Odorant-specific modes of signaling in mammalian olfaction

Position Review: Functional Selectivity in Mammalian Olfactory Receptors

There is increasing appreciation that G-protein-coupled receptors (GPCRs) can initiate diverse cellular responses by activating multiple G proteins, arrestins, and other biochemical effectors. Structurally different ligands targeting the same receptor are thought to stabilize the receptor in multiple distinct active conformations such that specific subsets of signaling effectors are engaged at the exclusion of others, creating a bias toward a particular outcome, which has been referred to as ligand-induced selective signaling, biased agonism, ligand-directed signaling, and functional selectivity, among others. The potential involvement of functional selectivity in mammalian olfactory signal transduction has received little attention, notwithstanding the fact that mammalian olfactory receptors comprise the largest family of mammalian GPCRs. This position review considers the possibility that, although such complexity in G-protein function may have been lost in the specialization of olfactory receptors to serve as sensory receptors, the ability of olfactory receptor neurons (ORNs) to function as signal integrators and growing appreciation that this functionality is widespread in the receptor population suggest otherwise. We pose that functional selectivity driving 2 opponent inputs have the potential to generate an output that reflects the balance of ligand-dependent signaling, the direction of which could be either suppressive or synergistic and, as such, needs to be considered as a mechanistic basis for signal integration in mammalian ORNs.

Perinatal exposure to diets with different n-6:n-3 fatty acid ratios affects olfactory tissue fatty acid composition

The olfactory mucosa (OM) and the olfactory bulb (OB) are responsible for the detection and processing of olfactory signals. Like the brain and retina, they contain high levels of n-3 and n-6 polyunsaturated fatty acids (PUFAs), which are essential for the structure and function of neuronal and non-neuronal cells. Since the influence of the maternal diet on olfactory lipid profiles of the offspring has been poorly explored, we examined the effects of feeding mice during the perinatal period with diets containing an adequate linoleic acid level but either deficient in α-linolenic acid (ALA) or supplemented in n-3 long-chain PUFAs on the lipid composition of dams and weaning offspring olfactory tissues. In both the OM and OB, the low n-3 ALA diet led to a marked reduction in n-3 PUFAs with a concomitant increase in n-6 PUFAs, whereas consumption of the high n-3 PUFA diet reduced n-6 PUFAs and increased n-3 PUFAs. Structural analysis showed that the molecular species profiles of the main phospholipid classes of olfactory tissues from weaning pups were markedly affected by the maternal diets. This study demonstrates that the PUFA status of olfactory tissues is sensitive to diet composition from the early stages of development.

Therapeutic potential of ectopic olfactory and taste receptors

Olfactory and taste receptors are expressed primarily in the nasal olfactory epithelium and gustatory taste bud cells, where they transmit real-time sensory signals to the brain. However, they are also expressed in multiple extra-nasal and extra-oral tissues, being implicated in diverse biological processes including sperm chemotaxis, muscle regeneration, bronchoconstriction and bronchodilatation, inflammation, appetite regulation and energy metabolism. Elucidation of the physiological roles of these ectopic receptors is revealing potential therapeutic and diagnostic applications in conditions including wounds, hair loss, asthma, obesity and cancers. This Review outlines current understanding of the diverse functions of ectopic olfactory and taste receptors and assesses their potential to be therapeutically exploited.

Odor-evoked inhibition of olfactory sensory neurons drives olfactory perception in Drosophila

Inhibitory response occurs throughout the nervous system, including the peripheral olfactory system. While odor-evoked excitation in peripheral olfactory cells is known to encode odor information, the molecular mechanism and functional roles of odor-evoked inhibition remain largely unknown. Here, we examined Drosophila olfactory sensory neurons and found that inhibitory odors triggered outward receptor currents by reducing the constitutive activities of odorant receptors, inhibiting the basal spike firing in olfactory sensory neurons. Remarkably, this odor-evoked inhibition of olfactory sensory neurons elicited by itself a full range of olfactory behaviors from attraction to avoidance, as did odor-evoked olfactory sensory neuron excitation. These results indicated that peripheral inhibition is comparable to excitation in encoding sensory signals rather than merely regulating excitation. Furthermore, we demonstrated that a bidirectional code with both odor-evoked inhibition and excitation in single olfactory sensory neurons increases the odor-coding capacity, providing a means of efficient sensory encoding.

