Differential modulation of agonist and antagonist structure activity relations for rat TRPV1 by cyclosporin A and other protein phosphatase inhibitors

TRPV1 Ion Channel: Structural Features, Activity Modulators, and Therapeutic Potential

Although TRPV1 ion channel has been attracting researchers' attention for many years, its functions in animal organisms, the principles of regulation, and the involvement in pathological processes have not yet been fully clarified. Mutagenesis experiments and structural studies have identified the structural features of the channel and binding sites for its numerous ligands; however, these studies are far from conclusion. This review summarizes recent achievements in the TRPV1 research with special focus on structural and functional studies of the channel and on its ligands, which are extremely diverse in their nature and interaction specificity to TRPV1. Particular attention was given to the effects of numerous endogenous agonists and antagonists that can fine-tune the channel sensitivity to its usual activators, such as capsaicin, heat, acids, or their combination. In addition to the pain sensing not covered in this review, the TRPV1 channel was found to be involved in the regulation of many important physiological and pathological processes and, therefore, can be considered as a promising therapeutic target in the treatment of various diseases, such as pneumonia, ischemia, diabetes, epilepsy, schizophrenia, psoriasis, etc.

Combination of a Rapidly Penetrating Agonist and a Slowly Penetrating Antagonist Affords Agonist Action of Limited Duration at the Cellular Level

The capsaicin receptor TRPV1 (transient receptor potential vanilloid 1) has been an object of intense interest for pharmacological development on account of its critical role in nociception. In the course of structure activity analysis, it has become apparent that TRPV1 ligands may vary dramatically in the rates at which they interact with TRPV1, presumably reflecting differences in their abilities to penetrate into the cell. Using a fast penetrating agonist together with an excess of a slower penetrating antagonist, we find that we can induce an agonist response of limited duration and, moreover, the duration of the agonist response remains largely independent of the absolute dose of agonist, as long as the ratio of antagonist to agonist is held constant. This general approach for limiting agonist duration under conditions in which absolute agonist dose is variable should have more general applicability.

Novel Radiolabeled Vanilloid with Enhanced Specificity for Human Transient Receptor Potential Vanilloid 1 (TRPV1)

Transient receptor potential vanilloid 1 (TRPV1) has emerged as a promising therapeutic target. While radiolabeled resiniferatoxin (RTX) has provided a powerful tool for characterization of vanilloid binding to TRPV1, TRPV1 shows 20-fold weaker binding to the human TRPV1 than to the rodent TRPV1. We now describe a tritium radiolabeled synthetic vanilloid antagonist, 1-((2-(4-(methyl-[³H])piperidin-1-yl-4-[³H])-6-(trifluoromethyl)pyridin-3-yl)methyl)-3-(3-oxo-3,4-dihydro-2H-benzo[b][1,4]oxazin-8-yl)urea ([³H]MPOU), that embodies improved absolute affinity for human TRPV1 and improved synthetic accessibility.

Inhibition of TRPV1 prevented skin irritancy induced by phenoxyethanol. A preliminary in vitro and in vivo study

BACKGROUND

Phenoxyethanol is a widely used preservative in personal care products. Transient receptor potential vanilloid 1 (TRPV1) on cell membrane is activated by TRPV1 agonist capsaicin resulting in an opening of the channel for calcium influx, which is linked with neurosensory sensations characterized by itching, burning and stinging of skin. Whether uncomfortable skin sensations caused by phenoxyethanol claimed by people having sensitive skin are also due to activation of TRPV1 has not been reported in the literature.

OBJECTIVE

The aim of this study was to evaluate whether the TRPV1 is involved in the induction of itching and burning sensation by phenoxyethanol.

