The control of chemotactic cell movement during Dictyostelium morphogenesis

Differential cell motion: A mathematical model of anterior posterior sorting

Here, we investigate how a subpopulation of cells can move through an aggregate of cells. Using a stochastic force-based model of Dictyostelium discoideum when the population is forming a slug, we simulate different strategies for prestalk cells to reliably move to the front of the slug while omitting interaction with the substrate thus ignoring the overall motion of the slug. Of the mechanisms that we simulated, prestalk cells being more directed is the best strategy followed by increased asymmetric motive forces for prestalk cells. The lifetime of the cell adhesion molecules, while not enough to produce differential motion, did modulate the results of the strategies employed. Finally, understanding and simulating the appropriate boundary conditions are essential to correctly predict the motion.

Network topology and movement cost, not updating mechanism, determine the evolution of cooperation in mobile structured populations

Evolutionary models are used to study the self-organisation of collective action, often incorporating population structure due to its ubiquitous presence and long-known impact on emerging phenomena. We investigate the evolution of multiplayer cooperation in mobile structured populations, where individuals move strategically on networks and interact with those they meet in groups of variable size. We find that the evolution of multiplayer cooperation primarily depends on the network topology and movement cost while using different stochastic update rules seldom influences evolutionary outcomes. Cooperation robustly co-evolves with movement on complete networks and structure has a partially detrimental effect on it. These findings contrast an established principle from evolutionary graph theory that cooperation can only emerge under some update rules and if the average degree is lower than the reward-to-cost ratio and the network far from complete. We find that group-dependent movement erases the locality of interactions, suppresses the impact of evolutionary structural viscosity on the fitness of individuals, and leads to assortative behaviour that is much more powerful than viscosity in promoting cooperation. We analyse the differences remaining between update rules through a comparison of evolutionary outcomes and fixation probabilities.

A survey on physarum polycephalum intelligent foraging behaviour and bio-inspired applications

In recent years, research on Physarum polycephalum has become more popular after Nakagaki (AIR 407: 6803-470, 2000) performed their famous experiment showing that Physarum was able to find the shortest route through a maze. Subsequent researches have confirmed the ability of Physarum-inspired algorithms to solve a wide range of real-world applications. In contrast to previous reviews that either focus on biological aspects or bio-inspired applications, here we present a comprehensive review that highlights recent Physarum polycephalum biological aspects, mathematical models, and Physarum bio-inspired algorithms and their applications. The novelty of this review stems from our exploration of Physarum intelligent behaviour in competition settings. Further, we have presented our new model to simulate Physarum in competition, where multiple Physarum interact with each other and with their environments. The bio-inspired Physarum in competition algorithms proved to have great potentials for future research.

ATP levels influence cell movement during the mound phase in Dictyostelium discoideum as revealed by ATP visualization and simulation

Cell migration plays an important role in multicellular organism development. The cellular slime mold Dictyostelium discoideum is a useful model organism for the study of cell migration during development. Although cellular ATP levels are known to determine cell fate during development, the underlying mechanism remains unclear. Here, we report that ATP-rich cells efficiently move to the central tip region of the mound against rotational movement during the mound phase. A simulation analysis based on an agent-based model reproduces the movement of ATP-rich cells observed in the experiments. These findings indicate that ATP-rich cells have the ability to move against the bulk flow of cells, suggesting a mechanism by which high ATP levels determine the cell fate of differentiation.

Pressure sensing through Piezo channels controls whether cells migrate with blebs or pseudopods

Cells migrating within the body perform vital functions in development and for defense and repair of tissues. In this dense environment, cells encounter mechanical forces and constraints not experienced when moving under buffer, and, accordingly, many change how they move. We find that gentle squashing, which mimics mechanical resistance, causes cells to move using blebs—a form of projection driven by fluid pressure—rather than pseudopods. This behavior depends on the Piezo stretch-operated ion channel in the cell membrane and calcium fluxes into the cell. Piezo is highly conserved and is required for light touch sensation; this work extends its functions into migrating cells.

Blebs and pseudopods can both power cell migration, with blebs often favored in tissues, where cells encounter increased mechanical resistance. To investigate how migrating cells detect and respond to mechanical forces, we used a “cell squasher” to apply uniaxial pressure to Dictyostelium cells chemotaxing under soft agarose. As little as 100 Pa causes a rapid (<10 s), sustained shift to movement with blebs rather than pseudopods. Cells are flattened under load and lose volume; the actin cytoskeleton is reorganized, with myosin II recruited to the cortex, which may pressurize the cytoplasm for blebbing. The transition to bleb-driven motility requires extracellular calcium and is accompanied by increased cytosolic calcium. It is largely abrogated in cells lacking the Piezo stretch-operated channel; under load, these cells persist in using pseudopods and chemotax poorly. We propose that migrating cells sense pressure through Piezo, which mediates calcium influx, directing movement with blebs instead of pseudopods.

Universal aspects of collective behavior in chemotactic systems

We investigate the collective dynamics of particles (e.g., microorganisms) interacting via chemotactic gradients. Specifically, we focus on continuum models for chemotaxis that include a damping of the chemical production with increasing local particle density and/or systems where the chemotactic sensitivity is reduced with increasing local concentration of the chemical. Using a recently introduced perturbative method [Phys. Rev. E 98, 020603 (2018)10.1103/PhysRevE.98.020603], we show that the onset of particle clustering in these systems is described by the universal Cahn-Hilliard (CH) model. On the one hand, this establishes particle-conserving models for chemotaxis as a further example for the universal class of nonequilibrium demixing phenomena we call active phase separation. On the other hand, the reduction to the CH model allows an analytical determination of suitable parameter ranges wherein, e.g., the transition to spatial density modulations is continuous and/or undesired blow-up solutions can be avoided. A comparison between the numerical solutions of the chemotaxis model and the derived CH model also provides the parameter range where the basic chemotaxis model behaves like other systems showing active phase separation, including the coarsening behavior in two spatial dimensions. Our approach highlights how basic principles of pattern formation theory allow the identification of common basic properties in different chemotaxis models.

