TRPM2 in the Brain: Role in Health and Disease

Roles of TRPM channels in glioma

Gliomas are the most common type of primary brain tumor. Despite advances in treatment, it remains one of the most aggressive and deadly tumor of the central nervous system (CNS). Gliomas are characterized by high malignancy, heterogeneity, invasiveness, and high resistance to radiotherapy and chemotherapy. It is urgent to find potential new molecular targets for glioma. The TRPM channels consist of TRPM1-TPRM8 and play a role in many cellular functions, including proliferation, migration, invasion, angiogenesis, etc. More and more studies have shown that TRPM channels can be used as new therapeutic targets for glioma. In this review, we first introduce the structure, activation patterns, and physiological functions of TRPM channels. Additionally, the pathological mechanism of glioma mediated by TRPM2, 3, 7, and 8 and the related signaling pathways are described. Finally, we discuss the therapeutic potential of targeting TRPM for glioma.

[Construction of HEK293T cell line stably expressing TRPM2 channel based on PiggyBac transposition system and its application in drug screening for cerebral ischemia and other diseases]

OBJECTIVES

To establish a cell line stably expressing the transient receptor potential melastatin 2 (TRPM2) channel for screening TRPM2 inhibitors based on PiggyBac transposition system.

METHODS

A plasmid PiggyBac-human TRPM2 (pPB-hTRPM2) eukaryotic expression vector was constructed using PiggyBac transposition system. The plasmid and a helper plasmid were co-transfected into HEK293T cells to express TRPM2, which was identified by fluorescence and patch-clamp assays. The high throughput screening performance was assessed with the Z ´ factor. Calcium imaging and patch clamp techniques were employed to assess the initial activity of eleven compound molecules, confirming the inhibitory effects of the primary molecules on TRPM2. The protective effect of the screened compounds on damaged cells was validated using the oxygen-glucose deprivation/reperfusion (OGD/R) injury model and CCK-8 kit. The level of cellular reactive oxygen species (ROS) was detected by flow cytometry. The neuroprotective effects of the compounds were evaluated using a transient middle cerebral artery occlusion (tMCAO) mouse model.

RESULTS

The HEK293T cells transfected with pPB-hTRPM2-EGFP showed high TRPM2 expression. Puromycin-resistant cells, selected through screening, exhibited robust fluorescence. Whole-cell patch results revealed that induced cells displayed classical TRPM2 current characteristics comparable to the control group, showing no significant differences (P>0.05). With a Z ´ factor of 0.5416 in calcium imaging, the model demonstrated suitability for high-throughput screening of TRPM2 inhibitors. Calcium imaging and electrophysiological experiments indicated that compound 6 significantly inhibited the TRPM2 channel. Further experiments showed that 1.0 μmol/L of compound 6 enhanced cell viability (P<0.05) and reduced the level of ROS (P<0.05) of SH-SY5Y under OGD/R injury. 0.3 and 1.0 mg/kg of compound 6 reduced the cerebral infarction volume in tMCAO mice (both P<0.05).

CONCLUSIONS

A stable TRPM2 gene expressing cell line has been successfully established using PiggyBac gene editing in this study. TRPM2 channel inhibitors were screened through calcium imaging and patch clamp techniques, and an inhibitor compound 6 was identified. This compound can alleviate cell damage after OGD/R by reducing cellular ROS levels and has a protective effect against cerebral ischemia-reperfusion injury in mice.

Evaluation of TRPM2 Channel-Mediated Autophagic Signaling Pathway in Hippocampus and Cortex Tissues of Rat Offspring Following Prenatal Exposure to Elevated Alcohol Levels

Fetal alcohol syndrome (FAS) can occur because of high amount of alcohol intake during pregnancy and is characterized by both physical and neurological problems. Children diagnosed with FAS have difficulties in learning, memory, and coordination. Hippocampus has a major role in memory and learning. We aimed to determine whether alcohol exposure during pregnancy had any effect on offspring by evaluating learning ability as well as oxidative stress and autophagy in the hippocampus and cortex tissues of litters. Attention was also paid to sex differences. To do so, TRPM2, Beclin1, p62, LC3B, IBA1, parvalbumin, GAD65, and mGluR5 expression levels were evaluated by immunohistochemistry. Lactate dehydrogenase (LDH), and malondialdehyde (MDA) levels, as well as total oxidant (TOS) and total antioxidant (TAS) status were determined by ELISA. Learning experiments were evaluated by the Morris water maze (MWM) test. Our findings demonstrated that IBA1, LC3B, GAD65, and mGluR5 expression levels were higher in female rats of the chronic alcohol exposure (CAE) model. Our IHC results revealed that TRPM2 expression levels were significantly increased in both males and females in the CAE group. Likewise, TAS was lower, and TOS was higher in CAE animals. Moreover, MWM outcomes supported a learning deficiency in CAE litters compared to controls and indicated that female offspring outperformed males in learning experiments. Therefore, our results revealed the detrimental effects of alcohol exposure during pregnancy on autophagy signaling in the hippocampus and cortex tissue of litters, which could affect the learning ability of animals.

