Genetic networks that regulate development in Dictyostelium cells

Inorganic Polyphosphate in Host and Microbe Biology

Inorganic polyphosphate (polyP) is produced by both bacteria and their eukaryotic hosts, and it appears to play multiple important roles in the interactions between those organisms. However, the detailed mechanisms of how polyP synthesis is regulated in bacteria, and how it influences both bacterial and host biology, remain largely unexplored. In this review, we examine recent developments in the understanding of how bacteria regulate the synthesis of polyP, what roles polyP plays in controlling virulence in pathogenic bacteria, and the effects of polyP on the mammalian immune system, as well as progress on developing drugs that may be able to target bacterial polyP synthesis as novel means of treating infectious disease.

A new mechanism for dendritic pattern formation in dense systems

Patterns are often formed when particles cluster: Since patterns reflect the connectivity of different types of material, the emergence of patterns affects the physical and chemical properties of systems and shares a close relationship to their macroscopic functions. A radial dendritic pattern (RDP) is observed in many systems such as snow crystals, polymer crystals and biological systems. Although most of these systems are considered as dense particle suspensions, the mechanism of RDP formation in dense particle systems is not yet understood. It should be noted that the diffusion limited aggregation model is not applicable to RDP formation in dense systems, but in dilute particle systems. Here, we propose a simple model that exhibits RDP formation in a dense particle system. The model potential for the inter-particle interaction is composed of two parts, a repulsive and an attractive force. The repulsive force is applied to all the particles all the time and the attractive force is exerted only among particles inside a circular domain, which expands at a certain speed as a wave front propagating from a preselected centre. It is found that an RDP is formed if the velocity of the wave front that triggers the attractive interaction is of the same order of magnitude as the time scale defined by the aggregation speed.

The C-module DNA-binding factor mediates expression of the dictyostelium aggregation-specific adenylyl cyclase ACA

Aggregation of Dictyostelium discoideum amoebae into multicellular structures is organized by cyclic AMP (cAMP), which acts as a chemoattractant, as a second messenger, and as a morphogen. Aggregation of D. discoideum cells depends on the expression of adenylyl cyclase ACA, which provides extracellular cAMP for signal relay and intracellular cAMP for the induction of genes required at multicellular stages. We have identified a DNA-binding activity specific for a highly A+T-enriched motif in the upstream region of the ACA-encoding gene, acaA. The factor shows DNA-binding characteristics very similar to those of C-module-binding factor (CbfA). Although CbfA was originally identified as a putative regulator of the activity of D. discoideum retrotransposon TRE5-A, it also was found to be essential for aggregation of D. discoideum cells. The identified DNA-binding activity was absent in mutant cells depleted of CbfA, and CbfA could be precipitated using an acaA promoter fragment. We propose that CbfA binds to the acaA promoter to provide a basal transcription activity that is required for induction of ACA expression after the onset of D. discoideum development.

CbfA, the C-module DNA-binding factor, plays an essential role in the initiation of Dictyostelium discoideum development

We recently isolated from Dictyostelium discoideum cells a DNA-binding protein, CbfA, that interacts in vitro with a regulatory element in retrotransposon TRE5-A. We have generated a mutant strain that expresses CbfA at <5% of the wild-type level to characterize the consequences for D. discoideum cell physiology. We found that the multicellular development program leading to fruiting body formation is highly compromised in the mutant. The cells cannot aggregate and stay as a monolayer almost indefinitely. The cells respond properly to prestarvation conditions by expressing discoidin in a cell density-dependent manner. A genomewide microarray-assisted expression analysis combined with Northern blot analyses revealed a failure of CbfA-depleted cells to induce the gene encoding aggregation-specific adenylyl cyclase ACA and other genes required for cyclic AMP (cAMP) signal relay, which is necessary for aggregation and subsequent multicellular development. However, the cbfA mutant aggregated efficiently when mixed with as few as 5% wild-type cells. Moreover, pulsing cbfA mutant cells developing in suspension with nanomolar levels of cAMP resulted in induction of acaA and other early developmental genes. Although the response was less efficient and slower than in wild-type cells, it showed that cells depleted of CbfA are able to initiate development if given exogenous cAMP signals. Ectopic expression of the gene encoding the catalytic subunit of protein kinase A restored multicellular development of the mutant. We conclude that sensing of cell density and starvation are independent of CbfA, whereas CbfA is essential for the pattern of gene expression which establishes the genetic network leading to aggregation and multicellular development of D. discoideum.

