The TRPV4 channel is a novel regulator of intracellular Ca2+ in human esophageal epithelial cells

Atractylodes macrocephala Koidz Alleviates Symptoms in Zymosan-Induced Irritable Bowel Syndrome Mouse Model through TRPV1, NaV1.5, and NaV1.7 Channel Modulation

(1) Background: Irritable bowel syndrome (IBS) is a common disease in the gastrointestinal (GI) tract. Atractylodes macrocephala Koidz (AMK) is known as one of the traditional medicines that shows a good efficacy in the GI tract. (2) Methods: We investigated the effect of AMK in a network pharmacology and zymosan-induced IBS animal model. In addition, we performed electrophysiological experiments to confirm the regulatory mechanisms related to IBS. (3) Results: Various characteristics of AMK were investigated using TCMSP data and various analysis systems. AMK restored the macroscopic changes and weight to normal. Colonic mucosa and inflammatory factors were reduced. These effects were similar to those of amitriptyline and sulfasalazine. In addition, transient receptor potential (TRP) V1, voltage-gated Na+ (NaV) 1.5, and NaV1.7 channels were inhibited. (4) Conclusion: These results suggest that AMK may be a promising therapeutic candidate for IBS management through the regulation of ion channels.

The potential effect of natural antioxidants on endothelial dysfunction associated with arterial hypertension

Arterial hypertension represents a leading cause of cardiovascular morbidity and mortality worldwide, and the identification of effective solutions for treating the early stages of elevated blood pressure (BP) is still a relevant issue for cardiovascular risk prevention. The pathophysiological basis for the occurrence of elevated BP and the onset of arterial hypertension have been widely studied in recent years. In addition, consistent progress in the development of novel, powerful, antihypertensive drugs and their appropriate applications in controlling BP have increased our potential for successfully managing disease states characterized by abnormal blood pressure. However, the mechanisms responsible for the disruption of endogenous mechanisms contributing to the maintenance of BP within a normal range are yet to be fully clarified. Recently, evidence has shown that several natural antioxidants containing active ingredients originating from natural plant extracts, used alone or in combination, may represent a valid solution for counteracting the development of arterial hypertension. In particular, there is evidence to show that natural antioxidants may enhance the viability of endothelial cells undergoing oxidative damage, an effect that could play a crucial role in the pathophysiological events accompanying the early stages of arterial hypertension. The present review aims to reassess the role of oxidative stress on endothelial dysfunction in the onset and progression of arterial hypertension and that of natural antioxidants in covering several unmet needs in the treatment of such diseases.

The traditional herbal medicines mixture, Banhasasim-tang, relieves the symptoms of irritable bowel syndrome via modulation of TRPA1, NaV1.5 and NaV1.7 channels

ETHNOPHARMACOLOGICAL RELEVANCE

The cause of irritable bowel syndrome (IBS), a functional gastrointestinal (GI) disorder, remains unclear. Banhasasim-tang (BHSST), a traditional herbal medicines mixture, mainly used to treat GI-related diseases, may have a potential in IBS treatment. IBS is characterized by abdominal pain as the main clinical symptom, which seriously affects the quality of life.

AIM OF THE STUDY

We conducted a study to evaluate the effectiveness of BHSST and its mechanisms of action in treating IBS.

MATERIALS AND METHODS

We evaluated the efficacy of BHSST in a zymosan-induced diarrhea-predominant animal model of IBS. Electrophysiological methods were used to confirm modulation of transient receptor potential (TRP) and voltage-gated Na⁺ (NaV) ion channels, which are associated mechanisms of action.

RESULTS

Oral administration of BHSST decreased colon length, increased stool scores, and increased colon weight. Weight loss was also minimized without affecting food intake. In mice administered with BHSST, the mucosal thickness was suppressed, making it similar to that of normal mice, and the degree of tumor necrosis factor-α was severely reduced. These effects were similar to those of the anti-inflammatory drug-sulfasalazine-and antidepressant-amitriptyline. Moreover, pain-related behaviors were substantially reduced. Additionally, BHSST inhibited TRPA1, NaV1.5, and NaV1.7 ion channels associated with IBS-mediated visceral hypersensitivity.

CONCLUSIONS

In summary, the findings suggest that BHSST has potential beneficial effects on IBS and diarrhea through the modulation of ion channels.

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

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

Activation of TRPV4 by lactate as a critical mediator of renal fibrosis in spontaneously hypertensive rats after moderate- and high-intensity exercise

