Eva C. Schwarz

Potent Inhibition of Ca2+ Release-activated Ca2+ Channels and T-lymphocyte Activation by the Pyrazole Derivative BTP2

Ca2+ entry through store-operated Ca2+ release-activated Ca2+ (CRAC) channels is essential for T-cell activation and proliferation. Recently, it has been shown that 3,5-bistrifluoromethyl pyrazole (BTP) derivatives are specific inhibitors of Ca2+-dependent transcriptional activity in T-cells (Trevillyan, J. M., Chiou, X. G., Chen, Y. W., Ballaron, S. J., Sheets, M. P., Smith, M. L., Wiedeman, P. E., Warrior, U., Wilkins, J., Gubbins, E. J., Gagne, G. D., Fagerland, J., Carter, G. W., Luly, J. R., Mollison, K. W., and Djuric, S. W. (2001) J. Biol. Chem. 276, 48118-48126). Whereas inhibition of Ca2+ signals was reported for BTP2 (Ishikawa, J., Ohga, K., Yoshino, T., Takezawa, R., Ichikawa, A., Kubota, H., and Yamada, T. (2003) J. Immunol. 170, 4441-4449), it was not found for BTP3 (Chen, Y., Smith, M. L., Chiou, G. X., Ballaron, S., Sheets, M. P., Gubbins, E., Warrior, U., Wilkins, J., Surowy, C., Nakane, M., Carter, G. W., Trevillyan, J. M., Mollison, K., and Djuric, S. W. (2002) Cell. Immunol. 220, 134-142). We show that BTP2 specifically inhibits CRAC channels in T-cells with an IC50 of ∼10 nm. It does not interfere with other mechanisms important for Ca2+ signals in T-cells, including Ca2+ pumps, mitochondrial Ca2+ signaling, endoplasmic reticulum Ca2+ release, and K+ channels. BTP2 inhibits Ca2+ signals in peripheral blood T-lymphocytes (in particular in CD4+ T-cells) and in human Jurkat T-cells. Inhibition of Ca2+ signals is independent of the stimulation method as Ca2+ entry was blocked following stimulation with anti-CD3, which activates the T-cell receptor, and also following stimulation with thapsigargin or inositol 1,4,5-trisphosphate. BTP2 also inhibited Ca2+-dependent gene expression (interleukins 2 and 5 and interferon γ) and proliferation of T-lymphocytes with similar IC50 values. BTP2 is the first potent and specific inhibitor of CRAC channels in primary T-lymphocytes. The inhibition of CRAC channels as well as Ca2+-dependent signal transduction with similar IC50 values in T-lymphocytes emphasizes the importance of CRAC channel activity during T-cell activation. Furthermore, BTP2 could prove to be a tool to finally unmask the molecular identity of CRAC channels.

T cell activation requires mitochondrial translocation to the immunological synapse

T helper (Th) cell activation is required for the adaptive immune response. Formation of the immunological synapse (IS) between Th cells and antigen-presenting cells is essential for Th cell activation. IS formation induces the polarization and redistribution of many signaling molecules; however, very little is known about organelle redistribution during IS formation in Th cells. We show that formation of the IS induced cytoskeleton-dependent mitochondrial redistribution to the immediate vicinity of the IS. Using total internal reflection microscopy, we found that upon stimulation, the distance between the IS and mitochondria was decreased to values <200 nm. Consequently, mitochondria close to the IS took up more Ca2+ than the ones farther away from the IS. The redistribution of mitochondria to the IS was necessary to maintain Ca2+ influx across the plasma membrane and Ca2+-dependent Th cell activation. Our results suggest that mitochondria are part of the signaling complex at the IS and that their localization close to the IS is required for Th cell activation.

Differential Redox Regulation of ORAI Ion Channels: A Mechanism to Tune Cellular Calcium Signaling

