Trafficking prerogatives of olfactory receptors

Olfactory dysfunction in COVID-19: new insights into the underlying mechanisms

The mechanisms of olfactory dysfunction in COVID-19 are still unclear. In this review, we examine potential mechanisms that may explain why the sense of smell is lost or altered. Among the current hypotheses, the most plausible is that death of infected support cells in the olfactory epithelium causes, besides altered composition of the mucus, retraction of the cilia on olfactory receptor neurons, possibly because of the lack of support cell-derived glucose in the mucus, which powers olfactory signal transduction within the cilia. This mechanism is consistent with the rapid loss of smell with COVID-19, and its rapid recovery after the regeneration of support cells. Host immune responses that cause downregulation of genes involved in olfactory signal transduction occur too late to trigger anosmia, but may contribute to the duration of the olfactory dysfunction.

Conserved C-terminal motifs in odorant receptors instruct their cell surface expression and cAMP signaling

The highly individual plasma membrane expression and cAMP signaling of odorant receptors have hampered their ligand assignment and functional characterization in test cell systems. Chaperones have been identified to support the cell surface expression of only a portion of odorant receptors, with mechanisms remaining unclear. The presence of amino acid motifs that might be responsible for odorant receptors' individual intracellular retention or cell surface expression, and thus, for cAMP signaling, is under debate: so far, no such protein motifs have been suggested. Here, we demonstrate the existence of highly conserved C-terminal amino acid motifs, which discriminate at least between class-I and class-II odorant receptors, with their numbers of motifs increasing during evolution, by comparing C-terminal protein sequences from 4808 receptors across eight species. Truncation experiments and mutation analysis of C-terminal motifs, largely overlapping with helix 8, revealed single amino acids and their combinations to have differential impact on the cell surface expression and on stimulus-dependent cAMP signaling of odorant receptors in NxG 108CC15 cells. Our results demonstrate class-specific and individual C-terminal motif equipment of odorant receptors, which instruct their functional expression in a test cell system, and in situ may regulate their individual cell surface expression and intracellular cAMP signaling.

The Sniffing Kidney: Roles for Renal Olfactory Receptors in Health and Disease

Olfactory receptors (ORs) represent the largest gene family in the human genome. Despite their name, functions exist for these receptors outside of the nose. Among the tissues known to take advantage of OR signaling is the kidney. From mouse to man, the list of renal ORs continues to expand, and they have now been linked to a variety of processes involved in the maintenance of renal homeostasis, including the modulation of blood pressure, response to acidemia, and the development of diabetes. In this review, we highlight the recent progress made on the growing appreciation for renal ORs in physiology and pathophysiology.

Modulation of the combinatorial code of odorant receptor response patterns in odorant mixtures

The perception of odors relies on combinatorial codes consisting of odorant receptor (OR) response patterns to encode odor identity. Modulation of these patterns by odorant interactions at ORs potentially explains several olfactory phenomena: mixture suppression, unpredictable sensory outcomes, and the perception of odorant mixtures as unique objects. We determined OR response patterns to 4 odorants and 3 binary mixtures in vivo in mice, identifying 30 responsive ORs. These patterns typically had a few strongly responsive ORs and a greater number of weakly responsive ORs. ORs responsive to an odorant were often unrelated sequences distributed across several OR subfamilies. Mixture responses predicted pharmacological interactions between odorants, which were tested in vitro by heterologous expression of ORs in cultured cells, providing independent evidence confirming odorant agonists for 13 ORs and identifying both suppressive and additive effects. This included 11 instances of antagonism of ORs by an odorant, 1 instance of additive responses to a binary mixture, 1 instance of suppression of a strong agonist by a weak agonist, and the discovery of an inverse agonist for an OR. Interactions between odorants at ORs are common even when the odorants are not known to interact perceptually in humans, and in some cases interactions at mouse ORs correlate with the ability of humans to perceive an odorant in a mixture.

Mammalian Taste Cells Express Functional Olfactory Receptors

The peripheral taste and olfactory systems in mammals are separate and independent sensory systems. In the current model of chemosensation, gustatory, and olfactory receptors are genetically divergent families expressed in anatomically distinct locations that project to disparate downstream targets. Although information from the 2 sensory systems merges to form the perception of flavor, the first cross talk is thought to occur centrally, in the insular cortex. Recent studies have shown that gustatory and olfactory receptors are expressed throughout the body and serve as chemical sensors in multiple tissues. Olfactory receptor cDNA has been detected in the tongue, yet the presence of physiologically functional olfactory receptors in taste cells has not yet been demonstrated. Here we report that olfactory receptors are functionally expressed in taste papillae. We found expression of olfactory receptors in the taste papillae of green fluorescent protein-expressing transgenic mice and, using immunocytochemistry and real-time quantitative polymerase chain reaction experiments, the presence of olfactory signal transduction molecules and olfactory receptors in cultured human fungiform taste papilla (HBO) cells. Both HBO cells and mouse taste papilla cells responded to odorants. Knockdown of adenylyl cyclase mRNA by specific small inhibitory RNA and pharmacological block of adenylyl cyclase eliminated these responses, leading us to hypothesize that the gustatory system may receive olfactory information in the periphery. These results provide the first direct evidence of the presence of functional olfactory receptors in mammalian taste cells. Our results also demonstrate that the initial integration of gustatory and olfactory information may occur as early as the taste receptor cells.