Moderate-intensity exercise training has been regarded a healthy way to alleviate kidney fibrosis by the transforming growth factor-beta (TGFβ) signaling pathway. However, the impact of different intensity exercise training on renal function is unknown, and the underlying mechanism is also unclear. The purpose of this study is to explore the effect of lactic acid in different intensity exercise training on renal fibrosis in spontaneous hypertension. Masson’s trichrome staining, immunohistochemistry, lactic acid kit, and Western blotting were applied on the excised renal tissue from six male Wistar–Kyoto rats (WKY) and 18 male spontaneously hypertensive rats (SHR), which were randomly divided into a sedentary hypertensive group (SHR), moderate-intensity exercise hypertensive group (SHR-M), and high-intensity exercise hypertensive group (SHR-H). The results revealed that renal and blood lactic acid, as well as the key fibrotic protein levels of transient receptor potential vanilloid 4 (TRPV4), TGFβ-1, phospho-Smad2/3 (p-Smad2/3), and connective tissue growth factor (CTGF), were significantly decreased in the SHR-M group when compared with the SHR and SHR-H groups. In further in vitro experiments, we selected normal rat kidney interstitial fibroblast (NRK-49F) cells. By immunofluorescence and Western blotting techniques, we found that TRPV4 antagonists (RN-1734) markedly inhibited lactate-induced fibrosis. In conclusion, compared with previous studies, high-intensity exercise training (HIET) can cause adverse effects (renal damage and fibrosis). High concentrations of lactic acid can aggravate renal fibrosis conditions via activating TRPV4-TGFβ1-SMAD2/3-CTGF-mediated renal fibrotic pathways in spontaneous hypertension. This finding might provide new ideas for treating hypertensive nephropathy with different intensity exercise in the future.

TRPV4 Stimulation Level Regulates Ca2+-Dependent Control of Human Corneal Endothelial Cell Viability and Survival

The functional contribution of transient receptor potential vanilloid 4 (TRPV4) expression in maintaining human corneal endothelial cells (HCEC) homeostasis is unclear. Accordingly, we determined the effects of TRPV4 gene and protein overexpression on responses modulating the viability and survival of HCEC. Q-PCR, Western blot, FACS analyses and fluorescence single-cell calcium imaging confirmed TRPV4 gene and protein overexpression in lentivirally transduced 12V4 cells derived from their parent HCEC-12 line. Although TRPV4 overexpression did not alter the baseline transendothelial electrical resistance (TEER), its cellular capacitance (Ccl) was larger than that in its parent. Scanning electron microscopy revealed that only the 12V4 cells developed densely packed villus-like protrusions. Stimulation of TRPV4 activity with GSK1016790A (GSK101, 10 µmol/L) induced larger Ca2+ transients in the 12V4 cells than those in the parental HCEC-12. One to ten nmol/L GSK101 decreased 12V4 viability, increased cell death rates and reduced the TEER, whereas 1 µmol/L GSK101 was required to induce similar effects in the HCEC-12. However, the TRPV4 channel blocker RN1734 (1 to 30 µmol/L) failed to alter HCEC-12 and 12V4 morphology, cell viability and metabolic activity. Taken together, TRPV4 overexpression altered both the HCEC morphology and markedly lowered the GSK101 dosages required to stimulate its 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).

Diverse Roles of TRPV4 in Macrophages: A Need for Unbiased Profiling

Transient receptor potential vanilloid 4 (TRPV4) is a non-selective mechanosensitive ion channel expressed by various macrophage populations. Recent reports have characterized the role of TRPV4 in shaping the activity and phenotype of macrophages to influence the innate immune response to pathogen exposure and inflammation. TRPV4 has been studied extensively in the context of inflammation and inflammatory pain. Although TRPV4 activity has been generally described as pro-inflammatory, emerging evidence suggests a more complex role where this channel may also contribute to anti-inflammatory activities. However, detailed understanding of how TRPV4 may influence the initiation, maintenance, and resolution of inflammatory disease remains limited. This review highlights recent insights into the cellular processes through which TRPV4 contributes to pathological conditions and immune processes, with a focus on macrophage biology. The potential use of high-throughput and omics methods as an unbiased approach for studying the functional outcomes of TRPV4 activation is also discussed.

A New Therapeutic Candidate for Cardiovascular Diseases: Berberine

Cardiovascular diseases (CVD) are the leading cause of death in the world. However, due to the limited effectiveness and potential adverse effects of current treatments, the long-term prognosis of CVD patients is still discouraging. In recent years, several studies have found that berberine (BBR) has broad application prospects in the prevention and treatment of CVD. Due to its effectiveness and safety for gastroenteritis and diarrhea caused by bacterial infections, BBR has been widely used in China and other Asian countries since the middle of the last century. The development of pharmacology also provides evidence for the multi-targets of BBR in treating CVD. Researches on CVD, such as arrhythmia, atherosclerosis, dyslipidemia, hypertension, ischemic heart disease, myocarditis and cardiomyopathy, heart failure, etc., revealed the cardiovascular protective mechanisms of BBR. This review systematically summarizes the pharmacological research progress of BBR in the treatment of CVD in recent years, confirming that BBR is a promising therapeutic option for CVD.

