NERVE LESIONS IN INDUSTRY—Atrophy of Disuse as a Confusing Element in Diagnosis; The Value of Electromyography

Traumatic peripheral nerve lesions characteristically result in denervation muscular atrophy. Atrophy of disuse may take place concomitantly, either proximal, adjacent to or distal to the denervation muscular atrophy. The degree of atrophy of disuse depends upon the severity of the nerve lesion. Clinically, it is difficult to determine where true denervation muscular atrophy ends and accompanying atrophy of disuse begins. In such circumstances a clinician may be misled into belief that the cause of so apparently extensive a lesion is elsewhere. The patient then is often submitted to other complex diagnostic procedures and treatments. This difficulty can usually be dissipated by the use of electromyography, for each specific type of muscular atrophy produces its own characteristic electromyographic changes. Disuse atrophy produces no changes in electrical activity, whereas denervation atrophy manifests itself by typical denervation activity. Moreover it is possible to determine what part of muscular atrophy in a given area is owing to damage to a nerve and what part is owing only to disuse without denervation.     

Electromyography; uses and limitations

A normal muscle at rest emits no detectable electric current, but in action, in diseases of the muscle and in denervation it emits electric impulses characteristic of these states. The impulses can be amplified and studied through the sonic and oscilloscopic patterns they create. These patterns are sufficiently different so that simple atrophy of disuse can be distinguished from the denervation that may be associated with it. Since denervation can be localized to individual muscles and thence to the nerves controlling them, electromyography serves much the same function as myelography, with comparable accuracy and with greater safety and simplicity. It aids in the diagnosis of several muscular diseases of children and adults. Because electromyographic changes due to injury do not appear until 18 to 21 days later, a study made soon after injury can either disclose or rule out preexisting lesions. Then a later study indicating denervation is objective evidence that any disability is due to the injury in question.     

ELECTROMYOGRAPHY—Uses and Limitations

A normal muscle at rest emits no detectable electric current, but in action, in diseases of the muscle and in denervation it emits electric impulses characteristic of these states. The impulses can be amplified and studied through the sonic and oscilloscopic patterns they create. These patterns are sufficiently different so that simple atrophy of disuse can be distinguished from the denervation that may be associated with it. Since denervation can be localized to individual muscles and thence to the nerves controlling them, electromyography serves much the same function as myelography, with comparable accuracy and with greater safety and simplicity. It aids in the diagnosis of several muscular diseases of children and adults.Because electromyographic changes due to injury do not appear until 18 to 21 days later, a study made soon after injury can either disclose or rule out preexisting lesions. Then a later study indicating denervation is objective evidence that any disability is due to the injury in question.     

Motoneurone Dysfunction in Patients with Hemiplegie Atrophy

THE nature of the mechanism responsible for the wasting of muscles in patients after lesions of the upper moto-neurone has been the subject of many studies^1,2 and various possibilities are summarized in Fig. 1. The simplest mechanism would be disuse alone; but atrophy might also result from disturbed blood flow in muscles of paralysed limbs or from a secondary arthritis. Another possibility is that an upper moto-neurone lesion deprives muscle of a trophic influence which is normally exerted, through an unspecified route, by the pre-central or postcentral gyrus. Finally, the upper motoneurone lesion may cause secondary changes in lower motoneurones which, in turn, affect muscles. The most widely held opinion is that wasting results from disuse only³, but we have findings which demonstrate that this supposition is incorrect. Instead it seems probable that the most important single factor in the genesis of atrophy is denervation of muscle fibres secondary to disturbed lower motoneurone function.     

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.     

Effects of Denervation and Simple Disuse on Rates of Oxidation and on Activities of Four Mitochondrial Enzymes in Type I Muscle

To differentiate the effect of muscle contractile activity from that of motor nerve on oxidative processes in type I muscle, oxidative processes were studied in muscle after immobilization and after denervation. The two processes led to similar atrophy of muscle weight and of the mean diameter of muscle fibers. Disuse of soleus muscle (type I) did not affect rates of oxidation of 14C-labeled substrates although these were reduced by disuse of the vastus lateralis (type II). Disuse of the soleus did not affect activities of several mitochondrial enzymes assayed by histochemical or biochemical methods. However, denervation of the soleus did lead to a fall in metabolic rates and enzyme activities. The activity of 3-hydroxybutyrate dehydrogenase fell more than did the activities of succinic dehydrogenase, lipoamide dehydrogenase, or cytochrome-c oxidase in both homogenates and in mitochondrial fractions. These results suggest nerve may regulate mitochondrial enzymes in type I muscle. The mechanism appears to be different from that which regulates oxidative processes in type II muscle.     

