TRPA1 and TRPV1 are differentially involved in heat nociception of mice

TRPV1 Channels in the Central Nervous System as Drug Targets

TRPV1 channels are polymodal cation channels located predominantly on primary afferent neurons that are activated by inflammatory mediators, capsaicin (the active component in chili peppers), and noxious heat. TRPV1 channel antagonists are potential new analgesic agents, but their development has been hindered by the finding that they also produce loss of thermal homeostasis and response to noxious heat. Results from recent studies of the TRPV1 channel indicate that it might be possible to develop TRPV1 channel antagonists that inhibit pain without affecting noxious heat sensation. TRPV1 channels are also present in the central nervous system (CNS) and have been implicated in learning, memory, and behaviour. TRPV1 channel modulators have been proposed to have possible therapeutic potential in the treatment of neurological and psychiatric conditions. However, further understanding of the role of TRPV1 channels in the CNS is required before therapeutic advances in the treatment of neuropsychiatric conditions with TRPV1 channel modulators can be made.

Role of thermosensitive transient receptor potential (TRP) channels in thermal preference of male and female mice

Transient Receptor Potential (TRP) ion channels are important for sensing environmental temperature. In rodents, TRPV4 senses warmth (25-34 °C), TRPV1 senses heat (>42 °C), TRPA1 putatively senses cold (<17 °C), and TRPM8 senses cool-cold (18-26 °C). We investigated if knockout (KO) mice lacking these TRP channels exhibited changes in thermal preference. Thermal preference was tested using a dual hot-cold plate with one thermoelectric surface set at 30 °C and the adjacent surface at a temperature of 15-45 °C in 5 °C increments. Blinded observers counted the number of times mice crossed through an opening between plates and the percentage of time spent on the 30 °C plate. In a separate experiment, observers blinded as to genotype also assessed the temperature at the location on a thermal gradient (1.83 m, 4-50 °C) occupied by the mouse at 5- or 10-min intervals over 2 h. Male and female wildtype mice preferred 30 °C and significantly avoided colder (15-20 °C) and hotter (40-45 °C) temperatures. Male TRPV1KOs and TRPA1KOs, and TRPV4KOs of both sexes, were similar, while female WTs, TRPV1KOs, TRPA1KOs and TRPM8KOs did not show significant thermal preferences across the temperature range. Male and female TRPM8KOs did not significantly avoid the coldest temperatures. Male mice (except for TRPM8KOs) exhibited significantly fewer plate crossings at hot and cold temperatures and more crossings at thermoneutral temperatures, while females exhibited a similar but non-significant trend. Occupancy temperatures along the thermal gradient exhibited a broad distribution that shrank somewhat over time. Mean occupancy temperatures (recorded at 90-120 min) were significantly higher for females (30-34 °C) compared to males (26-27 °C) of all genotypes, except for TRPA1KOs which exhibited no sex difference. The results indicate (1) sex differences with females (except TRPA1KOs) preferring warmer temperatures, (2) reduced thermosensitivity in female TRPV1KOs, and (3) reduced sensitivity to cold and innocuous warmth in male and female TRPM8KOs consistent with previous studies.

NCS-1 protein regulates TRPA1 channel through the PI3K pathway in breast cancer and neuronal cells

The physical and functional interaction between transient receptor potential channel ankyrin 1 (TRPA1) and neuronal calcium sensor 1 (NCS-1) was assessed. NCS-1 is a calcium (Ca2+) sensor found in many tissues, primarily neurons, and TRPA1 is a Ca2+ channel involved not only in thermal and pain sensation but also in conditions such as cancer and chemotherapy-induced peripheral neuropathy, in which NCS-1 is also a regulatory component.We explored the interactions between these two proteins by employing western blot, qRT-PCR, co-immunoprecipitation, Ca2+ transient monitoring with Fura-2 spectrophotometry, and electrophysiology assays in breast cancer cells (MDA-MB-231) with different levels of NCS-1 expression and neuroblastoma cells (SH-SY5Y).Our findings showed that the expression of TRPA1 was directly correlated with NCS-1 levels at both the protein and mRNA levels. Additionally, we found a physical and functional association between these two proteins. Physically, the NCS-1 and TRPA1 co-immunoprecipitate. Functionally, NCS-1 enhanced TRPA1-dependent Ca2+ influx, current density, open probability, and conductance, where the functional effects depended on PI3K. Conclusion: NCS-1 appears to act not only as a Ca2+ sensor but also modulates TRPA1 protein expression and channel function in a direct fashion through the PI3K pathway. These results contribute to understanding how Ca2+ homeostasis is regulated and provides a mechanism underlying conditions where Ca2+ dynamics are compromised, including breast cancer. With a cellular pathway identified, targeted treatments can be developed for breast cancer and neuropathy, among other related diseases.

