New insights into the roles of olfactory receptors in cardiovascular disease

Olfactory receptors (ORs) are G protein coupled receptors (GPCRs) with seven transmembrane domains that bind to specific exogenous chemical ligands and transduce intracellular signals. They constitute the largest gene family in the human genome. They are expressed in the epithelial cells of the olfactory organs and in the non-olfactory tissues such as the liver, kidney, heart, lung, pancreas, intestines, muscle, testis, placenta, cerebral cortex, and skin. They play important roles in the normal physiological and pathophysiological mechanisms. Recent evidence has highlighted a close association between ORs and several metabolic diseases. Cardiovascular diseases (CVDs) are the leading cause of morbidity and mortality globally. Furthermore, ORs play an essential role in the development and functional regulation of the cardiovascular system and are implicated in the pathophysiological mechanisms of CVDs, including atherosclerosis (AS), heart failure (HF), aneurysms, and hypertension (HTN). This review describes the specific mechanistic roles of ORs in the CVDs, and highlights the future clinical application prospects of ORs in the diagnosis, treatment, and prevention of the CVDs.

Arthropod repellent interactions with olfactory receptors and ionotropic receptors analyzed by molecular modeling

The main insect chemoreceptors are olfactory receptors (ORs), gustatory receptors (GRs) and ionotropic receptors (IRs). The odorant binding sites of many insect ORs appear to be occluded and inaccessible from the surface of the receptor protein, based on the three-dimensional structure of OR5 from the jumping bristletail Machilis hrabei (MhraOR5) and a survey of a sample of vinegar fly (Drosophila melanogaster) OR structures obtained from artificial intellegence (A.I.) modeling. Molecular dynamics simulations revealed that the occluded site can become accessible through tunnels that transiently open and close. The present study extends this analysis to examine seventeen ORs and one GR docking with ligands that have known valence: nine that signal attraction and nine that signal aversion. All but one of the receptors displayed occluded ligand binding sites analogous to MhraOR5, and docking software predicted the known attractant and repellent ligands will bind to the occluded sites. Docking of the repellent DEET was examined, and more than half of the OR ligand sites were predicted to bind DEET, including receptors that signal aversion as well as those that signal attraction. However, DEET may not actually have access to all the attractant binding sites. The larger size and lower flexibility of repellent molecules may restrict their passage through the tunnel bottlenecks, which could act as filters to select access to the ligand binding sites. In contrast to ORs and GRs, the IR ligand binding site is in an extracellular domain known to undergo a large conformational change from an open to a closed state. A.I. models of two D. melanogaster IRs of known valence and two blacklegged tick (Ixodes scapularis) IRs having unknown ligands were computationally tested for attractant and repellent binding. The ligand-binding sites in the closed state appear inaccessible to the protein surface, so attractants and repellents must bind initially at an accessible site in the open state before triggering the conformational change. In some IRs, repellent binding sites were identified at exterior sites adjacent to the ligand-binding site. These may be allosteric sites that, when occupied by repellents, can stabilize the open state of an attractant IR, or stabilize the closed state of an IR in the absence of its activating ligand. The model of D. melanogaster IR64a suggests a possible molecular mechanism for the activation of this IR by H+. The amino acids involved in this proposed mechanism are conserved in IR64a from several Dipteran pest species and disease vectors, potentially offering a route to discovery of new repellents that act via the allosteric site.

A single mutation in the mosquito (Aedes aegypti) olfactory receptor 8 causes loss of function to 1-octen-3-ol

The host-seeking behavior of mosquitoes have long been established to be primarily odor-mediated. In this process, olfactory receptors (Ors) play a critical role. 1-Octen-3-ol is a common volatile compound that is attractive to hematophagous arthropods such as mosquitos. The olfactory receptor 8 (AaOr8) on the tip of the stylet and maxillary palp of Aedes aegypti is tuned to 1-octen-3-ol, which is required for mosquitoes to quickly find blood vessels from a vertebrate host. However, little is known about the interaction of AaOr8 with 1-octen-3-ol which was studied in vivo and in silico in this study. The molecular binding poses and energies between ligands and the receptor were investigated. Three mutants of AaOr8 were cloned and compared with in vivo calcium imaging utilizing heterologous expression systems. As a result, our findings imply that a genetic disruption including targeted modification of Ors genes may be used to reduce mosquito bites.

Expanded olfactory system in ray-finned fishes capable of terrestrial exploration

BACKGROUND

Smell abilities differ greatly among vertebrate species due to distinct sensory needs, with exceptional variability reported in the number of olfactory genes and the size of the odour-processing regions of the brain. However, key environmental factors shaping genomic and phenotypic changes linked to the olfactory system remain difficult to identify at macroevolutionary scales. Here, we investigate the association between diverse ecological traits and the number of olfactory chemoreceptors in approximately two hundred ray-finned fishes.

RESULTS

We found independent expansions producing large gene repertoires in several lineages of nocturnal amphibious fishes, generally able to perform active terrestrial exploration. We reinforced this finding with on-purpose genomic and transcriptomic analysis of Channallabes apus, a catfish species from a clade with chemosensory-based aerial orientation. Furthermore, we also detected an augmented information-processing capacity in the olfactory bulb of nocturnal amphibious fishes by estimating the number of cells contained in this brain region in twenty-four actinopterygian species.

CONCLUSIONS

Overall, we report a convergent genomic and phenotypic magnification of the olfactory system in nocturnal amphibious fishes. This finding suggests the possibility of an analogous evolutionary event in fish-like tetrapod ancestors during the first steps of the water-to-land transition, favouring terrestrial adaptation through enhanced aerial orientation.

