Role of Phosphatidylinositol 3-Kinases in Chemotaxis in Dictyostelium

A meta-analysis indicates that the regulation of cell motility is a non-intrinsic function of chemoattractant receptors that is governed independently of directional sensing

Chemoattraction, defined as the migration of a cell toward a source of a chemical gradient, is controlled by chemoattractant receptors. Chemoattraction involves two basic activities, namely, directional sensing, a molecular mechanism that detects the direction of a source of chemoattractant, and actin-based motility, which allows the migration of a cell towards it. Current models assume first, that chemoattractant receptors govern both directional sensing and motility (most commonly inducing an increase in the migratory speed of the cells, i.e. chemokinesis), and, second, that the signaling pathways controlling both activities are intertwined. We performed a meta-analysis to reassess these two points. From this study emerge two main findings. First, although many chemoattractant receptors govern directional sensing, there are also receptors that do not regulate cell motility, suggesting that is the ability to control directional sensing, not motility, that best defines a chemoattractant receptor. Second, multiple experimental data suggest that receptor-controlled directional sensing and motility can be controlled independently. We hypothesize that this independence may be based on the existence of separated signalling modules that selectively govern directional sensing and motility in chemotactic cells. Together, the information gathered can be useful to update current models representing the signalling from chemoattractant receptors. The new models may facilitate the development of strategies for a more effective pharmacological modulation of chemoattractant receptor-controlled chemoattraction in health and disease.

Spatiotemporal dynamics of membrane surface charge regulates cell polarity and migration

During cell migration and polarization, numerous signal transduction and cytoskeletal components self-organize to generate localized protrusions. Although biochemical and genetic analyses have delineated many specific interactions, how the activation and localization of so many different molecules are spatiotemporally orchestrated at the subcellular level has remained unclear. Here we show that the regulation of negative surface charge on the inner leaflet of the plasma membrane plays an integrative role in the molecular interactions. Surface charge, or zeta potential, is transiently lowered at new protrusions and within cortical waves of Ras/PI3K/TORC2/F-actin network activation. Rapid alterations of inner leaflet anionic phospholipids-such as PI(4,5)P2, PI(3,4)P2, phosphatidylserine and phosphatidic acid-collectively contribute to the surface charge changes. Abruptly reducing the surface charge by recruiting positively charged optogenetic actuators was sufficient to trigger the entire biochemical network, initiate de novo protrusions and abrogate pre-existing polarity. These effects were blocked by genetic or pharmacological inhibition of key signalling components such as AKT and PI3K/TORC2. Conversely, increasing the negative surface charge deactivated the network and locally suppressed chemoattractant-induced protrusions or subverted EGF-induced ERK activation. Computational simulations involving excitable biochemical networks demonstrated that slight changes in feedback loops, induced by recruitment of the charged actuators, could lead to outsized effects on system activation. We propose that key signalling network components act on, and are in turn acted upon, by surface charge, closing feedback loops, which bring about the global-scale molecular self-organization required for spontaneous protrusion formation, cell migration and polarity establishment.

Identification of novel proteins in the Dictyostelium discoideum chemorepulsion pathway using REMI

Chemorepulsion, the biased migration of a cell away from a signal, is essential for many biological processes and the ability to manipulate chemorepulsion could lead to new therapeutics for a variety of diseases. However, little is known about eukaryotic cell chemorepulsion. Utilizing the model organism Dictyostelium discoideum, we previously identified an endogenous chemorepellent protein secreted by D. discoideum cells called AprA, and proteins involved in the AprA-induced chemorepulsion pathway including the G protein-coupled receptor GrlH, G beta and G protein alpha 8 protein subunits, protein kinase A, components of the mammalian target of rapamycin complex 2 (mTORC2), phospholipase A, PTEN and a PTEN-like phosphatase (CnrN), a retinoblastoma orthologue (RblA), extracellular signal-regulated kinase 1 (Erk1), p-21 activated protein kinase D (PakD), and the Ras proteins RasC and RasG. In this report, we used a genetic screen to identify 17 additional proteins involved in the AprA-induced chemorepulsion pathway .

Monitoring Phosphoinositide Fluxes and Effectors During Leukocyte Chemotaxis and Phagocytosis

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

Ras, PI3K and mTORC2 - three's a crowd?

The Ras oncogene is notoriously difficult to target with specific therapeutics. Consequently, there is interest to better understand the Ras signaling pathways to identify potential targetable effectors. Recently, the mechanistic target of rapamycin complex 2 (mTORC2) was identified as an evolutionarily conserved Ras effector. mTORC2 regulates essential cellular processes, including metabolism, survival, growth, proliferation and migration. Moreover, increasing evidence implicate mTORC2 in oncogenesis. Little is known about the regulation of mTORC2 activity, but proposed mechanisms include a role for phosphatidylinositol (3,4,5)-trisphosphate - which is produced by class I phosphatidylinositol 3-kinases (PI3Ks), well-characterized Ras effectors. Therefore, the relationship between Ras, PI3K and mTORC2, in both normal physiology and cancer is unclear; moreover, seemingly conflicting observations have been reported. Here, we review the evidence on potential links between Ras, PI3K and mTORC2. Interestingly, data suggest that Ras and PI3K are both direct regulators of mTORC2 but that they act on distinct pools of mTORC2: Ras activates mTORC2 at the plasma membrane, whereas PI3K activates mTORC2 at intracellular compartments. Consequently, we propose a model to explain how Ras and PI3K can differentially regulate mTORC2, and highlight the diversity in the mechanisms of mTORC2 regulation, which appear to be determined by the stimulus, cell type, and the molecularly and spatially distinct mTORC2 pools.