Denervation atrophy is independent from Akt and mTOR activation and is not rescued by myostatin inhibition

The purpose of our study was to compare two acquired muscle atrophies and the use of myostatin inhibition for their treatment. Myostatin naturally inhibits skeletal muscle growth by binding to ActRIIB, a receptor on the cell surface of myofibers. Because blocking myostatin in an adult wild-type mouse induces profound muscle hypertrophy, we applied a soluble ActRIIB receptor to models of disuse (limb immobilization) and denervation (sciatic nerve resection) atrophy. We found that treatment of immobilized mice with ActRIIB prevented the loss of muscle mass observed in placebo-treated mice. Our results suggest that this protection from disuse atrophy is regulated by serum and glucocorticoid-induced kinase (SGK) rather than by Akt. Denervation atrophy, however, was not protected by ActRIIB treatment, yet resulted in an upregulation of the pro-growth factors Akt, SGK and components of the mTOR pathway. We then treated the denervated mice with the mTOR inhibitor rapamycin and found that, despite a reduction in mTOR activation, there is no alteration of the atrophy phenotype. Additionally, rapamycin prevented the denervation-induced upregulation of the mTORC2 substrates Akt and SGK. Thus, our studies show that denervation atrophy is not only independent from Akt, SGK and mTOR activation but also has a different underlying pathophysiological mechanism than disuse atrophy.KEY WORDS: Skeletal muscle, Muscle atrophy pathophysiology, TGF-β signaling, Myostatin, Denervation atrophy     

Trophic interrelations at the neuromuscular junction as revealed by the use of botulinal neurotoxins

1. From denervation studies the trophic influence of the motor nerve on the muscle cell is well documented while little is known about the influence of the muscle on the nerve. Sectioning the axon invariably destroys the nerve terminals and produces nerve degeneration products which themselves may affect nerve and muscle properties. With regard to those difficulties we believe that the botulinal neurotoxins (BoTx) are valuable complements to denervation since they selectively interrupt impulse transmission across the synapse without damaging its morphology. 2. Paralysis of mouse or rat skeletal muscle in vivo with BoTx type A causes marked growth of motor nerve terminals. The sprouting terminals are rich in large dense-core synaptic vesicles containing various neuropeptides and they spontaneously release large quanta of ACh. Thus, it appears that paralysis by BoTx is a strong stimulus for motor nerve growth and the delivery of "trophic" substances to the nerve terminals. 3. Postsynaptically, in extrajunctional areas, paralysis by BoTx induces all the changes observed following denervation, i.e. atrophy, appearance of extra-junctional ACh receptors, TTX-resistant action potentials, a fall of resting membrane potential, fibrillation potentials and the disappearance of extrajunctional acetylcholinesterase activity. Endplate properties are, however, largely maintained. 4. BoTx blockade delays and prevents the retraction of polyneuronal innervation and motoneurone death during development. This supports the suggestion that the paralysed muscle secretes factors essential for growth and for the survival of motoneurones. 5. Like denervated muscle, BoTx paralysed ones, express a high endocytotic activity restricted to a segment in the endplate region.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of reinnervation on collagen synthesis in rat skeletal muscle.

The effect of reinnervation on the activities of prolyl 4-hydroxylase (PH) and galactosylhydroxylysyl glucosyltransferase (GGT), both enzymes of collagen biosynthesis, and on the concentration of hydroxyproline (Hyp) was studied in gastrocnemius, soleus, and tibialis anterior muscles of rat 19, 26, 40, and 61 days after crush denervation of the sciatic nerve. The GGT activity was elevated in denervated gastrocnemius and soleus muscles and the PH activity in gastrocnemius. Muscular Hyp concentration was increased in denervated tibialis anterior muscle. Both the PH and GGT activities and the Hyp concentration returned to the control level during the reinnervation period (19-61 days from the start of denervation). It seems that denervation atrophy of skeletal muscle is associated with an increased rate of muscular collagen biosynthesis and that during reinnervation collagen synthesis rate decreases despite accelerated muscular growth. The results thus suggest that innervation is a powerful suppressive regulator of muscular collagen biosynthesis.     

Effect of prior chronic contractile activity on mitochondrial function and apoptotic protein expression in denervated muscle.