Redox sensitivity of T cells decreases through ORAI Ca2+ channel subunit switching during T cell differentiation. Adapting to Oxidizing Environments Reactive oxygen species (ROS) were thought for many years to be only detrimental, causing damage to DNA and proteins. However, it has become clear that ROS, particularly H2O2, can act as intracellular signaling molecules that link cellular redox state to such processes as proliferation and differentiation. Bogeski et al. have uncovered a role for ROS in regulating calcium channel activity—and intracellular Ca2+ signals crucial to the immune response—in T lymphocytes. They found that activity of ORAI1 calcium channels was blocked by H2O2, whereas that of the related ORAI3 channels was not. Redox sensitivity decreased as naïve human T helper lymphocytes differentiated into effector T helper lymphocytes, which was associated with an increase in the abundance of mRNA encoding the insensitive ORAI3 protein. The authors suggest that changes in the specific complement of ORAI channels, and thereby sensitivity to ROS, could enable T lymphocytes to fine tune cellular responses in oxidizing environments such as those found during inflammation. Reactive oxygen species (ROS) are involved in many physiological and pathophysiological cellular processes. We used lymphocytes, which are exposed to highly oxidizing environments during inflammation, to study the influence of ROS on cellular function. Calcium ion (Ca2+) influx through Ca2+ release–activated Ca2+ (CRAC) channels composed of proteins of the ORAI family is essential for the activation, proliferation, and differentiation of T lymphocytes, but whether and how ROS affect ORAI channel function have been unclear. Here, we combined Ca2+ imaging, patch-clamp recordings, and measurements of cell proliferation and cytokine secretion to determine the effects of hydrogen peroxide (H2O2) on ORAI channel activity and human T helper lymphocyte (TH cell) function. ORAI1, but not ORAI3, channels were inhibited by oxidation by H2O2. The differential redox sensitivity of ORAI1 and ORAI3 channels depended mainly on an extracellularly located reactive cysteine, which is absent in ORAI3. TH cells became progressively less redox-sensitive after differentiation into effector cells, a shift that would allow them to proliferate, differentiate, and secrete cytokines in oxidizing environments. The decreased redox sensitivity of effector TH cells correlated with increased expression of Orai3 and increased abundance of several cytosolic antioxidants. Knockdown of ORAI3 with small-interfering RNA rendered effector TH cells more redox-sensitive. The differential expression of Orai isoforms between naïve and effector TH cells may tune cellular responses under oxidative stress.

Sustained activity of calcium release-activated calcium channels requires translocation of mitochondria to the plasma membrane.

A rise of the intracellular Ca2+ concentration has multiple signaling functions. Sustained Ca2+ influx across plasma membrane through calcium release-activated calcium (CRAC) channels is required for T-cell development in the thymus, gene transcription, and proliferation and differentiation of naïve T-cells into armed effectors cells. Intracellular Ca2+ signals are shaped by mitochondria, which function as a highly dynamic Ca2+ buffer. However, the precise role of mitochondria for Ca2+-dependent T-cell activation is unknown. Here we have shown that mitochondria are translocated to the plasma membrane as a consequence of Ca2+ influx and that this directed movement is essential to sustain Ca2+ influx through CRAC channels. The decreased distance between mitochondria and the plasma membrane enabled mitochondria to take up large amounts of inflowing Ca2+ at the plasma membrane, thereby preventing Ca2+-dependent inactivation of CRAC channels and sustaining Ca2+ signals. Inhibition of kinesin-dependent mitochondrial movement along microtubules abolished mitochondrial translocation and reduced sustained Ca2+ signals. Our results show how a directed movement of mitochondria is used to control important cellular functions such as Ca2+-dependent T-cell activation.

Calcium-dependent activation of T-lymphocytes

Activation of T-lymphocytes requires stimulation of T-cell receptors (TCR) and co-stimulatory signals. Among different signalling cascades, TCR engagement induces Ca2+ entry through plasma membrane Ca2+ channels, which is an indispensable step for T-cells to expand clonally and to acquire effector functions. The Ca2+ channels are activated by depletion of Ca2+ stores and are called Ca2+ release-activated Ca2+ (CRAC) channels. Ca2+ influx through CRAC channels is also controlled, directly or indirectly, by K+ channels, Ca2+-ATPases, mitochondria, endoplasmic reticulum and Ca2+ buffers. We review the functional implications of these transporters, organelles and buffers and develop a model of Ca2+ signal generation that depends mainly on their relative mutual localization. This model offers the possibility of controlling amplitude and kinetics of Ca2+ signals in T-cells. Decoding of various Ca2+ signals allows differential activation of the transcription factor families nuclear factor of activated T-cells (NFAT), nuclear factor-κB (NF-κB) and activator protein-1 (AP-1). Variation of amplitude and kinetics of Ca2+ signals thus is an important mechanism for modulating the specificity of T-cell responses.

TRP expression pattern and the functional importance of TRPC3 in primary human T-cells

TRP proteins form ion channels which are activated following receptor stimulation. In T-cell lines, expression data of TRP proteins have been published. However, almost no data about TRP expression is available in primary human T-cells. Using RT-PCR and quantitative RT-PCR, we compare the expression of TRP mRNA in 1) human peripheral blood lymphocytes, which are a mix of mostly mono-nuclear blood lymphocytes but contain other leucocytes, 2) a pure human CD4+ T-helper cell population in the resting (=naïve) and activated (=effector) state, and 3) two commonly used CD4+ Jurkat T-cell lines, E6-1 and parental. To mimic physiological cell stimulation, we analyzed TRP expression in primary human cells in a quantitative way over several days following formation of an immunological synapse through stimulation with antibody-coated beads. The TRP expression profile of primary human T-cells was significantly different from Jurkat T-cells. Among the TRP mRNAs of the TRPC, TRPM, and TRPV family, we found consistent expression of TRPC1, TRPC3, TRPV1, TRPM2, and TRPM7 in primary human CD4+ T-cells of all analyzed blood donors. Among these, TRPC3 and TRPM2 were strongly up-regulated following stimulation, but with different kinetics. We found that TRPC3 modulates Ca²+-dependent proliferation of primary CD4+ T-cells indicating that TRPC3 may be involved in Ca²+ homeostasis in T-cells besides the well-established STIM and ORAI proteins which are responsible for store-operated Ca²+ entry.

