Receptor-mediated calcium signal playing a nuclear third messenger in the activation of antigen-specific B cells

Regulation of nuclear calcium concentration

Transient increases in nuclear calcium concentration have been shown to activate gene expression and other nuclear processes. It has been suggested that nuclear calcium signals are controlled by a mechanism that is independent of calcium signalling in the cytosol. This would be possible if calcium diffusion is slow and a separate calcium release mechanism is localized to the nuclear region. Alternatively, the nuclear envelope could act as a diffusion barrier for calcium ions released either inside or outside the nucleus. It has also been proposed that inositol 1,4,5-trisphosphate (InsP3) can be generated inside the nucleus and that there are calcium release channels in the inner membrane of the nuclear envelope. Most of the experimental evidence supporting these hypotheses is based on the calibration of nuclear and cytosolic calcium concentrations. However, recent studies suggest that the local calibration of calcium indicators may not be accurate. We propose that nuclear calcium signals can be investigated by a different approach that does not rely on accurate calibration of indicators. We have developed calcium indicators that minimize facilitated calcium diffusion and are localized to either the nucleus or the cytosol. Using the diffusion coefficient of calcium ions, and measuring the delay between cytosolic and nuclear calcium increases, we show that the nuclear envelope is not a substantial barrier for calcium ions in PC12 (phaeochromocytoma) cells. This suggests that nuclear and cytosolic calcium signals equilibrate rapidly in these cells.

[Functional fluctuation]

The discovery of the double-helical structure of DNA, the elucidation of the genetic code, and the determination of the three-dimensional structure of several proteins are some of the outstanding achievements of biochemistry and life sciences in the latter half of the last century. Proteins play key roles in almost all the biological processes and the biological function of a protein depends on its conformation which is defined as the three-dimensional arrangement of the atoms of a molecule. The three-dimensional structure, however, is not rigid but fluctuated. Structural fluctuation plays an important role in bio-macromolecules. How about "functional fluctuation" in biological systems? The present review proposes that functional fluctuation is also very important for understanding the mechanism of supramolecules, biological processes in living cells, and the interaction between biological systems. This new theme is pretty well supported by our recent experiments for neuro-immune crosstalk, gene transfection with cationic liposomes, and cell signaling in embryonic stem cells.

Cleavage of integrin by mu-calpain during hypoxia in human endometrial cells

PROBLEM

The distribution and activation of mu-calpain and possible cleavage of integrin in human endometrial cells under hypoxic condition were investigated.

METHOD OF STUDY

Human endometrial epithelial and stromal cells were subjected to hypoxia, and subsequently used for immunostaining and western blot analysis.

RESULTS

The proform of mu-calpain was detected in the cytoplasm of normal cells, and displayed a substantial decrease after hypoxia. Conversely, the active form of mu-calpain was not detected in normal cells, but was abundant after hypoxia. The cytoplasmic domain of integrin beta3 was also detected in the cytoplasm of endometrial cells. Western blot analysis confirmed that both the proform of mu-calpain and the integrin beta3 cytoplasmic domain decreased during hypoxia.

CONCLUSIONS

Mu-calpain is activated in human endometrial cells during hypoxia and that subsequent cleavage of the integrin beta3 cytoplasmic domain may give some adverse effects to the function of human endometrium.

[Confocal laser scanning microscopy to study molecular mechanism of mast cell activation]

