TRPV4

Mechanosensitive TRPV4 channel guides maturation and organization of the bilayered mammary epithelium

Biophysical cues from the cell microenvironment are detected by mechanosensitive components at the cell surface. Such machineries convert physical information into biochemical signaling cascades within cells, subsequently leading to various cellular responses in a stimulus-dependent manner. At the surface of extracellular environment and cell cytoplasm exist several ion channel families that are activated by mechanical signals to direct intracellular events. One of such channel is formed by transient receptor potential cation channel subfamily V member, TRPV4 that is known to act as a mechanosensor in wide variaty of tissues and control ion-influx in a spatio-temporal way. Here we report that TRPV4 is prominently expressed in the stem/progenitor cell populations of the mammary epithelium and seems important for the lineage-specific differentiation, consequently affecting mechanical features of the mature mammary epithelium. This was evident by the lack of several markers for mature myoepithelial and luminal epithelial cells in TRPV4-depleted cell lines. Interestingly, TRPV4 expression is controlled in a tension-dependent manner and it also impacts differentation process dependently on the stiffness of the microenvironment. Furthermore, such cells in a 3D compartment were disabled to maintain normal mammosphere structures and displayed abnormal lumen formation, size of the structures and disrupted cellular junctions. Mechanosensitive TRPV4 channel therefore act as critical player in the homeostasis of normal mammary epithelium through sensing the physical environment and guiding accordingly differentiation and structural organization of the bilayered mammary epithelium.

The role of TRPV4 in programmed cell deaths

Programmed cell death is a major life activity of both normal development and disease. Necroptosis is early recognized as a caspase-independent form of programmed cell death followed obviously inflammation. Apoptosis is a gradually recognized mode of cell death that is characterized by a special morphological changes and unique caspase-dependent biological process. Ferroptosis, pyroptosis and autophagy are recently identified non-apoptotic regulated cell death that each has its own characteristics. The transient receptor potential vanilloid 4 (TRPV4) is a kind of nonselective calcium-permeable cation channel, which is received more and more attention in biology studies. It is widely expressed in human tissues and mainly located on the membrane of cells. Several researchers have identified that the influx Ca2+ from TRPV4 acts as a key role in the loss of cells by apoptosis, ferroptosis, necroptosis, pyroptosis and autophagy via mediating endoplasmic reticulum (ER) stress, oxidative stress and inflammation. This effect is bad for the normal function of organs on the one hand, on the other hand, it is benefit for anticancer activities. In this review, we will summarize the current discovery on the role and impact of TRPV4 in these programmed cell death pathological mechanisms to provide a new prospect of gene therapeutic target of related diseases.

Ouabain's Influence on TRPV4 Channels of Epithelial Cells: An Exploration of TRPV4 Activity, Expression, and Signaling Pathways

Ouabain, a substance originally obtained from plants, is now classified as a hormone because it is produced endogenously in certain animals, including humans. However, its precise effects on the body remain largely unknown. Previous studies have shown that ouabain can influence the phenotype of epithelial cells by affecting the expression of cell-cell molecular components and voltage-gated potassium channels. In this study, we conducted whole-cell clamp assays to determine whether ouabain affects the activity and/or expression of TRPV4 channels. Our findings indicate that ouabain has a statistically significant effect on the density of TRPV4 currents (dITRPV4), with an EC50 of 1.89 nM. Regarding treatment duration, dITRPV4 reaches its peak at around 1 h, followed by a subsequent decline and then a resurgence after 6 h, suggesting a short-term modulatory effect related to on TRPV4 channel activity and a long-term effect related to the promotion of synthesis of new TRPV4 channel units. The enhancement of dITRPV4 induced by ouabain was significantly lower in cells seeded at low density than in cells in a confluent monolayer, indicating that the action of ouabain depends on intercellular contacts. Furthermore, the fact that U73122 and neomycin suppress the effect caused by ouabain in the short term suggests that the short-term induced enhancement of dITRPV4 is due to the depletion of PIP2 stores. In contrast, the fact that the long-term effect is inhibited by PP2, wortmannin, PD, FR18, and IKK16 suggests that cSrc, PI3K, Erk1/2, and NF-kB are among the components included in the signaling pathways.

Fluid Osmolarity Modulates the Rate of Spontaneous Contraction of Lymphatic Vessels and Lymph Flow by Means of a Cooperation between TRPV and VRAC Channels

Lymphatic vessels are capable of sustaining lymph formation and propulsion via an intrinsic mechanism based on the spontaneous contraction of the lymphatic muscle in the wall of lymphatic collectors. Exposure to a hyper- or hypo-osmolar environment can deeply affect the intrinsic contraction rate and therefore alter lymph flow. In this work, we aimed at defining the putative receptors underlying such a response. Functional experiments were conducted in ex vivo rat diaphragmatic specimens containing spontaneously contracting lymphatic vessels that were exposed to either hyper- or hypo-osmolar solutions. Lymphatics were challenged with blockers to TRPV4, TRPV1, and VRAC channels, known to respond to changes in osmolarity and/or cell swelling and expressed by lymphatic vessels. Results show that the normal response to a hyperosmolar environment is a steady decrease in the contraction rate and lymph flow and can be prevented by blocking TRPV1 channels with capsazepine. The response to a hyposmolar environment consists of an early phase of an increase in the contraction rate, followed by a decrease. The early phase is abolished by blocking VRACs with DCPIB, while blocking TRPV4 mainly resulted in a delay of the early response. Overall, our data suggest that the cooperation of the three channels can shape the response of lymphatic vessels in terms of contraction frequency and lymph flow, with a prominent role of TRPV1 and VRACs.

Cannabidiol inhibits lung proliferation in monocrotaline-induced pulmonary hypertension in rats

Cannabidiol (CBD) is a safe and well-tolerated plant-derived drug with anti-proliferative properties. Pulmonary hypertension (PH) is a rapidly progressive and still incurable disease. CBD diminishes monocrotaline (MCT)-induced PH, including reduced right ventricular systolic pressure, pulmonary vascular hypertrophy, and right ventricular remodeling. The aim of our study was to investigate the effect of chronic administration of CBD (10 mg/kg once daily for 21 days) on selected remodeling parameters in the lung of MCT-induced PH rats. In MCT-induced PH, we found an increase in profibrotic parameters, e.g., transforming growth factor β1 (TGF-β1), galectin-3 (Gal-3), procollagen I, collagen I, C-propeptide, matrix metalloproteinase 9 (MMP-9) and an increased number of mast cells. In our study, we observed that the TGF-β1, Gal-3, procollagen I, collagen I, C-propeptide, and mast cell levels in lung tissue were decreased after CBD administration to MCT-treated rats. In summary, CBD treatment has an anti-proliferative effect on MCT-induced PH. Given the beneficial multidirectional effects of CBD on PH, we believe that CBD can be used as an adjuvant PH therapy, but this argument needs to be confirmed by clinical trials.

Dynamic evolution of transient receptor potential vanilloid (TRPV) ion channel family with numerous gene duplications and losses

The transient receptor potential vanilloid (TRPV) ion channel family is involved in multiple sensory and physiological functions including thermosensing and temperature-dependent neuroendocrine regulation. The objective of the present study was to investigate the number, origin and evolution of TRPV genes in metazoans, with special focus on the impact of the vertebrate whole-genome duplications (WGD). Gene searches followed by phylogenetic and synteny analyses revealed multiple previously undescribed TRPV genes. The common ancestor of Cnidaria and Bilateria had three TRPV genes that became four in the deuterostome ancestor. Two of these were lost in the vertebrate ancestor. The remaining two genes gave rise to two TRPV subfamilies in vertebrates, consisting of subtypes 1, 2, 3, 4, 9 and 5, 6, 7, 8, respectively. This gene expansion resulted from the two basal vertebrate WGD events (1R and 2R) and three local duplications before the radiation of gnathostomes. TRPV1, 4 and 5 have been retained in all gnathostomes investigated, presumably reflecting important functions. TRPV7 and 8 have been lost independently in various lineages but are still retained in cyclostomes, actinistians (coelacanth), amphibians, prototherians and basal actinopterygians (Polypteridae). TRPV3 and 9 are present in extant elasmobranchs, while TRPV9 was lost in the osteichthyan ancestor and TRPV3 in the actinopterygian ancestor. The coelacanth has retained the ancestral osteichthyan repertoire of TRPV1, 3, 4, 5, 7 and 8. TRPV2 arose in the tetrapod ancestor. Duplications of TRPV5 occurred independently in various lineages, such as cyclostomes, chondrichthyans, anuran amphibians, sauropsids, mammals (where the duplicate is called TRPV6), and actinopterygians (Polypteridae and Esocidae). After the teleost-specific WGD (3R) only TRPV1 retained its duplicate, whereas TRPV4 and 5 remained as single genes. Both 3R-paralogs of TRPV1 were kept in some teleost species, while one paralog was lost in others. The salmonid-specific WGD (4R) duplicated TRPV1, 4, and 5 leading to six TRPV genes. The largest number was found in Xenopus tropicalis with no less than 15 TRPV genes. This study provides a comprehensive evolutionary scenario for the vertebrate TRPV family, revealing additional TRPV types and proposing a phylogeny-based classification of TRPV across metazoans.

