Chemical and thermal sensitivity of medaka TRPA1 analyzed in heterologous expression system

Low temperature inhibits food intake via TRPA1 channel activation in Nile tilapia (Oreochromis niloticus)

Low temperatures significantly influence feeding behavior in ectothermic vertebrates, but the underlying mechanisms remain elusive. This study investigated the role of transient receptor potential ankyrin 1 (TRPA1) channels in mediating the appetite-suppressing effects of low temperature in Nile tilapia. TRPA1 was found to be highly expressed in the hypothalamus and co-localized with neuropeptide Y (NPY) neurons. Exposure to low temperatures reduced feeding frequency and increased TRPA1 expression. In vitro experiments demonstrated that low temperature and TRPA1 agonists induced calcium influx, which was blocked by a TRPA1 inhibitor. TRPA1 expression exhibited post-prandial increases and was downregulated by fasting. TRPA1 activation dose-dependently inhibited food intake, while its inhibition restored feeding suppressed by low temperature. TRPA1 activation downregulated orexigenic factors and upregulated anorexigenic factors through Ca2+/calmodulin-dependent pathways. These findings suggest that TRPA1 plays a crucial role in sensing low temperatures and regulating feeding behavior in tilapia.

Transient receptor potential (TRP) channels in Sebastes schlegelii: Genome-wide identification and ThermoTRP expression analysis under high-temperature

Transient Receptor Potential (TRP) channels, essential for sensing environmental stimuli, are widely distributed. Among them, thermosensory TRP channels play a crucial role in temperature sensing and regulation. Sebastes schlegelii, a significant aquatic economic species, exhibits sensitivity to temperature across multiple aspects. In this study, we identified 18 SsTRP proteins using whole-genome scanning. Motif analysis revealed motif 2 in all TRP proteins, with conserved motifs in subfamilies. TRP-related domains, anchored repeats, and ion-transmembrane domains were found. Chromosome analysis showed 18 TRP genes on 11 chromosomes and a scaffold. Phylogenetics classified SsTRPs into four subfamilies: TRPM, TRPA, TRPV, and TRPC. In diverse organisms, four monophyletic subfamilies were identified. Additionally, we identified key TRP genes with significantly upregulated transcription levels under short-term (30 min) and long-term (3 days) exposure at 24 °C (optimal elevated temperature) and 27 °C (critical high temperature). We propose that genes upregulated at 30 min may be involved in the primary response process of temperature sensing, while genes upregulated at 3 days may participate in the secondary response process of temperature perception. This study lays the foundation for understanding the regulatory mechanisms of TRPs responses to environmental stimuli in S. schlegelii and other fishes.

TRPA5 encodes a thermosensitive ankyrin ion channel receptor in a triatomine insect

As ectotherms, insects need heat-sensitive receptors to monitor environmental temperatures and facilitate thermoregulation. We show that TRPA5, a class of ankyrin transient receptor potential (TRP) channels absent in dipteran genomes, may function as insect heat receptors. In the triatomine bug Rhodnius prolixus (order: Hemiptera), a vector of Chagas disease, the channel RpTRPA5B displays a uniquely high thermosensitivity, with biophysical determinants including a large channel activation enthalpy change (72 kcal/mol), a high temperature coefficient (Q10 = 25), and in vitro temperature-induced currents from 53°C to 68°C (T0.5 = 58.6°C), similar to noxious TRPV receptors in mammals. Monomeric and tetrameric ion channel structure predictions show reliable parallels with fruit fly dTRPA1, with structural uniqueness in ankyrin repeat domains, the channel selectivity filter, and potential TRP functional modulator regions. Overall, the finding of a member of TRPA5 as a temperature-activated receptor illustrates the diversity of insect molecular heat detectors.

Identification and functional characterization of a novel TRPA1 gene from sea cucumber Apostichopus japonicus and interaction with miR-2013 in response to salt stress

