Ca2+-dependent induction of TRPM2 currents in hippocampal neurons

Extinction of contextual fear memory is facilitated in TRPM2 knockout mice

Transient receptor potential melastatin type 2 (TRPM2) is a nonselective cation channel involved in synaptic plasticity. We investigated its role in contextual fear conditioning and extinction of conditioned fear using Trpm2-deficient (Trpm2-/-) mice. Trpm2-/- mice exhibited reduced acquisition of contextual fear memory during conditioning but had an intact freezing response to conditioning context 24 h after conditioning. They also showed a reduced freezing response to extinction training, indicating facilitated extinction. Consistent with this, infusion of flufenamic acid (FFA), a TRPM2 antagonist, into the dentate gyrus (DG) of the hippocampus in fear-conditioned mice facilitated extinction of contextual fear. The enhanced extinction in Trpm2-/- and FFA-treated mice was associated with down-regulation of immediate-early genes (IEGs) including Npas4, c-Fos, Arc and Egr1 in the hippocampus after extinction training. Our results indicate that TRPM2 plays a positive role in retention of contextual fear memory by modulating neuronal activity in the hippocampus, and suggest that TRPM2 activity could potentially be targeted to strengthen extinction-based exposure therapies for post-traumatic stress disorder (PTSD).

A conserved mechanism couples cytosolic domain movements to pore gating in the TRPM2 channel

Transient Receptor Potential Melastatin 2 (TRPM2) cation channels contribute to immunocyte activation, insulin secretion, and central thermoregulation. TRPM2 opens upon binding cytosolic Ca2+ and ADP ribose (ADPR). We present here the 2.5 Å cryo-electronmicroscopy structure of TRPM2 from Nematostella vectensis (nvTRPM2) in a lipid nanodisc, complexed with Ca2+ and ADPR-2'-phosphate. Comparison with nvTRPM2 without nucleotide reveals that nucleotide binding-induced movements in the protein's three "core" layers deconvolve into a set of rigid-body rotations conserved from cnidarians to man. By covalently crosslinking engineered cysteine pairs we systematically trap the cytosolic layers in specific conformations and study effects on gate opening/closure. The data show that nucleotide binding in Layer 3 disrupts inhibitory intersubunit interactions, allowing rotation of Layer 2 which in turn expands the gate located in Layer 1. Channels trapped in that "activated" state are no longer nucleotide dependent, but are opened by binding of Ca2+ alone.

Identification of Ligands for Ion Channels: TRPM2

Transient receptor potential melastatin 2 (TRPM2) is a calcium-permeable, nonselective cation channel with a widespread distribution throughout the body. It is involved in many pathological and physiological processes, making it a potential therapeutic target for various diseases, including Alzheimer's disease, Parkinson's disease, and cancers. New analytical techniques are beneficial for gaining a deeper understanding of its involvement in disease pathogenesis and for advancing the drug discovery for TRPM2-related diseases. In this work, we present the application of collision-induced affinity selection mass spectrometry (CIAS-MS) for the direct identification of ligands binding to TRPM2. CIAS-MS circumvents the need for high mass detection typically associated with mass spectrometry of large membrane proteins. Instead, it focuses on the detection of small molecules dissociated from the ligand-protein-detergent complexes. This affinity selection approach consolidates all affinity selection steps within the mass spectrometer, resulting in a streamlined process. We showed the direct identification of a known TRPM2 ligand dissociated from the protein-ligand complex. We demonstrated that CIAS-MS can identify binding ligands from complex mixtures of compounds and screened a compound library against TRPM2. We investigated the impact of voltage increments and ligand concentrations on the dissociation behavior of the binding ligand, revealing a dose-dependent relationship.

TRPM2 - An adjustable thermostat

The Transient Receptor Potential Melastatin 2 (TRPM2) channel is a homotetrameric ligand-gated cation channel opened by the binding of cytosolic ADP ribose (ADPR) and Ca2+. In addition, strong temperature dependence of its activity has lately become a center of attention for both physiological and biophysical studies. TRPM2 temperature sensitivity has been affirmed to play a role in central and peripheral thermosensation, pancreatic insulin secretion, and immune cell function. On the other hand, a number of different underlying mechanisms have been proposed from studies in intact cells. This review summarizes available information on TRPM2 temperature sensitivity, with a focus on recent mechanistic insight obtained in a cell-free system. Those biophysical results outline TRPM2 as a channel with an intrinsically endothermic opening transition, a temperature threshold strongly modulated by cytosolic agonist concentrations, and a response steepness greatly enhanced through a positive feedback loop generated by Ca2+ influx through the channel's pore. Complex observations in intact cells and apparent discrepancies between studies using in vivo and in vitro models are discussed and interpreted in light of the intrinsic biophysical properties of the channel protein.

Pathophysiological Roles of Transient Receptor Potential (Trp) Channels and Zinc Toxicity in Brain Disease

Maintaining the correct ionic gradient from extracellular to intracellular space via several membrane-bound transporters is critical for maintaining overall cellular homeostasis. One of these transporters is the transient receptor potential (TRP) channel family that consists of six putative transmembrane segments systemically expressed in mammalian tissues. Upon the activation of TRP channels by brain disease, several cations are translocated through TRP channels. Brain disease, especially ischemic stroke, epilepsy, and traumatic brain injury, triggers the dysregulation of ionic gradients and promotes the excessive release of neuro-transmitters and zinc. The divalent metal cation zinc is highly distributed in the brain and is specifically located in the pre-synaptic vesicles as free ions, usually existing in cytoplasm bound with metallothionein. Although adequate zinc is essential for regulating diverse physiological functions, the brain-disease-induced excessive release and translocation of zinc causes cell damage, including oxidative stress, apoptotic cascades, and disturbances in energy metabolism. Therefore, the regulation of zinc homeostasis following brain disease is critical for the prevention of brain damage. In this review, we summarize recent experimental research findings regarding how TRP channels (mainly TRPC and TRPM) and zinc are regulated in animal brain-disease models of global cerebral ischemia, epilepsy, and traumatic brain injury. The blockade of zinc translocation via the inhibition of TRPC and TRPM channels using known channel antagonists, was shown to be neuroprotective in brain disease. The regulation of both zinc and TRP channels may serve as targets for treating and preventing neuronal death.

Transient Receptor Potential (TRP) Channels in Pain, Neuropsychiatric Disorders, and Epilepsy

Pharmacomodulation of membrane channels is an essential topic in the study of physiological conditions and disease status. Transient receptor potential (TRP) channels are one such family of nonselective cation channels that have an important influence. In mammals, TRP channels consist of seven subfamilies with a total of twenty-eight members. Evidence shows that TRP channels mediate cation transduction in neuronal signaling, but the full implication and potential therapeutic applications of this are not entirely clear. In this review, we aim to highlight several TRP channels which have been shown to mediate pain sensation, neuropsychiatric disorders, and epilepsy. Recent findings suggest that TRPM (melastatin), TRPV (vanilloid), and TRPC (canonical) are of particular relevance to these phenomena. The research reviewed in this paper validates these TRP channels as potential targets of future clinical treatment and offers patients hope for more effective care.

Transient receptor potential ion channels and cerebral stroke

METHODS

The databases Pubmed, and the National Library of Medicine were searched for literature. All papers on celebral stroke and transient receptor potential ion channels were considered.

RESULTS

Stroke is the second leading cause of death and disability, with an increasing incidence in developing countries. About 75 per cent of strokes are caused by occlusion of cerebral arteries, and substantial advances have been made in elucidating mechanisms how stroke affects the brain. Transient receptor potential (TRP) ion channels are calcium-permeable channels highly expressed in brain that drives Ca²⁺ entry into multiple cellular compartments. TRPC1/3/4/6, TRPV1/2/4, and TRPM2/4/7 channels have been implicated in stroke pathophysiology.

CONCLUSIONS

Although the precise mechanism of transient receptor potential ion channels in cerebral stroke is still unclear, it has the potential to be a therapeutic target for patients with stroke if developed appropriately. Hence, more research is needed to prove its efficacy in this context.

