.

Transcription and proteome changes involved in re-innervation muscle following nerve crush in rats

Severe peripheral nerve injury leads to the irreparable disruption of nerve fibers. This leads to disruption of synapses with the designated muscle, which consequently go through progressive atrophy and damage of muscle function. The molecular mechanism that underlies the re-innervation process has yet to be evaluated using proteomics or transcriptomics. In the present study, multi-dimensional data were therefore integrated with transcriptome and proteome profiles in order to investigate the mechanism of re-innervation in muscles. Two simulated nerve injury muscle models in the rat tibial nerve were compared: the nerve was either cut (denervated, DN group) or crushed but with the nerve sheath intact (re-innervated, RN group). The control group had a preserved and intact tibial nerve. At 4 weeks, the RN group showed better tibial nerve function and recovery of muscle atrophy compared to the DN group. As the high expression of Myh3, Postn, Col6a1 and Cfi, the RN group demonstrated superior re-innervation as well. Both differentially expressed genes (DEGs) and proteins (DEPs) were enriched in the peroxisome proliferator-activated receptors (PPARs) signaling pathway, as well as the energy metabolism. This study provides basic information regarding DEGs and DEPs during re-innervation-induced muscle atrophy. Furthermore, the crucial genes and proteins can be detected as possible treatment targets in the future.

Intramuscular Stem Cell Injection in Combination with Bioengineered Nerve Repair or Nerve Grafting Reduces Muscle Atrophy

BACKGROUND

Peripheral nerve injuries represent a clinical challenge, especially when they are accompanied by loss of neural tissue. In this study, the authors attempted to attain a better outcome after a peripheral nerve injury by both repairing the nerve lesion and treating the denervated muscle at the same time.

METHODS

Rat sciatic nerves were transected to create 10-mm gaps. Repair was performed in five groups (n = 5 rats for each), as follows: group 1, nerve repair using poly-3-hydroxybutyrate strips to connect the proximal and distal stumps, in combination with control growth medium injection in the gastrocnemius muscle; group 2, nerve repair with poly-3-hydroxybutyrate strip seeded with Schwann cell-like differentiated adipose stem cells (differentiated adipose stem cell strip) in combination with growth medium intramuscular injection; group 3, differentiated adipose stem cell strip in combination with intramuscular injection of differentiated adipose stem cells; group 4, repair using autograft (reverse sciatic nerve graft) in combination with intramuscular injection of growth medium; and group 5, autograft in combination with intramuscular injection of differentiated adipose stem cells. Six weeks after nerve injury, the effects of the stem cells on muscle atrophy were assessed.

RESULTS

Poly-3-hydroxybutyrate strips seeded with differentiated adipose stem cells showed a high number of βIII-tubulin-positive axons entering the distal stump and abundant endothelial cells. Group 1 animals exhibited more muscle atrophy than all the other groups, and group 5 animals had the greatest muscle weights and muscle fibers size.

CONCLUSION

Bioengineering nerve repair in combination with intramuscular stem cell injection is a promising technique to treat nerve lesions and associated muscle atrophy.

CLINICAL RELEVANCE STATEMENT

Nerve injuries and resulting muscle atrophy are a clinical challenge. To optimize functional recovery after a nerve lesion, the authors treated the nerve and muscle at the same time by using regenerative medicine with adipose stem cells and obtained encouraging results for future clinical applications.

Out of Control: The Role of the Ubiquitin Proteasome System in Skeletal Muscle during Inflammation

The majority of critically ill intensive care unit (ICU) patients with severe sepsis develop ICU-acquired weakness (ICUAW) characterized by loss of muscle mass, reduction in myofiber size and decreased muscle strength leading to persisting physical impairment. This phenotype results from a dysregulated protein homeostasis with increased protein degradation and decreased protein synthesis, eventually causing a decrease in muscle structural proteins. The ubiquitin proteasome system (UPS) is the predominant protein-degrading system in muscle that is activated during diverse muscle atrophy conditions, e.g., inflammation. The specificity of UPS-mediated protein degradation is assured by E3 ubiquitin ligases, such as atrogin-1 and MuRF1, which target structural and contractile proteins, proteins involved in energy metabolism and transcription factors for UPS-dependent degradation. Although the regulation of activity and function of E3 ubiquitin ligases in inflammation-induced muscle atrophy is well perceived, the contribution of the proteasome to muscle atrophy during inflammation is still elusive. During inflammation, a shift from standard- to immunoproteasome was described; however, to which extent this contributes to muscle wasting and whether this changes targeting of specific muscular proteins is not well described. This review summarizes the function of the main proinflammatory cytokines and acute phase response proteins and their signaling pathways in inflammation-induced muscle atrophy with a focus on UPS-mediated protein degradation in muscle during sepsis. The regulation and target-specificity of the main E3 ubiquitin ligases in muscle atrophy and their mode of action on myofibrillar proteins will be reported. The function of the standard- and immunoproteasome in inflammation-induced muscle atrophy will be described and the effects of proteasome-inhibitors as treatment strategies will be discussed.

Evolving Roles of Muscle-Resident Fibro-Adipogenic Progenitors in Health, Regeneration, Neuromuscular Disorders, and Aging

Normal skeletal muscle functions are affected following trauma, chronic diseases, inherited neuromuscular disorders, aging, and cachexia, hampering the daily activities and quality of life of the affected patients. The maladaptive accumulation of fibrous intramuscular connective tissue and fat are hallmarks of multiple pathologies where chronic damage and inflammation are not resolved, leading to progressive muscle replacement and tissue degeneration. Muscle-resident fibro-adipogenic progenitors are adaptable stromal cells with multilineage potential. They are required for muscle homeostasis, neuromuscular integrity, and tissue regeneration. Fibro-adipogenic progenitors actively regulate and shape the extracellular matrix and exert immunomodulatory functions via cross-talk with multiple other residents and non-resident muscle cells. Remarkably, cumulative evidence shows that a significant proportion of activated fibroblasts, adipocytes, and bone-cartilage cells, found after muscle trauma and disease, descend from these enigmatic interstitial progenitors. Despite the profound impact of muscle disease on human health, the fibrous, fatty, and ectopic bone tissues' origins are poorly understood. Here, we review the current knowledge of fibro-adipogenic progenitor function on muscle homeostatic integrity, regeneration, repair, and aging. We also discuss how scar-forming pathologies and disorders lead to dysregulations in their behavior and plasticity and how these stromal cells can control the onset and severity of muscle loss in disease. We finally explore the rationale of improving muscle regeneration by understanding and modulating fibro-adipogenic progenitors' fate and behavior.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

