The effect of external sodium concentration on sodium-calcium exchange in frog olfactory receptor cells

The physiological roles of anoctamin2/TMEM16B and anoctamin1/TMEM16A in chemical senses

Chemical senses allow animals to detect and discriminate a vast array of molecules. The olfactory system is responsible of the detection of small volatile molecules, while water dissolved molecules are detected by taste buds in the oral cavity. Moreover, many animals respond to signaling molecules such as pheromones and other semiochemicals through the vomeronasal organ. The peripheral organs dedicated to chemical detection convert chemical signals into perceivable information through the employment of diverse receptor types and the activation of multiple ion channels. Two ion channels, TMEM16B, also known as anoctamin2 (ANO2) and TMEM16A, or anoctamin1 (ANO1), encoding for Ca2+-activated Cl¯ channels, have been recently described playing critical roles in various cell types. This review aims to discuss the main properties of TMEM16A and TMEM16B-mediated currents and their physiological roles in chemical senses. In olfactory sensory neurons, TMEM16B contributes to amplify the odorant response, to modulate firing, response kinetics and adaptation. TMEM16A and TMEM16B shape the pattern of action potentials in vomeronasal sensory neurons increasing the interspike interval. In type I taste bud cells, TMEM16A is activated during paracrine signaling mediated by ATP. This review aims to shed light on the regulation of diverse signaling mechanisms and neuronal excitability mediated by Ca-activated Cl¯ channels, hinting at potential new roles for TMEM16A and TMEM16B in the chemical senses.

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.

The functional relevance of olfactory marker protein in the vertebrate olfactory system: a never-ending story

Olfactory marker protein (OMP) was first described as a protein expressed in olfactory receptor neurons (ORNs) in the nasal cavity. In particular, OMP, a small cytoplasmic protein, marks mature ORNs and is also expressed in the neurons of other nasal chemosensory systems: the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. While its expression pattern was more easily established, OMP's function remained relatively vague. To date, most of the work to understand OMP's role has been done using mice lacking OMP. This mostly phenomenological work has shown that OMP is involved in sharpening the odorant response profile and in quickening odorant response kinetics of ORNs and that it contributes to targeting of ORN axons to the olfactory bulb to refine the glomerular response map. Increasing evidence shows that OMP acts at the early stages of olfactory transduction by modulating the kinetics of cAMP, the second messenger of olfactory transduction. However, how this occurs at a mechanistic level is not understood, and it might also not be the only mechanism underlying all the changes observed in mice lacking OMP. Recently, OMP has been detected outside the nose, including the brain and other organs. Although no obvious logic has become apparent regarding the underlying commonality between nasal and extranasal expression of OMP, a broader approach to diverse cellular systems might help unravel OMP's functions and mechanisms of action inside and outside the nose.

The long tale of the calcium activated Cl- channels in olfactory transduction

Ca²⁺-activated Cl^- currents have been implicated in many cellular processes in different cells, but for many years, their molecular identity remained unknown. Particularly intriguing are Ca²⁺-activated Cl^- currents in olfactory transduction, first described in the early 90s. Well characterized electrophysiologically, they carry most of the odorant-induced receptor current in the cilia of olfactory sensory neurons (OSNs). After many attempts to determine their molecular identity, TMEM16B was found to be abundantly expressed in the cilia of OSNs in 2009 and having biophysical properties like those of the native olfactory channel. A TMEM16B knockout mouse confirmed that TMEM16B was indeed the olfactory Cl^- channel but also suggested a limited role in olfactory physiology and behavior. The question then arises of what the precise role of TMEM16b in olfaction is. Here we review the long story of this channel and its possible roles.

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 SLC24 gene family of Na⁺/Ca²⁺-K⁺ exchangers: from sight and smell to memory consolidation and skin pigmentation

Members of the SLC24 gene family encode K(+)-dependent Na(+)/Ca(2+) exchangers (NCKX) that utilize both the inward Na(+) and outward K(+) gradients to extrude Ca(2+) from cells. There are five human SLC24 genes that play a role in biological process as diverse as vision in retinal rod and cone photoreceptors, olfaction, skin pigmentation and at least three of the five genes are also widely expressed in the brain. Here I review the functional, physiological and structural features of NCKX proteins that have emerged in the past few years.

Characterization of a plasma membrane Ca(2+) ATPase expressed in olfactory receptor neurons of the moth Spodoptera littoralis

The response of insect olfactory receptor neurons (ORNs) involves an increase in intracellular Ca(2+) concentration, as in vertebrate ORNs. In order to decipher the Ca(2+) clearance mechanisms in insect ORNs, we have investigated the presence of a plasma membrane Ca(2+) ATPase (PMCA) in the peripheral olfactory system of the moth Spodoptera littoralis. From an analysis of a male antennal expressed-sequence-tag database combined with a strategy of 5'/3' rapid amplification of cDNA ends plus the polymerase chain reaction, we have cloned a full-length cDNA encoding a PMCA. In adult males, the PMCA transcript has been found in various tissues, including the antennae in which its presence has been detected in the sensilla trichodea, and in cultured ORNs. The PMCA gene is slightly expressed at the end of the pupal stage, reaches a maximum at emergence and is maintained at a high level during the adult period. Taken together, these results provide, for the first time, molecular evidence for the putative participation of a PMCA in signalling pathways responsible for the establishment and functioning of the insect peripheral olfactory system.

