Secreted Signaling Molecules at the Neuromuscular Junction in Physiology and Pathology

Signal transduction at the neuromuscular junction (NMJ) is affected in many human diseases, including congenital myasthenic syndromes (CMS), myasthenia gravis, Lambert–Eaton myasthenic syndrome, Isaacs’ syndrome, Schwartz–Jampel syndrome, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. The NMJ is a prototypic cholinergic synapse between the motor neuron and the skeletal muscle. Synaptogenesis of the NMJ has been extensively studied, which has also been extrapolated to further understand synapse formation in the central nervous system. Studies of genetically engineered mice have disclosed crucial roles of secreted molecules in the development and maintenance of the NMJ. In this review, we focus on the secreted signaling molecules which regulate the clustering of acetylcholine receptors (AChRs) at the NMJ. We first discuss the signaling pathway comprised of neural agrin and its receptors, low-density lipoprotein receptor-related protein 4 (Lrp4) and muscle-specific receptor tyrosine kinase (MuSK). This pathway drives the clustering of acetylcholine receptors (AChRs) to ensure efficient signal transduction at the NMJ. We also discuss three secreted molecules (Rspo2, Fgf18, and connective tissue growth factor (Ctgf)) that we recently identified in the Wnt/β-catenin and fibroblast growth factors (FGF) signaling pathways. The three secreted molecules facilitate the clustering of AChRs by enhancing the agrin-Lrp4-MuSK signaling pathway.

Skeletal Muscle Channelopathies

Skeletal muscle channelopathies are rare genetic neuromuscular conditions that include the nondystrophic myotonias and periodic paralyses. They cause disabling muscle symptoms and can limit educational potential, work opportunities, socialization, and quality of life. Effective therapy is available, making it essential to recognize and treat this group of disorders. Here, the authors highlight important aspects regarding diagnosis and management using illustrative case reports.

[Electrophysiological evidence of impaired neuromuscular junction in a case of phosphoglucomutase 1 deficiency manifesting fluctuating muscle weakness]

A 27 year-old Canadian man suffered from fluctuating muscle weakness in the past several years. The patient had a past history of intestinal bleeding, bifid uvula and hypothyroidism in his childhood. Repetitive nerve stimulation tests showed a decrement pattern in the left deltoid muscle. The single fiber electromyography of the left extensor digitorum muscle showed an increment of jitter. Both findings were improved by the edrophonium test. He was diagnosed as having phosphoglucomutase 1 (PGM1) deficiency, as the compound heterozygote mutation of the PGM1 gene was recognized in the whole-exome sequencing and the enzyme activity of PGM1 was defective in the biopsied muscle. Treatment with the galactose lead to improvement of the fluctuating muscle weakness and decremental pattern in the repetitive stimulation test. PGM1 deficiency should be listed in the differential diagnosis of the neuromuscular junction disorder, when the patient is seronegative for antibodies related with myasthenia gravis and shows symptoms or signs consistent with PGM1 deficiency.

Chronic TrkB agonist treatment in old age does not mitigate diaphragm neuromuscular dysfunction

Previously, we found that brain-derived neurotrophic factor (BDNF) signaling through the high-affinity tropomyosin-related kinase receptor subtype B (TrkB) enhances neuromuscular transmission in the diaphragm muscle. However, there is an age-related loss of this effect of BDNF/TrkB signaling that may contribute to diaphragm muscle sarcopenia (atrophy and force loss). We hypothesized that chronic treatment with 7,8-dihydroxyflavone (7,8-DHF), a small molecule BDNF analog and TrkB agonist, will mitigate age-related diaphragm neuromuscular transmission failure and sarcopenia in old mice. Adult male TrkBF616A mice (n = 32) were randomized to the following 6-month treatment groups: vehicle-control, 7,8-DHF, and 7,8-DHF and 1NMPP1 (an inhibitor of TrkB kinase activity in TrkBF616A mice) cotreatment, beginning at 18 months of age. At 24 months of age, diaphragm neuromuscular transmission failure, muscle-specific force, and fiber cross-sectional areas were compared across treatment groups. The results did not support our hypothesis in that chronic 7,8-DHF treatment did not improve diaphragm neuromuscular transmission or mitigate diaphragm muscle sarcopenia. Taken together, these results do not exclude a role for BDNF/TrkB signaling in aging-related changes in the diaphragm muscle, but they do not support the use of 7,8-DHF as a therapeutic agent to mitigate age-related neuromuscular dysfunction.

A single-fibre EMG Study of Neuromuscular Transmission in Migraine Patients

It is known that mutations of CACNA1A, which encodes a neuronal P/Q Ca2+ channel, are present in patients with familial hemiplegic migraine, and possibly in other types of migraine as well. This calcium channel is also involved in neuromuscular transmission. To assess if the single-fibre EMG (SFEMG) method can demonstrate a neuromuscular transmission deficit in migraine, a group of 26 patients with different types of migraine and 20 healthy control subjects were studied. The migraine patients were divided into three groups: 8 patients with migraine without aura (MoA), 12 with migraine with aura excluding visual aura (MA) and 6 with visual aura (VA). A SFEMG of the voluntarily activated extensor digitorum communis muscle was performed. The SFEMG results were normal in the healthy controls and the MoA group (migraine without aura). Slight neuromuscular transmission disturbances were present in 6/12 (50%) of patients with MA and in 1/6 (17%) of patients with VA. We suggest that abnormal neuromuscular transmission detectable by SFEMG may reflect a genetically determined dysfunction of the P/Q Ca2+ channels in a subgroup of migraineurs with aura.

