Frontrunners of T cell activation: Initial, localized Ca2+ signals mediated by NAADP and the type 1 ryanodine receptor

Imaging BiTE-Mediated Activation of Primary Human CD8+ T Cells

CD8+ T cells are a vital branch of the adaptive immune response. They are necessary for an effective antiviral and anticancer immunity, which is mediated through their cytotoxic effector functions. CD8+ T cell activation, proliferation, and effector functions are dependent on a complex network of Ca2+ signalling pathways involving both Ca2+ release and Ca2+ entry. Dysregulation of intracellular Ca2+ signalling pathways has been linked to immunodeficiencies including severe combined immunodeficiency syndrome (SCID), highlighting the need to study both Ca2+ dynamics and cytotoxic effector function in patient-derived CD8+ T cells. The following protocol describes a workflow for the isolation of primary human CD8+ T cells and their activation using bispecific T cell engagers (BiTEs). This allows for a simultaneous assessment of both Ca2+ dynamics and cytotoxicity through a target cell line presenting an epitope recognized by the BiTE.

Measuring NAADP-Evoked Ca2+ Release in Permeabilized T Cells

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.

Ca2+ puffs underlie adhesion-triggered Ca2+ microdomains in T cells

Ca2+ signalling is pivotal in T cell activation, an essential process in adaptive immune responses. Key to this activation are Ca2+ microdomains, which are transient increases in cytosolic Ca2+ concentration occurring within narrow regions between the endoplasmic reticulum (ER) and the plasma membrane (PM), lasting a few tens of milliseconds. Adhesion Dependent Ca2+ Microdomains (ADCM) rely on store-operated Ca2+ entry (SOCE) via the ORAI/STIM system. The nanometric scale at which these microdomains form poses challenges for direct experimental observation. Following the previous work of Gil et al. [1], which introduced a three-dimensional model of the ER-PM junction, this study combines a detailed description of the Ca2+ fluxes at the junction with stochastic dynamics of a cluster of D-myo-inositol 1,4,5 trisphosphate receptors (IP3R) located in the ER surrounding the junction. Because the consideration of Ca2+ release through the IP3R calls for the simulation of a portion of the cytoplasm considerably larger than the junction, our study also investigates the spatial distribution of PMCAs, revealing their likely localization outside the ER-PM junction. Simulations indicate that Ca2+ puffs implying the opening of 2-6 IP3Rs create ADCMs by provoking local depletions of ER Ca2+ stimulating Ca2+ entry through the ORAI1 channels. Such conditions allow the reproduction of the amplitude, duration and spatial extent of the observed ADCMs. By integrating advanced computational techniques with insights from experimental studies, our approach provides valuable information on the mechanisms governing early Ca2+ signalling in T cell activation, paving the way for a deeper understanding of immune responses.

MASTER-NAADP: a membrane permeable precursor of the Ca2+ mobilizing second messenger NAADP

Upon stimulation of membrane receptors, nicotinic acid adenine dinucleotide phosphate (NAADP) is formed as second messenger within seconds and evokes Ca2+ signaling in many different cell types. Here, to directly stimulate NAADP signaling, MASTER-NAADP, a Membrane permeAble, STabilized, bio-rEversibly pRotected precursor of NAADP is synthesized and release of its active NAADP mimetic, benzoic acid C-nucleoside, 2'-phospho-3'F-adenosine-diphosphate, by esterase digestion is confirmed. In the presence of NAADP receptor HN1L/JPT2 (hematological and neurological expressed 1-like protein, HN1L, also known as Jupiter microtubule-associated homolog 2, JPT2), this active NAADP mimetic releases Ca2+ and increases the open probability of type 1 ryanodine receptor. When added to intact cells, MASTER-NAADP initially evokes single local Ca2+ signals of low amplitude. Subsequently, also global Ca2+ signaling is observed in T cells, natural killer cells, and Neuro2A cells. In contrast, control compound MASTER-NADP does not stimulate Ca2+ signaling. Likewise, in cells devoid of HN1L/JPT2, MASTER-NAADP does not affect Ca2+ signaling, confirming that the product released from MASTER-NAADP is a bona fide NAADP mimetic.

The importance of Ca2+ microdomains for the adaptive immune response

Calcium signaling stands out as the most widespread and universally used signaling system and is of utmost importance for immunity. Controlled elevations in cytosolic and organellar Ca2+ concentrations in T cells control complex and essential effector functions including proliferation, differentiation, cytokine secretion, and cytotoxicity, among others. Additionally, disruptions in Ca2+ regulation in T cells contribute to diverse autoimmune, inflammatory, and immunodeficiency conditions. Among the initial intracellular signals, which occurring even before T cell receptor (TCR) stimulation are highly localized, spatially and temporally restricted so-called Ca2+ microdomains, caused by adhesion to extracellular matrix proteins (ECM proteins). The Ca2+ microdomains present both before and within the initial seconds following TCR stimulation are likely to play a crucial role in fine-tuning the downstream activity of T cell activation and thus, shaping an adaptive immune response. In this review, the emphasis is on the recent advances of adhesion-dependent Ca2+ microdomains (ADCM) in the absence of TCR stimulation, initial Ca2+ microdomains evoked by TCR stimulation (TDCM), the downstream signaling processes as well as possible therapeutic targets for interventions.

10th European Calcium Society symposium: The Ca2+-signaling toolkit in cell function, health and disease

The 10th European Calcium Society symposium, organized in Leuven, Belgium on November 15-17, 2023, focused on the role of Ca2+ signaling in cell function, health and disease. The symposium featured six scientific sessions, 16 invited speakers - of whom two were postdoctoral researchers - and 14 short talks. The talks covered various aspects of intracellular Ca2+ signaling and its implications in pathology. Each session was opened by one or more invited speakers, followed by a series of presentations from speakers selected from submitted abstracts. Through short talks, poster presentations, awards, and sustainable travel fellowships, the symposium also fostered opportunities for the active participation of early-career researchers. At least half of the short talks were allocated to early-career researchers, thereby offering a platform for the presentation of ongoing work and unpublished results. Presentations were also broadcast in real-time for online attendees. In this Meeting Review, we aim to capture the spirit of the meeting and discuss the main take-home messages that emerged during the symposium.

