omega-Conotoxin-sensitive, voltage-operated Ca2+ channels in insulin-secreting cells

The properties of voltage-operated Ca2+ channel subtypes were investigated in insulin-secreting RINm5F cells. Two types of channels were identified: a dihydropyridine-sensitive (L-type) channel, and an omega-conotoxin-sensitive (omega-type) channel. 125I-omega-Conotoxin bound with high affinity (Kd 46.7 pM) to a saturable number of binding sites (10.3 fmol/mg of protein). Toxin binding was not antagonized by L-type channel ligands, but was sensitive to Ca2+ and neomycin. 125I-omega-Conotoxin-labeled Ca2+ channels were recognized by autoantibodies of Lambert-Eaton myasthenic patients. These antibodies are known to be specific for the neuronal omega-type channel. High-voltage-activated Ca2+ currents, investigated with the patch-clamp technique, consisted of a major dihydropyridine-sensitive (L-type) component, and a minor fraction irreversibly blocked by omega-conotoxin. Depolarizing secretagogues, such as D-glyceraldehyde and alanine, induced Ca(2+)-dependent insulin secretion, which was attenuated by omega-conotoxin. Taken together, these results show that voltage-operated Ca2+ channels in insulin-secreting RINm5F cells are heterogeneous and, in particular, that an omega-type channel, pharmacologically, immunologically and electrophysiologically similar to the neuronal omega-type channel, is also expressed in endocrine cells where it might have a role in the control of hormone secretion.

Neuronal-type alpha-bungarotoxin receptors and the alpha 5-nicotinic receptor subunit gene are expressed in neuronal and nonneuronal human cell lines

alpha-Bungarotoxin (alpha Bgtx) is a toxin known to interact with muscle nicotinic receptors and with some neuronal nicotinic receptors. We show that alpha Bgtx binding sites are also expressed in nonmuscle and nonneuronal human cells, including small cell lung carcinoma and several epithelial cell lines. These receptors are immunologically related to the alpha Bgtx receptors of unknown function described in the nervous system and in the IMR32 neuroblastoma cell line and are distinct from muscle nicotinic receptors. We have also cloned from IMR32 cells the human alpha 5-nicotinic receptor subunit, which is supposed to participate in the formation of alpha Bgtx receptors. Transcripts corresponding to the alpha 5-subunit gene were found not only in neuroblastoma cells but also in all the cell lines expressing alpha Bgtx receptors, with the exception of the TE671 cell line, whose nicotinic receptor subunits are of the muscle type. We conclude that both alpha Bgtx receptors and the alpha 5-nicotinic subunit gene are not neuron-specific, as previously thought, but are expressed in a number of human cell lines of various origin.

Physiopathology of neuronal voltage-operated calcium channels

Voltage-operated calcium channels are multimeric transmembrane proteins crucially involved in control of calcium homeostasis. Multiple types of voltage-operated calcium channels have been described in both the nervous system and peripheral tissues. Different channels can be classified according to either their biophysical properties or their pharmacology, biochemical and molecular structure, and localization and functional role. Concentrating on neuronal cells, this paper reviews the different properties of low- and high-voltage activated channels, as well as various attempts to subdivide high-voltage activated channels into different subtypes (L, N, omega, P, etc.). The availability of selective drugs (such as dihydropyridines) and natural toxins (such as omega-Conotoxin, omega-agatoxin, and funnel-web spider toxins), which bind to specific channel subtypes, has greatly helped in channel classification. The emerging view is that there are many members of the family of voltage-operated calcium channels, each with its own molecular structure, a different pharmacology, a different localization, and possibly a different physiological role. Different calcium subtypes are selectively affected in human and animal diseases. The use of omega-Conotoxin has led to identification of the channel subtype (omega) specifically affected in Lambert-Eaton myasthenic syndrome (a human disease of neurotransmission), and has permitted development of new diagnostic approaches to the disease.

Presynaptic localization of omega-conotoxin-sensitive calcium channels at the frog neuromuscular junction

By the use of anti-omega-CTx antibodies in indirect immunofluorescence we demonstrated the presence of omega-CTx binding sites in the presynaptic compartment of frog nerve-muscle preparations. The images we obtained indicate that omega-CTx-sensitive channels are clustered at discrete sites corresponding in distribution to active zones.

