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

Ca2+-Activated Ion Channels Exert Opposite Effects in Different Signaling Compartments of Vomeronasal Sensory Neurons

In most mammals, conspecific chemical cues that drive innate social and sexual behavior are detected by the vomeronasal organ (VNO) and processed in the accessory olfactory bulb (AOB). Chemosensory stimulation of vomeronasal sensory neurons (VSNs) at their microvillous dendritic knobs triggers, first, a local signal transduction and amplification cascade and, second, transformation of that signal into action potential (AP) discharge at the soma. Both processes-signal transduction and AP generation-involve local Ca2+ elevations in the knob and soma, respectively. Here, we revisit the somewhat still controversial functions of Ca2+-activated ion channels in both VSN compartments. In acute mouse VNO slices (of either sex), focal photorelease of Ca2+ reveals that VSN knob and soma both act as independent Ca2+ signaling compartments, in which Ca2+ elevations exert opposite effects. While Ca2+ signals in the knob drive an excitatory inward current, Ca2+ elevations in the soma primarily activate hyperpolarizing outward currents that silence VSNs. A substantial fraction of the latter current is mediated by SK and/or BK channels. Notably, SK channel activity strongly affects VSN firing. Together, our study reveals a diverse composition of Ca2+-activated currents in VSN somata and uncovers an unexpected role of SK channels in dampening excitability and, thus, in controlling VSN-to-AOB information transfer.

Molecular, cellular, and developmental organization of the mouse vomeronasal organ at single cell resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors (VRs) and a population of canonical olfactory sensory neurons in the VNO. High-resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and VRs, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Noradrenaline modulates sensory information in mouse vomeronasal sensory neurons

During the induction of the body's alert state, the sympathetic system modulates sensory modalities and fine-tunes peripheral organs for improved stimulus detection. We explored noradrenaline (NA)'s role in modulating signaling in vomeronasal sensory neurons (VSNs), responsible for detecting pheromones and other semiochemicals. In current-clamp recordings, NA increased the firing frequency in response to natural stimuli of responsive VSNs and induced spiking activity in previously unresponsive neurons. Current injections into VSNs showed an increase in firing frequency during NA application. Combining transcriptomic analysis, electrophysiology, Ca2+ imaging, and a pharmacological approach, we identified alpha 1 adrenergic receptors as crucial for NA-induced firing frequency increases in VSNs. Immunohistochemistry revealed catecholaminergic fibers in the vomeronasal sensory epithelium, suggesting localized NA release near VSNs. This study unveils NA as a key regulator of VSN signaling, shedding light on the intricate interplay between the sympathetic nervous system and chemosensory processing, advancing our understanding of sensory modulation.

Molecular, Cellular, and Developmental Organization of the Mouse Vomeronasal organ at Single Cell Resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors and a population of canonical olfactory sensory neurons in the VNO. High resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and vomeronasal receptors, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Mechanisms of Glioblastoma Replication: Ca2+ Flares and Cl- Currents

Glioblastoma (GBM) is amongst the deadliest types of cancers, with no resolutive cure currently available. GBM cell proliferation in the patient's brain is a complex phenomenon controlled by multiple mechanisms. The aim of this study was to determine whether the ionic fluxes controlling cell duplication could represent a target for GBM therapy. In this work, we combined multi-channel Ca2+ and Cl- imaging, optical tweezers, electrophysiology, and immunohistochemistry to describe the role of ion fluxes in mediating the cell volume changes that accompany mitosis of U87 GBM cells. We identified three main steps: (i) in round GBM cells undergoing mitosis, during the transition from anaphase to telophase and cytokinesis, large Ca2+ flares occur, reaching values of 0.5 to 1 μmol/L; (ii) these Ca2+ flares activate Ca2+-dependent Cl- channels, allowing the entry of Cl- ions; and (iii) to maintain osmotic balance, GBM cells swell to complete mitosis. This sequence of steps was validated by electrophysiological experiments showing that Cl- channels are activated either directly or indirectly by Ca2+, and by additional live-cell imaging experiments. Cl- channel blockers with different molecular structures, such as niflumic acid and carbenoxolone, blocked GBM replication by arresting GBM cells in a round configuration. These results describe the central role of Ca2+ flares and Cl- fluxes during mitosis and show that inhibition of Ca2+-activated Cl- channels blocks GBM replication, opening the way to new approaches for the clinical treatment of GBM. Implications: Our work identifies ionic fluxes occurring during cell division as targets for devising novel therapies for glioblastoma treatment.

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.

