Functional coupling of TRPV4 cationic channel and large conductance, calcium-dependent potassium channel in human bronchial epithelial cell lines

Cough and itch: Common mechanisms of irritation in the throat and skin

Cough and itch are protective mechanisms in the body. Cough occurs as a reflex motor response to foreign body inhalation, while itch is a sensation that similarly evokes a scratch response to remove irritants from the skin. Both cough and itch can last for sustained periods, leading to debilitating chronic disorders that negatively impact quality of life. Understanding the parallels and differences between chronic cough and chronic itch may be paramount to developing novel therapeutic approaches. In this article, we identify connections in the mechanisms contributing to the complex cough and scratch reflexes and summarize potential shared therapeutic targets. An online search was performed using various search engines, including PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1983 to 2024. Articles were assessed for quality, and those relevant to the objective were analyzed and summarized. The literature demonstrated similarities in the triggers, peripheral and central nervous system processing, feedback mechanisms, immunologic mediators, and receptors involved in the cough and itch responses, with the neuronal sensitization processes exhibiting the greatest parallels between cough and itch. Given the substantial impact on quality of life, novel therapies targeting similar neuroimmune pathways may apply to both itch and cough and provide new avenues for enhancing their management.

Pharmacological activation of transient receptor potential vanilloid 4 promotes triggering of the swallowing reflex in rats

The swallowing reflex is an essential physiological reflex that allows food or liquid to pass into the esophagus from the oral cavity. Delayed triggering of this reflex is a significant health problem in patients with oropharyngeal dysphagia for which no pharmacological treatments exist. Transient receptor potential channels have recently been discovered as potential targets to facilitate triggering of the swallowing reflex. However, the ability of transient receptor potential vanilloid 4 (TRPV4) to trigger the swallowing reflex has not been studied. Here, we demonstrate the involvement of TRPV4 in triggering the swallowing reflex in rats. TRPV4 immunoreactive nerve fibers were observed in the superior laryngeal nerve (SLN)-innervated swallowing-related regions. Retrograde tracing with fluorogold revealed localization of TRPV4 on approximately 25% of SLN-afferent neurons in the nodose-petrosal-jugular ganglionic complex. Among them, approximately 49% were large, 35% medium, and 15% small-sized SLN-afferent neurons. Topical application of a TRPV4 agonist (GSK1016790A) to the SLN-innervated regions dose-dependently facilitated triggering of the swallowing reflex, with the highest number of reflexes triggered at a concentration of 250 μM. The number of agonist-induced swallowing reflexes was significantly reduced by prior topical application of a TRPV4 antagonist. These findings indicate that TRPV4 is expressed on sensory nerves innervating the swallowing-related regions, and that its activation by an agonist can facilitate swallowing. TRPV4 is a potential pharmacological target for the management of oropharyngeal dysphagia.

Modulation of transient receptor potential (TRP) channels by plant derived substances used in over-the-counter cough and cold remedies

BACKGROUND

Upper respiratory tract infections (URTIs) impact all age groups and have a significant economic and social burden on society, worldwide. Most URTIs are mild and self-limiting, but due to the wide range of possible causative agents, including Rhinovirus (hRV), Adenovirus, Respiratory Syncytial Virus (RSV), Coronavirus and Influenza, there is no single and effective treatment. Over-the-counter (OTC) remedies, including traditional medicines and those containing plant derived substances, help to alleviate symptoms including inflammation, pain, fever and cough.

PURPOSE

This systematic review focuses on the role of the major plant derived substances in several OTC remedies used to treat cold symptoms, with a particular focus on the transient receptor potential (TRP) channels involved in pain and cough.

METHODS

Literature searches were done using Pubmed and Web of Science, with no date limitations, using the principles of the PRISMA statement. The search terms used were 'TRP channel AND plant compound', 'cough AND plant compound', 'cough AND TRP channels AND plant compound', 'cough AND P2X3 AND plant compound' and 'P2X3 AND plant compound' where plant compound represents menthol or camphor or eucalyptus or turpentine or thymol.

RESULTS

The literature reviewed showed that menthol activates TRPM8 and may inhibit respiratory reflexes reducing irritation and cough. Menthol has a bimodal action on TRPA1, but inhibition may have an analgesic effect. Eucalyptus also activates TRPM8 and inhibits TRPA1 whilst down regulating P2X3, aiding in the reduction of cough, pain and airway irritation. Camphor inhibits TRPA1 and the activation of TRPM8 may add to the effects of menthol. Activation of TRPV1 by camphor, may also have an analgesic effect.

CONCLUSIONS

The literature suggests that these plant derived substances have multifaceted actions and can interact with the TRP 'cough' receptors. The plant derived substances used in cough and cold medicines have the potential to target multiple symptoms experienced during a cold.

Extracellular fluid viscosity enhances cell migration and cancer dissemination

Cells respond to physical stimuli, such as stiffness1, fluid shear stress2 and hydraulic pressure3,4. Extracellular fluid viscosity is a key physical cue that varies under physiological and pathological conditions, such as cancer5. However, its influence on cancer biology and the mechanism by which cells sense and respond to changes in viscosity are unknown. Here we demonstrate that elevated viscosity counterintuitively increases the motility of various cell types on two-dimensional surfaces and in confinement, and increases cell dissemination from three-dimensional tumour spheroids. Increased mechanical loading imposed by elevated viscosity induces an actin-related protein 2/3 (ARP2/3)-complex-dependent dense actin network, which enhances Na+/H+ exchanger 1 (NHE1) polarization through its actin-binding partner ezrin. NHE1 promotes cell swelling and increased membrane tension, which, in turn, activates transient receptor potential cation vanilloid 4 (TRPV4) and mediates calcium influx, leading to increased RHOA-dependent cell contractility. The coordinated action of actin remodelling/dynamics, NHE1-mediated swelling and RHOA-based contractility facilitates enhanced motility at elevated viscosities. Breast cancer cells pre-exposed to elevated viscosity acquire TRPV4-dependent mechanical memory through transcriptional control of the Hippo pathway, leading to increased migration in zebrafish, extravasation in chick embryos and lung colonization in mice. Cumulatively, extracellular viscosity is a physical cue that regulates both short- and long-term cellular processes with pathophysiological relevance to cancer biology.

TRP channel function in platelets and megakaryocytes: basic mechanisms and pathophysiological impact

Transient receptor potential (TRP) proteins form a superfamily of cation channels that are expressed in a wide range of tissues and cell types. During the last years, great progress has been made in understanding the molecular complexity and the functions of TRP channels in diverse cellular processes, including cell proliferation, migration, adhesion and activation. The diversity of functions depends on multiple regulatory mechanisms by which TRP channels regulate Ca²⁺ entry mechanisms and intracellular Ca²⁺ dynamics, either through membrane depolarization involving cation influx or store- and receptor-operated mechanisms. Abnormal function or expression of TRP channels results in vascular pathologies, including hypertension, ischemic stroke and inflammatory disorders through effects on vascular cells, including the components of blood vessels and platelets. Moreover, some TRP family members also regulate megakaryopoiesis and platelet production, indicating a complex role of TRP channels in pathophysiological conditions. In this review, we describe potential roles of TRP channels in megakaryocytes and platelets, as well as their contribution to diseases such as thrombocytopenia, thrombosis and stroke. We also critically discuss the potential of TRP channels as possible targets for disease prevention and treatment.

