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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Semin Cell Dev Biol. Author manuscript; available in PMC 2014 July 7.
Published in final edited form as:
PMCID: PMC4083813

Moving in the right direction: How eukaryotic cells migrate along chemical gradients


Many cells have the ability to grow or migrate towards chemical cues. Oriented growth and movement require detection of the external chemical gradient, transduction of signals, and reorganization of the cytoskeleton. Recent studies in Dictyostelium discoideum and mammalian neutrophils have revealed a complex signaling network that enables cells to migrate in chemical gradients.

1. Introduction

Many types of cells have an internal “compass” that enables them to sense chemicals in the environment and direct their movements (Fig. 1). This process referred to as chemotaxis or directed cell migration plays a critical role in a variety of biological processes. Dictyostelium amoebae rely on chemotaxis to find bacterial prey. When food is scarce, chemotactic responses towards secreted cyclic AMP (cAMP) allow the cells to aggregate into multicellular structures and produce spores that are resistant to harsh environmental conditions. During embryogenesis in metazoans, specific chemoattractants guide primordial germ cells to proper locations, mediate the formation of organs, and control the wiring of the nervous system. In the adult, chemotaxis is involved in immune cell trafficking, wound healing, tissue regeneration, and stem cell homing. Conversely, improper chemotaxis is the origin of many pathological conditions [1-3]. For example, during metastasis, chemotaxis is required for cancerous cells to escape the primary tumor, enter the circulation, and emigrate to specific tissues.

Fig. 1
Dictyostelium and neutrophils migrate along chemoattractant gradients. Left panel: Dictyostelium cells were placed on a chambered cover glass and exposed to a cAMP gradient generated by a micropipette. Right panel: Neutrophil-like HL-60 cells were placed ...

Chemotaxis initiates with the binding of chemoattractants (or chemorepellents) to specific receptors on the cell surface, which triggers a wide range of biochemical responses. These events influence the organization of the cytoskeleton in distinct ways at the front and the rear of the cell to bias migration. Chemotaxis can be conceptually divided into three distinctive yet interconnected processes: motility, directional sensing, and polarity (Fig. 2). Motility, in both Dictyostelium and mammalian neutrophils, involves periodic extension and retraction of pseudopodia, which coordinate with adhesion to propel cellular movement in random directions [4, 5]. Actin polymerization drives pseudopod formation at the leading edge whereas actomyosin filaments generate contractile forces at the sides and the rear. Directional sensing refers to the ability of chemotactic cells to determine the direction and proximity of external cues and bias the movement accordingly. This is generally achieved by converting shallow extracellular gradients of chemoattractant into steeper gradients of intracellular signals [6, 7]. Polarity refers to an elongated cell morphology, which is accompanied by the redistribution of cytoskeletal components and signaling molecules to either the anterior or the posterior of the cell. In polarized amoebae and neutrophils, the anterior becomes more sensitive to chemoattractants. When the direction of the gradient is changed, these cells maintain the original front and turn towards the new highest concentration rather than extend a new front [8]. It should be noted that neither motility nor polarity is required for a cell to sense direction. For example, cells treated with compounds that depolymerize the cytoskeleton, such as latrunculin A, are round and immobile, yet functional gradients of intracellular signaling molecules can still be established in response to external gradients [9, 10].

Fig. 2
Chemotaxis is composed of motility, directional sensing, and polarity. Motility involves the periodic extension and retraction of pseudopodia that drive cellular movement in the absence of chemoattractant gradients. Directional sensing is demonstrated ...

Observations of chemotaxing cells have led to a series of intriguing questions. How do cells convert chemoattractant gradients into intracellular signals? Which portions of the signaling network are involved in gradient detection and which control cell movements? What is the basis of the rhythmic production of pseudopodia? How are the temporal responses of cells to global chemotactic stimuli related to directed movement? Studies of chemotaxis in the two model cell types Dictyostelium amoebae and neutrophils have greatly facilitated our understanding of this process. Despite differences in the migratory behaviors and variations in the molecular details of the signaling pathways, many guidance mechanisms have turned out to be highly conserved among eukaryotic cells.

2. G-protein coupled receptors (GPCRs) detect chemoattractant gradient

Chemoattractants are detected by the seven-transmembrane-spanning serpentine receptors. In Dictyostelium, four cAMP receptors (cAR1-4) have been identified, which have different affinities for cAMP and are expressed at different stages of differentiation [11, 12]. The cARs signal through the heterotrimeric G-protein consisting of α2-, β-, and γ-subunits (there are only single copies of the Gβ and Gγ genes in Dictyostelium) [13]. Current evidence suggests that the β- and γ-subunits form a complex that mediates downstream signaling, whereas the α2-subunit is required for interaction with the receptor [14-16]. As shown by FRET analysis, binding of cAMP to the receptor leads to a rapid dissociation of the βγ-complex from the α2-subunit, which activates multiple downstream effectors that ultimately give rise to polarization and cellular movement [15, 16]. In neutrophils, chemoattractant receptors for ligands such as fMLP, LTB4, and C5a are predominantly linked to pertussis-sensitive Gαi proteins [17]. Signals from the Gαi-coupled receptor also seem to be mediated by the Gβγ subunits, but some studies have suggested that Gα12/13 plays a direct role in chemotaxis [18].

Studies of both Dictyostelium and neutrophils have shown that gradient sensing does not require redistribution of the receptor or G-protein. In Dictyostelium, cAR1-GFP and Gβ-GFP were found to distribute fairly evenly along the plasma membrane during random or chemotactic movements (Fig. 3) [19, 20]. Furthermore, FRET analysis revealed that the dissociation of the Gα- and βγ-subunits mirrors receptor occupancy and parallels the shallow external gradient [15]. Similarly, in neutrophils, the C5a receptor fused to GFP displays a uniform distribution on the plasma membrane in cells undergoing chemotaxis [21]. It is believed that evenly distributed receptors and G proteins enable a cell to constantly measure receptor occupancy across the entire perimeter so that it can respond swiftly to rapid changes in environmental conditions.