METHODS AND MATERIALS

The effect of phenoxyethanol on TRPV1 was assessed in vitro on HaCaT cells. The activation of TRPV1 and its inhibition by a TRPV1 antagonist were evaluated by cellular calcium influx. TRPV1 protein expression was also investigated by Western blot. In vivo in a split-face study, phenoxyethanol formulated at 1% was compared to a formulation additionally containing a TRPV1 antagonist. By applying the formulations to the nasolabial fold, the scores of phenoxyethanol-induced sensations were compared to those of the TRPV1 antagonist.

RESULTS

In vitro phenoxyethanol induced calcium influx in HaCaT cells in a dose-dependent manner after 20 min. This effect was abolished by a solution containing the TRPV1 antagonist trans-tert-butyl cyclohexanol (ID1609). Phenoxyethanol had no effect on the expression of TRPV1, whereas capsaicin caused a significant downregulation of this receptor in the same experiment. In vivo 1% phenoxyethanol induced a skin burning and itching sensation in a cohort of 60 of 243 Chinese female subjects being sensitive to phenoxyethanol discomfort. The uncomfortable skin sensations were significantly inhibited by ID1609.

CONCLUSIONS

Different to capsaicin, phenoxyethanol did not downregulate the expression of TRPV1 in HaCaT cells, suggesting that different regulatory mechanisms may exist for capsaicin and phenoxyethanol. Our experiments demonstrated that phenoxyethanol induces skin misperception and uncomfortable skin sensations like itching and burning comparable to capsaicin via activating TRPV1. The stimulation was inhibited by blocking TRPV1 with ID1609. The present data strengthened hitherto studies that TRPV1 plays a critical role in sensitive skin.

An HSV-based library screen identifies PP1α as a negative TRPV1 regulator with analgesic activity in models of pain

Transient receptor potential vanilloid 1 (TRPV1) is a pronociceptive cation channel involved in persistent inflammatory and neuropathic pain. Herpes simplex virus (HSV) vector expression of TRPV1 causes cell death in the presence of capsaicin, thereby completely blocking virus replication. Here we describe a selection system for negative regulators of TRPV1 based on rescue of virus replication. HSV-based coexpression of TRPV1 and a PC12 cell-derived cDNA library identified protein phosphatase 1α (PP1α) as a negative regulator of TRPV1, mimicking the activity of “poreless” (PL), a dominant-negative mutant of TRPV1. Vectors expressing PP1α or PL reduced thermal sensitivity following virus injection into rat footpads, but failed to reduce the nocifensive responses to menthol/icilin-activated cold pain or formalin, demonstrating that the activity identified in vitro is functional in vivo with a degree of specificity. This system should prove powerful for identifying other cellular factors that can inhibit ion channel activity.

Structural insight into tetrameric hTRPV1 from homology modeling, molecular docking, molecular dynamics simulation, virtual screening, and bioassay validations

The transient receptor potential vanilloid type 1 (TRPV1) is a heat-activated cation channel protein, which contributes to inflammation, acute and persistent pain. Antagonists of human TRPV1 (hTRPV1) represent a novel therapeutic approach for the treatment of pain. Developing various antagonists of hTRPV1, however, has been hindered by the unavailability of a 3D structure of hTRPV1. Recently, the 3D structures of rat TRPV1 (rTRPV1) in the presence and absence of ligand have been reported as determined by cryo-EM. rTRPV1 shares 85.7% sequence identity with hTRPV1. In the present work, we constructed and reported the 3D homology tetramer model of hTRPV1 based on the cryo-EM structures of rTRPV1. Molecular dynamics (MD) simulations, energy minimizations, and prescreen were applied to select and validate the best model of hTRPV1. The predicted binding pocket of hTRPV1 consists of two adjacent monomers subunits, which were congruent with the experimental rTRPV1 data and the cyro-EM structures of rTRPV1. The detailed interactions between hTRPV1 and its antagonists or agonists were characterized by molecular docking, which helped us to identify the important residues. Conformational changes of hTRPV1 upon antagonist/agonist binding were also explored by MD simulation. The different movements of compounds led to the different conformational changes of monomers in hTRPV1, indicating that TRPV1 works in a concerted way, resembling some other channel proteins such as aquaporins. We observed that the selective filter was open when hTRPV1 bound with an agonist during MD simulation. For the lower gate of hTRPV1, we observed large similarities between hTRPV1 bound with antagonist and with agonist. A five-point pharmacophore model based on several antagonists was established, and the structural model was used to screen in silico for new antagonists for hTRPV1. By using the 3D TRPV1 structural model above, the pilot in silico screening has begun to yield promising hits with activity as hTRPV1 antagonists, several of which showed substantial potency.