It is well established that odor-evoked excitation in olfactory sensory neurons (OSNs) encodes odor information. Here the authors report that odor-evoked inhibition in OSNs of Drosophila also encodes odor identity, and can in itself drive both attraction and avoidance behaviors.

Evidence for a shape-based recognition of odorants in vivo in the human nose from an analysis of the molecular mechanism of lily-of-the-valley odorants detection in the Lilial and Bourgeonal family using the C/Si/Ge/Sn switch strategy

We performed an analysis of possible mechanisms of ligand recognition in the human nose. The analysis is based on in vivo odor threshold determination and in vitro Ca2+ imaging assays with a C/Si/Ge/Sn switch strategy applied to the compounds Lilial and Bourgeonal, to differentiate between different molecular mechanisms of odorant detection. Our results suggest that odorant detection under threshold conditions is mainly based on the molecular shape, i.e. the van der Waals surface, and electrostatics of the odorants. Furthermore, we show that a single olfactory receptor type is responsible for odor detection of Bourgeonal at the threshold level in humans in vivo. Carrying out a QM analysis of vibrational energies contained in the odorants, there is no evidence for a vibration-based recognition.

Helional induces Ca2+ decrease and serotonin secretion of QGP-1 cells via a PKG-mediated pathway

The secretion, motility and transport by intestinal tissues are regulated among others by specialized neuroendocrine cells, the so-called enterochromaffin (EC) cells. These cells detect different luminal stimuli, such as mechanical stimuli, fatty acids, glucose and distinct chemosensory substances. The EC cells react to the changes in their environment through the release of transmitter molecules, most importantly serotonin, to mediate the corresponding physiological response. However, little is known about the molecular targets of the chemical stimuli delivered from consumed food, spices and cosmetics within EC cells. In this study, we evaluated the expression of the olfactory receptor (OR) 2J3 in the human pancreatic EC cell line QGP-1 at the mRNA and protein levels. Using ratiofluorometric Ca(2+) imaging experiments, we demonstrated that the OR2J3-specific agonist helional induces a transient dose-dependent decrease in the intracellular Ca(2+) levels. This Ca(2+) decrease is mediated by protein kinase G (PKG) on the basis that the specific pharmacological inhibition of PKG with Rp-8-pCPT-cGMPS abolished the helional-induced Ca(2+) response. Furthermore, stimulation of QGP-1 cells with helional caused a dose-dependent release of serotonin that was comparable with the release induced by the application of a direct PKG activator (8-bromo-cGMP). Taken together, our results demonstrate that luminal odorants can be detected by specific ORs in QGP-1 cells and thus cause the directed release of serotonin and a PKG-dependent decrease in intracellular Ca(2.)

Intranasal Oncolytic Virotherapy with CXCR4-Enhanced Stem Cells Extends Survival in Mouse Model of Glioma

The challenges to effective drug delivery to brain tumors are twofold: (1) there is a lack of non-invasive methods of local delivery and (2) the blood-brain barrier limits systemic delivery. Intranasal delivery of therapeutics to the brain overcomes both challenges. In mouse model of malignant glioma, we observed that a small fraction of intranasally delivered neural stem cells (NSCs) can migrate to the brain tumor site. Here, we demonstrate that hypoxic preconditioning or overexpression of CXCR4 significantly enhances the tumor-targeting ability of NSCs, but without altering their phenotype only in genetically modified NSCs. Modified NSCs deliver oncolytic virus to glioma more efficiently and extend survival of experimental animals in the context of radiotherapy. Our findings indicate that intranasal delivery of stem cell-based therapeutics could be optimized for future clinical applications, and allow for safe and repeated administration of biological therapies to brain tumors and other CNS disorders.