Salinity is important abiotic factor influencing sea cucumber aquaculture. This study aimed to identify and functional study of a novel transient receptor potential cation channel subfamily A member 1 (TRPA1) involved in salinity stress through interaction with miR-2013 in the sea cucumber. The full-length cDNA sequence was 1369 bp in length and encoded 138 amino acids. The TRPA1 homolog protein was a hydrophilic protein without a signal peptide and was predicted to a spatial structure of seven helices and eight random coils and two major ANK functional domains. Bioinformatic analysis and luciferase reporter assays confirmed TRPA1 as a target gene of miR-2013. Quantitative PCR revealed that miR-2013 was induced upregulation after salinity stress, while TRPA1 showed upregulated expression with maximum expression at 24 h. The expression of miR-2013 and TRPA1 was negatively regulated. Transfection experiments were conducted to validate the role of miR-2013 and TRPA1 in salinity response. The results showed that miR-2013 was upregulated and TRPA1 was downregulated after transfection with miR-2013 mimics, while miR-2013 was downregulated and TRPA1 was upregulated after transfection with miR-2013 inhibitor. Transfection with si-TRPA1 homolog resulted in upregulation of miR-2013 and downregulation of TRPA1 homolog. These findings suggest that miR-2013 can regulate the expression of TRPA1 under salt stress, and highlight the importance of miR-2013 and TRPA1 in salt stress response. miR-2013 mimics improved the survival rate, while miR-2013 inhibitor and si-TRPA1 reduced it. These findings suggest that miR-2013 and TRPA1 play important roles in sea cucumbers adaptation to salinity changes.

Temperature activated transient receptor potential ion channels from Antarctic fishes

Antarctic notothenioid fishes (cryonotothenioids) live in waters that range between -1.86°C and an extreme maximum +4°C. Evidence suggests these fish sense temperature peripherally, but the molecular mechanism of temperature sensation in unknown. Previous work identified transient receptor potential (TRP) channels TRPA1b, TRPM4 and TRPV1a as the top candidates for temperature sensors. Here, cryonotothenioid TRPA1b and TRPV1a are characterized using Xenopus oocyte electrophysiology. TRPA1b and TRPV1a showed heat-evoked currents with Q10s of 11.1 ± 2.2 and 20.5 ± 2.4, respectively. Unexpectedly, heat activation occurred at a threshold of 22.9 ± 1.3°C for TRPA1b and 32.1 ± 0.6°C for TRPV1a. These fish have not experienced such temperatures for at least 15 Myr. Either (1) another molecular mechanism underlies temperature sensation, (2) these fishes do not sense temperatures below these thresholds despite having lethal limits as low as 5°C, or (3) native cellular conditions modify the TRP channels to function at relevant temperatures. The effects of osmolytes, pH, oxidation, phosphorylation, lipids and accessory proteins were tested. No conditions shifted the activity range of TRPV1a. Oxidation in combination with reduced cholesterol significantly dropped activation threshold of TRPA1b to 11.3 ± 2.3°C, it is hypothesized the effect may be due to lipid raft disruption.

Two single-point mutations in Ankyrin Repeat one drastically change the threshold temperature of TRPV1

TRPV1 plays an important role in the thermosensory system; however, the mechanism controlling its heat activation property is not well understood. Here, we determine the heat activation properties of TRPV1 cloned from tailed amphibians, which prefer cooler environments, finding the threshold temperatures were approximately 10 °C lower compared with rat TRPV1 (rTRPV1). We find that two amino acid residues (Gln, Leu/Val) in the Ankyrin Repeat 1 (ANK1) region of the N-terminal domain are conserved among tailed amphibians and different from those (Arg, Lys) in rTRPV1. We observe the activation by heat in all urodelan TRPV1s is markedly elevated by substitution of these two amino acids. Conversely, reciprocal substitutions of rTRPV1 apparently lowers the high threshold temperature. Our studies demonstrate that tailed amphibians express TRPV1 with a reduced heat-activation threshold by substitution of two amino acid residues in the ANK1 region that likely contribute to cool-habitat selection.

Decreased heat sensitivity of lungfish TRPV1 revealed by the heterologous expression system

In this study, we focused on TRPV1 of African lungfish, Protopterus annectens. During drought at high temperature, African lungfish can survive by undergoing into aestivation in mud cocoons. Therefore, lungfish is considered to have some specialized heat-sensor, TRPV1, for heat tolerance. Further, lungfish which shares similarities with fishes and amphibians, is one of important species for investigating the fish-tetrapod transition. Since fish TRPV1 and tetrapod TRPV1 have some differences, character of lungfish TRPV1 attracts attention. Here, we first cloned TRPV1 paralogue from lungfish, Protopterus annectens (lfTRPV1) and determined the chemical and thermal sensitivities of lfTRPV1 by two-electrode voltage clamp method using frog oocytes. We detected activation of lfTRPV1 by acid and 2-APB, but capsaicin-induced activation was not observed. The sensitivity to acid of lfTRPV1 was similar to that of rat TRPV1 (rTRPV1), but the 2-APB sensitivity of lfTRPV1 was relatively weaker than rTRPV1. Heat stimulation up to 44 °C did not activate lfTRPV1 and the heat-activation was not detected even on acid condition of pH6. This dramatically decreased heat-sensitivity of TRPV1 may contribute the heat tolerance of African lungfish. Moreover, this might be the property of ancient tetrapod-type TRPV1 gene.

