Role of Nonneuronal TRPV4 Signaling in Inflammatory Processes

Structure- and Ligand-Based Virtual Screening for Identification of Novel TRPV4 Antagonists

Transient receptor potential vanilloid (TRPV) 4 is involved in signaling pathways specifically mediating pain and inflammation, making it a promising target for the treatment of various painful and inflammatory conditions. However, only one drug candidate targeting TRPV4 has entered the clinical trials. To identify potential TRPV4 inhibitors for drug development, we screened a library of ion channel-modulating compounds using both structure- and ligand-based virtual screening approaches. Since a high-resolution experimental structure of the human TRPV4 (hTRPV4) was not available during this study, we used a comparative model of hTRPV4 for the structure-based screening by molecular docking. The ligand-based virtual screening was performed using the pharmacophoric features of two known TRPV4 antagonists. Five potential hits were selected based on either the binding stability or the pharmacophore match, and their effect on hTRPV4 was tested using a FLIPRtetra assay. All tested compounds inhibited hTRPV4 at 30 µM, with compound Z1213735368 showing an IC50 of 8 µM at a concentration of 10 µM. Furthermore, natural stilbenoids, known to modulate other TRP channels, were evaluated for their hTRPV4 binding and inhibitory potential. The findings provide insight into the structural determinants of hTRPV4 modulation and may facilitate further efforts in developing therapeutic hTRPV4 ligands.

Tension at the gate: sensing mechanical forces at the blood-brain barrier in health and disease

Microvascular brain endothelial cells tightly limit the entry of blood components and peripheral cells into the brain by forming the blood-brain barrier (BBB). The BBB is regulated by a cascade of mechanical and chemical signals including shear stress and elasticity of the adjacent endothelial basement membrane (BM). During physiological aging, but especially in neurological diseases including multiple sclerosis (MS), stroke, small vessel disease, and Alzheimer's disease (AD), the BBB is exposed to inflammation, rigidity changes of the BM, and disturbed cerebral blood flow (CBF). These altered forces lead to increased vascular permeability, reduced endothelial reactivity to vasoactive mediators, and promote leukocyte transmigration. Whereas the molecular players involved in leukocyte infiltration have been described in detail, the importance of mechanical signalling throughout this process has only recently been recognized. Here, we review relevant features of mechanical forces acting on the BBB under healthy and pathological conditions, as well as the endothelial mechanosensory elements detecting and responding to altered forces. We demonstrate the underlying complexity by focussing on the family of transient receptor potential (TRP) ion channels. A better understanding of these processes will provide insights into the pathogenesis of several neurological disorders and new potential leads for treatment.

Very Low-Intensity Ultrasound Facilitates Glymphatic Influx and Clearance via Modulation of the TRPV4-AQP4 Pathway

Recently, the glymphatic system has been proposed as a mechanism for waste clearance from the brain parenchyma. Glymphatic dysfunction has previously been shown to be associated with several neurological diseases, including Alzheimer's disease, traumatic brain injury, and stroke. As such, it may serve as an important target for therapeutic interventions. In the present study, very low-intensity ultrasound (VLIUS) (center frequency, 1 MHz; pulse repetition frequency, 1 kHz; duty factor, 1%; spatial peak temporal average intensity [Ispta] = 3.68 mW cm2; and duration, 5 min) is found to significantly enhance the influx of cerebrospinal fluid tracers into the paravascular spaces of the brain, and further facilitate interstitial substance clearance from the brain parenchyma, including exogenous β-amyloid. Notably, no evidence of brain damage is observed following VLIUS stimulation. VLIUS may enhance glymphatic influx via the transient receptor potential vanilloid-4-aquaporin-4 pathway in astrocytes. This mechanism may provide insights into VLIUS-regulated glymphatic function that modifies the natural course of central nervous system disorders related to waste clearance dysfunction.

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.

