AChR phosphorylation and indirect inhibition of AChR function in seronegative MG

BACKGROUND: Approximately 10% to 20% of patients with autoimmune MG do not have antibodies to the acetylcholine receptor (AChR), so-called seronegative MG (SNMG). IgG antibodies from up to 70% of SNMG patients bind to the muscle-specific receptor tyrosine kinase, MuSK. The plasmas and non-IgG fractions from SNMG patients (and some with AChR antibodies) also contain a factor, perhaps an IgM antibody, that inhibits AChR function, but it is not clear how this factor acts and whether it is related to the MuSK IgG antibodies. METHODS: The authors studied 12 unselected SNMG plasmas and their non-IgG fractions; seven were positive for MuSK IgG antibodies. Ion flux assays, electrophysiology, phosphorylation, and kinase assays were used to look at mechanisms of action. RESULTS: Eight of the 12 plasmas and their non-IgG fractions inhibited AChR function, but the inhibitory activity was transient and did not correlate with the presence of MuSK IgG antibodies. Two of three plasmas added outside of a cell-attached patch pipette inhibited AChR function within the patch, and these two plasmas also increased AChR phosphorylation. CONCLUSIONS: The authors propose that a plasma factor(s) in SNMG patients, distinct from MuSK IgG antibodies, binds to a muscle membrane receptor and activates a second messenger pathway leading to AChR phosphorylation and reduced AChR function. Identifying the target for this factor should lead to improved diagnosis of MG in MuSK antibody-negative patients and may provide new insights into the function of the neuromuscular junction and pathophysiological mechanisms in MG.     

Clinical comparison of muscle-specific tyrosine kinase (MuSK) antibody-positive and -negative myasthenic patients

We assayed cryopreserved sera from 38 acetylcholine receptor (AChR) antibody-negative patients with myasthenia gravis (MG) who were followed clinically for muscle-specific tyrosine kinase (MuSK) antibodies and analyzed and compared their clinical characteristics. None of 13 sera from patients with purely ocular MG were positive. Sera from 10 of 25 patients (40%) with generalized MG were positive for MuSK antibodies. The age at onset of myasthenic symptoms was significantly earlier in MuSK antibody-positive patients (P = 0.02). MuSK antibodies were present in AChR antibody-negative patients of either gender, with virtually identical prevalence in women (41.2%) and men (37.5%). The distribution of weakness more commonly involved neck muscles in MuSK antibody-positive patients, and limb muscles in MuSK antibody-negative patients. Patients responded to immunosuppressive treatment regardless of whether MuSK antibody was present. We conclude that MuSK antibodies are present and diagnostically useful in a subset of myasthenic patients without AChR antibodies. Although the distribution of weakness differs somewhat depending on whether MuSK antibodies are present, responses to anticholinesterase and immunosuppressive treatments are similar.     

Histochemical findings of and fine structural changes in motor endplates in diseases with neuromuscular transmission abnormalities

We herein review the histochemical findings and fine structural changes of motor endplates associated with diseases causing neuromuscular transmission abnormalities. In anti-acetylcholine receptor (AChR) antibody-positive myasthenia gravis (MG), type 2 fiber atrophy is observed, and the motor endplates show a reduction in the nerve terminal area, simplification of the postsynaptic membrane, decreased number of acetylcholine receptors, and deposition of immune complexes. In anti-MuSK antibody-positive MG, the fine structure shows a decrease in the postsynaptic membrane length, but the secondary synaptic cleft is preserved. There is no decrease in the number of AChRs, and there are no deposits of immune complexes at the motor endplates. Patients with Lambert-Eaton myasthenic syndrome show type 2 fiber atrophy, their motor endplates show a decrease in both the mean postsynaptic area and postsynaptic membrane length in the brachial biceps muscle. Congenital myasthenic syndrome with episodic apnea is characterized only by small-sized synaptic vesicles; the postsynaptic area is preserved. In subjects with congenital myasthenic syndrome with acetylcholinesterase deficiency, quantitative electron microscopy reveals a significant decrease in the nerve terminal size and presynaptic membrane length; further, the Schwann cell processes extend into the primary synaptic cleft, and partially or completely occlude the presynaptic membrane. The postsynaptic folds are degenerated, and associated with pinocytotic vesicles and labyrinthine membranous networks. Patients with slow-channel congenital myasthenia syndrome show type 1 fiber predominance, and their junctional folds are typically degenerated with widened synaptic space and loss of AChRs. Patients with AChR deficiency syndrome caused by recessive mutations in AChR subunits also show type 1 fiber predominance, and while most junctional folds are normal, some are simplified and have smaller than normal endplates. Rapsin and MuSK mutations cause type 1 fiber predominance, and the small postsynaptic area is associated with AChR decrease.     

