An environmental sensor, TRPV4 is a novel regulator of intracellular Ca2+ in human synoviocytes

Roles of TRP and PIEZO receptors in autoimmune diseases

Autoimmune diseases are pathological autoimmune reactions in the body caused by various factors, which can lead to tissue damage and organ dysfunction. They can be divided into organ-specific and systemic autoimmune diseases. These diseases usually involve various body systems, including the blood, muscles, bones, joints and soft tissues. The transient receptor potential (TRP) and PIEZO receptors, which resulted in David Julius and Ardem Patapoutian winning the Nobel Prize in Physiology or Medicine in 2021, attracted people's attention. Most current studies on TRP and PIEZO receptors in autoimmune diseases have been carried out on animal model, only few clinical studies have been conducted. Therefore, this study aimed to review existing studies on TRP and PIEZO to understand the roles of these receptors in autoimmune diseases, which may help elucidate novel treatment strategies.

Transient receptor potential vanilloid 4 as a regulator of induced pluripotent stem cell chondrogenesis

Transient receptor potential vanilloid 4 (TRPV4) is a polymodal calcium-permeable cation channel that is highly expressed in cartilage and is sensitive to a variety of extracellular stimuli. The expression of this channel has been associated with the process of chondrogenesis in adult stem cells as well as several cell lines. Here, we used a chondrogenic reporter (Col2a1-GFP) in murine induced pluripotent stem cells (iPSCs) to examine the hypothesis that TRPV4 serves as both a marker and a regulator of chondrogenesis. Over 21 days of chondrogenesis, iPSCs showed significant increases in Trpv4 expression along with the standard chondrogenic gene markers Sox9, Acan, and Col2a1, particularly in the green fluorescent protein positive (GFP+) chondroprogenitor subpopulation. Increased gene expression for Trpv4 was also reflected by the presence of TRPV4 protein and functional Ca²⁺ signaling. Daily activation of TRPV4 using the specific agonist GSK1016790A resulted in significant increases in cartilaginous matrix production. An improved understanding of the role of TRPV4 in chondrogenesis may provide new insights into the development of new therapeutic approaches for diseases of cartilage, such as osteoarthritis, or channelopathies and hereditary disorders that affect cartilage during development. Harnessing the role of TRPV4 in chondrogenesis may also provide a novel approach for accelerating stem cell differentiation in functional tissue engineering of cartilage replacements for joint repair.

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

The transient receptor potential vanilloid 4 channel (TRPV4) belongs to the mammalian TRP superfamily of cation channels. TRPV4 is ubiquitously expressed, activated by a disparate array of stimuli, interacts with a multitude of proteins, and is modulated by a range of post-translational modifications, the majority of which we are only just beginning to understand. Not surprisingly, a great number of physiological roles have emerged for TRPV4, as have various disease states that are attributable to the absence, or abnormal functioning, of this ion channel. This review will highlight structural features of TRPV4, endogenous and exogenous activators of the channel, and discuss the reported roles of TRPV4 in health and disease.

Photobiomodulation with 630-nm LED radiation inhibits the proliferation of human synoviocyte MH7A cells possibly via TRPV4/PI3K/AKT/mTOR signaling pathway

Phototherapy has been used to treat postoperative pain and inflammatory response in rheumatoid arthritis. Confidence in this approach, however, is impaired by lack of understanding of the light-triggered cellular and molecular mechanisms. The purpose of this study was to characterize the response of human synoviocyte MH7A cells to visible LED red light in an attempt to elucidate the associated action mechanism. Human synoviocyte MH7A cells were treated with 630-nm LED light after stimulation of tumor necrosis factor-α (TNF-α). The effects of light radiation on cell proliferation and migration were detected by MTT assay and scratch test. The expressions of inflammatory cytokines were measured using RT-qPCR. This was followed by detection of the levels of extracellular proteins IL-6 and IL-8 after differential radiation. Furthermore, the expression levels and activation of proteins on PI3K/AKT/mTOR signaling pathway were examined with Western blot. In terms of the proliferation and migration, repeated radiation with LED red light (630 nm, 26 and 39 J/cm²) exerted an inhibitory effect on synoviocyte MH7A cells. Expression of inflammatory factors (IL-6, IL-1β, IL-8, and MMP-3) was reduced; meanwhile, the expression of anti-inflammatory factor IL-10 was promoted. At the protein level, treatment with 39 J/cm² of LED red light could decrease the level of extracellular protein (IL-6 and IL-8) and affect the expression and phosphorylation of proteins on TRPV4/PI3K/AKT/mTOR signaling pathway induced by TNF-α. These results demonstrated that LED red light (630 nm) inhibits proliferation and migration of MH7A cells. The growth-inhibiting effects of LED red light on human synoviocyte MH7A cells appear to be associated with regulation of the TRPV4/PI3K/AKT/mTOR signaling pathway.

