G protein signaling events are activated at the leading edge of chemotactic cells

Receptor-mediated activation of heterotrimeric G-proteins in living cells

Receptor-mediated activation of heterotrimeric GTP-binding proteins (G-proteins) was visualized in living Dictyostelium discoideum cells by monitoring fluorescence resonance energy transfer (FRET) between alpha- and beta- subunits fused to cyan and yellow fluorescent proteins. The G-protein heterotrimer rapidly dissociated and reassociated upon addition and removal of chemoattractant. During continuous stimulation, G-protein activation reached a dose-dependent steady-state level. Even though physiological responses subsided, the activation did not decline. Thus, adaptation occurs at another point in the signaling pathway, and occupied receptors, whether or not they are phosphorylated, catalyze the G-protein cycle. Construction of similar energy-transfer pairs of mammalian G-proteins should enable direct in situ mechanistic studies and applications such as drug screening and identifying ligands of newly found G-protein-coupled receptors.

An SH2-domain-containing kinase negatively regulates the phosphatidylinositol-3 kinase pathway

SHK1 is a novel dual-specificity kinase that contains an SH2 domain in its C-terminal region. We demonstrate that SHK1 is required for proper chemotaxis and phagocytosis. Mutant shk1 null cells lack polarity, move very slowly, and exhibit an elevated and temporally extended chemoattractant-mediated activation of the kinase Akt/PKB. GFP fusions of the PH domain of Akt/PKB or the PH-domain-containing protein CRAC, which become transiently associated with the plasma membrane after a global stimulation with a chemoattractant, remain associated with the plasma membrane for an extended period of time in shk1 null cells. These results suggest that SHK1 is a negative regulator of the PI3K (phosphatidylinositol-3 kinase) pathway. Furthermore, when a chemoattractant gradient is applied to a wild-type cell, these PH-domain-containing proteins and the F-actin-binding protein coronin localize to its leading edge, but in an shk1 null cell they become randomly associated with the plasma membrane and cortex, irrespective of the direction of the chemoattractant gradient, suggesting that SHK1 is required for the proper spatiotemporal control of F-actin levels in chemotaxing cells. Consistent with such functions, SHK1 is localized at the plasma membrane/cortex, and we show that its SH2 domain is required for this localization and the proper function of SHK1.

A role for myosin VII in dynamic cell adhesion

BACKGROUND

The initial stages of phagocytosis and cell motility resemble each other. The extension of a pseudopod at the leading edge of a migratory cell and the formation of a phagocytic cup are actin dependent, and each rely on the plasma membrane adhering to a surface during dynamic extension.

RESULTS

A myosin VII null mutant exhibited a drastic loss of adhesion to particles, consistent with the extent of an observed decrease in particle uptake. Additionally, cell-cell adhesion and the adhesion of the leading edge to the substratum during cell migration were defective in the myosin VII null cells. GFP-myosin VII rescued the phagocytosis defect of the null mutant and was distributed in the cytosol and recruited to the cortical cytoskeleton, where it appeared to be enriched at the tips of filopods. It was also localized to phagocytic cups, but only during the initial stages of particle engulfment. During migration, GFP-myosin VII is found at the leading edge of the cell.

CONCLUSIONS

Myosin VII plays an important role in mediating the initial binding of cells to substrata, a novel role for an unconventional myosin.

The cell biology of neuronal navigation

Morphogenesis of the nervous system requires the directed migration of postmitotic neurons to designated locations in the nervous system and the guidance of axon growth cones to their synaptic targets. Evidence suggests that both forms of navigation depend on common guidance molecules, surface receptors and signal transduction pathways that link receptor activation to cytoskeletal reorganization. Future challenges remain not only in identifying all the components of the signalling pathways, but also in understanding how these pathways achieve signal amplification and adaptation-two essential cellular processes for neuronal navigation.

Tortoise, a novel mitochondrial protein, is required for directional responses of Dictyostelium in chemotactic gradients

We have identified a novel gene, Tortoise (TorA), that is required for the efficient chemotaxis of Dictyostelium discoideum cells. Cells lacking TorA sense chemoattractant gradients as indicated by the presence of periodic waves of cell shape changes and the localized translocation of cytosolic PH domains to the membrane. However, they are unable to migrate directionally up spatial gradients of cAMP. Cells lacking Mek1 display a similar phenotype. Overexpression of Mek1 in torA- partially restores chemotaxis, whereas overexpression of TorA in mek1- does not rescue the chemotactic phenotype. Regardless of the genetic background, TorA overexpressing cells stop growing when separated from a substrate. Surprisingly, TorA-green fluorescent protein (GFP) is clustered near one end of mitochondria. Deletion analysis of the TorA protein reveals distinct regions for chemotactic function, mitochondrial localization, and the formation of clusters. TorA is associated with a round structure within the mitochondrion that shows enhanced staining with the mitochondrial dye Mitotracker. Cells overexpressing TorA contain many more of these structures than do wild-type cells. These TorA-containing structures resist extraction with Triton X-100, which dissolves the mitochondria. The characterization of TorA demonstrates an unexpected link between mitochondrial function, the chemotactic response, and the capacity to grow in suspension.