Some chemotactic mutants can be progress through development in chimeric populations

Cell migration in response to morphogen gradients affects morphogenesis. Chemotaxis towards adenosine 3', 5'-monophosphate (cAMP) is essential for the early stage of morphogenesis in the slime mold Dictyostelium discoideum. Here, we show that D. discoideum completes morphogenesis without cAMP-chemotaxis-dependent cell migration. The extracellular cAMP gradient is believed to cause cells to form a slug-shaped multicellular structure and fruiting body. The cAMP receptor, cAR1, was not expressed at the cell surface during these stages, correlating with reduced chemotactic activity. Gβ-null cells expressing temperature sensitive Gβ are unable to generate extracellular cAMP (Jin et al., 1998) and thus unable to aggregate and exhibit proper morphogenesis under restrictive temperature. However, when mixed with wild type cells ts-Gβ expressing gβ-null cells normally aggregated and exhibited normal morphogenesis under restrictive temperature. Furthermore, cells migrated after aggregation in a mixture containing wild-type cells. KI-5 cells, which do not show aggregation or morphogenesis, spontaneously migrated to a transplanted wild-type tip and underwent normal morphogenesis and cell differentiation; this was not observed in cells lacking tgrB1and tgrC1 cells adhesion molecules. Thus, cAMP gradient-dependent cell migration may not be required for multicellular pattern formation in late Dictyostelium development.

Evasion of phagotrophic predation by protist hosts and innate immunity of metazoan hosts by Legionella pneumophila

Legionella pneumophila is a ubiquitous environmental bacterium that has evolved to infect and proliferate within amoebae and other protists. It is thought that accidental inhalation of contaminated water particles by humans is what has enabled this pathogen to proliferate within alveolar macrophages and cause pneumonia. However, the highly evolved macrophages are equipped with more sophisticated innate defence mechanisms than are protists, such as the evolution of phagotrophic feeding into phagocytosis with more evolved innate defence processes. Not surprisingly, the majority of proteins involved in phagosome biogenesis (~80%) have origins in the phagotrophy stage of evolution. There are a plethora of highly evolved cellular and innate metazoan processes, not represented in protist biology, that are modulated by L. pneumophila, including TLR2 signalling, NF-κB, apoptotic and inflammatory processes, histone modification, caspases, and the NLRC-Naip5 inflammasomes. Importantly, L. pneumophila infects haemocytes of the invertebrate Galleria mellonella, kill G. mellonella larvae, and proliferate in and kill Drosophila adult flies and Caenorhabditis elegans. Although coevolution with protist hosts has provided a substantial blueprint for L. pneumophila to infect macrophages, we discuss the further evolutionary aspects of coevolution of L. pneumophila and its adaptation to modulate various highly evolved innate metazoan processes prior to becoming a human pathogen.

Nodal signaling has dual roles in fate specification and directed migration during germ layer segregation in zebrafish

During gastrulation, endodermal cells actively migrate to the interior of the embryo, but the signals that initiate and coordinate this migration are poorly understood. By transplanting ectopically induced endodermal cells far from the normal location of endoderm specification, we identified the inputs that drive internalization without the confounding influences of fate specification and global morphogenic movements. We find that Nodal signaling triggers an autocrine circuit for initiating endodermal internalization. Activation of the Nodal receptor directs endodermal specification through sox32 and also induces expression of more Nodal ligands. These ligands act in an autocrine fashion to initiate endodermal cell sorting. Our work defines an 'AND' gate consisting of sox32-dependent endodermal specification and Nodal ligand reception controlling endodermal cell sorting to the inner layer of the embryo at the onset of gastrulation.

Collective behaviour and swarm intelligence in slime moulds

The study of collective behaviour aims to understand how individual-level behaviours can lead to complex group-level patterns. Collective behaviour has primarily been studied in animal groups such as colonies of insects, flocks of birds and schools of fish. Although less studied, collective behaviour also occurs in microorganisms. Here, we argue that slime moulds are powerful model systems for solving several outstanding questions in collective behaviour. In particular, slime mould may hold the key to linking individual-level mechanisms to colony-level behaviours. Using well-established principles of collective animal behaviour as a framework, we discuss the extent to which slime mould collectives are comparable to animal groups, and we highlight some potentially fruitful areas for future research.

When Dicty Met Myco, a (Not So) Romantic Story about One Amoeba and Its Intracellular Pathogen

In recent years, Dictyostelium discoideum has become an important model organism to study the cell biology of professional phagocytes. This amoeba not only shares many molecular features with mammalian macrophages, but most of its fundamental signal transduction pathways are conserved in humans. The broad range of existing genetic and biochemical tools, together with its suitability for cell culture and live microscopy, make D. discoideum an ideal and versatile laboratory organism. In this review, we focus on the use of D. discoideum as a phagocyte model for the study of mycobacterial infections, in particular Mycobacterium marinum. We look in detail at the intracellular cycle of M. marinum, from its uptake by D. discoideum to its active or passive egress into the extracellular medium. In addition, we describe the molecular mechanisms that both the mycobacterial invader and the amoeboid host have developed to fight against each other, and compare and contrast with those developed by mammalian phagocytes. Finally, we introduce the methods and specific tools that have been used so far to monitor the D. discoideum-M. marinum interaction.

