Non-chemotactic Dictyostelium discoideum mutants with altered cGMP signal transduction

A Mutant of Dictyostelium discoideum, KI-Cell, as a Model of Collective Cell Migration Independent of Chemotaxis

Collective cell migration occurs when the orientation of cell polarity is aligned with each other in a group of cells. Such collective polarization depends on a reciprocal process between cell intrinsic mechanisms such as cell-cell adhesion and extracellular guidance mechanism such as wound healing and chemotaxis. As part of its development life cycle, individual single cells of Dictyostelium discoideum exhibit chemotaxis toward cAMP, which is secreted from a certain population of cells. During the formation of multicellular body by chemotaxis-dependent cell aggregation, D. discoideum is also known to relay on multiple cell-cell adhesion mechanisms. In particular, tail-following behavior at the contact site, called contact following of locomotion (CFL), plays a pivotal role on the formation of the multicellular body. However, whether and how CFL alone can lead to a formation of collective behavior was not well understood. KI cell is a mutant of D. discoideum that lacks all chemotactic activity. Yet, it can exhibit the CFL activity and show nontrivial collective cell migration. This mutant provides an excellent model system to analyze the mechanism of the CFL and the macroscopic phenomena brought by the CFL. This chapter describes protocols for using KI cell to understand the biophysics and cell biology behind the collective cell migration induced by CFL.

Dynamics of intracellular cGMP during chemotaxis in Dictyostelium cells

Cyclic guanosine 3',5'-monophosphate (cGMP) is a ubiquitous important second messenger involved in various physiological functions. Here, intracellular cGMP (cGMPi) was visualized in chemotactic Dictyostelium cells using the fluorescent probe, D-Green cGull. When wild-type cells were stimulated with a chemoattractant, fluorescence transiently increased, but guanylate cyclase-null cells did not show a change in fluorescence, suggesting that D-Green cGull is a reliable indicator of cGMPi. In the aggregation stage, the responses of cGMPi propagated in a wave-like fashion from the aggregation center. The oscillation of the cGMPi wave was synchronized almost in phase with those of other second messengers, such as the intracellular cAMP and Ca2+. The phases of these waves preceded those of the oscillations of actomyosin and cell velocity, suggesting that these second messengers are upstream of the actomyosin and chemotactic migration. An acute increase in cGMPi concentration released from membrane-permeable caged cGMP induced a transient shuttle of myosin II between the cytosol and cell cortex, suggesting a direct link between cGMP signaling and myosin II dynamics.

Polar pattern formation induced by contact following locomotion in a multicellular system

Biophysical mechanisms underlying collective cell migration of eukaryotic cells have been studied extensively in recent years. One mechanism that induces cells to correlate their motions is contact inhibition of locomotion, by which cells migrating away from the contact site. Here, we report that tail-following behavior at the contact site, termed contact following locomotion (CFL), can induce a non-trivial collective behavior in migrating cells. We show the emergence of a traveling band showing polar order in a mutant Dictyostelium cell that lacks chemotactic activity. We find that CFL is the cell-cell interaction underlying this phenomenon, enabling a theoretical description of how this traveling band forms. We further show that the polar order phase consists of subpopulations that exhibit characteristic transversal motions with respect to the direction of band propagation. These findings describe a novel mechanism of collective cell migration involving cell-cell interactions capable of inducing traveling band with polar order.

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.

Parallel signaling pathways regulate excitable dynamics differently for pseudopod formation in eukaryotic chemotaxis

In eukaryotic chemotaxis, parallel signaling pathways regulate the spatiotemporal pseudopod dynamics at the leading edge of a motile cell through characteristic dynamics of an excitable system; however, differences in the excitability and the physiological roles of individual pathways remain to be elucidated. Here we found that two different pathways, soluble guanylyl cyclase (sGC) and phosphatidylinositol 3-kinase (PI3K), exhibited similar all-or-none responses but different refractory periods by simultaneous observations of their excitable properties. Due to the shorter refractory period, sGC signaling responded more frequently to chemoattractants, leading to pseudopod formation with higher frequency. sGC excitability was regulated negatively by its product, cGMP, and cGMP-binding protein C (GbpC) through the suppression of F-actin polymerization, providing the underlying delayed negative feedback mechanism for the cyclical pseudopod formation. These results suggest that parallel pathways respond on different time-scales to environmental cues for chemotactic motility based on their intrinsic excitability. Key words: cGMP signaling, chemotaxis, excitability, pseudopod formation

Cell-alignment patterns in the collective migration of cells with polarized adhesion

Dictyostelium discoideum (Dd) utilizes inhomogeneities in the distribution of cell-cell adhesion molecules on cell membranes for collective cell migration. A simple example of an inhomogeneity is a front-side (leading-edge) polarization in the distribution at the early streaming stage. Experiments have shown that the polarized cell-cell adhesion induces side-by-side contact between cells [Beug et al., Nature (London) 274, 445 (1978)NATUAS0028-083610.1038/274445a0]. This result is counterintuitive, as one would expect cells to align front to front in contact with each other on the basis of front-side polarization. In this work, we theoretically examine whether front-side polarization induces side-by-side contact in collective cell migration. We construct a model for expressing cells with this polarization based on the two-dimensional cellular Potts model. By a numerical simulation with this model, we find cell-cell alignment wherein cells form lateral arrays with side-by-side contacts as observed in the experiments.

Self-generated chemotactic gradients-cells steering themselves

Chemotaxis is a fundamentally important part of biology, but we know very little about how gradients of chemoattractant are formed. One answer is self-generated gradients, in which the moving cells break down the attractant to provide their own gradient as they migrate. Here we discuss where self-generated gradients are known, how they can be recognized, and where they are likely to be found in the future.

Self-Generated Chemoattractant Gradients: Attractant Depletion Extends the Range and Robustness of Chemotaxis

Chemotaxis is fundamentally important, but the sources of gradients in vivo are rarely well understood. Here, we analyse self-generated chemotaxis, in which cells respond to gradients they have made themselves by breaking down globally available attractants, using both computational simulations and experiments. We show that chemoattractant degradation creates steep local gradients. This leads to surprising results, in particular the existence of a leading population of cells that moves highly directionally, while cells behind this group are undirected. This leading cell population is denser than those following, especially at high attractant concentrations. The local gradient moves with the leading cells as they interact with their surroundings, giving directed movement that is unusually robust and can operate over long distances. Even when gradients are applied from external sources, attractant breakdown greatly changes cells' responses and increases robustness. We also consider alternative mechanisms for directional decision-making and show that they do not predict the features of population migration we observe experimentally. Our findings provide useful diagnostics to allow identification of self-generated gradients and suggest that self-generated chemotaxis is unexpectedly universal in biology and medicine.

