Insight into mosquito GnRH-related neuropeptide receptor specificity revealed through analysis of naturally occurring and synthetic analogs of this neuropeptide family

Abstract

Adipokinetic hormone (AKH), corzazonin (CRZ) and the AKH/CRZ-related peptide (ACP) are peptides considered homologous to the vertebrate gonadotropin-releasing hormone (GnRH). All three Aedes aegypti GnRH-related neuropeptide receptors have been characterized and functionally deorphanized, which individually exhibit high specificity for their native ligands, which prompted us to investigate the contribution of ligand structures in conferring receptor specificity. In the current study, we designed a series of analogs based on the native ACP sequence in A. aegypti and screened them against the ACP receptor using a heterologous system to identify critical residues required for receptor activation. Specifically, analogs lacking the carboxy-terminal amidation, replacing aromatic residues, as well as truncated analogs were either completely inactive or had very low activities even at high concentration. The next most critical residues were the polar threonine in position 3 and the blocked amino-terminal pyroglutamate, with activity of the latter partially recovered using an alternatively blocked analog. ACP analogs with alanine substitutions at position 2 (valine), 5 (serine), 6 (arginine) and 7 (aspartic acid) positions were less detrimental as were replacements of charged residues. Interestingly, replacing asparagine with an alanine at position 9, creating a C-terminal WAA-amide, resulted in a 5-fold more active analog which may be useful as a lead superagonist compound. Similarly, we utilized this high-throughput approach against an A. aegypti AKH receptor (AKHR-IA) testing a number of mostly naturally-occurring AKH analogs from other insects to determine how substitutions of specific amino acids in the AKH ligand influences receptor activation. AKH analogs having single substitutions compared to the endogenous A. aegypti AKH revealed position 7 (serine) was well tolerated whereas changes to position 6 (proline) had pronounced effects, with receptor activity compromised nearly ten-fold. Substitution of position 3 (threonine) or analogs with combinations of substitutions were quite detrimental with a significant decrease in AKHR-IA activation. Interestingly, analogs with an asparagine residue at position seven displayed improved receptor activation compared to the native mosquito AKH. Collectively, these results advance our understanding of how two GnRH-related systems in A. aegypti sharing the most recent evolutionary origin sustain independence of function and signalling despite their relatively high degree of ligand and receptor homology.