Dancing to the tune of chemokines

Since their discovery 13 years ago, chemokines have emerged as the most important regulators of leukocyte trafficking. On target cells, chemokines bind to seven-transmembrane-domain receptors that are coupled to heterotrimeric Gi proteins. The common response of all cells to chemokine stimulation is chemotaxis. In addition, leukocyte activation triggers diverse signal transduction cascades; which cascade is triggered depends on the chemokine and receptor engaged. The selective activation of distinct pathways suggests that the receptors couple not only to G proteins but also to additional downstream effectors. This review discusses recent advances in the elucidation of the signal transduction that occurs in proximity to receptors and that leads to the early biochemical events in leukocyte activation.

Asymmetric cell division during animal development

Although most cells produce two equal daughters during mitosis, some can divide asymmetrically by segregating protein determinants into one of their two daughter cells. Interesting parallels exist between such asymmetric divisions and the polarity established in epithelial cells, and heterotrimeric G proteins might connect these aspects of cell polarity. The discovery of asymmetrically segregating proteins in vertebrates indicates that the results obtained in invertebrate model organisms might also apply to mammalian stem cells.

Spatial sensing in fibroblasts mediated by 3' phosphoinositides

The directed movement of fibroblasts towards locally released platelet-derived growth factor (PDGF) is a critical event in wound healing. Although recent studies have implicated polarized activation of phosphoinositide (PI) 3-kinase in G protein-mediated chemotaxis, the role of 3' PI lipids in tyrosine kinase-triggered chemotaxis is not well understood. Using evanescent wave microscopy and green fluorescent protein-tagged Akt pleckstrin homology domain (GFP-AktPH) as a molecular sensor, we show that application of a shallow PDGF gradient triggers a markedly steeper gradient in 3' PI lipids in the adhesion zone of fibroblasts. Polar GFP-AktPH gradients, as well as a new type of radial gradient, were measured from front to rear and from the periphery to the center of the adhesion zone, respectively. A strong spatial correlation between polarized 3' PI production and rapid membrane spreading implicates 3' PI lipids as a direct mediator of polarized migration. Analysis of the temporal changes of 3' PI gradients in the adhesion zone revealed a fast diffusion coefficient (0.5 microm(2)/s) and short lifetime of 3' PIs of <1 min. Together, this study suggests that the tyrosine kinase-coupled directional movement of fibroblasts and their radial membrane activity are controlled by local generation and rapid degradation of 3' PI second messengers.

Trimer carboxyl propeptide of collagen I produced by mature osteoblasts is chemotactic for endothelial cells

During the second phase of osteogenesis in vitro, rat osteoblasts secrete inducer(s) of chemotaxis and chemoinvasion of endothelial and tumor cells. We report here the characterization and purification from mature osteoblast conditioned medium of the agent chemotactic for endothelial cells. The chemoactive conditioned medium specifically induces directional migration of endothelial cells, not affecting the expression and activation of gelatinases, cell proliferation, and scattering. Directional migration induced in endothelial cells by conditioned medium from osteoblasts is inhibited by pertussis toxin, by blocking antibodies to integrins alpha(1), beta(1), and beta(3), and by antibodies to metalloproteinase 2 and 9. The biologically active purified protein has two sequences, coincident with the amino-terminal amino acids, respectively, of the alpha(1) and of the alpha(2) carboxyl propeptides of type I collagen, as physiologically produced by procollagen C proteinase. Antibodies to type I collagen and to the carboxyl terminus of alpha(1) or alpha(2) chains inhibit chemotaxis. The chemoattractant is the propeptide trimer carboxyl-terminal to type I collagen, and its activity is lost upon reduction. These data illustrate a previously unknown function for the carboxyl-terminal trimer, possibly relevant in promoting endothelial cell migration and vascularization of tissues producing collagen type I.

Roles of phospholipid signaling in chemoattractant-induced responses

Chemoattractants, including chemokines, play a central role in regulation of inflammatory reactions by attracting and activating leukocytes. These molecules have been found to regulate metabolism of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) via phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K). Recent studies of mouse lines that lack PLC-(beta)2, PLC-(beta)3, or PI3K(gamma) demonstrate that chemoattractants act through PLC-(beta)2 and PLC-(beta)3 to hydrolyze PtdIns(4,5)P(2) and through PI3K(gamma) to phosphorylate PtdIns(4,5)P(2) in mouse neutrophils. These studies also confirmed the importance and revealed new roles of these signaling pathways in chemoattractant-induced responses.

