Signal recognition and transduction in olfactory neurons

Transcriptomic and Gene Expression Analysis of Chemosensory Genes from White Grubs of Hylamorpha elegans (Coleoptera: Scarabaeidae), a Subterranean Pest in South America

Olfaction and gustation processes play key roles in the life cycle of insects, such as finding and accepting food sources, oviposition sites, and mates, among other fundamental aspects of insect development. In this context, chemosensory genes found in sensory organs (e.g., antennae and maxillary palps) are crucial for understanding insect behaviour, particularly the phytophagous behaviour of insect pests that attack economically important crops. An example is the scarab beetle Hylamorpha elegans, which feeds on the roots of several crops important for livestock in its larval stage. In this study, chemosensory gene candidates of H. elegans white grubs identified through the head transcriptome and phylogenetic and tissue-biased gene expression (antennae, head without antennae, and legs) have been reported. Overall, 47 chemosensory genes were identified (2 ORs, 1 GR, 11 IRs, 9 CSPs, and 24 OBPs). Gene expression analysis revealed the predominant presence of IRs in the legs, whereas ORs and the GR were present in the heads and/or antennae. Particularly, HeleOBP9 and HeleCSP2 were significantly expressed in the head but not in the antennae or legs; these and other genes are discussed as potential targets in the context of H. elegans management.

Insights Into Chemosensory Proteins From Non-Model Insects: Advances and Perspectives in the Context of Pest Management

Nowadays, insect chemosensation represents a key aspect of integrated pest management in the Anthropocene epoch. Olfaction-related proteins have been the focus of studies due to their function in vital processes, such ashost finding and reproduction behavior. Hence, most research has been based on the study of model insects, namely Drosophila melanogaster, Bombyx mori or Tribolium castaneum. Over the passage of time and the advance of new molecular techniques, insects considered non-models have been studied, contributing greatly to the knowledge of insect olfactory systems and enhanced pest control methods. In this review, a reference point for non-model insects is proposed and the concept of model and non-model insects is discussed. Likewise, it summarizes and discusses the progress and contribution in the olfaction field of both model and non-model insects considered pests in agriculture.

Functional characteristics of chemosensory proteins in the sawyer beetle Monochamus alternatus Hope

The Japanese pine sawyer, Monochamus alternatus Hope (Coleoptera: Cerambycidae), is a major pest of pines and it is also the key vector of the exotic pinewood nematode in China. In the present study, we cloned, expressed, and purified a chemosensory protein (CSP) in M. alternatus. We surveyed its expression in various developmental stages of male and female adult tissues and determined its binding affinities for different pine volatiles using a competitive binding fluorescence assay. A CSP known as CSP5 in M. alternatus was obtained from an antennal cDNA library and expressed in Escherichia coli. Quantitative reverse transcription polymerase chain reaction results indicated that the CSP5 gene was mainly expressed in male and female antennae. Competitive binding assays were performed to test the binding affinity of recombinant CSP5 to 13 odour molecules of pine volatiles. The results showed that CSP5 showed very strong binding abilities to myrcene, (+)-β-pinene, and (-)-isolongifolene, whereas the volatiles 2-methoxy-4-vinylphenol, p-cymene, and (+)-limonene oxide have relatively weak binding affinity at pH 5.0. Three volatiles myrcene, (+)-β-pinene, and (-)-isolongifolene may play crucial roles in CSP5 binding with ligands but this needs further study for confirmation. The sensitivity of insect to host plant volatiles can effectively be used to control and monitor the population through mass trapping as part of integrated pest management programs.

Identification of Differentially Expressed Genes In Deltamethrin-Resistant Culex pipiens quinquefasciatus

Culex quinquefasciatus is one of China's major house-dwelling mosquito species and an important vector of filariasis and encephalitis. Chemical treatments represent one of the most successful approaches for comprehensive mosquito prevention and control. However, the widespread use of chemical pesticides has led to the occurrence and development of insecticide resistance. Therefore, in-depth studies of resistance to insecticides are of vital importance. In this study, we performed a gene expression analysis to investigate genes from Cx. quinquefasciatus that may confer pyrethroid resistance. We aimed to understand the mechanisms of Cx. quinquefasciatus resistance to pyrethroid insecticides and provide insights into insect resistance management. Using a resistance bioassay, we determined the deltamethrin LC₅₀ values (lethal concentration required to kill 50% of the population) for Cx. quinquefasciatus larvae in the F₂₁, F₂₃, F₂₄, F₂₆, F₂₇, and F₃₀ generations. The 7 tested strains exhibited pesticide resistance that was 25.25 to 87.83 times higher than that of the SanYa strain. Moreover, the expression of the OBPjj7a (odorant-binding protein OBPjj7a), OBP28 (odorant-binding protein OBP28), and E2 (ubiquitin-conjugating enzyme) genes was positively correlated with deltamethrin resistance ( R² = 0.836, P = 0.011; R² = 0.788, P = 0.018; and R² = 0.850, P = 0.009, respectively) in Cx. quinquefasciatus. The expression of 4 additional genes, H/ACA, S19, SAR2, and PGRP, was not correlated with deltamethrin resistance. In summary, this study identified 3 Cx. quinquefasciatus genes with potential involvement in deltamethrin resistance, and these results may provide a theoretical basis for the control of mosquito resistance and insights into resistance detection.

