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

Petrol Note in Riesling - 1,1,6-Trimethyl-1,2-dihydronaphthalene (TDN) Selectively Activates Human Odorant Receptor OR8H1

Grapevine (Vitis vinifera) is one of the most important perennial fruit plants. The variety Riesling stands out by developing a characteristic petrol-like odor note during aging, elicited by the aroma compound 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN). The UV-dependent TDN contents differ largely among Rieslings grown in the northern versus the southern hemisphere. Highest TDN concentrations were found in Australian Rieslings, where TDN is a scoring ingredient. In contrast, in Rieslings from Europe, for example, TDN may be a tending cause of rejection. A human receptor for TDN has been unknown. Here, we report on the identification of OR8H1 as a TDN-selective odorant receptor, out of a library of 766 odorant receptor variants. OR8H1 is selectively tuned to six carbon ring structures, identified by screening a collection of 180 key food odorants, using a HEK-293 cell-based cAMP luminescence assay equipped with the GloSensor technology.

M2OR: a database of olfactory receptor-odorant pairs for understanding the molecular mechanisms of olfaction

Mammalian sense of smell is triggered by interaction between odorant molecules and a class of proteins, called olfactory receptors (ORs). These receptors, expressed at the surface of olfactory sensory neurons, encode myriad of distinct odors via a sophisticated activation pattern. However, determining the molecular recognition spectrum of ORs remains a major challenge. The Molecule to Olfactory Receptor database (M2OR, https://m2or.chemsensim.fr/) provides curated data that allows an easy exploration of the current state of the research on OR-molecule interaction. We have gathered a database of 75,050 bioassay experiments for 51 395 distinct OR-molecule pairs. Drawn from published literature and public databases, M2OR contains information about OR responses to molecules and their mixtures, receptor sequences and experimental details. Users can obtain information on the activity of a chosen molecule or a group of molecules, or search for agonists for a specific OR or a group of ORs. Advanced search allows for fine-grained queries using various metadata such as species or experimental assay system, and the database can be queried by multiple inputs via a batch search. Finally, for a given search query, users can access and download a curated aggregation of the experimental data into a binarized combinatorial code of olfaction.

Engineered odorant receptors illuminate structural principles of odor discrimination

A central challenge in olfaction is understanding how the olfactory system detects and distinguishes odorants with diverse physicochemical properties and molecular configurations. Vertebrate animals perceive odors via G protein-coupled odorant receptors (ORs). In humans, ~400 ORs enable the sense of smell. The OR family is composed of two major classes: Class I ORs are tuned to carboxylic acids while Class II ORs, representing the vast majority of the human repertoire, respond to a wide variety of odorants. How ORs recognize chemically diverse odorants remains poorly understood. A fundamental bottleneck is the inability to visualize odorant binding to ORs. Here, we uncover fundamental molecular properties of odorant-OR interactions by employing engineered ORs crafted using a consensus protein design strategy. Because such consensus ORs (consORs) are derived from the 17 major subfamilies of human ORs, they provide a template for modeling individual native ORs with high sequence and structural homology. The biochemical tractability of consORs enabled four cryoEM structures of distinct consORs with unique ligand recognition properties. The structure of a Class I consOR, consOR51, showed high structural similarity to the native human receptor OR51E2 and yielded a homology model of a related member of the human OR51 family with high predictive power. Structures of three Class II consORs revealed distinct modes of odorant-binding and activation mechanisms between Class I and Class II ORs. Thus, the structures of consORs lay the groundwork for understanding molecular recognition of odorants by the OR superfamily.

