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

Self-organization of the phosphatidylinositol lipids signaling system for random cell migration

Phosphatidylinositol (PtdIns) lipids have been identified as key signaling mediators for random cell migration as well as chemoattractant-induced directional migration. However, how the PtdIns lipids are organized spatiotemporally to regulate cellular motility and polarity remains to be clarified. Here, we found that self-organized waves of PtdIns 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] are generated spontaneously on the membrane of Dictyostelium cells in the absence of a chemoattractant. Characteristic oscillatory dynamics within the PtdIns lipids signaling system were determined experimentally by observing the phenotypic variability of PtdIns lipid waves in single cells, which exhibited characteristics of a relaxation oscillator. The enzymes phosphatase and tensin homolog (PTEN) and phosphoinositide-3-kinase (PI3K), which are regulators for PtdIns lipid concentrations along the membrane, were essential for wave generation whereas functional actin cytoskeleton was not. Defects in these enzymes inhibited wave generation as well as actin-based polarization and cell migration. On the basis of these experimental results, we developed a reaction-diffusion model that reproduced the characteristic relaxation oscillation dynamics of the PtdIns lipid system, illustrating that a self-organization mechanism provides the basis for the PtdIns lipids signaling system to generate spontaneous spatiotemporal signals for random cell migration and that molecular noise derived from stochastic fluctuations within the signaling components gives rise to the variability of these spontaneous signals.

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.

Glycogen synthase kinase-3 is required for efficient Dictyostelium chemotaxis

We present a new role for glycogen synthase kinase (GSK) in the regulation of aggregation and chemotaxis in Dictyostelium. GSK regulates two chemotactic pathways, PIP₃ and TORC2; hence, a loss of function of GSK leads to poor chemotaxis, an observation not previously seen when only one chemotactic pathway was targeted.

Glycogen synthase kinase-3 (GSK3) is a highly conserved protein kinase that is involved in several important cell signaling pathways and is associated with a range of medical conditions. Previous studies indicated a major role of the Dictyostelium homologue of GSK3 (gskA) in cell fate determination during morphogenesis of the fruiting body; however, transcriptomic and proteomic studies have suggested that GSK3 regulates gene expression much earlier during Dictyostelium development. To investigate a potential earlier role of GskA, we examined the effects of loss of gskA on cell aggregation. We find that cells lacking gskA exhibit poor chemotaxis toward cAMP and folate. Mutants fail to activate two important regulatory signaling pathways, mediated by phosphatidylinositol 3,4,5-trisphosphate (PIP₃) and target of rapamycin complex 2 (TORC2), which in combination are required for chemotaxis and cAMP signaling. These results indicate that GskA is required during early stages of Dictyostelium development, in which it is necessary for both chemotaxis and cell signaling.

Understanding eukaryotic chemotaxis: a pseudopod-centred view

Current descriptions of eukaryotic chemotaxis and cell movement focus on how extracellular signals (chemoattractants) cause new pseudopods to form. This 'signal-centred' approach is widely accepted but is derived mostly from special cases, particularly steep chemoattractant gradients. I propose a 'pseudopod-centred' explanation, whereby most pseudopods form themselves, without needing exogenous signals, and chemoattractants only bias internal pseudopod dynamics. This reinterpretation of recent data suggests that future research should focus on pseudopod mechanics, not signal processing.

An unconventional myosin required for cell polarization and chemotaxis

MyTH/FERM (myosin tail homology 4/band 4.1, ezrin, radixin, and moesin) myosins have roles in cellular adhesion, extension of actin-filled projections such as filopodia and stereocilia, and directional migration. The amoeba Dictyostelium discoideum expresses a simple complement of MyTH/FERM myosins, a class VII (M7) myosin required for cell-substrate adhesion and a unique myosin named MyoG. Mutants lacking MyoG exhibit a wide range of normal actin-based behaviors, including chemotaxis to folic acid, but have a striking defect in polarization and chemotaxis to cAMP. Although the myoG mutants respond to cAMP stimulation by increasing persistence and weakly increasing levels of cortical F-actin, they do not polarize; instead, they maintain a round shape and move slowly and randomly when exposed to a chemotactic gradient. The mutants also fail to activate and localize PI3K to the membrane closest to the source of chemoattractant. These data reveal a role for a MyTH/FERM myosin in mediating early chemotactic signaling and suggest that MyTH/FERM proteins have conserved roles in signaling and the generation of cell polarity.

Involvement of the cytoskeleton in controlling leading-edge function during chemotaxis

Cells activate signaling pathways at the site closest to the chemoattractant source that lead to pseudopod formation and directional movement up the gradient. We demonstrate that cytoskeletal components required for cortical tension, including MyoII and IQGAP/cortexillins help regulate the level and timing of leading-edge pathways.

