The electrochemical basis of odor transduction in vertebrate olfactory cilia

Cilia- and Flagella-Associated Protein 69 Regulates Olfactory Transduction Kinetics in Mice

Animals detect odorous chemicals through specialized olfactory sensory neurons (OSNs) that transduce odorants into neural electrical signals. We identified a novel and evolutionarily conserved protein, cilia- and flagella-associated protein 69 (CFAP69), in mice that regulates olfactory transduction kinetics. In the olfactory epithelium, CFAP69 is enriched in OSN cilia, where olfactory transduction occurs. Bioinformatic analysis suggests that a large portion of CFAP69 can form Armadillo-type α-helical repeats, which may mediate protein-protein interactions. OSNs lacking CFAP69, remarkably, displayed faster kinetics in both the on and off phases of electrophysiological responses at both the neuronal ensemble level as observed by electroolfactogram and the single-cell level as observed by single-cell suction pipette recordings. In single-cell analysis, OSNs lacking CFAP69 showed faster response integration and were able to fire APs more faithfully to repeated odor stimuli. Furthermore, both male and female mutant mice that specifically lack CFAP69 in OSNs exhibited attenuated performance in a buried food pellet test when a background of the same odor to the food pellet was present even though they should have better temporal resolution of coding olfactory stimulation at the peripheral. Therefore, the role of CFAP69 in the olfactory system seems to be to allow the olfactory transduction machinery to work at a precisely regulated range of response kinetics for robust olfactory behavior.SIGNIFICANCE STATEMENT Sensory receptor cells are generally thought to evolve to respond to sensory cues as fast as they can. This idea is consistent with mutational analyses in various sensory systems, where mutations of sensory receptor cells often resulted in reduced response size and slowed response kinetics. Contrary to this idea, we have found that there is a kinetic "damper" present in the olfactory transduction cascade of the mouse that slows down the response kinetics and, by doing so, it reduces the peripheral temporal resolution in coding odor stimuli and allows for robust olfactory behavior. This study should trigger a rethinking of the significance of the intrinsic speed of sensory transduction and the pattern of the peripheral coding of sensory stimuli.

Organization and Plasticity of Sodium Channel Expression in the Mouse Olfactory and Vomeronasal Epithelia

To understand the molecular basis of neuronal excitation in the mammalian olfactory system, we conducted a systematic analysis of the organization of voltage-gated sodium (Na_v) channel subunits in the main olfactory epithelium (MOE) and vomeronasal organ (VNO) of adult mice. We also analyzed changes in Na_v channel expression during development in these two systems and during regeneration of the MOE. Quantitative PCR shows that Na_v1.7 is the predominant isoform in both adult MOE and VNO. We detected pronounced immunoreactivity for Na_v1.7 and Na_v1.3 in axons of olfactory and vomeronasal sensory neurons (VSNs). Analysis of Na_v1.2 and Na_v1.6 revealed an unexpected subsystem-specific distribution. In the MOE, these Na_v channels are absent from olfactory sensory neurons (OSNs) but present in non-neuronal olfactory cell types. In the VNO, Na_v1.2 and Na_v1.6 are confined to VSNs, with Na_v1.2-immunoreactive somata solely present in the basal layer of the VNO. The subcellular localization of Na_v1.3 and Na_v1.7 in OSNs can change dramatically during periods of heightened plasticity in the MOE. During the first weeks of development and during regeneration of the olfactory epithelium following chemical lesion, expression of Na_v1.3 and Na_v1.7 is transiently enhanced in the somata of mature OSNs. Our results demonstrate a highly complex organization of Na_v channel expression in the mouse olfactory system, with specific commonalities but also differences between the MOE and the VNO. On the basis of their subcellular localization, Na_v1.3 and Na_v1.7 should play major roles in action potential propagation in both MOE and VNO, whereas Na_v1.2 and Na_v1.6 are specific to the function of VSNs. The plasticity of Na_v channel expression in OSNs during early development and recovery from injury could reflect important physiological requirements in a variety of activity-dependent mechanisms.

The native TRPP2-dependent channel of murine renal primary cilia

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening monogenic renal disease. ADPKD results from mutations in either of two proteins: polycystin-1 (also known as PC1 or PKD1) or transient receptor potential cation channel, subfamily P, member 2 (TRPP2, also known as polycystin-2, PC2, or PKD2). Each of these proteins is expressed in the primary cilium that extends from many renal epithelial cells. Existing evidence suggests that the cilium can promote renal cystogenesis, while PC1 and TRPP2 counter this cystogenic effect. To better understand the function of TRPP2, we investigated its electrophysiological properties in the native ciliary membrane. We recorded directly from the cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin. In one-third of cilia examined, a large-conductance channel was observed. The channel was not permeable to Cl¯ but conducted cations with permeability ratios P_K:P_Ca:P_Na of 1:0.55:0.14. The single-channel conductance ranged from 97 pS in typical physiological solutions to 189 pS in symmetrical 145 mM KCl. Open probability of the channel was very sensitive to membrane depolarization or increasing cytoplasmic free Ca²⁺ in the low micromolar range, with the open probability increasing in either case. Knocking out TRPP2 by CRISPR/Cas9 genome editing eliminated the channel current, establishing it as TRPP2 dependent. Possible mechanisms for activating the TRPP2-dependent channel in the renal primary cilium are discussed.

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.

The Ca2+-activated Cl- channel TMEM16B regulates action potential firing and axonal targeting in olfactory sensory neurons

TMEM16B is expressed in olfactory sensory neurons, but previous attempts to establish a physiological role in olfaction have been unsuccessful. Pietra et al. find that genetic ablation of TMEM16B results in defects in the olfactory behavior of mice and the cellular physiology of olfactory sensory neurons.

The Ca²⁺-activated Cl⁻ channel TMEM16B is highly expressed in the cilia of olfactory sensory neurons (OSNs). Although a large portion of the odor-evoked transduction current is carried by Ca²⁺-activated Cl⁻ channels, their role in olfaction is still controversial. A previous report (Billig et al. 2011. Nat. Neurosci.http://dx.doi.org/10.1038/nn.2821) showed that disruption of the TMEM16b/Ano2 gene in mice abolished Ca²⁺-activated Cl⁻ currents in OSNs but did not produce any major change in olfactory behavior. Here we readdress the role of TMEM16B in olfaction and show that TMEM16B knockout (KO) mice have behavioral deficits in odor-guided food-finding ability. Moreover, as the role of TMEM16B in action potential (AP) firing has not yet been studied, we use electrophysiological recording methods to measure the firing activity of OSNs. Suction electrode recordings from isolated olfactory neurons and on-cell loose-patch recordings from dendritic knobs of neurons in the olfactory epithelium show that randomly selected neurons from TMEM16B KO mice respond to stimulation with increased firing activity than those from wild-type (WT) mice. Because OSNs express different odorant receptors (ORs), we restrict variability by using a mouse line that expresses a GFP-tagged I7 OR, which is known to be activated by heptanal. In response to heptanal, we measure dramatic changes in the firing pattern of I7-expressing neurons from TMEM16B KO mice compared with WT: responses are prolonged and display a higher number of APs. Moreover, lack of TMEM16B causes a markedly reduced basal spiking activity in I7-expressing neurons, together with an alteration of axonal targeting to the olfactory bulb, leading to the appearance of supernumerary I7 glomeruli. Thus, TMEM16B controls AP firing and ensures correct glomerular targeting of OSNs expressing I7. Altogether, these results show that TMEM16B does have a relevant role in normal olfaction.

