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

Monitoring Phosphoinositide Fluxes and Effectors During Leukocyte Chemotaxis and Phagocytosis

The dynamic re-organization of cellular membranes in response to extracellular stimuli is fundamental to the cell physiology of myeloid and lymphoid cells of the immune system. In addition to maintaining cellular homeostatic functions, remodeling of the plasmalemma and endomembranes endow leukocytes with the potential to relay extracellular signals across their biological membranes to promote rolling adhesion and diapedesis, migration into the tissue parenchyma, and to ingest foreign particles and effete cells. Phosphoinositides, signaling lipids that control the interface of biological membranes with the external environment, are pivotal to this wealth of functions. Here, we highlight the complex metabolic transitions that occur to phosphoinositides during several stages of the leukocyte lifecycle, namely diapedesis, migration, and phagocytosis. We describe classical and recently developed tools that have aided our understanding of these complex lipids. Finally, major downstream effectors of inositides are highlighted including the cytoskeleton, emphasizing the importance of these rare lipids in immunity and disease.

A PKC-MARCKS-PI3K regulatory module links Ca2+ and PIP3 signals at the leading edge of polarized macrophages

The leukocyte chemosensory pathway detects attractant gradients and directs cell migration to sites of inflammation, infection, tissue damage, and carcinogenesis. Previous studies have revealed that local Ca²⁺ and PIP₃ signals at the leading edge of polarized leukocytes play central roles in positive feedback loop essential to cell polarization and chemotaxis. These prior studies showed that stimulation of the leading edge Ca²⁺ signal can strongly activate PI3K, thereby triggering a larger PIP₃ signal, but did not elucidate the mechanistic link between Ca²⁺ and PIP₃ signaling. A hypothesis explaining this link emerged, postulating that Ca²⁺-activated PKC displaces the MARCKS protein from plasma membrane PIP₂, thereby releasing sequestered PIP₂ to serve as the target and substrate lipid of PI3K in PIP₃ production. In vitro single molecule studies of the reconstituted pathway on lipid bilayers demonstrated the feasibility of this PKC-MARCKS-PI3K regulatory module linking Ca²⁺ and PIP₃ signals in the reconstituted system. The present study tests the model predictions in live macrophages by quantifying the effects of: (a) two pathway activators—PDGF and ATP that stimulate chemoreceptors and Ca²⁺ influx, respectively; and (b) three pathway inhibitors—wortmannin, EGTA, and Go6976 that inhibit PI3K, Ca²⁺ influx, and PKC, respectively; on (c) four leading edge activity sensors—AKT-PH-mRFP, CKAR, MARCKSp-mRFP, and leading edge area that report on PIP₃ density, PKC activity, MARCKS membrane binding, and leading edge expansion/contraction, respectively. The results provide additional evidence that PKC and PI3K are both essential elements of the leading edge positive feedback loop, and strongly support the existence of a PKC-MARCKS-PI3K regulatory module linking the leading edge Ca²⁺ and PIP₃ signals. As predicted, activators stimulate leading edge PKC activity, displacement of MARCKS from the leading edge membrane and increased leading edge PIP₃ levels, while inhibitors trigger the opposite effects. Comparison of the findings for the ameboid chemotaxis of leukocytes with recently published findings for the mesenchymal chemotaxis of fibroblasts suggests that some features of the emerging leukocyte leading edge core pathway (PLC-DAG-Ca²⁺-PKC-MARCKS-PIP₂-PI3K-PIP₃) may well be shared by all chemotaxing eukaryotic cells, while other elements of the leukocyte pathway may be specialized features of these highly optimized, professional gradient-seeking cells. More broadly, the findings suggest a molecular mechanism for the strong links between phospho-MARCKS and many human cancers.

Analytical Prediction of the Spatiotemporal Distribution of Chemoattractants around Their Source: Theory and Application to Complement-Mediated Chemotaxis

The ability of motile immune cells to detect and follow gradients of chemoattractant is critical to numerous vital functions, including their recruitment to sites of infection and-in emerging immunotherapeutic applications-to malignant tumors. Facilitated by a multitude of chemotactic receptors, the cells navigate a maze of stimuli to home in on their target. Distinct chemotactic processes direct this navigation at particular times and cell-target distances. The expedient coordination of this spatiotemporal hierarchy of chemotactic stages is the central element of a key paradigm of immunotaxis. Understanding this hierarchy is an enormous interdisciplinary challenge that requires, among others, quantitative insight into the shape, range, and dynamics of the profiles of chemoattractants around their sources. We here present a closed-form solution to a diffusion-reaction problem that describes the evolution of the concentration gradient of chemoattractant under various conditions. Our ready-to-use mathematical prescription captures many biological situations reasonably well and can be explored with standard graphing software, making it a valuable resource for every researcher studying chemotaxis. We here apply this mathematical model to characterize the chemoattractant cloud of anaphylatoxins that forms around bacterial and fungal pathogens in the presence of host serum. We analyze the spatial reach, rate of formation, and rate of dispersal of this locator cloud under realistic physiological conditions. Our analysis predicts that simply being small is an effective protective strategy of pathogens against complement-mediated discovery by host immune cells over moderate-to-large distances. Leveraging our predictions against single-cell, pure-chemotaxis experiments that use human immune cells as biosensors, we are able to explain the limited distance over which the cells recognize microbes. We conclude that complement-mediated chemotaxis is a universal, but short-range, homing mechanism by which chemotaxing immune cells can implement a last-minute course correction toward pathogenic microbes. Thus, the integration of theory and experiments provides a sound mechanistic explanation of the primary role of complement-mediated chemotaxis within the hierarchy of immunotaxis, and why other chemotactic processes are required for the successful recruitment of immune cells over large distances.

Localizing the lipid products of PI3Kγ in neutrophils

Class I phosphoinositide 3-kinases (PI3Ks) are important regulators of neutrophil migration in response to a range of chemoattractants. Their primary lipid products PtdIns(3,4,5)P₃ and PtdIns(3,4)P₂ preferentially accumulate near to the leading edge of migrating cells and are thought to act as an important cue organizing molecular and morphological polarization. We have investigated the distribution and accumulation of these lipids independently in mouse neutrophils using eGFP-PH reportersand electron microscopy (EM).

We found that authentic mouse neutrophils rapidly polarized their Class I PI3K signalling, as read-out by eGFP-PH reporters, both at the up-gradient leading edge in response to local stimulation with fMLP as well as spontaneously and randomly in response to uniform stimulation. EM studies revealed these events occurred at the plasma membrane, were dominated by accumulation of PtdIns(3,4,5)P₃, but not PtdIns(3,4)P₂, and were dependent on PI3Kγ and its upstream activation by both Ras and Gβγs.

The role of phosphoinositide 3-kinases in neutrophil migration in 3D collagen gels

The entry of neutrophils into tissue has been well characterised; however the fate of these cells once inside the tissue microenvironment is not fully understood. A variety of signal transduction pathways including those involving class I PI3 Kinases have been suggested to be involved in neutrophil migration. This study aims to determine the involvement of PI3 Kinases in chemokinetic and chemotactic neutrophil migration in response to CXCL8 and GM-CSF in a three-dimensional collagen gel, as a model of tissue. Using a three-dimensional collagen assay chemokinetic and chemotactic migration induced by CXCL8 was inhibited with the pan PI3 Kinase inhibitor wortmannin. Analysis of the specific Class I PI3 Kinase catalytic isoforms alpha, delta and gamma using the inhibitors PIK-75, PIK-294 and AS-605240 respectively indicated differential roles in CXCL8-induced neutrophil migration. PIK-294 inhibited both chemokinetic and chemotactic CXCL8-induced migration. AS-605240 markedly reduced CXCL8 induced chemokinetic migration but had no effect on CXCL8 induced chemotactic migration. In contrast PIK-75 inhibited chemotactic migration but not chemokinetic migration. At optimal concentrations of GM-CSF the inhibitors had no effect on the percentage of neutrophil migration in comparison to the control however at suboptimal concentrations wortmannin, AS-605240 and PIK-294 inhibited chemokinesis. This study suggests that PI3 Kinase is necessary for CXCL8 induced migration in a 3D tissue environment but that chemokinetic and chemotactic migration may be controlled by different isoforms with gamma shown to be important in chemokinesis and alpha important in chemotaxis. Neutrophil migration in response to suboptimal concentrations of GM-CSF is dependent on PI3 Kinase, particularly the gamma and delta catalytic isoforms.

