Stimulus-specific modulation of the cation channel TRPV4 by PACSIN 3

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.

TRPV4 Channel Modulators as Potential Drug Candidates for Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, resulting in defective chloride ion channels. This leads to thick, dehydrated mucus that severely disrupts mucociliary clearance in the respiratory system and triggers infection that eventually is the cause of death of CF patients. Current therapeutic strategies primarily focus on restoring CFTR function, blocking epithelial sodium channels to prevent mucus dehydration, or directly targeting mucus to reduce its viscosity. Among the ion channels expressed in ciliated bronchial epithelial cells, the transient receptor potential vanilloid 4 (TRPV4) channel emerges as a significant channel in CF pathogenesis. Activation of TRPV4 channels affects the regulation of airway surface liquid by modulating sodium absorption and intracellular calcium levels, which indirectly influences CFTR activity. TRPV4 is also involved in the regulatory volume decrease (RVD) process and enhances inflammatory responses in CF patients. Here, we combine current findings on TRPV4 channel modulation as a promising therapeutic approach for CF. Although limited studies have directly explored TRPV4 in CF, emerging evidence indicates that TRPV4 activation can significantly impact key pathological processes in the disease. Further investigation into TRPV4 modulators could lead to innovative treatments that alleviate severe respiratory complications and improve outcomes for CF patients.

Role of TRPV4 on vascular tone regulation in pathophysiological states

Vascular tone regulation is a key event in controlling blood flow in the body. Endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) help regulate the vascular tone. Abnormal vascular responsiveness to various stimuli, including constrictors and dilators, has been observed in pathophysiological states although EC and VSMC coordinate to maintain the exquisite balance between contraction and relaxation in vasculatures. Thus, investigating the mechanisms underlying vascular tone abnormality is very important in maintaining vascular health and treating vasculopathy. Increased intracellular free Ca2+ concentration ([Ca2+]i) is one of the major triggers initiating each EC and VSMC response. Transient receptor potential vanilloid family member 4 (TRPV4) is a Ca2+-permeable non-selective ion channel, which is activated by several stimuli, and is presented in both ECs and VSMCs. Therefore, TRPV4 plays an important role in vascular responses. Emerging evidence indicates the role of TRPV4 on the functions of ECs and VSMCs in various pathophysiological states, including hypertension, diabetes, and obesity. This review focused on the link between TRPV4 and the functions of ECs/VSMCs, particularly its role in vascular tone and responsiveness to vasoactive substances.

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

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

Pathophysiological Roles of the TRPV4 Channel in the Heart

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

Astrocytic TRPV4 Channels and Their Role in Brain Ischemia

Transient receptor potential cation channels subfamily V member 4 (TRPV4) are non-selective cation channels expressed in different cell types of the central nervous system. These channels can be activated by diverse physical and chemical stimuli, including heat and mechanical stress. In astrocytes, they are involved in the modulation of neuronal excitability, control of blood flow, and brain edema formation. All these processes are significantly impaired in cerebral ischemia due to insufficient blood supply to the tissue, resulting in energy depletion, ionic disbalance, and excitotoxicity. The polymodal cation channel TRPV4, which mediates Ca²⁺ influx into the cell because of activation by various stimuli, is one of the potential therapeutic targets in the treatment of cerebral ischemia. However, its expression and function vary significantly between brain cell types, and therefore, the effect of its modulation in healthy tissue and pathology needs to be carefully studied and evaluated. In this review, we provide a summary of available information on TRPV4 channels and their expression in healthy and injured neural cells, with a particular focus on their role in ischemic brain injury.

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

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

Transient Receptor Potential Vanilloid 4: a Double-Edged Sword in the Central Nervous System

Transient receptor potential vanilloid 4 (TRPV4) is a nonselective cation channel that can be activated by diverse stimuli, such as heat, mechanical force, hypo-osmolarity, and arachidonic acid metabolites. TRPV4 is widely expressed in the central nervous system (CNS) and participates in many significant physiological processes. However, accumulative evidence has suggested that deficiency, abnormal expression or distribution, and overactivation of TRPV4 are involved in pathological processes of multiple neurological diseases. Here, we review the latest studies concerning the known features of this channel, including its expression, structure, and its physiological and pathological roles in the CNS, proposing an emerging therapeutic strategy for CNS diseases.

Backbone NMR assignments of the extensive human and chicken TRPV4 N-terminal intrinsically disordered regions as important players in ion channel regulation

Transient receptor potential (TRP) channels are important pharmacological targets due to their ability to act as sensory transducers on the organismic and cellular level, as polymodal signal integrators and because of their role in numerous diseases. However, a detailed molecular understanding of the structural dynamics of TRP channels and their integration into larger cellular signalling networks remains challenging, in part due to the systematic absence of highly dynamic regions pivotal for channel regulation from available structures. In human TRP vanilloid 4 (TRPV4), a ubiquitously expressed homotetrameric cation channel involved in temperature, osmo- and mechano-sensation and in a multitude of (patho)physiological processes, the intrinsically disordered N-terminus encompasses 150 amino acids and thus represents > 17% of the entire channel sequence. Its deletion renders the channel significantly less excitable to agonists supporting a crucial role in TRPV4 activation and regulation. For a structural understanding and a comparison of its properties across species, we determined the NMR backbone assignments of the human and chicken TRPV4 N-terminal IDRs.