The Roles of Signaling in Cytoskeletal Changes, Random Movement, Direction-Sensing and Polarization of Eukaryotic Cells

Movement of cells and tissues is essential at various stages during the lifetime of an organism, including morphogenesis in early development, in the immune response to pathogens, and during wound-healing and tissue regeneration. Individual cells are able to move in a variety of microenvironments (MEs) (A glossary of the acronyms used herein is given at the end) by suitably adapting both their shape and how they transmit force to the ME, but how cells translate environmental signals into the forces that shape them and enable them to move is poorly understood. While many of the networks involved in signal detection, transduction and movement have been characterized, how intracellular signals control re-building of the cyctoskeleton to enable movement is not understood. In this review we discuss recent advances in our understanding of signal transduction networks related to direction-sensing and movement, and some of the problems that remain to be solved.

How cortical waves drive fission of motile cells

Cytokinesis-the division of a cell into two daughter cells-is a key step in cell growth and proliferation. It typically occurs in synchrony with the cell cycle to ensure that a complete copy of the genetic information is passed on to the next generation of daughter cells. In animal cells, cytokinesis commonly relies on an actomyosin contractile ring that drives equatorial furrowing and separation into the two daughter cells. However, also contractile ring-independent forms of cell division are known that depend on substrate-mediated traction forces. Here, we report evidence of an as yet unknown type of contractile ring-independent cytokinesis that we termed wave-mediated cytofission. It is driven by self-organized cortical actin waves that travel across the ventral membrane of oversized, multinucleated Dictyostelium discoideum cells. Upon collision with the cell border, waves may initiate the formation of protrusions that elongate and eventually pinch off to form separate daughter cells. They are composed of a stable elongated wave segment that is enclosed by a cell membrane and moves in a highly persistent fashion. We rationalize our observations based on a noisy excitable reaction-diffusion model in combination with a dynamic phase field to account for the cell shape and demonstrate that daughter cells emerging from wave-mediated cytofission exhibit a well-controlled size.

Caffeine inhibits PI3K and mTORC2 in Dictyostelium and differentially affects multiple other cAMP chemoattractant signaling effectors

Caffeine is commonly used in Dictyostelium to inhibit the synthesis of the chemoattractant cAMP and, therefore, its secretion and the autocrine stimulation of cells, in order to prevent its interference with the study of chemoattractant-induced responses. However, the mechanism through which caffeine inhibits cAMP synthesis in Dictyostelium has not been characterized. Here, we report the effects of caffeine on the cAMP chemoattractant signaling network. We found that caffeine inhibits phosphatidylinositol 3-kinase (PI3K) and mechanistic target of rapamycin complex 2 (mTORC2). Both PI3K and mTORC2 are essential for the chemoattractant-stimulated cAMP production, thereby providing a mechanism for the caffeine-mediated inhibition of cAMP synthesis. Our results also reveal that caffeine treatment of cells leads to an increase in cAMP-induced RasG and Rap1 activation, and inhibition of the PKA, cGMP, MyoII, and ERK1 responses. Finally, we observed that caffeine has opposite effects on F-actin and ERK2 depending on the assay and Dictyostelium strain used, respectively. Altogether, our findings reveal that caffeine considerably affects the cAMP-induced chemotactic signaling pathways in Dictyostelium, most likely acting through multiple targets that include PI3K and mTORC2.

Excitable dynamics of Ras triggers spontaneous symmetry breaking of PIP3 signaling in motile cells

Spontaneous cell movement is underpinned by an asymmetric distribution of signaling molecules including small G proteins and phosphoinositides on the cell membrane. However, the molecular network necessary for spontaneous symmetry breaking has not been fully elucidated. Here, we report that, in Dictyostelium discoideum, the spatiotemporal dynamics of GTP bound Ras (Ras-GTP) breaks the symmetry due its intrinsic excitability even in the absence of extracellular spatial cues and downstream signaling activities. A stochastic excitation of local and transient Ras activation induced phosphatidylinositol (3,4,5)-trisphosphate (PIP3) accumulation via direct interaction with Phosphoinositide 3-kinase (PI3K), causing tightly coupled traveling waves that propagated along the membrane. Comprehensive phase analysis of the waves of Ras-GTP and PIP3 metabolism-related molecules revealed the network structure of the excitable system including positive-feedback regulation of Ras-GTP by the downstream PIP3. A mathematical model reconstituted a series of the observed symmetry-breaking phenomena, illustrating the essential involvement of Ras excitability in the cellular decision-making process.

The Dictyostelium GSK3 kinase GlkA coordinates signal relay and chemotaxis in response to growth conditions

GSK3 plays a central role in orchestrating key biological signaling pathways, including cell migration. Here, we identify GlkA as a GSK3 family kinase with functions that overlap with and are distinct from those of GskA. We show that GlkA, as previously shown for GskA, regulates the cell's cytoskeleton through MyoII assembly and control of Ras and Rap1 function, leading to aberrant cell migration. However, there are both qualitative and quantitative differences in the regulation of Ras and Rap1 and their downstream effectors, including PKB, PKBR1, and PI3K, with glkA^- cells exhibiting a more severe chemotaxis phenotype than gskA^- cells. Unexpectedly, the severe glkA^- phenotypes, but not those of gskA^-, are only exhibited when cells are grown attached to a substratum but not in suspension, suggesting that GlkA functions as a key kinase of cell attachment signaling. Using proteomic iTRAQ analysis we show that there are quantitative differences in the pattern of protein expression depending on the growth conditions in wild-type cells. We find that GlkA expression affects the cell's proteome during vegetative growth and development, with many of these changes depending on whether the cells are grown attached to a substratum or in suspension. These changes include key cytoskeletal and signaling proteins known to be essential for proper chemotaxis and signal relay during the aggregation stage of Dictyostelium development.