Skeletal muscle loss is a detrimental side-effect of numerous chronic diseases that dramatically increases mortality and morbidity. The alteration of protein homeostasis is generally due to increased protein breakdown while, protein synthesis may also be down-regulated. The ubiquitin proteasome system (UPS) is a master regulator of skeletal muscle that impacts muscle contractile properties and metabolism through multiple levers like signaling pathways, contractile apparatus degradation, etc. Among the different actors of the UPS, the E3 ubiquitin ligases specifically target key proteins for either degradation or activity modulation, thus controlling both pro-anabolic or pro-catabolic factors. The atrogenes MuRF1/TRIM63 and MAFbx/Atrogin-1 encode for key E3 ligases that target contractile proteins and key actors of protein synthesis respectively. However, several other E3 ligases are involved upstream in the atrophy program, from signal transduction control to modulation of energy balance. Controlling E3 ligases activity is thus a tempting approach for preserving muscle mass. While indirect modulation of E3 ligases may prove beneficial in some situations of muscle atrophy, some drugs directly inhibiting their activity have started to appear. This review summarizes the main signaling pathways involved in muscle atrophy and the E3 ligases implicated, but also the molecules potentially usable for future therapies.

Mechanisms of IGF-1-Mediated Regulation of Skeletal Muscle Hypertrophy and Atrophy

Insulin-like growth factor-1 (IGF-1) is a key growth factor that regulates both anabolic and catabolic pathways in skeletal muscle. IGF-1 increases skeletal muscle protein synthesis via PI3K/Akt/mTOR and PI3K/Akt/GSK3β pathways. PI3K/Akt can also inhibit FoxOs and suppress transcription of E3 ubiquitin ligases that regulate ubiquitin proteasome system (UPS)-mediated protein degradation. Autophagy is likely inhibited by IGF-1 via mTOR and FoxO signaling, although the contribution of autophagy regulation in IGF-1-mediated inhibition of skeletal muscle atrophy remains to be determined. Evidence has suggested that IGF-1/Akt can inhibit muscle atrophy-inducing cytokine and myostatin signaling via inhibition of the NF-κΒ and Smad pathways, respectively. Several miRNAs have been found to regulate IGF-1 signaling in skeletal muscle, and these miRs are likely regulated in different pathological conditions and contribute to the development of muscle atrophy. IGF-1 also potentiates skeletal muscle regeneration via activation of skeletal muscle stem (satellite) cells, which may contribute to muscle hypertrophy and/or inhibit atrophy. Importantly, IGF-1 levels and IGF-1R downstream signaling are suppressed in many chronic disease conditions and likely result in muscle atrophy via the combined effects of altered protein synthesis, UPS activity, autophagy, and muscle regeneration.

Cellular and molecular features of neurogenic skeletal muscle atrophy

Neurogenic atrophy refers to the loss of muscle mass and function that results directly from injury or disease of the peripheral nervous system. Individuals with neurogenic atrophy may experience reduced functional status and quality of life and, in some circumstances, reduced survival. Distinct pathological findings on muscle histology can aid in diagnosis of a neurogenic cause for muscle dysfunction, and provide indicators for the chronicity of denervation. Denervation induces pleiotypic responses in skeletal muscle, and the molecular mechanisms underlying neurogenic muscle atrophy appear to share common features with other causes of muscle atrophy, including activation of FOXO transcription factors and corresponding induction of ubiquitin-proteasomal and lysosomal degradation. In this review, we provide an overview of histologic features of neurogenic atrophy and a summary of current understanding of underlying mechanisms.

Rodent Model of Muscular Atrophy for Sarcopenia Study

The hallmark symptom of sarcopenia is the loss of muscle mass and strength without the loss of overall body weight. Sarcopenia patients are likely to have worse clinical outcomes and higher mortality than do healthy individuals. The sarcopenia population shows an annual increase of ~0.8% in the population after age 50, and the prevalence rate is rapidly increasing with the recent worldwide aging trend. Based on International Classification of Diseases, Tenth Revision, a global classification of disease published by the World Health Organization, issued the disease code (M62.84) given to sarcopenia in 2016. Therefore, it is expected that the study of sarcopenia will be further activated based on the classification of disease codes in the aging society. Several epidemiological studies and meta-analyses have looked at the correlation between the prevalence of sarcopenia and several environmental factors. In addition, studies using cell lines and rodents have been done to understand the biological mechanism of sarcopenia. Laboratory rodent models are widely applicable in sarcopenia studies because of the advantages of time savings, cost saving, and various analytical applications that could not be used for human subjects. The rodent models that can be applied to the sarcopenia research are diverse, but a simple and fast method that can cause atrophy or aging is preferred. Therefore, we will introduce various methods of inducing muscular atrophy in rodent models to be applied to the study of sarcopenia.

Expression of genes in the skeletal muscle of individuals with cachexia/sarcopenia: A systematic review

Background

Cachexia occurs in individuals affected by chronic diseases in which systemic inflammation leads to fatigue, debilitation, decreased physical activity and sarcopenia. The pathogenesis of cachexia-associated sarcopenia is not fully understood.

Objectives

The aim of this systematic review is to summarize the current evidence on genes expressed in the skeletal muscles of humans with chronic disease-associated cachexia and/or sarcopenia (cases) compared to controls and to assess the strength of such evidence.

Methods

We searched PubMed, EMBASE and CINAHL using three concepts: cachexia/sarcopenia and associated symptoms, gene expression, and skeletal muscle.

Results

Eighteen genes were studied in at least three research articles, for a total of 27 articles analyzed in this review. Participants were approximately 60 years of age and majority male; sample size was highly variable. Use of comparison groups, matching criteria, muscle biopsy location, and definitions of cachexia and sarcopenia were not homogenous. None of the studies fulfilled all four criteria used to assess the quality of molecular analysis, with only one study powered on the outcome of gene expression. FOXO1 was the only gene significantly increased in cases versus healthy controls. No study found a significant decrease in expression of genes involved in autophagy, apoptosis or inflammation in cases versus controls. Inconsistent or non-significant findings were reported for genes involved in protein degradation, muscle differentiation/growth, insulin/insulin growth factor-1 or mitochondrial transcription.

Conclusion

Currently available evidence on gene expression in the skeletal muscles of humans with chronic disease-associated cachexia and/or sarcopenia is not powered appropriately and is not homogenous; therefore, it is difficult to compare results across studies and diseases.