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

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

Intracellular signaling events at the leading edge of migrating cells

Cell migration is an important facet of the life cycle of immune and other cell types. A complex set of events must take place at the leading edge of motile cells before these cells can migrate. Chemokines induce the motility of various cell types by activating multiple intracellular signaling pathways. These include the activation of chemokine receptors, which are coupled to the heterotrimeric G proteins. The release of G beta gamma subunits from chemokine receptors results in the recruitment to the plasma membrane, with subsequent activation of various down-stream signaling molecules. Among these molecules are the pleckstrin homology domain-containing proteins and the phosphoinositide 3-kinase gamma which phosphorylates phospholipids and activates members of the GTP exchange factors (GEFs). These GEFs facilitate the exchange of GTP for GDP in members of GTPases. The latter are important for reorganizing the cell cytoskeleton, and in inducing chemotaxis. Chemokines also induce the mobilization of intracellular calcium from intracellular stores. Second messengers such as inositol 1,4,5 trisphosphate, and cyclic adenosine diphosphate ribose are among those induced by chemokines. In addition, the G beta gamma subunits recruit members of the G protein-coupled receptor kinases, which phosphorylate chemokine receptors, resulting in desensitization and termination of the motility signals. This review will discuss the intracellular signaling pathways induced by chemokines, particularly those activated at the leading edge of migrating cells which lead to cell polarization, cytoskeleton reorganization and motility.

Dictyostelium: a model for regulated cell movement during morphogenesis

Dictyostelium has played an important role in unraveling the pathways that control cell movement and chemotaxis. Recent studies have started to elucidate the pathways that control cell sorting, morphogenesis, and the establishment of spatial patterning in this system. In doing so, they provide new insights into how cell movements within a multicellular organism are regulated and the importance of pathways that are similar to those that regulate chemotaxis of cells on two-dimensional surfaces during aggregation.

The control of chemotactic cell movement during Dictyostelium morphogenesis

Differential cell movement is an important mechanism in the development and morphogenesis of many organisms. In many cases there are indications that chemotaxis is a key mechanism controlling differential cell movement. This can be particularly well studied in the starvation-induced multicellular development of the social amoeba Dictyostelium discoideum. Upon starvation, up to 10(5) individual amoebae aggregate to form a fruiting body The cells aggregate by chemotaxis in response to propagating waves of cAMP, initiated by an aggregation centre. During their chemotactic aggregation the cells start to differentiate into prestalk and prespore cells, precursors to the stalk and spores that form the fruiting body. These cells enter the aggregate in a random order but then sort out to form a simple axial pattern in the slug. Our experiments strongly suggest that the multicellular aggregates (mounds) and slugs are also organized by propagating cAMP waves and, furthermore, that cell-type-specific differences in signalling and chemotaxis result in cell sorting, slug formation and movement.

Measuring dimensions: the regulation of size and shape

Over many years evidence has accumulated that plants and animals can regulate growth with reference to overall size rather than cell number. Thus, organs and organisms grow until they reach their characteristic size and shape and then they stop - they can even compensate for experimental manipulations that change, over several fold, cell number or average cell size. If the cell size is altered, the organism responds with a change in cell number and vice versa. We look at the Drosophila wing in more detail: here, both extracellular and intracellular regulators have been identified that link cell growth, division and cell survival to final organ size. We discuss a hypothesis that the local steepness of a morphogen gradient is a measure of length in one axis, a measure that is used to determine whether there will be net growth or not.

The molecular genetics of chemotaxis: sensing and responding to chemoattractant gradients

Chemotaxis plays a central role in various biological processes, such as the movement of neutrophils and macrophage during wound healing and in the aggregation of Dictyostelium cells. During the past few years, new understanding of the mechanisms controlling chemotaxis has been obtained through molecular genetic and biochemical studies of Dictyostelium and other experimental systems. This review outlines our present understanding of the signaling pathways that allow a cell to sense and respond to a chemoattractant gradient. In response to chemoattractants, cells either become polarized in the direction of the chemoattractant source, which results in the formation of a leading edge, or they reorient their polarity in the direction of the chemoattractant gradient and move with a stronger persistence up the gradient. Models are presented here to explain such directional responses. They include a localized activation of pathways at the leading edge and an "inhibition" of these pathways along the lateral edges of the cell. One of the primary pathways that may be responsible for such localized responses is the activation of phosphatidyl inositol-3 kinase (PI3K). Evidence suggests that a localized formation of binding sites for PH (pleckstrin homology) domain-containing proteins produced by PI3K leads to the formation of "activation domains" at the leading edge, producing a localized response.

Toward the functional analysis of the Dictyostelium discoideum genome

Dictyostelium discoideum is a useful model for molecular studies of cell biology and development. The 34-megabase Dictyostelium genome is currently being sequenced through the efforts of an international consortium. The genome is expected to encode 8-10,000 genes, including all those required for a free-living eukaryote capable of multicellular development. A complete description of the Dictyostelium genome will open the way toward the application of genome-based experimental approaches to studies of cell biology and development in this organism, and allow detailed physiological and evolutionary comparisons to other species.

In and out of the postsynaptic region: signalling proteins on the move

Reversible translocation of signalling proteins to and from their sites of action has emerged as an important theme in signal transduction. The recent findings of the stimulus-induced translocation of Ca2+-calmodulin-dependent protein kinase II (CaMKII) and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor to and from the postsynaptic region are model cases for understanding how the dynamic localization of signalling proteins is used to regulate signal transduction.

