The role of myosin heavy chain phosphorylation in Dictyostelium motility, chemotaxis and F-actin localization

Quantitative Motion Analysis in Two and Three Dimensions

This chapter describes 2D quantitative methods for motion analysis as well as 3D motion analysis and reconstruction methods. Emphasis is placed on the analysis of dynamic cell shape changes that occur through extension and retraction of force generating structures such as pseudopodia and lamellipodia. Quantitative analysis of these structures is an underutilized tool in the field of cell migration. Our intent, therefore, is to present methods that we developed in an effort to elucidate mechanisms of basic cell motility, directed cell motion during chemotaxis, and metastasis. We hope to demonstrate how application of these methods can more clearly define alterations in motility that arise due to specific mutations or disease and hence, suggest mechanisms or pathways involved in normal cell crawling and treatment strategies in the case of disease. In addition, we present a 4D tumorigenesis model for high-resolution analysis of cancer cells from cell lines and human cancer tissue in a 3D matrix. Use of this model led to the discovery of the coalescence of cancer cell aggregates and unique cell behaviors not seen in normal cells or normal tissue. Graphic illustrations to visually display and quantify cell shape are presented along with algorithms and formulae for calculating select 2D and 3D motion analysis parameters.

4D Tumorigenesis Model for Quantitating Coalescence, Directed Cell Motility and Chemotaxis, Identifying Unique Cell Behaviors, and Testing Anticancer Drugs

A 4D high-resolution computer-assisted reconstruction and motion analysis system has been developed and applied to the long-term (14-30 days) analysis of cancer cells migrating and aggregating within a 3D matrix. 4D tumorigenesis models more closely approximate the tumor microenvironment than 2D substrates and, therefore, are improved tools for elucidating the interactions within the tumor microenvironment that promote growth and metastasis. The model we describe here can be used to analyze the growth of tumor cells, aggregate coalescence, directed cell motility and chemotaxis, matrix degradation, the effects of anticancer drugs, and the behavior of immune and endothelial cells mixed with cancer cells. The information given in this chapter is also intended to acquaint the reader with computer-assisted methods and algorithms that can be used for high-resolution 3D reconstruction and quantitative motion analysis.

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.

TRPM7, the cytoskeleton and neuronal death

Ischemic stroke is one of the leading causes of disability and death in the world. Elucidation of the underlying mechanisms associated with neuronal death during this detrimental process has been of significant interest in the field of research. One principle component vital to the maintenance of cellular integrity is the cytoskeleton. Studies suggest that abnormalities at the level of this fundamental structure are directly linked to adverse effects on cellular well-being, including cell death. In recent years, evidence has also emerged regarding an imperative role for the transient receptor potential (TRP) family member TRPM7 in the mediation of excitotoxic-independent neuronal demise. In this review, we will elaborate on the current knowledge and unique properties associated with the functioning of this structure. In addition, we will deliberate the involvement of distinct mechanistic pathways during TRPM7-dependent cell death, including modifications at the level of the cytoskeleton.

Myosin heavy chain kinases play essential roles in Ca2+, but not cAMP, chemotaxis and the natural aggregation of Dictyostelium discoideum

Behavioral analyses of the deletion mutants of the four known myosin II heavy chain (Mhc) kinases of Dictyostelium discoideum revealed that all play a minor role in the efficiency of basic cell motility, but none play a role in chemotaxis in a spatial gradient of cAMP generated in vitro. However, the two kinases MhckA and MhckC were essential for chemotaxis in a spatial gradient of Ca(2+), shear-induced directed movement, and reorientation in the front of waves of cAMP during natural aggregation. The phenotypes of the mutants mhckA(-) and mhckC(-) were highly similar to that of the Ca(2+) channel/receptor mutant iplA(-) and the myosin II phosphorylation mutant 3XALA, which produces constitutively unphosphorylated myosin II. These results demonstrate that IplA, MhckA and MhckC play a selective role in chemotaxis in a spatial gradient of Ca(2+), but not cAMP, and suggest that Ca(2+) chemotaxis plays a role in the orientation of cells in the front of cAMP waves during natural aggregation.

