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.

alpha-Latrotoxin, acting via two Ca2+-dependent pathways, triggers exocytosis of two pools of synaptic vesicles

alpha-Latrotoxin stimulates three types of [(3)H]gamma-aminobutyric acid and [(14)C]glutamate release from synaptosomes. The Ca(2+)-independent component (i) is insensitive to SNAP-25 cleavage or depletion of vesicle contents by bafilomycin A1 and represents transmitter efflux mediated by alpha-latrotoxin pores. Two other components of release are Ca(2+)-dependent and vesicular but rely on distinct mechanisms. The fast receptor-mediated pathway (ii) involves intracellular Ca(2+) stores and acts upon sucrose-sensitive readily releasable vesicles; this mechanism is insensitive to inhibition of phosphatidylinositol 4-kinase (PI 4-kinase). The delayed pore-dependent exocytotic component (iii) is stimulated by Ca(2+) entering through alpha-latrotoxin pores; it requires PI 4-kinase and occurs mainly from depot vesicles. Lanthanum perturbs alpha-latrotoxin pores and blocks the two pore-mediated components (i, iii) but not the receptor-mediated release (ii). alpha-Latrotoxin mutant (LTX(N4C)) cannot form pores and stimulates only the Ca(2+)-dependent receptor-mediated amino acid exocytosis (ii) (detectable biochemically and electrophysiologically). These findings explain experimental data obtained by different laboratories and implicate the toxin receptors in the regulation of the readily releasable pool of synaptic vesicles. Our results also suggest that, similar to noradrenergic vesicles, amino acid-containing vesicles at some point in their cycle require PI 4-kinase.

alpha-latrotoxin forms calcium-permeable membrane pores via interactions with latrophilin or neurexin

In order to explore the mechanisms by which alpha-latrotoxin activates neurotransmitter release, we have characterized its effects by patch-clamp methods on cells heterologously expressing its receptors, latrophilin-1 or neurexin-Ialpha. Application of alpha-latrotoxin (1 nM) to cells expressing rat latrophilin or neurexin, but not mock-transfected cells, induced a cationic conductance. In cells expressing latrophilin, current development was slow in the absence of divalent cations, but was accelerated by Ca2+ or Mg2+. In cells expressing neurexin, alpha-latrotoxin did not elicit currents in the absence of Ca2+. The toxin-induced conductance was rectifying, persistent, permeable to monovalent and divalent cations, but blocked by La3+. Single-channel recording revealed a permanently open state, with the same unitary conductance irrespective of whether cells expressed latrophilin or neurexin. Therefore, while pore formation displayed differences consistent with the reported properties of alpha-latrotoxin binding to latrophilin and neurexin, the pores induced by alpha-latrotoxin had identical properties. These results suggest that after anchoring to either of its nerve terminal receptors, alpha-latrotoxin inserts into the membrane and constitutes a single type of transmembrane ion pore.

[Calcium channels and migraine]

Familial hemiplegic migraine is a hereditary form of migraine in which the aura includes a certain degree of motor deficit. A first gene responsible for this disease was located on chromosome 19 in 1993, and identified in 1996. It encodes the principal alpha 1-subunit of a potential-dependent calcium channel, the P/Q channel, which is selectively expressed in the nervous system. This channel is particularly rich in nerve terminals, where it contributes to the triggering and release of neuromediators. In patients with hemiplegic migraine, mis-sense mutations have been detected which result in a modification of channel function. Other mutations which lead to the synthesis of inactive protein truncations have been described in another disease, type 2 episodic ataxia. In the mouse, mutations in the same gene lead to different phenotypes (tottering mouse, leaner mouse). Finally, possible links between P/Q calcium channel dysfunction and migraine have been discussed.

