Philanthotoxin Analogues That Selectively Inhibit Ganglionic Nicotinic Acetylcholine Receptors with Exceptional Potency.

Philanthotoxin-433 (PhTX-433) is an active component of the venom from the Egyptian digger wasp, Philanthus triangulum. PhTX-433 nonselectively inhibits several excitatory ligand-gated ion channels, and we recently showed that its synthetic analogue, PhTX-343, exhibits strong selectivity for neuronal over muscle-type nicotinic acetylcholine receptors (nAChRs). Here, we examined the action of 17 analogues of PhTX-343 against ganglionic (α3ß4) and brain (α4ß2) nAChRs expressed in Xenopus oocytes by using a two-electrode voltage clamp at -100 mV. IC50 values for PhTX-343 inhibition of α3ß4 and α4ß2 receptors were 7.7 and 80 nM, respectively. All the studied analogues had significantly higher potency at α3ß4 nAChRs with IC50 values as low as 0.16 nM and with up to 91-fold selectivity for α3ß4 over α4ß2 receptors. We conclude that PhTX-343 analogues displaying both a saturated ring and an aromatic moiety in the hydrophobic headgroup of the molecule demonstrate exceptional potency and selectivity for α3ß4 nAChRs.

Insect toxins - selective pharmacological tools and drug/chemical leads.

Insect toxins comprise a diverse array of chemicals ranging from small molecules, polyamines and peptide toxins. Many target nervous system and neuromuscular ion channels and so rapidly affect the behaviour of animals to which the toxin is applied or injected. Other modes of action have also been identified. Wasps, bees, flies, beetles and ants generate a rich arsenal of channel-active toxins, some of which offer selective pharmacological probes that target particular ion channels, while others act on more than one type of channel. Philanthotoxins from the digger wasp have been fruitful in adding to our understanding of ligand-gated ion channels both in the nervous system and at neuromuscular junctions. Fire ants produce the toxic alkaloid solenopsin, a molecule which has stimulated attempts to generate synthetic compounds with insecticidal activity. Apamin from bee venom targets calcium-activated potassium channels, which can in turn influence the release of neuropeptides. Melittin, another bee venom component, is a membrane-acting peptide. The saliva of the assassin bug contains toxins that target the voltage-gated calcium channels of their insect prey. Certain beetles produce diamphotoxin, a haemolytic peptide toxin with traditional use as an arrow poison and others generate leptinotarsin with similar properties. Mastoparan is a powerful peptide toxin present in the venom of wasps. Its toxic actions can be engineered out leaving a potent antimicrobial molecule of interest. In this short review we describe the actions of selected insect toxins and evaluate their potential as neuroactive pharmacological tools, candidate lead molecules for insect control and therapeutic candidates with potential antimicrobial, antiviral and anti-cancer applications.

Block of nicotinic acetylcholine receptors by philanthotoxins is strongly dependent on their subunit composition.

Philanthotoxin-433 (PhTX-433) is an active component of the venom from the Egyptian digger wasp, Philanthus triangulum. PhTX-433 inhibits several excitatory ligand-gated ion channels, and to improve selectivity two synthetic analogues, PhTX-343 and PhTX-12, were developed. Previous work showed a 22-fold selectivity of PhTX-12 over PhTX-343 for embryonic muscle-type nicotinic acetylcholine receptors (nAChRs) in TE671 cells. We investigated their inhibition of different neuronal nAChR subunit combinations as well as of embryonic muscle receptors expressed in Xenopus oocytes. Whole-cell currents in response to application of acetylcholine alone or co-applied with PhTX analogue were studied by using two-electrode voltage-clamp. α3ß4 nAChRs were most sensitive to PhTX-343 (IC50 = 12 nM at -80 mV) with α4ß4, α4ß2, α3ß2, α7 and α1ß1γδ being 5, 26, 114, 422 and 992 times less sensitive. In contrast α1ß1γδ was most sensitive to PhTX-12 along with α3ß4 (IC50 values of 100 nM) with α4ß4, α4ß2, α3ß2 and α7 being 3, 3, 26 and 49 times less sensitive. PhTX-343 inhibition was strongly voltage-dependent for all subunit combinations except α7, whereas this was not the case for PhTX-12 for which weak voltage dependence was observed. We conclude that PhTX-343 mainly acts as an open-channel blocker of nAChRs with strong subtype selectivity.