OTRPC4, a nonselective cation channel that confers sensitivity to extracellular osmolarity

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.

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.

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.

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.

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.

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.

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.

Tissue-specific expression of TRP channel genes in the mouse and its variation in three different mouse strains

Background

The purpose of this work was to study the gene expression of transient receptor potential (TRP) channels in the mouse. The application of a standardized and quantitative technique, TaqMan RT-PCR, should give information about the pattern and relative importance of TRP channels for murine tissues and cell types. To verify data sets with an independent method, we studied the occurrence of some of the transcripts by in situ hybridization.

Results

We have characterized the mRNA expression of 22 TRP channels in the mouse with a focus on nerve and muscle tissues. This is the first study to describe the expression profiles of all channel isoforms of the four related Group 1 subfamilies (TRPC, TRPV, TRPM and TRPA) with a standardized and quantitative technique. Comparisons of transcript abundance showed a consistent dominance of TRPM7 and TRPC3 in most tissues. We further observed characteristic patterns and differences in gene expression of individual channels ranging over three orders of magnitude. The overall level of TRP channel mRNAs was highest in brain areas followed by kidney, lung, reproductive organs and muscle. In brain TRPM3 and TRPM7 dominated and 19 other isoforms were detected. In lung and kidney TRPV4, TRPV5 and TRPM7 were found in highest levels. TRPM7, TRPC3, TRPC6 and TRPM3 mRNAs were characteristically present in all tested muscle tissues. Most data obtained with the C57Bl/10 mouse strain were confirmed with Balb/c and NOD mice. However, TRPC3, C6, TRPM7, M3, TRPV2 and V4 expression showed marked differences in the three tested mouse strains. In situ hybridization revealed co-expression of transcripts on the cellular level and widely confirmed the data obtained with RT-PCR.

Conclusion

Transcripts coding for members of the TRPC, TRPV, TRPM and TRPA subfamilies of TRP cation channels are present in a broad spectrum of murine tissues. Several channel isoforms often coexist in a specific tissue or cell type. TRP channel expression does not show typical tissue specific dominance of individual members as is known from other ion channel families. Mouse strain specific variations of TRP channel expression indicate that genetic background or physiological requirements considerably influence expression levels.

Ca2+ influx through mechanosensitive channels inhibits neurite outgrowth in opposition to other influx pathways and release from intracellular stores

Ca2+ signals are known to be important regulators of neurite outgrowth and steering. Here we show that inhibiting Ca2+ influx through stretch-activated channels using various compounds, including a highly specific peptide isolated from Grammostola spatulata spider venom (GsMTx4), strongly accelerates the rate of neurite extension on diverse substrata and within the intact spinal cord. Consistent with the presence of stretch-activated channels, we show that Ca2+ influx is triggered by hypotonic solutions, which can be partially blocked by GsMTx4. Finally, chelating local, but not global, Ca2+ signals prevents the acceleration that is normally produced by GsMTx4. Blocking Ca2+ influx through other channel types has little or opposite effects, but release from intracellular stores is required for maximal acceleration. Together, our data suggest that Ca2+ functions at distinct microdomains in growth cones, with influx through mechanosensitive channels acting to inhibit outgrowth in opposition to influx through other plasma membrane channels and release from stores.

TRP channels in C. elegans

The TRP (transient receptor potential) superfamily of cation channels is present in all eukaryotes, from yeast to mammals. Many TRP channels have been studied in the nematode Caenorhabditis elegans, revealing novel biological functions, regulatory modes, and mechanisms of localization. C. elegans TRPV channels function in olfaction, mechanosensation, osmosensation, and activity-dependent gene regulation. Their activity is regulated by G protein signaling and polyunsaturated fatty acids. C. elegans TRPPs related to human polycystic kidney disease genes are expressed in male-specific neurons. The KLP-6 kinesin directs TRPP channels to cilia, where they may interact with F0/F1 ATPases. A sperm-specific TRPC channel, TRP-3, is required for fertilization. Upon sperm activation, TRP-3 translocates from an intracellular compartment to the plasma membrane to allow store-operated Ca2+ entry. The TRPM channels GON-2 and GTL-2 regulate Mg2+ homeostasis and Mg2+ uptake by intestinal cells; GON-2 is also required for gonad development. The TRPML CUP-5 promotes normal lysosome biogenesis and prevents apoptosis. Dynamic, precise expression of TRP proteins generates a remarkable range of cellular functions.

Physiological volume regulation by spermatozoa

Maturing spermatozoa passing through the epididymis experience increasing osmolality in the luminal environment and mature cells are stored in fluids hyper-osmotic to serum. When ejaculated into the female tract, they encounter a hypo-osmotic challenge which initiates the process of regulatory volume decrease (RVD). Defects in RVD result in hindrance of mucus penetration in man and failure of utero-tubal passage in mice. Epididymal sperm from the mouse and cynomolgus monkey and ejaculated sperm from man and monkey have been isolated and dispersed in media with osmolalities mimicking those of uterine fluid or cervical mucus. The effects of specific and broad-spectrum ion channel blockers indicate the involvement of separate K+ and Cl- channels as well as organic osmolytes in physiological sperm RVD, with mechanisms developed during epididymal maturation. Western blotting and immuno-cytochemistry identify and localise some of these channels which play a crucial role in fertilisation in vivo and could be targets for post-testicular contraception.