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.

Signaling via class IA Phosphoinositide 3-kinases (PI3K) in human, breast-derived cell lines

We have addressed the differential roles of class I Phosphoinositide 3-kinases (PI3K) in human breast-derived MCF10a (and iso-genetic derivatives) and MDA-MB 231 and 468 cells. Class I PI3Ks are heterodimers of p110 catalytic (α, β, δ and γ) and p50-101 regulatory subunits and make the signaling lipid, phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) that can activate effectors, eg protein kinase B (PKB), and responses, eg migration. The PtdIns(3,4,5)P3-3-phosphatase and tumour-suppressor, PTEN inhibits this pathway. p110α, but not other p110s, has a number of onco-mutant variants that are commonly found in cancers. mRNA-seq data shows that MCF10a cells express p110β>>α>δ with undetectable p110γ. Despite this, EGF-stimulated phosphorylation of PKB depended upon p110α-, but not β- or δ- activity. EGF-stimulated chemokinesis, but not chemotaxis, was also dependent upon p110α, but not β- or δ- activity. In the presence of single, endogenous alleles of onco-mutant p110α (H1047R or E545K), basal, but not EGF-stimulated, phosphorylation of PKB was increased and the effect of EGF was fully reversed by p110α inhibitors. Cells expressing either onco-mutant displayed higher basal motility and EGF-stimulated chemokinesis.This latter effect was, however, only partially-sensitive to PI3K inhibitors. In PTEN(-/-) cells, basal and EGF-stimulated phosphorylation of PKB was substantially increased, but the p110-dependency was variable between cell types. In MDA-MB 468s phosphorylation of PKB was significantly dependent on p110β, but not α- or δ- activity; in PTEN(-/-) MCF10a it remained, like the parental cells, p110α-dependent. Surprisingly, loss of PTEN suppressed basal motility and EGF-stimulated chemokinesis. These results indicate that; p110α is required for EGF signaling to PKB and chemokinesis, but not chemotaxis; onco-mutant alleles of p110α augment signaling in the absence of EGF and may increase motility, in part, via acutely modulating PI3K-activity-independent mechanisms. Finally, we demonstrate that there is not a universal mechanism that up-regulates p110β function in the absence of PTEN.

Loss of phosphoinositide 3-kinase P110γ is protective in the acute phase but detrimental in the resolution phase of hapten-induced colitis

BACKGROUND

Pharmacologic inhibition or genetic ablation of phosphoinositide 3-kinase gamma (PI3Kγ) has been shown to be protective against experimental colitis. However, the role of PI3Kγ in the resolution phase of colitis remains unexplored. In this study, we assess the effects of genetic knockout of PI3Kγ on the induction and resolution of colitis induced by the hapten trinitrobenzene sulfonic acid (TNBS).

METHODS

Colitis was induced in wild-type C57/Bl6 or PI3Kγ-/- mice by intrarectal administration of 2.5 mg of TNBS in 50% ethanol. Body weights were monitored daily, and colon tissues were collected at days 3, 7, or 14 after treatment, and colitis was assessed using disease activity and histologic damage scores, measurement of tissue myeloperoxidase and neutrophil infiltration, and local cytokine production.

RESULTS

Mice lacking PI3Kγ were significantly protected from disease during the acute phase (day 3) of TNBS colitis. However, PI3Kγ-/- mice have difficulty resolving acute inflammation because they failed to restore lost weight and had significantly elevated histologic damage scores and tissue myeloperoxidase levels at days 7 and 14 after TNBS administration compared with wild-type controls. This phenomenon was dependent on presensitization with TNBS and seems to involve an inability to clear invading bacteria, resulting in the generation of a persistent inflammatory cytokine response.

CONCLUSIONS

This study confirms that PI3Kγ plays a role in the induction of colitis. However, PI3Kγ is also required for the resolution of intestinal damage following acute inflammation. This must be taken into consideration before the inhibition of PI3Kγ can be used as a treatment for disorders such as inflammatory bowel disease.

Fer kinase limits neutrophil chemotaxis toward end target chemoattractants

Neutrophil recruitment and directional movement toward chemotactic stimuli are important processes in innate immune responses. This study examines the role of Fer kinase in neutrophil recruitment and chemotaxis to various chemoattractants in vitro and in vivo. Mice targeted with a kinase-inactivating mutation (Fer(DR/DR)) or wild type (WT) were studied using time-lapse intravital microscopy to examine leukocyte recruitment and chemotaxis in vivo. In response to keratinocyte-derived cytokine, no difference in leukocyte chemotaxis was observed between WT and Fer(DR/DR) mice. However, in response to the chemotactic peptide WKYMVm, a selective agonist of the formyl peptide receptor, a 2-fold increase in leukocyte emigration was noted in Fer(DR/DR) mice (p < 0.05). To determine whether these defects were due to Fer signaling in the endothelium or other nonhematopoietic cells, bone marrow chimeras were generated. WKYMVm-induced leukocyte recruitment in chimeric mice (WT bone marrow to Fer(DR/DR) recipients or vice versa) was similar to WT mice, suggesting that Fer kinase signaling in both leukocytes and endothelial cells serves to limit chemotaxis. Purified Fer(DR/DR) neutrophils demonstrated enhanced chemotaxis toward end target chemoattractants (WKYMVm and C5a) compared with WT using an under-agarose gel chemotaxis assay. These defects were not observed in response to intermediate chemoattractants (keratinocyte-derived cytokine, MIP-2, or LTB(4)). Increased WKYMVm-induced chemotaxis of Fer(DR/DR) neutrophils correlated with sustained PI3K activity and reduced reliance on the p38 MAPK pathway compared with WT neutrophils. Together, these data identify Fer as a novel inhibitory kinase for neutrophil chemotaxis toward end target chemoattractants through modulation of PI3K activity.

Molecular mechanisms of spontaneous and directed mast cell motility

Migration is a fundamental function of immune cells, and a role for Ca(2+) in immune cell migration has been an interest of scientific investigations for many decades. Mast cells are the major effector cells in IgE-mediated immune responses, and cross-linking of IgE-FcεRI complexes at the mast cell surface by antigen activates a signaling cascade that causes mast cell activation, resulting in Ca(2+) mobilization and granule exocytosis. These cells are known to accumulate at sites of inflammation in response to parasite and bacterial infections. Using real-time imaging, we monitored chemotactic migration of RBL and rat BMMCs in response to a gradient of soluble multivalent antigen. Here, we show that Ca(2+) influx via Orai1 plays an important role in regulating spontaneous motility and directional migration of mast cells toward antigen via IgER complexes. Inhibition of Ca(2+) influx or knockdown of the Ca(2+) entry channel protein Orai1 by shRNA causes inhibition of both of these processes. In addition, a mutant Syk- shows impaired spontaneous motility and chemotaxis toward antigen that is rescued by expression of Syk. Our findings identify a novel Ca(2+) influx-mediated, Orai1-dependent mechanism for mast cell migration.

Vav1 in differentiation of tumoral promyelocytes

The multidomain protein Vav1, in addition to promote the acquisition of maturation related properties by normal hematopoietic cells, is a key player in the ATRA- and PMA-induced completion of the differentiation program of tumoral myeloid precursors derived from APL. This review is focussed on the role of Vav1 in differentiating promyelocytes, as part of interconnected networks of functionally related proteins ended to regulate different aspects of myeloid maturation. The role of Vav1 in determining actin cytoskeleton reorganization alternative to the best known function as a GEF for small G proteins is discussed, as well as the binding of Vav1 with cytoplasmic and nuclear signaling molecules which provides a new perspective in the modulation of nuclear architecture and activity. In particular, new hints are provided on the ability of Vav1 to determine the nuclear amount of proteins implicated in modulating mRNA production and stability and in regulating the ATRA-dependent protein expression also by direct interaction with transcription factors known to drive the ATRA-induced maturation of myeloid cells. The reviewed findings summarize the major advances in the understanding of additional, non conventional functions connected with the vast interactive potential of Vav1.