A Lipidomics- and Transcriptomics-Based Analysis of the Intestine of Genetically Obese (ob/ob) and Diabetic (db/db) Mice: Links with Inflammation and Gut Microbiota

Obesity is associated with a cluster of metabolic disorders, chronic low-grade inflammation, altered gut microbiota, increased intestinal permeability, and alterations of the lipid mediators of the expanded endocannabinoid (eCB) signaling system, or endocannabinoidome (eCBome). In the present study, we characterized the profile of the eCBome and related oxylipins in the small and large intestines of genetically obese (ob/ob) and diabetic (db/db) mice to decipher possible correlations between these mediators and intestinal inflammation and gut microbiota composition. Basal lipid and gene expression profiles, measured by LC/MS-MS-based targeted lipidomics and qPCR transcriptomics, respectively, highlighted a differentially altered intestinal eCBome and oxylipin tone, possibly linked to increased mRNA levels of inflammatory markers in db/db mice. In particular, the duodenal levels of several 2-monoacylglycerols and N-acylethanolamines were increased and decreased, respectively, in db/db mice, which displayed more pronounced intestinal inflammation. To a little extent, these differences were explained by changes in the expression of the corresponding metabolic enzymes. Correlation analyses suggested possible interactions between eCBome/oxylipin mediators, cytokines, and bacterial components and bacterial taxa closely related to intestinal inflammation. Collectively, this study reveals that db/db mice present a higher inflammatory state in the intestine as compared to ob/ob mice, and that this difference is associated with profound and potentially adaptive or maladaptive, and partly intestinal segment-specific alterations in eCBome and oxylipin signaling. This study opens the way to future investigations on the biological role of several poorly investigated eCBome mediators and oxylipins in the context of obesity and diabetes-induced gut dysbiosis and inflammation.

Localization of TRP Channels in Healthy Oral Mucosa from Human Donors

The oral cavity is exposed to a remarkable range of noxious and innocuous conditions, including temperature fluctuations, mechanical forces, inflammation, and environmental and endogenous chemicals. How such changes in the oral environment are sensed is not completely understood. Transient receptor potential (TRP) ion channels are a diverse family of molecular receptors that are activated by chemicals, temperature changes, and tissue damage. In non-neuronal cells, TRP channels play roles in inflammation, tissue development, and maintenance. In somatosensory neurons, TRP channels mediate nociception, thermosensation, and chemosensation. To assess whether TRP channels might be involved in environmental sensing in the human oral cavity, we investigated their distribution in human tongue and hard palate biopsies. TRPV3 and TRPV4 were expressed in epithelial cells with inverse expression patterns where they likely contribute to epithelial development and integrity. TRPA1 immunoreactivity was present in fibroblasts, immune cells, and neuronal afferents, consistent with known roles of TRPA1 in sensory transduction and response to damage and inflammation. TRPM8 immunoreactivity was found in lamina propria and neuronal subpopulations including within the end bulbs of Krause, consistent with a role in thermal sensation. TRPV1 immunoreactivity was identified in intraepithelial nerve fibers and end bulbs of Krause, consistent with roles in nociception and thermosensation. TRPM8 and TRPV1 immunoreactivity in end bulbs of Krause suggest that these structures contain a variety of neuronal afferents, including those that mediate nociception, thermosensation, and mechanotransduction. Collectively, these studies support the role of TRP channels in oral environmental surveillance and response.

Lipopolysaccharide-Induced Model of Neuroinflammation: Mechanisms of Action, Research Application and Future Directions for Its Use

Despite advances in antimicrobial and anti-inflammatory therapies, inflammation and its consequences still remain a significant problem in medicine. Acute inflammatory responses are responsible for directly life-threating conditions such as septic shock; on the other hand, chronic inflammation can cause degeneration of body tissues leading to severe impairment of their function. Neuroinflammation is defined as an inflammatory response in the central nervous system involving microglia, astrocytes, and cytokines including chemokines. It is considered an important cause of neurodegerative diseases, such as Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. Lipopolysaccharide (LPS) is a strong immunogenic particle present in the outer membrane of Gram-negative bacteria. It is a major triggering factor for the inflammatory cascade in response to a Gram-negative bacteria infection. The use of LPS as a strong pro-inflammatory agent is a well-known model of inflammation applied in both in vivo and in vitro studies. This review offers a summary of the pathogenesis associated with LPS exposure, especially in the field of neuroinflammation. Moreover, we analyzed different in vivo LPS models utilized in the area of neuroscience. This paper presents recent knowledge and is focused on new insights in the LPS experimental model.