A transient receptor potential vanilloid 4-dependent mechanism of hyperalgesia is engaged by concerted action of inflammatory mediators

The transient receptor potential vanilloid 4 (TRPV4) is a primary afferent transducer that plays a crucial role in neuropathic hyperalgesia for osmotic and mechanical stimuli, as well as in inflammatory mediator-induced hyperalgesia for osmotic stimuli. In view of the clinical importance of mechanical hyperalgesia in inflammatory states, the present study investigated the role of TRPV4 in mechanical hyperalgesia induced by inflammatory mediators and the second-messenger pathways involved. Intradermal injection of either the inflammogen carrageenan or a soup of inflammatory mediators enhanced the nocifensive paw-withdrawal reflex elicited by hypotonic or mechanical stimuli in rat. Spinal administration of TRPV4 antisense oligodeoxynucleotide blocked the enhancement without altering baseline nociceptive threshold. Similarly, in TRPV4(-/-) knock-out mice, inflammatory soup failed to induce any significant mechanical or osmotic hyperalgesia. In vitro investigation showed that inflammatory mediators engage the TRPV4-mediated mechanism of sensitization by direct action on dissociated primary afferent neurons. Additional behavioral observations suggested that multiple mediators are necessary to achieve sufficient activation of the cAMP pathway to engage the TRPV4-dependent mechanism of hyperalgesia. In addition, direct activation of protein kinase A or protein kinase C epsilon, two pathways that mediate inflammation-induced mechanical hyperalgesia, also induced hyperalgesia for both hypotonic and mechanical stimuli that was decreased by TRPV4 antisense and absent in TRPV4(-/-) mice. We conclude that TRPV4 plays a crucial role in the mechanical hyperalgesia that is generated by the concerted action of inflammatory mediators present in inflamed tissues.

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.

A role for AQP5 in activation of TRPV4 by hypotonicity: concerted involvement of AQP5 and TRPV4 in regulation of cell volume recovery

Regulation of cell volume in response to changes in osmolarity is critical for cell function and survival. However, the molecular basis of osmosensation and regulation of cell volume are not clearly understood. We have examined the mechanism of regulatory volume decrease (RVD) in salivary gland cells and report a novel association between osmosensing TRPV4 (transient receptor potential vanalloid 4) and AQP5 (aquaporin 5), which is required for regulating water permeability and cell volume. Exposure of salivary gland cells and acini to hypotonicity elicited an increase in cell volume and activation of RVD. Hypotonicity also activated Ca2+ entry, which was required for subsequent RVD. Ca2+ entry was associated with a distinct nonselective cation current that was activated by 4alphaPDD and inhibited by ruthenium red, suggesting involvement of TRPV4. Consistent with this, endogenous TRPV4 was detected in cells and in the apical region of acini along AQP5. Importantly, acinar cells from mice lacking either TRPV4 or AQP5 displayed greatly reduced Ca2+ entry and loss of RVD in response to hypotonicity, although the extent of cell swelling was similar. Expression of N terminus-deleted AQP5 suppressed TRPV4 activation and RVD but not cell swelling. Furthermore, hypotonicity increased the association and surface expression of AQP5 and TRPV4. Both these effects and RVD were reduced by actin depolymerization. These data demonstrate that (i) activation of TRPV4 by hypotonicity depends on AQP5, not on cell swelling per se, and (ii) TRPV4 and AQP5 concertedly control regulatory volume decrease. These data suggest a potentially important role for TRPV4 in salivary gland function.

Calcium permeability and flux through osmosensory transduction channels of isolated rat supraoptic nucleus neurons

Hypertonic stimuli delivered into the supraoptic nucleus provoke neuropeptide release from the somata of magnocellular neurosecretory cells (MNCs) in the presence of tetrodotoxin, suggesting that such stimuli can increase intracellular calcium concentration ([Ca2+]i) in the absence of action potentials. We therefore examined whether the stretch-inhibited cation (SIC) channels of MNCs can mediate calcium influx. Whole-cell recordings were made in MNCs isolated from the supraoptic nuclei of adult rats. Measurements of reversal potentials in different solutions revealed that the current induced by a suction-evoked decrease in cell volume (ISIC) displays a selectivity sequence for monovalent cations of K+>Cs+>Na+>NMDG+. The permeability of SIC channels to Ca2+, relative to Na+, was approximately 5. In the presence of physiological concentrations of external Na+ and K+, the amplitude of inward ISIC was reduced dose-dependently by external Ca2+ with an IC50 of 4.9 mM. This was not due to reduced suction-evoked volume changes or to an accumulation of [Ca2+]i. Confocal imaging of cytoplasmic Calcium Green-1 fluorescence revealed that activation of ISIC significantly increases [Ca2+]i in physiological solutions. This effect is absent in Ca2+-free solution, or when Gd3+ (300 microM) is added to Ca2+-containing solution. Part of this effect is inhibited in the presence of dantrolene (10 microM) and heparin (4 mg/mL), suggesting that release from intracellular Ca2+ stores participates in suction-evoked Ca2+ signalling. These observations indicate that SIC channels are highly permeable to Ca2+, mediate significant Ca2+ entry and release of Ca2+ from internal stores under conditions when the volume of MNCs is decreased.

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.

