Calcium-activated chloride current amplifies the response to urine in mouse vomeronasal sensory neurons

Interaction between TRP channels and anoctamins

Anoctamin 1 (ANO1) binds to transient receptor potential (TRP) channels (protein-protein interaction) and then is activated by TRP channels (functional interaction). TRP channels are non-selective cation channels that are expressed throughout the body and play roles in multiple physiological functions. Studies on TRP channels increased after the identification of TRP vanilloid 1 (TRPV1) in 1997. Calcium-activated chloride channel anoctamin 1 (ANO1, also called TMEM16A and DOG1) was identified in 2008. ANO1 plays a major role in TRP channel-mediated functions, as first shown in 2014 with the demonstration of a protein-protein interaction between TRPV4 and ANO1. In cells that co-express TRP channels and ANO1, calcium entering cells through activated TRP channels causes ANO1 activation. Therefore, in many tissues, the physiological functions related to TRP channels are modulated through chloride flux associated with ANO1 activation. In this review, we summarize the latest understanding of TRP-ANO1 interactions, particularly interaction of ANO1 with TRPV4, TRP canonical 6 (TRPC6), TRPV3, TRPV1, and TRPC2 in the salivary glands, blood vessels, skin keratinocytes, primary sensory neurons, and vomeronasal organs, respectively.

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.

TMEM16A and TMEM16B Modulate Pheromone-Evoked Action Potential Firing in Mouse Vomeronasal Sensory Neurons

The mouse vomeronasal system controls several social behaviors. Pheromones and other social cues are detected by sensory neurons in the vomeronasal organ (VNO). Stimuli activate a transduction cascade that leads to membrane potential depolarization, increase in cytosolic Ca²⁺ level, and increased firing. The Ca²⁺-activated chloride channels TMEM16A and TMEM16B are co-expressed within microvilli of vomeronasal neurons, but their physiological role remains elusive. Here, we investigate the contribution of each of these channels to vomeronasal neuron firing activity by comparing wild-type (WT) and knock-out (KO) mice. Performing loose-patch recordings from neurons in acute VNO slices, we show that spontaneous activity is modified by Tmem16a KO, indicating that TMEM16A, but not TMEM16B, is active under basal conditions. Upon exposure to diluted urine, a rich source of mouse pheromones, we observe significant changes in activity. Vomeronasal sensory neurons (VSNs) from Tmem16a cKO and Tmem16b KO mice show shorter interspike intervals (ISIs) compared with WT mice, indicating that both TMEM16A and TMEM16B modulate the firing pattern of pheromone-evoked activity in VSNs.

Cyclophosphamide has Long-Term Effects on Proliferation in Olfactory Epithelia

Chemotherapy patients often experience chemosensory changes during and after drug therapy. The chemotherapy drug, cyclophosphamide (CYP), has known cytotoxic effects on sensory and proliferating cells of the taste system. Like the taste system, cells in the olfactory epithelia undergo continuous renewal. Therefore, we asked if a single injection of 75 mg/kg CYP would affect cell proliferation in the anterior dorsomedial region of the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) from 0 to 125 days after injection. Both epithelia showed a decrease in Ki67-labeled cells compared to controls at day 1 and no Ki67+ cells at day 2 postinjection. In the sensory layer of the MOE, cell proliferation began to recover 4 days after CYP injection and by 6 days, the rate of proliferation was significantly greater than controls. Ki67+ cells peaked 30 days postinjection, then declined to control levels at day 45. Similar temporal sequences of initial CYP-induced suppression of cell proliferation followed by elevated rates peaking 30-45 days postinjection were seen in the sustentacular layer of the MOE and all 3 areas (sensory, sustentacular, marginal) of the VNO. CYP affected proliferation in the sensory layer of the MOE more than the sustentacular layer and all 3 areas of the VNO. These findings suggest that chemotherapy involving CYP is capable of affecting cell renewal of the olfactory system and likely contributes to clinical loss of function during and after chemotherapy.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Ca2+-activated Cl- currents in the murine vomeronasal organ enhance neuronal spiking but are dispensable for male-male aggression

