Anandamide and arachidonic acid use epoxyeicosatrienoic acids to activate TRPV4 channels

Determinants of 4 alpha-phorbol sensitivity in transmembrane domains 3 and 4 of the cation channel TRPV4

TRPV4, a Ca(2+)-permeable member of the vanilloid subgroup of the transient receptor potential (TRP) channels, is activated by cell swelling and moderate heat (>27 degrees C) as well as by diverse chemical compounds including synthetic 4 alpha-phorbol esters, the plant extract bisandrographolide A, and endogenous epoxyeicosatrienoic acids (EETs; 5,6-EET and 8,9-EET). Previous work identified a tyrosine residue located in the first half of putative transmembrane segment 3 (TM3) as a crucial determinant for the activation of TRPV4 by its most specific agonist 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), suggesting that 4 alpha-PDD interacts with the channel through its transmembrane segments. To obtain insight in the 4 alpha-PDD-binding site and in the mechanism of ligand-dependent TRPV4 activation, we investigated the consequences of specific point mutations in TM4 on the sensitivity of the channel to different chemical and physical stimuli. Mutations of two hydrophobic residues in the central part of TM4 (Leu(584) and Trp(586)) caused a severe reduction of the sensitivity of the channel to 4 alpha-PDD, bisandrographolide A, and heat, whereas responses to cell swelling, arachidonic acid, and 5,6-EET remained unaffected. In contrast, mutations of two residues in the C-terminal part of TM4 (Tyr(591) and Arg(594)) affected channel activation of TRPV4 by all stimuli, suggesting an involvement in channel gating rather than in interaction with agonists. Based on a comparison of the responses of WT and mutant TRPV4 to 4 alpha-PDD and different 4 alpha-phorbol esters, we conclude that the length of the fatty acid moiety determines the ligand binding affinity and propose a model for the interaction between 4 alpha-phorbol esters and the TM3/4 region of TRPV4.

Upregulation of TRPC1 in the development of cardiac hypertrophy

The importance of Ca(2+) entry in the cardiac hypertrophic response is well documented, but the actual Ca(2+) entry channels remain unknown. Transient receptor potential (TRP) proteins are thought to form either homo- or heteromeric Ca(2+) entry channels that are involved in the proliferation and differentiation of various cells. The purpose of this study was to explore the potential involvement of TRP channels in the development of cardiac hypertrophy. The mRNA and protein expression of several TRP channel subunits were evaluated using hearts from abdominal aortic-banded (AAB) rats. Although TRPs C1, C3, C5, and C6 were constitutively expressed, only TRPC1 expression was significantly increased in the hearts of AAB rats compared to sham-operated rats. Using primary cultures of neonatal rat cardiomyocytes, we detected increases in the expression of TRPC1, brain natriuretic peptide (BNP), and atrial natriuretic factor (ANF), as well as increases in store-operated Ca(2+) entry (SOCE) and cell surface area, following endothelin-1 (ET-1) treatment. Silencing of the TRPC1 gene via small interfering RNA (siRNA) attenuated SOCE and prevented ET-1-, angiotensin-II (AT II)-, and phenylephrine (PE)-induced cardiac hypertrophy. In HEK 293T cells, overexpression of TRPC1 augmented SOCE, leading to an increase in nuclear factor of activated T cells (NFAT) promoter activity, while co-transfection with dominant-negative forms of TRPC1 suppressed it. In conclusion, TRPC1 functions in Ca(2+) influx, and its upregulation is involved in the development of cardiac hypertrophy; moreover, it plays an important role in the regulation of the signaling pathways that govern cardiac hypertrophy. These findings establish TRPC1 as a functionally important regulator of cardiac hypertrophy.

Spectrally and spatially resolved fluorescence lifetime imaging in living cells: TRPV4-microfilament interactions

Time- and space-correlated single photon counting method has been used to demonstrate the interactions of cation channel "transient receptor potential vanilloid 4" (TRPV4) and microfilaments. Living cells co-expressing TRPV4-CFP and actin-YFP, when excited for the donor molecules (CFP) exhibited an emission peak at 527 nm and decrease of the lifetime in the wavelength band 460-490 nm; corresponding to resonance energy transfer to YFP. CFP fluorescence decay was fitted best by a dual mode decay model. Considering the average lifetime of the donor, both in the presence and absence of acceptor yielded an apparent FRET efficiency of approximately 20%. This is rather high placing the minimum distance of chromophores in the two fluorescent proteins in the range of 4 nm. Thus, this study shows for the first time that TRPV4 and actin intimately associate within living cells. The significance of this finding for cell volume regulation is highlighted.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

Involvement of transient receptor potential canonical 1 (TRPC1) in angiotensin II-induced vascular smooth muscle cell hypertrophy

Angiotensin II (Ang II) induces vascular smooth muscle cell (VSMC) hypertrophy as one of the major events leading to atherosclerosis. Increased Ca(2+) entry is an important stimulus for VSMC hypertrophy, but the association with Ang II remains to be determined. Transient receptor potential canonical 1 (TRPC1) forms store-operated Ca(2+) (SOC) channels that are involved in Ca(2+) homeostasis. Our aim was to ascertain the potential involvement of TRPC1 in Ang II-induced VSMC hypertrophy. For this purpose, we used cultured human coronary artery smooth muscle cells (hCASMCs). Store-operated Ca(2+) entry (SOCE) increased in the Ang II-induced hypertrophied cells, and SOC channel blocker inhibited the Ang II-induced hypertrophic response. Although hCASMCs constitutively expressed TRPC1, C3, C4, C5, and C6, only TRPC1 increased in response to Ang II stimulation. TRPC1 siRNA decreased SOCE and prevented Ang II-induced hypertrophy. We found NF-kappaB binding sites in the 5'-regulatory region of the human TRPC1 gene. An electrophoretic mobility shift assay showed that Ang II increased the TRPC1 promoter's NF-kappaB binding activity. Co-treatment with NF-kappaB decoy oligonucleotides not only reduced TRPC1 expression, but also inhibited the hypertrophic responses. In conclusion, our data suggest that Ang II and subsequent NF-kappaB activation induces hCASMC hypertrophy through an enhancement of TRPC1 expression.

A shifted repertoire of endocannabinoid genes in the zebrafish (Danio rerio)

The zebrafish has served as a model organism for developmental biology. Sequencing its genome has expanded zebrafish research into physiology and drug-development testing. Several cannabinoid pharmaceuticals are in development, but expression of endocannabinoid receptors and enzymes remains unknown in this species. We conducted a bioinformatics analysis of the zebrafish genome using 17 human endocannabinoid genes as a reference set. Putative zebrafish orthologs were identified in filtered BLAST searches as reciprocal best hits. Orthology was confirmed by three in silico methods: phylogenetic testing, synteny analysis, and functional mapping. Zebrafish expressed orthologs of cannabinoid receptor 1, transient receptor potential channel vanilloid receptor 4, GPR55 receptor, fatty acid amide hydrolase 1, monoacylglycerol lipase, NAPE-selective phospholipase D, abhydrolase domain-containing protein 4, and diacylglycerol lipase alpha and beta; and paired paralogs of cannabinoid receptor 2, fatty acid amide hydrolase 2, peroxisome proliferator-activated receptor alpha, prostaglandin-endoperoxide synthase 2, and transient receptor potential cation channel subtype A1. Functional mapping suggested the orthologs of transient receptor potential vanilloid receptor 1 and peroxisome proliferator-activated receptor gamma lack specific amino acids critical for cannabinoid ligand binding. No orthologs of N-acylethanolamine acid amidase or protein tyrosine phosphatase, non-receptor type 22 were identified. In conclusion, the zebrafish genome expresses a shifted repertoire of endocannabinoid genes. In vitro analyses are warranted before using zebrafish for cannabinoid development testing.

