Effects of the ß2-adrenoceptor agonist, albuterol, in a mouse model of anti-MuSK myasthenia gravis

The Mechanism of α2 adrenoreceptor-dependent Modulation of Neurotransmitter Release at the Neuromuscular Junctions

α2-Adrenoreceptors (ARs) are main Gi-protein coupled autoreceptors in sympathetic nerve terminals and targets for dexmedetomidine (DEX), a widely used sedative. We hypothesize that α2-ARs are also potent regulators of neuromuscular transmission via G protein-gated inwardly rectifying potassium (GIRK) channels. Using extracellular microelectrode recording of postsynaptic potentials, we found DEX-induced inhibition of spontaneous and evoked neurotransmitter release as well as desynchronization of evoked exocytotic events in the mouse diaphragm neuromuscular junction. These effects were suppressed by SKF-86,466, a selective α2-AR antagonist. An activator of GIRK channels ML297 had the same effects on neurotransmitter release as DEX. By contrast, inhibition of GIRK channels with tertiapin-Q prevented the action of DEX on evoked neurotransmitter release, but not on spontaneous exocytosis. The synaptic vesicle exocytosis is strongly dependent on Ca2+ influx through voltage-gated Ca2+ channels (VGCCs), which can be negatively regulated via α2-AR - GIRK channel axis. Indeed, inhibition of P/Q-, L-, N- or R-type VGCCs prevented the inhibitory action of DEX on evoked neurotransmitter release; antagonists of P/Q- and N-type channels also suppressed the DEX-mediated desynchronization of evoked exocytotic events. Furthermore, inhibition of P/Q-, L- or N-type VGCCs precluded the frequency decrease of spontaneous exocytosis upon DEX application. Thus, α2-ARs acting via GIRK channels and VGCCs (mainly, P/Q- and N-types) exert inhibitory effect on the neuromuscular communication by attenuating and desynchronizing evoked exocytosis. In addition, α2-ARs can suppress spontaneous exocytosis through GIRK channel-independent, but VGCC-dependent pathway.

The emerging spectrum of fetal acetylcholine receptor antibody-related disorders (FARAD)

In utero exposure to maternal antibodies targeting the fetal acetylcholine receptor isoform (fAChR) can impair fetal movement, leading to arthrogryposis multiplex congenita (AMC). Fetal AChR antibodies have also been implicated in apparently rare, milder myopathic presentations termed fetal acetylcholine receptor inactivation syndrome (FARIS). The full spectrum associated with fAChR antibodies is still poorly understood. Moreover, since some mothers have no myasthenic symptoms, the condition is likely underreported, resulting in failure to implement effective preventive strategies. Here we report clinical and immunological data from a multicentre cohort (n = 46 cases) associated with maternal fAChR antibodies, including 29 novel and 17 previously reported with novel follow-up data. Remarkably, in 50% of mothers there was no previously established myasthenia gravis (MG) diagnosis. All mothers (n = 30) had AChR antibodies and, when tested, binding to fAChR was often much greater than that to the adult AChR isoform. Offspring death occurred in 11/46 (23.9%) cases, mainly antenatally due to termination of pregnancy prompted by severe AMC (7/46, 15.2%), or during early infancy, mainly from respiratory failure (4/46, 8.7%). Weakness, contractures, bulbar and respiratory involvement were prominent early in life, but improved gradually over time. Facial (25/34; 73.5%) and variable peripheral weakness (14/32; 43.8%), velopharyngeal insufficiency (18/24; 75%) and feeding difficulties (16/36; 44.4%) were the most common sequelae in long-term survivors. Other unexpected features included hearing loss (12/32; 37.5%), diaphragmatic paresis (5/35; 14.3%), CNS involvement (7/40; 17.5%) and pyloric stenosis (3/37; 8.1%). Oral salbutamol used empirically in 16/37 (43.2%) offspring resulted in symptom improvement in 13/16 (81.3%). Combining our series with all previously published cases, we identified 21/85 mothers treated with variable combinations of immunotherapies (corticosteroids/intravenous immunoglobulin/plasmapheresis) during pregnancy either for maternal MG symptom control (12/21 cases) or for fetal protection (9/21 cases). Compared to untreated pregnancies (64/85), maternal treatment resulted in a significant reduction in offspring deaths (P < 0.05) and other complications, with treatment approaches involving intravenous immunoglobulin/ plasmapheresis administered early in pregnancy most effective. We conclude that presentations due to in utero exposure to maternal (fetal) AChR antibodies are more common than currently recognized and may mimic a wide range of neuromuscular disorders. Considering the wide clinical spectrum and likely diversity of underlying mechanisms, we propose 'fetal acetylcholine receptor antibody-related disorders' (FARAD) as the most accurate term for these presentations. FARAD is vitally important to recognize, to institute appropriate management strategies for affected offspring and to improve outcomes in future pregnancies. Oral salbutamol is a symptomatic treatment option in survivors.

