TRPing to the Point of Clarity: Understanding the Function of the Complex TRPV4 Ion Channel

Aqp4a and Trpv4 mediate regulatory cell volume increase for swimming maintenance of marine fish spermatozoa

Volume regulation is essential for cell homeostasis and physiological function. Amongst the sensory molecules that have been associated with volume regulation is the transient receptor potential vanilloid 4 (TRPV4), which is a non-selective cation channel that in conjunction with aquaporins, typically controls regulatory volume decrease (RVD). Here we show that the interaction between orthologous AQP4 (Aqp4a) and TRPV4 (Trpv4) is important for regulatory volume increase (RVI) in post-activated marine fish spermatozoa under high osmotic stress. Based upon electrophysiological, volumetric, and in vivo and ex vivo functional experiments using the pharmacological and immunological inhibition of Aqp4a and Trpv4 our model suggests that upon ejaculation and exposure to the hypertonic seawater, spermatozoon shrinkage is initially mediated by water efflux through Aqp1aa in the flagellar tail. The shrinkage results in an increase in intracellular Ca2+ concentration, and the activation of sperm motility and a Na+/K+/2Cl- (NKCC1) cotransporter. The activity of NKCC1 is required for the initiation of cell swelling, which secondarily activates the Aqp4a-Trpv4 complex to facilitate the influx of water via Aqp4a-M43 and Ca2+ via Trpv4 and L-type channels for the mediation of RVI. The inhibitory experiments show that blocking of each of these events prevents either shrinkage or RVI. Our data thus reveal that post-activated marine fish spermatozoa are capable of initiating RVI under a high hypertonic stress, which is essential for the maintenance of sperm motility.

TRP Channels in Excitotoxicity

Glutamate excitotoxicity is a central mechanism contributing to cellular dysfunction and death in various neurological disorders and diseases, such as stroke, traumatic brain injury, epilepsy, schizophrenia, addiction, mood disorders, Huntington's disease, Alzheimer's disease, Parkinson's disease, multiple sclerosis, pathologic pain, and even normal aging-related changes. This detrimental effect emerges from glutamate binding to glutamate receptors, including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, N-methyl-d-aspartate receptors, kainate receptors, and GluD receptors. Thus, excitotoxicity could be prevented by targeting glutamate receptors and their downstream signaling pathways. However, almost all the glutamate receptor antagonists failed to attenuate excitotoxicity in human patients, mainly due to the limited understanding of the underlying mechanisms regulating excitotoxicity. Transient receptor potential (TRP) channels serve as ancient cellular sensors capable of detecting and responding to both external and internal stimuli. The study of human TRP channels has flourished in recent decades since the initial discovery of mammalian TRP in 1995. These channels have been found to play pivotal roles in numerous pathologic conditions, including excitotoxicity. In this review, our focus centers on exploring the intricate interactions between TRP channels and glutamate receptors in excitotoxicity.

TRPV4 in adipose tissue ameliorates diet-induced obesity by promoting white adipocyte browning

The induction of adipocyte browning to increase energy expenditure is a promising strategy to combat obesity. Transient receptor potential channel V4 (TRPV4) functions as a nonselective cation channel in various cells and plays physiological roles in osmotic and thermal sensations. However, the function of TRPV4 in energy metabolism remains controversial. This study revealed the role of TRPV4 in adipose tissue in the development of obesity. Adipose-specific TRPV4 overexpression protected mice against diet-induced obesity (DIO) and promoted white fat browning. TRPV4 overexpression was also associated with decreased adipose inflammation and improved insulin sensitivity. Mechanistically, TRPV4 could directly promote white adipocyte browning via the AKT pathway. Consistently, adipose-specific TRPV4 knockout exacerbated DIO with impaired thermogenesis and activated inflammation. Corroborating our findings in mice, TRPV4 expression was low in the white adipose tissue of obese people. Our results positioned TRPV4 as a potential regulator of obesity and energy expenditure in mice and humans.

5,6-diHETE lactone (EPA-L) mediates hypertensive microvascular dilation by activating the endothelial GPR-PLC-IP3 signaling pathway

