High-throughput microarray detection of olfactory receptor gene expression in the mouse

Epigenetic programming of stochastic olfactory receptor choice

The mammalian sense of smell relies upon a vast array of receptor proteins to detect odorant compounds present in the environment. The proper deployment of these receptor proteins in olfactory sensory neurons is orchestrated by a suite of epigenetic processes that remodel the olfactory genes in differentiating neuronal progenitors. The goal of this review is to elucidate the central role of gene regulatory processes acting in neuronal progenitors of olfactory sensory neurons that lead to a singular expression of an odorant receptor in mature olfactory sensory neurons. We begin by describing the principal features of odorant receptor gene expression in mature olfactory sensory neurons. Next, we delineate our current understanding of how these features emerge from multiple gene regulatory mechanisms acting in neuronal progenitors. Finally, we close by discussing the key gaps in our understanding of how these regulatory mechanisms work and how they interact with each other over the course of differentiation.

Insertion of a neomycin selection cassette in the Amigo1 locus alters gene expression in the olfactory epithelium leading to region-specific defects in olfactory receptor neuron development

During development of the nervous system, neurons connect to one another in a precisely organized manner. Sensory systems provide a good example of this organization, whereby the composition of the outside world is represented in the brain by neuronal maps. Establishing correct patterns of neural circuitry is crucial, as inaccurate map formation can lead to severe disruptions in sensory processing. In rodents, olfactory stimuli modulate a wide variety of behaviors essential for survival. The formation of the olfactory glomerular map is dependent on molecular cues that guide olfactory receptor neuron axons to broad regions of the olfactory bulb and on cell adhesion molecules that promote axonal sorting into specific synaptic units in this structure. Here, we demonstrate that the cell adhesion molecule Amigo1 is expressed in a subpopulation of olfactory receptor neurons, and we investigate its role in the precise targeting of olfactory receptor neuron axons to the olfactory bulb using a genetic loss-of-function approach in mice. While ablation of Amigo1 did not lead to alterations in olfactory sensory neuron axonal targeting, our experiments revealed that the presence of a neomycin resistance selection cassette in the Amigo1 locus can lead to off-target effects that are not due to loss of Amigo1 expression, including unexpected altered gene expression in olfactory receptor neurons and reduced glomerular size in the ventral region of the olfactory bulb. Our results demonstrate that insertion of a neomycin selection cassette into the mouse genome can have specific deleterious effects on the development of the olfactory system and highlight the importance of removing antibiotic resistance cassettes from genetic loss-of-function mouse models when studying olfactory system development.

From odor to oncology: non-canonical odorant receptors in cancer

Odorant receptors, traditionally associated with olfaction as chemoreceptors, have been increasingly recognized for their presence and diverse functions in various non-nasal tissues throughout the body. Beyond their roles in sensory perception, emerging evidence suggests a compelling interplay between odorant receptors and cancer progression as well. Alongside the canonical GPCR odorant receptors, dysregulation of non-canonical odorant receptors such as trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and membrane-spanning 4A family (MS4As) has been observed in various cancer types, suggesting their contributions to cancer progression. The roles of these non-canonical chemoreceptors in cancer are complex, with some receptors promoting tumorigenesis and others acting as tumor-suppressing factors upon activation, depending on the cancer type. These findings shed light on the potential of non-canonical odorant receptors as therapeutic targets and prognostic markers in cancer, inviting further exploration to unravel their precise mechanisms of action and implications in cancer biology. In this review, we provide a comprehensive overview of the intricate relationships between these chemoreceptors and various types of cancer, potentially paving the way for innovative odor-based therapeutics. Ultimately, this review discusses the potential development of novel therapeutic strategies targeting these non-canonical chemoreceptors.

A histological protocol for quantifying the birthrates of specific subtypes of olfactory sensory neurons in mice

Mammals typically have hundreds of distinct olfactory sensory neuron subtypes, each defined by expression of a specific odorant receptor gene, which undergo neurogenesis throughout life at rates that can depend on olfactory experience. Here, we present a protocol to quantify the birthrates of specific neuron subtypes via the simultaneous detection of corresponding receptor mRNAs and 5-ethynyl-2'-deoxyuridine. For preparation prior to beginning the protocol, we detail procedures for generating odorant receptor-specific riboprobes and experimental mouse olfactory epithelial tissue sections. For complete details on the use and execution of this protocol, please refer to van der Linden et al. (2020).1.

Pseudotime analysis reveals novel regulatory factors for multigenic onset and monogenic transition of odorant receptor expression

During their maturation from horizontal basal stem cells, olfactory sensory neurons (OSNs) are known to select exactly one out of hundreds of olfactory receptors (ORs) and express it on their surface, a process called monogenic selection. Monogenic expression is preceded by a multigenic phase during which several OR genes are expressed in a single OSN. Here, we perform pseudotime analysis of a single cell RNA-Seq dataset of murine olfactory epithelium to precisely align the multigenic and monogenic expression phases with the cell types occurring during OSN differentiation. In combination with motif analysis of OR gene cluster-associated enhancer regions, we identify known and novel transcription (co-)factors (Ebf1, Lhx2, Ldb1, Fos and Ssbp2) and chromatin remodelers (Kdm1a, Eed and Zmynd8) associated with OR expression. The inferred temporal order of their activity suggests novel mechanisms contributing to multigenic OR expression and monogenic selection.

A 3D transcriptomics atlas of the mouse nose sheds light on the anatomical logic of smell

The sense of smell helps us navigate the environment, but its molecular architecture and underlying logic remain understudied. The spatial location of odorant receptor genes (Olfrs) in the nose is thought to be independent of the structural diversity of the odorants they detect. Using spatial transcriptomics, we create a genome-wide 3D atlas of the mouse olfactory mucosa (OM). Topographic maps of genes differentially expressed in space reveal that both Olfrs and non-Olfrs are distributed in a continuous and overlapping fashion over at least five broad zones in the OM. The spatial locations of Olfrs correlate with the mucus solubility of the odorants they recognize, providing direct evidence for the chromatographic theory of olfaction. This resource resolves the molecular architecture of the mouse OM and will inform future studies on mechanisms underlying Olfr gene choice, axonal pathfinding, patterning of the nervous system, and basic logic for the peripheral representation of smell.

The sorption/chromatography hypothesis of olfactory discrimination: The rise, fall, and rebirth of a Phoenix

Herein, I discuss the enduring mystery of the receptor layout in the vertebrate olfactory system. Since the awarding of the 2004 Nobel Prize to Axel and Buck for their discovery of the gene family that encodes olfactory receptors, our field has enjoyed a golden era. Despite this Renaissance, an answer to one of the most fundamental questions for any sensory system-what is the anatomical logic of its receptor array?-eludes us, still, for olfaction! Indeed, the only widely debated hypothesis, finding its origins in the musing of another Nobel laureate Sir Edgar Adrian, has it that the vertebrate nose organizes its receptors according to the "sorptive" properties of their ligands. This idea, known as the "sorption" or "chromatography" hypothesis, enjoys considerable support despite being controversial. Here, I review the history of the hypothesis-its rises and falls-and discuss the latest data and future prospects for this perennial idea whose history I liken to the mythical Phoenix.