The role of PI3K/protein kinase B (PKB/c-akt) in migration and homing of hematopoietic stem and progenitor cells

PURPOSE OF REVIEW

Hematopoietic stem cell (HSC) transplantation is the most powerful treatment modality for a variety of hematological disorders. Successful hematopoietic recovery after transplantation depends on optimal homing of HSCs to the bone marrow and subsequent lodging in the HSC niche. The molecular mechanisms underlying bone marrow homing are, thus far, incompletely understood. This review focuses on recent studies that extended our understanding of how the phosphatidylinositol-3-kinase (PI3K)/protein kinase B (PKB/c-akt) signaling module can regulate migration and homing of HSCs.

RECENT FINDINGS

In addition to regulation of HSC maintenance and lineage development, it has recently become apparent that the PI3K/PKB signaling module plays a critical role in regulation of migration and adhesion of hematopoietic stem and progenitor cells. Activation of this signaling pathway enhances firm adhesion, reduces migration and inhibits bone marrow homing, whereas inhibition of PKB conversely induces bone marrow homing.

SUMMARY

These findings clearly implicate the PI3K/PKB signaling module in playing a critical role in regulation of bone marrow homing, suggesting that pharmacological modulation of this signaling molecule prior to transplantation may provide a clinical means of improving engraftment levels and accelerating hematopoietic recovery.

Understanding the structure and function of the immunological synapse

The immunological synapse has been an area of very active scientific interest over the last decade. Surprisingly, much about the synapse remains unknown or is controversial. Here we review some of these current issues in the field: how the synapse is defined, its potential role in T-cell function, and our current understanding about how the synapse is formed.

PI3-kinase p110α mediates β1 integrin-induced Akt activation and membrane protrusion during cell attachment and initial spreading

Integrin-mediated cell adhesion activates several signaling effectors, including phosphatidylinositol 3-kinase (PI3K), a central mediator of cell motility and survival. To elucidate the molecular mechanisms of this important pathway the specific members of the PI3K family activated by different integrins have to be identified. Here, we studied the role of PI3K catalytic isoforms in β1 integrin-induced lamellipodium protrusion and activation of Akt in fibroblasts. Real-time total internal reflection fluorescence imaging of the membrane-substrate interface demonstrated that β1 integrin-mediated attachment induced rapid membrane spreading reaching essentially maximal contact area within 5-10 min. This process required actin polymerization and involved activation of PI3K. Isoform-selective pharmacological inhibition identified p110α as the PI3K catalytic isoform mediating both β1 integrin-induced cell spreading and Akt phosphorylation. A K756L mutation in the membrane-proximal part of the β1 integrin subunit, known to cause impaired Akt phosphorylation after integrin stimulation, induced slower cell spreading. The initial β1 integrin-regulated cell spreading as well as Akt phosphorylation were sensitive to the tyrosine kinase inhibitor PP2, but were not dependent on Src family kinases, FAK or EGF/PDGF receptor transactivation. Notably, cells expressing a Ras binding-deficient p110α mutant were severely defective in integrin-induced Akt phosphorylation, but exhibited identical membrane spreading kinetics as wild-type p110α cells. We conclude that p110α mediates β1 integrin-regulated activation of Akt and actin polymerization important for survival and lamellipodia dynamics. This could contribute to the tumorigenic properties of cells expressing constitutively active p110α.

Differential regulation of protrusion and polarity by PI3K during neutrophil motility in live zebrafish

Cell polarity is crucial for directed migration. Here we show that phosphoinositide 3-kinase (PI(3)K) mediates neutrophil migration in vivo by differentially regulating cell protrusion and polarity. The dynamics of PI(3)K products PI(3,4,5)P(3)-PI(3,4)P(2) during neutrophil migration were visualized in living zebrafish, revealing that PI(3)K activation at the leading edge is critical for neutrophil motility in intact tissues. A genetically encoded photoactivatable Rac was used to demonstrate that localized activation of Rac is sufficient to direct migration with precise temporal and spatial control in vivo. Similar stimulation of PI(3)K-inhibited cells did not direct migration. Localized Rac activation rescued membrane protrusion but not anteroposterior polarization of F-actin dynamics of PI(3)K-inhibited cells. Uncoupling Rac-mediated protrusion and polarization suggests a paradigm of two-tiered PI(3)K-mediated regulation of cell motility. This work provides new insight into how cell signaling at the front and back of the cell is coordinated during polarized cell migration in intact tissues within a multicellular organism.

International Union of Basic and Clinical Pharmacology. LXXIII. Nomenclature for the formyl peptide receptor (FPR) family

Formyl peptide receptors (FPRs) are a small group of seven-transmembrane domain, G protein-coupled receptors that are expressed mainly by mammalian phagocytic leukocytes and are known to be important in host defense and inflammation. The three human FPRs (FPR1, FPR2/ALX, and FPR3) share significant sequence homology and are encoded by clustered genes. Collectively, these receptors bind an extraordinarily numerous and structurally diverse group of agonistic ligands, including N-formyl and nonformyl peptides of different composition, that chemoattract and activate phagocytes. N-formyl peptides, which are encoded in nature only by bacterial and mitochondrial genes and result from obligatory initiation of bacterial and mitochondrial protein synthesis with N-formylmethionine, is the only ligand class common to all three human receptors. Surprisingly, the endogenous anti-inflammatory peptide annexin 1 and its N-terminal fragments also bind human FPR1 and FPR2/ALX, and the anti-inflammatory eicosanoid lipoxin A4 is an agonist at FPR2/ALX. In comparison, fewer agonists have been identified for FPR3, the third member in this receptor family. Structural and functional studies of the FPRs have produced important information for understanding the general pharmacological principles governing all leukocyte chemoattractant receptors. This article aims to provide an overview of the discovery and pharmacological characterization of FPRs, to introduce an International Union of Basic and Clinical Pharmacology (IUPHAR)-recommended nomenclature, and to discuss unmet challenges, including the mechanisms used by these receptors to bind diverse ligands and mediate different biological functions.

Temperature pretreatment alters the polarization response of human neutrophils to the chemoattractant N-formyl-Met-Leu-Phe

Neutrophils present a polarized morphology upon stimulation of chemoattractants, which play a vital role in host-defense mechanisms. Many studies have been published on neutrophil polarization, in which three different temperatures pretreatment (4 degrees C, 25 degrees C and 37 degrees C) have been used. However, no study has investigated whether different temperature pretreatments affect neutrophil polarization. In the current study, we examined the effects of 4 degrees C, 25 degrees C and 37 degrees C pretreatment temperatures on short-term (1 or 3 min) chemoattractant-induced polarization. Human neutrophils were polarized upon the stimulation of N-formyl-Met-Leu-Phe (fMLP) after pretreated by different temperature. The morphological changes of the neutrophils were investigated under the microscopy. The F-actin polymerization was determined by immunological histological chemistry. There were more head-tail polarized cells (>50% of the cells) in the 25 degrees C and 37 degrees C pretreatment groups than in the 4 degrees C group (32.4%). The average lengths of the pseudopod were 3.2 +/- 1.1 microm (n = 17), 5.3 +/- 2.1 microm (n = 12) and 7.4 +/- 2.7 microm (n = 21) in the 4 degrees C, 25 degrees C and 37 degrees C pretreatment groups, respectively; the 4 degrees C and 37 degrees C pretreatment groups were statistically different (P < 0.05). Additionally, there was a statistically significant difference in the pseudopod extension rate among the three groups, as well as the Lamellipod percentage between the 4 degrees C group and the other two groups within 1 min of stimulation with fMLP. This study demonstrates that different temperature pretreatments affect neutrophil polarization upon short-term stimulation with fMLP.

