TRPM2 is a Ca2+-permeable cation channel activated by ADP-ribose (ADPR) and oxidative stress, yet the relative contributions of its two nucleotide-binding domains, MHR1/2 and NUDT9H, remain incompletely understood. Here, we quantitatively determine the affinities of the isolated human TRPM2 MHR1/2 and NUDT9H domains for ADPR, 2'-deoxy-dADPR (dADPR), and 8-Br-cADPR using biophysical approaches. The MHR1/2 domain binds ADPR with high affinity (Kd ≈ 0.5 μM), whereas the NUDT9H domain displays substantially lower affinity (Kd ≈ 192 μM), revealing a difference of nearly three orders of magnitude. Mutational analysis demonstrates that alterations in MHR1/2 strongly affect ligand binding and channel activation, while mutations within NUDT9H that markedly reduce ligand affinity exert only modest effects on gating. In parallel, we quantify intracellular ADPR concentrations in resting and hydrogen peroxide-stimulated cells and find that they remain well below the affinity required for substantial NUDT9H occupancy. Together, our findings indicate that high-affinity binding to the MHR1/2 domain is sufficient to drive TRPM2 activation under physiological conditions, whereas the NUDT9H domain likely contributes to maintaining the structural integrity of the channel rather than directly mediating ligand-dependent activation. These results provide a quantitative framework for understanding ligand-dependent TRPM2 regulation in cells.

Abstract Enzymatically active macrodomains of (+)ss-RNA viruses mediate immune evasion by countering ADP-ribosylation and are therefore promising druggable targets. Here we report testing of ADP / ADP-ribose analogues for their ability to inhibit Mac1 of SARS-CoV-2, measurement of the affinity of active compounds and characterization of their binding mode by cocrystallization, uncovering critical molecular determinants of protein-ligand interaction. Key findings of the resulting structure-activity relationship (SAR) include that inhibitory potency is improved by either replacing the distal ribose of ADP-ribose by a small alkyl group or the adenine N7 by carbon. Based on insights from the SAR, we show β-methyl-GS-441524-diphosphate as nanomolar inhibitor that exhibits >1000-fold selectivity over human MacroD1 and MacroD2. Addition of C11-acyloxybenzyl (AB)-masking groups yields a membrane permeable, lipophilic prodrug that inhibits SARS-CoV-2 in cell culture (EC50 0.06 µM) while exhibiting low cytotoxicity (CC50 > 50 µM). Replacement of the terminal methyl phosphate with an ethyl phosphonate increases stability of the prodrug with little effect on toxicity and antiviral potency (EC50 = 0.03 µM), making it a membrane-permeable nucleotide-based prodrug against viral macrodomains.

Unsupervised deep image prior (DIP) addresses shortcomings of training data requirements and limited generalization associated with supervised deep learning. The performance of DIP depends on the network architecture and the stopping point of its iterative process. Optimizing these parameters for a new image requires time, restricting DIP application in domains where many images need to be processed. Focusing on fluorescence microscopy data, we hypothesize that similar images share comparable optimal parameter configurations for DIP-based denoising, potentially enabling optimization-free DIP for fluorescence microscopy. We generated a calibration (n=110) and validation set (n=55) of semantically different images from an open-source dataset for a network architecture search targeted towards ideal U-net architectures and stopping points. The calibration set represented our transfer basis. The validation set enabled the assessment of which image similarity criterion yields the best results. We then implemented AUTO-DIP, a pipeline for automatic parameter transfer, and compared it to the originally published DIP configuration (baseline) and a state-of-the-art image-specific variational denoising approach. We show that a parameter transfer from the calibration dataset to a test image based on only image metadata similarity (e.g., microscope type, imaged specimen) leads to similar and better performance than a transfer based on quantitative image similarity measures. AUTO-DIP outperforms the baseline DIP (DIP with original DIP parameters) as well as the variational denoising approaches for several open-source test datasets of varying complexity, particularly for very noisy inputs. Applications to locally acquired fluorescence microscopy images further proved superiority of AUTO-DIP.