Endothelial microvascular dysfunction affects multi-organ pathologic processes that contribute to increased vascular tone and is at the base of impaired metabolic and cardiovascular diseases. The vascular dilation impaired by nitric oxide (NO) deficiency in such dysfunctional endothelium is often balanced by endothelial-derived hyperpolarizing factors (EDHFs), which play a critical role in managing vascular tone. Our latest research has uncovered a new group of lactone oxylipins produced in the polyunsaturated fatty acids (PUFAs) CYP450 epoxygenase pathway, significantly affecting vascular dilation. The lactone oxylipin, derived from arachidonic acid (5,6-diHET lactone, AA-L), has been previously shown to facilitate vasodilation dependent on the endothelium in isolated human microvessels. The administration of the lactone oxylipin derived from eicosapentaenoic acid (5,6-diHETE lactone, EPA-L) to hypertensive rats demonstrated a significant decrease in blood pressure and improvement in the relaxation of microvessels. However, the molecular signaling processes that underlie these observations were not fully understood. The current study delineates the molecular pathways through which EPA-L promotes endothelium-dependent vascular dilation. In microvessels from hypertensive individuals, it was found that EPA-L mediates endothelium-dependent vasodilation while the signaling pathway was not dependent on NO. In vitro studies on human endothelial cells showed that the hyperpolarization mediated by EPA-L relies on G-protein-coupled receptor (GPR)-phospholipase C (PLC)-IP3 signaling that further activates calcium-dependent potassium flux. The pathway was confirmed using a range of inhibitors and cells overexpressing GPR40, where a specific antagonist reduced the calcium levels and outward currents induced by EPA-L. The downstream AKT and endothelial NO synthase (eNOS) phosphorylations were non-significant. These findings show that the GPR-PLC-IP3 pathway is a key mediator in the EPA-L-triggered vasodilation of arterioles. Therefore, EPA-L is identified as a significant lactone-based PUFA metabolite that contributes to endothelial and vascular health.

Modulating TRPV4 Channel Activity in Pro-Inflammatory Macrophages within the 3D Tissue Analog

Investigating macrophage plasticity emerges as a promising strategy for promoting tissue regeneration and can be exploited by regulating the transient receptor potential vanilloid 4 (TRPV4) channel. The TRPV4 channel responds to various stimuli including mechanical, chemical, and selective pharmacological compounds. It is well documented that treating cells such as epithelial cells and fibroblasts with a TRPV4 agonist enhances the Ca2+ influx to the cells, which leads to secretion of pro-inflammatory cytokines, while a TRPV4 antagonist reduces both Ca2+ influx and pro-inflammatory cytokine secretion. In this work, we investigated the effect of selective TRPV4 modulator compounds on U937-differentiated macrophages encapsulated within three-dimensional (3D) matrices. Despite offering a more physiologically relevant model than 2D cultures, pharmacological treatment of macrophages within 3D collagen matrices is largely overlooked in the literature. In this study, pro-inflammatory macrophages were treated with an agonist, 500 nM of GSK1016790A (TRPV4(+)), and an antagonist, 10 mM of RN-1734 (TRPV4(-)), to elucidate the modulation of the TRPV4 channel at both cellular and extracellular levels. To evaluate macrophage phenotypic alterations within 3D collagen matrices following TRPV4 modulator treatment, we employed structural techniques (SEM, Masson's trichrome, and collagen hybridizing peptide (CHP) staining), quantitative morphological measures for phenotypic assessment, and genotypic methods such as quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC). Our data reveal that pharmacological modulation of the macrophage TRPV4 channel alters the cytoskeletal structure of macrophages and influences the 3D structure encapsulating them. Moreover, we proved that treating macrophages with a TRPV4 agonist and antagonist enhances the expression of pro- and anti-inflammatory genes, respectively, leading to the upregulation of surface markers CD80 and CD206. In the TRPV4(-) group, the CD206 gene and CD206 surface marker were significantly upregulated by 9- and 2.5-fold, respectively, compared to the control group. These findings demonstrate that TRPV4 modulation can be utilized to shift macrophage phenotype within the 3D matrix toward a desired state. This is an innovative approach to addressing inflammation in musculoskeletal tissues.

Dual function of the choroid plexus: Cerebrospinal fluid production and control of brain ion homeostasis

The choroid plexus is a small monolayered epithelium located in the brain ventricles and serves to secrete the cerebrospinal fluid (CSF) that envelops the brain and fills the central ventricles. The CSF secretion is sustained with a concerted effort of a range of membrane transporters located in a polarized fashion in this tissue. Prominent amongst these are the Na+/K+-ATPase, the Na+,K+,2Cl- cotransporter (NKCC1), and several HCO3- transporters, which together support the net transepithelial transport of the major electrolytes, Na+ and Cl-, and thus drive the CSF secretion. The choroid plexus, in addition, serves an important role in keeping the CSF K+ concentration at a level compatible with normal brain function. The choroid plexus Na+/K+-ATPase represents a key factor in the barrier-mediated control of the CSF K+ homeostasis, as it increases its K+ uptake activity when faced with elevated extracellular K+ ([K+]o). In certain developmental or pathological conditions, the NKCC1 may revert its net transport direction to contribute to CSF K+ homeostasis. The choroid plexus ion transport machinery thus serves dual, yet interconnected, functions with its contribution to electrolyte and fluid secretion in combination with its control of brain K+ levels.

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.

