BACKGROUND: Nasal hyperreactivity (NHR) is prevalent in all chronic upper airway inflammatory phenotypes, including allergic rhinitis (AR) and chronic rhinosinusitis with nasal polyps (CRSwNP). Although NHR in patients with non-allergic rhinitis is mediated by neuronal pathways, AR and CRSwNP are mainly characterized by type 2 inflammation. METHODS: Eighteen healthy controls and 45 patients with symptomatic AR/CRSwNP underwent a cold, dry air (CDA) provocation test for objective diagnosis of NHR. Before and after, questionnaires were filled out and nasal secretions and biopsies were collected. Markers for neurogenic inflammation (substance P, calcitonin gene-related peptide, neurokinin A), epithelial activation (IL-33), and histamine were measured in secretions by ELISA; and expression of neuronal markers PGP9.5, TRPV1, and TRPM8 was studied in biopsies by RT-q-PCR. Effects of histamine on TRPV1/A1 were studied with Ca2+-imaging using murine trigeminal neurons. RESULTS: CDA-provocation reduced peak nasal inspiratory flow (PNIF) of patients with subjective NHR but not of non-NHR controls/ patients (p

In order to understand protein function, the field of structural biology makes extensive use of cryogenic electron microscopy (cryo-EM), a technique that enables structure determination at atomic resolution following embedding of protein particles in vitreous ice. Considering the profound effects of temperature on macromolecule function, an important—but often neglected-question is how the frozen particles relate to the actual protein conformations at physiological temperatures. In a recent study, Hu et al. compare structures of the cation channel TRPM4 “frozen” at 4 °C versus 37 °C, revealing how temperature critically affects the binding of activating Ca2+ ions and other channel modulators.

Host-derived succinate accumulates in the airways during bacterial infection. Here, we show that luminal succinate activates murine tracheal brush (tuft) cells through a signaling cascade involving the succinate receptor 1 (SUCNR1), phospholipase Cβ2, and the cation channel transient receptor potential channel subfamily M member 5 (TRPM5). Stimulated brush cells then trigger a long-range Ca 2+ wave spreading radially over the tracheal epithelium through a sequential signaling process. First, brush cells release acetylcholine, which excites nearby cells via muscarinic acetylcholine receptors. From there, the Ca 2+ wave propagates through gap junction signaling, reaching also distant ciliated and secretory cells. These effector cells translate activation into enhanced ciliary activity and Cl − secretion, which are synergistic in boosting mucociliary clearance, the major innate defense mechanism of the airways. Our data establish tracheal brush cells as a central hub in triggering a global epithelial defense program in response to a danger-associated metabolite.

BACKGROUND:The transient receptor potential ankyrin 1 (TRPA1) cation channels function as broadly-tuned sensors of noxious chemicals in many species. Recent studies identified four functional TRPA1 isoforms in Drosophila melanogaster (dTRPA1(A) to (D)), but their responses to non-electrophilic chemicals are yet to be fully characterized.METHODS:We determined the behavioral responses of adult flies to the mammalian TRPA1 non-electrophilic activators citronellal and menthol, and characterized the effects of these compounds on all four dTRPA1 channel isoforms using intracellular Ca2+ imaging and whole-cell patch-clamp recordings.RESULTS:Wild type flies avoided citronellal and menthol in an olfactory test and this behavior was reduced in dTrpA1 mutant flies. Both compounds activate all dTRPA1 isoforms in the heterologous expression system HEK293T, with the following sensitivity series: dTRPA1(C) = dTRPA1(D) > dTRPA1(A) ≫ dTRPA1(B) for citronellal and dTRPA1(A) > dTRPA1(D) > dTRPA1(C) > dTRPA1(B) for menthol.CONCLUSIONS:dTrpA1 was required for the normal avoidance of Drosophila melanogaster towards citronellal and menthol. All dTRPA1 isoforms are activated by both compounds, but the dTRPA1(B) is consistently the least sensitive. We discuss how these findings may guide further studies on the physiological roles and the structural bases of chemical sensitivity of TRPA1 channels.

