Species differences and molecular determinant of TRPA1 cold sensitivity

Ion channels of cold transduction and transmission

Thermosensation requires the activation of a unique collection of ion channels and receptors that work in concert to transmit thermal information. It is widely accepted that transient receptor potential melastatin 8 (TRPM8) activation is required for normal cold sensing; however, recent studies have illuminated major roles for other ion channels in this important somatic sensation. In addition to TRPM8, other TRP channels have been reported to contribute to cold transduction mechanisms in diverse sensory neuron populations, with both leak- and voltage-gated channels being identified for their role in the transmission of cold signals. Whether the same channels that contribute to physiological cold sensing also mediate noxious cold signaling remains unclear; however, recent work has found a conserved role for the kainite receptor, GluK2, in noxious cold sensing across species. Additionally, cold-sensing neurons likely engage in functional crosstalk with nociceptors to give rise to cold pain. This Review will provide an update on our understanding of the relationship between various ion channels in the transduction and transmission of cold and highlight areas where further investigation is required.

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.

TRPA1-Related Diseases and Applications of Nanotherapy

Transient receptor potential (TRP) channels, first identified in Drosophila in 1969, are multifunctional ion channels expressed in various cell types. Structurally, TRP channels consist of six membrane segments and are classified into seven subfamilies. Transient receptor potential ankyrin 1 (TRPA1), the first member of the TRPA family, is a calcium ion affinity non-selective cation channel involved in sensory transduction and responds to odors, tastes, and chemicals. It also regulates temperature and responses to stimuli. Recent studies have linked TRPA1 to several disorders, including chronic pain, inflammatory diseases, allergies, and respiratory problems, owing to its activation by environmental toxins. Mutations in TRPA1 can affect the sensory nerves and microvasculature, potentially causing nerve pain and vascular problems. Understanding the function of TRPA1 is important for the development of treatments for these diseases. Recent developments in nanomedicines that target various ion channels, including TRPA1, have had a significant impact on disease treatment, providing innovative alternatives to traditional disease treatments by overcoming various adverse effects.

Evidence that the cold- and menthol-sensing functions of the human TRPM8 channel evolved separately

Transient receptor potential melastatin 8 (TRPM8) is a temperature- and menthol-sensitive ion channel that contributes to diverse physiological roles, including cold sensing and pain perception. Clinical trials targeting TRPM8 have faced repeated setbacks predominantly due to the knowledge gap in unraveling the molecular underpinnings governing polymodal activation. A better understanding of the molecular foundations between the TRPM8 activation modes may aid the development of mode-specific, thermal-neutral therapies. Ancestral sequence reconstruction was used to explore the origins of TRPM8 activation modes. By resurrecting key TRPM8 nodes along the human evolutionary trajectory, we gained valuable insights into the trafficking, stability, and function of these ancestral forms. Notably, this approach unveiled the differential emergence of cold and menthol sensitivity over evolutionary time, providing a fresh perspective on complex polymodal behavior. These studies provide a paradigm for understanding polymodal behavior in TRPM8 and other proteins with the potential to enhance our understanding of sensory receptor biology and pave the way for innovative therapeutic interventions.

Transient Receptor Potential Channels: Multiple Modulators of Peripheral Neuropathic Pain in Several Rodent Models

Neuropathic pain, a prevalent chronic condition in clinical settings, has attracted widespread societal attention. This condition is characterized by a persistent pain state accompanied by affective and cognitive disruptions, significantly impacting patients' quality of life. However, current clinical therapies fall short of addressing its complexity. Thus, exploring the underlying molecular mechanism of neuropathic pain and identifying potential targets for intervention is highly warranted. The transient receptor potential (TRP) receptors, a class of widely distributed channel proteins, in the nervous system, play a crucial role in sensory signaling, cellular calcium regulation, and developmental influences. TRP ion channels are also responsible for various sensory responses including heat, cold, pain, and stress. This review highlights recent advances in understanding TRPs in various rodent models of neuropathic pain, aiming to uncover potential therapeutic targets for clinical management.

Intragastric administration of cinnamaldehyde induces changes in body temperature via TRPA1

The transient receptor potential (TRP) channel family, including TRPA1, is known to be involved in temperature sensing and response. Previous studies have shown that intragastric administration of cinnamaldehyde (a typical TRPA1 agonist) can change body temperature, but the role of TRPA1 in this response is not clear. In this study, we found that intragastric administration of cinnamaldehyde increased in the intrascapular brown adipose tissue (IBAT) and rectal temperatures. However, this effect was not observed in TRPA1 knockout mice, suggesting that TRPA1 is involved in these temperature changes. Intravenous cinnamaldehyde also increased IBAT and rectal temperatures, only in the presence of TRPA1. We also explored the contribution of the vagus nerve to these temperature changes and found that it played a limited role. These results suggest that cinnamaldehyde can affect body temperature through TRPA1 activation, with the vagus nerve having a minor influence.

Identification of transient receptor potential channel genes from the swimming crab, Portunus Trituberculatus, and their expression profiles under acute temperature stress

BACKGROUND

Temperature is an important environment factor that is critical to the survival and growth of crustaceans. However, the mechanisms by which crustaceans detect changes in temperature are still unclear. The transient receptor potential (TRP) channels are non-selective cation channels well known for properties in temperature sensation. However, comprehensive understandings on TRP channels as well as their temperature sensing functions are still lacking in crustaceans.

RESULTS

In this study, a total of 26 TRP genes were identified in the swimming crab, Portunus trituberculatus, which can be classified into TRPA, TRPC, TRPP, TRPM, TRPML, TRPN and TRPV. Tissue expression analysis revealed a wide distribution of these TRP genes in P. trituberculatus, and antennules, neural tissues, and ovaries were the most commonly expressed tissues. To investigate the responsiveness of TRP genes to the temperature change, 18 TRPs were selected to detect their expression after high and low temperature stress. The results showed that 12 TRPs showed induced gene expression in both high and low temperature groups, while 3 were down-regulated in the low temperature group, and 3 showed no change in expression in either group.

CONCLUSIONS

This study characterized the TRP family genes in P. trituberculatus, and explored their involvement in response to temperature stress. Our results will enhance overall understanding of crustacean TRP channels and their possible functions.

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.

Thermosensation and TRP Channels

Somatosensory neurons can sense external temperature by converting sensation of temperature information to neural activity via afferent input to the central nervous system. Various populations of somatosensory neurons have specialized gene expression, including expression of thermosensitive transient receptor potential (TRP) ion channels. Thermosensitive TRP channels are responsible for thermal transduction at the peripheral ends of somatosensory neurons and can sense a wide range of temperatures. Here we focus on several thermosensitive TRP channels including TRPV1, TRPV4, TRPM2, TRPM3, TRPM8, TRPC5, and TRPA1 in sensory neurons. TRPV3, TRPV4, and TRPC5 are also involved in somatosensation in nonneuronal cells and tissues. In particular, we discuss whether skin senses ambient temperatures through TRPV3 and TRPV4 activation in skin keratinocytes and the involvement of TRPM2 expressed by hypothalamic neurons in thermosensation in the brain.