Visualizing muscle cell migration in situ

BACKGROUND

Cell migration has been studied extensively by manipulating and observing cells bathed in putative chemotactic or chemokinetic agents on planar substrates. This environment differs from that in vivo and, consequently, the cells can behave abnormally. Embryo slices provide an optically accessible system for studying cellular navigation pathways during development. We extended this system to observe the migration of muscle precursors from the somite into the forelimb, their cellular morphology, and the localization of green fluorescent protein (GFP)-tagged adhesion-related molecules under normal and perturbed conditions.

RESULTS

Muscle precursors initiated migration synchronously and migrated in broad, rather than highly defined, regions. Bursts of directed migration were followed by periods of meandering or extension and retraction of cell protrusions. Although paxillin did not localize to discernible intracellular structures, we found that alpha-actinin localized to linear, punctate structures, and the alpha5 integrin to some focal complexes and/or vesicle-like concentrations. Alterations in the expression of adhesion molecules inhibited migration. The muscle precursors migrating in situ formed unusually large, long-lived protrusions that were polarized in the direction of migration. Unlike wild-type Rac, a constitutively active Rac localized continuously around the cell surface and promoted random protrusive activity and migration.

CONCLUSIONS

The observation of cellular migration and the dynamics of molecular organization at high temporal and spatial resolution in situ is feasible. Migration from the somite to the wing bud is discontinuous and not highly stereotyped. In situ, local activation of Rac appears to produce large protrusions, which in turn, leads to directed migration. Adhesion can also regulate migration.

Role of Rac in controlling the actin cytoskeleton and chemotaxis in motile cells

We have used the chemotactic ability of Dictyostelium cells to examine the roles of Rho family members, known regulators of the assembly of F-actin, in cell movement. Wild-type cells polarize with a leading edge enriched in F-actin toward a chemoattractant. Overexpression of constitutively active Dictyostelium Rac1B(61L) or disruption of DdRacGAP1, which encodes a Dictyostelium Rac1 GAP, induces membrane ruffles enriched with actin filaments around the perimeter of the cell and increased levels of F-actin in resting cells. Whereas wild-type cells move linearly toward the cAMP source, Rac1B(61L) and Ddracgap1 null cells make many wrong turns and chemotaxis is inefficient, which presumably results from the unregulated activation of F-actin assembly and pseudopod extension. Cells expressing dominant-negative DdRac1B(17N) do not have a well-defined F-actin-rich leading edge and do not protrude pseudopodia, resulting in very poor cell motility. From these studies and assays examining chemoattractant-mediated F-actin assembly, we suggest DdRac1 regulates the basal levels of F-actin assembly, its dynamic reorganization in response to chemoattractants, and cellular polarity during chemotaxis.

Point mutations identify a conserved region of the saccharomyces cerevisiae AFR1 gene that is essential for both the pheromone signaling and morphogenesis functions

Mating pheromone receptors activate a G protein signal pathway that leads to the conjugation of the yeast Saccharomyces cerevisiae. This pathway also induces the production of Afr1p, a protein that negatively regulates pheromone receptor signaling and is required to form pointed projections of new growth that become the site of cell fusion during mating. Afr1p lacks strong similarity to any well-characterized proteins to help predict how it acts. Therefore, we investigated the relationship between the different functions of Afr1p by isolating and characterizing seven mutants that were defective in regulating pheromone signaling. The AFR1 mutants were also defective when expressed as fusions to STE2, the alpha-factor receptor, indicating that the mutant Afr1 proteins are defective in function and not in co-localizing with receptors. The mutant genes contained four distinct point mutations that all occurred between codons 254 and 263, identifying a region that is critical for AFR1 function. Consistent with this, we found that the corresponding region is very highly conserved in the Afr1p homologs from the yeasts S. uvarum and S. douglasii. In contrast, there were no detectable effects on pheromone signaling caused by deletion or overexpression of YER158c, an open reading frame with overall sequence similarity to Afr1p that lacks this essential region. Interestingly, all of the AFR1 mutants showed a defect in their ability to form mating projections that was proportional to their defect in regulating pheromone signaling. This suggests that both functions may be due to the same action of Afr1p. Thus, these studies identify a specific region of Afr1p that is critical for its function in both signaling and morphogenesis.

Semaphorin 3A is a chemoattractant for cortical apical dendrites

The apical dendrites of pyramidal neurons integrate inputs from various cortical layers and are central to information processing. Here we show that the growth of apical dendrites towards the pial surface is regulated by a diffusible chemoattractant present at high levels near the marginal zone. A major component of this signal is semaphorin 3A (Sema3A), which was previously characterized as a chemorepellant for cortical axons. Soluble guanylate cyclase is asymmetrically localized to the developing apical dendrite, and is required for the chemoattractive effect of Sema3A. Thus the asymmetric localization of soluble guanylate cyclase confers distinct Sema3A responses to axons and dendrites. These observations reveal a mechanism by which a single chemotropic signal can pattern both axons and dendrites during development.

