Direct Gating of the TRPM2 Channel by cADPR via Specific Interactions with the ADPR Binding Pocket

Neurological-related proteomic profiling in plasma of children with metabolic healthy and unhealthy overweight/obesity

OBJECTIVE

Children with overweight/obesity (OW/OB) exhibit poor cardiometabolic health, yet mechanisms influencing brain health remain unclear. We examined the differences in neurological-related circulating proteins in plasma among children with metabolically healthy obesity (MHO) and metabolically unhealthy obesity (MUO) and the association with metabolic syndrome markers.

METHODS

In this cross-sectional study, we included 84 Caucasian children (39% girls), aged 10.1 ± 1.1 years, from the ActiveBrains project (NCT02295072). A ninety-two-protein targeted approach using Olink's® technology was used.

RESULTS

We identified distinct concentrations of CD38, LAIR2, MANF and NRP2 proteins in MHO compared with MUO. Moreover, individual metabolic syndrome (MS) markers were linked to nine proteins (CD38, CPM, EDA2R, IL12, JAMB, KYNU, LAYN, MSR1 and SMOC2) in children with OW/OB. These proteins play crucial roles in diverse biological processes (e.g., angiogenesis, cholesterol transport, nicotinamide adenine dinucleotide (NAD+) catalysis and maintenance of blood-brain barrier) related to brain health.

CONCLUSION

Our proteomics study suggests that cardiometabolic health (represented by MHO/MUO or individual MS markers) is associated with the concentration in plasma of several proteins involved in brain health. Larger-scale studies are needed to contrast/confirm these findings, with CD38 standing out as a particularly noteworthy and robust discovery.

CD38 and extracellular NAD+ regulate the development and maintenance of Hp vaccine-induced CD4+ TRM in the gastric epithelium

Tissue-resident memory T cells (TRM) can be induced by infection and vaccination, and play a key role in maintaining long-term protective immunity against mucosal pathogens. Our studies explored the key factors and mechanisms affecting the differentiation, maturation, and stable residence of gastric epithelial CD4+ TRM induced by Helicobacter pylori (Hp) vaccine and optimized Hp vaccination to promote the generation and residence of TRM. Cluster of differentiation (CD)38 regulated mitochondrial activity and enhanced transforming growth factor-β signal transduction to promote the differentiation and residence of gastric epithelial CD4+ TRM by mediating the expression of CD105. Extracellular nucleotides influenced the long-term maintenance of TRM in gastric epithelium by the P2X7 receptor (P2RX7). Vitamin D3 and Gram-positive enhancer matrix (GEM) particles as immune adjuvants combined with Hp vaccination promoted the production of CD69+CD103+CD4+ TRM.

Bidirectional regulation mechanism of TRPM2 channel: role in oxidative stress, inflammation and ischemia-reperfusion injury

Transient receptor potential melastatin 2 (TRPM2) is a non-selective cation channel that exhibits Ca2+ permeability. The TRPM2 channel is expressed in various tissues and cells and can be activated by multiple factors, including endogenous ligands, Ca2+, reactive oxygen species (ROS) and temperature. This article reviews the multiple roles of the TRPM2 channel in physiological and pathological processes, particularly on oxidative stress, inflammation and ischemia-reperfusion (I/R) injury. In oxidative stress, the excessive influx of Ca2+ caused by the activation of the TRPM2 channel may exacerbate cellular damage. However, under specific conditions, activating the TRPM2 channel can have a protective effect on cells. In inflammation, the activation of the TRPM2 channel may not only promote inflammatory response but also inhibit inflammation by regulating ROS production and bactericidal ability of macrophages and neutrophils. In I/R, the activation of the TRPM2 channel may worsen I/R injury to various organs, including the brain, heart, kidney and liver. However, activating the TRPM2 channel may protect the myocardium from I/R injury by regulating calcium influx and phosphorylating proline-rich tyrosine kinase 2 (Pyk2). A thorough investigation of the bidirectional role and regulatory mechanism of the TRPM2 channel in these physiological and pathological processes will aid in identifying new targets and strategies for treatment of related diseases.

Nicotinamide N-Methyltransferase (NNMT): A New Hope for Treating Aging and Age-Related Conditions

The complex process of aging leads to a gradual deterioration in the function of cells, tissues, and the entire organism, thereby increasing the risk of disease and death. Nicotinamide N-methyltransferase (NNMT) has attracted attention as a potential target for combating aging and its related pathologies. Studies have shown that NNMT activity increases over time, which is closely associated with the onset and progression of age-related diseases. NNMT uses S-adenosylmethionine (SAM) as a methyl donor to facilitate the methylation of nicotinamide (NAM), converting NAM into S-adenosyl-L-homocysteine (SAH) and methylnicotinamide (MNA). This enzymatic action depletes NAM, a precursor of nicotinamide adenine dinucleotide (NAD+), and generates SAH, a precursor of homocysteine (Hcy). The reduction in the NAD+ levels and the increase in the Hcy levels are considered important factors in the aging process and age-related diseases. The efficacy of RNA interference (RNAi) therapies and small-molecule inhibitors targeting NNMT demonstrates the potential of NNMT as a therapeutic target. Despite these advances, the exact mechanisms by which NNMT influences aging and age-related diseases remain unclear, and there is a lack of clinical trials involving NNMT inhibitors and RNAi drugs. Therefore, more in-depth research is needed to elucidate the precise functions of NNMT in aging and promote the development of targeted pharmaceutical interventions. This paper aims to explore the specific role of NNMT in aging, and to evaluate its potential as a therapeutic target.

Coupling enzymatic activity and gating in an ancient TRPM chanzyme and its molecular evolution

Channel enzymes represent a class of ion channels with enzymatic activity directly or indirectly linked to their channel function. We investigated a TRPM2 chanzyme from choanoflagellates that integrates two seemingly incompatible functions into a single peptide: a channel module activated by ADP-ribose with high open probability and an enzyme module (NUDT9-H domain) consuming ADP-ribose at a remarkably slow rate. Using time-resolved cryogenic-electron microscopy, we captured a complete series of structural snapshots of gating and catalytic cycles, revealing the coupling mechanism between channel gating and enzymatic activity. The slow kinetics of the NUDT9-H enzyme module confers a self-regulatory mechanism: ADPR binding triggers NUDT9-H tetramerization, promoting channel opening, while subsequent hydrolysis reduces local ADPR, inducing channel closure. We further demonstrated how the NUDT9-H domain has evolved from a structurally semi-independent ADP-ribose hydrolase module in early species to a fully integrated component of a gating ring essential for channel activation in advanced species.

