Signaling between intracellular Ca2+ stores and depletion-activated Ca2+ channels generates [Ca2+]i oscillations in T lymphocytes

A fluorometric method for estimating the calcium content of internal stores

The concentration of Ca2+ in intracellular stores is an important factor in many aspects of Ca2+ signaling, including the generation of Ca2+ spikes, oscillations and waves, control of mitochondrial respiration, and activation of store-operated Ca2+ channels. Here we describe a consistent method for estimating the content of stores, based on the release of stored Ca2+ by thapsigargin (TG) or ionomycin (IO). Once released from stores, Ca2+ elevates [Ca2+]i transiently before it is pumped across the plasma membrane. If the dependence of the pump rate on [Ca2+]i is known, then the kinetics and amplitude of the Ca2+ transient allows the total amount of releasable Ca2+ to be estimated. We develop this quantitative approach and validate its use in human T cells, in which the Ca2+ clearance rate is an approximately linear function of [Ca2+]i. Our results support the assumption that the ER Ca2+ leak in resting T cells is unregulated, i.e. its rate is proportional to luminal [Ca2+]. The characteristic time constant for basal Ca2+ release is 110-140 s, comparable to that for activation of Ca2+ release-activated Ca2+ (CRAC) channels by TG and consistent with the dependence of ICRAC on store depletion. This method for estimating store content may be useful for quantifying the overlap between functionally distinct stores and for defining the relation between store content and cellular responses.

Are some minis multiquantal?

The amplitude distribution of miniature postsynaptic currents (minis) in many central neurons has a large variance and positive skew, but the sources of this variance and skew are unresolved. Recently it has been proposed that spontaneous Ca2+ influx into a presynaptic bouton with multiple release sites could cause spontaneous multiquantal minis by synchronizing release at all sites in the bouton, accounting for both the large variance and skew of the mini distribution. We tested this hypothesis by evoking minis with internally perfused, buffered Ca2+ and the secretagogue alpha-latrotoxin, both in the absence of external Ca2+. With these manipulations, the synchronized release model predicts that the mini distribution should collapse to a Gaussian distribution with a reduced coefficient of variation. Contrary to this expectation, we find that mini amplitude distributions under these conditions retain a large variance and positive skew and are indistinguishable from amplitude distributions of depolarization-evoked minis, strongly suggesting that minis are uniquantal.

Oligomycin inhibits store-operated channels by a mechanism independent of its effects on mitochondrial ATP

Inhibitors of mitochondrial oxidative metabolism have been proposed to interfere with Ca2+ influx mediated by store-operated channels (SOC), secondary to their effects on ATP production. We assessed SOC activity by 45Ca2+ influx and fluorimetric measurements of free Ca2+ or Mn2+ quench in thapsigargin-treated Chinese hamster ovary cells and Jurkat T-cells, and additionally by electrophysiological measurements of the Ca2+-release-activated Ca2+ current (Icrac) in Jurkat T-cells. Various mitochondrial antagonists were confirmed to inhibit SOC. However, the following evidence supported the proposal that oligomycin, in particular, exerts an inhibitory effect on SOC in addition to its known actions on mitochondria and Na+-pump activity: (i) the concentrations of oligomycin required to inhibit SOC-mediated Ca2+ influx or Icrac (half-inhibitory concentration approximately 2 microM) were nearly 50-fold higher than the concentrations that blocked mitochondrial ATP production; (ii) the rank order of potency of oligomycins A, B and C for decreasing SOC-mediated Ca2+ influx or Icrac differed from that known for inhibition of mitochondrial function; (iii) oligomycin blocked Icrac under voltage clamp and with intracellular Na+ and K+ concentrations fixed by dialysis from the patch pipette, arguing that the effect was not secondary to membrane polarization or pump activity; and (iv) fixing the cytosolic ATP concentration by dialysis from the patch pipette attenuated rotenone- but not oligomycin-mediated inhibition of Icrac. Oligomycin also blocked volume-activated Cl- currents, a profile common to some other known blockers of SOC that are not known mitochondrial inhibitors. These findings raise the possibility that oligomycin interacts directly with SOC, and thus may extend the known pharmacological profile for this type of Ca2+-influx pathway.

Mitochondrial regulation of store-operated calcium signaling in T lymphocytes

Mitochondria act as potent buffers of intracellular Ca2+ in many cells, but a more active role in modulating the generation of Ca2+ signals is not well established. We have investigated the ability of mitochondria to modulate store-operated or "capacitative" Ca2+ entry in Jurkat leukemic T cells and human T lymphocytes using fluorescence imaging techniques. Depletion of the ER Ca2+ store with thapsigargin (TG) activates Ca2+ release-activated Ca2+ (CRAC) channels in T cells, and the ensuing influx of Ca2+ loads a TG-insensitive intracellular store that by several criteria appears to be mitochondria. Loading of this store is prevented by carbonyl cyanide m-chlorophenylhydrazone or by antimycin A1 + oligomycin, agents that are known to inhibit mitochondrial Ca2+ import by dissipating the mitochondrial membrane potential. Conversely, intracellular Na+ depletion, which inhibits Na+-dependent Ca2+ export from mitochondria, enhances store loading. In addition, we find that rhod-2 labels mitochondria in T cells, and it reports changes in Ca2+ levels that are consistent with its localization in the TG-insensitive store. Ca2+ uptake by the mitochondrial store is sensitive (threshold is <400 nM cytosolic Ca2+), rapid (detectable within 8 s), and does not readily saturate. The rate of mitochondrial Ca2+ uptake is sensitive to extracellular [Ca2+], indicating that mitochondria sense Ca2+ gradients near CRAC channels. Remarkably, mitochondrial uncouplers or Na+ depletion prevent the ability of T cells to maintain a high rate of capacitative Ca2+ entry over prolonged periods of >10 min. Under these conditions, the rate of Ca2+ influx in single cells undergoes abrupt transitions from a high influx to a low influx state. These results demonstrate that mitochondria not only buffer the Ca2+ that enters T cells via store-operated Ca2+ channels, but also play an active role in modulating the rate of capacitative Ca2+ entry.

