Autoantibody to dihydropyridine receptor in myasthenia gravis

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.

Can a rare form of myasthenia gravis shed additional light on disease mechanisms?

A healthy 43-year-old physician developed gradually progressive and fluctuating fatigable muscle weakness involving ocular, limb, bulbar and respiratory muscles, with episodic acute respiratory failure, eventually necessitating intermittent non-invasive respiratory support (NIV). A mild short episode occurred 15 years earlier with complete resolution. Electromyography (EMG) studies and acetylcholine receptor (AchR) antibodies were repeatedly non-diagnostic. The diagnosis of myasthenia gravis (MG) was finally confirmed by direct measurement of diaphragmatic strength using magnetic nerve stimulation providing clear cut evidence of significant fatigable weakness and the demonstration of muscle-specific kinase (MuSK) serum antibodies using a novel cell-based assay. The cluster of several atypical features and lack of response to commonly used treatment modalities prompted a search for a unifying mechanism and better understanding of the underlying pathophysiology. Review of the literature suggested a possible impairment of excitation-contraction coupling with malfunction of a signaling protein downstream to the AchR, without an accompanying impairment of electrical transmission. This postulated mechanism, resulting in a disturbance of calcium signaling, explained the unusual features in this patient's illness and led to treatment with salbutamol and ephedrine and to significant symptomatic improvement not achieved by any other treatment.

Early effect of tacrolimus in improving excitation-contraction coupling in myasthenia gravis

OBJECTIVES

Tacrolimus (FK506) is a macrolide T-cell immunomodulator used to treat myasthenia gravis (MG). Besides immunosuppression, tacrolimus has been reported to have the potential to increase muscle strength by enhancing ryanodine receptor (RyR) function. However, few attempts have been made to demonstrate the early effect of tacrolimus as an RyR enhancer in clinical investigation.

METHODS

In 20 MG patients, masseteric compound muscle action potential (CMAP) and mandibular movement-related potentials (MRPs) were recorded simultaneously after stimulating the trigeminal motor nerve with a needle electrode. The excitation-contraction (E-C) coupling time (ECCT) was calculated by the latency difference between CMAP and MRP. Bite force was measured using a pressure-sensitive sheet. Serial assessments of % decrement in masseteric repetitive nerve stimulation (RNS), ECCT and bite force were performed before and within 4 weeks of tacrolimus (3 mg day(-1)) treatment. The median (mean, range) interval of assessment was 2 (2.4, 1-4) weeks. We also measured serum antibodies against RyR, acetylcholine receptor and muscle-specific receptor tyrosine kinase.

RESULTS

Bite force increased after tacrolimus treatment accompanying clinical improvement assessed by Myasthenia Gravis Foundation of America classification, but the bite force difference did not reach statistical significance. Wilcoxon matched-pairs signed-ranks test detected a significant ECCT shortening in 12 patients assessed after 1-2 weeks of tacrolimus treatment as well as in eight patients assessed after 3-4 weeks. In contrast, masseteric CMAP and % decrement showed no significant changes after short-term tacrolimus treatment.

CONCLUSIONS

Tacrolimus induces ECCT shortening accompanying clinical improvement despite no improvement in % decrement within 2 weeks.

SIGNIFICANCE

This early effect of tacrolimus may imply a pharmacological enhancement of RyR function to improve E-C coupling in MG.

Three types of striational antibodies in myasthenia gravis

Myasthenia gravis (MG) is caused by antibodies that react mainly with the acetylcholine receptor on the postsynaptic site of the neuromuscular junction. A wide range of clinical presentations and associated features allow MG to be classified into subtypes based on autoantibody status. Striational antibodies, which react with epitopes on the muscle proteins titin, ryanodine receptor (RyR), and Kv1.4, are frequently found in MG patients with late-onset and thymoma. Antititin and anti-RyR antibodies are determined by enzyme-linked immunosorbent assay or immunoblot. More recently, a method for the detection of anti-Kv1.4 autoantibodies has become available, involving 12-15% of all MG patients. The presence of striational antibodies is associated with more severe disease in all MG subgroups. Anti-Kv1.4 antibody is a useful marker for the potential development of lethal autoimmune myocarditis and response to calcineurin inhibitors. Detection of striational antibodies provides more specific and useful clinical information in MG patients.

The auto-antigen repertoire in myasthenia gravis

Myasthenia Gravis (MG) is an antibody-mediated autoimmune disorder affecting the postsynaptic membrane of the neuromuscular junction (NMJ). MG is characterized by an impaired signal transmission between the motor neuron and the skeletal muscle cell, caused by auto-antibodies directed against NMJ proteins. The auto-antibodies target the nicotinic acetylcholine receptor (nAChR) in about 90% of MG patients. In approximately 5% of MG patients, the muscle specific kinase (MuSK) is the auto-antigen. In the remaining 5% of MG patients, however, antibodies against the nAChR or MuSK are not detectable (idiopathic MG, iMG). Although only the anti-nAChR and anti-MuSK auto-antibodies have been demonstrated to be pathogenic, several other antibodies recognizing self-antigens can also be found in MG patients. Various auto-antibodies associated with thymic abnormalities have been reported, as well as many non-MG-specific auto-antibodies. However, their contribution to the cause, pathology and severity of the disease is still poorly understood. Here, we comprehensively review the reported auto-antibodies in MG patients and discuss their role in the pathology of this autoimmune disease.

Research advancement in immunopathogenesis of myasthenia gravis

Myasthenia gravis (MG) is an autoimmune neuromuscular junction disease mediated by antibodies against the acetylcholine receptor (AChR). The etiology and immunopathogenesis of MG remain unclear. Recent research has shown the involvement of autoantibodies, lymphocytes, cytokines and chemokines, in the pathogenesis of MG. Systematic factors are also demonstrated, such as inheritance and endocrine. This review indicates the research development in immunopathogenesis of MG.