The activation of T cells is the fundamental on switch for the adaptive immune system. Ca(2+) signaling is essential for T cell activation and starts as initial, short-lived, localized Ca(2+) signals. The second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) forms rapidly upon T cell activation and stimulates early Ca(2+) signaling. We developed a high-resolution imaging technique using multiple fluorescent Ca(2+) indicator dyes to characterize these early signaling events and investigate the channels involved in NAADP-dependent Ca(2+) signals. In the first seconds of activation of either primary murine T cells or human Jurkat cells with beads coated with an antibody against CD3, we detected Ca(2+) signals with diameters close to the limit of detection and that were close to the activation site at the plasma membrane. In Jurkat cells in which the ryanodine receptor (RyR) was knocked down or in primary T cells from RyR1(-/-) mice, either these early Ca(2+) signals were not detected or the number of signals was markedly reduced. Local Ca(2+) signals observed within 20 ms upon microinjection of Jurkat cells with NAADP were also sensitive to RyR knockdown. In contrast, TRPM2 (transient receptor potential channel, subtype melastatin 2), a potential NAADP target channel, was not required for the formation of initial Ca(2+) signals in primary T cells. Thus, through our high-resolution imaging method, we characterized early Ca(2+) release events in T cells and obtained evidence for the involvement of RyR and NAADP in such signals.

Differential scanning fluorimetry (DSF) using the inherent fluorescence of proteins (nDSF) is a popular technique to evaluate thermal protein stability in different conditions (e.g. buffer, pH). In many cases, ligand binding increases thermal stability of a protein and often this can be detected as a clear shift in nDSF experiments. Here, we evaluate binding affinity quantification based on thermal shifts. We present four protein systems with different binding affinity ligands, ranging from nM to high μM. Our study suggests that binding affinities determined by isothermal analysis are in better agreement with those from established biophysical techniques (ITC and MST) compared to apparent Kds obtained from melting temperatures. In addition, we describe a method to optionally fit the heat capacity change upon unfolding ($$\Delta {C}_{p}$$ Δ C p ) during the isothermal analysis. This publication includes the release of a web server for easy and accessible application of isothermal analysis to nDSF data.

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

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 0 -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 0 -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.

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 Ca 2+ -permeable transient receptor potential melastatin subfamily 2 (TRPM2) channel contains a homologous NUDT9-homology (NUDT9H) domain and is activated by ADPR. Sustained Ca 2+ 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 0deoxy-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 0 -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.

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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.