Chemotactic sorting to cAMP in the multicellular stages of Dictyostelium development

From environmental sensing to developmental control: cognitive evolution in dictyostelid social amoebas

Dictyostelid social amoebas respond to starvation by self-organizing into multicellular slugs that migrate towards light to construct spore-bearing structures. These behaviours depend on excitable networks that enable amoebas to produce propagating waves of the chemoattractant cAMP, and to respond by directional movement. cAMP additionally regulates cell differentiation throughout development, with differentiation and cell movement being coordinated by interaction of the stalk inducer c-di-GMP with the adenylate cyclase that generates cAMP oscillations. Evolutionary studies indicate how the manifold roles of cAMP in multicellular development evolved from a role as intermediate for starvation-induced encystation in the unicellular ancestor. A merger of this stress response with the chemotaxis excitable networks yielded the developmental complexity and cognitive capabilities of extant Dictyostelia.

This article is part of the theme issue ‘Basal cognition: conceptual tools and the view from the single cell’.

Intracellular ATP levels influence cell fates in Dictyostelium discoideum differentiation

Multicellular organisms contain various differentiated cells. Fate determination of these cells remains a fundamental issue. The cellular slime mold Dictyostelium discoideum is a useful model organism for studying differentiation; it proliferates as single cells in nutrient-rich conditions, which aggregate into a multicellular body upon starvation, subsequently differentiating into stalk cells or spores. The fates of these cells can be predicted in the vegetative phase: Cells expressing higher and lower levels of omt12 differentiate into stalk cells and spores, respectively. However, omt12 is merely a marker gene and changes in its expression do not influence the cell fate, and determinant factors remain unknown. In this study, we analyzed cell fate determinants in the stalk-destined and spore-destined cells that were sorted based on omt12 expression. Luciferase assay demonstrated higher levels of intracellular ATP in the stalk-destined cells than in the spore-destined cells. Live-cell observation during development using ATP sensor probes revealed that cells with higher ATP levels differentiated into stalk cells. Furthermore, reducing the ATP level by treating with an inhibitor of ATP production changed the differentiation fates of the stalk-destined cells to spores. These results suggest that intracellular ATP levels influence cell fates in D. discoideum differentiation.

Tissue self-organization based on collective cell migration by contact activation of locomotion and chemotaxis

Despite their central role in multicellular organization, navigation rules that dictate cell rearrangement remain largely undefined. Contact between neighboring cells and diffusive attractant molecules are two of the major determinants of tissue-level patterning; however, in most cases, molecular and developmental complexity hinders one from decoding the exact governing rules of individual cell movement. A primordial example of tissue patterning by cell rearrangement is found in the social amoeba Dictyostelium discoideum where the organizing center or the "tip" self-organizes as a result of sorting of differentiating prestalk and prespore cells. By employing microfluidics and microsphere-based manipulation of navigational cues at the single-cell level, here we uncovered a previously overlooked mode of Dictyostelium cell migration that is strictly directed by cell-cell contact. The cell-cell contact signal is mediated by E-set Ig-like domain-containing heterophilic adhesion molecules TgrB1/TgrC1 that act in trans to induce plasma membrane recruitment of the SCAR complex and formation of dendritic actin networks, and the resulting cell protrusion competes with those induced by chemoattractant cAMP. Furthermore, we demonstrate that both prestalk and prespore cells can protrude toward the contact signal as well as to chemotax toward cAMP; however, when given both signals, prestalk cells orient toward the chemoattractant, whereas prespore cells choose the contact signal. These data suggest a model of cell sorting by competing juxtacrine and diffusive cues, each with potential to drive its own mode of collective cell migration.

Collective cell migration of Dictyostelium without cAMP oscillations at multicellular stages

In Dictyostelium discoideum, a model organism for the study of collective cell migration, extracellular cyclic adenosine 3',5'-monophosphate (cAMP) acts as a diffusible chemical guidance cue for cell aggregation, which has been thought to be important in multicellular morphogenesis. Here we revealed that the dynamics of cAMP-mediated signaling showed a transition from propagating waves to steady state during cell development. Live-cell imaging of cytosolic cAMP levels revealed that their oscillation and propagation in cell populations were obvious for cell aggregation and mound formation stages, but they gradually disappeared when multicellular slugs started to migrate. A similar transition of signaling dynamics occurred with phosphatidylinositol 3,4,5-trisphosphate signaling, which is upstream of the cAMP signal pathway. This transition was programmed with concomitant developmental progression. We propose a new model in which cAMP oscillation and propagation between cells, which are important at the unicellular stage, are unessential for collective cell migration at the multicellular stage.

Theaflavins from black tea affect growth, development, and motility in Dictyostelium discoideum

Theaflavins, flavonoids found in black tea, exhibit a variety of health-promoting activities, but the mechanisms by which they act are not clear. Here, we assess the effects of black tea extract and isolated theaflavins on Dictyostelium discoideum, a model organism exhibiting an unusual life cycle relying on conserved pathways involved in human disease. Dictyostelium has been used to characterize the activities of numerous bioactive small molecules, including catechins, from which theaflavins are produced during the preparation of black tea. We show that theaflavins block growth, development, and motility in Dictyostelium, results that suggest catechins and theaflavins exert similar activities in this organism.

Glutathione upregulates cAMP signalling via G protein alpha 2 during the development of Dictyostelium discoideum

Despite the importance of glutathione in Dictyostelium, the role of glutathione synthetase (gshB/GSS) has not been clearly investigated. In this study, we observed that increasing glutathione content by constitutive expression of gshB leads to mound-arrest and defects in 3',5'-cyclic adenosine monophosphate (cAMP)-mediated aggregation and developmental gene expression. The overexpression of gpaB encoding G protein alpha 2 (Gα2), an essential component of the cAMP signalling pathway, results in a phenotype similar to that caused by gshB overexpression, whereas gpaB knockdown in gshB-overexpressing cells partially rescues the above-mentioned phenotypic defects. Furthermore, Gα2 is highly enriched at the plasma membrane of gshB-overexpressing cells compared to wild-type cells. Therefore, our findings suggest that glutathione upregulates cAMP signalling via Gα2 modulation during Dictyostelium development.