Mechanosensory Signaling in Astrocytes

Mechanosensitivity is a well-known feature of astrocytes, however, its underlying mechanisms and functional significance remain unclear. There is evidence that astrocytes are acutely sensitive to decreases in cerebral perfusion pressure and may function as intracranial baroreceptors, tuned to monitor brain blood flow. This study investigated the mechanosensory signaling in brainstem astrocytes, as these cells reside alongside the cardiovascular control circuits and mediate increases in blood pressure and heart rate induced by falls in brain perfusion. It was found that mechanical stimulation-evoked Ca2+ responses in astrocytes of the rat brainstem were blocked by (1) antagonists of connexin channels, connexin 43 (Cx43) blocking peptide Gap26, or Cx43 gene knock-down; (2) antagonists of TRPV4 channels; (3) antagonist of P2Y1 receptors for ATP; and (4) inhibitors of phospholipase C or IP3 receptors. Proximity ligation assay demonstrated interaction between TRPV4 and Cx43 channels in astrocytes. Dye loading experiments showed that mechanical stimulation increased open probability of carboxyfluorescein-permeable membrane channels. These data suggest that mechanosensory Ca2+ responses in astrocytes are mediated by interaction between TRPV4 and Cx43 channels, leading to Cx43-mediated release of ATP which propagates/amplifies Ca2+ signals via P2Y1 receptors and Ca2+ recruitment from the intracellular stores. In astrocyte-specific Cx43 knock-out mice the magnitude of heart rate responses to acute increases in intracranial pressure was not affected by Cx43 deficiency. However, these animals displayed lower heart rates at different levels of cerebral perfusion, supporting the hypothesis of connexin hemichannel-mediated release of signaling molecules by astrocytes having an excitatory action on the CNS sympathetic control circuits.SIGNIFICANCE STATEMENT There is evidence suggesting that astrocytes may function as intracranial baroreceptors that play an important role in the control of systemic and cerebral circulation. To function as intracranial baroreceptors, astrocytes must possess a specialized membrane mechanism that makes them exquisitely sensitive to mechanical stimuli. This study shows that opening of connexin 43 (Cx43) hemichannels leading to the release of ATP is the key central event underlying mechanosensory Ca2+ responses in astrocytes. This astroglial mechanism plays an important role in the autonomic control of heart rate. These data add to the growing body of evidence suggesting that astrocytes function as versatile surveyors of the CNS metabolic milieu, tuned to detect conditions of potential metabolic threat, such as hypoxia, hypercapnia, and reduced perfusion.

Weak acids induce PGE2 production in human oesophageal cells: novel mechanisms underlying GERD symptoms

The role of weak acids with pH values in the range of 4–7 has been implicated in the symptoms of gastroesophageal reflux disease (GERD). Prostaglandin E₂ (PGE₂) is associated with heartburn symptom in GERD patients; however, the precise productive mechanisms remain unclear. In this study, we revealed that exposure to weak acids increases PGE₂ production with a peak at pH 4–5, slightly in human normal oesophageal cells (Het-1A), and robustly in oesophageal squamous carcinoma cells (KYSE-270). Release of PGE₂ from the oesophageal mucosa was augmented by weak acid treatment in rat. Chenodeoxycholic acid (CDCA), a bile acid, upregulated cyclooxygenase-2 (COX-2) expression in Het-1A and KYSE-270 and induced PGE₂ production in KYSE-270 cells. Weak acid-induced PGE₂ production was significantly inhibited by cytosolic phospholipase A2 (cPLA2), ERK, and transient receptor potential cation channel subfamily V member 4 (TRPV4), a pH-sensing ion channel, inhibitors. Hangeshashinto, a potent inhibitor of COX-2, strongly decreased weak acid- and CDCA-induced PGE₂ levels in KYSE-270. These results indicated that weak acids induce PGE₂ production via TRPV4/ERK/cPLA2 in oesophageal epithelial cells, suggesting a role in GERD symptoms like heartburn. Interventions targeting pH values up to 5 may be necessary for the treatment of GERD.

Gap junctions amplify TRPV4 activation-initiated cell injury via modification of intracellular Ca2+ and Ca2+-dependent regulation of TXNIP

The elevated intracellular Ca²⁺ and oxidative stress are well-reported mechanisms behind renal tubular epithelial injury initiated by various insults. Given that TRPV4 and connexin43 (Cx43) channels are activated by a wide range of stimuli and regulate both intracellular Ca²⁺ and redox status, we speculated an involvement of these channels in renal tubular cell injury. Here, we tested this possibility and explored the potential underlying mechanisms. Our results demonstrated that exposure of renal tubular epithelial cells to aminoglycoside G418 led to cell death, which was attenuated by both TRPV4 and gap junction (Gj) inhibitor. Activation of TRPV4 caused cell damage, which was associated with an early increase in Cx43 expression and function. Inhibition of Cx43 with chemical inhibitor or siRNA largely prevented TRPV4 activation-induced cell damage. Further analysis revealed that TRPV4 agonists elicited a rise in intracellular Ca²⁺ and caused a Ca²⁺-dependent elevation in TXNIP (a negative regulator of the antioxidant thioredoxin). In the presence of Gj inhibitor, however, these effects of TRPV4 were largely prevented. The depletion of intracellular Ca²⁺ with Ca²⁺ chelator BAPTA-AM or downregulation of TXNIP with siRNA significantly alleviated TRPV4 activation-initiated cell injury. Collectively, our results point to a critical involvement of TRPV4/Cx43 channel interaction in renal tubular cell injury through mechanisms involving a synergetic induction of intracellular Ca²⁺ and oxidative stress. Channel interactions could be an important mechanism underlying cell injury. Targeting channels could have therapeutic potential for the treatment of acute tubular cell injury.