Time-resolved role of P2X4 and P2X7 during CD8+ T cell activation

CD8+ T cells are a crucial part of the adaptive immune system, responsible for combating intracellular pathogens and tumor cells. The initial activation of T cells involves the formation of highly dynamic Ca2+ microdomains. Recently, purinergic signaling was shown to be involved in the formation of the initial Ca2+ microdomains in CD4+ T cells. In this study, the role of purinergic cation channels, particularly P2X4 and P2X7, in CD8+ T cell signaling from initial events to downstream responses was investigated, focusing on various aspects of T cell activation, including Ca2+ microdomains, global Ca2+ responses, NFAT-1 translocation, cytokine expression, and proliferation. While Ca2+ microdomain formation was significantly reduced in the first milliseconds to seconds in CD8+ T cells lacking P2X4 and P2X7 channels, global Ca2+ responses over minutes were comparable between wild-type (WT) and knockout cells. However, the onset velocity was reduced in P2X4-deficient cells, and P2X4, as well as P2X7-deficient cells, exhibited a delayed response to reach a certain level of free cytosolic Ca2+ concentration ([Ca2+]i). NFAT-1 translocation, a crucial transcription factor in T cell activation, was also impaired in CD8+ T cells lacking P2X4 and P2X7. In addition, the expression of IFN-γ, a major pro-inflammatory cytokine produced by activated CD8+ T cells, and Nur77, a negative regulator of T cell activation, was significantly reduced 18h post-stimulation in the knockout cells. In line, the proliferation of T cells after 3 days was also impaired in the absence of P2X4 and P2X7 channels. In summary, the study demonstrates that purinergic signaling through P2X4 and P2X7 enhances initial Ca2+ events during CD8+ T cell activation and plays a crucial role in regulating downstream responses, including NFAT-1 translocation, cytokine expression, and proliferation on multiple timescales. These findings suggest that targeting purinergic signaling pathways may offer potential therapeutic interventions.

DARTS: an open-source Python pipeline for Ca2+ microdomain analysis in live cell imaging data

Ca2+ microdomains play a key role in intracellular signaling processes. For instance, they mediate the activation of T cells and, thus, the initial adaptive immune system. They are, however, also of utmost importance for activation of other cells, and a detailed understanding of the dynamics of these spatially localized Ca2+ signals is crucial for a better understanding of the underlying signaling processes. A typical approach to analyze Ca2+ microdomain dynamics is live cell fluorescence microscopy imaging. Experiments usually involve imaging a larger number of cells of different groups (for instance, wild type and knockout cells), followed by a time consuming image and data analysis. With DARTS, we present a modular Python pipeline for efficient Ca2+ microdomain analysis in live cell imaging data. DARTS (Deconvolution, Analysis, Registration, Tracking, and Shape normalization) provides state-of-the-art image postprocessing options like deep learning-based cell detection and tracking, spatio-temporal image deconvolution, and bleaching correction. An integrated automated Ca2+ microdomain detection offers direct access to global statistics like the number of microdomains for cell groups, corresponding signal intensity levels, and the temporal evolution of the measures. With a focus on bead stimulation experiments, DARTS provides a so-called dartboard projection analysis and visualization approach. A dartboard projection covers spatio-temporal normalization of the bead contact areas and cell shape normalization onto a circular template that enables aggregation of the spatiotemporal information of the microdomain detection results for the individual cells of the cell groups of interest. The dartboard visualization allows intuitive interpretation of the spatio-temporal microdomain dynamics at the group level. The application of DARTS is illustrated by three use cases in the context of the formation of initial Ca2+ microdomains after cell stimulation. DARTS is provided as an open-source solution and will be continuously extended upon the feedback of the community. Code available at: 10.5281/zenodo.10459243.

T cell Ca2+ microdomains through the lens of computational modeling

Cellular Ca2+ signaling is highly organized in time and space. Locally restricted and short-lived regions of Ca2+ increase, called Ca2+ microdomains, constitute building blocks that are differentially arranged to create cellular Ca2+ signatures controlling physiological responses. Here, we focus on Ca2+ microdomains occurring in restricted cytosolic spaces between the plasma membrane and the endoplasmic reticulum, called endoplasmic reticulum-plasma membrane junctions. In T cells, these microdomains have been finely characterized. Enough quantitative data are thus available to develop detailed computational models of junctional Ca2+ dynamics. Simulations are able to predict the characteristics of Ca2+ increases at the level of single channels and in junctions of different spatial configurations, in response to various signaling molecules. Thanks to the synergy between experimental observations and computational modeling, a unified description of the molecular mechanisms that create Ca2+ microdomains in the first seconds of T cell stimulation is emerging.

Adhesion to laminin-1 and collagen IV induces the formation of Ca2+ microdomains that sensitize mouse T cells for activation

During an immune response, T cells migrate from blood vessel walls into inflamed tissues by migrating across the endothelium and through extracellular matrix (ECM). Integrins facilitate T cell binding to endothelial cells and ECM proteins. Here, we report that Ca²⁺ microdomains observed in the absence of T cell receptor (TCR)/CD3 stimulation are initial signaling events triggered by adhesion to ECM proteins that increase the sensitivity of primary murine T cells to activation. Adhesion to the ECM proteins collagen IV and laminin-1 increased the number of Ca²⁺ microdomains in a manner dependent on the kinase FAK, phospholipase C (PLC), and all three inositol 1,4,5-trisphosphate receptor (IP₃R) subtypes and promoted the nuclear translocation of the transcription factor NFAT-1. Mathematical modeling predicted that the formation of adhesion-dependent Ca²⁺ microdomains required the concerted activity of two to six IP₃Rs and ORAI1 channels to achieve the increase in the Ca²⁺ concentration in the ER-plasma membrane junction that was observed experimentally and that required SOCE. Further, adhesion-dependent Ca²⁺ microdomains were important for the magnitude of the TCR-induced activation of T cells on collagen IV as assessed by the global Ca²⁺ response and NFAT-1 nuclear translocation. Thus, adhesion to collagen IV and laminin-1 sensitizes T cells through a mechanism involving the formation of Ca²⁺ microdomains, and blocking this low-level sensitization decreases T cell activation upon TCR engagement.

Enzymology of Ca2+-Mobilizing Second Messengers Derived from NAD: From NAD Glycohydrolases to (Dual) NADPH Oxidases

Nicotinamide adenine dinucleotide (NAD) and its 2'-phosphorylated cousin NADP are precursors for the enzymatic formation of the Ca²⁺-mobilizing second messengers adenosine diphosphoribose (ADPR), 2'-deoxy-ADPR, cyclic ADPR, and nicotinic acid adenine dinucleotide phosphate (NAADP). The enzymes involved are either NAD glycohydrolases CD38 or sterile alpha toll/interleukin receptor motif containing-1 (SARM1), or (dual) NADPH oxidases (NOX/DUOX). Enzymatic function(s) are reviewed and physiological role(s) in selected cell systems are discussed.

Meta-analysis of NAD(P)(H) quantification results exhibits variability across mammalian tissues

Nicotinamide Adenine Dinucleotide (NAD+) plays an important role in energy metabolism and signaling pathways controlling crucial cellular functions. The increased interest in NAD+ metabolism and NAD+-boosting therapies has reinforced the necessity for accurate NAD+ quantification. To examine the published NAD(P)(H) measures across mammalian tissues, we performed a meta-analysis of the existing data. An Ovid MEDLINE database search identified articles with NAD(P)(H) quantification results obtained from mammalian tissues published between 1961 and 2021. We screened 4890 records and extracted quantitative data, as well as the quantification methods, pre-analytical conditions, and subject characteristics. The extracted physiological NAD(P)(H) concentrations in various tissues from mice, rats, and humans, revealed an important inter- and intra-method variability that extended to recent publications. This highlights the relatively poor potential for cross-experimental analyses for NAD(P)(H) quantitative data and the importance of standardization for NAD(P)(H) quantification methods and pre-analytical procedures for future preclinical and clinical studies.