Progress and perspectives in signal transduction, actin dynamics, and movement at the cell and tissue level: lessons from Dictyostelium

Movement of cells and tissues is a basic biological process that is used in development, wound repair, the immune response to bacterial invasion, tumour formation and metastasis, and the search for food and mates. While some cell movement is random, directed movement stimulated by extracellular signals is our focus here. This involves a sequence of steps in which cells first detect extracellular chemical and/or mechanical signals via membrane receptors that activate signal transduction cascades and produce intracellular signals. These intracellular signals control the motile machinery of the cell and thereby determine the spatial localization of the sites of force generation needed to produce directed motion. Understanding how force generation within cells and mechanical interactions with their surroundings, including other cells, are controlled in space and time to produce cell-level movement is a major challenge, and involves many issues that are amenable to mathematical modelling.

Evolutionary reconstruction of pattern formation in 98 Dictyostelium species reveals that cell-type specialization by lateral inhibition is a derived trait

Background

Multicellularity provides organisms with opportunities for cell-type specialization, but requires novel mechanisms to position correct proportions of different cell types throughout the organism. Dictyostelid social amoebas display an early form of multicellularity, where amoebas aggregate to form fruiting bodies, which contain only spores or up to four additional cell-types. These cell types will form the stalk and support structures for the stalk and spore head. Phylogenetic inference subdivides Dictyostelia into four major groups, with the model organism D. discoideum residing in group 4. In D. discoideum differentiation of its five cell types is dominated by lateral inhibition-type mechanisms that trigger scattered cell differentiation, with tissue patterns being formed by cell sorting.

Results

To reconstruct the evolution of pattern formation in Dictyostelia, we used cell-type specific antibodies and promoter-reporter fusion constructs to investigate pattern formation in 98 species that represent all groupings. Our results indicate that in all early diverging Dictyostelia and most members of groups 1–3, cells differentiate into maximally two cell types, prestalk and prespore cells, with pattern formation being dominated by position-dependent transdifferentiation of prespore cells into prestalk cells. In clade 2A, prestalk and stalk cell differentiation are lost and the prespore cells construct an acellular stalk. Group 4 species set aside correct proportions of prestalk and prespore cells early in development, and differentiate into up to three more supporting cell types.

Conclusions

Our experiments show that positional transdifferentiation is the ancestral mode of pattern formation in Dictyostelia. The early specification of a prestalk population equal to the number of stalk cells is a derived trait that emerged in group 4 and a few late diverging species in the other groups. Group 4 spore masses are larger than those of other groups and the differentiation of supporting cell types by lateral inhibition may have facilitated this increase in size. The signal DIF-1, which is secreted by prespore cells, triggers differentiation of supporting cell types. The synthesis and degradation of DIF-1 were shown to be restricted to group 4. This suggests that the emergence of DIF-1 signalling caused increased cell-type specialization in this group.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-5-34) contains supplementary material, which is available to authorized users.

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

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

Evolution of developmental cyclic adenosine monophosphate signaling in the Dictyostelia from an amoebozoan stress response

The Dictyostelid social amoebas represent one of nature's several inventions of multicellularity. Though normally feeding as single cells, nutrient stress triggers the collection of amoebas into colonies that form delicately shaped fruiting structures in which the cells differentiate into spores and up to three cell types to support the spore mass. Cyclic adenosine monophosphate (cAMP) plays a very dominant role in controlling morphogenesis and cell differentiation in the model species Dictyostelium discoideum. As a secreted chemoattractant cAMP coordinates cell movement during aggregation and fruiting body morphogenesis. Secreted cAMP also controls gene expression at different developmental stages, while intracellular cAMP is extensively used to transduce the effect of other stimuli that control the developmental program. In this review, I present an overview of the different roles of cAMP in the model D. discoideum and I summarize studies aimed to resolve how these roles emerged during Dictyostelid evolution.

Cell type-specific filamin complex regulation by a novel class of HECT ubiquitin ligase is required for normal cell motility and patterning

Differential cell motility, which plays a key role in many developmental processes, is perhaps most evident in examples of pattern formation in which the different cell types arise intermingled before sorting out into discrete tissues. This is thought to require heterogeneities in responsiveness to differentiation-inducing signals that result in the activation of cell type-specific genes and 'salt and pepper' patterning. How differential gene expression results in cell sorting is poorly defined. Here we describe a novel gene (hfnA) that provides the first mechanistic link between cell signalling, differential gene expression and cell type-specific sorting in Dictyostelium. HfnA defines a novel group of evolutionarily conserved HECT ubiquitin ligases with an N-terminal filamin domain (HFNs). HfnA expression is induced by the stalk differentiation-inducing factor DIF-1 and is restricted to a subset of prestalk cells (pstO). hfnA(-) pstO cells differentiate but their sorting out is delayed. Genetic interactions suggest that this is due to misregulation of filamin complex activity. Overexpression of filamin complex members phenocopies the hfnA(-) pstO cell sorting defect, whereas disruption of filamin complex function in a wild-type background results in pstO cells sorting more strongly. Filamin disruption in an hfnA(-) background rescues pstO cell localisation. hfnA(-) cells exhibit altered slug phototaxis phenotypes consistent with filamin complex hyperactivity. We propose that HfnA regulates filamin complex activity and cell type-specific motility through the breakdown of filamin complexes. These findings provide a novel mechanism for filamin regulation and demonstrate that filamin is a crucial mechanistic link between responses to differentiation signals and cell movement in patterning based on 'salt and pepper' differentiation and sorting out.

Evolutionary crossroads in developmental biology: Dictyostelium discoideum

Dictyostelium discoideum belongs to a group of multicellular life forms that can also exist for long periods as single cells. This ability to shift between uni- and multicellularity makes the group ideal for studying the genetic changes that occurred at the crossroads between uni- and multicellular life. In this Primer, I discuss the mechanisms that control multicellular development in Dictyostelium discoideum and reconstruct how some of these mechanisms evolved from a stress response in the unicellular ancestor.