Odorant G protein-coupled receptors as potential therapeutic targets for adult diffuse gliomas: a systematic analysis and review

Odorant receptors (ORs) account for about 60% of all human G protein-coupled receptors (GPCRs). OR expression outside of the nose has functions distinct from odor perception, and may contribute to the pathogenesis of disorders including brain diseases and cancers. Glioma is the most common adult malignant brain tumor and requires novel therapeutic strategies to improve clinical outcomes. Here, we outlined the expression of brain ORs and investigated OR expression levels in glioma. Although most ORs were not ubiquitously expressed in gliomas, a subset of ORs displayed glioma subtype-specific expression. Moreover, through systematic survival analysis on OR genes, OR51E1 (mouse Olfr558) was identified as a potential biomarker of unfavorable overall survival, and OR2C1 (mouse Olfr15) was identified as a potential biomarker of favorable overall survival in isocitrate dehydrogenase (IDH) wild-type glioma. In addition to transcriptomic analysis, mutational profiles revealed that somatic mutations in OR genes were detected in > 60% of glioma samples. OR5D18 (mouse Olfr1155) was the most frequently mutated OR gene, and OR5AR1 (mouse Olfr1019) showed IDH wild-type-specific mutation. Based on this systematic analysis and review of the genomic and transcriptomic profiles of ORs in glioma, we suggest that ORs are potential biomarkers and therapeutic targets for glioma.

Ectopic Odorant Receptor Responding to Flavor Compounds: Versatile Roles in Health and Disease

Prompted by the ground-breaking discovery of the rodent odorant receptor (OR) gene family within the olfactory epithelium nearly 30 years ago, followed by that of OR genes in cells of the mammalian germ line, and potentiated by the identification of ORs throughout the body, our appreciation for ORs as general chemoreceptors responding to odorant compounds in the regulation of physiological or pathophysiological processes continues to expand. Ectopic ORs are now activated by a diversity of flavor compounds and are involved in diverse physiological phenomena varying from adipogenesis to myogenesis to hepatic lipid accumulation to serotonin secretion. In this review, we outline the key biological functions of the ectopic ORs responding to flavor compounds and the underlying molecular mechanisms. We also discuss research opportunities for utilizing ectopic ORs as therapeutic strategies in the treatment of human disease as well as challenges to be overcome in the future. The recognition of the potent function, signaling pathway, and pharmacology of ectopic ORs in diverse tissues and cell types, coupled with the fact that they belong to G protein-coupled receptors, a highly druggable protein family, unequivocally highlight the potential of ectopic ORs responding to flavor compounds, especially food-derived odorant compounds, as a promising therapeutic strategy for various diseases.

Paternal Finasteride Treatment Can Influence the Testicular Transcriptome Profile of Male Offspring-Preliminary Study

(1) Background: Hormone-dependent events that occur throughout spermatogenesis during postnatal testis maturation are significant for adult male fertility. Any disturbances in the T/DHT ratio in male progeny born from females fertilized by finasteride-treated male rats (F0:Fin) can result in the impairment of testicular physiology. The goal of this work was to profile the testicular transcriptome in the male filial generation (F1:Fin) from paternal F0:Fin rats. (2) Methods: The subject material for the study were testis from immature and mature male rats born from females fertilized by finasteride-treated rats. Testicular tissues from the offspring were used in microarray analyses. (3) Results: The top 10 genes having the highest and lowest fold change values were mainly those that encoded odoriferous (Olfr: 31, 331, 365, 633, 774, 814, 890, 935, 1109, 1112, 1173, 1251, 1259, 1253, 1383) and vomeronasal (Vmn1r: 50, 103, 210, 211; Vmn2r: 3, 23, 99) receptors and RIKEN cDNA 5430402E10, also known as odorant-binding protein. (4) Conclusions: Finasteride treatment of male adult rats may cause changes in the testicular transcriptome of their male offspring, leading to a defective function of spermatozoa in response to odorant-like signals, which are recently more and more often noticed as significant players in male fertility.

The Zonal Organization of Odorant Receptor Gene Choice in the Main Olfactory Epithelium of the Mouse

A mature olfactory sensory neuron (OSN) of the main olfactory epithelium (MOE) typically expresses one allele of one odorant receptor (OR) gene. It is widely thought that the great majority of the 1,141 intact mouse OR genes are expressed in one of four MOE zones (or bands or stripes), which are largely non-overlapping. Here, we develop a multiplex method to map, in 3D and MOE-wide, the expression areas of multiple OR genes in individual, non-genetically modified mice by three-color fluorescence in situ hybridization, semi-automated image segmentation, and 3D reconstruction. We classify the expression areas of 68 OR genes into 9 zones. These zones are highly overlapping and strikingly complex when viewed in 3D reconstructions. There could well be more zones. We propose that zones reflect distinct OSN types that are each restricted in their choice to a subset of the OR gene repertoire.

Odorant and Taste Receptors in Sperm Chemotaxis and Cryopreservation: Roles and Implications in Sperm Capacitation, Motility and Fertility

Sperm chemotaxis, which guide sperm toward oocyte, is tightly associated with sperm capacitation, motility, and fertility. However, the molecular mechanism of sperm chemotaxis is not known. Reproductive odorant and taste receptors, belong to G-protein-coupled receptors (GPCR) super-family, cause an increase in intracellular Ca²⁺ concentration which is pre-requisite for sperm capacitation and acrosomal reaction, and result in sperm hyperpolarization and increase motility through activation of Ca²⁺-dependent Cl^¯ channels. Recently, odorant receptors (ORs) in olfactory transduction pathway were thought to be associated with post-thaw sperm motility, freeze tolerance or freezability and cryo-capacitation-like change during cryopreservation. Investigation of the roles of odorant and taste receptors (TRs) is important for our understanding of the freeze tolerance or freezability mechanism and improve the motility and fertility of post-thaw sperm. Here, we reviewed the roles, mode of action, impact of odorant and taste receptors on sperm chemotaxis and post-thaw sperm quality.

Genetic Background Effects on the Expression of an Odorant Receptor Gene

There are more than 1000 odorant receptor (OR) genes in the mouse genome. Each olfactory sensory neuron expresses only one of these genes, in a monoallelic fashion. The transcript abundance of homologous OR genes vary between distinct mouse strains. Here we analyzed the expression of the OR gene Olfr17 (also named P2) in different genomic contexts. Olfr17 is expressed at higher levels in the olfactory epithelium from 129 mice than from C57BL/6 (B6) mice. However, we found that in P2-IRES-tauGFP knock-in mice, the transcript levels of the 129 Olfr17 allele are highly reduced when compared to the B6 Olfr17 allele. To address the mechanisms involved in this variation we compared the 5′ region sequence and DNA methylation patterns of the B6 and 129 Olfr17 alleles. Our results show that genetic variations in cis regulatory regions can lead to differential DNA methylation frequencies in these OR gene alleles. They also show that expression of the Olfr17 alleles is largely affected by the genetic background, and suggest that in knock-in mice, expression can be affected by epigenetic modifications in the region of the targeted locus.