Higenamine inhibits acute and chronic inflammatory pain through modulation of TRPV4 channels

Pain is the cardinal symptom of many debilitating diseases and results in heavy health and economic burdens worldwide. Asarum (Asarum sieboldii Miq.) is a commonly used analgesic in Chinese medicine. However, the analgesic components and mechanisms of asarum in acute and chronic pain mice model remain unknown. In this study, we first generated asarum water extract and confirmed strong analgesic properties in mice in both the acute thermal and mechanical pain models, as well as in the complete Freund's adjuvant (CFA) induced chronic inflammatory pain model. Second, we identified higenamine as a major component of asarum and found that higenamine significantly inhibited thermal and mechanical induced acute pain and CFA induced chronic inflammatory pain. Then, using Trpv4-/- mice, we found that TRPV4 is necessary for CFA induced thermal and mechanical allodynia, and demonstrated that higenamine analgesia in the CFA model is partly through TRPV4 channel inhibition. Finally, we found that GSK1016790A, a TRPV4 agonist, induced calcium response was significantly inhibited by higenamine in both cultured DRG neurons and TRPV4 transfected HEK293 cells. Consistent with calcium imaging results, higenamine pretreatment also dose-dependently inhibited GSK1016790A induced acute pain. Taken together, our behavior and calcium imaging results demonstrate that the asarum component higenamine inhibits acute and chronic inflammatory pain by modulation of TRPV4 channels.

Human Transient Receptor Potential Ankyrin 1 Channel: Structure, Function, and Physiology

The transient receptor potential ion channel TRPA1 is a Ca2+-permeable nonselective cation channel widely expressed in sensory neurons, but also in many nonneuronal tissues typically possessing barrier functions, such as the skin, joint synoviocytes, cornea, and the respiratory and intestinal tracts. Here, the primary role of TRPA1 is to detect potential danger stimuli that may threaten the tissue homeostasis and the health of the organism. The ability to directly recognize signals of different modalities, including chemical irritants, extreme temperatures, or osmotic changes resides in the characteristic properties of the ion channel protein complex. Recent advances in cryo-electron microscopy have provided an important framework for understanding the molecular basis of TRPA1 function and have suggested novel directions in the search for its pharmacological regulation. This chapter summarizes the current knowledge of human TRPA1 from a structural and functional perspective and discusses the complex allosteric mechanisms of activation and modulation that play important roles under physiological or pathophysiological conditions. In this context, major challenges for future research on TRPA1 are outlined.

The human TRPA1 intrinsic cold and heat sensitivity involves separate channel structures beyond the N-ARD domain

TRP channels sense temperatures ranging from noxious cold to noxious heat. Whether specialized TRP thermosensor modules exist and how they control channel pore gating is unknown. We studied purified human TRPA1 (hTRPA1) truncated proteins to gain insight into the temperature gating of hTRPA1. In patch-clamp bilayer recordings, ∆1-688 hTRPA1, without the N-terminal ankyrin repeat domain (N-ARD), was more sensitive to cold and heat, whereas ∆1-854 hTRPA1, also lacking the S1-S4 voltage sensing-like domain (VSLD), gained sensitivity to cold but lost its heat sensitivity. In hTRPA1 intrinsic tryptophan fluorescence studies, cold and heat evoked rearrangement of VSLD and the C-terminus domain distal to the transmembrane pore domain S5-S6 (CTD). In whole-cell electrophysiology experiments, replacement of the CTD located cysteines 1021 and 1025 with alanine modulated hTRPA1 cold responses. It is proposed that hTRPA1 CTD harbors cold and heat sensitive domains allosterically coupled to the S5-S6 pore region and the VSLD, respectively.