Therapeutic potential of transient receptor potential (TRP) channels in psychiatric disorders

Psychiatric disorders such as Bipolar disorder, Anxiety, Major depressive disorder, Schizophrenia, Attention-deficit/hyperactivity disorder, as well as neurological disorders such as Migraine, are linked by the evidence of altered calcium homeostasis. The disturbance of intra-cellular calcium homeostasis disrupts the activity of numerous ion channels including transient receptor potential (TRP) channels. TRP channel families comprise non-selective calcium-permeable channels that have been implicated in variety of physiological processes in the brain, as well as in the pathogenesis of psychiatric disorders. Through a comprehensive review of current research and experimentation, this investigation elucidates the role of TRP channels in psychiatric disorders. Furthermore, this review discusses about the exploration of epigenetics and TRP channels in psychiatric disorders.

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.

Ion Channel Genes in Painful Neuropathies

Neuropathic pain (NP) is a typical symptom of peripheral nerve disorders, including painful neuropathy. The biological mechanisms that control ion channels are important for many cell activities and are also therapeutic targets. Disruption of the cellular mechanisms that govern ion channel activity can contribute to pain pathophysiology. The voltage-gated sodium channel (VGSC) is the most researched ion channel in terms of NP; however, VGSC impairment is detected in only <20% of painful neuropathy patients. Here, we discuss the potential role of the other peripheral ion channels involved in sensory signaling (transient receptor potential cation channels), neuronal excitation regulation (potassium channels), involuntary action potential generation (hyperpolarization-activated cyclic nucleotide-gated channels), thermal pain (anoctamins), pH modulation (acid sensing ion channels), and neurotransmitter release (calcium channels) related to pain and their prospective role as therapeutic targets for painful neuropathy.

The metabolic and functional roles of sensory nerves in adipose tissues

Homeostatic regulation of adipose tissue is critical for the maintenance of energy balance and whole-body metabolism. The peripheral nervous system provides bidirectional neural communication between the brain and adipose tissue, thereby providing homeostatic control. Most research on adipose innervation and nerve functions has been limited to the sympathetic nerves and their neurotransmitter norepinephrine. In recent years, more work has focused on adipose sensory nerves, but the contributions of subsets of sensory nerves to metabolism and the specific roles contributed by sensory neuropeptides are still understudied. Advances in imaging of adipose innervation and newer tissue denervation techniques have confirmed that sensory nerves contribute to the regulation of adipose functions, including lipolysis and browning. Here, we summarize the historical and latest findings on the regulation, function and plasticity of adipose tissue sensory nerves that contribute to metabolically important processes such as lipolysis, vascular control and sympathetic axis cross-talk.

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.

Intracellular hydrogen peroxide produced by 6-hydroxydopamine is a trigger for nigral dopaminergic degeneration of rats via rapid influx of extracellular Zn2

To elucidate the mechanism and significance of 6-hydroxydopamine (6-OHDA)-induced Zn²⁺ toxicity, which is involved in neurodegeneration in the substantia nigra pars compacta (SNpc) of rats, we postulated that intracellular hydrogen peroxide (H₂O₂) produced by 6-OHDA is a trigger for intracellular Zn²⁺ dysregulation in the SNpc. Intracellular H₂O₂ level elevated by 6-OHDA in the SNpc was completely inhibited by co-injection of GBR 13069 dihydrochloride (GBR), a dopamine reuptake inhibitor, suggesting that 6-OHDA taken up through dopamine transporters produces H₂O₂ in the intercellular compartment of dopaminergic neurons. When the SNpc was perfused with H₂O₂, glutamate accumulated in the extracellular compartment and the accumulation was inhibited in the presence of N-(p-amylcinnamoyl)anthranilic acid (ACA), a blocker of the transient receptor potential melastatin 2 (TRPM2) channels. In addition to 6-OHDA, H₂O₂ also induced intracellular Zn²⁺ dysregulation via AMPA receptor activation followed by nigral dopaminergic degeneration. Furthermore, 6-OHDA-induced nigral dopaminergic degeneration was completely inhibited by co-injection of either HYDROP, an intracellular H₂O₂ scavenger or GBR into the SNpc. The present study indicates that H₂O₂ is produced by 6-OHDA taken up through dopamine transporters in the SNpc, is retrogradely transported to presynaptic glutamatergic terminals, activates TRPM2 channels, accumulates glutamate in the extracellular compartment, and induces intracellular Zn²⁺ dysregulation via AMPA receptor activation, resulting in nigral dopaminergic degeneration prior to movement disorder. It is likely that intracellular H₂O₂, but not extracellular H₂O₂, is a key trigger for nigral dopaminergic degeneration via intracellular Zn²⁺ dysregulation.

Role of TRP ion channels in cerebral circulation and neurovascular communication

The dynamic regulation of blood flow is essential for meeting the high metabolic demands of the brain and maintaining brain function. Cerebral blood flow is regulated primarily by 1) the intrinsic mechanisms that determine vascular contractility and 2) signals from neurons and astrocytes that alter vascular contractility. Stimuli from neurons and astrocytes can also initiate a signaling cascade in the brain capillary endothelium to increase regional blood flow. Recent studies provide evidence that TRP channels in endothelial cells, smooth muscle cells, neurons, astrocytes, and perivascular nerves control cerebrovascular contractility and cerebral blood flow. TRP channels exert their functional effects either through cell membrane depolarization or by serving as a Ca2+ influx pathway. Endothelial cells and astrocytes also maintain the integrity of the blood-brain barrier. Both endothelial cells and astrocytes express TRP channels, and an increase in endothelial TRP channel activity has been linked with a disrupted endothelial barrier function. Therefore, TRP channels can play a potentially important role in regulating blood-brain barrier integrity. Here, we review the regulation of cerebrovascular contractility by TRP channels under healthy and disease conditions and their potential roles in maintaining blood-brain barrier function.