Olfactory Sensitivity in Mammalian Species

Olfaction enables most mammalian species to detect and discriminate vast numbers of chemical structures called odorants and pheromones. The perception of such chemical compounds is mediated via two major olfactory systems, the main olfactory system and the vomeronasal system, as well as minor systems, such as the septal organ and the Grueneberg ganglion. Distinct differences exist not only among species but also among individuals in terms of their olfactory sensitivity; however, little is known about the mechanisms that determine these differences. In research on the olfactory sensitivity of mammals, scientists thus depend in most cases on behavioral testing. In this article, we reviewed scientific studies performed on various mammalian species using different methodologies and target chemical substances. Human and non-human primates as well as rodents and dogs are the most frequently studied species. Olfactory threshold studies on other species do not exist with the exception of domestic pigs. Olfactory testing performed on seals, elephants, and bats focused more on discriminative abilities than on sensitivity. An overview of olfactory sensitivity studies as well as olfactory detection ability in most studied mammalian species is presented here, focusing on comparable olfactory detection thresholds. The basics of olfactory perception and olfactory sensitivity factors are also described.

Super-resolution imaging of ciliary microdomains in isolated olfactory sensory neurons using a custom two-color stimulated emission depletion microscope

We performed stimulated emission depletion (STED) imaging of isolated olfactory sensory neurons (OSNs) using a custom-built microscope. The STED microscope uses a single pulsed laser to excite two separate fluorophores, Atto 590 and Atto 647N. A gated timing circuit combined with temporal interleaving of the different color excitation/STED laser pulses filters the two channel detection and greatly minimizes crosstalk. We quantified the instrument resolution to be ∼81 and ∼44  nm, for the Atto 590 and Atto 647N channels. The spatial separation between the two channels was measured to be under 10 nm, well below the resolution limit. The custom-STED microscope is incorporated onto a commercial research microscope allowing brightfield, differential interference contrast, and epifluorescence imaging on the same field of view. We performed immunolabeling of OSNs in mice to image localization of ciliary membrane proteins involved in olfactory transduction. We imaged Ca2+-permeable cyclic nucleotide gated (CNG) channel (Atto 594) and adenylyl cyclase type III (ACIII) (Atto 647N) in distinct cilia. STED imaging resolved well-separated subdiffraction limited clusters for each protein. We quantified the size of each cluster to have a mean value of 88±48  nm and 124±43  nm, for CNG and ACIII, respectively. STED imaging showed separated clusters that were not resolvable in confocal images.

Anatomical and molecular consequences of Unilateral Naris Closure on two populations of olfactory sensory neurons expressing defined odorant receptors

Mammalian olfactory sensory neurons (OSNs), the primary elements of the olfactory system, are located in the olfactory epithelium lining the nasal cavity. Exposed to the environment, their lifespan is short. Consequently, OSNs are regularly regenerated and several reports show that activity strongly modulates their development and regeneration: the peripheral olfactory system can adjust to the amount of stimulus through compensatory mechanisms. Unilateral naris occlusion (UNO) was frequently used to investigate this mechanism at the entire epithelium level. However, there is little data regarding the effects of UNO at the cellular level, especially on individual neuronal populations expressing a defined odorant receptor. Here, using UNO during the first three postnatal weeks, we analyzed the anatomical and molecular consequences of sensory deprivation in OSNs populations expressing the MOR23 and M71 receptors. The density of MOR23-expressing neurons is decreased in the closed side while UNO does not affect the density of M71-expressing neurons. Using Real Time qPCR on isolated neurons, we observed that UNO modulates the transcript levels for transduction pathway proteins (odorant receptors, CNGA2, PDE1c). The transcripts modulated by UNO will differ between populations depending on the receptor expressed. These results suggest that sensory deprivation will have different effects on different OSNs' populations. As a consequence, early experience will shape the functional properties of OSNs differently depending on the type of odorant receptor they express.

Properties of an optogenetic model for olfactory stimulation

KEY POINTS

In olfactory research it is difficult to deliver stimuli with defined intensity and duration to olfactory sensory neurons. Expression of channelrhodopsin 2 (ChR2) in olfactory sensory neurons provides a means to activate these neurons with light flashes. Appropriate mouse models are available. The present study explores the suitability of an established olfactory marker protein (OMP)/ChR2-yellow fluorescent protein (YFP) mouse model for ex vivo experimentation. Expression of ChR2 in sensory neurons of the main olfactory epithelium, the septal organ and vomeronasal organ is characterized. Expression pattern of ChR2 in olfactory receptor neurons and the properties of light responses indicate that light stimulation does not impact on signal transduction in the chemosensory cilia. Light-induced electro-olfactograms are characterized with light flashes of different intensities, durations and frequencies. The impact of light-induced afferent stimulation on the olfactory bulb is examined with respect to response amplitude, polarity and low-pass filtering.

ABSTRACT

For the examination of sensory processing, it is helpful to deliver stimuli in precisely defined temporal and spatial patterns with accurate control of stimulus intensity. This is challenging in experiments with the mammalian olfactory system because airborne odorants have to be transported into the intricate sensory structures of the nose and must dissolve in mucus to be detected by sensory neurons. Defined and reproducible activity can be generated in olfactory sensory neurons that express the light-gated ion channel channelrhodopsin 2 (ChR2). The neurons can be stimulated by light flashes in a controlled fashion by this optogenetic approach. Here we examined the application of an olfactory marker protein (OMP)/ChR2-yellow fluorescent protein (YFP) model for ex vivo exploration of the olfactory epithelium and the olfactory bulb of the mouse. We studied the expression patterns of ChR2 in the main olfactory system, the vomeronasal system, and the septal organ, and we found that ChR2 is absent from the sensory cilia of olfactory sensory neurons. In the olfactory epithelium, we characterized light-induced electro-olfactograms with respect to peripheral encoding of stimulus intensity, stimulus duration and stimulus frequency. In acute slices of the olfactory bulb, we identified specific aspects of the ChR2-induced input signal, concerning its dynamic range, its low-pass filter property and its response to prolonged stimulation. Our study describes the performance of the OMP/ChR2-YFP model for ex vivo experimentation on the peripheral olfactory system and documents its versatility and its limitations for olfactory research.

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.

