Calcium-dependent activation of T-lymphocytes

Enhanced and sustained T cell activation in response to fluid shear stress

The efficacy of T cell therapies in treating solid tumors is limited by poor in vivo persistence, proliferation, and cytotoxicity, which can be attributed to limited and variable ex vivo activation. Herein, we present a 10-day kinetic profile of T cells subjected to fluid shear stress (FSS) ex vivo, with and without stimulation utilizing bead-conjugated anti-CD3/CD28 antibodies. We demonstrate that mechanical stimulation via FSS combined with bead-bound anti-CD3/CD28 antibodies yields a synergistic effect, resulting in amplified and sustained downstream signaling (NF-κB, c-Fos, and NFAT), expression of activation markers (CD69 and CD25), proliferation and production of pro-inflammatory cytokines (IFN-γ, TNF-α, and IL-2). This study represents the first characterization of the dynamic response of primary T cells to FSS. Collectively, our findings underscore the critical role of mechanosensitive ion channel-mediated mechanobiological signaling in T cell activation and fitness, enabling the development of strategies to address the current challenges associated with poor immunotherapy outcomes.

Rapid metabolic regulation of a novel arginine methylation of KCa3.1 attenuates T cell exhaustion

T cell exhaustion is linked to persistent antigen exposure and perturbed activation events, correlating with poor disease prognosis. Tumor-mediated T cell exhaustion is well documented; however, how the nutrient-deprived tumor niche affects T cell receptor (TCR) activation is largely unclear. We show that methionine metabolism licenses optimal TCR signaling by regulating the protein arginine methylome, and limiting methionine availability during early TCR signaling promotes subsequent T cell exhaustion. We discovered a novel arginine methylation of a Ca 2+ -activated potassium transporter, KCa3.1, prevention of which results in increased Ca 2+ -mediated NFAT1 activation, NFAT1 promoter occupancy, and T cell exhaustion. Furthermore, methionine supplementation reduces nuclear NFAT1 in tumor-infiltrating T cells and augments their anti-tumor activity. These findings demonstrate metabolic regulation of T cell exhaustion determined during TCR engagement.

P2X7-Mediated Antigen-Independent Activation of CD8+ T Cells Promotes Salt-Sensitive Hypertension

BACKGROUND

CD8+ T cells (CD8Ts) have been implicated in hypertension. However, the specific mechanisms are not fully understood. In this study, we explore the contribution of the P2X7 (purinergic receptor P2X7) receptor to CD8T activation and subsequent promotion of sodium retention in the kidney.

METHODS

We used mouse models of hypertension. Wild type were used as genetic controls, OT1 and Rag2/OT1 mice were utilized to determine antigen dependency, and P2X7-knockout mice were studied to define the role of P2X7 in activating CD8Ts and promoting hypertension. Blood pressure was monitored continuously and kidneys were obtained at different experimental end points. Freshly isolated CD8Ts from mice for activation assays and ATP stimulation. CD8T activation-induced promotion of sodium retention was explored in cocultures of CD8Ts and mouse DCTs.

RESULTS

We found that OT1 and Rag2/OT1 mice, which are nonresponsive to common antigens, still developed hypertension and CD8T-activation in response to deoxycorticosterone acetate/salt treatment, similar to wild-type mice. Further studies identified the P2X7 receptor on CD8Ts as a possible mediator of this antigen-independent activation of CD8Ts in hypertension. Knockout of the P2X7 receptor prevented calcium influx and cytokine production in CD8Ts. This finding was associated with reduced CD8T-DCT stimulation, reversal of excessive salt retention in DCTs, and attenuated development of salt-sensitive hypertension.

CONCLUSIONS

Our findings suggest a novel mechanism by which CD8Ts are activated in hypertension to exacerbate salt retention and infer that the P2X7 receptor on CD8Ts may represent a new therapeutic target to attenuate T-cell-mediated immunopathology in hypertension.

Dietary restriction to optimize T cell immunity is an ancient survival strategy conserved in vertebrate evolution

Recent advances highlight a key role of transient fasting in optimizing immunity of human and mouse. However, it remains unknown whether this strategy is independently acquired by mammals during evolution or instead represents gradually evolved functions common to vertebrates. Using a tilapia model, we report that T cells are the main executors of the response of the immune system to fasting and that dietary restriction bidirectionally modulates T cell immunity. Long-term fasting impaired T cell immunity by inducing intense autophagy, apoptosis, and aberrant inflammation. However, transient dietary restriction triggered moderate autophagy to optimize T cell response by maintaining homeostasis, alleviating inflammation and tissue damage, as well as enhancing T cell activation, proliferation and function. Furthermore, AMPK is the central hub linking fasting and autophagy-controlled T cell immunity in tilapia. Our findings demonstrate that dietary restriction to optimize immunity is an ancient strategy conserved in vertebrate evolution, providing novel perspectives for understanding the adaptive evolution of T cell response.

The link between immunity and hypertension in the kidney and heart

Hypertension is the primary cause of cardiovascular disease, which is a leading killer worldwide. Despite the prevalence of this non-communicable disease, still between 90% and 95% of cases are of unknown or multivariate cause ("essential hypertension"). Current therapeutic options focus primarily on lowering blood pressure through decreasing peripheral resistance or reducing fluid volume, but fewer than half of hypertensive patients can reach blood pressure control. Hence, identifying unknown mechanisms causing essential hypertension and designing new treatment accordingly are critically needed for improving public health. In recent years, the immune system has been increasingly implicated in contributing to a plethora of cardiovascular diseases. Many studies have demonstrated the critical role of the immune system in the pathogenesis of hypertension, particularly through pro-inflammatory mechanisms within the kidney and heart, which, eventually, drive a myriad of renal and cardiovascular diseases. However, the precise mechanisms and potential therapeutic targets remain largely unknown. Therefore, identifying which immune players are contributing to local inflammation and characterizing pro-inflammatory molecules and mechanisms involved will provide promising new therapeutic targets that could lower blood pressure and prevent progression from hypertension into renal or cardiac dysfunction.

Chlorpyrifos exposure induces calcium-dependent necrosis in carp (Cyprinus carpio) lymphocytes via the inhibition of T cell receptor gamma (TCR γ)

The insecticide chlorpyrifos plays an important role in agricultural production and is widely used because of its excellent insecticidal ability. However, the mechanism by which chlorpyrifos causes lymphocyte death remains unclear. In this study, transcriptomic techniques were used to analyze the head kidney tissues of carp (Cyprinus carpio) treated with chlorpyrifos. Subsequently, we screened out differentially expressed genes (DEGs) and performed the corresponding processing in the head kidney lymphocyte. Then, the intracellular calcium content and necrosis were detected by fluorescence staining, real-time fluorescence quantitative PCR, and flow cytometry. Our results showed that the expression of T cell receptor gamma (TCR γ) was significantly decreased, and TCR γ was inhibited after chlorpyrifos treatment. Also, TCR γ significantly increased the abundance of calcium channel messenger RNA (mRNA). To verify this result, we established the TCR γ overexpression group and found that the reverse results were observed in TCR γ of in the overexpression group. The results of cytoplasmic calcium concentration detection, calcium staining, and flow cytometry confirmed the conclusion of increased calcium in the cytoplasm. The function of TCR γ significantly enhanced the mRNA expression levels of necrosis-related genes, and this conclusion was evidenced by the result of necrotic flow detection. Our results showed that chlorpyrifos could inhibit TCR γ in carp lymphocytes and induce calcium-dependent necrosis.