Characterizing novel olfactory receptors expressed in the murine renal cortex

The kidney uses specialized G protein-coupled receptors, including olfactory receptors (ORs), to act as sensors of molecules and metabolites. In the present study, we cloned and studied seven renal ORs, which we previously found to be expressed in the murine renal cortex. As most ORs are orphan receptors, our goal was to identify ligands for these ORs in the hope that this will guide future research into their functional roles. We identified novel ligands for two ORs: Olfr558 and Olfr90. For Olfr558, we confirmed activation by previously reported ligands and identified 16 additional carboxylic acids that activated this OR. The strongest activation of Olfr558 was produced by butyric, cyclobutanecarboxylic, isovaleric, 2-methylvaleric, 3-methylvaleric, 4-methylvaleric, and valeric acids. The primary in vivo source of both butyric and isovaleric acids is gut microbial metabolism. We also identified 14 novel ligands that activated Olfr90, the strongest of which were 2-methyl-4-propyl-1,3-oxathiane, 1-octen-3-ol, 2-octanol, and 3-octanol. Interestingly, 8 of these 14 ligands are of fungal origin. We also investigated the tissue distribution of these receptors and found that they are each found in a subset of "nonsensory" tissues. Finally, we examined the putative human orthologs of Olfr558 and Olfr90 and found that the human ortholog of Olfr558 (OR51E1) has a similar ligand profile, indicating that the role of this OR is likely evolutionarily conserved. In summary, we examined seven novel renal ORs and identified new ligands for Olfr558 and Olfr90, which imply that both of these receptors serve to detect metabolites produced by microorganisms.

A bi-functional IL-6-HaloTag® as a tool to measure the cell-surface expression of recombinant odorant receptors and to facilitate their activity quantification

The functional cell surface expression of recombinant odorant receptors typically has been investigated by expressing N-terminally extended, "tagged" receptors in test cell systems, using antibody-based immunocytochemistry or flow cytometry, and by measuring odorant/receptor-induced cAMP signaling, mostly by an odorant/receptor-induced and cAMP signaling-dependent transcriptional activation of a luciferase-based luminescence assay. In the present protocol, we explain a method to measure the cell-surface expression and signaling of recombinant odorant receptors carrying a bi-functional, N-terminal 'IL-6-HaloTag®'. IL-6, being a secreted cytokine, facilitates functional cell surface expression of recombinant HaloTag®-odorant receptors, and the HaloTag® protein serves as a highly specific acceptor for cell-impermeant or cell-permeant, fluorophore-coupled ligands, which enable the quantification of odorant receptor expression by antibody-independent, chemical live-cell staining and flow cytometry. Here, we describe how to measure the cell surface expression of recombinant IL-6-HaloTag®-odorant receptors in HEK-293 cells or NxG 108CC15 cells, by live-cell staining and flow cytometry, and how to measure an odorant-induced activation of these receptors by the fast, real-time, luminescence-based GloSensor® cAMP assay.

IL-6-HaloTag® enables live-cell plasma membrane staining, flow cytometry, functional expression, and de-orphaning of recombinant odorant receptors

The assignment of cognate odorant/agonist pairs is a prerequisite for an understanding of odorant coding at the receptor level. However, the identification of new ligands for odorant receptors (ORs) in cell-based assays has been challenging, due to their individual and rather sub-optimal plasma membrane expression, as compared with other G protein-coupled receptors. Accessory proteins, such as the chaperone RTP1S, or Ric8b, have improved the surface expression of at least a portion of ORs. Typically, recombinant ORs carry N-terminal tags, which proved helpful for their functional membrane expression. The most common tag is the 'Rho-tag', representing an N-terminal part of rhodopsin, but also 'Lucy-' or 'Flag-tag' extensions have been described. Here, we used a bi-functional N-terminal tag, called 'interleukin 6 (IL-6)-HaloTag®', with IL-6 facilitating functional cell surface expression of recombinant ORs, and the HaloTag® protein, serving as a highly specific acceptor for cell-impermeant or cell-permeant, fluorophore-coupled ligands, which enable the quantification of odorant receptor expression by live-cell flow cytometry. Our experiments revealed on average an about four-fold increased surface expression, a four-fold higher signaling amplitude, and a significantly higher potency of odorant-induced cAMP signaling of six different human IL-6-HaloTag®-ORs across five different receptor families in NxG 108CC15 cells, as compared to their Rho-tag-HaloTag® constructs. We observed similar results in HEK-293 cells. Moreover, screening an IL-6-HaloTag®-odorant receptor library with allyl phenyl acetate, revealed both known receptors as best responders for this compound. In summary, the IL-6-HaloTag® represents a promising tool for the de-orphaning of ORs.

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.

Trafficking of ciliary G protein-coupled receptors

In the last decade highly conserved cellular appendages called cilia have enjoyed a renewed interest from basic, biomedical scientists, and clinicians alike. This interest has grown upon the elucidation that cilia throughout the body serve as important sensory and signaling centers in both development and adult homeostasis. Furthermore, the identification of several rare genetic disorders associated with cilia dysfunction has broadened the field. However, even though their potential role in human health and disease is now recognized many basic questions about their functions remain. This chapter seeks to explore the trafficking of cilia-specific G protein-coupled receptors (GPCRs) and discusses several model systems in which this has been explored. We open the chapter by briefly discussing cilia and GPCRs then begin discussing some aspects of rhodopsin trafficking, arguably the most well studied of cilia GPCRs. We continue with sections on neuronal cilia and olfactory cilia receptor trafficking. Finally, we conclude with the emerging area of dynamic ciliary GPCR trafficking and speculate about future directions and some of the questions that remain for ciliary GPCRs.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

The state of the art of odorant receptor deorphanization: a report from the orphanage

The odorant receptors (ORs) provide our main gateway to sensing the world of volatile chemicals. This involves a complex encoding process in which multiple ORs, each of which detects its own set of odorants, work as an ensemble to produce a distributed activation code that is presumably unique to each odorant. One marked challenge to decoding the olfactory code is OR deorphanization, the identification of a set of activating odorants for a particular receptor. Here, we survey various methods used to try to express defined ORs of interest. We also suggest strategies for selecting odorants for test panels to evaluate the functional expression of an OR. Integrating these tools, while retaining awareness of their idiosyncratic limitations, can provide a multi-tiered approach to OR deorphanization, spanning the initial discovery of a ligand to vetting that ligand in a physiologically relevant setting.