The Emerging Pro-Algesic Profile of Transient Receptor Potential Vanilloid Type 4

Transient receptor potential vanilloid type 4 (TRPV4) channels are Ca²⁺-permeable non-selective cation channels which mediate a wide range of physiological functions and are activated and modulated by a diverse array of stimuli. One of this ion channel's least discussed functions is in relation to the generation and maintenance of certain pain sensations. However, in the two decades which have elapsed since the identification of this ion channel, considerable data has emerged concerning its function in mediating pain sensations. TRPV4 is a mediator of mechanical hyperalgesia in the various contexts in which a mechanical stimulus, comprising trauma (at the macro-level) or discrete extracellular pressure or stress (at the micro-level), results in pain. TRPV4 is also recognised as constituting an essential component in mediating inflammatory pain. It also plays a role in relation to many forms of neuropathic-type pain, where it functions in mediating mechanical allodynia and hyperalgesia.Here, we review the role of TRPV4 in mediating pain sensations.

Role of Known Transient Receptor Potential Vanilloid Channels in Modulating Cardiac Mechanobiology

The transient receptor potential (TRP) channels have been described in almost every mammalian cell type. Several members of the Vanilloid (TRPV) subtype have been found to play important roles in modulating cardiac structure and function through Ca²⁺ handling in response to systemic and local mechanobiological cues. In this review, we will consider the most studied TRPV channels in the cardiovascular field; transient receptor potential vanilloid 1 as a modulator of cardiac hypertrophy; transient receptor potential vanilloid 2 as a structural and functional protein; transient receptor potential vanilloid 3 in the development of hypertrophy and myocardial fibrosis; and transient receptor potential vanilloid 4 in its roles modulating the fibrotic and functional responses of the heart to pressure overload. Lastly, we will also review the potential overlapping roles of these channels with other TRP proteins as well as the advances in translational and clinical arenas associated with TRPV channels.

Involvement of TRPV4 in changes in rapidly inactivating potassium channels in the early stage of pilocarpine-induced status epilepticus in mice

The rapidly inactivating potassium current (I_A ) is important in controlling neuronal action potentials. Altered I_A function and K⁺ channel expression have been found in epilepsy, and activation of the transient receptor potential vanilloid 4 (TRPV4) channel is involved in epilepsy pathogenesis. This study examined whether TRPV4 affects Kv4.2 and K⁺ channel interacting protein (KCHIP) expression and I_A changes following pilocarpine-induced status epilepticus (PISE) in mice. Herein, hippocampal protein levels of Kv4.2 and KCHIP2 increased 3 h-3 d and decreased 7-30 d; that of KCHIP1 increased 3-24 h and decreased 3-30 d post-PISE. The TRPV4 antagonist HC-067047 attenuated the increased protein levels of Kv4.2 and KCHIP2 but not that of KCHIP1 post-PISE. The TRPV4 agonist GSK1016790A increased hippocampal protein levels of Kv4.2 and KCHIP2 but had no effect on KCHIP1 expression. HC-067047 attenuated the increased I_A in hippocampal pyramidal neurons 24 h and 3 d post-PISE. GSK1016790A increased I_A in hippocampal pyramidal neurons, shifting the voltage-dependent inactivation curve toward depolarization. The GSK1016790A-induced increase of I_A was blocked by protein kinase A and calcium/calmodulin-dependent kinase II antagonists but was unaffected by protein kinase C antagonists. We conclude that TRPV4 activation may be responsible for the increases of Kv4.2 and KCHIP2 expression in hippocampi and I_A in hippocampal pyramidal neurons in PISE mice, which are likely compensatory measures for hyperexcitability at the early stage of epilepsy.

Hypo-Osmotic Loading Induces Expression of IL-6 in Nucleus Pulposus Cells of the Intervertebral Disc Independent of TRPV4 and TRPM7

Painful intervertebral disc (IVD) degeneration is an age-related process characterized by reduced tissue osmolarity, increased catabolism of the extracellular matrix, and elevated levels of pro-inflammatory molecules. With the aging population and constantly rising treatment costs, it is of utmost importance to identify potential therapeutic targets and new pharmacological treatment strategies for low back pain. Transient receptor potential (TRP) channels are a family of Ca²⁺ permeable cell membrane receptors, which can be activated by multitude of stimuli and have recently emerged as contributors to joint disease, but were not investigated closer in the IVD. Based on the gene array screening, TRPC1, TRPM7, and TRPV4 were overall the most highly expressed TRP channels in bovine IVD cells. We demonstrated that TRPV4 gene expression was down-regulated in hypo-osmotic condition, whereas its Ca²⁺ flux increased. No significant differences in Ca²⁺ flux and gene expression were observed for TRPM7 between hypo- and iso-osmotic groups. Upon hypo-osmotic stimulation, we overall identified via RNA sequencing over 3,000 up- or down-regulated targets, from which we selected aggrecan, ADAMTS9, and IL-6 and investigated whether their altered gene expression is mediated through either the TRPV4 or TRPM7 channel, using specific activators and inhibitors (GSK1016790A/GSK2193874 for TRPV4 and Naltriben/NS8593 for TRPM7). GSK1016790A induced the expression of IL-6 under iso-osmotic condition, alike to hypo-osmotic stimulation alone, indicating that this effect might be TRPV4-mediated. However, using the TRPV4 blocker GSK2193874 failed to prevent the increase of IL-6 under hypo-osmotic condition. A treatment with TRPM7-activator did not cause significant changes in the gene expression of tested targets. In conclusion, while TRPV4 and TRPM7 are likely involved in osmosensing in the IVD, neither of them mediates hypo-osmotically-induced gene expression changes of aggrecan, ADAMTS9, and IL-6.

TRPV4 plays an important role in rat prefrontal cortex changes induced by acute hypoxic exercise

OBJECTIVE

This study aims to investigate the effects of TRPV4 on acute hypoxic exercise-induced central fatigue, in order to explore the mechanism in central for exercise capacity decline of athletes in the early stage of altitude training.

METHODS

120 male Wistar rats were randomly divided into 12 groups: 4 normoxia groups (quiet group, 5-level group, 8-level group, exhausted group), 4 groups at simulated 2500 m altitude (grouping as before), 4 groups at simulated 4500 m altitude (grouping as before), 10 in each group. With incremental load movement, materials were drawn corresponding to the load. Intracellular calcium ion concentration was measured by HE staining, enzyme-linked immunosorbent assay, immunohistochemistry, RT-qPCR, Fluo-4/AM and Fura-2/AM fluorescence staining.