Role of TRPA1 in Painful Cold Hypersensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that plays a pivotal role in pain generation after exposure to irritant chemicals and is involved in the sensation of a wide variety of pathological pain. TRPA1 was first reported to be sensitive to noxious cold, but its intrinsic cold sensitivity still remains under debate. To address this issue, we focused on cold hypersensitivity induced by oxaliplatin, a platinum-based chemotherapeutic drug, as a peculiar adverse symptom of acute peripheral neuropathy. We and other groups have shown that oxaliplatin enhances TRPA1 sensitivity to its chemical agonists and reactive oxygen species (ROS). Our in vitro and animal model studies revealed that oxaliplatin, or its metabolite oxalate, inhibits hydroxylation of a proline residue within the N-terminus of human TRPA1 (hTRPA1) via inhibition of prolyl hydroxylase domain-containing protein (PHD), which induces TRPA1 sensitization to ROS. Although hTRPA1 is insensitive to cold, PHD inhibition endows hTRPA1 with cold sensitivity through sensing the small amount of ROS produced after exposure to cold. Hence, we propose that PHD inhibition can unveil the cold sensitivity of hTRPA1 by converting ROS signaling into cold sensitivity. Furthermore, in this review, we summarize the role of TRPA1 in painful cold hypersensitivity during peripheral vascular impairment.

Temperature-Dependent Activation of Thermosensitive Transient Receptor Potential Channels

Temperature detection is essential for the survival and perpetuation of any species. Thermoreceptors in the skin sense the body temperature and also the temperatures of the ambient air and the objects. In 1997, Dr. David Julius and his colleagues found that a receptor expressed in small-diameter primary sensory neurons was activated by capsaicin (the pungent chemical in hot pepper). This receptor was also activated by temperature above 42 °C. That was the first time that a thermal receptor in primary sensory neurons has been identified. This receptor is named transient receptor potential vanilloid 1 (TRPV1). Now, 11 thermosensitive TRP channels are known. In this chapter, we summarize the reports and analyze thermosensitive TRP channels in a variety of ways to clarify the activation mechanisms by which temperature changes are sensed.

Canine, mouse, and human transient receptor potential ankyrin 1 (TRPA1) channels show different sensitivity to menthol or cold stimulation

Transient receptor potential ankyrin 1 (TRPA1) is a nonselective cation channel that is activated by a variety of stimuli and acts as a nociceptor. Mouse and human TRPA1 exhibit different reactivity to some stimuli, including chemicals such as menthol as well as cold stimuli. The cold sensitivity of TRPA1 in mammalian species is controversial. Here, we analyzed the reactivity of heterologously expressed canine TRPA1 as well as the mouse and human orthologs to menthol or cold stimulation in Ca2+-imaging experiments. Canine and human TRPA1 exhibited a similar response to menthol, that is, activation in a concentration-dependent manner, even at the high concentration range in contrast to the mouse ortholog, which did not respond to high concentration of menthol. In addition, the response during the removal of menthol was different; mouse TRPA1-expressing cells exhibited a typical response with a rapid and clear increase in [Ca2+]i ("off-response"), whereas [Ca2+]i in human TRPA1-expressing cells was dramatically decreased by the washout of menthol and [Ca2+]i in canine TRPA1-expressing cells was slightly decreased. Finally, canine TRPA1 as well as mouse and human TRPA1 were activated by cold stimulation (below 19-20°C). The sensitivity to cold stimulation differed between these species, that is, human TRPA1 activated at higher temperatures compared with the canine and mouse orthologs. All of the above responses were suppressed by the selective TRPA1 inhibitor HC-030031. Because the concentration-dependency and "off-response" of menthol as well as the cold sensitivity were not uniform among these species, studies of canine TRPA1 might be useful for understanding the species-specific functional properties of mammalian TRPA1.

TRPA1 Antagonist LY3526318 Inhibits the Cinnamaldehyde-Evoked Dermal Blood Flow Increase: Translational Proof of Pharmacology

Transient receptor potential Ankyrin 1 (TRPA1) is an ion channel expressed by sensory neurons, where it mediates pain signaling. Consequently, it has emerged as a promising target for novel analgesics, yet, to date, no TRPA1 antagonists have been approved for clinical use. In the present translational study, we utilized dermal blood flow changes evoked by TRPA1 agonist cinnamaldehyde as a target engagement biomarker to investigate the in vivo pharmacology of LY3526318, a novel TRPA1 antagonist. In rats, LY3526318 (1, 3, and 10 mg/kg, p.o.) dose-dependently reduced the cutaneous vasodilation typically observed following topical application of 10% v/v cinnamaldehyde. The inhibition was significant at the site of cinnamaldehyde application and also when including an adjacent area of skin. Similarly, in a cohort of 16 healthy human volunteers, LY3526318 administration (10, 30, and 100 mg, p.o.) dose-dependently reduced the elevated blood flow surrounding the site of 10% v/v cinnamaldehyde application, with a trend toward inhibition at the site of application. Comparisons between both species reveal that the effects of LY3526318 on the cinnamaldehyde-induced dermal blood flow are greater in rats relative to humans, even when adjusting for cross-species differences in potency of the compound at TRPA1. Exposure-response relationships suggest that a greater magnitude response may be observed in humans if higher antagonist concentrations could be achieved. Taken together, these results demonstrate that cinnamaldehyde-evoked changes in dermal blood flow can be utilized as a target engagement biomarker for TRPA1 activity and that LY3526318 antagonizes the ion channel both in rats and humans.

Hesperetin-7-O-glucoside/β-cyclodextrin Inclusion Complex Induces Acute Vasodilator Effect to Inhibit the Cold Sensation Response during Localized Cold-Stimulate Stress in Healthy Human Subjects: A Randomized, Double-Blind, Crossover, and Placebo-Controlled Study

Hesperetin, a citrus flavonoid, exerts vasodilation and is expected to improve endothelial function and alleviate cold sensation by activating nervous system thermal transduction pathways. In this randomized, double-blind, crossover, and placebo-controlled study, the purpose was to assess the effect of an orally administered highly bioavailable soluble inclusion complex of hesperetine-7-O-glucoside with β-cyclodextrin (HEPT7G/βCD; SunActive® HES/HCD) on cold sensation response during localized cold-stimulated stress in healthy humans. A significant (p ≤ 0.05) dose-dependent increase in skin cutaneous blood flow following relatively small doses of HEPT7G/βCD inclusion complex ingestion was confirmed, which led to a relatively effective recovery of peripheral skin temperature. The time delay of an increase in blood flow during rewarming varied significantly between low- and high-dose HEPT7G/βCD inclusion complex consumption (e.g., 150 mg and 300 mg contain 19.5 mg and 39 mg of HEPT7G, respectively). In conclusion, the substantial alteration in peripheral skin blood flow observed during local cooling stress compared to placebo suggested that deconjugated hesperetin metabolites may have a distinct capacity for thermoregulatory control of human skin blood flow to maintain a constant body temperature during cold stress exposure via cutaneous vasodilation and vasoconstriction systems.