Separation of gating properties from permeation and block in mslo large conductance Ca-activated K+ channels

In this and the following paper we have examined the kinetic and steady-state properties of macroscopic mslo Ca-activated K+ currents in order to interpret these currents in terms of the gating behavior of the mslo channel. To do so, however, it was necessary to first find conditions by which we could separate the effects that changes in Ca2+ concentration or membrane voltage have on channel permeation from the effects these stimuli have on channel gating. In this study we investigate three phenomena which are unrelated to gating but are manifest in macroscopic current records: a saturation of single channel current at high voltage, a rapid voltage-dependent Ca2+ block, and a slow voltage-dependent Ba2+ block. Where possible methods are described by which these phenomena can be separated from the effects that changes in Ca2+ concentration and membrane voltage have on channel gating. Where this is not possible, some assessment of the impact these effects have on gating parameters determined from macroscopic current measurements is provided. We have also found that without considering the effects of Ca2+ and voltage on channel permeation and block, macroscopic current measurements suggest that mslo channels do not reach the same maximum open probability at all Ca2+ concentrations. Taking into account permeation and blocking effects, however, we find that this is not the case. The maximum open probability of the mslo channel is the same or very similar over a Ca2+ concentration range spanning three orders of magnitude indicating that over this range the internal Ca2+ concentration does not limit the ability of the channel to be activated by voltage.

Differential activation of transcription factors induced by Ca2+ response amplitude and duration

An increase in the intracellular calcium ion concentration ([Ca2+]i) controls a diverse range of cell functions, including adhesion, motility, gene expression and proliferation. Calcium signalling patterns can occur as single transients, repetitive oscillations or sustained plateaux, but it is not known whether these patterns are responsible for encoding the specificity of cellular responses. We report here that the amplitude and duration of calcium signals in B lymphocytes controls differential activation of the pro-inflammatory transcriptional regulators NF-kappaB, c-Jun N-terminal kinase (JNK) and NFAT. NF-kappaB and JNK are selectively activated by a large transient [Ca2+]i rise, whereas NFAT is activated by a low, sustained Ca2+ plateau. Differential activation results from differences in the Ca2+ sensitivities and kinetic behaviour of the three pathways. Our results show how downstream effectors can decode information contained in the amplitude and duration of Ca2+ signals, revealing a mechanism by which a multifunctional second messenger such as Ca2+ can achieve specificity in signalling to the nucleus.

Different nuclear signals are activated by the B cell receptor during positive versus negative signaling

It is not known how immunogenic versus tolerogenic cellular responses are signaled by receptors such as the B cell antigen receptor (BCR). Here we compare BCR signaling in naive cells that respond positively to foreign antigen and self-tolerant cells that respond negatively to self-antigen. In naive cells, foreign antigen triggered a large biphasic calcium response and activated nuclear signals through NF-AT, NF-kappa B, JNK, and ERK/pp90rsk. In tolerant B cells, self-antigen stimulated low calcium oscillations and activated NF-AT and ERK/pp90rsk but not NF-kappa B or JNK. Self-reactive B cells lacking the phosphatase CD45 did not exhibit calcium oscillations or ERK/pp90rsk activation, nor did they repond negatively to self-antigen. These data reveal striking biochemical differences in BCR signaling to the nucleus during positive selection by foreign antigens and negative selection by self-antigens.

Ca2+ entry induced by cyclic ADP-ribose in intact T-lymphocytes

Cyclic ADP-ribose (cADPr) is a potent Ca2+-mobilizing natural compound (Lee, H. C., Walseth, T. F., Bratt, G. T., Hayes, R. N., and Clapper, D. L. (1989) J. Biol. Chem. 264, 1608-1615) which has been shown to release Ca2+ from an intracellular store of permeabilized T-lymphocytes (Guse, A. H., Silva, C. P., Emmrich, F., Ashamu, G., Potter, B. V. L., and Mayr, G. W. (1995) J. Immunol. 155, 3353-3359). Microinjection of cADPr into intact single T lymphocytes dose dependently induced repetitive but irregular Ca2+ spikes which were almost completely dependent on the presence of extracellular Ca2+. The Ca2+ spikes induced by cADPr could be blocked either by co-injection of cADPr with the specific antagonist 8-NH2-cADPr, by omission of Ca2+ from the medium, or by superfusion of the cells with Zn2+ or SK-F 96365. Ratiometric digital Ca2+ imaging revealed that single Ca2+ spikes were initiated at several sites ("hot spots") close to the plasma membrane. These hot spots then rapidly formed a circular zone of high Ca2+ concentration below the plasma membrane which subsequently propagated like a closing optical diaphragm into the center of the cell. Taken together these data indicate a role for cADPr in Ca2+ entry in T-lymphocytes.

