Odorant receptor-based discovery of natural repellents of human lice

Cuticular extracts induce aggregation in head lice

The human head lice Pediculus humanus capitis (De Geer) (Phthiraptera: Pediculidae) are strict, obligate human ectoparasites that spends their entire life cycle in the host and cause skin irritation and derived infections. Despite the health-related importance, few studies have evaluated the chemical communication among these insects. Here, we evaluate the response of lice of both sexes to cuticular extracts using two solvents of different polarity (hexane and methanol). Cuticular extracts that elicited an attraction response towards head lice were analysed by gas chromatography-mass spectrometry (GC-MS) to determine the cuticular lipid profile. Both lice sexes were attracted to the hexane extracts but not the methanol extracts, suggesting the non-polarity of the compounds present in the cuticle. Chemical analyses of hexane extracts from males and females showed high similarity in major compounds. This study provides the first evidence that lice respond to cuticle extracts, which may be important to understand aggregation behaviour.

Biosensors for Odor Detection: A Review

Animals can easily detect hundreds of thousands of odors in the environment with high sensitivity and selectivity. With the progress of biological olfactory research, scientists have extracted multiple biomaterials and integrated them with different transducers thus generating numerous biosensors. Those biosensors inherit the sensing ability of living organisms and present excellent detection performance. In this paper, we mainly introduce odor biosensors based on substances from animal olfactory systems. Several instances of organ/tissue-based, cell-based, and protein-based biosensors are described and compared. Furthermore, we list some other biological materials such as peptide, nanovesicle, enzyme, and aptamer that are also utilized in odor biosensors. In addition, we illustrate the further developments of odor biosensors.

Conserved Odorant Receptors Involved in Nonanal-Induced Female Attractive Behavior in Two Spodoptera Species

Moths rely on plant volatiles to locate appropriate plants for feeding and laying eggs. While extensive research has been conducted on the global agricultural pests, Spodoptera frugiperda and Spodoptera litura, their molecular mechanisms for detecting plant volatiles remain mostly unknown. Here, we have demonstrated that nonanal, a common plant volatile, is attractive for both virgin and gravid females of the two species. Second, we have identified a conserved odorant receptor clade (SfruOR47 clade) that is primarily tuned to nonanal. Finally, by three-dimensional (3D) structure prediction, molecular docking, and site-directed mutagenesis, we have revealed that the His57 and Glu61 residues, also shared by other six orthologous ORs, are essential for nonanal binding in SfruOR47 and SlituOR9, indicating the conserved structure and function of ORs in the SfruOR47 clade. These findings offer novel insights into the molecular mechanisms and evolutionary aspects of moth behavior in response to plant volatiles.

Essential Oils in Urban Insect Management-A Review

The allures of city life have culminated in the rise of urban populations resulting in conditions that promote the establishment of certain insect pests. Globally, the public health significance of these urban insect pests is enormous, ranging from billions of dollars to loss of lives. Most chemical insecticides no longer provide the anticipated level of control, and significant insecticide resistance has been reported. Therefore, there has been a spike in interest for alternatives to conventional insecticides. Among them, natural products from plants such as essential oils (EOs) and essential oil components (EOCs) have enjoyed the most attention owing to widespread reports of efficacy and toxicity even against insecticide-resistant urban insects. Yet, there is no comprehensive synthesis on the extent and impact of the management of urban insects using EOs or EOCs. Such a review is highly relevant since it provides a means to assess the extent of progress made, shortfalls, limitations, and prospects. More so, we hope it can be used to make informed decisions and develop relevant policies reliably. We present the ranges of insecticidal effects of EOs, EOCs, and commercially available EO-based products from laboratory and field studies. Finally, we discuss the gaps in our knowledge and prospects for the sustainable use of EOs.