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Intranasal delivery of NSCs is a promising platform for glioma therapy

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Hypoxia or CXCR4 overexpression enhances NSC migration to glioma

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Oncolytic viruses loaded in CXCR4-enhanced NSCs improve animal survival

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Non-invasive intranasal NSC-based therapies warrant clinical translation

Phosphoinositide-3-Kinase Is the Primary Mediator of Phosphoinositide-Dependent Inhibition in Mammalian Olfactory Receptor Neurons

Odorants inhibit as well as excite primary olfactory receptor neurons (ORNs) in many animal species. Growing evidence suggests that inhibition of mammalian ORNs is mediated by phosphoinositide (PI) signaling through activation of phosphoinositide 3-kinase (PI3K), and that canonical adenylyl cyclase III signaling and PI3K signaling interact to provide the basis for ligand-induced selective signaling. As PI3K is known to act in concert with phospholipase C (PLC) in some cellular systems, the question arises as to whether they work together to mediate inhibitory transduction in mammalian ORNs. The present study is designed to test this hypothesis. While we establish that multiple PLC isoforms are expressed in the transduction zone of rat ORNs, that odorants can activate PLC in ORNs in situ, and that pharmacological blockade of PLC enhances the excitatory response to an odorant mixture in some ORNs in conjunction with PI3K blockade, we find that by itself PLC does not account for an inhibitory response. We conclude that PLC does not make a measurable independent contribution to odor-evoked inhibition, and that PI3K is the primary mediator of PI-dependent inhibition in mammalian ORNs.

Identification of a Novel Gnao-Mediated Alternate Olfactory Signaling Pathway in Murine OSNs

It is generally agreed that in olfactory sensory neurons (OSNs), the binding of odorant molecules to their specific olfactory receptor (OR) triggers a cAMP-dependent signaling cascade, activating cyclic-nucleotide gated (CNG) channels. However, considerable controversy dating back more than 20 years has surrounded the question of whether alternate signaling plays a role in mammalian olfactory transduction. In this study, we demonstrate a specific alternate signaling pathway in Olfr73-expressing OSNs. Methylisoeugenol (MIEG) and at least one other known weak Olfr73 agonist (Raspberry Ketone) trigger a signaling cascade independent from the canonical pathway, leading to the depolarization of the cell. Interestingly, this pathway is mediated by Gnao activation, leading to Cl(-) efflux; however, the activation of adenylyl cyclase III (ACIII), the recruitment of Ca(2+) from extra-or intracellular stores, and phosphatidylinositol 3-kinase-dependent signaling (PI signaling) are not involved. Furthermore, we demonstrated that our newly identified pathway coexists with the canonical olfactory cAMP pathway in the same OSN and can be triggered by the same OR in a ligand-selective manner. We suggest that this pathway might reflect a mechanism for odor recognition predominantly used in early developmental stages before olfactory cAMP signaling is fully developed. Taken together, our findings support the existence of at least one odor-induced alternate signal transduction pathway in native OSNs mediated by Olfr73 in a ligand-selective manner.

Contrasting responses within a single neuron class enable sex-specific attraction in Caenorhabditis elegans

Animals find mates and food, and avoid predators, by navigating to regions within a favorable range of available sensory cues. How are these ranges set and recognized? Here we show that male Caenorhabditis elegans exhibit strong concentration preferences for sex-specific small molecule cues secreted by hermaphrodites, and that these preferences emerge from the collective dynamics of a single male-specific class of neurons, the cephalic sensory neurons (CEMs). Within a single worm, CEM responses are dissimilar, not determined by anatomical classification and can be excitatory or inhibitory. Response kinetics vary by concentration, suggesting a mechanism for establishing preferences. CEM responses are enhanced in the absence of synaptic transmission, and worms with only one intact CEM show nonpreferential attraction to all concentrations of ascaroside for which CEM is the primary sensor, suggesting that synaptic modulation of CEM responses is necessary for establishing preferences. A heterogeneous concentration-dependent sensory representation thus appears to allow a single neural class to set behavioral preferences and recognize ranges of sensory cues.

