Inhibition of low- and high-threshold Ca2+ channels of human neuroblastoma IMR32 cells by Lambert-Eaton myasthenic syndrome (LEMS) IgGs

Effects of exposure to gradient magnetic fields emitted by nuclear magnetic resonance devices on clonogenic potential and proliferation of human hematopoietic stem cells

This study investigates effects of gradient magnetic fields (GMFs) emitted by magnetic resonance imaging (MRI) devices on hematopoietic stem cells. Field measurements were performed to assess exposure to GMFs of staff working at 1.5 T and 3 T MRI units. Then an exposure system reproducing measured signals was realized to expose in vitro CD34+ cells to GMFs (1.5 T-protocol and 3 T-protocol). CD34+ cells were obtained by Fluorescence Activated Cell Sorting from six blood donors and three MRI-exposed workers. Blood donor CD34+ cells were exposed in vitro for 72 h to 1.5 T or 3 T-protocol and to sham procedure. Cells were then cultured and evaluated in colony forming unit (CFU)-assay up to 4 weeks after exposure. Results showed that in vitro GMF exposure did not affect cell proliferation but instead induced expansion of erythroid and monocytes progenitors soon after exposure and for the subsequent 3 weeks. No decrease of other clonogenic cell output (i.e., CFU-granulocyte/erythroid/macrophage/megakaryocyte and CFU-granulocyte/macrophage) was noticed, nor exposed CD34+ cells underwent the premature exhaustion of their clonogenic potential compared to sham-exposed controls. On the other hand, pilot experiments showed that CD34+ cells exposed in vivo to GMFs (i.e., samples from MRI workers) behaved in culture similarly to sham-exposed CD34+ cells, suggesting that other cells and/or microenvironment factors might prevent GMF effects on hematopoietic stem cells in vivo. Accordingly, GMFs did not affect the clonogenic potential of umbilical cord blood CD34+ cells exposed in vitro together with the whole mononuclear cell fraction.

Herpes Simplex Virus type-1 infection induces synaptic dysfunction in cultured cortical neurons via GSK-3 activation and intraneuronal amyloid-β protein accumulation

Increasing evidence suggests that recurrent Herpes Simplex Virus type 1 (HSV-1) infection spreading to the CNS is a risk factor for Alzheimer’s Disease (AD) but the underlying mechanisms have not been fully elucidated yet. Here we demonstrate that in cultured mouse cortical neurons HSV-1 induced Ca²⁺-dependent activation of glycogen synthase kinase (GSK)-3. This event was critical for the HSV-1-dependent phosphorylation of amyloid precursor protein (APP) at Thr668 and the following intraneuronal accumulation of amyloid-β protein (Aβ). HSV-1-infected neurons also exhibited: i) significantly reduced expression of the presynaptic proteins synapsin-1 and synaptophysin; ii) depressed synaptic transmission. These effects depended on GSK-3 activation and intraneuronal accumulation of Aβ. In fact, either the selective GSK-3 inhibitor, SB216763, or a specific antibody recognizing Aβ (4G8) significantly counteracted the effects induced by HSV-1 at the synaptic level. Moreover, in neurons derived from APP KO mice and infected with HSV-1 Aβ accumulation was not found and synaptic protein expression was only slightly reduced when compared to wild-type infected neurons. These data further support our contention that HSV-1 infections spreading to the CNS may contribute to AD phenotype.

Role of methionine 35 in the intracellular Ca2+ homeostasis dysregulation and Ca2+-dependent apoptosis induced by amyloid beta-peptide in human neuroblastoma IMR32 cells

Amyloid beta-peptide (Abeta) plays a fundamental role in the pathogenesis of Alzheimer's disease. We recently reported that the redox state of the methionine residue in position 35 of amyloid beta-peptide (Abeta) 1-42 (Met35) strongly affects the peptide's ability to trigger apoptosis and is thus a major determinant of its neurotoxicity. Dysregulation of intracellular Ca(2+) homeostasis resulting in the activation of pro-apoptotic pathways has been proposed as a mechanism underlying Abeta toxicity. Therefore, we investigated correlations between the Met35 redox state, Abeta toxicity, and altered intracellular Ca(2+) signaling in human neuroblastoma IMR32 cells. Cells incubated for 6-24 h with 10 microM Abeta1-42 exhibited significantly increased KCl-induced Ca(2+) transient amplitudes and resting free Ca(2+) concentrations. Nifedipine-sensitive Ca(2+) current densities and Ca(v)1 channel expression were markedly enhanced by Abeta1-42. None of these effects were observed when cells were exposed to Abeta containing oxidized Met35 (Abeta1-42(Met35-Ox)). Cell pre-treatment with the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (1 microM) or the Ca(v)1 channel blocker nifedipine (5 microM) significantly attenuated Abeta1-42-induced apoptosis but had no effect on Abeta1-42(Met35-Ox) toxicity. Collectively, these data suggest that reduced Met35 plays a critical role in Abeta1-42 toxicity by rendering the peptide capable of disrupting intracellular Ca(2+) homeostasis and thereby provoking apoptotic cell death.