From odor to oncology: non-canonical odorant receptors in cancer

Odorant receptors, traditionally associated with olfaction as chemoreceptors, have been increasingly recognized for their presence and diverse functions in various non-nasal tissues throughout the body. Beyond their roles in sensory perception, emerging evidence suggests a compelling interplay between odorant receptors and cancer progression as well. Alongside the canonical GPCR odorant receptors, dysregulation of non-canonical odorant receptors such as trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and membrane-spanning 4A family (MS4As) has been observed in various cancer types, suggesting their contributions to cancer progression. The roles of these non-canonical chemoreceptors in cancer are complex, with some receptors promoting tumorigenesis and others acting as tumor-suppressing factors upon activation, depending on the cancer type. These findings shed light on the potential of non-canonical odorant receptors as therapeutic targets and prognostic markers in cancer, inviting further exploration to unravel their precise mechanisms of action and implications in cancer biology. In this review, we provide a comprehensive overview of the intricate relationships between these chemoreceptors and various types of cancer, potentially paving the way for innovative odor-based therapeutics. Ultimately, this review discusses the potential development of novel therapeutic strategies targeting these non-canonical chemoreceptors.

Slow Inactivation of Sodium Channels Contributes to Short-Term Adaptation in Vomeronasal Sensory Neurons

Adaptation plays an important role in sensory systems as it dynamically modifies sensitivity to allow the detection of stimulus changes. The vomeronasal system controls many social behaviors in most mammals by detecting pheromones released by conspecifics. Stimuli activate a transduction cascade in vomeronasal neurons that leads to spiking activity. Whether and how these neurons adapt to stimuli is still debated and largely unknown. Here, we measured short-term adaptation performing current-clamp whole-cell recordings by using diluted urine as a stimulus, as it contains many pheromones. We measured spike frequency adaptation in response to repeated identical stimuli of 2-10 s duration that was dependent on the time interval between stimuli. Responses to paired current steps, bypassing the signal transduction cascade, also showed spike frequency adaptation. We found that voltage-gated Na+ channels in VSNs undergo slow inactivation processes. Furthermore, recovery from slow inactivation of voltage-gated Na+ channels occurs in several seconds, a time scale similar to that measured during paired-pulse adaptation protocols, suggesting that it partially contributes to short-term spike frequency adaptation. We conclude that vomeronasal neurons do exhibit a time-dependent short-term spike frequency adaptation to repeated natural stimuli and that slow inactivation of Na+ channels contributes to this form of adaptation. These findings not only increase our knowledge about adaptation in the vomeronasal system, but also raise the question of whether slow inactivation of Na+ channels may play a role in other sensory systems.

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.

Principles of odor coding in vertebrates and artificial chemosensory systems

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

Convergent vomeronasal system reduction in mammals coincides with convergent losses of calcium signalling and odorant-degrading genes

The vomeronasal system (VNS) serves crucial functions for detecting olfactory clues often related to social and sexual behaviour. Intriguingly, two of the main components of the VNS, the vomeronasal organ (VNO) and the accessory olfactory bulb, are regressed in aquatic mammals, several bats and primates, likely due to adaptations to different ecological niches. To detect genomic changes that are associated with the convergent reduction of the VNS, we performed the first systematic screen for convergently inactivated protein-coding genes associated with convergent VNS reduction, considering 106 mammalian genomes. Extending previous studies, our results support that Trpc2, a cation channel that is important for calcium signalling in the VNO, is a predictive molecular marker for the presence of a VNS. Our screen also detected the convergent inactivation of the calcium-binding protein S100z, the aldehyde oxidase Aox2 that is involved in odorant degradation, and the uncharacterized Mslnl gene that is expressed in the VNO and olfactory epithelium. Furthermore, we found that Trpc2 and S100z or Aox2 are also inactivated in otters and Phocid seals for which no morphological data about the VNS are available yet. This predicts a VNS reduction in these semi-aquatic mammals. By examining the genomes of 115 species in total, our study provides a detailed picture of how the convergent reduction of the VNS coincides with gene inactivation in placental mammals. These inactivated genes provide experimental targets for studying the evolution and biological significance of the olfactory system under different environmental conditions.

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.

Comprehensive behavioral analyses of anoctamin1/TMEM16A-conditional knockout mice

AIMS

Anoctamin-1 (TMEM16A) is a calcium-activated chloride channel that is involved in numerous physiological conditions. Its role has been identified in electrophysiological and histological studies of genetic knockout animals. Recent cellular localization studies have shown that anoctamin-1 is co-expressed with presynaptic proteins, therefore its role in presynaptic terminals has been suggested. However, behavioral studies are lacking because conventional knockouts of anoctamin-1 are lethal after birth. In this study, we explored the role of anoctamin-1 in presynaptic terminals by analyzing the behavior of mice with conditional knockouts of anoctamin-1 in synapsin1-expressing cells.

