A contextual framework for characterizing motility and chemotaxis mutants in Dictyostelium discoideum

Challenges in tissue engineering - towards cell control inside artificial scaffolds

Control of living cells is vital for the survival of organisms. Each cell inside an organism is exposed to diverse external mechano-chemical cues, all coordinated in a spatio-temporal pattern triggering individual cell functions. This complex interplay between external chemical cues and mechanical 3D environments is translated into intracellular signaling loops. Here, we describe how external mechano-chemical cues control cell functions, especially cell migration, and influence intracellular information transport. In particular, this work focuses on the quantitative analysis of (1) intracellular vesicle transport to understand intracellular state changes in response to external cues, (2) cellular sensing of external chemotactic cues, and (3) the cells' ability to migrate in 3D structured environments, artificially fabricated to mimic the 3D environment of tissue in the human body.

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.

Extracellular and intracellular factors regulating the migration direction of a chemotactic cell in traveling-wave chemotaxis

This report presents a simple model that describes the motion of a single Dictyostelium discoideum cell exposed to a traveling wave of cyclic adenosine monophosphate (cAMP). The model incorporates two types of responses to stimulation by cAMP: the changes in the polarity and motility of the cell. The periodic change in motility is assumed to be induced by periodic cAMP stimulation on the basis of previous experimental studies. Consequently, the net migration of the cell occurs in a particular direction with respect to wave propagation, which explains the migration of D. discoideum cells in aggregation. The wave period and the difference between the two response times are important parameters that determine the direction of migration. The theoretical prediction compared with experiments presented in another study. The transition from the single-cell state of the population of D. discoideum cells to the aggregation state is understood to be a specific example of spontaneous breakage of symmetry in biology.

Diverse and dynamic sources and sinks in gradient formation and directed migration

The traditional view of directional cell migration within a tissue is that it requires a gradient of a soluble attractive chemical that is stable in space and time due to the presence of a source and a sink. However, advances in live imaging technology and the ability to study cell migration in vivo have revealed that endogenous sources and sinks are typically far more varied and complex. Both sources and sinks can be made up of multiple tissues. During long-range migrations, cells tend to divide up their trajectories and follow different source signals in each segment. When a single source signal is used repeatedly in each segment, its expression is dynamically controlled. Source signals can also originate locally from neighboring migrating cells. Sinks are important in some cases but not all, to sculpt a permissive migratory path or allow cells to move from one intermediate target to another. Migrating cells themselves can function as sinks to create a gradient out of an initially uniform chemoattractant. These diverse ways of building sources and sinks allow different cell types to navigate distinct trajectories through the same embryo even as the whole embryo undergoes the dramatic changes in cell number, position, arrangement and fate that are the essence of development.

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.

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.

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.

A quorum-sensing factor in vegetative Dictyostelium discoideum cells revealed by quantitative migration analysis

Background

Many cells communicate through the production of diffusible signaling molecules that accumulate and once a critical concentration has been reached, can activate or repress a number of target genes in a process termed quorum sensing (QS). In the social amoeba Dictyostelium discoideum, QS plays an important role during development. However little is known about its effect on cell migration especially in the growth phase.

Methods and Findings

To investigate the role of cell density on cell migration in the growth phase, we use multisite timelapse microscopy and automated cell tracking. This analysis reveals a high heterogeneity within a given cell population, and the necessity to use large data sets to draw reliable conclusions on cell motion. In average, motion is persistent for short periods of time (), but normal diffusive behavior is recovered over longer time periods. The persistence times are positively correlated with the migrated distances. Interestingly, the migrated distance decreases as well with cell density. The adaptation of cell migration to cell density highlights the role of a secreted quorum sensing factor (QSF) on cell migration. Using a simple model describing the balance between the rate of QSF generation and the rate of QSF dilution, we were able to gather all experimental results into a single master curve, showing a sharp cell transition between high and low motile behaviors with increasing QSF.

Conclusion

This study unambiguously demonstrates the central role played by QSF on amoeboid motion in the growth phase.