Uptake of fluorescent gentamicin by vertebrate sensory cells in vivo

Aminoglycoside uptake in the inner ear remains poorly understood. We subcutaneously injected a fluorescently-conjugated aminoglycoside, gentamicin-Texas Red (GTTR), to investigate the in vivo uptake of GTTR in the inner ear of several vertebrates, and in various murine sensory cells using confocal microscopy. In bullfrogs, GTTR uptake was prominent in mature hair cells, but not in immature hair cells. Avian hair cells accrued GTTR more rapidly at the base of the basilar papilla. GTTR was associated with the hair bundle; and, in guinea pigs and mice, somatic GTTR fluorescence was initially diffuse before punctate (endosomal) fluorescence could be observed. A baso-apical gradient of intracellular GTTR uptake in guinea pig cochleae could only be detected at early time points (<3h). In 21-28 day mice, cochlear GTTR uptake was greatly reduced compared to guinea pigs, 6-day-old mice, or mice treated with ethacrynic acid. In mice, GTTR was also rapidly taken up, and retained, in the kidney, dorsal root and trigeminal ganglia. In linguinal and vibrissal tissues rapid GTTR uptake cleared over a period of several days. The preferential uptake of GTTR by mature saccular, and proximal hair cells resembles the pattern of aminoglycoside-induced hair cell death in bullfrogs and chicks. Differences in the degree of GTTR uptake in hair cells of different species suggests variation in serum levels, clearance rates from serum, and/or the developmental and functional integrity of the blood-labyrinth barrier. GTTR uptake by hair cells in vivo suggests that GTTR has potential to elucidate aminoglycoside transport mechanisms into the inner ear, and as a bio-tracer for in vivo pharmacokinetic studies.

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.

Translation of salt retention to central activation of the sympathetic nervous system in hypertension

1. Increased dietary salt increases blood pressure in many hypertensive individuals, producing salt-sensitive hypertension (SSH). The cause is unknown, but a major component appears to be activation of the sympathetic nervous system. The purpose of this short review is to present one hypothesis to explain how increased dietary salt increases sympathetic activity in SSH. 2. It is proposed that increased salt intake causes salt retention and raises plasma sodium chloride (NaCl) concentrations, which activate sodium/osmoreceptors to trigger sympathoexcitation. Moreover, we suggest that small and often undetectable increases in osmolality can drive significant sympathoexcitation, because the gain of the relationship between osmolality and increased sympathetic activity is enhanced. Multiple factors may contribute to this facilitation, including inappropriately elevated levels of angiotensin II or aldosterone, changes in gene expression or synaptic plasticity and increased sodium concentrations in cerebrospinal fluid. 3. Future studies are required to delineate the brain sites and mechanisms of action and interaction of osmolality and these amplification factors to elicit sustained sympathoexcitation in SSH.

Cytoplasmic and intra-nuclear binding of gentamicin does not require endocytosis

Understanding the cellular mechanism(s) by which the oto- and nephrotoxic aminoglycoside antibiotics penetrate cells, and the precise intracellular distribution of these molecules, will enable identification of aminoglycoside-sensitive targets, and potential uptake blockers. Clones of two kidney cell lines, OK and MDCK, were treated with the aminoglycoside gentamicin linked to the fluorophore Texas Red (GTTR). As in earlier reports, endosomal accumulation was observed in live cells, or cells fixed with formaldehyde only. However, delipidation of fixed cells revealed GTTR fluorescence in cytoplasmic and nuclear compartments. Immunolabeling of both GTTR and unconjugated gentamicin corresponded to the cytoplasmic distribution of GTTR fluorescence. Intra-nuclear GTTR binding co-localized with labeled RNA in the nucleoli and trans-nuclear tubules. Cytoplasmic and nuclear distribution of GTTR was quenched by phosphatidylinositol-bisphosphate (PIP2), a known ligand for gentamicin. Cytoplasmic and nuclear GTTR binding increased over time (at 37 degrees C, or on ice to inhibit endocytosis), and was serially competed off by increasing concentrations of unconjugated gentamicin, i.e., GTTR binding is saturable. In contrast, little or no reduction of endocytotic GTTR uptake was observed when cells were co-incubated with up to 4 mg/mL unconjugated gentamicin. Thus, cytoplasmic and nuclear GTTR uptake is time-dependent, weakly temperature-dependent and saturable, suggesting that it occurs via an endosome-independent mechanism, implicating ion channels, transporters or pores in the plasma membrane as bioregulatory routes for gentamicin entry into cells.

WNK kinases influence TRPV4 channel function and localization

TRPV4, a renally expressed nonselective cation channel of the transient receptor potential (TRP) family, is gated by hypotonicity. Kinases of the WNK family influence expression and function of the thiazide-sensitive Na+-Cl- cotransporter, and monogenic human hypertension has been linked to mutations in the gene coding for WNK4. Along with TRPV4, WNK isoforms are highly expressed in the distal nephron. We show here that coexpression of WNK4 downregulates TRPV4 function in human embryonic kidney (HEK-293) cells and that this effect is mediated via decreased cell surface expression of TRPV4; total abundance of TRPV4 in whole cell lysates is unaffected. The effect of the related kinase WNK1 on TRPV4 function and surface expression was similar to that of WNK4. Disease-causing point mutations in WNK4 abrogate, but do not eliminate, the inhibitory effect on TRPV4 function. In contrast to wild-type WNK4, a kinase-dead WNK4 point mutant failed to influence TRPV4 trafficking; however, deletion of the entire WNK4 kinase domain did not blunt the effect of WNK4 on localization of TRPV4. Deletion of the extreme COOH-terminal putative coiled-coil domain of WNK4 abolished its effect. In immunoprecipitation experiments, we were unable to detect direct interaction between TRPV4 and either WNK kinase. In aggregate, these data indicate that TRPV4 is functionally regulated by WNK family kinases at the level of cell surface expression. Because TRPV4 and WNK kinases are coexpressed in the distal nephron in vivo and because there is a tendency toward hypercalcemia in TRPV4-/- mice, we speculate that this pathway may impact systemic Ca2+ balance. In addition, because WNK kinases and TRPV4 are activated by anisotonicity, they may comprise elements of an osmosensing or osmotically responsive signal transduction cascade in the distal nephron.

