Electrophysiological analysis of neuromuscular synaptic function in myasthenia gravis patients and animal models

Calcium-binding protein 7 expressed in muscle negatively regulates age-related degeneration of neuromuscular junctions in mice

The neuromuscular junction (NMJ) forms centrally in myotubes and, as the only synapse between motor neuron and myotube, are indispensable for motor activity. The midmuscle formation of NMJs, including midmuscle-restricted expression of NMJ-related genes, is governed by the muscle-specific kinase (MuSK). However, mechanisms underlying MuSK-mediated signaling are unclear. Here, we find that the Calcium-binding protein 7 (Cabp7) gene shows midmuscle-restricted expression, and muscle-specific depletion of Cabp7 in mice accelerated age-related NMJ degeneration, muscle weakness/atrophy, and motor dysfunction. Surprisingly, forced expression in muscle of CIP, an inhibitory peptide of the negative regulator of NMJ formation cyclin-dependent kinase 5 (Cdk5), restored NMJ integrity and muscle strength, and healed muscle atrophy in muscle-specific Cabp7-deficient mice, which showed increased muscle expression of the Cdk5 activator p25. These findings together demonstrate that MuSK-mediated signaling induces muscle expression of Cabp7, which suppresses age-related NMJ degeneration likely by attenuating p25 expression, providing insights into prophylactic/therapeutic intervention against age-related motor dysfunction.

Morphological and functional alterations of neuromuscular synapses in a mouse model of ACTA1 congenital myopathy

Mutations in skeletal muscle α-actin (Acta1) cause myopathies. In a mouse model of congenital myopathy, heterozygous Acta1 (H40Y) knock-in (Acta1+/Ki) mice exhibit features of human nemaline myopathy, including premature lethality, severe muscle weakness, reduced mobility, and the presence of nemaline rods in muscle fibers. In this study, we investigated the impact of Acta1 (H40Y) mutation on the neuromuscular junction (NMJ). We found that the NMJs were markedly fragmented in Acta1+/Ki mice. Electrophysiological analysis revealed a decrease in amplitude but increase in frequency of miniature end-plate potential (mEPP) at the NMJs in Acta1+/Ki mice, compared with those in wild type (Acta1+/+) mice. Evoked end-plate potential (EPP) remained similar at the NMJs in Acta1+/Ki and Acta1+/+ mice, but quantal content was increased at the NMJs in Acta1+/Ki, compared with Acta1+/+ mice, suggesting a homeostatic compensation at the NMJs in Acta1+/Ki mice to maintain normal levels of neurotransmitter release. Furthermore, short-term synaptic plasticity of the NMJs was compromised in Acta1+/Ki mice. Together, these results demonstrate that skeletal Acta1 H40Y mutation, albeit muscle-origin, leads to both morphological and functional defects at the NMJ.

Opposite Regulation of Homer Signal at the NMJ Postsynaptic Micro Domain between Slow- and Fast-Twitch Muscles in an Experimentally Induced Autoimmune Myasthenia Gravis (EAMG) Mouse Model

Accelerated postsynaptic remodelling and disturbance of neuromuscular transmission are common features of autoimmune neurodegenerative diseases. Homer protein isoform expression, crosslinking activity and neuromuscular subcellular localisation are studied in mouse hind limb muscles of an experimentally induced autoimmune model of Myasthenia Gravis (EAMG) and correlated to motor end plate integrity. Soleus (SOL), extensor digitorum longus (EDL) and gastrocnemius (GAS) skeletal muscles are investigated. nAChR membrane clusters were studied to monitor neuromuscular junction (NMJ) integrity. Fibre-type cross-sectional area (CSA) analysis is carried out in order to determine the extent of muscle atrophy. Our findings clearly showed that crosslinking activity of Homer long forms (Homer 1b/c and Homer2a/b) are decreased in slow-twitch and increased in fast-twitch muscle of EAMG whereas the short form of Homer that disrupts Homer crosslinking (Homer1a) is upregulated in slow-twitch muscle only. Densitometry analysis showed a 125% increase in Homer protein expression in EDL, and a 45% decrease in SOL of EAMG mice. In contrast, nAChR fluorescence pixel intensity decreased in endplates of EAMG mice, more distinct in type-I dominant SOL muscle. Morphometric CSA of EAMG vs. control (CTR) revealed a significant reduction in EDL but not in GAS and SOL. Taken together, these results indicate that postsynaptic Homer signalling is impaired in slow-twitch SOL muscle from EAMG mice and provide compelling evidence suggesting a functional coupling between Homer and nAChR, underscoring the key role of Homer in skeletal muscle neurophysiology.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Intravenous magnesium sulfate inducing acute respiratory failure in a patient with myasthenia gravis

A woman in her 90s with a background of myasthenia gravis and atrial fibrillation presented to hospital following a fall. While in the emergency department it was noted that she was in atrial fibrillation with a fast-ventricular response and as part of her management was given intravenous magnesium. Following this she developed acute respiratory failure and required intubation and ventilation. The patient recovered quickly and was extubated in the intensive care unit the next day. On subsequent days, the patient received two further doses of intravenous magnesium before the link was identified. On both of these occasions she again developed respiratory failure which were managed with non-invasive ventilation. This case highlights the importance of all members of the team being aware of the drugs that can induce a myasthenic crisis. It also stimulates further research into the development of a guide of how to safely treat symptomatic hypomagnesaemia in patients with myasthenia gravis.

A bioassay for neuromuscular junction-restricted complement activation by myasthenia gravis acetylcholine receptor antibodies

BACKGROUND

Myasthenia gravis (MG) is an autoimmune neuromuscular disorder hallmarked by fluctuating fatigable muscle weakness. Most patients have autoantibodies against acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ). These are thought to have three possible pathogenic mode-of-actions: 1) cross-linking and endocytosis of AChRs, 2) direct block of AChRs and 3) complement activation. The relative contributions of these mechanisms to synaptic block and muscle weakness of individual patients cannot be determined. It likely varies between patients and perhaps also with disease course, depending on the nature of the circulating AChR antibodies.

