Abstract
Adipokinetic hormone (AKH) is a highly researched insect neuropeptide that induces the mobilization of carbohydrates and lipids from the fat body at times of high physical activity, such as flight and locomotion. As a naturally occurring ligand, AKH has undergone quite a number of amino acid changes throughout evolution, and in some insect species multiple AKHs are present. AKH acts by binding to a rhodopsin-like G protein-coupled receptor, which is related to the vertebrate gonadotropin-releasing hormone receptors. In the current study, we have cloned AKH receptors (AKHRs) from seven different species, covering a wide phylogenetic range of insect orders: the fruit fly, Drosophila melanogaster, and the yellow fever mosquito, Aedes aegypti (Diptera); the red flour beetle, Tribolium castaneum, and the large pine weevil, Hylobius abietis (Coleoptera); the honeybee, Apis mellifera (Hymenoptera); the pea aphid, Acyrthosiphon pisum (Hemiptera); and the desert locust, Schistocerca gregaria (Orthoptera). The agonistic activity of different insect AKHs, including the respective endogenous AKHs, at these receptors was tested with a bioluminescence-based assay in Chinese hamster ovary cells. All receptors were activated by their endogenous ligand in the nanomolar range. Based on our data, we can refute the previously formulated hypothesis that a functional AKH signaling system is absent in the beneficial species, Apis mellifera. Furthermore, our data also suggest that some of the investigated AKH receptors, such as the mosquito AKHR, are more selective for the endogenous (conspecific) ligand, while others, such as the locust AKHR, are more promiscuous and can be activated by AKHs from many other insects. This information will be of high importance when further analyzing the potential use of AKHRs as targets for developing novel pest control agents.
Highlights
Adipokinetic hormone was first isolated from the corpus cardiacum (CC)—a paired endocrine gland situated near the brain—of the locusts, Locusta migratoria and Schistocerca gregaria [1]
Adipokinetic hormone (AKH) closely resemble the red pigment concentrating hormones (RPCH) of Crustacea; Panbo-RPCH is synthesized in decapod crustaceans, as well as in a number of insect orders [20] they are grouped as the AKH/RPCH family of peptides, which are considered arthropod homologues of the vertebrate gonadotropin-releasing hormone (GnRH) peptide family [21,22,23]
Our results show that the AKH receptor of S. gregaria is the least selective of the tested receptors, whereas the AKH receptors of the evolutionarily
Summary
Adipokinetic hormone was first isolated from the corpus cardiacum (CC)—a paired endocrine gland situated near the brain—of the locusts, Locusta migratoria and Schistocerca gregaria [1]. Other reports have stated a functional role for AKH in stimulating locomotor activity, heart rate and other myotropic contractions, and in inhibiting protein and lipid synthesis in diverse insect species [3,11,12,13]. All AKHs are derived from precursor proteins and are preceded by a secretory signal peptide and followed by an AKH precursor-related peptide, for which the function is currently still unknown [16]. Some insects, such as the locusts, L. migratoria and S. gregaria, have multiple AKH precursor genes encoded in their genome, resulting in the presence of multiple mature AKHs in their CC [19]. AKHs closely resemble the red pigment concentrating hormones (RPCH) of Crustacea; Panbo-RPCH is synthesized in decapod crustaceans, as well as in a number of insect orders [20] they are grouped as the AKH/RPCH family of peptides, which are considered arthropod homologues of the vertebrate gonadotropin-releasing hormone (GnRH) peptide family [21,22,23]
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