Resiniferatoxin Mediated Ablation of TRPV1+ Neurons Removes TRPA1 as Well

Hydrogen Sulfide (H2S)/Polysulfides (H2Sn) Signalling and TRPA1 Channels Modification on Sulfur Metabolism

Hydrogen sulfide (H2S) and polysulfides (H2Sn, n ≥ 2) produced by enzymes play a role as signalling molecules regulating neurotransmission, vascular tone, cytoprotection, inflammation, oxygen sensing, and energy formation. H2Sn, which have additional sulfur atoms to H2S, and other S-sulfurated molecules such as cysteine persulfide and S-sulfurated cysteine residues of proteins, are produced by enzymes including 3-mercaptopyruvate sulfurtransferase (3MST). H2Sn are also generated by the chemical interaction of H2S with NO, or to a lesser extent with H2O2. S-sulfuration (S-sulfhydration) has been proposed as a mode of action of H2S and H2Sn to regulate the activity of target molecules. Recently, we found that H2S/H2S2 regulate the release of neurotransmitters, such as GABA, glutamate, and D-serine, a co-agonist of N-methyl-D-aspartate (NMDA) receptors. H2S facilitates the induction of hippocampal long-term potentiation, a synaptic model of memory formation, by enhancing the activity of NMDA receptors, while H2S2 achieves this by activating transient receptor potential ankyrin 1 (TRPA1) channels in astrocytes, potentially leading to the activation of nearby neurons. The recent findings show the other aspects of TRPA1 channels-that is, the regulation of the levels of sulfur-containing molecules and their metabolizing enzymes. Disturbance of the signalling by H2S/H2Sn has been demonstrated to be involved in various diseases, including cognitive and psychiatric diseases. The physiological and pathophysiological roles of these molecules will be discussed.

TRPA1 and TPRV1 Ion Channels Are Required for Contact Lens-Induced Corneal Parainflammation and Can Modulate Levels of Resident Corneal Immune Cells

Purpose

Contact lens wear can induce corneal parainflammation involving CD11c+ cell responses (24 hours), γδ T cell responses (24 hours and 6 days), and IL-17-dependent Ly6G+ cell responses (6 days). Topical antibiotics blocked these CD11c+ responses. Because corneal CD11c+ responses to bacteria require transient receptor potential (TRP) ion-channels (TRPA1/TRPV1), we determined if these channels mediate lens-induced corneal parainflammation.

Methods

Wild-type mice were fitted with contact lenses for 24 hours or 6 days and compared to lens wearing TRPA1 (-/-) or TRPV1 (-/-) mice or resiniferatoxin (RTX)-treated mice. Contralateral eyes were not fitted with lenses. Corneas were examined for major histocompatibility complex (MHC) class II+, CD45+, γδ T, or TNF-α+ cell responses (24 hours) or Ly6G+ responses (6 days) by quantitative imaging. The quantitative PCR (qPCR) determined cytokine gene expression.

Results

Lens-induced increases in MHC class II+ cells after 24 hours were abrogated in TRPV1 (-/-) but not TRPA1 (-/-) mice. Increases in CD45+ cells were unaffected. Increases in γδ T cells after 24 hours of wear were abrogated in TRPA1 (-/-) and TRPV1 (-/-) mice, as were 6 day Ly6G+ cell responses. Contralateral corneas of TRPA1 (-/-) and TRPV1 (-/-) mice showed reduced MHC class II+ and γδ T cells at 24 hours. RTX inhibited lens-induced parainflammatory phenotypes (24 hours and 6 days), blocked lens-induced TNF-α and IL-18 gene expression, TNF-α+ cell infiltration (24 hours), and reduced baseline MHC class II+ cells.

Conclusions

TRPA1 and TRPV1 mediate contact lens-induced corneal parainflammation after 24 hours and 6 days of wear and can modulate baseline levels of resident corneal immune cells.

Anatomical Analysis of Transient Potential Vanilloid Receptor 1 (Trpv1+) and Mu-Opioid Receptor (Oprm1+) Co-expression in Rat Dorsal Root Ganglion Neurons

Primary afferent neurons of the dorsal root ganglia (DRG) transduce peripheral nociceptive signals and transmit them to the spinal cord. These neurons also mediate analgesic control of the nociceptive inputs, particularly through the μ-opioid receptor (encoded by Oprm1). While opioid receptors are found throughout the neuraxis and in the spinal cord tissue itself, intrathecal administration of μ-opioid agonists also acts directly on nociceptive nerve terminals in the dorsal spinal cord resulting in marked analgesia. Additionally, selective chemoaxotomy of cells expressing the TRPV1 channel, a nonselective calcium-permeable ion channel that transduces thermal and inflammatory pain, yields profound pain relief in rats, canines, and humans. However, the relationship between Oprm1 and Trpv1 expressing DRG neurons has not been precisely determined. The present study examines rat DRG neurons using high resolution multiplex fluorescent in situ hybridization to visualize molecular co-expression. Neurons positive for Trpv1 exhibited varying levels of expression for Trpv1 and co-expression of other excitatory and inhibitory ion channels or receptors. A subpopulation of densely labeled Trpv1+ neurons did not co-express Oprm1. In contrast, a population of less densely labeled Trpv1+ neurons did co-express Oprm1. This finding suggests that the medium/low Trpv1 expressing neurons represent a specific set of DRG neurons subserving the opponent processes of both transducing and inhibiting nociceptive inputs. Additionally, the medium/low Trpv1 expressing neurons co-expressed other markers implicated in pathological pain states, such as Trpa1 and Trpm8, which are involved in chemical nociception and cold allodynia, respectively, as well as Scn11a, whose mutations are implicated in familial episodic pain. Conversely, none of the Trpv1+ neurons co-expressed Spp1, which codes for osteopontin, a marker for large diameter proprioceptive neurons, validating that nociception and proprioception are governed by separate neuronal populations. Our findings support the hypothesis that the population of Trpv1 and Oprm1 coexpressing neurons may explain the remarkable efficacy of opioid drugs administered at the level of the DRG-spinal synapse, and that this subpopulation of Trpv1+ neurons is responsible for registering tissue damage.