TRPV4

TRPV4 is a non-selective cation channel subunit expressed in a wide variety of tissues. TRP channels are formed by a tetrameric complex of channel subunits. The available evidence suggests that TRPV4 cannot form heteromultimers with other TRPV isoforms, and that TRPV4-containing channels are homotetramers. These channels have a characteristic outwardly rectifying current-voltage relation, and are 5-10 times more permeable for Ca2+ than for Na+. TRPV4 can be activated by a wide range of stimuli including physical (cell swelling, heat, mechanical stimulation) and chemical stimuli (endocannabinoids, arachidonic acid, and, surprisingly, 4alpha-phorbol esters). Activation by swelling and endocannabinoids involves cytochrome P450 epoxygenase-dependent arachidonic acid metabolism to the epoxyeicosatrienoic acids (EETs). Heat and 4alpha-phorbol esters also seem to share a common mechanism of activation, but the endogenous messenger involved in the response to heat has not yet been identified. Ca2+ acting from the intracellular side can have both potentiating and inhibitory effects on channel activity and is involved in channel activation and inactivation. Given its wide expression and the variety of activatory stimuli, TRPV4 is likely to play a number of physiological roles. Studies with TRPV4(-/-) mice suggest a role for the channel in the regulation of body osmolarity, mechanosensation, temperature sensing, vascular regulation and, possibly, hearing.

TRP channels activated by extracellular hypo-osmoticity in epithelia

TRP (transient receptor potential) channels comprise a superfamily of non-selective cation channels with at least seven subfamilies. The variety of subfamilies corresponds to the differences in the activation mechanisms and functions. TRPM3 (TRP melastatin 3) and TRPV4 (TRP vanilloid 3) have been characterized as cation channels activated by extracellular hypo-osmoticity. In addition, TRPV4 is activated by metabolites of arachidonic acid as well as α-isomers of phorbol esters known to be ineffective in stimulating proteins of the protein kinase C family. TRPM3 is responsive to sphingosine derivatives. The detection of splice variants with probably different activation mechanisms supports the idea that TRPM3 may have diverse cellular functions depending on the expression of a particular variant. The expression of TRPV4 in many epithelial cell types raised the question of the role of TRPV4 in epithelial physiology. Single-cell experiments as well as approaches using epithelial layers show that multiple cellular responses are triggered by TRPV4 activation and subsequent elevation of intracellular calcium. The TRPV4-induced responses increasing transcellular ion flux as well as paracellular permeability may allow the cells to adjust to changes in extracellular osmolarity. In summary, TRPV4 plays a central role in epithelial homoeostasis by modulating epithelial barrier function.

Transient Receptor Potential Channels in Mechanosensing and Cell Volume Regulation

Transient receptor potential (TRP) channels are unique cellular sensors responding to a wide variety of extra- and intracellular signals, including mechanical and osmotic stress. In recent years, TRP channels from multiple subfamilies have been added to the list of mechano- and/or osmosensitive channels, and it is becoming increasingly apparent that Ca(2+) influx via TRP channels plays a crucial role in the response to mechanical and osmotic perturbations in a wide range of cell types. Although the events translating mechanical and osmotic stimuli into regulation of TRP channels are still incompletely understood, the specific mechanisms employed vary between different TRP isoforms, and probably include changes in the tension and/or curvature of the lipid bilayer, changes in the cortical cytoskeleton, and signaling events such as lipid metabolism and protein phosphorylation/dephosphorylation. This chapter describes candidate mechanosensitive channels from mammalian TRP subfamilies, discusses inherent and technical issues potentially confounding evaluation of mechano- and/or osmosensitivity, and presents methods relevant to the study of TRP channel regulation by mechanical and osmotic stimuli and involvement in cell volume regulation.

TRP channels and lipids: from Drosophila to mammalian physiology

The transient receptor potential (TRP) ion channel family was the last major ion channel family to be discovered. The prototypical member (dTRP) was identified by a forward genetic approach in Drosophila, where it represents the transduction channel in the photoreceptors, activated downstream of a Gq-coupled PLC. In the meantime 29 vertebrate TRP isoforms are recognized, distributed amongst seven subfamilies (TRPC, TRPV, TRPM, TRPML, TRPP, TRPA, TRPN). They subserve a wide range of functions throughout the body, most notably, though by no means exclusively, in sensory transduction and in vascular smooth muscle. However, their precise physiological roles and mechanism of activation and regulation are still only gradually being revealed. Most TRP channels are subject to multiple modes of regulation, but a common theme amongst the TRPC/V/M subfamilies is their regulation by lipid messengers. Genetic evidence supports an excitatory role of diacylglycerol (DAG) for the dTRP's, although curiously only DAG metabolites (PUFAs) have been found to activate the Drosophila channels. TRPC2,3,6 and 7 are widely accepted as DAG-activated channels, although TRPC3 can also be regulated via a store-operated mechanism. More recently PIP2 has been shown to be required for activity of TRPV5, TRPM4,5,7 and 8, whilst it may inhibit TRPV1 and the dTRPs. Although compelling evidence for a direct interaction of DAG with the TRPC channels is lacking, mutagenesis studies have identified putative PIP2-interacting domains in the C-termini of several TRPV and TRPM channels.

Ca2+ refilling of the endoplasmic reticulum is largely preserved albeit reduced Ca2+ entry in endothelial cells

In this study the relationship between the efficiency of endoplasmic reticulum (ER) Ca2+ refilling and the extent of Ca2+ entry was investigated in endothelial cells. ER and mitochondrial Ca2+ concentration were measured using genetically encoded Ca2+ sensors, while the amount of entering Ca2+ was controlled by varying either the extracellular Ca2+ or the electrical driving force for Ca2+ by changing the plasma membrane potential. In the absence of an agonist, ER Ca2+ replenishment was fully accomplished even if the Ca2+ concentration applied was reduced from 2 to 0.5mM. A similar strong efficiency of ER Ca2+ refilling was obtained under condition of plasma membrane depolarization. However, in the presence of histamine, ER Ca2+ refilling depended on mitochondrial Ca2+ transport and was more susceptible to membrane depolarization. Store-operated Ca2+ entry (SOCE), was strongly reduced under low Ca2+ and depolarizing conditions but increased if ER Ca2+ uptake was blocked or if ER Ca2+ was released continuously by IP(3). A correlation of the kinetics of ER Ca2+refilling with cytosolic Ca2+ signals revealed that termination of SOCE is a rapid event that is not delayed compared to ER refilling. Our data indicate that ER refilling occurs in priority to, and independently from the cytosolic Ca2+ elevation upon Ca2+ entry and that this important process is widely achieved even under conditions of diminished Ca2+entry.