Nervous regulation: beta-2-adrenergic signaling in immune homeostasis, cancer immunotherapy, and autoimmune diseases

Beta-2-adrenergic receptor (β2-AR) mediates neural signaling from the sympathetic nervous system (SNS) to the immune system to modulate immunogenic and immunosuppressive responses for maintaining immune homeostasis. β2-AR regulates various cellular activities on the innate and adaptive immune cells through differential signaling to modulate activation, proliferation, differentiation, and cytokine production. This signaling pathway has been found to be critical for regulating anti-tumor immune responses and autoimmune responses. Recently, β2-AR has also been implicated in the mobilization of immune cells in peripheral blood and ex-vivo expansion of cytotoxic T cells from donor blood that has clinical implications for improving cancer immunotherapy. This review attempts to provide a comprehensive overview of the established and emerging roles of β2-AR signaling in immune homeostasis, cancer immunotherapy, and autoimmune diseases.

Effect of salbutamol on neuromuscular junction function and structure in a mouse model of DOK7 congenital myasthenia

Congenital myasthenic syndromes (CMS) are characterized by fatigable muscle weakness resulting from impaired neuromuscular transmission. β2-adrenergic agonists are an effective treatment for DOK7-CMS. DOK7 is a component within the AGRN-LRP4-MUSK-DOK7 signalling pathway that is key for the formation and maintenance of the synaptic structure of the neuromuscular junction (NMJ). The precise mechanism of action of β2-adrenergic agonists at the NMJ is not fully understood. In this study, we investigated whether β2-adrenergic agonists improve both neurotransmission and structural integrity of the NMJ in a mouse model of DOK7-CMS. Ex-vivo electrophysiological techniques and microscopy of the NMJ were used to study the effect of salbutamol, a β2-adrenergic agonist, on synaptic structure and function. DOK7-CMS model mice displayed a severe phenotype with reduced weight gain and perinatal lethality. Salbutamol treatment improved weight gain and survival in DOK7 myasthenic mice. Model animals had fewer active NMJs, detectable by endplate recordings, compared with age-matched wild-type littermates. Salbutamol treatment increased the number of detectable NMJs during endplate recording. Correspondingly, model mice had fewer acetylcholine receptor-stained NMJs detected by fluorescent labelling, but following salbutamol treatment an increased number were detectable. The data demonstrate that salbutamol can prolong survival and increase NMJ number in a severe model of DOK7-CMS.

Heterogeneity in myasthenia gravis: considerations for disease management

Introduction: Myasthenia gravis is a rare disease of the neuromuscular junction and a prototype of B cell-driven immunopathology. Pathogenic antibodies target post-synaptic transmembrane proteins, most commonly the nicotinic acetylcholine receptor and the muscle-specific tyrosine kinase, inducing end-plate alterations and neuromuscular transmission impairment. Several clinical subtypes are distinct on the basis of associated antibodies, age at symptom onset, thymus pathology, genetic factors, and weakness distribution. These subtypes have distinct pathogenesis that can account for different responses to treatment. Conventional therapy is based on the use of symptomatic agents, steroids, immunosuppressants and thymectomy. Of late, biologics have emerged as effective therapeutic options.Areas covered: In this review, we will discuss the management of myasthenia gravis in relation to its phenotypic and biological heterogeneity, in the light of recent advances in the disease immunopathology, new diagnostic tools, and results of clinical trialsExpert opinion: Clinical management is shaped on serological subtype, and patient age at onset, lifestyle and comorbidities, balancing therapeutic needs and safety. Although reliable biomarkers predictive of clinical and biologic outcome are still lacking, recent developments promise a more effective and safe treatment. Disease subtyping according to serological testing and immunopathology is crucial to the appropriateness of clinical management.