Enhancement of calcium signaling and proliferation responses in activated human T lymphocytes. Inhibitory effects of K+ channel block by charybdotoxin depend on the T cell activation state

T cell receptor (TCR) stimulation, leading to T cell activation and ultimately to cell proliferation and differentiation, evokes elevations of [Ca2+]i with a high variability between individual T lymphocytes. We have used Ca(2+)-imaging of Fura-2 loaded cells to study the origin of the variation in Ca2+ signals and its consequences for the final cellular response. We found that, compared to resting cells, the percentage of responding cells and the average amplitude of the Ca2+ signal upon TCR re-stimulation by PHA increases in the first 5 days of T cell activation and declines thereafter, with more pronounced [Ca2+]i oscillations in later stages. In parallel, an enhancement of T cell proliferation is observed. Stronger stimulation of the TCR/CD3 complex by co-crosslinking CD3 with CD4/CD8 molecules evokes oscillating Ca2+ responses irrespective of the activation state, indicating that the basic capacity for Ca2+ signaling is essentially the same in resting and activated cells. Nevertheless, also the amplitude of the CD3+CD4/8 response shows a transient additional increase during the first days of T cell activation. Experiments with the K+ channel blocker charybdotoxin (CTX) indicate that [Ca2+]i oscillations depend critically on K+ channel functioning, but suppression of these oscillations by CTX does not significantly affect the average amplitude of the Ca2+ signal nor PHA-induced proliferation. However, when applied during the first 4-5 days of activation, CTX reduces in addition the average level of the TCR evoked Ca2+ response and inhibits subsequent proliferation.

Dual T cell receptor T cells have a decreased sensitivity to physiological ligands due to reduced density of each T cell receptor

A considerable fraction of T cells express two distinct T cell receptors (TCR), mainly due to expression of two TCR alpha chains. It has been suggested that such dual-TCR cells could have a role in autoimmunity. However, as such cells express less of each TCR, they could be less sensitive to their physiological ligand, i.e. peptide plus major histocompatibility complex molecules (MHC). We tested this hypothesis in a transgenic TCR model in which most T cells express different amounts of the transgene-encoded TCR, due to expression of endogenous TCR alpha chains. Five Th1 clones derived from lambda2(315) immunoglobulin light chain-specific TCR-transgenic mice expressed different levels of the transgene-encoded TCR, ranging from approximately 10,000 to approximately 50,000 TCR per cell. Cytosolic Ca2+ mobilization in single T cells from these clones elicited by lambda2(315) peptide-pulsed, I-Ed-expressing antigen-presenting cells, correlated linearly with the relative transgene-encoded TCR expression. The peptide requirement for half-maximal T cell proliferation showed a similar correlation, with low TCR levels requiring higher peptide concentration. Corroborative evidence was obtained by deployment of short-term polyclonal CD4+ lines from TCR-transgenic mice. Such lines had reduced early (Ca2+ mobilization) and late (lymphokine and proliferation) responses, compared with T cell lines from recombination-deficient TCR-transgenic severe combined immunodeficiency mice (which express only a single transgene-encoded TCR). Taken together, the Ca2+ responses increase gradually with increasing TCR expression per cell, similar to the previously described analog Ca2+ signaling elicited by increasing amounts of peptide/MHC [Røtnes et al., Eur. J. Immunol. 1994. 24: 851]. Surprisingly small reductions in TCR expression per cell reduce T cell responsiveness. This suggests that dual-TCR T cells are immunologically less effective than single-TCR T cells.

A capacitative calcium current in cultured skeletal muscle cells is mediated by the calcium-specific leak channel and inhibited by dihydropyridine compounds

Calcium stores from cultured skeletal muscle cells were depleted using cyclopiazonic acid (CPA), a reversible inhibitor of Ca2+-ATPases at the sarcoplasmic reticulum. Store depletion led to activation of the calcium-specific leak channel, as assayed using single-channel patch clamp analysis and rates of manganese influx and quenching of fura-2 fluorescence. Two novel dihydropyridine compounds inhibited this single-channel leak channel activity, the resting and depletion-induced manganese influx, and refilling of the CPA-depleted intracellular calcium store. These compounds represent the first antagonists for a calcium leak channel and for a channel that mediates a capacitative current. The development of the skeletal muscle capacitative current was inhibited by genistein, a tyrosine kinase inhibitor, but was not affected by okadaic acid, a phosphatase inhibitor, or econazole. Thus, the capacitative current in cultured skeletal muscle cells was mediated by the calcium leak channel and was inhibited by pharmacological antagonists and may provide a model system for uncovering the complete set of signals leading from store depletion to channel activation.

Regulation of inositol trisphosphate-induced membrane currents in Xenopus oocytes by a Jurkat cell calcium influx factor