Compensatory plasticity in the olfactory epithelium: age, timing, and reversibility

Like other biological systems, olfaction responds "homeostatically" to enduring change in the stimulus environment. This adaptive mechanism, referred to as compensatory plasticity, has been studied almost exclusively in developing animals. Thus it is unknown if this phenomenon is limited to ontogenesis and irreversible, characteristics common to some other forms of plasticity. Here we explore the effects of odor deprivation on the adult mouse olfactory epithelium (OE) using nasal plugs to eliminate nasal airflow unilaterally. Plugs were in place for 2-6 wk after which electroolfactograms (EOGs) were recorded from the occluded and open sides of the nasal cavity. Mean EOG amplitudes were significantly greater on the occluded than on the open side. The duration of plugging did not affect the results, suggesting that maximal compensation occurs within 2 wk or less. The magnitude of the EOG difference between the open and occluded side in plugged mice was comparable to adults that had undergone surgical naris occlusion as neonates. When plugs were removed after 4 wk followed by 2 wk of recovery, mean EOG amplitudes were not significantly different between the always-open and previously plugged sides of the nasal cavity suggesting that this form of plasticity is reversible. Taken together, these results suggest that compensatory plasticity is a constitutive mechanism of olfactory receptor neurons that allows these cells to recalibrate their stimulus-response relationship to fit the statistics of their current odor environment.

SnapShot: Sensing and Signaling by Cilia

Primary cilia are cellular appendages that coordinate diverse sensory and signaling activities. They are important for proper mammalian development, adult tissue homeostasis, and vision and odorant detection, and their dysfunction contributes to disease pathology and developmental defects.

Odorant receptors signaling instructs the development and plasticity of the glomerular map

The olfactory system provides a great opportunity to explore the mechanisms that underlie the formation and function of neural circuits because of the simplicity of its structure. Olfactory sensory neurons (OSNs) located in the peripheral olfactory epithelium (OE) take part in the initial formation and function of glomeruli in the olfactory bulb (OB) inside the central nervous system. Glomeruli are key in the process of transduction of olfactory information, as they constitute a map in the OB that sorts the different types of odorant inputs. This odorant categorization allows proper olfactory perception, and it is achieved through the anatomical organization and function of the different glomerular circuits. Once formed, glomeruli keep the capacity to undergo diverse plasticity processes, which is unique among the different neural circuits of the central nervous system. In this context, through the expression and function of the odorant receptors (ORs), OSNs perform two of the most important roles in the olfactory system: transducing odorant information to the nervous system and initiating the development of the glomerular map to organize olfactory information. This review addresses essential information that has emerged in recent years about the molecular basis of these processes.

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.

Evaluation of the role of g protein-coupled receptor kinase 3 in desensitization of mouse odorant receptors in a Mammalian cell line and in olfactory sensory neurons

Thousands of odors are sensed and discriminated by G protein-coupled odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs). G protein-coupled receptor kinases (GRKs) may have a role in desensitization of ORs. However, whether ORs are susceptible to agonist-dependent desensitization and whether GRKs affect odorant responsiveness of OSNs are currently unknown. Here we show that GRK3 attenuated the agonist responsiveness of a specific mouse odorant receptor for eugenol (mOR-EG) upon agonist pretreatment in HEK293 cells, but GRK3 did not affect the response amplitude or the recovery kinetics upon repeated agonist stimulation. We performed electrophysiological recordings of single OSNs which expressed mOR-EG and green fluorescent protein (GFP) in the presence or absence of GRK3. The kinetics and amplitude of agonist responsiveness of individual GFP-labeled mOR-EG neurons were not significantly affected by the absence of GRK3. These results indicate that the role of GRK3 in attenuating ORs responsiveness in OSNs may have been overestimated.

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.

GCY-8, PDE-2, and NCS-1 are critical elements of the cGMP-dependent thermotransduction cascade in the AFD neurons responsible for C. elegans thermotaxis

Certain thermoreceptor neurons are sensitive to tiny thermal fluctuations (0.01°C or less) and maintain their sensitivity across a wide range of ambient temperatures through a process of adaptation, but understanding of the biochemical basis for this performance is rudimentary. Prior studies of the AFD thermoreceptor in Caenorhabditis elegans revealed a signaling cascade that depends on a trio of receptor guanylate cyclases (rGCs), GCY-8, GCY-18, and GCY-23, and gives rise to warming-activated thermoreceptor currents (ThRCs) carried by cyclic GMP–gated ion channels. The threshold for ThRC activation adapts to the ambient temperature through an unknown calcium-dependent process. Here, we use in vivo whole-cell patch-clamp recording from AFD to show that loss of GCY-8, but not of GCY-18 or GCY-23, reduces or eliminates ThRCs, identifying this rGC as a crucial signaling element. To learn more about thermotransduction and adaptation, we used behavioral screens and analysis of gene expression patterns to identify phosphodiesterases (PDEs) likely to contribute to thermotransduction. Deleting PDE-2 decouples the threshold for ThRC activation from ambient temperature, altering adaptation. We provide evidence that the conserved neuronal calcium sensor 1 protein also regulates the threshold for ThRC activation and propose a signaling network to account for ThRC activation and adaptation. Because PDEs play essential roles in diverse biological processes, including vertebrate phototransduction and olfaction, and regulation of smooth muscle contractility and cardiovascular function, this study has broad implications for understanding how extraordinary sensitivity and dynamic range is achieved in cyclic nucleotide–based signaling networks.

Postnatal odorant exposure induces peripheral olfactory plasticity at the cellular level

Mammalian olfactory sensory neurons (OSNs) form the primary elements of the olfactory system. Inserted in the olfactory mucosa lining of the nasal cavity, they are exposed to the environment and their lifespan is brief. Several reports say that OSNs are regularly regenerated during the entire life and that odorant environment affects the olfactory epithelium. However, little is known about the impact of the odorant environment on OSNs at the cellular level and more precisely in the context of early postnatal olfactory exposure. Here we exposed MOR23-green fluorescent protein (GFP) and M71-GFP mice to lyral or acetophenone, ligands for MOR23 or M71, respectively. Daily postnatal exposure to lyral induces plasticity in the population of OSNs expressing MOR23. Their density decreases after odorant exposure, whereas the amount of MOR23 mRNA and protein remain stable in the whole epithelium. Meanwhile, quantitative PCR indicates that each MOR23 neuron has higher levels of olfactory receptor transcripts and also expresses more CNGA2 and phosphodiesterase 1C, fundamental olfactory transduction pathway proteins. Transcript levels return to baseline after 4 weeks recovery. Patch-clamp recordings reveal that exposed MOR23 neurons respond to lyral with higher sensitivity and broader dynamic range while the responses' kinetics were faster. These effects are specific to the odorant-receptor pair lyral-MOR23: there was no effect of acetophenone on MOR23 neurons and no effect of acetophenone and lyral on the M71 population. Together, our results clearly demonstrate that OSNs undergo specific anatomical, molecular, and functional adaptation when chronically exposed to odorants in the early stage of life.