Evolutionary trajectory of TRPM2 channel activation by adenosine diphosphate ribose and calcium

Ion channel activation upon ligand gating triggers a myriad of biological events and, therefore, evolution of ligand gating mechanism is of fundamental importance. TRPM2, a typical ancient ion channel, is activated by adenosine diphosphate ribose (ADPR) and calcium and its activation has evolved from a simple mode in invertebrates to a more complex one in vertebrates, but the evolutionary process is still unknown. Molecular evolutionary analysis of TRPM2s from more than 280 different animal species has revealed that, the C-terminal NUDT9-H domain has evolved from an enzyme to a ligand binding site for activation, while the N-terminal MHR domain maintains a conserved ligand binding site. Calcium gating pattern has also evolved, from one Ca2+-binding site as in sea anemones to three sites as in human. Importantly, we identified a new group represented by olTRPM2, which has a novel gating mode and fills the missing link of the channel gating evolution. We conclude that the TRPM2 ligand binding or activation mode evolved through at least three identifiable stages in the past billion years from simple to complicated and coordinated. Such findings benefit the evolutionary investigations of other channels and proteins.

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.

ROS-Activated TRPM2 Channel: Calcium Homeostasis in Cardiovascular/renal System and Speculation in Cardiorenal Syndrome

The transient receptor potential melastatin 2 (TRPM2) channel is a nonselective calcium channel that is sensitive to oxidative stress (OS), and is widely expressed in multiple organs, such as the heart, kidney, and brain, which is inextricably related to calcium dyshomeostasis and downstream pathological events. Due to the increasing global burden of kidney or cardiovascular diseases (CVDs), safe and efficient drugs specific to novel targets are imperatively needed. Notably, investigation of the possibility to regard the TRPM2 channel as a new therapeutic target in ROS-related CVDs or renal diseases is urgently required because the roles of the TRPM2 channel in heart or kidney diseases have not received enough attention and thus have not been fully elaborated. Therefore, we aimed to review the involvement of the TRPM2 channel in cardiovascular disorders related to kidney or typical renal diseases and attempted to speculate about TRPM2-mediated mechanisms of cardiorenal syndrome (CRS) to provide representative perspectives for future research about novel and effective therapeutic strategies.

TRPM2 as a conserved gatekeeper determines the vulnerability of DA neurons by mediating ROS sensing and calcium dyshomeostasis

Different dopaminergic (DA) neuronal subgroups exhibit distinct vulnerability to stress, while the underlying mechanisms are elusive. Here we report that the transient receptor potential melastatin 2 (TRPM2) channel is preferentially expressed in vulnerable DA neuronal subgroups, which correlates positively with aging in Parkinson's Disease (PD) patients. Overexpression of human TRPM2 in the DA neurons of C. elegans resulted in selective death of ADE but not CEP neurons in aged worms. Mechanistically, TRPM2 activation mediates FZO-1/CED-9-dependent mitochondrial hyperfusion and mitochondrial permeability transition (MPT), leading to ADE death. In mice, TRPM2 knockout reduced vulnerable substantia nigra pars compacta (SNc) DA neuronal death induced by stress. Moreover, the TRPM2-mediated vulnerable DA neuronal death pathway is conserved from C. elegans to toxin-treated mice model and PD patient iPSC-derived DA neurons. The vulnerable SNc DA neuronal loss is the major symptom and cause of PD, and therefore the TRPM2-mediated pathway serves as a promising therapeutic target against PD.

The Safety and Antiaging Effects of Nicotinamide Mononucleotide in Human Clinical Trials: an Update

The importance of nicotinamide adenine dinucleotide (NAD+) in human physiology is well recognized. As the NAD+ concentration in human skin, blood, liver, muscle, and brain are thought to decrease with age, finding ways to increase NAD+ status could possibly influence the aging process and associated metabolic sequelae. Nicotinamide mononucleotide (NMN) is a precursor for NAD+ biosynthesis, and in vitro/in vivo studies have demonstrated that NMN supplementation increases NAD+ concentration and could mitigate aging-related disorders such as oxidative stress, DNA damage, neurodegeneration, and inflammatory responses. The promotion of NMN as an antiaging health supplement has gained popularity due to such findings; however, since most studies evaluating the effects of NMN have been conducted in cell or animal models, a concern remains regarding the safety and physiological effects of NMN supplementation in the human population. Nonetheless, a dozen human clinical trials with NMN supplementation are currently underway. This review summarizes the current progress of these trials and NMN/NAD+ biology to clarify the potential effects of NMN supplementation and to shed light on future study directions.

Cell death induction and protection by activation of ubiquitously expressed anion/cation channels. Part 3: the roles and properties of TRPM2 and TRPM7

Cell volume regulation (CVR) is a prerequisite for animal cells to survive and fulfill their functions. CVR dysfunction is essentially involved in the induction of cell death. In fact, sustained normotonic cell swelling and shrinkage are associated with necrosis and apoptosis, and thus called the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. Since a number of ubiquitously expressed ion channels are involved in the CVR processes, these volume-regulatory ion channels are also implicated in the NVI and AVD events. In Part 1 and Part 2 of this series of review articles, we described the roles of swelling-activated anion channels called VSOR or VRAC and acid-activated anion channels called ASOR or PAC in CVR and cell death processes. Here, Part 3 focuses on therein roles of Ca2+-permeable non-selective TRPM2 and TRPM7 cation channels activated by stress. First, we summarize their phenotypic properties and molecular structure. Second, we describe their roles in CVR. Since cell death induction is tightly coupled to dysfunction of CVR, third, we focus on their participation in the induction of or protection against cell death under oxidative, acidotoxic, excitotoxic, and ischemic conditions. In this regard, we pay attention to the sensitivity of TRPM2 and TRPM7 to a variety of stress as well as to their capability to physicall and functionally interact with other volume-related channels and membrane enzymes. Also, we summarize a large number of reports hitherto published in which TRPM2 and TRPM7 channels are shown to be involved in cell death associated with a variety of diseases or disorders, in some cases as double-edged swords. Lastly, we attempt to describe how TRPM2 and TRPM7 are organized in the ionic mechanisms leading to cell death induction and protection.

