Regulation of leukocyte locomotion by Ca2+

Breast Cancer: An Examination of the Potential of ACKR3 to Modify the Response of CXCR4 to CXCL12

Upon binding with the chemokine CXCL12, the chemokine receptor CXCR4 has been shown to promote breast cancer progression. This process, however, can be affected by the expression of the atypical chemokine receptor ACKR3. Given ACKR3's ability to form heterodimers with CXCR4, we investigated how dual expression of both receptors differed from their lone expression in terms of their signalling pathways. We created single and double CXCR4 and/or ACKR3 Chinese hamster ovary (CHO) cell transfectants. ERK and Akt phosphorylation after CXCL12 stimulation was assessed and correlated with receptor internalization. Functional consequences in cell migration and proliferation were determined through wound healing assays and calcium flux. Initial experiments showed that CXCR4 and ACKR3 were upregulated in primary breast cancer and that CXCR4 and ACKR3 could form heterodimers in transfected CHO cells. This co-expression modified CXCR4's Akt activation after CXCL12's stimulation but not ERK phosphorylation (p < 0.05). To assess this signalling disparity, receptor internalization was assessed and it was observed that ACKR3 was recycled to the surface whilst CXCR4 was degraded (p < 0.01), a process that could be partially inhibited with a proteasome inhibitor (p < 0.01). Internalization was also assessed with the ACKR3 agonist VUF11207, which caused both CXCR4 and ACKR3 to be degraded after internalization (p < 0.05 and p < 0.001), highlighting its potential as a dual targeting drug. Interestingly, we observed that CXCR4 but not ACKR3, activated calcium flux after CXCL12 stimulation (p < 0.05) and its co-expression could increase cellular migration (p < 0.01). These findings suggest that both receptors can signal through ERK and Akt pathways but co-expression can alter their kinetics and internalization pathways.

Forms and functions of store-operated calcium entry mediators, STIM and Orai

Calcium signals arise by multiple mechanisms, including mechanisms of release of intracellular stored Ca²⁺, and the influx of Ca²⁺ through channels in the plasma membrane. One mechanism that links these two sources of Ca²⁺ is store-operated Ca²⁺ entry, the most commonly encountered version of which involves the extensively studied calcium-release-activated Ca²⁺ (CRAC) channel. The minimal and essential molecular components of the CRAC channel are the STIM proteins that function as Ca²⁺ sensors in the endoplasmic reticulum, and the Orai proteins that comprise the pore forming subunits of the CRAC channel. CRAC channels are known to play significant roles in a wide variety of physiological functions. This review discusses the multiple forms of STIM and Orai proteins encountered in mammalian cells, and discusses some specific examples of how these proteins modulate or mediate important physiological processes.

The functions of store-operated calcium channels

Store-operated calcium channels provide calcium signals to the cytoplasm of a wide variety of cell types. The basic components of this signaling mechanism include a mechanism for discharging Ca²⁺ stores (commonly but not exclusively phospholipase C and inositol 1,4,5-trisphosphate), a sensor in the endoplasmic reticulum that also serves as an activator of the plasma membrane channel (STIM1 and STIM2), and the store-operated channel (Orai1, 2 or 3). The advent of mice genetically altered to reduce store-operated calcium entry globally or in specific cell types has provided important tools to understand the functions of these widely encountered channels in specific and clinically important physiological systems. This review briefly discusses the history and cellular properties of store-operated calcium channels, and summarizes selected studies of their physiological functions in specific physiological or pathological contexts. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Cell migration at the interface of a dual chemical-mechanical gradient