OEsophageal Ion Transport Mechanisms and Significance Under Pathological Conditions

Ion transporters play an important role in several physiological functions, such as cell volume regulation, pH homeostasis and secretion. In the oesophagus, ion transport proteins are part of the epithelial resistance, a mechanism which protects the oesophagus against reflux-induced damage. A change in the function or expression of ion transporters has significance in the development or neoplastic progression of Barrett’s oesophagus (BO). In this review, we discuss the physiological and pathophysiological roles of ion transporters in the oesophagus, highlighting transport proteins which serve as therapeutic targets or prognostic markers in eosinophilic oesophagitis, BO and esophageal cancer. We believe that this review highlights important relationships which might contribute to a better understanding of the pathomechanisms of esophageal diseases.

The Pivotal Role of TRP Channels in Homeostasis and Diseases throughout the Gastrointestinal Tract

The transient receptor potential (TRP) channels superfamily are a large group of proteins that play crucial roles in cellular processes. For example, these cation channels act as sensors in the detection and transduction of stimuli of temperature, small molecules, voltage, pH, and mechanical constrains. Over the past decades, different members of the TRP channels have been identified in the human gastrointestinal (GI) tract playing multiple modulatory roles. Noteworthy, TRPs support critical functions related to the taste perception, mechanosensation, and pain. They also participate in the modulation of motility and secretions of the human gut. Last but not least, altered expression or activity and mutations in the TRP genes are often related to a wide range of disorders of the gut epithelium, including inflammatory bowel disease, fibrosis, visceral hyperalgesia, irritable bowel syndrome, and colorectal cancer. TRP channels could therefore be promising drug targets for the treatment of GI malignancies. This review aims at providing a comprehensive picture of the most recent advances highlighting the expression and function of TRP channels in the GI tract, and secondly, the description of the potential roles of TRPs in relevant disorders is discussed reporting our standpoint on GI tract–TRP channels interactions.

Transient receptor potential vanilloid 4 is involved in the upregulation of connexin expression following pilocarpine-induced status epilepticus in mice

Epilepsy is characterized by spontaneous seizures. Changes in the expression of the connexins (Cxs) have been reported to be involved in epileptogenesis. It has previously been shown that the transient receptor potential vanilloid 4 (TRPV4) plays an important role in the modulation of neuronal excitability, and that application of a TRPV4 antagonist blocks hyperthermia-induced seizures. Accordingly, in the present study, we sought to explore whether TRPV4 is involved in the regulation of Cx expression following pilocarpine-induced status epilepticus (PISE) in mice. We observed that TRPV4 protein levels in hippocampi increased 3 h to 30 d following PISE, peaking 1-3 d after induction, and that pre-application of the TRPV4 antagonist HC-067047 increased the latency to develop SE induced by pilocarpine and reduced the success rate of PISE preparation. We demonstrated that Cx43 protein levels followed a time profile similar to that of TRPV4, and further showed that the increase in Cx43 protein levels on 3 d post-PISE was markedly attenuated by HC-067047. In contrast, the corresponding increase in Cx32 protein levels lagged substantially behind, and these levels were unaffected by HC-067047. Similarly, the TRPV4 agonist GSK1016790A increased the mRNA and protein levels of Cx43, but not those of Cx32. We thus conclude that the upregulation of Cx43 expression by TRPV4 may be involved in the pathophysiology of epilepsy.

Rho-Associated Coiled-Coil Kinase (ROCK) in Molecular Regulation of Angiogenesis

Identified as a major downstream effector of the small GTPase RhoA, Rho-associated coiled-coil kinase (ROCK) is a versatile regulator of multiple cellular processes. Angiogenesis, the process of generating new capillaries from the pre-existing ones, is required for the development of various diseases such as cancer, diabetes and rheumatoid arthritis. Recently, ROCK has attracted attention for its crucial role in angiogenesis, making it a promising target for new therapeutic approaches. In this review, we summarize recent advances in understanding the role of ROCK signaling in regulating the permeability, migration, proliferation and tubulogenesis of endothelial cells (ECs), as well as its functions in non-ECs which constitute the pro-angiogenic microenvironment. The therapeutic potential of ROCK inhibitors in angiogenesis-related diseases is also discussed.

TRPV4 Channel-Regulated ATP Release Contributes to γ-Irradiation-Induced Production of IL-6 and IL-8 in Epidermal Keratinocytes

External stimuli, such as radiation, induce inflammatory cytokine and chemokine production in skin, but the mechanisms involved are not completely understood. We previously showed that the P2Y11 nucleotide receptor, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) all participate in interleukin (IL)-6 production induced by γ-irradiation. Here, we focused on the transient receptor potential vanilloid 4 (TRPV4) channel, which is expressed in skin keratinocytes and has been reported to play a role in inflammation. We found that irradiation of human epidermal keratinocytes HaCaT cells with 5 Gy of γ-rays (¹³⁷Cs: 0.75 Gy/min) induced IL-6 and IL-8 production. HaCaT cells treated with TRPV4 channel agonist GSK1016790A also showed increased IL-6 and IL-8 production. In both cases, IL-6/IL-8 production was not increased at 24 h after stimulation, but was increased at 48 h. ATP was released from cells exposed to γ-irradiation or TRPV4 channel agonist, and the release was suppressed by TRPV4 channel inhibitors. The γ-irradiation-induced increase in IL-6 and IL-8 production was suppressed by apyrase (ecto-nucleotidase), NF157 (selective P2Y11 receptor antagonist) and SB203580 (p38 MAPK inhibitor). GSK1016790A-induced inhibitor of kappa B-alpha (IκBα) decomposition, which causes NF-κB activation was suppressed by NF157 and SB203580, and γ-irradiation-induced IκBα decomposition was suppressed by TRPV4 channel inhibitors. Our results suggest that γ-irradiation of keratinocytes induces ATP release via activation of the TRPV4 channel, and then ATP activates P2Y11 receptor and p38 MAPK-NF-κB signaling, resulting in IL-6/IL-8 production.