Neuroprotection against supra-lethal 'stroke in a dish' insults by an anti-excitotoxic receptor antagonist cocktail

The goal of this study was to identify cocktails of drugs able to protect cultured rodent cortical neurons against increasing durations of oxygen-glucose deprivation (OGD). As expected, a cocktail composed of an NMDA and AMPA receptor antagonists and a voltage gated Ca²⁺ channel blocker (MK-801, CNQX and nifedipine, respectively) provided complete neuroprotection against mild OGD. Increasingly longer durations of OGD necessitated increasing the doses of MK-801 and CNQX, until these cocktails ultimately failed to provide neuroprotection against supra-lethal OGD, even at maximal drug concentrations. Surprisingly, supplementation of any of these cocktails with blockers of TRPM7 channels for increasing OGD durations was not neuroprotective, unless these blockers possessed the ability to inhibit NMDA receptors. Supplementation of the maximally effective cocktail with other NMDA receptor antagonists augmented neuroprotection, suggesting insufficient NMDAR blockade by MK-801. Substitution of MK-801 in cocktails with high concentrations of a glycine site NMDA receptor antagonist caused the greatest improvements in neuroprotection, with the more potent SM-31900 superior to L689,560. Substitution of CQNX in cocktails with AMPA receptor antagonists at high concentrations also improved neuroprotection, particularly with the combination of SYM 2206 and NBQX. The most neuroprotective cocktail was thus composed of SM-31900, SYM2206, NBQX, nifedipine and the antioxidant trolox. Thus, the cumulative properties of antagonist potency and concentration in a cocktail dictate neuroprotective efficacy. The central target of supra-lethal OGD is excitotoxicity, which must be blocked to the greatest extent possible to minimize ion influx.

Functional coupling of TRPM2 and extrasynaptic NMDARs exacerbates excitotoxicity in ischemic brain injury

Excitotoxicity induced by NMDA receptor (NMDAR) activation is a major cause of neuronal death in ischemic stroke. However, past efforts of directly targeting NMDARs have unfortunately failed in clinical trials. Here, we reveal an unexpected mechanism underlying NMDAR-mediated neurotoxicity, which leads to the identification of a novel target and development of an effective therapeutic peptide for ischemic stroke. We show that NMDAR-induced excitotoxicity is enhanced by physical and functional coupling of NMDAR to an ion channel TRPM2 upon ischemic insults. TRPM2-NMDAR association promotes the surface expression of extrasynaptic NMDARs, leading to enhanced NMDAR activity and increased neuronal death. We identified a specific NMDAR-interacting motif on TRPM2 and designed a membrane-permeable peptide to uncouple the TRPM2-NMDAR interaction. This disrupting peptide protects neurons against ischemic injury in vitro and protects mice against ischemic stroke in vivo. These findings provide an unconventional strategy to mitigate excitotoxic neuronal death without directly targeting NMDARs.

Distribution and Assembly of TRP Ion Channels

In the last several decades, a large family of ion channels have been identified and studied intensively as cellular sensors for diverse physical and/or chemical stimuli. Named transient receptor potential (TRP) channels, they play critical roles in various aspects of cellular physiology. A large number of human hereditary diseases are found to be linked to TRP channel mutations, and their dysregulations lead to acute or chronical health problems. As TRP channels are named and categorized mostly based on sequence homology rather than functional similarities, they exhibit substantial functional diversity. Rapid advances in TRP channel study have been made in recent years and reported in a vast body of literature; a summary of the latest advancements becomes necessary. This chapter offers an overview of current understandings of TRP channel distribution and subunit assembly.

A Systemic Review of the Integral Role of TRPM2 in Ischemic Stroke: From Upstream Risk Factors to Ultimate Neuronal Death

Ischemic stroke causes a heavy health burden worldwide, with over 10 million new cases every year. Despite the high prevalence and mortality rate of ischemic stroke, the underlying molecular mechanisms for the common etiological factors of ischemic stroke and ischemic stroke itself remain unclear, which results in insufficient preventive strategies and ineffective treatments for this devastating disease. In this review, we demonstrate that transient receptor potential cation channel, subfamily M, member 2 (TRPM2), a non-selective ion channel activated by oxidative stress, is actively involved in all the important steps in the etiology and pathology of ischemic stroke. TRPM2 could be a promising target in screening more effective prophylactic strategies and therapeutic medications for ischemic stroke.

TRPM2 in ischemic stroke: Structure, molecular mechanisms, and drug intervention

Ischemic stroke has a high lethality rate worldwide, and novel treatments are limited. Calcium overload is considered to be one of the mechanisms of cerebral ischemia. Transient receptor potential melastatin 2 (TRPM2) is a reactive oxygen species (ROS)-sensitive calcium channel. Cerebral ischemia-induced TRPM2 activation triggers abnormal intracellular Ca2+ accumulation and cell death, which in turn causes irreversible brain damage. Thus, TRPM2 has emerged as a new therapeutic target for ischemic stroke. This review provides data on the expression, structure, and function of TRPM2 and illustrates its cellular and molecular mechanisms in ischemic stroke. Natural and synthetic TRPM2 inhibitors (both specific and nonspecific) are also summarized. The three-dimensional protein structure of TRPM2 has been identified, and we speculate that molecular simulation techniques will be essential for developing new drugs that block TRPM2 channels. These insights about TRPM2 may be the key to find potent therapeutic approaches for the treatment of ischemic stroke.

Hydrogen peroxide and phosphoinositide metabolites synergistically regulate a cation current to influence neuroendocrine cell bursting

In various neurons, including neuroendocrine cells, non-selective cation channels elicit plateau potentials and persistent firing. Reproduction in the marine snail Aplysia californica is initiated when the neuroendocrine bag cell neurons undergo an afterdischarge, that is, a prolonged period of enhanced excitability and spiking during which egg-laying hormone is released into the blood. The afterdischarge is associated with both the production of hydrogen peroxide (H₂ O₂ ) and activation of phospholipase C (PLC), which hydrolyses phosphatidylinositol-4,5-bisphosphate into diacylglycerol (DAG) and inositol trisphosphate (IP₃ ). We previously demonstrated that H₂ O₂ gates a voltage-dependent cation current and evokes spiking in bag cell neurons. The present study tests if DAG and IP₃ impact the H₂ O₂ -induced current and excitability. In whole-cell voltage-clamped cultured bag cell neurons, bath-application of 1-oleoyl-2-acetyl-sn-glycerol (OAG), a DAG analogue, enhanced the H₂ O₂ -induced current, which was amplified by the inclusion of IP₃ in the pipette. A similar outcome was produced by the PLC activator, N-(3-trifluoromethylphenyl)-2,4,6-trimethylbenzenesulfonamide. In current-clamp, OAG or OAG plus IP₃ , elevated the frequency of H₂ O₂ -induced bursting. PKC is also triggered during the afterdischarge; when PKC was stimulated with phorbol 12-myristate 13-acetate, it caused a voltage-dependent inward current with a reversal potential similar to the H₂ O₂ -induced current. Furthermore, PKC activation followed by H₂ O₂ reduced the onset latency and increased the duration of action potential firing. Finally, inhibiting nicotinamide adenine dinucleotide phosphate oxidase with 3-benzyl-7-(2-benzoxazolyl)thio-1,2,3-triazolo[4,5-d]pyrimidine diminished evoked bursting in isolated bag cell neuron clusters. These results suggest that reactive oxygen species and phosphoinostide metabolites may synergize and contribute to reproductive behaviour by promoting neuroendocrine cell firing. KEY POINTS: Aplysia bag cell neurons secrete reproductive hormone during a lengthy burst of action potentials, known as the afterdischarge. During the afterdischarge, phospholipase C (PLC) hydrolyses phosphatidylinositol-4,5-bisphosphate into diacylglycerol (DAG) and inositol trisphosphate (IP₃ ). Subsequent activation of protein kinase C (PKC) leads to H₂ O₂ production. H₂ O₂ evokes a voltage-dependent inward current and action potential firing. Both a DAG analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG), and IP₃ enhance the H₂ O₂ -induced current, which is mimicked by the PLC activator, N-(3-trifluoromethylphenyl)-2,4,6-trimethylbenzenesulfonamide. The frequency of H₂ O₂ -evoked afterdischarge-like bursting is augmented by OAG or OAG plus IP₃ . Stimulating PKC with phorbol 12-myristate 13-acetate shortens the latency and increases the duration of H₂ O₂ -induced bursts. The nicotinamide adenine dinucleotide phosphate oxidase inhibitor, 3-benzyl-7-(2-benzoxazolyl)thio-1,2,3-triazolo[4,5-d]pyrimidine, attenuates burst firing in bag cell neuron clusters.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Research progress on transient receptor potential melastatin 2 channel in nervous system diseases

Transient receptor potential M2 (TRPM2) ion channel is a non-selective cationic channel that can permeate calcium ions, and plays an important role in neuroinflammation, ischemic reperfusion brain injury, neurodegenerative disease, neuropathic pain, epilepsy and other neurological diseases. In ischemic reperfusion brain injury, TRPM2 mediates neuronal death by modulating the different subunits of glutamate N-methyl-D-aspartic acid receptor in response to calcium/zinc signal. In Alzheimer's disease, TRPM2 is activated by reactive oxygen species generated by β-amyloid peptide to form a malignant positive feedback loop that induces neuronal death and is involved in the pathological process of glial cells by promoting inflammatory response and oxidative stress. In epilepsy, the TRPM2-knockout alleviates epilepsy induced neuronal degeneration by inhibiting autophagy and apoptosis related proteins. The roles of TRPM2 channel in the pathogenesis of various central nervous system diseases and its potential drug development and clinical application prospects are summarized in this review.

