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

Simulations of active zone structure and function at mammalian NMJs predict that loss of calcium channels alone is not sufficient to replicate LEMS effects

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune-mediated neuromuscular disease thought to be caused by autoantibodies against P/Q-type voltage-gated calcium channels (VGCCs), which attack and reduce the number of VGCCs within transmitter release sites (active zones; AZs) at the neuromuscular junction (NMJ), resulting in neuromuscular weakness. However, patients with LEMS also have antibodies to other neuronal proteins, and about 15% of patients with LEMS are seronegative for antibodies against VGCCs. We hypothesized that a reduction in the number of P/Q-type VGCCs alone is not sufficient to explain LEMS effects on transmitter release. Here, we used a computational model to study a variety of LEMS-mediated effects on AZ organization and transmitter release constrained by electron microscopic, pharmacological, immunohistochemical, voltage imaging, and electrophysiological observations. We show that models of healthy AZs can be modified to predict the transmitter release and short-term facilitation characteristics of LEMS and that in addition to a decrease in the number of AZ VGCCs, disruption in the organization of AZ proteins, a reduction in AZ number, a reduction in the amount of synaptotagmin, and the compensatory expression of L-type channels outside the remaining AZs are important contributors to LEMS-mediated effects on transmitter release. Furthermore, our models predict that antibody-mediated removal of synaptotagmin in combination with disruption in AZ organization alone could mimic LEMS effects without the removal of VGCCs (a seronegative model). Overall, our results suggest that LEMS pathophysiology may be caused by a collection of pathological alterations to AZs at the NMJ, rather than by a simple loss of VGCCs.NEW & NOTEWORTHY We used a computational model of the active zone (AZ) in the mammalian neuromuscular junction to investigate Lambert-Eaton myasthenic syndrome (LEMS) pathophysiology. This model suggests that disruptions in presynaptic active zone organization and protein content (particularly synaptotagmin), beyond the simple removal of presynaptic calcium channels, play an important role in LEMS pathophysiology.

Lambert-Eaton myasthenic syndrome: mouse passive-transfer model illuminates disease pathology and facilitates testing therapeutic leads

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder caused by antibodies directed against the voltage-gated calcium channels that provide the calcium ion flux that triggers acetylcholine release at the neuromuscular junction. To study the pathophysiology of LEMS and test candidate therapeutic strategies, a passive-transfer animal model has been developed in mice, which can be created by daily intraperitoneal injections of LEMS patient serum or IgG into mice for 2-4 weeks. Results from studies of the mouse neuromuscular junction have revealed that each synapse has hundreds of transmitter release sites but that the probability for release at each one is likely to be low. LEMS further reduces this low probability such that transmission is no longer effective at triggering a muscle contraction. The LEMS-mediated attack reduces the number of presynaptic calcium channels, disorganizes transmitter release sites, and results in the homeostatic upregulation of other calcium channel types. Symptomatic treatment is focused on increasing the probability of release from dysfunctional release sites. Current treatment uses the potassium channel blocker 3,4-diaminopyridine (DAP) to broaden the presynaptic action potential, providing more time for calcium channels to open. Current research is focused on testing new calcium channel gating modifiers that work synergistically with DAP.

Retinal glycoprotein enrichment by concanavalin a enabled identification of novel membrane autoantigen synaptotagmin-1 in equine recurrent uveitis

Complete knowledge of autoantigen spectra is crucial for understanding pathomechanisms of autoimmune diseases like equine recurrent uveitis (ERU), a spontaneous model for human autoimmune uveitis. While several ERU autoantigens were identified previously, no membrane protein was found so far. As there is a great overlap between glycoproteins and membrane proteins, the aim of this study was to test whether pre-enrichment of retinal glycoproteins by ConA affinity is an effective tool to detect autoantigen candidates among membrane proteins. In 1D Western blots, the glycoprotein preparation allowed detection of IgG reactions to low abundant proteins in sera of ERU patients. Synaptotagmin-1, a Ca2+-sensing protein in synaptic vesicles, was identified as autoantigen candidate from the pre-enriched glycoprotein fraction by mass spectrometry and was validated as a highly prevalent autoantigen by enzyme-linked immunosorbent assay. Analysis of Syt1 expression in retinas of ERU cases showed a downregulation in the majority of ERU affected retinas to 24%. Results pointed to a dysregulation of retinal neurotransmitter release in ERU. Identification of synaptotagmin-1, the first cell membrane associated autoantigen in this spontaneous autoimmune disease, demonstrated that examination of tissue fractions can lead to the discovery of previously undetected novel autoantigens. Further experiments will address its role in ERU pathology.