Ca²⁺-activated Cl^- currents have been observed in many physiological processes, including sensory transduction in mammalian olfaction. The olfactory vomeronasal (or Jacobson's) organ (VNO) detects molecular cues originating from animals of the same species or from predators. It then triggers innate behaviors such as aggression, mating, or flight. In the VNO, Ca²⁺-activated Cl^- channels (CaCCs) are thought to amplify the initial pheromone-evoked receptor potential by mediating a depolarizing Cl^- efflux. Here, we confirmed the co-localization of the Ca²⁺-activated Cl^- channels anoctamin 1 (Ano1, also called TMEM16A) and Ano2 (TMEM16B) in microvilli of apically and basally located vomeronasal sensory neurons (VSNs) and their absence in supporting cells of the VNO. Both channels were expressed as functional isoforms capable of giving rise to Ca²⁺-activated Cl^- currents. Although these currents persisted in the VNOs of mice lacking Ano2, they were undetectable in olfactory neuron-specific Ano1 knockout mice irrespective of the presence of Ano2 The loss of Ca²⁺-activated Cl^- currents resulted in diminished spontaneous and drastically reduced pheromone-evoked spiking of VSNs. Although this indicated an important role of anoctamin channels in VNO signal amplification, the lack of this amplification did not alter VNO-dependent male-male territorial aggression in olfactory Ano1/Ano2 double knockout mice. We conclude that Ano1 mediates the bulk of Ca²⁺-activated Cl^- currents in the VNO and that Ano2 plays only a minor role. Furthermore, vomeronasal signal amplification by CaCCs appears to be dispensable for the detection of male-specific pheromones and for near-normal aggressive behavior in mice.

Inferior Olivary TMEM16B Mediates Cerebellar Motor Learning

Ca²⁺-activated ion channels shape membrane excitability and Ca²⁺ dynamics in response to cytoplasmic Ca²⁺ elevation. Compared to the Ca²⁺-activated K⁺ channels, known as BK and SK channels, the physiological importance of Ca²⁺-activated Cl^- channels (CaCCs) in neurons has been largely overlooked. Here we report that CaCCs coexist with BK and SK channels in inferior olivary (IO) neurons that send climbing fibers to innervate cerebellar Purkinje cells for the control of motor learning and timing. Ca²⁺ influx through the dendritic high-threshold voltage-gated Ca²⁺ channels activates CaCCs, which contribute to membrane repolarization of IO neurons. Loss of TMEM16B expression resulted in the absence of CaCCs in IO neurons, leading to markedly diminished action potential firing of IO neurons in TMEM16B knockout mice. Moreover, these mutant mice exhibited severe cerebellar motor learning deficits. Our findings thus advance the understanding of the neurophysiology of CaCCs and the ionic basis of IO neuron excitability.

Type 3 inositol 1,4,5-trisphosphate receptor is dispensable for sensory activation of the mammalian vomeronasal organ

Signal transduction in sensory neurons of the mammalian vomeronasal organ (VNO) involves the opening of the canonical transient receptor potential channel Trpc2, a Ca²⁺-permeable cation channel that is activated by diacylglycerol and inhibited by Ca²⁺-calmodulin. There has been a long-standing debate about the extent to which the second messenger inositol 1,4,5-trisphosphate (InsP₃) and type 3 InsP₃ receptor (InsP₃R3) are involved in the opening of Trpc2 channels and in sensory activation of the VNO. To address this question, we investigated VNO function of mice carrying a knockout mutation in the Itpr3 locus causing a loss of InsP₃R3. We established a new method to monitor Ca²⁺ in the endoplasmic reticulum of vomeronasal sensory neurons (VSNs) by employing the GFP-aequorin protein sensor erGAP2. We also performed simultaneous InsP₃ photorelease and Ca²⁺ monitoring experiments, and analysed Ca²⁺ dynamics, sensory currents, and action potential or field potential responses in InsP₃R3-deficient VSNs. Disruption of Itpr3 abolished or minimized the Ca²⁺ transients evoked by photoactivated InsP₃, but there was virtually no effect on sensory activation of VSNs. Therefore, InsP₃R3 is dispensable for primary chemoelectrical transduction in mouse VNO. We conclude that InsP₃R3 is not required for gating of Trpc2 in VSNs.

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.

Elevated Cytosolic Cl- Concentrations in Dendritic Knobs of Mouse Vomeronasal Sensory Neurons

In rodents, the vomeronasal system controls social and sexual behavior. However, several mechanistic aspects of sensory signaling in the vomeronasal organ remain unclear. Here, we investigate the biophysical basis of a recently proposed vomeronasal signal transduction component-a Ca(2+)-activated Cl(-) current. As the physiological role of such a current is a direct function of the Cl(-) equilibrium potential, we determined the intracellular Cl(-) concentration in dendritic knobs of vomeronasal neurons. Quantitative fluorescence lifetime imaging of a Cl(-)-sensitive dye at the apical surface of the intact vomeronasal neuroepithelium revealed increased cytosolic Cl(-) levels in dendritic knobs, a substantially lower Cl(-) concentration in vomeronasal sustentacular cells, and an apparent Cl(-) gradient in vomeronasal neurons along their dendritic apicobasal axis. Together, our data provide a biophysical basis for sensory signal amplification in vomeronasal neuron microvilli by opening Ca(2+)-activated Cl(-) channels.