The trishanku gene and terminal morphogenesis in Dictyostelium discoideum

Multicellular development in the social amoeba Dictyostelium discoideum is triggered by starvation. It involves a series of morphogenetic movements, among them being the rising of the spore mass to the tip of the stalk. The process requires precise coordination between two distinct cell types-presumptive (pre-) spore cells and presumptive (pre-) stalk cells. Trishanku (triA) is a gene expressed in prespore cells that is required for normal morphogenesis. The triA(-) mutant shows pleiotropic effects that include an inability of the spore mass to go all the way to the top. We have examined the cellular behavior required for the normal ascent of the spore mass. Grafting and mixing experiments carried out with tissue fragments and cells show that the upper cup, a tissue that derives from prestalk cells and anterior-like cells (ALCs), does not develop properly in a triA(-) background. A mutant upper cup is unable to lift the spore mass to the top of the fruiting body, likely due to defective intercellular adhesion. If wild-type upper cup function is provided by prestalk and ALCs, trishanku spores ascend all the way. Conversely, Ax2 spores fail to do so in chimeras in which the upper cup is largely made up of mutant cells. Besides proving that under these conditions the wild-type phenotype of the upper cup is necessary and sufficient for terminal morphogenesis in D. discoideum, this study provides novel insights into developmental and evolutionary aspects of morphogenesis in general. Genes that are active exclusively in one cell type can elicit behavior in a second cell type that enhances the reproductive fitness of the first cell type, thereby showing that morphogenesis is a cooperative process.

Forming patterns in development without morphogen gradients: scattered differentiation and sorting out

Few mechanisms provide alternatives to morphogen gradients for producing spatial patterns of cells in development. One possibility is based on the sorting out of cells that initially differentiate in a salt and pepper mixture and then physically move to create coherent tissues. Here, we describe the evidence suggesting this is the major mode of patterning in Dictyostelium. In addition, we discuss whether convergent evolution could have produced a conceptually similar mechanism in other organisms.

cAMP-dependent signal pathways in unicellular eukaryotes

The review summarizes current data about mechanisms of signal transduction with participation of cAMP (cyclic adenosine monophosphate) and elements of the complex cAMP-protein kinase A (PKA) signal pathway in unicellular eukaryotes. Conceptions of evolutionary origin of eukaryotic signal transduction systems are developed.

A model for cell type localization in the migrating slug of Dictyostelium discoideum based on differential chemotactic sensitivity to cAMP and differential sensitivity to suppression of chemotaxis by ammonia

The three basic cell types in the migrating slug of Dictyostelium discoideum show differential chemotactic response to cyclic AMP (cAMP) and differential sensitivity to suppression of the chemotaxis by ammonia.The values of these parameters indicate a progressive maturation of chemotactic properties during the transdifferentiation of slug cell types.We present a model that explains the localization of the three cell types within the slug based on these chemotactic differences and on the maturation of their chemotactic properties.

Cell type specificity of a diffusible inducer is determined by a GATA family transcription factor

One poorly understood mechanism of developmental patterning involves the intermingled differentiation of different cell types that then sort out to generate pattern. Examples of this are known in nematodes and vertebrates, and in Dictyostelium it is the major mechanism. However, a general problem with this mechanism is the possibility that different inducers are required for each cell type that arises independently of positional information. Consistent with this idea, in Dictyostelium the signalling molecule DIF acts as a position-independent signal and was thought only to regulate the differentiation of a single cell type (pstO). The results presented here challenge this idea. In a novel genetic selection to isolate genes required for DIF signal transduction, we found a mutant (dimC(-)) that is a hypomorphic allele of a GATA family transcription factor (gtaC). gtaC expression is directly regulated by DIF, and GtaC rapidly translocates to the nucleus in response to DIF. gtaC(-) null cells showed some hallmark DIF signalling defects. Surprisingly, other aspects of the mutant were distinct from those of other DIF signalling mutants, suggesting that gtaC regulates a subset of DIF responses. For example, pstO cell differentiation appeared normal. However, we found that pstB cells were mislocalised and the pstB-derived basal disc was much reduced or missing. These defects are due to a failure to respond to DIF as they are phenocopied in other DIF signalling mutants. These findings therefore identify a novel small-molecule-activated GATA factor that is required to regulate the cell type-specific effects of DIF. They also reveal that a non-positional signal can regulate the differentiation of multiple cell types through differential interpretation in receiving cells.

Regulation of ammonia homeostasis by the ammonium transporter AmtA in Dictyostelium discoideum

Ammonia has been shown to function as a morphogen at multiple steps during the development of the cellular slime mold Dictyostelium discoideum; however, it is largely unknown how intracellular ammonia levels are controlled. In the Dictyostelium genome, there are five genes that encode putative ammonium transporters: amtA, amtB, amtC, rhgA, and rhgB. Here, we show that AmtA regulates ammonia homeostasis during growth and development. We found that cells lacking amtA had increased levels of ammonia/ammonium, whereas their extracellular ammonia/ammonium levels were highly decreased. These results suggest that AmtA mediates the excretion of ammonium. In support of a role for AmtA in ammonia homeostasis, AmtA mRNA is expressed throughout the life cycle, and its expression level increases during development. Importantly, AmtA-mediated ammonia homeostasis is critical for many developmental processes. amtA(-) cells are more sensitive to NH(4)Cl than wild-type cells in inhibition of chemotaxis toward cyclic AMP and of formation of multicellular aggregates. Furthermore, even in the absence of exogenously added ammonia, we found that amtA(-) cells produced many small fruiting bodies and that the viability and germination of amtA(-) spores were dramatically compromised. Taken together, our data clearly demonstrate that AmtA regulates ammonia homeostasis and plays important roles in multiple developmental processes in Dictyostelium.