TRP Channels in Stroke

Ischemic stroke is a devastating disease that affects millions of patients worldwide. Unfortunately, there are no effective medications for mitigating brain injury after ischemic stroke. TRP channels are evolutionally ancient biosensors that detect external stimuli as well as tissue or cellular injury. To date, many members of the TRP superfamily have been reported to contribute to ischemic brain injury, including the TRPC subfamily (1, 3, 4, 5, 6, 7), TRPV subfamily (1, 2, 3, 4) and TRPM subfamily (2, 4, 7). These TRP channels share structural similarities but have distinct channel functions and properties. Their activation during ischemic stroke can be beneficial, detrimental, or even both. In this review, we focus on discussing the interesting features of stroke-related TRP channels and summarizing the underlying cellular and molecular mechanisms responsible for their involvement in ischemic brain injury.

[Sevoflurane alleviates ventilator-induced lung injury in rats by down-regulating the TRPV4/C-PLA2 signaling pathway]

OBJECTIVE

To explore the mechanism underlying the protective effect of sevoflurane against ventilator-induced lung injury (VILI).

METHODS

Thirty-two SD rats were randomized into mechanical ventilation (MV) group, MV+sevoflurane group (MS group), MV+sevoflurane+transient receptor potential vanillate subtype 4 (TRPV4) agonist group (MST group) and MV+ sevoflurane + vehicle group (MSV group). Arachidonic acid (AA) in the lung tissues was quantified with ELISA. TRPV4, cytoplasmic phospholipase A2 (C-PLA2) and myosin light chain kinase (MLCK) protein expressions were detected by Western blotting. Lung injury in the rats was evaluated by assessing MLCK protein expression level, pulmonary permeability index, lung wet/dry ratio, leukocyte count in the bronchoalveolar lavage fluid (BALF), myeloperoxidase content in lung tissue, and histological score of the lungs.

RESULTS

The rats in MV group showed significantly increased TRPV4 and C-PLA2 expression levels in the lung tissues with increased lung permeability and obvious lung inflammation compared with those in the other 3 groups (P < 0.05). No significant differences were found in the parameters associated with lung injuries between MS group and MSV group. Compared with those in MST group, the rats in MS group and MSV group showed significantly reduced AA production and TRPV4 and C-PLA2 expressions in the lungs (P < 0.05) with alleviated lung hyper-permeability and inflammation (P < 0.05).

CONCLUSION

Sevoflurane protects against VILI in rats by down-regulating the TRPV4/C-PLA2 signaling pathway.

Alleviation of allergic conjunctivitis by (±)5(6)-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid in mice

Background: Allergic conjunctivitis (AC) is a common ophthalmologic disorder that causes symptoms that often reduces a patient's quality of life (QOL). We investigated the effects of the eicosapentaenoic acid metabolite (±)5(6)-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid ((±)5(6)-DiHETE) on AC using a mouse model. Methods: BALB/c mice were sensitized with two injections of short ragweed pollen in alum, challenged fifth with pollen in eyedrops. The clinical signs and tear volume were evaluated at 15 min after the final challenge. Histamine-induced ocular inflammation model was prepared by instilling histamine onto the surface of the eye. Fifteen minutes after histamine application, tear volume was measured using the Schirmer tear test. Miles assay was performed to investigate vascular permeability. To cause scratching behavior 10 μg of serotonin was injected in the cheek. Results: Repeated topical application of pollen induced conjunctivitis, accompanied by eyelid edema and tearing in mice. Pollen application typically degranulates mast cells and recruits eosinophils to the conjunctiva. Intraperitoneal administration of 300 μg/kg of (±)5(6)-DiHETE significantly inhibited pollen-induced symptoms. The administration of (±)5(6)-DiHETE also attenuated mast cell degranulation and eosinophil infiltration into the conjunctiva. To assess the effects of (±)5(6)-DiHETE on the downstream pathway of mast cell activation in AC, we used a histamine-induced ocular inflammation model. Topical application of 4 μg/eye histamine caused eyelid edema and tearing and increased vascular permeability, as indicated by Evans blue dye extravasation. Intraperitoneal administration of 300 μg/kg or topical administration of 1 μg/eye (±)5(6)-DiHETE inhibited histamine-induced manifestations. Finally, we assessed the effects of (±)5(6)-DiHETE on itching. An intradermal injection of 10 μg serotonin in the cheek caused scratching behavior in mice. Intraperitoneal administration of 300 μg/kg (±)5(6)-DiHETE significantly inhibited serotonin-induced scratching. Conclusion: Thus, (±)5(6)-DiHETE treatment broadly suppressed AC pathology and could be a novel treatment option for AC.

Mechanosensitive Ion Channels: Their Physiological Importance and Potential Key Role in Cancer

Mechanosensitive ion channels comprise a broad group of proteins that sense mechanical extracellular and intracellular changes, translating them into cation influx to adapt and respond to these physical cues. All cells in the organism are mechanosensitive, and these physical cues have proven to have an important role in regulating proliferation, cell fate and differentiation, migration and cellular stress, among other processes. Indeed, the mechanical properties of the extracellular matrix in cancer change drastically due to high cell proliferation and modification of extracellular protein secretion, suggesting an important contribution to tumor cell regulation. In this review, we describe the physiological significance of mechanosensitive ion channels, emphasizing their role in cancer and immunity, and providing compelling proof of the importance of continuing to explore their potential as new therapeutic targets in cancer research.