RESULTS

(1) Hypoxic 2-5 groups showed obvious venous congestion, with symptoms similar to normoxia-8 group; Hypoxic 2-8 groups showed meningeal loosening edema, infra-meningeal venous congestion, with symptoms similar to normoxia-exhausted group and hypoxic 1-exhaused group. (2) For 5,6-EET, regardless of normoxic or hypoxic environment, significant or very significant differences existed between each exercise load group (normoxic - 5 level 20.58 ± 0.66 pg/mL, normoxic - 8 level 23.15 ± 0.46 pg/mL, normoxic - exhausted 26.66 ± 0.71 pg/mL; hypoxic1-5 level 21.72 ± 0.43 pg/mL, hypoxic1-8 level 24.73 ± 0.69 pg/mL, hypoxic 1-exhausted 28.68 ± 0.48 pg/mL; hypoxic2-5 level 22.75 ± 0.20 pg/mL, hypoxic2-8 level 25.62 ± 0.39 pg/mL, hypoxic 2-exhausted 31.03 ± 0.41 pg/mL) and quiet group in the same environment(normoxic-quiet 18.12 ± 0.65 pg/mL, hypoxic 1-quiet 19.94 ± 0.43 pg/mL, hypoxic 2-quiet 21.72 ± 0.50 pg/mL). The 5,6-EET level was significantly or extremely significantly increased in hypoxic 1 environment and hypoxic 2 environment compared with normoxic environment under the same load. (3) With the increase of exercise load, expression of TRPV4 in the rat prefrontal cortex was significantly increased; hypoxic exercise groups showed significantly higher TRPV4 expression than the normoxic group. (4) Calcium ion concentration results showed that in the three environments, 8 level group (normoxic-8 190.93 ± 6.11 nmol/L, hypoxic1-8 208.92 ± 6.20 nmol/L, hypoxic2-8 219.13 ± 4.57 nmol/L) showed very significant higher concentration compared to quiet state in the same environment (normoxic-quiet 107.11 ± 0.49 nmol/L, hypoxic 1-quiet 128.48 ± 1.51 nmol/L, hypoxic 2-quiet 171.71 ± 0.84 nmol/L), and the exhausted group in the same environment (normoxic-exhausted 172.51 ± 3.30 nmol/L, hypoxic 1-exhausted 164.54 ± 6.01 nmol/L, hypoxic 2-exhausted 154.52 ± 1.80 nmol/L) had significant lower concentration than 8-level group; hypoxic2-8 had significant higher concentration than normoxic-8.

CONCLUSION

Acute hypoxic exercise increases the expression of TRPV4 channel in the prefrontal cortex of the brain. For a lower ambient oxygen concentration, expression of TRPV4 channel is higher, suggesting that TRPV4 channel may be one important mechanism involved in calcium overload in acute hypoxic exercise.

Morin induces endothelium-dependent relaxation by activating TRPV4 channels in rat mesenteric arteries

Morin, a natural flavonol, has been reported to have beneficial pharmacological effects. Although its vascular protective effects have been studied, little is known about its effects on the mesenteric artery and the underlying mechanisms. Transient receptor potential vanilloid type 4 (TRPV4) channels are one of the most important Ca²⁺-permeable cation channels in vascular endothelial cells and play an important role in regulating rat mesenteric vascular tone. In the present study, the myogenic effects of morin were investigated using wire and pressure myography in the isolated mesenteric artery. Morin induced endothelium-dependent relaxation of isolated rat mesenteric arteries in a concentration-dependent manner. In addition, morin stimulated relaxation by activating TRPV4-mediated Ca²⁺ influx without affecting the nitric oxide (NO), hydrogen peroxide (H₂O₂), cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) pathways. In primary cultured rat mesenteric artery endothelial cells and over-expressing TRPV4 HEK 293 cells, the TRPV4 inhibitor HC067047 significantly reduced the morin-induced increase in intracellular Ca²⁺ concentration. Furthermore, in rats with hypertension induced by N^Ꞷ-nitro-L-arginine methyl ester (L-NAME), oral administration of morin (50 mg/kg/day) decreased systolic blood pressure. In L-NAME-induced hypertensive rats, morin significantly improved the relaxation response of the arteries to acetylcholine. Thus, we demonstrated that morin induces endothelium-dependent relaxation in the rat mesenteric artery by acting on TRPV4 channels to mediate Ca²⁺ influx and attenuate blood pressure in L-NAME-induced hypertension, thereby highlighting the potential of morin in the treatment of hypertension.

Stroke and the neurovascular unit: glial cells, sex differences, and hypertension

A functional neurovascular unit (NVU) is central to meeting the brain's dynamic metabolic needs. Poststroke damage to the NVU within the ipsilateral hemisphere ranges from cell dysfunction to complete cell loss. Thus, understanding poststroke cell-cell communication within the NVU is of critical importance. Loss of coordinated NVU function exacerbates ischemic injury. However, particular cells of the NVU (e.g., astrocytes) and those with ancillary roles (e.g., microglia) also contribute to repair mechanisms. Epidemiological studies support the notion that infarct size and recovery outcomes are heterogeneous and greatly influenced by modifiable and nonmodifiable factors such as sex and the co-morbid condition common to stroke: hypertension. The mechanisms whereby sex and hypertension modulate NVU function are explored, to some extent, in preclinical laboratory studies. We present a review of the NVU in the context of ischemic stroke with a focus on glial contributions to NVU function and dysfunction. We explore the impact of sex and hypertension as modifiable and nonmodifiable risk factors and the underlying cellular mechanisms that may underlie heterogeneous stroke outcomes. Most of the preclinical investigative studies of poststroke NVU dysfunction are carried out primarily in male stroke models lacking underlying co-morbid conditions, which is very different from the human condition. As such, the evolution of translational medicine to target the NVU for improved stroke outcomes remains elusive; however, it is attainable with further research.

The TRPV4-TAZ mechanotransduction signaling axis in matrix stiffness- and TGFβ1-induced epithelial-mesenchymal transition

Introduction

The implantation of biomaterials into soft tissue leads to the development of foreign body response, a non-specific inflammatory condition that is characterized by the presence of fibrotic tissue. Epithelial-mesenchymal transition (EMT) is a key event in development, fibrosis, and oncogenesis. Emerging data support a role for both a mechanical signal and a biochemical signal in EMT. We hypothesized that transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive channel, is a mediator of EMT.

Methods

Normal human primary epidermal keratinocytes (NHEKs) were seeded on collagen-coated plastic plates or varied stiffness polyacrylamide gels in the presence or absence of TGFβ1, Immunofluorescence, immunoblot, and polymerase chain reaction analysis were performed to determine expression level of EMT markers and signaling proteins. Knock-down of TRPV4 function was achieved by siRNA transfection or by GSK2193874 treatment.

Results

We found that knock-down of TRPV4 blocked both matrix stiffness- and TGFβ1-induced EMT in NHEKs. In a murine skin fibrosis model, TRPV4 deletion resulted in decreased expression of the mesenchymal marker, α-SMA, and increased expression of epithelial marker, E-cadherin. Mechanistically, our data showed that: i) TRPV4 was essential for the nuclear translocation of TAZ in response to matrix stiffness and TGFβ1; ii) Antagonism of TRPV4 inhibited both matrix stiffness-induced and TGFβ1-induced expression of TAZ proteins; and iii) TRPV4 antagonism suppressed both matrix stiffness-induced and TGFβ1-induced activation of Smad2/3, but not of AKT.

Conclusions

These data identify a novel role for TRPV4-TAZ mechanotransduction signaling axis in regulating EMT in NHEKs in response to both matrix stiffness and TGFβ1.

Augmented astrocyte microdomain Ca2+ dynamics and parenchymal arteriole tone in angiotensin II-infused hypertensive mice

Hypertension is an important contributor to cognitive decline but the underlying mechanisms are unknown. Although much focus has been placed on the effect of hypertension on vascular function, less is understood of its effects on nonvascular cells. Because astrocytes and parenchymal arterioles (PA) form a functional unit (neurovascular unit), we tested the hypothesis that hypertension-induced changes in PA tone concomitantly increases astrocyte Ca²⁺ . We used cortical brain slices from 8-week-old mice to measure myogenic responses from pressurized and perfused PA. Chronic hypertension was induced in mice by 28-day angiotensin II (Ang II) infusion; PA resting tone and myogenic responses increased significantly. In addition, chronic hypertension significantly increased spontaneous Ca²⁺ events within astrocyte microdomains (MD). Similarly, a significant increase in astrocyte Ca²⁺ was observed during PA myogenic responses supporting enhanced vessel-to-astrocyte signaling. The transient potential receptor vanilloid 4 (TRPV4) channel, expressed in astrocyte processes in contact with blood vessels, namely endfeet, respond to hemodynamic stimuli such as increased pressure/flow. Supporting a role for TRPV4 channels in aberrant astrocyte Ca²⁺ dynamics in hypertension, cortical astrocytes from hypertensive mice showed augmented TRPV4 channel expression, currents and Ca²⁺ responses to the selective channel agonist GSK1016790A. In addition, pharmacological TRPV4 channel blockade or genetic deletion abrogated enhanced hypertension-induced increases in PA tone. Together, these data suggest chronic hypertension increases PA tone and Ca²⁺ events within astrocytes MD. We conclude that aberrant Ca²⁺ events in astrocyte constitute an early event toward the progression of cognitive decline.