Intracellular chloride concentration of the mouse vomeronasal neuron

Background

The vomeronasal organ (VNO) is specialized in detecting pheromone and heterospecific cues in the environment. Recent studies demonstrate the involvement of multiple ion channels in VNO signal transduction, including the calcium-activated chloride channels (CACCs). Opening of CACCs appears to result in activation of VNO neuron through outflow of Cl⁻ ions. However, the intracellular Cl⁻ concentration remains undetermined.

Results

We used the chloride ion quenching dye, MQAE, to measure the intracellular Cl⁻ concentration of VNO neuron in live VNO slices. The resting Cl⁻ concentration in the VNO neurons is measured at 84.73 mM. Urine activation of the VNO neurons causes a drop in Cl⁻ concentration, consistent with the notion of an efflux of Cl⁻ to depolarize the cells. Similar observation is made for VNO neurons from mice with deletion of the transient receptor potential canonical channel 2 (TRPC2), which have a resting Cl⁻ concentrations at 81 mM.

Conclusions

The VNO neurons rest at high intracellular Cl⁻ concentration, which can lead to depolarization of the cell when chloride channels open. These results also provide additional support of TRPC2-independent pathway of VNO activation.

TRICK or TRP? What Trpc2(-/-) mice tell us about vomeronasal organ mediated innate behaviors

The vomeronasal organ (VNO) plays an important role in mediating semiochemical communications and social behaviors in terrestrial species. Genetic knockout of individual components in the signaling pathways has been used to probe vomeronasal functions, and has provided much insights into how the VNO orchestrates innate behaviors. However, all data do not agree. In particular, knocking out Trpc2, a member of the TRP family of non-selective cationic channel thought to be the main transduction channel in the VNO, results in a number of fascinating behavioral phenotypes that have not been observed in other animals whose vomeronasal function is disrupted. Recent studies have identified signaling pathways that operate in parallel of Trpc2, raising the possibility that Trpc2 mutant animals may display neomorphic behaviors. In this article, I provide a critical analysis of emerging evidence to reconcile the discrepancies and discuss their implications.

Proteomic analysis of cellular protein expression profiles in response to grass carp reovirus infection

Grass carp (Ctenopharyngodon idella) hemorrhagic disease, caused by grass carp reovirus (GCRV), is emerging as a serious problem in grass carp aquaculture. To better understand the molecular responses to GCRV infection, two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization tandem mass spectroscopy were performed to investigate altered proteins in C. idella kidney (CIK) cells. Differentially expressed proteins in mock infected CIK cells and GCRV-infected CIK cells were compared. Twenty-three differentially expressed spots were identified (22 upregulated spots and 1 downregulated spot), which included cytoskeleton proteins, macromolecular biosynthesis-associated proteins, stress response proteins, signal transduction proteins, energy metabolism-associated proteins and ubiquitin proteasome pathway-associated proteins. Moreover, 10 of the corresponding genes of the differentially expressed proteins were quantified by real-time reverse transcription polymerase chain reaction to examine their transcriptional profiles. The T cell internal antigen 1 (TIA1) and Ras-GTPase-activating SH3-domain-binding protein1 (G3BP1) of the cellular stress granule pathway from grass carp C. idella (designated as CiTIA1 and CiG3BP1) were upregulated and downregulated during GCRV infection, respectively. The full-length cDNA of CiTIA1 was 2753 bp, with an open reading frame (ORF) of 1155bp, which encodes a putative 385-amino acid protein. The 2271 bp full-length cDNA of CiG3BP1 comprised an ORF of 1455 bp that encodes a putative 485-amino acid protein. Phylogenetic analysis revealed that the complete ORFs of CiTIA1 and CiG3BP1 were very similar to zebrafish and well-characterized mammalian homologs. The expressions of the cellular proteins CiTIA1 and CiG3BP1 in response to GCRV were validated by western blotting, which indicated that the GCRV should unlink TIA1 aggregation and stress granule formation. This study provides useful information on the proteomic and cellular stress granule pathway's responses to GCRV infection, which adds to our understanding of viral pathogenesis.

Conditional knockout of TMEM16A/anoctamin1 abolishes the calcium-activated chloride current in mouse vomeronasal sensory neurons

TMEM16A is an essential component of Ca²⁺-activated Cl⁻ currents in mouse vomeronasal sensory neurons.