The IplA Ca2+ channel of Dictyostelium discoideum is necessary for chemotaxis mediated through Ca2+, but not through cAMP, and has a fundamental role in natural aggregation

During aggregation of Dictyostelium discoideum, nondissipating, symmetrical, outwardly moving waves of cAMP direct cells towards aggregation centers. It has been assumed that the spatial and temporal characteristics of the front and back of each cAMP wave regulate both chemokinesis and chemotaxis. However, during the period preceding aggregation, cells acquire not only the capacity to chemotax in a spatial gradient of cAMP, but also in a spatial gradient of Ca(2+). The null mutant of the putative IplA Ca(2+) channel gene, iplA(-), undergoes normal chemotaxis in spatial gradients of cAMP and normal chemokinetic responses to increasing temporal gradients of cAMP, both generated in vitro. However, iplA(-) cells lose the capacity to undergo chemotaxis in response to a spatial gradient of Ca(2+), suggesting that IplA is either the Ca(2+) chemotaxis receptor or an essential component of the Ca(2+) chemotaxis regulatory pathway. In response to natural chemotactic waves generated by wild-type cells, the chemokinetic response of iplA(-) cells to the temporal dynamics of the cAMP wave is intact, but the capacity to reorient in the direction of the aggregation center at the onset of each wave is lost. These results suggest that transient Ca(2+) gradients formed between cells at the onset of each natural cAMP wave augment reorientation towards the aggregation center. If this hypothesis proves correct, it will provide a more complex contextual framework for interpreting D. discoideum chemotaxis.

Nhe1 is essential for potassium but not calcium facilitation of cell motility and the monovalent cation requirement for chemotactic orientation in Dictyostelium discoideum

In Dictyostelium discoideum, extracellular K+ or Ca2+ at a concentration of 40 or 20 mM, respectively, facilitates motility in the absence or presence of a spatial gradient of chemoattractant. Facilitation results in maximum velocity, cellular elongation, persistent translocation, suppression of lateral pseudopod formation, and myosin II localization in the posterior cortex. A lower threshold concentration of 15 mM K+ or Na or 5 mM Ca2+ is required for chemotactic orientation. Although the common buffer solutions used by D. discoideum researchers to study chemotaxis contain sufficient concentrations of cations for chemotactic orientation, the majority contain insufficient levels to facilitate motility. Here it has been demonstrated that Nhe1, a plasma membrane protein, is required for K+ but not Ca2+ facilitation of cell motility and for the lower K+ but not Ca2+ requirement for chemotactic orientation.

The effects of extracellular calcium on motility, pseudopod and uropod formation, chemotaxis, and the cortical localization of myosin II in Dictyostelium discoideum

Extracellular Ca(++), a ubiquitous cation in the soluble environment of cells both free living and within the human body, regulates most aspects of amoeboid cell motility, including shape, uropod formation, pseudopod formation, velocity, and turning in Dictyostelium discoideum. Hence it affects the efficiency of both basic motile behavior and chemotaxis. Extracellular Ca(++) is optimal at 10 mM. A gradient of the chemoattractant cAMP generated in the absence of added Ca(++) only affects turning, but in combination with extracellular Ca(++), enhances the effects of extracellular Ca(++). Potassium, at 40 mM, can partially substitute for Ca(++). Mg(++), Mn(++), Zn(++), and Na(+) cannot. Extracellular Ca(++), or K(+), also induce the cortical localization of myosin II in a polar fashion. The effects of Ca(++), K(+) or a cAMP gradient do not appear to be similarly mediated by an increase in the general pool of free cytosolic Ca(++). These results suggest a model, in which each agent functioning through different signaling systems, converge to affect the cortical localization of myosin II, which in turn effects the behavioral changes leading to efficient cell motility and chemotaxis. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

