Sodium channel polypeptides in central nervous systems of various insects identified with site directed antibodies

Genetic Dissection of Aversive Associative Olfactory Learning and Memory in Drosophila Larvae

Memory formation is a highly complex and dynamic process. It consists of different phases, which depend on various neuronal and molecular mechanisms. In adult Drosophila it was shown that memory formation after aversive Pavlovian conditioning includes—besides other forms—a labile short-term component that consolidates within hours to a longer-lasting memory. Accordingly, memory formation requires the timely controlled action of different neuronal circuits, neurotransmitters, neuromodulators and molecules that were initially identified by classical forward genetic approaches. Compared to adult Drosophila, memory formation was only sporadically analyzed at its larval stage. Here we deconstruct the larval mnemonic organization after aversive olfactory conditioning. We show that after odor-high salt conditioning larvae form two parallel memory phases; a short lasting component that depends on cyclic adenosine 3’5’-monophosphate (cAMP) signaling and synapsin gene function. In addition, we show for the first time for Drosophila larvae an anesthesia resistant component, which relies on radish and bruchpilot gene function, protein kinase C activity, requires presynaptic output of mushroom body Kenyon cells and dopamine function. Given the numerical simplicity of the larval nervous system this work offers a unique prospect for studying memory formation of defined specifications, at full-brain scope with single-cell, and single-synapse resolution.

Learning and memory helps organisms to predict and adapt to events in their environment. Gained experience leaves traces of memory in the nervous system. Yet, memory formation in vertebrates and invertebrates is a highly complex and dynamic process that consists of different phases, which depend on various neuronal and molecular mechanisms. To understand which changes occur in a brain when it learns, we applied a reductionist approach. Instead of studying complex cases, we analyzed learning and memory in Drosophila larvae that have a simple brain that is genetically and behaviorally accessible and consists of only about 10,000 neurons. Drosophila larvae are able to learn to associate an odor with punishing high salt concentrations. It is therefore possible to correlate changes in larval behavior with molecular events in identifiable neurons after classical olfactory conditioning. We show that under these circumstances larvae form two parallel memory phases; a short lasting component (lSTM) that is molecularly conserved throughout the animal kingdom as it depends on the classical cAMP pathway. In parallel they establish a larval anesthesia resistant memory (lARM) that relies on a different molecular signal. lARM has not been described in larvae before.

Dynamics of learning-related cAMP signaling and stimulus integration in the Drosophila olfactory pathway

Functional imaging with genetically encoded calcium and cAMP reporters was used to examine the signal integration underlying learning in Drosophila. Dopamine and octopamine modulated intracellular cAMP in spatially distinct patterns in mushroom body neurons. Pairing of neuronal depolarization with subsequent dopamine application revealed a synergistic increase in cAMP in the mushroom body lobes, which was dependent on the rutabaga adenylyl cyclase. This synergy was restricted to the axons of mushroom body neurons, and occurred only following forward pairing with time intervals similar to those required for behavioral conditioning. In contrast, forward pairing of neuronal depolarization and octopamine produced a subadditive effect on cAMP. Finally, elevating intracellular cAMP facilitated calcium transients in mushroom body neurons, suggesting that cAMP elevation is sufficient to induce presynaptic plasticity. These data suggest that rutabaga functions as a coincidence detector in an intact neuronal circuit, with dopamine and octopamine bidirectionally influencing the generation of cAMP.

An insecticidal peptide from the theraposid Brachypelma smithi spider venom reveals common molecular features among spider species from different genera

The soluble venom of the Mexican theraposid spider Brachypelma smithi was screened for insecticidal peptides based on toxicity to house crickets. An insecticidal peptide, named Bs1 (which stands for Brachypelma smithi toxin 1) was obtained in homogeneous form after the soluble venom was fractionated using reverse-phase and cation-exchange chromatography. It contains 41 amino acids cross-linked by three disulfide bridges. Its sequence is similar to an insecticidal peptide isolated from the theraposid spider Ornithoctonus huwena from China, and another from the hexathelid spider Macrothelegigas from Japan, indicating that they are phylogenetically related. A cDNA library was prepared from the venomous glands of B. smithi and the gene that code for Bs1 was cloned. Sequence analysis of the nucleotides of Bs1 showed similarities to that of the hexathelid spider from Japan proving additional evidence for close genetic relationship between these spider peptides. The mRNAs of these toxins code for signal peptides that are processed at the segment rich in acidic and basic residues. Their C-terminal amino acids are amidated. However, they contain only a glycine residue at the most C-terminal position, without the presence of additional basic amino acid residues, normally required for post-translation processing of other toxins reported in the literature. The possible mechanism of action of Bs1 was investigated using several ion channels as putative receptors. Bs1 had minor, but significant effects on the Para/tipE insect ion channel, which could indirectly correlate with the observed lethal activity to crickets.