Excitable Signal Transduction Networks in Directed Cell Migration

Although directed migration of eukaryotic cells may have evolved to escape nutrient depletion, it has been adopted for an extensive range of physiological events during development and in the adult organism. The subversion of these movements results in disease, such as cancer. Mechanisms of propulsion and sensing are extremely diverse, but most eukaryotic cells move by extending actin-filled protrusions termed macropinosomes, pseudopodia, or lamellipodia or by extension of blebs. In addition to motility, directed migration involves polarity and directional sensing. The hundreds of gene products involved in these processes are organized into networks of parallel and interconnected pathways. Many of these components are activated or inhibited coordinately with stimulation and on each spontaneously extended protrusion. Moreover, these networks display hallmarks of excitability, including all-or-nothing responsiveness and wave propagation. Cellular protrusions result from signal transduction waves that propagate outwardly from an origin and drive cytoskeletal activity. The range of the propagating waves and hence the size of the protrusions can be altered by lowering or raising the threshold for network activation, with larger and wider protrusions favoring gliding or oscillatory behavior over amoeboid migration. Here, we evaluate the variety of models of excitable networks controlling directed migration and outline critical tests. We also discuss the utility of this emerging view in producing cell migration and in integrating the various extrinsic cues that direct migration.

Role of the small GTPase Rap1 in signal transduction, cell dynamics and bacterial infection

Rap1 belongs to the Ras family of small GTPases, which are involved in a multitude of cellular signal transduction pathways and have extensively been linked to cancer biogenesis and metastasis. The small GTPase is activated in response to various extracellular and intracellular cues. Rap1 has conserved functions in Dictyostelium discoideum amoeba and mammalian cells, which are important for cell polarity, substrate and cell-cell adhesion and other processes that involve the regulation of cytoskeletal dynamics. Moreover, our recent study has shown that Rap1 is required for the formation of the replication-permissive vacuole of an intracellular bacterial pathogen. Here we review the function and regulation of Rap1 in these distinct processes, and we discuss the underlying signal transduction pathways.

A Gα-Stimulated RapGEF Is a Receptor-Proximal Regulator of Dictyostelium Chemotaxis

Chemotaxis, or directional movement toward extracellular chemical gradients, is an important property of cells that is mediated through G-protein-coupled receptors (GPCRs). Although many chemotaxis pathways downstream of Gβγ have been identified, few Gα effectors are known. Gα effectors are of particular importance because they allow the cell to distinguish signals downstream of distinct chemoattractant GPCRs. Here we identify GflB, a Gα2 binding partner that directly couples the Dictyostelium cyclic AMP GPCR to Rap1. GflB localizes to the leading edge and functions as a Gα-stimulated, Rap1-specific guanine nucleotide exchange factor required to balance Ras and Rap signaling. The kinetics of GflB translocation are fine-tuned by GSK-3 phosphorylation. Cells lacking GflB display impaired Rap1/Ras signaling and actin and myosin dynamics, resulting in defective chemotaxis. Our observations demonstrate that GflB is an essential upstream regulator of chemoattractant-mediated cell polarity and cytoskeletal reorganization functioning to directly link Gα activation to monomeric G-protein signaling.

Signaling in chemotactic amoebae remains spatially confined to stimulated membrane regions

Recent work has demonstrated that the receptor-mediated signaling system in chemotactic amoeboid cells shows typical properties of an excitable system. Here, we delivered spatially confined stimuli of the chemoattractant cAMP to the membrane of differentiated Dictyostelium discoideum cells to investigate whether localized receptor stimuli can induce the spreading of excitable waves in the G-protein-dependent signal transduction system. By imaging the spatiotemporal dynamics of fluorescent markers for phosphatidylinositol (3,4,5)-trisphosphate (PIP₃), PTEN and filamentous actin, we observed that the activity of the signaling pathway remained spatially confined to the stimulated membrane region. Neighboring parts of the membrane were not excited and no receptor-initiated spatial spreading of excitation waves was observed. To generate localized cAMP stimuli, either particles that carried covalently bound cAMP molecules on their surface were brought into contact with the cell or a patch of the cell membrane was aspirated into a glass micropipette to shield this patch against freely diffusing cAMP molecules in the surrounding medium. Additionally, the binding site of the cAMP receptor was probed with different surface-immobilized cAMP molecules, confirming results from earlier ligand-binding studies.