How a cell crawls and the role of cortical myosin II

The movements of Dictyostelium discoideum amoebae translocating on a glass surface in the absence of chemoattractant have been reconstructed at 5-second intervals and motion analyzed by employing 3D-DIAS software. A morphometric analysis of pseudopods, the main cell body, and the uropod provides a comprehensive description of the basic motile behavior of a cell in four dimensions (4D), resulting in a list of 18 characteristics. A similar analysis of the myosin II phosphorylation mutant 3XASP reveals a role for the cortical localization of myosin II in the suppression of lateral pseudopods, formation of the uropod, cytoplasmic distribution of cytoplasm in the main cell body, and efficient motility. The results of the morphometric analysis suggest that pseudopods, the main cell body, and the uropod represent three motility compartments that are coordinated for efficient translocation. It provides a contextual framework for interpreting the effects of mutations, inhibitors, and chemoattractants on the basic motile behavior of D. discoideum. The generality of the characteristics of the basic motile behavior of D. discoideum must now be tested by similar 4D analyses of the motility of amoeboid cells of higher eukaryotic cells, in particular human polymorphonuclear leukocytes.

PTEN is a mechanosensing signal transducer for myosin II localization in Dictyostelium cells

To investigate the role of PTEN in regulation of cortical motile activity, especially in myosin II localization, eGFP-PTEN and mRFP-myosin II were simultaneously expressed in Dictyostelium cells. PTEN and myosin II co-localized at the posterior of migrating cells and furrow region of dividing cells. In suspension culture, PTEN knockout (pten(-)) cells became multinucleated, and myosin II significantly decreased in amount at the furrow. During pseudopod retraction and cell aspiration by microcapillary, PTEN accumulated at the tips of pseudopods and aspirated lobes prior to the accumulation of myosin II. In pten(-) cells, only a small amount of myosin II accumulated at the retracting pseudopods and aspirated cell lobes. PTEN accumulated at the retracting pseudopods and aspirated lobes even in myosin II null cells and latrunculin B-treated cells though in reduced amounts, indicating that PTEN accumulates partially depending on myosin II and cortical actin. Accumulation of PTEN prior to myosin II suggests that PTEN is an upstream component in signaling pathway to localize myosin II, possibly with mechanosensing signaling loop where actomyosin-driven contraction further augments accumulation of PTEN and myosin II by a positive feedback mechanism.

cAMP oscillations during aggregation of Dictyostelium

For many years it has been known that developing cells of Dictyostelium discoideum show periodic surges as they aggregate. When it was discovered that the cells were responding chemotactically to cAMP gradients produced within the populations, experiments were carried out that demonstrated similar periodic changes in the concentration of extracellular cAMP. Moreover, homogenous populations of developed cells held in suspension could be shown to respond to cAMP by changes in cell shape. Such suspensions showed spontaneous oscillations in light scattering as well as cAMP levels as the result of entrainment of the cells. The molecular components necessary for the pulsatile release of cAMP were uncovered by analyzing the behavior of a large number of strains with defined mutations isolated from saturation mutagenic screens. Subsequent genetic and biochemical studies established the connections between a dozen proteins essential for spontaneous oscillations. Computer simulations of a molecular circuit based on these results showed that it is able to account for the temporal and quantitative aspects of the oscillatory system. The circuit also appears to be coupled to the construction and dismantling of the actin/myosin cortical layer that ensures that pseudopods are restricted to the anterior of cells during chemotaxis and that the cells do not back-track when the natural wave is behind them. Since the same molecular clock controls both signal production and signal response, these behaviors are always kept strictly in phase.