Developmental decisions in Dictyostelium discoideum

Dictyostelium discoideum is an excellent system in which to study developmental decisions. Synchronous development is triggered by starvation and rapidly generates a limited number of cell types. Genetic and image analyses have revealed the elegant intricacies associated with this simple development system. Key signaling pathways identified as regulating cell fate decisions are likely to be conserved with metazoa and are providing insight into differentiation decisions under circumstances where considerable cell movement takes place during 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 demonstration of pattern formation without positional information in Dictyostelium

Although positional information, conveyed by morphogen gradients, is a widely accepted way of forming patterns during development, an alternative method is conceivable, based on the intermingled differentiation of cells with different fates, followed by their sorting into discrete pattern elements. It has been proposed that Dictyostelium prestalk and prespore cells behave in this way at the mound stage of development. However, it has been difficult to conclusively demonstrate that they initially differentiate intermingled, because rapid cell movement within the mound makes it impossible to be sure where prestalk and prespore cells originate. We have taken a novel approach to address this problem by blocking cell movement at different stages in development, using the actin-depolymerizing drug, latrunculin-A. Prestalk and prespore cells differentiate with essentially normal efficiency and timing in such paralyzed structures. When movement is blocked sufficiently early, the major cell types all subsequently differentiate at scattered positions throughout the aggregate, and even in the streams leading into it. Our work strongly supports the idea that the prestalk/prespore pattern in Dictyostelium forms without positional information and demonstrate that latrunculin-A may provide a useful tool for the investigation of patterning in other organisms.

Extracellular matrix family proteins that are potential targets of Dd-STATa in Dictyostelium discoideum

Dd-STATa is a functional Dictyostelium homologue of metazoan STAT (signal transducers and activators of transcription) proteins, which is activated by cAMP and is thereby translocated into the nuclei of anterior tip cells of the prestalk region of the slug. By using in situ hybridization analyses, we found that the SLF308 cDNA clone, which contains the ecmF gene that encodes a putative extracellular matrix protein and is expressed in the anterior tip cells, was greatly down-regulated in the Dd-STATa-null mutant. Disruption of the ecmF gene, however, resulted in almost no phenotypic change. The absence of any obvious mutant phenotype in the ecmF-null mutant could be due to a redundancy of similar genes. In fact, a search of the Dictyostelium whole genome database demonstrates the existence of an additional 16 homologues, all of which contain a cellulose-binding module. Among these homologues, four genes show Dd-STATa-dependent expression, while the others are Dd-STATa-independent. We discuss the potential role of Dd-STATa in morphogenesis via its effect on the interaction between cellulose and these extracellular matrix family proteins.

Efficient system-wide coordination in noisy environments

Many natural and social systems display global organization and coordination without centralized control. The origin of this global coordination is a topic of great current interest. Here we investigate a density-classification task as a model system for coordination and information processing in decentralized systems. We show that sophisticated strategies, selected under idealized conditions, are not robust to environmental changes. We also demonstrate that a simple heuristic is able to successfully complete the classification task under a broad range of environmental conditions. Our findings hint at the possibility that complex networks and ecologically efficient rules coevolve over time.

A cAMP receptor-like G protein-coupled receptor with roles in growth regulation and development

Dictyostelium discoideum uses G protein-mediated signal transduction for many vegetative and developmental functions, suggesting the existence of G protein-coupled receptors (GPCRs) other than the four known cyclic adenosine monophosphate (cAMP) receptors (cAR1-4). Sequences of the cAMP receptors were used to identify Dictyostelium genes encoding cAMP receptor-like proteins, CrlA-C. Limited sequence identity between these putative GPCRs and the cAMP receptors suggests the Crl receptors are unlikely to be receptors for cAMP. The crl genes are expressed at various times during growth and the developmental life cycle. Disruption of individual crl genes did not impair chemotactic responses to folic acid or cAMP or alter cAMP-dependent aggregation. However, crlA(-) mutants grew to a higher cell density than did wild-type cells and high-copy-number crlA expression vectors were detrimental to cell viability, suggesting that CrlA is a negative regulator of cell growth. In addition, crlA(-) mutants produce large aggregates with delayed anterior tip formation indicating a role for the CrlA receptor in the development of the anterior prestalk cell region. The scarcity of GFP-expressing crlA(-) mutants in the anterior prestalk cell region of chimeric organisms supports a cell-autonomous role for the CrlA receptor in prestalk cell differentiation.

A bZIP/bRLZ transcription factor required for DIF signaling in Dictyostelium

The intermingled differentiation and sorting out of Dictyostelium prestalk-O and prespore cells requires the diffusible signaling molecule DIF-1, and provides an example of a spatial information-independent patterning mechanism. To further understand this patterning process, we used genetic selection to isolate mutants in the DIF-1 response pathway. The disrupted gene in one such mutant, dimA(-), encodes a bZIP/bRLZ transcription factor, which is required for every DIF-1 response investigated. Furthermore, the dimA(-) mutant shows strikingly similar developmental defects to the dmtA(-) mutant, which is specifically defective in DIF-1 synthesis. However, key differences exist: (1) the dmtA(-) mutant responds to DIF-1 but does not produce DIF-1; (2) the dimA(-) mutant produces DIF-1 but does not respond to DIF-1; and (3) the dimA(-) mutant exhibits cell autonomous defects in cell type differentiation. These results suggest that dimA encodes the key transcriptional regulator required to integrate DIF-1 signaling and subsequent patterning in Dictyostelium.

