PKCα mediates acetylcholine-induced activation of TRPV4-dependent calcium influx in endothelial cells

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.

Epac-protein kinase C alpha signaling in purinergic P2X3R-mediated hyperalgesia after inflammation

Protein kinase C alpha plays a major role in mediating Epac-dependent enhancement of purinergic P2X3R activity in dorsal root ganglion neurons after inflammation.

Sensitization of purinergic P2X3 receptors (P2X3Rs) is a major mechanism contributing to injury-induced exaggerated pain responses. We showed in a previous study that cyclic adenosine monophosphate (cAMP)–dependent guanine nucleotide exchange factor 1 (Epac1) in rat sensory dorsal root ganglia (DRGs) is upregulated after inflammatory injury, and it plays a critical role in P2X3R sensitization by activating protein kinase C epsilon (PKCε) inside the cells. protein kinase C epsilon has been established as the major PKC isoform mediating injury-induced hyperalgesic responses. On the other hand, the role of PKCα in receptor sensitization was seldom considered. Here, we studied the participation of PKCα in Epac signaling in P2X3R-mediated hyperalgesia. The expression of both Epac1 and Epac2 and the level of cAMP in DRGs are greatly enhanced after complete Freund adjuvant (CFA)–induced inflammation. The expression of phosphorylated PKCα is also upregulated. Complete Freund adjuvant (CFA)–induced P2X3R-mediated hyperalgesia is not only blocked by Epac antagonists but also by the classical PKC isoform inhibitors, Go6976, and PKCα-siRNA. These CFA effects are mimicked by the application of the Epac agonist, 8-(4-chlorophenylthio)-2 -O-methyl-cAMP (CPT), in control rats, further confirming the involvement of Epacs. Because the application of Go6976 prior to CPT still reduces CPT-induced hyperalgesia, PKCα is downstream of Epacs to mediate the enhancement of P2X3R responses in DRGs. The pattern of translocation of PKCα inside DRG neurons in response to CPT or CFA stimulation is distinct from that of PKCε. Thus, in contrast to prevalent view, PKCα also plays an essential role in producing complex inflammation-induced receptor-mediated hyperalgesia.

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.

Mechanosensitive transient receptor potential vanilloid 4 regulates Dermatophagoides farinae-induced airway remodeling via 2 distinct pathways modulating matrix synthesis and degradation

Contributions of mechanical signals to airway remodeling during asthma are poorly understood. Transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive ion channel, has been implicated in cardiac and pulmonary fibrosis; however, its role in asthma remains elusive. Employing a Dermatophagoides farinae-induced asthma model, we report here that TRPV4-knockout mice were protected from D. farinae-induced airway remodeling. Furthermore, lung fibroblasts that were isolated from TRPV4-knockout mice showed diminished differentiation potential compared with wild-type mice. Fibroblasts from asthmatic lung exhibited increased TRPV4 activity and enhanced differentiation potential compared with normal human lung fibroblasts. Of interest, TGF-β1 treatment enhanced TRPV4 activation in a PI3K-dependent manner in normal human lung fibroblasts in vitro Mechanistically, TRPV4 modulated matrix remodeling in the lung via 2 distinct but dependent pathways: one enhances matrix deposition by fibrotic gene activation, whereas the other slows down matrix degradation by increased plasminogen activator inhibitor 1. Of importance, both pathways are regulated by Rho/myocardin-related transcription factor-A and contribute to fibroblast differentiation and matrix remodeling in the lung. Thus, our results support a unique role for TRPV4 in D. farinae-induced airway remodeling and warrant further studies in humans for it to be used as a novel therapeutic target in the treatment of asthma.-Gombedza, F., Kondeti, V., Al-Azzam, N., Koppes, S., Duah, E., Patil, P., Hexter, M., Phillips, D., Thodeti, C. K., Paruchuri, S. Mechanosensitive transient receptor potential vanilloid 4 regulates Dermatophagoides farinae-induced airway remodeling via 2 distinct pathways modulating matrix synthesis and degradation.

Hydrogen sulfide-induced vasodilation mediated by endothelial TRPV4 channels

Hydrogen sulfide (H₂S) is a recently described gaseous vasodilator produced within the vasculature by the enzymes cystathionine γ-lyase and 3-mercaptopyruvate sulfurtransferase. Previous data demonstrate that endothelial cells (EC) are the source of endogenous H₂S production and are required for H₂S-induced dilation. However, the signal transduction pathway activated by H₂S within EC has not been elucidated. TRPV4 and large-conductance Ca²⁺-activated K channels (BK channels) are expressed in EC. H₂S-induced dilation is inhibited by luminal administration of iberiotoxin and disruption of the endothelium. Calcium influx through TRPV4 may activate these endothelial BK channels (eBK). We hypothesized that H₂S-mediated vasodilation involves activation of TRPV4 within the endothelium. In pressurized, phenylephrine-constricted mesenteric arteries, H₂S elicited a dose-dependent vasodilation blocked by inhibition of TRPV4 channels (GSK2193874A, 300 nM). H₂S (1 μM) increased TRPV4-dependent (1.8-fold) localized calcium events in EC of pressurized arteries loaded with fluo-4 and Oregon Green. In pressurized EC tubes, H₂S (1 μM) and the TRPV4 activator, GSK101679A (30 nM), increased calcium events 1.8- and 1.5-fold, respectively. H₂S-induced an iberiotoxin-sensitive outward current measured using whole cell patch-clamp techniques in freshly dispersed EC. H₂S increased K⁺ currents from 10 to 30 pA/pF at +150 mV. Treatment with Na₂S increased the level of sulfhydration of TRPV4 channels in aortic ECs. These results demonstrate that H₂S-mediated vasodilation involves activation of TRPV4-dependent Ca²⁺ influx and BK channel activation within EC. Activation of TRPV4 channels appears to cause calcium events that result in the opening of eBK channels, endothelial hyperpolarization, and subsequent vasodilation.

