Molecular and functional characterization of GABA receptor subunits GRD and LCCH3 from human body louse Pediculus humanus humanus .

Human louse Pediculus humanus is a cosmopolitan obligatory blood-feeding ectoparasite causing pediculosis and transmitting many bacterial pathogens. Control of infestation is difficult due to the developed resistance to insecticides that mainly target GABA (γ-amino butyric acid) receptors. Previous work showed that Pediculus humanus humanus (Phh) GABA receptor subunit RDL is the target of lotilaner, a synthetic molecule of the isoxazoline chemical class. To enhance our understanding of how insecticides act on GABA receptors, two other GABA receptors subunits were cloned and characterized: three variants of Phh-grd and one variant of Phh-lcch3 Relative mRNA expression levels of Phh-rdl , Phh-grd and Phh-lcch3 revealed that they were expressed throughout the developmental stages (eggs, larvae, adults) and in the different parts of adult lice (head, thorax and abdomen). When expressed individually in the Xenopus oocyte heterologous expression system, Phh-GRD1, Phh-GRD2, Phh-GRD3 and Phh-LCCH3 were unable to reconstitute functional channels, whereas the subunit combinations Phh-GRD1/Phh-LCCH3, Phh-GRD1/Phh-RDL and Phh-LCCH3/Phh-RDL responded to GABA in a concentration-dependent manner. The three heteromeric receptors were similarly sensitive to the antagonistic effect of picrotoxin and fipronil, while Phh-GRD1/Phh-RDL and Phh-LCCH3/Phh-RDL were respectively about 2.5-fold and 5-fold more sensitive to ivermectin than Phh-GRD1/Phh-LCCH3. Moreover, the heteropentameric receptor constituted by Phh-GRD1/Phh-LCCH3 was found to be permeable and highly sensitive to the extracellular sodium concentration. These findings provided valuable additions to our knowledge of the complex nature of GABA receptors in human louse that could help in understanding the resistance pattern to commonly used pediculicides Significance Statement Human louse is an ectoparasite that causes pediculosis and transmits several bacterial pathogens. Emerging strains developed resistance to the commonly used insecticides, especially those targeting GABA receptors. To understand the molecular mechanisms underlying this resistance, two subunits of GABA receptors were cloned and described: Phh-grd and Phh-lcch3 The heteromeric receptor reconstituted with the two subunits was functional in Xenopus oocytes and sensitive to commercially available insecticides. Moreover, both subunits were transcribed throughout the parasite lifecycle.