Biological Activity of the Alternative Promoters of the Dictyostelium discoideum Adenylyl Cyclase A Gene

Amoebae of the Dictyostelium discoideum species form multicellular fruiting bodies upon starvation. Cyclic adenosine monophosphate (cAMP) is used as intercellular signalling molecule in cell-aggregation, cell differentiation and morphogenesis. This molecule is synthesized by three adenylyl cyclases, one of which, ACA, is required for cell aggregation. The gene coding for ACA (acaA) is transcribed from three different promoters that are active at different developmental stages. Promoter 1 is active during cell-aggregation, promoters 2 and 3 are active in prespore and prestalk tip cells at subsequent developmental stages. The biological relevance of acaA expression from each of the promoters has been studied in this article. The acaA gene was expressed in acaA-mutant cells, that do not aggregate, under control of each of the three acaA promoters. acaA expression under promoter 1 control induced cell aggregation although subsequent development was delayed, very small fruiting bodies were formed and cell differentiation genes were expressed at very low levels. Promoter 2-driven acaA expression induced the formation of small aggregates and small fruiting bodies were formed at the same time as in wild-type strains and differentiation genes were also expressed at lower levels. Expression of acaA from promoter 3 induced aggregates and fruiting bodies formation and their size and the expression of differentiation genes were more similar to that of wild-type cells. Expression of acaA from promoters 1 and 2 in AX4 cells also produced smaller structures. In conclusion, the expression of acaA under control of the aggregation-specific Promoter 1 is able to induce cell aggregation in acaA-mutant strains. Expression from promoters 2 and 3 also recovered aggregation and development although promoter 3 induced a more complete recovery of fruiting body formation.

Tuning active emulsion dynamics via surfactants and topology

We study water-in-oil emulsion droplets, running the Belousov-Zhabotinsky reaction, that form a new type of synthetic active matter unit. These droplets, stabilised by surfactants dispersed in the oil medium, are capable of internal chemical oscillations and self-propulsion. Here we present studies of networks of such self-propelled chemical oscillators and show that the resulting dynamics depend strongly on the topology of the active matter units and their connections. The chemical oscillations can couple via the exchange of promoter and inhibitor type of reaction intermediates across the droplets under precise conditions of surfactant bilayer formation between the droplets. The self-emerging synchronization dynamics are then characterized by the topology of the oscillator networks. Further, we show that the chemical oscillations inside the droplets cause oscillatory speed variations in the motion of individual droplets, extending our previous studies on such swimmers. Finally, we demonstrate that qualitatively new types of self-propelled motion can occur when simple droplet networks, for example two droplets connected by a bilayer, are set into motion. Altogether, these results lead to exciting possibilities in future studies of autonomous active matter.

Dictyostelium discoideum SecG interprets cAMP-mediated chemotactic signals to influence actin organization

Tight control of actin cytoskeletal dynamics is essential for proper cell function and survival. Arf nucleotide binding-site opener (ARNO), a mammalian guanine nucleotide exchange factor for Arf, has been implicated in actin cytoskeletal regulation but its exact role is still unknown. To explore the role of ARNO in this regulation as well as in actin-mediated processes, the Dictyostelium discoideum homolog, SecG, was examined. SecG peaks during aggregation and mound formation. The overexpression of SecG arrests development at the mound stage. SecG overexpressing (SecG OE) cells fail to stream during aggregation. Although carA is expressed, SecG OE cells do not chemotax toward cAMP, indicating SecG is involved in the cellular response to cAMP. This chemotactic defect is specific to cAMP-directed chemotaxis, as SecG OE cells chemotax to folate without impairment and exhibit normal cell motility. The chemotactic defects of the SecG mutants may be due to an impaired cAMP response as evidenced by altered cell polarity and F-actin polymerization after cAMP stimulation. Cells overexpressing SecG have increased filopodia compared to wild type cells, implying that excess SecG causes abnormal organization of F-actin. The general function of the cytoskeleton, however, is not disrupted as the SecG OE cells exhibit proper cell-substrate adhesion. Taken together, the results suggest proper SecG levels are needed for appropriate response to cAMP signaling in order to coordinate F-actin organization during development.

Dictyostelium discoideum developmental cycle (DDDC) assay: a tool for Hg toxicity assessment and soil health screening

The social amoeba Dictyostelium discoideum has been proposed for assessing stress responses to pollutants in soil and it has already been successfully employed in the aquatic environment. Presently, we developed the DDDC assay (D. discoideum developmental cycle assay) for both soil toxicity assessment and soil health screening. The DDDC assay is primarily aimed at determining the capacity of D. discoideum to undergo its developmental programme forming a fruiting body, measured in terms of fruiting body formation inhibition and fruiting body size factor, which may be considered an indication of its ecological fitness (potential for spore dispersal). A second objective of the solid phase DDDC assay is to identify potential mechanisms of toxic action on the developmental cycle, for which three checkpoints are examined: (a) aggregation arrest, (b) migration arrest, and (c) culmination arrest. Presently, conditions for the DDDC assay such as soil texture, soil water content, soil pH, food availability and incubation time were investigated and optimized. In addition, both solid and liquid phase variants of the DDDC assay were applied to assess the toxicity of Hg, at regulatory concentrations. The developmental cycle and ecological fitness were affected from the exposure to 0.3 mg Hg/kg dry-wt soil onwards. The DDDC assay has been shown to be a high sensitivity test.