Cholinergic hyperactivity in patients with myasthenia gravis with MuSK antibodies: A neurophysiological study

OBJECTIVE

Patients with myasthenia gravis associated with muscle-specific tyrosine kinase antibodies (MuSK-MG) often manifest signs of cholinergic hyperactivity with standard doses of acetylcholinesterase inhibitors (AChE-Is). Aim of the study was to investigate whether repetitive compound muscle action potential (R-CMAP), the neurophysiological correlate of cholinergic hyperactivity, was present in MuSK-MG irrespective of AChE-I treatment.

METHODS

Patients with confirmed diagnosis of MuSK-MG were consecutively enrolled during follow-up visits, from January 2019 to April 2020. All these subjects underwent the same neurophysiological protocol, including motor nerve conduction studies and repetitive nerve stimulation. In patients taking pyridostigmine, neurophysiological testing was performed at least 12 hours after the last dose. For comparison, the presence of R-CMAP was investigated in 20 consecutive acetylcholine receptor antibody positive myasthenia gravis (AChR-MG) patients.

RESULTS

We enrolled 25 MuSK-MG patients (20 females), aged 16-79 years at the study time, with disease duration ranging 0.6-48.8 years (median: 17.7 years). R-CMAP was detected in 12/25 (48%) MuSK-MG cases and in none of the AChR-MG controls (p = 0.0003). In the MuSK-MG population, a history of muscle cramps and fasciculations, during low-dose pyridostigmine therapy, was significantly more frequent in R-CMAP positive than in R-CMAP negative patients (100% vs 31%, p = 0.001). At the time of the study, the proportion of patients still symptomatic for MG was higher among R-CMAP positive cases (92% vs 23%, p = 0.0005).

CONCLUSIONS

Cholinergic hyperactivity is a relatively common finding in MuSK-MG patients, independent of AChE-I treatment, and may constitute an intrinsic feature of the disease.

SIGNIFICANCE

R-CMAP detection can represent a useful diagnostic clue for MuSK-MG and predicts poor tolerance to AChE-Is.

A novel DOK7 mutation causing congenital myasthenic syndrome with limb-girdle weakness: case series of three family members

Congenital myasthenia syndrome (CMS) is a group of heterogeneous diseases affecting the neuromuscular endplate. CMS has a considerably different phenotypic presentations, with the onset time ranging from early infancy to late adulthood. Here, we report a case of a CMS due to a new DOK7 mutation in a 28-year-old man and two of his sisters, who have a pure limb-girdle weakness. DOK7 CMS has a varying presentation. Typically, the onset occurs in childhood with ptosis, bulbar symptoms, difficulty walking, weakness, and gait abnormality. This case sheds light on a novel DOK7 gene mutation with a unique presentation of CMS and provides insight into its unique phenotypic presentation.

Maintenance of contractile force of the hind limb muscles by the somato-lumbar sympathetic reflexes

This study aimed to clarify whether the reflex excitation of muscle sympathetic nerves induced by contractions of the skeletal muscles modulates their contractility. In anesthetized rats, isometric tetanic contractions of the triceps surae muscles were induced by electrical stimulation of the intact tibial nerve before and after transection of the lumbar sympathetic trunk (LST), spinal cord, or dorsal roots. The amplitude of the tetanic force (TF) was reduced by approximately 10% at 20 min after transection of the LST, spinal cord, or dorsal roots. The recorded postganglionic sympathetic nerve activity from the lumbar gray ramus revealed that both spinal and supraspinal reflexes were induced in response to the contractions. Repetitive electrical stimulation of the cut peripheral end of the LST increased the TF amplitude. Our results indicated that the spinal and supraspinal somato-sympathetic nerve reflexes induced by contractions of the skeletal muscles contribute to the maintenance of their own contractile force.

