Transient increases in cytosolic free calcium appear to be required for the migration of adherent human neutrophils

Alterations in Purkinje cell spines of calbindin D-28 k and parvalbumin knock-out mice

The second messenger Ca2+ is known to act in a broad spectrum of fundamental cell processes, including modifications of cell shape and motility, through the intermediary of intracellular calcium-binding proteins. The possible impact of the lack of the intracellular soluble Ca2+-binding proteins parvalbumin (PV) and calbindin D-28 k (CB) was tested on spine morphology and topology in Purkinje cell dendrites of genetically modified mice. Three different genotypes were studied, i.e. PV or CB single knock-out (PV-/-, CB-/-) and PV and CB double knock-out mice (PV-/-CB-/-). Purkinje cells were microinjected with Lucifer Yellow and terminal dendrites scanned at high resolution with a confocal laser microscope followed by three-dimensional (3-D) reconstruction. The absence of PV had no significant effect on spine morphology, whereas the absence of CB resulted in a slight increase of various spine parameters, most notably spine length. In double knock-out mice, the absence of both PV and CB entailed a doubling of spine length, an increase in spine volume and spine surface, a higher spine density along the dendrites, as well as a more clustered spine distribution. In all three genotypes, a reduction in the number of stubby spines was observed compared with wild-type animals. These results suggest a morphological compensation for the lack of the soluble calcium buffers in the cytoplasm of Purkinje cell dendritic spines. The increase in various spine parameters, particularly volume, may counteract the lack of the calcium buffers, such as to adjust Ca2+-transients at the transitional zone between spines and dendrites.

Nitric oxide induces dose-dependent CA(2+) transients and causes temporal morphological hyperpolarization in human neutrophils

We exposed adherent neutrophils to the nitric oxide (NO)-radical donors S-nitroso-N-acetylpenicillamine (SNAP), S-nitrosoglutathione (GSNO), and sodium nitroprusside (SNP) to study the role of NO in morphology and Ca(2+) signaling. Parallel to video imaging of cell morphology and migration in neutrophils, changes in intracellular free Ca(2+) ([Ca(2+)](i)) were assessed by ratio imaging of Fura-2. NO induced a rapid and persistent morphological hyperpolarization followed by migrational arrest that usually lasted throughout the 10-min experiments. Addition of 0.5-800 microM SNAP caused concentration-dependent elevation of [Ca(2+)](i) with an optimal effect at 50 microM. This was probably induced by NO itself, because no change in [Ca(2+)](i) was observed after treatment with NO donor byproducts, i.e. D-penicillamine, glutathione, or potassium cyanide. Increasing doses of SNAP (>/=200 microM) attenuated the Ca(2+) response to the soluble chemotactic stimulus formyl-methionyl-leucyl-phenylalanine (fMLP), and both NO- and fMLP-induced Ca(2+) transients were abolished at 800 microM SNAP or more. In kinetic studies of fluorescently labeled actin cytoskeleton, NO markedly reduced the F-actin content and profoundly increased cell area. Immunoblotting to investigate the formation of nitrotyrosine residues in cells exposed to NO donors did not imply nitrosylation, nor could we mimic the effects of NO with the cell permeant form of cGMP, i.e., 8-Br-cGMP. Hence these processes were probably not the principal NO targets. In summary, NO donors initially increased neutrophil morphological alterations, presumably due to an increase in [Ca(2+)](i), and thereafter inhibited such shape changes. Our observations demonstrate that the effects of NO donors are important for regulation of cellular signaling, i.e., Ca(2+) homeostasis, and also affect cell migration, e.g., through effects on F-actin turnover. Our results are discussed in relation to the complex mechanisms that govern basic cell shape changes, required for migration.

Inhibition of interleukin-8-activated human neutrophil chemotaxis by thapsigargin in a calcium- and cyclic AMP-dependent way

Chemotactic migration of human neutrophils, induced by interleukin-8 (IL-8) or other activators, was inhibited by thapsigargin in the high nanomolar range. The degree of inhibition depended on the type of activator. Other inhibitors of Ca(2+)-ATPases associated with intracellular calcium stores, such as cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone, equally inhibited IL-8-activated migration. Inhibition of migration by thapsigargin and the other ATPase inhibitors occurred only in the presence of extracellular Ca2+; migration was not inhibited in the presence of EGTA. La3+ reversed thapsigargin-induced inhibition to a large degree; other calcium channel blockers gave a partial reversal (econazole, verapamil, and SK&F 96365) or had no effect (gadolinium chloride and Ni2+). Using electroporated cells and Ca buffers, it was shown that inhibition started at about 0.2 microM and was complete at a cytosolic Ca concentration of about 2 microM. It appears that under certain conditions the thapsigargin-induced influx of extracellular calcium, causing relatively high local calcium concentrations, initiates or permits a process which may be detrimental to chemotactic migration. Cyclic AMP (cAMP; adenosine 3',5'-cyclic monophosphate) is probably involved in this process, because thapsigargin increased the cAMP level and cAMP inhibited IL-8-activated migration in a calcium-dependent way. The hypothesis that cAMP is involved in the effect of thapsigargin on migration is supported by the finding that very low concentrations of thapsigargin stimulate neutrophil migration in the absence of other chemoattractants. The results suggest that thapsigargin causes a (compartmentalized) increase in cAMP, which results in a calcium-dependent modulation of migration.

Migration of human vascular smooth muscle cells involves serum-dependent repeated cytosolic calcium transients

Migration of vascular smooth muscle cells (VSMC) is a key event in the formation of neointima during atherosclerosis. Fura-2 loaded VSMCs were used to investigate calcium homeostasis during cell migration. Multiple spontaneous transient increases in cytosolic free calcium [Ca(2+)](i)were observed in single human VSMCs migrating on type I collagen. Such [Ca(2+)](i)transients were dependent on the presence of serum or PDGF-BB. Removal of serum, or loading cells with BAPTA, abolished the transients and decreased cell migration speed. The transients were not affected by disruption of cell polarization by dihydrocytochalasin B. Adhesion was used to investigate the specific role of cell-substrate interactions in the generation of transients. Transients are seen in VSMCs adhering either on collagen or on poly-L-lysine, suggesting that generation of transients is not strictly dependent on integrins. Buffering [Ca(2+)](i) with BAPTA led to accumulation of (beta)1 integrins at the cellular tail, and to increased release of integrin on the extracellular matrix. These results demonstrate a role for [Ca(2+)](i) transients in the rapid, serum-dependent migration of VSMCs. These [Ca(2+)](i)transients are present in migrating VSMCs only when two simultaneous events occur: (1) substrate independent spreading and (2) stimulation of cells by serum components such as PDGF-BB.

Ways of looking at calcium

What we understand about signalling pathways depends very much on the ways we can measure them. I review ways of measuring calcium and explore how changes in methods have led to new ways of thinking about calcium signals. I also suggest how the ways we have of looking at calcium will influence the analysis of other signalling pathways that, until now, have not been studied with the spatiotemporal precision available to those studying calcium signalling.

Calcium signaling in the developing Xenopus myotome

Embryonic Xenopus myocytes generate spontaneous calcium (Ca(2+)) transients during differentiation in culture. Suppression of these transients disrupts myofibril organization and the formation of sarcomeres through an identified signal transduction cascade. Since transients often occur during myocyte polarization and migration in culture, we hypothesized they might play additional roles in vivo during tissue formation. We have tested this hypothesis by examining Ca(2+) dynamics in the intact Xenopus paraxial mesoderm as it differentiates into the mature myotome. We find that Ca(2+) transients occur in cells of the developing myotome with characteristics remarkably similar to those in cultured myocytes. Transients produced within the myotome are correlated with somitogenesis as well as myocyte maturation. Since transients arise from intracellular stores in cultured myocytes, we examined the functional distribution of both IP(3) and ryanodine receptors in the intact myotome by eliciting Ca(2+) elevations in response to photorelease of caged IP(3) and superfusion of caffeine, respectively. As in culture, transients in vivo depend on Ca(2+) release from ryanodine receptor (RyR) stores, and blocking RyR during development interferes with somite maturation.