Social experience influences thermal sensitivity: lessons from an amphibious mangrove fish

Understanding the factors affecting the capacity of ectothermic fishes to cope with warming temperature is critical given predicted climate change scenarios. We know that a fish's social environment introduces plasticity in how it responds to high temperature. However, the magnitude of this plasticity and the mechanisms underlying socially modulated thermal responses are unknown. Using the amphibious hermaphroditic mangrove rivulus fish Kryptolebias marmoratus as a model, we tested three hypotheses: (1) social stimulation affects physiological and behavioural thermal responses of isogenic lineages of fish; (2) social experience and acute social stimulation result in distinct physiological and behavioural responses; and (3) a desensitization of thermal receptors is responsible for socially modulated thermal responses. To test the first two hypotheses, we measured the temperature at which fish emerged from the water (i.e. pejus temperature) upon acute warming with socially naive isolated fish and with fish that were raised alone and then given a short social experience prior to exposure to increasing temperature (i.e. socially experienced fish). Our results did not support our first hypothesis as fish socially stimulated by mirrors during warming (i.e. acute social stimulation) emerged at similar temperatures to isolated fish. However, in support of our second hypothesis, a short period of prior social experience resulted in fish emerging at a higher temperature than socially naive fish suggesting an increase in pejus temperature with social experience. To test our third hypothesis, we exposed fish that had been allowed a brief social interaction and naive fish to capsaicin, an agonist of TRPV1 thermal receptors. Socially experienced fish emerged at significantly higher capsaicin concentrations than socially naive fish suggesting a desensitization of their TRPV1 thermal receptors. Collectively, our data indicate that past and present social experiences impact the behavioural response of fish to high temperature. We also provide novel data suggesting that brief periods of social experience affect the capacity of fish to perceive warm temperature.

Does etodolac affect TRPA1 functionality in vivo in human?

OBJECTIVES

In preclinical research, etodolac, a non-steroidal anti-inflammatory drug, affected transient receptor potential ankyrin 1 (TRPA1) activation. Yet, whether the in vitro interaction between etodolac and TRPA1 translates to altered TRPA1 functionality in vivo in human remains to be investigated.

METHODS

A randomized, double-blinded, celecoxib-controlled study was conducted to assess the effect of etodolac on TRPA1-mediated dermal blood flow (DBF) changes on the forearm of 15 healthy, male volunteers aged between 18 and 45 years. Over four study visits, separated by at least five days wash-out, a single or four-fold dose of etodolac 200 mg or celecoxib 200 mg was administered orally. Two hours post-dose, TRPA1 functionality was evaluated by assessing cinnamaldehyde-induced DBF changes. DBF changes were quantified and expressed in Perfusion Units (PUs) using laser Doppler imaging during 60 min post-cinnamaldehyde application. The corresponding area under the curve (AUC_0-60min) was calculated as summary measure. Statistical analysis was performed using Linear mixed models with post-hoc Dunnett.

RESULTS

Neither the single dose of etodolac nor celecoxib inhibited the cinnamaldehyde-induced DBF changes compared to no treatment (AUC_0-60min ± SEM of 17,751 ± 1,514 PUs*min and 17,532 ± 1,706 PUs*min vs. 19,274 ± 1,031 PUs*min, respectively, both p=1.00). Similarly, also a four-fold dose of both compounds failed to inhibit the cinnamaldehyde-induced DBF changes (19,235 ± 1,260 PUs*min and 19,367 ± 1,085 PUs*min vs. 19,274 ± 1,031 PUs*min, respectively, both p=1.00).

CONCLUSIONS

Etodolac did not affect the cinnamaldehyde-induced DBF changes, suggesting that it does not alter TRPA1 functionality in vivo in human.

"ThermoTRP" Channel Expression in Cancers: Implications for Diagnosis and Prognosis (Practical Approach by a Pathologist)

Temperature-sensitive transient receptor potential (TRP) channels (so-called "thermoTRPs") are multifunctional signaling molecules with important roles in cell growth and differentiation. Several "thermoTRP" channels show altered expression in cancers, though it is unclear if this is a cause or consequence of the disease. Regardless of the underlying pathology, this altered expression may potentially be used for cancer diagnosis and prognostication. "ThermoTRP" expression may distinguish between benign and malignant lesions. For example, TRPV1 is expressed in benign gastric mucosa, but is absent in gastric adenocarcinoma. TRPV1 is also expressed both in normal urothelia and non-invasive papillary urothelial carcinoma, but no TRPV1 expression has been seen in invasive urothelial carcinoma. "ThermoTRP" expression can also be used to predict clinical outcomes. For instance, in prostate cancer, TRPM8 expression predicts aggressive behavior with early metastatic disease. Furthermore, TRPV1 expression can dissect a subset of pulmonary adenocarcinoma patients with bad prognosis and resistance to a number of commonly used chemotherapeutic agents. This review will explore the current state of this rapidly evolving field with special emphasis on immunostains that can already be added to the armoire of diagnostic pathologists.

Molecular architecture and gating mechanisms of the Drosophila TRPA1 channel

The transient receptor potential channel subfamily A member 1 (TRPA1) ion channel is an evolutionary conserved polymodal sensor responding to noxious temperature or chemical stimuli. Notably, the thermosensitivity of TRPA1 varies among different species or even different isoforms in the same species. However, the underlying molecular basis of its thermo-gating remains largely unknown. Here, we determine the structures of a heat-sensitive isoform of TRPA1 in Drosophila melanogaster in two distinct conformations with cryo-samples prepared at 8 °C. Large conformational changes are observed in the ankyrin repeat domain (ARD) and the coiled-coil domain between the two states. Remarkably, all 17 ankyrin repeats are mapped in the newly resolved conformation, forming a propeller-like architecture. Two intersubunit interfaces are identified in the amino (N)-terminal domain, and play vital roles during both heat and chemical activation as shown by electrophysiological analysis. With cryo-samples prepared at 35 °C, only one conformation is resolved, suggesting possible state transitions during heat responses. These findings provide a basis for further understanding how the ARD regulates channel functions, and insights into the gating mechanism of TRPA1.

Regulation of ThermoTRP Channels by PIP2 and Cholesterol

Transient receptor potential (TRP) ion channels are proteins that are expressed by diverse tissues and that play pivotal functions in physiology. These channels are polymodal and are activated by several stimuli. Among TRPs, some members of this family of channels respond to changes in ambient temperature and are known as thermoTRPs. These proteins respond to heat or cold in the noxious range and some of them to temperatures considered innocuous, as well as to mechanical, osmotic, and/or chemical stimuli. In addition to this already complex ability to respond to different signals, the activity of these ion channels can be fine-tuned by lipids. Two lipids well known to modulate ion channel activity are phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol. These lipids can either influence the function of these proteins through direct interaction by binding to a site in the structure of the ion channel or through indirect mechanisms, which can include modifying membrane properties, such as curvature and rigidity, by regulating their expression or by modulating the actions of other molecules or signaling pathways that affect the physiology of ion channels. Here, we summarize the key aspects of the regulation of thermoTRP channels by PIP2 and cholesterol.

