Spectrofluorimetric and image recordings of spontaneous and lectin-induced cytosolic calcium oscillations in Jurkat T cells

Differential recruitment of alpha2beta1 and alpha4beta1 integrins to lipid rafts in Jurkat T lymphocytes exposed to collagen type IV and fibronectin

Collagen type IV (CnIV) and fibronectin (Fn) were used as ligands to study the distribution of alpha(2)beta(1) and alpha(4)beta(1) integrins in low-density, detergent-resistant microdomains (DRM) of Jurkat lymphocytes. CnIV-coated microspheres induced (optical trapping) the redistribution of GM(1)-associated fluorescence from the cell periphery to the area of contact. This was not observed in cells treated with beta-methyl cyclodextrin (MCD). Fn- or bovine serum albumin-coated microspheres did not modify the peripheral distribution of fluorescence. These observations were confirmed by confocal microscopy. Western blot analysis of cells exposed to surfaces coated with CnIV revealed that the alpha(2)-subunit was initially present at low levels in DRM, became strongly associated after 40 min, and returned to basal levels after 75 min. Fn induced a slight recruitment of the beta(1)-integrin alpha(4)-subunit in DRM after 5 and 10 min, followed by a return to basal levels. Neither CnIV nor Fn triggered significant changes in the distribution of the beta(1)-subunit in DRM. Fn- and CnIV-coated microspheres or surfaces coated with these ligands triggered a MCD-sensitive mobilization of Ca(2)(+). MCD did not alter the state of the Ca(2)(+) reserves. The differential distributions of the alpha(2)beta(1) and alpha(4)beta(1) integrins in DRM may provide one additional step in the regulation of outside-in signaling involving these integrins.

VCAM-1 is internalized by a clathrin-related pathway in human endothelial cells but its alpha 4 beta 1 integrin counter-receptor remains associated with the plasma membrane in human T lymphocytes

Lymphocyte extravasation involves a step(s) of de-adhesion to allow trans- and subendothelial migration in response to inflammatory signals. We show here that ligated VCAM-1 was rapidly internalized (t1/2 14.5 min) in ECV 304 endothelial cells and in TNF-alpha-primed human umbilical vein-derived endothelial cells (t1/2 11.2 min). The process required energy (ATP), intracellular Ca2+, an intact cytoskeletal network and active protein kinases. The internalization of VCAM-1 involved a clathrin-dependent pathway based on the observations that 1) it was inhibited in cells treated with lysosomotropic agents or with a hypertonic concentration of sucrose, and 2) internalized VCAM-1 colocalized with clathrin. In contrast, the cross-linked alpha 4 beta 1 integrin counter-receptor of VCAM-1 remained associated with the plasma membrane of purified peripheral T and Jurkat cells. Our results suggest a model where VCAM-1 would initially participate in the retention of T cells to the endothelium by binding alpha 4 beta 1 integrin. Lymphocyte de-adhesion would be facilitated as a result of the internalization of VCAM-1. The persistent cell surface expression of alpha 4 beta 1 integrin would allow the migrating T cells to interact with and receive signal(s) from its fibronectin ligand of the extracellular matrix.

Clustering the adhesion molecules VLA-4 (CD49d/CD29) in Jurkat T cells or VCAM-1 (CD106) in endothelial (ECV 304) cells activates the phosphoinositide pathway and triggers Ca2+ mobilization

Ligation of very late antigen (VLA)-4 (alpha 4 beta 1 integrin) with a cross-linked anti-alpha 4 subunit monoclonal antibody (mAb) triggered a biphasic Ca2+ response in Jurkat cell populations and in peripheral human lymphocytes. Cross-linking vascular cell adhesion molecule (VCAM)-1 (the counter-receptor of VLA-4) in ECV 304 endothelial cells generated a biphasic Ca2+ response. Tumor necrosis factor-alpha-primed human umbilical cord vascular endothelial cells also responded to the cross-linked mAb with a biphasic Ca2+ profile. Ligated VLA-4 (Jurkat cells) or VCAM-1 (ECV 304) stimulated the production of myo-inositol 1,4,5-trisphosphate. ECV 304 cells induced a biphasic Ca2+ response in Fura2-loaded Jurkat cells, whereas a transient response was observed when Jurkat cells were added to Fura2-loaded ECV 304 cells. The Ca2+ responses in these experiments involved VLA-4/VCAM-1 interactions since they were significantly reduced (approximately 80%) by prior treatment of the target cells with the relevant noncross-linked mAb. Close contact between the cells triggered mutual Ca2+ signaling as shown by spectrofluorimetric and confocal microscopy time-dependent recordings. Fibronectin and its CS-1 fragment (V25) triggered a sustained Ca2+ response in Jurkat cells (confocal microscopy). Our results suggest that the VLA-4 and VCAM-1 adhesion molecules can transduce a signal that involves activation of the phosphoinositide pathway and the mobilization of Ca2+.

