Signaling by olfactory receptor neurons near threshold

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.

Nonreciprocal interactions give rise to fast cilium synchronization in finite systems

Motile cilia beat in an asymmetric fashion in order to propel the surrounding fluid. When many cilia are located on a surface, their beating can synchronize such that their phases form metachronal waves. Here, we computationally study a model where each cilium is represented as a spherical particle, moving along a tilted trajectory with a position-dependent active driving force and a position-dependent internal drag coefficient. The model thus takes into account all the essential broken symmetries of the ciliary beat. We show that taking into account the near-field hydrodynamic interactions, the effective coupling between cilia even over an entire beating cycle can become nonreciprocal: The phase of a cilium is more strongly affected by an adjacent cilium on one side than by a cilium at the same distance in the opposite direction. As a result, synchronization starts from a seed at the edge of a group of cilia and propagates rapidly across the system, leading to a synchronization time that scales proportionally to the linear dimension of the system. We show that a ciliary carpet is characterized by three different velocities: the velocity of fluid transport, the phase velocity of metachronal waves, and the group velocity of order propagation. Unlike in systems with reciprocal coupling, boundary effects are not detrimental for synchronization, but rather enable the formation of the initial seed.

General Chemical Reaction Network Theory for Olfactory Sensing Based on G-Protein-Coupled Receptors: Elucidation of Odorant Mixture Effects and Agonist-Synergist Threshold

This work presents a general chemical reaction network theory for olfactory sensing processes that employ G-protein-coupled receptors as olfactory receptors (ORs). The theory can be applied to general mixtures of odorants and an arbitrary number of ORs. Reactions of ORs with G-proteins, in both the presence and absence of odorants, are explicitly considered. A unique feature of the theory is the definition of an odor activity vector consisting of strengths of odorant-induced signals from ORs relative to those due to background G-protein activity in the absence of odorants. It is demonstrated that each component of the odor activity defined this way reduces to a Michaelis-Menten form capable of accounting for cooperation or competition effects between different odorants. The main features of the theory are illustrated for a two-odorant mixture. Known and potential mixture effects, such as suppression, shadowing, inhibition, and synergy, are quantitatively described. Effects of relative values of rate constants, basal activity, and G-protein concentration are also demonstrated.

Determination of the nanoscale electrical properties of olfactory receptor hOR1A1 and their dependence on ligand binding: Towards the development of capacitance-operated odorant biosensors

The transduction of odorant binding into cellular signaling by olfactory receptors (ORs) is not understood and knowing its mechanism would enable developing new pharmacology and biohybrid electronic detectors of volatile organic compounds bearing high sensitivity and selectivity. The electrical characterization of ORs in bulk experiments is subject to microscopic models and assumptions. We have directly determined the nanoscale electrical properties of ORs immobilized in a fixed orientation, and their change upon odorant binding, using electrochemical scanning tunneling microscopy (EC-STM) in near-physiological conditions. Recordings of current versus time, distance, and electrochemical potential allows determining the OR impedance parameters and their dependence with odorant binding. Our results allow validating OR structural-electrostatic models and their functional activation processes, and anticipating a novel macroscopic biosensor based on ORs.

Low signaling efficiency from receptor to effector in olfactory transduction: A quantified ligand-triggered GPCR pathway

G protein-coupled receptor (GPCR) signaling is ubiquitous. As an archetype of this signaling motif, rod phototransduction has provided many fundamental, quantitative details, including a dogma that one active GPCR molecule activates a substantial number of downstream G protein/enzyme effector complexes. However, rod phototransduction is light-activated, whereas GPCR pathways are predominantly ligand-activated. Here, we report a detailed study of the ligand-triggered GPCR pathway in mammalian olfactory transduction, finding that an odorant-receptor molecule when (one-time) complexed with its most effective odorants produces on average much less than one downstream effector. Further experiments gave a nominal success probability of tentatively ∼10-4 (more conservatively, ∼10-2 to ∼10-5). This picture is potentially more generally representative of GPCR signaling than is rod phototransduction, constituting a paradigm shift.