Fig. 3
Fluorescent images depicting the localization of signaling components. Top panel: In migrating Dictyostelium cells, the cAMP receptor cAR1 and Gβ subunit distribute evenly along the plasma membrane. Middle panel: PI3K is recruited to the leading ...

3. Signaling pathways steer chemotaxis

Despite the uniform distribution of the GPCRs, a number of downstream signaling events are restricted to either the high (the side of the cell experiencing higher concentrations of chemoattractants) or the low side of the cell. In this way, the initial small difference in receptor occupancy and G-protein activation is greatly amplified by conversion into sharply localized internal responses (Fig. 3). Important pathways (Fig. 4) responsible for signal amplification and guidance of cell migration will be discussed in detail in the following sections.

Fig. 4
Signaling networks control chemotaxis. Depicted is a network of signaling events triggered by cAMP to control front projection and back contraction in Dictyostelium. At the front, binding of cAMP to GPCR leads to the activation of RasG and RasC, which ...

3.1 Phosphatidylinositol-3,4,5-triphosphate (PIP3) signaling in chemotaxis

The observation that proteins containing the pleckstrin homology (PH) domain specifically localize to the leading edge of chemotaxing amoebae and neutrophils provided the first clue into how signals become amplified [9, 22]. Translocation of these proteins to the plasma membrane reflects the local accumulation of PIP3. In Dictyostelium, the level of PIP3 is controlled by two classes of enzymes: PI3K (phosphatidylinositol 3-kinase), which produces PIP3 by phosphorylating PI(4,5)P2, and PTEN (phosphatase and tensin homologue deleted on chromosome ten), which catalyzes the reverse reaction [23-26]. When cells are migrating, PI3K is recruited to the front of the cell, conversely, PTEN falls off the membrane at the front and localizes to the lateral sides and the back. As a result of this reciprocal distribution, PIP3 accumulates specifically at the leading edge (Fig. 3). The mechanisms that control the cellular localization and activity of PI3K and PTEN are therefore central to our understanding of signal amplification and directional sensing.

The Dictyostelium genome encodes five class I PI3Ks. PI3K1 and PI3K2 are responsible for most of the PIP3 production in response to cAMP [23, 25, 27]. PI3K activation requires localization to the plasma membrane. Membrane-targeted PI3Ks are active even in the absence of chemoattractants [23, 25]. All five PI3Ks contain a Ras-binding domain (RBD). PI3K1 and PI3K2 carrying point mutations in the RBD fail to rescue the defects in PIP3 production in the pi3k1/pi3k2 cell, suggesting that PI3K receives additional signal from the Ras family small GTPases [25]. Studies of a collection of knockout mutants implicated that RasG is an upstream regulator of PI3K in Dictyostelium [10, 28], but it is unlikely to be the membrane recruiter. The N-terminal domains of PI3K1 and PI3K2, which lack the RBD, are sufficient for recruitment of GFP to the plasma membrane in response to stimulus [25]. In addition to signals from the receptor, a positive feedback loop involving PI3K and F-actin has also been proposed to regulate the localization of PI3K [28]. It should be noted that F-actin is not absolutely required for PI3K activation since PIP3 gradient remains largely intact in cells treated with inhibitors of actin polymerization [9]. The interaction of Dictyostelium PTEN with the plasma membrane requires a N-terminal PI(4,5)P2 binding motif [29]. Mutant version of PTEN lacking the PIP2 binding motif does not bind to the plasma membrane nor does it rescue the phenotypes of pten cells. Some studies suggested that PLC, through its ability to degrade PI(4,5)P2, regulates PTEN distribution by preferentially removing PTEN binding sites at the leading edge [30].

In neutrophils, PIP3 accumulation dependents on the activities of PI3Kγ, a class IB PI3K, the 5′-phosphatase SHIP-1, and the 3′-phosphatase PTEN [31-34]. PI3Kγ appears to be regulated by the synergistic action of Gβγ-subunits and Ras proteins [35]. In addition, a large body of work indicated that Rac proteins and components of the F-actin networks form part of a positive-feedback loop to stimulate PIP3 production at the leading edge [36-38]. The localization of PTEN in moving neutrophils is currently unresolved. Although some studies localized PTEN to the posterior of the cell, others found it to be evenly distributed in the cytosol [18, 32, 39].

Accumulating evidence suggests that the localized accumulation of PIP3 provides an important signal for directed cell migration. First, the internal PIP3 gradient is a highly conserved signature of chemotactic signaling observed in many cell types. Second, PIP3 is a strong candidate for a directional sensing cue since cells treated with inhibitors of F-actin are still able to localize PIP3 when exposed to a gradient. Third, chemotaxis is impaired in cells with genetic loss of PI3Ks or treated with PI3K inhibitors [17, 25, 26, 40]. Finally, a collection of data has linked PIP3 production to actin polymerization and pseudopod formation. In Dictyostelium cells lacking PTEN and mouse neutrophils lacking SHIP-1, excess PIP3 diffuses away from the leading edge, resulting in unrestricted pseudopodia extensions, a severe loss of cell polarity and defects in chemotaxis [24, 34]. Furthermore, delivery of PIP3 to or activation of PI3K in neutrophils is sufficient to trigger actin polymerization, induce polarity and initiate migration [36, 41].