Role of Oxidative Stress and Ca2+ Signaling in Psychiatric Disorders

Psychiatric disorders are caused by complex and diverse factors, and numerous mechanisms have been proposed for the pathogenesis of these disorders. Accumulating evidence suggests that oxidative stress is one of the general factors involved in the pathogenesis/pathophysiology of major psychiatric disorders, including bipolar disorder, depression, anxiety disorder, and schizophrenia. Indeed, some clinical trials have shown improvement of the symptoms of these disorders by antioxidant supplementation. However, the molecular basis for the relationship between oxidative stress and the pathogenesis of psychiatric disorders remains largely unknown. In general, Ca2+ channels play central roles in neuronal functions, including neuronal excitability, neurotransmitter release, synaptic plasticity, and gene regulation, and genes that encode Ca2+ channels have been found to be associated with psychiatric disorders. Notably, a class of Ca2+-permeable transient receptor potential (TRP) cation channels is activated by changes in cellular redox status, whereby these TRP channels can link oxidative stress to Ca2+ signals. Given the unique characteristic of redox-sensitive TRP channels, these channels could be a target for delineating the pathogenesis or pathophysiology of psychiatric disorders. In this review, we summarize the outcomes of clinical trials for antioxidant treatment in patients with psychiatric disorders and the current insights into the physiological/pathological significance of redox-sensitive TRP channels in the light of neural functions, including behavioral phenotypes, and discuss the potential role of TRP channels in the pathogenesis of psychiatric disorders. Investigation of redox-sensitive TRP channels may lead to the development of novel therapeutic strategies for the treatment of psychiatric disorders.

KATP and TRPM2-like channels couple metabolic status to resting membrane potential of octopus neurons in the mouse ventral cochlear nucleus

ATP-sensitive potassium (K_ATP) channels and transient receptor potential melastatin 2 (TRPM2) channels are commonly expressed both pre- and postsynaptically in the central nervous system (CNS). We hypothesized that K_ATP and TRPM2 may couple metabolic status to the resting membrane potential of octopus neurons of the mouse ventral cochlear nucleus (VCN). Therefore, we studied the expression of K_ATP channels and TRPM2 channels in octopus cells by immunohistochemical techniques and their contribution to neuronal electrical properties by the electrophysiological patch clamp technique. In immunohistochemical staining of octopus cells, labelling with Kir6.2 and SUR1 antibodies was strong, and labelling with the SUR2 antibody was moderate, but labelling with Kir6.1 was very weak. Octopus cells had intense staining with TRPM2 antibodies. In patch clamp recordings, bath application of K_ATP channel agonists H₂O₂ (880 μM), ATZ (1 mM), cromakalim (50 μM), diazoxide (200 μM), NNC 55-0118 and NN 414 separately resulted in hyperpolarizations of resting potential to different extents. Application of 8-Bro-cADPR (50 μM), a specific antagonist of TRPM2 channels, in the presence of H₂O₂ (880 μM) resulted in further hyperpolarization by approximately 1 mV. The amplitudes of H₂O₂-induced outward K_ATP currents and ADPR-induced inward currents were 206.1 ± 31.5 pA (n = 4) and 136.8 ± 22.4 pA, respectively, at rest. Their respective reversal potentials were -77 ± 2.6 mV (n = 3) and -6.3 ± 2.9 (n = 3) and -6.3 ± 2.9 (n = 3). In conclusion, octopus cells appear to possess both K_ATP channels and TRPM2-like channels. K_ATP might largely be constituted by SUR1-Kir6.2 subunits and SUR2-Kir6.2 subunits. Both K_ATP and TRPM2-like channels might have a modulatory action in setting the membrane potential.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.

Modulation of the excitability of stellate neurons in the ventral cochlear nucleus of mice by TRPM2 channels

Oxidative stress-induced Ca²⁺ permeable transient receptor potential melastatin 2 (TRPM2) channels are expressed at high levels in the brain, appear to link neuronal excitability to cellular metabolism, and are involved in the pathogenesis of neurodegenerative disorders. We aimed to study the electrophysiological properties of TRPM2 channels in stellate cells of the mouse ventral cochlear nucleus (VCN) using molecular, immunohistochemical and electrophysiological approaches. In the present study, the real time PCR analysis revealed the presence of the TRPM2 mRNA in the mouse VCN tissue. Cell bodies of stellate cells were moderately labeled with TRPM2 antibodies using immunohistochemical staining. Stellate cells were sensitive to intracellular ADP-ribose (ADPR), a TRPM2 agonist. Upon the application of ADPR, the resting membrane potential of the stellate cells was significantly depolarized, shifting from -61.2 ± 0.9 mV to -57.0 ± 0.8 mV (P < 0.001; n = 21), and the firing rate significantly increased (P < 0.001, n = 6). When the pipette solution contained ADPR (300 μM) and the TRPM2 antagonists flufenamic acid (FFA) (100 μM), N-(p-amylcinnamoyl) anthranilic acid (ACA) (50 μM) and 8-bromo-cADP-Ribose (8-Br-cADPR) (50 μM), the membrane potential shifted in a hyperpolarizing direction. ADPR did not significantly change the resting membrane potential and action potential firing rate of stellate cells from TRPM2-/- mice. In conclusion, the results obtained using these molecular, immunohistochemical and electrophysiological approaches reveal the expression of functional TRPM2 channels in stellate neurons of the mouse VCN. TRPM2 might exert a significant modulatory effect on setting the level of resting excitability.

Characterization and Optimization of the Novel Transient Receptor Potential Melastatin 2 Antagonist tatM2NX

Transient receptor potential melastatin 2 (TRPM2) is a calcium-permeable channel activated by adenosine diphosphate ribose metabolites and oxidative stress. TRPM2 contributes to neuronal injury in the brain caused by stroke and cardiac arrest among other diseases including pain, inflammation, and cancer. However, the lack of specific inhibitors hinders the study of TRPM2 in brain pathophysiology. Here, we present the design of a novel TRPM2 antagonist, tatM2NX, which prevents ligand binding and TRPM2 activation. We used mutagenesis of tatM2NX to determine the structure-activity relationship and antagonistic mechanism on TRPM2 using whole-cell patch clamp and Calcium imaging in human embryonic kidney 293 cells with stable human TRPM2 expression. We show that tatM2NX inhibits over 90% of TRPM2 channel currents at concentrations as low as 2 μM. Moreover, tatM2NX is a potent antagonist with an IC50 of 396 nM. Our results from tatM2NX mutagenesis indicate that specific residues within the tatM2NX C terminus are required to confer antagonism on TRPM2. Therefore, the peptide tatM2NX represents a new tool for the study of TRPM2 function in cell biology and enhances our understanding of TRPM2 in disease. SIGNIFICANCE STATEMENT: TatM2NX is a potent TRPM2 channel antagonist with the potential for clinical benefit in neurological diseases. This study characterizes interactions of tatM2NX with TRPM2 and the mechanism of action using structure-activity analysis.