Sensory Transduction in Photoreceptors and Olfactory Sensory Neurons: Common Features and Distinct Characteristics

The past decades have seen tremendous progress in our understanding of the function of photoreceptors and olfactory sensory neurons, uncovering the mechanisms that determine their properties and, ultimately, our ability to see and smell. This progress has been driven to a large degree by the powerful combination of physiological experimental tools and genetic manipulations, which has enabled us to identify the main molecular players in the transduction cascades of these sensory neurons, how their properties affect the detection and discrimination of stimuli, and how diseases affect our senses of vision and smell. This review summarizes some of the common and unique features of photoreceptors and olfactory sensory neurons that make these cells so exciting to study.

Ciliary chemosensitivity is enhanced by cilium geometry and motility

Cilia are hairlike organelles involved in both sensory functions and motility. We discuss the question of whether the location of chemical receptors on cilia provides an advantage in terms of sensitivity and whether motile sensory cilia have a further advantage. Using a simple advection-diffusion model, we compute the capture rates of diffusive molecules on a cilium. Because of its geometry, a non-motile cilium in a quiescent fluid has a capture rate equivalent to a circular absorbing region with ∼4× its surface area. When the cilium is exposed to an external shear flow, the equivalent surface area increases to ∼6×. Alternatively, if the cilium beats in a non-reciprocal way in an otherwise quiescent fluid, its capture rate increases with the beating frequency to the power of 1/3. Altogether, our results show that the protruding geometry of a cilium could be one of the reasons why so many receptors are located on cilia. They also point to the advantage of combining motility with chemical reception.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Role of anaxonic local neurons in the crossover to continuously varying exponents for avalanche activity

Local anaxonic neurons with graded potential release are important ingredients of nervous systems, present in the olfactory bulb system of mammalians and in the human visual system, as well as in arthropods and nematodes. We develop a neuronal network model including both axonic and anaxonic neurons and monitor the activity tuned by the following parameters: the decay length of the graded potential in local neurons, the fraction of local neurons, the largest eigenvalue of the adjacency matrix, and the range of connections of the local neurons. Tuning the fraction of local neurons, we derive the phase diagram including two transition lines: a critical line separating subcritical and supercritical regions, characterized by power-law distributions of avalanche sizes and durations, and a bifurcation line. We find that the overall behavior of the system is controlled by a parameter tuning the relevance of local neuron transmission with respect to the axonal one. The statistical properties of spontaneous activity are affected by local neurons at large fractions and on the condition that the graded potential transmission dominates the axonal one. In this case the scaling properties of spontaneous activity exhibit continuously varying exponents, rather than the mean-field branching model universality class.

The cyclic AMP signaling pathway in the rodent main olfactory system

Odor perception begins with the detection of odorant molecules by the main olfactory epithelium located in the nasal cavity. Odorant molecules bind to and activate a large family of G-protein-coupled odorant receptors and trigger a cAMP-mediated transduction cascade that converts the chemical stimulus into an electrical signal transmitted to the brain. Morever, odorant receptors and cAMP signaling plays a relevant role in olfactory sensory neuron development and axonal targeting to the olfactory bulb. This review will first explore the physiological response of olfactory sensory neurons to odorants and then analyze the different components of cAMP signaling and their different roles in odorant detection and olfactory sensory neuron development.

Establishing and regulating the composition of cilia for signal transduction

The primary cilium is a hair-like surface-exposed organelle of the eukaryotic cell that decodes a variety of signals - such as odorants, light and Hedgehog morphogens - by altering the local concentrations and activities of signalling proteins. Signalling within the cilium is conveyed through a diverse array of second messengers, including conventional signalling molecules (such as cAMP) and some unusual intermediates (such as sterols). Diffusion barriers at the ciliary base establish the unique composition of this signalling compartment, and cilia adapt their proteome to signalling demands through regulated protein trafficking. Much progress has been made on the molecular understanding of regulated ciliary trafficking, which encompasses not only exchanges between the cilium and the rest of the cell but also the shedding of signalling factors into extracellular vesicles.

