The effect of myasthenic syndrome antibody on presynaptic calcium channels in the mouse

Presynaptic Paraneoplastic Disorders of the Neuromuscular Junction: An Update

The neuromuscular junction (NMJ) is the target of a variety of immune-mediated disorders, usually classified as presynaptic and postsynaptic, according to the site of the antigenic target and consequently of the neuromuscular transmission alteration. Although less common than the classical autoimmune postsynaptic myasthenia gravis, presynaptic disorders are important to recognize due to the frequent association with cancer. Lambert Eaton myasthenic syndrome is due to a presynaptic failure to release acetylcholine, caused by antibodies to the presynaptic voltage-gated calcium channels. Acquired neuromyotonia is a condition characterized by nerve hyperexcitability often due to the presence of antibodies against proteins associated with voltage-gated potassium channels. This review will focus on the recent developments in the autoimmune presynaptic disorders of the NMJ.

Lung cancer prediction in Lambert-Eaton myasthenic syndrome in a prospective cohort

To evaluate the Dutch-English Lambert-Eaton Myasthenic Syndrome (LEMS) Tumour Association Prediction (DELTA-P) score in a prospective cohort of patients with newly diagnosed LEMS to assess the clinical validity of this tool in a real-world setting. Clinical features from 87 patients with LEMS, occurring within three months from disease onset, were collated to produce a DELTA-P score for each patient. Lung cancer was detected in 44/87 (51%) LEMS patients. Weight loss ≥ 5%, tobacco use at LEMS onset and age at onset ≥ 50 years were independent predictors for the development of small-cell lung cancer (SCLC) in LEMS patients in multivariable analysis. Median DELTA-P scores were significantly higher in SCLC-LEMS patients (3.5, 95% CI 3 to 4) compared to non-tumour-LEMS (2, 95% CI 1 to 2) (P < 0.0001). Higher DELTA-P scores increased the risk of SCLC stepwise (score 0 = 0%, 1 = 18.8%, 2 = 45%, 3 = 55.5%, 4 = 85.7%, 5 = 87.5%, 6 = 100%). The area under the curve of the receiver operating curve was 82.5% (95% CI 73.9% to 91%). The DELTA-P cancer prediction score, calculated at the time of LEMS diagnosis, is an effective tool for cancer screening in an independent, prospective study setting.

Long-term survival in paraneoplastic Lambert-Eaton myasthenic syndrome

Objective:

To establish whether improved tumor survival in patients with Lambert-Eaton myasthenic syndrome (LEMS) and small-cell lung cancer (SCLC) was due to known prognostic risk factors or an effect of LEMS independently, perhaps as a result of circulating factors.

Methods:

We undertook a prospective observational cohort study of patients with LEMS attending Nottingham University Hospitals, UK, or via the British Neurological Surveillance Unit. In parallel, patients with a new diagnosis of biopsy-proven SCLC were enrolled, examined for neurologic illness, and followed up until death or study end.

Results:

Between May 2005 and November 2014, we recruited 31 patients with LEMS and SCLC and 279 patients with SCLC without neurologic illness. Allowing for known SCLC survival prognostic factors of disease extent, age, sex, performance status, and sodium values, multivariate Cox regression analysis showed that the presence of LEMS with SCLC conferred a significant survival advantage independently of the other prognostic variables (hazard ratio 1.756, 95% confidence interval 1.137–2.709, p = 0.011).

Conclusions:

Improved SCLC tumor survival seen in patients with LEMS and SCLC may not be due solely to lead time bias, given that survival advantage remains after allowing for other prognostic factors and that the same degree of survival advantage is not seen in patients with paraneoplastic neurologic syndromes other than LEMS presenting before SCLC diagnosis.

Decreased intracellular [Ca2+ ] coincides with reduced expression of Dhprα1s, RyR1, and diaphragmatic dysfunction in a rat model of sepsis

INTRODUCTION

Sepsis can cause decreased diaphragmatic contractility. Intracellular calcium as a second messenger is central to diaphragmatic contractility. However, changes in intracellular calcium concentration ([Ca²⁺ ]) and the distribution and co-localization of relevant calcium channels [dihydropyridine receptors, (DHPRα1s) and ryanodine receptors (RyR1)] remain unclear during sepsis. In this study we investigated the effect of changed intracellular [Ca²⁺ ] and expression and distribution of DHPRα1s and RyR1 on diaphragm function during sepsis.

METHODS

We measured diaphragm contractility and isolated diaphragm muscle cells in a rat model of sepsis. The distribution and co-localization of DHPRα1s and RyR1 were determined using immunohistochemistry and immunofluorescence, whereas intracellular [Ca²⁺ ] was measured by confocal microscopy and fluorescence spectrophotometry.

RESULTS

Septic rat diaphragm contractility, expression of DHPRα1s and RyR1, and intracellular [Ca²⁺ ] were significantly decreased in the rat sepsis model compared with controls.

DISCUSSION

Decreased intracellular [Ca²⁺ ] coincides with diaphragmatic contractility and decreased expression of DHPRα1s and RyR1 in sepsis. Muscle Nerve 56: 1128-1136, 2017.

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.

The association between Lambert-Eaton myasthenic syndrome and small cell lung carcinoma

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder mediated by autoantibodies to voltage-gated calcium channels. The disorder is diagnosed clinically on the basis of a triad of symptoms (proximal muscle weakness, hyporeflexia, and autonomic disturbance), supported by electrophysiological findings and the presence of autoantibodies. Between 40% and 62% of patients diagnosed with LEMS are found to have small-cell lung cancer (SCLC), almost all of whom develop neurological symptoms before their cancer is diagnosed. Prompt identification of LEMS and appropriate screening for SCLC is key to improving the outcome of both conditions. Here we review the pathophysiology and clinical management of LEMS, focusing particularly on the relationship with SCLC.

Structural and functional maturation of active zones in large synapses

Virtually all functions of the nervous system rely upon synapses, the sites of communication between neurons and between neurons and other cells. Synapses are complex structures, each one comprising hundreds of different types of molecules working in concert. They are organized by adhesive and scaffolding molecules that align presynaptic vesicular release sites, namely, active zones, with postsynaptic neurotransmitter receptors, thereby allowing rapid and reliable intercellular communication. Most synapses are relatively small, and acting alone exerts little effect on their postsynaptic partners. Some, however, are much larger and stronger, reliably driving the postsynaptic cell to its action potential threshold, acting essentially as electrical relays of excitation. These large synapses are among the best understood, and two of these are the subject of this review, namely, the vertebrate neuromuscular junction and the calyx of Held synapse in the mammalian auditory pathway of the brain stem. Both synapses undergo through a complex and well-coordinated maturation process, during which time the molecular elements and the biophysical properties of the secretory machinery are continuously adjusted to the synapse size and to the functional requirements. We here review the morphological and functional changes occurring during postnatal maturation, noting particular similarities and differences between these two large synapses.