Ca2+ signaling regulates ecmB expression, cell differentiation and slug regeneration in Dictyostelium

Ca(2+) regulates cell differentiation and morphogenesis in a diversity of organisms and dysregulation of Ca(2+) signal transduction pathways leads to many cellular pathologies. In Dictyostelium Ca(2+) induces ecmB expression and stalk cell differentiation in vitro. Here we have analyzed the pattern of ecmB expression in intact and bisected slugs and the effect of agents that affect Ca(2+) levels or antagonize calmodulin (CaM) on this expression pattern. We have shown that Ca(2+) and CaM regulate ecmB expression and pstAB/pstB cell differentiation in vivo. Agents that increase intracellular Ca(2+) levels increased ecmB expression and/or pstAB and pstB cell differentiation, while agents that decrease intracellular Ca(2+) or antagonize CaM decreased it. In isolated slug tips agents that affect Ca(2+) levels and antagonize CaM had differential effect on ecmB expression and cell differentiation in the anterior versus posterior zones. Agents that increase intracellular Ca(2+) levels increased the number of ecmB expressing cells in the anterior region of slugs, while agents that decrease intracellular Ca(2+) levels or antagonize CaM activity increased the number of ecmB expressing cells in the posterior. We have also demonstrated that agents that affect Ca(2+) levels or antagonize CaM affect cells motility and regeneration of shape in isolated slug tips and backs and regeneration of tips in isolated slug backs. To our knowledge, this is the first study detailing the pattern of ecmB expression in regenerating slugs as well as the role of Ca(2+) and CaM in the regeneration process and ecmB expression.

Colchicine affects cell motility, pattern formation and stalk cell differentiation in Dictyostelium by altering calcium signaling

Previous work, verified here, showed that colchicine affects Dictyostelium pattern formation, disrupts morphogenesis, inhibits spore differentiation and induces terminal stalk cell differentiation. Here we show that colchicine specifically induces ecmB expression and enhances accumulation of ecmB-expressing cells at the posterior end of multicellular structures. Colchicine did not induce a nuclear translocation of DimB, a DIF-1 responsive transcription factor in vitro. It also induced terminal stalk cell differentiation in a mutant strain that does not produce DIF-1 (dmtA-) and after the treatment of cells with DIF-1 synthesis inhibitor cerulenin (100 μM). This suggests that colchicine induces the differentiation of ecmB-expressing cells independent of DIF-1 production and likely through a signaling pathway that is distinct from the one that is utilized by DIF-1. Depending on concentration, colchicine enhanced random cell motility, but not chemotaxis, by 3-5 fold (10-50 mM colchicine, respectively) through a Ca(2+)-mediated signaling pathway involving phospholipase C, calmodulin and heterotrimeric G proteins. Colchicine's effects were not due to microtubule depolymerization as other microtubule-depolymerizing agents did not have these effects. Finally normal morphogenesis and stalk and spore cell differentiation of cells treated with 10 mM colchicine were rescued through chelation of Ca2+ by BAPTA-AM and EDTA and calmodulin antagonism by W-7 but not PLC inhibition by U-73122. Morphogenesis or spore cell differentiation of cells treated with 50 mM colchicine could not be rescued by the above treatments but terminal stalk cell differentiation was inhibited by BAPTA-AM, EDTA and W-7, but not U-73122. Thus colchicine disrupts morphogenesis and induces stalk cell differentiation through a Ca(2+)-mediated signaling pathway involving specific changes in gene expression and cell motility.

Paxillin and phospholipase D interact to regulate actin-based processes in Dictyostelium discoideum

The actin cytoskeleton forms a membrane-associated network whose proper regulation is essential for numerous processes, including cell differentiation, proliferation, adhesion, chemotaxis, endocytosis, exocytosis, and multicellular development. In this report, we show that in Dictyostelium discoideum, paxillin (PaxB) and phospholipase D (PldB) colocalize and coimmunoprecipitate, suggesting that they interact physically. Additionally, the phenotypes observed during development, cell sorting, and several actin-required processes, including cyclic AMP (cAMP) chemotaxis, cell-substrate adhesion, actin polymerization, phagocytosis, and exocytosis, reveal a genetic interaction between paxB and pldB, suggesting a functional interaction between their gene products. Taken together, our data point to PldB being a required binding partner of PaxB during processes involving actin reorganization.

The Dictyostelium discoideum acaA gene is transcribed from alternative promoters during aggregation and multicellular development

BACKGROUND

Extracellular cAMP is a key extracellular signaling molecule that regulates aggregation, cell differentiation and morphogenesis during multi-cellular development of the social amoeba Dictyostelium discoideum. This molecule is produced by three different adenylyl cyclases, encoded by the genes acaA, acrA and acgA, expressed at different stages of development and in different structures.

METHODOLOGY/PRINCIPAL FINDINGS

This article describes the characterization of the promoter region of the acaA gene, showing that it is transcribed from three different alternative promoters. The distal promoter, promoter 1, is active during the aggregation process while the more proximal promoters are active in tip-organiser and posterior regions of the structures. A DNA fragment containing the three promoters drove expression to these same regions and similar results were obtained by in situ hybridization. Analyses of mRNA expression by quantitative RT-PCR with specific primers for each of the three transcripts also demonstrated their different temporal patterns of expression.

CONCLUSIONS/SIGNIFICANCE

The existence of an aggregation-specific promoter can be associated with the use of cAMP as chemo-attractant molecule, which is specific for some Dictyostelium species. Expression at late developmental stages indicates that adenylyl cyclase A might play a more important role in post-aggregative development than previously considered.