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.

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.

Function and therapeutic potential of transient receptor potential ankyrin 1 in fibrosis

The transient receptor potential (TRP) protein superfamily is a special group of cation channels expressed in different cell types and signaling pathways. In this review, we focus on TRPA1 (transient receptor potential ankyrin 1), an ion channel in this family that exists in the cell membrane and shows a different function from other TRP channels. TRPA1 usually has a special activation effect that can induce cation ions, especially calcium ions, to flow into activated cells. In this paper, we review the role of TRPA1 in fibroblasts. To clarify the relationship between fibroblasts and TRPA1, we have also paid special attention to the interactions between TRPA1 and inflammatory factors leading to fibroblast activation. TRPA1 has different functions in the fibrosis process in different organs, and there have also been interesting discussions of the mechanism of TRPA1 in fibroblasts. Therefore, this review aims to describe the function of TRP channels in controlling fibrosis through fibroblasts in different organ inflammatory and immune-mediated diseases. We attempt to prove that TRPA1 is a target for fibrosis. In fact, some clinical trials have already proven that TRPA1 is a potential adjuvant therapy for treating fibrosis.

A cross-taxonomic perspective on the integration of temperature cues in vertebrate seasonal neuroendocrine pathways

The regulation of seasonality has been an area of interest for decades, yet global climate change has created extra urgency in the quest to understand how sensory circuits and neuroendocrine control systems interact to generate flexibility in biological timekeeping. The capacity of temperature to alter endogenous or photoperiod-regulated neuroendocrine mechanisms driving seasonality, either as a direct cue or through temperature-dependent effects on energy and metabolism, is at the heart of this phenological flexibility. However, until relatively recently, little research had been done on the integration of temperature information in canonical seasonal neuroendocrine pathways, particularly in vertebrates. We review recent advances from research in vertebrates that deepens our understanding of how temperature cues are perceived and integrated into seasonal hypothalamic thyroid hormone (TH) signaling, which is a critical regulator of downstream seasonal phenotypic changes such as those regulated by the BPG (brain-pituitary-gonadal) axis. Temperature perception occurs through cutaneous transient receptor potential (TRP) neurons, though sensitivity of these neurons varies markedly across taxa. Although photoperiod is the dominant cue used to trigger seasonal physiology or entrain circannual clocks, across birds, mammals, fish, reptiles and amphibians, seasonality appears to be temperature sensitive and in at least some cases this appears to be related to phylogenetically conserved TH signaling in the hypothalamus. Nevertheless, the exact mechanisms through which temperature modulates seasonal neuroendocrine pathways remains poorly understood.

TRP channels: a journey towards a molecular understanding of pain

The perception of nociceptive signals, which are translated into pain, plays a fundamental role in the survival of organisms. Because pain is linked to a negative sensation, animals learn to avoid noxious signals. These signals are detected by receptors, which include some members of the transient receptor potential (TRP) family of ion channels that act as transducers of exogenous and endogenous noxious cues. These proteins have been in the focus of the field of physiology for several years, and much knowledge of how they regulate the function of the cell types and organs where they are expressed has been acquired. The last decade has been especially exciting because the 'resolution revolution' has allowed us to learn the molecular intimacies of TRP channels using cryogenic electron microscopy. These findings, in combination with functional studies, have provided insights into the role played by these channels in the generation and maintenance of pain.

TRP channels in thermosensation

The ability to sense external temperature is assumed by somatosensory neurons, in which temperature information is converted to neural activity by afferent input to the central nervous system. Somatosensory neurons consist of various populations with specialized gene expression, including thermosensitive transient receptor potential ion channels (thermo-TRPs). Thermo-TRPs are responsible for thermal transduction at the peripheral ends of somatosensory neurons and over a wide range of temperatures. In this review, we focus on several thermo-TRPs expressed in sensory neurons: TRPV1, TRPV4, TRPM2, TRPM3, TRPM8, TRPC5, and TRPA1. TRPV3, TRPV4, and TRPC5 expressed in non-neuronal cells that are also involved in somatosensation are also discussed, whereas TRPM2 and TRPM8 are involved in thermosensation in the brain.

Current and Emerging Pharmacotherapeutic Interventions for the Treatment of Peripheral Nerve Disorders

Peripheral nerve disorders are caused by a range of different aetiologies. The range of causes include metabolic conditions such as diabetes, obesity and chronic kidney disease. Diabetic neuropathy may be associated with severe weakness and the loss of sensation, leading to gangrene and amputation in advanced cases. Recent studies have indicated a high prevalence of neuropathy in patients with chronic kidney disease, also known as uraemic neuropathy. Immune-mediated neuropathies including Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy may cause significant physical disability. As survival rates continue to improve in cancer, the prevalence of treatment complications, such as chemotherapy-induced peripheral neuropathy, has also increased in treated patients and survivors. Notably, peripheral neuropathy associated with these conditions may be chronic and long-lasting, drastically affecting the quality of life of affected individuals, and leading to a large socioeconomic burden. This review article explores some of the major emerging clinical and experimental therapeutic agents that have been investigated for the treatment of peripheral neuropathy due to metabolic, toxic and immune aetiologies.

Combining Topical Agonists With the Recording of Event-Related Brain Potentials to Probe the Functional Involvement of TRPM8, TRPA1 and TRPV1 in Heat and Cold Transduction in the Human Skin

TRP channels play a central role in the transduction of thermal and nociceptive stimuli by free nerve endings. Most of the research on these channels has been conducted in vitro or in vivo in nonhuman animals and translation of these results to humans must account for potential experimental biases and interspecific differences. This study aimed at evaluating the involvement of TRPM8, TRPA1 and TRPV1 channels in the transduction of heat and cold stimuli by the human thermonociceptive system. For this purpose, we evaluated the effects of topical agonists of these 3 channels (menthol, cinnamaldehyde and capsaicin) on the event-related brain potentials (ERPs) elicited by phasic thermal stimuli (target temperatures: 10°C, 42°C, and 60°C) selected to activate cold Aδ thermoreceptors, warm sensitive C thermoreceptors and heat sensitive Aδ polymodal nociceptors. Sixty-four participants were recruited, 16 allocated to each agonist solution group (20% menthol, 10% cinnamaldehyde, .025% capsaicin and 1% capsaicin). Participants were treated sequentially with the active solution on one forearm and vehicle only on the other forearm for 20 minutes. Menthol decreased the amplitude and increased the latency of cold and heat ERPs. Cinnamic aldehyde decreased the amplitude and increased the latency of heat but not cold ERPs. Capsaicin decreased the amplitude and increased the latency of heat ERPs and decreased the amplitude of the N2P2 complex of the cold ERPs without affecting the earlier N1 wave or the latencies of the peaks. These findings are compatible with previous evidence indicating that TRPM8 is involved in innocuous cold transduction and that TRPV1 and TRPA1 are involved in noxious heat transduction in humans. PERSPECTIVE: By chemically modulating TRPM8, TRPA1 and TRPV1 reactivity (key molecules in the transduction of temperature) and assessing how this affected EEG responses to the activation of cold thermoreceptors and heat nociceptors, we aimed at confirming the role of these channels in a functional healthy human model.