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

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

Mini-review: Dissecting receptor-mediated stimulation of TRPV4 in nociceptive and inflammatory pathways

Transient Receptor Potential Vanilloid 4 (TRPV4) is a polymodal, non-selective cation channel that detects thermal, mechanical, and environmental cues and contributes to a range of diverse physiological processes. The effects of chronic TRPV4 stimulation and gain-of-function genetic mutations suggest that TRPV4 may also be a valuable therapeutic target for pathophysiological events including neurogenic inflammation, peripheral neuropathies, and impaired wound healing. There has been significant interest in defining how and where TRPV4 may promote inflammation and pain. Endogenous stimuli such as osmotic stress and lipid binding are established TRPV4 activators. The TRP channel family is also well-known to be controlled by 'receptor-operated' pathways. For example, G protein-coupled receptors (GPCRs) expressed by primary afferent neurons or other cells in inflammatory pathways utilize TRPV4 as an effector protein to amplify nociceptive and inflammatory signaling. Contributing to disorders including arthritis, neuropathies, and pulmonary edema, GPCRs such as the protease-activated receptor PAR2 mediate activation of kinase signaling cascades to increase TRPV4 phosphorylation, resulting in sensitization and enhanced neuronal excitability. Phospholipase activity also leads to production of polyunsaturated fatty acid lipid mediators that directly activate TRPV4. Consistent with the contribution of TRPV4 to disease, pharmacological inhibition or genetic ablation of TRPV4 can diminish receptor-mediated inflammatory events. This review outlines how receptor-mediated signaling is a major endogenous driver of TRPV4 gating and discusses key signaling pathways and emerging TRPV4 modulators such as the mechanosensitive Piezo1 ion channel. A collective understanding of how endogenous stimuli can influence TRPV4 function is critical for future therapeutic endeavors to modulate this channel.

Innate Immune Signaling and Role of Glial Cells in Herpes Simplex Virus- and Rabies Virus-Induced Encephalitis

The environment of the central nervous system (CNS) represents a double-edged sword in the context of viral infections. On the one hand, the infectious route for viral pathogens is restricted via neuroprotective barriers; on the other hand, viruses benefit from the immunologically quiescent neural environment after CNS entry. Both the herpes simplex virus (HSV) and the rabies virus (RABV) bypass the neuroprotective blood-brain barrier (BBB) and successfully enter the CNS parenchyma via nerve endings. Despite the differences in the molecular nature of both viruses, each virus uses retrograde transport along peripheral nerves to reach the human CNS. Once inside the CNS parenchyma, HSV infection results in severe acute inflammation, necrosis, and hemorrhaging, while RABV preserves the intact neuronal network by inhibiting apoptosis and limiting inflammation. During RABV neuroinvasion, surveilling glial cells fail to generate a sufficient type I interferon (IFN) response, enabling RABV to replicate undetected, ultimately leading to its fatal outcome. To date, we do not fully understand the molecular mechanisms underlying the activation or suppression of the host inflammatory responses of surveilling glial cells, which present important pathways shaping viral pathogenesis and clinical outcome in viral encephalitis. Here, we compare the innate immune responses of glial cells in RABV- and HSV-infected CNS, highlighting different viral strategies of neuroprotection or Neuroinflamm. in the context of viral encephalitis.

Trpv4 regulates Nlrp3 inflammasome via SIRT1/PGC-1α pathway in a cuprizone-induced mouse model of demyelination