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a highly potent Ca2+ releasing second messenger with a distinct role during early T cell activation. While studies have proposed NAADP signaling in T cells as a target for treating autoimmune diseases, there are currently no pharmacologic inhibitors available for clinical trial. This is in part due to the fact, that investigating NAADP-evoked Ca2+ signals in T cells currently requires laborious techniques such as microinjections. We have combined recent advances on NAADP signaling to develop a protocol for measuring NAADP-evoked Ca2+ release from permeabilized T cells, that can be used to further characterize this pathway in T cells and validate new pharmacologic agents.

Transient receptor potential melastatin 2 (TRPM2) is a multifunctional non-selective Ca2+-permeable cation channel expressed in numerous immune cells. 2'-Deoxy-adenosine diphosphoribose (2d-ADPR) has been identified as a superagonist of TRPM2 channels that induces higher whole-cell currents and requires a lower intracellular Ca2+ concentration for activation. 2d-ADPR can be produced from 2'-deoxy-nicotinamide adenine dinucleotide (2d-NAD) by the enzyme CD38 in vitro under physiological conditions.Here, we describe a two-dimensional HPLC method, suitable for the quantification of endogenous 2d-ADPR and 2d-NAD. The results demonstrated that 25 million Jurkat T cells per sample were sufficient to quantify endogenous 2d-ADPR and 2d-NAD.

Enzymatically active viral macrodomains of (+)ss-RNA viruses mediate immune evasion by countering ADP-ribosylation and are therefore promising druggable targets. We tested ADP / ADP-ribose analogs for inhibition of Mac1 of SARS-CoV-2 and determined affinity and binding mode of active compunds. The SAR showed, that replacement of either the distal ribose of ADPR by a small alkyl group or the adenine N7 by carbon improved inhibitory potency, resulting in the development of β-methyl-GS-441524-diphosphate, a nanomolar inhibitor that exhibits high selectivity (>1000-fold) over human MacroD1 and MacroD2. Addition of C 11 -acyloxybenzyl (AB)-masking groups yielded a membrane permeable, lipophilic prodrug that inhibits SARS-CoV-2 in cell culture (EC 50 0.06 μM) while exhibiting low cytotoxicity (CC 50 > 50 μM). Replacement of the terminal methyl phosphate with an ethyl phosphonate increased stability of the prodrug with little effect on toxicity and antiviral potency (EC 50 = 0.03 μM), making it the first membrane-permeable nucleotide-based prodrug against viral macrodomains.

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.

Bitter taste receptors (T2Rs) are G protein-coupled receptors originally detected in the gustatory system. More recently, T2Rs have been shown to be expressed in extra-oral cells eliciting non-gustatory functions. Emerging evidence has suggested a potential role for T2R signaling in diverse pathophysiological conditions, including cancer. The aim of the present study was to evaluate the expression of T2R14 in pancreatic ductal adenocarcinoma (PDAC) and to assess its involvement in the anticancer effects induced by apigenin, a natural ligand of T2R14. For this purpose, T2R14 expression was explored in PDAC tumor tissue and tumor-derived cell lines. Using the cell lines expressing the highest levels of T2R14, its effects on chemoresponsiveness and migration upon activation with apigenin were investigated in vitro. To the best of our knowledge, the present study was the first to confirm the expression of the T2R family member T2R14 in PDAC. Patients with relatively high levels of T2R14 expression exhibited significantly prolonged overall survival compared with that of patients with low T2R14 expression. Furthermore, novel functions for apigenin were revealed; notably, apigenin was shown to elicit cytotoxic, anti-migratory and chemosensitizing effects to 5-fluoruracil (5-FU) and to 5-FU, leucovorin, irinotecan and oxaliplatin in pancreatic cancer cells. In conclusion, the present study extended the evidence for the anticancer effects of apigenin and strongly indicated the functional relevance of T2R14 in PDAC, even though their respective underlying pathways appear to be independent of each other.

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.