Upregulation of RCAN1.4 by HIF1α alleviates OGD-induced inflammatory response in astrocytes

OBJECTIVE

Ischemic stroke is a leading cause of human mortality and long-term disability worldwide. As one of the main forms of regulator of calcineurin 1 (RCAN1), the contribution of RCAN1.4 in diverse biological and pathological conditions has been implicated. But the role of RCAN1.4 in ischemic stroke progression remains elusive. This study is to explore the expression changes and roles of RCAN1.4 in ischemic stroke as well as the underlying mechanisms for these changes and effects of RCAN1.4 in ischemic stroke.

METHODS

Middle cerebral artery occlusion model in C57BL/6J mice and oxygen-glucose deprivation (OGD) model in primary astrocytes were performed to induce the cerebral ischemic stroke. The expression pattern of RCAN1.4 was assessed using real-time quantitative PCR and western blotting in vivo and in vitro. Mechanistically, the underlying mechanism for the elevation of RCAN1.4 in the upstream was investigated. Lentiviruses were administrated, and the effect of RCAN1.4 in postischemic inflammation was clearly clarified.

RESULTS

Here we uncovered that RCAN1.4 was dramatically increased in mouse ischemic brains and OGD-induced primary astrocytes. HIF1α, activated upon OGD, significantly upregulated RCAN1.4 gene expression through specifically binding to the RCAN1.4 promoter region and activating its promoter activity. The functional hypoxia-responsive element (HRE) was located between -254 and -245 bp in the RCAN1.4 promoter region. Moreover, elevated RCAN1.4 alleviated the release of pro-inflammatory cytokines TNFα, IL1β, IL6 and reduced expression of iNOS, COX2 in primary astrocytes upon OGD, whereas RCAN1.4 silencing has the opposite effect. Of note, RCAN1.4 overexpression inhibited OGD-induced NF-κB activation in primary astrocytes, leading to decreased degradation of IκBα and reduced nuclear translocation of NF-κB/p65.

INTERPRETATION

Our results reveal a novel mechanism underscoring the upregulation of RCAN1.4 by HIF1α and the protective effect of RCAN1.4 against postischemic inflammation, suggesting its significance as a promising therapeutic target for ischemic stroke treatment.

Calcium regulation of T cell metabolism

T cells are an essential component of the immune system that provide antigen-specific acute and long lasting immune responses to infections and tumors, ascertain the maintenance of immunological tolerance and, on the flipside, mediate autoimmunity in a variety of diseases. The activation of T cells through antigen recognition by the T cell receptor (TCR) results in transient and sustained Ca2+ signals that are shaped by the opening of Ca2+ channels in the plasma membrane and cellular organelles. The dynamic regulation of intracellular Ca2+ concentrations controls a variety of T cell functions on the timescale of seconds to days after signal initiation. Among the more recently identified roles of Ca2+ signaling in T cells is the regulation of metabolic pathways that control the function of many T cell subsets. In this review, we discuss how Ca2+ regulates several metabolic programs in T cells such as the activation of AMPK and the PI3K-AKT-mTORC1 pathway, aerobic glycolysis, mitochondrial metabolism including tricarboxylic acid (TCA) cycle function and oxidative phosphorylation (OXPHOS), as well as lipid metabolism.

Efficient Non-Epigenetic Activation of HIV Latency through the T-Cell Receptor Signalosome

Human immunodeficiency virus type-1 (HIV-1) can either undergo a lytic pathway to cause productive systemic infections or enter a latent state in which the integrated provirus remains transcriptionally silent for decades. The ability to latently infect T-cells enables HIV-1 to establish persistent infections in resting memory CD4+ T-lymphocytes which become reactivated following the disruption or cessation of intensive drug therapy. The maintenance of viral latency occurs through epigenetic and non-epigenetic mechanisms. Epigenetic mechanisms of HIV latency regulation involve the deacetylation and methylation of histone proteins within nucleosome 1 (nuc-1) at the viral long terminal repeats (LTR) such that the inhibition of histone deacetyltransferase and histone lysine methyltransferase activities, respectively, reactivates HIV from latency. Non-epigenetic mechanisms involve the nuclear restriction of critical cellular transcription factors such as nuclear factor-kappa beta (NF-κB) or nuclear factor of activated T-cells (NFAT) which activate transcription from the viral LTR, limiting the nuclear levels of the viral transcription transactivator protein Tat and its cellular co-factor positive transcription elongation factor b (P-TEFb), which together regulate HIV transcriptional elongation. In this article, we review how T-cell receptor (TCR) activation efficiently induces NF-κB, NFAT, and activator protein 1 (AP-1) transcription factors through multiple signal pathways and how these factors efficiently regulate HIV LTR transcription through the non-epigenetic mechanism. We further discuss how elongation factor P-TEFb, induced through an extracellular signal-regulated kinase (ERK)-dependent mechanism, regulates HIV transcriptional elongation before new Tat is synthesized and the role of AP-1 in the modulation of HIV transcriptional elongation through functional synergy with NF-κB. Furthermore, we discuss how TCR signaling induces critical post-translational modifications of the cyclin-dependent kinase 9 (CDK9) subunit of P-TEFb which enhances interactions between P-TEFb and the viral Tat protein and the resultant enhancement of HIV transcriptional elongation.

Endoplasmic Reticulum Calcium Pumps and Tumor Cell Differentiation

Endoplasmic reticulum (ER) calcium homeostasis plays an essential role in cellular calcium signaling, intra-ER protein chaperoning and maturation, as well as in the interaction of the ER with other organelles. Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA enzymes) that generate by active, ATP-dependent transport, a several thousand-fold calcium ion concentration gradient between the cytosol (low nanomolar) and the ER lumen (high micromolar). SERCA enzymes are coded by three genes that by alternative splicing give rise to several isoforms, which can display isoform-specific calcium transport characteristics. SERCA expression levels and isoenzyme composition vary according to cell type, and this constitutes a mechanism whereby ER calcium homeostasis is adapted to the signaling and metabolic needs of the cell, depending on its phenotype, its state of activation and differentiation. As reviewed here, in several normal epithelial cell types including bronchial, mammary, gastric, colonic and choroid plexus epithelium, as well as in mature cells of hematopoietic origin such as pumps are simultaneously expressed, whereas in corresponding tumors and leukemias SERCA3 expression is selectively down-regulated. SERCA3 expression is restored during the pharmacologically induced differentiation of various cancer and leukemia cell types. SERCA3 is a useful marker for the study of cell differentiation, and the loss of SERCA3 expression constitutes a previously unrecognized example of the remodeling of calcium homeostasis in tumors.

