Local and global changes in cytosolic free calcium in neutrophils during chemotaxis and phagocytosis

Localisation of Intracellular Signals and Responses during Phagocytosis

Phagocytosis is one of the most polarised of all cellular activities. Both the stimulus (the target for phagocytosis) and the response (its internalisation) are focussed at just one part of the cell. At the locus, and this locus alone, pseudopodia form a phagocytic cup around the particle, the cytoskeleton is rearranged, the plasma membrane is reorganised, and a new internal organelle, the phagosome, is formed. The effect of signals from the stimulus must, thus, both be complex and yet be restricted in space and time to enable an effective focussed response. While many aspects of phagocytosis are being uncovered, the mechanism for the restriction of signalling or the effects of signalling remains obscure. In this review, the details of the problem of restricting chemical intracellular signalling are presented, with a focus on diffusion into the cytosol and of signalling lipids along the plasma membrane. The possible ways in which simple diffusion is overcome so that the restriction of signalling and effective phagocytosis can be achieved are discussed in the light of recent advances in imaging, biophysics, and cell biochemistry which together are providing new insights into this area.

Efferocytosis requires periphagosomal Ca2+-signaling and TRPM7-mediated electrical activity

Efficient clearance of apoptotic cells by phagocytosis, also known as efferocytosis, is fundamental to developmental biology, organ physiology, and immunology. Macrophages use multiple mechanisms to detect and engulf apoptotic cells, but the signaling pathways that regulate the digestion of the apoptotic cell cargo, such as the dynamic Ca²⁺ signals, are poorly understood. Using an siRNA screen, we identify TRPM7 as a Ca²⁺-conducting ion channel essential for phagosome maturation during efferocytosis. Trpm7-targeted macrophages fail to fully acidify or digest their phagosomal cargo in the absence of TRPM7. Through perforated patch electrophysiology, we demonstrate that TRPM7 mediates a pH-activated cationic current necessary to sustain phagosomal acidification. Using mice expressing a genetically-encoded Ca²⁺ sensor, we observe that phagosome maturation requires peri-phagosomal Ca²⁺-signals dependent on TRPM7. Overall, we reveal TRPM7 as a central regulator of phagosome maturation during macrophage efferocytosis.

Efficient removal of apoptotic cells by phagocytosis underlies tissue development, wound repair, host defense and organ homeostasis. Here, authors identify TRPM7 as a regulator of cargo acidification and Ca2+ signaling during apoptotic cell clearance.

Neutrophil Mechanosignaling Promotes Integrin Engagement With Endothelial Cells and Motility Within Inflamed Vessels

Neutrophils are the most motile of mammalian cells, a feature that enables them to protect the host against the rapid spread of pathogens from tissue into the circulatory system. A critical process is the recruitment of neutrophils to inflamed endothelium within post-capillary venules. This occurs through cooperation between at least four families of adhesion molecules and G-protein coupled signaling receptors. These adhesion molecules convert the drag force induced by blood flow acting on the cell surface into bond tension that resists detachment. A common feature of selectin-glycoprotein tethering and integrin-ICAM bond formation is the mechanics by which force acting on these specific receptor-ligand pairs influences their longevity, strength, and topographic organization on the plasma membrane. Another distinctly mechanical aspect of neutrophil guidance is the capacity of adhesive bonds to convert external mechanical force into internal biochemical signals through the transmission of force from the outside-in at focal sites of adhesive traction on inflamed endothelium. Within this region of the plasma membrane, we denote the inflammatory synapse, Ca²⁺ release, and intracellular signaling provide directional cues that guide actin assembly and myosin driven motive force. This review provides an overview of how bond formation and outside-in signaling controls neutrophil recruitment and migration relative to the hydrodynamic shear force of blood flow.

Extension of chemotactic pseudopods by nonadherent human neutrophils does not require or cause calcium bursts

Global bursts in free intracellular calcium (Ca²⁺) are among the most conspicuous signaling events in immune cells. To test the common view that Ca²⁺ bursts mediate rearrangement of the actin cytoskeleton in response to the activation of G protein-coupled receptors, we combined single-cell manipulation with fluorescence imaging and monitored the Ca²⁺ concentration in individual human neutrophils during complement-mediated chemotaxis. By decoupling purely chemotactic pseudopod formation from cell-substrate adhesion, we showed that physiological concentrations of anaphylatoxins, such as C5a, induced nonadherent human neutrophils to form chemotactic pseudopods but did not elicit Ca²⁺ bursts. By contrast, pathological or supraphysiological concentrations of C5a often triggered Ca²⁺ bursts, but pseudopod protrusion stalled or reversed in such cases, effectively halting chemotaxis, similar to sepsis-associated neutrophil paralysis. The maximum increase in cell surface area during pseudopod extension in pure chemotaxis was much smaller-by a factor of 8-than the known capacity of adherent human neutrophils to expand their surface. Because the measured rise in cortical tension was not sufficient to account for this difference, we attribute the limited deformability to a reduced ability of the cytoskeleton to generate protrusive force in the absence of cell adhesion. Thus, we hypothesize that Ca²⁺ bursts in neutrophils control a mechanistic switch between two distinct modes of cytoskeletal organization and dynamics. A key element of this switch appears to be the expedient coordination of adhesion-dependent lock or release events of cytoskeletal membrane anchors.

Topographical interrogation of the living cell surface reveals its role in rapid cell shape changes during phagocytosis and spreading

Dramatic and rapid changes in cell shape are perhaps best exemplified by phagocytes, such as neutrophils. These cells complete the processes of spreading onto surfaces, and phagocytosis within 100 s of stimulation. Although these cell shape changes are accompanied by an apparent large increase in cell surface area, the nature of the membrane “reservoir” for the additional area is unclear. One proposal is that the wrinkled cell surface topography (which forms micro-ridges on the neutrophil surface) provides the resource for neutrophils to expand their available surface area. However, it has been problematic to test this proposal in living cells because these surface structures are sub-light microscopic. In this paper, we report the development of a novel approach, a variant of FRAP (fluorescent recovery after photo-bleaching) modified to interrogate the diffusion path-lengths of membrane associated molecules. This approach provides clear evidence that the cell surface topography changes dramatically during neutrophil shape change (both locally and globally) and can be triggered by elevating cytosolic Ca²⁺.

