TRPV4 plays an evolutionary conserved role in the transduction of osmotic and mechanical stimuli in live animals

Roles for TRPV4 in disease: A discussion of possible mechanisms

The transient receptor potential vanilloid 4 (TRPV4) ion channel is a ubiquitously expressed Ca2+-permeable ion channel that controls intracellular calcium ([Ca2+]i) homeostasis in various types of cells. The physiological roles for TRPV4 are tissue specific and the mechanisms behind this specificity remain mostly unclarified. It is noteworthy that mutations in the TRPV4 channel have been associated to a broad spectrum of congenital diseases, with most of these mutations mainly resulting in gain-of-function. Mutations have been identified in human patients showing a variety of phenotypes and symptoms, mostly related to skeletal and neuromuscular disorders. Since TRPV4 is so widely expressed throughout the body, it comes as no surprise that the literature is growing in evidence linking this protein to malfunction in systems other than the skeletal and neuromuscular. In this review, we summarize the expression patterns of TRPV4 in several tissues and highlight findings of recent studies that address critical structural and functional features of this channel, particularly focusing on its interactions and signaling pathways related to Ca2+ entry. Moreover, we discuss the roles of TRPV4 mutations in some diseases and pinpoint some of the mechanisms underlying pathological states where TRPV4's malfunction is prominent.

Recent advances on the structure and the function relationships of the TRPV4 ion channel

The members of the superfamily of Transient Receptor Potential (TRP) ion channels are physiologically important molecules that have been studied for many years and are still being intensively researched. Among the vanilloid TRP subfamily, the TRPV4 ion channel is an interesting protein due to its involvement in several essential physiological processes and in the development of various diseases. As in other proteins, changes in its function that lead to the development of pathological states, have been closely associated with modification of its regulation by different molecules, but also by the appearance of mutations which affect the structure and gating of the channel. In the last few years, some structures for the TRPV4 channel have been solved. Due to the importance of this protein in physiology, here we discuss the recent progress in determining the structure of the TRPV4 channel, which has been achieved in three species of animals (Xenopus tropicalis, Mus musculus, and Homo sapiens), highlighting conserved features as well as key differences among them and emphasizing the binding sites for some ligands that play crucial roles in its regulation.

Investigation of local stimulation effects of embedding PGLA at Zusanli (ST36) acupoint in rats based on TRPV2 and TRPV4 ion channels

Introduction

Acupoint Catgut Embedding (ACE) is an extended and developed form of traditional acupuncture that serves as a composite stimulation therapy for various diseases. However, its local stimulation effects on acupoints remain unclear. Acupuncture can activate mechanically sensitive calcium ion channels, TRPV2 and TRPV4, located on various cell membranes, promoting Ca2+ influx in acupoint tissues to exert effects. Whether ACE can form mechanical physical stimulation to regulate these channels and the related linkage effect requires validation.

Methods

This study investigates the influence of TRPV2 and TRPV4 ion channels on the local stimulation effects of ACE by embedding PGLA suture at the Zusanli (ST36) acupoint in rats and using TRPV2 and TRPV4 inhibitors. Flow cytometry, immunofluorescence, Western blot, and Real-time quantitative PCR were employed to detect intracellular Ca2+ fluorescence intensity, the expression of macrophage (Mac) CD68 and mast cell (MC) tryptase, as well as the protein and mRNA expression of TRPV2 and TRPV4 in acupoint tissues after PGLA embedding.

Results

The results indicate that ACE using PGLA suture significantly increases the mRNA and protein expression of TRPV2 and TRPV4, Ca2+ fluorescence intensity, and the expression of Mac CD68 and MC tryptase in acupoint tissues, with these effects diminishing over time. The increasing trends are reduced after using inhibitors, particularly when both inhibitors are used simultaneously. Furthermore, correlation analysis shows that embedding PGLA suture at the ST36 acupoint regulates Mac and MC functions through Ca2+ signaling involving not only TRPV2 and TRPV4 but multiple pathways.

Discussion

These results suggest that embedding PGLA suture at the ST36 acupoint generates mechanical physical stimulation and regulates TRPV2 and TRPV4 ion channels, which couple with Ca2+ signaling to form a linkage effect that gradually weakens over time. This provides new reference data for further studies on the stimulation effects and clinical promotion of ACE.

Control of fluid intake in dehydrated rats and evolution of sodium appetite

The objective of the present work is to examine from a new perspective the existence of causal factors not predicted by the classical theory that thirst and sodium appetite are two distinct motivations. For example, we ask why water deprivation induces sodium appetite, thirst is not "water appetite", and intracellular dehydration potentially causes sodium appetite. Contrary to the classical theory, we suggest that thirst first, and sodium appetite second, designate a temporal sequence underlying the same motivation. The single motivation becomes an "intervenient variable" a concept borrowed from the literature, fully explained in the text, between causes of dehydration (extracellular, intracellular, or both together), and respective behavioral responses subserved by hindbrain-dependent inhibition (e.g., lateral parabrachial nucleus) and forebrain facilitation (e.g., angiotensin II). A corollary is homology between rat sodium appetite and marine teleost thirst-like motivation that we name "protodipsia". The homology argument rests on similarities between behavior (salty water intake) and respective neuroanatomical as well as functional mechanisms. Tetrapod origin in a marine environment provides additional support for the homology. The single motivation hypothesis is also consistent with ingestive behaviors in nature given similarities (e.g., thirst producing brackish water intake) between the behavior of the laboratory rat and wild animals, rodents included. The hypotheses of single motivation and homology might explain why hyperosmotic rats, or eventually any other hyperosmotic tetrapod, shows paradoxical signs of sodium appetite. They might also explain how ingestive behaviors determined by dehydration and subserved by hindbrain inhibitory mechanisms contributed to tetrapod transition from sea to land.

Engineering magnetic nanosystem for TRPV1 and TRPV4 channel activation

Recently, physical tools for remotely stimulating mechanical force-sensitive and temperature-sensitive proteins to regulate intracellular pathways have opened up novel and exciting avenues for basic research and clinical applications. Among the numerous modes of physical stimulation, magnetic stimulation is significantly attractive for biological applications due to the advantages of depth penetration and spatial-temporally controlled transduction. Herein, the physicochemical parameters (e.g., shape, size, composition) that influence the magnetic properties of magnetic nanosystems as well as the characteristics of transient receptor potential vanilloid-1 (TRPV1) and transient receptor potential vanilloid-4 (TRPV4) channels are systematically summarized, which offer opportunities for magnetic manipulation of cell fate in a precise and effective manner. In addition, representative regulatory applications involving magnetic nanosystem-based TRPV1 and TRPV4 channel activation are highlighted, both at the cellular level and in animal models. Furthermore, perspectives on the further development of this magnetic stimulation mode are commented on, with emphasis on scientific limitations and possible directions for exploitation. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

The Multifaceted Functions of TRPV4 and Calcium Oscillations in Tissue Repair

The transient receptor potential vanilloid 4 (TRPV4) specifically functions as a mechanosensitive ion channel and is responsible for conveying changes in physical stimuli such as mechanical stress, osmotic pressure, and temperature. TRPV4 enables the entry of cation ions, particularly calcium ions, into the cell. Activation of TRPV4 channels initiates calcium oscillations, which trigger intracellular signaling pathways involved in a plethora of cellular processes, including tissue repair. Widely expressed throughout the body, TRPV4 can be activated by a wide array of physicochemical stimuli, thus contributing to sensory and physiological functions in multiple organs. This review focuses on how TRPV4 senses environmental cues and thereby initiates and maintains calcium oscillations, critical for responses to organ injury, tissue repair, and fibrosis. We provide a summary of TRPV4-induced calcium oscillations in distinct organ systems, along with the upstream and downstream signaling pathways involved. In addition, we delineate current animal and disease models supporting TRPV4 research and shed light on potential therapeutic targets for modulating TRPV4-induced calcium oscillation to promote tissue repair while reducing tissue fibrosis.