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.

Calcium signaling of in situ chondrocytes in articular cartilage under compressive loading: Roles of calcium sources and cell membrane ion channels

Mechanical loading on articular cartilage can induce many physical and chemical stimuli on chondrocytes residing in the extracellular matrix (ECM). Intracellular calcium ([Ca²⁺ ]_i ) signaling is among the earliest responses of chondrocytes to physical stimuli, but the [Ca²⁺ ]_i signaling of in situ chondrocytes in loaded cartilage is not fully understood due to the technical challenges in [Ca²⁺ ]_i imaging of chondrocytes in a deforming ECM. This study developed a novel bi-directional microscopy loading device that enables the record of transient [Ca²⁺ ]_i responses of in situ chondrocytes in loaded cartilage. It was found that compressive loading significantly promoted [Ca²⁺ ]_i signaling in chondrocytes with faster [Ca²⁺ ]_i oscillations in comparison to the non-loaded cartilage. Seven [Ca²⁺ ]_i signaling pathways were further investigated by treating the cartilage with antagonists prior to and/or during the loading. Removal of extracellular Ca²⁺ ions completely abolished the [Ca²⁺ ]_i responses of in situ chondrocytes, suggesting the indispensable role of extracellular Ca²⁺ sources in initiating the [Ca²⁺ ]_i signaling in chondrocytes. Depletion of intracellular Ca²⁺ stores, inhibition of PLC-IP₃ pathway, and block of purinergic receptors on plasma membrane led to significant reduction in the responsive rate of cells. Three types of ion channels that are regulated by different physical signals, TRPV4 (osmotic and mechanical stress), T-type VGCCs (electrical potential), and mechanical sensitive ion channels (mechanical loading) all demonstrated critical roles in controlling the [Ca²⁺ ]_i responses of in situ chondrocyte in the loaded cartilage. This study provided new knowledge about the [Ca²⁺ ]_i signaling and mechanobiology of chondrocytes in its natural residing environment. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:730-738, 2018.

Premature contractions of the bladder are suppressed by interactions between TRPV4 and SK3 channels in murine detrusor PDGFRα+ cells

During filling, urinary bladder volume increases dramatically with little change in pressure. This is accomplished by suppressing contractions of the detrusor muscle that lines the bladder wall. Mechanisms responsible for regulating detrusor contraction during filling are poorly understood. Here we describe a novel pathway to stabilize detrusor excitability involving platelet-derived growth factor receptor-α positive (PDGFRα⁺) interstitial cells. PDGFRα⁺ cells express small conductance Ca²⁺-activated K⁺ (SK) and TRPV4 channels. We found that Ca²⁺ entry through mechanosensitive TRPV4 channels during bladder filling stabilizes detrusor excitability. GSK1016790A (GSK), a TRPV4 channel agonist, activated a non-selective cation conductance that coupled to activation of SK channels. GSK induced hyperpolarization of PDGFRα⁺ cells and decreased detrusor contractions. Contractions were also inhibited by activation of SK channels. Blockers of TRPV4 or SK channels inhibited currents activated by GSK and increased detrusor contractions. TRPV4 and SK channel blockers also increased contractions of intact bladders during filling. Similar enhancement of contractions occurred in bladders of Trpv4 ^-/- mice during filling. An SK channel activator (SKA-31) decreased contractions during filling, and rescued the overactivity of Trpv4 ^-/- bladders. Our findings demonstrate how Ca²⁺ influx through TRPV4 channels can activate SK channels in PDGFRα⁺ cells and prevent bladder overactivity during filling.

TRPV4 channels contribute to renal myogenic autoregulation in neonatal pigs

Myogenic response, a phenomenon in which resistance size arteries and arterioles swiftly constrict or dilate in response to an acute elevation or reduction, respectively, in intravascular pressure is a key component of renal autoregulation mechanisms. Although it is well established that the renal system is functionally immature in neonates, mechanisms that regulate neonatal renal blood flow (RBF) remain poorly understood. In this study, we investigated the hypothesis that members of the transient receptor potential vanilloid (TRPV) channels are molecular components of renal myogenic constriction in newborns. We show that unlike TRPV1-3, TRPV4 channels are predominantly expressed in neonatal pig preglomerular vascular smooth muscle cells (SMCs). Intracellular Ca²⁺ concentration ([Ca²⁺]_i) elevation induced by osmotic cell swelling was attenuated by TRPV4, L-type Ca²⁺, and stretch-activated Ca²⁺ channel blockers but not phospholipase A₂ inhibitor. Blockade of TRPV4 channels reversed steady-state myogenic tone and inhibited pressure-induced membrane depolarization, [Ca²⁺]_i elevation, and constriction in distal interlobular arteries. A step increase in arterial pressure induced efficient autoregulation of renal cortical perfusion and total RBF in anesthetized and mechanically ventilated neonatal pigs. Moreover, intrarenal arterial infusion of the TRPV4 channel blockers HC 067047 and RN 1734 attenuated renal autoregulation in the pigs. These data suggest that renal myogenic autoregulation is functional in neonates. Our findings also indicate that TRPV4 channels are mechanosensors in neonatal pig preglomerular vascular SMCs and contribute to renal myogenic autoregulation.

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.

TRPV4 regulates insulin mRNA expression and INS-1E cell death via ERK1/2 and NO-dependent mechanisms

TRPV4 is a Ca²⁺-permeable, nonselective cation channel. Recently, TRPV4 was implicated in controlling peripheral insulin sensitivity, insulin secretion and apoptosis of pancreatic beta cells. Here, we characterize the role and potential mechanisms of TRPV4 in regulating insulin mRNA expression and cell death in insulin producing INS-1E cells and rat pancreatic islets. TRPV4 protein production was downregulated by siRNA. Intracellular calcium level was measured using Fluo-3 AM. Gene expression was studied by real-time PCR. Phosphorylation of extracellular signal-regulated kinase (ERK1 and ERK2) was detected by Western blot. Nitric oxide (NO) production was assessed by chemiluminescent reaction. Reactive oxygen species (ROS) level was analysed using a fluorogenic dye (DCFDA). Cell death was evaluated by determination of cytoplasmic histone-associated DNA fragments. Downregulation of TRPV4 neither affected insulin mRNA expression nor INS-1E cell growth. By contrast, pharmacological TRPV4 activation by 100nmol/l GSK1016790A increased Ca²⁺ levels in INS-1E cells and enhanced insulin mRNA expression after 1 and 3h, whereas a suppression of insulin mRNA expression was detected after 24h incubation. GSK1016790A increased ERK1/2 phosphorylation and NO production but not ROS production. Pharmacological blockade of ERK1/2 attenuated GSK1016790A-induced insulin mRNA expression. Inhibition of NO synthesis by l-NAME failed to affect insulin mRNA expression in GSK1016790A treated INS-1E cells. Furthermore, inhibition of NO production attenuated GSK1016790A-induced INS-1E cell death. In pancreatic islets, 100nmol/l GSK1016790A increased insulin mRNA levels after 3h without inducing cytotoxicity after 24h. In conclusion, TRPV4 differently regulates insulin mRNA expression in INS-1E cells via ERK1/2 and NO-dependent mechanisms.

Modulation of the TRPV4 ion channel as a therapeutic target for disease

Transient Receptor Potential Vanilloid 4 (TRPV4) is a broadly expressed, polymodally gated ion channel that plays an important role in many physiological and pathophysiological processes. TRPV4 knockout mice and several synthetic pharmacological compounds that selectively target TRPV4 are now available, which has allowed detailed investigation in to the therapeutic potential of this ion channel. Results from animal studies suggest that TRPV4 antagonism has therapeutic potential in oedema, pain, gastrointestinal disorders, and lung diseases such as cough, bronchoconstriction, pulmonary hypertension, and acute lung injury. A lack of observed side-effects in vivo has prompted a first-in-human trial for a TRPV4 antagonist in healthy participants and stable heart failure patients. If successful, this would open up an exciting new area of research for a multitude of TRPV4-related pathologies. This review will discuss the known roles of TRPV4 in disease, and highlight the possible implications of targeting this important cation channel for therapy.