Phosphoribosyltransferases and Their Roles in Plant Development and Abiotic Stress Response

Glycosylation is a widespread glycosyl modification that regulates gene expression and metabolite bioactivity in all life processes of plants. Phosphoribosylation is a special glycosyl modification catalyzed by phosphoribosyltransferase (PRTase), which functions as a key step in the biosynthesis pathway of purine and pyrimidine nucleotides, histidine, tryptophan, and coenzyme NAD(P)⁺ to control the production of these essential metabolites. Studies in the past decades have reported that PRTases are indispensable for plant survival and thriving, whereas the complicated physiological role of PRTases in plant life and their crosstalk is not well understood. Here, we comprehensively overview and critically discuss the recent findings on PRTases, including their classification, as well as the function and crosstalk in regulating plant development, abiotic stress response, and the balance of growth and stress responses. This review aims to increase the understanding of the role of plant PRTase and also contribute to future research on the trade-off between plant growth and stress response.

Use the force, fluke: Ligand-independent gating of Schistosoma mansoni ion channel TRPMPZQ

The parasitic flatworm ion channel, TRPMPZQ, is a non-selective cation channel that mediates Ca2+ entry and membrane depolarization when activated by the anthelmintic drug, praziquantel (PZQ). TRPMPZQ is conserved in all platyhelminth genomes scrutinized to date, with the sensitivity of TRPMPZQ in any particular flatworm correlating with the overall sensitivity of the worm to PZQ. Conservation of this channel suggests it plays a role in flatworm physiology, but the nature of the endogenous cues that activate this channel are currently unknown. Here, we demonstrate that TRPMPZQ is activated in a ligand-independent manner by membrane stretch, with the electrophysiological signature of channel opening events being identical whether evoked by negative pressure, or by PZQ. TRPMPZQ is therefore a multimodal ion channel gated by both physical and chemical cues. The mechanosensitivity of TRPMPZQ is one route for endogenous activation of this ion channel that holds relevance for schistosome physiology given the persistent pressures and mechanical cues experienced throughout the parasite life cycle.

TIR domain-associated nucleotides with functions in plant immunity and beyond

TIR (Toll/interlukin-1 receptor) domains are found in archaea, bacteria and eukaryotes, featured in proteins generally associated with immune functions. In plants, they are found in a large group of NLRs (nucleotide-binding leucine-rich repeat receptors), NLR-like proteins and TIR-only proteins. They are also present in effector proteins from phytopathogenic bacteria that are associated with suppression of host immunity. TIR domains from plants and bacteria are enzymes that cleave NAD+ (nicotinamide adenine dinucleotide, oxidized form) and other nucleotides. In dicot plants, TIR-derived signalling molecules activate downstream immune signalling proteins, the EDS1 (enhanced disease susceptibility 1) family proteins, and in turn helper NLRs. Recent work has brought major advances in understanding how TIR domains work, how they produce signalling molecules and how these products signal.

TRPM2 Mediates Hepatic Ischemia-Reperfusion Injury via Ca2+-Induced Mitochondrial Lipid Peroxidation through Increasing ALOX12 Expression

Hepatic ischemia-reperfusion (IR) injury is a serious clinical problem that complicates liver resection and transplantation. Despite recent advances in understanding of the pathophysiology of hepatic IR injury, effective interventions and therapeutics are still lacking. Here, we examined the role of transient receptor potential melastatin 2 (TRPM2), a Ca²⁺-permeable, non-selective cation channel, in mediating hepatic IR injury. Our data showed that TRPM2 deficiency attenuated IR-induced liver dysfunction, inflammation, and cell death in mice. Moreover, RNA sequencing analysis indicated that TRPM2-induced IR injury occurs via ferroptosis-related pathways. Consistently, as a ferroptosis inducer, (1S,3R)-RSL3 treatment induced mitochondrial dysfunction in hepatocytes and a TRPM2 inhibitor suppressed this. Interestingly, TRPM2-mediated calcium influx caused mitochondrial calcium accumulation via the mitochondrial Ca²⁺-selective uniporter and increased the expression level of arachidonate 12-lipoxygenase (ALOX12), which results in mitochondrial lipid peroxidation during hepatic IR injury. Furthermore, hepatic IR injury-induced ferroptosis was obviously relieved by a TRPM2 inhibitor or calcium depletion, both in vitro and in vivo. Collectively, these findings demonstrate a crucial role for TRPM2-mediated ferroptosis in hepatic IR injury via increased Ca²⁺-induced ALOX12 expression, indicating that pharmacological inhibition of TRPM2 may provide an effective therapeutic strategy for hepatic IR injury-related diseases, such as during liver resection and transplantation.

Coupling enzymatic activity and gating in an ancient TRPM chanzyme and its molecular evolution

The canonical ion channels gated by chemical ligands use the free energy of agonist binding to open the channel pore, returning to a closed state upon agonist departure. A unique class of ion channels, known as channel-enzymes (chanzymes), possess additional enzymatic activity that is directly or indirectly linked to their channel function. Here we investigated a TRPM2 chanzyme from choanoflagellates, an evolutionary ancestor of all metazoan TRPM channels, which integrates two seemingly incompatible functions into a single peptide: a channel module activated by ADP ribose (ADPR) with high open probability and an enzyme module (NUDT9-H domain) consuming ADPR at a remarkably slow rate. Using time-resolved cryo- electron microscopy (cryo-EM), we captured a complete series of structural snapshots of the gating and catalytic cycles, revealing the coupling mechanism between channel gating and enzymatic activity. Our results showed that the slow kinetics of the NUDT9-H enzyme module confers a novel self-regulatory mechanism, whereby the enzyme module modulates channel gating in a binary manner. Binding of ADPR to NUDT9-H first triggers tetramerization of the enzyme modules, promoting channel opening, while the subsequent hydrolysis reaction reduces local ADPR availability, inducing channel closure. This coupling enables the ion-conducting pore to alternate rapidly between open and closed states, avoiding Mg 2+ and Ca 2+ overload. We further demonstrated how the NUDT9-H domain has evolved from a structurally semi-independent ADPR hydrolase module in early species TRPM2 to a fully integrated component of a gating ring essential for channel activation in advanced species TRPM2. Our study demonstrated an example of how organisms can adapt to their environments at the molecular level.