Targeting Transient Receptor Potential (TRP) Channels, Mas-Related G-Protein-Coupled Receptors (Mrgprs), and Protease-Activated Receptors (PARs) to Relieve Itch

Itch (pruritus) is a sensation in the skin that provokes the desire to scratch. The sensation of itch is mediated through a subclass of primary afferent sensory neurons, termed pruriceptors, which express molecular receptors that are activated by itch-evoking ligands. Also expressed in pruriceptors are several types of Transient Receptor Potential (TRP) channels. TRP channels are a diverse class of cation channels that are responsive to various somatosensory stimuli like touch, pain, itch, and temperature. In pruriceptors, TRP channels can be activated through intracellular signaling cascades initiated by pruritogen receptors and underly neuronal activation. In this review, we discuss the role of TRP channels TRPA1, TRPV1, TRPV2, TRPV3, TRPV4, TRPM8, and TRPC3/4 in acute and chronic pruritus. Since these channels often mediate itch in association with pruritogen receptors, we also discuss Mas-related G-protein-coupled receptors (Mrgprs) and protease-activated receptors (PARs). Additionally, we cover the exciting therapeutic targets amongst the TRP family, as well as Mrgprs and PARs for the treatment of pruritus.

Transient Receptor Potential Vanilloid 4-Dependent Microglial Function in Myelin Injury and Repair

Microglia are found pathologically at all stages of multiple sclerosis (MS) lesion development and are hypothesized to contribute to both inflammatory injury and neuroprotection in the MS brain. Transient receptor potential vanilloid 4 (TRPV4) channels are widely expressed, play an important role as environmental sensors, and are involved in calcium homeostasis for a variety of cells. TRPV4 modulates myeloid cell phagocytosis in the periphery and microglial motility in the central nervous system. We hypothesized that TRPV4 deletion would alter microglia phagocytosis in vitro and lessen disease activity and demyelination in experimental autoimmune encephalitis (EAE) and cuprizone-induced demyelination. We found that genetic deletion of TRPV4 led to increased microglial phagocytosis in vitro but did not alter the degree of demyelination or remyelination in the cuprizone mouse model of MS. We also found no difference in disease in EAE following global or microglia-specific deletion of Trpv4. Additionally, lesioned and normal appearing white matter from MS brains exhibited similar TRPV4 expression compared to healthy brain tissue. Taken together, these findings indicate that TRPV4 modulates microglial activity but does not impact disease activity in mouse models of MS, suggesting a muted and/or redundant role in MS pathogenesis.

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

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

Hydrophobic gating in bundle-crossing ion channels: a case study of TRPV4

Transmembrane ion channels frequently regulate ion permeation by forming bundle crossing of the pore-lining helices when deactivated. The resulting physical constriction is believed to serve as the de facto gate that imposes the major free energy barrier to ion permeation. Intriguingly, many ion channels also contain highly hydrophobic inner pores enclosed by bundle crossing, which can undergo spontaneous dewetting and give rise to a "vapor barrier" to block ion flow even in the absence of physical constriction. Using atomistic simulations, we show that hydrophobic gating and bundle-crossing mechanisms co-exist and complement one and another in the human TRPV4 channel. In particular, a single hydrophilic mutation in the lower pore can increase pore hydration and reduce the ion permeation free energy barrier by about half without affecting the bundle crossing. We believe that hydrophobic gating may play a key role in other bundle-crossing ion channels with hydrophobic inner pores.

Ion channels in cancer-induced bone pain: from molecular mechanisms to clinical applications

Cancer-induced bone pain (CIBP) caused by bone metastasis is one of the most prevalent diseases, and current treatments rely primarily on opioids, which have significant side effects. However, recent developments in pharmaceutical science have identified several new mechanisms for CIBP, including the targeted modification of certain ion channels and receptors. Ion channels are transmembrane proteins, which are situated on biological cell membranes, which facilitate passive transport of inorganic ions across membranes. They are involved in various physiological processes, including transmission of pain signals in the nervous system. In recent years, there has been an increasing interest in the role of ion channels in chronic pain, including CIBP. Therefore, in this review, we summarize the current literature on ion channels, related receptors, and drugs and explore the mechanism of CIBP. Targeting ion channels and regulating their activity might be key to treating pain associated with bone cancer and offer new treatment avenues.

TET1-TRPV4 Signaling Contributes to Bone Cancer Pain in Rats

Bone cancer pain (BCP) is excruciating for cancer patients, with limited clinical treatment options and significant side effects, due to the complex and unclear pathogenesis of bone cancer pain. Peripheral sensitization in dorsal root ganglion (DRG) neurons is a recognized cellular mechanism for bone cancer pain. The pathological mechanism of chronic pain is increasingly being affected by epigenetic mechanisms. In this study, we unbiasedly showed that the DNA hydroxymethylase ten-eleven translocation 1 (TET1) expression was significantly increased in the L4-6 DRG of BCP rats and ten-eleven translocation 2 (TET2) expression did not change significantly. Notably, TET1 inhibition by intrathecal injection of Bobcat339 (a TET1 inhibitor) effectively relieved mechanical hyperalgesia in BCP rats. Peripheral sensitization in chronic pain relies on the activation and overexpression of ion channels on neurons. Here, we demonstrated that TRPV4, one of the transient receptor potential ion channel family members, was significantly elevated in the L4-6 DRG of BCP rats. In addition, TRPV4 inhibition by intrathecal injection of HC067047 (a TRPV4 inhibitor) also significantly attenuated mechanical hyperalgesia in BCP rats. Interestingly, we found that TET1 inhibition downregulated TRPV4 expression in the L4-6 DRG of BCP rats. As a result, these findings suggested that TET1 may contribute to bone cancer pain by upregulating TRPV4 expression in the L4-6 DRG of BCP rats and that TET1 or TRPV4 may become therapeutic targets for bone cancer pain.