Congenital diseases of feline muscle and neuromuscular junction

Although muscle diseases occur relatively rarely in cats, a number of congenital feline myopathies have been described over the last 20 years and are reviewed in this paper. Some of them have been reported exclusively in specific breeds, including the hypokalaemic myopathy of Burmese cats, type IV glycogen storage disease in Norwegian Forest cats, or the myopathy of Devon Rex. Other congenital disorders of muscle and neuromuscular junction such as myotonia congenita, dystrophin-deficient hypertrophic feline muscular dystrophy, laminin α2 deficiency, or congenital myasthenia gravis may occur in any cat. A systematic approach is essential in order to efficiently obtain a timely diagnosis in cats showing signs of muscle disease. After a thorough clinical examination, this approach includes blood analyses (eg, serum concentration of muscle enzymes), electrophysiology where available (electromyography, nerve conduction studies), and sampling of muscle biopsies for histological, histochemical and immunohistochemical evaluation. When available, detection of healthy carriers of these genetic disorders is important to eliminate the gene mutations from breeding families. Clinicians regularly receiving feline patients must have a good knowledge of congenital feline myopathies and the features which enable a diagnosis to be made and prognosis given. Besides preserving or restoring the well-being of the myopathic patient, rapid and efficient information and counselling of the breeders are of central importance in order to prevent the recurrence of the problem in specific breeding lines.

Acetylcholinesterase deficiency contributes to neuromuscular junction dysfunction in type 1 diabetic neuropathy

Diabetic neuropathy is associated with functional and morphological changes of the neuromuscular junction (NMJ) associated with muscle weakness. This study examines the effect of type 1 diabetes on NMJ function. Swiss Webster mice were made diabetic with three interdaily ip injections of streptozotocin (STZ). Mice were severely hyperglycemic within 7 days after the STZ treatment began. Whereas performance of mice on a rotating rod remained normal, the twitch tension response of the isolated extensor digitorum longus to nerve stimulation was reduced significantly at 4 wk after the onset of STZ-induced hyperglycemia. This mechanical alteration was associated with increased amplitude and prolonged duration of miniature end-plate currents (mEPCs). Prolongation of mEPCs was not due to expression of the embryonic acetylcholine receptor but to reduced muscle expression of acetylcholine esterase (AChE). Greater sensitivity of mEPC decay time to the selective butyrylcholinesterase (BChE) inhibitor PEC suggests that muscle attempts to compensate for reduced AChE levels by increasing expression of BChE. These alterations of AChE are attributed to STZ-induced hyperglycemia since similar mEPC prolongation and reduced AChE expression were found for db/db mice. The reduction of muscle end-plate AChE activity early during the onset of STZ-induced hyperglycemia may contribute to endplate pathology and subsequent muscle weakness during diabetes.

Signaling and aging at the neuromuscular synapse: lessons learnt from neuromuscular diseases

The neuromuscular junction (NMJ) is a specialized synapse between motor neurons and skeletal muscle with a complex signaling network that assures highly reliable neuromuscular transmission. Diseases of the NMJ cause skeletal muscle fatigue and include inherited and acquired disorders that affect presynaptic, intrasynaptic or postsynaptic components. Moreover, fragmentation of the NMJ contributes to sarcopenia, the loss of muscle mass during aging. Studies from recent years indicate that the formation and stabilization of NMJs differs between various muscles and that this difference affects their response under pathological conditions. This review summarizes the most important mechanisms involved in the development, maintenance and dysfunction of the NMJ and it discusses their significance in myasthenic disorders and aging and as targets for possible future treatment of NMJ dysfunction.

Paraneoplastic muscle disease

In paraneoplastic muscle disease, the malignancy may remotely affect neuromuscular transmission or incite muscle inflammation or necrosis. In several of these diseases, an autoimmune basis for the muscle disease has been established and has become a defining feature. These paraneoplastic muscle diseases may be the first manifestation of a malignancy, and their diagnosis thus demands a vigilant search for an underlying tumor. This article is focused on inflammatory and necrotizing myopathies and disorders of neuromuscular transmission that may arise in the setting of malignancy and are considered paraneoplastic phenomena.

[Neurophysiologic assessment of disorders affecting the neuromuscular junction]

The neuromuscular junction (NMJ) is a specialized synapse with a complex structural organization. Muscle contraction involves several steps: (1) nerve conduction to depolarize the motor nerve terminals, (2) opening of voltage-gated calcium channels (VGCCs) in the presynaptic membrane, (3) generation of endplate potential in the postsynaptic membrane via acetylcholine receptors, (4) depolarization of muscle sodium channels, and (5) excitation-contraction (E-C) coupling. Each step can be affected by various diseases. Guillain-Barre syndrome involves distal axons and possibly the presynaptic NMJ. The abnormalities can be detected by nerve conduction studies and single-fiber electromyography (SFEMG). Myasthenia gravis (MG) with anti-acetylcholine receptor (AChR) antibodies, is the most common NMJ disorder, and ~5% of myasthenia patients are positive for anti-muscle specific kinase (MuSK) antibodies. Patterns and severity of neuromuscular transmission failure detected by repetitive nerve stimulation test and SFEMG are somewhat different in AChR-MG and MuSK-MG. Excitation-contraction (E-C) coupling can be affected by MG, possibly via antibodies against ryanodine receptors. The E-C coupling time can be assessed with an accelerometer. Lambert-Eaton myasthenic syndrome is caused by antibodies against presynaptic VGCCs. This review will focus on neurophysiological testing, including SFEMG, and measurements of E-C coupling time with an accelerometer. In addition to confirming or excluding the diagnosis, these techniques can provide new insights into the pathophysiology of a variety of neuromuscular disorders.

Myasthenic syndrome due to defects in rapsyn: Clinical and molecular findings in 39 patients

BACKGROUND

Pathogenic mutations in rapsyn result in endplate acetylcholine receptor (AChR) deficiency and are a common cause of postsynaptic congenital myasthenic syndromes.

METHODS

Clinical, electrophysiologic, pathologic, and molecular studies were done in 39 patients.