Molecular mechanism of active zone organization at vertebrate neuromuscular junctions

Organization of presynaptic active zones is essential for development, plasticity, and pathology of the nervous system. Recent studies indicate a trans-synaptic molecular mechanism that organizes the active zones by connecting the pre- and the postsynaptic specialization. The presynaptic component of this trans-synaptic mechanism is comprised of cytosolic active zone proteins bound to the cytosolic domains of voltage-dependent calcium channels (P/Q-, N-, and L-type) on the presynaptic membrane. The postsynaptic component of this mechanism is the synapse organizer (laminin β2) that is expressed by the postsynaptic cell and accumulates specifically on top of the postsynaptic specialization. The pre- and the postsynaptic components interact directly between the extracellular domains of calcium channels and laminin β2 to anchor the presynaptic protein complex in front of the postsynaptic specialization. Hence, the presynaptic calcium channel functions as a scaffolding protein for active zone organization and as an ion-conducting channel for synaptic transmission. In contrast to the requirement of calcium influx for synaptic transmission, the formation of the active zone does not require the calcium influx through the calcium channels. Importantly, the active zones of adult synapses are not stable structures and require maintenance for their integrity. Furthermore, aging or diseases of the central and peripheral nervous system impair the active zones. This review will focus on the molecular mechanisms that organize the presynaptic active zones and summarize recent findings at the neuromuscular junctions and other synapses.

Calcium channels link the muscle-derived synapse organizer laminin β2 to Bassoon and CAST/Erc2 to organize presynaptic active zones

Synapse formation requires the organization of presynaptic active zones, the synaptic vesicle release sites, in precise apposition to postsynaptic neurotransmitter receptor clusters; however, the molecular mechanisms responsible for these processes remain unclear. Here, we show that P/Q-type and N-type voltage-dependent calcium channels (VDCCs) play essential roles as scaffolding proteins in the organization of presynaptic active zones. The neuromuscular junction of double knock-out mice for P/Q- and N-type VDCCs displayed a normal size but had significantly reduced numbers of active zones and docked vesicles and featured an attenuation of the active-zone proteins Bassoon, Piccolo, and CAST/Erc2. Consistent with this phenotype, direct interactions of the VDCC β1b or β4 subunits and the active zone-specific proteins Bassoon or CAST/Erc2 were confirmed by immunoprecipitation. A decrease in the number of active zones caused by a loss of presynaptic VDCCs resembled the pathological conditions observed in the autoimmune neuromuscular disorder Lambert-Eaton myasthenic syndrome. At the synaptic cleft of double knock-out mice, we also observed a decrease of the synaptic organizer laminin β2 protein, an extracellular ligand of P/Q- and N-type VDCCs. However, the transcription level of laminin β2 did not decrease in double knock-out mice, suggesting that the synaptic accumulation of laminin β2 protein required its interaction with presynaptic VDCCs. These results suggest that presynaptic VDCCs link the target-derived synapse organizer laminin β2 to active-zone proteins and function as scaffolding proteins to anchor active-zone proteins to the presynaptic membrane.

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.

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.

Alpha-ketoisocaproic acid increases phosphorylation of intermediate filament proteins from rat cerebral cortex by mechanisms involving Ca2+ and cAMP

We have previously described that alpha-ketoisocaproic acid (KIC), the main metabolite accumulating in maple syrup urine disease (MSUD), increased the in vitro phosphorylation of cytoskeletal proteins in cerebral cortex of 17- and 21-day-old rats through NMDA glutamatergic receptors. In the present study we investigated the protein kinases involved in the effects of KIC on the phosphorylating system associated with the cytoskeletal fraction and provided an insight on the mechanisms involved in such effects. Results showed that 1 mM KIC increased the in vitro incorporation of 32P into intermediate filament (IF) proteins in slices of 21-day-old rats at shorter incubation times (5 min) than previously reported. Furthermore, this effect was prevented by 10 microM KN-93 and 10 microM H-89, indicating that KIC treatment increased Ca2+/calmodulin- (PKCaMII) and cAMP- (PKA) dependent protein kinases activities, respectively. Nifedipine (100 microM), a blocker of voltage-dependent calcium channels (VDCC), DL-AP5 (100 microM), a NMDA glutamate receptor antagonist and BAPTA-AM (50 microM), a potent intracellular Ca2+ chelator, were also able to prevent KIC-induced increase of in vitro phosphorylation of IF proteins. In addition, KIC treatment was able to significantly increase the intracellular cAMP levels. This data support the view that KIC increased the activity of the second messenger-dependent protein kinases PKCaMII and PKA through intracellular Ca2+ levels. Considering that hyperphosphorylation of cytoskeletal proteins is related to neurodegeneration it is presumed that the Ca2+-dependent hyperphosphorylation of IF proteins caused by KIC may be involved to the neuropathology of MSUD patients.