Myasthenia gravis and myasthenic syndromes

More than a decade ago myasthenic symptoms were observed in rabbits immunized with acetylcholine receptor (AChR) [119] and AChR deficiency was found at the neuromuscular junction in human myasthenia gravis (MG) [36]. By 1977 the autoimmune character of MG and the pathogenic role of AChR antibodies had been established by several measures. These included the demonstration of circulating AChR antibodies in nearly 90% of patients with MG [87], passive transfer with IgG of several features of the disease from human to mouse [149], localization of immune complexes (IgG and complement) on the postsynaptic membrane [30], and the beneficial effects of plasmapheresis [20, 123]. Substantial subsequent progress has occurred in understanding the structure and function of AChR and its interaction with AChR antibodies. The relationships of the concentration, specificities, and functional properties of the antibodies to the clinical state in MG have been carefully analyzed, and the mechanisms by which AChR antibodies impair neuromuscular transmission have been further investigated. The clinical classification of MG has been refined, the role of the thymus gland in the disease has been further clarified, and new information has become available on transient neonatal MG. The prognosis for generalized MG is improving, but there is still no consensus on its optimal management. Novel therapeutic approaches to MG are now being explored in animal models. Recognition of the autoimmune origin of acquired MG also implied that myasthenic disorders occurring in a genetic or congenital setting had a different cause. As a result, a number of congenital myasthenic syndromes have come to be recognized and investigated. Finally, an acquired disorder of neuromuscular transmission different from MG, the Lambert-Eaton myasthenic syndrome, has also been shown to have an autoimmune basis. In this syndrome, active zone particles of the presynaptic membrane are direct or indirect targets of the pathogenic autoantibodies.     

Myasthenia gravis with muscle‐specific tyrosine kinase antibodies: A narrative review

Growing evidence provides new insights about myasthenia gravis (MG) with antibodies against muscle‐specific tyrosine kinase (MuSK‐MG), including its pathogenesis, clinical and electrophysiological manifestations, and treatment. Data now support the presence of both presynaptic and postsynaptic dysfunction in MuSK‐MG. This is 1 of many key differences between MuSK‐MG and acetylcholine receptor antibody‐MG (AChR‐MG), especially as it pertains to potential therapeutic implications. In comparison to AChR‐MG, MuSK‐MG is generally more refractory to treatment. However, because MuSK‐MG is better understood and more readily recognized today, there are more reports of a relatively benign course. The most effective immunotherapies for MuSK‐MG are corticosteroids, plasmapheresis, and rituximab. With appropriate therapy, most patients with MuSK‐MG achieve minimal manifestation status or better on the postintervention status outlined by the Myasthenia Gravis Foundation of America. A minority of patients remain refractory to treatment, and optimal management for this group remains a considerable challenge. Muscle Nerve 58 : 344–358, 2018     

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

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

Patterns and severity of neuromuscular transmission failure in seronegative myasthenia gravis

OBJECTIVES: To compare the clinical and electrophysiological features of myasthenia gravis (MG) patients with (seropositive) or without (seronegative) antibodies to acetylcholine receptor. To investigate whether antibodies to muscle specific kinase (MuSK) and ryanodine receptor (RyR) are associated with particular features. METHODS: Clinical profiles and single fibre electromyography (SFEMG) in the extensor digitorum communis (EDC) were reviewed in consecutive 57 seropositive and 13 seronegative patients. Antibodies to MuSK and RyR were measured by immunoassays. RESULTS: Of the 13 seronegative patients, four (31%) were positive for MuSK antibodies and seven (54%) were positive for RyR antibodies, including all four MuSK positive patients. Clinical features were similar at presentation for seropositive and seronegative patients, but MuSK positive patients frequently developed myasthenic crises. Despite the similar clinical severities at the time of examination, the proportion with positive jitter (93% of seropositive patients, 50% of MuSK positive patients, and 44% of MuSK negative patients) and the extent of jitter (mean consecutive difference: 76 micros in seropositive patients, 36 micros in MuSK positive patients, and 30 micros in MuSK negative patients) were less in seronegative MG patients compared with seropositive MG patients. CONCLUSIONS: Seronegative MG is heterogeneous with respect to the presence of antibodies to MuSK. Impairment of neuromuscular synaptic transmission in EDC is less marked in seronegative than seropositive MG despite the similar clinical severity. This discrepancy may partly reflect the distribution of affected muscles in seronegative patients, but it is possible that other factors, such as impaired excitation-contraction coupling resulting from RyR antibodies, contribute to the clinical phenotype.     