Autoimmune channelopathies of the nervous system

Ion channels are complex transmembrane proteins that orchestrate the electrical signals necessary for normal function of excitable tissues, including the central nervous system, peripheral nerve, and both skeletal and cardiac muscle. Progress in molecular biology has allowed cloning and expression of genes that encode channel proteins, while comparable advances in biophysics, including patch-clamp electrophysiology and related techniques, have made the functional assessment of expressed proteins at the level of single channel molecules possible. The role of ion channel defects in the pathogenesis of numerous disorders has become increasingly apparent over the last two decades. Neurological channelopathies are frequently genetically determined but may also be acquired through autoimmune mechanisms. All of these autoimmune conditions can arise as paraneoplastic syndromes or independent from malignancies. The pathogenicity of autoantibodies to ion channels has been demonstrated in most of these conditions, and patients may respond well to immunotherapies that reduce the levels of the pathogenic autoantibodies. Autoimmune channelopathies may have a good prognosis, especially if diagnosed and treated early, and if they are non-paraneoplastic. This review focuses on clinical, pathophysiologic and therapeutic aspects of autoimmune ion channel disorders of the nervous system.

Treatment for Lambert-Eaton myasthenic syndrome

BACKGROUND

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder of neuromuscular transmission. Treatments attempt to overcome the harmful autoimmune process, or improve residual neuromuscular transmission

OBJECTIVES

The objective was to examine the efficacy of treatment in Lambert-Eaton myasthenic syndrome.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group Specialized Register (12 October 2010), the Cochrane Central Register of Controlled Trials (CENTRAL) (12 October 2010, Issue 4 2010 in the Cochrane Library), MEDLINE (January 1966 to September 2010) and EMBASE (January 1980 to September 2010).

SELECTION CRITERIA

All randomised or quasi-randomised trials of adults and children with a diagnosis of Lambert-Eaton myasthenic syndrome, with or without small-cell lung cancer, receiving any form of pharmacological or physical treatment.

DATA COLLECTION AND ANALYSIS

All authors independently assessed studies for inclusion and extracted data. Study authors were contacted for missing information when possible.

MAIN RESULTS

Four controlled trials of 3,4-diaminopyridine compared with placebo in a total of 54 participants with Lambert-Eaton myasthenic syndrome were eligible: three cross-over trials and one parallel group. Two were added at this update. One of these trials also assessed pyridostigmine in conjunction with 3,4-diaminopyridine. A further cross-over trial compared intravenous immunoglobulin (IVIg) to placebo in nine participants.Four trials of 3,4-diaminopyridine reported significant improvement in the primary outcome, muscle strength score, or myometric limb measurement for between hours and a week following treatment, and significant improvement in resting compound muscle action potential (CMAP) amplitude following 3,4-diaminopyridine, compared with placebo.A meta-analysis of the primary endpoint showed Quantitative Myasthenia Gravis (QMG) muscle score assessed between three and eight days was likely to improve by a mean of 2.44 points (95% confidence interval 3.6 to 1.22). Meta-analysis of the secondary endpoint CMAP amplitude also showed a mean improvement of 1.36 mV (95% confidence interval 0.99 to 1.72) over the same period. The risk of bias was determined to be low, and quality of evidence moderate to high.A single cross-over trial reported significant improvement in myometric limb strength and non-significant improvement in mean resting CMAP amplitude with IVIg compared to placebo. Clinical improvement lasted for up to eight weeks.

AUTHORS' CONCLUSIONS

Limited but moderate to high quality evidence from randomised controlled trials showed that over days 3,4-diaminopyridine, or for up to 8 weeks IVIg, improved muscle strength scores and CMAP amplitudes in participants with Lambert-Eaton myasthenic syndrome. There are insufficient data at present to quantify this effect. Other possible treatments have not been tested in randomised controlled trials.

Update on treatment options for Lambert-Eaton myasthenic syndrome: focus on use of amifampridine

In Lambert-Eaton myasthenic syndrome (LEMS), antibodies against presynaptic voltage-gated calcium channels reduce the quantal release of acetylcholine, causing muscle weakness and autonomic dysfunction. More than half of the affected patients have associated small cell lung cancer, and thorough screening for an underlying malignancy is crucial. The mainstay of treatment for LEMS is symptomatic but immunotherapy is needed in more severely affected patients. Symptomatic therapies aim at increasing the concentration of acetylcholine at the muscle endplate. While acetylcholinesterase inhibitors were the first drugs to be used for the amelioration of symptoms, 3,4-diaminopyridine (3,4-DAP, amifampridine) has been shown to be more effective. 3,4-DAP blocks presynaptic potassium channels, thereby prolonging the action potential and increasing presynaptic calcium concentrations. This then results in increased quantal release of acetylcholine. The efficacy of 3,4-DAP for increasing muscle strength and resting compound muscle action potentials has been demonstrated by four placebo-controlled trials. Side effects are usually mild, and the most frequently reported are paresthesias. The most common serious adverse events are epileptic seizures. 3,4-DAP is currently the treatment of choice in patients with Lambert-Eaton myasthenic syndrome.

Macromolecular connections of active zone material to docked synaptic vesicles and presynaptic membrane at neuromuscular junctions of mouse

Electron tomography was used to view macromolecules composing active zone material (AZM) in axon terminals at mouse neuromuscular junctions. Connections of the macromolecules to each other, to calcium channels in the presynaptic membrane, and to synaptic vesicles docked on the membrane prior to fusing with it during synaptic transmission were similar to those of AZM macromolecules at frog neuromuscular junctions previously examined by electron tomography and support the hypothesis that AZM regulates vesicle docking and fusion. A species difference in the arrangement of AZM relative to docked vesicles may help account for a greater vesicle-presynaptic membrane contact area during docking and a greater probability of fusion during synaptic transmission in mouse. Certain AZM macromolecules in mouse were connected to synaptic vesicles contacting the presynaptic membrane at sites where fusion does not occur. These secondary docked vesicles had a different relationship to the membrane and AZM macromolecules than primary docked vesicles, consistent with their having a different AZM-regulated behavior.