TRP channels

The TRP (Transient Receptor Potential) superfamily of cation channels is remarkable in that it displays greater diversity in activation mechanisms and selectivities than any other group of ion channels. The domain organizations of some TRP proteins are also unusual, as they consist of linked channel and enzyme domains. A unifying theme in this group is that TRP proteins play critical roles in sensory physiology, which include contributions to vision, taste, olfaction, hearing, touch, and thermo- and osmosensation. In addition, TRP channels enable individual cells to sense changes in their local environment. Many TRP channels are activated by a variety of different stimuli and function as signal integrators. The TRP superfamily is divided into seven subfamilies: the five group 1 TRPs (TRPC, TRPV, TRPM, TRPN, and TRPA) and two group 2 subfamilies (TRPP and TRPML). TRP channels are important for human health as mutations in at least four TRP channels underlie disease.

Modulation of temperature-sensitive TRP channels

Animals sense temperature--either cold or hot--by the direct activation of temperature-sensitive members of the TRP family of ion channels, the thermo-TRPs. To date, six TRP channels--TRPV1-4, TRPM8 and TRPA1--have been reported to be directly activated by heat and to be involved in thermosensation. Temperature sensing can be modulated by phosphorylation of intracellular residues by protein kinases or by insertion of new channels into the cell membrane. In this review we provide a brief overview of the properties of thermo-TRPs, and we summarise signalling pathways involved in their regulation.

Protease-activated receptor 2 sensitizes the transient receptor potential vanilloid 4 ion channel to cause mechanical hyperalgesia in mice

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR2) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR2-mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). Immunoreactive TRPV4 was coexpressed by rat dorsal root ganglia (DRG) neurons with PAR2, substance P (SP) and calcitonin gene-related peptide (CGRP), mediators of pain transmission. In PAR2-expressing cell lines that either naturally expressed TRPV4 (bronchial epithelial cells) or that were transfected to express TRPV4 (HEK cells), pretreatment with a PAR2 agonist enhanced Ca2+ and current responses to the TRPV4 agonists phorbol ester 4alpha-phorbol 12,13-didecanoate (4alphaPDD) and hypotonic solutions. PAR2-agonist similarly sensitized TRPV4 Ca2+ signals and currents in DRG neurons. Antagonists of phospholipase Cbeta and protein kinases A, C and D inhibited PAR2-induced sensitization of TRPV4 Ca2+ signals and currents. 4alphaPDD and hypotonic solutions stimulated SP and CGRP release from dorsal horn of rat spinal cord, and pretreatment with PAR2 agonist sensitized TRPV4-dependent peptide release. Intraplantar injection of PAR2 agonist caused mechanical hyperalgesia in mice and sensitized pain responses to the TRPV4 agonists 4alphaPDD and hypotonic solutions. Deletion of TRPV4 prevented PAR2 agonist-induced mechanical hyperalgesia and sensitization. This novel mechanism, by which PAR2 activates a second messenger to sensitize TRPV4-dependent release of nociceptive peptides and induce mechanical hyperalgesia, may underlie inflammatory hyperalgesia in diseases where proteases are activated and released.

Different expression patterns of TRP genes in murine B and T lymphocytes

A prolonged increase in the intracellular calcium concentration ([Ca2+]i) is essential for lymphocyte activation that includes cell proliferation and differentiation. This increase in [Ca2+]i results from Ca2+ release from the intracellular store and the subsequent Ca2+ influx from the extracellular environment via calcium channels located on the plasma membrane. Although transient receptor potential (TRP) channels have been reported to play important roles in the [Ca2+]i increase in lymphocytes, the function of these channels in lymphocyte activation remains unknown. Here, we report the comprehensive expression profile of TRP channel gene families including TRPC, TRPV, and TRPM in the murine immune system. RT-PCR analysis revealed different expression patterns of the TRP channel genes in B and T lymphocytes isolated from the spleen. Therefore, our results provide an appropriate reference of TRP gene expression in murine lymphocytes.

PLA2 and TRPV4 channels regulate endothelial calcium in cerebral arteries

We previously demonstrated that endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations in cerebral arteries are significantly reduced by inhibitors of PLA(2). In this study we examined possible mechanisms by which PLA(2) regulates endothelium-dependent dilation, specifically whether PLA(2) is involved in endothelial Ca(2+) regulation through stimulation of TRPV4 channels. Studies were carried out with middle cerebral arteries (MCA) or freshly isolated MCA endothelial cells (EC) of male Long-Evans rats. Nitro-l-arginine methyl ester (l-NAME) and indomethacin were present throughout. In pressurized MCA, luminally delivered UTP produced increased EC intracellular Ca(2+) concentration ([Ca(2+)](i)) and MCA dilation. Incubation with PACOCF(3), a PLA(2) inhibitor, significantly reduced both EC [Ca(2+)](i) and dilation responses to UTP. EC [Ca(2+)](i) was also partially reduced by a transient receptor potential vanilloid (TRPV) channel blocker, ruthenium red. Manganese quenching experiments demonstrated Ca(2+) influx across the luminal and abluminal face of the endothelium in response to UTP. Interestingly, PLA(2)-sensitive Ca(2+) influx occurred primarily across the abluminal face. Luminal application of arachidonic acid, the primary product of PLA(2) and a demonstrated activator of certain TRPV channels, increased both EC [Ca(2+)](i) and MCA diameter. TRPV4 mRNA and protein was demonstrated in the endothelium by RT-PCR and immunofluorescence, respectively. Finally, application of 4alpha-phorbol 12,13-didecanoate (4alphaPDD), a TRPV4 channel activator, produced an increase in EC [Ca(2+)](i) that was significantly reduced in the presence of ruthenium red. We conclude that PLA(2) is involved in EC Ca(2+) regulation through its regulation of TRPV4 channels. Furthermore, the PLA(2)-sensitive component of Ca(2+) influx may be polarized to the abluminal face of the endothelium.

Anandamide mediates hyperdynamic circulation in cirrhotic rats via CB(1) and VR(1) receptors

BACKGROUND AND PURPOSE

Hyperdynamic circulation and mesenteric hyperaemia are found in cirrhosis. To delineate the role of endocannabinoids in these changes, we examined the cardiovascular effects of anandamide, AM251 (CB(1) antagonist), AM630 (CB(2) antagonist) and capsazepine (VR1 antagonist), in a rat model of cirrhosis.

EXPERIMENTAL APPROACH

Cirrhosis was induced by bile duct ligation. Controls underwent sham operation. Four weeks later, diameters of mesenteric arteriole and venule (intravital microscopy), arterial pressure, cardiac output, systemic vascular resistance and superior mesenteric artery (SMA) flow were measured after anandamide, AM251 (with or without anandamide), AM630 and capsazepine administration. CB(1), CB(2) and VR1 receptor expression in SMA was assessed by western blot and RT-PCR.

KEY RESULTS

Anandamide increased mesenteric vessel diameter and flow, and cardiac output in cirrhotic rats, but did not affect controls. Anandamide induced a triphasic arterial pressure response in controls, but this pattern differed markedly in cirrhotic rats. Pre-administration of AM251 blocked the effects of anandamide. AM251 (without anandamide) increased arterial pressure and systemic vascular resistance, constricted mesenteric arterioles, decreased SMA flow and changed cardiac output in a time-dependent fashion in cirrhotic rats. Capsazepine decreased cardiac output and mesenteric arteriolar diameter and flow, and increased systemic vascular resistance in cirrhotic rats, but lacked effect in controls. Expression of CB(1) and VR1 receptor proteins were increased in cirrhotic rats. AM630 did not affect any cardiovascular parameter in either group.

CONCLUSIONS AND IMPLICATIONS

These data suggest that endocannabinoids contribute to hyperdynamic circulation and mesenteric hyperaemia in cirrhosis, via CB(1)- and VR1-mediated mechanisms.