TRPM2 contributes to neuroinflammation and cognitive deficits in a cuprizone-induced multiple sclerosis model via NLRP3 inflammasome

Multiple sclerosis (MS) is a disease of the central nervous system (CNS) that is characterized by demyelination, axonal injury and neurological deterioration. Few medications are available for progressive MS, which is associated with neuroinflammation confined to the CNS compartment. Transient receptor potential melastatin 2 (TRPM2) is a calcium-permeable, non-selective cation channel that plays pathological roles in a wide range of neuroinflammatory diseases; however, the underlying molecular mechanisms of TRPM2 remain elusive. Here, we established a cuprizone model that presents hallmark MS pathologies to investigate the role of TRPM2 in progressive MS. We demonstrated that genetic deletion of TRPM2 yields protection from the cuprizone-induced demyelination, synapse loss, microglial activation, NLRP3 inflammasome activation and proinflammatory cytokines production and ultimately leads to an improvement in cognitive decline. Furthermore, we showed that the pharmacological inhibition of NLRP3 ameliorated the demyelination, neuroinflammation and cognitive impairment in the model with no additive effects on the TRPM2 KO mice. Taken together, these results indicated that TRPM2 plays important roles in regulating neuroinflammation in progressive MS via NLRP3 inflammasome, and the results shed light on TRPM2's potential role as a therapeutic target for MS.

Contribution of non-selective membrane channels and receptors in epilepsy

Overcoming refractory epilepsy's resistance to the combination of antiepileptic drugs (AED), mitigating side effects, and preventing sudden unexpected death in epilepsy are critical goals for therapy of this disorder. Current therapeutic strategies are based primarily on neurocentric mechanisms, overlooking the participation of astrocytes and microglia in the pathophysiology of epilepsy. This review is focused on a set of non-selective membrane channels (permeable to ions and small molecules), including channels and ionotropic receptors of neurons, astrocytes, and microglia, such as: the hemichannels formed by Cx43 and Panx1; the purinergic P2X7 receptors; the transient receptor potential vanilloid (TRPV1 and TRPV4) channels; calcium homeostasis modulators (CALHMs); transient receptor potential canonical (TRPC) channels; transient receptor potential melastatin (TRPM) channels; voltage-dependent anion channels (VDACs) and volume-regulated anion channels (VRACs), which all have in common being activated by epileptic activity and the capacity to exacerbate seizure intensity. Specifically, we highlight evidence for the activation of these channels/receptors during epilepsy including neuroinflammation and oxidative stress, discuss signaling pathways and feedback mechanisms, and propose the functions of each of them in acute and chronic epilepsy. Studying the role of these non-selective membrane channels in epilepsy and identifying appropriate blockers for one or more of them could provide complementary therapies to better alleviate the disease.

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.

Serotonin and chronic hypoxic pulmonary hypertension activate a NADPH oxidase 4 and TRPM2 dependent pathway for pulmonary arterial smooth muscle cell proliferation and migration

5-Hydroxytryptamine (5-HT)-dependent signaling mediated through its transporters and receptors plays important roles in chronic hypoxic pulmonary hypertension (CHPH), which is associated with aberrant reactive oxygen species (ROS) production. NADPH oxidase 4 (NOX4) is one of the major sources of ROS in pulmonary vasculature, and has been implicated in the development of PH. NOX4 generates H2O2, which can activate the transient receptor potential melastatin 2 (TRPM2) channels, providing Ca2+ signals for cell proliferation and migration. However, the connection between 5-HT, NOX4, ROS and TRPM2 in the context of PH has not been established. Here we examined the level of 5-HT and expression of NOX4 and TRPM2, and their roles in pulmonary arterial smooth muscle cells (PASMCs) proliferation and migration. NOX4 and TRPM2 were upregulated in pulmonary arteries of CHPH rats, which were associated with elevated levels of 5-HT and ROS, and enhanced proliferation and migration in PASMCs. The increase in ROS, and the enhanced proliferation and migration of PASMCs from CHPH rats were mimicked by treating normoxic PASMCs with 5-HT. 5-HT; and CH-induced ROS production were reversed by catalase, the NOX1/NOX4 inhibitor GKT137831, and Nox4 siRNA. 5-HT and H2O2 elicited Ca2+ responses were significantly augmented in CHPH PASMCs; and the augmented Ca2+ responses were obliterated by the 2-Aminoethoxydiphenyl borate (2-APB) and Trpm2-specific siRNA. Moreover, 5-HT and CH-induced proliferation and migration were suppressed by Nox4 or Trpm2 siRNA; and simultaneous transfection of both siRNA did not cause further inhibition. These results suggest that the 5-HT and CH-induced PASMC proliferation and migration were mediated, at least in part, by TRPM2 via activation of NOX4-dependent ROS production; and revealed a novel NOX4-ROS-TRPM2 signaling pathway for the pathogenesis of CHPH.