Structural and Functional Coupling of Calcium-Activated BK Channels and Calcium-Permeable Channels Within Nanodomain Signaling Complexes

Biochemical and functional studies of ion channels have shown that many of these integral membrane proteins form macromolecular signaling complexes by physically associating with many other proteins. These macromolecular signaling complexes ensure specificity and proper rates of signal transduction. The large-conductance, Ca²⁺-activated K⁺ (BK) channel is dually activated by membrane depolarization and increases in intracellular free Ca²⁺ ([Ca²⁺]_i). The activation of BK channels results in a large K⁺ efflux and, consequently, rapid membrane repolarization and closing of the voltage-dependent Ca²⁺-permeable channels to limit further increases in [Ca²⁺]_i. Therefore, BK channel-mediated K⁺ signaling is a negative feedback regulator of both membrane potential and [Ca²⁺]_i and plays important roles in many physiological processes and diseases. However, the BK channel formed by the pore-forming and voltage- and Ca²⁺-sensing α subunit alone requires high [Ca²⁺]_i levels for channel activation under physiological voltage conditions. Thus, most native BK channels are believed to co-localize with Ca²⁺-permeable channels within nanodomains (a few tens of nanometers in distance) to detect high levels of [Ca²⁺]_i around the open pores of Ca²⁺-permeable channels. Over the last two decades, advancement in research on the BK channel’s coupling with Ca²⁺-permeable channels including recent reports involving NMDA receptors demonstrate exemplary models of nanodomain structural and functional coupling among ion channels for efficient signal transduction and negative feedback regulation. We hereby review our current understanding regarding the structural and functional coupling of BK channels with different Ca²⁺-permeable channels.

Molecular characterization of transient receptor potential vanilloid 4 (TRPV4) gene transcript variant mRNA of chum salmon Oncorhynchus keta in response to salinity or temperature changes

Transient receptor potential vanilloid 4 (TRPV4) is an osmosensory cation channel that respond to an increase in cell volume and participates in various physiological functions. Among organisms in aquatic environments, euryhaline teleost is are suitable experimental models to study ion channel proteins related to physiological functions involving osmosensing. Among the studies of various regulatory molecules that mediate osmotic regulation in fish, however, information is lacking, particularly on the TRP family. This study investigated the structural characteristics of theTRPV4 gene of chum salmon (Oncorhynchus keta) and their responses to changes in salinity and temperature. Interestingly, TRPV4 generates transcript variants of the intron-retention form through alternative splicing, resulting in a frameshift leading to the generation of transcripts of different structures. In particular, TRPV4 x1 and TRPV x2 mRNAs were predominant in the gill and skin including at the lateral line. The expression levels of chum salmon TRPV4 x1 were significantly increased with increase in salinity and temperature, whereas TRPV4 x2 mainly responded to temperature decrease. Overall, these results demonstrate for the first time the effects of salinity and temperature on the expression of two salmonid TRPV4 transcript variants, suggesting their contribution to the regulation of hydromineral balance.

TRPV4 channels mediate the mechanoresponse in retinal microglia

The physiological and neurological correlates of plummeting brain osmolality during edema, traumatic CNS injury, and severe ischemia are compounded by neuroinflammation. Using multiple approaches, we investigated how retinal microglia respond to challenges mediated by increases in strain, osmotic gradients, and agonists of the stretch-activated cation channel TRPV4. Dissociated and intact microglia were TRPV4-immunoreactive and responded to the selective agonist GSK1016790A and substrate stretch with altered motility and elevations in intracellular calcium ([Ca2+ ]i ). Agonist- and hypotonicity-induced swelling was associated with a nonselective outwardly rectifying cation current, increased [Ca2+ ]i , and retraction of higher-order processes. The antagonist HC067047 reduced the extent of hypotonicity-induced microglial swelling and inhibited the suppressive effects of GSK1016790A and hypotonicity on microglial branching. Microglial TRPV4 signaling required intermediary activation of phospholipase A2 (PLA2), cytochrome P450, and epoxyeicosatrienoic acid production (EETs). The expression pattern of vanilloid thermoTrp genes in retinal microglia was markedly different from retinal neurons, astrocytes, and cortical microglia. These results suggest that TRPV4 represents a primary retinal microglial sensor of osmochallenges under physiological and pathological conditions. Its activation, associated with PLA2, modulates calcium signaling and cell architecture. TRPV4 inhibition might be a useful strategy to suppress microglial overactivation in the swollen and edematous CNS.

Structural determinants of TRPV4 inhibition and identification of new antagonists with antiviral activity

Background and Purpose

The transient receptor potential vanilloid 4 (TRPV4) cation channel participates in multiple physiological processes and is also at the core of different diseases, making this channel an interesting pharmacological target with therapeutic potential. However, little is known about the structural elements governing its inhibition.

Experimental Approach

We have now combined in silico drug discovery and molecular dynamics simulation based on Xenopus tropicalis xTRPV4 structure with functional studies measuring cell Ca²⁺ influx mediated by human TRPV4 channel to characterize the binding site of known TRPV4 inhibitors and to identify novel small molecule channel modulators.

Key Results

We have found that the inhibitor HC067047 binds to a pocket conformed by residues from S2–S3 linker (xTRPV4‐D542), S4 (xTRPV4‐M583 and Y587 and S5 (xTRPV4‐D609 and F613). This pocket was also used for structure‐based virtual screening in the search of novel channel modulators. Forty potential hits were selected based on the lower docking scores (from ~250,000 compounds) and their effect upon TRPV4 functionally tested. Three were further analysed for stability using molecular dynamics simulation and functionally tested on TRPV4 channels carrying mutations in the binding pocket. Compound NSC151066, shown to require residue xTRPV4‐M583 for its inhibitory effect, presented an IC₅₀ of 145 nM and demonstrated to be an effective antiviral against Zika virus with a potency similar to HC067047.

Conclusion and Implications

Together, we propose structural insights into the inhibition of TRPV4 and how this information can be used for the design of novel channel modulators.

LINKED ARTICLES

This article is part of a themed issue on Structure Guided Pharmacology of Membrane Proteins (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.14/issuetoc

Cytochrome P450 Epoxygenase-Dependent Activation of TRPV4 Channel Participates in Enhanced Serotonin-Induced Pulmonary Vasoconstriction in Chronic Hypoxic Pulmonary Hypertension

Transient receptor potential vanilloid 4 (TRPV4) is a multi-functional non-selective channel expressed in pulmonary vasculatures. TRPV4 contributes to serotonin- (5-HT-) induced pulmonary vasoconstriction and is responsible in part for the enhanced 5-HT response in pulmonary arteries (PAs) of chronic hypoxia mice. Epoxyeicosatrienoic acid (EET) is an endogenous agonist of TRPV4 and is known to regulate vasoreactivity. The levels of EETs, the expression of cytochrome P450 (CYP) epoxygenase for EET production, and epoxide hydrolase for EET degradation are altered by chronic hypoxia. Here, we examined the role of EET-dependent TRPV4 activation in the 5-HT-mediated PA contraction. In PAs of normoxic mice, inhibition of TRPV4 with a specific inhibitor HC-067047 caused a decrease in the sensitivity of 5-HT-induced PA contraction without affecting the maximal contractile response. Application of the cytochrome P450 epoxygenase inhibitor MS-PPOH had no effect on the vasoreactivity to 5-HT. In contrast, inhibition of CYP epoxygenase or TRPV4 both attenuated the 5-HT-elicited maximal contraction to a comparable level in PAs of chronic hypoxic mice. Moreover, the inhibitory effect of MS-PPOH on the 5-HT-induced contraction was obliterated in PAs of chronic hypoxic trpv4^−/− mice. These results suggest that TRPV4 contributes to the enhanced 5-HT-induced vasoconstriction in chronic hypoxic PAs, in part via the CYP-EET-TRPV4 pathway. Our results further support the notion that manipulation of TRPV4 function may offer a novel therapeutic strategy for the treatment of hypoxia-related pulmonary hypertension.