Calcium channels, neuromuscular synaptic transmission and neurological diseases

Voltage-dependent calcium channels are essential in neuronal signaling and synaptic transmission, and their functional alterations underlie numerous human disorders whether monogenic (e.g., ataxia, migraine, etc.) or autoimmune. We review recent work on Ca(V)2.1 or P/Q channelopathies, mostly using neuromuscular junction preparations, and focus specially on the functional hierarchy among the calcium channels recruited to mediate neurotransmitter release when Ca(V)2.1 channels are mutated or depleted. In either case, synaptic transmission is greatly compromised; evidently, none of the reported functional replacements with other calcium channels compensates fully.

Paraneoplastic neurological autoimmunity and survival in small-cell lung cancer

The autoimmune disorder of Lambert-Eaton myasthenic syndrome (LEMS) associates with small cell lung carcinoma (SCLC) in 50-60% of cases. It has been postulated that patients who harbour paraneoplastic neurological syndromes such as LEMS have an improved tumour prognosis compared to other patients with the tumour but without neurological deficit. In this intermediate report of an ongoing prospective study, 100 consecutive patients with biopsy-proven SCLC underwent full neurological examination and serum was taken for autoantibody analysis. Antibodies to voltage-gated calcium channels were detected in 10 patients, however only 4 had clinical and electrophysiological features of LEMS, 1 had limbic encephalitis, whilst the remaining 5 had no neurological signs. A further 6 patients had onconeural antibodies; only one had a paraneoplastic syndrome, sensory neuropathy. The median survival of the four antibody positive LEMS patients (19.6 months) was considerably greater than that for the antibody negative (8.9 months) or antibody positive patients as a whole (10.5 months). Although preliminary, these results suggest that functionally effective antibodies present in the sera of patients with LEMS may confer a survival advantage.

Lambert-Eaton myasthenic syndrome: search for alternative autoimmune targets and possible compensatory mechanisms based on presynaptic calcium homeostasis

The Lambert-Eaton myasthenic syndrome (LEMS) is a disease of neuromuscular transmission in which autoantibodies against the P/Q-type voltage-gated calcium channel (VGCC) at the presynaptic nerve terminal play a major role in decreasing quantal release of acetylcholine (ACh), resulting in skeletal muscle weakness and autonomic symptoms. It is associated with cancer, particularly small-cell lung carcinoma (SCLC), in 50-60% of LEMS patients; the nerve terminal and carcinoma cells apparently share a common antigen (VGCC), suggesting an immunological cross-reactivity that may lead to the neurological abnormality. Non-tumor LEMS has a strong association with HLA-DR3-B8. In approximately 15% of LEMS patients, no anti-P/Q-type VGCC antibodies are found, suggesting recognition of other targets(s). The VGCC-associated protein synaptotagmin could be one candidate, because it acts as an exocytotic calcium receptor, is implicated in fast ACh release; its N-terminus is exposed extracellularly during exocytosis and it is expressed in SCLC. Antibodies against synaptotagmin-1 were detected in both anti-VGCC-positive and -negative LEMS patients (20%), and it can be immunogenic, allowing induction of an animal model of LEMS. Another candidate target is the M1-type presynaptic muscarinic ACh receptor (M1 mAChR), also expressed extracellularly on motor nerve terminals; it modulates cholinergic transmission, linking to P/Q-type VGCC. In our series of 25 LEMS patients with and without SCLC, anti-M1 mAChR antibodies were prevalent in both anti-VGCC-positive and -negative LEMS patients. Autonomic symptoms seemed more frequent in the latter; serum from one of them passively transferred LEMS-type electrophysiological defects to mice. As a compensatory mechanism, researchers in Oxford suggested a shift in the dependence of ACh release from the P/Q-type to other types of VGCC. We have also focused on G protein-coupled mAChRs and neurotrophins, which may affect both P/Q-type VGCC and clathrin-independent "kiss-and-run" synaptic vesicle recycling (fast-mode of endocytosis) via protein kinase C activation. We hypothesize that these signaling cascades help to compensate for the immune-mediated defects in calcium entry in LEMS, compensation that may frequently be restricted by the coincident anti-M1 mAChR antibodies in this disease.

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.

Autoimmune Channelopathies and Related Neurological Disorders

Ion channels are crucial elements in neuronal signaling and synaptic transmission, and defects in their function are known to underlie rare genetic disorders, including some forms of epilepsy. A second class of channelopathies, characterized by autoantibodies against ligand- and voltage-gated ion channels, cause a variety of defects in peripheral neuromuscular and ganglionic transmission. There is also emerging evidence for autoantibody-mediated mechanisms in subgroups of patients with central nervous system disorders, particularly those involving defects in cognition or sleep and often associated with epilepsy. In all autoimmune channelopathies, the relationship between autoantibody specificity and clinical phenotype is complex. But with this new information, autoimmune channelopathies are detected and treated with increasing success, and future research promises new insights into the mechanisms of dysfunction at neuronal synapses and the determinants of clinical phenotype.

Decrement pattern in Lambert–Eaton myasthenic syndrome is different from myasthenia gravis

The decrement pattern at low rates of repetitive nerve stimulation in myasthenia gravis (MG) is characterized by a decrease of compound muscle action potential size within the first 4-5 responses. With subsequent stimuli, compound muscle action potential size either increases or does not change. Following an observation that the pattern of decrement might be different in patients with Lambert-Eaton myasthenic syndrome (LEMS), we retrospectively studied traces from eight LEMS patients and 14 patients with seropositive generalized MG, calculating decrement percent from first to fourth and from first to ninth compound muscle action potential. In the LEMS patients, compound muscle action potential amplitude decreased progressively from first to ninth stimulus at 2, 3 or 5Hz in all traces but one. In contrast, MG patients demonstrated the expected improvement after the initial decrement in all traces except one. In the evaluation of patients suspected of having myasthenia gravis, the finding of progressive decrement pattern at low rates of repetitive nerve stimulation may alert the electromyographer to the possibility of Lambert-Eaton syndrome and prompt the performance of further electrodiagnostic tests.