Reactive Oxygen Species-Induced TRPM2-Mediated Ca2+ Signalling in Endothelial Cells

Endothelial cells form the innermost layer of blood vessels with a fundamental role as the physical barrier. While regulation of endothelial cell function by reactive oxygen species (ROS) is critical in physiological processes such as angiogenesis, endothelial function is a major target for interruption by oxidative stress resulting from generation of high levels of ROS in endothelial cells by various pathological factors and also release of ROS by neutrophils. TRPM2 is a ROS-sensitive Ca²⁺-permeable channel expressed in endothelial cells of various vascular beds. In this review, we provide an overview of the TRPM2 channel and its role in mediating ROS-induced Ca²⁺ signaling in endothelial cells. We discuss the TRPM2-mediated Ca²⁺ signaling in vascular endothelial growth factor-induced angiogenesis and in post-ischemic neovascularization. In particular, we examine the accumulative evidence that supports the role of TRPM2-mediated Ca²⁺ signaling in endothelial cell dysfunction caused by various oxidative stress-inducing factors that are associated with tissue inflammation, obesity and diabetes, as well as air pollution. These findings provide new, mechanistic insights into ROS-mediated regulation of endothelial cells in physiology and diseases.

Methamphetamine and HIV-Tat Protein Synergistically Induce Oxidative Stress and Blood-Brain Barrier Damage via Transient Receptor Potential Melastatin 2 Channel

Synergistic impairment of the blood-brain barrier (BBB) induced by methamphetamine (METH) and HIV-Tat protein increases the risk of HIV-associated neurocognitive disorders (HAND) in HIV-positive METH abusers. Studies have shown that oxidative stress plays a vital role in METH- and HIV-Tat-induced damage to the BBB but have not clarified the mechanism. This study uses the human brain microvascular endothelial cell line hCMEC/D3 and tree shrews to investigate whether the transient receptor potential melastatin 2 (TRPM2) channel, a cellular effector of the oxidative stress, might regulate synergistic damage to the BBB caused by METH and HIV-Tat. We showed that METH and HIV-Tat damaged the BBB in vitro, producing abnormal cell morphology, increased apoptosis, reduced protein expression of the tight junctions (TJ) including Junctional adhesion molecule A (JAMA) and Occludin, and a junctional associated protein Zonula occludens 1 (ZO1), and increased the flux of sodium fluorescein (NaF) across the hCMEC/D3 cells monolayer. METH and HIV-Tat co-induced the oxidative stress response, reducing catalase (CAT), glutathione peroxidase (GSH-PX), and superoxide dismutase (SOD) activity, as well as increased reactive oxygen species (ROS) and malonaldehyde (MDA) level. Pretreatment with n-acetylcysteine amide (NACA) alleviated the oxidative stress response and BBB damage characterized by improving cell morphology, viability, apoptosis levels, TJ protein expression levels, and NaF flux. METH and HIV-Tat co-induced the activation and high protein expression of the TRPM2 channel, however, early intervention using 8-Bromoadenosine-5′-O-diphosphoribose (8-Br-ADPR), an inhibitor of TPRM2 channel, or TRPM2 gene knockdown attenuated the BBB damage. Oxidative stress inhibition reduced the activation and high protein expression of the TRPM2 channel in the in vitro model, which in turn reduced the oxidative stress response. Further, 8-Br-ADPR attenuated the effects of METH and HIV-Tat on the BBB in tree shrews—namely, down-regulated TJ protein expression and increased BBB permeability to Evans blue (EB) and NaF. In summary, the TRPM2 channel can regulate METH- and HIV-Tat-induced oxidative stress and BBB injury, giving the channel potential for developing drug interventions to reduce BBB injury and neuropsychiatric symptoms in HIV-infected METH abusers.

Ca2+ homeostasis in brain microvascular endothelial cells

Blood brain barrier (BBB) is formed by the brain microvascular endothelial cells (BMVECs) lining the wall of brain capillaries. Its integrity is regulated by multiple mechanisms, including up/downregulation of tight junction proteins or adhesion molecules, altered Ca²⁺ homeostasis, remodeling of cytoskeleton, that are confined at the level of BMVECs. Beside the contribution of BMVECs to BBB permeability changes, other cells, such as pericytes, astrocytes, microglia, leukocytes or neurons, etc. are also exerting direct or indirect modulatory effects on BBB. Alterations in BBB integrity play a key role in multiple brain pathologies, including neurological (e.g. epilepsy) and neurodegenerative disorders (e.g. Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis etc.). In this review, the principal Ca²⁺ signaling pathways in brain microvascular endothelial cells are discussed and their contribution to BBB integrity is emphasized. Improving the knowledge of Ca²⁺ homeostasis alterations in BMVECa is fundamental to identify new possible drug targets that diminish/prevent BBB permeabilization in neurological and neurodegenerative disorders.