Population-based high-throughput toxicity screen of human iPSC-derived cardiomyocytes and neurons

In this study, we establish a population-based human induced pluripotent stem cell (hiPSC) drug screening platform for toxicity assessment. After recruiting 1,000 healthy donors and screening for high-frequency human leukocyte antigen (HLA) haplotypes, we identify 13 HLA-homozygous "super donors" to represent the population. These "super donors" are also expected to represent at least 477,611,135 of the global population. By differentiating these representative hiPSCs into cardiomyocytes and neurons we show their utility in a high-throughput toxicity screen. To validate hit compounds, we demonstrate dose-dependent toxicity of the hit compounds and assess functional modulation. We also show reproducible in vivo drug toxicity results using mouse models with select hit compounds. This study shows the feasibility of using a population-based hiPSC drug screening platform to assess cytotoxicity, which can be used as an innovative tool to study inter-population differences in drug toxicity and adverse drug reactions in drug discovery applications.

Atomistic mechanisms of human TRPA1 activation by electrophile irritants through molecular dynamics simulation and mutual information analysis

The ion channel TRPA1 is a promiscuous chemosensor, with reported response to a wide spectrum of noxious electrophilic irritants, as well as cold, heat, and mechanosensation. It is also implicated in the inception of itch and pain and has hence been investigated as a drug target for novel analgesics. The mechanism of electrophilic activation for TRPA1 is therefore of broad interest. TRPA1 structures with the pore in both open and closed states have recently been published as well as covalent binding modes for electrophile agonists. However, the detailed mechanism of coupling between electrophile binding sites and the pore remains speculative. In addition, while two different cysteine residues (C621 and C665) have been identified as critical for electrophile bonding and activation, the bound geometry has only been resolved at C621. Here, we use molecular dynamics simulations of TRPA1 in both pore-open and pore-closed states to explore the allosteric link between the electrophile binding sites and pore stability. Our simulations reveal that an open pore is structurally stable in the presence of open ‘pockets’ in the C621/C665 region, but rapidly collapses and closes when these pockets are shut. Binding of electrophiles at either C621 or C665 provides stabilisation of the pore-open state, but molecules bound at C665 are shown to be able to rotate in and out of the pocket, allowing for immediate stabilisation of transient open states. Finally, mutual information analysis of trajectories reveals an informational path linking the electrophile binding site pocket to the pore via the voltage-sensing-like domain, giving a detailed insight into the how the pore is stabilized in the open state.

Treating Duchenne Muscular Dystrophy: The Promise of Stem Cells, Artificial Intelligence, and Multi-Omics

Muscular dystrophies are chronic and debilitating disorders caused by progressive muscle wasting. Duchenne muscular dystrophy (DMD) is the most common type. DMD is a well-characterized genetic disorder caused by the absence of dystrophin. Although some therapies exist to treat the symptoms and there are ongoing efforts to correct the underlying molecular defect, patients with muscular dystrophies would greatly benefit from new therapies that target the specific pathways contributing directly to the muscle disorders. Three new advances are poised to change the landscape of therapies for muscular dystrophies such as DMD. First, the advent of human induced pluripotent stem cells (iPSCs) allows researchers to design effective treatment strategies that make up for the gaps missed by conventional “one size fits all” strategies. By characterizing tissue alterations with single-cell resolution and having molecular profiles for therapeutic treatments for a variety of cell types, clinical researchers can design multi-pronged interventions to not just delay degenerative processes, but regenerate healthy tissues. Second, artificial intelligence (AI) will play a significant role in developing future therapies by allowing the aggregation and synthesis of large and disparate datasets to help reveal underlying molecular mechanisms. Third, disease models using a high volume of multi-omics data gathered from diverse sources carry valuable information about converging and diverging pathways. Using these new tools, the results of previous and emerging studies will catalyze precision medicine-based drug development that can tackle devastating disorders such as DMD.

Transient Receptor Potential (TRP) and Thermoregulation in Animals: Structural Biology and Neurophysiological Aspects

This review presents and analyzes recent scientific findings on the structure, physiology, and neurotransmission mechanisms of transient receptor potential (TRP) and their function in the thermoregulation of mammals. The aim is to better understand the functionality of these receptors and their role in maintaining the temperature of animals, or those susceptible to thermal stress. The majority of peripheral receptors are TRP cation channels formed from transmembrane proteins that function as transductors through changes in the membrane potential. TRP are classified into seven families and two groups. The data gathered for this review include controversial aspects because we do not fully know the mechanisms that operate the opening and closing of the TRP gates. Deductions, however, suggest the intervention of mechanisms related to G protein-coupled receptors, dephosphorylation, and ligands. Several questions emerge from the review as well. For example, the future uses of these data for controlling thermoregulatory disorders and the invitation to researchers to conduct more extensive studies to broaden our understanding of these mechanisms and achieve substantial advances in controlling fever, hyperthermia, and hypothermia.

Transient Receptor Potential Melastatin 2 Thermosensitive Neurons in the Preoptic Area Involved in Menopausal Hot Flashes in Ovariectomized Mice

INTRODUCTION

Menopausal hot flashes are related to hypothalamic preoptic area (POA) dysfunction. Thermosensitive transient receptor potential channels (ThermoTRPs) are involved in temperature sensing and regulation of thermosensitive neurons (TSNs) in the POA. Whether ThermoTRP-TSNs in the POA, particularly the non-noxious thermoreceptor, transient receptor potential melastatin 2 (TRPM2), are involved in the occurrence of hot flashes is still unclear.

METHODS

Twenty wild-type and 50 Trpm2-Cre adult female mice were randomly divided into sham (SHAM) and ovariectomy (OVX) groups. In the POA, ERα, ERβ, GPR30, TRPA1, TRPM8, TRPM2, and TRPV1 expression was detected by Western blot or/and quantitative real-time polymerase chain reaction and the number of TSNs expressing TRPM2 (TRPM2-TSNs) by immunofluorescence. Before and after TRPM2-TSN activation/inhibition, back (BST) and tail skin temperature (TST) and the proportion of glutamatergic and GABAergic neurons among TRPM2-TSNs were recorded.