Ca2+-dependent regulation of synaptic SNARE complex assembly via a calmodulin- and phospholipid-binding domain of synaptobrevin

Synaptic core complex formation is an essential step in exocytosis, and assembly into a superhelical structure may drive synaptic vesicle fusion. To ascertain how Ca(2+) could regulate this process, we examined calmodulin binding to recombinant core complex components. Surface plasmon resonance and pull-down assays revealed Ca(2+)-dependent calmodulin binding (K(d) = 500 nM) to glutathione S-transferase fusion proteins containing synaptobrevin (VAMP 2) domains but not to syntaxin 1 or synaptosomal-associated protein of 25 kDa (SNAP-25). Deletion mutations, tetanus toxin cleavage, and peptide synthesis localized the calmodulin-binding domain to VAMP(77-94), immediately C-terminal to the tetanus toxin cleavage site (Q(76)-F(77)). In isolated synaptic vesicles, Ca(2+)/calmodulin protected native membrane-inserted VAMP from proteolysis by tetanus toxin. Assembly of a (35)S-SNAP-25, syntaxin 1 GST-VAMP(1-96) complex was inhibited by Ca(2+)/calmodulin, but assembly did not mask subsequent accessibility of the calmodulin-binding domain. The same domain contains a predicted phospholipid interaction site. SPR revealed calcium-independent interactions between VAMP(77-94) and liposomes containing phosphatidylserine, which blocked calmodulin binding. Circular dichroism spectroscopy demonstrated that the calmodulin/phospholipid-binding peptide displayed a significant increase in alphahelical content in a hydrophobic environment. These data provide insight into the mechanisms by which Ca(2+) may regulate synaptic core complex assembly and protein interactions with membrane bilayers during exocytosis.

Humoral immunity against glutamic acid decarboxylase and tyrosine phosphatase IA-2 in Lambert-Eaton myasthenic syndrome

Some beta-cell-specific autoantigens also are present in the central nervous system. Furthermore, stiff man syndrome, an autoimmune neurological disease, is frequently associated with diabetes and shares with this one an anti-GAD and IA-2 humoral immunoreactivity. We wondered whether these autoantibodies could be found in other neurological diseases with a present or supposed autoimmune origin. So, anti-GAD65 (GAD65A) and anti-IA-2 (IA-2A) autoantibodies were assayed in various neurological diseases. There was a higher prevalence of such antibodies in Lambert-Eaton myasthenic syndrome (LEMS) (GAD65A, 35%; IA-2A, 21%; double positivity, 18%) compared to amyotrophic lateral sclerosis (18%, 12%, and 12%, respectively) and multiple sclerosis (10%, 3%, and 3%, respectively). In LEMS, the humoral reaction was more frequent and/or appeared earlier in the paraneoplastic forms. The detection of such autoantibodies in patients with small-cell lung carcinoma (SCLC) without LEMS suggests that these autoantigens, GAD65 and IA-2, could be produced by SCLC tissue.

Synaptotagmins in membrane traffic: which vesicles do the tagmins tag?

The aim of this review is to give a broad picture of what is actually known about the synaptotagmin family. Synaptotagmin I is an abundant synaptic vesicle and secretory granule protein in neurons and endocrine cells which plays a key role in Ca(2+)-induced exocytosis. It belongs to the large family of C2 domain-proteins as it contains two internal repeats that have homology to the C2 domain of protein kinase C. Eleven synaptotagmin genes have been described in rat and mouse. Except for synaptotagmin I, and by analogy synaptotagmin II, the functions of these proteins are unknown. In this review we focus on data obtained on the various isoforms without exhaustively discussing the role of synaptotagmin I in neurotransmission. Numerous in vitro interactions of synaptotagmin I with key components of the exocytosis-endocytosis machinery have been reported. Additional data concerning the other synaptotagmins are now becoming available and are reviewed here. Only interactions which have been described for several synaptotagmins, are mentioned. It is unlikely that a single isoform displays all of these potential interactions in vivo and probably the subcellular distribution of the protein will favor some of them and preclude others. Therefore, to discuss the putative role of the various synaptotagmins we have examined in detail published data concerning their localization.