Ca2+ signaling in T lymphocytes: the interplay of the endoplasmic reticulum, mitochondria, membrane potential, and CRAC channels on transcription factor activation

T cell receptor stimulation initiates a cascade of reactions that cause an increase in intracellular calcium (Ca2+) concentration mediated through inositol 1,4,5-trisphosphate (IP3). To understand the basic mechanisms by which the immune response in T cells is activated, it is useful to understand the signaling pathways that contain important targets for drugs in a quantitative fashion. A computational model helps us to understand how the selected elements in the pathways interact with each other, and which component plays the crucial role in systems. We have developed a mathematical model to explore the mechanism for controlling transcription factor activity, which regulates gene expression, by the modulation of calcium signaling triggered during T cell activation. The model simulates the activation and modulation of Ca2+ release-activated Ca2+ (CRAC) channels by mitochondrial dynamics and depletion of endoplasmic reticulum (ER) store, and also includes membrane potential in T-cells. The model simulates the experimental finding that increases in Ca2+ current enhances the activation of transcription factors and the Ca2+ influx through CRAC is also essential for the NFAT and NFκB activation. The model also suggests that plasma membrane Ca2+-ATPase (PMCA) controls a majority of the extrusion of Ca2+ and modulates the activation of CRAC channels. Furthermore, the model simulations explain how the complex interaction of the endoplasmic reticulum, membrane potential, mitochondria, and ion channels such as CRAC channels control T cell activation.

Indirect Measurement of CRAC Channel Activity Using NFAT Nuclear Translocation by Flow Cytometry in Jurkat Cells

Flow cytometry is a powerful technology to assess the presence of NFAT in the nuclei after CRAC channel activation. Here we described a simplified procedure for the analysis of CRAC channel activity using NFAT nuclear translocation by flow cytometry, based on the isolation of Jurkat E6-1 cell nuclei.

Leucocyte protein Trojan, a possible regulator of apoptosis

Trojan is a leucocyte-specific protein, cloned from chicken embryonic thymocyte cDNA library. The molecule is a type I transmembrane protein with an extracellular CCP domain, followed by two FN3 domains. Its cytoplasmic tail is predicted to possess a MAPK docking and a PKA phosphorylation sites. Trojan has been proposed to have an anti-apoptotic role based on its differential expression on developing thymocyte subpopulations. Using a chicken cell line, our in vitro studies showed that upon apoptosis induction, Trojan expression rises dramatically on the surface of surviving cells and gradually decreases towards its normal levels as cells recover. When sorted based on their expression levels of Trojan, cells with high expression appeared less susceptible to apoptotic induction than those bearing no or low levels of Trojan on their surface. The mechanism by which the molecule exerts its function is yet to be discovered. We found that cells overexpressing Trojan from a cDNA plasmid show elevated steady-state levels of intracellular calcium, suggesting the molecule is able to transmit cytoplasmic signals. The mechanistic nature of Trojan-induced signalling is a target of future investigation. In this article, we conducted a series of experiments that suggest Trojan as an anti-apoptotic regulator.

Pressure Induced Changes in Adaptive Immune Function in Belugas (Delphinapterus leucas); Implications for Dive Physiology and Health

Increased pressure, associated with diving, can alter cell function through several mechanisms and has been shown to impact immune functions performed by peripheral blood mononuclear cells (PBMC) in humans. While marine mammals possess specific adaptations which protect them from dive related injury, it is unknown how their immune system is adapted to the challenges associated with diving. The purpose of this study was to measure PBMC activation (IL2R expression) and Concanavalin A induced lymphocyte proliferation (BrdU incorporation) in belugas following in vitro pressure exposures during baseline, Out of Water Examination (OWE) and capture/release conditions. Beluga blood samples (n = 4) were obtained from animals at the Mystic Aquarium and from free ranging animals in Alaska (n = 9). Human blood samples (n = 4) (Biological Specialty Corporation) were run for comparison. In vivo catecholamines and cortisol were measured in belugas to characterize the neuroendocrine response. Comparison of cellular responses between controls and pressure exposed cells, between conditions in belugas, between belugas and humans as well as between dive profiles, were run using mixed generalized linear models (α = 0.05). Cortisol was significantly higher in Bristol Bay belugas and OWE samples as compared with baseline for aquarium animals. Both IL2R expression and proliferation displayed significant pressure induced changes, and these responses varied between conditions in belugas. Both belugas and humans displayed increased IL2R expression, while lymphocyte proliferation decreased for aquarium animals and increased for humans and Bristol Bay belugas. Results suggest beluga PBMC function is altered during diving and changes may represent dive adaptation as the response differs from humans, a non-dive adapted mammal. In addition, characteristics of a dive (i.e., duration, depth) as well as neuroendocrine activity can alter the response of beluga cells, potentially impacting the ability of animals to fight infection or avoid dive related pathologies.

Macroautophagy inhibition maintains fragmented mitochondria to foster T cell receptor-dependent apoptosis

Mitochondrial dynamics and functionality are linked to the autophagic degradative pathway under several stress conditions. However, the interplay between mitochondria and autophagy upon cell death signalling remains unclear. The T-cell receptor pathway signals the so-called activation-induced cell death (AICD) essential for immune tolerance regulation. Here, we show that this apoptotic pathway requires the inhibition of macroautophagy. Protein kinase-A activation downstream of T-cell receptor signalling inhibits macroautophagy upon AICD induction. This leads to the accumulation of damaged mitochondria, which are fragmented, display remodelled cristae and release cytochrome c, thereby driving apoptosis. Autophagy-forced reactivation that clears the Parkin-decorated mitochondria is as effective in inhibiting apoptosis as genetic interference with cristae remodelling and cytochrome c release. Thus, upon AICD induction regulation of macroautophagy, rather than selective mitophagy, ensures apoptotic progression.

The effects of telmisartan on the nuclear factor of activated T lymphocytes signalling pathway in hypertensive patients

HYPOTHESIS

Previous studies provide links between the nuclear factor of activated T lymphocytes (NFAT) signalling pathway and the development of hypertension. Our preliminary studies indicate that telmisartan can block Kv1.3 potassium channels and effectively inhibit potassium current densities, along with Kv1.3 mRNA and protein expression levels. This paper aims to investigate whether telmisartan has an inhibitory effect on the NFAT signalling pathway after activation and proliferation of peripheral blood T lymphocytes in Kazakh patients with essential hypertension (EH) from Xinjiang, China.

MATERIALS AND METHODS

T lymphocytes were isolated using the immunomagnetic cell sorting method (MACS). The mRNA expression of NFATc1, IL-6 and TNF-α was measured by quantitative polymerase chain reaction (qRT-PCR) and relative protein levels were evaluated by Western blot. T cell samples from 50 hypertensive Kazakh patients from Xinjiang were randomly divided into control, telmisartan, cyclosporin A (CsA), VIVIT, and 4-aminopytidine (4-AP) groups. Peripheral blood T lymphocytes were first activated and proliferated in vitro, then incubated for 48 h under different treatment conditions before determination of protein and mRNA expression of NFATc1, IL-6, and TNF-α by Western blot and qRT-PCR analyses, respectively.