Odor coding in the mammalian olfactory epithelium

Noses are extremely sophisticated chemical detectors allowing animals to use scents to interpret and navigate their environments. Odor detection starts with the activation of odorant receptors (ORs), expressed in mature olfactory sensory neurons (OSNs) populating the olfactory mucosa. Different odorants, or different concentrations of the same odorant, activate unique ensembles of ORs. This mechanism of combinatorial receptor coding provided a possible explanation as to why different odorants are perceived as having distinct odors. Aided by new technologies, several recent studies have found that antagonist interactions also play an important role in the formation of the combinatorial receptor code. These findings mark the start of a new era in the study of odorant-receptor interactions and add a new level of complexity to odor coding in mammals.

Human neonates prefer colostrum to mature milk: Evidence for an olfactory bias toward the "initial milk"?

OBJECTIVES

Colostrum is the initial milk secretion which ingestion by neonates warrants their adaptive start in life. Colostrum is accordingly expected to be attractive to newborns. The present study aims to assess whether colostrum is olfactorily attractive for 2-day-old newborns when presented against mature milk or a control.

METHODS

The head-orientation of waking newborns was videotaped in three experiments pairing the odors of: (a) colostrum (sampled on postpartum day 2, not from own mother) and mature milk (sampled on average on postpartum day 32, not from own mother) (n tested newborns = 15); (b) Colostrum and control (water; n = 9); and (c) Mature milk and control (n = 13).

RESULTS

When facing the odors of colostrum and mature milk, the infants turned their nose significantly longer toward former (32.8 vs 17.7% of a 120-s test). When exposed to colostrum against the control, they responded in favor of colostrum (32.9 vs 16.6%). Finally, when the odor of mature milk was presented against the control, their response appeared undifferentiated (26.7 vs 28.6%).

CONCLUSIONS

These results indicate that human newborns can olfactorily differentiate conspecific lacteal fluids sampled at different lactation stages. They prefer the odor of the mammary secretion - colostrum - collected at the lactation stage that best matches the postpartum age of their own mother. These results are discussed in the context of the earliest mother-infant chemo-communication. Coinciding maternal emission and offspring reception of chemosignals conveyed in colostrum may be part of the sensory precursors of attunement between mothers and infants.

Immunocytochemical Localization of Olfactory-signaling Molecules in Human and Rat Spermatozoa

Expression of olfactory receptors (ORs) in non-olfactory tissues has been widely reported over the last 20 years. Olfactory marker protein (OMP) is highly expressed in mature olfactory sensory neurons (mOSNs) of the olfactory epithelium. It is involved in the olfactory signal transduction pathway, which is mediated by well-conserved components, including ORs, olfactory G protein (Golf), and adenylyl cyclase 3 (AC3). OMP is widely expressed in non-olfactory tissues with an apparent preference for motile cells. We hypothesized that OMP is expressed in compartment-specific locations and co-localize with an OR, Golf, and AC3 in rat epididymal and human-ejaculated spermatozoa. We used immunocytochemistry to examine the expression patterns of OMP and OR6B2 (human OR, served as positive olfactory control) in experimentally induced modes of activation and determine whether there are any observable differences in proteins expression during the post-ejaculatory stages of spermatozoal functional maturation. We found that OMP was expressed in compartment-specific locations in human and rat spermatozoa. OMP was co-expressed with Golf and AC3 in rat spermatozoa and with OR6B2 in all three modes of activation (control, activated, and hyperactivated), and the mode of activation changed the co-expression pattern in acrosomal-reacted human spermatozoa. These observations suggest that OMP expression is a reliable indicator of OR-mediated chemoreception, may be used to identify ectopically expressed ORs, and could participate in second messenger signaling cascades that mediate fertility.

Innate immune signaling in the olfactory epithelium reduces odorant receptor levels: modeling transient smell loss in COVID-19 patients

Post-infectious anosmias typically follow death of olfactory sensory neurons (OSNs) with a months-long recovery phase associated with parosmias. While profound anosmia is the leading symptom associated with COVID-19 infection, many patients regain olfactory function within days to weeks without distortions. Here, we demonstrate that sterile induction of anti-viral type I interferon signaling in the mouse olfactory epithelium is associated with diminished odor discrimination and reduced odor-evoked local field potentials. RNA levels of all class I, class II, and TAAR odorant receptors are markedly reduced in OSNs in a non-cell autonomous manner. We find that people infected with COVID-19 rate odors with lower intensities and have odor discrimination deficits relative to people that tested negative for COVID-19. Taken together, we propose that inflammatory-mediated loss of odorant receptor expression with preserved circuit integrity accounts for the profound anosmia and rapid recovery of olfactory function without parosmias caused by COVID-19.

Transcriptome Analysis of Zebrafish Olfactory Epithelium Reveal Sexual Differences in Odorant Detection

Animals have evolved a large number of olfactory receptor genes in their genome to detect numerous odorants in their surrounding environments. However, we still know little about whether males and females possess the same abilities to sense odorants, especially in fish. In this study, we used deep RNA sequencing to examine the difference of transcriptome between male and female zebrafish olfactory epithelia. We found that the olfactory transcriptomes between males and females are highly similar. We also found evidence of some genes showing differential expression or alternative splicing, which may be associated with odorant-sensing between sexes. Most chemosensory receptor genes showed evidence of expression in the zebrafish olfactory epithelium, with a higher expression level in males than in females. Taken together, our results provide a comprehensive catalog of the genes mediating olfactory perception and pheromone-evoked behavior in fishes.

TRPM5-expressing Microvillous Cells Regulate Region-specific Cell Proliferation and Apoptosis During Chemical Exposure

The mammalian main olfactory epithelium (MOE) is exposed to a wide spectrum of external chemicals during respiration and relies on adaptive plasticity to maintain its structural and functional integrity. We previously reported that the chemo-responsive and cholinergic transient receptor potential channel M5 (TRPM5)-expressing-microvillous cells (MCs) in the MOE are required for maintaining odor-evoked electrophysiological responses and olfactory-guided behavior during two-week exposure to an inhaled chemical mixture. Here, we investigated the underlying factors by assessing the potential modulatory effects of TRPM5-MCs on MOE morphology and cell proliferation and apoptosis, which are important for MOE maintenance. In the posterior MOE of TRPM5-GFP mice, we found that two-week chemical exposure induced a significant increase in Ki67-expressing proliferating basal stem cells without a significant reduction in the thickness of the whole epithelium or mature olfactory sensory neuron (OSN) layer. This adaptive increase in stem cell proliferation was missing in chemical-exposed transcription factor Skn-1a knockout (Skn-1a^-/-) mice lacking TRPM5-MCs. In addition, a greater number of isolated OSNs from chemical-exposed Skn-1a^-/- mice displayed unhealthily high levels of resting intracellular Ca²⁺. Intriguingly, in the anterior MOE where we found a higher density of TRPM5-MCs, chemical-exposed TRPM5-GFP mice exhibited a time-dependent increase in apoptosis and a loss of mature OSNs without a significant increase in proliferation or neurogenesis to compensate for OSN loss. Together, our data suggest that TRPM5-MC-dependent region-specific upregulation of cell proliferation in the majority of the MOE during chemical exposure contributes to the adaptive maintenance of OSNs and olfactory function.

