Structure-activity relationship of adenosine 5'-diphosphoribose at the transient receptor potential melastatin 2 (TRPM2) channel: rational design of antagonists

Design and synthesis of a library of C8-substituted sulfamidoadenosines to probe bacterial permeability

Gram-negative bacteria pose a major challenge in antibiotic drug discovery because their cell envelope presents a permeability barrier that affords high intrinsic resistance to small-molecule drugs. The identification of correlations between chemical structure and Gram-negative permeability would thus enable development of predictive tools to facilitate antibiotic discovery. Toward this end, have advanced a library design paradigm in which various chemical scaffolds are functionalized at different regioisomeric positions using a uniform reagent set. This design enables decoupling of scaffold, regiochemistry, and substituent effects upon Gram-negative permeability of these molecules. Building upon our recent synthesis of a library of C2-substituted sulfamidoadenosines, we have now developed an efficient synthetic route to an analogous library of regioisomeric C8-substituted congeners. The C8 library samples a region of antibiotic-relevant chemical space that is similar to that addressed by the C2 library, but distinct from that sampled by a library of analogously substituted oxazolidinones. Selected molecules were tested for accumulation in Escherichia coli in a pilot analysis, setting the stage for full comparative evaluation of these libraries in the future.

2'-deoxy-ADPR activates human TRPM2 faster than ADPR and thereby induces higher currents at physiological Ca2+ concentrations

TRPM2 is a Ca2+ permeable, non-selective cation channel in the plasma membrane that is involved in the innate immune response regulating, for example, chemotaxis in neutrophils and cytokine secretion in monocytes and macrophages. The intracellular adenine nucleotides ADP-ribose (ADPR) and 2'-deoxy-ADPR (2dADPR) activate the channel, in combination with their co-agonist Ca2+. Interestingly, activation of human TRPM2 (hsTRPM2) by 2dADPR is much more effective than activation by ADPR. However, the underlying mechanism of the nucleotides' differential effect on the channel is not yet fully understood. In this study, we performed whole-cell patch clamp experiments with HEK293 cells heterologously expressing hsTRPM2. We show that 2dADPR has an approx. 4-fold higher Ca2+ sensitivity than ADPR (EC50 = 190 and 690 nM). This allows 2dADPR to activate the channel at lower and thus physiological intracellular Ca2+ concentrations. Kinetic analysis of our data reveals that activation by 2dADPR is faster than activation by ADPR. Mutation in a calmodulin binding N-terminal IQ-like motif in hsTRPM2 completely abrogated channel activation by both agonists. However, mutation of a single amino acid residue (W1355A) in the C-terminus of hsTRPM2, at a site of extensive inter-domain interaction, resulted in slower activation by 2dADPR and neutralized the difference in rate of activation between the two agonists. Taken together, we propose a mechanism by which 2dADPR induces higher hsTRPM2 currents than ADPR by means of faster channel activation. The finding that 2dADPR has a higher Ca2+ sensitivity than ADPR may indicate that 2dADPR rather than ADPR activates hsTRPM2 in physiological contexts such as the innate immune response.

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.

Identified Isosteric Replacements of Ligands' Glycosyl Domain by Data Mining

Biologically equivalent replacements of key moieties in molecules rationalize scaffold hopping, patent busting, or R-group enumeration. Yet, this information may depend upon the expert-defined space, and might be subjective and biased toward the chemistries they get used to. Most importantly, these practices are often informatively incomplete since they are often compromised by a try-and-error cycle, and although they depict what kind of substructures are suitable for the replacement occurrence, they fail to explain the driving forces to support such interchanges. The protein data bank (PDB) encodes a receptor-ligand interaction pattern and could be an optional source to mine structural surrogates. However, manual decoding of PDB has become almost impossible and redundant to excavate the bioisosteric know-how. Therefore, a text parsing workflow has been developed to automatically extract the local structural replacement of a specific structure from PDB by finding spatial and steric interaction overlaps between the fragments in endogenous ligands and particular ligand fragments. Taking the glycosyl domain for instance, a total of 49 520 replacements that overlap on nucleotide ribose were identified and categorized based on their SMILE codes. A predominately ring system, such as aliphatic and aromatic rings, was observed; yet, amide and sulfonamide replacements also occur. We believe these findings may enlighten medicinal chemists on the structure design and optimization of ligands using the bioisosteric replacement strategy.

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

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

Synthesis and Characterization of NUC-7738, an Aryloxy Phosphoramidate of 3'-Deoxyadenosine, as a Potential Anticancer Agent

3'-Deoxyadenosine (3'-dA, Cordycepin, 1) is a nucleoside analogue with anticancer properties, but its clinical development has been hampered due to its deactivation by adenosine deaminase (ADA) and poor cellular uptake due to low expression of the human equilibrative transporter (hENT1). Here, we describe the synthesis and characterization of NUC-7738 (7a), a 5'-aryloxy phosphoramidate prodrug of 3'-dA. We show in vitro evidence that 7a is an effective anticancer drug in a panel of solid and hematological cancer cell lines, showing its preferential cytotoxic effects on leukemic stem cells. We found that unlike 3'-dA, the activity of 7a was independent of hENT1 and kinase activity. Furthermore, it was resistant to ADA metabolic deactivation. Consistent with these findings, 7a showed increased levels of intracellular 3'-deoxyadenosine triphosphate (3'-dATP), the active metabolite. Mechanistically, levels of intracellular 3'-dATP were strongly associated with in vitro potency. NUC-7738 is now in Phase II, dose-escalation study in patients with advanced solid tumors.