Phoneutria nigriventer ω-Phonetoxin IIA: A new tool for anti-calcium channel autoantibody assays in Lambert–Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is a neurological autoimmune disease in which downregulation of voltage-gated calcium channels (VGCCs) leads to reduced acetylcholine release from motoneuron terminals. 70% of cases are paraneoplastic and rapid diagnosis of LEMS can result in early detection of the underlying tumor. Serological assays based on the capacity of autoantibodies to precipitate VGCCs labeled with radioligands provide valuable data. We have established a novel assay using the spider venom peptide 125I-omega-Phonetoxin IIA (125I-omegaPtxIIA). 125I-omegaPtxIIA labeled recombinant Cav2.1 and Cav2.2 channels and endogenous VGCCs in rat brain membranes. Autoantibodies that immunoprecipitate a 125I-omegaPtxIIA/channel complex were detected in 26/31 (84%) LEMS patients. The patients that were seropositive in the 125I-omegaPtxIIA assay corresponded precisely to the population that was positive for Cav2.1 and/or Cav2.2 antibodies detected using two different omega-conotoxins. Thus, the 125I-omegaPtxIIA assay detects a broader spectrum of autoantibody specificities than current omega-conotoxin-based assays.

Treatment for Lambert-Eaton myasthenic syndrome

BACKGROUND

Lambert-Eaton myasthenic syndrome is an autoimmune presynaptic disorder of neuromuscular transmission. Treatments attempt to overcome the harmful autoimmune process, or to improve residual neuromuscular transmission, in order to reverse muscle weakness.

OBJECTIVES

The objective was to examine the efficacy of treatment in Lambert-Eaton myasthenic syndrome.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group trials register (December 2004), MEDLINE (January 1966 to December 2004) and EMBASE (January 1980 to December 2004), and checked bibliographies and contacted authors to identify additional published or unpublished data.

SELECTION CRITERIA

All randomised or quasi-randomised trials of adults and children with a diagnosis of Lambert-Eaton myasthenic syndrome, with or without small-cell lung cancer, receiving any form of pharmacological or physical treatment. The primary outcome measure was change in muscle strength scale score (Quantitative Myasthenia Gravis score), or limb muscle strength measured by myometry. The secondary outcome measure was improvement in the mean amplitude of the resting compound muscle action potentials. The mean amplitude used was the mean of all muscles tested.

DATA COLLECTION AND ANALYSIS

We identified three randomised controlled trials.

MAIN RESULTS

Two controlled trials of the effects of 3,4-diaminopyridine compared with placebo in a total of 38 patients with Lambert-Eaton myasthenic syndrome were eligible, one of which was of crossover design. A third crossover trial compared intravenous immunoglobulin treatment to placebo in nine patients. Two trials of 3,4-diaminopyridine reported a significant improvement in muscle strength score, or myometric limb measurement following treatment, and a significant improvement in resting compound muscle action potential amplitude following 3,4-diaminopyridine, compared with placebo.A meta-analysis of the primary endpoint results was not possible because of marked differences in primary outcome measures. However, a meta-analysis of the secondary endpoint was possible. The overall weighted mean difference was 1.80 mV (95% confidence interval 0.82 to 2.78), favouring treatment.A crossover trial reported a significant improvement in myometric limb strength and a non-significant improvement in change in the mean resting compound muscle action potential amplitude when patients received intravenous immunoglobulin compared to placebo infusions. Clinical improvement lasted for up to eight weeks.

AUTHORS' CONCLUSIONS

Limited evidence from randomised controlled trials showed that either 3,4-diaminopyridine or intravenous immunoglobulin improved muscle strength scores and compound muscle action potential amplitudes in patients with Lambert-Eaton myasthenic syndrome. There are insufficient data at present to quantify this treatment effect. Other possible treatments have not been tested in randomised controlled trials.

Presynaptic effects of immunoglobulin G from patients with Lambert-Eaton myasthenic syndrome: Their neutralization by intravenous immunoglobulins

Intravenous immunoglobulin (IVIg) treatment improves muscle strength in Lambert-Eaton myasthenic syndrome (LEMS), but its specific mode of action is unknown. We have delineated its mode of action on neuromuscular blocking properties of LEMS IgG. The effect of sera and purified IgG from six patients with LEMS on evoked quantal release was investigated after direct application to the motor nerve terminal by the perfused macro-patch-clamp electrode in mouse hemidiaphragms. The effect of LEMS IgG was analyzed alone and after coincubation with different concentrations of IVIg or its Fab fragments. All LEMS sera and purified LEMS IgG fractions taken before IVIg treatment inhibited evoked quantal release in a dose-dependent manner. When LEMS IgG was coincubated with a therapeutic IVIg preparation, presynaptic inhibitory activity of LEMS IgG was diminished in a dose-dependent fashion. Monovalent Fab fragments were as effective in neutralizing the activity of LEMS IgG as whole IVIg. These direct neutralizing effects of IVIg may explain its therapeutic efficacy.

Alzheimer's amyloid peptides mediate hypoxic up-regulation of L-type Ca2+ channels

We examined the effects of chronic hypoxia on recombinant human L-type Ca2+ channel alpha1C subunits stably expressed in HEK 293 cells, using whole-cell patch-clamp recordings. Current density was dramatically increased following 24 h exposure to chronic hypoxia (CH), and membrane channel protein levels were enhanced. CH also increased the levels of Alzheimer's amyloid beta peptides (AbetaPs), determined immunocytochemically. Pharmacological prevention of AbetaP production (via exposure to inhibitors of secretase enzymes that are required to cleave AbetaP from its precursor protein) prevented hypoxic augmentation of currents, as did inhibition of vesicular trafficking with bafilomycin A1. The enhancing effect of AbetaPs or CH were abolished following incubation with the monoclonal 3D6 antibody, raised against the extracellular N' terminus of AbetaP. Immunolocalization and immunoprecipitation studies provided compelling evidence that AbetaPs physically associated with the alpha1C subunit, and this association was promoted by hypoxia. These data suggest an important role for AbetaPs in mediating the increase in Ca2+ channel activity following CH and show that AbetaPs act post-transcriptionally to promote alpha1C subunit insertion into (and/or retention within) the plasma membrane. Such an action will likely contribute to the Ca2+ dyshomeostasis of Alzheimer's disease and may contribute to the mechanisms underlying the known increased incidence of this neurodegenerative disease following hypoxic episodes.