The Dictyostelium LIM domain-containing protein LIM2 is essential for proper chemotaxis and morphogenesis

We have identified limB, a gene encoding a novel LIM domain-containing protein, LIM2, in a screen for genes required for morphogenesis. limB null cells aggregate, although poorly, but they are unable to undergo morphogenesis, and the aggregates arrest at the mound stage. limB null cells exhibit an aberrant actin cytoskeleton and have numerous F-actin-enriched microspikes. The cells exhibit poor adhesion to a substratum and do not form tight cell-cell agglomerates in suspension. Furthermore, limB null cells are unable to properly polarize in chemoattractant gradients and move very poorly. Expression of limB from a prestalk-specific but not a prespore-specific promoter complements the morphogenetic defects of the limB null strain, suggesting that the limB null cell developmental defect results from an inability to properly sort prestalk cells. LIM2 protein is enriched in the cortex of wild-type cells, although it does not colocalize with the actin cytoskeleton. Our analysis indicates that LIM2 is a new regulatory protein that functions to control rearrangements of the actin cytoskeleton and is required for cell motility and chemotaxis. Our findings may be generally applicable to understanding pathways that control cell movement and morphogenesis in all multicellular organisms. Structure function studies on the LIM domains are presented.

Actin cytoskeleton and cell signaling

The role of the actin cytoskeleton in the function of eukaryotic cells is ubiquitous. Regulation of actin polymerization allows cells to control their shape, to move, divide, secrete, and phagocytose. Actin filaments provide strength, connections to other cells and the extracellular matrix, paths for intracellular transport and a scaffold for generating force. Recently, a number of signal transduction pathways have been identified that regulate actin polymerization and contractility. GTP-binding proteins, protein kinases, phosphoinositide kinases, and protein phosphatases all play important roles in determining the location and extent of actin polymerization and contractility of actin/myosin filaments. These pathways allow cells to respond to extracellular signals to regulate movement, the tone of vascular smooth muscle cells, secretion, and phagocytosis. Some pathogens use signal transduction pathways that regulate actin polymerization to invade cells. The signal transduction pathways that regulate actin-dependent events are the focus of this review.

The actin cytoskeleton of Dictyostelium: a story told by mutants

Actin-binding proteins are effectors of cell signalling and coordinators of cellular behaviour. Research on the Dictyostelium actin cytoskeleton has focused both on the elucidation of the function of bona fide actin-binding proteins as well as on proteins involved in signalling to the cytoskeleton. A major part of this work is concerned with the analysis of Dictyostelium mutants. The results derived from these investigations have added to our understanding of the role of the actin cytoskeleton in growth and development. Furthermore, the studies have identified several cellular and developmental stages that are particularly sensitive to an unbalanced cytoskeleton. In addition, use of GFP fusion proteins is revealing the spatial and temporal dynamics of interactions between actin-associated proteins and the cytoskeleton.

Localization of the G protein betagamma complex in living cells during chemotaxis

Gradients of chemoattractants elicit signaling events at the leading edge of a cell even though chemoattractant receptors are uniformly distributed on the cell surface. In highly polarized Dictyostelium discoideum amoebas, membrane-associated betagamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) were localized in a shallow anterior-posterior gradient. A uniformly applied chemoattractant generated binding sites for pleckstrin homology (PH) domains on the inner surface of the membrane in a pattern similar to that of the Gbetagamma subunits. Loss of cell polarity resulted in uniform distribution of both the Gbetagamma subunits and the sensitivity of PH domain recruitment. These observations indicate that Gbetagamma subunits are not sufficiently localized to restrict signaling events to the leading edge but that their distribution may determine the relative chemotactic sensitivity of polarized cells.

Polarization of chemoattractant receptor signaling during neutrophil chemotaxis

Morphologic polarity is necessary for chemotaxis of mammalian cells. As a probe of intracellular signals responsible for this asymmetry, the pleckstrin homology domain of the AKT protein kinase (or protein kinase B), tagged with the green fluorescent protein (PHAKT-GFP), was expressed in neutrophils. Upon exposure of cells to chemoattractant, PHAKT-GFP is recruited selectively to membrane at the cell's leading edge, indicating an internal signaling gradient that is much steeper than that of the chemoattractant. Translocation of PHAKT-GFP is inhibited by toxin-B from Clostridium difficile, indicating that it requires activity of one or more Rho guanosine triphosphatases.

How WASP-family proteins and the Arp2/3 complex convert intracellular signals into cytoskeletal structures

In most cells, the structure of the actin cytoskeleton is regulated by Rho-family G proteins. Recent work has outlined a highly conserved signaling pathway from G protein activation to actin assembly. The key downstream components are WASP family proteins - adaptor molecules that bind multiple signaling and cytoskeletal proteins - and the Arp2/3 complex - a multi-functional protein complex that nucleates and crosslinks actin filaments.