Mucociliary clearance through coordinated ciliary beating is a major innate defense removing pathogens from the lower airways, but the pathogen sensing and downstream signaling mechanisms remain unclear. We identified virulence-associated formylated bacterial peptides that potently stimulated ciliary-driven transport in the mouse trachea. This innate response was independent of formyl peptide and taste receptors but depended on key taste transduction genes. Tracheal cholinergic chemosensory cells expressed these genes, and genetic ablation of these cells abrogated peptide-driven stimulation of mucociliary clearance. Trpm5-deficient mice were more susceptible to infection with a natural pathogen, and formylated bacterial peptides were detected in patients with chronic obstructive pulmonary disease. Optogenetics and peptide stimulation revealed that ciliary beating was driven by paracrine cholinergic signaling from chemosensory to ciliated cells operating through muscarinic M3 receptors independently of nerves. We provide a cellular and molecular framework that defines how tracheal chemosensory cells integrate chemosensation with innate defense.

The transient receptor potential ankyrin (TRPA) channels are Ca2+-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH2 terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca2+, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Key points 25‐Hydroxyvitamin D (25OHD) is a partial agonist of TRPV1 whereby 25OHD can weakly activate TRPV1 yet antagonize the stimulatory effects of the full TRPV1 agonists capsaicin and oleoyl dopamine. 25OHD binds to TRPV1 within the same vanilloid binding pocket as capsaicin. 25OHD inhibits the potentiating effects of PKC‐mediated TRPV1 activity. 25OHD reduces T‐cell activation and trigeminal neuron calcium signalling mediated by TRPV1 activity. These results provide evidence that TRPV1 is a novel receptor for the biological actions of vitamin D in addition to the well‐documented effects of vitamin D upon the nuclear vitamin D receptor. The results may have important implications for our current understanding of certain diseases where TRPV1 and vitamin D deficiency have been implicated, such as chronic pain and autoimmune diseases, such as type 1 diabetes. AbstractThe capsaicin receptor TRPV1 plays an important role in nociception, inflammation and immunity and its activity is regulated by exogenous and endogenous lipophilic ligands. As vitamin D is lipophilic and involved in similar biological processes as TRPV1, we hypothesized that it directly regulates TRPV1 activity and function. Our calcium imaging and electrophysiological data demonstrate that vitamin D (25‐hydroxyvitamin D (25OHD) and 1,25‐hydroxyvitamin D (1,25OHD)) can weakly activate TRPV1 at physiologically relevant concentrations (100 nM). Furthermore, both 25OHD and 1,25OHD can inhibit capsaicin‐induced TRPV1 activity (IC50 = 34.3 ± 0.2 and 11.5 ± 0.9 nM, respectively), but not pH‐induced TRPV1 activity, suggesting that vitamin D interacts with TRPV1 in the same region as the TRPV1 agonist capsaicin. This hypothesis is supported by our in silico TRPV1 structural modelling studies, which place 25OHD in the same binding region as capsaicin. 25OHD also attenuates PKC‐dependent TRPV1 potentiation via interactions with a known PKC phospho‐acceptor residue in TRPV1. To provide evidence for a physiological role for the interaction of vitamin D with TRPV1, we employed two different cellular models known to express TRPV1: mouse CD4+ T‐cells and trigeminal neurons. Our results indicate that 25OHD reduces TRPV1‐induced cytokine release from T‐cells and capsaicin‐induced calcium activity in trigeminal neurons. In summary, we provide evidence that vitamin D is a novel endogenous regulator of TRPV1 channel activity that may play an important physiological role in addition to its known effects through the canonical nuclear vitamin D receptor pathway.

Transient Receptor Potential ion channels (TRPs) have been described as polymodal sensors, being responsible for transducing a wide variety of stimuli, and being involved in sensory functions such as chemosensation, thermosensation, mechanosensation, and photosensation. Mechanical and chemical stresses exerted on the membrane can be transduced by specialized proteins into meaningful intracellular biochemical signaling, resulting in physiological changes. Of particular interest are compounds that can change the local physical properties of the membrane, thereby affecting nearby proteins, such as TRP channels, which are highly sensitive to the membrane environment. In this review, we provide an overview of the current knowledge of TRP channel activation as a result of changes in the membrane properties induced by amphipathic structural lipidic components such as cholesterol and diacylglycerol, and by exogenous amphipathic bacterial endotoxins.