Epigenetic silencing of OR and TAS2R genes expression in human orbitofrontal cortex at early stages of sporadic Alzheimer's disease

Modulation of brain olfactory (OR) and taste receptor (TASR) expression was recently reported in neurological diseases. However, there is still limited evidence of these genes' expression in the human brain and the transcriptional regulation mechanisms involved remain elusive. We explored the possible expression and regulation of selected OR and TASR in the human orbitofrontal cortex (OFC) of sporadic Alzheimer's disease (AD) and non-demented control specimens using quantitative real-time RT-PCR and ELISA. Global H3K9me3 amounts were measured on OFC total histone extracts, and H3K9me3 binding at each chemoreceptor locus was examined through native chromatin immunoprecipitation. To investigate the potential interactome of the repressive histone mark H3K9me3 in OFC specimens, native nuclear complex co-immunoprecipitation (Co-IP) was combined with reverse phase-liquid chromatography coupled to mass spectrometry analysis. Interaction between H3K9me3 and MeCP2 was validated by reciprocal Co-IP, and global MeCP2 levels were quantitated. We found that OR and TAS2R genes are expressed and markedly downregulated in OFC at early stages of sporadic AD, preceding the progressive reduction in their protein levels and the appearance of AD-associated neuropathology. The expression pattern did not follow disease progression suggesting transcriptional regulation through epigenetic mechanisms. We discovered an increase of OFC global H3K9me3 levels and a substantial enrichment of this repressive signature at ORs and TAS2Rs proximal promoter at early stages of AD, ultimately lost at advanced stages. We revealed the interaction between H3K9me3 and MeCP2 at early stages and found that MeCP2 protein is increased in sporadic AD. Findings suggest MeCP2 might be implicated in OR and TAS2R transcriptional regulation through interaction with H3K9me3, and as an early event, it may uncover a novel etiopathogenetic mechanism of sporadic AD.

Odorant-responsive biological receptors and electronic noses for volatile organic compounds with aldehyde for human health and diseases: A perspective review

Volatile organic compounds (VOCs) are gaseous chemicals found in ambient air and exhaled breath. In particular, highly reactive aldehydes are frequently found in polluted air and have been linked to various diseases. Thus, extensive studies have been carried out to elucidate disease-specific aldehydes released from the body to develop potential biomarkers for diagnostic purposes. Mammals possess innate sensory systems, such as receptors and ion channels, to detect these VOCs and maintain physiological homeostasis. Recently, electronic biosensors such as the electronic nose have been developed for disease diagnosis. This review aims to present an overview of natural sensory receptors that can detect reactive aldehydes, as well as electronic noses that have the potential to diagnose certain diseases. In this regard, this review focuses on eight aldehydes that are well-defined as biomarkers in human health and disease. It offers insights into the biological aspects and technological advances in detecting aldehyde-containing VOCs. Therefore, this review will aid in understanding the role of aldehyde-containing VOCs in human health and disease and the technological advances for improved diagnosis.

Modeling of Olfactory Receptors

Olfactory receptors (ORs) form the largest subfamily within class A G protein-coupled receptors (GPCRs). No experimental structural data of any OR is available to date. Homology modeling has become a popular strategy to propose plausible OR models, in order to study the structure-function relationships of the receptors and to aid the discovery and development of ligands capable of modulating receptor activity. In this chapter, we provide a general guideline for OR structure construction, including the collection of candidate templates, structure-based sequence alignment, 3D structure construction, ligand docking, and molecular dynamic simulation.

Chromosome-scale genome assembly of the African giant pouched rat (Cricetomys ansorgei) and evolutionary analysis reveals evidence of olfactory specialization

The Southern giant pouched rat, Cricetomys ansorgei, is a large rodent best known for its ability to detect landmines using its impressive sense of smell. Their powerful chemosensory abilities enable subtle discrimination of chemical social signals, and female pouched rats demonstrate a unique reproductive physiology hypothesized to be mediated by pheromonal mechanisms. Thus, C. ansorgei represents a novel mammalian model for chemosensory physiology, social behavior, and pheromonal control of reproductive physiology. We present the first chromosome-scale genomic sequence of the pouched rat encoding 22,671 protein coding genes, including 1571 olfactory receptors, and provide a glance into the evolutionary history of this species. Functional enrichment analysis reveals genetic expansions specific to the pouched rat are enriched for functions related to olfactory specialization. Overall, this assembly is of reference-quality, and will serve as a useful and informative genomic sequence on which we can confidently base future molecular research involving the pouched rat.

BarH1 regulates odorant-binding proteins expression and olfactory perception of Monochamus alternatus Hope

Insect odorant-binding proteins (OBPs) are a class of small soluble proteins that can be found in various tissues wherein binding and transport of small molecules are required. Thus, OBPs are not only involved in typical olfactory function by specific activities with odorants but also participate in other physiological processes in non-chemosensory tissues. To better understand the complex biological functions of OBPs, it is necessary to study the transcriptional regulation of their expression patterns. In this paper, an apparent gradient expression pattern of Obp19, that was highly and specifically expressed in antennae and played an essential role in the detection of camphene, was defined in the antennae of the Japanese pine sawyer. Further, the transcription factor BarH1, that also presented gradient expression pattern in antennae, was found to regulate expression of Obp19 directly through binding to its upstream DNA sequence. The condition of BarH1 gene silence, the gene expression levels of Obp19 significantly decreased. At the same time, additional olfactory genes also were regulated and thus influence camphene reception. These findings provide us an opportunity to incorporate Obps in the gene regulatory networks of insects, which contribute to a better understanding of the multiplicity and diversity of OBPs and the olfactory mediated behaviors.

Contribution of odorant binding proteins to olfactory detection of (Z)-11-hexadecenal in Helicoverpa armigera

Helicoverpa armigera utilizes (Z)-11-hexadecenal (Z11-16:Ald) as its major sex pheromone component. Three pheromone binding proteins (PBPs) and two general odorant binding proteins (GOBPs) are abundantly expressed in the male antennae of H. armigera. However, their precise roles in the olfactory detection of Z11-16:Ald remain enigmatic. To answer this question, we first synthesized the antibody against HarmOR13, an olfactory receptor (OR) primarily responding to Z11-16:Ald and mapped the local associations between PBPs/GOBPs and HarmOR13. Immunostaining showed that HarmPBPs and HarmGOBPs were localized in the supporting cells of trichoid sensilla and basiconic sensilla respectively. In particular, HarmPBP1 and HarmPBP2 were colocalized in the cells surrounding the olfactory receptor neurons (ORNs) expressing HarmOR13. Next, using two noninterfering binary expression tools, we heterologously expressed HarmPBP1, HarmPBP2 and HarmOR13 in Drosophila T1 sensilla to validate the functional interplay between PBPs and HarmOR13. We found that the addition of HarmPBP1 or HarmPBP2, not HarmPBP3, significantly increased HarmOR13's response to Z11-16:Ald. However, the presence of either HarmPBP1 or HarmPBP2 was ineffective to change the tuning breadth of HarmOR13 and modulate the response kinetics of this receptor. Taken together, this work demonstrates both HarmPBP1 and HarmPBP2 are involved in Z11-16:Ald detection. Our results support the idea that PBPs can contribute to the peripheral olfactory sensitivity but do little in modulating the selectivity and the response kinetics of corresponding ORs.