Implications for human odor sensing revealed from the statistics of odorant-receptor interactions

Binding of odorants to olfactory receptors (ORs) elicits downstream chemical and neural signals, which are further processed to odor perception in the brain. Recently, Mainland and colleagues have measured more than 500 pairs of odorant-OR interaction by a high-throughput screening assay method, opening a new avenue to understanding the principles of human odor coding. Here, using a recently developed minimal model for OR activation kinetics, we characterize the statistics of OR activation by odorants in terms of three empirical parameters: the half-maximum effective concentration EC₅₀, the efficacy, and the basal activity. While the data size of odorants is still limited, the statistics offer meaningful information on the breadth and optimality of the tuning of human ORs to odorants, and allow us to relate the three parameters with the microscopic rate constants and binding affinities that define the OR activation kinetics. Despite the stochastic nature of the response expected at individual OR-odorant level, we assess that the confluence of signals in a neuron released from the multitude of ORs is effectively free of noise and deterministic with respect to changes in odorant concentration. Thus, setting a threshold to the fraction of activated OR copy number for neural spiking binarizes the electrophysiological signal of olfactory sensory neuron, thereby making an information theoretic approach a viable tool in studying the principles of odor perception.

Despite the decades of research, quantitative details of human olfaction have remained largely unexplored. However, a high-throughput measurement has recently been carried out to produce dose-response data between a set of odorants and a repertoire of human olfactory receptors. We characterized each pair of odorant-receptor interaction in terms of EC50, efficacy, and basal level, a strategy often adopted in biochemical, pharmacological sciences to describe the response of receptors to cognate ligands. The distributions of EC50 values and efficacies acquired from the analysis provide glimpses into how human olfactory receptors are tuned to odorants. Specifically, the response of human ORs is optimized around ∼ 100μM of odorant. Next, the efficacies of OR responses to odorants are bi-exponentially distributed, which indicates that the strength of odorant-OR interaction is classified into strong and weak subgroups. By showing that the stochastic response of individual receptor to odorant can effectively be binarized at cellular level through olfactory processes, we also provide a theoretical basis for an information theoretical approach in studying the principles of odor perception.

Tracking of unfamiliar odors is facilitated by signal amplification through anoctamin 2 chloride channels in mouse olfactory receptor neurons

Many animals follow odor trails to find food, nesting sites, or mates, and they require only faint olfactory cues to do so. The performance of a tracking dog, for instance, poses the question on how the animal is able to distinguish a target odor from the complex chemical background around the trail. Current concepts of odor perception suggest that animals memorize each odor as an olfactory object, a percept that enables fast recognition of the odor and the interpretation of its valence. An open question still is how this learning process operates efficiently at the low odor concentrations that typically prevail when animals inspect an odor trail. To understand olfactory processing under these conditions, we studied the role of an amplification mechanism that boosts signal transduction at low stimulus intensities, a process mediated by calcium‐gated anoctamin 2 chloride channels. Genetically altered Ano2^−/− mice, which lack these channels, display an impaired cue‐tracking behavior at low odor concentrations when challenged with an unfamiliar, but not with a familiar, odor. Moreover, recordings from the olfactory epithelium revealed that odor coding lacks sensitivity and temporal resolution in anoctamin 2‐deficient mice. Our results demonstrate that the detection of an unfamiliar, weak odor, as well as its memorization as an olfactory object, require signal amplification in olfactory receptor neurons. This process may contribute to the phenomenal tracking abilities of animals that follow odor trails.