Activation of TRPV1 mediates thymic stromal lymphopoietin release via the Ca2+/NFAT pathway in airway epithelial cells

The airway epithelium is exposed to a range of irritants in the environment that are known to trigger inflammatory response such as asthma. Transient receptor potential vanilloid 1 (TRPV1) is a Ca(2+)-permeable cation channel critical for detecting noxious stimuli by sensory neurons. Recently increasing evidence suggests TRPV1 is also crucially involved in the pathophysiology of asthma on airway epithelium in human. Here we report that in airway epithelial cells TRPV1 activation potently induces allergic cytokine thymic stromal lymphopoietin (TSLP) release. TSLP induction by protease-activated receptor (PAR)-2 activation is also partially mediated by TRPV1 channels.

Activation of the TRPV1 cation channel contributes to stress-induced astrocyte migration

Astrocytes provide metabolic, structural, and synaptic support to neurons in normal physiology and also contribute widely to pathogenic processes in response to stress or injury. Reactive astrocytes can undergo cytoskeletal reorganization and increase migration through changes in intracellular Ca(2+) mediated by a variety of potential modulators. Here we tested whether migration of isolated retinal astrocytes following mechanical injury (scratch wound) involves the transient receptor potential vanilloid-1 channel (TRPV1), which contributes to Ca(2+)-mediated cytoskeletal rearrangement and migration in other systems. Application of the TRPV1-specific antagonists, capsazepine (CPZ) or 5'-iodoresiniferatoxin (IRTX), slowed migration by as much as 44%, depending on concentration. In contrast, treatment with the TRPV1-specific agonists, capsaicin (CAP) or resiniferatoxin (RTX) produced only a slight acceleration over a range of concentrations. Chelation of extracellular Ca(2+) with EGTA (1 mM) slowed astrocyte migration by 35%. Ratiometric imaging indicated that scratch wound induced a sharp 20% rise in astrocyte Ca(2+) that dissipated with distance from the wound. Treatment with IRTX both slowed and dramatically reduced the scratch-induced Ca(2+) increase. Both CPZ and IRTX influenced astrocyte cytoskeletal organization, especially near the wound edge. Taken together, our results indicate that astrocyte mobilization in response to mechanical stress involves influx of extracellular Ca(2+) and cytoskeletal changes in part mediated by TRPV1 activation.