Pheromones are substances released from animals that, when detected by the vomeronasal organ of other individuals of the same species, affect their physiology and behavior. Pheromone binding to receptors on microvilli on the dendritic knobs of vomeronasal sensory neurons activates a second messenger cascade to produce an increase in intracellular Ca²⁺ concentration. Here, we used whole-cell and inside-out patch-clamp analysis to provide a functional characterization of currents activated by Ca²⁺ in isolated mouse vomeronasal sensory neurons in the absence of intracellular K⁺. In whole-cell recordings, the average current in 1.5 µM Ca²⁺ and symmetrical Cl⁻ was −382 pA at −100 mV. Ion substitution experiments and partial blockade by commonly used Cl⁻ channel blockers indicated that Ca²⁺ activates mainly anionic currents in these neurons. Recordings from inside-out patches from dendritic knobs of mouse vomeronasal sensory neurons confirmed the presence of Ca²⁺-activated Cl⁻ channels in the knobs and/or microvilli. We compared the electrophysiological properties of the native currents with those mediated by heterologously expressed TMEM16A/anoctamin1 or TMEM16B/anoctamin2 Ca²⁺-activated Cl⁻ channels, which are coexpressed in microvilli of mouse vomeronasal sensory neurons, and found a closer resemblance to those of TMEM16A. We used the Cre–loxP system to selectively knock out TMEM16A in cells expressing the olfactory marker protein, which is found in mature vomeronasal sensory neurons. Immunohistochemistry confirmed the specific ablation of TMEM16A in vomeronasal neurons. Ca²⁺-activated currents were abolished in vomeronasal sensory neurons of TMEM16A conditional knockout mice, demonstrating that TMEM16A is an essential component of Ca²⁺-activated Cl⁻ currents in mouse vomeronasal sensory neurons.

From sexual attraction to maternal aggression: when pheromones change their behavioural significance

This article is part of a Special Issue "Chemosignals and Reproduction". This paper reviews the role of chemosignals in the socio-sexual interactions of female mice, and reports two experiments testing the role of pup-derived chemosignals and the male sexual pheromone darcin in inducing and promoting maternal aggression. Female mice are attracted to urine-borne male pheromones. Volatile and non-volatile urine fractions have been proposed to contain olfactory and vomeronasal pheromones. In particular, the male-specific major urinary protein (MUP) MUP20, darcin, has been shown to be rewarding and attractive to females. Non-urinary male chemosignals, such as the lacrimal protein ESP1, promote lordosis in female mice, but its attractive properties are still to be tested. There is evidence indicating that ESP1 and MUPs are detected by vomeronasal type 2 receptors (V2R). When a female mouse becomes pregnant, she undergoes dramatic changes in her physiology and behaviour. She builds a nest for her pups and takes care of them. Dams also defend the nest against conspecific intruders, attacking especially gonadally intact males. Maternal behaviour is dependent on a functional olfactory system, thus suggesting a role of chemosignals in the development of maternal behaviour. Our first experiment demonstrates, however, that pup chemosignals are not sufficient to induce maternal aggression in virgin females. In addition, it is known that vomeronasal stimuli are needed for maternal aggression. Since MUPs (and other molecules) are able to promote intermale aggression, in our second experiment we test if the attractive MUP darcin also promotes attacks on castrated male intruders by lactating dams. Our findings demonstrate that the same chemosignal, darcin, promotes attraction or aggression according to female reproductive state.

Calmodulin-dependent activation and inactivation of anoctamin calcium-gated chloride channels

Calcium-dependent chloride channels serve critical functions in diverse biological systems. Driven by cellular calcium signals, the channels codetermine excitatory processes and promote solute transport. The anoctamin (ANO) family of membrane proteins encodes three calcium-activated chloride channels, named ANO 1 (also TMEM16A), ANO 2 (also TMEM16B), and ANO 6 (also TMEM16F). Here we examined how ANO 1 and ANO 2 interact with Ca²⁺/calmodulin using nonstationary current analysis during channel activation. We identified a putative calmodulin-binding domain in the N-terminal region of the channel proteins that is involved in channel activation. Binding studies with peptides indicated that this domain, a regulatory calmodulin-binding motif (RCBM), provides two distinct modes of interaction with Ca²⁺/calmodulin, one at submicromolar Ca²⁺ concentrations and one in the micromolar Ca²⁺ range. Functional, structural, and pharmacological data support the concept that calmodulin serves as a calcium sensor that is stably associated with the RCBM domain and regulates the activation of ANO 1 and ANO 2 channels. Moreover, the predominant splice variant of ANO 2 in the brain exhibits Ca²⁺/calmodulin-dependent inactivation, a loss of channel activity within 30 s. This property may curtail ANO 2 activity during persistent Ca²⁺ signals in neurons. Mutagenesis data indicated that the RCBM domain is also involved in ANO 2 inactivation, and that inactivation is suppressed in the retinal ANO 2 splice variant. These results advance the understanding of Ca²⁺ regulation in anoctamin Cl⁻ channels and its significance for the physiological function that anoctamin channels subserve in neurons and other cell types.