Antagonism in olfactory receptor neurons and its implications for the perception of odor mixtures

Natural environments feature mixtures of odorants of diverse quantities, qualities and complexities. Olfactory receptor neurons (ORNs) are the first layer in the sensory pathway and transmit the olfactory signal to higher regions of the brain. Yet, the response of ORNs to mixtures is strongly non-additive, and exhibits antagonistic interactions among odorants. Here, we model the processing of mixtures by mammalian ORNs, focusing on the role of inhibitory mechanisms. We show how antagonism leads to an effective ‘normalization’ of the ensemble ORN response, that is, the distribution of responses of the ORN population induced by any mixture is largely independent of the number of components in the mixture. This property arises from a novel mechanism involving the distinct statistical properties of receptor binding and activation, without any recurrent neuronal circuitry. Normalization allows our encoding model to outperform non-interacting models in odor discrimination tasks, leads to experimentally testable predictions and explains several psychophysical experiments in humans.

When ordering in a coffee shop, you probably recognize and enjoy the aroma of freshly roasted coffee beans. But as well as coffee, you can also smell the croissants behind the counter and maybe even the perfume or cologne of the person next to you. Each of these scents consists of a collection of chemicals, or odorants. To distinguish between the aroma of coffee and that of croissants, your brain must group the odorants appropriately and then keep the groups separate from each other.

This is not a trivial task. Odorants bind to proteins called odorant receptors found on the surface of cells in the nose called olfactory receptor neurons. But each odorant does not have its own dedicated receptor. Instead, a single odorant will bind to multiple types of odorant receptors, and thus, each olfactory receptor neuron may respond to multiple odorants. So how does the brain encode mixtures of odorants in a way that allows us to distinguish one aroma from another?

Reddy, Zak et al. have developed a computational model to explain how this process works. The model assumes that an odorant triggers a response in an olfactory receptor neuron via two steps. First, the odorant binds to an odorant receptor. Second, the bound odorant activates the receptor. But the odorant that binds most strongly to a receptor will not necessarily be the odorant that is best at activating that receptor.

This allows a phenomenon called competitive antagonism to occur. This is when one odorant in a mixture binds more strongly to a receptor than the other odorants, but only weakly activates that receptor. In so doing, the strongly bound odorant prevents the other odorants from binding to and activating the receptor. This helps tame the dominating influence of background odors, which might otherwise saturate the responses of individual olfactory receptor neurons.

Reddy, Zak et al. show that processes such as competitive antagonism enable olfactory receptor neurons to encode all of the odors within a mixture. The model can explain various phenomena observed in experiments and it adds to our understanding of how the brain generates our sense of smell. The model may also be relevant to other biological systems that must filter weak signals from a dominant background. These include the immune system, which must distinguish a small set of foreign proteins from the much larger number of proteins that make up our bodies.

Simultaneous Loss of NCKX4 and CNG Channel Desensitization Impairs Olfactory Sensitivity

In vertebrate olfactory sensory neurons (OSNs), Ca²⁺ plays key roles in both mediating and regulating the olfactory response. Ca²⁺ enters OSN cilia during the response through the olfactory cyclic nucleotide-gated (CNG) channel and stimulates a depolarizing chloride current by opening the olfactory Ca²⁺-activated chloride channel to amplify the response. Ca²⁺ also exerts negative regulation on the olfactory transduction cascade, through mechanisms that include reducing the CNG current by desensitizing the CNG channel via Ca²⁺/calmodulin (CaM), to reduce the response. Ca²⁺ is removed from the cilia primarily by the K⁺-dependent Na⁺/Ca²⁺ exchanger 4 (NCKX4), and the removal of Ca²⁺ leads to closure of the chloride channel and response termination. In this study, we investigate how two mechanisms conventionally considered negative regulatory mechanisms of olfactory transduction, Ca²⁺ removal by NCKX4, and desensitization of the CNG channel by Ca²⁺/CaM, interact to regulate the olfactory response. We performed electro-olfactogram (EOG) recordings on the double-mutant mice, NCKX4^-/-;CNGB1^ΔCaM, which are simultaneously lacking NCKX4 (NCKX4^-/-) and Ca²⁺/CaM-mediated CNG channel desensitization (CNGB1^ΔCaM). Despite exhibiting alterations in various response attributes, including termination kinetics and adaption properties, OSNs in either NCKX4^-/- mice or CNGB1^ΔCaM mice show normal resting sensitivity, as determined by their unchanged EOG response amplitude. We found that OSNs in NCKX4^-/-;CNGB1^ΔCaM mice displayed markedly reduced EOG amplitude accompanied by alterations in other response attributes. This study suggests that what are conventionally considered negative regulatory mechanisms of olfactory transduction also play a role in setting the resting sensitivity in OSNs.