MAP kinases have different functions in Dictyostelium G protein-mediated signaling

Extracellular signal regulated kinases (ERKs) are a class of MAP kinases that function in many signaling pathways in eukaryotic cells and in some cases, a single stimulus can activate more than one ERK suggesting functional redundancy or divergence from a common pathway. Dictyostelium discoideum encodes only two MAP kinases, ERK1 and ERK2, that both function during the developmental life cycle. To determine if ERK1 and ERK2 have overlapping functions, chemotactic and developmental phenotypes of erk1(-) and erk2(-) mutants were assessed with respect to G protein-mediated signal transduction pathways. ERK1 was specifically required for Galpha5-mediated tip morphogenesis and inhibition of folate chemotaxis but not for cAMP-stimulated chemotaxis or cGMP accumulation. ERK2 was the primary MAPK phosphorylated in response to folate or cAMP stimulation. Cell growth was not altered in erk1(-), erk2(-) or erk1(-)erk2(-) mutants but each mutant displayed a different pattern of cell sorting in chimeric aggregates. The distribution of GFP-ERK1 or GFP-ERK2 fusion proteins in the cytoplasm and nucleus was not grossly altered in cells stimulated with cAMP or folate. These results suggest ERK1 and ERK2 have different roles in G protein-mediated signaling during growth and development.

DIF-1 induces the basal disc of the Dictyostelium fruiting body

The polyketide DIF-1 induces Dictyostelium amoebae to form stalk cells in culture. To better define its role in normal development, we examined the phenotype of a mutant blocking the first step of DIF-1 synthesis, which lacks both DIF-1 and its biosynthetic intermediate, dM-DIF-1 (des-methyl-DIF-1). Slugs of this polyketide synthase mutant (stlB(-)) are long and thin and rapidly break up, leaving an immotile prespore mass. They have approximately 30% fewer prestalk cells than their wild-type parent and lack a subset of anterior-like cells, which later form the outer basal disc. This structure is missing from the fruiting body, which perhaps in consequence initiates culmination along the substratum. The lower cup is rudimentary at best and the spore mass, lacking support, slips down the stalk. The dmtA(-) methyltransferase mutant, blocked in the last step of DIF-1 synthesis, resembles the stlB(-) mutant but has delayed tip formation and fewer prestalk-O cells. This difference may be due to accumulation of dM-DIF-1 in the dmtA(-) mutant, since dM-DIF-1 inhibits prestalk-O differentiation. Thus, DIF-1 is required for slug migration and specifies the anterior-like cells forming the basal disc and much of the lower cup; significantly the DIF-1 biosynthetic pathway may supply a second signal - dM-DIF-1.

Evolution of size and pattern in the social amoebas

A fundamental goal of biology is to understand how novel phenotypes evolved through changes in existing genes. The Dictyostelia or social amoebas represent a simple form of multicellularity, where starving cells aggregate to build fruiting structures. This review summarizes efforts to provide a framework for investigating the genetic changes that generated novel morphologies in the Dictyostelia. The foundation is a recently constructed molecular phylogeny of the Dictyostelia, which was used to examine trends in the evolution of novel forms and in the divergence of genes that shape these forms. There is a major trend towards the formation of large unbranched fruiting bodies, which is correlated with the use of cyclic AMP (cAMP) as a secreted signal to coordinate cell aggregation. The role of cAMP in aggregation arose through co‐option of a pathway that originally acted to coordinate fruiting body formation. The genotypic changes that caused this innovation and the role of dynamic cAMP signaling in defining fruiting body size and pattern throughout social amoeba evolution are discussed. BioEssays 29:635–644, 2007. © 2007 Wiley Periodicals, Inc.

Mechanisms of Gradient Detection: A Comparison of Axon Pathfinding with Eukaryotic Cell Migration

The detection of gradients of chemotactic cues is a common task for migrating cells and outgrowing axons. Eukaryotic gradient detection employs a spatial mechanism, meaning that the external gradient has to be translated into an intracellular signaling gradient, which affects cell polarization and directional movement. The sensitivity of gradient detection is governed by signal amplification and adaptation mechanisms. Comparison of the major signal transduction pathways underlying gradient detection in three exemplary chemotaxing cell types, Dictyostelium, neutrophils, and fibroblasts and in neuronal growth cones, reveals conserved mechanisms such as localized PI3 kinase/PIP3 signaling and a common output, the regulation of the cytoskeleton by Rho GTPases. Local protein translation plays a role in directional movement of both fibroblasts and neuronal growth cones. Ca(2+) signaling is prominently involved in growth cone gradient detection. The diversity of signaling between different cell types and its functional implications make sense in the biological context.

Disruption of the ifkA and ifkB genes results in altered cell adhesion, morphological defects and a propensity to form pre-stalk O cells during development of Dictyostelium

IfkA and ifkB are two GCN2-like genes present in Dictyostelium. Disruption of either gene alone results in subtle developmental defects. However, disruption of ifkA and ifkB within the same strain results in severe morphological and patterning defects in the developing double null cells. The mutant cells aggregate in streams that give tightly clumped mounds. Fingers form from the mounds but remain attached to one another, especially at their bases. The fingers culminate to give fused and entangled structures lacking proper stalk but containing some spores. The morphological defects are consistent with an enhanced cell-cell and cell-substrate adhesiveness of the developing double null cells, which may result in inappropriate cell contacts and altered cell motility and sorting properties. In ifkA/ifkB nulls, cell type proportioning and patterning is altered in favor of ALC/pstO cell types. The bias toward the ALC/pstO cell types may be due, in part, to the nuclear localization of the transcription factor STATc in growing ifkA/ifkB null cells. STATc normally becomes localized to the nucleus during finger formation and only within the pre-stalk O zone. The precocious nuclear localization seen in the mutant cells may predispose the cells to a ALC/pstO cell fate. The findings indicate that IfkA and IfkB have redundant functions in Dictyostelium morphogenesis that involve maintaining proper cell-cell and cell-substrate adhesion and the equilibrium between different cell types for proper spatial patterning.