Biological soliton in multicellular movement

Solitons have been observed in various physical phenomena. Here, we show that the distinct characteristics of solitons are present in the mass cell movement of non-chemotactic mutants of the cellular slime mould Dictyostelium discoideum. During starvation, D. discoideum forms multicellular structures that differentiate into spore or stalk cells and, eventually, a fruiting body. Non-chemotactic mutant cells do not form multicellular structures; however, they do undergo mass cell movement in the form of a pulsatile soliton-like structure (SLS). We also found that SLS induction is mediated by adhesive cell-cell interactions. These observations provide novel insights into the mechanisms of biological solitons in multicellular movement.

A RabGAP regulates life-cycle duration via trimeric G-protein cascades in Dictyostelium discoideum

Background

The life-cycle of cellular slime molds comprises chronobiologically regulated processes. During the growth phase, the amoeboid cells proliferate at a definite rate. Upon starvation, they synthesize cAMP as both first and second messengers in signalling pathways and form aggregates, migrating slugs, and fruiting bodies, consisting of spores and stalk cells, within 24 h. In Dictyostelium discoideum, because most growth-specific events cease during development, proliferative and heterochronic mutations are not considered to be interrelated and no genetic factor governing the entire life-cycle duration has ever been identified.

Methodology/Principal Findings

Using yeast 2-hybrid library screening, we isolated a Dictyostelium discoideum RabGAP, Dd Rbg-3, as a candidate molecule by which the Dictyostelium Gα2 subunit directs its effects. Rab GTPase-activating protein, RabGAP, acts as a negative regulator of Rab small GTPases, which orchestrate the intracellular membrane trafficking involved in cell proliferation. Deletion mutants of Dd rbg-3 exhibited an increased growth rate and a shortened developmental period, while an overexpression mutant demonstrated the opposite effects. We also show that Dd Rbg-3 interacts with 2 Gα subunits in an activity-dependent manner in vitro. Furthermore, both human and Caenorhabditis elegans rbg-3 homologs complemented the Dd rbg-3–deletion phenotype in D. discoideum, indicating that similar pathways may be generally conserved in multicellular organisms.

Conclusions/Significance

Our findings suggest that Dd Rbg-3 acts as a key element regulating the duration of D. discoideum life-span potentially via trimeric G-protein cascades.

Arachidonic acid enhances caffeine-induced cell death via caspase-independent cell death

Caffeine is a globally consumed psychostimulant but can be fatal to cells at overdose exposures. Although caspase-dependent apoptosis plays a role in caffeine-induced cell death, the responsible intracellular signalling cascade remains incompletely understood. The cellular slime mould, Dictyostelium discoideum, does not possess caspase-dependent apoptotic machinery. Here, we observed that ablation of D. discoideumplaA, which encodes a phospholipase A2 (PLA₂) homolog, leads to a decreased rate of cell death under high caffeine concentrations and to enhanced cell death with the addition of arachidonic acid. Moreover, the inhibition of PLA₂ activity lead to a recovery of the survival rate in caspase-inhibited Hela cervical carcinoma cells under high caffeine concentrations, indicating that caffeine-induced cell death is enhanced via PLA₂-dependent signalling. Our results indicate that arachidonic acid may be a general second messenger that negatively regulates caffeine tolerance via a caspase-independent cell death cascade, which leads to multiple effects in eukaryotic cells.

Open access microfluidic device for the study of cell migration during chemotaxis

Cells sense and interpret chemical gradients, and respond by localized responses that lead to directed migration. An open microfluidic device (OMD) was developed to provide quantitative information on both the gradient and morphological changes that occurred as cells crawled through various microfabricated channels. This device overcame problems that many current devices have been plagued with, such as complicated cell loading, media evaporation and channel blockage by air bubbles. We used a micropipette to set up stable gradients formed by passive diffusion and thus avoided confounding cellular responses produced by shear forces. Two versions of the OMD are reported here: one device that has channels with widths of 6, 8, 10 and 12 μm, while the other has two large 100 μm channels to minimize cellular interaction with lateral walls. These experiments compared the migration rates and qualitative behavior of Dictyostelium discoideum cells responding to measurable cAMP and folic acid gradients in small and large channels. We report on the influence that polarity has on a cell's ability to migrate when confined in a channel. Polarized cells that migrated to cAMP were significantly faster than the unpolarized cells that crawled toward folic acid. Unpolarized cells in wide channels often strayed off course, yet migrated faster than unpolarized cells in confined channels. Cells in channels farthest from the micropipette migrated through the channels at rates similar to cells in channels with higher concentrations, suggesting that cell speed was independent of mean concentration. Lastly, it was found that the polarized cells could easily change migration direction even when only the leading edge of the cell was exposed to a lateral gradient.

Costars, a Dictyostelium protein similar to the C-terminal domain of STARS, regulates the actin cytoskeleton and motility

Through analysis of a chemotaxis mutant obtained from a genetic screen in Dictyostelium discoideum, we have identified a new gene involved in regulating cell migration and have named it costars (cosA). The 82 amino acid Costars protein sequence appears highly conserved among diverse species, and significantly resembles the C-terminal region of the striated muscle activator of Rho signaling (STARS), a mammalian protein that regulates the serum response factor transcriptional activity through actin binding and Rho GTPase activation. The cosA-null (cosA(-)) cells formed smooth plaques on bacterial lawns, produced abnormally small fruiting bodies when developed on the non-nutrient agar and displayed reduced migration towards the cAMP source in chemotactic assays. Analysis of cell motion in cAMP gradients revealed decreased speed but wild-type-like directional persistence of cosA(-) cells, suggesting a defect in the cellular machinery for motility rather than for chemotactic orientation. Consistent with this notion, cosA(-) cells exhibited changes in the actin cytoskeleton, showing aberrant distribution of F-actin in fluorescence cell staining and an increased amount of cytoskeleton-associated actin. Excessive pseudopod formation was also noted in cosA(-) cells facing chemoattractant gradients. Expressing cosA or its human counterpart mCostars eliminated abnormalities of cosA(-) cells. Together, our results highlight a role for Costars in modulating actin dynamics and cell motility.

Differentiation-inducing factor-1 and -2 function also as modulators for Dictyostelium chemotaxis

BACKGROUND

In the early stages of development of the cellular slime mold Dictyostelium discoideum, chemotaxis toward cAMP plays a pivotal role in organizing discrete cells into a multicellular structure. In this process, a series of signaling molecules, such as G-protein-coupled cell surface receptors for cAMP, phosphatidylinositol metabolites, and cyclic nucleotides, function as the signal transducers for controlling dynamics of cytoskeleton. Differentiation-inducing factor-1 and -2 (DIF-1 and DIF-2) were originally identified as the factors (chlorinated alkylphenones) that induce Dictyostelium stalk cell differentiation, but it remained unknown whether the DIFs had any other physiologic functions.