Synaptotagmin I and IV define distinct populations of neuronal transport vesicles

Mammalian synaptotagmins constitute a multigene family of at least 11 membrane proteins. We have characterized synaptotagmin IV using antibodies directed against the C2A domain of the protein. Antibodies reacted specifically with a protein band that migrated as a 41-44 kDa doublet. Synaptotagmin IV expression was regulated throughout development. A strong decrease in the amount detected by Western blotting occurred between postnatal day 5 and adulthood, in agreement with studies on the expression of synaptotagmin IV transcripts. In subcellular fractionation, synaptotagmin IV was not detected in the synaptic vesicle-enriched fraction. Immunofluorescence microscopy was concordant with this finding. In 6-day-old rat cerebellum and cultured hippocampal neurons the subcellular distribution of synaptotagmin IV was clearly different from that of synaptotagmin I. Synaptotagmin IV displayed a punctate non-polarized distribution on neuronal extensions, whereas synaptotagmin I staining was essentially synaptic. Synaptotagmin IV staining was also observed in the soma in strong perinuclear fluorescent puncta superimposed on that of Golgi/TGN markers. Furthermore, synaptotagmin IV was seen in the proximal part of the growth cone domain and not in the microfilament-rich region which includes filopodia. Co-localizations with the adhesion molecules vinculin and zyxin at the proximal part of growth cones were observed. Synaptotagmin IV may thus be involved in the regulation of specific membrane-trafficking pathways during brain development.

Direct interaction between synaptotagmin and the intracellular loop I-II of neuronal voltage-sensitive sodium channels

Synaptotagmin, a synaptic vesicle protein involved in Ca(2+)-regulated exocytosis, displayed direct high affinity interaction with neuronal sodium channels. Monoclonal antibodies directed against synaptotagmins I and II adsorbed in a concentration-dependent and -specific manner [(3)H]saxitoxin prelabeled sodium channels extracted with detergent from nerve endings. Conversely, co-immunoprecipitation of synaptotagmin was achieved by antibodies against sodium channel subunits. Consistent with the co-immunoprecipitation assays, solubilized [(3)H]saxitoxin-prelabeled sodium channels were trapped on immobilized maltose binding protein (MBP)-synaptotagmin I. In vitro recombinant protein assays were employed to identify the interaction site of synaptotagmin I, which was located on the cytoplasmic loop between domains I and II of the sodium channel alphaIIA subunit. The co-immunoprecipitated synaptotagmin-sodium channel complexes were found to be Ca(2+)-dependent; this effect was mimicked by Ba(2+) and Sr(2+) but not Mg(2+). Finally the complex was shown to be distinct from the synaptotagmin-SNARE protein complex that can selectively interact with presynaptic calcium channels (N and P/Q types). Thus, our findings demonstrate an unexpected and direct interaction between sodium channels and synaptotagmin. The Ca(2+)-regulated association between sodium channels and a protein implicated in vesicular fusion may have intriguing consequences for the establishment and regulation of neuronal excitability.

Calcium-dependent dissociation of synaptotagmin from synaptic SNARE complexes

The formation of the synaptic core (SNARE) complex constitutes a crucial step in synaptic vesicle fusion at the nerve terminal. The interaction of synaptotagmin I with this complex potentially provides a means of conferring Ca2+-dependent regulation of exocytosis. However, the subcellular compartments in which interactions occur and their modulation by Ca2+ influx remain obscure. Sodium dodecyl sulfate (SDS)-resistant core complexes, associated with synaptotagmin I, were enriched in rat brain fractions containing plasma membranes and docked synaptic vesicles. Depolarization of synaptosomes triggered [3H]GABA release and Ca2+-dependent dissociation of synaptotagmin from the core complex. In perforated synaptosomes, synaptotagmin dissociation was induced by Ca2+ (30-300 microM) but not Sr2+ (1 mM); it apparently required intact membrane bilayers but did not result in disassembly of trimeric SNARE complexes. Synaptotagmin was not associated with unstable v-SNARE/t-SNARE complexes, present in fractions containing synaptic vesicles and cytoplasm. These complexes acquired SDS resistance when N-ethylmaleimide-sensitive fusion protein (NSF) was inhibited with N-ethylmaleimide or adenosine 5'-O-(3-thiotriphosphate), suggesting that constitutive SNARE complex disassembly occurs in undocked synaptic vesicles. Our findings are consistent with models in which the Ca2+ triggered release of synaptotagmin precedes vesicle fusion. NSF may then dissociate ternary core complexes captured by endocytosis and recycle/prime individual SNARE proteins.