Moving forward moving backward: directional sorting of chemotactic cells due to size and adhesion differences

Differential movement of individual cells within tissues is an important yet poorly understood process in biological development. Here we present a computational study of cell sorting caused by a combination of cell adhesion and chemotaxis, where we assume that all cells respond equally to the chemotactic signal. To capture in our model mesoscopic properties of biological cells, such as their size and deformability, we use the Cellular Potts Model, a multiscale, cell-based Monte Carlo model. We demonstrate a rich array of cell-sorting phenomena, which depend on a combination of mescoscopic cell properties and tissue level constraints. Under the conditions studied, cell sorting is a fast process, which scales linearly with tissue size. We demonstrate the occurrence of "absolute negative mobility", which means that cells may move in the direction opposite to the applied force (here chemotaxis). Moreover, during the sorting, cells may even reverse the direction of motion. Another interesting phenomenon is "minority sorting", where the direction of movement does not depend on cell type, but on the frequency of the cell type in the tissue. A special case is the cAMP-wave-driven chemotaxis of Dictyostelium cells, which generates pressure waves that guide the sorting. The mechanisms we describe can easily be overlooked in studies of differential cell movement, hence certain experimental observations may be misinterpreted.

A cysteine-rich extracellular protein containing a PA14 domain mediates quorum sensing in Dictyostelium discoideum

Much remains to be understood about quorum-sensing factors that allow cells to sense their local density. Dictyostelium discoideum is a simple eukaryote that grows as single-celled amoebae and switches to multicellular development when food becomes limited. As the growing cells reach a high density, they begin expressing discoidin genes. The cells secrete an unknown factor, and at high cell densities the concomitant high levels of the factor induce discoidin expression. We report here the enrichment of discoidin-inducing complex (DIC), an approximately 400-kDa protein complex that induces discoidin expression during growth and development. Two proteins in the DIC preparation, DicA1 and DicB, were identified by sequencing proteolytic digests. DicA1 and DicB were expressed in Escherichia coli and tested for their ability to induce discoidin during growth and development. Recombinant DicB was unable to induce discoidin expression, while recombinant DicA1 was able to induce discoidin expression. This suggests that DicA1 is an active component of DIC and indicates that posttranslational modification is dispensable for activity. DicA1 mRNA is expressed in vegetative and developing cells. The mature secreted form of DicA1 has a molecular mass of 80 kDa and has a 24-amino-acid cysteine-rich repeat that is similar to repeats in Dictyostelium proteins, such as the extracellular matrix protein ecmB/PstA, the prespore cell-inducing factor PSI, and the cyclic AMP phosphodiesterase inhibitor PDI. Together, the data suggest that DicA1 is a component of a secreted quorum-sensing signal regulating discoidin gene expression during Dictyostelium growth and development.

LrrA, a novel leucine-rich repeat protein involved in cytoskeleton remodeling, is required for multicellular morphogenesis in Dictyostelium discoideum

Cell sorting by differential cell adhesion and movement is a fundamental process in multicellular morphogenesis. We have identified a Dictyostelium discoideum gene encoding a novel protein, LrrA, which composes almost entirely leucine-rich repeats (LRRs) including a putative leucine zipper motif. Transcription of lrrA appeared to be developmentally regulated with robust expression during vegetative growth and early development. lrrA null cells generated by homologous recombination aggregated to form loose mounds, but subsequent morphogenesis was blocked without formation of the apical tip. The cells adhered poorly to a substratum and did not form tight cell-cell agglomerates in suspension; in addition, they were unable to polarize and exhibit chemotactic movement in the submerged aggregation and Dunn chamber chemotaxis assays. Fluorescence-conjugated phalloidin staining revealed that both vegetative and aggregation competent lrrA(-) cells contained numerous F-actin-enriched microspikes around the periphery of cells. Quantitative analysis of the fluorescence-stained F-actin showed that lrrA(-) cells exhibited a dramatically increase in F-actin as compared to the wild-type cells. When developed together with wild-type cells, lrrA(-) cells were unable to move to the apical tip and sorted preferentially to the rear and lower cup regions. These results indicate that LrrA involves in cytoskeleton remodeling, which is needed for normal chemotactic aggregation and efficient cell sorting during multicellular morphogenesis, particularly in the formation of apical tip.

A Phytophthora infestans G-protein beta subunit is involved in sporangium formation

The heterotrimeric G-protein pathway regulates cellular responses to a wide range of extracellular signals in virtually all eukaryotes. It also controls various developmental processes in the oomycete plant pathogen Phytophthora infestans, as was concluded from previous studies on the role of the G-protein alpha-subunit PiGPA1 in this organism. The expression of the P. infestans G-protein beta-subunit gene Pigpb1 was induced in nutrient-starved mycelium before the onset of sporangium formation. The gene was hardly expressed in mycelium incubated in rich growth medium. The introduction of additional copies of Pigpb1 into the genome led to silencing of the gene and resulted in transformants deficient in PiGPB1. These Pigpb1-silenced mutants formed very few asexual spores (sporangia) when cultured in rye sucrose medium and produced a denser mat of aerial mycelium than the wild type. Partially Pigpb1-silenced mutants showed intermediate phenotypes with regard to sporulation, and a relatively large number of their sporangia were malformed. The results show that PiGPB1 is important for vegetative growth and sporulation and, therefore, for the pathogenicity of this organism.