Identification of suitable reference genes for gene expression studies in rat skeletal muscle following sciatic nerve crush injury

Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) is a molecular biological method used to assess gene expression characterized by high simplicity, effectiveness, specificity and sensitivity. The selection of a suitable reference gene for normalization is critical for the accuracy of quantitative results. Peripheral nerve injury is a common clinical disorder that affects multiple tissues and organs, including peripheral nerves, neurons and the innervated muscles. Numerous genes are differentially expressed in skeletal muscles during muscle denervation and reinnervation following peripheral nerve injury. The identification of a suitable reference gene in innervated muscles following nerve injury may improve the understanding of the alterations in gene expression in the processes of peripheral nerve repair and regeneration. Therefore, in the present study, by using a rat sciatic nerve crush model, the expression levels of various housekeeping genes were examined. In particular, the expression levels of 13 housekeeping genes, including 18S ribosomal RNA, actin β, ankyrin repeat domain 27, cyclophilin A, GAPDH, hypoxanthine phosphoribosyltransferase 1 (HPRT1), mitochondrial ribosomal protein L10, phosphoglycerate kinase 1, RPTOR independent companion of mammalian target of rapamycin complex 2, TATA-box binding protein, ubiquitin C, UBX domain protein 11 and tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein ζ, were investigated in gastrocnemius muscles. The geNorm and NormFinder analyses suggested that the expression level of HPRT1 was particularly stable in gastrocnemius muscles following rat sciatic nerve crush injury. Therefore, HPRT1 may be used as a reference gene for the normalization of gene expression data generated by RT-qPCR.

The adipose tissue stromal vascular fraction secretome enhances the proliferation but inhibits the differentiation of myoblasts

Background

Adipose tissue is an excellent source for isolation of stem cells for treating various clinical conditions including injuries to the neuromuscular system. Many previous studies have focused on differentiating these adipose stem cells (ASCs) towards a Schwann cell-like phenotype (dASCs), which can enhance axon regeneration and reduce muscle atrophy. However, the stromal vascular fraction (SVF), from which the ASCs are derived, also exerts broad regenerative potential and might provide a faster route to clinical translation of the cell therapies for treatment of neuromuscular disorders.

Methods

The aim of this study was to establish the effects of SVF cells on the proliferation and differentiation of myoblasts using indirect co-culture experiments. A Growth Factor PCR Array was used to compare the secretomes of SVF and dASCs, and the downstream signaling pathways were investigated.

Results

SVF cells, unlike culture-expanded dASCs, expressed and secreted hepatocyte growth factor (HGF) at concentrations sufficient to enhance the proliferation of myoblasts. Pharmacological inhibitor studies revealed that the signal is mediated via ERK1/2 phosphorylation and that the effect is significantly reduced by the addition of 100 pM Norleual, a specific HGF inhibitor. When myoblasts were differentiated into multinucleated myotubes, the SVF cells reduced the expression levels of fast-type myosin heavy chain (MyHC2) suggesting an inhibition of the differentiation process.

Conclusions

In summary, this study shows the importance of HGF as a mediator of the SVF effects on myoblasts and provides further evidence for the importance of the secretome in cell therapy and regenerative medicine applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1096-6) contains supplementary material, which is available to authorized users.

Paralytic and nonparalytic muscle adaptations to exercise training versus high-protein diet in individuals with long-standing spinal cord injury

This study compares the effects of an 8-wk isocaloric high-protein (HP) diet versus a combination exercise (Comb-Ex) regimen on paralytic vastus lateralis (VL) and nonparalytic deltoid muscle in individuals with long-standing spinal cord injury (SCI). Fiber-type distribution, cross-sectional area (CSA), levels of translation initiation signaling proteins (Erk-1/2, Akt, p70S6K1, 4EBP1, RPS6, and FAK), and lean thigh mass were analyzed at baseline and after the 8-wk interventions. A total of 11 participants (C5-T12 levels, 21.8 ± 6.3 yr postinjury; 6 Comb-Ex and 5 HP diet) completed the study. Comb-Ex training occurred 3 days/wk and consisted of upper body resistance training (RT) in addition to neuromuscular electrical stimulation (NMES)-induced-RT for paralytic VL muscle. Strength training was combined with high-intensity arm-cranking exercises (1-min intervals at 85-90%, V̇o2peak) for improving cardiovascular endurance. For the HP diet intervention, protein and fat each comprised 30%, and carbohydrate comprised 40% of total energy. Clinical tests and muscle biopsies were performed 24 h before and after the last exercise or diet session. The Comb-Ex intervention increased Type IIa myofiber distribution and CSA in VL muscle and Type I and IIa myofiber CSA in deltoid muscle. In addition, Comb-Ex increased lean thigh mass, V̇o2peak, and upper body strength ( P < 0.05). These results suggest that exercise training is required to promote favorable changes in paralytic and nonparalytic muscles in individuals with long-standing SCI, and adequate dietary protein consumption alone may not be sufficient to ameliorate debilitating effects of paralysis. NEW & NOTEWORTHY This study is the first to directly compare the effects of an isocaloric high-protein diet and combination exercise training on clinical and molecular changes in paralytic and nonparalytic muscles of individuals with long-standing spinal cord injury. Our results demonstrated that muscle growth and fiber-type alterations can best be achieved when the paralyzed muscle is sufficiently loaded via neuromuscular electrical stimulation-induced resistance training.

Transcutaneous carbon dioxide application with hydrogel prevents muscle atrophy in a rat sciatic nerve crush model

The acceleration of nerve regeneration remains a clinical challenge. We previously demonstrated that transcutaneous CO₂ application using a novel hydrogel increases the oxygen concentration in local tissue via an "artificial Bohr effect" with the potential to prevent muscle atrophy. In this study, we investigated the effect of transcutaneous CO₂ administration on limb function after peripheral nerve injury in a rat sciatic nerve injury model. In total, 73 Sprague-Dawley rats were divided into a sham group, a control group (crush injury to sciatic nerve and no treatment) or a CO₂ group (crush injury with transcutaneous CO₂ application). CO₂ was administered percutaneously for 20 min five times per week. Scores for the sciatic function index and pinprick test were significantly higher in the CO₂ group than control group. The muscle wet weight ratios of the tibialis anterior and soleus muscles were higher in the CO₂ group than control group. Electrophysiological examination showed that the CO₂ group had higher compound motor action potential amplitudes and shorter distal motor latency than the control group. Histological examination of the soleus muscle sections at postoperative week 2 showed shorter fiber diameter in the control group than in the CO₂ group. The mRNA expression of Atrogin-1 and MuRF-1 was lower, mRNA expression of VEGF and myogenin and MyoD was higher in CO₂ group at postoperative week 2 compared to the control group.