Transient receptor potential vanilloid 4-mediated disruption of the alveolar septal barrier: a novel mechanism of acute lung injury

Disruption of the alveolar septal barrier leads to acute lung injury, patchy alveolar flooding, and hypoxemia. Although calcium entry into endothelial cells is critical for loss of barrier integrity, the cation channels involved in this process have not been identified. We hypothesized that activation of the vanilloid transient receptor potential channel TRPV4 disrupts the alveolar septal barrier. Expression of TRPV4 was confirmed via immunohistochemistry in the alveolar septal wall in human, rat, and mouse lung. In isolated rat lung, the TRPV4 activators 4alpha-phorbol-12,13-didecanoate and 5,6- or 14,15-epoxyeicosatrienoic acid, as well as thapsigargin, a known activator of calcium entry via store-operated channels, all increased lung endothelial permeability as assessed by measurement of the filtration coefficient, in a dose- and calcium-entry dependent manner. The TRPV antagonist ruthenium red blocked the permeability response to the TRPV4 agonists, but not to thapsigargin. Light and electron microscopy of rat and mouse lung revealed that TRPV4 agonists preferentially produced blebs or breaks in the endothelial and epithelial layers of the alveolar septal wall, whereas thapsigargin disrupted interendothelial junctions in extraalveolar vessels. The permeability response to 4alpha-phorbol-12,13-didecanoate was absent in TRPV4(-/-) mice, whereas the response to thapsigargin remained unchanged. Collectively, these findings implicate TRPV4 in disruption of the alveolar septal barrier and suggest its participation in the pathogenesis of acute lung injury.

Transient receptor potential vanilloid 1 is required for intrinsic osmoreception in organum vasculosum lamina terminalis neurons and for normal thirst responses to systemic hyperosmolality

Recent studies have indicated that members of the transient receptor potential vanilloid (TRPV) family of cation channels are required for the generation of normal osmoregulatory responses, yet the mechanism of osmosensory transduction in primary osmoreceptor neurons of the CNS remains to be defined. Indeed, despite ample evidence suggesting that the organum vasculosum lamina terminalis (OVLT) serves as the primary locus of the brain for the detection of osmotic stimuli, evidence that neurons in the OVLT are intrinsically osmosensitive has remained elusive. Here we show that murine OVLT neurons are intrinsically sensitive to increases in the osmolality of the extracellular fluid. Hypertonic conditions provoked increases in membrane cation conductance that resulted in the generation of an inward current, depolarizing osmoreceptor potentials, and enhanced action potential discharge. Moreover, we found that this osmosensory signal transduction cascade was absent in OVLT neurons from TRPV1 knock-out (TRPV1-/-) mice and that responses of wild type (WT) OVLT neurons could be blocked by ruthenium red, an inhibitor of TRPV channels. Finally, TRPV1-/- mice showed significantly attenuated water intake in response to systemic hypertonicity compared with WT controls. These findings indicate that OVLT neurons act as primary osmoreceptors and that a product of the trpv1 gene is required for osmosensory transduction.

Ca2+-independent phospholipase A2 enhances store-operated Ca2+ entry in dystrophic skeletal muscle fibers

Duchenne muscular dystrophy is caused by deficiency of dystrophin and leads to progressive weakness. It has been proposed that the muscle degeneration occurring in this disease is caused by increased Ca2+ influx due to enhanced activity of cationic channels that are activated either by stretch of the plasma membrane (stretch-activated channels) or by Ca2+-store depletion (store-operated channels). Using both cytosolic Ca2+ measurements with Fura-2 and the manganese quench method, we show here that store-operated Ca2+ entry is greatly enhanced in dystrophic skeletal flexor digitorum brevis fibers isolated from mdx5cv mice, a mouse model of Duchenne muscular dystrophy. Moreover, we show for the first time that store-operated Ca2+ entry in these fibers is under the control of the Ca2+-independent phospholipase A2 and that the exaggerated Ca2+ influx can be completely attenuated by inhibitors of this enzyme. Enhanced store-operated Ca2+ entry in dystrophic fibers is likely to be due to a near twofold overexpression of Ca2+-independent phospholipase A2. The Ca2+-independent phospholipase A2 pathway therefore appears as an attractive target to reduce excessive Ca2+ influx and subsequent degeneration occurring in dystrophic fibers.

Transient receptor potential ion channels as participants in thermosensation and thermoregulation

Living organisms must evaluate changes in environmental and internal temperatures to mount appropriate physiological and behavioral responses conducive to survival. Classical physiology has provided a wealth of information regarding the specialization of thermosensory functions among subclasses of peripheral sensory neurons and intrinsically thermosensitive neurons within the hypothalamus. However, until recently, the molecular mechanisms by which these cells carry out thermometry have remained poorly understood. The demonstration that certain ion channels of the transient receptor potential (TRP) family can be activated by increases or decreases in ambient temperature, along with the recognition of their heterogeneous expression patterns and heterogeneous temperature sensitivities, has led investigators to evaluate these proteins as candidate endogenous thermosensors. Much of this work has involved one specific channel, TRP vanilloid 1 (TRPV1), which is both a receptor for capsaicin and related pungent vanilloid compounds and a "heat receptor," capable of directly depolarizing neurons in response to temperatures >42 degrees C. Evidence for a contribution of TRPV1 to peripheral thermosensation has come from pharmacological, physiological, and genetic approaches. In contrast, although capsaicin-sensitive mechanisms clearly influence core body temperature regulation, the specific contribution of TRPV1 to this process remains a matter of debate. Besides TRPV1, at least six additional thermally sensitive TRP channels have been identified in mammals, and many of these also appear to participate in thermosensation. Moreover, the identification of invertebrate TRP channels, whose genetic ablation alters thermally driven behaviors, makes it clear that thermosensation represents an evolutionarily conserved role of this ion channel family.

Trp ion channels and temperature sensation

The abilities to sense environmental and internal temperatures are required for survival, both for maintenance of homeostasis and for avoidance of tissue-damaging noxious temperatures. Vertebrates can sense external physical stimuli via specialized classes of neurons in the peripheral nervous system that project to the skin. Temperature-sensitive neurons can be divided into two classes: innocuous thermosensors (warm or cool) and noxious thermonociceptors (hot or cold). ThermoTRPs, a subset of the transient receptor potential family of ion channels, which are expressed in sensory nerve endings and in skin, respond to distinct thermal thresholds. In this review, we examine the extent to which thermoTRPs are responsible for providing a molecular basis for thermal sensation.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Neurophysiological, neuroimmunological, and neuroendocrine basis of pruritus

Pruritus (itch) can be defined as an unpleasant cutaneous sensation associated with the immediate desire to scratch. Recent findings have identified potential classes of endogenous "itch mediators" and establish a modern concept for the pathophysiology of pruritus. First, there in no universal peripheral itch mediator, but disease-specific sets of involved mediators. Second, numerous mediators of skin cells can activate and sensitize pruritic nerve endings, and even modulate their growth. Our knowledge of itch processing in the spinal cord and the involved centers in the central nervous system is rapidly growing. This review summarizes the current information about the significance of neuron-skin interactions, ion channels, neuropeptides, proteases, cannabinoids, opioids, kinins, cytokines, biogenic amines, neurotransmitters, and their receptors in the pathobiology of pruritus. A deeper understanding of these circuits is required for the development of novel antipruritic strategies.