G protein-coupled receptors: what a difference a 'partner' makes

G protein-coupled receptors (GPCRs) are important cell signaling mediators, involved in essential physiological processes. GPCRs respond to a wide variety of ligands from light to large macromolecules, including hormones and small peptides. Unfortunately, mutations and dysregulation of GPCRs that induce a loss of function or alter expression can lead to disorders that are sometimes lethal. Therefore, the expression, trafficking, signaling and desensitization of GPCRs must be tightly regulated by different cellular systems to prevent disease. Although there is substantial knowledge regarding the mechanisms that regulate the desensitization and down-regulation of GPCRs, less is known about the mechanisms that regulate the trafficking and cell-surface expression of newly synthesized GPCRs. More recently, there is accumulating evidence that suggests certain GPCRs are able to interact with specific proteins that can completely change their fate and function. These interactions add on another level of regulation and flexibility between different tissue/cell-types. Here, we review some of the main interacting proteins of GPCRs. A greater understanding of the mechanisms regulating their interactions may lead to the discovery of new drug targets for therapy.

A cleavable N-terminal signal peptide promotes widespread olfactory receptor surface expression in HEK293T cells

Olfactory receptors (ORs) are G protein-coupled receptors that detect odorants in the olfactory epithelium, and comprise the largest gene family in the genome. Identification of OR ligands typically requires OR surface expression in heterologous cells; however, ORs rarely traffic to the cell surface when exogenously expressed. Therefore, most ORs are orphan receptors with no known ligands. To date, studies have utilized non-cleavable rhodopsin (Rho) tags and/or chaperones (i.e. Receptor Transporting Protein, RTP1S, Ric8b and G_αolf) to improve surface expression. However, even with these tools, many ORs still fail to reach the cell surface. We used a test set of fifteen ORs to examine the effect of a cleavable leucine-rich signal peptide sequence (Lucy tag) on OR surface expression in HEK293T cells. We report here that the addition of the Lucy tag to the N-terminus increases the number of ORs reaching the cell surface to 7 of the 15 ORs (as compared to 3/15 without Rho or Lucy tags). Moreover, when ORs tagged with both Lucy and Rho were co-expressed with previously reported chaperones (RTP1S, Ric8b and G_αolf), we observed surface expression for all 15 receptors examined. In fact, two-thirds of Lucy-tagged ORs are able to reach the cell surface synergistically with chaperones even when the Rho tag is removed (10/15 ORs), allowing for the potential assessment of OR function with only an 8-amino acid Flag tag on the mature protein. As expected for a signal peptide, the Lucy tag was cleaved from the mature protein and did not alter OR-ligand binding and signaling. Our studies demonstrate that widespread surface expression of ORs can be achieved in HEK293T cells, providing promise for future large-scale deorphanization studies.

Pharmacology of mammalian olfactory receptors

Mammalian species have evolved a large and diverse number of odorant receptors (ORs). These proteins comprise the largest family of G-protein-coupled receptors (GPCRs) known, amounting to ~1,000-different receptors in the rodent. From the perspective of olfactory coding, the availability of such a vast number of chemosensory receptors poses several fascinating questions; in addition, such a large repertoire provides an attractive biological model to study ligand-receptor interactions. The limited functional expression of these receptors in heterologous systems, however, has greatly hampered attempts to deorphanize them. We have employed a successful approach that combines electrophysiological and imaging techniques to analyze the response profiles of single sensory neurons. Our approach has enabled us to characterize the "odor space" of a population of native aldehyde receptors and the molecular range of a genetically engineered receptor, OR-I7.

Functional assay of mammalian and insect olfactory receptors using Xenopus oocytes

The large number of olfactory receptors (ORs) expressed by various mammalian and insect species, as well as the large number of potential odorant ligands, has made the pairing of odorants with receptors -(de-orphaning) exceedingly difficult. These efforts are further complicated by difficulties in expressing ORs in many standard expression systems. Xenopus laevis oocytes offer a versatile expression platform for the de-orphaning and functional characterization of ORs. Two-electrode voltage clamp electrophysiology is a common and relatively straightforward approach to the functional assay of receptors expressed in Xenopus oocytes, and this technique has been discussed extensively in the literature. However, laboratories that are new to the use of Xenopus oocytes are often stymied by some of the peculiarities of the Xenopus oocyte expression system. We discuss some of the key methodological issues in Xenopus care, oocyte -isolation and receptor expression, with a focus on using this expression system to study the ORs of mammals and insects.

Human olfactory receptors: recombinant expression in the baculovirus/Sf9 insect cell system, functional characterization, and odorant identification

Cell surface expression of recombinant olfactory receptors (ORs) is a major limitation in characterizing their functional nature. We have shown that the recombinant expression of a human OR, OR 17-210, in the baculovirus/Sf9 insect cell system allows this protein to be expressed at the cell surface. We used Ca(2+) imaging to demonstrate that recombinant OR 17-210 produces cellular activities upon odorant stimulation with ketones. Furthermore, this expression and functional system has been used to show that the preincubation of Human Odorant Binding Protein 2A decrease the calcium response of OR 17-210 following stimulation by acetophenone and beta ionone.