Evolving roles of CD38 metabolism in solid tumour microenvironment

Given that plenty of clinical findings and reviews have already explained in detail on the progression of CD38 in multiple myeloma and haematological system tumours, here we no longer give unnecessary discussion on the above progression. Though therapeutic antibodies have been regarded as a greatest breakthrough in multiple myeloma immunotherapies due to the durable anti-tumour responses in the clinic, but the role of CD38 in the immunologic regulation and evasion of non-hematopoietic solid tumours are just initiated and controversial. Therefore, we will focus on the bio-function of CD38 enzymatic substrates or metabolites in the variety of non-hematopoietic malignancies and the potential therapeutic value of targeting the CD38-NAD+ or CD38-cADPR/ADPR signal axis. Though limited, we review some ongoing researches and clinical trials on therapeutic approaches in solid tumour as well.

Synthesis and biological evaluation of novel photo-clickable adenosine and cyclic ADP-ribose analogs: 8-N3-2'-O-propargyladenosine and 8-N3-2'-O-propargyl-cADPR

A photo-clickable analog of adenosine was devised and synthesized in which the photoactive functional group (8-azidoadenosine) and the click moiety (2'-O-propargyl-ether) were compactly combined within the structure of the adenosine nucleoside itself. We synthesized 8-N3-2'-O-propargyl adenosine in four steps starting from adenosine. This photo-clickable adenosine was 5'-phosphorylated and coupled to nicotinamide mononucleotide to form the NAD analog 8-N3-2'-O-propargyl-NAD. This NAD analog was recognized by Aplysia californica ADP-ribosyl cyclase and enzymatically cyclized producing 8-N3-2'-O-propargyl cyclic ADP-ribose. Photo-clickable cyclic-ADP-ribose analog was envisioned as a probe to label cyclic ADP-ribose binding proteins. The monofunctional 8-N3-cADPR has previously been shown to be an antagonist of cADPR-induced calcium release [T.F. Walseth et. al., J. Biol. Chem (1993) 268, 26686-26691]. 2'-O-propargyl-cADPR was recognized as an agonist which elicited Ca2+ release when added at low concentration to sea urchin egg homogenates. The bifunctional 8-N3-2'-O-propargyl cyclic ADP-ribose did not elicit Ca2+ release at low concentration or impact cyclic ADP-ribose mediated Ca2+ release either when added to sea urchin egg homogenates or when microinjected into cultured human U2OS cells. The photo-clickable adenosine will none-the-less be a useful scaffold for synthesizing photo-clickable probes for identifying proteins that interact with a variety of adenosine nucleotides.

The Nucleotide Revolution: Immunity at the Intersection of Toll/Interleukin-1 Receptor Domains, Nucleotides, and Ca2

The discovery of the enzymatic activity of the toll/interleukin-1 receptor (TIR) domain protein SARM1 five years ago preceded a flood of discoveries regarding the nucleotide substrates and products of TIR domains in plants, animals, bacteria, and archaea. These discoveries into the activity of TIR domains coincide with major advances in understanding the structure and mechanisms of NOD-like receptors and the mutual dependence of pattern recognition receptor- and effector-triggered immunity (PTI and ETI, respectively) in plants. It is quickly becoming clear that TIR domains and TIR-produced nucleotides are ancestral signaling molecules that modulate immunity and that their activity is closely associated with Ca²⁺ signaling. TIR domain research now bridges the separate disciplines of molecular plant- and animal-microbe interactions, neurology, and prokaryotic immunity. A cohesive framework for understanding the role of enzymatic TIR domains in diverse organisms will help unite the research of these disparate fields. Here, we review known products of TIR domains in plants, animals, bacteria, and archaea and use context gained from animal and prokaryotic TIR domain systems to present a model for TIR domains, nucleotides, and Ca²⁺ at the intersection of PTI and ETI in plant immunity. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

The identification of the key residues E829 and R845 involved in transient receptor potential melastatin 2 channel gating

Transient receptor potential melastatin 2 (TRPM2), a non-selective cation channel, is involved in many physiological and pathological processes, including temperature sensing, synaptic plasticity regulation, and neurodegenerative diseases. However, the gating mechanism of TRPM2 channel is complex, which hinders its functional research. With the discovery of the Ca2+ binding site in the S2–S3 domain of TRPM2 channel, more and more attention has been drawn to the role of the transmembrane segments in channel gating. In this study, we focused on the D820-F867 segment around the S2 domain, and identified the key residues on it. Functional assays of the deletion mutants displayed that the deletions of D820-W835 and L836-P851 destroyed channel function totally, indicating the importance of these two segments. Sequence alignments on them found three polar and charged residues with high conservation (D820, E829, and R845). D820A, E829A, and R845A which removed the charge and the side chain of the residues were tested by 500 μM adenosine diphosphate-ribose (ADPR) or 50 mM Ca2+. E829A and R845A affected the characteristic of channel currents, while D820A behaved similarly to WT, indicating the participations of E829 and R845 in channel gating. The charge reversing mutants, E829K and R845D were then constructed and the electrophysiological tests showed that E829A and E829K made the channel lose function. Interestingly, R845A and R845D exhibited an inactivation process when using 500 μM ADPR, but activated normally by 50 mM Ca2+. Our data suggested that the negative charge at E829 took a vital part in channel activation, and R845 increased the stability of the Ca2+ combination in S2-S3 domain, thus guaranteeing the opening of TRPM2 channel. In summary, our identification of the key residues E829 and R845 in the transmembrane segments of TRPM2. By exploring the gating process of TRPM2 channel, our work helps us better understand the mechanism of TRPM2 as a potential biomarker in neurodegenerative diseases, and provides a new approach for the prediction, diagnosis, and prognosis of neurodegenerative diseases.