Dictyostelium uses ether-linked inositol phospholipids for intracellular signalling

Inositol phospholipids are critical regulators of membrane biology throughout eukaryotes. The general principle by which they perform these roles is conserved across species and involves binding of differentially phosphorylated inositol head groups to specific protein domains. This interaction serves to both recruit and regulate the activity of several different classes of protein which act on membrane surfaces. In mammalian cells, these phosphorylated inositol head groups are predominantly borne by a C38:4 diacylglycerol backbone. We show here that the inositol phospholipids of Dictyostelium are different, being highly enriched in an unusual C34:1e lipid backbone, 1-hexadecyl-2-(11Z-octadecenoyl)-sn-glycero-3-phospho-(1'-myo-inositol), in which the sn-1 position contains an ether-linked C16:0 chain; they are thus plasmanylinositols. These plasmanylinositols respond acutely to stimulation of cells with chemoattractants, and their levels are regulated by PIPKs, PI3Ks and PTEN. In mammals and now in Dictyostelium, the hydrocarbon chains of inositol phospholipids are a highly selected subset of those available to other phospholipids, suggesting that different molecular selectors are at play in these organisms but serve a common, evolutionarily conserved purpose.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Degradation of activated K-Ras orthologue via K-Ras-specific lysine residues is required for cytokinesis

Mammalian cells encode three closely related Ras proteins, H-Ras, N-Ras, and K-Ras. Oncogenic K-Ras mutations frequently occur in human cancers, which lead to dysregulated cell proliferation and genomic instability. However, mechanistic role of the Ras isoform regulation have remained largely unknown. Furthermore, the dynamics and function of negative regulation of GTP-loaded K-Ras have not been fully investigated. Here, we demonstrate RasG, the Dictyostelium orthologue of K-Ras, is targeted for degradation by polyubiquitination. Both ubiquitination and degradation of RasG were strictly associated with RasG activity. High resolution tandem mass spectrometry (LC-MS/MS) analysis indicated that RasG ubiquitination occurs at C-terminal lysines equivalent to lysines found in human K-Ras but not in H-Ras and N-Ras homologues. Substitution of these lysine residues with arginines (4KR-RasG) diminished RasG ubiquitination and increased RasG protein stability. Cells expressing 4KR-RasG failed to undergo proper cytokinesis and resulted in multinucleated cells. Ectopically expressed human K-Ras undergoes polyubiquitin-mediated degradation in Dictyostelium, whereas human H-Ras and a Dictyostelium H-Ras homologue (RasC) are refractory to ubiquitination. Our results indicate the existence of GTP-loaded K-Ras orthologue-specific degradation system in Dictyostelium, and further identification of the responsible E3-ligase may provide a novel therapeutic approach against K-Ras-mutated cancers.

A novel human receptor involved in bitter tastant detection identified using Dictyostelium discoideum

Detection of substances tasting bitter to humans occurs in diverse organisms including the social amoeba Dictyostelium discoideum. To establish a molecular mechanism for bitter tastant detection in Dictyostelium, we screened a mutant library for resistance to a commonly used bitter standard, phenylthiourea. This approach identified a G-protein-coupled receptor mutant, grlJ(-), which showed a significantly increased tolerance to phenylthiourea in growth, survival and movement. This mutant was not resistant to a structurally dissimilar potent bitter tastant, denatonium benzoate, suggesting it is not a target for at least one other bitter tastant. Analysis of the cell-signalling pathway involved in the detection of phenylthiourea showed dependence upon heterotrimeric G protein and phosphatidylinositol 3-kinase activity, suggesting that this signalling pathway is responsible for the cellular effects of phenylthiourea. This is further supported by a phenylthiourea-dependent block in the transient cAMP-induced production of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in wild-type but not grlJ(-) cells. Finally, we have identified an uncharacterized human protein γ-aminobutyric acid (GABA) type B receptor subunit 1 isoform with weak homology to GrlJ that restored grlJ(-) sensitivity to phenylthiourea in cell movement and PIP3 regulation. Our results thus identify a novel pathway for the detection of the standard bitter tastant phenylthiourea in Dictyostelium and implicate a poorly characterized human protein in phenylthiourea-dependent cell responses.

Two distinct functions for PI3-kinases in macropinocytosis

Class-1 PI3-kinases are major regulators of the actin cytoskeleton, whose precise contributions to chemotaxis, phagocytosis and macropinocytosis remain unresolved. We used systematic genetic ablation to examine this question in growing Dictyostelium cells. Mass spectroscopy shows that a quintuple mutant lacking the entire genomic complement of class-1 PI3-kinases retains only 10% of wild-type PtdIns(3,4,5)P3 levels. Chemotaxis to folate and phagocytosis of bacteria proceed normally in the quintuple mutant but macropinocytosis is abolished. In this context PI3-kinases show specialized functions, only one of which is directly linked to gross PtdIns(3,4,5)P3 levels: macropinosomes originate in patches of PtdIns(3,4,5)P3, with associated F-actin-rich ruffles, both of which depend on PI3-kinase 1/2 (PI3K1/2) but not PI3K4, whereas conversion of ruffles into vesicles requires PI3K4. A biosensor derived from the Ras-binding domain of PI3K1 suggests that Ras is activated throughout vesicle formation. Binding assays show that RasG and RasS interact most strongly with PI3K1/2 and PI3K4, and single mutants of either Ras have severe macropinocytosis defects. Thus, the fundamental function of PI3-kinases in growing Dictyostelium cells is in macropinocytosis where they have two distinct functions, supported by at least two separate Ras proteins.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

The green tea catechin epigallocatechin gallate (EGCG) blocks cell motility, chemotaxis and development in Dictyostelium discoideum

Catechins, flavanols found at high levels in green tea, have received significant attention due to their potential health benefits related to cancer, autoimmunity and metabolic disease, but little is known about the mechanisms by which these compounds affect cellular behavior. Here, we assess whether the model organism Dictyostelium discoideum is a useful tool with which to characterize the effects of catechins. Epigallocatechin gallate (EGCG), the most abundant and potent catechin in green tea, has significant effects on the Dictyostelium life cycle. In the presence of EGCG aggregation is delayed, cells do not stream and development is typically stalled at the loose aggregate stage. The developmental effects very likely result from defects in motility, as EGCG reduces both random movement and chemotaxis of Dictyostelium amoebae. These results suggest that catechins and their derivatives may be useful tools with which to better understand cell motility and development in Dictyostelium and that this organism is a useful model to further characterize the activities of catechins.