Leucocyte Rheology at Baseline and after Activation in Post-Phlebitic Syndrome

Objective:

To evaluate leucocyte rheology, expressed as leucocyte filtration, polymorphonuclear leucocyte (PMN) membrane fluidity and cytosolic Ca2+ concentration in subjects with post-phlebitic leg syndrome (PPS).

Methods:

In 22 subjects with PPS we determined leucocyte filtration [unfractionated, mononuclear (MN) and PMN cells], employing the St George Filtrometer, PMN membrane fluidity using the fluorescent probe 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) and PMN cytosolic Ca2+ concentration using the fluorescent probe Fura 2-AM. Subsequently we determined the same PMN parameters after in vitro activation with 4-phorbol 12-myristate 13-acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP).

Results:

At baseline we observed a difference in the filtration parameters of unfractionated and MN cells and an increase in PMN cytosolic Ca2+ concentration. After activation, a significant variation in PMN filtration parameters was evident both in normals and in PPS subjects, although in subjects with PPS this variation, especially with PMA, was significantly greater. We found a decrease in PMN membrane fluidity and an increase in PMN cytosolic Ca2+ concentration only in subjects with PPS.

Conclusion:

These results suggest that there is a functional alteration of systemic leucocytes in PPS, in which the mechanisms are not yet clear.

Ca2+ dynamics correlates with phenotype and function in primary human neutrophils

Central to the immune defense function of neutrophils is to sense, to move and to kill. Neutrophils acquire distinct cellular states necessary to fulfill these functions each associated with a particular phenotype. The cells constituting the neutrophil population are presumably not synchronized with respect to their actual state, e.g. due to maturity or preactivation. It is also likely that they exhibit a different degree of phenotypic plasticity (that is, the ability to switch to a particular state). Calcium is known to play a crucial role in neutrophils such as for cell motility. The present study focuses on characterizing the cell-to-cell variability at the morphological as well as at the level of calcium dynamics by studying single primary human neutrophils. We apply long-term multivariate live cell imaging to (i) characterize neutrophil phenotypes of different functional states, (ii) analyze the distribution of cells being in these states and, (iii) study the individual intracellular calcium response simultaneously with shape changes. We are able to differentiate the five distinct subpopulations of neutrophils based on quantitative parameters of cell morphology and motility. As a major result, we demonstrate that the calcium dynamics of individual cells correlates with their respective functional state. Finally, we see a number of cells that undergo spontaneous phenotypic changes from one cellular state to another. These events are preceded either by exhibiting the calcium dynamics of the future state or by switching to the respective calcium dynamics in parallel to switching the morphology. Based on our results we conclude that specific calcium dynamics carries crucial information for the function and phenotype of neutrophils.

Chemokines, selectins and intracellular calcium flux: temporal and spatial cues for leukocyte arrest

Leukocyte trafficking to acute sites of injury or infection requires spatial and temporal cues that fine tune precise sites of firm adhesion and guide migration to endothelial junctions where they undergo diapedesis to sites of insult. Many detailed studies on the location and gradient of chemokines such as IL-8 and other CXCR ligands reveal that their recognition shortly after selectin-mediated capture and rolling exerts acute effects on integrin activation and subsequent binding to their ligands on the endothelium, which directs firm adhesion, adhesion strengthening, and downstream migration. In this process, G-protein coupled receptor (GPCR) signaling has been found to play an integral role in activating and mobilizing intracellular stores of calcium, GTPases such as Rap-1 and Rho and cytokeletal proteins such as Talin and F-actin to facilitate cell polarity and directional pseudopod formation. A critical question remaining is how intracellular Ca(2+) flux from CRAC channels such as Orai1 synergizes with cytosolic stores to mediate a rapid flux which is critical to the onset of PMN arrest and polarization. Our review will highlight a specific role for calcium as a signaling messenger in activating focal clusters of integrins bound to the cytoskeleton which allows the cell to attain a migratory phenotype. The precise interplay between chemokines, selectins, and integrins binding under the ubiquitous presence of shear stress from blood flow provides an essential cooperative signaling mechanism for effective leukocyte recruitment.

The role of CD38 in Fcγ receptor (FcγR)-mediated phagocytosis in murine macrophages

Phagocytosis is a crucial event in the immune system that allows cells to engulf and eliminate pathogens. This is mediated through the action of immunoglobulin (IgG)-opsonized microbes acting on Fcγ receptors (FcγR) on macrophages, which results in sustained levels of intracellular Ca(2+) through the mobilization of Ca(2+) second messengers. It is known that the ADP-ribosyl cyclase is responsible for the rise in Ca(2+) levels after FcγR activation. However, it is unclear whether and how CD38 is involved in FcγR-mediated phagocytosis. Here we show that CD38 is recruited to the forming phagosomes during phagocytosis of IgG-opsonized particles and produces cyclic-ADP-ribose, which acts on ER Ca(2+) stores, thus allowing an increase in FcγR activation-mediated phagocytosis. Ca(2+) data show that pretreatment of J774A.1 macrophages with 8-bromo-cADPR, ryanodine, blebbistatin, and various store-operated Ca(2+) inhibitors prevented the long-lasting Ca(2+) signal, which significantly reduced the number of ingested opsonized particles. Ex vivo data with macrophages extracted from CD38(-/-) mice also shows a reduced Ca(2+) signaling and phagocytic index. Furthermore, a significantly reduced phagocytic index of Mycobacterium bovis BCG was shown in macrophages from CD38(-/-) mice in vivo. This study suggests a crucial role of CD38 in FcγR-mediated phagocytosis through its recruitment to the phagosome and mobilization of cADPR-induced intracellular Ca(2+) and store-operated extracellular Ca(2+) influx.