Modulators of Transient Receptor Potential (TRP) Channels as Therapeutic Options in Lung Disease

The lungs are essential for gas exchange and serve as the gateways of our body to the external environment. They are easily accessible for drugs from both sides, the airways and the vasculature. Recent literature provides evidence for a role of Transient Receptor Potential (TRP) channels as chemosensors and essential members of signal transduction cascades in stress-induced cellular responses. This review will focus on TRP channels (TRPA1, TRPC6, TRPV1, and TRPV4), predominantly expressed in non-neuronal lung tissues and their involvement in pathways associated with diseases like asthma, cystic fibrosis, chronic obstructive pulmonary disease (COPD), lung fibrosis, and edema formation. Recently identified specific modulators of these channels and their potential as new therapeutic options as well as strategies for a causal treatment based on the mechanistic understanding of molecular events will also be evaluated.

Cellular Mechanism Underlying Hydrogen Sulfide Mediated Epithelial K+ Secretion in Rat Epididymis

As a novel gasotransmitter, hydrogen sulfide (H₂S) elicits various physiological actions including smooth muscle relaxation and promotion of transepithelial ion transport. However, the pro-secretory function of H₂S in the male reproductive system remains largely unclear. The aim of this study is to elucidate the possible roles of H₂S in modulating rat epididymal intraluminal ionic microenvironment essential for sperm storage. The results revealed that endogenous H₂S-generating enzymes cystathionine β-synthetase (CBS) and cystathionine γ-lyase (CSE) were both expressed in rat epididymis. CBS located predominantly in epithelial cells whilst CSE expressed primarily in smooth muscle cells. The relative expression level of CBS and CSE escalated from caput to cauda regions of epididymis, which was paralleled to the progressively increasing production of endogenous H₂S. The effect of H₂S on epididymal epithelial ion transportation was investigated using short-circuit current (I_SC), measurement of intracellular ion concentration and in vivo rat epididymal microperfusion. Our data showed that H₂S induced transepithelial K⁺ secretion via adenosine triphosphate-sensitive K⁺ (K_ATP) channel and large conductance Ca²⁺-activated K⁺ (BK_Ca) channel. Transient receptor potential vanilloid 4 (TRPV4) channel-mediated Ca²⁺ influx was implicated in the activation of BK_Ca channel. In vivo studies further demonstrated that H₂S promoted K⁺ secretion in rat epididymal epithelium. Inhibition of endogenous H₂S synthesis caused a significant decrease in K⁺ concentration of cauda epididymal intraluminal fluid. Moreover, our data demonstrated that high extracellular K⁺ concentration actively depressed the motility of cauda epididymal sperm in a pH-independent manner. Collectively, the present study demonstrated that H₂S was vital to the formation of high K⁺ concentration in epididymal intraluminal fluid by promoting the transepithelial K⁺ secretion, which might contribute to the maintenance of the cauda epididymal sperm in quiescent dormant state before ejaculation.

Novel mechanisms regulating endothelial barrier function in the pulmonary microcirculation

The pulmonary epithelial and vascular endothelial cell layers provide two sequential physical and immunological barriers that together form a semi-permeable interface and prevent alveolar and interstitial oedema formation. In this review, we focus specifically on the continuous endothelium of the pulmonary microvascular bed that warrants strict control of the exchange of gases, fluid, solutes and circulating cells between the plasma and the interstitial space. The present review provides an overview of emerging molecular mechanisms that permit constant transcellular exchange between the vascular and interstitial compartment, and cause, prevent or reverse lung endothelial barrier failure under experimental conditions, yet with a clinical perspective. Based on recent findings and at times seemingly conflicting results we discuss emerging paradigms of permeability regulation by altered ion transport as well as shifts in the homeostasis of sphingolipids, angiopoietins and prostaglandins.

Transient Receptor Potential Channels and Chronic Airway Inflammatory Diseases: A Comprehensive Review

Chronic airway inflammatory diseases remain a major problem worldwide, such that there is a need for additional therapeutic targets and novel drugs. Transient receptor potential (TRP) channels are a group of non-selective cation channels expressed throughout the body that are regulated by various stimuli. TRP channels have been identified in numerous cell types in the respiratory tract, including sensory neurons, airway epithelial cells, airway smooth muscle cells, and fibroblasts. Different types of TRP channels induce cough in sensory neurons via the vagus nerve. Permeability and cytokine production are also regulated by TRP channels in airway epithelial cells, and these channels also contribute to the modulation of bronchoconstriction. TRP channels may cooperate with other TRP channels, or act in concert with calcium-dependent potassium channels and calcium-activated chloride channel. Hence, TRP channels could be the potential therapeutic targets for chronic airway inflammatory diseases. In this review, we aim to discuss the expression profiles and physiological functions of TRP channels in the airway, and the roles they play in chronic airway inflammatory diseases.

Loss of barrier integrity in alveolar epithelial cells downregulates ENaC expression and activity via Ca2+ and TRPV4 activation

The epithelial Na channel (ENaC) plays an essential role in lung physiology by modulating the amount of liquid lining the respiratory epithelium. Here, we tested the effect of breaking alveolar epithelial cell barrier integrity on ENaC expression and function. We found that either mechanical wounding by scratching the monolayer or disruption of tight junction with EDTA induced a ~ 50% decrease of α,β and γENaC mRNA expression and an 80% reduction of ENaC short-circuit current (I_sc) at 6 h. Scratching the cell monolayer generated a Ca²⁺ wave that spread from the margin of the scratch to distant cells. Pretreatment with BAPTA-AM, an intracellular Ca²⁺ chelator, abolished the effect of mechanical wounding and EDTA on αENaC mRNA expression, suggesting that [Ca²⁺]_i is important for this modulation. We tested the hypothesis that a mechanosensitive channel such as TRPV4, a cationic channel known to increase [Ca²⁺]_i, could mediate this effect. Activation of the channel with the TRPV4 specific agonist GSK-1016790A (GSK) decreased αENAC mRNA expression and almost completely abolished ENaC I_sc. Pretreatment of alveolar epithelial cells with HC-067047 (HC0), a specific TRPV4 antagonist, reduced the extent of αENAC mRNA downregulation by mechanical wounding and EDTA. Altogether, our results suggest that mechanical stress induced by wounding or TRPV4-mediated loss of tight junction increases [Ca²⁺]_i and elicits a Ca²⁺ wave that affects ENaC expression and function away from the site of injury. These data are important to better understand how Ca²⁺ signaling affects lung liquid clearance in injured lungs.

The TRPV4 channel links calcium influx to DDX3X activity and viral infectivity

Ion channels are well placed to transduce environmental cues into signals used by cells to generate a wide range of responses, but little is known about their role in the regulation of RNA metabolism. Here we show that the TRPV4 cation channel binds the DEAD-box RNA helicase DDX3X and regulates its function. TRPV4-mediated Ca²⁺ influx releases DDX3X from the channel and drives DDX3X nuclear translocation, a process that involves calmodulin (CaM) and the CaM-dependent kinase II. Genetic depletion or pharmacological inhibition of TRPV4 diminishes DDX3X-dependent functions, including nuclear viral export and translation. Furthermore, TRPV4 mediates Ca²⁺ influx and nuclear accumulation of DDX3X in cells exposed to the Zika virus or the purified viral envelope protein. Consequently, targeting of TRPV4 reduces infectivity of dengue, hepatitis C and Zika viruses. Together, our results highlight the role of TRPV4 in the regulation of DDX3X-dependent control of RNA metabolism and viral infectivity.

The ion channel TRPV4 senses many environmental cues, but its role in virus infection is not known. Here, Doñate-Macián et al. show that Zika virus induces TRPV4-mediated Ca²⁺ influx into cells, resulting in the nuclear accumulation of the DDX3X RNA helicase, which increases virus replication.