Transient receptor potential melastatin 2 (TRPM2) is a Ca2+-permeable, non-selective cation channel involved in diverse physiological processes such as immune response, apoptosis and body temperature sensing. TRPM2 is activated by ADP-ribose (ADPR) and 2′-deoxy-ADPR in a Ca2+-dependent manner. While two species-specific binding sites exist for ADPR, a binding site for 2′-deoxy-ADPR is not known yet. Here, we report the crystal structure of the MHR1/2 domain of TRPM2 from zebrafish (Danio rerio) and show binding of both ligands to this domain. We identified a so-far unrecognized Zn2+-binding domain that was not resolved in previous cryo-EM structures and that is conserved in most TRPM channels. In combination with patch clamp experiments, we comprehensively characterize the effect of the Zn2+-binding domain on TRPM2 activation. Our results provide insight into a conserved structural element essential for channel activity.

The pulp of human teeth contains a population of self-renewing stem cells that can regulate the functions of immune cells. When applied to patients, these cells can protect tissues from damage by excessive inflammation. We confirm that dental pulp cells effectively inhibit the proliferation and activation of cytotoxic T cells in vitro, and show that they carry high levels of CD73, a key enzyme in the conversion of pro-inflammatory extracellular ATP to immunosuppressive adenosine. Given their accessibility and abundance, as well as their potential for allogeneic administration, dental pulp cells provide a valuable source for immunomodulatory therapy.

Transient receptor potential melastatin 2 (TRPM2) is a Ca 2+ -permeable, nonselective cation channel involved in diverse physiological processes such as immune response, apoptosis, and body temperature sensing. TRPM2 is activated by ADP-ribose (ADPR) and 2′-deoxy-ADPR in a Ca 2+ -dependent manner. While two distinct binding sites exist for ADPR that exert different functions dependent on the species, the involvement of either binding site regarding the superagonistic effect of 2′-deoxy-ADPR is not clear yet. Here, we report the crystal structure of the MHR1/2 domain of TRPM2 from zebrafish ( Danio rerio ), and show that both ligands bind to this domain and activate the channel. We identified a so far unrecognized Zn 2+ -binding domain that was not resolved in previous cryo-EM structures and that is conserved in most TRPM channels. In combination with patch clamp experiments we comprehensively characterize the effect of the Zn 2+ -binding domain on TRPM2 activation. Our results provide insight into a conserved motif essential for structural integrity and channel activity.

NAADP-evoked Ca2+ release through type 1 ryanodine receptors (RYR1) is a major mechanism underlying the earliest signals in T cell activation, which are the formation of Ca2+ microdomains. In our characterization of the molecular machinery underlying NAADP action, we identified an NAADP-binding protein, called hematological and neurological expressed 1-like protein (HN1L) [also known as Jupiter microtubule-associated homolog 2 (JPT2)]. Gene deletion of Hn1l/Jpt2 in human Jurkat and primary rat T cells resulted in decreased numbers of initial Ca2+ microdomains and delayed the onset and decreased the amplitude of global Ca2+ signaling. Photoaffinity labeling demonstrated direct binding of NAADP to recombinant HN1L/JPT2. T cell receptor/CD3-dependent coprecipitation of HN1L/JPT2 with RYRs and colocalization of these proteins suggest that HN1L/JPT2 connects NAADP formation with the activation of RYR channels within the first seconds of T cell activation. Thus, HN1L/JPT2 enables NAADP to activate Ca2+ release from the endoplasmic reticulum through RYR.

Nudix hydrolase 9 (NUDT9) is a member of the nucleoside linked to another moiety X (NUDIX) protein superfamily, which hydrolyses a broad spectrum of organic pyrophosphates from metabolic processes. ADP-ribose (ADPR) has been the only known endogenous substrate accepted by NUDT9 so far. The Ca2+-permeable transient receptor potential melastatin subfamily 2 (TRPM2) channel contains a homologous NUDT9-homology (NUDT9H) domain and is activated by ADPR. Sustained Ca2+ influx via ADPR-activated TRPM2 triggers apoptotic mechanisms. Thus, a precise regulation of cellular ADPR levels by NUDT9 is essential. A detailed characterization of the enzyme-substrate interaction would help to understand the high substrate specificity of NUDT9. Here, we analysed ligand binding to NUDT9 using a variety of biophysical techniques. We identified 2 '-deoxy-ADPR as an additional substrate for NUDT9. Similar enzyme kinetics and binding affinities were determined for the two ligands. The high-affinity binding was preserved in NUDT9 containing the mutated NUDIX box derived from the human NUDT9H domain. NMR spectroscopy indicated that ADPR and 2 '-deoxy-ADPR bind to the same binding site of NUDT9. Backbone resonance assignment and subsequent molecular docking allowed further characterization of the binding pocket. Substantial conformational changes of NUDT9 upon ligand binding were observed which might allow for the development of NUDT9-based ADPR fluorescence resonance energy transfer sensors that may help with the analysis of ADPR signalling processes in cells in the future.