Identification of odorant binding proteins and chemosensory proteins in Microplitis mediator as well as functional characterization of chemosensory protein 3

Odorant binding proteins (OBPs) and chemosensory proteins (CSPs) play important roles in transporting semiochemicals through the sensillar lymph to olfactory receptors in insect antennae. In the present study, twenty OBPs and three CSPs were identified from the antennal transcriptome of Microplitis mediator. Ten OBPs (MmedOBP11-20) and two CSPs (MmedCSP2-3) were newly identified. The expression patterns of these new genes in olfactory and non-olfactory tissues were investigated by real-time quantitative PCR (qPCR) measurement. The results indicated that MmedOBP14, MmedOBP18, MmedCSP2 and MmedCSP3 were primarily expressed in antennae suggesting potential olfactory roles in M. mediator. However, other genes including MmedOBP11-13, 15-17, 19-20 appeared to be expressed at higher levels in body parts than in antennae. Focusing on the functional characterization of MmedCSP3, immunocytochemistry and fluorescent competitive binding assays were conducted indoors. It was found that MmedCSP3 was specifically located in the sensillum lymph of olfactory sensilla basiconca type 2. The recombinant MmedCSP3 could bind several types of host insects odors and plant volatiles. Interestingly, three sex pheromone components of Noctuidae insects, cis-11-hexadecenyl aldehyde (Z11-16: Ald), cis-11-hexadecanol (Z11-16: OH), and trans-11-tetradecenyl acetate (E11-14: Ac), showed high binding affinities (Ki = 17.24-18.77 μM). The MmedCSP3 may be involved in locating host insects. Our data provide a base for further investigating the physiological roles of OBPs and CSPs in M. mediator, and extend the function of MmedCSP3 in chemoreception of M. mediator.

Structure-based discovery of potentially active semiochemicals for Cydia pomonella (L.)

The development of physiologically active semiochemicals is largely limited by the labor-consuming searching process. How to screen active semiochemicals efficiently is of significance to the extension of behavior regulation in pest control. Here pharmacophore modeling and shape-based virtual screening were combined to predict candidate ligands for Cydia pomonella pheromone binding protein 1 (CpomPBP1). Out of the predicted compounds, ETrME displayed the highest affinity to CpomPBP1. Further studies on the interaction between CpomPBP1 and ETrME, not only depicted the binding mode, but also revealed residues providing negative and positive contributions to the ETrME binding. Moreover, key residues involved in interacting with ETrME of CpomPBP1 were determined as well. These findings were significant to providing insights for the future searching and optimization of active semiochemicals.

Structural insights into Cydia pomonella pheromone binding protein 2 mediated prediction of potentially active semiochemicals

Given the advantages of behavioral disruption application in pest control and the damage of Cydia pomonella, due progresses have not been made in searching active semiochemicals for codling moth. In this research, 31 candidate semiochemicals were ranked for their binding potential to Cydia pomonella pheromone binding protein 2 (CpomPBP2) by simulated docking, and this sorted result was confirmed by competitive binding assay. This high predicting accuracy of virtual screening led to the construction of a rapid and viable method for semiochemicals searching. By reference to binding mode analyses, hydrogen bond and hydrophobic interaction were suggested to be two key factors in determining ligand affinity, so is the length of molecule chain. So it is concluded that semiochemicals of appropriate chain length with hydroxyl group or carbonyl group at one head tended to be favored by CpomPBP2. Residues involved in binding with each ligand were pointed out as well, which were verified by computational alanine scanning mutagenesis. Progress made in the present study helps establish an efficient method for predicting potentially active compounds and prepares for the application of high-throughput virtual screening in searching semiochemicals by taking insights into binding mode analyses.

Three amino acid residues of an odorant-binding protein are involved in binding odours in Loxostege sticticalis L

Odorant-binding proteins (OBPs) play an important role in insect olfactory processes and are thought to be responsible for the transport of pheromones and other semiochemicals across the sensillum lymph to the olfactory receptors within the antennal sensilla. As an important general odorant binding protein in the process of olfactory recognition, LstiGOBP1 of Loxostege sticticalis L. has been shown to have good affinity to various plant volatiles. However, the binding specificity of LstiGOBP1 should be further explored in order to better understand the olfactory recognition mechanism of L. sticticalis. In this study, real-time PCR experiments indicated that LstiGOBP1 was expressed primarily in adult antennae. Homology modelling and molecular docking were then conducted on the interactions between LstiGOBP1 and 1-heptanol to understand the interactions between LstiGOBP1 and their ligands. Hydrogen bonds formed by amino acid residues might be crucial for the ligand-binding specificity on molecular docking, a hypothesis that was tested by site-directed mutagenesis. As predicted binding sites for LstiGOBP1, Thr15, Trp43 and Val14 were replaced by alanine to determine the changes in binding affinity. Finally, fluorescence assays revealed that the mutants Thr15 and Trp43 had significantly decreased binding affinity to most odours; in mutants that had two-site mutations, the binding to the six odours that were tested was completely abolished. This result indicates that Thr15 and Trp43 were involved in binding these compounds, possibly by forming multiple hydrogen bonds with the functional groups of the ligands. These results provide new insights into the detailed chemistry of odours' interactions with proteins.

Construction and analysis of antennal cDNA library from rice striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae), and expression profiles of putative odorant-binding protein and chemosensory protein genes

In this study, we constructed a high-quality cDNA library from the antennae of the Chilo suppressalis (Walker) (Lepidoptera: Pyralidae). A total of 1,235 colonies with inserts greater than 0.7 kb were sequenced and analyzed. Homology searching coupled with bioinformatics analysis identified 15 and 7 cDNA sequences, respectively, encoding putative odorant-binding proteins (OBPs) and chemosensory proteins (CSPs). A phylogenetic tree of CsupCSPs showed that each CsupCSP has orthologs in Manduca sexta and Bombyx mori with strong bootstrapping support. One CSP was either very specific or more related to the CSPs of another species than to conspecific CSP. The expression profiles of the OBPs and CSPs in different tissues were measured by real-time quantitative PCR. The results revealed that of the 11 OBP genes, the transcript levels of CsupOBP1, CsupOBP5, and CsupOBP7 were higher in both male and female antennae than those in other tissues. And CsupCSP7 was highly expressed in both male and female antennae. Based on these results, the possible physiological functions of CsupOBPs and CsupCSPs were discussed.