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.

Salvianolic acid B inhibiting chemically and mechanically activated Piezo1 channels as a mechanism for ameliorating atherosclerosis

BACKGROUND AND PURPOSE

Salvianolic acid B (SalB) is effective for treating cardiovascular diseases. However, its therapeutic molecular mechanisms remain unclear. Mechanosensitive Piezo1 channels play important roles in vascular biology, although its pharmacological properties are poorly defined. Here, we aimed to identify novel Piezo1 inhibitors and gain insights into their mechanisms of action.

EXPERIMENTAL APPROACH

Intracellular Ca²⁺ ions were measured in human umbilical vein endothelial cells (HUVECs), murine liver endothelial cells (MLECs), THP-1 and RAW264.7 cell lines, and bone marrow-derived macrophages (BMDMs). Isometric tensions in mouse thoracic aorta were recorded. Shear-stress assays with HUVECs were conducted. Patch-clamp recordings with mechanical stimulation were performed with HUVECs in whole-cell mode. Foam cell formation was induced by treating BMDMs with oxidised low-density lipoprotein (oxLDL). Atherosclerotic plaque assays were performed with Ldlr^-/- and Piezo1 genetically depleted mice on a high-fat diet.

KEY RESULTS

We discovered that SalB inhibited Yoda1-induced Ca²⁺ influx in HUVECs and MLECs. Similar results were observed in macrophage cell lines and BMDMs. Furthermore, we demonstrated that SalB inhibited Yoda1 and mechanically activated currents. SalB restrained Yoda1-induced aortic ring relaxation and inhibited HUVECs alignment in the direction of shear stress. Additionally, we found that Yoda1 enhanced the formation of foam cells, which was reversed by SalB and SalB inhibited the formation of atherosclerotic plaques and was insensitive to Piezo1 genetically depletion.

CONCLUSION AND IMPLICATIONS

Our study provides novel mechanistic insights into the inhibitory role of SalB against Piezo1 channels and improves our understanding of SalB in preventing atherosclerotic lesions.

TRPM2 Non-Selective Cation Channels in Liver Injury Mediated by Reactive Oxygen Species

TRPM2 channels admit Ca²⁺ and Na⁺ across the plasma membrane and release Ca²⁺ and Zn²⁺ from lysosomes. Channel activation is initiated by reactive oxygen species (ROS), leading to a subsequent increase in ADP-ribose and the binding of ADP-ribose to an allosteric site in the cytosolic NUDT9 homology domain. In many animal cell types, Ca²⁺ entry via TRPM2 channels mediates ROS-initiated cell injury and death. The aim of this review is to summarise the current knowledge of the roles of TRPM2 and Ca²⁺ in the initiation and progression of chronic liver diseases and acute liver injury. Studies to date provide evidence that TRPM2-mediated Ca²⁺ entry contributes to drug-induced liver toxicity, ischemia–reperfusion injury, and the progression of non-alcoholic fatty liver disease to cirrhosis, fibrosis, and hepatocellular carcinoma. Of particular current interest are the steps involved in the activation of TRPM2 in hepatocytes following an increase in ROS, the downstream pathways activated by the resultant increase in intracellular Ca²⁺, and the chronology of these events. An apparent contradiction exists between these roles of TRPM2 and the role identified for ROS-activated TRPM2 in heart muscle and in some other cell types in promoting Ca²⁺-activated mitochondrial ATP synthesis and cell survival. Inhibition of TRPM2 by curcumin and other “natural” compounds offers an attractive strategy for inhibiting ROS-induced liver cell injury. In conclusion, while it has been established that ROS-initiated activation of TRPM2 contributes to both acute and chronic liver injury, considerable further research is needed to elucidate the mechanisms involved, and the conditions under which pharmacological inhibition of TRPM2 can be an effective clinical strategy to reduce ROS-initiated liver injury.

TRP channels in COVID-19 disease: Potential targets for prevention and treatment

Coronavirus disease 2019 [COVID-19] is a global health threat caused by severe acute respiratory syndrome coronavirus 2 [SARS-CoV2] that requires two proteins for entry: angiotensin-converting enzyme 2 [ACE2] and -membrane protease serine 2 [TMPRSS2]. Many patients complain from pneumonia, cough, fever, and gastrointestinal (GI) problems. Notably, different TRP channels are expressed in various tissues infected by SARS-CoV-2. TRP channels are cation channels that show a common architecture with high permeability to calcium [Ca²⁺] in most sub-families. Literature review shed light on the possible role of TRP channels in COVID-19 disease. TRP channels may take part in inflammation, pain, fever, anosmia, ageusia, respiratory, cardiovascular, GI and neurological complications related to COVID-19. Also, TRP channels could be the targets for many active compounds that showed effectiveness against SARS-CoV-2. Desensitization or blocking TRP channels by antibodies, aptamers, small molecules or venoms can be an option for COVID-19 prevention and future treatment. This review provides insights into the involvement of TRP channels in different symptoms and mechanisms of SARS-CoV-2 , potential treatments targeting these channels and highlights missing gaps in literature.