TRPM3 in Brain (Patho)Physiology

Already for centuries, humankind is driven to understand the physiological and pathological mechanisms that occur in our brains. Today, we know that ion channels play an essential role in the regulation of neural processes and control many functions of the central nervous system. Ion channels present a diverse group of membrane-spanning proteins that allow ions to penetrate the insulating cell membrane upon opening of their channel pores. This regulated ion permeation results in different electrical and chemical signals that are necessary to maintain physiological excitatory and inhibitory processes in the brain. Therefore, it is no surprise that disturbances in the functions of cerebral ion channels can result in a plethora of neurological disorders, which present a tremendous health care burden for our current society. The identification of ion channel-related brain disorders also fuel the research into the roles of ion channel proteins in various brain states. In the last decade, mounting evidence has been collected that indicates a pivotal role for transient receptor potential (TRP) ion channels in the development and various physiological functions of the central nervous system. For instance, TRP channels modulate neurite growth, synaptic plasticity and integration, and are required for neuronal survival. Moreover, TRP channels are involved in numerous neurological disorders. TRPM3 belongs to the melastatin subfamily of TRP channels and represents a non-selective cation channel that can be activated by several different stimuli, including the neurosteroid pregnenolone sulfate, osmotic pressures and heat. The channel is best known as a peripheral nociceptive ion channel that participates in heat sensation. However, recent research identifies TRPM3 as an emerging new player in the brain. In this review, we summarize the available data regarding the roles of TRPM3 in the brain, and correlate these data with the neuropathological processes in which this ion channel may be involved.

The Role of TRP Channels and PMCA in Brain Disorders: Intracellular Calcium and pH Homeostasis

Brain disorders include neurodegenerative diseases (NDs) with different conditions that primarily affect the neurons and glia in the brain. However, the risk factors and pathophysiological mechanisms of NDs have not been fully elucidated. Homeostasis of intracellular Ca²⁺ concentration and intracellular pH (pH_i) is crucial for cell function. The regulatory processes of these ionic mechanisms may be absent or excessive in pathological conditions, leading to a loss of cell death in distinct regions of ND patients. Herein, we review the potential involvement of transient receptor potential (TRP) channels in NDs, where disrupted Ca²⁺ homeostasis leads to cell death. The capability of TRP channels to restore or excite the cell through Ca²⁺ regulation depending on the level of plasma membrane Ca²⁺ ATPase (PMCA) activity is discussed in detail. As PMCA simultaneously affects intracellular Ca²⁺ regulation as well as pH_i, TRP channels and PMCA thus play vital roles in modulating ionic homeostasis in various cell types or specific regions of the brain where the TRP channels and PMCA are expressed. For this reason, the dysfunction of TRP channels and/or PMCA under pathological conditions disrupts neuronal homeostasis due to abnormal Ca²⁺ and pH levels in the brain, resulting in various NDs. This review addresses the function of TRP channels and PMCA in controlling intracellular Ca²⁺ and pH, which may provide novel targets for treating NDs.

Medicinal chemistry perspective of TRPM2 channel inhibitors: where we are and where we might be heading?

Transient receptor potential melastatin 2 (TRPM2) is a Ca²⁺- permeable nonselective cation channel that is involved in diverse biological functions as a cellular sensor for oxidative stress and temperature. It has been considered a promising therapeutic target for the treatment of ischemia/reperfusion (IR) injury, inflammation, cancer, and neurodegenerative diseases. Development of highly potent and selective TRPM2 inhibitors and validation of their use in relevant disease models will advance drug discovery. In this review, we describe the molecular structures and gating mechanism of the TRPM2 channel, and offer a comprehensive review of advances in the discovery of TRPM2 inhibitors. Furthermore, we analyze the properties of reported TRPM2 inhibitors with an emphasis on how specific inhibitors targeting this channel could be better developed.

TRPM2 channel-mediated cell death: An important mechanism linking oxidative stress-inducing pathological factors to associated pathological conditions

Oxidative stress resulting from the accumulation of high levels of reactive oxygen species is a salient feature of, and a well-recognised pathological factor for, diverse pathologies. One common mechanism for oxidative stress damage is via the disruption of intracellular ion homeostasis to induce cell death. TRPM2 is a non-selective Ca2+-permeable cation channel with a wide distribution throughout the body and is highly sensitive to activation by oxidative stress. Recent studies have collected abundant evidence to show its important role in mediating cell death induced by miscellaneous oxidative stress-inducing pathological factors, both endogenous and exogenous, including ischemia/reperfusion and the neurotoxicants amyloid-β peptides and MPTP/MPP+ that cause neuronal demise in the brain, myocardial ischemia/reperfusion, proinflammatory mediators that disrupt endothelial function, diabetogenic agent streptozotocin and diabetes risk factor free fatty acids that induce loss of pancreatic β-cells, bile acids that damage pancreatic acinar cells, renal ischemia/reperfusion and albuminuria that are detrimental to kidney cells, acetaminophen that triggers hepatocyte death, and nanoparticles that injure pericytes. Studies have also shed light on the signalling mechanisms by which these pathological factors activate the TRPM2 channel to alter intracellular ion homeostasis leading to aberrant initiation of various cell death pathways. TRPM2-mediated cell death thus emerges as an important mechanism in the pathogenesis of conditions including ischemic stroke, neurodegenerative diseases, cardiovascular diseases, diabetes, pancreatitis, chronic kidney disease, liver damage and neurovascular injury. These findings raise the exciting perspective of targeting the TRPM2 channel as a novel therapeutic strategy to treat such oxidative stress-associated diseases.