Moving towards a better understanding of chemotaxis

Eukaryotic cells are thought to move across supporting surfaces through a combination of coordinated processes: polarisation; extension of dynamic protrusions from a leading edge; adhesion-associated stabilisation of some protrusions; centripetal pulling against those leading adhesions; and de-adhesion at the rear. Gradients of extracellular ligands can be detected by cells and then used to guide them either towards the source (in the case of a chemoattractant) or away from the source (in the case of a chemorepellent)--such migration is termed chemotaxis. Recent work suggests that chemotaxis probably emerges from the ability of cells to spatially encode extracellular gradients of ligands, a process for which phosphoinositide 3'-kinase (PI3K) signals alone are insufficient, and to use that vectorial information to bias movement by enhancing the survival, and not the formation, of the protrusions that experience the greatest stimulation.

Changing directions in the study of chemotaxis

Chemotaxis--the guided movement of cells in chemical gradients--probably first emerged in our single-celled ancestors and even today is recognizably similar in neutrophils and amoebae. Chemotaxis enables immune cells to reach sites of infection, allows wounds to heal and is crucial for forming embryonic patterns. Furthermore, the manipulation of chemotaxis may help to alleviate disease states, including the metastasis of cancer cells. This review discusses recent results concerning how cells orientate in chemotactic gradients and the role of phosphatidylinositol-3,4,5-trisphosphate, what produces the force for projecting pseudopodia and a new role for the endocytic cycle in movement.

Immunostaining evidence for PI(4,5)P2 localization at the leading edge of chemoattractant-stimulated HL-60 cells

It is well known that in fMLP-stimulated neutrophils, phosphatidyl inositol 3,4,5-trisphosphate [PI(3,4,5)P3] localizes at the leading edge of the cells. However, no effort has been made to study the PI 4,5-bisphosphate [PI(4,5)P2] distribution in these cells. In fact, it has been suggested that PI(4,5)P2 is unlikely to localize, as its basal level is orders of magnitude higher than that of PI(3,4,5)P3. We developed an optimized immunostaining protocol for studying the endogenous distribution of PI(4,5)P2 in neutrophil-like HL-60 cells. We show that PI(4,5)P2 localizes sharply at the leading edge with an intensity gradient similar to that for PI(3,4,5)P3. The enzymes for the production of PI(4,5)P2, namely, PI5KIalpha and PI5KIgamma, were also found to localize at the leading edge, further supporting our finding that PI(4,5)P2 localizes at the leading edge. These results imply that complementary regulation of PI3K and phosphate and tensin homolog (PTEN) is not the sole or dominant mechanism of PI(3,4,5)P3 polarization in HL-60 cells.

Phosphatidylinositol 3- and 4-phosphate modulate actin filament reorganization in guard cells of day flower

Phosphatidylinositol 3-kinases (PtdIns 3-kinases) that produce phosphatidylinositol (3,4,5) triphosphate (PtdIns(3,4,5)P(3)) are considered to be important regulators of actin dynamics in animal cells. In plants, neither PtdIns(3,4,5)P(3) nor the enzyme that produces this lipid has been reported. However, a PtdIns 3-kinase that produces phosphatidylinositol 3-phosphate (PtdIns3P) has been identified, suggesting that PtdIns3P, instead of PtdIns(3,4,5)P(3), regulates actin dynamics in plant cells. Phosphatidylinositol 4-kinase (PtdIns 4-kinase) is closely associated with the actin cytoskeleton in plant cells, suggesting a role for this lipid kinase and its product phosphatidylinositol 4-phosphate (PtdIns4P) in actin-related processes. Here, we investigated whether or not PtdIns3P or PtdIns4P plays a role in actin reorganization induced by a plant hormone abscisic acid (ABA) in guard cells of day flower (Commelina communis). ABA-induced changes in actin filaments were inhibited by LY294002 (LY) and wortmannin (WM), inhibitors of PtdIns3P and PtdIns4P synthesis. Expression of PtdIns3P- and PtdIns4P-binding domains also inhibited ABA-induced actin reorganization in a manner similar to LY and WM. These results suggest that PtdIns3P and PtdIns4P regulate actin dynamics in guard cells. Furthermore, we demonstrate that PtdIns3P exerts its effect on actin dynamics, at least in part, via generation of reactive oxygen species (ROS) in response to ABA.

PI(3)Kgamma has an important context-dependent role in neutrophil chemokinesis

The directional movement of cells in a gradient of external stimulus is termed chemotaxis and is important in many aspects of development and differentiated cell function. Phophoinositide 3-kinases (PI(3)Ks) are thought to have critical roles within the gradient-sensing machinery of a variety of highly motile cells, such as mammalian phagocytes, allowing these cells to respond quickly and efficiently to shallow gradients of soluble stimuli. Our analysis of mammalian neutrophil migration towards ligands such as fMLP shows that, although PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3) accumulate in a PI(3)Kgamma-dependent fashion at the up-gradient leading-edge, this signal is not required for efficient gradient-sensing and gradient-biased movement. PI(3)Kgamma activity is however, a critical determinant of the proportion of cells that can move, that is, respond chemokinetically, in reaction to fMLP. Furthermore, this dependence of chemokinesis on PI(3)Kgamma activity is context dependent, both with respect to the state of priming of the neutrophils and the type of surface on which they are migrating. We propose this effect of PI(3)Kgamma is through roles in the regulation of some aspects of neutrophil polarization that are relevant to movement, such as integrin-based adhesion and the accumulation of polymerized (F)-actin at the leading-edge.

Class IA phosphatidylinositide 3-kinases, rather than p110 gamma, regulate formyl-methionyl-leucyl-phenylalanine-stimulated chemotaxis and superoxide production in differentiated neutrophil-like PLB-985 cells

Class I PI3Ks, through the formation of phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P(3)), are thought of as essential elements of the neutrophil response to chemotactic factors. Moreover, the recent development of PI3K-deficient mice and isoform-specific inhibitors enabled examinations of the contribution of the distinct PI3K isoforms in neutrophil activation. However, the results of these various studies are conflicting, and the exact role of the different PI3K isoforms is not yet clearly established, particularly in human cells. In the present study, we used a different approach to assess the role of the distinct PI3K isoforms in response to the chemotactic agent fMLP. We inhibited PI3K activities by the transient expression following nucleofection of dominant negative mutants of either p85alpha or p110gamma in the human myeloid cell line PLB-985, which can be induced to express a neutrophil-like phenotype. The data obtained with this approach showed that the production of PI(3,4,5)P(3) triggered by fMLP is biphasic, with a peak of production observed in a short time period that entirely depends on p110gamma activity, and a delayed phase that is mediated by class I(A) PI3K. We also provide evidence that the PI3K-dependent functional responses (i.e., superoxide production and chemotaxis) induced by the chemotactic factor mainly involve PI3K I(A) and, by implication, the delayed phase of PI(3,4,5)P(3) production, whereas p110gamma and the early peak of PI(3,4,5)P(3) do not play major roles in the initiation or the control of these responses.

Galectin-8 binds specific β1 integrins and induces polarized spreading highlighted by asymmetric lamellipodia in Jurkat T cells

Integrin-mediated encounters of T cells with extracellular cues lead these cells to adhere to a variety of substrates and acquire a spread phenotype needed for their tissue incursions. We studied the effects of galectin-8 (Gal-8), a beta-galactoside binding lectin, on Jurkat T cells. Immobilized Gal-8 bound alpha1beta1, alpha3beta1 and alpha5beta1 but not alpha2beta1 and alpha4beta1 and adhered these cells with similar kinetics to immobilized fibronectin (FN). Function-blocking experiments with monoclonal anti-integrin antibodies suggested that alpha5beta1 is the main mediator of cell adhesion to this lectin. Gal-8, but not FN, induced extensive cell spreading frequently leading to a polarized phenotype characterized by an asymmetric lamellipodial protrusion. These morphological changes involved actin cytoskeletal rearrangements controlled by PI3K, Rac-1 and ERK1/2 activity. Gal-8-induced Rac-1 activation and binding to alpha1 and alpha5 integrins have not been described in any other cellular system. Strikingly, Gal-8 was also a strong stimulus on Jurkat cells in suspension, triggering ERK1/2 activation that in most adherent cells is instead dependent on cell attachment. In addition, we found that patients with systemic lupus erythematosus (SLE), a prototypic autoimmune disorder, produce Gal-8 autoantibodies that impede both its binding to integrins and cell adhesion. These are the first function-blocking autoantibodies reported for a member of the galectin family. These results indicate that Gal-8 constitutes a novel extracellular stimulus for T cells, able to bind specific beta1 integrins and to trigger signaling pathways conducive to cell spreading. Gal-8 could modulate a wide range of T cell-driven immune processes that eventually become altered in autoimmune disorders.