METHODOLOGY/PRINCIPAL FINDINGS

To further elucidate the functions of DIFs, in the present study we investigated their effects on chemotaxis under various conditions. Quite interestingly, in shallow cAMP gradients, DIF-1 suppressed chemotaxis whereas DIF-2 promoted it greatly. Analyses with various mutants revealed that DIF-1 may inhibit chemotaxis, at least in part, via GbpB (a phosphodiesterase) and a decrease in the intracellular cGMP concentration ([cGMP](i)). DIF-2, by contrast, may enhance chemotaxis, at least in part, via RegA (another phosphodiesterase) and an increase in [cGMP](i). Using null mutants for DimA and DimB, the transcription factors that are required for DIF-dependent prestalk differentiation, we also showed that the mechanisms for the modulation of chemotaxis by DIFs differ from those for the induction of cell differentiation by DIFs, at least in part.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that DIF-1 and DIF-2 function as negative and positive modulators for Dictyostelium chemotaxis, respectively. To our knowledge, this is the first report in any organism of physiologic modulators (small molecules) for chemotaxis having differentiation-inducing activity.

Switching direction in electric-signal-induced cell migration by cyclic guanosine monophosphate and phosphatidylinositol signaling

Switching between attractive and repulsive migration in cell movement in response to extracellular guidance cues has been found in various cell types and is an important cellular function for translocation during cellular and developmental processes. Here we show that the preferential direction of migration during electrotaxis in Dictyostelium cells can be reversed by genetically modulating both guanylyl cyclases (GCases) and the cyclic guanosine monophosphate (cGMP)-binding protein C (GbpC) in combination with the inhibition of phosphatidylinositol-3-OH kinases (PI3Ks). The PI3K-dependent pathway is involved in cathode-directed migration under a direct-current electric field. The catalytic domains of soluble GCase (sGC) and GbpC also mediate cathode-directed signaling via cGMP, whereas the N-terminal domain of sGC mediates anode-directed signaling in conjunction with both the inhibition of PI3Ks and cGMP production. These observations provide an identification of the genes required for directional switching in electrotaxis and suggest that a parallel processing of electric signals, in which multiple-signaling pathways act to bias cell movement toward the cathode or anode, is used to determine the direction of migration.

Signaling pathways mediating chemotaxis in the social amoeba, Dictyostelium discoideum

Chemotaxis, or cell migration guided by chemical cues, is critical for a multitude of biological processes in a diverse array of organisms. Dictyostelium discoideum amoebae rely on chemotaxis to find food and to survive starvation conditions, and we have taken advantage of this system to study the molecular regulation of this vital cell behavior. Previous work has identified phosphoinositide signaling as one mechanism which may contribute to directional sensing and actin polymerization during chemotaxis; a mechanism which is conserved in mammalian neutrophils. In this review, we will discuss recent data on genes and pathways governing directional sensing and actin polymerization, with a particular emphasis on contributions from our laboratory.

Regulation of growth and differentiation in Dictyostelium

In general, growth and differentiation are mutually exclusive, but they are cooperatively regulated during the course of development. Thus, the process of a cell's transition from growth to differentiation is of general importance not only for the development of organisms but also for the initiation of malignant transformation, in which this process is reversed. The cellular slime mold Dictyostelium, a wonderful model organism, grows and multiplies as long as nutrients are supplied, and its differentiation is triggered by starvation. A strict checkpoint (growth/differentiation transition or GDT point), from which cells start differentiating in response to starvation, has been specified in the cell cycle of D. discoideum Ax-2 cells. Accordingly, integration of GDT point-specific events with starvation-induced events is needed to understand the mechanism regulating GDTs. A variety of intercellular and intracellular signals are involved positively or negatively in the initiation of differentiation, making a series of cross-talks. As was expected from the presence of GDT points, the cell's positioning in cell masses and subsequent cell-type choices occur depending on the cell's phase in the cell cycle at the onset of starvation. Since novel and somewhat unexpected multiple functions of mitochondria in cell movement, differentiation, and pattern formation have been well realized in Dictyostelium cells, they are reviewed in this article.

Chemoattractant-stimulated calcium influx in Dictyostelium discoideum does not depend on cGMP

Chemoattractant stimulation of Dictyostelium cells leads to the opening of calcium channels in the plasma membrane, causing extracellular calcium to flux into the cell. The genetically uncharacterised mutants stmF and KI8 show strongly altered chemoattractant-stimulated cGMP responses. The aberrant calcium influx in these strains has provided evidence that the chemoattractant-stimulated calcium influx is potentiated by cGMP. We have tested this hypothesis in genetically defined mutants by measuring the calcium influx in a strain that lacks intracellular cGMP due to the disruption of two guanylyl cyclases, and in a strain with increased cGMP levels caused by the disruption of two cGMP-degrading phosphodiesterases. The results reveal that the calcium influx stimulated by cAMP or folic acid is essentially identical in these strains. We conclude that cGMP is not involved in chemoattractant-stimulated calcium influx.

Phosducin-like proteins in Dictyostelium discoideum: implications for the phosducin family of proteins

Retinal phosducin is known to sequester transducin Gbetagamma, thereby modulating transducin activity. Phosducin is a member of a family of phosducin-like proteins (PhLP) found in eukaryotes. Phylogeny of 33 phosducin-like proteins from metazoa, plants and lower eukaryotes identified three distinct groups named phosducin-I-III. We discovered three phlp genes in Dictyostelium, each encoding a phosducin-like protein of a different group. Disruption of the phlp1 gene strongly impaired G-protein signalling, apparently due to mislocalization of Gbetagamma in phlp1-null cells. GFP-Gbeta and GFP-Ggamma are membrane associated in wild-type cells, but cytosolic in phlp1-null cells. Phlp2 disruption is lethal due to a synchronous collapse of the cells after 16-17 cell divisions. Phlp3 disruptants show no abnormal phenotype. These results establish a role for phosducin-like proteins in facilitating folding, localization or function of proteins, in addition to modulating G-protein signalling.

Multiple signalling pathways connect chemoattractant receptors and calcium channels in Dictyostelium

Dictyostelium mutants expressing aequorin were used to study and compare the roles of heterotrimeric G-proteins and the second messengers IP3 and cGMP in regulating folate- and cAMP receptor-activated [Ca2+]i signals. The calcium responses of vegetative cells to folate were dramatically impaired in Gbeta and Galpha4 null mutants but were restored with altered kinetics and temperature-sensitivity in Gbeta null mutants overexpressing wild type and temperature-sensitive Gbeta isoforms. Folic acid receptors thus mediate changes in [Ca2+]i via a Galpha4betagamma-dependent pathway. Neither folate nor cAMP-induced [Ca2+]i signals were significantly altered in PLC null transformants, but [Ca2+]i changes elicited by both attractants were significantly prolonged in two stmF mutants lacking cGMP-specific phosphodiesterase activity. This confirms an important role of cGMP in regulating receptor-activated Ca2+ uptake and/or extrusion systems. This cGMP-dependent part of the Ca2+ response to cAMP stimuli was developmentally down-regulated and all but disappeared by the time the cells reached full aggregation competence after 8 h of starvation. The results suggest that folate and cAMP receptor-activated [Ca2+]i signals are regulated in a complex manner via multiple signalling pathways, one that is G-protein- and cGMP-dependent (present at the vegetative and early poststarvation stage) and another that is G-protein-independent (dominant in fully aggregation-competent cells at approximately 8 h poststarvation).