Therapeutic potential of ectopic olfactory and taste receptors

Olfactory and taste receptors are expressed primarily in the nasal olfactory epithelium and gustatory taste bud cells, where they transmit real-time sensory signals to the brain. However, they are also expressed in multiple extra-nasal and extra-oral tissues, being implicated in diverse biological processes including sperm chemotaxis, muscle regeneration, bronchoconstriction and bronchodilatation, inflammation, appetite regulation and energy metabolism. Elucidation of the physiological roles of these ectopic receptors is revealing potential therapeutic and diagnostic applications in conditions including wounds, hair loss, asthma, obesity and cancers. This Review outlines current understanding of the diverse functions of ectopic olfactory and taste receptors and assesses their potential to be therapeutically exploited.

Alternative polyadenylation produces multiple 3' untranslated regions of odorant receptor mRNAs in mouse olfactory sensory neurons

BACKGROUND

Odorant receptor genes constitute the largest gene family in mammalian genomes and this family has been extensively studied in several species, but to date far less attention has been paid to the characterization of their mRNA 3' untranslated regions (3'UTRs). Given the increasing importance of UTRs in the understanding of RNA metabolism, and the growing interest in alternative polyadenylation especially in the nervous system, we aimed at identifying the alternative isoforms of odorant receptor mRNAs generated through 3'UTR variation.

RESULTS

We implemented a dedicated pipeline using IsoSCM instead of Cufflinks to analyze RNA-Seq data from whole olfactory mucosa of adult mice and obtained an extensive description of the 3'UTR isoforms of odorant receptor mRNAs. To validate our bioinformatics approach, we exhaustively analyzed the 3'UTR isoforms produced from 2 pilot genes, using molecular approaches including northern blot and RNA ligation mediated polyadenylation test. Comparison between datasets further validated the pipeline and confirmed the alternative polyadenylation patterns of odorant receptors. Qualitative and quantitative analyses of the annotated 3' regions demonstrate that 1) Odorant receptor 3'UTRs are longer than previously described in the literature; 2) More than 77% of odorant receptor mRNAs are subject to alternative polyadenylation, hence generating at least 2 detectable 3'UTR isoforms; 3) Splicing events in 3'UTRs are restricted to a limited subset of odorant receptor genes; and 4) Comparison between male and female data shows no sex-specific differences in odorant receptor 3'UTR isoforms.

CONCLUSIONS

We demonstrated for the first time that odorant receptor genes are extensively subject to alternative polyadenylation. This ground-breaking change to the landscape of 3'UTR isoforms of Olfr mRNAs opens new avenues for investigating their respective functions, especially during the differentiation of olfactory sensory neurons.

Kirrel2 is differentially required in populations of olfactory sensory neurons for the targeting of axons in the olfactory bulb

The formation of olfactory maps in the olfactory bulb (OB) is crucial for the control of innate and learned mouse behaviors. Olfactory sensory neurons (OSNs) expressing a specific odorant receptor project axons into spatially conserved glomeruli within the OB and synapse onto mitral cell dendrites. Combinatorial expression of members of the Kirrel family of cell adhesion molecules has been proposed to regulate OSN axonal coalescence; however, loss-of-function experiments have yet to establish their requirement in this process. We examined projections of several OSN populations in mice that lacked either Kirrel2 alone, or both Kirrel2 and Kirrel3. Our results show that Kirrel2 and Kirrel3 are dispensable for the coalescence of MOR1-3-expressing OSN axons to the most dorsal region (DI) of the OB. In contrast, loss of Kirrel2 caused MOR174-9- and M72-expressing OSN axons, projecting to the DII region, to target ectopic glomeruli. Our loss-of-function approach demonstrates that Kirrel2 is required for axonal coalescence in subsets of OSNs that project axons to the DII region and reveals that Kirrel2/3-independent mechanisms also control OSN axonal coalescence in certain regions of the OB.

Sense of Smell: Structural, Functional, Mechanistic Advancements and Challenges in Human Olfactory Research

Olfaction, the sense of smell detects and discriminate odors as well as social cues which influence our innate responses. The olfactory system in human beings is found to be weak as compared to other animals; however, it seems to be very precise. It can detect and discriminate millions of chemical moieties (odorants) even in minuscule quantities. The process initiates with the binding of odorants to specialized olfactory receptors, encoded by a large family of Olfactory Receptor (OR) genes belonging to the G-protein-coupled receptor superfamily. Stimulation of ORs converts the chemical information encoded in the odorants, into respective neuronal action-potentials which causes depolarization of olfactory sensory neurons. The olfactory bulb relays this signal to different parts of the brain for processing. Odors are encrypted using a combinatorial approach to detect a variety of chemicals and encode their unique identity. The discovery of functional OR genes and proteins provided an important information to decipher the genomic, structural and functional basis of olfaction. ORs constitute 17 gene families, out of which 4 families were reported to contain more than hundred members each. The olfactory machinery is not limited to GPCRs; a number of non- GPCRs is also employed to detect chemosensory stimuli. The article provides detailed information about such olfaction machinery, structures, transduction mechanism, theories of odor perception, and challenges in the olfaction research. It covers the structural, functional and computational studies carried out in the olfaction research in the recent past.

Evolution of Gustatory Receptor Gene Family Provides Insights into Adaptation to Diverse Host Plants in Nymphalid Butterflies

The host plant range of herbivorous insects is a major aspect of insect–plant interaction, but the genetic basis of host range expansion in insects is poorly understood. In butterflies, gustatory receptor genes (GRs) play important roles in host plant selection by ovipositing females. Since several studies have shown associations between the repertoire sizes of chemosensory gene families and the diversity of resource use, we hypothesized that the increase in the number of genes in the GR family is associated with host range expansion in butterflies. Here, we analyzed the evolutionary dynamics of GRs among related species, including the host generalist Vanessa cardui and three specialists. Although the increase of the GR repertoire itself was not observed, we found that the gene birth rate of GRs was the highest in the lineage leading to V. cardui compared with other specialist lineages. We also identified two taxon-specific subfamilies of GRs, characterized by frequent lineage-specific duplications and higher non-synonymous substitution rates. Together, our results suggest that frequent gene duplications in GRs, which might be involved in the detection of plant secondary metabolites, were associated with host range expansion in the V. cardui lineage. These evolutionary patterns imply that the capability to perceive various compounds during host selection was favored during adaptation to diverse host plants.

