The missense of smell: functional variability in the human odorant receptor repertoire

Targeting Odorant Receptors in Adipose Tissue with Food-Derived Odorants: A Novel Approach to Obesity Treatment

Odorant receptors (ORs) have long been thought to serve as chemosensors located on the cilia of olfactory sensory neurons (OSNs) in the olfactory epithelium, where they recognize odorant molecules and comprise the largest family of seven transmembrane-domain G protein-coupled receptors (GPCRs). Over the last three decades, accumulating evidence has suggested that ORs are distributed in a variety of peripheral tissues beyond their supposed typical tissue expression in the olfactory epithelium. These ectopic ORs play a role in regulating various cellular, physiological, and pathophysiological phenomena in the body, such as regulation of hypertension, hepatic glucose production, cancer development, and chronic skin disease. Adipose tissue, the key organ in regulating obesity and energy metabolism, has been reported to take advantage of ectopic OR-mediated signaling. In this review, we summarize and provide an in-depth analysis of the current research on the key biological functions of adipose tissue ORs in response to food-derived odorants, as well as the molecular mechanisms underlying their activity.

Conserved pattern-based classification of human odorant receptor multigene family

Conserved protein-coding sequences are critical for maintaining protein function across species. Odorant receptors (ORs), a large poorly understood multigene family responsible for odor detection, lack comprehensive classification methods that reflect their functional diversity. In this study, we propose a new approach called conserved motif-based classification (CMC) for classifying ORs based on amino acid sequence similarities within conserved motifs. Specifically, we focused on three well-conserved motifs: MAYDRYVAIC in TM3, KAFSTCASH in TM6, and PMLNPFIY in TM7. Using an unsupervised clustering technique, we classified human ORs (hORs) into two main clusters with six sub-clusters. CMC partly reflects previously identified subfamilies, revealing altered residue positions among the sub-clusters. These altered positions interacted with specific residues within or adjacent to the transmembrane domain, suggesting functional implications. Furthermore, we found that the CMC correlated with both ligand responses and ectopic expression patterns, highlighting its relevance to OR function. This conserved motif-based classification will help in understanding the functions and features that are not understood by classification based solely on entire amino acid sequence similarity.

Beyond flavor: the versatile roles of eugenol in health and disease

Eugenol, a phenylpropanoid compound, is found in various dietary resources and medicinal plants. From a historical perspective, eugenol is widely employed as a flavoring agent in the food and fragrance industries. Here, this review mainly focuses on recent advances in eugenol with respect to its versatile physiological roles in health and disease and discusses the mechanisms. Emerging evidence has highlighted that eugenol exhibits multiple biological activities in cancer, diabetes, obesity, cardiovascular diseases, and neurodegenerative diseases. It also has analgesic, anti-inflammatory, and antioxidant qualities and has lethal or inhibiting effects on various viruses, bacteria, fungi, and parasites. The manuscript also contains some patents that have been filed thus far regarding the production and application of eugenol. Overall, these benefits make eugenol a promising nutritional supplement which fulfils its historical function as a flavoring agent, opening up new possibilities for the creation of therapeutic agents for the treatment of disease.

Molecular determinants of olfactory receptor activation: Comparative analysis of Olfr205 and Olfr740 family member responses to indole

Indole is widely present in nature and contributes significantly to the smell of flowers and animal excretion. However, the odor perception mechanism for indole is unclear, despite previous reports suggesting that it activates the Olfr740 family of receptors. In this study, we successfully identified another receptor, Olfr205, that is responsive to indole. Molecular model construction and binding pocket analysis predicted that the A202 residue in transmembrane helix 5 of Olfr205 forms a crucial hydrogen bond with indole, facilitating receptor activation. Additionally, G112 in transmembrane helix 3 of the Olfr740 family is involved in indole activation of receptors. Finally, our mutant function assay showed that substitution of A202 in Olfr205 and G112 in Olfr740 with other amino acids significantly decreased the receptor response to indole, which provides robust evidence to confirm the docking results. In summary, our study is the first to reveal that Olfr205 is an olfactory receptor distinct from those in the Olfr740 family that is activated by indole. Moreover, these receptors display different indole-binding mechanisms. This study sheds light on molecular binding mechanisms and contributes to a deeper understanding of indole perception.

Geosmin, a Food- and Water-Deteriorating Sesquiterpenoid and Ambivalent Semiochemical, Activates Evolutionary Conserved Receptor OR11A1

Geosmin, a ubiquitous volatile sesquiterpenoid of microbiological origin, is causative for deteriorating the quality of many foods, beverages, and drinking water, by eliciting an undesirable "earthy/musty" off-flavor. Moreover, and across species from worm to human, geosmin is a volatile, chemosensory trigger of both avoidance and attraction behaviors, suggesting its role as semiochemical. Volatiles typically are detected by chemosensory receptors of the nose, which have evolved to best detect ecologically relevant food-related odorants and semiochemicals. An insect receptor for geosmin was recently identified in flies. A human geosmin-selective receptor, however, has been elusive. Here, we report on the identification and characterization of a human odorant receptor for geosmin, with its function being conserved in orthologs across six mammalian species. Notably, the receptor from the desert-dwelling kangaroo rat showed a more than 100-fold higher sensitivity compared to its human ortholog and detected geosmin at low nmol/L concentrations in extracts from geosmin-producing actinomycetes.

Kinetic Model for the Desensitization of G Protein-Coupled Receptor

Desensitization of G-protein-coupled receptors (GPCR) is a general regulatory mechanism adopted by biological organisms against overstimulation of G protein signaling. Although the details of the mechanism are extensively studied, it is not easy to gain an overarching understanding of the process constituted by a multitude of molecular events with vastly differing time scales. To offer a semiquantitative yet predictive understanding of the mechanism, we formulate a kinetic model for the G protein signaling and desensitization by considering essential biochemical steps from ligand binding to receptor internalization. The internalization, followed by receptor depletion from the plasma membrane, attenuates the downstream signal. Together with the kinetic model and its full numerics of the expression derived for the dose-response relation, an approximated form of the expression clarifies the role played by the individual biochemical processes and allows us to identify four distinct regimes for the downregulation that emerge from the balance between phosphorylation, dephosphorylation, and the cellular level of β-arrestin.