In the immune system, mast cells are a key cell type in the pathogenesis of immunoglobulin E (IgE)-dependent hypersensitivity reactions. Engagement of the high-affinity IgE receptors by multivalent antigens initiates the downstream activation of signal-transducing enzymes and evokes degranulation and cytokine production via an increase in the intracellular Ca2+ concentration. In addition, mast cells also play a prominent role in non-IgE-mediated hypersensitivity reactions. Mast cells are closely apposed to nerves in vivo and are likely to be regulated functionally by nerves. However, the molecular mechanisms for mast cell activation in an IgE-dependent and -independent manner have not been fully clarified. Confocal laser scanning microscopy has played an essential role in cell biology by allowing visualization of specific intracellular signaling molecules with high spatiotemporal resolution in living cells. We have studied intracellular movements of Ca2+ using a specific fluorescent probe and several types of signaling molecules using derivatives of green fluorescent protein in a living single mast cell using a microscopic strategy. We here describe our imaging analysis of the calcium signals to the nucleus, the movement of secretory granules in the degranulation process, and the nucleocytoplasmic shuttling of mitogen-activated protein kinase in mast cells. Further, we demonstrate that direct communication between mast cells and nerves occurs. These findings provide useful information from a new perspective to understand the molecular mechanisms of allergic reaction and inflammation.

Live Cell Imaging to Study Signaling Molecules in Allergic Reactions

Mast cells are widely distributed throughout the body, predominantly near blood vessels and nerves, and express effector functions in allergic reactions, inflammatory diseases, and host defense. The activation of mast cells results in secretion of the preformed chemical mediators in their granules by a regulated process of exocytosis and leads to synthesis and secretion of lipid mediators and cytokines. Their soluble factors contribute to allergic inflammation. Mast cells are associated with hypersensitivity reactions, not only in the classical immunoglobulin E (IgE)-dependent mechanism but also in an IgE-independent manner. In particular, investigations of potential anatomical and functional interactions between mast cells and the nervous system have recently attracted great interest. To understand these molecular mechanisms in mast cell activation, the ability to visualize, track, and quantify molecules and events in living mast cells is an essential and powerful tool. Recent dramatic advances in imaging technology and labeling techniques have enabled us to carry out these tasks with high spatiotemporal resolution using confocal laser scanning microscopes, green fluorescent protein and its derivatives, and image analysis systems. Here we review our investigations of the dynamic processes of intracellular signaling molecules, cellular structure, and interactions with neurons in mast cells to provide basic and valuable information for allergy and clinical immunology using these new imaging methods.

Bi-directional relationship of in vitro mast cell-nerve communication observed by confocal laser scanning microscopy

Communication between nerves and mast cells is a prototypic demonstration of neuro-immune interaction. Recently, we used an in vitro co-culture approach comprising cultured murine superior cervical ganglia (SCG) and rat basophilic leukemia (RBL) cells to study this interaction. Previously, we concentrated mainly on the activation signal from neurites to mast cells (RBL). However, it is proposed that mast cell-nerve communication is not a one-sided relationship but a bi-directional one. In the present work, we studied the communication from mast cells to neurites. We observed that binding of anti-IgE receptor antibodies to mast cells increases calcium ion concentration [Ca2+]i in SCG neurites. This indicates that mast cell-nerve communication is bi-directional. Confocal fluorescence microscopic images indicated that [Ca2+]i in neurites increased after an increase of [Ca2+]i in mast cells. The lag-time of neurite activation was several times longer than that of mast cell activation. The correlation coefficient between the lag-times for mast cell and nerve activation was calculated to be 0.81. In addition, the fluorescence images showed that calcium signals in SCG neurites were able to extend to a long distance (100-200 microm) from the site where mast cells (RBL) attached to neurites.

Atomic force microscopy to study direct neurite-mast cell (RBL) communication in vitro

Communication between nerves and mast cells is a prototypic demonstration of neuroimmune interaction. We used an in vitro co-culture approach comprising cultured murine superior cervical ganglia (SCG) and rat basophilic leukemia (RBL-2H3) cells. Atomic force microscopy (AFM) showed how neurites attached to a pseudopodium or a cell body of an RBL cell. After stimulation of SCG neurites with bradykinin or scorpion venom, RBL cells attached to neurites spread and flattened, and several discharged granules (0. 5-1.0 microm in diameter) were found on the surface of the RBL cells. A neurokinin (NK)-1 receptor (i.e. substance P receptor) antagonist prevented the RBL degranulation. The results showed that activation of the SCG neurites with bradykinin or scorpion venom was able to elicit degranulation in RBL cells which were attached to neurites.