Ammonia differentially suppresses the cAMP chemotaxis of anterior-like cells and prestalk cells in Dictyostelium discoideum

A drop assay for chemotaxis to cAMP confirms that both anterior-like cells (ALC) and prestalk cells (pst cells) respond to cAMP gradients. We present evidence that the chemotactic response of both ALC and pst cells is suppressed by ammonia, but a higher concentration of ammonia is required to suppress the response in pst cells. ALC show a chemotactic response to cAMP when moving on a substratum of prespore cells in isolated slug posteriors incubated under oxygen. ALC chemotaxis on a prespore cell substratum is suppressed by the same concentration of ammonia that suppresses ALC chemotaxis on the agar substratum in drop assays. Chemotaxis suppression is mediated by the unprotonated (NH3) species of ammonia. The observed suppression, by ammonia, of ALC chemotaxis to cAMP supports our earlier hypothesis that ammonia is the tip-produced suppressor of such chemotaxis. We discuss implications of ammonia sensitivity of pst cells and ALC with regard to the movement and localization of ALC and pst cells in the slug and to the roles played by ALC in fruiting body formation. In addition, we suggest that a progressive decrease in sensitivity to ammonia is an important part of the maturation of ALC into pst cells.

Spatial expression patterns of genes involved in cyclic AMP responses in Dictyostelium discoideum development

The spatial expression patterns of genes involved in cyclic adenosine monophosphate (cAMP) responses during morphogenesis in Dictyostelium discoideum were analyzed by in situ hybridization. Genes encoding adenylyl cyclase A (ACA), cAMP receptor 1, G-protein alpha2 and beta subunits, cytosolic activator of ACA (CRAC and Aimless), catalytic subunit of protein kinase A (PKA-C) and cAMP phosphodiesterases (PDE and REG-A) were preferentially expressed in the anterior prestalk (tip) region of slugs, which acts as an organizing center. MAP kinase ERK2 (extracellular signal-regulated kinase-2) mRNA, however, was enriched in the posterior prespore region. At the culmination stage, the expression of ACA, CRAC and PKA-C mRNA increased in prespore cells in contrast with the previous stage. However, no alteration in the site of expression was observed for the other mRNA analyzed. Based on these findings, two and four classes of expression patterns were catalogued for these genes during the slug and culmination stages, respectively. Promoter analyses of genes in particular classes should enhance understanding of the regulation of dynamic and coordinated gene expression during morphogenesis.

Inducible nuclear translocation of a STAT protein in Dictyostelium prespore cells: implications for morphogenesis and cell-type regulation

Dd-STATa, the Dictyostelium STAT (signal transducer and activator of transcription) protein, is selectively localised in the nuclei of a small subset of prestalk cells located in the slug tip. Injection of cAMP into the extracellular spaces in the rear of the slug induces rapid nuclear translocation of a Dd-GFP:STATa fusion protein in prespore cells surrounding the site of injection. This suggests that cAMP signals that emanate from the tip direct the localised nuclear accumulation of Dd-STATa. It also shows that prespore cells are competent to respond to cAMP, by Dd-STATa activation, and it implies that cAMP signalling is in some way limiting in the rear of the slug. Co-injection of a specific inhibitor of the cAR1 serpentine cAMP receptor almost completely prevents the cAMP-induced nuclear translocation, showing that most or all of the cAMP signal is transduced by cAR1. Dd-GFP:STATa also rapidly translocates into the nuclei of cells adjoining the front and back cut edges when a slug is bisected. Less severe mechanical disturbances, such as pricking the rear of a slug with an unfilled micropipette, also cause a more limited nuclear translocation of Dd-GFP:STATa. We propose that these signalling events form part of a repair mechanism that is activated when the migrating slug suffers mechanical damage.

tip genes act in parallel pathways of early Dictyostelium development

Analysis of Dictyostelium strains carrying null mutations in tipA showed a primary defect in cell sorting and the formation of tips on the developing mound. To study the process affected in tipA- mutants further, other mutants with a similar phenotype were isolated and characterized. These studies showed three new Dictyostelium genes: tipB, tipC, and tipD. All the tip mutants aggregate into larger than average mounds, which split up and form many lips on their surfaces. Furthermore, each mutant exhibits reduced or aberrant cell-sorting behavior, never makes migrating slugs, and has severely reduced fruiting body and spore production. The mRNA of each tip gene is present in vegetative cells and does not vary significantly with development. Prespore and prestalk gene expression is reduced or delayed in the tip mutants indicating cell type differentiation is dependent on the function of these genes. Developing mutant cells in chimeric mixtures with wildtype cells demonstrated that the defects in each tip mutant behave cell autonomously. The overexpression of TipA in a tipB- background and the overexpression of TipB in a tipA- background significantly improved the morphogenesis of these mutants. These were the only situations in which the expression of one tip gene could compensate for the lack of a different tip gene. Except for the tipA-/tipB- strain, double mutations in the tip genes have additive effects, causing a more severe mutant phenotype with defects earlier in development than single mutants. The tipA-/tipB- double mutant does not show additive effects and is very similar to the tipA-single mutant. Analysis of the effects of double mutations and overexpression indicates that members of this class of genes appear to act through parallel pathways of differentiation and tip formation in early Dictyostelium development. Furthermore, TipA and TipB appear to have some overlapping functions or are involved in the same pathway. The multitipped phenotype observed in all the mutants may be a general result of perturbing early developmental events such as cell type differentiation and cell type proportioning.

Three-dimensional in vivo analysis of Dictyostelium mounds reveals directional sorting of prestalk cells and defines a role for the myosin II regulatory light chain in prestalk cell sorting and tip protrusion

During cell sorting in Dictyostelium, we observed that GFP-tagged prestalk cells (ecmAO-expressing cells) moved independently and directionally to form a cluster. This is consistent with a chemotaxis model for cell sorting (and not differential adhesion) in which a long-range signal attracts many of the prestalk cells to the site of cluster formation. Surprisingly, the ecmAO prestalk cluster that we observed was initially found at a random location within the mound of this Ax3 strain, defining an intermediate sorting stage not widely reported in Dictyostelium. The cluster then moved en masse to the top of the mound to produce the classic, apical pattern of ecmAO prestalk cells. Migration of the cluster was also directional, suggesting the presence of another long-range guidance cue. Once at the mound apex, the cluster continued moving upward leading to protrusion of the mound's tip. To investigate the role of the cluster in tip protrusion, we examined ecmAO prestalk-cell sorting in a myosin II regulatory light chain (RLC) null in which tips fail to form. In RLC-null mounds, ecmAO prestalk cells formed an initial cluster that began to move to the mound apex, but then arrested as a vertical column that extended from the mound's apex to its base. Mixing experiments with wild-type cells demonstrated that the RLC-null ecmAO prestalk-cell defect is cell autonomous. These observations define a specific mechanism for myosin's function in tip formation, namely a mechanical role in the upward movement of the ecmAO prestalk cluster. The wild-type data demonstrate that cell sorting can occur in two steps, suggesting that, in this Ax3 strain, spatially and temporally distinct cues may guide prestalk cells first to an initial cluster and then later to the tip.