Thermosensitive TRP ion channels mediate cytosolic calcium response in human synoviocytes

The transient receptor potential (TRP) channels are important membrane sensors, responding to thermal, chemical, osmotic, or mechanical stimuli by activation of calcium and sodium fluxes. In this study, three distinct TRP channels were detected and their role established in mediating cytosolic free calcium concentration ([Ca2+]cyt) response in tumor-derived SW982 synoviocytes and primary cultures of human synovial cells from patients with inflammatory arthropathies. As shown by fura-2 ratio measurements while cells were incubated in a temperature-regulated chamber, significant [Ca2+]cytelevation was elicited by rapid changes in bath temperature, application of TRPV1 receptor agonists capsaicin and resiniferatoxin, or a cold receptor stimulator, icilin. Temperature thresholds for calcium response were determined to be 12 ± 1°C for cold and 28 ± 2°C for heat activation. Temperature increases or decreases beyond these thresholds resulted in a significant rise in the magnitude of [Ca2+]cytspikes. Observed changes in [Ca2+]cytwere completely abolished in calcium-free medium and thus resulted from direct calcium entry through TRP channels rather then by activation of voltage-dependent calcium channels. Two heat sensitive channels, TRPV1 and TRPV4, and a cold-sensitive channel, TRPA1, were detected by RT-PCR. Minimal mRNA for TRPV3 or TRPM8 was amplified. The RT-PCR results support the data obtained with the [Ca2+]cytmeasurements. We propose that the TRP channels are functionally expressed in human synoviocytes and may play a critical role in adaptive or pathological changes in articular surfaces during arthritic inflammation.

TRPV4‐mediated regulation of epithelial permeability

TRPV4 is a calcium-permeable channel activated by extracellular hypotonicity, polyunsaturated fatty acids, phorbol esters, and heat. We show that TRPV4 is localized in the basolateral membrane of the mouse mammary cell line HC11. Activation of TRPV4 caused an increase in the intracellular Ca(2+) concentration through influx of extracellular Ca(2+), triggering two independent chains of events: 1) a rapid increase in transcellular conductance through the activation of apical large conductance calcium-activated (BK) potassium channels that were blockable by paxilline; 2) a slow increase in paracellular permeability for small solutes. The latter effect was accompanied by a down-regulation of the tight junctional proteins claudin -1, -3, -4, -5, -7, and -8 and by dramatic changes in tight junction morphology, including frequent large breaks in the tight junction strands. This dual modulation of epithelial permeability after TRPV4 activation may be involved in regulating the tonicity across mammary gland epithelia. TRPV4 activation may also be responsible for exudation and edema formation during inflammation processes.-Reiter, B., Kraft, R., Günzel, D., Zeissig, S., Schulzke, J-D., Fromm, M., Harteneck, C. TRPV4-mediated regulation of epithelial permeability.

Solid-phase combinatorial approach for the optimization of soluble epoxide hydrolase inhibitors

A 192-member library of N,N'-disubstituted urea inhibitors was synthesized by a solid-phase method. The ureas were tested for their inhibitory activities against recombinant human soluble epoxide hydrolase. Simple carbocyclic or para/meta-substituted phenyl groups showed inhibition potencies that were equal to or greater than adamantane-based sEH inhibitors, while the presence of bulky or ionizable groups close to the urea group dramatically decreased their activities.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.

Receptor-induced activation of Drosophila TRP gamma by polyunsaturated fatty acids

Cellular calcium homeostasis is regulated by hormones and neurotransmitters, resulting in the activation of a variety of proteins, in particular, channel proteins of the plasma membrane and of intracellular compartments. Such channels are, for example, TRP channels of the TRPC protein family that are activated by various mediators from receptor-stimulated signaling cascades. In Drosophila, two TRPC channels, TRP and TRPL, are involved in phototransduction. In addition, a third Drosophila TRPC channel, TRPgamma, has been identified and described as an auxiliary subunit of TRPL. Beyond it, our data show that heterologously expressed TRPgamma formed a receptor-activated, outwardly rectifying cation channel independent from TRPL co-expression. Analysis of the activation mechanism revealed that TRPgamma is activated by various polyunsaturated fatty acids generated in a phospholipase C- and phospholipase A(2)-dependent manner. The most potent activator of TRPgamma, the stable analogue of arachidonic acid, 5,8,11,14-eicosatetraynoic acid, induced currents in single channel recordings. Here we show that upon heterologous expression TRPgamma forms a homomeric channel complex that is activated by polyunsaturated fatty acids as mediators of receptor-dependent signaling pathways. Reverse transcription PCR analysis showed that TRPgamma is expressed in Drosophila heads and bodies. Its body-wide expression pattern and its activation mechanism suggest that TRPgamma forms a fly cation channel responsible for the regulation of intracellular calcium in a variety of hormonal signaling cascades.

Bisandrographolide from Andrographis paniculata activates TRPV4 channels

Many transient receptor potential (TRP) channels are activated or blocked by various compounds found in plants; two prominent examples include the activation of TRPV1 channels by capsaicin and the activation of TRPM8 channels by menthol. We sought to identify additional plant compounds that are active on other types of TRP channels. We screened a library of extracts from 50 Chinese herbal plants using a calcium-imaging assay to find compounds active on TRPV3 and TRPV4 channels. An extract from the plant Andrographis paniculata potently activated TRPV4 channels. The extract was fractionated further, and the active compound was identified as bisandrographolide A (BAA). We used purified compound to characterize the activity of BAA on certain TRPV channel subtypes. Although BAA activated TRPV4 channels with an EC(50) of 790-950 nm, it did not activate or block activation of TRPV1, TRPV2, or TRPV3 channels. BAA activated a large TRPV4-like current in immortalized mouse keratinocytes (308 cells) that have been shown to express TRPV4 protein endogenously. This compound also activated TRPV4 currents in cell-free outside-out patches from HEK293T cells overexpressing TRPV4 cDNA, suggesting that BAA can activate the channel in a membrane-delimited manner. Another related compound, andrographolide, found in abundance in the plant Andrographis was unable to activate or block activation of TRPV4 channels. These experiments show that BAA activates TRPV4 channels, and we discuss the possibility that activation of TRPV4 by BAA could play a role in some of the effects of Andrographis extract described in traditional medicine.

Inhibition of soluble epoxide hydrolase reduces LPS-induced thermal hyperalgesia and mechanical allodynia in a rat model of inflammatory pain

Soluble epoxide hydrolases catalyze the hydrolysis of epoxides in acyclic systems. In man this enzyme is the product of a single copy gene (EPXH-2) present on chromosome 8. The human sEH is of interest due to emerging roles of its endogenous substrates, epoxygenated fatty acids, in inflammation and hypertension. One of the consequences of inhibiting sEH in rodent inflammation models is a profound decrease in the production of pro-inflammatory and proalgesic lipid metabolites including prostaglandins. This prompted us to hypothesize that sEH inhibitors may have antinociceptive properties. Here we tested if sEH inhibitors can reduce inflammatory pain. Hyperalgesia was induced by intraplantar LPS injection and sEH inhibitors were delivered topically. We found that two structurally dissimilar but equally potent sEH inhibitors can be delivered through the transdermal route and that sEH inhibitors effectively attenuate thermal hyperalgesia and mechanical allodynia in rats treated with LPS. In addition we show that epoxydized arachidonic acid metabolites, EETs, are also effective in attenuating thermal hyperalgesia in this model. In parallel with the observed biological activity metabolic analysis of oxylipids showed that inhibition of sEH resulted with a decrease in PGD2 levels and sEH generated degradation products of linoleic and arachidonic acid metabolites with a concomitant increase in epoxides of linoleic acid. These data show that inhibition of sEH may become a viable therapeutic strategy to attain analgesia.