A Near-Complete Spatial Map of Olfactory Receptors in the Mouse Main Olfactory Epithelium

Different regions of the mammalian nose smell different odors. In the mouse olfactory system, spatially regulated expression of >1000 olfactory receptors (ORs) along the dorsomedial-ventrolateral (DV) axis forms a topological map in the main olfactory epithelium (MOE). However, the locations of most ORs along the DV axis are currently unknown. By sequencing mRNA of 12 isolated MOE pieces, we mapped out the DV locations-as quantified by "zone indices" on a scale of 1-5-of 1033 OR genes with an estimated error of 0.3 zone indices. Our map covered 81% of all intact OR genes and 99.4% of the total OR mRNA abundance. Spatial regulation tended to vary gradually along chromosomes. We further identified putative non-OR genes that may exhibit spatial expression along the DV axis.

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.

Hydrogen Sulfide Specifically Alters NAD(P)H Quinone Dehydrogenase 1 (NQO1) Olfactory Neurons in the Rat

The regions of the olfactory epithelium affected by hydrogen sulfide (H₂S) toxicity in the rat present a striking similarity with the developmental olfactory zone 1 described in the mouse. This zone which is the only region containing neurons expressing NAD(P)H quinone dehydrogenase 1 (NQO1) is involved in complex behavioral responses in rodents, and other mammals, triggered by specific olfactory stimuli. We therefore sought to determine whether (1) olfactory neurons expressing NQO1 are located in the same regions in the rats and in the mice and (2) there is an overlap between olfactory neurons expressing this protein and those affected by the toxicity of H₂S. Rats were exposed to H₂S - 200 ppm during 3 h, three consecutive days- and displayed symmetric acute segmental necrosis of the neurons and sustentacular cells of the olfactory epithelium in the dorsomedial nasal cavity. We found that expression of NQO1 in Sprague-Dawley rats spatially recapitulated that of the mouse. The degree of agreement or overlap between these two populations of neurons (necrosis vs. NQO1 expression) reached 80.2%. Although the underlying mechanisms accounted for the high sensitivity for NQO1 neurons -or the relative protection of non NQO1 neurons- to sulfide toxicity remain to be established, this observation is offering an intriguing approach that could be used to acutely suppress the pool of neural cells in olfactory zone I and to understand the mechanisms of toxicity and protection of other populations of neurons exposed to sulfide.

Immobility responses are induced by photoactivation of single glomerular species responsive to fox odour TMT

Fox odour 2,4,5-trimethyl thiazoline (TMT) is known to activate multiple glomeruli in the mouse olfactory bulb (OB) and elicits strong fear responses. In this study, we screened TMT-reactive odourant receptors and identified Olfr1019 with high ligand reactivity and selectivity, whose glomeruli are located in the posterodorsal OB. In the channelrhodopsin knock-in mice for Olfr1019, TMT-responsive olfactory-cortical regions were activated by photostimulation, leading to the induction of immobility, but not aversive behaviour. Distribution of photoactivation signals was overlapped with that of TMT-induced signals, but restricted to the narrower regions. In the knockout mice, immobility responses were reduced, but not entirely abolished likely due to the compensatory function of other TMT-responsive glomeruli. Our results demonstrate that the activation of a single glomerular species in the posterodorsal OB is sufficient to elicit immobility responses and that TMT-induced fear may be separated into at least two different components of immobility and aversion.

The olfactory bulb is arranged in glomeruli defined by their olfactory receptor expression. The authors identify an olfactory receptor for fox odour, TMT, and show that activation of the glomerulus expressing that receptor in mice leads to immobility, but does not lead to other fear behaviours.

Differentially expressed genes in the testicular tissues of adenylyl cyclase 3 knockout mice

Adenylate cyclase 3 (AC3) is an important component of the cyclic adenosine 3',5'-monophosphate (cAMP) signaling pathway and converts adenosine triphosphate into cAMP. Male mice with AC3 deletion (AC3^-/-) are sterile. However, the mechanical mechanism remains unclear. By TUNEL staining, we found that cell apoptosis in the testicular tissues of AC3^-/- mice increased significantly compared with that in the wild-type (AC3^+/+) mice. Differentially expressed genes regulated by AC3 in the testicular tissues were identified by gene chip hybridization. We observed that the expression of 693 genes was altered in the testicular tissues of AC3^-/- mice, including 330 up-regulated and 363 down-regulated gene expression with fold changes higher than 2 (≥2) as the standards. Furthermore, part of these differentially expressed genes was verified by the real-time fluorescence quantification PCR and immunofluorescent staining. The expression levels of the genes related to olfactory receptors, cell apoptosis, transcriptional activity, defensive reaction, cell adhesion, cell death, and immunoreactions were significantly altered in the testicular tissues of AC3^-/- mice compared with AC3^+/+ mice. In addition, the corresponding Ca²⁺, cAMP, and cell adhesion signaling pathways, as well as the signaling pathways related to axon guidance and cell interaction, were altered significantly in the AC3^-/- mice. These data would help elucidate the general understanding of the mechanisms underlying the sterility in AC3^-/- male mice.

Olfactory Receptors in Semen and in the Male Tract: From Proteome to Proteins

The estimated number of testicular olfactory receptors (ORs) in mammals range between 20 and 66. Previous data reported the role of hOR17-4 and mOR23 in sperm-oocyte chemiotaxis. Proteomic analysis was performed to understand which are the ORs expressed in seminal plasma. Seminal samples by four fertile men were analyzed by an Ultimate 3000Nano/Micro-HPLC apparatus coupled with an LTQ-Orbitrap XL hybrid mass spectrometer. Western blot analysis confirmed the expression of three identified ORs. The expression of ORs in sperm cells, testis, and epididymis was evaluated by confocal microscopy analysis. In seminal plasma eight different ORs were identified by proteomics and three ORs have been confirmed by western blot. Confocal microscopy analysis revealed that OR4S1, OR4C13, and OR1I1 are expressed on the surface of sperm cells. In testicular tissue, OR4S1 and OR1I1 are expressed in spermatocytes and spermatids and OR4C13 is expressed throughout all the tubules. In patients with spermatocyte maturation arrest OR4S1 and OR1I1 expression was reduced and a weak positivity for OR4C13 was detected in the spermatogonia. OR4S1, OR4C13, and OR1I1 had intense and diffuse staining in the epididymis. This study initiated a new methodology for screening OR repertoire in sperms, testis and epididymis. Our results open new insights into OR involvement in sperm maturation and migration.