Changes in the magnitude and distribution of forces at different Dictyostelium developmental stages

The distribution of forces exerted by migrating Dictyostelium amebae at different developmental stages was measured using traction force microscopy. By using very soft polyacrylamide substrates with a high fluorescent bead density, we could measure stresses as small as 30 Pa. Remarkable differences exist both in term of the magnitude and distribution of forces in the course of development. In the vegetative state, cells present cyclic changes in term of speed and shape between an elongated form and a more rounded one. The forces are larger in this first state, especially when they are symmetrically distributed at the front and rear edge of the cell. Elongated vegetative cells can also present a front-rear asymmetric force distribution with the largest forces in the crescent-shaped rear of the cell (uropod). Pre-aggregating cells, once polarized, only present this last kind of asymmetric distribution with the largest forces in the uropod. Except for speed, no cycle is observed. Neither the force distribution of pre-aggregating cells nor their overall magnitude are modified during chemotaxis, the later being similar to the one of vegetative cells (F(0) approximately 6 nN). On the contrary, both the force distribution and overall magnitude is modified for the fast moving aggregating cells. In particular, these highly elongated cells exert lower forces (F(0) approximately 3 nN). The location of the largest forces in the various stages of the development is consistent with the myosin II localization described in the literature for Dictyostelium (Yumura et al.,1984. J Cell Biol 99:894-899) and is confirmed by preliminary experiments using a GFP-myosin Dictyostelium strain.

The regulation of cell motility and chemotaxis by phospholipid signaling

Phosphoinositide 3-kinase (PI3K), PTEN and localized phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] play key roles in chemotaxis, regulating cell motility by controlling the actin cytoskeleton in Dictyostelium and mammalian cells. PtdIns(3,4,5)P3, produced by PI3K, acts via diverse downstream signaling components, including the GTPase Rac, Arf-GTPases and the kinase Akt (PKB). It has become increasingly apparent, however, that chemotaxis results from an interplay between the PI3K-PTEN pathway and other parallel pathways in Dictyostelium and mammalian cells. In Dictyostelium, the phospholipase PLA2 acts in concert with PI3K to regulate chemotaxis, whereas phospholipase C (PLC) plays a supporting role in modulating PI3K activity. In adenocarcinoma cells, PLC and the actin regulator cofilin seem to provide the direction-sensing machinery, whereas PI3K might regulate motility.

Linking Ras to myosin function: RasGEF Q, a Dictyostelium exchange factor for RasB, affects myosin II functions

Ras guanine nucleotide exchange factor (GEF) Q, a nucleotide exchange factor from Dictyostelium discoideum, is a 143-kD protein containing RasGEF domains and a DEP domain. We show that RasGEF Q can bind to F-actin, has the potential to form complexes with myosin heavy chain kinase (MHCK) A that contain active RasB, and is the predominant exchange factor for RasB. Overexpression of the RasGEF Q GEF domain activates RasB, causes enhanced recruitment of MHCK A to the cortex, and leads to cytokinesis defects in suspension, phenocopying cells expressing constitutively active RasB, and myosin-null mutants. RasGEF Q⁻ mutants have defects in cell sorting and slug migration during later stages of development, in addition to cell polarity defects. Furthermore, RasGEF Q⁻ mutants have increased levels of unphosphorylated myosin II, resulting in myosin II overassembly. Collectively, our results suggest that starvation signals through RasGEF Q to activate RasB, which then regulates processes requiring myosin II.

Directional sensing during chemotaxis

Cells have the innate ability to sense and move towards a variety of chemoattractants. We investigate the pathways by which cells sense and respond to chemoattractant gradients. We focus on the model system Dictyostelium and compare our understanding of chemotaxis in this system with recent advances made using neutrophils and other mammalian cell types, which share many molecular components and signaling pathways with Dictyostelium. This review also examines models that have been proposed to explain how cells are able to respond to small differences in ligand concentrations between the anterior leading edge and posterior of the cell. In addition, we highlight the overlapping functions of many signaling components in diverse processes beyond chemotaxis, including random cell motility and cell division.