Plasma membrane organization is essential for balancing competing pseudopod- and uropod-promoting signals during neutrophil polarization and migration

Exposure of neutrophils to chemoattractant induces cell polarization and migration. These behaviors require the asymmetric activation of distinct signaling pathways and cytoskeletal elements in the protruding pseudopod at the front of cells and the retracting uropod at the rear. An important outstanding question is, how does the organization of the plasma membrane participate in establishing asymmetry during polarization and migration? To answer this question, we investigated the function of cholesterol, a lipid known to influence membrane organization. Using controlled cholesterol depletion, we found that a cholesterol-dependent membrane organization enabled cell polarization and migration by promoting uropod function and suppressing ectopic pseudopod formation. At a mechanistic level, we showed that cholesterol was directly required for suppressing inappropriate activation of the pseudopod-promoting Gi/PI3-kinase signaling pathway. Furthermore, cholesterol was required for dampening Gi-dependent negative feedback on the RhoA signaling pathway, thus enabling RhoA activation and uropod function. Our findings suggest a model in which a cholesterol-dependent membrane organization plays an essential role in the establishment of cellular asymmetry by balancing the activation and segregating the localization of competing pseudopod- and uropod-inducing signaling pathways during neutrophil polarization and migration.

Leukocytes on the move with phosphoinositide 3-kinase and its downstream effectors

Cell signalling mediators derived from membrane phospholipids are frequent participants in biological processes. The family of phosphoinositide 3-kinases (PI3Ks) phosphorylate the membrane lipid phosphatidylinositol, generating second messengers that direct diverse responses. These PI3K products are fundamental for leukocyte migration or chemotaxis, a pivotal event during the immune response. This system is therefore of significant biomedical interest. This review focuses on the biochemistry and signalling pathways of PI3K, with particular emphasis on chemokine (chemotactic cytokine)-directed responses. The key objectives of chemotaxis are motility and direction. The latter--direction--requires distinct events at the front and back of a cell. In light of this, the coordinated localisation of signalling factors, an event choreographed by a sharp intracellular gradient of PI3K-derived products, is a common theme.

Activation of human neutrophils by granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor alpha: role of phosphatidylinositol 3-kinase

Stimulation of human neutrophils with granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or tumor necrosis factor alpha (TNF) resulted in phosphorylation of Akt, the potency being GM-CSF > G-CSF = TNF, which was inhibited by wortmannin. The findings indicated that phosphatidylinositol 3-kinase (PI3K) is activated by these cytokines. The possible participation of PI3K in activation of neutrophil functions induced by these cytokines was explored with PI3K inhibitors (wortmannin and LY294002). Superoxide release and adherence induced by GM-CSF or TNF were inhibited by PI3K inhibitors. Actin reorganization and morphological changes induced by G-CSF or GM-CSF were also inhibited by wortmannin, whereas these responses induced by TNF were unaffected by wortmannin. These findings suggested that PI3K is differentially involved in cytokine-mediated activation of neutrophil functions depending on the cytokines used. The results also showed that activation of extracellular signal-regulated kinase, but not p38 mitogen-activated protein kinase, induced by these cytokines is partly mediated by PI3K activation.

Controlled pseudopod extension of human neutrophils stimulated with different chemoattractants

The formation of pseudopods and lamellae after ligation of chemoattractant sensitive G-protein coupled receptors (GPCRs) is essential for chemotaxis. Here, pseudopod extension was stimulated with chemoattractant delivered from a micropipet. The chemoattractant diffusion and convection mass transport were considered, and it is shown that when the delivery of chemoattractant was limited by diffusion there was a strong chemoattractant gradient along the cell surface. The diffusion-limited delivery of chemoattractant from a micropipet allowed for maintaining an almost constant chemoattractant concentration at the leading edge of single pseudopods during their growth. In these conditions, the rate of pseudopod extension was dependent on the concentration of chemoattractant in the pipet delivering chemoattractant. The pseudopod extension induced using micropipets was oscillatory even in the presence of a constant delivery of chemoattractant. This oscillatory pseudopod extension was controlled by activated RhoA and its downstream effector kinase ROCK and was abolished after the inhibition of RhoA activation with Clostridium botulinium C3 exoenzyme (C3) or the blocking of ROCK activation with Y-27632. The ability of the micropipet assay to establish a well-defined chemoattractant distribution around the activated cell over a wide range of molecular weights of the used chemoattractants allowed for comparison of the effect of chemoattractant stimulation on the dynamics of pseudopod growth. Pseudopod growth was stimulated using N-formylated peptide (N-formyl-methionyl-leucyl-phenylalanine (fMLP)), platelet activating factor (PAF), leukotriene B4 (LTB(4)), C5a anaphylotoxin (C5a), and interleukin-8 (IL-8), which represent the typical ligands for G-protein coupled chemotactic receptors. The dependence of the rate of pseudopod extension on the concentration of these chemoattractants and their equimolar mixture was measured and shown to be similar for all chemoattractants. The inhibition of the activity of phosphoinositide-3 kinase (PI3K) with wortmannin showed that 72%-80% of the rate of pseudopod extension induced with N-formyl-methionyl-leucyl-phenylalanine, platelet activating factor, and leukotriene B4 was phosphoinositide-3 kinase-dependent, in contrast to 55% of the rate of pseudopod extension induced with interleukin-8. The dependence of the rate of pseudopod extension on the concentration of individual chemoattractants and their equimolar mixture suggests that there is a common rate-limiting mechanism for the polymerization of cytoskeletal F-actin in the pseudopod region induced by G-protein coupled chemoattractant receptors.

Flavonoid apigenin inhibits motility and invasiveness of carcinoma cellsin vitro

Investigations of the mechanisms of the cancer-preventive activity of apigenin (4',5,7,-trihydroxyflavone), a plant-derived, anti-carcinogenic flavonoid, showed its interference with cell proliferation, survival, and gap junctional coupling. We used a model based on non-invasive HeLa wild-type cells and their connexin43 (Cx43) transfected counterparts to correlate the effect of apigenin on tumour cell invasiveness with its influence on cell motility. Both cell lines displayed similar motile properties in control conditions. Apigenin treatment resulted in a significant and reversible inhibition of translocation of both HeLa wild-type cells and HeLa Cx43 transfectants. The effect of apigenin on cell proliferation was less pronounced especially at low apigenin concentration, whereas its influence on cell motility correlated with the reduction of the invasive potential of HeLa Cx43 cells as shown by an invasion assay based on the confrontation of tumour cell spheroids with chick embryo heart fragments. HeLa Cx43 cells were highly invasive in controls, but did not invade the heart tissue at tumour cell aggregate-fibroblast capsule interfaces in the presence of apigenin and failed to fully engulf these heart fragments. Because the motility of chick heart fibroblasts was only slightly affected by apigenin, these observations indicate that apigenin exerts its anti-invasive effect on HeLa cells predominantly via a specific inhibition of tumour cell motility. This inhibitory effect of apigenin on tumour cell invasiveness in vitro demonstrates that apigenin may exert its anti-tumorigenic effect in vivo via inhibition of tumour cell penetration of the healthy tissue.