Calcium channelopathies: voltage-gated calcium channels

Since the initial identification of native calcium currents, significant progress has been made towards our understanding of the molecular and cellular contributions of voltage-gated calcium channels in multiple physiological processes. Moreover, we are beginning to comprehend their pathophysiological roles through both naturally occurring channelopathies in humans and mice and through targeted gene deletions. The data illustrate that small perturbations in voltage-gated calcium channel function induced by genetic alterations can affect a wide variety of mammalian developmental, physiological and behavioral functions. At least in those instances wherein the channelopathies can be attributed to gain-of-function mechanisms, the data point towards new therapeutic strategies for developing highly selective calcium channel antagonists.

Modulation of Ca(v)1 and Ca(v)2.2 channels induced by nitric oxide via cGMP-dependent protein kinase

The unconventional gaseous transmitter nitric oxide (NO) markedly influences most of mechanisms involved in the regulation of intracellular Ca2+ homeostasis. In excitable cells, Ca2+ signaling mainly depends on the activity of voltage-gated Ca2+ channels (VGCCs). In the present paper, we will review data from our laboratory and others characterizing NO-induced modulation of Ca(v)1 (L-type) and Ca(v)2.2 (N-type) channels. In particular, we will explore experimental evidence indicating that NO's inhibition of channel gating is produced via cGMP-dependent protein kinase and examine some of the numerous cell functions that are potentially influenced by the action of NO on Ca2+ channels.

Effect of maprotiline on Ca2+ movement in human neuroblastoma cells

In human neuroblastoma IMR32 cells, the effect of the anti-depressant maprotiline on baseline intracellular Ca2+ concentrations ([Ca2+]i) was explored by using the Ca2+-sensitive probe fura-2. Maprotiline at concentrations greater than 100 microM caused a rapid rise in [Ca2+]i in a concentration-dependent manner (EC50 = 200 microM). Maprotiline-induced [Ca2+]i rise was reduced by 50% by removal of extracellular Ca2+. Maprotiline-induced [Ca2+]i rises were inhibited by half by nifedipine, but was unaffected by verapamil or diiltiazem. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which the increasing effect of maprotiline on [Ca2+]i was abolished. U73122, an inhibitor of phospholipase C, did not affect maprotiline-induced [Ca2+]i rises. These findings suggest that in human neuroblastoma cells, maprotiline increases [Ca2+]i by stimulating extracellular Ca2+ influx and also by causing intracellular Ca2+ release from the endoplasmic reticulum via a phospholiase C-independent manner.

cGMP/protein kinase G-dependent inhibition of N-type Ca2+ channels induced by nitric oxide in human neuroblastoma IMR32 cells

Although data from our laboratory and others suggest that nitric oxide (NO) exerts an overall inhibitory action on high-voltage-activated Ca2+ channels, conflicting observations have been reported regarding its effects on N-type channels. We performed whole-cell and cell-attached patch-clamp recordings in IMR32 cells to clarify the functional role of NO in the modulation of N channels of human neuronal cells. During depolarizing steps to +10 mV from V(h) = -90 mV, the NO donor, sodium nitroprusside (SNP; 200 microm), reduced macroscopic N currents by 34% (p < 0.01). The magnitude of inhibition was similar at all voltages tested (range, -40 to +50 mV). No significant inhibition was observed when SNP was applied together with the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium salt (300 microm), or after cell treatment with the guanylate cyclase inhibitor, 1H-[1,2,4] oxadiazole [4,3-a] quinoxalin-1-one (10 microm). 8-bromoguanosine-cGMP (8-Br-cGMP) (400 microm) mimicked the effects of SNP, reducing Ba2+ currents by 37% (p < 0.001). Cell treatment with the protein kinase G (PKG) inhibitor KT5823 (1 microm) or guanosine 3',5'-cyclic monophosphorothioate, 8-(4-chloro-phenylthio)-Rp-isomer, triethylammonium salt (20 microm) virtually abolished the effects of 8-Br-cGMP. At the single-channel level, 8-Br-cGMP reduced the channel open probability by 59% and increased both the mean shut time and the null sweep probability, but it had no significant effects on channel conductance, mean open time, or latency of first openings. These data suggest that NO inhibits N-channel gating through cGMP and PKG. The consequent decrease in Ca2+ influx through these channels may affect different neuronal functions, including neurotransmitter release.