Pharmacological Inhibition of Transient Receptor Potential Melastatin 2 (TRPM2) Channels Attenuates Diabetes-induced Cognitive Deficits in Rats: A Mechanistic Study

Background:

Diabetes is a chronic metabolic disorder affecting the central nervous system. A growing body of evidence has depicted that high glucose level leads to the activation of the transient receptor potential melastatin 2 (TRPM2) channels. However, there are no studies targeting TRPM2 channels in diabetes-induced cognitive decline using a pharmacological approach.

Objective:

The present study intended to investigate the effects of 2-aminoethoxydiphenyl borate (2-APB), a TRPM2 inhibitor, in diabetes-induced cognitive impairment.

Methods:

Streptozotocin (STZ, 50 mg/kg, i.p.) was used to induce diabetes in rats. Animals were randomly divided into the treatment group, model group and age-matched control and pre se group. 2-APB treatment was given for three weeks to the animals. After 10 days of behavioural treatment, parameters were performed. Animals were sacrificed at 10th week of diabetic induction and the hippocampus and cortex were isolated. After that, protein and mRNA expression study was performed in the hippocampus. Acetylcholinesterase (AchE) activity was done in the cortex.

Results: :

Our study showed the 10th week diabetic animals developed cognitive impairment, which was evident from the behavioural parameters. Diabetic animals depicted an increase in the TRPM2 mRNA and protein expression in the hippocampus as well as increased AchE activity in the cortex. However, memory associated proteins were down-regulated, namely Ca2+/calmodulin-dependent protein kinase II (CaMKII-Thr286), glycogen synthase kinase 3 beta (GSK-3β-Ser9), cAMP response element-binding protein (CREB-Ser133), and postsynaptic density protein 95 (PSD-95). Gene expression of parvalbumin, calsequestrin and brain-derived neurotrophic factor (BDNF) were down-regulated while mRNA level of calcineurin A/ protein phosphatase 3 catalytic subunit alpha (PPP3CA) was upregulated in the hippocampus of diabetic animals. A three-week treatment with 2-APB significantly ameliorated the alteration in behavioural cognitive parameters in diabetic rats. Moreover, 2-APB also down-regulated the expression of TRPM2 mRNA and protein in the hippocampus as well as AchE activity in the cortex of diabetic animals as compared to diabetic animals. Moreover, the 2-APB treatment also upregulated the CaMKII (Thr-286), GSK-3β (Ser9), CREB (Ser133), and PSD-95 expression and mRNA levels of parvalbumin, calsequestrin, and BDNF while mRNA level of calcineurin A was down-regulated in the hippocampus of diabetic animals.

Conclusion: :

This study confirms the ameliorative effect of TRPM2 channel inhibitor in the diabetes- induced cognitive deficits. Inhibition of TRPM2 channels reduced the calcium associated downstream signaling and showed a neuroprotective effect of TRPM2 channels in diabetesinduced cognitive impairment.

Interrelation between ROS and Ca2+ in aging and age-related diseases

Calcium (Ca2+) and reactive oxygen species (ROS) are versatile signaling molecules coordinating physiological and pathophysiological processes. While channels and pumps shuttle Ca2+ ions between extracellular space, cytosol and cellular compartments, short-lived and highly reactive ROS are constantly generated by various production sites within the cell. Ca2+ controls membrane potential, modulates mitochondrial adenosine triphosphate (ATP) production and affects proteins like calcineurin (CaN) or calmodulin (CaM), which, in turn, have a wide area of action. Overwhelming Ca2+ levels within mitochondria efficiently induce and trigger cell death. In contrast, ROS comprise a diverse group of relatively unstable molecules with an odd number of electrons that abstract electrons from other molecules to gain stability. Depending on the type and produced amount, ROS act either as signaling molecules by affecting target proteins or as harmful oxidative stressors by damaging cellular components. Due to their wide range of actions, it is little wonder that Ca2+ and ROS signaling pathways overlap and impact one another. Growing evidence suggests a crucial implication of this mutual interplay on the development and enhancement of age-related disorders, including cardiovascular and neurodegenerative diseases as well as cancer.

Transient Receptor Potential Melastatin 2 (TRPM2) Inhibition by Antioxidant, N-Acetyl-l-Cysteine, Reduces Global Cerebral Ischemia-Induced Neuronal Death

A variety of pathogenic mechanisms, such as cytoplasmic calcium/zinc influx, reactive oxygen species production, and ionic imbalance, have been suggested to play a role in cerebral ischemia induced neurodegeneration. During the ischemic state that occurs after stroke or heart attack, it is observed that vesicular zinc can be released into the synaptic cleft, and then translocated into the cytoplasm via various cation channels. Transient receptor potential melastatin 2 (TRPM2) is highly distributed in the central nervous system and has high sensitivity to oxidative damage. Several previous studies have shown that TRPM2 channel activation contributes to neuroinflammation and neurodegeneration cascades. Therefore, we examined whether anti-oxidant treatment, such as with N-acetyl-l-cysteine (NAC), provides neuroprotection via regulation of TRPM2, following global cerebral ischemia (GCI). Experimental animals were then immediately injected with NAC (150 mg/kg/day) for 3 and 7 days, before sacrifice. We demonstrated that NAC administration reduced activation of GCI-induced neuronal death cascades, such as lipid peroxidation, microglia and astroglia activation, free zinc accumulation, and TRPM2 over-activation. Therefore, modulation of the TRPM2 channel can be a potential therapeutic target to prevent ischemia-induced neuronal death.