Hydrogen Peroxide Gates a Voltage-Dependent Cation Current in Aplysia Neuroendocrine Cells

Nonselective cation channels promote persistent spiking in many neurons from a diversity of animals. In the hermaphroditic marine-snail, Aplysia californica, synaptic input to the neuroendocrine bag cell neurons triggers various cation channels, causing an ∼30 min afterdischarge of action potentials and the secretion of egg-laying hormone. During the afterdischarge, protein kinase C is also activated, which in turn elevates hydrogen peroxide (H₂O₂), likely by stimulating nicotinamide adenine dinucleotide phosphate oxidase. The present study investigated whether H₂O₂ regulates cation channels to drive the afterdischarge. In single, cultured bag cell neurons, H₂O₂ elicited a prolonged, concentration- and voltage-dependent inward current, associated with an increase in membrane conductance and a reversal potential of ∼+30 mV. Compared with normal saline, the presence of Ca²⁺-free, Na⁺-free, or Na⁺/Ca²⁺-free extracellular saline, lowered the current amplitude and left-shifted the reversal potential, consistent with a nonselective cationic conductance. Preventing H₂O₂ reduction with the glutathione peroxidase inhibitor, mercaptosuccinate, enhanced the H₂O₂-induced current, while boosting glutathione production with its precursor, N-acetylcysteine, or adding the reducing agent, dithiothreitol, lessened the response. Moreover, the current generated by the alkylating agent, N-ethylmaleimide, occluded the effect of H₂O₂ The H₂O₂-induced current was inhibited by tetrodotoxin as well as the cation channel blockers, 9-phenanthrol and clotrimazole. In current-clamp, H₂O₂ stimulated burst firing, but this was attenuated or prevented altogether by the channel blockers. Finally, H₂O₂ evoked an afterdischarge from whole bag cell neuron clusters recorded ex vivo by sharp-electrode. H₂O₂ may regulate a cation channel to influence long-term changes in activity and ultimately reproduction.SIGNIFICANCE STATEMENT Hydrogen peroxide (H₂O₂) is often studied in a pathological context, such as ischemia or inflammation. However, H₂O₂ also physiologically modulates synaptic transmission and gates certain transient receptor potential channels. That stated, the effect of H₂O₂ on neuronal excitability remains less well defined. Here, we examine how H₂O₂ influences Aplysia bag cell neurons, which elicit ovulation by releasing hormones during an afterdischarge. These neuroendocrine cells are uniquely identifiable and amenable to recording as individual cultured neurons or a cluster from the nervous system. In both culture and the cluster, H₂O₂ evokes prolonged, afterdischarge-like bursting by gating a nonselective voltage-dependent cationic current. Thus, H₂O₂, which is generated in response to afterdischarge-associated second messengers, may prompt the firing necessary for hormone secretion and procreation.

TRPM2 channel: A novel target for alleviating ischaemia-reperfusion, chronic cerebral hypo-perfusion and neonatal hypoxic-ischaemic brain damage

The transient receptor potential melastatin-related 2 (TRPM2) channel, a reactive oxygen species (ROS)-sensitive cation channel, has been well recognized for being an important and common mechanism that confers the susceptibility to ROS-induced cell death. An elevated level of ROS is a salient feature of ischaemia-reperfusion, chronic cerebral hypo-perfusion and neonatal hypoxia-ischaemia. The TRPM2 channel is expressed in hippocampus, cortex and striatum, the brain regions that are critical for cognitive functions. In this review, we examine the recent studies that combine pharmacological and/or genetic interventions with using in vitro and in vivo models to demonstrate a crucial role of the TRPM2 channel in brain damage by ischaemia-reperfusion, chronic cerebral hypo-perfusion and neonatal hypoxic-ischaemia. We also discuss the current understanding of the underlying TRPM2-dependent cellular and molecular mechanisms. These new findings lead to the hypothesis of targeting the TRPM2 channel as a potential novel therapeutic strategy to alleviate brain damage and cognitive dysfunction caused by these conditions.

TRP Channels in Nociception and Pathological Pain

Thermal and noxious stimuli are detected by specialized nerve endings, which transform the stimuli into electrical signals and transmit the signals into central nervous system to facilitate the perception of temperature and pain. Several members within the transient receptor potential (TRP) channel family serve as the sensors for temperature and noxious stimuli and are involved in the development of pathological pain, especially inflammatory pain. Various inflammatory mediators can sensitize and modulate the activation threshold of TRP channels and result in the development of inflammatory pain behaviors. A brief review of the role of TRP channels in nociception and the modulatory mechanisms of TRP channels by inflammatory mediators, focusing on TRPV1, TRPA1, and TRPM2, will be presented. Recent advances in the development of therapeutic strategies targeting against TRP channels will also be reviewed.

Protective Effects of Duloxetine against Cerebral Ischemia-Reperfusion Injury via Transient Receptor Potential Melastatin 2 Inhibition

Activation of transient receptor potential melastatin 2 (TRPM2), an oxidative stress-sensitive Ca2+-permeable channel, contributes to the aggravation of cerebral ischemia-reperfusion (CIR) injury. Recent studies indicated that treatment with the antidepressant duloxetine for 24 hours (long term) attenuates TRPM2 activation in response to oxidative stress in neuronal cells. To examine the direct effects of antidepressants on TRPM2 activation, we examined their short-term (0-30 minutes) treatment effects on H2O2-induced TRPM2 activation in TRPM2-expressing human embryonic kidney 293 cells using the Ca2+ indicator fura-2. Duloxetine exerted the strongest inhibitory effects on TRPM2 activation among the seven antidepressants tested. These inhibitory effects appeared to be due to the inhibition of H2O2-induced TRPM2 activation via an open-channel blocking-like mechanism, because duloxetine reduced the sustained phase but not the initial phase of increases in intracellular Ca2+ concentrations. In a whole-cell patch-clamp study, duloxetine reduced the TRPM2-mediated inward current during the channel opening state. We also examined the effects of duloxetine in a mouse model of CIR injury. The administration of duloxetine to wild-type mice attenuated CIR injury, similar to that in Trpm2 knockout (KO) mice. The administration of duloxetine did not reduce CIR injury further in Trpm2 KO mice, suggesting that it exerts neuroprotective effects against CIR injury by inhibiting TRPM2 activation. Regarding drug repositioning, duloxetine may be a useful drug in reperfusion therapy for ischemic stroke because it has already been used clinically in therapeutics for several disorders, including depression.

Identification of Inhibitory Ca2+ Binding Sites in the Upper Vestibule of the Yeast Vacuolar TRP Channel

By vacuolar patch-clamp and Ca²⁺ imaging experiments, we show that the yeast vacuolar transient receptor potential (TRPY) channel 1 is activated by cytosolic Ca²⁺ and inhibited by Ca²⁺ from the vacuolar lumen. The channel is cooperatively affected by vacuolar Ca²⁺ (Hill coefficient, 1.5), suggesting that it may accommodate a Ca²⁺ receptor that can bind two calcium ions. Alanine scanning of six negatively charged amino acid residues in the transmembrane S5 and S6 linker, facing the vacuolar lumen, revealed that two aspartate residues, 401 and 405, are essential for current inhibition and direct binding of ⁴⁵Ca²⁺. Expressed in HEK-293 cells, a significant fraction of TRPY1, present in the plasma membrane, retained its Ca²⁺ sensitivity. Based on these data and on homology with TRPV channels, we conclude that D401 and D405 are key residues within the vacuolar vestibule of the TRPY1 pore that decrease cation access or permeation after Ca²⁺ binding.

•

The yeast vacuolar TRPY1 channel is inhibited by vacuolar Ca²⁺

•

Aspartate residues D401A and D405A are essential for Ca²⁺-mediated inhibition

•

Aspartate residues D401 and D405 are essential for direct Ca²⁺ binding

•

Ca²⁺ binding to D401 and D405 within vacuolar pore vestibule mediates inhibition

The TRPM2 channel nexus from oxidative damage to Alzheimer's pathologies: An emerging novel intervention target for age-related dementia

Alzheimer's disease (AD), an age-related neurodegenerative condition, is the most common cause of dementia among the elder people, but currently there is no treatment. A number of putative pathogenic events, particularly amyloid β peptide (Aβ) accumulation, are believed to be early triggers that initiate AD. However, thus far targeting Aβ generation/aggregation as the mainstay strategy of drug development has not led to effective AD-modifying therapeutics. Oxidative damage is a conspicuous feature of AD, but this remains poorly defined phenomenon and mechanistically ill understood. The TRPM2 channel has emerged as a potentially ubiquitous molecular mechanism mediating oxidative damage and thus plays a vital role in the pathogenesis and progression of diverse neurodegenerative diseases. This article will review the emerging evidence from recent studies and propose a novel 'hypothesis' that multiple TRPM2-mediated cellular and molecular mechanisms cascade Aβ and/or oxidative damage to AD pathologies. The 'hypothesis' based on these new findings discusses the prospect of considering the TRPM2 channel as a novel therapeutic target for intervening AD and age-related dementia.