The human macrophage sodium channel NaV1.5 regulates mycobacteria processing through organelle polarization and localized calcium oscillations

Phagocytosis and intracellular processing of mycobacteria by macrophages are complex cellular processes that require spatial and temporal coordination of particle uptake, organelle movement, activation of signaling pathways, and channel-mediated ionic flux. Recent work demonstrated that human macrophage NaV1.5, an intracellular voltage-gated sodium channel expressed on late endosomes, enhances endosomal acidification and phagocytosis. Here, using bacillus Camille-Guerin (BCG) as a model of mycobacterial infection, we examined how this channel regulates phagocytosis and phagosome maturation in human macrophages. Knockdown of NaV1.5 reduced high capacity uptake of labeled BCG. BCG-containing, NaV1.5-expressing cells demonstrated localization of NaV1.5 and Rab-7 positive endosomes and mitochondria to periphagosome regions that was not observed in NaV1.5-deficient cells. Knockdown of the channel reduced the initial calcium response following bacterial challenge and prevented the generation of prolonged and localized calcium oscillations during phagosome maturation. Inhibition of the mitochondrial Na(+) /Ca(2+) exchanger also prevented prolonged calcium oscillations during phagosome maturation. These results suggest that NaV1.5 and mitochondrial-dependent calcium signaling regulate mycobacteria phagocytosis and phagosome maturation in human macrophages through spatial-temporal coordination of calcium signaling within a unique subcellular region.

Essential role for calcium waves in migration of human vascular smooth muscle cells

Vascular smooth muscle cell (SMC) migration is characterized by extension of the lamellipodia at the leading edge, lamellipodial attachment to substrate, and release of the rear (uropod) of the cell, all of which enable forward movement. However, little is known regarding the role of intracellular cytosolic Ca(2+) concentration ([Ca(2+)](i)) in coordinating these distinct activities of migrating SMCs. The objective of our study was to determine whether regional changes of Ca(2+) orchestrate the migratory cycle in human vascular SMCs. We carried out Ca(2+) imaging using digital fluorescence microscopy of fura-2 loaded human smooth muscle cells. We found that motile SMCs exhibited Ca(2+) waves that characteristically swept from the rear of polarized cells toward the leading edge. Ca(2+) waves were less evident in nonpolarized, stationary cells, although acute stimulation of these SMCs with the agonists platelet-derived growth factor-BB or histamine could elicit transient rise of [Ca(2+)](i). To investigate a role for Ca(2+) waves in the migratory cycle, we loaded cells with the Ca(2+) chelator BAPTA, which abolished Ca(2+) waves and significantly reduced retraction, supporting a causal role for Ca(2+) in initiation of retraction. However, lamellipod motility was still evident in BAPTA-loaded cells. The incidence of Ca(2+) oscillations was reduced when Ca(2+) release from intracellular stores was disrupted with the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin or by treatment with the inositol 1,4,5-trisphosphate receptor blocker 2-aminoethoxy-diphenyl borate or xestospongin C, implicating Ca(2+) stores in generation of waves. We conclude that Ca(2+) waves are essential for migration of human vascular SMCs and can encode cell polarity.

Acute-phase protein α1-antitrypsin inhibits neutrophil calpain I and induces random migration

A rapid recruitment of neutrophils to sites of injury or infection is a hallmark of the inflammatory response and is required for effective host defense against pathogenic stimuli. However, neutrophil-mediated inflammation can also lead to chronic tissue destruction; therefore, a better understanding of the mechanisms underlying neutrophil influx and activation is of critical importance. We have previously shown that the acute phase protein α1-antitrypsin (AAT) inhibits neutrophil chemotaxis. In this study, we examine mechanisms related to the effect of AAT on neutrophil responses. We report a previously unknown function of AAT to inactivate calpain I (μ-calpain) and to induce a rapid cell polarization and random migration. These effects of AAT coincided with a transient rise in intracellular calcium, increase in intracellular lipids, activation of the Rho GTPases, Rac1 and Cdc42, and extra-cellular signal-regulated kinase (ERK1/2). Furthermore, AAT caused a significant inhibition of nonstimulated as well as formyl-met-leu-phe (fMLP)-stimulated neutrophil adhesion to fibronectin, strongly inhibited lipopolysaccharide-induced IL-8 release and slightly delayed neutrophil apoptosis. The results presented here broaden our understanding of the regulation of calpain-related neutrophil functional activities, and provide the impetus for new studies to define the role of AAT and other acute phase proteins in health and disease.

Mechanisms of failed apoptotic cell clearance by phagocyte subsets in cardiovascular disease

Recent evidence in humans indicate that defective phagocytic clearance of dying cells is linked to progression of advanced atherosclerotic lesions, the precursor to atherothrombosis, ischemic heart disease, and leading cause of death in the industrialized world. During atherogenesis, apoptotic cell turnover in the vascular wall is counterbalanced by neighboring phagocytes with high clearance efficiency, thereby limiting cellularity and maintaining lesion integrity. However, as lesions mature, phagocytic removal of apoptotic cells (efferocytosis) becomes defective, leading to secondary necrosis, expansion of plaque necrotic cores, and susceptibility to rupture. Recent genetic causation studies in experimental rodents have implicated key molecular regulators of efferocytosis in atherosclerotic progression. These include MER tyrosine kinase (MERTK), milk fat globule-EGF factor 8 (MFGE8), and complement C1q. At the cellular level, atheromata are infiltrated by a heterogenous population of professional phagocytes, comprised of monocytes, differentiated macrophages, and CD11c(+) dendritic-like cells. Each cell type is characterized by disparate clearance efficiencies and varying activities of key phagocytic signaling molecules. It is in this context that we outline a working model whereby plaque necrosis and destabilization is jointly promoted by (1) direct inhibition of core phagocytic signaling pathways and (2) expansion of phagocyte subsets with poor clearance capacity. Towards identifying targets for promoting efficient apoptotic cell clearance and resolving inflammation in atherosclerosis and during ischemic heart disease and post myocardial infarction, this review will discuss potential in vivo suppressors of efferocytosis at each stage of clearance and how these putative interventional targets may differentially affect uptake at the level of vascular phagocyte subsets.