Genetic- and diet-induced ω-3 fatty acid enrichment enhances TRPV4-mediated vasodilation in mice

TRPV4 channel activation in endothelial cells leads to vasodilation, while impairment of TRPV4 activity is implicated in vascular dysfunction. Strategies that increase TRPV4 activity could enhance vasodilation and ameliorate vascular disorders. Here, we show that supplementation with eicosapentaenoic acid (EPA), an ω-3 polyunsaturated fatty acid known to have beneficial cardiovascular effects, increases TRPV4 activity in human endothelial cells of various vascular beds. Mice carrying the C. elegans FAT-1 enzyme, which converts ω-6 to ω-3 polyunsaturated fatty acids, display higher EPA content and increased TRPV4-mediated vasodilation in mesenteric arteries. Likewise, mice fed an EPA-enriched diet exhibit enhanced and prolonged TRPV4-dependent vasodilation in an endothelial cell-specific manner. We also show that EPA supplementation reduces TRPV4 desensitization, which contributes to the prolonged vasodilation. Neutralization of positive charges in the TRPV4 N terminus impairs the effect of EPA on channel desensitization. These findings highlight the beneficial effects of manipulating fatty acid content to enhance TRPV4-mediated vasodilation.

Cell Confluence Modulates TRPV4 Channel Activity in Response to Hypoxia

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

Endosomal trafficking in schizophrenia

Schizophrenia is a severe and heritable neuropsychiatric disorder, which arises due to a combination of common genetic variation, rare loss of function variation, and copy number variation. Functional genomic evidence has been used to identify candidate genes affected by this variation, which revealed biological pathways that may be disrupted in schizophrenia. Understanding the contributions of these pathways are critical next steps in understanding schizophrenia pathogenesis. A number of genes involved in endocytosis are implicated in schizophrenia. In this review, we explore the history of endosomal trafficking in schizophrenia and highlight new endosomal candidate genes. We explore the function of these candidate genes and hypothesize how their dysfunction may contribute to schizophrenia.

Role of mechanosensitive channels/receptors in atherosclerosis

Mechanical forces are critical physical cues that can affect numerous cellular processes regulating the development, tissue maintenance, and functionality of cells. The contribution of mechanical forces is especially crucial in the vascular system where it is required for embryogenesis and for maintenance of physiological function in vascular cells including aortic endothelial cells, resident macrophages, and smooth muscle cells. Emerging evidence has also identified a role of these mechanical cues in pathological conditions of the vascular system such as atherosclerosis and associated diseases like hypertension. Of the different mechanotransducers, mechanosensitive ion channels/receptors are gaining prominence due to their involvement in numerous physiological and pathological conditions. However, only a handful of potential mechanosensory ion channels/receptors have been shown to be involved in atherosclerosis, and their precise role in disease development and progression remains poorly understood. Here, we provide a comprehensive account of recent studies investigating the role of mechanosensitive ion channels/receptors in atherosclerosis. We discuss the different groups of mechanosensitive proteins and their specific roles in inflammation, endothelial dysfunction, macrophage foam cell formation, and lesion development, which are crucial for the development and progression of atherosclerosis. Results of the studies discussed here will help in developing an understanding of the current state of mechanobiology in vascular diseases, specifically in atherosclerosis, which may be important for the development of innovative and targeted therapeutics for this disease.

Activation of TRPV4 Induces Exocytosis and Ferroptosis in Human Melanoma Cells

TRPV4 (transient receptor potential vanilloid 4), a calcium permeable TRP ion channel, is known to play a key role in endocytosis. However, whether it contributes to exocytosis remains unclear. Here, we report that activation of TRPV4 induced massive exocytosis in both melanoma A375 cell and heterologous expression systems. We show here that, upon application of TRPV4-specific agonists, prominent vesicle priming from endoplasmic reticulum (ER) was observed, followed by morphological changes of mitochondrial crista may lead to cell ferroptosis. We further identified interactions between TRPV4 and folding/vesicle trafficking proteins, which were triggered by calcium entry through activated TRPV4. This interplay, in turn, enhanced TRPV4-mediated activation of folding and vesicle trafficking proteins to promote exocytosis. Our study revealed a signaling mechanism underlying stimulus-triggered exocytosis in melanoma and highlighted the role of cellular sensor TRPV4 ion channel in mediating ferroptosis.

Multiubiquitination of TRPV4 reduces channel activity independent of surface localization

Ubiquitin (Ub)-mediated regulation of plasmalemmal ion channel activity canonically occurs via stimulation of endocytosis. Whether ubiquitination can modulate channel activity by alternative mechanisms remains unknown. Here, we show that the transient receptor potential vanilloid 4 (TRPV4) cation channel is multiubiquitinated within its cytosolic N-terminal and C-terminal intrinsically disordered regions (IDRs). Mutagenizing select lysine residues to block ubiquitination of the N-terminal but not C-terminal IDR resulted in a marked elevation of TRPV4-mediated intracellular calcium influx, without increasing cell surface expression levels. Conversely, enhancing TRPV4 ubiquitination via expression of an E3 Ub ligase reduced TRPV4 channel activity but did not decrease plasma membrane abundance. These results demonstrate Ub-dependent regulation of TRPV4 channel function independent of effects on plasma membrane localization. Consistent with ubiquitination playing a key negative modulatory role of the channel, gain-of-function neuropathy-causing mutations in the TRPV4 gene led to reduced channel ubiquitination in both cellular and Drosophila models of TRPV4 neuropathy, whereas increasing mutant TRPV4 ubiquitination partially suppressed channel overactivity. Together, these data reveal a novel mechanism via which ubiquitination of an intracellular flexible IDR domain modulates ion channel function independently of endocytic trafficking and identify a contributory role for this pathway in the dysregulation of TRPV4 channel activity by neuropathy-causing mutations.