An overview of carbonic anhydrases and membrane channels of synoviocytes in inflamed joints

The highly aggressive fibroblast-like synoviocytes (FLSs) are inflammatory mediators involved in synovial joint destruction. Membrane channels and transporters are essential components of the cell migration apparatus and are involved in various cellular functions. Although evidence is emerging that cell migration is a physiological/pathological process, the mechanism of highly dynamic synoviocytes linked to the membrane channels and carbonic anhydrases (CAs) in inflamed joints is only partially understood. In this review, topics covered will give a brief overview of CAs and the membrane channels of synoviocytes. We have also systematically focused on the role of FLS channels and transporters under various conditions, including rheumatoid arthritis (RA), to understand the pathophysiology of the migration of synoviocytes as inflammatory mediators in joints.

Transient Receptor Potential vanilloid 4 ion channel in C-fibres is involved in mechanonociception of the normal and inflamed joint

The Transient Receptor Potential vanilloid 4 ion channel (TRPV4) is an important sensor for osmotic and mechanical stimuli in the musculoskeletal system, and it is also involved in processes of nociception. In this study we investigated the putative role of TRPV4 ion channels in joint pain. In anesthetized rats we recorded from mechanosensitive nociceptive A∂- and C-fibres supplying the medial aspect of the knee joint. The intraarticular injection of the TRPV4 antagonist RN-1734 into the knee joint reduced the responses of C-fibres of the normal joint to noxious mechanical stimulation and the responses of the sensitized C-fibres of the acutely inflamed joint to innocuous and noxious mechanical stimulation. The responses of nociceptive A∂-fibres were not significantly altered by RN-1734. The intraarticular application of the TRPV4 agonists 4αPDD, GSK 1016790 A, and RN-1747 did not consistently alter the responses of A∂- and C-fibres to mechanical stimulation of the joint nor did they induce ongoing activity. We conclude that TRPV4 ion channels are involved in the responses of C-fibres to noxious mechanical stimulation of the normal joint, and in the enhanced sensitivity of C-fibres to mechanical stimulation of the joint during inflammation of the joint.

Protein informatics combined with multiple data sources enriches the clinical characterization of novel TRPV4 variant causing an intermediate skeletal dysplasia

Background

Transient receptor potential cation channel subfamily V member 4 (TRPV4) is an ion channel permeable to Ca²⁺ that is sensitive to physical, hormonal, and chemical stimuli. This protein is expressed in many cell types, including osteoclasts, chondrocytes, and sensory neurons. As such, pathogenic variants of this gene are associated with skeletal dysplasias and neuromuscular disorders. Pathogenesis of these phenotypes is not yet completely understood, but it is known that genotype–phenotype correlations for TRPV4 pathogenic variants often are not present.

Methods

Newly characterized, suspected pathogenic variant in TRPV4 was analyzed using protein informatics and personalized protein‐level molecular studies, genomic exome analysis, and clinical study.

Results

This statement is demonstrated in the family of our proband, a 47‐year‐old female having the novel c.2401A>G (p.K801E) variant of TRPV4. We discuss the common symptoms between the proband, her father, and her daughter, and compare her phenotype to known TRPV4‐associated skeletal dysplasias.

Conclusions

Protein informatics and molecular modeling are used to confirm the pathogenicity of the unique TRPV4 variant found in this family. Multiple data were combined in a comprehensive manner to give complete overall perspective on the patient disease and prognosis.