The role of mechanical forces in plant morphogenesis

The shape of an organism relies on a complex network of genetic regulations and on the homeostasis and distribution of growth factors. In parallel to the molecular control of growth, shape changes also involve major changes in structure, which by definition depend on the laws of mechanics. Thus, to understand morphogenesis, scientists have turned to interdisciplinary approaches associating biology and physics to investigate the contribution of mechanical forces in morphogenesis, sometimes re-examining theoretical concepts that were laid out by early physiologists. Major advances in the field have notably been possible thanks to the development of computer simulations and live quantitative imaging protocols in recent years. Here, we present the mechanical basis of shape changes in plants, focusing our discussion on undifferentiated tissues. How can growth be translated into a quantified geometrical output? What is the mechanical basis of cell and tissue growth? What is the contribution of mechanical forces in patterning?

Eukaryotic chemotaxis

During eukaryotic chemotaxis, external chemical gradients guide the crawling motion of cells. This process plays an important role in a large variety of biological systems and has wide ranging medical implications. New experimental techniques including confocal microscopy and microfluidics have advanced our understanding of chemotaxis while numerical modeling efforts are beginning to offer critical insights. In this short review, we survey the current experimental status of the field by dividing chemotaxis into three distinct "modules": directional sensing, polarity and motility. For each module, we attempt to point out potential new directions of research and discuss how modeling studies interact with experimental investigations.

Ca2+ chemotaxis in Dictyostelium discoideum

Using a newly developed microfluidic chamber, we have demonstrated in vitro that Ca(2+) functions as a chemoattractant of aggregation-competent Dictyostelium discoideum amoebae, that parallel spatial gradients of cAMP and Ca(2+) are more effective than either alone, and that cAMP functions as a stronger chemoattractant than Ca(2+). Effective Ca(2+) gradients are extremely steep compared with effective cAMP gradients. This presents a paradox because there is no indication to date that steep Ca(2+) gradients are generated in aggregation territories. However, given that Ca(2+) chemotaxis is co-acquired with cAMP chemotaxis during development, we speculate on the role that Ca(2+) chemotaxis might have and the possibility that steep, transient Ca(2+) gradients are generated during natural aggregation in the interstitial regions between cells.

External and internal constraints on eukaryotic chemotaxis

Chemotaxis, the chemically guided movement of cells, plays an important role in several biological processes including cancer, wound healing, and embryogenesis. Chemotacting cells are able to sense shallow chemical gradients where the concentration of chemoattractant differs by only a few percent from one side of the cell to the other, over a wide range of local concentrations. Exactly what limits the chemotactic ability of these cells is presently unclear. Here we determine the chemotactic response of Dictyostelium cells to exponential gradients of varying steepness and local concentration of the chemoattractant cAMP. We find that the cells are sensitive to the steepness of the gradient as well as to the local concentration. Using information theory techniques, we derive a formula for the mutual information between the input gradient and the spatial distribution of bound receptors and also compute the mutual information between the input gradient and the motility direction in the experiments. A comparison between these quantities reveals that for shallow gradients, in which the concentration difference between the back and the front of a 10-mum-diameter cell is <5%, and for small local concentrations (<10 nM) the intracellular information loss is insignificant. Thus, external fluctuations due to the finite number of receptors dominate and limit the chemotactic response. For steeper gradients and higher local concentrations, the intracellular information processing is suboptimal and results in a smaller mutual information between the input gradient and the motility direction than would have been predicted from the ligand-receptor binding process.

Cell speed, persistence and information transmission during signal relay and collective migration

Collective migration is a key feature of the social amoebae Dictyostelium discoideum, where the binding of chemoattractants leads to the production and secretion of additional chemoattractant and the relay of the signal to neighboring cells. This then guides cells to migrate collectively in a head-to-tail fashion. We used mutants that were defective in signal relay to elucidate which quantitative metrics of cell migration are most strongly affected by signal relay and collective motion. We show that neither signal relay nor collective motion markedly impact the speed of cell migration. Cells maintained a preferred overall direction of motion for several minutes with similar persistence, regardless of whether or not they were attracted to moving neighbors, moving collectively in contact with their neighbors, or simply following a fixed exogenous signal. We quantitatively establish that signal relay not only increases the number of cells that respond to a chemotactic signal, but most remarkably, also transmits information about the location of the source accurately over large distances, independently of the strength of the exogenous signal. We envision that signal relay has a similar key role in the migration of a variety of chemotaxing mammalian cells that can relay chemoattractant signals.