Prediction of the key binding site of odorant-binding protein of Holotrichia oblita Faldermann (Coleoptera: Scarabaeida)

The scarab beetle Holotrichia oblita Faldermann (Coleoptera: Scarabaeidae) is a predominant underground pest in the northern parts of China, and its larvae (grubs) cause great economic losses because of its wide range of host plants and covert habitats. Environmentally friendly strategies for controlling adults would have novel and broad potential applications. One potential pest management measure is the regulation of olfactory chemoreception to control target insect pests. In the process of olfactory recognition, odorant-binding proteins (OBPs) are believed to carry hydrophobic odorants from the environment to the surface of olfactory receptor neurons. To obtain a better understanding of the relationship between OBP structures and their ligands, homology modelling and molecular docking have been conducted on the interaction between HoblOBP1 and hexyl benzoate in the present study. Based on the results, site-directed mutagenesis and binding experiments were combined to describe the binding sites of HoblOBP1 and to explore its ligand-binding mechanism. After homology modelling of HoblOBP1, it was found that the three-dimensional structure of HoblOBP1 consists of six α-helices and three disulphide bridges that connect the helices, and the hydrophobic pockets are both composed of five helices. Based on the docking study, we found that van der Waals interactions and hydrophobic interactions are both important in the bonding between HoblOBP1 and hexyl benzoate. Intramolecular residues formed the hydrogen bonds in the C terminus of the protein and the bonds are crucial for the ligand-binding specificity. Finally, MET48, ILE80 and TYR111 are binding sites predicted for HoblOBP1. Using site-directed mutagenesis and fluorescence assays, it was found that ligands could not be recognized by mutant of Tyr111. A possible explanation is that the compound could not be recognized by the mutant, and remains in the binding cavity because of the loss of the intramolecular hydrogen bonding that acts as a holder. So we believe that Tyr111 of HoblOBP1 is a key binding site. We also believe that Ile80A is a very important binding site, especially to some ligands.

Functional characterization and immunolocalization of odorant binding protein 1 in the lucerne plant bug, Adelphocoris lineolatus (GOEZE)

In the insect phylum, the relationships between individuals and their environment are often modulated by chemical communication. Odorant binding proteins (OBPs) are widely and robustly expressed in insect olfactory organs and play a key role in chemosensing and transporting hydrophobic odorants across the sensillum lymph to the olfactory receptor neuron. In this study, a novel OBP gene (AlinOBP1) in the lucerne plant bug, Adelphocoris lineolatus was identified, cloned and expressed. Real-time PCR results indicated that the expression level of AlinOBP1 gene differed in each developmental stage (from first instar to adult) and was predominantly expressed in the antennae of adults. The expression level of AlinOBP1 was 1.91 times higher in male antennae than in female antennae. The binding properties of AlinOBP1 with 114 odorants were measured using a fluorescence probe, N-phenyl-1-naphthylamine (1-NPN), with fluorescence competitive binding. The results revealed that AlinOBP1 exhibits high binding abilities with two major putative pheromone components, ethyl butyrate and trans-2-hexenyl butyrate. In addition, it was observed that six volatiles released from cotton, octanal, nonanal, decanal, 2-ethyl-1-hexanol, β-caryophyllene and β-ionone also bind to AlinOBP1. Immunocytochemistry analysis showed that AlinOBP1 was expressed in the sensillum lymph of sensilla trichodica and sensilla basiconca. Our results demonstrate that AlinOBP1 may function as a carrier in the chemoperception of the lucerne plant bug.

Molecular characterization of a phospholipase C beta potentially involved in moth olfactory transduction

To clarify the role of phospholipase C (PLC) in insect olfactory transduction, we have undertaken its molecular identification in the moth Spodoptera littoralis. From the analysis of a male antennal expressed sequence tag library, we succeeded in cloning a full-length cDNA encoding a PLC that belongs to the cluster of PLC-beta subtypes. In adult males, the PLC-beta transcript was located predominantly in brain and antennae where its presence was detected in the olfactory sensilla trichodea. Moreover, PLC-beta was expressed in antennae at the beginning of the pupal stage, then reached a maximum at the end of this stage and was maintained at this level during the adult period. Taken together, these results provided molecular evidence for the putative participation of a PLC-beta in signaling pathways responsible for the establishment and the functioning of insect olfactory system.

Cloning, expression and functional analysis of a general odorant-binding protein 2 gene of the rice striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae)

A full-length cDNA encoding a general odorant binding protein 2 (GOBP2) was cloned from the antennae of the rice striped stem borer, Chilo suppressalis (Lepidoptera: Pyralidae), by the combination of reverse transcription PCR (RT-PCR) and rapid amplification of cDNA ends PCR (RACE-PCR). The cDNA contains a 489 bp open reading frame, which encodes a 162 amino acid protein, termed as Ch. suppressalis GOBP2 (CsupGOBP2). CsupGOBP2 is similar in the number of amino acids and protein sequence to GOBP2s in other species of Lepidoptera. RT-PCR results showed that CsupGOBP2 mRNA was highly expressed in the adult antennae of both females and males, as was CsupGOBP2 protein as revealed by Western blot analysis. CsupGOBP2 expressed in Escherichia coli was purified by affinity chromatography, refolding and gel filtration from the inclusion body. Fluorescence emission spectra and competitive binding assays by using N-phenyl-1-naphthylamine as first binding ligand and odorants as potential competitors revealed that the CsupGOBP2 protein has significant affinity to cis-11-hexadecenal (Z11-16:Ald), the main component of Ch. suppressalis pheromone and to laurinaldehyd and benzaldehyde, two general plant volatile aldehydes.