An efficient nonlinear hybridization chain reaction-based sensitive fluorescent assay for in situ estimation of calcium channel protein expression on bone marrow cells

A sensitive and highly efficient approach to monitor the expression of proteins on live cells was urgently needed to demonstrate its factor and mechanism and most important for clinical diagnostics and molecular biology. Herein, we developed a simple and highly efficient strategy, nonlinear hybridization chain reaction (nonlinear HCR), for the sensitive determination of proteins on live cells with transient receptor potential vanilloid 4 (TRPV4) and RAW264.7 cells as a model. Unlike the normal hybridization chain reaction (HCR) with multiplicative amplification, an exponential amplified fluorescent response could be obtained in theory based on the proposed nonlinear HCR. As a result, the nonlinear HCR generated a significant enhancement about 3 times compared with the normal HCR and 10 times compared with the directly immunofluorescence assay. Based on the proposed nonlinear HCR, the fluorescent signals increased with the concentration of TRPV4 in the range from 10 pg/mL to 100 ng/mL with a detection limit of 2.8 pg/mL, which would be useful for the sensitive detection of proteins in cell lysis or on cell surface. At the same time, the significant improvements via nonlinear HCR were achieved in the fluorescent imaging system compared with traditional immunofluorescence staining and normal HCR, proving the significant value of nonlinear HCR-based amplification strategy. Success in the establishment of the highly efficient nonlinear HCR strategy offered a simple and sensitive approach to demonstrate the concentration of special proteins on cell and other proteins and nucleotide potentially, revealing a simple and efficient technology for research fields of clinical diagnostics and molecular biology.

ATP triggers a robust intracellular [Ca2+ ]-mediated signalling pathway in human synovial fibroblasts

NEW FINDINGS

What is the central question of this study? What are the main [Ca²⁺ ]_i signalling pathways activated by ATP in human synovial fibroblasts? What is the main finding and its importance? In human synovial fibroblasts ATP acts through a linked G-protein (G_q ) and phospholipase C signalling mechanism to produce IP₃ , which then markedly enhances release of Ca²⁺ from the endoplasmic reticulum. These results provide new information for the detection of early pathophysiology of arthritis.

ABSTRACT

In human articular joints, synovial fibroblasts (HSFs) have essential physiological functions that include synthesis and secretion of components of the extracellular matrix and essential articular joint lubricants, as well as release of paracrine substances such as ATP. Although the molecular and cellular processes that lead to a rheumatoid arthritis (RA) phenotype are not fully understood, HSF cells exhibit significant changes during this disease progression. The effects of ATP on HSFs were studied by monitoring changes in intracellular Ca²⁺ ([Ca²⁺ ]_i ), and measuring electrophysiological properties. ATP application to HSF cell populations that had been enzymatically released from 2-D cell culture revealed that ATP (10-100 μm), or its analogues UTP or ADP, consistently produced a large transient increase in [Ca²⁺ ]_i . These changes (i) were initiated by activation of the P₂ Y purinergic receptor family, (ii) required G_q -mediated signal transduction, (iii) did not involve a transmembrane Ca²⁺ influx, but instead (iv) arose almost entirely from activation of endoplasmic reticulum (ER)-localized inositol 1,4,5-trisphosphate (IP₃ ) receptors that triggered Ca²⁺ release from the ER. Corresponding single cell electrophysiological studies revealed that these ATP effects (i) were insensitive to [Ca²⁺ ]_o removal, (ii) involved an IP₃ -mediated intracellular Ca²⁺ release process, and (iii) strongly turned on Ca²⁺ -activated K⁺ current(s) that significantly hyperpolarized these cells. Application of histamine produced very similar effects in these HSF cells. Since ATP is a known paracrine agonist and histamine is released early in the inflammatory response, these findings may contribute to identification of early steps/defects in the initiation and progression of RA.

Na+ entry through heteromeric TRPC4/C1 channels mediates (-)Englerin A-induced cytotoxicity in synovial sarcoma cells