RESULTS

Compared with SHAM, the expression of ERα, ERβ, TRPM2, and TRPM8 in the POA of the OVX group decreased, with a significantly larger change range for TRPM2 than TRPM8. In addition, the number of TRPM2-TSNs showing TRPA1, TRPM8, and TRPV1 expression in the OVX group decreased, and the proportion of glutamatergic and GABAergic neurons in TRPM2-TSNs decreased and increased, respectively. Meanwhile, BST and TST increased. After activating or inhibiting TRPM2-TSNs, the proportions of glutamatergic and GABAergic neurons in TRPM2-TSNs changed, along with the BST and TST.

CONCLUSION

In menopause, the abnormal quantity and function of TRPM2-TSNs in the POA is key for the development of hot flashes, characterized by an imbalance in heat dissipation and production due to the corresponding imbalance in glutamatergic and GABAergic neurons.

Pleiotropic Effects of Kappa Opioid Receptor-Related Ligands in Non-human Primates

The kappa opioid receptor (KOR)-related ligands have been demonstrated in preclinical studies for several therapeutic potentials. This chapter highlights (1) how non-human primates (NHP) studies facilitate the research and development of ligands targeting the KOR, (2) effects of the endogenous opioid peptide, dynorphin A-(1-17), and its analogs in NHP, and (3) pleiotropic effects and therapeutic applications of KOR-related ligands. In particular, synthetic ligands targeting the KOR have been extensively studied in NHP in three therapeutic areas, i.e., the treatment for itch, pain, and substance use disorders. As the KORs are widely expressed in the peripheral and central nervous systems, pleiotropic effects of KOR-related ligands, such as discriminative stimulus effects, neuroendocrine effects (e.g., prolactin release and stimulation of hypothalamic-pituitary-adrenal axis), and diuresis, in NHP are discussed. Centrally acting KOR agonists are known to produce adverse effects including dysphoria, hallucination, and sedation. Nonetheless, with strategic advances in medicinal chemistry, three classes of KOR-related agonists, i.e., peripherally restricted KOR agonists, mixed KOR/mu opioid receptor partial agonists, and G protein-biased KOR agonists, warrant additional NHP studies to improve our understanding of their functional efficacy, selectivity, and tolerability. Pharmacological studies in NHP which carry high translational significance will facilitate future development of KOR-based medications.

Mini-review: The nociceptive sensory functions of the polymodal receptor Transient Receptor Potential Ankyrin Type 1 (TRPA1)

Over the last 17 years since its cloning in 2003, the receptor-channel TRPA1 has received increasing attention due to its polymodal features and prominent role in pain signaling in a variety of human disease states. While evidence has been accumulating for non-neuronal TRPA1 expression, it is the presence of this channel in nociceptive nerve endings which has taken centre stage, due to its potential clinical ramifications. As a consequence, we shall focus in this review on the sensory functions of TRPA1 related to its expression in the peripheral nervous system. While substantial research has been focused on the putative role of TRPA1 in detecting irritant compounds, noxious cold and mechanical stimuli, the current overall picture is, to some extent, still cloudy. The chemosensory function of the channel is well demonstrated, as well as its involvement in the detection of oxidative and nitrosative stress; however, the other sensory features of TRPA1 have not been fully elucidated yet. The current state of the experimental evidence for these physiological roles of TRPA1 in mammals, and particularly in humans, will be discussed in this review.

Thermal Sensitivity of Heat Sensor TRPA1 Correlates With Temperatures Inducing Heat Avoidance Behavior in Terrestrial Ectotherms

Temperature is an essential environmental factor that controls an organism’s performances. As ectothermic animals largely rely on external heat sources for adjusting their body temperature, thermal perception is a primary process of behavioral thermoregulation. Transient receptor potential ankyrin 1 (TRPA1) is a heat sensitive ion channel in most non-mammalian species, and its heat activation has been suggested to induce heat avoidance behaviors in ectothermic animals. However, associations between TRPA1 and ecologically relevant temperatures have not been investigated, and the analyses including diverse taxa will provide robust support for understanding the associations. Here, I conducted extensive literature review, and assembled published data on thermal threshold of TRPA1 and three physiological parameters: the experimental voluntary maximum (EVM), which is body temperatures when heat avoidance behaviors are induced; the critical thermal maximum (CTmax), which is a point in temperature beyond which an organism becomes incapacitated; and average body temperature (Tmean) recorded in the field. Then, I examined the relationships between thermal threshold of TRPA1 and each of the three physiological parameters. As phylogenetically closely related species tend to show similar trait values among species, I conducted the regression analyses by accounting for phylogenetic distances among species. This study supports previous research by affirming that thermal threshold of TRPA1 is substantially correlated with body temperature that the animals escaped from the heat source, represented here as EVM. Nevertheless, thermal threshold of TRPA1 showed a statistically insignificant correlation with CTmax and Tmean. The results suggest that although thermal threshold of TRPA1 is evolutionarily labile, its associations with EVM is highly conserved among diverse terrestrial ectotherms. Therefore, thermal threshold of TRPA1 could be a useful parameter to evaluate species vulnerability to thermal stress particularly in the recent climate warming scenario.

Satellite glial cells in sensory ganglia express functional transient receptor potential ankyrin 1 that is sensitized in neuropathic and inflammatory pain

Transient receptor potential ankyrin 1 (TRPA1) is well documented as an important molecule in pain hypersensitivity following inflammation and nerve injury and in many other cellular biological processes. Here, we show that TRPA1 is expressed not only by sensory neurons of the dorsal root ganglia (DRG) but also in their adjacent satellite glial cells (SGCs), as well as nonmyelinating Schwann cells. TRPA1 immunoreactivity is also detected in various cutaneous structures of sensory neuronal terminals, including small and large caliber cutaneous sensory fibers and endings. The SGC-expressed TRPA1 is functional. Like DRG neurons, dissociated SGCs exhibit a robust response to the TRPA1-selective agonist allyl isothiocyanate (AITC) by an increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i). These responses are abolished by the TRPA1 antagonist HC030031 and are absent in SGCs and neurons from global TRPA1 null mice. SGCs and neurons harvested from DRG proximal to painful tissue inflammation induced by plantar injection of complete Freund’s adjuvant show greater AITC-evoked elevation of [Ca²⁺]_i and slower recovery compared to sham controls. Similar TRPA1 sensitization occurs in both SGCs and neurons during neuropathic pain induced by spared nerve injury. Together, these results show that functional TRPA1 is expressed by sensory ganglia SGCs, and TRPA1 function in SGCs is enhanced after both peripheral inflammation and nerve injury, and suggest that TRPA1 in SGCs may contribute to inflammatory and neuropathic pain.

