Antigen receptor-mediated calcium signals in B cells as revealed by confocal fluorescence microscopy

Active pharmaceutical ingredients and mechanisms underlying phasic myometrial contractions stimulated with the saponin extract from Paris polyphylla Sm. var. yunnanensis used for abnormal uterine bleeding

BACKGROUND

Total steroidal saponins of Paris polyphylla Sm. var. yunnanensis (TSSP) have been widely used in China for the treatment of abnormal uterine bleeding (AUB). But until now, the main active constituents and the mechanisms underlying the pharmacological actions on uterine activity have not been described.

METHODS

Total steroidal saponins were extracted with EtOH and purified by chromatography. In vitro isometric contraction studies were performed using myometrial strips from estrogen-primed or pregnant rats. Intracellular calcium was monitored under a confocal microscope using Fluo-3 AM-loaded myometrial cells.

RESULTS

TSSP dose-dependently induced phasic myometrial contractions in vitro. Experiments with calcium channel blockers or kinase inhibitors demonstrated that the TSSP-stimulated myometrial contraction was mediated by an increase in [Ca(2+)](i) via influx of extracellular calcium and release of intracellular calcium. Through bioassay-guided separation, it was found that total spirostanol saponins exhibited contractile activity in myometrium and Pennogenin-3-O-alpha-L-arabinofuranosyl(1-->4)[alpha-L-rhamnopyranosyl(1-->2)]-beta-D-glucopyranoside (PARG) was identified as the active ingredient of TSSP. Furthermore, the contractile response of rat myometrium to PARG was significantly enhanced with advancing pregnancy.

CONCLUSIONS

These data provide evidence that myometrial contractility stimulated by TSSP results from [Ca(2+)](i) increase and supports the possibility that some spirostanol gylcosides may represent a new type of contractile agonist for the uterus.

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.

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.

Heme oxygenase-1 (HO-1) protein induction in a mouse model of asthma

BACKGROUND AND OBJECTIVE

Carbon monoxide (CO) is known to be present in measurable quantities in the exhalation of asthmatic patients. Corticosteroid treatment resulted in a decrease in exhaled CO levels in asthmatic patients, raising the possibility that an increase in exhaled CO concentration reflects inflammation of the asthmatic airway. Heme oxygenase-1 (HO-1) protein, also called HSP32, is the rate-limiting enzyme in the catabolism of heme to biliverdin, free iron and CO. However, it is unknown whether an expression of HO-1 within the lung tissue is related to allergic airway inflammation. We studied the expression of HO-1 in lung tissue and bronchoalveolar lavage cells in a mouse model of asthma.

METHODS

Ovalbumin (OVA)-sensitized C57BL/6 mice were challenged with aerosolized OVA. HO-1 positive cells were identified by immunostaining in lung tissue and bronchoalveolar lavage fluid (BALF) after the challenge.

RESULTS

HO-1 positive cell numbers increased in the subepithelium of the bronchi after OVA challenge. In cytospin preparations from BALF after OVA challenge, HO-1 was localized to alveolar macrophages. Inside the macrophages, HO-1 reactivity was expressed in the cytoplasm, and the perinuclear region in particular.

CONCLUSION

The expression of HO-1 is increased within the lung tissue in allergic airway inflammation. Measurement of HO-1 activity may be clinically useful in the management of asthma.

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.

Nuclear and cytosolic calcium changes in osteoclasts stimulated with ATP and integrin-binding peptide

Cytosolic calcium modulates the activity of osteoclasts, large multinucleate cells that resorb bone. Nuclear events, such as gene transcription, are also calcium-regulated in these cells, and fluorescence imaging has suggested that calcium signals produced by some stimuli are specifically targeted to, or amplified within, osteoclast nuclei. We used two alternative techniques of dye loading to examine the changes of intracellular calcium induced in rat osteoclasts by three stimuli. Osteoclasts loaded with the calcium indicator Fura-2 by the acetoxymethyl (AM) ester technique appeared to display marked nuclear calcium amplification. During stimulation with integrin-binding peptides, ATP, or high extracellular calcium, fluorescence ratios recorded from the nuclei rose higher than did ratios recorded from extranuclear regions. In contrast, nuclear calcium amplification was not observed after AM loading in the presence of the anion transport inhibitor sulfinpyrazone, nor in osteoclasts injected with Fura-2 conjugated to a high MW dextran. In these cells, nuclear fluorescence ratios were equal to the extranuclear values at all times: upon stimulation by an agonist, the nuclear and cytosolic calcium concentrations increased by the same amount. The calcium changes seen in stimulated osteoclasts can no longer be taken as evidence for the general validity of the phenomenon of nuclear calcium amplification.