Cellular responses to FMLP challenging: a mini-review

FMLP (N-formyl-methionyl-leucyl-phenylalanine) and other N-formylpeptides are powerful "activators" of polymorphonuclear and mononuclear phagocytes, but they are also active on other cell types. Present knowledge about formylpeptide receptors and the relevant tools for their imaging and the study of their dynamics are briefly discussed. The main responses elicited by FMLP in granulocytes are cell polarisation, the generation of reactive oxygen species, the production of arachidonic acid metabolites, and the release of lysosomal enzymes. The transduction cascades involved and the agents able to modulate these responses are reviewed. Homologous desensitization and heterologous desensitization of the FMLP-receptor following ligation of other chemokine receptors are also outlined. Finally, the receptor expression and the pharmacological and toxic actions of FMLP upon other tissues and organs, and its actions on the developing embryo, are illustrated.

Regulation of cell movement is mediated by stretch-activated calcium channels

Intracellular calcium regulates many of the molecular processes that are essential for cell movement. It is required for the production of actomyosin-based contractile forces, the regulation of the structure and dynamics of the actin cytoskeletons, and the formation and disassembly of cell-substratum adhesions. Calcium also serves as a second messenger in many biochemical signal-transduction pathways. However, despite the pivotal role of calcium in motile processes, it is not clear how calcium regulates overall cell movement. Here we show that transient increases in intracellular calcium, [Ca2+]i, during the locomotion of fish epithelial keratocytes, occur more frequently in cells that become temporarily 'stuck' to the substratum or when subjected to mechanical stretching. We find that calcium transients arise from the activation of stretch-activated calcium channels, which triggers an influx of extracellular calcium. In addition, the subsequent increase in [Ca2+]i is involved in detachment of the rear cell margin. Thus, we have defined a mechanism by which cells can detect and transduce mechanical forces into biochemical signals that can modulate locomotion.

C-reactive protein inhibits chemotactic peptide-induced p38 mitogen-activated protein kinase activity and human neutrophil movement

Serum levels of the acute-phase reactant, C-reactive protein (CRP), increase dramatically during acute inflammatory episodes. CRP inhibits migration of neutrophils toward the chemoattractant, f-Met-Leu-Phe (fMLP) and therefore acts as an anti-inflammatory agent. Since tyrosine kinases are involved in neutrophil migration and CRP has been shown to decrease phosphorylation of some neutrophil proteins, we hypothesized that CRP inhibits neutrophil chemotaxis via inhibition of MAP kinase activity. The importance of p38 MAP kinase in neutrophil movement was determined by use of the specific p38 MAP kinase inhibitor, SB203580. CRP and SB203580 both blocked random and fMLP-directed neutrophil movement in a concentration-dependent manner. Additionally, extracellular signal-regulated MAP kinase (ERK) was not involved in fMLP-induced neutrophil movement as determined by use of the MEK-specific inhibitor, PD98059. Blockade of ERK with PD98059 did not inhibit chemotaxis nor did it alter the ability of CRP or SB203580 to inhibit fMLP-induced chemotaxis. More importantly, CRP inhibited fMLP-induced p38 MAP kinase activity in a concentration-dependent manner as measured by an in vitro kinase assay. Impressively, CRP-mediated inhibition of p38 MAP kinase activity correlated with CRP-mediated inhibition of fMLP-induced chemotaxis (r = -0.7144). These data show that signal transduction through p38 MAP kinase is necessary for neutrophil chemotaxis and that CRP intercedes through this pathway in inhibiting neutrophil movement.

Dependence of superoxide anion production on extracellular and intracellular calcium ions and protein kinase C in PMA-stimulated bovine neutrophils

The involvement of both intracellular and extracellular calcium, as well as the activation of protein kinase C (PKC), in phorbol myristate acetate (PMA)-stimulated respiratory burst in bovine neutrophils has been studied. PMA significantly stimulated the superoxide anion production by these cells. The increased production of superoxide anion was inhibited by BAPTA/AM, an intracellular calcium ([Ca2+]i) chelator, but not affected by EGTA, an extracellular calcium ([Ca2+]0) chelator. PMA also induced PKC activation, and a PKC inhibitor, calphostin C, blocked the stimulatory effect of PMA on superoxide anion production by the neutrophils. Therefore, we conclude that PMA-induced respiratory burst in bovine neutrophils is [Ca2+]i- but not [Ca2+]0-dependent, and also requires PKC activation.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.

Calcium waves induced by large voltage pulses in fish keratocytes

Intracellular calcium waves in fish keratocytes are induced by the application of electric field pulses with amplitudes between 55 and 120 V/cm and full width at half-maximum of 65-100 ms. Calcium concentrations were imaged using two-photon excited fluorescence microscopy (Denk et al., 1990 Science. 248:73-76; Williams et al. 1994 FASEB J. 8:804-813) and the ratiometric calcium indicator indo-1. The applied electric field pulses induced waves with fast calcium rise times and slow decays, which nucleated in the lamellipodium at the hyperpolarized side of the cells and, less frequently, at the depolarized side. The effectiveness of wave generation was determined by the change induced in the membrane potential, which is about half the field strength times the cell width in the direction of the field. Stimulation of waves began at voltage drops across the cell above 150 mV and saturated at voltage drops above 300 mV, where almost all cells exhibited a wave. Waves were not induced in low-calcium media and were blocked by the nonselective calcium channel blockers cobalt chloride and verapamil, but not by specific organic antagonists of voltage-sensitive calcium channel conductance. Thapsigargin stopped wave propagation in the cell body, indicating that calcium release from intracellular stores is necessary. Thus a voltage pulse stimulates Ca2+ influx through calcium channels in the plasma membrane, and if the intracellular calcium concentration reaches a threshold, release from intracellular stores is induced, creating a propagating wave. These observations and the measured parameters (average velocity approximately 66 micron/s and average rise time approximately 68 ms) are consistent with a wave amplification model in which[equation, see text] determines the effective diffusivity of the propagating molecules, D approximately 300 micron2/s (Meyer, 1991. Cell. 64:675-678).

Calpain regulates actin remodeling during cell spreading

Previous studies suggest that the Ca2+-dependent proteases, calpains, participate in remodeling of the actin cytoskeleton during wound healing and are active during cell migration. To directly test the role that calpains play in cell spreading, several NIH-3T3- derived clonal cell lines were isolated that overexpress the biological inhibitor of calpains, calpastatin. These cells stably overexpress calpastatin two- to eightfold relative to controls and differ from both parental and control cell lines in morphology, spreading, cytoskeletal structure, and biochemical characteristics. Morphologic characteristics of the mutant cells include failure to extend lamellipodia, as well as abnormal filopodia, extensions, and retractions. Whereas wild-type cells extend lamellae within 30 min after plating, all of the calpastatin-overexpressing cell lines fail to spread and assemble actin-rich processes. The cells genetically altered to overexpress calpastatin display decreased calpain activity as measured in situ or in vitro. The ERM protein ezrin, but not radixin or moesin, is markedly increased due to calpain inhibition. To confirm that inhibition of calpain activity is related to the defect in spreading, pharmacological inhibitors of calpain were also analyzed. The cell permeant inhibitors calpeptin and MDL 28, 170 cause immediate inhibition of spreading. Failure of the intimately related processes of filopodia formation and lamellar extension indicate that calpain is intimately involved in actin remodeling and cell spreading.