PIP3 signaling is believed to be transduced by effector proteins that bind selectively to its head group. As noted before such proteins often contain PH domains and are recruited to the cell's leading edge. In Dictyostelium, CRAC (cytosolic regulator of adenylyl cyclase), PhdA (PH domain-containing protein A), and PKBA (protein kinase B A) are three well-characterized PIP3 binding proteins and a recent study identified three additional ones, designated PhdB, PhdG, and PhdI [9, 26, 42, 43]. CRAC is required for the process of signal relay, in which extracellular cAMP stimulates further production and release of cAMP [44, 45]. Cells lacking PhdA, PhdB, or PhdG all show reduced chemotaxis [26, 42]. PKBA, also known as Akt, is a serine/threonine protein kinase. cAMP stimulates the activity of PKBA, which in turn mediates the phosphorylation of a series of signaling and cytoskeletal proteins [43, 46, 47]. Deletion of PKBA in pten cell rescues the chemotaxis defects seen in pten cell. Even though the level of PIP3 remains elevated in the double knockout cell, pseudopod projection is confined to the leading edge, suggesting that PKBA is a critical downstream effector of PIP3 in regulating the cytoskeletal activity [47]. Recently, a recursive-learning algorithm was developed to predict PIP3 responsive PH-domain containing proteins [48]. This algorithm predicted as many as twenty potential PIP3 binding proteins in Dictyostelium. Characterization of these new proteins is currently in progress. A number of PIP3-binding proteins have also been identified in neutrophils, many of which are regulators for small GTPases of the Rac, Rho, and Cdc42 family. PIP3 may signal through these proteins to modulate the cytoskeleton [40, 49]

3.2 PIP3-independent pathways in chemotaxis

Although PIP3 clearly plays an important role in cell polarity and migration, inhibiting the pathway does not always block chemotaxis. PI3K inhibitors such as LY294002 and wortmannin produce only partial defects, and in some cases, cells can recover from the treatment and resume efficient chemotaxis [50]. Dictyostelium cells lacking all five PI3Ks are still able to carry out chemotaxis [51]. Studies in neutrophils have revealed a similar pattern of results. In one study it was shown that the dependence on PI3Kγ activity is context dependent, with respect to the state of priming of the neutrophils and the type of surface on which they are migrating [31]. These observations led to the search for signaling pathways that act independently or in parallel with the PIP3 pathway.

3.2.1 TORC2

Target of rapamycin complex 2 (TORC2) is a conserved serine/threonine kinase complex, which has been shown to regulate cytoskeleton-based events in many cell types [52-55]. Pianissimo (PiaA, the mRictor homolog), an essential component of the Dictyostelium TORC2, was identified fourteen years ago in a genetic screen for mutants that failed to aggregate upon starvation [56]. It was demonstrated only recently that TORC2 regulates Dictyostelium chemotaxis and the cell-to-cell relay of cAMP signal through the phosphorylation of two Akt/PKB kinases, PKBA and PKBR1 [46, 57-59]. In response to chemoattractant stimulation, both PKBs are transiently phosphorylated within their hydrophobic motifs by TORC2 [46]. In migrating cells, these phosphorylation events are restricted to the leading edge, suggesting that they may play a role in regulating the actin cytoskeleton. Indeed, PKB activity was found to mediate the phosphorylation of the regulators of the Rac and Rho family small GTPases [46, 47]. The activation of PKBA, as noted in section 3.1, requires an additional interaction with PIP3. In contrast, PIP3 is not required for the phosphorylation and activation of PKBR1 [46, 57, 60]. Furthermore, cells lacking PiaA or PKBR1 exhibit more severe defects in chemotaxis compared to cells lacking PKBA, implying that the PIP3-independent TORC2-PKBR1 pathway provides the major PKB activity required for migration [46]. TORC2 has also been found to regulate chemotaxis in neutrophils. A recent study showed that, as in Dictyostelium, TORC2 signaling promotes chemoattractant-induced production of cAMP in neutrophils. cAMP accumulation in the cell body modulates the RhoA-ROCK-myosin II pathway, which in turn controls back retraction [61]. Neutrophils lacking TORC2 activity display a loss of polarity and a strong defect in chemotaxis. Interestingly, TORC2 stimulates cAMP production through PKC but not the PIP3-responsive substrate Akt, suggesting that this may also be a PIP3-independent process [61].

How do cells transduce chemoattractant stimuli to TORC2 activation? In Dictyostelium, three recent studies provided strong evidence that a Ras family small GTPase, RasC, exerts both spatial and temporal control of TORC2 during chemotaxis [46, 57, 58]. Cells deleted of RasC exhibit attenuated TORC2 signaling. The expression of persistently activated forms of RasC prolong the time course of TORC2 mediated activation of PKBR1 and the phosphorylation of PKB substrates. Paralleling these changes pseudopodial activity is increased and mislocalized and chemotaxis is impaired [57]. Furthermore, the effects of activated RasC can be suppressed by deletion of PiaA and addition of immunopurified TORC2 to membranes containing activated RasC and PKBR1 reconstitute the phosphorylation event in the absence of chemoattractant stimuli [57]. In neutrophils, it remains a mystery how chemoattractants are relayed within the cell to activate TORC2. Chemoattractant-stimulated Ras activation has been observed in human neutrophils [62, 63] and it will be interesting to learn whether Ras plays a similar role in neutrophil chemotaxis to activate TORC2.

3.2.2 Other PIP3-independent pathways

An unbiased genetic screen in Dictyostelium for mutants that were selectively impaired in chemotaxis when PI3K was inhibited led to the identification of a gene with homology to patatin-like phospholipase A2 (PLA2) [64]. The role of this gene in chemotaxis was further supported by work using PLA2 inhibitors [65]. The simultaneous loss of PI3K and PLA2 activities causes a stronger chemotactic defect and a further reduction in receptor-mediated actin polymerization than does the loss of either activity alone, suggesting that PLA2 and PI3K may act in parallel to mediate chemotaxis [64, 65]. PLA2 specifically cleaves the second acyl chain of phospholipids to produce fatty acids, predominantly arachidonic acid (AA) and lysophospholipids. It was shown that chemoattractants stimulation leads to a rapid and transient production of AA derivatives in Dictyostelium [64]. The pathway leading to PLA2 activation and the exact function of its lipid products have yet to be determined.