A model for cGMP signal transduction in Dictyostelium in perspective of 25 years of cGMP research

The chemoattactant mediated cGMP response of Dictyostelium cells was discovered about twenty-five years ago. Shortly thereafter, guanylyl cyclases, cGMP-phosphodiesterases and cGMP-binding proteins were detected already in lysates, but the encoding genes were discovered only very recently. The deduced proteins appear to be very different from proteins with the same function in metazoa. In this review we discuss these new findings in perspective of the previously obtained biochemical and functional data on cGMP in Dictyostelium.

Identification and characterization of two unusual cGMP-stimulated phoshodiesterases in dictyostelium

Recently, we recognized two genes, gbpA and gbpB, encoding putative cGMP-binding proteins with a Zn(2+)-hydrolase domain and two cyclic nucleotide binding domains. The Zn(2+)-hydrolase domains belong to the superfamily of beta-lactamases, also harboring a small family of class II phosphodiesterases from bacteria and lower eukaryotes. Gene inactivation and overexpression studies demonstrate that gbpA encodes the cGMP-stimulated cGMP-phosphodiesterase that was characterized biochemically previously and was shown to be involved in chemotaxis. cAMP neither activates nor is a substrate of GbpA. The gbpB gene is expressed mainly in the multicellular stage and seems to encode a dual specificity phosphodiesterase with preference for cAMP. The enzyme hydrolyses cAMP approximately 9-fold faster than cGMP and is activated by cAMP and cGMP with a K(A) value of approximately 0.7 and 2.3 microM, respectively. Cells with a deletion of the gbpB gene have increased basal and receptor stimulated cAMP levels and are sporogeneous. We propose that GbpA and GbpB hydrolyze the substrate in the Zn(2+)-hydrolase domain, whereas the cyclic nucleotide binding domains mediate activation. The human cGMP-stimulated cAMP/cGMP phosphodiesterase has similar biochemical properties, but a completely different topology: hydrolysis takes place by a class I catalytic domain and GAF domains mediate cGMP activation.

Myosin II dynamics in Dictyostelium: determinants for filament assembly and translocation to the cell cortex during chemoattractant responses

In the simple amoeba Dictyostelium discoideum, myosin II filament assembly is regulated primarily by the action of a set of myosin heavy chain (MHC) kinases and by MHC phosphatase activity. Chemoattractant signals acting via G-protein coupled receptors lead to rapid recruitment of myosin II to the cell cortex, but the structural determinants on myosin necessary for translocation and the second messengers upstream of MHC kinases and phosphatases are not well understood. We report here the use of GFP-myosin II fusions to characterize the domains necessary for myosin II filament assembly and cytoskeletal recruitment during responses to global stimulation with the developmental chemoattractant cAMP. Analysis performed with GFP-myosin fusions, and with latrunculin A-treated cells, demonstrated that F-actin binding via the myosin motor domain together with concomitant filament assembly mediates the rapid cortical translocation observed in response to chemoattractant stimulation. A "headless" GFP-myosin construct lacking the motor domain was unable to translocate to the cell cortex in response to chemoattractant stimulation, suggesting that myosin motor-based motility may drive translocation. This lack of localization contrasts with previous work demonstrating accumulation of the same construct in the cleavage furrow of dividing cells, suggesting that recruitment signals and interactions during cytokinesis differ from those during chemoattractant responses. Evaluating upstream signaling, we find that iplA null mutants, devoid of regulated calcium fluxes during chemoattractant stimulation, display full normal chemoattractant-stimulated myosin assembly and translocation. These results indicate that calcium transients are not necessary for chemoattractant-regulated myosin II filament assembly and translocation.

Identification of a novel type of cGMP phosphodiesterase that is defective in the chemotactic stmF mutants

StmF mutants are chemotactic mutants that are defective in a cGMP phosphodiesterase (PDE) activity. We identified a novel gene, PdeD, that harbors two cyclic nucleotide-binding domains and a metallo-beta-lactamase homology domain. Similar to stmF mutants, pdeD-null mutants displayed extensively streaming aggregates, prolonged elevation of cGMP levels after chemotactic stimulation, and reduced cGMP-PDE activity. PdeD transcripts were lacking in stmF mutant NP377, indicating that this mutant carries a PdeD lesion. Expression of a PdeD-YFP fusion protein in pdeD-null cells restored the normal cGMP response and showed that PdeD resides in the cytosol. When purified by immunoprecipitation, the PdeD-YFP fusion protein displayed cGMP-PDE activity, which was retained in a truncated construct that contained only the metallo-beta-lactamase domain.

A novel cGMP signalling pathway mediating myosin phosphorylation and chemotaxis in Dictyostelium

Chemotactic stimulation of Dictyostelium cells results in a transient increase in cGMP levels, and transient phosphorylation of myosin II heavy and regulatory light chains. In Dictyostelium, two guanylyl cyclases and four candidate cGMP-binding proteins (GbpA- GbpD) are implicated in cGMP signalling. GbpA and GbpB are homologous proteins with a Zn2+-hydrolase domain. A double gbpA/gbpB gene disruption leads to a reduction of cGMP-phosphodiesterase activity and a 10-fold increase of basal and stimulated cGMP levels. Chemotaxis in gbpA(-)B(-) cells is associated with increased myosin II phosphorylation compared with wild-type cells; formation of lateral pseudopodia is suppressed resulting in enhanced chemotaxis. GbpC is homologous to GbpD, and contains Ras, MAPKKK and Ras-GEF domains. Inactivation of the gbp genes indicates that only GbpC harbours high affinity cGMP-binding activity. Myosin phosphorylation, assembly of myosin in the cytoskeleton as well as chemotaxis are severely impaired in mutants lacking GbpC and GbpD, or mutants lacking both guanylyl cyclases. Thus, a novel cGMP signalling cascade is critical for chemotaxis in Dictyostelium, and plays a major role in myosin II regulation during this process.