The sesquiterpene (-)Englerin A (EA) is an organic compound from the plant Phyllanthus engleri which acts via heteromeric TRPC4/C1 channels to cause cytotoxicity in some types of cancer cell but not normal cells. Here we identified selective cytotoxicity of EA in human synovial sarcoma cells (SW982 cells) and investigated the mechanism. EA induced cation channel current (Icat) in SW982 cells with biophysical characteristics of heteromeric TRPC4/C1 channels. Inhibitors of homomeric TRPC4 channels were weak inhibitors of the Icat and EA-induced cytotoxicity whereas a potent inhibitor of TRPC4/C1 channels (Pico145) strongly inhibited Icat and cytotoxicity. Depletion of TRPC1 converted Icat into a current with biophysical and pharmacological properties of homomeric TRPC4 channels and depletion of TRPC1 or TRPC4 suppressed the cytotoxicity of EA. A Na⁺/K⁺-ATPase inhibitor (ouabain) potentiated EA-induced cytotoxicity and direct Na⁺ loading by gramicidin-A caused Pico145-resistant cytotoxicity in the absence of EA. We conclude that EA has a potent cytotoxic effect on human synovial sarcoma cells which is mediated by heteromeric TRPC4/C1 channels and Na⁺ loading.

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.

Hypotonic stress promotes ATP release, reactive oxygen species production and cell proliferation via TRPV4 activation in rheumatoid arthritis rat synovial fibroblasts

Rheumatoid arthritis (RA) is a chronic and systemic autoimmune-disease with complex and unclear etiology. Hypotonicity of synovial fluid is a typical characteristic of RA, which may play pivotal roles in RA pathogenesis. In this work, we studied the responses of RA synovial fibroblasts to hypotonic stress in vitro and further explored the underlying mechanisms. Data showed that hyposmotic solutions significantly triggered increases in cytosolic calcium concentration ([Ca²⁺]_c) of synoviocytes. Subsequently, it caused rapid release of ATP, as well as remarkable production of intracellular reactive oxygen species (ROS). Meanwhile, hypotonic stimulus promoted the proliferation of synovial fibroblasts. These effects were almost abolished by calcium-free buffer and significantly inhibited by gadolinium (III) chloride (a mechanosensitive Ca²⁺ channel blocker) and ruthenium red (a transient receptor potential vanilloid 4 (TRPV4) blocker). 4α-phorbol 12,13-didecanoate, a specific agonist of TRPV4, also mimicked hypotonic shock-induced responses shown above. In contrast, voltage-gated channel inhibitors verapamil and nifedipine had little influences on these responses. Furthermore, RT-PCR and western blotting evidently detected TRPV4 expression at mRNA and protein level in isolated synoviocytes. Taken together, our results indicated that hypotonic stimulus resulted in ATP release, ROS production, and cell proliferation depending on Ca²⁺ entry through activation of TRPV4 channel in synoviocytes.

Cellular electrophysiological principles that modulate secretion from synovial fibroblasts

Rheumatoid arthritis (RA) is a progressive disease that affects both pediatric and adult populations. The cellular basis for RA has been investigated extensively using animal models, human tissues and isolated cells in culture. However, many aspects of its aetiology and molecular mechanisms remain unknown. Some of the electrophysiological principles that regulate secretion of essential lubricants (hyaluronan and lubricin) and cytokines from synovial fibroblasts have been identified. Data sets describing the main types of ion channels that are expressed in human synovial fibroblast preparations have begun to provide important new insights into the interplay among: (i) ion fluxes, (ii) Ca²⁺ release from the endoplasmic reticulum, (iii) intercellular coupling, and (iv) both transient and longer duration changes in synovial fibroblast membrane potential. A combination of this information, knowledge of similar patterns of responses in cells that regulate the immune system, and the availability of adult human synovial fibroblasts are likely to provide new pathophysiological insights.

Cartilage-Specific Knockout of the Mechanosensory Ion Channel TRPV4 Decreases Age-Related Osteoarthritis

Osteoarthritis (OA) is a progressive degenerative disease of articular cartilage and surrounding tissues, and is associated with both advanced age and joint injury. Biomechanical factors play a critical role in the onset and progression of OA, yet the mechanisms through which physiologic or pathologic mechanical signals are transduced into a cellular response are not well understood. Defining the role of mechanosensory pathways in cartilage during OA pathogenesis may yield novel strategies or targets for the treatment of OA. The transient receptor potential vanilloid 4 (TRPV4) ion channel transduces mechanical loading of articular cartilage via the generation of intracellular calcium ion transients. Using tissue-specific, inducible Trpv4 gene-targeted mice, we demonstrate that loss of TRPV4-mediated cartilage mechanotransduction in adulthood reduces the severity of aging-associated OA. However, loss of chondrocyte TRPV4 did not prevent OA development following destabilization of the medial meniscus (DMM). These results highlight potentially distinct roles of TRPV4-mediated cartilage mechanotransduction in age-related and post-traumatic OA, and point to a novel disease-modifying strategy to therapeutically target the TRPV4-mediated mechanotransduction pathway for the treatment of aging-associated OA.