RESULTS

There were no significant differences in cardiovascular risk factors among the patients with samples assigned to the five groups (p > 0.05). Expression of NFATc1, IL-6, and TNF-α mRNA and protein was significantly reduced in T lymphocytes in all treatment groups (telmisartan, CsA, VIVIT, and 4-AP) compared with controls.

CONCLUSIONS

Antihypertensive function and inhibitory effects of telmisartan on the T lymphocyte NFAT signalling pathway are unlikely to affect the normal immune function of hypertensive patients. Telmisartan may exert anti-inflammatory effects by inhibition of the NFAT signalling pathway in the T lymphocytes of hypertensive patients.

Aluminum Chloride Induces Osteoblasts Apoptosis via Disrupting Calcium Homeostasis and Activating Ca(2+)/CaMKII Signal Pathway

Aluminum promotes osteoblast (OB) apoptosis. Apoptosis is induced by the disordered calcium homeostasis. Therefore, to investigate the relationship between Al-induced OB apoptosis and calcium homeostasis, calvarium OBs from neonatal rats (3-4 days) were cultured and exposed to 0.048-mg/mL Al(3+) or 0.048-mg/mL Al(3+) combined with 5 μM BAPTA-AM (OBs were pretreated with 5 μM BAPTA-AM for 1 h, then added 0.048 mg/mL Al(3+)), respectively. Then OB apoptosis rate, intracellular calcium ions concentration ([Ca(2+)]i), mRNA expression level of calmodulin (CaM), and protein expression levels of CaM and p-CaMKII in OBs were examined. The result showed that AlCl3 increased OB apoptosis rate, and [Ca(2+)]i and p-CaMKII expression levels and decreased CaM expression levels, whereas BAPTA-AM relieved the effects. These results proved that AlCl3 induced OB apoptosis by disrupting the intracellular Ca(2+) homeostasis and activating the Ca(2+)/CaMKII signal pathway. Our findings can provide new insights for revealing the apoptosis mechanism of OBs exposed to AlCl3.

CRAC channels, calcium, and cancer in light of the driver and passenger concept

Advances in next-generation sequencing allow very comprehensive analyses of large numbers of cancer genomes leading to an increasingly better characterization and classification of cancers. Comparing genomic data predicts candidate genes driving development, growth, or metastasis of cancer. Cancer driver genes are defined as genes whose mutations are causally implicated in oncogenesis whereas passenger mutations are defined as not being oncogenic. Currently, a list of several hundred cancer driver mutations is discussed including prominent members like TP53, BRAF, NRAS, or NF1. According to the vast literature on Ca(2+) and cancer, Ca(2+) signals and the underlying Ca(2+) channels and transporters certainly influence the development, growth, and metastasis of many cancers. In this review, I focus on the calcium release-activated calcium (CRAC) channel genes STIM and Orai and their role for cancer development, growth, and metastasis. STIM and Orai genes are being discussed in the context of current cancer concepts with a focus on the driver-passenger hypothesis. One result of this discussion is the hypothesis that a driver analysis of Ca(2+) homeostasis-related genes should not be carried out by looking at isolated genes. Rather a pool of “Ca(2+) genes” might be considered to act as one potential cancer driver. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Inositol 1,4,5 trisphosphate receptor 1 is a key player of human myoblast differentiation

Cytosolic Ca(2+) signals are fundamental for the early and late steps of myoblast differentiation and are, as in many cells, generated by Ca(2+) release from internal stores as well as by plasma membrane Ca(2+) entry. Our recent studies identified the store-operated Ca(2+) channels, Orai1 and TRPC1&C4, as crucial for the early steps of human myogenesis and for the late fusion events. In the present work, we assessed the role of the inositol-1,4,5 tris-phosphate receptor (IP3R) type 1 during human myoblast differentiation. We demonstrated, using siRNA strategy that IP3R1 is required for the expression of muscle-specific transcription factors such as myogenin and MEF2 (myocyte enhancer factor 2), and for the formation of myotubes. The knockdown of IP3R1 strongly reduced endogenous spontaneous Ca(2+) transients, and attenuated store-operated Ca(2+) entry. As well, two Ca(2+)-dependent key enzymes of muscle differentiation, NFAT and CamKII are down-regulated upon siIP3R1 treatment. On the contrary, the overexpression of IP3R1 accelerated myoblasts differentiation. These findings identify Ca(2+) release mediated by IP3R1 as an essential mechanism during the early steps of myoblast differentiation.

Stable expression and function of the inositol 1,4,5-triphosphate receptor requires palmitoylation by a DHHC6/selenoprotein K complex

Calcium (Ca(2+)) is a secondary messenger in cells and Ca(2+) flux initiated from endoplasmic reticulum (ER) stores via inositol 1,4,5-triphosphate (IP3) binding to the IP3 receptor (IP3R) is particularly important for the activation and function of immune cells. Previous studies demonstrated that genetic deletion of selenoprotein K (Selk) led to decreased Ca(2+) flux in a variety of immune cells and impaired immunity, but the mechanism was unclear. Here we show that Selk deficiency does not affect receptor-induced IP3 production, but Selk deficiency through genetic deletion or low selenium in culture media leads to low expression of the IP3R due to a defect in IP3R palmitoylation. Bioinformatic analysis of the DHHC (letters represent the amino acids aspartic acid, histidine, histidine, and cysteine in the catalytic domain) family of enzymes that catalyze protein palmitoylation revealed that one member, DHHC6, contains a predicted Src-homology 3 (SH3) domain and DHHC6 is localized to the ER membrane. Because Selk is also an ER membrane protein and contains an SH3 binding domain, immunofluorescence and coimmunoprecipitation experiments were conducted and revealed DHHC6/Selk interactions in the ER membrane that depended on SH3/SH3 binding domain interactions. DHHC6 knockdown using shRNA in stably transfected cell lines led to decreased expression of the IP3R and impaired IP3R-dependent Ca(2+) flux. Mass spectrophotometric and bioinformatic analyses of the IP3R protein identified two palmitoylated cysteine residues and another potentially palmitoylated cysteine, and mutation of these three cysteines to alanines resulted in decreased IP3R palmitoylation and function. These findings reveal IP3R palmitoylation as a critical regulator of Ca(2+) flux in immune cells and define a previously unidentified DHHC/Selk complex responsible for this process.