Quantitative analysis of the behavior of Dictyostelium discoideum amoebae: stringency of pteridine reception

A convenient, sensitive, quantitative assay for the measurement of chemotaxis of populations of D. discoideum vegetative amoebae is presented. A strategy for determining the boundary of the bulk of a population of migrating amoebae was devised and is described. This assay employs a dynamic gradient and is independent of deaminase activity. Measurements of chemoattractant capabilities of various pteridines, folates, and mixtures of folate fragments are reported. 2-Amino 4-quinazolinone, a pterin analog without the pyrazine ring nitrogens, is chemotactic. Lumazine, deaminated pterin, inhibits chemotaxis towards pterin but not towards folic acid. Deaminofolic acid is a chemoattractant as are mixtures of lumazine plus aminobenzoylglutamic acid or deaminopteroic acid plus various amino acids. Separately, the components of these mixtures exhibit no ability to stimulate chemotaxis. These mixtures are of fragments that together comprise most of the folate structure. Our results are in accord with separate receptors for pterin vs. folic acid and with a high stringency for pterin reception but a relative tolerance for folate reception. The possibility of using such mixtures to investigate the requirements of various parts of the folate structure for competent signalling is discussed.

A cell number-counting factor regulates group size in Dictyostelium by differentially modulating cAMP-induced cAMP and cGMP pulse sizes

A secreted counting factor (CF), regulates the size of Dictyostelium discoideum fruiting bodies in part by regulating cell-cell adhesion. Aggregation and the expression of adhesion molecules are mediated by relayed pulses of cAMP. Cells also respond to cAMP with a short cGMP pulse. We find that CF slowly down-regulates the cAMP-induced cGMP pulse by inhibiting guanylyl cyclase activity. A 1-min exposure of cells to purified CF increases the cAMP-induced cAMP pulse. CF does not affect the cAMP receptor or its interaction with its associated G proteins or the translocation of the cytosolic regulator of adenylyl cyclase to the membrane in response to cAMP. Pulsing streaming wild-type cells with a high concentration of cAMP results in the formation of small groups, whereas reducing cAMP pulse size with exogenous cAMP phosphodiesterase during stream formation causes cells to form large groups. Altering the extracellular cAMP pulse size does not phenocopy the effects of CF on the cAMP-induced cGMP pulse size or cell-cell adhesion, indicating that CF does not regulate cGMP pulses and adhesion via CF's effects on cAMP pulses. The results suggest that regulating cell-cell adhesion, the cGMP pulse size, or the cAMP pulse size can control group size and that CF regulates all three of these independently.

Lysophosphatidic acid- and Gbeta-dependent activation of Dictyostelium MAP kinase ERK2

Exogenous lysophosphatidic acid (LPA) has been shown to evoke a chemotactic response in aggregative cells of the social amoeba Dictyostelium discoideum. In this paper, we demonstrate that extracellular LPA is also able to induce activation of mitogen-activated protein (MAP) kinase DdERK2 (extracellular signal regulated kinase 2) in these cells. This activation is independent of cyclic AMP receptors, yet fully dependent on the single Gbeta subunit, hinting to the presence of functional heptahelical LPA receptors in a primitive eukaryote. We did not observe LPA-dependent cyclic GMP accumulation, which suggests that the pathways for LPA-induced and "classical" chemotaxis of D. discoideum cells are substantially different.

Guanylate cyclase in Dictyostelium discoideum with the topology of mammalian adenylate cyclase

The core of adenylate and guanylate cyclases is formed by an intramolecular or intermolecular dimer of two cyclase domains arranged in an antiparallel fashion. Metazoan membrane-bound adenylate cyclases are composed of 12 transmembrane spanning regions, and two cyclase domains which function as a heterodimer and are activated by G-proteins. In contrast, membrane-bound guanylate cyclases have only one transmembrane spanning region and one cyclase domain, and are activated by extracellular ligands to form a homodimer. In the cellular slime mould, Dictyostelium discoideum, membrane-bound guanylate cyclase activity is induced after cAMP stimulation; a G-protein-coupled cAMP receptor and G-proteins are essential for this activation. We have cloned a Dictyostelium gene, DdGCA, encoding a protein with 12 transmembrane spanning regions and two cyclase domains. Sequence alignment demonstrates that the two cyclase domains are transposed, relative to these domains in adenylate cyclases. DdGCA expressed in Dictyostelium exhibits high guanylate cyclase activity and no detectable adenylate cyclase activity. Deletion of the gene indicates that DdGCA is not essential for chemotaxis or osmo-regulation. The knock-out strain still exhibits substantial guanylate cyclase activity, demonstrating that Dictyostelium contains at least one other guanylate cyclase.

Tortoise, a novel mitochondrial protein, is required for directional responses of Dictyostelium in chemotactic gradients

We have identified a novel gene, Tortoise (TorA), that is required for the efficient chemotaxis of Dictyostelium discoideum cells. Cells lacking TorA sense chemoattractant gradients as indicated by the presence of periodic waves of cell shape changes and the localized translocation of cytosolic PH domains to the membrane. However, they are unable to migrate directionally up spatial gradients of cAMP. Cells lacking Mek1 display a similar phenotype. Overexpression of Mek1 in torA- partially restores chemotaxis, whereas overexpression of TorA in mek1- does not rescue the chemotactic phenotype. Regardless of the genetic background, TorA overexpressing cells stop growing when separated from a substrate. Surprisingly, TorA-green fluorescent protein (GFP) is clustered near one end of mitochondria. Deletion analysis of the TorA protein reveals distinct regions for chemotactic function, mitochondrial localization, and the formation of clusters. TorA is associated with a round structure within the mitochondrion that shows enhanced staining with the mitochondrial dye Mitotracker. Cells overexpressing TorA contain many more of these structures than do wild-type cells. These TorA-containing structures resist extraction with Triton X-100, which dissolves the mitochondria. The characterization of TorA demonstrates an unexpected link between mitochondrial function, the chemotactic response, and the capacity to grow in suspension.

Ca(2+) signalling is not required for chemotaxis in Dictyostelium

Dictyostelium cells can move rapidly towards a source of cyclic-AMP (cAMP). This chemoattractant is detected by G-protein-linked receptors, which trigger a signalling cascade including a rapid influx of Ca(2+). We have disrupted an inositol 1,4,5-trisphosphate (InsP(3)) receptor-like gene, iplA, to produce null cells in which Ca(2+) entry in response to chemoattractants is abolished, as is the normal increase in free cytosolic Ca(2+) ([Ca(2+)](c)) that follows chemotactic stimulation. However, the resting [Ca(2+)](c) is similar to wild type. This mutant provides a test for the role of Ca(2+) influx in both chemotaxis and the signalling cascade that controls it. The production of cyclic-GMP and cAMP, and the activation of the MAP kinase, DdERK2, triggered from the cAMP receptor, are little perturbed in the mutant; mobilization of actin into the cytoskeleton also follows similar kinetics to wild type. Mutant cells chemotax efficiently towards cAMP or folic acid and their sensitivity to cAMP is similar to wild type. Finally, they move at similar speeds to wild-type cells, with or without chemoattractant. We conclude that Ca(2+) signalling is not necessary for chemotaxis to cAMP.