The role of calcium signaling in phagocytosis

Immune cells kill microbes by engulfing them in a membrane-enclosed compartment, the phagosome. Phagocytosis is initiated when foreign particles bind to receptors on the membrane of phagocytes. The best-studied phagocytic receptors, those for Igs (FcgammaR) and for complement proteins (CR), activate PLC and PLD, resulting in the intracellular production of the Ca(2+)-mobilizing second messengers InsP3 and S1P, respectively. The ensuing release of Ca(2+) from the ER activates SOCE channels in the plasma and/or phagosomal membrane, leading to sustained or oscillatory elevations in cytosolic Ca(2+) concentration. Cytosolic Ca(2+) elevations are required for efficient ingestion of foreign particles by some, but not all, phagocytic receptors and stringently control the subsequent steps involved in the maturation of phagosomes. Ca(2+) is required for the solubilization of the actin meshwork that surrounds nascent phagosomes, for the fusion of phagosomes with granules containing lytic enzymes, and for the assembly and activation of the superoxide-generating NADPH oxidase complex. Furthermore, Ca(2+) entry only occurs at physiological voltages and therefore, requires the activity of proton channels that counteract the depolarizing action of the phagocytic oxidase. The molecules that mediate Ca(2+) ion flux across the phagosomal membrane are still unknown but likely include the ubiquitous SOCE channels and possibly other types of Ca(2+) channels such as LGCC and VGCC. Understanding the molecular basis of the Ca(2+) signals that control phagocytosis might provide new, therapeutic tools against pathogens that subvert phagocytic killing.

Purinergic regulation of neutrophil chemotaxis

Chemotaxis allows polymorphonuclear neutrophils (PMN) to rapidly reach infected and inflamed sites. However, excessive influx of PMN damages host tissues. Better knowledge of the mechanisms that control PMN chemotaxis may lead to improved treatments of inflammatory diseases. Recent findings suggest that ATP and adenosine are involved in PMN chemotaxis. Therefore, these purinergic signaling processes may be suitable targets for novel therapeutic approaches to ameliorate host tissue damage.

Observation of calcium microdomains at the uropod of living morphologically polarized human neutrophils using flash lamp-based fluorescence microscopy

The present study outlines improved strategies for ratiometric imaging of cell calcium using a flash lamp-based excitation method and its application to neutrophil polarization. A brief (approximately 6 micros) and intense flash was used to excite the Fluo-4 and Fura Red calcium dye combination in morphologically polarized human neutrophils. These illumination conditions do not allow the dye or calcium ions to diffuse significant distances during the exposure period. Buffer conditions such as pH, pyruvate concentration, and glucose levels were adjusted to more faithfully replicate these parameters in sepsis patients. Fluorescence images at both dyes' emission wavelengths were simultaneously collected using a Dual-View apparatus and an ICCD camera. The ratiometric images, when viewed as single frames or averaged image stacks, clearly demonstrated high calcium probe ratios at the uropod and comparatively low ratios at the cell body that were not evident using conventional imaging methods with longer exposure times. Calcium signaling at the uropod is likely associated with cytoskeletal remodeling during cell motility.

Mechano-chemical signaling maintains the rapid movement of Dictyostelium cells

The survival of Dictyostelium cells depends on their ability to efficiently chemotax, either towards food or to form multicellular aggregates. Although the involvement of Ca2+ signaling during chemotaxis is well known, it is not clear how this regulates cell movement. Previously, fish epithelial keratocytes have been shown to display transient increases in intracellular calcium ([Ca2+]i) that are mediated by stretch-activated calcium channels (SACs), which play a role in retraction of the cell body [J. Lee, A. Ishihara, G. Oxford, B. Johnson, and K. Jacobson, Regulation of cell movement is mediated by stretch-activated calcium channels. Nature, 1999. 400(6742): p. 382-6.]. To investigate the involvement of SACs in Dictyostelium movement we performed high resolution calcium imaging in wild-type (NC4A2) Dictyostelium cells to detect changes in [Ca2+]i. We observed small, brief, Ca2+ transients in randomly moving wild-type cells that were dependent on both intracellular and extracellular sources of calcium. Treatment of cells with the SAC blocker gadolinium (Gd3+) inhibited transients and decreased cell speed, consistent with the involvement of SACs in regulating Dictyostelium motility. Additional support for SAC activity was given by the increase in frequency of Ca2+ transients when Dictyostelium cells were moving on a more adhesive substratum or when they were mechanically stretched. We conclude that mechano-chemical signaling via SACs plays a major role in maintaining the rapid movement of Dictyostelium cells.

Leukocyte membrane "expansion": a central mechanism for leukocyte extravasation

The infiltration of inflamed tissues by leukocytes is a key event in the development and progression of inflammation. Although individual cytokines, which coordinate extravasation, have become the targets for therapy, a mechanism that is common to white cell extravasation, regardless of the specific molecular mechanism involved, would represent a more attractive therapeutic target. Such a target may be represented by the events underlying the spreading of leukocytes on the endothelium, which is a necessary prelude to extravasation. This leukocyte "spreading" involves an apparent increase in the cell surface area. The aim of this review is to examine whether the mechanism underlying the apparent expansion of plasma membrane surface area during leukocyte extravasation could be an "Achilles' heel," which is amenable to therapeutic intervention. In this short review, we evaluate the models proposed for the mechanism of membrane "expansion" and discuss recent data, which point to a mechanism of membrane "unwrinkling." The molecular pathway for the unwrinkling of the leukocyte plasma membrane may involve Ca2+ activation of mu-calpain and cleavage of cytoskeletal linkage molecules such as talin and ezrin. This route could be common to all extravasation signals and thus, represents a potential target for anti-inflammatory therapy.