Dosage et spécificité d’autoanticorps anti-canaux calcium dans le syndrome myasthénique de Lambert-Eaton

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune channelopathy in which patients produce autoantibodies directed against voltage-gated calcium channels. Autoantibodies down-regulate calcium channels resulting in reduced transmitter release, which in turn leads to muscular weakness and autonomic dysfunction. LEMS is paraneoplastic in 60-70% of patients, most frequently associated with small cell lung carcinoma (SCLC). SCLC lines express many neuronal and neuroendocrine proteins including neuronal calcium channels of the Cav2 family (P/Q and N-type channels). It is thus likely that the paraneoplastic form of LEMS is the consequence of an anti-tumoral immune response and the production of antibodies that cross-react with identical or homologous antigens in nerve terminals. Neurological symptoms generally appear several Months before detection of the tumor. Consequently correct diagnosis of LEMS is crucial as it can allow early treatment of a particularly aggressive carcinoma. Based on published studies, our laboratory has set-up serological assays for LEMS autoantibodies as an aid to diagnosis. Calcium channels in detergent extracts of rat brain or cerebellum membranes were labeled with radioligands specific for N-type (125I-omega conotoxin GVIA) or P/Q-type (125I-omega conotoxin MVIIC) calcium channels. Autoantibodies that immunoprecipitate the ligand/channel complex can thus be titrated. Analysis of 31 LEMS sera revealed the presence of anti-N type channel antibodies in 58% and anti-P/Q type channel antibodies in 74% of patients with titres ranging from 90 to 2950 pM. Only 5 patients were seronegative in both tests, thus a combination of the two assays reliably detected autoantibodies in 26/31 (84%) patients.

Paraneoplastic neurologic syndromes

Cancer can affect the nervous system through many metastatic and nonmetastatic mechanisms, including side effects of cancer treatment, infections, coagulopathy, and metabolic or nutritional deficits. Paraneoplastic neurologic disorders (PND) are an extensive group of syndromes that cannot be explained by any of these complications and may affect any part of the nervous system. PND often develop before the presence of a cancer is known and their recognition may lead to the tumor diagnosis.

Treatment for Lambert-Eaton myasthenic syndrome

BACKGROUND

Lambert-Eaton myasthenic syndrome is an autoimmune presynaptic disorder of neuromuscular transmission. Treatments have attempted to overcome the harmful autoimmune process, or to improve residual neuromuscular transmission, in order to reverse the principal neurological symptom of muscle weakness.

OBJECTIVES

The objective was to examine the efficacy of all forms of treatment in Lambert-Eaton myasthenic syndrome.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group specialised trials register (September 2002), MEDLINE (January 1966 to November 2002) and EMBASE (January 1980 to November 2002). We checked the bibliographies in reports of the randomised trials and contacted authors to identify additional published or unpublished data.

SELECTION CRITERIA

Types of studies: all randomised or quasi-randomised trials.

TYPES OF PARTICIPANTS

all adults and children with a diagnosis of Lambert-Eaton myasthenic syndrome, with or without small-cell lung cancer. Types of interventions: any form of medical (pharmacological or physical) treatment. Types of outcome measures: Primary: change in the muscle strength scale score (Quantitative Myasthenia Gravis score), or limb muscle strength measured by myometry. Secondary: improvement in the mean amplitude of the resting compound muscle action potentials. The mean amplitude used was the mean of all muscles tested.

DATA COLLECTION AND ANALYSIS

We identified three randomised controlled trials. Individual patient data were only available for one trial.

MAIN RESULTS

The three eligible trials included two controlled trials of the effects of 3,4-diaminopyridine compared with placebo in a total of 38 patients with Lambert-Eaton myasthenic syndrome, one of which was of crossover design. A third crossover trial compared intravenous immunoglobulin treatment to placebo in nine patients with Lambert-Eaton myasthenic syndrome. A meta-analysis of the primary endpoint results of these trials was not possible because of differences in comparisons and endpoints and, in two trials, lack of individual patient data. EFFECTS OF 3,4-DIAMINOPYRIDINE: Two trials of 3,4-diaminopyridine reported a significant improvement in the primary endpoint of muscle strength score, or myometric limb measurement following treatment. Both trials also reported a significant improvement in the secondary endpoint of resting compound muscle action potential amplitude following 3,4-diaminopyridine, compared with placebo. A meta-analysis of the primary endpoint results was not possible because of marked differences in these two trials regarding primary outcome measures. However, a meta-analysis of the secondary endpoint (improvement in the amplitude of the mean resting compound muscle action potential) was possible. It was necessary to assume a known correlation (similarity) of the paired responses for each individual in the two treatment periods in order to properly allow for the crossover design of one of the two trials (the correlation coefficient was assumed to be 0.5 in calculations). Using this approach, meta-analysis revealed a significant overall benefit in compound muscle action potential amplitude after 3,4-diaminopyridine treatment. The overall weighted mean difference was 1.80 mV (95% confidence interval 0.82 to 2.78), favouring treatment. These results were not sensitive to the assumption made because the overall benefit estimated was still significant when the correlation was assumed to be less than 0.1. EFFECTS OF INTRAVENOUS IMMUNOGLOBULIN: A crossover trial reported a significant improvement in the primary outcome measure of myometric limb strength when patients received intravenous immunoglobulin compared to placebo infusions. This trial also demonstrated an improvement in the secondary outcome measure of change in the mean resting compound muscle action potential amplitude following intravenous immunoglobulin, but this improvement did not reach significance. Clinical improvement lasted for up to eight weeks.

REVIEWER'S CONCLUSIONS

Limited evidence from randomised controlled trials showed that either 3,4-diaminopyridine or intravenous immunoglobulin improved muscle strength scores and compound muscle action potential amplitudes in patients with Lambert-Eaton myasthenic syndrome. There are insufficient data at present to quantify this treatment effect. Other possible treatments, such as plasma exchange, steroids and immunosuppressive agents have not been tested in randomised controlled trials.

Passive transfer of Lambert-Eaton syndrome to mice induces dihydropyridine sensitivity of neuromuscular transmission

Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic disorder in which autoantibodies apparently target the voltage-gated Ca2+ channels that regulate acetylcholine (ACh) release at motor nerve terminals. P/Q-type Ca2+ channels are primarily involved in ACh release at mammalian neuromuscular junctions. Passive transfer of LEMS to mice by repeated administration of plasma from LEMS patients reduces the amplitude of the perineurial P/Q-type current, and unmasks a dihydropyridine (DHP)-sensitive L-type Ca2+ current at the motor nerve terminal. The present study sought to determine if this DHP-sensitive component contributes to ACh release. Mice were treated for 30 days with plasma from healthy human controls or patients with LEMS. For some studies, diaphragms from naive mice were incubated with LEMS or control human plasma for 2 or 24 h. End-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) were recorded from neuromuscular junctions in the hemidiaphragm. Treatment of mice with LEMS plasma evoked the characteristic electrophysiological signs of LEMS: reduced quantal content and facilitation of EPP amplitudes at high-frequency stimulation. Quantal content was also reduced in muscles incubated acutely with LEMS plasma. Nimodipine, a DHP-type blocker of L-type Ca2+ channels, did not significantly affect the quantal content of muscles treated for 2 or 24 h with either control or LEMS plasma, or following chronic treatment with control plasma. However, following 30 days treatment with LEMS plasma, nimodipine significantly reduced the remaining quantal content to 57.7 +/- 3.3 % of pre-nimodipine control levels. Thus, DHP-sensitive Ca2+ channels become involved in synaptic transmission at the mouse neuromuscular junction after chronic, but not acute treatment with LEMS plasma. However, reductions in quantal release of ACh occur even after very short periods of exposure to LEMS plasma. As such, development of the L-type Ca2+ channel contribution to ACh release during passive transfer of LEMS appears to occur only after quantal release is significantly impaired for an extended duration, suggesting that an adaptive response of the ACh release apparatus occurs in LEMS.

Autonomic dysfunction in Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder characterized by muscle weakness and autonomic dysfunction. Recent ex vivo and in vitro studies demonstrate that autoantibodies to the P/Q-subtype of voltage-gated calcium channel inhibit transmitter release from parasympathetic, sympathetic, and enteric neurons, a mechanism likely to underlie the widespread autonomic dysfunction in LEMS. This review summarizes clinical studies characterizing the autonomic symptoms and signs in LEMS and the effectiveness of treatment in alleviating these symptoms. Serological assays and in vitro pharmacologic and electrophysiologic studies are also discussed.

Long term outcome in Lambert-Eaton myasthenic syndrome without lung cancer

OBJECTIVES

To determine the prognosis in patients with Lambert-Eaton myasthenic syndrome (LEMS) without small cell lung cancer (SCLC), and to analyse longitudinal clinical, electrophysiological, and immunological data on each patient to establish prognostic factors for long term outcome.

METHODS

The retrospective and part prospective study of 47 patients with LEMS was undertaken from data recorded during visits to a specialist neuromuscular clinic. Serial measurements of muscle strength score in shoulder abduction, elbow extension and hip flexion, compound muscle action potential (CMAP) amplitude, and postcontraction increment in abductor digiti minimi (ADM), and anti-P/Q-type voltage gated calcium channel (VGCC) antibody titre were made at each visit.

RESULTS

Muscle strength scores were improved in 88% of patients after a median duration of immunosuppressive treatment of 6 years (range 1.3 to 17 years); anti-VGCC antibody titres fell in 52% after treatment; and mean resting CMAP amplitude improved from 2.7 mV initially to 8.8 mV after 2 years of treatment p<0.001). Initial pretreatment anti-VGCC antibody titre did not correlate significantly with either CMAP amplitude, CMAP increment, or clinical score: from serial measurements made during follow up, significant correlation between antibody titre and CMAP amplitude was seen in only two patients. Sustained clinical remission was achieved by 20 (43%) of whom only four remained in remission without the need for immunosuppression. Using a Cox proportional hazards model, the only independent predictor of sustained clinical remission was initial pretreatment clinical score (p=0.03). Lymphoma presented in three patients during the study.

CONCLUSIONS

The prognosis in patients with LEMS without SCLC is favourable, although patients often need significant doses of immunosuppressive treatment to remain clinically stable. Only initial clinical muscle strength measurements and not anti-VGCC antibody titres or electrophysiological recordings are predictive of long term outcome.

Paraneoplastic syndromes of the spinal cord, nerve, and muscle

Cancer can affect the peripheral nervous system by non-metastatic, sometimes immune-mediated mechanisms. Recognition of these paraneoplastic syndromes is important because it can lead to the detection of the tumor, and also helps to avoid unnecessary studies to determine the cause of the neurologic symptoms in patients with cancer. Many paraneoplastic syndromes of the peripheral nervous system are not associated with serum antibodies that serve as markers of paraneoplasia. For this group of disorders the diagnosis depends on the clinician's index of suspicion and conventional electrophysiologic and laboratory tests. Treatment of the tumor, immunotherapy, or both may improve some of these syndromes. This review focuses on paraneoplastic syndromes of the spinal cord, peripheral nerve, and muscle.

Antibodies to calcium channel and synaptotagmin in Lambert-Eaton myasthenic syndrome

In the Lambert-Eaton myasthenic syndrome (LEMS), an autoimmune disease that is often associated with lung cancer and characterized by reduced quantal release of acetylcholine from the motor nerve terminal, our studies to search for the target of LEMS antibodies have brought the voltage-gated calcium channel (VGCC) into relief. Among multiple types of VGCCs, the P/Q-type was highly recognized by LEMS antibodies. Using synthetic peptides or recombinant proteins as antigens, the study specified the S5-S6 linker regions in 3 of 4 domains as immunodominant sites in the molecular structure of P/Q-type VGCC alpha1 subunit. Synaptotagmin, one of the functionally VGCC-associated synaptic proteins, was also found to be an immunogen in the pathogenesis of LEMS.

Lambert-Eaton myasthenic syndrome as an autoimmune calcium-channelopathy

Lambert-Eaton myasthenic syndrome (LEMS), often associated with small cell lung carcinoma (SCLC), is a disease of neuromuscular transmission in which antibodies directed against voltage-gated calcium channel (VGCC) in the motor nerve terminal play a crucial role in causing a deficient quantal release of acetylcholine. We focused attention on the P/Q-type VGCC, against which a majority of LEMS patients carry the specific antibody. Since the P/Q-type VGCC expresses in SCLC, the motor nerve terminal and SCLC may share a common VGCC antigen. In search for antigenic sites at the molecular level, We employed peptides or recombinant protein corresponding to the S5-S6 linker of each of four domains forming the alpha 1A subunit and tested their antigenicity. As the result, we specified the domain II, III and IV as immunodominant sites by the induction of an immune-mediated animal model of LEMS and the assay for antibodies in LEMS patients. Also, by use of peptides or recombinant protein corresponding to the synaptotagmin I, we found that in this VGCC-associated protein, the segment which exposes extracellularly during exocytosis can be antigenic for LEMS.