Endo-Lysosomal Two-Pore Channels and Their Protein Partners

Two-pore channels are ion channels expressed on acidic organelles such as the various vesicles that constitute the endo-lysosomal system. They are permeable to Ca2+ and Na+ and activated by the second messenger NAADP as well as the phosphoinositide, PI(3,5)P2 and/or voltage. Here, we review the proteins that interact with these channels including recently identified NAADP receptors.

NAADP-Evoked Ca2+ Signaling: The DUOX2-HN1L/JPT2-Ryanodine Receptor 1 Axis

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca²⁺ mobilizing second messenger known to date. Major steps elucidating metabolism and Ca²⁺ mobilizing activity of NAADP are reviewed, with emphasis on a novel redox cycle between the inactive reduced form, NAADPH, and the active oxidized form, NAADP. Oxidation from NAADPH to NAADP is catalyzed in cell free system by (dual) NADPH oxidases NOX5, DUOX1, and DUOX2, whereas reduction from NAADP to NAADPH is catalyzed by glucose 6-phosphate dehydrogenase. Using different knockout models for NOX and DUOX isozymes, DUOX2 was identified as NAADP forming enzyme in early T-cell activation.Recently, receptors or binding proteins for NAADP were identified: hematological and neurological expressed 1-like protein (HN1L)/Jupiter microtubule associated homolog 2 (JPT2) and Lsm12 are small cytosolic proteins that bind NAADP. In addition, they interact with NAADP-sensitive Ca²⁺ channels, such as ryanodine receptor type 1 (RYR1) or two-pore channels (TPC).Due to its role as Ca²⁺ mobilizing second messenger in T cells, NAADP's involvement in inflammation is also reviewed. In the central nervous system (CNS), NAADP regulates autoimmunity because NAADP antagonism affects a couple of T-cell migration and re-activation events, e.g. secretion of the pro-inflammatory cytokine interleukin-17. Further, the role of NAADP in transdifferentiation of IL-17-producing Th17 cells into T regulatory type 1 cells in vitro and in vivo is discussed.

NAADP Signaling: New Kids on the Block

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a universal Ca²⁺ mobilizing second messenger essential for initiation of Ca²⁺ signaling. Recently, novel molecular mechanisms of both its rapid formation upon receptor stimulation and its mode of action were discovered. Dual NADPH oxidase 2 (DUOX2) and hematological and neurological expressed 1-like protein (HN1L)/Jupiter microtubule-associated homolog 2 (JPT2) were discovered as NAADP-forming enzyme and NAADP receptor/binding protein—the new kids on the block. These novel aspects are reviewed and integrated into the previous view of NAADP signaling.

Development and validation of UV chromatographic method for quantification of copper and copper nanoparticles in different matrices and pharmaceutical products

Background

The applications of Cu and CuNPs based on the earth-abundant and inexpensive Cu metal have generated a great deal of interest in recent years, including medical applications. A novel, specific, precise, accurate and sensitive reverse-phase high-performance liquid chromatography (RP-HPLC) method with UV detection has been developed and validated to quantify copper (Cu) and copper nanoparticles (CuNPs) in different biological matrices and pharmaceutical products.

Methods

The developed method has been validated for linearity, precision, sensitivity, specificity and accuracy. Cu concentration was detected in pharmaceutical products without an extraction process. Moreover, liver, serum and muscle tissues were used as biological matrices. High Cu recovery in biological samples was afforded by using citric acid as a green chelating agent, exact extraction time and pH adjustment. Cu pharmaceutical and biological samples were eluted by acetonitrile: ammonium acetate (50 mM) with 0.5 mg/ml EDTA (30:70 v:v) as an isocratic mobile phase. EDTA reacted with Cu ions forming a Cu-EDTA coloured complex, separated through the C18 column and detected by UV at 310 nm.

Results

The developed method was specific with a short retention time of 4.95 min. It achieved high recovery from 100.3% to 109.9% in pharmaceutical samples and 96.8–105.7% in biological samples. The precision RSD percentage was less than two. The method was sensitive by achieving low detection limits (DL) and quantification limits (QL).

Conclusion

The validated method was efficient and economical for detecting Cu and CuNPs by readily available chemicals as EDTA and Citric acid with C18 column, which present the best results on RP-HPLC.

Three-Dimensional Model of Sub-Plasmalemmal Ca2+ Microdomains Evoked by T Cell Receptor/CD3 Complex Stimulation

Ca²⁺ signalling plays an essential role in T cell activation, which is a key step to start an adaptive immune response. During the transition from a quiescent to a fully activated state, Ca²⁺ microdomains of reduced spatial and temporal extents develop in the junctions between the plasma membrane and the endoplasmic reticulum (ER). These microdomains rely on Ca²⁺ entry from the extracellular medium, via the ORAI1/STIM1/STIM2 system that mediates store operated Ca²⁺ entry Store operated calcium entry. The mechanism leading to local store depletion and subsequent Ca²⁺ entry depends on the activation state of the cells. The initial, smaller microdomains are triggered by D-myo-inositol 1,4,5-trisphosphate (IP₃) signalling in response to T cell adhesion. T cell receptor (TCR)/CD3 stimulation then initiates nicotinic acid adenine dinucleotide phosphate signalling, which activates ryanodine receptors (RYR). We have recently developed a mathematical model to elucidate the spatiotemporal Ca²⁺ dynamics of the microdomains triggered by IP₃ signalling in response to T cell adhesion (Gil et al., 2021). This reaction-diffusion model describes the evolution of the cytosolic and endoplasmic reticulum Ca²⁺ concentrations in a three-dimensional ER-PM junction and was solved using COMSOL Multiphysics. Modelling predicted that adhesion-dependent microdomains result from the concerted activity of IP₃ receptors and pre-formed ORAI1-STIM2 complexes. In the present study, we extend this model to include the role of RYRs rapidly after TCR/CD3 stimulation. The involvement of STIM1, which has a lower K_D for Ca²⁺ than STIM2, is also considered. Detailed 3D spatio-temporal simulations show that these Ca²⁺ microdomains rely on the concerted opening of ∼7 RYRs that are simultaneously active in response to the increase in NAADP induced by T cell stimulation. Opening of these RYRs provoke a local depletion of ER Ca²⁺ that triggers Ca²⁺ flux through the ORAI1 channels. Simulations predict that RYRs are most probably located around the junction and that the increase in junctional Ca²⁺ concentration results from the combination between diffusion of Ca²⁺ released through the RYRs and Ca²⁺ entry through ORAI1 in the junction. The computational model moreover provides a tool allowing to investigate how Ca²⁺ microdomains occur, extend and interact in various states of T cell activation.