A wide range of pheromone-stimulated sexual and reproductive behaviors in female mice depend on G protein Gαo

BACKGROUND

Optimal reproductive fitness is essential for the biological success and survival of species. The vomeronasal organ is strongly implicated in the display of sexual and reproductive behaviors in female mice, yet the roles that apical and basal vomeronasal neuron populations play in controlling these gender-specific behaviors remain largely unclear.

RESULTS

To dissect the neural pathways underlying these functions, we genetically inactivated the basal vomeronasal organ layer using conditional, cell-specific ablation of the G protein Gαo. Female mice mutant for Gαo show severe alterations in sexual and reproductive behaviors, timing of puberty onset, and estrous cycle. These mutant mice are insensitive to reproductive facilitation stimulated by male pheromones that accelerate puberty and induce ovulation. Gαo-mutant females exhibit a striking reduction in sexual receptivity or lordosis behavior to males, but gender discrimination seems to be intact. These mice also show a loss in male scent preference, which requires a learned association for volatile olfactory signals with other nonvolatile ownership signals that are contained in the high molecular weight fraction of male urine. Thus, Gαo impacts on both instinctive and learned social responses to pheromones.

CONCLUSIONS

These results highlight that sensory neurons of the Gαo-expressing vomeronasal subsystem, together with the receptors they express and the molecular cues they detect, control a wide range of fundamental mating and reproductive behaviors in female mice.

A wide range of pheromone-stimulated sexual and reproductive behaviors in female mice depend on G protein Gαo

Background

Optimal reproductive fitness is essential for the biological success and survival of species. The vomeronasal organ is strongly implicated in the display of sexual and reproductive behaviors in female mice, yet the roles that apical and basal vomeronasal neuron populations play in controlling these gender-specific behaviors remain largely unclear.

Results

To dissect the neural pathways underlying these functions, we genetically inactivated the basal vomeronasal organ layer using conditional, cell-specific ablation of the G protein Gαo. Female mice mutant for Gαo show severe alterations in sexual and reproductive behaviors, timing of puberty onset, and estrous cycle. These mutant mice are insensitive to reproductive facilitation stimulated by male pheromones that accelerate puberty and induce ovulation. Gαo-mutant females exhibit a striking reduction in sexual receptivity or lordosis behavior to males, but gender discrimination seems to be intact. These mice also show a loss in male scent preference, which requires a learned association for volatile olfactory signals with other nonvolatile ownership signals that are contained in the high molecular weight fraction of male urine. Thus, Gαo impacts on both instinctive and learned social responses to pheromones.

Conclusions

These results highlight that sensory neurons of the Gαo-expressing vomeronasal subsystem, together with the receptors they express and the molecular cues they detect, control a wide range of fundamental mating and reproductive behaviors in female mice.

Structure and function of TMEM16 proteins (anoctamins)

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

Estradiol rapidly modulates odor responses in mouse vomeronasal sensory neurons

In rodents, many social behaviors are driven by the sense of smell. The vomeronasal organ (VNO), part of the accessory olfactory system mediates many of these chemically driven behaviors. The VNO is heavily vascularized, and is readily accessible to circulating peptide or steroid hormones. Potentially, this allows circulating hormones to alter behavior through modulating the output of the primary sensory neurons in the VNO, the vomeronasal sensory neurons (VSNs). Based on this, we hypothesized that steroid hormones, in particular 17β-estradiol, would modulate activity of VSNs. In this paper, we show that the estrogen receptors, GPR30 and ERα, were present in VSNs and that estradiol may be synthesized locally in the VNO. Our results also showed that 17β-estradiol decreased responses of isolated VSNs to dilute urine, a potent natural stimulus, with respect to current amplitudes and depolarization. Further, 17β-estradiol increased the latency of the first action potential (AP) and the AP amplitude. Additionally, calcium responses to sulfated steroids (present in the low molecular weight fraction of urine) that act as ligands for apical vomeronasal receptors were decreased by 17β-estradiol. In conclusion, we show that estradiol modulates odorant responses mediated by VSNs and hence paves the way for future studies to better understand the mechanisms by which odorant mediated behavior is altered by endocrine status of the animal.

