Kristel Vuerinckx

More than two decades of research on insect neuropeptide GPCRs: an overview

This review focuses on the state of the art on neuropeptide receptors in insects. Most of these receptors are G protein-coupled receptors (GPCRs) and are involved in the regulation of virtually all physiological processes during an insects life. More than 20 years ago a milestone in invertebrate endocrinology was achieved with the characterization of the first insect neuropeptide receptor, i.e., the Drosophila tachykinin-like receptor. However, it took until the release of the Drosophila genome in 2000 that research on neuropeptide receptors boosted. In the last decade a plethora of genomic information of other insect species also became available, leading to a better insight in the functions and evolution of the neuropeptide signaling systems and their intracellular pathways. It became clear that some of these systems are conserved among all insect species, indicating that they fulfill crucial roles in their physiological processes. Meanwhile, other signaling systems seem to be lost in several insect orders or species, suggesting that their actions were superfluous in those insects, or that other neuropeptides have taken over their functions. It is striking that the deorphanization of neuropeptide GPCRs gets much attention, but the subsequent unraveling of the intracellular pathways they elicit, or their physiological functions are often hardly examined. Especially in insects besides Drosophila this information is scarce if not absent. And although great progress made in characterizing neuropeptide signaling systems, even in Drosophila several predicted neuropeptide receptors remain orphan, awaiting for their endogenous ligand to be determined. The present review gives a précis of the insect neuropeptide receptor research of the last two decades. But it has to be emphasized that the work done so far is only the tip of the iceberg and our comprehensive understanding of these important signaling systems will still increase substantially in the coming years.

Characterisation and tissue distribution of the PISCF allatostatin receptor in the red flour beetle, Tribolium castaneum

The insect PISCF/allatostatins (ASTs) are pleiotropic peptides that are involved in the regulation of juvenile hormone biosynthesis, are myoinhibitory on the gut and the heart, and suppress feeding in various insects, but their roles in beetles are poorly understood. To provide further insight into the significance of PISCF/ASTs in beetles, the PISCF/AST receptor from Tribolium castaneum has been characterised and its tissue distribution determined. The biological activity of the T. castaneum PISCF/AST (Trica-AS) was also investigated. The Trica-AS receptor shows high sequence homology to other insect PISCF/AST receptors, which are related to the mammalian somatostatin/opioid receptors, a family of G protein-coupled receptors. The Trica-AS receptor was activated in a dose-dependent manner by both Trica-AS and T. castaneum allatostatin double C (Trica-ASTCC) as well as Manduca sexta-allatostatin (Manse-AS). Other allatoregulatory peptides (a FLG/AST, a MIP/AST and an allatotropin) and somatostatin(14) were inactive on this receptor. Receptor transcript levels in tissues, determined by qRT-PCR, were highest in the head and the gut, with variable amounts in the fat body and reproductive organs. There were measurable differences in receptor levels of the head, fat body and reproductive organs between males and females. There was also a widespread distribution of Trica-AS in various tissues of T. castaneum. The Trica-AS peptide precursor was most abundant in the head and there was a significant difference between levels in the heads and reproductive organs of males and females. Whole mount immunocytochemistry localised Trica-AS in the median and lateral neurosecretory cells of the brain, in the corpus cardiacum and throughout the ventral nerve cord. The peptide was also present in midgut neurosecretory cells, but no immunostaining was detected in the reproductive organs or Malpighian tubules. The widespread distribution of both Trica-AS and its receptor suggest this peptide may have multiple roles in beetles. However, Trica-AS had no effect on the spontaneous contractions of the gut or ovaries of T. castaneum but this peptide did stimulate the release of proteases from the anterior midgut of another beetle, Tenebrio molitor. The activation of the Trica-AS receptor by Trica-ASTCC implies a physiological role for this peptide in beetles, which remains to be identified.

Developmental‐ and food‐dependent foraging transcript levels in the desert locust

Drastic changes in the environment during a lifetime require developmental and physiological flexibility to ensure animal survival. Desert locusts, Schistocerca gregaria, live in an extremely changeable environment, which alternates between periods of rainfall and abundant food and periods of drought and starvation. In order to survive, locusts display an extreme form of phenotypic plasticity that allows them to rapidly cope with these changing conditions by converting from a cryptic solitarious phase to a swarming, voracious gregarious phase. To accomplish this, locusts possess different conserved mediators of phenotypic plasticity. Recently, attention has been drawn to the possible roles of protein kinases in this process. In addition to cyclic AMP‐dependent protein kinase (PKA), also cyclic GMP‐dependent protein kinase (PKG), which was shown to be involved in changes of food‐related behavior in a variety of insects, has been associated with locust phenotypic plasticity. In this article, we study the transcript levels of the S. gregaria orthologue of the foraging gene that encodes a PKG in different food‐related, developmental and crowding conditions. Transcript levels of the S. gregaria foraging orthologue are highest in different parts of the gut and differ between isolated and crowd‐reared locusts. They change when the availability of food is altered, display a distinct pattern with higher levels after a moult and decrease with age during postembryonic development.

RNA interference mortality points to noncircadian functions for the clock gene in the desert locust Schistocerca gregaria

One of the core genes in the circadian regulation network is clock (clk). By forming a heterodimer with CYCLE (CYC) that binds on an E‐box in the promoter region, it induces the transcription of other elements in the circadian transcriptional feedback loops and different clock output genes. In contrast to other insects, a clk double‐stranded RNA (dsRNA) treatment is lethal in adults and fifth instar nymphs of the desert locust, Schistocerca gregaria, in a dose‐dependent manner. Clk knock down fifth instar nymphs are able to undergo their imaginal moult but, depending on the amount of dsRNA, it takes them longer than the controls to reach adulthood. As adults, clk knock down animals do not develop their fat body and ovaries like the control animals. Therefore, we tested the expression of different genes involved in energy metabolism and reproduction to see the effect of the clk RNA interference knock down. Surprisingly, the expression of the vitellogenin gene was up‐regulated in the clk knock down females who did not appear to invest their energy in egg development. Taken together, our results point out that the clk gene in the desert locust has an additional function in development besides its established role in maintaining the circadian rhythms in the brain.