Control of cell polarity and motility by the PtdIns(3,4,5)P3 phosphatase SHIP1

bFGF regulates PI3-kinase-Rac1-JNK pathway and promotes fibroblast migration in wound healing

Fibroblast proliferation and migration play important roles in wound healing. bFGF is known to promote both fibroblast proliferation and migration during the process of wound healing. However, the signal transduction of bFGF-induced fibroblast migration is still unclear, because bFGF can affect both proliferation and migration. Herein, we investigated the effect of bFGF on fibroblast migration regardless of its effect on fibroblast proliferation. We noticed involvement of the small GTPases of the Rho family, PI3-kinase, and JNK. bFGF activated RhoA, Rac1, PI3-kinase, and JNK in cultured fibroblasts. Inhibition of RhoA did not block bFGF-induced fibroblast migration, whereas inhibition of Rac1, PI3-kinase, or JNK blocked the fibroblast migration significantly. PI3-kinase-inhibited cells down-regulated the activities of Rac1 and JNK, and Rac1-inhibited cells down-regulated JNK activity, suggesting that PI3-kinase is upstream of Rac1 and that JNK is downstream of Rac1. Thus, we concluded that PI3-kinase, Rac1, and JNK were essential for bFGF-induced fibroblast migration, which is a novel pathway of bFGF-induced cell migration.

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.

Proteomic identification of phosphatidylinositol (3,4,5) triphosphate-binding proteins in Dictyostelium discoideum

Phosphatidylinositol (3,4,5)-triphosphate (PtdInsP(3)) mediates intracellular signaling for directional sensing and pseudopod extension at the leading edge of migrating cells during chemotaxis. How this PtdInsP(3) signal is translated into remodeling of the actin cytoskeleton is poorly understood. Here, using a proteomics approach, we identified multiple PtdInsP(3)-binding proteins in Dictyostelium discoideum, including five pleckstrin homology (PH) domain-containing proteins. Two of these, the serine/threonine kinase Akt/protein kinase B and the PH domain-containing protein PhdA, were previously characterized as PtdInsP(3)-binding proteins. In addition, PhdB, PhdG, and PhdI were identified as previously undescribed PH domain-containing proteins. Specific PtdInsP(3) interactions with PhdB, PhdG, and PhdI were confirmed using an in vitro lipid-binding assay. In cells, PhdI associated with the plasma membrane in a manner dependent on both the PH domain and PtdInsP(3). Consistent with this finding, PhdI located to the leading edge in migrating cells. In contrast, PhdG was found in the cytosol in WT cells. However, when PtdInsP(3) was overproduced in pten(-) cells, PhdG located to the plasma membrane, suggesting its weak affinity for PtdInsP(3). PhdB was found to bind to the plasma membrane via both PtdInsP(3)-dependent and -independent mechanisms. The PtdInsP(3)-independent interaction was mediated by the middle domain, independent of the PH domain. In migrating cells, the majority of PhdB was found at the lagging edge. Finally, we deleted the genes encoding PhdB and PhdG and demonstrated that both proteins are required for efficient chemotaxis. Thus, this study advances our understanding of the PtdInsP(3)-mediated signaling mechanisms that control directed cell migration in chemotaxis.

The PtdIns(3,4)P(2) phosphatase INPP4A is a suppressor of excitotoxic neuronal death

Phosphorylated derivatives of phosphatidylinositol, collectively referred to as phosphoinositides, occur in the cytoplasmic leaflet of cellular membranes and regulate activities such as vesicle transport, cytoskeletal reorganization and signal transduction. Recent studies have indicated an important role for phosphoinositide metabolism in the aetiology of diseases such as cancer, diabetes, myopathy and inflammation. Although the biological functions of the phosphatases that regulate phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) have been well characterized, little is known about the functions of the phosphatases regulating the closely related molecule phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P(2)). Here we show that inositol polyphosphate phosphatase 4A (INPP4A), a PtdIns(3,4)P(2) phosphatase, is a suppressor of glutamate excitotoxicity in the central nervous system. Targeted disruption of the Inpp4a gene in mice leads to neurodegeneration in the striatum, the input nucleus of the basal ganglia that has a central role in motor and cognitive behaviours. Notably, Inpp4a(-/-) mice show severe involuntary movement disorders. In vitro, Inpp4a gene silencing via short hairpin RNA renders cultured primary striatal neurons vulnerable to cell death mediated by N-methyl-d-aspartate-type glutamate receptors (NMDARs). Mechanistically, INPP4A is found at the postsynaptic density and regulates synaptic NMDAR localization and NMDAR-mediated excitatory postsynaptic current. Thus, INPP4A protects neurons from excitotoxic cell death and thereby maintains the functional integrity of the brain. Our study demonstrates that PtdIns(3,4)P(2), PtdIns(3,4,5)P(3) and the phosphatases acting on them can have distinct regulatory roles, and provides insight into the unique aspects and physiological significance of PtdIns(3,4)P(2) metabolism. INPP4A represents, to our knowledge, the first signalling protein with a function in neurons to suppress excitotoxic cell death. The discovery of a direct link between PtdIns(3,4)P(2) metabolism and the regulation of neurodegeneration and involuntary movements may aid the development of new approaches for the treatment of neurodegenerative disorders.

PI3K inhibition in inflammation: Toward tailored therapies for specific diseases

In the past decade, the availability of genetically modified animals has enabled the discovery of interesting roles for phosphatidylinositol 3-kinase-gamma (PI3Kgamma) and -delta (PI3Kdelta) in different cell types orchestrating innate and adaptive immune responses. Therefore, these PI3K isoforms appear to be attractive drug targets for the treatment of diseases caused by unrestrained immune reactions. Currently, pharmacological targeting of PI3Kgamma and/or PI3Kdelta represents one of the most promising challenges for companies interested in the development of novel safe treatments for inflammatory diseases. In this review we provide a general outline of PI3Kgamma- and PI3Kdelta-specific functions in distinct subsets of inflammatory cells. We also discuss the therapeutic impact of novel compounds targeting PI3Kgamma, PI3Kdelta or both, in mouse models of autoimmune disorders (systemic lupus erythematosus (SLE) and rheumatoid arthritis), respiratory diseases (allergic asthma and chronic obstructive pulmonary disease) and cardiovascular dysfunctions (atherosclerosis and myocardial infarction).