Blocking TRPV4 Ameliorates Osteoarthritis by Inhibiting M1 Macrophage Polarization via the ROS/NLRP3 Signaling Pathway

Osteoarthritis (OA) is a low-level inflammatory disease in which synovial macrophage M1 polarization exacerbates the progression of synovitis and OA. Notedly, the ROS (reactive oxygen species) level in macrophages is intimately implicated in macrophage M1 polarization. TRPV4 (transient receptor potential channel subfamily V member 4), as an ion channel, plays a pivotal role in oxidative stress and inflammation. In this study, we investigated the role of TRPV4 in OA progression and M1 macrophage polarization. Male adult Sprague-Dawley (SD) rats underwent a medial meniscus radial transection operation to create an OA model in vivo and RAW 264.7 cells were intervened with 100 ng/mL LPS (lipopolysaccharide) to induce M1-polarized macrophages in vitro. We demonstrated that the infiltration of M1 synovial macrophages and the expression of TRPV4 were increased significantly in OA synovium. In addition, intra-articular injection of HC067074 (a specific inhibitor of TRPV4) alleviated the progression of rat OA and significantly decreased synovial macrophage M1 polarization. Further mechanisms suggested that ROS production by M1 macrophages was decreased after TRPV4 inhibition. In addition, NLRP3 (pyrin domain containing protein 3) as a downstream effector of ROS in M1-polarized macrophage, was significantly suppressed following TRPV4 inhibition. In conclusion, this study discovered that inhibition of TRPV4 delays OA progression by inhibiting M1 synovial macrophage polarization through the ROS/NLRP3 pathway.

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.

TRPV4 and PIEZO Channels Mediate the Mechanosensing of Chondrocytes to the Biomechanical Microenvironment

Articular cartilage and their chondrocytes are physiologically submitted to diverse types of mechanical cues. Chondrocytes produce and maintain the cartilage by sensing and responding to changing mechanical loads. TRPV4 and PIEZOs, activated by mechanical cues, are important mechanosensing molecules of chondrocytes and have pivotal roles in articular cartilage during health and disease. The objective of this review is to introduce the recent progress indicating that the mechanosensitive ion channels, TRPV4 and PIEZOs, are involved in the chondrocyte sensing of mechanical and inflammatory cues. We present a focus on the important role of TRPV4 and PIEZOs in the mechanotransduction regulating diverse chondrocyte functions in the biomechanical microenvironment. The review synthesizes the most recent advances in our understanding of how mechanical stimuli affect various cellular behaviors and functions through differentially activating TRPV4 and PIEZO ion channels in chondrocyte. Advances in understanding the complex roles of TRPV4/PIEZO-mediated mechanosignaling mechanisms have the potential to recapitulate physiological biomechanical microenvironments and design cell-instructive biomaterials for cartilage tissue engineering.

Mechanogenetics: harnessing mechanobiology for cellular engineering

'Mechanogenetics,' a new field at the convergence of mechanobiology and synthetic biology, presents an innovative strategy to treat, repair, or restore diseased cells and tissues by harnessing mechanical signal transduction pathways to control gene expression. While the role of mechanical forces in regulating development, homeostasis, and disease is well established, only recently have we identified the specific mechanosensors and downstream signaling pathways involved in these processes. Simultaneously, synthetic biological systems are developing increasingly sophisticated approaches of controlling mammalian cellular responses. Continued mechanistic refinement and identification of how cellular mechanosensors respond to homeostatic and pathological mechanical forces, combined with synthetic tools to integrate and respond to these inputs, promises to extend the development of new therapeutic approaches for treating disease.

Mechanosensory and mechanotransductive processes mediated by ion channels in articular chondrocytes: Potential therapeutic targets for osteoarthritis

Articular cartilage consists of an extracellular matrix including many proteins as well as embedded chondrocytes. Articular cartilage formation and function are influenced by mechanical forces. Hind limb unloading or simulated microgravity causes articular cartilage loss, suggesting the importance of the healthy mechanical environment in articular cartilage homeostasis and implying a significant role of appropriate mechanical stimulation in articular cartilage degeneration. Mechanosensitive ion channels participate in regulating the metabolism of articular chondrocytes, including matrix protein production and extracellular matrix synthesis. Mechanical stimuli, including fluid shear stress, stretch, compression and cell swelling and decreased mechanical conditions (such as simulated microgravity) can alter the membrane potential and regulate the metabolism of articular chondrocytes via transmembrane ion channel-induced ionic fluxes. This process includes Ca2+ influx and the resulting mobilization of Ca2+ that is due to massive released Ca2+ from stores, intracellular cation efflux and extracellular cation influx. This review brings together published information on mechanosensitive ion channels, such as stretch-activated channels (SACs), voltage-gated Ca2+ channels (VGCCs), large conductance Ca2+-activated K+ channels (BKCa channels), Ca2+-activated K+ channels (SKCa channels), voltage-activated H+ channels (VAHCs), acid sensing ion channels (ASICs), transient receptor potential (TRP) family channels, and piezo1/2 channels. Data based on epithelial sodium channels (ENaCs), purinergic receptors and N-methyl-d-aspartate (NMDA) receptors are also included. These channels mediate mechanoelectrical physiological processes essential for converting physical force signals into biological signals. The primary channel-mediated effects and signaling pathways regulated by these mechanosensitive ion channels can influence the progression of osteoarthritis during the mechanosensory and mechanoadaptive process of articular chondrocytes.

TRPing on Cell Swelling - TRPV4 Senses It

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

Reprint of: Mechanosensitive ion channels in cell migration

Cellular processes are initiated and regulated by different stimuli, including mechanical forces. Cell membrane mechanosensors represent the first step towards the conversion of mechanical stimuli to a biochemical or electrical response. Mechanosensitive (MS) ion channels form a growing family of ion gating channels that respond to direct physical force or plasma membrane deformations. A number of calcium (Ca²⁺) permeable MS channels are known to regulate the initiation, direction, and persistence of cell migration during development and tumour progression. While the evidence that links individual MS ion channels to cell migration is growing, a unified analysis of the molecular mechanisms regulated downstream of MS ion channel activation is lacking. In this review, we describe the MS ion channel families known to regulate cell migration. We discuss the molecular mechanisms that act downstream of MS ion channels with an emphasis on Ca²⁺ mediated processes. Finally, we propose the future directions and impact of MS ion channel activity in the field of cell migration.

Mechanosensitive ion channels in cell migration

Cellular processes are initiated and regulated by different stimuli, including mechanical forces. Cell membrane mechanosensors represent the first step towards the conversion of mechanical stimuli to a biochemical or electrical response. Mechanosensitive (MS) ion channels form a growing family of ion gating channels that respond to direct physical force or plasma membrane deformations. A number of calcium (Ca2+) permeable MS channels are known to regulate the initiation, direction, and persistence of cell migration during development and tumour progression. While the evidence that links individual MS ion channels to cell migration is growing, a unified analysis of the molecular mechanisms regulated downstream of MS ion channel activation is lacking. In this review, we describe the MS ion channel families known to regulate cell migration. We discuss the molecular mechanisms that act downstream of MS ion channels with an emphasis on Ca2+ mediated processes. Finally, we propose the future directions and impact of MS ion channel activity in the field of cell migration.