In Dictyostelium cells that are allowed to progress further in their developmental program, the lack of PI3K and PLA2 activities can be compensated by a pathway involving the soluble guanylyl cyclase (sGC), which is responsible for the synthesis of the second messenger cyclic GMP (cGMP) [66]. cGMP promotes the formation of myosin filaments at the back and the lateral sides of the cell through the cGMP-binding protein GbpC [67, 68]. Cells expressing catalytically deficient sGC mutants migrate slowly and have more lateral pseudopods compared to wild-type cells.

4. A mechanistic understanding of chemotaxis

Over the years, investigations of chemotaxis in amoeboid cells have revealed some salient, consistent features of the process. First, a large portion of the signaling network downstream of the receptor and G-protein (Fig. 4) appears to act in a coordinated fashion not only when stimulated but also during spontaneous extension of pseudopod. For example, chemoattractants trigger activation of Ras proteins and PI3Ks and loss of PTEN and myosin II from the cell surface; the same events occur at the tips of pseudopodia formed spontaneously in the absence of stimuli [69, 70]. Moreover, recent studies have shown that the cortical cytoskeleton and parts of the signaling network behave as an excitable medium. Observations by total internal reflection fluorescence microscopy (TIRFM) reveal propagating waves of recruitment of actin binding proteins to the cell cortex, local Ras activation, and PIP3 accumulation [71-73]. We speculate that this excitable biochemical network underlies the random motility of cells. Second, directional sensing is linked to a general characteristic of chemotactic systems referred to as adaptation. Cells respond to increases in receptor occupancy, adapt when occupancy is held constant, and respond again when occupancy is increased further or when the stimulus is removed and reapplied after a period of deadaptation [74-76]. Most cellular responses adapt, including activation of Ras proteins, production of cAMP, cGMP, and PIP3, phosphorylation of PKB, polymerization of actin, and changes of cell shape. Third, chemoattractant-mediated responses are actually biphasic in both Dictyostelium and neutrophils (Fig. 5A) [77-79]. That is, there is an initial peak that declines sharply, followed by broader, weaker responses which occur over the next several minutes. When visualized with a PIP3 biosensor, the PH domain from CRAC (Fig. 5A), the first response is seen to occur uniformly around the perimeter and then disappear during a “cringe”. The second response consists of a series of intermittent patches at the tips of projections on the spreading cells.

Fig. 5
Chemotactic responses and the LEGI-BEN model. (A) Top: Schematic of a biphasic response triggered by a global stimulation. Bottom: A biphasic response of PIP3 production in cells expressing GFP-labeled PHcrac (B) In the LEGI model, chemotactic stimuli ...

Several general classes of models have been put forth to explain these observations [80]. The Local Excitation-Global Inhibition (LEGI) model (Fig. 5B) was proposed to explain the temporal and spatial responses of immobilized cells to chemoattractants [81]. The LEGI model accounts for perfect adaptation displayed by cells exposed to uniform step increases in chemoattractants and also explains the persistent, directional responses displayed by immobilized cells towards spatial gradients. However, it does not amplify the external gradient nor does it account for the dynamic behavior of migrating cells or the development of stable polarity. A variety of schemes have attempted to model the spontaneous acquisition of polarity and the excitability of signaling events in cells [36, 41, 71, 72, 82-85].

In the LEGI-Biased Excitable Network hypothesis (LEGI-BEN) [71], upstream signaling components including chemoattractant receptor and G-protein, and an inhibitor, read the stimulus and produce a Response Regulator (RR) as described by the LEGI module. RR serves as an input that biases a downstream excitable biochemical network that spontaneously generates pseudopodia (Fig. 5C). The time scale of the RR is a few minutes, corresponding to the rate of adaptation, whereas the time constant of the excitable network is much faster. Fig. 5D shows one simulation produced by implementation of this hypothesis. Assuming that the patches of biochemical activity are correlated with cellular projections, the LEGI-BEN model can account for most of the observed behaviors of chemotaxing cells. First, the simulated cells display spontaneous activity in the absence of stimulation. The excitable network serves as a “clock” that regulates the extension of projections. Second, the simulated cells respond and adapt to uniform stimuli and display persistent directional sensing in gradients. Third, since the RR and the excitable network have different time constants, the responses to stimuli are biphasic. Interestingly, the extraordinary sensitivity of chemotaxing cells is a natural consequence of the model since the excitable network amplifies small differences in the level of RR. Forth, alterations of the feedback loops within the excitable network can give rise to alternate behaviors. For example, enhancing the positive feedback causes spontaneous and stimulated activities to be greatly prolonged and exaggerated, a behavior that has been described for pten cells and cells expressing constitutively activated RasC and RasG [10, 24, 57].

5. Conclusion

Eukaryotic cells have developed a robust and sophisticated machinery to guide their movements in complicated environmental conditions. Multiple signaling pathways act in parallel to convert extracellular chemical gradients into localized intracellular signals. A large portion of the signaling network appears to be excitable, and this excitability underlies the random motility of cells and is biased by chemoattractants in directed migration. Notwithstanding the significant progress we have made in the study of chemotaxis, many new questions have been raised. What is the molecular mechanism of adaptation? From which point does the receptor and G-protein enter the excitable signaling network? What are the components that make up the positive and negative feedback loops? What is the mechanism of wave generation and propagation and what is the relationship between the waves and cell motility? To what extent do different signaling pathways regulate chemotaxis? It is no doubt that answers to these questions will provide further insights into this fascinating area of cell biology.