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.

PKCα regulates TMEM16A-mediated Cl⁻ secretion in human biliary cells

TMEM16A is a newly identified Ca(2+)-activated Cl(-) channel in biliary epithelial cells (BECs) that is important in biliary secretion. While extracellular ATP stimulates TMEM16A via binding P2 receptors and increasing intracellular Ca(2+) concentration ([Ca(2+)]i), the regulatory pathways have not been elucidated. Protein kinase C (PKC) contributes to ATP-mediated secretion in BECs, although its potential role in TMEM16A regulation is unknown. To determine whether PKCα regulates the TMEM16A-dependent membrane Cl(-) transport in BECs, studies were performed in human biliary Mz-cha-1 cells. Addition of extracellular ATP induced a rapid translocation of PKCα from the cytosol to the plasma membrane and activation of whole cell Ca(2+)-activated Cl(-) currents. Currents demonstrated outward rectification and reversal at 0 mV (properties consistent with TMEM16A) and were inhibited by either molecular (siRNA) or pharmacologic (PMA or Gö6976) inhibition of PKCα. Intracellular dialysis with recombinant PKCα activated Cl(-) currents with biophysical properties identical to TMEM16A in control cells but not in cells after transfection with TMEM16A siRNA. In conclusion, our studies demonstrate that PKCα is coupled to ATP-stimulated TMEM16A activation in BECs. Targeting this ATP-Ca(2+)-PKCα signaling pathway may represent a therapeutic strategy to increase biliary secretion and promote bile formation.

P2Y1 Receptor Activation of the TRPV4 Ion Channel Enhances Purinergic Signaling in Satellite Glial Cells

Transient receptor potential (TRP) ion channels of peripheral sensory pathways are important mediators of pain, itch, and neurogenic inflammation. They are expressed by primary sensory neurons and by glial cells in the central nervous system, but their expression and function in satellite glial cells (SGCs) of sensory ganglia have not been explored. SGCs tightly ensheath neurons of sensory ganglia and can regulate neuronal excitability in pain and inflammatory states. Using a modified dissociation protocol, we isolated neurons with attached SGCs from dorsal root ganglia of mice. SGCs, which were identified by expression of immunoreactive Kir4.1 and glutamine synthetase, were closely associated with neurons, identified using the pan-neuronal marker NeuN. A subpopulation of SGCs expressed immunoreactive TRP vanilloid 4 (TRPV4) and responded to the TRPV4-selective agonist GSK1016790A by an influx of Ca(2+) ions. SGCs did not express functional TRPV1, TRPV3, or TRP ankyrin 1 channels. Responses to GSK1016790A were abolished by the TRPV4 antagonist HC067047 and were absent in SGCs from Trpv4(-/-) mice. The P2Y1-selective agonist 2-methylthio-ADP increased [Ca(2+)]i in SGCs, and responses were prevented by the P2Y1-selective antagonist MRS2500. P2Y1 receptor-mediated responses were enhanced in TRPV4-expressing SGCs and HEK293 cells, suggesting that P2Y1 couples to and activates TRPV4. PKC inhibitors prevented P2Y1 receptor activation of TRPV4. Our results provide the first evidence for expression of TRPV4 in SGCs and demonstrate that TRPV4 is a purinergic receptor-operated channel in SGCs of sensory ganglia.

TRPV4 channel activation selectively inhibits tumor endothelial cell proliferation

Endothelial cell proliferation is a critical event during angiogenesis, regulated by both soluble factors and mechanical forces. Although the proliferation of tumor cells is studied extensively, little is known about the proliferation of tumor endothelial cells (TEC) and its contribution to tumor angiogenesis. We have recently shown that reduced expression of the mechanosensitive ion channel TRPV4 in TEC causes aberrant mechanosensitivity that result in abnormal angiogenesis. Here, we show that TEC display increased proliferation compared to normal endothelial cells (NEC). Further, we found that TEC exhibit high basal ERK1/2 phosphorylation and increased expression of proliferative genes important in the G1/S phase of the cell cycle. Importantly, pharmacological activation of TRPV4, with a small molecular activator GSK1016790A (GSK), significantly inhibited TEC proliferation, but had no effect on the proliferation of NEC or the tumor cells (epithelial) themselves. This reduction in TEC proliferation by TRPV4 activation was correlated with a decrease in high basal ERK1/2 phosphorylation. Finally, using a syngeneic tumor model revealed that TRPV4 activation, with GSK, significantly reduced endothelial cell proliferation in vivo. Our findings suggest that TRPV4 channels regulate tumor angiogenesis by selectively inhibiting tumor endothelial cell proliferation.