Direct Neurite-Mast Cell Communication In Vitro Occurs Via the Neuropeptide Substance P

Communication between nerves and mast cells is a prototypic demonstration of neuroimmune interaction. However, whether mast cell activation occurs as a direct response to neuronal activation or requires an intermediary cell is unclear. Addressing this issue, we used an in vitro coculture approach comprising cultured murine superior cervical ganglia and rat leukemia basophilic cells (RBLs; possesses properties of mucosal-type mast cells). Following loading with the calcium fluorophore, Fluo-3, neurite-RBL units (separated by <50 nm) were examined by confocal laser scanning microscopy. Addition of bradykinin, or scorpion venom, dose-dependently elicited neurite activation (i.e., Ca2+ mobilization) and, after a lag period, RBL Ca2+ mobilization. Neither bradykinin nor scorpion venom had any direct effect on the RBLs in the absence of neurites. Addition of a neutralizing substance P Ab or a neurokinin (NK)-1 receptor antagonist, but not an NK-2 receptor antagonist, dose-dependently prevented the RBL activation that resulted as a consequence of neural activation by either bradykinin or scorpion venom. These data illustrate that nerve-mast cell cross-talk can occur in the absence of an intermediary transducing cell and that the neuropeptide substance P, operating via NK-1 receptors, is an important mediator of this communication. Our findings have implications for the neuroimmune signaling cascades that are likely to occur during airways inflammation, intestinal hypersensitivity, and other conditions in which mast cells feature.

Nuclear translocation of green fluorescent protein-nuclear factor kappaB with a distinct lag time in living cells

A highly fluorescent mutant form of the green fluorescent protein (GFP) has been fused to the human nuclear factor kappaB (NF-kappaB) p50 and p105 (p50/IkappaB gamma), a precursor protein of NF-kappaB p50. GFP-p50 and GFP-p105 were expressed in monkey COS-7 cells and human HeLa cells. Translocation of these chimeric proteins was observed by confocal laser scanning microscopy. GFP-p50 (without IkappaB gamma) in the transfected cells resided in the nucleus. On the other hand, GFP-p105 (GFP-p50 with IkappaB gamma) localized only in the cytoplasm before stimulation and translocated to the nucleus with stimulant specificity similar to that of native NF-kappaB/IkappaB. In addition, the translocation of NF-kappaB to the nucleus had a distinct lag time (a quiescent time) in the target cells. The lag time lasted 10-20 min after stimulation with hydrogen peroxide or tumor necrosis factor alpha. It was suggested that this might be due to the existence of a limiting step where NF-kappaB is released from NF-kappaB/IkappaB by the proteasome.

Mercuric chloride induces increases in both cytoplasmic and nuclear free calcium ions through a protein phosphorylation-linked mechanism

The mechanism of the lymphocyte stimulatory action of sulfhydryl group-reactive mercuric ions was studied with respect to its potential ability to induce a protein tyrosine phosphorylation-linked signal for mobilization of free Ca2+ into cytoplasm and nucleus of the cell. Exposure of human leukamic T cell line (Jurkat) cells to high (1 mM) and low (0.01 mM) concentrations of HgCl2 induced tyrosine phosphorylation of multiple proteins in a concentration-dependent manner. Confocal microscopy directly visualized the time course localization of Ca2+ inside the cells after exposure to HgCl2. The onset and level of Ca2+ mobilization following HgCl2 exposure were in parallel to those of protein tyrosine phosphorylation. Interestingly, by either concentration of HgCl2, Ca2+ was mobilized in both cytoplasm and nucleus almost simultaneously, and the level of Ca2+ mobilization in the nucleus was more than that in the cytoplasm. All the HgCl2-mediated Ca2+ mobilization was prevented by addition of protein kinase inhibitor staurosporin prior to HgCl2. These results suggest that heavy metal stress triggers a protein tyrosine phosphorylation-linked signal that leads to a nuclear event-dominant Ca2+ mobilization.