Adrenoceptors Modulate Cholinergic Synaptic Transmission at the Neuromuscular Junction

Adrenoceptor activators and blockers are widely used clinically for the treatment of cardiovascular and pulmonary disorders. More recently, adrenergic agents have also been used to treat neurodegenerative diseases. Recent studies indicate a location of sympathetic varicosities in close proximity to neuromuscular junctions. The pressing question is whether there could be any effects of endo- or exogenous catecholamines on cholinergic neuromuscular transmission. It was shown that the pharmacological stimulation of adrenoceptors, as well as sympathectomy, can affect both acetylcholine release from motor nerve terminals and the functioning of postsynaptic acetylcholine receptors. In this review, we discuss the recent data regarding the effects of adrenergic drugs on neurotransmission at the neuromuscular junction. The elucidation of the molecular mechanisms by which the clinically relevant adrenomimetics and adrenoblockers regulate quantal acetylcholine release from the presynaptic nerve terminals and postsynaptic sensitivity may help in the design of highly effective and well-tolerated sympathomimetics for treating a number of neurodegenerative diseases accompanied by synaptic defects.

Adrenaline Facilitates Synaptic Transmission by Synchronizing Release of Acetylcholine Quanta from Motor Nerve Endings

The long history of studies on the effect of catecholamines on synaptic transmission does not answer the main question about the mechanism of their action on quantal release in the neuromuscular junction. Currently, interest in catecholamines has increased not only because of their widespread use in the clinic for the treatment of cardiovascular and pulmonary diseases but also because of recent data on their possible use for the treatment of certain neurodegenerative diseases, muscle weakness and amyotrophic sclerosis. Nevertheless, the effects and mechanisms of catecholamines on acetylcholine release remain unclear. We investigated the action of noradrenaline and adrenaline on the spontaneous and evoked quantal secretion of acetylcholine in the neuromuscular junction of the rat soleus muscle. Noradrenaline (10 μM) did not change the spontaneous acetylcholine quantal release, the number of released quanta after nerve stimulation, or the timing of the quantal secretion. However, adrenaline at the same concentration increased spontaneous secretion by 40%, increased evoked acetylcholine quantal release by 62%, and synchronized secretion. These effects differ from those previously described by us in the synapses of the frog cutaneous pectoris muscle and mouse diaphragm. This indicates specificity in catecholamine action that depends on the functional type of muscle and the need to take the targeted type of muscle into account in clinical practice.

Spinal muscular atrophy - insights and challenges in the treatment era

Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease caused by deletion or mutation of SMN1. Four subtypes exist, characterized by different clinical severities. New therapeutic approaches have become available in the past few years, dramatically changing the natural history of all SMA subtypes, including substantial clinical improvement with the severe and advanced SMA type 1 variant. Trials have now demonstrated that phenotypic rescue is even more dramatic when pre-symptomatic patients are treated, and emerging real-world data are demonstrating the benefits of intervention even in the chronic phase of the condition. Here, we critically review how the field is rapidly evolving in response to the new therapies and questions that the new treatments have posed, including the effects of treatment at different ages and stages of disease, new phenotypes and long-term outcomes in patients who would not have survived without treatment, and decisions of who to treat and when. We also discuss how the outcomes associated with different timing of therapeutic intervention are contributing to our understanding of the biology and pathogenesis of SMA.

Sympathomimetics regulate quantal acetylcholine release at neuromuscular junctions through various types of adrenoreceptors