RESULTS

In all but one patient, the disease presented in the first 2 years of life. In 9 patients, the myasthenic symptoms included constant or episodic ophthalmoparesis, and 1 patient had a pure limb-girdle phenotype. More than one-half of the patients experienced intermittent exacerbations. Long-term follow-up was available in 25 patients after start of cholinergic therapy: 21 became stable or were improved and 2 of these became asymptomatic; 3 had a progressive course; and 1 died in infancy. In 7 patients who had endplate studies, the average counts of AChR per endplate and the synaptic response to ACh were less reduced than in patients harboring low AChR expressor mutations. Eight patients were homozygous and 23 heterozygous for the common p.N88K mutation. Six mutations, comprising 3 missense mutations, an in-frame deletion, a splice-site mutation, and a nonsense mutation, are novel. Homozygosity for p.N88K was associated with varying grades of severity. No genotype-phenotype correlations were observed except in 8 Near-Eastern patients homozygous for the promoter mutation (c.-38A>G), who had a mild course.

CONCLUSIONS

All but 1 patient presented early in life and most responded to cholinergic agonists. With early diagnosis and therapy, rapsyn deficiency has a benign course in most patients. There was no consistent phenotype-genotype correlation except for an E-box mutation associated with jaw deformities.

Monozygous twins with neuromuscular transmission defects at opposite sides of the motor endplate

Disorders affecting the postsynaptic side of the neuromuscular junction include autoimmune myasthenia gravis (MG) as well as some of the congenital myasthenic syndromes (CMS). Lambert-Eaton myasthenic syndrome (LEMS) is an acquired autoimmune neuromuscular disorder in which autoantibodies are directed against the presynaptic calcium channels. Here we describe two monozygous twin brothers: case 1 was diagnosed with an indeterminate form of acquired postsynaptic neuromuscular junction defect at age 32 and case 2 with LEMS at age 47. Case 1 presented clinically with mild generalized myasthenic weakness, neurophysiological examination revealed disturbed neuromuscular transmission along with probable myositis and serum analysis regarding antibodies against the acetylcholine receptor and muscle-specific tyrosine kinase was negative. Case 2 presented with proximal muscle fatigue accompanied by areflexia at rest and antibodies against the P/Q-type voltage-gated calcium channels were present. Neurophysiologically, case 2 had reduced baseline compound motor action potential amplitudes on neurography, decrement on low-frequency repetitive nerve stimulation (RNS) and pathological increment on high frequency RNS. To our knowledge this is the first case report of its kind and adds an intriguing contrast to the more common diagnosis of CMS in monozygous twins.

Neuromuscular junction in health and disease

A number of illnesses and other factors can affect the function of the neuromuscular junction (NMJ). These may have an affect at pre- or post-junctional sites. This review outlines the anatomy and the physiology of the NMJ. It also describes the mechanisms and physiological basis of many of the disorders of the NMJ. Finally, the importance of these disorders in anaesthetic practice is discussed.

Rab3a deletion reduces vesicle docking and transmitter release at the mouse diaphragm synapse

Rab3a is a small GTP binding protein associated with presynaptic vesicles that is thought to regulate vesicle targeting to active zones. Although this rab3a function implies that vesicle docking and action potential-evoked release might be inhibited in rab3a gene-deleted synapses, such inhibition has never been demonstrated. To investigate vesicle docking at the neuromuscular junction of rab3a gene-deleted (rab3a(-)) mice, we performed electron microscopy analysis of the diaphragm slow-fatigue (type I) synapses. We found a significant (26%) reduction in the number of vesicles docked to the presynaptic membrane in rab3a(-) terminals, although intraterminal vesicles were not affected. Aiming to detect possible changes in quantal release due to rab3a gene deletion, we minimized the variability between preparations employing focal recordings of synaptic responses from visualized type I endplates. We found a significant decrease in both evoked (27% reduction in quantal content) and spontaneous (28% reduction in mini frequency) quantal release. The decrease in the evoked release produced by rab3a deletion was most pronounced at reduced extracellular Ca(2+) concentrations (over 50% decrease at 0.5 and 0.2 mM Ca(2+)). By manipulating extracellular calcium, we demonstrated that calcium cooperativity is not altered in rab3a(-) synapses, however calcium sensitivity of quantal release is affected. Thus, we demonstrated that rab3a positively regulates docking and basal quantal release at the mouse neuromuscular junction. This result is consistent with the proposed role of rab3a in trafficking and targeting vesicles to the active zones.

Outcome of acetylcholinesterase deficiency for neuromuscular functioning

Acetylcholinesterase (AChE) plays an essential role in neuromuscular transmission, therefore it is surprising that AChE knockout (KO) mice could live to the adulthood. Neuromuscular functioning in KO and normal (wild type, WT) mice were studied, at different age (1.5-, 4- and 9-month-old). Hindlimb muscle force productions in response to nerve or muscle electric stimulation were recorded in situ and in vitro. Our results show that contrary to WT mice, 1.5-, 4- and 9-month-old KO mice exhibited a decreased in tetanic force during short periods (500 ms) of repetitive nerve stimulations (tetanic fade). Nevertheless submaximal muscle forces in response to single or repetitive nerve stimulation were increased (potentiation) in 1.5-, 4- and 9-month-old KO mice as compared to WT mice (p<0.05). Tetanic fade and potentiation were absent when muscles were directly stimulated, indicating neuromuscular transmission alterations in KO mice. Contrary to younger mice, muscle weight and maximal tetanic force in response to repetitive nerve stimulation were not reduced in 4- and 9-month-old KO mice as compared to WT mice (p>0.05). In conclusion AChE deficit leads to marked neuromuscular alterations in hind limb muscle functioning and a prominent symptom is the lack of resistance to fatigue.

Mice Deficient for the Vesicular Acetylcholine Transporter Are Myasthenic and Have Deficits in Object and Social Recognition

An important step for cholinergic transmission involves the vesicular storage of acetylcholine (ACh), a process mediated by the vesicular acetylcholine transporter (VAChT). In order to understand the physiological roles of the VAChT, we developed a genetically altered strain of mice with reduced expression of this transporter. Heterozygous and homozygous VAChT knockdown mice have a 45% and 65% decrease in VAChT protein expression, respectively. VAChT deficiency alters synaptic vesicle filling and affects ACh release. Whereas VAChT homozygous mutant mice demonstrate major neuromuscular deficits, VAChT heterozygous mice appear normal in that respect and could be used for analysis of central cholinergic function. Behavioral analyses revealed that aversive learning and memory are not altered in mutant mice; however, performance in cognitive tasks involving object and social recognition is severely impaired. These observations suggest a critical role of VAChT in the regulation of ACh release and physiological functions in the peripheral and central nervous system.