The formalin test does not probe inflammatory pain but excitotoxicity in rodent skin

The most widely used formalin test to screen antinociceptive drug candidates is still apostrophized as targeting inflammatory pain, in spite of strong opposing evidence published. In our rat skin-nerve preparation ex vivo, recording from all classes of sensory single-fibers (n = 32), 30 units were transiently excited by formaldehyde concentrations 1-100 mM applied to receptive fields (RFs) for 3 min, C and Aδ-fibers being more sensitive (1-30 mM) than Aβ-fibers. From 30 mM on, ~1% of the concentration usually injected in vivo, all RFs were defunctionalized and conduction in an isolated sciatic nerve preparation was irreversibly blocked. Thus, formaldehyde, generated a state of 'anesthesia dolorosa' in the RFs in so far as after a quiescent interphase all fibers with unmyelinated terminals developed a second phase of vigorous discharge activity which correlated well in time course and magnitude with published pain-related behaviors. Sural nerve filament recordings in vivo confirmed that higher formalin concentrations (> 42 mM) have to be injected to the skin to induce this second phase of discharge. Patch-clamp and calcium-imaging confirmed TRPA1 as the primary transducer of formaldehyde (10 mM) effects on mouse sensory neurons. However, stimulated CGRP release from isolated skin of TRPA1+/+ and TRPA1-/- mice showed a convergence of the saturating concentration-response curves at 100 mM formaldehyde, which did not occur with nerve and trachea preparations. Finally, skin-nerve recordings from C and Aδ-fibers of TRPA1-/- mice revealed a massive reduction in formaldehyde (30 mM)-evoked discharge. However, the remaining activity was still biphasic, thus confirming additional unspecific excitotoxic actions of the fixative that diffuses along still excitable axons as previously published. The multiplicity of formaldehyde's actions requires extensive discussion and literature review, leading to a fundamental reevaluation of the formalin test.

Nobel somatosensations and pain

The Nobel prices 2021 for Physiology and Medicine have been awarded to David Julius and Ardem Patapoutian "for their discoveries of receptors for temperature and touch", TRPV1 and PIEZO1/2. The present review tells the past history of the capsaicin receptor, covers further selected TRP channels, TRPA1 in particular, and deals with mechanosensitivity in general and mechanical hyperalgesia in particular. Other achievements of the laureates and translational aspects of their work are shortly treated.

Transient receptor potential ankyrin 1 channel: An evolutionarily tuned thermosensor

The discovery of the role of the transient receptor potential ankyrin 1 (TRPA1) channel as a polymodal detector of cold and pain-producing stimuli almost two decades ago catalyzed the consequent identification of various vertebrate and invertebrate orthologues. In different species, the role of TRPA1 has been implicated in numerous physiological functions, indicating that the molecular structure of the channel exhibits evolutionary flexibility. Until very recently, information about the critical elements of the temperature-sensing molecular machinery of thermosensitive ion channels such as TRPA1 had lagged far behind information obtained from mutational and functional analysis. Current developments in single-particle cryo-electron microscopy are revealing precisely how the thermosensitive channels operate, how they might be targeted with drugs, and at which sites they can be critically regulated by membrane lipids. This means that it is now possible to resolve a huge number of very important pharmacological, biophysical and physiological questions in a way we have never had before. In this review, we aim at providing some of the recent knowledge on the molecular mechanisms underlying the temperature sensitivity of TRPA1. We also demonstrate how the search for differences in temperature and chemical sensitivity between human and mouse TRPA1 orthologues can be a useful approach to identifying important domains with a key role in channel activation.

Properties of Thermal Analgesia in a Human Chronic Low Back Pain Model

Purpose

For years, heat has been used for comfort and analgesia is recommended as a first-line therapy in many clinical guidelines. Yet, there are questions that remain about the actual effectiveness of heat for a condition as common as chronic low back pain, and factors such as time of onset, optimal temperature, and duration of effect.