Lambert-Eaton antibodies inhibit Ca2+ currents but paradoxically increase exocytosis during stimulus trains in bovine adrenal chromaffin cells

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease that affects neurotransmitter release at peripheral synapses. LEMS antibodies inhibit Ca2+ currents in excitable cells, but it is not known whether there are additional effects on stimulus-secretion coupling. The effect of LEMS antibodies on Ca2+ currents and exocytosis was studied in bovine adrenal chromaffin cells using whole-cell voltage clamp in perforated-patch recordings. Purified LEMS IgGs from five patients inhibited N- and P/Q-type Ca2+ current components to different extents. The reduction in Ca2+ current resulted in smaller exocytotic responses to single depolarizing pulses, but the normal relationship between integrated Ca2+ entry and exocytosis (Enisch and Nowycky, 1996) was preserved. The hallmark of LEMS is a large potentiation of neuromuscular transmission after high-frequency stimulation. In chromaffin cells, stimulus trains can induce activity-dependent enhancement of the Ca2+-exocytosis relationship. Enhancement during trains occurs most frequently when pulses are brief and evoke very small amounts of Ca2+ entry (Engisch et al., 1997). LEMS antibody treatment increased the percentage of trains eliciting enhancement through two mechanisms: (1) by reducing Ca2+ entry and (2) through a Ca2+-independent effect on the process of enhancement. This leads to a paradoxical increase in the amount of exocytosis during stimulus trains, despite inhibition of Ca2+ currents.

Lambert-Eaton antibodies inhibit Ca2+ currents but paradoxically increase exocytosis during stimulus trains in bovine adrenal chromaffin cells

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease that affects neurotransmitter release at peripheral synapses. LEMS antibodies inhibit Ca2+ currents in excitable cells, but it is not known whether there are additional effects on stimulus-secretion coupling. The effect of LEMS antibodies on Ca2+ currents and exocytosis was studied in bovine adrenal chromaffin cells using whole-cell voltage clamp in perforated-patch recordings. Purified LEMS IgGs from five patients inhibited N- and P/Q-type Ca2+ current components to different extents. The reduction in Ca2+ current resulted in smaller exocytotic responses to single depolarizing pulses, but the normal relationship between integrated Ca2+ entry and exocytosis (Enisch and Nowycky, 1996) was preserved. The hallmark of LEMS is a large potentiation of neuromuscular transmission after high-frequency stimulation. In chromaffin cells, stimulus trains can induce activity-dependent enhancement of the Ca2+-exocytosis relationship. Enhancement during trains occurs most frequently when pulses are brief and evoke very small amounts of Ca2+ entry (Engisch et al., 1997). LEMS antibody treatment increased the percentage of trains eliciting enhancement through two mechanisms: (1) by reducing Ca2+ entry and (2) through a Ca2+-independent effect on the process of enhancement. This leads to a paradoxical increase in the amount of exocytosis during stimulus trains, despite inhibition of Ca2+ currents.

Passive transfer of Lambert-Eaton myasthenic syndrome induces dihydropyridine sensitivity of ICa in mouse motor nerve terminals

Mice were injected for 30 days with plasma from three patients with Lambert-Eaton Myasthenic Syndrome (LEMS). Recordings were made from the perineurial sheath of motor axon terminals of triangularis sterni muscle preparations. The objective was to characterize pharmacologically the identity of kinetically distinct, defined potential changes associated with motor nerve terminal Ca2+ currents (ICa) that were affected by LEMS autoantibodies. ICa elicited at 0.01 Hz were significantly reduced in amplitude by approximately 35% of control in LEMS-treated nerve terminals. During 10-Hz stimulation, ICa amplitude was unchanged in LEMS-treated motor nerve terminals, but was depressed in control. During 20- or 100-Hz trains, facilitation of ICa occurred in LEMS-treated nerve terminals whereas in control, no facilitation occurred during the trains at 20 Hz and marked depression occurred at 100 Hz. Saturation for amplitude and duration of ICa in control terminals occurred at 2 and 4-6 mM extracellular Ca2+, respectively; in LEMS-treated terminals, the extracellular Ca2+ concentration had to increase by two to three times of control to cause saturation. Amplitude of the two components of ICa observed when the preparation was exposed to 50 microM 3,4-diaminopyridine and 1 mM tetraethylammonium were both reduced by LEMS plasma treatment. The fast component (ICa,s) was reduced by 35%, whereas the slow component (ICa, s) was reduced by 37%. omega-Agatoxin IVA (omega-Aga-IVA; 0.15 microM) and omega-conotoxin-MVIIC (omega-CTx-MVIIC; 5 microM) completely blocked ICa in control motor nerve terminals. The same concentrations of toxins were 20-30% less effective in blocking ICa in LEMS-treated terminals. The residual ICa remaining after treatment with omega-Aga-IVA or omega-CTx-MVIIC was blocked by 10 microM nifedipine and 10 microM Cd2+. Thus LEMS plasma appears to downregulate omega-Aga-IVA-sensitive (P-type) and/or omega-CTx-MVIIC-sensitive (Q-type) Ca2+ channels in murine motor nerve terminals, whereas dihydropyridine (DHP)-sensitive (L-type) Ca2+ channels are unmasked in these terminals. Acute exposure (90 min) of rat forebrain synaptosomes to LEMS immunoglobulins (Igs; 4 mg/ml) did not alter the binding of [3H]-nitrendipine or [125I]-omega-conotoxin-GVIA (-omega-CgTx GVIA) when compared with synaptosomes incubated with an equivalent concentration of control Igs. Conversely, LEMS Igs significantly decreased the Bmax for [3H]-verapamil to approximately 45% of control. The apparent affinity of verapamil (KD) for the remaining receptors was not significantly altered. Thus acute exposure of isolated central nerve terminals to LEMS Igs does not increase DHP sensitivity, whereas it reduces the number of binding sites for verapamil but not for nitrendipine or omega-CgTx-GVIA. These results suggest that chronic but not acute exposure to LEMS Igs either upregulates or unmasks DHP-sensitive Ca2+ channels in motor nerve endings.