Evidence that intracellular Ca2+ mediates the effect of α-ketoisocaproic acid on the phosphorylating system of cytoskeletal proteins from cerebral cortex of immature rats

In this study we investigated the involvement of Ca2+ on the effects of alpha-ketoisocaproic acid (KIC), the main metabolite accumulating in maple syrup urine disease (MSUD), on the phosphorylating system associated with the intermediate filament (IF) proteins in slices from cerebral cortex of 9-day-old rats. We first observed that KIC significantly decreased the in vitro phosphorylation of IF proteins in brain slices. KIC-induced dephosphorylation was mediated especially by the protein phosphatase PP2B, a Ca2+-dependent protein phosphatase, but also by PP2A. We also demonstrated the involvement of Ca2+-dependent mechanisms in the KIC effects using the specific L-voltage-dependent Ca2+ channels (L-VDCC) inhibitor nifedipine, the NMDA antagonist DL-AP5 and the intracellular Ca2+ chelator BAPTA-AM. Blockage of Ca2+ channels or chelating intracellular Ca2+ completely prevented the effects of KIC on the phosphorylating system associated to IF proteins. In addition, we verified that KIC increased 45Ca2+ uptake in brain slices after 3 and 30 min incubation. Taken together, our present data indicate that KIC increase intracellular Ca2+ levels, probably promoting the activation of calcineurin. These results might be associated with the increased dephosphorylation of the IF proteins in slices of cerebral cortex of immature rats exposed to KIC at similar concentrations from those found in blood and tissues of patients with MSUD.

Immune dysregulation and self‐reactivity in schizophrenia: Do some cases of schizophrenia have an autoimmune basis?

Schizophrenia affects 1% of the world's population, but its cause remains obscure. Numerous theories have been proposed regarding the cause of schizophrenia, ranging from developmental or neurodegenerative processes or neurotransmitter abnormalities to infectious or autoimmune processes. In this review, findings suggestive of immune dysregulation and reactivity to self in patients with schizophrenia are examined with reference to criteria for defining whether or not a human disease is autoimmune in origin. Associations with other autoimmune diseases and particular MHC haplotypes, increased serum levels of autoantibodies, and in vivo and in vitro replication of some of the functional and ultrastructural abnormalities of schizophrenia by transfer of autoantibodies from the sera of patients with schizophrenia suggest that, in some patients at least, autoimmune mechanisms could play a role in the development of disease. Recent findings regarding specific autoimmune responses directed against neurotransmitter receptors in the brain in patients with schizophrenia will also be reviewed.

Lambert-Eaton myasthenic syndrome: A rare manifestation of paraneoplastic syndrome in ovarian cancer: Case report

Introduction Paraneoplastic neurological syndromes are rare non-metastatic complications of cancer that have an immune-mediated etiology. The Lambert-Eaton myasthenic syndrome (LEMS) is a neuromuscular disorder, often associated with small cell lung carcinoma (SCLC), which is characterized by reduced quantal release of acetylcholine from the motor nerve terminals. Lambert-Eaton Myasthenic Syndrome The Lambert-Eaton Myasthenic Syndrome (LEMS) is characterized by proximal muscle weakness initially affecting gait, autonomic symptoms (dry mouth, constipation, erectile failure) and augmentation of strength during initial voluntary activation. Symptomatic treatment of the junctional disorder is based on cholinergic drugs, immunosuppression, immunomodulation and physical therapy useful in case of unsuccessful antineoplastic therapy. Case report A rare case of ovarian cancer with Eaton-Lambert syndrome is reported. A 50-year-old woman was admitted to the gynecologic department, complaining of weakness and pain in her arms and shoulders. Physical therapy resulted in partial improvement. Treatment of paraneoplastic syndrome markedly improves the quality of life of cancer patients. Patients presenting with this syndrome should undergo a careful evaluation for the presence of an occult malignancy.

Effects of pyrethroids on voltage-sensitive calcium channels: a critical evaluation of strengths, weaknesses, data needs, and relationship to assessment of cumulative neurotoxicity