Olfactory receptors impact pathophysiological processes of lung diseases in bronchial epithelial cells

BACKGROUND

Therapeutic options for steroid-resistant non-type 2 inflammation in obstructive lung diseases are limited. Bronchial epithelial cells are key in the pathogenesis by releasing the central proinflammatory cytokine interleukine-8 (IL-8). Olfactory receptors (ORs) are expressed in various cell types. This study examined the drug target potential of ORs by investigating their impact on associated pathophysiological processes in lung epithelial cells.

METHODS

Experiments were performed in the A549 cell line and in primary human bronchial epithelial cells. OR expression was investigated using RT-PCR, Western blot, and immunocytochemical staining. OR-mediated effects were analyzed by measuring 1) intracellular calcium concentration via calcium imaging, 2) cAMP concentration by luminescence-based assays, 3) wound healing by scratch assays, 4) proliferation by MTS-based assays, 5) cellular vitality by Annexin V/PI-based FACS staining, and 6) the secretion of IL-8 in culture supernatants by ELISA.

RESULTS

By screening 100 potential OR agonists, we identified two, Brahmanol and Cinnamaldehyde, that increased intracellular calcium concentrations. The mRNA and proteins of the corresponding receptors OR2AT4 and OR2J3 were detected. Stimulation of OR2J3 with Cinnamaldehyde reduced 1) IL-8 in the absence and presence of bacterial and viral pathogen-associated molecular patterns (PAMPs), 2) proliferation, and 3) wound healing but increased cAMP. In contrast, stimulation of OR2AT4 by Brahmanol increased wound healing but did not affect cAMP and proliferation. Both ORs did not influence cell vitality.

CONCLUSION

ORs might be promising drug target candidates for lung diseases with non-type 2 inflammation. Their stimulation might reduce inflammation or prevent tissue remodeling by promoting wound healing.

A pattern recognition artificial olfactory system based on human olfactory receptors and organic synaptic devices

Neuromorphic sensors, designed to emulate natural sensory systems, hold the promise of revolutionizing data extraction by facilitating rapid and energy-efficient analysis of extensive datasets. However, a challenge lies in accurately distinguishing specific analytes within mixtures of chemically similar compounds using existing neuromorphic chemical sensors. In this study, we present an artificial olfactory system (AOS), developed through the integration of human olfactory receptors (hORs) and artificial synapses. This AOS is engineered by interfacing an hOR-functionalized extended gate with an organic synaptic device. The AOS generates distinct patterns for odorants and mixtures thereof, at the molecular chain length level, attributed to specific hOR-odorant binding affinities. This approach enables precise pattern recognition via training and inference simulations. These findings establish a foundation for the development of high-performance sensor platforms and artificial sensory systems, which are ideal for applications in wearable and implantable devices.

Decoding Molecular Mechanism Underlying Human Olfactory Receptor OR8D1 Activation by Sotolone Enantiomers

Sotolone, a chiral compound, plays an important role in the food industry. Herein, (R)-/(S)-sotolone were separated to determine their odor characteristics and thresholds in air (R-form: smoky, burned, herb, and green aroma, 0.0514 μg/m3; S-form: sweet, milk, acid, and nutty aroma, 0.0048 μg/m3). OR8D1 responses to (R)-/(S)-sotolone were detected in a HEK293 cell-based luminescence assay. (S)-Sotolone was a more potent agonist than (R)-sotolone (EC50 values of 84.98 ± 1.05 and 167.20 ± 0.25 μmol/L, respectively). Molecular dynamics simulations and molecular mechanics Poisson-Boltzmann surface area analyses confirmed that the combination of (S)-sotolone and OR8D1 was more stable than that of (R)-sotolone. Odorant docking, multiple sequence alignments, site-directed mutagenesis, and functional studies with recombinant odorant receptors (ORs) in a cell-based luminescence assay identified 11 amino-acid residues that influence the enantioselectivity of OR8D1 toward sotolone significantly and that N2065.46 was indispensable to the activation of OR8D1 by (S)-sotolone.

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.

What Does the Human Olfactory System Do, and How Does It Do It?

Historically, the human sense of smell has been regarded as the odd stepchild of the senses, especially compared to the sensory bravado of seeing, touching, and hearing. The idea that the human olfaction has little to contribute to our experience of the world is commonplace, though with the emergence of COVID-19 there has rather been a sea change in this understanding. An ever increasing body of work has convincingly highlighted the keen capabilities of the human nose and the sophistication of the human olfactory system. Here, we provide a concise overview of the neuroscience of human olfaction spanning the last 10-15 years, with focus on the peripheral and central mechanisms that underlie how odor information is processed, packaged, parceled, predicted, and perturbed to serve odor-guided behaviors. We conclude by offering some guideposts for harnessing the next decade of olfactory research in all its shapes and forms.

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.

The Role of the Olfactory System in Obesity and Metabolism in Humans: A Systematic Review and Meta-Analysis

Obesity, linked to chronic diseases, poses a global health challenge. While the role of the olfactory system in energy homeostasis is well-documented in rodents, its role in metabolism regulation and obesity in humans remains understudied. This review examines the interplay between olfactory function and metabolic alterations in human obesity and the effects of bariatric surgery on olfactory capabilities in humans. Adhering to PRISMA guidelines, a systematic review and meta-analysis was conducted, focusing exclusively on original human studies. From 51 articles, 14 were selected for the meta-analysis. It was found that variations in olfactory receptor genes influence the susceptibility to odors and predisposition to weight gain and poor eating habits. Bariatric surgery, particularly sleeve gastrectomy, shows significant improvements in olfactory function (SMD 2.37, 95% CI [0.96, 3.77], I = 92%, p = 0.001), especially regarding the olfactory threshold (SMD -1.65, 95% CI [-3.03, -0.27], I = 81%, p = 0.02). There is a bidirectional relationship between olfactory function and metabolism in humans. Bariatric surgery improves olfactory perception in obese patients, but it is still unclear if impacting the olfactory system directly affects eating behavior and the energy balance. However, these findings open novel avenues for future studies addressing the olfactory system as a novel target to alter systemic metabolism in humans.