The functional interactions of a Jurkat cell-derived calcium influx factor (CIF) with Ins(1,4,5)P3 were examined by microinjection and voltage-clamp recording of current responses in Xenopus oocytes. CIF, which stimulates Ca2+ entry directly on microinjection, was active at dilutions at which it had no direct effect by augmenting both initial rapid Ins(1,4,5)P3-mediated Ca2+ discharge-activated currents and later sustained Ca2+ entry-activated currents. Augmented initial membrane currents were 3-5-fold greater in peak amplitude than currents evoked by injection of the same dose of Ins(1,4,5)P3 alone. The augmented initial response was not decreased by removal of extracellular Ca2+, suggesting that there is potentiation of Ins(1,4,5)P3-mediated discharge from intracellular Ca2+ stores. However, the augmentation of Ins(1,4,5)P3-mediated discharge cannot be due to an enhanced production of endogenous Ins(1,4,5)P3 because maximal Ins(1,4,5)P3-activated currents saturate (approx. 500 nA) with supramaximal levels of Ins(1,4,5)P3 (10-50 microM). Depletion of Ca2+ stores, by pretreatment with thapsigargin or by prior injection with the Ins(1,4,5)P3 receptor antagonist heparin, abolished membrane currents elicited by Ins(1,4,5)P3/CIF co-injection, further suggesting that the Ins(1,4,5)P3 receptor was the target for the initial-current potentiating actions of CIF. In this regard, CIF also induced augmented initial currents with co-injection of either Ins(2,4,5)P3 or Ins(1,3,4,5)P4. The augmentation of Ins(1,4,5)P3-mediated currents by CIF was bell-shaped with regard to Ins(1,4,5)P3 concentration, reminiscent of the regulatory influence of Ca2+ on Ins(1,4,5)P3 responses. Co-injection of Ins(1,4,5)P3 and CIF also augmented (2-3-fold) later current responses arising from sustained Ca2+ entry. The augmented late-current responses were not due to enhanced Ca2+ store depletion because supramaximal levels of Ins(1,4,5)P3 (50 microM) or injection of the poorly metabolized Ins(1,4,5)P3 analogue, Ins(2,4,5)P3, cannot activate the same magnitude of Ca(2+)-entry-dependent currents. These results suggest that CIF at low levels interacts with Ins(1,4,5)P3 to sensitize two pathways of Ca2+ signalling: initial discharge and later Ca2+ entry. Thus under physiological conditions CIF might be more potent as a co-messenger than as a direct Ca2+ entry signal and might provide a novel type of direct feedback regulation between the stores-activated influx pathway and the Ins(1,4,5)P3 receptor. Moreover these results suggest that CIF modulation of the receptor for Ins(1,4,5)P3 may underlie control of both augmentation of discharge and Ca2+ entry, as has been predicted from the conformational coupling model of Ca2+ entry.

Differentiation of the human monocytic cell line U937 results in an upregulation of the calcium release-activated current, ICRAC

1. Single cell fura-2 fluorescence measurements and whole-cell patch clamp recordings were used to investigate the effects of macrophage-like differentiation, induced by dibutyryl cAMP (dbcAMP), on Ca2+ influx triggered by Ca2+ store depletion in the human monocytic cell line, U937. 2. In differentiated cells, the rise in intracellular [Ca2+] following store depletion by thapsigargin (TG) in nominally Ca(2+)-free solution was 94% greater and the [Ca2+]i rise on subsequent re-addition of external Ca2+ (2 mM) was 292% greater than in undifferentiated cells. 3. Under conditions where [Ca2+]i was buffered by BAPTA, TG-induced store depletion failed to activate a detectable inward Ca2+ current in undifferentiated U937 cells. Under identical conditions, store depletion of differentiated U937 cells generated an inwardly rectifying Ca(2+)-selective current which showed no reversal from -140 to +30 mV and was blocked by 1 microM external La3+; characteristics of the calcium release-activated Ca2+ current (ICRAC) identified in other cells. 4. We conclude that U937 cells show a differentiation-dependent upregulation of a store-mediated Ca2+ entry pathway, identified as ICRAC, which is not correlated with the small associated increase in the size of TG-sensitive Ca2+ pools.

Evidence for a non-capacitative Ca2+ entry during [Ca2+] oscillations

Current models for the agonist-induced activation of Ca2+ entry from the extracellular medium in non-excitable cells generally emphasize a capacitative mechanism whereby Ca2+ entry is activated simply as a result of the emptying of intracellular Ca2+ stores, without any direct involvement of inositol phosphates. To date, the activation and control of Ca2+ entry have generally been studied under conditions where the agonist-sensitive stores undergo a profound and sustained depletion. However, responses under more normal physiological conditions typically involve the cyclical release and refilling of the stores associated with oscillations in [Ca2+], and the nature and control of entry under these conditions has received relatively little attention. In this study, using isolated cells from the exocrine avian nasal gland as a model system, we show that: (a) the agonist-enhanced rate of Mn2+ quench is independent of the cyclical emptying and refilling of the agonist-sensitive Ca2+ pool during oscillations; (b) the Ca2+ entry pathway is maintained in an activated state for extended periods following inhibition of oscillations under conditions in which agonist-sensitive stores can be shown to be full; (c) no Ca2+ entry could be detected in oscillating cells in experiments that followed a definitive protocol for the demonstration of capacitative entry; and (d) on initial exposure to low agonist concentrations, activation of Ca2+ entry preceded any detectable release of Ca2+ from the stores. We conclude that the essential characteristics of the control of Ca2+ entry during oscillations are incompatible with current capacitative models.

Calcium-dependent potentiation of store-operated calcium channels in T lymphocytes

The depletion of intracellular Ca2+ stores triggers the opening of Ca2+ release-activated Ca2+ (CRAC) channels in the plasma membrane of T lymphocytes. We have investigated the additional role of extracellular Ca2+ (Ca02+) in promoting CRAC channel activation in Jurkat leukemic T cells. Ca2+ stores were depleted with 1 microM thapsigargin in the nominal absence of Ca02+ with 12 mM EGTA or BAPTA in the recording pipette. Subsequent application of Ca02+ caused ICRAC to appear in two phases. The initial phase was complete within 1 s and reflects channels that were open in the absence of Ca02+. The second phase consisted of a severalfold exponential increase in current amplitude with a time constant of 5-10 s; we call this increase Ca(2+)-dependent potentiation, or CDP. The shape of the current-voltage relation and the inferred single-channel current amplitude are unchanged during CDP, indicating that CDP reflects an alteration in channel gating rather than permeation. The extent of CDP is modulated by voltage, increasing from approximately 50% at +50 mV to approximately 350% at -75 mV in the presence of 2 mM Ca02+. The voltage dependence of CDP also causes ICRAC to increase slowly during prolonged hyperpolarizations in the constant presence of Ca02+. CDP is not affected by exogenous intracellular Ca2+ buffers, and Ni2+, a CRAC channel blocker, can cause potentiation. Thus, the underlying Ca2+ binding site is not intracellular. Ba2+ has little or no ability to potentiate CRAC channels. These results demonstrate that the store-depletion signal by itself triggers only a small fraction of capacitative Ca2+ entry and establish Ca2+ as a potent cofactor in this process. CDP confers a previously unrecognized voltage dependence and slow time dependence on CRAC channel activation that may contribute to the dynamic behavior of ICRAC.