Either main or accessory olfactory system signaling can mediate the rewarding effects of estrous female chemosignals in sexually naive male mice

A long-held view has been that interest of male mice in female body odors reflects an activation of reward circuits in the male brain following their detection by the vomeronasal organ (VNO) and processing via the accessory olfactory system. We found that adult, sexually naive male mice acquired a conditioned place preference (CPP) after repeatedly receiving estrous female urine on the nose and being placed in an initially nonpreferred chamber with soiled estrous bedding on the floor. CPP was not acquired in control mice that received saline on the nose before being placed in a nonpreferred chamber with clean bedding. Robust acquisition of a CPP using estrous female odors as the reward persisted in separate groups of mice in which VNO-accessory olfactory function was disrupted by bilateral lesioning of the accessory olfactory bulb (AOB) or in which main olfactory function was disrupted by zinc sulfate lesions of the main olfactory epithelium (MOE). By contrast, no CPP was acquired for estrous odors in males that received combined AOB and MOE lesions. Either the main or the accessory olfactory system suffices to mediate the rewarding effects of estrous female odors in the male mouse, even in the absence of prior mating experience. The main olfactory system is part of the circuitry that responds to chemosignals involved in motivated behavior, a role that may be particularly important for humans who lack a functional accessory olfactory system.

The genomic basis of vomeronasal-mediated behaviour

The vomeronasal organ (VNO) is a chemosensory subsystem found in the nose of most mammals. It is principally tasked with detecting pheromones and other chemical signals that initiate innate behavioural responses. The VNO expresses subfamilies of vomeronasal receptors (VRs) in a cell-specific manner: each sensory neuron expresses just one or two receptors and silences all the other receptor genes. VR genes vary greatly in number within mammalian genomes, from no functional genes in some primates to many hundreds in rodents. They bind semiochemicals, some of which are also encoded in gene families that are coexpanded in species with correspondingly large VR repertoires. Protein and peptide cues that activate the VNO tend to be expressed in exocrine tissues in sexually dimorphic, and sometimes individually variable, patterns. Few chemical ligand–VR–behaviour relationships have been fully elucidated to date, largely due to technical difficulties in working with large, homologous gene families with high sequence identity. However, analysis of mouse lines with mutations in genes involved in ligand–VR signal transduction has revealed that the VNO mediates a range of social behaviours, including male–male and maternal aggression, sexual attraction, lordosis, and selective pregnancy termination, as well as interspecific responses such as avoidance and defensive behaviours. The unusual logic of VR expression now offers an opportunity to map the specific neural circuits that drive these behaviours.

Functional coupling of TRPC2 cation channels and the calcium-activated anion channels in rat thyroid cells: implications for iodide homeostasis

The initial step in a synthesis of thyroid hormones is the uptake of iodide from the circulation. Iodide (I(-)) is transported into thyroid cells via a Na(+)/I(-) symporter (NIS), which is electrogenic and thus sensitive to alterations in membrane potential (V(m)). I(-) is then released to the lumen of thyroid follicles where the hormones are synthesised and stored. The mechanisms of I(-) release to follicle lumen are poorly characterised. Our whole-cell voltage clamp recordings revealed the presence of a Ca(2+) activated Cl(-) current (CaCC) in Fisher rat thyroid cell line 5 (FRTL-5). Transcripts of anoctamin 1 (ANO1) and anoctamin 10 (ANO10), putative molecular constituents of CaCC, were detected. The anion channels underlying CaCC are highly permeable to I(-). Both niflumic acid (NFA) and 2-aminoethyl diphenylborinate (2-APB), antagonists of CaCC and transient receptor potential channels, respectively, inhibited CaCC. Canonical transient receptor potential channel 2 (TRPC2) is the only TRPC member present in FRTL-5 cells. The activation rate of CaCC was markedly slower in shTRPC2 knock-down cells, indicating that Ca(2+) entry via TRPC2 contributes to CaCC activation. The uptake of iodide was enhanced and the resting V(m) was more depolarised in TRPC2 knock-down cells. We suggest that the interplay between TRPC2 and ANO1 may have dual effects on iodide transport, modulating I(-) release via ANO channels and I(-) uptake via the V(m) sensitive NIS.

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.

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.