The studies of the interaction between the sympathetic and motor nervous systems are extremely relevant due to therapy for many neurodegenerative and cardiovascular disorders involving adrenergic compounds. Evidences indicate close contact between sympathetic varicosities and neuromuscular synapses. This raises questions about the effects of catecholamines on synaptic transmission. The currently available information is contradictory, and the types of adrenoreceptors responsible for modulation of neurotransmitter release have not been identified in mammalian neuromuscular synapses. Our results have shown that the α1A, α1B, α2A, α2B, α2C, and β1 adrenoreceptor subtypes are expressed in mouse diaphragm muscle containing neuromuscular synapses and sympathetic varicosities. Pharmacological stimulation of adrenoreceptors affects both spontaneous and evoked acetylcholine quantal secretion. Agonists of the α1, α2 and β1 adrenoreceptors decrease spontaneous release. Activation of the α2 and β1 adrenoreceptors reduces the number of acetylcholine quanta released in response to a nerve stimulus (quantal content), but an agonist of the β2 receptors increases quantal content. Activation of α2 and β2 adrenoreceptors alters the kinetics of acetylcholine quantal release by desynchronizing the neurosecretory process. Specific blockers of these receptors eliminate the effects of the specific agonists. The action of blockers on quantal acetylcholine secretion indicates possible action of endogenous catecholamines on neuromuscular transmission. Elucidating the molecular mechanisms by which clinically utilized adrenomimetics and adrenoblockers regulate synaptic vesicle release at the motor axon terminal will lead to the creation of improved and safer sympathomimetics for the treatment of various neurodegenerative diseases with synaptic defects.

MuSK antibodies, lessons learned from poly- and monoclonality

Muscle-specific kinase (MuSK) plays a critical role in establishing and maintaining neuromuscular synapses. Antibodies derived from immunizing animals with MuSK were important tools to help detect MuSK and its activity. The role of antibodies in MuSK-related research got an extra dimension when autoantibodies to MuSK were found to cause myasthenia gravis (MG) in 2001. Active immunization with MuSK or passive transfer of polyclonal purified IgG(4) fractions from patients reproduced myasthenic muscle weakness in a range of animal models. Polyclonal patient-purified autoantibodies were furthermore found to block agrin-Lrp4-MuSK signaling, explaining the synaptic disassembly, failure of neuromuscular transmission and ultimately muscle fatigue observed in vivo. MuSK autoantibodies are predominantly of the IgG4 subclass. Low levels of other subclass MuSK antibodies coexist, but their role in the pathogenesis is unclear. Patient-derived monoclonal antibodies revealed that MuSK antibody subclass and valency alters their functional effects and possibly their pathogenicity. Interestingly, recombinant functional bivalent MuSK antibodies might even have therapeutic potential for a variety of neuromuscular disorders, due to their agonistic nature on the MuSK signaling cascade. Thus, MuSK antibodies have proven to be helpful tools to study neuromuscular junction physiology, contributed to our understanding of the pathophysiology of MuSK MG and might be used to treat neuromuscular disorders. The source of MuSK antibodies and consequently their (mixed) polyclonal or monoclonal nature were important confounding factors in these experiments. Here we review the variety of MuSK antibodies described thus far, the insights they have given us and their potential for the future.

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.

Sympathomimetics regulate neuromuscular junction transmission through TRPV1, P/Q- and N-type Ca2+ channels

Increasing evidence indicates that, first, the sympathetic nervous system interacts extensively with both vasculature and skeletal muscle fibers near neuromuscular junctions (NMJs) and, second, its neurotransmitter, noradrenaline, influences myofiber molecular composition and function and motor innervation. Since sympathomimetic agents have been reported to improve NMJ transmission, we examined whether two in clinical use, salbutamol and clenbuterol, affect the motor axon terminal via extracellular Ca2+ and molecular targets, such as TRPV1 and P/Q- and N-type voltage-activated Ca2+ channels. Electrophysiological recordings in ex-vivo preparations of peroneal nerves and lumbricalis muscles from young adult mice focused on spontaneous miniature end-plate potentials and singly and repetitively evoked end-plate potentials. Adding one dose of salbutamol or clenbuterol to the nerve/muscle preparation or repeatedly administering salbutamol to a mouse for 4 weeks increased spontaneous and evoked synaptic vesicle release but induced a steep decline in EPP amplitude in response to repetitive nerve stimulation. These effects were mediated primarily by ω-agatoxin IVA-sensitive P/Q-type and secondarily by ω-conotoxin GVIA-sensitive N-type Ca2+ channels. Presynaptic arvanil-sensitive TRPV1 channels seem to regulate Ca2+ at the motor neuron terminal at rest, while putative presynaptic β-adrenergic receptors may mediate sympathomimetic and catecholamine effects on presynaptic Ca2+ channels during NMJ activation.