Cell migration plays a critical role in numerous physiological processes, such as wound healing, response to inflammation, and cancer metastasis. In recent years, accumulating evidence indicates that cell movement is regulated not only by chemical signals but also by mechanical stimuli. In this study, the primary goal is to identify whether a chemical or mechanical stimulus plays the decisive role in directing cell migration. Measuring the motility of cells when they are presented with a combination of chemical and mechanical cues will provide insight into the complex physiological phenomena that guide and direct migration. A novel polyacrylamide hydrogel was designed with an interfacial region where the chemical and mechanical properties varied in opposing directions. One side of the interface was stiff (high Young's modulus) with a low protein concentration, whereas the other side of the interface was compliant (low Young's modulus) with a high protein concentration. The chemical gradient was created by varying the collagen (type I) concentration and the mechanical gradient was introduced by changing the extent of cross-linking in the polymer. The length of the interface with opposing chemical-mechanical profiles was found to be approximately 100 mum. Our results demonstrate that when Balb/c 3T3 fibroblasts were presented with a choice, they either migrated preferentially toward the high-collagen-compliant (low Young's modulus) side of the interfacial region or remained on the high-collagen region, suggesting a more dominant role for chemical stimuli in directing fibroblast locomotion.

Chemotaxis assays for eukaryotic cells

Chemotaxis is a complex response of a cell to an external stimulus. It involves detecting and measuring the concentration of the chemoattractant, biochemical transmission of the information, and the motility and adhesive changes associated with the response. This unit describes a number of chemotaxis assays that can be used to identify chemoattractants individually and in large-scale screenings, to distinguish chemotaxis from chemokinesis, and to analyze cellular behavioral and biochemical responses. Some of these assays such as the filter, under agarose, and small population assays, can be used to monitor the behavior of large groups of cells; the bridge, pipet, and upshift assays can be used to analyze the responses of single cells.

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.

Cell migration: mechanisms of rear detachment and the formation of migration tracks

Cell migration is central to many biological and pathological processes, including embryogenesis, tissue repair and regeneration as well as cancer and the inflammatory response. In general, cell migration can be usefully conceptualized as a cyclic process. The initial response of a cell to a migration-promoting agent is to polarize and extend protrusions in the direction of migration. These protrusions can be large, broad lamellipodia or spike-like filopodia, are usually driven by actin polymerization, and are stabilized by adhering to the extracellular matrix (ECM) via transmembrane receptors of the integrin family linked to the actin cytoskeleton. These adhesions serve as traction sites for migration as the cell moves forward over them, and they must be disassembled at the cell rear, allowing it to detach. The mechanisms of rear detachment and the regulatory processes involved are not well understood. The disassembly of adhesions that is required for detachment depends on a coordinated interaction of actin and actin-binding proteins, signaling molecules and effector enzymes including proteases, kinases and phosphatases. Originally, the biochemically regulated processes leading to rear detachment of migrating cells were thought not to be necessarily accompanied by any loss of cell material. However, it has been shown that during rear detachment long tubular extensions, the retracting fibers, are formed and that "membrane ripping" occurs at the cell rear. By this process, a major fraction of integrin-containing cellular material is left behind forming characteristic migration tracks that exactly mark the way a cell has taken.

Signal transduction of chemokine platelet factor 4 in human erythroleukemia cells

Previous data have demonstrated that CXC-chemokine platelet factor 4 (PF4) inhibits the proliferation of the human erythroleukemia cell line (HEL). However, the mechanism of action is unclear at present. The signaling transduction induced by PF4 in the HEL was compared with that induced by transforming growth factor beta1 (TGF-beta1), which is also a potent inhibitor of HEL growth. It was found that PF4 had no inhibitory effect on intracellular calcium levels in resting HEL cells. When HEL cells were stimulated with interleukin-3 (IL-3), a rapid increase in the intracellular level of free calcium occurred within 15 to 20 seconds, and this increase was followed by a sustained increase that gradually declined until resting levels were reached 30 to 40 minutes later. PF4 dramatically decreased the transient rise of [Ca2+] and protein kinase C (PKC) activity of HEL cells induced by IL-3. However, PF4 had no inhibitory effect on PKC activation in resting HEL cells. Furthermore, PF4 was found to down-regulate significantly protein tyrosine kinase (PTK) activity. In contrast, TGF-beta1 induced an increase in intracellular free calcium concentration and PKC and PTK activity in HEL cells. Furthermore, PF4 significantly increased the messenger RNA (mRNA) level of p21waf1 in HEL cells. These data demonstrate that PF4 acts on HEL cells through a signaling transduction pathway, which is different from that of TGF-beta1 and is related to the up-regulatory mRNA level of p21waf1 in HEL cells.