The impact of Nucleofection® on the activation state of primary human CD4 T cells

Gene transfer into primary human CD4 T lymphocytes is a critical tool in studying the mechanism of T cell-dependent immune responses and human immunodeficiency virus-1 (HIV-1) infection. Nucleofection® is an electroporation technique that allows efficient gene transfer into primary human CD4 T cells that are notoriously resistant to traditional electroporation. Despite its popularity in immunological research, careful characterization of its impact on the physiology of CD4 T cells has not been documented. Herein, using freshly-isolated primary human CD4 T cells, we examine the effects of Nucleofection® on CD4 T cell morphology, intracellular calcium levels, cell surface activation markers, and transcriptional activity. We find that immediately after Nucleofection®, CD4 T cells undergo dramatic morphological changes characterized by wrinkled and dilated plasma membranes before recovering 1h later. The intracellular calcium level also increases after Nucleofection®, peaking after 1h before recovering 8h post transfection. Moreover, Nucleofection® leads to increased expression of T cell activation markers, CD154 and CD69, for more than 24h, and enhances the activation effects of phytohemagglutinin (PHA) stimulation. In addition, transcriptional activity is increased in the first 24h after Nucleofection®, even in the absence of exogenous stimuli. Therefore, Nucleofection® significantly alters the activation state of primary human CD4 T cells. The effect of transferred gene products on CD4 T cell function by Nucleofection® should be assessed after sufficient resting time post transfection or analyzed in light of the activation caveats mentioned above.

Downregulation of inhibitory SRC homology 2 domain-containing phosphatase-1 (SHP-1) leads to recovery of T cell responses in elderly

BACKGROUND

Immune responses are generally impaired in aged mammals. T cells have been extensively studied in this context due to the initial discovery of their reduced proliferative capacity with aging. The decreased responses involve altered signaling events associated with the early steps of T cell activation. The underlying causes of these changes are not fully understood but point to alterations in assembly of the machinery for T cell activation. Here, we have tested the hypothesis that the T cell pool in elderly subjects displayed reduced functional capacities due to altered negative feedback mechanisms that participate in the regulation of the early steps of T cell activation. Such conditions tip the immune balance in favor of altered T cell activation and a related decreased response in aging.

RESULTS

We present evidence that the tyrosine phosphatase SHP-1, a key regulator of T cell signal transduction machinery is, at least in part, responsible for the impaired T cell activation in aging. We used tyrosine-specific mAbs and Western blot analysis to show that a deregulation of the Csk/PAG loop in activated T cells from elderly individuals favored the inactive form of tyrosine-phosphorylated Lck (Y505). Confocal microscopy analysis revealed that the dynamic movements of these regulatory proteins in lipid raft microdomains was altered in T cells of aged individuals. Enzymic assays showed that SHP-1 activity was upregulated in T cells of aged donors, in contrast to young subjects. Pharmacological inhibition of SHP-1 resulted in recovery of TCR/CD28-dependent lymphocyte proliferation and IL-2 production of aged individuals to levels approaching those of young donors. Significant differences in the active (Y394) and inactive (Y505) phosphorylation sites of Lck in response to T cell activation were observed in elderly donors as compared to young subjects, independently of CD45 isoform expression.

CONCLUSIONS

Our data suggest that the role of SHP-1 in T cell activation extends to its increased effect in negative feedback in aging. Modulation of SHP-1 activity could be a target to restore altered T cell functions in aging. These observations could have far reaching consequences for improvement of immunosenescence and its clinical consequences such as infections, altered response to vaccination.

The calcium feedback loop and T cell activation: how cytoskeleton networks control intracellular calcium flux

During T cell activation, the engagement of a T cell with an antigen-presenting cell (APC) results in rapid cytoskeletal rearrangements and a dramatic increase of intracellular calcium (Ca(2+)) concentration, downstream to T cell antigen receptor (TCR) ligation. These events facilitate the organization of an immunological synapse (IS), which supports the redistribution of receptors, signaling molecules and organelles towards the T cell-APC interface to induce downstream signaling events, ultimately supporting T cell effector functions. Thus, Ca(2+) signaling and cytoskeleton rearrangements are essential for T cell activation and T cell-dependent immune response. Rapid release of Ca(2+) from intracellular stores, e.g. the endoplasmic reticulum (ER), triggers the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels, residing in the plasma membrane. These channels facilitate a sustained influx of extracellular Ca(2+) across the plasma membrane in a process termed store-operated Ca(2+) entry (SOCE). Because CRAC channels are themselves inhibited by Ca(2+) ions, additional factors are suggested to enable the sustained Ca(2+) influx required for T cell function. Among these factors, we focus here on the contribution of the actin and microtubule cytoskeleton. The TCR-mediated increase in intracellular Ca(2+) evokes a rapid cytoskeleton-dependent polarization, which involves actin cytoskeleton rearrangements and microtubule-organizing center (MTOC) reorientation. Here, we review the molecular mechanisms of Ca(2+) flux and cytoskeletal rearrangements, and further describe the way by which the cytoskeletal networks feedback to Ca(2+) signaling by controlling the spatial and temporal distribution of Ca(2+) sources and sinks, modulating TCR-dependent Ca(2+) signals, which are required for an appropriate T cell response. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Impaired calcium entry into cells is associated with pathological signs of zinc deficiency

Zinc is an essential trace element whose deficiency gives rise to specific pathological signs. These signs occur because an essential metabolic function is impaired as the result of failure to form or maintain a specific metal-ion protein complex. Although zinc is a component of many essential metalloenzymes and transcription factors, few of these have been identified with a specific sign of incipient zinc deficiency. Zinc also functions as a structural component of other essential proteins. Recent research with Swiss murine fibroblasts, 3T3 cells, has shown that zinc deficiency impairs calcium entry into cells, a process essential for many cell functions, including proliferation, maturation, contraction, and immunity. Impairment of calcium entry and the subsequent failure of cell proliferation could explain the growth failure associated with zinc deficiency. Defective calcium uptake is associated with impaired nerve transmission and pathology of the peripheral nervous system, as well as the failure of platelet aggregation and the bleeding tendency of zinc deficiency. There is a strong analogy between the pathology of genetic diseases that result in impaired calcium entry and other signs of zinc deficiency, such as decreased and cyclic food intake, taste abnormalities, abnormal water balance, skin lesions, impaired reproduction, depressed immunity, and teratogenesis. This analogy suggests that failure of calcium entry is involved in these signs of zinc deficiency as well.

Molecular mechanisms of treg-mediated T cell suppression

CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) can suppress other immune cells and, thus, are critical mediators of peripheral self-tolerance. On the one hand, Tregs avert autoimmune disease and allergies. On the other hand, Tregs can prevent immune reactions against tumors and pathogens. Despite the importance of Tregs, the molecular mechanisms of suppression remain incompletely understood and controversial. Proliferation and cytokine production of CD4(+)CD25(-) conventional T cells (Tcons) can be inhibited directly by Tregs. In addition, Tregs can indirectly suppress Tcon activation via inhibition of the stimulatory capacity of antigen presenting cells. Direct suppression of Tcons by Tregs can involve immunosuppressive soluble factors or cell contact. Different mechanisms of suppression have been described, so far with no consensus on one universal mechanism. Controversies might be explained by the fact that different mechanisms may operate depending on the site of the immune reaction, on the type and activation state of the suppressed target cell as well as on the Treg activation status. Further, inhibition of T cell effector function can occur independently of suppression of proliferation. In this review, we summarize the described molecular mechanisms of suppression with a particular focus on suppression of Tcons and rapid suppression of T cell receptor-induced calcium (Ca(2+)), NFAT, and NF-κB signaling in Tcons by Tregs.