The cation channel TRPA1 transduces a myriad of noxious chemical stimuli into nociceptor electrical excitation and neuropeptide release, leading to pain and neurogenic inflammation. Despite emergent evidence that TRPA1 is regulated by the membrane environment, it remains unknown whether this channel localizes in membrane microdomains or whether it interacts with cholesterol. Using total internal reflection fluorescence microscopy and density gradient centrifugation we found that mouse TRPA1 localizes preferably into cholesterol-rich domains and functional experiments revealed that cholesterol depletion decreases channel sensitivity to chemical agonists. Moreover, we identified two structural motifs in transmembrane segments 2 and 4 involved in mTRPA1-cholesterol interactions that are necessary for normal agonist sensitivity and plasma membrane localization. We discuss the impact of such interactions on TRPA1 gating mechanisms, regulation by the lipid environment, and role of this channel in sensory membrane microdomains, all of which helps to understand the puzzling pharmacology and pathophysiology of this channel.

This chapter focuses on transient receptor potential (TRP) proteins, which compose a superfamily of cation channels that play increasingly acknowledged roles in the pathophysiology of all of the vertebrate systems, including the nervous and immune systems. It argues that the chemosensory functions of neuronal TRP channels may serve in physiological conditions as triggers of regulatory loops that, through protective reflexes, result in behavioral and humoral responses that limit the secondary injury induced by overactivation of the immune system. The neuroimmune interactions arise not only from an intense biochemical cross-talk between neurons and immune cells, but also from the overlap in the sensory functions of these cells. As exciting and important as it is, the study of neuroimmune interactions seems to be in its infancy, mainly due to insufficient interaction between immunologists and neuroscientists. The gastrointestinal system provides for good examples of the implication of neuronal TRP channels in neuroimmune interactions.

Acute neurogenic inflammation and pain associated to bacterial infection have been traditionally ascribed to sensitization and activation of sensory nerve afferents secondary to immune cell stimulation. However, we recently showed that lipopolysaccharides (LPS) directly activate the Transient Receptor Potential channels TRPA1 in sensory neurons and TRPV4 in airway epithelial cells. Here we investigated whether LPS activates other sensory TRP channels expressed in sensory neurons. Using intracellular Ca2+ imaging and patch-clamp we determined the effects of LPS on recombinant TRPV1, TRPV2, TRPM3 and TRPM8, heterologously expressed in HEK293T cells. We found that LPS activates TRPV1, although with lower potency than for TRPA1. Activation of TRPV1 by LPS was not affected by mutations of residues required for activation by electrophilic agents or by diacylglycerol and capsaicin. On the other hand, LPS weakly activated TRPM3, activated TRPM8 at 25 degrees C, but not at 35 degrees C, and was ineffective on TRPV2. Experiments performed in mouse dorsal root ganglion (DRG) neurons revealed that genetic ablation of Trpa1 did not abolish the responses to LPS, but remain detected in 30% of capsaicin-sensitive cells. The population of neurons responding to LPS was dramatically lower in double Trpa1/Trpv1 KO neurons. Our results show that, in addition to TRPA1, other TRP channels in sensory neurons can be targets of LPS, suggesting that they may contribute to trigger and regulate innate defenses against gram-negative bacterial infections.

Acrolein is a toxic and highly reactive unsaturated aldehyde, often found in cigarette smoke and vehicle exhaust gases. Likewise, acrolein derived from cyclophosphamide-treated patients constitutes the major culprit of bladder irritation during chemotherapy in cancer patient. Although, initially, its toxicity and inflammatory properties have been related to the activation of the transient receptor potential A1 (TRPA1) in nociceptive neurons, recent evidences suggests that other receptor may also play a role in acrolein-induced toxicity. Here we show that, acrolein induces chemical irritation and nocifensive response in the absence of TRPA1. Ratiometric calcium measurements and patch-clamp demonstrate that acrolein activates TRPV1 but, unlike TRPA1 that desensitizes immediately after activation, acrolein-induced activation of TRPV1 is prolonged in time. Furthermore, we identify the N-terminal amino acid residue C157 as key for acrolein-induced TRPV1 activation. Taken together, our results reveal a mechanism underlying the major role of TRPV1 as mediator for the acrolein-induced toxicity, unveiling TRPV1 as a potential therapeutic target in a wide spectrum of noxious conditions, from exposure to smoke to cancer treatment.