5-Stabilized phosphatidylinositol 3,4,5-trisphosphate analogues bind Grp1 PH, inhibit phosphoinositide phosphatases, and block neutrophil migration

Metabolically stabilized analogues of PtdIns(3,4,5)P3 have shown long-lived agonist activity for cellular events and selective inhibition of lipid phosphatase activity. We describe an efficient asymmetric synthesis of two 5-phosphatase-resistant analogues of PtdIns(3,4,5)P3, the 5-methylene phosphonate (MP) and 5-phosphorothioate (PT). Furthermore, we illustrate the biochemical and biological activities of five stabilized PtdIns(3,4,5)P3 analogues in four contexts. First, the relative binding affinities of the 3-MP, 3-PT, 5-MP, 5-PT, and 3,4,5-PT3 analogues to the Grp1 PH domain are shown, as determined by NMR spectroscopy. Second, the enzymology of the five analogues is explored, showing the relative efficiency of inhibition of SHIP1, SHIP2, and phosphatase and tensin homologue deleted on chromosome 10 (PTEN), as well as the greatly reduced ability of these phosphatases to process these analogues as substrates as compared to PtdIns(3,4,5)P3. Third, exogenously delivered analogues severely impair complement factor C5a-mediated polarization and migration of murine neutrophils. Finally, the new analogues show long-lived agonist activity in mimicking insulin action in sodium transport in A6 cells.

Polarization of migrating monocytic cells is independent of PI 3-kinase activity

Background

Migration of mammalian cells is a complex cell type and environment specific process. Migrating hematopoietic cells assume a rapid amoeboid like movement when exposed to gradients of chemoattractants. The underlying signaling mechanisms remain controversial with respect to localization and distribution of chemotactic receptors within the plasma membrane and the role of PI 3-kinase activity in cell polarization.

Methodology/Principal Findings

We present a novel model for the investigation of human leukocyte migration. Monocytic THP-1 cells transfected with the α_2A-adrenoceptor (α_2AAR) display comparable signal transduction responses, such as calcium mobilization, MAP-kinase activation and chemotaxis, to the noradrenaline homlogue UK 14'304 as when stimulated with CCL2, which binds to the endogenous chemokine receptor CCR2. Time-lapse video microcopy reveals that chemotactic receptors remain evenly distributed over the plasma membrane and that their internalization is not required for migration. Measurements of intramolecular fluorescence resonance energy transfer (FRET) of α_2AAR-YFP/CFP suggest a uniform activation of the receptors over the entire plasma membrane. Nevertheless, PI 3-kinse activation is confined to the leading edge. When reverting the gradient of chemoattractant by moving the dispensing micropipette, polarized monocytes – in contrast to neutrophils – rapidly flip their polarization axis by developing a new leading edge at the previous posterior side. Flipping of the polarization axis is accompanied by re-localization of PI-3-kinase activity to the new leading edge. However, reversal of the polarization axis occurs in the absence of PI 3-kinase activation.

Conclusions/Significance

Accumulation and internalization of chemotactic receptors at the leading edge is dispensable for cell migration. Furthermore, uniformly distributed receptors allow the cells to rapidly reorient and adapt to changes in the attractant cue. Polarized monocytes, which display typical amoeboid like motility, can rapidly develop a new leading edge facing the highest chemoattractant concentration at any site of the plasma membrane, including the uropod. The process appears to be independent of PI 3-kinase activity.

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.

Molecular regulators of leucocyte chemotaxis during inflammation

A fundamental feature of any immune response is the movement of leucocytes from one site in the body to another to provide effector functions. Therefore, elucidating the molecular mechanisms underlying the migration of leucocytes from the blood to tissues is critical to our understanding of immune function during inflammation. The classic steps of leucocyte trafficking involve leucocyte tethering and rolling on vessel walls of the vasculature, followed by firm adhesion to the endothelium. Recent evidence suggests that upon adhering, leucocytes crawl within the vessels before transmigrating across vessel walls and crawling into targeted tissues. The directed nature of the crawling events is orchestrated by a complex array of soluble factors and molecular regulators in combination with the local intravascular and extracellular environment. In fact, this process is known as chemotaxis and orientates cell movement in relation to the ligand gradient. Several signalling pathways have been proposed to be involved in this gradient-sensing and amplification process, but the best studied, discussed in detail here, is the phosphatidylinositol 3-kinase pathway. Substantial progress has been made in understanding how cells roll and adhere in blood vessels; however, how cells crawl in blood vessels, emigrate, and then crawl in tissues has received much less attention. Therefore, the focus of this review is to provide recent insights into molecular mechanisms and cellular processes that mediate leucocyte crawling in blood vessels and tissues during the inflammatory response.

Inhibitory Role of Sphingosine 1-Phosphate Receptor 2 in Macrophage Recruitment during Inflammation

Macrophage recruitment to sites of inflammation is an essential step in host defense. However, the mechanisms preventing excessive accumulation of macrophages remain relatively unknown. The lysophospholipid sphingosine 1-phosphate (S1P) promotes T and B cell egress from lymphoid organs by acting on S1P receptor 1 (S1P1R). More recently, S1P5R was shown to regulate NK cell mobilization during inflammation, raising the possibility that S1P regulates the trafficking of other leukocyte lineages. In this study, we show that S1P2R inhibits macrophage migration in vitro and that S1P2R-deficient mice have enhanced macrophage recruitment during thioglycollate peritonitis. We identify the signaling mechanisms used by S1P2R in macrophages, involving the second messenger cAMP and inhibition of Akt phosphorylation. In addition, we show that the phosphoinositide phosphatase and tensin homolog deleted on chromosome 10, which has been suggested to mediate S1P2R effects in other cell types, does not mediate S1P2R inhibition in macrophages. Our results suggest that S1P serves as a negative regulator of macrophage recruitment by inhibiting migration in these cells and identify an additional facet to the regulation of leukocyte trafficking by S1P.