Potentiation of TRPV3 channel function by unsaturated fatty acids

Transient receptor potential vanilloid (TRPV) channels are polymodal detectors of multiple environmental factors, including temperature, pH, and pressure. Inflammatory mediators enhance TRPV function through multiple signaling pathways. The lipoxygenase and epoxygenase products of arachidonic acid (AA) metabolism have been shown to directly activate TRPV1 and TRPV4, respectively. TRPV3 is a thermosensitive channel with an intermediate temperature threshold of 31-39 degrees C. We have previously shown that TRPV3 is activated by 2-aminoethoxydiphenyl borate (2APB). Here we show that AA and other unsaturated fatty acids directly potentiate 2APB-induced responses of TRPV3 expressed in HEK293 cells, Xenopus oocytes, and mouse keratinocytes. The AA-induced potentiation is observed in intracellular Ca2+ measurement, whole-cell and two-electrode voltage clamp studies, as well as single channel recordings of excised inside-out and outside-out patches. The fatty acid-induced potentiation is not blocked by inhibitors of protein kinase C and thus differs from that induced by the kinase. The potentiation does not require AA metabolism but is rather mimicked by non-metabolizable analogs of AA. These results suggest a novel mechanism regulating the TRPV3 response to inflammation, which differs from TRPV1 and TRPV4, and involves a direct action of free fatty acids on the channel.

The cannabinoid WIN 55,212-2 inhibits transient receptor potential vanilloid 1 (TRPV1) and evokes peripheral antihyperalgesia via calcineurin

Cannabinoids can evoke antihyperalgesia and antinociception at a peripheral site of action. However, the signaling pathways mediating these effects are not clearly understood. We tested the hypothesis that certain cannabinoids directly inhibit peripheral capsaicin-sensitive nociceptive neurons by dephosphorylating and desensitizing transient receptor potential vanilloid 1 (TRPV1) via a calcium calcineurin-dependent mechanism. Application of the cannabinoid WIN 55,212-2 (WIN) to cultured trigeminal (TG) neurons or isolated skin biopsies rapidly and significantly inhibited capsaicin-activated inward currents and neuropeptide exocytosis by a mechanism requiring the presence of extracellular calcium. The inhibitory effect did not involve activation of G protein-coupled cannabinoid receptors, because neither pertussis toxin nor GDPbetaS treatments altered the WIN effect. However, application of WIN-activated calcineurin, as measured by nuclear translocation of the nuclear factor of activated T cells (NFAT)c4 transcription factor, dephosphorylated TRPV1. The WIN-induced desensitization of TRPV1 was mediated by calcineurin, because the application of structurally distinct calcineurin antagonists (calcineurin autoinhibitory peptide and cyclosporine/cyclophilin complex) abolished WIN-induced inhibition of capsaicin-evoked inward currents and neuropeptide exocytosis. This mechanism also contributed to peripheral antinociceptive/antihyperalgesic effects of WIN because pretreatment with the calcineurin antagonist calcineurin autoinhibitory peptide (CAIP) significantly reduced peripherally mediated WIN effects in two behavioral models. Collectively, these data demonstrate that cannabinoids such as WIN directly inhibit TRPV1 functional activities via a calcineurin pathway that represents a mechanism of cannabinoid actions at peripheral sites.

Evidence for a Functional Role of Endothelial Transient Receptor Potential V4 in Shear Stress–Induced Vasodilatation

OBJECTIVE

Ca2+-influx through transient receptor potential (TRP) channels was proposed to be important in endothelial function, although the precise role of specific TRP channels is unknown. Here, we investigated the role of the putatively mechanosensitive TRPV4 channel in the mechanisms of endothelium-dependent vasodilatation.

METHODS AND RESULTS

Expression and function of TRPV4 was investigated in rat carotid artery endothelial cells (RCAECs) by using in situ patch-clamp techniques, single-cell RT-PCR, Ca2+ measurements, and pressure myography in carotid artery (CA) and Arteria gracilis. In RCAECs in situ, TRPV4 currents were activated by the selective TRPV4 opener 4alpha-phorbol-12,13-didecanoate (4alphaPDD), arachidonic acid, moderate warmth, and mechanically by hypotonic cell swelling. Single-cell RT-PCR in endothelial cells demonstrated mRNA expression of TRPV4. In FURA-2 Ca2+ measurements, 4alphaPDD increased [Ca2+]i by &140 nmol/L above basal levels. In pressure myograph experiments in CAs and A gracilis, 4alphaPDD caused robust endothelium-dependent and strictly endothelium-dependent vasodilatations by &80% (K(D) 0.3 microL), which were suppressed by the TRPV4 blocker ruthenium red (RuR). Shear stress-induced vasodilatation was similarly blocked by RuR and also by the phospholipase A2 inhibitor arachidonyl trifluoromethyl ketone (AACOCF3). 4alphaPDD produced endothelium-derived hyperpolarizing factor (EDHF)-type responses in A gracilis but not in rat carotid artery. Shear stress did not produce EDHF-type vasodilatation in either vessel type.

CONCLUSIONS

Ca2+ entry through endothelial TRPV4 channels triggers NO- and EDHF-dependent vasodilatation. Moreover, TRPV4 appears to be mechanistically important in endothelial mechanosensing of shear stress.

Receptor-independent actions of cannabinoids on cell membranes: Focus on endocannabinoids

Cannabinoids are a structurally diverse group of mostly lipophilic molecules that bind to cannabinoid receptors. In fact, endogenous cannabinoids (endocannabinoids) are a class of signaling lipids consisting of amides and esters of long-chain polyunsaturated fatty acids. They are synthesized from lipid precursors in plasma membranes via Ca(2+) or G-protein-dependent processes and exhibit cannabinoid-like actions by binding to cannabinoid receptors. However, endocannabinoids can produce effects that are not mediated by these receptors. In pharmacologically relevant concentrations, endocannabinoids modulate the functional properties of voltage-gated ion channels including Ca(2+) channels, Na(+) channels, various types of K(+) channels, and ligand-gated ion channels such as serotonin type 3, nicotinic acetylcholine, and glycine receptors. In addition, modulatory effects of endocannabinoids on other ion-transporting membrane proteins such as transient potential receptor-class channels, gap junctions and transporters for neurotransmitters have also been demonstrated. Furthermore, functional properties of G-protein-coupled receptors for different types of neurotransmitters and neuropeptides are altered by direct actions of endocannabinoids. Although the mechanisms of these effects are currently not clear, it is likely that these direct actions of endocannabinoids are due to their lipophilic structures. These findings indicate that additional molecular targets for endocannabinoids exist and that these targets may represent novel sites for cannabinoids to alter either the excitability of the neurons or the response of the neuronal systems. This review focuses on the results of recent studies indicating that beyond their receptor-mediated effects, endocannabinoids alter the functions of ion channels and other integral membrane proteins directly.