CLINICAL SIGNIFICANCE

Transcutaneous CO₂ application has the therapeutic potential to accelerate the recovery of muscle atrophy in peripheral nerve injury. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1653-1658, 2018.

Sternopygus macrurus electric organ transcriptome and cell size exhibit insensitivity to short-term electrical inactivity

Electrical activity is an important regulator of cellular function and gene expression in electrically excitable cell types. In the weakly electric teleost fish Sternopygus macrurus, electrocytes, i.e., the current-producing cells of the electric organ, derive from a striated muscle lineage. Mature electrocytes are larger than muscle fibers, do not contain sarcomeres, and are driven continuously at frequencies higher than those exerted on muscle cells. Previous work showed that the removal of electrical activity by spinal cord transection (ST) for two and five weeks led to an upregulation of some sarcomeric proteins and a decrease in electrocyte size. To test whether changes in gene transcription preceded these phenotypic changes, we determined the sensitivity of electrocyte gene expression to electrical inactivity periods of two and five days after ST. Whole tissue gene expression profiles using deep RNA sequencing showed minimal alterations in the levels of myogenic transcription factor and sarcomeric transcripts after either ST period. Moreover, while analysis of differentially expressed genes showed a transient upregulation of genes associated with proteolytic mechanisms at two days and an increase in mRNA levels of cytoskeletal genes at five days after electrical silencing, electrocyte size was not affected. Electrical inactivity also resulted in the downregulation of genes that were classified into enriched clusters associated with functions of axon migration and synapse structure. Overall, these data demonstrate that unlike tissues in the myogenic lineage in other vertebrate species, regulation of gene transcription and cell size in the muscle-like electrocytes of S. macrurus is highly insensitive to short-term electrical inactivity. Moreover, together with data obtained from control and long-term ST studies, the present data suggest that neural input might influence post-transcriptional processes to affect the mature electrocyte phenotype.

Recovery of upper extremity function following endoscopically assisted contralateral C7 transfer for obstetrical brachial plexus injury

Transfer of the contralateral C7 nerve for reconstruction of the brachial plexus in infants with obstetrical brachial plexus injury has rarely been reported. We developed a new endoscopy-assisted technique via the prevertebral (retroesophageal) route for the transfer of the contralateral C7 nerve in reconstruction of the brachial plexus. The reconstruction was performed in 20 infants (14 boys and six girls). Motor recovery was assessed using the Gilbert and Raimondi scales. The Narakas Sensory Grading System was used to evaluate hand sensation. The mean follow-up period was 45 months (SD 18.2). Of the 20 children, nine had contralateral C7 transfer to lower nerve roots, two had transfer to upper nerve roots and nine had transfer to both upper and lower roots. The postoperative shoulder and elbow functions were good or satisfactory according to the Gilbert classification in all children whose preoperative scores were poor. All patients with lower roots reconstruction (9) had satisfactory hand function. A total of 15 children had a Narakas score of S3. Our technique enables safe contralateral C7 transfer to the avulsed roots in severe obstetrical brachial plexus injury infants with a satisfactory functional recovery.

LEVEL OF EVIDENCE

Level IV.

Altered Satellite Cell Responsiveness and Denervation Implicated in Progression of Rotator-Cuff Injury

Background

Rotator-cuff injury (RCI) is common and painful; even after surgery, joint stability and function may not recover. Relative contributions to atrophy from disuse, fibrosis, denervation, and satellite-cell responsiveness to activating stimuli are not known.

Methods and Findings

Potential contributions of denervation and disrupted satellite cell responses to growth signals were examined in supraspinatus (SS) and control (ipsilateral deltoid) muscles biopsied from participants with RCI (N = 27). Biopsies were prepared for explant culture (to study satellite cell activity), immunostained to localize Pax7, BrdU, and Semaphorin 3A in satellite cells, sectioning to study blood vessel density, and western blotting to measure the fetal (γ) subunit of acetylcholine receptor (γ-AchR). Principal component analysis (PCA) for 35 parameters extracted components identified variables that contributed most to variability in the dataset. γ-AchR was higher in SS than control, indicating denervation. Satellite cells in SS had a low baseline level of activity (Pax7+ cells labelled in S-phase) versus control; only satellite cells in SS showed increased proliferative activity after nitric oxide-donor treatment. Interestingly, satellite cell localization of Semaphorin 3A, a neuro-chemorepellent, was greater in SS (consistent with fiber denervation) than control muscle at baseline. PCAs extracted components including fiber atrophy, satellite cell activity, fibrosis, atrogin-1, smoking status, vascular density, γAchR, and the time between symptoms and surgery. Use of deltoid as a control for SS was supported by PCA findings since “muscle” was not extracted as a variable in the first two principal components. SS muscle in RCI is therefore atrophic, denervated, and fibrotic, and has satellite cells that respond to activating stimuli.

Conclusions

Since SS satellite cells can be activated in culture, a NO-donor drug combined with stretching could promote muscle growth and improve functional outcome after RCI. PCAs suggest indices including satellite cell responsiveness, atrogin-1, atrophy, and innervation may predict surgical outcome.

Denervation-Induced Activation of the Ubiquitin-Proteasome System Reduces Skeletal Muscle Quantity Not Quality

It is well known that the ubiquitin-proteasome system is activated in response to skeletal muscle wasting and functions to degrade contractile proteins. The loss of these proteins inevitably reduces skeletal muscle size (i.e., quantity). However, it is currently unknown whether activation of this pathway also affects function by impairing the muscle’s intrinsic ability to produce force (i.e., quality). Therefore, the purpose of this study was twofold, (1) document how the ubiquitin-proteasome system responds to denervation and (2) identify the physiological consequences of these changes. To induce soleus muscle atrophy, C57BL6 mice underwent tibial nerve transection of the left hindlimb for 7 or 14 days (n = 6–8 per group). At these time points, content of several proteins within the ubiquitin-proteasome system were determined via Western blot, while ex vivo whole muscle contractility was specifically analyzed at day 14. Denervation temporarily increased several key proteins within the ubiquitin-proteasome system, including the E3 ligase MuRF1 and the proteasome subunits 19S, α7 and β5. These changes were accompanied by reductions in absolute peak force and power, which were offset when expressed relative to physiological cross-sectional area. Contrary to peak force, absolute and relative forces at submaximal stimulation frequencies were significantly greater following 14 days of denervation. Taken together, these data represent two keys findings. First, activation of the ubiquitin-proteasome system is associated with reductions in skeletal muscle quantity rather than quality. Second, shortly after denervation, it appears the muscle remodels to compensate for the loss of neural activity via changes in Ca²⁺ handling.