Functional reconstitution of olfactory receptor complex on baculovirus

Despite that recent progress in genomics has elucidated the genomic structure of the olfactory receptors (ORs), most of them are still orphan receptors. The low expression level of ORs in heterologous cells has hampered many attempts to establish cell biological OR assay systems. Recently, we demonstrated that certain G protein-coupled receptors, such as the leukotriene B4 receptor or the dopamine D1 receptor, were efficiently reconstituted on baculovirus budding from infected Sf9 cells. The budded virus (BV) was shown to be mostly free of exogenous proteins other than those related to viral infection, resulting in low-noise assay conditions. Taking advantage of these conditions, we attempted to reconstitute OR complexes on BV. Sf9 cells were coinfected with recombinant baculoviruses harboring the cDNAs encoding adenylyl cyclase, trimeric G-protein, and the receptor: mOR-EG or S6. The coexpression of these proteins was detected by western blot, and the agonist- or antagonist-dependent receptor response was confirmed using ligand-dependent cyclic AMP production. These results demonstrated the successful reconstitution of functional OR complex on BV. Additionally, the expression of OR8B3 on BV, one of human orphan ORs, was also confirmed. This BV expression system is expected to be a highly effective tool for screening unknown ligands for ORs.

Functional architecture of olfactory ionotropic glutamate receptors

VIDEO ABSTRACT

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate chemical communication between neurons at synapses. A variant iGluR subfamily, the Ionotropic Receptors (IRs), was recently proposed to detect environmental volatile chemicals in olfactory cilia. Here, we elucidate how these peripheral chemosensors have evolved mechanistically from their iGluR ancestors. Using a Drosophila model, we demonstrate that IRs act in combinations of up to three subunits, comprising individual odor-specific receptors and one or two broadly expressed coreceptors. Heteromeric IR complex formation is necessary and sufficient for trafficking to cilia and mediating odor-evoked electrophysiological responses in vivo and in vitro. IRs display heterogeneous ion conduction specificities related to their variable pore sequences, and divergent ligand-binding domains function in odor recognition and cilia localization. Our results provide insights into the conserved and distinct architecture of these olfactory and synaptic ion channels and offer perspectives into the use of IRs as genetically encoded chemical sensors.

Chemical stress induces the unfolded protein response in olfactory sensory neurons

More than any other neuron, olfactory sensory neurons are exposed to environmental insults. Surprisingly, their only documented response to damaging stress is apoptosis and subsequent replacement by new neurons. However, they expressed unfolded protein response genes, a transcriptionally regulated defense mechanism activated by many types of insults. The unfolded protein response transcripts Xbp1, spliced Xbp1, Chop (Ddit3), and BiP (Hspa5) were decreased when external access of stressors was reduced by blocking a nostril (naris occlusion). These transcripts and Nrf2 (Nfe2l2) were increased by systemic application of tunicamycin or the selective olfactotoxic chemical methimazole. Methimazole's effects overcame naris occlusion, and the unfolded protein response was independent of odor-evoked neuronal activity. Chemical stress is therefore a major and chronic activator of the unfolded protein response in olfactory sensory neurons. Stress-dependent repression of the antiapoptotic gene Bcl2 was absent, however, suggesting a mechanism for disconnecting the UPR from apoptosis and tolerating a chronic unfolded protein response. Environmental stressors also affect both the sustentacular cells that support the neurons and the respiratory epithelia, because naris occlusion decreased expression of the xenobiotic chemical transformation enzyme Cyp2a5 in sustentacular cells, and both naris occlusion and methimazole altered the abundance of the antibacterial lectin Reg3g in respiratory epithelia.

PHR1 is a vesicle-bound protein abundantly expressed in mature olfactory neurons

OBJECTIVES/HYPOTHESIS

To characterize the role of Phr1, a gene highly expressed in primary sensory neurons where it encodes an integral membrane protein with an N-terminal pleckstrin homology domain and a C-terminal transmembrane domain, in the olfactory system.

METHODS

We studied the immunelocalization of the PHR1 protein in mouse olfactory epithelium both at steady state and during regeneration following methyl bromide (MeBr) exposure using scanning confocal microscopy. Additionally, we examined the electrophysiologic role of Phr1 in olfaction and short-term olfactory adaptation.

RESULTS

We found that PHR1 is abundantly and specifically expressed in olfactory neurons. It is widely distributed in punctate, vesiculated organelles throughout the cell bodies, axons, and glomeruli of primary olfactory neurons but is specifically excluded from the olfactory cilia. In the regenerating olfactory epithelium, PHR1 expression appears at 14 days following MeBr ablation coinciding with the onset of olfactory neuron maturity. Despite the abundant and specific expression throughout the olfactory neurons, mice lacking Phr1 did not exhibit differences in the distribution of the components of olfactory signal transduction system, the rate of olfactory regeneration following MeBr exposure, olfactory function, or short-term adaptation to odors.

CONCLUSIONS

Phr1 is widely and abundantly expressed throughout mature olfactory neurons and other primary sensory neurons, but its absence does not appear to affect olfactory morphology, regeneration, sensory function, or adaptation. The exact function of Phr1 remains to be discovered.

Molecular tuning of odorant receptors and its implication for odor signal processing

The discovery of the odorant receptor (OR) family by Buck and Axel in 1991 provided a quantum jump in our understanding of olfactory function. However, the study of the responsiveness of ORs to odor ligands was challenging due to the difficulties in deorphanizing the receptors. In this manuscript, we review recent findings of OR responsiveness that have come about through improved OR deorphanization methods, site-directed mutagenesis, structural modeling studies, and studies of OR responses in situ in olfactory sensory neurons. Although there has been a major leap in our understanding of receptor-ligand interactions and how these contribute to the input to the olfactory system, an improvement of our understanding of receptor structure and dynamics and interactions with intracellular and extracellular proteins is necessary.

