Karina Kiilerich Hansen:
Discovery of Novel Neuropeptide Signaling Systems in Arthropods

Date: 15-08-2011    Supervisor: Cornelis J. P. Grimmelikhuijzen

Neuropeptides are important signaling molecules expressed by neurons and together with their G protein coupled receptors (GPCRs) they steer central physiological processes such as reproduction, development, and behaviour. The sequencing of several arthropod genomes has made it possible to perform genome-wide analyses to identify and characterize novel neuropeptide and GPCR systems, and additionally to investigate receptor/ligand evolution and co-evolution. In this thesis, I present five studies all focussed on the exploration of peptide signaling systems in arthropods.

The related neuropeptides oxytocin and vasopressin are involved in water homeostasis and reproduction in mammals. Previously, an oxytocin/vasopressin-related peptide was discovered in the locust Locusta migratoria but in the sequenced genomes of the fruitfly, mosquitoes, the honeybee, and silkworm the oxytocin/vasopressin-related peptide and receptor was absent. In the first study of this thesis an oxytocin/vasopressinrelated peptide, designated inotocin, and the cognate GPCR were identified in the newly sequenced insect genomes from the flour beetle Tribolium castaneum and the wasp Nasonia vitripennis. In addition, the inotocin hormonal system was identified in the water flea Daphnia pulex (crustacean) showing conservation of this peptide signaling system in the arthropod phylum.

In the second study, a novel hormonal system structurally intermediate between the adipokinetic hormone (AKH) and the corazonin system was identified, and because of the structural relationship the peptide was named AKH/corazonin-related peptide (ACP). Since both the ACP peptide and the ACP receptor are structurally and phylogenetically related to the AKH system and the corazonin system this is an example of receptor/ligand co-evolution. Functional analysis showed that the ACP receptor is exclusively activated by the ACP peptide demonstrating that the ACP system is an independent hormonal system. The ACP system is present in some but not in all sequenced insect genomes and consequently it belongs to the group of variable neuropeptides.

The genome from the insect Pea aphid Acyrthosiphon pisum has been sequenced and the analyses including annotations of GPCR encoding genes from this genome are presented in the third study. We have identified 42 neuropeptide GPCR encoding genes in the genome of Acyrthosiphon pisum.

Recently, a novel neuropeptide, CCHamide, was identified in the silkworm Bombyx mori. In the fourth study we demonstrate that all insects with a sequenced genome have two CCHamide genes; one encoding CCHamide-1 and the other CCHamide-2. Two GPCRs from Drosophila melanogaster were identified to be the CCHamide receptors, each of them specific to either CCHamide peptide. In the crustacean Daphnia pulex and the chelicerata Ixodes scapularis only one CCHamide peptide and one CCHamide receptor could be identified indicating that a duplication of the CCHamide hormonal system arose after the split of crustaceans and hexapods, approximately 410 million years ago.

Previously, a novel neuropeptide, RYamide, had been discovered. In the last study of this thesis we identify the RYamide receptor from Tribolium castaneum and Drosophila melanogaster. Phylogenetic analysis revealed that the RYamide receptors are not related to mammalian neuropeptide Y receptors. Altogether, the work in this thesis describes novel peptide signaling systems in arthropods and provides new insights into their evolution.