Molecular basis of the remarkable species selectivity of an insecticidal sodium channel toxin from the African spider Augacephalus ezendami.

The inexorable decline in the armament of registered chemical insecticides has stimulated research into environmentally-friendly alternatives. Insecticidal spider-venom peptides are promising candidates for bioinsecticide development but it is challenging to find peptides that are specific for targeted pests. In the present study, we isolated an insecticidal peptide (Ae1a) from venom of the African spider Augacephalus ezendami (family Theraphosidae). Injection of Ae1a into sheep blowflies (Lucilia cuprina) induced rapid but reversible paralysis. In striking contrast, Ae1a was lethal to closely related fruit flies (Drosophila melanogaster) but induced no adverse effects in the recalcitrant lepidopteran pest Helicoverpa armigera. Electrophysiological experiments revealed that Ae1a potently inhibits the voltage-gated sodium channel BgNaV1 from the German cockroach Blattella germanica by shifting the threshold for channel activation to more depolarized potentials. In contrast, Ae1a failed to significantly affect sodium currents in dorsal unpaired median neurons from the American cockroach Periplaneta americana. We show that Ae1a interacts with the domain II voltage sensor and that sensitivity to the toxin is conferred by natural sequence variations in the S1-S2 loop of domain II. The phyletic specificity of Ae1a provides crucial information for development of sodium channel insecticides that target key insect pests without harming beneficial species.

Melanocortin-1 receptor-mediated signalling pathways activated by NDP-MSH and HBD3 ligands.

Binding of melanocortin peptide agonists to the melanocortin-1 receptor of melanocytes results in eumelanin production, whereas binding of the agouti signalling protein inverse agonist results in pheomelanin synthesis. Recently, a novel melanocortin-1 receptor ligand was reported. A ß-defensin gene mutation was found to be responsible for black coat colour in domestic dogs. Notably, the human equivalent, ß-defensin 3, was found to bind with high affinity to the melanocortin-1 receptor; however, the action of ß-defensin as an agonist or antagonist was unknown. Here, we use in vitro assays to show that ß-defensin 3 is able to act as a weak partial agonist for cAMP signalling in human embryonic kidney (HEK) cells expressing human melanocortin-1 receptor. ß-defensin 3 is also able to activate MAPK signalling in HEK cells stably expressing either wild type or variant melanocortin-1 receptors. We suggest that ß-defensin 3 may be a novel melanocortin-1 receptor agonist involved in regulating melanocyte responses in humans.