A caffeine/ryanodine-sensitive Ca2+ pool is involved in triggering spontaneous variations of Ca2+ in Jurkat T lymphocytes by a Ca(2+)-induced Ca2+ release (CICR) mechanism

Caffeine and ryanodine triggered an increase in [Ca2+]i (73 +/- 22 and 61 +/- 18 nM, respectively) in Jurkat cell populations that was independent of external Ca2+. In individual cells, caffeine and ryanodine induced Ca2+ spikes. Jurkat cell populations initially exposed to caffeine did not respond further to ryanodine and vice versa, suggesting an overlap of the Ca2+ pool that was contained within the thapsigargin-sensitive Ca2+ reserve. [3H]ryanodine bound to a single class of sites of Jurkat microsomes (KD, 18.4 +/- 5.7 nM; Bmax, 24.3 +/- 7.7 fmol/mg protein). Photolytic release (Nitr5) of caged Ca2+ induced a time-dependent increase of Ca2+ in individual Jurkat cells. The profile of the release of Ca2+ was characterized, 1) by a kinetic (0.55 +/- 0.07 nM s-1) slower than the Ca2+ response to caffeine (3.93 +/- 0.66 nM s-1) or to ryanodine (3.96 +/- 0.94 nM s-1), 2) by a release of Ca2+ (131 +/- 43 nM) that slowly returned to baseline and during which low amplitude oscillations were present (room temperature) or Ca2+ spikes (37 degrees C) and, 3) by a lack of dependency on an influx of Ca2+. Inhibitors of CICR (ruthenium red and 1-octanol) prevented the photolysis-dependent increase in [Ca2+]i but not the InsP3-dependent Ca2+ response. Our data suggest that Jurkat T cells possess at least two Ca2+ pools, one that is sensitive to InsP3 and one that is insensitive. These two Ca2+ pools may be involved in a CICR that generates spontaneous Ca2+ spikes and oscillations in these cells.

The influence of 50-Hz magnetic fields on cytoplasmic Ca2+ oscillations in human leukemia T-cells

We have studied the effects of 50-Hz 100-microT rms magnetic fields on intracellular Ca2+ concentration in the Jurkat T lymphocyte variant E6.1 using fluorescent probes Indo-1 and Fura-2. We found, however, that the pattern of intracellular Ca2+ fluctuations also depended on the agent used for cell attachment, in our case the polypeptide poly-L-lysine. In order to isolate possible effects of magnetic field exposure from those of poly-L-lysine, the action of polypeptide on cytosolic Ca2+ was studied as well. It was found that a 10(-7)% concentration of polypeptide triggered prolonged Ca2+ spiking. Higher (10(-4)%) concentrations induced rapid increases in intracellular Ca2+ followed by high, unstable Ca2+ levels. The response of these cells to the monoclonal antibody anti-CD3 was also inhomogeneous, similar to one caused by poly-L-lysine. The effect of magnetic field exposure was studied on cells initially exhibiting (1) non-oscillating, low Ca2+ concentration and (2) prolonged Ca2+ concentration oscillations. In case (1) the result was negative. In case (2), statistically significant changes were found: the oscillation amplitude was reduced on average by 30%, and the frequency composition was shifted towards higher frequencies.

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.