Insights into chemosensory genes of Pagiophloeus tsushimanus adults using transcriptome and qRT-PCR analysis

Pagiophloeus tsushimanus is a new, destructive, and monophagous weevil pest that thrives on Cinnamomum camphora, found in Shanghai. The functions of chemosensory genes involved in the host location and intraspecific communication of P. tsushimanus remain unknown. The male-female transcriptomes of P. tsushimanus adults were assembled using Illumina sequencing, and we focused on all chemosensory genes in transcriptomes. In general, 58,088 unigenes with a mean length of 1018.19 bp were obtained. In total, 39 odorant binding proteins (OBPs), 10 chemosensory proteins (CSPs), 22 olfactory receptors (ORs), 16 gustatory receptors (GRs), eight ionotropic receptors (IRs), and five sensory neuron membrane proteins (SNMPs) were identified. PtsuOBPs comprised four subfamilies (20 Minus-C, one Plus-C, two Dimer, and 15 Classic). Both PtsuOBPs and PtsuCSPs contained a highly conserved sequence motif of cysteine residues. PtsuORs including one olfactory receptor co-receptors (Ptsu/Orco) comprised seven predicted transmembrane domains. Phylogenetic analysis revealed that PtsuOBPs, PtsuCSPs, and PtsuORs in P. tsushimanus exhibited low homology compared to other insect species. The results of tissue- and sex-specific expression patterns indicated that PtsuOBPs and PtsuORs were highly abundant in the antennae; whereas, PtsuCSPs were not only highly abundant in antennae, but also abdominal apexes, wings, and legs. In conclusion, these results enrich the gene database of P. tsushimanus, which may serve as a basis for identifying novel targets to disrupt olfactory key genes and may provide a reverse validation method to identify attractants for formulating potential eco-friendly control strategies for this pest.

The mutational landscape of human olfactory G protein-coupled receptors

BACKGROUND

Olfactory receptors (ORs) constitute a large family of sensory proteins that enable us to recognize a wide range of chemical volatiles in the environment. By contrast to the extensive information about human olfactory thresholds for thousands of odorants, studies of the genetic influence on olfaction are limited to a few examples. To annotate on a broad scale the impact of mutations at the structural level, here we analyzed a compendium of 119,069 natural variants in human ORs collected from the public domain.

RESULTS

OR mutations were categorized depending on their genomic and protein contexts, as well as their frequency of occurrence in several human populations. Functional interpretation of the natural changes was estimated from the increasing knowledge of the structure and function of the G protein-coupled receptor (GPCR) family, to which ORs belong. Our analysis reveals an extraordinary diversity of natural variations in the olfactory gene repertoire between individuals and populations, with a significant number of changes occurring at the structurally conserved regions. A particular attention is paid to mutations in positions linked to the conserved GPCR activation mechanism that could imply phenotypic variation in the olfactory perception. An interactive web application (hORMdb, Human Olfactory Receptor Mutation Database) was developed for the management and visualization of this mutational dataset.

CONCLUSION

We performed topological annotations and population analysis of natural variants of human olfactory receptors and provide an interactive application to explore human OR mutation data. We envisage that the utility of this information will increase as the amount of available pharmacological data for these receptors grow. This effort, together with ongoing research in the study of genetic changes in other sensory receptors could shape an emerging sensegenomics field of knowledge, which should be considered by food and cosmetic consumer product manufacturers for the benefit of the general population.

Immobilization of Olfactory Receptors Carried by Nanosomes onto a Gold Sensor Surface

Mammalian olfactory receptors (ORs) constitute the largest family of G-protein-coupled receptors, with up to about 1000 different genes per species, each having specific odorant ligands. ORs could be used as sensing elements of highly specific and sensitive bioelectronic hybrid devices such as bioelectronic noses. After optimized immobilization onto the device, natural ORs provide molecular recognition of various odors with their intrinsic sensitivity, discrimination, and detection properties. However, the main difficulties are related to the low expression level of recombinant ORs, their stability and potential loss of activity. Such drawbacks can be successfully overcome in bioelectronic noses integrating nanosomes (nanometric membrane vesicles carrying ORs) that are stably immobilized through a specific antibody. The advantages of such a platform rely on the fact that ORs stay in the natural membrane environment, and thus preserve their full activity. Thanks to their small sizes, nanosomes offer potential for micro- and nano-scale sensor development. In this paper, we summarize the key elements regarding nanosomes production and manipulation and provide an example of their immobilization onto a gold sensor surface. Rat ORI7 is used as a representative OR that can be functionally expressed in Saccharomyces cerevisiae. The receptor was not purified but only nanosomes were prepared. Nanosomes were immobilized onto functionalized gold surface using the anti-I7 antibody. Utilization of the antibody provides enrichment of ORI7 on the sensor surface but also uniform and appropriate orientation of the receptors. These features are crucial in optimization of bioelectronic nose' analytical performances.

Expression pattern of olfactory receptor genes in human cumulus cells as an indicator for competent oocyte selection

Odorant or olfactory receptors are mainly localized in the olfactory epithelium for the perception of different odors. Interestingly, many ectopic olfactory receptors with low expression levels have recently been found in nonolfactory tissues to involve in local functions. Therefore, we investigated the probable role of the olfactory signaling pathway in the surrounding microenvironment of oocyte. This study included 22 women in intracytoplasmic sperm injection cycle. The expression of olfactory target molecules in cumulus cells surrounding the growing and mature oocytes was evaluated by Western blotting and real-time polymerase chain reaction. Additionally, integrated bioinformatics analyses were carried out and 6 ectopic olfactory receptors were selected for further evaluation. The initiation of olfactory transduction cascade in cumulus cells of competent oocytes was confirmed by analyzing the expression of adenylyl cyclase type 3 and olfactory market protein. Moreover, the expression pattern of the selected olfactory receptors was evaluated and OR10H2 was selected due to a high level of expression in mature fertile oocytes. We suggested that OR10H2 could be considered as a reliable biomarker for oocyte selection in assisted reproduction technique programs. However, further studies are required to elucidate the role of olfactory transduction cascade in embryo quality and implantation.