Phosphoinositide-dependent protein kinase (PDK) activity regulates phosphatidylinositol 3,4,5-trisphosphate-dependent and -independent protein kinase B activation and chemotaxis

Chemotactic cells must sense shallow extracellular gradients and produce localized intracellular responses. We previously showed that the temporal and spatial activation of two protein kinase B (PKB) homologues, PkbA and PkbR1, in Dictyostelium discoideum by phosphorylation of activation loops (ALs) and hydrophobic motifs had important roles in chemotaxis. We found that hydrophobic motif phosphorylation depended on regulation of TorC2 (target of rapamycin complex 2); however, the regulation of AL phosphorylation remains to be determined at a molecular level. Here, we show that two PDK (phosphoinositide-dependent protein kinase) homologues, PdkA and PdkB, function as the key AL kinases. Cells lacking both PdkA and PdkB are defective in PKB activation, chemotaxis, and fruiting body formation upon nutrient deprivation. The pleckstrin homology domain of PdkA is sufficient to localize it to the membrane, but transient activation of PdkA is independent of PIP(3) as well as TorC2 and dispensable for full function. These results confirm the importance of the TorC2-PDK-PKB pathway in chemotaxis and point to a novel mechanism of regulation of PDKs by chemoattractant.

Manipulation of neutrophil-like HL-60 cells for the study of directed cell migration

Many cells undergo directed cell migration in response to external cues in a process known as chemotaxis. This ability is essential for many single-celled organisms to hunt and mate, the development of multicellular organisms, and the functioning of the immune system. Because of their relative ease of manipulation and their robust chemotactic abilities, the neutrophil-like cell line (HL-60) has been a powerful system to analyze directed cell migration. In this chapter, we describe the maintenance and transient transfection of HL-60 cells and explain how to analyze their behavior with two standard chemotactic assays (micropipette and EZ-TAXIS). Finally, we demonstrate how to fix and stain the actin cytoskeleton of polarized cells for fluorescent microscopy imaging.

PI3K signaling in neutrophils

PI3Ks play important roles in the signaling pathways used by a wide variety of cell surface receptors on neutrophils. Class IB PI3K plays a major role in the initial generation of PtdIns(3,4,5)P₃ by Gi-coupled G-protein coupled receptors (GPCRs) (e.g., receptors for fMLP, C5a, LTB₄). Class IA PI3Ks generate PtdIns(3,4,5)P₃ downstream of receptors which directly or indirectly couple to protein tyrosine kinases such as integrins, FcγRs, cytokine receptors, and GPCRs. The PtdIns(3,4,5)P₃ made by Class I PI3Ks regulates the activity of several different effector proteins, many of which are plasma membrane GEFs or GAPs for small GTPases. Class III PI3K generates PtdIns(3)P in the phagosome membrane and plays an important role in efficient assembly of the NADPH oxidase at this location. Much still remains to be discovered about the molecular details that govern activation of PI3Ks and the mechanisms by which these enzymes regulate complex cellular processes, such as neutrophil spreading, chemotaxis, phagocytosis, and killing of pathogens. However, it is clear from recent use of transgenic mouse models and isoform-selective PI3K inhibitors that these pathways are important in regulating neutrophil recruitment to sites of infection and damage in vivo. Thus, PI3K pathways may present novel opportunities for selective inhibition in some inflammatory pathologies.

The G protein betagamma subunit mediates reannealing of adherens junctions to reverse endothelial permeability increase by thrombin

The inflammatory mediator thrombin proteolytically activates protease-activated receptor (PAR1) eliciting a transient, but reversible increase in vascular permeability. PAR1-induced dissociation of Gα subunit from heterotrimeric Gq and G12/G13 proteins is known to signal the increase in endothelial permeability. However, the role of released Gβγ is unknown. We now show that impairment of Gβγ function does not affect the permeability increase induced by PAR1, but prevents reannealing of adherens junctions (AJ), thereby persistently elevating endothelial permeability. We observed that in the naive endothelium Gβ1, the predominant Gβ isoform is sequestered by receptor for activated C kinase 1 (RACK1). Thrombin induced dissociation of Gβ1 from RACK1, resulting in Gβ1 interaction with Fyn and focal adhesion kinase (FAK) required for FAK activation. RACK1 depletion triggered Gβ1 activation of FAK and endothelial barrier recovery, whereas Fyn knockdown interrupted with Gβ1-induced barrier recovery indicating RACK1 negatively regulates Gβ1-Fyn signaling. Activated FAK associated with AJ and stimulated AJ reassembly in a Fyn-dependent manner. Fyn deletion prevented FAK activation and augmented lung vascular permeability increase induced by PAR1 agonist. Rescuing FAK activation in fyn^−/− mice attenuated the rise in lung vascular permeability. Our results demonstrate that Gβ1-mediated Fyn activation integrates FAK with AJ, preventing persistent endothelial barrier leakiness.

The signaling mechanisms underlying cell polarity and chemotaxis

Chemotaxis--the directed movement of cells in a gradient of chemoattractant--is essential for neutrophils to crawl to sites of inflammation and infection and for Dictyostelium discoideum (D. discoideum) to aggregate during morphogenesis. Chemoattractant-induced activation of spatially localized cellular signals causes cells to polarize and move toward the highest concentration of the chemoattractant. Extensive studies have been devoted to achieving a better understanding of the mechanism(s) used by a neutrophil to choose its direction of polarity and to crawl effectively in response to chemoattractant gradients. Recent technological advances are beginning to reveal many fascinating details of the intracellular signaling components that spatially direct the cytoskeleton of neutrophils and D. discoideum and the complementary mechanisms that make the cell's front distinct from its back.

P2X1 ion channels promote neutrophil chemotaxis through Rho kinase activation

ATP, released at the leading edge of migrating neutrophils, amplifies chemotactic signals. The aim of our study was to investigate whether neutrophils express ATP-gated P2X(1) ion channels and whether these channels could play a role in chemotaxis. Whole-cell patch clamp experiments showed rapidly desensitizing currents in both human and mouse neutrophils stimulated with P2X(1) agonists, alphabeta-methylene ATP (alphabetaMeATP) and betagammaMeATP. These currents were strongly impaired or absent in neutrophils from P2X(1)(-/-) mice. In Boyden chamber assays, alphabetaMeATP provoked chemokinesis and enhanced formylated peptide- and IL-8-induced chemotaxis of human neutrophils. This agonist similarly increased W-peptide-induced chemotaxis of wild-type mouse neutrophils, whereas it had no effect on P2X(1)(-/-) neutrophils. In human as in mouse neutrophils, alphabetaMeATP selectively activated the small RhoGTPase RhoA that caused reversible myosin L chain phosphorylation. Moreover, the alphabetaMeATP-elicited neutrophil movements were prevented by the two Rho kinase inhibitors, Y27632 and H1152. In a gradient of W-peptide, P2X(1)(-/-) neutrophils migrated with reduced speed and displayed impaired trailing edge retraction. Finally, neutrophil recruitment in mouse peritoneum upon Escherichia coli injection was enhanced in wild-type mice treated with alphabetaMeATP, whereas it was significantly impaired in the P2X(1)(-/-) mice. Thus, activation of P2X(1) ion channels by ATP promotes neutrophil chemotaxis, a process involving Rho kinase-dependent actomyosin-mediated contraction at the cell rear. These ion channels may therefore play a significant role in host defense and inflammation.