Nerve-associated transient receptor potential ion channels can contribute to intrinsic resistance to bacterial adhesion in vivo

The cornea of the eye differs from other mucosal surfaces in that it lacks a viable bacterial microbiome and by its unusually high density of sensory nerve endings. Here, we explored the role of corneal nerves in preventing bacterial adhesion. Pharmacological and genetic methods were used to inhibit the function of corneal sensory nerves or their associated transient receptor potential cation channels TRPA1 and TRPV1. Impacts on bacterial adhesion, resident immune cells, and epithelial integrity were examined using fluorescent labeling and quantitative confocal imaging. TRPA1/TRPV1 double gene-knockout mice were more susceptible to adhesion of environmental bacteria and to that of deliberately-inoculated Pseudomonas aeruginosa. Supporting the involvement of TRPA1/TRPV1-expressing corneal nerves, P. aeruginosa adhesion was also promoted by treatment with bupivacaine, or ablation of TRPA1/TRPV1-expressing nerves using RTX. Moreover, TRPA1/TRPV1-dependent defense was abolished by enucleation which severs corneal nerves. High-resolution imaging showed normal corneal ultrastructure and surface-labeling by wheat-germ agglutinin for TRPA1/TRPV1 knockout murine corneas, and intact barrier function by absence of fluorescein staining. P. aeruginosa adhering to corneas after perturbation of nerve or TRPA1/TRPV1 function failed to penetrate the surface. Single gene-knockout mice showed roles for both TRPA1 and TRPV1, with TRPA1^-/- more susceptible to P. aeruginosa adhesion while TRPV1^-/- corneas instead accumulated environmental bacteria. Corneal CD45+/CD11c+ cell responses to P. aeruginosa challenge, previously shown to counter bacterial adhesion, also depended on TRPA1/TRPV1 and sensory nerves. Together, these results demonstrate roles for corneal nerves and TRPA1/TRPV1 in corneal resistance to bacterial adhesion in vivo and suggest that the mechanisms involve resident immune cell populations.

Pain Treatment in the Companion Canine Model to Validate Rodent Results and Incentivize the Transition to Human Clinical Trials

One of the biggest challenges for analgesic drug development is how to decide if a potential analgesic candidate will work in humans. What preclinical data are the most convincing, incentivizing and most predictive of success? Such a predicament is not unique to analgesics, and the pain field has certain advantages over drug development efforts in areas like neuropsychiatry where the etiological origins are either unknown or difficult to ascertain. For pain, the origin of the problem frequently is known, and the causative peripheral tissue insult might be observable. The main conundrum centers around evaluation of translational cell- and rodent-based results. While cell and rodent models are undeniably important first steps for screening, probing mechanism of action, and understanding factors of adsorption, distribution metabolism and excretion, two questions arise from such studies. First, are they reliable indicators of analgesic performance of a candidate drug in human acute and chronic pain? Second, what additional model systems might be capable of increasing translational confidence? We address this second question by assessing, primarily, the companion canine model, which can provide particularly strong predictive information for candidate analgesic agents in humans. This statement is mainly derived from our studies with resiniferatoxin (RTX) a potent TRPV1 agonist but also from protein therapeutics using a conjugate of Substance P and saporin. Our experience, to date, is that rodent models might be very well suited for acute pain translation, but companion canine models, and other large animal studies, can augment initial discovery research using rodent models for neuropathic or chronic pain. The larger animal models also provide strong translational predictive capacity for analgesic performance in humans, better predict dosing parameters for human trials and provide insight into behavior changes (bladder, bowel, mood, etc.) that are not readily assessed in laboratory animals. They are, however, not without problems that can be encountered with any experimental drug treatment or clinical trial. It also is important to recognize that pain treatment is a major veterinary concern and is an intrinsically worthwhile endeavor for animals as well as humans.

Neuronal and non-neuronal transient receptor potential ankyrin 1 mediates UVB radiation-induced skin inflammation in mice

AIMS

Neuronal and non-neuronal TRPA1 channel plays an active role in the pathogenesis of several skin inflammatory diseases. Although a recent study identified the TRPA1 channel activation upon UVB exposure, its role in inflammatory, oxidative, and proliferative processes underlying UVB radiation-induced sunburn was not yet fully understood. We evaluated the TRPA1 channel contribution in inflammatory, oxidative, and proliferative states on skin inflammation induced by UVB radiation in mice.

MAIN METHODS

TRPA1 role was evaluated from inflammatory (ear edema, myeloperoxidase, and N-acetyl-β-D-glycosaminidase activities, histological changes, and cytokines levels), proliferative (epidermal hyperplasia, PCNA, and TRPA1 levels), and oxidative (reactive oxygen intermediates measure, H₂O₂ quantification, and NADPH oxidase activity) parameters caused by UVB radiation single (0.5 J/cm²) or repeated (0.1 J/cm²) exposure. We verified the contribution of non-neuronal and neuronal TRPA1 on UVB radiation-induced inflammatory parameters using RTX-denervation (50 μg/kg s.c.).

KEY FINDINGS

TRPA1 blockade by the selective antagonist Lanette® N HC-030031 reduced all parameters induced by UVB radiation single (at concentration of 1%) or repeated (at concentration of 0.1%) exposure. We evidenced an up-regulation of the TRPA1 protein after UVB radiation repeated exposure, which was blocked by topical Lanette® N HC-030031 (0.1%). By RTX-denervation, we verified that non-neuronal TRPA1 also interferes in some inflammatory parameters induction. However, cutaneous nerve fibers seem to be most needed in the development of UVB radiation-induced inflammatory processes.

SIGNIFICANCE

We propose the TRPA1 channel participates in the UVB radiation-induced sunburn in mice, and it could be a promising therapeutic target to treat skin inflammatory disorders.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

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.