Materials and Methods

A randomized double-blinded controlled trial was designed to compare the analgesic response to heat delivered via pulses at 45°C (experimental group, N=49) to steady heat at 37°C (control group, N=51) in subjects with longstanding low back pain. Treatment lasted 30 minutes with follow-up out to four hours. The hypothesis was that the experimental group would experience a higher degree of analgesia compared to the control group. Time of onset and duration of effect were also measured.

Results

Both groups were similar in average duration of pain (10.3 years). The primary outcome measure was pain reduction at 30 minutes after the end of treatment, using a 10-points numeric pain scale. Reduction in pain was greater for the experimental group than the control group (difference in mean reduction = 0.72, 95% CI 0.15–1.29, p = 0.014). Statistically significant differences in pain levels were observed from the first measure at 5 minutes of treatment through 120 minutes after completion of treatment. Reduction in pain associated movement was greater in the active heat group than the placebo group (p = 0.04).

Conclusion

High-level pulsed heat (45°C) produced significantly more analgesia as compared to steady heat at 37°C at the primary end point and for an additional 2 hours after treatment. The onset of analgesia was rapid, <5 minutes of treatment. The results of this trial provide insight into the mechanisms and properties of thermal analgesia that are not well understood in a chronic low back pain model.

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Proximal C-Terminus Serves as a Signaling Hub for TRPA1 Channel Regulation via Its Interacting Molecules and Supramolecular Complexes

Our understanding of the general principles of the polymodal regulation of transient receptor potential (TRP) ion channels has grown impressively in recent years as a result of intense efforts in protein structure determination by cryo-electron microscopy. In particular, the high-resolution structures of various TRP channels captured in different conformations, a number of them determined in a membrane mimetic environment, have yielded valuable insights into their architecture, gating properties and the sites of their interactions with annular and regulatory lipids. The correct repertoire of these channels is, however, organized by supramolecular complexes that involve the localization of signaling proteins to sites of action, ensuring the specificity and speed of signal transduction events. As such, TRP ankyrin 1 (TRPA1), a major player involved in various pain conditions, localizes into cholesterol-rich sensory membrane microdomains, physically interacts with calmodulin, associates with the scaffolding A-kinase anchoring protein (AKAP) and forms functional complexes with the related TRPV1 channel. This perspective will contextualize the recent biochemical and functional studies with emerging structural data with the aim of enabling a more thorough interpretation of the results, which may ultimately help to understand the roles of TRPA1 under various physiological and pathophysiological pain conditions. We demonstrate that an alteration to the putative lipid-binding site containing a residue polymorphism associated with human asthma affects the cold sensitivity of TRPA1. Moreover, we present evidence that TRPA1 can interact with AKAP to prime the channel for opening. The structural bases underlying these interactions remain unclear and are definitely worth the attention of future studies.

Human and Mouse TRPA1 Are Heat and Cold Sensors Differentially Tuned by Voltage

Transient receptor potential ankyrin 1 channel (TRPA1) serves as a key sensor for reactive electrophilic compounds across all species. Its sensitivity to temperature, however, differs among species, a variability that has been attributed to an evolutionary divergence. Mouse TRPA1 was implicated in noxious cold detection but was later also identified as one of the prime noxious heat sensors. Moreover, human TRPA1, originally considered to be temperature-insensitive, turned out to act as an intrinsic bidirectional thermosensor that is capable of sensing both cold and heat. Using electrophysiology and modeling, we compare the properties of human and mouse TRPA1, and we demonstrate that both orthologues are activated by heat, and their kinetically distinct components of voltage-dependent gating are differentially modulated by heat and cold. Furthermore, we show that both orthologues can be strongly activated by cold after the concurrent application of voltage and heat. We propose an allosteric mechanism that could account for the variability in TRPA1 temperature responsiveness.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

What is new about mild temperature sensing? A review of recent findings

The superfamily of Transient Receptor Potential (TRP) channels is composed by a group of calcium-permeable ionic channels with a generally shared topology. The thermoTRP channels are a subgroup of 11 members, found in the TRPA, TRPV, TRPC, and TRPM subfamilies. Historically, members of this subgroup have been classified as cold, warm or hot-specific temperature sensors. Recently, new experimental results have shown that the role that has been given to the thermoTRPs in thermosensation is not necessarily strict. In addition, it has been shown that these channels activate over temperature ranges, which can have variations depending on the species and the interaction with a specific biological context. Investigation of these interactions could help to elucidate the mechanisms of activation by temperature, which remains uncertain. Abbreviations: Cryo-EM: Cryogenic electron microscopy; DRG: Dorsal root ganglia; H: Human; ROS: Reactive Oxygen Species; TG: Trigeminal ganglia; TRP: Transient Receptor Potential; TRPA: TRP ankyrin; TRPV: TRP vanilloid; TRPC: TRP canonical; TRPM: TRP melastatin.