Acetylcholine receptors loss and postsynaptic damage in MuSK antibody-positive myasthenia gravis

Muscle-specific tyrosine kinase (MuSK) antibodies are found in some patients with "seronegative" myasthenia gravis (MG), but how they cause myasthenic symptoms is not clear. We visualized acetylcholine receptors (AChRs) and complement component 3 (C3) in muscle biopsies from 10 Japanese MG patients with MuSK antibodies, compared with 42 with AChR antibodies. The AChR density was not significantly decreased in MuSK antibody (Ab)-positive end-plates compared with AChR antibody-positive end-plates, and C3 was detected in only two of eight MuSK Ab-positive patients. MuSK antibodies do not appear to cause substantial AChR loss, complement deposition, or morphological damage. Effects on MuSK function need to be explored.     

Is the decremental pattern in Lambert–Eaton syndrome different from that in myasthenia gravis?

ObjectiveWe reviewed our experience to determine if the decremental pattern during low frequency repetitive nerve stimulation (LF-RNS) distinguishes between the Lambert–Eaton myasthenic syndrome (LEMS) and myasthenia gravis (MG).MethodsLF-RNS studies were reviewed from 34 LEMS and 44 MG patients, 4 of whom had antibodies to muscle specific kinase (MuSK). In each train we calculated the ratio between the early and the later decrement. Receiver-operator characteristic curves were calculated to determine the ratio that best distinguished between LEMS and MG.ResultsThe late decrement was more often greater in LEMS and the converse was true in MG, but with some overlap in values in individual patients. A late decrement more than 102% of the early decrement discriminated between LEMS and MG in 90% of studies. The decremental pattern in MG patients with MuSK antibodies resembled that in LEMS.ConclusionWhen the decrement becomes progressively greater during low frequency RNS, the patient is more likely to have LEMS than MG, and in MG, is more likely to have MuSK antibodies.SignificanceA progressive decrement in patients otherwise felt to have MG should prompt further clinical, serological and electrodiagnostic tests. Further studies are needed to assess the decremental pattern in MuSK MG.     

Autoantibodies against M1 muscarinic acetylcholine receptor in myasthenic disorders

The Lambert–Eaton myasthenic syndrome (LEMS), often associated with small-cell lung carcinoma (SCLC), is a disorder of acetylcholine (ACh) release from motor nerve terminals. In most patients, it is caused by autoantibodies against the P/Q-type voltage-gated calcium channels (VGCC) that trigger ACh release. However, these antibodies are not detected in approximately 15% of clinically and electrophysiologically typical cases. The M1-type pre-synaptic muscarinic ACh receptor (M1 mAChR) modulates cholinergic neuromuscular transmission by linking to P/Q-type VGCC, and may partially compensate for the reduced calcium entry. Immunoblotting against solubilized human M1 mAChR, we detected autoantibodies in: (a) 14 of 20 (70%) anti-VGCC-positive LEMS patients; (b) all five anti-VGCC-negative LEMS patients, one of whose serum had previously passively transferred LEMS-type electrophysiological defects to mice; (c) all five LEMS patients with autonomic symptoms; (d) seven of 25 (28%) myasthenia gravis (MG) patients in whom increased ACh release partially compensates for post-synaptic defects; (e) none of 10 SCLC patients without LEMS. Although not proving primary pathogenicity of anti-M1 mAChR antibodies, the present results highlight their potential to affect synaptic compensatory mechanisms, more in LEMS than MG.     

Passive and active immunization models of MuSK-Ab positive myasthenia: electrophysiological evidence for pre and postsynaptic defects