Mechanical stretch: physiological and pathological implications for human vascular endothelial cells

Vascular endothelial cells are subjected to hemodynamic forces such as mechanical stretch due to the pulsatile nature of blood flow. Mechanical stretch of different intensities is detected by mechanoreceptors on the cell surface which enables the conversion of external mechanical stimuli to biochemical signals in the cell, activating downstream signaling pathways. This activation may vary depending on whether the cell is exposed to physiological or pathological stretch intensities. Substantial stretch associated with normal physiological functioning is important in maintaining vascular homeostasis as it is involved in the regulation of cell structure, vascular angiogenesis, proliferation and control of vascular tone. However, the elevated pressure that occurs with hypertension exposes cells to excessive mechanical load, and this may lead to pathological consequences through the formation of reactive oxygen species, inflammation and/or apoptosis. These processes are activated by downstream signaling through various pathways that determine the fate of cells. Identification of the proteins involved in these processes may help elucidate novel mechanisms involved in vascular disease associated with pathological mechanical stretch and could provide new insight into therapeutic strategies aimed at countering the mechanisms’ negative effects.

Blockade or deletion of transient receptor potential vanilloid 4 (TRPV4) is not protective in a murine model of sepsis

Sepsis is a systemic inflammatory response triggered by microbial infection that can cause cardiovascular collapse, insufficient tissue perfusion and multi-organ failure. The cation channel transient receptor potential vanilloid 4 (TRPV4) is expressed in vascular endothelium and causes vasodilatation, but excessive TRPV4 activation leads to profound hypotension and circulatory collapse - key features of sepsis pathogenesis. We hypothesised that loss of TRPV4 signaling would protect against cardiovascular dysfunction in a mouse model of sepsis (endotoxaemia). Multi-parameter monitoring of conscious systemic haemodynamics (by radiotelemetry probe), mesenteric microvascular blood flow (laser speckle contrast imaging) and blood biochemistry (iSTAT blood gas analysis) was carried out in wild type (WT) and TRPV4 knockout (KO) mice. Endotoxaemia was induced by a single intravenous injection of lipopolysaccharide (LPS; 12.5 mg/kg) and systemic haemodynamics monitored for 24 h. Blood flow recording was then conducted under terminal anaesthesia after which blood was obtained for haematological/biochemical analysis. No significant differences were observed in baseline haemodynamics or mesenteric blood flow. Naïve TRPV4 KO mice were significantly acidotic relative to WT counterparts. Following induction of sepsis, all mice became significantly hypotensive, though there was no significant difference in the degree of hypotension between TRPV4 WT and KO mice. TRPV4 KO mice exhibited a higher sepsis severity score. While septic WT mice became significantly hypernatraemic relative to the naïve state, this was not observed in septic KO mice. Mesenteric blood flow was inhibited by topical application of the TRPV4 agonist GSK1016790A in naïve WT mice, but enhanced 24 h following LPS injection. Contrary to the initial hypothesis, loss of TRPV4 signaling (either through gene deletion or pharmacological antagonism) did not attenuate sepsis-induced cardiovascular dysfunction: in fact, pathology appeared to be modestly exaggerated in mice lacking TRPV4. Local targeting of TRPV4 signalling may be more beneficial than global inhibition in sepsis treatment.

Activation of mechanosensitive ion channel TRPV4 normalizes tumor vasculature and improves cancer therapy

Tumor vessels are characterized by abnormal morphology and hyper-permeability that together cause inefficient delivery of chemotherapeutic agents. Although VEGF has been established as a critical regulator of tumor angiogenesis, the role of mechanical signaling in the regulation of tumor vasculature or tumor endothelial cell (TEC) function is not known. Here, we show that the mechanosensitive ion channel TRPV4 regulates tumor angiogenesis and tumor vessel maturation via modulation of TEC mechanosensitivity. We found that TEC exhibit reduced TRPV4 expression and function, which is correlated with aberrant mechanosensitivity towards ECM stiffness, increased migration and abnormal angiogenesis by TEC. Further, syngeneic tumor experiments revealed that the absence of TRPV4 induced increased vascular density, vessel diameter and reduced pericyte coverage resulting in enhanced tumor growth in TRPV4 KO mice. Importantly, overexpression or pharmacological activation of TRPV4 restored aberrant TEC mechanosensitivity, migration and normalized abnormal angiogenesis in vitro by modulating Rho activity. Finally, a small molecule activator of TRPV4, GSK1016790A, in combination with anti-cancer drug Cisplatin, significantly reduced tumor growth in WT mice by inducing vessel maturation. Our findings demonstrate TRPV4 channels to be critical regulators of tumor angiogenesis and represent a novel target for anti-angiogenic and vascular normalization therapies.

TRPV1 and TRPV4 channels: potential therapeutic targets for ischemic conditioning-induced cardioprotection

Besides the involvement of TRPV channels in exhibiting various cellular functions including thermoregulation, pain perception, maintenance of bone homeostasis and gastrointestinal function; certain studies have also implicated the putative role of these channels in mediating ischemic conditioning-induced cardioprotection. The potential role of TRPV1 channels in different forms of ischemic conditioning (pre/post/remote)-induced cardioprotection has been described by employing TRPV1 knockout mice and various pharmacological modulators. The cardioprotective effects of TRPV1 activation during ischemic conditioning have been linked with increased CGRP, substance P release and augmented ALOX expression. Furthermore, the role of TRPV4 channels in mediating preconditioning-induced preservation of vascular function in terms restoring NO- and further improving EDH(F)-mediated endothelial relaxation has been described. The present review discusses the putative role of TRPV1 and TRPV4 channels in mediating different forms of conditioning (pre/post/remote)-induced cardioprotection along with the possible mechanisms. Future perspectives have also been described to fully understand the cascade of signaling and contribution of TRPV channel activation during myocardial ischemic conditioning.