MAIN METHODS

Using a synapsin1-Cre system, we selectively ablated anoctamin-1 in synapsin1 expressing cells. The mice were used in the behavioral experiments when they were between 6 and 9 months of age.

KEY FINDINGS

The mice with the conditional knockout of anoctamin-1 in synapsin1-expressing cells displayed impaired social behavior. In addition, the mice showed depressive-like behavior and decreased weight. However, these animals displayed normal locomotor activity, cognitive function, and motor coordination.

SIGNIFICANCE

These results suggested that anoctamin-1 is involved in psychiatric behavior because of its role in the regulation of synaptic transmission in presynaptic terminals.

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.

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

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

Calcium-regulated chloride channel anoctamin-1 is present in the suprachiasmatic nuclei of rats

Calcium-regulated chloride channel (CaCC) anoctamin-1 has been recently identified in neurons. In neurons, which express the Na-K-2Cl cotransporter, activation of CaCCs increases firing frequency, by reversion between the Cl equilibrium potential and the membrane resting potential, leading to membrane depolarization by Cl extrusion from the cell. Although there are no reports of CaCCs present in the suprachiasmatic nuclei (SCN), the fact that Na-K-2Cl cotransporter is present in SCN neurons, where it has been shown to be involved in the excitatory effects of γ-aminobutyric acid, together with the increase of neuronal firing rate induced by release of intracellular Ca after administration of 100 nM ryanodine, leads us to determine whether CaCCs are present in the SCN. Immunohistochemistry and western blots show the expression of the CaCCs anoctamin-1 protein. Quantitative PCR demonstrated the expression of anoctamin-1 mRNA in the SCN. These results clearly indicate the presence of CaCC in SCN of rats.

Coupling of TRPV6 and TMEM16A in epithelial principal cells of the rat epididymis

Principal cells regulate the ionic environment of the epididymal lumen via unknown mechanisms. Gao et al. use electrophysiological and pharmacological tools to characterize rat principal cells and reveal a TRPV6-mediated calcium conductance and TMEM16A-mediated calcium-activated chloride conductance.

The epididymis establishes a congenial environment for sperm maturation and protection. Its fluid is acidic, and the calcium concentration is low and declines along the length of the epididymal tubule. However, our knowledge of ionic currents and mechanisms of calcium homeostasis in rat epididymal epithelial cells remains enigmatic. In this study, to better understand calcium regulation in the epididymis, we use the patch-clamp method to record from single rat cauda epididymal principal cells. We detect a constitutively active Ca²⁺ current with characteristics that match the epithelial calcium channel TRPV6. Electrophysiological and pharmacological data also reveal a constitutively active calcium-activated chloride conductance (CaCC). Removal of extracellular calcium attenuates not only the TRPV6-like conductance, but also the CaCC. Lanthanide block is time dependent such that the TRPV6-like component is inhibited first, followed by the CaCC. The putative CaCC blocker niflumic acid partially inhibits whole-cell currents, whereas La³⁺ almost abolishes whole-cell currents in principal cells. Membrane potential measurements reveal an interplay between La³⁺-sensitive ion channels and those that are sensitive to the specific TMEM16A inhibitor tannic acid. In vivo perfusion of the cauda epididymal tubule shows a substantial rate of Ca²⁺ reabsorption from the luminal side, which is dose-dependently suppressed by ruthenium red, a putative blocker of epithelial Ca²⁺ channels and CaCC. Finally, we discover messenger RNA for both TRPV6 and TMEM16A in the rat epididymis and show that their proteins colocalize in the apical membrane of principal cells. Collectively, these data provide evidence for a coupling mechanism between TRPV6 and TMEM16A in principal cells that may play an important role in the regulation of calcium homeostasis in the epididymis.

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.