High-precision mapping reveals the structure of odor coding in the human brain

Odor perception is inherently subjective. Previous work has shown that odorous molecules evoke distributed activity patterns in olfactory cortices, but how these patterns map on to subjective odor percepts remains unclear. In the present study, we collected neuroimaging responses to 160 odors from 3 individual subjects (18 h per subject) to probe the neural coding scheme underlying idiosyncratic odor perception. We found that activity in the orbitofrontal cortex (OFC) represents the fine-grained perceptual identity of odors over and above coarsely defined percepts, whereas this difference is less pronounced in the piriform cortex (PirC) and amygdala. Furthermore, the implementation of perceptual encoding models enabled us to predict olfactory functional magnetic resonance imaging responses to new odors, revealing that the dimensionality of the encoded perceptual spaces increases from the PirC to the OFC. Whereas encoding of lower-order dimensions generalizes across subjects, encoding of higher-order dimensions is idiosyncratic. These results provide new insights into cortical mechanisms of odor coding and suggest that subjective olfactory percepts reside in the OFC.

Molecular mechanisms of caramel-like odorant-olfactory receptor interactions based on a computational chemistry approach

Molecular mechanisms of caramel-like odorant-olfactory receptor interactions were investigated based on molecular docking and molecular dynamics simulations. The transmembrane regions TM-3, TM-5 and TM-6 of receptors were main contributors of amino acid residues in the docking. Molecular docking results showed that hydrogen bonding and pi-pi stacking were the key forces for the stabilization of caramel-like odorants. The binding energies were positively correlated with the molecular weight of caramel-like odorants. Residues Asn155 (84%, OR2W1), Asn206 (86%, OR8D1), Ser155 (77%, OR8D1), Asp179 (87%, OR5M3), Val182 (84%, OR2J2) and Tyr260 (94%, OR2J2) with high frequencies played an important role in the complexes formation. Odorants 4-hydroxy-5-methylfuran-3(2H)-one (16#) and methylglyoxal (128#) were screened by molecular field-based similarity analysis, which tended to bind to the receptors OR1G1 and OR52H1 respectively, resulting a caramel-like aroma perception. The obtained results are useful for better understanding the perception of caramel-like odorants and their high-throughput screening.

Emergence and influence of sequence bias in evolutionarily malleable, mammalian tandem arrays

BACKGROUND

The radiation of mammals at the extinction of the dinosaurs produced a plethora of new forms-as diverse as bats, dolphins, and elephants-in only 10-20 million years. Behind the scenes, adaptation to new niches is accompanied by extensive innovation in large families of genes that allow animals to contact the environment, including chemosensors, xenobiotic enzymes, and immune and barrier proteins. Genes in these "outward-looking" families are allelically diverse among humans and exhibit tissue-specific and sometimes stochastic expression.

RESULTS

Here, we show that these tandem arrays of outward-looking genes occupy AT-biased isochores and comprise the "tissue-specific" gene class that lack CpG islands in their promoters. Models of mammalian genome evolution have not incorporated the sharply different functions and transcriptional patterns of genes in AT- versus GC-biased regions. To examine the relationship between gene family expansion, sequence content, and allelic diversity, we use population genetic data and comparative analysis. First, we find that AT bias can emerge during evolutionary expansion of gene families in cis. Second, human genes in AT-biased isochores or with GC-poor promoters experience relatively low rates of de novo point mutation today but are enriched for non-synonymous variants. Finally, we find that isochores containing gene clusters exhibit low rates of recombination.

CONCLUSIONS

Our analyses suggest that tolerance of non-synonymous variation and low recombination are two forces that have produced the depletion of GC bases in outward-facing gene arrays. In turn, high AT content exerts a profound effect on their chromatin organization and transcriptional regulation.

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).

Predictive value of abbreviated olfactory tests in prodromal Parkinson disease

There is disagreement in the literature whether olfaction may show specific impairments in Parkinson Disease (PD) and if olfactory tests comprised of selected odors could be more specific for diagnosis. We sought to validate previously proposed subsets of the University of Pennsylvania Smell Identification Test (UPSIT) odors for predicting conversion to PD in an independent, prodromal cohort. Conversion to PD was assessed in 229 participants in the Parkinson At Risk Study who completed baseline olfactory testing with the UPSIT and up to 12 years of clinical and imaging evaluations. No commercially available or proposed subset performed better than the full 40-item UPSIT. The proposed "PD-specific" subsets also did not perform better than expected by chance. We did not find evidence for selective olfactory impairment in Parkinson disease. Shorter odor identification tests, including commercially available 10-12 item tests, may have utility for ease of use and cost, but not for superior predictive value.

Acute Myeloid Leukemia Expresses a Specific Group of Olfactory Receptors

Acute myeloid leukemia (AML) is the most common form of acute leukemia in adults, with a 5-year overall survival rate of approximately 30%. Despite recent advances in therapeutic options, relapse remains the leading cause of death and poor survival outcomes. New drugs benefit specific small subgroups of patients with actionable therapeutic targets. Thus, finding new targets with greater applicability should be pursued. Olfactory receptors (ORs) are seven transmembrane G-protein coupled receptors preferentially expressed in sensory neurons with a critical role in recognizing odorant molecules. Recent studies have revealed ectopic expression and putative function of ORs in nonolfactory tissues and pathologies, including AML. Here, we investigated OR expression in 151 AML samples, 6400 samples of 15 other cancer types, and 11,200 samples of 51 types of healthy tissues. First, we identified 19 ORs with a distinct and major expression pattern in AML, which were experimentally validated by RT-PCR in an independent set of 13 AML samples, 13 healthy donors, and 8 leukemia cell lines. We also identified an OR signature with prognostic potential for AML patients. Finally, we found cancer-related genes coexpressed with the ORs in the AML samples. In summary, we conducted an extensive study to identify ORs that can be used as novel biomarkers for the diagnosis of AML and as potential drug targets.