Heterodimerization of Chemoreceptors TAS1R3 and mGlu2 in Human Blood Leukocytes

The expression of canonical chemosensory receptors of the tongue, such as the heteromeric sweet taste (TAS1R2/TAS1R3) and umami taste (TAS1R1/TAS1R3) receptors, has been demonstrated in many extra-oral cells and tissues. Gene expression studies have revealed transcripts for all TAS1 and metabotropic glutamate (mGlu) receptors in different types of immune cells, where they are involved, for example, in the chemotaxis of human neutrophils and the protection of T cells from activation-induced cell death. Like other class-C G protein-coupling receptors (GPCRs), TAS1Rs and mGlu receptors form heteromers within their families. Since mGlu receptors and TAS1R1/TAS1R3 share the same ligand, monosodium glutamate (MSG), we hypothesized their hitherto unknown heteromerization across receptor families in leukocytes. Here we show, by means of immunocytochemistry and co-IP/Western analysis, that across class-C GPCR families, mGlu2 and TAS1R3 co-localize and heterodimerize in blood leukocytes. Expressing the recombinant receptors in HEK-293 cells, we validated their heterodimerization by bioluminescence resonance energy transfer. We demonstrate MSG-induced, mGlu2/TAS1R3 heteromer-dependent gain-of-function and pertussis toxin-sensitive signaling in luminescence assays. Notably, we show that mGlu2/TAS1R3 is necessary and sufficient for MSG-induced facilitation of N-formyl-methionyl-leucyl-phenylalanine-stimulated IL-8 secretion in neutrophils, using receptor-specific antagonists. In summary, our results demonstrate mGlu2/TAS1R3 heterodimerization in leukocytes, suggesting cellular function-tailored chemoreceptor combinations to modulate cellular immune responses.

Human Olfactory Receptor Sensor for Odor Reconstitution

Among the five human senses, light, sound, and force perceived by the eye, ear, and skin, respectively are physical phenomena, and therefore can be easily measured and expressed as objective, univocal, and simple digital data with physical quantity. However, as taste and odor molecules perceived by the tongue and nose are chemical phenomena, it has been difficult to express them as objective and univocal digital data, since no reference chemicals can be defined. Therefore, while the recording, saving, transmitting to remote locations, and replaying of human visual, auditory, and tactile information as digital data in digital devices have been realized (this series of data flow is defined as DX (digital transformation) in this review), the DX of human taste and odor information is not yet in the realization stage. Particularly, since there are at least 400,000 types of odor molecules and an infinite number of complex odors that are mixtures of these molecules, it has been considered extremely difficult to realize "human olfactory DX" by converting all odors perceived by human olfaction into digital data. In this review, we discuss the current status and future prospects of the development of "human olfactory DX", which we believe can be realized by utilizing odor sensors that employ the olfactory receptors (ORs) that support human olfaction as sensing molecules (i.e., human OR sensor).

Modeling the Orthosteric Binding Site of the G Protein-Coupled Odorant Receptor OR5K1

With approximately 400 encoding genes in humans, odorant receptors (ORs) are the largest subfamily of class A G protein-coupled receptors (GPCRs). Despite its high relevance and representation, the odorant-GPCRome is structurally poorly characterized: no experimental structures are available, and the low sequence identity of ORs to experimentally solved GPCRs is a significant challenge for their modeling. Moreover, the receptive range of most ORs is unknown. The odorant receptor OR5K1 was recently and comprehensively characterized in terms of cognate agonists. Here, we report two additional agonists and functional data of the most potent compound on two mutants, L104^3.32 and L255^6.51. Experimental data was used to guide the investigation of the binding modes of OR5K1 ligands into the orthosteric binding site using structural information from AI-driven modeling, as recently released in the AlphaFold Protein Structure Database, and from homology modeling. Induced-fit docking simulations were used to sample the binding site conformational space for ensemble docking. Mutagenesis data guided side chain residue sampling and model selection. We obtained models that could better rationalize the different activity of active (agonist) versus inactive molecules with respect to starting models and also capture differences in activity related to minor structural differences. Therefore, we provide a model refinement protocol that can be applied to model the orthosteric binding site of ORs as well as that of GPCRs with low sequence identity to available templates.