Cardiac and vascular KATP channels in rats are activated by endogenous epoxyeicosatrienoic acids through different mechanisms

We have reported that epoxyeicosatrienoic acids (EETs), the cytochrome P450 (CYP) epoxygenase metabolites of arachidonic acid (AA), are potent sarcolemmal ATP-sensitive K+ (KATP) channel activators. However, activation of cardiac and vascular KATP channels by endogenously produced EETs under physiological intracellular conditions has not been demonstrated and direct comparison of the mechanisms whereby EETs activate the KATP channels in cardiac myocytes versus vascular smooth muscle cells has not been made. In this study, we examined the effects of AA on KATP channels in freshly isolated cardiac myocytes from rats, wild-type (WT) and transgenic mice overexpressing CYP2J2 cDNA, and mesenteric arterial smooth muscle cells from rats. We also compared the activation of cardiac and vascular KATP channels by extracellularly and intracellularly applied 11,12-EET. We found that 1 microm AA enhanced KATP channel activities in both cardiac and vascular smooth muscle cells, and the AA effects were inhibited by preincubation with CYP epoxygenase inhibitors. Baseline cardiac KATP current densities in CYP2J2 transgenic mice were 190% higher than those of WT mice, and both were reduced to similar levels by CYP epoxygenase inhibition. Western blot analysis showed that expression of Kir6.2 and SUR2A was similar between WT and CYP2J2 transgenic hearts. 11,12-EET (5 microm) applied intracellularly enhanced the KATP currents by 850% in cardiac myocytes, but had no effect in vascular smooth muscle cells. In contrast, 11,12-EET (5 microm) applied extracellularly increased KATP currents by 520% in mesenteric arterial smooth muscle cells, but by only 209% in cardiac myocytes. Preincubation with 100 microm m-iodobenzylguanidine or 5 microm myristoylated PKI amide did not alter the activation of cardiac KATP channels by 5 microm 11,12-EET, but significantly inhibited activation of vascular KATP channels. Moreover, EET only enhanced the inward component of cardiac KATP currents, but activated both the inward and outward components of vascular KATP currents. Our results indicate that endogenously derived CYP metabolites of AA potently activate cardiac and vascular KATP channels. EETs regulate cardiac electrophysiology and vascular tone by KATP channel activation, albeit through different mechanisms: the cardiac KATP channels are directly activated by EETs, whereas activation of the vascular KATP channels by EETs is protein kinase A dependent.

Small molecule vanilloid TRPV1 receptor antagonists approaching drug status: can they live up to the expectations?

The cloning of the transient receptor potential vanilloid type-1 (TRPV1) receptor initiated the discovery of potent small molecule antagonists, many of which are in preclinical phase or already undergoing clinical trials. While animal experiments imply a therapeutic value for these compounds as novel analgesic-antiphlogistic drugs, new findings with TRPV1 deficient (trpv1 -/-) mice signal troubles for TRPV1 antagonists as clinical research gains impetus. An emerging concept with important implications for drug development is that TRPV1 may be differentially regulated under physiological and pathological conditions. If so, it is conceivable that such TRPV1 ligands can be synthesized that specifically target TRPV1 in diseased (e.g. inflamed or neoplastic) tissues but spare TRPV1 that subserves its physiological functions in healthy organs. This review explores the current status of this field and seeks an answer to the question how these new discoveries could be factored into TRPV1 drug discovery and development.

Frontiers in pruritus research: scratching the brain for more effective itch therapy

This Review highlights selected frontiers in pruritus research and focuses on recently attained insights into the neurophysiological, neuroimmunological, and neuroendocrine mechanisms underlying skin-derived itch (pruritogenic pruritus), which may affect future antipruritic strategies. Special attention is paid to newly identified itch-specific neuronal pathways in the spinothalamic tract that are distinct from pain pathways and to CNS regions that process peripheral pruritogenic stimuli. In addition, the relation between itch and pain is discussed, with emphasis on how the intimate contacts between these closely related yet distinct sensory phenomena may be exploited therapeutically. Furthermore, newly identified or unduly neglected intracutaneous itch mediators (e.g., endovanilloids, proteases, cannabinoids, opioids, neurotrophins, and cytokines) and relevant receptors (e.g., vanilloid receptor channels and proteinase-activated, cannabinoid, opioid, cytokine, and new histamine receptors) are discussed. In summarizing promising new avenues for managing itch more effectively, we advocate therapeutic approaches that strive for the combination of peripherally active antiinflammatory agents with drugs that counteract chronic central itch sensitization.

An introduction to TRP channels

The aim of this review is to provide a basic framework for understanding the function of mammalian transient receptor potential (TRP) channels, particularly as they have been elucidated in heterologous expression systems. Mammalian TRP channel proteins form six-transmembrane (6-TM) cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Selected functional properties of TRP channels from each subfamily are summarized in this review. Although a single defining characteristic of TRP channel function has not yet emerged, TRP channels may be generally described as calcium-permeable cation channels with polymodal activation properties. By integrating multiple concomitant stimuli and coupling their activity to downstream cellular signal amplification via calcium permeation and membrane depolarization, TRP channels appear well adapted to function in cellular sensation. Our review of recent literature implicating TRP channels in neuronal growth cone steering suggests that TRPs may function more widely in cellular guidance and chemotaxis. The TRP channel gene family and its nomenclature, the encoded proteins and alternatively spliced variants, and the rapidly expanding pharmacology of TRP channels are summarized in online supplemental material.

Swelling-activated ion channels: functional regulation in cell-swelling, proliferation and apoptosis

Cell volume regulation is one of the most fundamental homeostatic mechanisms and essential for normal cellular function. At the same time, however, many physiological mechanisms are associated with regulatory changes in cell size meaning that the set point for cell volume regulation is under physiological control. Thus, cell volume is under a tight and dynamic control and abnormal cell volume regulation will ultimately lead to severe cellular dysfunction, including alterations in cell proliferation and cell death. This review describes the different swelling-activated ion channels that participate as key players in the maintenance of normal steady-state cell volume, with particular emphasis on the intracellular signalling pathways responsible for their regulation during hypotonic stress, cell proliferation and apoptosis.

beta-adrenoceptor mediated responses in rat pulmonary artery: putative role of TASK-1 related K channels

The effect of isoprenaline on tone, cyclic adenosine 3':5' monophosphate (cAMP), and smooth muscle membrane potential (E ( m )) were assessed in rat isolated pulmonary arteries. N(omega)-nitro-L-arginine methyl ester (10.0 microM) or removal of endothelium partially inhibited relaxant responses to isoprenaline, but glibenclamide (10.0 microM) and indomethacin (10.0 microM) did not. While Rp-8-Br-cAMP (30.0 microM), tetraethylammonium (0.3 & 1.0 mM), 4-aminopyridine (100 microM), anandamide (10.0 microM), charybdotoxin (0.1 microM), ouabain (100 microM), and barium chloride (100 microM), incompletely blocked relaxation to isoprenaline, cyclopiazonic acid (1.0 microM), apamin (3.0 microM) and zinc acetate (300 microM) were without effect. Increasing extracellular K(+) ([K(+)](e)) inhibited relaxant responses to isoprenaline, completely abolishing the response at 30 mM [K+](e). Vasorelaxant effects of isoprenaline were significantly attenuated in buffer pH 6.4, and concomitant presence of Rp-8-Br-cAMP (30.0 microM) in pH 6.4 produced significant additive inhibition when compared to pH 6.4 without Rp-8-Br-cAMP. Isoprenaline increased cAMP turnover (1.55+/-0.24 fold; mean +/- SEM), which was inhibited by propranolol (1.0 microM). Resting E ( m ) of smooth muscle cells was -63.0+/-0.50 mV, and isoprenaline (1.0 microM) produced hyperpolarisation (-73.3+/-0.80 mV). While glibenclamide failed to affect isoprenaline-induced hyperpolarisation, ICI 118,551 (1.0 microM), anandamide or buffer pH 6.4 prevented it, and barium chloride and oubain combined caused partial inhibition. Isoprenaline-mediated relaxation seems to arise from several processes, including the generation of nitric oxide, the cAMP-cascade and, more importantly, a hyperpolarisation that is not due to activation of ATP-sensitive K channels but possibly of two-pore domain K channels of the TASK family.