Olfactory Receptors in Non-Chemosensory Organs: The Nervous System in Health and Disease

Olfactory receptors (ORs) and down-stream functional signaling molecules adenylyl cyclase 3 (AC3), olfactory G protein α subunit (Gαolf), OR transporters receptor transporter proteins 1 and 2 (RTP1 and RTP2), receptor expression enhancing protein 1 (REEP1), and UDP-glucuronosyltransferases (UGTs) are expressed in neurons of the human and murine central nervous system (CNS). In vitro studies have shown that these receptors react to external stimuli and therefore are equipped to be functional. However, ORs are not directly related to the detection of odors. Several molecules delivered from the blood, cerebrospinal fluid, neighboring local neurons and glial cells, distant cells through the extracellular space, and the cells’ own self-regulating internal homeostasis can be postulated as possible ligands. Moreover, a single neuron outside the olfactory epithelium expresses more than one receptor, and the mechanism of transcriptional regulation may be different in olfactory epithelia and brain neurons. OR gene expression is altered in several neurodegenerative diseases including Parkinson’s disease (PD), Alzheimer’s disease (AD), progressive supranuclear palsy (PSP) and sporadic Creutzfeldt-Jakob disease (sCJD) subtypes MM1 and VV2 with disease-, region- and subtype-specific patterns. Altered gene expression is also observed in the prefrontal cortex in schizophrenia with a major but not total influence of chlorpromazine treatment. Preliminary parallel observations have also shown the presence of taste receptors (TASRs), mainly of the bitter taste family, in the mammalian brain, whose function is not related to taste. TASRs in brain are also abnormally regulated in neurodegenerative diseases. These seminal observations point to the need for further studies on ORs and TASRs chemoreceptors in the mammalian brain.

Lysine-specific demethylase-1 (LSD1) is compartmentalized at nuclear chromocenters in early post-mitotic cells of the olfactory sensory neuronal lineage

Mammalian olfaction depends on the development of specialized olfactory sensory neurons (OSNs) that each express one odorant receptor (OR) protein from a large family of OR genes encoded in the genome. The lysine-specific demethylase-1 (LSD1) protein removes activating H3K4 or silencing H3K9 methylation marks at gene promoters and is required for proper OR regulation. We show that LSD1 protein exhibits variable organization within nuclei of developing OSNs, and tends to consolidate into a single dominant compartment at the edges of chromocenters within nuclei of early post-mitotic cells of the mouse olfactory epithelium (MOE). Using an immortalized cell line derived from developing olfactory placode, we show that consolidation of LSD1 appears to be cell-cycle regulated, with a peak occurrence in early G1. LSD1 co-compartmentalizes with CoREST, a protein known to collaborate with LSD1 to carry out a variety of chromatin-modifying functions. We show that LSD1 compartments co-localize with 1-3 OR loci at the exclusion of most OR genes, and commonly associate with Lhx2, a transcription factor involved in OR regulation. Together, our data suggests that LSD1 is sequestered into a distinct nuclear space that might restrict a histone-modifying function to a narrow developmental time window and/or range of OR gene targets.

Gene Expression Profiles of Main Olfactory Epithelium in Adenylyl Cyclase 3 Knockout Mice

Adenylyl Cyclase 3 (AC3) plays an important role in the olfactory sensation-signaling pathway in mice. AC3 deficiency leads to defects in olfaction. However, it is still unknown whether AC3 deficiency affects gene expression or olfactory signal transduction pathways within the main olfactory epithelium (MOE). In this study, gene microarrays were used to screen differentially expressed genes in MOE from AC3 knockout (AC3^−/−) and wild-type (AC3^+/+) mice. The differentially expressed genes identified were subjected to bioinformatic analysis and verified by qRT-PCR. Gene expression in the MOE from AC3^−/− mice was significantly altered, compared to AC3^+/+ mice. Of the 41266 gene probes, 3379 had greater than 2-fold fold change in expression levels between AC3^−/− and AC3^+/+ mice, accounting for 8% of the total gene probes. Of these genes, 1391 were up regulated, and 1988 were down regulated, including 425 olfactory receptor genes, 99 genes that are specifically expressed in the immature olfactory neurons, 305 genes that are specifically expressed in the mature olfactory neurons, and 155 genes that are involved in epigenetic regulation. Quantitative RT-PCR verification of the differentially expressed epigenetic regulation related genes, olfactory receptors, ion transporter related genes, neuron development and differentiation related genes, lipid metabolism and membrane protein transport etc. related genes showed that P75NTR, Hinfp, Gadd45b, and Tet3 were significantly up-regulated, while Olfr370, Olfr1414, Olfr1208, Golf, Faim2, Tsg101, Mapk10, Actl6b, H2BE, ATF5, Kirrrel2, OMP, Drd2etc. were significantly down-regulated. In summary, AC3 may play a role in proximal olfactory signaling and play a role in the regulation of differentially expressed genes in mouse MOE.

Effect of 50 Hz Extremely Low-Frequency Electromagnetic Fields on the DNA Methylation and DNA Methyltransferases in Mouse Spermatocyte-Derived Cell Line GC-2

Previous studies have shown that the male reproductive system is one of the most sensitive organs to electromagnetic radiation. However, the biological effects and molecular mechanism are largely unclear. Our study was designed to elucidate the epigenetic effects of 50 Hz ELF-EMF in vitro. Mouse spermatocyte-derived GC-2 cell line was exposed to 50 Hz ELF-EMF (5 min on and 10 min off) at magnetic field intensity of 1 mT, 2 mT, and 3 mT with an intermittent exposure for 72 h. We found that 50 Hz ELF-EMF exposure decreased genome-wide methylation at 1 mT, but global methylation was higher at 3 mT compared with the controls. The expression of DNMT1 and DNMT3b was decreased at 1 mT, and 50 Hz ELF-EMF can increase the expression of DNMT1 and DNMT3b of GC-2 cells at 3 mT. However, 50 Hz ELF-EMF had little influence on the expression of DNMT3a. Then, we established DNA methylation and gene expression profiling and validated some genes with aberrant DNA methylation and expression at different intensity of 50 Hz ELF-EMF. These results suggest that the alterations of genome-wide methylation and DNMTs expression may play an important role in the biological effects of 50 Hz ELF-EMF exposure.

Identification of novel post-transcriptional features in olfactory receptor family mRNAs

Olfactory receptor (Olfr) genes comprise the largest gene family in mice. Despite their importance in olfaction, how most Olfr mRNAs are regulated remains unexplored. Using RNA-seq analysis coupled with analysis of pre-existing databases, we found that Olfr mRNAs have several atypical features suggesting that post-transcriptional regulation impacts their expression. First, Olfr mRNAs, as a group, have dramatically higher average AU-content and lower predicted secondary structure than do control mRNAs. Second, Olfr mRNAs have a higher density of AU-rich elements (AREs) in their 3'UTR and upstream open reading frames (uORFs) in their 5 UTR than do control mRNAs. Third, Olfr mRNAs have shorter 3' UTR regions and with fewer predicted miRNA-binding sites. All of these novel properties correlated with higher Olfr expression. We also identified striking differences in the post-transcriptional features of the mRNAs from the two major classes of Olfr genes, a finding consistent with their independent evolutionary origin. Together, our results suggest that the Olfr gene family has encountered unusual selective forces in neural cells that have driven them to acquire unique post-transcriptional regulatory features. In support of this possibility, we found that while Olfr mRNAs are degraded by a deadenylation-dependent mechanism, they are largely protected from this decay in neural lineage cells.