Properties of skeletal muscle in the teleost Sternopygus macrurus are unaffected by short-term electrical inactivity

Skeletal muscle is distinguished from other tissues on the basis of its shape, biochemistry, and physiological function. Based on mammalian studies, fiber size, fiber types, and gene expression profiles are regulated, in part, by the electrical activity exerted by the nervous system. To address whether similar adaptations to changes in electrical activity in skeletal muscle occur in teleosts, we studied these phenotypic properties of ventral muscle in the electric fish Sternopygus macrurus following 2 and 5 days of electrical inactivation by spinal transection. Our data show that morphological and biochemical properties of skeletal muscle remained largely unchanged after these treatments. Specifically, the distribution of type I and type II muscle fibers and the cross-sectional areas of these fiber types observed in control fish remained unaltered after each spinal transection survival period. This response to electrical inactivation was generally reflected at the transcript level in real-time PCR and RNA-seq data by showing little effect on the transcript levels of genes associated with muscle fiber type differentiation and plasticity, the sarcomere complex, and pathways implicated in the regulation of muscle fiber size. Data from this first study characterizing the acute influence of neural activity on muscle mass and sarcomere gene expression in a teleost are discussed in the context of comparative studies in mammalian model systems and vertebrate species from different lineages.

Altered Metabolic Homeostasis in Amyotrophic Lateral Sclerosis: Mechanisms of Energy Imbalance and Contribution to Disease Progression

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the death of motor neurones, which leads to paralysis and death in an average of 3 years following diagnosis. The cause of ALS is unknown, but there is substantial evidence that metabolic factors, including nutritional state and body weight, affect disease progression and survival. This review provides an overview of the characteristics of metabolic dysregulation in ALS focusing on mechanisms that lead to disrupted energy supply (at a whole-body and cellular level) and altered energy expenditure. We discuss how a decrease in energy supply occurs in parallel with an increase in energy demand and leads to a state of chronic energy deficit which has a negative impact on disease outcome in ALS. We conclude by presenting potential and tested strategies to compensate for, or correct this energy imbalance, and speculate on promising areas for further research.

Heightened TWEAK-NF-κB signaling and inflammation-associated fibrosis in paralyzed muscles of men with chronic spinal cord injury

Individuals with long-standing spinal cord injury (SCI) often present with extreme muscle atrophy and impaired glucose metabolism at both the skeletal muscle and whole body level. Persistent inflammation and increased levels of proinflammatory cytokines in the skeletal muscle are potential contributors to dysregulation of glucose metabolism and atrophy; however, to date no study has assessed the effects of long-standing SCI on their expression or intracellular signaling in the paralyzed muscle. In the present study, we assessed the expression of genes (TNFαR, TNFα, IL-6R, IL-6, TWEAK, TWEAK R, atrogin-1, and MuRF1) and abundance of intracellular signaling proteins (TWEAK, TWEAK R, NF-κB, and p-p65/p-50/105) that are known to mediate inflammation and atrophy in skeletal muscle. In addition, based on the effects of muscle inflammation on promotion of skeletal muscle fibrosis, we assessed the degree of fibrosis between myofibers and fascicles in both groups. For further insight into the distribution and variability of muscle fiber size, we also analyzed the frequency distribution of SCI fiber size. Resting vastus lateralis (VL) muscle biopsy samples were taken from 11 men with long-standing SCI (≈22 yr) and compared with VL samples from 11 able-bodied men of similar age. Our results demonstrated that chronic SCI muscle has heightened TNFαR and TWEAK R gene expression and NF-κB signaling (higher TWEAK R and phospho-NF-κB p65) and fibrosis, along with substantial myofiber size heterogeneity, compared with able-bodied individuals. Our data suggest that the TWEAK/TWEAK R/NF-κB signaling pathway may be an important mediator of chronic inflammation and fibrotic adaptation in SCI muscle.

MiR-206 Attenuates Denervation-Induced Skeletal Muscle Atrophy in Rats Through Regulation of Satellite Cell Differentiation via TGF-β1, Smad3, and HDAC4 Signaling

Background

Denervation-induced skeletal muscle atrophy results in significant biochemical and physiological changes potentially leading to devastating outcomes including increased mortality. Effective treatments for skeletal muscle diseases are currently not available. Muscle-specific miRNAs, such as miR-206, play an important role in the regulation of muscle regeneration.

The aim of the present study was to examine the beneficial effects of miR-206 treatment during the early changes in skeletal muscle atrophy, and to study the underlying signaling pathways in a rat skeletal muscle atrophy model.

Material/Methods

The rat denervation-induced skeletal muscle atrophy model was established. miRNA-206 was overexpressed with or without TGF-β1 inhibitor in the rats. The mRNA and protein expression of HDAC4, TGF-β1, and Smad3 was determined by real-time PCR and western blot. The gastrocnemius muscle cross-sectional area and relative muscle mass were measured. MyoD1, TGF-β1, and Pax7 were determined by immunohistochemical staining.

Results

After sciatic nerve surgical transection, basic muscle characteristics, such as relative muscle weight, deteriorated continuously during a 2-week period. Injection of miR-206 (30 μg/rat) attenuated morphological and physiological deterioration of muscle characteristics, prevented fibrosis effectively, and inhibited the expression of TGF-β1 and HDAC4 as assessed 2 weeks after denervation. Moreover, miR-206 treatment increased the number of differentiating (MyoD1⁺/Pax7⁺) satellite cells, thereby protecting denervated muscles from atrophy. Interestingly, the ability of miR-206 to govern HDAC4 expression and to attenuate muscle atrophy was weakened after pharmacological blockage of the TGF-β1/Smad3 axis.

Conclusions

TGF-β1/Smad3 signaling pathway is one of the crucial signaling pathways by which miR-206 counteracts skeletal muscle atrophy by affecting proliferation and differentiation of satellite cells. miR-206 may be a potential target for development of a new strategy for treatment of patients with early denervation-induced skeletal muscle atrophy.