In response to directional stimulation by a chemoattractant, cells rapidly activate a series of signaling pathways at the site closest to the chemoattractant source that leads to F-actin polymerization, pseudopod formation, and directional movement up the gradient. Ras proteins are major regulators of chemotaxis in Dictyostelium; they are activated at the leading edge, are required for chemoattractant-mediated activation of PI3K and TORC2, and are one of the most rapid responders, with activity peaking at ∼3 s after stimulation. We demonstrate that in myosin II (MyoII) null cells, Ras activation is highly extended and is not restricted to the site closest to the chemoattractant source. This causes elevated, extended, and spatially misregulated activation of PI3K and TORC2 and their effectors Akt/PKB and PKBR1, as well as elevated F-actin polymerization. We further demonstrate that disruption of specific IQGAP/cortexillin complexes, which also regulate cortical mechanics, causes extended activation of PI3K and Akt/PKB but not Ras activation. Our findings suggest that MyoII and IQGAP/cortexillin play key roles in spatially and temporally regulating leading-edge activity and, through this, the ability of cells to restrict the site of pseudopod formation.

Direct detection of guidance receptor activity during border cell migration

Guidance receptor signaling is crucial for steering migrating cells. Despite this, we generally lack direct measurements of such signaling. Border cells in Drosophila migrate as a tightly associated group, but dynamically, with front and rear cells exchanging places. They use the receptor tyrosine kinase (RTK) PDGF/VEGF receptor (PVR) as a guidance receptor, perceiving the attractant Pvf1. Here we determine the spatial distribution of PVR signaling by generating an antibody that specifically detects activated PVR in situ. PVR activity is very low in migrating border cells, due to strong activity of cellular phosphatases. Measurements of signal at the cell cortex show variability but a strong bias for both total active PVR and specific activity of PVR to be elevated at the front versus side of the leading cell, often with several-fold difference in signal levels. This polarized active PVR signal requires the E3 ubiquitin ligase Cbl and the recycling regulator Rab11, indicating a dependency on receptor trafficking. The endogenous ligand gradient contributes to shaping of signaling by increasing the specific activity of PVR toward the source in front cells. Surprisingly, signaling is also elevated at the back versus the side of rear cells. This distally polarized distribution of active PVR is ligand independent. Thus the actual guidance signal transmitted in border cells appears to integrate perceived ligand distribution with cell polarity or cell orientation with respect to the cluster. A general implication is that both group configuration and extrinsic cues can directly modulate guidance receptor signaling during collective cell migration.

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.

A Rap/phosphatidylinositol 3-kinase pathway controls pseudopod formation [corrected]

GbpD, a guanine exchange factor specific for Rap1, has been implicated in adhesion, cell polarity, and chemotaxis of Dictyostelium cells. Here it is shown that activated Rap1 directly binds to PI3K. The activation of PI3K by Rap1 and RasG regulates basal and chemoattractant-stimulated PIP3 levels and pseudopod formation.

GbpD, a Dictyostelium discoideum guanine exchange factor specific for Rap1, has been implicated in adhesion, cell polarity, and chemotaxis. Cells overexpressing GbpD are flat, exhibit strongly increased cell-substrate attachment, and extend many bifurcated and lateral pseudopodia. Phg2, a serine/threonine-specific kinase, mediates Rap1-regulated cell-substrate adhesion, but not cell polarity or chemotaxis. In this study we demonstrate that overexpression of GbpD in pi3k1/2-null cells does not induce the adhesion and cell morphology phenotype. Furthermore we show that Rap1 directly binds to the Ras binding domain of PI3K, and overexpression of GbpD leads to strongly enhanced PIP3 levels. Consistently, upon overexpression of the PIP3-degradating enzyme PTEN in GbpD-overexpressing cells, the strong adhesion and cell morphology phenotype is largely lost. These results indicate that a GbpD/Rap/PI3K pathway helps control pseudopod formation and cell polarity. As in Rap-regulated pseudopod formation in Dictyostelium, mammalian Rap and PI3K are essential for determining neuronal polarity, suggesting that the Rap/PI3K pathway is a conserved module regulating the establishment of cell polarity.

Group migration and signal relay in Dictyostelium

The ability of cells to migrate directionally in gradients of chemoattractant is a fundamental biological response that is essential for the survival of the social amoebae Dictyostelium discoideum. In Dictyostelium, cAMP is the most potent chemoattractant and the detection, synthesis, and degradation of cAMP is exquisitely regulated. Interestingly, as Dictyostelium cells migrate directionally, they do so in a head-to-tail fashion, forming characteristic streams. This group behavior is acquired through the relay of the cAMP signals to neighboring cells. This chapter describes experimental procedures used to obtain synchronized populations of chemotactically competent cells and to assess their streaming behavior. In addition, we provide a detailed account of the method used to measure the ability of chemoattractants to directly stimulate adenylyl cyclase activity. Together, these techniques provide a way to combine cell biological and biochemical approaches to the study of signal relay.