The human odorant receptor OR10A6 is tuned to the pheromone of the commensal fruit fly Drosophila melanogaster

All living things speak chemistry. The challenge is to reveal the vocabulary, the odorants that enable communication across phylogenies and to translate them to physiological, behavioral, and ecological function. Olfactory receptors (ORs) interface animals with airborne odorants. Expression in heterologous cells makes it possible to interrogate single ORs and to identify cognate ligands. The cosmopolitan, anthropophilic strain of the vinegar fly Drosophila melanogaster depends on human resources and housing for survival. Curiously, humans sense the pheromone (Z)-4-undecenal (Z4-11Al) released by single fly females. A screening of all human ORs shows that the most highly expressed OR10A6 is tuned to Z4-11Al. Females of an ancestral African fly strain release a blend of Z4-11Al and Z4-9Al that produces a different aroma, which is how we distinguish these fly strains by nose. That flies and humans sense Z4-11Al via dedicated ORs shows how convergent evolution shapes communication channels between vertebrate and invertebrate animals.

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Humans sense the sex pheromone Z411-Al released by single Drosophila melanogaster females

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The most highly expressed human olfactory receptor OR10A6 is tuned to Z411-Al

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An African fly strain emits two aldehydes, which we distinguish from Z411-Al by nose

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Convergent evolution shapes chemical communication between phylogenies

A genetic platform for functionally profiling odorant receptors in olfactory cilia ex vivo

The molecular basis for odor perception in humans remains enigmatic because of the difficulty in studying odorant receptors (ORs) outside their native environment. Efforts toward OR expression and functional profiling have been met with limited success because of the poor efficiency of their cell surface expression in vitro. Structures protruding from the surface of olfactory sensory neurons called cilia contain all of the components of the olfactory signal transduction machinery and can be placed in an ex vivo plate assay to rapidly measure odor-specific responses. Here, we describe an approach using cilia isolated from the olfactory sensory neurons of mice expressing two human ORs, OR1A1 and OR5AN1, that showed 10- to 100-fold more sensitivity to ligands as compared to previous assays. A single mouse can produce enough olfactory cilia for up to 4000 384-well assay wells, and isolated cilia can be stored frozen and thus preserved. This pipeline offers a sensitive and highly scalable ex vivo odor-screening platform that has the potential to decode human olfaction.

Hot Spot Mutagenesis Improves the Functional Expression of Unique Mammalian Odorant Receptors

Vertebrate animals detect odors through olfactory receptors (ORs), members of the G protein-coupled receptor (GPCR) family. Due to the difficulty in the heterologous expression of ORs, studies of their odor molecule recognition mechanisms have progressed poorly. Functional expression of most ORs in heterologous cells requires the co-expression of their chaperone proteins, receptor transporting proteins (RTPs). Yet, some ORs were found to be functionally expressed without the support of RTP (RTP-independent ORs). In this study, we investigated whether amino acid residues highly conserved among RTP-independent ORs improve the functional expression of ORs in heterologous cells. We found that a single amino acid substitution at one of two sites (N_BW3.39 and 3.43) in their conserved residues (E and L, respectively) significantly improved the functional expression of ORs in heterologous cells. E^3.39 and L^3.43 also enhanced the membrane expression of RTP-dependent ORs in the absence of RTP. These changes did not alter the odorant responsiveness of the tested ORs. Our results showed that specific sites within transmembrane domains regulate the membrane expression of some ORs.

Key Food Furanones Furaneol and Sotolone Specifically Activate Distinct Odorant Receptors

Furanones formed during the Maillard reaction often are natural aroma-determining compounds found in numerous foods. Prominent economically relevant representatives are the structural homologues Furaneol and sotolone, which are important natural flavoring compounds because of their distinct caramel- and seasoning-like odor qualities. These, however, cannot be predicted by the odorants' molecular shape, rather their receptors' activation parameters help to decipher the encoding of odor quality. Here, the distinct odor qualities of Furaneol and sotolone suggested an activation of at least two out of our ca. 400 different odorant receptor types, which are the molecular biosensors of our chemical sense of olfaction. While an odorant receptor has been identified for sotolone, a receptor specific for Furaneol has been elusive. Using a bidirectional screening approach employing 616 receptor variants and 187 key food odorants in a HEK-293 cell-based luminescence assay, we newly identified OR5M3 as a receptor specifically activated by Furaneol and homofuraneol.

