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

Ticks represent a major health issue for humans and domesticated animals. Assessing the expression patterns of the tick’s central nervous system, known as the synganglion, is an important step in understanding tick physiology and in managing tick-borne diseases. Neuron-specific genes like the cys-loop ligand-gated ion channels (cys-loop LGICs) are important pharmacological targets of acaricides. Here, 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 related to biological processes or functions associated with neurotransmission and located in neurones or the synaptic membrane, including most of the cys-loop LGICs. As a first step of a functional study of cysLGICs, we cloned the predicted sequence of the resistance to dieldrin (RDL) subunit homologue, and functionally reconstituted the first GABA-gated receptor of Ixodes ricinus using a hetrologous expression approach. A phylogenetic study was performed for the nicotinic acetylcholine receptors (nAChRs) and for other cys-loop LGICs respectively, showing tick-specific expansions of some types of receptors (Histamine-gated, GluCls).