Cadaverine and Spermine Elicit Ca2+ Uptake in Human CP Cells via a Trace Amine-Associated Receptor 1 Dependent Pathway

The choroid plexus (CP) constitutes a barrier between the blood and the cerebrospinal fluid (CSF) which regulates the exchange of substances between these two fluids through mechanisms that are not completely understood. Polyamines as spermine, spermidine and putrescine are produced by all cells and are present in the CSF. Interestingly, their levels are altered in some neuronal disorders as Alzheimer's and Parkinson's diseases, thus increasing the interest in their signalling in the central nervous system (CNS). Cadaverine, on the other hand, is synthetized by the intestinal microbiome, suggesting that the presence of this bacterial metabolite in the CSF requires that it is up taken to the CNS across brain barriers. We knew that polyamines are detected by the olfactory signalling cascade operating at the CP, but the receptor involved had not been identified. The zebrafish TAAR13c was the only receptor known to bind a polyamine-cadaverine. Thus, we searched for a human receptor with homology to TAAR13c and found that some human TAARs including TAAR1 showed great homology. Then, we confirmed the expression of TAAR1 mRNA and protein in a human cell line of the CP, and in human CP samples. Calcium imaging assays after TAAR1 knockdown in these cells with a specific siRNA against TAAR1 showed a consistent reduction in the responses of these cells to cadaverine and spermidine, but not to spermine, suggesting that TAAR1 is activated by cadaverine and spermidine, but not spermine.

Effect of OBPs on the response of olfactory receptors

The technique of two-electrode voltage-clamp (TEVC) recording from the heterologous expression system of olfactory receptors (ORs) in Xenopus laevis oocytes has been widely used to deorphanize insect ORs, that is to identify specific ligands for each of them. However, there is a controversial issue on whether ORs are activated by the odorant/OBP complex or the odorant alone. The mechanism of interaction among odorants, odorant-binding proteins (OBPs) and ORs remains largely unknown, due to the limitations in the use of scientific and innovative methods. In this chapter, the modified Xenopus oocytes expression system combined with TEVC technique is used to approach this issue. We describe the experimental strategies and provide detailed protocols for recording the signals generated by ORs in response to odorant/OBP complex at different concentrations. Results obtained by this approach have revealed that the presence of OBPs in the system affects the selectivity and sensitivity responses of ORs. Such studies help understanding the molecular mechanism of odorant detection in peripheral nervous system.

A unified nomenclature for vertebrate olfactory receptors

BACKGROUND

Olfactory receptors (ORs) are G protein-coupled receptors with a crucial role in odor detection. A typical mammalian genome harbors ~ 1000 OR genes and pseudogenes; however, different gene duplication/deletion events have occurred in each species, resulting in complex orthology relationships. While the human OR nomenclature is widely accepted and based on phylogenetic classification into 18 families and further into subfamilies, for other mammals different and multiple nomenclature systems are currently in use, thus concealing important evolutionary and functional insights.

RESULTS

Here, we describe the Mutual Maximum Similarity (MMS) algorithm, a systematic classifier for assigning a human-centric nomenclature to any OR gene based on inter-species hierarchical pairwise similarities. MMS was applied to the OR repertoires of seven mammals and zebrafish. Altogether, we assigned symbols to 10,249 ORs. This nomenclature is supported by both phylogenetic and synteny analyses. The availability of a unified nomenclature provides a framework for diverse studies, where textual symbol comparison allows immediate identification of potential ortholog groups as well as species-specific expansions/deletions; for example, Or52e5 and Or52e5b represent a rat-specific duplication of OR52E5. Another example is the complete absence of OR subfamily OR6Z among primate OR symbols. In other mammals, OR6Z members are located in one genomic cluster, suggesting a large deletion in the great ape lineage. An additional 14 mammalian OR subfamilies are missing from the primate genomes. While in chimpanzee 87% of the symbols were identical to human symbols, this number decreased to ~ 50% in dog and cow and to ~ 30% in rodents, reflecting the adaptive changes of the OR gene superfamily across diverse ecological niches. Application of the proposed nomenclature to zebrafish revealed similarity to mammalian ORs that could not be detected from the current zebrafish olfactory receptor gene nomenclature.

CONCLUSIONS

We have consolidated a unified standard nomenclature system for the vertebrate OR superfamily. The new nomenclature system will be applied to cow, horse, dog and chimpanzee by the Vertebrate Gene Nomenclature Committee and its implementation is currently under consideration by other relevant species-specific nomenclature committees.

Convergent degeneration of olfactory receptor gene repertoires in marine mammals

Background

Olfactory receptors (ORs) can bind odor molecules and play a crucial role in odor sensation. Due to the frequent gains and losses of genes during evolution, the number of OR members varies greatly among different species. However, whether the extent of gene gains/losses varies between marine mammals and related terrestrial mammals has not been clarified, and the factors that might underlie these variations are unknown.

Results

To address these questions, we identified more than 10,000 members of the OR family in 23 mammals and classified them into 830 orthologous gene groups (OGGs) and 281 singletons. Significant differences occurred in the number of OR repertoires and OGGs among different species. We found that all marine mammals had fewer OR genes than their related terrestrial lineages, with the fewest OR genes found in cetaceans, which may be closely related to olfactory degradation. ORs with more gene duplications or loss events tended to be under weaker purifying selection. The average gain and loss rates of OR genes in terrestrial mammals were higher than those of mammalian gene families, while the average gain and loss rates of OR genes in marine mammals were significantly lower and much higher than those of mammalian gene families, respectively. Additionally, we failed to detect any one-to-one orthologous genes in the focal species, suggesting that OR genes are not well conserved among marine mammals.

Conclusions

Marine mammals have experienced large numbers of OR gene losses compared with their related terrestrial lineages, which may result from the frequent birth-and-death evolution under varied functional constrains. Due to their independent degeneration, OR genes present in each lineage are not well conserved among marine mammals. Our study provides a basis for future research on the olfactory receptor function in mammals from the perspective of evolutionary trajectories.