Irritable bowel syndrome: update on colonic neuromuscular dysfunction and treatment

Recent discoveries regarding the relatively autonomous workings of the enteric nervous system have expanded our understanding of the pathophysiology of irritable bowel syndrome (IBS). However, the heterogeneity of the pathogenesis of IBS continues to create unique challenges for clinicians who care for these patients. Advances in our understanding of the structure and functions of the brain-gut axis and its interplay with other potentially important factors, such as genetic predisposition, inflammation, and psychological unrest, have led to new developments in the field of targeted pharmacotherapy for IBS sufferers. Therapies designed specifically to modulate gut motility, secretion, and/or sensation have been created and introduced into the marketplace in recent years, and additional designer formulations are in various stages of development. Concurrently, new discoveries of potentially beneficial effects of agents approved for other, often non-gastroenterologic, conditions continue to be reported. This article reviews the accumulating body of evidence supporting the importance of neuromuscular dysfunction as a central cause of IBS symptoms and provides a rationale for the discussion of current and future drug development.

Neuromuscular disorders in critical illness

Neuromuscular disorders in the background of critical illness are under diagnosed. Standardized screening for weakness in the intensive care unit (ICU) setting is uncommon and persistent weakness as a sequel of critical illness is usually not recognized by physicians in the ICU for whom survival from acute illness is the primary outcome. The spectrum of illness ranges from isolated nerve entrapment with focal pain or weakness, to disuse muscle atrophy with mild weakness, and to severe myopathy or neuropathy with associated severe, prolonged weakness. This update focuses on neuromuscular disorders occurring in the critical care set up associated with diffuse and severe weakness.

The control of neuromuscular transmission in health and disease

Multiple techniques are available to study failure of neuromuscular transmission. Electrophysiological techniques used in patients are well suited to detect failure of neuromuscular transmission; however, these methods offer little insight into the mechanisms underlying failure of transmission. More detailed techniques that are better suited for studying the underlying mechanisms can be performed in animal models of neuromuscular disease. However, it is often difficult to compare studies using different techniques to measure neuromuscular transmission. In this review, I discuss different techniques that are available to study failure of neuromuscular transmission. The strengths and weaknesses of various techniques are compared using several diseases as examples. The review concludes with a discussion of mechanisms that may contribute to failure of neuromuscular transmission during repetitive stimulation.

Nitric oxide modulates neuromuscular transmission during hypoxia in rat diaphragm

Hypoxia impairs neuromuscular transmission in the rat diaphragm. In previous studies, we have shown that nitric oxide (NO) plays a role in force modulation of the diaphragm under hypoxic conditions. The role of NO, a neurotransmitter, on neurotransmission in skeletal muscle under hypoxic conditions is unknown. The effects of the NO synthase (NOS) inhibitor nomega-nitro-L-arginine (L-NNA, 1 mM) and the NO donor spermine NONOate (Sp-NO, 1 mM) were evaluated on neurotransmission failure during nonfatiguing and fatiguing contractions of the rat diaphragm under hypoxic (PO2 approximately 5.8 kPa) and hyperoxic conditions (PO2 approximately 64.0 kPa). Hypoxia impaired force generated by both muscle stimulation at 40 HZ (P40M) and by nerve stimulation at 40 HZ (P40N). The effect of hypoxia in the latter was more pronounced. L-NNA increased P40N whereas Sp-NO decreased P40N during hypoxia. In contrast, neither L-NNA nor Sp-NO affected P40N during hyperoxia. L-NNA only slightly reduced neurotransmission failure during fatiguing contractions under hyperoxic conditions. Consequently, neurotransmission failure assessed by comparing force loss during repetitive nerve simulation and superimposed direct muscle stimulation was more pronounced in hypoxia, which was alleviated by L-NNA and aggravated by Sp-NO. These data provide insight in the underlying mechanisms of hypoxia-induced neurotransmission failure. This is important as respiratory muscle failure may result from hypoxia in vivo.

Anti-disialosyl antibodies mediate selective neuronal or Schwann cell injury at mouse neuromuscular junctions

The human paralytic neuropathy, Miller Fisher syndrome (MFS) is associated with autoantibodies specific for disialosyl epitopes on gangliosides GQ1b, GT1a, and GD3. Since these gangliosides are enriched in synaptic membranes, anti-ganglioside antibodies may target neuromuscular junctions (NMJs), thereby contributing to disease symptoms. We have shown previously that at murine NMJs, anti-disialosyl antibodies induce an alpha-latrotoxin-like effect, electrophysiologically characterized by transient massive increase of spontaneous neurotransmitter release followed by block of evoked release, resulting in paralysis of the muscle preparation. Morphologically, motor nerve terminal damage, as well as perisynaptic Schwann cell (pSC) death is observed. The relative contributions of neuronal and pSC injury to the paralytic effect and subsequent repair are unknown. In this study, we have examined the ability of subsets of anti-disialosyl antibodies to discriminate between the neuronal and glial elements of the NMJ and thereby induce either neuronal injury or pSC death. Most antibodies reactive with GD3 induced pSC death, whereas antibody reactivity with GT1a correlated with the extent of nerve terminal injury. Motor nerve terminal injury resulted in massive uncontrolled exocytosis with paralysis. However, pSC ablation induced no acute (within 1 h) electrophysiological or morphological changes to the underlying nerve terminal. These data suggest that at mammalian NMJs, acute pSC injury or ablation has no major deleterious influence on synapse function. Our studies provide evidence for highly selective targeting of mammalian NMJ membranes, based on ganglioside composition, that can be exploited for examining axonal-glial interactions both in disease states and in normal NMJ homeostasis.