Skeletal muscle is highly adaptable in response to increases and decreases in contractile activity. The purpose of this study was to determine whether the preconditioning of skeletal muscle has a protective effect against subsequent denervation-induced apoptotic protein expression. To investigate this, we chronically stimulated the tibialis anterior and extensor digitorum longus muscles for 7 days (10 Hz, 3 h/day) before 7 days of denervation. Denervation reduced total cytochrome-c oxidase activity by 39%, which was likely a consequence of a decrease in subsarcolemmal (SS) mitochondria. This decrease in the SS subfraction was prevented by prior chronic stimulation and, as a result, maintained total mitochondrial content at control levels. The expression of Bax was elevated 2.2-fold by denervation, and prior chronic stimulation did not attenuate this increase. This produced a increase in the Bax-to-Bcl-2 ratio, indicating greater muscle apoptotic susceptibility. Denervation also decreased state 3 respiration in SS and intermyofibrillar mitochondria and elevated state 4 reactive oxygen species production within both mitochondrial subfractions. These changes were not prevented by prior chronic stimulation. Furthermore, the antioxidant protein MnSOD was also reduced by denervation, whereas Beclin-1 was markedly elevated. This suggests that autophagic cell death could also play a significant part in denervation-induced muscle atrophy. Thus, despite prior chronic stimulation, denervation increases the apoptotic susceptibility of skeletal muscle by altering the Bax-to-Bcl-2 ratio, by increasing reactive oxygen species production, and by reducing the expression of MnSOD. Whether a more extensive stimulation paradigm would be more effective in attenuating apoptosis before muscle disuse remains to be determined.     

Variations of Glucocorticoid Receptors in Intact or Denervated Muscles: Lack of Cause-Effect Relationship with Muscle Atrophy in the Rat

Abstract

The process of muscular atrophy following denervation has been tentatively ascribed to the influence of glucocorticoids (G) because of the rapid increase of cytosolic G receptors (R^G) after sciatic nerve section. It appears however that the level of muscular atrophy is similar: 1. in slow or fast-twitch muscles in spite of huge variations in R^G; 2. in intact or adrenalectomized (ADR-X) rats. Moreover, the protein muscle profile of intact or ADR-X rats after gel electrophoresis is similar but drastically decreased after 3 weeks of denervation. We conclude that there is no causeeffect relationship between muscle atrophy and R^G elevation after nerve section.     

Denervation-induced mitochondrial dysfunction and autophagy in skeletal muscle of apoptosis-deficient animals

Skeletal muscle undergoes remarkable adaptations in response to chronic decreases in contractile activity, such as a loss of muscle mass, decreases in both mitochondrial content and function, as well as the activation of apoptosis. Although these adaptations are well known, questions remain regarding the signaling pathways that mediated these changes. Autophagy is an organelle turnover pathway that could contribute to these adaptations. The purpose of this study was to determine whether denervation-induced muscle disuse would result in the activation of autophagy gene expression in both wild-type (WT) and Bax/Bak double knockout (DKO) animals, which display an attenuated apoptotic response. Denervation caused a reduction in muscle mass for WT and DKO animals; however, there was a 40% attenuation in muscle atrophy in DKO animals. Mitochondrial state 3 respiration was significantly reduced, and reactive oxygen species production was increased by two- to threefold in both WT and DKO animals. Apoptotic markers, including cytosolic AIF and DNA fragmentation, were elevated in WT, but not in DKO animals following denervation. Autophagy proteins including LC3II, ULK1, ATG7, p62, and Beclin1 were increased similarly following denervation for both WT and DKO. Interestingly, denervation markedly increased the localization of LC3II to subsarcolemmal mitochondria, and this was more pronounced in the DKO animals. Thus denervation-induced muscle disuse activates both apoptotic and autophagic signaling pathways in muscle, and autophagic protein expression does not exhibit a compensatory increase in the presence of attenuated apoptosis. However, the absence of Bax and Bak may represent a potential signal to trigger mitophagy in muscle.     