PACSIN proteins in vivo: Roles in development and physiology

Protein kinase C and casein kinase substrate in neurons (PACSINs), or syndapins (synaptic dynamin‐associated proteins), are a family of proteins involved in the regulation of cell cytoskeleton, intracellular trafficking and signalling. Over the last twenty years, PACSINs have been mostly studied in the in vitro and ex vivo settings, and only in the last decade reports on their function in vivo have emerged. We first summarize the identification, structure and cellular functions of PACSINs, and then focus on the relevance of PACSINs in vivo. During development in various model organisms, PACSINs participate in diverse processes, such as neural crest cell development, gastrulation, laterality development and neuromuscular junction formation. In mouse, PACSIN2 regulates angiogenesis during retinal development and in human, PACSIN2 associates with monosomy and embryonic implantation. In adulthood, PACSIN1 has been extensively studied in the brain and shown to regulate neuromorphogenesis, receptor trafficking and synaptic plasticity. Several genetic studies suggest a role for PACSIN1 in the development of schizophrenia, which is also supported by the phenotype of mice depleted of PACSIN1. PACSIN2 plays an essential role in the maintenance of intestinal homeostasis and participates in kidney repair processes after injury. PACSIN3 is abundant in muscle tissue and necessary for caveolar biogenesis to create membrane reservoirs, thus controlling muscle function, and has been linked to certain genetic muscular disorders. The above examples illustrate the importance of PACSINs in diverse physiological or tissue repair processes in various organs, and associations to diseases when their functions are disturbed.

The Emerging Pro-Algesic Profile of Transient Receptor Potential Vanilloid Type 4

Transient receptor potential vanilloid type 4 (TRPV4) channels are Ca²⁺-permeable non-selective cation channels which mediate a wide range of physiological functions and are activated and modulated by a diverse array of stimuli. One of this ion channel's least discussed functions is in relation to the generation and maintenance of certain pain sensations. However, in the two decades which have elapsed since the identification of this ion channel, considerable data has emerged concerning its function in mediating pain sensations. TRPV4 is a mediator of mechanical hyperalgesia in the various contexts in which a mechanical stimulus, comprising trauma (at the macro-level) or discrete extracellular pressure or stress (at the micro-level), results in pain. TRPV4 is also recognised as constituting an essential component in mediating inflammatory pain. It also plays a role in relation to many forms of neuropathic-type pain, where it functions in mediating mechanical allodynia and hyperalgesia.Here, we review the role of TRPV4 in mediating pain sensations.

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.

TRPV4 channels mediate the mechanoresponse in retinal microglia

The physiological and neurological correlates of plummeting brain osmolality during edema, traumatic CNS injury, and severe ischemia are compounded by neuroinflammation. Using multiple approaches, we investigated how retinal microglia respond to challenges mediated by increases in strain, osmotic gradients, and agonists of the stretch-activated cation channel TRPV4. Dissociated and intact microglia were TRPV4-immunoreactive and responded to the selective agonist GSK1016790A and substrate stretch with altered motility and elevations in intracellular calcium ([Ca2+ ]i ). Agonist- and hypotonicity-induced swelling was associated with a nonselective outwardly rectifying cation current, increased [Ca2+ ]i , and retraction of higher-order processes. The antagonist HC067047 reduced the extent of hypotonicity-induced microglial swelling and inhibited the suppressive effects of GSK1016790A and hypotonicity on microglial branching. Microglial TRPV4 signaling required intermediary activation of phospholipase A2 (PLA2), cytochrome P450, and epoxyeicosatrienoic acid production (EETs). The expression pattern of vanilloid thermoTrp genes in retinal microglia was markedly different from retinal neurons, astrocytes, and cortical microglia. These results suggest that TRPV4 represents a primary retinal microglial sensor of osmochallenges under physiological and pathological conditions. Its activation, associated with PLA2, modulates calcium signaling and cell architecture. TRPV4 inhibition might be a useful strategy to suppress microglial overactivation in the swollen and edematous CNS.

Ca2+ Signaling by TRPV4 Channels in Respiratory Function and Disease

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

Unstructural Biology of TRP Ion Channels: The Role of Intrinsically Disordered Regions in Channel Function and Regulation

The first genuine high-resolution single particle cryo-electron microscopy structure of a membrane protein determined was a transient receptor potential (TRP) ion channel, TRPV1, in 2013. This methodical breakthrough opened up a whole new world for structural biology and ion channel aficionados alike. TRP channels capture the imagination due to the sheer endless number of tasks they carry out in all aspects of animal physiology. To date, structures of at least one representative member of each of the six mammalian TRP channel subfamilies as well as of a few non-mammalian families have been determined. These structures were instrumental for a better understanding of TRP channel function and regulation. However, all of the TRP channel structures solved so far are incomplete since they miss important information about highly flexible regions found mostly in the channel N- and C-termini. These intrinsically disordered regions (IDRs) can represent between a quarter to almost half of the entire protein sequence and act as important recruitment hubs for lipids and regulatory proteins. Here, we analyze the currently available TRP channel structures with regard to the extent of these "missing" regions and compare these findings to disorder predictions. We discuss select examples of intra- and intermolecular crosstalk of TRP channel IDRs with proteins and lipids as well as the effect of splicing and post-translational modifications, to illuminate their importance for channel function and to complement the prevalently discussed structural biology of these versatile and fascinating proteins with their equally relevant 'unstructural' biology.