Activation-dependent and biphasic electromagnetic field effects: model based on cooperative enzyme kinetics in cellular signaling

Experiments on filed exposure effects of extremely-low-frequency electric and magnetic fields (EMFs) on biological systems have shown that, in many cases, the biological-functional status is of fundamental importance for an effective interaction. For example, studies of calcium uptake regulation in cells of the immune system, particularly in T lymphocytes, have revealed that, depending on the degree of cellular activation, either stimulatory, inhibitory, or no field exposure effects are observed for identical field parameters. A brief summary of the experimental findings is given, and a theoretical approach is presented that accounts in a qualitative manner for EMF exposure effects 1) that depend on the degree of cellular activation and 2) that exhibit a biphasic response behavior (stimulation/ inhibition). In the model, biochemical stimulation of the cell results in activation of specific signaling pathways that regulate calcium dynamics in the cell (calcium release from intracellular calcium stores and capacitative calcium entry). We assume that, controlled by these pathways, a specific EMF-sensitive enzyme system becomes activated. The activated enzyme, in turn, exhibits a feedback control on the signal processes, thus leading to a modulation of calcium entry. This modulation may affect other cellular processes that are calcium dependent (e.g., DNA synthesis). Magnetic field exposure is assumed to alter the kinetics of a specific step within the enzyme-reaction cycle in accord with the radical-pair mechanism, although the formulism is not restricted to this specific example. Results show that inclusion of cooperative steps within the enzyme-reaction cycle provides a theoretical basis that enables a simple description of a biphasic response behavior to EMF exposure.

Capacitative calcium entry

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Characterization of T cell mutants with defects in capacitative calcium entry: genetic evidence for the physiological roles of CRAC channels

Prolonged Ca2+ influx is an essential signal for the activation of T lymphocytes by antigen. This influx is thought to occur through highly selective Ca2+ release-activated Ca2+ (CRAC) channels that are activated by the depletion of intracellular Ca2+ stores. We have isolated mutants of the Jurkat human T cell line NZdipA to explore the molecular mechanisms that underlie capacitative Ca2+ entry and to allow a genetic test of the functions of CRAC channels in T cells. Five mutant cell lines (CJ-1 through CJ-5) were selected based on their failure to express a lethal diphtheria toxin A chain gene and a lacZ reporter gene driven by NF-AT, a Ca(2+)- and protein kinase C-dependent transcription factor. The rate of Ca2+ influx evoked by thapsigargin was reduced to varying degrees in the mutant cells whereas the dependence of NF-AT/lacZ gene transcription on [Ca2+]i was unaltered, suggesting that the transcriptional defect in these cells is caused by a reduced level of capacitative Ca2+ entry. We examined several factors that determine the rate of Ca2+ entry, including CRAC channel activity, K(+)-channel activity, and Ca2+ clearance mechanisms. The only parameter found to be dramatically altered in most of the mutant lines was the amplitude of the Ca2+ current (ICRAC), which ranged from 1 to 41% of that seen in parental control cells. In each case, the severity of the ICRAC defect was closely correlated with deficits in Ca2+ influx rate and Ca(2-)-dependent gene transcription. Behavior of the mutant cells provides genetic evidence for several roles of ICRAC in T cells. First, mitogenic doses of ionomycin appear to elevate [Ca2+]i primarily by activating CRAC channels. Second, ICRAC promotes the refilling of empty Ca2+ stores. Finally, CRAC channels are solely responsible for the Ca2+ influx that underlies antigen-mediated T cell activation. These mutant cell lines may provide a useful system for isolating, expressing, and exploring the functions of genes involved in capacitative Ca2+ entry.

Intracellular calcium signals in response to bradykinin in individual neuroblastoma cells

The Ca indicator fura 2 was used to study the modulation of cytoplasmic Ca by bradykinin (Bk) in single N1E-115 murine neuroblastoma cells. Increases in cytoplasmic Ca in response to Bk were mediated by the B2 receptor subtype. Responses to high concentrations of Bk (1-100 nM) were homogeneous and characterized by a rapidly rising transient that decayed to baseline in the continued presence of agonist, with a half-time of 15 s. Responses to low concentrations of Bk (100-500 pM) were more heterogeneous, with longer latencies and often with oscillations. Pretreatment with thapsigargin for 20 min prevented the Ca response, showing that the Ca change results from intracellular Ca release. Removal of external Ca had little effect on the response to Bk, indicating that the agonist does not activate Ca influx. The extent of Ca release and refilling after Bk was tested with ionomycin. A saturating dose of Bk (20 nM) mobilizes > 90% of stored Ca within 30 s, and this is replaced slowly. Replacement of external Na by N-methyl-D-glucamine to block Na/Ca exchange affected the Ca response, causing decreases in latency and in the period of Ca oscillations and increases in overall duration and peak amplitude of the response.

