Encoding of Odorants by Olfactory Sensory Neurons

The olfactory system relies on a combinatorial code where a given odorant receptor (OR) detects multiple odorants, and a given odorant is detected by multiple ORs (Malnic, Hirono et al. 1999). Prior attempts to decipher the code have emphasized linking genetic sequence to functional profile, but this approach has led to deorphanization of only ~85 out of ~1200 ORs in mouse (Zhang and Firestein 2007). With such a narrow window onto the combinatorial code, even the deorphaned ORs effectively remain stranded. High throughput calcium imaging of olfactory sensory neurons (OSNs) can provide the missing context. With this method, it is possible to survey the population response patterns while still preserving information on the individual receptive fields that contribute to the ensemble. I have used this technique to gain a more comprehensive view of the combinatorial code. Octanal is an odorant capable of recruiting many OSNs, but how functionally diverse are they? Screening with a panel of odorants made the subdivisions among this large suite of OSNs clear, revealing that nearly half uniquely parse the test panel. Expanding upon this, I show that such rare response patterns can be used like a fingerprint to assess, via physiology, that an OSN expresses a given OR. Population level analysis of the combinatorial code led me to two driving concepts. One is that the OR repertoire, despite its diversity, is nevertheless markedly constrained in its ability to discriminate certain series of odorants. For example, an OSN cannot respond to an alcohol and acid without also responding to an aldehyde. Exploring potential mechanisms, I used designer aldehydes that were trapped in an intermediate polar anchor state. I found that a previously discounted binding mode correlated with the ability of OSNs to selectively respond to aldehydes while excluding alcohols. The other key finding is that odorants can often adopt high energy conformations when activating OSNs. Initially, this was noted for aromatic odorants during a general screen. To probe the phenomenon in greater detail, I used a series of cyclized compounds that mimic rarely assumed states of the flexible tail of octanal. Comparing the activation strength of each analog to that elicited by unconstrained octanal demonstrated extensive co-recognition. This suggests that the flexibility of octanal contributes to its promiscuity in terms of recruiting a high number of OSNs. This study led to the realization that rings could often be treated as merely preserving a particular trajectory of a hydrocarbon backbone. Guided by this concept, I developed new panels with odorants that previously would have been considered discrepant. Hedione is an odorant where a ring imparts specialized geometry that greatly impacts perception. Yet at the OR combinatorial code level, I found that the ring was not critical and flexible but related odorants were still effective. I also demonstrated that OSNs readily accept odorants where an aromatic ring has been substituted with specific alkyl fragments. Thus, aromatic rings too, despite their unique electronics, are sometimes better viewed from a strictly architectural perspective. Using population analysis to identify what the ORs deem the important features of odorants can clarify the trends that sculpt the combinatorial code. This knowledge can help us consolidate seemingly broad receptive fields to better understand what information the OR repertoire extracts from the external chemical environment

A Painful Trp Can Be a Bonding Experience

The receptive field of the TRPA1 nociceptor is remarkably expansive when compared to other chemodetectors such as odorant receptors. The identification of a unique mechanism utilized by TRPA1 helps clarify how this protein can efficiently alert the cell to an array of reactive chemical agents, regardless of their structure

Functional odor classification through a medicinal chemistry approach

Crucial for any hypothesis about odor coding is the classification and prediction of sensory qualities in chemical compounds. The relationship between perceptual quality and molecular structure has occupied olfactory scientists throughout the 20th century, but details of the mechanism remain elusive. Odor molecules are typically organic compounds of low molecular weight that may be aliphatic or aromatic, may be saturated or unsaturated, and may have diverse functional polar groups. However, many molecules conforming to these characteristics are odorless. One approach recently used to solve this problem was to apply machine learning strategies to a large set of odors and human classifiers in an attempt to find common and unique chemical features that would predict a chemical’s odor. We use an alternative method that relies more on the biological responses of olfactory sensory neurons and then applies the principles of medicinal chemistry, a technique widely used in drug discovery. We demonstrate the effectiveness of this strategy through a classification for esters, an important odorant for the creation of flavor in wine. Our findings indicate that computational approaches that do not account for biological responses will be plagued by both false positives and false negatives and fail to provide meaningful mechanistic data. However, the two approaches used in tandem could resolve many of the paradoxes in odor perception

Applying medicinal chemistry strategies to understand odorant discrimination

Associating an odorant’s chemical structure with its percept is a long-standing challenge. One hindrance may come from the adoption of the organic chemistry scheme of molecular description and classification. Chemists classify molecules according to characteristics that are useful in synthesis or isolation, but which may be of little importance to a biological sensory system. Accordingly, we look to medicinal chemistry, which emphasizes biological function over chemical form, in an attempt to discern which among the many molecular features are most important for odour discrimination. Here we use medicinal chemistry concepts to assemble a panel of molecules to test how heteroaromatic ring substitution of the benzene ring will change the odour percept of acetophenone. This work allows us to describe an extensive rule in odorant detection by mammalian olfactory receptors. Whereas organic chemistry would have predicted the ring size and composition to be key features, our work reveals that the topological polar surface area is the key feature for the discrimination of these odorants