Inhibition of mucin secretion via glucocorticoid-induced regulation of calcium-related proteins in mouse lung

Calcium is important for physiological functioning in many tissues and is essential in mucus secretion and muscle contraction. Intracellular concentrations of calcium are regulated by calcium-related proteins, such as transient receptor potential cation channel subfamily V member 4 (TRPV 4), TRPV6, Calbindin-D9k (CaBP-9k), sodium-calcium exchanger (NCX1), and plasma membrane Ca2+ ATPase 1 (PMCA1). In this study, the relationship between secretion of pulmonary mucus and calcium regulation was investigated. To confirm the effect of steroid hormones, immature mice were injected with estrogen (E2) or progesterone (P4), and mature mice were injected with dexamethasone (DEX). Subsequently, the location and expression of TRPV4, TRPV6, CaBP-9k, NCX1, and PMCA1 in lung tissue were examined. Periodic acid-Schiff staining was performed to investigate functional aspects of the protein expression. There were no significant differences in calcium-related gene expression in E2- and P4-treated mice, but TRPV4, NCX1, and PMCA1 were increased in DEX-treated mice and were recovered by RU486 treatment. DEX induces the expression of calcium-related proteins through the glucocorticoid receptor-mediated pathway and may involve decreased mucin secretion in the bronchiole. TRPV4, TRPV6, CaBP-9k, NCX1, and PMCA1 were specifically expressed in Clara and alveolar type 2 cells of mouse lung. CC10, a marker of Clara cells, was decreased by DEX. In addition, mucin secretion, which is a functional aspect of this cell, was also decreased by DEX treatment. Control of calcium-related gene expression may affect the control of mucus secretion in the lung. Such a control mechanism can form the basis of studies into diseases such as inflammation attributable to mucus secretion abnormalities, coughing, and respiratory disorders and distress.

TRPV4: a Sensor for Homeostasis and Pathological Events in the CNS

Transient receptor potential vanilloid type 4 (TRPV4) was originally described as a calcium-permeable nonselective cation channel. TRPV4 is now recognized as a polymodal ionotropic receptor: it is a broadly expressed, nonselective cation channel (permeable to calcium, potassium, magnesium, and sodium) that plays an important role in a multitude of physiological processes. TRPV4 is involved in maintaining homeostasis, serves as an osmosensor and thermosensor, can be activated directly by endogenous or exogenous chemical stimuli, and can be activated or sensitized indirectly via intracellular signaling pathways. Additionally, TRPV4 is upregulated in a variety of pathological conditions. In this review, we focus on the role of TRPV4 in mediating homeostasis and pathological events in the central nervous system (CNS). This review is composed of three parts. Section 1 describes the role of TRPV4 in maintaining homeostatic processes, including the volume of body water, ionic concentrations, volume, and the temperature. Section 2 describes the effects of activation and inhibition of TRPV4 in the CNS. Section 3 focuses on the role of TRPV4 during pathological events in CNS.

The role of TRPV4 in fibrosis

The transient receptor potential vanilloid 4 (TRPV4) is a highly Ca²⁺-permeable non-selective cation channel in TRPV family. Accumulating evidence hints that TRPV4 play a significant role in a wide diversity of pathologic changes. Fibrosis is a kind of chronic disease which was characterized by the formation of excessive accumulation of extracellular matrix (ECM) components in tissues and organs. In recent years, a growing body of studies showed that TRPV4 acted as a crucial regulator in the progression of fibrosis including myocardial fibrosis, cystic fibrosis, pulmonary fibrosis, hepatic fibrosis and pancreatic fibrosis, suggesting TRPV4 may be a potential therapeutic vehicle in fibrotic diseases. However, the mechanisms by which TRPV4 regulates fibrosis are still undefined. In this review, firstly, we intend to sum up the collective knowledge of TRPV4. Then we provided the latent mechanism between TRPV4 and fibrosis. We also elaborated the distinct signaling pathways focus on TRPV4 with fibrosis. Finally, we discussed its potential as a novel therapeutic target for fibrosis.

Silica nanoparticles inhibit the cation channel TRPV4 in airway epithelial cells

BACKGROUND

Silica nanoparticles (SiNPs) have numerous beneficial properties and are extensively used in cosmetics and food industries as anti-caking, densifying and hydrophobic agents. However, the increasing exposure levels experienced by the general population and the ability of SiNPs to penetrate cells and tissues have raised concerns about possible toxic effects of this material. Although SiNPs are known to affect the function of the airway epithelium, the molecular targets of these particles remain largely unknown. Given that SiNPs interact with the plasma membrane of epithelial cells we hypothesized that they may affect the function of Transient Receptor Potential Vanilloid 4 (TRPV4), a cation-permeable channel that regulates epithelial barrier function. The main aims of this study were to evaluate the effects of SiNPs on the activation of TRPV4 and to determine whether these alter the positive modulatory action of this channel on the ciliary beat frequency in airway epithelial cells.

RESULTS

Using fluorometric measurements of intracellular Ca²⁺ concentration ([Ca²⁺]_i) we found that SiNPs inhibit activation of TRPV4 by the synthetic agonist GSK1016790A in cultured human airway epithelial cells 16HBE and in primary cultured mouse tracheobronchial epithelial cells. Inhibition of TRPV4 by SiNPs was confirmed in intracellular Ca²⁺ imaging and whole-cell patch-clamp experiments performed in HEK293T cells over-expressing this channel. In addition to these effects, SiNPs were found to induce a significant increase in basal [Ca²⁺]_i, but in a TRPV4-independent manner. SiNPs enhanced the activation of the capsaicin receptor TRPV1, demonstrating that these particles have a specific inhibitory action on TRPV4 activation. Finally, we found that SiNPs abrogate the increase in ciliary beat frequency induced by TRPV4 activation in mouse airway epithelial cells.

CONCLUSIONS

Our results show that SiNPs inhibit TRPV4 activation, and that this effect may impair the positive modulatory action of the stimulation of this channel on the ciliary function in airway epithelial cells. These findings unveil the cation channel TRPV4 as a primary molecular target of SiNPs.

Structural determinants of 5',6'-epoxyeicosatrienoic acid binding to and activation of TRPV4 channel

TRPV4 cation channel activation by cytochrome P450-mediated derivatives of arachidonic acid (AA), epoxyeicosatrienoic acids (EETs), constitute a major mechanisms of endothelium-derived vasodilatation. Besides, TRPV4 mechano/osmosensitivity depends on phospholipase A₂ (PLA₂) activation and subsequent production of AA and EETs. However, the lack of evidence for a direct interaction of EETs with TRPV4 together with claims of EET-independent mechanical activation of TRPV4 has cast doubts on the validity of this mechanism. We now report: 1) The identification of an EET-binding pocket that specifically mediates TRPV4 activation by 5',6'-EET, AA and hypotonic cell swelling, thereby suggesting that all these stimuli shared a common structural target within the TRPV4 channel; and 2) A structural insight into the gating of TRPV4 by a natural agonist (5',6'-EET) in which K535 plays a crucial role, as mutant TRPV4-K535A losses binding of and gating by EET, without affecting GSK1016790A, 4α-phorbol 12,13-didecanoate and heat mediated channel activation. Together, our data demonstrates that the mechano- and osmotransducing messenger EET gates TRPV4 by a direct action on a site formed by residues from the S2-S3 linker, S4 and S4-S5 linker.

TRPV4: Molecular Conductor of a Diverse Orchestra

Transient receptor potential vanilloid type 4 (TRPV4) is a calcium-permeable nonselective cation channel, originally described in 2000 by research teams led by Schultz (Nat Cell Biol 2: 695-702, 2000) and Liedtke (Cell 103: 525-535, 2000). TRPV4 is now recognized as being a polymodal ionotropic receptor that is activated by a disparate array of stimuli, ranging from hypotonicity to heat and acidic pH. Importantly, this ion channel is constitutively expressed and capable of spontaneous activity in the absence of agonist stimulation, which suggests that it serves important physiological functions, as does its widespread dissemination throughout the body and its capacity to interact with other proteins. Not surprisingly, therefore, it has emerged more recently that TRPV4 fulfills a great number of important physiological roles and that various disease states are attributable to the absence, or abnormal functioning, of this ion channel. Here, we review the known characteristics of this ion channel's structure, localization and function, including its activators, and examine its functional importance in health and disease.