2D and 3D quantitative analysis of cell motility and cytoskeletal dynamics

2D- and 3D-Dynamic Image Analysis Systems (2D- and 3D-DIAS) for quantitative analysis of cell motility and chemotaxis are described. Particular attention is given to protocols that have proven useful in the quantitation of cell shape changes and pseudopod dynamics during basic cell motility (i.e. crawling in the absence of a chemotactic or other type of extracellular signal) and directed motion. In addition, methods provided, highlight the applicability of this approach to the accurate phenotypic characterizations of cytoskeletal mutations in Dictyostelium discoideum, cytoskeletal alterations in metastatic cells, and cytoskeletal defects in chemotactically defective polymorphonuclear neutrophils.

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.

Light microscopy to image and quantify cell movement

For decades, Dictyostelium discoideum has been an efficacious and attractive model system for the study of cell motility, primarily because cells become highly motile during the transition from growth phase to aggregation competence and because the haploid genome is readily amenable to mutation. These crawling amoebae, as well as other motile cells such as polymorphonuclear neutrophils (PMNs), extend pseudopodia, retract pseudopodia, and translocate across a substratum even in the absence of chemoattractant. This phenomenon, referred to as basic motile behavior, has been investigated in Dictyostelium through analysis of cytoskeletal mutants. Likewise, many chemotactic signal transduction pathways and networks have been inferred from studies of Dictyostelium mutants. However, before concluding from mutational analyses that a particular molecule or protein plays a role in chemotaxis, it is imperative to first precisely define its contribution, if any, to basic motile behavior. Here, we describe two-dimensional and three-dimensional technologies that can be coupled with 2D and 3D Dynamic Image Analysis System (2D and 3D-DIAS) software for the analysis of cell motility, shape changes, pseudopod formation, and localization of tagged molecules during basic motile behavior. In addition, we describe a method to analyze the 3D trajectories of microspheres attached to the surface of crawling Dictyostelium cells. We include information on microscopy, image acquisition techniques, and computer hardware that could be reproduced in a typical laboratory setting for motion analysis using 2D and 3D-DIAS software. Finally, we highlight features available in DIAS that have proven insightful in identifying defects in basic motile behavior exhibited by various cytoskeletal and putative signal transduction mutants.

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.

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

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

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.

Oscillatory signaling and network responses during the development of Dictyostelium discoideum

Periodic biological variations reflect interactions among molecules and cells, or even organisms. The Dictyostelium cAMP oscillatory circuit is a highly robust example. cAMP oscillations in Dictyostelium arise intracellularly by a complex interplay of activating and inhibiting pathways, are transmitted extracellularly, and synchronize an entire local population. Once established, cAMP signal-relay persists stably for hours. On a two-dimensional surface, >100,000 cells may form a single coordinated territory. In suspension culture, >10(10) cells can oscillate in harmony. This review focuses on molecular mechanisms that cyclically activate and attenuate signal propagation and on chemotactic responses to oscillatory wave progression.

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.