Identification of Kv1.1 expression by murine CD4-CD8- thymocytes. A role for voltage-dependent K+ channels in murine thymocyte development

The patch-clamp recording technique and RNA-polymerase chain reaction were used to identify the voltage-dependent K+ channels expressed by murine fetal and adult CD4-CD8- thymocytes. Two distinct currents, encoded by the genes Kv1.1 and Kv1.3 were identified based upon their biophysical and pharmacologic characteristics and confirmed with RNA-polymerase chain reaction. Peptide blockers of Kv1.1 and Kv1.3 gene products were also applied to a murine fetal thymic organ culture system to investigate the developmental role of these K+ channels. Dendrotoxin (DTX) and charybdotoxin (CTX), antagonists of Kv1.1 and Kv1.3 channels, respectively, decreased thymocyte yields in organ culture without affecting thymocyte viability. DTX-treated thymi contained 56 +/- 8% (n = 8 experiments), and CTX-treated thymi contained 74 +/- 4% (n = 7 experiments) as many thymocytes as untreated lobes. DTX and CTX also altered the developmental progression of thymocytes in fetal organ culture. These data provide the first evidence of Kv1.1 expression in a lymphoid cell and indicate that thymocyte voltage-dependent K+ channels are critical to thymocyte preclonal expansion and/or maturation.

Calcium response of helper T lymphocytes to antigen-presenting cells in a single-cell assay

We developed a dynamic, single-cell assay involving alternating differential interference contrast and fluorescence microscopy, together with digital imaging, for both viewing the physical interaction of live helper T lymphocytes (Th cells) with antigen-presenting cells (APCs) and monitoring the increases in the intracellular free calcium concentration of the Th cell, an early event in Th cell activation. We obtained Th-APC conjugates by allowing the Th cells to migrate toward and interact with APCs that either settled nearby or had been micromanipulated in close proximity to the Th cells. Th cell motility played an important role in initiating Th-APC contacts but not in determining the Th cell calcium response. We found that the intracellular calcium responses of individual Th cells are heterogeneous and an all-or-none phenomenon, independent of antigen concentration. However, the fraction of Th-APC conjugates involving responding Th cells is an increasing function of the antigen concentration. Finally, we measured some characteristics of the developing Th-APC contact area. We used all of these data together with previously developed mathematical models to estimate that only 1 to 20 major histocompatibility class II-antigen complexes are required in the initial Th-APC contact area to elicit a Th cell calcium response.

InsP3-induced Ca2+ excitability of the endoplasmic reticulum

Oscillations in intracellular Ca2+ can be induced by a variety of cellular signalling processes (Woods et al., 1986; Berridge 1988; Jacob et al., 1988) and appear to play a role in secretion (Stojilković et al., 1994), fertilization (Miyazaki et al., 1993), and smooth muscle contraction (Iino and Tsukioka, 1994). Recently, great progress has been made in understanding the mechanisms involved in a particular class of Ca2+ oscillation, associated with the second messenger inositol 1,4,5-trisphosphate (InsP3) (Berridge, 1993). Working in concert with intracellular Ca2+, InsP3 controls Ca2+ release via the InsP3 receptor in the endoplasmic reticulum (ER) (Berridge and Irvine, 1989). The IP3 receptor is regulated by its coagonists InsP3 and Ca2+, which both activate and inhibit Ca2+ release (Finch et al., 1991; Bezprozvanny et al., 1991; De Young and Keizer, 1992). These processes, together with the periodic activation of Ca2+ uptake into the ER, have been identified as key features in the mechanism of InsP3-induced Ca2+ oscillations in pituitary gonadotrophs (Li et al., 1994), Xenopus laevis oocytes (Lechleiter and Clapham, 1992; Atri et al., 1993), and other cell types (Keizer and De Young, 1993). Earlier discussions and models of InsP3-induced Ca2+ oscillations focused on the nature and number of internal releasable pools of Ca2+ (Goldbeter et al., 1990; Swillens and Mercan, 1990; Somogyi and Stucki, 1991), the importance of oscillations in InsP3 (Meyer and Stryer, 1988), and other issues not based on detailed experimental findings in specific cells types.

Slow calcium-dependent inactivation of depletion-activated calcium current. Store-dependent and -independent mechanisms

Feedback regulation of Ca2+ release-activated Ca2+ (CRAC) channels was studied in Jurkat leukemic T lymphocytes using whole cell recording and [Ca2+]i measurement techniques. CRAC channels were activated by passively depleting intracellular Ca2+ stores in the absence of extracellular Ca2+. Under conditions of moderate intracellular Ca2+ buffering, elevating [Ca2+]o to 22 mM initiated an inward current through CRAC channels that declined slowly with a half-time of approximately 30 s. This slow inactivation was evoked by a rise in [Ca2+]i, as it was effectively suppressed by an elevated level of EFTA in the recording pipette that prevented increases in [Ca2+]i. Blockade of Ca2+ uptake into stores by thapsigargin with or without intracellular inositol 1,4,5-trisphosphate reduced the extent of slow inactivation by approximately 50%, indicating that store refilling normally contributes significantly to this process. The store-independent (thapsigargin-insensitive) portion of slow inactivation was largely prevented by the protein phosphatase inhibitor, okadaic acid, and by a structurally related compound, 1-norokadaone, but not by calyculin A nor by cyclosporin A and FK506 at concentrations that fully inhibit calcineurin (protein phosphatase 2B) in T cells. These results argue against the involvement of protein phosphatases 1, 2A, 2B, or 3 in store-independent inactivation. We conclude that calcium acts through at least two slow negative feedback pathways to inhibit CRAC channels. Slow feedback inhibition of CRAC current is likely to play important roles in controlling the duration and dynamic behavior of receptor-generated Ca2+ signals.