Crosstalk between regulatory elements in disordered TRPV4 N-terminus modulates lipid-dependent channel activity

Intrinsically disordered regions (IDRs) are essential for membrane receptor regulation but often remain unresolved in structural studies. TRPV4, a member of the TRP vanilloid channel family involved in thermo- and osmosensation, has a large N-terminal IDR of approximately 150 amino acids. With an integrated structural biology approach, we analyze the structural ensemble of the TRPV4 IDR and the network of antagonistic regulatory elements it encodes. These modulate channel activity in a hierarchical lipid-dependent manner through transient long-range interactions. A highly conserved autoinhibitory patch acts as a master regulator by competing with PIP2 binding to attenuate channel activity. Molecular dynamics simulations show that loss of the interaction between the PIP2-binding site and the membrane reduces the force exerted by the IDR on the structured core of TRPV4. This work demonstrates that IDR structural dynamics are coupled to TRPV4 activity and highlights the importance of IDRs for TRP channel function and regulation.

Structure of human TRPV4 in complex with GTPase RhoA

Transient receptor potential (TRP) channel TRPV4 is a polymodal cellular sensor that responds to moderate heat, cell swelling, shear stress, and small-molecule ligands. It is involved in thermogenesis, regulation of vascular tone, bone homeostasis, renal and pulmonary functions. TRPV4 is implicated in neuromuscular and skeletal disorders, pulmonary edema, and cancers, and represents an important drug target. The cytoskeletal remodeling GTPase RhoA has been shown to suppress TRPV4 activity. Here, we present a structure of the human TRPV4-RhoA complex that shows RhoA interaction with the membrane-facing surface of the TRPV4 ankyrin repeat domains. The contact interface reveals residues that are mutated in neuropathies, providing an insight into the disease pathogenesis. We also identify the binding sites of the TRPV4 agonist 4α-PDD and the inhibitor HC-067047 at the base of the S1-S4 bundle, and show that agonist binding leads to pore opening, while channel inhibition involves a π-to-α transition in the pore-forming helix S6. Our structures elucidate the interaction interface between hTRPV4 and RhoA, as well as residues at this interface that are involved in TRPV4 disease-causing mutations. They shed light on TRPV4 activation and inhibition and provide a template for the design of future therapeutics for treatment of TRPV4-related diseases.

Pathophysiological Roles of the TRPV4 Channel in the Heart

The transient receptor potential vanilloid 4 (TRPV4) channel is a non-selective cation channel that is mostly permeable to calcium (Ca²⁺), which participates in intracellular Ca²⁺ handling in cardiac cells. It is widely expressed through the body and is activated by a large spectrum of physicochemical stimuli, conferring it a role in a variety of sensorial and physiological functions. Within the cardiovascular system, TRPV4 expression is reported in cardiomyocytes, endothelial cells (ECs) and smooth muscle cells (SMCs), where it modulates mitochondrial activity, Ca²⁺ homeostasis, cardiomyocytes electrical activity and contractility, cardiac embryonic development and fibroblast proliferation, as well as vascular permeability, dilatation and constriction. On the other hand, TRPV4 channels participate in several cardiac pathological processes such as the development of cardiac fibrosis, hypertrophy, ischemia-reperfusion injuries, heart failure, myocardial infarction and arrhythmia. In this manuscript, we provide an overview of TRPV4 channel implications in cardiac physiology and discuss the potential of the TRPV4 channel as a therapeutic target against cardiovascular diseases.

TRPV4 expression in the renal tubule is necessary for maintaining whole body K+ homeostasis

The Ca2+-permeable transient receptor potential vanilloid type 4 (TRPV4) channel serves as the sensor of tubular flow, thus being well suited to govern mechanosensitive K+ transport in the distal renal tubule. Here, we directly tested whether the TRPV4 function is significant in affecting K+ balance. We used balance metabolic cage experiments and systemic measurements with different K+ feeding regimens [high (5% K+), regular (0.9% K+), and low (<0.01% K+)] in newly created transgenic mice with selective TRPV4 deletion in the renal tubule (TRPV4fl/fl-Pax8Cre) and their littermate controls (TRPV4fl/fl). Deletion was verified by the absence of TRPV4 protein expression and lack of TRPV4-dependent Ca2+ influx. There were no differences in plasma electrolytes, urinary volume, and K+ levels at baseline. In contrast, plasma K+ levels were significantly elevated in TRPV4fl/fl-Pax8Cre mice on high K+ intake. K+-loaded knockout mice exhibited lower urinary K+ levels than TRPV4fl/fl mice, which was accompanied by higher aldosterone levels by day 7. Moreover, TRPV4fl/fl-Pax8Cre mice had more efficient renal K+ conservation and higher plasma K+ levels in the state of dietary K+ deficiency. H+-K+-ATPase levels were significantly increased in TRPV4fl/fl-Pax8Cre mice on a regular diet and especially on a low-K+ diet, pointing to augmented K+ reabsorption in the collecting duct. Consistently, we found a significantly faster intracellular pH recovery after intracellular acidification, as an index of H+-K+-ATPase activity, in split-opened collecting ducts from TRPV4fl/fl-Pax8Cre mice. In summary, our results demonstrate an indispensable prokaliuretic role of TRPV4 in the renal tubule in controlling K+ balance and urinary K+ excretion during variations in dietary K+ intake. NEW & NOTEWORTHY The mechanoactivated transient receptor potential vanilloid type 4 (TRPV4) channel is expressed in distal tubule segments, where it controls flow-dependent K+ transport. Global TRPV4 deficiency causes impaired adaptation to variations in dietary K+ intake. Here, we demonstrate that renal tubule-specific TRPV4 deletion is sufficient to recapitulate the phenotype by causing antikaliuresis and higher plasma K+ levels in both states of K+ load and deficiency.