Selective profiling of N- and C-terminal nucleotide-binding sites in a TRPM2 channel

Transient receptor potential melastatin 2 (TRPM2) is a homotetrameric Ca²⁺-permeable cation channel important for the immune response, body temperature regulation, and insulin secretion, and is activated by cytosolic Ca²⁺ and ADP ribose (ADPR). ADPR binds to two distinct locations, formed by large N- and C-terminal cytosolic domains, respectively, of the channel protein. In invertebrate TRPM2 channels, the C-terminal site is not required for channel activity but acts as an active ADPR phosphohydrolase that cleaves the activating ligand. In vertebrate TRPM2 channels, the C-terminal site is catalytically inactive but cooperates with the N-terminal site in channel activation. The precise functional contributions to channel gating and the nucleotide selectivities of the two sites in various species have not yet been deciphered. For TRPM2 of the sea anemone Nematostella vectensis (nvTRPM2), catalytic activity is solely attributable to the C-terminal site. Here, we show that nvTRPM2 channel gating properties remain unaltered upon deletion of the C-terminal domain, indicating that the N-terminal site is single-handedly responsible for channel gating. Exploiting such functional independence of the N- and C-terminal sites, we selectively measure their affinity profiles for a series of ADPR analogues, as reflected by apparent affinities for channel activation and catalysis, respectively. Using site-directed mutagenesis, we confirm that the same N-terminal site observed in vertebrate TRPM2 channels was already present in ancient cnidarians. Finally, by characterizing the functional effects of six amino acid side chain truncations in the N-terminal site, we provide first insights into the mechanistic contributions of those side chains to TRPM2 channel gating.

The Molecular Basis of COVID-19 Pathogenesis, Conventional and Nanomedicine Therapy

In late 2019, a new member of the Coronaviridae family, officially designated as "severe acute respiratory syndrome coronavirus 2" (SARS-CoV-2), emerged and spread rapidly. The Coronavirus Disease-19 (COVID-19) outbreak was accompanied by a high rate of morbidity and mortality worldwide and was declared a pandemic by the World Health Organization in March 2020. Within the Coronaviridae family, SARS-CoV-2 is considered to be the third most highly pathogenic virus that infects humans, following the severe acute respiratory syndrome coronavirus (SARS-CoV) and the Middle East respiratory syndrome coronavirus (MERS-CoV). Four major mechanisms are thought to be involved in COVID-19 pathogenesis, including the activation of the renin-angiotensin system (RAS) signaling pathway, oxidative stress and cell death, cytokine storm, and endothelial dysfunction. Following virus entry and RAS activation, acute respiratory distress syndrome develops with an oxidative/nitrosative burst. The DNA damage induced by oxidative stress activates poly ADP-ribose polymerase-1 (PARP-1), viral macrodomain of non-structural protein 3, poly (ADP-ribose) glycohydrolase (PARG), and transient receptor potential melastatin type 2 (TRPM2) channel in a sequential manner which results in cell apoptosis or necrosis. In this review, blockers of angiotensin II receptor and/or PARP, PARG, and TRPM2, including vitamin D3, trehalose, tannins, flufenamic and mefenamic acid, and losartan, have been investigated for inhibiting RAS activation and quenching oxidative burst. Moreover, the application of organic and inorganic nanoparticles, including liposomes, dendrimers, quantum dots, and iron oxides, as therapeutic agents for SARS-CoV-2 were fully reviewed. In the present review, the clinical manifestations of COVID-19 are explained by focusing on molecular mechanisms. Potential therapeutic targets, including the RAS signaling pathway, PARP, PARG, and TRPM2, are also discussed in depth.

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.

The Discovery of Novel ACA Derivatives as Specific TRPM2 Inhibitors that Reduce Ischemic Injury Both In Vitro and In Vivo

The transient receptor potential melastatin 2 (TRPM2) channel is associated with ischemia/reperfusion injury, inflammation, cancer, and neurodegenerative diseases. However, the limit of specific inhibitors impedes the development of TRPM2-targeted therapeutic agents. To discover more potent and selective TRPM2 inhibitors, 59 N-(p-amylcinnamoyl) anthranilic acid (ACA) derivatives were synthesized and evaluated using calcium imaging and electrophysiology approaches. Systematic structure-activity relationship studies resulted in some potent compounds inhibiting the TRPM2 channel with sub-micromolar half-maximal inhibitory concentration values. Among them, the preferred compound A23 exhibited TRPM2 selectivity over TRPM8 and TRPV1 channels as well as phospholipase A2 and showed neuroprotective activity in vitro. Following pharmacokinetic studies, A23 was further evaluated in a transient middle cerebral artery occlusion model in vivo, which significantly reduced cerebral infarction. These data indicate that A23 might serve as a useful tool for TRPM2-related research as well as a lead compound for the development of therapeutic agents for ischemic injury.

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

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

Enhanced production of cordycepin in Ophiocordyceps sinensis using growth supplements under submerged conditions

