Structure-activity relationship of ETH during ecdysis in the tobacco hornworm, Manduca sexta

In insects, ecdysis or shedding of the old cuticle, consists of a series of behaviors that are regulated by the coordinated actions of a number of neuropeptides, one of which is ecdysis triggering hormone (ETH). ETH acts directly on central pattern generators of the abdominal ganglia to trigger onset of pre-ecdysis behaviors, as well as indirectly to activate release of eclosion hormone, thereby inducing onset of ecdysis behaviors through a cGMP-mediated mechanism. We assessed the minimal C-terminal amino acids required for biological activity of ETH, by assessing: (i) onset of pre-ecdysis and ecdysis behaviors in vivo, after injection of peptide analogs, (ii) onset of fictive pre-ecdysis and ecdysis motor patterns in vitro, as recorded extracellularly, after incubation of the CNS with the peptide analogs, and (iii) accumulation of cGMP within cells of the abdominal ganglia, as assessed immunohistochemically. Amidation of ETH at the C-terminus was required to elicit a biological response in vivo and in vitro, as well as an accumulation of cGMP within the CNS. The five amino acid amidated C-terminus of ETH (NIPRMamide) was the minimal moiety able to induce a robust pre-ecdysis response in vivo and in vitro, while a seven amino acid core (NKNIPRMa) was required for induction of ecdysis, including accumulation of cGMP immunoreactivity within the CNS. Analogs smaller than 12 amino acids in length were only active at very high concentrations in vivo, suggesting that smaller fragments might be susceptible to hemolymph degradation. Some alanine substitutions or removal of internal amino acids altered the activity of ETH, as well as the time of onset of ecdysis behaviors, suggesting that internal amino acids play a role in maintaining proper folding of the peptide for successful binding or activity at the ETH receptor.