Mosquito odorant receptor sensitive to natural spatial repellents and inhibitory compounds

Previously, we have identified an odorant receptor (OR) from the southern house mosquito Culex quinquefasciatus, CquiOR32, which responded to both odorants (agonists) and inhibitory compounds (antagonists). CquiOR32/CquiOrco-expressing oocytes responded to methyl salicylate and other odorants with inward (regular) currents but gave currents in the reverse direction when challenged with eucalyptol and other inhibitors. To determine whether hitherto unknown ORs show this intrareceptor inhibition, we have now examined two other receptors in the same cluster, CquiOR27 and CquiOR28. We cloned and tested four variants of CquiOR28, but none of the 250 compounds in our panel of odorants, including an Orco ligand candidate (OLC12), elicited inward or upward deflections of the current traces. By contrast, CquiOR27/CquiOrco-expressing oocytes gave robust, dose-dependent inward currents when challenged with γ-octalactone and other odorants. On the other hand, octylamine and other phenolic compounds elicited dose-dependent currents in the reverse direction. When stimulatory and inhibitory compounds were presented in binary mixtures, γ-octalactone-elicited inward currents were attenuated in a dose-dependent manner according to the concentration of octylamine. As part of our chemical ecology approach, we tested the repellency activity of the most potent ligands in the surface landing and feeding assay and a newly reported hand-in cage assay. Protection elicited by γ-octalactone did not differ significantly from that of DEET at the same dose. In the hand-in cage assay, a cream formulation of γ-octalactone showed 97.0 ± 1.3% protection, with 47.6 ± 8.3% and 1.4 ± 0.7% landings per trial in the hands covered with a control and γ-octalactone cream, respectively (N = 8, p = 0.0078, Wilcoxon matched-pairs signed-rank test).

The closer the better: Sensory tools and host-association in blood-sucking insects

Many hematophagous insects acquire medical and veterinary relevance because they transmit disease causing pathogens to humans. Hematophagy is only fulfilled once a blood feeder successfully locates a vertebrate host by means of fine sensory systems. In nature, blood-sucking insects can exploit environments with differential association with their hosts. Given the relevance of the sensory systems during host searching, we review the current state of knowledge of the sensory machinery of four blood-sucking insects: human lice, bed bugs, kissing bugs and mosquitoes. Each one is representative of highly anthropophilic behaviours and a different degree of association with human hosts. We compare the number, arrangement and functional type of cuticular sensory structures dispersed on the main sensory organs. We also compare the genetic machinery potentially involved in the detection of host stimuli. Finally, we discuss the sensory diversity of the insects studied here.

Mouthpart sensory structures of the human head louse Pediculus humanus capitis

Head lice are exclusive human parasitic blood-sucking insects. Distributed worldwide among school-age children, this parasitosis generates scalp irritation and sometimes social prejudice. Understanding how head lice detect and perceive their human hosts is crucial to control transmission. Here, we describe the sensory structures present on the mouthparts of Pediculus humanus capitis and their possible contribution to the feeding decision-making process. On the anterior zone of the clypeus around the haustellum two morphological types of sensilla, invariable in location and number, were identified: fourteen short clypeus bristles (SCB) and six long clypeus bristles (LCB). During feeding these structures contact the host skin but not its blood. Located antero-dorsally on the everted haustellum and between the epipharyngeal teeth, a third sensillar type was identified: about four short peg epipharyngeal (SPE) sensilla. These structures are bathed with the incoming blood, when head lice feed, so may have a gustatory role. In behavioural experiments antennectomy of lice did not interfere with feeding behaviour, suggesting that the sensory structures on the mouthparts could be involved in host assessment.

Identification of a General Odorant Receptor for Repellents in the Asian Corn Borer Ostrinia furnacalis

Attractants and repellents are considered to be an environment-friendly approach for pest management. Odorant receptors (ORs), which are located on the dendritic membranes of olfactory sensory neurons in insects, are essential genes for recognizing attractants and repellents. In the Asian corn borer, Ostrinia furnacalis, ORs that respond to sex pheromones have been characterized, but general ORs for plant odorants, especially for repellents, have not been identified. Nonanal is a plant volatile of maize that could result in avoidance of the oviposition process for female adults in O. furnacalis. In this study, we identified a female-biased OR that responds to nonanal using a Xenopus oocyte expression system. In addition, we found that OfurOR27 was also sensitive to two other compounds, octanal and 1-octanol. Behavioral analysis showed that octanal and 1-octanol also caused female avoidance of oviposition. Our results indicated that OfurOR27 is an OR that is sensitive to repellents. Moreover, the two newly identified repellents may help to develop a chemical ecology approach for pest control in O. furnacalis.