The membrane proteome of sensory cilia to the depth of olfactory receptors

In the nasal cavity, the nonmotile cilium of olfactory sensory neurons (OSNs) constitutes the chemosensory interface between the ambient environment and the brain. The unique sensory organelle facilitates odor detection for which it includes all necessary components of initial and downstream olfactory signal transduction. In addition to its function in olfaction, a more universal role in modulating different signaling pathways is implicated, for example, in neurogenesis, apoptosis, and neural regeneration. To further extend our knowledge about this multifunctional signaling organelle, it is of high importance to establish a most detailed proteome map of the ciliary membrane compartment down to the level of transmembrane receptors. We detached cilia from mouse olfactory epithelia via Ca(2+)/K(+) shock followed by the enrichment of ciliary membrane proteins at alkaline pH, and we identified a total of 4,403 proteins by gel-based and gel-free methods in conjunction with high resolution LC/MS. This study is the first to report the detection of 62 native olfactory receptor proteins and to provide evidence for their heterogeneous expression at the protein level. Quantitative data evaluation revealed four ciliary membrane-associated candidate proteins (the annexins ANXA1, ANXA2, ANXA5, and S100A5) with a suggested function in the regulation of olfactory signal transduction, and their presence in ciliary structures was confirmed by immunohistochemistry. Moreover, we corroborated the ciliary localization of the potassium-dependent Na(+)/Ca(2+) exchanger (NCKX) 4 and the plasma membrane Ca(2+)-ATPase 1 (PMCA1) involved in olfactory signal termination, and we detected for the first time NCKX2 in olfactory cilia. Through comparison with transcriptome data specific for mature, ciliated OSNs, we finally delineated the membrane ciliome of OSNs. The membrane proteome of olfactory cilia established here is the most complete today, thus allowing us to pave new avenues for the study of diverse molecular functions and signaling pathways in and out of olfactory cilia and thus to advance our understanding of the biology of sensory organelles in general.

Postnatal odorant exposure induces peripheral olfactory plasticity at the cellular level

Mammalian olfactory sensory neurons (OSNs) form the primary elements of the olfactory system. Inserted in the olfactory mucosa lining of the nasal cavity, they are exposed to the environment and their lifespan is brief. Several reports say that OSNs are regularly regenerated during the entire life and that odorant environment affects the olfactory epithelium. However, little is known about the impact of the odorant environment on OSNs at the cellular level and more precisely in the context of early postnatal olfactory exposure. Here we exposed MOR23-green fluorescent protein (GFP) and M71-GFP mice to lyral or acetophenone, ligands for MOR23 or M71, respectively. Daily postnatal exposure to lyral induces plasticity in the population of OSNs expressing MOR23. Their density decreases after odorant exposure, whereas the amount of MOR23 mRNA and protein remain stable in the whole epithelium. Meanwhile, quantitative PCR indicates that each MOR23 neuron has higher levels of olfactory receptor transcripts and also expresses more CNGA2 and phosphodiesterase 1C, fundamental olfactory transduction pathway proteins. Transcript levels return to baseline after 4 weeks recovery. Patch-clamp recordings reveal that exposed MOR23 neurons respond to lyral with higher sensitivity and broader dynamic range while the responses' kinetics were faster. These effects are specific to the odorant-receptor pair lyral-MOR23: there was no effect of acetophenone on MOR23 neurons and no effect of acetophenone and lyral on the M71 population. Together, our results clearly demonstrate that OSNs undergo specific anatomical, molecular, and functional adaptation when chronically exposed to odorants in the early stage of life.

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.

Developmental expression of the calcium-activated chloride channels TMEM16A and TMEM16B in the mouse olfactory epithelium

Calcium-activated chloride channels are involved in several physiological processes including olfactory perception. TMEM16A and TMEM16B, members of the transmembrane protein 16 family (TMEM16), are responsible for calcium-activated chloride currents in several cells. Both are present in the olfactory epithelium of adult mice, but little is known about their expression during embryonic development. Using immunohistochemistry we studied their expression in the mouse olfactory epithelium at various stages of prenatal development from embryonic day (E) 12.5 to E18.5 as well as in postnatal mice. At E12.5, TMEM16A immunoreactivity was present at the apical surface of the entire olfactory epithelium, but from E16.5 became restricted to a region near the transition zone with the respiratory epithelium, where localized at the apical part of supporting cells and in their microvilli. In contrast, TMEM16B immunoreactivity was present at E14.5 at the apical surface of the entire olfactory epithelium, increased in subsequent days, and localized to the cilia of mature olfactory sensory neurons. These data suggest different functional roles for TMEM16A and TMEM16B in the developing as well as in the postnatal olfactory epithelium. The presence of TMEM16A at the apical part and in microvilli of supporting cells is consistent with a role in the regulation of the chloride ionic composition of the mucus covering the apical surface of the olfactory epithelium, whereas the localization of TMEM16B to the cilia of mature olfactory sensory neurons is consistent with a role in olfactory signal transduction.

Calcium-activated chloride channels do not contribute to the odorant transduction current in the marine teleost Isacia conceptionis

This study compared the contribution of the Ca²⁺-activated Cl⁻ conductance to the electroolfactogram (EOG) evoked by different odorant classes between the marine Cabinza grunt Isacia conceptionis and rainbow trout Oncorhynchus mykiss. The Ca²⁺-activated Cl⁻ channel blocker niflumic acid significantly diminished odorant responses in O. mykiss, but had no effect on the EOG in I. conceptionis, supporting the notion that Ca²⁺-activated Cl⁻ channels may not operate as odorant transduction current amplifiers in this marine teleost.

Unraveling adaptation in eukaryotic pathways: lessons from protocells

Eukaryotic adaptation pathways operate within wide-ranging environmental conditions without stimulus saturation. Despite numerous differences in the adaptation mechanisms employed by bacteria and eukaryotes, all require energy consumption. Here, we present two minimal models showing that expenditure of energy by the cell is not essential for adaptation. Both models share important features with large eukaryotic cells: they employ small diffusible molecules and involve receptor subunits resembling highly conserved G-protein cascades. Analyzing the drawbacks of these models helps us understand the benefits of energy consumption, in terms of adjustability of response and adaptation times as well as separation of cell-external sensing and cell-internal signaling. Our work thus sheds new light on the evolution of adaptation mechanisms in complex systems.

Adaptation is a common feature in sensory systems, well familiar to us from light and dark adaptation of our visual system. Biological cells, ranging from bacteria to complex eukaryotes, including single-cell organisms and human sensory receptors, adopt different strategies to fulfill this property. However, all of them require substantial amounts of energy to adapt. Here, we compare the different biological strategies and design two minimal models which allow adaptation without requiring energy consumption. Schemes similar to the ones we proposed in our minimal models could have been adopted by ancient protocells, that have evolved into the pathways we now know and study. Analyzing our models can thus help elucidate the advantages brought to the cells by consumption of energy, including the bypassing of hard-wired cell parameters such as diffusion constants with increased control over time scales.