Vasculo-Neuronal Coupling: Retrograde Vascular Communication to Brain Neurons

Continuous cerebral blood flow is essential for neuronal survival, but whether vascular tone influences resting neuronal function is not known. Using a multidisciplinary approach in both rat and mice brain slices, we determined whether flow/pressure-evoked increases or decreases in parenchymal arteriole vascular tone, which result in arteriole constriction and dilation, respectively, altered resting cortical pyramidal neuron activity. We present evidence for intercellular communication in the brain involving a flow of information from vessel to astrocyte to neuron, a direction opposite to that of classic neurovascular coupling and referred to here as vasculo-neuronal coupling (VNC). Flow/pressure increases within parenchymal arterioles increased vascular tone and simultaneously decreased resting pyramidal neuron firing activity. On the other hand, flow/pressure decreases evoke parenchymal arteriole dilation and increased resting pyramidal neuron firing activity. In GLAST-CreERT2; R26-lsl-GCaMP3 mice, we demonstrate that increased parenchymal arteriole tone significantly increased intracellular calcium in perivascular astrocyte processes, the onset of astrocyte calcium changes preceded the inhibition of cortical pyramidal neuronal firing activity. During increases in parenchymal arteriole tone, the pyramidal neuron response was unaffected by blockers of nitric oxide, GABA_A, glutamate, or ecto-ATPase. However, VNC was abrogated by TRPV4 channel, GABA_B, as well as an adenosine A₁ receptor blocker. Differently to pyramidal neuron responses, increases in flow/pressure within parenchymal arterioles increased the firing activity of a subtype of interneuron. Together, these data suggest that VNC is a complex constitutive active process that enables neurons to efficiently adjust their resting activity according to brain perfusion levels, thus safeguarding cellular homeostasis by preventing mismatches between energy supply and demand.

SIGNIFICANCE STATEMENT

We present evidence for vessel-to-neuron communication in the brain slice defined here as vasculo-neuronal coupling. We showed that, in response to increases in parenchymal arteriole tone, astrocyte intracellular Ca²⁺ increased and cortical neuronal activity decreased. On the other hand, decreasing parenchymal arteriole tone increased resting cortical pyramidal neuron activity. Vasculo-neuronal coupling was partly mediated by TRPV4 channels as genetic ablation, or pharmacological blockade impaired increased flow/pressure-evoked neuronal inhibition. Increased flow/pressure-evoked neuronal inhibition was blocked in the presence of adenosine A1 receptor and GABA_B receptor blockade. Results provide evidence for the concept of vasculo-neuronal coupling and highlight the importance of understanding the interplay between basal CBF and resting neuronal activity.

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.

TRPV currents and their role in the nociception and neuroplasticity

Transient receptor potential channels sensitive to vanilloids (TRPVs) are group of ion channels which are sensitive to various tissue damaging signals and their activation is generally perceived as pain. Therefore, they are generally named as nociceptors. Understanding their activation and function as well as their interaction with intracellular pathways is crucial for the development of pharmacological interference in order to reduce pain perception. The current review summarizes basic facts in regard to TRPV and discusses their relevance in the sensing of (pain-) signals and their intracellular processing, focussing on their modulation of the intracellular calcium ([Ca(2+)]i) signal. Furthermore we discuss the basic mechanisms how the modification of [Ca(2+)]i through TRPV might induce long-term-potentiation (LTP) or long-term- depression (LTD) and from "memories" of pain. Understanding of these mechanisms is needed to localize the best point of interference for pharmacological treatment. Therefore, high attention is given to highlight physiological and pathological processes and their interaction with significant modulators and their roles in neuroplasticity and pain modulation.

Calcium-permeable ion channels in the kidney

Calcium ions (Ca(2+)) are crucial for a variety of cellular functions. The extracellular and intracellular Ca(2+) concentrations are thus tightly regulated to maintain Ca(2+) homeostasis. The kidney, one of the major organs of the excretory system, regulates Ca(2+) homeostasis by filtration and reabsorption. Approximately 60% of the Ca(2+) in plasma is filtered, and 99% of that is reabsorbed by the kidney tubules. Ca(2+) is also a critical signaling molecule in kidney development, in all kidney cellular functions, and in the emergence of kidney diseases. Recently, studies using genetic and molecular biological approaches have identified several Ca(2+)-permeable ion channel families as important regulators of Ca(2+) homeostasis in kidney. These ion channel families include transient receptor potential channels (TRP), voltage-gated calcium channels, and others. In this review, we provide a brief and systematic summary of the expression, function, and pathological contribution for each of these Ca(2+)-permeable ion channels. Moreover, we discuss their potential as future therapeutic targets.

Ocular transient receptor potential channel function in health and disease

Transient receptor potential (TRP) channels sense and transduce environmental stimuli into Ca(2+) transients that in turn induce responses essential for cell function and adaptation. These non-selective channels with variable Ca(2+) selectivity are grouped into seven different subfamilies containing 28 subtypes based on differences in amino acid sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells where they affect a host of stress-induced regulatory responses essential for normal vision maintenance. This article reviews our current knowledge about the expression, function and regulation of TRPs in different eye tissues. We also describe how under certain conditions TRP activation can induce responses that are maladaptive to ocular function. Furthermore, the possibility of an association between TRP mutations and disease is considered. These findings contribute to evidence suggesting that drug targeting TRP channels may be of therapeutic benefit in a clinical setting. We point out issues that must be more extensively addressed before it will be possible to decide with certainty that this is a realistic endeavor. Another possible upshot of future studies is that disease process progression can be better evaluated by profiling changes in tissue specific functional TRP subtype activity as well as their gene and protein expression.

Possible involvement of TRPV1 and TRPV4 in nociceptive stimulation- induced nocifensive behavior and neuroendocrine response in mice

Members of the transient receptor potential (TRP) family of ion channels play important roles in inflammation and pain. Here, we showed that both TRPV1 and TRPV4 might contribute to biphasic nocifensive behavior and neuroendocrine response following a formalin test. We subcutaneously injected saline, formalin, or the TRPV4 agonist, 4α-phorbol 12,13-didecanoate (4α-PDD) into one hindpaw of wild-type (WT), TRPV1-deficient (Trpv1(-/-)), and TRPV4-deficient (Trpv4(-/-)) mice to investigate nocifensive behaviors (phase I [0-10 min] and phase II [10-60 min]) and Fos expression in the dorsal horn of the spinal cord and other brain regions related to pain, in the paraventricular nucleus (PVN), paraventricular nucleus of the thalamus, the medial habenular nucleus, the medial nucleus of the amygdala and capsular part of the central amygdala. Subcutaneous (s.c.) injection of formalin caused less nocifensive behavior in Trpv1(-/-) and Trpv4(-/-) mice than in WT mice during phase I. In phase II, however, formalin induced less nocifensive behavior only in the Trpv1(-/-) mice, but not in the Trpv4(-/-) mice, relative to WT mice. The number of Fos-like immunoreactive (LI) neurons in laminae I-II of the dorsal horn increased in all types of mice 90 min after s.c. injection of formalin; however, there was no difference in the other regions between saline- and formalin-treated mice. Furthermore, s.c. injection of 4α-PDD did not induce nociceptive behavior nor influence the number of Fos-LI neurons in the all above mentioned regions in any of the mice. These results suggest that TRPV4-mediated nociceptive information from the peripheral tissue excluding the spinal pathway might be involved the formalin behavioral response during phase I. Only TRPV1 might regulate the formalin behavioral response in peripheral neuron.