Phosphorylation of actin-related protein 2 (Arp2) is required for normal development and cAMP chemotaxis in Dictyostelium

Phosphorylation of the actin-related protein 2 (Arp2) subunit of the Arp2/3 complex on evolutionarily conserved threonine and tyrosine residues was recently identified and shown to be necessary for nucleating activity of the Arp2/3 complex and membrane protrusion of Drosophila cells. Here we use the Dictyostelium diploid system to replace the essential Arp2 protein with mutants that cannot be phosphorylated at Thr-235/6 and Tyr-200. We found that aggregation of the resulting mutant cells after starvation was substantially slowed with delayed early developmental gene expression and that chemotaxis toward a cAMP gradient was defective with loss of polarity and attenuated F-actin assembly. Chemotaxis toward cAMP was also diminished with reduced cell speed and directionality and shorter pseudopod lifetime when Arp2 phosphorylation mutant cells were allowed to develop longer to a responsive state similar to that of wild-type cells. However, clathrin-mediated endocytosis and chemotaxis under agar to folate in vegetative cells were only subtly affected in Arp2 phosphorylation mutants. Thus, phosphorylation of threonine and tyrosine is important for a subset of the functions of the Arp2/3 complex, in particular an unexpected major role in regulating development.

A matricellular protein and EGF-like repeat signalling in the social amoebozoan Dictyostelium discoideum

Matricellular proteins interact with the extracellular matrix (ECM) and modulate cellular processes by binding to cell surface receptors and initiating intracellular signal transduction. Their association with the ECM and the ability of some members of this protein family to regulate cell motility have opened up new avenues of research to investigate their functions in normal and diseased cells. In this review, we summarize the research on CyrA, an ECM calmodulin-binding protein in Dictyostelium. CyrA is proteolytically cleaved into smaller EGF-like (EGFL) repeat containing cleavage products during development. The first EGFL repeat of CyrA binds to the cell surface and activates a novel signalling pathway that modulates cell motility in this model organism. The similarity of CyrA to the most well-characterized matricellular proteins in mammals allows it to be designated as the first matricellular protein identified in Dictyostelium.

Daydreamer, a Ras effector and GSK-3 substrate, is important for directional sensing and cell motility

Daydreamer (DydA), a new Mig10/RIAM/lamellipodin family adaptor protein, is a Ras effector required for cell polarization and directional movement during chemotaxis. DydA is phosphorylated by glycogen synthase kinase-3, which is required for some, but not all, of DydA's functions. gskA⁻ cells exhibit very strong chemotactic phenotypes, a subset of which are exhibited by dydA⁻ cells.

How independent signaling pathways are integrated to holistically control a biological process is not well understood. We have identified Daydreamer (DydA), a new member of the Mig10/RIAM/lamellipodin (MRL) family of adaptor proteins that localizes to the leading edge of the cell. DydA is a putative Ras effector that is required for cell polarization and directional movement during chemotaxis. dydA⁻ cells exhibit elevated F-actin and assembled myosin II (MyoII), increased and extended phosphoinositide-3-kinase (PI3K) activity, and extended phosphorylation of the activation loop of PKB and PKBR1, suggesting that DydA is involved in the negative regulation of these pathways. DydA is phosphorylated by glycogen synthase kinase-3 (GSK-3), which is required for some, but not all, of DydA's functions, including the proper regulation of PKB and PKBR1 and MyoII assembly. gskA⁻ cells exhibit very strong chemotactic phenotypes, as previously described, but exhibit an increased rate of random motility. gskA⁻ cells have a reduced MyoII response and a reduced level of phosphatidylinositol (3,4,5)-triphosphate production, but a highly extended recruitment of PI3K to the plasma membrane and highly extended kinetics of PKB and PKBR1 activation. Our results demonstrate that GSK-3 function is essential for chemotaxis, regulating multiple substrates, and that one of these effectors, DydA, plays a key function in the dynamic regulation of chemotaxis.

Delineating the core regulatory elements crucial for directed cell migration by examining folic-acid-mediated responses

Dictyostelium discoideum shows chemotaxis towards folic acid (FA) throughout vegetative growth, and towards cAMP during development. We determined the spatiotemporal localization of cytoskeletal and signaling molecules and investigated the FA-mediated responses in a number of signaling mutants to further our understanding of the core regulatory elements that are crucial for cell migration. Proteins enriched in the pseudopods during chemotaxis also relocalize transiently to the plasma membrane during uniform FA stimulation. In contrast, proteins that are absent from the pseudopods during migration redistribute transiently from the PM to the cytosol when cells are globally stimulated with FA. These chemotactic responses to FA were also examined in cells lacking the GTPases Ras C and G. Although Ras and phosphoinositide 3-kinase activity were significantly decreased in Ras G and Ras C/G nulls, these mutants still migrated towards FA, indicating that other pathways must support FA-mediated chemotaxis. We also examined the spatial movements of PTEN in response to uniform FA and cAMP stimulation in phospholipase C (PLC) null cells. The lack of PLC strongly influences the localization of PTEN in response to FA, but not cAMP. In addition, we compared the gradient-sensing behavior of polarized cells migrating towards cAMP to that of unpolarized cells migrating towards FA. The majority of polarized cells make U-turns when the cAMP gradient is switched from the front of the cell to the rear. Conversely, unpolarized cells immediately extend pseudopods towards the new FA source. We also observed that plasma membrane phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] levels oscillate in unpolarized cells treated with Latrunculin-A, whereas polarized cells had stable plasma membrane PtdIns(3,4,5)P3 responses toward the chemoattractant gradient source. Results were similar for cells that were starved for 4 hours, with a mixture of polarized and unpolarized cells responding to cAMP. Taken together, these findings suggest that similar components control gradient sensing during FA- and cAMP-mediated motility, but the response of polarized cells is more stable, which ultimately helps maintain their directionality.