Phosphoinositide and Erk signaling pathways mediate activity-driven rodent olfactory sensory neuronal survival and stress mitigation

Olfactory sensory neurons (OSNs) are the initial site for olfactory signal transduction. Therefore, their survival is essential to olfactory function. In the current study, we demonstrated that while odorant stimulation promoted rodent OSN survival, it induced generation of reactive oxygen species in a dose- and time-dependent manner as well as loss of membrane potential and fragmentation of mitochondria. The MEK-Erk pathway played a critical role in mediating these events, as its inhibition decreased odorant stimulation-dependent OSN survival and exacerbated intracellular stress measured by reactive oxygen species generation and heat-shock protein 70 expression. The phosphoinositide pathway, rather than the cyclic AMP pathway, mediated the odorant-induced activation of the MEK-Erk pathway. These findings provide important insights into the mechanisms of activity-driven OSN survival, the role of the phosphoinositide pathway in odorant signaling, and demonstrate that odorant detection and odorant stimulation-mediated survival proceed via independent signaling pathways. This mechanism, which permits independent regulation of odorant detection from survival signaling, may be advantageous if not diminished by repeated or prolonged odor exposure. We investigated the role of odorant stimulation in generating cellular stress and the molecular mechanisms mitigating such stress and promoting neuronal survival. Odorant stimulation promoted olfactory sensory neuron (OSN) survival and also induced intracellular oxidative stress, which was exacerbated when MEK/Erks pathway was inhibited. Sensory stimulation simultaneously activated at least two parallel pathways, the AC/cAMP cascade responsible for odorant detection, and phosphoinositide hydrolysis to promote odorant stimulation-dependent neuronal survival odorants may activate parallel signaling cascades to mediate sensory detection and sensory stimulation-dependent survival. AC, adenylyl cyclase; cAMP, cyclic adenosine monophosphate; Erk, extracellular signal-regulated kinase; MEK, MAPK/ERK kinase.

Characterizing olfactory binary mixture interactions in Fischer 344 rats using behavioral reaction times

Response times provide essential subthreshold perceptual data that extend beyond accuracy alone. Behavioral reaction times (RTs) were used to characterize rats' ability to detect individual odorants in a series of complimentary binary odorant mixture ratios. We employed an automated, liquid-dilution olfactometer to train Fischer 344 rats (N = 8) on an odor identification task using nonreinforced probe trials. Binary mixture ratios composed of aliphatic odorants (citral and octanol) were arranged such that relative contributions of the 2 components varied systematically by a factor of 1% (v/v). Odorant concentrations for the target (S+), control (S-), and mixture (S+:S-) odorants were presented relative to threshold for each rat. Rats were initially trained to respond by licking at a spout to obtain liquid reward for either citral or octanol as the reinforced target (S+) odorant. After achieving 100% accuracy, rats were transferred to variable ratio (VR 2) reinforcement for correct responding. Nonreinforced probe trials (2 per block of 22 trials) were tested for each mixture ratio and recorded as either S+ (rats lick-responded in the presence of the mixture) or S- (rats refrained from licking), thereby indicating detection of the trained, S+ odorant. To determine the perceived salience for each ratio, RTs (latency from odorant onset to lick response) were recorded for each trial. Consistent with previous studies, RTs for both odorants were shortest (~150-200ms) when the probe trials consisted of a single, monomolecular component. Binary mixtures that contained as little as 1% of the S-, nontarget odorant, however, were sufficiently different perceptually to increase behavioral RTs (i.e., rats hesitated longer before responding); RTs changed systematically as a function of the binary ratio. Interestingly, the rate of RT change was dependent on which odorant served as the S+, suggesting an asymmetric interaction between the 2 odorants. The data demonstrate the value of behavioral RT as a sensitive measure of suprathreshold perceptual responding.