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.

Role of Nonneuronal TRPV4 Signaling in Inflammatory Processes

Transient receptor potential (TRP) ion channels are important signaling components in nociceptive and inflammatory pathways. This is attributed to their ability to function as polymodal sensors of environmental stimuli (chemical and mechanical) and as effector molecules in receptor signaling pathways. TRP vanilloid 4 (TRPV4) is a nonselective cation channel that is activated by multiple endogenous stimuli including shear stress, membrane stretch, and arachidonic acid metabolites. TRPV4 contributes to many important physiological processes and dysregulation of its activity is associated with chronic conditions of metabolism, inflammation, peripheral neuropathies, musculoskeletal development, and cardiovascular regulation. Mechanosensory and receptor- or lipid-mediated signaling functions of TRPV4 have historically been attributed to central and peripheral neurons. However, with the development of potent and selective pharmacological tools, transgenic mice and improved molecular and imaging techniques, many new roles for TRPV4 have been revealed in nonneuronal cells. In this chapter, we discuss these recent findings and highlight the need for greater characterization of TRPV4-mediated signaling in nonneuronal cell types that are either directly associated with neurons or indirectly control their excitability through release of sensitizing cellular factors. We address the integral role of these cells in sensory and inflammatory processes as well as their importance when considering undesirable on-target effects that may be caused by systemic delivery of TRPV4-selective pharmaceutical agents for treatment of chronic diseases. In future, this will drive a need for targeted drug delivery strategies to regulate such a diverse and promiscuous protein.

Activation of Transient Receptor Potential Vanilloid 4 Promotes the Proliferation of Stem Cells in the Adult Hippocampal Dentate Gyrus

Neurogenesis plays an important role in adult hippocampal function, and this process can be modulated by intracellular calcium. The activation of transient receptor potential vanilloid 4 (TRPV4) induces an increase in intracellular calcium concentration, but whether neurogenesis can be modulated by TRPV4 activation remains unclear. Here, we report that intracerebroventricular injection of the TRPV4 agonist GSK1016790A for 5 days enhanced the proliferation of stem cells in the hippocampal dentate gyrus (DG) of adult mice without affecting neurite growth, differentiation, or survival of newborn cells. GSK1016790A induced increases in the hippocampal protein levels of cyclin-dependent kinase (CDK) 6, CDK2, cyclin E1, and cyclin A2 but did not affect CDK4 and cyclin D1 expression. The phosphorylation of retinoblastoma protein (Rb) in hippocampi was enhanced in GSK1016790A-injected mice compared with control mice. Moreover, hippocampal protein levels of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) phosphorylation were enhanced by GSK1016790A. Finally, GSK1016790A-enhanced proliferation was markedly blocked by a MAPK/ERK kinase or p38 MAPK antagonist (U0126 or SB203580, respectively). The increased protein levels of CDK2 and CDK6, as well as those of cyclin E1 and cyclin A2, in GSK1016790A-injected mice were substantially reduced by co-injection of U0126 or SB203580. We conclude that TRPV4 activation results in the proliferation of stem cells in the adult hippocampal DG, which is likely mediated through ERK1/2 and p38 MAPK signaling to increase the expression of CDKs (CDK6 and CDK2) and cyclins (cyclin E1 and A2), phosphorylate Rb consequently, and accelerate the cell cycle ultimately.

Selective potentiation of 2-APB-induced activation of TRPV1-3 channels by acid

Temperature-sensitive TRP channels are important for responses to pain and inflammation, to both of which tissue acidosis is a major contributing factor. However, except for TRPV1, acid-sensing by other ThermoTRP channels remains mysterious. We show here that unique among TRPV1–3 channels, TRPV3 is directly activated by protons from cytoplasmic side. This effect is very weak and involves key cytoplasmic residues L508, D512, S518, or A520. However, mutations of these residues did not affect a strong proton induced potentiation of TRPV3 currents elicited by the TRPV1–3 common agonist, 2-aminoethoxydiphenyl borate (2-APB), no matter if the ligand was applied from extracellular or cytoplasmic side. The acid potentiation was common among TRPV1–3 and only seen with 2-APB-related ligands. Using ¹H-nuclear magnetic resonance to examine the solution structures of 2-APB and its analogs, we observed striking structural differences of the boron-containing compounds at neutral/basic as compared to acidic pH, suggesting that a pH-dependent configuration switch of 2-APB-based drugs may underlie their functionality. Supporting this notion, protons also enhanced the inhibitory action of 2-APB on TRPM8. Collectively, our findings reveal novel insights into 2-APB action on TRP channels, which should facilitate the design of new drugs for these channels.