Increasing evidence has demonstrated that the Nod-like receptor pyrin domain containing 3 (Nlrp3) inflammasome overactivated during demyelinating disorders. It has been implicated that transient receptor potential type 4 (Trpv4) is regarded as a polymodal ionotropic receptor that plays an important role in a multitude of pathological conditions, including inflammation. The aim of this study was to investigate whether the Trpv4 channel regulates Nlrp3 inflammasome in the corpus callosum of mice with demyelination. Our results showed that CPZ treatment significantly increased the expression of Trpv4, activated Nlrp3 inflammasome, reduced peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) and decreased mitochondrial function. siRNA-mediated Nlrp3 knockdown inhibited glial activation and alleviated demyelination. Whereas knockdown of Trpv4 by siRNA markedly ameliorated Nlrp3 inflammasome activation and restored mitochondrial function as well as reducing the level of reactive oxygen species (ROS). Meanwhile, glial activation, demyelination and behavioral impairment induced by CPZ were also alleviated by siRNA-mediated Trpv4 knockdown. Furthermore, immunoprecipitation and use of a lysine acetylation assay showed that Sirtuin1 (SIRT1) mediated the PGC-1α deacetylation, which is involved in Nlrp3 inflammasome activation. These findings suggest that Trpv4 regulates mitochondrial function through the SIRT1/PGC-1α pathway, which further trigger Nlrp3 inflammasome activation in the CPZ-induced demyelination in mice.

Integrated Transcriptomic and Proteomic Analysis of Human Eccrine Sweat Glands Identifies Missing and Novel Proteins

The eccrine sweat gland is an exocrine gland that is involved in the secretion of sweat for control of temperature. Malfunction of the sweat glands can result in disorders such as miliaria, hyperhidrosis and bromhidrosis. Understanding the transcriptome and proteome of sweat glands is important for understanding their physiology and role in diseases. However, no systematic transcriptome or proteome analysis of sweat glands has yet been reported. Here, we isolated eccrine sweat glands from human skin by microdissection and performed RNA-seq and proteome analysis. In total, ∼138,000 transcripts and ∼6,100 proteins were identified. Comparison of the RNA-seq data of eccrine sweat glands to other human tissues revealed the closest resemblance to the cortex region of kidneys. The proteome data showed enrichment of proteins involved in secretion, reabsorption, and wound healing. Importantly, protein level identification of the calcium ion channel TRPV4 suggests the importance of eccrine sweat glands in re-epithelialization of wounds and prevention of dehydration. We also identified 2 previously missing proteins from our analysis. Using a proteogenomic approach, we identified 7 peptides from 5 novel genes, which we validated using synthetic peptides. Most of the novel proteins were from short open reading frames (sORFs) suggesting that many sORFs still remain to be annotated in the human genome. This study presents the first integrated analysis of the transcriptome and proteome of the human eccrine sweat gland and would become a valuable resource for studying sweat glands in physiology and disease.

TRPV4-induced inflammatory response is involved in neuronal death in pilocarpine model of temporal lobe epilepsy in mice

Activation of transient receptor potential vanilloid 4 (TRPV4) induces neuronal injury. TRPV4 activation enhances inflammatory response and promotes the proinflammatory cytokine release in various types of tissue and cells. Hyperneuroinflammation contributes to neuronal damage in epilepsy. Herein, we examined the contribution of neuroinflammation to TRPV4-induced neurotoxicity and its involvement in the inflammation and neuronal damage in pilocarpine model of temporal lobe epilepsy in mice. Icv. injection of TRPV4 agonist GSK1016790A (GSK1016790A-injected mice) increased ionized calcium binding adapter molecule-1 (Iba-1) and glial fibrillary acidic protein (GFAP) protein levels and Iba-1-positive (Iba-1⁺) and GFAP-positive (GFAP⁺) cells in hippocampi, which indicated TRPV4-induced microglial cell and astrocyte activation. The protein levels of nucleotide-binding oligomerization domain-like receptor pyrin domain containing 3 (NLRP3) inflammasome components NLRP3, apoptosis-related spotted protein (ASC) and cysteinyl aspartate-specific protease-1 (caspase-1) were increased in GSK1016790A-injected mice, which indicated NLRP3 inflammasome activation. GSK1016790A also increased proinflammatory cytokine IL-1β, TNF-α and IL-6 protein levels, which were blocked by caspase-1 inhibitor Ac-YVAD-cmk. GSK1016790A-induced neuronal death was attenuated by Ac-YVAD-cmk. Icv. injection of TRPV4-specific antagonist HC-067047 markedly increased the number of surviving cells 3 d post status epilepticus in pilocarpine model of temporal lobe epilepsy in mice (pilocarpine-induced status epilepticus, PISE). HC-067047 also markedly blocked the increase in Iba-1 and GFAP protein levels, as well as Iba-1⁺ and GFAP⁺ cells 3 d post-PISE. Finally, the increased protein levels of NLRP3, ASC and caspase-1 as well as IL-1β, TNF-α and IL-6 were markedly blocked by HC-067047. We conclude that TRPV4-induced neuronal death is mediated at least partially by enhancing the neuroinflammatory response, and this action is involved in neuronal injury following status epilepticus.