Bidirectional Role of β2-Adrenergic Receptor in Autoimmune Diseases

Disorder of the sympathetic nervous system (SNS) is closely related to the pathogenesis of various autoimmune diseases (ADs). Catecholamine triggered beta2-adrenergic receptor (β2-AR) signaling is important in creating a bidirectional response in the progression of ADs due to factors including diverse expression patterns, single nucleotide polymorphisms (SNPs), biased signals, and desensitization of β2-AR, as well as different subtypes of Gα binding to β2-AR. In this review, we summarize the actions of β2-AR signaling in regulating the functions of immunocytes and in the pathogenesis of ADs, and the application of β2-AR agonists or antagonists in treating major types of ADs is also discussed. We suggest that restoring the immune balance via a soft regulation of the expression or activation of β2-AR is one of the promising therapeutic strategies for systematic ADs.

Beta-2 Adrenergic Receptor Agonists Enhance AChR Clustering in C2C12 Myotubes: Implications for Therapy of Myasthenic Disorders

Background:

Congenital myasthenic syndromes (CMS) are a group of inherited neuromuscular transmission disorders causing fatiguable muscle weakness. ADRB2 agonists have been observed to provide therapeutic benefit where destabilisation of NMJ structures is part of the underlying pathology, such as in DOK7, COLQ and MuSK CMS as well as in slow channel syndrome. However, very little is known about the molecular mechanisms underlying the effects of ADRB2 agonists in CMS.

Objective:

In vitro investigation into whether an ADRB2 agonist affects the AChR clustering pathway and has the potential to increase the number and stability of AChR clusters.

Methods:

Cultured C2C12 mouse myotubes overexpressing the common DOK7 frameshift mutation c.1124_1127dupTGCC were incubated with salbutamol sulphate and the effect on AChR cluster numbers were investigated. Moreover, agrin-induced AChR clusters in C2C12 WT cells were left to disperse after agrin-wash-off, and the effects of incubation with salbutamol sulphate on AChR cluster numbers were explored.

Results:

Salbutamol sulphate induced a significant increase in the number of AChR clusters formed on C2C12 cells overexpressing c.1124_1127dupTGCC. Furthermore, significantly more clusters remained in C2C12 WT myotubes incubated with salbutamol sulphate following agrin wash-off.

Conclusions:

The results suggest that ADRB2 agonists directly affect proteins located at the neuromuscular junction and exert a stabilising effect on AChR clusters.

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.

Myasthenia gravis with antibodies to MuSK: an update

Myasthenia gravis with antibodies to the muscle-specific tyrosine kinase (MuSK⁺ MG) is a rare disease with distinctive pathogenic mechanisms and clinical features. An acute onset and predominant bulbar muscle weakness are very common and highly suggestive of the disease. On the other hand, a more indolent course, atypical ocular presentation, and signs of cholinergic hyperactivity may complicate the diagnosis. Though MuSK⁺ MG is still a severe disease, over the years we have observed a steady reduction in the rate of respiratory crisis and a significant improvement in the clinical outcome, both likely related to earlier diagnosis and timely treatment. Despite the improved management, MuSK⁺ MG patients tend to remain dependent on long-term immunosuppressive treatment and may develop permanent disabling weakness. In uncontrolled studies, B cell depletion with rituximab proved effective in most patients with refractory disease, inducing prolonged clinical responses associated with a sustained reduction of serum antibody levels. Promising results from experimental studies and case reports suggest that both 3,4-diaminopyridine and albuterol may be effective as symptomatic agents.

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.

Longitudinal epitope mapping in MuSK myasthenia gravis: implications for disease severity

Muscle weakness in MuSK myasthenia gravis (MG) is caused predominantly by IgG4 antibodies which block MuSK signalling and destabilize neuromuscular junctions. We determined whether the binding pattern of MuSK IgG4 antibodies change throughout the disease course ("epitope spreading"), and affect disease severity or treatment responsiveness. We mapped the MuSK epitopes of 255 longitudinal serum samples of 53 unique MuSK MG patients from three independent cohorts with ELISA. Antibodies against the MuSK Iglike-1 domain determine disease severity. Epitope spreading outside this domain did not contribute to disease severity nor to pyridostigmine responsiveness. This provides a rationale for epitope specific treatment strategies.