Functional TRPV2 and TRPV4 channels in human cardiac c-kit(+) progenitor cells

The cellular physiology and biology of human cardiac c-kit(+) progenitor cells has not been extensively characterized and remains an area of active research. This study investigates the functional expression of transient receptor potential vanilloid (TRPV) and possible roles for this ion channel in regulating proliferation and migration of human cardiac c-kit(+) progenitor cells. We found that genes coding for TRPV2 and TRPV4 channels and their proteins are significantly expressed in human c-kit(+) cardiac stem cells. Probenecid, an activator of TRPV2, induced an increase in intracellular Ca(2+) (Ca(2+) i ), an effect that may be attenuated or abolished by the TRPV2 blocker ruthenium red. The TRPV4 channel activator 4α-phorbol 12-13-dicaprinate induced Ca(2+) i oscillations, which can be inhibited by the TRPV4 blocker RN-1734. The alteration of Ca(2+) i by probenecid or 4α-phorbol 12-13-dicprinate was dramatically inhibited in cells infected with TRPV2 short hairpin RNA (shRNA) or TRPV4 shRNA. Silencing TRPV2, but not TRPV4, significantly reduced cell proliferation by arresting cells at the G0/G1 boundary of the cell cycle. Cell migration was reduced by silencing TRPV2 or TRPV4. Western blot revealed that silencing TRPV2 decreased expression of cyclin D1, cyclin E, pERK1/2 and pAkt, whereas silencing TRPV4 only reduced pAkt expression. Our results demonstrate for the first time that functional TRPV2 and TRPV4 channels are abundantly expressed in human cardiac c-kit(+) progenitor cells. TRPV2 channels, but not TRPV4 channels, participate in regulating cell cycle progression; moreover, both TRPV2 and TRPV4 are involved in migration of human cardiac c-kit(+) progenitor cells.

Protease-Activated Receptor-2 Up-Regulates Transient Receptor Potential Vanilloid 4 Function in Mouse Esophageal Keratinocyte

BACKGROUND

The reflux of pancreatic-duodenal fluids is implicated in the pathophysiology of proton-pump inhibitor-resistant gastroesophageal reflux disease (GERD). Protease-activated receptor-2 (PAR-2) is activated by proteases, the pancreatic enzyme, trypsin, and the activated PAR-2 enhances transient receptor potential vanilloid 4 (TRPV4) function in neurons. TRPV4 stimulates ATP exocytosis in conjunction with the vesicular nucleotide transporter, which mediates mechano-transduction and vagal stimulation. The aim of the present study was to verify whether the activated PAR-2 up-regulates TRPV4 function in mouse esophageal keratinocytes, which may link to the pathophysiology in PPI-resistant GERD.

METHODS

TRPV4 and PAR-2 expressions were detected by RT-PCR, immunostaining, and western blotting in mouse esophageal keratinocytes. The functional response of TRPV4 to esophageal keratinocytes was analyzed using a Ca(2+) imaging system. Cellular ATP release was examined by luciferase-luciferin reaction. TRPV4 phosphorylation was studied by immunoprecipitation and western blotting.

RESULTS

PAR-2 and TRPV4 mRNAs and proteins were expressed in esophageal keratinocytes. Pre-treatment with trypsin significantly increased the responses to TRPV4 activator in esophageal keratinocytes, probably via the phosphorylation of serine residue of TRPV4 by protein kinase C and resulted in cellular ATP release from the cells.

CONCLUSIONS

Activated PAR-2 with trypsin exposure up-regulated TRPV4 function and increased ATP release in mouse esophageal keratinocytes. This mechanism might be related to the pathophysiology of GERD, especially non-erosive GERD.

Basal cells express functional TRPV4 channels in the mouse nasal epithelium

Basal cells in the nasal epithelium (olfactory and airway epithelia) are stem/progenitor cells that are capable of dividing, renewing and differentiating into specialized cells. These stem cells can sense their biophysical microenvironment, but the underlying mechanism of this process remains unknown. Here, we demonstrate the prominent expression of the transient receptor potential vanilloid type 4 (TRPV4) channel, a Ca²⁺-permeable channel that is known to act as a sensor for hypo-osmotic and mechanical stresses, in the basal cells of the mouse nasal epithelium. TRPV4 mRNA was expressed in the basal portions of the prenatal mouse nasal epithelium, and this expression continued into adult mice. The TRPV4 protein was also detected in the basal layers of the nasal epithelium in wild-type but not in TRPV4-knockout (TRPV4-KO) mice. The TRPV4-positive immunoreactions largely overlapped with those of keratin 14 (K14), a marker of basal cells, in the airway epithelium, and they partially overlapped with those of K14 in the olfactory epithelium. Ca²⁺ imaging analysis revealed that hypo-osmotic stimulation and 4α-phorbol 12,13 didecanoate (4α-PDD), both of which are TRPV4 agonists, caused an increase in the cytosolic Ca²⁺ concentration in a subset of primary epithelial cells cultured from the upper parts of the nasal epithelium of the wild-type mice. This response was barely noticeable in cells from similar parts of the epithelium in TRPV4-KO mice. Finally, there was no significant difference in BrdU-labeled proliferation between the olfactory epithelia of wild-type and TRPV4-KO mice under normal conditions. Thus, TRPV4 channels are functionally expressed in basal cells throughout the nasal epithelium and may act as sensors for the development and injury-induced regeneration of basal stem cells.