The Sensory Machinery of the Head Louse Pediculus humanus capitis: From the Antennae to the Brain

Insect antennae are sophisticated sensory organs, usually covered with sensory structures responsible for the detection of relevant signals of different modalities coming from the environment. Despite the relevance of the head louse Pediculus humanus capitis as a human parasite, the role of its antennal sensory system in the highly dependent relation established with their hosts has been barely studied. In this work, we present a functional description of the antennae of these hematophagous insects by applying different approaches, including scanning electron microscopy (SEM), anterograde antennal fluorescent backfills, and behavioral experiments with intact or differentially antennectomized lice. Results constitute a first approach to identify and describe the head louse antennal sensilla and to determine the role of the antenna in host recognition. SEM images allowed us to identify a total of 35-40 sensilla belonging to seven different morphological types that according to their external architecture are candidates to bear mechano-, thermo-, hygro-, or chemo-receptor functions. The anterograde backfills revealed a direct neural pathway to the ipsilateral antennal lobe, which includes 8-10 glomerular-like diffuse structures. In the two-choice behavioral experiments, intact lice chose scalp chemicals and warm surfaces (i.e., 32°C) and avoided wet substrates. Behavioral preferences disappeared after ablation of the different flagellomeres of their antenna, allowing us to discuss about the location and function of the different identified sensilla. This is the first study that integrates morphological and behavioral aspects of the sensory machinery of head lice involved in host perception.

Identification and functional characterization of an odorant receptor in pea aphid, Acyrthosiphon pisum

The sensitive olfactory system is necessary for survival of insects. Odorant receptors (ORs) are located on the dendrites of olfactory receptor neurons and play a critical role in odor detection. Insect ORs are functionally analyzed via heterologous expression in a Xenopus oocyte system using a two-electrode voltage-clamp (TEVC) electrophysiological recording. Here, we have identified a novel OR in the pea aphid, Acyrthosiphon pisum, then we cloned and named it ApisOR4. We analyzed the ApisOR4 tissue expression patterns and found expression only in antennae tissues. Further functional analysis using TEVC revealed that ApisOR4 is broadly tuned to eight volatiles, which elicit electrophysiological response in pea aphid antennae. This study provides an initial functional analysis of aphid ORs and identifies candidate volatiles to be used in developing new strategies for aphid control.

Reverse chemical ecology-based approach leading to the accidental discovery of repellents for Rhodnius prolixus, a vector of Chagas diseases refractory to DEET

Rhodnius prolixus is one of the most important vectors of Chagas disease in Central and South America for which repellents and attractants are sorely needed. Repellents like DEET, picaridin, and IR3535 are widely used as the first line of defense against mosquitoes and other vectors, but they are ineffective against R. prolixus. Our initial goal was to identify in R. prolixus genome odorant receptors sensitive to putative sex pheromones. We compared gene expression of 21 ORs in the R. prolixus genome, identified 4 ORs enriched in male (compared with female) antennae. Attempts to de-orphanize these ORs using the Xenopus oocyte recording system showed that none of them responded to putative sex pheromone constituents. One of the them, RproOR80, was sensitive to 4 compounds in our panel of 109 odorants, namely, 2-heptanone, γ-octalactone, acetophenone, and 4-methylcychohexanol. Interestingly, these compounds, particularly 4-methylcyclohexanol, showed strong repellency activity as indicated not only by a significant decrease in residence time close to a host, but also by a remarkable reduction in blood intake. 4-Methylcyclohexanol-elicited repellency activity was abolished in RNAi-treated insects. In summary, our search for pheromone receptors led to the discovery of repellents for R. prolixus.