TRPA1/NOX in the soma of trigeminal ganglion neurons mediates migraine-related pain of glyceryl trinitrate in mice

Glyceryl trinitrate is administered as a provocative test for migraine pain. Glyceryl trinitrate causes prolonged mechanical allodynia in rodents, which temporally correlates with delayed glyceryl trinitrate-evoked migraine attacks in patients. However, the underlying mechanism of the allodynia evoked by glyceryl trinitrate is unknown. The proalgesic transient receptor potential ankyrin 1 (TRPA1) channel, expressed by trigeminal nociceptors, is sensitive to oxidative stress and is targeted by nitric oxide or its by-products. Herein, we explored the role of TRPA1 in glyceryl trinitrate-evoked allodynia. Systemic administration of glyceryl trinitrate elicited in the mouse periorbital area an early and transient vasodilatation and a delayed and prolonged mechanical allodynia. The systemic, intrathecal or local administration of selective enzyme inhibitors revealed that nitric oxide, liberated from the parent drug by aldehyde dehydrogenase 2 (ALDH2), initiates but does not maintain allodynia. The central and the final phases of allodynia were respectively associated with generation of reactive oxygen and carbonyl species within the trigeminal ganglion. Allodynia was absent in TRPA1-deficient mice and was reversed by TRPA1 antagonists. Knockdown of neuronal TRPA1 by intrathecally administered antisense oligonucleotide and selective deletion of TRPA1 from sensory neurons in Advillin-Cre; Trpa1^fl/fl mice revealed that nitric oxide-dependent oxidative and carbonylic stress generation is due to TRPA1 stimulation, and resultant NADPH oxidase 1 (NOX1) and NOX2 activation in the soma of trigeminal ganglion neurons. Early periorbital vasodilatation evoked by glyceryl trinitrate was attenuated by ALDH2 inhibition but was unaffected by TRPA1 blockade. Antagonists of the calcitonin gene-related peptide receptor did not affect the vasodilatation but partially inhibited allodynia. Thus, although both periorbital allodynia and vasodilatation evoked by glyceryl trinitrate are initiated by nitric oxide, they are temporally and mechanistically distinct. While vasodilatation is due to a direct nitric oxide action in the vascular smooth muscle, allodynia is a neuronal phenomenon mediated by TRPA1 activation and ensuing oxidative stress. The autocrine pathway, sustained by TRPA1 and NOX1/2 within neuronal cell bodies of trigeminal ganglia, may sensitize meningeal nociceptors and second order trigeminal neurons to elicit periorbital allodynia, and could be of relevance for migraine-like headaches evoked by glyceryl trinitrate in humans.

TRPV1 mediates inflammation and hyperplasia in imiquimod (IMQ)-induced psoriasiform dermatitis (PsD) in mice

BACKGROUND

Transient Receptor Potential Vanilloid 1 (TRPV1) is known to mediate itch and neurogenic inflammation, but the role of TRPV1 in psoriasiform dermal inflammation is poorly understood.

OBJECTIVE

To investigate the function of TRPV1 in imiquimod (IMQ)-induced psoriasiform dermatitis (PsD) in mice.

METHODS

Following daily treatment of topical IMQ cream for consecutive 5 days in C57BL/6 wide-type (WT) and TRPV1 gene knockout (KO) mice, we assessed the psoriasis severity index (PSI) scores, transepidermal water loss (TEWL), dermal inflammatory infiltrates, as well as gene expression levels for psoriasis related genes in mouse skin lesions.

RESULTS

Compared with WT mice, the clinical and TEWL scores, the extent of skin hyperplasia, the area of Munro microabscesses (MM) and angiogenesis of psoriasis were all significantly decreased in TRPV1 KO mice triggered with IMQ, suggesting a reduction in skin inflammation and barrier defects. In addition, the infiltration of CD45⁺ leukocytes, mast cells as well as CD3⁺ T cells was all reduced in the IMQ-treated skin of TRPV1 KO mice. Quantitative Real-time PCR (RT-qPCR) revealed that expression levels of IL-1β, IL-6, IL-23, S100A8 were decreased while IL-10 was increased in TRPV1 KO mice.

CONCLUSIONS

In summary, key markers of psoriatic inflammation and epidermal hyperplasia are reduced in TRPV1 KO mice, indicating the involvement of TRPV1 in the psoriasiform inflammation and suggesting its potential as a therapeutic target.

Participation of transient receptor potential vanilloid 1 in paclitaxel-induced acute visceral and peripheral nociception in rodents

The clinical use of paclitaxel as a chemotherapeutic agent is limited by the severe acute and chronic hypersensitivity caused when it is administered via intraperitoneal or intravenous routes. Thus far, evidence has suggested that transient receptor potential vanilloid-1 (TRPV1) has a key role in the chronic neuropathy induced by paclitaxel. Despite this, the role of TRPV1 in paclitaxel -related acute nociception, especially the development of visceral nociception, has not been evaluated. Thus, the goal of this study was to evaluate the participation of TRPV1 in a model of acute nociception induced by paclitaxel in rats and mice. A single intraperitoneal (i.p.) paclitaxel administration (1 mg/kg, i.p.) produced an immediate visceral nociception response 1 h after administration, caused mechanical and heat hypersensitivity, and diminished burrowing behaviour 24 h after administration. These nociceptive responses were reduced by SB-366791 treatment (0.5 mg/kg, i.p., a TRPV1 antagonist). In addition, TRPV1-positive sensory fibre ablation (using resiniferatoxin, 200 µg/kg, s.c.) reduced visceral nociception and mechanical or heat hypersensitivity caused by paclitaxel injection. Similarly, TRPV1 deficient mice showed a pronounced reduction in mechanical allodynia to paclitaxel acute injection and did not develop heat hypersensitivity. Moreover, 24 h after its injection, paclitaxel induced chemical hypersensitivity to capsaicin (a TRPV1 agonist, 0.01 nmol/site) and increased TRPV1 immunoreactivity in the dorsal root ganglion and sciatic nerve. In conclusion, TRPV1 is involved in mechanical and heat hypersensitivity and spontaneous-pain behaviour induced 24 h after a single paclitaxel injection. This receptor is also involved in visceral nociception induced immediately after paclitaxel administration.