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.

Shaping up for shipping out: PLCgamma signaling of morphology changes in EGF-stimulated fibroblast migration

For effective migration, cells must establish an asymmetry in cell/substratum biophysical interactions permitting cellular protrusive and contractile motive forces to produce net cell body translocation; often this is superficially manifested as a polarized cell shape. This change is most easily noted for epithelial cells, which typically undergo a mesenchymal transition prior to rapid motility, and for hematopoietic cells, which must transition from non-adherent to adherent states. These two situations entail dramatic changes that also involve cell-cell contact and differentiation-related changes, and thus introduce confounding events and signals in defining control elements. Hence, a simpler biochemical and biophysical model system may be useful for gaining fundamental insights into the underlying mechanisms. Fortunately, even relatively "uniform" fibroblasts also undergo an initial shape change to commence locomotion. Investigators have recently begun to probe underlying signals that contribute to the reorganization of the actin cytoskeleton. We describe here a model for fibroblast shape changes involved in epidermal growth factor (EGF) stimulation of motility, focusing on signals through EGF receptor (EGFR) -mediated pathways influencing cytoskeletal organization and cell/substratum adhesion. We present new data addressing specifically phospholipase C-gamma (PLCgamma) pathway activation of actin-modifying proteins, including gelsolin, that contributes to these changes and promotes cell migration by increasing the fraction of cells in a motility-permissive morphology and the time spent in such a state.

Morphogenetic cell movement in Dictyostelium

Dictyostelium morphogenesis starts with the chemotactic aggregation of starving individual cells. The cells move in response to propagating waves of the chemoattractant cyclic AMP initiated by cells in the aggregation centre. During aggregation the cells begin to differentiate into several types with different signalling and chemotactic properties. These cell types sort out from each other to form an axial pattern in the slug. There is now good evidence that periodic chemotactic signals not only control aggregation, but also later stages of morphogenesis. These signals take the form of target patterns, spirals, multi-armed spirals and scroll waves. I will discuss their role in the control of cell movement during mound and slug formation and in the formation of the fruiting body.

Rho GTPases control migration and polarization of adhesion molecules and cytoskeletal ERM components in T lymphocytes

Motile lymphocytes adopt a polarized morphology with different adhesion molecules (ICAM, CD43 and CD44) and ERM actin-binding proteins concentrated on the uropod, a slender posterior appendage with important functions in cell-cell interactions and lymphocyte recruitment. We have studied the role of Rho family of GTPases (Rho, Rac and Cdc42) in the control of lymphocyte polarity and migration by analyzing the effects of exogenously introduced Rho GTPase mutants. Transfection of T cell lines that constitutively display a polarized motile morphology with activated mutants of RhoA, Rac1 and Cdc42 impaired cell polarization. A guanosine nucleotide exchange factor for Rac, Tiam-1, induced the same effect as activated Rac1. Conversely, dominant negative forms of the three GTP-binding proteins induced a polarized phenotype in constitutively round-shaped T cells with redistribution of ICAM-3 and moesin to the uropod in an integrin-dependent manner. On the other hand, overexpression of dominant negative Cdc42 and activated mutants of all three Rho GTPases significantly inhibited SDF-1alpha-induced T cell chemotaxis. Together, these data demonstrate that Rho GTPases regulate lymphocyte polarization and chemokine-induced migration, and underscore the key role of Cdc42 in lymphocyte directional migration.

Orientation of chemotactic cells and growth cones: models and mechanisms

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

A novel Ras-interacting protein required for chemotaxis and cyclic adenosine monophosphate signal relay in Dictyostelium

We have identified a novel Ras-interacting protein from Dictyostelium, RIP3, whose function is required for both chemotaxis and the synthesis and relay of the cyclic AMP (cAMP) chemoattractant signal. rip3 null cells are unable to aggregate and lack receptor activation of adenylyl cyclase but are able, in response to cAMP, to induce aggregation-stage, postaggregative, and cell-type-specific gene expression in suspension culture. In addition, rip3 null cells are unable to properly polarize in a cAMP gradient and chemotaxis is highly impaired. We demonstrate that cAMP stimulation of guanylyl cyclase, which is required for chemotaxis, is reduced approximately 60% in rip3 null cells. This reduced activation of guanylyl cyclase may account, in part, for the defect in chemotaxis. When cells are pulsed with cAMP for 5 h to mimic the endogenous cAMP oscillations that occur in wild-type strains, the cells will form aggregates, most of which, however, arrest at the mound stage. Unlike the response seen in wild-type strains, the rip3 null cell aggregates that form under these experimental conditions are very small, which is probably due to the rip3 null cell chemotaxis defect. Many of the phenotypes of the rip3 null cell, including the inability to activate adenylyl cyclase in response to cAMP and defects in chemotaxis, are very similar to those of strains carrying a disruption of the gene encoding the putative Ras exchange factor AleA. We demonstrate that aleA null cells also exhibit a defect in cAMP-mediated activation of guanylyl cyclase similar to that of rip3 null cells. A double-knockout mutant (rip3/aleA null cells) exhibits a further reduction in receptor activation of guanylyl cyclase, and these cells display almost no cell polarization or movement in cAMP gradients. As RIP3 preferentially interacts with an activated form of the Dictyostelium Ras protein RasG, which itself is important for cell movement, we propose that RIP3 and AleA are components of a Ras-regulated pathway involved in integrating chemotaxis and signal relay pathways that are essential for aggregation.