Salicylic Acid Enhances Heat Stress Resistance of Pleurotus ostreatus (Jacq.) P. Kumm through Metabolic Rearrangement

Pleurotus ostreatus (Jacq.) P. Kumm is cultivated worldwide, and its growth is seriously threatened by heat stress. Here, we performed a comprehensive analysis to investigate the influence of the phytohormone salicylic acid (SA) in P. ostreatus under HS. The results showed that the hyphal growth recovery rate and the antioxidant capacity of P. ostreatus increased with exogenous SA application (0.01 mmol/L and 0.05 mmol/L) after HS treatment. Metabolomic and transcriptomic analyses showed that SA application (0.05 mmol/L) weakened central carbon metabolism to allow cells to survive HS efficiently. In addition, SA shifted glycolysis to one-carbon metabolism to produce ROS scavengers (GSH and NADPH) and reduced ROS production by altering mitochondrial metabolism. SA also maintained nucleotide homeostasis, led to membrane lipid remodeling, activated the MAPK pathway, and promoted the synthesis of cell-wall components. This study provides a reference for further study of SA in microorganisms.

cADPR Does Not Activate TRPM2

cADPR is a second messenger that releases Ca²⁺ from intracellular stores via the ryanodine receptor. Over more than 15 years, it has been controversially discussed whether cADPR also contributes to the activation of the nucleotide-gated cation channel TRPM2. While some groups have observed activation of TRPM2 by cADPR alone or in synergy with ADPR, sometimes only at 37 °C, others have argued that this is due to the contamination of cADPR by ADPR. The identification of a novel nucleotide-binding site in the N-terminus of TRPM2 that binds ADPR in a horseshoe-like conformation resembling cADPR as well as the cADPR antagonist 8-Br-cADPR, and another report that demonstrates activation of TRPM2 by binding of cADPR to the NUDT9H domain raised the question again and led us to revisit the topic. Here we show that (i) the N-terminal MHR1/2 domain and the C-terminal NUDT9H domain are required for activation of human TRPM2 by ADPR and 2'-deoxy-ADPR (2dADPR), (ii) that pure cADPR does not activate TRPM2 under a variety of conditions that have previously been shown to result in channel activation, (iii) the cADPR antagonist 8-Br-cADPR also inhibits activation of TRPM2 by ADPR, and (iv) cADPR does not bind to the MHR1/2 domain of TRPM2 while ADPR does.

Sarm1 activation produces cADPR to increase intra-axonal Ca++ and promote axon degeneration in PIPN

Cancer patients frequently develop chemotherapy-induced peripheral neuropathy (CIPN), a painful and long-lasting disorder with profound somatosensory deficits. There are no effective therapies to prevent or treat this disorder. Pathologically, CIPN is characterized by a "dying-back" axonopathy that begins at intra-epidermal nerve terminals of sensory neurons and progresses in a retrograde fashion. Calcium dysregulation constitutes a critical event in CIPN, but it is not known how chemotherapies such as paclitaxel alter intra-axonal calcium and cause degeneration. Here, we demonstrate that paclitaxel triggers Sarm1-dependent cADPR production in distal axons, promoting intra-axonal calcium flux from both intracellular and extracellular calcium stores. Genetic or pharmacologic antagonists of cADPR signaling prevent paclitaxel-induced axon degeneration and allodynia symptoms, without mitigating the anti-neoplastic efficacy of paclitaxel. Our data demonstrate that cADPR is a calcium-modulating factor that promotes paclitaxel-induced axon degeneration and suggest that targeting cADPR signaling provides a potential therapeutic approach for treating paclitaxel-induced peripheral neuropathy (PIPN).

Structural basis of ALMT1-mediated aluminum resistance in Arabidopsis

The plant aluminum (Al)-activated malate transporter ALMT1 mediates the efflux of malate to chelate the Al in acidic soils and underlies the plant Al resistance. Here we present cryo-electron microscopy (cryo-EM) structures of Arabidopsis thaliana ALMT1 (AtALMT1) in the apo, malate-bound, and Al-bound states at neutral and/or acidic pH at up to 3.0 Å resolution. The AtALMT1 dimer assembles an anion channel and each subunit contains six transmembrane helices (TMs) and six cytosolic α-helices. Two pairs of Arg residues are located in the center of the channel pore and contribute to malate recognition. Al binds at the extracellular side of AtALMT1 and induces conformational changes of the TM1-2 loop and the TM5-6 loop, resulting in the opening of the extracellular gate. These structures, along with electrophysiological measurements, molecular dynamic simulations, and mutagenesis study in Arabidopsis, elucidate the structural basis for Al-activated malate transport by ALMT1.

A novel mechanism of imeglimin-mediated insulin secretion via the cADPR-TRP channel pathway

AIMS/INTRODUCTION

Imeglimin is a novel oral hypoglycemic agent that improves blood glucose levels through multiple mechanisms of action including the enhancement of glucose-stimulated insulin secretion (GSIS), however, the details of this mechanism have not been clarified. In the process of GSIS, activation of the transient receptor potential melastatin 2 (TRPM2) channel, a type of non-selective cation channel (NSCCs) in β-cells, promotes plasma membrane depolarization. The present study aimed to examine whether imeglimin potentiates GSIS via the TRPM2 channel in β-cells.

MATERIALS AND METHODS

Pancreatic islets were isolated by collagenase digestion from male wild-type and TRPM2-knockout (KO) mice. Insulin release and nicotinamide adenine dinucleotide (NAD+ ) production in islets were measured under static incubation. NSCC currents in mouse single β-cells were measured by patch-clamp experiments.

RESULTS

Batch-incubation studies showed that imeglimin enhanced GSIS at stimulatory 16.6 mM glucose, whereas it did not affect basal insulin levels at 2.8 mM glucose. Imeglimin increased the glucose-induced production of NAD+ , a precursor of cADPR, in islets and the insulinotropic effects of imeglimin were attenuated by a cADPR inhibitor 8-Br-cADPR. Furthermore, imeglimin increased NSCC current in β-cells, and abolished this current in TRPM2-KO mice. Imeglimin did not potentiate GSIS in the TRPM2-KO islets, suggesting that imeglimin's increase of NSCC currents through the TRPM2 channel is causally implicated in its insulin releasing effects.

CONCLUSIONS

Imeglimin may activate TRPM2 channels in β-cells via the production of NAD+ /cADPR, leading to the potentiation of GSIS. Developing approaches to stimulate cADPR-TRPM2 signaling provides a potential therapeutic tool to treat type 2 diabetes.