Classification of potassium and chlorine ionic currents in retinal ganglion cell line (RGC-5) by whole-cell patch clamp

Retinal ganglion cell line (RGC-5) has been widely used as a valuable model for studying pathophysiology and physiology of retinal ganglion cells in vitro. However, the electrophysiological characteristics, especially a thorough classification of ionic currents in the cell line, remain to be elucidated in details. In the present study, we determined the resting membrane potential (RMP) in RGC-5 cell line and then identified different types of ionic currents by using the whole-cell patch-clamp technique. The RMP recorded in the cell line was between −30 and −6 mV (−17.6 ± 2.6 mV, n = 10). We observed the following voltage-gated ion channel currents: (1) inwardly rectifying Cl− current (ICl,ir), which could be blocked by Zn2+; (2) Ca2+-activated Cl− current (ICl,Ca), which was sensitive to extracellular Ca2+ and could be inhibited by disodium 4,4’-diisothiocyanatostilbene-2,2’-disulfonate; (3) inwardly rectifying K+ currents (IK1), which could be blocked by Ba2+; (4) a small amount of delayed rectifier K+ current (IK). On the other hand, the voltage-gated sodium channels current (INa) and transient outward potassium channels current (IA) were not observed in this cell line. These results further characterize the ionic currents in the RGC-5 cell line and are beneficial for future studies especially on ion channel (patho)physiology and pharmacology in the RGC-5 cell line.

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.

Paradoxical contribution of SK3 and GIRK channels to the activation of mouse vomeronasal organ

The vomeronasal organ (VNO) is essential for intraspecies communication in many terrestrial vertebrates. The ionic mechanisms of VNO activation remain unclear. We found that the calcium-activated potassium channel SK3 and the G protein-activated potassium channel GIRK are part of an independent pathway for VNO activation. In slice preparations, the potassium channels attenuated inward currents carried by TRPC2 and calcium-activated chloride channels (CACCs). In intact tissue preparations, paradoxically, the potassium channels enhanced urine-evoked inward currents. This discrepancy resulted from the loss of a high concentration of lumenal potassium, which enabled the influx of potassium ions to depolarize the VNO neurons in vivo. Both Sk3 (also known as Kcnn3) and Girk1 (also known as Kcnj3) homozygous null mice showed deficits in mating and aggressive behaviors, and the deficiencies in Sk3(-/-) mice were exacerbated by Trpc2 knockout. Our results suggest that VNO activation is mediated by TRPC2, CACCs and two potassium channels, all of which contributed to the in vivo depolarization of VNO neurons.

ATP excites mouse vomeronasal sensory neurons through activation of P2X receptors

Purinergic signaling through activation of P2X and P2Y receptors is critically important in the chemical senses. In the mouse main olfactory epithelium (MOE), adenosine 5'-triphosphate (ATP) elicits an increase in intracellular calcium ([Ca(2+)](I)) and reduces the responsiveness of olfactory sensory neurons to odorants through activation of P2X and P2Y receptors. We investigated the role of purinergic signaling in vomeronasal sensory neuron (VSN)s from the mouse vomeronasal organ (VNO), an olfactory organ distinct from the MOE that responds to many conspecific chemical cues. Using a combination of calcium imaging and patch-clamp electrophysiology with isolated VSNs, we demonstrated that ATP elicits an increase in [Ca(2+)](I) and an inward current with similar EC(50)s. Neither adenosine nor the P2Y receptor ligands adenosine 5'-diphosphate, uridine 5'-triphosphate, and uridine-5'-disphosphate could mimic either effect of ATP. Moreover, the increase in [Ca(2+)](I) required the presence of extracellular calcium and the inward current elicited by ATP was partially blocked by the P2X receptor antagonists pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate. Consistent with the activation of P2X receptors, we detected gene expression of the P2X1 and 3 receptors in the VNO by Reverse transcription polymerase chain reaction (RT-PCR). When co-delivered with dilute urine, a natural stimulus, ATP significantly increased the inward current above that elicited by dilute urine or ATP alone. Mechanical stimulation of the VNO induced the release of ATP, detected by luciferin-luciferase luminometry, and this release of ATP was completely abolished in the presence of the connexin/pannexin hemichannel blocker, carbenoxolone. We conclude that the release of ATP could occur during the activity of the vasomotor pump that facilitates the movement of chemicals into the VNO for detection by VSNs. This mechanism could lead to a global increase in excitability and the chemosensory response in VSNs through activation of P2X receptors.

From genes to social communication: molecular sensing by the vomeronasal organ

The ability to distinguish molecular cues emitted by other individuals is a fundamental feature of social interactions such as finding and identifying a mate, establishing social hierarchies, and initiating interspecies defensive behaviors. In rodents, this ability involves the vomeronasal organ (VNO), a distinct chemoreceptive structure that is part of the olfactory system. Recent insights have led to unprecedented progress in identifying ligand and receptor families underlying vomeronasal recognition, characterizing the behavioral consequences caused by VNO activation, and defining higher neural circuits underlying the initiation of instinctive behaviors such as aggression. Here, we review such findings and discuss future areas for investigation, including large-scale mapping studies, immune system-VNO interactions, in vivo recording of neural activity, and optogenetic alteration of sexual and social behaviors.