Investigation of nasal CO₂ receptor transduction mechanisms in wild-type and GC-D knockout mice

The main olfactory system of mice contains a small subset of olfactory sensory neurons (OSNs) that are stimulated by CO₂. The objective of this study was to record olfactory receptor responses to a range of CO₂ concentrations to further elucidate steps in the proposed CO₂ transduction pathway in mice. Electro-olfactograms (EOGs) were recorded before and after inhibiting specific steps in the CO₂ transduction pathway with topically applied inhibitors. Inhibition of extracellular carbonic anhydrase (CA) did not significantly affect EOG responses to CO₂ but did decrease EOG responses to several control odorants. Inhibition of intracellular CA or cyclic nucleotide-gated channels attenuated EOG responses to CO₂, confirming the role of these components in CO₂ sensing in mice. We also show that, like canonical OSNs, CO₂-sensitive OSNs depend on Ca²⁺-activated Cl⁻ channels for depolarization of receptor neurons. Lastly, we found that guanylyl cyclase-D knockout mice were still able to respond to CO₂, indicating that other pathways may exist for the detection of low concentrations of nasal CO₂. We discuss these findings as they relate to previous studies on CO₂-sensitive OSNs in mice and other animals.

Comparing thoracic and intra-nasal pressure transients to monitor active odor sampling during odor-guided decision making in the mouse

BACKGROUND

Recording of physiological parameters in behaving mice has seen an immense increase over recent years driven by, for example, increased miniaturization of recording devices. One parameter particularly important for odorant-driven behaviors is the breathing frequency, since the latter dictates the rate of odorant delivery to the nasal cavity and the olfactory receptor neurons located therein.

NEW METHOD

Typically, breathing patterns are monitored by either measuring the breathing-induced temperature or pressure changes in the nasal cavity. Both require the implantation of a nasal cannula and tethering of the mouse to either a cable or tubing. To avoid these limitations we used an implanted pressure sensor which reads the thoracic pressure and transmits the data telemetrically, thus making it suitable for experiments which require a freely moving animal.

RESULTS

Mice performed a Go/NoGo odorant-driven behavioral task with the implanted pressure sensor, which proved to work reliably to allow recording of breathing signals over several weeks from a given animal.

COMPARISON TO EXISTING METHOD(S)

We simultaneously recorded the thoracic and nasal pressure changes and found that measuring the thoracic pressure change yielded similar results compared to measurements of nasal pressure changes.

CONCLUSION

Telemetrically recorded breathing signals are a feasible method to monitor odorant-guided behavioral changes in breathing rates. Its advantages are most significant when recording from a freely moving animal over several weeks. The advantages and disadvantages of different methods to record breathing patterns are discussed.

Channel properties of the splicing isoforms of the olfactory calcium-activated chloride channel Anoctamin 2

Anoctamin (ANO)2 (or TMEM16B) forms a cell membrane Ca(2+)-activated Cl(-) channel that is present in cilia of olfactory receptor neurons, vomeronasal microvilli, and photoreceptor synaptic terminals. Alternative splicing of Ano2 transcripts generates multiple variants with the olfactory variants skipping exon 14 and having alternative splicing of exon 4. In the present study, 5' rapid amplification of cDNA ends analysis was conducted to characterize the 5' end of olfactory Ano2 transcripts, which showed that the most abundant Ano2 transcripts in the olfactory epithelium contain a novel starting exon that encodes a translation initiation site, whereas transcripts of the publically available sequence variant, which has an alternative and longer 5' end, were present in lower abundance. With two alternative starting exons and alternative splicing of exon 4, four olfactory ANO2 isoforms are thus possible. Patch-clamp experiments in transfected HEK293T cells expressing these isoforms showed that N-terminal sequences affect Ca(2+) sensitivity and that the exon 4-encoded sequence is required to form functional channels. Coexpression of the two predominant isoforms, one with and one without the exon 4 sequence, as well as coexpression of the two rarer isoforms showed alterations in channel properties, indicating that different isoforms interact with each other. Furthermore, channel properties observed from the coexpression of the predominant isoforms better recapitulated the native channel properties, suggesting that the native channel may be composed of two or more splicing isoforms acting as subunits that together shape the channel properties.

Ultrasensitive detection of amines by a trace amine-associated receptor

The mammalian main olfactory pathway detects volatile chemicals using two families of G-protein-coupled receptors: a large repertoire of canonical odorant receptors and a much smaller set of trace amine-associated receptors (TAARs). The TAARs are evolutionarily conserved in vertebrates, including humans, suggesting an indispensible role in olfaction. However, little is known about the functional properties of TAARs when expressed in native olfactory sensory neurons. Here we describe experiments using gene targeting, electrophysiology, and optical imaging to study the response properties of TAAR-expressing sensory neurons and their associated glomeruli in mice. We show that olfactory sensory neurons that express a subset of the TAAR repertoire are preferentially responsive to amines. In addition, neurons expressing specific TAARs, TAAR3 or TAAR4, are highly sensitive and are also broadly tuned-responding to structurally diverse amines. Surprisingly, we find that TAAR4 is exquisitely sensitive, with apparent affinities for a preferred ligand, phenylethylamine, rivaling those seen with mammalian pheromone receptors. We provide evidence that this unprecedented sensitivity is mediated via receptor coupling to the canonical odorant transduction cascade. The data suggest that the TAARs are evolutionarily retained in the olfactory receptor repertoire to mediate high-sensitivity detection of a biologically relevant class of odorous stimuli.

Common dynamical features of sensory adaptation in photoreceptors and olfactory sensory neurons

Sensory systems adapt, i.e., they adjust their sensitivity to external stimuli according to the ambient level. In this paper we show that single cell electrophysiological responses of vertebrate olfactory receptors and of photoreceptors to different input protocols exhibit several common features related to adaptation, and that these features can be used to investigate the dynamical structure of the feedback regulation responsible for the adaptation. In particular, we point out that two different forms of adaptation can be observed, in response to steps and to pairs of pulses. These two forms of adaptation appear to be in a dynamical trade-off: the more adaptation to a step is close to perfect, the slower is the recovery in adaptation to pulse pairs and viceversa. Neither of the two forms is explained by the dynamical models currently used to describe adaptation, such as the integral feedback model.

Temporal sampling, resetting, and adaptation orchestrate gradient sensing in sperm

Sperm use temporal sampling, resetting of intracellular calcium level, and adaptation of their sensitivity to respond to a wide range of chemoattractant concentrations during their voyage toward the egg.

Sperm, navigating in a chemical gradient, are exposed to a periodic stream of chemoattractant molecules. The periodic stimulation entrains Ca²⁺ oscillations that control looping steering responses. It is not known how sperm sample chemoattractant molecules during periodic stimulation and adjust their sensitivity. We report that sea urchin sperm sampled molecules for 0.2–0.6 s before a Ca²⁺ response was produced. Additional molecules delivered during a Ca²⁺ response reset the cell by causing a pronounced Ca²⁺ drop that terminated the response; this reset was followed by a new Ca²⁺ rise. After stimulation, sperm adapted their sensitivity following the Weber–Fechner law. Taking into account the single-molecule sensitivity, we estimate that sperm can register a minimal gradient of 0.8 fM/µm and be attracted from as far away as 4.7 mm. Many microorganisms sense stimulus gradients along periodic paths to translate a spatial distribution of the stimulus into a temporal pattern of the cell response. Orchestration of temporal sampling, resetting, and adaptation might control gradient sensing in such organisms as well.