SIGNIFICANCE STATEMENT

Sensory receptor cells maintain high sensitivity at rest. Although the mechanisms responsible for setting the resting sensitivity of sensory receptor cells are not well understood, it has generally been assumed that the sensitivity is set primarily by how effectively the components in the activation cascade of sensory transduction can be stimulated. Our findings in mouse olfactory sensory neurons suggest that mechanisms that are primarily responsible for terminating the olfactory response are also critical for proper resting sensitivity.

Kinetic Model for the Activation of Mammalian Olfactory Receptor

The sense of smell is triggered by binding of odorants to a set of olfactory receptors (ORs), the activation of which generates specific patterns of neuronal signals in olfactory bulbs. Despite a long history of research and speculations, very little is known about the actual mechanism of OR activation. In particular, there is virtually no theoretical framework capable of describing the kinetics of olfactory activation at a quantitative level. Based on the fact that mammalian ORs belong to a class of G-protein coupled receptors (GPCRs) and utilizing the information available from recent studies on other types of GPCRs with known structural data, we construct a minimal kinetic model for mammalian olfactory activation, obtaining a new expression for the signal strength as a function of odorant and G-protein concentrations and defining this as odor activity (OA). The parametric dependence of OA on equilibrium dissociation and rate constants provides a new comprehensive means to describe how odorant-OR binding kinetics affects the odor signal, and offers new quantitative criteria for classifying agonistic, partially agonistic, and antagonistic (or inverse agonistic) behavior. The dependence of OA on the concentration of G-proteins also suggests a new experimental method to determine key equilibrium constants for odorant-OR and G-protein-OR association/dissociation processes.

Cyclic-nucleotide-gated cation current and Ca2+-activated Cl current elicited by odorant in vertebrate olfactory receptor neurons

Olfactory transduction in vertebrate olfactory receptor neurons (ORNs) involves primarily a cAMP-signaling cascade that leads to the opening of cyclic-nucleotide-gated (CNG), nonselective cation channels. The consequent Ca²⁺ influx triggers adaptation but also signal amplification, the latter by opening a Ca²⁺-activated Cl channel (ANO2) to elicit, unusually, an inward Cl current. Hence the olfactory response has inward CNG and Cl components that are in rapid succession and not easily separable. We report here success in quantitatively separating these two currents with respect to amplitude and time course over a broad range of odorant strengths. Importantly, we found that the Cl current is the predominant component throughout the olfactory dose-response relation, down to the threshold of signaling to the brain. This observation is very surprising given a recent report by others that the olfactory-signal amplification effected by the Ca²⁺-activated Cl current does not influence the behavioral olfactory threshold in mice.

Spontaneously active NaV1.5 sodium channels may underlie odor sensitivity

The olfactory system is remarkably sensitive to airborne odor molecules, but precisely how very low odor concentrations bordering on just a few molecules per olfactory sensory neuron can trigger graded changes in firing is not clear. This report reexamines signaling in olfactory sensory neurons in light of the recent account of NaV1.5 sodium channel-mediated spontaneous firing. Using a model of spontaneous channel activity, the study shows how even submillivolt changes in membrane potential elicited by odor are expected to cause meaningful changes in NaV1.5-dependent firing. The results suggest that the random window currents of NaV1.5 channels may underpin not only spontaneous firing in olfactory sensory neurons but the cellular response to odor as well, thereby ensuring the robustness and sensitivity of signaling that is especially important for low odor concentrations.