Altered Ca2+ homeostasis in polymorphonuclear leukocytes from chronic myeloid leukaemia patients

Background

In polymorphonuclear leukocytes (PMNL), mobilization of calcium ions is one of the early events triggered by binding of chemoattractant to its receptors. Besides chemotaxis, a variety of other functional responses are dependent on calcium ion mobilization. PMNL from chronic myeloid leukaemia (CML) patients that were morphologically indistinguishable from normal PMNL were found to be defective in various functions stimulated by a chemoattractant – fMLP. To study the mechanism underlying defective functions in CML PMNL, we studied calcium mobilization in CML PMNL in response to two different classical chemoattractants, fMLP and C5a.

Results

Release of calcium estimated by flow cytometry and spectrofluorimetry using fluo-3 as an indicator showed that the [Ca²⁺]_i levels were lower in CML PMNL as compared to those in normal PMNL. But, both normal and CML PMNL showed maximum [Ca²⁺]_i in response to fMLP and C5a at 10 sec and 30 sec, respectively. Spectrofluorimetric analysis of the total calcium release in chemoattractant treated PMNL indicated more and faster efflux of [Ca²⁺]_i in CML PMNL as compared to normal PMNL.

Conclusion

Fine-tuning of Ca²⁺ homeostasis was altered in CML PMNL. The altered Ca²⁺ homeostasis may contribute to the defective functions of CML PMNL.

Ca2+ release from host intracellular stores and related signal transduction during Campylobacter jejuni 81-176 internalization into human intestinal cells

Campylobacter jejuni is the leading bacterial cause of human diarrhoeal disease in many parts of the world, including the USA. The ability of C. jejuni to invade the host intestinal epithelium is an important determinant of virulence. A common theme among pathogenic invasive micro-organisms is their ability to usurp the eukaryotic cell-signalling systems both to allow for invasion and to trigger disease pathogenesis. Ca(2+) is very important in a great variety of eukaryotic cell-signalling processes (e.g. calmodulin-activated enzymes, nuclear transcriptional upregulation, and cytoskeletal rearrangements). This study analyses the effects of Ca(2+) availability on invasion of human INT407 intestinal epithelial cells by C. jejuni strain 81-176. The ability of C. jejuni to invade INT407 cells was not blocked by chelation of any remaining extracellular Ca(2+) from host cells incubated in Ca(2+)-free, serum-free media. In contrast, C. jejuni invasion was markedly reduced either by chelating host intracellular Ca(2+) with 1,2-bis-(2-)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA, AM) or by blocking the release of Ca(2+) from intracellular stores with dantrolene or U73122. Moreover, Bay K8644, a plasma-membrane Ca(2+)-channel agonist, was observed to stimulate C. jejuni invasion, presumably by increasing host intracellular free Ca(2+) levels. Measurement of host-cell cytosolic Ca(2+) via spectrofluorimetry and fluorescence microscopy revealed an increase in Ca(2+) from 10 min post-infection. Monolayer pretreatment with either a calmodulin antagonist or a specific inhibitor of protein kinase C was found to cause a marked reduction in C. jejuni invasion, suggesting roles for these Ca(2+)-activated modulators in signal-transduction events involved in C. jejuni invasion. These results demonstrate that C. jejuni induces the mobilization of Ca(2+) from host intracellular stores, which is an essential step in the invasion of intestinal cells by this pathogen.

Ratio imaging instrumentation

Using ratio imaging to obtain quantitative information from microscope images is a powerful tool that has been used successfully in numerous studies. Although ratio imaging reduces the effects of many parameters that can interfere with accurate measurements, it is not a panacea. In designing a ratio imaging experiment, all of the potential problems discussed in this chapter must be considered. Undoubtedly, other problems that were not discussed can also interfere with accurate and meaningful measurements. Many of the problems discussed here were observed in the authors' laboratories. In our experience there are no standard routines or methods that can foresee every problem before it has been encountered. Good experimental design can minimize problems, but the investigator must continue to be alert. Progress in instrumentation continues to overcome some of the difficulties encountered in ratio imaging. CCD cameras with 12- to 14-bit pixel depth are being used more frequently, and several confocal microscope manufacturers are now also using 12-bit digitization. The dramatic increase in the use of confocal microscopes over the past decade is now causing microscope manufacturers to more critically evaluate the effect of axial chromatic aberration in objectives, and recent designs to minimize this problem are being implemented. Other developments such as the use of AOTFs to attenuate laser lines extend the applicability of ratio imaging. Ratio imaging is clearly applicable to a wide range of cell biological problems beyond its widespread use for measuring ion concentrations. Imaginative but careful use of this technique should continue to provide novel insights into the properties of cells.

Chemotactic signaling pathways in neutrophils: from receptor to actin assembly

In this review, we present an overview of the signaling elements between neutrophil chemotactic receptors and the actin cytoskeleton that drives cell motility. From receptor-ligand interactions, activation of heterotrimeric G-proteins, their downstream effectors PLC and PI-3 kinase, the activation of small GTPases of the Rho family, and their regulation of particular cytoskeletal regulatory proteins, we describe pathways specific to the chemotaxing neutrophil and elements documented to be important for neutrophil function.