Involvement of the glucose-regulated protein 94 (Dd-GRP94) in starvation response of Dictyostelium discoideum cells

Upon deprivation of nutrients, Dictyostelium discoideum Ax-2 cells arrest proliferation and initiate a metamorphosed developmental program including induction of altered gene expressions which are necessary for differentiation. In Ax-2 cells, we found out a member of Hsp90 family usually contained in the endoplasmic reticulum (ER), Dd-GRP94 (Dictyostelium discoideum glucose-regulated protein 94). In general, GRP94 are induced either by glucose-depletion or by depletion of Ca(2+) in intracellular Ca(2+) stores. Unexpectedly, however, the expression of Dd-grp94 was greatly reduced within 60 min of starvation. Dd-grp94-overexpressing cells (GRP94(OE) cells) collected without forming distinct aggregation streams, and never formed normal fruiting bodies. Also, prespore differentiation as well as maturation into spores and stalk cells were particularly impaired in the GRP94(OE) cells. Thus Dd-GRP94 seems to be crucial in late differentiation as well as in starvation response.

Dictyostelium development-socializing through cAMP

Starving Dictyostelium amoebae use cAMP as a chemoattractant to gather into aggregates, as a hormone-like signal to induce cell differentiation, and as an intracellular messenger to control stalk- and spore cell maturation and germination of spores. In this chapter we describe the respective roles of the three adenylyl cyclases ACA, ACB and ACG in controlling cAMP signaling during development and we discuss how cAMP signals are processed by the cells to trigger the large repertoire of gene regulatory events that is under control of this signal molecule.

A cell's sense of direction

In eukaryotic cells directional sensing is mediated by heterotrimeric guanine nucleotide-binding protein (G protein)-linked signaling pathways. In Dictyostelium discoideum amoebae and mammalian leukocytes, the receptors and G-protein subunits are uniformly distributed around the cell perimeter. Chemoattractants induce the transient appearance of binding sites for several pleckstrin homology domain-containing proteins on the inner face of the membrane. In gradients of attractant these sites are persistently present on the side of the cell facing the higher concentration, even in the absence of a functional actin cytoskeleton or cell movement. Thus, the cell senses direction by spatially regulating the activity of the signal transduction pathway.

Regulation of Dictyostelium protein-tyrosine phosphatase-3 (PTP3) through osmotic shock and stress stimulation and identification of pp130 as a PTP3 substrate

Osmotic shock and growth-medium stimulation of Dictyostelium cells results in rapid cell rounding, a reduction in cell volume, and a rearrangement of the cytoskeleton that leads to resistance to osmotic shock. Osmotic shock induces the activation of guanylyl cyclase, a rise in cGMP mediating the phosphorylation of myosin II, and the tyrosine phosphorylation of actin and the approximately 130-kDa protein (p130). We present data suggesting that signaling pathways leading to these different responses are, at least in part, independent. We show that a variety of stresses induce the Ser/Thr phosphorylation of the protein-tyrosine phosphatase-3 (PTP3). This modification does not alter PTP3 catalytic activity but correlates with its translocation from the cytosol to subcellular structures that co-localize to endosomal vesicles. This translocation is independent of PTP3 activity. Mutation of the catalytically essential Cys to a Ser results in inactive PTP3 that forms a stable complex with tyrosine-phosphorylated p130 (pp130) in vivo and in vitro, suggesting that PTP3 has a substrate specificity for pp130. The data suggest that stresses activate several interacting signaling pathways controlled by Ser/Thr and Tyr phosphorylation, which, along with the activation of guanylyl cyclase, mediate the ability of this organism to respond to adverse changes in the external environment.

Responding to attraction: chemotaxis and chemotropism in Dictyostelium and yeast

Polarized growth in response to external signals is essential for both the internal organization of cells and generation of complex multicellular structures during development. Oriented growth or movement requires specific detection of an external cue, reorganization of the cytoskeleton and subsequent growth or movement. Genetic approaches in both the budding yeast Saccharomyces cerevisiae and the social amoeba Dictyostelium discoideum have shed light on the molecular and cellular aspects of growth or movement towards an external signal. This review discusses the mechanisms and signalling pathways that enable yeast and Dictyostelium cells to translate external signals into directed growth and movement, respectively.

Selection of gbeta subunits with point mutations that fail to activate specific signaling pathways in vivo: dissecting cellular responses mediated by a heterotrimeric G protein in Dictyostelium discoideum

In Dictyostelium discoideum, a unique Gbeta subunit is required for a G protein-coupled receptor system that mediates a variety of cellular responses. Binding of cAMP to cAR1, the receptor linked to the G protein G2, triggers a cascade of responses, including activation of adenylyl cyclase, gene induction, actin polymerization, and chemotaxis. Null mutations of the cAR1, Galpha2, and Gbeta genes completely impair all these responses. To dissect specificity in Gbetagamma signaling to downstream effectors in living cells, we screened a randomly mutagenized library of Gbeta genes and isolated Gbeta alleles that lacked the capacity to activate some effectors but retained the ability to regulate others. These mutant Gbeta subunits were able to link cAR1 to G2, to support gene expression, and to mediate cAMP-induced actin polymerization, and some were able to mediate to chemotaxis toward cAMP. None was able to activate adenylyl cyclase, and some did not support chemotaxis. Thus, we separated in vivo functions of Gbetagamma by making point mutations on Gbeta. Using the structure of the heterotrimeric G protein displayed in the computer program CHAIN, we examined the positions and the molecular interactions of the amino acids substituted in each of the mutant Gbetas and analyzed the possible effects of each replacement. We identified several residues that are crucial for activation of the adenylyl cyclase. These residues formed an area that overlaps but is not identical to regions where bovine Gtbetagamma interacts with its regulators, Galpha and phosducin.

Dictyostelium TRFA homologous to yeast Ssn6 is required for normal growth and early development

The TPR (tetratricopeptide repeat) family became widespread during evolution, having been found from bacteria to mammals. By means of restriction enzyme-mediated integration, we have identified a Dictyostelium gene (trfA) highly homologous to a Saccharomyces cerevisiae gene encoding a TPR protein, Ssn6 (Cyc8), which functions as a global transcriptional repressor for diverse genes. The deduced amino acid sequence of the Dictyostelium gene product, TRFA, contains 10 consecutive TPR units as well as Gln repeats, Asn repeats, and a region rich in Glu, Lys, Ser, and Thr. The sequences of some of the 10 TPR units in TRFA are more than 70% identical to the corresponding units in Ssn6. The trfA- cells produced smooth plaques on a bacterial lawn and failed to aggregate normally when starved on a plain agar plate. Individual trfA- cells also failed to correctly respond to cAMP, although the adenylyl cyclase of trfA- cells was expressed upon starvation and activated by stimulation with cAMP as in the wild-type cells. When cultured in a rich medium in suspension, they grew more slowly and stopped growing at a lower density than the wild-type cells. Furthermore, they divided into cells of various sizes and tended to be much smaller than the wild-type cells. These pleiotropic defects of the trfA- cells suggest the possibility that Dictyostelium TRFA may regulate the transcription of diverse genes required for normal growth and early development.