Intracellular free Ca2+ movements in cultured cardiac myocytes as shown by rapid scanning confocal microscopy

Two-dimensional images of intracellular free Ca2+ movements in cultured cardiac myocytes were obtained at 33-ms intervals with a Ca(2+)-sensitive fluorescence probe, fluo-3, and a rapid scanning confocal laser microscope, a prototype of Nikon RCM8000. The cells used were isolated from the ventricular myocardium of neonatal mice, cultured for approximately 72 h and loaded with fluo-3. One type of cytoplasmic Ca2+ movement observed was a simultaneous increase in [Ca2+] throughout the cytoplasm, termed a "spike"; another type was a local increase in [Ca2+] propagating in the cytoplasm, termed a "wave." Cells with either spike or wave or both types of movements were observed. Tetrodotoxin (TTX) 10(-5) M, nicardipine 10(-6) M, and increased extracellular potassium concentration (40 mM) selectively inhibited spike, and ryanodine 10(-6) M and cyclopiazonic acid (CPA) 3 x 10(-6) M selectively inhibited wave. These results indicate that spike was triggered by depolarization-induced Ca2+ influx across the sarcolemma, whereas wave was a propagating local increase in Ca2+ due to Ca2+ release from the sarcoplasmic reticulum (SR). On spike, nuclear [Ca2+] was shown to increase and decrease synchronously with cytoplasmic [Ca2+], with a delay and slower time course.

Restricted propagation of cytoplasmic Ca2+ oscillation into the nucleus in guinea pig cardiac myocytes as revealed by rapid scanning confocal microscopy and indo-1

Two-dimensional images of cytoplasmic and nuclear free Ca2+ movements in cardiac myocytes were obtained at 67-msec intervals using a Ca(2+)-sensitive fluorescence probe, indo-1, and a rapid scanning confocal laser microscope, Nikon RCM8000. Isolated guinea pig ventricular cells were loaded with indo-1 and stimulated at 0.5 Hz through patch pipettes. On stimulation, nuclear Ca2+ concentration ([Ca2+]) was observed to rise and fall following cytoplasmic [Ca2+] with an obvious delay. Application of isoproterenol significantly increased the peak [Ca2+] on stimulation in both the cytoplasm and nucleus with no substantial change in the basal [Ca2+]; the increase in peak [Ca2+] produced by application of isoproterenol was larger in the cytoplasm than in the nucleus. Under a low [Na+] condition, the basal [Ca2+] was increased from the control values in both the cytoplasm and nucleus; no difference in basal [Ca2+] was observed between the two regions. The increase in peak [Ca2+] by low [Na+] in the cytoplasm was significantly larger than that in the nucleus. When the cells were voltage clamped at 0 mV for 3 sec, no difference in the steady state [Ca2+] was observed between the cytoplasm and nucleus. Nuclear [Ca2+] was also observed to increase following a Ca2+ wave, a local increase in [Ca2+] propagating within the cytoplasm, with a delay. Thus, we demonstrated in isolated myocardial cells that cytoplasmic Ca2+ movements, although hampered by the nuclear envelope, are propagated into the nucleus, a mechanism through which factors affecting cytoplasmic Ca2+ may influence intranuclear events.

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.

Secretagogue-induced calcium wave shows higher and prolonged transients of nuclear calcium concentration in mast cells

To clarify the mechanism of secretagogue (compound 48/80)-induced calcium signaling in rat peritoneal mast cells, we analyzed serial confocal calcium images with high spatial and temporal resolution using different Ca(2+)-probes. The Ca(2+)-wave began at the periphery of the cytoplasm, and then spread to the center of the nucleus. Nuclear [Ca2+]i was clearly higher than cytoplasmic [Ca2+]i. The heterogeneity of [Ca2+]i continued until about 2 min after degranulation. These results suggest the existence of an intranuclear Ca(2+)-store which possesses a Ca(2+)-releasing mechanism similar to that in the cytoplasm.

Intracellular calcium mobilization by inositol 1,4,5-trisphosphate: intracellular movements and compartmentalization

Intracellular calcium ion (Ca2+) changes in NIH-3T3 fibroblasts responding to inositol 1,4,5-trisphosphate (IP3) injections have been monitored using high resolution digital imaging of the calcium indicator Fura-2. Ester loaded and microinjected indicator report radically different patterns of Ca2+ change during the IP3 response. These differences arise from intracellular compartmentalization of the ester loaded indicator which can seriously distort reported Ca2+ levels. Prominent among these aberrant responses is a signal in which Ca2+ levels in the cell nucleus appear to exceed those in the rest of the cell, and an apparent slowing of the Ca2+ recovery time-course throughout the cell when temperature is increased. Similar behavior is observed in other cell types. Judicious use of both loading techniques can provide information on Ca2+ movements into organelles that might otherwise escape detection. The Ca2+ rise normally measured in bulk or integrated single cell measurements is a complex mix of cytosol/nucleus and organellar changes. Much, if not all, of the observable organellar change is an accumulation, not release, of Ca2+ following the IP3 injection. The Golgi apparatus is a conspicuous early site for this accumulation, and mitochondria show a large, temperature sensitive uptake that is capable of limiting the maximal Ca2+ change during the response.