The Two Main Olfactory Receptor Families in Drosophila, ORs and IRs: A Comparative Approach

Most insect species rely on the detection of olfactory cues for critical behaviors for the survival of the species, e.g., finding food, suitable mates and appropriate egg-laying sites. Although insects show a diverse array of molecular receptors dedicated to the detection of sensory cues, two main types of molecular receptors have been described as responsible for olfactory reception in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). Although both receptor families share the role of being the first chemosensors in the insect olfactory system, they show distinct evolutionary origins and several distinct structural and functional characteristics. While ORs are seven-transmembrane-domain receptor proteins, IRs are related to the ionotropic glutamate receptor (iGluR) family. Both types of receptors are expressed on the olfactory sensory neurons (OSNs) of the main olfactory organ, the antenna, but they are housed in different types of sensilla, IRs in coeloconic sensilla and ORs in basiconic and trichoid sensilla. More importantly, from the functional point of view, they display different odorant specificity profiles. Research advances in the last decade have improved our understanding of the molecular basis, evolution and functional roles of these two families, but there are still controversies and unsolved key questions that remain to be answered. Here, we present an updated review on the advances of the genetic basis, evolution, structure, functional response and regulation of both types of chemosensory receptors. We use a comparative approach to highlight the similarities and differences among them. Moreover, we will discuss major open questions in the field of olfactory reception in insects. A comprehensive analysis of the structural and functional convergence and divergence of both types of receptors will help in elucidating the molecular basis of the function and regulation of chemoreception in insects.

Cryo-EM structure of the insect olfactory receptor Orco

The olfactory system must recognize and discriminate amongst an enormous variety of chemicals in the environment. To contend with such diversity, insects have evolved a family of odorant-gated ion channels comprised of a highly conserved co-receptor (Orco) and a divergent odorant receptor (OR) that confers chemical specificity. Here, we present the single-particle cryo-electron microscopy structure of an Orco homomer from the parasitic fig wasp Apocrypta bakeri at 3.5 Å resolution, providing structural insight into this receptor family. Orco possesses a novel channel architecture, with four subunits symmetrically arranged around a central pore that diverges into four lateral conduits that open to the cytosol. The Orco tetramer has few inter-subunit interactions within the membrane and is bound together by a small cytoplasmic anchor domain. The minimal sequence conservation among ORs maps largely to the pore and anchor domain, shedding light on how the architecture of this receptor family accommodates its remarkable sequence diversity and facilitates the evolution of odour tuning.

Citrus medica: nutritional, phytochemical composition and health benefits – a review

Health benefits and products ofCitrus medica.

Molecular basis of peripheral olfactory sensing during oviposition in the behavior of the parasitic wasp Anastatus japonicus

