Abstract
The corpora cardiaca of the Indian stick insect, Carausius morosus, synthesize two decapeptide neuropeptides of the adipokinetic hormone (AKH) family, both of which can increase the trehalose levels in the hemolymph when the stick insect is ligated between the head and the thorax. Here, we use two biological assays to assess the potencies of 19 AKH analogs in ligated C. morosus: the carbohydrate-mobilizing assay measures the change in the levels of circulating carbohydrates following injection of a substance, while the semi-exposed heart assay measures a change in heart beat rate after the peptide is applied onto the heart. With the endogenous AKH (Carmo-HrTH-II) as lead peptide, we report here on seven naturally-occurring AKH peptides (bioanalogs) selected for testing because of a single or double amino acid replacement, or for being octapeptides. Single amino acid substitutions by an alanine residue at all positions of Carmo-HrTH-II, as well as analogs modified at the termini were also investigated to give a comprehensive view of ligand-receptor interaction at the physiological level in a hemimetabolous insect that practices thanatosis (feigning death). Only small changes are elicited in the bioassays, but the results from the two tests are comparable bar one or two anomalies. Results show that analogs modified at the termini have no or reduced activity. Regarding structural requirements of a ligand, the C. morosus AKH receptor appears to be strict: octapeptides are not preferred and many of the decapeptide analogs failed to reach 50% activity relative to Carmo-HrTH-II. The data implies that the AKH receptor in C. morosus mostly does not tolerate shorter peptides and single amino acid replacements in most places of the native AKH peptide. This information is important if environmentally friendly insect-specific pesticides are made based on an insect AKH as lead peptide: stick insects that are normally not viewed as pest insects may not be easily targeted by cross-reactive AKH mimetics directed at harmful insects, due to the very specific amino acid requirements to activate the C. morosus AKH receptor.
Highlights
Insects are well-known for their diversity and abundance and for their influence on the biosphere and human life
A ventral incision of the abdomen exposed the ventral cavity with the internal organs; a few drops of stick insect saline were added to the exposed part
The peptides of the adipokinetic hormone (AKH)/RPCH family are mainly known for their involvement in the regulation of energy metabolism, the mobilization of stored fuel metabolites, the AKHs are pleiotropic (Gäde, 1997; Marco and Gäde, 2020)
Summary
Insects are well-known for their diversity and abundance and for their influence on the biosphere and human life. There is a strong interest in developing environmentally friendly insecticides that are selective and affect only the target (pest) species instead of all insects. The development of hormone-like compounds that can be used in specific drug design to act as targeted pesticides are being considered (Altstein et al, 2000; Gäde and Goldsworthy, 2003; Audsley and Down, 2015). A well-researched family of hormones are the adipokinetic hormones (AKHs) which are synthesized and released from the retrocerebral corpora cardiaca (CC); the AKHs are mainly tasked with mobilizing fuels (energy-rich metabolites) from fat body stores into the hemolymph and are identified as putative targets to develop new insecticides (see review by Marco and Gäde, 2020). The AKH peptide family is generally characterized by peptides having (a) a chain length of 8–10 amino acids; (b) post-translationally modified termini: a pGlu residue at the N-terminus and a carboxyamide at the C-terminus; (c) either a Leu, Ile, Val or Phe residue at position 2; (d) a Thr or Asn residue at position 3; (e) an aromatic Phe or Tyr residue at position 4; (f) the branched amino acids Thr or Ser at position 5; (g) the aromatic residue Trp at position 8; (h) the simple amino acid Gly at position 9, and (i) variable amino acids at positions 6, 7, and 10 (see Gäde, 2009)
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