Polymodal roles of transient receptor potential channels in the control of ocular function

Maintenance of intracellular Ca²⁺ levels at orders of magnitude below those in the extracellular environment is a requisite for preserving cell viability. Membrane channels contribute to such control through modulating their time-dependent opening and closing behaviour. Such regulation requires Ca²⁺ to serve as a second messenger mediating receptor control of numerous life-sustaining responses. Transient receptor potential (TRP) channels signal transduce a wide variety of different sensory stimuli to induce responses modulating cellular function. These channels are non-selective cation channels with variable Ca²⁺ selectivity having extensive sequence homology. They constitute a superfamily made up of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Some of these TRP channel isotypes are expressed in the eye and localized to both neuronal and non-neuronal cell membranes. Their activation generates intracellular Ca²⁺ transients and other downstream-linked signalling events that affect numerous responses required for visual function. As there is an association between changes in functional TRP expression in various ocular diseases, there are efforts underway to determine if these channels can be used as drug targets to reverse declines in ocular function. We review here our current knowledge about the expression, function and regulation of TRPs in different eye tissues in health and disease. Furthermore, some of the remaining hurdles are described to developing safe and efficacious TRP channel modulators for use in a clinical setting.

Effects of Adipokines and Insulin on Intracellular pH, Calcium Concentration, and Responses to Hypo-Osmolarity in Human Articular Chondrocytes from Healthy and Osteoarthritic Cartilage

OBJECTIVE

To evaluate the effects of adipokines and insulin on intracellular calcium concentration ([Ca(2+)]i) and pH (pHi) in human articular chondrocytes from healthy (CHC) and osteoarthritic cartilage (COC).

DESIGN

pHi and [Ca(2+)]i were measured using BCECF and Fura-2 fluorometric probes in CHC and COC under control conditions and following a hypotonic shock. The effects of interleukin-1β (IL1β), tumor necrosis factor-α (TNFα), insulin, leptin, resistin, and adiponectin were assessed.

RESULTS

pHi was lower in COC than in CHC. Only IL1β β decreased pHi in both cell types; all the agents enhanced pHi recovery following an ammonium prepulse in CHC, effect that was attenuated by Na(+)-H(+) exchanger inhibitors, but they had no effect in COC. Hypotonic shock (HTS) caused a pHi increase, which was significantly smaller in COC. All the hormones attenuated this response and the effect of IL1β was greater. The basal [Ca(2+)]i was similar in COC and CHC; IL1β, TNFα, and insulin increased the [Ca(2+)]i, but leptin, resistin, and adiponectin did not. These effects were greater in COC. This [Ca(2+)]i increase was dependent on extracellular Ca(2+) and attenuated by Na(+)-Ca(2+) exchanger inhibitors. HTS caused a [Ca(2+)]i increase, which was inhibited by transient receptor potential vanilloid blockers and attenuated by all the hormones tested with the exception of adiponectin.

CONCLUSIONS

These findings may help explain the association between obesity and osteoarthritis, in which these hormones are altered. The responses of CHC and COC are different, which suggests that a modification of pH and Ca(2+) homeostasis is part of the osteoarthritis pathophysiology.

Brain transient receptor potential channels and stroke

Transient receptor potential (TRP) channels have been increasingly implicated in the pathological mechanisms of CNS disorders. TRP expression has been detected in neurons, astrocytes, oligodendrocytes, microglia, and ependymal cells as well as in the cerebral vascular endothelium and smooth muscle. In stroke, TRPC3/4/6, TRPM2/4/7, and TRPV1/3/4 channels have been found to participate in ischemia-induced cell death, whereas other TRP channels, in particular those expressed in nonneuronal cells, have been less well studied. This review summarizes the current knowledge on the expression and functions of the TRP channels in various cell types in the brain and our current understanding of TRP channels in stroke pathophysiology. In an aging society, the occurrence of stroke is expected to increase steadily, and there is an urgent requirement to improve the current stroke management strategy. Therefore, elucidating the roles of TRP channels in stroke could shed light on the development of novel therapeutic strategies and ultimately improve stroke outcome.

Angiotensin II induces membrane trafficking of natively expressed transient receptor potential vanilloid type 4 channels in hypothalamic 4B cells

Transient receptor potential vanilloid family type 4 (TRPV4) channels are expressed in central neuroendocrine neurons and have been shown to be polymodal in other systems. We previously reported that in the rodent, a model of dilutional hyponatremia associated with hepatic cirrhosis, TRPV4 expression is increased in lipid rafts from the hypothalamus and that this effect may be angiotensin dependent. In this study, we utilized the immortalized neuroendocrine rat hypothalamic 4B cell line to more directly test the effects of angiotensin II (ANG II) on TRPV4 expression and function. Our results demonstrate the expression of corticotropin-releasing factor (CRF) transcripts, for sex-determining region Y (SRY) (male genotype), arginine vasopressin (AVP), TRPV4, and ANG II type 1a and 1b receptor in 4B cells. After a 1-h incubation in ANG II (100 nM), 4B cells showed increased TRPV4 abundance in the plasma membrane fraction, and this effect was prevented by the ANG II type 1 receptor antagonist losartan (1 μM) and by a Src kinase inhibitor PP2 (10 μM). Ratiometric calcium imaging experiments demonstrated that ANG II incubation potentiated TRPV4 agonist (GSK 1016790A, 20 nM)-induced calcium influx (control 18.4 ± 2.8% n = 5 and ANG II 80.5 ± 2.4% n = 5). This ANG II-induced increase in calcium influx was also blocked by 1 μM losartan and 10 μM PP2 (losartan 26.4 ± 3.8% n = 5 and PP2 19.7 ± 3.9% n = 5). Our data suggests that ANG II can increase TRPV4 channel membrane expression in 4B cells through its action on AT1R involving a Src kinase pathway.

TRPV4 activation mediates flow-induced nitric oxide production in the rat thick ascending limb

Nitric oxide (NO) regulates renal function. Luminal flow stimulates NO production in the thick ascending limb (TAL). Transient receptor potential vanilloid 4 (TRPV4) is a mechano-sensitive channel activated by luminal flow in different types of cells. We hypothesized that TRPV4 mediates flow-induced NO production in the rat TAL. We measured NO production in isolated, perfused rat TALs using the fluorescent dye DAF FM. Increasing luminal flow from 0 to 20 nl/min stimulated NO from 8 ± 3 to 45 ± 12 arbitrary units (AU)/min (n = 5; P < 0.05). The TRPV4 antagonists, ruthenium red (15 μmol/l) and RN 1734 (10 μmol/l), blocked flow-induced NO production. Also, luminal flow did not increase NO production in the absence of extracellular calcium. We also studied the effect of luminal flow on NO production in TALs transduced with a TRPV4shRNA. In nontransduced TALs luminal flow increased NO production by 47 ± 17 AU/min (P < 0.05; n = 5). Similar to nontransduced TALs, luminal flow increased NO production by 39 ± 11 AU/min (P < 0.03; n = 5) in TALs transduced with a control negative sequence-shRNA while in TRPV4shRNA-transduced TALs, luminal flow did not increase NO production (Δ10 ± 15 AU/min; n = 5). We then tested the effect of two different TRPV4 agonists on NO production in the absence of luminal flow. 4α-Phorbol 12,13-didecanoate (1 μmol/l) enhanced NO production by 60 ± 11 AU/min (P < 0.002; n = 7) and GSK1016790A (10 ηmol/l) increased NO production by 52 ± 15 AU/min (P < 0.03; n = 5). GSK1016790A (10 ηmol/l) did not stimulate NO production in TRPV4shRNA-transduced TALs. We conclude that activation of TRPV4 channels mediates flow-induced NO production in the rat TAL.

Analysis of responses to the TRPV4 agonist GSK1016790A in the pulmonary vascular bed of the intact-chest rat

The transient receptor potential vanilloid 4 (TRPV4) channel is a nonselective cation channel expressed on many cell types, including the vascular endothelium and smooth muscle cells. TRPV4 channels play a role in regulating vasomotor tone and capillary permeability. The present study was undertaken to investigate responses to the TRPV4 agonist GSK101790A on the pulmonary and systemic vascular beds in the rat. Intravenous injection of GSK1016790A at doses of 2-10 μg/kg produced dose-dependent decreases in systemic arterial pressure, small decreases in pulmonary arterial pressure, and small increases in cardiac output, and responses were not altered by the cyclooxygenase inhibitor meclofenamate or the cytochrome P-450 inhibitor miconazole. Injection of GSK1016790A at a dose of 12 μg/kg iv produced cardiovascular collapse that was reversible in some animals. GSK1016790A produced dose-related decreases in pulmonary and systemic arterial pressure when baseline tone in the pulmonary vascular bed was increased with U-46619. After treatment with the nitric oxide synthase (NOS) inhibitor N-nitro-l-arginine methyl ester, GSK1016790A produced larger decreases in systemic arterial pressure and dose-dependent increases in pulmonary arterial pressure followed by a small decrease. These results demonstrate that GSK1016790A has vasodilator activity in pulmonary and systemic vascular beds and that when NOS is inhibited, GSK1016790A produced pulmonary vasoconstrictor responses that were attenuated by the L-type Ca(2+) channel antagonist isradipine. The presence of TRPV4 immunoreactivity was observed in small pulmonary arteries and airways. The present data indicate that responses to TRPV4 are modulated differently by NOS in pulmonary and systemic vascular beds and are attenuated by the TRPV4 antagonist GSK2193874.