Calcium signalling and cell proliferation

The orderly sequence of events that constitutes the cell cycle is carefully regulated. A part of this regulation depends upon the ubiquitous calcium signalling system. Many growth factors utilize the messenger inositol trisphosphate (InsP3) to set up prolonged calcium signals, often organized in an oscillatory pattern. These repetitive calcium spikes require both the entry of external calcium and its release from internal stores. One function of this calcium signal is to activate the immediate early genes responsible for inducing resting cells (G0) to re-enter the cell cycle. It may also promote the initiation of DNA synthesis at the G1/S transition. Finally, calcium contributes to the completion of the cell cycle by stimulating events at mitosis. The role of calcium in cell proliferation is highlighted by the increasing number of anticancer therapies and immunosuppressant drugs directed towards this calcium signalling pathway.

Intracellular Ca2+ oscillations and membrane potential fluctuations in intact human T lymphocytes: role of K+ channels in Ca2+ signaling

In intact human T lymphocytes, voltage-gated K+ [K(V)] channels and Ca(2+)-activated K+ [K(Ca)] channels have been recorded using the patch clamp technique in the cell-attached configuration. The reversal potential of the voltage-gated current with high K+ solution in the pipette gives a measure for the cell membrane potential (VM). The open probability of the K(Ca) channels gives a measure for intracellular Ca2+ concentration ([Ca2+]i). By simultaneous recording of both types of K+ channels, the interaction of VM and [Ca2+]i in T lymphocytes was investigated. It was demonstrated that VM fluctuates under resting conditions in a 20 mV range around an average value of -60 mV. In response to T cell receptor stimulation by PHA, rises in [Ca2+]i occur, which vary between cells from transient or sustained elevations to Ca2+ oscillations, in parallel with amplification of the hyperpolarizing deflections of VM. The correlation between VM and [Ca2+]i suggests that Ca2+ oscillations are modulated by positive feedback between Ca2+ influx, [Ca2+]i and VM mediated by K(Ca) channels and by intrinsic VM fluctuations caused by negative feedback between VM and the K(V) channel. Differences in the ratio between K(Ca) and K(V) channel numbers can account for the variability in Ca2+ responses between cells. The results predict periodic K(V) channel activity at rest and alternating K(V) and K(Ca) channel activity during Ca2+ signaling, which was consistent with subsequent observations.

The intracellular Ca2+ concentration optimal for T cell activation is quite different after ionomycin or CD3 stimulation

The relationship between the initial increase of intracellular Ca2+ concentration ([Ca2+]i) (measured at the single-cell level with an imaging system) and the ensuing proliferation was examined in a human T cell clone stimulated by a phorbol ester in combination with ionomycin, thapsigargin or an anti-CD3 mAb (monoclonal antibody against the CD3 molecule, UCHT1). From the responses to various ionomycin concentrations, one can define a range of [Ca2+]i values (400-900 nM) which appears optimal for T cell proliferation; lower [Ca2+]i values are suboptimal, higher values are cytotoxic. It was then examined if the [Ca2+]i requirements were similar following anti-CD3 stimulation. [Ca2+]i oscillations elicited by a concentration of UCHT1 (1/1,000) optimal for mitogenicity fall precisely within the 400-900 nM range. However, very low concentrations of UCHT1 (1/100,000) which evoke barely detectable [Ca2+]i responses still cause the cells to proliferate. The possibility that the lower [Ca2+]i requirements observed following anti-CD3 stimulation was due to [Ca2+]i oscillations was tested under conditions which prevented the appearance of these oscillations. It turns out that an oscillatory Ca2+ signal is not more mitogenic than a sustained augmentation of [Ca2+]i. Finally, it was examined if overstimulation via CD3 could have toxic consequences similar to those elicited after ionomycin overstimulation. Large transient [Ca2+]i responses can be observed following anti-CD3 stimulation in appropriate conditions, and namely in T cells pretreated with interleukin-2. These [Ca2+]i augmentations are not cytotoxic. A role for the plasmalemmal Ca2+ pump in the prevention of cytotoxicity can be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid inactivation of depletion-activated calcium current (ICRAC) due to local calcium feedback

Rapid inactivation of Ca2+ release-activated Ca2+ (CRAC) channels was studied in Jurkat leukemic T lymphocytes using whole-cell patch clamp recording and [Ca2+]i measurement techniques. In the presence of 22 mM extracellular Ca2+, the Ca2+ current declined with a biexponential time course (time constants of 8-30 ms and 50-150 ms) during hyperpolarizing pulses to potentials more negative than -40 mV. Several lines of evidence suggest that the fast inactivation process is Ca2+ but not voltage dependent. First, the speed and extent of inactivation are enhanced by conditions that increase the rate of Ca2+ entry through open channels. Second, inactivation is substantially reduced when Ba2+ is present as the charge carrier. Third, inactivation is slowed by intracellular dialysis with BAPTA (12 mM), a rapid Ca2+ buffer, but not by raising the cytoplasmic concentration of EGTA, a slower chelator, from 1.2 to 12 mM. Recovery from fast inactivation is complete within 200 ms after repolarization to -12 mV. Rapid inactivation is unaffected by changes in the number of open CRAC channels or global [Ca2+]i. These results demonstrate that rapid inactivation of ICRAC results from the action of Ca2+ in close proximity to the intracellular mouths of individual channels, and that Ca2+ entry through one CRAC channel does not affect neighboring channels. A simple model for Ca2+ diffusion in the presence of a mobile buffer predicts multiple Ca2+ inactivation sites situated 3-4 nm from the intracellular mouth of the pore, consistent with a location on the CRAC channel itself.