Mitochondrial Ca2+ uptake and release influence metabotropic and ionotropic cytosolic Ca2+ responses in rat oligodendrocyte progenitors

1. Many physiologically important activities of oligodendrocyte progenitor cells (O-2A cells), including proliferation, migration and differentiation, are regulated by cytosolic Ca2+ signals. However, little is known concerning the mechanisms of Ca2+ signalling in this cell type. We have studied the interactions between Ca2+ entry, Ca2+ release from endoplasmic reticulum and Ca2+ regulation by mitochondria in influencing cytosolic Ca2+ responses in O-2A cells. 2. Methacholine (MCh; 100 microM) activated Ca2+ waves that propagated from several initiation sites along O-2A processes. 3. During a Ca2+ wave evoked by MCh, mitochondrial membrane potential was often either depolarized (21 % of mitochondria) or hyperpolarized (20 % of mitochondria), as measured by changes in the fluorescence of 5,5',6,6'-tetrachloro-1,1',3, 3'-tetraethylbenzimidazole carbocyanine iodide (JC-1). 4. Stimulation with kainate (100 microM) evoked a slowly rising, sustained cytosolic Ca2+ elevation in O-2A cells. This also, in some cases, resulted in either a depolarization (15 % of mitochondria) or hyperpolarization (12 % of mitochondria) of mitochondrial membrane potential. 5. Simultaneous measurement of cytosolic (fluo-3 AM) and mitochondrial (rhod-2 AM) Ca2+ responses revealed that Ca2+ elevations in the cytosol evoked by either MCh or kainate were translated into long-lasting Ca2+ elevations in subpopulations of mitochondria. In some mitochondria, Ca2+ signals appeared to activate Ca2+ release into the cytosol. 6. Inhibition of the mitochondrial Na+-Ca2+ exchanger by CGP-37157 (25 microM) decreased kainate Ca2+ response amplitude and increased the rate of return of the response to basal Ca2+ levels. 7. Thus, both ionotropic and metabotropic stimulation evoke changes in mitochondrial membrane potential and Ca2+ levels in O-2A cells. Ca2+ uptake into some mitochondria is activated by Ca2+ entry into cells or release from stores. Mitochondrial Ca2+ release appears to play a key role in shaping kainate-evoked Ca2+ responses.

Regulation of neural cell adhesion molecule polysialylation: evidence for nontranscriptional control and sensitivity to an intracellular pool of calcium

The up- and downregulation of polysialic acid-neural cell adhesion molecule (PSA-NCAM) expression on motorneurons during development is associated respectively with target innervation and synaptogenesis, and is regulated at the level of PSA enzymatic biosynthesis involving specific polysialyltransferase activity. The purpose of this study has been to describe the cellular mechanisms by which that regulation might occur. It has been found that developmental regulation of PSA synthesis by ciliary ganglion motorneurons is not reflected in the levels of polysialyltransferase-1 (PST) or sialyltransferase-X (STX) mRNA. On the other hand, PSA synthesis in both the ciliary ganglion and the developing tectum appears to be coupled to the concentration of calcium in intracellular compartments. This study documents a calcium dependence of polysialyltransferase activity in a cell-free assay over the range of 0.1-1 mM, and a rapid sensitivity of new PSA synthesis, as measured in a pulse-chase analysis of tissue explants, to calcium ionophore perturbation of intracellular calcium levels. Moreover, the relevant calcium pool appears to be within a specific intracellular compartment that is sensitive to thapsigargin and does not directly reflect the level of cytosolic calcium. Perturbation of other major second messenger systems, such as cAMP and protein kinase-dependent pathways, did not affect polysialylation in the pulse chase analysis. These results suggest that the shuttling of calcium to different pools within the cell can result in the rapid regulation of PSA synthesis in developing tissues.

Oscillatory pericellular proteolysis and oxidant deposition during neutrophil locomotion

To better understand the mechanism of leukocyte migration in complex environments, model extracellular matrices were prepared using gelatin, Hanks' solution, Bodipy-BSA (fluorescent upon proteolysis), and dihydrotetramethylrosamine or hydroethidine (fluorescent upon oxidation). Using quantitative microfluorometry, neutrophil-mediated extracellular pulses of reactive oxygen metabolites (ROMs) and pericellular proteolysis were periodically observed showing that these functions occur as quantal bursts. However, chronic granulomatous disease neutrophils, which do not produce ROMs, did not display ROM deposition. Matrices show an alternating pattern of green (proteolytic) and red (oxidative) fluorescence, indicating these functions are out of phase. Electric fields phase-matched with metabolic oscillations, which increase the amplitude of intracellular NAD(P)H oscillations, increase ROM deposition and pericellular proteolysis; this further supports the link between intracellular chemical oscillators and extracellular functions. This phase relationship may allow ROMs to inactivate protease inhibitors, followed by protease activation.

Distribution of plasmalemmal Ca(2+)-pump and caveolin in the corneal epithelium during the wound healing process

PURPOSE

Caveolae are small plasmalemmal invaginations which are assumed to play various physiological functions. In the present study, distribution of two caveolae-specific proteins, the plasmalemmal Ca(2+)-pump and caveolin, was examined in the corneal epithelium in the normal state and after artificial wounding.

METHODS

A central epithelial ablation was made in the mouse cornea by a razor blade. After various intervals, the corneas were excised, fixed, and rapidly frozen. The specimens were subjected to immunofluorescence microscopy and immunoelectron microscopy, using antibodies against the plasmalemmal Ca(2+)-pump or caveolin.

RESULTS

In the normal corneal epithelium, both plasmalemmal Ca(2+)-pump and caveolin were observed along the cell surface by immunofluorescence microscopy, and were localized to caveolae by immunogold electron microscopy. In the regenerating epithelium, 12-18 h after injury, plasmalemmal Ca(2+)-pump was seen as many dots in the cytoplasm by immunofluorescence microscopy; in contrast, caveolin persisted along the cell surface. Immunoelectron microscopy revealed that the labeling for the plasmalemmal Ca(2+)-pump was located around membranous structures in the cytoplasm and was scarce along the plasma membrane, while caveolin remained in caveolae. The Ca(2+)-pump regained normal distribution when the wound was closed. By quantitation in electron micrographs, the number of caveolae per unit plasma membrane length was found to be decreased in the wounded corneal epithelium.

CONCLUSIONS

The present results indicate that caveolae undergo compositional modification during the wound healing process of the corneal epithelium. Considering putative caveolar functions, the phenomenon may be related to possible fluctuations of the intracellular Ca(2+)-concentration in the regenerating epithelium.

Proximity oscillations of complement type 4 (alphaX beta2) and urokinase receptors on migrating neutrophils

Migrating neutrophils utilize beta2 integrins for substrate attachment and urokinase receptors (uPAR) to focus pericellular proteolysis. Our studies show that CR3 associates with uPAR on resting cells, whereas uPAR associates with CR4 at lamellipodia of migrating cells. Using resonance energy transfer (RET) microscopy, we show that the molecular proximity between CR4 and uPAR oscillates on migrating cells, thus suggesting that CR4 molecules periodically bind/release uPAR. Cell contact with fibrinogen, endothelial cells, chemotactic factors and indomethacin, and treatment with sub-optimal doses of signal transduction inhibitors, affect the oscillations' period, amplitude, and/or waveform. The oscillations were indistinguishable in period and 180 degrees out-of-phase with cytosolic NAD(P)H autofluorescence oscillations. Thus, CR4 and CR3 identify a neutrophil's axis of migration and CR4 may restrain uPAR at lamellipodia. Oscillations in signal transduction and energy metabolism may coordinate cell adherence, local proteolysis, oxidant release, actin assembly, and cell extension.

Controlling the molecular motor of neutrophil chemotaxis

Our defence against microbes depends largely on the ability of neutrophils to migrate from the blood stream to sites of infection. Although the ability of animal cells to move may be primitive, and also fundamental for a number of phenomena in biology, the cellular mechanism by which neutrophils are able to move rapidly towards the infection remains an enigma. Even though the structures of the receptors involved have been sequenced and many of the molecules involved in neutrophil adherence and traction identified, the essential mechanisms that control and regulate the neutrophil motor remain obscure. Here, an outline of the fundamental inadequacies in our current understanding is given, along with some recent developments that promise to produce some significant advances.

Effects of buffering intracellular free calcium on neutrophil migration through three-dimensional matrices

Repeated transient increases in intracellular free calcium levels ([Ca2-]i) are required for polymorphonuclear neutrophil migration on two-dimensional surfaces coated with fibronectin or vitronectin. Cells in which [Ca2+]i is buffered with quin2 become stuck on these substrates. Neutrophils migrating through the extracellular matrix in vivo encounter these and other substrates in a three-dimensional architecture that may alter the spatial distribution of adhesion receptors in contact with the matrix. In this study, we used fluorescence confocal microscopy to obtain moving three-dimensional images of neutrophils migrating through a biological tissue (human amnion) in the presence and absence of [Ca2+]i-buffering with quin2. In the absence of buffering, [Ca2+]i transients similar to those seen in cells migrating in two-dimensions were observed. [Ca2+]i-buffered neutrophils were able to migrate into the matrix, but they became attached firmly to the substrate at the rear of the cell, resulting in a drastically elongated morphology. Immunofluorescence revealed that neutrophils adhered to regions of the matrix that contained fibronectin. RGD-containing peptides and antibodies that block integrin adhesion receptors for fibronectin and vitronectin were able to rescue the migration of quin2-treated cells through three-dimensional gels containing fibronectin and vitronectin. These data show that neutrophils migrating through physiologically relevant, three-dimensional matrices undergo repetitive increases in [Ca2+]i that are required for integrin-mediated detachment from the matrix.