Visualization of Ras-PI3K interaction in the endosome using BiFC

Recent studies indicate the importance of spatiotemporal regulation in the diversity and specificity of intracellular signaling. Here, we show that Ras-PI3K signaling plays an important role in the local regulation of phosphatidylinositol metabolism in the endosome through live-cell imaging by using a bimolecular fluorescence complementation technique, in which molecular interaction is indicated by fluorescence emission. Using several possible combinations of Ras and the Ras-binding domain, we identified an optimal set of probe molecules that yielded the most significant increase in fluorescence intensity between the active and inactive forms of Ras. This combination revealed that, among the Ras effectors tested, phosphatidylinositol 3-kinase (PI3K) was specifically implicated in signaling in the endosome. We also found that full length PI3K was recruited to the endosome in EGF- and Ras-dependent manners, which appears to be essential for the activation of PI3K in this compartment. Taken together, these findings demonstrate that the spatiotemporal regulation of Ras-PI3K signaling may dictate the activation of PI3K and subsequent downstream signaling in the endosome.

Mammalian phosphoinositide kinases and phosphatases

Phosphoinositides are lipids that are present in the cytoplasmic leaflet of a cell's plasma and internal membranes and play pivotal roles in the regulation of a wide variety of cellular processes. Phosphoinositides are molecularly diverse due to variable phosphorylation of the hydroxyl groups of their inositol rings. The rapid and reversible configuration of the seven known phosphoinositide species is controlled by a battery of phosphoinositide kinases and phosphoinositide phosphatases, which are thus critical for phosphoinositide isomer-specific localization and functions. Significantly, a given phosphoinositide generated by different isozymes of these phosphoinositide kinases and phosphatases can have different biological effects. In mammals, close to 50 genes encode the phosphoinositide kinases and phosphoinositide phosphatases that regulate phosphoinositide metabolism and thus allow cells to respond rapidly and effectively to ever-changing environmental cues. Understanding the distinct and overlapping functions of these phosphoinositide-metabolizing enzymes is important for our knowledge of both normal human physiology and the growing list of human diseases whose etiologies involve these proteins. This review summarizes the structural and biological properties of all the known mammalian phosphoinositide kinases and phosphoinositide phosphatases, as well as their associations with human disorders.

A tissue-scale gradient of hydrogen peroxide mediates rapid wound detection in zebrafish

Barrier structures (for example, epithelia around tissues and plasma membranes around cells) are required for internal homeostasis and protection from pathogens. Wound detection and healing represent a dormant morphogenetic program that can be rapidly executed to restore barrier integrity and tissue homeostasis. In animals, initial steps include recruitment of leukocytes to the site of injury across distances of hundreds of micrometres within minutes of wounding. The spatial signals that direct this immediate tissue response are unknown. Owing to their fast diffusion and versatile biological activities, reactive oxygen species, including hydrogen peroxide (H(2)O(2)), are interesting candidates for wound-to-leukocyte signalling. Here we probe the role of H(2)O(2) during the early events of wound responses in zebrafish larvae expressing a genetically encoded H(2)O(2) sensor. This reporter revealed a sustained rise in H(2)O(2) concentration at the wound margin, starting approximately 3 min after wounding and peaking at approximately 20 min, which extended approximately 100-200 microm into the tail-fin epithelium as a decreasing concentration gradient. Using pharmacological and genetic inhibition, we show that this gradient is created by dual oxidase (Duox), and that it is required for rapid recruitment of leukocytes to the wound. This is the first observation, to our knowledge, of a tissue-scale H(2)O(2) pattern, and the first evidence that H(2)O(2) signals to leukocytes in tissues, in addition to its known antiseptic role.

Simultaneous loss of the DLC1 and PTEN tumor suppressors enhances breast cancer cell migration

The phosphatase and tensin homolog (PTEN) gene is a tumor suppressor frequently deleted or mutated in sporadic tumors of the breast, prostate, endometrium and brain. The protein acts as a dual specificity phosphatase for lipids and proteins. PTEN loss confers a growth advantage to cells, protects from apoptosis and favors cell migration. The deleted in liver cancer 1 (DLC1) gene has emerged as a novel tumor suppressor downregulated in a variety of tumor types including those of the breast. DLC1 contains a Rho GTPase activating domain that is involved in the inhibition of cell proliferation, migration and invasion. To investigate how simultaneous loss of PTEN and DLC1 contributes to cell transformation, we downregulated both proteins by RNA interference in the non-invasive MCF7 breast carcinoma cell line. Joint depletion of PTEN and DLC1 resulted in enhanced cell migration in wounding and chemotactic transwell assays. Interestingly, both proteins were found to colocalize at the plasma membrane and interacted physically in biochemical pulldowns and coimmunoprecipitations. We therefore postulate that the concerted local inactivation of signaling pathways downstream of PTEN and DLC1, respectively, is required for the tight control of cell migration.

Bringing up the rear: defining the roles of the uropod

Renewed interest in cell shape has been prompted by a recent flood of evidence that indicates that cell polarity is essential for the biology of motile cells. The uropod, a protrusion at the rear of amoeboid motile cells such as leukocytes, exemplifies the importance of morphology in cell motility. Remodelling of cell shape by uropod-interfering agents disturbs cell migration. But even though the mechanisms by which uropods regulate cell migration are beginning to emerge, their functional significance remains enigmatic.

Sequential regulation of DOCK2 dynamics by two phospholipids during neutrophil chemotaxis

During chemotaxis, activation of the small guanosine triphosphatase Rac is spatially regulated to organize the extension of membrane protrusions in the direction of migration. In neutrophils, Rac activation is primarily mediated by DOCK2, an atypical guanine nucleotide exchange factor. Upon stimulation, we found that DOCK2 rapidly translocated to the plasma membrane in a phosphatidylinositol 3,4,5-trisphosphate-dependent manner. However, subsequent accumulation of DOCK2 at the leading edge required phospholipase D-mediated synthesis of phosphatidic acid, which stabilized DOCK2 there by means of interaction with a polybasic amino acid cluster, resulting in increased local actin polymerization. When this interaction was blocked, neutrophils failed to form leading edges properly and exhibited defects in chemotaxis. Thus, intracellular DOCK2 dynamics are sequentially regulated by distinct phospholipids to localize Rac activation during neutrophil chemotaxis.