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

Lambert-Eaton myasthenic syndrome is an autoimmune disease that impairs neuromuscular transmission. Several studies suggest that neurotransmitter release is reduced by an immune response directed against the calcium channel complex of nerve terminals. The immunoglobulin G fractions from Lambert-Eaton myasthenic syndrome patients immunoprecipitate solubilized neuronal N- and P/Q-type channels and in certain cases brain, skeletal and cardiac muscle L-type channels [El Far O. et al. (1995) J. Neurochem. 64, 1696-1702; Lennon V. A. and Lambert E. H. (1989) Mayo Clin. Proc. 64, 1498-1504; Sher E. et al. (1989) Lancet ii, 640-643; Suenaga A. et al. (1996) Muscle Nerve 19, 1166-1168]. These channel immunoprecipitation assays are considered as useful for the diagnosis of this syndrome. In this study, we demonstrate that two predominant neuronal voltage-dependent calcium channel beta subunits (beta3 and beta4, of mol. wt 58,000) are general targets of Lambert-Eaton myasthenic syndrome autoantibodies. Of 20 disease sera tested, 55% were able to immunoprecipitate 35S-labeled beta subunits. All five patients affected with small-cell lung carcinoma were positive for the beta-subunit immunoprecipitation assay. Interestingly, only a fraction of the beta-subunit-positive sera was also able to immunoprecipitate N- and P/Q-type channels, suggesting that several of the beta-subunit epitopes are masked in native channels. In accordance with this observation, we found that several beta-positive sera were able to prevent the interaction between calcium channel alpha1 and beta subunits in vitro. In cases where sera were able to immunoprecipitate beta subunits, N- and P/Q-type channels, the immunoprecipitation of both channel types was either partially or entirely mediated by beta-subunit antibodies. Our results suggest that assays based on the immunoprecipitation of beta subunits can be used as an additional test to assist in the diagnosis of Lambert-Eaton myasthenic syndrome.

Interactions between proteins implicated in exocytosis and voltage-gated calcium channels

Neurotransmitter release from synaptic vesicles is triggered by voltage-gated calcium influx through P/Q-type or N-type calcium channels. Purification of N-type channels from rat brain synaptosomes initially suggested molecular interactions between calcium channels and two key proteins implicated in exocytosis: synaptotagmin I and syntaxin 1. Co-immunoprecipitation experiments were consistent with the hypothesis that both N- and P/Q-type calcium channels, but not L-type channels, are associated with the 7S complex containing syntaxin 1, SNAP-25, VAMP and synaptotagmin I or II. Immunofluorescence confocal microscopy at the frog neuromuscular junction confirmed that calcium channels, syntaxin 1 and SNAP-25 are co-localized at active zones of the presynaptic plasma membrane where transmitter release occurs. Experiments with recombinant proteins were performed to map synaptic protein interaction sites on the alpha 1A subunit, which forms the pore of the P/Q-type calcium channel. In vitro-translated 35S-synaptotagmin I bound to a site located on the cytoplasmic loop linking homologous domains II and III of the alpha 1A subunit. This direct link would target synaptotagmin, a putative calcium sensor for exocytosis, to a microdomain of calcium influx close to the channel mouth. Cysteine string proteins (CSPs) contain a J-domain characteristic of molecular chaperones that cooperate with Hsp70. They are located on synaptic vesicles and thought to be involved in modulating the activity of presynaptic calcium channels. CSPs were found to bind to the same domain of the calcium channel as synaptotagmin, and also to associate with VAMP. CSPs may act as molecular chaperones in association with Hsp70 to direct assembly or dissociation of multiprotein complexes at the calcium channel.