A robust neuromuscular system protects rat and human skeletal muscle from sarcopenia

Declining muscle mass and function is one of the main drivers of loss of independence in the elderly. Sarcopenia is associated with numerous cellular and endocrine perturbations, and it remains challenging to identify those changes that play a causal role and could serve as targets for therapeutic intervention. In this study, we uncovered a remarkable differential susceptibility of certain muscles to age-related decline. Aging rats specifically lose muscle mass and function in the hindlimbs, but not in the forelimbs. By performing a comprehensive comparative analysis of these muscles, we demonstrate that regional susceptibility to sarcopenia is dependent on neuromuscular junction fragmentation, loss of motoneuron innervation, and reduced excitability. Remarkably, muscle loss in elderly humans also differs in vastus lateralis and tibialis anterior muscles in direct relation to neuromuscular dysfunction. By comparing gene expression in susceptible and non-susceptible muscles, we identified a specific transcriptomic signature of neuromuscular impairment. Importantly, differential molecular profiling of the associated peripheral nerves revealed fundamental changes in cholesterol biosynthetic pathways. Altogether our results provide compelling evidence that susceptibility to sarcopenia is tightly linked to neuromuscular decline in rats and humans, and identify dysregulation of sterol metabolism in the peripheral nervous system as an early event in this process.

Implantation of muscle satellite cells overexpressing myogenin improves denervated muscle atrophy in rats

This study evaluated the effect of muscle satellite cells (MSCs) overexpressing myogenin (MyoG) on denervated muscle atrophy. Rat MSCs were isolated and transfected with the MyoG-EGFP plasmid vector GV143. MyoG-transfected MSCs (MTMs) were transplanted into rat gastrocnemius muscles at 1 week after surgical denervation. Controls included injections of untransfected MSCs or the vehicle only. Muscles were harvested and analyzed at 2, 4, and 24 weeks post-transplantation. Immunofluorescence confirmed MyoG overexpression in MTMs. The muscle wet weight ratio was significantly reduced at 2 weeks after MTM injection (67.17±6.79) compared with muscles injected with MSCs (58.83±5.31) or the vehicle (53.00±7.67; t=2.37, P=0.04 and t=3.39, P=0.007, respectively). The muscle fiber cross-sectional area was also larger at 2 weeks after MTM injection (2.63×10³±0.39×10³) compared with MSC injection (1.99×10³±0.58×10³) or the vehicle only (1.57×10³±0.47×10³; t=2.24, P=0.049 and t=4.22, P=0.002, respectively). At 4 and 24 weeks post-injection, the muscle mass and fiber cross-sectional area were similar across all three experimental groups. Immunohistochemistry showed that the MTM group had larger MyoG-positive fibers. The MTM group (3.18±1.13) also had higher expression of MyoG mRNA than other groups (1.41±0.65 and 1.03±0.19) at 2 weeks after injection (t=2.72, P=0.04). Transplanted MTMs delayed short-term atrophy of denervated muscles. This approach can be optimized as a novel stand-alone therapy or as a bridge to surgical re-innervation of damaged muscles.

Circulating protein synthesis rates reveal skeletal muscle proteome dynamics

Here, we have described and validated a strategy for monitoring skeletal muscle protein synthesis rates in rodents and humans over days or weeks from blood samples. We based this approach on label incorporation into proteins that are synthesized specifically in skeletal muscle and escape into the circulation. Heavy water labeling combined with sensitive tandem mass spectrometric analysis allowed integrated synthesis rates of proteins in muscle tissue across the proteome to be measured over several weeks. Fractional synthesis rate (FSR) of plasma creatine kinase M-type (CK-M) and carbonic anhydrase 3 (CA-3) in the blood, more than 90% of which is derived from skeletal muscle, correlated closely with FSR of CK-M, CA-3, and other proteins of various ontologies in skeletal muscle tissue in both rodents and humans. Protein synthesis rates across the muscle proteome generally changed in a coordinate manner in response to a sprint interval exercise training regimen in humans and to denervation or clenbuterol treatment in rodents. FSR of plasma CK-M and CA-3 revealed changes and interindividual differences in muscle tissue proteome dynamics. In human subjects, sprint interval training primarily stimulated synthesis of structural and glycolytic proteins. Together, our results indicate that this approach provides a virtual biopsy, sensitively revealing individualized changes in proteome-wide synthesis rates in skeletal muscle without a muscle biopsy. Accordingly, this approach has potential applications for the diagnosis, management, and treatment of muscle disorders.

PDLIM7 is a novel target of the ubiquitin ligase Nedd4-1 in skeletal muscle

Skeletal muscle atrophy remains a complication occurring both as a natural response to muscle disuse and as a pathophysiological response to illness such as diabetes mellitus and nerve injury, such as traumatic muscle denervation. The ubiquitin-proteasome system (UPS) is the predominant proteolytic machinery responsible for atrophy of skeletal muscle, and Nedd4-1 (neural precursor cell-expressed developmentally down-regulated 4-1) is one of a series of E3 ubiquitin ligases identified to mediate inactivity-induced muscle wasting. Targets of Nedd4-1 mediated ubiquitination in skeletal muscle remain poorly understood. In the present study, we identified PDLIM7 (PDZ and LIM domain 7, Enigma), a member of the PDZ-LIM family of proteins, as a novel target of Nedd4-1 in skeletal muscle. The PDZ-LIM family of proteins is known to regulate muscle development and function. We show that Nedd4-1 expression in muscle atrophied by denervation is co-incident with a decrease in PDLIM7 and that PDLIM7 protein levels are stabilized in denervated muscle of Nedd4-1 skeletal muscle-specific knockout mice (SMS-KO). Exogenous PDLIM7 and Nedd4-1 transfected into human embryonic kidney (HEK)293 cells co-immunoprecipitate through binding between the PY motif of PDLIM7 and the second and third WW domains of Nedd4-1 and endogenous PDLIM7 and Nedd4-1 interact in the cytoplasm of differentiated C2C12 myotubes, leading to PDLIM7 ubiquitination. These results identify PDLIM7 as a bona fide skeletal muscle substrate of Nedd4-1 and suggest that this interaction may underlie the progression of skeletal muscle atrophy. This offers a novel therapeutic target that could be potentially used to attenuate muscle atrophy.

Microarray Analysis Gene Expression Profiles in Laryngeal Muscle After Recurrent Laryngeal Nerve Injury

OBJECTIVES

The pathophysiology of recurrent laryngeal nerve (RLN) transection injury is rare in that it is characteristically followed by a high degree of spontaneous reinnervation, with reinnervation of the laryngeal adductor complex (AC) preceding that of the abducting posterior cricoarytenoid (PCA) muscle. Here, we aim to elucidate the differentially expressed myogenic factors following RLN injury that may be at least partially responsible for the spontaneous reinnervation.