The differential preference of scorpion α-toxins for insect or mammalian sodium channels: Implications for improved insect control

Receptor site-3 on voltage-gated sodium channels is targeted by a variety of structurally distinct toxins from scorpions, sea anemones, and spiders whose typical action is the inhibition of sodium current inactivation. This site interacts allosterically with other topologically distinct receptors that bind alkaloids, lipophilic polyether toxins, pyrethroids, and site-4 scorpion toxins. These features suggest that design of insecticides with specificity for site-3 might be rewarding due to the positive cooperativity with other toxins or insecticidal agents. Yet, despite the central role of scorpion alpha-toxins in envenomation and their vast use in the study of channel functions, molecular details on site-3 are scarce. Scorpion alpha-toxins vary greatly in preference for sodium channels of insects and mammals, and some of them are highly active on insects. This implies that despite its commonality, receptor site-3 varies on insect vs. mammalian channels, and that elucidation of these differences could potentially be exploited for manipulation of toxin preference. This review provides current perspectives on (i) the classification of scorpion alpha-toxins, (ii) their mode of interaction with sodium channels and pharmacological divergence, (iii) molecular details on their bioactive surfaces and differences associated with preference for channel subtypes, as well as (iv) a summary of the present knowledge about elements involved in constituting receptor site-3. These details, combined with the variations in allosteric interactions between site-3 and the other receptor sites on insect and mammalian sodium channels, may be useful in new strategies of insect control and future design of anti-insect selective ligands.

A steroid hormone affects sodium channel expression in Manduca central neurons

Neuronal differentiation is characterized by stereotypical sequences of membrane channel and receptor acquisition. This is regulated by the coordinated interactions of a variety of developmental mechanisms, one of which is the control by steroid hormones. We have used the metamorphosis of the holometabolous insect, Manduca sexta, as a model to study effects of 20-hydroxyecdysone on the maturation of thoracic neuron membrane channel expression. To test for direct hormone action, neurons were dissociated into primary cell culture on the first day of pupal life. In situ hybridization demonstrated that the amount of expression of the acetylcholine receptor alpha subunit, MARA1, was not affected by 20-hydroxyecdysone. Immunocytochemistry with an antibody directed against the SP19 segment of voltage-gated sodium channels revealed no effect of 20-hydroxyecdysone treatment during the first 6 days in culture. SP19 sodium channel protein was evenly distributed along all neurites. In contrast, after 8 days in culture, 20-hydroxyecdysone increased the amount of SP19 protein expression and strongly affected its distribution in differentiating neurons. In the presence of 20-hydroxyecdysone, patches of high densities of SP19 sodium channel protein were found in growth cones close to the base of filopodia. This is a further step toward unraveling the blend of membrane proteins under the control of steroids during the development of the central nervous system of postembryonic Manduca. Our results, taken together with previous studies, indicate that 20-hydroxyecdysone does not affect the expression of potassium membrane current or of the nicotinic acetylcholine receptor but instead regulates the amplitude of the calcium membrane current and the amount and distribution of SP19 sodium channel protein.

Localization of cardiac sodium channels in caveolin-rich membrane domains: regulation of sodium current amplitude