A temperature-sensitive adenylyl cyclase mutant of Dictyostelium

Dictyostelium development starts with the chemotactic aggregation of up to 10(6) amoebae in response to propagating cAMP waves. cAMP is produced by the aggregation stage adenylyl cyclase (ACA) and cells lacking ACA (aca null) cannot aggregate. Temperature-sensitive mutants of ACA were selected from a population of aca null cells transformed with a library of ACA genes, a major segment of which had been amplified by error-prone PCR. One mutant (tsaca2) that can complement the aggregation null phenotype of aca null cells at 22 degrees C but not at 28 degrees C was characterized in detail. The basal catalytic activity of the enzyme in this mutant was rapidly and reversibly inactivated at 28 degrees C. Using this mutant strain we show that cell movement in aggregates and mounds is organized by propagating waves of cAMP. Synergy experiments between wild-type and tsaca2 cells, shifted to the restrictive temperature at various stages of development, showed that ACA plays an important role in the control of cell sorting and tip formation.

The Dictyostelium LIM domain-containing protein LIM2 is essential for proper chemotaxis and morphogenesis

We have identified limB, a gene encoding a novel LIM domain-containing protein, LIM2, in a screen for genes required for morphogenesis. limB null cells aggregate, although poorly, but they are unable to undergo morphogenesis, and the aggregates arrest at the mound stage. limB null cells exhibit an aberrant actin cytoskeleton and have numerous F-actin-enriched microspikes. The cells exhibit poor adhesion to a substratum and do not form tight cell-cell agglomerates in suspension. Furthermore, limB null cells are unable to properly polarize in chemoattractant gradients and move very poorly. Expression of limB from a prestalk-specific but not a prespore-specific promoter complements the morphogenetic defects of the limB null strain, suggesting that the limB null cell developmental defect results from an inability to properly sort prestalk cells. LIM2 protein is enriched in the cortex of wild-type cells, although it does not colocalize with the actin cytoskeleton. Our analysis indicates that LIM2 is a new regulatory protein that functions to control rearrangements of the actin cytoskeleton and is required for cell motility and chemotaxis. Our findings may be generally applicable to understanding pathways that control cell movement and morphogenesis in all multicellular organisms. Structure function studies on the LIM domains are presented.

Light affects cAMP signaling and cell movement activity in Dictyostelium discoideum

The multicellular, slug stage of the slime mould Dictyostelium discoideum lacks specific sensory cells and organs but can nevertheless respond in a very sensitive manner to external stimuli such as temperature and light. Within the migrating slug, the behavior of up to 100,000 individual amoebae is coordinated by cAMP mediated cell-cell signaling and chemotaxis. We report here the striking result that light directly modulates the cAMP cell-cell signaling system. Light-induced secretion of cAMP from the slug tips decreased the period length of optical density waves and speeded up cell movement. A local effect of light on cAMP release within the slug tip could modulate cell movement within the slug and thus control its phototactic turning and orientation toward a light source.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.

Folic acid stimulation of the Galpha4 G protein-mediated signal transduction pathway inhibits anterior prestalk cell development in Dictyostelium

In Dictyostelium discoideum, several G proteins are known to mediate the transduction of signals that direct chemotactic movement and regulate developmental morphogenesis. The G protein alpha subunit encoded by the Galpha4 gene has been previously shown to be required for chemotactic responses to folic acid, proper developmental morphogenesis, and spore production. In this study, cells overexpressing the wild type Galpha4 gene, due to high copy gene dosage (Galpha4HC), were found to be defective in the ability to form the anterior prestalk cell region, express prespore- and prestalk-cell specific genes, and undergo spore formation. In chimeric organisms, Galpha4HC prespore cell-specific gene expression and spore production were rescued by the presence of wild-type cells, indicating that prespore cell development in Galpha4HC cells is limited by the absence of an intercellular signal. Transplanted wild-type tips were sufficient to rescue Galpha4HC prespore cell development, suggesting that the rescuing signal originates from the anterior prestalk cells. However, the deficiencies in prestalk-specific gene expression were not rescued in the chimeric organisms. Furthermore, Galpha4HC cells were localized to the prespore region of these chimeric organisms and completely excluded from the anterior prestalk region, suggesting that the Galpha4 subunit functions cell-autonomously to prevent anterior prestalk cell development. The presence of exogenous folic acid during vegetative growth and development delayed anterior prestalk cell development in wild-type but not galpha4 null mutant aggregates, indicating that folic acid can inhibit cell-type-specific differentiation by stimulation of the Galpha4-mediated signal transduction pathway. The results of this study suggest that Galpha4-mediated signals can regulate cell-type-specific differentiation by promoting prespore cell development and inhibiting anterior prestalk-cell development.