Non-inositol 1,4,5-trisphosphate (IP3) receptor IP3-binding proteins

Conventionally, myo-D-inositol 1, 4,5-trisphosphate (IP₃) is thought to exert its second messenger effects through the gating of IP₃R Ca²⁺ release channels, located in Ca²⁺-storage organelles like the endoplasmic reticulum. However, there is considerable indirect evidence to support the concept that IP₃ might interact with other, non-IP₃R proteins within cells. To explore this possibility further, the Protein Data Bank was searched using the term "IP3". This resulted in the retrieval of 203 protein structures, the majority of which were members of the IP₃R/ryanodine receptor superfamily of channels. Only 49 of these structures were complexed with IP₃. These were inspected for their ability to interact with the carbon-1 phosphate of IP₃, since this is the least accessible phosphate group of its precursor, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). This reduced the number of structures retrieved to 35, of which 9 were IP₃Rs. The remaining 26 structures represent a diverse range of proteins, including inositol-lipid metabolizing enzymes, signal transducers, PH domain containing proteins, cytoskeletal anchor proteins, the TRPV4 ion channel, a retroviral Gag protein and fibroblast growth factor 2. Such proteins may impact on IP₃ signalling and its effects on cell-biology. This represents an area open for exploration in the field of IP₃ signalling.

TRPV4 channel is involved in HSV-2 infection in human vaginal epithelial cells through triggering Ca2+ oscillation

Herpes simplex virus (HSV) infection induces a rapid and transient increase in intracellular calcium concentration ([Ca2+]i), which plays a critical role in facilitating viral entry. T-type calcium channel blockers and EGTA, a chelate of extracellular Ca2+, suppress HSV-2 infection. But the cellular mechanisms mediating HSV infection-activated Ca2+ signaling have not been completely defined. In this study we investigated whether the TRPV4 channel was involved in HSV-2 infection in human vaginal epithelial cells. We showed that the TRPV4 channel was expressed in human vaginal epithelial cells (VK2/E6E7). Using distinct pharmacological tools, we demonstrated that activation of the TRPV4 channel induced Ca2+ influx, and the TRPV4 channel worked as a Ca2+-permeable channel in VK2/E6E7 cells. We detected a direct interaction between the TRPV4 channel protein and HSV-2 glycoprotein D in the plasma membrane of VK2/E6E7 cells and the vaginal tissues of HSV-2-infected mice as well as in phallic biopsies from genital herpes patients. Pretreatment with specific TRPV4 channel inhibitors, GSK2193874 (1-4 μM) and HC067047 (100 nM), or gene silence of the TRPV4 channel not only suppressed HSV-2 infectivity but also reduced HSV-2-induced cytokine and chemokine generation in VK2/E6E7 cells by blocking Ca2+ influx through TRPV4 channel. These results reveal that the TRPV4 channel works as a Ca2+-permeable channel to facilitate HSV-2 infection in host epithelial cells and suggest that the design and development of novel TRPV4 channel inhibitors may help to treat HSV-2 infections.

Blockage of TRPV4 Downregulates the Nuclear Factor-Kappa B Signaling Pathway to Inhibit Inflammatory Responses and Neuronal Death in Mice with Pilocarpine-Induced Status Epilepticus