Ca(2+) signalling is not required for chemotaxis in Dictyostelium

Dictyostelium cells can move rapidly towards a source of cyclic-AMP (cAMP). This chemoattractant is detected by G-protein-linked receptors, which trigger a signalling cascade including a rapid influx of Ca(2+). We have disrupted an inositol 1,4,5-trisphosphate (InsP(3)) receptor-like gene, iplA, to produce null cells in which Ca(2+) entry in response to chemoattractants is abolished, as is the normal increase in free cytosolic Ca(2+) ([Ca(2+)](c)) that follows chemotactic stimulation. However, the resting [Ca(2+)](c) is similar to wild type. This mutant provides a test for the role of Ca(2+) influx in both chemotaxis and the signalling cascade that controls it. The production of cyclic-GMP and cAMP, and the activation of the MAP kinase, DdERK2, triggered from the cAMP receptor, are little perturbed in the mutant; mobilization of actin into the cytoskeleton also follows similar kinetics to wild type. Mutant cells chemotax efficiently towards cAMP or folic acid and their sensitivity to cAMP is similar to wild type. Finally, they move at similar speeds to wild-type cells, with or without chemoattractant. We conclude that Ca(2+) signalling is not necessary for chemotaxis to cAMP.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

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.

Tumor invasion: role of growth factor-induced cell motility

Cancer progression to the invasive and metastatic stage represents the most formidable barrier to successful treatment. To develop rational therapies, we must determine the molecular bases of these transitions. Cell motility is one of the defining characteristics of invasive tumors, enabling tumors to migrate into adjacent tissues or transmigrate limiting basement membranes and extracellular matrices. Invasive tumor cells have been demonstrated to present dysregulated cell motility in response to extracellular signals from growth factors and cytokines. Recent findings suggest that this growth factor receptor-mediated motility is one of the most common aberrations in tumor cells leading to invasiveness and represents a cellular behavior distinct from-adhesion-related haptokinetic and haptotactic migration. This review focuses on the emerging understanding of the biochemical and biophysical foundations of growth factor-induced cell motility and tumor cell invasiveness, and the implications for development of targeted agents, with particular emphasis on signaling from the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptors, as these have most often been associated with tumor invasion. The nascent models highlight the roles of various intracellular signaling pathways including phospholipase C-gamma (PLC gamma), phosphatidylinositol (PI)3'-kinase, mitogen-activated protein (MAP) kinase, and actin cytoskeleton-related events. Development of novel agents against tumor invasion will require not only a detailed appreciation of the biochemical regulatory elements of motility but also a paradigm shift in our approach to and assessment of cancer therapy.

Integrin signalling in neutrophils and macrophages

Integrins have been characterized extensively as adhesion receptors capable of transducing signals inside the cell. In myelomonocytic cells, integrin-mediated adhesive interactions regulate different selective cell responses, such as transmigration into the inflammatory site, cytokine secretion, production or reactive oxygen intermediates, degranulation and phagocytosis. In the last few years, great progress has been made in elucidating mechanisms of signal transduction by integrins in neutrophils and macrophages. This review summarises the current information on the role of integrins in regulating myelomonocytic cell functions and highlights the signalling pathways activated by integrin engagement in these cells. Also, exploiting the current knowledge of mechanisms of integrin signal transduction in other cell types, we propose a model to explain how integrins transduce signals inside neutrophils and macrophages, and how signaling pathways leading to regulation of selective cell functions may be coordinated.

Rho-kinase in human neutrophils: a role in signalling for myosin light chain phosphorylation and cell migration

The role of a Rho-associated coiled-coil forming kinase in migration of neutrophils has been investigated. Rho-associated coiled-coil forming kinase I was expressed in human neutrophils. Chemotactic peptide led to a Rho-associated coiled-coil forming kinase-dependent increase in phosphorylation of myosin light chain. This was determined with the help of an antibody directed against serine 19-phosphorylated myosin light chain and an inhibitor of Rho-associated coiled-coil forming kinase (Y-27632). Y-27632 suppressed myosin light chain phosphorylation and chemotactic peptide-induced development of cell polarity and locomotion with similar potency (ED50 0.5-1.1 microM). The data strongly suggest that a Rho-associated coiled-coil forming kinase isoform, activated in human neutrophils exposed to chemotactic peptide, is important for motile functions of these cells.