  1. Many cells have the ability to grow or migrate towards chemical cues.
  2. Chemotaxis can be conceptually divided into the processes of motility, directional sensing, and polarity.
  3. A complex signaling network converts extracellular chemical gradients into localized intracellular signals to guide chemotaxis.
  4. A large portion of the signaling network appears to be excitable, and this excitability underlies the random motility of cells and is biased by chemoattractants in directed migration.


The authors wish to acknowledge members of the Devreotes lab, in particular Yulia Artemenko for allowing the inclusion of her unpublished data. We apologize to any colleagues whose work was not mentioned due to space restriction. This work is supported by National Institutes of Health grants GM28007 and GM34933 to P.N.D. and Helen Hay Whitney postdoctoral fellowship to H.C.


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1. Laird DJ, von Andrian UH, Wagers AJ. Stem cell trafficking in tissue development, growth, and disease. Cell. 2008;132:612–30. [PubMed]
2. Jin T, Xu X, Hereld D. Chemotaxis, chemokine receptors and human disease. Cytokine. 2008;44:1–8. [PMC free article] [PubMed]
3. Condeelis J, Singer RH, Segall JE. The great escape: when cancer cells hijack the genes for chemotaxis and motility. Annu Rev Cell Dev Biol. 2005;21:695–718. [PubMed]
4. Andrew N, Insall RH. Chemotaxis in shallow gradients is mediated independently of PtdIns 3-kinase by biased choices between random protrusions. Nat Cell Biol. 2007;9:193–200. [PubMed]
5. Bosgraaf L, Van Haastert PJ. The ordered extension of pseudopodia by amoeboid cells in the absence of external cues. PLoS One. 2009;4:e5253. [PMC free article] [PubMed]
6. Swaney KF, Huang CH, Devreotes PN. Eukaryotic chemotaxis: a network of signaling pathways controls motility, directional sensing, and polarity. Annu Rev Biophys. 2010;39:265–89. [PMC free article] [PubMed]
7. Janetopoulos C, Firtel RA. Directional sensing during chemotaxis. FEBS Lett. 2008;582:2075–85. [PMC free article] [PubMed]
8. Devreotes P, Janetopoulos C. Eukaryotic chemotaxis: distinctions between directional sensing and polarization. J Biol Chem. 2003;278:20445–8. [PubMed]
9. Parent CA, Blacklock BJ, Froehlich WM, Murphy DB, Devreotes PN. G protein signaling events are activated at the leading edge of chemotactic cells. Cell. 1998;95:81–91. [PubMed]
10. Zhang S, Charest PG, Firtel RA. Spatiotemporal regulation of Ras activity provides directional sensing. Curr Biol. 2008;18:1587–93. [PMC free article] [PubMed]
11. Milne JL, Kim JY, Devreotes PN. Chemoattractant receptor signaling: G protein-dependent and -independent pathways. Adv Second Messenger Phosphoprotein Res. 1997;31:83–104. [PubMed]
12. Kim JY, Borleis JA, Devreotes PN. Switching of chemoattractant receptors programs development and morphogenesis in Dictyostelium: receptor subtypes activate common responses at different agonist concentrations. Dev Biol. 1998;197:117–28. [PubMed]
13. Chen MY, Insall RH, Devreotes PN. Signaling through chemoattractant receptors in Dictyostelium. Trends Genet. 1996;12:52–7. [PubMed]
14. Hynes TR, Tang L, Mervine SM, Sabo JL, Yost EA, Devreotes PN, et al. Visualization of G protein betagamma dimers using bimolecular fluorescence complementation demonstrates roles for both beta and gamma in subcellular targeting. J Biol Chem. 2004;279:30279–86. [PubMed]
15. Janetopoulos C, Jin T, Devreotes P. Receptor-mediated activation of heterotrimeric G-proteins in living cells. Science. 2001;291:2408–11. [PubMed]
16. Xu X, Meier-Schellersheim M, Jiao X, Nelson LE, Jin T. Quantitative imaging of single live cells reveals spatiotemporal dynamics of multistep signaling events of chemoattractant gradient sensing in Dictyostelium. Mol Biol Cell. 2005;16:676–88. [PMC free article] [PubMed]
17. Wang F. The signaling mechanisms underlying cell polarity and chemotaxis. Cold Spring Harb Perspect Biol. 2009;1:a002980. [PMC free article] [PubMed]
18. Xu J, Wang F, Van Keymeulen A, Herzmark P, Straight A, Kelly K, et al. Divergent signals and cytoskeletal assemblies regulate self-organizing polarity in neutrophils. Cell. 2003;114:201–14. [PubMed]
19. Xiao Z, Zhang N, Murphy DB, Devreotes PN. Dynamic distribution of chemoattractant receptors in living cells during chemotaxis and persistent stimulation. J Cell Biol. 1997;139:365–74. [PMC free article] [PubMed]
20. Jin T, Zhang N, Long Y, Parent CA, Devreotes PN. Localization of the G protein betagamma complex in living cells during chemotaxis. Science. 2000;287:1034–6. [PubMed]