The TWEAK-Fn14 system: breaking the silence of cytokine-induced skeletal muscle wasting

The occurrence of skeletal muscle atrophy, a devastating complication of a large number of disease states and inactivity/disuse conditions, provides a never ending quest to identify novel targets for its therapy. Proinflammatory cytokines are considered the mediators of muscle wasting in chronic diseases; however, their role in disuse atrophy has just begun to be elucidated. An inflammatory cytokine, tumor necrosis factor (TNF)- like weak inducer of apoptosis (TWEAK), has recently been identified as a potent inducer of skeletal muscle wasting. TWEAK activates various proteolytic pathways and stimulates the degradation of myofibril protein both in vitro and in vivo. Moreover, TWEAK mediates the loss of skeletal muscle mass and function in response to denervation, a model of disuse atrophy. Adult skeletal muscle express very low to minimal levels of TWEAK receptor, Fn14. Specific catabolic conditions such as denervation, immobilization, or unloading rapidly increase the expression of Fn14 in skeletal muscle which in turn stimulates the TWEAK activation of various catabolic pathways leading to muscle atrophy. In this article, we have discussed the emerging roles and the mechanisms of action of TWEAK-Fn14 system in skeletal muscle with particular reference to different models of muscle atrophy and injury and its potential to be used as a therapeutic target for prevention of muscle loss.     

Regulation of dihydropyridine receptor gene expression in mouse skeletal muscles by stretch and disuse

This study examined dihydropyridine receptor (DHPR) gene expression in mouse skeletal muscles during physiological adaptations to disuse. Disuse was produced by three in vivo models—denervation, tenotomy, and immobilization—and DHPR _1s mRNA was measured by quantitative Northern blot. After 14-day simultaneous denervation of the soleus (Sol), tibialis anterior (TA), extensor digitorum longus (EDL), and gastrocnemius (Gastr) muscles by sciatic nerve section, DHPR mRNA increased preferentially in the Sol and TA (+1.6-fold), whereas it increased in the EDL (+1.6-fold) and TA (+1.8-fold) after selective denervation of these muscles by peroneal nerve section. It declined in all muscles (–1.3- to –2.6-fold) after 14-day tenotomy, which preserves nerve input but removes mechanical tension. Atrophy was comparable in denervated and tenotomized muscles. These results suggest that factor(s) in addition to inactivity per se, muscle phenotype, or associated atrophy can regulate DHPR gene expression. To test the contribution of passive tension to this regulation, we subjected the same muscles to disuse by limb immobilization in a maximally dorsiflexed position. DHPR _1s mRNA increased in the stretched muscles (Sol, +2.3-fold; Gastr, +1.5-fold) and decreased in the shortened muscles (TA, –1.4-fold; EDL, –1.3-fold). The effect of stretch was confirmed in vitro. DHPR protein did not change significantly after 4-day immobilization, suggesting that additional levels of regulation may exist. These results demonstrate that DHPR _1s gene expression is regulated as an integral part of the adaptive response of skeletal muscles to disuse in both slow- and fast-twitch muscles and identify passive tension as an important signal for its regulation in vivo. dihydropyridine receptor mRNA; decreased use; passive tension; denervation; tenotomy; hindlimb immobilization     

Specialization of mitochondrial and vascular oxidant modulated VEGFR in the denervated skeletal muscle

Denervation of skeletal muscles results in timely muscular inflammation and muscle-T cell interaction, the cellular events might orchestrate a local circuit involved with IL-1β and IL-15. In the present study, by a combination assay of nerve–muscle preparation, western blot, immuno-precipitation, and radioactive of enzyme activity, we confirmed that mitochondrial and vascular oxidants were considerably up-regulated following gastrocnemius denervation, which was due to gradual decay in mitochondrial biogenesis and XO pathway and accompanied by strengthened IL-1β-VEGFR-2 and IL-15-VEGFR-1 signaling. Intriguingly, these alterations could be triggered by the early established muscular inflammation. In contrast, with prolonged muscle denervation, settings of organelle interconnection were ultimately conveyed by ER bound PTP1B, which promoted VEGFR-1 signaling and contributed to VEGFR-2 activation, and the process could be modulated by mitochondrial and vascular oxidant. Importantly, VEGFR-2 could rescue the disruption of MuSK activity and AchR cluster exerted by IL-1β and IL-15, with PGC-1α and XO involvement. Altogether, extensive network centered on VEGFR-2 signaling was essentially contributed to early recovery processes regarding muscle denervation. Increasing knowledge of this mechanism might open up a conduit for functional response to muscle atrophy, and enable the development of better agents to combat the related disorders.     