Functional expression of transient receptor potential melastatin- and vanilloid-related channels in pulmonary arterial and aortic smooth muscle

Transient receptor potential melastatin- (TRPM) and vanilloid-related (TRPV) channels are nonselective cation channels pertinent to diverse physiological functions. Multiple TRPM and TRPV channel subtypes have been identified and cloned in different tissues. However, their information in vascular tissue is scant. In this study, we sought to identify TRPM and TRPV channel subtypes expressed in rat deendothelialized intralobar pulmonary arteries (PAs) and aorta. With RT-PCR, mRNA of TRPM2, TRPM3, TRPM4, TRPM7, and TRPM8 of TRPM family and TRPV1, TRPV2, TRPV3, and TRPV4 of TRPV family were detected in both PAs and aorta. Quantitative real-time RT-PCR showed that TRPM8 and TRPV4 were the most abundantly expressed TRPM and TRPV subtypes, respectively. Moreover, Western blot analysis verified expression of TRPM2, TRPM8, TRPV1, and TRPV4 proteins in both types of vascular tissue. To examine the functional activities of these channels, we monitored intracellular Ca(2+) transients ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMCs) and aortic smooth muscle cells (ASMCs). The TRPM8 agonist menthol (300 muM) and the TRPV4 agonist 4alpha-phorbol 12,13-didecanoate (1 muM) evoked significant increases in [Ca(2+)](i) in PASMCs and ASMCs. These Ca(2+) responses were abolished in the absence of extracellular Ca(2+) or the presence of 300 muM Ni(2+) but were unaffected by 1 muM nifedipine, suggesting Ca(2+) influx via nonselective cation channels. Hence, for the first time, our results indicate that multiple functional TRPM and TRPV channels are coexpressed in rat intralobar PAs and aorta. These novel Ca(2+) entry pathways may play important roles in the regulation of pulmonary and systemic circulation.

Glycosylation of the osmoresponsive transient receptor potential channel TRPV4 on Asn-651 influences membrane trafficking

We identified a consensus N-linked glycosylation motif within the pore-forming loop between the fifth and sixth transmembrane segments of the osmoresponsive transient receptor potential (TRP) channel TRPV4. Mutation of this residue from Asn to Gln (i.e., TRPV4(N651Q)) resulted in loss of a slower migrating band on anti-TRPV4 immunoblots and a marked reduction in lectin-precipitable TRPV4 immunoreactivity. HEK293 cells transiently transfected with the mutant TRPV4(N651Q) exhibited increased calcium entry in response to hypotonic stress relative to wild-type TRPV4 transfectants. This increase in hypotonicity responsiveness was associated with an increase in plasma membrane targeting of TRPV4(N651Q) relative to wild-type TRPV4 in both HEK293 and COS-7 cells but had no effect on overall channel abundance in whole cell lysates. Residue N651 of TRPV4 is immediately adjacent to the pore-forming loop. Although glycosylation in this vicinity has not been reported for a TRP channel, the structurally related hexahelical hyperpolarization-activated cyclic nucleotide-gated channel, HCN2, and the voltage-gated potassium channel, human ether-a-go-go-related (HERG), share a nearly identically situated and experimentally confirmed N-linked glycosylation site which promotes rather than limits channel insertion into the plasma membrane. These data point to a potentially conserved structural and functional feature influencing membrane trafficking across diverse members of the voltage-gated-like ion channel superfamily.

TRPV4 Forms a Novel Ca 2+ Signaling Complex With Ryanodine Receptors and BK Ca Channels

Vasodilatory factors produced by the endothelium are critical for the maintenance of normal blood pressure and flow. We hypothesized that endothelial signals are transduced to underlying vascular smooth muscle by vanilloid transient receptor potential (TRPV) channels. TRPV4 message was detected in RNA from cerebral artery smooth muscle cells. In patch-clamp experiments using freshly isolated cerebral myocytes, outwardly rectifying whole-cell currents with properties consistent with those of expressed TRPV4 channels were evoked by the TRPV4 agonist 4alpha-phorbol 12,13-didecanoate (4alpha-PDD) (5 micromol/L) and the endothelium-derived arachidonic acid metabolite 11,12 epoxyeicosatrienoic acid (11,12 EET) (300 nmol/L). Using high-speed laser-scanning confocal microscopy, we found that 11,12 EET increased the frequency of unitary Ca2+ release events (Ca2+ sparks) via ryanodine receptors located on the sarcoplasmic reticulum of cerebral artery smooth muscle cells. EET-induced Ca2+ sparks activated nearby sarcolemmal large-conductance Ca2+-activated K+ (BKCa) channels, measured as an increase in the frequency of transient K+ currents (referred to as "spontaneous transient outward currents" [STOCs]). 11,12 EET-induced increases in Ca2+ spark and STOC frequency were inhibited by lowering external Ca2+ from 2 mmol/L to 10 micromol/L but not by voltage-dependent Ca2+ channel inhibitors, suggesting that these responses require extracellular Ca2+ influx via channels other than voltage-dependent Ca2+ channels. Antisense-mediated suppression of TRPV4 expression in intact cerebral arteries prevented 11,12 EET-induced smooth muscle hyperpolarization and vasodilation. Thus, we conclude that TRPV4 forms a novel Ca2+ signaling complex with ryanodine receptors and BKCa channels that elicits smooth muscle hyperpolarization and arterial dilation via Ca2+-induced Ca2+ release in response to an endothelial-derived factor.