ACAP-A/B are ArfGAP homologs in dictyostelium involved in sporulation but not in chemotaxis

Arfs and Arf GTPase-activating proteins (ArfGAPs) are regulators of membrane trafficking and actin dynamics in mammalian cells. In this study, we identified a primordial Arf, ArfA, and two ArfGAPs (ACAP-A/B) containing BAR, PH, ArfGAP and Ankyrin repeat domains in the eukaryote Dictyostelium discoideum. In vitro, ArfA has similar nucleotide binding properties as mammalian Arfs and, with GTP bound, is a substrate for ACAP-A and B. We also investigated the physiological functions of ACAP-A/B by characterizing cells lacking both ACAP-A and B. Although ACAP-A/B knockout cells showed no defects in cell growth, migration or chemotaxis, they exhibited abnormal actin protrusions and ∼50% reduction in spore yield. We conclude that while ACAP-A/B have a conserved biochemical mechanism and effect on actin organization, their role in migration is not conserved. The absence of an effect on Dictyostelium migration may be due to a specific requirement for ACAPs in mesenchymal migration, which is observed in epithelial cancer cells where most studies of mammalian ArfGAPs were performed.

Introduction

The phosphoinositide-3-kinase (PI3K) family of lipid kinases has been well conserved from yeast to mammals. In this evolutionary perspective on the PI3K family, we discuss the prototypical properties of PI3Ks: 1) the utilization of sparse but specifically localized lipid substrates; 2) the nucleation signaling complexes at membrane-targeted sites; and 3) the integration of intracellular signaling with extracellular cues. Together, these three core properties serve to establish order within the entropic environment of the cell. Many human diseases, including cancer and diabetes, are the direct result of loss or defects in one or more of these core properties, putting much hope in the clinical use of PI3K inhibitors singly and in combination to restore order within diseased tissues.

Flotillins are involved in the polarization of primitive and mature hematopoietic cells

Background

Migration of mature and immature leukocytes in response to chemokines is not only essential during inflammation and host defense, but also during development of the hematopoietic system. Many molecules implicated in migratory polarity show uniform cellular distribution under non-activated conditions, but acquire a polarized localization upon exposure to migratory cues.

Methodology/Principal Findings

Here, we present evidence that raft-associated endocytic proteins (flotillins) are pre-assembled in lymphoid, myeloid and primitive hematopoietic cells and accumulate in the uropod during migration. Furthermore, flotillins display a polarized distribution during immunological synapse formation. Employing the membrane lipid-order sensitive probe Laurdan, we show that flotillin accumulation in the immunological synapse is concomittant with membrane ordering in these regions.

Conclusions

Together with the observation that flotillin polarization does not occur in other polarized cell types such as polarized epithelial cells, our results suggest a specific role for flotillins in hematopoietic cell polarization. Based on our results, we propose that in hematopoietic cells, flotillins provide intrinsic cues that govern segregation of certain microdomain-associated molecules during immune cell polarization.

Chemotaxis: finding the way forward with Dictyostelium

Understanding cell migration is centrally important to modern cell biology. However, despite years of study, progress has been hindered by experimental limitations and the complexity of the process. This has led to the popularity of Dictyostelium discoideum, with its experimentally-friendly lifestyle and small, haploid genome, as a tool to dissect the pathways involved in migration. This humble amoeba is now established at the centre of dramatic changes in our understanding of cell movement. In this review we describe the recent reinterpretation of the role of phosphatidylinositol trisphosphate (PIP(3)) and other intracellular messengers that connect signalling and migration, and the transition to models of chemotaxis driven by multiple, intertwined signalling pathways. In shallow gradients, pseudopods are generated with random directions, and we discuss how chemotaxis can operate by biasing this process. Overall we describe how Dictyostelium has the potential to unlock many fundamental questions in the cell motility field.

Navigation of chemotactic cells by parallel signaling to pseudopod persistence and orientation

The mechanism of chemotaxis is one of the most interesting issues in modern cell biology. Recent work shows that shallow chemoattractant gradients do not induce the generation of pseudopods, as has been predicted in many models. This poses the question of how else cells can steer towards chemoattractants. Here we use a new computational algorithm to analyze the extension of pseudopods by Dictyostelium cells. We show that a shallow gradient of cAMP induces a small bias in the direction of pseudopod extension, without significantly affecting parameters such as pseudopod frequency or size. Persistent movement, caused by alternating left/right splitting of existing pseudopodia, amplifies the effects of this bias by up to 5-fold. Known players in chemotactic pathways play contrasting parts in this mechanism; PLA2 and cGMP signal to the cytoskeleton to regulate the splitting process, while PI 3-kinase and soluble guanylyl cyclase mediate the directional bias. The coordinated regulation of pseudopod generation, orientation and persistence by multiple signaling pathways allows eukaryotic cells to detect extremely shallow gradients.