Expression of Nav1.6 sodium channels by Schwann cells at neuromuscular junctions: Role in the motor endplate disease phenotype

In addition to their role in action potential generation and fast synaptic transmission in neurons, voltage-dependent sodium channels can also be active in glia. Terminal Schwann cells (TSCs) wrap around the nerve terminal arborization at the neuromuscular junction, which they contribute to shape during development and in the postdenervation processes. Using fluorescent in situ hybridization (FISH), immunofluorescence, and confocal microscopy, we detected the neuronal Nav1.6 sodium channel transcripts and proteins in TSCs in normal adult rats and mice. Nav1.6 protein co-localized with the Schwann cell marker S-100 but was not detected in the SV2-positive nerve terminals. The med phenotype in mice is due to a mutation in the SCN8A gene resulting in loss of Nav1.6 expression. It leads to early onset in postnatal life of defects in neuromuscular transmission with minimal alteration of axonal conduction. Strikingly, in mutant mice, the nonmyelinated pre-terminal region of axons showed abundant sprouting at neuromuscular junctions, and most of the alpha-bungarotoxin-labeled endplates were devoid of S-100- or GFAP-positive TSCs. Using specific antibodies against the Nav1.2 and Nav1.6 sodium channels, ankyrin G and Caspr 1, and a pan sodium channel antibody, we found that a similar proportion of ankyrin G-positive nodes of Ranvier express sodium channels in mutant and wild-type animals and that nodal expression of Nav1.2 persists in med mice. Our data supports the hypothesis that the lack of expression of Nav1.6 in Schwann cells at neuromuscular junctions might play a role in the med phenotype.

Neuromuscular junction channelopathies: a brief overview

The neuromuscular junction lacks the protection of the blood-nerve barrier and is vulnerable to antibody-mediated disorders. In myasthenia gravis (MG), 85% of patients have IgG antibodies to acetylcholine receptors (AChRs). About half the remaining patients have IgG antibodies to Muscle Specific Kinase (MuSK), an AChR-associated transmembrane post-synaptic protein concerned in AChR aggregation. Bulbar weakness is typically predominant in this form of MG, and females are more often affected. The Lambert-Eaton Myasthenic Syndrome (LEMS) can occur in a paraneoplastic form (P-LEMS) usually with small cell lung cancer, or in a non-paraneoplastic form (NP-LEMS). In both, IgG antibodies to nerve terminal voltage-gated calcium channels (VGCCs), detectable in over 90% of patients, lead to VGCC loss and impaired quantal release of transmitter and may be implicated in the occasionally associated cerebellar ataxia. Neuromyotonia (NMT) and Cramp-Fasciculation syndrome (C-FS) are manifestations of peripheral nerve hyperexcitability and share some clinical and electromyographic features. Antibodies to voltage-gated potassium channels (VGKCs) are present in about 40% of NMT patients, but less frequently in C-FS, and appear to cause loss of functional VGKCs. They may also be implicated in the Maladie de Morvan and limbic encephalitis that can associate with NMT: The antibodies described here provide valuable aids to diagnosis and management. The Congenital Myasthenic Syndromes are a group of genetically determined heterogeneous disorders, usually recessively inherited. The commonest mutation sites appear to be the acetylcholine receptor epsilon-subunit and rapsyn.

Advances in the diagnosis of neuromuscular junction disorders

Disorders of the neuromuscular junction have a wide range of clinical presentations, which frequently poses a diagnostic challenge to evaluating clinicians. This article describes the tests used in the diagnosis of diseases of the neuromuscular junction, reviews the evidence supporting the use of each test, and proposes guidelines for their efficient utilization. A focused review of the literature was employed. Reports from four main categories of diagnostic tests (pharmacologic, electrodiagnostic, immunologic, and miscellaneous tests) were reviewed, and the sensitivity and specificity of each test in the diagnosis of specific neuromuscular junction diseases were examined. The clinical presentation determines which diagnostic tests should be utilized in individual cases of suspected neuromuscular junction disease. However, knowledge of the sensitivity and specificity of each test can help to focus the diagnostic evaluation and maximize the diagnostic yield of each test.

Normal values for single fiber EMG parameters of frontalis muscle in healthy subjects older than 70 years

OBJECTIVE

Single fiber EMG (SFEMG) is a potent electrophysiological method to evaluate impaired neuromuscular transmission, and allows sensitive diagnosis of neuromuscular transmission abnormalities such as myasthenia gravis. The jitter and fiber density values are different for various muscles and age groups and the reference values increase with age. In this study, we evaluated the reference values of jitter and fiber density of frontalis muscle in healthy subjects older than 70 years.

METHODS

We evaluated the jitter and fiber density of frontalis muscle in 32 healthy subjects. Twenty-two of them were between 70 and 79 years old (mean +/- SD, 73.9 +/- 1.7), and 10 of them were older than 80 years (mean +/- SD, 82.2 +/- 1.2).

RESULTS

Normal limit of jitter (95% confidence limit) was calculated as 40.4 micros for healthy subjects between 70 and 79 years old and 43.7 micros for healthy subjects older than 80 years and normal limit of fiber density (95% confidence limit) were calculated as 1.90 for subjects between 70 and 79 years old and 2.14 for subjects older than 80 years.

CONCLUSIONS

We designated the reference values of jitter and fiber density for frontalis muscle in healthy subjects older than 70 years. Our reference values may have value to diagnose neuromuscular transmission abnormalities in elderly patients.

SIGNIFICANCE

SFEMG is sensitive for neuromuscular transmission abnormalities and it is important to know the reference values of frontalis muscle in healthy subjects older than 70 years.

[Neurophysiological studies of the neuromuscular junction]

INTRODUCTION

The role played by neurophysiological studies (NPS) in the diagnosis of diseases affecting neuromuscular transmission (NMT) is based on the study of the failure of muscle fibres to achieve a sufficient degree of depolarisation for the junction potential to reach the appropriate threshold and attain a muscular action potential. This totally or partially blocked impulse will give rise to different types of responses in neurophysiological tests.