Hyperpolarisation-activated cyclic nucleotide-gated channels regulate the spontaneous firing rate of olfactory receptor neurons and affect glomerular formation in mice

Olfactory receptor neurons (ORNs), which undergo lifelong neurogenesis, have been studied extensively to understand how neurons form precise topographical networks. Neural projections from ORNs are principally guided by the genetic code, which directs projections from ORNs that express a specific odorant receptor to the corresponding glomerulus in the olfactory bulb. In addition, ORNs utilise spontaneous firing activity to establish and maintain the neural map. However, neither the process of generating this spontaneous activity nor its role as a guidance cue in the olfactory bulb is clearly understood. Utilising extracellular unit-recordings in mouse olfactory epithelium slices, we demonstrated that the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels in the somas of ORNs depolarise their membranes and boost their spontaneous firing rates by sensing basal cAMP levels; the odorant-sensitive cyclic nucleotide-gated (CNG) channels in cilia do not. The basal cAMP levels were maintained via the standing activation of β-adrenergic receptors. Using a Tet-off system to over-express HCN4 channels resulted in the enhancement of spontaneous ORN activity and dramatically reduced both the size and number of glomeruli in the olfactory bulb. This phenotype was rescued by the administration of doxycycline. These findings suggest that cAMP plays different roles in cilia and soma and that basal cAMP levels in the soma are directly converted via HCN channels into a spontaneous firing frequency that acts as an intrinsic guidance cue for the formation of olfactory networks.

Phosphorylation of adenylyl cyclase III at serine1076 does not attenuate olfactory response in mice

Feedback inhibition of adenylyl cyclase III (ACIII) via Ca(2+)-induced phosphorylation has long been hypothesized to contribute to response termination and adaptation of olfactory sensory neurons (OSNs). To directly determine the functional significance of this feedback mechanism for olfaction in vivo, we genetically mutated serine(1076) of ACIII, the only residue responsible for Ca(2+)-induced phosphorylation and inhibition of ACIII (Wei et al., 1996, 1998), to alanine in mice. Immunohistochemistry and Western blot analysis showed that the mutation affects neither the cilial localization nor the expression level of ACIII in OSNs. Electroolfactogram analysis showed no differences in the responses between wild-type and mutant mice to single-pulse odorant stimulations or in several stimulation paradigms for adaptation. These results suggest that phosphorylation of ACIII on serine(1076) plays a far less important role in olfactory response attenuation than previously thought.

General olfactory sensitivity database (GOSdb): candidate genes and their genomic variations

Genetic variations in olfactory receptors likely contribute to the diversity of odorant-specific sensitivity phenotypes. Our working hypothesis is that genetic variations in auxiliary olfactory genes, including those mediating transduction and sensory neuronal development, may constitute the genetic basis for general olfactory sensitivity (GOS) and congenital general anosmia (CGA). We thus performed a systematic exploration for auxiliary olfactory genes and their documented variation. This included a literature survey, seeking relevant functional in vitro studies, mouse gene knockouts and human disorders with olfactory phenotypes, as well as data mining in published transcriptome and proteome data for genes expressed in olfactory tissues. In addition, we performed next-generation transcriptome sequencing (RNA-seq) of human olfactory epithelium and mouse olfactory epithelium and bulb, so as to identify sensory-enriched transcripts. Employing a global score system based on attributes of the 11 data sources utilized, we identified a list of 1,680 candidate auxiliary olfactory genes, of which 450 are shortlisted as having higher probability of a functional role. For the top-scoring 136 genes, we identified genomic variants (probably damaging single nucleotide polymorphisms, indels, and copy number deletions) gleaned from public variation repositories. This database of genes and their variants should assist in rationalizing the great interindividual variation in human overall olfactory sensitivity (http://genome.weizmann.ac.il/GOSdb).

Bestrophin-encoded Ca²⁺-activated Cl⁻ channels underlie a current with properties similar to the native current in the moth Spodoptera littoralis olfactory receptor neurons

Responses of insect olfactory receptor neurons (ORNs) involve an entry of Ca²⁺ through olfactory heterodimeric receptor complexes. In moths, the termination of ORN responses was found to strongly depend on the external Ca²⁺ concentration through the activation of unknown Ca²⁺-dependent Cl⁻ channels. We thus investigated the molecular identity of these Cl⁻ channels. There is compelling evidence that bestrophins form Cl⁻ channels when expressed in heterologous systems. Here we provide evidence that antennae of the moth Spodoptera littoralis express three transcripts encoding proteins with hallmarks of bestrophins. One of these transcripts, SlitBest1b, is expressed in ORNs. The heterologous expression of SlitBest1b protein in CHO-K1 cells yielded a Ca²⁺-activated Cl⁻ current that shares electrophysiological properties with the native Ca²⁺-activated Cl⁻ current of ORNs. Both currents are anionic, present similar dependence on the intracellular Ca²⁺ concentration, partly inactivate over time, have the same anion permeability sequence, the same sequence of inhibitory efficiency of blockers, the same almost linear I–V relationships and finally both currents do not depend on the cell volume. Therefore, our data suggest that SlitBest1b is a good candidate for being a molecular component of the olfactory Ca²⁺-activated Cl⁻ channel and is likely to constitute part of the insect olfactory transduction pathway. A different function (e.g. regulation of other proteins, maintenance of the anionic homeostasis in the sensillar lymph) and a different role (e.g. involvement in the olfactory system development) cannot be excluded however.

Neuropeptide receptors provide a signalling pathway for trigeminal modulation of olfactory transduction

The mammalian olfactory epithelium contains olfactory receptor neurons and trigeminal sensory endings. The former mediate odor detection, the latter the detection of irritants. The two apparently parallel chemosensory systems are in reality interdependent in various well-documented ways. Psychophysical studies have shown that virtually all odorants can act as irritants, and that most irritants have an odor. Thus, the sensory perception of odorants and irritants is based on simultaneous input from the two systems. Moreover, functional interactions between the olfactory system and the trigeminal system exist on both peripheral and central levels. Here we examine the impact of trigeminal stimulation on the odor response of olfactory receptor neurons. Using an odorant with low trigeminal potency (phenylethyl alcohol) and a non-odorous irritant (CO(2) ), we have explored this interaction in psychophysical experiments with human subjects and in electroolfactogram (EOG) recordings from rats. We have demonstrated that simultaneous activation of the trigeminal system attenuates the perception of odor intensity and distorts the EOG response. On the molecular level, we have identified a route for this cross-modal interaction. The neuropeptide calcitonin-gene related peptide (CGRP), which is released from trigeminal sensory fibres upon irritant stimulation, inhibits the odor response of olfactory receptor neurons. CGRP receptors expressed by these neurons mediate this neuromodulatory effect. This study demonstrates a site of trigeminal-olfactory interaction in the periphery. It reveals a pathway for trigeminal impact on olfactory signal processing that influences odor perception.