Control of cell type proportioning in Dictyostelium discoideum by differentiation-inducing factor as determined by in situ hybridization

We have determined the proportions of the prespore and prestalk regions in Dictyostelium discoideum slugs by in situ hybridization with a large number of prespore- and prestalk-specific genes. Microarrays were used to discover genes expressed in a cell type-specific manner. Fifty-four prespore-specific genes were verified by in situ hybridization, including 18 that had been previously shown to be cell type specific. The 36 new genes more than doubles the number of available prespore markers. At the slug stage, the prespore genes hybridized to cells uniformly in the posterior 80% of wild-type slugs but hybridized to the posterior 90% of slugs lacking the secreted alkylphenone differentiation-inducing factor 1 (DIF-1). There was a compensatory twofold decrease in prestalk cells in DIF-less slugs. Removal of prespore cells resulted in cell type conversion in both wild-type and DIF-less anterior fragments. Thus, DIF-1 appears to act in concert with other processes to establish cell type proportions.

Dictyostelium discoideum developmentally regulated genes whose expression is dependent on MADS box transcription factor SrfA

The MADS box transcription factor SrfA is required for spore differentiation in Dictyostelium discoideum. srfA null strains form rounded spores that do not resist adverse environmental conditions. Five genes whose expression is dependent on SrfA have been isolated by differential hybridization. One of these genes, sigC, is identical to phg1b, previously characterized in mutants with altered adhesive properties and found to encode a nine-transmembrane-domain protein. This gene is transcribed into two mRNAs as the result of alternative splicing of two internal exons. The slower-migrating mRNA codes for a shorter protein that lacks the first transmembrane fragment and is not expressed in srfA null strains. The other four genes (sigA, sigB, sigD, and 45D) are expressed only during late developmental stages. In situ hybridization experiments showed that expression of sigA, sigB, and sigD is restricted to the sorus of developing structures. sigA codes for a homologue of malate dehydrogenase that converts pyruvate to malate to replenish the tricarboxylic acid cycle. sigB encodes a protein with significant similarity to the GP63 metalloproteinase of Leishmania, leishmanolysin. The sequence of SigD is highly similar to that of several spore coat proteins of D. discoideum, and it may play a role in that structure. The gene 45D codes for an RNA-binding protein homologue whose expression is also dependent on the GATA transcription factor stalky (StkA). The expression of sigB is also dependent on both SrfA and StkA. The expression of 45D, but not of sigA, sigB, sigC, and sigD, can be induced in srfA null cells by constitutive protein kinase A activation. Strains in which either sigA, sigB, or sigD is disrupted were isolated and found to form spores that are not detectably different from those of wild-type strains.

Revamp a model-status and prospects of the Dictyostelium genome project

International efforts are underway that aim at determining the complete genome sequence of the social amoeba Dictyostelium discoideum. As strategy, a whole chromosome shotgun (WCS) approach was chosen and each of the six Dictyostelium chromosomes was assigned to project partners. The project is well advanced, chromosome 2 was recently published, and it is expected that the sequences of chromosomes 1 and 6 and a gene catalogue for the complete genome will be available at the end of this year. The genome sequence, together with powerful molecular genetic tools, will undoubtedly further accelerate Dictyostelium research into a number of fundamental biological processes that are common to a wide range of eukaryotes. Furthermore, it will constitute the basis for genome-wide functional analyses. The integration of results from these studies should ultimately lead to a better understanding of the complex networks that govern cellular behavior and development.

Genome-wide expression analyses of gene regulation during early development of Dictyostelium discoideum

Using genome-wide microarrays, we recognized 172 genes that are highly expressed at one stage or another during multicellular development of Dictyostelium discoideum. When developed in shaken suspension, 125 of these genes were expressed if the cells were treated with cyclic AMP (cAMP) pulses at 6-min intervals between 2 and 6 h of development followed by high levels of exogenous cAMP. In the absence of cAMP treatment, only three genes, carA, gbaB, and pdsA, were consistently expressed. Surprisingly, 14 other genes were induced by cAMP treatment of mutant cells lacking the activatable adenylyl cyclase, ACA. However, these genes were not cAMP induced if both of the developmental adenylyl cyclases, ACA and ACR, were disrupted, showing that they depend on an internal source of cAMP. Constitutive activity of the cAMP-dependent protein kinase PKA was found to bypass the requirement of these genes for adenylyl cyclase and cAMP pulses, demonstrating the critical role of PKA in transducing the cAMP signal to early gene expression. In the absence of constitutive PKA activity, expression of later genes was strictly dependent on ACA in pulsed cells.