Anastatus japonicus is a parasitic wasp and natural enemy of the litchi pest Tessaratoma papillosa, and for decades in China, A. japonicus has been mass-reared inside the eggs of Antheraea pernyi to control T. papillosa. A series of experiments was performed to explore the olfactory mechanism underlying the oviposition behavior of A. japonicus. First, a transcriptomic analysis was performed on the antennae of A. japonicus, and the resulting assemblies led to the generation of 70,473 unigenes. Subsequently, 21,368 unigenes were matched to known proteins, 48 odorant receptors (ORs) (including Orco) and 13 antennal ionotropic receptors (IRs) (including the co-receptors IR8a and IR25a) were identified and predicted to form complete open reading frames (ORFs). The FPKM (fragments per Kb per million reads) values and RT-PCR results showed that AjapOrco, AjapOR10, AjapOR27, AjapOR33 and AjapOR35 were either highly abundant or expressed specifically in the olfactory organs. Furthermore, AjapOrco silencing resulted in a significant decrease in both the parasitism rate and the host-seeking time of A. japonicus, whereas dsRNA injection showed that IR8a and IR25a did not produce significant behavioral changes, suggesting that the oviposition behavior of A. japonicus is more reliant on OR-based pathways than IR-based pathways. Our previous GC-MS data derived twenty-nine compounds which were abundent from these host plants and host insects. We performed electrophysiological and oviposition assays on A. japonicus, and eight odorants were found to elicit a significant electroantennogram (EAG) response. Among these odorants, β-Caryophyllene, Undecane, (E)-α-Farnesene (+)-Aromadendrene and Cis-3-Hexen-ol had strong attractant effects on oviposition, whereas 2-Ethyl-1-Hexan-ol, Ethyl Acetate and α-Caryophyllene had a strong repellant effects. Thus, these chemicals might influence oviposition guidance/repulsion behavior in A. japonicus. To further explore the target ORs that are tuned to the functional odorants, the nine candidate ORs described above were silenced by RNA interference, and the results showed that a large decrease in the EAG response of all the tested functional odorants in the AjapOrco-silencing group. In addition, the AjapOR35-silencing group showed a significant decrease in the EAG response to β-Caryophyllene and (E)-α-Farnesene, indicating that AjapOR35 is tuned to these two oviposition attractants β-Caryophyllene and (E)-α-Farnesene. Further binary-choice oviposition assays showed that the oviposition attractant effect of β-Caryophyllene and (E)-α-Farnesene vanished after AjapOR35 was silenced, indicating that the emission of these attractants from host plants can guide A. japonicus to locate eggs for ovipositioning and indicated that AjapOR35 is correlated with the olfactory detection oviposition behavior of this species. This study provides a better understanding of the molecular basis and functional chemicals underlying the oviposition behavior of A. japonicus, and the results may help improve biocontrol approaches.

Arrestant Effect of Human Scalp Components on Head Louse (Phthiraptera: Pediculidae) Behavior

Relevant evidence has shown that parasites process host-related information using chemical, visual, tactile, or auditory cues. However, the cues that are involved in the host-parasite interaction between Pediculus humanus capitis (De Geer 1767) and humans have not been identified yet. In this work, we studied the effect of human scalp components on the behavior of adult head lice. Filter paper segments were rubbed on volunteers' scalps and then placed in the experimental arena, where adult head lice were individually tested. The movement of the insects was recorded for each arena using the software EthoVision. Average movement parameters were calculated for the treatments in the bioassays such as total distance, velocity, number of times a head louse crossed between zones of the arena, and time in each zone of the arena. We found that scalp components induced head lice to decrease average locomotor activity and to remain arrested on the treated paper. The effect of the ageing of human scalp samples in the response of head lice was not statistically significant (i.e., human scalp samples of 4, 18, 40, and 60 h of ageing did not elicit a significant change in head louse behavior). When we analyzed the effect of the sex in the response of head lice to human scalp samples, males demonstrated significant differences. Our results showed for the first time the effect of host components conditioning head lice behavior. We discuss the role of these components in the dynamic of head lice infestation.

Identification of repellent odorants to the body louse, Pediculus humanus corporis, in clove essential oil

The control of body lice is an important issue for human health and welfare because lice act as vectors of disease such as typhus, relapsing fever, and trench fever. Body lice exhibit avoidance behavior to some essential oils, including clove essential oil. Therefore, odorants containing clove essential oil components may potentially be useful in the development of repellents to body lice. However, such odorants that induce avoidance behavior in body lice have not yet been identified from clove essential oil. Here, we established an analysis method to evaluate the avoidance behavior of body lice to specific odorants. The behavioral analysis of the body lice in response to clove essential oil and its constituents revealed that eugenol, a major component of clove essential oil, has strong repellent effect on body lice, whereas the other components failed to induce obvious avoidance behavior. A comparison of the repellent effects of eugenol with those of other structurally related odorants revealed possible moieties that are important for the avoidance effects to body lice. The repellent effect of eugenol to body lice was enhanced by combining it with the other major component of clove essential oil, β-caryophyllene. We conclude that a synthetic blend of eugenol and β-caryophyllene is the most effective repellent to body lice. This finding will be valuable as the potential use of eugenol as body lice repellent.