21. Servant G, Weiner OD, Neptune ER, Sedat JW, Bourne HR. Dynamics of a chemoattractant receptor in living neutrophils during chemotaxis. Mol Biol Cell. 1999;10:1163–78. [PMC free article] [PubMed]
22. Servant G, Weiner OD, Herzmark P, Balla T, Sedat JW, Bourne HR. Polarization of chemoattractant receptor signaling during neutrophil chemotaxis. Science. 2000;287:1037–40. [PMC free article] [PubMed]
23. Huang YE, Iijima M, Parent CA, Funamoto S, Firtel RA, Devreotes P. Receptor-mediated regulation of PI3Ks confines PI(3,4,5)P3 to the leading edge of chemotaxing cells. Mol Biol Cell. 2003;14:1913–22. [PMC free article] [PubMed]
24. Iijima M, Devreotes P. Tumor suppressor PTEN mediates sensing of chemoattractant gradients. Cell. 2002;109:599–610. [PubMed]
25. Funamoto S, Meili R, Lee S, Parry L, Firtel RA. Spatial and temporal regulation of 3-phosphoinositides by PI 3-kinase and PTEN mediates chemotaxis. Cell. 2002;109:611–23. [PubMed]
26. Funamoto S, Milan K, Meili R, Firtel RA. Role of phosphatidylinositol 3′ kinase and a downstream pleckstrin homology domain-containing protein in controlling chemotaxis in dictyostelium. J Cell Biol. 2001;153:795–810. [PMC free article] [PubMed]
27. Zhou K, Pandol S, Bokoch G, Traynor-Kaplan AE. Disruption of Dictyostelium PI3K genes reduces [32P]phosphatidylinositol 3,4 bisphosphate and [32P]phosphatidylinositol trisphosphate levels, alters F-actin distribution and impairs pinocytosis. J Cell Sci. 1998;111(Pt 2):283–94. [PubMed]
28. Sasaki AT, Chun C, Takeda K, Firtel RA. Localized Ras signaling at the leading edge regulates PI3K, cell polarity, and directional cell movement. J Cell Biol. 2004;167:505–18. [PMC free article] [PubMed]
29. Iijima M, Huang YE, Luo HR, Vazquez F, Devreotes PN. Novel mechanism of PTEN regulation by its phosphatidylinositol 4,5-bisphosphate binding motif is critical for chemotaxis. J Biol Chem. 2004;279:16606–13. [PubMed]
30. Kortholt A, King JS, Keizer-Gunnink I, Harwood AJ, Van Haastert PJ. Phospholipase C regulation of phosphatidylinositol 3,4,5-trisphosphate-mediated chemotaxis. Mol Biol Cell. 2007;18:4772–9. [PMC free article] [PubMed]
31. Ferguson GJ, Milne L, Kulkarni S, Sasaki T, Walker S, Andrews S, et al. PI(3)Kgamma has an important context-dependent role in neutrophil chemokinesis. Nat Cell Biol. 2007;9:86–91. [PubMed]
32. Li Z, Hannigan M, Mo Z, Liu B, Lu W, Wu Y, et al. Directional sensing requires G beta gamma-mediated PAK1 and PIX alpha-dependent activation of Cdc42. Cell. 2003;114:215–27. [PubMed]
33. Heit B, Robbins SM, Downey CM, Guan Z, Colarusso P, Miller BJ, et al. PTEN functions to ‘prioritize’ chemotactic cues and prevent ‘distraction’ in migrating neutrophils. Nat Immunol. 2008;9:743–52. [PubMed]
34. Nishio M, Watanabe K, Sasaki J, Taya C, Takasuga S, Iizuka R, et al. Control of cell polarity and motility by the PtdIns(3,4,5)P3 phosphatase SHIP1. Nat Cell Biol. 2007;9:36–44. [PubMed]
35. Suire S, Condliffe AM, Ferguson GJ, Ellson CD, Guillou H, Davidson K, et al. Gbetagammas and the Ras binding domain of p110gamma are both important regulators of PI(3)Kgamma signalling in neutrophils. Nat Cell Biol. 2006;8:1303–9. [PubMed]
36. Weiner OD, Neilsen PO, Prestwich GD, Kirschner MW, Cantley LC, Bourne HR. A PtdInsP(3)- and Rho GTPase-mediated positive feedback loop regulates neutrophil polarity. Nat Cell Biol. 2002;4:509–13. [PMC free article] [PubMed]
37. Welch HC, Coadwell WJ, Ellson CD, Ferguson GJ, Andrews SR, Erdjument-Bromage H, et al. P-Rex1, a PtdIns(3,4,5)P3- and Gbetagamma-regulated guanine-nucleotide exchange factor for Rac. Cell. 2002;108:809–21. [PubMed]
38. Yoo SK, Deng Q, Cavnar PJ, Wu YI, Hahn KM, Huttenlocher A. Differential regulation of protrusion and polarity by PI3K during neutrophil motility in live zebrafish. Dev Cell. 2010;18:226–36. [PMC free article] [PubMed]
39. Lacalle RA, Gomez-Mouton C, Barber DF, Jimenez-Baranda S, Mira E, Martinez AC, et al. PTEN regulates motility but not directionality during leukocyte chemotaxis. J Cell Sci. 2004;117:6207–15. [PubMed]
40. Stephens L, Milne L, Hawkins P. Moving towards a better understanding of chemotaxis. Curr Biol. 2008;18:R485–94. [PubMed]
41. Wang F, Herzmark P, Weiner OD, Srinivasan S, Servant G, Bourne HR. Lipid products of PI(3)Ks maintain persistent cell polarity and directed motility in neutrophils. Nat Cell Biol. 2002;4:513–8. [PubMed]
42. Zhang P, Wang Y, Sesaki H, Iijima M. Proteomic identification of phosphatidylinositol (3,4,5) triphosphate-binding proteins in Dictyostelium discoideum. Proc Natl Acad Sci U S A. 2010;107:11829–34. [PubMed]