Endothelial TRPV4 channels prevent tumor growth and metastasis via modulation of tumor angiogenesis and vascular integrity

Transient receptor potential vanilloid 4 (TRPV4) is a ubiquitously expressed polymodally activated ion channel. TRPV4 has been implicated in tumor progression; however, the cell-specific role of TRPV4 in tumor growth, angiogenesis, and metastasis is unknown. Here, we generated endothelial-specific TRPV4 knockout (TRPV4^ECKO) mice by crossing TRPV4^lox/lox mice with Tie2-Cre mice. Tumor growth and metastasis were significantly increased in a syngeneic Lewis lung carcinoma tumor model of TRPV4^ECKO mice compared to TRPV4^lox/lox mice. Multiphoton microscopy, dextran leakage, and immunohistochemical analysis revealed increased tumor angiogenesis and metastasis that were correlated with aberrant leaky vessels (increased width and reduced pericyte and VE-cadherin coverage). Mechanistically, increases in VEGFR2, p-ERK, and MMP-9 expression and DQ gelatinase activity were observed in the TRPV4^ECKO mouse tumors. Our results demonstrated that endothelial TRPV4 is a critical modulator of vascular integrity and tumor angiogenesis and that deletion of TRPV4 promotes tumor angiogenesis, growth, and metastasis.

Transient receptor potential vanilloid 4 channel participates in mouse ventricular electrical activity

The TRPV4 channel is a calcium-permeable channel (PCa/PNa ∼ 10). Its expression has been reported in ventricular myocytes, where it is involved in several cardiac pathological mechanisms. In this study, we investigated the implication of TRPV4 in ventricular electrical activity. Left ventricular myocytes were isolated from trpv4+/+ and trpv4-/- mice. TRPV4 membrane expression and its colocalization with L-type calcium channels (Cav1.2) was confirmed using Western blot biotinylation, immunoprecipitation, and immunostaining experiments. Then, electrocardiograms (ECGs) and patch-clamp recordings showed shortened QTc and action potential (AP) duration in trpv4-/- compared with trpv4+/+ mice. Thus, TRPV4 activator GSK1016790A produced a transient and dose-dependent increase in AP duration at 90% of repolarization (APD90) in trpv4+/+ but not in trpv4-/- myocytes or when combined with TRPV4 inhibitor GSK2193874 (100 nM). Hence, GSK1016790A increased calcium transient (CaT) amplitude in trpv4+/+ but not in trpv4-/- myocytes, suggesting that TRPV4 carries an inward Ca2+ current in myocytes. Conversely, TRPV4 inhibitor GSK2193874 (100 nM) alone reduced APD90 in trpv4+/+ but not in trpv4-/- myocytes, suggesting that TRPV4 prolongs AP duration in basal condition. Finally, introducing TRPV4 parameters in a mathematical model predicted the development of an inward TRPV4 current during repolarization that increases AP duration and CaT amplitude, in accord with what was found experimentally. This study shows for the first time that TRPV4 modulates AP and QTc durations. It would be interesting to evaluate whether TRPV4 could be involved in long QT-mediated ventricular arrhythmias.NEW & NOTEWORTHY Transient receptor potential vanilloid 4 (TRPV4) is expressed at the membrane of mouse ventricular myocytes and colocalizes with non-T-tubular L-type calcium channels. Deletion of trpv4 gene in mice results in shortened QT interval on electrocardiogram and reduced action potential duration of ventricular myocytes. Pharmacological activation of TRPV4 channel leads to increased action potential duration and increased calcium transient amplitude in trpv4-/- but not in trpv4-/- ventricular myocytes. To the contrary, TRPV4 channel pharmacological inhibition reduces action potential duration in trpv4+/+ but not in trpv4-/- myocytes. Integration of TRPV4 channel in a computational model of mouse action potential shows that the channel carries an inward current contributing to slowing down action potential repolarization and to increase calcium transient amplitude, similarly to what is observed experimentally. This study highlights for the first time the involvement of TRPV4 channel in ventricular electrical activity.

Dental Pulp-Derived Mesenchymal Stem Cells for Modeling Genetic Disorders

A subpopulation of mesenchymal stem cells, developmentally derived from multipotent neural crest cells that form multiple facial tissues, resides within the dental pulp of human teeth. These stem cells show high proliferative capacity in vitro and are multipotent, including adipogenic, myogenic, osteogenic, chondrogenic, and neurogenic potential. Teeth containing viable cells are harvested via minimally invasive procedures, based on various clinical diagnoses, but then usually discarded as medical waste, indicating the relatively low ethical considerations to reuse these cells for medical applications. Previous studies have demonstrated that stem cells derived from healthy subjects are an excellent source for cell-based medicine, tissue regeneration, and bioengineering. Furthermore, stem cells donated by patients affected by genetic disorders can serve as in vitro models of disease-specific genetic variants, indicating additional applications of these stem cells with high plasticity. This review discusses the benefits, limitations, and perspectives of patient-derived dental pulp stem cells as alternatives that may complement other excellent, yet incomplete stem cell models, such as induced pluripotent stem cells, together with our recent data.

Modulating the Mechanical Activation of TRPV4 at the Cell-Substrate Interface

Ion channels activated by mechanical inputs are important force sensing molecules in a wide array of mammalian cells and tissues. The transient receptor potential channel, TRPV4, is a polymodal, nonselective cation channel that can be activated by mechanical inputs but only if stimuli are applied directly at the interface between cells and their substrate, making this molecule a context-dependent force sensor. However, it remains unclear how TRPV4 is activated by mechanical inputs at the cell-substrate interface, which cell intrinsic and cell extrinsic parameters might modulate the mechanical activation of the channel and how mechanical activation differs from TRPV4 gating in response to other stimuli. Here we investigated the impact of substrate mechanics and cytoskeletal components on mechanically evoked TRPV4 currents and addressed how point mutations associated with TRPV4 phosphorylation and arthropathy influence mechanical activation of the channel. Our findings reveal distinct regulatory modulation of TRPV4 from the mechanically activated ion channel PIEZO1, suggesting the mechanosensitivity of these two channels is tuned in response to different parameters. Moreover, our data demonstrate that the effect of point mutations in TRPV4 on channel activation are profoundly dependent on the gating stimulus.

Implications of Transient Receptor Potential Cation Channels in Migraine Pathophysiology

Migraine is a common and debilitating headache disorder. Although its pathogenesis remains elusive, abnormal trigeminal and central nervous system activity is likely to play an important role. Transient receptor potential (TRP) channels, which transduce noxious stimuli into pain signals, are expressed in trigeminal ganglion neurons and brain regions closely associated with the pathophysiology of migraine. In the trigeminal ganglion, TRP channels co-localize with calcitonin gene-related peptide, a neuropeptide crucially implicated in migraine pathophysiology. Many preclinical and clinical data support the roles of TRP channels in migraine. In particular, activation of TRP cation channel V1 has been shown to regulate calcitonin gene-related peptide release from trigeminal nerves. Intriguingly, several effective anti-migraine therapies, including botulinum neurotoxin type A, affect the functions of TRP cation channels. Here, we discuss currently available data regarding the roles of major TRP cation channels in the pathophysiology of migraine and the therapeutic applicability thereof.

The role of TRPV4 channels in ocular function and pathologies

Transient potential receptor vanilloid 4 (TRPV4) is an ion channel responsible for sensing osmotic and mechanical signals, which in turn regulates calcium signaling across cell membranes. TRPV4 is widely expressed throughout the body, and plays an important role in normal physiological function, as well as different pathologies, however, its role in the eye is not well known. In the eye, TRPV4 is expressed in various tissues, such as the retina, corneal epithelium, ciliary body, and the lens. In this review, we provide an overview on TRPV4 structure, activation, mutations, and summarize the current knowledge of TRPV4 function and signaling mechanisms in various locations throughout the eye, as well as its role in ocular diseases, such as glaucoma and diabetic retinopathy. Based on the available data, we highlight the therapeutic potential of TRPV4 as well as the shortcomings of current research. Finally, we provide future perspectives on the implications of targeting TRPV4 to treat various ocular pathologies.