Neuropathy-causing TRPV4 mutations disrupt TRPV4-RhoA interactions and impair neurite extension

TRPV4 is a cell surface-expressed calcium-permeable cation channel that mediates cell-specific effects on cellular morphology and function. Dominant missense mutations of TRPV4 cause distinct, tissue-specific diseases, but the pathogenic mechanisms are unknown. Mutations causing peripheral neuropathy localize to the intracellular N-terminal domain whereas skeletal dysplasia mutations are in multiple domains. Using an unbiased screen, we identified the cytoskeletal remodeling GTPase RhoA as a TRPV4 interactor. TRPV4-RhoA binding occurs via the TRPV4 N-terminal domain, resulting in suppression of TRPV4 channel activity, inhibition of RhoA activation, and extension of neurites in vitro. Neuropathy but not skeletal dysplasia mutations disrupt TRPV4-RhoA binding and cytoskeletal outgrowth. However, inhibition of RhoA restores neurite length in vitro and in a fly model of TRPV4 neuropathy. Together these results identify RhoA as a critical mediator of TRPV4-induced cell structure changes and suggest that disruption of TRPV4-RhoA binding may contribute to tissue-specific toxicity of TRPV4 neuropathy mutations.

TRPV4 dominant mutations cause neuropathy. Here, the authors show that TRPV4 binds and interacts with RhoA, modulating the actin cytoskeleton. Neuropathy-causing mutations of TRPV4 disrupt this complex, leading to RhoA activation and impairment of neurite extension in cultured cells and flies.

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.

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.

TRPV4 integrates matrix mechanosensing with Ca2+ signaling to regulate extracellular matrix remodeling

In healthy connective tissues, mechanosensors trigger the generation of Ca²⁺ signals, which enable cells to maintain the structure of the fibrillar collagen matrix through actomyosin contractile forces. Transient receptor potential vanilloid type 4 (TRPV4) is a mechanosensitive Ca²⁺ -permeable channel that, when expressed in cell-matrix adhesions of the plasma membrane, regulates extracellular matrix (ECM) remodeling. In high prevalence disorders such as fibrosis and tumor metastasis, dysregulated matrix remodeling is associated with disruptions of Ca²⁺ homeostasis and TRPV4 function. Here, we consider that ECM polymers transmit cell-activating mechanical signals to TRPV4 in cell adhesions. When activated, TRPV4 regulates fibrillar collagen remodeling, thereby altering the mechanical properties of the ECM. In this review, we integrate functionally connected processes of matrix remodeling to highlight how TRPV4 in cell adhesions and matrix mechanics are reciprocally regulated through Ca²⁺ signaling.

Applications of Transition-Metal-Catalyzed Asymmetric Allylic Substitution in Total Synthesis of Natural Products: An Update

Metal-catalyzed asymmetric allylic substitution (AAS) reaction is one of the most synthetically useful reactions catalyzed by metal complexes for the formation of carbon-carbon and carbon-heteroatom bonds. It comprises the substitution of allylic substrates with a wide range of nucleophiles or S_N 2'-type allylic substitution, which results in the formation of the above-mentioned bonds with high levels of enantioselective induction. AAS reaction tolerates a broad range of functional groups, thus has been successfully applied in the asymmetric synthesis of a wide range of optically pure compounds. This reaction has been extensively used in the total synthesis of several complex molecules, especially natural products. In this review, we try to highlight the applications of metal (Pd, Ir, Mo, or Cu)-catalyzed AAS reaction in the total synthesis of the biologically active natural products, as a key step, updating the subject from 2003 till date.

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.

Polymodal Sensory Transduction in Mouse Corneal Epithelial Cells

Purpose

Contact lenses, osmotic stressors, and chemical burns may trigger severe discomfort and vision loss by damaging the cornea, but the signaling mechanisms used by corneal epithelial cells (CECs) to sense extrinsic stressors are not well understood. We therefore investigated the mechanisms of swelling, temperature, strain, and chemical transduction in mouse CECs.

Methods

Intracellular calcium imaging in conjunction with electrophysiology, pharmacology, transcript analysis, immunohistochemistry, and bioluminescence assays of adenosine triphosphate (ATP) release were used to track mechanotransduction in dissociated CECs and epithelial sheets isolated from the mouse cornea.

Results

The transient receptor potential vanilloid (TRPV) transcriptome in the mouse corneal epithelium is dominated by Trpv4, followed by Trpv2, Trpv3, and low levels of Trpv1 mRNAs. TRPV4 protein was localized to basal and intermediate epithelial strata, keratocytes, and the endothelium in contrast to the cognate TRPV1, which was confined to intraepithelial afferents and a sparse subset of CECs. The TRPV4 agonist GSK1016790A induced cation influx and calcium elevations, which were abolished by the selective blocker HC067047. Hypotonic solutions, membrane strain, and moderate heat elevated [Ca²⁺]_CEC with swelling- and temperature-, but not strain-evoked signals, sensitive to HC067047. GSK1016790A and swelling evoked calcium-dependent ATP release, which was suppressed by HC067027 and the hemichannel blocker probenecid.