Antagonistic interactions between odorants alter human odor perception

The olfactory system uses hundreds of odorant receptors (ORs), the largest group of the G-protein-coupled receptor (GPCR) superfamily, to detect a vast array of odorants. Each OR is activated by specific odorous ligands, and like other GPCRs, antagonism can block activation of ORs. Recent studies suggest that odorant antagonisms in mixtures influence olfactory neuron activities, but it is unclear how this affects perception of odor mixtures. In this study, we identified a set of human ORs activated by methanethiol and hydrogen sulfide, two potent volatile sulfur malodors, through large-scale heterologous expression. Screening odorants that block OR activation in heterologous cells identified a set of antagonists, including β-ionone. Sensory evaluation in humans revealed that β-ionone reduced the odor intensity and unpleasantness of methanethiol. Additionally, suppression was not observed when methanethiol and β-ionone were introduced simultaneously to different nostrils. Our study supports the hypothesis that odor sensation is altered through antagonistic interactions at the OR level.

Bitter Odorants and Odorous Bitters: Toxicity and Human TAS2R Targets

Flavor is perceived through the olfactory, taste, and trigeminal systems, mediated by designated GPCRs and channels. Signal integration occurs mainly in the brain, but some cross-reactivities occur at the receptor level. Here, we predict potential bitterness and taste receptors targets for thousands of odorants. BitterPredict and BitterIntense classifiers suggest that 3-9% of flavor and food odorants have bitter taste, but almost none are intensely bitter. About 14% of bitter molecules are expected to have an odor. Bitterness is more common for unpleasant smells such as fishy, amine, and ammoniacal, while non-bitter odorants often have pleasant smells. Experimental toxicity values suggest that fishy ammoniac smells are more toxic than pleasant smells, regardless of bitterness. TAS2R14 is predicted as the main bitter receptor for odorants, confirmed by in vitro profiling of 10 odorants. The activity of bitter odorants may have implications for physiology due to ectopic expression of taste and smell receptors.

Identification and Characterization of Proteins That Are Involved in RTP1S-Dependent Transport of Olfactory Receptors

Olfaction is mediated via olfactory receptors (ORs) that are expressed on the cilia membrane of olfactory sensory neurons in the olfactory epithelium. The functional expression of most ORs requires the assistance of receptor-transporting proteins (RTPs). We examined the interactome of RTP1S and OR via proximity biotinylation. Deubiquitinating protein VCIP135, the F-actin-capping protein sub-unit alpha-2, and insulin-like growth factor 2 mRNA-binding protein 2 were biotinylated via AirID fused with OR, RTP1S-AirID biotinylated heat shock protein A6 (HSPA6), and double-stranded RNA-binding protein Staufen homolog 2 (STAU2). Co-expression of HSPA6 partially enhanced the surface expression of Olfr544. The surface expression of Olfr544 increased by 50-80%. This effect was also observed when RTP1S was co-expressed. Almost identical results were obtained from the co-expression of STAU2. The interactions of HSPA6 and STAU2 with RTP1S were examined using a NanoBit assay. The results show that the RTP1S N-terminus interacted with the C-terminal domain of HSP6A and the N-terminal domain of STAU2. In contrast, OR did not significantly interact with STAU2 and HSPA6. Thus, HSP6A and STAU2 appear to be involved in the process of OR traffic through interaction with RTP1S.

Advanced investigation of a putative adsorption process of nine non key food odorants (non-KFOs) on the broadly tuned human olfactory receptor OR2W1: Statistical physics modeling and molecular docking study

In the present paper, statistical physics formalism was used to understand the olfactory perception via the investigation of dose-olfactory response curves of a putative adsorption process of nine non key food odorants (non-KFOs) on the broadly tuned human olfactory receptor OR2W1, in order to quantitative characterize the interactions between the nine studied non-KFOs, i. e., furfuryl sulfide, furfuryl disulfide, benzyl methyl disulfide, furfuryl methyl disulfide, benzyl methyl sulfide, 1-phenylethanethiol, benzyl mercaptan, furfuryl methyl sulfide and 3-phenylpropanol molecules and OR2W1 binding sites at a molecular level. Two advanced adsorption models have been proposed: the advanced monolayer monoenergy model (monolayer model with identical and independent olfactory receptor binding sites) (Model 1) and the advanced monolayer model with two independent types of olfactory receptor binding sites (Model 2). It was concluded that the monolayer monoenergy model was selected as the most adequate model to fit the experimental dose-olfactory response curves tabulated in literature. Actually, the numerical values of the three fitted physico-chemical parameters (RM1, n and C1) were obtained by a non-linear regression. Indeed, modeling results suggested that the number of docked non-KFOs per OR2W1 binding site n values (1.24 < n < 1.94) was always superior to 1, which indicated the non-parallel orientation of the studied odorants on the olfactory receptor and the multi-molecular adsorption mechanism. The estimated molar adsorption energy ΔEa values (ranged from 6.07 to 12.16 kJ/mol) for the nine olfactory systems confirmed the physical the exothermic characters of the adsorption process since ΔEa values were lower than 40 kJ/mol and positive. Furthermore, these estimated parameters were applied to characterize stereographically and energetically the interaction between the nine non-KFOs and OR2W1 through the determination of the human receptor binding site size distributions (RSDs) and the adsorption energy distributions (AEDs), which were spread out from 0.25 to 6.50 nm and from 0 to 22.50 kJ/mol, respectively. The docking computation between these nine non-KFOs and OR2W1 proved that the estimated binding affinities were belonged to the adsorption energies spectrum in general and the specific adsorption energy band or the molecular vibration modes limited spectrum (between 2.50 kJ/mol and 17 kJ/mol) (approximate olfactory band).