A Renal Olfactory Receptor Aids in Kidney Glucose Handling

International audienceOlfactory receptors (ORs) are G protein-coupled receptors which serve important sensory functions beyond their role as odorant detectors in the olfactory epithelium. Here we describe a novel role for one of these ORs, Olfr1393, as a regulator of renal glucose handling. Olfr1393 is specifically expressed in the kidney proximal tubule, which is the site of renal glucose reabsorption. Olfr1393 knockout mice exhibit urinary glucose wasting and improved glucose tolerance, despite euglycemia and normal insulin levels. Consistent with this phenotype, Olfr1393 knockout mice have a significant decrease in luminal expression of Sglt1, a key renal glucose transporter, uncovering a novel regulatory pathway involving Olfr1393 and Sglt1. In addition, by utilizing a large scale screen of over 1400 chemicals we reveal the ligand profile of Olfr1393 for the first time, offering new insight into potential pathways of physiological regulation for this novel signaling pathway. Olfactory receptors (ORs) are seven transmembrane domain G protein-coupled receptors (GPCRs) that serve as the chemical sensors of smell in the olfactory epithelium (OE). While these receptors were originally thought to be restricted to the nose 1 , it is now appreciated that ORs and other sensory receptors are found in a variety of other tissues where they play important physiological functions 2–9. We previously reported that at least 10 different ORs as well as their downstream signaling components (adenylate cyclase 3 and the olfactory G protein) are expressed in the kidney, and that one of these renal ORs contributes to blood pressure regulation 6,10. However, the functions of the remaining renal ORs have remained a mystery. In this study, we report for the first time the localization, ligand profile and physiological relevance of renal Olfactory Receptor 1393 (Olfr1393). We find that Olfr1393 localizes to all three segments of the renal proximal tubule, which is the site of renal glucose reabsorption. Typically, an individual's entire blood volume is filtered ~50 times/day, and because glucose is neither protein-bound nor complexed with macromolecules, it is freely filtered by the glomerulus 11,12. Under normal conditions, the proximal tubule reabsorbs the entirety of the ~180 grams of glucose filtered per day from the ultra-filtrate, such that no glucose is detected in the final urine. Glucose reabsorption in the proximal tubule is mediated by two apical sodium-glucose co-transporters: Sglt2 (SCL5A2) and Sglt1 (SLC5A1) 11–14. The low affinity , high-flux transporter, Sglt2, is localized to the apical membrane of the early proximal tubule (S1 and S2) and handles > 90% of all glucose reabsorption. The remaining glucose is cleared from the lumen by the high affinity, low-flux transporter, Sglt1, in the straight proximal tubule (S3). While these transporters have been extensively characterized and explored as potential drug targets for type II diabetes 11,15,16 , the understanding of their regulation within the proximal tubule is limited 17. Here, we report that Olfr1393 knockout (KO) mice present with euglycemic glycosuria and improved glucose tolerance. Consistent with this, we observe an altered distribution of Sglt1 in the proximal tubule of KO animals, implicating Olfr1393 as a novel contributor to renal glucose handling. Additionally, when we began these studies, Olfr1393 was an 'orphan' receptor with no known ligands; therefore, we undertook a ligand screen and identified 8 ligands for Olfr1393. In sum, these studies have uncovered a novel role for a renal OR in kidney glucose handling , and have identified a novel mechanism for potential physiologic regulation of Sglt1

A G protein/cAMP signal cascade is required for axonal convergence into olfactory glomeruli

The mammalian odorant receptors (ORs) comprise a large family of G protein-coupled receptors that are critical determinants of both the odorant response profile and the axonal identity of the olfactory sensory neurons in which they are expressed. Although the pathway by which ORs activate odor transduction is well established, the mechanism by which they direct axons into proper glomerular relationships remains unknown. We have developed a gain-of-function approach by using injection of retroviral vectors into the embryonic olfactory epithelium to study the ORs′ contribution to axon guidance. By ectopically expressing ORs, we demonstrate that functional OR proteins induce axonal coalescence. Furthermore, ectopic expression of Gα mutants reveals that activation of the signal transduction cascade is sufficient to cause axonal convergence into glomeruli. Analysis of Gα subunit expression indicates that development and odorant transduction use separate transduction pathways. Last, we establish that the generation of cAMP through adenylyl cyclase 3 is necessary to establish proper axonal identity. Our data point to a model in which axonal sorting is accomplished by OR stimulation of cAMP production by coupling to Gαs

Aldehyde Recognition and Discrimination by Mammalian Odorant Receptors via Functional Group-Specific Hydration Chemistry

The mammalian odorant receptors (ORs) form a chemical-detecting interface between the atmosphere and the nervous system. This large gene family is composed of hundreds of membrane proteins predicted to form as many unique small molecule binding niches within their G-protein coupled receptor (GPCR) framework, but very little is known about the molecular recognition strategies they use to bind and discriminate between small molecule odorants. Using rationally designed synthetic analogs of a typical aliphatic aldehyde, we report evidence that among the ORs showing specificity for the aldehyde functional group, a significant percentage detect the aldehyde through its ability to react with water to form a 1,1-<i>geminal</i> (<i>gem</i>)-diol. Evidence is presented indicating that the rat OR-I7, an often-studied and modeled OR known to require the aldehyde function of octanal for activation, is likely one of the <i>gem</i>-diol activated receptors. A homology model based on an activated GPCR X-ray structure provides a structural hypothesis for activation of OR-I7 by the <i>gem</i>-diol of octanal