Spatiotemporal analysis of calcium dynamics in the nucleus of hamster oocytes

1. Subcellular Ca2+ dynamics inside and around the nucleus of immature hamster oocytes were analysed with confocal Ca2+ imaging. 2. The ratio value between emission intensity of two injected fluorescent Ca2+ indicators, Calcium Green and Fura Red, was almost uniform over the entire oocyte, suggesting that nucleoplasmic Ca2+ concentration ([Ca2+]n) is comparable to cytoplasmic Ca2+ concentration ([Ca2+]c) at the resting state. 3. When Ca2+ was iontophoretically injected into the nucleoplasm or the perinuclear cytoplasm, it diffused across the nuclear envelope (NE), and perinuclear [Ca2+]c and [Ca2+]n reached the same level within 2 s, although the NE worked as a weak but detectable barrier for Ca2+ diffusion. 4. Inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release from the NE through the inner membrane was not detected, even when a large amount of IP3 was delivered in close proximity to the inner nuclear membrane. 5. When an oocyte was uniformly stimulated by photolysis of caged IP3, a Ca2+ rise was initiated in the perinuclear cytoplasm. The [Ca2+]n rise was always delayed with respect to, but rapidly equilibrated with, the [Ca2+]c rise. 6. Clusters of the endoplasmic reticulum were located in the perinuclear cytoplasm and served as the trigger zone of IP3-induced Ca2+ release. 7. The results indicate that the [Ca2+]n rise occurs as the consequence of the influx of Ca2+ which was released in the perinuclear cytoplasm, not Ca2+ release from NE to the nucleoplasm.

Quantitative and realtime correlation between receptor aggregation and intracellular calcium signal transduction

Quantitative correlation between intracellular calcium signals and hapten density or molecular size of antigens was studied for two cell lines; hapten-specific murine B cells (TP67.21) and rat basophilic leukemia cells (RBL-2H3) with hapten-specific IgE. Magnitude of the induced calcium signal in both cells exhibited the same dependence on hapten density of antigen molecules and there existed an optimal hapten density which induced the maximum amount of calcium signal for both cells. However, they responded differently to antigens of various molecular size. In contrast to TP67.21 cells which showed larger response to larger antigen molecules, RBL-2H3 cells showed the largest response to the smallest antigen. This may possibly suggest that there exists an optimal structure of receptor aggregates for each cell. Calcium signal induced in each cell by multivalent antigen was rapidly abrogated by addition of excess hapten and this abrogation occurred both in transmembrane influx and the release from intracellular stores. We directly observed the mobilization of receptor molecules during this calcium signal abrogation at single cell level by using two fluorescent calcium probes, whose fluorescence wavelength ranges have least overlap, and confocal microscopy. During this abrogation, large clusters of receptor molecules were not affected by hapten molecules. We, therefore, conclude that these large clusters are inactive in the induction of calcium signal and smaller clusters of receptor molecules are necessary for calcium signal induction.

Single-cell observation of calcium signals in T cells and antigen-presenting cells during antigen presentation

Intracellular calcium ion mobilization in T-cell hybridomas and antigen-presenting cells (APC) during the interaction was observed using confocal fluorescence microscopy. No calcium signal was detected in non-activated T-cell hybridomas by antigen presentation. However, in activated T-cell hybridomas, intracellular calcium ion concentration rapidly increased by antigen presentation and thereafter apoptosis was induced. On the contrary, during the interaction with T-cell hybridomas, calcium signal was induced in APCs irrespective of the activation of T-cell hybridomas. Chemical modification of APCs with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, which is known to induce T-cell unresponsiveness during antigen presentation, inhibited cap formation of surface MHC class II molecules and suppressed calcium signals during the interaction with T-cell hybridomas.