Sequence-specific protein interaction with a transcriptional enhancer involved in the autoregulated expression of cAMP receptor 1 in Dictyostelium

Major stages of Dictyostelium development are regulated by secreted, extracellular cAMP through activation of a serpentine receptor family. During early development, oscillations of extracellular cAMP mobilize cells for aggregation; later, continuous exposure to higher extracellular cAMP concentrations downregulates early gene expression and promotes cytodifferentiation and cell-specific gene expression. The cAMP receptor 1 gene CAR1 has two promoters that are differentially responsive to these extracellular cAMP stimuli. The early CAR1 promoter is induced by nM pulses of cAMP, which in turn are generated by CAR1-dependent activation of adenylyl cyclase (AC). Higher, non-fluctuating concentrations of cAMP will adapt this AC stimulus-response, repress the activated early promoter and induce the dormant late promoter. We now identify a critical element of the pulse-induced CAR1 promoter and a nuclear factor with sequence-specific interaction. Mutation of four nucleotides within the element prevents both in vitro protein binding and in vivo expression of an otherwise fully active early CAR1 promoter and multimerization of the wild-type, but not mutant, sequence will confer cAMP regulation to a quiescent heterologous promoter. These cis and trans elements, thus, constitute a part of the molecular response to the cAMP transmembrane signal cascade that regulates early development of Dictyostelium.

Chemotactic and osmotic signals share a cGMP transduction pathway in Dictyostelium discoideum

In the ameboid eukaryote Dictyostelium discoideum, chemotactic stimulation by cAMP induces an increase of intracellular cGMP and subsequently the phosphorylation of myosin heavy chain II. Resistance to high osmotic stress also requires transient increases of intracellular cGMP and phosphorylation of myosin heavy chain II, although the kinetics is much slower than for chemotaxis. To examine if chemotaxis and osmotic stress share common signaling components we systematically analyzed the osmotic cGMP response and survival in chemotactic mutants with altered cGMP signaling. Null mutants with deletions of cell surface cAMP receptors or the associated GTP-binding proteins Galpha2 and Gbeta show no cAMP-induced cGMP response and chemotaxis; in contrast, osmotic stress induces the normal cGMP accumulation and survival. The same result was obtained with the non-chemotactic mutant KI-10, which lacks the activation of guanylyl cyclase by cAMP. This indicates that these components are required for chemotaxis but not osmotic cGMP signaling and survival. The potential guanylyl cyclase null mutant KI-8 shows no chemotaxis, no osmotic cGMP increase and reduced survival in high osmolarity. Two types of cGMP-binding protein mutants, KI-4 and KI-7, also show reduced tolerance during high osmotic stress. Taken together, these observations clarify that chemotactic and osmotic signals are detected by different mechanisms, but share a cGMP signaling pathway.

cGMP potentiates receptor-stimulated Ca2+ influx in Dictyostelium discoideum

Binding of extracellular cAMP to surface receptors induces at least two responses in Dictyostelium discoideum, the G-protein-dependent activation of guanylyl cyclase, and the opening of a plasma membrane Ca2+ channel. Some experiments suggest that intracellular cGMP opens the Ca2+ channel, while others demonstrate that the channel can open in the absence of functional G-proteins (and thus in the absence of cGMP formation). We have analysed 45Ca2+ uptake in three mutants with altered cGMP formation. Mutant stmF shows a prolonged cGMP response due to deletion of an intracellular phosphodiesterase. Uptake of receptor-stimulated 45Ca2+ is enhanced about two-fold in this mutant if compared to wild-type cells, suggesting that cGMP regulates the opening of the channel. Mutant KI-7 has very low levels of surface cAMP receptors, but nevertheless an enhanced receptor-stimulated cGMP response due to a defect in the turn-off of guanylyl cyclase. This mutant shows poor receptor-stimulated 45Ca2+ uptake, suggesting that cGMP alone is not sufficient to open the Ca2+ channel. Finally, mutant KI-8 has no cGMP due to the absence of nearly all guanylyl cyclase activity. The mutant shows significant but reduced 45Ca2+ uptake (19% of wild-type; 60% if corrected for the reduced level of surface cAMP receptors), suggesting that the channel can open in the absence of cGMP. Taken together, the results demonstrate that receptor-stimulated Ca2+ influx is not directly induced by cGMP formation; it can occur in the absence of cGMP, but is potentiated two- to four-fold by cGMP.

A novel cytosolic regulator, Pianissimo, is required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase in Dictyostelium

Genetic analysis was applied to identify novel genes involved in G protein-linked pathways controlling development. Using restriction enzyme-mediated integration (REMI), we have identified a new gene, Pianissimo (PiaA), involved in cAMP signaling in Dictyostelium discoideum. PiaA encodes a 130-kD cytosolic protein required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase. In piaA- null mutants, neither chemoattractant stimulation of intact cells nor GTPgammaS treatment of lysates activates the enzyme; constitutive expression of PiaA reverses these defects. Cytosols of wild-type cells that contain Pia protein reconstitute the GTPgammaS stimulation of adenylyl cyclase activity in piaA- lysates, indicating that Pia is directly involved in the activation. Pia and CRAC, a previously identified cytosolic regulator, are both essential for activation of the enzyme as lysates of crac- piaA- double mutants require both proteins for reconstitution. Homologs of PiaA are found in Saccharomyces cerevisiae and Schizosaccaromyces pombe; disruption of the S. cerevisiae homolog results in lethality. We propose that homologs of Pia and similar modes of regulation of these ubiquitous G protein-linked pathways are likely to exist in higher eukaryotes.

Chemotaxis to cAMP and slug migration in Dictyostelium both depend on migA, a BTB protein

Chemotaxis in natural aggregation territories and in a chamber with an imposed gradient of cyclic AMP (cAMP) was found to be defective in a mutant strain of Dictyostelium discoideum that forms slugs unable to migrate. This strain was selected from a population of cells mutagenized by random insertion of plasmids facilitated by introduction of restriction enzyme (a method termed restriction enzyme-mediated integration). We picked this strain because it formed small misshapen fruiting bodies. After isolation of portions of the gene as regions flanking the inserted plasmid, we were able to regenerate the original genetic defect in a fresh host and show that it is responsible for the developmental defects. Transformation of this recapitulated mutant strain with a construct carrying the full-length migA gene and its upstream regulatory region rescued the defects. The sequence of the full-length gene revealed that it encodes a novel protein with a BTB domain near the N terminus that may be involved in protein-protein interactions. The migA gene is expressed at low levels in all cells during aggregation and then appears to be restricted to prestalk cells as a consequence of rapid turnover in prespore cells. Although migA- cells have a dramatically reduced chemotactic index to cAMP and an abnormal pattern of aggregation in natural waves of cAMP, they are completely normal in size, shape, and ability to translocate in the absence of any chemotactic signal. They respond behaviorally to the rapid addition of high levels of cAMP in a manner indicative of intact circuitry connecting receptor occupancy to restructuring of the cytoskeleton. Actin polymerization in response to cAMP is also normal in the mutant cells. The defects at both the aggregation and slug stage are cell autonomous. The MigA protein therefore is necessary for efficiently assessing chemical gradients, and its absence results in defective chemotaxis and slug migration.