Molecular receptive range variation among mouse odorant receptors for aliphatic carboxylic acids

The ability of mammals to identify and distinguish among many thousands of different odorants suggests a combinatorial use of odorant receptors, with each receptor detecting multiple odorants and each odorant interacting with multiple receptors. Numerous receptors may be devoted to the sampling of particularly important regions of odor space. In this study, we explore the similarities and differences in the molecular receptive ranges of four mouse odorant receptors (MOR23-1, MOR31-4, MOR32-11 and MOR40-4), which have previously been identified as receptors for aliphatic carboxylic acids. Each receptor was expressed in Xenopus oocytes, along with Galpha(olf) and the cystic fibrosis transmembrane regulator to allow electrophysiological assay of receptor responses. We find that even though these receptors are relatively unrelated, there is extensive overlap among their receptive ranges. That is, these receptors sample a similar region of odor space. However, the receptive range of each receptor is unique. Thus, these receptors contribute to the depth of coverage of this small region of odor space. Such a group of receptors with overlapping, but distinct receptive ranges, may participate in making fine distinctions among complex mixtures of closely related odorant compounds.

Human olfactory receptor families and their odorants

The human nose detects volatile chemical stimuli by at least three different receptor families: odorant receptors, trace amine-associated receptors, and vomeronasal type-1 receptors. As G protein-coupled receptors, all of the few functionally characterized olfactory receptors share major functional features: when expressed in heterologous cell systems, they 1) respond to odorants of certain chemical groups, e.g., amines, aliphatic carboxylic acids or aldehydes, floral or fruity odorants, including certain key-food odorants, and putative pheromones, and 2) transduce their signals to intracellular cAMP signaling. However, little is known yet about specific differences in the functional designation of the three olfactory receptor families. Recently, two heterologous cell systems expressing olfactory signaling molecules have been developed. Different screening strategies will shed light on the yet sparsely available odorant specificity profiles and structure-function relationships of olfactory receptors, as well as the structure-activity relationships of their odorants.

Predicting the receptive range of olfactory receptors

Although the family of genes encoding for olfactory receptors was identified more than 15 years ago, the difficulty of functionally expressing these receptors in an heterologous system has, with only some exceptions, rendered the receptive range of given olfactory receptors largely unknown. Furthermore, even when successfully expressed, the task of probing such a receptor with thousands of odors/ligands remains daunting. Here we provide proof of concept for a solution to this problem. Using computational methods, we tune an electronic nose to the receptive range of an olfactory receptor. We then use this electronic nose to predict the receptors' response to other odorants. Our method can be used to identify the receptive range of olfactory receptors, and can also be applied to other questions involving receptor–ligand interactions in non-olfactory settings.

A key goal in biology is to identify specific ligands for specific receptors. One example is where the ligand is a drug. In turn, in the olfactory system the ligand is the odorant that binds to olfactory receptors. There are many olfactory receptor types, and which odorants will activate which receptors remains largely unknown. One way to answer this is to systematically vary the molecular features of ligands and to measure the olfactory receptor response. However, the vast number of molecular features and their combinations renders such an effort potentially unsolvable. Here, rather than looking at the trees (each molecular feature), we looked at the forest (the smell they generate). We used a device called an electronic nose that generates a patterned response to odorants. We then obtained the response to a set of odorants that are known to activate a particular olfactory receptor, and we used this pattern to predict the response of that receptor to other odorants. We found that, on average in three out of four we could predict the response of olfactory receptors. This result provides a new method for probing the olfactory system, and also suggests a novel method for identifying potential drugs.

Searching for the ligands of odorant receptors

Through the sense of smell mammals can detect and discriminate between a large variety of odorants present in the surrounding environment. Odorants bind to a large repertoire of odorant receptors located in the cilia of olfactory sensory neurons of the nose. Each olfactory neuron expresses one single type of odorant receptor, and neurons expressing the same type of receptor project their axons to one or a few glomeruli in the olfactory bulb, creating a map of odorant receptor inputs. The information is then passed on to other regions of the brain, leading to odorant perception. To understand how the olfactory system discriminates between odorants, it is necessary to determine the odorant specificities of individual odorant receptors. These studies are complicated by the extremely large size of the odorant receptor family and by the poor functional expression of these receptors in heterologous cells. This article provides an overview of the methods that are currently being used to investigate odorant receptor-ligand interactions.

Deorphanizing vertebrate olfactory receptors: Recent advances in odorant-response assays

Olfactory receptors (ORs) comprise the largest multigene G protein-coupled receptor families in organisms from fish to primates, and play a critical role in recognizing thousands of odorant molecules. Recent achievement of functional OR expression in heterologous cells led to identification of ligands for some ORs, revealing a combinatorial receptor coding scheme in the olfactory sensory system. Using the functional assay, the odorant-binding site in ORs has been elucidated, showing that a binding pocket constructed by transmembrane helices provides the molecular basis for agonist and antagonist specificity. To retrospectively identify ORs that recognize a particular odorant of interest, two functional cloning strategies have been developed: one is a strategy wherein OR genes are amplified from single olfactory neurons that show odorant responsiveness in Ca(2+) imaging, and another is an approach based on glomerular activity by combining in vivo bulbar Ca(2+) imaging and retrograde dye labeling of innervating olfactory neurons. The conventional ligand-screening approach and the functional cloning strategies in an odorant-directed manner have allowed us to match ORs to the cognate odorants both in vitro and in vivo.