Peripheral Mechanobiology of Touch-Studies on Vertebrate Cutaneous Sensory Corpuscles

The vertebrate skin contains sensory corpuscles that are receptors for different qualities of mechanosensitivity like light brush, touch, pressure, stretch or vibration. These specialized sensory organs are linked anatomically and functionally to mechanosensory neurons, which function as low-threshold mechanoreceptors connected to peripheral skin through Aβ nerve fibers. Furthermore, low-threshold mechanoreceptors associated with Aδ and C nerve fibers have been identified in hairy skin. The process of mechanotransduction requires the conversion of a mechanical stimulus into electrical signals (action potentials) through the activation of mechanosensible ion channels present both in the axon and the periaxonal cells of sensory corpuscles (i.e., Schwann-, endoneurial- and perineurial-related cells). Most of those putative ion channels belong to the degenerin/epithelial sodium channel (especially the family of acid-sensing ion channels), the transient receptor potential channel superfamilies, and the Piezo family. This review updates the current data about the occurrence and distribution of putative mechanosensitive ion channels in cutaneous mechanoreceptors including primary sensory neurons and sensory corpuscles.

TRPV4-Mediated Regulation of the Blood Brain Barrier Is Abolished During Inflammation

Blood-brain barrier (BBB) dysfunction is critically involved in determining the extent of several central nervous systems (CNS) pathologies and here in particular neuroinflammatory conditions. Inhibiting BBB breakdown could reduce the level of vasogenic edema and the number of immune cells invading the CNS, thereby counteracting neuronal injury. Transient receptor potential (TRP) channels have an important role as environmental sensors and constitute attractive therapeutic targets that are involved in calcium homeostasis during pathologies of the CNS. Transient receptor potential vanilloid 4 (TRPV4) is a calcium permeable, non-selective cation channel highly expressed in endothelial cells. As it is involved in the regulation of the blood brain barrier permeability and consequently cerebral edema formation, we anticipated a regulatory role of TRPV4 in CNS inflammation and subsequent neuronal damage. Here, we detected an increase in transendothelial resistance in mouse brain microvascular endothelial cells (MbMECs) after treatment with a selective TRPV4 inhibitor. However, this effect was abolished after the addition of IFNγ and TNFα indicating that inflammatory conditions override TRPV4-mediated permeability. Accordingly, we did not observe a protection of Trpv4-deficient mice when compared to wildtype controls in a preclinical model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), and no differences in infarct sizes following transient middle cerebral artery occlusion (tMCAO), the experimental stroke model, which leads to an acute postischemic inflammatory response. Furthermore, Evans Blue injections did not show differences in alterations of the blood brain barrier (BBB) permeability between genotypes in both animal models. Together, TRPV4 does not regulate brain microvascular endothelial permeability under inflammation.

TRPV4 Mediates Acute Bladder Responses to Bacterial Lipopolysaccharides

Urinary tract infections (UTI) affect a large proportion of the population, causing among other symptoms, more frequent and urgent micturition. Previous studies reported that the gram-negative bacterial wall component lipopolysaccharides (LPS) trigger acute epithelial and bladder voiding responses, but the underlying mechanisms remain unknown. The cation channel TRPV4 is implicated in the regulation of the bladder voiding. Since TRPV4 is activated by LPS in airway epithelial cells, we sought to determine whether this channel plays a role in LPS-induced responses in urothelial cells (UCs). We found that human-derived UCs display a fast increase in intracellular Ca²⁺ concentration upon acute application of Escherichia coli LPS. Such responses were detected also in freshly isolated mouse UCs, and found to be dependent on TRPV4, but not to require the canonical TLR4 signaling pathway of LPS detection. Confocal microscopy experiments revealed that TRPV4 is dispensable for LPS-induced nuclear translocation of NF-κB in mouse UCs. On the other hand, quantitative RT PCR determinations showed an enhanced LPS-induced production of proinflammatory cytokines in TRPV4-deficient UCs. Cystometry experiments in anesthetized wild type mice revealed that acute intravesical instillation of LPS rapidly increases voiding frequency. This effect was not observed in TRPV4-deficient animals, but was largely preserved in Tlr4 KO and Trpa1 KO mice. Our results suggest that activation of TRPV4 by LPS in UCs regulates the proinflammatory response and contributes to LPS-induced increase in voiding frequency. These findings further support the concept that TRP channels are sensors of LPS, mediating fast innate immunity mechanisms against gram-negative bacteria.

Portal vein innervation underlying the pressor effect of water ingestion with and without cold stress

Water-induced pressor response appears mediated through the activation of transient receptor potential channel TRPV4 on hepatic portal circulation in animals. We sought to elucidate the mechanism of portal vein signaling in this response. Forty-five rats were divided into four groups: control rats without water ingestion (WI), control rats with WI, portal vein denervation rats with WI (PVDWI), and TRPV4 antagonist-treated rats with WI (anti-TRPV4WI). Cardiovascular responses were monitored throughout the experiments. Data analysis was performed using descriptive methods and spectral and cross-spectral analysis of blood pressure variability (BPV) and heart rate variability (HRV). Key results showed that at baseline (PreCS) before cold stress trial (CS), WI elicited robust pressor and tachycardia responses accompanied by spectral power changes, in particular, increases of low-frequency BPV (LFBPV) and very-LFBPV (VLFBPV), but decrease of very-low-frequency HRV. PVDWI, likewise, elicited pressor and tachycardia responses accompanied by increases of high-frequency BPV, high-frequency HRV, LFBPV, low-frequency HRV, and VLFBPV. When compared with WI at PreCS, WI at CS elicited pressor and tachycardia responses accompanied by increases of high-frequency BPV, LFBPV, and VLFBPV, whereas in WI, the CS-evoked pressor response and the accompanied LFBPV and VLFBPV increases were all tended augmented by PVDWI. When compared with WI and PVDWI at both PreCS and CS, however, anti-TRPV4WI attenuated their pressor responses and attenuated their increased LFBPV, VLFBPV, and very-low-frequency HRV. The results indicate that the portal vein innervation is critical for a buffering mechanism in splanchnic sympathetic activation and water-induced pressor response.

Soft Tissue: A Possible Source of Pain Pre and Post Minimally Invasive Spine Surgery

STUDY DESIGN

Case studies.

OBJECTIVES

To demonstrate that muscle generated pain (MGP) may be a cause of pain in patients who have undergone minimally invasive spine surgery (MISS).

METHODS

A physical examination including electrical stimulation of putative pain generating muscles to identify the presence of lowered thresholds for depolarization of muscle nociceptors, and an examination of strength and flexibility of key muscles in the upper and lower body, may identify multiple etiologies of MGP. Treatment of identified muscles consisted of muscle/tendon injections to identified sensitized muscles followed by exercises incorporating relaxation limbering and stretching.