Schwann cell TRPA1 mediates neuroinflammation that sustains macrophage-dependent neuropathic pain in mice

It is known that transient receptor potential ankyrin 1 (TRPA1) channels, expressed by nociceptors, contribute to neuropathic pain. Here we show that TRPA1 is also expressed in Schwann cells. We found that in mice with partial sciatic nerve ligation, TRPA1 silencing in nociceptors attenuated mechanical allodynia, without affecting macrophage infiltration and oxidative stress, whereas TRPA1 silencing in Schwann cells reduced both allodynia and neuroinflammation. Activation of Schwann cell TRPA1 evoked NADPH oxidase 1 (NOX1)-dependent H2O2 release, and silencing or blocking Schwann cell NOX1 attenuated nerve injury-induced macrophage infiltration, oxidative stress and allodynia. Furthermore, the NOX2-dependent oxidative burst, produced by macrophages recruited to the perineural space activated the TRPA1-NOX1 pathway in Schwann cells, but not TRPA1 in nociceptors. Schwann cell TRPA1 generates a spatially constrained gradient of oxidative stress, which maintains macrophage infiltration to the injured nerve, and sends paracrine signals to activate TRPA1 of ensheathed nociceptors to sustain mechanical allodynia.

Menthol evokes Ca2+ signals and induces oxidative stress independently of the presence of TRPM8 (menthol) receptor in cancer cells

Menthol is a naturally occurring monoterpene alcohol possessing remarkable biological properties including antipruritic, analgesic, antiseptic, anti-inflammatory and cooling effects. Here, we examined the menthol-evoked Ca²⁺ signals in breast and prostate cancer cell lines. The effect of menthol (50–500 µM) was predicted to be mediated by the transient receptor potential ion channel melastatin subtype 8 (TRPM8). However, the intensity of menthol-evoked Ca²⁺ signals did not correlate with the expression levels of TRPM8 in breast and prostate cancer cells indicating a TRPM8-independent signaling pathway. Menthol-evoked Ca²⁺ signals were analyzed in detail in Du 145 prostate cancer cells, as well as in CRISPR/Cas9 TRPM8-knockout Du 145 cells. Menthol (500 µM) induced Ca²⁺ oscillations in both cell lines, thus independent of TRPM8, which were however dependent on the production of inositol trisphosphate. Results based on pharmacological tools point to an involvement of the purinergic pathway in menthol-evoked Ca²⁺ responses. Finally, menthol (50–500 µM) decreased cell viability and induced oxidative stress independently of the presence of TRPM8 channels, despite that temperature-evoked TRPM8-mediated inward currents were significantly decreased in TRPM8-knockout Du 145 cells compared to wild type Du 145 cells.

Supraspinal-selective TRPV1 desensitization induced by intracerebroventricular treatment with resiniferatoxin

The transient receptor potential vanilloid type 1 (TRPV1) is a thermosensitive cation channel that triggers heat pain in the periphery. Long-term desensitization of TRPV1, which can be induced by excess amounts of agonists, has been a method for investigating the physiological relevance of TRPV1-containing neuronal circuits, and desensitization induced by various routes of administration, including systemic, intrathecal and intraganglionic, has been demonstrated in rodents. In the present study, we examined the effect of intracerebroventricular (i.c.v.) treatment with an ultrapotent TRPV1 agonist, resiniferatoxin (RTX), on nociception and the analgesic effect of acetaminophen, which is known to mediate the activation of central TRPV1. I.c.v. administration of RTX a week before the test did not affect the licking/biting response to intraplantar injection of RTX (RTX test), suggesting that such i.c.v. treatment spares the function of TRPV1 at the hindpaw. Mice that had been i.c.v.-administered RTX also exhibited normal nociceptive responses in the formalin test and the tail pressure test, but acetaminophen failed to induce analgesia in those mice in any of the tests. These results suggest that i.c.v. administration of RTX leads to brain-selective TRPV1 desensitization in mice.

Targeting breast cancer cells by MRS1477, a positive allosteric modulator of TRPV1 channels

There is convincing epidemiological and experimental evidence that capsaicin, a potent natural transient receptor potential cation channel vanilloid member 1 (TRPV1) agonist, has anticancer activity. However, capsaicin cannot be given systemically in large doses, because of its induction of acute pain and neurological inflammation. MRS1477, a dihydropyridine derivative acts as a positive allosteric modulator of TRPV1, if added together with capsaicin, but is ineffective, if given alone. Addition of MRS1477 evoked Ca²⁺ signals in MCF7 breast cancer cells, but not in primary breast epithelial cells. This indicates that MCF7 cells not only express functional TRPV1 channels, but also produce endogenous TRPV1 agonists. We investigated the effects of MRS1477 and capsaicin on cell viability, caspase-3 and -9 activities and reactive oxygen species production in MCF7 cells. The fraction of apoptotic cells was increased after 3 days incubation with capsaicin (10 μM) paralleled by increased reactive oxygen species production and caspase activity. These effects were even more pronounced, when cells were incubated with MRS1477 (2 μM) either alone or together with CAPS (10 μM). Capsazepine, a TRPV1 blocker, inhibited both the effect of capsaicin and MRS1477. Whole-cell patch clamp recordings revealed that capsaicin-evoked TRPV1-mediated current density levels were increased after 3 days incubation with MRS1477 (2 μM). However, the tumor growth in MCF7 tumor-bearing immunodeficient mice was not significantly decreased after treatment with MRS1477 (10 mg/ kg body weight, i.p., injection twice a week). In conclusion, in view of a putative in vivo treatment with MRS1477 or similar compounds further optimization is required.