Preventive effect of dietary quercetin on disuse muscle atrophy by targeting mitochondria in denervated mice

Quercetin is a major dietary flavonoid in fruits and vegetables. We aimed to clarify the preventive effect of dietary quercetin on disuse muscle atrophy and the underlying mechanisms. We established a mouse denervation model by cutting the sciatic nerve in the right leg (SNX surgery) to lack of mobilization in hind-limb. Preintake of a quercetin-mixed diet for 14 days before SNX surgery prevented loss of muscle mass and atrophy of muscle fibers in the gastrocnemius muscle (GM). Phosphorylation of Akt, a key phosphorylation pathway of suppression of protein degradation, was activated in the quercetin-mixed diet group with and without SNX surgery. Intake of a quercetin-mixed diet suppressed the generation of hydrogen peroxide originating from mitochondria and elevated mitochondrial peroxisome proliferator-activated receptor-γ coactivator 1α mRNA expression as well as NADH dehydrogenase 4 expression in the GM with SNX surgery. Quercetin and its conjugated metabolites reduced hydrogen peroxide production in the mitochondrial fraction obtained from atrophied muscle. In C2C12 myotubes, quercetin reached the mitochondrial fraction. These findings suggest that dietary quercetin can prevent disuse muscle atrophy by targeting mitochondria in skeletal muscle tissue through protecting mitochondria from decreased biogenesis and reducing mitochondrial hydrogen peroxide release, which can be related to decreased hydrogen peroxide production and/or improvements on antioxidant capacity of mitochondria.     

Observations on the fate of muscle fibres temporarily isolated by transection of a muscle belly

Summary Transection of the sternohyoid muscle of the rat has been used as an experimental situation in which the fate of the portions of fibre isolated from their nerve supply by the lesion can be studied. Sections from the muscle were stained to demonstrate oxidative and glycolytic enzymes and cholinesterase. Biopsies performed after periods of up to ten weeks after operation revealed a series of changes that suggested that after passing through the early stages of denervation atrophy recovery of the fibres took place. There was no indication that new motor end-plates were formed among the isolated fibres and it was concluded that communication had been reestablished with innervated fibres and that this reunion had been followed by a redetermination of the metabolic activity of the isolated fibres.I would like to thank Professor J. Z. Young for his advice and encouragement and Mrs. C. A. Joseph for her histochemical preparations.     

Activating adenosine A1 receptor accelerates PC12 cell injury via ADORA1/PKC/KATP pathway after intermittent hypoxia exposure

Skeletal muscle atrophy occurs in different catabolic conditions and mostly accompanied with upregulation of Muscle ring finger 1 (MuRF1) gene which is one of the master regulatory genes in muscle atrophy. Taurine amino acid is widely distributed in different tissues and has anti-inflammatory and antioxidant effects. This study aimed to investigate the potential influence of taurine on muscle atrophy induced by reduced mechanical loading. Twenty-eight Albino mice were used, and divided equally into four groups: group I (control); group II (immobilization); group III (immobilization?+?taurine); and group IV (taurine). Quadriceps muscle sections were taken for histopathology, immunohistochemical analysis of caspase 3 expression, and qRT-PCR of MuRF1 gene. Our data revealed Zenker necrosis associated with axonal injury of the nerve trunk of the immobilized muscle together with increase of caspase 3 expression and upregulation of MuRF1 gene. While, taurine supplementation alleviated the muscular and neural tissues damage associated with disuse skeletal muscle atrophy through downregulation of MuRF1 gene and decrease of tissue caspase 3 expression. In conclusion, taurine may be helpful to counteract apoptosis and up-regulated MuRF1 gene expression related to muscle atrophy, which might be hopeful for a large number of patients.     

Activation of the vasoactive intestinal peptide 2 receptor modulates normal and atrophying skeletal muscle mass and force.

Of the two known vasoactive intestinal peptide receptors (VPAC1R and VPAC2R), the VPAC2R is expressed in skeletal muscle. To evaluate the function of the VPAC2R in the physiological control of skeletal muscle mass, we utilized the VPAC1R selective agonist [K15,R16,L27]VIP(1-7) GRF(8-27)-NH2 and the VPAC2R selective agonist Ro-25-1553 to treat mice and rats undergoing either nerve damage-, corticosteroid-, or disuse-induced skeletal muscle atrophy. These analyses demonstrated that activation of VPAC2R, but not VPAC1R, reduced the loss of skeletal muscle mass and force during conditions of skeletal muscle atrophy resulting from corticosteroid administration, denervation, casting-induced disuse, increased skeletal muscle mass, and force of nonatrophying muscles. These studies indicate that VPAC2R agonists may have utility for the treatment of skeletal muscle-wasting diseases.