Monoacylglycerols activate TRPV1--a link between phospholipase C and TRPV1

Phospholipase C-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate generates diacylglycerol, inositol 1,4,5-trisphosphate and protons, all of which can regulate TRPV1 activity via different mechanisms. Here we explored the possibility that the diacylglycerol metabolites 2-arachidonoylglycerol and 1-arachidonoylglycerol, and not metabolites of these monoacylglycerols, activate TRPV1 and contribute to this signaling cascade. 2-Arachidonoylglycerol and 1-arachidonoylglycerol activated native TRPV1 on vascular sensory nerve fibers and heterologously expressed TRPV1 in whole cells and inside-out membrane patches. The monoacylglycerol lipase inhibitors methylarachidonoyl-fluorophosphonate and JZL184 prevented the metabolism of deuterium-labeled 2-arachidonoylglycerol and deuterium-labeled 1-arachidonoylglycerol in arterial homogenates, and enhanced TRPV1-mediated vasodilator responses to both monoacylglycerols. In mesenteric arteries from TRPV1 knock-out mice, vasodilator responses to 2-arachidonoylglycerol were minor. Bradykinin and adenosine triphosphate, ligands of phospholipase C-coupled membrane receptors, increased the content of 2-arachidonoylglycerol in dorsal root ganglia. In HEK293 cells expressing the phospholipase C-coupled histamine H1 receptor, exposure to histamine stimulated the formation of 2-AG, and this effect was augmented in the presence of JZL184. These effects were prevented by the diacylglycerol lipase inhibitor tetrahydrolipstatin. Histamine induced large whole cell currents in HEK293 cells co-expressing TRPV1 and the histamine H1 receptor, and the TRPV1 antagonist capsazepine abolished these currents. JZL184 increased the histamine-induced currents and tetrahydrolipstatin prevented this effect. The calcineurin inhibitor ciclosporin and the endogenous "entourage" compound palmitoylethanolamide potentiated the vasodilator response to 2-arachidonoylglycerol, disclosing TRPV1 activation of this monoacylglycerol at nanomolar concentrations. Furthermore, intracerebroventricular injection of JZL184 produced TRPV1-dependent antinociception in the mouse formalin test. Our results show that intact 2-arachidonoylglycerol and 1-arachidonoylglycerol are endogenous TRPV1 activators, contributing to phospholipase C-dependent TRPV1 channel activation and TRPV1-mediated antinociceptive signaling in the brain.

Altered pharmacology of native rodent spinal cord TRPV1 after phosphorylation

BACKGROUND AND PURPOSE

Evidence suggests that phosphorylation of TRPV1 is an important component underlying its aberrant activation in pathological pain states. To date, the detailed pharmacology of diverse TRPV1 receptor agonists and antagonists has yet to be reported for native TRPV1 under phosphorylating conditions. Our goal was to optimize a relatively high-throughput methodology to allow pharmacological characterization of the native TRPV1 receptor using a spinal cord neuropeptide release assay under naive and phosphorylating states.

EXPERIMENTAL APPROACH

Herein, we describe characterization of rodent TRPV1 by measurement of CGRP release from acutely isolated lumbar (L1-L6) spinal cord using a 96-well technique that combines use of native, adult tissue with quantitation of CGRP release by ELISA.

KEY RESULTS

We have studied a diverse panel of TRPV1 agonists and antagonists under basal and phosphorylating conditions. We show that TRPV1-mediated CGRP release is evoked, in a temperature-dependent manner, by a PKC activator, phorbol 12,13-dibutyrate (PDBu); and that treatment with PDBu increases the potency and efficacy of known TRPV1 chemical agonists, in an agonist-specific manner. We also show that the pharmacological profile of diverse TRPV1 antagonists is dependent on whether the stimulus is PDBu or capsaicin. Of note, HPPB was identified as an antagonist of capsaicin-evoked, but a potentiator of PDBu-evoked, CGRP release.

CONCLUSIONS AND IMPLICATIONS

Our findings indicate that both TRPV1 agonist and antagonist profiles can be differentially altered by PKC activation. These findings may offer new insights for targeting TRPV1 in pain states.

Structure-activity relationships of vanilloid receptor agonists for arteriolar TRPV1

BACKGROUND AND PURPOSE

The transient receptor potential vanilloid 1 (TRPV1) plays a role in the activation of sensory neurons by various painful stimuli and is a therapeutic target. However, functional TRPV1 that affect microvascular diameter are also expressed in peripheral arteries and we attempted to characterize this receptor.

EXPERIMENTAL APPROACH

Sensory TRPV1 activation was measured in rats by use of an eye wiping assay. Arteriolar TRPV1-mediated smooth muscle specific responses (arteriolar diameter, changes in intracellular Ca(2+)) were determined in isolated, pressurized skeletal muscle arterioles obtained from the rat and wild-type or TRPV1(-/-) mice and in canine isolated smooth muscle cells. The vascular pharmacology of the TRPV1 agonists (potency, efficacy, kinetics of action and receptor desensitization) was determined in rat isolated skeletal muscle arteries.