Deep sequencing of the murine olfactory receptor neuron transcriptome

The ability of animals to sense and differentiate among thousands of odorants relies on a large set of olfactory receptors (OR) and a multitude of accessory proteins within the olfactory epithelium (OE). ORs and related signaling mechanisms have been the subject of intensive studies over the past years, but our knowledge regarding olfactory processing remains limited. The recent development of next generation sequencing (NGS) techniques encouraged us to assess the transcriptome of the murine OE. We analyzed RNA from OEs of female and male adult mice and from fluorescence-activated cell sorting (FACS)-sorted olfactory receptor neurons (ORNs) obtained from transgenic OMP-GFP mice. The Illumina RNA-Seq protocol was utilized to generate up to 86 million reads per transcriptome. In OE samples, nearly all OR and trace amine-associated receptor (TAAR) genes involved in the perception of volatile amines were detectably expressed. Other genes known to participate in olfactory signaling pathways were among the 200 genes with the highest expression levels in the OE. To identify OE-specific genes, we compared olfactory neuron expression profiles with RNA-Seq transcriptome data from different murine tissues. By analyzing different transcript classes, we detected the expression of non-olfactory GPCRs in ORNs and established an expression ranking for GPCRs detected in the OE. We also identified other previously undescribed membrane proteins as potential new players in olfaction. The quantitative and comprehensive transcriptome data provide a virtually complete catalogue of genes expressed in the OE and present a useful tool to uncover candidate genes involved in, for example, olfactory signaling, OR trafficking and recycling, and proliferation.

Mammalian olfactory receptors: molecular mechanisms of odorant detection, 3D-modeling, and structure-activity relationships

This chapter describes the main characteristics of olfactory receptor (OR) genes of vertebrates, including generation of this large multigenic family and pseudogenization. OR genes are compared in relation to evolution and among species. OR gene structure and selection of a given gene for expression in an olfactory sensory neuron (OSN) are tackled. The specificities of OR proteins, their expression, and their function are presented. The expression of OR proteins in locations other than the nasal cavity is regulated by different mechanisms, and ORs display various additional functions. A conventional olfactory signal transduction cascade is observed in OSNs, but individual ORs can also mediate different signaling pathways, through the involvement of other molecular partners and depending on the odorant ligand encountered. ORs are engaged in constitutive dimers. Ligand binding induces conformational changes in the ORs that regulate their level of activity depending on odorant dose. When present, odorant binding proteins induce an allosteric modulation of OR activity. Since no 3D structure of an OR has been yet resolved, modeling has to be performed using the closest G-protein-coupled receptor 3D structures available, to facilitate virtual ligand screening using the models. The study of odorant binding modes and affinities may infer best-bet OR ligands, to be subsequently checked experimentally. The relationship between spatial and steric features of odorants and their activity in terms of perceived odor quality are also fields of research that development of computing tools may enhance.

2,4,6-trichloroanisole is a potent suppressor of olfactory signal transduction

We investigated the sensitivity of single olfactory receptor cells to 2,4,6-trichloroanisole (TCA), a compound known for causing cork taint in wines. Such off-flavors have been thought to originate from unpleasant odor qualities evoked by contaminants. However, we here show that TCA attenuates olfactory transduction by suppressing cyclic nucleotide-gated channels, without evoking odorant responses. Surprisingly, suppression was observed even at extremely low (i.e., attomolar) TCA concentrations. The high sensitivity to TCA was associated with temporal integration of the suppression effect. We confirmed that potent suppression by TCA and similar compounds was correlated with their lipophilicity, as quantified by the partition coefficient at octanol/water boundary (pH 7.4), suggesting that channel suppression is mediated by a partitioning of TCA into the lipid bilayer of plasma membranes. The rank order of suppression matched human recognition of off-flavors: TCA equivalent to 2,4,6-tribromoanisole, which is much greater than 2,4,6-trichlorophenol. Furthermore, TCA was detected in a wide variety of foods and beverages surveyed for odor losses. Our findings demonstrate a potential molecular mechanism for the reduction of flavor.

Phosphoinositide 3-kinase dependent inhibition as a broad basis for opponent coding in Mammalian olfactory receptor neurons

Phosphoinositide 3-kinase (PI3K) signaling has been implicated in mediating inhibitory odorant input to mammalian olfactory receptor neurons (ORNs). To better understand the breadth of such inhibition in odor coding, we screened a panel of odorants representing different chemical classes, as well as odorants known to occur in a natural odor object (tomato), for their ability to rapidly activate PI3K-dependent inhibitory signaling. Odorants were screened on dissociated native rat ORNs before and after pre-incubation with the PI3K-isoform specific blockers AS252424 and TGX221. Many different odorants increased their excitatory strength for particular ORNs following PI3K blockade in a manner consistent with activating PI3K-dependent inhibitory signaling in those cells. The PI3K-dependent inhibitory odorants overlapped with conventional excitatory odorants, but did not share the same bias, indicating partial partitioning of the odor space. Finding that PI3K-dependent inhibition can be activated by a wide range of otherwise conventional excitatory odorants strongly implies PI3K-dependent inhibition provides a broad basis for opponent coding in mammalian ORNs.