Fluid-induced lung injury-role of TRPV4 channels

Administration of bolus intravenous fluid is associated with respiratory dysfunction and increased mortality, findings with no clear mechanistic explanation. The objective of this study was to examine whether bolus intravenous (i.v.) fluid administration results in acute lung injury in a rat model and further, to examine whether this injury is associated with transient receptor potential vallinoid (TRPV)4 channel function and endothelial inflammatory response. Healthy male Sprague-Dawley rats were administered 60 ml/kg 0.9% saline i.v. over 30 min. Manifestation of acute lung injury was assessed by lung physiology, morphology, and markers of inflammation. The role of TRPV4 channels in fluid-induced lung injury was subsequently examined by the administration of ruthenium red (RR) in this established rat model and again in TRPV4 KO mice. In endothelial cell culture, permeability and P-selectin expression were measured following TRPV4 agonist with and without antagonist; 0.9% saline resulted in an increase in lung water, lavage protein and phospholipase A₂, and plasma angiopoietin-2, with worsening in arterial blood oxygen (PaO₂), lung elastance, surfactant activity, and lung histological injury score. These effects were ameliorated following i.v. fluid in rats receiving RR. TRPV4 KO mice did not develop lung edema. Expression of P-selectin increased in endothelial cells following administration of a TRPV4 agonist, which was ameliorated by simultaneous addition of RR. Bolus i.v. 0.9% saline resulted in permeability pulmonary edema. Data from ruthenium red, TRPV4 KO mice, and endothelial cell culture suggest activation of TRPV4 and release of angiopoietin 2 and P-selectin as the central mechanism.

Emerging roles of calcium-activated K channels and TRPV4 channels in lung oedema and pulmonary circulatory collapse

It has been suggested that the transient receptor potential cation (TRP) channel subfamily V (vanilloid) type 4 (TRPV4) and intermediate conductance calcium-activated potassium (KCa3.1) channels contribute to endothelium-dependent vasodilation. Here, we summarize very recent evidence for a synergistic interplay of TRPV4 and KCa3.1 channels in lung disease. Among the endothelial Ca²⁺ -permeable TRPs, TRPV4 is best characterized and produces arterial dilation by stimulating Ca²⁺ -dependent nitric oxide synthesis and endothelium-dependent hyperpolarization. Besides these roles, some TRP channels control endothelial/epithelial barrier functions and vascular integrity, while KCa3.1 channels provide the driving force required for Cl^- and water transport in some cells and most secretory epithelia. The three conditions, increased pulmonary venous pressure caused by left heart disease, high inflation pressure and chemically induced lung injury, may lead to activation of TRPV4 channels followed by Ca²⁺ influx leading to activation of KCa3.1 channels in endothelial cells ultimately leading to acute lung injury. We find that a deficiency in KCa3.1 channels protects against TRPV4-induced pulmonary arterial relaxation, fluid extravasation, haemorrhage, pulmonary circulatory collapse and cardiac arrest in vivo. These data identify KCa3.1 channels as crucial molecular components in downstream TRPV4 signal transduction and as a potential target for the prevention of undesired fluid extravasation, vasodilatation and pulmonary circulatory collapse.

Obesogenic and Diabetogenic Effects of High-Calorie Nutrition Require Adipocyte BK Channels

Elevated adipose tissue expression of the Ca²⁺- and voltage-activated K⁺ (BK) channel was identified in morbidly obese men carrying a BK gene variant, supporting the hypothesis that K⁺ channels affect the metabolic responses of fat cells to nutrients. To establish the role of endogenous BKs in fat cell maturation, storage of excess dietary fat, and body weight (BW) gain, we studied a gene-targeted mouse model with global ablation of the BK channel (BK^L1/L1) and adipocyte-specific BK-deficient (adipoqBK^L1/L2) mice. Global BK deficiency afforded protection from BW gain and excessive fat accumulation induced by a high-fat diet (HFD). Expansion of white adipose tissue-derived epididymal BK^L1/L1 preadipocytes and their differentiation to lipid-filled mature adipocytes in vitro, however, were improved. Moreover, BW gain and total fat masses of usually superobese ob/ob mice were significantly attenuated in the absence of BK, together supporting a central or peripheral role for BKs in the regulatory system that controls adipose tissue and weight. Accordingly, HFD-fed adipoqBK^L1/L2 mutant mice presented with a reduced total BW and overall body fat mass, smaller adipocytes, and reduced leptin levels. Protection from pathological weight gain in the absence of adipocyte BKs was beneficial for glucose handling and related to an increase in body core temperature as a result of higher levels of uncoupling protein 1 and a low abundance of the proinflammatory interleukin-6, a common risk factor for diabetes and metabolic abnormalities. This suggests that adipocyte BK activity is at least partially responsible for excessive BW gain under high-calorie conditions, suggesting that BK channels are promising drug targets for pharmacotherapy of metabolic disorders and obesity.

Channels and Volume Changes in the Life and Death of the Cell

Volume changes deviating from original cell volume represent a major challenge for cellular homeostasis. Cell volume may be altered either by variations in the external osmolarity or by disturbances in the transmembrane ion gradients that generate an osmotic imbalance. Cells respond to anisotonicity-induced volume changes by active regulatory mechanisms that modify the intracellular/extracellular concentrations of K(+), Cl(-), Na(+), and organic osmolytes in the direction necessary to reestablish the osmotic equilibrium. Corrective osmolyte fluxes permeate across channels that have a relevant role in cell volume regulation. Channels also participate as causal actors in necrotic swelling and apoptotic volume decrease. This is an overview of the types of channels involved in either corrective or pathologic changes in cell volume. The review also underlines the contribution of transient receptor potential (TRP) channels, notably TRPV4, in volume regulation after swelling and describes the role of other TRPs in volume changes linked to apoptosis and necrosis. Lastly we discuss findings showing that multimers derived from LRRC8A (leucine-rich repeat containing 8A) gene are structural components of the volume-regulated Cl(-) channel (VRAC), and we underline the intriguing possibility that different heteromer combinations comprise channels with different intrinsic properties that allow permeation of the heterogenous group of molecules acting as organic osmolytes.

Modulation of TRPV4 by diverse mechanisms

Transient receptor potential ion channels (TRP) are a superfamily of non-selective ion channels which are opened in response to a diverse range of stimuli. The TRP vanilloid 4 (TRPV4) ion channel is opened in response to heat, mechanical stimuli, hypo-osmolarity and arachidonic acid metabolites. However, recently TRPV4 has been identified as an ion channel that is modulated by, and opened by intracellular signalling cascades from other receptors and signalling pathways. Although TRPV4 knockout mice show relatively mild phenotypes, some mutations in TRPV4 cause severe developmental abnormalities, such as the skeletal dyplasia and arthropathy. Regulated TRPV4 function is also essential for healthy cardiovascular system function as a potent agonist compromises endothelial cell function, leading to vascular collapse. A better understanding of the signalling mechanisms that modulate TRPV4 function is necessary to understand its physiological roles. Post translational modification of TRPV4 by kinases and other signalling molecules can modulate TRPV4 opening in response to stimuli such as mechanical and hyposmolarity and there is an emerging area of research implicating TRPV4 as a transducer of these signals as opposed to a direct sensor of the stimuli. Due to its wide expression profile, TRPV4 is implicated in multiple pathophysiological states. TRPV4 contributes to the sensation of pain due to hypo-osmotic stimuli and inflammatory mechanical hyperalsgesia, where TRPV4 sensitizaton by intracellular signalling leads to pain behaviors in mice. In the vasculature, TRPV4 is a regulator of vessel tone and is implicated in hypertension and diabetes due to endothelial dysfunction. TRPV4 is a key regulator of epithelial and endothelial barrier function and signalling to and opening of TRPV4 can disrupt these critical protective barriers. In respiratory function, TRPV4 is involved in cystic fibrosis, cilary beat frequency, bronchoconstriction, chronic obstructive pulmonary disease, pulmonary hypertension, acute lung injury, acute respiratory distress syndrome and cough.In this review we highlight how modulation of TRPV4 opening is a vital signalling component in a range of tissues and why understanding of TRPV4 regulation in the body may lead to novel therapeutic approaches to treating a range of disease states.