Chemotaxis: movement, direction, control

This review focuses on basic principles of motility in different cell types, formation of the specific cell structures that enable directed migration, and how external signals are transduced into cells and coupled to the motile machinery. Feedback mechanisms and their potential role in maintenance of internal chemotactic gradients and persistence of directed migration are highlighted.

Migrating fibroblasts reorient directionality by a metastable, PI3K-dependent mechanism

Mesenchymal cell migration as exhibited by fibroblasts is distinct from amoeboid cell migration and is characterized by dynamic competition among multiple protrusions, which determines directional persistence and responses to spatial cues. Localization of phosphoinositide 3-kinase (PI3K) signaling is thought to play a broadly important role in cell motility, yet the context-dependent functions of this pathway have not been adequately elucidated. By mapping the spatiotemporal dynamics of cell protrusion/retraction and PI3K signaling monitored by total internal reflection fluorescence microscopy, we show that randomly migrating fibroblasts reorient polarity through PI3K-dependent branching and pivoting of protrusions. PI3K inhibition did not affect the initiation of newly branched protrusions, nor did it prevent protrusion induced by photoactivation of Rac. Rather, PI3K signaling increased after, not before, the onset of local protrusion and was required for the lateral spreading and stabilization of nascent branches. During chemotaxis, the branch experiencing the higher chemoattractant concentration was favored, and, thus, the cell reoriented so as to align with the external gradient.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.

Prolyl oligopeptidase, inositol phosphate signalling and lithium sensitivity

Inhibition of prolyl oligopeptidase (PO) elevates inositol phosphate (IP) signalling and reduces cell sensitivity to lithium (Li⁺). This review discusses recent evidence that shows PO acts via the multiple inositol polyphosphate phosphatase (MIPP) to regulate gene expression. As a consequence, PO inhibition causes both a transient, rapid increase in I(1,4,5)P₃ and a long-term elevation of IP signalling. This pathway is evolutionary conserved, being present in both the social amoeba Dictyostelium and human cell systems, and has potential implications for mental health.

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

The WD40 repeat protein WDR26 binds Gβγ and promotes Gβγ-dependent signal transduction and leukocyte migration

The Gβγ subunits of heterotrimeric G proteins transmit signals to control many cellular processes, including leukocyte migration. Gβγ signaling may regulate and be regulated by numerous signaling partners. Here, we reveal that WDR26, a member of the WD40 repeat protein family, directly bound free Gβγ in vitro, and formed a complex with endogenous Gβγ in Jurkat T cells stimulated by the chemokine SDF1α. Suppression of WDR26 by siRNAs selectively inhibited Gβγ-dependent phospholipase Cβ and PI3K activation, and attenuated chemotaxis in Jurkat T cells and differentiated HL60 cells in vitro and Jurkat T cell homing to lymphoid tissues in scid mice. Similarly, disruption of the WDR26/Gβγ interaction via expression of a WDR26 deletion mutant impaired Gβγ signaling and Jurkat T cell migration, indicating that the function of WDR26 depends on its binding to Gβγ. Additional data show that WDR26 also controlled RACK1, a negative regulator, in binding Gβγ and inhibiting leukocyte migration. Collectively, these experiments identify WDR26 as a novel Gβγ-binding protein that is required for the efficacy of Gβγ signaling and leukocyte migration.

PI3K and chemotaxis: a priming issue?

Although the spatiotemporal activation of phosphoinositide 3-kinases (PI3Ks) at the leading edge of chemotaxing cells represents a key marker of polarity, both Dictyostelium discoideum and neutrophils lacking measurable PI3K activity can still migrate directionally under certain conditions. Evidence from various papers suggests that the differentiation state of cells or their priming status can consolidate otherwise contradictory findings.

Post-bioprinting processing methods to improve cell viability and pattern fidelity in heterogeneous tissue test systems

Bioprinted tissue test systems show promise as a powerful tool for studying cell-cell interaction in heterogeneous, tissue-like co-culture. Several challenges were encountered while attempting to consistently fabricate samples with high viability and pattern fidelity. This paper evaluates four methods for processing samples after bioprinting but prior to adding media for incubation. These methods, composed of various combinations of three techniques meant to promote cell hydration, are evaluated with respect to sample viability and pattern preservation. In the best performing method, Hank's Balanced Salt Solution was applied immediately after fabrication and a collagen overlayer was applied one hour thereafter. The success of this method highlights the ability of the collagen substrate to absorb moisture, which promotes cell health without disturbing the cell's printed location. An addendum to the main study is an investigation of the limits of an HP26 print cartridge to deposit cells at a faster rate for the purpose of creating cell layers with densities that approach confluence.