Arachidonic acid is a chemoattractant for Dictyostelium discoideum cells

Cyclic AMP (cAMP)is a natural chemoattractant of the social amoeba Dictyostelium discoideum. It is detected by cell surface cAMP receptors. Besides a signalling cascade involving phosphatidylinositol 3,4,5-trisphosphate (PIP3), Ca2+ signalling has been shown to have a major role in chemotaxis. Previously, we have shown that arachidonic acid (AA) induces an increase in the cytosolic Ca2+ concentration by causing the release of Ca2+ from intracellular stores and activating influx of extracellular Ca2+. Here we report that AA is a chemoattractant for D. discoideum cells differentiated for 8-9 h. Motility towards a glass capillary filled with an AA solution was dose-dependent and qualitatively comparable to cAMP-induced chemotaxis. Ca2+ played an important role in AA chemotaxis of wild-type Ax2 as ethyleneglycol-bis(b-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA) added to the extracellular buffer strongly inhibited motility. In the HM1049 mutant whose iplA gene encoding a putative Ins(1,4,5)P3 -receptor had been knocked out, chemotaxis was only slightly affected by EGTA. Chemotaxis in the presence of extracellular Ca2+ was similar in both strains. Unlike cAMP, addition of AA to a cell suspension did not change cAMP or cGMP levels. A model for AA chemotaxis based on the findings in this and previous work is presented.

Exploring the control circuit of cell migration by mathematical modeling

We have developed a top-down, rule-based mathematical model to explore the basic principles that coordinate mechanochemical events during animal cell migration, particularly the local-stimulation-global-inhibition model suggested originally for chemotaxis. Cells were modeled as a shape machine that protrudes or retracts in response to a combination of local protrusion and global retraction signals. Using an optimization algorithm to identify parameters that generate specific shapes and migration patterns, we show that the mechanism of local stimulation global inhibition can readily account for the behavior of Dictyostelium under a large collection of conditions. Within this collection, some parameters showed strong correlation, indicating that a normal phenotype may be maintained by complementation among functional modules. In addition, comparison of parameters for control and nocodazole-treated Dictyostelium identified the most prominent effect of microtubules as regulating the rates of retraction and protrusion signal decay, and the extent of global inhibition. Other changes in parameters can lead to profound transformations from amoeboid cells into cells mimicking keratocytes, neurons, or fibroblasts. Thus, a simple circuit of local stimulation-global inhibition can account for a wide range of cell behaviors. A similar top-down approach may be applied to other complex problems and combined with molecular manipulations to define specific protein functions.

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.

Biased random walk by stochastic fluctuations of chemoattractant-receptor interactions at the lower limit of detection

Binding of ligand to its receptor is a stochastic process that exhibits fluctuations in time and space. In chemotaxis, this leads to a noisy input signal. Therefore, in a gradient of chemoattractant, the cell may occasionally experience a "wrong" gradient of occupied receptors. We obtained a simple equation for P(pos), the probability that half of the cell closest to the source of chemoattractant has higher receptor occupancy than the opposite half of the cell. P(pos) depends on four factors, the gradient property delC/sq. root of C, the receptor characteristic R(t)/K(D), a time-averaging constant I, and nonreceptor noise sigma(B). We measured chemotaxis of Dictyostelium cells to known shallow gradients of cAMP and obtained direct estimates for these constants. Furthermore, we observed that in shallow gradients, the measured chemotaxis index is correlated with P(pos), which suggests that chemotaxis in shallow gradients is a pure biased random walk. From the observed chemotaxis and derived time-averaging constant, we deduce that the gradient transducing second messenger has a lifetime of 2-8 s and a diffusion rate constant of approximately 1 microm(2)/s. Potential candidates for such second messengers are discussed.

Towards a molecular understanding of human diseases using Dictyostelium discoideum

The social amoeba Dictyostelium discoideum is increasingly being used as a simple model for the investigation of problems that are relevant to human health. This article focuses on several recent examples of Dictyostelium-based biomedical research, including the analysis of immune-cell disease and chemotaxis, centrosomal abnormalities and lissencephaly, bacterial intracellular pathogenesis, and mechanisms of neuroprotective and anti-cancer drug action. The combination of cellular, genetic and molecular biology techniques that are available in Dictyostelium often makes the analysis of these problems more amenable to study in this system than in mammalian cell culture. Findings that have been made in these areas using Dictyostelium have driven research in mammalian systems and have established Dictyostelium as a powerful model for human-disease analysis.