Calcium signalling in individual T-cells measured by confocal microscopy

Laser-scanning confocal microscopy was used in conjunction with a highly fluorescent Ca2+ indicator fluo-3 to visualize real-time alterations in the intracellular Ca2+ concentration ([Ca2+]i) in individual living Jurkat T-cells during the first minutes of activation by phytohaemagglutinin (PHA) at the physiological temperature (37 degrees C). With a delay of 30-120 s, PHA induced a strong [Ca2+]i peak in the micromolar range (1-3 microM). The rise in [Ca2+]i lasted for 1-2 minutes, and was followed by a sustained plateau of elevated [Ca2+]i in the 0.2-0.5 microM range. Some cells (10-20%) responded with additional asynchronous 0.5-1.5 microM peaks during the plateau phase. These oscillations continued for 10-20 minutes. The spans of the peaks ranged from 30 to 100 s, intervals between peaks varied from 60 to 300 s. It was shown that the initial [Ca2+]i peak was associated with Ca2+ mobilisation from internal sources, whereas the plateau was maintained by an influx of Ca2+ from external medium. In K(+)-rich medium or in the presence of quinine, a K+ channel blocker, no secondary response to PHA-activation characterised by an elevated plateau was observed. The data suggest that the Ca2+ influx was dependent on the membrane potential and/or the extracellular K(+)-concentration. Optical sectioning showed that the intracellular Ca2+ distributed almost homogeneously throughout the cell volume both in control and in PHA-stimulated cells including those exhibiting Ca2+ oscillations. This suggests that Ca2+ signals are localized not only in cytoplasm at the cell plasma membrane but can be also transferred directly into the nucleus.

Ca2+ mobilization in physiologically stimulated single T cells gradually increases with peptide concentration (analog signaling)

We have investigated Ca2+ mobilization in single T cells stimulated with their physiological ligand, i.e. antigenic peptide bound to major histocompatibility complex (MHC) molecules on antigen-presenting cells (APC). Fibroblasts expressing I-Ed class II molecules were pulsed with a peptide derived from the lambda 2(315) immunoglobulin light chain. Onto such antigen-pulsed fibroblasts were sedimented cloned Th1 cells loaded with Fura-2. Changes in cytosolic Ca2+ concentration in single T cells were continually monitored by use of an imaging system based on fluorometry. Ca2+ mobilization was both peptide-specific and MHC-restricted. Within seconds of the initial APC-T cell contact, a Ca2+ spike could be observed. The Ca2+ response gradually declined over a 25-min period, during which oscillations were noted. Various parameters characterizing the magnitude of the Ca2+ response (latency, increase rate, max and mean Ca2+ increase, frequency and period of oscillations) all correlated with the amount of peptide used for pulsing the fibroblasts. Thus, Ca2+ mobilization in single T cells appears not to be an all or none phenomenon. Rather, activation is incremental (analog signaling), the degree of Ca2+ mobilization probably being related to the number of stimulatory peptide-MHC complexes on the surface of the APC. The extent of calcium mobilization and lymphokine production (interleukin (IL)-2, IL-3, interferon-gamma) correlated, at least at the population level.

Intracellular calcium dependence of gene expression in single T lymphocytes

In T lymphocytes, intracellular Ca2+ concentration ([Ca2+]i) rises within seconds of T-cell antigen-receptor stimulation and initiates the synthesis and secretion of interleukin 2, a cytokine essential for T-cell proliferation and the immune response. Using video-imaging techniques, we tracked [Ca2+]i signals in individual T cells and measured subsequent expression of a beta-galactosidase reporter gene (lacZ) controlled by the NF-AT element of the interleukin 2 enhancer. [Ca2+]i spikes elicited by monoclonal antibody binding to the CD3 epsilon subunit of the T-cell receptor were positively correlated with gene expression, but varied widely between individual cells and were therefore difficult to relate quantitatively to lacZ expression. The [Ca2+]i dependence of NF-AT-regulated gene expression was determined by elevating [Ca2+]i with either thapsigargin or ionomycin and then "clamping" [Ca2+]i to various, stable levels by altering either extracellular [Ca2+] or extracellular [K+]. Raising [Ca2+]i from resting levels of 70 nM to between 200 nM and 1.6 microM increased the fraction of cells expressing lacZ, with Kd approximately 1 microM. Activation of protein kinase C enhanced the [Ca2+]i sensitivity of gene expression (Kd = 210 nM), whereas stimulation of protein kinase A inhibited [Ca2+]i-dependent gene expression. The experiments described here provide single-cell measurements linking a second messenger to gene expression in individual cells.

