Antibodies against the beta subunit of voltage-dependent calcium channels in Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disease characterized by proximal muscle weakness, loss of tendon reflexes, and autonomic dysfunction. Muscle weakness usually starts in the upper legs and can progress to oculobulbar and in severe cases respiratory muscles. P/Q-type voltage-gated calcium channels (VGCCs) localized in the presynaptic motor nerve terminal and in the autonomic nervous system are targeted by antibodies in LEMS patients. These antibodies can be detected in about 90% of patients, and the presence of decrement and increment upon repetitive nerve stimulation is also a highly sensitive diagnostic test. Rapid diagnosis is important because of the association with SCLC in 50%-60% of patients, which stresses the need for vigorous tumor screening after diagnosis. Clinical parameters can predict tumor probability and guide frequency of tumor screening. Treatment of the tumor as well as symptomatic treatment and immunosuppression can effectively control symptoms in the majority of patients.

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.

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.

Protein partners of the calcium channel β subunit highlight new cellular functions

Calcium plays a key role in cell signalling by its intervention in a wide range of physiological processes. Its entry into cells occurs mainly via voltage-gated calcium channels (VGCC), which are found not only in the plasma membrane of excitable cells but also in cells insensitive to electrical signals. VGCC are composed of different subunits, α1, β, α2δ and γ, among which the cytosolic β subunit (Cavβ) controls the trafficking of the channel to the plasma membrane, its regulation and its gating properties. For many years, these were the main functions associated with Cavβ. However, a growing number of proteins have been found to interact with Cavβ, emphasizing the multifunctional role of this versatile protein. Interestingly, some of the newly assigned functions of Cavβ are independent of its role in the regulation of VGCC, and thus further increase its functional roles. Based on the identity of Cavβ protein partners, this review emphasizes the diverse cellular functions of Cavβ and summarizes both past findings as well as recent progress in the understanding of VGCC.

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 ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

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.

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.

Auxiliary subunits: essential components of the voltage-gated calcium channel complex

Voltage-gated calcium channels are important mediators of several physiological processes, including neuronal excitability and muscle contraction. At the molecular level, the channels are composed of four subunits--the pore forming alpha(1) subunit and the auxiliary alpha(2)delta, beta and gamma subunits. The auxiliary subunits modulate the trafficking and the biophysical properties of the alpha(1) subunit. In the past several years there has been an acceleration of our understanding of the auxiliary subunits, primarily because of their molecular characterization and the availability of spontaneous and targeted mouse mutants. These studies have revealed the crucial role of the subunits in the functional effects that are mediated by voltage-gated calcium channels.

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.

Reversibility of the Ca(2+) channel alpha(1)-beta subunit interaction

The auxiliary beta subunit importantly regulates voltage-dependent Ca(2+) channel activity through an interaction with the AID domain, a binding site located in the cytoplasmic I-II linker of the ion-conducting alpha(1) subunit. In the present study, we used two synthetic peptides corresponding to partial sequences of the I-II linker of alpha(1A) (AID(A)-peptides) as tools to disrupt the alpha(1)-beta interaction. In vitro binding experiments confirmed that these peptides exhibit a reasonable affinity to the neuronal beta(3) subunit to serve this purpose, although they failed to prevent immunoprecipitation of native N- and P/Q-type channels by anti-beta(3) antibodies. Together, our results (i) provide evidence for the reversibility of channel subunit association suggesting that the disruption of the alpha(1)-beta interaction may be a possible mechanism for Ca(2+) channel regulation in vivo, and (ii) support a model whereby the alpha(1)-beta association is based on multiple interaction sites.

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.

Functional properties of a new voltage-dependent calcium channel alpha(2)delta auxiliary subunit gene (CACNA2D2)

We have positionally cloned and characterized a new calcium channel auxiliary subunit, alpha(2)delta-2 (CACNA2D2), which shares 56% amino acid identity with the known alpha(2)delta-1 subunit. The gene maps to the critical human tumor suppressor gene region in chromosome 3p21.3, showing very frequent allele loss and occasional homozygous deletions in lung, breast, and other cancers. The tissue distribution of alpha(2)delta-2 expression is different from alpha(2)delta-1, and alpha(2)delta-2 mRNA is most abundantly expressed in lung and testis and well expressed in brain, heart, and pancreas. In contrast, alpha(2)delta-1 is expressed predominantly in brain, heart, and skeletal muscle. When co-expressed (via cRNA injections) with alpha(1B) and beta(3) subunits in Xenopus oocytes, alpha(2)delta-2 increased peak size of the N-type Ca(2+) currents 9-fold, and when co-expressed with alpha(1C) or alpha(1G) subunits in Xenopus oocytes increased peak size of L-type channels 2-fold and T-type channels 1.8-fold, respectively. Anti-peptide antibodies detect the expression of a 129-kDa alpha(2)delta-2 polypeptide in some but not all lung tumor cells. We conclude that the alpha(2)delta-2 gene encodes a functional auxiliary subunit of voltage-gated Ca(2+) channels. Because of its chromosomal location and expression patterns, CACNA2D2 needs to be explored as a potential tumor suppressor gene linking Ca(2+) signaling and lung, breast, and other cancer pathogenesis. The homologous location on mouse chromosome 9 is also the site of the mouse neurologic mutant ducky (du), and thus, CACNA2D2 is also a candidate gene for this inherited idiopathic generalized epilepsy syndrome.

The I-II loop of the Ca2+ channel alpha1 subunit contains an endoplasmic reticulum retention signal antagonized by the beta subunit

The auxiliary beta subunit is essential for functional expression of high voltage-activated Ca2+ channels. This effect is partly mediated by a facilitation of the intracellular trafficking of alpha1 subunit toward the plasma membrane. Here, we demonstrate that the I-II loop of the alpha1 subunit contains an endoplasmic reticulum (ER) retention signal that severely restricts the plasma membrane incorporation of alpha1 subunit. Coimmunolabeling reveals that the I-II loop restricts expression of a chimera CD8-I-II protein to the ER. The beta subunit reverses the inhibition imposed by the retention signal. Extensive deletion of this retention signal in full-length alpha1 subunit facilitates the cell surface expression of the channel in the absence of beta subunit. Our data suggest that the beta subunit favors Ca2+ channel plasma membrane expression by inhibiting an expression brake contained in beta-binding alpha1 sequences.