Noradrenergic inhibition and voltage-dependent facilitation of omega-conotoxin-sensitive Ca channels in insulin-secreting RINm5F cells

We found that, besides dihydropyridine-sensitive Ca channels, insulin-secreting RINm5F cells also contain a minority (15-25%) of omega-conotoxin (omega-CgTx)-sensitive channels that show a high-affinity binding to [125I] omega-CgTx (Kd 51 pM). Noradrenaline (NA, 10 microM) slows down Ca-channel activation in these cells and produces a sizeable reduction of Ca currents that is relieved by strong pre-conditioning depolarizations (facilitation). The action of NA is mimicked by intracellular application of GTP-gamma-S and is prevented by pertussis toxin (PTX) or by cell pre-incubation with omega-CgTx. This suggests specific noradrenergic inhibition of omega-CgTx-sensitive Ca channels that is modulated by membrane potentials and PTX-sensitive G-protein activation.

Effect of salbutamol on potassium influx in erythrocytes of allergic subjects and investigation of beta 2 receptor autoantibodies

Recent experimental and clinical reports have demonstrated that beta-adrenergic blockade impairs and beta-adrenergic stimulation enhances in vivo extrarenal potassium uptake in man. In some allergic patients extrarenal potassium disposal in vivo was reduced compared with normal subjects. In the present study we report that in vitro salbutamol induced potassium uptake by red blood cells may be reduced in some atopic patients. By using a ligand binding assay on cultured human A431 cells, we tried to determine whether in the sera of these atopic subjects there could be anti beta-adrenergic receptor autoantibodies. The results suggest that the observed reduced response to salbutamol of atopics' red blood cells does not depend on autoantibody activity.

Voltage-operated calcium channels in small cell lung carcinoma cell lines: pharmacological, functional, and immunological properties

Different subtypes of voltage-operated calcium channels (VOCCs) are expressed in different tissues and can be distinguished by functional and pharmacological criteria. One type of high voltage-activated calcium channel, specifically recognized by the peptide neurotoxin omega-conotoxin (omega CTx), is expressed only in neurons. Seven different human small cell lung carcinoma (SCC) cell lines were also found to bind 125I-omega CTx. The binding was specific, saturable, and of high affinity. 125I-omega CTx binding was not antagonized by the calcium channel ligands verapamil, nitrendipine, and diltiazem. There was a correlation between the amount of toxin binding and the detection of depolarization-induced calcium fluxes studied with the fluorimetric probe Fura2. Fura2 experiments also demonstrated that, in addition to omega CTx-sensitive calcium channels, SCC cell lines also expressed omega CTx-insensitive calcium channels, which were antagonized by nitrendipine and verapamil. 125I-omega CTx-labeled VOCCs from SCC cells were, furthermore, precipitated by anti-VOCC autoantibodies obtained from patients affected by the Lambert-Eaton myasthenic syndrome, a neuromuscular disease often associated with SCC. The present findings further indicate the presence of neuronal molecules with important biological function on SCC plasma membrane and add new insights into the pathogenetic mechanism of autoimmune neurological paraneoplastic diseases, like Lambert-Eaton myasthenic syndrome.

Ca currents in human neuroblastoma IMR32 cells: kinetics, permeability and pharmacology

We have investigated the kinetics, permeability and pharmacological properties of Ca channels in in vitro differentiated IMR32 human neuroblastoma cells. The low-threshold (LVA, T) Ca current activated positive to -50 mV and inactivated fully within 100 ms in a voltage-dependent manner. This current persisted in the presence of 3.2 microM omega-conotoxin (omega-CgTx) or 40 microM Cd and showed a weaker sensitivity to Ni and amiloride than in other neurons. The high-threshold Ca currents (HVA,L and N) turned on positive to -30 mV, and inactivated slowly and incompletely during pulses of 200 ms duration. The amplitude of the HVA currents and the number of 125I-omega-CgTx binding sites increased markedly during cell differentiation. In agreement with recent reports, 6.4 microM omega-CgTx blocked only about 85% of the Ba currents through HVA channels in 50% of the cells. Residual omega-CgTx-resistant currents proved to be more sensitive to dihydropyridines (DHP) than total HVA currents. Bay K 8644 (1 microM) had a clear agonistic action on omega-CgTx-resistant currents and was preferred to other Ca antagonists for identifying HVA DHP-sensitive channels. Compared to the omega-CgTx-sensitive, the DHP-sensitive currents turned on at slightly more negative potentials and showed a weaker sensitivity to voltage. The two HVA currents were otherwise hardly distinguishable in terms of activation/inactivation kinetics, Ca/Ba permeability and sensitivity to holding potentials. This suggests that currently used criteria for identifying multiple types of neuronal Ca channels (T;L,N) may be widely misleading if not supported by pharmacological assays.