NEW METHOD

We developed a new bioassay which specifically enables functional characterization and quantification of complement-mediated synaptic damage at NMJs, without interference of the other pathogenic mechanisms. To this end, we pre-incubated mouse hemi-diaphragm muscle-nerve preparations with mAb35-hG1, a humanized rat AChR monoclonal and subsequently exposed the preparation to normal human serum as a complement source. NMJ-restricted effects were studied.

RESULTS

Clearly NMJ-restricted damage occurred. With immunohistology we showed complement deposition at NMJs, and synaptic electrophysiological measurements demonstrated transmission block. In whole-muscle contraction experiments we quantified the effect and characterized its onset and progression during the incubation with normal human serum.

COMPARISON WITH EXISTING METHODS

With this new assay the complement-mediated component of myasthenic NMJ pathology can be studied separately.

CONCLUSIONS

Our assay will be of importance in detailed mechanistic studies of local complement activation at NMJs, investigations of new complement inhibitors, and laboratory pre-screening of therapeutic efficacy for individual MG patients to optimize care with clinically approved complement inhibitors.

The Safety Factor for Neuromuscular Transmission: Effects of Dimethylsulphoxide, Cannabinoids and Synaptic Homeostasis

BackgroundIn myasthenia gravis, impaired postsynaptic sensitivity to acetylcholine results in failure of neuromuscular transmission and fatiguing muscle weakness.ObjectiveDevelop an ex vivo muscle contraction assay to test cannabinoids and other substances that might act on the myasthenic neuromuscular junction to restore control of the muscle.MethodsTubocurarine was added to an ex vivo, mouse phrenic nerve-hemidiaphragm muscle preparation to reduce acetylcholine sensitivity. This produced a myasthenia-like decrement in twitch force during a train of 10 nerve impulses (3 / sec). Endplate potential (EPP) recordings were used to confirm and extend the findings.ResultsSurprisingly, addition to the bath of dimethylsulphoxide (DMSO), at concentrations as low as 0.1%(v/v), partially reversed the decrement in nerve-evoked force. Intracellular electrophysiology, conducted in the presence of tubocurarine, showed that DMSO increased the amplitudes of both the spontaneous miniature EPP (MEPP) and the (nerve-evoked) EPP. In the absence of tubocurarine (synaptic potentials at physiological levels), an adaptive fall in quantal content negated the DMSO-induced rise in EPP amplitude. The effects of cannabinoid receptor agonists (solubilized with DMSO) in the contraction assay do not support their further exploration as useful therapeutic agents for myasthenia gravis. CP 55,940 (a dual agonist for cannabinoid receptor types 1 and 2) reversed the beneficial effects of DMSO.Conclusions:We demonstrate a powerful effect of DMSO upon quantal amplitude that might mislead pharmacological studies of synaptic function wherever DMSO is used as a drug vehicle. Our results also show that compounds targeting impaired neuromuscular transmission should be tested under myasthenic-like conditions, so as to avoid confounding effects of synaptic homeostasis.

Diagnosis of Myasthenia Gravis

The diagnosis of autoimmune Myasthenia Gravis (MG) remains clinical and rests on the history and physical findings of fatigable, fluctuating muscle weakness in a specific distribution. Ancillary bedside tests and laboratory methods help confirm the synaptic disorder, define its type and severity, classify MG according to the causative antibodies, and assess the effect of treatment objectively. We present an update on the tests used in the diagnosis and follow-up of MG and the suggested approach for their application.

Functional monovalency amplifies the pathogenicity of anti-MuSK IgG4 in myasthenia gravis

An expanding group of autoimmune diseases is now recognized to be hallmarked by pathogenic IgG4 autoantibodies. IgG4 has the unique ability to exchange Fab-arms, rendering it bispecific and functionally monovalent. Here we show that autoantibody functional monovalency significantly amplifies the pathogenicity of IgG4 autoantibodies using patient-derived monoclonal antibodies in an in vivo model of MuSK myasthenia gravis. Therefore, subclass switching to predominant IgG4 autoantibodies is a critical step in the development of MuSK myasthenia gravis. This new mechanism in autoimmunity is also potentially relevant to 29 other IgG4-mediated autoimmune diseases known to date, allergy and other disease settings where IgG4 antibodies contribute to pathology.

Human immunoglobulin (Ig) G4 usually displays antiinflammatory activity, and observations of IgG4 autoantibodies causing severe autoimmune disorders are therefore poorly understood. In blood, IgG4 naturally engages in a stochastic process termed “Fab-arm exchange” in which unrelated IgG4s exchange half-molecules continuously. The resulting IgG4 antibodies are composed of two different binding sites, thereby acquiring monovalent binding and inability to cross-link for each antigen recognized. Here, we demonstrate that this process amplifies autoantibody pathogenicity in a classic IgG4-mediated autoimmune disease: muscle-specific kinase (MuSK) myasthenia gravis. In mice, monovalent anti-MuSK IgG4s caused rapid and severe myasthenic muscle weakness, whereas the same antibodies in their parental bivalent form were less potent or did not induce a phenotype. Mechanistically this could be explained by opposing effects on MuSK signaling. Isotype switching to IgG4 in an autoimmune response thereby may be a critical step in the development of disease. Our study establishes functional monovalency as a pathogenic mechanism in IgG4-mediated autoimmune disease and potentially other disorders.

