Two types of muscarinic acetylcholine receptors in Drosophila and other arthropods

Abstract

Muscarinic acetylcholine receptors (mAChRs) play a central role in the mammalian nervous system. These receptors are G protein-coupled receptors (GPCRs), which are activated by the agonists acetylcholine and muscarine, and blocked by a variety of antagonists. Mammals have five mAChRs (m1-m5). In this study, we cloned two structurally related GPCRs from the fruit fly Drosophila melanogaster, which, after expression in Chinese hamster ovary cells, proved to be muscarinic acetylcholine receptors. One mAChR (the A-type; encoded by gene CG4356) is activated by acetylcholine (EC50, 5 × 10(-8) M) and muscarine (EC50, 6 × 10(-8) M) and blocked by the classical mAChR antagonists atropine, scopolamine, and 3-quinuclidinyl-benzilate (QNB), while the other (the B-type; encoded by gene CG7918) is also activated by acetylcholine, but has a 1,000-fold lower sensitivity to muscarine, and is not blocked by the antagonists. A- and B-type mAChRs were also cloned and functionally characterized from the red flour beetle Tribolium castaneum. Recently, Haga et al. (Nature 2012, 482: 547-551) published the crystal structure of the human m2 mAChR, revealing 14 amino acid residues forming the binding pocket for QNB. These residues are identical between the human m2 and the D. melanogaster and T. castaneum A-type mAChRs, while many of them are different between the human m2 and the B-type receptors. Using bioinformatics, one orthologue of the A-type and one of the B-type mAChRs could also be found in all other arthropods with a sequenced genome. Protostomes, such as arthropods, and deuterostomes, such as mammals and other vertebrates, belong to two evolutionarily distinct lineages of animal evolution that split about 700 million years ago. We found that animals that originated before this split, such as cnidarians (Hydra), had two A-type mAChRs. From these data we propose a model for the evolution of mAChRs.

Peptide-gated ion channels and the simple nervous system of Hydra

“…Similarly, in Hydra, there are also ample immunohistochemical, biochemical and functional data to indicate their presence (Kass-Simon and Pierobon, 2007;Pierobon, 2012) and two putative metabotropic acetylcholine receptors (mAchRs) have been identified (Collin et al, 2013). Ionotropic receptors for small-molecule transmitters have, however, not been studied at the molecular level in Hydra (see below); we will therefore concentrate our discussion of neurotransmitters on neuropeptides.…”

“…The genome of H. magnipapillata contains two mAchRs (Collin et al, 2013) and 17 genes coding for nicotinic acetylcholine receptor (nAchR) subunits , strongly suggesting that H. magnipapillata uses acetylcholine for fast transmission. Expression of five of the nAchRs could be detected by RT-PCR, revealing a predominant expression in the head region.…”

The A- and B-type muscarinic acetylcholine receptors from Drosophila melanogaster couple to different second messenger pathways

“…We have recently found that Drosophila melanogaster and other insects have two types of mAChRs, named A and B. The A-type mAChRs are pharmacologically very similar to the mammalian m1-m5 receptors, being activated by the agonists acetylcholine and muscarine and inhibited by the antagonists atropine, 3-quinuclidinylbenzilate (QNB), and scopolamine [5]. The B-type mAChRs, however, have a pharmacology that is quite different from the mammalian mAChRs.…”

“…The B-type mAChRs, however, have a pharmacology that is quite different from the mammalian mAChRs. They have a high sensitivity to acetylcholine, but a 1000-fold lower sensitivity to muscarine than the A-type receptors and are not blocked by the classical antagonists [5].…”

Nitric oxide modulates a swimmeret beating rhythm in the crayfish

“…Nicotine increases the burst frequency in active preparations that show rhythmic bursts of motor neurone spikes. Because muscarinic receptors belong to a superfamily of G-protein-coupled receptors (Collin et al, 2013), carbachol could induce rhythmic bursts of motor neurone spikes via the activation of cAMP. The finding in this study that the burst frequency of the swimmeret motor neurones elicited by cAMP was facilitated by co-application of L-arginine suggests that the NO-cGMP signaling pathway affects cAMP activity.…”