Characterization of the MEK5-ERK5 module in human neutrophils and its relationship to ERK1/ERK2 in the chemotactic response

The role of the extracellular signal-regulated kinase (ERK) 1 and ERK2 in the neutrophil chemotactic response remains to be identified since a previously used specific inhibitor of MEK1 and MEK2, PD98059, that was used to provide evidence for a role of ERK1 and ERK2 in regulating chemotaxis, has recently been reported to also inhibit MEK5. This issue is made more critical by our present finding that human neutrophils express mitogen-activated protein (MAP) kinase/ERK kinase (MEK)5 and ERK5 (Big MAP kinase), and that their activities were stimulated by the bacterial tripeptide, formyl methionyl-leucyl-phenylalanine (fMLP). Dose response studies demonstrated a bell-shaped profile of fMLP-stimulated MEK5 and ERK5 activation, but this was left-shifted when compared with the profile of fMLP-stimulated chemotaxis. Kinetics studies demonstrated increases in kinase activity within 2 min, peaking at 3-5 min, and MEK5 activation was more persistent than that of ERK5. There were some similarities as well as differences in the pattern of activation between fMLP-stimulated ERK1 and ERK2, and MEK5-ERK5 activation. The up-regulation of MEK5-ERK5 activities was dependent on phosphatidylinositol 3-kinase. Studies with the recently described specific MEK inhibitor, PD184352, at concentrations that inhibited ERK1 and ERK2 but not ERK5 activity demonstrate that the ERK1 and ERK2 modules were involved in regulating fMLP-stimulated chemotaxis and chemokinesis. Our data suggest that the MEK5-ERK5 module is likely to regulate neutrophil responses at very low chemoattractant concentrations whereas at higher concentrations, a shift to the ERK1/ERK2 and p38 modules is apparent.

A requirement of MAPKAPK2 in the uropod localization of PTEN during FMLP-induced neutrophil chemotaxis

The directionality control in chemotaxis is the result of a reciprocal regulation of PI3-kinase and PTEN subcellular localization. MK2(-/-) neutrophils have a directionality loss in fMLP-induced chemotaxis. We found that in polarized WT neutrophils PTEN was localized in the uropod region. However, MK2(-/-) neutrophils or p38 MAPK inhibitor-SB203580-pretreated WT neutrophils showed a disrupted PTEN subcellular localization. Some PTEN was localized at the leading edge of the polarized neutrophils, which may lower the concentration of PI3-kinase lipid product PtdIns(3,4,5)P3 required for directionality sensing. FMLP-stimulated MK2(-/-) neutrophils or SB203580-pretreated WT neutrophils also had disrupted F-actin polarization. F-actin polymerization inhibitor lantrunculin-B disrupted the polarization of PTEN, but not PtdIns(3,4,5)P3. The results suggest that PTEN uropod polarization is F-actin polymerization-dependent and may be through the effect of MK2 on F-actin polarization.

Signaling through the Leukocyte Integrin LFA-1 in T Cells Induces a Transient Activation of Rac-1 That Is Regulated by Vav and PI3K/Akt-1

Integrin LFA-1 is a receptor that is able to transmit multiple intracellular signals in leukocytes. Herein we show that LFA-1 induces a potent and transient increase in the activity of the small GTPase Rac-1 in T cells. Maximal Rac-1 activity peaked 10-15 min after LFA-1 stimulation and rapidly declined to basal levels at longer times. We have identified Vav, a guanine nucleotide exchange factor for Rac-1, and PI3K/Akt, as regulators of the activation and inactivation phases of the activity of Rac-1, respectively, in the context of LFA-1 signaling based on the following experimental evidence: (i) LFA-1 induced activation of Vav and PI3K/Akt with kinetics consistent with a regulatory role for these molecules on Rac-1, (ii) overexpression of a constitutively active Vav mutant induces activation of Rac independently of LFA-1 stimulation whereas overexpression of a dominant-negative Vav mutant blocks LFA-1-mediated Rac activation, (iii) pharmacological inhibition of PI3K/Akt prevented the fall in the activity of Rac-1 after its initial activation but had no effect on Vav activity, and (iv) overexpression of a dominant-negative or a constitutively active Akt-1 induced or inhibited, respectively, Rac-1 activity. Finally, we show that T cells with a sustained Rac activity have impaired capacity to elongate onto ICAM-1. These results demonstrate that down-regulation of the activity of this GTPase is a requirement for the regulation of T cell morphology and motility and highlight the importance of temporal regulation of the signaling triggered from this integrin.

Signaling to migration in neutrophils: importance of localized pathways

Neutrophils, a major type of blood leukocytes, are indispensable for host defense of bacterial infections. Directed migration in a gradient of chemotactic stimuli enables these cells to rapidly find the site of infection and destroy the invading pathogens. Chemotactic factors bind to seven-transmembrane-domain receptors and activate heterotrimeric G-proteins. Downstream of these proteins a complex interrelated signaling network is activated in human neutrophils. Stimulation of phospholipase C beta results in activation of protein kinase C isoforms and increases in cytosolic calcium. Activation of the enzyme phosphoinositide 3-kinase results in increased production of phosphatidylinositol 3,4,5-trisphosphate and phosphatidyl 3,4-bisphosphate. In addition, small GTP-binding proteins of the Rho family, the mitogen-activated protein kinase cascade, tyrosine kinases and protein phosphatases are activated. The enzyme phosphoinositide 3-kinase and the small cytosolic GTP-binding proteins Rho and Rac emerge as key regulators of neutrophil migration. A steep internal gradient of phosphatidylinositol 3,4,5-trisphosphate, with a high concentration in the leading lamellae, is thought to regulate polarized actin polymerization and formation of protrusions, together with Rac which may be more directly involved in initiating actin reorganization. Rho may regulate localized myosin activation, tail retraction, cell body traction and dynamics of adhesion. The impact of these different signaling pathways on reversible actin polymerization, development of polarity, reversible adhesion and migration, and the putative targets of these pathways in neutrophils, are reviewed in this article. Insight into mechanisms regulating migration of neutrophils could potentially lead to novel therapeutic strategies for counteracting chronic activation of neutrophils which leads to tissue damage.

Novel pathways of F-actin polymerization in the human neutrophil

Recently we demonstrated the existence of a phosphatidylinositol 3-kinase (PI3K)-independent F-actin polymerization during neutrophil pseudopod extension. Here we examine the use of the PI3K-dependent and PI3K-independent pathways of activation by the N-formyl peptide receptor and the chemokine receptors, and the priming of the 2 pathways by granulocyte-macrophage colony-stimulating factor (GM-CSF) and insulin. The inhibition of PI3K activity with wortmannin showed that rate of pseudopod extension stimulated with N-formyl-Met-Leu-Phe (fMLP was mostly dependent on PI3K, while the rate of interleukin-8 (IL-8)-stimulated pseudopod extension was less dependent on PI3K. The incubation of cells with either GM-CSF or insulin increased the rate of pseudopod extension by 50% when the cells were stimulated with IL-8 but not with fMLP. The stimulation with IL-8 phosphorylated the PI3K regulatory subunit. This phosphorylation was enhanced by GM-CSF, which increased PI3K activity and total phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) production. The effect of GM-CSF was blocked with wortmannin. In contrast, insulin did not increase p85 phosphorylation and did not enhance PI3K activity or PtdIns(3,4,5)P3 production. The effect of insulin was insensitive to wortmannin; however, it was blocked by an Src homology 2 (SH2)-binding peptide. These data indicate that priming of IL-8 activation with GM-CSF was mediated via the PI3Ks of class IA, while priming with insulin used a PI3K-independent pathway.