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.

The Na(+)/Ca(2+) exchanger NCKX4 governs termination and adaptation of the mammalian olfactory response

Sensory perception requires accurate encoding of stimulus information by sensory receptor cells. We identified NCKX4, a potassium-dependent Na(+)/Ca(2+) exchanger, as being necessary for rapid response termination and proper adaptation of vertebrate olfactory sensory neurons (OSNs). Nckx4(-/-) (also known as Slc24a4) mouse OSNs displayed substantially prolonged responses and stronger adaptation. Single-cell electrophysiological analyses revealed that the majority of Na(+)-dependent Ca(2+) exchange in OSNs relevant to sensory transduction is a result of NCKX4 and that Nckx4(-/-) mouse OSNs are deficient in encoding action potentials on repeated stimulation. Olfactory-specific Nckx4(-/-) mice had lower body weights and a reduced ability to locate an odorous source. These results establish the role of NCKX4 in shaping olfactory responses and suggest that rapid response termination and proper adaptation of peripheral sensory receptor cells tune the sensory system for optimal perception.

Olfactory receptor neuron responses coding for rapid odour sampling

Vertebrate olfactory receptor neurons (ORNs) are stimulated in a rhythmic manner in vivo, driven by delivery of odorants to the nasal cavity carried by the inhaled air, making olfaction a sense where animals can control the frequency of stimulus delivery. How ORNs encode repeated stimulation at resting, low breathing frequencies and at increased sniffing frequencies is not known, nor is it known if the olfactory transduction cascade is accurate and fast enough to follow high frequency stimulation. We investigated mouse olfactory responses to stimulus frequencies mimicking odorant exposure during low (2Hz) and high (5Hz) frequency sniffing. ORNs reliably follow low frequency stimulations with high fidelity by generating bursts of action potentials at each stimulation at intermediate odorant concentrations, but fail to do so at high odorant concentrations. Higher stimulus frequencies across all odorant concentrations reduced the likelihood of action potential generation, increased the latency of response, and decreased there liability of encoding the onset of stimulation. Thus an increase in stimulus frequency degrades and at high odorant concentrations entirely prevents action potential generation in individual ORNs, causing reduced signalling to the olfactory bulb. These results demonstrate that ORNs do not simply relay timing and concentration of an odorous stimulus, but also process and modulate the stimulus in a frequency-dependent manner which is controlled by the chosen sniffing rate.

Spatial distribution of calcium-gated chloride channels in olfactory cilia

Background

In vertebrate olfactory receptor neurons, sensory cilia transduce odor stimuli into changes in neuronal membrane potential. The voltage changes are primarily caused by the sequential openings of two types of channel: a cyclic-nucleotide-gated (CNG) cationic channel and a calcium-gated chloride channel. In frog, the cilia are 25 to 200 µm in length, so the spatial distributions of the channels may be an important determinant of odor sensitivity.

Principal Findings

To determine the spatial distribution of the chloride channels, we recorded from single cilia as calcium was allowed to diffuse down the length of the cilium and activate the channels. A computational model of this experiment allowed an estimate of the spatial distribution of the chloride channels. On average, the channels were concentrated in a narrow band centered at a distance of 29% of the ciliary length, measured from the base of the cilium. This matches the location of the CNG channels determined previously. This non-uniform distribution of transduction proteins is consistent with similar findings in other cilia.

Conclusions

On average, the two types of olfactory transduction channel are concentrated in the same region of the cilium. This may contribute to the efficient detection of weak stimuli.

Olfactory response termination involves Ca2+-ATPase in vertebrate olfactory receptor neuron cilia

In vertebrate olfactory receptor neurons (ORNs), odorant-induced activation of the transduction cascade culminates in production of cyclic AMP, which opens cyclic nucleotide–gated channels in the ciliary membrane enabling Ca²⁺ influx. The ensuing elevation of the intraciliary Ca²⁺ concentration opens Ca²⁺-activated Cl⁻ channels, which mediate an excitatory Cl⁻ efflux from the cilia. In order for the response to terminate, the Cl⁻ channel must close, which requires that the intraciliary Ca²⁺ concentration return to basal levels. Hitherto, the extrusion of Ca²⁺ from the cilia has been thought to depend principally on a Na⁺–Ca²⁺ exchanger.