Conclusions

These results demonstrate that cation influx via TRPV4 transduces osmotic and thermal but not strain inputs to CECs and promotes hemichannel-dependent ATP release. The TRPV4-hemichannel-ATP signaling axis might modulate corneal pain induced by excessive mechanical, osmotic, and chemical stimulation.

The Zinc-Finger Domain Containing Protein ZC4H2 Interacts with TRPV4, Enhancing Channel Activity and Turnover at the Plasma Membrane

The Ca²⁺-permeable Transient Receptor Potential channel vanilloid subfamily member 4 (TRPV4) is involved in a broad range of physiological processes, including the regulation of systemic osmotic pressure, bone resorption, vascular tone, and bladder function. Mutations in the TRPV4 gene are the cause of a spectrum of inherited diseases (or TRPV4-pathies), which include skeletal dysplasias, arthropathies, and neuropathies. There is little understanding of the pathophysiological mechanisms underlying these variable disease phenotypes, but it has been hypothesized that disease-causing mutations affect interaction with regulatory proteins. Here, we performed a mammalian protein-protein interaction trap (MAPPIT) screen to identify proteins that interact with the cytosolic N terminus of human TRPV4, a region containing the majority of disease-causing mutations. We discovered the zinc-finger domain-containing protein ZC4H2 as a TRPV4-interacting protein. In heterologous expression experiments, we found that ZC4H2 increases both the basal activity of human TRPV4 as well as Ca²⁺ responses evoked by ligands or hypotonic cell swelling. Using total internal reflection fluorescence (TIRF) microscopy, we further showed that ZC4H2 accelerates TRPV4 turnover at the plasma membrane. Overall, these data demonstrate that ZC4H2 is a positive modulator of TRPV4, and suggest a link between TRPV4 and ZC4H2-associated rare disorders, which have several neuromuscular symptoms in common with TRPV4-pathies.

Elevated TRPV4 Levels Contribute to Endothelial Damage and Scarring in Experimental Spinal Cord Injury

Currently, the role of transient receptor potential vanilloid type 4 (TRPV4), a nonselective cation channel in the pathology of spinal cord injury (SCI), is not recognized. Herein, we report the expression and contribution of TRPV4 in the pathology of scarring and endothelial and secondary damage after SCI. TRPV4 expression increased during the inflammatory phase in female rats after SCI and was expressed primarily by cells at endothelial-microglial junctions. Two-photon microscopy of intracellular-free Ca2+ levels revealed a biphasic increase at similar time points after SCI. Expression of TRPV4 at the injury epicenter, but not intracellular-free Ca2+, progressively increases with the severity of the injury. Activation of TRPV4 with specific agonist altered the organization of endothelial cells, affected tight junctions in the hCMEC/D3 BBB cell line in vitro, and increases the scarring in rat spinal cord as well as induced endothelial damage. By contrast, suppression of TRPV4 with a specific antagonist or in female Trpv4 KO mouse attenuated inflammatory cytokines and chemokines, prevented the degradation of tight junction proteins, and preserve blood-spinal cord barrier integrity, thereby attenuate the scarring after SCI. Likewise, secondary damage was reduced, and behavioral outcomes were improved in Trpv4 KO mice after SCI. These results suggest that increased TRPV4 expression disrupts endothelial cell organization during the early inflammatory phase of SCI, resulting in tissue damage, vascular destabilization, blood-spinal cord barrier breakdown, and scarring. Thus, TRPV4 inhibition/knockdown represents a promising therapeutic strategy to stabilize/protect endothelial cells, attenuate nociception and secondary damage, and reduce scarring after SCI.SIGNIFICANCE STATEMENT TRPV4, a calcium-permeable nonselective cation channel, is widely expressed in both excitable and nonexcitable cells. Spinal cord injury (SCI) majorly caused by trauma/accidents is associated with changes in osmolarity, mechanical injury, and shear stress. After SCI, TRPV4 was increased and were found to be linked with the severity of injury at the epicenter at the time points that were reported to be critical for repair/treatment. Activation of TRPV4 was damaging to endothelial cells that form the blood-spinal cord barrier and thus contributes to scarring (glial and fibrotic). Importantly, inhibition/knockdown of TRPV4 prevented these effects. Thus, the manipulation of TRPV4 signaling might lead to new therapeutic strategies or combinatorial therapies to protect endothelial cells and enhance repair after SCI.

Structural insights into the gating mechanisms of TRPV channels

Transient Receptor Potential channels from the vanilloid subfamily (TRPV) are a group of cation channels modulated by a variety of endogenous stimuli as well as a range of natural and synthetic compounds. Their roles in human health make them of keen interest, particularly from a pharmacological perspective. However, despite this interest, the complexity of these channels has made it difficult to obtain high resolution structures until recently. With the cryo-EM resolution revolution, TRPV channel structural biology has blossomed to produce dozens of structures, covering every TRPV family member and a variety of approaches to examining channel modulation. Here, we review all currently available TRPV structures and the mechanistic insights into gating that they reveal.