TRP Channels Role in Pain Associated With Neurodegenerative Diseases

Transient receptor potential (TRP) are cation channels expressed in both non-excitable and excitable cells from diverse tissues, including heart, lung, and brain. The TRP channel family includes 28 isoforms activated by physical and chemical stimuli, such as temperature, pH, osmotic pressure, and noxious stimuli. Recently, it has been shown that TRP channels are also directly or indirectly activated by reactive oxygen species. Oxidative stress plays an essential role in neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases, and TRP channels are involved in the progression of those diseases by mechanisms involving changes in the crosstalk between Ca2+ regulation, oxidative stress, and production of inflammatory mediators. TRP channels involved in nociception include members of the TRPV, TRPM, TRPA, and TRPC subfamilies that transduce physical and chemical noxious stimuli. It has also been reported that pain is a complex issue in patients with Alzheimer's and Parkinson's diseases, and adequate management of pain in those conditions is still in discussion. TRPV1 has a role in neuroinflammation, a critical mechanism involved in neurodegeneration. Therefore, some studies have considered TRPV1 as a target for both pain treatment and neurodegenerative disorders. Thus, this review aimed to describe the TRP-dependent mechanism that can mediate pain sensation in neurodegenerative diseases and the therapeutic approach available to palliate pain and neurodegenerative symptoms throughout the regulation of these channels.

Transient receptor potential melastatin 2 contributes to neuroinflammation and negatively regulates cognitive outcomes in a pilocarpine-induced mouse model of epilepsy

Neuroinflammation contributes to the generation of epileptic seizures and is associate with neuropathology and comorbidities. Transient receptor potential melastatin 2 (TRPM2) expresses in various cell types in the brain. It plays a pathological role in a wide range of neuroinflammatory diseases, but has yet been studied in epilepsy. Here, a temporal lobe epilepsy model was generated by pilocarpine administration in mice. At 24 h, knockout (KO) TRPM2 alleviated the level of neuroinflammation, showing a reduction of IL-1β, TNF-α, CXCL2 and IL-6 mRNA production, NLRP3, ASC, and Caspase-1 protein expression and glial activation. Moreover, KO TRPM2 alleviated neurodegeneration, concurrent with reduced Beclin-1 and ATG5 protein expression. Later, KO TRPM2 ameliorated the epilepsy-induced psychological disorders, with improved performance in the open-field, Y maze and novel object recognition test. Together, these results suggest that TRPM2 facilitates epilepsy-related brain injury and may shed light on its potential as a therapeutic target for epilepsy-associated neuropathology and comorbidities.

Therapeutic potential of pharmacological agents targeting TRP channels in CNS disorders

Central nervous system (CNS) disorders like Alzheimer's disease (AD), Parkinson disease (PD), stroke, epilepsy, depression, and bipolar disorder have a high impact on both medical and social problems due to the surge in their prevalence. All of these neuronal disorders share some common etiologies including disruption of Ca²⁺ homeostasis and accumulation of misfolded proteins. These misfolded proteins further disrupt the intracellular Ca²⁺ homeostasis by disrupting the activity of several ion channels including transient receptor potential (TRP) channels. TRP channel families include non-selective Ca²⁺ permeable channels, which act as cellular sensors activated by various physio-chemical stimuli, exogenous, and endogenous ligands responsible for maintaining the intracellular Ca²⁺ homeostasis. TRP channels are abundantly expressed in the neuronal cells and disturbance in their activity leads to various neuronal diseases. Under the pathological conditions when the activity of TRP channels is perturbed, there is a disruption of the neuronal homeostasis through increased inflammatory response, generation of reactive oxygen species, and mitochondrial dysfunction. Therefore, there is a potential of pharmacological interventions targeting TRP channels in CNS disorders. This review focuses on the role of TRP channels in neurological diseases; also, we have highlighted the current insights into the pharmacological modulators targeting TRP channels.

One Single Nucleotide Polymorphism of the TRPM2 Channel Gene Identified as a Risk Factor in Bipolar Disorder Associates with Autism Spectrum Disorder in a Japanese Population