TRPM2 Is Not Required for T-Cell Activation and Differentiation

Antigen recognition by the T-cell receptor induces a cytosolic Ca²⁺ signal that is crucial for T-cell function. The Ca²⁺ channel TRPM2 (transient receptor potential cation channel subfamily M member 2) has been shown to facilitate influx of extracellular Ca²⁺ through the plasma membrane of T cells. Therefore, it was suggested that TRPM2 is involved in T-cell activation and differentiation. However, these results are largely derived from in vitro studies using T-cell lines and non-physiologic means of TRPM2 activation. Thus, the relevance of TRPM2-mediated Ca²⁺ signaling in T cells remains unclear. Here, we use TRPM2-deficient mice to investigate the function of TRPM2 in T-cell activation and differentiation. In response to TCR stimulation in vitro, Trpm2^-/- and WT CD4⁺ and CD8⁺ T cells similarly upregulated the early activation markers NUR77, IRF4, and CD69. We also observed regular proliferation of Trpm2^-/- CD8⁺ T cells and unimpaired differentiation of CD4⁺ T cells into Th1, Th17, and Treg cells under specific polarizing conditions. In vivo, Trpm2^-/- and WT CD8⁺ T cells showed equal specific responses to Listeria monocytogenes after infection of WT and Trpm2^-/- mice and after transfer of WT and Trpm2^-/- CD8⁺ T cells into infected recipients. CD4⁺ T-cell responses were investigated in the model of anti-CD3 mAb-induced intestinal inflammation, which allows analysis of Th1, Th17, Treg, and Tr1-cell differentiation. Here again, we detected similar responses of WT and Trpm2^-/- CD4⁺ T cells. In conclusion, our results argue against a major function of TRPM2 in T-cell activation and differentiation.

P2X4 and P2X7 are essential players in basal T cell activity and Ca2+ signaling milliseconds after T cell activation

Initial T cell activation is triggered by the formation of highly dynamic, spatiotemporally restricted Ca²⁺ microdomains. Purinergic signaling is known to be involved in Ca²⁺ influx in T cells at later stages compared to the initial microdomain formation. Using a high-resolution Ca²⁺ live-cell imaging system, we show that the two purinergic cation channels P2X4 and P2X7 not only are involved in the global Ca²⁺ signals but also promote initial Ca²⁺ microdomains tens of milliseconds after T cell stimulation. These Ca²⁺ microdomains were significantly decreased in T cells from P2rx4^-/- and P2rx7^-/- mice or by pharmacological inhibition or blocking. Furthermore, we show a pannexin-1-dependent activation of P2X4 in the absence of T cell receptor/CD3 stimulation. Subsequently, upon T cell receptor/CD3 stimulation, ATP release is increased and autocrine activation of both P2X4 and P2X7 then amplifies initial Ca²⁺ microdomains already in the first second of T cell activation.

Acidic Ca2+ stores and immune-cell function

Acidic organelles act as intracellular Ca²⁺ stores; they actively sequester Ca²⁺ in their lumina and release it to the cytosol upon activation of endo-lysosomal Ca²⁺ channels. Recent data suggest important roles of endo-lysosomal Ca²⁺ channels, the Two-Pore Channels (TPCs) and the TRPML channels (mucolipins), in different aspects of immune-cell function, particularly impacting membrane trafficking, vesicle fusion/fission and secretion. Remarkably, different channels on the same acidic vesicles can couple to different downstream physiology. Endo-lysosomal Ca²⁺ stores can act under different modalities, be they acting alone (via local Ca²⁺ nanodomains around TPCs/TRPMLs) or in conjunction with the ER Ca²⁺ store (to either promote or suppress global ER Ca²⁺ release). These different modalities impinge upon functions as broad as phagocytosis, cell-killing, anaphylaxis, immune memory, thrombostasis, and chemotaxis.

Dual NADPH oxidases DUOX1 and DUOX2 synthesize NAADP and are necessary for Ca2+ signaling during T cell activation

The formation of Ca2+ microdomains during T cell activation is initiated by the production of nicotinic acid adenine dinucleotide phosphate (NAADP) from its reduced form NAADPH. The reverse reaction—NAADP to NAADPH—is catalyzed by glucose 6-phosphate dehydrogenase (G6PD). Here, we identified NADPH oxidases NOX and DUOX as NAADP-forming enzymes that convert NAADPH to NAADP under physiological conditions in vitro. T cells express NOX1, NOX2, and, to a minor extent, DUOX1 and DUOX2. Local and global Ca2+ signaling were decreased in mouse T cells with double knockout of Duoxa1 and Duoxa2 but not with knockout of Nox1 or Nox2. Ca2+ microdomains in the first 15 s upon T cell activation were significantly decreased in Duox2−/− but not in Duox1−/− T cells, whereas both DUOX1 and DUOX2 were required for global Ca2+ signaling between 4 and 12 min after stimulation. Our findings suggest that a DUOX2- and G6PD-catalyzed redox cycle rapidly produces and degrades NAADP through NAADPH as an inactive intermediate.

Trans-Ned 19-Mediated Antagonism of Nicotinic Acid Adenine Nucleotide-Mediated Calcium Signaling Regulates Th17 Cell Plasticity in Mice

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca2+ mobilizing agent and its inhibition proved to inhibit T-cell activation. However, the impact of the NAADP signaling on CD4+ T-cell differentiation and plasticity and on the inflammation in tissues other than the central nervous system remains unclear. In this study, we used an antagonist of NAADP signaling, trans-Ned 19, to study the role of NAADP in CD4+ T-cell differentiation and effector function. Partial blockade of NAADP signaling in naïve CD4+ T cells in vitro promoted the differentiation of Th17 cells. Interestingly, trans-Ned 19 also promoted the production of IL-10, co-expression of LAG-3 and CD49b and increased the suppressive capacity of Th17 cells. Moreover, using an IL-17A fate mapping mouse model, we showed that NAADP inhibition promotes conversion of Th17 cells into regulatory T cells in vitro and in vivo. In line with the results, we found that inhibiting NAADP ameliorates disease in a mouse model of intestinal inflammation. Thus, these results reveal a novel function of NAADP in controlling the differentiation and plasticity of CD4+ T cells.

Time-Dependent Image Restoration of Low-SNR Live-Cell Ca2 Fluorescence Microscopy Data

Live-cell Ca2+ fluorescence microscopy is a cornerstone of cellular signaling analysis and imaging. The demand for high spatial and temporal imaging resolution is, however, intrinsically linked to a low signal-to-noise ratio (SNR) of the acquired spatio-temporal image data, which impedes on the subsequent image analysis. Advanced deconvolution and image restoration algorithms can partly mitigate the corresponding problems but are usually defined only for static images. Frame-by-frame application to spatio-temporal image data neglects inter-frame contextual relationships and temporal consistency of the imaged biological processes. Here, we propose a variational approach to time-dependent image restoration built on entropy-based regularization specifically suited to process low- and lowest-SNR fluorescence microscopy data. The advantage of the presented approach is demonstrated by means of four datasets: synthetic data for in-depth evaluation of the algorithm behavior; two datasets acquired for analysis of initial Ca2+ microdomains in T-cells; finally, to illustrate the transferability of the methodical concept to different applications, one dataset depicting spontaneous Ca2+ signaling in jGCaMP7b-expressing astrocytes. To foster re-use and reproducibility, the source code is made publicly available.

NAADP: From Discovery to Mechanism

Nicotinic acid adenine dinucleotide 2'-phosphate (NAADP) is a naturally occurring nucleotide that has been shown to be involved in the release of Ca²⁺ from intracellular stores in a wide variety of cell types, tissues and organisms. Current evidence suggests that NAADP may function as a trigger to initiate a Ca²⁺ signal that is then amplified by other Ca²⁺ release mechanisms. A fundamental question that remains unanswered is the identity of the NAADP receptor. Our recent studies have identified HN1L/JPT2 as a high affinity NAADP binding protein that is essential for the modulation of Ca²⁺ channels.