Understanding autoimmunity: The ion channel perspective

Ion channels are integral membrane proteins that orchestrate the passage of ions across the cell membrane and thus regulate various key physiological processes of the living system. The stringently regulated expression and function of these channels hold a pivotal role in the development and execution of various cellular functions. Malfunction of these channels results in debilitating diseases collectively termed channelopathies. In this review, we highlight the role of these proteins in the immune system with special emphasis on the development of autoimmunity. The role of ion channels in various autoimmune diseases is also listed out. This comprehensive review summarizes the ion channels that could be used as molecular targets in the development of new therapeutics against autoimmune disorders.

Calcium channels: the potential therapeutic targets for inflammatory bone destruction of rheumatoid arthritis

INTRODUCTION

Inflammatory bone resorption causes progressive joint destruction which ultimately leads to functional disability in rheumatoid arthritis (RA). The primary cell responsible for bone resorption is the osteoclast, which means it is a potential therapeutic target against bone destruction. In fact, experimental and clinical findings suggest that blockade of osteoclast differentiation and function is highly effective in inhibiting bone destruction in RA.

DISCUSSION AND CONCLUSION

In this report, we show several lines of experimental evidence which suggest that a variety of Ca(2+) channels are essential in osteoclast differentiation and function, and present a hypothesis that modulation of Ca(2+) channels is a highly effective therapeutic strategy in preventing osteoclast-induced structural damage in RA.

Ion channel expression and function in normal and osteoarthritic human synovial fluid progenitor cells

Osteoarthritis (OA) is a chronic disease affecting the cartilage of over 15% of Canadians. Synovial fluid mesenchymal progenitor cells (sfMPCs) are present in joints and are thought to contribute to healing. OA sfMPCs have a greater proliferative ability but decreased chondrogenic potential. However, little is known about the factors influencing/regulating the differences between normal and OA sfMPCs. Recently, our lab has shown that sfMPC chondrogenic differentiation in vitro is favorably biased toward a similar osmotic environment as they experience in vivo. The current study now examines the expression and functionality of a variety of ion channels in sfMPCs derived from normal individuals and early OA patients. Results indicated that there is differential ion channel regulation at the functional level and expression level in early OA sfMPCs. All ion channels were upregulated in early OA compared to normal sfMPCs with the exception of KCNMA1 at the mRNA level. At the protein level, TRPV4 was over expressed in early OA sfMPCs, while KCNJ12 and KCNMA1 were unchanged between normal and early OA sfMPCs. At the functional level, the inward rectifying potassium channel was under expressed in early OA sfMPCs, however the membrane potential was unchanged between normal and early OA sfMPCs. In the synovial environment itself, a number of differences in ion concentration between normal and early OA synovial fluid were observed. These findings suggest that normal and OA progenitor cells demonstrate functional differences in how they interact with the synovial ion environment.

Regulation of TRP channels by steroids: Implications in physiology and diseases

While effects of different steroids on the gene expression and regulation are well established, it is proven that steroids can also exert rapid non-genomic actions in several tissues and cells. In most cases, these non-genomic rapid effects of steroids are actually due to intracellular mobilization of Ca(2+)- and other ions suggesting that Ca(2+) channels are involved in such effects. Transient Receptor Potential (TRP) ion channels or TRPs are the largest group of non-selective and polymodal ion channels which cause Ca(2+)-influx in response to different physical and chemical stimuli. While non-genomic actions of different steroids on different ion channels have been established to some extent, involvement of TRPs in such functions is largely unexplored. In this review, we critically analyze the literature and summarize how different steroids as well as their metabolic precursors and derivatives can exert non-genomic effects by acting on different TRPs qualitatively and/or quantitatively. Such effects have physiological repercussion on systems such as in sperm cells, immune cells, bone cells, neuronal cells and many others. Different TRPs are also endogenously expressed in diverse steroid-producing tissues and thus may have importance in steroid synthesis as well, a process which is tightly controlled by the intracellular Ca(2+) concentrations. Tissue and cell-specific expression of TRP channels are also regulated by different steroids. Understanding of the crosstalk between TRP channels and different steroids may have strong significance in physiological, endocrinological and pharmacological context and in future these compounds can also be used as potential biomedicine.

