The molecular targets of ivermectin and lotilaner in the human louse Pediculus humanus humanus: New prospects for the treatment of pediculosis

Control of infestation by cosmopolitan lice (Pediculus humanus) is increasingly difficult due to the transmission of parasites resistant to pediculicides. However, since the targets for pediculicides have no been identified in human lice so far, their mechanisms of action remain largely unknown. The macrocyclic lactone ivermectin is active against a broad range of insects including human lice. Isoxazolines are a new chemical class exhibiting a strong insecticidal potential. They preferentially act on the γ-aminobutyric acid (GABA) receptor made of the resistant to dieldrin (RDL) subunit and, to a lesser extent on glutamate-gated chloride channels (GluCls) in some species. Here, we addressed the pediculicidal potential of isoxazolines and deciphered the molecular targets of ivermectin and the ectoparasiticide lotilaner in the human body louse species Pediculus humanus humanus. Using toxicity bioassays, we showed that fipronil, ivermectin and lotilaner are efficient pediculicides on adult lice. The RDL (Phh-RDL) and GluCl (Phh-GluCl) subunits were cloned and characterized by two-electrode voltage clamp electrophysiology in Xenopus laevis oocytes. Phh-RDL and Phh-GluCl formed functional homomeric receptors respectively gated by GABA and L-glutamate with EC₅₀ values of 16.0 μM and 9.3 μM. Importantly, ivermectin displayed a super agonist action on Phh-GluCl, whereas Phh-RDL receptors were weakly affected. Reversally, lotilaner strongly inhibited the GABA-evoked currents in Phh-RDL with an IC₅₀ value of 40.7 nM, whereas it had no effect on Phh-GluCl. We report here for the first time the insecticidal activity of isoxazolines on human ectoparasites and reveal the mode of action of ivermectin and lotilaner on GluCl and RDL channels from human lice. These results emphasize an expected extension of the use of the isoxazoline drug class as new pediculicidal agents to tackle resistant-louse infestations in humans.     

Mucosal layers and related nerve fibres in non-chagasic and chagasic human colon—a quantitative immunohistochemical study

Chagasic megacolon is accompanied by extensive myenteric and, simultaneously, moderate submucosal neuron loss. Here, we examined changes of the innervation pattern of the lamina propria (LP) and muscularis mucosae (MM). Two alternating sets of cryosections were taken from seven non-chagasic colonic and seven chagasic megacolonic specimens (the latter included both the dilated megacolonic and the non-dilated transitional oral and anal zones) and were immunohistochemically triple-stained for smooth-muscle actin (SMA), synaptophysin (SYN) and glial acid protein S100 and, alternatively, for SMA, vasoactive intestinal peptide (VIP) and somatostatin (SOM). Subsequent image analysis and statistical evaluation of nervous tissue profile areas revealed that, in LP, the most extreme differences (i.e. increase in thickness or decrease in nerve, glia and muscle tissue profile area, respectively) compared with control values occurred in the dilated megacolonic zone itself. In contrast, the most extreme differences in the MM were in the anal-to-megacolonic zone (except the profile area of muscle tissue, which was lowest in the megacolonic zone). This parallels our previous results in the external muscle coat. A partial and selective survival of VIP-immunoreactive in contrast to SOM-immunoreactive nerve fibres was observed in both mucosal layers investigated. Thus, VIPergic nerve elements might be crucial for the maintenance of the mucosal barrier. The differential changes of neural tissue parameters in LP and MM might reflect a multifactorial rather than a pure neurogenic development of megacolon in chronic Chagas’ disease.     

Colicin import and pore formation: a system for studying protein transport across membranes?