Conditioned odor aversion induces social anxiety towards females in wild-type and TrpC2 knockout male mice

Female-emitted pheromonal inputs possess an intrinsic rewarding value for conspecific males, promoting approach and investigation of the potential mating partner. In mice these inputs are detected mainly by the vomeronasal organ (VNO) and the main olfactory epithelium (MOE). We investigated the role of VNO-mediated inputs in experience-dependent plasticity of reproductive responses. We applied a sex-specific conditioned odor aversion (COA) paradigm on adult, wild-type (WT) male mice and on male mice impaired in VNO-mediated signal transduction (TrpC2^-/- ). We found that WT males, which underwent COA to female-soiled bedding, lost their innate preference to female odors and presented lower motivation to approach a sexually receptive female. COA also abolished the testosterone surge normally seen following exposure to female odors. Moreover, the conditioned males displayed impairments in copulatory behaviors, which lasted for several weeks. Surprisingly, these males also exhibited phobic behaviors towards receptive females, including freezing and fleeing responses. In contrast, WT males which underwent COA specifically to male pheromones showed no change in olfactory preference and only a marginally significant elevation in intermale aggression. Finally, we show that TrpC2^-/- males were able to acquire aversion to female-soiled bedding and presented similar behavioral alterations following COA in their responses to female cues. Our results demonstrate that the intrinsic rewarding value of female pheromones can be overridden through associative olfactory learning, which occurs independently of VNO inputs, probably through MOE signaling.

NHERF1 in Microvilli of Vomeronasal Sensory Neurons

In most mammals, the vomeronasal system detects a variety of (semio)chemicals that mediate olfactory-driven social and sexual behaviors. Vomeronasal chemosensation depends on G protein-coupled receptors (V1R, V2R, and FPR-rs) that operate at remarkably low stimulus concentrations, thus, indicating a highly sensitive and efficient signaling pathway. We identified the PDZ domain-containing protein, Na⁺/H⁺ exchanger regulatory factor-1 (NHERF1), as putative molecular organizer of signal transduction in vomeronasal neurons. NHERF1 is a protein that contains 2 PDZ domains and a carboxy-terminal ezrin-binding domain. It localizes to microvilli of vomeronasal sensory neurons and interacts with V1Rs. Furthermore, NHERF1 and Gα_i2 are closely colocalized. These findings open up new aspects of the functional organization and regulation of vomeronasal signal transduction by PDZ scaffolding proteins.

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.

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.

Signaling mechanisms and behavioral function of the mouse basal vomeronasal neuroepithelium

The vomeronasal organ (VNO) is a sensory organ that is found in most terrestrial vertebrates and that is principally implicated in the detection of pheromones. The VNO contains specialized sensory neurons organized in a pseudostratified neuroepithelium that recognize chemical signals involved in initiating innate behavioral responses. In rodents, the VNO neuroepithelium is segregated into two distinct zones, apical and basal. The molecular mechanisms involved in ligand detection by apical and basal VNO sensory neurons differ extensively. These two VNO subsystems express different subfamilies of vomeronasal receptors and signaling molecules, detect distinct chemosignals, and project to separate regions of the accessory olfactory bulb (AOB). The roles that these olfactory subdivisions play in the control of specific olfactory-mediated behaviors are largely unclear. However, analysis of mutant mouse lines for signal transduction components together with identification of defined chemosensory ligands has revealed a fundamental role of the basal part of the mouse VNO in mediating a wide range of instinctive behaviors, such as aggression, predator avoidance, and sexual attraction. Here we will compare the divergent functions and synergies between the olfactory subsystems and consider new insights in how higher neural circuits are defined for the initiation of instinctive behaviors.

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.

TRPs in olfaction

The mammalian olfactory system has become an excellent model system to understand the function of transient receptor potential (TRP) channels within their native cellular and circuit environment. The discovery that the canonical TRP channel TRPC2 is highly expressed in sensory neurons of the vomeronasal organ (VNO) has led to major advances in our understanding of the cellular and molecular processes underlying signal transduction of pheromones and other molecular cues that play an essential role in the control of instinctive decisions and innate social behaviors. TRPC2 knockout mice provide a striking example that the loss of function of a single gene can cause severe alterations in a variety of social interactions including the display of aggression, social dominance, and sexual behaviors. There is mounting evidence that TRPC2 is not the only TRP channel expressed in cells of the olfactory system but that other TRP channel subtypes such as TRPC1, TRPC4, TRPC6, TRPM4, and TRPM5 could also play important functional roles in mammalian olfaction. Here, I review such findings and discuss future areas for investigation.