Localization of G protein alpha subunits and morphology of receptor neurons in olfactory and vomeronasal epithelia in Reeve's turtle, Geoclemys reevesii

Most vertebrates have two nasal epithelia: the olfactory epithelium (OE) and the vomeronasal epithelium (VNE). The apical surfaces of OE and VNE are covered with cilia and microvilli, respectively. In rodents, signal transduction pathways involve G alpha olf and G alpha i2/G alpha o in OE and VNE, respectively. Reeve's turtles (Geoclemys reevesii) live in a semiaquatic environment. The aim of this study was to investigate the localization of G proteins and the morphological characteristics of OE and VNE in Reeve's turtle. In-situ hybridization analysis revealed that both G alpha olf and G alpha o are expressed in olfactory receptor neurons (ORNs) and vomeronasal receptor neurons (VRNs). Immunocytochemistry of G alpha olf/s and G alpha o revealed that these two G proteins were located at the apical surface, cell bodies, and axon bundles in ORNs and VRNs. Electron microscopic analysis revealed that ORNs had both cilia and microvilli on the apical surface of the same neuron, whereas VRNs had only microvilli. Moreover G alpha olf/s was located on only the cilia of OE, whereas G alpha o was not located on cilia but on microvilli. Both G alpha olf/s and G alpha o were located on microvilli of VNE. These results imply that, in Reeve's turtle, both G alpha olf/s and G alpha o function as signal transduction molecules for chemoreception in ORNs and VRNs.

Electro-olfactograms are present when odorous stimuli have not been perceived

After chemical stimulation of the human olfactory epithelium it is possible to record a negative response (electro-olfactogram, EOG) which is interpreted as the summated generator potential of olfactory neurons. The aim of the present investigation was to test whether the EOG is present when olfactory stimuli have not been perceived. Stimulation was performed with vanillin and eugenol at supraliminal and subliminal levels. Twelve healthy volunteers participated in the experiments. Stimuli were applied at an interstimulus interval of approximately 60s. Although recordings were successful in 4 of the 12 subjects, for both stimulants EOG could be obtained even when the stimuli had not been perceived by the subjects. EOG recordings in response to supra- and subliminal stimuli exhibited no major differences, except for the onset of the EOG in response to subliminal eugenol-stimuli which were prolonged compared to supraliminal stimulation. All in all, the present data provide a physiological basis for the subliminal influence of odorous stimuli on human behavior.

Molecular cloning and bacterial expression of pheromone binding protein in the antennae of Helicoverpa armigera (Hübner)

A cDNA clone coding for pheromone binding protein was isolated from the antennae of Helicoverpa armigera by RT-PCR and (5'/3')-RACE technique. The full-length of H. armigera pheromone binding protein (HarmPBP) was 952 bp, possessing 162 amino acid residues including a signal peptide of 20 amino acids. Its predicted molecular weight and isoelectric point were 18.26 kDa and 5.23, respectively. This deduced amino acid sequence shared some common structural features with odorant-binding proteins from several moth species, including the six conserved cysteine motif, a typical characteristic of insect's odorant-binding proteins. Northern blot showed that HarmPBP is specifically expressed in the antennae of Helicoverpa armigera and more abundantly expressed in male than female. During the antennal development, HarmPBP is first expressed about 4 days prior to adult eclosion and rises to a plateau 2 days prior to adult eclosion. In order to obtain sufficient PBP for further determining its biochemical and physiological properties, a bacterical expression vector of PBP was constructed and successfully expressed in Escherichia coli. The recombinant PBP was shown to cross-react with an anti-PBP antiserum from Antheraea polyphemus. Polyclonal antibodies against HarmPBP were used to mark the distribution of the protein in olfactory sensilla. Very strong labeling was observed in the sensillum lymph of the hair lumen and of the sensillum-lymph cavity. In the male, HarmPBP is expressed in sensilla trichodea and not in sensilla basiconica, while in the female, it is expressed both in sensilla basiconica and sensilla trichodea.

OMP gene deletion results in an alteration in odorant-induced mucosal activity patterns

Previous behavioral work, using a complex five-odorant identification task, demonstrated that olfactory marker protein (OMP) is critically involved in odor processing to the extent that its loss results in an alteration in odorant quality perception. Exactly how the lack of OMP exerts its influence on the perception of odorant quality is unknown. However, there is considerable neurophysiological evidence that different odorants produce different spatiotemporal patterns of neural activity at the level of the mucosa and that these patterns predict the psychophysically determined perceptual relationship among odorants. In this respect, OMP gene deletion is known to result in a constellation of physiologic defects (i.e., marked reduction in the electroolfactogram (EOG) and altered response and recovery kinetics) that would be expected to alter the odorant-induced spatiotemporal activity patterns that are characteristic of different odorants. This, in turn, would be expected to alter the spatiotemporal patterning of information that results from the mucosal projection onto the bulb, thereby changing odorant quality perception. To test the hypothesis that odorant-induced mucosal activity patterns are altered in mice lacking the gene for OMP, we optically recorded the fluorescent changes in response to odorant stimulation from both the septum and turbinates of both OMP-null and control mice using a voltage-sensitive dye (di-4-ANEPPS Molecular Probes, Eugene, OR) and a Dalsa 120 x 120, 12-bit CCD camera. To maintain continuity with the previous behavioral work, the odorants 2-propanol, citral, carvone, ethylacetoacetate, and propyl acetate were again used. Each odorant was randomly presented to each mucosal surface in a Latin-Square design. The results of this study demonstrated that, for both mouse strains, there do indeed exist different spatiotemporal activity patterns for different odorants. More importantly, however, these patterns significantly differed between OMP-null and control mice. That is, although the general regions of characteristic activity for different odorants were the same in both mouse strains, the patterns in the null animals were degraded relative to controls. These data suggest therefore that the alterations in mucosal activity may serve as the substrate for the behaviorally observed changes in odorant quality perception in the null mutant.

Cloning, expression and immunocytochemical localization of a general odorant-binding protein gene from Helicoverpa armigera (Hübner)

A cDNA clone coding for general odorant-binding protein2 was isolated from the antenna of Helicoverpa armigera by RT-PCR and (5'/3')-RACE technique. Results of sequencing and structural analyses showed that the full-length of GOBP2Harm was 636 bp, possessing 162 amino acid residues and a signal peptide of 21 amino acids. Its predicted molecular weight and isoelectric point were 18.2 kDa and 5.21, respectively. This deduced amino acid sequence shared some common structural features with odorant-binding proteins from several moth species, including the six conserved cysteine motif, typical of insect's OBPs. Northern blot showed that GOBP2Harm is specifically expressed in the antenna of Helicoverpa armigera at similar levels in both sexes. In order to obtain sufficient GOBP2 for further determining its biochemical and physiological properties, a bacterical expression vector of GOBP2 was constructed and successfully expressed. The protein was obtained mainly as insoluble inclusion bodies, that, however, could be solubilized and refolded. The rGOBP2 was purified by affinity chromatography and gel filtration. The rGOBP2 was shown to cross-react with an anti-GOBP antiserum from Antheraea polyphemus. Finally, polyclonal antibodies against GOBP2Harm were used to mark the distribution of the protein in olfactory sensilla and were tested by immuno-electron microscopy. In the male, GOBP2Harm is mainly expressed in sensilla basiconica, while in the female, it is equally expressed in sensilla basiconica and in sensilla trichodea.