Cell Guidance by 3D-Gradients in Hydrogel Matrices: Importance for Biomedical Applications

Concentration gradients of soluble and matrix-bound guidance cues in the extracellular matrix direct cell growth in native tissues and are of great interest for design of biomedical scaffolds and on implant surfaces. The focus of this review is to demonstrate the importance of gradient guidance for cells as it would be desirable to direct cell growth onto/into biomedical devices. Many studies have been described that illustrate the production and characterization of surface gradients, but three dimensional (3D)-gradients that direct cellular behavior are not well investigated. Hydrogels are considered as synthetic replacements for native extracellular matrices as they share key functions such as 2D- or 3D-solid support, fibrous structure, gas- and nutrition permeability and allow storage and release of biologically active molecules. Therefore this review focuses on current studies that try to implement soluble or covalently-attached gradients of growth factors, cytokines or adhesion sequences into 3D-hydrogel matrices in order to control cell growth, orientation and migration towards a target. Such gradient architectures are especially desirable for wound healing purposes, where defined cell populations need to be recruited from the blood stream and out of the adjacent tissue, in critical bone defects, for vascular implants or neuronal guidance structures where defined cell populations should be guided by appropriate signals to reach their proper positions or target tissues in order to accomplish functional repair.

Correlated waves of actin filaments and PIP3 in Dictyostelium cells

Chemotaxis-deficient amiB-null mutant Dictyostelium cells show two distinct movements: (1) they extend protrusions randomly without net displacements; (2) they migrate persistently and unidirectionally in a keratocyte-like manner. Here, we monitored the intracellular distribution of phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)) to gain insight into roles PIP(3) plays in those spontaneous motilities. In keratocyte-like cells, PIP(3) showed convex distribution over the basal membrane, with no anterior enrichment. In stalled cells, as well as in wild type cells, PIP(3) repeated wave-like changes, including emergence, expansion and disappearance, on the basal membrane. The waves induced lamellipodia when they approached the cell edge, and the advancing speed of the waves was comparable to the migration speed of the keratocyte-like cells. LY294002, an inhibitor of PI3 kinase, abolished PIP(3) waves in stalled cells and stopped keratocyte-like cells. These results together suggested that keratocyte-like cells are "surfing" on the PIP(3) waves by coupling steady lamellipodial protrusions to the PIP(3) waves. Simultaneous live observation of actin filaments and PIP(3) in wild type or stalled amiB(-) cells indicated that the PIP(3) waves were correlated with wave-like distributions of actin filaments. Most notably, PIP(3) waves often followed actin waves, suggesting that PIP(3) induces local depolymerization of actin filaments. Consistent with this idea, cortical accumulation of PIP(3) was often correlated with local retraction of the periphery. We propose that the waves of PIP(3) and actin filaments are loosely coupled with each other and play important roles in generating spontaneous cell polarity.

Myeloid-specific deletion of tumor suppressor PTEN augments neutrophil transendothelial migration during inflammation

Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) is a second messenger that is involved in a number of cell activities including cell growth, proliferation, and motility. PIP(3) is produced by PI3K and regulated by PTEN (phosphatase and tensin homolog deleted on chromosome 10) and SHIP lipid phosphatases. Evidence from our experiments shows that enhanced PIP(3) production results in elevated neutrophil recruitment under inflammatory conditions. However, the mechanism of this elevation is not well understood. We used intravital video microscopy to investigate neutrophil recruitment in the cremaster venules of wild-type and PTEN knockout (KO) mice. Neutrophil transmigration was augmented in PTEN KO mice 4 h after TNF-alpha intrascrotal injection. PTEN KO neutrophils also showed significantly enhanced transmigration 2 h after MIP-2 intrascrotal injection, an effect that dramatically decreased when PI3K or Src kinase inhibitor treatments preceded MIP-2 stimulation. Similarly, fMLP superfusion of the cremaster muscle lead to enhanced emigration in PTEN KO mice. The observed elevation in neutrophil emigration was likely caused by increased speed of crawling, crossing the venular wall, and migrating through the muscular tissue in PTEN KO mice because the effect of PTEN depletion on neutrophil rolling or adhesion was minimal. Interestingly, chemoattractant-induced release of gelatinase and elastase was also elevated in PTEN null neutrophils, providing a potential mechanism for the enhanced neutrophil migration in the PTEN KO mice. Collectively, these results demonstrate that PTEN deletion in neutrophils enhances their invasivity and recruitment to inflamed sites more likely by raising the cell physical capability to cross the vascular and tissue barriers.

How human leukocytes track down and destroy pathogens: lessons learned from the model organism Dictyostelium discoideum

Human leukocytes, including macrophages and neutrophils, are phagocytic immune cells that capture and engulf pathogens and subsequently destroy them in intracellular vesicles. To accomplish this vital task, these leukocytes utilize two basic cell behaviors-chemotaxis for chasing down infectious pathogens and phagocytosis for destroying them. The molecular mechanisms controlling these behaviors are not well understood for immune cells. Interestingly, a soil amoeba, Dictyostelium discoideum, uses these same behaviors to pursue and injest its bacterial food source and to organize its multi-cellular development. Consequently, studies of this model system have provided and will continue to provide us with mechanistic insights into the chemotaxis and phagocytosis of immune cells. Here, we review recent research in these areas that have been conducted in the Chemotaxis Signal Section of NIAID's Laboratory of Immunogenetics.