The blockage of transient receptor potential vanilloid 4 (TRPV4) inhibits inflammation and reduces hippocampal neuronal injury in a pilocarpine-induced mouse model of temporal lobe epilepsy. However, the underlying mechanisms remain largely unclear. NF-κB signaling pathway is responsible for the inflammation and neuronal injury during epilepsy. Here, we explored whether TRPV4 blockage could affect the NF-κB pathway in mice with pilocarpine-induced status epilepticus (PISE). Application of a TRPV4 antagonist markedly attenuated the PISE-induced increase in hippocampal HMGB1, TLR4, phospho (p)-IκK (p-IκK), and p-IκBα protein levels, as well as those of cytoplasmic p-NF-κB p65 (p-p65) and nuclear NF-κB p65 and p50; in contrast, the application of GSK1016790A, a TRPV4 agonist, showed similar changes to PISE mice. Administration of the TLR4 antagonist TAK-242 or the NF-κB pathway inhibitor BAY 11-7082 led to a noticeable reduction in the hippocampal protein levels of cleaved IL-1β, IL-6 and TNF, as well as those of cytoplasmic p-p65 and nuclear p65 and p50 in GSK1016790A-injected mice. Finally, administration of either TAK-242 or BAY 11-7082 greatly increased neuronal survival in hippocampal CA1 and CA2/3 regions in GSK1016790A-injected mice. Therefore, TRPV4 activation increases HMGB1 and TLR4 expression, leading to IκK and IκBα phosphorylation and, consequently, NF-κB activation and nuclear translocation. The resulting increase in pro-inflammatory cytokine production is responsible for TRPV4 activation-induced neuronal injury. We conclude that blocking TRPV4 can downregulate HMGB1/TLR4/IκK/κBα/NF-κB signaling following PISE onset, an effect that may underlie the anti-inflammatory response and neuroprotective ability of TRPV4 blockage in mice with PISE.

The Ca2+ channel TRPV4 is dispensable for Ca2+ influx and cell volume regulation during hypotonic stress response in human keratinocyte cell lines

Cell swelling as a result of hypotonic stress is counteracted in mammalian cells by a process called regulatory volume decrease (RVD). We have recently discovered that RVD of human keratinocytes requires the LRRC8 volume-regulated anion channel (VRAC) and that Ca²⁺ exerts a modulatory function on RVD. However, the ion channel that is responsible for Ca²⁺ influx remains unknown. We investigated in this study whether the Ca²⁺-permeable TRPV4 ion channel, which functions as cell volume sensor in many cell types, may be involved in cell volume regulation during hypotonic stress response of human keratinocytes. We interfered with TRPV4 function in two human keratinocyte cell lines (HaCaT and NHEK-E6/E7) by using two TRPV4-specific inhibitors (RN1734 and GSK2193874), and by creating a CRISPR/Cas9-mediated genetic TRPV4^-/- knockout in HaCaT cells. We employed electrophysiological patch clamp analysis, fluorescence-based Ca²⁺ imaging and cell volume measurements to determine the functional importance of TRPV4. We could show that both hypotonic stress and direct activation of TRPV4 by the specific agonist GSK1016790A triggered intracellular Ca²⁺ response. Strikingly, the Ca²⁺ increase upon hypotonic stress was neither affected by genetic knockout of TRPV4 in HaCaT cells nor by pharmacological inhibition of TRPV4 in both keratinocyte cell lines. Accordingly, hypotonicity-induced cell swelling, downstream activation of VRAC currents as well as subsequent RVD were unaffected both in TRPV4 inhibitor-treated keratinocytes and in HaCaT-TRPV4^-/- cells. In summary, our study shows that keratinocytes do not require TRPV4 for coping with hypotonic stress, which implies the involvement of other, yet unidentified Ca²⁺ channels.

Transient Receptor Potential (TRP) Channels in Airway Toxicity and Disease: An Update

Our respiratory system is exposed to toxicants and pathogens from both sides: the airways and the vasculature. While tracheal, bronchial and alveolar epithelial cells form a natural barrier in the airways, endothelial cells protect the lung from perfused toxic compounds, particulate matter and invading microorganism in the vascular system. Damages induce inflammation by our immune response and wound healing by (myo)fibroblast proliferation. Members of the transient receptor potential (TRP) superfamily of ion channel are expressed in many cells of the respiratory tract and serve multiple functions in physiology and pathophysiology. TRP expression patterns in non-neuronal cells with a focus on TRPA1, TRPC6, TRPM2, TRPM5, TRPM7, TRPV2, TRPV4 and TRPV6 channels are presented, and their roles in barrier function, immune regulation and phagocytosis are summarized. Moreover, TRP channels as future pharmacological targets in chronic obstructive pulmonary disease (COPD), asthma, cystic and pulmonary fibrosis as well as lung edema are discussed.

Lysophosphatidic acid as a CSF lipid in posthemorrhagic hydrocephalus that drives CSF accumulation via TRPV4-induced hyperactivation of NKCC1

Background

A range of neurological pathologies may lead to secondary hydrocephalus. Treatment has largely been limited to surgical cerebrospinal fluid (CSF) diversion, as specific and efficient pharmacological options are lacking, partly due to the elusive molecular nature of the CSF secretion apparatus and its regulatory properties in physiology and pathophysiology.

Methods

CSF obtained from patients with subarachnoid hemorrhage (SAH) and rats with experimentally inflicted intraventricular hemorrhage (IVH) was analyzed for lysophosphatidic acid (LPA) by alpha-LISA. We employed the in vivo rat model to determine the effect of LPA on ventricular size and brain water content, and to reveal the effect of activation and inhibition of the transient receptor potential vanilloid 4 (TRPV4) ion channel on intracranial pressure and CSF secretion rate. LPA-mediated modulation of TRPV4 was determined with electrophysiology and an ex vivo radio-isotope assay was employed to determine the effect of these modulators on choroid plexus transport.