Gold Surface Functionalization and Patterning for Specific Immobilization of Olfactory Receptors Carried by Nanosomes

There is substantial interest in engineering solid supports to achieve functional immobilization of membrane receptors both for investigation of their biological function and for the development of novel biosensors. Three simple and practical strategies for immobilization of a human olfactory receptor carried by nanosomes are presented. The basis of the functionalization of solid gold surfaces is a self-assembled monolayer (SAM) containing biotinyl groups. Biotinyl groups are subsequently used to attach neutravidin and then biotinylated monoclonal antibody directed against the receptor to allow its specific grafting. Surface plasmon resonance technique is implemented for real-time monitoring of step-by-step surface functionalization and, in addition, for testing the functional response of immobilized olfactory receptors. We show that OR1740 is functional when immobilized via a tag attached to its C-terminus, but not via its N-terminus. Finally, we demonstrate that gold surfaces can be patterned by the SAMs tested using microcontact printing. AFM images of immobilized nanosomes onto a patterned surface suggest that small nanosomes flatten and fuse into larger vesicles but do not merge into a continuous layer. The whole study emphasizes the outstanding performances of the BAT/PEGAT SAM, which could be useful for developing on-a-chip sensor formats for membrane receptor investigations and use.

Loss of CNGB1 protein leads to olfactory dysfunction and subciliary cyclic nucleotide-gated channel trapping

Olfactory receptor neurons (ORNs) employ a cyclic nucleotide-gated (CNG) channel to generate a receptor current in response to an odorant-induced rise in cAMP. This channel contains three types of subunits, the principal CNGA2 subunit and two modulatory subunits (CNGA4 and CNGB1b). Here, we have analyzed the functional relevance of CNGB1 for olfaction by gene targeting in mice. Electro-olfactogram responses of CNGB1-deficient (CNGB1-/-) mice displayed a reduced maximal amplitude and decelerated onset and recovery kinetics compared with wild-type mice. In a behavioral test, CNGB1-/- mice exhibited a profoundly decreased olfactory performance. Electrophysiological recordings revealed that ORNs of CNGB1-/- mice weakly expressed a CNG current with decreased cAMP sensitivity, very rapid flicker-gating behavior and no fast modulation by Ca2+-calmodulin. Co-immunoprecipitation confirmed the presence of a CNGA2/CNGA4 channel in the olfactory epithelium of CNGB1-/- mice. This CNGA2/CNGA4 channel was targeted to the plasma membrane of olfactory knobs, but failed to be trafficked into olfactory cilia. Interestingly, we observed a similar trafficking defect in mice deficient for the CNGA4 subunit. In conclusion, these results demonstrate that CNGB1 has a dual function in vivo. First, it endows the olfactory CNG channel with a variety of biophysical properties tailored to the specific requirements of olfactory transduction. Second, together with the CNGA4 subunit, CNGB1 is needed for ciliary targeting of the olfactory CNG channel.

Visualizing odorant receptor trafficking in living cells down to the single-molecule level

Despite the importance of trafficking for regulating G protein-coupled receptor signaling, for many members of the seven transmembrane helix protein family, such as odorant receptors, little is known about this process in live cells. Here, the complete life cycle of the human odorant receptor OR17-40 was directly monitored in living cells by ensemble and single-molecule imaging, using a double-labeling strategy. While the overall, intracellular trafficking of the receptor was visualized continuously by using a GFP tag, selective imaging of cell surface receptors was achieved by pulse-labeling an acyl carrier protein tag. We found that OR17-40 efficiently translocated to the plasma membrane only at low expression, whereas at higher biosynthesis the receptor accumulated in intracellular compartments. Receptors in the plasma membrane showed high turnover resulting from constitutive internalization along the clathrin pathway, even in the absence of ligand. Single-molecule microscopy allowed monitoring of the early, dynamic processes in odorant receptor signaling. Although mobile receptors initially diffused either freely or within domains of various sizes, binding of an agonist or an antagonist increased partitioning of receptors into small domains of approximately 190 nm, which likely are precursors of clathrin-coated pits. The binding of a ligand, therefore, resulted in modulation of the continuous, constitutive internalization. After endocytosis, receptors were directed to early endosomes for recycling. This unique mechanism of continuous internalization and recycling of OR17-40 might be instrumental in allowing rapid recovery of odor perception.

Heterodimerization and surface localization of G protein coupled receptors

G protein coupled receptors (GPCRs) are one of the largest human gene families, and are targets for many important therapeutic drugs. Over the last few years, there has been a major paradigm shift in our understanding of how these receptors function. Formerly, GPCRs were thought to exist as monomers that, upon agonist occupation, activated a heterotrimeric G protein to alter the concentrations of specific second messengers. Until recently, this relatively linear cascade has been the standard paradigm for signaling by these molecules. However, it is now clear that this model is not adequate to explain many aspects of GPCR function. We now know that many, if not most, GPCRs form homo- and/or hetero-oligomeric complexes and interact directly with intracellular proteins in addition to G proteins. It now appears that many GPCRs may not function independently, but might more accurately be described as subunits of large multi-protein signaling complexes. These observations raise many important new questions; some of which include: (1) how many functionally and pharmacologically distinct receptor subtypes exist in vivo? (2) Which GPCRs physically associate, and in what stochiometries? (3) What are the roles of individual subunits in binding ligand and activating responses? (4) Are the pharmacological or signaling properties of GPCR heterodimers different from monomers? Since these receptors are the targets for a large number of clinically useful compounds, such information is likely to be of direct therapeutic importance, both in understanding how existing drugs work, but also in discovering novel compounds to treat disease.