TRPV4 as a therapeutic target for joint diseases

Biomechanical factors play a critical role in regulating the physiology as well as the pathology of multiple joint tissues and have been implicated in the pathogenesis of osteoarthritis. Therefore, the mechanisms by which cells sense and respond to mechanical signals may provide novel targets for the development of disease-modifying osteoarthritis drugs (DMOADs). Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+)-permeable cation channel that serves as a sensor of mechanical or osmotic signals in several musculoskeletal tissues, including cartilage, bone, and synovium. The importance of TRPV4 in joint homeostasis is apparent in patients harboring TRPV4 mutations, which result in the development of a spectrum of skeletal dysplasias and arthropathies. In addition, the genetic knockout of Trpv4 results in the development of osteoarthritis and decreased osteoclast function. In engineered cartilage replacements, chemical activation of TRPV4 can reproduce many of the anabolic effects of mechanical loading to accelerate tissue growth and regeneration. Overall, TRPV4 plays a key role in transducing mechanical, pain, and inflammatory signals within joint tissues and thus is an attractive therapeutic target to modulate the effects of joint diseases. In pathological conditions in the joint, when the delicate balance of TRPV4 activity is altered, a variety of different tools could be utilized to directly or indirectly target TRPV4 activity.

Trafficking of ThermoTRP Channels

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

The NADPH oxidase inhibitor diphenyleneiodonium activates the human TRPA1 nociceptor

Transient receptor potential ankyrin 1 (TRPA1) is a Ca(2+)-permeable nonselective cation channel expressed in neuronal and nonneuronal cells and plays an important role in acute and inflammatory pain. Here, we show that an NADPH oxidase (NOX) inhibitor, diphenyleneiodonium (DPI), functions as a TRPA1 activator in human embryonic kidney cells expressing human TRPA1 (HEK-TRPA1) and in human fibroblast-like synoviocytes. Application of DPI at 0.03-10 μM induced a Ca(2+) response in HEK-TRPA1 cells in a concentration-dependent manner. The Ca(2+) response was effectively blocked by a selective TRPA1 antagonist, HC-030031 (HC). In contrast, DPI had no effect on HEK cells expressing TRPV1-V4 or TRPM8. Four other NOX inhibitors, apocynin (APO), VAS2870 (VAS), plumbagin, and 2-acetylphenothiazine, also induced a Ca(2+) response in HEK-TRPA1 cells, which was inhibited by pretreatment with HC. In the presence of 5 mM glutathione, the Ca(2+) response to DPI was effectively reduced. Moreover, mutation of cysteine 621 in TRPA1 substantially inhibited the DPI-induced Ca(2+) response, while it did not inhibit the APO- and VAS-induced responses. The channel activity was induced by DPI in excised membrane patches with both outside-out and inside-out configurations. Internal application of neomycin significantly inhibited the DPI-induced inward currents. In inflammatory synoviocytes with TRPA1, DPI evoked a Ca(2+) response that was sensitive to HC. In mice, intraplantar injection of DPI caused a pain-related response which was inhibited by preadministration with HC. Taken together, our findings demonstrate that DPI and other NOX inhibitors activate human TRPA1 without mediating NOX.

Temporomandibular joint pain: a critical role for Trpv4 in the trigeminal ganglion