This study demonstrates that caveolae, omega-shaped membrane invaginations, are involved in cardiac sodium channel regulation by a mechanism involving the alpha subunit of the stimulatory heterotrimeric G-protein, Galpha(s), via stimulation of the cell surface beta-adrenergic receptor. Stimulation of beta-adrenergic receptors with 10 micromol/L isoproterenol in the presence of a protein kinase A inhibitor increased the whole-cell sodium current by a "direct" cAMP-independent G-protein mechanism. The addition of antibodies against caveolin-3 to the cell's cytoplasm via the pipette solution abrogated this direct G protein-induced increase in sodium current, whereas antibodies to caveolin-1 or caveolin-2 did not. Voltage-gated sodium channel proteins were found to associate with caveolin-rich membranes obtained by detergent-free buoyant density separation. The purity of the caveolar membrane fraction was verified by Western blot analyses, which indicated that endoplasmic/sarcoplasmic reticulum, endosomal compartments, Golgi apparatus, clathrin-coated vesicles, and sarcolemmal membranes were excluded from the caveolin-rich membrane fraction. Additionally, the sodium channel was found to colocalize with caveolar membranes by immunoprecipitation, indirect immunofluorescence, and immunogold transmission electron microscopy. These results suggest that stimulation of beta-adrenergic receptors, and thereby Galpha(s), promotes the presentation of cardiac sodium channels associated with caveolar membranes to the sarcolemma.

Variations in receptor site-3 on rat brain and insect sodium channels highlighted by binding of a funnel-web spider delta-atracotoxin

Delta-atracotoxins (delta-ACTXs) from Australian funnel-web spiders differ structurally from scorpion alpha-toxins (Sc(alpha)Tx) but similarly slow sodium current inactivation and compete for their binding to sodium channels at receptor site-3. Characterization of the binding of 125I-labelled delta-ACTX-Hv1a to various sodium channels reveals a decrease in affinity for depolarized (0 mV; Kd=6.5 +/- 1.4 nm) vs.polarized (-55 mV; Kd=0.6 +/- 0.2 nm) rat brain synaptosomes. The increased Kd under depolarized conditions correlates with a 4.3-fold reduction in the association rate and a 1.8-increase in the dissociation rate. In comparison, Sc(alpha)Tx binding affinity decreased 33-fold under depolarized conditions due to a 48-fold reduction in the association rate. The binding of 125I-labelled delta-ACTX-Hv1a to rat brain synaptosomes is inhibited competitively by classical Sc(alpha)Txs and allosterically by brevetoxin-1, similar to Sc(alpha)Tx binding. However, in contrast with classical Sc(alpha)Txs, 125I-labelled delta-ACTX-Hv1a binds with high affinity to cockroach Na+ channels (Kd=0.42 +/- 0.1 nm) and is displaced by the Sc(alpha)Tx, Lqh(alpha)IT, a well-defined ligand of insect sodium channel receptor site-3. However, delta-ACTX-Hv1a exhibits a surprisingly low binding affinity to locust sodium channels. Thus, unlike Sc(alpha)Txs, which are capable of differentiating between mammalian and insect sodium channels, delta-ACTXs differentiate between various insect sodium channels but bind with similar high affinity to rat brain and cockroach channels. Structural comparison of delta-ACTX-Hv1a to Sc(alpha)Txs suggests a similar putative bioactive surface but a 'slimmer' overall shape of the spider toxin. A slimmer shape may ease the interaction with the cockroach and mammalian receptor site-3 and facilitate its association with different conformations of the rat brain receptor, correlated with closed/open and slow-inactivated channel states.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Structural implications on the interaction of scorpion alpha-like toxins with the sodium channel receptor site inferred from toxin iodination and pH-dependent binding

The alpha-like toxin from the venom of the scorpion Leiurus quinquestriatus hebraeus (Lqh III) binds with high affinity to receptor site 3 on insect sodium channels but does not bind to rat brain synaptosomes. The binding affinity of Lqh III to cockroach neuronal membranes was fivefold higher at pH 6.5 than at pH 7.5. This correlated with an increase in the electropositive charge on the toxin surface resulting from protonation of its four histidines. Radioiodination of Tyr(14) of Lqh III abolished its binding to locust but not cockroach sodium channels, whereas the noniodinated toxin bound equally well to both neuronal preparations. Radioiodination of Tyr(10) or Tyr(21) of the structurally similar alpha-toxin from L. quinquestriatus hebraeus (LqhalphaIT), as well as their substitution by phenylalanine, had only minor effects on binding to cockroach neuronal membranes. However, substitution of Tyr(21), but not Tyr(14), by leucine decreased the binding affinity of LqhalphaIT approximately 87-fold. Thus, Tyr(14) is involved in the bioactivity of Lqh III to locust receptor site 3 and is not crucial for the binding of LqhalphaIT to this site. In turn, the aromatic ring of Tyr(21) takes part in the bioactivity of LqhalphaIT to insects. These results highlight subtle architectural variations between locust and cockroach receptor site 3, in addition to previous results demonstrating the competence of Lqh III to differentiate between insect and mammalian sodium channel subtypes.