Antipruritic effects of transient heat stimulation on histaminergic and nonhistaminergic itch

BACKGROUND

Chronic itch is notoriously difficult to treat. Counterstimuli are able to inhibit itch, but this principle is difficult to apply in clinical practice, and the mechanisms behind counterstimulation-induced itch suppression in humans are unclear.

OBJECTIVES

Firstly, to analyse the stimulus-response effects of transient heat stimuli on histaminergic and nonhistaminergic itch, and secondly, to investigate whether the antipruritic effect depends on homotopic (peripheral mediation) or heterotopic (central mediation) counterstimulation relative to the itch provocation site.

METHODS

Eighteen healthy volunteers participated (eight female, mean age 25·7 ± 0·8 years). Itch was evoked on premarked areas of the volar forearms, by either histamine (1% solution) or cowhage (35-40 spicules). In addition to the itch provocations (experiment 1), 5-s homotopic heat stimuli at 32, 40, 45 or 50 °C were applied. In experiment 2, heat stimuli were applied either homotopically, intrasegmentally (next to the provocation site) or extrasegmentally (dorsal forearm). Itch intensity was evaluated throughout the procedures using a digital visual analogue scale.

RESULTS

Homotopic counterstimuli inhibited histaminergic itch by 41·3% at 45 °C (P < 0·01) and by 76·7% at 50 °C (P < 0·001). Cowhage-induced itch was less prone to counterstimulation and was significantly diminished only at 50 °C, by 43·6% (P = 0·009). Counterstimulations applied heterotopically were not able to inhibit itch significantly.

CONCLUSIONS

Itch pathway-specific effects of counterstimuli were observed between homo- and heterotopic stimulation. Histaminergic itch was robustly inhibited by short-term homotopic noxious heat stimuli for up to 10 min. Nonhistaminergic itch was only weakly inhibited. The inhibitory effects exerted by the short-term heat stimuli only occurred following homotopic counterstimulation.

Effect of single-nucleotide polymorphisms in TRPV1 on burning pain and capsaicin sensitivity in Japanese adults

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that is expressed in the sensory neurons and responds to various noxious stimuli including heat and capsaicin. The molecular properties of TRPV1 have been clearly examined; however, there are obvious individual differences in human sensitivity to thermal stimuli and capsaicin. Here, we examined the possibility that different genome sequence of human TRPV1 caused the different sensitivity to heat or capsaicin. The sensitivities to burning pain and capsaicin of Japanese adult subjects were compared with their TRPV1 genome sequence, and we detected 6 single-nucleotide polymorphisms and 11 single-nucleotide polymorphisms related to burning pain and capsaicin sensitivity, respectively. In particular, homozygous I585V, a single-nucleotide polymorphism with amino acid substitution, significantly related to higher capsaicin sensitivity.

TRPA1 channels: molecular sentinels of cellular stress and tissue damage

TRPA1 is a non-selective cation channel expressed in mammalian peripheral pain receptors, with a major role in chemonociception. TRPA1 has also been implicated in noxious cold and mechanical pain sensation. TRPA1 has an ancient origin and plays important functions in lower organisms, including thermotaxis, mechanotransduction and modulation of lifespan. Here we highlight the role of TRPA1 as a multipurpose sensor of harmful signals, including toxic bacterial products and UV light, and as a sensor of stress and tissue damage. Sensing roles span beyond the peripheral nervous system to include major barrier tissues: gut, skin and lung. Tissue injury, environmental irritants and microbial pathogens are danger signals that can threaten the health of organisms. These signals lead to the coordinated activation of the nociceptive and the innate immune system to provide a homeostatic response trying to re-establish physiological conditions including tissue repair. Activation of TRPA1 participates in protective neuroimmune interactions at multiple levels, sensing ROS and bacterial products and triggering the release of neuropeptides. However, an exaggerated response to danger signals is maladaptive and can lead to the development of chronic inflammatory conditions.