Tests of the chromatographic theory of olfaction with highly soluble odors: a combined electro-olfactogram and computational fluid dynamics study in the mouse

The idea that the vertebrate nasal cavity operates like a gas chromatograph to separate and discriminate odors, referred to herein as the ‘chromatographic theory’ (CT), has a long and interesting history. Though the last decade has seen renewed interest in the notion, its validity remains in question. Here we examine a necessary condition of the theory: a correlation between nasal odor deposition patterns based on mucus solubility and the distribution of olfactory sensory neuron odotypes. Our recent work in the mouse failed to find such a relationship even across large sorption gradients within the olfactory epithelium (OE). However, these studies did not test extremely soluble odorants or low odor concentrations, factors that could explain our inability to find supporting evidence for the CT. The current study combined computational fluid dynamics (CFD) simulations of odor sorption patterns and electro-olfactogram (EOG) measurements of olfactory sensory neuron responses. The odorants tested were at the extremes of mucus solubility and at a range of concentrations. Results showed no relationship between local odor sorption patterns and EOG response maps. Together, results again failed to support a necessary condition of the CT casting further doubt on the viability of this classical odor coding mechanism.

Summary: This paper casts doubt on the classical chromatographic theory of olfaction, showing there is no correlation between olfactory receptor spatial layout and odor solubility patterns, a necessary condition of the theory.

Bcl11b controls odorant receptor class choice in mice

Each olfactory sensory neuron (OSN) expresses a single odorant receptor (OR) gene from the class I or class II repertoire in mice. The mechanisms that regulate OR class choice in OSNs remain unknown. Here, we show that the transcription factor Bcl11b determines the OR class to be expressed in OSNs. Both loss- and gain-of-function analyses demonstrate that class I is a default fate of OSNs and that Bcl11b dictates a class II OR choice by suppressing the effect of the J-element, a class I-OR enhancer. We further demonstrate that OSN-specific genetic manipulations of Bcl11b bias the OR class choice, generating mice with "class I-dominant" and "class II-dominant" noses, which display contrasting innate olfactory behaviors to two distinct aversive odorants. Overall, these findings reveal a unique transcriptional mechanism mediating a binary switch for OR class choice that is crucial to both the anatomical and functional organization of the olfactory system.

Chronic kidney disease and the olfactory system

Alterations in the sense of smell (dysosmia, anosmia, hyposmia) are frequently experienced by patients with chronic kidney disease. However, currently, the aetiology and consequences are poorly understood, with no effective treatments available to address such impairment. In general, the capacity of olfactory perception is affected in patients with chronic kidney disease (even in those who have not undergone dialysis therapy), and whether these alterations improve after dialysis is disputed. Patients in peritoneal dialysis and haemodialysis have the same olfactory perception defects. Kidney transplantation improves olfactory perception, and one important consequence of such impairment is the potential impact on the patient's nutritional status.

Suppression of olfactory signal transduction by insecticides

2,4,6-Trichloroanisole (TCA) is a well-known, potent off-flavour compound present in various foods and beverages. TCA has been hypothesised to be a universal cause of flavour loss experienced in daily life. Here, however, we show that titres for the suppression of olfactory transducer channels caused by low-quality bananas are much higher than those for that caused by the TCA itself contained in the banana. We resurveyed other components of low-quality bananas and found that bananas also contain an insecticide (chlorpyrifos), and that it suppresses olfactory transducer channels. Other insecticides also suppressed olfactory transducer channels. Hence, even after passing safety examinations, certain insecticides may decrease the quality of foods and beverages by reducing their intrinsic scents.

Transcriptomics reveal the molecular underpinnings of chemosensory proteins in Chlorops oryzae

BACKGROUND

Chemosensory proteins are a family of insect-specific chemical sensors that sense specific chemical cues and regulate insect behavior. Chemosensory proteins have been identified and analyzed in many insect species, such as Drosophila melanogaster, Bactrocera dorsalis and Calliphora stygia. This research has revealed that these proteins play a crucial role in insect orientation, predation and oviposition. However, little is known about the chemosensory proteins of Chlorops oryzae, a major pest of rice crops throughout Asia.

RESULTS

Comparative transcription analysis of the genes of Chlorops oryzae larvae, pupae and adults identified a total of 104 chemosensory genes, including 25 odorant receptors (ORs), 26 odorant-binding proteins (OBPs), 19 ionotropic receptors (IRs), 23 gustatory receptors (GRs) and 11 sensory neuron membrane proteins (SNMPs). The sequences of these candidate chemosensory genes were confirmed and used to construct phylogenetic trees. Quantitative real-time PCR (qRT-PCR) confirmed that the expression of candidate OR genes in different developmental stages was consistent with the fragments per kilobase per million fragments (FPKM) values of differentially expressed genes (DEGs).

CONCLUSIONS

The identification of chemosensory genes in C. oryzae provides a foundation for the investigation of the function of chemosensory proteins in this species, which, in turn, could allow the development of new, improved methods of controlling this pest.

Drosophila melanogaster odorant receptors as volatile compound detectors in forensic science: a proof-of-concept study

The ability to detect and identify substances based on the volatile compounds (odors) they emit is relied upon heavily for numerous investigative purposes. Animals have an innate olfactory sensitivity and selectivity that out-performs current instrumentation. This has led to immense interest in their employment as chemical sensors for a range of applications, including forensic science, both as whole organisms and as sensing elements in biosensors. Using electrophysiological and calcium imaging assays, this research examined the response of Drosophila melanogaster olfactory receptors (ORs) to odor compounds significant in forensic science and assessed their potential utility as volatile compound sensors. This investigation illustrated the different sensitivities, selectivities, and sensing features of individual ORs and demonstrated that their employment for detection purposes is feasible. While further research expanding on this study will be required to demonstrate the performance characteristics that an OR-based detection system will ultimately possess, this research provides an encouraging first step towards the goal of utilizing isolated biological ORs as volatile compound sensors in forensic science.