Modulation of T cell activation by localized K⁺ accumulation at the immunological synapse--a mathematical model

The response of T cells to antigens (T cell activation) is marked by an increase in intracellular Ca²⁺ levels. Voltage-gated and Ca²⁺-dependent K⁺ channels control the membrane potential of human T cells and regulate Ca²⁺ influx. This regulation is dependent on proper accumulation of K⁺ channels at the immunological synapse (IS) a signaling zone that forms between a T cell and antigen presenting cell. It is believed that the IS provides a site for regulation of the activation response and that K⁺ channel inhibition occurs at the IS, but the underlying mechanisms are unknown. A mathematical model was developed to test whether K⁺ efflux through K⁺ channels leads to an accumulation of K⁺ in the IS cleft, ultimately reducing K⁺ channel function and intracellular Ca²⁺ concentration ([Ca²⁺](i)). Simulations were conducted in models of resting and activated T cell subsets, which express different levels of K⁺ channels, by varying the K⁺ diffusion constant and the spatial localization of K⁺ channels at the IS. K⁺ accumulation in the IS cleft was calculated to increase K⁺ concentration ([K⁺]) from its normal value of 5.0 mM to 5.2-10.0 mM. Including K⁺ accumulation in the model of the IS reduced calculated K⁺ current by 1-12% and consequently, reduced calculated [Ca²⁺](i) by 1-28%. Significant reductions in K⁺ current and [Ca²⁺](i) only occurred in activated T cell simulations when most K⁺ channels were centrally clustered at the IS. The results presented show that the localization of K⁺ channels at the IS can produce a rise in [K⁺] in the IS cleft and lead to a substantial decrease in K⁺ currents and [Ca²⁺](i) in activated T cells thus providing a feedback inhibitory mechanism during T cell activation.

CaMKII targets Bcl10 in T-cell receptor induced activation of NF-κB

Recognition of antigen by T- or B-cell receptors leads to formation of an immunological synapse and initiation of signalling events that collaborate to determine the nature of the adaptive immune response. Activation of NF-κB transcription factors has a key role in regulation of numerous genes with important functions in immune responses and inflammation and is of great importance for lymphocyte activation and differentiation. The activation of NF-κB depends on changes in intracellular Ca(2+) levels, and both calmodulin (CaM) and a CaM-dependent kinase, CaMKII, help regulate NF-κB activation after T-cell receptor (TCR) stimulation, but the mechanisms are not well characterized. Here we have analyzed the functional role of CaMKII in the signalling pathway from the TCR to activation of IKK, the kinase that phosphorylates the NF-κB inhibitor IκB. We show that CaMKII is recruited to the immunological synapse where it interacts with and phosphorylates the signalling adaptor protein Bcl10. Furthermore, phosphorylation of the CARD domain of Bcl10 by CaMKII regulates the interactions within the important Carma1, Bcl10, Malt1 signalling complex and the essential signal induced ubiquitinations of Bcl10 and IKKγ. We propose a novel mechanism whereby Ca(2+) signals can be integrated at the immunological synapse through CaMKII-dependent phosphorylation of Bcl10.

Selenoprotein K knockout mice exhibit deficient calcium flux in immune cells and impaired immune responses

Selenoprotein K (Sel K) is a selenium-containing protein for which no function has been identified. We found that Sel K is an endoplasmic reticulum transmembrane protein expressed at relatively high levels in immune cells and is regulated by dietary selenium. Sel K(-/-) mice were generated and found to be similar to wild-type controls regarding growth and fertility. Immune system development was not affected by Sel K deletion, but specific immune cell defects were found in Sel K(-/-) mice. Receptor-mediated Ca(2+) flux was decreased in T cells, neutrophils, and macrophages from Sel K(-/-) mice compared with controls. Ca(2+)-dependent functions including T cell proliferation, T cell and neutrophil migration, and Fcγ receptor-mediated oxidative burst in macrophages were decreased in cells from Sel K(-/-) mice compared with that in cells from controls. West Nile virus infections were performed, and Sel K(-/-) mice exhibited decreased viral clearance in the periphery and increased viral titers in brain. Furthermore, West Nile virus-infected Sel K(-/-) mice demonstrated significantly lower survival (2 of 23; 8.7%) compared with that of wild-type controls (10 of 26; 38.5%). These results establish Sel K as an endoplasmic reticulum-membrane protein important for promoting effective Ca(2+) flux during immune cell activation and provide insight into molecular mechanisms by which dietary selenium enhances immune responses.

The non-selective monovalent cationic channels TRPM4 and TRPM5

Transient Receptor Potential (TRP) proteins are non-selective cationic channels with a consistent Ca(2+)-permeability, except for TRPM4 and TRPM5 that are not permeable to this ion. However, Ca(2+) is a major regulator of their activity since both channels are activated by a rise in internal Ca(2+). Thus TRPM4 and TRPM5 are responsible for most of the Ca(2+)-activated non-selective cationic currents (NSC(Ca)) recorded in a large variety of tissues. Their activation induces cell-membrane depolarization that modifies the driving force for ions as well as activity of voltage gated channels and thereby strongly impacts cell physiology. In the last few years, the ubiquitously expressed TRPM4 channel has been implicated in insulin secretion, the immune response, constriction of cerebral arteries, the activity of inspiratory neurons and cardiac dysfunction. Conversely, TRPM5 whose expression is more restricted, has until now been mainly implicated in taste transduction.

Understanding the temporal codes of intra-cellular signals

The health of organisms and cells depends on appropriate responses to diverse internal and external cues, stimuli, or challenges, such as changes in hormone or cytokine levels, or exposure to a pathogen. Cellular responses must be tailored to the identity and intensity of the stimulus and therefore intra-cellular signals must carry information about both. However, signaling mediators often form intricate networks that react to multiple stimuli yet manage to produce stimulus-specific responses. The multi-functionality ('functional pleiotropism') of signaling nodes suggests that biological networks have evolved ways of passing physiologically relevant stimulus information through shared channels. Increasing evidence supports the notion that this is achieved in part through temporal regulation of signaling mediators' activities. The present challenge is to identify the features of temporal activity profile that represent information about a given stimulus and understand how cells read the temporal codes to control their responses.

Neuroinflammation in Huntington's disease

Huntington's disease (HD) is a monogenic neurodegenerative disease characterized by abnormal motor movements, personality changes and early death. In contrast to other neurodegenerative diseases, very little is known about the role of neuroinflammation in HD. While the current data clearly demonstrate the existence of inflammatory processes in HD pathophysiology, the question of whether neuroinflammation is purely reactive or might actively participate in disease pathogenesis is currently a matter of ongoing research and debate. This review will try to shed some light on the current state of research in this area and provide an outlook on potential future developments.