Antibodies directed against the post-synaptic neuromuscular junction protein, muscle specific kinase (MuSK) are found in a small proportion of generalized myasthenia gravis (MuSK-MG) patients. MuSK is a receptor tyrosine kinase which is essential for clustering of the acetylcholine receptors (AChRs) at the neuromuscular junction, but the mechanisms by which MuSK antibodies (MuSK-Abs) affect neuromuscular transmission are not clear. Experimental models of MuSK-MG have been described but there have been no detailed electrophysiological studies and no comparisons between the MuSK-MG and the typical form with AChR-Abs (AChR-MG). Here we studied the electrophysiology of neuromuscular transmission after immunization against MuSK compared with immunization against AChR, and also after passive transfer of IgG from MuSK-MG or AChR-MG patients. Overt clinical weakness was observed in 6/10 MuSK-immunized and 3/9 AChR-immunized mice but not in those injected with patients' IgG. Miniature endplate potentials (MEPPS) were reduced in all weak mice consistent with the reduction in postsynaptic AChRs that was found. However, whereas there was an increase in the quantal release of acetylcholine (ACh) in the weak AChR-immunized mice, no such increase was found in the weak MuSK-immunized mice. Similar trends were found after the passive transfer of purified IgG antibodies from MuSK-MG or AChR-MG patients. Preliminary results showed that MuSK expression was considerably higher at the neuromuscular junctions of the masseter (facial) than in the gastrocnemius (leg) with no reduction in MuSK immunostaining at the neuromuscular junctions. Overall, these results suggest that MuSK antibodies act in at least two ways. Firstly by indirectly affecting MuSK's ability to maintain the high density of AChRs and secondly by interfering with a compensatory presynaptic mechanism that regulates quantal release and helps to preserve neuromuscular function. These results raise questions about how MuSK is involved in retrograde signaling, and the combination of post-synaptic defects with lack of presynaptic compensation may begin to explain the more severe disease in MuSK-MG patients.     

Neurophysiologic assessment of disorders affecting the neuromuscular junction

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

Effect of sera from AChR-antibody negative myasthenia gravis patients on AChR and MuSK in cell cultures

A proportion of patients with myasthenia gravis (MG) do not have antibodies to the acetylcholine receptor (AChR). Some of these patients have antibodies to muscle specific kinase (MuSK), whereas others have neither antibody (seronegative MG, SNMG). Both MuSK antibody positive MG (MuSK-MG) and SNMG are antibody-mediated diseases but how they cause neuromuscular junction failure is not clear. One possibility is that they reduce the clustering and expression of AChRs. We looked at the effects of MuSK-MG and SNMG sera/IgG on surface AChR distribution and expression, and AChR subunit and MuSK mRNA by quantitative RT-PCR, in TE671 and C2C12 myotubes. In TE671 cells MuSK-MG sera reduced AChR expression by about 20%, but had no effect on AChR subunit or MuSK mRNA expression. In C2C12 myotubes, MuSK-MG sera caused a reduction in the number of agrin-induced clusters, but the clusters became larger and there was no significant effect on total surface AChR numbers or AChR subunit or MuSK mRNA. By contrast, SNMG sera not only reduced AChR numbers by about 20% in TE671 cells, but modestly upregulated AChR gamma subunit expression in TE671 cells and both AChR gamma subunit and MuSK expression in C2C12 myotubes. Thus, although these results have, disappointingly, demonstrated little effect of MuSK antibodies on AChR expression, they do imply that SNMG antibodies act on AChR-associated pathways.     

Specificity of Lambert-Eaton Myasthenic Syndrome immunoglobulin for nerve terminal calcium channels

Lambert-Eaton Myasthenic Syndrome (LEMS) is a presynaptic, neuromuscular disorder characterized by impaired nerve-evoked release of ACh. Repetitive nerve stimulation, which increases the probability of quantal release, improves the transmission defect. An autoantibody to Ca2+ channels of presynaptic motor nerve terminals is thought to mediate the pathogenesis of this disease. The goal of the present study was to examine the specificity of LEMS autoantibodies for nerve terminal Ca2+ channels as compared to other voltage-sensitive ion channels in nerve terminals, and to determine if non-specific membrane damage contributed to the pathogenesis of LEMS. The ion channel specificity of LEMS autoantibody was assessed by comparing the ability of acute application of IgG isolated from the plasma of a patient with LEMS to reduce depolarization-dependent uptake of 45Ca2+ and 22Na+ into or efflux of 86Rb+ from rat forebrain synaptosomes. The clinical diagnosis of LEMS was confirmed electrophysiologically by treatment of mice for 30 days with plasma (1.5 ml/day) taken from this patient. Characteristic reduction of quantal content elicited at 1 Hz and facilitation at 20 Hz was observed in mice treated with LEMS plasma compared to those treated with control plasma. One s, K(+)-stimulated 45Ca2+ uptake was inhibited 36.5 +/- 14.5% and 44.5 +/- 9.8% by acute application of 2 and 4 mg/ml LEMS IgG, respectively; IgG from patients with small cell carcinoma of the lung (SCC) had no effect on 45Ca2+ entry. The same concentrations of LEMS IgG affected neither voltage-dependent uptake of 22Na+ into veratridine-depolarized synaptosomes nor 86Rb+ efflux from K(+)-depolarized synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Autoantibodies to low-density lipoprotein receptor-related protein 4 in myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction, where acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low-density lipoprotein (LDL) receptor-related protein 4 (Lrp4) are essential. About 80% and 0% to 10% of patients with generalized MG have autoantibodies to AChR and MuSK, respectively, but pathogenic factors are elusive in others. Here we show that a proportion of AChR antibody-negative patients have autoantibodies to Lrp4. These antibodies inhibit binding of Lrp4 to its ligand and predominantly belong to the immunoglobulin G1 (IgG1) subclass, a complement activator. These findings together indicate the involvement of Lrp4 antibodies in the pathogenesis of AChR antibody-negative MG.     