Expression pattern in the antennae of a newly isolated lepidopteran Gq protein alpha subunit cDNA

From the antennae of the moth Mamestra brassicae, we have identified a lepidopteran G protein alpha subunit belonging to the Gq family, through immunological detection in crude antennal extract and antennal primary cell cultures, followed by molecular cloning. The complete cDNA sequence (1540 bp) contains an open reading frame encoding a protein of 353 amino acids. This deduced sequence possesses all of the characteristics of the Gq family and shares a very high degree of amino-acid sequence identity with vertebrate (80% with mouse or human Gqalpha) and invertebrate subunits (varying between 60 and 87% for Gqalpha from organisms as diverse as sponge and Drosophila). The expression pattern of the Gq subunit in adult antennae was associated with the olfactory sensilla suggesting a specific role in olfaction. These data provide molecular evidence for a component of the phosphoinositide signaling pathway in moth antennae: this G protein alpha subunit may be involved in the olfaction transduction process through interaction with G-protein-coupled receptors, stimulating the phospholipase C mediated second messenger pathway.

Cross-talk between olfactory second messenger pathways

The second messengers 3'-5'-cyclic-monophosphate (cAMP) and inositol 1,4,5-trisphosphate (InsP3) have been implicated in olfactory signal transduction in various species. The results of the present study provide evidence that the two olfactory second messenger pathways in rat olfactory neurons do not work independently but rather show a functional antagonism: whereas inhibition of phospholipase C (PLC) in isolated olfactory cilia by U-73122 led to an augmentation of odor-induced cAMP signaling, activation of the phosphoinositol pathway resulted in attenuation of odor-induced cAMP formation. Furthermore, this study indicates that elevated cAMP levels cause suppression of odor-induced InsP3 signaling, whereas inhibition of adenylate cyclase (AC) by cisN-(2-phenylcyclopentyl)azacylotridec-1-en-2-amine (MDL-12,330 A) results in potentiation of odor-induced InsP3 formation. Concerning the molecular mechanism involved in cross-interaction, the experimental data indicate that the observed antagonism of elevated cAMP is based on inhibition of PLC activation rather than on stimulation of InsP3 degradation. As blockage of the endogenous protein kinase A (PKA) prevented the inhibitory effect of cAMP, the suppression of odor-induced InsP3 signaling by cAMP may be mediated by a PKA-controlled reaction.

Cellular and molecular constituents of olfactory sensation in vertebrates

Since the discovery of odorant-activated adenylate cyclase in the olfactory receptor cilia, research into the olfactory perception of vertebrates has rapidly expanded. Recent studies have shown how the odor discrimination starts at the receptor level: each of 700-1000 types of the olfactory neurons in the neural olfactory epithelium contains a single type of odor receptor protein. Although the receptors have relatively low specific affinities for odorants, excitation of different types of receptors forms an excitation pattern specific to each odorant in the glomerular layer of the olfactory bulb. It was demonstrated that adenosine 3',5'-cyclic monophosphate (cAMP) is very likely the sole second messenger for olfactory transduction. It was also demonstrated that the affinity of the cyclic nucleotide-gated channel for cAMP regulated by Ca(2+)/calmodulin is solely responsible for the adaptation of the cell. However, many other regulatory components were found in the transduction cascade. Regulated by Ca(2+) and/or the protein-phosphorylation, many of them may serve for the adaptation of the cell, probably on a longer time scale. It may be important to consider the resensitization as a part of this adaptation, as well as to collect kinetic data of each reaction to gain further insight into the olfactory mechanism.

Responsiveness of olfactory neurons to distinct aliphatic aldehydes

The responsiveness of isolated olfactory sensory neurons to stimulation with aliphatic aldehydes of varying chain length (5–10 hydrogenated carbon atoms) was investigated by means of Ca(2+)imaging. More than half the cells examined were responsive to aliphatic aldehydes. Individual cells did not react or reacted to one or multiple aldehydes; in the latter case, cells only reacted to aldehydes of consecutive carbon chain lengths. The largest proportion of cells responded to octanal. It was also demonstrated that a structural difference as small as one hydrogenated carbon atom was detectable by the olfactory neurons. Neurons were increasingly able to discriminate between two aldehydes as the difference in chain length between the two increased. Discrimination between aldehydes with longer carbon chains was reduced. Although the odorants examined belong to a distinct chemical class and differ only slightly in structure, individual olfactory sensory neurons showed quite different receptive properties.

Hybrid systems for virtual screening: interest of fuzzy clustering applied to olfaction

Kohonen neural networks, also known as Self Organizing Map (SOM), offer a useful 2D representation of the compound distribution inside a large chemical database. This distribution results from the compound organization in a molecular diversity hyperspace derived from a large set of molecular descriptors. Fuzzy techniques based on the "concept of partial truth" reveal to be also a valuable tool for the direct exploitation of chemical databases or SOM. In such cases a fuzzy clustering algorithm is used. In this paper, a complete hybrid system, combining SOM and fuzzy clustering, is applied. As example, a series of olfactory compounds was selected. The complexity of such information is that a same compound may exhibit different odors. It is shown how fuzzy logic helps to have a better understanding of the organization of the compounds. These hybrid systems, using simultaneously SOM and fuzzy clustering, are foreseen as powerful tools for "virtual pre-screening".