Cell sorting by differential cell motility: a model for pattern formation in Dictyostelium

In the slug stage of the cellular slime mold Dictyostelium discoideum, prespore cells and four types of prestalk cells show a well-defined spatial distribution in a migrating slug. We have developed a continuous mathematical model for the distribution pattern of these cell types based on the balance of force in individual cells. In the model, cell types are assumed to have different properties in cell motility, i.e. different motive force, the rate of resistance against cell movement, and diffusion coefficient. Analysis of the stationary solution of the model shows that combination of these parameters and slug speed determines the three-dimensional shape of a slug and cell distribution pattern within it. Based on experimental data of slug motive force and velocity measurements, appropriate sets of parameters were chosen so that the cell-type distribution at stationary state matches the distribution in real slugs. With these parameters, we performed numerical calculation of the model in two-dimensional space using a moving particle method. The results reproduced many of the basic features of slug morphogenesis, i.e. cell sorting, translocation of the prestalk region, elongation of the slug, and its steady migration.

The cyclase-associated protein CAP as regulator of cell polarity and cAMP signaling in Dictyostelium

Cyclase-associated protein (CAP) is an evolutionarily conserved regulator of the G-actin/F-actin ratio and, in yeast, is involved in regulating the adenylyl cyclase activity. We show that cell polarization, F-actin organization, and phototaxis are altered in a Dictyostelium CAP knockout mutant. Furthermore, in complementation assays we determined the roles of the individual domains in signaling and regulation of the actin cytoskeleton. We studied in detail the adenylyl cyclase activity and found that the mutant cells have normal levels of the aggregation phase-specific adenylyl cyclase and that receptor-mediated activation is intact. However, cAMP relay that is responsible for the generation of propagating cAMP waves that control the chemotactic aggregation of starving Dictyostelium cells was altered, and the cAMP-induced cGMP production was significantly reduced. The data suggest an interaction of CAP with adenylyl cyclase in Dictyostelium and an influence on signaling pathways directly as well as through its function as a regulatory component of the cytoskeleton.

Modelling of Dictyostelium discoideum slug migration

The development of most multicellular organisms involves differential movement of cells resulting in the formation of tissues. The principles governing these movements are poorly understood. One exception is the formation of the slug in the social amoebae Dictyostelium discoideum. The slug forms by the chemotactic aggregation of up to 10(5) starving cells, it is motile and migrates in response to light and temperature gradients to the surface of the soil to form a fruiting body consisting of a stalk supporting a spore head. Slug migration and behaviour result from coordinated chemotactic movement of the individual cells in the slug. Waves of a chemoattractant, most likely cAMP, are periodically initiated in the tip of the slug and propagate towards the back of the slug resulting in periodic forward movement of individual cells as well as the whole slug. Here we develop a model to investigate how wave propagation and cell movement interacts to result in migration and shape changes of the slug. The slug tissue is modelled as an incompressible liquid, in which waves of chemoattractant are generated in an excitable manner. The liquid is "active", i.e. it is able to generate body forces in response to the gradients of the chemoattractant. These forces lead to the formation of flows (representing chemotactically moving cells) and result in slug movement and shape changes. The model provides a theoretical framework for the understanding of the interactions between cell-cell signalling and cell movement, which govern slug behaviour and tissue morphogenesis.

Modeling the early stages of vascular network assembly

In vertebrates, networks of capillary vessels supply tissues with nutrients. Capillary patterns are closely mimicked by endothelial cells cultured on basement membrane proteins that allow single randomly dispersed cells to self-organize into vascular networks. Here we provide a model including chemoattraction as the fundamental mechanism for cell-to-cell communication in order to identify key parameters in the complexity of the formation of vascular patterns. By flanking biological experiments, theoretical insights and numerical simulations, we provide strong evidence that endothelial cell number and the range of activity of a chemoattractant factor regulate vascular network formation and size. We propose a mechanism linking the scale of formed endothelial structures to the range of cell-to-cell interaction mediated by the release of chemoattractants.

Possible role of contact following in the generation of coherent motion of Dictyostelium cells

After aggregation by chemotaxis, cells of the cellular slime mold Dictyostelium discoideum form a multicellular structure and show coherent motion such as vortices. Here, we present a mathematical model to explain both aggregation and coherent motion of cells in two-dimensional space. The model incorporates chemotactic response of cells and the cell's property, called "contact following", to follow the other cells with which they are in contact. Analytical study and computer simulation using the model show that with contact following, cells form circular clusters within which cell rotation occurs. Unidirectional cell motion in a long belt of cells is another type of solution of the model. Besides, contact following has an effect to accelerate cell cluster merging. By considering the mechanism of cell movement, possible explanations of contact following are proposed.

Cell movement patterns during gastrulation in the chick are controlled by positive and negative chemotaxis mediated by FGF4 and FGF8

During gastrulation in amniotes, epiblast cells ingress through the primitive streak and migrate away to form endodermal, mesodermal, and extraembryonic structures. Here we analyze the detailed movement trajectories of cells emerging at different anterior-posterior positions from the primitive streak, using in vivo imaging of the movement of GFP-tagged streak cells. Cells emerging at different anterior-posterior positions from the streak show characteristic cell migration patterns, in response to guidance signals from neighboring tissues. Streak cells are attracted by sources of FGF4 and repelled by sources of FGF8. The observed movement patterns of anterior streak cells can be explained by an FGF8-mediated chemorepulsion of cells away from the streak followed by chemoattraction toward an FGF4 signal produced by the forming notochord.

Visualizing PI3 kinase-mediated cell-cell signaling during Dictyostelium development

BACKGROUND

Starving amoebae of Dictyostelium discoideum communicate by relaying extracellular cAMP signals, which direct chemotactic movement, resulting in the aggregation of thousands of cells into multicellular aggregates. Both cAMP relay and chemotaxis require the activation of PI3 kinase signaling. The spatiotemporal dynamics of PI3 kinase signaling can be followed in individual cells via the cAMP-induced membrane recruitment of a GFP-tagged PH domain-containing protein, CRAC, which is required for the activation of adenylylcyclase.