RESULTS

Postsurgical pain was eliminated and mobility restored in both presented cases replicating success in prior published studies.

CONCLUSIONS

Understanding the pathophysiological mechanisms of muscle pain may facilitate the evaluation and treatment of MGP in MISS patients diagnosed with failed back surgery syndrome.

Transcription factor Sp4 is required for hyperalgesic state persistence

Understanding how painful hypersensitive states develop and persist beyond the initial hours to days is critically important in the effort to devise strategies to prevent and/or reverse chronic painful states. Changes in nociceptor transcription can alter the abundance of nociceptive signaling elements, resulting in longer-term change in nociceptor phenotype. As a result, sensitized nociceptive signaling can be further amplified and nocifensive behaviors sustained for weeks to months. Building on our previous finding that transcription factor Sp4 positively regulates the expression of the pain transducing channel TRPV1 in Dorsal Root Ganglion (DRG) neurons, we sought to determine if Sp4 serves a broader role in the development and persistence of hypersensitive states in mice. We observed that more than 90% of Sp4 staining DRG neurons were small to medium sized, primarily unmyelinated (NF200 neg) and the majority co-expressed nociceptor markers TRPV1 and/or isolectin B4 (IB4). Genetically modified mice (Sp4+/-) with a 50% reduction of Sp4 showed a reduction in DRG TRPV1 mRNA and neuronal responses to the TRPV1 agonist-capsaicin. Importantly, Sp4+/- mice failed to develop persistent inflammatory thermal hyperalgesia, showing a reversal to control values after 6 hours. Despite a reversal of inflammatory thermal hyperalgesia, there was no difference in CFA-induced hindpaw swelling between CFA Sp4+/- and CFA wild type mice. Similarly, Sp4+/- mice failed to develop persistent mechanical hypersensitivity to hind-paw injection of NGF. Although Sp4+/- mice developed hypersensitivity to traumatic nerve injury, Sp4+/- mice failed to develop persistent cold or mechanical hypersensitivity to the platinum-based chemotherapeutic agent oxaliplatin, a non-traumatic model of neuropathic pain. Overall, Sp4+/- mice displayed a remarkable ability to reverse the development of multiple models of persistent inflammatory and neuropathic hypersensitivity. This suggests that Sp4 functions as a critical control point for a network of genes that conspire in the persistence of painful hypersensitive states.

Cannabinoid Ligands Targeting TRP Channels

Transient receptor potential (TRP) channels are a group of membrane proteins involved in the transduction of a plethora of chemical and physical stimuli. These channels modulate ion entry, mediating a variety of neural signaling processes implicated in the sensation of temperature, pressure, and pH, as well as smell, taste, vision, and pain perception. Many diseases involve TRP channel dysfunction, including neuropathic pain, inflammation, and respiratory disorders. In the pursuit of new treatments for these disorders, it was discovered that cannabinoids can modulate a certain subset of TRP channels. The TRP vanilloid (TRPV), TRP ankyrin (TRPA), and TRP melastatin (TRPM) subfamilies were all found to contain channels that can be modulated by several endogenous, phytogenic, and synthetic cannabinoids. To date, six TRP channels from the three subfamilies mentioned above have been reported to mediate cannabinoid activity: TRPV1, TRPV2, TRPV3, TRPV4, TRPA1, and TRPM8. The increasing data regarding cannabinoid interactions with these receptors has prompted some researchers to consider these TRP channels to be “ionotropic cannabinoid receptors.” Although CB1 and CB2 are considered to be the canonical cannabinoid receptors, there is significant overlap between cannabinoids and ligands of TRP receptors. The first endogenous agonist of TRPV1 to be discovered was the endocannabinoid, anandamide (AEA). Similarly, N-arachidonyl dopamine (NADA) and AEA were the first endogenous TRPM8 antagonists discovered. Additionally, Δ⁹-tetrahydrocannabinol (Δ⁹-THC), the most abundant psychotropic compound in cannabis, acts most potently at TRPV2, moderately modulates TRPV3, TRPV4, TRPA1, and TRPM8, though Δ⁹-THC is not reported to modulate TRPV1. Moreover, TRP receptors may modulate effects of synthetic cannabinoids used in research. One common research tool is WIN55,212-2, a CB1 agonist that also exerts analgesic effects by desensitizing TRPA1 and TRPV1. In this review article, we aim to provide an overview and classification of the cannabinoid ligands that have been reported to modulate TRP channels and their therapeutic potential.

Inhaled calcium salts inhibit tobacco smoke-induced inflammation by modulating expression of chemokines and cytokines

Tobacco smoke-induced lung inflammation in patients with chronic obstructive pulmonary disease (COPD) worsens with disease progression and acute exacerbations caused by respiratory infections. Chronic therapies to manage COPD center on bronchodilators to improve lung function and inhaled corticosteroids (ICS) to help reduce the risk of exacerbations. Novel therapies are needed that reduce the underlying inflammation associated with COPD and the inflammation resulting from respiratory infections that worsen disease. The lung is lined with airway surface liquid (ASL), a rheologically active material that provides an innate defense for the airway against inhaled particulate and is continuously cleared from the airways by mucociliary clearance. The rheological properties of the ASL can be altered by changes in airway hydration and by cations, such as calcium, that interact with electronegative glycoproteins. The effect of inhaled salts on inflammation resulting from tobacco smoke exposure was studied to determine if cations could be used to alter the properties of the ASL and reduce inflammation. Inhaled calcium salts, but not sodium or magnesium salts, reduced cellular inflammation and key chemokines and cytokines that were induced by tobacco smoke exposure. Similar anti-inflammatory effects of calcium salts were observed using in vitro cultures of human monocyte derived macrophages and human bronchial epithelial cells. The data suggest that inhaled calcium salts may act broadly on both biophysical and biological pathways to reduce pulmonary inflammation.

Ménière’s Disease Pathophysiology: Endolymphatic Sac Immunohistochemical Study of Aquaporin-2, V2R Vasopressin Receptor, NKCC2, and TRPV4

Objectives

Endolymphatic sac (ELS) pathophysiology in Ménière’s disease (MD) remains poorly understood. We identified from the literature a group of proteins expressed on the ELS and involved in endolymph volume regulation: aquaporin-2 (AQP2), vasopressin receptor V2R, sodium potassium chloride cotransporter 2 (NKCC2), and transient receptor potential cation channel V4 (TRPV4). Our objective was to determine whether their ELS expression was altered in MD, to better understand the pathophysiology of endolymphatic hydrops.

Study Design

Prospective case-control study.

Setting

Tertiary care center.

Subjects

Twenty-four patients with definite MD undergoing endolymphatic duct blockage surgery were recruited, as well as 23 controls with no history of MD undergoing surgery for vestibular schwannoma (VS).

Methods

ELS biopsies and blood samples for plasma arginine vasopressin (AVP) were obtained. Immunohistochemistry for AQP2, V2R, NKCC2, and TRPV4 was performed. Slides were scanned digitally for highly sensitive pixel density analysis by specialized software (VIS; Visiopharm).

Results

Global scores generated by the software represent total and relative protein expression density of 3 staining intensity levels, exclusively on ELS epithelium. AQP2 expression density was significantly elevated in MD compared to VS ( P = .003). There was no significant difference in plasma AVP, V2R, NKCC2, and TRPV4 expression.