Olfactory reception in invertebrates

Recent progress in understanding the principles and mechanisms in olfaction is the result of multidisciplinary research efforts that explored chemosensation by using a variety of model organisms. Studies on invertebrates, notably nematodes, insects, and crustaceans, to which diverse experimental approaches can be applied, have greatly helped elucidate various aspects of olfactory signaling. From the converging results of genetic, molecular, and physiological studies, a common set of chemosensory mechanisms emerges. Recognition and discrimination of odorants as well as chemo-electrical transduction and processing of olfactory signals appear to be mediated by fundamentally similar mechanisms in phylogenetically diverse animals. The common challenge of organisms to decipher the world of odors was apparently met by a phylogenetically conserved strategy. Thus, comparative studies should continue to provide important contributions toward an understanding of the sense of smell.

Concentration tuning mediated by spare receptor capacity in olfactory sensory neurons: A theoretical study

The olfactory system is capable of detecting odorants at very low concentrations. Physiological experiments have demonstrated odorant sensitivities down to the picomolar range in preparations from the sensory epithelium. However, the contemporary model for olfactory signal transduction provides that odorants bind to olfactory receptors with relatively low specificity and consequently low affinity, making this detection of low-concentration odorants theoretically difficult to understand. We employ a computational model to demonstrate how olfactory sensory neuron (OSN) sensitivity can be tuned by modulation of receptor-effector coupling and/or by other mechanisms regulating spare receptor capacity, thus resolving this conundrum. The EC10-90 intensity tuning ranges (ITRs) of whole olfactory glomeruli and postsynaptic mitral cells are considerably broader than the commensurate ITRs of individual OSNs. These data are difficult to reconcile with certain contemporary hypotheses that convergent OSNs in mammals exhibit a homogeneous population of olfactory receptors and identical tuning for odor stimuli. We show that heterogeneity in spare receptor capacities within a convergent OSN population can increase the ITR (EC10-90) of a convergent population of OSNs regardless of the presence or absence of a diversity of receptor expression within the population. The modulation of receptor-effector coupling has been observed in OSNs; other mechanisms for cellular regulation of spare receptor capacity are also highly plausible (e.g., quantitative regulation of the relative expression levels of receptor and effector proteins). We present a model illustrating that these processes can underlie both how OSNs come to exhibit high sensitivity to odorant stimuli without necessitating increased ligand-receptor binding affinities or specificities and how a population of convergent OSNs could exhibit a broader concentration sensitivity than its individual constituent neurons, even given a population expressing identical odorant receptors. The regulation of spare receptor capacity may play an important role in the olfactory system's ability to reliably detect low odor concentrations, discriminate odor intensities, and segregate this intensity information from representations of odor quality.

Responses of Xenopus laevis water nose to water-soluble and volatile odorants

Using the whole-cell mode of the patch-clamp technique, we recorded action potentials, voltage-activated cationic currents, and inward currents in response to water-soluble and volatile odorants from receptor neurons in the lateral diverticulum (water nose) of the olfactory sensory epithelium of Xenopus laevis. The resting membrane potential was -46.5 +/- 1.2 mV (mean +/- SEM, n = 68), and a current injection of 1-3 pA induced overshooting action potentials. Under voltage-clamp conditions, a voltage-dependent Na+ inward current, a sustained outward K+ current, and a Ca2+-activated K+ current were identified. Application of an amino acid cocktail induced inward currents in 32 of 238 olfactory neurons in the lateral diverticulum under voltage-clamp conditions. Application of volatile odorant cocktails also induced current responses in 23 of 238 olfactory neurons. These results suggest that the olfactory neurons respond to both water-soluble and volatile odorants. The application of alanine or arginine induced inward currents in a dose-dependent manner. More than 50% of the single olfactory neurons responded to multiple types of amino acids, including acidic, neutral, and basic amino acids applied at 100 microM or 1 mM. These results suggest that olfactory neurons in the lateral diverticulum have receptors for amino acids and volatile odorants.

Cyclic AMP diffusion coefficient in frog olfactory cilia

Cyclic AMP (cAMP) is one of the intracellular messengers that mediate odorant signal transduction in vertebrate olfactory cilia. Therefore, the diffusion coefficient of cAMP in olfactory cilia is an important factor in the transduction of the odorous signal. We have employed the excised cilium preparation from the grass frog (Rana pipiens) to measure the cAMP diffusion coefficient. In this preparation an olfactory cilium is drawn into a patch pipette and a gigaseal is formed at the base of the cilium. Subsequently the cilium is excised, allowing bath cAMP to diffuse into the cilium and activate the cyclic nucleotide-gated channels on the plasma membrane. In order to estimate the cAMP diffusion coefficient, we analyzed the kinetics of the currents elicited by step changes in the bath cAMP concentration in the absence of cAMP hydrolysis. Under such conditions, the kinetics of the cAMP-activated currents has a simple dependence on the diffusion coefficient. From the analysis we have obtained a cAMP diffusion coefficient of 2.7 +/- 0.2. 10(-6) cm2 s-1 for frog olfactory cilia. This value is similar to the expected value in aqueous solution, suggesting that there are no significant diffusional barriers inside olfactory cilia. At cAMP concentrations higher than 5 microM, diffusion slowed considerably, suggesting the presence of buffering by immobile cAMP binding sites. A plausible physiological function of such buffering sites would be to prolong the response of the cell to strong stimuli.

Expression of olfactory receptors during development in Xenopus laevis

A coordinated expression of tissue- and cell-specific genes during development is required to establish the complex functional organization of the vertebrate olfactory system. Owing to the unique features of its olfactory system and the well-characterized phases of its development, Xenopus laevis was chosen as a model organism to study the onset and the temporal and spatial patterns of expression of olfactory-specific genes. Using RT-PCR and in situ hybridization, it was found that expression of Xenopus olfactory marker protein and of class I receptors, which are thought to be responsible for the perception of water-soluble odorants, was detectable as early as stage 32, less than 2 days after fertilization. In contrast, expression of class II receptors, which are thought to recognize airborne odours, was not detected before stage 49, approximately 12 days after fertilization. The results indicate that the expression of olfactory receptors and marker protein is governed by temporally regulated cues during development.