RESULTS

We show that polarized periodic CRAC-GFP translocation occurs during the aggregation and mound stages of development in response to periodic cAMP signals. The duration of CRAC translocation to the membrane is determined by the duration of the rising phase of the cAMP signal. The system shows rapid adaptation and responds to the rate of change of the extracellular cAMP concentration. When the cells are in close contact, it takes 10 s for the signal to propagate from one cell to the next. In slugs, all cells show a permanent polarized PI3 kinase signaling in their leading edge, which is dependent on cell-cell contact.

CONCLUSIONS

Measuring the redistribution of GFP-tagged CRAC has enabled us to study the dynamics of PI3 kinase-mediated cell-cell communication at the individual cell level in the multicellular stages of Dictyostelium development. This approach should also be useful to study the interactions between cell-cell signaling, cell polarization, and movement in the development of other organisms.

A novel Dictyostelium gene encoding multiple repeats of adhesion inhibitor-like domains has effects on cell-cell and cell-substrate adhesion

The Dictyostelium protein AmpA (adhesion modulation protein A) is encoded by the gene originally identified by the D11 cDNA clone. AmpA contains repeated domains homologous to a variety of proteins that influence cell adhesion. The protein accumulates during development, reaching a maximal level at the finger stage. Much of the AmpA protein is found extracellularly during development, and in culminants, AmpA is found in association with anterior-like cells. Characterization of an ampA- strain generated by gene replacement reveals a significant increase in cell-cell clumping when cells are starved in nonnutrient buffer suspensions. Developing ampA- cells are also more adhesive to the underlying substrate and are delayed in developmental progression, with the severity of the delay increasing as cells are grown in the presence of bacteria or on tissue culture dishes rather than in suspension culture. Reintroduction of the ampA gene rescues the developmental defects of ampA- cells; however, expression of additional copies of the gene in wild-type cells results in more severe developmental delays and decreased clumping in suspension culture. We propose that the AmpA protein functions as an anti-adhesive to limit cell-cell and cell-substrate adhesion during development and thus facilitates cell migration during morphogenesis.

Cell-cell adhesion and signal transduction duringDictyosteliumdevelopment

The development of the non-metazoan eukaryote Dictyostelium discoideum displays many of the features of animal embryogenesis, including regulated cell-cell adhesion. During early development, two proteins, DdCAD-1 and csA, mediate cell-cell adhesion between amoebae as they form a loosely packed multicellular mass. The mechanism governing this process is similar to epithelial sheet sealing in animals. Although cell differentiation can occur in the absence of cell contact, regulated cell-cell adhesion is an important component of Dictyostelium morphogenesis, and a third adhesion molecule, gp150, is required for multicellular development past the aggregation stage.

Cell-cell junctions that appear to be adherens junctions form during the late stages of Dictyostelium development. Although they are not essential to establish the basic multicellular body plan, these junctions are required to maintain the structural integrity of the fruiting body. The Dictyostelium β-catenin homologue Aardvark (Aar) is present in adherens junctions, which are lost in its absence. As in the case of its metazoan counterparts, Aar also has a function in cell signalling and regulates expression of the pre-spore gene psA.

It is becoming clear that cell-cell adhesion is an integral part of Dictyostelium development. As in animals, cell adhesion molecules have a mechanical function and may also interact with the signal-transduction processes governing morphogenesis.

Propagating chemoattractant waves coordinate periodic cell movement inDictyosteliumslugs

Migration and behaviour of Dictyostelium slugs results from coordinated movement of its constituent cells. It has been proposed that cell movement is controlled by propagating waves of cAMP as during aggregation and in the mound. We report the existence of optical density waves in slugs; they are initiated in the tip and propagate backwards. The waves reflect periodic cell movement and are mediated by cAMP, as injection of cAMP or cAMP phosphodiesterase disrupts wave propagation and results in effects on cell movement and, therefore, slug migration. Inhibiting the function of the cAMP receptor cAR1 blocks wave propagation, showing that the signal is mediated by cAR1. Wave initiation is strictly dependent on the tip; in decapitated slugs no new waves are initiated and slug movement stops until a new tip regenerates. Isolated tips continue to migrate while producing waves. We conclude from these observations that the tip acts as a pacemaker for cAMP waves that coordinate cell movement in slugs.Movies available on-line

cAMP receptor affinity controls wave dynamics, geometry and morphogenesis inDictyostelium

Serpentine G-protein-coupled cAMP receptors are key components in the detection and relay of the extracellular cAMP waves that control chemotactic cell movement during Dictyostelium development. During development the cells sequentially express four closely related cAMP receptors of decreasing affinity. In this study, we investigated the effect of cAMP receptor type and affinity on the dynamics of cell-cell signalling in vivo, by measuring the dynamics of wave initiation and propagation in a variety of cAMP receptor mutants. We found that receptor affinity controls the frequency of wave initiation, but it does not determine wave propagation velocity, thus resulting in dramatic changes in wave geometry. In the limiting case, the affinity of the receptor is so low that waves can still be initiated but no stable centres form - thus, the cells cannot aggregate. In mounds, expression of low affinity receptors results in slow concentric waves instead of the normally observed multi-armed spiral waves. Under these conditions there is no rotational cell movement and the hemispherical mounds cannot transform into slugs. These results highlight the importance of receptor number and affinity in the proper control of cell-cell signalling dynamics required for the successful completion of development.