Cytosolic free Ca(2+) changes and calpain activation are required for beta integrin-accelerated phagocytosis by human neutrophils

Phagocytosis of microbes coated with opsonins such as the complement component C3bi is the key activity of neutrophils. However, the mechanism by which opsonins enhance the rate of phagocytosis by these cells is unknown and has been difficult to study, partly because of the problem of observing and quantifying the events associated with phagocytosis. In this study, C3bi-opsonized particles were presented to neutrophils with a micromanipulator, so that the events of binding, pseudopod cup formation, engulfment, and completion of phagocytosis were clearly defined and distinguished from those involved with chemotaxis. Using this approach in combination with simultaneous phase contrast and Ca(2+) imaging, the temporal relationship between changes in cytosolic free Ca(2+) concentration and phagocytosis were correlated. Here we show that whereas small, localized Ca(2+) changes occur at the site of particle attachment and cup formation as a result of store release, rapid engulfment of the particle required a global change in cytosolic free Ca(2+) which resulted from Ca(2+) influx. This latter rise in cytosolic free Ca(2+) concentration also liberated a fraction of beta2 integrin receptors which were initially immobile on the neutrophil surface, as demonstrable by both fluorescence recovery after laser bleaching and by visualization of localized beta2 integrin labelling. Inhibitors of calpain activation prevented both the Ca(2+)-induced liberation of beta2 integrin and the rapid stage of phagocytosis, despite the persistence of the global Ca(2+) signal. Therefore, we propose that Ca(2+) activation of calpain causes beta2 integrin liberation, and that this signal plays a key role in the acceleration of beta2 integrin-mediated phagocytosis.

Growth factors but not gap junctions play a role in injury-induced Ca2+ waves in epithelial cells

This paper characterizes the early responses of epithelial cells to injury. Ca2+ is an important early messenger that transiently increases in the cytoplasm of cells in response to external stimuli. Its elevation leads to the regulation of signaling pathways responsible for the downstream events important for wound repair, such as cell migration and proliferation. Live cell imaging in combination with confocal laser scanning microscopy of fluo-3 AM loaded cells was performed. We found that mechanical injury in a confluent region of cells creates an elevation in Ca2+ that is immediately initiated at the wound edge and travels as a wave to neighboring cells, with [Ca2+]i returning to background levels within two minutes. Addition of epidermal growth factor (EGF), but not platelet-derived growth factor-BB, resulted in increased [Ca2+]i, and EGF specifically enhanced the amplitude and duration of the injury-induced Ca2+ wave. Propagation of the Ca2+ wave was dependent on intracellular Ca2+ stores, as was demonstrated using both thapsigargin and Ca2+ chelators (EGTA and BAPTA/AM). Injury-induced Ca2+ waves were not mediated via gap junctions, as the gap-junction inhibitors 1-heptanol and 18α-glycyrrhetinic acid did not alter wave propagation, nor did the cells recover in photobleaching experiments. Additional studies also demonstrated that the wave could propagate across an acellular region. The propagation of the injury-induced Ca2+ wave occurs via diffusion of an extracellular mediator, most probably via a nucleotide such as ATP or UTP, that is released upon cell damage.

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Granulocyte integrins before and after activation in acute ischaemic stroke

We examined in 19 subjects with acute ischaemic stroke (AIS) the PMN integrin pattern (CD11a, CD11b, CD11c, CD18), using indirect immunofluorescence and adopting a flow cytometer, at baseline and during activation, prolonged for 5 and 15 min, with 4-phorbol 12-myristate 13-acetate (PMA). At baseline, an increase in the expression of CD11c and CD18 and a decrease in the CD11b were evident in AIS subjects compared to normals. After activation, we found in normals a constant and significant increase of all PMN adhesive molecules, while in AIS subjects, we found an increase in CD11b and CD18, a decrease in CD11a and no variation in CD11c. While the basal upregulation of CD11c and CD18 may depend on the PMN spontaneous activation or on the increase of cytokines, the decrease of CD11b may be due to its self-consumption. After activation, the decrease in CD11a noted in AIS may be related to its cleavage or to an altered integrin phosphorylation/dephosphorylation balance.

Polymorphonuclear integrins, membrane fluidity, and cytosolic Ca(2+) content after activation in essential hypertension

The purpose of this research was to obtain further information about the role of polymorphonuclear leukocytes in essential hypertension. These cells could be involved in the pathogenesis of organ injury. Thirty subjects (14 men and 16 women) with essential hypertension were enrolled. In these subjects we determined, at baseline and after in vitro activation with 4-phorbol 12-myristate 13-acetate and N:-formyl-methionyl-leucyl-phenylalanine, the polymorphonuclear leukocyte membrane fluidity, obtained by labeling the cells with 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3, 5-hexatriene, cytosolic Ca(2+) concentration, obtained by marking the cells with Fura 2-AM, and integrin pattern (CD11a, CD11b, CD11c, and CD18), by using the indirect immunofluorescence with a flow cytometer. At baseline there was no difference in membrane fluidity between normal subjects and hypertensives, whereas hypertensives showed an increase in cytosolic Ca(2+) content and an increase of the phenotypical expression of CD11a, CD11b, and CD18. In normal subjects and in hypertensives, after activation, no variation was found in membrane fluidity and cytosolic Ca(2+) content. In normal subjects, after activation, we observed a significant increase of the expression of all adhesion molecules, whereas in hypertensives we found an increase of the expression of CD11b, CD11c, and CD18 but also a decrease of CD11a. The behavior of the polymorphonuclear leukocyte integrin profile may have several explanations, and in particular, the trend of CD11a after chemotactic activation may be related to its cleavage or to an altered integrin phosphorylation/dephosphorylation balance hypothetically present in this clinical condition.

Acute ischemic stroke : polymorphonuclear leukocyte membrane fluidity and cytosolic Ca(2+) concentration at baseline and after chemotactic activation

BACKGROUND AND PURPOSE

Several reports have considered the role of systemic leukocytes in acute ischemic stroke (AIS). Initially, greater attention was focused on the leukocyte count and subsequently on their adhesiveness, aggregation, rheology, and activation. The aim of this study was the evaluation of certain polymorphonuclear leukocyte (PMN) parameters, reflecting their rheology and activation, in subjects with AIS.

METHODS

In a group of 19 subjects with AIS and in a control group of 18 subjects with asymptomatic vascular atherosclerotic disease, we evaluated the PMN membrane fluidity and cytosolic Ca(2+) concentration at baseline and after in vitro chemotactic activation with 4-phorbol 12-myristate 13-acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP).