Chemotaxis of MDCK-F cells toward fibroblast growth factor-2 depends on transient receptor potential canonical channel 1

Movement toward the source of a chemoattractant gradient is a basic cellular property in health and disease. Enhanced migration during metastasis involves deregulated growth factor signaling. Growth factor stimulation and cell migration converge both on the important second messenger Ca(2+). To date, the molecular identification of Ca(2+) entry pathways activated by growth factors during chemotaxis is still an open issue. We investigated the involvement of the nonselective Ca(2+) channel TRPC1 (transient receptor potential canonical 1) in FGF-2 guided chemotaxis by means of time-lapse video microscopy and by functional Ca(2+) measurements. To specifically address TRPC1 function in transformed MDCK cells we altered the expression levels by siRNA or overexpression. We report that TRPC1 channels are required for the orientation of transformed MDCK cells in FGF-2 gradients because TRPC1 knockdown or pharmacological blockade prevented chemotaxis. Stimulation with FGF-2 triggered an immediate Ca(2+) influx via TRPC1 channels that depended on phospholipase C and phosphatidylinositol 3-kinase signaling. Impeding this Ca(2+) influx abolished chemotaxis toward FGF-2. This functional connection correlated with clustering of FGF receptors and TRPC1 channels as was observed by immunolabeling. These findings show the important interplay between growth factor signaling and Ca(2+) influx in chemotaxis.

Phosphatidylinositol 3-kinase facilitates microtubule-dependent membrane transport for neuronal growth cone guidance

The activity of PI3K is necessary for polarized cell motility. To guide extending axons, environmental cues polarize the growth cone via asymmetric generation of Ca(2+) signals and subsequent intracellular mechanical events, including membrane trafficking and cytoskeletal reorganization. However, it remains unclear how PI3K is involved in such events for axon guidance. Here, we demonstrate that PI3K plays a permissive role in growth cone turning by facilitating microtubule (MT)-dependent membrane transport. Using embryonic chick dorsal root ganglion neurons in culture, attractive axon turning was induced by Ca(2+) elevations on one side of the growth cone by photolyzing caged Ca(2+) or caged inositol 1,4,5-trisphosphate. We show that PI3K activity was required downstream of Ca(2+) signals for growth cone turning. Attractive Ca(2+) signals, generated with caged Ca(2+) or caged inositol 1,4,5-trisphosphate, triggered asymmetric transport of membrane vesicles from the center to the periphery of growth cones in a MT-dependent manner. This centrifugal vesicle transport was abolished by PI3K inhibitors, suggesting that PI3K is involved in growth cone attraction at the level of membrane trafficking. Consistent with this observation, immunocytochemistry showed that PI3K inhibitors reduced MTs in the growth cone peripheral domain. Time-lapse imaging of EB1 on the plus-end of MTs revealed that MT advance into the growth cone peripheral domain was dependent on PI3K activity: inhibition of the PI3K signaling pathway attenuated MT advance, whereas exogenous phosphatidylinositol 3,4,5-trisphosphate, the product of PI3K-catalyzed reactions, promoted MT advance. This study demonstrates the importance of PI3K-dependent membrane trafficking in chemotactic cell migration.

Chemotactic activation of Dictyostelium AGC-family kinases AKT and PKBR1 requires separate but coordinated functions of PDK1 and TORC2

Protein kinases AKT and PKBR1 of Dictyostelium belong to the AGC protein kinase superfamily. AKT and PKBR1 are phosphorylated at similar sites by phosphoinositide-dependent kinase 1 (PDK1) and TORC2 kinases; however, they have different subcellular localizing domains. AKT has a phosphoinositide 3-kinase (PI3K)/phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)]-regulated PH (pleckstrin homology) domain whereas PKBR1 is myristoylated and persistently membrane localized. Using strains defective for PI3K/PtdIns(3,4,5)P(3)-, PDK1- and TORC2-signaling or strains that express phospho-site mutants of AKT and PKBR1, we dissect the different roles of PI3K/PtdIns(3,4,5)P(3), PDK1 and TORC2. We show that activation of AKT and PKBR1 requires PDK1-site phosphorylation, but that phosphorylation by TORC2 is insufficient for AKT or PKBR1 activation. However, PDK1-site phosphorylation is dependent on phosphorylation by TORC2, which suggests that there is regulatory coordination among PDK1, TORC2 and their phospho-site targets. This defines a separate input for signaling in control of chemotaxis and dependency on PDK1 function. We also demonstrate that PDK1 in Dictyostelium functions independently of PI3K/PtdIns(3,4,5)P(3). Finally, we show that AKT and PKBR1 exhibit substrate selectivity and identify two novel lipid-interacting proteins preferentially phosphorylated by AKT. Despite certain similarities, AKT and PKBR1 have distinct regulatory paths that impact activation and effector targeting, with PDK1 serving a central role.

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.

A mathematical modelling framework for understanding chemorepulsive signal transduction in Dictyostelium

Chemorepulsion is the process by which an organism or a cell moves in the direction of decreasing chemical concentration. While a few experimental studies have been performed, no mathematical models exist for this process. In this paper we have modelled gradient sensing, the first subprocess of chemorepulsion, in Dictyostelium discoideum-a well characterized model eukaryotic system. We take the first steps towards achieving a comprehensive mechanistic understanding of chemorepulsion in this system. We have used, as a basis, the biochemical network of the Keizer-Gunnink et al. (2007) to develop the mathematical modelling framework. This network describes the underlying pathways of chemorepellent gradient sensing in D. discoideum. Working within this modelling framework we address whether the postulated interactions of the pathways and species in this network can lead to a chemorepulsive response. We also analyse the possible role of additional regulatory effects (such as additional receptor regulation of enzymes in this network) and if this is necessary to achieve this behaviour. Thus we have investigated the receptor regulation of important enzymes and feedback effects in the network. This modelling framework generates important insights into and testable predictions regarding the role of key components and feedback loops in regulating chemorepulsive gradient sensing, and what factors might be important for generating a chemorepulsive response; it serves as a first step towards a comprehensive mechanistic understanding of this process.