The role of TRPA1 channels in thermosensation

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal nonselective cation channel sensitive to different physical and chemical stimuli. TRPA1 is associated with many important physiological functions in different species and thus is involved in different degrees of evolution. TRPA1 acts as a polymodal receptor for the perceiving of irritating chemicals, cold, heat, and mechanical sensations in various animal species. Numerous studies have supported many functions of TRPA1, but its temperature-sensing function remains controversial. Although TRPA1 is widely distributed in both invertebrates and vertebrates, and plays a crucial role in tempreture sensing, the role of TRPA1 thermosensation and molecular temperature sensitivity are species-specific. In this review, we summarize the temperature-sensing role of TRPA1 orthologues in terms of molecular, cellular, and behavioural levels.

Cloning and functional characterization of medaka TRPV4

Medaka, Oryzias latipes is distributed in fresh water of South Asia. To study roles of TRPV4 in osmosensing and adaption mechanism of medaka during changes in salinity environment, we isolated the cDNA for TRPV4 from medaka (olTRPV4) and characterized it. The electrophysiological analysis using Xenopus oocytes revealed that olTRPV4 can be activated by a TRPV4-specific agonist, GSK1016790A and acid at pH 5. Further, olTRPV4 was sensitive to 2-APB. Although warm temperatures activate mammalian TRPV4, olTRPV4 was activated by cold and hot stimulation. The threshold for cold activation was determined as 13.13 ± 0.60 °C, and the heat-activation threshold was 40.26 ± 0.25 °C. Further, when olTRPV4-expressing oocytes were stimulated by hypotonic solution, an apparent activation was observed. We further found that the expression of this hypotonic sensor, olTRPV4 was significantly down-regulated in gills but up-regulated in brains of sea water-acclimated medaka. Results demonstrated that olTRPV4 must function as an osmosensor and play important roles in adaption mechanism of medaka fish in various salinity environments.

OUP accepted manuscript

Animals rely on their sensory systems to inform them of ecologically relevant environmental variation. In the Southern Ocean, the thermal environment has remained between −1.9 and 5 °C for 15 Myr, yet we have no knowledge of how an Antarctic marine organism might sense their thermal habitat as we have yet to discover a thermosensitive ion channel that gates (opens/closes) below 10 °C. Here, we investigate the evolutionary dynamics of transient receptor potential (TRP) channels, which are the primary thermosensors in animals, within cryonotothenioid fishes—the dominant fish fauna of the Southern Ocean. We found cryonotothenioids have a similar complement of TRP channels as other teleosts (∼28 genes). Previous work has shown that thermosensitive gating in a given channel is species specific, and multiple channels act together to sense the thermal environment. Therefore, we combined evidence of changes in selective pressure, gene gain/loss dynamics, and the first sensory ganglion transcriptome in this clade to identify the best candidate TRP channels that might have a functional dynamic range relevant for frigid Antarctic temperatures. We concluded that TRPV1a, TRPA1b, and TRPM4 are the likeliest putative thermosensors, and found evidence of diversifying selection at sites across these proteins. We also put forward hypotheses for molecular mechanisms of other cryonotothenioid adaptations, such as reduced skeletal calcium deposition, sensing oxidative stress, and unusual magnesium homeostasis. By completing a comprehensive and unbiased survey of these genes, we lay the groundwork for functional characterization and answering long-standing thermodynamic questions of thermosensitive gating and protein adaptation to low temperatures.

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.

The diversity in sensitivity of TRPA1 and TRPV1 of various animals to polyphenols

The perception of tastes is sensed by the receptors that stimulate sensory cells. We previously reported that TRPA1 and TRPV1 channels expressed in the oral cavity of mammals, are activated by the auto-oxidized product of epigallocatechin gallate (oxiEGCG), a major astringent catechin in green tea. Here, we investigated and compared the sensitivity of TRPA1 and TRPV1 from various animals to astringent polyphenols. We selected three polyphenols, oxiEGCG, tannic acid and myricetin. HEK293T cells expressing TRPA1 or TRPV1 from mammal, bird, reptile, amphibian, and fish, were analyzed for their activation by the Ca²⁺-imaging. We found the apparent diversity in the polyphenol-sensitivity among various animals. Mammalian TRPs showed relatively higher sensitivity to polyphenols, and especially, human TRPA1 and TRPV1 could be activated by all of three polyphenols at 20 μM. Reptile TRP channels, however, were insensitive to any polyphenols examined. Moreover, the polyphenol-sensitivity of zebrafish TRPA1 and TRPV1 was quite different from that of medaka TRP channels. Since many polyphenols are present in plants and the sensing of polyphenols using TRP channels in the oral cavity might cause astringent taste, the observed diversity of the polyphenol-sensitivity of TRP channels might be involved in the divergence in the food habit of various animals.