The Mouse Levator Auris Longus Muscle: An Amenable Model System to Study the Role of Postsynaptic Proteins to the Maintenance and Regeneration of the Neuromuscular Synapse

The neuromuscular junction (NMJ) is the peripheral synapse that controls the coordinated movement of many organisms. The NMJ is also an archetypical model to study synaptic morphology and function. As the NMJ is the primary target of neuromuscular diseases and traumatic injuries, the establishment of suitable models to study the contribution of specific postsynaptic muscle-derived proteins on NMJ maintenance and regeneration is a permanent need. Considering the unique experimental advantages of the levator auris longus (LAL) muscle, here we present a method allowing for efficient electroporation-mediated gene transfer and subsequent detailed studies of the morphology and function of the NMJ and muscle fibers. Also, we have standardized efficient facial nerve injury protocols to analyze LAL muscle NMJ degeneration and regeneration. Our results show that the expression of a control fluorescent protein does not alter either the muscle structural organization, the apposition of the pre- and post-synaptic domains, or the functional neurotransmission parameters of the LAL muscle NMJs; in turn, the overexpression of MuSK, a major regulator of postsynaptic assembly, induces the formation of ectopic acetylcholine receptor clusters. Our NMJ denervation experiments showed complete reinnervation of LAL muscle NMJs four weeks after facial nerve injury. Together, these experimental strategies in the LAL muscle constitute effective methods to combine protein expression with accurate analyses at the levels of structure, function, and regeneration of the NMJ.

Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability

Myasthenia gravis (MG) is a disease of the postsynaptic neuromuscular junction (NMJ) where nicotinic acetylcholine (ACh) receptors (AChRs) are targeted by autoantibodies. Search for other pathogenic antigens has detected the antibodies against muscle-specific tyrosine kinase (MuSK) and low-density lipoprotein-related protein 4 (Lrp4), both causing pre- and post-synaptic impairments. Agrin is also suspected as a fourth pathogen. In a complex NMJ organization centering on MuSK: (1) the Wnt non-canonical pathway through the Wnt-Lrp4-MuSK cysteine-rich domain (CRD)-Dishevelled (Dvl, scaffold protein) signaling acts to form AChR prepatterning with axonal guidance; (2) the neural agrin-Lrp4-MuSK (Ig1/2 domains) signaling acts to form rapsyn-anchored AChR clusters at the innervated stage of muscle; (3) adaptor protein Dok-7 acts on MuSK activation for AChR clustering from “inside” and also on cytoskeleton to stabilize AChR clusters by the downstream effector Sorbs1/2; (4) the trans-synaptic retrograde signaling contributes to the presynaptic organization via: (i) Wnt-MuSK CRD-Dvl-β catenin-Slit 2 pathway; (ii) Lrp4; and (iii) laminins. The presynaptic Ca²⁺ homeostasis conditioning ACh release is modified by autoreceptors such as M1-type muscarinic AChR and A2A adenosine receptors. The post-synaptic structure is stabilized by: (i) laminin-network including the muscle-derived agrin; (ii) the extracellular matrix proteins (including collagen Q/perlecan and biglycan which link to MuSK Ig1 domain and CRD); and (iii) the dystrophin-associated glycoprotein complex. The study on MuSK ectodomains (Ig1/2 domains and CRD) recognized by antibodies suggested that the MuSK antibodies were pathologically heterogeneous due to their binding to multiple functional domains. Focussing one of the matrix proteins, biglycan which functions in the manner similar to collagen Q, our antibody assay showed the negative result in MG patients. However, the synaptic stability may be impaired by antibodies against MuSK ectodomains because of the linkage of biglycan with MuSK Ig1 domain and CRD. The pathogenic diversity of MG is discussed based on NMJ signaling molecules.

β2-Adrenergic receptor agonists ameliorate the adverse effect of long-term pyridostigmine on neuromuscular junction structure

Acetylcholine receptor deficiency is the most common form of the congenital myasthenic syndromes, a heterogeneous collection of genetic disorders of neuromuscular transmission characterized by fatiguable muscle weakness. Most patients with acetylcholine receptor deficiency respond well to acetylcholinesterase inhibitors; however, in some cases the efficacy of acetylcholinesterase inhibitors diminishes over time. Patients with acetylcholine receptor deficiency can also benefit from the addition of a β2-adrenergic receptor agonist to their medication. The working mechanism of β2-adrenergic agonists in myasthenic patients is not fully understood. Here, we report the long-term follow-up for the addition of β2-adrenergic agonists for a cohort of patients with acetylcholine receptor deficiency on anticholinesterase medication that demonstrates a sustained quantitative improvement. Coincidently we used a disease model to mirror the treatment of acetylcholine receptor deficiency, and demonstrate improved muscle fatigue, improved neuromuscular transmission and improved synaptic structure resulting from the addition of the β2-adrenergic agonist salbutamol to the anticholinesterase medication pyridostigmine. Following an initial improvement in muscle fatiguability, a gradual decline in the effect of pyridostigmine was observed in mice treated with pyridostigmine alone (P < 0.001). Combination therapy with pyridostigmine and salbutamol counteracted this decline (P < 0.001). Studies of compound muscle action potential decrement at high nerve stimulation frequencies (P < 0.05) and miniature end-plate potential amplitude analysis (P < 0.01) showed an improvement in mice following combination therapy, compared to pyridostigmine monotherapy. Pyridostigmine alone reduced postsynaptic areas (P < 0.001) and postsynaptic folding (P < 0.01). Combination therapy increased postsynaptic area (P < 0.001) and promoted the formation of postsynaptic junctional folds (P < 0.001), in particular in fast-twitch muscles. In conclusion, we demonstrate for the first time how the improvement seen in patients from adding salbutamol to their medication can be explained in an experimental model of acetylcholine receptor deficiency, the most common form of congenital myasthenic syndrome. Salbutamol enhances neuromuscular junction synaptic structure by counteracting the detrimental effects of long-term acetylcholinesterase inhibitors on the postsynaptic neuromuscular junction. The results have implications for both autoimmune and genetic myasthenias where anticholinesterase medication is a standard treatment.