AIMS

To analyse the different NPS as diagnostic methods in diseases that affect NMT.

DEVELOPMENT

The article offers a review of the concept of the safety margin at the neuromuscular junction and a description of the most common neurophysiological techniques currently in use--repetitive stimulation, as well as conventional or single fibre electromyography (EMG) with voluntary activation or axonal electrical activation. The most frequent findings in diseases affecting NMT are also discussed.

CONCLUSIONS

NPS will be useful to confirm or reject the clinical diagnosis, to exclude other concomitant neuromuscular diseases, to establish whether the process is pre- or post-synaptic, to monitor the clinical course of the disease (when it is both natural or in response to the medical or surgical treatment) and also to enable the physician to determine the status of NMT in cases of clinical remission, as well as to detect subclinical disorders. Single fibre EMG studies are the most sensitive method of neurophysiological diagnosis when dealing with these diseases.

Over-expression of tau results in defective synaptic transmission in Drosophila neuromuscular junctions

We have shown that over-expression of human tau (0N3R) in Drosophila larval motor neurons causes significant morphological and functional disruption to the neuromuscular junctions (NMJs). Tau-expressing NMJs are reduced in size with irregular and abnormal bouton structure. Immunocytochemical analysis shows that the abnormal NMJs still retain synaptotagmin expression and form active zones. Functionally, the NMJs exhibit abnormal endo/exocytosis as revealed by incorporation of the styryl dye FM1-43. Electrophysiological studies showed that with low frequency stimulation (1 Hz), evoked synaptic potentials produced from tau over-expressing motor neurons were indistinguishable from wild type, however, following high frequency stimulation (50 Hz), evoked synaptic potentials were significantly decreased. Analysis of the number and distribution of mitochondria showed that motor neurons over-expressing tau had a significant reduction in functional mitochondria in the presynaptic terminal. Collapsing the mitochondrial membrane potential in wild type larvae phenocopied the effects of tau over-expression on synaptic transmission. Our results demonstrate that tau over-expression in vivo cause a synaptic dysfunction, which may be caused by a reduced complement of functional mitochondria.

Clinical classification of patellofemoral pain syndrome: guidelines for non-operative treatment

The patellofemoral pain syndrome (PFPS) remains a challenging musculoskeletal entity encountered by clinicians. Reviewing the literature, conflicting data seem to exist regarding the effect of non-operative treatment in PFPS patients. A possible explanation may be lack of a clear classification system of patients with PFPS. It is our opinion that the term PFPS still is a 'wastebasket', which probably comprises several different entities. Therefore, it seems important to subdivide this broad group of patients into different categories with a specific rehabilitation approach. In this study, we introduce a classification system, which reflects a consensus reached by the European Rehabilitation Panel. This classification system should help the clinicians to identify the cause(s) of patellofemoral pain, and consequently help to select the most appropriate non-operative treatment. The authors are aware that no rehabilitation protocol will work for all PFPS patients, since the underlying mosaic of pathophysiology and tissue-healing responses are unique. Therefore, the aim of this study with a classification system was to guide the clinician through clinical examination in order to develop a non-operative treatment protocol, specific for each individual with PFPS.

Neuromuscular rehabilitation and electrodiagnosis. 3. Diseases of muscles and neuromuscular junction

This self-directed learning module highlights formation of a differential diagnosis as well as electrodiagnostic evaluation for those patients who present with the common complaint of weakness. It is part of the chapter on neuromuscular rehabilitation and electrodiagnosis in the Self-Directed Physiatric Education Program for practitioners and trainees in physical medicine and rehabilitation. This article specifically focuses on the common symptoms and typical clinical findings that allow the clinician to narrow the differential diagnosis. This is followed by the diagnostic evaluation, with emphasis on the technical aspects and interpretation of electrodiagnostic studies.

OVERALL ARTICLE OBJECTIVE

To summarize the clinical presentation and electrodiagnostic findings in persons with disorders of muscle or disorders of the neuromuscular junction.

Myasthenia gravis: diagnostic mimics

The clinical hallmark of myasthenia gravis (MG) is fluctuating, painless weakness of muscles that most often affect extraocular, lower bulbar, or limb musculature. Predicting the probability of successful treatment for the patient assumes that the physician has made an accurate diagnosis. In this review, the practical differential diagnosis of MG is reviewed from the perspective of conditions (at presentation of symptoms and signs) that may mimic the disorder. The differential diagnosis includes disorders that limit eye movements (with or without associated diplopia), cause false-positive laboratory studies, and mimic MG but have normal eye movements. The differential diagnosis includes disorders that affect the upper brainstem, cranial nerves, neuromuscular junction, muscles, or local orbit anatomy. Nonneurological systemic diseases (i.e., encephalopathy, sepsis) can produce fluctuating ptosis or eye movements that can occasionally be confused with MG. Although MG is considered often in the differential diagnosis of weakness or fatigue symptoms that lack a correlate on neurological examination (subjective fatigue, breakaway weakness, chronic fatigue syndrome), MG is almost never found.

Hypercapnic impairment of neuromuscular function is related to afferent depression

Acetazolamide (ACZ), a carbonic anhydrase inhibitor, results in altered neuromuscular function secondary to depressed afferent transmission in intact humans. One effect of ACZ is hypercapnia. Thus, to test if the neuromuscular depression observed following ACZ treatment is related to elevated CO(2), human subjects ( n=10) were exposed to 15 min of room air (0% CO(2)) or hypercapnia (7% inspired CO(2)), and neuromuscular function was evaluated. Isometric force (36.8 to 31.1 N) and peak-to-peak electromyographic amplitude (EMG, 1.5 to 1.0 mV) associated with an Achilles tendon tap, and soleus H(max):M(max) ratio (69.0 to 62.2%) were depressed, while EMG latency (34.8 to 39.8 ms) was increased by hypercapnia. Reflex recovery profiles (following a conditioning tap to the contralateral Achilles tendon), motor nerve conduction velocity, amplitude of the maximum M-wave, and peak twitch tension at M(max) were unaltered by hypercapnia. We conclude that elevated CO(2) impairs neuromuscular function through effects on afferent transmission or synaptic integrity between type Ia fibers of the muscle spindle and the alpha motor neuron, without affecting the muscle spindle, efferent conduction or skeletal muscle force-generating capacity.