Neglected wardens: T lymphocyte ryanodine receptors

Ryanodine receptors (RyRs) are intracellular Ca²⁺ release channels ubiquitously expressed in various cell types. RyRs were extensively studied in striated muscle cells due to their crucial role in muscle contraction. In contrast, the role of RyRs in Ca²⁺ signalling and functions in non-excitable cells, such as T lymphocytes, remains poorly understood. Expression of different isoforms of RyRs was shown in primary T cells and T cell lines. In T cells, RyRs co-localize with the plasmalemmal store-operated Ca²⁺ channels of the Orai family and endoplasmic reticulum Ca²⁺ sensing Stim family proteins and are activated by store-operated Ca²⁺ entry and pyridine nucleotide metabolites, the intracellular second messengers generated upon stimulation of T cell receptors. Experimental data indicate that together with d-myo-inositol 1,4,5-trisphosphate receptors, RyRs regulate intercellular Ca²⁺ dynamics by controlling Ca²⁺ concentration within the lumen of the endoplasmic reticulum and, consequently, store-operated Ca²⁺ entry. Gain-of-function mutations, genetic deletion or pharmacological inhibition of RyRs alters T cell Ca²⁺ signalling and effector functions. The picture emerging from the collective data shows that RyRs are the essential regulators of T cell Ca²⁺ signalling and can be potentially used as molecular targets for immunomodulation or T cell-based diagnostics of the disorders associated with RyRs dysregulation.

Three-Dimensional Model of Sub-Plasmalemmal Ca2+ Microdomains Evoked by the Interplay Between ORAI1 and InsP3 Receptors

Ca²⁺ signaling plays an essential role in T cell activation, which is a key step to start an adaptive immune response. During the transition from a quiescent to a fully activated state, Ca²⁺ microdomains characterized by reduced spatial and temporal extents are observed in the junctions between the plasma membrane (PM) and the endoplasmic reticulum (ER). Such Ca²⁺ responses can also occur in response to T cell adhesion to other cells or extracellular matrix proteins in otherwise unstimulated T cells. These non-TCR/CD3-dependent Ca²⁺ microdomains rely on d-myo-inositol 1,4,5-trisphosphate (IP₃) signaling and subsequent store operated Ca²⁺ entry (SOCE) via the ORAI/STIM system. The detailed molecular mechanism of adhesion-dependent Ca²⁺ microdomain formation remains to be fully elucidated. We used mathematical modeling to investigate the spatiotemporal characteristics of T cell Ca²⁺ microdomains and their molecular regulators. We developed a reaction-diffusion model using COMSOL Multiphysics to describe the evolution of cytosolic and ER Ca²⁺ concentrations in a three-dimensional ER-PM junction. Equations are based on a previously proposed realistic description of the junction, which is extended to take into account IP₃ receptors (IP₃R) that are located next to the junction. The first model only considered the ORAI channels and the SERCA pumps. Taking into account the existence of preformed clusters of ORAI1 and STIM2, ORAI1 slightly opens in conditions of a full ER. These simulated Ca²⁺ microdomains are too small as compared to those observed in unstimulated T cells. When considering the opening of the IP₃Rs located near the junction, the local depletion of ER Ca²⁺ allows for larger Ca²⁺ fluxes through the ORAI1 channels and hence larger local Ca²⁺ concentrations. Computational results moreover show that Ca²⁺ diffusion in the ER has a major impact on the Ca²⁺ changes in the junction, by affecting the local Ca²⁺ gradients in the sub-PM ER. Besides pointing out the likely involvement of the spontaneous openings of IP₃Rs in the activation of SOCE in conditions of T cell adhesion prior to full activation, the model provides a tool to investigate how Ca²⁺ microdomains extent and interact in response to T cell receptor activation.

Spatio-temporal feature learning with reservoir computing for T-cell segmentation in live-cell [Formula: see text] fluorescence microscopy

Advances in high-resolution live-cell [Formula: see text] imaging enabled subcellular localization of early [Formula: see text] signaling events in T-cells and paved the way to investigate the interplay between receptors and potential target channels in [Formula: see text] release events. The huge amount of acquired data requires efficient, ideally automated image processing pipelines, with cell localization/segmentation as central tasks. Automated segmentation in live-cell cytosolic [Formula: see text] imaging data is, however, challenging due to temporal image intensity fluctuations, low signal-to-noise ratio, and photo-bleaching. Here, we propose a reservoir computing (RC) framework for efficient and temporally consistent segmentation. Experiments were conducted with Jurkat T-cells and anti-CD3 coated beads used for T-cell activation. We compared the RC performance with a standard U-Net and a convolutional long short-term memory (LSTM) model. The RC-based models (1) perform on par in terms of segmentation accuracy with the deep learning models for cell-only segmentation, but show improved temporal segmentation consistency compared to the U-Net; (2) outperform the U-Net for two-emission wavelengths image segmentation and differentiation of T-cells and beads; and (3) perform on par with the convolutional LSTM for single-emission wavelength T-cell/bead segmentation and differentiation. In turn, RC models contain only a fraction of the parameters of the baseline models and reduce the training time considerably.

The role of CD38 in HIV infection

The widely-expressed molecule CD38 is a single-stranded type II transmembrane glycoprotein that is mainly involved in regulating the differentiation and activation state of the cell. CD38 has broad and complex functions, including enzymatic activity, intercellular signal transduction, cell activation, cytokine production, receptor function and adhesion activity, and it plays an important role in the physiological and pathological processes of many diseases. Many studies have shown that CD38 is related to the occurrence and development of HIV infection, and CD38 may regulate its progression through different mechanisms. Therefore, investigating the role of CD38 in HIV infection and the potential signaling pathways that are involved may provide a new perspective on potential treatments for HIV infection. In the present review, the current understanding of the roles CD38 plays in HIV infection are summarized. In addition, the specific role of CD38 in the process of HIV infection of human CD4⁺ T lymphocytes is also discussed.

Essential requirement for JPT2 in NAADP-evoked Ca2+ signaling

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger that releases Ca²⁺ from acidic organelles through the activation of two-pore channels (TPCs) to regulate endolysosomal trafficking events. NAADP action is mediated by NAADP-binding protein(s) of unknown identity that confer NAADP sensitivity to TPCs. Here, we used a "clickable" NAADP-based photoprobe to isolate human NAADP-binding proteins and identified Jupiter microtubule-associated homolog 2 (JPT2) as a TPC accessory protein required for endogenous NAADP-evoked Ca²⁺ signaling. JPT2 was also required for the translocation of a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pseudovirus through the endolysosomal system. Thus, JPT2 is a component of the NAADP receptor complex that is essential for TPC-dependent Ca²⁺ signaling and control of coronaviral entry.

HN1L/JPT2: A signaling protein that connects NAADP generation to Ca2+ microdomain formation

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.