Molecular Surgery Concept from Bench to Bedside: A Focus on TRPV1+ Pain-Sensing Neurons

"Molecular neurosurgery" is emerging as a new medical concept, and is the combination of two partners: (i) a molecular neurosurgery agent, and (ii) the cognate receptor whose activation results in the selective elimination of a specific subset of neurons in which this receptor is endogenously expressed. In general, a molecular surgery agent is a selective and potent ligand, and the target is a specific cell type whose elimination is desired through the molecular surgery procedure. These target cells have the highest innate sensitivity to the molecular surgery agent usually due to the highest receptor density being in their plasma membrane. The interaction between the ligand and its receptor evokes an overactivity of the receptor. If the receptor is a ligand-activated non-selective cation channel, the overactivity of receptor leads to excess Ca²⁺ and Na⁺ influx into the cell and finally cell death. One of the best known examples of such an interaction is the effect of ultrapotent vanilloids on TRPV1-expressing pain-sensing neurons. One intrathecal resiniferatoxin (RTX) dose allows for the receptor-mediated removal of TRPV1+ neurons from the peripheral nervous system. The TRPV1 receptor-mediated ion influx induces necrotic processes, but only in pain-sensing neurons, and usually within an hour. Besides that, target-specific apoptotic processes are also induced. Thus, as a nano-surgery scalpel, RTX removes the neurons responsible for generating pain and inflammation from the peripheral nervous system providing an option in clinical management for the treatment of morphine-insensitive pain conditions. In the future, the molecular surgery concept can also be exploited in cancer research for selectively targeting the specific tumor cell.

Endogenous TRPV1 stimulation leads to the activation of the inositol phospholipid pathway necessary for sustained Ca2+ oscillations

Sensory neuron subpopulations as well as breast and prostate cancer cells express functional transient receptor potential vanilloid type 1 (TRPV1) ion channels; however little is known how TRPV1 activation leads to biological responses. Agonist-induced activation of TRPV1 resulted in specific spatiotemporal patterns of cytoplasmic Ca²⁺ signals in breast and prostate cancer-derived cells. Capsaicin (CAPS; 50μM) evoked intracellular Ca²⁺ oscillations and/or intercellular Ca²⁺ waves in all cell lines. As evidenced in prostate cancer Du 145 cells, oscillations were largely dependent on the expression of functional TRPV1 channels in the plasma membrane, phospholipase C activation and on the presence of extracellular Ca²⁺ ions. Concomitant oscillations of the mitochondrial matrix Ca²⁺ concentration resulted in mitochondria energization evidenced by increased ATP production. CAPS-induced Ca²⁺ oscillations also occurred in a subset of sensory neurons, yet already at lower CAPS concentrations (1μM). Stimulation of ectopically expressed TRPV1 channels in CAPS-insensitive NIH-3T3 cells didn't provoke CAPS-triggered Ca²⁺ oscillations; rather it resulted in low-magnitude, long-lasting elevations of the cytosolic Ca²⁺ concentration. This indicates that sole TRPV1 activation is not sufficient to generate Ca²⁺ oscillations. Instead the initial TRPV1-mediated signal leads to the activation of the inositol phospholipid pathway. This in turn suffices to generate a biologically relevant frequency-modulated Ca²⁺ signal.

TRPV1: A Target for Rational Drug Design

Transient Receptor Potential Vanilloid 1 (TRPV1) is a non-selective, Ca²⁺ permeable cation channel activated by noxious heat, and chemical ligands, such as capsaicin and resiniferatoxin (RTX). Many compounds have been developed that either activate or inhibit TRPV1, but none of them are in routine clinical practice. This review will discuss the rationale for antagonists and agonists of TRPV1 for pain relief and other conditions, and strategies to develop new, better drugs to target this ion channel, using the newly available high-resolution structures.

Neuronal TRPV1 activation regulates alveolar bone resorption by suppressing osteoclastogenesis via CGRP

The transient receptor potential vanilloid 1 (TRPV1) channel is abundantly expressed in peripheral sensory neurons where it acts as an important polymodal cellular sensor for heat, acidic pH, capsaicin, and other noxious stimuli. The oral cavity is densely innervated by afferent sensory neurons and is a highly specialized organ that protects against infections as well as physical, chemical, and thermal stresses in its capacity as the first part of the digestive system. While the function of TRPV1 in sensory neurons has been intensively studied in other organs, its physiological role in periodontal tissues is unclear. In this study we found that Trpv1^−/− mice developed severe bone loss in an experimental model of periodontitis. Chemical ablation of TRPV1-expressing sensory neurons recapitulated the phenotype of Trpv1^−/− mice, suggesting a functional link between neuronal TRPV1 signaling and periodontal bone loss. TRPV1 activation in gingival nerves induced production of the neuropeptide, calcitonin gene-related peptide (CGRP), and CGRP treatment inhibited osteoclastogenesis in vitro. Oral administration of the TRPV1 agonist, capsaicin, suppressed ligature-induced bone loss in mice with fewer tartrate-resistant acid phosphatase (TRAP)-positive cells in alveolar bone. These results suggest that neuronal TRPV1 signaling in periodontal tissue is crucial for the regulation of osteoclastogenesis via the neuropeptide CGRP.