Induction of optical density waves and chemotactic cell movement in Dictyostelium discoideum by microinjection of cAMP pulses

The development of most multicellular organisms involves coordinated cell movement. The early aggregation of Dictyostelium cells has been shown to be mediated by chemotactic movement to propagating waves of cAMP. We have proposed that propagating waves of a chemoattractant, most likely cAMP, also control the movement of cells in mounds and slugs. We have now used periodic pressure injection of pulses of cAMP in the extracellular space of aggregation streams, mounds, and slugs to investigate whether these signals can be relayed and control cell movement, using quantitative digital time-lapse microscopy. Our major findings are (1) short (0.1 s) pulses of cAMP (10(7) molecules) were able to elicit optical density (OD) waves in fields of aggregating amoebae. They propagate from the micropipet outward and interact with endogenous OD waves. (2) Periodic injection of cAMP pulses into aggregation streams blocked the pulses coming from the center and led to the rapid accumulation of cells downstream of the pipet around the pipet. (3) Injection of pulses of cAMP into mounds elicited OD waves, which propagated from the pipet outward and interacted with the endogenous waves, indicating that the same propagator carries them. (4) Periodic microinjection of cAMP in the prespore zone of slugs led to accumulation of anterior-like cells around the micropipet followed by tip formation. Furthermore, the cAMP signal could control the spacing of the endogenous sorting pattern. These results strongly support the hypothesis that the optical density waves observed during early development up to the mound stage represent propagating cAMP waves. They suggest furthermore that cAMP is the morphogen that controls cell movements in slugs.

Possible cooperation of differential adhesion and chemotaxis in mound formation of Dictyostelium

In the mound stage of Dictyostelium discoideum, pre-stalk cells sort and form a tip at the apex. How this pattern forms is as yet unknown. A cellular level model allows us to simulate both differential cell adhesion and chemotaxis, to show that with differential adhesion only, pre-stalk cells move to the surface of the mound but form no tip. With chemotaxis driven by an outgoing circular wave only, a tip forms but contains both pre-stalk and pre-spore cells. Only for a narrow range of relative strengths between differential adhesion and chemotaxis can both mechanisms work in concert to form a tip containing only pre-stalk cells. The simulations provide a method to determine the processes necessary for patterning and suggest a series of further experiments.

A novel Dictyostelium cell surface protein important for both cell adhesion and cell sorting

A mutant of Dictyostelium that is aberrant in the process of tip formation (dtfA-: defective in tip formation A) has been isolated by gene tagging. The dtfA gene is predicted to encode a protein of 163 kDa. There are no extensive sequence homologies between DTFA and previously identified proteins, but four short N-terminal sequence motifs show partial homology to repeats found in mammalian mucins. Immunofluorescence reveals a lattice-like arrangement of DTFA protein at the cell surface. When developing on a bacterial lawn, cells of the mutant strain (dtfA- cells) aggregate to form tight mounds, but development then becomes arrested. When developed in the absence of nutrients, a fraction of dtfA- cells complete development, but there is a long delay at the tight mound stage and the culminants that eventually form are aberrant. In such dtfA- mounds the prestalk cells fail to move to the apex on cue and so tip formation is delayed. dtfA- cells also show a conditional defect in early development, in that they are unable to aggregate when plated at low density. In addition dtfA- cells do not agglomerate efficiently when shaken in suspension. In combination, these results suggest that DTFA may form part of a cell-cell adhesion system that is needed both for optimal aggregation and for efficient cell sorting during multicellular development. The DTFA protein also appears to be important during cell growth, because cytokinesis is defective and the actin cytoskeleton aberrant in growing dtfA- cells.

Propagating waves control Dictyostelium discoideum morphogenesis

The morphogenesis of Dictyostelium results from the coordinated movement of starving cells to form a multicellular aggregate (mound) which transforms into a motile slug and finally a fruiting body. Cells differentiate in the mound and sort out to form an organised pattern in the slug and fruiting body. During aggregation, cell movement is controlled by propagating waves of the chemo-attractant cAMP. We show that mounds are also organised by propagating waves. Their geometry changes from target or single armed spirals during aggregation to multi-armed spiral waves in the mound. Some mounds develop transiently into rings in which multiple propagating wave fronts can still be seen. We model cell sorting in the mound stage assuming cell type specific differences in cell movement speed and excitability. This sorting feeds back on the wave geometry to generate twisted scroll waves in the slug. Slime mould morphogenesis can be understood in terms of wave propagation directing chemotactic cell movement.

Coexpression of a constitutively active plasma membrane calcium pump with GFP identifies roles for intracellular calcium in controlling cell sorting during morphogenesis in Dictyostelium

To examine the potential role of calcium in regulating Dictyostelium development, we reduced free cytosolic and total cell Ca2+ in Dictyostelium cells by expressing a constitutively active form of a human erythrocyte plasma membrane calcium pump. The pump-expressing cells lacked a thapsigargin-mediated increase in cytoplasmic calcium, consistent with a reduced level of total cellular Ca2+. During aggregation, the cells initially formed a large number of aggregation centers, many of which coalesced to form mounds that were smaller than those of wild-type cells, and the cells did not exhibit the normal formation of elongated aggregation streams. The majority of the mounds either arrested at this stage with the formation of small protrusions or formed very aberrant finger-like structures, indicating an essential role for cellular calcium in morphogenesis. We used pump and wild-type cells differentially labeled by expressing different wavelength (green and blue) forms of green fluorescent protein and three-dimensional (3-D) reconstruction of serial fluorescent imaging to visualize the movement of pump and wild-type cells within the aggregate. The results showed that the pump cells exhibited very aberrant cell movement and sorting within the forming mound, suggesting that the reduced cytosolic calcium affects movement required for tip formation. When allowed to form chimeric organisms with wild-type cells, pump cells preferentially localized to two bands, one at the prestalk/prespore boundary and the other in the very posterior of the organism, suggesting that pump cells are unable to properly sort. Expression of the calcium pump had little effect on the induction of prestalk- or prespore-specific genes, whereas extended treatment with EGTA blocked induction of both classes of cell-type-specific genes. Our results suggest a role for intracellular Ca2+ in controlling cell sorting and morphogenesis in Dictyostelium.