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.

Upregulation of SK3 and IK1 channels contributes to the enhanced endothelial calcium signaling and the preserved coronary relaxation in obese Zucker rats

Background and Aims

Endothelial small- and intermediate-conductance K_Ca channels, SK3 and IK1, are key mediators in the endothelium-derived hyperpolarization and relaxation of vascular smooth muscle and also in the modulation of endothelial Ca²⁺ signaling and nitric oxide (NO) release. Obesity is associated with endothelial dysfunction and impaired relaxation, although how obesity influences endothelial SK3/IK1 function is unclear. Therefore we assessed whether the role of these channels in the coronary circulation is altered in obese animals.

Methods and Results

In coronary arteries mounted in microvascular myographs, selective blockade of SK3/IK1 channels unmasked an increased contribution of these channels to the ACh- and to the exogenous NO- induced relaxations in arteries of Obese Zucker Rats (OZR) compared to Lean Zucker Rats (LZR). Relaxant responses induced by the SK3/IK1 channel activator NS309 were enhanced in OZR and NO- endothelium-dependent in LZR, whereas an additional endothelium-independent relaxant component was found in OZR. Fura2-AM fluorescence revealed a larger ACh-induced intracellular Ca²⁺ mobilization in the endothelium of coronary arteries from OZR, which was inhibited by blockade of SK3/IK1 channels in both LZR and OZR. Western blot analysis showed an increased expression of SK3/IK1 channels in coronary arteries of OZR and immunohistochemistry suggested that it takes place predominantly in the endothelial layer.

Conclusions

Obesity may induce activation of adaptive vascular mechanisms to preserve the dilator function in coronary arteries. Increased function and expression of SK3/IK1 channels by influencing endothelial Ca²⁺ dynamics might contribute to the unaltered endothelium-dependent coronary relaxation in the early stages of obesity.

Trafficking of ThermoTRP Channels

ThermoTRP channels (thermoTRPs) define a subfamily of the transient receptor potential (TRP) channels that are activated by changes in the environmental temperature, from noxious cold to injurious heat. Acting as integrators of several stimuli and signalling pathways, dysfunction of these channels contributes to several pathological states. The surface expression of thermoTRPs is controlled by both, the constitutive and regulated vesicular trafficking. Modulation of receptor surface density during pathological processes is nowadays considered as an interesting therapeutic approach for management of diseases, such as chronic pain, in which an increased trafficking is associated with the pathological state. This review will focus on the recent advances trafficking of the thermoTRP channels, TRPV1, TRPV2, TRPV4, TRPM3, TRPM8 and TRPA1, into/from the plasma membrane. Particularly, regulated membrane insertion of thermoTRPs channels contributes to a fine tuning of final channel activity, and indeed, it has resulted in the development of novel therapeutic approaches with successful clinical results such as disruption of SNARE-dependent exocytosis by botulinum toxin or botulinomimetic peptides.

AKAP150-dependent cooperative TRPV4 channel gating is central to endothelium-dependent vasodilation and is disrupted in hypertension

Endothelial cell dysfunction, characterized by a diminished response to endothelial cell-dependent vasodilators, is a hallmark of hypertension. TRPV4 channels play a major role in endothelial-dependent vasodilation, a function mediated by local Ca(2+) influx through clusters of functionally coupled TRPV4 channels rather than by a global increase in endothelial cell Ca(2+). We showed that stimulation of muscarinic acetylcholine receptors on endothelial cells of mouse arteries exclusively activated TRPV4 channels that were localized at myoendothelial projections (MEPs), specialized regions of endothelial cells that contact smooth muscle cells. Muscarinic receptor-mediated activation of TRPV4 depended on protein kinase C (PKC) and the PKC-anchoring protein AKAP150, which was concentrated at MEPs. Cooperative opening of clustered TRPV4 channels specifically amplified Ca(2+) influx at MEPs. Cooperativity of TRPV4 channels at non-MEP sites was much lower, and cooperativity at MEPs was greatly reduced by chelation of intracellular Ca(2+) or AKAP150 knockout, suggesting that Ca(2+) entering through adjacent channels underlies the AKAP150-dependent potentiation of TRPV4 activity. In a mouse model of angiotensin II-induced hypertension, MEP localization of AKAP150 was disrupted, muscarinic receptor stimulation did not activate TRPV4 channels, cooperativity among TRPV4 channels at MEPs was weaker, and vasodilation in response to muscarinic receptor stimulation was reduced. Thus, endothelial-dependent dilation of resistance arteries is enabled by MEP-localized AKAP150, which ensures the proximity of PKC to TRPV4 channels and the coupled channel gating necessary for efficient communication from endothelial to smooth muscle cells in arteries. Disruption of this molecular assembly may contribute to altered blood flow in hypertension.