Scorpion alpha-like toxins, toxic to both mammals and insects, differentially interact with receptor site 3 on voltage-gated sodium channels in mammals and insects

alpha-Like toxins, a unique group designated among the scorpion alpha-toxin class that inhibit sodium channel inactivation, are highly toxic to mice but do not compete for alpha-toxin binding to receptor site 3 on rat brain sodium channels. We analysed the sequence of a new alpha-like toxin, which was also highly active on insects, and studied its action and binding on both mammalian and insect sodium channels. Action of the alpha-like toxin on isolated cockroach axon is similar to that of an alpha-toxin, and the radioactive toxin binds with a high affinity to insect sodium channels. Other sodium channel neurotoxins interact competitively or allosterically with the insect alpha-like toxin receptor site, similarly to alpha-toxins, suggesting that the alpha-like toxin receptor site is closely related to receptor site 3. Conversely, on rat brain sodium channels, specific binding of 125I-alpha-like toxin could not be detected, although at high concentration it inhibits sodium current inactivation on rat brain sodium channels. The difficulty in measuring binding to rat brain channels may be attributed to low-affinity binding due to the acidic properties of the alpha-like toxins that also impair the interaction with receptor site 3. The results suggest that alpha-like toxins bind to a distinct receptor site on sodium channels that is differentially related to receptor site 3 on mammalian and insect sodium channels.

The putative bioactive surface of insect-selective scorpion excitatory neurotoxins

Scorpion neurotoxins of the excitatory group show total specificity for insects and serve as invaluable probes for insect sodium channels. However, despite their significance and potential for application in insect-pest control, the structural basis for their bioactivity is still unknown. We isolated, characterized, and expressed an atypically long excitatory toxin, Bj-xtrIT, whose bioactive features resembled those of classical excitatory toxins, despite only 49% sequence identity. With the objective of clarifying the toxic site of this unique pharmacological group, Bj-xtrIT was employed in a genetic approach using point mutagenesis and biological and structural assays of the mutant products. A primary target for modification was the structurally unique C-terminal region. Sequential deletions of C-terminal residues suggested an inevitable significance of Ile73 and Ile74 for toxicity. Based on the bioactive role of the C-terminal region and a comparison of Bj-xtrIT with a Bj-xtrIT-based model of a classical excitatory toxin, AaHIT, a conserved surface comprising the C terminus is suggested to form the site of recognition with the sodium channel receptor.

The insect voltage-gated sodium channel as target of insecticides

Examination of the function, chemistry, and pharmacology of the voltage-gated insect sodium channel (ISC) reveals that the ISC closely resembles its vertebrate counterpart in electrophysiology and ion conductance, primary structure and allocation of all functional domains, and its pharmacological diversity and flexibility exhibited by the occurrence of different allosterically coupled receptor-binding sites for various neurotoxicants. The toxicants include several groups of insecticides, namely DDT and its analogues, pyrethroids, N-alkylamides, and dihydropyrazoles, which affect channel gating and ion permeability. Despite their similarity, the insect and vertebrate channels are pharmacologically distinguishable, as revealed by the responsiveness of the heterologously expressed Drosophila para clone to channel modifiers and blockers and the occurrence of the insect-selective sodium channel neurotoxins derived from arachnid venoms presently used for the design of recombinant baculovirus-mediated selective bioinsecticides. The pharmacological specificity of the ISC may lead to the design of insect-selective toxicants, and its pharmacological flexibility may direct the use of ISC insecticides for resistance management. Insecticide resistance [such as knockdown resistance (KDR)] is acquired by natural selection and operated by increased metabolism, channel mutagenesis, or both. The resistance issue can be dealt with in several ways. One is by simultaneous application of low doses of synergistic, allosterically coupled mixtures (thus delaying or preventing the onset of resistance). An alternative is to replace an insecticide to which resistance was acquired by channel mutation with a different ISC toxicant to which increased susceptibility was conferred by the same mutation. Such a possibility was exemplified by a significant increase in susceptibility to N-alkylamides, as well as an insect-selective neurotoxin revealed by KDR insects. Third, both of these methods can be combined. Thus owing to its pharmacological uniqueness, the ISC may serve as a high-priority target for future selective and resistance-manageable insecticides.