TRPV1 dysfunction in cystinosis patients harboring the homozygous 57 kb deletion

Cystinosis is a rare autosomal recessive disorder characterized by lysosomal cystine accumulation due to loss of function of the lysosomal cystine transporter (CTNS). The most common mutation in cystinosis patients of Northern Europe consists of a 57-kb deletion. This deletion not only inactivates the CTNS gene but also extends into the non-coding region upstream of the start codon of the TRPV1 gene, encoding the capsaicin- and heat-sensitive ion channel TRPV1. To evaluate the consequences of the 57-kb deletion on functional TRPV1 expression, we compared thermal, mechanical and chemical sensitivity of cystinosis patients with matched healthy controls. Whereas patients heterozygous for the 57-kb deletion showed normal sensory responses, homozygous subjects exhibited a 60% reduction in vasodilation and pain evoked by capsaicin, as well as an increase in heat detection threshold. Responses to cold, mechanical stimuli or cinnamaldehyde, an agonist of the related nociceptor channel TRPA1, were unaltered. We conclude that cystinosis patients homozygous for the 57-kb deletion exhibit a strong reduction of TRPV1 function, leading to sensory deficiencies akin to the phenotype of TRPV1-deficient mice. These deficits may account for the reported sensory alterations and thermoregulatory deficits in these patients, and provide a paradigm for life-long TRPV1 deficiency in humans.

TRPV1: A Target for Rational Drug Design

Transient Receptor Potential Vanilloid 1 (TRPV1) is a non-selective, Ca²⁺ permeable cation channel activated by noxious heat, and chemical ligands, such as capsaicin and resiniferatoxin (RTX). Many compounds have been developed that either activate or inhibit TRPV1, but none of them are in routine clinical practice. This review will discuss the rationale for antagonists and agonists of TRPV1 for pain relief and other conditions, and strategies to develop new, better drugs to target this ion channel, using the newly available high-resolution structures.

Human TRPA1 is a heat sensor displaying intrinsic U-shaped thermosensitivity

Thermosensitive Transient Receptor Potential (TRP) channels are believed to respond to either cold or heat. In the case of TRP subtype A1 (TRPA1), there seems to be a species-dependent divergence in temperature sensation as non-mammalian TRPA1 is heat-sensitive whereas mammalian TRPA1 is sensitive to cold. It has been speculated but never experimentally proven that TRPA1 and other temperature-sensitive ion channels have the inherent capability of responding to both cold and heat. Here we show that redox modification and ligands affect human TRPA1 (hTRPA1) cold and heat sensing properties in lipid bilayer and whole-cell patch-clamp recordings as well as heat-evoked TRPA1-dependent calcitonin gene-related peptide (CGRP) release from mouse trachea. Studies of purified hTRPA1 intrinsic tryptophan fluorescence, in the absence of lipid bilayer, consolidate hTRPA1 as an intrinsic bidirectional thermosensor that is modified by the redox state and ligands. Thus, the heat sensing property of TRPA1 is conserved in mammalians, in which TRPA1 may contribute to sensing warmth and uncomfortable heat in addition to noxious cold.

Neuronal cell lines as model dorsal root ganglion neurons: A transcriptomic comparison

BACKGROUND

Dorsal root ganglion neuron-derived immortal cell lines including ND7/23 and F-11 cells have been used extensively as in vitro model systems of native peripheral sensory neurons. However, while it is clear that some sensory neuron-specific receptors and ion channels are present in these cell lines, a systematic comparison of the molecular targets expressed by these cell lines with those expressed in intact peripheral neurons is lacking.

RESULTS

In this study, we examined the expression of RNA transcripts in the human neuroblastoma-derived cell line, SH-SY5Y, and two dorsal root ganglion hybridoma cell lines, F-11 and ND7/23, using Illumina next-generation sequencing, and compared the results with native whole murine dorsal root ganglions. The gene expression profiles of these three cell lines did not resemble any specific defined dorsal root ganglion subclass. The cell lines lacked many markers for nociceptive sensory neurons, such as the Transient receptor potential V1 gene, but expressed markers for both myelinated and unmyelinated neurons. Global gene ontology analysis on whole dorsal root ganglions and cell lines showed similar enrichment of biological process terms across all samples.