Expression and molecular pharmacology of the mouse CRTH2 receptor

Prostaglandin D2 (PGD2), the predominant prostanoid produced by activated mast cells, is implicated in a variety of allergic diseases. PGD2 exerts its effects through two G-protein coupled receptors, DP and CRTH2. PGD2 mediates chemotaxis of eosinophils, basophils, and Th2 cells via CRTH2-evoked signaling, suggesting a role for this receptor in allergic disease. To characterize the mouse CRTH2 ortholog (mCRTH2), we amplified the mCRTH2 receptor gene and expressed it in HEK293 cells. Saturation ligand binding isotherms demonstrated high-affinity binding of [3H]PGD2, with a Kd of 8.8 +/- 0.8 nM. Competition binding assays with a panel unlabeled prostanoids demonstrated an order of affinity of 13,14-dihydro-15-keto-PGD2 (DK-PGD2) >or= 15-deoxy-Delta12,14-PGJ2 (15d-PGJ2) >or= PGD2 >or= PGJ2. [3H]PGD2 binding was also displaced by the nonsteroidal anti-inflammatory drug indomethacin, with a Ki value of 1.04 +/- 0.13 microM. No [3H]PGD2 displacement was detected using fluribrofen, ibuprofen, or aspirin as competitors at concentrations of up to 30 microM. PGD2, DK-PGD2, 15d-PGJ2, and indomethacin each inhibited intracellular cAMP generation in stable transfectant ER293/mCRTH2 cells through a pertussis toxin (PTX) sensitive pathway, consistent with mCRTH2 coupling to a Gi heterotrimeric G-protein. Activation of mCRTH2 elicited chemotaxis of ER293/mCRTH2 cells in response to PGD2, indomethacin, and 15d-PGJ2. mCRTH2-dependent chemotaxis was inhibited by PTX and wortmannin, indicating dependence on Gi and PI 3-kinase signal transduction pathways. These data provide the first pharmacological and functional characterization of the mouse CRTH2 receptor.

Eukaryotic chemotaxis: distinctions between directional sensing and polarization

Directional sensing and polarization are fundamental cellular responses that play a central role in health and disease. In this review we define each process and evaluate a series of models previously proposed to explain these phenomena. New findings show that directional sensing by G protein-coupled receptors is localized at a discrete step in the signaling pathway downstream of G protein activation but upstream of the accumulation of PIP3. Local levels of PIP3, whether triggered by chemoattractants, particle binding, or spontaneous events, determine the sites of new actin-filled projections. Robust control of the temporal and spatial levels of PIP3 is achieved by reciprocal regulation of PI3K and PTEN. These observations suggest that a local excitation-global inhibition model can account for the localization of PI3K and PTEN and thereby explain directional sensing. However, elements of other models, including positive feedback and the reaction of the cytoskeleton, must be invoked to account for polarization.

Microtubule-disruption-induced and chemotactic-peptide-induced migration of human neutrophils: implications for differential sets of signalling pathways

Neutrophil granulocytes rely on a functional actin network for directed migration. Microtubule disassembly does not impair receptor-linked chemotaxis, instead it induces development of polarity and chemokinesis in neutrophils concomitant with polarized distribution of alpha-actinin and F-actin. Cells stimulated with colchicine, which disassembles microtubules, migrate with a speed comparable to cells exposed to chemotactic peptide. We investigated signalling pathways involved in colchicine-induced neutrophil polarization and migration. Colchicine-induced development of polarity was insensitive to treatment with pertussis toxin, in contrast to chemotactic-peptide-induced shape changes, which were completely abolished by this treatment. Thus, colchicine does not appear to act via activating heterotrimeric G(i) proteins. Colchicine does also not seem to act via phosphatidylinositol 3-kinase, as it failed to induce phosphorylation of its downstream target Akt and the potent phosphatidylinositol 3-kinase inhibitor wortmannin failed to inhibit colchicine-induced shape changes. By contrast, wortmannin significantly reduced chemotactic-peptide-induced shape changes. However, the Rho-kinase inhibitor Y-27632 (10 micro M) inhibited colchicine-induced development of polarity by 95+/-3% (n=5) and chemokinesis by 76+/-9% (n=3), which suggests that the Rho-Rho-kinase pathway has a crucial role in polarity and migration. Indeed, treatment of cells with colchicine induced a significant increase in membrane-bound Rho-kinase II, which is indicative of activation of this protein. This membrane translocation could be prevented by taxol, which stabilizes microtubules. Colchicine also induced a marked increase in myosin light chain phosphorylation, which could be suppressed by Y-27632 and by taxol. In summary, we provide evidence that microtubule disassembly induces in neutrophils a selective activation of Rho-kinase, bypassing activation of heterotrimeric Gi proteins and phosphatidylinositol 3-kinase. This process is sufficient for induction of chemokinesis and mediates increased phosphorylation of myosin light chain and accumulation of F-actin and alpha-actinin in the leading edge.

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.

An intracellular signaling hierarchy determines direction of migration in opposing chemotactic gradients

Neutrophils must follow both endogenous and bacterial chemoattractant signals out of the vasculature and through the interstitium to arrive at a site of infection. By necessity, in the setting of multiple chemoattractants, the neutrophils must prioritize, favoring end target chemoattractants (e.g., fMLP and C5a) emanating from the site of infection over intermediary endogenous chemoattractants (e.g., IL-8 and LTB4) encountered en route to sites of infection. In this study, we propose a hierarchical model of two signaling pathways mediating the decision-making process of the neutrophils, which allows end target molecules to dominate over intermediary chemoattractants. In an under agarose assay, neutrophils predominantly migrated toward end target chemoattractants via p38 MAPK, whereas intermediary chemoattractant-induced migration was phosphoinositide 3-kinase (PI3K)/Akt dependent. When faced with competing gradients of end target and intermediary chemoattractants, Akt activation was significantly reduced within neutrophils, and the cells migrated preferentially toward end target chemoattractants even at 1/1,000th that of intermediary chemoattractants. End target molecules did not require chemotactic properties, since the p38 MAPK activator, LPS, also inhibited Akt and prevented migration to intermediary chemoattractants. p38 MAPK inhibitors not only reversed this hierarchy, such that neutrophils migrated preferentially toward intermediary chemoattractants, but also allowed neutrophils to be drawn out of a local end target chemoattractant environment and toward intermediary chemoattractants unexpectedly in an exaggerated (two- to fivefold) fashion. This was entirely related to significantly increased magnitude and duration of Akt activation. Finally, end target chemoattractant responses were predominantly Mac-1 dependent, whereas nondominant chemoattractants used primarily LFA-1. These data provide support for a two pathway signaling model wherein the end target chemoattractants activate p38 MAPK, which inhibits intermediary chemoattractant-induced PI3K/Akt pathway, establishing an intracellular signaling hierarchy.

Temporal and spatial regulation of chemotaxis

The ability to sense and respond to shallow gradients of extracellular signals is remarkably similar in Dictyostelium discoideum amoebae and mammalian leukocytes. Chemoattractant receptors and G proteins are fairly evenly distributed along the cell surface. Receptor occupancy generates local excitatory and global inhibitory processes that balance to control the chemotactic response. Uniform stimuli transiently recruit PI3Ks to, and release PTEN from, the plasma membrane, while gradients of chemoattractant cause the two enzymes to bind to the membrane at the front and back of the cell, respectively. Interference with PI3Ks alters chemotaxis, and disruption of PTEN broadens PI localization and actin polymerization in parallel. Thus, counteracting signals from the upstream elements of the pathway converge to regulate the key enzymes of PI metabolism, localize these lipids, and direct pseudopod formation.

A PtdInsP(3)- and Rho GTPase-mediated positive feedback loop regulates neutrophil polarity

When presented with a gradient of chemoattractant, many eukaryotic cells respond with polarized accumulation of the phospholipid PtdIns(3,4,5)P(3). This lipid asymmetry is one of the earliest readouts of polarity in neutrophils, Dictyostelium discoideum and fibroblasts. However, the mechanisms that regulate PtdInsP(3) polarization are not well understood. Using a cationic lipid shuttling system, we have delivered exogenous PtdInsP(3) to neutrophils. Exogenous PtdInsP(3) elicits accumulation of endogenous PtdInsP(3) in a positive feedback loop that requires endogenous phosphatidylinositol-3-OH kinases (PI(3)Ks) and Rho family GTPases. This feedback loop is important for establishing PtdInsP(3) polarity in response to both chemoattractant and to exogenous PtdInsP(3); it may function through a self-organizing pattern formation system. Emergent properties of positive and negative regulatory links between PtdInsP(3) and Rho family GTPases may constitute a broadly conserved module for the establishment of cell polarity during eukaryotic chemotaxis.