Recent developments in vascular endothelial cell transient receptor potential channels

Among the 28 identified and unique mammalian TRP (transient receptor potential) channel isoforms, at least 19 are expressed in vascular endothelial cells. These channels appear to participate in a diverse range of vascular functions, including control of vascular tone, regulation of vascular permeability, mechanosensing, secretion, angiogenesis, endothelial cell proliferation, and endothelial cell apoptosis and death. Malfunction of these channels may result in disorders of the human cardiovascular system. All TRP channels, except for TRPM4 and TRPM5, are cation channels that allow Ca2+ influx. However, there is a daunting diversity in the mode of activation and regulation in each case. Specific TRP channels may be activated by different stimuli such as vasoactive agents, oxidative stress, mechanical stimuli, and heat. TRP channels may then transform these stimuli into changes in the cytosolic Ca2+, which are eventually coupled to various vascular responses. Evidence has been provided to suggest the involvement of at least the following TRP channels in vascular function: TRPC1, TRPC4, TRPC6, and TRPV1 in the control of vascular permeability; TRPC4, TRPV1, and TRPV4 in the regulation of vascular tone; TRPC4 in hypoxia-induced vascular remodeling; and TRPC3, TRPC4, and TRPM2 in oxidative stress-induced responses. However, in spite of the large body of data available, the functional role of many endothelial TRP channels is still poorly understood. Elucidating the mechanisms regulating the different endothelial TRP channels, and the associated development of drugs selectively to target the different isoforms, as a means to treat cardiovascular disease should, therefore, be a high priority.

An N-terminal variant of Trpv1 channel is required for osmosensory transduction

Body fluid homeostasis requires the release of arginine-vasopressin (AVP, an antidiuretic hormone) from the neurohypophysis. This release is controlled by specific and highly sensitive 'osmoreceptors' in the hypothalamus. Indeed, AVP-releasing neurons in the supraoptic nucleus (SON) are directly osmosensitive, and this osmosensitivity is mediated by stretch-inhibited cation channels. However, the molecular nature of these channels remains unknown. Here we show that SON neurons express an N-terminal splice variant of the transient receptor potential vanilloid type-1 (Trpv1), also known as the capsaicin receptor, but not full-length Trpv1. Unlike their wild-type counterparts, SON neurons in Trpv1 knockout (Trpv1(-/-)) mice could not generate ruthenium red-sensitive increases in membrane conductance and depolarizing potentials in response to hyperosmotic stimulation. Moreover, Trpv1(-/-) mice showed a pronounced serum hyperosmolality under basal conditions and severely compromised AVP responses to osmotic stimulation in vivo. These results suggest that the Trpv1 gene may encode a central component of the osmoreceptor.

TRP channels in airway smooth muscle as therapeutic targets

Cation channels are of fundamental importance in regulating the function of airway smooth cells especially bronchoconstriction in response to spasmogens, and are therefore key players in the pathogenesis of asthma. To date, the identity of these cation channels remains a mystery. However, the recently emerged transient receptor potential (TRP) cation channel family has provided several promising channel candidates. The identification of the key TRP channels involved in regulating airway smooth muscle contractility, and therefore airway tone, could provide new and exciting prospects for the development of novel therapies for the treatment of airway diseases such as asthma.

Human TRPV4 channel splice variants revealed a key role of ankyrin domains in multimerization and trafficking

The TRPV4 cation channel exhibits a topology consisting of six predicted transmembrane domains (TM) with a putative pore loop between TM5 and TM6 and intracellular N- and C-tails, the former containing at least three ankyrin domains. Functional transient receptor potential (TRP) channels are supposed to result following the assembly of four subunits. However, the rules governing subunit assembly and protein domains implied in this process are only starting to emerge. The ankyrin, TM, and the C-tail domains have been identified as important determinants of the oligomerization process. We now describe the maturation and oligomerization of five splice variants of the TRPV4 channel. The already known TRPV4-A and TRPV4-B (delta384-444) variants and the new TRPV4-C (delta237-284), TRPV4-D (delta27-61), and TRPV4-E (delta237-284 and delta384-444) variants. All alternative spliced variants involved deletions in the cytoplasmic N-terminal region, affecting (except for TRPV4-D) the ankyrin domains. Subcellular localization, fluorescence resonance energy transfer, co-immunoprecipitation, glycosylation profile, and functional analysis of these variants permitted us to group them into two classes: group I (TRPV4-A and TRPV4-D) and group II (TRPV4-B, TRPV4-C, and TRPV4-E). Group I, unlike group II variants, were correctly processed, homo- and heteromultimerized in the endoplasmic reticulum, and were targeted to the plasma membrane where they responded to typical TRPV4 stimuli. Our results suggest that: 1) TRPV4 biogenesis involves core glycosylation and oligomerization in the endoplasmic reticulum followed by transfer to the Golgi apparatus for subsequent maturation; 2) ankyrin domains are necessary for oligomerization of TRPV4; and 3) lack of TRPV4 oligomerization determines its accumulation in the endoplasmic reticulum.