Interactions between presynaptic calcium channels and proteins implicated in synaptic vesicle trafficking and exocytosis

Monoclonal antibodies were generated by immunizing mice with chick brain synaptic membranes and screening for immunoprecipitation of solubilized omega conotoxin GVIA receptors (N-type calcium channels). Antibodies against two synaptic proteins (p35--syntaxin 1 and p58--synaptotagmin) were produced and used to purify and characterize a ternary complex containing N-type channels associated with these two proteins. These results provided the first evidence for a specific interaction between presynaptic calcium channels and SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. Immunoprecipitation experiments supported the conclusion that syntaxin 1/SNAP-25/VAMP/synaptotagmin I or II complexes associate with N-type, P/Q-type, but not L-type calcium channels from rat brain nerve terminals. Immunofluorescent confocal microscopy at the frog neuromuscular junction was consistent with the co-localization of syntaxin 1, SNAP-25, and calcium channels, all of which are predominantly expressed at active zones of the presynaptic plasma membrane facing post-synaptic folds rich in acetylcholine receptors. The interaction of proteins implicated in calcium-dependent exocytosis with presynaptic calcium channels may locate the sensor(s) that trigger vesicle fusion within a microdomain of calcium entry.

Interaction of cysteine string proteins with the alpha1A subunit of the P/Q-type calcium channel

Cysteine string proteins (Csps) are J-domain chaperone proteins anchored at the surface of synaptic vesicles. Csps are involved in neurotransmitter release and may modulate presynaptic calcium channel activity, although the molecular mechanisms are unknown. Interactions between Csps, proteins of the synaptic core (SNARE) complex, and P/Q-type calcium channels were therefore explored. Co-immunoprecipitation suggested that Csps occur in complexes containing synaptobrevin (VAMP), but not syntaxin 1, SNAP-25, nor P/Q-type calcium channels labeled with 125I-omega-conotoxin MVIIC. However binding experiments with 35S-labeled Csp1 demonstrated an interaction (apparent KD = 700 nM at pH 7.4 and 4 degreesC) with a fusion protein containing a segment of the cytoplasmic loop linking homologous domains II-III of the alpha1A calcium channel subunit (BI isoform, residues 780-969). Binding was specific as it was displaced by unlabeled Csp1, and no interactions were detected with fusion proteins containing other calcium channel domains, VAMP, or syntaxin 1A. A Csp binding site on the P/Q-type calcium channel is thus located within the 200 residue synaptic protein interaction site that can also bind syntaxin I, SNAP-25, and synaptotagmin I. Csp may act as a molecular chaperone to direct assembly or disassembly of exocytotic complexes at the calcium channel.

Binding of chimeric analogs of omega-conotoxin MVIIA and MVIIC to the N- and P/Q-type calcium channels

Despite their high sequence homology, the peptide neurotoxins omega-conotoxin MVIIA and MVIIC selectively block N- and P/Q-type calcium channels, respectively. To study the recognition mechanism of calcium channel subtypes, two chimeric analogs of omega-conotoxin MVIIA and MVIIC were synthesized by exchanging their N- and C-terminal halves. Binding assay for both N- and P/Q-type calcium channels showed that amino acid residues restricted to the N-terminal half are important for the recognition of N-type channels, whereas essential residues for P/Q-type channel recognition are widely spread over the whole omega-conotoxin molecule.

Immunoassays fail to detect antibodies against neuronal calcium channels in amyotrophic lateral sclerosis serum

Recent studies suggested that autoantibodies that bind to voltage-dependent calcium channels and activate calcium entry may play a role in the progressive degeneration of motoneurons in sporadic amyotrophic lateral sclerosis. Immunoassays were performed to assess autoantibody titer in patients with amyotrophic lateral sclerosis or Lambert-Eaton myasthenic syndrome, a disease in which the presence of anti-calcium channel antibodies is well documented. Based on immunoprecipitation assays for antibodies against N-type calcium channels, only 8% (2/25) of amyotrophic lateral sclerosis patients had marginally positive titers, whereas 58% (18/31) of patients with Lambert-Eaton myasthenic syndrome had positive titers. Enzyme-linked immunosorbent assays with purified neuronal N-type calcium channels revealed immunoreactivity in 2 of 25 amyotrophic lateral sclerosis sera and 12 of 31 Lambert-Eaton myasthenic syndrome sera, which is not compatible with suggestions that enzyme-linked immunosorbent assay is a more sensitive technique for the detection of autoantibodies in amyotrophic lateral sclerosis. Furthermore, based on immunoprecipitation assays, amyotrophic lateral sclerosis sera were totally negative for antibodies against L-type calcium channels from skeletal muscle or brain. These data do not support the hypothesis that an autoimmune response against calcium channels plays a primary role in amyotrophic lateral sclerosis.