Volume sensing in the transient receptor potential vanilloid 4 ion channel is cell type-specific and mediated by an N-terminal volume-sensing domain

Many retinal diseases are associated with pathological cell swelling, but the underlying etiology remains to be established. A key component of the volume-sensitive machinery, the transient receptor potential vanilloid 4 (TRPV4) ion channel, may represent a sensor and transducer of cell swelling, but the molecular link between the swelling and TRPV4 activation is unresolved. Here, our results from experiments using electrophysiology, cell volumetric measurements, and fluorescence imaging conducted in murine retinal cells and Xenopus oocytes indicated that cell swelling in the physiological range activated TRPV4 in Müller glia and Xenopus oocytes, but required phospholipase A₂ (PLA₂) activity exclusively in Müller cells. Volume-dependent TRPV4 gating was independent of cytoskeletal rearrangements and phosphorylation. Our findings also revealed that TRPV4-mediated transduction of volume changes is dependent by its N terminus, more specifically by its distal-most part. We conclude that the volume sensitivity and function of TRPV4 in situ depend critically on its functional and cell type-specific interactions.

Homozygous TRPV4 mutation causes congenital distal spinal muscular atrophy and arthrogryposis

Objective

To identify the genetic cause of disease in a form of congenital spinal muscular atrophy and arthrogryposis (CSMAA).

Methods

A 2-year-old boy was diagnosed with arthrogryposis multiplex congenita, severe skeletal abnormalities, torticollis, vocal cord paralysis, and diminished lower limb movement. Whole-exome sequencing (WES) was performed on the proband and family members. In silico modeling of protein structure and heterologous protein expression and cytotoxicity assays were performed to validate pathogenicity of the identified variant.

Results

WES revealed a homozygous mutation in the TRPV4 gene (c.281C>T; p.S94L). The identification of a recessive mutation in TRPV4 extends the spectrum of mutations in recessive forms of the TRPV4-associated disease. p.S94L and other previously identified TRPV4 variants in different protein domains were compared in structural modeling and functional studies. In silico structural modeling suggests that the p.S94L mutation is in the disordered N-terminal region proximal to important regulatory binding sites for phosphoinositides and for PACSIN3, which could lead to alterations in trafficking and/or channel sensitivity. Functional studies by Western blot and immunohistochemical analysis show that p.S94L increased TRPV4 activity-based cytotoxicity and resultant decreased TRPV4 expression levels, therefore involves a gain-of-function mechanism.

Conclusions

This study identifies a novel homozygous mutation in TRPV4 as a cause of the recessive form of CSMAA.

Recurrent activations of transient receptor potential vanilloid-1 and vanilloid-4 promote cellular proliferation and migration in esophageal squamous cell carcinoma cells

Some members of the transient receptor potential vanilloid (TRPV) subfamily of cation channels are thermosensitive. Earlier studies have revealed the distribution and functions of these thermo‐TRPVs (TRPV1–4) in various organs, but their expression and function in the human esophagus are not fully understood. Here, we probed for the expression of the thermo‐TRPVs in one nontumor human esophageal squamous cell line and two esophageal squamous cell carcinoma (ESCC) cell lines. TRPV1, TRPV2, and TRPV4 proteins were found to be upregulated in ESCC cells, while TRPV3 was not detectable in any of these cell lines. Subsequently, channel function was evaluated via monitoring of Ca²⁺ transients by Ca²⁺ imaging and nonselective cation channel currents were recorded by whole‐cell patch clamp. We found that TRPV4 was activated by heat at 28 °C–35 °C, whereas TRPV1 and TRPV2 were activated by higher, noxious temperatures (44 °C and 53 °C, respectively). Furthermore, TRPV1 was activated by capsaicin (EC₅₀ = 20.32 μm), and this effect was antagonized by AMG9810; TRPV2 was activated by a newly developed cannabinoid compound, O1821, and inhibited by tranilast. In addition, TRPV4 was activated by hypotonic solutions (220 m Osm), and this effect was abolished by ruthenium red. The effects of TRPV1 and TRPV4 on ESCC were also explored. Our data, for the first time, showed that the overactivation of TRPV1 and TRPV4 promoted the proliferation and/or migration of ESCC cells. In summary, TRPV1, TRPV2, and TRPV4 were functionally expressed in human esophageal squamous cells, and thermo‐TRPVs might play an important role in the development of ESCC.

Structural Basis of TRPV4 N Terminus Interaction with Syndapin/PACSIN1-3 and PIP2

Transient receptor potential (TRP) channels are polymodally regulated ion channels. TRPV4 (vanilloid 4) is sensitized by PIP₂ and desensitized by Syndapin3/PACSIN3, which bind to the structurally uncharacterized TRPV4 N terminus. We determined the nuclear magnetic resonance structure of the Syndapin3/PACSIN3 SH3 domain in complex with the TRPV4 N-terminal proline-rich region (PRR), which binds as a class I polyproline II (PPII) helix. This PPII conformation is broken by a conserved proline in a cis conformation. Beyond the PPII, we find that the proximal TRPV4 N terminus is unstructured, a feature conserved across species thus explaining the difficulties in resolving it in previous structural studies. Syndapin/PACSIN SH3 domain binding leads to rigidification of both the PRR and the adjacent PIP₂ binding site. We determined the affinities of the TRPV4 N terminus for PACSIN1, 2, and 3 SH3 domains and PIP₂ and deduce a hierarchical interaction network where Syndapin/PACSIN binding influences the PIP₂ binding site but not vice versa.