PACSINs bind to the TRPV4 cation channel. PACSIN 3 modulates the subcellular localization of TRPV4

TRPV4 is a cation channel that responds to a variety of stimuli including mechanical forces, temperature, and ligand binding. We set out to identify TRPV4-interacting proteins by performing yeast two-hybrid screens, and we isolated with the avian TRPV4 amino terminus the chicken orthologues of mammalian PACSINs 1 and 3. The PACSINs are a protein family consisting of three members that have been implicated in synaptic vesicular membrane trafficking and regulation of dynamin-mediated endocytotic processes. In biochemical interaction assays we found that all three murine PACSIN isoforms can bind to the amino terminus of rodent TRPV4. No member of the PACSIN protein family was able to biochemically interact with TRPV1 and TRPV2. Co-expression of PACSIN 3, but not PACSINs 1 and 2, shifted the ratio of plasma membrane-associated versus cytosolic TRPV4 toward an apparent increase of plasma membrane-associated TRPV4 protein. A similar shift was also observable when we blocked dynamin-mediated endocytotic processes, suggesting that PACSIN 3 specifically affects the endocytosis of TRPV4, thereby modulating the subcellular localization of the ion channel. Mutational analysis shows that the interaction of the two proteins requires both a TRPV4-specific proline-rich domain upstream of the ankyrin repeats of the channel and the carboxyl-terminal Src homology 3 domain of PACSIN 3. Such a functional interaction could be important in cell types that show distribution of both proteins to the same subcellular regions such as renal tubule cells where the proteins are associated with the luminal plasma membrane.

Endothelium-Derived Epoxyeicosatrienoic Acids and Vascular Function

Epoxyeicosatrienoic acids (EETs) are epoxides of arachidonic acid generated by cytochrome P450 (CYP) epoxygenases. The activation of CYP epoxygenases in endothelial cells is an important step in the NO and prostacyclin-independent vasodilatation of several vascular beds, and EETs have been identified as an endothelium-derived hyperpolarizing factor. However, EETs also exert membrane potential-independent effects and modulate several signaling cascades that affect endothelial cell proliferation and angiogenesis. This review summarizes the role of CYP-derived EETs in endothelium-derived hyperpolarizing factor-mediated responses and highlights the evidence indicating that EETs are important second messengers involved in endothelial cell signaling pathways related to angiogenesis.

Calcium-activated potassium channels and the regulation of vascular tone

Different calcium signals in the endothelium and smooth muscle target different types of Ca2+-sensitive K+ channels to modulate vascular function. These differential calcium signals and targets represent multilayered opportunities for prevention and/or treatment of vascular dysfunctions.

Transient receptor potential vanilloid 4 regulates aquaporin-5 abundance under hypotonic conditions

Aquaporin-5 (AQP5) is expressed in epithelia of lung, cornea, and various secretory glands, sites where extracellular osmolality is known to fluctuate. Hypertonic aquaporin (AQP) induction has been described, but little is known about the effects of a hypotonic environment on AQP abundance. We report that, when mouse lung epithelial cells were exposed to hypotonic medium, a dose-responsive decrease in AQP5 abundance was observed. Hypotonic reduction of AQP5 was blocked by ruthenium red, methanandamide, and miconazole, agents that inhibit the cation channel transient receptor potential vanilloid (TRPV) 4 present in lung epithelial cells. Several observations indicate that TRPV4 participates in hypotonic reduction of AQP5, including a requirement for extracellular calcium to achieve AQP5 reduction; an increase in intracellular calcium in mouse lung epithelial (MLE) cells after hypotonic stimulation; and reduction of AQP5 abundance after addition of the TRPV4 agonist 4alpha-Phorbol-12,13-didecanoate (4alpha-PDD). Similarly, addition of hypotonic PBS to mouse trachea in vivo decreased AQP5 within 1 h, an effect blocked by ruthenium red. To confirm a functional interaction, AQP5 was expressed in control or TRPV4-expressing human embryonic kidney (HEK) cells. Hypotonic reduction of AQP5 was observed only in the presence of TRPV4 and was blocked by ruthenium red. Combined with earlier studies, these observations indicate that AQP5 abundance is tightly regulated along a range of osmolalities and that AQP5 reduction by extracellular hypotonicity can be mediated by TRPV4. These findings have direct relevance to regulation of membrane water permeability and water homeostasis in epithelia of the lung and other organs.

The transient receptor potential vanilloid 1: role in airway inflammation and disease

The transient receptor potential vanilloid 1 (TRPV1) is an excitatory cation channel, rather selectively expressed in a subpopulation of nociceptive, primary sensory neurons that promote neurogenic inflammation via neuropeptide release. TRPV1 is activated by noxious temperature, low extracellular pH and diverse lipid derivatives, and is uniquely sensitive to vanilloid molecules, including capsaicin. TRPV1 expression and sensitivity is highly regulated by diverse G protein-coupled and tyrosine kinase receptors. Other exogenous or endogenous chemical agents, including reactive oxygen species, ethanol and hydrogen sulphide sensitize/activate TRPV1. In the airways, TRPV1 agonists cause cough, bronchoconstriction, microvascular leakage, hyperreactivity and hypersecretion. Patients with asthma and chronic obstructive pulmonary disease are more sensitive to the tussive effect of TRPV1 agonists and TRPV1 activation may contribute to respiratory symptoms caused by acidic media present in the airways during asthma exacerbation, gastroesophageal reflux induced asthma or in other conditions. TRPV1 antagonists may be useful in the treatment of these diseases.

Functional expression of transient receptor potential vanilloid 4 in the mouse cochlea

Transient receptor potential vanilloid 4, the Ca2+-permeable cation channel has been proposed as an osmosensitive and a mechanosensitive channel. We investigated functional expression of transient receptor potential vanilloid 4 in inner hair cells, outer hair cells, and spiral ganglion neurons of the mouse cochlea. Transient receptor potential vanilloid 4 mRNA and protein were expressed in inner hair cells, outer hair cells, and spiral ganglion neurons on the basis of the findings of reverse transcriptase-polymerase chain reaction, single-cell reverse transcriptase-polymerase chain reaction, and immunohistochemistry, whereas they were negative in transient receptor potential vanilloid 4-/- mice cochleae. Hypotonic stimulation and 4-alpha-phorbol 12,13-didecanoate, a transient receptor potential vanilloid 4 synthetic activator, increased the intracellular Ca2+ concentrations in wild-type outer hair cells, whereas in transient receptor potential vanilloid 4-/- mice, outer hair cells failed to exhibit a Ca2+ response to both stimulations. In conclusion, transient receptor potential vanilloid 4 may function as an osmosensory and a mechanosensory receptor in the cochlea.