An in vitro force measurement assay to study the early mechanical interaction between corneal fibroblasts and collagen matrix

An in vitro force measurement assay has been developed to quantify the forces exerted by single corneal fibroblasts during the early interaction with a collagen matrix. Corneal fibroblasts were sparsely seeded on top of collagen matrices whose stiffness was predetermined by micromanipulation with calibrated fine glass microneedles. The forces exerted by individual cells were calculated from time-lapse videomicroscopic recordings of the 2-D elastic distortion of the matrix. In additional experiments, the degree of permanent reorganization of the collagen matrices was assessed by lysing the cells with 1% Triton X-100 solution at the end of a 2-hour incubation and recording the subsequent relaxation. The data suggest that a cell can exert comparable centripetal force during either extension of a cell process or partial retraction of an extended pseudopodia. The rates of force associated with pseudopodial extension and partial retraction were 0.180 +/- 0.091 (x 10(-8)) N/min (n = 8 experiments) and 0.213 +/- 0.063 (x 10(-8)) N/min (n = 8 experiments), respectively. Rupture of pseudopodial adhesion associated with cell locomotion causes a release of force on the matrix and a complete recoil of the pseudopodia concerned; a simultaneous release of force on the matrix was also observed at the opposite end of the cell. Lysis of cells resulted in 84 +/- 18% relaxation of the matrix, suggesting that little permanent remodeling of matrix is produced by the actions of isolated migrating cells.

How do injured cells communicate with the surviving cell monolayer?

Mechanically scratching cell monolayers relieves contact inhibition and induces surviving cells near the wound edge to move and proliferate. The present work was designed to test whether surviving cells passively respond to newly available space, or whether cells are actively stimulated by signals from injured cells nearby. We monitored intracellular free Ca2+ ([Ca2+]i) while scratching confluent monolayers of bovine pulmonary endothelial cells and mouse mammary epithelial cells. Within seconds after wounding, a transient elevation of [Ca2+]i was observed in surviving cells. In endothelial cells, the [Ca2+]i elevation propagated into the monolayer for a distance of 10 to 12 cell rows at a speed of 20 to 28 microm/second. The amplitude of the wave of [Ca2+]i was reduced as it propagated into the monolayer, but the velocity of the wave was nearly constant. Cells that experienced the [Ca2+]i elevation had intact plasma membranes, and survived for over 24 hours post wounding. Removing extracellular Ca2+ decreased the amplitude by two-thirds and reduced the propagation rate by half, suggesting that Ca2+ influx contributed to the increased [Ca2+]i. To determine how [Ca2+]i waves were stimulated, we blocked extracellular communication by fluid perfusion or intercellular communication by breaks in the monolayer. In bovine pulmonary artery endothelial cultures, the [Ca2+]i wave passed over breaks in the monolayer, and was prevented from traveling upstream in a perfusion chamber. Conditioned media from injured cells also elevated [Ca2+]i in unwounded reporter cultures. In mouse mammary epithelial monolayers with established cell-cell contacts, the [Ca2+]i wave passed over breaks in the monolayer, but was only partially prevented from traveling upstream during perfusion. These experiments showed that mechanical wounds lead to long distance, [Ca2+]i-dependent communication between the injured cells and the surviving cell monolayer through at least two mechanisms: first, extracellular release of a chemical stimulus from wounded cells that diffused to neighboring cells (present in both monolayers); second, transmission of an intercellular signal through cell-cell junctions (present in the mammary epithelial monolayers). Thus, mechanical injury provided a direct, chemical stimulus to nearby cells which have not themselves been damaged.

Integrin-mediated calcium signaling and regulation of cell adhesion by intracellular calcium

Integrins are ubiquitous trans-membrane adhesion molecules that mediate the interaction of cells with the extracellular matrix (ECM). Integrins link cells to the ECM by interacting with the cell cytoskeleton. In cases such as leukocyte binding, integrins mediate cell-cell interactions and cell-ECM interactions. Recent research indicates that integrins also function as signal transduction receptors, triggering a number of intracellular signaling pathways that regulate cell behavior and development. A number of integrins are known to stimulate changes in intracellular calcium levels, resulting in integrin activation. Although changes in intracellular calcium regulate a vast number of cellular functions, this review will discuss the stimulation of calcium signaling by integrins and the role of intracellular calcium in the regulation of integrin-mediated adhesion.

Relationship between the chemiluminescent response of bovine neutrophils and changes in intracellular free calcium concentration

The relationship between luminol dependent chemiluminescent (LDCL) response and changes in intracellular free Ca2+ concentrations in the bovine neutrophils was evaluated. LDCL responses and changes in intracellular Ca2+ concentrations of neutrophils were clearly detected by the stimulation with opsonized zymosan (OPZ), concanavalin A(ConA), heat-aggregated IgG (H-agg.IgG) and phorbol myristate acetate (PMA). Patterns of LDCL responses and intracellular Ca2+ of neutrophils showed characteristic features for each stimulant. PMA was a weak stimulant of the intracellular Ca2+ concentration, whereas it was a strong stimulant of LDCL response. Con A strongly stimulated an increase in the intracellular Ca2+ concentration, but was a weak stimulant of LDCL response. LDCL response of intracellular Ca(2+)-depleted neutrophils treated with ionomycin, stimulated with each stimulant was inhibited markedly without extracellular Ca2+. The sustained phase of intracellular Ca2+ concentrations stimulated with OPZ was inhibited significantly (P < 0.05) by the preincubation with anti-CD18 antibody, whereas the transient phase of intracellular Ca2+ concentrations was not inhibited. These results indicate that LDCL response is regulated at least in part by the elevation of the intracellular Ca2+, and a rise in intracellular Ca2+ concentration, which may be mediated by specific receptors appears to be essential in the LDCL response of bovine neutrophils.

Neutrophil activation by adhesion: mechanisms and pathophysiological implications

Neutrophil adhesion plays an essential role in the formation of an inflammatory exudate. Moreover, adhesion activates selective neutrophil functions and regulates the cell response to additional stimuli. In this review we summarize the information available on adhesion molecules involved in neutrophil adhesion to endothelial cells and extracellular matrix proteins and the experimental approaches which have been developed to block neutrophil adhesion and neutrophil mediated tissue damage. We also address the mechanisms of activation of selective neutrophil functions by adhesion molecules and, in particular the mechanisms of signal transduction by neutrophil integrins. On the basis of recent results obtained in our and other laboratories we propose a model hypothesizing mechanisms of signaling by neutrophil integrins involved in regulation of selective functions.

Insulin and ATP stimulate actin polymerization in U937 cells by a wortmannin-sensitive mechanism

ATP and insulin stimulate increases in phosphatidylinositol (3,4,5)-trisphosphate levels in myeloid-derived U937 cells. Quantification of FITC-phalloidin binding by fluorescence-activated cell sorting reveals that both ATP and insulin stimulate actin polymerization with distinctive kinetics in U937 cells. The response to ATP is rapid and dose-dependent with an EC50 of 200 nM, and is abolished by pre-incubation with the Ca2+ chelator BAPTA-AM. At 800 nM concentration, wortmannin, a potent inhibitor of phosphoinositide 3-kinase (PI3K), blocks the late, but not the early phase of actin polymerization stimulated by 100 nM ATP. Responses elicited by 10 micrograms/ml insulin are slower, smaller and more transient than responses to ATP, and are inhibited by preincubation with 100 nM wortmannin. Actin polymerization can also be stimulated by thapsigargin, but not by phorbol ester, providing further evidence for a role for Ca2+ in actin polymerization. These data implicate distinct Ca2+ and PI3K-mediated pathways in the regulation of actin polymerization.