Mammalian pheromones

Mammalian pheromones control a myriad of innate social behaviors and acutely regulate hormone levels. Responses to pheromones are highly robust, reproducible, and stereotyped and likely involve developmentally predetermined neural circuits. Here, I review several facets of pheromone transduction in mammals, including (a) chemosensory receptors and signaling components of the main olfactory epithelium and vomeronasal organ involved in pheromone detection; (b) pheromone-activated neural circuits subject to sex-specific and state-dependent modulation; and (c) the striking chemical diversity of mammalian pheromones, which range from small, volatile molecules and sulfated steroids to large families of proteins. Finally, I review (d) molecular mechanisms underlying various behavioral and endocrine responses, including modulation of puberty and estrous; control of reproduction, aggression, suckling, and parental behaviors; individual recognition; and distinguishing of own species from predators, competitors, and prey. Deconstruction of pheromone transduction mechanisms provides a critical foundation for understanding how odor response pathways generate instinctive behaviors.

Of pheromones and kairomones: what receptors mediate innate emotional responses?

Some chemicals elicit innate emotionally laden behavioral responses. Pheromones mediate sexual attraction, parental care or agonistic confrontation, whereas predators' kairomones elicit defensive behaviors in their preys. This essay explores the hypothesis that the detection of these semiochemicals relies on highly specific olfactory and/or vomeronasal receptors. The V1R, V2R, and formyl-peptide vomeronasal receptors bind their ligands in highly specific and sensitive way, thus being good candidates for pheromone- or kairomone-detectors (e.g., secreted and excreted proteins, peptides and lipophilic volatiles). The olfactory epithelium also expresses specific receptors, for example trace amine-associated receptors (TAAR) and guanylyl cyclase receptors (GC-D and other types), some of which bind kairomones and putative pheromones. However, most of the olfactory neurons express canonical olfactory receptors (ORs) that bind many ligands with different affinity, being not suitable for mediating responses to pheromones and kairomones. In this respect, trimethylthiazoline (TMT) is considered a fox-derived kairomone for mice and rats, but it seems to be detected by canonical ORs. Therefore, we have reassessed the kairomonal nature of TMT by analyzing the behavioral responses of outbred (CD1) and inbred mice (C57BL/J6) to TMT. Our results confirm that both mouse strains avoid TMT, which increases immobility in C57BL/J6, but not CD1 mice. However, mice of both strains sniff at TMT throughout the test and show no trace of TMT-induced contextual conditioning (immobility or avoidance). This suggests that TMT is not a kairomone but, similar to a loud noise, in high concentrations it induces aversion and stress as unspecific responses to a strong olfactory stimulation.

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.

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.

Responses to sulfated steroids of female mouse vomeronasal sensory neurons

The rodent vomeronasal organ plays an important role in many social behaviors. Using the calcium imaging technique with the dye fluo-4 we measured intracellular calcium concentration changes induced by the application of sulfated steroids to neurons isolated from the vomeronasal organ of female mice. We found that a mix of 10 sulfated steroids from the androgen, estrogen, pregnanolone, and glucocorticoid families induced a calcium response in 71% of neurons. Moreover, 31% of the neurons responded to a mix composed of 3 glucocorticoid-derived compounds, and 28% responded to a mix composed of 3 pregnanolone-derived compounds. Immunohistochemistry showed that neurons responding to sulfated steroids expressed phosphodiesterase 4A, a marker specific for apical neurons expressing V1R receptors. None of the neuron that responded to 1 mix responded also to the other, indicating a specificity of the responses. Some neurons responded to more than 1 individual component of the glucocorticoid-derived mix tested at high concentration, suggesting that these neurons are broadly tuned, although they still displayed strong specificity, remaining unresponsive to high concentrations of the ineffective compounds.

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.

Contribution of pheromones processed by the main olfactory system to mate recognition in female mammals

Until recently it was widely believed that the ability of female mammals (with the likely exception of women) to identify and seek out a male breeding partner relied on the detection of non-volatile male pheromones by the female's vomeronasal organ (VNO) and their subsequent processing by a neural circuit that includes the accessory olfactory bulb (AOB), vomeronasal amygdala, and hypothalamus. Emperical data are reviewed in this paper that demonstrate the detection of volatile pheromones by the main olfactory epithelium (MOE) of female mice which, in turn, leads to the activation of a population of glomeruli and abutting mitral cells in the main olfactory bulb (MOB). Anatomical results along with functional neuroanatomical data demonstrate that some of these MOB mitral cells project to the vomeronasal amygdala. These particular MOB mitral cells were selectively activated (i.e., expressed Fos protein) by exposure to male as opposed to female urinary volatiles. A similar selectivity to opposite sex urinary volatiles was also seen in mitral cells of the AOB of female mice. Behavioral data from female mouse, ferret, and human are reviewed that implicate the main olfactory system, in some cases interacting with the accessory olfactory system, in mate recognition.

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.