METHODS

F344 male rats underwent RLN injury (n = 12) or sham surgery (n = 12). One week after RLN injury, larynges were harvested following euthanasia. The mRNA was extracted from PCA and AC muscles bilaterally, and microarray analysis was performed using a full rat genome array.

RESULTS

Microarray analysis of denervated AC and PCA muscles demonstrated dramatic differences in gene expression profiles, with 205 individual probes that were differentially expressed between the denervated AC and PCA muscles and only 14 genes with similar expression patterns.

CONCLUSIONS

The differential expression patterns of the AC and PCA suggest different mechanisms of reinnervation. The PCA showed the gene patterns of Wallerian degeneration, while the AC expressed the gene patterns of reinnervation by adjacent axonal sprouting. This finding may reveal important therapeutic targets applicable to RLN and other peripheral nerve injuries.

Transcriptional Pathways Associated with Skeletal Muscle Changes after Spinal Cord Injury and Treadmill Locomotor Training

The genetic and molecular events associated with changes in muscle mass and function after SCI and after the implementation of candidate therapeutic approaches are still not completely known. The overall objective of this study was to identify key molecular pathways activated with muscle remodeling after SCI and locomotor training. We implemented treadmill training in a well-characterized rat model of moderate SCI and performed genome wide expression profiling on soleus muscles at multiple time points: 3, 8, and 14 days after SCI. We found that the activity of the protein ubiquitination and mitochondrial function related pathways was altered with SCI and corrected with treadmill training. The BMP pathway was differentially activated with early treadmill training as shown by Ingenuity Pathway Analysis. The expression of several muscle mass regulators was modulated by treadmill training, including Fst, Jun, Bmpr2, Actr2b, and Smad3. In addition, key players in fatty acids metabolism (Lpl and Fabp3) responded to both SCI induced inactivity and reloading with training. The decrease in Smad3 and Fst early after the initiation of treadmill training was confirmed by RT-PCR. Our data suggest that TGFβ/Smad3 signaling may be mainly involved in the decrease in muscle mass observed with SCI, while the BMP pathway was activated with treadmill training.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.

Far-Infrared Therapy Promotes Nerve Repair following End-to-End Neurorrhaphy in Rat Models of Sciatic Nerve Injury

This study employed a rat model of sciatic nerve injury to investigate the effects of postoperative low-power far-infrared (FIR) radiation therapy on nerve repair following end-to-end neurorrhaphy. The rat models were divided into the following 3 groups: (1) nerve injury without FIR biostimulation (NI/sham group); (2) nerve injury with FIR biostimulation (NI/FIR group); and (3) noninjured controls (normal group). Walking-track analysis results showed that the NI/FIR group exhibited significantly higher sciatic functional indices at 8 weeks after surgery (P < 0.05) compared with the NI/sham group. The decreased expression of CD4 and CD8 in the NI/FIR group indicated that FIR irradiation modulated the inflammatory process during recovery. Compared with the NI/sham group, the NI/FIR group exhibited a significant reduction in muscle atrophy (P < 0.05). Furthermore, histomorphometric assessment indicated that the nerves regenerated more rapidly in the NI/FIR group than in the NI/sham group; furthermore, the NI/FIR group regenerated neural tissue over a larger area, as well as nerve fibers of greater diameter and with thicker myelin sheaths. Functional recovery, inflammatory response, muscular reinnervation, and histomorphometric assessment all indicated that FIR radiation therapy can accelerate nerve repair following end-to-end neurorrhaphy of the sciatic nerve.

Morphological Proof of nerve regeneration after long-term defects of rat sciatic nerves

Unsatisfactory efficacy of clinical cure for long-term delayed injuries and other disadvantages such as the low regeneration rate and speed of axotomized neurons and the questionable reinnervation ability of atrophic target organ lead to inaction to the long-term delayed injuries. Here we attempted to use autologous nerve to bridge a long-term delayed 10-mm defect in SD rats based on some previous positive messages of basic and clinical research. In this study, for experimental groups, the rat sciatic nerve had been transected leaving a 10-mm defect, which was maintained for 3 or 6 months before implantation with the autologous graft. The non-grafted animals served as negative control. Measuring with electrophysiological and histological techniques, we find: (1) A number of long-term axotomized neurons survived and sustained certain degree of axonal regenerative capacity; (2) A few denervated Schwann cells survived and retained their ability to provide trophic support and myelinate axons in at least 6 months; (3) the chronically denervated muscle can partially be reinnervated by regenerated axons. But the quantity and the quality of the regenerated nerve fibers and the reinnervated muscle fibers were all poor. Thus these observations provide new positive morphological proof of nerve regeneration after long-term defects and further studies will be needed to increase the survival rate and the regenerative speed of long-term chronic axotomized neurons, enhance the support provided by denervated distal stumps and protect the target muscle.

Morphological differences in skeletal muscle atrophy of rats with motor nerve and/or sensory nerve injury

Skeletal muscle atrophy occurs after denervation. The present study dissected the rat left ventral root and dorsal root at L_4-6 or the sciatic nerve to establish a model of simple motor nerve injury, sensory nerve injury or mixed nerve injury. Results showed that with prolonged denervation time, rats with simple motor nerve injury, sensory nerve injury or mixed nerve injury exhibited abnormal behavior, reduced wet weight of the left gastrocnemius muscle, decreased diameter and cross-sectional area and altered ultrastructure of muscle cells, as well as decreased cross-sectional area and increased gray scale of the gastrocnemius muscle motor end plate. Moreover, at the same time point, the pathological changes were most severe in mixed nerve injury, followed by simple motor nerve injury, and the changes in simple sensory nerve injury were the mildest. These findings indicate that normal skeletal muscle morphology is maintained by intact innervation. Motor nerve injury resulted in larger damage to skeletal muscle and more severe atrophy than sensory nerve injury. Thus, reconstruction of motor nerves should be considered first in the clinical treatment of skeletal muscle atrophy caused by denervation.