Allosteric modulation of a human odorant receptor

Odor perception is first determined by how the myriad of environmental volatiles are detected at the periphery of the olfactory system. The combinatorial activation of dedicated odorant receptors generates enough encoding power for the discrimination of tens of thousands of odorants. Recent studies have revealed that odorant receptors undergo widespread inhibitory modulation of their activity when presented with mixtures of odorants, a property likely required to maintain discrimination and ensure sparsity of the code for complex mixtures. Here, we establish the role of human OR5AN1 in the detection of musks and identify distinct odorants capable of enhancing its activity in binary mixtures. Chemical and pharmacological characterization indicate that specific α-β unsaturated aliphatic aldehydes act as positive allosteric modulators. Sensory experiments show decreased odor detection threshold in humans, suggesting that allosteric modulation of odorant receptors is perceptually relevant and likely adds another layer of complexity to how odors are encoded in the peripheral olfactory system.

Olfactory abstraction: a communicative and metacognitive account

The usual puzzle raised about olfaction is that of a deficit of abstraction: smells, by contrast notably with colours, do not easily lend themselves to abstract categories and labels. Some studies have argued that the puzzle is culturally restricted and that abstraction is more common outside urban Western societies. Here, I argue that the puzzle is misconstrued and should be reversed: given that odours are constantly changing and that their commonalities are difficult for humans to identify, what is surprising is not that abstract terms are rare, but that they should be used at all for olfaction. Given the nature of the olfactory environment and our cognitive equipment, concrete labels referring to sources seem most adaptive. To explain the use and presence of abstract terms, we need to examine their social and communicative benefits. Here these benefits are spelt out as securing a higher agreement among individuals varying in their olfactory experiences as well as the labels they use, as well as feeling a heightened sense of confidence in one's naming capacities. This article is part of the theme issue 'Concepts in interaction: social engagement and inner experiences'.

Genetic variation in the human olfactory receptor OR5AN1 associates with the perception of musks

Humans have significant individual variations in odor perception, derived from their experience or sometimes from differences in the olfactory receptor (OR) gene repertoire. In several cases, the genetic variation of a single OR affects the perception of its cognate odor ligand. Musks are widely used for fragrance and are known to demonstrate specific anosmia. It, however, remains to be elucidated whether the OR polymorphism contributes to individual variations in musk odor perception. Previous studies reported that responses of the human musk receptor OR5AN1 to a variety of musks in vitro correlated well with perceptual sensitivity to those odors in humans and that the mouse ortholog, Olfr1440 (MOR215-1), plays a critical role in muscone perception. Here, we took advantage of genetic variation in OR5AN1 to examine how changes in receptor sensitivity are associated with human musk perception. We investigated the functional differences between OR5AN1 variants in an in vitro assay and measured both perceived intensity and detection threshold in human subjects with different OR5AN1 genotypes. Human subjects homozygous for the more sensitive L289F allele had a lower detection threshold for muscone and found macrocyclic musks to be more intense than subjects homozygous for the reference allele. These results demonstrate that the genetic variation in OR5AN1 contributes to perceptual differences for some musks. In addition, we found that the more functional variant of OR5A1, a receptor involved in β-ionone perception, is associated with the less functional variant of OR5AN1, suggesting that the perceived intensities of macrocyclic musks and β-ionone are inversely correlated.

Genetic and functional odorant receptor variation in the Homo lineage

Humans, Neanderthals, and Denisovans independently adapted to a wide range of geographic environments and their associated food odors. Using ancient DNA sequences, we explored the in vitro function of thirty odorant receptor genes in the genus Homo. Our extinct relatives had highly conserved olfactory receptor sequence, but humans did not. Variations in odorant receptor protein sequence and structure may have produced variation in odor detection and perception. Variants led to minimal changes in specificity but had more influence on functional sensitivity. The few Neanderthal variants disturbed function, whereas Denisovan variants increased sensitivity to sweet and sulfur odors. Geographic adaptations may have produced greater functional variation in our lineage, increasing our olfactory repertoire and expanding our adaptive capacity. Our survey of olfactory genes and odorant receptors suggests that our genus has a shared repertoire with possible local ecological adaptations.

An odorant receptor that senses four classes of musk compounds

Musk was originally identified in male musk deer and other mammals to mark territories and attract females. In humans, musk compounds are widely used in perfumes and consumer products for their superior perceptual odor quality.^1,2,3,4,5 Strikingly diverse natural and synthetic chemicals have exhibited a similar "musky" odor, which has resulted in diverse models of musk odor perception and raises questions regarding the simplistic associations between chemical features and odor quality. Scientists' lack of understanding of this principle has hampered the design of a novel musk compound. Here, we functionally identified the odorant receptor, OR5A2, as a receptor for the musky odor of diverse musk compounds. First, we discovered that engineered OR5A2 with enhanced expression in heterologous cells is sensitive to and selective of musk compounds in all four structural classes. Second, the clarified functional variation of OR5A2 accounts for the reported association between genetic variation and perception in a musk compound. Finally, the revealed ligand selectivity of OR5A2 provides insight into developing a trained model to use machine learning-based virtual screening on candidates for a new musk compound. We propose that OR5A2 contributes to the long-sought gateway of sensing musk compounds and generating their unique odor quality.

Olfactory perception of 5α-androst-16-en-3-one: Data obtained in the residents of central Russia

This dataset describes olfactory perception of androstenone (5α-androst-16-en-3-one) in residents of central Russia (n=807, 9-84 years old). Based on previous studies, 3.13 × 10-3% androstenone was utilized for sensory testing. A modified two-alternative choice task with "no-difference" options was applied as an odor discrimination procedure. The participants rated the perceived odor intensity and pleasantness on 3-point categorical scales. The subsequent task was free odor identification. We used a survey to determine characteristics of our study population such as age, sex, ethnicity, olfactory abilities, smoking habits, respiratory tract diseases, perfume use, blood group. To examine sex differences in characteristics of participants, as well as in their perception of androstenone, we have applied one-way ANOVA for age variable and Chi-square tests for categorical variables. The dataset will be useful for assessing the impact of the aforementioned factors on the perception of volatile steroids. A wide range of researchers studying verbal descriptors of specific odors and cross-cultural differences in olfactory perception may reuse this dataset. Primarily, this dataset may benefit meat industry, as androstenone, which is perceived as an unpleasant odor by about a quarter of the population, significantly contributes to pork flavor.