In this study, we show using simultaneous suction pipette recording and Ca²⁺-sensitive dye fluorescence measurements that in fire salamander ORNs, withdrawal of external Na⁺ from the solution bathing the cilia, which incapacitates Na⁺–Ca²⁺exchange, has only a modest effect on the recovery of the electrical response and the accompanying decay of intraciliary Ca²⁺ concentration. In contrast, exposure of the cilia to vanadate or carboxyeosin, a manipulation designed to block Ca²⁺-ATPase, has a substantial effect on response recovery kinetics. Therefore, we conclude that Ca²⁺-ATPase contributes to Ca²⁺ extrusion in ORNs, and that Na⁺–Ca²⁺exchange makes only a modest contribution to Ca²⁺ homeostasis in this species.

Mechanism of olfactory masking in the sensory cilia

Olfactory masking has been used to erase the unpleasant sensation in human cultures for a long period of history. Here, we show a positive correlation between the human masking and the odorant suppression of the transduction current through the cyclic nucleotide-gated (CNG) and Ca2+-activated Cl- (Cl(Ca)) channels. Channels in the olfactory cilia were activated with the cytoplasmic photolysis of caged compounds, and their sensitiveness to odorant suppression was measured with the whole cell patch clamp. When 16 different types of chemicals were applied to cells, cyclic AMP (cAMP)-induced responses (a mixture of CNG and Cl(Ca) currents) were suppressed widely with these substances, but with different sensitivities. Using the same chemicals, in parallel, we measured human olfactory masking with 6-rate scoring tests and saw a correlation coefficient of 0.81 with the channel block. Ringer's solution that was just preexposed to the odorant-containing air affected the cAMP-induced current of the single cell, suggesting that odorant suppression occurs after the evaporation and air/water partition of the odorant chemicals at the olfactory mucus. To investigate the contribution of Cl(Ca), the current was exclusively activated by using the ultraviolet photolysis of caged Ca, DM-nitrophen. With chemical stimuli, it was confirmed that Cl(Ca) channels were less sensitive to the odorant suppression. It is interpreted, however, that in the natural odorant response the Cl(Ca) is affected by the reduction of Ca2+ influx through the CNG channels as a secondary effect. Because the signal transmission between CNG and Cl(Ca) channels includes nonlinear signal-boosting process, CNG channel blockage leads to an amplified reduction in the net current. In addition, we mapped the distribution of the Cl(Ca) channel in living olfactory single cilium using a submicron local [Ca2+]i elevation with the laser photolysis. Cl(Ca) channels are expressed broadly along the cilia. We conclude that odorants regulate CNG level to express masking, and Cl(Ca) in the cilia carries out the signal amplification and reduction evenly spanning the entire cilia. The present findings may serve possible molecular architectures to design effective masking agents, targeting olfactory manipulation at the nano-scale ciliary membrane.

Limits of calcium clearance by plasma membrane calcium ATPase in olfactory cilia

Background

In any fine sensory organelle, a small influx of Ca²⁺ can quickly elevate cytoplasmic Ca²⁺. Mechanisms must exist to clear the ciliary Ca²⁺ before it reaches toxic levels. One such organelle has been well studied: the vertebrate olfactory cilium. Recent studies have suggested that clearance from the olfactory cilium is mediated in part by plasma membrane Ca²⁺-ATPase (PMCA).

Principal Findings

In the present study, electrophysiological assays were devised to monitor cytoplasmic free Ca²⁺ in single frog olfactory cilia. Ca²⁺ was allowed to enter isolated cilia, either through the detached end or through membrane channels. Intraciliary Ca²⁺ was monitored via the activity of ciliary Ca²⁺-gated Cl⁻ channels, which are sensitive to free Ca²⁺ from about 2 to 10 µM. No significant effect of MgATP on intraciliary free Ca²⁺ could be found. Carboxyeosin, which has been used to inhibit PMCA, was found to substantially increase a ciliary transduction current activated by cyclic AMP. This increase was ATP-independent.

Conclusions

Alternative explanations are suggested for two previous experiments taken to support a role for PMCA in ciliary Ca²⁺ clearance. It is concluded that PMCA in the cilium plays a very limited role in clearing the micromolar levels of intraciliary Ca²⁺ produced during the odor response.

The electrochemical basis of odor transduction in vertebrate olfactory cilia

Most vertebrate olfactory receptor neurons share a common G-protein-coupled pathway for transducing the binding of odorant into depolarization. The depolarization involves 2 currents: an influx of cations (including Ca2+) through cyclic nucleotide-gated channels and a secondary efflux of Cl- through Ca2+-gated Cl- channels. The relation between stimulus strength and receptor current shows positive cooperativity that is attributed to the channel properties. This cooperativity amplifies the responses to sufficiently strong stimuli but reduces sensitivity and dynamic range. The odor response is transient, and prolonged or repeated stimulation causes adaptation and desensitization. At least 10 mechanisms may contribute to termination of the response; several of these result from an increase in intraciliary Ca2+. It is not known to what extent regulation of ionic concentrations in the cilium depends on the dendrite and soma. Although many of the major mechanisms have been identified, odor transduction is not well understood at a quantitative level.