Delta-atracotoxins from Australian funnel-web spiders compete with scorpion alpha-toxin binding on both rat brain and insect sodium channels

Atracotoxins are novel peptide toxins from the venom of Australian funnel-web spiders that slow sodium current inactivation in a similar manner to scorpion alpha-toxins. To analyse their interaction with known sodium channel neurotoxin receptor sites we determined their effect on scorpion toxin, batrachotoxin and saxitoxin binding. Nanomolar concentrations of delta-atracotoxin-Hv1 and delta-atracotoxin-Ar1 completely inhibited the binding of the scorpion alpha-toxin AaH II to rat brain synaptosomes as well as the binding of LqhalphaIT, a scorpion alpha-toxin highly active on insects, to cockroach neuronal membranes. Moreover, delta-atracotoxin-Hv1 cooperatively enhanced batrachotoxin binding to rat brain synaptosomes in an analogous fashion to scorpion alpha-toxins. Thus the delta-atracotoxins represent a new class of toxins which bind to both mammalian and insect sodium channels at sites similar to, or partially overlapping with, the receptor binding sites of scorpion alpha-toxins.

Na+-Dependent neuritic spikes initiate Ca2+-dependent somatic plateau action potentials in insect dorsal paired median neurons

The origin of plateau action potentials was studied in short-term cultures of dorsal paired median (DPM) neurons dissociated from the terminal abdominal ganglion of the cockroach, Periplaneta americana. Spontaneous plateau action potentials were recorded by intracellular microelectrodes in cell bodies that had neurite stumps. These action potentials featured a fast initial depolarization followed by a plateau. However, only fast spikes of short duration were observed when the cell was hyperpolarized from the resting membrane potential. These two different components of the action potentials could be separated by applying depolarizing current pulses from a hyperpolarized holding potential. Application of 200 nM tetrodotoxin (TTX) abolished both fast and slow phases, but depolarization to the original resting potential by steady current injection triggered slow monophasic action potentials that could be blocked by 3 mM CoCl2. In contrast, DPM neurons without neurites were not spontaneously active. In these cells, calcium-dependent slow monophasic action potentials were only recorded immediately after impalement or with current pulse stimulation. Immunocytochemical observations showed that dorsal unpaired median (DUM) neuron cell bodies, which are known to exhibit spontaneous sodium-dependent action potentials, reacted with an antibody directed against a synthetic peptide corresponding to the SP19 segment of voltage-activated sodium channels. In contrast, the antibody did not stain DPM neuron cell bodies but gave intense, patchy staining only in the neurite. Whole cell patch-clamp experiments performed on isolated DPM neuron cell bodies without a neurite revealed the presence of an inward current that did not inactivate completly within the duration of the test pulse. This current was insensitive to both 100 nM TTX and sodium-free saline. It was defined as a high-voltage-activated calcium current according to its high threshold of activation (-30 mV) and its sensitivity to 1 mM CdCl2 and 100 nM omega-conotoxin GVIA. Our findings demonstrate that spontaneous sodium-dependent spikes arising from the neurite are required to initiate slow somatic calcium-dependent action potentials in DPM neurons.

New toxins acting on sodium channels from the scorpion Leiurus quinquestriatus hebraeus suggest a clue to mammalian vs insect selectivity

Two new toxins were purified from Leiurus quinquestriatus hebraeus (Lqh) scorpion venom, Lqh II and Lqh III. Lqh II sequence reveals only two substitutions, as compared to AaH II, the most active scorpion alpha-toxin on mammals from Androctounus australis Hector. Lqh III shares 80% sequence identity with the alpha-like toxin Bom III from Buthus occitanus mardochei. Using bioassays on mice and cockroach coupled with competitive binding studies with 125I-labeled scorpion alpha-toxins on rat brain and cockroach synaptosomes, the animal selectivity was examined. Lqh II has comparable activity to mammals as AaH II, but reveals significantly higher activity to insects attributed to its C-terminal substitution, and competes at low concentration for binding on both mammalian and cockroach sodium channels. Lqh II thus binds to receptor site 3 on sodium channels. Lqh III is active on both insects and mammals but competes for binding only on cockroach. The latter indicates that Lqh III binds to a distinct receptor site. Thus, Lqh II and Lqh III represent two different scorpion toxin groups, the alpha- and alpha-like toxins, respectively, according to the structural and pharmacological criteria. These new toxins may serve as a lead for clarification of the structural basis for insect vs mammal selectivity of scorpion toxins.