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

We have studied receptor-mediated calcium signals in antigen-specific B cells (trinitrophenol-specific B cell clone, TP67.21) using a confocal fluorescence microscope with an argon ion laser (488 nm) and a He-Cd laser (325 nm). Confocal fluorescence images of fluo-3 loaded B cells, excited by an argon ion laser, became much brighter and more nonhomogeneous than those before antigen stimulation. Time-dependent fluorescence changes in intensities were abrupt and quite similar to the patterns of the intracellular calcium ion concentration [Ca2+]i observed by a conventional fluorescence microscope using fura-2. From the morphological patterns of the calcium images, the parts of the bright fluorescence seemed to belong to the nucleus in B cells. To confirm the above events we measured the confocal fluorescence images of the nucleus. From the fluorescence images of co-loaded Hoechst 33342 (a DNA-specific fluorescent probe), which excited by a He-Cd laser, the brighter parts of the fluo-3 fluorescence intensities were identified to the nucleus in B cells. This suggested the possibility that the increased intranuclear calcium ions may play a nuclear third messenger in B cells.

Inositol trisphosphate and calcium signalling

Inositol trisphosphate is a second messenger that controls many cellular processes by generating internal calcium signals. It operates through receptors whose molecular and physiological properties closely resemble the calcium-mobilizing ryanodine receptors of muscle. This family of intracellular calcium channels displays the regenerative process of calcium-induced calcium release responsible for the complex spatiotemporal patterns of calcium waves and oscillations. Such a dynamic signalling pathway controls many cellular processes, including fertilization, cell growth, transformation, secretion, smooth muscle contraction, sensory perception and neuronal signalling.

Cytosolic and intranuclear calcium signals in rat basophilic leukemia cells as revealed by a confocal fluorescence microscope

A confocal fluorescence microscope with an argon-ion laser (488 nm) and a He-Cd laser (325 nm) was used to study spatial heterogeneity of the calcium signals in rat basophilic leukemia 2H3 cloned cell line (RBL-2H3). After stimulation with antigen (2,4-dinitrophenol-conjugated bovine serum albumin), fluo-3-fluorescence intensities increased in individual RBL-2H3 cells with different lag times. Time-dependent profiles of the fluo-3-fluorescence intensities resembled closely the patterns of the sequential fluorescence-ratio images of fura-2, which were used to measure the intracellular free-calcium concentration ([Ca2+]i) in individual RBL-2H3 cells using a conventional fluorescence microscope. The present results obtained using the confocal fluorescence microscope showed spatial heterogeneities of fluo-3-fluorescence intensities, suggesting the existence of spatial heterogeneity of [Ca2+]i in RBL-2H3 cells. That is, the results showed that calcium signals first occurred transiently at pseudopodia in RBL-2H3 cells, then the signals transferred to the central parts of the cells. In addition, from the fluorescence images of co-loaded Hoechst 33342 (bisbenzimide H 33342, a DNA-specific probe) which were produced by excitation with a He-Cd laser, it was found that the fluorescence images of the nucleus were quite similar to those of the calcium signals mentioned above. This suggested that the receptor-mediated calcium signals were transferred not only to the cytoplasm but also to the nucleus.

Single cell observation of calcium signals in class II specific killer T cells

A confocal fluorescence microscope with an argon ion laser and a digital imaging fluorescence microscope were used to study calcium signals in class II-specific killer T cells after interaction with target cells. Here, we used two kinds of I-Ak specific allogeneic killer T cell clones, QM11 (CD8+) and QM56 (CD4+), and target cells, B10.A spleen cells (I-Ak) and B cell hybridomas (TP67.21, I-Ak). After interaction with target cells, the intracellular free calcium ion concentration ([Ca2+]i) in QM11 cells (CD8+) increased rapidly with short lag-times (several seconds). The [Ca2+]i in QM56 cells (CD4+), however, increased with much longer lag-times (hundreds of seconds). The results were consistent with previous findings that the cytolytic activity of QM11 cells (CD8+) was much greater than that of QM56 cells (CD4+).

A fluorescent molecular rotor probes the kinetic process of degranulation of mast cells

A confocal fluorescence microscope was used to study the exocytotic secretory processes of mast cells in combination with an fluorescent molecular rotor, 9-(dicyanovinyl)julolidine (DCVJ). DCVJ is known to be an unique fluorescent dye which increases its quantum yield with decreasing intramolecular rotation. Here, DCVJ-loaded peritoneal rat mast cells were stimulated with compound 48/80 and their fluorescence images were compared with fluorescence calcium images of fluo-3-loaded mast cells. Subsequent to transient increases in intracellular free calcium ion concentration, DCVJ fluorescence increased dramatically in the cytoplasm and formed a ring-like structure around the nucleus, suggesting the possibility that the dye bound to the proteins composing the cytoskeletal architecture. Furthermore, the increases of DCVJ fluorescence intensities were mostly blocked in the presence of cytochalasin D (10 microM). However, fluo-3 fluorescence intensities still increased after addition of compound 48/80.