The group migration of Dictyostelium cells is regulated by extracellular chemoattractant degradation

Starvation of Dictyostelium induces a developmental program in which cells form an aggregate that eventually differentiates into a multicellular structure. The aggregate formation is mediated by directional migration of individual cells that quickly transition to group migration in which cells align in a head-to-tail manner to form streams. Cyclic AMP acts as a chemoattractant and its production, secretion, and degradation are highly regulated. A key protein is the extracellular phosphodiesterase PdsA. In this study we examine the role and localization of PdsA during chemotaxis and streaming. We find that pdsA(-) cells respond chemotactically to a narrower range of chemoattractant concentrations compared with wild-type (WT) cells. Moreover, unlike WT cells, pdsA(-) cells do not form streams at low cell densities and form unusual thick and transient streams at high cell densities. We find that the intracellular pool of PdsA is localized to the endoplasmic reticulum, which may provide a compartment for storage and secretion of PdsA. Because we find that cAMP synthesis is normal in cells lacking PdsA, we conclude that signal degradation regulates the external cAMP gradient field generation and that the group migration behavior of these cells is compromised even though their signaling machinery is intact.

Neutrophils establish rapid and robust WAVE complex polarity in an actin-dependent fashion

Asymmetric intracellular signals enable cells to migrate in response to external cues. The multiprotein WAVE (also known as SCAR or WASF) complex activates the actin-nucleating Arp2/3 complex [1-4] and localizes to propagating "waves," which direct actin assembly during neutrophil migration [5, 6]. Here, we observe similar WAVE complex dynamics in other mammalian cells and analyze WAVE complex dynamics during establishment of neutrophil polarity. Earlier models proposed that spatially biased generation [7] or selection of protrusions [8] enables chemotaxis. These models require existing morphological polarity to control protrusions. We show that spatially biased generation and selection of WAVE complex recruitment also occur in morphologically unpolarized neutrophils during development of their first protrusions. Additionally, several mechanisms limit WAVE complex recruitment during polarization and movement: Intrinsic cues restrict WAVE complex distribution during establishment of polarity, and asymmetric intracellular signals constrain it in morphologically polarized cells. External gradients can overcome both intrinsic biases and control WAVE complex localization. After latrunculin-mediated inhibition of actin polymerization, addition and removal of agonist gradients globally recruits and releases the WAVE complex from the membrane. Under these conditions, the WAVE complex no longer polarizes, despite the presence of strong external gradients. Thus, actin polymer and the WAVE complex reciprocally interact during polarization.

The mood stabiliser lithium suppresses PIP3 signalling in Dictyostelium and human cells

Bipolar mood disorder (manic depression) is a major psychiatric disorder whose molecular origins are unknown. Mood stabilisers offer patients both acute and prophylactic treatment, and experimentally, they provide a means to probe the underlying biology of the disorder. Lithium and other mood stabilisers deplete intracellular inositol and it has been proposed that bipolar mood disorder arises from aberrant inositol (1,4,5)-trisphosphate [IP(3), also known as Ins(1,4,5)P(3)] signalling. However, there is no definitive evidence to support this or any other proposed target; a problem exacerbated by a lack of good cellular models. Phosphatidylinositol (3,4,5)-trisphosphate [PIP(3), also known as PtdIns(3,4,5)P(3)] is a prominent intracellular signal molecule within the central nervous system (CNS) that regulates neuronal survival, connectivity and synaptic function. By using the genetically tractable organism Dictyostelium, we show that lithium suppresses PIP(3)-mediated signalling. These effects extend to the human neutrophil cell line HL60. Mechanistically, we show that lithium attenuates phosphoinositide synthesis and that its effects can be reversed by overexpression of inositol monophosphatase (IMPase), consistent with the inositol-depletion hypothesis. These results demonstrate a lithium target that is compatible with our current knowledge of the genetic predisposition for bipolar disorder. They also suggest that lithium therapy might be beneficial for other diseases caused by elevated PIP(3) signalling.

Regulation of Dictyostelium morphogenesis by RapGAP3

Rap1 is a key regulator of cell adhesion and cell motility in Dictyostelium. Here, we identify a Rap1-specific GAP protein (RapGAP3) and provide evidence that Rap1 signaling regulates cell-cell adhesion and cell migration within the multicellular organism. RapGAP3 mediates the deactivation of Rap1 at the late mound stage of development and plays an important role in regulating cell sorting during apical tip formation, when the anterior-posterior axis of the organism is formed, by controlling cell-cell adhesion and cell migration. The loss of RapGAP3 results in a severely altered morphogenesis of the multicellular organism at the late mound stage. Direct measurement of cell motility within the mound shows that rapGAP3(-) cells have a reduced speed of movement and, compared to wild-type cells, have a reduced motility towards the apex. rapGAP3(-) cells exhibit some increased EDTA/EGTA sensitive cell-cell adhesion at the late mound stage. RapGAP3 transiently and rapidly translocates to the cell cortex in response to chemoattractant stimulation, which is dependent on F-actin polymerization. We suggest that the altered morphogenesis and the cell-sorting defect of rapGAP3(-) cells may result in reduced directional movement of the mutant cells to the apex of the mound.