Characterization of blood pressure and endothelial function in TRPV4-deficient mice with l-NAME- and angiotensin II-induced hypertension

Transient receptor potential vanilloid type 4 (TRPV4) is an endothelial Ca²⁺ entry channel contributing to endothelium‐mediated dilation in conduit and resistance arteries. We investigated the role of TRPV4 in the regulation of blood pressure and endothelial function under hypertensive conditions. TRPV4‐deficient (TRPV4^−/−) and wild‐type (WT) control mice were given l‐NAME (0.5 g/L) in drinking water for 7 days or subcutaneously infused with angiotensin (Ang) II (600 ng/kg per minute) for 14 days, and blood pressure measured by radiotelemetry. TRPV4^−/− mice had a lower baseline mean arterial pressure (MAP) (12‐h daytime MAP, 94 ± 2 vs. 99 ± 2 mmHg in WT controls). l‐NAME treatment induced a slightly greater increase in MAP in TRPV4^−/− mice (day 7, 13 ± 4%) compared to WT controls (6 ± 2%), but Ang II‐induced increases in MAP were similar in TRPV4^−/− and WT mice (day 14, 53 ± 6% and 37 ± 11%, respectively, P < 0.05). Chronic infusion of WT mice with Ang II reduced both acetylcholine (ACh)‐induced dilation (dilation to 10⁻⁵ mol/L ACh, 71 ± 5% vs. 92 ± 2% of controls) and the TRPV4 agonist GSK1016790A‐induced dilation of small mesenteric arteries (10⁻⁸ mol/L GSK1016790A, 14 ± 5% vs. 77 ± 7% of controls). However, Ang II treatment did not affect ACh dilation in TRPV4^−/− mice. Mechanistically, Ang II did not significantly alter either TRPV4 total protein expression in mesenteric arteries or TRPV4 agonist‐induced Ca²⁺ response in mesenteric endothelial cells in situ. These results suggest that TRPV4 channels play a minor role in blood pressure regulation in l‐NAME‐ but not Ang II‐induced hypertension, but may be importantly involved in Ang II‐induced endothelial dysfunction.

We investigated the role of transient receptor potential vanilloid type 4 (TRPV4) in regulating vascular tone in vivo under normal conditions and in response to hypertensive challenges. Our results indicate that TRPV4 channels may play a complex role in the control of vascular tone and endothelial function under stress.

Cysteinyl leukotrienes regulate endothelial cell inflammatory and proliferative signals through CysLT₂ and CysLT₁ receptors

Cysteinyl leukotrienes (cys-LTs), LTC₄, LTD₄, LTE₄ are potent inflammatory lipid mediators that act through two distinct G-protein-coupled receptors, CysLT₁R and CysLT₂R. Although cys-LTs are shown to induce vascular leakage and atherosclerosis, the molecular mechanism by which cys-LTs modulate endothelial function is not known. Here, we show that cys-LTs (LTC₄ and LTD₄) induce robust calcium influx in human umbilical vein endothelial cells (HUVECs) through CysLT₂R, but not CysLT₁R. Further, cys-LT treatment induced endothelial cell (EC) contraction leading to monolayer disruption via CysLT₂R/Rho kinase dependent pathway. Furthermore, stimulation with cys-LTs potentiated TNFα-induced VCAM-1 expression and leukocyte recruitment to ECs through CysLT₂R. In contrast, we found that both LTC₄ and LTD₄ stimulated EC proliferation through CysLT₁R. Taken together, these results suggest that cys-LTs induce endothelial inflammation and proliferation via CysLT₂R/Rho kinase and CysLT₁R/Erk dependent pathways, respectively, which play critical role in the etiology of cardiovascular diseases such as atherosclerosis and myocardial infarction.

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.

Glial cell-expressed mechanosensitive channel TRPV4 mediates infrasound-induced neuronal impairment

Vibroacoustic disease, a progressive and systemic disease, mainly involving the central nervous system, is caused by excessive exposure to low-frequency but high-intensity noise generated by various heavy transportations and machineries. Infrasound is a type of low-frequency noise. Our previous studies demonstrated that infrasound at a certain intensity caused neuronal injury in rats but the underlying mechanism(s) is still largely unknown. Here, we showed that glial cell-expressed TRPV4, a Ca(2+)-permeable mechanosensitive channel, mediated infrasound-induced neuronal injury. Among different frequencies and intensities, infrasound at 16 Hz and 130 dB impaired rat learning and memory abilities most severely after 7-14 days exposure, a time during which a prominent loss of hippocampal CA1 neurons was evident. Infrasound also induced significant astrocytic and microglial activation in hippocampal regions following 1- to 7-day exposure, prior to neuronal apoptosis. Moreover, pharmacological inhibition of glial activation in vivo protected against neuronal apoptosis. In vitro, activated glial cell-released proinflammatory cytokines IL-1β and TNF-α were found to be key factors for this neuronal apoptosis. Importantly, infrasound induced an increase in the expression level of TRPV4 both in vivo and in vitro. Knockdown of TRPV4 expression by siRNA or pharmacological inhibition of TRPV4 in cultured glial cells decreased the levels of IL-1β and TNF-α, attenuated neuronal apoptosis, and reduced TRPV4-mediated Ca(2+) influx and NF-κB nuclear translocation. Finally, using various antagonists we revealed that calmodulin and protein kinase C signaling pathways were involved in TRPV4-triggered NF-κB activation. Thus, our results provide the first evidence that glial cell-expressed TRPV4 is a potential key factor responsible for infrasound-induced neuronal impairment.