ORAI-mediated calcium influx in T cell proliferation, apoptosis and tolerance.

Ca(2+) homeostasis controls a diversity of cellular processes including proliferation and apoptosis. A very important aspect of Ca(2+) signaling is how different Ca(2+) signals are translated into specific cell functions. In T cells, Ca(2+) signals are induced following the recognition of antigen by the T cell receptor and depend mainly on Ca(2+) influx through store-operated CRAC channels, which are mediated by ORAI proteins following their activation by STIM proteins. The complete absence of Ca(2+) influx caused by mutations in Stim1 and Orai1 leads to severe immunodeficiency. Here we summarize how Ca(2+) signals are tuned to regulate important T cell functions as proliferation, apoptosis and tolerance, the latter one being a special state of immune cells in which they can no longer respond properly to an otherwise activating stimulus. Perturbations of Ca(2+) signaling may be linked to immune suppressive diseases and autoimmune diseases.

Calcium dependence of T cell proliferation following focal stimulation

Clonal T cell expansion through proliferation is a central process of the adaptive immune response. Apoptosis of activated T cells is required to avoid chronic inflammation. T cell proliferation and apoptosis are often analyzed with stimuli that do not induce formation of a functional immunological synapse. Here we analyze the Ca2+ dependence of proliferation and apoptosis in primary human CD4+ T cells following stimulation with anti‐CD3/anti‐CD28‐coated beads, which induce a tight interaction similar to the immunological synapse. We found this focal stimulation to be much more efficient for stimulating IL‐2 production and proliferation than non‐focal TCR stimuli. Surprising little Ca2+ entry through Ca2+ channels was required for T cell proliferation. Transient free intracellular calcium concentration ([Ca2+]i) elevations of up to 220 nM from a baseline level of around 40 nM were sufficient for maximal proliferation in primary human CD4+ T cells. We also show that proliferation was very Ca2+ sensitive in the range 90–120 nM, whereas apoptosis was basically constant for [Ca2+]i levels of 90–120 nM. We conclude that very small changes in [Ca2+]i can dramatically change the ratio between proliferation and apoptosis, thus keeping the balance between overshooting and inefficient immune responses.

Calcium, cancer and killing: the role of calcium in killing cancer cells by cytotoxic T lymphocytes and natural killer cells.

Killing cancer cells by cytotoxic T lymphocytes (CTL) and by natural killer (NK) cells is of vital importance. Cancer cell proliferation and apoptosis depend on the intracellular Ca(2+) concentration, and the expression of numerous ion channels with the ability to control intracellular Ca(2+) concentrations has been correlated with cancer. A rise of intracellular Ca(2+) concentrations is also required for efficient CTL and NK cell function and thus for killing their targets, in this case cancer cells. Here, we review the data on Ca(2+)-dependent killing of cancer cells by CTL and NK cells. In addition, we discuss emerging ideas and present a model how Ca(2+) may be used by CTL and NK cells to optimize their cancer cell killing efficiency. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

ORAI1 Ca2+ Channels Control Endothelin-1-Induced Mitogenesis and Melanogenesis in Primary Human Melanocytes

UV radiation of the skin triggers keratinocytes to secrete endothelin-1 (ET-1) that binds to endothelin receptors on neighboring melanocytes. Melanocytes respond with a prolonged increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), which is necessary for proliferation and melanogenesis. A major fraction of the Ca(2+) signal is caused by entry through Ca(2+)-permeable channels of unknown identity in the plasma membrane. ORAI Ca(2+) channels are molecular determinants of Ca(2+) release-activated Ca(2+) (CRAC) channels and are expressed in many tissues. Here, we show that ORAI1-3 and their activating partners stromal interaction molecules 1 and 2 (STIM1 and STIM2) are expressed in human melanocytes. Although ORAI1 is the predominant ORAI isoform, STIM2 mRNA expression exceeds STIM1. Inhibition of ORAI1 by 2-aminoethoxydiphenyl borate (2-APB) or downregulation of ORAI1 by small interfering RNA (siRNA) reduced Ca(2+) entry and CRAC current amplitudes in activated melanocytes. In addition, suppression of ORAI1 caused reduction in the ET-1-induced cellular viability, melanin synthesis, and tyrosinase activity. Our results imply a role for ORAI1 channels in skin pigmentation and their potential involvement in UV-induced stress responses of the human skin.