Complement 5a controls motility of murine microglial cells in vitro via activation of an inhibitory G-protein and the rearrangement of the actin cytoskeleton

Microglial cells respond to most pathological events by rapid transformation from a quiescent to an activated phenotype characterized by increased cytotoxicity and motile activity. To investigate the regulation of microglial motility by different inflammatory mediators, we studied cultured murine microglia by time-lapse video microscopy and a computer-based motility assay. Microglial cells exhibited a high resting motility. The acute application of complement 5a (C5a) immediately induced intense ruffling of microglial membranes followed by lamellipodia extension within few seconds, while formyl-Met-Leu-Phe-OH, bacterial endotoxin (lipopolysaccharide) or inflammatory cytokines did not increase motility. This process was accompanied by a rapid rearrangement of the actin cytoskeleton as demonstrated by labelling with fluorescein isothiocyanate-phalloidin and could be inhibited by cytochalasin B. A GTP-binding protein was involved in the signal cascade, since pertussis toxin inhibited motility and actin assembly in response to C5a. Chemotactic migration in a gradient of C5a was also completely blocked by pertussis toxin and cytochalasin B. The C5a-induced motility reaction was accompanied by an increase in intracellular calcium ([Ca2+]i) as measured by a Fluo-3 based imaging system. Ca2+ transients were, however, not a prerequisite for triggering the increase in motility; motility could be repeatedly evoked by C5a in nominally Ca(2+)-free solution, while Ca2+ signals occurred only upon the first stimulation. Moreover, conditions mimicking intracellular Ca2+ transients, like incubation with thapsigargin or Ca2+ ionophore A23187, were not able to induce any motility reaction, suggesting that Ca2+ transients are not necessary for, but are associated with, microglial motility. Motile activity was shown to be restricted to a defined concentration range of [Ca2+]i as revealed by lowering [Ca2+]i with BAPTA-AM or increasing [Ca2+]i with A23187. Since complement factors are released at pathological sites, this signal cascade could serve to increase motility and to direct microglial cells to the lesioned or damaged area by means of a G-protein-dependent pathway and via the rearrangement of the actin cytoskeleton.

Intracellular Ca2+ fluctuations modulate the rate of neuronal migration

Transient elevations of intracellular Ca2+ levels play critical roles in neuronal development, but such elevations have not been demonstrated in migrating neurons. Here, we show that the amplitude and frequency components of Ca2+ fluctuations are correlated positively with the rate of granule cell movement in cerebellar microexplant cultures. Moreover, depression of the amplitude and frequency components of Ca2+ fluctuations by blockade of Ca2+ influx across the plasma membrane results in a reversible retardation of cell movement. These results indicate that the combination of amplitude and frequency components of intracellular Ca2+ fluctuations may provide an intracellular signal controlling the rate of neuronal cell migration.

Ca2+ signalling mechanisms of the beta 2 integrin on neutrophils: involvement of phospholipase C gamma 2 and Ins(1,4,5)P3

Engagement of beta 2 integrins triggers a tyrosine kinase-dependent intracellular mobilization and influx of Ca2+ in human neutrophils. However, the transduction pathway involved in generating this Ca2+ signal is obscure. In the present study we identified phospholipase C gamma 2 (PLC gamma 2) as one of the major proteins that was phosphorylated on tyrosine in response to beta 2 integrin activation. This beta 2 integrin-induced phosphorylation of PLC gamma 2 occurred in parallel with an increased accumulation of Ins(1,4,5)P3. The relevance of these observations for the beta 2 integrin-induced Ca2+ signal was investigated using an inhibitor of PLC signalling pathways, 1-(6-{[17 beta-3-methoxyoestra-1,3.5(10)-trien-17-yl] amino}hexyl)-1H-pyrrole-2,5-dione(U73122). U73122 dose-dependently (IC50, approx. 0.15 microM) inhibited both the beta 2 integrin-induced release of Ca2+ from intracellular stores and the subsequent influx of Ca2+ across the plasma membrane. These effects were not observed with the inactive analogue 1-(6-{[17 beta-3-methoxyoestra-1,3,5(10)-trien-17-yl] amino}hexyl)-pyrrolidine-2,5-dione (U73343). To gain further support for an involvement of PLC-induced Ins(1,4,5)P3 formation in the beta 2 integrin-induced Ca2+ signal, we searched for the molecular event(s) underlying the effects of U73122. Our experiments revealed that U73122 had no effect on either beta 2 integrin-induced tyrosine phosphorylation of PLC gamma 2 (or any of the other proteins) or on the formation of Ins(1,4,5)P3, but it reduced the Ins(1,4,5)P3-induced release of 45Ca2+ from intracellular stores of electropermeabilized cells. Taken together, the present data suggest that the beta 2 integrin-induced Ca2+ signal in human neutrophils is generated through activation of a PLC gamma 2-dependent pathway.

Localised and global cytosolic Ca2+ changes in neutrophils during engagement of Cd11b/CD18 integrin visualised using confocal laser scanning reconstruction

A confocal laser scanning technique was used to provide optical sections in the vertical plane through living neutrophils during engagement of integrin and shape change. This has permitted the visualisation of cytosolic free Ca2+ rises localised to the sites of integrin immobilisation. These localised Ca2+ changes resulted from the release of Ca2+ from intracellular stores, and were not inhibited by removal of extracellular Ca2+ or by the Ca2+ channel blocking nickel ions. After integrin engagement and initial cell shape changes, a second rapid phase of cell spreading occurred, which was accompanied by global Ca2+ signalling bursts that continued sporadically after maximum spreading. This global signalling was driven mainly by Ca2+ influx from the extracellular medium. We propose that the localised and global Ca2+ signalling triggered by integrin engagement results from a common underlying mechanism and these signals are important for neutrophil shape change and extravasation.

Intracellular pH regulation during spreading of human neutrophils

The regulation of the intracelluar pH (pHi) during spreading of human neutrophils was studied by a combination of fluorescence imaging and video microscopy. Spreading on adhesive substrates caused a rapid and sustained cytosolic alkalinization. This pHi increase was prevented by the omission of external Na+, suggesting that it results from the activation of Na+/H+ exchange. Spreading-induced alkalinization was also precluded by the compound HOE 694 at concentrations that selectively block the NHE-1 isoform of the Na+H+ antiporter. Inhibition of Na+/H+ exchange by either procedure unmasked a sizable cytosolic acidification upon spreading, indicative of intracellular acid production. The excess acid generation was caused, at least in part, by the activation of the respiratory burst, since the acidification closely correlated with superoxide production, measured in single spreading neutrophils with dihydrorhodamine-123, and little acid production was observed in the presence of diphenylene iodonium, a blocker of the NADPH oxidase. Moreover, neutrophils from chronic granulomatous disease patients, which do not produce superoxide, failed to acidify. Comparable pHi changes were observed when beta 2 integrins were selectively activated during spreading on surfaces coated with anti-CD18 antibodies. When integrin engagement was precluded by pretreatment with soluble anti-CD18 antibody, the pHi changes associated with spreading on fibrinogen were markedly reduced. Inhibition of microfilament assembly with cytochalasin D precluded spreading and concomitantly abolished superoxide production and the associated pHi changes, indicating that cytoskeletal reorganization and/or an increase in the number of adherence receptors engaged are required for the responses. Neutrophils spread normally when the oxidase was blocked or when pHi was clamped near physiological values with nigericin. Spreading, however, was strongly inhibited when pHi was clamped at acidic values. Our results indicate that neutrophils release superoxide upon spreading, generating a burst of intracellular acid production. The concomitant activation of the Na+/H+ antiport not only prevents the deleterious effects of the acid released by the NADPH oxidase, but induces a net cytosolic alkalinization. Since several functions of neutrophils are inhibited at an acidic pHi, the coordinated activation of pHi regulatory mechanisms along with the oxidase is essential for sustained microbicidal activity.