43. Meili R, Ellsworth C, Lee S, Reddy TB, Ma H, Firtel RA. Chemoattractant-mediated transient activation and membrane localization of Akt/PKB is required for efficient chemotaxis to cAMP in Dictyostelium. EMBO J. 1999;18:2092–105. [PubMed]
44. Kriebel PW, Barr VA, Rericha EC, Zhang G, Parent CA. Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge. J Cell Biol. 2008;183:949–61. [PMC free article] [PubMed]
45. Insall R, Kuspa A, Lilly PJ, Shaulsky G, Levin LR, Loomis WF, et al. CRAC, a cytosolic protein containing a pleckstrin homology domain, is required for receptor and G protein-mediated activation of adenylyl cyclase in Dictyostelium. J Cell Biol. 1994;126:1537–45. [PMC free article] [PubMed]
46. Kamimura Y, Xiong Y, Iglesias PA, Hoeller O, Bolourani P, Devreotes PN. PIP3-independent activation of TorC2 and PKB at the cell's leading edge mediates chemotaxis. Curr Biol. 2008;18:1034–43. [PMC free article] [PubMed]
47. Tang M, Iijima M, Kamimura Y, Chen L, Long Y, Devreotes P. Disruption of PKB signaling restores polarity to cells lacking tumor suppressor PTEN. Mol Biol Cell. 2011;22:437–47. [PMC free article] [PubMed]
48. Park WS, Heo WD, Whalen JH, O'Rourke NA, Bryan HM, Meyer T, et al. Comprehensive identification of PIP3-regulated PH domains from C. elegans to H. sapiens by model prediction and live imaging. Mol Cell. 2008;30:381–92. [PMC free article] [PubMed]
49. Krugmann S, Anderson KE, Ridley SH, Risso N, McGregor A, Coadwell J, et al. Identification of ARAP3, a novel PI3K effector regulating both Arf and Rho GTPases, by selective capture on phosphoinositide affinity matrices. Mol Cell. 2002;9:95–108. [PubMed]
50. Loovers HM, Postma M, Keizer-Gunnink I, Huang YE, Devreotes PN, van Haastert PJ. Distinct roles of PI(3,4,5)P3 during chemoattractant signaling in Dictyostelium: a quantitative in vivo analysis by inhibition of PI3-kinase. Mol Biol Cell. 2006;17:1503–13. [PMC free article] [PubMed]
51. Hoeller O, Kay RR. Chemotaxis in the absence of PIP3 gradients. Curr Biol. 2007;17:813–7. [PubMed]
52. Jacinto E, Loewith R, Schmidt A, Lin S, Ruegg MA, Hall A, et al. Mammalian TOR complex 2 controls the actin cytoskeleton and is rapamycin insensitive. Nat Cell Biol. 2004;6:1122–8. [PubMed]
53. Sarbassov DD, Ali SM, Kim DH, Guertin DA, Latek RR, Erdjument-Bromage H, et al. Rictor, a novel binding partner of mTOR, defines a rapamycin-insensitive and raptor-independent pathway that regulates the cytoskeleton. Curr Biol. 2004;14:1296–302. [PubMed]
54. Schmidt A, Kunz J, Hall MN. TOR2 is required for organization of the actin cytoskeleton in yeast. Proc Natl Acad Sci U S A. 1996;93:13780–5. [PubMed]
55. Cybulski N, Hall MN. TOR complex 2: a signaling pathway of its own. Trends Biochem Sci. 2009;34:620–7. [PubMed]
56. Chen MY, Long Y, Devreotes PN. A novel cytosolic regulator, Pianissimo, is required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase in Dictyostelium. Genes Dev. 1997;11:3218–31. [PubMed]
57. Cai H, Das S, Kamimura Y, Long Y, Parent CA, Devreotes PN. Ras-mediated activation of the TORC2-PKB pathway is critical for chemotaxis. J Cell Biol. 2010;190:233–45. [PMC free article] [PubMed]
58. Charest PG, Shen Z, Lakoduk A, Sasaki AT, Briggs SP, Firtel RA. A Ras signaling complex controls the RasC-TORC2 pathway and directed cell migration. Dev Cell. 2010;18:737–49. [PMC free article] [PubMed]
59. Lee S, Comer FI, Sasaki A, McLeod IX, Duong Y, Okumura K, et al. TOR complex 2 integrates cell movement during chemotaxis and signal relay in Dictyostelium. Mol Biol Cell. 2005;16:4572–83. [PMC free article] [PubMed]
60. Meili R, Ellsworth C, Firtel RA. A novel Akt/PKB-related kinase is essential for morphogenesis in Dictyostelium. Curr Biol. 2000;10:708–17. [PubMed]
61. Liu L, Das S, Losert W, Parent CA. mTORC2 regulates neutrophil chemotaxis in a cAMP-and RhoA-dependent fashion. Dev Cell. 2010;19:845–57. [PMC free article] [PubMed]
62. Worthen GS, Avdi N, Buhl AM, Suzuki N, Johnson GL. FMLP activates Ras and Raf in human neutrophils. Potential role in activation of MAP kinase. J Clin Invest. 1994;94:815–23. [PMC free article] [PubMed]
63. Zheng L, Eckerdal J, Dimitrijevic I, Andersson T. Chemotactic peptide-induced activation of Ras in human neutrophils is associated with inhibition of p120-GAP activity. J Biol Chem. 1997;272:23448–54. [PubMed]
64. Chen L, Iijima M, Tang M, Landree MA, Huang YE, Xiong Y, et al. PLA2 and PI3K/PTEN pathways act in parallel to mediate chemotaxis. Dev Cell. 2007;12:603–14. [PMC free article] [PubMed]
65. van Haastert PJ, Keizer-Gunnink I, Kortholt A. Essential role of PI3-kinase and phospholipase A2 in Dictyostelium discoideum chemotaxis. J Cell Biol. 2007;177:809–16. [PMC free article] [PubMed]