TRPV4 mediates pain-related behavior induced by mild hypertonic stimuli in the presence of inflammatory mediator

The ligand-gated ion channel, TRPV4, functions as a transducer of hypotonic stimuli in primary afferent nociceptive neurons and contributes to inflammatory and neuropathic pain. Hypertonic saline also stimulates primary afferent nociceptors and the injection of mild hypertonic saline (2-5%) is widely used as an experimental model of pain in humans. Therefore, we tested whether TRPV4 participates in the transduction of hypertonic stimuli. Intradermal injection of 2% (607 mOsm) or 10% (3,250 mOsm) saline solution in the hind paw of rats induced a concentration-dependent pain-related behavior, flinching. Sensitization with prostaglandin E(2) (PGE(2)) caused a 7-fold increase in the number of flinches induced by 2% saline but failed to increase those caused by 10% saline. Spinal administration of antisense oligodeoxynucleotides to TRPV4 caused a 46% decrease in the number of flinches induced by 2% saline, but there was no change in flinching induced by 10% saline. Similarly, only the nociceptive behavior caused by 2% saline was reduced in TRPV4(-/-) knockout mice. The TRPV4-mediated nociceptive behaviors induced by hyper- and hypotonic stimuli were dependent on Src tyrosine kinase. We suggest TRPV4 is a transducer in primary afferents that mediates nociceptive behavior induced by small increases or decreases in osmolarity. Such changes in osmolarity might contribute to pain in inflammatory and neuropathic states.

Multiple eicosanoid-activated nonselective cation channels regulate B-lymphocyte adhesion to integrin ligands

Arachidonic acid (AA) is a substrate for a variety of proinflammatory mediators, which are generated by cyclooxygenases (COXs), lipoxygenases (LOXs), and cytochrome P-450 (CYP450) enzymes. COX (e.g., PGs and prostacyclins) and LOX (e.g., leukotrienes) products have well-established proinflammatory roles; however, little is known about the functions of CYP450 products in leukocytes. We previously found that mechanical strain generated by subjecting lymphocytes to hypotonic challenge triggered AA production and that two CYP450 products of AA, 5,6-epoxyeicosatrienoic acid (5,6-EET) and 20-hydroxyeicosatetraenoic acid (20-HETE), as well as a product of LOX, 5-(S)-hydroperoxyeicosatetrenoic acid (5-HPETE), induced Ca(2+) entry into primary B cells. The main goal of the present studies, therefore, was to define the biophysically properties of eicosanoid-activated channels responsible for Ca(2+) entry and the physiological consequences of activating these channels, including their role in mechanical signaling. We found that 5,6-EET, 20-HETE, and 5-HPETE each activated distinct Ca(2+)-permeant nonselective cation channels (NSCCs) in primary B cells. These NSCCs each regulate plasma membrane potential and B-cell adhesion to integrin ligands ICAM-1 and VCAM-1. Thus our data demonstrate that proinflammatory mediators produced in response to osmotic and/or physical stress play a direct role in regulating the B-cell membrane potential and their adhesion to specific ECM proteins. These results not only have important implications for understanding normal mechanisms of B-cell activation, differentiation, and trafficking but also point to novel targets for modulating the pathogenesis of B-cell-mediated inflammatory diseases.

A rapid, nongenomic pathway facilitates the synaptic transmission induced by retinoic acid at the developing synapse

We have previously shown that retinoic acid (RA), a factor highly expressed in spinal cord, rapidly and specifically enhances the spontaneous acetylcholine release at developing neuromuscular synapses in Xenopus cell culture, using whole-cell patch-clamp recording. We have now further investigated the underlying mechanisms that are involved in RA-induced facilitation on the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by RA. The prompt RA-enhanced SSC frequency was not abolished when Ca2+ was eliminated from the culture medium or there was bath application of the pharmacological Ca2+ channel inhibitor Cd2+, indicating that Ca2+ influx through voltage-activated Ca2+ channels are not required. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] or ryanodine receptors effectively blocked the increase of SSC frequency elicited by RA. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase) and with the phospholipase Cgamma (PLCgamma) inhibitor U73122, abolished RA-induced facilitation of synaptic transmission. Preincubation of the cultures with pharmacological inhibitors, either genistein, a broad-spectrum tyrosine kinase inhibitor, or PP2, which predominantly inhibits the Src family of nonreceptor tyrosine kinase, completely abolished RA-induced synaptic facilitation. Taken collectively, these results suggest that RA elicits Ca2+ release from Ins1,4,5P3 and/or ryanodine-sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PLCgamma/PI 3-kinase signaling cascades and Src tyrosine kinase activation, leading to an enhancement of spontaneous transmitter release.

Modulation of the Ca2 permeable cation channel TRPV4 by cytochrome P450 epoxygenases in vascular endothelium