Actin polymerization in human eosinophils, unlike human neutrophils, depends on intracellular calcium mobilization

Eosinophils represent major effector cells in the allergic inflammation. In contrast to neutrophils, the mechanism of eosinophil activation during the inflammatory response is poorly understood. In this study, the relation between calcium fluxes, chemotaxis, and actin polymerization in eosinophils from healthy non-atopic donors was investigated. Pre-incubation of eosinophils with the intracellular calcium chelator BAPTA dose-dependently prevented an increase in the intracellular calcium concentration ([Ca2+]i), whereas the depletion of extracellular calcium in the test medium had no effect. The chemotactic response of eosinophils, which was measured by the modified boyden chamber technique upon stimulation with RANTES, C5a and PAF, was dose-dependently inhibited by the chelation of intracellular calcium as well as inactivation of the cells in Ca2+ -depleted medium. To evaluate whether other cell functions which are involved in the migratory response of eosinophils might be dependent on intracellular and extracellular calcium, actin polymerization was investigated. Flow-cytometric measurement of F-actin with NBD-phallacidin revealed that actin polymerization in human eosinophils in response to RANTES, C5a, and PAF was dose-dependently inhibited by the intracellular calcium chelator BAPTA. Since it is well known that actin polymerization in neutrophils is not affected by chelation of intracellular calcium, actin polymerization in these cells was investigated under the same conditions as for eosinophils. In contrast to eosinophils, BAPTA did not inhibit actin polymerization in neutrophils. In summary, these data demonstrate that intracellular calcium fluxes represent a prerequisite for eosinophil chemotaxis and actin polymerization in human eosinophils. Furthermore, regulation of actin polymerization in eosinophils differed from that of neutrophils on the level of intracellular calcium fluxes.

Ca(2+)-dependence of conoid extrusion in Toxoplasma gondii tachyzoites

The role of Ca2+ in conoid extrusion was investigated in isolated Toxoplasma gondii tachyzoites by treatment with Ca(2+)-ionophores, Ca(2+)-chelating agents and an inhibitor of the Ca(2+)-ATPase at the endoplasmic reticulum. The results were evaluated by light phase-contrast microscopy and electron microscopy. Ionomycin (0.5-1 microM) caused an immediate and sustained extrusion of the conoid in up to 80% of the tachyzoites, depending on the concentrations of ionophore and Ca2+ in the medium. However, over 50% of the tachyzoites extruded the conoid when treated with ionomycin in Ca(2+)-free saline complemented with EGTA. The effect of ionomycin was reversible and could be induced a second time in about half of the responsive population. Similar results were obtained with A23187. Conoid extrusion induced by ionomycin in Ca(2+)-free medium was almost completely abolished when the tachyzoites were previously loaded with a permeable compound known to chelate intracellular Ca2+ (BAPTA/AM; 25 microM). On the other hand, exposure of tachyzoites to the Ca(2+)-ATPase inhibitor thapsigargin (0.5-1 microM) produced significant extrusion of the conoid. Tachyzoites loaded with BAPTA/AM as well as those treated with ionomycin, i.e. with conoids paralyzed in opposite positions, had a diminished capacity to invade cultured epithelial cells. A substantial reduction in the response to stimulation by ionomycin was found also in parasites treated with cytochalasin-D, a drug that depolymerizes actin-filaments. The results suggest that Ca(2+)-release from internal stores may act as a key signal to activate a mechanism of conoid extrusion probably mediated, at least in part, by actin-filaments.

Acidic pH enhances the invasive behavior of human melanoma cells

As a consequence of poor perfusion and elevated acid production, the extracellular pH (pHex) of tumors is generally acidic. Despite this, most in vitro experiments are still performed at the relatively alkaline pHex of 7.4. This is significant, because slight changes in pHex can have profound effects on cell phenotype. In this study we examined the effects of mildly acidic conditions on the in vitro invasive potential of two human melanoma cell lines; the highly invasive C8161, and poorly invasive A375P. We observed that culturing of either cell line at acidic pH (6.8) caused dramatic increases in both migration and invasion, as measured with the Membrane Invasion Culture System (MICS). This was not due to a direct effect of pH on the invasive machinery, since cells cultured at normal pH (7.4) and tested at acidic pH did not exhibit increased invasive potential. Similarly, cells cultured at acidic pH were more aggressive than control cells when tested at the same medium pH. These data indicate that culturing of cells at mildly acidic pH induces them to become more invasive. Since acid pH will affect the intracellular pH (pHin) and intracellular calcium ([Ca2+]in), we examined the effect of these parameters on invasion. While changes in [Ca2+]in were not consistent with invasive potential, the changes in pHin were. While these conditions decrease the overall amount of gelatinases A and B secreted by these cells, there is a consistent and significant increase in the proportion of the activated form of gelatinase B.

F-actin network formation in tethers and in pseudopods stimulated by chemoattractant

Micropipets are used either to deliver a given concentration of the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) to a local region of a human neutrophil or to create a membrane tether. Pseudopods, which have a cylindrical shape and grow at a constant rate, are formed in either case. After reaching a maximum extension, they retract, even in the presence of chemoattractant. As a pseudopod grows, cell granules begin to penetrate the pseudopod region to a "boundary" that defines a distance to the pseudopod's leading edge that is almost constant. The exclusion of granules from this domain indicates that it is filled with a dense network. The formation of this network involves the plasma membrane because pseudopod growth ceases when a membrane tether is pulled away from the leading edge. The rate of pseudopod growth depends on fMLP concentration just as the number of occupied N-formyl peptide receptors depends on this concentration. The experimental data are explained by assuming that F-actin network is formed next to the plasma membrane. The newly formed network displaces the membrane and the dominant process in the network region then becomes F-actin depolymerization. The rate of pseudopod growth is determined by the rate of the process leading to network formation. This process is apparently an enzymatic type of reaction. It has a positive enthalpy change and, therefore, is endothermic.

Human intestinal intraepithelial lymphocytes have potent chemotactic activity

BACKGROUND & AIMS

Inflammation is created by the movement of leukocytes toward soluble attractants (chemokines). The aim of this study was to identify the chemokines that attract intestinal lymphocytes.

METHODS

Intraepithelial and lamina propria lymphocytes were isolated from human jejunal mucosa of healthy individuals and cultured in interleukin (IL) 2 for 3 days. Migration was assessed using the Boyden transwell assay. Increases in cytoplasmic calcium ion concentration ([Ca2+]i) were measured from changes in fluorescence in Fura-2-loaded lymphocytes on exposure to the cytokines.

RESULTS

A large number of intraepithelial lymphocytes migrated toward IL-8 and RANTES (regulated on activation, normal T cell expressed and secreted), and a small number migrated toward IL-10. In contrast, only a few lamina propria lymphocytes migrated toward IL-8, and none responded to the other chemokines tested. The chemokines IL-8 and IL-10, but not RANTES, increased [Ca2+]i in intraepithelial lymphocytes; this calcium mobilization was not affected by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, indicating that calcium is released from intracellular sources. None of the factors caused calcium mobilization by lamina propria lymphocytes.

CONCLUSIONS

This study shows that intraepithelial lymphocytes have greater migratory and calcium-mobilizing activity than lamina propria lymphocytes and that these two activities do not correlate with each other.

Does cytosolic free Ca2+ signal neutrophil chemotaxis in response to formylated chemotactic peptide?

Cytosolic free Ca2+ concentration was measured and imaged in human neutrophils moving towards a source of formylated peptide in a micropipette held close to the cells. Under these conditions, neutrophils changed shape and displayed chemotaxis without significant or persistent global or localised elevations in cytosolic free Ca2+. A rear-to-front persistent Ca2+ gradient of less than 0.5 nM/micron was present in the migrating neutrophils, until they reached the zone of higher peptide concentration, when an abrupt rise in cytosolic free Ca2+ concentration was triggered and chemotaxis stopped. Small localised rises in cytosolic free Ca2+, which were occasionally observed during neutrophil manoeuvring, were attributed to the effect of local deformation of the neutrophil membrane, since deformation of the membrane with a blunt micropipette caused similar Ca2+ changes. These data suggest that neutrophil chemotaxis towards a source of formylated peptide occurs without significant changes in Ca2+ signalling.