The action of Lambert-Eaton myasthenic syndrome immunoglobulin G on cloned human voltage-gated calcium channels

In the Lambert-Eaton myasthenic syndrome (LEMS), immunoglobulin G (IgG) autoantibodies to presynaptic voltage-gated calcium channels (VGCCs) at the neuromuscular junction lead to a reduction in nerve-evoked release of neurotransmitter and muscle weakness. We have examined the action of LEMS IgGs on cloned human VGCCs stably expressed in transfected human embryonic kidney (HEK293) cell lines: 10-13 (alpha(1A-2), alpha(2b)delta, beta(4a)) and C2D7 (alpha(1B-1), alpha(2b)delta, beta(1b)). All LEMS IgGs studied showed surface binding to [(125)I]-omega-CTx-MVIIC-labeled VGCCs in the alpha(1A) cell line and two of six IgGs showed surface binding to [(125)I]-omega-CTx-GVIA-labeled VGCCs in the alpha(1B) cell line. We next studied the effect of LEMS IgGs (2 mg/ml) on whole-cell calcium currents in the alpha(1A) and alpha(1B) cell lines. Overnight treatment of alpha(1A) (10-13) cells with LEMS IgGs led to a significant reduction in peak current density without alteration of the current-voltage relationship or the voltage dependence of steady-state inactivation. In contrast, LEMS IgGs did not reduce peak current density in the alpha(1B) cell line. Overall these data demonstrate the specificity of LEMS IgGs for the alpha(1A) cell line and suggest that LEMS IgGs bind to and downregulate VGCCs in this cell line. Although several LEMS IgGs can be shown to bind to the alpha(1B) (C2D7) cell line, no functional effects were seen on this channel.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

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.

Human autoantibodies specific for the alpha1A calcium channel subunit reduce both P-type and Q-type calcium currents in cerebellar neurons

The pharmacological properties of voltage-dependent calcium channel (VDCC) subtypes appear mainly to be determined by the alpha1 pore-forming subunit but, whether P-and Q-type VDCCs are encoded by the same alpha1 gene presently is unresolved. To investigate this, we used IgG antibodies to presynaptic VDCCs at motor nerve terminals that underlie muscle weakness in the autoimmune Lambert-Eaton myasthenic syndrome (LEMS). We first studied their action on changes in intracellular free Ca2+ concentration [Ca2+]i in human embryonic kidney (HEK293) cell lines expressing different combinations of human recombinant VDCC subunits. Incubation for 18 h with LEMS IgG (2 mg/ml) caused a significant dose-dependent reduction in the K+-stimulated [Ca2+]i increase in the alpha1A cell line but not in the alpha1B, alpha1C, alpha1D, and alpha1E cell lines, establishing the alpha1A subunit as the target for these autoantibodies. Exploiting this specificity, we incubated cultured rat cerebellar neurones with LEMS IgG and observed a reduction in P-type current in Purkinje cells and both P- and Q-type currents in granule cells. These data are consistent with the hypothesis that the alpha1A gene encodes for the pore-forming subunit of both P-type and Q-type VDCCs.