A new approach to insect-pest control--combination of neurotoxins interacting with voltage sensitive sodium channels to increase selectivity and specificity

Voltage-sensitive sodium channels are responsible for the generation of electrical signals in most excitable tissues and serve as specific targets for many neurotoxins. At least seven distinct classes of neurotoxins have been designated on the basis of physiological activity and competitive binding studies. Although the characterization of the neurotoxin receptor sites was predominantly performed using vertebrate excitable preparations, insect neuronal membranes were shown to possess similar receptor sites. We have demonstrated that the two mutually competing anti-insect excitatory and depressant scorpion toxins, previously suggested to occupy the same receptor site, bind to two distinct receptors on insect sodium channels. The latter provides a new approach to their combined use in insect control strategy. Although the sodium channel receptor sites are topologically separated, there are strong allosteric interactions among them. We have shown that the lipid-soluble sodium channel activators, veratridine and brevetoxin, reveal divergent allosteric modulation on scorpion alpha-toxins binding at homologous receptor sites on mammalian and insect sodium channels. The differences suggest a functionally important structural distinction between these channel subtypes. The differential allosteric modulation may provide a new approach to increase selective activity of pesticides on target organisms by simultaneous application of allosterically interacting drugs, designed on the basis of the selective toxins. Thus, a comparative study of neurotoxin receptor sites on mammalian and invertebrate sodium channels may elucidate the structural features involved in the binding and activity of the various neurotoxins, and may offer new targets and approaches to the development of highly selective pesticides.

Expression and distribution of voltage-sensitive sodium channels in pyrethroid-susceptible and pyrethroid-resistant Musca domestica

Knockdown resistance (kdr) to pyrethroid insecticides has been found in numerous insect species. kdr causes nerve insensitivity by altering the primary target of these insecticides, the voltage-sensitive sodium channel. In Musca domestica, cloning and sequencing of susceptible, kdr, and super-kdr alleles of the sodium channel gene (Msc) homologous to the Drosophila melanogaster para channel gene has revealed point mutations. The conservation of the nature and of the position of these mutations strongly suggests a role in the kdr mechanism. To determine if these mutations are associated with modifications of channel expression in adult flies, we investigated the localization of the Msc transcripts, and the size and the tissue distribution of the channel protein in pyrethroid-susceptible and super-kdr strains. Msc channels were mainly found in the cortical regions of the central nervous system with additional labeling in some neuronal processes and in the eyes. No qualitative or quantitative difference was observed between the strains. In immunoblotting experiments, anti-Msc antibodies bound to only one polypeptide of 260 kDa in adult brain. No differences were found in antibody staining between susceptible and pyrethroid-resistant strains. These results were correlated with those on Drosophila melanogaster, for which two sodium channel genes have been identified.

Biochemical and pharmacological characterization of a depressant insect toxin from the venom of the scorpion Buthacus arenicola

A depressant toxin active on insects, Buthacus arenicola IT2, was isolated from the venom of the North African scorpion B. arenicola and its structural and pharmacological properties were investigated. B. arenicola IT2 is a single polypeptide of 61 amino acid residues, including 8 half-cystines but no methionine and histidine, with a molecular mass of 6835 Da. Its amino acid sequence is 79-95% identical to other depressant toxins from scorpions. When injected into the cockroach Blatella germanica, B. arenicola IT2 induced a slow depressant flaccid paralysis with a LD50 of 175 ng. B. arenicola IT2 has two non-interacting binding sites in cockroach neuronal membranes: one of high affinity (Kd1 = 0.11 +/- 0.04 nM) and low capacity (Bmax1 = 2.2 +/- 0.6 pmol/mg), and one of low affinity (Kd2 = 24 +/- 7 nM) and high capacity (Bmax2 = 226 +/- 92 pmol/mg). Its binding to these two sites was completely inhibited by Leiurus quinquestriatus quinquestriatus IT2, a depressant toxin from L. quinquestriatus quinquestriatus. Reciprocal-binding experiments between B. arenicola IT2 and the excitatory insect-toxin A. australis Hector IT revealed competition between the two toxins for the high-affinity sites of B. arenicola IT2. B. arenicola IT2 has a higher affinity than L. quinquestriatus hebraeus IT2, a depressant toxin from L. quinquestriatus hebraeus. Thus, B. arenicola IT2 represents an interesting tool to study the receptor site for depressant toxins on insect sodium channels.