Nonadaptive regulation of ERK2 in Dictyostelium: implications for mechanisms of cAMP relay

It is assumed that ERK2 in Dictyostelium is subject to adaptive regulation in response to constant extracellular ligand stimulation. We now show, to the contrary, that ERK2 remains active under continuous stimulation, differing from most ligand-activated pathways in chemotactically competent Dictyostelium and other cells. We show that the upstream phosphorylation pathway, responsible for ERK2 activation, transiently responds to receptor stimulation, whereas ERK2 dephosphorylation (deactivation) is inhibited by continuous stimulation. We argue that the net result of these two regulatory actions is a persistently active ERK2 pathway when the extracellular ligand (i.e., cAMP) concentration is held constant and that oscillatory production/destruction of secreted cAMP in chemotaxing cells accounts for the observed oscillatory activity of ERK2. We also show that pathways controlling seven-transmembrane receptor (7-TMR) ERK2 activation/deactivation function independently of G proteins and ligand-induced production of intracellular cAMP and the consequent activation of PKA. Finally, we propose that this regulation enables ERK2 to function both in an oscillatory manner, critical for chemotaxis, and in a persistent manner, necessary for gene expression, as secreted ligand concentration increases during later development. This work redefines mechanisms of ERK2 regulation by 7-TMR signaling in Dictyostelium and establishes new implications for control of signal relay during chemotaxis.

Is there a pilot in a pseudopod?

The concept of pilot pseudopodia is reconsidered 30 years after its inauguration (Gerisch, G., Hülser, D., Malchow, D., Wick, U., 1975. Cell communication by periodic cyclic-AMP pulses. Phil. Trans. R. Soc. Lond. B 272, 181-192). The original hypothesis stated that protruding pseudopodia serve as dynamic sensory organelles that aid a cell in perceiving variations of chemoattractant concentration and, consequently, in navigation during chemotaxis. This influential idea is reevaluated in the light of recent findings about the mechanisms governing chemotactic cell motility, morphology and dynamics of pseudopodia, and about molecular constituents and regulators of pseudopod extension and retraction. It is proposed that stimulation by a chemoattractant modulates speed of pseudopod protrusion and thereby increases cell elongation. Elongation further enhances chemotactic sensitivity of the cell to shallow chemoattractant gradients, reinforces cell polarization, and finally leads to suppression of lateral pseudopodia and continuation of cell migration in the gradient direction.

Initiation of convergence and extension movements of lateral mesoderm during zebrafish gastrulation

Embryonic morphogenesis is accomplished by cellular movements, rearrangements, and cell fate inductions. Vertebrate gastrulation entails morphogenetic processes that generate three germ layers, endoderm, mesoderm, and ectoderm, shaped into head, trunk, and tail. To understand how cell migration mechanistically contributes to tissue shaping during gastrulation, we examined migration of lateral mesoderm in the zebrafish. Our results illustrate that cell behaviors, different from mediolaterally oriented cell intercalation, also promote convergence and extension (C&E). During early gastrulation, upon internalization, individually migrating mesendodermal cells contribute to the elongation of the mesoderm by moving animally, without dorsal movement. Convergence toward dorsal starts later, by 70% epiboly (7.7 hpf). Depending on location along the Animal-Vegetal axis, an animal or vegetal bias is added to the dorsalward movement, so that paths fan out and the lateral mesoderm both converges and extends. Onset of convergence is independent of noncanonical Wnt signaling but is delayed when Stat3 signaling is compromised. To understand which aspects of motility are controlled by guidance cues, we measured turning behavior of lateral mesodermal cells. We show that cells exhibit directional preference, directionally-regulated speed, and turn toward dorsal when off-course. We estimate that ectoderm could supply from a fraction to all the dorsalward displacement seen in mesoderm cells. Using mathematical modeling, we demonstrate that directional preference is sufficient to account for mesoderm convergence and extension, and that, at minimum, two sources of guidance cues could orient cell paths realistically if located in the dorsal midline.