The homeobox-containing gene Wariai regulates anterior-posterior patterning and cell-type homeostasis in Dictyostelium

We have identified a Dictyostelium gene, Wariai (Wri), that encodes a protein with a homeobox and seven ankyrin repeats; both domains are required for function. A null mutation results in a more than doubling of the size of the prestalk O (pstO) compartment, one of the anterior prestalk compartments lying along the anterior-posterior axis of the migrating slug. There is a concomitant decrease in the more posterior prespore domain and no change in the more anterior prestalk A (pstA) and prestalk AB (pstAB) domains. wri null cells also have a morphological defect consistent with an increase in the pstO cell population. Wri itself is preferentially expressed in the pstA but not the pstO compartment, raising the possibility that Wri regulation of pstO compartment size is nonautonomous. Analysis of chimeric organisms is consistent with this model. Development in Dictyostelium is highly regulative, with cells within the prestalk and prespore populations being able to transdifferentiate into other cells to maintain proper cell-type proportioning. Our results suggest that Wri controls cell-type proportioning, possibly by functioning as a negative regulator of a pathway mediating pstO cell differentiation and controlling the mechanism of homeostasis regulating the size of one or more of the cell-type compartments. Our results also suggest that homeobox gene regulation of anterior-posterior axis patterning may have evolved prior to the evolution of metazoans.

Null mutations of the Dictyostelium cyclic nucleotide phosphodiesterase gene block chemotactic cell movement in developing aggregates

Extracellular cAMP is a critical messenger in the multicellular development of the cellular slime mold Dictyostelium discoideum. The levels of cAMP are controlled by a cyclic nucleotide phosphodiesterase (PDE) that is secreted by the cells. The PDE gene (pdsA) is controlled by three promoters that permit expression during vegetative growth, during aggregation, and in prestalk cells of the older structures. Targeted disruption of the gene aborts development, and complementation with a modified pdsA restores development. Two distinct promoters must be used for full complementation, and an inhibitory domain of the PDE must be removed. We took advantage of newly isolated PDE-null cells and the natural chimerism of the organism to ask whether the absence of PDE affected individual cell behavior. PDE-null cells aggregated with isogenic wild-type cells in chimeric mixtures, but could not move in a coordinated manner in mounds. The wild-type cells move inward toward the center of the mound, leaving many of the PDE-null cells at the periphery of the aggregate. During the later stages of development, PDE-null cells in the chimera segregate to regions which correspond to the prestalk region and the rear of the slug. Participation in the prespore/spore population returns with the restoration of a modified pdsA to the null cells.

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.

Sorting of the initial cell types in Dictyostelium is dependent on the tipA gene

About 8 hr after the initiation of development in Dictyostelium discoideum, a few randomly scattered cells express prestalk specific genes and subsequently sort out to the top of the aggregate where they form a tip. The tip elongates and forms the anterior of the migrating slug before differentiating into a stalk which supports the ball of spores in a mature fruiting body. Using REMI mutagenesis we isolated a mutant strain, AK244, in which the initial aggregate subdivides to give a highly papillated surface. This mutant fails to form slugs and appears to have a defect in sorting of prestalk cells. The disrupted gene, tipA, encodes a novel 83-kDa protein and is preferentially expressed in PST-O cells after the cell types have sorted out. Mutant strains that lack TipA express the prestalk-specific gene ecmA at reduced levels and form very few spores. These defects cannot be overcome by developing the mutant cells in the presence of wild-type cells. Thus, TipA acts in a cell-autonomous manner at an early stage in development. Using strains carrying reporter constructs, we found that mutant cells expressing a prestalk marker remain dispersed in the aggregates. Prespore cells appear to sort such that the base is free of cells expressing cell-type-specific markers. Even after 20 hr of development, when wild-type cells are undergoing terminal differentiation, prestalk cells in tipA mutants form very small clumps, most of which fail to sort to the periphery or the tops of aggregates. The tipA gene appears to play an essential role in the sorting of the initial cell types.

Interacting signaling pathways controlling multicellular development in Dictyostelium

cAMP functions as the key extracellular signaling molecule controlling Dictyostelium development acting through classic G-protein-coupled/serpentine receptors. Whereas aggregation is controlled by nanomolar pulses of cAMP, a more continuous micromolar signal controls multicellular differentiation by activating a transcriptional cascade via a receptor-mediated but non G-protein-coupled pathway. Potential mechanisms by which extracellular cAMP functions to differentially control aggregation followed by morphogenesis and cell-type differentiation are discussed. This review also summarizes new findings elucidating pathways controlling cell-type regulation in this organism, including signaling cascades mediated by glycogen synthase kinase 3 and cAMP-dependent protein kinase, key regulators of cell-type differentiation in metazoans, and newly identified transcription factors.

Mutations in the Dictyostelium heterotrimeric G protein alpha subunit G alpha5 alter the kinetics of tip morphogenesis

Tip morphogenesis during the Dictyostelium developmental life cycle is a process by which prestalk cells sort to form the anterior region of the multicellular organism. We show that the temporal regulation of this morphological process is dependent on the copy number of the Dictyostelium G alpha5 gene. Tip formation is delayed in aggregates of g alpha5 null mutant cells and accelerated in aggregates overexpressing the G alpha5 gene compared to tip formation in wild-type cells. The onset of cell-type-specific gene expression associated with mound formation and tip morphogenesis is also temporally altered in G alpha5 mutants. Tip morphogenesis in chimeric organisms of G alpha5 mutants and wild-type cells is dependent on the copy number of the G alpha5 gene, indicating that G alpha5 function plays an integral role in the intercellular signaling of this stage of development. The G alpha5 gene encodes a G alpha subunit that has 51% identity to the Dictyostelium G alpha4 subunit. Like the G alpha4 gene, the G alpha5 gene is expressed in a subset of cells distributed throughout the multicellular organism, with a distribution that is similar to the anterior-like cell population. Amino acid substitutions in the G alpha5 subunit analogous to substitutions altering guanine nucleotide binding and hydrolysis in other G alpha subunits had no apparent effect on the rate of tip formation when a single copy of the mutant gene was used to replace the wild-type gene. Overexpression of these mutant G alpha5 genes by increased gene dosage resulted in cell death, suggesting that high levels of the altered subunits have detrimental effects during vegetative growth.