Results

Elevated levels of LPA were observed in CSF obtained from patients with subarachnoid hemorrhage (SAH) and from rats with experimentally-inflicted intraventricular hemorrhage (IVH). Intraventricular administration of LPA caused elevated brain water content and ventriculomegaly in experimental rats, via its action as an agonist of the choroidal transient receptor potential vanilloid 4 (TRPV4) channel. TRPV4 was revealed as a novel regulator of ICP in experimental rats via its ability to modulate the CSF secretion rate through its direct activation of the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC1) implicated in CSF secretion.

Conclusions

Together, our data reveal that a serum lipid present in brain pathologies with hemorrhagic events promotes CSF hypersecretion and ensuing brain water accumulation via its direct action on TRPV4 and its downstream regulation of NKCC1. TRPV4 may therefore be a promising future pharmacological target for pathologies involving brain water accumulation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00361-9.

Cell Confluence Modulates TRPV4 Channel Activity in Response to Hypoxia

Transient receptor potential vanilloid 4 (TRPV4) is a polymodal Ca²⁺-permeable channel involved in various hypoxia-sensitive pathophysiological phenomena. Different tools are available to study channel activity, requiring cells to be cultured at specific optimal densities. In the present study, we examined if cell density may influence the effect of hypoxia on TRPV4 activity. Transiently TRPV4-transfected HEK293T cells were seeded at low or high densities corresponding to non-confluent or confluent cells, respectively, on the day of experiments, and cultured under in vitro normoxia or hypoxia. TRPV4-mediated cytosolic Ca²⁺ responses, single-channel currents, and Ca²⁺ influx through the channel were measured using Ca²⁺ imaging/microspectrofluorimetric assay, patch-clamp, and Bioluminescence Resonance Energy Transfer (BRET), respectively. TRPV4 plasma membrane translocation was studied using confocal microscopy, biotinylation of cell surface proteins, and BRET. Our results show that hypoxia exposure has a differential effect on TRPV4 activation depending on cell confluence. At low confluence levels, TRPV4 response is increased in hypoxia, whereas at high confluence levels, TRPV4 response is strongly inhibited, due to channel internalization. Thus, cell density appears to be a crucial parameter for TRPV4 channel activity.

Effects of butyrate− on ruminal Ca2+ transport: evidence for the involvement of apically expressed TRPV3 and TRPV4 channels

The ruminal epithelium absorbs large quantities of NH₄⁺ and Ca²⁺. A role for TRPV3 has emerged, but data on TRPV4 are lacking. Furthermore, short-chain fatty acids (SCFA) stimulate ruminal Ca²⁺ and NH₄⁺ uptake in vivo and in vitro, but the pathway is unclear. Sequencing of the bovine homologue (bTRPV4) revealed 96.79% homology to human TRPV4. Two commercial antibodies were tested using HEK-293 cells overexpressing bTRPV4, which in ruminal protein detected a weak band at the expected ~ 100 kDa and several bands ≤ 60 kDa. Immunofluorescence imaging revealed staining of the apical membrane of the stratum granulosum for bTRPV3 and bTRPV4, with cytosolic staining in other layers of the ruminal epithelium. A similar expression pattern was observed in a multilayered ruminal cell culture which developed resistances of > 700 Ω · cm² with expression of zonula occludens-1 and claudin-4. In Ussing chambers, 2-APB and the TRPV4 agonist GSK1016790A stimulated the short-circuit current across native bovine ruminal epithelia. In whole-cell patch-clamp recordings on HEK-293 cells, bTRPV4 was shown to be permeable to NH₄⁺, K⁺, and Na⁺ and highly sensitive to GSK1016790A, while effects of butyrate⁻ were insignificant. Conversely, bTRPV3 was strongly stimulated by 2-APB and by butyrate⁻ (pH 6.4 > pH 7.4), but not by GSK1016790A. Fluorescence calcium imaging experiments suggest that butyrate⁻ stimulates both bTRPV3 and bTRPV4. While expression of bTRPV4 appears to be weaker, both channels are candidates for the ruminal transport of NH₄⁺ and Ca²⁺. Stimulation by SCFA may involve cytosolic acidification (bTRPV3) and cell swelling (bTRPV4).