The calcium signaling enzyme CD38 - a paradigm for membrane topology defining distinct protein functions

CD38 is a single-pass transmembrane enzyme catalyzing the synthesis of two nucleotide second messengers, cyclic ADP-ribose (cADPR) from NAD and nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP. The former mediates the mobilization of the endoplasmic Ca²⁺-stores in response to a wide range of stimuli, while NAADP targets the endo-lysosomal stores. CD38 not only possesses multiple enzymatic activities, it also exists in two opposite membrane orientations. Type III CD38 has the catalytic domain facing the cytosol and is responsible for producing cellular cADPR. The type II CD38 has an opposite orientation and is serving as a surface receptor mediating extracellular functions such as cell adhesion and lymphocyte activation. Its ecto-NADase activity also contributes to the recycling of external NAD released by apoptosis. Endocytosis can deliver surface type II CD38 to endo-lysosomes, which acidic environment favors the production of NAADP. This article reviews the rationale and evidence that have led to CD38 as a paradigm for membrane topology defining distinct functions of proteins. Also described is the recent discovery of a hitherto unknown cADPR-synthesizing enzyme, SARM1, ushering in a new frontier in cADPR-mediated Ca²⁺-signaling.

Structural and functional basis of the selectivity filter as a gate in human TRPM2 channel

Transient receptor potential melastatin 2 (TRPM2), a Ca²⁺-permeable cation channel, is gated by intracellular adenosine diphosphate ribose (ADPR), Ca²⁺, warm temperature, and oxidative stress. It is critically involved in physiological and pathological processes ranging from inflammation to stroke to neurodegeneration. At present, the channel's gating and ion permeation mechanisms, such as the location and identity of the selectivity filter, remain ambiguous. Here, we report the cryo-electron microscopy (cryo-EM) structure of human TRPM2 in nanodisc in the ligand-free state. Cryo-EM map-guided computational modeling and patch-clamp recording further identify a quadruple-residue motif as the ion selectivity filter, which adopts a restrictive conformation in the closed state and acts as a gate, profoundly contrasting with its widely open conformation in the Nematostella vectensis TRPM2. Our study reveals the gating of human TRPM2 by the filter and demonstrates the feasibility of using cryo-EM in conjunction with computational modeling and functional studies to garner structural information for intrinsically dynamic but functionally important domains.

Probing the Ca2+ mobilizing properties on primary cortical neurons of a new stable cADPR mimic

Cyclic adenosine diphosphate ribose (cADPR) is a second messenger involved in the Ca²⁺ homeostasis. Its chemical instability prompted researchers to tune point by point its structure, obtaining stable analogues featuring interesting biological properties. One of the most challenging derivatives is the cyclic inosine diphosphate ribose (cIDPR), in which the hypoxanthine isosterically replaces the adenine. As our research focuses on the synthesis of N1 substituted inosines, in the last few years we have produced new flexible cIDPR analogues, where the northern ribose has been replaced by alkyl chains. Interestingly, some of them mobilized Ca²⁺ ions in PC12 cells. To extend our SAR studies, herein we report on the synthesis of a new stable cIDPR derivative which contains the 2″S,3″R dihydroxypentyl chain instead of the northern ribose. Interestingly, the new cyclic derivative and its open precursor induced an increase in intracellular calcium concentration ([Ca²⁺]_i) with the same efficacy of the endogenous cADPR in rat primary cortical neurons.

Risks and rewards of targeting NAD+ homeostasis in the brain

Strategies to correct declining nicotinamide adenine dinucleotide (NAD⁺) levels in neurological disease and biological ageing are promising therapeutic candidates. These strategies include supplementing with NAD⁺ precursors, small molecule activation of NAD⁺ biosynthetic enzymes, and treatment with small molecule inhibitors of NAD⁺ consuming enzymes such as CD38, SARM1 or members of the PARP family. While these strategies have shown efficacy in animal models of neurological disease, each of these has the mechanistic potential for adverse events that could preclude their preclinical use. Here, we discuss the implications of these strategies for treating neurological diseases, including potential off-target effects that may be unique to the brain.

The Role of TRPM2 in Endothelial Function and Dysfunction

The transient receptor potential (TRP) melastatin-like subfamily member 2 (TRPM2) is a non-selective calcium-permeable cation channel. It is expressed by many mammalian tissues, including bone marrow, spleen, lungs, heart, liver, neutrophils, and endothelial cells. The best-known mechanism of TRPM2 activation is related to the binding of ADP-ribose to the nudix-box sequence motif (NUDT9-H) in the C-terminal domain of the channel. In cells, the production of ADP-ribose is a result of increased oxidative stress. In the context of endothelial function, TRPM2-dependent calcium influx seems to be particularly interesting as it participates in the regulation of barrier function, cell death, cell migration, and angiogenesis. Any impairments of these functions may result in endothelial dysfunction observed in such conditions as atherosclerosis or hypertension. Thus, TRPM2 seems to be an attractive therapeutic target for the conditions connected with the increased production of reactive oxygen species. However, before the application of TRPM2 inhibitors will be possible, some issues need to be resolved. The main issues are the lack of specificity, poor membrane permeabilization, and low stability in in vivo conditions. The article aims to summarize the latest findings on a role of TRPM2 in endothelial cells. We also show some future perspectives for the application of TRPM2 inhibitors in cardiovascular system diseases.

The intrinsic role and mechanism of tumor expressed-CD38 on lung adenocarcinoma progression

It has been recently reported that CD38 expressed on tumor cells of multiple murine and human origins could be upregulated in response to PD-L1 antibody therapy, which led to dysfunction of tumor-infiltrating CD8+ T immune cells due to increasing the production of adenosine. However, the role of tumor expressed-CD38 on neoplastic formation and progression remains elusive. In the present study, we aimed to delineate the molecular and biochemical function of the tumor-associated CD38 in lung adenocarcinoma progression. Our clinical data showed that the upregulation of tumor-originated CD38 was correlated with poor survival of lung cancer patients. Using multiple in vitro assays we found that the enzymatic activity of tumor expressed-CD38 facilitated lung cancer cell migration, proliferation, colony formation, and tumor development. Consistently, our in vivo results showed that inhibition of the enzymatic activity or antagonizing the enzymatic product of CD38 resulted in the similar inhibition of tumor proliferation and metastasis as CD38 gene knock-out or mutation. At biochemical level, we further identified that cADPR, the mainly hydrolytic product of CD38, was responsible for inducing the opening of TRPM2 iron channel leading to the influx of intracellular Ca2+ and then led to increasing levels of NRF2 while decreasing expression of KEAP1 in lung cancer cells. These findings suggested that malignant lung cancer cells were capable of using cADPR catalyzed by CD38 to facilitate tumor progression, and blocking the enzymatic activity of CD38 could be represented as an important strategy for preventing tumor progression.