Lambert-Eaton syndrome antibodies inhibit acetylcholine release and P/Q-type Ca2+ channels in electric ray nerve endings

1. The types of voltage-dependent calcium channels (VDCCs) present in the cholinergic terminals isolated from the electric organ of the ray, Narke japonica, were characterized on the basis of their pharmacological sensitivity to specific antagonists. Inhibition of these channel types by autoantibodies from patients with the Lambert-Eaton syndrome (LES) was then studied to determine the specificity of the pathogenic IgG. 2. In normal untreated synaptosomal preparations, maximal doses of N- and P and/or Q-type Ca2+ channel antagonists, omega-conotoxin GVIA and omega-agatoxin IVA, inhibited depolarization-evoked ACh release by 47 % and 43 %, respectively. Calciseptine, an L-type VDCC antagonist, caused a 20 % reduction in the release. This indicates that the exocytotic release process is predominantly mediated by N- and P/Q-type VDCCs. 3. LES IgG or sera caused an inhibition of ACh release by 39-45 % in comparison with the control antibody-treated preparations. The ionomycin-induced ACh release, however, was not altered by the antibodies. Additionally, the same LES antibodies inhibited whole-cell calcium currents (ICa) in bovine adrenal chromaffin cells. Thus, the pathogenic antibodies exert their action on VDCCs present in the synaptosomes. 4. The efficacy of three Ca2+ channel antagonists in blocking ACh release was determined in preparations pretreated with LES IgG. omega-Agatoxin IVA produced only an additional 3-5 % reduction in release beyond that obtained with LES antibodies. Despite the pretreatment with LES IgG, omega-conotoxin GVIA and calciseptine inhibited the release to nearly their control levels. 5. These results indicate that LES antibodies mainly downregulate P/Q-type Ca2+ channels which contribute to presynaptic transmitter release from the cholinergic nerve terminals of electric organ. 6. The present findings are consistent with the hypothesis that P/Q-type VDCCs at the neuromuscular junction are the target of LES antibodies and that their inhibition by the antibodies produces the characteristic neuromuscular defect in this disease.

Effect of Lambert-Eaton myasthenic syndrome antibodies on autonomic neurons in the mouse

Somatic muscle weakness and autonomic symptoms characterize the autoimmune Lambert-Eaton myasthenic syndrome (LEMS). The former results from IgG autoantibody-mediated down-regulation of P/Q-type voltage-gated calcium channels at motor nerve terminals and consequent reduction in acetylcholine release; the basis for the autonomic symptoms is unknown. Using omega-conotoxins GVIA and MVIIC and omega-agatoxin IVA that block N-, Q-, and P-type channels, we investigated ex vivo the calcium channels subserving transmitter release from postganglionic parasympathetic neurons in the bladder and from postganglionic sympathetic neurons in the vas deferens of mice injected with IgG from LEMS patients or from controls. Calcium influx through N-, P-, and Q-type channels subserved transmitter release from parasympathetic and sympathetic neurons in control mice. In test mice, the component of transmitter release subserved by P-type channels was abolished by four of four LEMS IgG preparations, that subserved by Q-type channels was significantly reduced by three, and that subserved by N-type channels by one. Thus, LEMS IgG impairs transmitter release from parasympathetic and sympathetic neurons through down-regulation of one or more subtypes of voltage-gated calcium channels. The results suggest that antibody-mediated interference with specific ion channel function may also underlie autonomic dysfunction occurring in other autoimmune diseases.

Voltage-dependent binding and calcium channel current inhibition by an anti-alpha 1D subunit antibody in rat dorsal root ganglion neurones and guinea-pig myocytes

1. The presence of calcium channel alpha 1D subunit mRNA in cultured rat dorsal root ganglion (DRG) neurones and guinea-pig cardiac myocytes was demonstrated using the reverse transcriptase-polymerase chain reaction. 2. An antipeptide antibody targeted at a region of the voltage-dependent calcium channel alpha 1D subunit C-terminal to the pore-forming SS1-SS2 loop in domain IV (amino acids 1417-1434) only bound to this exofacial epitope if the DRG neurones and cardiac myocytes were depolarized with 30 mM K+. 3. Incubation of cells under depolarizing conditions for 2-4 h with the antibody resulted in a maximal inhibition of inward current density of 49% (P < 0.005) for DRGs and 30% (P < 0.05) for cardiac myocytes when compared with controls. 4. S-(-)-Bay K 8644 (1 microM) enhanced calcium channel currents in DRGs by 75 +/- 19% (n = 5) in neurones incubated under depolarizing conditions with antibody that had been preabsorbed with its immunizing peptide (100 micrograms ml-1). This was significantly (P < 0.05) larger than the enhancement by S-(-)-Bay K 8644 that was seen with cells incubated under identical conditions but with antibody alone, which was 15 +/- 4% (n = 5). 5. These results demonstrate the presence of calcium channel alpha 1D subunits in rat DRG neurones and guinea-pig cardiac myocytes. They also show that amino acids 1417-1434 of the alpha 1D subunit are only exposed to the extracellular face of the membrane following depolarization and that the binding of an antibody to these amino acids attenuates calcium channel current and reduces the ability of S-(-)-Bay K 8644 to enhance this current, indicating that it is an L-type current that is attenuated.