The role of cGMP and the rear of the cell in Dictyostelium chemotaxis and cell streaming

During chemotaxis, pseudopod extensions lead the cell towards the source of attractant. The role of actin-filled pseudopodia at the front of the cell is well recognized, whereas the function of the rear of the cell in chemotaxis and cell-cell interactions is less well known. Dictyostelium cell aggregation is mediated by outwardly propagating waves of extracellular cAMP that induce chemotaxis and cell-cell contacts, resulting in streams of cells moving towards the aggregation centre. Wild-type cells efficiently retract pseudopodia in the rear of the cell during the rising flank of the cAMP wave and have a quiescent cell posterior. This polarization largely remains during the declining flank, which causes cells to continue their chemotactic movement towards the aggregation centre and to form stable streams of moving cells. The dominance of the leading-edge pseudopod rescues chemotaxis during the rising flank of the wave, but the cells move in random directions after the peak of the wave has passed. As a consequence, cell-cell contacts cannot be maintained, and the cell streams break up. The results show that a quiescent rear of the cell increases the efficiency of directional movement and is essential to maintain stable cell-cell contacts.

PTEN plays a role in the suppression of lateral pseudopod formation during Dictyostelium motility and chemotaxis

It has been suggested that the phosphatydylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] phosphatase and tensin homolog PTEN plays a fundamental role in Dictyostelium discoideum chemotaxis. To identify that role, the behavior of a pten(-) mutant was quantitatively analyzed using two-dimensional and three-dimensional computer-assisted methods. pten(-) cells were capable of polarizing and translocating in the absence of attractant, and sensing and responding to spatial gradients, temporal gradients and natural waves of attractant. However, all of these responses were compromised (i.e. less efficient) because of the fundamental incapacity of pten(-) cells to suppress lateral pseudopod formation and turning. This defect was equally manifested in the absence, as well as presence, of attractant. PTEN, which is constitutively localized in the cortex of polarized cells, was found essential for the attractant-stimulated increase in cortical myosin II and F-actin that is responsible for the increased suppression of pseudopods during chemotaxis. PTEN, therefore, plays a fundamental role in the suppression of lateral pseudopod formation, a process essential for the efficiency of locomotion and chemotaxis, but not in directional sensing.

Big roles for small GTPases in the control of directed cell movement

Small GTPases are involved in the control of diverse cellular behaviours, including cellular growth, differentiation and motility. In addition, recent studies have revealed new roles for small GTPases in the regulation of eukaryotic chemotaxis. Efficient chemotaxis results from co-ordinated chemoattractant gradient sensing, cell polarization and cellular motility, and accumulating data suggest that small GTPase signalling plays a central role in each of these processes as well as in signal relay. The present review summarizes these recent findings, which shed light on the molecular mechanisms by which small GTPases control directed cell migration.

TOS9 regulates white-opaque switching in Candida albicans

In Candida albicans, the a1-alpha2 complex represses white-opaque switching, as well as mating. Based upon the assumption that the a1-alpha2 corepressor complex binds to the gene that regulates white-opaque switching, a chromatinimmunoprecipitation-microarray analysis strategy was used to identify 52 genes that bound to the complex. One of these genes, TOS9, exhibited an expression pattern consistent with a "master switch gene." TOS9 was only expressed in opaque cells, and its gene product, Tos9p, localized to the nucleus. Deletion of the gene blocked cells in the white phase, misexpression in the white phase caused stable mass conversion of cells to the opaque state, and misexpression blocked temperature-induced mass conversion from the opaque state to the white state. A model was developed for the regulation of spontaneous switching between the opaque state and the white state that includes stochastic changes of Tos9p levels above and below a threshold that induce changes in the chromatin state of an as-yet-unidentified switching locus. TOS9 has also been referred to as EAP2 and WOR1.