Characterization of new G protein-coupled adenine receptors in mouse and hamster

The nucleobase adenine has previously been reported to activate G protein-coupled receptors in rat and mouse. Adenine receptors (AdeR) thus constitute a new family of purine receptors, for which the designation "P0-receptors" has been suggested. We now describe the cloning and characterization of two new members of the AdeR family from mouse (MrgA10, termed mAde1R) and hamster (cAdeR). Both receptors were expressed in Sf9 insect cells, and radioligand binding studies were performed using [(3)H]adenine. Specific binding of the radioligand was detected in transfected, but not in untransfected cells, and K D values of 286 nM (mAde1R, B max 1.18 pmol/mg protein) and 301 nM (cAdeR, B max 17.7 pmol/mg protein), respectively, were determined. A series of adenine derivatives was investigated in competition binding assays. Minor structural modifications generally led to a reduction or loss of affinity, with one exception: 2-fluoroadenine was at least as potent as adenine itself at the cAdeR. Structure-activity relationships at all AdeR orthologs and subtypes investigated so far were similar, but not identical. For functional analyses, the cAdeR was homologously expressed in Chinese hamster ovary (CHO) cells, while the mAde1R was heterologously expressed in 1321N1 astrocytoma cells. Like the previously described AdeRs from rat (rAdeR) and mouse (mAde2R), the mAde1R (EC50 9.77 nM) and the cAdeR (EC50 51.6 nM) were coupled to inhibition of adenylate cyclase. In addition, the cAdeR from hamster expressed in CHO cells produced an increase in intracellular calcium concentrations (EC50 6.24 nM) and was found to be additionally coupled to Gq proteins.

TRPV4 channel contributes to serotonin-induced pulmonary vasoconstriction and the enhanced vascular reactivity in chronic hypoxic pulmonary hypertension

Transient receptor potential vanilloid 4 (TRPV4) is a mechanosensitive channel in pulmonary arterial smooth muscle cells (PASMCs). Its upregulation by chronic hypoxia is associated with enhanced myogenic tone, and genetic deletion of trpv4 suppresses the development of chronic hypoxic pulmonary hypertension (CHPH). Here we further examine the roles of TRPV4 in agonist-induced pulmonary vasoconstriction and in the enhanced vasoreactivity in CHPH. Initial evaluation of TRPV4-selective antagonists HC-067047 and RN-1734 in KCl-contracted pulmonary arteries (PAs) of trpv4(-/-) mice found that submicromolar HC-067047 was devoid of off-target effect on pulmonary vasoconstriction. Inhibition of TRPV4 with 0.5 μM HC-067047 significantly reduced the sensitivity of serotonin (5-HT)-induced contraction in wild-type (WT) PAs but had no effect on endothelin-1 or phenylephrine-activated response. Similar shift in the concentration-response curve of 5-HT was observed in trpv4(-/-) PAs, confirming specific TRPV4 contribution to 5-HT-induced vasoconstriction. 5-HT-induced Ca(2+) response was attenuated by HC-067047 in WT PASMCs but not in trpv4(-/-) PASMCs, suggesting TRPV4 is a major Ca(2+) pathway for 5-HT-induced Ca(2+) mobilization. Nifedipine also attenuated 5-HT-induced Ca(2+) response in WT PASMCs but did not cause further reduction in the presence of HC-067047, suggesting interdependence of TRPV4 and voltage-gated Ca(2+) channels in the 5-HT response. Chronic exposure (3-4 wk) of WT mice to 10% O2 caused significant increase in 5-HT-induced maximal contraction, which was partially reversed by HC-067047. In concordance, the enhancement of 5-HT-induced contraction was significantly reduced in PAs of CH trpv4(-/-) mice and HC-067047 had no further effect on the 5-HT induced response. These results suggest unequivocally that TRPV4 contributes to 5-HT-dependent pharmaco-mechanical coupling and plays a major role in the enhanced pulmonary vasoreactivity to 5-HT in CHPH.

Arachidonic acid-induced dilation in human coronary arterioles: convergence of signaling mechanisms on endothelial TRPV4-mediated Ca2+ entry

BACKGROUND

Arachidonic acid (AA) and/or its enzymatic metabolites are important lipid mediators contributing to endothelium-derived hyperpolarizing factor (EDHF)-mediated dilation in multiple vascular beds, including human coronary arterioles (HCAs). However, the mechanisms of action of these lipid mediators in endothelial cells (ECs) remain incompletely defined. In this study, we investigated the role of the transient receptor potential vanilloid 4 (TRPV4) channel in AA-induced endothelial Ca(2+) response and dilation of HCAs.