TRP functions in the broncho-pulmonary system

The current understanding of the role of transient receptor potential (TRP) channels in the airways and lung was initially based on the localization of a series of such channels in a subset of sensory nerve fibers of the respiratory tract. Soon after, TRP channel expression and function have been identified in respiratory nonneuronal cells. In these two locations, TRPs regulate physiological processes aimed at integrating different stimuli to maintain homeostasis and to react to harmful agents and tissue injury by building up inflammatory responses and repair processes. There is no doubt that TRPs localized in the sensory network contribute to airway neurogenic inflammation, and emerging evidence underlines the role of nonneuronal TRPs in orchestrating inflammation and repair in the respiratory tract. However, recent basic and clinical studies have offered clues regarding the contribution of neuronal and nonneuronal TRPs in the mechanism of asthma, chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, cough, and other respiratory diseases.

How does rhinovirus cause the common cold cough?

Cough is a protective reflex to prevent aspiration and can be triggered by a multitude of stimuli. The commonest form of cough is caused by upper respiratory tract infection and has no benefit to the host. The virus hijacks this natural defence mechanism in order to propagate itself through the population. Despite the resolution of the majority of cold symptoms within 2 weeks, cough can persist for some time thereafter. Unfortunately, the mechanism of infectious cough brought on by pathogenic viruses, such as human rhinovirus, during colds, remains elusive despite the extensive work that has been undertaken. For socioeconomic reasons, it is imperative we identify the mechanism of cough. There are several theories which have been proposed as the causative mechanism of cough in rhinovirus infection, encompassing a range of different processes. Those of which hold most promise are physical disruption of the epithelial lining, excess mucus production and an inflammatory response to rhinovirus infection which may be excessive. And finally, neuronal modulation, the most convincing hypothesis, is thought to potentiate cough long after the original stimulus has been cleared. All these hypotheses will be briefly covered in the following sections.

Genetic deficit of KCa 3.1 channels protects against pulmonary circulatory collapse induced by TRPV4 channel activation

BACKGROUND AND PURPOSE

The intermediate conductance calcium/calmodulin-regulated K⁺ channel K_Ca 3.1 produces hyperpolarizing K⁺ currents that counteract depolarizing currents carried by transient receptor potential (TRP) channels, and provide the electrochemical driving force for Cl^- and fluid movements. We investigated whether a deficiency in K_Ca 3.1 (K_Ca 3.1^-/- ) protects against fatal pulmonary circulatory collapse in mice after pharmacological activation of the calcium-permeable TRP subfamily vanilloid type 4 (TRPV4) channels.

EXPERIMENTAL APPROACH

An opener of TRPV4 channels, GSK1016790A, was infused in wild-type (wt) and K_Ca 3.1^-/- mice; haemodynamic parameters, histology and pulmonary vascular reactivity were measured; and patch clamp was performed on pulmonary arterial endothelial cells (PAEC).

KEY RESULTS

In wt mice, GSK1016790A decreased right ventricular and systemic pressure leading to a fatal circulatory collapse that was accompanied by increased protein permeability, lung haemorrhage and fluid extravasation. In contrast, K_Ca 3.1^-/- mice exhibited a significantly smaller drop in pressure to GSK1016790A infusion, no haemorrhage and fluid water extravasation, and the mice survived. Moreover, the GSK1016790A-induced relaxation of pulmonary arteries of K_Ca 3.1^-/- mice was significantly less than that of wt mice. GSK1016790A induced TRPV4 currents in PAEC from wt and K_Ca 3.1^-/- mice, which co-activated K_Ca 3.1 and disrupted membrane resistance in wt PAEC, but not in K_Ca 3.1^-/- PAEC.

CONCLUSIONS AND IMPLICATIONS

Our findings show that a genetic deficiency of K_Ca 3.1 channels prevented fatal pulmonary circulatory collapse and reduced lung damage caused by pharmacological activation of calcium-permeable TRPV4 channels. Therefore, inhibition of K_Ca 3.1channels may have therapeutic potential in conditions characterized by abnormal high endothelial calcium signalling, barrier disruption, lung oedema and pulmonary circulatory collapse.

TRPV4: physiological role and therapeutic potential in respiratory diseases

Members of the family of transient receptor potential (TRP) channels have been implicated in the pathophysiology of a host of lung diseases. The role of these multimodal cation channels in lung homeostasis is thought to stem from their ability to respond to changes in mechanical stimuli (i.e., shear and stretch), as well as to various protein and lipid mediators. The vanilloid subfamily member, TRPV4, which is highly expressed in the majority of lung cell types, is well positioned for critical involvement in several pulmonary conditions, including edema formation, control of pulmonary vascular tone, and the lung response to local or systemic inflammatory insults. In recent years, several pharmacological inhibitors of TRPV4 have been developed, and the current generation of compounds possess high affinity and specificity for TRPV4. As such, we have now entered a time where the therapeutic potential of TRPV4 inhibitors can be systematically examined in a variety of lung diseases. Due to this fact, this review seeks to describe the current state of the art with respect to the role of TRPV4 in pulmonary homeostasis and disease, and to highlight the current and future roles of TRPV4 inhibitors in disease treatment. We will first focus on genera aspects of TRPV4 structure and function, and then will discuss known roles for TRPV4 in pulmonary diseases, including pulmonary edema formation, pulmonary hypertension, and acute lung injury. Finally, both promising aspects and potential pitfalls of the clinical use of TRPV4 inhibitors will be examined.

Effect of osmotic stress on the expression of TRPV4 and BKCa channels and possible interaction with ERK1/2 and p38 in cultured equine chondrocytes

The metabolic activity of articular chondrocytes is influenced by osmotic alterations that occur in articular cartilage secondary to mechanical load. The mechanisms that sense and transduce mechanical signals from cell swelling and initiate volume regulation are poorly understood. The purpose of this study was to investigate how the expression of two putative osmolyte channels [transient receptor potential vanilloid 4 (TRPV4) and large-conductance Ca2+-activated K+ (BKCa)] in chondrocytes is modulated in different osmotic conditions and to examine a potential role for MAPKs in this process. Isolated equine articular chondrocytes were subjected to anisosmotic conditions, and TRPV4 and BKCa channel expression and ERK1/2 and p38 MAPK protein phosphorylation were investigated using Western blotting. Results indicate that the TRPV4 channel contributes to the early stages of hypo-osmotic stress, while the BKCa channel is involved in responding to elevated intracellular Ca2+ and mediating regulatory volume decrease. ERK1/2 is phosphorylated by hypo-osmotic stress ( P < 0.001), and p38 MAPK is phosphorylated by hyperosmotic stress ( P < 0.001). In addition, this study demonstrates the importance of endogenous ERK1/2 phosphorylation in TRPV4 channel expression, where blocking ERK1/2 by a specific inhibitor (PD98059) prevented increased levels of the TRPV4 channel in cells exposed to hypo-osmotic stress and decreased TRPV4 channel expression to below control levels in iso-osmotic conditions ( P < 0.001).