The regulation of myosin II in Dictyostelium

Dictyostelium conventional myosin (myosin II) is an abundant protein that plays a role in various cellular processes such as cytokinesis, cell protrusion and development. This review will focus on the signal transduction pathways that regulate myosin II during cell movement. Myosin II appears to have two modes of action in Dictyostelium: local stabilization of the cytoskeleton by myosin filament association to the actin meshwork (structural mode) and force generation by contraction of actin filaments (motor mode). Some processes, such as cell movement under restrictive environment, require only the structural mode of myosin. However, cytokinesis in suspension and uropod retraction depend on motor activity as well. Myosin II can self-assemble into bipolar filaments. The formation of these filaments is negatively regulated by heavy chain phosphorylation through the action of a set of novel alpha kinases and is relatively well understood. However, only recently it has become clear that the formation of bipolar filaments and their translocation to the cortex are separate events. Translocation depends on filamentous actin, and is regulated by a cGMP pathway and possibly also by the cAMP phosphodiesterase RegA and the p21-activated kinase PAKa. Myosin motor activity is regulated by phosphorylation of the regulatory light chain through myosin light chain kinase A. Unlike conventional light chain kinases, this enzyme is not regulated by calcium but is activated by cGMP-induced phosphorylation via an upstream kinase and subsequent autophosphorylation.

TRPM7, a novel regulator of actomyosin contractility and cell adhesion

Actomyosin contractility regulates various cell biological processes including cytokinesis, adhesion and migration. While in lower eukaryotes, alpha-kinases control actomyosin relaxation, a similar role for mammalian alpha-kinases has yet to be established. Here, we examined whether TRPM7, a cation channel fused to an alpha-kinase, can affect actomyosin function. We demonstrate that activation of TRPM7 by bradykinin leads to a Ca(2+)- and kinase-dependent interaction with the actomyosin cytoskeleton. Moreover, TRPM7 phosphorylates the myosin IIA heavy chain. Accordingly, low overexpression of TRPM7 increases intracellular Ca2+ levels accompanied by cell spreading, adhesion and the formation of focal adhesions. Activation of TRPM7 induces the transformation of these focal adhesions into podosomes by a kinase-dependent mechanism, an effect that can be mimicked by pharmacological inhibition of myosin II. Collectively, our results demonstrate that regulation of cell adhesion by TRPM7 is the combined effect of kinase-dependent and -independent pathways on actomyosin contractility.

Intracellular role of adenylyl cyclase in regulation of lateral pseudopod formation during Dictyostelium chemotaxis

Cyclic AMP (cAMP) functions as the extracellular chemoattractant in the aggregation phase of Dictyostelium development. There is some question, however, concerning what role, if any, it plays intracellularly in motility and chemotaxis. To test for such a role, the behavior of null mutants of acaA, the adenylyl cyclase gene that encodes the enzyme responsible for cAMP synthesis during aggregation, was analyzed in buffer and in response to experimentally generated spatial and temporal gradients of extracellular cAMP. acaA- cells were defective in suppressing lateral pseudopods in response to a spatial gradient of cAMP and to an increasing temporal gradient of cAMP. acaA- cells were incapable of chemotaxis in natural waves of cAMP generated by majority control cells in mixed cultures. These results indicate that intracellular cAMP and, hence, adenylyl cyclase play an intracellular role in the chemotactic response. The behavioral defects of acaA- cells were surprisingly similar to those of cells of null mutants of regA, which encodes the intracellular phosphodiesterase that hydrolyzes cAMP and, hence, functions opposite adenylyl cyclase A (ACA). This result is consistent with the hypothesis that ACA and RegA are components of a receptor-regulated intracellular circuit that controls protein kinase A activity. In this model, the suppression of lateral pseudopods in the front of a natural wave depends on a complete circuit. Hence, deletion of any component of the circuit (i.e., RegA or ACA) would result in the same chemotactic defect.