Nature and Action of Antibodies in Myasthenia Gravis

This article discusses antibodies associated with immune-mediated myasthenia gravis and the pathologic action of these antibodies at the neuromuscular junctions of skeletal muscle. To explain how these antibodies act, we consider the physiology of neuromuscular transmission with emphasis on 4 features: the structure of the neuromuscular junction; the roles of postsynaptic acetylcholine receptors and voltage-gated Na⁺ channels and in converting the chemical signal from the nerve terminal into a propagated action potential on the muscle fiber that triggers muscle contraction; the safety factor for neuromuscular transmission; and how the safety factor is reduced in different forms of autoimmune myasthenia gravis.

Muscle specific kinase protects dystrophic mdx mouse muscles from eccentric contraction-induced loss of force-producing capacity

KEY POINTS

Adeno-associated viral vector was used to elevate the expression of muscle specific kinase (MuSK) and rapsyn (a cytoplasmic MuSK effector protein) in the tibialis anterior muscle of wild-type and dystrophic (mdx) mice. In mdx mice, enhanced expression of either MuSK or rapsyn ameliorated the acute loss of muscle force associated with strain injury. Increases in sarcolemmal immunolabelling for utrophin and β-dystroglycan suggest a mechanism for the protective effect of MuSK in mdx muscles. MuSK also caused subtle changes to the structure and function of the neuromuscular junction, suggesting novel roles for MuSK in muscle physiology and pathophysiology.

ABSTRACT

Muscle specific kinase (MuSK) has a well-defined role in stabilizing the developing mammalian neuromuscular junction, but MuSK might also be protective in some neuromuscular diseases. In the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy, limb muscles are especially fragile. We injected the tibialis anterior muscle of 8-week-old mdx and wild-type (C57BL10) mice with adeno-associated viral vectors encoding either MuSK or rapsyn (a cytoplasmic MuSK effector protein) fused to green fluorescent protein (MuSK-GFP and rapsyn-GFP, respectively). Contralateral muscles injected with empty vector served as controls. One month later mice were anaesthetized with isoflurane and isometric force-producing capacity was recorded from the distal tendon. MuSK-GFP caused an unexpected decay in nerve-evoked tetanic force, both in wild-type and mdx muscles, without affecting contraction elicited by direct electrical stimulation of the muscle. Muscle fragility was probed by challenging muscles with a strain injury protocol consisting of a series of four strain-producing eccentric contractions in vivo. When applied to muscles of mdx mice, eccentric contraction produced an acute 27% reduction in directly evoked muscle force output, affirming the susceptibility of mdx muscles to strain injury. mdx muscles overexpressing MuSK-GFP or rapsyn-GFP exhibited significantly milder force deficits after the eccentric contraction challenge (15% and 14%, respectively). The protective effect of MuSK-GFP in muscles of mdx mice was associated with increased immunolabelling for utrophin and β-dystroglycan in the sarcolemma. Elevating the expression of MuSK or rapsyn revealed several distinct synaptic and extrasynaptic effects, suggesting novel roles for MuSK signalling in muscle physiology and pathophysiology.

Congenital Myasthenic Syndromes or Inherited Disorders of Neuromuscular Transmission: Recent Discoveries and Open Questions

Congenital myasthenic syndromes (CMS) form a heterogeneous group of rare diseases characterized by fatigable muscle weakness. They are genetically-inherited and caused by defective synaptic transmission at the cholinergic neuromuscular junction (NMJ). The number of genes known to cause CMS when mutated is currently 30, and the relationship between fatigable muscle weakness and defective functions is quite well-understood for many of them. However, some of the most recent discoveries in individuals with CMS challenge our knowledge of the NMJ, where the basis of the pathology has mostly been investigated in animal models. Frontier forms between CMS and congenital myopathy, which have been genetically and clinically identified, underline the poorly understood interplay between the synaptic and extrasynaptic molecules in the neuromuscular system. In addition, precise electrophysiological and histopathological investigations of individuals with CMS suggest an important role of NMJ plasticity in the response to CMS pathogenesis. While efficient drug-based treatments are already available to improve neuromuscular transmission for most forms of CMS, others, as well as neurological and muscular comorbidities, remain resistant. Taken together, the available pathological data point to physiological issues which remain to be understood in order to achieve precision medicine with efficient therapeutics for all individuals suffering from CMS.

Autoimmune Channelopathies at Neuromuscular Junction

The neuromuscular junction, also called myoneural junction, is a site of chemical communication between a nerve fiber and a muscle cell. There are many types of channels at neuromuscular junction that play indispensable roles in neuromuscular signal transmission, such as voltage-gated calcium channels and voltage-gated potassium channels on presynaptic membrane, and acetylcholine receptors on post-synaptic membrane. Over the last two decades, our understanding of the role that autoantibodies play in neuromuscular junction disorders has been greatly improved. Antibodies against these channels cause a heterogeneous group of diseases, such as Lambert-Eaton syndrome, Isaacs' syndrome and myasthenia gravis. Lambert-Eaton syndrome is characterized by late onset of fatigue, skeletal muscle weakness, and autonomic symptoms. Patients with Isaacs' syndrome demonstrate muscle cramps and fasciculation. Myasthenia gravis is the most common autoimmune neuromuscular junction channelopathy characterized by fluctuation of muscle weakness. All these disorders have a high risk of tumor. Although these channelopathies share some common features, they differ for clinical features, antibodies profile, neurophysiological features, and treatments. The purpose of this review is to give a comprehensive insight on recent advances in autoimmune channelopathies at the neuromuscular junction.