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.

Two Decades of Evolution of Our Understanding of the Transient Receptor Potential Melastatin 2 (TRPM2) Cation Channel

The transient receptor potential melastatin (TRPM) family belongs to the superfamily of TRP ion channels. It consists of eight family members that are involved in a plethora of cellular functions. TRPM2 is a homotetrameric Ca²⁺-permeable cation channel activated upon oxidative stress and is important, among others, for body heat control, immune cell activation and insulin secretion. Invertebrate TRPM2 proteins are channel enzymes; they hydrolyze the activating ligand, ADP-ribose, which is likely important for functional regulation. Since its cloning in 1998, the understanding of the biophysical properties of the channel has greatly advanced due to a vast number of structure–function studies. The physiological regulators of the channel have been identified and characterized in cell-free systems. In the wake of the recent structural biochemistry revolution, several TRPM2 cryo-EM structures have been published. These structures have helped to understand the general features of the channel, but at the same time have revealed unexplained mechanistic differences among channel orthologues. The present review aims at depicting the major research lines in TRPM2 structure-function. It discusses biophysical properties of the pore and the mode of action of direct channel effectors, and interprets these functional properties on the basis of recent three-dimensional structural models.

Enzymology of extracellular NAD metabolism

Extracellular NAD represents a key signaling molecule in different physiological and pathological conditions. It exerts such function both directly, through the activation of specific purinergic receptors, or indirectly, serving as substrate of ectoenzymes, such as CD73, nucleotide pyrophosphatase/phosphodiesterase 1, CD38 and its paralog CD157, and ecto ADP ribosyltransferases. By hydrolyzing NAD, these enzymes dictate extracellular NAD availability, thus regulating its direct signaling role. In addition, they can generate from NAD smaller signaling molecules, like the immunomodulator adenosine, or they can use NAD to ADP-ribosylate various extracellular proteins and membrane receptors, with significant impact on the control of immunity, inflammatory response, tumorigenesis, and other diseases. Besides, they release from NAD several pyridine metabolites that can be taken up by the cell for the intracellular regeneration of NAD itself. The extracellular environment also hosts nicotinamide phosphoribosyltransferase and nicotinic acid phosphoribosyltransferase, which inside the cell catalyze key reactions in NAD salvaging pathways. The extracellular forms of these enzymes behave as cytokines, with pro-inflammatory functions. This review summarizes the current knowledge on the extracellular NAD metabolome and describes the major biochemical properties of the enzymes involved in extracellular NAD metabolism, focusing on the contribution of their catalytic activities to the biological function. By uncovering the controversies and gaps in their characterization, further research directions are suggested, also to better exploit the great potential of these enzymes as therapeutic targets in various human diseases.

NAD+ metabolism and its roles in cellular processes during ageing

Nicotinamide adenine dinucleotide (NAD⁺) is a coenzyme for redox reactions, making it central to energy metabolism. NAD⁺ is also an essential cofactor for non-redox NAD⁺-dependent enzymes, including sirtuins, CD38 and poly(ADP-ribose) polymerases. NAD⁺ can directly and indirectly influence many key cellular functions, including metabolic pathways, DNA repair, chromatin remodelling, cellular senescence and immune cell function. These cellular processes and functions are critical for maintaining tissue and metabolic homeostasis and for healthy ageing. Remarkably, ageing is accompanied by a gradual decline in tissue and cellular NAD⁺ levels in multiple model organisms, including rodents and humans. This decline in NAD⁺ levels is linked causally to numerous ageing-associated diseases, including cognitive decline, cancer, metabolic disease, sarcopenia and frailty. Many of these ageing-associated diseases can be slowed down and even reversed by restoring NAD⁺ levels. Therefore, targeting NAD⁺ metabolism has emerged as a potential therapeutic approach to ameliorate ageing-related disease, and extend the human healthspan and lifespan. However, much remains to be learnt about how NAD⁺ influences human health and ageing biology. This includes a deeper understanding of the molecular mechanisms that regulate NAD⁺ levels, how to effectively restore NAD⁺ levels during ageing, whether doing so is safe and whether NAD⁺ repletion will have beneficial effects in ageing humans.

Roles of NAD+ and Its Metabolites Regulated Calcium Channels in Cancer

Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor for redox enzymes, but also moonlights as a regulator for ion channels, the same as its metabolites. Ca²⁺ homeostasis is dysregulated in cancer cells and affects processes such as tumorigenesis, angiogenesis, autophagy, progression, and metastasis. Herein, we summarize the regulation of the most common calcium channels (TRPM2, TPCs, RyRs, and TRPML1) by NAD⁺ and its metabolites, with a particular focus on their roles in cancers. Although the mechanisms of NAD⁺ metabolites in these pathological processes are yet to be clearly elucidated, these ion channels are emerging as potential candidates of alternative targets for anticancer therapy.

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.

Structure-Function Relationship of TRPM2: Recent Advances, Contradictions, and Open Questions

When in a particular scientific field, major progress is rapidly reached after a long period of relative stand-still, this is often achieved by the development or exploitation of new techniques and methods. A striking example is the new insights brought into the understanding of the gating mechanism of the transient receptor potential melastatin type 2 cation channel (TRPM2) by cryogenic electron microscopy structure analysis. When conventional methods are complemented by new ones, it is quite natural that established researchers are not fully familiar with the possibilities and limitations of the new method. On the other hand, newcomers may need some assistance in perceiving the previous knowledge in detail; they may not realize that some of their interpretations are at odds with previous results and need refinement. This may in turn trigger further studies with new and promising perspectives, combining the promises of several methodological approaches. With this review, I aim to give a comprehensive overview on functional data of several orthologous of TRPM2 that are nicely explained by structural studies. Moreover, I wish to point out some functional contradictions raised by the structural data. Finally, some open questions and some lines of possible future experimental approaches shall be discussed.