Structural and in Vitro Functional Characterization of a Menthyl TRPM8 Antagonist Indicates Species-Dependent Regulation

TRPM8 antagonists derived from its cognate ligand, (-)-menthol, are underrepresented. We determine the absolute stereochemistry of a well-known TRPM8 antagonist, (-)-menthyl 1, using VCD and 2D NMR. We explore 1 for its antagonist effects of the human TRPM8 (hTRPM8) orthologue to uncover species-dependent inhibition versus rat channels. (-)-Menthyl 1 inhibits menthol- and icilin-evoked Ca²⁺ responses at hTRPM8 with IC₅₀ values of 805 ± 200 nM and 1.8 ± 0.6 μM, respectively, while more potently inhibiting agonist responses at the rat orthologue (rTRPM8 IC₅₀ (menthol) = 117 ± 18 nM, IC₅₀ (icilin) = 521 ± 20 nM). Whole-cell patch-clamp recordings of hTRPM8 confirm the 1 inhibition of menthol-stimulated currents, with an IC₅₀ of 700 ± 200 nM. We demonstrate that 1 possesses ≥400-fold selectivity for hTRPM8 versus hTRPA1/hTRPV1. (-)-menthyl 1 can be used as a novel chemical tool to study hTRPM8 pharmacology and differences in species commonly used in drug discovery.

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.

Enhanced Sensory Coding in Mouse Vibrissal and Visual Cortex through TRPA1

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel, broadly expressed throughout the body. Despite its expression in the mammalian brain, little is known about the contribution of TRPA1 to cortical function. Here, we characterize how TRPA1 affects sensory information processing in two cortical areas in mice: the primary vibrissal (whisker) somatosensory cortex (vS1) and the primary visual cortex (V1). In vS1, local activation of TRPA1 by allyl isothiocyanate (AITC) increases the ongoing activity of neurons and their evoked response to vibrissal stimulation, producing a positive gain modulation. The gain modulation is reversed by TRPA1 inhibitor HC-030031 and is absent in TRPA1 knockout mice. Similarly, in V1, TRPA1 activation increases the gain of direction and orientation selectivity. Linear decoding of V1 population activity confirms faster and more reliable encoding of visual signals under TRPA1 activation. Overall, our findings reveal a physiological role for TRPA1 in enhancing sensory signals in the mammalian cortex.

Pharmacological modulation of brain activation to non-noxious stimulation in a cynomolgus macaque model of peripheral nerve injury

In vivo neuroimaging could be utilized as a noninvasive tool for elaborating the CNS mechanism of chronic pain and for elaborating mechanisms of potential analgesic therapeutics. A model of unilateral peripheral neuropathy was developed in the cynomolgus macaque, a species that is phylogenetically close to humans. Nerve entrapment was induced by placing a 4 mm length of polyvinyl cuff around the left common sciatic nerve. Prior to nerve injury, stimulation of the foot with a range of non-noxious von Frey filaments (1, 4, 8, 15, and 26 g) did not evoke brain activation as observed with functional magnetic resonance imaging (fMRI). Two weeks after injury, stimulation of the ipsilateral foot with non-noxious filaments activated the contralateral insula/secondary somatosensory cortex (Ins/SII) and anterior cingulate cortex (ACC). By contrast, no activation was observed with stimulation of the contralateral foot. Robust bilateral activation of thalamus was observed three to five weeks after nerve injury. Treatment with the clinical analgesic pregabalin reduced evoked activation of Ins/SII, thalamus and ACC whereas treatment with the NK1 receptor antagonist aprepitant reduced activation of the ipsilateral (left) thalamus. Twelve to 13 weeks after nerve injury, treatment with pregabalin reduced evoked activation of all regions of interest (ROI). By contrast, brain activation persisted in most ROI, except the ACC, following aprepitant treatment. Activation of the contralateral Ins/SII and bilateral thalamus was observed six months after nerve injury and pregabalin treatment suppressed activation of these nuclei. The current findings demonstrated persistent changes in CNS neurons following nerve injury as suggested by activation with non-painful mechanical stimulation. Furthermore, it was possible to functionally distinguish between a clinically efficacious analgesic drug, pregabalin, from a drug that has not demonstrated significant clinical analgesic efficacy, aprepitant. In vivo neuroimaging in the current nonhuman model could enhance translatability.

Atractylodin Produces Antinociceptive Effect through a Long-Lasting TRPA1 Channel Activation

Atractylodin (ATR) is a bioactive component found in dried rhizomes of Atractylodes lancea (AL) De Candolle. Although AL has accumulated empirical evidence for the treatment of pain, the molecular mechanism underlying the anti-pain effect of ATR remains unclear. In this study, we found that ATR increases transient receptor potential ankyrin-1 (TRPA1) single-channel activity in hTRPA1 expressing HEK293 cells. A bath application of ATR produced a long-lasting calcium response, and the response was completely diminished in the dorsal root ganglion neurons of TRPA1 knockout mice. Intraplantar injection of ATR evoked moderate and prolonged nociceptive behavior compared to the injection of allyl isothiocyanate (AITC). Systemic application of ATR inhibited AITC-induced nociceptive responses in a dose-dependent manner. Co-application of ATR and QX-314 increased the noxious heat threshold compared with AITC in vivo. Collectively, we concluded that ATR is a unique agonist of TRPA1 channels, which produces long-lasting channel activation. Our results indicated ATR-mediated anti-nociceptive effect through the desensitization of TRPA1-expressing nociceptors.

Interactions between Chemesthesis and Taste: Role of TRPA1 and TRPV1

In addition to the sense of taste and olfaction, chemesthesis, the sensation of irritation, pungency, cooling, warmth, or burning elicited by spices and herbs, plays a central role in food consumption. Many plant-derived molecules demonstrate their chemesthetic properties via the opening of transient receptor potential ankyrin 1 (TRPA1) and transient receptor potential vanilloid 1 (TRPV1) channels. TRPA1 and TRPV1 are structurally related thermosensitive cation channels and are often co-expressed in sensory nerve endings. TRPA1 and TRPV1 can also indirectly influence some, but not all, primary taste qualities via the release of substance P and calcitonin gene-related peptide (CGRP) from trigeminal neurons and their subsequent effects on CGRP receptor expressed in Type III taste receptor cells. Here, we will review the effect of some chemesthetic agonists of TRPA1 and TRPV1 and their influence on bitter, sour, and salt taste qualities.

Human Induced Pluripotent Stem Cells as a Screening Platform for Drug-Induced Vascular Toxicity

Evaluation of potential vascular injury is an essential part of the safety study during pharmaceutical development. Vascular liability issues are important causes of drug termination during preclinical investigations. Currently, preclinical assessment of vascular toxicity primarily relies on the use of animal models. However, accumulating evidence indicates a significant discrepancy between animal toxicity and human toxicity, casting doubt on the clinical relevance of animal models for such safety studies. While the causes of this discrepancy are expected to be multifactorial, species differences are likely a key factor. Consequently, a human-based model is a desirable solution to this problem, which has been made possible by the advent of human induced pluripotent stem cells (iPSCs). In particular, recent advances in the field now allow the efficient generation of a variety of vascular cells (e.g., endothelial cells, smooth muscle cells, and pericytes) from iPSCs. Using these cells, different vascular models have been established, ranging from simple 2D cultures to highly sophisticated vascular organoids and microfluidic devices. Toxicity testing using these models can recapitulate key aspects of vascular pathology on molecular (e.g., secretion of proinflammatory cytokines), cellular (e.g., cell apoptosis), and in some cases, tissue (e.g., endothelium barrier dysfunction) levels. These encouraging data provide the rationale for continuing efforts in the exploration, optimization, and validation of the iPSC technology in vascular toxicology.