Cordycepin is a crucial bioactive compound produced by the fungus Cordyceps spp. Its therapeutic potential has been recognized for a wide range of biological properties such as anticancer, anti-diabetic, antidepressant, antioxidant, immunomodulation, etc. Moreover, its human random clinical trials depicted a promising anti-inflammatory activity that reduced the airway inflammation remarkably in asthmatic patients. But its overexploitation and low production of cordycepin in naturally growing biomass are insufficient to meet its existing market demand for its therapeutic use. Therefore, strategies for enhancement of cordycepin production in Cordyceps spp. are warranted. However, specifically, wild type Ophiocordyceps sinensis possesses a very low content of cordycepin and has restricted growth in natural mycelial biomass. To overcome these limitations, this study attempted to enhance cordycepin production in its mycelial biomass in vitro under submerged conditions by adding various growth supplements. The effect of these growth supplements was evaluated by reversed-phase high-performance liquid chromatography (RP-HPLC) which demonstrated that among nucleosides- hypoxanthine and adenosine; amino acids-glycine and glutamine; plant hormones- 1-naphthaleneacetic acid (NAA) and 3-indoleacetic acid (IAA); vitamin-thiamine (B1) from each group of growth supplements yielded a higher amount of cordycepin with 466.48 ± 3.88, 380.23 ± 1.78, 434.97 ± 2.32, 269.78 ± 2.92, 227.61 ± 2.34, 226.02 ± 1.69 and 185.26 ± 2.35 mg/L respectively as compared to control with 13.66 ± 0.64 mg/L. Further, at the transcriptional level, quantitative real time-polymerase chain reaction (qRT-PCR) analysis of genes associated with metabolism and cordycepin biosynthesis depicted significant upregulation of major downstream genes- NT5E, RNR, purA, and ADEK which corroborated well with RP-HPLC analysis. Taken together, the present study identified growth supplements as potential precursors to activate the cordycepin biosynthesis pathway leading to improved cordycepin production in O. sinensis.

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.

25 Years of Collaboration with A Genius: Deciphering Adenine Nucleotide Ca2+ Mobilizing Second Messengers Together with Professor Barry Potter

Ca²⁺-mobilizing adenine nucleotide second messengers cyclic adenosine diphosphoribose, (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), adenosine diphosphoribose (ADPR), and 2'deoxy-ADPR were discovered since the late 1980s. They either release Ca²⁺ from endogenous Ca²⁺ stores, e.g., endoplasmic reticulum or acidic organelles, or evoke Ca²⁺ entry by directly activating a Ca²⁺ channel in the plasma membrane. For 25 years, Professor Barry Potter has been one of the major medicinal chemists in this topical area, designing and contributing numerous analogues to develop structure-activity relationships (SAR) as a basis for tool development in biochemistry and cell biology and for lead development in proof-of-concept studies in disease models. With this review, I wish to acknowledge our 25-year-long collaboration on Ca²⁺-mobilizing adenine nucleotide second messengers as a major part of Professor Potter's scientific lifetime achievements on the occasion of his retirement in 2020.

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.

Asymmetric Total Synthesis of C9'-epi-Sinefungin

The natural nucleoside (+)-sinefungin, structurally similar to cofactor S-adenosyl-l-methionine, inhibits various SAM-dependent methyltransferases (MTs). Access to sinefungin analogues could serve as the basis for the rational design of small molecule methyltransferase inhibitors. We developed a route to the unnatural C9' epimer of sinefungin that employed a diastereoselective Overman rearrangement to install the key C6' amino stereocenter. The ability for late-stage modification is highlighted, opening an avenue for the discovery of new MT inhibitors.

Synthesis of cordycepin: Current scenario and future perspectives

Cordyceps genus, such as C. militaris and C. kyushuensis, is a source of a rare traditional Chinese medicine that has been used for the treatment of numerous chronic and malignant diseases. Cordycepin, 3'-deoxyadenosine, is a major active compound found in most Cordyceps. Cordycepin exhibits a variety of biological activities, including anti-tumor, immunomodulation, antioxidant, and anti-aging, among others, which could be applied in health products, medicine, cosmeceutical etc. fields. This review focuses on the synthesis methods for cordycepin. The current methods for cordycepin synthesis involve chemical synthesis, microbial fermentation, in vitro synthesis and biosynthesis; however, some defects are unavoidable and the production is still far from the demand of cordycepin. For the future study of cordycepin synthesis, based on the illumination of cordycepin biosynthesis pathway, genetical engineering of the Cordyceps strain or introducing microbes by virtue of synthetic biology will be the great potential strategies for cordycepin synthesis. This review will aid the future synthesis of the valuable cordycepin.

Synthesis of phosphonoacetate analogues of the second messenger adenosine 5'-diphosphate ribose (ADPR)

Adenosine 5′-diphosphate ribose (ADPR) is an intracellular signalling molecule generated from nicotinamide adenine dinucleotide (NAD⁺). Synthetic ADPR analogues can shed light on the mechanism of activation of ADPR targets and their downstream effects. Such chemical biology studies, however, are often challenging due to the negatively charged pyrophosphate that is also sensitive to cellular pyrophosphatases. Prior work on an initial ADPR target, the transient receptor potential cation channel TRPM2, showed complete pyrophosphate group replacement to be a step too far in maintaining biological activity. Thus, we designed ADPR analogues with just one of the negatively charged phosphate groups removed, by employing a phosphonoacetate linker. Synthesis of two novel phosphonoacetate ADPR analogues is described via tandem N,N′-dicyclohexylcarbodiimide coupling to phosphonoacetic acid. Neither analogue, however, showed significant agonist or antagonist activity towards TRPM2, underlining the importance of a complete pyrophosphate motif in activation of this particular receptor.

Pyrophosphate replacement using phosphonoacetate isosteres – tools to study biological targets of ADPR.