The Shwachman-Bodian-Diamond syndrome gene encodes an RNA-binding protein that localizes to the pseudopod ofDictyosteliumamoebae during chemotaxis

The Shwachman-Bodian-Diamond syndrome (SBDS) is an autosomal disorder with multisystem defects. The Shwachman-Bodian-Diamond syndrome gene (SBDS), which contains mutations in a majority of SBDS patients, encodes a protein of unknown function, although it has been strongly implicated in RNA metabolism. There is also some evidence that it interacts with molecules that regulate cytoskeletal organization. Recently, it has been demonstrated by computer-assisted methods that the single behavioral defect of polymorphonuclear leukocytes (PMNs) of SBDS patients is the incapacity to orient correctly in a spatial gradient of chemoattractant. We considered using the social amoeba Dictyostelium discoideum, a model for PMN chemotaxis, an excellent system for elucidating the function of the SBDS protein. We first identified the homolog of SBDS in D. discoideum and found that the amino acids that are altered in human disease were conserved. Given that several proteins involved in chemotactic orientation localize to the pseudopods of cells undergoing chemotaxis, we tested whether the SBDS gene product did the same. We produced an SBDS-GFP chimeric in-frame fusion gene, and generated transformants either with multiple ectopic insertions of the fusion gene or multiple copies of a non-integrated plasmid carrying the fusion gene. In both cases, the SBDS-GFP protein was dispersed equally through the cytoplasm and pseudopods of cells migrating in buffer. However, we observed differential enrichment of SBDS in the pseudopods of cells treated with the chemoattractant cAMP, suggesting that the SBDS protein may play a role in chemotaxis. In light of these results, we discuss how SBDS might function during chemotaxis.

RasC plays a role in transduction of temporal gradient information in the cyclic-AMP wave of Dictyostelium discoideum

To define the role that RasC plays in motility and chemotaxis, the behavior of a rasC null mutant, rasC-, in buffer and in response to the individual spatial, temporal, and concentration components of a natural cyclic AMP (cAMP) wave was analyzed by using computer-assisted two-dimensional and three-dimensional motion analysis systems. These quantitative studies revealed that rasC- cells translocate at the same velocity and exhibit chemotaxis up spatial gradients of cAMP with the same efficiency as control cells. However, rasC- cells exhibit defects in maintaining anterior-posterior polarity along the substratum and a single anterior pseudopod when translocating in buffer in the absence of an attractant. rasC- cells also exhibit defects in their responses to both the increasing and decreasing temporal gradients of cAMP in the front and the back of a wave. These defects result in the inability of rasC- cells to exhibit chemotaxis in a natural wave of cAMP. The inability to respond normally to temporal gradients of cAMP results in defects in the organization of the cytoskeleton, most notably in the failure of both F actin and myosin II to exit the cortex in response to the decreasing temporal gradient of cAMP in the back of the wave. While the behavioral defect in the front of the wave is similar to that of the myoA-/myoF- myosin I double mutant, the behavioral and cytoskeletal defects in the back of the wave are similar to those of the S13A myosin II regulatory light-chain phosphorylation mutant. Expression array data support the premise that the behavioral defects exhibited by the rasC- mutant are the immediate result of the absence of RasC function.

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.

Sphingosine-1-phosphate plays a role in the suppression of lateral pseudopod formation during Dictyostelium discoideum cell migration and chemotaxis

Sphingosine-1-phosphate (S-1-P) is a bioactive lipid that plays a role in diverse biological processes. It functions both as an extracellular ligand through a family of high-affinity G-protein-coupled receptors, and intracellularly as a second messenger. A growing body of evidence has implicated S-1-P in controlling cell movement and chemotaxis in cultured mammalian cells. Mutant D. discoideum cells, in which the gene encoding the S-1-P lyase had been specifically disrupted by homologous recombination, previously were shown to be defective in pseudopod formation, suggesting that a resulting defect might exist in motility and/or chemotaxis. To test this prediction, we analyzed the behavior of mutant cells in buffer, and in both spatial and temporal gradients of the chemoattractant cAMP, using computer-assisted 2-D and 3-D motion analysis systems. Under all conditions, S-1-P lyase null mutants were unable to suppress lateral pseudopod formation like wild-type control cells. This resulted in a reduction in velocity in buffer and spatial gradients of cAMP. Mutant cells exhibited positive chemotaxis in spatial gradients of cAMP, but did so with lowered efficiency, again because of their inability to suppress lateral pseudopod formation. Mutant cells responded normally to simulated temporal waves of cAMP but mimicked the temporal dynamics of natural chemotactic waves. The effect must be intracellular since no homologs of the S-1-P receptors have been identified in the Dictyostelium genome. The defects in the S-1-P lyase null mutants were similar to those seen in mutants lacking the genes for myosin IA, myosin IB, and clathrin, indicating that S-1-P signaling may play a role in modulating the activity or organization of these cytoskeletal elements in the regulation of lateral pseudopod formation.