Quantitative assessment of olfactory receptors activity in immobilized nanosomes: a novel concept for bioelectronic nose

We describe how mammalian olfactory receptors (ORs) could be used as sensing elements of highly specific and sensitive bioelectronic noses. An OR and an appropriate G(alpha) protein were co-expressed in Saccharomyces cerevisiae cells from which membrane nanosomes were prepared, and immobilized on a sensor chip. By Surface Plasmon Resonance, we were able to quantitatively evaluate OR stimulation by an odorant, and G protein activation. We demonstrate that ORs in nanosomes discriminate between odorant ligands and unrelated odorants, as in whole cells. This assay also provides the possibility for quantitative assessment of the coupling efficiency of the OR with different G(alpha) subunits, without the interference of the cellular transduction pathway. Our findings will be useful to develop a new generation of electronic noses for detection and discrimination of volatile compounds, particularly amenable to micro- and nano-sensor formats.

Ric-8B promotes functional expression of odorant receptors

Odorants are detected by a large family of odorant receptors (ORs) expressed in the nose. The information provided by the ORs is transmitted to specific regions of the brain, leading to odorant perception. The determination of the odorant specificities of the different ORs will contribute to the understanding of how odorants are discriminated by the olfactory system. However, to date only a few ORs have been linked to odorants they recognize, because ORs are poorly expressed on the cell surface of heterologous cells. Here we show that Ric-8B, a putative guanine nucleotide exchange factor for Galphaolf, promotes efficient heterologous expression of ORs. Our results also show that Ric-8B enhances accumulation of Galphaolf at the cell periphery, indicating that it promotes functional OR expression by improving the efficiency of OR coupling to Galphaolf. Expression systems containing Galphaolf and Ric-8B should contribute to the functional characterization of ORs.

Odorant receptors and olfactory-like signaling mechanisms in mammalian sperm

Since their discovery in 1991, members of the odorant receptor (OR) family have been found in various ectopic tissues, including testis and sperm. It took, however, more than a decade for the first mammalian testicular ORs to be functionally characterized and implicated in a reproductively relevant scenario. Activation of hOR17-4 and mOR23 in human and mouse sperm, respectively, mediates distinct flagellar motion patterns and chemotactic behavior in various bioassays. For hOR17-4, receptor function and downstream signal transduction events are shown to be subject to pharmacological manipulation. Further insight into the basic principles that govern sperm OR operation as well as into the molecular logic that underlies OR-mediated signaling could set the stage for pioneering future applications in procreation and/or contraception.

Functional analysis of a mammalian odorant receptor subfamily

Phylogenetic analysis groups mammalian odorant receptors into two broad classes and numerous subfamilies. These subfamilies are proposed to reflect functional organization. Testing this idea requires an assay allowing detailed functional characterization of odorant receptors. Here we show that a variety of Class I and Class II mouse odorant receptors can be functionally expressed in Xenopus laevis oocytes. Receptor constructs included the N-terminal 20 residues of human rhodopsin and were co-expressed with Galphaolf and the cystic fibrosis transmembrane regulator to allow electrophysiological measurement of receptor responses. For most mouse odorant receptors tested, these conditions were sufficient for functional expression. Co-expression of accessory proteins was required to allow functional surface expression of some mouse odorant receptors. We used this assay to examine the receptive ranges of all members of the mouse odorant receptor 42 (MOR42) subfamily. MOR42-1 responded to dicarboxylic acids, preferring a 10-12 carbon chain length. MOR42-2 responded to monocarboxylic acids (7-10 carbons). MOR42-3 responded to dicarboxylic acids (8-10 carbons) and monocarboxylic acids (10-12 carbons). Thus, the receptive range of each receptor was unique. However, overlap between the individual receptive ranges suggests that the members of this subfamily form one contiguous subfamily receptive range, suggesting that odorant receptor subfamilies do constitute functional units.

Atypical membrane topology and heteromeric function of Drosophila odorant receptors in vivo

Drosophila olfactory sensory neurons (OSNs) each express two odorant receptors (ORs): a divergent member of the OR family and the highly conserved, broadly expressed receptor OR83b. OR83b is essential for olfaction in vivo and enhances OR function in vitro, but the molecular mechanism by which it acts is unknown. Here we demonstrate that OR83b heterodimerizes with conventional ORs early in the endomembrane system in OSNs, couples these complexes to the conserved ciliary trafficking pathway, and is essential to maintain the OR/OR83b complex within the sensory cilia, where odor signal transduction occurs. The OR/OR83b complex is necessary and sufficient to promote functional reconstitution of odor-evoked signaling in sensory neurons that normally respond only to carbon dioxide. Unexpectedly, unlike all known vertebrate and nematode chemosensory receptors, we find that Drosophila ORs and OR83b adopt a novel membrane topology with their N-termini and the most conserved loops in the cytoplasm. These loops mediate direct association of ORs with OR83b. Our results reveal that OR83b is a universal and integral part of the functional OR in Drosophila. This atypical heteromeric and topological design appears to be an insect-specific solution for odor recognition, making the OR/OR83b complex an attractive target for the development of highly selective insect repellents to disrupt olfactory-mediated host-seeking behaviors of insect disease vectors.

This study reveals a novel membrane topology for olfactory receptors in Drosophila and details the molecular mechanisms of receptor localization at the sensory cilia.