Interaction of SNARE complexes with P/Q-type calcium channels in rat cerebellar synaptosomes

P- and Q-type calcium channels, which trigger rapid neurotransmitter release at many mammalian synapses, are blocked by omega-conotoxin MVIIC. 125I-omega-Conotoxin MVIIC binding to rat cerebellar synaptosomes was not displaced by omega-conotoxins GVIA or MVIIA (Ki > 1 microM), which are selective for N-type calcium channels. Solubilized 125I-omega-conotoxin MVIIC receptors were specifically recognized by antibodies directed against alpha1A calcium channel subunits, proteins known to constitute a pore with P/Q-like channel properties. Antibodies against syntaxin 1, SNAP 25, and VAMP 2 (synaptobrevin) each immunoprecipitated a similar fraction (20-40%) of omega-conotoxin MVIIC receptors. Immunoprecipitation was not additive, suggesting that heterotrimeric (SNARE) complexes containing these three proteins interact with P/Q-type calcium channels. Immobilized monoclonal anti-syntaxin antibodies retained alpha1A calcium channel subunits of 220, 180 and 160 kDa monitored by immunoblotting with site directed antibodies. Synaptotagmin was detected in channel-associated complexes, but not synaptophysin, Rab 3A nor rat cysteine string protein. Trimeric SNARE complexes are implicated in calcium-dependent exocytosis, a process thought to be regulated by synaptotagmin. Our results indicate that these proteins interact with P/Q-type calcium channels, which may optimize their location within domains of calcium influx.

Cellular localization of synaptotagmin I, II, and III mRNAs in the central nervous system and pituitary and adrenal glands of the rat

Three isoforms of synaptotagmin, a synaptic vesicle protein involved in neurotransmitter release, have been characterized in the rat, although functional differences between these isoforms have not been reported. In situ hybridization was used to define the localization of synaptotagmin I, II, and III transcripts in the rat CNS and pituitary and adrenal glands. Each of the three synaptotagmin genes has a unique expression pattern. The synaptotagmin III gene is expressed in most neurons, but transcripts are much less abundant than the products of the synaptotagmin I and II genes. A majority of neurons in the forebrain expressed both synaptotagmin I and III mRNAs while synaptotagmin II gene expression was confined to subsets of neurons in layers IV-VI of the cerebral cortex, in the dentate granule cell region, the hilus, and the CA1-CA3 areas of the hippocampus. In the cerebellum, all three transcripts were visualized in the granule cell layer. Furthermore, synaptotagmin I probes revealed striking differences between distinct populations of neurons, as in addition to moderate labeling of granule cells, much more prominent hybridization signals were detected on scattered cell bodies likely to be Golgi interneurons. In the most caudal part of the brain, synaptotagmin II transcripts were abundant and were coexpressed with synaptotagmin III mRNAs. This pattern was found in putative motoneurons of the spinal cord, suggesting that the two isoforms might be involved in exocytosis at the neuromuscular junction. Only synaptotagmin I mRNAs were detected in the anterior and intermediate pituitary and in adrenal medullary cells. These data reveal an unexpectedly subtle segregation of the expression of synaptotagmin genes and the existence of multiple combinations of synaptotagmin isoforms which may provide diversity in the regulation of neurosecretion.