Immune complex-induced integrin activation and L-plastin phosphorylation require protein kinase A

Integrins in resting leukocytes are poorly adhesive, and cell activation is required to induce integrin-mediated adhesion. We recently demonstrated a close correlation between phosphorylation of Ser(5) in L-plastin (LPL), a leukocyte-specific 67-kDa actin bundling protein, and activation of alpha(M)beta(2)-mediated adhesion in polymorphonuclear neutrophils (PMN) (Jones, S. L., Wang, J., Turck, C. W., and Brown, E. J. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 9331-9336). However, the kinase that phosphorylates LPL Ser(5) has not been identified. We found that cAMP-dependent protein kinase (PKA), but not a variety of other serine kinases, can specifically phosphorylate LPL and LPL-derived peptides on Ser(5) in vitro. The cell-permeable cAMP analog 8-bromo-cAMP and the adenylate cyclase activator forskolin both induce LPL phosphorylation in cells. Two PKA inhibitors, H89 and KT5720, inhibited immune complex (IC)-stimulated LPL phosphorylation as well as IC-induced activation of alpha(M)beta(2)-mediated adhesion in PMN. The dose response of H89 inhibition of PMN adhesion correlated with its inhibition of LPL phosphorylation in response to IC. IC stimulation also transiently increased intracellular cAMP concentration in PMN. Thus, PKA functions in an integrin activation pathway initiated by IC binding to Fcgamma receptors in addition to its better known role as a negative regulator of cell activation by G protein-coupled receptors. In contrast, LPL Ser(5) phosphorylation and PMN adhesion induced by formylmethionyl-leucylphenylalanine or phorbol myristate acetate were not affected by PKA inhibitors, suggesting that a different kinase(s) is responsible for LPL phosphorylation in response to these agonists. Phosphoinositidyl 3-kinase also is required for FcgammaR but not formylmethionyl-leucylphenylalanine- or phorbol myristate acetate-induced LPL phosphorylation and activation of alpha(M)beta(2). Two phosphoinositidyl 3-kinase inhibitors blocked FcgammaR-induced cAMP accumulation, demonstrating that this kinase acts upstream of PKA. These data demonstrate a necessary role for PKA in IC-induced integrin activation and LPL phosphorylation.

The Dictyostelium genome project: an invitation to species hopping

Dictyostelium allows some of the general problems of eukaryotic biology to be addressed by using molecular genetic tools that are more normally associated with yeast. The genome project, now underway, marks an important increase in the attractiveness of Dictyostelium as an experimental organism and will invite increased 'species hopping' by experimenters. We provide a brief guide to the problems that are being addressed in Dictyostelium, to the genome project itself and to the molecular genetic tools available for its exploitation.

DAip1, a Dictyostelium homologue of the yeast actin-interacting protein 1, is involved in endocytosis, cytokinesis, and motility

The 64-kD protein DAip1 from Dictyostelium contains nine WD40-repeats and is homologous to the actin-interacting protein 1, Aip1p, from Saccharomyces cerevisiae, and to related proteins from Caenorhabditis, Physarum, and higher eukaryotes. We show that DAip1 is localized to dynamic regions of the cell cortex that are enriched in filamentous actin: phagocytic cups, macropinosomes, lamellipodia, and other pseudopodia. In cells expressing green fluorescent protein (GFP)-tagged DAip1, the protein rapidly redistributes into newly formed cortical protrusions. Functions of DAip1 in vivo were assessed using null mutants generated by gene replacement, and by overexpressing DAip1. DAip1-null cells are impaired in growth and their rates of fluid-phase uptake, phagocytosis, and movement are reduced in comparison to wild-type rates. Cytokinesis is prolonged in DAip1-null cells and they tend to become multinucleate. On the basis of similar results obtained by DAip1 overexpression and effects of latrunculin-A treatment, we propose a function for DAip1 in the control of actin depolymerization in vivo, probably through interaction with cofilin. Our data suggest that DAip1 plays an important regulatory role in the rapid remodeling of the cortical actin meshwork.

A cell's sense of direction

In eukaryotic cells directional sensing is mediated by heterotrimeric guanine nucleotide-binding protein (G protein)-linked signaling pathways. In Dictyostelium discoideum amoebae and mammalian leukocytes, the receptors and G-protein subunits are uniformly distributed around the cell perimeter. Chemoattractants induce the transient appearance of binding sites for several pleckstrin homology domain-containing proteins on the inner face of the membrane. In gradients of attractant these sites are persistently present on the side of the cell facing the higher concentration, even in the absence of a functional actin cytoskeleton or cell movement. Thus, the cell senses direction by spatially regulating the activity of the signal transduction pathway.