Lipopolysaccharides (LPS), the major components of the wall of gram-negative bacteria, trigger powerful defensive responses in the airways via mechanisms thought to rely solely on the Toll-like receptor 4 (TLR4) immune pathway. Here we show that airway epithelial cells display an increase in intracellular Ca2+ concentration within seconds of LPS application. This response occurs in a TLR4-independent manner, via activation of the transient receptor potential vanilloid 4 cation channel (TRPV4). We found that TRPV4 mediates immediate LPS-induced increases in ciliary beat frequency and the production of bactericidal nitric oxide. Upon LPS challenge TRPV4-deficient mice display exacerbated ventilatory changes and recruitment of polymorphonuclear leukocytes into the airways. We conclude that LPS-induced activation of TRPV4 triggers signaling mechanisms that operate faster and independently from the canonical TLR4 immune pathway, leading to immediate protective responses such as direct antimicrobial action, increase in airway clearance, and the regulation of the inflammatory innate immune reaction.

The TRPA1 cation channel functions as a broadly-tuned chemonociceptor in many species. Recent studies identified several splice variants of Drosophila TRPA1 (from dTRPA1(A) to (D)). These were reported to display distinct chemical and thermal sensitivities and distinct expression patterns. Some differences in the chemical sensitivities between insect and mammalian orthologues have also been reported. Given the usefulness of Drosophila as animal model for studying TRPA1 chemosensation in vivo, we re-evaluated the effects of three mammalian TRPA1 modulators (nicotine, citronellal and menthol) on dTRPA1 splice variants in HEK293T cells. Using intracellular Ca2+ fluorimetry and whole-cell patch-clamp, we confirmed that the electrophilic agonist allyl isothiocyanate activates all splice variants, whereas nicotine activates all variants except dTRPA1(B). Citronellal robustly activated all splice variants. The activation by nicotine and citronellal could be inhibited by the TRPA1 antagonist HC030031. On the other hand, menthol did not activate any dTRPA1 isoform. Our results demonstrate that the mammalian heterologous expression system HEK293T can be used for functional studies on insect TRPA1 channels, and that the chemosensory properties of these channels may strongly differ from those of the mammalian homologues. Our findings serve as starting point for further structure-function studies for the determination of the structural bases of chemical sensitivity in TRPA1 channels.

The overall perception of flavor results from the integration of taste, smell, and somatosensory information streaming out of specialized receptor cells located in the oronasal cavities. Several members of the transient receptor potential family of cation channels contribute to the signal transduction of chemical stimuli. All bona fide TRP channel chemosensors contribute to flavor detection by acting on epithelial cells and/or sensory nerve endings in the mucosa of the nose, mouth, and throat. Chemical activation of these channels results in a very obvious, but yet obscure, sensory modality called trigeminality or chemesthesis, which is related to the perception of texture, temperature, and pungency. These sensations arise when chemical compounds activate receptor cells associated with other senses that mediate touch, thermal perception, and pain. In this chapter we illustrate the huge diversity of chemical agonists of TRP channels and underscore the need of more basic research on this amazing family of molecular sensors, which are very likely to hold the key for better understanding of human sensory pathophysiology.

The TRPA1 ion channel functions as a broadly-tuned chemonociceptor in many species. In the fruit fly, Drosophila melanogaster, dTRPA1(A) is known to contribute to chemosensation for its expression in a subset of gustatory receptor neurons in the labral sense organ and in the labellum. These neurons also express a gustatory receptor Gr66a and mediate the avoidance of non-volatile compounds that are recognized by mammalian bitter taste receptors. In this study we aimed to further characterize the chemosensory properties of dTRPA1(A). This channel was reported to mediate the aversion towards citronellal, but to be activated indirectly through a G protein/PLC signalling cascade. We expressed the TRPA1(A) in a mammalian heterologous expression system. By use of the whole cell patch clamp technique, we were able to show that Citronellal activates TRPA1(A) in HEK293T cells. It has been recently shown that TRPA1 plays a role in the acute mammalian nocifensive responses to bacterial lipopolysaccharides (LPS). We hypothesized that dTRPA1 endows Drosophila fruit flies with the ability to detect LPS and to adopt avoiding behaviours towards this compound. Behavioural experiments showed that D. melanogaster flies avoid laying eggs on food contaminated with bacteria through mechanisms that are partly dependent on dTRPA1 activation. By use of the whole cell patch clamp technique we could illustrate the activation of dTRPA1(A) currents in HEK293T cells. Altogether, these experiments indicate for a direct role of TRPA1 in the detection and avoidance of a wide variety of noxious chemicals in D. melanogaster.