Teaming with NAADP

NAADP is a potent Ca2+-mobilizing messenger linked to numerous Ca2+-dependent effects across the natural world. On 18 February 2021, delegates attended a virtual symposium to discuss studies of the mechanisms of action of NAADP, including the work of Gunaratne et al and Roggenkamp et al in this issue of Science Signaling.

Mechanisms and functions of calcium microdomains produced by ORAI channels, d-myo-inositol 1,4,5-trisphosphate receptors, or ryanodine receptors

With the discovery of local Ca²⁺ signals in the 1990s the concept of 'elementary Ca²⁺ signals' and 'fundamental Ca²⁺ signals' was developed. While 'elementary Ca²⁺signals' relate to optical signals gained by activity of small clusters of Ca²⁺channels, 'fundamental signals' describe such optical signals that arise from opening of single Ca²⁺channels. In this review, we discuss (i) concepts of local Ca²⁺ signals and Ca²⁺ microdomains, (ii) molecular mechanisms underlying Ca²⁺ microdomains, (iii) functions of Ca²⁺ microdomains, and (iv) mathematical modelling of Ca²⁺ microdomains. We focus on Ca²⁺ microdomains produced by ORAI channels, D-myo-inositol 1,4,5-trisphosphate receptors, or ryanodine receptors. In summary, research on local Ca²⁺ signals in different cell models aims to better understand how cells use the Ca²⁺ toolkit to produce Ca²⁺ microdomains as relevant signals for specific cellular responses, but also how local Ca²⁺ signals as building blocks merge into global Ca²⁺ signaling.

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.

The influence of sex, genotype, and dose on serum and hippocampal cytokine levels in juvenile mice developmentally exposed to a human-relevant mixture of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are pervasive environmental contaminants implicated as risk factors for neurodevelopmental disorders (NDDs). Immune dysregulation is another NDD risk factor, and developmental PCB exposures are associated with early life immune dysregulation. Studies of the immunomodulatory effects of PCBs have focused on the higher-chlorinated congeners found in legacy commercial mixtures. Comparatively little is known about the immune effects of contemporary, lower-chlorinated PCBs. This is a critical data gap given recent reports that lower-chlorinated congeners comprise >70% of the total PCB burden in serum of pregnant women enrolled in the MARBLES study who are at increased risk for having a child with an NDD. To examine the influence of PCBs, sex, and genotype on cytokine levels, mice were exposed throughout gestation and lactation to a PCB mixture in the maternal diet, which was based on the 12 most abundant PCBs in sera from MARBLES subjects. Using multiplex array, cytokines were quantified in the serum and hippocampus of weanling mice expressing either a human gain-of-function mutation in ryanodine receptor 1 (T4826I mice), a human CGG premutation repeat expansion in the fragile X mental retardation gene 1 (CGG mice), or both mutations (DM mice). Congenic wildtype (WT) mice were used as controls. There were dose-dependent effects of PCB exposure on cytokine concentrations in the serum but not hippocampus. Differential effects of genotype were observed in the serum and hippocampus. Hippocampal cytokines were consistently elevated in T4826I mice and also in WT animals for some cytokines compared to CGG and DM mice, while serum cytokines were usually elevated in the mutant genotypes compared to the WT group. Males had elevated levels of 19 cytokines in the serum and 4 in the hippocampus compared to females, but there were also interactions between sex and genotype for 7 hippocampal cytokines. Only the chemokine CCL5 in the serum showed an interaction between PCB dose, genotype, and sex. Collectively, these findings indicate differential influences of PCB exposure and genotype on cytokine levels in serum and hippocampal tissue of weanling mice. These results suggest that developmental PCB exposure has chronic effects on baseline serum, but not hippocampal, cytokine levels in juvenile mice.

NAADP/SERCA3-Dependent Ca2+ Stores Pathway Specifically Controls Early Autocrine ADP Secretion Potentiating Platelet Activation

RATIONALE

Ca²⁺ signaling is a key and ubiquitous actor of cell organization and its modulation controls many cellular responses. SERCAs (sarco-endoplasmic reticulum Ca²⁺-ATPases) pump Ca²⁺ into internal stores that play a major role in the cytosolic Ca²⁺ concentration rise upon cell activation. Platelets exhibit 2 types of SERCAs, SERCA2b and SERCA3 (SERCA3 deficient mice), which may exert specific roles, yet ill-defined. We have recently shown that Ca²⁺ mobilization from SERCA3-dependent stores was required for full platelet activation in weak stimulation conditions.

OBJECTIVE

To uncover the signaling mechanisms associated with Ca²⁺ mobilization from SERCA3-dependent stores leading to ADP secretion.

METHODS AND RESULTS

Using platelets from wild-type or Serca3-deficient mice, we demonstrated that an early (within 5-10 s following stimulation) secretion of ADP specifically dependent on SERCA3 stored Ca²⁺ is exclusively mobilized by nicotinic acid adenosine dinucleotide-phosphate (NAADP): both Ca²⁺ mobilization from SERCA3-dependent stores and primary ADP secretion are blocked by the NAADP receptor antagonist Ned-19, and reciprocally both are stimulated by permeant NAADP. In contrast, Ca²⁺ mobilization from SERCA3-dependent stores and primary ADP secretion were unaffected by inhibition of the production of IP3 (inositol-1,4,5-trisphosphate) by phospholipase-C and accordingly were not stimulated by permeant IP3.

CONCLUSIONS

Upon activation, an NAADP/SERCA3 Ca²⁺ mobilization pathway initiates an early ADP secretion, potentiating platelet activation, and a secondary wave of ADP secretion driven by both an IP3/SERCA2b-dependent Ca²⁺ stores pathway and the NAADP/SERCA3 pathway. This does not exclude that Ca²⁺ mobilized from SERCA3 stores may also enhance platelet global reactivity to agonists. Because of its modulating effect on platelet activation, this NAADP-SERCA3 pathway may be a relevant target for anti-thrombotic therapy. Graphic Abstract: A graphic abstract is available for this article.

Contribution of NAADP to Glutamate-Evoked Changes in Ca2+ Homeostasis in Mouse Hippocampal Neurons