TRPM2 mediates mitochondria-dependent apoptosis of melanocytes under oxidative stress

Abnormal mitochondrial calcium accumulation plays a critical role in oxidative stress-induced apoptosis of melanocytes. Transient receptor potential cation channel subfamily M member 2 (TRPM2) is a calcium channel sensitive to oxidative stress. However, whether TRPM2 participates in melanocyte apoptosis under oxidative stress was unknown before. In the present study, we initially found that hydrogen peroxide (H₂O₂) induced the demethylation of the promoter region in TRPM2 gene and increased the expression of TRPM2 in normal human melanocytes (NHMs). Meanwhile, TRPM2 was overexpressed in lesional melanocytes of vitiligo that is a skin disease caused by melanocyte loss under oxidative stress. Furthermore, either TRPM2 inhibitors or TRPM2 shRNA could ameliorate H₂O₂-induced apoptosis, mitochondrial reactive oxygen species (ROS) accumulation and mitochondrial membrane potential (MMP) loss in NHMs, which was similar to the effects of an anti-oxidant. More importantly, TRPM2 mediated the calcium influx into the cytoplasm and the mitochondria of NHMs exposed to H₂O₂, and a specific mitochondrial Ca²⁺ uptake inhibitor Ruthenium 360 (Ru360) could also protect NHMs from apoptosis and mitochondrial damages caused by H₂O₂. Taken together, our findings demonstrate that oxidative stress promotes the expression of TRPM2 and thus facilitates mitochondria-dependent apoptosis of melanocytes by increasing calcium influx. Our study indicates that TRPM2 is a potential target for protecting melanocytes against oxidative damages in vitiligo.

TRPM2 confers susceptibility to social stress but is essential for behavioral flexibility

Transient receptor potential melastatin 2 (TRPM2) is a Ca²⁺-permeable, nonselective cation channel and a member of the TRP channel superfamily that acts as a sensor of intracellular redox states. TRPM2 is widely distributed in many tissues and highly expressed in the brain, but the physiological roles of TRPM2 in the central nervous system remain unclear. In this study, TRPM2-deficient mice were examined in a series of behavioral tests. TRPM2-deficient mice did not significantly differ from wild-type littermates in muscle strength, light/dark transition test, rotarod, elevated plus maze, social interaction, prepulse inhibition, Y-maze, forced swim test, cued and contextual fear conditioning, and tail suspension test. In the Barnes circular maze, TRPM2-deficient mice learned the fixed escape box position at similar extent to wild-type littermates, suggesting normal reference memory. However, performance of the first reversal trial and probe test were significantly impaired in TRPM2-deficient mice. In the T-maze delayed alternation task, TRPM2 deficiency significantly reduced choice accuracy. These results indicate that TRPM2-deficient mice shows behavioral inflexibility. Meanwhile, social avoidance induced by repeated social defeat stress was significantly attenuated in TRPM2-deficient mice, suggesting that TRPM2 deficiency confers stress resiliency. Our findings indicate that TRPM2 plays an essential role in maintaining behavioral flexibility but it increases susceptibility to stress.

TRPM2 in the Brain: Role in Health and Disease

Transient receptor potential (TRP) proteins have been implicated in several cell functions as non-selective cation channels, with about 30 different mammalian TRP channels having been recognized. Among them, TRP-melastatin 2 (TRPM2) is particularly involved in the response to oxidative stress and inflammation, while its activity depends on the presence of intracellular calcium (Ca²⁺). TRPM2 is involved in several physiological and pathological processes in the brain through the modulation of multiple signaling pathways. The aim of the present review is to provide a brief summary of the current insights of TRPM2 role in health and disease to focalize our attention on future potential neuroprotective strategies.

TRPM2: a candidate therapeutic target for treating neurological diseases

Transient receptor potential melastatin 2 (TRPM2) is a calcium (Ca²⁺)-permeable non-selective cation channel belonging to the TRP ion channel family. Oxidative stress-induced TRPM2 activation provokes aberrant intracellular Ca²⁺ accumulation and cell death in a variety of cell types, including neurons. Aberrant TRPM2 function has been implicated in several neurological disorders including ischemia/stroke, Alzheimer's disease, neuropathic pain, Parkinson's disease and bipolar disorder. In addition to research identifying a role for TRPM2 in disease, progress has been made in the identification of physiological functions of TRPM2 in the brain, including recent evidence that TRPM2 is necessary for the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term depression, an important form of synaptic plasticity at glutamate synapses. Here, we summarize recent evidence on the role of TRPM2 in the central nervous system (CNS) in health and disease and discuss the potential therapeutic implications of targeting TRPM2. Collectively, these studies suggest that TRPM2 represents a prospective novel therapeutic target for neurological disorders.

The role of TRPM2 channels in neurons, glial cells and the blood-brain barrier in cerebral ischemia and hypoxia

Stroke is one of the major causes of mortality and morbidity worldwide, yet novel therapeutic treatments for this condition are lacking. This review focuses on the roles of the transient receptor potential melastatin 2 (TRPM2) ion channels in cellular damage following hypoxia-ischemia and their potential as a future therapeutic target for stroke. Here, we highlight the complex molecular signaling that takes place in neurons, glial cells and the blood-brain barrier following ischemic insult. We also describe the evidence of TRPM2 involvement in these processes, as shown from numerous in vitro and in vivo studies that utilize genetic and pharmacological approaches. This evidence implicates TRPM2 in a broad range of pathways that take place every stage of cerebral ischemic injury, thus making TRPM2 a promising target for drug development for stroke and other neurodegenerative conditions of the central nervous system.

Multiple molecular mechanisms form a positive feedback loop driving amyloid β42 peptide-induced neurotoxicity via activation of the TRPM2 channel in hippocampal neurons

Emerging evidence supports an important role for the ROS-sensitive TRPM2 channel in mediating age-related cognitive impairment in Alzheimer’s disease (AD), particularly neurotoxicity resulting from generation of excessive neurotoxic Aβ peptides. Here we examined the elusive mechanisms by which Aβ₄₂ activates the TRPM2 channel to induce neurotoxicity in mouse hippocampal neurons. Aβ₄₂-induced neurotoxicity was ablated by genetic knockout (TRPM2-KO) and attenuated by inhibition of the TRPM2 channel activity or activation through PARP-1. Aβ₄₂-induced neurotoxicity was also inhibited by treatment with TPEN used as a Zn²⁺-specific chelator. Cell imaging revealed that Aβ₄₂-induced lysosomal dysfunction, cytosolic Zn²⁺ increase, mitochondrial Zn²⁺ accumulation, loss of mitochondrial function, and mitochondrial generation of ROS. These effects were suppressed by TRPM2-KO, inhibition of TRPM2 or PARP-1, or treatment with TPEN. Bafilomycin-induced lysosomal dysfunction also resulted in TRPM2-dependent cytosolic Zn²⁺ increase, mitochondrial Zn²⁺ accumulation, and mitochondrial generation of ROS, supporting that lysosomal dysfunction and accompanying Zn²⁺ release trigger mitochondrial Zn²⁺ accumulation and generation of ROS. Aβ₄₂-induced effects on lysosomal and mitochondrial functions besides neurotoxicity were also suppressed by inhibition of PKC and NOX. Furthermore, Aβ₄₂-induced neurotoxicity was prevented by inhibition of MEK/ERK. Therefore, our study reveals multiple molecular mechanisms, including PKC/NOX-mediated generation of ROS, activation of MEK/ERK and PARP-1, lysosomal dysfunction and Zn²⁺ release, mitochondrial Zn²⁺ accumulation, loss of mitochondrial function, and mitochondrial generation of ROS, are critically engaged in forming a positive feedback loop that drives Aβ₄₂-induced activation of the TRPM2 channel and neurotoxicity in hippocampal neurons. These findings shed novel and mechanistic insights into AD pathogenesis.