Efgartigimod improves muscle weakness in a mouse model for muscle-specific kinase myasthenia gravis

Myasthenia gravis is hallmarked by fatigable muscle weakness resulting from neuromuscular synapse dysfunction caused by IgG autoantibodies. The variant with muscle-specific kinase (MuSK) autoantibodies is characterized by prominent cranial and bulbar weakness and a high frequency of respiratory crises. The majority of MuSK MG patients requires long-term immunosuppressive treatment, but the result of these treatments is considered less satisfactory than in MG with acetylcholine receptor antibodies. Emergency treatments are more frequently needed, and many patients develop permanent facial weakness and nasal speech. Therefore, new treatment options would be welcome. The neonatal Fc receptor protects IgG from lysosomal breakdown, thus prolonging IgG serum half-life. Neonatal Fc receptor antagonism lowers serum IgG levels and thus may act therapeutically in autoantibody-mediated disorders. In MuSK MG, IgG4 anti-MuSK titres closely correlate with disease severity. We therefore tested efgartigimod (ARGX-113), a new neonatal Fc receptor blocker, in a mouse model for MuSK myasthenia gravis. This model involves 11 daily injections of purified IgG4 from MuSK myasthenia gravis patients, resulting in overt myasthenic muscle weakness and, consequently, body weight loss. Daily treatment with 0.5 mg efgartigimod, starting at the fifth passive transfer day, reduced the human IgG4 titres about 8-fold, despite continued daily injection. In muscle strength and fatigability tests, efgartigimod-treated myasthenic mice outperformed control myasthenic mice. Electromyography in calf muscles at endpoint demonstrated less myasthenic decrement of compound muscle action potentials in efgartigimod-treated mice. These substantial in vivo improvements of efgartigimod-treated MuSK MG mice following a limited drug exposure period were paralleled by a tendency of recovery at neuromuscular synaptic level (in various muscles), as demonstrated by ex vivo functional studies. These synaptic improvements may well become more explicit upon longer drug exposure. In conclusion, our study shows that efgartigimod has clear therapeutic potential in MuSK myasthenia gravis and forms an exciting candidate drug for many autoantibody-mediated neurological and other disorders.

How many nerve fibres can be separated as donor from an integral nerve trunk when reconstructing a peripheral nerve trauma with amplification method by artificial biochitin conduit?

Using portion of a nearby nerve trunk to reconstruct a severe nerve lesion by artificial biodegradable chitin conduit is the core practicable method based on peripheral nerve amplification regeneration. However, the quantitative influences on skeletal muscle function corresponding to the injury of the donated nerve fibres were not previously reported. Here, we aimed to explore the compensative capacity in tibialis anterior muscles of rats with the models of acute tibialis anterior nerve branch injuries. The tibialis anterior branch of deep peroneal nerve was transected in various levels each time. Both the decreased treads of maximal compound muscle action potential (CMAP) amplitude and complete tetanic tension of the tibialis anterior muscle in rats were similar with the increasing numbers of damaged nerve fibres, which showed two S-shaped curves. When the nerve injury level was less than approximately 10%, the skeletal muscle function remained normal through complete compensation of motor endplates. As the injury degree went from 10% to 85%, the muscle function was partially impaired due to the broken compensation of motor endplates. When the nerve injury level was over approximately 85%, the skeletal muscle function was totally lost. It suggests that within a certain level of nerve injury, the skeletal muscle function maintained basically unchanged via complete compensation of motor endplates. Such nerve fibres may be used as donor nerve to repair peripheral nerve injury.

Animal Models of the Neuromuscular Junction, Vitally Informative for Understanding Function and the Molecular Mechanisms of Congenital Myasthenic Syndromes

The neuromuscular junction is the point of contact between motor nerve and skeletal muscle, its vital role in muscle function is reliant on the precise location and function of many proteins. Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders of neuromuscular transmission with 30 or more implicated proteins. The use of animal models has been instrumental in determining the specific role of many CMS-related proteins. The mouse neuromuscular junction (NMJ) has been extensively studied in animal models of CMS due to its amenability for detailed electrophysiological and histological investigations and relative similarity to human NMJ. As well as their use to determine the precise molecular mechanisms of CMS variants, where an animal model accurately reflects the human phenotype they become useful tools for study of therapeutic interventions. Many of the animal models that have been important in deconvolving the complexities of neuromuscular transmission and revealing the molecular mechanisms of disease are highlighted.

Cannabinoid-induced increase of quantal size and enhanced neuromuscular transmission

Cannabinoids exert dynamic control over many physiological processes including memory formation, cognition and pain perception. In the central nervous system endocannabinoids mediate negative feedback of quantal transmitter release following postsynaptic depolarization. The influence of cannabinoids in the peripheral nervous system is less clear and might have broad implications for the therapeutic application of cannabinoids. We report a novel cannabinoid effect upon the mouse neuromuscular synapse: acutely increasing synaptic vesicle volume and raising the quantal amplitudes. In a mouse model of myasthenia gravis the cannabinoid receptor agonist WIN 55,212 reversed fatiguing failure of neuromuscular transmission, suggesting future therapeutic potential. Our data suggest an endogenous pathway by which cannabinoids might help to regulate transmitter release at the neuromuscular junction.

Passive transfer models of myasthenia gravis with muscle-specific kinase antibodies

Myasthenia gravis (MG) with antibodies to muscle-specific kinase (MuSK) is characterized by fluctuating fatigable weakness. In MuSK MG, involvement of bulbar muscles, neck, and shoulder and respiratory weakness are more prominent than in acetylcholine receptor (AChR) MG. MuSK autoantibodies are mainly of the IgG4 subclass, and as such are unable to activate complement, have low affinity for Fc receptors, and are functionally monovalent. Therefore, the pathogenicity of IgG4 MuSK autoantibodies was initially questioned. A broad collection of in vitro active immunization and passive transfer models has been developed that have shed light on the pathogenicity of MuSK autoantibodies. Passive transfer studies with purified IgG4 from MuSK MG patients confirmed that IgG4 is sufficient to reproduce clear clinical, electrophysiological, and histological signs of myasthenia. In vitro experiments revealed that MuSK IgG4 autoantibodies preferably bind the first Ig-like domain of MuSK, correlate with disease severity, and interfere with the association between MuSK and low-density lipoprotein receptor-related protein 4 and collagen Q. Some patients have additional IgG1 MuSK autoantibodies, but their role in the disease is unclear. Altogether, this provides a rationale for epitope-specific or IgG4-specific treatment strategies for MuSK MG and emphasizes the importance of the development of different experimental models.