International Union of Basic and Clinical Pharmacology. LXXXV: calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) are widely expressed in various tissues and implicated in physiological processes such as sensory transduction, epithelial secretion, and smooth muscle contraction. Transmembrane proteins with unknown function 16 (TMEM16A) has recently been identified as a major component of CaCCs. Detailed molecular analysis of TMEM16A will be needed to understand its structure-function relationships. The role this channel plays in physiological systems remains to be established and is currently a subject of intense investigation.

G protein G(alpha)o is essential for vomeronasal function and aggressive behavior in mice

The rodent vomeronasal organ (VNO) mediates the regulation of species-specific and interspecies social behaviors. We have used gene targeting to examine the role of the G protein Gαo, encoded by the gene Gnao1, in vomeronasal function. We used the Cre-loxP system to delete Gαo in those cells that express olfactory marker protein, which includes all vomeronasal sensory neurons of the basal layer of the VNO sensory epithelium. Using electrophysiology and calcium imaging, we show that the conditional null mice exhibit strikingly reduced sensory responses in V2R receptor-expressing vomeronasal sensory neurons to specific molecular cues, including MHC1 antigens, major urinary proteins, and exocrine gland-secreting peptide. Gαo is also vital for vomeronasal sensing of two N-formylated mitochondrially encoded peptides derived from NADH dehydrogenase 1. Furthermore, we show that Gαo is an essential requirement for the display of male-male territorial aggression as well as maternal aggression in mice. Finally, we show that Gαo-dependent maternal aggression can be induced by major urinary proteins. These cellular and behavioral phenotypes identify Gαo as the primary G-protein α-subunit mediating the detection of peptide and protein pheromones by sensory neurons of the VNO.

Requirement of calcium-activated chloride channels in the activation of mouse vomeronasal neurons

In terrestrial vertebrates, the vomeronasal organ (VNO) detects and transduces pheromone signals. VNO activation is thought to be mediated by the transient receptor potential C2 (TRPC2) channel. The aberrant behavioural phenotypes observed in TRPC2−/− mice are generally attributed to the lost VNO function. Recently, calcium-activated chloride channels have been shown to contribute to VNO activation. Here we show that CACCs can be activated in VNO slice preparations from the TRPC2−/− mice and this activation is blocked by pharmacological agents that inhibit intracellular Ca²⁺ release. Urine-evoked Cl⁻ current is sufficient to drive spiking changes in VNO neurons from both wild-type (WT) and TRPC2−/− mice. Moreover, blocking Cl⁻ conductance essentially abolishes VNO activation in WT neurons. These results suggest a TRPC2-independent signalling pathway in the VNO and the requirement of calcium-activated chloride channels currents to mediate pheromone activation. Our data further suggest that TRPC2−/− mice retain partial VNO function.

The vomeronasal organ detects pheromones, which are thought to activate TRPC2 channels on the surface of vomeronasal neurons. Using TRPC2 knockout mice, the authors show that urinary pheromones can also activate these neurons via calcium-activated chloride channels, suggesting a TRPC2-independent pathway for sensing pheromones.

Calcium activates a chloride conductance likely involved in olfactory receptor neuron repolarization in the moth Spodoptera littoralis

The response of insect olfactory receptor neurons (ORNs) to odorants involves the opening of Ca(2+)-permeable channels, generating an increase in intracellular Ca(2+) concentration. Here, we studied the downstream effect of this Ca(2+) rise in cultured ORNs of the moth Spodoptera littoralis. Intracellular dialysis of Ca(2+) from the patch pipette in whole-cell patch-clamp configuration activated a conductance with a K(1/2) of 2.8 microm. Intracellular and extracellular anionic and cationic substitutions demonstrated that Cl(-) carries this current. The anion permeability sequence I(-) > NO(3)(-) > Br(-) > Cl(-) > CH(3)SO(3)(-) >> gluconate(-) of the Ca(2+)-activated Cl(-) channel suggests a weak electrical field pore of the channel. The Ca(2+)-activated current partly inactivated over time and did not depend on protein kinase C (PKC) and CaMKII activity or on calmodulin. Application of Cl(-) channel blockers, flufenamic acid, 5-nitro-2-(3-phenylpropylamino) benzoic acid, or niflumic acid reversibly blocked the Ca(2+)-activated current. In addition, lowering Cl(-) concentration in the sensillar lymph bathing the ORN outer dendrites caused a significant delay in pheromone response termination in vivo. The present work identifies a new Cl(-) conductance activated by Ca(2+) in insect ORNs presumably required for ORN repolarization.