Temporomandibular joint disorder (TMJD) is known for its mastication-associated pain. TMJD is medically relevant because of its prevalence, severity, chronicity, the therapy-refractoriness of its pain, and its largely elusive pathogenesis. Against this background, we sought to investigate the pathogenetic contributions of the calcium-permeable TRPV4 ion channel, robustly expressed in the trigeminal ganglion sensory neurons, to TMJ inflammation and pain behavior. We demonstrate here that TRPV4 is critical for TMJ-inflammation-evoked pain behavior in mice and that trigeminal ganglion pronociceptive changes are TRPV4-dependent. As a quantitative metric, bite force was recorded as evidence of masticatory sensitization, in keeping with human translational studies. In Trpv4(-/-) mice with TMJ inflammation, attenuation of bite force was significantly less than in wildtype (WT) mice. Similar effects were seen with systemic application of a specific TRPV4 inhibitor. TMJ inflammation and mandibular bony changes were apparent after injections of complete Freund adjuvant but were remarkably independent of the Trpv4 genotype. It was intriguing that, as a result of TMJ inflammation, WT mice exhibited significant upregulation of TRPV4 and phosphorylated extracellular-signal-regulated kinase (ERK) in TMJ-innervating trigeminal sensory neurons, which were absent in Trpv4(-/-) mice. Mice with genetically-impaired MEK/ERK phosphorylation in neurons showed resistance to reduction of bite force similar to that of Trpv4(-/-) mice. Thus, TRPV4 is necessary for masticatory sensitization in TMJ inflammation and probably functions upstream of MEK/ERK phosphorylation in trigeminal ganglion sensory neurons in vivo. TRPV4 therefore represents a novel pronociceptive target in TMJ inflammation and should be considered a target of interest in human TMJD.

Stimulation of human TRPA1 channels by clinical concentrations of the antirheumatic drug auranofin

Gold compounds, which were widely used to treat rheumatoid arthritis, have been recently used as experimental agents for tumor treatment. Transient receptor potential (TRP) ankyrin repeat 1 (TRPA1) is a Ca2+-permeable ion channel that senses acute and inflammatory pain signals. Electrophilic compounds such as mustard oil and cinnamaldehyde activate TRPA1 by interacting with TRPA1 cysteine residues. Here we investigate the effects of the gold compound auranofin (AUR) on TRPA1 channels. Intracellular Ca2+ and whole cell patch-clamp recordings were performed on human embryonic kidney cells transiently expressed with TRPA1, TRP melastatin 8 (TRPM8), and vanilloid type TRP (TRPV1–4) channels. AUR stimulated TRPA1 in a concentration-dependent manner with a half-maximum potency of around 1.0 μM. The AUR-induced response was effectively blocked by HC030031, a TRPA1 antagonist. On the other hand, AUR failed to activate TRPM8 and TRPV1–4 channels, which are highly expressed in sensory neurons as nociceptors. The stimulatory effect on TRPA1 channels depended on the C414, C421, C621, and C633 cysteine residues and not on the inhibition of thioredoxin reductase by AUR. Moreover, AUR effectively activated TRPA1 channels expressed in human differentiated neuroblastoma cell lines. The study shows that AUR is a potent stimulator of TRPA1 channels.

TRPV4 partially participates in proliferation of human brain capillary endothelial cells

AIMS

The vanilloid type 4 transient receptor potential channel (TRPV4) is a potential environmental sensor to multiple stimuli in many types of cells. In this study, we show that TRPV4 activated by 4α-phorbol 12,13-didecanoate (4αPDD) and hypo-osmotic stimulation (HOS) is a regulator of intracellular calcium ([Ca(2+)](i)) in human brain capillary endothelial cells (HBCEs), and its activation can partially regulate cell proliferation of HBCEs.

MAIN METHODS

The expression of TRPV4 in HBCEs was analyzed at the mRNA and protein levels. The function of TRPV4 in HBCEs was evaluated using a TRPV4 agonist, 4αPDD, and HOS while measuring [Ca(2+)](i) and membrane currents.

KEY FINDINGS

Analysis of the mRNA transcripts of the TRPV subfamily revealed that TRPV2 and TRPV4 were expressed in HBCEs. Immunoreactivity to the TRPV4 protein was also detected in HBCEs, which were positively stained by von Willebrand factor and CD31. When 4αPDD was applied, [Ca(2+)](i) in HBCEs was elevated in a concentration-dependent manner. In addition, exposure of HBCEs to HOS at 228mOsm induced an elevation of [Ca(2+)](i). Application of 4αPDD also activated non-selective cation currents (NSCCs). Pretreatment of HBCEs with short interference RNA targeting TRPV4 (siRNA) significantly reduced the 4αPDD-induced elevation of [Ca(2+)](i). When HBCEs were treated for 24h with concentrations of 4αPDD between 0.3 and 3 μM, the cell proliferation was potentiated in a concentration-dependent manner. The potentiation was partially inhibited in HBCEs treated with siRNA.

SIGNIFICANCE

These data suggest that endogenous TRPV4, which functions as a regulator of [Ca(2+)](i) in HBCEs, partially controls cell proliferation.