Scorpion toxins affecting sodium current inactivation bind to distinct homologous receptor sites on rat brain and insect sodium channels

Sodium channels posses receptor sites for many neurotoxins, of which several groups were shown to inhibit sodium current inactivation. Receptor sites that bind alpha- and alpha-like scorpion toxins are of particular interest since neurotoxin binding at these extracellular regions can affect the inactivation process at intramembranal segments of the channel. We examined, for the first time, the interaction of different scorpion neurotoxins, all affecting sodium current inactivation and toxic to mammals, with alpha-scorpion toxin receptor sites on both mammalian and insect sodium channels. As specific probes for rat and insect sodium channels, we used the radiolabeled alpha-scorpion toxins AaH II and LqhalphaIT, the most active alpha-toxins on mammals and insect, respectively. We demonstrate that the different scorpion toxins may be classified to several groups, according to their in vivo and in vitro activity on mammalian and insect sodium channels. Analysis of competitive binding interaction reveal that each group may occupy a distinct receptor site on sodium channels. The alpha-mammal scorpion toxins and the anti-insect Lqh alphaIT bind to homologous but not identical receptor sites on both rat brain and insect sodium channels. Sea anemone toxin ATX II, previously considered to share receptor site 3 with alpha-scorpion toxins, is suggested to bind to a partially overlapping receptor site with both AaH II and Lqh alphaIT. Competitive binding interactions with other scorpion toxins suggest the presence of a putative additional receptor site on sodium channels, which may bind a unique group of these scorpion toxins (Bom III and IV), active on both mammals and insects. We suggest the presence of a cluster of receptor sites for scorpion toxins that inhibit sodium current inactivation, which is very similar on insect and rat brain sodium channels, in spite of the structural and pharmacological differences between them. The sea anemone toxin ATX II is also suggested to bind within this cluster.

Alpha-scorpion toxins binding on rat brain and insect sodium channels reveal divergent allosteric modulations by brevetoxin and veratridine

At least six topologically separated neurotoxin receptor sites have been identified on sodium channels that reveal strong allosteric interactions among them. We have studied the allosteric modulation induced by veratridine, binding to receptor site 2, and brevetoxin PbTx-1, occupying receptor site 5, on the binding of alpha-scorpion toxins at receptor site 3, on three different neuronal sodium channels: rat brain, locust, and cockroach synaptosomes. We used 125I-AaH II, the most active alpha-scorpion toxin on vertebrates, and 125I-Lqh alpha IT, shown to have high activity on insects, as specific probes for receptor site 3 in rat brain and insect sodium channels. Our results reveal that brevetoxin PbTx-1 generates three types of effects at receptor site 3:1) negative allosteric modulation in rat brain sodium channels, 2) positive modulation in locust sodium channels, and 3) no effect on cockroach sodium channel. However, PbTx-1 activates sodium channels in cockroach axon similarly to its activity in other preparation. Veratridine positively modulates both rat brain and locust sodium channels but had no effect on alpha-toxin binding in cockroach. The dramatic differences in allosteric modulations in each sodium channel subtype suggest structural differences in receptor sites for PbTx-1 and/or at the coupling regions with alpha-scorpion toxin receptor sites in the different sodium channels, which can be detected by combined application of specific channel modifiers and may elucidate the dynamic gating activity and the mechanism of allosteric interactions among various neurotoxin receptors.