OMP gene deletion causes an elevation in behavioral threshold sensitivity

To characterize the behavioral consequences of OMP gene deletion on odor processing we assessed the ability of OMP-null animals to acquire an air vs odor discrimination for five odorants, and determined whether OMP-null animals differed from controls in their threshold sensitivity to the odorant propanol. On average, control and OMP-null animals did not differ in the number of testing sessions needed to achieve criterion performance on each discrimination problem (2.04 vs 1.68, respectively; t=0.83, p=0.41). However, null animals were significantly less sensitive to the odorant propanol (3.01 x 10(-8) vs 1.06 x 10(-5), respectively; t=4.09, p=0.015). These in vivo behavioral results provide support for the hypothesis that OMP plays a modulatory role in the odor detection/signal transduction process.

Expression and functional analysis of olfactory receptors

The olfactory system recognizes and discriminates myriads of odorants of diverse molecular structure. This task is supposed to be accomplished by a large array of seven-transmembrane domain receptors encoded by a multigene family. Although circumstantial evidence suggests that the identified genes encode odorant receptors, unequivocal proof requires demonstration that the resulting proteins should be able to interact with odorous molecules and couple via G proteins onto second messenger cascades. This goal can be achieved by heterologous expression of receptor proteins in surrogate eucaryotic cells, although the task is complicated by the diversity of putative odorous ligands and the large size of the receptor family. Employing the baculovirus/Sf9 cell system it was found that receptor proteins can be expressed at high levels. Stimulating receptor-expressing Sf9 cells with a mixture of numerous odorous compounds elicited a significant and dose-dependent second messenger response, which was never observed in control cells. Assaying a large panel of odorous compounds, including representatives of different odor classes and compounds of different chemical classes revealed that distinct receptor subtypes respond to certain odorants but not to others. Graded responses to only a subset of odorants indicate that the heterologous expressed receptor types have a selective but relatively broad ligand specificity. The easily manipulated bacterial system was employed to produce olfactory receptor proteins in large quantities. It was solubilized from inclusion bodies and upon reconstitution in liposomes displayed specific interaction with odor ligands.

Phosphorylation and inhibition of olfactory adenylyl cyclase by CaM kinase II in Neurons: a mechanism for attenuation of olfactory signals

Acute desensitization of olfactory signaling is a critical property of the olfactory system that allows animals to detect and respond to odorants. Correspondingly, an important feature of odorant-stimulated cAMP increases is their transient nature, a phenomenon that may be attributable to the unique regulatory properties of the olfactory adenylyl cyclase (AC3). AC3 is stimulated by receptor activation and inhibited by Ca2+ through Ca2+/calmodulin kinase II (CaMKII) phosphorylation at Ser-1076. Since odorant-stimulated cAMP increases are accompanied by elevated intracellular Ca2+, CaMKII inhibition of AC3 may contribute to termination of olfactory signaling. To test this hypothesis, we generated a polyclonal antibody specific for AC3 phosphorylated at Ser-1076. A brief exposure of mouse olfactory cilia or primary olfactory neurons to odorants stimulated phosphorylation of AC3 at Ser-1076. This phosphorylation was blocked by inhibitors of CaMKII, which also ablated cAMP decreases associated with odorant-stimulated cAMP transients. These data define a novel mechanism for termination of olfactory signaling that may be important in olfactory responses.

Chemosensory function and dysfunction

Taste and smell are fundamental sensory systems essential in nutrition and food selection, for the hedonic and sensory experience of food, for efficient metabolism, and, in general, for the maintenance of a good quality of life. The gustatory and olfactory systems demonstrate a diversity of transduction mechanisms, and during the last decade, considerable progress has been made toward our understanding of the basic mechanisms of taste and smell. Understanding normal chemosensory function helps clarify the molecular events that underlie taste and smell disorders. At least 2,000,000 Americans suffer from chemosensory disorders--a number that is likely to grow as the aging segment of the population increases. Smell disorders are more frequent than taste disturbances, due to the vulnerability and anatomical distinctiveness of the olfactory system, and because a decline in olfactory function is part of the normal aging process. Common gustatory and olfactory complaints are due to a number of medications, to upper respiratory infections, to nasal and paranasal sinus diseases, and to damage to peripheral nerves supplying taste and smell. Most chemosensory complaints have an identifiable cause. Although diagnosis of taste and smell disorders has improved considerably over the last two decades, treatment of these disorders is still limited to conditions with discernible and reversible causes. Future research is needed for a better understanding of chemosensory mechanisms, establishing improved diagnostic procedures, and disseminating knowledge on chemosensory disorders among practitioners and the general public.

An isoform of the rod photoreceptor cyclic nucleotide-gated channel beta subunit expressed in olfactory neurons

Sensory transduction in olfactory neurons involves the activation of a cyclic nucleotide-gated (CNG) channel by cAMP. Previous studies identified a CNG channel alpha subunit (CNG2) and a beta subunit (CNG5), which when heterologously expressed form a channel with properties similar but not identical to those of native olfactory neurons. We have cloned a new type of CNG channel beta subunit (CNG4. 3) from rat olfactory epithelium. CNG4.3 derives from the same gene as the rod photoreceptor beta subunit (CNG4.1) but lacks the long, glutamic acid-rich domain found in the N terminus of CNG4.1. Northern blot and in situ hybridization revealed that CNG4.3 is expressed specifically in olfactory neurons. Expression of CNG4.3 in human embryonic kidney 293 cells did not lead to detectable currents. Coexpression of CNG4.3 with CNG2 induced a current with significantly increased sensitivity for cAMP whereas cGMP affinity was not altered. Additionally, CNG4.3 weakened the outward rectification of the current in the presence of extracellular Ca2+, decreased the relative permeability for Ca2+, and enhanced the sensitivity for L-cis diltiazem. Upon coexpression of CNG2, CNG4.3, and CNG5, a conductance with a cAMP sensitivity greater than that of either the CNG2/CNG4.3 or the CNG2/CNG5 channel and near that of native olfactory channel was observed. Our data suggest that CNG4.3 forms a subunit of the native olfactory CNG channel. The expression of various CNG4 isoforms in retina and olfactory epithelium indicates that the CNG4 subunit may be necessary for normal function of both photoreceptor and olfactory CNG channels.