METHODS AND RESULTS

AA induced concentration-dependent dilation in isolated HCAs. The dilation was largely abolished by the TRPV4 antagonist RN-1734 and by inhibition of endothelial Ca(2+)-activated K(+) channels. In native and TRPV4-overexpressing human coronary artery ECs (HCAECs), AA increased intracellular Ca(2+) concentration ([Ca(2+)]i), which was mediated by TRPV4-dependent Ca(2+) entry. The AA-induced [Ca(2+)]i increase was inhibited by cytochrome P450 (CYP) inhibitors. Surprisingly, the CYP metabolites of AA, epoxyeicosatrienoic acids (EETs), were much less potent activators of TRPV4, and CYP inhibitors did not affect EET production in HCAECs. Apart from its effect on [Ca(2+)]i, AA induced endothelial hyperpolarization, and this effect was required for Ca(2+) entry through TRPV4. AA-induced and TRPV4-mediated Ca(2+) entry was also inhibited by the protein kinase A inhibitor PKI. TRPV4 exhibited a basal level of phosphorylation, which was inhibited by PKI. Patch-clamp studies indicated that AA activated TRPV4 single-channel currents in cell-attached and inside-out patches of HCAECs.

CONCLUSIONS

AA dilates HCAs through a novel mechanism involving endothelial TRPV4 channel-dependent Ca(2+) entry that requires endothelial hyperpolarization, PKA-mediated basal phosphorylation of TRPV4, and direct activation of TRPV4 channels by AA.

Role of impaired endothelial cell Ca(2+) signaling in uteroplacental vascular dysfunction during diabetic rat pregnancy

Diabetes mellitus in pregnancy is associated with impaired endothelium-mediated dilatation of maternal arteries, although the underlying cellular mechanisms remain unknown. In this study, we hypothesized that diabetes during rat gestation attenuates agonist-induced uterine vasodilation through reduced endothelial cell (EC) Ca(2+) elevations and impaired smooth muscle cell (SMC) hyperpolarization and SMC intracellular Ca(2+) concentration ([Ca(2+)]i) responses. Diabetes was induced by an injection of streptozotocin to second-day pregnant rats and confirmed by the development of maternal hyperglycemia. Control rats were injected with a citrate buffer. Fura-2-based measurements of SMC [Ca(2+)]i or microelectrode recordings of SMC membrane potential were performed concurrently with dilator responses to ACh in uteroplacental arteries from control and diabetic pregnant rats. Basal levels of EC [Ca(2+)]i and ACh-induced EC [Ca(2+)]i elevations in pressurized vessels and small EC sheets were studied as well. Diabetes reduced ACh-induced vasodilation due to a markedly impaired EDHF-mediated response. Diminished vasodilation to ACh was associated with attenuated SMC hyperpolarization and [Ca(2+)]i responses. Basal levels of EC [Ca(2+)]i and ACh-induced EC [Ca(2+)]i elevations were significantly reduced by diabetes. In conclusion, these data demonstrate that reduced endothelium-mediated hyperpolarization contributes to attenuated uteroplacental vasodilation and SMC [Ca(2+)]i responses to ACh in diabetic pregnancy. Impaired endothelial Ca(2+) signaling is in part responsible for endothelial dysfunction in the uterine resistance vasculature of diabetic rats. Pharmacological improvement of EC Ca(2+) handling may provide an important strategy for the restoration of endothelial function and enhancement of maternal blood flow in human pregnancies complicated by diabetes.

Protease-activated receptor 2 (PAR2) protein and transient receptor potential vanilloid 4 (TRPV4) protein coupling is required for sustained inflammatory signaling

G protein-coupled receptors of nociceptive neurons can sensitize transient receptor potential (TRP) ion channels, which amplify neurogenic inflammation and pain. Protease-activated receptor 2 (PAR(2)), a receptor for inflammatory proteases, is a major mediator of neurogenic inflammation and pain. We investigated the signaling mechanisms by which PAR(2) regulates TRPV4 and determined the importance of tyrosine phosphorylation in this process. Human TRPV4 was expressed in HEK293 cells under control of a tetracycline-inducible promoter, allowing controlled and graded channel expression. In cells lacking TRPV4, the PAR(2) agonist stimulated a transient increase in [Ca(2+)](i). TRPV4 expression led to a markedly sustained increase in [Ca(2+)](i). Removal of extracellular Ca(2+) and treatment with the TRPV4 antagonists Ruthenium Red or HC067047 prevented the sustained response. Inhibitors of phospholipase A(2) and cytochrome P450 epoxygenase attenuated the sustained response, suggesting that PAR(2) generates arachidonic acid-derived lipid mediators, such as 5',6'-EET, that activate TRPV4. Src inhibitor 1 suppressed PAR(2)-induced activation of TRPV4, indicating the importance of tyrosine phosphorylation. The TRPV4 tyrosine mutants Y110F, Y805F, and Y110F/Y805F were expressed normally at the cell surface. However, PAR(2) was unable to activate TRPV4 with the Y110F mutation. TRPV4 antagonism suppressed PAR(2) signaling to primary nociceptive neurons, and TRPV4 deletion attenuated PAR(2)-stimulated neurogenic inflammation. Thus, PAR(2) activation generates a signal that induces sustained activation of TRPV4, which requires a key tyrosine residue (TRPV4-Tyr-110). This mechanism partly mediates the proinflammatory actions of PAR(2).