Activation of endogenous TRPV1 fails to induce overstimulation-based cytotoxicity in breast and prostate cancer cells but not in pain-sensing neurons

Vanilloids including capsaicin and resiniferatoxin are potent transient receptor potential vanilloid type 1 (TRPV1) agonists. TRPV1 overstimulation selectively ablates capsaicin-sensitive sensory neurons in animal models in vivo. The cytotoxic mechanisms are based on strong Na(+) and Ca(2+) influx via TRPV1 channels, which leads to mitochondrial Ca(2+) accumulation and necrotic cell swelling. Increased TRPV1 expression levels are also observed in breast and prostate cancer and derived cell lines. Here, we examined whether potent agonist-induced overstimulation mediated by TRPV1 might represent a means for the eradication of prostate carcinoma (PC-3, Du 145, LNCaP) and breast cancer (MCF7, MDA-MB-231, BT-474) cells in vitro. While rat sensory neurons were highly vanilloid-sensitive, normal rat prostate epithelial cells were resistant in vivo. We found TRPV1 to be expressed in all cancer cell lines at mRNA and protein levels, yet protein expression levels were significantly lower compared to sensory neurons. Treatment of all human carcinoma cell lines with capsaicin didn't lead to overstimulation cytotoxicity in vitro. We assume that the low vanilloid-sensitivity of prostate and breast cancer cells is associated with low expression levels of TRPV1, since ectopic TRPV1 expression rendered them susceptible to the cytotoxic effect of vanilloids evidenced by plateau-type Ca(2+) signals, mitochondrial Ca(2+) accumulation and Na(+)- and Ca(2+)-dependent membrane disorganization. Moreover, long-term monitoring revealed that merely the ectopic expression of TRPV1 stopped cell proliferation and often induced apoptotic processes via strong activation of caspase-3 activity. Our results indicate that specific targeting of TRPV1 function remains a putative strategy for cancer treatment.

Ion channel TRPV1-dependent activation of PTP1B suppresses EGFR-associated intestinal tumorigenesis

The intestinal epithelium has a high rate of turnover, and dysregulation of pathways that regulate regeneration can lead to tumor development; however, the negative regulators of oncogenic events in the intestinal epithelium are not fully understood. Here we identified a feedback loop between the epidermal growth factor receptor (EGFR), a known mediator of proliferation, and the transient receptor potential cation channel, subfamily V, member 1 (TRPV1), in intestinal epithelial cells (IECs). We found that TRPV1 was expressed by IECs and was intrinsically activated upon EGFR stimulation. Subsequently, TRPV1 activation inhibited EGFR-induced epithelial cell proliferation via activation of Ca2+/calpain and resulting activation of protein tyrosine phosphatase 1B (PTP1B). In a murine model of multiple intestinal neoplasia (Apc(Min/+) mice), TRPV1 deficiency increased adenoma formation, and treatment of these animals with an EGFR kinase inhibitor reversed protumorigenic phenotypes, supporting a functional association between TRPV1 and EGFR signaling in IECs. Administration of a TRPV1 agonist suppressed intestinal tumorigenesis in Apc(Min/+) mice, similar to--as well as in conjunction with--a cyclooxygenase-2 (COX-2) inhibitor, which suggests that targeting both TRPV1 and COX-2 has potential as a therapeutic approach for tumor prevention. Our findings implicate TRPV1 as a regulator of growth factor signaling in the intestinal epithelium through activation of PTP1B and subsequent suppression of intestinal tumorigenesis.

Intraganglionic signaling as a novel nasal-meningeal pathway for TRPA1-dependent trigeminovascular activation by inhaled environmental irritants

Headache is the most common symptom associated with air pollution, but little is understood about the underlying mechanism. Nasal administration of environmental irritants activates the trigeminovascular system by a TRPA1-dependent process. This report addresses questions about the anatomical pathway involved and the function of TRP channels in this pathway. TRPV1 and TRPA1 are frequently co-localized and interact to modulate function in sensory neurons. We demonstrate here that resiniferatoxin ablation of TRPV1 expressing neurons significantly reduces meningeal blood flow responses to nasal administration of both TRPV1 and TRPA1 agonists. Accordingly resiniferatoxin also significantly reduces TRPV1 and CGRP immunostaining and TRPV1 and TRPA1 message levels in trigeminal ganglia. Sensory neurons of the trigeminal ganglia innervate the nasal epithelium and the meninges, but the mechanism and anatomical route by which nasal administration evokes meningeal vasodilatation is unclear. Double retrograde labeling from the nose and meninges reveals no co-localization of fluorescent label, however nasal and meningeal labeled cells are located in close proximity to each other within the trigeminal ganglion. Our data demonstrate that TRPV1 expressing neurons are important for TRPA1 responses in the nasal-meningeal pathway. Our data also suggest that the nasal-meningeal pathway is not primarily by axon reflex, but may instead result from intraganglionic transmission.

TRPA1 receptor stimulation by hydrogen peroxide is critical to trigger hyperalgesia and inflammation in a model of acute gout

Acute gout attacks produce severe joint pain and inflammation associated with monosodium urate (MSU) crystals leading to oxidative stress production. The transient potential receptor ankyrin 1 (TRPA1) is expressed by a subpopulation of peptidergic nociceptors and, via its activation by endogenous reactive oxygen species, including hydrogen peroxide (H2O2), contributes to pain and neurogenic inflammation. The aim of this study was to investigate the role of TRPA1 in hyperalgesia and inflammation in a model of acute gout attack in rodents. Inflammatory parameters and mechanical hyperalgesia were measured in male Wistar rats and in wild-type (Trpa1(+/+)) or TRPA1-deficient (Trpa1(-/-)) male mice. Animals received intra-articular (ia, ankle) injection of MSU. The role of TRPA1 was assessed by receptor antagonism, gene deletion or expression, sensory fiber defunctionalization, and calcitonin gene-related peptide (CGRP) release. We found that nociceptor defunctionalization, TRPA1 antagonist treatment (via ia or oral administration), and Trpa1 gene ablation abated hyperalgesia and inflammatory responses (edema, H2O2 generation, interleukin-1β release, and neutrophil infiltration) induced by ia MSU injection. In addition, we showed that MSU evoked generation of H2O2 in synovial tissue, which stimulated TRPA1 producing CGRP release and plasma protein extravasation. The MSU-elicited responses were also reduced by the H2O2-detoxifying enzyme catalase and the reducing agent dithiothreitol. TRPA1 activation by MSU challenge-generated H2O2 mediates the entire inflammatory response in an acute gout attack rodent model, thus strengthening the role of the TRPA1 receptor and H2O2 production as potential targets for treatment of acute gout attacks.