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.

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

The esophageal epithelium has sensory properties that enable it to sustain normal barrier function. Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+)-permeable channel that is activated by extracellular hypotonicity, polyunsaturated fatty acids, phorbol esters, and elevated temperature. We found that TRPV4 is expressed in both human esophageal tissue and in HET-1A cells, a human esophageal epithelial cell line. Specific activation of TRPV4 by the phorbol ester 4α-phorbol 12,13-didecanoate (4α-PDD) increased intracellular Ca(2+) in a subset of HET-1A cells. Elevated temperature strongly potentiated this effect at low concentrations of 4α-PDD, and all of the responses were inhibited by the TRPV antagonist ruthenium red. TRPV4 activation differentially affected cell proliferation and cell viability; HET-1A cell proliferation was increased by 1 μM 4α-PDD, whereas higher concentrations (10 μM and 30 μM) significantly decreased cell viability. Transient TRPV4 activation triggered ATP release in a concentration-dependent manner via gap-junction hemichannels, including pannexin 1 and connexin 43. Furthermore, TRPV4 activation for 24 h did not increase the production of interleukin 8 (IL-8) but reduced IL-1β-induced IL-8 production. Small-interference RNA targeted to TRPV4 significantly attenuated all of the 4α-PDD-induced responses in HET-1A cells. Collectively, these findings suggest that TRPV4 is a novel regulator of Ca(2+)-dependent signaling pathways linked to cell proliferation, cell survival, ATP release, and IL-8 production in human esophageal epithelial cells.

TRPM3 channel stimulated by pregnenolone sulphate in synovial fibroblasts and negatively coupled to hyaluronan

Background

Calcium-permeable channels are known to have roles in many mammalian cell types but the expression and contribution of such ion channels in synovial cells is mostly unknown. The objective of this study was to investigate the potential relevance of Transient Receptor Potential Melastatin 3 (TRPM3) channel to fibroblast-like synoviocytes (FLSs) of patients with rheumatoid arthritis.

Methods

The study used RT-PCR and immunofluorescence to detect mRNA and protein. Intracellular calcium measurement detected channel activity in a FLS cell-line and primary cultures of FLSs from patients with rheumatoid arthritis. Enzyme-linked immunosorbent assays measured hyaluronan.

Results

Endogenous expression of TRPM3 was detected. Previously reported stimulators of TRPM3 sphingosine and pregnenolone sulphate evoked sustained elevation of intracellular calcium in FLSs. The FLS cell-line showed an initial transient response to sphingosine which may be explained by TRPV4 channels but was not observed in FLSs from patients. Blocking antibody targeted to TRPM3 inhibited sustained sphingosine and pregnenolone sulphate responses. Secretion of hyaluronan, which contributes adversely in rheumatoid arthritis, was suppressed by pregnenolone sulphate in FLSs from patients and the effect was blocked by anti-TRPM3 antibody.

Conclusions

The data suggest that FLSs of patients with rheumatoid arthritis express TRPM3-containing ion channels that couple negatively to hyaluronan secretion and can be stimulated by pharmacological concentrations of pregnenolone sulphate.

Transient receptor potential vanilloid 4: The sixth sense of the musculoskeletal system?

The critical discovery in the past two decades of the transient receptor potential (TRP) superfamily of ion channels has revealed the potential mechanisms by which cells sense diverse stimuli beyond the prototypical "five senses," identifying ion channels that are gated by heat, cold, mechanical loading, osmolarity, and other physical and chemical stimuli. TRP vanilloid 4 (TRPV4) is a Ca(2+)-permeable nonselective cation channel that appears to play a mechanosensory or osmosensory role in several musculoskeletal tissues. In articular cartilage, TRPV4 exhibits osmotic sensitivity, controlling cellular volume recovery, and other physiologic responses to osmotic stress. TRPV4 is expressed in both osteoblasts and osteoclasts, and the absence of TRPV4 prevents disuse-induced bone loss. TRPV4 activation promotes chondrogenesis by inducing SOX9 transcription, whereas a TRPV4 gain-of-function mutation leads to a developmental skeletal dysplasia, suggesting a critical role for TRPV4 in skeletal development. These studies provide mounting evidence for a regulatory role for the sensory channel TRPV4 in control of musculoskeletal tissues.