Lipid products of PI(3)Ks maintain persistent cell polarity and directed motility in neutrophils

In gradients of external chemo-attractant, mammalian neutrophilic leukocytes (neutrophils) and Dictyostelium discoideum amoebae adopt a polarized morphology and selectively accumulate lipid products of phosphatidylinositol-3-OH kinases (PI(3)Ks), including PtdIns(3,4,5)P(3), at their up-gradient edges; the internal PtdIns(3,4,5)P(3) gradient substantially exceeds that of the external attractant. An accompanying report presents evidence for a positive feedback loop that amplifies the gradient of internal signal: PtdIns(3,4,5)P(3) at the leading edge stimulates its own accumulation by inducing activation of one or more Rho GTPases (Rac, Cdc42, and/or Rho), which in turn increase PtdIns(3,4,5)P(3) accumulation. Here we show that interruption of this feedback by treatment with PI(3)K inhibitors reduces the size and stability of pseudopods and causes cells to migrate in jerky trajectories that deviate more from the up-gradient direction than do those of controls. Moreover, amplification of the internal PtdIns(3,4,5)P(3) gradient is markedly impaired by latrunculin or jasplakinolide, toxins that inhibit polymerization or depolymerization of actin, respectively. Thus reciprocal interplay between PtdIns(3,4,5)P(3) and polymerized actin initiates and maintains the asymmetry of intracellular signals responsible for cell polarity and directed motility.

Differentiated HL-60 cells are a valid model system for the analysis of human neutrophil migration and chemotaxis

We have carried out a detailed comparison of the motile properties of differentiated HL-60 cells and human peripheral blood neutrophils. We compared the effects of chemotactic stimuli and of inhibitors of signalling proteins on morphology, chemokinesis and chemotaxis of neutrophils and differentiated HL-60 cells using videomicroscopy and a filter assay for chemotaxis. We also assessed expression of signalling and cytoskeletal proteins using Western blotting. Chemotactic peptide induced a front-tail polarity in HL-60 cells comparable to that of neutrophils. Chemokinetic and chemotactic responses to chemotactic peptide were also very similar for both cell types, concerning mean speed of migration, the fraction of migrated cells and the concentration of stimulus optimal for activation. The cytokine interleukin-8 was in contrast clearly less effective in activating motile responses of differentiated HL-60 cells as compared to neutrophils. An important functional role of Rho-activated kinases and phosphatidylinositol 3-kinase in motile responses of HL-60 cells, consistent with their upregulation during differentiation, could be confirmed using inhibitors with specificity for the corresponding enzymes. The only difference observed here between HL-60 cells and neutrophils concerned the differential effects of a protein kinase C inhibitor.In summary, the results presented here show that differentiated HL-60 cells, stimulated with chemotactic peptide, are a valid model system to study molecular mechanisms of neutrophil emigration.

Chemotactic factor-induced recruitment and activation of Tec family kinases in human neutrophils. Implication of phosphatidynositol 3-kinases

The importance of the tyrosine phosphorylation cascades in the initiation and regulation of the functional responsiveness of human neutrophils is well established. On the other hand, the link between the G protein-coupled receptors (to which the receptors for chemotactic factors belong) and the activation of tyrosine kinases is very poorly characterized. Based on previous observations indicating that the stimulation of tyrosine phosphorylation was sensitive to inhibition by the phosphatidylinositol 3-kinase inhibitor wortmannin and the recent description of pleckstrin homology domain-containing tyrosine kinases (the Tec family), we have examined the potential implication of the latter in the responses of human neutrophils to chemotactic factors. The results obtained indicate firstly that several members of the Tec family of tyrosine kinases are expressed in human neutrophils, including Tec, Btk, and Bmx. Stimulation of the cells with fMet-Leu-Phe led to a rapid activation of Tec as indicated by its translocation to a membrane fraction and to increases in its in situ level of tyrosine phosphorylation and its capacity to tyrosine phosphorylate itself or an exogenous substrate (SAM68-GST) in in vitro kinase assays. The activation of Tec was inhibited by pertussis toxin as well as by wortmannin. The results of this study provide direct evidence for the implication of Tec family kinases in the responses of human neutrophils to chemotactic factors. They also suggest that one of the links between G protein-coupled receptors and tyrosine kinases depends on the activation of phosphatidylinositol 3-kinase and the generation of phosphatidylinositol 3,4,5-trisphosphate.

Roles of PI3Ks in leukocyte chemotaxis and phagocytosis

Through selective disruption of phosphoinositide 3-kinase (PI3K) activity and the use of green fluorescent protein tagged derivatives of domains capable of specifically binding the lipid products of PI3Ks in vivo, it has been shown that this family of signalling enzymes have vital and distinct roles in chemotaxis, phagocytosis and phagosome maturation in leukocytes.

Regulation of cell polarity during eukaryotic chemotaxis: the chemotactic compass

Phosphatidylinositol 3-kinase lipid products and the Rho GTPases play a central role in transmitting information from chemotactic receptors to the effectors of cell polarity, and recent advances in the field have allowed us to understand these roles more clearly. Emergent properties of positive and negative regulation of these molecules may account for the establishment of cell polarity during chemotaxis for a wide range of cells from Dictyostelium to fibroblasts to neutrophils.

Effects of wortmannin on airways inflammation induced by allergen in actively sensitised Brown Norway rats

We have investigated the effect of wortmannin, a potent and selective inhibitor of phosphatidylinositol-3-kinase, on the immediate-type allergic response and the late phase pulmonary inflammation induced by allergen challenge in the ovalbumin-sensitised Brown Norway rat. Intratracheal (i.t.) instillation of ovalbumin induced dose-related bronchoconstrictor responses. Administration of wortmannin (1, 10 or 100 microg kg(-1) i.t., 1 h prior to challenge) induced a marked and dose-dependent inhibition of ovalbumin-induced bronchospasm (ED(50) ca. 5 microg kg(-1) i.t.). At similar doses, wortmannin also suppressed the bronchoconstrictor responses to 5-hydroxytryptamine and methacholine but the degree of blockade of these spasmogens (1.4-1.9-fold) was less than that of ovalbumin (>20-fold). Wortmannin, given intratracheally 1 h prior to allergen challenge, also suppressed the increases in bronchoalveolar lavage fluid leukocyte numbers and eosinophil peroxidase activity measured 24 h post challenge. However, relatively high doses were necessary (ED(50) ca. 100 microg kg(-1) i.t.). The potency of wortmannin was increased when dosed 1 h prior to and 24 h after allergen challenge and the readout was 48 h after challenge (ED(50) 3-5 microg kg(-1) i.t.). Thus, wortmannin is a potent inhibitor of the bronchoconstrictor response induced by allergen in the airways of actively sensitised Brown Norway rats. Inhibition of phosphatidylinositol-3-kinase, an obligatory step in mast cell activation in response to allergen, is the presumed mechanism of action. The fact that similar doses of wortmannin do not suppress the late response to allergen suggests a minimal role for the mast cell in generating the late response to allergen in this model. The striking increase in potency to inhibit the late response when dosed 1 h prior to and 24 h after allergen challenge with the readout taken at 48 h may represent an effect of wortmannin to suppress the migration of leukocytes.

PIP3, PIP2, and cell movement--similar messages, different meanings?

The inositol lipids PI(4,5)P(2) and PI(3,4,5)P(3) are important regulators of actin polymerization, but their different temporal and spatial dynamics suggest that they perform separate roles. PI(3,4,5)P(3) seems to act as an instructive second messenger, inducing local actin polymerization. PI(4,5)P(2) appears to be present at too high a concentration and homogeneous a distribution to fulfil a similar role. Instead, we suggest that PI(4,5)P(2) acts permissively, restricting new actin polymerization to the region of the plasma membrane.