Chemotaxis: signalling the way forward

During random locomotion, human neutrophils and Dictyostelium discoideum amoebae repeatedly extend and retract cytoplasmic processes. During directed cell migration--chemotaxis--these pseudopodia form predominantly at the leading edge in response to the local accumulation of certain signalling molecules. Concurrent changes in actin and myosin enable the cell to move towards the stimulus. Recent studies are beginning to identify an intricate network of signalling molecules that mediate these processes, and how these molecules become localized in the cell is now becoming clear.

Phg2, a kinase involved in adhesion and focal site modeling in Dictyostelium

The amoeba Dictyostelium is a simple genetic system for analyzing substrate adhesion, motility and phagocytosis. A new adhesion-defective mutant named phg2 was isolated in this system, and PHG2 encodes a novel serine/threonine kinase with a ras-binding domain. We compared the phenotype of phg2 null cells to other previously isolated adhesion mutants to evaluate the specific role of each gene product. Phg1, Phg2, myosin VII, and talin all play similar roles in cellular adhesion. Like myosin VII and talin, Phg2 also is involved in the organization of the actin cytoskeleton. In addition, phg2 mutant cells have defects in the organization of the actin cytoskeleton at the cell-substrate interface, and in cell motility. Because these last two defects are not seen in phg1, myoVII, or talin mutants, this suggests a specific role for Phg2 in the control of local actin polymerization/depolymerization. This study establishes a functional hierarchy in the roles of Phg1, Phg2, myosinVII, and talin in cellular adhesion, actin cytoskeleton organization, and motility.

Cell polarity and Dictyostelium development

Cell polarity is essential for unicellular and multicellular stages of Dictyostelium development. Chemotaxis during early development requires each cell to rapidly reorganize its cytoskeleton to point towards a source of cAMP. This involves a balance between local induction of F-actin polymerization and suppression of pseudopods that point in other directions. Both the lipid phosphatidylinositol (3,4,5) trisphosphate and the soluble signal cGMP have been implicated in these processes, in addition to conserved and novel proteins. During later development cells adopt newly discovered, alternative modes of movement and interact through adhesion molecules. Finally, cells polarize secretion to particular regions of their surface.

Release of a potent polymorphonuclear leukocyte chemoattractant is regulated by white-opaque switching in Candida albicans

Previous studies employing transmembrane assays suggested that Candida albicans and related species, as well as Saccharomyces cerevisiae, release chemoattractants for human polymorphonuclear leukocytes (PMNs). Because transmembrane assays do not definitively distinguish between chemokinesis and chemotaxis, single-cell chemotaxis assays were used to confirm these findings and test whether mating-type or white-opaque switching affects the release of attractant. Our results demonstrate that C. albicans, C. dubliniensis, C. tropicalis, C. parapsilosis, and C. glabrata release bona fide chemoattractants for PMNs. S. cerevisiae, however, releases a chemokinetic factor but not a chemoattractant. Characterization of the C. albicans chemoattractant revealed that it is a peptide of approximately 1 kDa. Whereas the mating type of C. albicans did not affect the release of chemoattractant, switching did. White-phase cells released chemoattractant, but opaque-phase cells did not. Since the opaque phase of C. albicans represents the mating-competent phenotype, it may be that opaque-phase cells selectively suppress the release of chemoattractant to facilitate mating.