RESULTS

From the obtained data, it is evident that at baseline only PMN membrane fluidity distinguishes control subjects from AIS subjects. After PMN activation with PMA and fMLP, prolonged for 5 and 15 minutes, we found an increase in PMN cytosolic Ca(2+) concentration and a decrease in PMN membrane fluidity only in subjects with AIS.

CONCLUSIONS

These findings emphasize that in subjects with AIS a functional alteration of systemic PMN cells is clearly expressed during chemotactic activation, although the mechanism of this abnormality is not yet explained.

Leukocyte rheology before and after chemotactic activation in some venous diseases

OBJECTIVE

to evaluate leukocyte rheology, polymorphonuclear leukocyte (PMN) membrane fluidity and cytosolic Ca2+ concentration in subjects with post-phlebitic leg syndrome (PPS) and acute deep-venous leg thrombosis (DVT).

SUBJECTS

twenty-two subjects with leg PPS and 14 subjects with leg DVT.

METHODS

we evaluated the leukocyte filtration (unfractionated, mononuclear cells (MN) and PMN), the PMN membrane fluidity and the PMN cytosolic Ca2+ concentration. Subsequently, we evaluated the same PMN variables after in vitro chemotactic activation with 4-phorbol 12-myristate 13-acetate (PMA) and N -formyl-methionyl-leucyl-phenylalanine (fMLP).

RESULTS

at baseline we observed a significant difference in the filtration variables of unfractionated and MN cells and in PMN cytosolic Ca2+ concentration. After activation, in normal subjects and subjects with PPS and DVT, a significant variation in PMN filtration at 5 and 15 minutes was evident. In normal subjects, no variation was present in PMN membrane fluidity or cytosolic Ca2+ concentration after activation. In subjects with PPS and DVT, we found a decrease in PMN membrane fluidity and an increase in PMN cytosolic Ca2+ concentration. After PMN activation (at 5 and 15 min) Delta% of IRFR distinguished normal subjects from subjects with PPS and DVT, while no difference was found in Delta% of membrane fluidity or cytosolic Ca2+ concentration.

CONCLUSIONS

there is a functional alteration of leukocytes in these patients whose mechanisms are not yet clear.

Orientation of chemotactic cells and growth cones: models and mechanisms

A model is proposed for an amplification step in chemotactically sensitive cells or growth cones that accounts for their extraordinary directional sensitivity. It is assumed that cells have an intrinsic pattern forming system that generates the signals for extension of filopods and lamellipods. An external signal such as a graded cue is assumed to impose some directional preference onto the pattern formed. According to the model, a saturating, self-enhancing reaction is coupled with two antagonistic reactions. One antagonist equilibrates rapidly over the whole cell, causing competition between different surface elements of the cell cortex for activation. It will be won by those cortical regions of the cell that are exposed to the highest concentrations of the external graded cues. The second antagonistic reaction is assumed to act more locally and has a longer time constant. It causes a destabilization of peaks after they have formed. While the total activated area on the cell surface is maintained, the disappearance of some hot spots allows the formation of new ones, preferentially at positions specified by the actual external guiding signal. Computer simulations show that the model accounts for the highly dynamic behaviour of chemotactic cells and growth cones. In the absence of external signals, maxima of the internal signals emerge at random positions and disappear after some time. Travelling waves or oscillations in counter phase can emerge on the cell cortex, in agreement with observations reported in the literature. In other ranges of parameters, the model accounts for the generation of a stable cell polarity.

Molecular basis of the interaction of Salmonella with the intestinal mucosa

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed.

Yersinia pseudotuberculosis-induced calcium signaling in neutrophils is blocked by the virulence effector YopH

Pathogenic species of the genus Yersinia evade the bactericidal functions of phagocytes. This evasion is mediated through their virulence effectors, Yops, which act within target cells. In this study we investigated the effect of Yersinia pseudotuberculosis on Ca2+ signaling in polymorphonuclear neutrophils. The intracellular free calcium concentration in single adherent human neutrophils was monitored during bacterial infection and, in parallel, the encounter between the bacteria and cells was observed. When a plasmid-cured strain was used for infection, adherence of a single bacterium to the cellular surface induced a beta1 integrin-dependent transient increase in the intracellular concentration of free calcium. This was, however, not seen with Yop-expressing wild-type bacteria, which adhered to the cell surface without generating any Ca2+ signal. Importantly, the overall Ca2+ homeostasis was not affected by the wild-type strain; the Ca2+ signal mediated by the G-protein-coupled formyl-methionyl-leucyl-phenylalanine receptor was still functioning. Hence, the blocking effect was restricted to certain receptors and their signaling pathways. The use of different Yop mutant strains revealed that the protein tyrosine phosphatase YopH was responsible for the inhibition. This virulence determinant has previously been implicated in very rapid Yersinia-mediated effects on target cells as the key effector in the blockage of phagocytic uptake. The present finding, that Y. pseudotuberculosis, via YopH, specifically inhibits a self-induced immediate-early Ca2+ signal in neutrophils, offers more-detailed information concerning the effectiveness of this virulence effector and implies an effect on Ca2+-dependent, downstream signals.

Mechanisms of IL-8-induced Ca2+ signaling in human neutrophil granulocytes

Interleukin-8 (IL-8) plays an important role in the activation of neutrophil granulocytes. Although intracellular Ca2+ signals are essential in this process, they have not been studied in great detail so far. Here, we have measured IL-8-induced Ca2+ signals in single human neutrophil granulocytes using the Ca2+ indicator dye FURA-2 AM and we have investigated the signal transduction that leads to these Ca2+ signals with various pharmacological tools. Our results indicate that IL-8-induced Ca2+ signals consist of at least two components. An initial fast component was followed by a smaller and more persistent one. The initial Ca2+ signal was independent of extracellular Ca2+. It required the activation of phospholipase C via a pertussis toxin sensitive G-protein and was due to activation of IP3 receptor-coupled Ca2+ release channels. The late phase of the Ca2+ signal was suppressed when extracellular Ca2+ was removed suggesting that it was generated by Ca2+ influx through Ca2+ release-activated Ca2+ (CRAC) channels. This Ca2+ influx may prolong IL-8-induced Ca2+ signals during granulocyte 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.