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger that evokes calcium release from intracellular organelles by the engagement of calcium release channels, including members of the Transient Receptor Potential (TRP) family, such as TRPML1, the (structurally) related Two Pore Channel type 1 (TPC1) and TPC2 channels as well as Ryanodine Receptors type 1 (RYR1; Guse, 2012). NAADP evokes calcium release from acidic calcium stores of many cell types (Guse, 2012), and NAADP-sensitive Ca²⁺ stores have been described in hippocampal neurons of the rat (Bak et al., 1999; McGuinness et al., 2007). Glutamate triggers Ca²⁺-mediated neuronal excitotoxicity in inflammation-induced neurodegenerative pathologies such as Multiple Sclerosis (MS; Friese et al., 2014), and when applied extracellularly to neurons glutamate can elevate NAADP levels in these cells. Accordingly, glutamate-evoked Ca²⁺ signals from intracellular organelles were inhibited by preventing organelle acidification (Pandey et al., 2009). Analysis of reported RNA sequencing experiments of cultured hippocampal neurons revealed the abundance of Mcoln1 (encoding TRPML1), Tpcn1, and Tpcn2 (encoding TPC1 and TPC2, respectively) as potential NAADP target channels in these cells. Transcripts encoding Ryr1 were not found in contrast to Ryr2 and Ryr3. To study the contribution of NAADP signaling to glutamate-evoked calcium transients in murine hippocampal neurons we used the NAADP antagonists Ned-19 (Naylor et al., 2009) and BZ194 (Dammermann et al., 2009). Our results show that both NAADP antagonists significantly reduce glutamate-evoked calcium transients. In addition to extracellular glutamate application, we studied synchronized calcium oscillations in the cells of the neuronal cultures evoked by addition of the GABA_A receptor antagonist bicuculline. Pretreatment with Ned-19 (50 μM) or BZ194 (100 μM) led to an increase in the frequency of bicuculline-induced calcium oscillations at the cost of calcium transient amplitudes. Interestingly, Ned-19 triggered a rise in intracellular calcium concentrations 25 min after bicuculline stimulation, leading to the question whether NAADP acts as a neuroprotective messenger in hippocampal neurons. Taken together, our results are in agreement with the concept that NAADP signaling significantly contributes to glutamate evoked Ca²⁺ rise in hippocampal neurons and to the amplitude and frequency of synchronized Ca²⁺ oscillations triggered by spontaneous glutamate release events.

Nicotinic acid adenine dinucleotide phosphate activates two-pore channel TPC1 to mediate lysosomal Ca2+ release in endothelial colony-forming cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most recently discovered Ca²⁺ -releasing messenger that increases the intracellular Ca²⁺ concentration by mobilizing the lysosomal Ca²⁺ store through two-pore channels 1 (TPC1) and 2 (TPC2). NAADP-induced lysosomal Ca²⁺ release regulates multiple endothelial functions, including nitric oxide release and proliferation. A sizeable acidic Ca²⁺ pool endowed with TPC1 is also present in human endothelial colony-forming cells (ECFCs), which represent the only known truly endothelial precursors. Herein, we sought to explore the role of the lysosomal Ca²⁺ store and TPC1 in circulating ECFCs by harnessing Ca²⁺ imaging and molecular biology techniques. The lysosomotropic agent, Gly-Phe β-naphthylamide, and nigericin, which dissipates the proton gradient which drives Ca²⁺ sequestration by acidic organelles, caused endogenous Ca²⁺ release in the presence of a replete inositol-1,4,5-trisphosphate (InsP₃ )-sensitive endoplasmic reticulum (ER) Ca²⁺ pool. Likewise, the amount of ER releasable Ca²⁺ was reduced by disrupting lysosomal Ca²⁺ content. Liposomal delivery of NAADP induced a transient Ca²⁺ signal that was abolished by disrupting the lysosomal Ca²⁺ store and by pharmacological and genetic blockade of TPC1. Pharmacological manipulation revealed that NAADP-induced Ca²⁺ release also required ER-embedded InsP₃ receptors. Finally, NAADP-induced lysosomal Ca²⁺ release was found to trigger vascular endothelial growth factor-induced intracellular Ca²⁺ oscillations and proliferation, while it did not contribute to adenosine-5'-trisphosphate-induced Ca²⁺ signaling. These findings demonstrated that NAADP-induced TPC1-mediated Ca²⁺ release can selectively be recruited to induce the Ca²⁺ response to specific cues in circulating ECFCs.

NAADP-regulated two-pore channels drive phagocytosis through endo-lysosomal Ca2+ nanodomains, calcineurin and dynamin

Macrophages clear pathogens by phagocytosis and lysosomes that fuse with phagosomes are traditionally regarded as to a source of membranes and luminal degradative enzymes. Here, we reveal that endo-lysosomes act as platforms for a new phagocytic signalling pathway in which FcγR activation recruits the second messenger NAADP and thereby promotes the opening of Ca²⁺ -permeable two-pore channels (TPCs). Remarkably, phagocytosis is driven by these local endo-lysosomal Ca²⁺ nanodomains rather than global cytoplasmic or ER Ca²⁺ signals. Motile endolysosomes contact nascent phagosomes to promote phagocytosis, whereas endo-lysosome immobilization prevents it. We show that TPC-released Ca²⁺ rapidly activates calcineurin, which in turn dephosphorylates and activates the GTPase dynamin-2. Finally, we find that different endo-lysosomal Ca²⁺ channels play diverse roles, with TPCs providing a universal phagocytic signal for a wide range of particles and TRPML1 being only required for phagocytosis of large targets.

The Endoplasmic Reticulum-Plasma Membrane Junction: A Hub for Agonist Regulation of Ca2+ Entry

Stimulation of cell-surface receptors induces cytosolic Ca²⁺ ([Ca²⁺]_i) increases that are detected and transduced by effector proteins for regulation of cell function. Intracellular Ca²⁺ release, via endoplasmic reticulum (ER) proteins inositol 1,4,5-trisphosphate receptors (IP₃R) and ryanodine receptors (RyR), and Ca²⁺ influx, via store-operated Ca²⁺ entry (SOCE), contribute to the increase in [Ca²⁺]_i The amplitude, frequency, and spatial characteristics of the [Ca²⁺]_i increases are controlled by the compartmentalization of proteins into signaling complexes such as receptor-signaling complexes and SOCE complexes. Both complexes include protein and lipid components, located in the plasma membrane (PM) and ER. Receptor signaling initiates in the PM via phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and culminates with the activation of IP₃R in the ER. Conversely, SOCE is initiated in the ER by Ca²⁺-sensing stromal interaction molecule (STIM) proteins, which then interact with PM channels Orai1 and TRPC1 to activate Ca²⁺ entry. This review will address how ER-PM junctions serve a central role in agonist regulation of SOCE.

Calcium signalling in T cells

Calcium (Ca²⁺) signalling is of paramount importance to immunity. Regulated increases in cytosolic and organellar Ca²⁺ concentrations in lymphocytes control complex and crucial effector functions such as metabolism, proliferation, differentiation, antibody and cytokine secretion and cytotoxicity. Altered Ca²⁺ regulation in lymphocytes leads to various autoimmune, inflammatory and immunodeficiency syndromes. Several types of plasma membrane and organellar Ca²⁺-permeable channels are functional in T cells. They contribute highly localized spatial and temporal Ca²⁺ microdomains that are required for achieving functional specificity. While the mechanistic details of these Ca²⁺ microdomains are only beginning to emerge, it is evident that through crosstalk, synergy and feedback mechanisms, they fine-tune T cell signalling to match complex immune responses. In this article, we review the expression and function of various Ca²⁺-permeable channels in the plasma membrane, endoplasmic reticulum, mitochondria and endolysosomes of T cells and their role in shaping immunity and the pathogenesis of immune-mediated diseases.