Endothelial TRPV4 channels in lung edema and injury

The alveolo-capillary barrier is relatively impermeable, and facilitates gas exchange via the large alveolar surface in the lung. Disruption of alveolo-capillary barrier leads to accumulation of edema fluid in lung injury. Studies in animal models of various forms of lung injury provide evidence that TRPV4 channels play a critical role in disruption of the alveolo-capillary barrier and pathogenesis of lung injury. TRPV4 channels from capillary endothelial cells, alveolar epithelial cells, and immune cells have been implicated in the pathogenesis of lung injury. Recent studies in endothelium-specific TRPV4 knockout mice point to a central role for endothelial TRPV4 channels in lung injury. In this chapter, we review the findings on the pathological roles of endothelial TRPV4 channels in different forms of lung injury and future directions for further investigation.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

TRPV4 Mechanotransduction in Fibrosis

Fibrosis is an irreversible, debilitating condition marked by the excessive production of extracellular matrix and tissue scarring that eventually results in organ failure and disease. Differentiation of fibroblasts to hypersecretory myofibroblasts is the key event in fibrosis. Although both soluble and mechanical factors are implicated in fibroblast differentiation, much of the focus is on TGF-β signaling, but to date, there are no specific drugs available for the treatment of fibrosis. In this review, we describe the role for TRPV4 mechanotransduction in cardiac and lung fibrosis, and we propose TRPV4 as an alternative therapeutic target for fibrosis.

A possible follow-up method for diabetic heart failure patients

INTRODUCTION

Plasma osmolarity is maintained through various mechanisms. The osmolarity of the aqueous humor around the crystalline lens is correlated with plasma osmolarity. A vacuole can be formed in the lens upon changes in osmolarity. The sodium-glucose cotransporter 2 inhibitors (SGLT2i) are new in the treatment of heart failure. They can cause osmotic diuresis but do not affect plasma osmolarity.

OBJECTIVE

It is unclear if the presence or absence of lens vacuole changes can monitor diabetic heart failure and SGLT2i treatment efficacy.

METHODS

Web of Science, PubMed and Scopus databases were searched for relevant articles about osmolarity, diabetes, transient receptor potential vanilloid channel, diabetic heart failure, lens vacuoles up to May 2021.

MAIN MESSAGE

The effect of SGLT2i on osmosis underlies its benefit to heart failure, but this in turn affects many other mechanisms. Failure to experience osmolarity changes will reduce the negative changes in terms of heart failure affected by osmolarity. A practical observable method is needed.

CONCLUSIONS

There is a possibility of using lens vacuoles in the follow-up of diabetic heart failure patients.

TRPing on Cell Swelling - TRPV4 Senses It

The transient receptor potential vanilloid 4 channel (TRPV4) is a non-selective cation channel that is widely expressed and activated by a range of stimuli. Amongst these stimuli, changes in cell volume feature as a prominent regulator of TRPV4 activity with cell swelling leading to channel activation. In experimental settings based on abrupt introduction of large osmotic gradients, TRPV4 activation requires co-expression of an aquaporin (AQP) to facilitate such cell swelling. However, TRPV4 readily responds to cell volume increase irrespectively of the molecular mechanism underlying the cell swelling and can, as such, be considered a sensor of increased cell volume. In this review, we will discuss the proposed events underlying the molecular coupling from cell swelling to channel activation and present the evidence of direct versus indirect swelling-activation of TRPV4. With this summary of the current knowledge of TRPV4 and its ability to sense cell volume changes, we hope to stimulate further experimental efforts in this area of research to clarify TRPV4’s role in physiology and pathophysiology.

TRP channels in health and disease at a glance

The transient receptor potential (TRP) channel superfamily consists of a large group of non-selective cation channels that serve as cellular sensors for a wide spectrum of physical and environmental stimuli. The 28 mammalian TRPs, categorized into six subfamilies, including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPML (mucolipin) and TRPP (polycystin), are widely expressed in different cells and tissues. TRPs exhibit a variety of unique features that not only distinguish them from other superfamilies of ion channels, but also confer diverse physiological functions. Located at the plasma membrane or in the membranes of intracellular organelles, TRPs are the cellular safeguards that sense various cell stresses and environmental stimuli and translate this information into responses at the organismal level. Loss- or gain-of-function mutations of TRPs cause inherited diseases and pathologies in different physiological systems, whereas up- or down-regulation of TRPs is associated with acquired human disorders. In this Cell Science at a Glance article and the accompanying poster, we briefly summarize the history of the discovery of TRPs, their unique features, recent advances in the understanding of TRP activation mechanisms, the structural basis of TRP Ca2+ selectivity and ligand binding, as well as potential roles in mammalian physiology and pathology.

Ca2+ Signaling by TRPV4 Channels in Respiratory Function and Disease

Members of the transient receptor potential (TRP) superfamily are broadly expressed in our body and contribute to multiple cellular functions. Most interestingly, the fourth member of the vanilloid family of TRP channels (TRPV4) serves different partially antagonistic functions in the respiratory system. This review highlights the role of TRPV4 channels in lung fibroblasts, the lung endothelium, as well as the alveolar and bronchial epithelium, during physiological and pathophysiological mechanisms. Data available from animal models and human tissues confirm the importance of this ion channel in cellular signal transduction complexes with Ca²⁺ ions as a second messenger. Moreover, TRPV4 is an excellent therapeutic target with numerous specific compounds regulating its activity in diseases, like asthma, lung fibrosis, edema, and infections.