Signalling by CXC-chemokine receptors 1 and 2 expressed in CHO cells: a comparison of calcium mobilization, inhibition of adenylyl cyclase and stimulation of GTPgammaS binding induced by IL-8 and GROalpha

The effect of interleukin-8 (IL-8) and growth-related oncogene alpha (GROalpha) on [35S]-guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding, forskolin-stimulated cyclic AMP accumulation and cytosolic calcium concentration were determined in recombinant CHO cells expressing HA-tagged CXC-chemokine receptors 1 and 2 (CXCR1 and CXCR2). Radioligand binding assays confirmed that the binding profiles of the recombinant receptors were similar to those of the native proteins. IL-8 displaced [125I]-IL-8 binding to CXCR1 and CXCR2 with pKi values of 8.89+/-0.05 and 9.27+/-0.03, respectively. GROalpha, a selective CXCR2 ligand, had a pKi value of 9.66+/-0.39 at CXCR2 but a pKi>8 at CXCR1. Calcium mobilization experiments were also consistent with previous reports on native receptors. Activation of both receptors resulted in stimulation of [35S]GTPgammaS binding and inhibition of adenylyl cyclase. A comparison of the functional data at CXCRI showed that a similar potency order (IL-8> >GROalpha) was obtained in all three assays. However, at CXCR2 whilst the potency orders for calcium mobilization and inhibition of adenylyl cyclase were similar (IL-8 > or = GROalpha), the order was reversed for stimulation of [35S]GTPgammaS binding (GROalpha > IL-8). All of the functional responses at both receptors were inhibited by pertussis toxin (PTX), suggesting coupling to a Gi/Go protein. However, the calcium mobilization induced by IL-8 at CXCR1 was not fully inhibited by PTX, suggesting an interaction with a G-protein of the Gq family. Our results with pertussis toxin also suggested that, in the [35S]GTPgammaS binding assay, CXCR1 displays some constitutive activity. Thus, we have characterized the binding and several functional responses at HA-tagged CXCRs 1 and 2 and have shown that their pharmacology agrees well with that of the native receptors. We also have preliminary evidence that CXCR1 displays constitutive activity in our cell line and that CXCR2 may traffic between different PTX sensitive G-proteins.

Regulation of integrin-mediated adhesion during cell migration

Migrating cells form dynamic and highly regulated adhesive interactions with their environment. In particular, integrin-mediated adhesions to the extracellular matrix (ECM) play a central role in cell migration. This review focuses on recent advances in understanding the adhesive mechanisms that regulate cell detachment at the rear of migrating fibroblasts and neutrophils. The contribution of several key adhesive regulators is discussed, including myosin mediated cell contractility, tyrosine phosphorylation, rho, calcium fluxes, and calpain. A challenge for future investigation will be to determine how adhesive events are spatially and temporally coordinated to promote productive directional cell movements.

Epidermal growth factor receptor-mediated motility in fibroblasts

Cell motility is induced by many growth factors acting through cognate receptors with intrinsic tyrosine kinase activity (RPTK). However, most of the links between receptor activation and the biophysical processes of cell motility remain undeciphered. We have focused on the mechanisms by which the EGF receptor (EGFR) actuates fibroblast cell motility in an attempt to define this integrated process in one system. Our working model is that divergent, but interconnected pathways lead to the biophysical processes necessary for cell motility: cytoskeleton reorganization, membrane extension, formation of new adhesions to substratum, cell contraction, and release of adhesions at the rear. We postulate that for any given growth factor some of the pathways/processes will be actively signaled and rate-limiting, while others will be permissive due to background low-level activation. Certain couplings have been defined, such as PLCgamma and actin modifying proteins being involved in cytoskeletal reorganization and lamellipod extension and MEK being implicated in detachment from substratum. Others are suggested by complementary investigations in integrin-mediated motility, including rac in membrane protrusion, rho in new adhesions, myosin II motors in contraction, and calpain in detachment, but have yet to be placed in growth factor-induced motility. Our model postulates that many biochemical pathways will be shared between chemokinetic and haptokinetic motility but that select pathways will be activated only during RPTK-enhanced motility.