Interaction of calmodulin with Bcl10 modulates NF-kappaB activation

Calcium signals resulting from antigen receptor activation are important in determining the responses of a T or B lymphocyte to an antigen. Calmodulin (CaM), a multi-functional sensor of intracellular calcium (Ca(2+)) signals in cells, is required in the pathway from the T cell receptor (TCR) to activation of the key transcription factor NF-kappaB. Here we searched for a partner in direct interaction with CaM in the pathway, and found that CaM interacts specifically with the signaling adaptor Bcl10. The binding is Ca(2+) dependent and of high affinity, with a K(d) of approximately 160 nM. Proximity of CaM and Bcl10 in vivo is induced by increases in the intracellular Ca(2+) level. The interaction is localized to the CARD domain of Bcl10, which interacts with the CARD domain of the upstream signaling partner Carma1. Binding of CaM to Bcl10 is shown to inhibit the ability of Bcl10 to interact with Carma1, an interaction that is required for signaling from the TCR to NF-kappaB. Furthermore, a mutant of Bcl10 with reduced binding to CaM shows increased activation of an NF-kappaB reporter, which is further enhanced by activating stimuli. We propose a novel mechanism whereby the Ca(2+) sensor CaM regulates T cell responses to antigens by binding to Bcl10, thereby modulating its interaction with Carma1 and subsequent activation of NF-kappaB.

Human TMEM174 that is highly expressed in kidney tissue activates AP-1 and promotes cell proliferation

Mitogen-activated protein kinase (MAPK) cascades play an important role in regulation of AP-1 activity through the phosphorylation of distinct substrates. In the present study, we identified a novel protein, TMEM174, whose RNA transcripts are highly expressed in human kidney tissue. TMEM174 is comprised of 243 amino acids, and contains two predicted transmembrane helices which determine its subcellular localization in endoplasmic reticulum and influences its functions. Over-expression of TMME174 enhanced the transcriptional activity of AP-1 and promoted cell proliferation, whereas the truncated mutant TMEM174DeltaTM without the transmembrane regions did not retain these functions. The possible mechanism of activation of AP-1 by TMEM174 was further examined. Our results suggest the potential role of TMEM174 in renal development and physiological function.

Correlating short-term Ca(2+) responses with long-term protein expression after activation of single T cells

In order to elucidate the dynamics of cellular processes that are induced in context with intercellular communication, defined events along the signal transduction cascade and subsequent activation steps have to be analyzed on the level of individual cells and correlated with each other. Here we present an approach that allows the initiation of cell-cell or cell-particle interactions and the analysis of cellular reactions within various regimes while the identity of each individual cell is preserved. It utilizes dielectrophoresis (DEP) and microfluidics in a lab-on-chip system. With high spatial and temporal precision we contacted single T cells with functionalized microbeads and monitored their immediate cytosolic Ca(2+) response. After this, the cells were released from the chip system and cultivated further. Expression of the activation marker molecule CD69 was analyzed the next day and correlated with the previously recorded Ca(2+) signal for each individual cell. We found a significant difference in the patterns of Ca(2+) traces between activated and non-activated cells, which shows that Ca(2+) signals in T cells can provide early information about a later reaction of the cell. Although T cells are non-excitable cells, we also observed irregular Ca(2+) transients upon exposure to the DEP field only. These Ca(2+) signals depended on exposure time, electric field strength and field frequency. By minimizing their occurrence rate, we could identify experimental conditions that caused the least interference with the physiology of the cell.

Stromal interaction molecules 1 and 2 are key regulators of autoreactive T cell activation in murine autoimmune central nervous system inflammation

Calcium (Ca(2+)) signaling in T lymphocytes is essential for a variety of functions, including the regulation of differentiation, gene transcription, and effector functions. A major Ca(2+) entry pathway in nonexcitable cells, including T cells, is store-operated Ca(2+) entry (SOCE), wherein depletion of intracellular Ca(2+) stores upon receptor stimulation causes subsequent influx of extracellular Ca(2+) across the plasma membrane. Stromal interaction molecule (STIM) 1 is the Ca(2+) sensor in the endoplasmic reticulum, which controls this process, whereas the other STIM isoform, STIM2, coregulates SOCE. Although the contribution of STIM molecules and SOCE to T lymphocyte function is well studied in vitro, their significance for immune processes in vivo has remained largely elusive. In this study, we studied T cell function in mice lacking STIM1 or STIM2 in a model of myelin-oligodendrocyte glycoprotein (MOG(35-55))-induced experimental autoimmune encephalomyelitis (EAE). We found that STIM1 deficiency significantly impaired the generation of neuroantigen-specific T cell responses in vivo with reduced Th1/Th17 responses, resulting in complete protection from EAE. Mice lacking STIM2 developed EAE, but the disease course was ameliorated. This was associated with a reduced clinical peak of disease. Deficiency of STIM2 was associated with an overall reduced proliferative capacity of lymphocytes and a reduction of IFN-gamma/IL-17 production by neuroantigen-specific T cells. Neither STIM1 nor STIM2 deficiency altered the phenotype or function of APCs. These findings reveal a crucial role of STIM-dependent pathways for T cell function and activation under autoimmune inflammatory conditions, establishing them as attractive new molecular therapeutic targets for the treatment of inflammatory and autoimmune disorders.

Localization of Kv1.3 channels in the immunological synapse modulates the calcium response to antigen stimulation in T lymphocytes

The immunological synapse (IS), a highly organized structure that forms at the point of contact between a T cell and an APC, is essential for the proper development of signaling events, including the Ca(2+) response. Kv1.3 channels control Ca(2+) homeostasis in human T cells and move into the IS upon Ag presentation. However, the process involved in channel accumulation in the IS and the functional implications of this localization are not yet known. Here we define the movement of Kv1.3 into the IS and study whether Kv1.3 localization into the IS influences Ca(2+) signaling in Jurkat T cells. Crosslinking of the channel protein with an extracellular Ab limits Kv1.3 mobility and accumulation at the IS. Moreover, Kv1.3 recruitment to the IS does not involve the transport of newly synthesized channels and it does not occur through recycling of membrane channels. Kv1.3 localization in the IS modulates the Ca(2+) response. Blockade of Kv1.3 movement into the IS by crosslinking significantly increases the amplitude of the Ca(2+) response triggered by anti-CD3/anti-CD28-coated beads, which induce the formation of the IS. On the contrary, the Ca(2+) response induced by TCR stimulation without the formation of the IS with soluble anti-CD3/anti-CD28 Abs is unaltered. The results presented herein indicate that, upon Ag presentation, membrane-incorporated Kv1.3 channels move along the plasma membrane to localize in the IS. This localization is important to control the amplitude of the Ca(2+) response, and disruption of this process can account for alterations of downstream Ca(2+)-dependent signaling events.