CONCLUSIONS

This paper provides insights into the receptor repertoire expressed in common dorsal root ganglion neuron-derived cell lines compared with whole murine dorsal root ganglions, and illustrates the limits and potentials of these cell lines as tools for neuropharmacological exploration.

TRPA1 as a drug target--promise and challenges

The transient receptor potential ankyrin 1 (TRPA1) channel is a nonselective cation channel belonging to the superfamily of transient receptor potential (TRP) channels. It is predominantly expressed in sensory neurons and serves as an irritant sensor for a plethora of electrophilic compounds. Recent studies suggest that TRPA1 is involved in pain, itch, and respiratory diseases, and TRPA1 antagonists have been actively pursued as therapeutic agents. Here, we review the recent progress, unsettled issues, and challenges in TRPA1 research and drug discovery.

TRPM3 in temperature sensing and beyond

TRPM3, also known as melastatin 2 (MLSN2), LTRPC3 (long TRPC3) and KIAA1616, is a member of the TRPM subfamily of transient receptor potential (TRP) ion channels. The channel was originally identified as a volume-regulated ion channel that can be activated upon reduction of the extracellular osmolality. Later, the channel was proposed to be involved in the modulation of insulin release in pancreatic islets. However, new evidence has uncovered a role of TRPM3 as a thermosensitive nociceptor channel implicated in the detection of noxious heat. The channel is functionally expressed in a subset of neurons of the somatosensory system and can be activated by heat. The purpose of the present review is to summarize existing knowledge of the expression, biophysics and pharmacology of TRPM3 and to serve as a guide for future studies aimed at improving the understanding of the mechanism of thermosensation and proposed physiological functions of TRPM3.

Transient receptor potential (TRP) channels: a clinical perspective

Transient receptor potential (TRP) channels are important mediators of sensory signals with marked effects on cellular functions and signalling pathways. Indeed, mutations in genes encoding TRP channels are the cause of several inherited diseases in humans (the so-called 'TRP channelopathies') that affect the cardiovascular, renal, skeletal and nervous systems. TRP channels are also promising targets for drug discovery. The initial focus of research was on TRP channels that are expressed on nociceptive neurons. Indeed, a number of potent, small-molecule TRPV1, TRPV3 and TRPA1 antagonists have already entered clinical trials as novel analgesic agents. There has been a recent upsurge in the amount of work that expands TRP channel drug discovery efforts into new disease areas such as asthma, cancer, anxiety, cardiac hypertrophy, as well as obesity and metabolic disorders. A better understanding of TRP channel functions in health and disease should lead to the discovery of first-in-class drugs for these intractable diseases. With this review, we hope to capture the current state of this rapidly expanding and changing field.

UVB radiation generates sunburn pain and affects skin by activating epidermal TRPV4 ion channels and triggering endothelin-1 signaling

At our body surface, the epidermis absorbs UV radiation. UV overexposure leads to sunburn with tissue injury and pain. To understand how, we focus on TRPV4, a nonselective cation channel highly expressed in epithelial skin cells and known to function in sensory transduction, a property shared with other transient receptor potential channels. We show that following UVB exposure mice with induced Trpv4 deletions, specifically in keratinocytes, are less sensitive to noxious thermal and mechanical stimuli than control animals. Exploring the mechanism, we find that epidermal TRPV4 orchestrates UVB-evoked skin tissue damage and increased expression of the proalgesic/algogenic mediator endothelin-1. In culture, UVB causes a direct, TRPV4-dependent Ca(2+) response in keratinocytes. In mice, topical treatment with a TRPV4-selective inhibitor decreases UVB-evoked pain behavior, epidermal tissue damage, and endothelin-1 expression. In humans, sunburn enhances epidermal expression of TRPV4 and endothelin-1, underscoring the potential of keratinocyte-derived TRPV4 as a therapeutic target for UVB-induced sunburn, in particular pain.