Clinical aspects of neuromuscular transmission disorders

Autoimmune disorders of neuromuscular transmission are caused by antibodies (abs) directed against membrane proteins at the motor end-plate. Myasthenia gravis (MG) is due, in most cases, to abs against the nicotinic acetylcholine receptor (AChR). Anti-AChR-positive MG actually includes different disease entities: weakness can be confined to extrinsic ocular muscles or can be generalized; patients with generalized MG (G-MG) can be subdivided on the basis of age of onset, HLA association and thymic pathology. About 15% of G-MG patients are anti-AChR-negative; in a proportion of these cases serum abs against the muscle- specific kinase (MuSK) are found. Anti-MuSK-positive MG is characterized by predominant involvement of bulbar muscles and very low frequency of thymic pathology. The Lambert-Eaton myasthenic syndrome (LEMS) is caused by abs against voltage-gated calcium channels at nerve terminal. LEMS is characterized by muscle weakness and autonomic disturbances and it is paraneoplastic in over 50% of the cases. In neuromyotonia and cramp-fasciculation syndrome, that are thought to be due to anti-voltage-gated potassium channel abs, signs of peripheral nerve hyperexcitability can be associated with CNS features.     

Antibodies against Wnt receptor of muscle-specific tyrosine kinase in myasthenia gravis

Muscle-specific tyrosine kinase (MuSK) antibodies are detected in a proportion of myasthenia gravis (MG) patients who are negative for acetylcholine receptor (AChR) antibodies and have prominent bulbar weakness and crises. In the MuSK ectodomains, the immunoglobulin-like 1 and 2 domains (Ig1/2) mediate the agrin–Lrp4–MuSK signaling and the cysteine-rich domain (CRD) mediates the Wnt–MuSK–Dishevelled signaling; both contribute to AChR clustering. Immunoblotting against recombinant proteins showed MuSK Ig1/2 antibodies in 33 anti-AChR-negative MG patients; 10 patients of them (30%) were additionally positive for MuSK CRD antibodies. The result suggests that MuSK antibodies have heterogeneity in their binding to functional domains of MuSK.     

Seronegative generalised myasthenia gravis: clinical features,antibodies, and their targets

Myasthenia gravis (MG) is a well-recognised disorder of neuromuscular transmission that can be diagnosed by the presence of antibodies to the acetylcholine receptor (AChR). However, some patients (about 15%) with generalised MG do not have detectable AChR antibodies. There is some evidence, however, that this "seronegative" MG is an antibody-mediated disorder. Plasma from patients with the disorder seems to contain various distinct humoral factors: IgG antibodies that reversibly inhibit AChR function; a non-IgG (possibly IgM) factor that indirectly inhibits AChR function; and an IgG antibody against the muscle-specific kinase (MuSK). The presence of antibodies against MuSK appears to define a subgroup of patients with seronegative MG who have predominantly localised, in many cases bulbar, muscle weaknesses (face, tongue, pharynx, etc) and reduced response to conventional immunosuppressive treatments. Moreover, muscle wasting may be present, which prevents complete response to these therapies.     

Action of Lambert-Eaton myasthenic syndrome IgG at mouse motor nerve terminals

We have studied the electrophysiological effects of IgG obtained from four patients with Lambert-Eaton myasthenic syndrome (LEMS) (two with small cell carcinoma), using the mouse passive transfer model. Mice received LEMS or control IgG or plasma, 10 to 60 mg daily. Microelectrode intracellular recordings were made from diaphragm muscle. LEMS IgG and plasma decreased end-plate potential quantal content similarly, confirming IgG as the active factor. LEMS IgG was equally effective in C5-deficient mice, indicating that late complement components are not required. The time course of decline and recovery of quantal content closely followed that of the human IgG in the mouse serum, with time to half-maximal effect of about 1.5 days in each case. Binding/dissociation of IgG or down/up regulation of the antigenic determinants, possibly Ca2+ channels, has a half-life of between 2 and 36 hours. The results confirm our concept that IgG antibody to nerve terminal determinants underlies the disorder of transmitter release in LEMS.