Efficiency of T-cell costimulation by CD80 and CD86 cross-linking correlates with calcium entry

Costimulation is a fundamental principle of T-cell activation. In addition to T-cell receptor engagement, the interaction between CD80 and/or CD86 with CD28 and/or cytotoxic T-lymphocyte antigen 4 (CTLA-4) receptors is required to regulate T-cell activation and tolerance. While the importance of costimulation is clearly established, the exact molecular mechanism is unknown. We demonstrate that T-cell proliferation and the ability of CD8(+) T-effector cells to kill were enhanced slightly by CD80 but dramatically by CD86 costimulation. To further analyse the cellular process of costimulation, we developed a single-cell assay to analyse Ca(2+) signals following costimulation with bi-specific antibodies. We found that this stimulation method worked in every human T-cell that was analysed, making it one of the most efficient T-cell activation methods to date for primary human T cells. The enhanced proliferation and killing by costimulation was paralleled by an increase of Ca(2+) influx following CD86 costimulation and it was dependent on CD28/CTLA-4 expression. The enhanced Ca(2+) influx following CD86 costimulation was abrogated by an antibody that interfered with CD28 function. The differences in Ca(2+) influx between CD80 and CD86 costimulation were not dependent on the depletion of Ca(2+) stores but were eliminated by the application of 10 mum 2-aminoethyldiphenyl borate which has recently been shown to enhance stromal interaction molecule 2 (STIM2)-dependent Ca(2+) entry while reducing STIM1-dependent Ca(2+) entry. Our data indicate that differences in the efficiency of costimulation are linked to differences in Ca(2+) entry.

Formation of STIM and Orai complexes: puncta and distal caps

In the last few years, great progress has been made in understanding how stromal interacting molecule 1 (STIM1), a protein containing a calcium sensor that is located in the endoplasmic reticulum, and Orai1, a protein that forms a calcium channel in the plasma membrane, interact and give rise to store-operated calcium entry. Pharmacological depletion of calcium stores leads to the formation of clusters containing STIM and Orai that appear to be sites for calcium influx. Similar puncta are also produced in response to physiological stimuli in immune cells. In T cells engaged with antigen-presenting cells, clusters containing STIM and Orai accumulate at the immunological synapse. We recently discovered that in activated T cells, STIM1 and Orai1 also accumulate in cap-like structures opposite the immune synapse at the distal pole of the cell. Both caps and puncta are long-lived stable structures containing STIM1 and Orai1 in close proximity. The function of puncta as sites of calcium influx is clear. We speculate that the caps may provide a secondary site of calcium entry. Alternatively, they may serve as a source of preformed channel complexes that move to new immune synapses as T cells repeatedly engage antigen-presenting cells.

LFA-1-dependent Ca2+ entry following suboptimal T cell receptor triggering proceeds without mobilization of intracellular Ca2+

A surge in cytosolic calcium ion concentration by entry of extracellular Ca2+ is a hallmark of T cell activation. According to store-operated Ca2+ entry mechanism, the Ca2+ entry is preceded by activation of phospholipase C-gamma1 (PLC-gamma1) and the consequent mobilization of intracellular Ca2+. Using membrane vesicles expressing the mouse class I major histocompatibility complex, i.e. Ld plus costimulatory ligands, i.e. B7-1 and intercellular adhesion molecule-1 along with 2C T cell receptor transgenic T cells, we investigated the roles of CD28 and LFA-1 (lymphocyte function-associated antigen-1) in the activation of PLC-gamma1 and Ca2+ signaling. Both CD28 and LFA-1 made significant and comparable contributions to the activation of PLC-gamma1 as gauged by the level of its phosphorylation at tyrosine 783. In contrast, their roles in Ca2+ signaling were quite distinct so that LFA-1/intercellular adhesion molecule-1 interaction exerted a determining role, whereas CD28/B7-1 interaction played only a minimal role. In particular, when the T cells were activated by suboptimal T cell receptor stimulation, LFA-1 played an indispensable role in the Ca2+ signaling. Further experiments using Ca2+-free medium demonstrated that the entry of extracellular Ca2+ was not always accompanied by mobilization of intracellular Ca2+. Thus, intracellular Ca2+ mobilization was hardly detected under the condition that LFA-1 played the indispensable role in the entry of extracellular Ca2+, while a distinct level of intracellular Ca2+ mobilization was readily detected under the condition that LFA-1 played only the supporting role. These results ensure the unique role of LFA-1 in T cell Ca2+ signaling and reveal that LFA-1-dependent Ca2+ entry proceeds via a mechanism separate from store-operated Ca2+ entry.

Determination of apparent calcium affinity for endogenously expressed human sarco(endo)plasmic reticulum calcium-ATPase isoform SERCA3

The sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) play a crucial role in regulating free cytosolic Ca2+ concentration in diverse cell types. It has been shown that recombinant SERCA3, when measured in heterologous systems, exhibits low apparent affinity for Ca2+; however, Ca2+ affinity of native SERCA3 in an endogenous setting has not been examined. Such a measurement is complicated, because SERCA3 is always coexpressed with the housekeeping isoform SERCA2b. We used a fluorescence-based assay for monitoring continuous Ca2+ uptake into microsomes to examine the properties of endogenous human SERCA3 and SERCA2b. The kinetic parameters were derived using a cooperative two-component uptake model for Ca2+ activation, and the values assigned to SERCA3 were confirmed using the highly specific human SERCA3 inhibitory antibody PL/IM430. First, using recombinant human SERCA3 and SERCA2b proteins transiently expressed in HEK-293 cells, we confirmed the previously observed low apparent Ca2+ affinity for SERCA3 compared with SERCA2b (1.10 ± 0.04 vs. 0.26 ± 0.01 μM), and using mixtures of recombinant protein isoforms, we validated the two-component uptake model. Then we determined apparent Ca2+ affinity for SERCA proteins present endogenously in cultured Jurkat T lymphocytes and freshly isolated human tonsil lymphocytes. The apparent Ca2+ affinity in these two preparations was 1.04 ± 0.07 and 1.1 ± 0.2 μM for SERCA3 and 0.27 ± 0.02 and 0.26 ± 0.01 μM for SERCA2b, respectively. Our data demonstrate, for the first time, that affinity for Ca2+ is inherently lower for SERCA3 expressed in situ than for other SERCA isoforms.

Activation of the brain-specific neurogranin gene in murine T-cell lymphomas by proviral insertional mutagenesis

Neurogranin (Nrgn) is a highly expressed brain-specific protein, which sequesters calmodulin at low Ca(2+)-levels. We report here on retroviral activation of the Nrgn gene in tumors induced by the T-cell lymphomagenic SL3-3 murine leukemia virus. We have performed a systematic expression analysis of Nrgn in various mouse tissues and SL3-3 induced T-cell tumors. This demonstrated that insertional activation of Nrgn increased RNA and protein expression levels to that observed in brain. Furthermore, elevated Nrgn expression was also observed in some T-cell tumors with no detected provirus integrations into this genomic region. The presented data demonstrate that Nrgn can be produced at high levels outside the brain, and suggest a novel oncogenic role in T-cell lymphomas in mice.