Ca2+Channels and Synaptic Transmission at the Adult, Neonatal, and P/Q-Type Deficient Neuromuscular Junction

Different types of voltage-activated Ca(2+) channels have been established based on their molecular structure and pharmacological and biophysical properties. One of them, the P/Q-type, is the main channel involved in nerve-evoked neurotransmitter release at neuromuscular junctions and the immunological target in Eaton-Lambert Syndrome. At adult neuromuscular junctions, L- and N-type Ca(2+) channels become involved in transmitter release only under certain experimental or pathological conditions. In contrast, at neonatal rat neuromuscular junctions, nerve-evoked synaptic transmission depends jointly on both N- and P/Q-type channels. Synaptic transmission at neuromuscular junctions of the ataxic P/Q-type Ca(2+) channel knockout mice is also dependent on two different types of channels, N- and R-type. At both neonatal and P/Q knockout junctions, the K(+)-evoked increase in miniature endplate potential frequency was not affected by N-type channel blockers, but strongly reduced by both P/Q- and R-type channel blockers. These differences could be accounted for by a differential location of the channels at the release site, being either P/Q- or R-type Ca(2+) channels located closer to the release site than N-type Ca(2+) channels. Thus, Ca(2+) channels may be recruited to mediate neurotransmitter release where P/Q-type channels seem to be the most suited type of Ca(2+) channel to mediate exocytosis at neuromuscular junctions.

Neuropathology considerations: clinical and SEMG/biofeedback applications

SEMG investigation protocols are described within the neurological framework. A neuroanatomic pathway is utilized including muscular disease, neuromuscular junction conditions, peripheral neuropathy, radiculopahy, myleopathy, brainstem disorders, cerebellar disorders, subcortical and cortical disease. SEMG findings within the clinical presentation of those pathologies are aimed at improving the diagnostic process and serve to focus the SEMG neuromuscular reeducation (biofeedback) component of the overall treatment plan.

[Current neurophysiological tests and revised JSCN technical standards for clinical EEG]

The purpose of this lecture is to review the development of current neurophysiology and the revised standard of society for clinical EEG. 1. The improvement of neurophysiological tests. 1) EEG and evoked potential: EEG and evoked potential testing includes the routine EEG recording, EEG monitoring in surgical operation, all night sleep polygraph for the diagnosis of sleep apnea syndrome and many kinds of brain evoked potentials. Especially, the P300 component in the ERP(event-related evoked potential) is useful for the testing of essential brain functions. 2) EMG and evoked EMG: These tests are applied for the diagnosis of neurogenic, myogenic and neuromuscular junction disorder, and also the single fiber EMG using micro needle electrode is useful for the diagnosis of myasthenia gravis. Motor and sensory nerve conduction velocity are calculated from the latency of evoked EMGs. Furthermore, the distribution of these conduction velocities in many nerve fibers is measured by the collision technique. 3) Other tests: Near-infrared spectroscopy for the testing of brain functions has made rapid progress, and the transcranial magnetic stimulation method has come to be used for evaluation of functional diseases in the pyramidal tract, cerebellum and the spinal cord. 2. The revised JSCN technical standards for clinical EEG. The revised recording conditions of ECI(electro cerebral inactivity: flat EEG) in brain death are the focus of this lecture.

Electromyographic approach to neuromuscular junction disorders repetitive nerve stimulation and single-fiber electromyography

Understanding the physiology of the NMJ aids the electromyographer in understanding how NMJ testing was developed. Practicing the RNS protocol on healthy subjects helps the electromyographer gain greater comfort with the implementation of a test that is not performed daily. The practice parameter proposed by the AAEM has established guidelines for testing and for diagnosing NMJ disorders using RNS and single-fiber EMG.

Decrement in area of muscle responses to repetitive nerve stimulation

Measurement of the decremental muscle response to repetitive nerve stimulation (RNS) has low yields for the diagnosis of neuromuscular transmission defects compared with single fiber electromyography (SFEMG). We compared area and amplitude of muscle responses to RNS in 87 patients and 30 controls, using SFEMG as the reference standard. Decrement of response area provided additional diagnostic yields of 5.3% to 30% depending on the muscle examined and disease severity, and is recommended as a diagnostic adjunct to measurement of amplitude decrement during RNS.

Acetazolamide acts on neuromuscular transmission abnormalities found in some migraineurs

Mild subclinical impairment of neuromuscular transmission can be detected with single-fibre electromyography (SFEMG) in subgroups of patients suffering from migraine and could be due to dysfunctioning Ca2+-channels on motor axons controlling stimulation-induced acetylcholine release. Acetazolamide, which is thought to ameliorate ion channel function, was shown effective in familial hemiplegic migraine and episodic ataxia type 2, both of which are associated with mutations of the neuronal Ca2+-channel gene CACNA1A, as well as in aura status. We treated therefore in an open pilot study five non-hemiplegic migraineurs showing mild SFEMG abnormalities with acetazolamide for several weeks. This was followed by a normalization of SFEMG recordings in all patients and by clinical improvement in four. These results support the assumption that the subclinical impairment of neuromuscular transmission found in certain migraineurs might be due to dysfunctioning Ca2+-channels.