The TRPV4 Agonist GSK1016790A Regulates the Membrane Expression of TRPV4 Channels

TRPV4 is a non-selective cation channel that tunes the function of different tissues including the vascular endothelium, lung, chondrocytes, and neurons. GSK1016790A is the selective and potent agonist of TRPV4 and a pharmacological tool that is used to study the TRPV4 physiological function in vitro and in vivo. It remains unknown how the sensitivity of TRPV4 to this agonist is regulated. The spatial and temporal dynamics of receptors are the major determinants of cellular responses to stimuli. Membrane translocation has been shown to control the response of several members of the transient receptor potential (TRP) family of ion channels to different stimuli. Here, we show that TRPV4 stimulation with GSK1016790A caused an increase in [Ca²⁺]_i that is stable for a few minutes. Single molecule analysis of TRPV4 channels showed that the density of TRPV4 at the plasma membrane is controlled through two modes of membrane trafficking, complete, and partial vesicular fusion. Further, we show that the density of TRPV4 at the plasma membrane decreased within 20 min, as they translocate to the recycling endosomes and that the surface density is dependent on the release of calcium from the intracellular stores and is controlled via a PI3K, PKC, and RhoA signaling pathway.

TRPV4 Inhibition Improved Myelination and Reduced Glia Reactivity and Inflammation in a Cuprizone-Induced Mouse Model of Demyelination

The inhibition of demyelination and the promotion of remyelination are both considerable challenges in the therapeutic process for many central nervous system (CNS) diseases. Increasing evidence has demonstrated that neuroglial activation and neuroinflammation are responsible for myelin sheath damage during demyelinating disorders. It has been revealed that the nonselective cation channel transient receptor potential vanilloid 4 (TRPV4) profoundly affects a variety of physiological processes, including inflammation. However, its roles and mechanisms in demyelination have remained unclear. Here, for the first time, we found that there was a significant increase in TRPV4 in the corpus callosum in a demyelinated mouse model induced by cuprizone (CPZ). RN-1734, a TRPV4-antagonist, clearly alleviated demyelination and inhibited glial activation and the production of tumor necrosis factor α (TNF-α) and interleukin 1β (IL-1β) without altering the number of olig2-positive cells. In vitro, RN-1734 treatment clearly inhibited the influx of calcium and decreased the levels of IL-1β and TNF-α in lipopolysaccharide (LPS)-activated microglial cells by suppressing NF-κB P65 phosphorylation. Apoptosis of oligodendrocyte induced by LPS-activated microglia was also alleviated by RN-1734. The results suggest that activation of TRPV4 in microglia is involved in oligodendrocyte apoptosis through the activation of the NF-κB signaling pathway, thus revealing a new mechanism of CNS demyelination.

TRP Channels as Sensors of Bacterial Endotoxins

The cellular and systemic effects induced by bacterial lipopolysaccharides (LPS) have been solely attributed to the activation of the Toll-like receptor 4 (TLR4) signalling cascade. However, recent studies have shown that LPS activates several members of the Transient Receptor Potential (TRP) family of cation channels. Indeed, LPS induces activation of the broadly-tuned chemosensor TRPA1 in sensory neurons in a TLR4-independent manner, and genetic ablation of this channel reduced mouse pain and inflammatory responses triggered by LPS and the gustatory-mediated avoidance to LPS in fruit flies. LPS was also shown to activate TRPV4 channels in airway epithelial cells, an effect leading to an immediate production of bactericidal nitric oxide and to an increase in ciliary beat frequency. In this review, we discuss the role of TRP channels as sensors of bacterial endotoxins, and therefore, as crucial players in the timely detection of invading gram-negative bacteria.