Decoding and deorphanizing an olfactory map

Sequence-Based Prediction of Olfactory Receptor Responses

Computational prediction of how strongly an olfactory receptor (OR) responds to various odors can help in bridging the widening gap between the large number of receptors that have been sequenced and the small number of experiments measuring their responses. Previous efforts in this area have predicted the responses of a receptor to some odors, using the known responses of the same receptor to other odors. Here, we present a method to predict the responses of a receptor without any known responses by using available data about the responses of other conspecific receptors and their sequences. We applied this method to ORs in insects Drosophila melanogaster (both adult and larva) and Anopheles gambiae and to mouse and human ORs. We found the predictions to be in significant agreement with the experimental measurements. The method also provides clues about the response-determining positions within the receptor sequences.

Expansion of vomeronasal receptor genes (OlfC) in the evolution of fright reaction in Ostariophysan fishes

Ostariophysans are the most diverse group of freshwater fishes and feature a pheromone-elicited fright reaction. However, the genetic basis of fright reaction is unclear. Here, we compared vomeronasal type 2 receptor-like (OlfC) genes from fishes having and lacking fright reaction, to provide insight into evolution of pheromonal olfaction in fishes. We found OlfC genes expanded remarkably in ostariophysans having fright reaction compared with fishes lacking fright reaction. Phylogenetic analysis indicates OlfC subfamily 9 expanded specifically in ostariophysans having fright reaction. Principle component and phylogenetic logistic regression analysis partitioned fishes by ecotype (having or lacking fright reaction) and identified OlfC subfamily 9 as being an important factor for fright reaction. Expression levels of expanded OlfC subfamily genes after fright reaction in zebrafish changed more than did genes that had not expanded. Furthermore, evidence of positive selection was found in the expanded OlfC proteins in ostariophysan fishes having fright reaction. These results provide new insight into the genetic basis of fright reaction in ostariophysan fish and will enable future research into the mechanism of action of OlfC proteins.

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.

High-Throughput Odorant Receptor Deorphanization Via Phospho-S6 Ribosomal Protein Immunoprecipitation and mRNA Profiling

We describe an approach for the high-throughput surveying of odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs) that have been activated by specific odorants. When OSNs are activated, there is a molecular signature in the form of a phosphorylated-S6 (pS6) ribosomal subunit. By the immunoprecipitation of the protein-RNA complex containing pS6, we identify the OR mRNA species expressed in these activated OSNs. The one neuron - one receptor rule (mature OSN expresses a single unique OR) allows for the identification of the collection of ORs that responded toward the tested odorant. Here we detail the procedure of (1) odor stimulation, (2) tissue harvesting, (3) immunoprecipitation, and (4) mRNA profiling for the high-throughput deorphanization of ORs in vivo.

Conscious olfaction: Content, function, and localization

The target article's emphasis on olfaction is a welcome reminder of the multimodal nature of conscious experience. Here, I explore the distinctive and even unique attributes of our sense of smell from the point of view of their bearing on and fit with a subcortical locus of sensory experience.