Fatigue and abnormal neuromuscular transmission in Kennedy's disease

We describe a patient with Kennedy's disease (X-linked bulbospinal neuronopathy) who experienced leg muscle fatigue with long-distance running. The patient also reported muscle twitching involving the face and extremities and long-standing muscle cramps. Aside from mild facial and tongue weakness (and fasciculations), his examination was normal, including completely preserved muscle strength in the extremities. Electrodiagnostic evaluation revealed evidence for a chronic motor axonopathy/neuronopathy and abnormal sensory nerve action potentials. In addition, repetitive nerve stimulation studies were normal, but neuromuscular jitter tested in the same muscle was markedly abnormal. The normal clinical strength and repetitive nerve stimulation studies in a muscle showing markedly increased neuromuscular jitter suggested a mechanism for this patient's symptoms of muscle fatigue, related to failure of neuromuscular transmission at a critical number of endplates during extremes of physical activity.

Neuromuscular junction changes in aged rat thyroarytenoid muscle

Dynamic remodeling of neuromuscular junction (NMJ) structure is postulated as a cause of age-related muscular atrophy. Direct study of NMJ morphology in laryngeal muscles is important to our understanding of age-related decrements in voice and swallowing. The morphology of NMJs was studied in a rat model to compare young and old specimens of thyroarytenoid muscle--a muscle critical to airway protection and phonation. Fluorescent, triple-label immunohistochemical analysis and confocal microscopic visualization were used to analyze the structure of NMJs. We found that laryngeal NMJs underwent significant changes that were similar to those observed after denervation. Specifically, the axon terminal area was significantly reduced, there were a number of postsynaptic acetylcholine receptor areas unoccupied by nerve terminals, and there was increased variability in end plate architecture in the old muscles. The results of this study increase our understanding of the age-related morphological changes in the larynx, and may serve as a baseline to test the effectiveness of future interventions.

Electrodiagnostic approach to the patient with suspected neuromuscular junction disorder

The electrodiagnostic (EDX) examination in patients with suspected Neuromuscular Junction (NMJ) disorder constitutes the most advanced and complex type of EDX studies. Understanding the anatomy and physiology of neuromuscular transmission is prerequisite for the comprehension and planning of EDX studies in patients with suspected NMJ disorders. In addition to routine nerve conduction studies and conventional needle electromyography (EMG), the EDX studies, which are most useful in the diagnosis of NMJ disorders, include repetitive nerve stimulation and single fiber EMG.

[Statistical analysis of repetitive nerve stimulation test]

INTRODUCTION

Statistical properties of Repetitive Nerve Stimulation test (RNS) are studied for different variables (forth to first potentials ratio of CMAP) together with two new variables (trends of area and amplitude for series of CMAP) OBJECTIVE: To compare the variables commonly used, in order to define the best statistical properties and, in the other hand, investigate if the new variables (trends in area and amplitude) could be useful for the study of neuromuscular transmission pathology.

SUBJECTS AND METHODS

Statistical properties of voltage (V4/V1(V)), area of CMAP (V4/V1(a)) and slopes of regression lines of voltage (m(V)) and area (m(a)) measures are determined for runs of 3, 10, 15 and 20 Hz series of stimuli in 13 men and 17 women healthy volunteers.

RESULTS

1) The sample fits well to a normal distribution; 2) The variables that measure tendencies in CMAP series, can be reasonably fitted to linear regression functions; 3) Differences between sex and age have been not found; 4) Variables measuring tendencies have been the most stables, specially m(a); 5) The variable that fits better to normal distribution is m(V); 6) All variables behave linearly with respect frequency of stimulation.

CONCLUSIONS

1) Neuromuscular transmission is a particularly stable physiologic function; 2) The most used variable in clinical neurophysiology laboratories, V4/V1(V) have the worst statistical properties, showing a very significant variability; 3) Trends measures (m(a)) and m(V)) show very satisfactory statistical properties; this implies that these could be very useful for the study of neuromuscular transmission diseases.

Computer simulation of jitter phenomenon in neuromuscular transmission disorders

Neuromuscular jitter is a sensitive measure of the safety factor of neuromuscular transmission. Nevertheless, the actual relationship between jitter and the safety factor is unknown because these parameters have not been simultaneously measured. In order to explore the theoretical relationship between them, a computer model of mammalian neuromuscular transmission has been developed. If the safety factor is expressed as the absolute value of the natural logarithm of the probability of a block, the model predicts a double exponential relationship between the safety factor and jitter, except when the percentage of blocks is greater than 90%. In that case, the jitter value decreases. Simulation of acetylcholinesterase inhibition shows that this treatment decreases the neuromuscular transmission blocks more effectively in postsynaptic than in presynaptic disorders. In contrast, when the percentage of blocks is greater than 60%, the jitter value increases in both conditions.

Loss-of-function EA2 mutations are associated with impaired neuromuscular transmission

OBJECTIVE

To examine the functional consequences of episodic ataxia type 2 (EA2)-causing nonsense and missense mutations in vitro and to characterize the basis of fluctuating weakness in patients with E2A.

BACKGROUND

Mutations in CACNA1A encoding the Ca(v)2.1 calcium channel subunit cause EA2 through incompletely understood mechanisms. Although the Ca(v)2.1 subunit is important for neurotransmission at the neuromuscular junction, weakness has not been considered a feature of EA2.

METHODS

The disease-causing mutations in three unrelated patients with EA2 and fluctuating weakness were identified by mutation screening and sequencing. Mutant constructs harboring mutations R1281X, F1406C, R1549X were transfected into COS7 cells and expressed for patch clamp studies. Single-fiber electromyography (SFEMG) was performed in patients to examine synaptic transmission at the neuromuscular junction.

RESULTS

Functional studies in COS7 cells of nonsense and missense EA2 mutants demonstrated markedly decreased current densities compared with wild type. SFEMG demonstrated jitter and blocking in these patients with EA2, compared with normal subjects and three patients with SCA-6.

CONCLUSION

EA2-causing missense and nonsense mutations in CACNA1A produced mutant channels with diminished whole cell calcium channel activity in vitro due to loss of function. Altered biophysical properties or reduced efficiency of plasma membrane targeting of mutant channels may contribute to abnormal neuromuscular transmission, manifesting as myasthenic syndrome.