Olfactory processing: a time and place for everything

The behavioral effects of pharmacologically desynchronizing neuronal firing in the brain of the honeybee provide new evidence that the oscillatory synchronization of neuronal activity plays an important role in fine olfactory discrimination.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

Bioelectronic noses: a status report. Part I

The present state of the art to record or mimic electronically the human senses of olfaction and taste is characterized. In this part I, an introduction to our present understanding in the development of electronic and bioelectronic noses is given. Finally the natural olfactory system is described in detail.

Elements of the olfactory signaling pathways in insect antennae

Owing to their enormous ability to recognize airborne molecules, insects have long been used as model systems for studying various aspects of olfaction. Modern biological techniques have opened new avenues for exploring the molecular mechanisms underlying the complex signaling processes in chemosensory neurons. Biochemical and molecular analyses have allowed the identification of molecular elements of the olfactory reaction pathways and have shed light on mechanisms that account for the sensitivity and specificity of the chemosensory system.

cAMP production in rabbit carotid body: role of adenosine

In the present study, we have investigated the possible role of adenosine in the hypoxia-mediated increase in adenosine 3',5'-cyclic monophosphate (cAMP) in the carotid body. cAMP levels in rabbit carotid bodies superfused in vitro for 10 min were increased in the presence of adenosine (100 microM and 1.0 mM; maximum increase = 127%, P < 0.01). These effects were reduced by the nonspecific adenosine-receptor antagonist 1,3-dipropyl-8[p-sulfophenyl]xanthine (DPSPX; 10 microM). The specific A2-receptor agonist 2-[4'(2-carboxymethyl)phenylethylamino]-5'-N-ethylcarboxamido adenosine (CGS-21680; 100 nM) also elevated carotid body cAMP levels, an effect that was blocked by the specific A2-antagonist 3,7-dimethyl-L-propargyl-xanthine (DMPX; 50 microM). Hypoxia-evoked elevations in cAMP were potentiated in the presence of the adenosine-uptake inhibitor dipyridamole (100 nM) and blocked by exposure to adenosine-receptor antagonists. Our data suggest that the rabbit carotid body contains specific adenosine receptors (A2 subtype) that are positively coupled to adenylate cyclase and that increases in cAMP associated with hypoxia are mediated by the release of endogenous adenosine.

Selectivity and response characteristics of human olfactory neurons

Transduction mechanisms were investigated in human olfactory neurons by determining characteristics of odorant-induced changes in intracellular calcium concentration ([Ca2+]i). Olfactory neurons were freshly isolated from nasal biopsies, allowed to attach to coverslips, and loaded with the calcium-sensitive indicator fura-2. Changes in [Ca2+]i were studied in response to exposure to individual odors, or odorant mixtures composed to distinguish between transduction pathways mediated by adenosine 3'5'-monophosphate (cAMP; mix A) or inositol 1,4,5-trisphosphate (InsP3; mix B). Overall, 52% of biopsies produced one or more odorant-responsive olfactory neurons, whereas 24% of all olfactory neurons tested responded to odorant exposure with a change in [Ca2+]i. As in olfactory neurons from other species, the data suggest that odorant exposure elicited calcium influx via second-messenger pathways involving cAMP or InsP3. Unlike olfactory neurons from other species that have been tested, some human olfactory neurons responded to odorants with decreases in [Ca2+]i. Also in contrast with olfactory neurons from other species, human olfactory neurons were better able to discriminate between odorant mixtures in that no neuron responded to more than one type of odor or mixture. These results suggest the presence of a previously unreported type of olfactory transduction mechanism, and raise the possibility that coding of odor qualities in humans may be accomplished to some degree differently than in other vertebrates, with the olfactory neuron itself making a greater contribution to the discrimination process.

Expression of an olfactory receptor in Escherichia coli: purification, reconstitution, and ligand binding

An olfactory receptor has been expressed in bacterial cells as a fusion protein with glutathione S-transferase (GST). Overexpression of receptor protein yielding about 10% of the cell protein was achieved with mutants lacking the N-terminus and the first transmembrane region or with mutants carrying three positively charged residues in the first intracellular loop. The overexpressed fusion protein accumulated in inclusion bodies and could be solubilized in detergent. It was purified by metal chelation chromatography based on a C-terminal 6-histidine tag, and the GST portion was removed after proteolytic cleavage. The purified receptor was reconstituted into lipid vesicles and specific binding of odor ligands was shown by photoaffinity labeling and tryptophan fluorescence measurements. Thus, for the first time, an odorant receptor/ligand pair becomes available in large amounts for biophysical and screening studies.

Existence of multiple receptors in single neurons: responses of single bullfrog olfactory neurons to many cAMP-dependent and independent odorants

The responses of single bullfrog olfactory neurons to various odorants were measured with the whole-cell patch clamp which offers direct information on cellular events and with the ciliary recording technique to obtain stable quantitative data from many neurons. A large portion of single olfactory neurons (about 64% and 79% in the whole-cell recording and in the ciliary recording, respectively) responded to many odorants with quite diverse molecular structures, including both odorants previously indicated to be cAMP-dependent (increasing) and independent odorants. One odorant elicited a response in many cells; e.g. hedione and citralva elicited the response in 100% and 92% of total neurons examined with the ciliary recording technique. To confirm that a single neuron carries different receptors or transduction pathways, the cross-adaptation technique was applied to single neurons. Application of hedione to a single neuron after desensitization of the current in response to lyral or citralva induced an inward current with a similar magnitude to that applied alone. It was suggested that most single olfactory neurons carry multiple receptors and at least dual transduction pathways.