Mechanisms involved in abdominal nociception induced by either TRPV1 or TRPA1 stimulation of rat peritoneum

Abdominal pain is a frequent symptom of peritoneal cavity irritation, but little is known about the role of the receptors for irritant substances, transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1), in this painful condition. Thus, we investigated the abdominal nociception caused by peritoneal stimulation with TRPV1 (capsaicin) and TRPA1 (allyl isothiocyanate, AITC) agonists and their mechanisms in rats. The intraperitoneal (i.p.) injection of either capsaicin or AITC (0.03-10 mg/kg) induced short-term (up to 20 min) and dose-dependent abdominal nociception, and also produced c-fos expression in spinal afferents of the dorsal horn. TRPV1 antagonism prevented (94 ± 4% inhibition) nociception induced by capsaicin but not by AITC. In contrast, the TRPA1 antagonism almost abolished AITC-induced nociception (95 ± 2% inhibition) without altering the capsaicin response. Moreover, nociception induced by either capsaicin or AITC was reduced by the desensitisation of TRPV1-positive sensory fibres with resiniferatoxin (73 ± 18 and 76 ± 15% inhibitions, respectively) and by the NK1 receptor antagonist aprepitant (56 ± 5 and 53 ± 8% inhibitions, respectively). Likewise, the i.p. injections of capsaicin or AITC increased the content of substance P in the peritoneal fluid. Nevertheless, neither the mast cell membrane stabiliser cromoglycate, nor the H1 antagonist promethazine, nor depletion of peritoneal macrophages affected abdominal nociception induced either by capsaicin or AITC. Accordingly, neither capsaicin nor AITC increased the histamine content in the peritoneal fluid or provoked peritoneal mast cell degranulation in vitro. Collectively, our findings suggest that TRPV1 and TRPA1 stimulation in the peritoneum produces abdominal nociception that is mediated by sensory fibres activation.

Functionally important amino acid residues in the transient receptor potential vanilloid 1 (TRPV1) ion channel--an overview of the current mutational data

This review aims to create an overview of the currently available results of site-directed mutagenesis studies on transient receptor potential vanilloid type 1 (TRPV1) receptor. Systematization of the vast number of data on the functionally important amino acid mutations of TRPV1 may provide a clearer picture of this field, and may promote a better understanding of the relationship between the structure and function of TRPV1. The review summarizes information on 112 unique mutated sites along the TRPV1, exchanged to multiple different residues in many cases. These mutations influence the effect or binding of different agonists, antagonists, and channel blockers, alter the responsiveness to heat, acid, and voltage dependence, affect the channel pore characteristics, and influence the regulation of the receptor function by phosphorylation, glycosylation, calmodulin, PIP2, ATP, and lipid binding. The main goal of this paper is to publish the above mentioned data in a form that facilitates in silico molecular modelling of the receptor by promoting easier establishment of boundary conditions. The better understanding of the structure-function relationship of TRPV1 may promote discovery of new, promising, more effective and safe drugs for treatment of neurogenic inflammation and pain-related diseases and may offer new opportunities for therapeutic interventions.

Divalent heavy metal cations block the TRPV1 Ca(2+) channel

Transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel involved in pain sensation and in a wide range of non-pain-related physiological and pathological conditions. The aim of the present study was to explore the effects of selected heavy metal cations on the function of TRPV1. The cations ranked in the following sequence of pore-blocking activity: Co(2+) [half-maximal inhibitory concentration (IC(50)) = 13 μM] > Cd(2+) (I (50) = 38 μM) > Ni(2+) (IC(50) = 62 μM) > Cu(2+) (IC(50) = 200 μM). Zn(2+) proved to be a weak (IC(50) = 27 μM) and only partial inhibitor of the channel function, whereas Mg(2+), Mn(2+) and La(3+) did not exhibit any substantial effect. Co(2+), the most potent channel blocker, was able not only to compete with Ca(2+) but also to pass with it through the open channel of TRPV1. In response to heat activation or vanilloid treatment, Co(2+) accumulation was verified in TRPV1-transfected cell lines and in the TRPV1+ dorsal root ganglion neurons. The inhibitory effect was also demonstrated in vivo. Co(2+) applied together with vanilloid agonists attenuated the nocifensive eye wipe response in mice. Different rat TRPV1 pore point mutants (Y627W, N628W, D646N and E651W) were created that can validate the binding site of previously used channel blockers in agonist-evoked (45)Ca(2+) influx assays in cells expressing TRPV1. The IC(50) of Co(2+) on these point mutants were determined to be reasonably comparable to those on the wild type, which suggests that divalent cations passing through the TRPV1 channel use the same negatively charged amino acids as Ca(2+).

Transient receptor potential ankyrin 1 channel localized to non-neuronal airway cells promotes non-neurogenic inflammation

Background

The transient receptor potential ankyrin 1 (TRPA1) channel, localized to airway sensory nerves, has been proposed to mediate airway inflammation evoked by allergen and cigarette smoke (CS) in rodents, via a neurogenic mechanism. However the limited clinical evidence for the role of neurogenic inflammation in asthma or chronic obstructive pulmonary disease raises an alternative possibility that airway inflammation is promoted by non-neuronal TRPA1.