Transient receptor potential (TRP) channels in the airway: role in airway disease

Over the last few decades, there has been an explosion of scientific publications reporting the many and varied roles of transient receptor potential (TRP) ion channels in physiological and pathological systems throughout the body. The aim of this review is to summarize the existing literature on the role of TRP channels in the lungs and discuss what is known about their function under normal and diseased conditions. The review will focus mainly on the pathogenesis and symptoms of asthma and chronic obstructive pulmonary disease and the role of four members of the TRP family: TRPA1, TRPV1, TRPV4 and TRPM8. We hope that the article will help the reader understand the role of TRP channels in the normal airway and how their function may be changed in the context of respiratory disease.

KCa and Ca(2+) channels: the complex thought

Potassium channels belong to the largest and the most diverse super-families of ion channels. Among them, Ca(2+)-activated K(+) channels (KCa) comprise many members. Based on their single channel conductance they are divided into three subfamilies: big conductance (BKCa), intermediate conductance (IKCa) and small conductance (SKCa; SK1, SK2 and SK3). Ca(2+) channels are divided into two main families, voltage gated/voltage dependent Ca(2+) channels and non-voltage gated/voltage independent Ca(2+) channels. Based on their electrophysiological and pharmacological properties and on the tissue where there are expressed, voltage gated Ca(2+) channels (Cav) are divided into 5 families: T-type, L-type, N-type, P/Q-type and R-type Ca(2+). Non-voltage gated Ca(2+) channels comprise the TRP (TRPC, TRPV, TRPM, TRPA, TRPP, TRPML and TRPN) and Orai (Orai1 to Orai3) families and their partners STIM (STIM1 to STIM2). A depolarization is needed to activate voltage-gated Ca(2+) channels while non-voltage gated Ca(2+) channels are activated by Ca(2+) depletion of the endoplasmic reticulum stores (SOCs) or by receptors (ROCs). These two Ca(2+) channel families also control constitutive Ca(2+) entries. For reducing the energy consumption and for the fine regulation of Ca(2+), KCa and Ca(2+) channels appear associated as complexes in excitable and non-excitable cells. Interestingly, there is now evidence that KCa-Ca(2+) channel complexes are also found in cancer cells and contribute to cancer-associated functions such as cell proliferation, cell migration and the capacity to develop metastases. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

The TRPV4 channel

The widely distributed TRPV4 cationic channel participates in the transduction of both physical (osmotic, mechanical, and heat) and chemical (endogenous, plant-derived, and synthetic ligands) stimuli. In this chapter we will review TRPV4 expression, biophysics, structure, regulation, and interacting partners as well as physiological and pathological insights obtained in TRPV4 animal models and human genetic studies.

TRPV4-mediated calcium influx and ciliary activity in human native airway epithelial cells

The transient receptor potential vanilloid 4 (TRPV4) is a calcium permeable ion channel expressed in airway epithelial cells. Based on studies of cell lines and animals, TRPV4 has been suggested to play a role in the regulation of ciliary beat frequency (CBF). Whether the same is true for human ciliated epithelial cells is not known. Therefore, the aim was to examine the expression and function of TRPV4 in human native nasal epithelial cells. Expression of TRPV4 mRNA in nasal epithelial cells and in the cell lines BEAS2B and 16HBE was confirmed by quantitative real-time PCR. A marked apical TRPV4 immunoreactivity was observed in nasal epithelial cells using immunocytochemistry. Responses to pharmacological modulation of TRPV4 were assessed with calcium imaging and CBF measurements. The TRPV4 agonist GSK1016790A produced concentration-dependent calcium responses in TRPV4-expressing HEK293, BEAS2B and 16HBE cells, and the TRPV4 antagonist HC067047 caused a rightward shift of the GSK1016790A concentration-response curves. Nasal epithelial cells responded to the TRPV4 agonist GSK1016790A with increased intracellular calcium signals and increased CBF, followed by cessation of ciliary beating and cell death. These effects were prevented or inhibited by the TRPV4 antagonist HC067047, the TRP channel blocker ruthenium red or removal of extracellular calcium. We conclude that TRPV4 is expressed in human primary nasal epithelial cells and modulates epithelial calcium levels and CBF. Thus, TRPV4 may participate in mucociliary clearance and airway protection. However, exaggerated activation of TRPV4 may result in epithelial cell death.

Chondrocyte channel transcriptomics: do microarray data fit with expression and functional data?

To date, a range of ion channels have been identified in chondrocytes using a number of different techniques, predominantly electrophysiological and/or biomolecular; each of these has its advantages and disadvantages. Here we aim to compare and contrast the data available from biophysical and microarray experiments. This letter analyses recent transcriptomics datasets from chondrocytes, accessible from the European Bioinformatics Institute (EBI). We discuss whether such bioinformatic analysis of microarray datasets can potentially accelerate identification and discovery of ion channels in chondrocytes. The ion channels which appear most frequently across these microarray datasets are discussed, along with their possible functions. We discuss whether functional or protein data exist which support the microarray data. A microarray experiment comparing gene expression in osteoarthritis and healthy cartilage is also discussed and we verify the differential expression of 2 of these genes, namely the genes encoding large calcium-activated potassium (BK) and aquaporin channels.

TRPV4 and the regulation of vascular tone

Recent studies have introduced the importance of transient receptor potential vanilloid subtype 4 (TRPV4) channels in the regulation of vascular tone. TRPV4 channels are expressed in both endothelium and vascular smooth muscle cells and can be activated by numerous stimuli including mechanical (eg, shear stress, cell swelling, and heat) and chemical (eg, epoxyeicosatrienoic acids, endocannabinoids, and 4α-phorbol esters). In the brain, TRPV4 channels are primarily localized to astrocytic endfeet processes, which wrap around blood vessels. Thus, TRPV4 channels are strategically localized to sense hemodynamic changes and contribute to the regulation of vascular tone. TRPV4 channel activation leads to smooth muscle cell hyperpolarization and vasodilation. Here, we review recent findings on the cellular mechanisms underlying TRPV4-mediated vasodilation; TRPV4 channel interaction with other proteins including transient receptor potential channel 1, small conductance (K(Ca)2.3), and large conductance (K(Ca)1.1) calcium-activated potassium-selective channels; and the importance of caveolin-rich domains for these interactions to take place.

Phosphatidylinositol-4,5-biphosphate-dependent rearrangement of TRPV4 cytosolic tails enables channel activation by physiological stimuli

Most transient receptor potential (TRP) channels are regulated by phosphatidylinositol-4,5-biphosphate (PIP2), although the structural rearrangements occurring on PIP2 binding are currently far from clear. Here we report that activation of the TRP vanilloid 4 (TRPV4) channel by hypotonic and heat stimuli requires PIP2 binding to and rearrangement of the cytosolic tails. Neutralization of the positive charges within the sequence (121)KRWRK(125), which resembles a phosphoinositide-binding site, rendered the channel unresponsive to hypotonicity and heat but responsive to 4α-phorbol 12,13-didecanoate, an agonist that binds directly to transmembrane domains. Similar channel response was obtained by depletion of PIP2 from the plasma membrane with translocatable phosphatases in heterologous expression systems or by activation of phospholipase C in native ciliated epithelial cells. PIP2 facilitated TRPV4 activation by the osmotransducing cytosolic messenger 5'-6'-epoxyeicosatrienoic acid and allowed channel activation by heat in inside-out patches. Protease protection assays demonstrated a PIP2-binding site within the N-tail. The proximity of TRPV4 tails, analyzed by fluorescence resonance energy transfer, increased by depleting PIP2 mutations in the phosphoinositide site or by coexpression with protein kinase C and casein kinase substrate in neurons 3 (PACSIN3), a regulatory molecule that binds TRPV4 N-tails and abrogates activation by cell swelling and heat. PACSIN3 lacking the Bin-Amphiphysin-Rvs (F-BAR) domain interacted with TRPV4 without affecting channel activation or tail rearrangement. Thus, mutations weakening the TRPV4-PIP2 interacting site and conditions that deplete PIP2 or restrict access of TRPV4 to PIP2--in the case of PACSIN3--change tail conformation and negatively affect channel activation by hypotonicity and heat.