Current Treatment, Emerging Translational Therapies, and New Therapeutic Targets for Autoimmune Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune disease associated with the production of autoantibodies against 1) the skeletal muscle acetylcholine receptor; 2) muscle-specific kinase, a receptor tyrosine kinase critical for the maintenance of neuromuscular synapses; 3) low-density lipoprotein receptor-related protein 4, an important molecular binding partner for muscle-specific kinase; and 4) other muscle endplate proteins. In addition to the profile of autoantibodies, MG may be classified according the location of the affected muscles (ocular vs generalized), the age of symptom onset, and the nature of thymic pathology. Immunopathologic events leading to the production of autoantibodies differ in the various disease subtypes. Advances in our knowledge of the immunopathogenesis of the subtypes of MG will allow for directed utilization of the ever-growing repertoire of therapeutic agents that target distinct nodes in the immune pathway relevant to the initiation and maintenance of autoimmune disease. In this review, we examine the pathogenesis of MG subtypes, current treatment options, and emerging new treatments and therapeutic targets.

Overcoming challenges in the diagnosis and treatment of myasthenia gravis

In recent years, the discovery of new autoantigens and the use of sensitive assays have expanded the clinical spectrum of myasthenia gravis (MG). In particular, antibodies binding to clustered acetylcholine receptors and to the low-density lipoprotein receptor-related protein 4 have not only bridged a significant gap in diagnosis but also have relevant clinical implications. MG management includes different therapeutic options, from symptomatic agents as the only therapy in mildly affected cases to combined long-term immunosuppression and thymectomy in patients with severe disabling disease. MG biological diversity can influence the response to therapies and should be taken into account when planning treatment. Biologic agents are promising, though their use is currently limited to patients with refractory disease.

Anticholinesterase Therapy Worsening Head Drop and Limb Weakness Due to a Novel DOK7 Mutation

Dok-7 myasthenia is an autosomal recessive congenital myasthenic syndrome due to DOK7 mutations. Anticholinesterase therapy is ineffective and may worsen the weakness in patients with Dok-7 myasthenia or few other forms of congenital myasthenic syndromes. We describe a 31-year-old man previously diagnosed with seronegative myasthenia gravis. Repetitive stimulation of the right spinal accessory nerve showed 51% decrement. Needle electromyography revealed myopathic changes in clinically affected muscles. Muscle biopsy was normal. The patient was referred to us for worsening weakness after taking pyridostigmine. We searched for DOK7 mutations and identified compound heterozygous mutations of a common c.1124_1127dupTGCC mutation and a novel splice site mutation, c.772+2_+4delinsCCGGGCAGGCGGGCA. Discontinuation of pyridostigmine improved weakness. He further regained strength with oral albuterol therapy and decrement was reduced to 25%. Worsening of symptoms with anticholinesterase therapy in patients with "seronegative myasthenia gravis" should prompt clinicians to consider a possibility of congenital myasthenic syndromes to avoid unnecessary use of immunosuppressive therapy. Patients with Dok-7 myasthenia respond well to oral albuterol treatment.

Collagen Q and anti-MuSK autoantibody competitively suppress agrin/LRP4/MuSK signaling

MuSK antibody-positive myasthenia gravis (MuSK-MG) accounts for 5 to 15% of autoimmune MG. MuSK and LRP4 are coreceptors for agrin in the signaling pathway that causes clustering of acetylcholine receptor (AChR). MuSK also anchors the acetylcholinesterase (AChE)/collagen Q (ColQ) complex to the synaptic basal lamina. We previously reported that anti-MuSK antibodies (MuSK-IgG) block binding of ColQ to MuSK and cause partial endplate AChE deficiency in mice. We here analyzed the physiological significance of binding of ColQ to MuSK and block of this binding by MuSK-IgG. In vitro plate-binding assay showed that MuSK-IgG blocked MuSK-LRP4 interaction in the presence of agrin. Passive transfer of MuSK-IgG to Colq-knockout mice attenuated AChR clustering, indicating that lack of ColQ is not the key event causing defective clustering of AChR in MuSK-MG. In three MuSK-MG patients, the MuSK antibodies recognized the first and fourth immunoglobulin-like domains (Ig1 and Ig4) of MuSK. In two other MuSK-MG patients, they recognized only the Ig4 domain. LRP4 and ColQ also bound to the Ig1 and Ig4 domains of MuSK. Unexpectedly, the AChE/ColQ complex blocked MuSK-LRP4 interaction and suppressed agrin/LRP4/MuSK signaling. Quantitative analysis showed that MuSK-IgG suppressed agrin/LRP4/MuSK signaling to a greater extent than ColQ.