TRPV4 is a broadly expressed Ca2+-permeable cation channel in the vanilloid subfamily of transient receptor potential channels. TRPV4 gates in response to a large variety of stimuli, including cell swelling, warm temperatures, the synthetic phorbol ester 4alpha-phorbol 12,13-didecanoate (4alpha-PDD), and the endogenous lipid arachidonic acid (AA). Activation by cell swelling and AA requires cytochrome P450 (CYP) epoxygenase activity to convert AA to epoxyeicosatrienoic acids (EETs) such as 5,6-EET, 8,9-EET, which both act as direct TRPV4 agonists. To evaluate the role of TRPV4 and its modulation by the CYP pathway in vascular endothelial cells, we performed Ca2+ imaging and patch-clamp measurements on mouse aortic endothelial cells (MAECs) isolated from wild-type and TRPV4(-/-) mice. All TRPV4-activating stimuli induced robust Ca2+ responses in wild-type MAECs but not in MAECs isolated from TRPV4(-/-) mice. Upregulation of CYP2C expression by preincubation with nifedipine enhanced the responses to AA and cell swelling in wild-type MAECs, whereas responses to other stimuli remained unaffected. Conversely, inhibition of CYP2C9 activity with sulfaphenazole abolished the responses to AA and hypotonic solution (HTS). Moreover, suppression of EET hydrolysis using 1-adamantyl-3-cyclo-hexylurea or indomethacin, inhibitors of soluble epoxide hydrolases (sEHs), and cyclooxygenases, respectively, enhanced the TRPV4-dependent responses to AA, HTS, and EETs but not those to 4alpha-PDD or heat. Together, our data establish that CYP-derived EETs modulate the activity of TRPV4 channels in endothelial cells and shows the unraveling of novel modulatory pathways via CYP2C modulation and sEH inhibition.

A possible unifying principle for mechanosensation

Of Aristotle's five senses, we know that sight, smell and much of taste are initiated by ligands binding to G-protein-coupled receptors; however, the mechanical sensations of touch and hearing remain without a clear understanding of their molecular basis. Recently, the relevant force-transducing molecules--the mechanosensitive ion channels--have been identified. Such channel proteins purified from bacteria sense forces from the lipid bilayer in the absence of other proteins. Recent evidence has shown that lipids are also intimately involved in opening and closing the mechanosensitive channels of fungal, plant and animal species.

Determining the functional role of TRPC channels in primary cells

Although the TRPC members of the mammalian transient receptor potential TRP cation channel family were the first to be described in 1995, the depth of knowledge of TRPC channels has fallen behind that of their counterparts in the TRPV and TRPM subfamilies in the intervening years. The complexities and controversies of TRPC channel composition and regulation have hindered their progress as therapeutic targets in the drug discovery environment to date, however embracing these challenges as opportunities may bring TRPC channels to the forefront of the discovery of novel therapies for many diseases. These challenges and opportunities of exploring TRPC channels as therapeutic targets are highlighted and discussed in this review with respect to respiratory diseases.

Feeling the pressure in mammalian somatosensation

Mechanoreceptor cells of the somatosensory system initiate the perception of touch and pain. Molecules required for mechanosensation have been identified from invertebrate neurons, and recent functional studies indicate that ion channels of the transient receptor potential and degenerin/epithelial Na+ channel families are likely to be transduction channels. The expression of related channels in mammalian somatosensory neurons has fueled the notion that these channels mediate mechanotransduction in vertebrates; however, genetic disruption and heterologous expression have not yet revealed a direct role for any of these candidates in somatosensory mechanotransduction. Thus, new systems are needed to define the function of these ion channels in somatosensation and to pinpoint molecules or signaling pathways that underlie mechanotransduction in vertebrates.

Sensing with TRP channels

Drosophila melanogaster flies carrying the trp (transient receptor potential) mutation are rapidly blinded by bright light, because of the absence of a Ca2+-permeable ion channel in their photoreceptors. The identification of the trp gene and the search for homologs in yeast, flies, worms, zebrafish and mammals has led to the discovery of a large superfamily of related cation channels, named TRP channels. Activation of TRP channels is highly sensitive to a variety of chemical and physical stimuli, allowing them to function as dedicated biological sensors that are essential in processes such as vision, taste, tactile sensation and hearing.

TRPV4 and the mammalian kidney

Transient receptor potential vanilloid 4 (TRPV4) was identified as the mammalian homologue of the Caenorhabditis elegans osmosensory channel protein, OSM-9. In mammals, TRPV4 is activated by a variety of stimuli including thermal stress, fatty acid metabolites, and hypotonicity. Two distinct mechanisms have been described through which TRPV4 may be activated by hypotonicity: one involves the Src family of nonreceptor protein tyrosine kinases, whereas a second is mediated via arachidonic acid metabolites. TRPV4 likely plays a role in systemic osmoregulation; accordingly, it is expressed in the blood-brain barrier-deficient osmosensory nuclei of the hypothalamus. TRPV4 is also abundantly expressed in the kidney, and its precisely demarcated distribution along the kidney tubule permits speculation about a physiological role in this tissue. TRPV4-expressing and TRPV4-negative tubule segments co-exist at all levels of the kidney, from the cortex through the inner medulla. It is conceivable that basolaterally expressed TRPV4 transmits signals arising in the interstitium (e.g, changing tonicity) to more-distal tubule segments where "fine-tuning" of the incipient urine takes place.

TRPV4 plays an evolutionary conserved role in the transduction of osmotic and mechanical stimuli in live animals

The TRPV4 ion channel, previously named vanilloid receptor-related osmotically activated channel (VR-OAC), functions in vivo in the transduction of osmotic and mechanical stimuli. In trpv4 null mice, TRPV4 was found to be necessary for the maintenance of systemic osmotic equilibrium, and for normal thresholds in response to noxious mechanical stimuli. In a Caenorhabditis elegans TRPV mutant transgenic for mammalian TRPV4, the mammalian transgene was directing the osmotic and mechanical avoidance response in the context of the ASH 'nociceptive' neurone. Molecular mechanisms of gating of TRPV4 in vivo are not known at this point and have to be determined.