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.

p110gamma and p110delta isoforms of phosphoinositide 3-kinase differentially regulate natural killer cell migration in health and disease

The mechanisms that regulate NK cell trafficking are unclear. Phosphoinositide-3 kinases (PI3K) control cell motility and the p110gamma and p110delta isoforms are mostly expressed in leukocytes, where they transduce signals downstream of G protein coupled receptors (GPCR) or tyrosine kinase receptors, respectively. Here, we set out to determine the relative contribution of p110gamma and p110delta to NK cell migration in mice. Using a combination of single-cell imaging analysis of transgenic cells reporting on PI3K activity in real time and small molecule inhibitors of p110gamma and p110delta, we show here that the tyrosine-kinase coupled p110delta is linked to GPCR signaling and, depending on the GPCR, may even be preferentially activated over p110gamma. Using gene-targeted mice, we showed that both isoforms were essential for NK cell chemotaxis to CXCL12 and to CCL3 and, in vivo, for normal NK cell migration during pregnancy and to the inflamed peritoneum. By contrast, only p110delta was indispensable for chemotaxis to S1P and CXCL10 and for NK cell distribution throughout lymphoid and nonlymphoid tissues and for extravasation to tumors. These results implicate p110delta downstream of GPCRs in NK cells and highlight its nonredundant role as a key regulator of NK cell trafficking in health and disease.

Mechanisms of chemokine and antigen-dependent T-lymphocyte navigation

T-lymphocyte trafficking is targeted to specific organs by selective molecular interactions depending on their differentiation and functional properties. Specific chemokine receptors have been associated with organ-specific trafficking of memory and effector T-cells, as well as the recirculation of naïve T-cells to secondary lymphoid organs. In addition to the acquisition of tissue-selective integrins and chemokine receptors, an additional level of specificity for T-cell trafficking into the tissue is provided by specific recognition of antigen displayed by the endothelium involving the TCRs (T-cell antigen receptors) and co-stimulatory receptors. Activation of PI3K (phosphoinositide 3-kinase) is a robust signalling event shared by most chemokine receptors as well as the TCR and co-stimulatory receptors, contributing to several aspects of T-lymphocyte homing as well as actin reorganization and other components of the general migratory machinery. Accordingly, inhibition of PI3K has been considered seriously as a potential therapeutic strategy by which to combat various T-lymphocyte-dependent pathologies, including autoimmune and inflammatory diseases, as well as to prevent transplant rejection. However, there is substantial evidence for PI3K-independent mechanisms that facilitate T-lymphocyte migration. In this regard, several other signalling-pathway components, including small GTPases, PLC (phospholipase C) and PKC (protein kinase C) isoforms, have also been implicated in T-lymphocyte migration in response to chemokine stimulation. The present review will therefore examine the PI3K-dependent and -independent signal-transduction pathways involved in T-cell migration during distinct modes of T-cell trafficking in response to either chemokines or the TCR and co-stimulatory molecules.

Phosphoinositide 3-kinases in cell migration

Cell migration is essential for many biological processes in animals and is a complex highly co-ordinated process that involves cell polarization, actin-driven protrusion and formation and turnover of cell adhesions. The PI3K (phosphoinositide 3-kinase) family of lipid kinases regulate cell migration in many different cell types, both through direct binding of proteins to their lipid products and indirectly through crosstalk with other pathways, such as Rho GTPase signalling. Emerging evidence suggests that the involvement of PI3Ks at different stages of migration varies even within one cell type, and is dependent on the combination of external stimuli, as well as on the signalling status of the cell. In addition, it appears that different PI3K isoforms have distinct roles in cell polarization and migration. This review describes how PI3K signalling is regulated by pro-migratory stimuli, and the diverse ways in which PI3K-mediated signal transduction contributes to different aspects of cell migration.

Upsides and downsides to polarity and asymmetric cell division in leukemia

The notion that polarity regulators can act as tumor suppressors in epithelial cells is now well accepted. The function of these proteins in lymphocytes is less well explored, and their possible function as suppressors of leukemia has had little attention so far. We review the literature on lymphocyte polarity and the growing recognition that polarity proteins have an important function in lymphocyte function. We then describe molecular relationships between the polarity network and signaling pathways that have been implicated in leukemogenesis, which suggest mechanisms by which the polarity network might impact on leukemogenesis. We particularly focus on the possibility that disruption of polarity might alter asymmetric cell division (ACD), and that this might be a leukemia-initiating event. We also explore the converse possibility that leukemic stem cells might be produced or maintained by ACD, and therefore that Dlg, Scribble and Lgl might be important regulators of this process.

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.

Chemotaxis in neutrophil-like HL-60 cells

Asymmetric localization of intracellular proteins and signals directs movement during axon guidance, endothelial cell invasion, and immune cell migration. In these processes, cell movement is guided by external chemical cues in a process known as chemotaxis. In particular, leukocyte migration in the innate immune system has been studied in the human neutrophil-like cell line (HL-60). Here, we describe the maintenance and transfection of HL-60 cells and explain how to analyze their behavior with two standard chemotactic assays. Finally, we demonstrate how to fix and stain the actin cytoskeleton of polarized cells for fluorescent microscopy imaging.

Spatio-temporal dynamics of phosphatidylinositol-3,4,5-trisphosphate signalling

Many effects of insulin, insulin-like growth factors and other receptor stimuli are mediated via the phospholipid second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP(3)). PIP(3) is formed by the activity of phosphoinositide 3-kinases in the plasma membrane, where it serves to recruit signalling proteins. These proteins coordinate complex events leading to changes in cell metabolism, growth, movement and survival. Over the past decade, new techniques for measurements of PIP(3) in the plasma membrane of individual living cells have markedly improved our understanding of the role of this messenger in a variety of cellular processes. This review summarises the mechanisms involved in formation and degradation of PIP(3) in insulin-responsive cells, how PIP(3) can be measured in individual cells as well as accumulating evidence that the plasma membrane PIP(3) concentration undergoes complex spatio-temporal patterns in many types of cells, with particular emphasis on autocrine insulin-induced PIP(3) oscillations in pancreatic beta-cells.