Biochemical Evidence for a Putative Inositol 1,3,4,5-Tetrakisphosphate Receptor in the Olfactory System of Atlantic Salmon (Salmo salar)

Olfactory receptor neurons in Atlantic salmon (Salmo salar) appear to use a phosphoinositide-directed phospholipase C (PLC) in odorant signal transduction. The consequences of odor-activated PLC depend on its product, inositol 1,4,5-trisphosphate (IP₃). Therefore, a plasma membrane rich (PMR) fraction, previously characterized from salmon olfactory rosettes, was used to study binding sites for IP₃ and its phosphorylation product, inositol 1,3,4,5-tetrakisphosphate (IP₄). Binding sites for IP₃ were present at the lower limit for detection in the PMR fraction but were abundant in a microsomal fraction. Binding sites for IP₄ were abundant in the PMR fraction and thus colocalized in the same subcellular fraction with odorant receptors for amino acids and bile acids. Binding of IP₄ was saturable and high affinity (K_d = 83 nM). The rank order for potency of inhibition of IP₄ by other inositol polyphosphates (InsP_x) followed the phosphorylation number with InsP₆ > InsP₅ > other InsP₄ isomers > InsP₃ isomers > InsP₂ isomers, with the latter showing no activity. The consequences of PLC activity in this system may be dictated in part by a putative receptor for IP₄.

Phosphoinositide 3-kinase-dependent antagonism in mammalian olfactory receptor neurons

Phosphoinositide signaling, in particular, phosphoinositide 3-kinase (PI3K) signaling, has been implicated in mediating inhibitory odorant input to mammalian olfactory receptor neurons (ORNs). To better understand this phenomenon we investigated PI3K-dependent inhibition between single odorant pairs. The concentration-dependent inhibition of the response of native rat ORNs to octanol by citral is PI3K dependent; blocking PI3K activity with the β and γ isoform-specific inhibitors AS252424 (5-[5-(4-fluoro-2-hydroxy-phenyl)-furan-2-ylmethylene]-thiazolidine-2,4-dione) and TGX221(7-methyl-2-(4-morpholinyl)-9-[1-(phenylamino)ethyl]-4H-pyrido [1,2-a]pyrimidin-4-one) eliminated or strongly reduced the inhibition. Interestingly, blocking PI3K also changed the apparent agonist strength of the otherwise noncompetitive antagonist citral. The excitation evoked by citral after blocking PI3K, could be suppressed by the adenylate cyclase III (ACIII) blockers MDL12330A (cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine hydrochloride) and SQ22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine], indicating that citral could also activate ACIII, presumably through the canonical olfactory receptor (OR). The G-protein G(β)γ subunit blockers suramin (8,8'-[carbonylbis[imino-3,1-phenylen ecarbonylimino(4-methyl-3,1-phenylene)carbonylimino]]bis-1,3,5-naphthalenetrisulfonic acid), gallein (3',4',5',6'-tetrahydroxyspiro[isobenzofuran-1(3H),9'-(9H)xanthen]-3-one), and M119 (cyclohexanecarboxylic acid [2-(4,5,6-trihydroxy-3-oxo-3H-xanthen-9-yl)-(9CI)]) suppressed citral's inhibition of the response to octanol, indicating that the activation of PI3K by citral was G-protein dependent, consistent with the idea that inhibition acts via the canonical OR. Lilial similarly antagonized the response to isoamyl acetate in other ORNs, indicating the effect generalizes to at least one other odorant pair. The ability of methyl-isoeugenol, limonene, α-pinene, isovaleric acid, and isosafrole to inhibit the response of other ORNs to IBMX (3-isobutyl-1-methylxanthine)/forskolin in a PI3K-dependent manner argues the effect generalizes to yet other structurally dissimilar odorants. Our findings collectively raise the interesting possibility that the OR serves as a molecular logic gate when mammalian ORNs are activated by natural, complex mixtures containing both excitatory and inhibitory odorants.