Ca(2+)- and calcineurin-dependent recycling of an integrin to the front of migrating neutrophils

Chemoattractants stimulate neutrophil migration by activating signalling pathways including repeated transient increases in intracellular free calcium, [Ca2+]i. A motile neutrophil sends out many pseudopods, some of which adhere to the substrate; to continue moving forward the cell must release these attachments. Adhesion can be actively regulated, and neutrophils in which [Ca2+]i transients are inhibited become stuck on fibronectin or vitronectin, extracellular matrix proteins that neutrophils encounter in vivo. Function-blocking antibodies to beta 3 integrins or the alpha v beta 3 heterodimer restore motility on vitronectin to [Ca2+]i-buffered cells (B. Hendey, M.A.L., E. Marcantonio and F.R.M., manuscript submitted), indicating that an alpha v beta 3-like integrin is responsible for the [Ca2+]i-sensitive adhesion. We show that the density of alpha v beta 3 integrins in the adherent membrane of neutrophils migrating on vitronectin is much higher at the leading edge than at the rear, but [Ca2+]i buffering or inhibition of Ca(2+)-calmodulin-activated protein phosphatase 2B (calcineurin) leads to accumulation of alpha v beta 3 on the adherent surface at the rear of the cell. We show that the polarized distribution of alpha v beta 3 integrins in migrating neutrophils is maintained by [Ca2+]i-dependent release of adhesion followed by endocytosis of these integrins and recycling to the leading edge.

Activation of the respiratory burst in human eosinophils by chemotaxins requires intracellular calcium fluxes

Eosinophils represent major effector cells in the allergic inflammatory response. Following activation, these cells are capable of mediating tissue damage, particularly by the release of reactive oxygen species. In this study, the role of extracellular and intracellular calcium in the induction of the respiratory burst of human eosinophils was investigated in healthy non-atopic individuals. Pre-incubation of Fura-2-loaded eosinophils with the intracellular calcium chelator 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid prevented the increase of the [Ca++]i following stimulation by RANTES, C5a and PAF, in concentration-dependent fashion, whereas depletion of extracellular calcium in the test medium by ethyl=eneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid was ineffective. To investigate the potential role of extracellular and intracellular calcium on the production of reactive oxygen species, flow-cytometric measurement of H2O2 production by dihydrorhodamine 123 and lucigenin-dependent chemiluminescence were carried out. Chelation of both intracellular and extracellular calcium prevented production of reactive oxygen species after stimulation with C5a, PAF, or RANTES. However, production of reactive oxygen species after stimulation by phorbol myristate acetate, which bypasses post-receptor events by direct activation of protein kinase C, was prevented only after chelation of intracellular but not extracellular calcium. This suggested a Ca(++)-sensitive form of protein kinase C in the activation process of the respiratory burst. These data demonstrate that intracellular and extracellular calcium represent a prerequisite of chemotaxin-induced activation of the respiratory burst in human eosinophils. Thus, intracellular calcium seems to play a central role in the modulation of the respiratory burst in eosinophils and might therefore be an interesting target for drugs that interfere with calcium homeostasis and reduce the tissue destructive power of eosinophils.

Interleukin-7 activates intestinal lymphocytes

Human intestinal lymphocytes, particularly intraepithelial lymphocytes, proliferate minimally to some agents, like mitogens and stimuli of the CD3 pathway. This in vitro finding may be due, in part, to a loss of factors found in vivo. Three T-cell growth factors, IL-7, IL-9, and IL-12, were tested for their ability to stimulate the proliferation of intestinal lymphocytes. Both intraepithelial lymphocytes and lamina propria lymphocytes proliferated more vigorously to IL-7 than to IL-9 or IL-12, and only IL-7 increased stimulation through the CD3 pathway. The IL-7-induced response was IL-2-dependent: IL-2 receptors appeared on both intestinal lymphocyte types, and antibody to the IL-2 receptor blocked IL-7-induced proliferation. Both CD4+ and CD8+ T-cell subsets responded to this cytokine as shown by phenotype-depletion experiments and constancy in the CD4/CD8 ratios after culture with IL-7. In addition, the T-cell receptor alpha beta and gamma delta subsets responded equally well to IL-7. This newly described selective proliferative response of intestinal lymphocytes to IL-7, but not to IL-9 or IL-12, requires no preactivation and may enhance growth in vivo.

Different calcium-dependent pathways control fertilisation-triggered glycoside release and the cortical contraction in ascidian eggs

Fertilisation of ascidian eggs induces the rapid release of a cell surface N-acetylglycosaminidase that blocks sperm binding to vitelline coat sperm receptors resulting in a block to polyspermy. Fertilisation also triggers a large contraction of the egg (thus stimulating ooplasmic segregation) that is completed within 5 min of insemination. In eggs of the ascidian Phallusia mammillata, glycosidase release and cortical contractions are blocked by BAPTA-AM [bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester], a cell-permeant calcium chelator, indicating that both processes are probably dependent on a rise in intracellular calcium levels. Both glycosidase release and the cortical contraction are induced by treatment of the egg with the protein synthesis inhibitor emetine, while only the glycosidase release is induced by isoproterenol, carbachol or acetylcholine. Previous work with ryanodine demonstrated that ryanodine also caused glycosidase release but not the cortical contraction. Inversely, activation by ionomycin in calcium-free sea water causes cortical contractions but not glycosidase release. Thus the two processes can be activated independently. Dextran-coupled (10 kDa) calcium green-1 injected eggs show an increase in intracellular calcium 30-40 s before the cortical contraction is triggered by fertilisation or ionomycin-induced activation. This confirms previous findings that the cortical contraction is a consequence of the activation calcium wave triggered by the sperm. The extracellular calcium requirement for the glycosidase release suggests that calcium influx may be more important for this phase of egg activation. Thus activation of ascidian eggs appears to involve two independent pathways involving calcium.

Calcium transients in growth cones and axons of cultured Helisoma neurons in response to conditioning factors

Accumulating evidence indicates that cytosolic calcium levels regulate growth cone motility and neurite extension. The purpose of this study was to determine if intracellular calcium levels also influence the initiation of neurite extension induced by growth-promoting factors. An in vitro preparation of axotomized neurons that can be maintained in the absence of growth-promoting factors was utilized. The distal axons of cultured Helisoma neurons plated into defined medium do not extend neurites until they are exposed to Helisoma brain-conditioned medium. This provided the opportunity to study the intracellular changes associated with neurite extension. Cytosolic calcium levels were monitored with the calcium-sensitive dye fura 2 at the distal axon. In control medium calcium levels in the distal axon were constant. However, transient elevations in cytosolic calcium in the axonal growth cone occurred after addition of conditioned medium and coincident with the initiation of neurite extension. Application of calcium channel blockers showed that the transients resulted from calcium influx across the neuronal membrane. The transients, however, were not required for neurite extension, although they did influence the rate and extent of neurite outgrowth. Simultaneous extracellular patch recordings demonstrated that the calcium transients were correlated temporally with an increase in rhythmic spontaneous electrical activity of cells, suggesting that conditioned medium influences ionic membrane properties of these neurons.

Control of the alpha 5 beta 1 integrin/fibronectin interaction in vitro by the serine/threonine protein phosphatase calcineurin

Using Chinese hamster ovary cell lysate, an in vitro assay has been developed to study the interaction of fibronectin with the alpha 5 beta 1 integrin in a cytosolic environment. In our solid phase assay, 96-well microtiter plates were coated with fibronectin in which cell lysate was incubated. A dose-dependent binding of the fibronectin receptor onto the coated plastic was immunodetected by specific polyclonal antibodies raised against the alpha 5 beta 1 integrin. Both soluble fibronectin and PB1, a monoclonal antibody raised against the fibronectin receptor, competed with the alpha 5 beta 1 integrin for binding to the fibronectin-coated plastic. General phosphatase inhibitors used during cell lysis completely abolished the fibronectin/integrin interaction in the assay, indicating that the affinity of the fibronectin receptor might be modulated by a protein phosphatase activity. Furthermore, in this assay, the interaction between the fibronectin receptor and its substrate in a cytosolic environment required intracellular calcium. Additionally, the action of more specific phosphatase inhibitors and the inhibition of the integrin/fibronectin interaction by a monoclonal antibody raised against the calcium/calmodulin-dependent protein phosphatase calcineurin suggested that calcineurin allowed the interaction between the alpha 5 beta 1 integrin and fibronectin. Metabolical labeling experiments showed that alpha 5 beta 1 itself was not the target of phosphorylation/dephosphorylation cascades involving calcineurin and leading to the modulation of integrin affinity. Taken together, these results showed that in vitro one substrate of the serine/threonine protein phosphatase calcineurin regulates the alpha 5 beta 1 integrin affinity by interacting with a yet unidentified effector.