Functional expression of olfactory receptors using cell-free expression system for biomimetic sensors towards odorant detection

How to obtain sufficient functional olfactory receptors and coupled to transducers with high efficiency are crucial to biomimetic olfactory-based biosensors. In this study, a cell-free expression system was employed to prepare functional olfactory receptors that were utilized as sensitive elements for biomimetic olfactory receptor-based biosensors. A nematode olfactory receptor, ODR-10, was used as a model of olfactory receptors and expressed in Escherichia coli (E. coli) cell-free expression system. To improve the coupling efficiency and obtain the on-chip purification, a His₆-tag was expressed as a fusion to ODR-10, which makes the expressed ODR-10 capable of selectively binding to the sensor surface modified with anti-His₆-tag aptamers. Electrolyte-insulator-semiconductor (EIS) sensors were utilized as the transducer to measure the capacitance changes induced by the ODR-10 responding to its natural ligand, diacetyl. The results show that ODR-10 was successfully expressed using E. coli cell-free expression system and the expression could be promoted by adding 0.5% Brij58 as the detergent. Capacitance measurement results indicate that this olfactory receptor-based biosensor can detect diacetyl with high specificity and sensitivity. Concentration-dependent linear response to diacetyl ranging from 0.01 nM to 1 nM was obtained. The detection limit was as low as 0.01 nM. All the results demonstrated that the cell-free expression system provides a new approach for the preparation of functional olfactory receptors, which have great potential to be applied in biomimetic sensors towards chemical sensing.

Expressional and functional interactions of two Apis cerana cerana olfactory receptors

Apis cerana cerana relies on its sensitive olfactory system to perform foraging activities in the surrounding environment. Olfactory receptors (ORs) are a primary requirement for odorant recognition and coding. However, the molecular recognition of volatile compounds with ORs in A. cerana cerana is still not clear. Hence, in the present study, we achieved transient transfection and cell surface expression of A. cerana cerana ORs (AcerOr1 and AcerOr2; AcerOr2 is orthologous to the co-receptor) in Spodoptera frugiperda (Sf9) cells. AcerOr2 narrowly responded to N-(4-ethylphenyl)-2-((4-ethyl-5-(3-pyridinyl)-4H-1,2,4-triazol-3-yl) thio) acetamide (VUAA1), whereas AcerOr1 was sensitive to eugenol, lauric acid, ocimene, 1-nonanol, linolenic acid, hexyl acetate, undecanoic acid, 1-octyl alcohol, and nerol. Of the compounds tested, AcerOr1 showed the highest sensitivity to these odorants with EC₅₀ values of 10⁻⁷ and 10⁻⁸ M, and AcerOr2 recognized VUAA1 with higher sensitivity [EC₅₀ = (6.621 ± 0.26) × 10⁻⁸]. These results indicate that AcerOr2 is an essential gene for olfactory signaling, and AcerOr1 is a broadly tuned receptor. We discovered ligands that were useful for probing receptor activity during odor stimulation and validated three of them by electroantennography. The response increased with concentration of the odorant. The present study provides insight into the mechanism of olfactory discrimination in A. cerana cerana.

Overlapping but distinct topology for zebrafish V2R-like olfactory receptors reminiscent of odorant receptor spatial expression zones

Background

The sense of smell is unrivaled in terms of molecular complexity of its input channels. Even zebrafish, a model vertebrate system in many research fields including olfaction, possesses several hundred different olfactory receptor genes, organized in four different gene families. For one of these families, the initially discovered odorant receptors proper, segregation of expression into distinct spatial subdomains within a common sensory surface has been observed both in teleost fish and in mammals. However, for the remaining three families, little to nothing was known about their spatial coding logic. Here we wished to investigate, whether the principle of spatial segregation observed for odorant receptors extends to another olfactory receptor family, the V2R-related OlfC genes. Furthermore we thought to examine, how expression of OlfC genes is integrated into expression zones of odorant receptor genes, which in fish share a single sensory surface with OlfC genes.

Results

To select representative genes, we performed a comprehensive phylogenetic study of the zebrafish OlfC family, which identified a novel OlfC gene, reduced the number of pseudogenes to 1, and brought the total family size to 60 intact OlfC receptors. We analyzed the spatial pattern of OlfC-expressing cells for seven representative receptors in three dimensions (height within the epithelial layer, horizontal distance from the center of the olfactory organ, and height within the olfactory organ). We report non-random distributions of labeled neurons for all OlfC genes analysed. Distributions for sparsely expressed OlfC genes are significantly different from each other in nearly all cases, broad overlap notwithstanding. For two of the three coordinates analyzed, OlfC expression zones are intercalated with those of odorant receptor zones, whereas in the third dimension some segregation is observed.

Conclusion

Our results show that V2R-related OlfC genes follow the same spatial logic of expression as odorant receptors and their expression zones intermingle with those of odorant receptor genes. Thus, distinctly different expression zones for individual receptor genes constitute a general feature shared by teleost and tetrapod V2R/OlfC and odorant receptor families alike.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4740-8) contains supplementary material, which is available to authorized users.

An overview of an artificial nose system

The present review describes recent advances in the development of an artificial nose system based on olfactory receptors and various sensing platforms. The kind of artificial nose, the production of olfactory receptors, the sensor platform for signal conversion and the application of the artificial nose system based on olfactory receptors and various sensing platforms are presented. The associated transduction modes are also discussed. The paper presents a review of the latest achievements and a critical evaluation of the state of the art in the field of artificial nose systems.

Gallein, a Gβγ subunit signalling inhibitor, inhibits metastatic spread of tumour cells expressing OR51E2 and exposed to its odorant ligand

Objective

We previously reported that the olfactory receptor OR51E2, overexpressed in LNCaP prostate cancer cells, promotes cell invasiveness upon stimulation of its agonist β-ionone, and this phenomenon increases metastatic spread. Furthermore, we showed that the induced cell invasiveness involves a PI3 kinase dependent signalling pathway. We report here the results of a new investigation to address whether gallein, a small inhibitor of G protein βγ subunit interaction with PI3 kinase, can inhibit β-ionone effects both in vitro and in vivo.

Results

We demonstrate that gallein can inhibit the β-ionone-induced cell invasiveness in vitro, as well as the spread of metastases in vivo. LNCaP cell invasiveness, assessed using spheroid cultures in collagen gels in vitro, was increased by β-ionone and the effect was reversed by co-administration of gallein. LNCaP tumour cells, subcutaneously inoculated to immunodeficient mice, generated more metastases in vivo when β-ionone was applied through the skin. Furthermore, the intraperitoneal injection of gallein inhibited this increased metastasis spread. Our results thus support the role of OR51E2 in the β-ionone observed effects, and suggest that gallein could be a potential new agent in personalized medicine of the tumours expressing OR51E2.

Electronic supplementary material

The online version of this article (10.1186/s13104-017-2879-z) contains supplementary material, which is available to authorized users.