Chemoattractant-mediated transient activation and membrane localization of Akt/PKB is required for efficient chemotaxis to cAMP in Dictyostelium

Chemotaxis-competent cells respond to a variety of ligands by activating second messenger pathways leading to changes in the actin/myosin cytoskeleton and directed cell movement. We demonstrate that Dictyostelium Akt/PKB, a homologue of mammalian Akt/PKB, is very rapidly and transiently activated by the chemoattractant cAMP. This activation takes place through G protein-coupled chemoattractant receptors via a pathway that requires homologues of mammalian p110 phosphoinositide-3 kinase. pkbA null cells exhibit aggregation-stage defects that include aberrant chemotaxis, a failure to polarize properly in a chemoattractant gradient and aggregation at low densities. Mechanistically, we demonstrate that the PH domain of Akt/PKB fused to GFP transiently translocates to the plasma membrane in response to cAMP with kinetics similar to those of Akt/PKB kinase activation and is localized to the leading edge of chemotaxing cells in vivo. Our results indicate Akt/PKB is part of the regulatory network required for sensing and responding to the chemoattractant gradient that mediates chemotaxis and aggregation.

Dynamics of a chemoattractant receptor in living neutrophils during chemotaxis

Persistent directional movement of neutrophils in shallow chemotactic gradients raises the possibility that cells can increase their sensitivity to the chemotactic signal at the front, relative to the back. Redistribution of chemoattractant receptors to the anterior pole of a polarized neutrophil could impose asymmetric sensitivity by increasing the relative strength of detected signals at the cell's leading edge. Previous experiments have produced contradictory observations with respect to receptor location in moving neutrophils. To visualize a chemoattractant receptor directly during chemotaxis, we expressed a green fluorescent protein (GFP)-tagged receptor for a complement component, C5a, in a leukemia cell line, PLB-985. Differentiated PLB-985 cells, like neutrophils, adhere, spread, and polarize in response to a uniform concentration of chemoattractant, and orient and crawl toward a micropipette containing chemoattractant. Recorded in living cells, fluorescence of the tagged receptor, C5aR-GFP, shows no apparent increase anywhere on the plasma membrane of polarized and moving cells, even at the leading edge. During chemotaxis, however, some cells do exhibit increased amounts of highly folded plasma membrane at the leading edge, as detected by a fluorescent probe for membrane lipids; this is accompanied by an apparent increase of C5aR-GFP fluorescence, which is directly proportional to the accumulation of plasma membrane. Thus neutrophils do not actively concentrate chemoattractant receptors at the leading edge during chemotaxis, although asymmetrical distribution of membrane may enrich receptor number, relative to adjacent cytoplasmic volume, at the anterior pole of some polarized cells. This enrichment could help to maintain persistent migration in a shallow gradient of chemoattractant.

A Cdc24p-Far1p-Gbetagamma protein complex required for yeast orientation during mating

Oriented cell growth requires the specification of a site for polarized growth and subsequent orientation of the cytoskeleton towards this site. During mating, haploid Saccharomyces cerevisiae cells orient their growth in response to a pheromone gradient overriding an internal landmark for polarized growth, the bud site. This response requires Cdc24p, Far1p, and a heterotrimeric G-protein. Here we show that a two- hybrid interaction between Cdc24p and Gbeta requires Far1p but not pheromone-dependent MAP-kinase signaling, indicating Far1p has a role in regulating the association of Cdc24p and Gbeta. Binding experiments demonstrate that Cdc24p, Far1p, and Gbeta form a complex in which pairwise interactions can occur in the absence of the third protein. Cdc24p localizes to sites of polarized growth suggesting that this complex is localized. In the absence of CDC24-FAR1-mediated chemotropism, a bud site selection protein, Bud1p/Rsr1p, is essential for morphological changes in response to pheromone. These results suggest that formation of a Cdc24p-Far1p-Gbetagamma complex functions as a landmark for orientation of the cytoskeleton during growth towards an external signal.

Cell polarization: chemotaxis gets CRACKing

An early stage in the establishment of cell polarity during chemotaxis of Dictyostelium dicoideum has been identified by a recent study; the new results also show that the development of cell polarity does not rely upon cytoskeletal rearrangement, and may use a spatial sensing mechanism.

Responding to attraction: chemotaxis and chemotropism in Dictyostelium and yeast

Polarized growth in response to external signals is essential for both the internal organization of cells and generation of complex multicellular structures during development. Oriented growth or movement requires specific detection of an external cue, reorganization of the cytoskeleton and subsequent growth or movement. Genetic approaches in both the budding yeast Saccharomyces cerevisiae and the social amoeba Dictyostelium discoideum have shed light on the molecular and cellular aspects of growth or movement towards an external signal. This review discusses the mechanisms and signalling pathways that enable yeast and Dictyostelium cells to translate external signals into directed growth and movement, respectively.