Functional importance of NUDT9H domain and N-terminal ADPR-binding pocket in two species variants of vertebrate TRPM2 channels

There are at least two different principles of how ADP-ribose (ADPR) induces activation of TRPM2 channels. In human TRPM2, gating requires the C-terminal NUDT9H domain as ADPR-binding module, whereas in sea anemone, NUDT9H is dispensable and binding of ADPR occurs N-terminally. Zebrafish TRPM2 needs both, the N-terminal ADPR-binding pocket and NUDT9H. Our aim was to pinpoint the relative functional contributions of NUDT9H and the N-terminal ADPR-binding pocket in zebrafish TRPM2, to identify fundamental mechanisms of ADPR-directed gating. We show that the NUDT9H domains of human and zebrafish TRPM2 are interchangeable since chimeras generate ADPR-sensitive channels. A point mutation at a highly conserved position within NUDT9H induces loss-of-function in both vertebrate channels. The substrate specificity of zebrafish TRPM2 corresponds to that of sea anemone TRPM2, indicating gating by the proposed N-terminal ADPR-binding pocket. However, a point mutation in this region abolishes ADPR activation also in human TRPM2. These findings provide functional evidence for an uniform N-terminal ADPR-binding pocket in TRPM2 of zebrafish and sea anemone with modified function in human TRPM2. The structural importance of NUDT9H in vertebrate TRPM2 can be associated with a single amino acid residue which is not directly involved in the binding of ADPR.

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.

Integrated Target-Based and Phenotypic Screening Approaches for the Identification of Anti-Tubercular Agents That Bind to the Mycobacterial Adenylating Enzyme MbtA

Iron is essential for the pathogenicity and virulence of Mycobacterium tuberculosis, which synthesises salicyl-capped siderophores (mycobactins) to acquire this element from the host. MbtA is the adenylating enzyme that catalyses the initial reaction of mycobactin biosynthesis and is solely expressed by mycobacteria. A 3200-member library comprised of lead-like, structurally diverse compounds was screened against M. tuberculosis for whole-cell inhibitory activity. A set of 846 compounds that inhibited the tubercle bacilli growth were then tested for their ability to bind to MbtA using a fluorescence-based thermal shift assay and NMR-based Water-LOGSY and saturation transfer difference (STD) experiments. We identified an attractive hit molecule, 5-hydroxyindol-3-ethylamino-(2-nitro-4-trifluoromethyl)benzene (5), that bound with high affinity to MbtA and produced a MIC90 value of 13 μm. The ligand was docked into the MbtA crystal structure and displayed an excellent fit within the MbtA active pocket, adopting a binding mode different from that of the established MbtA inhibitor Sal-AMS.

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.

Synthesis of Terminal Ribose Analogues of Adenosine 5'-Diphosphate Ribose as Probes for the Transient Receptor Potential Cation Channel TRPM2

TRPM2 (transient receptor potential cation channel, subfamily M, member 2) is a nonselective cation channel involved in the response to oxidative stress and in inflammation. Its role in autoimmune and neurodegenerative diseases makes it an attractive pharmacological target. Binding of the nucleotide adenosine 5′-diphosphate ribose (ADPR) to the cytosolic NUDT9 homology (NUDT9H) domain activates the channel. A detailed understanding of how ADPR interacts with the TRPM2 ligand binding domain is lacking, hampering the rational design of modulators, but the terminal ribose of ADPR is known to be essential for activation. To study its role in more detail, we designed synthetic routes to novel analogues of ADPR and 2′-deoxy-ADPR that were modified only by removal of a single hydroxyl group from the terminal ribose. The ADPR analogues were obtained by coupling nucleoside phosphorimidazolides to deoxysugar phosphates. The corresponding C2″-based analogues proved to be unstable. The C1″- and C3″-ADPR analogues were evaluated electrophysiologically by patch-clamp in TRPM2-expressing HEK293 cells. In addition, a compound with all hydroxyl groups of the terminal ribose blocked as its 1″-β-O-methyl-2″,3″-O-isopropylidene derivative was evaluated. Removal of either C1″ or C3″ hydroxyl groups from ADPR resulted in loss of agonist activity. Both these modifications and blocking all three hydroxyl groups resulted in TRPM2 antagonists. Our results demonstrate the critical role of these hydroxyl groups in channel activation.

A synthetic cyclitol-nucleoside conjugate polyphosphate is a highly potent second messenger mimic

A densely functionalised phosphorylated chiro-inositol-nucleoside ether conjugate constructed from cyclic fragments is the most potent IP₃ receptor ligand discovered.

Reactions that form sec–sec ethers are well known, but few lead to compounds with dense functionality around the O-linkage. Replacement of the α-glucopyranosyl unit of adenophostin A, a potent d-myo-inositol 1,4,5-trisphosphate (IP₃R) agonist, with a d-chiro-inositol surrogate acting substantially as a pseudosugar, leads to “d-chiro-inositol adenophostin”. At its core, this cyclitol-nucleoside trisphosphate comprises an ether linkage between the axial 1-hydroxyl position of d-chiro-inositol and the 3′-hydroxyl group of an adenosine ribose sugar. A divergent synthesis of d-chiro-inositol adenophostin has been achieved. Key features of the synthetic strategy to produce a triol for phosphorylation include a new selective mono-tosylation of racemic 1,2:4,5-di-O-isopropylidene-myo-inositol using tosyl imidazole; subsequent conversion of the product into separable camphanate ester derivatives, one leading to a chiral myo-inositol triflate used as a synthetic building block and the other to l-1-O-methyl-myo-inositol [l-(+)-bornesitol] to assign the absolute configuration; the nucleophilic coupling of an alkoxide of a ribose pent-4-ene orthoester unit with a structurally rigid chiral myo-inositol triflate derivative, representing the first sec–sec ether formation between a cyclitol and ribose. Reaction of the coupled product with a silylated nucleobase completes the assembly of the core structure. Further protecting group manipulation, mixed O- and N-phosphorylation, and subsequent removal of all protecting groups in a single step achieves the final product, avoiding a separate N6 protection/deprotection strategy. d-chiro-Inositol adenophostin evoked Ca²⁺ release through IP₃Rs at lower concentrations than adenophostin A, hitherto the most potent known agonist of IP₃Rs.