Mechanics regulates ATP-stimulated collective calcium response in fibroblast cells

Cells constantly sense their chemical and mechanical environments. We study the effect of mechanics on the ATP-induced collective calcium response of fibroblast cells in experiments that mimic various tissue environments. We find that closely packed two-dimensional cell cultures on a soft polyacrylamide gel (Young's modulus E = 690 Pa) contain more cells exhibiting calcium oscillations than cultures on a rigid substrate (E = 36 000 Pa). Calcium responses of cells on soft substrates show a slower decay of calcium level relative to those on rigid substrates. Actin enhancement and disruption experiments for the cell cultures allow us to conclude that actin filaments determine the collective Ca(2+) oscillatory behaviour in the culture. Inhibition of gap junctions results in a decrease of the oscillation period and reduced correlation of calcium responses, which suggests additional complexity of signalling upon cell-cell contact. Moreover, the frequency of calcium oscillations is independent of the rigidity of the substrate but depends on ATP concentration. We compare our results with those from similar experiments on individual cells. Overall, our observations show that collective chemical signalling in cell cultures via calcium depends critically on the mechanical environment.

Distinct signaling of Drosophila chemoreceptors in olfactory sensory neurons

In Drosophila, olfactory sensory neurons (OSNs) rely primarily on two types of chemoreceptors, odorant receptors (Ors) and ionotropic receptors (Irs), to convert odor stimuli into neural activity. The cellular signaling of these receptors in their native OSNs remains unclear because of the difficulty of obtaining intracellular recordings from Drosophila OSNs. Here, we developed an antennal preparation that enabled the first recordings (to our knowledge) from targeted Drosophila OSNs through a patch-clamp technique. We found that brief odor pulses triggered graded inward receptor currents with distinct response kinetics and current-voltage relationships between Or- and Ir-driven responses. When stimulated with long-step odors, the receptor current of Ir-expressing OSNs did not adapt. In contrast, Or-expressing OSNs showed a strong Ca(2+)-dependent adaptation. The adaptation-induced changes in odor sensitivity obeyed the Weber-Fechner relation; however, surprisingly, the incremental sensitivity was reduced at low odor backgrounds but increased at high odor backgrounds. Our model for odor adaptation revealed two opposing effects of adaptation, desensitization and prevention of saturation, in dynamically adjusting odor sensitivity and extending the sensory operating range.

Co-expression of anoctamins in cilia of olfactory sensory neurons

Vertebrates can sense and identify a vast array of chemical cues. The molecular machinery involved in chemodetection and transduction is expressed within the cilia of olfactory sensory neurons. Currently, there is only limited information available on the distribution and density of individual signaling components within the ciliary compartment. Using super-resolution microscopy, we show here that cyclic-nucleotide-gated channels and calcium-activated chloride channels of the anoctamin family are localized to discrete microdomains in the ciliary membrane. In addition to ANO2, a second anoctamin, ANO6, also localizes to ciliary microdomains. This observation, together with the fact that ANO6 and ANO2 co-localize, indicates a role for ANO6 in olfactory signaling. We show that both ANO2 and ANO6 can form heteromultimers and that this heteromerization alters the recombinant channels' physiological properties. Thus, we provide evidence for interaction of ANO2 and ANO6 in olfactory cilia, with possible physiological relevance for olfactory signaling.