Assays for chemotaxis and chemoattractant-stimulated TorC2 activation and PKB substrate phosphorylation in Dictyostelium

Chemotaxis is a highly coordinated biological system where chemoattractants trigger multiple signal transduction pathways which act in concert to bring about directed migration. A signaling pathway acting through PIP(3), which accumulates at the leading edge of the cell, has been extensively characterized. However, chemotaxis still remains in cells depleted of PIP(3), suggesting there are PIP(3)-independent pathways. We have identified a pathway involving TorC2-PKBR1 as well as another containing PLA2 activity that act in parallel with PIP(3). Activation of PKBR1, a myristoylated Protein Kinase B homolog, is dependent on TorC2 (Rapamycin-insensitive Tor complex 2) kinase but is completely independent of PIP(3). In response to chemoattractant, PKBs rapidly phosphorylate at least eight proteins, including Talin B, PI4P 5-kinase, two RasGefs, and a RhoGap. These studies help to link the signaling pathways to specific effectors and provide a more complete understanding of chemotaxis.

Single-molecule imaging techniques to visualize chemotactic signaling events on the membrane of living Dictyostelium cells

In this chapter, we describe methods to monitor signaling events at the single-molecule level on the membrane of living cells by using total internal reflection fluorescence microscopy (TIRFM). The techniques provide a powerful tool for elucidating the stochastic properties of signaling molecules involved in chemotaxis of the cellular slime mold Dictyostelium discoideum. Taking cAMP receptor 1 (cAR1) as an example of a target protein for single-molecule imaging, we describe the experimental setup of TIRFM, a method for labeling cAR1 with a fluorescent dye, and a method for investigating the receptor's lateral mobility. We discuss how the developmental progression of cells modulates both cAR1 behavior and the phenotypic variability in cAR1 mobility for different cell populations.

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.

Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge

Chemoattractant signaling induces the polarization and directed movement of cells secondary to the activation of multiple effector pathways. In addition, chemotactic signals can be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms underlying such remarkable features remain ill defined. We show that the asymmetrical distribution of adenylyl cyclase (ACA) at the back of Dictyostelium discoideum cells, an essential determinant of their ability to migrate in a head-to-tail fashion, requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. We also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. We propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis.

The GPI-anchored superoxide dismutase SodC is essential for regulating basal Ras activity and for chemotaxis of Dictyostelium discoideum

A genetic screen for Dictyostelium mutant displaying high level of constitutive phosphatidylinositol (3,4,5)-trisphosphate led to the finding that the glycosylphosphatidylinositol (GPI)-anchored superoxide dismutase SodC regulates small GTPase Ras. Cells that lack SodC exhibited constitutively high levels of active Ras, more membrane localization of GFP-PHcrac, and defects in chemoattractant sensing, cell polarization and motility. These defects of SodC-lacking cells were partially restored by expression of wild-type SodC but not by the catalytically inactive mutant SodC (H245R, H247Q). Furthermore, an inhibition of PI3K activity in SodC-deficient cells by LY294002 only partially restored chemoattractant sensing and cell polarization, consistent with the fact that SodC-deficient cells have aberrantly high level of active Ras, which functions upstream of PI3K. A higher level of active GFP-RasG was observed in SodC-deficient cells, which significantly decreased upon incubation of SodC-deficient cells with the superoxide scavenger XTT. Having constitutively high levels of active Ras proteins and more membrane localization of GFP-PHcrac, SodC-deficient cells exhibited severe defects in chemoattractant sensing, cell polarization and motility.

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.

Spatiotemporal regulation of Ras activity provides directional sensing

Cells' ability to detect and orient themselves in chemoattractant gradients has been the subject of numerous studies, but the underlying molecular mechanisms remain largely unknown [1]. Ras activation is the earliest polarized response to chemoattractant gradients downstream from heterotrimeric G proteins in Dictyostelium, and inhibition of Ras signaling results in directional migration defects [2]. Activated Ras is enriched at the leading edge, promoting the localized activation of key chemotactic effectors, such as PI3K and TORC2 [2-5]. To investigate the role of Ras in directional sensing, we studied the effect of its misregulation by using cells with disrupted RasGAP activity. We identified an ortholog of mammalian NF1, DdNF1, as a major regulator of Ras activity in Dictyostelium. We show that disruption of nfaA leads to spatially and temporally unregulated Ras activity, causing cytokinesis and chemotaxis defects. By using unpolarized, latrunculin-treated cells, we show that tight regulation of Ras is important for gradient sensing. Together, our findings suggest that Ras is part of the cell's compass and that the RasGAP-mediated regulation of Ras activity affects directional sensing.