Morphological polarization of human polymorphonuclear leucocytes in response to three different chemoattractants: an effector response independent of calcium rise and tyrosine kinases

Chemoattractants such as interleukin-8, C5a and N-formylmethionyl-leucyl-phenylalanine induce a cytosolic calcium rise involved in triggering the secretory functions of human polymorphonuclear leucocytes. We studied the possible role of calcium rise in membrane ruffling, actin polymerization, filamentous actin distribution, and morphological polarization, which are all events contributing to chemotaxis. Membrane ruffling was assessed by right-angle light-scatter changes, the cellular content of polymerized actin by fluorescence of bodipy phallacidin, the intracellular distribution of filamentous actin by fluorescence microscopy and image digitization, and morphological polarization by scanning electron microscopy. Pretreatment of polymorphonuclear leucocytes with 50 microM BAPTA/AM, an intracellular calcium chelator, lowered the basal level in cell calcium and inhibited the transient calcium rise stimulated by 2 nM interleukin-8, 2 nM C5a, and 10 nM N-formylmethionyl-leucyl-phenylalanine. However, BAPTA pretreatment of polymorphonuclear leucocytes did not modify membrane ruffling, actin polymerization, filamentous actin distribution, and morphological polarization stimulated by chemoattractants. Downstream effectors may be protein tyrosine kinases. However, the tyrosine kinase inhibitor tyrphostin did not affect the cytoskeletal characteristics elicited by chemoattractants. Taken together, our results suggest that the transductional pathway leading to cytoskeleton organization and morphological polarization of polymorphonuclear leucocytes is different from that leading to secretion.

Intracellular calcium levels correlate with speed and persistent forward motion in migrating neutrophils

The relationship between cytosolic free calcium concentration ([Ca2+]i) and human neutrophil motility was studied by video microscopy. Neutrophils stimulated by a uniform concentration of an N-formylated peptide chemoattractant (f-Met-Leu-Phe) were tracked during chemokinetic migration on albumin, fibronectin, and vitronectin. [Ca2+]i buffering with quin2 resulted in significant decreases in mean speed on albumin. To further characterize the relationship between [Ca2+]i changes and motility we carried out a cross-correlation analysis of [Ca2+]i with several motility parameters. Cross-correlations between [Ca2+]i and each cell's speed, angle changes, turn strength, and persistent forward motion revealed (i) a positive correlation between [Ca2+]i and cell speed (p < 0.05), (ii) no significant correlation between turns and calcium spikes, and (iii) the occurrence of turns during periods of low speed. Significant negative correlations between [Ca2+]i and angle change were noted on the high adhesion substrates vitronectin and fibronectin but not on the low adhesion substrate albumin. These data imply that there is a general temporal relationship between [Ca2+]i, speed, and persistent motion. However, the correlations are not sufficiently strong to imply that changes in [Ca2+]i are required proximal signals for velocity changes.

Influence of nifedipine on plasma membrane fluidity and oxidative burst of polymorphonuclear leucocytes

It has been demonstrated that the calcium antagonist nifedipine inhibits the reactive oxygen species (ROS) production by polymorphonuclear leucocytes (PMNLs) activated with phorbol myristate acetate (PMA), but the mechanism underlying this effect is still unknown. In the present study we investigated the influence of nifedipine on the PMNL plasma membrane using 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5,hexatriene (TMA-DPH) fluorescence polarization (P) and on PMA- and N-formyl-methionyl-leucyl-phenylalanine (FMLP)-induced ROS production, measured by luminol-dependent chemiluminescence (CL). The plasma membrane fluidity of untreated PMNLs, expressed as P, was 0.371 +/- 0.008. After preincubation of 15 min, nifedipine induced a significant change in P values only at a concentration of 10(-4) M (P = 0.00018). After preincubation of 60 min significant changes in P values were also observed at concentrations of 10(-6) M (P = 0.023) and 10(-7) M (P = 0.023). PMA-induced ROS production by PMNLs was markedly inhibited by nifedipine. Nifedipine also determined a striking change in the FMLP-induced CL response, characterized by both an overall inhibition of PMNL activity and a modification of the kinetics of the oxidative burst (rapid increase in ROS production followed by a pronounced drop in the PMNL response). Such a pattern was found at concentrations of 10(-4) M (preincubation time: 15 min), 10(-6) M and 10(-7) M (preincubation time: 60 min). These findings indicate that nifedipine directly interacts with the PMNLs by inducing a marked decrease in plasma membrane fluidity and an inhibition of the oxidative burst.

Cell membrane stretch in osteoclasts triggers a self-reinforcing Ca2+ entry pathway

Many cell types respond to mechanical membrane perturbation with intracellular Ca2+ responses. Stretch-activated (SA) ion channels may be involved in such responses. We studied the occurrence as well as the underlying mechanisms of cell membrane stretch-evoked responses in fetal chicken osteoclasts using separate and simultaneous patch-clamp and Ca2+ imaging measurements. In the present paper, evidence is presented showing that such responses involve a self-reinforcing mechanism including SA channel activity, Ca(2+)-activated K+ (KCa) channel activity, membrane potential changes and local and general intracellular Ca2+ ([Ca2+]i) increases. The model we propose is that during membrane stretch, both SA channels and KCa channels open at membrane potential values near the resting membrane potential. SA channel characterization showed that these SA channels are permeable to Ca2+. During membrane stretch, Ca2+ influx through SA channels and hyperpolarization due to KCa channel activity serve as positive feedback, leading ultimately to a Ca2+ wave and cell membrane hyperpolarization. This self-reinforcing mechanism is turned off upon SA channel closure after cessation of membrane stretch. We suggest that this Ca2+ entry mechanism plays a role in regulation of osteoclast activity.

Dynamics of triton-insoluble and triton-soluble F-actin pools in calcium-activated human polymorphonuclear leukocytes: evidence for regulation by gelsolin

Gelsolin, a Ca++ activated, 90 kd actin binding protein, can regulate actin polymerization in polymorphonuclear leukocytes (PMNs) via severing of filaments to dissolve gels or by capping of filament ends to limit polymerization. In Triton-lysed PMNs, 30% of gelsolin is bound to the Triton-soluble F-actin (TSF) pool and none is bound to the Triton-insoluble F-actin (TIF) pool. Calcium-activated PMNs exhibit concurrent temporal and quantitative TIF growth and TSF and total F-actin loss. To determine if gelsolin plays a role in regulating TSF pool size, we monitored gelsolin-actin interactions and TIF, TSF and G-actin content at 5 second intervals in PMNs activated with the calcium ionophore, ionomycin. Actin pools were measured by NBDphallacidin binding and by gel scans and expressed relative to basal; gelsolin-actin interactions were measured as change in the amount of EGTA-resistant gelsolin:actin (G:A) complexes and by immunoblot quantification of gelsolin in actin pools. In basal PMNs, 33% of PMN gelsolin is bound in 1:1 EGTA-resistant G:A complexes and TSF and TIF retain 30% and 0% of PMN gelsolin, respectively. By 20 seconds after ionomycin addition, TSF decreases, TIF increases and a fraction of gelsolin repartitions from the TSF to the TIF pool. At maximum change (60 seconds), total F-actin (TIF + TSF) and TSF decrease and TIF increases by 25%; gelsolin is bound to both TSF and TIF (35% of total gelsolin in each pool), and 1:1 EGTA-resistant G:A complexes increase from 33% to 70%. No changes occur in cells activated by ionomycin in the absence of Ca++. The data show Ca++ activated TIF growth and TSF loss are temporally and quantitatively associated with an increase in the percent of gelsolin bound to actin and the translocation of gelsolin from TSF to TIF. This is unique, since no other PMN activator is known to repartition gelsolin into TIF actin. Further, the Ca++ activated initial increase in TIF concurrent with a fall in TSF without a change in total F-actin or G-actin content suggest that TIF grows initially only by TSF annealing/cross-linking to TIF. Gelsolin may regulate these events.