Phosphorylation of adenylyl cyclase III at serine1076 does not attenuate olfactory response in mice

Feedback inhibition of adenylyl cyclase III (ACIII) via Ca(2+)-induced phosphorylation has long been hypothesized to contribute to response termination and adaptation of olfactory sensory neurons (OSNs). To directly determine the functional significance of this feedback mechanism for olfaction in vivo, we genetically mutated serine(1076) of ACIII, the only residue responsible for Ca(2+)-induced phosphorylation and inhibition of ACIII (Wei et al., 1996, 1998), to alanine in mice. Immunohistochemistry and Western blot analysis showed that the mutation affects neither the cilial localization nor the expression level of ACIII in OSNs. Electroolfactogram analysis showed no differences in the responses between wild-type and mutant mice to single-pulse odorant stimulations or in several stimulation paradigms for adaptation. These results suggest that phosphorylation of ACIII on serine(1076) plays a far less important role in olfactory response attenuation than previously thought.

Expression patterns of anoctamin 1 and anoctamin 2 chloride channels in the mammalian nose

Calcium-activated chloride channels are expressed in chemosensory neurons of the nose and contribute to secretory processes and sensory signal transduction. These channels are thought to be members of the family of anoctamins (alternative name: TMEM16 proteins), which are opened by micromolar concentrations of intracellular Ca(2+). Two family members,ANO 1 (TMEM16A) and ANO 2 (TMEM16B), are expressed in the various sensory and respiratory tissues of the nose.We have examined the tissue specificity and sub-cellular localization of these channels in the nasal respiratory epithelium and in the five chemosensory organs of the nose: the main olfactory epithelium, the septal organ of Masera, the vomeronasal organ, the Grueneberg ganglion and the trigeminal system. We have found that the two channels show mutually exclusive expression patterns. ANO 1 is present in the apical membranes of various secretory epithelia in which it is co-localized with the water channel aquaporin 5. It has also been detected in acinar cells and duct cells of subepithelial glands and in the supporting cells of sensory epithelia. In contrast, ANO 2 expression is restricted to chemosensory neurons in which it has been detected in microvillar and ciliary surface structures. The different expression patterns of ANO 1 and ANO 2 have been observed in the olfactory, vomeronasal and respiratory epithelia. No expression has been detected in the Grueneberg ganglion or trigeminal sensory fibers. On the basis of this differential expression, we derive the main functional features of ANO 1 and ANO 2 chloride channels in the nose and suggest their significance for nasal physiology.

Calcium-activated chloride channels in the apical region of mouse vomeronasal sensory neurons

The rodent vomeronasal organ plays a crucial role in several social behaviors. Detection of pheromones or other emitted signaling molecules occurs in the dendritic microvilli of vomeronasal sensory neurons, where the binding of molecules to vomeronasal receptors leads to the influx of sodium and calcium ions mainly through the transient receptor potential canonical 2 (TRPC2) channel. To investigate the physiological role played by the increase in intracellular calcium concentration in the apical region of these neurons, we produced localized, rapid, and reproducible increases in calcium concentration with flash photolysis of caged calcium and measured calcium-activated currents with the whole cell voltage-clamp technique. On average, a large inward calcium-activated current of −261 pA was measured at −50 mV, rising with a time constant of 13 ms. Ion substitution experiments showed that this current is anion selective. Moreover, the chloride channel blockers niflumic acid and 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid partially inhibited the calcium-activated current. These results directly demonstrate that a large chloride current can be activated by calcium in the apical region of mouse vomeronasal sensory neurons. Furthermore, we showed by immunohistochemistry that the calcium-activated chloride channels TMEM16A/anoctamin1 and TMEM16B/anoctamin2 are present in the apical layer of the vomeronasal epithelium, where they largely colocalize with the TRPC2 transduction channel. Immunocytochemistry on isolated vomeronasal sensory neurons showed that TMEM16A and TMEM16B coexpress in the neuronal microvilli. Therefore, we conclude that microvilli of mouse vomeronasal sensory neurons have a high density of calcium-activated chloride channels that may play an important role in vomeronasal transduction.

Recordings from cultured newt olfactory receptor cells

Freshly dissociated olfactory receptor cells (ORCs) are commonly used in electrophysiological research investigations of the physicochemical mechanisms of olfactory signal transduction. Because the morphology of cultured cells clearly becomes worse over time, the ORCs are examined traditionally within several days after dissociation. However, there has been a major concern that cells are affected soon after dissociation. To gain a better understanding of the reliability of data obtained from solitary cells, we obtained electrical data during the lifetime of single ORCs dissociated from the newt. The time course for the deterioration could be revealed by monitoring the membrane properties during culture. Although the number of living cells that were identified by trypan blue extrusion declined day by day, the remaining cells retained morphology and their fundamental electrical features until day 19. In some cells, the cilia and dendrite were observed until day 21, and the bipolar morphology until day 31. The fundamental features of cell excitation were maintained during culture without showing remarkable changes when they retained morphological features. The results suggest that electrical properties of cells are almost unchanged within several days. Furthermore, the dissociated newt ORCs can be used for several weeks that are almost comparable to the intrinsic lifetime of the ORCs in vivo.

A dynamical feedback model for adaptation in the olfactory transduction pathway

Olfactory transduction exhibits two distinct types of adaptation, which we denote multipulse and step adaptation. In terms of measured transduction current, multipulse adaptation appears as a decrease in the amplitude of the second of two consecutive responses when the olfactory neuron is stimulated with two brief pulses. Step adaptation occurs in response to a sustained steplike stimulation and is characterized by a return to a steady-state current amplitude close to the prestimulus value, after a transient peak. In this article, we formulate a dynamical model of the olfactory transduction pathway, which includes the kinetics of the CNG channels, the concentration of Ca ions flowing through them, and the Ca-complexes responsible for the regulation. Based on this model, a common dynamical explanation for the two types of adaptation is suggested. We show that both forms of adaptation can be well described using different time constants for the kinetics of Ca ions (faster) and the kinetics of the feedback mechanisms (slower). The model is validated on experimental data collected in voltage-clamp conditions using different techniques and animal species.

Computing with neural synchrony

Neurons communicate primarily with spikes, but most theories of neural computation are based on firing rates. Yet, many experimental observations suggest that the temporal coordination of spikes plays a role in sensory processing. Among potential spike-based codes, synchrony appears as a good candidate because neural firing and plasticity are sensitive to fine input correlations. However, it is unclear what role synchrony may play in neural computation, and what functional advantage it may provide. With a theoretical approach, I show that the computational interest of neural synchrony appears when neurons have heterogeneous properties. In this context, the relationship between stimuli and neural synchrony is captured by the concept of synchrony receptive field, the set of stimuli which induce synchronous responses in a group of neurons. In a heterogeneous neural population, it appears that synchrony patterns represent structure or sensory invariants in stimuli, which can then be detected by postsynaptic neurons. The required neural circuitry can spontaneously emerge with spike-timing-dependent plasticity. Using examples in different sensory modalities, I show that this allows simple neural circuits to extract relevant information from realistic sensory stimuli, for example to identify a fluctuating odor in the presence of distractors. This theory of synchrony-based computation shows that relative spike timing may indeed have computational relevance, and suggests new types of neural network models for sensory processing with appealing computational properties.