Engineered agrin attenuates the severity of experimental autoimmune myasthenia gravis

INTRODUCTION

Agrin is essential for the formation and maintenance of neuromuscular junctions (NMJs). NT-1654 is a C-terminal fragment of mouse neural agrin. In this study, we determined the effects of NT-1654 on the severity of experimental autoimmune myasthenia gravis (EAMG).

METHODS

EAMG was induced in female Lewis rats by immunization with the Torpedo acetylcholine receptor (tAChR) and complete Freund's adjuvant (CFA). NT-1654 was dissolved in phosphate-buffered saline (PBS) and injected daily subcutaneously into tAChR immunized rats during the first 10 days after immunization, and then every other day for the following 20 days.

RESULTS

We showed that NT-1654 attenuated clinical severity, effectively promoted the clustering of AChRs at NMJs, and alleviated the impairment of NMJ transmission and the reduction of muscle-specific kinase (MuSK) in EAMG rats.

DISCUSSION

We demonstrated that NT-1654 attenuated clinical severity, effectively promoted the clustering of AChRs at NMJs, and alleviated the impairment of NMJ transmission and the reduction of muscle-specific kinase (MuSK) in EAMG rats. Muscle Nerve 57: 814-820, 2018.

Neuromuscular synapse electrophysiology in myasthenia gravis animal models

The neuromuscular junction (NMJ) forms the synaptic connection between a motor neuron and a skeletal muscle fiber. In order to achieve a sustained muscle contraction, this synapse has to reliably transmit motor neuronal action potentials onto the muscle fiber. To guarantee successful transmission even during intense activation of the NMJ, a safety factor of neuromuscular transmission exists. In the neuromuscular disorder myasthenia gravis (MG), autoantibodies are directed against acetylcholine receptors or, in the rarer variants, against other postsynaptic NMJ proteins. This causes loss of functional acetylcholine receptors, which compromises the safety factor of neuromuscular transmission, leading to the typical fatigable muscle weakness of MG. With intracellular microelectrode measurement of (miniature) endplate potentials at NMJs in ex vivo nerve-muscle preparations from MG animal models, these functional synaptic defects have been determined in much detail. Here, we describe the electrophysiological events at the normal NMJ and the pathoelectrophysiology at NMJs of animal models for MG.

Cardiac troponin T and fast skeletal muscle denervation in ageing

BACKGROUND

Ageing skeletal muscle undergoes chronic denervation, and the neuromuscular junction (NMJ), the key structure that connects motor neuron nerves with muscle cells, shows increased defects with ageing. Previous studies in various species have shown that with ageing, type II fast-twitch skeletal muscle fibres show more atrophy and NMJ deterioration than type I slow-twitch fibres. However, how this process is regulated is largely unknown. A better understanding of the mechanisms regulating skeletal muscle fibre-type specific denervation at the NMJ could be critical to identifying novel treatments for sarcopenia. Cardiac troponin T (cTnT), the heart muscle-specific isoform of TnT, is a key component of the mechanisms of muscle contraction. It is expressed in skeletal muscle during early development, after acute sciatic nerve denervation, in various neuromuscular diseases and possibly in ageing muscle. Yet the subcellular localization and function of cTnT in skeletal muscle is largely unknown.

METHODS

Studies were carried out on isolated skeletal muscles from mice, vervet monkeys, and humans. Immunoblotting, immunoprecipitation, and mass spectrometry were used to analyse protein expression, real-time reverse transcription polymerase chain reaction was used to measure gene expression, immunofluorescence staining was performed for subcellular distribution assay of proteins, and electromyographic recording was used to analyse neurotransmission at the NMJ.

RESULTS

Levels of cTnT expression in skeletal muscle increased with ageing in mice. In addition, cTnT was highly enriched at the NMJ region-but mainly in the fast-twitch, not the slow-twitch, muscle of old mice. We further found that the protein kinase A (PKA) RIα subunit was largely removed from, while PKA RIIα and RIIβ are enriched at, the NMJ-again, preferentially in fast-twitch but not slow-twitch muscle in old mice. Knocking down cTnT in fast skeletal muscle of old mice: (i) increased PKA RIα and reduced PKA RIIα at the NMJ; (ii) decreased the levels of gene expression of muscle denervation markers; and (iii) enhanced neurotransmission efficiency at NMJ.

CONCLUSIONS

Cardiac troponin T at the NMJ region contributes to NMJ functional decline with ageing mainly in the fast-twitch skeletal muscle through interfering with PKA signalling. This knowledge could inform useful targets for prevention and therapy of age-related decline in muscle function.

Immunization of mice with LRP4 induces myasthenia similar to MuSK-associated myasthenia gravis

Since the first report of experimental animal models of myasthenia gravis (MG) with autoantibodies against low-density lipoprotein receptor-related protein 4 (LRP4), there have not been any major reports replicating the pathogenicity of anti-LRP4 antibodies (Abs). Recent clinical studies have cast doubt on the specificity and pathogenicity of anti-LRP4 antibodies for MG, highlighting the need for further research. In this study, we purified antigens corresponding to the extracellular region of human LRP4 stably expressed with chaperones in 293 cells and used these antigens to immunize female A/J mice. Immunization with LRP4 protein caused mice to develop myasthenia having similar electrophysiological and histological features as are observed in MG patients with circulating Abs against muscle-specific kinase (MuSK). Our results clearly demonstrate that active immunization of mice with LRP4 proteins causes myasthenia similar to the MG induced by anti-MuSK Abs. Further experimental and clinical studies are required to prove the pathogenicity of anti-LRP4 Abs in MG patients.