The chemotaxis defect of Shwachman-Diamond Syndrome leukocytes

Shwachman-Diamond Syndrome (SDS) is a rare autosomal recessive, multisystem disorder presenting in childhood with intermittent neutropenia and pancreatic insufficiency. It is characterized by recurrent infections independent of neutropenia, suggesting a functional neutrophil defect. While mutations at a single gene locus (SBDS) appear to be responsible for SDS in a majority of patients, the function of that gene and a specific defect in SDS neutrophil behavior have not been elucidated. Therefore, employing 2D and 3D computer-assisted motion analysis systems, we have analyzed the basic motile behavior and chemotactic responsiveness of individual polymorphonuclear leukocytes (PMNs) of 14 clinically diagnosed SDS patients. It is demonstrated that the basic motile behavior of SDS PMNs is normal in the absence of chemoattractant, that SDS PMNs respond normally to increasing and decreasing temporal gradients of the chemoattractant fMLP, and that SDS PMNs exhibit a normal chemokinetic response to a spatial gradient of fMLP. fMLP receptors were also distributed uniformly through the plasma membrane of SDS PMNs as in control PMNs. SDS PMNs, however, were incapable of orienting in and chemotaxing up a spatial gradient of fMLP. This unique defect in orientation was manifested by the PMNs of every SDS patient tested. The PMNs of an SDS patient who had received an allogenic hematopoietic stem cell transplant, as well as PMNs from a cystic fibrosis patient, oriented normally. These results suggest that the defect in SDS PMNs is in a specific pathway emanating from the fMLP receptor that is involved exclusively in regulating orientation in response to a spatial gradient of fMLP. This pathway must function in parallel with additional pathways, intact in SDS patients, that emanate from the fMLP receptor and regulate responses to temporal rather than spatial changes in receptor occupancy.

Human polymorphonuclear leukocytes respond to waves of chemoattractant, like Dictyostelium

It has been assumed that the natural chemotactic signal that attracts human polymorphonuclear leukocytes (PMNs) over long distances to sites of infection is in the form of a standing spatial gradient of chemoattractant. We have questioned this assumption on the grounds, first, that standing spatial gradients may not be stable over long distances for long periods of time and, second, that in the one animal cell chemotaxis system in which the natural chemotactic signal has been described in space and time, aggregation of Dicytostelium discoideum, the signal is in the form of an outwardly relayed, nondissipating wave of attractant. Here, it is demonstrated that PMNs alter their behavior in each of the four phases of a wave of PMN chemoattractant, fashioned after the Dictyostelium wave, in a manner similar to Dictyostelium. These results demonstrate that PMNs have all of the machinery to respond to a natural wave of attractant, providing support to the hypothesis that the natural signal that attracts PMNs over large distances to sites of infection in the human body may also be in the form of a wave.

Constitutively active protein kinase A disrupts motility and chemotaxis in Dictyostelium discoideum

The deletion of the gene for the regulatory subunit of protein kinase A (PKA) results in constitutively active PKA in the pkaR mutant. To investigate the role of PKA in the basic motile behavior and chemotaxis of Dictyostelium discoideum, pkaR mutant cells were subjected to computer-assisted two- and three-dimensional motion analysis. pkaR mutant cells crawled at only half the speed of wild-type cells in buffer, chemotaxed in spatial gradients of cyclic AMP (cAMP) but with reduced efficiency, were incapable of suppressing lateral pseudopods in the front of temporal waves of cAMP, a requirement for natural chemotaxis, did not exhibit the normal velocity surge in response to the front of a wave, and were incapable of chemotaxing toward an aggregation center in natural waves generated by wild-type cells that made up the majority of cells in mixed cultures. Many of the behavioral defects appeared to be the result of the constitutively ovoid shape of the pkaR mutant cells, which forced the dominant pseudopod off the substratum and to the top of the cell body. The behavioral abnormalities that pkaR mutant cells shared with regA mutant cells are discussed by considering the pathway ERK2 perpendicular RegA perpendicular [cAMP] --> PKA, which emanates from the front of a wave. The results demonstrate that cells must suppress PKA activity in order to elongate along a substratum, suppress lateral-pseudopod formation, and crawl and chemotax efficiently. The results also implicate PKA activation in dismantling cell polarity at the peak and in the back of a natural cAMP wave.