Immune cells at sites of inflammation are continuously activated by local antigens and cytokines, and regulatory mechanisms must be enacted to control inflammation. The stepwise hydrolysis of extracellular ATP by ectonucleotidases CD39 and CD73 generates adenosine, a potent immune suppressor. Here we report that human effector CD8 T cells contribute to adenosine production by releasing CD73-containing extracellular vesicles upon activation. These extracellular vesicles have AMPase activity, and the resulting adenosine mediates immune suppression independently of regulatory T cells. In addition, we show that extracellular vesicles isolated from the synovial fluid of patients with juvenile idiopathic arthritis contribute to T cell suppression in a CD73-dependent manner. Our results suggest that the generation of adenosine upon T cell activation is an intrinsic mechanism of human effector T cells that complements regulatory T cell-mediated suppression in the inflamed tissue. Finally, our data underscore the role of immune cell-derived extracellular vesicles in the control of immune responses.

CD38 is the major NAD + -hydrolyzing ecto-enzyme in most mammals. As a type II transmembrane protein, CD38 is also a promising target for the immunotherapy of multiple myeloma (MM). Nanobodies are single immunoglobulin variable domains from heavy chain antibodies that naturally occur in camelids. Using phage display technology, we isolated 13 mouse CD38-specific nanobodies from immunized llamas and produced these as recombinant chimeric mouse IgG2a heavy chain antibodies (hcAbs). Sequence analysis assigned these hcAbs to five distinct families that bind to three non-overlapping epitopes of CD38. Members of families 4 and 5 inhibit the GDPR-cyclase activity of CD38. Members of families 2, 4 and 5 effectively induce complement-dependent cytotoxicity against CD38-expressing tumor cell lines, while all families effectively induce antibody dependent cellular cytotoxicity. Our hcAbs present unique tools to assess cytotoxicity mechanisms of CD38-specific hcAbs in vivo against tumor cells and potential off-target effects on normal cells expressing CD38 in syngeneic mouse tumor models, i.e. in a fully immunocompetent background.

The nucleotides ATP and NAD+ are released from stressed cells as endogenous danger signals. Ecto-enzymes in the tumor microenvironment hydrolyze these inflammatory nucleotides to immunosuppressive adenosine, thereby, hampering anti-tumor immune responses. The NAD+ hydrolase CD38 is expressed at high levels on the cell surface of multiple myeloma (MM) cells. Daratumumab, a CD38-specific monoclonal antibody promotes cytotoxicity against MM cells. With long CDR3 loops, nanobodies and nanobody-based heavy chain antibodies (hcAbs) might bind to cavities on CD38 and thereby inhibit its enzyme activity more potently than conventional antibodies. The goal of our study was to establish assays for monitoring the enzymatic activities of CD38 on the cell surface of tumor cells and to assess the effects of CD38-specific antibodies on these activities. We monitored the enzymatic activity of CD38-expressing MM and other tumor cell lines, using fluorometric and HPLC assays. Our results showed that daratumumab and hcAb MU1067 inhibit the ADPR cyclase but not the NAD+ hydrolase activity of CD38-expressing MM cells. We conclude that neither clinically approved daratumumab nor recently developed nanobody-derived hcAbs provide a second mode of action against MM cells. Thus, there remains a quest for "double action" CD38-inhibitory antibodies.

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, also sensitive to cellular pyrophosphatases, and 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.