P. aeruginosa LPS stimulates calcium signaling and chloride secretion via CFTR in human bronchial epithelial cells

BACKGROUND

Pseudomonas aeruginosa airway infection is associated with a high mortality rate in cystic fibrosis. Lipopolysaccharide (LPS), a main constituent of the outer membrane of P. aeruginosa, is responsible for activation of innate immune response but its role on airway epithelium ion transport, is not well known. The aim of this study was to determine the role for P. aeruginosa LPS in modulating chloride secretion and intracellular calcium in the human bronchial epithelial cell line, 16HBE14o-.

METHODS

We used intracellular calcium imaging and short-circuit current measurement upon exposure of cells to P. aeruginosa LPS.

RESULTS

Apical LPS stimulated intracellular calcium release and calcium entry and enhanced chloride secretion. This latter effect was significantly inhibited by CFTR(inh)-172 and BAPTA-AM (intracellular Ca(2+) chelator).

CONCLUSIONS

Our data provides evidence for a new role of P. aeruginosa LPS in stimulating calcium entry and release and a subsequent chloride secretion via CFTR in human bronchial epithelium.

The negative feedback regulation of TRPV4 Ca2+ ion channel function by its C-terminal cytoplasmic domain

The transient receptor potential vanilloid 4 (TRPV4) cation channel, a member of the TRP vanilloid subfamily, is expressed in a broad range of tissues where it participates in the generation of a Ca(2+) signal and/or depolarization of the membrane potential. Regulation of the abundance of TRPV4 at the cell surface is critical in osmo- and mechanotransduction. In this review, we discussed that the potential effect of Ca(2+) occurs via its action at an intracellular site in the C-terminus of the channel protein by the effect of the modulation on TRPV4 (such as 824 Ser residue phosphorylation), and its regulation for TRPV4 functions related with cell surface spread, wound healing or its polarity reorientation through its differential affinity with actin or tubulin.

Advanced in vitro approach to study neurovascular coupling mechanisms in the brain microcirculation

An understanding of the signalling events underlying neurovascular coupling mechanisms in the brain is a crucial step in the development of novel therapeutic approaches for the treatment of cerebrovascular-associated disorders. In this study we present an enhanced in vitro brain slice preparation from male Wistar rat cortical slices that incorporates haemodynamic variables (flow and pressure) into parenchymal arterioles resulting in the development of myogenic tone (28% from maximum dilatation). Moreover, we characterized flow-induced vascular responses, resulting in various degrees of vasoconstrictions and the response to 10 mM K(+) or astrocytic activation with the mGluR agonist, t-ACPD (100 μM), resulting in vasodilatations of 33.6±4.7% and 38.6±4.6%, respectively. Using fluorescence recovery, we determined perfusate velocity to calculate diameter changes under different experimental pH conditions. Using this approach, we demonstrate no significant differences between diameter changes measured using videomicroscopy or predicted from the velocity values obtained using fluorescence recovery after photobleaching. The model is further validated by demonstrating our ability to cannulate arterioles in two brain regions (cortex and supraoptic nucleus of the hypothalamus). Altogether, we believe this is the first study demonstrating successful cannulation and perfusion of parenchymal arterioles while monitoring/estimating luminal diameter and pressure under conditions where flow rates are controlled.

Hypotonicity-induced TRPV4 function in renal collecting duct cells: modulation by progressive cross-talk with Ca2+-activated K+ channels

The mouse cortical collecting duct (CCD) M-1 cells were grown to confluency on coverslips to assess the interaction between TRPV4 and Ca(2+)-activated K(+) channels. Immunocytochemistry demonstrated strong expression of TRPV4, along with the CCD marker, aquaporin-2, and the Ca(2+)-activated K(+) channels, the small conductance SK3 (K(Ca)2.3) channel and large conductance BKα channel (K(Ca)1.1). TRPV4 overexpression studies demonstrated little physical dependency of the K(+) channels on TRPV4. However, activation of TRPV4 by hypotonic swelling (or GSK1016790A, a selective agonist) or inhibition by the selective antagonist, HC-067047, demonstrated a strong dependency of SK3 and BK-α activation on TRPV4-mediated Ca(2+) influx. Selective inhibition of BK-α channel (Iberiotoxin) or SK3 channel (apamin), thereby depolarizing the cells, further revealed a significant dependency of TRPV4-mediated Ca(2+) influx on activation of both K(+) channels. It is concluded that a synergistic cross-talk exists between the TRPV4 channel and SK3 and BK-α channels to provide a tight functional regulation between the channel groups. This cross-talk may be progressive in nature where the initial TRPV4-mediated Ca(2+) influx would first activate the highly Ca(2+)-sensitive SK3 channel which, in turn, would lead to enhanced Ca(2+) influx and activation of the less Ca(2+)-sensitive BK channel.

Functional apical large conductance, Ca2+-activated, and voltage-dependent K+ channels are required for maintenance of airway surface liquid volume

Large conductance, Ca(2+)-activated, and voltage-dependent K(+) (BK) channels control a variety of physiological processes in nervous, muscular, and renal epithelial tissues. In bronchial airway epithelia, extracellular ATP-mediated, apical increases in intracellular Ca(2+) are important signals for ion movement through the apical membrane and regulation of water secretion. Although other, mainly basolaterally expressed K(+) channels are recognized as modulators of ion transport in airway epithelial cells, the role of BK in this process, especially as a regulator of airway surface liquid volume, has not been examined. Using patch clamp and Ussing chamber approaches, this study reveals that BK channels are present and functional at the apical membrane of airway epithelial cells. BK channels open in response to ATP stimulation at the apical membrane and allow K(+) flux to the airway surface liquid, whereas no functional BK channels were found basolaterally. Ion transport modeling supports the notion that apically expressed BK channels are part of an apical loop current, favoring apical Cl(-) efflux. Importantly, apical BK channels were found to be critical for the maintenance of adequate airway surface liquid volume because continuous inhibition of BK channels or knockdown of KCNMA1, the gene coding for the BK α subunit (KCNMA1), lead to airway surface dehydration and thus periciliary fluid height collapse revealed by low ciliary beat frequency that could be fully rescued by addition of apical fluid. Thus, apical BK channels play an important, previously unrecognized role in maintaining adequate airway surface hydration.

Arresting a transient receptor potential (TRP) channel: beta-arrestin 1 mediates ubiquitination and functional down-regulation of TRPV4

β-Arrestins, originally discovered to desensitize activated G protein-coupled receptors, (aka seven-transmembrane receptors, 7TMRs) also mediate 7TMR internalization and G protein-independent signaling via these receptors. More recently, several regulatory roles of β-arrestins for atypical 7TMRs and non-7TM receptors have emerged. Here, we uncover an entirely novel regulatory role of β-arrestins in cross-talk between the angiotensin receptor (AT1aR) and a member of the transient receptor potential (TRP) ion channel family, TRPV4. AT1aR and TRPV4 form a constitutive complex in the plasma membrane, and angiotensin stimulation leads to recruitment of β-arrestin 1 to this complex. Surprisingly, angiotensin stimulation results in ubiquitination of TRPV4, a process that requires β-arrestin 1, and subsequently to internalization and functional down-regulation of TRPV4. β-Arrestin 1 interacts with, and acts as an adaptor for AIP4, an E3 ubiquitin ligase responsible for TRPV4 ubiquitination. Thus, our data provide the first evidence of a functional link between β-arrestins and TRPV4 and uncovers an entirely novel mechanism to maintain appropriate intracellular Ca²⁺ concentration to avoid excessive Ca²⁺ signaling.