TRPV channels as thermosensory receptors in epithelial cells

Temperature-sensitive transient receptor potential vanilloid (TRPV) ion channels are critical contributors to normal pain and temperature sensation and therefore represent attractive targets for pain therapy. When these channels were first discovered, most attention was focused on their potential contributions to direct thermal activation of peripheral sensory neurons. However, recent anatomical, physiological, and behavioral studies have provided evidence that TRPV channels expressed in skin epithelial cells may also contribute to thermosensation in vitro and in vivo. Here, we review these studies and speculate on possible communication mechanisms from cutaneous epithelial cells to sensory neurons.

TRPV4 as osmosensor: a transgenic approach

The transient receptor potential vanilloid 4 (TRPV4) ion channel was named initially vanilloid-receptor-related osmotically activated channel (VR-OAC). Preliminary answers to the question, "What is the function of the trpv4 gene in live animals ?" are highlighted briefly in this review. In trpv4 null mice, TRPV4 is necessary for the maintenance of osmotic equilibrium, and in Caenorhabditis elegans transgenic for mammalian TRPV4, TRPV4 directs the osmotic avoidance response in the context of the ASH "nociceptive" neuron. The molecular mechanisms of gating of TRPV4 in vivo need to be determined; in particular, whether TRPV4 in live animals is gated via phosphorylation of defined amino-acid residues or more directly through the osmotic stimulus itself.

Drosophila TRP channels

The transient receptor potential (TRP) superfamily comprises a large group of related cation channels that display surprising diversity in the specific modes of activation and cation selectivities. However, a unifying theme is that many TRP channels play important roles in sensory physiology. The superfamily includes 28 mammalian members, which are subdivided into multiple subfamilies. Each of these subfamilies is represented by at least one of the 13 members in Drosophila, suggesting common evolutionary relationships. In recent years it has become clear that TRP channels in flies and mammals participate in similar sensory modalities. These include, but are not limited to, hearing, thermosensation, and certain specialized types of vision. With the recent flurry of new studies, 9 out of the 13 TRPs have been addressed in various contexts. As a result, the repertoire of biological roles attributed to Drosophila TRPs has increased considerably and is likely to lead to many additional surprises over the next few years.

TRPV4 exhibits a functional role in cell-volume regulation

Tight regulation of the cell volume is important for the maintenance of cellular homeostasis. In a hypotonic environment, cells swell owing to osmosis. With many vertebrate cells, swelling is followed by an active reduction of volume, a process called regulatory volume decrease (RVD). A possible participant in RVD is the non-selective cation channel TRPV4, a member of the TRP superfamily that has been shown to react to hypotonic stimuli with a conductance for Ca2+. As a model for cell-volume regulation, we used a human keratinocyte cell line (HaCaT) that produces TRPV4 endogenously. When HaCaT cells were exposed to a hypotonic solution (200 mOsm) maximal swelling was followed by RVD. During swelling and volume regulation, a strong Ca2+ influx was measured. Gd3+, an inhibitor of TRPV4, blocked RVD of HaCaT cells and the accompanying rise of cytosolic Ca2+. To define the role of TRPV4 in volume regulation, a TRPV4-EGFP fusion protein was produced in CHO cells. CHO cells are unable to undergo RVD under hypotonic conditions and do not produce TRPV4 endogenously. Fluorescence imaging revealed that recombinant TRPV4 was localized to the cell membrane. Production of TRPV4 enabled CHO cells to undergo typical RVD after hypo-osmolarity-induced cell swelling. RVD of TRPV4-transfected CHO cells was significantly reduced by Gd3+ treatment or in Ca2+-free solution. Taken together, these results show a direct participation of TRPV4 in RVD.

TRP channels in mechanosensation

Channels of the TRP superfamily have sensory roles in a wide variety of receptor cells, especially in mechanosensation. In some cases, the channels appear to be directly activated by mechanical force; in others, they appear to be downstream of a messenger pathway initiated by force on a non-channel sensor. A remaining challenge for most of these mechanosensory TRPs is to clarify the specific mechanism of activation.

The mechanosensitive nature of TRPV channels

Transient receptor potential vanilloid (TRPV) channels are widely expressed in both sensory and nonsensory cells. Whereas the channels display a broad diversity to activation by chemical and physical stimuli, activation by mechanical stimuli is common to many members of this group in both lower and higher organisms. Genetic screening in Caenorhabditis elegans has demonstrated an essential role for two TRPV channels in sensory neurons. OSM-9 and OCR-2, for example, are essential for both osmosensory and mechanosensory (nose-touch) behaviors. Likewise, two Drosophila TRPV channels, NAN and IAV, have been shown to be critical for hearing by the mechanosensitive chordotonal organs located in the fly's antennae. The mechanosensitive nature of the channels appears to be conserved in higher organisms for some TRPV channels. Two vertebrate channels, TRPV2 and TRPV4, are sensitive to hypotonic cell swelling, shear stress/fluid flow (TRPV4), and membrane stretch (TRPV2). In the osmosensing neurons of the hypothalamus (circumventricular organs), TRPV4 appears to function as an osmoreceptor, or part of an osmoreceptor complex, in control of vasopressin release, whereas in inner ear hair cells and vascular baroreceptors a mechanosensory role is suggestive, but not demonstrated. Finally, in many nonsensory cells expressing TRPV4, such as vascular endothelial cells and renal tubular epithelial cells, the channel exhibits well-developed local mechanosensory transduction processes where both cell swelling and shear stress/fluid flow lead to channel activation. Hence, many TRPV channels, or combinations of TRPV channels, display a mechanosensitive nature that underlies multiple mechanosensitive processes from worms to mammals.