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.

Microfluidic switching system for analyzing chemotaxis responses of wortmannin-inhibited HL-60 cells

The chemotaxis of phosphoinositide kinase-3 (PI3K)-inhibited differentiated HL-60 cells stably expressing CXCR2 was studied in a microfluidic switching gradient device that can generate stable and well-defined forward and reverse gradients. Wortmannin, a widely used PI3K inhibitor, was added during cell preparation and the experiment process. The studies quantify the chemotaxis gradient and the effects of a change in the direction of a CXCL-8 gradient on cell migration. PI3K-inhibited HL-60 cells migrated more efficiently toward the gradient before gradient switching than after, as measured by the effective chemotactic index. The inhibited HL-60 cells also showed that inadequate polarization, slower response time, and reduced cell populations can follow the gradient change. We observed that the role of PI3K in directing cellular response to gradient reversal was important in cell polarization and directional sensing associated with gradient switching.

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.

PI3-kinase signaling contributes to orientation in shallow gradients and enhances speed in steep chemoattractant gradients

Dictyostelium cells that chemotax towards cAMP produce phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] at the leading edge, which has been implicated in actin reorganization and pseudopod extension. However, in the absence of PtdIns(3,4,5)P(3) signaling, cells will chemotax via alternative pathways. Here we examined the potential contribution of PtdIns(3,4,5)P(3) to chemotaxis of wild-type cells. The results show that steep cAMP gradients (larger than 10% concentration difference across the cell) induce strong PtdIns(3,4,5)P(3) patches at the leading edge, which has little effect on the orientation but strongly enhances the speed of the cell. Using a new sensitive method for PtdIns(3,4,5)P(3) detection that corrects for the volume of cytosol in pixels at the boundary of the cell, we show that, in shallow cAMP gradient (less than 5% concentration difference across the cell), PtdIns(3,4,5)P(3) is still somewhat enriched at the leading edge. Cells lacking PI3-kinase (PI3K) activity exhibit poor chemotaxis in these shallow gradients. Owing to the reduced speed and diminished orientation of the cells in steep and shallow gradients, respectively, cells lacking PtdIns(3,4,5)P(3) signaling require two- to six-fold longer times to reach a point source of chemoattractant compared with wild-type cells. These results show that, although PI3K signaling is dispensable for chemotaxis, it gives the wild type an advantage over mutant cells.

Chemotaxis, chemokine receptors and human disease

Cell migration is involved in diverse physiological processes including embryogenesis, immunity, and diseases such as cancer and chronic inflammatory disease. The movement of many cell types is directed by extracellular gradients of diffusible chemicals. This phenomenon, referred to as "chemotaxis", was first described in 1888 by Leber who observed the movement of leukocytes toward sites of inflammation. We now know that a large family of small proteins, chemokines, serves as the extracellular signals and a family of G-protein-coupled receptors (GPCRs), chemokine receptors, detects gradients of chemokines and guides cell movement in vivo. Currently, we still know little about the molecular machineries that control chemokine gradient sensing and migration of immune cells. Fortunately, the molecular mechanisms that control these fundamental aspects of chemotaxis appear to be evolutionarily conserved, and studies in lower eukaryotic model systems have allowed us to form concepts, uncover molecular components, develop new techniques, and test models of chemotaxis. These studies have helped our current understanding of this complicated cell behavior. In this review, we wish to mention landmark discoveries in the chemotaxis research field that shaped our current understanding of this fundamental cell behavior and lay out key questions that remain to be addressed in the future.

Parallel phosphatidylinositol 3-kinase (PI3K)-dependent and Src-dependent pathways lead to CXCL8-mediated Rac2 activation and chemotaxis

The requirement for phosphatidylinositol 3-kinase (PI3K) in the establishment of cell polarity and motility in a number of cell types has recently come into question. In this study, we demonstrate that inhibition of PI3K by wortmannin in neutrophil-like differentiated HL60 cells expressing CXCR2 resulted in reduced cell motility but normal chemotaxis in response to a gradient of CXCL8. However, wortmannin inhibition of PI3K did impair the ability of cells to re-orient their polarity and respond quickly to a change in the direction of the CXCL8 gradient. We hypothesized that Src-regulated ELMO-Dock2-Rac2 activation mediates chemotaxis in the absence of PI3K activity. Inhibition of Src with the small molecule inhibitor, PP2, or inhibition of Dock2 by shRNA knockdown confirmed the functional role of Src and Dock2 in regulating chemotaxis when PI3K was inhibited. Moreover, neutrophils isolated from bone marrow of hck(-/-)fgr(-/-)lyn(-/-) mice exhibited much more severe inhibition of chemotaxis when PI3K was blocked with wortmannin as compared with neutrophils isolated from bone marrow of wild-type mice. Thus, PI3K and Src-ELMO-Dock2 pathways work in parallel to activate Rac2 and modulate chemotaxis in response to a CXCL8 gradient in neutrophils.