Action of Ca2+ agonists/antagonists in mammalian peripheral neurons

The action of several ligands on the low- (LVA,T) and high-threshold (HVA,L and N) Ca channels of adult rat sensory neurons and human neuroblastoma IMR32 cells has been investigated. In both cell types, 40 microM Cd2+ and 6.4 microM /omega-Conotoxin (omega-CgTx) selectively blocked the HVA channels, sparing the majority of LVA channels that were antagonized by amiloride and Ni2+. In 50% of the cells, however, /omega-CgTx spared also a 15% of HVA channels that proved to be sensitive to BAY K 8644. The agonistic action of BAY K 8644 on [omega-CgTx-resistant HVA channels caused a large Ba current increase, prolonged current deactivation and acceleration of HVA channels inactivation that was particularly evident in adult rat DRG.

Specificity of calcium channel autoantibodies in Lambert-Eaton myasthenic syndrome

Autoantibodies that interfere with neurotransmitter release by affecting the function of voltage-operated calcium channels (VOCCs) have been found in patients with Lambert-Eaton myasthenic syndrome (LES). To find out the nature of the antigen to which these autoantibodies bind, tests were done with omega-conotoxin, which blocks some types of VOCCs. LES antibodies were able to immunoprecipitate VOCCs prepared from the human neuronal cell line IMR32 which were pre-labelled with the specific VOCC ligand omega-conotoxin. LES autoantibodies are also able to specifically down-regulate the expression of VOCCs in IMR32 cells. A new radioimmunoassay for the quantitative detection of LES antibodies was developed and found to be of value in distinguishing LES patients from patients with myasthenia gravis and some other neurological disorders.

Human neuroblastoma cells acquire regulated secretory properties and different sensitivity to Ca2+ and alpha-latrotoxin after exposure to differentiating agents

IMR-32 human neuroblastoma cells are unable to release [3H]dopamine in response to secretagogues. However, they express a normal complement of membrane receptors and ion channels which are efficiently coupled to second messenger production. In the present study we took advantage of the ability of this cell line to differentiate in vitro in the presence of either dibutyrryl-cAMP or 5-bromodeoxyuridine, to analyze any developmentally regulated changes in its secretory properties. Uptake, storage, and release of [3H]dopamine were studied biochemically and by autoradiography. The calcium ionophore ionomycin, phorbol 12-myristate 13-acetate and the presynaptic acting neurotoxin alpha-latrotoxin were used in both control and differentiated cells as secretagogue agents. The presence of secretory organelles was investigated by electron microscopy; the expression of secretory organelle markers, such as chromogranin/secretogranin proteins (secretory proteins) and synaptophysin (membrane protein), was detected by Western blotting and immunofluorescence. The results obtained indicate that IMR-32 cells acquire regulated secretory properties after in vitro drug-induced differentiation: (a) they assemble "de novo" secretory organelles, as revealed by electron microscopy and detection of secretory organelle markers, and (b) they are able to store [3H]dopamine and to release the neurotransmitter in response to secretagogue stimuli. Furthermore, secretagogue sensitivity was found to be different, depending on the differentiating agent. In fact, dibutyrryl-cAMP treated cells release [3H]dopamine in response to alpha-latrotoxin, but not in response to ionomycin, whereas 5-bromodeoxyuridine treated cells release the neurotransmitter in response to both secretagogues. All together these results suggest that IMR-32 cells represent an adequate model for studying the development of the secretory apparatus in cultured human neurons.

Adenosine receptors linked to adenylate cyclase activity in human neuroblastoma cells: modulation during cell differentiation