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).

Quantitative assessment of leading edge adhesion: reattachment kinetics modulated by injury-derived intracellular calcium predict wound closure rates in endothelial monolayers

Migrating cells continually develop new substrate attachments at the leading edge (LE) in order to maintain traction for movement. This study evaluates the relationship between LE adhesion and wound closure by modulating injury-derived intracellular free Ca2+ ([Ca2+]i) signaling in endothelial cell (EC) monolayers following scrape-wounding. These data show that brief treatment with increased extracellular Ca2+ ([Ca2+]e) during wounding accelerated wound area closure rates by 50-65%, while brief treatments with calcium influx inhibitors reduced rates by 30-50%. Fura-2 studies in wounded monolayers indicated supranormal [Ca2+]e during wounding increased (by 52%), while influx-inhibitors decreased (by 36%) the percentage of cells exhibiting elevated plateau [Ca2+]i levels. Quantitative time-lapse interference reflection microscopy (IRM) together with indirect alphavbeta3 integrin immunofluorescence was used to measure the effects of 100 microM Gd3+ and 5 mM [Ca2+]e treatment on fractional LE adhesion after wounding. Influx inhibition blocked development of increased injury-derived LE adhesion. Measurements indicated a linear relationship (r2 = 0.99, 0.98) between LE adhesion, development rates (quantified as an association rate constant) and steady state wound closure rates. Changes in filopodial activity, as indicated by phase contrast microscopy, did not correlate with changes in wound closure rates, but an association existed between the percentile peak [Ca2+]i response and the initiation of filopodial activity, suggesting a role for filopodia in mediating Ca2+-sensitive acceleration. Taken together, our data suggest that injury-derived [Ca2+]i signaling may regulate wound closure rates by an adhesion-mediated mechanism.

The effect of thimerosal on neutrophil migration: a comparison with the effect on calcium mobilization and CD11b expression

The sulfhydryl-reactive compound thimerosal caused a chemotactic stimulation of neutrophil migration at low concentrations and inhibition of chemoattractant-stimulated chemotaxis at high concentrations. Thiosalicylic acid, an analog of thimerosal devoid of mercury, also stimulated migration at low concentrations and caused inhibition at higher concentrations, though the inhibitory effect was less pronounced than that of thimerosal. These results indicate that the stimulatory effect of thimerosal on migration is due to the thiosalicylic acid moiety of the molecule. In contrast with thimerosal which, especially at higher concentrations than required for optimal stimulation of migration, caused an increase in cytosolic free calcium ([Ca2+]i), thiosalicylic acid had no effect on [Ca2+]i of the neutrophil. This suggests that the presence of mercury is decisive for the calcium-mobilizing effect, but not for stimulation of migration, and that mobilization of calcium and activation of migration are not related. Thimerosal caused a strong increase of CD11b expression in neutrophils in suspension, especially at inhibitory concentrations, while thiosalicylic acid had no effect on CD11b expression. This could mean (but does not prove) that CD11b expression is more related to the calcium-mobilizing effect of thimerosal than to its stimulation of migration.

Regulation of cell migration by the calcium-dependent protease calpain

Integrin receptors play an important role during cell migration by mediating linkages and transmitting forces between the extracellular matrix and the actin cytoskeleton. The mechanisms by which these linkages are regulated and released during migration are not well understood. We show here that cell-permeable inhibitors of the calcium-dependent protease calpain inhibit both beta1 and beta3 integrin-mediated cell migration. Calpain inhibition specifically stabilizes peripheral focal adhesions, increases adhesiveness, and decreases the rate of cell detachment. Furthermore, these inhibitors alter the fate of integrin receptors at the rear of the cell during migration. A Chinese hamster ovary cell line expressing low levels of calpain I also shows reduced migration rates with similar morphological changes, further implicating calpain in this process. Taken together, the data suggest that calpain inhibition modulates cell migration by stabilizing cytoskeletal linkages and decreasing the rate of retraction of the cell's rear. Inhibiting calpain-mediated proteolysis may therefore be a potential therapeutic approach to control pathological cell migration such as tumor metastasis.