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TRPV4 is expressed in basal stem cells of the nasal airway and olfactory epithelium.

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TRPV4 expression appears in the nasal epithelium during the late prenatal stages.

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TRPV4 activation causes an increase in cytosolic Ca²⁺ concentration.

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TRPV4 may be involved in a variety of cellular functions in progenitor/stem cells.

Berberine via suppression of transient receptor potential vanilloid 4 channel improves vascular stiffness in mice

Berberine, as an alkaloid found in many Chinese herbs, improves vascular functions in patients with cardiovascular diseases. We determined the effects of berberine in hypertension and vascular ageing, and elucidated the underlying mechanisms. In isolated aortas, berberine dose-dependently elicited aortic relaxation. In cultured cells, berberine induced the relaxation of vascular smooth muscle cells (VSMCs). Overexpression of transient receptor potential vanilloid 4 (TRPV4) channel by genetic approaches abolished the berberine-induced reduction in intracellular Ca²⁺ concentration in VSMCs and attenuated berberine-elicited vessel dilation in mice aortas. In deoxycorticosterone acetate (DOCA)-induced hypertensive model, treatment of mice with berberine or RN-1734, a pharmacological inhibitor of TRPV4, significantly decreased systemic blood pressure (BP) in control mice or mice infected with an adenovirus vector. However, berberine-induced effects of lowering BP were reversed by overexpressing TRPV4 in mice by infecting with adenovirus. Furthermore, long-term administration of berberine decreased mean BP and pulse BP, increased artery response to vasodilator and reduced vascular collagen content in aged mice deficient in apolipoprotein E (Apoe-KO), but not in Apoe-KO old mice with lentivirus-mediated overexpression of TRPV4 channel. In conclusion, berberine induces direct vasorelaxation to lower BP and reduces vascular stiffness in aged mice through suppression of TRPV4.

Emerging functions of pannexin 1 in the eye

Pannexin 1 (Panx1) is a high-conductance, voltage-gated channel protein found in vertebrates. Panx1 is widely expressed in many organs and tissues, including sensory systems. In the eye, Panx1 is expressed in major divisions including the retina, lens and cornea. Panx1 is found in different neuronal and non-neuronal cell types. The channel is mechanosensitive and responds to changes in extracellular ATP, intracellular calcium, pH, or ROS/nitric oxide. Since Panx1 channels operate at the crossroad of major signaling pathways, physiological functions in important autocrine and paracrine feedback signaling mechanisms were hypothesized. This review starts with describing in depth the initial Panx1 expression and localization studies fostering functional studies that uncovered distinct roles in processing visual information in subsets of neurons in the rodent and fish retina. Panx1 is expressed along the entire anatomical axis from optical nerve to retina and cornea in glia, epithelial and endothelial cells as well as in neurons. The expression and diverse localizations throughout the eye points towards versatile functions of Panx1 in neuronal and non-neuronal cells, implicating Panx1 in the crosstalk between immune and neural cells, pressure related pathological conditions like glaucoma, wound repair or neuronal cell death caused by ischemia. Summarizing the literature on Panx1 in the eye highlights the diversity of emerging Panx1 channel functions in health and disease.

TRPV4: new therapeutic target for inflammatory bowel diseases

The transient receptor potential vanilloid-4 (TRPV4) belongs to a family of ion channels and can be activated by warm temperature, hypotonicity, cell swelling or lipid mediators of the arachidonic cascade. The metabolites or events responsible for TRPV4 activation are associated with inflammation, arguing in favor of a role for this receptor in inflammatory diseases. The first studies have focused their attention on the role of TRPV4 in neurons and endothelial cells but TRPV4 cellular distribution is widespread, particularly in the gastrointestinal tract. Herein, we review a number of studies demonstrating the expression of TRPV4 in the gut, the regulation of its expression and functions by inflammatory mediators in that organ and the consequences of TRPV4 activation or inhibition in the intestine. We further discuss the relevance of considering this receptor as a potential target for therapeutic development in inflammatory bowel diseases.

The TRPV4 channel

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

Functional TRPV and TRPM channels in human preadipocytes

Preadipocytes are widely used as an in vitro model to investigate proliferation, adipogenic differentiation, and lipodystrophy; however, cellular physiology and biology are not fully understood in human preadipocytes. The present study was to investigate the expression of transient receptor potential (TRP) channels in human preadipocytes and their potential roles in regulating proliferation and adipogenic differentiation using approaches of confocal microscopy, whole-cell patch voltage-clamp, reverse transcription polymerase chain reaction, Western blot, etc. We found that TRPV2, TRPV4, and TRPM7 channels were abundantly expressed in human preadipocytes. The intracellular Ca(2+) transient activated by the TRPV2 activator probenecid was reversed or prevented by ruthenium red, a TRPV2 blocker. The TRPV4 channel activator, 4α-phorbol 12-13-dicaprinate, enhanced intracellular Ca(2+) oscillations, and the effect was inhibited by the TRPV4 blocker RN-1734. TRPM7 current was recorded with dialysis of Mg(2+)-free pipette solution, which was inhibited by the TRP channel blocker 2-aminoethoxydiphenyl borate and enhanced by acidic extracellular pH. Silencing TRPV2 or TRPM7, but not TRPV4, significantly reduced cell proliferation via inhibiting cyclin D1, cyclin E, and p-ERK1/2. Interestingly, individually silencing these three channels decreased adipogenic differentiation of human preadipocytes by reducing p-Akt kinase. Our results demonstrate for the first time that functional TRPV2, TRPV4, and TRPM7 channels are abundantly expressed in human preadipocytes. TRPV2 and TRPM7, but not TRPV4, regulate cell proliferation via activating cyclin D1, cyclin E, and p-ERK1/2, while they are all involved in adipogenesis in human preadipocytes via phosphorylating Akt kinase.