Fundamental principles of the olfactory code

Sensory coding represents a basic principle of all phyla in nature: species attempt to perceive their natural surroundings and to make sense of them. Ultimately, sensory coding is the only way to allow a species to make the kinds of crucial decisions that lead to a behavioral response. In this manner, animals are able to detect numerous parameters, ranging from temperature and humidity to light and sound to volatile or non-volatile chemicals. Most of these environmental cues represent a clearly defined stimulus array that can be described along a single physical parameter, such as wavelength or frequency; odorants, in contrast, cannot. The odor space encompasses an enormous and nearly infinite number of diverse stimuli that cannot be classified according to their positions along a single dimension. Hence, the olfactory system has to encode and translate the vast odor array into an accurate neural map in the brain. In this review, we will outline the relevant steps of the olfactory code and describe its progress along the olfactory pathway, i.e., from the peripheral olfactory organs to the first olfactory center in the brain and then to the higher processing areas where the odor perception takes place, enabling an organism to make odor-guided decisions. We will focus mainly on studies from the vinegar fly Drosophila melanogaster, but we will also indicate similarities to and differences from the olfactory system of other invertebrate species as well as of the vertebrate world.

Coordinated shift of olfactory amino acid responses and V2R expression to an amphibian water nose during metamorphosis

All olfactory receptors identified in teleost fish are expressed in a single sensory surface, whereas mammalian olfactory receptor gene families segregate into different olfactory organs, chief among them the main olfactory epithelium expressing ORs and TAARs, and the vomeronasal organ expressing V1Rs and V2Rs. A transitional stage is embodied by amphibians, with their vomeronasal organ expressing more 'modern', later diverging V2Rs, whereas more 'ancient', earlier diverging V2Rs are expressed in the main olfactory epithelium. During metamorphosis, the main olfactory epithelium of Xenopus tadpoles transforms into an air-filled cavity (principal cavity, air nose), whereas a newly formed cavity (middle cavity) takes over the function of a water nose. We report here that larval expression of ancient V2Rs is gradually lost from the main olfactory epithelium as it transforms into the air nose. Concomitantly, ancient v2r gene expression begins to appear in the basal layers of the newly forming water nose. We observe the same transition for responses to amino acid odorants, consistent with the hypothesis that amino acid responses may be mediated by V2R receptors.

Diversity and regulatory impact of copy number variation in the primate Macaca fascicularis

BACKGROUND

Copy number variations (CNVs) are a significant source of genetic diversity and commonly found in mammalian genomes. We have generated a genome-wide CNV map for Cynomolgus monkeys (Macaca fascicularis). This crab-eating macaque is the closest animal model to humans that is used in biomedical research.

RESULTS

We show that Cynomolgus monkey CNVs are in general much smaller in size than gene loci and are specific to the population of origin. Genome-wide expression data from five vitally important organs demonstrates that CNVs in close proximity to transcription start sites associate strongly with expression changes. Among these eQTL genes we find an overrepresentation of genes involved in metabolism, receptor activity, and transcription.

CONCLUSION

These results provide evidence that CNVs shape tissue transcriptomes in monkey populations, potentially offering an adaptive advantage. We suggest that this genetic diversity should be taken into account when using Cynomolgus macaques as models.

Olfactory signaling components and olfactory receptors are expressed in tubule cells of the human kidney

Cells of the renal tubule system are in direct contact with compounds dissolved in the urine, such as short chain fatty acids (SCFA). Murine OR78, a member of the olfactory receptor (OR) family, is involved in SCFA-related regulation of renal blood pressure in mice. It is still unclear whether OR signaling has an impact on human renal physiology. In our study, we showed that OR51E1 and OR11H7, both of which can be activated by the SCFA isovaleric acid, are expressed in the HK-2 human proximal tubule cell line. We observed a transient increase in intracellular Ca²⁺ when isovaleric acid and 4-methylvaleric acid were added to HK-2 cells. The isovaleric acid-induced response was dependent on extracellular Ca²⁺ and adenylyl cyclase (AC) activation. Furthermore, we demonstrated that the canonical olfactory signaling components Gα_olf and ACIII are co-localized with OR51E1. The number of cells responding to isovaleric acid correlated with the presence of primary cilia on HK-2 cells. OR51E1 protein expression was confirmed in the tubule system of human kidney tissue. Our study is the first to show the expression of ORs and olfactory signaling components in human kidney cells. Additionally, we discuss ORs as potential modulators of the renal physiology.

After oxidation, zinc nanoparticles lose their ability to enhance responses to odorants

Electrical responses of olfactory sensory neurons to odorants were examined in the presence of zinc nanoparticles of various sizes and degrees of oxidation. The zinc nanoparticles were prepared by the underwater electrical discharge method and analyzed by atomic force microscopy and X-ray photoelectron spectroscopy. Small (1.2 ± 0.3 nm) zinc nanoparticles significantly enhanced electrical responses of olfactory neurons to odorants. After oxidation, however, these small zinc nanoparticles were no longer capable of enhancing olfactory responses. Larger zinc oxide nanoparticles (15 nm and 70 nm) also did not modulate responses to odorants. Neither zinc nor zinc oxide nanoparticles produced olfactory responses when added without odorants. The enhancement of odorant responses by small zinc nanoparticles was explained by the creation of olfactory receptor dimers initiated by small zinc nanoparticles. The results of this work will clarify the mechanisms for the initial events in olfaction, as well as to provide new ways to alleviate anosmia related to the loss of olfactory receptors.

Bioelectronic nose: Current status and perspectives

A characteristic feature of human and animal organs of smell is the ability to identify hundreds of thousands of odours. It is accompanied by particular smell sensations, which are a basic source of information about odour mixture. The main structural elements of biological smell systems are the olfactory receptors. Small differences in a structure of odorous molecules (odorants) can lead to significant change of odour, which is due to the fact that each of the olfactory receptors is coded with different gene and usually corresponds to different type of odour. Discovery and characterisation of the gene family coding the olfactory receptors contributed to the elaboration and development of the electronic smell systems, the so-called bioelectronic noses. The olfactory receptors are employed as a biological element in this type of instruments. An electronic system includes a converter part, which allows measurement and processing of generated signals. A suitable data analysis system is also required to visualise the results. Application potentialities of the bioelectronic noses are focused on the fields of economy and science where highly selective and sensitive analysis of odorous substances is required. The paper presents a review of the latest achievements and critical evaluation of the state of art in the field of bioelectronic noses.