Triggering of calcium signals in antigen-specific B-cells on the supported lipid monolayers

Using supported lipid monolayers we have studied here calcium signals in antigen-specific B-cells (TNP-specific B-cell hybridomas, TP67.21) triggered by lipid hapten (TNP-Cap-DPPE). Stimulation of the B-cell hybridomas (TP67.21) with a supported DPPC monolayers containing 1% TNP-Cap-DPPE increased the intracellular free calcium ion concentration [Ca2+]i in B-cells. None of B-cells responded to a DPPC monolayers without lipid hapten (TNF-Cap-DPPE). Triggering for calcium signals was clearly dependent on the fluidity of the lipid monolayers. Solid DPPC and DSPC monolayers triggered the calcium signals more efficiently than the fluid DMPC monolayers did. These calcium signals became apparently more efficient in the presence of cholesterol. All of these results suggested that the rigidity of cross-linking for antigen receptors (mIgM) may be a crucial role for triggering calcium signals in B-cells.

Confocal fluorescence microscopy for studying thapsigargin-induced bivalent-cation entry into B cells

We studied thapsigargin-induced bivalent-cation entry into antigen-specific B cells (TP67.21) with a confocal fluorescence microscope. Confocal fluorescence images of fluo-3-loaded B cells showed that thapsigargin-stimulated Ca2+ signals were transferred not only to the cytoplasm but also to the nucleus. In the absence of external Ca2+ ions, the free Ca2+ concentrations both in the cytosol and in the nucleus declined to basal levels by 5 min after addition of thapsigargin. However, subsequent addition of Ca2+ in the external medium made the fluo-3 (fura-2) fluorescence intensity rise, reflecting the fact that Ca2+ accumulated again in the nucleus as well as in the cytoplasm. Then, we added Ba2+ and Mn2+ instead of Ca2+, because Ba2+ and Mn2+ are known to enter via Ca2+ channels. The addition of Ba2+ and Mn2+ in the external medium quenched the fluo-3 fluorescence both in the nucleus and in the cytoplasm of B cells. This suggested the possibility that the increase in intranuclear Ca2+ after thapsigargin stimulation may come from the cytoplasm, not from the nuclear stores.

Thapsigargin-induced nuclear calcium signals in rat basophilic leukaemia cells

By a confocal fluorescence microscope with an argon-ion laser (488 nm) and a He-Cd laser (325 nm) we have studied thapsigargin-induced calcium signals in individual rat basophilic leukaemia (RBL-2H3) cells. In the presence or absence of external calcium ions, thapsigargin-induced calcium signals were transferred to the nucleus as well as to the cytoplasm of RBL-2H3 cells. The calcium signals were generally much stronger in the nucleus than in the cytoplasm. However, some of the RBL-2H3 cells had apparently reduced nuclear calcium signals. They had a basophil-like bilobed (multilobed) nucleus, although most RBL-2H3 cells had a mast-cell-like monolobed nucleus. In the cells with a bilobed nucleus, IgE-receptor-mediated calcium signals were neither transferred to the nucleus nor to the cytoplasm. The results gave a new insight into the understanding of the mechanism of the nuclear calcium signals in RBL-2H3 cells.

Relationship between [Ca2+] changes in nucleus and cytosol

The free calcium concentration in nucleus ([Ca2+]n) and in cytoplasm ([Ca2+]c) of single cells were estimated by confocal laser microscopy using the Ca(2+)-indicator Indo-1. It is shown that in various cell types a nucleo-cytosolic Ca(2+)-gradient is present at rest and during stimulation. The direction and the extent of the nucleo-cytosolic Ca(2+)-gradient may vary with the cell type, differentiation status, phosphorylation conditions and also with the type of agonist. Evidence is given for the role of extra- and intranuclear storage sites as well as for Ca(2+)-influx. Finally potential artefactual interference with the measurements is discussed.