Multiple myosin II heavy chain kinases: roles in filament assembly control and proper cytokinesis in Dictyostelium

Myosin II filament assembly in Dictyostelium discoideum is regulated via phosphorylation of residues located in the carboxyl-terminal portion of the myosin II heavy chain (MHC) tail. A series of novel protein kinases in this system are capable of phosphorylating these residues in vitro, driving filament disassembly. Previous studies have demonstrated that at least three of these kinases (MHCK A, MHCK B, and MHCK C) display differential localization patterns in living cells. We have created a collection of single, double, and triple gene knockout cell lines for this family of kinases. Analysis of these lines reveals that three MHC kinases appear to represent the majority of cellular activity capable of driving myosin II filament disassembly, and reveals that cytokinesis defects increase with the number of kinases disrupted. Using biochemical fractionation of cytoskeletons and in vivo measurements via fluorescence recovery after photobleaching (FRAP), we find that myosin II overassembly increases incrementally in the mutants, with the MHCK A(-)/B(-)/C(-) triple mutant showing severe myosin II overassembly. These studies suggest that the full complement of MHC kinases that significantly contribute to growth phase and cytokinesis myosin II disassembly in this organism has now been identified.

Computer-assisted analysis of filopod formation and the role of myosin II heavy chain phosphorylation in Dictyostelium

To investigate the role played by filopodia in the motility and chemotaxis of amoeboid cells, a computer-assisted 3D reconstruction and motion analysis system, DIAS 4.0, has been developed. Reconstruction at short time intervals of Dictyostelium amoebae migrating in buffer or in response to chemotactic signals, revealed that the great majority of filopodia form on pseudopodia, not on the cell body; that filopodia on the cell body originate primarily on pseudopodia and relocate; and that filopodia on the uropod are longer and more stable than those located on other portions of the cell. When adjusting direction through lateral pseudopod formation in a spatial gradient of chemoattractant, the temporal and spatial dynamics of lateral pseudopodia suggest that filopodia may be involved in stabilizing pseudopodia on the substratum while the decision is being made by a cell either to turn into a pseudopodium formed in the correct direction (up the gradient) or to retract a pseudopodium formed in the wrong direction (down the gradient). Experiments in which amoebae were treated with high concentrations of chemoattractant further revealed that receptor occupancy plays a role both in filopod formation and retraction. As phosphorylation-dephosphorylation of myosin II heavy chain (MHC) plays a role in lateral pseudopod formation, turning and chemotaxis, the temporal and spatial dynamics of filopod formation were analyzed in MHC phosphorylation mutants. These studies revealed that MHC phosphorylation-dephosphorylation plays a role in the regulation of filopod formation during cell migration in buffer and during chemotaxis. The computer-assisted technology described here for reconstructing filopodia at short time intervals in living cells, therefore provides a new tool for investigating the role filopodia play in the motility and chemotaxis of amoeboid cells.

RasGEF-containing proteins GbpC and GbpD have differential effects on cell polarity and chemotaxis in Dictyostelium

The regulation of cell polarity plays an important role in chemotaxis. Previously, two proteins termed GbpC and GbpD were identified in Dictyostelium, which contain RasGEF and cyclic nucleotide binding domains. Here we show that gbpC-null cells display strongly reduced chemotaxis, because they are unable to polarise effectively in a chemotactic gradient. However, gbpD-null mutants exhibit the opposite phenotype: cells display improved chemotaxis and appear hyperpolar, because cells make very few lateral pseudopodia, whereas the leading edge is continuously remodelled. Overexpression of GbpD protein results in severely reduced chemotaxis. Cells extend many bifurcated and lateral pseudopodia, resulting in the absence of a leading edge. Furthermore, cells are flat and adhesive owing to an increased number of substrate-attached pseudopodia. This GbpD phenotype is not dependent on intracellular cGMP or cAMP, like its mammalian homolog PDZ-GEF. Previously we showed that GbpC is a high-affinity cGMP-binding protein that acts via myosin II. We conclude that cGMP activates GbpC, mediating the chemoattractant-induced establishment of cell polarity through myosin. GbpD induces the formation of substrate-attached pseudopodia, resulting in increased attachment and suppression of polarity.