Alteration in Intracellular Zn2+ Homeostasis as a Result of TRPM2 Channel Activation Contributes to ROS-Induced Hippocampal Neuronal Death

Transient receptor potential melastatin-related 2 (TRPM2) channel, a molecular sensor for reactive oxygen species (ROS), plays an important role in cognitive dysfunction associated with post-ischemia brain damage thought to result from ROS-induced TRPM2-dependent neuronal death during reperfusion. Emerging evidence further suggests that an alteration in the Zn²⁺ homeostasis is critical in ROS-induced TRPM2-dependent neuronal death. Here we applied genetic and pharmacological interventions to define the role of TRPM2 channel in ROS-induced neuronal death and explore the mechanisms contributing in the alteration in intracellular Zn²⁺ homeostasis in mouse hippocampal neurons. Exposure of neurons to 30–300 μM H₂O₂ for 2–24 h caused concentration/duration-dependent neuronal death, which was significantly suppressed, but not completely prevented, by TRPM2-knockout (TRPM2-KO) and pharmacological inhibition of the TRPM2 channel. H₂O₂-induced neuronal death was also attenuated by treatment with TPEN acting as a Zn²⁺ selective chelator. Single cell imaging demonstrated that H₂O₂ evoked a prominent increase in the intracellular Zn²⁺ concentration, which was completely prevented by TPEN as well as TRPM2-KO and inhibition of the TRPM2 channel. Furthermore, H₂O₂ induced lysosomal Zn²⁺ release and lysosomal dysfunction, and subsequent mitochondrial Zn²⁺ accumulation that provokes mitochondrial dysfunction and ROS generation. These H₂O₂-induced lysosomal/mitochondrial effects were prevented by TRPM2-KO or TPEN. Taken together, our results provide evidence to show that a dynamic alteration in the intracellular Zn²⁺ homeostasis as a result of activation of the TRPM2 channel contributes to ROS-induced hippocampal neuronal death.

Contributions of space-clamp errors to apparent time-dependent loss of Mg2+ block induced by NMDA

N-methyl-d-aspartate receptors (NMDARs) govern synaptic plasticity, development, and neuronal response to insult. Prolonged activation of NMDARs such as during an insult may activate secondary currents or modulate Mg²⁺ sensitivity, but the conditions under which these occur are not fully defined. We reexamined the effect of prolonged NMDAR activation in juvenile mouse hippocampal slices. NMDA (10 μM) elicited current with the expected negative-slope conductance in the presence of 1.2 mM Mg²⁺ However, several minutes of continued NMDA exposure elicited additional inward current at -70 mV. A higher concentration of NMDA (100 µM) elicited the current more rapidly. The additional current was not dependent on Ca²⁺, network activity, or metabotropic NMDAR function and did not persist on agonist removal. Voltage ramps revealed no alteration of either reversal potential or NMDA-elicited conductance between -30 mV and +50 mV. The result was a more linear NMDA current-voltage relationship. The current linearization was also induced in interneurons and in mature dentate granule neurons but not immature dentate granule cells, dissociated cultured hippocampal neurons, or nucleated patches excised from CA1 pyramidal neurons. Comparative simulations of NMDA application to a CA1 pyramidal neuron and to a cultured neuron revealed that linearization can be explained by space-clamp errors arising from gradual recruitment of distal dendritic NMDARs. We conclude that persistent secondary currents do not strongly contribute to NMDAR responses in juvenile mouse hippocampus and careful discernment is needed to exclude contributions of clamp artifacts to apparent secondary currents.NEW & NOTEWORTHY We report that upon sustained activation of NMDARs in juvenile mouse hippocampal neurons there is apparent loss of Mg²⁺ block at negative membrane potentials. However, the phenomenon is explained by loss of dendritic voltage clamp, leading to a linear current-voltage relationship. Our results give a specific example of how spatial voltage errors in voltage-clamp recordings can readily be misinterpreted as biological modulation.

Transient receptor potential melastatin 2 channels (TRPM2) mediate neonatal hypoxic-ischemic brain injury in mice

Transient receptor potential melastatin 2 (TRPM2), a calcium-permeable non-selective cation channel, is reported to mediate brain damage following ischemic insults in adult mice. However, the role of TRPM2 channels in neonatal hypoxic-ischemic brain injury remains unknown. We hypothesize that TRPM2^+/- and TRPM2^-/- neonatal mice have reduced hypoxic-ischemic brain injury. To study the effect of TRPM2 on neonatal brain damage, we used 2,3,5-triphenyltetrazolium chloride (TTC) staining to assess the infarct volume and whole brain imaging to assess morphological changes in the brain. In addition, we also evaluated neurobehavioral outcomes for sensorimotor function 7days following hypoxic-ischemic brain injury. We report that the infarct volumes were significantly smaller and behavioral outcomes were improved in both TRPM2^+/- and TRPM2^-/- mice compared to that of wildtype mice. Next, we found that TRPM2-null mice showed reduced dephosphorylation of GSK-3β following hypoxic ischemic injury unlike sham mice. TRPM2^+/- and TRPM2^-/- mice also had reduced activation of astrocytes and microglia in ipsilateral hemispheres, compared to wildtype mice. These findings suggest that TRPM2 channels play an essential role in mediating hypoxic-ischemic brain injury in neonatal mice. Genetically eliminating TRPM2 channels can provide neuroprotection against hypoxic-ischemic brain injury and this effect is elicited in part through regulation of GSK-3β.

The overexpressed functional transient receptor potential channel TRPM2 in oral squamous cell carcinoma

TRPM2, one member of the transient receptor potential (TRP) protein super-family, is a Ca²⁺-permeable channel that is activated by oxidative stress and confers susceptibility to cell death. In the human tongue specimens of carcinoma and the tongue carcinoma SCC cell lines, we observed the enhanced expression of TRPM2. By means of the whole-cell electrophysiological recording, the ADPR-induced currents mediated by TRPM2 were recorded in cultured SCC9 cells. Moreover, after H₂O₂ treatment for 24 hours, the apoptotic number of SCC9 cells was significantly increased. However, the selectively knocked-down TRPM2 with the small interfering RNA technique inhibited the survival and migration of the SCC9 cancer cells, which was independent of the p53-p21 pathway, since the expression of p21 was enhanced after TRPM2 knockdown. Furthermore, the sub-cellular localization of TRPM2 was remarkably different between cancerous and non-cancerous cells. A significant amount of the TRPM2 proteins were located in the nuclei in cancer cells. All these data suggest that TRPM2 is essential for the survival and migration of SCC cancer cells and may be a potential target for the selective treatment of tongue cancer.

Ovariectomy-Induced Mitochondrial Oxidative Stress, Apoptosis, and Calcium Ion Influx Through TRPA1, TRPM2, and TRPV1 Are Prevented by 17β-Estradiol, Tamoxifen, and Raloxifene in the Hippocampus and Dorsal Root Ganglion of Rats

Relative 17β-estradiol (E2) deprivation and excessive production of mitochondrial oxygen free radicals (OFRs) with a high amount of Ca²⁺ influx TRPA1, TRPM2, and TRPV1 activity is one of the main causes of neurodegenerative disease in postmenopausal women. In addition to the roles of tamoxifen (TMX) and raloxifene (RLX) in cancer and bone loss treatments, regulator roles in Ca²⁺ influx and mitochondrial oxidative stress in neurons have not been reported. The aim of this study was to evaluate whether TMX and RLX interactions with TRPA1, TRPM2, and TRPV1 in primary hippocampal (HPC) and dorsal root ganglion (DRG) neuron cultures of ovariectomized (OVX) rats. Forty female rats were divided into five groups: a control group, an OVX group, an OVX+E2 group, an OVX+TMX group, and an OVX+RLX group. The OVX+E2, OVX+TMX, and OVX+RLX groups received E2, TMX, and RLX, respectively, for 14 days after the ovariectomy. E2, ovariectomy-induced TRPA1, TRPM2, and TRPV1 current densities, as well as accumulation of cytosolic free Ca²⁺ in the neurons, were returned to the control levels by E2, TMX, and RLX treatments. In addition, E2, TMX, and RLX via modulation of TRPM2 and TRPV1 activity reduced ovariectomy-induced mitochondrial membrane depolarization, apoptosis, and cytosolic OFR production. TRPM2, TRPV1, PARP, and caspase-3 and caspase-9 expressions were also decreased in the neurons by the E2, TMX, and RLX treatments. In conclusion, we first reported the molecular effects of E2, TMX, and RLX on TRPA1, TRPM2, and TRPV1 channel activation in the OVX rats. In addition, we observed neuroprotective effects of E2, RLX, and TMX on oxidative and apoptotic injuries of the hippocampus and peripheral pain sensory neurons (DRGs) in the OVX rats. Graphical Abstract Possible molecular pathways of involvement of DEX in cerebral ischemia-induced apoptosis, oxidative stress, and calcium accumulation through TRPA1, TRPM2 and TRPV1 in the hippocampus and DRG neurons of rats. The N domain of the TRPM2 contains ADP-ribose (ADPR) pyrophosphate enzyme, which is separately activated by ADPR and oxidative stress, although the channel is reversibly inhibited by N-(p-amylcinnamoyl) anthranilic acid (ACA). The TRPV1 is also activated by mitochondrial oxidative stress and capsaicin, and it is blocked by capsazepine (CPZ). TRPA1 is also activated by oxidative stress it is inhibited by AP18. Increased cytosolic Ca²⁺ concentration through TRPA1, TRPM2 and TRPV1 in ovariectomized (OVX) rats may lead to neuronal toxicity, reactive oxygen species (ROS) processes, and eventual cell death. 17β-Estradiol (E2), tamoxifen (TMX), and raloxifene (RLX) reduced oxidative stress, apoptosis (including caspase-3 and caspase-9), mitochondrial membrane depolarization, and Ca²⁺ influx through the inhibition of TRPA1, TRPM2 and TRPV1 activation.