The role of apocrine released proteins in the post-testicular regulation of human sperm function

A unifying hypothesis is presented postulating an apocrine release of several seminal proteins which mix and reaggregate in seminal fluid, thereby eventually forming particles designated either as "prostasomes", "vesiculosomes" or "seminosomes". The term "aposomes" should be restricted to the blebs released from secretory cells in the rat dorsal prostate and coagulating gland. Three different proteins present in human seminosomes along with the respective antibodies have been used to identify the localization, function and hypothetical interaction with spermatozoa. The proteins were (1) seminal vesicle-derived fibronectin, (2) prostate-derived 5'-nucleotidase and (3) a hitherto unidentified 100 kD membrane protein from epididymis, seminal vesicle and prostate. I. Fibronectin is an extracellular matrix protein which is also secreted from the seminal vesicles participating in the formation of the seminal clot. Immunofluorescence and immunoelectron microscopy revealed a relatively broad distribution pattern of fibronectin immunoreactivity on spermatozoa from different donors. Adding a fibronectin antiserum at a moderate dilution to vital spermatozoa in vitro resulted in a significant increase in sperm motility. Purified plasma fibronectin added at various concentrations to a vital sperm preparation was found to inhibit sperm motility in a dose-dependent manner. Measurement of calcium fluxes in individual sperm in the presence of fibronectin showed a significant increase. These findings point to a possible post-testicular regulatory function of seminal fibronectin. 2.5'-Nucleotidase (5'-NT) is an enzyme that hydrolyzes nucleotides such as AMP or IMP into inorganic phosphate and the respective nucleoside. The highest amount and activity of 5'-nucleotidase was present in glandular cells of the prostate; much less was detected in seminal vesicles and epididymis. On spermatozoa, the enzyme was localized on the outer leaflet of the plasma membrane covering the acrosomal region. Addition of purified enzyme to an in vitro incubation system of spermatozoa had no effect on sperm motility. A slight reduction of overall motility, however, was observed after addition of 5'-NT antibody to the spermatozoa. When 5'-nucleotidase inhibitors and adenosine channel antagonists were added to the sperm incubation system, a clear-cut inhibition of sperm motility occurred in a dose-dependent manner. This result is interpreted as indicating a significant role of ecto-5'-nucleotidase in the regulation of sperm motility. 3. A polyvalent antiserum against native human prostasomes recognized antigens in the range of 10-14 kD and of approximately 100 kD, respectively, in seminal fluid and prostate homogenates. Immunohistochemical studies revealed the presence of respective antigens in the epididymis, seminal vesicles and the prostate. Immunoelectron microscopy of ultracryo-sections showed labeling both of the apical plasma membrane in the prostate, as well as intraluminal secretory particles indicating the apocrine i.e. plasma-membrane bounded release of these particles. The secretory elements are termed "seminosomes". An affinity-purified fraction within the antiserum recognizes a 100 kD protein which is present both in the apical plasma membrane of the male genital glands, but also in the sperm head and principal piece of human spermatozoa. Incubation of spermatozoa with seminosomes and the respective purified antiserum had no effect on sperm motility. This is in contradistinction to former reports on motility increase induced by the so-called prostasomes.