Olfactory Receptors as an Emerging Chemical Sensing Scaffold

Chemical biosensors are an increasingly ubiquitous part of our lives. Beyond enzyme-coupled assays, recent synthetic biology advances now allow us to hijack more complex biosensing systems to respond to difficult to detect analytes, such as chemical small molecules. Here, we briefly overview recent advances in the biosensing of small molecules, including nucleic acid aptamers, allosteric transcription factors, and two-component systems. We then look more closely at a recently developed chemical sensing system, G protein-coupled receptor (GPCR)-based sensors. Finally, we consider the chemical sensing capabilities of the largest GPCR subfamily, olfactory receptors (ORs). We examine ORs' role in nature, their potential as a biomedical target, and their ability to detect compounds not amenable for detection using other biological scaffolds. We conclude by evaluating the current challenges, opportunities, and future applications of GPCR- and OR-based sensors.

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

Emerging targets signaling for inflammation in Parkinson's disease drug discovery

Parkinson's disease (PD) patients not only show motor features such as bradykinesia, tremor, and rigidity but also non-motor features such as anxiety, depression, psychosis, memory loss, attention deficits, fatigue, sexual dysfunction, gastrointestinal issues, and pain. Many pharmacological treatments are available for PD patients; however, these treatments are partially or transiently effective since they only decrease the symptoms. As these therapies are unable to restore dopaminergic neurons and stop the development of Parkinson's disease, therefore, the need for an effective therapeutic approach is required. The current review summarizes novel targets for PD, that can be utilized to identify disease-modifying treatments.

Effects of 3-octen-2-one on human olfactory receptor responses to vanilla flavor

Most of the odors that humans perceive daily are complex odors. It is believed that the modulation, enhancement, and suppression of overall complex odors are caused by interactions between odor molecules. In this study, to understand the interaction between odor molecules at the level of human olfactory receptor responses, the effects of 3-octen-2-one, which has been shown to modulate vanilla flavors, were analyzed using a human olfactory receptor sensor that uses all human olfactory receptors (388 types) as sensing molecules. As a result, the response intensity of 1 common receptor (OR1D2) was synergistically enhanced in vanilla flavor with 3-octen-2-one compared with vanilla flavor, and the response of 1 receptor (OR5K1) to vanilla flavor was completely suppressed. These results strongly suggested that the response of human olfactory receptors to complex odors is enhanced or suppressed by relatively few other odor molecules.

More than meets the AI: The possibilities and limits of machine learning in olfaction

Can machine learning crack the code in the nose? Over the past decade, studies tried to solve the relation between chemical structure and sensory quality with Big Data. These studies advanced computational models of the olfactory stimulus, utilizing artificial intelligence to mine for clear correlations between chemistry and psychophysics. Computational perspectives promised to solve the mystery of olfaction with more data and better data processing tools. None of them succeeded, however, and it matters as to why this is the case. This article argues that we should be deeply skeptical about the trend to black-box the sensory system's biology in our theories of perception. Instead, we need to ground both stimulus models and psychophysical data on real causal-mechanistic explanations of the olfactory system. The central question is: Would knowledge of biology lead to a better understanding of the stimulus in odor coding than the one utilized in current machine learning models? That is indeed the case. Recent studies about receptor behavior have revealed that the olfactory system operates by principles not captured in current stimulus-response models. This may require a fundamental revision of computational approaches to olfaction, including its psychological effects. To analyze the different research programs in olfaction, we draw on Lloyd's "Logic of Research Questions," a philosophical framework which assists scientists in explicating the reasoning, conceptual commitments, and problems of a modeling approach in question.

Humans and the Olfactory Environment: A Case of Gene-Culture Coevolution?

As hunter-gatherers, humans used their sense of smell to identify plants and animals, to find their way within a foraging area, or to distinguish each other by gender, age, kinship, or social dominance. Because women gathered while men hunted, the sexes evolved different sensitivities to plant and animal odors. They also ended up emitting different odors. Male odors served to intimidate rival males or assert dominance. With the rise of farming and sedentism, humans no longer needed their sense of smell to find elusive food sources or to orient themselves within a large area. Odors now came from a narrower range of plants and animals. Meanwhile, body odor was removed through bathing to facilitate interactions in enclosed spaces. This new phenotype became the template for the evolution of a new genotype: less sensitivity to odors of wild plants and animals, lower emissions of male odors, and a more negative response to them. Further change came with the development of fragrances to reodorize the body and the home. This new olfactory environment coevolved with the ability to represent odors in the mind, notably for storage in memory, for vicarious re-experiencing, or for sharing with other people through speech and writing.

Interactions among key residues regulate mammalian odorant receptor trafficking

Odorant receptors (ORs) expressed in mammalian olfactory sensory neurons are essential for the sense of smell. However, structure-function studies of many ORs are hampered by unsuccessful heterologous expression. To understand and eventually overcome this bottleneck, we performed heterologous expression and functional assays of over 80 OR variants and chimeras. Combined with literature data and machine learning, we found that the transmembrane domain 4 (TM4) and its interactions with neighbor residues are important for OR functional expression. The data highlight critical roles of T^4.62 therein. ORs that fail to reach the cell membrane can be rescued by modifications in TM4. Consequently, such modifications in MOR256-3 (Olfr124) also alter OR responses to odorants. T161^4.62 P causes the retention of MOR256-3 in the endoplasmic reticulum (ER), while T161^4.62 P/T148^4.49 A reverses the retention and makes receptor trafficking to cell membrane. This study offers new clues toward wide-range functional studies of mammalian ORs.