Receptive range analysis of a mouse odorant receptor subfamily

Mammals deploy a large array of odorant receptors (ORs) to detect and distinguish a vast number of odorant molecules. ORs vary widely in the type of odorant structures recognized and in the breadth of molecular receptive range (MRR), with some ORs recognizing a small group of closely related molecules and other ORs recognizing a wide range of structures. While closely related ORs have been shown to have similar MRRs, the functional relationships among less closely related ORs are unclear. We screened a small group of ORs with a diverse odorant panel to identify a new odorant-OR pairing (unsaturated aldehydes and MOR263-3). We then extensively screened MOR263-3 and a series of additional MORs related to MOR263-3 in various ways. MORs related by phylogenetic analysis (several other members of the MOR263 subfamily) had MRRs that overlapped with the MRR of MOR263-3, even with amino acid identity as low as 48% (MOR263-2). MOR171-17, predicted to be functionally related to MOR263-3 by an alternative bioinformatic analysis, but with only 39% amino acid identity, had a distinct odorant specificity. Our results support the use of phylogenetic analysis to predict functional relationships among ORs with relatively low amino acid identity. We screened a small group of mouse odorant receptors (MORs) with a diverse odorant panel to identify a new odorant-OR pairing (unsaturated aldehydes and MOR263-3), then extensively screened a series of additional MORs related to MOR263-3 in various ways. MORs related by phylogenetic analysis had odorant specificities that overlapped with that of MOR263-3, but MOR171-17, predicted to be functionally related to MOR263-3 by an alternative bioinformatic analysis, had a distinct odorant specificity.

Deep sequencing of the olfactory epithelium reveals specific chemosensory receptors are expressed at sexual maturity in the European eel Anguilla anguilla

Vertebrate genomes encode a diversity of G protein-coupled receptor (GPCR) that belong to large gene families and are used by olfactory systems to detect chemical cues found in the environment. It is not clear however, if individual receptors from these large gene families have evolved roles that are specific to certain life stages. Here, we used deep sequencing to identify differentially expressed receptor transcripts in the olfactory epithelia (OE) of freshwater, seawater and sexually mature male eels (Anguilla anguilla). This species is particularly intriguing because of its complex life cycle, extreme long-distance migrations and early-branching position within the teleost phylogeny. In the A. anguillaOE, we identified full-length transcripts for 13, 112, 6 and 38 trace amine-associated receptors, odorant receptors (OR) and type I and type II vomeronasal receptors (V1R and V2R). Most of these receptors were expressed at similar levels at different life stages and a subset of OR and V2R-like transcripts was more abundant in sexually mature males suggesting that ORs and V2R-like genes are important for reproduction. We also identified a set of GPCR signal transduction genes that were differentially expressed indicating that eels make use of different GPCR signal transduction genes at different life stages. The finding that a diversity of chemosensory receptors is expressed in the olfactory epithelium and that a subset is differentially expressed suggests that most receptors belonging to large chemosensory gene families have functions that are important at multiple life stages, while a subset has evolved specific functions at different life stages.

Olfactory marker protein expression is an indicator of olfactory receptor-associated events in non-olfactory tissues

Olfactory receptor (OR)-associated events are mediated by well-conserved components in the olfactory epithelium, including olfactory G-protein (G_olf), adenylate cyclase III (ACIII), and olfactory marker protein (OMP). The expression of ORs has recently been observed in non-olfactory tissues where they are involved in monitoring extracellular chemical cues. The large number of OR genes and their sequence similarities illustrate the need to find an effective and simple way to detect non-olfactory OR-associated events. In addition, expression profiles and physiological functions of ORs in non-olfactory tissues are largely unknown. To overcome limitations associated with using OR as a target protein, this study used OMP with G_olf and ACIII as targets to screen for potential OR-mediated sensing systems in non-olfactory tissues. Here, we show using western blotting, real-time PCR, and single as well as double immunoassays that ORs and OR-associated proteins are co-expressed in diverse tissues. The results of immunohistochemical analyses showed OMP (+) cells in mouse heart and in the following cells using the corresponding marker proteins c-kit, keratin 14, calcitonin, and GFAP in mouse tissues: interstitial cells of Cajal of the bladder, medullary thymic epithelial cells of the thymus, parafollicular cells of the thyroid, and Leydig cells of the testis. The expression of ORs in OMP (+) tissues was analyzed using a refined microarray analysis and validated with RT-PCR and real-time PCR. Three ORs (olfr544, olfr558, and olfr1386) were expressed in the OMP (+) cells of the bladder and thyroid as shown using a co-immunostaining method. Together, these results suggest that OMP is involved in the OR-mediated signal transduction cascade with olfactory canonical signaling components between the nervous and endocrine systems. The results further demonstrate that OMP immunohistochemical analysis is a useful tool for identifying expression of ORs, suggesting OMP expression is an indicator of potential OR-mediated chemoreception in non-olfactory systems.

G protein-coupled receptors in cardiac biology: old and new receptors

G protein-coupled receptors (GPCRs) are seven-transmembrane-spanning proteins that mediate cellular and physiological responses. They are critical for cardiovascular function and are targeted for the treatment of hypertension and heart failure. Nevertheless, current therapies only target a small fraction of the cardiac GPCR repertoire, indicating that there are many opportunities to investigate unappreciated aspects of heart biology. Here, we offer an update on the contemporary view of GPCRs and the complexities of their signalling, and review the roles of the 'classical' GPCRs in cardiovascular physiology and disease. We then provide insights into other GPCRs that have been less extensively studied in the heart, including orphan, odorant and taste receptors. We contend that these novel cardiac GPCRs contribute to heart function in health and disease and thereby offer exciting opportunities to therapeutically modulate heart function.

Identification and characterization of novel renal sensory receptors

Recent studies have highlighted the important roles that “sensory” receptors (olfactory receptors, taste receptors, and orphan “GPR” receptors) play in a variety of tissues, including the kidney. Although several studies have identified important roles that individual sensory receptors play in the kidney, there has not been a systematic analysis of the renal repertoire of sensory receptors. In this study, we identify novel renal sensory receptors belonging to the GPR (n = 76), olfactory receptor (n = 6), and taste receptor (n = 11) gene families. A variety of reverse transcriptase (RT)- PCR screening strategies were used to identify novel renal sensory receptors, which were subsequently confirmed using gene-specific primers. The tissue-specific distribution of these receptors was determined, and the novel renal ORs were cloned from whole mouse kidney. Renal ORs that trafficked properly in vitro were screened for potential ligands using a dual-luciferase ligand screen, and novel ligands were identified for Olfr691. These studies demonstrate that multiple sensory receptors are expressed in the kidney beyond those previously identified. These results greatly expand the known repertoire of renal sensory receptors. Importantly, the mRNA of many of the receptors identified in this study are expressed highly in the kidney (comparable to well-known and extensively studied renal GPCRs), and in future studies it will be important to elucidate the roles that these novel renal receptors play in renal physiology.