The transient receptor potential melastatin 2 (TRPM2) is a non-specific cation channel, resulting in Ca²⁺ influx at warm temperatures from 34 °C to 47 °C, thus including the body temperature range in mammals. TRPM2 channels are activated by β-NAD⁺, ADP-ribose (ADPR), cyclic ADPR, and 2′-deoxyadenosine 5′-diphosphoribose. It has been shown that TRPM2 cation channels and CD38, a type II or type III transmembrane protein with ADP-ribosyl cyclase activity, simultaneously play a role in heat-sensitive and NAD⁺ metabolite-dependent intracellular free Ca²⁺ concentration increases in hypothalamic oxytocinergic neurons. Subsequently, oxytocin (OT) is released to the brain. Impairment of OT release may induce social amnesia, one of the core symptoms of autism spectrum disorder (ASD). The risk of single nucleotide polymorphisms (SNPs) and variants of TRPM2 have been reported in bipolar disorder, but not in ASD. Therefore, it is reasonable to examine whether SNPs or haplotypes in TRPM2 are associated with ASD. Here, we report a case-control study with 147 ASD patients and 150 unselected volunteers at Kanazawa University Hospital in Japan. The sequence-specific primer-polymerase chain reaction method together with fluorescence correlation spectroscopy was applied. Of 14 SNPs examined, one SNP (rs933151) displayed a significant p-value (OR = 0.1798, 95% CI = 0.039, 0.83; Fisher’s exact test; p = 0.0196). The present research data suggest that rs93315, identified as a risk factor for bipolar disorder, is a possible association factor for ASD.

Ion channels as potential redox sensors in lysosomes

Lysosomes are central organelles that recycle materials and energy to maintain intracellular homeostasis. Lysosomes are capable of sensing environmental cues such as nutrition to regulate their function accordingly. Whether lysosomes can sense redox signaling, however, was unclear. Here in this review, we summarized recent evidence of lysosomal ion channel as redox sensors for this organelle. We also discussed their roles in lysosomal diseases that features imbalanced redox.

TRPM2 channel regulates cytokines production in astrocytes and aggravates brain disorder during lipopolysaccharide-induced endotoxin sepsis

Sepsis is one of the most significant challenges in intensive care units, which is associated with increased morbidity and mortality. Sepsis-associated encephalopathy (SAE) is a severe complication which can cause death and serious disabilities. Calcium signaling in astrocyte is essential for cellular activation and the potential resolution of infection or inflammation in SAE patients. The transient receptor potential melastatin 2 (TRPM2) channel has been identified as a unique fusion of a Ca2+-permeable nonselective cation channel, which plays an important role in inflammation and immune response. Because of its role as an oxidative stress sensor in astrocytes, we investigated the function of TRPM2 in inflammation mediators (interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α) release, Bcl-2/E1B-19 K-interacting protein 3 (BNIP3), apoptosis inducing factor (AIF) and Endonuclease G (Endo G) expression. We showed that TRPM2-KO mice, when intraperitoneally (i.p) injected with LPS, exhibited better neurologic assessment scores and decreased inflammatory injury in hippocampal neurons compared with wild-type (WT) mice. The absence of TRPM2 triggered less production of inflammatory mediators (IL-1β, IL-6, TNF-α) and decreased apoptosis related proteins (BNIP3, AIF, Endo G) expressions in response to LPS induced sepsis. Furthermore, TRPM2-deficient astrocytes (transfected with TRPM2 siRNA) upon LPS stimulation also induced decreased IL-1β, IL-6 and TNF-α level. Our data suggested that decreased production of inflammatory cytokines and apoptosis related proteins with TRPM2 deletion could regulate inflammatory stress and decrease inflammatory injury in hippocampal neurons, and consequently, ameliorate brain disorder.

Ligand recognition and gating mechanism through three ligand-binding sites of human TRPM2 channel

TRPM2 is critically involved in diverse physiological processes including core temperature sensing, apoptosis, and immune response. TRPM2’s activation by Ca²⁺ and ADP ribose (ADPR), an NAD⁺-metabolite produced under oxidative stress and neurodegenerative conditions, suggests a role in neurological disorders. We provide a central concept between triple-site ligand binding and the channel gating of human TRPM2. We show consecutive structural rearrangements and channel activation of TRPM2 induced by binding of ADPR in two indispensable locations, and the binding of Ca²⁺ in the transmembrane domain. The 8-Br-cADPR—an antagonist of cADPR—binds only to the MHR1/2 domain and inhibits TRPM2 by stabilizing the channel in an apo-like conformation. We conclude that MHR1/2 acts as a orthostatic ligand-binding site for TRPM2. The NUDT9-H domain binds to a second ADPR to assist channel activation in vertebrates, but not necessary in invertebrates. Our work provides insights into the gating mechanism of human TRPM2 and its pharmacology.

Novel CaM-binding motif in its NudT9H domain contributes to temperature sensitivity of TRPM2

TRPM2 is a non-selective, Ca2+-permeable cation channel, which plays a role in cell death but also contributes to diverse immune cell functions. In addition, TRPM2 contributes to the control of body temperature and is involved in perception of non-noxious heat and thermotaxis. TRPM2 is regulated by many factors including Ca2+, ADPR, 2'-deoxy-ADPR, Ca2+-CaM, and temperature. However, the molecular basis for the temperature sensitivity of TRPM2 as well as the interplay between the regulatory factors is still not understood. Here we identify a novel CaM-binding site in the unique NudT9H domain of TRPM2. Using a multipronged biophysical approach we show that binding of Ca2+-CaM to this site occurs upon partial unfolding at temperatures >35 °C and prevents further thermal destabilization. In combination with patch-clamp measurements of full-length TRPM2 our results suggest a role of this CaM-binding site in the temperature sensitivity of TRPM2. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.