Ca2+ influx during agonist and Ins(2,4,5)P3-evoked Ca2+ oscillations in HeLa epithelial cells

1. Histamine-stimulated [Ca2+]i oscillations were studied in > 162 HeLa cells using the techniques of Ca2+ imaging, patch clamp and single-cell indo-1 fluorescence. 2. [Ca2+]i oscillations in HeLa cells were acutely dependent on extracellular Ca2+ and were also blocked by the extracellular addition of Cd2+ (100 microM). The Mn2+ quench technique, using fura-2 fluorescence, demonstrated that agonist-stimulated Ca2+ oscillations were associated with an increase in plasma membrane Mn2+ permeability. However, no cyclic fluctuations in Mn2+ influx were resolved over the period of [Ca2+]i spiking. 3. In whole-cell patch clamped cells an imposed potential of +80 mV was shown to block the thapsigargin-induced Ca2+ influx. Histamine and Ins(2,4,5)P3-induced [Ca2+]i oscillations were tested for the phase dependence on extracellular Ca2+ by rapidly switching the membrane potential to +80 mV, reversibly blocking Ca2+ influx. Ca2+ spikes were abolished by steps to +80 mV made at the point of spike initiation but not by steps made after development of the rapid rising phase of the spike. Steps of membrane potential to +80 mV, for increasing periods of time during the interspike period, increased the latency to the next [Ca2+]i spike by up to a maximum of approximately 150% of the control interspike interval. 4. It is concluded that the extracellular Ca2+ dependence of the histamine-induced [Ca2+]i oscillations is due to a crucial role of Ca2+ influx during spike initiation and an additional important role in setting the interspike interval. The results obtained can be interpreted in terms of a constant stimulated Ca2+ influx during [Ca2+]i spiking.

Calcium entry activated by store depletion in human umbilical vein endothelial cells

We have used the patch clamp technique combined with simultaneous measurement of intracellular Ca2+ to record ionic currents activated by depletion of intracellular Ca(2+)-stores in endothelial cells from human umbilical veins. Two protocols were used to release Ca2+ from intracellular stores, i.e. loading of the cells via the patch pipette with Ins(1,4,5)P3, and extracellular application of thapsigargin. Ins(1,4,5)P3 (10 microM) evoked a transient increase in [Ca2+]i in cells exposed to Ca(2+)-free extracellular solutions. A subsequent reapplication of extracellular Ca2+ induced an elevation of [Ca2+]i. These changes in [Ca2+]i were very reproducible. The concomitant membrane currents were neither correlated in time nor in size with the changes in [Ca2+]i. Similar changes in [Ca2+]i and membrane currents were observed if the Ca(2+)-stores were depleted with thapsigargin. Activation of these currents was prevented and holding currents at -40 mV were small if store depletion was induced in the presence of 50 microM NPPB. This identifies the large currents, which are activated as a consequence of store-depletion, as mechanically activated Cl- currents, which have been described previously [1,2]. Loading the cells with Ins(1,4,5)P3 together with 10 mM BAPTA induced only a very short lasting Ca2+ transient, which was not accompanied by activation of a detectable current, even in a 10 mM Ca(2+)-containing extracellular solution. Also thapsigargin does not activate any membrane current if the pipette solution contains 10 mM BAPTA (ruptured patches). The contribution of Ca(2+)-influx to the membrane current during reapplication of 10 mM extracellular calcium to thapsigargin-pretreated cells was estimated from the first time derivative of the corresponding Ca2+ transients at different holding potentials. These current values showed strong inward rectification, with a maximal amplitude of 1.0 +/- 0.3 pA at -80 mV (n = 8; membrane capacitance 59 +/- 9 pF).(ABSTRACT TRUNCATED AT 250 WORDS)

Na+/Ca2+ exchange-mediated calcium entry in human lymphocytes

Regulation of cytosolic Ca2+ and cytosolic Na+ is critical for lymphocyte cation homeostasis and function. To examine the influence of cytosolic Na+ on Ca2+ regulation in human peripheral blood lymphocytes, Ca2+ entry and cytosolic Ca2+ (measured with fura-2) were monitored in cells in which cytosolic Na+ was increased and/or the Na+ gradient was decreased by reduction of external Na+ concentration. Ouabain-treated cells (0.1 mM for 30 min at 37 degrees C), suspended in Na(+)-free medium, showed a 30-65% increase in Ca2+ uptake compared to cells in 140 mM Na+ medium. Enhanced Ca2+ influx was entirely dependent on ouabain pretreatment and reversal of the Na+ gradient. Na pump inhibition or Na ionophore addition and subsequent exposure to Na(+)-free medium resulted in a sustained elevation of cytosolic Ca2+. As preincubation of cells in Ca(2+)-free medium further enhanced the ouabain-dependent increase in cytosolic Ca2+, the effects of the microsomal Ca(2+)-ATPase inhibitor thapsigargin on Ca2+ influx and cytosolic Ca2+ were studied. Thapsigargin stimulated Ca2+ entry following ouabain pretreatment and reversal of the Na+ gradient; the effects of thapsigargin were retained in the presence of LaCl3, a potent inhibitor of store-dependent calcium influx pathways. These results show lymphocytes demonstrate Na+/Ca2+ exchange activity and suggest the Na+/Ca2+ exchanger modulates cytosolic Ca2+ following intracellular Ca2+ store depletion.