Ordered patterns of cell shape and orientational correlation during spontaneous cell migration

Background

In the absence of stimuli, most motile eukaryotic cells move by spontaneously coordinating cell deformation with cell movement in the absence of stimuli. Yet little is known about how cells change their own shape and how cells coordinate the deformation and movement. Here, we investigated the mechanism of spontaneous cell migration by using computational analyses.

Methodology

We observed spontaneously migrating Dictyostelium cells in both a vegetative state (round cell shape and slow motion) and starved one (elongated cell shape and fast motion). We then extracted regular patterns of morphological dynamics and the pattern-dependent systematic coordination with filamentous actin (F-actin) and cell movement by statistical dynamic analyses.

Conclusions/Significance

We found that Dictyostelium cells in both vegetative and starved states commonly organize their own shape into three ordered patterns, elongation, rotation, and oscillation, in the absence of external stimuli. Further, cells inactivated for PI3-kinase (PI3K) and/or PTEN did not show ordered patterns due to the lack of spatial control in pseudopodial formation in both the vegetative and starved states. We also found that spontaneous polarization was achieved in starved cells by asymmetric localization of PTEN and F-actin. This breaking of the symmetry of protein localization maintained the leading edge and considerably enhanced the persistence of directed migration, and overall random exploration was ensured by switching among the different ordered patterns. Our findings suggest that Dictyostelium cells spontaneously create the ordered patterns of cell shape mediated by PI3K/PTEN/F-actin and control the direction of cell movement by coordination with these patterns even in the absence of external stimuli.

Signal relay during chemotaxis

The ability of cells to migrate in response to external cues, a process known as chemotaxis, is a fundamental phenomenon in biology. It is exhibited by a wide variety of cell types in the context of embryogenesis, angiogenesis, inflammation, wound healing and many other complex physiological processes. Here, we discuss the signals that control the directed migration of the social amoebae Dictyostelium discoideum both as single cells and in the context of group migration. This multi-cellular organism has served as an excellent model system to decipher amoeboid-like leukocyte migration and has played a key role in establishing signalling paradigms in the chemotaxis field. We envision that Dictyostelium will continue to bring forward basic knowledge as we seek to understand the mechanisms regulating group cell migration.

IGF-I secreted by osteoblasts acts as a potent chemotactic factor for osteoblasts

Osteoblast recruitment to the site of future bone formation is essential for skeletal development, bone remodeling and fracture healing. A number of factors associated with bone tissue have been reported to induce directional migration of osteoblasts but the mechanism remains to be clarified. In this study, to explore a major chemotactic factor(s) for osteoblasts, we examined the serum-free medium conditioned by MC3T3-E1 osteoblast-like cells for its ability to induce osteoblast migration. Employing sequential chromatography and tandem mass spectrometry analysis, we purified and identified IGF-I as a potent chemotactic factor from the conditioned medium. IGF-I induced cell migration of both MC3T3-E1 cells and primary mouse osteoblasts, and checkerboard analysis revealed that IGF-I markedly induced directional migration (chemotaxis) of osteoblasts. Neutralization of mouse IGF-I with monoclonal antibodies resulted in delayed osteoblast monolayer wound healing and cellular polarization but addition of human IGF-I reversed these effects. IGF-I also promoted cell spreading on fibronectin in an integrin beta1-dependent manner. IGF-I induced Akt and Rac activation and localized accumulation of phosphatidylinositol 3,4,5-triphosphate (PtdIns (3,4,5)P3) at the membrane in osteoblasts. The phosphatidyl inositol 3 kinase (PI3K) inhibitor LY294002 inhibited IGF-I-induced cell migration and wound healing. Together, the results suggest that IGF-I secreted from osteoblasts in the bone tissue is a potent chemotactic factor that may play a major role in recruitment of osteoblasts during bone formation.

Protein kinase A regulates 3-phosphatidylinositide dynamics during platelet-derived growth factor-induced membrane ruffling and chemotaxis

Spatial regulation of the cAMP-dependent protein kinase (PKA) is required for chemotaxis in fibroblasts; however, the mechanism(s) by which PKA regulates the cell migration machinery remain largely unknown. Here we report that one function of PKA during platelet-derived growth factor (PDGF)-induced chemotaxis was to promote membrane ruffling by regulating phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) dynamics. Inhibition of PKA activity dramatically altered membrane dynamics and attenuated formation of peripheral membrane ruffles in response to PDGF. PKA inhibition also significantly decreased the number and size of PIP(3)-rich membrane ruffles in response to uniform stimulation and to gradients of PDGF. This ruffling defect was quantified using a newly developed method, based on computer vision edge-detection algorithms. PKA inhibition caused a marked attenuation in the bulk accumulation of PIP(3) following PDGF stimulation, without effects on PI3-kinase (PI3K) activity. The deficits in PIP(3) dynamics correlated with a significant inhibition of growth factor-induced membrane recruitment of endogenous Akt and Rac activation in PKA-inhibited cells. Simultaneous inhibition of PKA and Rac had an additive inhibitory effect on growth factor-induced ruffling dynamics. Conversely, the expression of a constitutively active Rac allele was able to rescue the defect in membrane ruffling and restore the localization of a fluorescent PIP(3) marker to membrane ruffles in PKA-inhibited cells, even in the absence of PI3K activity. These data demonstrate that, like Rac, PKA contributes to PIP(3) and membrane dynamics independently of direct regulation of PI3K activity and suggest that modulation of PIP(3)/3-phosphatidylinositol (3-PI) lipids represents a major target for PKA in the regulation of PDGF-induced chemotactic events.

Understanding PTEN regulation: PIP2, polarity and protein stability

The PTEN tumour suppressor is a lipid and protein phosphatase that inhibits phosphoinositide 3-kinase (PI3K)-dependent signalling by dephosphorylating phosphatidylinositol 3,4,5-trisphosphate (PtdInsP(3)). Here, we discuss the concept of PTEN as an 'interfacial enzyme', which exists in a high activity state when bound transiently at membrane surfaces containing its substrate and other acidic lipids, such as PtdIns(4,5)P(2) and phosphatidylserine (PtdSer). This mechanism ensures that PTEN functions in a spatially restricted manner, and may explain its involvement in forming the gradients of PtdInsP(3), which are necessary for generating and/or sustaining cell polarity during motility, in developing neurons and in epithelial tissues. Coordinating PTEN activity with alternative mechanisms of PtdInsP(3) metabolism, by the tightly regulated SHIP 5-phoshatases, synthesizing the independent second messenger PtdIns(3,4)P(2), may also be important for cellular polarization in some cell types. Superimposed on this interfacial mechanism are additional post-translational regulatory processes, which generally act to reduce PTEN activity. Oxidation of the active site cysteine residue by reactive oxygen species and phosphorylation of serine/threonine residues at sites in the C-terminus of the protein inhibit PTEN. These phosphorylation sites also appear to play a role in regulating both stability and localization of PTEN, as does ubiquitination of PTEN. Because genetic studies in mice show that the level of expression of PTEN in an organism profoundly influences tumour susceptibility, factors that regulate PTEN, localization, activity and turnover should be important in understanding its biological functions as a tumour suppressor.