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.

Wound-induced calcium waves in alveolar type II cells

Alveolar type II epithelial (ATII) cells repopulate the alveolus after acute lung injury. We hypothesized that injury would initiate signals in nearby survivors. When rat ATII monolayers were wounded, elevations in intracellular free Ca2+ concentration ([Ca2+]i) began at the edge of the wound and propagated outward as a wave for at least 300 microns. The [Ca2+]i wave was due to both influx of extracellular Ca2+ and release of intracellular Ca2+ stores. Reducing Ca2+ influx with brief treatments of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid or Gd3+ reduced both the amplitude and the apparent speed. Draining intracellular Ca2+ stores by pretreatment with cyclopiazonic acid eliminated the [Ca2+]i wave. Therefore, the [Ca2+]i wave depended critically on intracellular Ca2+ stores. [Ca2+]i elevations propagated over a break in the monolayer, suggesting that extracellular pathways were involved. Furthermore, extracellular factors from injured cells elevated [Ca2+]i in uninjured cultures. We conclude that wounding produces a [Ca2+]i wave in surviving cells and part of this response is mediated by soluble factors released into the extracellular space during injury.

Calcium homeostasis and yeast phagocytosis in hemocytes of the colonial ascidian Botryllus schlosseri

This paper reports the effects of drugs affecting the homeostasis of cytosolic-free calcium on in vitro yeast phagocytosis by hemocytes of the colonial ascidian Botryllus schlosseri. Significant inhibition of phagocytosis is observed after exposure of hemocytes to 10 microM or higher concentrations of thimerosal, which is known to deplete intracellular calcium stores in mammalian cells. The two calcium channel blockers nifedipine and verapamil significantly decrease the phagocytic index, the minimum effective concentrations being 10 and 50 microM, respectively. As these substances have no effects at lower concentrations, they probably act through the inhibition of Ca(2+)-ATPase activity, required to restock intracellular calcium stores, due to their interaction with calmodulin. Analogously, pimozide, which suppresses ATPase activity by interacting with calmodulin, and thapsigargin, which inhibits Ca(2+)-ATPase activity, significantly reduce the phagocytic index. Moreover, nifedipine, by altering cytosolic calcium homeostasis, also lowers the production of superoxide anion associated with phagocytosis. Results indicate that in ascidians, as in mammals, a rise in intracellular calcium is required for phagocyte activation and induction of the respiratory burst.

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.

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.

Effects of calcium blockers on the cytosolic calcium, H2O2 production and elastase release in human neutrophils

Activated neutrophils are assumed to be one plausible cause of tissue injury in the ischaemic and reperfused myocardium. We studied the inhibitory effects of the calcium antagonists felodipine, nimodipine and verapamil on human neutrophil activation in order to elucidate the mechanisms underlying their myocardioprotective effects and to determine whether calcium antagonists with different chemical structures vary in their effect on neutrophil activation. Neutrophils were stimulated with formyl-Met-Leu-Phe (0.1 microM) or by phorbol myristate acetate (0.16 microM), and the rise in cytosolic calcium and the H2O2 production were determined. For felodipine, the inhibitory effect on granulocyte elastase release was also studied. The calcium antagonists reduced formyl-Met-Leu-Phe and phorbol myristate acetate-induced neutrophil activation in a concentration-dependent manner, the order of potency being: felodipine > nimodipine > verapamil. For felodipine, the IC50 (concentration causing 50% reduction) values were 3 x 10(-6) and 2 x 10(-6) M for the formyl-Met-Leu-Phe-induced cytosolic calcium increase and H2O2 production, respectively. The IC50-value for the phorbol myristate acetate-induced cytosolic calcium increase was 6 x 10(-6) and for H2O2 production 4 x 10(-6) M. For formyl-Met-Leu-Phe-induced granulocyte elastase release, the IC50-value was 5 x 10(-6) M. The inhibitory effect of felodipine on the phorbol myristate acetate-induced granulocyte elastase release did not exceed 50%. Nimodipine was a less potent inhibitor than felodipine for both formyl-Met-Leu-Phe- and phorbol myristate acetate-induced cell activities. Verapamil was even less potent than the other two agents. The present study demonstrates that felodipine potentially suppresses neutrophil activation at micromolar concentrations. However, this observation should not be directly extrapolated to explain the tissue protection by the compounds without evidence of profound local accumulation.

Responses of tumor cell pseudopod protrusion to changes in medium osmolality

The potential involvement of osmotically generated force in protrusion of tumor cell pseudopods was examined during a micropipette assay. Experiments were performed on single A2058 melanoma cells activated by a micropipette filled with soluble type IV collagen. Previous observations suggested that tumor cell pseudopod protrusion induced by type IV collagen took place in distinct, separable phases: an initial bleb (first phase) caused by localized Ca2+-activated actin filament severing resulting in an osmotic flux followed by an extension with an irregular shape (second phase) which required G protein-mediated actin polymerization (Dong et al., 1994, Microvasc. Res., 47:55-67). Presently we studied cell pseudopod protrusion in response to the changes in chemoattractant osmolality. Reduction of attractant osmolality by 20-25% from its baseline value (297 mmol/ kg) resulted in an increase in pseudopod length by 50% apparent in the initial phase. Increases in attractant osmolality by 25-30% from the baseline value arrested pseudopod protrusion significantly during both initial and later phases. Using a dual-pipette method, such osmotic influence on the cell pseudopod protrusion was shown to be only a local effect in a small region where the cell surface was stimulated by the micropipette. While forces derived from actin polymerization and osmotic pressure have been proposed to cause protrusion in general, our results suggested that osmotically generated force is more apparent in the initial phase of the pseudopod formation.

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.

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.