Recombinant calcium channel is recognized by Lambert-Eaton myasthenic syndrome antibodies

Mapping autoantigen epitopes: molecular insights into autoantibody-associated disorders of the nervous system

Background

Our knowledge of autoantibody-associated diseases of the central (CNS) and peripheral (PNS) nervous systems has expanded greatly over the recent years. A number of extracellular and intracellular autoantigens have been identified, and there is no doubt that this field will continue to expand as more autoantigens are discovered as a result of improved clinical awareness and methodological practice. In recent years, interest has shifted to uncover the target epitopes of these autoantibodies.

Main body

The purpose of this review is to discuss the mapping of the epitope targets of autoantibodies in CNS and PNS antibody-mediated disorders, such as N-methyl-D-aspartate receptor (NMDAR), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), leucine-rich glioma-inactivated protein 1 (Lgi1), contactin-associated protein-like 2 (Caspr2), myelin oligodendrocyte glycoprotein (MOG), aquaporin-4 (AQP4), 65 kDa glutamic acid decarboxylase (GAD65), acetylcholine receptor (AChR), muscle-specific kinase (MuSK), voltage-gated calcium channel (VGCC), neurofascin (NF), and contactin. We also address the methods used to analyze these epitopes, the relevance of their determination, and how this knowledge can inform studies on autoantibody pathogenicity. Furthermore, we discuss triggers of autoimmunity, such as molecular mimicry, ectopic antigen expression, epitope spreading, and potential mechanisms for the rising number of double autoantibody-positive patients.

Conclusions

Molecular insights into specificity and role of autoantibodies will likely improve diagnosis and treatment of CNS and PNS neuroimmune diseases.

A single-fibre EMG Study of Neuromuscular Transmission in Migraine Patients

It is known that mutations of CACNA1A, which encodes a neuronal P/Q Ca2+ channel, are present in patients with familial hemiplegic migraine, and possibly in other types of migraine as well. This calcium channel is also involved in neuromuscular transmission. To assess if the single-fibre EMG (SFEMG) method can demonstrate a neuromuscular transmission deficit in migraine, a group of 26 patients with different types of migraine and 20 healthy control subjects were studied. The migraine patients were divided into three groups: 8 patients with migraine without aura (MoA), 12 with migraine with aura excluding visual aura (MA) and 6 with visual aura (VA). A SFEMG of the voluntarily activated extensor digitorum communis muscle was performed. The SFEMG results were normal in the healthy controls and the MoA group (migraine without aura). Slight neuromuscular transmission disturbances were present in 6/12 (50%) of patients with MA and in 1/6 (17%) of patients with VA. We suggest that abnormal neuromuscular transmission detectable by SFEMG may reflect a genetically determined dysfunction of the P/Q Ca2+ channels in a subgroup of migraineurs with aura.

Pathogenic immune mechanisms at the neuromuscular synapse: the role of specific antibody-binding epitopes in myasthenia gravis

Autoantibodies against three different postsynaptic antigens and one presynaptic antigen at the neuromuscular junction are known to cause myasthenic syndromes. The mechanisms by which these antibodies cause muscle weakness vary from antigenic modulation and complement-mediated membrane damage to inhibition of endogenous ligand binding and blocking of essential protein-protein interactions. These mechanisms are related to the autoantibody titre, specific epitopes on the target proteins and IgG autoantibody subclass. We here review the role of specific autoantibody-binding epitopes in myasthenia gravis, their possible relevance to the pathophysiology of the disease and potential implications of epitope mapping knowledge for new therapeutic strategies.

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.

Small-cell lung cancer-associated autoantibodies: potential applications to cancer diagnosis, early detection, and therapy

Small-cell lung cancer (SCLC) is the most aggressive lung cancer subtype and lacks effective early detection methods and therapies. A number of rare paraneoplastic neurologic autoimmune diseases are strongly associated with SCLC. Most patients with such paraneoplastic syndromes harbor high titers of antibodies against neuronal proteins that are abnormally expressed in SCLC tumors. These autoantibodies may cross-react with the nervous system, possibly contributing to autoimmune disease development. Importantly, similar antibodies are present in many SCLC patients without autoimmune disease, albeit at lower titers. The timing of autoantibody development relative to cancer and the nature of the immune trigger remain to be elucidated. Here we review what is currently known about SCLC-associated autoantibodies, and describe a recently developed mouse model system of SCLC that appears to lend itself well to the study of the SCLC-associated immune response. We also discuss potential clinical applications for these autoantibodies, such as SCLC diagnosis, early detection, and therapy.

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.