Pore-forming colicins are a family of protein toxins (M_r40–70kDa) produced by Escherichia coli and related bacteria. They are bactericidal by virtue of their ability to form ion channels in the inner membrane of target cells. They provide a useful means of studying questions such as toxin action, polypeptide translocation across and into membranes, voltage-gated channels and receptor function. These colicins bind to a receptor in the outer membrane before being translocated across the cell envelope with the aid of helper proteins that belong to nutrient-uptake systems and the so-called‘Tol’proteins, the function of which has not yet been properly defined. A distinct domain appears to be associated with each of three steps (receptor binding, translocation and formation of voltage-gated channels). The Tol-dependent uptake pathway is described here. The structures and interactions of TolA, B, Q and R have by now been quite clearly defined. Transmembrane α-helix interactions are required for the functional assembly of the E. coli Tol complex, which is preferentially located at contact sites between the inner and outer membranes. The number of colicin translocation sites is about 1000 per cell. The role and the involvement of the OmpF porin (with colicins A and N) have been described in a recent study on the structural and functional interactions of a colicin-resistant mutant of OmpF. The X-ray crystal structure of the channel-forming fragment of colicin A and that of the entire colicin la have provided the basis for biophysical and site-directed muta-genesis studies. Thanks to this powerful combination, it has been established that the interaction with the receptor in the outer membrane leads to a very substantial conformational change, as a result of which the N-terminal domains of colicins interact with the lumen of the OmpF pore and then with the C-terminal domain of TolA. A molten globular conformation of colicins probably constitutes the intermediate translocation/insertion competent state. Once the pore has formed, the polypeptide chain spans the whole cell envelope. Three distinct steps occur in the last stage of the process: (i) fast binding of the C-terminal domain to the outer face of the cytoplasmic membrane; (ii) a slow insertion of the polypeptide chain into the outer face of the inner membrane in the absence of Δψ and (iii) a profound reorganization of the helix association, triggered by the transmembrane potential and resulting in the formation of the colicin channel.     

PHOTOACCLIMATION IN THE TROPICAL CORALLINE ALGA HYDROLITHON ONKODES (RHODOPHYTA, CORALLINACEAE) FROM A FRENCH POLYNESIAN REEF

Pigment concentration, in vivo absorption, and photosynthetic parameters of the coralline alga Hydrolithon onkodes (Heydrich) Penrose and Woelkerling were compared among samples from a lagoon and from a reef crest of Tahiti Island. Four groups of specimens were considered, differing in their natural exposure to PAR. For specimens collected from the lagoon, the tissues from low-light samples had significantly higher pigment concentration, particularly chl a and phycobilins, compared with the high-light exposed plants that contained more total carotenoids. The in vivo absorption spectra normalized to chl a (called a* values) also revealed differences. The low-light samples had a reduced absorption capacity and a well-marked phycobilin absorption signature, whereas sunlit samples showed a greater absorption at wavelengths absorbed mainly by chl a and carotenoids. The decrease of a* when pigment concentration increased is interpreted as a consequence of the pigment packaging. Significantly lower α (chlorophyll basis) and higher E_k values were found in the shaded plants. The values of P _max for the four groups of specimens were not significantly different. The samples showed various degrees of photoinhibition depending on the light exposure during growth, and this effect was more pronounced in the shaded plants. The specimens from the reef crest deviate from the general model presented for the lagoon samples and show a mix of sun- and shade-exposed characteristics. We have shown that the coralline alga H. onkodes responds to its light environment, probably by acclimation rather than ecotypic genetic variation, by adjusting its physiology, but some morphological differences are also involved. Photoacclimation can explain partly the wide distribution of this species over the reef ecosystem and its major contribution to the building of the reef.     

Evidence for rapid limitation of the I element copy number in a genome submitted to several generations of I-R hybrid dysgenesis in Drosophila melanogaster

Summary When Drosophila melanogaster males coming from a class of strains known as inducer are crossed with females from the complementary class (reactive), a quite specific kind of sterility is observed in the F₁ female progeny (denoted SF). The inducer chromosomes differ from the reactive chromosomes by the presence of a transposable element (called the I factor) that is responsible for the induction of this dysgenic symptom. In the germ line of dysgenic females, up to 100% of the reactive chromosomes may be contaminated, i.e. they acquire I factor(s) owing to very frequent replicative transpositions. A contaminated reactive stock was obtained by reconstructing the reactive genotype in the offspring of SF females and its kinetics of invasion by I elements was followed in the successive inbred dysgenic generations. The results show that the mean copy number of I elements increased very quickly up to the level of inducer strains and then stayed in equilibrium even though the dysgenic state was perpetuated by selection for SF sterility at every generation. The possible mechanisms of this copy number limitation are discussed.