tsunami, the Dictyostelium homolog of the Fused kinase, is required for polarization and chemotaxis

In a forward genetic screen for chemotaxis mutants in Dictyostelium discoideum, we identified a loss-of-function mutation, designated tsunami, encoding a homolog of the Fused kinase. Cells lacking tsuA function could not effectively perform chemotaxis and were unable to become polarized or correctly orient pseudopods in chemotactic gradients. While tsuA(-) cells were able to couple receptor occupancy to phosphatidylinositol (3,4,5) trisphosphate (PIP3) production and actin polymerization, the PIP3 response was prolonged and basal F-actin levels were increased. Interestingly, TsuA localizes to the microtubule network and puncta mainly found at the cell periphery. Analysis of the gene uncovered a novel C-terminal domain that we designated the Tsunami Homology (TH) domain. Both the kinase domain and the TH domain are required to rescue the phenotypic defects of tsuA(-) cells. While kinase activity is not required for localization to microtubules, the TH domain is essential. Thus, localization of kinase activity to microtubules is critical for TsuA function. We propose that functions in association with the microtubule network may underlie the divergent roles of Fused kinase proteins in different organisms.

A spatially restricted increase in receptor mobility is involved in directional sensing during Dictyostelium discoideum chemotaxis

The directed cell migration towards a chemotactic source, chemotaxis, involves three complex and interrelated processes: directional sensing, cell polarization and motility. Directional sensing allows migrating eukaryotic cells to chemotax in extremely shallow gradients (<2% across the cell body) of the chemoattractant. Although directional sensing has been observed as spatially restricted responses along the plasma membrane, our understanding of the ;compass' of the cell that controls the gradient-induced translocation of proteins during chemotactic movements is still largely lacking. Until now, the dynamical behaviour and mobility of the chemoattractant-receptor molecule has been neglected in models describing the directional sensing mechanisms. Here, we show by single-molecule microscopy an agonist-induced increase in the mobile fraction of cAMP-receptor at the leading edge of chemotacting Dictyostelium discoideum cells. The onset of receptor mobility was correlated to the uncoupling and activation of the Galpha2-protein. A finite-element simulation showed that the increase in mobile fraction of the activated receptor enabled the amplified generation of activated Gbetagamma-dimers at the leading edge of the cell, faithfully representing a primary linear amplification step in directional sensing. We propose here that modulation of the receptor mobility is directly involved in directional sensing and provides a new mechanistic basis for the primary amplification step in current theoretical models that describe directional sensing.

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.

Stochastic control of spontaneous signal generation for gradient sensing in chemotaxis

Chemotaxis is characterized by spontaneous cellular behavior. This spontaneity results, in part, from the stochasticity of intracellular reactions. Spontaneous and random migration of chemotactic cells is regulated by spontaneously generated signals, namely transient local increases in the level of phosphoinositol-3,4,5-triphosphate (PIP3 pulses). In this study, we attempted to elucidate the mechanisms that generate these PIP3 pulses and how the pulses contribute to gradient sensing during chemotaxis. To this end, we constructed a simple biophysical model of intracellular signal transduction consisting of an inositol phospholipid signaling pathway and small GTPases. Our theoretical analysis revealed that an excitable system can emerge from the non-linear dynamics of the model, and that stochastic reactions allow the system to spontaneously become excited, which was corresponded to the PIP3 pulses. Based on these results, we framed a hypothesis of the gradient sensing; a chemical gradient spatially modifies a potential barrier for excitation and then PIP3 pulses are preferentially generated on the side of the cell exposed to the higher chemical concentration. We then validated our hypothesis using stochastic simulations of the signal transduction.

Chemotaxis, chemokine receptors and human disease

Cell migration is involved in diverse physiological processes including embryogenesis, immunity, and diseases such as cancer and chronic inflammatory disease. The movement of many cell types is directed by extracellular gradients of diffusible chemicals. This phenomenon, referred to as "chemotaxis", was first described in 1888 by Leber who observed the movement of leukocytes toward sites of inflammation. We now know that a large family of small proteins, chemokines, serves as the extracellular signals and a family of G-protein-coupled receptors (GPCRs), chemokine receptors, detects gradients of chemokines and guides cell movement in vivo. Currently, we still know little about the molecular machineries that control chemokine gradient sensing and migration of immune cells. Fortunately, the molecular mechanisms that control these fundamental aspects of chemotaxis appear to be evolutionarily conserved, and studies in lower eukaryotic model systems have allowed us to form concepts, uncover molecular components, develop new techniques, and test models of chemotaxis. These studies have helped our current understanding of this complicated cell behavior. In this review, we wish to mention landmark discoveries in the chemotaxis research field that shaped our current understanding of this fundamental cell behavior and lay out key questions that remain to be addressed in the future.