How does the brain compute? Traditional theories of neural computation describe the operating function of neurons in terms of average firing rates, with the timing of spikes bearing little information. However, numerous studies have shown that spike timing can convey information and that neurons are highly sensitive to synchrony in their inputs. Here I propose a simple spike-based computational framework, based on the idea that stimulus-induced synchrony can be used to extract sensory invariants (for example, the location of a sound source), which is a difficult task for classical neural networks. It relies on the simple remark that a series of repeated coincidences is in itself an invariant. Many aspects of perception rely on extracting invariant features, such as the spatial location of a time-varying sound, the identity of an odor with fluctuating intensity, the pitch of a musical note. I demonstrate that simple synchrony-based neuron models can extract these useful features, by using spiking models in several sensory modalities.

A selective PMCA inhibitor does not prolong the electroolfactogram in mouse

Background

Within the cilia of vertebrate olfactory receptor neurons, Ca²⁺ accumulates during odor transduction. Termination of the odor response requires removal of this Ca²⁺, and prior evidence suggests that both Na⁺/Ca²⁺ exchange and plasma membrane Ca²⁺-ATPase (PMCA) contribute to this removal.

Principal Findings

In intact mouse olfactory epithelium, we measured the time course of termination of the odor-induced field potential. Replacement of mucosal Na⁺ with Li⁺, which reduces the ability of Na⁺/Ca²⁺ exchange to expel Ca²⁺, prolonged the termination as expected. However, treating the epithelium with the specific PMCA inhibitor caloxin 1b1 caused no significant increase in the time course of response termination.

Conclusions

Under these experimental conditions, PMCA does not contribute detectably to the termination of the odor response.

Functional evidence of multidrug resistance transporters (MDR) in rodent olfactory epithelium

BACKGROUND

P-glycoprotein (Pgp) and multidrug resistance-associated protein (MRP1) are membrane transporter proteins which function as efflux pumps at cell membranes and are considered to exert a protective function against the entry of xenobiotics. While evidence for Pgp and MRP transporter activity is reported for olfactory tissue, their possible interaction and participation in the olfactory response has not been investigated.

PRINCIPAL FINDINGS

Functional activity of putative MDR transporters was assessed by means of the fluorometric calcein acetoxymethyl ester (calcein-AM) accumulation assay on acute rat and mouse olfactory tissue slices. Calcein-AM uptake was measured as fluorescence intensity changes in the presence of Pgp or MRP specific inhibitors. Epifluorescence microscopy measured time course analysis in the olfactory epithelium revealed significant inhibitor-dependent calcein uptake in the presence of each of the selected inhibitors. Furthermore, intracellular calcein accumulation in olfactory receptor neurons was also significantly increased in the presence of either one of the Pgp or MRP inhibitors. The presence of Pgp or MRP1 encoding genes in the olfactory mucosa of rat and mouse was confirmed by RT-PCR with appropriate pairs of species-specific primers. Both transporters were expressed in both newborn and adult olfactory mucosa of both species. To assess a possible involvement of MDR transporters in the olfactory response, we examined the electrophysiological response to odorants in the presence of the selected MDR inhibitors by recording electroolfactograms (EOG). In both animal species, MRPs inhibitors induced a marked reduction of the EOG magnitude, while Pgp inhibitors had only a minor or no measurable effect.

CONCLUSIONS

The findings suggest that both Pgp and MRP transporters are functional in the olfactory mucosa and in olfactory receptor neurons. Pgp and MRPs may be cellular constituents of olfactory receptor neurons and represent potential mechanisms for modulation of the olfactory response.

Odorant-induced responses recorded from olfactory receptor neurons using the suction pipette technique

Animals sample the odorous environment around them through the chemosensory systems located in the nasal cavity. Chemosensory signals affect complex behaviors such as food choice, predator, conspecific and mate recognition and other socially relevant cues. Olfactory receptor neurons (ORNs) are located in the dorsal part of the nasal cavity embedded in the olfactory epithelium. These bipolar neurons send an axon to the olfactory bulb (see Fig. 1, Reisert & Zhao, originally published in the Journal of General Physiology) and extend a single dendrite to the epithelial border from where cilia radiate into the mucus that covers the olfactory epithelium. The cilia contain the signal transduction machinery that ultimately leads to excitatory current influx through the ciliary transduction channels, a cyclic nucleotide-gated (CNG) channel and a Ca(2+)-activated Cl(-) channel (Fig. 1). The ensuing depolarization triggers action potential generation at the cell body. In this video we describe the use of the "suction pipette technique" to record odorant-induced responses from ORNs. This method was originally developed to record from rod photoreceptors and a variant of this method can be found at jove.com modified to record from mouse cone photoreceptors. The suction pipette technique was later adapted to also record from ORNs. Briefly, following dissociation of the olfactory epithelium and cell isolation, the entire cell body of an ORN is sucked into the tip of a recording pipette. The dendrite and the cilia remain exposed to the bath solution and thus accessible to solution changes to enable e.g. odorant or pharmacological blocker application. In this configuration, no access to the intracellular environment is gained (no whole-cell voltage clamp) and the intracellular voltage remains free to vary. This allows the simultaneous recording of the slow receptor current that originates at the cilia and fast action potentials fired by the cell body. The difference in kinetics between these two signals allows them to be separated using different filter settings. This technique can be used on any wild type or knockout mouse or to record selectively from ORNs that also express GFP to label specific subsets of ORNs, e.g. expressing a given odorant receptor or ion channel.

Physiological mechanisms of signal termination in biological systems

Studies on the regulation of cellular activity mainly focus on signal generation, but termination of signalling is an equally important factor, which prevents inappropriate activity. This paper reviews the mechanisms, which can cause termination of signalling, and provides examples that illustrate the importance of these processes. Inactivation of voltage-gated Na(+) channels and the photoactivated rhodopsin molecule is caused by rapid conformational rearrangements. Negative feedback can also contribute to the termination of signalling for various mechanisms, including plasma membrane ion channels or cAMP signal generation. In immune cells, the tyrosine-based inhibitory motif (ITIM)-containing molecules are essential negative regulatory components. Desensitization of G-protein-coupled receptors can occur with homologous and heterologous mechanisms, mediated by β-arrestin molecules and second messenger-induced kinases respectively. In NF-κB signalling, resynthetized IκB and other enzymes form negative feedback loops. GTPase-activating proteins are also dedicated to termination of signalling, because they can switch off the small G proteins by increasing their endogenous GTP hydrolysis. In many systems, signal termination is a result of a combined action of several different mechanisms, which underlines the importance of these processes.