Pre- and postsynaptic blockade of neuromuscular transmission by Miller-Fisher syndrome IgG at mouse motor nerve terminals

Miller-Fisher syndrome, a variant of an acute inflammatory neuropathy is often associated with serum antibodies to the ganglioside GQ1b, but the pathogenic role of these antibodies and other serum factors is unclear. We here investigated the effect of highly purified immunoglobulin G (IgG) from patients with typical Miller-Fisher syndrome, recording quantal endplate currents by means of a perfused macro-patch-clamp electrode on hemidiaphragms of adult mice. The GQ1b-positive and the GQ1b-negative Miller-Fisher IgG as well as its monovalent Fab-fragments depressed evoked quantal release in a fast and fully reversible, concentration and voltage dependent manner. The time-course of quantal release was changed with the late releases becoming more frequent. The extent of depression of release followed a Michaelis-Menten kinetic and depended on the extracellular calcium concentration. In addition the amplitude of quanta was reduced postsynaptically. IgG and sera from healthy subjects had no effect. Our results indicate that in Miller-Fisher syndrome, IgG antibodies to an undetermined antigen depress the release process, most likely by interfering with the presynaptic Ca2+ inflow or by interacting with proteins of the exocytotic apparatus, and prevent the activation of postsynaptic channels. Antibodies thus seem to be one pathogenic factor for muscle weakness in Miller-Fisher syndrome and our findings may explain why muscle strength recovers rapidly after therapeutical plasmapheresis.

Long-term follow-up of Lambert-Eaton syndrome treated with intravenous immunoglobulin

Recent reports have shown that patients with Lambert-Eaton myasthenic syndrome (LEMS) improve transiently after high-dose intravenous immunoglobulin (IVIG) administration. Information about the usefulness of IVIG for long-term treatment is rather scanty. Our findings demonstrate the efficacy of monthly IVIG courses at a dose of 0.4 g/kg/day for 5 days, in a 41-year-old patient with LEMS without detectable malignancy. Improvement in limb strength, peak expiratory flow rate, and electrophysiological parameters, as well as clinical signs following IVIG, was evident as early as 7 days after the first course and is still maintained at 24-months follow-up.

Down-regulation of non-L-, non-N-type (Q-like) Ca2+ channels by Lambert-Eaton myasthenic syndrome (LEMS) antibodies in rat insulinoma RINm5F cells

The action exerted on non-L-, non-N-type (Q-like) Ca 2+ channels by immunoglobulins G (IgGs) obtained from two patients with Lambert-Eaton myasthenic syndrome (LEMS) was investigated in the rat insulinoma RINm5F cell line. LEMS IgGs reduced by 30-36% the whole-cell Ba2+ currents through Q-like Ca2+ channels at +10 mV without significantly modifying their voltage dependence and activation kinetics. Single- and multiple-channel recordings in cell-attached and outside-out patches of cells treated with LEMS IgGs showed no significant changes of the channel elementary properties but rather a decreased number of active channels per patch. This suggests that Q-like current depression by LEMS autoantibodies is mostly due to a down-regulation of functioning Ca2+ channels. In agreement with previous observations, LEMS IgGs also reduced by 20-33% the dihydropyridine-sensitive (L-type) Ba2+ current. The suggested down-regulation of Q-like channels by LEMS IgGs in RINm5F cells may have a functional correlation with the depressive action of LEMS autoantibodies on the P/Q-type Ca2+ channels controlling acetylcholine release from mammalian neuromuscular junctions.

Antibodies to recombinant synaptotagmin and calcium channel subtypes in Lambert-Eaton myasthenic syndrome

Several proteins have been postulated as possible targets of immune attack in Lambert-Eaton myasthenic syndrome (LEMS). Heterogeneity of autoantibodies in sera from 20 LEMS patients was studied by comparing their reactivity to synaptotagmin, a synaptic vesicle protein, and voltage-gated calcium channels (VGCCs). Six patients' sera (1 with small cell lung carcinoma (SCLC)) contained antibodies specifically recognizing the recombinant synaptotagmin on immunoblots. Thirteen (11 with SCLC) and 16 (11 with SCLC and 1 with poorly differentiated cell carcinoma in the lung) patients' sera immunoprecipitated omega-conotoxin GVIA-labeled N-type and omega-conotoxin MVIIC-labeled Q-type VGCCs, respectively. Three of 6 synaptotagmin-positive sera had cross-reactivity with N and/or Q subtypes of VGCC; the remaining 3 showed no cross-reactivity with VGCCs. Results indicate that LEMS sera are heterogeneous in the spectrum of containing antibodies, and suggest that this heterogeneity reflects the immune response to various synaptic proteins including not only multiple VGCCs but also synaptosecretory complex proteins.

Decreased calcium currents in motor nerve terminals of mice with Lambert-Eaton myasthenic syndrome

1. The effects of immunoglobulin G (IgG) from patients with Lambert-Eaton myasthenic syndrome on Ca2+ currents in mammalian motor nerve terminals are unknown. Therefore, we recorded these currents in phrenic nerves of mice injected with serum from either LEMS patients, myasthenia gravis patients, or healthy control individuals. 2. In control preparations, the endplate currents induced by repetitive stimulation at > or = 20 Hz were depressed as expected. However, in the LEMS animals quantal content decreased and either depression did not occur or synaptic facilitation occurred. 3. Ca2+ currents were smaller in LEMS animals. At 0.5 Hz stimulation frequency, normalized Ca2+ currents in LEMS animals were 57 +/- 14% of those in control. At higher frequencies, Ca2+ currents become smaller in control but not in LEMS animals. 4. Ca2+ currents in controls were unaffected by addition of nifedipine but were reduced by 37% upon addition of omega-conotoxin GVIA. In LEMS animals, however, the currents were depressed by 43% by nifedipine but were unaffected by omega-conotoxin GVIA. Thus, LEMS is associated with reduced Ca2+ currents and a shift to dihydropyridine sensitivity.

Immunopathology of the Lambert-Eaton myasthenic syndrome

LEMS is an antibody-mediated autoimmune disease that can occur in isolation, or as a paraneoplastic disorder in association with SCLC (60% of patients). The underlying defect is a reduction in the quantal release of the neurotransmitter ACh from the presynaptic nerve terminal at the neuromuscular junction. Experimental evidence indicates the autoantibodies are directed against nerve terminal VGCCs causing down-regulation in the number of functional channels by cross-linkage. Functional VGCCs have been detected in SCLC cell lines. In cancer-associated LEMS it appears likely that antibodies initially provoked by tumour VGCCs cross-react with VGCCs at the nerve terminal, causing the clinical disorder. Antibodies against L-, N- and P-/Q- subtypes of the calcium channels have been identified and radioimmunoassays have been developed to help diagnose the disease. Using peptide toxin 125I-omega-CmTx MVIIC to label P-/Q-type VGCC solubilised from human cerebellum, positive antibody titres can be detected in 85% of patients. However, autoantibodies in LEMS are heterogenous; the antigenic targets include different VGCC subtypes, the intracellular beta subunit and the synaptic vesicle protein synaptotagmin. The disease phenotype may reflect the diversity and titre of these different antibodies.