Different substrate specificities of the two ADPR binding sites in TRPM2 channels of Nematostella vectensis and the role of IDPR

NvTRPM2 (Nematostella vectensis Transient Receptor Potential Melastatin 2), the species variant of the human apoptosis-related cation channel hTRPM2, is gated by ADP-ribose (ADPR) independently of the C-terminal NUDT9H domain that mediates ADPR-directed gating in hTRPM2. The decisive binding site in NvTRPM2 is likely to be identical with the N-terminal ADPR binding pocket in zebra fish DrTRPM2. Our aim was a characterization of this binding site in NvTRPM2 with respect to its substrate specificity, in comparison to the classical ADPR interaction site within NUDT9H that is highly homologous in hTRPM2 and NvTRPM2, although only in NvTRPM2, catalytic (ADPRase) activity is conserved. With various ADPR analogues, key differences of the two sites were identified. Particularly, two reported antagonists on hTRPM2 were agonists on NvTRPM2. Moreover, IDP-ribose (IDPR) induced currents both in hTRPM2 and NvTRPM2 but not in NvTRPM2 mutants in which NUDT9H was absent. Thus, IDPR acts on NUDT9H rather than N-terminally, revealing a regulatory function of NUDT9H in NvTRPM2 opposed to that in hTRPM2. We propose that IDPR competitively inhibits the ADPRase function of NUDT9H and evokes ADPR accumulation. The findings provide important insights into the structure-function relationship of NvTRPM2 and will allow further characterization of the novel ADPR interaction site.

Unpacking the Pathogen Box-An Open Source Tool for Fighting Neglected Tropical Disease

The Pathogen Box is a 400-strong collection of drug-like compounds, selected for their potential against several of the world's most important neglected tropical diseases, including trypanosomiasis, leishmaniasis, cryptosporidiosis, toxoplasmosis, filariasis, schistosomiasis, dengue virus and trichuriasis, in addition to malaria and tuberculosis. This library represents an ensemble of numerous successful drug discovery programmes from around the globe, aimed at providing a powerful resource to stimulate open source drug discovery for diseases threatening the most vulnerable communities in the world. This review seeks to provide an in-depth analysis of the literature pertaining to the compounds in the Pathogen Box, including structure-activity relationship highlights, mechanisms of action, related compounds with reported activity against different diseases, and, where appropriate, discussion on the known and putative targets of compounds, thereby providing context and increasing the accessibility of the Pathogen Box to the drug discovery community.

Adenine nucleotides as paracrine mediators and intracellular second messengers in immunity and inflammation

Adenine nucleotides (AdNs) play important roles in immunity and inflammation. Extracellular AdNs, such as adenosine triphosphate (ATP) or nicotinamide adenine dinucleotide (NAD) and their metabolites, act as paracrine messengers by fine-tuning both pro- and anti-inflammatory processes. Moreover, intracellular AdNs derived from ATP or NAD play important roles in many cells of the immune system, including T lymphocytes, macrophages, neutrophils and others. These intracellular AdNs are signaling molecules that transduce incoming signals into meaningful cellular responses, e.g. activation of immune responses against pathogens.

Synthesis of [15 N4 ] purine labeled cytokinin glycosides derived from zeatins and topolins with 9-β-d, 7-β-d-glucopyranosyl, or 9-β-d-ribofuranosyl group

Synthesis of [¹⁵ N₄ ] purine labeled cytokinine glycosides derived from zeatins and topolins containing a 9-β-d, 7-β-d-glucopyranosyl, or 9-β-d-ribofuranosyl group is described. These N⁶ -substituted adenine derivatives are intended as internal analytic standards for phytohormone analysis. All labeled compounds were prepared from 6-chloro[¹⁵ N₄ ]purine (1). The equilibrium reaction of 1 with acetobromo-α-d-glucose gave isomeric 7-β-d (3) and 9-β-d (4) chloro glucosyl precursors, which were treated with the corresponding amines to get desired labeled cytokinin 7-β-d (6) and 9-β-d (5) glucopyranosides. Cytokinins containing 9-β-d-ribofuranosyl group (8) were obtained by direct enzymatic transglycosylation reaction of cytokinins (7) prepared from 6-chloro[¹⁵ N₄ ] purine (1).