Synaptic Homeostasis and Its Immunological Disturbance in Neuromuscular Junction Disorders

In the neuromuscular junction, postsynaptic nicotinic acetylcholine receptor (nAChR) clustering, trans-synaptic communication and synaptic stabilization are modulated by the molecular mechanisms underlying synaptic plasticity. The synaptic functions are based presynaptically on the active zone architecture, synaptic vesicle proteins, Ca²⁺ channels and synaptic vesicle recycling. Postsynaptically, they are based on rapsyn-anchored nAChR clusters, localized sensitivity to ACh, and synaptic stabilization via linkage to the extracellular matrix so as to be precisely opposed to the nerve terminal. Focusing on neural agrin, Wnts, muscle-specific tyrosine kinase (a mediator of agrin and Wnts signalings and regulator of trans-synaptic communication), low-density lipoprotein receptor-related protein 4 (the receptor of agrin and Wnts and participant in retrograde signaling), laminin-network (including muscle-derived agrin), extracellular matrix proteins (participating in the synaptic stabilization) and presynaptic receptors (including muscarinic and adenosine receptors), we review the functional structures of the synapse by making reference to immunological pathogenecities in postsynaptic disease, myasthenia gravis. The synapse-related proteins including cortactin, coronin-6, caveolin-3, doublecortin, R-spondin 2, amyloid precursor family proteins, glia cell-derived neurotrophic factor and neurexins are also discussed in terms of their possible contribution to efficient synaptic transmission at the neuromuscular junction.

On-chip, multisite extracellular and intracellular recordings from primary cultured skeletal myotubes

In contrast to the extensive use of microelectrode array (MEA) technology in electrophysiological studies of cultured neurons and cardiac muscles, the vast field of skeletal muscle research has yet to adopt the technology. Here we demonstrate an empowering MEA technology for high quality, multisite, long-term electrophysiological recordings from cultured skeletal myotubes. Individual rat skeletal myotubes cultured on micrometer sized gold mushroom-shaped microelectrode (gMμE) based MEA tightly engulf the gMμEs, forming a high seal resistance between the myotubes and the gMμEs. As a consequence, spontaneous action potentials generated by the contracting myotubes are recorded as extracellular field potentials with amplitudes of up to 10 mV for over 14 days. Application of a 10 ms, 0.5-0.9 V voltage pulse through the gMμEs electroporated the myotube membrane, and transiently converted the extracellular to intracellular recording mode for 10-30 min. In a fraction of the cultures stable attenuated intracellular recordings were spontaneously produced. In these cases or after electroporation, subthreshold spontaneous potentials were also recorded. The introduction of the gMμE-MEA as a simple-to-use, high-quality electrophysiological tool together with the progress made in the use of cultured human myotubes opens up new venues for basic and clinical skeletal muscle research, preclinical drug screening, and personalized medicine.

Pathogenesis of myasthenia gravis: update on disease types, models, and mechanisms

Myasthenia gravis is an autoimmune disease of the neuromuscular junction (NMJ) caused by antibodies that attack components of the postsynaptic membrane, impair neuromuscular transmission, and lead to weakness and fatigue of skeletal muscle. This can be generalised or localised to certain muscle groups, and involvement of the bulbar and respiratory muscles can be life threatening. The pathogenesis of myasthenia gravis depends upon the target and isotype of the autoantibodies. Most cases are caused by immunoglobulin (Ig)G1 and IgG3 antibodies to the acetylcholine receptor (AChR). They produce complement-mediated damage and increase the rate of AChR turnover, both mechanisms causing loss of AChR from the postsynaptic membrane. The thymus gland is involved in many patients, and there are experimental and genetic approaches to understand the failure of immune tolerance to the AChR. In a proportion of those patients without AChR antibodies, antibodies to muscle-specific kinase (MuSK), or related proteins such as agrin and low-density lipoprotein receptor-related protein 4 (LRP4), are present. MuSK antibodies are predominantly IgG4 and cause disassembly of the neuromuscular junction by disrupting the physiological function of MuSK in synapse maintenance and adaptation. Here we discuss how knowledge of neuromuscular junction structure and function has fed into understanding the mechanisms of AChR and MuSK antibodies. Myasthenia gravis remains a paradigm for autoantibody-mediated conditions and these observations show how much there is still to learn about synaptic function and pathological mechanisms.

Autoimmune synaptopathies

Autoantibodies targeting proteins at the neuromuscular junction are known to cause several distinct myasthenic syndromes. Recently, autoantibodies targeting neurotransmitter receptors and associated proteins have also emerged as a cause of severe, but potentially treatable, diseases of the CNS. Here, we review the clinical evidence as well as in vitro and in vivo experimental evidence that autoantibodies account for myasthenic syndromes and autoimmune disorders of the CNS by disrupting the functional or structural integrity of synapses. Studying neurological and psychiatric diseases of autoimmune origin may provide new insights into the cellular and circuit mechanisms underlying a broad range of CNS disorders.