Conclusion

This original study evaluates simultaneous in situ expression of AQP2, V2R, NKCC2, and TRPV4 on the human ELS in MD, with a control group. Our results show only AQP2 upregulation on the ELS of patients with MD. We suggest a constitutively increased expression of AQP2 in MD, independent of its regulatory axis (AVP-V2R). Acquired regulator sequence mutations could support this model.

The pain threshold of high-threshold mechanosensitive receptors subsequent to maximal eccentric exercise is a potential marker in the prediction of DOMS associated impairment

Background

Delayed-onset muscle soreness (DOMS) refers to dull pain and discomfort in people after participating in exercise, sport or recreational physical activities. The aim of this study was to detect underlying mechanical thresholds in an experimental model of DOMS.

Methods

Randomised study to detect mechanical pain thresholds in a randomised order following experimentally induced DOMS of the non-dominant arm in healthy participants. Main outcome was the detection of the pressure pain threshold (PPT), secondary thresholds included mechanical detection (MDT) and pain thresholds (MPT), pain intensity, pain perceptions and the maximum isometric voluntary force (MIVF).

Results

Twenty volunteers (9 female and 11 male, age 25.2 ± 3.2 years, weight 70.5 ± 10.8 kg, height 177.4 ± 9.4 cm) participated in the study. DOMS reduced the PPT (at baseline 5.9 ± 0.4 kg/cm²) by a maximum of 1.5 ± 1.4 kg/cm² (-24%) at 48 hours (p < 0.001). This correlated with the decrease in MIVF (r = -0.48, p = 0.033). Whereas subjective pain was an indicator of the early 48 hours, the PPT was still present after 72 hours (r = 0.48, p = 0.036). Other mechanical thresholds altered significantly due to DOMS, but did show no clinically or physiologically remarkable changes.

Conclusions

Functional impairment following DOMS seems related to the increased excitability of high-threshold mechanosensitive nociceptors. The PPT was the most valid mechanical threshold to quantify the extent of dysfunction. Thus PPT rather than pain intensity should be considered a possible marker indicating the athletes’ potential risk of injury.

Merkel cells and Meissner's corpuscles in human digital skin display Piezo2 immunoreactivity

The transformation of mechanical energy into electrical signals is the first step in mechanotransduction in the peripheral sensory nervous system and relies on the presence of mechanically gated ion channels within specialized sensory organs called mechanoreceptors. Piezo2 is a vertebrate stretch-gated ion channel necessary for mechanosensitive channels in mammalian cells. Functionally, it is related to light touch, which has been detected in murine cutaneous Merkel cell-neurite complexes, Meissner-like corpuscles and lanceolate nerve endings. To the best of our knowledge, the occurrence of Piezo2 in human cutaneous mechanoreceptors has never been investigated. Here, we used simple and double immunohistochemistry to investigate the occurrence of Piezo2 in human digital glabrous skin. Piezo2 immunoreactivity was detected in approximately 80% of morphologically and immunohistochemically characterized (cytokeratin 20⁺ , chromogranin A⁺ and synaptophisin⁺ ) Merkel cells. Most of them were in close contact with Piezo2^- nerve fibre profiles. Moreover, the axon, but not the lamellar cells, of Meissner's corpuscles was also Piezo2⁺ , but other mechanoreceptors, i.e. Pacinian or Ruffini's corpuscles, were devoid of immunoreactivity. Piezo2 was also observed in non-nervous tissue, especially the basal keratinocytes, endothelial cells and sweat glands. The present results demonstrate the occurrence of Piezo2 in cutaneous sensory nerve formations that functionally work as slowly adapting (Merkel cells) and rapidly adapting (Meissner's corpuscles) low-threshold mechanoreceptors and are related to fine and discriminative touch but not to vibration or hard touch. These data offer additional insight into the molecular basis of mechanosensing in humans.

When size matters: transient receptor potential vanilloid 4 channel as a volume-sensor rather than an osmo-sensor

KEY POINTS

Mammalian cells are frequently exposed to stressors causing volume changes. The transient receptor potential vanilloid 4 (TRPV4) channel translates osmotic stress into ion flux. The molecular mechanism coupling osmolarity to TRPV4 activation remains elusive. TRPV4 responds to isosmolar cell swelling and osmolarity translated via different aquaporins. TRPV4 functions as a volume-sensing ion channel irrespective of the origin of the cell swelling.

ABSTRACT

Transient receptor potential channel 4 of the vanilloid subfamily (TRPV4) is activated by a diverse range of molecular cues, such as heat, lipid metabolites and synthetic agonists, in addition to hyposmotic challenges. As a non-selective cation channel permeable to Ca²⁺ , it transduces physical stress in the form of osmotic cell swelling into intracellular Ca²⁺ -dependent signalling events. Its contribution to cell volume regulation might include interactions with aquaporin (AQP) water channel isoforms, although the proposed requirement for a TRPV4-AQP4 macromolecular complex remains to be resolved. To characterize the elusive mechanics of TRPV4 volume-sensing, we expressed the channel in Xenopus laevis oocytes together with AQP4. Co-expression with AQP4 facilitated the cell swelling induced by osmotic challenges and thereby activated TRPV4-mediated transmembrane currents. Similar TRPV4 activation was induced by co-expression of a cognate channel, AQP1. The level of osmotically-induced TRPV4 activation, although proportional to the degree of cell swelling, was dependent on the rate of volume changes. Importantly, isosmotic cell swelling obtained by parallel activation of the co-expressed water-translocating Na⁺ /K⁺ /2Cl^- cotransporter promoted TRPV4 activation despite the absence of the substantial osmotic gradients frequently employed for activation. Upon simultaneous application of an osmotic gradient and the selective TRPV4 agonist GSK1016790A, enhanced TRPV4 activation was observed only with subsaturating stimuli, indicating that the agonist promotes channel opening similar to that of volume-dependent activation. We propose that, contrary to the established paradigm, TRPV4 is activated by increased cell volume irrespective of the molecular mechanism underlying cell swelling. Thus, the channel functions as a volume-sensor, rather than as an osmo-sensor.

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.

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.

Regulation of Piezo2 Mechanotransduction by Static Plasma Membrane Tension in Primary Afferent Neurons

The Piezo2 channel is a newly identified mammalian mechanical transducer that confers rapidly adapting mechanically activated (RA-MA) currents in primary afferent neurons. The Piezo2 channels sense rapid membrane displacement, but it is not clear whether they are sensitive to osmotic swelling, which slowly increases static plasma membrane tension (SPMT). Here, we show that SPMT exerts a profound impact on the mechanical sensitivity of RA-MA channels in primary afferent neurons. RA-MA currents are greatly enhanced, and the mechanical threshold was reduced in both primary afferent neurons of rat dorsal root ganglia (DRG) and HEK293 cells heterologously expressing Piezo2 when these cells undergo osmotic swelling to increase SPMT. Osmotic swelling switches the kinetics of RA-MA currents to the slowly adapting type in both cultured DRG neurons and HEK293 cells heterologously expressing Piezo2. The potentiation of RA-MA currents is abolished when cultured DRG neurons are treated with cytochalasin D, an actin filament disruptor that prevents SPMT of cultured DRG neurons from an increase by osmotic swelling. Osmotic swelling significantly increases DRG neuron mechano-excitability such that a subthreshold mechanical stimulus can result in action potential firing. Behaviorally, the mechanical hind paw withdrawal threshold in rats is reduced following the injection of a hypotonic solution, but this osmotic effect is abolished when cytochalasin D or Gd(3+) is co-administered with the hypo-osmotic solution. Taken together, our findings suggest that Piezo2-mediated mechanotransduction is regulated by SPMT in primary afferent neurons. Because SPMT can be changed by multiple biological factors, our findings may have broad implications in mechanical sensitivity under physiological and pathological conditions.