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

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

Identification of a Novel EF-Loop in the N-terminus of TRPM2 Channel Involved in Calcium Sensitivity

As an oxidative stress sensor, transient receptor potential melastatin 2 (TRPM2) channel is involved in many physiological and pathological processes including warmth sensing, ischemia injury, inflammatory diseases and diabetes. Intracellular calcium is critical for TRPM2 channel activation and the IQ-like motif in the N-terminus has been shown to be important by mediating calmodulin binding. Sequence analysis predicted two potential EF-loops in the N-terminus of TRPM2. Site-directed mutagenesis combining with functional assay showed that substitution with alanine of several residues, most of which are conserved in the typical EF-loop, including D267, D278, D288, and E298 dramatically reduced TRPM2 channel currents. By further changing the charges or side chain length of these conserved residues, our results indicate that the negative charge of D267 and the side chain length of D278 are critical for calcium-induced TRPM2 channel activation. G272I mutation also dramatically reduced the channel currents, suggesting that this site is critical for calcium-induced TRPM2 channel activation. Furthermore, D267A mutant dramatically reduced the currents induced by calcium alone compared with that by ADPR, indicating that D267 residue in D267–D278 motif is the most important site for calcium sensitivity of TRPM2. In addition, inside-out recordings showed that mutations at D267, G272, D278, and E298 had no effect on single-channel conductance. Taken together, our data indicate that D267–D278 motif in the N-terminus as a novel EF-loop is critical for calcium-induced TRPM2 channel activation.

Structure of a TRPM2 channel in complex with Ca2+ explains unique gating regulation

Transient receptor potential melastatin 2 (TRPM2) is a Ca²⁺-permeable cation channel required for immune cell activation, insulin secretion, and body heat control. TRPM2 is activated by cytosolic Ca²⁺, phosphatidyl-inositol-4,5-bisphosphate and ADP ribose. Here, we present the ~3 Å resolution electron cryo-microscopic structure of TRPM2 from Nematostella vectensis, 63% similar in sequence to human TRPM2, in the Ca²⁺-bound closed state. Compared to other TRPM channels, TRPM2 exhibits unique structural features that correlate with its function. The pore is larger and more negatively charged, consistent with its high Ca²⁺ selectivity and larger conductance. The intracellular Ca²⁺ binding sites are connected to the pore and cytosol, explaining the unusual dependence of TRPM2 activity on intra- and extracellular Ca²⁺. In addition, the absence of a post-filter motif is likely the cause of the rapid inactivation of human TRPM2. Together, our cryo-EM and electrophysiology studies provide a molecular understanding of the unique gating mechanism of TRPM2.

TRPM2: a candidate therapeutic target for treating neurological diseases

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

Design, synthesis and biological activities of 2,3-dihydroquinazolin-4(1H)-one derivatives as TRPM2 inhibitors

Transient receptor potential melastatin 2 (TRPM2), a Ca²⁺-permeable cationic channel, plays critical roles in insulin release, cytokine production, body temperature regulation and cell death as a reactive oxygen species (ROS) and temperature sensor. However, few TRPM2 inhibitors have been reported, especially TRP-subtype selective inhibitors, which hampers the investigation and validation of TRPM2 as a drug target. To discover novel TRPM2 inhibitors, 3D similarity-based virtual screening method was employed, by which 2,3-dihydroquinazolin-4(1H)-one derivative H1 was identified as a TRPM2 inhibitor. A series of novel 2,3-dihydroquinazolin-4(1H)-one derivatives were subsequently synthesized and characterized. Their inhibitory activities against the TRPM2 channel were evaluated by calcium imaging and electrophysiology approaches. Some of the compounds exhibited significant inhibitory activity, especially D9 which showed an IC₅₀ of 3.7 μM against TRPM2 and did not affect the TRPM8 channel. The summarized structure-activity relationship (SAR) provides valuable insights for further development of specific TRPM2 targeted inhibitors.

Selective inhibition of TRPM2 channel by two novel synthesized ADPR analogues

Transient receptor potential melastatin-2 (TRPM2) channel critical for monitoring internal body temperature is implicated in the pathological processes such as neurodegeneration. However, lacking selective and potent TRPM2 inhibitors impedes investigation and validation of the channel as a drug target. To discover novel and selective TRPM2 inhibitors, a series of adenosine 5'-diphosphoribose analogues were synthesized, and their activities and selectivity were evaluated. Whole-cell patch-clamp recordings were employed for screen and evaluation of synthesized compounds. Two compounds, 7i and 8a, were identified as TRPM2 inhibitors with IC50 of 5.7 and 5.4 μm, respectively. Both 7i and 8a inhibited TRPM2 current without affecting TRPM7, TRPM8, TRPV1 and TRPV3. These two TRPM2 inhibitors can serve as new pharmacological tools for further investigation and validation of TRPM2 channel as a drug target, and the summarized structure-activity relationship (SAR) may also provide insights into further improving existing inhibitors as potential lead compounds.

2'-Deoxyadenosine 5'-diphosphoribose is an endogenous TRPM2 superagonist

Transient receptor potential melastatin 2 (TRPM2) is a ligand-gated Ca²⁺-permeable nonselective cation channel. Whereas physiological stimuli, such as chemotactic agents, evoke controlled Ca²⁺ signals via TRPM2, pathophysiological stimuli such as reactive oxygen species and genotoxic stress result in prolonged TRPM2-mediated Ca²⁺ entry and, consequently, apoptosis. To date, adenosine 5'-diphosphoribose (ADPR) has been assumed to be the main agonist for TRPM2. Here we show that 2'-deoxy-ADPR was a significantly better TRPM2 agonist, inducing 10.4-fold higher whole-cell currents at saturation. Mechanistically, this increased activity was caused by a decreased rate of inactivation and higher average open probability. Using high-performance liquid chromatography (HPLC) and mass spectrometry, we detected endogenous 2'-deoxy-ADPR in Jurkat T lymphocytes. Consistently, cytosolic nicotinamide mononucleotide adenylyltransferase 2 (NMNAT-2) and nicotinamide adenine dinucleotide (NAD)-glycohydrolase CD38 sequentially catalyzed the synthesis of 2'-deoxy-ADPR from nicotinamide mononucleotide (NMN) and 2'-deoxy-ATP in vitro. Thus, 2'-deoxy-ADPR is an endogenous TRPM2 superagonist that may act as a cell signaling molecule.