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

Study of the electrophysiological function of the neuromuscular junction (NMJ) is instrumental in the understanding of the symptoms and pathophysiology of myasthenia gravis (MG), an autoimmune disorder characterized by fluctuating and fatigable muscle weakness. Most patients have autoantibodies to the acetylcholine receptor at the NMJ. However, in recent years autoantibodies to other crucial postsynaptic membrane proteins have been found in previously 'seronegative' MG patients. Electromyographical recording of compound and single-fibre muscle action potentials provides a crucial in vivo method to determine neuromuscular transmission failure while ex vivo (miniature) endplate potential recordings can reveal the precise synaptic impairment. Here we will review these electrophysiological methods used to assess NMJ function and discuss their application and typical results found in the diagnostic and experimental study of patients and animal models of the several forms of MG.

The neuromuscular junction: measuring synapse size, fragmentation and changes in synaptic protein density using confocal fluorescence microscopy

The neuromuscular junction (NMJ) is the large, cholinergic relay synapse through which mammalian motor neurons control voluntary muscle contraction. Structural changes at the NMJ can result in neurotransmission failure, resulting in weakness, atrophy and even death of the muscle fiber. Many studies have investigated how genetic modifications or disease can alter the structure of the mouse NMJ. Unfortunately, it can be difficult to directly compare findings from these studies because they often employed different parameters and analytical methods. Three protocols are described here. The first uses maximum intensity projection confocal images to measure the area of acetylcholine receptor (AChR)-rich postsynaptic membrane domains at the endplate and the area of synaptic vesicle staining in the overlying presynaptic nerve terminal. The second protocol compares the relative intensities of immunostaining for synaptic proteins in the postsynaptic membrane. The third protocol uses Fluorescence Resonance Energy Transfer (FRET) to detect changes in the packing of postsynaptic AChRs at the endplate. The protocols have been developed and refined over a series of studies. Factors that influence the quality and consistency of results are discussed and normative data are provided for NMJs in healthy young adult mice.

Guidelines for pre-clinical animal and cellular models of MuSK-myasthenia gravis

Muscle-specific tyrosine kinase (MuSK) autoantibodies are the hallmark of a form of myasthenia gravis (MG) that can challenge the neurologist and the experimentalist. The clinical disease can be difficult to treat effectively. MuSK autoantibodies affect the neuromuscular junction in several ways. When added to muscle cells in culture, MuSK antibodies disperse acetylcholine receptor clusters. Experimental animals actively immunized with MuSK develop MuSK autoantibodies and muscle weakness. Weakness is associated with reduced postsynaptic acetylcholine receptor numbers, reduced amplitudes of miniature endplate potentials and endplate potentials, and failure of neuromuscular transmission. Similar impairments have been found in mice injected with IgG from MG patients positive for MuSK autoantibody (MuSK-MG). The active and passive models have begun to reveal the mechanisms by which MuSK antibodies disrupt synaptic function at the neuromuscular junction, and should be valuable in developing therapies for MuSK-MG. However, translation into new and improved treatments for patients requires procedures that are not too cumbersome but suitable for examining different aspects of MuSK function and the effects of potential therapies. Study design, conduct and analysis should be carefully considered and transparently reported. Here we review what has been learnt from animal and culture models of MuSK-MG, and offer guidelines for experimental design and conduct of studies, including sample size determination, randomization, outcome parameters and precautions for objective data analysis. These principles may also be relevant to the increasing number of other antibody-mediated diseases that are now recognized.