Methodology/Principal Findings

By using Real-Time PCR and calcium imaging, we found that cultured human airway cells, including fibroblasts, epithelial and smooth muscle cells express functional TRPA1 channels. By using immunohistochemistry, TRPA1 staining was observed in airway epithelial and smooth muscle cells in sections taken from human airways and lung, and from airways and lung of wild-type, but not TRPA1-deficient mice. In cultured human airway epithelial and smooth muscle cells and fibroblasts, acrolein and CS extract evoked IL-8 release, a response selectively reduced by TRPA1 antagonists. Capsaicin, agonist of the transient receptor potential vanilloid 1 (TRPV1), a channel co-expressed with TRPA1 by airway sensory nerves, and acrolein or CS (TRPA1 agonists), or the neuropeptide substance P (SP), which is released from sensory nerve terminals by capsaicin, acrolein or CS), produced neurogenic inflammation in mouse airways. However, only acrolein and CS, but not capsaicin or SP, released the keratinocyte chemoattractant (CXCL-1/KC, IL-8 analogue) in bronchoalveolar lavage (BAL) fluid of wild-type mice. This effect of TRPA1 agonists was attenuated by TRPA1 antagonism or in TRPA1-deficient mice, but not by pharmacological ablation of sensory nerves.

Conclusions

Our results demonstrate that, although either TRPV1 or TRPA1 activation causes airway neurogenic inflammation, solely TRPA1 activation orchestrates an additional inflammatory response which is not neurogenic. This finding suggests that non-neuronal TRPA1 in the airways is functional and potentially capable of contributing to inflammatory airway diseases.

Involvement of the glutamatergic system in the nociception induced intrathecally for a TRPA1 agonist in rats

The transient receptor potential ankyrin 1 (TRPA1) is expressed in peripheral and spinal terminals of sensory neurons, jointly to the vanilloid receptor (TRPV1). A relevant peripheral role of TRPA1 receptor has been implicated in a variety of processes, including the detection of noxious cold, and diverse painful stimulus, but the functional role of TRPA1 receptor in nociceptive transmission at spinal cord in vivo is poorly known. Therefore, the aim of this study was to evaluate whether the glutamatergic system is involved in the transmission of nociceptive stimulus induced for a TRPA1 agonist in the rat spinal cord. We observed that cinnamaldehyde, a TRPA1 agonist, on spinal cord synaptosomes leads to an increase in [Ca(2+)](i) and a rapid release of glutamate, but was not able to change the specific [(3)H]-glutamate binding. In addition, spinally administered cinnamaldehyde produced heat hyperalgesia and mechanical allodynia in rats. This behavior was reduced by the co-injection (i.t.) of camphor (TRPA1 antagonist) or MK-801 (N-methyl-D-aspartate (NMDA) receptor antagonist) to cinnamaldehyde. Besides, the pretreatment with resiniferatoxin (RTX), a potent TRPV1 agonist, abolished the cinnamaldehyde-induced heat hyperalgesia. Here, we showed that intrathecal RTX results in a decrease in TRPA1 and TRPV1 immunoreactivity in dorsal root ganglion. Collectively, our results demonstrate the pertinent participation of spinal TRPA1 in the possible enhancement of glutamatergic transmission of nociceptive signals leading to increase of the hypersensitivity, here observed as heat hyperalgesia. Then the modulation of spinal TRPA1 might be a valuable target in painful conditions associated with central pain hypersensitivity.

Transient receptor potential ankyrin 1: emerging pharmacology and indications for cardiovascular biology

Transient receptor potential anykrin 1 (TRPA1) is a member of the TRP superfamily, representing the sole member of the TRPA subfamily. It has many identified endogenous and exogenous agonists, comprising largely of chemical irritants and products of oxidative stress. Classically located on sensory neurone endings, TRPA1 has developed a strong presence in pain and inflammatory studies, where it is now becoming an intriguing clinical drug target. TRPA1 is increasingly recognized in a growing number of neuronal and non-neuronal locations with expanding expression and activity profiles providing evidence of a role for TRPA1 in other systems. Interest in discovering the pharmacological and functional roles of TRPA1 is increasing and diversifying into many areas. Historically, compounds now known as TRPA1 agonists have demonstrated cardiovascular activity, modulating activities in both the heart and the vasculature. Now TRPA1 has been identified as the receptor via which these compounds can act, these studies are being revisited and expanded on using current techniques. It is therefore timely to review the current knowledge of TRPA1 receptor presence and activities of relevance to the cardiovascular system, summarizing findings to date and identifying potential areas for future investigation.

Transient receptor potential channel A1 and noxious cold responses in rat cutaneous nociceptors

The role of transient receptor potential channel A1 (TRPA1) in noxious cold sensation remains unclear. Some data support the hypothesis that TRPA1 is a transducer of noxious cold whilst other data contest it. In this study we investigated the role of TRPA1 in cold detection in cutaneous nociceptors in vivo using complementary experimental approaches. We used noxious withdrawal reflex electromyography, and single fibre recordings in vivo, to test the hypothesis that TRPA1-expressing primary afferents mediate noxious cold responses in anaesthetised rats. TRPV1 and TRPM8 agonists sensitise their cognate receptors to heat and cold stimuli respectively. Herein we show that the TRPA1 agonist cinnamaldehyde applied to the skin in anaesthetised rats did not sensitise noxious cold evoked hind limb withdrawal. In contrast, cinnamaldehyde did sensitise the C fibre-mediated noxious heat withdrawal, indicated by a significant drop in the withdrawal temperature. TRPA1 agonist thus sensitised the noxious reflex withdrawal to heat, but not cold. Thermal stimuli also sensitise transient receptor potential (TRP) channels to agonist. Activity evoked by capsaicin in teased primary afferent fibres showed a significant positive correlation with receptive field temperature, in both normal and Freund's complete adjuvant-induced cutaneous inflammation. Altering the temperature of the receptive field did not modulate TRPA1 agonist evoked-activity in cutaneous primary afferents, in either normal or inflamed skin. In addition, block of the TRPA1 channel with Ruthenium Red did not inhibit cold evoked activity in either cinnamaldehyde sensitive or insensitive cold responsive nociceptors. In cinnamaldehyde-sensitive-cold-sensitive afferents, although TRPA1 agonist-evoked activity was totally abolished by Ruthenium Red, cold evoked activity was unaffected by channel blockade. We conclude that these results do not support the hypothesis that TRPA1-expressing cutaneous afferents play an important role in noxious cold responses.