Detecting pathogens and mounting immune responses upon infection is crucial for animal health. However, these responses come at a high metabolic price (McKean and Lazzaro, 2011, Kominsky et al., 2010), and avoiding pathogens before infection may be advantageous. The bacterial endotoxins lipopolysaccharides (LPS) are important immune system infection cues (Abbas et al., 2014), but it remains unknown whether animals possess sensory mechanisms to detect them prior to infection. Here we show that Drosophila melanogaster display strong aversive responses to LPS and that gustatory neurons expressing Gr66a bitter receptors mediate avoidance of LPS in feeding and egg laying assays. We found the expression of the chemosensory cation channel dTRPA1 in these cells to be necessary and sufficient for LPS avoidance. Furthermore, LPS stimulates Drosophila neurons in a TRPA1-dependent manner and activates exogenous dTRPA1 channels in human cells. Our findings demonstrate that flies detect bacterial endotoxins via a gustatory pathway through TRPA1 activation as conserved molecular mechanism.

Asthma may be induced by chemical sensitisers, via mechanisms that are still poorly understood. This type of asthma is characterised by airway hyperreactivity (AHR) and little airway inflammation. Since potent chemical sensitisers, such as toluene-2,4-diisocyanate (TDI), are also sensory irritants, it is suggested that chemical-induced asthma relies on neuro-immune mechanisms.We investigated the involvement of transient receptor potential channels (TRP) A1 and V1, major chemosensors in the airways, and mast cells, known for their ability to communicate with sensory nerves, in chemical-induced AHR.In vitro intracellular calcium imaging and patch-clamp recordings in TRPA1- and TRPV1-expressing Chinese hamster ovarian cells showed that TDI activates murine TRPA1, but not TRPV1. Using an in vivo model, in which an airway challenge with TDI induces AHR in TDI-sensitised C57Bl/6 mice, we demonstrated that AHR does not develop, despite successful sensitisation, in Trpa1 and Trpv1 knockout mice, and wild-type mice pretreated with a TRPA1 blocker or a substance P receptor antagonist. TDI-induced AHR was also abolished in mast cell deficient Kit(Wsh) (/Wsh) mice, and in wild-type mice pretreated with the mast cell stabiliser ketotifen, without changes in immunological parameters.These data demonstrate that TRPA1, TRPV1 and mast cells play an indispensable role in the development of TDI-elicited AHR.

Neurogenic inflammation and pain associated to bacterial infection have been ascribed to sensitization and activation of sensory nerve afferents. We have recently unveiled a role of Transient Receptor Potential (TRP) A1 channel as sensor of lipopolysaccharide (LPS) in nociceptive neurons. However, here we show that responses to LPS are still detected in 30% of dorsal root ganglion neurons isolated from Trpa1 KO mice. The proportion of cells responding to LPS was dramatically lower in double Trpa1/Trpv1 KO neurons. Using intracellular calcium imaging and patch-clamp in a recombinant expression system, we studied the effects of LPS on TRPV1, TRPV2, TRPM3 and TRPM8 heterologously expressed in HEK cells. In isolated sensory neurons, we compared the activation of TRPV1 and TRPA1 by LPS. In contrast, LPS was ineffective on TRPV2, weakly activated TRPM3 and activated TRPM8 at 25°C but not at 35°C. Our results indicate that, in addition to TRPA1, other TRP channels in sensory neurons can be targets of LPS, raising the possibility that they may also contribute to trigger and regulate innate immune responses.