The Food Quality Protection Act of 1996 requires that the U.S. Environmental Protection Agency conduct cumulative risk assessments for classes of pesticides that have a common mode or mechanism of action. For the pyrethroid insecticides, disruption of voltage-sensitive sodium channel function is generally accepted as the mechanism underlying acute neurotoxicity. However, data exist which suggest that voltage-sensitive calcium (Ca(2+)) channels (VSCC) may also be important targets of pyrethroid action. VSCC are important to neuronal function during development and for neurotransmitter release, gene expression, and electrical excitability in the nervous system. Disruption of these and other processes mediated by VSCC can result in neurotoxicity. If effects on VSCC are demonstrated to contribute to pyrethroid neurotoxicity, then such effects will have to be considered when making decisions regarding cumulative risk of exposure to this class of compounds. This document provides a critical review of the data related to the hypothesis that VSCC are important targets of pyrethroid effects. Data supporting effects of pyrethroids on VSCC have been generated by several different laboratories using different techniques and biological preparations. Thus, the many reports of effects on VSCC provide evidence that pyrethroids may interact with VSCC. However, evidence to support a role of VSCC in pyrethroid neurotoxicity is based entirely on in vitro observations, and numerous limitations exist in these data, including: (1) lack of defined concentration-response relationships, with some effects observed only at relatively high concentrations, (2) the use of indirect measures of VSCC function, (3) data from nonmammalian species, (4) data from studies that have not been peer-reviewed, (5) the need for replication of some effects, and (6) inconsistent or contradictory results from different laboratories/preparations. Thus, at the present time, it is premature to conclude that effects on VSCC play an important role in the acute neurotoxicity of pyrethroid insecticides in mammals. To demonstrate that VSCC are important targets of pyrethroid neurotoxicity in mammals, in vivo studies supporting a role for pyrethroid effects on VSCC are needed. Additional support could be provided by demonstration of direct effects of pyrethroid compounds on mammalian neuronal VSCC in vitro, including demonstration that concentration-response relationships are similar, or greater, in sensitivity to effects of pyrethroids on voltage-sensitive sodium channels. If such effects were to be demonstrated, the rationale for considering VSCC as targets of pyrethroid compounds when assessing cumulative risk would be strengthened. However, at the present time, the data available neither support nor refute conclusively the hypothesis that effects on VSCC are important to the acute neurotoxicity of pyrethroids.

Ontogeny of voltage-sensitive calcium channel alpha(1A) and alpha(1E) subunit expression and synaptic function in rat central nervous system

Immunohistochemical expression in the neocortex, hippocampus and cerebellum of the alpha(1A) or alpha(1E) subunit of the voltage-sensitive Ca(2+) channel was examined in Long-Evans hooded rats on gestational day 18 and postnatal days 1, 4, 7, 10, 14, 21, 90, 360 and 720. On gestational day 18 and postnatal day 1, alpha(1A) immunoreactivity was more dense in the neocortex and hippocampus than the cerebellum. By postnatal day 7, levels of alpha(1A) immunoreactivity increased dramatically in the cerebellum, while in neocortex, alpha(1A) immunoreactivity became more sparse, which approached the more diffuse pattern of cellular staining in the mature brain. Expression of alpha(1E) in the neocortex, hippocampus and cerebellum was much less dense than alpha(1A) between gestational day 18 and postnatal day 4. There was also significant alpha(1E) immunoreactivity in the mossy fibers of the hippocampus and in dendrites of Purkinje cells of the cerebellum. Depolarization-dependent 45Ca(2+) influx was examined in rat brain synaptosomes on postnatal days 4, 7, 10, 14, 21 and >60. In neocortical and hippocampal synaptosomes, 45Ca(2+) influx increased steadily with age and reached adult levels by postnatal day 10. In cerebellar synaptosomes, 45Ca(2+) influx was constant across all ages, except for a spike in activity which was observed on postnatal day 21. In neocortical and hippocampal synaptosomes, 100 nM omega-conotoxin MVIIC significantly inhibited 45Ca(2+) influx on postnatal day 10 and 14, respectively, or after. In cerebellar synaptosomes, influx was inhibited by omega-conotoxin MVIIC only on postnatal day 10 or prior. On postnatal day 7, 45Ca(2+) influx was not inhibited in neocortical and hippocampal synaptosomes by a combination of 10 microM nifedipine, 1 microM omega-conotoxin GVIA and 1 microM omega-conotoxin MVIIC, suggesting that an 'insensitive' flux predominates at this age. Overall, the results suggest that expression of voltage-sensitive Ca(2+) channels during development is dynamic and is important in central nervous system development.

Linear B-cell epitopes in Lambert-Eaton myasthenic syndrome defined by cell-free synthetic peptide binding

Lambert-Eaton Myasthenic Syndrome (LEMS) is a rare autoimmune disorder characterized by autoantibodies that bind to P/Q voltage-gated calcium channels at the neuromuscular junction. Using cell-free direct peptide binding in 12 LEMS patients, we find linear B-cell epitopes that reside on the extracellular peptide linkers between the S5-S6 transmembrane peptides of Domains II and IV. A major epitope is located within the S5-S6 linker peptide of Domain IV defined by the acidic amino acid sequence EDEDSDEDE.