Mechanisms of regulation of olfactory transduction and adaptation in the olfactory cilium

Olfactory adaptation is a fundamental process for the functioning of the olfactory system, but the underlying mechanisms regulating its occurrence in intact olfactory sensory neurons (OSNs) are not fully understood. In this work, we have combined stochastic computational modeling and a systematic pharmacological study of different signaling pathways to investigate their impact during short-term adaptation (STA). We used odorant stimulation and electroolfactogram (EOG) recordings of the olfactory epithelium treated with pharmacological blockers to study the molecular mechanisms regulating the occurrence of adaptation in OSNs. EOG responses to paired-pulses of odorants showed that inhibition of phosphodiesterases (PDEs) and phosphatases enhanced the levels of STA in the olfactory epithelium, and this effect was mimicked by blocking vesicle exocytosis and reduced by blocking cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and vesicle endocytosis. These results suggest that G-coupled receptors (GPCRs) cycling is involved with the occurrence of STA. To gain insights on the dynamical aspects of this process, we developed a stochastic computational model. The model consists of the olfactory transduction currents mediated by the cyclic nucleotide gated (CNG) channels and calcium ion (Ca²⁺)-activated chloride (CAC) channels, and the dynamics of their respective ligands, cAMP and Ca²⁺, and it simulates the EOG results obtained under different experimental conditions through changes in the amplitude and duration of cAMP and Ca²⁺ response, two second messengers implicated with STA occurrence. The model reproduced the experimental data for each pharmacological treatment and provided a mechanistic explanation for the action of GPCR cycling in the levels of second messengers modulating the levels of STA. All together, these experimental and theoretical results indicate the existence of a mechanism of regulation of STA by signaling pathways that control GPCR cycling and tune the levels of second messengers in OSNs, and not only by CNG channel desensitization as previously thought.

High density and ligand affinity confer ultrasensitive signal detection by a guanylyl cyclase chemoreceptor

Guanylyl cyclases (GCs), which synthesize the messenger cyclic guanosine 3',5'-monophosphate, control several sensory functions, such as phototransduction, chemosensation, and thermosensation, in many species from worms to mammals. The GC chemoreceptor in sea urchin sperm can decode chemoattractant concentrations with single-molecule sensitivity. The molecular and cellular underpinnings of such ultrasensitivity are not known for any eukaryotic chemoreceptor. In this paper, we show that an exquisitely high density of 3 × 10(5) GC chemoreceptors and subnanomolar ligand affinity provide a high ligand-capture efficacy and render sperm perfect absorbers. The GC activity is terminated within 150 ms by dephosphorylation steps of the receptor, which provides a means for precise control of the GC lifetime and which reduces "molecule noise." Compared with other ultrasensitive sensory systems, the 10-fold signal amplification by the GC receptor is surprisingly low. The hallmarks of this signaling mechanism provide a blueprint for chemical sensing in small compartments, such as olfactory cilia, insect antennae, or even synaptic boutons.

The scaffold protein MUPP1 regulates odorant-mediated signaling in olfactory sensory neurons

The olfactory signal transduction cascade transforms odor information into electrical signals by a cAMP-based amplification mechanism. The mechanisms underlying the very precise temporal and spatial organization of the relevant signaling components remains poorly understood. Here, we identify, using co-immunoprecipitation experiments, a macromolecular assembly of signal transduction components in mouse olfactory neurons, organized through MUPP1. Disruption of the PDZ signaling complex, through use of an inhibitory peptide, strongly impaired odor responses and changed the activation kinetics of olfactory sensory neurons. In addition, our experiments demonstrate that termination of the response is dependent on PDZ-based scaffolding. These findings provide new insights into the functional organization, and regulation, of olfactory signal transduction.