Characterization of neuromuscular synapse function abnormalities in multiple Duchenne muscular dystrophy mouse models

Duchenne muscular dystrophy (DMD) is an X-linked myopathy caused by dystrophin deficiency. Dystrophin is present intracellularly at the sarcolemma, connecting actin to the dystrophin-associated glycoprotein complex. Interestingly, it is enriched postsynaptically at the neuromuscular junction (NMJ), but its synaptic function is largely unknown. Utrophin, a dystrophin homologue, is also concentrated at the NMJ, and upregulated in DMD. It is possible that the absence of dystrophin at NMJs in DMD causes neuromuscular transmission defects that aggravate muscle weakness. We studied NMJ function in mdx mice (lacking dystrophin) and wild type mice. In addition, mdx/utrn(+/-) and mdx/utrn(-/-) mice (lacking utrophin) were used to investigate influences of utrophin levels. The three Duchenne mouse models showed muscle weakness when comparatively tested in vivo, with mdx/utrn(-/-) mice being weakest. Ex vivo muscle contraction and electrophysiological studies showed a reduced safety factor of neuromuscular transmission in all models. NMJs had ~ 40% smaller miniature endplate potential amplitudes compared with wild type, indicating postsynaptic sensitivity loss for the neurotransmitter acetylcholine. However, nerve stimulation-evoked endplate potential amplitudes were unchanged. Consequently, quantal content (i.e. the number of acetylcholine quanta released per nerve impulse) was considerably increased. Such a homeostatic compensatory increase in neurotransmitter release is also found at NMJs in myasthenia gravis, where autoantibodies reduce acetylcholine receptors. However, high-rate nerve stimulation induced exaggerated endplate potential rundown. Study of NMJ morphology showed that fragmentation of acetylcholine receptor clusters occurred in all models, being most severe in mdx/utrn(-/-) mice. Overall, we showed mild 'myasthenia-like' neuromuscular synaptic dysfunction in several Duchenne mouse models, which possibly affects muscle weakness and degeneration.

Forced expression of muscle specific kinase slows postsynaptic acetylcholine receptor loss in a mouse model of MuSK myasthenia gravis

We investigated the influence of postsynaptic tyrosine kinase signaling in a mouse model of muscle‐specific kinase (MuSK) myasthenia gravis (MG). Mice administered repeated daily injections of IgG from MuSK MG patients developed impaired neuromuscular transmission due to progressive loss of acetylcholine receptor (AChR) from the postsynaptic membrane of the neuromuscular junction. In this model, anti‐MuSK‐positive IgG caused a reduction in motor endplate immunolabeling for phosphorylated Src‐Y418 and AChR β‐subunit‐Y390 before any detectable loss of MuSK or AChR from the endplate. Adeno‐associated viral vector (rAAV) encoding MuSK fused to enhanced green fluorescent protein (MuSK‐EGFP) was injected into the tibialis anterior muscle to increase MuSK synthesis. When mice were subsequently challenged with 11 daily injections of IgG from MuSK MG patients, endplates expressing MuSK‐EGFP retained more MuSK and AChR than endplates of contralateral muscles administered empty vector. Recordings of compound muscle action potentials from myasthenic mice revealed less impairment of neuromuscular transmission in muscles that had been injected with rAAV‐MuSK‐EGFP than contralateral muscles (empty rAAV controls). In contrast to the effects of MuSK‐EGFP, forced expression of rapsyn‐EGFP provided no such protection to endplate AChR when mice were subsequently challenged with MuSK MG IgG. In summary, the immediate in vivo effect of MuSK autoantibodies was to suppress MuSK‐dependent tyrosine phosphorylation of proteins in the postsynaptic membrane, while increased MuSK synthesis protected endplates against AChR loss. These results support the hypothesis that reduced MuSK kinase signaling initiates the progressive disassembly of the postsynaptic membrane scaffold in this mouse model of MuSK MG.

Myf5 and Myogenin in the development of thymic myoid cells - Implications for a murine in vivo model of myasthenia gravis

Myasthenia gravis (MG) is caused by autoantibodies against the neuromuscular junction of striated muscle. Most MG patients have autoreactive T- and B-cells directed to the acetylcholine receptor (AChR). To achieve immunologic tolerance, developing thymocytes are normally eliminated after recognition of self-antigen-derived peptides. Presentation of muscle-specific antigens is likely achieved through two pathways: on medullary thymic epithelial cells and on medullary dendritic cells cross-presenting peptides derived from a unique population of thymic myoid cells (TMC). Decades ago, it has been hypothesized that TMC play a key role in the induction of immunological tolerance towards skeletal muscle antigens. However, an experimental model to address this postulate has not been available. To generate such a model, we tested the hypothesis that the development of TMC depends on myogenic regulatory factors. To this end, we utilized Myf5-deficient mice, which lack the first wave of muscle cells but form normal skeletal muscles later during development, and Myogenin-deficient mice, which fail to form differentiated myofibers. We demonstrate for the first time that Myf5- and Myogenin-deficient mice showed a partial or complete, respectively, loss of TMC in an otherwise regularly structured thymus. To overcome early postnatal lethality of muscle-deficient, Myogenin-knockout mice we transplanted Myogenin-deficient fetal thymuses into Foxn1(nu/nu) mice that lack their own thymus anlage. We found that the transplants are functional but lack TMC. In combination with established immunization strategies (utilizing AChR or Titin), this model should enable us in the future testing the hypothesis that TMC play an indispensable role in the development of central tolerance towards striated muscle antigens.

Functional Connectivity under Optogenetic Control Allows Modeling of Human Neuromuscular Disease

Capturing the full potential of human pluripotent stem cell (PSC)-derived neurons in disease modeling and regenerative medicine requires analysis in complex functional systems. Here we establish optogenetic control in human PSC-derived spinal motorneurons and show that co-culture of these cells with human myoblast-derived skeletal muscle builds a functional all-human neuromuscular junction that can be triggered to twitch upon light stimulation. To model neuromuscular disease we incubated these co-cultures with IgG from myasthenia gravis patients and active complement. Myasthenia gravis is an autoimmune disorder that selectively targets neuromuscular junctions. We saw a reversible reduction in the amplitude of muscle contractions, representing a surrogate marker for the characteristic loss of muscle strength seen in this disease. The ability to recapitulate key aspects of disease pathology and its symptomatic treatment suggests that this neuromuscular junction assay has significant potential for modeling of neuromuscular disease and regeneration.