Reduction of calcium currents by Lambert-Eaton syndrome sera: motoneurons are preferentially affected, and L-type currents are spared

Previous work has demonstrated that Lambert-Eaton syndrome (LES) antibodies reduce calcium currents in nonneuronal cells and neurons and reduce the amplitude of extracellularly recorded currents at mouse motor nerve terminals. We compared effects of LES sera on whole-cell currents of cultured nerve and muscle. LES sera more strongly reduced calcium currents in motoneurons than in sensory neurons. Motoneuronal potassium currents were unaffected. The sera minimally affected calcium currents in skeletal and cardiac muscle. In motoneurons, both low voltage-activated (LVA) and high voltage-activated (HVA) components of calcium current were decreased, demonstrating that the sera targeted more than one calcium channel type. The HVA current remaining in LES-treated motoneurons was little affected by micromolar omega-conotoxin MVIIC but was reduced > 70% by micromolar nimodipine. This pharmacological profile contrasts with untreated cells and suggest that LES sera primarily spare L-type currents in motoneurons.

Lambert-Eaton myasthenic syndrome immunoglobulins react with multiple types of calcium channels in small-cell lung carcinoma

Barium currents through voltage-gated calcium (Ca2+) channels were studied in the small-cell lung carcinoma cell line NCI-H345 using patch clamp techniques. Pharmacological dissection of whole-cell barium currents revealed that 23% of the current was sensitive to nitrendipine, 35% to omega-conotoxin GVIA, and between 10 and 39% to omega-Aga-IVA. This implies that these cells express L-, N-, and P-type calcium channels. Only large cells expressed current that was sensitive to omega-Aga-IVA. The size dependency of this P-type channel expression may reflect the cell cycle stage. Cell-attached recordings revealed three unitary conductances: 5 to 6 pS, 10 to 12 pS, and 20 to 23 pS. The largest conductance channel (20-23 pS) was sensitive to Bay K 8644 and is presumed to represent L-type calcium channels. The frequency of observing the medium conductance channel (10-12 pS) was reduced by exposure to omega-conotoxin GVIA and may represent N-type channels. Incubation of cells with Lambert-Eaton myasthenic syndrome IgG for 24 to 48 hours removed up to 71% of the whole-cell current. Incubation with control human IgG (normal or myasthenia gravis) had no effect. Lambert-Eaton myasthenic syndrome IgG did not selectively target one "presynaptic" type of calcium channel, but rather appeared to target many of the calcium channel types that are expressed on small-cell lung carcinoma cells.

Lambert-Eaton sera reduce low-voltage and high-voltage activated Ca2+ currents in murine dorsal root ganglion neurons

Voltage-gated Ca2+ channels are categorized as either high-voltage activated (HVA) or low-voltage activated (LVA), and a subtype (or subtypes) of HVA Ca2+ channels link the presynaptic depolarization to rapid neuro-transmitter release. Reductions in transmitter release are characteristic of the autoimmune disorder, Lambert-Eaton syndrome (LES). Because antibodies from LES patients reduce Ca2+ influx in a variety of cell types and disrupt the intramembrane organization of active zones at neuromuscular synapses, specificity of LES antibodies for the Ca2+ channels that control transmitter release has been suggested as the mechanism for disease. We tested sera from four patients with LES. Serum samples from three of the four patients reduced both the maximal LVA and HVA Ca2+ conductances in murine dorsal root ganglion neurons. Thus, even though LES is expressed as a neuromuscular and autonomic disorder, our studies suggest that Ca2+ channels may be broadly affected in LES patients. To account for the specificity of disease expression, we suggest that incapacitation of only a fraction of the Ca2+ channels clustered at active zones would severely depress transmitter release. In particular, if several Ca2+ channels in a cluster are normally required to open simultaneously before transmitter release becomes likely, the loss of a few active zone Ca2+ channels would exponentially reduce the probability of transmitter release. This model may explain why LES is expressed as a neuromuscular disorder and can account for a clinical hallmark of LES, facilitation of neuromuscular transmission produced by vigorous voluntary effort.

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.