Does Cyclic ADP-Ribose (cADPR) Activate the Non-selective Cation Channel TRPM2?

TRPM2 is a non-selective, Ca²⁺-permeable cation channel widely expressed in immune cells. It is firmly established that the channel can be activated by intracellular adenosine 5′-diphosphoribose (ADPR). Until recent cryo-EM structures have exhibited an additional nucleotide binding site in the N-terminus of the channel, this activation was thought to occur via binding to a C-terminal domain of the channel that is highly homologous to the ADPR pyrophosphatase NudT9. Over the years it has been controversially discussed whether the Ca²⁺ mobilizing second messenger cyclic ADP ribose (cADPR) might also directly activate Ca²⁺ entry via TRPM2. Here we will review the status of this discussion.

CD38: A Potential Therapeutic Target in Cardiovascular Disease

Substantial research has demonstrated the association between cardiovascular disease and the dysregulation of intracellular calcium, ageing, reduction in nicotinamide adenine dinucleotide NAD+ content, and decrease in sirtuin activity. CD38, which comprises the soluble type, type II, and type III, is the main NADase in mammals. This molecule catalyses the production of cyclic adenosine diphosphate ribose (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), and adenosine diphosphate ribose (ADPR), which stimulate the release of Ca²⁺, accompanied by NAD+ consumption and decreased sirtuin activity. Therefore, the relationship between cardiovascular disease and CD38 has been attracting increased attention. In this review, we summarize the structure, regulation, function, targeted drug development, and current research on CD38 in the cardiac context. More importantly, we provide original views about the as yet elusive mechanisms of CD38 action in certain cardiovascular disease models. Based on our review, we predict that CD38 may serve as a novel therapeutic target in cardiovascular disease in the future.

Molecular Regulations and Functions of the Transient Receptor Potential Channels of the Islets of Langerhans and Insulinoma Cells

Insulin secretion from the β-cells of the islets of Langerhans is triggered mainly by nutrients such as glucose, and incretin hormones such as glucagon-like peptide-1 (GLP-1). The mechanisms of the stimulus-secretion coupling involve the participation of the key enzymes that metabolize the nutrients, and numerous ion channels that mediate the electrical activity. Several members of the transient receptor potential (TRP) channels participate in the processes that mediate the electrical activities and Ca²⁺ oscillations in these cells. Human β-cells express TRPC1, TRPM2, TRPM3, TRPM4, TRPM7, TRPP1, TRPML1, and TRPML3 channels. Some of these channels have been reported to mediate background depolarizing currents, store-operated Ca²⁺ entry (SOCE), electrical activity, Ca²⁺ oscillations, gene transcription, cell-death, and insulin secretion in response to stimulation by glucose and GLP1. Different channels of the TRP family are regulated by one or more of the following mechanisms: activation of G protein-coupled receptors, the filling state of the endoplasmic reticulum Ca²⁺ store, heat, oxidative stress, or some second messengers. This review briefly compiles our current knowledge about the molecular mechanisms of regulations, and functions of the TRP channels in the β-cells, the α-cells, and some insulinoma cell lines.

Predisposition to Alzheimer's and Age-Related Brain Pathologies by PM2.5 Exposure: Perspective on the Roles of Oxidative Stress and TRPM2 Channel

Accumulating epidemiological evidence supports that chronic exposure to ambient fine particular matters of <2.5 μm (PM2.5) predisposes both children and adults to Alzheimer’s disease (AD) and age-related brain damage leading to dementia. There is also experimental evidence to show that PM2.5 exposure results in early onset of AD-related pathologies in transgenic AD mice and development of AD-related and age-related brain pathologies in healthy rodents. Studies have also documented that PM2.5 exposure causes AD-linked molecular and cellular alterations, such as mitochondrial dysfunction, synaptic deficits, impaired neurite growth, neuronal cell death, glial cell activation, neuroinflammation, and neurovascular dysfunction, in addition to elevated levels of amyloid β (Aβ) and tau phosphorylation. Oxidative stress and the oxidative stress-sensitive TRPM2 channel play important roles in mediating multiple molecular and cellular alterations that underpin AD-related cognitive dysfunction. Documented evidence suggests critical engagement of oxidative stress and TRPM2 channel activation in various PM2.5-induced cellular effects. Here we discuss recent studies that favor causative relationships of PM2.5 exposure to increased AD prevalence and AD- and age-related pathologies, and raise the perspective on the roles of oxidative stress and the TRPM2 channel in mediating PM2.5-induced predisposition to AD and age-related brain damage.

A structural overview of the ion channels of the TRPM family

The TRPM (transient receptor potential melastatin) family belongs to the superfamily of TRP cation channels. The TRPM subfamily is composed of eight members that are involved in diverse biological functions such as temperature sensing, inflammation, insulin secretion, and redox sensing. Since the first cloning of TRPM1 in 1998, tremendous progress has been made uncovering the function, structure, and pharmacology of this family. Complete structures of TRPM2, TRPM4, and TRPM8, as well as a partial structure of TRPM7, have been determined by cryo-EM, providing insights into their channel assembly, ion permeation, gating mechanisms, and structural pharmacology. Here we summarize the current knowledge about channel structure, emphasizing general features and principles of the structure of TRPM channels discovered since 2017. We also discuss some of the key unresolved issues in the field, including the molecular mechanisms underlying voltage and temperature dependence, as well as the functions of the TRPM channels' C-terminal domains.

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.

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.

New Linear Precursors of cIDPR Derivatives as Stable Analogs of cADPR: A Potent Second Messenger with Ca2+-Modulating Activity Isolated from Sea Urchin Eggs

Herein, we report on the synthesis of a small set of linear precursors of an inosine analogue of cyclic ADP-ribose (cADPR), a second messenger involved in Ca2+ mobilization from ryanodine receptor stores firstly isolated from sea urchin eggs extracts. The synthesized compounds were obtained starting from inosine and are characterized by an N1-alkyl chain replacing the "northern" ribose and a phosphate group attached at the end of the N1-alkyl chain and/or 5'-sugar positions. Preliminary Ca2+ mobilization assays, performed on differentiated C2C12 cells, are reported as well.