Thermoregulation in fish

Thermoregulation is critical for survival and animals therefore employ strategies to keep their body temperature within a physiological range. As ectotherms, fish exclusively rely on behavioral strategies for thermoregulation. Different species of fish seek out their specific optimal temperatures through thermal navigation by biasing behavioral output based on experienced environmental temperatures. Like other vertebrates, fish sense water temperature using thermoreceptors in trigeminal and dorsal root ganglia neurons that innervate the skin. Recent research in larval zebrafish has revealed how neural circuits subsequently transform this sensation of temperature into thermoregulatory behaviors. Across fish species, thermoregulatory strategies rely on a modulation of swim vigor based on current temperature and a modulation of turning based on temperature change. Interestingly, temperature preferences are not fixed but depend on other environmental cues and internal states. The following review is intended as an overview on the current knowledge as well as open questions in fish thermoregulation.

Functional evolution of vertebrate sensory receptors

Sensory receptors enable animals to perceive their external world, and functional properties of receptors evolve to detect the specific cues relevant for an organism's survival. Changes in sensory receptor function or tuning can directly impact an organism's behavior. Functional tests of receptors from multiple species and the generation of chimeric receptors between orthologs with different properties allow for the dissection of the molecular basis of receptor function and identification of the key residues that impart functional changes in different species. Knowledge of these functionally important sites facilitates investigation into questions regarding the role of epistasis and the extent of convergence, as well as the timing of sensory shifts relative to other phenotypic changes. However, as receptors can also play roles in non-sensory tissues, and receptor responses can be modulated by numerous other factors including varying expression levels, alternative splicing, and morphological features of the sensory cell, behavioral validation can be instrumental in confirming that responses observed in heterologous systems play a sensory role. Expression profiling of sensory cells and comparative genomics approaches can shed light on cell-type specific modifications and identify other proteins that may affect receptor function and can provide insight into the correlated evolution of complex suites of traits. Here we review the evolutionary history and diversity of functional responses of the major classes of sensory receptors in vertebrates, including opsins, chemosensory receptors, and ion channels involved in temperature-sensing, mechanosensation and electroreception.

Unique high sensitivity to heat of axolotl TRPV1 revealed by the heterologous expression system

The thermosensation mechanism plays critical roles in various animals living in different thermal environment. We focused on an axolotl, which is a tailed amphibian originally from Lake Xochimilco area in the Vally of Mexico, and examined its behavior response to heat stimulation. Mild heat at 33 °C induced noxious locomotive activity to axolotls, but the noxious response of another tailed amphibian, Iberian ribbed newt, was not observed at 33 °C. To explore the mechanism for the temperature sensitivity of axolotls, we isolated a cDNA of TRPV1. Using the degenerate primer PCR method, we identified the DNA fragment encoding axolotl TRPV1 (axTRPV1), and then cloned a full-length cDNA. We studied the chemical and thermal sensitivities of axTRPV1 by two-electrode voltage clamp method using Xenopus oocyte expression system. Capsaicin, acid, and 2-aminoethoxydiphenylborane apparently activated axTRPV1 channels in a dose-dependent manner. The analysis of thermal sensitivity showed that axTRPV1 was significantly activated by heat but not by cold. The average temperature threshold for heat-activation was 30.95 ± 0.12 °C. This thermal activation threshold of axTRPV1 is unique and significantly low, when compared with the known thresholds of TRPV1s from various animals. Further, this threshold of axTRPV1 is well consistent with the observation of heat-induced behavior of axolotls at 33 °C, demonstrating that axolotl shows noxious response to mild heat mediated through axTRPV1.

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.

Functional properties of axolotl transient receptor potential ankyrin 1 revealed by the heterologous expression system

Transient receptor potential ankyrin 1 (TRPA1) from tetrapod vertebrates except rodents are activated by high temperature with a relatively clear threshold. Our recent investigation suggested that a gradual heat activation without clear threshold might be a common feature for TRPA1 of fish. To approach which animal first acquires TRPA1 as a threshold detector instead of a gradual heat sensor, here, we focused on TRPA1 from axolotls (Ambystoma mexicanum). We isolated a full-length cDNA of axolotl transient receptor potential ankyrin 1 (axTRPA1) and studied the functional properties by two-electrode voltage clamp method using Xenopus oocytes. Allyl isothiocyanate, caffeine, methyl anthranilate and carvacrol activated axTRPA1 channels. The results indicated that axTRPA1 is heat activated with the average threshold of 39.7°C, suggesting that axTRPA1 already has acquired the functional property of land animals.