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.

Transient receptor potential vanilloid 4 mediates hypotonicity-induced enhancement of synaptic transmission in hippocampal slices

AIM AND METHODS

Changes in cerebrospinal fluid osmotic pressure modulate brain excitability. Transient receptor potential vanilloid 4 (TRPV4), which is sensitive to hypotonic stimulation, is expressed in hippocampus. The present study investigated the effect of hypotonic stimulation on hippocampal synaptic transmission and the role of TRPV4 in hypotonicity-action using electrophysiological recording and pharmacological technique.

RESULTS

Accompanied with the decrease in paired pulse facilitation, field excitatory postsynaptic potential (fEPSP) was enhanced by hypotonicity and TRPV4 agonist 4α-PDD in hippocampal slices, which was sensitive to TRPV4 antagonist HC-067047. Hypotonicity-induced increase in fEPSP was blocked by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, but not N-methyl-d-aspartate receptor or N- or P/Q-type voltage-gated calcium channel antagonist. High voltage-gated calcium current (ICa ) in hippocampal CA3 pyramidal neurons was not affected by hypotonicity. AMPA-activated current (IAMPA ) in hippocampal CA1 pyramidal neurons was increased by hypotonicity and 4α-PDD, which was attenuated by HC-067047. Inhibition of protein kinase C or protein kinase A markedly attenuated hypotonicity-increased IAMPA , whereas antagonism of calcium/calmodulin-dependent protein kinase II had no such effect.

CONCLUSION

TRPV4 is involved in hypotonicity-induced enhancement of hippocampal synaptic transmission, which may be mediated through promoting presynaptic glutamate release and increasing postsynaptic AMPA receptor function.

Astrocyte regulation of cerebral vascular tone

Cerebral blood flow is controlled by two crucial processes, cerebral autoregulation (CA) and neurovascular coupling (NVC) or functional hyperemia. Whereas CA ensures constant blood flow over a wide range of systemic pressures, NVC ensures rapid spatial and temporal increases in cerebral blood flow in response to neuronal activation. The focus of this review is to discuss the cellular mechanisms by which astrocytes contribute to the regulation of vascular tone in terms of their participation in NVC and, to a lesser extent, CA. We discuss evidence for the various signaling modalities by which astrocytic activation leads to vasodilation and vasoconstriction of parenchymal arterioles. Moreover, we provide a rationale for the contribution of astrocytes to pressure-induced increases in vascular tone via the vasoconstrictor 20-HETE (a downstream metabolite of arachidonic acid). Along these lines, we highlight the importance of the transient receptor potential channel of the vanilloid family (TRPV4) as a key molecular determinant in the regulation of vascular tone in cerebral arterioles. Finally, we discuss current advances in the technical tools available to study NVC mechanisms in the brain as it relates to the participation of astrocytes.

Activation of Transient Receptor Potential Vanilloid 4 Increases NMDA-Activated Current in Hippocampal Pyramidal Neurons

The glutamate excitotoxicity, mediated through N-methyl-d-aspartate receptors (NMDARs), plays an important role in cerebral ischemia injury. Transient receptor potential vanilloid 4 (TRPV4) can be activated by multiple stimuli that may happen during stroke. The present study evaluated the effect of TRPV4 activation on NMDA-activated current (I_NMDA) and that of blocking TRPV4 on brain injury after focal cerebral ischemia in mice. We herein report that activation of TRPV4 by 4α-PDD and hypotonic stimulation increased I_NMDA in hippocampal CA1 pyramidal neurons, which was sensitive to TRPV4 antagonist HC-067047 and NMDAR antagonist AP-5, indicating that TRPV4 activation potentiates NMDAR response. In addition, the increase in I_NMDA by hypotonicity was sensitive to the antagonist of NMDAR NR2B subunit, but not of NR2A subunit. Furthermore, antagonists of calcium/calmodulin-dependent protein kinase II (CaMKII) significantly attenuated hypotonicity-induced increase in I_NMDA, while antagonists of protein kinase C or casein kinase II had no such effect, indicating that phosphorylation of NR2B subunit by CaMKII is responsible for TRPV4-potentiated NMDAR response. Finally, we found that intracerebroventricular injection of HC-067047 after 60 min middle cerebral artery occlusion reduced the cerebral infarction with at least a 12 h efficacious time-window. These findings indicate that activation of TRPV4 increases NMDAR function, which may facilitate glutamate excitotoxicity. Closing TRPV4 may exert potent neuroprotection against cerebral ischemia injury through many mechanisms at least including the prevention of NMDAR-mediated glutamate excitotoxicity.

Prolonged high fat/alcohol exposure increases TRPV4 and its functional responses in pancreatic stellate cells

The present study investigated transient receptor potential vanilloid type 4 (TRPV4) ion channels in pancreatic stellate cells (PSCs) isolated from rats with high-fat and alcohol diet (HFA)-induced chronic pancreatitis. TRPV4 is a calcium-permeable nonselective ion channel responsive to osmotic changes, alcohol metabolites arachidonic acid, anandamide, their derivatives, and injury-related lipid mediators. Male Lewis rats were fed HFA for 6-8 wk before isolation and primary culture of PSCs. Control PSCs were harvested from rats fed standard chow. Immunoreactivity for cytoskeletal protein activation product α-smooth muscle actin (α-SMA) and platelet-derived growth factor receptor-β subunit (PDGFR-β) characterized the cells as PSCs. TRPV4 expression increased in PSCs of HFA-fed rats and control cultures after alcohol treatment (50 mM). Cell responses to activation of inducible TRPV4 were assessed with live cell calcium imaging. Threefold increased and sustained intracellular calcium mobilization responses occurred in 70% of pancreatic stellate cells from HFA-fed rats in response to TRPV4 activators arachidonic acid, lipid second messenger, phorbol ester 4 α-phorbol 12,13-didecanoate (4αPDD), and 50% hypoosmotic media compared with relatively unresponsive PSCs from control rats. Activation responses were attenuated by nonselective TRPV channel blocker ruthenium red. Tumor necrosis factor-α (TNF-α, 1 ng/ml, 16 h) increased responses to 4αPDD in control PSCs. These findings implicate TRPV4-mediated calcium responses inducible after HFA exposure and inflammation in reactive responses of activated PSCs that impair pancreatic function, such as responsiveness to cytokines and the deposition of collagen fibrosis that precipitates ductal blockage and pain.

Novel insights into TRPV4 function in the kidney

Kidneys are complex highly organized paired organs of nearly one million nephrons each. They rigorously process about 180 l of plasma daily to keep whole body homeostasis. To effectively perform such a titanic work, kidneys rely on mechanisms able to sense dynamic changes in composition and flow rates of protourine along the renal tubule. It is envisioned that Ca(2+)-permeable transient receptor potential (TRP) channels, and specifically mechanosensitive TRPV4, can serve to interpret these external mechanical cues in the form of elevated intracellular Ca(2+) concentration. This, in turn, initiates multiple cellular responses and adaptation mechanisms. The current review summarizes up-to-date knowledge about the sites of TRPV4 expression in renal tissue as well as discusses the functional role of the channel in cellular responses to hypotonicity and tubular flow. We will also provide insights as to how TRPV4 fits into classical polycystin mechanosensory complex in cilia and will speculate about previously underappreciated clinical implication of pharmacological TRPV4 targeting in treatment of polycystic kidney disease.