Ubiquitously expressed GPCR membrane-trafficking orthologs

Olfactory receptors are a diverse set of G-protein-coupled receptors (GPCRs) that localize to cellular plasma membranes in the olfactory epithelium. Associated trafficking proteins often assist in targeting these GPCRs to the membrane, facilitating function. One such trafficking protein has been isolated as a mutant defective for both odorant response and proper receptor localization in Caenorhabditis elegans. This gene (ODR-4) allows for functional expression of olfactory receptors in heterologous cells that are otherwise incapable of targeting. We have isolated a full-length human cDNA that is homologous to the C. elegans gene at the protein level across nearly the entire gene by using a novel RecA-based gene enrichment procedure. This sequence is homologous to a family of orthologs that share predicted structural features, indicating a conserved function. The gene was expressed in 41 of 44 human, mouse, and rat tissues, suggesting an important role in trafficking olfactory and other GPCRs.

Differentially expressed transcripts from phenotypically identified olfactory sensory neurons

In comparing purified mouse olfactory sensory neurons (OSNs) with neighboring cells, we identified 54 differentially expressed transcripts. One-third of the transcripts encode proteins with no known function, but the others have functions that correlate with challenges faced by OSNs. The OSNs expressed a diversity of signaling protein genes, including stomatin (Epb7.2), S100A5, Ddit3, Sirt2, CD81, Sdc2, Omp, and Ptpla. The elaboration of dendrites, cilia, and axons that places OSNs in contact with diverse cell types and signals presumably also requires large investments in cytoskeletal-associated proteins, lipid biosynthesis, and energy production. Several of the genes encode proteins that participate in these biological processes, including ATP5g3, Ndufa9, Sqrdl, Mdh1, Got1, beta-2 tubulin, Capza1, Bin3, Tom1, Acl6, and similar to O-MACS. Three transcripts had restricted expression patterns. Similar to O-MACS and Gstm2 had zonally restricted expression patterns in OSNs and sustentacular cells but not in Bowman's glands, suggesting that zonality can be differentially regulated by cell type. The mosaic expression pattern of S100A5 in approximately 70% of OSNs predicts that it is coexpressed with a subset of odorant receptors. We captured four abundant transcripts, Cyp2a4, similar to Cyp2g1, Gstm2, and Cbr2, that encode xenobiotic metabolizing enzymes expressed by sustentacular cells or Bowman's glands, reinforcing the interpretation that clearance of xenobiotic compounds is a major function of these cells. Within the olfactory epithelium, Cbr2 is a new anatomical marker for sustentacular cells. We also discovered that Reg3g is a marker for respiratory epithelium.

Specialized membrane-localized chaperones prevent aggregation of polytopic proteins in the ER

The integral endoplasmic reticulum (ER) membrane protein Shr3p is required for proper plasma membrane localization of amino acid permeases (AAPs) in yeast. In the absence of Shr3p AAPs are uniquely retained in the ER with each of their twelve membrane-spanning segments correctly inserted in the membrane. Here, we show that the membrane domain of Shr3p specifically prevents AAPs from aggregating, and thus, plays a critical role in assisting AAPs to fold and correctly attain tertiary structures required for ER exit. Also, we show that the integral ER proteins, Gsf2p, Pho86p, and Chs7p, function similarly to Shr3p. In cells individually lacking one of these components only their cognate substrates, hexose transporters, phosphate transporters, and chitin synthase-III, respectively, aggregate and consequently fail to exit the ER membrane. These findings indicate that polytopic membrane proteins depend on specialized membrane-localized chaperones to prevent inappropriate interactions between membrane-spanning segments as they insert and fold in the lipid bilayer of the ER membrane.

Functional expression of olfactory receptors in yeast and development of a bioassay for odorant screening

The functional expression of olfactory receptors (ORs) is a primary requirement to examine the molecular mechanisms of odorant perception and coding. Functional expression of the rat I7 OR and its trafficking to the plasma membrane was achieved under optimized experimental conditions in the budding yeast Saccharomyces cerevisiae. The membrane expression of the receptor was shown by Western blotting and immunolocalization methods. Moreover, we took advantage of the functional similarities between signal transduction cascades of G protein-coupled receptor in mammalian cells and the pheromone response pathway in yeast to develop a novel biosensor for odorant screening using luciferase as a functional reporter. Yeasts were engineered to coexpress I7 OR and mammalian G(alpha) subunit, to compensate for the lack of endogenous Gpa1 subunit, so that stimulation of the receptor by its ligands activates a MAP kinase signaling pathway and induces luciferase synthesis. The sensitivity of the bioassay was significantly enhanced using mammalian G(olf) compared to the G(alpha15) subunit, resulting in dose-dependent responses of the system. The biosensor was probed with an array of odorants to demonstrate that the yeast-borne I7 OR retains its specificity and selectivity towards ligands. The results are confirmed by functional expression and bioluminescence response of human OR17-40 to its specific ligand, helional. Based on these findings, the bioassay using the luciferase reporter should be amenable to simple, rapid and inexpensive odorant screening of hundreds of ORs to provide insight into olfactory coding mechanisms.

RTP Family Members Induce Functional Expression of Mammalian Odorant Receptors

Transport of G protein-coupled receptors (GPCRs) to the cell surface membrane is critical in order for the receptors to recognize their ligands. However, mammalian GPCR odorant receptors (ORs), when heterologously expressed in cells, are poorly expressed on the cell surface. Here we show that the transmembrane proteins RTP1 and RTP2 promote functional cell surface expression of ORs expressed in HEK293T cells. Genes encoding these proteins are expressed specifically in olfactory neurons. These proteins are associated with OR proteins and enhance the OR responses to odorants. Similar although weaker effects were seen with a third protein, REEP1. These findings suggest that RTP1 and RTP2 in particular play significant roles in the translocation of ORs to the plasma membrane as well as in the functioning of ORs. We have used this approach to identify active odorant ligands for ORs, providing a platform for screening the chemical selectivity of the large OR family.