Ligand-induced activation of human TRPM2 requires the terminal ribose of ADPR and involves Arg1433 and Tyr1349

TRPM2 (transient receptor potential channel, subfamily melastatin, member 2) is a Ca2+-permeable non-selective cation channel activated by the binding of adenosine 5'-diphosphoribose (ADPR) to its cytoplasmic NUDT9H domain (NUDT9 homology domain). Activation of TRPM2 by ADPR downstream of oxidative stress has been implicated in the pathogenesis of many human diseases, rendering TRPM2 an attractive novel target for pharmacological intervention. However, the structural basis underlying this activation is largely unknown. Since ADP (adenosine 5'-diphosphate) alone did not activate or antagonize the channel, we used a chemical biology approach employing synthetic analogues to focus on the role of the ADPR terminal ribose. All novel ADPR derivatives modified in the terminal ribose, including that with the seemingly minor change of methylating the anomeric-OH, abolished agonist activity at TRPM2. Antagonist activity improved as the terminal substituent increasingly resembled the natural ribose, indicating that gating by ADPR might require specific interactions between hydroxyl groups of the terminal ribose and the NUDT9H domain. By mutating amino acid residues of the NUDT9H domain, predicted by modelling and docking to interact with the terminal ribose, we demonstrate that abrogating hydrogen bonding of the amino acids Arg1433 and Tyr1349 interferes with activation of the channel by ADPR. Taken together, using the complementary experimental approaches of chemical modification of the ligand and site-directed mutagenesis of TRPM2, we demonstrate that channel activation critically depends on hydrogen bonding of Arg1433 and Tyr1349 with the terminal ribose. Our findings allow for a more rational design of novel TRPM2 antagonists that may ultimately lead to compounds of therapeutic potential.

Identification of the ADPR binding pocket in the NUDT9 homology domain of TRPM2

Activation of the transient receptor potential melastatin 2 (TRPM2) channel occurs during the response to oxidative stress under physiological conditions as well as in pathological processes such as ischemia and diabetes. Accumulating evidence indicates that adenosine diphosphate ribose (ADPR) is the most important endogenous ligand of TRPM2. However, although it is known that ADPR binds to the NUDT9 homology (NUDT9-H) domain in the intracellular C-terminal region, the molecular mechanism underlying ADPR binding and activation of TRPM2 remains unknown. In this study, we generate a structural model of the NUDT9-H domain and identify the binding pocket for ADPR using induced docking and molecular dynamics simulation. We find a subset of 11 residues-H1346, T1347, T1349, L1379, G1389, S1391, E1409, D1431, R1433, L1484, and H1488-that are most likely to directly interact with ADPR. Results from mutagenesis and electrophysiology approaches support the predicted binding mechanism, indicating that ADPR binds tightly to the NUDT9-H domain, and suggest that the most significant interactions are the van der Waals forces with S1391 and L1484, polar solvation interaction with E1409, and electronic interactions (including π-π interactions) with H1346, T1347, Y1349, D1431, and H1488. These findings not only clarify the roles of a range of newly identified residues involved in ADPR binding in the TRPM2 channel, but also reveal the binding pocket for ADPR in the NUDT9-H domain, which should facilitate structure-based drug design for the TRPM2 channel.

Designer small molecules to target calcium signalling

Synthetic compounds open up new avenues to interrogate and manipulate intracellular Ca2+ signalling pathways. They may ultimately lead to drug-like analogues to intervene in disease. Recent advances in chemical biology tools available to probe Ca2+ signalling are described, with a particular focus on those synthetic analogues from our group that have enhanced biological understanding or represent a step towards more drug-like molecules. Adenophostin (AdA) is the most potent known agonist at the inositol 1,4,5-trisphosphate receptor (IP3R) and synthetic analogues provide a binding model for receptor activation and channel opening. 2-O-Modified inositol 1,4,5-trisphosphate (IP3) derivatives that are partial agonists at the IP3R reveal key conformational changes of the receptor upon ligand binding. Biphenyl polyphosphates illustrate that simple non-inositol surrogates can be engineered to give prototype IP3R agonists or antagonists and act as templates for protein co-crystallization. Cyclic adenosine 5'-diphosphoribose (cADPR) can be selectively modified using total synthesis, generating chemically and biologically stable tools to investigate Ca2+ release via the ryanodine receptor (RyR) and to interfere with cADPR synthesis and degradation. The first neutral analogues with a synthetic pyrophosphate bioisostere surprisingly retain the ability to release Ca2+, suggesting a new route to membrane-permeant tools. Adenosine 5'-diphosphoribose (ADPR) activates the Ca2+-, Na+- and K+-permeable transient receptor potential melastatin 2 (TRPM2) cation channel. Synthetic ADPR analogues provide the first structure-activity relationship (SAR) for this emerging messenger and the first functional antagonists. An analogue based on the nicotinic acid motif of nicotinic acid adenine dinucleotide phosphate (NAADP) antagonizes NAADP-mediated Ca2+ release in vitro and is effective in vivo against induced heart arrhythmia and autoimmune disease, illustrating the therapeutic potential of targeted small molecules.