How reflux causes symptoms: reflux perception in gastroesophageal reflux disease

In gastroesophageal reflux disease (GERD) symptoms arise due to reflux of gastric content into the oesophagus. However, the relation between magnitude and onset of reflux and symptom generation in GERD patients is far from simple; gastroesophageal reflux occurs several times a day in everyone and the majority of reflux episodes remains asymptomatic. This review aims to address the question how reflux causes symptoms, focussing on factors leading to enhanced reflux perception. We will highlight esophageal sensitivity variance between subtypes of GERD, which is influenced by peripheral sensitization of primary afferents, central sensitization of spinal dorsal horn neurons, impaired mucosal barrier function and genetic factors. We will also discuss the contribution of specific refluxate characteristics to reflux perception, including acidity, and the role of bile, pepsin and gas and proximal extent. Further understanding of reflux perception might improve GERD treatment, especially in current partial responders to therapy.

Hypoxia-inducible factor-1α (HIF1α) switches on transient receptor potential ankyrin repeat 1 (TRPA1) gene expression via a hypoxia response element-like motif to modulate cytokine release

Transient receptor potential ankyrin repeat 1 (TRPA1) forms calcium (Ca²⁺)- and zinc (Zn²⁺)-permeable ion channels that sense noxious substances. Despite the biological and clinical importance of TRPA1, there is little knowledge of the mechanisms that lead to transcriptional regulation of TRPA1 and of the functional role of transcriptionally induced TRPA1. Here we show induction of TRPA1 by inflammatory mediators and delineate the underlying molecular mechanisms and functional relevance. In human fibroblast-like synoviocytes, key inflammatory mediators (tumor necrosis factor-α and interleukin-1α) induced TRPA1 gene expression via nuclear factor-κB signaling and downstream activation of the transcription factor hypoxia-inducible factor-1α (HIF1α). HIF1α unexpectedly acted by binding to a specific hypoxia response element-like motif and its flanking regions in the TRPA1 gene. The induced TRPA1 channels, which were intrinsically activated by endogenous hydrogen peroxide and Zn²⁺, suppressed secretion of interleukin-6 and interleukin-8. The data suggest a previously unrecognized HIF1α mechanism that links inflammatory mediators to ion channel expression.

TRPV4 in porcine lens epithelium regulates hemichannel-mediated ATP release and Na-K-ATPase activity

In several tissues, transient receptor potential vanilloid 4 (TRPV4) channels are involved in the response to hyposmotic challenge. Here we report TRPV4 protein in porcine lens epithelium and show that TRPV4 activation is an important step in the response of the lens to hyposmotic stress. Hyposmotic solution (200 mosM) elicited ATP release from intact lenses and TRPV4 antagonists HC 067047 and RN 1734 prevented the release. In isosmotic solution, the TRPV4 agonist GSK1016790A (GSK) elicited ATP release. When propidium iodide (PI) (MW 668) was present in the bathing medium, GSK and hyposmotic solution both increased PI entry into the epithelium of intact lenses. Increased PI uptake and ATP release in response to GSK and hyposmotic solution were abolished by a mixture of agents that block connexin and pannexin hemichannels, 18α-glycyrrhetinic acid and probenecid. Increased Na-K-ATPase activity occurred in the epithelium of lenses exposed to GSK and 18α-glycyrrhetinic acid + probenecid prevented the response. Hyposmotic solution caused activation of Src family kinase and increased Na-K-ATPase activity in the lens epithelium and TRPV4 antagonists prevented the response. Ionomycin, which is known to increase cytoplasmic calcium, elicited ATP release, the magnitude of which was no greater when lenses were exposed simultaneously to ionomycin and hyposmotic solution. Ionomycin-induced ATP release was significantly reduced in calcium-free medium. TRPV4-mediated calcium entry was examined in Fura-2-loaded cultured lens epithelium. Hyposmotic solution and GSK both increased cytoplasmic calcium that was prevented by TRPV4 antagonists. The cytoplasmic calcium rise in response to hyposmotic solution or GSK was abolished when calcium was removed from the bathing solution. The findings are consistent with hyposmotic shock-induced TRPV4 channel activation which triggers hemichannel-mediated ATP release. The results point to TRPV4-mediated calcium entry that causes a cytoplasmic calcium increase which is an essential early step in the mechanism used by the lens to sense and respond to hyposmotic stress.