Functional analysis of human olfactory receptors with a high basal activity using LNCaP cell line

Humans use a family of more than 400 olfactory receptors (ORs) to detect odorants. However, deorphanization of ORs is a critical issue because the functional properties of more than 80% of ORs remain unknown, thus, hampering our understanding of the relationship between receptor function and perception. HEK293 cells are the most commonly used heterologous expression system to determine the function of a given OR; however, they cannot functionally express a majority of ORs probably due to a lack of factor(s) required in cells in which ORs function endogenously. Interestingly, ORs have been known to be expressed in a variety of cells outside the nose and play critical physiological roles. These findings prompted us to test the capacity of cells to functionally express a specific repertoire of ORs. In this study, we selected three cell lines that endogenously express functional ORs. We demonstrated that human prostate carcinoma (LNCaP) cell lines successfully identified novel ligands for ORs that were not recognized when expressed in HEK293 cells. Further experiments suggested that the LNCaP cell line was effective for functional expression of ORs, especially with a high basal activity, which impeded the sensitive detection of ligand-mediated activity of ORs. This report provides an efficient functional assay system for a specific repertoire of ORs that cannot be characterized in current cell systems.

From musk to body odor: Decoding olfaction through genetic variation

The olfactory system combines input from multiple receptor types to represent odor information, but there are few explicit examples relating olfactory receptor (OR) activity patterns to odor perception. To uncover these relationships, we performed genome-wide scans on odor-perception phenotypes for ten odors in 1000 Han Chinese and validated results for six of these odors in an ethnically diverse population (n = 364). In both populations, consistent with previous studies, we replicated three previously reported associations (β-ionone/OR5A1, androstenone/OR7D4, cis-3-hexen-1-ol/OR2J3 LD-band), but not for odors containing aldehydes, suggesting that olfactory phenotype/genotype studies are robust across populations. Two novel associations between an OR and odor perception contribute to our understanding of olfactory coding. First, we found a SNP in OR51B2 that associated with trans-3-methyl-2-hexenoic acid, a key component of human underarm odor. Second, we found two linked SNPs associated with the musk Galaxolide in a novel musk receptor, OR4D6, which is also the first human OR shown to drive specific anosmia to a musk compound. We noticed that SNPs detected for odor intensity were enriched with amino acid substitutions, implying functional changes of odor receptors. Furthermore, we also found that the derived alleles of the SNPs tend to be associated with reduced odor intensity, supporting the hypothesis that the primate olfactory gene repertoire has degenerated over time. This study provides information about coding for human body odor, and gives us insight into broader mechanisms of olfactory coding, such as how differential OR activation can converge on a similar percept.

Physico-chemical investigations of human olfactory receptors OR10G4 and OR2B11 activated by vanillin, ethyl vanillin, coumarin and quinoline molecules using statistical physics method

This research work is a contribution to understand the olfaction mechanism at a molecular level of vanillin, ethyl vanillin, coumarin and quinoline molecules using a modeling of a putative adsorption process by analytical model established by statistical physics formalism. A statistical physics modeling using the monolayer model with identical and independent binding sites of the responses of the two human olfactory receptors OR10G4 and OR2B11 showed that vanillin and quinoline were adsorbed with a mixed non-parallel and parallel orientation on OR10G4 and on OR2B11, respectively. However, ethyl vanillin and coumarin were anchored with a total non-parallel orientation. The adsorption energy values collected from data analysis, which were ranged from 12.51 to 20.91 kJ/mol, confirmed that the adsorption of vanillin and ethyl vanillin on OR10G4 and the adsorption of coumarin and quinoline on OR2B11were exothermic and were based on physical interactions. Furthermore, the dose-olfactory response curves of vanillin, ethyl vanillin, coumarin and quinoline provided access to OR10G4 and OR2B11 steric characterization via the calculation of the studied olfactory receptors site size distributions (RSDs). Indeed, vanillin, ethyl vanillin, coumarin and quinoline RSDs are spread from 0.3 to 12 nm, from 0.5 to 12 nm, from 0.40 to 12 nm and from 0.14 to 12 nm, respectively, with a maximum at 1.55 nm, 2.11 nm, 2.50 and 1.13 nm, respectively. Lastly, the physico-chemical model parameters can be used for the energetic characterization to confirm the physical nature of the vanillin/ethyl vanillin-OR10G4 and the coumarin/quinoline-OR2B11 interactions and to determine an olfactory band of order of 12 kJ/mol [11-23 kJ/mol], 10 kJ/mol [14-24 kJ/mol], 7 kJ/mol [9-16 kJ/mol], 15 kJ/mol [13-28 kJ/mol] for vanillin, ethyl vanillin, coumarin and quinoline, respectively, through the determination of the adsorption energy distributions (AEDs).

The key food odorant receptive range of broadly tuned receptor OR2W1

Mammals perceive a multitude of odorants by their chemical sense of olfaction, a high-dimensional stimulus-detection system, with hundreds of narrowly or broadly tuned receptors, enabling pattern recognition by the brain. Cognate receptor-agonist information, however, is sparse, and the role of broadly tuned odorant receptors for encoding odor quality remains elusive. Here, we screened IL-6-HaloTag®-OR2W1 and haplotypes against 187 out of 230 defined key food odorants using the GloSensor™ system in HEK-293 cells, yielding 48 new agonists. Altogether, key food odorants represent about two-thirds of now 153 reported agonists of OR2W1, the highest number of agonists known for a mammalian odorant receptor. In summary, we characterized OR2W1 as a human odorant receptor, with a chemically diverse but exclusive receptive range, complementary to chemical subgroups covered by evolutionary younger, highly selective receptors. Our data suggest OR2W1 to be suited for participating in the detection of many foodborne odorants.

Sniffing the human body volatile hexadecanal blocks aggression in men but triggers aggression in women

In terrestrial mammals, body volatiles can effectively trigger or block conspecific aggression. Here, we tested whether hexadecanal (HEX), a human body volatile implicated as a mammalian-wide social chemosignal, affects human aggression. Using validated behavioral paradigms, we observed a marked dissociation: Sniffing HEX blocked aggression in men but triggered aggression in women. Next, using functional brain imaging, we uncovered a pattern of brain activity mirroring behavior: In both men and women, HEX increased activity in the left angular gyrus, an area implicated in perception of social cues. HEX then modulated functional connectivity between the angular gyrus and a brain network implicated in social appraisal (temporal pole) and aggressive execution (amygdala and orbitofrontal cortex) in a sex-dependent manner consistent with behavior: increasing connectivity in men but decreasing connectivity in women. These findings implicate sex-specific social chemosignaling at the mechanistic heart of human aggressive behavior.