Current understanding on the neurophysiology of behavioral thermoregulation

Temperature influence on the physiology and biochemistry of living organisms has long been recognized, which propels research in the field of thermoregulation. With the cloning and characterization of the transient receptor potential (TRP) ion channels as the principal temperature sensors of the mammalian somatosensory neurons, the understanding, at a molecular level, of thermosensory and thermoregulatory mechanisms became promising. Because thermal environment can be extremely hostile (temperature range on earth's surface is from ∼ −69°C to 58°C), living organisms developed an array of thermoregulatory strategies to guarantee survival, which include both autonomic mechanisms, which aim at increasing or decreasing heat exchange between body, and ambient and behavioral strategies. The knowledge regarding neural mechanisms involved in autonomic thermoregulatory strategies has progressed immensely compared to the knowledge on behavioral thermoregulation. This review aims at collecting the up-to-date knowledge on the neural basis for behavioral thermoregulation in mammals in order to point out perspectives and deployment of this research field.

Damage to lens fiber cells causes TRPV4-dependent Src family kinase activation in the epithelium

The bulk of the lens consists of tightly packed fiber cells. Because mature lens fibers lack mitochondria and other organelles, lens homeostasis relies on a monolayer of epithelial cells at the anterior surface. The detection of various signaling pathways in lens epithelial cells suggests they respond to stimuli that influence lens function. Focusing on Src Family Kinases (SFKs) and Transient Receptor Potential Vanilloid 4 (TRPV4), we tested whether the epithelium can sense and respond to an event that occurs in fiber mass. The pig lens was subjected to localized freeze-thaw (FT) damage to fibers at posterior pole then the lens was incubated for 1-10 min in Krebs solution at 37 °C. Transient SFK activation in the epithelium was detectable at 1 min. Using a western blot approach, the ion channel TRPV4 was detected in the epithelium but was sparse or absent in fiber cells. Even though TRPV4 expression appears low at the actual site of FT damage to the fibers, SFK activation in the epithelium was suppressed in lenses subjected to FT damage then incubated with the TRPV4 antagonist HC067047 (10 μM). Na,K-ATPase activity was examined because previous studies report changes of Na,K-ATPase activity associated with SFK activation. Na,K-ATPase activity doubled in the epithelium removed from FT-damaged lenses and the response was prevented by HC067047 or the SFK inhibitor PP2 (10 μM). Similar changes were observed in response to fiber damage caused by injection of 5 μl hyperosmotic NaCl or mannitol solution beneath the surface of the posterior pole. The findings point to a TRPV4-dependent mechanism that enables the epithelial cells to detect remote damage in the fiber mass and respond within minutes by activating SFK and increasing Na,K-ATPase activity. Because TRPV4 channels are mechanosensitive, we speculate they may be stimulated by swelling of the lens structure caused by damage to the fibers. Increased Na,K-ATPase activity gives the lens greater capacity to control ion concentrations in the fiber mass and the Na,K-ATPase response may reflect the critical contribution of the epithelium to lens ion homeostasis.

Biomechanical Origins of Muscle Stem Cell Signal Transduction

Skeletal muscle, the most abundant and widespread tissue in the human body, contracts upon receiving electrochemical signals from the nervous system to support essential functions such as thermoregulation, limb movement, blinking, swallowing and breathing. Reconstruction of adult muscle tissue relies on a pool of mononucleate, resident muscle stem cells, known as "satellite cells", expressing the paired-box transcription factor Pax7 necessary for their specification during embryonic development and long-term maintenance during adult life. Satellite cells are located around the myofibres in a niche at the interface of the basal lamina and the host fibre plasma membrane (i.e., sarcolemma), at a very low frequency. Upon damage to the myofibres, quiescent satellite cells are activated and give rise to a population of transient amplifying myogenic progenitor cells, which eventually exit the cell cycle permanently and fuse to form new myofibres and regenerate the tissue. A subpopulation of satellite cells self-renew and repopulate the niche, poised to respond to future demands. Harnessing the potential of satellite cells relies on a complete understanding of the molecular mechanisms guiding their regulation in vivo. Over the past several decades, studies revealed many signal transduction pathways responsible for satellite cell fate decisions, but the niche cues driving the activation and silencing of these pathways are less clear. Here we explore the scintillating possibility that considering the dynamic changes in the biophysical properties of the skeletal muscle, namely stiffness, and the stretch and shear forces to which a myofibre can be subjected to may provide missing information necessary to gain a full understanding of satellite cell niche regulation.

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

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

Osmolarity influences chondrocyte repair after injury in human articular cartilage

Background

The purpose was to determine the influence of irrigation solution osmolarity on articular chondrocytes survival and metabolic state following mechanical injury.

Methods

Osteochondral explants were harvested from patients undergoing total knee arthroplasty for osteoarthritis and then cut through their full thickness to establish mechanical injury models. Cartilage explants were incubated in irrigation solutions (saline and balanced salt) with different osmolarities (180, 280, 380, 580 mOsm/L) for 2 h. The percentage of cell death (100 × number of dead cells/number of dead and live cells) was quantified with the laser confocal microscopy. The terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay was performed to detect apoptosis index of injured cartilage. The contents of proteoglycan elution were determined by spectrophotometer at 530 nm, and HIF-1α and type II collagen mRNA yields were quantified with real-time PCR.

Results

In situ dead chondrocytes were mainly localized to the superficial tangential region of injured cartilage edge after mechanical injury. The percentage of cell death was decreased, and proteoglycan elution was gradually reduced with the increasing of osmolarity. The apoptosis indices of TUNEL assay in different osmolarities had no significant difference (P = 0.158). HIF-1α and type II collagen mRNA yields were the least for chondrocytes exposed to 180 mOsm/L medium and were the greatest for chondrocytes exposed to 380 mOsm/L medium. Compared with the saline group, the cell death of superficial zone was significantly decreased (P = 0.001) and contents of proteoglycan elution were also significantly decreased (P = 0.045) in the balanced salt. HIF-1α (P = 0.017) and type II collagen (P = 0.034) mRNA yields in the chondrocytes exposed to the balanced salt were significantly more than the saline group.

Conclusion

The osmolarity of irrigation solutions plays an important role in the survival and metabolic state of chondrocytes following mechanical injury, and the chondrocyte death is not caused by apoptosis. Increasing osmolarity of irrigation solutions may be chondroprotective with decreasing the chondrocyte death, reducing inhibition of metabolism and proteoglycan elution, ultimately preventing cartilage degeneration and promoting integrative repair.