Commemorating John F. MacDonald and the Art of Being a Mentor

John F. MacDonald was a close friend and mentor whose life was ended far too soon on April 22, 2014. To those who knew him, John was an endearing blend of fiery Scotsman, compassionate socialist, dedicated family man, and tireless investigator. Those close to him valued his loyalty and friendship, relished his biting wit, and puzzled at his self-deprecating manner. His career spanned a remarkable period of discovery from the early identification of excitatory amino acid, to the molecular cloning and characterization of glutamate receptors and the elucidation of mechanisms responsible for regulating their function. A true pioneer in each of these areas, John's research has had a lasting impact on our understanding of excitatory synaptic transmission and its plasticity. Our intent in commemorating John's work is to focus on some notable discoveries that highlight the impact and innovative aspects of John's work. In doing so, we also wish to highlight just how greatly our understanding of the glutamate transmitter systems has advanced since the late 1970s, when John first launched his independent neuroscience career.

Sensing of redox status by TRP channels

Cellular redox status is maintained by the balance between series of antioxidant systems and production of reactive oxygen/nitrogenous species. Cells utilize this redox balance to mediate diverse physiological functions. Transient receptor potential (TRP) channels are non-selective cation channels that act as biosensors for environmental and noxious stimuli, such as capsaicin and allicin, as well as changes in temperature and conditions inside the cell. TRP channels also have an emerging role as essential players in detecting cellular redox status to regulate cellular signals mediating physiological phenomena. Reactive species activate TRP channels either directly through oxidative amino acid modifications or indirectly through second messengers. For instance, TRPA1, TRPV1 and TRPC5 channels are directly activated by oxidizing agents through cysteine modification; whereas, TRPM2 channel is indirectly activated by production of ADP-ribose. One intriguing property of several TRP channels is susceptibility to both oxidizing and reducing stimuli, suggesting TRP channels could potentially act as a bidirectional sensor for detecting deviations in redox status. In this review, we discuss the unique chemical physiologies of redox sensitive TRP channels and their physiological significance in Ca(2+) signaling.

The Transient Receptor Potential Melastatin 2 (TRPM2) Channel Contributes to β-Amyloid Oligomer-Related Neurotoxicity and Memory Impairment

In Alzheimer's disease, accumulation of soluble oligomers of β-amyloid peptide is known to be highly toxic, causing disturbances in synaptic activity and neuronal death. Multiple studies relate these effects to increased oxidative stress and aberrant activity of calcium-permeable cation channels leading to calcium imbalance. The transient receptor potential melastatin 2 (TRPM2) channel, a Ca(2+)-permeable nonselective cation channel activated by oxidative stress, has been implicated in neurodegenerative diseases, and more recently in amyloid-induced toxicity. Here we show that the function of TRPM2 is augmented by treatment of cultured neurons with β-amyloid oligomers. Aged APP/PS1 Alzheimer's mouse model showed increased levels of endoplasmic reticulum stress markers, protein disulfide isomerase and phosphorylated eukaryotic initiation factor 2α, as well as decreased levels of the presynaptic marker synaptophysin. Elimination of TRPM2 in APP/PS1 mice corrected these abnormal responses without affecting plaque burden. These effects of TRPM2 seem to be selective for β-amyloid toxicity, as ER stress responses to thapsigargin or tunicamycin in TRPM2(-/-) neurons was identical to that of wild-type neurons. Moreover, reduced microglial activation was observed in TRPM2(-/-)/APP/PS1 hippocampus compared with APP/PS1 mice. In addition, age-dependent spatial memory deficits in APP/PS1 mice were reversed in TRPM2(-/-)/APP/PS1 mice. These results reveal the importance of TRPM2 for β-amyloid neuronal toxicity, suggesting that TRPM2 activity could be potentially targeted to improve outcomes in Alzheimer's disease.

SIGNIFICANCE STATEMENT

Transient receptor potential melastatin 2 (TRPM2) is an oxidative stress sensing calcium-permeable channel that is thought to contribute to calcium dysregulation associated with neurodegenerative diseases, including Alzheimer's disease. Here we show that oligomeric β-amyloid, the toxic peptide in Alzheimer's disease, facilitates TRPM2 channel activation. In mice designed to model Alzheimer's disease, genetic elimination of TRPM2 normalized deficits in synaptic markers in aged mice. Moreover, the absence of TRPM2 improved age-dependent spatial memory deficits observed in Alzheimer's mice. Our results reveal the importance of TRPM2 for neuronal toxicity and memory impairments in an Alzheimer's mouse model and suggest that TRPM2 could be targeted for the development of therapeutic agents effective in the treatment of dementia.

Flufenamic acid prevents behavioral manifestations of salicylate-induced tinnitus in the rat

INTRODUCTION

Tinnitus is defined as a phantom auditory sensation, the perception of sound in the absence of external acoustic stimulation. Given that flufenamic acid (FFA) blocks TRPM2 cation channels, resulting in reduced neuronal excitability, we aimed to investigate whether FFA suppresses the behavioral manifestation of sodium salicylate (SSA)-induced tinnitus in rats.

MATERIAL AND METHODS

Tinnitus was evaluated using a conditioned lick suppression model of behavioral testing. Thirty-one Wistar rats, randomly divided into four treatment groups, were trained and tested in the behavioral experiment: (1) control group: DMSO + saline (n = 6), (2) SSA group: DMSO + SSA (n = 6), (3) FFA group: FFA (66 mg/kg bw) + saline (n = 9), (4) FFA + SSA group: FFA (66 mg/kg bw) + SSA (400 mg/kg bw) (n = 10). Localization of TRPM2 to the plasma membrane of cochlear nucleus neurons was demonstrated by confocal microscopy.

RESULTS

Pavlovian training resulted in strong suppression of licking, having a mean value of 0.05 ±0.03 on extinction day 1, which is below the suppression training criterion level of 0.20 in control tinnitus animals. The suppression rate for rats having both FFA (66 mg/kg bw) and SSA (400 mg/kg bw) injections was significantly lower than that for the rats having SSA injections (p < 0.01).

CONCLUSIONS

We suggest that SSA-induced tinnitus could possibly be prevented by administration of a TRPM2 ion channel antagonist, FFA at 66 mg/kg bw.

Ruling out pyridine dinucleotides as true TRPM2 channel activators reveals novel direct agonist ADP-ribose-2'-phosphate

ADP-ribose-2′-phosphate acts as a direct agonist of TRPM2, whereas NAD, NAAD, and NAADP do not.