The order of exercise during concurrent training for rehabilitation does not alter acute genetic expression, mitochondrial enzyme activity or improvements in muscle function

Concurrent exercise combines different modes of exercise (e.g., aerobic and resistance) into one training protocol, providing stimuli meant to increase muscle strength, aerobic capacity and mass. As disuse is associated with decrements in strength, aerobic capacity and muscle size concurrent training is an attractive modality for rehabilitation. However, interference between the signaling pathways may result in preferential improvements for one of the exercise modes. We recruited 18 young adults (10 ♂, 8 ♀) to determine if order of exercise mode during concurrent training would differentially affect gene expression, protein content and measures of strength and aerobic capacity after 2 weeks of knee-brace induced disuse. Concurrent exercise sessions were performed 3x/week for 6 weeks at gradually increasing intensities either with endurance exercise preceding (END>RES) or following (RES>END) resistance exercise. Biopsies were collected from the vastus lateralis before, 3 h after the first exercise bout and 48 h after the end of training. Concurrent exercise altered the expression of genes involved in mitochondrial biogenesis (PGC-1α, PRC, PPARγ), hypertrophy (PGC-1α4, REDD2, Rheb) and atrophy (MuRF-1, Runx1), increased electron transport chain complex protein content, citrate synthase and mitochondrial cytochrome c oxidase enzyme activity, muscle mass, maximum isometric strength and VO_2peak. However, the order in which exercise was completed (END>RES or RES>END) only affected the protein content of mitochondrial complex II subunit. In conclusion, concurrent exercise training is an effective modality for the rehabilitation of the loss of skeletal muscle mass, maximum strength, and peak aerobic capacity resulting from disuse, regardless of the order in which the modes of exercise are performed.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Skeletal muscle atrophy and the E3 ubiquitin ligases MuRF1 and MAFbx/atrogin-1

Muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/atrogin-1 were identified more than 10 years ago as two muscle-specific E3 ubiquitin ligases that are increased transcriptionally in skeletal muscle under atrophy-inducing conditions, making them excellent markers of muscle atrophy. In the past 10 years much has been published about MuRF1 and MAFbx with respect to their mRNA expression patterns under atrophy-inducing conditions, their transcriptional regulation, and their putative substrates. However, much remains to be learned about the physiological role of both genes in the regulation of mass and other cellular functions in striated muscle. Although both MuRF1 and MAFbx are enriched in skeletal, cardiac, and smooth muscle, this review will focus on the current understanding of MuRF1 and MAFbx in skeletal muscle, highlighting the critical questions that remain to be answered.

Mechanosensitivity may be enhanced in skeletal muscles of spinal cord-injured versus able-bodied men

We investigated the effects of an acute bout of neuromuscular electrical stimulation-induced resistance exercise (NMES-RE) on intracellular signaling pathways involved in translation initiation and mechanical loading-induced muscle hypertrophy in spinal cord-injured (SCI) versus able-bodied (AB) individuals. AB and SCI individuals completed 90 isometric knee extension contractions at 30% of maximum voluntary or evoked contraction, respectively. Muscle biopsies were collected before, and 10 and 60 min after NMES-RE. Protein levels of α7- and β1-integrin, phosphorylated and total GSK-3α/β, S6K1, RPS6, 4EBP1, and FAK were assessed by immunoblotting. SCI muscle appears to be highly sensitive to muscle contraction even several years after the injury, and in fact it may be more sensitive to mechanical stress than AB muscle. Heightened signaling associated with muscle mechanosensitivity and translation initiation in SCI muscle may be an attempted compensatory response to offset elevated protein degradation in atrophied SCI muscle. .

Skeletal muscle dysfunction in idiopathic pulmonary arterial hypertension

Despite improvements in survival with disease-targeted therapies, the majority of patients with pulmonary arterial hypertension (PAH) have persistent exercise intolerance that results from impaired cardiac function and skeletal muscle dysfunction. Our intent was to understand the molecular mechanisms mediating skeletal muscle dysfunction in PAH. A total of 12 patients with PAH and 10 matched control subjects were assessed. Patients with PAH demonstrated diminished exercise capacity (lower oxygen uptake max, lower anaerobic threshold and higher minute ventilation/CO2) compared with control subjects. Quadriceps muscle cross-sectional area was significantly smaller in patients with PAH. The vastus lateralis muscle was biopsied to enable muscle fiber morphometric assessment and to determine expression levels/activation of proteins regulating (1) muscle mass, (2) mitochondria biogenesis and shaping machinery, and (3) excitation-contraction coupling. Patients with PAH demonstrated a decreased type I/type II muscle fiber ratio, with a smaller cross-sectional area in the type I fibers. Diminished AKT and p70S6 kinase phosphorylation, with increased atrogin-1 and muscle RING-finger protein-1 transcript levels, were evident in the PAH muscle, suggesting engagement of cellular signaling networks stimulating ubiquitin-proteasome-mediated proteolysis of muscle, with concurrent depression of networks mediating muscle hypertrophy. Although there were no differences in expression/activation of proteins associated with mitochondrial biogenesis or fission (MTCO2 [cytochrome C oxidase subunit II]/succinate dehydrogenase flavoprotein subunit A, mitochondrial transcription factor A, nuclear respiratory factor-1/dynamin-related protein 1 phosphorylation), protein levels of a positive regulator of mitochondrial fusion, Mitofusin2, were significantly lower in patients with PAH. Patients with PAH demonstrated increased phosphorylation of ryanodine receptor 1 receptors, suggesting that altered sarcoplasmic reticulum Ca(++) sequestration may impair excitation-contraction coupling in the PAH muscle. These data suggest that muscle dysfunction in PAH results from a combination of muscle atrophy and intrinsically impaired contractility.

The Biology of Long-Term Denervated Skeletal Muscle

This review concentrates on the biology of long-term denervated muscle, especially as it relates to newer techniques for restoring functional mass. After denervation, muscle passes through three stages: 1) immediate loss of voluntary function and rapid loss of mass, 2) increasing atrophy and loss of sarcomeric organization, and 3) muscle fiber degeneration and replacement of muscle by fibrous connective tissue and fat. Parallel to the overall program of atrophy and degeneration is the proliferation and activation of satellite cells, and the appearance of neomyogenesis within the denervated muscle. Techniques such as functional electrical stimulation take advantage of this capability to restore functional mass to a denervated muscle.

ICU-acquired weakness: mechanisms of disability

PURPOSE OF REVIEW

ICU-acquired weakness (ICUAW) is now recognized as a major complication of critical illness. There is no doubt that ICUAW is prevalent - some might argue ubiquitous - after critical illness, but its true role, the interaction with preexisting nerve and muscle lesions as well as its contribution to long-term functional disability, remains to be elucidated.

RECENT FINDINGS

In this article, we review the current state-of-the-art of the basic pathophysiology of nerve and muscle weakness after critical illness and explore the current literature on ICUAW with a special emphasis on the most important mechanisms of weakness.

SUMMARY

Variable contributions of structural and functional changes likely contribute to both early and late myopathy and neuropathy, although the specifics of the temporality of both processes, and the influence patient comorbidities, age, and nature of the ICU insult have on them, remain to be determined.