Evolution, Expression, and Function of Nonneuronal Ligand-Gated Chloride Channels in Drosophila melanogaster

Alkaline taste sensation through the alkaliphile chloride channel in Drosophila

The sense of taste is an important sentinel governing what should or should not be ingested by an animal, with high pH sensation playing a critical role in food selection. Here we explore the molecular identities of taste receptors detecting the basic pH of food using Drosophila melanogaster as a model. We identify a chloride channel named alkaliphile (Alka), which is both necessary and sufficient for aversive taste responses to basic food. Alka forms a high-pH-gated chloride channel and is specifically expressed in a subset of gustatory receptor neurons (GRNs). Optogenetic activation of alka-expressing GRNs is sufficient to suppress attractive feeding responses to sucrose. Conversely, inactivation of these GRNs causes severe impairments in the aversion to high pH. Altogether, our discovery of Alka as an alkaline taste receptor lays the groundwork for future research on alkaline taste sensation in other animals.

Transcriptome of the synganglion in the tick Ixodes ricinus and evolution of the cys-loop ligand-gated ion channel family in ticks

BACKGROUND

Ticks represent a major health issue for humans and domesticated animals. Exploring the expression landscape of the tick's central nervous system (CNS), known as the synganglion, would be an important step in understanding tick physiology and in managing tick-borne diseases, but studies on that topic are still relatively scarce. Neuron-specific genes like the cys-loop ligand-gated ion channels (cys-loop LGICs, or cysLGICs) are important pharmacological targets of acaricides. To date their sequence have not been well catalogued for ticks, and their phylogeny has not been fully studied.

RESULTS

We carried out the sequencing of transcriptomes of the I. ricinus synganglion, for adult ticks in different conditions (unfed males, unfed females, and partially-fed females). The de novo assembly of these transcriptomes allowed us to obtain a large collection of cys-loop LGICs sequences. A reference meta-transcriptome based on synganglion and whole body transcriptomes was then produced, showing high completeness and allowing differential expression analyses between synganglion and whole body. Many of the genes upregulated in the synganglion were associated with neurotransmission and/or localized in neurons or the synaptic membrane. As the first step of a functional study of cysLGICs, we cloned the predicted sequence of the resistance to dieldrin (RDL) subunit homolog, and functionally reconstituted the first GABA-gated receptor of Ixodes ricinus. A phylogenetic study was performed for the nicotinic acetylcholine receptors (nAChRs) and other cys-loop LGICs respectively, revealing tick-specific expansions of some types of receptors (especially for Histamine-like subunits and GluCls).

CONCLUSIONS

We established a large catalogue of genes preferentially expressed in the tick CNS, including the cysLGICs. We discovered tick-specific gene family expansion of some types of cysLGIC receptors, and a case of intragenic duplication, suggesting a complex pattern of gene expression among different copies or different alternative transcripts of tick neuro-receptors.

Zinc is an important inter-kingdom signal between the host and microbe

Zinc (Zn) is an essential trace element in living organisms and plays a vital role in the regulation of both microbial virulence and host immune responses. A growing number of studies have shown that zinc deficiency or the internal Zn concentration does not meet the needs of animals and microbes, leading to an imbalance in zinc homeostasis and intracellular signalling pathway dysregulation. Competition for zinc ions (Zn²⁺) between microbes and the host exists in the use of Zn²⁺ to maintain cell structure and physiological functions. It also affects the interplay between microbial virulence factors and their specific receptors in the host. This review will focus on the role of Zn in the crosstalk between the host and microbe, especially for changes in microbial pathogenesis and nociceptive neuron-immune interactions, as it may lead to new ways to prevent or treat microbial infections.

secCl is a cys-loop ion channel necessary for the chloride conductance that mediates hormone-induced fluid secretion in Drosophila

Organisms use circulating diuretic hormones to control water balance (osmolarity), thereby avoiding dehydration and managing excretion of waste products. The hormones act through G-protein-coupled receptors to activate second messenger systems that in turn control the permeability of secretory epithelia to ions like chloride. In insects, the chloride channel mediating the effects of diuretic hormones was unknown. Surprisingly, we find a pentameric, cys-loop chloride channel, a type of channel normally associated with neurotransmission, mediating hormone-induced transepithelial chloride conductance. This discovery is important because: 1) it describes an unexpected role for pentameric receptors in the membrane permeability of secretory epithelial cells, and 2) it suggests that neurotransmitter-gated ion channels may have evolved from channels involved in secretion.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

The orphan pentameric ligand-gated ion channel pHCl-2 is gated by pH and regulates fluid secretion in Drosophila Malpighian tubules

Pentameric ligand-gated ion channels (pLGICs) constitute a large protein superfamily in metazoa whose role as neurotransmitter receptors mediating rapid, ionotropic synaptic transmission has been extensively studied. Although the vast majority of pLGICs appear to be neurotransmitter receptors, the identification of pLGICs in non-neuronal tissues and homologous pLGIC-like proteins in prokaryotes points to biological functions, possibly ancestral, that are independent of neuronal signalling. Here, we report the molecular and physiological characterization of a highly divergent, orphan pLGIC subunit encoded by the pHCl-2 (CG11340) gene, in Drosophila melanogaster We show that pHCl-2 forms a channel that is insensitive to a wide array of neurotransmitters, but is instead gated by changes in extracellular pH. pHCl-2 is expressed in the Malpighian tubules, which are non-innervated renal-type secretory tissues. We demonstrate that pHCl-2 is localized to the apical membrane of the epithelial principal cells of the tubules and that loss of pHCl-2 reduces urine production during diuresis. Our data implicate pHCl-2 as an important source of chloride conductance required for proper urine production, highlighting a novel role for pLGICs in epithelial tissues regulating fluid secretion and osmotic homeostasis.