A review of environmental odor quantification and qualification methods: The question of objectivity in sensory analysis

For several years, various issues have up surged linked to odor nuisances with impacts on health and economic concerns. As awareness grew, recent development in instrumental techniques and sensorial analysis have emerged offering efficient and complementary approaches regarding environmental odor monitoring and control. While chemical analysis faces several obstacles, the sensory approach can help overcome them. Therefore, this latter may be considered as subjective, putting the reliability of the studies at risk. This paper is a review of the most commonly sensory methodology used for quantitative and qualitative environmental assessment of odor intensity (OI), odor concentration (OC), odor nature (ON) and hedonic tone (HT). For each of these odor dimensions, the assessment techniques are presented and compared: panel characteristics are discussed; laboratory and field studies are considered and the objectivity of the results is debated. For odor quantification, the use of a reference scale for OI assessment offers less subjectivity than other techniques but at the expense of ease-of-use. For OC assessment, the use of dynamic olfactometry was shown to be the least biased. For odor qualification, the ON description was less subjective when a reference-based lexicon was used but at the expense of simplicity, cost, and lesser panel-training requirements. Only when assessing HT was subjectivity an accepted feature because it reflects the impacted communities' acceptance of odorous emissions. For all discussed dimensions, field studies were shown to be the least biased due to the absence of air sampling, except for OC, where the dispersion modeling approach also showed great potential. In conclusion, this paper offers the reader a guide for environmental odor sensory analysis with the capacity to choose among different methods depending on the study nature, expectations, and capacities.

Machine Learning Assisted Approach for Finding Novel High Activity Agonists of Human Ectopic Olfactory Receptors

Olfactory receptors (ORs) constitute the largest superfamily of G protein-coupled receptors (GPCRs). ORs are involved in sensing odorants as well as in other ectopic roles in non-nasal tissues. Matching of an enormous number of the olfactory stimulation repertoire to its counterpart OR through machine learning (ML) will enable understanding of olfactory system, receptor characterization, and exploitation of their therapeutic potential. In the current study, we have selected two broadly tuned ectopic human OR proteins, OR1A1 and OR2W1, for expanding their known chemical space by using molecular descriptors. We present a scheme for selecting the optimal features required to train an ML-based model, based on which we selected the random forest (RF) as the best performer. High activity agonist prediction involved screening five databases comprising ~23 M compounds, using the trained RF classifier. To evaluate the effectiveness of the machine learning based virtual screening and check receptor binding site compatibility, we used docking of the top target ligands to carefully develop receptor model structures. Finally, experimental validation of selected compounds with significant docking scores through in vitro assays revealed two high activity novel agonists for OR1A1 and one for OR2W1.

Molecular reconstruction of recurrent evolutionary switching in olfactory receptor specificity

Olfactory receptor repertoires exhibit remarkable functional diversity, but how these proteins have evolved is poorly understood. Through analysis of extant and ancestrally reconstructed drosophilid olfactory receptors from the Ionotropic receptor (Ir) family, we investigated evolution of two organic acid-sensing receptors, Ir75a and Ir75b. Despite their low amino acid identity, we identify a common ‘hotspot’ in their ligand-binding pocket that has a major effect on changing the specificity of both Irs, as well as at least two distinct functional transitions in Ir75a during evolution. Moreover, we show that odor specificity is refined by changes in additional, receptor-specific sites, including those outside the ligand-binding pocket. Our work reveals how a core, common determinant of ligand-tuning acts within epistatic and allosteric networks of substitutions to lead to functional evolution of olfactory receptors.

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.

Breeding Tomato Hybrids for Flavour: Comparison of GWAS Results Obtained on Lines and F1 Hybrids

Tomato flavour is an important goal for breeders. Volatile organic compounds (VOCs) are major determinants of tomato flavour. Although most tomato varieties for fresh market are F1 hybrids, most studies on the genetic control of flavour-related traits are performed on lines. We quantified 46 VOCs in a panel of 121 small fruited lines and in a test cross panel of 165 hybrids (the previous panel plus 44 elite cherry tomato lines crossed with a common line). High and consistent heritabilities were assessed for most VOCs in the two panels, and 65% of VOC contents were strongly correlated between lines and hybrids. Additivity was observed for most VOCs. We performed genome wide association studies (GWAS) on the two panels separately, along with a third GWAS on the test cross subset carrying only F1 hybrids corresponding to the line panel. We identified 205, 183 and 138 associations, respectively. We identified numerous overlapping associations for VOCs belonging to the same metabolic pathway within each panel; we focused on seven chromosome regions with clusters of associations simultaneously involved in several key VOCs for tomato aroma. The study highlighted the benefit of testcross panels to create tasty F1 hybrid varieties.

Olfactory Optogenetics: Light Illuminates the Chemical Sensing Mechanisms of Biological Olfactory Systems

The mammalian olfactory system has an amazing ability to distinguish thousands of odorant molecules at the trace level. Scientists have made great achievements on revealing the olfactory sensing mechanisms in decades; even though many issues need addressing. Optogenetics provides a novel technical approach to solve this dilemma by utilizing light to illuminate specific part of the olfactory system; which can be used in all corners of the olfactory system for revealing the olfactory mechanism. This article reviews the most recent advances in olfactory optogenetics devoted to elucidate the mechanisms of chemical sensing. It thus attempts to introduce olfactory optogenetics according to the structure of the olfactory system. It mainly includes the following aspects: the sensory input from the olfactory epithelium to the olfactory bulb; the influences of the olfactory bulb (OB) neuron activity patterns on olfactory perception; the regulation between the olfactory cortex and the olfactory bulb; and the neuromodulation participating in odor coding by dominating the olfactory bulb. Finally; current challenges and future development trends of olfactory optogenetics are proposed and discussed.