TRPV4 channels mediate cardiac fibroblast differentiation by integrating mechanical and soluble signals

The phenotypic switch underlying the differentiation of cardiac fibroblasts into hypersecretory myofibroblasts is critical for cardiac remodeling following myocardial infarction. Myofibroblasts facilitate wound repair in the myocardium by secreting and organizing extracellular matrix (ECM) during the wound healing process. However, the molecular mechanisms involved in myofibroblast differentiation are not well known. TGF-β has been shown to promote differentiation and this, combined with the robust mechanical environment in the heart, lead us to hypothesize that the mechanotransduction and TGF-β signaling pathways play active roles in the differentiation of cardiac fibroblasts to myofibroblasts. Here, we show that the mechanosensitve ion channel TRPV4 is required for TGF-β1-induced differentiation of cardiac fibroblasts into myofibroblasts. We found that the TRPV4-specific antagonist AB159908 and siRNA knockdown of TRPV4 significantly inhibited TGFβ1-induced differentiation as measured by incorporation of α-SMA into stress fibers. Further, we found that TGF-β1-induced myofibroblast differentiation was dependent on ECM stiffness, a response that was attenuated by TRPV4 blockade. Finally, TGF-β1 treated fibroblasts exhibited enhanced TRPV4 expression and TRPV4-mediated calcium influx compared to untreated controls. Taken together these results suggest for the first time that the mechanosensitive ion channel, TRPV4, regulates cardiac fibroblast differentiation to myofibroblasts by integrating signals from TGF-β1 and mechanical factors.

Neurogenic hypertension and elevated vertebrobasilar arterial resistance: is there a causative link?

There is evidence of sympathetic overdrive in a significant proportion of patients with essential hypertension and an animal model of the condition, the spontaneously hypertensive rat (SHR). The reasons for this remain elusive. However, there is also evidence of narrowing of the arteries supplying the brainstem in the SHR and hypertensive humans. In this review, we discuss the possible role of brainstem hypoperfusion in driving increased sympathetic activity and hypertension.

Vascular Hypoxic Preconditioning Relies on TRPV4-Dependent Calcium Influx and Proper Intercellular Gap Junctions Communication

Objective—

We investigated the impact of hypoxia-reoxygenation on endothelial relaxation and aimed to clarify the role of transient receptor potential cation channels V4 (TRPV4) and gap junctions in the protective effect associated with hypoxic preconditioning on the vascular function.

Methods and Results—

By mimicking ischemia-reperfusion in C57BL/6 male mice in vivo, we documented a reduced NO-mediated relaxation and an increased endothelium-derived hyperpolarization (EDH[F])-mediated relaxation. Hypoxic preconditioning, however, restored NO relaxation and further improved the EDH(F) response. We also examined specifically 2 major effectors of the EDH(F) pathway, transient receptor potential cation channels V4 and connexins. We found that in endothelial cells, expression and activity of transient receptor potential cation channels V4 were increased by hypoxic stimuli independently of preconditioning which was interestingly associated with an increase of structural caveolar component caveolin-1 at membrane locations. Gap junctions, however, seemed to directly support EDH(F)-driven preconditioning as connexin 40 and connexin 43 expression increased and as in vivo carbenoxolone treatment completely inhibited the EDH(F) pathway and significantly reduced the protection afforded by preconditioning for the concomitant NO-mediated relaxation.

Conclusion—

Our work provides evidence on how transient receptor potential cation channels V4 and connexins might participate in preserving vasorelaxation under hypoxia and restoring the NO-mediated pathway in hypoxic preconditioning conditions pointing out caveolae as a common signaling location.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Moderate hypothermia attenuates α(1)-adrenoceptor-mediated contraction in isolated rat aorta: the role of the endothelium

Moderate hypothermia (25-31 °C) may have a significant influence on vascular tone. We investigated the cellular mechanisms by which moderate hypothermia alters α-adrenoceptor-mediated contraction in rat thoracic aortae. Cyclooxygenase inhibition by indomethacin; nitric oxide (NO) synthase inhibition by L-NAME; potassium channel and endothelium-derived hyperpolarizing factor (EDHF) inhibition by glibenclamide and TEA; G protein inhibition by pertussis toxin; α₂-adrenergic inhibition by yohimbine; and β-adrenergic inhibition by propranolol were assessed for their effect on the contractile response to the α1-adrenoceptor agonist phenylephrine (Phe) in combination with moderate hypothermia (25 °C). Moderate hypothermia produced a shift to the right for the Phe concentration-response curves in endothelium-intact (E+) and endothelium-denuded (E-) aortic rings. The maximal response to Phe in E+ rings was significantly decreased (P<0.05) at 25 °C compared to 38 °C, whereas there was no significant difference in E- rings. Hypothermia-induced vasorelaxation in E+ rings was attenuated (P<0.05) following combined pretreatment with L-NAME (10⁻⁴ M) and indomethacin (10⁻⁵ M), whereas other inhibitors had no significant effect. Importantly, the addition of TEA to rings that were pretreated with L-NAME and indomethacin exhibited no further attenuation (P>0.05) of hypothermia-induced vasorelaxation. The concentrations of cGMP and cAMP, as measured by radioimmunoassay, were significantly increased (P<0.05) in E+ rings at 25 °C compared to those at 38 °C, whereas there were no significant differences (P>0.05) in E- rings. The present study demonstrated that rat aortic endothelium is stimulated during moderate hypothermia and that the NO-cGMP and prostacyclin (PGI₂)-cAMP pathways represent endothelium-dependent mechanisms of hypothermia-induced vasorelaxation. In contrast, EDHF may not be associated with hypothermia-induced vasorelaxation.