Low expression of ryanodine receptor 2 is associated with poor prognosis in thyroid carcinoma

Genetic alterations are vital in the progression of thyroid carcinoma. Ryanodine receptor 2 (RyR2) is reported to serve an important role in several types of human carcinoma. However, the expression and effect of RyR2 in thyroid carcinoma remain unknown. Therefore, the present study analyzed the status of RyR2 in thyroid carcinoma using bioinformatics tools. The mRNA profiles of thyroid carcinoma were downloaded from The Cancer Genome Atlas. RyR2 was distinguished as a differentially expressed gene that has not been reported in thyroid carcinoma. Further analysis indicated that there was selective downregulation of RyR2 expression in thyroid carcinoma tissues compared with that in normal thyroid tissues. Survival analysis showed that RyR2 expression was associated with poorer disease-free survival (DFS) for all patients with thyroid carcinoma. Univariate analysis revealed that a low expression of RyR2 was significantly associated with lymphatic metastasis, extracapsular extension, and the Tumor-Node-Metastasis stage. Cox analysis demonstrated that RyR2 was an independent prognostic factor in thyroid carcinoma for DFS. The biological processes and signaling pathways of RyR2 were reviewed with Gene Set Enrichment Analysis. In conclusion, the present study has revealed that RyR2 is downregulated in thyroid carcinoma, and that low expression of RyR2 is associated with poor prognosis in patients with thyroid carcinoma. RyR2 may therefore serve as a promising tumor suppressor gene in thyroid carcinoma.

The synthesis and characterization of a clickable-photoactive NAADP analog active in human cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca²⁺ mobilizing second messenger which triggers Ca²⁺ release in both sea urchin egg homogenates and in mammalian cells. The NAADP binding protein has not been identified and the regulation of NAADP mediated Ca²⁺ release remains controversial. To address this issue, we have synthesized an NAADP analog in which 3-azido-5-azidomethylbenzoic acid is attached to the amino group of 5-(3-aminopropyl)-NAADP to produce an NAADP analog which is both a photoaffinity label and clickable. This 'all-in-one-clickable' NAADP (AIOC-NAADP) elicited Ca²⁺ release when microinjected into cultured human SKBR3 cells at low concentrations. In contrast, it displayed little activity in sea urchin egg homogenates where very high concentrations were required to elicit Ca²⁺ release. In mammalian cell homogenates, incubation with low concentrations of [³²P]AIOC-NAADP followed by irradiation with UV light resulted in labeling 23 kDa protein(s). Competition between [³²P]AIOC-NAADP and increasing concentrations of NAADP demonstrated that the labeling was selective. We show that this label recognizes and selectively photodervatizes the 23 kDa NAADP binding protein(s) in cultured human cells identified in previous studies using [³²P]5-N₃-NAADP.

Histamine induces intracellular Ca2+ oscillations and nitric oxide release in endothelial cells from brain microvascular circulation

The neuromodulator histamine is able to vasorelax in human cerebral, meningeal and temporal arteries via endothelial histamine 1 receptors (H₁ Rs) which result in the downstream production of nitric oxide (NO), the most powerful vasodilator transmitter in the brain. Although endothelial Ca ²⁺ signals drive histamine-induced NO release throughout the peripheral circulation, the mechanism by which histamine evokes NO production in human cerebrovascular endothelial cells is still unknown. Herein, we exploited the human cerebral microvascular endothelial cell line, hCMEC/D3, to assess the role of intracellular Ca ²⁺ signaling in histamine-induced NO release. To achieve this goal, hCMEC/D3 cells were loaded with the Ca ²⁺ - and NO-sensitive dyes, Fura-2/AM and DAF-FM/AM, respectively. Histamine elicited repetitive oscillations in intracellular Ca ²⁺ concentration in hCMEC/D3 cells throughout a concentration range spanning from 1 pM up to 300 μM. The oscillatory Ca ²⁺ response was suppressed by the inhibition of H ₁ Rs with pyrilamine, whereas H ₁ R was abundantly expressed at the protein level. We further found that histamine-induced intracellular Ca ²⁺ oscillations were initiated by endogenous Ca ²⁺ mobilization through inositol-1,4,5-trisphosphate- and nicotinic acid dinucleotide phosphate-sensitive channels and maintained over time by store-operated Ca ²⁺ entry. In addition, histamine evoked robust NO release that was prevented by interfering with the accompanying intracellular Ca ²⁺ oscillations, thereby confirming that the endothelial NO synthase is recruited by Ca ²⁺ spikes also in hCMEC/D3 cells. These data provide the first evidence that histamine evokes NO production from human cerebrovascular endothelial cells through intracellular Ca ²⁺ oscillations, thereby shedding novel light on the mechanisms by which this neuromodulator controls cerebral blood flow.

Durable Interactions of T Cells with T Cell Receptor Stimuli in the Absence of a Stable Immunological Synapse

T cells engage in two modes of interaction with antigen-presenting surfaces: stable synapses and motile kinapses. Although it is surmised that durable interactions of T cells with antigen-presenting cells involve synapses, in situ 3D imaging cannot resolve the mode of interaction. We have established in vitro 2D platforms and quantitative metrics to determine cell-intrinsic modes of interaction when T cells are faced with spatially continuous or restricted stimulation. All major resting human T cell subsets, except memory CD8 T cells, spend more time in the kinapse mode on continuous stimulatory surfaces. Surprisingly, we did not observe any concordant relationship between the mode and durability of interaction on cell-sized stimulatory spots. Naive CD8 T cells maintain kinapses for more than 3 hr before leaving stimulatory spots, whereas their memory counterparts maintain synapses for only an hour before leaving. Thus, durable interactions do not require stable synapses.

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Naive T cells spend more time in the motile kinapse state

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Only human memory CD8 T cells spend more time in the stable synapse state

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Kinapses do not reduce durability of interaction with cell-sized stimulatory spots

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Spatial restriction of TCR stimulation does not force formation of a synapse

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) Induces Intracellular Ca2+ Release through the Two-Pore Channel TPC1 in Metastatic Colorectal Cancer Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) gates two-pore channels 1 and 2 (TPC1 and TPC2) to elicit endo-lysosomal (EL) Ca²⁺ release. NAADP-induced EL Ca²⁺ signals may be amplified by the endoplasmic reticulum (ER) through the Ca²⁺-induced Ca²⁺ release mechanism (CICR). Herein, we aimed at assessing for the first time the role of EL Ca²⁺ signaling in primary cultures of human metastatic colorectal carcinoma (mCRC) by exploiting Ca²⁺ imaging and molecular biology techniques. The lysosomotropic agent, Gly-Phe β-naphthylamide (GPN), and nigericin, which dissipates the ΔpH which drives Ca²⁺ refilling of acidic organelles, caused massive Ca²⁺ release in the presence of a functional inositol-1,4,5-trisphosphate (InsP₃)-sensitive ER Ca²⁺ store. Liposomal delivery of NAADP induced a transient Ca²⁺ release that was reduced by GPN and NED-19, a selective TPC antagonist. Pharmacological and genetic manipulations revealed that the Ca²⁺ response to NAADP was triggered by TPC1, the most expressed TPC isoform in mCRC cells, and required ER-embedded InsP₃ receptors. Finally, NED-19 and genetic silencing of TPC1 reduced fetal calf serum-induced Ca²⁺ signals, proliferation, and extracellular signal-regulated kinase and Akt phoshorylation in mCRC cells. These data demonstrate that NAADP-gated TPC1 could be regarded as a novel target for alternative therapies to treat mCRC.