An evolutionary conserved olfactory receptor for foodborne and semiochemical alkylpyrazines

Molecular recognition is a fundamental principle in biological systems. The olfactory detection of both food and predators via ecological relevant odorant cues are abilities of eminent evolutionary significance for many species. Pyrazines are such volatile cues, some of which act as both human-centered key food odorants (KFOs) and semiochemicals. A pyrazine-selective odorant receptor has been elusive. Here we screened 2,3,5-trimethylpyrazine, a KFO and semiochemical, and 2,5-dihydro-2,4,5-trimethylthiazoline, an innate fear-associated non-KFO, against 616 human odorant receptor variants, in a cell-based luminescence assay. OR5K1 emerged as sole responding receptor. Tested against a comprehensive collection of 178 KFOs, we newly identified 18 pyrazines and (2R/2S)-4-methoxy-2,5-dimethylfuran-3(2H)-one as agonists. Notably, OR5K1 orthologs in mouse and domesticated species displayed a human-like, potency-ranked activation pattern of pyrazines, suggesting a domestication-led co-evolution of OR5K1 and its orthologs. In summary, OR5K1 is a specialized olfactory receptor across mammals for the detection of pyrazine-based key food odors and semiochemicals.

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.

Structural instability and divergence from conserved residues underlie intracellular retention of mammalian odorant receptors

Mammalian odorant receptors are a diverse and rapidly evolving set of G protein-coupled receptors expressed in olfactory cilia membranes. Most odorant receptors show little to no cell surface expression in nonolfactory cells due to endoplasmic reticulum retention, which has slowed down biochemical studies. Here we provide evidence that structural instability and divergence from conserved residues of individual odorant receptors underlie intracellular retention using a combination of large-scale screening of odorant receptors cell surface expression in heterologous cells, point mutations, structural modeling, and machine learning techniques. We demonstrate the importance of conserved residues by synthesizing consensus odorant receptors that show high levels of cell surface expression similar to conventional G protein-coupled receptors. Furthermore, we associate in silico structural instability with poor cell surface expression using molecular dynamics simulations. We propose an enhanced evolutionary capacitance of olfactory sensory neurons that enable the functional expression of odorant receptors with cryptic mutations.

Real-time In Vitro Monitoring of Odorant Receptor Activation by an Odorant in the Vapor Phase

Olfactory perception begins with the interaction of odorants with odorant receptors (OR) expressed by olfactory sensory neurons (OSN). Odor recognition follows a combinatorial coding scheme, where one OR can be activated by a set of odorants and one odorant can activate a combination of ORs. Through such combinatorial coding, organisms can detect and discriminate between a myriad of volatile odor molecules. Thus, an odor at a given concentration can be described by an activation pattern of ORs, which is specific to each odor. In that sense, cracking the mechanisms that the brain uses to perceive odor requires the understanding odorant-OR interactions. This is why the olfaction community is committed to "de-orphanize" these receptors. Conventional in vitro systems used to identify odorant-OR interactions have utilized incubating cell media with odorant, which is distinct from the natural detection of odors via vapor odorants dissolution into nasal mucosa before interacting with ORs. Here, we describe a new method that allows for real-time monitoring of OR activation via vapor-phase odorants. Our method relies on measuring cAMP release by luminescence using the Glosensor assay. It bridges current gaps between in vivo and in vitro approaches and provides a basis for a biomimetic volatile chemical sensor.

Molecular Basis of Mammalian Odor Discrimination: A Status Report

Humans have 396 unique, intact olfactory receptors (ORs), G-protein coupled receptors (GPCRs) containing receptor-specific binding sites; other mammals have more. Activation of these transmembrane proteins by an odorant initiates a signaling cascade, evoking an action potential leading to perception of a smell. Because the number of distinguishable odorants vastly exceeds the number of ORs, research has focused on mechanisms of recognition and signaling processes for classes of odorants. In this review, selected recent examples will be presented of "deorphaned" mammalian receptors, where the OR ligands (odorants) as well as key aspects of receptor-odorant interactions were identified using odorant-mediated receptor activation data together with site-directed mutagenesis and molecular modeling. Based on cumulative evidence from OR deorphaning and olfactory receptor neuron activation studies, a receptor-ligand docking model rather than an alternative bond vibration model is suggested to best explain the molecular basis of the exquisitely sensitive odor discrimination in mammals.