cGMP as second messenger during Dictyostelium chemotaxis

The chemoattractant cAMP induces directed cell locomotion in Dictyostelium cells. Several second messenger pathways are activated upon binding of cAMP to G-protein-coupled receptors, including adenylyl cyclase, guanylyl cyclase, phospholipase C, and the opening of plasma membrane Ca2+ channels. These second messenger responses are unaltered in many chemotactic mutants, except for the cGMP response. Activation of guanylyl cyclase depends on G-proteins and is regulated by a cGMP-binding protein in a complex manner. This cGMP-binding protein also mediates intracellular functions of cGMP to activate a PKC-related kinase that phosphorylates myosin II heavy chain, thereby allowing myosin filaments to rearrange during cell movement.

Genetics of phototaxis in a model eukaryote, Dictyostelium discoideum

The life cycle of Dictyostelium discoideum offers a unique opportunity to study signal transduction in eukaryotic cells at both the unicellular and multicellular levels of organization. Adding to the already extensive knowledge of the unicellular stages, classical and molecular genetics have begun to unravel transduction of signals controlling morphogenesis and behaviour (phototaxis and thermotaxis) in the multicellular 'slug' stage of the life cycle. Distributed over all seven genetic linkage groups are probably about 20, but possibly as many as 55, genes of importance for slug behaviour. The encoded proteins appear from pharmacological studies and mutant phenotypes to govern transduction pathways involving the intracellular second messengers cyclic AMP, cyclic GMP, IP3 and Ca2+. Pathways from the photo- and thermoreceptors converge first with each other and thence, at the level of the second messengers, with those from extracellular tip activation (cyclic AMP) and inhibition (Slug Turning Factor and/or ammonia and/or adenosine) signals that control slug movement and morphogenesis.

A mutation in the cAMP signaling pathway affects sexual development of Dictyostelium discoideum

Amoebae of cellular slime molds have two developmental modes, asexual fruiting body formation and sexual macrocyst formation. How developmental choice is made is an interesting subject of wide importance. Light exposure and dry conditions are favorable for asexual development, while conditions of darkness and high humidity are so for sexual development. In Dictyostelium discoideum, the latter conditions enhance zygote formation, which determines the fate of surrounding cells for sexual development. Here, a mutant (TMC1) defective in the post-fusion aggregation of cells during sexual development is described. This mutant is also aggregationless in asexual development, and the level of cyclic adenosine monophosphate (cAMP) receptor is reduced. Correspondingly, a series of existing mutants with defects in cAMP signaling pathways showed the same sexual phenotype as TMC1. These results suggest that molecular mechanisms of development are shared by the two alternative developmental modes.

A discrete cell model with adaptive signalling for aggregation of Dictyostelium discoideum

Dictyostelium discoideum (Dd) is a widely studied model system from which fundamental insights into cell movement, chemotaxis, aggregation and pattern formation can be gained. In this system aggregation results from the chemotactic response by dispersed amoebae to a travelling wave of the chemoattractant cAMP. We have developed a model in which the cells are treated as discrete points in a continuum field of the chemoattractant, and transduction of the extracellular cAMP signal into the intracellular signal is based on the G protein model developed by Tang & Othmer. The model reproduces a number of experimental observations and gives further insight into the aggregation process. We investigate different rules for cell movement the factors that influence stream formation the effect on aggregation of noise in the choice of the direction of movement and when spiral waves of chemoattractant and cell density are likely to occur. Our results give new insight into the origin of spiral waves and suggest that streaming is due to a finite amplitude instability.

An increase in cytosolic Ca2+ delays cAMP oscillations in Dictyostelium cells

We have shown that calmidazolium (R24571) causes a transient increase in the cytosolic free Ca2+ concentration ([Ca2+]i) in Dictyostelium discoideum [Schlatterer and Schaloske (1996) Biochem. J. 313, 661-667]. Here we have used R24571 to artifically increase [Ca2+]i during light-scattering oscillations and have found that, depending on the time of addition during the oscillatory cycle, R24571 suppressed cAMP synthesis and delayed the next spike for several minutes. Addition of Ca2+ to the medium, which also elevates [Ca2+]i, induced phase delays and resulted in a similar phase response curve as R24571. The magnitude of the phase delay was correlated with the point during the oscillatory cycle at which Ca2+ was added, indicating that an artificial increase in [Ca2+]i also resets the phase of the intrinsic oscillator.

Regulation of guanylyl cyclase by a cGMP-binding protein during chemotaxis in Dictyostelium discoideum

Chemoattractants transiently activate guanylyl cyclase in Dictyostelium discoideum cells. Mutant analysis demonstrates that the produced cGMP plays an essential role in chemotactic signal transduction, controlling the actomyosin-dependent motive force. Guanylyl cyclase activity is associated with the particulate fraction of a cell homogenate. The addition of the cytosol stimulates guanylyl cyclase activity, whereas the cytosol plus ATP/Mg2+ inhibits enzyme activity. We have analyzed the regulation of guanylyl cyclase in chemotactic mutants and present evidence that a cGMP-binding protein mediates both stimulation and ATP-dependent inhibition of guanylyl cyclase. Upon chromatography of cytosolic proteins, cGMP binding activity co-elutes with both guanylyl cyclase-stimulating and ATP-dependent-inhibiting activities. In addition, ATP-dependent inhibition of guanylyl cyclase activity is enhanced by the cGMP analogue 8-Br-cGMP, suggesting that a cGMP-binding protein regulates guanylyl cyclase activity. Mutant KI-4 has an aberrant cGMP binding activity with very low Kd and shows a very small chemoattractant-mediated cGMP response; the cytosol from this mutant does not stimulate guanylyl cyclase. In contrast to KI-4, the aberrant cGMP binding activity of mutant KI-7 has a very high Kd and chemoattractants induce a prolonged cGMP response. The cytosol of this mutant stimulates guanylyl cyclase activity, but ATP does not inhibit the enzyme. Thus, two previously isolated chemotactic mutants are defective in the activation and inhibition of guanylyl cyclase, respectively. The positive and negative regulation of guanylyl cyclase by its product cGMP may well explain how cells process the temporospatial information of chemotactic signals, which is necessary for sensing the direction of the chemoattractant.