How does your kidney smell? Emerging roles for olfactory receptors in renal function

Olfactory receptors (ORs) are chemosensors that are responsible for one's sense of smell. In addition to this specialized role in the nose, recent evidence suggests that ORs are also found in a variety of additional tissues including the kidney. As this list of renal ORs continues to expand, it is becoming clear that they play important roles in renal and whole-body physiology, including a novel role in blood pressure regulation. In this review, we highlight important considerations that are crucial when studying ORs and present the current literature on renal ORs and their emerging relevance in maintaining renal function.

OR2W3 sequence variants are unlikely to cause inherited retinal diseases

Because of its formidable throughput, whole exome sequencing (WES) is significantly increasing the power of investigations in ophthalmic genetics. However, when applied to Mendelian conditions, WES results often contain many false positives, e.g. candidate mutations that are unrelated to the disease. For instance, highly polymorphic genes such as olfactory receptor genes carry a plethora of both common and rare alleles that are part of the normal set of variations of the human genome. Following a WES-based study, the heterozygous missense variant p.R142W in the olfactory receptor gene OR2W3 was recently reported as a pathogenic mutation causing autosomal dominant retinitis pigmentosa (RP). This variant, however, was not scored against data contained in public WES repositories, indicating that p.R142W is present in ~1 in 6500 control individuals. Therefore, if it really was pathogenic, it would be responsible for a percentage of dominant RP cases corresponding to the double of those recorded so far worldwide, or 2/3 of all RP cases (dominant, recessive, and X-linked). We therefore conclude that this sequence variant, and hence the OR2W3 gene, do not cause RP. Prompted by these findings and based on simple principles of population genetics, we suggest that WES studies should consider DNA variants as the possible cause of dominant RP only if they are present in less than 1:100,000 individuals from the general population. In addition, we propose that DNA variants belonging to highly polymorphic genes should be carefully analyzed at the functional level before inferring their pathogenicity, in RP or other genetic diseases.

Immunochemical strategy for quantification of G-coupled olfactory receptor proteins on natural nanovesicles

Cell membrane proteins are involved in a variety of biochemical pathways and therefore constitute important targets for therapy and development of new drugs. Bioanalytical platforms and binding assays using these membrane protein receptors for drug screening or diagnostic require the construction of well-characterized liposome and lipid bilayer arrays that act as support to prevent protein denaturation during biochip processing. Quantification of the protein receptors in the lipid membrane arrays is a key issue in order to produce reproducible and well-characterized chips. Herein, we report a novel immunochemical analytical approach for the quantification of membrane proteins (i.e., G-protein-coupled receptor, GPCR) in nanovesicles (NVs). The procedure allows direct determination of tagged receptors (i.e., c-myc tag) without any previous protein purification or extraction steps. The immunochemical method is based on a microplate ELISA format and quantifies this tag on proteins embedded in NVs with detectability in the picomolar range, using protein bioconjugates as reference standards. The applicability of the method is demonstrated through the quantification of the c-myc-olfactory receptor (OR, c-myc-OR1740) in the cell membrane NVs. The reported method opens the possibility to develop well-characterized drug-screening platforms based on G-coupled proteins embedded on membranes.

Decrease in olfactory and taste receptor expression in the dorsolateral prefrontal cortex in chronic schizophrenia

We have recently identified up- or down-regulation of the olfactory (OR) and taste (TASR) chemoreceptors in the human cortex in several neurodegenerative diseases, raising the possibility of a general deregulation of these genes in neuropsychiatric disorders. In this study, we explore the possible deregulation of OR and TASR gene expression in the dorsolateral prefrontal cortex in schizophrenia. We used quantitative polymerase chain reaction on extracts from postmortem dorsolateral prefrontal cortex of subjects with chronic schizophrenia (n = 15) compared to control individuals (n = 14). Negative symptoms were evaluated premortem by the Positive and Negative Syndrome and the Clinical Global Impression Schizophrenia Scales. We report that ORs and TASRs are deregulated in the dorsolateral prefrontal cortex in schizophrenia. Seven out of eleven ORs and four out of six TASRs were down-regulated in schizophrenia, the most prominent changes of which were found in genes from the 11p15.4 locus. The expression did not associate with negative symptom clinical scores or the duration of the illness. However, most ORs and all TASRs inversely associated with the daily chlorpromazine dose. This study identifies for the first time a decrease in brain ORs and TASRs in schizophrenia, a neuropsychiatric disease not linked to abnormal protein aggregates, suggesting that the deregulation of these receptors is associated with altered cognition of these disorders. In addition, the influence of antipsychotics on the expression of ORs and TASRs in schizophrenia suggests that these receptors could be involved in the mechanism of action or side effects of antipsychotics.

Density peaks of paralog pairs in human and mouse genomes

Paralog gene trees, which reflect the increase of genomic complexity in the evolution, can be complicated and ambiguous. A simpler complementary approach is analysis of density distribution of paralog pairs. It can reveal general features of genome evolution, which may be hidden in the forest of gene trees. It is known that distribution of human paralog pairs along the axis of protein divergence between pair members forms two main peaks. Here I show that there are three main peaks in the mouse genome. Thus, the multimodality of paralog pair distribution seems to be a fundamental feature of mammalian genomes. Despite the great diversity of domains presented in small amounts or in multidomain architectures with a few predominant domains, both in human and mouse the first peak consists mostly of gene pairs with zinc finger domains or olfactory receptor domain. In the mouse the olfactory receptor predominates, which stipulates the three-peak distribution (since in the olfactory receptors the second peak is closer to the first peak than in other genes). The mammalian-wide zinc finger orthologs are biased towards the second peak. Thus, the marsupial orthologs are nearly absent in the first peak of human and mouse. The gene pairs in the first peak show a lower ratio of nonsynonymous to synonymous substitutions, which suggests that their evolution is more constrained. The plausible explanation is that they are in subfunctionalization state (partition of initial function of ancestral gene), whereas the second peak contains gene pairs that are already in neofunctionalization state (acquiring of novel functions). These data suggest that the adaptive radiation of mammals was accompanied by a burst of duplication of zinc finger genes, which are located in the first (most recent) peak of paralog pairs.