Anti-Ca2+ channel antibody attenuates Ca2+ currents and mimics cerebellar ataxia in vivo

Voltage-gated Ca(2+) channels (VGCCs) are membrane proteins that determine the activity and survival of neurons, and mutations in the P/Q-type VGCCs are known to cause cerebellar ataxia. VGCC dysfunction may also underlie acquired peripheral and central nervous system diseases associated with small-cell lung cancer, including Lambert-Eaton myasthenic syndrome (LEMS) and paraneoplastic cerebellar ataxia (PCA). The pathogenic role of anti-VGCC antibody in LEMS is well established. Although anti-VGCC antibody is also found in a significant fraction of PCA patients, its contribution to PCA is unclear. Using a polyclonal peptide antibody against a major immunogenic region in P/Q-type VGCCs (the extracellular Domain-III S5-S6 loop), we demonstrated that such antibody was sufficient to inhibit VGCC function in neuronal and recombinant VGCCs, alter cerebellar synaptic transmission, and confer the phenotype of cerebellar ataxia. Our data support the hypothesis that anti-VGCC antibody may play a significant role in the pathogenesis of cerebellar dysfunction in PCA.

Lambert-Eaton myasthenic syndrome as an autoimmune calcium channelopathy

Lambert-Eaton myasthenic syndrome, often associated with small-cell lung carcinoma, is a disease of neuromuscular transmission in which antibodies directed against voltage-gated calcium channel (VGCC)(P/Q-type) in the motor nerve terminal play a crucial role in causing a deficient quantal release of acetylcholine. The motor nerve terminal and carcinoma cell may share a common antigen. The study using synthetic peptides and recombinant protein specified the extracellular S5-S6 linker regions in 3 of 4 domains as immunodominant sites in the molecular structure of P/Q-type VGCC alpha1 subunit. Also, the study by use of peptides and recombinant protein corresponding to synaptotagmin I suggested that in this functionally VGCC-associated presynaptic protein, the segment which exposes extracellularly during exocytosis can be immunogenic for the syndrome.

Nonvasculitic autoimmune inflammatory meningoencephalitis

Caselli and colleagues described five cases with encephalopathy, progressive cognitive decline, ataxia, abnormal CSF studies and steroid responsiveness, and proposed the term non-vasculitic autoimmune inflammatory meningoencephalitis (NAIM). Many of these cases had brain biopsy showing mild leptomeningeal perivascular lymphocytic inflammation, however, none of the cases had a post-mortem. Nonvasculitic autoimmune mediated meningoencephalitis has been described in patients with Sjogren's syndrome, systemic lupus erythematosus and, more recently, with Hashimoto's disease. The present study is the first post-mortem report of a case with a clinical diagnosis of NAIM. Neuropathological examination revealed a panencephalitis with intact vessel walls. T and B immunostaining showed a mixture of T and B cells. The findings were not consistent with other reported findings in collagen vascular diseases including Sjogren's syndrome, CNS vasculitis or Hashimoto's encephalopathy. There was no evidence of neoplasia, bacteria, acid-fast bacilli, fungi or atypical infectious agents. This is the first post-mortem report of a case with a clinical diagnosis of NAIM and demonstrates a panencephalitis without evidence of a vasculitis. The pathology seems unique, however, Sjogren and Hashimoto's encephalopathy might be variants of NAIM.

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.

Abnormal intracellular ca(2+)homeostasis and disease

A whole range of cell functions are regulated by the free cytosolic Ca(2+)concentration. Activator Ca(2+)from the extracellular space enters the cell through various types of Ca(2+)channels and sometimes the Na(+)/Ca(2+)-exchanger, and is actively extruded from the cell by Ca(2+)pumps and Na(+)/Ca(2+)-exchangers. Activator Ca(2+)can also be released from internal Ca(2+)stores through inositol trisphosphate or ryanodine receptors and is taken up into these organelles by means of Ca(2+)pumps. The resulting Ca(2+)signal is highly organized in space, frequency and amplitude because the localization and the integrated free cytosolic Ca(2+)concentration over time contain specific information. Mutations or functional abnormalities in the various Ca(2+)transporters, which in vitro seem to induce trivial functional alterations, therefore, often lead to a plethora of diseases. Skeletal-muscle pathology can be caused by mutations in ryanodine receptors (malignant hyperthermia, porcine stress syndrome, central-core disease), dihydropyridine receptors (familial hypokalemic periodic paralysis, malignant hyperthermia, muscular dysgenesis) or Ca(2+)pumps (Brody disease). Ca(2+)-pump mutations in cutaneous epidermal keratinocytes and cochlear hair cells lead to, skin diseases (Darier and Hailey-Hailey) and hearing/vestibular problems respectively. Mutated Ca(2+)channels in the photoreceptor plasma membrane cause vision problems. Hemiplegic migraine, spinocerebellar ataxia type-6, one form of episodic ataxia and some forms of epilepsy can be due to mutations in plasma-membrane Ca(2+)channels, while antibodies against these channels play a pathogenic role in all patients with the Lambert-Eaton myasthenic syndrome and may be of significance in sporadic amyotrophic lateral sclerosis. Brain inositol trisphosphate receptors have been hypothesized to contribute to the pathology in opisthotonos mice, manic-depressive illness and perhaps Alzheimer's disease. Various abnormalities in Ca(2+)-handling proteins have been described in heart during aging, hypertrophy, heart failure and during treatment with immunosuppressive drugs and in diabetes mellitus. In some instances, disease-causing mutations or abnormalities provide us with new insights into the cell biology of the various Ca(2+)transporters.

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.