Properties of omega conotoxin MVIIC receptors associated with alpha 1A calcium channel subunits in rat brain

Solubilized 125I-omega conotoxin MVIIC receptors from rat cerebellum were immunoprecipitated by antibodies directed against the calcium channel alpha 1A subunit. Anti-alpha 1A antibodies recognized a 240-220, 180 and 160 kDa proteins in immunoblots of cerebellar membranes. Disuccinimidyl suberate cross-linked 125I-omega conotoxin MVIIC to an alpha 2 delta-like 200-180 kDa subunit, which migrated at 150-140 kDa after disulfide reduction. These observations are consistent with a heteromeric structure in which high affinity omega conotoxin MVIIC binding sites formed by alpha 1A subunits are located in close proximity to peripheral alpha 2 subunits.

Binding of Lambert-Eaton myasthenic syndrome IgG to synaptosomal proteins does not correlate with an inhibition of calcium uptake

The immunochemical and functional properties of IgG fractions from patients with Lambert-Eaton myasthenic syndrome (LEMS) were examined in chick and rat synaptosomes. LEMS IgG immunoprecipitated 125I-omega conotoxin GVIA (125I-omega CgTx) labeled N-type calcium channels solubilized from both tissues, and reacted with a 65 kDa protein band in immunoblots of rat synaptosomes. Depolarization-induced 45Ca2+ influx into chick synaptosomes was partially inhibited by omega CgTx, whereas influx into rat synaptosomes was insensitive to omega CgTx. No effect of LEMS sera or IgG on 45Ca2+ uptake was apparent in either preparation.

Inositol phosphate regulation of voltage-dependent calcium channels in cerebellar granule neurons

The effects of intracellularly applied inositol phosphates on voltage-dependent calcium channel currents were assessed in rat cerebellar neurons using the whole-cell recording configuration of the patch-clamp technique. Intraneuronal perfusion of 10 microM inositol 1,4,5-trisphosphate (IP3) increased the amplitude of currents elicited by depolarization from a holding potential of -40 mV. IP3 did not modify current activation, but shifted the steady-state inactivation curve toward more positive values. The dose-response curve indicated an EC50 of 0.5 microM for IP3. Inositol 1,3,4,5-tetrakisphosphate (IP4), but not inositol 4,5,-bisphosphate, mimicked the effect of IP3. The effect of IP3 persisted in the presence of 100 micrograms/ml heparin and did not depend on intracellular calcium mobilization, as similar responses were not produced by 10 mM caffeine or by intrapipette calcium buffering at pCa 6 instead of pCa 7.7. Preincubation with omega-conotoxin led to a 55% inhibition of barium current; however, inhibition was reversed by IP3, which reestablished the control current amplitude. These results imply that IP3 and IP4 can elicit calcium entry by modifying both the gating characteristics and the pharmacological properties of voltage-dependent calcium channels.

Polypeptide components of the apamin receptor associated with a calcium activated potassium channel

Photoaffinity labeling of rat brain membranes with [125I]ANPAA-apamin incorporated radioactivity into polypeptides of 86 and 59 kDa and occasionally a more weakly labeled component of 45 kDa. These polypeptides were immunoprecipitated with anti-apamin antibodies and treated with glycosidases. Neither the 86 nor the 59 kDa polypeptide appeared to be N-glycosylated. Partial proteolytic mapping of affinity labeled polypeptides with chymotrypsin or V8 protease generated an identical pattern. These results suggest that the 59 and 45 kDa components are not additional subunits of an oligomeric protein but result from cleavage of the 86 kDa polypeptide.

Subtypes of voltage-sensitive calcium channels in cultured rat brain neurons

Subtypes of voltage-sensitive calcium channels have been investigated in cultured rat brain neurons using two classes of specific probes, dihydropyridine compounds and omega-conotoxin. Membranes prepared from cultured neurons contain specific binding sites for [3H]PN200-110, a dihydropyridine antagonist, and for 125I-omega-conotoxin with a stoichiometry of about 1:1. A depolarization induced 45Ca2+ influx into intact brain neurons was partially inhibited by a dihydropyridine antagonist, nifedipine and stimulated by a dihydropyridine agonist, Bay K8644. This dihydropyridine sensitive 45Ca2+ flux was insensitive to omega-conotoxin at concentrations which saturate the specific toxin binding sites indicating that in cultured brain neurons, dihydropyridine-sensitive calcium channels are not sensitive to omega-conotoxin.