Transient receptor potential melastatin 2 (TRPM2), a Ca²⁺-permeable cation channel implicated in postischemic neuronal cell death, leukocyte activation, and insulin secretion, is activated by intracellular ADP ribose (ADPR). In addition, the pyridine dinucleotides nicotinamide-adenine-dinucleotide (NAD), nicotinic acid–adenine-dinucleotide (NAAD), and NAAD-2′-phosphate (NAADP) have been shown to activate TRPM2, or to enhance its activation by ADPR, when dialyzed into cells. The precise subset of nucleotides that act directly on the TRPM2 protein, however, is unknown. Here, we use a heterologously expressed, affinity-purified–specific ADPR hydrolase to purify commercial preparations of pyridine dinucleotides from substantial contaminations by ADPR or ADPR-2′-phosphate (ADPRP). Direct application of purified NAD, NAAD, or NAADP to the cytosolic face of TRPM2 channels in inside-out patches demonstrated that none of them stimulates gating, or affects channel activation by ADPR, indicating that none of these dinucleotides directly binds to TRPM2. Instead, our experiments identify for the first time ADPRP as a true direct TRPM2 agonist of potential biological interest.

A dual role of transient receptor potential melastatin 2 channel in cytotoxicity induced by silica nanoparticles

Silica nanoparticles (NPs) have remarkable applications. However, accumulating evidence suggests NPs can cause cellular toxicity by inducing ROS production and increasing intracellular Ca(2+) ([Ca(2+)]i), but the underlying molecular mechanism is largely unknown. Transient receptor potential melastatin 2 (TRPM2) channel is known to be a cellular redox potential sensor that provides an important pathway for increasing the [Ca(2+)]i under oxidative stress. In this study, we examined the role of TRPM2 channel in silica NPs-induced oxidative stress and cell death. By quantitation of cell viability, ROS production, [Ca(2+)]i, and protein identification, we showed that TRPM2 channel is required for ROS production and Ca(2+) increase induced by silica NPs through regulating NADPH oxidase activity in HEK293 cells. Strikingly, HEK293 cells expressing low levels of TRPM2 were more susceptible to silica NPs than those expressing high levels of TRPM2. Macrophages from young mice showed significantly lower TRPM2 expression than those from senescent mice and had significantly lower viability after silica NPs exposure than those from senescent ones. Taken together, these findings demonstrate for the first time that TRPM2 channel acts as an oxidative stress sensor that plays a dual role in silica NPs-induced cytotoxicity by differentially regulating the NADPH oxidase activity and ROS generation.

COX-2-Derived Prostaglandin E2 Produced by Pyramidal Neurons Contributes to Neurovascular Coupling in the Rodent Cerebral Cortex

Vasodilatory prostaglandins play a key role in neurovascular coupling (NVC), the tight link between neuronal activity and local cerebral blood flow, but their precise identity, cellular origin and the receptors involved remain unclear. Here we show in rats that NMDA-induced vasodilation and hemodynamic responses evoked by whisker stimulation involve cyclooxygenase-2 (COX-2) activity and activation of the prostaglandin E2 (PgE2) receptors EP2 and EP4. Using liquid chromatography-electrospray ionization-tandem mass spectrometry, we demonstrate that PgE2 is released by NMDA in cortical slices. The characterization of PgE2 producing cells by immunohistochemistry and single-cell reverse transcriptase-PCR revealed that pyramidal cells and not astrocytes are the main cell type equipped for PgE2 synthesis, one third expressing COX-2 systematically associated with a PgE2 synthase. Consistent with their central role in NVC, in vivo optogenetic stimulation of pyramidal cells evoked COX-2-dependent hyperemic responses in mice. These observations identify PgE2 as the main prostaglandin mediating sensory-evoked NVC, pyramidal cells as their principal source and vasodilatory EP2 and EP4 receptors as their targets.

SIGNIFICANCE STATEMENT

Brain function critically depends on a permanent spatiotemporal match between neuronal activity and blood supply, known as NVC. In the cerebral cortex, prostaglandins are major contributors to NVC. However, their biochemical identity remains elusive and their cellular origins are still under debate. Although astrocytes can induce vasodilations through the release of prostaglandins, the recruitment of this pathway during sensory stimulation is questioned. Using multidisciplinary approaches from single-cell reverse transcriptase-PCR, mass spectrometry, to ex vivo and in vivo pharmacology and optogenetics, we provide compelling evidence identifying PgE2 as the main prostaglandin in NVC, pyramidal neurons as their main cellular source and the vasodilatory EP2 and EP4 receptors as their main targets. These original findings will certainly change the current view of NVC.

COX-2-Derived Prostaglandin E2 Produced by Pyramidal Neurons Contributes to Neurovascular Coupling in the Rodent Cerebral Cortex

Vasodilatory prostaglandins play a key role in neurovascular coupling (NVC), the tight link between neuronal activity and local cerebral blood flow, but their precise identity, cellular origin and the receptors involved remain unclear. Here we show in rats that NMDA-induced vasodilation and hemodynamic responses evoked by whisker stimulation involve cyclooxygenase-2 (COX-2) activity and activation of the prostaglandin E2 (PgE2) receptors EP2 and EP4. Using liquid chromatography-electrospray ionization-tandem mass spectrometry, we demonstrate that PgE2 is released by NMDA in cortical slices. The characterization of PgE2 producing cells by immunohistochemistry and single-cell reverse transcriptase-PCR revealed that pyramidal cells and not astrocytes are the main cell type equipped for PgE2 synthesis, one third expressing COX-2 systematically associated with a PgE2 synthase. Consistent with their central role in NVC, in vivo optogenetic stimulation of pyramidal cells evoked COX-2-dependent hyperemic responses in mice. These observations identify PgE2 as the main prostaglandin mediating sensory-evoked NVC, pyramidal cells as their principal source and vasodilatory EP2 and EP4 receptors as their targets.

SIGNIFICANCE STATEMENT

Brain function critically depends on a permanent spatiotemporal match between neuronal activity and blood supply, known as NVC. In the cerebral cortex, prostaglandins are major contributors to NVC. However, their biochemical identity remains elusive and their cellular origins are still under debate. Although astrocytes can induce vasodilations through the release of prostaglandins, the recruitment of this pathway during sensory stimulation is questioned. Using multidisciplinary approaches from single-cell reverse transcriptase-PCR, mass spectrometry, to ex vivo and in vivo pharmacology and optogenetics, we provide compelling evidence identifying PgE2 as the main prostaglandin in NVC, pyramidal neurons as their main cellular source and the vasodilatory EP2 and EP4 receptors as their main targets. These original findings will certainly change the current view of NVC.

Brain transient receptor potential channels and stroke

Transient receptor potential (TRP) channels have been increasingly implicated in the pathological mechanisms of CNS disorders. TRP expression has been detected in neurons, astrocytes, oligodendrocytes, microglia, and ependymal cells as well as in the cerebral vascular endothelium and smooth muscle. In stroke, TRPC3/4/6, TRPM2/4/7, and TRPV1/3/4 channels have been found to participate in ischemia-induced cell death, whereas other TRP channels, in particular those expressed in nonneuronal cells, have been less well studied. This review summarizes the current knowledge on the expression and functions of the TRP channels in various cell types in the brain and our current understanding of TRP channels in stroke pathophysiology. In an aging society, the occurrence of stroke is expected to increase steadily, and there is an urgent requirement to improve the current stroke management strategy. Therefore, elucidating the roles of TRP channels in stroke could shed light on the development of novel therapeutic strategies and ultimately improve stroke outcome.

TRPM2 channel deficiency prevents delayed cytosolic Zn2+ accumulation and CA1 pyramidal neuronal death after transient global ischemia

Transient ischemia is a leading cause of cognitive dysfunction. Postischemic ROS generation and an increase in the cytosolic Zn²⁺ level ([Zn²⁺]_c) are critical in delayed CA1 pyramidal neuronal death, but the underlying mechanisms are not fully understood. Here we investigated the role of ROS-sensitive TRPM2 (transient receptor potential melastatin-related 2) channel. Using in vivo and in vitro models of ischemia–reperfusion, we showed that genetic knockout of TRPM2 strongly prohibited the delayed increase in the [Zn²⁺]_c, ROS generation, CA1 pyramidal neuronal death and postischemic memory impairment. Time-lapse imaging revealed that TRPM2 deficiency had no effect on the ischemia-induced increase in the [Zn²⁺]_c but abolished the cytosolic Zn²⁺ accumulation during reperfusion as well as ROS-elicited increases in the [Zn²⁺]_c. These results provide the first evidence to show a critical role for TRPM2 channel activation during reperfusion in the delayed increase in the [Zn²⁺]_c and CA1 pyramidal neuronal death and identify TRPM2 as a key molecule signaling ROS generation to postischemic brain injury.