TRPV4: a Sensor for Homeostasis and Pathological Events in the CNS

Transient receptor potential vanilloid type 4 (TRPV4) was originally described as a calcium-permeable nonselective cation channel. TRPV4 is now recognized as a polymodal ionotropic receptor: it is a broadly expressed, nonselective cation channel (permeable to calcium, potassium, magnesium, and sodium) that plays an important role in a multitude of physiological processes. TRPV4 is involved in maintaining homeostasis, serves as an osmosensor and thermosensor, can be activated directly by endogenous or exogenous chemical stimuli, and can be activated or sensitized indirectly via intracellular signaling pathways. Additionally, TRPV4 is upregulated in a variety of pathological conditions. In this review, we focus on the role of TRPV4 in mediating homeostasis and pathological events in the central nervous system (CNS). This review is composed of three parts. Section 1 describes the role of TRPV4 in maintaining homeostatic processes, including the volume of body water, ionic concentrations, volume, and the temperature. Section 2 describes the effects of activation and inhibition of TRPV4 in the CNS. Section 3 focuses on the role of TRPV4 during pathological events in CNS.

The role of TRPV4 in fibrosis

The transient receptor potential vanilloid 4 (TRPV4) is a highly Ca²⁺-permeable non-selective cation channel in TRPV family. Accumulating evidence hints that TRPV4 play a significant role in a wide diversity of pathologic changes. Fibrosis is a kind of chronic disease which was characterized by the formation of excessive accumulation of extracellular matrix (ECM) components in tissues and organs. In recent years, a growing body of studies showed that TRPV4 acted as a crucial regulator in the progression of fibrosis including myocardial fibrosis, cystic fibrosis, pulmonary fibrosis, hepatic fibrosis and pancreatic fibrosis, suggesting TRPV4 may be a potential therapeutic vehicle in fibrotic diseases. However, the mechanisms by which TRPV4 regulates fibrosis are still undefined. In this review, firstly, we intend to sum up the collective knowledge of TRPV4. Then we provided the latent mechanism between TRPV4 and fibrosis. We also elaborated the distinct signaling pathways focus on TRPV4 with fibrosis. Finally, we discussed its potential as a novel therapeutic target for fibrosis.

Drugs Affecting TRP Channels

Chronic obstructive pulmonary disease (COPD) and asthma are both common respiratory diseases that are associated with airflow reduction/obstruction and pulmonary inflammation. Whilst drug therapies offer adequate symptom control for many mild to moderate asthmatic patients, severe asthmatics and COPD patients symptoms are often not controlled, and in these cases, irreversible structural damage occurs with disease progression over time. Transient receptor potential (TRP) channels, in particular TRPV1, TRPA1, TRPV4 and TRPM8, have been implicated with roles in the regulation of inflammation and autonomic nervous control of the lungs. Evidence suggests that inflammation elevates levels of activators and sensitisers of TRP channels and additionally that TRP channel expression may be increased, resulting in excessive channel activation. The enhanced activity of these channels is thought to then play a key role in the propagation and maintenance of the inflammatory disease state and neuronal symptoms such as bronchoconstriction and cough. For TRPM8 the evidence is less clear, but as with TRPV1, TRPA1 and TRPV4, antagonists are being developed by multiple companies for indications including asthma and COPD, which will help in elucidating their role in respiratory disease.

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.

Transient receptor potential vanilloid 4 (TRPV4) silencing in Helicobacter pylori-infected human gastric epithelium

BACKGROUND

Helicobacter pylori (HP) infection induces methylation silencing of specific genes in gastric epithelium. Various stimuli activate the nonselective cation channel TRPV4, which is expressed in gastric epithelium where it detects mechanical stimuli and promotes ATP release. As CpG islands in TRPV4 are methylated in HP-infected gastric epithelium, we evaluated HP infection-dependent changes in TRPV4 expression in gastric epithelium.

MATERIALS AND METHODS

Human gastric biopsy samples, a human gastric cancer cell line (AGS), and a normal gastric epithelial cell line (GES-1) were used to detect TRPV4 mRNA and protein expression by RT-PCR and Western blotting, respectively. Ca²⁺ imaging was used to evaluate TRPV4 ion channel activity. TRPV4 methylation status was assessed by methylation-specific PCR (MSP). ATP release was measured by a luciferin-luciferase assay.

RESULTS

TRPV4 mRNA and protein were detected in human gastric biopsy samples and in GES-1 cells. MSP and demethylation assays showed TRPV4 methylation silencing in AGS cells. HP coculture directly induced methylation silencing of TRPV4 in GES-1 cells. In human samples, HP infection was associated with TRPV4 methylation silencing that recovered after HP eradication in a time-dependent manner.

CONCLUSION

HP infection-dependent DNA methylation suppressed TRPV4 expression in human gastric epithelia, suggesting that TRPV4 methylation may be involved in HP-associated dyspepsia.

TRPV4 regulates insulin mRNA expression and INS-1E cell death via ERK1/2 and NO-dependent mechanisms