Sodium channel distribution in a spider mechanosensory organ

A site-directed antibody was used immunocytochemically to measure the distribution of sodium channels in the tissues of a spider mechanoreceptor organ. The VS-3 slit sense organ contains 7-8 pairs of bipolar sensory neurons; these neurons are representative of a wide range of arthropod mechanoreceptors. Sensory transduction is thought to occur at the tips of the dendrites and to cause action potentials that are regeneratively conducted to the cell bodies, although it has not been possible to confirm this by direct intracellular recordings from the dendrites. Wholemount preparations were labelled by immunofluorescence and thin sections were immunogold labelled, using an antibody to the highly conserved SP19 sequence of the voltage-activated sodium channel. Labelling for sodium channels was found in the neurons and in their surrounding glial cells. Both cytoplasm and membranes were labelled, but immunogold particles were clearly aligned along cell membranes, indicating that the majority of labelling represented membrane-bound sodium channels. Channel density in the dendrites was similar to the axons and higher than in the cell bodies, supporting the idea of active conduction in the sensory dendrites. Labelling in glial cell membranes was indistinguishable from the neighboring neurons, suggesting a significant role for sodium channels in the functions of these supporting cells.

Immunocytochemical localization of sodium channels in an insect central nervous system using a site-directed antibody

Antibodies to channel proteins and specific peptide sequences have been previously used to localize voltage-activated sodium channels in the rat brain. Here we describe the first localization of sodium channels in an insect nervous system using a site-directed antibody. The mesothoracic ganglion of the cockroach was stained with an antibody to the highly conserved SP19 sequence. Antibody labelling was visualized by light microscopy using the avidin/biotin method on wax sections, and transmission electron microscopy of immunogold-labelled thin sections. Central ganglia of insects contain clearly separated regions of cell bodies, synaptic neuropil, axon tracts, and nerves. Antibody staining by light microscopy was limited to neurons, and was intense in axons throughout the ganglion and nerves. Staining was also strong in the cytoplasm, but not the nuclei, of many neuronal cell bodies. Neuropil regions were relatively lightly labelled. These findings can be correlated with the known electrophysiology of the ganglion. Electron microscopy detected sodium channels in areas surrounding axons, probably including axon membranes and enveloping glial cell membranes. Axonal mitochondria were also heavily labelled, suggesting a sodium channel transport function for these organelles.

Transcription analysis of the para gene by in situ hybridization and immunological characterization of its expression product in wild-type and mutant strains of Drosophila

In Drosophila, the para gene has been shown to encode a functional voltage-dependent sodium channel. We used a cDNA clone to study the distribution of its transcripts by in situ mRNA hybridization on adult fly sections. These transcripts are found in cortical regions of the central nervous system and in the eyes. On immunoblots, antibodies raised against expression products of part of the gene recognize a polypeptide of M(r) approximately 270,000 in head membranes. Immunolocalization experiments indicate that anti-para antibodies bind to cortical regions of the brain and give heavy signals in the eyes. Immunohistochemistry was also performed on napts and seits1, two mutant Drosophila strains known to be defective in sodium channel activity. Only napts flies displayed a decrease in the expression of the para protein.

Binding of an alpha scorpion toxin to insect sodium channels is not dependent on membrane potential

The insect-specific Lqh alpha IT toxin resembles alpha scorpion toxins affecting mammals by its amino acid sequence and effects on sodium conductance. The present study reveals that Lqh alpha IT does not bind to rat brain membranes and possesses in locust neuronal membranes a single class of high affinity (Kd = 1.06 +/- 0.15 nM) and low capacity (Bmax = 0.7 +/- 0.19 pmol/mg protein) binding sites. The latter are: (1) distinct from binding sites of other sodium channel neurotoxins; (2) inhibited by sea anemone toxin II; (3) cooperatively interacting with veratridine; (4) not dependent on membrane potential, in contrast to the binding sites of alpha toxins in vertebrate systems. These data suggest the occurrence of (a) conformational-structural differences between insect and mammal sodium channels and (b) the animal group specificity and pharmacological importance of the alpha scorpion toxins.

Solubilization and characterization of the insect neuronal sodium channel

Locust neuronal sodium channels were solubilized by 1% cholate and 0.2% Triton X-100, and their functionality was monitored by [3H]saxitoxin (STX) binding assays. About 40% of STX binding activity was recovered in the solubilized fraction without affecting affinity (Kd = 0.5 nM) and the time and temperature dependent STX binding activity was significantly stabilized in the presence of 20 nM STX. Partial purification by an anion exchange resin yielded a 20% recovery and a 3.5 times increase in the specific STX binding activity. Identification of the locust solubilized sodium channels by immunoprecipitation and radiophosphorylation revealed a Mr of 245,000 on SDS-PAGE. The present solubilized preparation will enable the study of the unique pharmacology of insect sodium channels.