The olfactory transcriptomes of mice

The olfactory (OR) and vomeronasal receptor (VR) repertoires are collectively encoded by 1700 genes and pseudogenes in the mouse genome. Most OR and VR genes were identified by comparative genomic techniques and therefore, in many of those cases, only their protein coding sequences are defined. Some also lack experimental support, due in part to the similarity between them and their monogenic, cell-specific expression in olfactory tissues. Here we use deep RNA sequencing, expression microarray and quantitative RT-PCR in both the vomeronasal organ and whole olfactory mucosa to quantify their full transcriptomes in multiple male and female mice. We find evidence of expression for all VR, and almost all OR genes that are annotated as functional in the reference genome, and use the data to generate over 1100 new, multi-exonic, significantly extended receptor gene annotations. We find that OR and VR genes are neither equally nor randomly expressed, but have reproducible distributions of abundance in both tissues. The olfactory transcriptomes are only minimally different between males and females, suggesting altered gene expression at the periphery is unlikely to underpin the striking sexual dimorphism in olfactory-mediated behavior. Finally, we present evidence that hundreds of novel, putatively protein-coding genes are expressed in these highly specialized olfactory tissues, and carry out a proof-of-principle validation. Taken together, these data provide a comprehensive, quantitative catalog of the genes that mediate olfactory perception and pheromone-evoked behavior at the periphery.

The sense of smell in mice involves the detection of odors and pheromones by many hundreds of olfactory and vomeronasal receptors. The genes that encode these receptors account for around 5% of the whole gene catalog, but they are poorly understood because they are very similar to each other, and are thought to be turned on randomly in only a small number of cells. Here we use multiple gene expression technologies to curate and measure the activity of all the genes involved in the detection of odors and find evidence of many new ones. We show that most genes encoding olfactory and vomeronasal receptors have complex, multi-exonic structures that generate different isoforms. We find that some receptors are consistently more abundant in the nose than others, which suggests they are not turned on randomly. This may explain why mice are particularly sensitive to some odors, but less attuned to others. We find that overall males and females differ very little in gene expression, despite having altered behavioral responses to the same odors. Thus diversity in receptor expression can explain differences in odor sensitivity, but does not appear to dictate whether sex pheromones are differentially detected by males or females.

Mesencephalic dopaminergic neurons express a repertoire of olfactory receptors and respond to odorant-like molecules

Background

The mesencephalic dopaminergic (mDA) cell system is composed of two major groups of projecting cells in the Substantia Nigra (SN) (A9 neurons) and the Ventral Tegmental Area (VTA) (A10 cells). Selective degeneration of A9 neurons occurs in Parkinson’s disease (PD) while abnormal function of A10 cells has been linked to schizophrenia, attention deficit and addiction. The molecular basis that underlies selective vulnerability of A9 and A10 neurons is presently unknown.

Results

By taking advantage of transgenic labeling, laser capture microdissection coupled to nano Cap-Analysis of Gene Expression (nanoCAGE) technology on isolated A9 and A10 cells, we found that a subset of Olfactory Receptors (OR)s is expressed in mDA neurons. Gene expression analysis was integrated with the FANTOM5 Helicos CAGE sequencing datasets, showing the presence of these ORs in selected tissues and brain areas outside of the olfactory epithelium. OR expression in the mesencephalon was validated by RT-PCR and in situ hybridization. By screening 16 potential ligands on 5 mDA ORs recombinantly expressed in an heterologous in vitro system, we identified carvone enantiomers as agonists at Olfr287 and able to evoke an intracellular Ca²⁺ increase in solitary mDA neurons. ORs were found expressed in human SN and down-regulated in PD post mortem brains.

Conclusions

Our study indicates that mDA neurons express ORs and respond to odor-like molecules providing new opportunities for pharmacological intervention in disease.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-729) contains supplementary material, which is available to authorized users.

A lifetime of neurogenesis in the olfactory system

Neurogenesis continues well beyond embryonic and early postnatal ages in three areas of the nervous system. The subgranular zone supplies new neurons to the dentate gyrus of the hippocampus. The subventricular zone supplies new interneurons to the olfactory bulb, and the olfactory neuroepithelia generate new excitatory sensory neurons that send their axons to the olfactory bulb. The latter two areas are of particular interest as they contribute new neurons to both ends of a first-level circuit governing olfactory perception. The vomeronasal organ and the main olfactory epithelium comprise the primary peripheral olfactory epithelia. These anatomically distinct areas share common features, as each exhibits extensive neurogenesis well beyond the juvenile phase of development. Here we will discuss the effect of age on the structural and functional significance of neurogenesis in the vomeronasal and olfactory epithelia, from juvenile to advanced adult ages, in several common model systems. We will next discuss how age affects the regenerative capacity of these neural stem cells in response to injury. Finally, we will consider the integration of newborn neurons into an existing circuit as it is modified by the age of the animal.

A novel SCFA receptor, the microbiota, and blood pressure regulation

The maintenance of blood pressure homeostasis is a complex process which is carefully regulated by a variety of inputs. We recently identified two sensory receptors (Olfactory receptor 78 and G protein couple receptor 41) as novel regulators of blood pressure. Both Olfr78 and Gpr41 are receptors for short chain fatty acids (SCFAs), and we showed that propionate (a SCFA) modifies blood pressure in a manner which is differentially modulated by the absence of either Olfr78 or Gpr41. In addition, propionate modifies renin release in an Olfr78-dependent manner. Our study also demonstrated that antibiotic treatment modulates blood pressure in Olfr78 null mice, indicating that SCFAs produced by the gut microbiota likely influence blood pressure regulation. In this addendum, we summarize the findings of our recent study and provide a perspective on the implications of the interactions between the gut microbiota and blood pressure control.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.

Promotion of cancer cell invasiveness and metastasis emergence caused by olfactory receptor stimulation

Olfactory receptors (ORs) are expressed in the olfactory epithelium, where they detect odorants, but also in other tissues with additional functions. Some ORs are even overexpressed in tumor cells. In this study, we identified ORs expressed in enterochromaffin tumor cells by RT-PCR, showing that single cells can co-express several ORs. Some of the receptors identified were already reported in other tumors, but they are orphan (without known ligand), as it is the case for most of the hundreds of human ORs. Thus, genes coding for human ORs with known ligands were transfected into these cells, expressing functional heterologous ORs. The in vitro stimulation of these cells by the corresponding OR odorant agonists promoted cell invasion of collagen gels. Using LNCaP prostate cancer cells, the stimulation of the PSGR (Prostate Specific G protein-coupled Receptor), an endogenously overexpressed OR, by β-ionone, its odorant agonist, resulted in the same phenotypic change. We also showed the involvement of a PI3 kinase γ dependent signaling pathway in this promotion of tumor cell invasiveness triggered by OR stimulation. Finally, after subcutaneous inoculation of LNCaP cells into NSG immunodeficient mice, the in vivo stimulation of these cells by the PSGR agonist β-ionone significantly enhanced metastasis emergence and spreading.