TRPV4 is a Ca²⁺-permeable, nonselective cation channel. Recently, TRPV4 was implicated in controlling peripheral insulin sensitivity, insulin secretion and apoptosis of pancreatic beta cells. Here, we characterize the role and potential mechanisms of TRPV4 in regulating insulin mRNA expression and cell death in insulin producing INS-1E cells and rat pancreatic islets. TRPV4 protein production was downregulated by siRNA. Intracellular calcium level was measured using Fluo-3 AM. Gene expression was studied by real-time PCR. Phosphorylation of extracellular signal-regulated kinase (ERK1 and ERK2) was detected by Western blot. Nitric oxide (NO) production was assessed by chemiluminescent reaction. Reactive oxygen species (ROS) level was analysed using a fluorogenic dye (DCFDA). Cell death was evaluated by determination of cytoplasmic histone-associated DNA fragments. Downregulation of TRPV4 neither affected insulin mRNA expression nor INS-1E cell growth. By contrast, pharmacological TRPV4 activation by 100nmol/l GSK1016790A increased Ca²⁺ levels in INS-1E cells and enhanced insulin mRNA expression after 1 and 3h, whereas a suppression of insulin mRNA expression was detected after 24h incubation. GSK1016790A increased ERK1/2 phosphorylation and NO production but not ROS production. Pharmacological blockade of ERK1/2 attenuated GSK1016790A-induced insulin mRNA expression. Inhibition of NO synthesis by l-NAME failed to affect insulin mRNA expression in GSK1016790A treated INS-1E cells. Furthermore, inhibition of NO production attenuated GSK1016790A-induced INS-1E cell death. In pancreatic islets, 100nmol/l GSK1016790A increased insulin mRNA levels after 3h without inducing cytotoxicity after 24h. In conclusion, TRPV4 differently regulates insulin mRNA expression in INS-1E cells via ERK1/2 and NO-dependent mechanisms.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

Deciphering physiological role of the mechanosensitive TRPV4 channel in the distal nephron

Long-standing experimental evidence suggests that epithelial cells in the renal tubule are able to sense osmotic and pressure gradients caused by alterations in ultrafiltrate flow by elevating intracellular Ca(2+) concentration. These responses are viewed as critical regulators of a variety of processes ranging from transport of water and solutes to cellular growth and differentiation. A loss in the ability to sense mechanical stimuli has been implicated in numerous pathologies associated with systemic imbalance of electrolytes and to the development of polycystic kidney disease. The molecular mechanisms conferring mechanosensitive properties to epithelial tubular cells involve activation of transient receptor potential (TRP) channels, such as TRPV4, allowing direct Ca(2+) influx to increase intracellular Ca(2+) concentration. In this review, we critically analyze the current evidence about signaling determinants of TRPV4 activation by luminal flow in the distal nephron and discuss how dysfunction of this mechanism contributes to the progression of polycystic kidney disease. We also review the physiological relevance of TRPV4-based mechanosensitivity in controlling flow-dependent K(+) secretion in the distal renal tubule.

TRPV4 participates in the establishment of trailing adhesions and directional persistence of migrating cells

Calcium signaling participates in different cellular processes leading to cell migration. TRPV4, a non-selective cation channel that responds to mechano-osmotic stimulation and heat, is also involved in cell migration. However, the mechanistic involvement of TRPV4 in cell migration is currently unknown. We now report that expression of the mutant channel TRPV4-(121)AAWAA (lacking the phosphoinositide-binding site (121)KRWRK(125) and the response to physiological stimuli) altered HEK293 cell migration. Altered migration patterns included periods of fast and persistent motion followed by periods of stalling and turning, and the extension of multiple long cellular protrusions. TRPV4-WT overexpressing cells showed almost complete loss of directionality with frequent turns, no progression, and absence of long protrusions. Traction microscopy revealed higher tractions forces in the tail of TRPV4-(121)AAWAA than in TRPV4-WT expressing cells. These results are consistent with a defective and augmented tail retraction in TRPV4-(121)AAWAA- and TRPV4-WT-expressing cells, respectively. The activity of calpain, a protease implicated in focal adhesion (FA) disassembly, was decreased in TRPV4-(121)AAWAA compared with TRPV4-WT-expressing cells. Consistently, larger focal adhesions were seen in TRPV4-(121)AAWAA compared with TRPV4-WT-expressing HEK293 cells, a result that was also reproduced in T47D and U87 cells. Similarly, overexpression of the pore-dead mutant TRPV4-M680D resumed the TRPV4-(121)AAWAA phenotype presenting larger FA. The migratory phenotype obtained in HEK293 cells overexpressing TRPV4-(121)AAWAA was mimicked by knocking-down TRPC1, a cationic channel that participates in cell migration. Together, our results point to the participation of TRPV4 in the dynamics of trailing adhesions, a function that may require the interplay of TRPV4 with other cation channels or proteins present at the FA sites.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.

TRPV4: physiological role and therapeutic potential in respiratory diseases

Members of the family of transient receptor potential (TRP) channels have been implicated in the pathophysiology of a host of lung diseases. The role of these multimodal cation channels in lung homeostasis is thought to stem from their ability to respond to changes in mechanical stimuli (i.e., shear and stretch), as well as to various protein and lipid mediators. The vanilloid subfamily member, TRPV4, which is highly expressed in the majority of lung cell types, is well positioned for critical involvement in several pulmonary conditions, including edema formation, control of pulmonary vascular tone, and the lung response to local or systemic inflammatory insults. In recent years, several pharmacological inhibitors of TRPV4 have been developed, and the current generation of compounds possess high affinity and specificity for TRPV4. As such, we have now entered a time where the therapeutic potential of TRPV4 inhibitors can be systematically examined in a variety of lung diseases. Due to this fact, this review seeks to describe the current state of the art with respect to the role of TRPV4 in pulmonary homeostasis and disease, and to highlight the current and future roles of TRPV4 inhibitors in disease treatment. We will first focus on genera aspects of TRPV4 structure and function, and then will discuss known roles for TRPV4 in pulmonary diseases, including pulmonary edema formation, pulmonary hypertension, and acute lung injury. Finally, both promising aspects and potential pitfalls of the clinical use of TRPV4 inhibitors will be examined.