The phosphatidylinositol 3-kinase inhibitor wortmannin markedly reduces chemotactic peptide-induced locomotion and increases in cytoskeletal actin in human neutrophils

To define a possible role of the enzyme phosphatidylinositol 3-kinase (PI 3-kinase) in motile functions of neutrophils, we have used a potent inhibitor of this enzyme, [1S-(1alpha,6b alpha,9a beta,11alpha,11bbeta)]-1-(acetyloxy)-1,6b,7,8,9a,10,11 ,11b-octahydro-1-(methoxymethyl)-9a,11b-dimethyl-3H-furo[4,3,2-de]indeno [4,5-h]-2-benzopyran-3,6,9-trione (wortmannin). Wortmannin markedly attenuated chemotactic peptide-induced development of polarity, locomotion and increases in cytoskeletal actin and alpha-actinin in human neutrophils at low, nM, concentrations (ED50 = 4-40 nM; 0.4-3 pmol/10(6) cells). The increase in cytoskeletal actin induced by phorbol-12-myristate-13-acetate in contrast was not affected by wortmannin (18 pmol/10[6] cells). Moreover, the increase in total F-actin induced by an incubation for 1 min with chemotactic peptide was much less sensitive to wortmannin than increases in cytoskeletal actin; 80 pmol/10(6) cells were necessary for half-maximal inhibition. Wortmannin thus appears to primarily affect F-actin organization, rather than polymerization. Inhibition of development of polarity by wortmannin correlated with inhibition of production of phosphatidylinositol 3,4,5-trisphosphate. According to our findings, activation of a wortmannin-sensitive target, very likely PI 3-kinase, is required for optimal chemotactic peptide-induced neutrophil motility.

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.

Suppression of polarity, locomotion and F-actin levels of Walker carcinosarcoma cells by the inhibitor CI-959

Locomotor activity of tumor cells is an important factor for the capacity for invasion and metastasis. Therefore, inhibitors interfering with cellular mechanisms regulating spontaneous cell locomotion are of particular interest for cancer therapy. CI-959, a new benzothiophene cell activation inhibitor, has the capacity to suppress spontaneous polarity and locomotion of Walker carcinosarcoma cells. Suppression of polarity and locomotion was closely associated with a reduction in the relative amount of F-actin. The mechanisms involved in suppression of motility are Ca2(+)-independent and not related to cell-substratum adhesion. Walker carcinosarcoma cells appear to be able to locomote at very low (nM) levels of free [Ca2+]i.

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.

Polarity of T cell shape, motility, and sensitivity to antigen

T cell activation requires contact with APCs. We used optical techniques to demonstrate T cell polarity on the basis of shape, motility, and localized sensitivity to antigen. An intracellular Ca2+ clamp showed that T cell shape and motility are extremely sensitive to changes in [Ca2+]i (Kd = 200 nM), with immobilization and rounding occurring via a calcineurin-independent pathway. Ca2+ dependent immobilization prolonged T cell contact with the antigen-presenting B cell; buffering the [Ca2+]i signal prevented the formation of stable cell pairs. Optical tweezers revealed spatial T cell sensitivity to antigen by controlling placement on the T cell surface of either B cells or alpha-CD3 MAb-coated beads. T cells were 4-fold more sensitive to contact made at the leading edge of the T cell compared with the tail. We conclude that motile T cells are polarized antigen sensors that respond physically to [Ca2+]i signals to stabilize their interaction with APCs.

Microtubules can modulate pseudopod activity from a distance inside macrophages

Microtubules are thought to influence cell shape as structural components of an integrated cytoskeletal matrix. Here we show that microtubules can affect the dynamics of macrophage pseudopodia without being integrated into their structure. Macrophages landing on glass surfaces spread within 15 min into flattened circular cells with radial symmetry, and the radial distribution of microtubules reflected this symmetry. Depolymerization of microtubules using nocodazole, colchicine, or vinblastine did not inhibit macrophage spreading or the early establishment of radial symmetry. Shortly after spreading, however, macrophages without microtubules gradually became asymmetric, assuming irregular, lobed profiles. The asymmetry resulted from exaggerated protrusion and retraction of pseudopodia, with net retraction overall. This loss of radial symmetry could be inhibited by treatment of initially spread cells with cytochalasin D, indicating that the change in cell shape was mediated by the actin cytoskeleton. Intact microtubules suppressed the exaggerated pseudopod movements, even when they were separated by a distance from the cell margin. In cells treated with taxol, microtubules remained clustered near the cell center after spreading, yet the dynamics of pseudopodia at the cell margin were reduced and cells maintained a circular profile. Similarly, in cells treated with low concentrations of nocodazole, a much reduced microtubule cytoskeleton nonetheless suppressed pseudopod dynamics. We propose that microtubules act to stabilize cell shape at a distance from the cell edge via a biochemical intermediate that affects the structure or function of the microfilament system.