66. Veltman DM, Keizer-Gunnik I, Van Haastert PJ. Four key signaling pathways mediating chemotaxis in Dictyostelium discoideum. J Cell Biol. 2008;180:747–53. [PMC free article] [PubMed]
67. Bosgraaf L, Russcher H, Smith JL, Wessels D, Soll DR, Van Haastert PJ. A novel cGMP signalling pathway mediating myosin phosphorylation and chemotaxis in Dictyostelium. Embo J. 2002;21:4560–70. [PubMed]
68. Bosgraaf L, Waijer A, Engel R, Visser AJ, Wessels D, Soll D, et al. RasGEF-containing proteins GbpC and GbpD have differential effects on cell polarity and chemotaxis in Dictyostelium. J Cell Sci. 2005;118:1899–910. [PubMed]
69. Sasaki AT, Janetopoulos C, Lee S, Charest PG, Takeda K, Sundheimer LW, et al. G protein-independent Ras/PI3K/F-actin circuit regulates basic cell motility. J Cell Biol. 2007;178:185–91. [PMC free article] [PubMed]
70. Pramanik MK, Iijima M, Iwadate Y, Yumura S. PTEN is a mechanosensing signal transducer for myosin II localization in Dictyostelium cells. Genes Cells. 2009;14:821–34. [PubMed]
71. Xiong Y, Huang CH, Iglesias PA, Devreotes PN. Cells navigate with a local-excitation, global-inhibition-biased excitable network. Proc Natl Acad Sci U S A. 2010;107:17079–86. [PubMed]
72. Weiner OD, Marganski WA, Wu LF, Altschuler SJ, Kirschner MW. An actin-based wave generator organizes cell motility. PLoS Biol. 2007;5:e221. [PubMed]
73. Gerisch G, Bretschneider T, Muller-Taubenberger A, Simmeth E, Ecke M, Diez S, et al. Mobile actin clusters and traveling waves in cells recovering from actin depolymerization. Biophys J. 2004;87:3493–503. [PubMed]
74. Dinauer MC, Steck TL, Devreotes PN. Cyclic 3′,5′-AMP relay in Dictyostelium discoideum V. Adaptation of the cAMP signaling response during cAMP stimulation. J Cell Biol. 1980;86:554–61. [PMC free article] [PubMed]
75. Dinauer MC, Steck TL, Devreotes PN. Cyclic 3′,5′-AMP relay in Dictyostelium discoideum IV. Recovery of the cAMP signaling response after adaptation to cAMP. J Cell Biol. 1980;86:545–53. [PMC free article] [PubMed]
76. Zigmond SH, Sullivan SJ. Sensory adaptation of leukocytes to chemotactic peptides. J Cell Biol. 1979;82:517–27. [PMC free article] [PubMed]
77. Chen L, Janetopoulos C, Huang YE, Iijima M, Borleis J, Devreotes PN. Two phases of actin polymerization display different dependencies on PI(3,4,5)P3 accumulation and have unique roles during chemotaxis. Mol Biol Cell. 2003;14:5028–37. [PMC free article] [PubMed]
78. Postma M, Roelofs J, Goedhart J, Gadella TW, Visser AJ, Van Haastert PJ. Uniform cAMP stimulation of Dictyostelium cells induces localized patches of signal transduction and pseudopodia. Mol Biol Cell. 2003;14:5019–27. [PMC free article] [PubMed]
79. Norgauer J, Krutmann J, Dobos GJ, Traynor-Kaplan AE, Oades ZG, Schraufstatter IU. Actin polymerization, calcium-transients, and phospholipid metabolism in human neutrophils after stimulation with interleukin-8 and N-formyl peptide. J Invest Dermatol. 1994;102:310–4. [PubMed]
80. Iglesias PA, Devreotes PN. Navigating through models of chemotaxis. Curr Opin Cell Biol. 2008;20:35–40. [PubMed]
81. Kutscher B, Devreotes P, Iglesias PA. Local excitation, global inhibition mechanism for gradient sensing: an interactive applet. Sci STKE. 2004;2004:pl3. [PubMed]
82. Bretschneider T, Anderson K, Ecke M, Muller-Taubenberger A, Schroth-Diez B, Ishikawa-Ankerhold HC, et al. The three-dimensional dynamics of actin waves, a model of cytoskeletal self-organization. Biophys J. 2009;96:2888–900. [PubMed]
83. Hecht I, Kessler DA, Levine H. Transient localized patterns in noise-driven reaction-diffusion systems. Phys Rev Lett. 2010;104:158301. [PMC free article] [PubMed]
84. Meinhardt H. Orientation of chemotactic cells and growth cones: models and mechanisms. J Cell Sci. 1999;112(Pt 17):2867–74. [PubMed]
85. Inoue T, Meyer T. Synthetic activation of endogenous PI3K and Rac identifies an AND-gate switch for cell polarization and migration. PLoS One. 2008;3:e3068. [PMC free article] [PubMed]
86. Kamimura Y, Devreotes PN. Phosphoinositide-dependent protein kinase (PDK) activity regulates phosphatidylinositol 3,4,5-trisphosphate-dependent and -independent protein kinase B activation and chemotaxis. J Biol Chem. 2010;285:7938–46. [PMC free article] [PubMed]
87. Jeon TJ, Lee DJ, Merlot S, Weeks G, Firtel RA. Rap1 controls cell adhesion and cell motility through the regulation of myosin II. J Cell Biol. 2007;176:1021–33. [PMC free article] [PubMed]
88. Mondal S, Bakthavatsalam D, Steimle P, Gassen B, Rivero F, Noegel AA. Linking Ras to myosin function: RasGEF Q, a Dictyostelium exchange factor for RasB, affects myosin II functions. J Cell Biol. 2008;181:747–60. [PMC free article] [PubMed]