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.

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.

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.

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.

Role of 3'-phosphoinositides in oncogenic KRAS-induced modulation of shape and motility of airway epithelial cells

The authors' previous study demonstrated that oncogenic KRAS modulates the shape and motility of airway epithelial cells. To explore detailed mechanism mediating these events, the possible involvement of phosphatidylinositides (PIP) was investigated. The intracellular localization of PIP was visualized with a pleckstrin homology domain-enhanced green fluorescent protein (EGFP) construct. PIP accumulated at the leading edges of polarizing epithelial cells, while they co-localized with cortical actin at cell-cell contacts, suggesting that PIP play important roles in the cytoskeletal organization. Transduction of oncogenic KRAS induced multiple pseudopodia and disrupted cortical actin, enhancing motility. A mitogen activated protein kinase kinase (MEK) inhibitor reduced the accumulation of PIP at membranes and development of pseudopodia, and restored stable cortical actin, reducing the motility. A phosphoinositide 3-kinase (PI3K) inhibitor also reduced accumulation of PIP at membranes, formation of pseudopodia and motility, but its effect on cortical actin was indistinct. The KRAS V12/S35 mutant, activating only the MEK pathway, induced multiple pseudopodia and disrupted the cortical actin. The KRAS V12/C40 mutant, activating only the PI3K pathway, also induced pseudopodia, but its effect on cortical actin was obscure. Taken together, oncogenic KRAS could cause the accumulation of PIP via the PI3K and MEK pathways and modulate the cell shape and migration.

Spontaneous phosphoinositide 3-kinase signaling dynamics drive spreading and random migration of fibroblasts

During directed cell migration (chemotaxis), cytoskeletal dynamics are stimulated and spatially biased by phosphoinositide 3-kinase (PI3K) and other signal transduction pathways. Live-cell imaging using total internal reflection fluorescence (TIRF) microscopy revealed that, in the absence of soluble cues, 3'-phosphoinositides are enriched in a localized and dynamic fashion during active spreading and random migration of mouse fibroblasts on adhesive surfaces. Surprisingly, we found that PI3K activation is uncoupled from classical integrin-mediated pathways and feedback from the actin cytoskeleton. Inhibiting PI3K significantly impairs cell motility, both in the context of normal spreading and when microtubules are dissociated, which induces a dynamic protrusion phenotype as seen by TIRF in our cells. Accordingly, during random migration, 3'-phosphoinositides are frequently localized to regions of membrane protrusion and correlate quantitatively with the direction and persistence of cell movement. These results underscore the importance of localized PI3K signaling not only in chemotaxis but also in basal motility/migration of fibroblasts.

Spatiotemporal regulation of Ras-GTPases during chemotaxis

Many eukaryotic cells can elicit intracellular signaling relays to produce pseudopodia and move up to the chemoattractant gradient (chemotaxis) or move randomly in the absence of extracellular stimuli and nutrients (random movement). A precise spatiotemporal regulation of Ras-GTPases, such as Ras and Rap, is crucial to induce pseudopodia formation and cellular adhesion during the chemotaxis and random movement. Here, we describe biochemical and real-time imaging methods for using Dictyostelium to understand the signaling events important for chemotaxis and random cell movement. The chapter includes (1) a biochemical method to assess Ras and Rap1 activation in response to chemoattractant, (2) an imaging method to detect endogenous Ras and Rap1 activation in moving cells, and (3) a simultaneous imaging method to decipher the precise order and localization of these signaling events. With a combination of powerful Dictyostelium genetics, these methods will facilitate to elucidate a dynamic activation of Ras proteins and their inter relay with other signaling molecules during chemotaxis and random movement.

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.

Dictyostelium Dock180-related RacGEFs regulate the actin cytoskeleton during cell motility

Cell motility of amoeboid cells is mediated by localized F-actin polymerization that drives the extension of membrane protrusions to promote forward movements. We show that deletion of either of two members of the Dictyostelium Dock180 family of RacGEFs, DockA and DockD, causes decreased speed of chemotaxing cells. The phenotype is enhanced in the double mutant and expression of DockA or DockD complements the reduced speed of randomly moving DockD null cells' phenotype, suggesting that DockA and DockD are likely to act redundantly and to have similar functions in regulating cell movement. In this regard, we find that overexpressing DockD causes increased cell speed by enhancing F-actin polymerization at the sites of pseudopod extension. DockD localizes to the cell cortex upon chemoattractant stimulation and at the leading edge of migrating cells and this localization is dependent on PI3K activity, suggesting that DockD might be part of the pathway that links PtdIns(3,4,5)P(3) production to F-actin polymerization. Using a proteomic approach, we found that DdELMO1 is associated with DockD and that Rac1A and RacC are possible in vivo DockD substrates. In conclusion, our work provides a further understanding of how cell motility is controlled and provides evidence that the molecular mechanism underlying Dock180-related protein function is evolutionarily conserved.