In IMR32 neuroblastoma cells, the two adenosine receptor agonists N6-R-phenylisopropyladenosine and 5'-N-ethylcarboxamidoadenosine dose-dependently stimulated membrane adenylate cyclase activity with potencies consistent with the presence of adenosine receptors of the A2-subtype. The S enantiomer of N6-R-phenylisopropyladenosine induced a significantly lower stimulation of adenylate cyclase, accordingly to its lower ability to activate adenosine receptors. These effects were selectively counteracted by the adenosine receptor antagonist theophylline and, conversely, were not affected by the A1-adenosine receptor selective blocker 8-cyclopentyl-1,3-dipropylxanthine. No adenosine receptors belonging to the A1-subtype seem, therefore, to be present in this cell line, as also shown by the lack of inhibitory activity of N6-R-phenylisopropyladenosine on both basal and forskolin-stimulated adenylate cyclase activity. Activation of A2-receptors did not modify intracellular basal calcium levels, did not influence calcium influx through voltage-dependent calcium channels and did not modify calcium influx and redistribution induced by muscarinic receptor activation. Prolonged exposure of cells to either N6-R-phenylisopropyladenosine or 5'-N-ethylcarboxamidoadenosine was associated with a small but significant degree of morphological differentiation, comparable to that induced by dibutyryl cAMP, and therefore presumably related to the prolonged increase of intracellular cAMP levels elicited by the two adenosine agonists. After cellular differentiation induced with either dibutyryl cAMP or 5-bromodeoxyuridine, a selective desensitization of A2-receptor stimulated adenylate cyclase activity was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Omega-conotoxin binding and effects on calcium channel function in human neuroblastoma and rat pheochromocytoma cell lines

Binding of omega-conotoxin, a peptide toxin specific for some subtypes of voltage-operated calcium channels (VOCCs), was investigated in IMR32 neuroblastoma and PC12 pheochromocytoma cell lines. In both cell types, binding was specific, saturable and of high affinity. Association was rapid and dissociation almost non-existent. Dihydropyridines and verapamil failed to affect toxin binding, while high concentrations of CaCl2 completely antagonized it. Depolarization with high K+ induced a [Ca2+]i rise (revealed by the fura-2 fluorimetric technique) that consisted of an initial (0.5-1 min) peak followed by a prolonged (several minutes) plateau phase. omega-Conotoxin blocked mainly the first phase, while the dihydropyridine Ca2+ channel blocker, nitrendipine, primarily affected the plateau. This result suggests that in the two cell lines investigated, omega-conotoxin acts mainly on a subgroup of VOCCs that is resistant to dihydropyridines.

Intracellular calcium homeostasis in a human neuroblastoma cell line: modulation by depolarization, cholinergic receptors, and alpha-latrotoxin

Intracellular calcium homeostasis and its modulation by different agents was studied in control and differentiated IMR32 human neuroblastoma cells by using the Ca2+-sensitive fluorescent dye quin2. The results obtained demonstrate the existence in IMR32 cells of (a) voltage-dependent, verapamil sensitive, Ca2+ channels, which are expressed before differentiation; (b) muscarinic receptors whose activation triggers both Ca2+ influx and Ca2+ redistribution from intracellular stores, whereas nicotinic receptors and alpha-bungarotoxin binding sites do not; and (c) receptors for alpha-latrotoxin (the major toxin of the black widow spider venom), which are well-known markers of the neuronal presynaptic membrane. Up to now, no cell lines of human origin sensitive to this toxin have been identified. These results confirm that IMR32 cells are very convenient model cells for studying specific aspects of the neurochemistry and neurobiology of the human neuron at the molecular and cellular levels.

Two-tiered regulation of spatially patterned engrailed gene expression during Drosophila embryogenesis

A regulatory cascade, initiated during the syncytial stage of embryogenesis, culminates in the striped pattern of engrailed gene expression at the cellular blastoderm stage. The early regulatory genes, for example the pair-rule genes, are expressed transiently and as their products decay a distinct regulatory programme involving segment polarity genes takes over. This late programme maintains and perhaps modifies the striped pattern of engrailed expression through interactions that may involve cell communication.

Effects of long-term in vitro exposure to aluminum, cadmium or lead on differentiation and cholinergic receptor expression in a human neuroblastoma cell line

Neurotoxicity of long-term exposure to lead, aluminum and cadmium has been studied in vitro on the human neuroblastoma cell line IMR32 by measuring cytotoxicity, and the effects on neuronal-specific characteristics such as nitrite outgrowth and expression of cholinergic receptors as parameters of toxicity. Cytotoxicity was highest with cadmium, intermediate with lead and lowest with aluminum exposure. Lead, but not cadmium and aluminum, interfered with neurite growth. The expression of alpha-bungarotoxin binding sites and muscarinic receptors was markedly increased by cadmium and not affected by aluminum exposure. Lead induced only an increase of toxin binding sites. These in vitro modifications are discussed in relation to the possible use of neuronal cell lines for detecting neurotoxic effects of heavy metals.