Interplay among cGMP, cAMP, and Ca2+ in living olfactory sensory neurons in vitro and in vivo

The mechanism of cGMP production in olfactory sensory neurons (OSNs) is poorly understood, although this messenger takes part in several key processes such as adaptation, neuronal development, and long-term cellular responses to odorant stimulation. Many aspects of the regulation of cGMP in OSNs are still unknown or highly controversial, such as its subcellular heterogeneity, mechanism of coupling to odorant receptors and downstream targets. Here, we have investigated the dynamics and the intracellular distribution of cGMP in living rat OSNs in culture transfected with a genetically encoded sensor for cGMP. We demonstrate that OSNs treated with pharmacological stimuli able to activate membrane or soluble guanylyl cyclase (sGC) presented an increase in cGMP in the entire neuron, from cilia-dendrite to the axon terminus-growth cone. Upon odorant stimulation, a rise in cGMP was again found in the entire neuron, including the axon terminus, where it is locally synthesized. The odorant-dependent rise in cGMP is due to sGC activation by nitric oxide (NO) and requires an increase of cAMP. The link between cAMP and NO synthase appears to be the rise in cytosolic Ca(2+) concentration elicited by either plasma membrane Ca(2+) channel activation or Ca(2+) mobilization from stores via the guanine nucleotide exchange factor Epac. Finally, we show that a cGMP rise can elicit both in vitro and in vivo the phosphorylation of nuclear CREB, suggesting that this signaling pathway may be relevant for both local events (pathfinding, neurotransmitter release) and more distal processes involving gene expression regulation.

Nav1.7 is the predominant sodium channel in rodent olfactory sensory neurons

Background

Voltage-gated sodium channel Na_v1.7 is preferentially expressed in dorsal root ganglion (DRG) and sympathetic neurons within the peripheral nervous system. Homozygous or compound heterozygous loss-of-function mutations in SCN9A, the gene which encodes Na_v1.7, cause congenital insensitivity to pain (CIP) accompanied by anosmia. Global knock-out of Na_v1.7 in mice is neonatal lethal reportedly from starvation, suggesting anosmia. These findings led us to hypothesize that Na_v1.7 is the main sodium channel in the peripheral olfactory sensory neurons (OSN, also known as olfactory receptor neurons).

Methods

We used multiplex PCR-restriction enzyme polymorphism, in situ hybridization and immunohistochemistry to determine the identity of sodium channels in rodent OSNs.

Results

We show here that Na_v1.7 is the predominant sodium channel transcript, with low abundance of other sodium channel transcripts, in olfactory epithelium from rat and mouse. Our in situ hybridization data show that Na_v1.7 transcripts are present in rat OSNs. Immunostaining of Na_v1.7 and Na_v1.6 channels in rat shows a complementary accumulation pattern with Na_v1.7 in peripheral presynaptic OSN axons, and Na_v1.6 primarily in postsynaptic cells and their dendrites in the glomeruli of the olfactory bulb within the central nervous system.

Conclusions

Our data show that Na_v1.7 is the dominant sodium channel in rat and mouse OSN, and may explain anosmia in Na_v1.7 null mouse and patients with Na_v1.7-related CIP.

Unitary response of mouse olfactory receptor neurons

The sense of smell begins with odorant molecules binding to membrane receptors on the cilia of olfactory receptor neurons (ORNs), thereby activating a G protein, G(olf), and the downstream effector enzyme, an adenylyl cyclase (ACIII). Recently, we have found in amphibian ORNs that an odorant-binding event has a low probability of activating sensory transduction at all; even when successful, the resulting unitary response apparently involves a single active Gα(olf)-ACIII molecular complex. This low amplification is in contrast to rod phototransduction in vision, the best-quantified G-protein signaling pathway, where each photoisomerized rhodopsin molecule is well known to produce substantial amplification by activating many G-protein, and hence effector-enzyme, molecules. We have now carried out similar experiments on mouse ORNs, which offer, additionally, the advantage of genetics. Indeed, we found the same low probability of transduction, based on the unitary olfactory response having a fairly constant amplitude and similar kinetics across different odorants and randomly encountered ORNs. Also, consistent with our picture, the unitary response of Gα(olf)(+/-) ORNs was similar to WT in amplitude, although their Gα(olf)-protein expression was only half of normal. Finally, from the action potential firing, we estimated that ≤19 odorant-binding events successfully triggering transduction in a WT mouse ORN will lead to signaling to the brain.