KEY RESULTS

Capsaicin evoked a constrictor response in isolated arteries similar to that mediated by noradrenaline, this was absent in arteries from TRPV1 knockout mice and competitively inhibited by TRPV1 antagonist AMG9810. Capsaicin increased intracellular Ca(2+) in the arteriolar wall and in isolated smooth muscle cells. The TRPV1 agonists evoked similar vascular constrictions (MSK-195 and JYL-79) or were without effect (resiniferatoxin and JYL-273), although all increased the number of responses (sensory activation) in the eye wiping assay. Maximal doses of all agonists induced complete desensitization (tachyphylaxis) of arteriolar TRPV1 (with the exception of capsaicin). Responses to the partial agonist JYL-1511 suggested 10% TRPV1 activation is sufficient to evoke vascular tachyphylaxis without sensory activation.

CONCLUSIONS AND IMPLICATIONS

Arteriolar TRPV1 have different pharmacological properties from those located on sensory neurons in the rat.

TRPV1 activation is not an all-or-none event: TRPV1 partial agonism/antagonism and its regulatory modulation

TRPV1 has emerged as a promising therapeutic target for pain as well as a broad range of other conditions such as asthma or urge incontinence. The identification of resiniferatoxin as an ultrapotent ligand partially able to dissect the acute activation of TRPV1 from subsequent desensitization and the subsequent intense efforts in medicinal chemistry have revealed that TRPV1 affords a dramatic landscape of opportunities for pharmacological manipulation. While agonism and antagonism have represented the primary directions for drug development, the pharmacological complexity of TRPV1 affords additional opportunities. Partial agonism/partial antagonism, its modulation by signaling pathways, variable desensitization, and slow kinetics of action can all be exploited through drug design.

Regulation of the benzamil-insensitive salt taste receptor by intracellular Ca2+, protein kinase C, and calcineurin

The regulation of the benzamil (Bz)-insensitive salt taste receptor was investigated by intracellular Ca2+ ([Ca2+]i), protein kinase C (PKC), and the Ca2+-dependent serine-threonine phosphatase, calcineurin (PP2B), by monitoring chorda tympani taste nerve responses to 0.1 M NaCl solutions containing Bz (5x10(-6) M) and resiniferatoxin (RTX; 0-10x10(-6) M) in Sprague-Dawley rats and in wild-type (WT) and transient receptor potential vanilloid-1 knockout (TRPV1 KO) mice. In rats and WT mice, RTX increased the NaCl+Bz chorda tympani responses between 0.25x10(-6) and 1x10(-6) M and inhibited the responses above 1x10(-6) M. Decreasing taste receptor cell (TRC) [Ca2+]i with BAPTA loading, activation of PKC with 4alpha-phorbol-12,13-didecanoate (PMA), or inhibition of PP2B by cyclosporin A or FK-506, enhanced the magnitude of the Bz-insensitive NaCl chorda tympani responses in the presence of RTX and either minimized or completely eliminated the decrease in the chorda tympani response>1x10(-6) M RTX. In contrast, increasing TRC [Ca2+]i with ionomycin inhibited Bz-insensitive NaCl chorda tympani responses in the presence of RTX. No effect of the cited modulators was observed on the chorda tympani responses in WT mice and rats in the presence of TRPV1 blocker SB-366791 (1x10(-6) M) or in TRPV1 KO mice. 32P-labeling demonstrated direct phosphorylation of TRPV1 or TRPV1t in anterior lingual epithelium by PMA, cyclosporin A, or FK-506. PMA also enhanced the RTX-sensitive unilateral apical Na+ flux in polarized fungiform TRC in vitro. We conclude that TRPV1 or its variant TRPV1t is phosphorylated and dephosphorylated by PKC and PP2B, respectively, and either sensitizes or desensitizes the Bz-insensitive NaCl chorda tympani responses to RTX stimulation.

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.