Nociceptive and pro-inflammatory effects of dimethylallyl pyrophosphate via TRPV4 activation

BACKGROUND AND PURPOSE

Sensory neuronal and epidermal transient receptor potential ion channels (TRPs) serve an important role as pain sensor molecules. While many natural and synthetic ligands for sensory TRPs have been identified, little is known about the endogenous activator for TRPV4. Recently, we reported that endogenous metabolites produced by the mevalonate pathway regulate the activities of sensory neuronal TRPs. Here, we show that dimethylallyl pyrophosphate (DMAPP), a substance produced by the same pathway is an activator of TRPV4.

EXPERIMENTAL APPROACH

We examined the effects of DMAPP on sensory TRPs using Ca²⁺ imaging and whole-cell electrophysiology experiments with a heterologous expression system (HEK293T cells transfected with individual TRP channels), cultured sensory neurons and keratinocytes. We then evaluated nociceptive behavioural and inflammatory changes upon DMAPP administration in mice in vivo.

KEY RESULTS

In the HEK cell heterologous expression system, cultured sensory neurons and keratinocytes, µM concentrations of DMAPP activated TRPV4. Agonistic and antagonistic potencies of DMAPP for other sensory TRP channels were examined and activation of TRPV3 by camphor was found to be inhibited by DMAPP. In vivo assays, intraplantar injection of DMAPP acutely elicited nociceptive flinches that were prevented by pretreatment with TRPV4 blockers, indicating that DMAPP is a novel pain-producing molecule through TRPV4 activation. Further, DMAPP induced acute inflammation and noxious mechanical hypersensitivities in a TRPV4-dependent manner.

CONCLUSIONS AND IMPLICATIONS

Overall, we found a novel sensory TRP acting metabolite and suggest that its use may help to elucidate the physiological role of TRPV4 in nociception and associated inflammation.

Apigenin, a plant-derived flavone, activates transient receptor potential vanilloid 4 cation channel

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+) -permeable channel with multiple modes of activation. Apigenin is a plant-derived flavone, which has potential preventive effects on the development of cardiovascular disease. We set out to explore the effects of apigenin on TRPV4 channel activity and its role in vasodilatation.

EXPERIMENTAL APPROACH

The effects of apigenin (0.01-30 µM) on TPRV4 channels were investigated in HEK293 cells over-expressing TRPV4, rat primary cultured mesenteric artery endothelial cells (MAECs) and isolated small mesenteric arterial segments using whole-cell patch clamp, fluorescent Ca(2+) imaging, intracellular recording and pressure myography.

KEY RESULTS

Whole-cell patch clamp and fluorescent Ca(2+) imaging in HEK cells over-expressing TRPV4 showed that apigenin concentration-dependently stimulated the TRPV4-mediated cation current and Ca(2+) influx. In MAECs, apigenin stimulated Ca(2+) influx in a concentration-dependent manner. These increases in cation current and Ca(2+) influx were markedly inhibited by TRPV4-specific blockers and siRNAs. Furthermore, pressure myography and intracellular recording in small third-order mesenteric arteries showed that apigenin dose-dependently evoked smooth muscle cell membrane hyperpolarization and subsequent vascular dilatation, which were significantly inhibited by TRPV4-specific blockers. TRPV4 blocker or charybdotoxin (200 nM) plus apamin (100 nM) diminished the apigenin-induced dilatation.

CONCLUSION AND IMPLICATIONS

This is the first study to demonstrate the selective stimulation of TRPV4 by apigenin. Apigenin was found to activate TRPV4 channels in a dose-dependent manner in HEK cells over-expressing TRPV4 and in native endothelial cells. In rat small mesenteric arteries, apigenin acts on TRPV4 in endothelial cells to induce EDHF-mediated vascular dilatation.

The increased activity of TRPV4 channel in the astrocytes of the adult rat hippocampus after cerebral hypoxia/ischemia

The polymodal transient receptor potential vanilloid 4 (TRPV4) channel, a member of the TRP channel family, is a calcium-permeable cationic channel that is gated by various stimuli such as cell swelling, low pH and high temperature. Therefore, TRPV4-mediated calcium entry may be involved in neuronal and glia pathophysiology associated with various disorders of the central nervous system, such as ischemia. The TRPV4 channel has been recently found in adult rat cortical and hippocampal astrocytes; however, its role in astrocyte pathophysiology is still not defined. In the present study, we examined the impact of cerebral hypoxia/ischemia (H/I) on the functional expression of astrocytic TRPV4 channels in the adult rat hippocampal CA1 region employing immunohistochemical analyses, the patch-clamp technique and microfluorimetric intracellular calcium imaging on astrocytes in slices as well as on those isolated from sham-operated or ischemic hippocampi. Hypoxia/ischemia was induced by a bilateral 15-minute occlusion of the common carotids combined with hypoxic conditions. Our immunohistochemical analyses revealed that 7 days after H/I, the expression of TRPV4 is markedly enhanced in hippocampal astrocytes of the CA1 region and that the increasing TRPV4 expression coincides with the development of astrogliosis. Additionally, adult hippocampal astrocytes in slices or cultured hippocampal astrocytes respond to the TRPV4 activator 4-alpha-phorbol-12,-13-didecanoate (4αPDD) by an increase in intracellular calcium and the activation of a cationic current, both of which are abolished by the removal of extracellular calcium or exposure to TRP antagonists, such as Ruthenium Red or RN1734. Following hypoxic/ischemic injury, the responses of astrocytes to 4αPDD are significantly augmented. Collectively, we show that TRPV4 channels are involved in ischemia-induced calcium entry in reactive astrocytes and thus, might participate in the pathogenic mechanisms of astroglial reactivity following ischemic insult.

Endocannabinoids and the cardiovascular response to stress

Stress activates the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system (SNS), resulting in cardiovascular responses. The endocannabinoid system (ECS), a ubiquitously expressed lipid signalling system, modulates both HPA and SNS activity. The purpose of this review is to explore the possible involvement/role of the ECS in the cardiovascular response to stress. The ECS has numerous cardiovascular effects including modulation of blood pressure, heart rate, the baroreflex, and direct vascular actions. It is also involved in a protective manner in response to stressors in cardiac preconditioning, and various stressors (for example, pain, orthostasis and social stress) increase plasma levels of endocannabinoids. Given the multitude of vascular effects of endocannabinoids, this is bound to have consequences. Beneficial effects of ECS upregulation could include cardioprotection, vasodilatation, CB(2)-mediated anti-inflammatory effects and activation of peroxisome proliferator-activated receptors. Negative effects of endocannabinoids could include mediation of the effects of glucocorticoids, CB(1)-mediated metabolic changes, and metabolism to vasoconstrictor products. It is also likely that there is a central role for the ECS in modulating cardiovascular activity via the HPA and SNS. However, much more work is required to fully integrate the role of the ECS in mediating many of the physiological responses to stress, including cardiovascular responses.

Lymphocyte TRPV 1-4 gene expression and MIF blood levels in a young girl clinically diagnosed with HSAN IV

OBJECTIVE

Patients with congenital insensitivity to pain are unable to sense pain and temperature. They undergo many injuries, inflammatory state, and infections. Various mutations in the neurotrophic tyrosine kinase receptor gene have been implicated in this disorder. We measured the leukocyte expression of transient receptor potential vanilloid (TRPV) 1-4 genes and the blood macrophage migration inhibitory factor (MIF) concentration in a young girl clinically diagnosed with congenital insensitivity to pain. The investigation may help to define the interplay between nerve growth factor and TRPV 1-4 channels and between these sensors and MIF in this disease, and in broader terms in nociception.

METHODS

TRPV 1-4 gene expression (real-time polymerase chain reaction) and MIF concentration (enzyme-linked immunosorbent assay) were determined in the blood of the girl, her family, and control participants. Statistical analysis of gene expression was carried out between samples and controls with a mathematical model based on the correction for exact polymerase chain reaction efficiencies, and the mean crossing point deviation between samples and controls.

RESULTS

The TRPV 1--4 gene expression rates did not significantly differ from the values found in the control group. TRPV1 was almost doubly upregulated. MIF levels were much higher than the reference value.

DISCUSSION

The high increase in the MIF concentration (likely due to the chronic or recurrent inflammatory state) may have contributed to the normal expression of TRPV 1-4 and to the relative upregulation of TRPV1. The role of this cytokine on the expression of these genes deserves further investigation.