The roles of PTEN in development, physiology and tumorigenesis in mouse models: a tissue-by-tissue survey

In 1997, PTEN (phosphatase and tensin homologue deleted on chromosome 10, 10q23.3) was identified as an important tumor suppressor gene that is inactivated in a wide variety of human cancers. Ever since, PTEN's function has been extensively studied, and huge progress has been made in understanding PTEN's role in normal physiology and disease. In this review, we will systematically summarize the important data that have been gained from gene inactivation studies in mice and will put these data into physiological context using a tissue-by-tissue approach. We will cover mice exhibiting complete and constitutive inactivation of Pten as well as a large number of strains in which Pten has been conditionally deleted in specific tissues. We hope to highlight not only the tumor suppressive function of Pten but also its roles in embryogenesis and in the maintenance of the normal physiological functions of many organ systems.

PIP3-independent activation of TorC2 and PKB at the cell's leading edge mediates chemotaxis

BACKGROUND

Studies show that high phosphotidylinositol 3,4,5-trisphosphate (PIP(3)) promotes cytoskeletal rearrangements and alters cell motility and chemotaxis, possibly through activation of protein kinase Bs (PKBs). However, chemotaxis can still occur in the absence of PIP(3), and the identities of the PIP(3)-independent pathways remain unknown.

RESULTS

Here, we outline a PIP(3)-independent pathway linking temporal and spatial activation of PKBs by Tor complex 2 (TorC2) to the chemotactic response. Within seconds of stimulating Dictyostelium cells with chemoattractant, two PKB homologs, PKBA and PKBR1, mediate transient phosphorylation of at least eight proteins, including Talin, PI4P 5-kinase, two Ras GEFs, and a RhoGap. Surprisingly, all of the substrates are phosphorylated with normal kinetics in cells lacking PI 3-kinase activity. Cells deficient in TorC2 or PKB activity show reduced phosphorylation of the endogenous substrates and are impaired in chemotaxis. The PKBs are activated through phosphorylation of their hydrophobic motifs via TorC2 and subsequent phosphorylation of their activation loops. These chemoattractant-inducible events are restricted to the cell's leading edge even in the absence of PIP(3). Activation of TorC2 depends on heterotrimeric G protein function and intermediate G proteins, including Ras GTPases.

CONCLUSIONS

The data lead to a model where cytosolic TorC2, encountering locally activated small G protein(s) at the leading edge of the cell, becomes activated and phosphorylates PKBs. These in turn phosphorylate a series of signaling and cytoskeletal proteins, thereby regulating directed migration.

Activation and membrane binding of retinal protein kinase Balpha/Akt1 is regulated through light-dependent generation of phosphoinositides

Akt is a phospholipid-binding protein and the downstream effector of the phosphoinositide 3-kinase (PI3K) pathway. Akt has three isoforms: Akt1, Akt2, and Akt3. All of these isoforms are expressed in rod photoreceptor cells, but the individual functions of each isoform are not known. In this study, we found that light induces the activation of Akt1. The membrane binding of Akt1 to rod outer segments (ROS) is insulin receptor (IR)/PI3K-dependent as demonstrated by reduced binding of Akt1 to ROS membranes of photoreceptor-specific IR knockout mice. Membrane binding of Akt1 is mediated through its Pleckstrin homology (PH) domain. To determine whether binding of the PH domain of Akt1 to photoreceptor membranes is regulated by light, various green fluorescent protein (GFP)/Akt1-PH domain fusion proteins were expressed in rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The R25C mutant PH domain of Akt1, which does not bind phosphoinositides, failed to associate with plasma membranes in a light-dependent manner. This study suggests that light-dependent generation of phosphoinositides regulates the activation and membrane binding of Akt1 in vivo. Our results also suggest that actin cytoskeletal organization may be regulated through light-dependent generation of phosphoinositides.

Double deficiency of tetraspanins CD9 and CD81 alters cell motility and protease production of macrophages and causes chronic obstructive pulmonary disease-like phenotype in mice

CD9 and CD81 are closely related tetraspanins that regulate cell motility and signaling by facilitating the organization of multimolecular membrane complexes, including integrins. We show that CD9 and CD81 are down-regulated in smoking-related inflammatory response of a macrophage line, RAW264.7. When functions of CD9 and CD81 were ablated with monoclonal antibody treatment, small interfering RNA transfection, or gene knock-out, macrophages were less motile and produced larger amounts of matrix metalloproteinase (MMP)-2 and MMP-9 than control cells in vitro. In line with this, CD9/CD81 double-knock-out mice spontaneously developed pulmonary emphysema, a major pathological component of chronic obstructive pulmonary disease (COPD). The mutant lung contained an increased number of alveolar macrophages with elevated activities of MMP-2 and MMP-9 and progressively displayed enlarged airspace and disruption of elastic fibers in the alveoli. Secretory cell metaplasia, a finding similar to goblet cell metaplasia in cigarette smokers, was also observed in the epithelium of terminal bronchioles. With aging, the double-knockout mice showed extrapulmonary phenotypes, including weight loss, kyphosis, and osteopenia. These results suggest that the tetraspanins CD9 and CD81 regulate cell motility and protease production of macrophages and that their dysfunction may underlie the progression of COPD.