Model for receptor-controlled cytosolic calcium oscillations and for external influences on the signal pathway

The external stimulation of many cells by a hormone, for example, often leads to an oscillating cytosolic calcium concentration. This periodic behavior is now designated the cytosolic calcium oscillator. A theoretical model is presented that describes this behavior on the basis of inositol(1,4,5)trisphosphate-induced calcium oscillations. In contrast to other models only a single positive feedback loop is taken into account to obtain oscillations. The model includes important innovations compared to other approaches. It includes the contribution of extracellular calcium and its modification after the stimulation of the cell. Furthermore, the signal pathway that leads to cytosolic calcium oscillations is described in more detail than in other models. This enables investigations on the influence of additional parameters like external electromagnetic fields on the signal transduction pathway. The model and the calculations are based on the theory of nonlinear self-sustained oscillators.

Cytosolic calcium oscillations induced by hepatocyte growth factor (HGF) in single fura-2-loaded cultured hepatocytes: effects of extracellular calcium and protein kinase C

Hepatocyte growth factor (HGF) induced the periodic fluctuations of cytosolic calcium concentration ([Ca2+]i) in primary cultured rat hepatocytes, which were dependent on extracellular calcium. The HGF-induced [Ca2+]i oscillations were suppressed by the pretreatment with phorbol 12-myristate 13-acetate (PMA). Administration of PMA during oscillations also caused their blockade, but the subsequent addition of protein kinase C (PKC) inhibitor H-7 reversed the inhibitory effects of PMA, thereby resulting in the resumption of the oscillatory responses. Moreover, the prior exposure to H-7 caused apparent increases in [Ca2+]i spike peaks elicited by HGF. These results suggest a negative modulation via PKC in HGF-induced repetitive [Ca2+]i transients. The absence of HGF-induced oscillations after the thapsigargin treatment indicates that the agonist-sensitive intracellular Ca2+ pool plays a crucial role in the [Ca2+] oscillations.

Effects of ACTH and angiotensin II on cytosolic calcium in cultured adrenal glomerulosa cells. Role of cAMP production in the ACTH effect

We have used microspectrofluorometry and video imaging techniques in order to study and compare the changes in intracellular calcium concentrations [( Ca2+]i) of individual Fura-2 loaded glomerulosa cells cultured for three days and stimulated either with angiotensin II (AT), K+, or adrenocorticotropin (ACTH). As previously demonstrated for freshly isolated cells, K+ ion induces an immediate increase in [Ca2+]i, although AT induces a biphasic response, characterized by an initial transient spike, followed by a sustained plateau. In this study, we demonstrate, for the first time, that ACTH is able to induce a [Ca2+]i increase in cultured glomerulosa cells from rat and bovine sources. Moreover, it is clear that the pattern of [Ca2+]i increase elicited by ACTH is different from that observed with AT. In most cases, addition of ACTH leads to a slow increase in [Ca2+]i after a long latency period ranging from 10-15 min, which could be correlated to cAMP time-production. The present results show that: (a) in the absence of extracellular Ca2+, ACTH does not increase [Ca2+]i; (b) the response develops slowly and cases immediately after [Ca2+]e depletion or addition of calcium channel blockers, such as nifedipine or omega-conotoxin; (c) the addition of the calcium channel agonist Bay K 8644 enhances the ACTH response; (d) the cAMP analog, 8-Br-cAMP, induces an increase in [Ca2+]i similar to that observed with ACTH, which is also dependent of the presence of calcium in the extracellular medium; (e) time-production of ACTH-induced cAMP follows quite well the increase in [Ca2+]i; (f) Bay K 8644 also enhances the 8-Br-cAMP induced increase in [Ca2+]i; and (g) ACTH-induced Cai response is inhibited by the specific protein kinase A blocker, HA1004. These observations, combined with previous results obtained on the effects of ACTH on calcium currents and action potentials, suggest that the [Ca2+]i increase induced by ACTH results from a calcium influx through dihydropyridine and omega-conotoxin sensitive calcium channels, which need to be phosphorylated by cAMP for full activation. The use of video-imaging techniques has allowed us to examine the spatial distribution of changes in [Ca2+]i in single cells. The ability to simultaneously record images of a number of cells confirm the heterogeneity of cellular responses, and corroborate results obtained through photocounting only. Our results indicate that ACTH initially increases [Ca2+]i locally beneath the cell membrane and throughout the cell thereafter, whereas angiotensin II elicits a more prominent effect in certain regions of the cell and eventually extends to the entire cell surface.