Thermosensation involving thermo-TRPs

The transient receptor potential (TRP) channels constitute a superfamily of large ion channels that are activated by a wide range of chemical, mechanical and thermal stimuli. TRP channels with temperature sensitivity are called thermo-TRPs. They are involved in diverse physiological functions through their detection of external environmental temperature and internal body temperature. Each thermo-TRP has its own characteristic temperature threshold for activation. As a group, they cover temperatures ranging from cold to nociceptive high temperatures. Recently, many studies have identified the functions of thermo-TRPs residing in deep organs where they are exposed to body temperature. Importantly, temperature thresholds of thermo-TRPs can be regulated by physiological factors enabling their function at relatively constant body temperature. Moreover, several thermo-TRPs are reportedly engaged in body temperature regulation. This review will summarize the current understanding of thermo-TRPs, including their roles in thermosensation and functional regulation of physiological responses at body temperature and the regulation of body temperature.

Quaternary Lidocaine Derivatives: Past, Present, and Future

Local anesthetics have the advantage of complete analgesia with fewer side effects compared to systemic analgesics. However, their clinical use is limited due to their short duration of action. Thus, local anesthetics with fast onset, long duration of action, selective nociceptive block, and low local and systemic toxicity are highly desirable. In the past electrophysiological studies, quaternary lidocaine derivatives (QLDs) showed these characteristics. Here, we review electrophysiological properties of QLDs and their pharmacodynamic characteristics to shed light on potential problems.

Angiotensin Type 2 Receptors: Painful, or Not?

Pain in response to various types of acute injury can be a protective stimulus to prevent the organism from using the injured part and allow tissue repair and healing. On the other hand, neuropathic pain, defined as ‘pain caused by a lesion or disease of the somatosensory nervous system’, is a debilitating pathology. The TRPA1 neurons in the Dorsal Root Ganglion (DRG) respond to reactive oxygen species (ROS) and induce pain. In acute nerve injury and inflammation, macrophages infiltrating the site of injury undergo an oxidative burst, and generate ROS that promote tissue repair and induce pain via TRPA1. The latter discourages using the injured limb, with a lack of movement helping wound healing. In chronic inflammation caused by diabetes, cancer etc., ROS levels increase systemically and modulate TRPA1 neuronal functions and cause debilitating neuropathic pain. It is important to distinguish between drug targets that elicit protective vs. debilitating pain when developing effective drugs for neuropathic pain. In this context, the connection of the Angiotensin type 2 receptor (AT₂R) to neuropathic pain presents an interesting dilemma. Several lines of evidence show that AT₂R activation promotes anti-inflammatory and anti-nociceptive signaling, tissue repair, and suppresses ROS in chronic inflammatory models. Conversely, some studies suggest that AT₂R antagonists are anti-nociceptive and therefore AT₂R is a drug target for neuropathic pain. However, AT₂R expression in nociceptive neurons is lacking, indicating that neuronal AT₂R is not involved in neuropathic pain. It is also important to consider that Novartis terminated their phase II clinical trial (EMPHENE) to validate that AT₂R antagonist EMA401 mitigates post-herpetic neuralgia. This trial, conducted in Australia, United Kingdom, and a number of European and Asian countries in 2019, was discontinued due to pre-clinical drug toxicity data. Moreover, early data from the trial did not show statistically significant positive outcomes. These facts suggest that may AT₂R not be the proper drug target for neuropathic pain in humans and its inhibition can be harmful.

Pain pathways and potential new targets for pain relief

Pain is an unpleasant sensory and emotional experience that affects a sizable percentage of people on a daily basis. Sensory neurons known as nociceptors built specifically to detect damaging stimuli can be found throughout the body. They transmit information about noxious stimuli from mechanical, thermal, and chemical sources to the central nervous system and higher brain centers via electrical signals. Nociceptors express various channels and receptors such as voltage-gated sodium and calcium channels, transient receptor potential channels, and opioid receptors that allow them to respond in a highly specific manner to noxious stimuli. Attenuating the pain response can be achieved by inhibiting or altering the expression of these pain targets. Achieving a deeper understanding of how these receptors can be affected at the molecular level can lead to the development of novel pain therapies. This review will discuss the mechanisms of pain, introduce the various receptors that are responsible for detecting pain, and future directions in pharmacological therapies.

A Non-covalent Ligand Reveals Biased Agonism of the TRPA1 Ion Channel

The TRPA1 ion channel is activated by electrophilic compounds through the covalent modification of intracellular cysteine residues. How non-covalent agonists activate the channel and whether covalent and non-covalent agonists elicit the same physiological responses are not understood. Here, we report the discovery of a non-covalent agonist, GNE551, and determine a cryo-EM structure of the TRPA1-GNE551 complex, revealing a distinct binding pocket and ligand-interaction mechanism. Unlike the covalent agonist allyl isothiocyanate, which elicits channel desensitization, tachyphylaxis, and transient pain, GNE551 activates TRPA1 into a distinct conducting state without desensitization and induces persistent pain. Furthermore, GNE551-evoked pain is relatively insensitive to antagonist treatment. Thus, we demonstrate the biased agonism of TRPA1, a finding that has important implications for the discovery of effective drugs tailored to different disease etiologies.

Transient receptor potential channels in pulmonary chemical injuries and as countermeasure targets

The lung is highly sensitive to chemical injuries caused by exposure to threat agents in industrial or transportation accidents, occupational exposures, or deliberate use as weapons of mass destruction (WMD). There are no antidotes for the majority of the chemical threat agents and toxic inhalation hazards despite their use as WMDs for more than a century. Among several putative targets, evidence for transient receptor potential (TRP) ion channels as mediators of injury by various inhalational chemical threat agents is emerging. TRP channels are expressed in the respiratory system and are essential for homeostasis. Among TRP channels, the body of literature supporting essential roles for TRPA1, TRPV1, and TRPV4 in pulmonary chemical injuries is abundant. TRP channels mediate their function through sensory neuronal and nonneuronal pathways. TRP channels play a crucial role in complex pulmonary pathophysiologic events including, but not limited to, increased intracellular calcium levels, signal transduction, recruitment of proinflammatory cells, neurogenic inflammatory pathways, cough reflex, hampered mucus clearance, disruption of the integrity of the epithelia, pulmonary edema, and fibrosis. In this review, we summarize the role of TRP channels in chemical threat agents-induced pulmonary injuries and how these channels may serve as medical countermeasure targets for broader indications.