Synthetic activation of endogenous PI3K and Rac identifies an AND-gate switch for cell polarization and migration

Phosphatidylinositol 3-OH kinase (PI3K) has been widely studied as a principal regulator of cell polarization, migration, and chemotaxis [1], [2], [3], [4]. Surprisingly, recent studies showed that mammalian neutrophils and Dictyostelium discoideum cells can polarize and migrate in the absence of PI3K activity [5], [6], [7]. Here we directly probe the roles of PI3K and its downstream effector, Rac, in HL-60 neutrophils by using a chemical biology approach whereby the endogenously present enzymes are synthetically activated in less than one minute [8], [9], [10]. We show that uniform activation of endogenous PI3K is sufficient to polarize previously unpolarized neutrophils and trigger effective cell migration. After a delay following symmetrical phosphatidylinositol (3,4,5)-triphosphate (PIP₃) production, a polarized distribution of PIP₃ was induced by positive feedback requiring actin polymerization. Pharmacological studies argue that this process does not require receptor-coupled trimeric G proteins. Contrary to the current working model, rapid activation of endogenous Rac proteins triggered effective actin polymerization but failed to feed back to PI3K to generate PIP₃ or induce cell polarization. Thus, the increase in PIP₃ concentration at the leading edge is generated by positive feedback with an AND gate logic with a PI3K-Rac-actin polymerization pathway as a first input and a PI3K initiated non-Rac pathway as a second input. This AND-gate control for cell polarization can explain how Rac can be employed for both PI3K-dependent and -independent signaling pathways coexisting in the same cell.

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.

Down-regulation of PKCzeta expression inhibits chemotaxis signal transduction in human lung cancer cells

Metastasis is the major cause of mortality in lung cancer. Chemotaxis plays a vital role in cancer cell metastasis. In the current study, we reported that epidermal growth factor (EGF) induced a robust chemotaxis of A549 and H1299 cells, two representative human non-small cell lung cancer (NSCLC) cells. Chelerythrine chloride, an inhibitor of all protein kinase C (PKC) isozymes, significantly reduced the chemotactic capacity of NSCLC cells while inhibitors of classical or novel PKC isozymes, such as Gö6976, calphostin C, or Gö6850, showed no effect, which suggested that atypical PKC might be involved in the chemotactic process of NSCLC cells. EGF-elicited translocation and phosphorylation of atypical PKCzeta, indicating that EGF could activate PKCzeta. Treatment with a PKCzeta specific inhibitor, a myristoylated pseudosubstrate, blocked the chemotaxis in a dose-dependent manner, further confirming that atypical PKCzeta was required for NSCLC chemotaxis. Mechanistic studies suggested that PKCzeta was regulated by phosphatidylinositol 3 kinase (PI3K)/Akt. Furthermore, PKCzeta-mediated chemotaxis by regulating actin polymerization and cell adhesion. Taken together, our study suggested that PKCzeta was required in NSCLC cell chemotaxis, thus could be used as a target to develop anti-lung cancer metastasis therapies.

Rapid turnover rate of phosphoinositides at the front of migrating MDCK cells

Phosphoinositides (PtdInss) play key roles in cell polarization and motility. With a series of biosensors based on Förster resonance energy transfer, we examined the distribution and metabolism of PtdInss and diacylglycerol (DAG) in stochastically migrating Madin-Darby canine kidney (MDCK) cells. The concentrations of phosphatidylinositol (4,5)-bisphosphate, phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), phosphatidylinositol (3,4)-bisphosphate, and DAG were higher at the plasma membrane in the front of the cell than at the plasma membrane of the rear of the cell. The difference in the concentrations of PtdInss was estimated to be less than twofold between the front and rear of the migrating MDCK cells. To decode the spatial activities of PtdIns metabolic enzymes from the obtained concentration maps of PtdInss, we developed a one-dimensional reaction diffusion model of PtdIns metabolism. In this model, the activities of phosphatidylinositol monophosphate 5-kinase, phosphatidylinositol 3-kinase, phospholipase C, and PIP(3) 5-phosphatases were higher at the plasma membrane of the front than at the plasma membrane of the rear of the cell. This result suggests that, although the difference in the steady-state level of PtdInss is less than twofold, PtdInss were more rapidly turned over at the front than the rear of the migrating MDCK cells.

SHIP-1 increases early oxidative burst and regulates phagosome maturation in macrophages

Although the inositol phosphatase SHIP-1 is generally thought to inhibit signaling for Fc receptor-mediated phagocytosis, the product of its activity, phosphatidylinositol 3,4 bisphosphate (PI(3,4)P(2)), has been implicated in activation of the NADPH oxidase. This suggests that SHIP-1 positively regulates the generation of reactive oxygen species after phagocytosis. To examine how SHIP-1 activity contributes to Fc receptor-mediated phagocytosis, we measured and compared phospholipid dynamics, membrane trafficking, and the oxidative burst in macrophages from SHIP-1-deficient and wild-type mice. SHIP-1-deficient macrophages showed significantly elevated ratios of PI(3,4,5)P(3) to PI(3,4)P(2) on phagosomal membranes. Imaging reactive oxygen intermediate activities in phagosomes revealed decreased early NADPH oxidase activity in SHIP-1-deficient macrophages. SHIP-1 deficiency also altered later stages of phagosome maturation, as indicated by the persistent elevation of PI(3)P and the early localization of Rab5a to phagosomes. These direct measurements of individual organelles indicate that phagosomal SHIP-1 enhances the early oxidative burst through localized alteration of the membrane 3'-phosphoinositide composition.

Effective clearance of intracellular Leishmania major in vivo requires Pten in macrophages

Leishmaniases are a major international public health problem, and macrophages are crucial for host resistance to this parasite. To determine if phosphatase and tensin homologue deleted on chromosome ten (Pten), a negative regulator of the PI3K pathway, plays a role in macrophage-mediated resistance to Leishmania, we generated C57BL/6 mice lacking Pten specifically in macrophages (LysMCrePten(flox/flox) mice). Examination of lesions resulting from Leishmania major infection showed that LysMCrePten(flox/flox) mice were more susceptible to the parasite than wild-type (WT) mice in the early phase of the infection, but were eventually able to eliminate the pathogen. In vitro Pten-deficient macrophages showed a reduced ability to kill parasites in response to IFN-gamma treatment, possibly because the mutant cells exhibited decreased TNF secretion that correlated with reductions in inducible nitric oxide synthase expression and nitric oxide production. In response to various TLR ligands, Pten-deficient macrophages produced less TNF and IL-12 but more IL-10 than WT cells. However, analysis of cells in the lymph nodes draining L. major inoculation sites indicated that both LysMCrePten(flox/flox) and WT mice developed normal Th1 responses following L. major infection, in line with the ability of LysMCrePten(flox/flox) mice to eventually eliminate the parasite. Our results indicate that the efficient clearance of intracellular parasites requires Pten in macrophages.