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 polymorphonuclear leukocytes reduces their adhesion to P-selectin and causes redistribution of ligands for P-selectin on their surfaces

In acute inflammatory responses, selectins mediate initial rolling of neutrophils (PMNs) along the endothelial surface. This is followed by tight adhesion that requires activation-dependent up-regulation of CD11/CD18 integrins on PMNs. For emigration to occur, the initial bonds that are established at the endothelial surface must be disengaged. We show that activation of PMNs results in their detachment from P-selectin, a glycoprotein expressed at the surface of inflamed endothelium that mediates initial tethering of PMNs. Loosening of the bond occurs when PMNs are activated by platelet-activating factor, which is coexpressed with P-selectin, or by other signaling molecules. The time course of reduced adhesion to P-selectin, when compared to up-regulation of CD11/CD18 integrins, suggests that "bond trading" may occur as activated PMNs transmigrate in vivo. Activation of PMNs did not alter binding of fluid-phase P-selectin, indicating that the ligand(s) for P-selectin is not shed or internalized. Using microspheres coated with P-selectin, we found that ligands for P-selectin were randomly distributed over the surfaces of rounded, unactivated PMNs. An antibody against P-selectin glycoprotein ligand-1 (PSGL-1) completely inhibited binding of P-selectin-coated beads suggesting that P-selectin glycoprotein ligand-1 is the critical binding site in this assay. In contrast to the dispersed pattern on unactivated PMNs, the ligands for P-selectin were localized on the uropods of activated, polarized cells. Pretreating PMNs with cytochalasin D before activation prevented the change in cell shape, the redistribution of binding sites for P-selectin-coated beads, and the decrease in cellular adhesiveness for P-selectin. These experiments indicate that the distribution of ligands for P-selectin is influenced by cellular activation and by cytoskeletal interactions, and that redistribution of these ligands may influence adhesive interactions. Activation of PMNs may cause loosening or disengagement of bonds between P-selectin and its ligands, facilitating transendothelial migration.

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.

Membrane ruffling and signal transduction

One of the earliest structural changes observed in cells in response to many extracellular factors is membrane ruffling: the formation of motile cell surface protrusions containing a meshwork of newly polymerized actin filaments. It is becoming clear that actin reorganization is an integral part of early signal transduction pathways, and that many signalling molecules interact with the actin cytoskeleton. The small GTP-binding protein Rac is a key regulator of membrane ruffling, and proteins that can regulate Rac activity, such as Bcr, are likely to act on this signalling pathway. In addition, several previously characterized signal transducing molecules are implicated in the membrane-ruffling response, including Ras, the adaptor protein Grb2, phosphatidyl inositol 3-kinase, phospholipase A2 and phorbol ester-responsive proteins. Changes in polyphosphoinositide metabolism and intracellular Ca2+ levels may also play a role. A number of actin-binding and organizing proteins localize to membrane ruffles and are potential targets for these signal transducing molecules.

Cytokine control of cell motility: modulation and mediation by the extracellular matrix

Cytokines are multifunctional regulators of cell behaviour affecting such diverse activities as cell proliferation, gene expression and motility. Matrix macromolecules influence a similarly wide range of cell functions. A review of the available literature suggests that cytokines may affect cell motility by (a) directly influencing the motility apparatus, and (b) indirectly as a consequence of the altered expression of genes coding for matrix macromolecules, their respective cell surface receptors and matrix degrading enzymes and their inhibitors. Conversely, the composition and supramolecular organisation of the matrix plays a central role in defining cellular response to potentially multifunctional cytokines. Such complex and reciprocal interactions between cytokines and the matrix elicit both positive and negative reiterative feedback loops which must be taken into account when interpreting the results of migration assays in vitro and extrapolating them to in vivo processes.