Surrogate hosts: protozoa and invertebrates as models for studying pathogen-host interactions

Animal models, primary cell culture systems and permanent cell lines have provided important information on virulence properties of pathogenic microorganisms. Recently, it has been shown that some inherent limitations of such models can be circumvented by using non-vertebrate hosts such as Caenorhabditis elegans, Drosophila melanogaster and Dictyostelium discoideum. These new models are helpful to follow infection processes at the molecular level. Persuasive support comes from the fact that processes such as phagocytosis, cell signaling or innate immunity can be studied in these surrogate hosts. This review describes the establishment and application of each of the three aforementioned and genetically tractable hosts. In addition, we will report on a number of approaches that led to the identification of host cell factors which influence the susceptibility of the hosts to infection.

Sequence and analysis of chromosome 2 of Dictyostelium discoideum

The genome of the lower eukaryote Dictyostelium discoideum comprises six chromosomes. Here we report the sequence of the largest, chromosome 2, which at 8 megabases (Mb) represents about 25% of the genome. Despite an A + T content of nearly 80%, the chromosome codes for 2,799 predicted protein coding genes and 73 transfer RNA genes. This gene density, about 1 gene per 2.6 kilobases (kb), is surpassed only by Saccharomyces cerevisiae (one per 2 kb) and is similar to that of Schizosaccharomyces pombe (one per 2.5 kb). If we assume that the other chromosomes have a similar gene density, we can expect around 11,000 genes in the D. discoideum genome. A significant number of the genes show higher similarities to genes of vertebrates than to those of other fully sequenced eukaryotes. This analysis strengthens the view that the evolutionary position of D. discoideum is located before the branching of metazoa and fungi but after the divergence of the plant kingdom, placing it close to the base of metazoan evolution.

CulB, a putative ubiquitin ligase subunit, regulates prestalk cell differentiation and morphogenesis in Dictyostelium spp

Dictyostelium amoebae accomplish a starvation-induced developmental process by aggregating into a mound and forming a single fruiting body with terminally differentiated spores and stalk cells. culB was identified as the gene disrupted in a developmental mutant with an aberrant prestalk cell differentiation phenotype. The culB gene product appears to be a homolog of the cullin family of proteins that are known to be involved in ubiquitin-mediated protein degradation. The culB mutants form supernumerary prestalk tips atop each developing mound that result in the formation of multiple small fruiting bodies. The prestalk-specific gene ecmA is expressed precociously in culB mutants, suggesting that prestalk cell differentiation occurs earlier than normal. In addition, when culB mutant cells are mixed with wild-type cells, they display a cell-autonomous propensity to form stalk cells. Thus, CulB appears to ensure that the proper number of prestalk cells differentiate at the appropriate time in development. Activation of cyclic AMP-dependent protein kinase (PKA) by disruption of the regulatory subunit gene (pkaR) or by overexpression of the catalytic subunit gene (pkaC) enhances the prestalk/stalk cell differentiation phenotype of the culB mutant. For example, culB- pkaR- cells form stalk cells without obvious multicellular morphogenesis and are more sensitive to the prestalk O (pstO) cell inducer DIF-1. The sensitized condition of PKA activation reveals that CulB may govern prestalk cell differentiation in Dictyostelium, in part by controlling the sensitivity of cells to DIF-1, possibly by regulating the levels of one or more proteins that are rate limiting for prestalk differentiation.

Altered cell-type proportioning in Dictyostelium lacking adenosine monophosphate deaminase

The proportions of prespore and prestalk cells in Dictyostelium discoideum are regulated so that they are size invariant and can adjust when the ratio is perturbed. We have found that disruption of the gene amdA that encodes AMP deaminase results in a significantly increased proportion of prestalk cells. Strains lacking AMP deaminase form short, thick stalks and glassy sori with less than 5% the normal number of spores. The levels of prestalk-specific mRNAs in amdA(-) cells are more than twice as high as those in wild-type strains and prespore-specific mRNAs are reduced. Using an ecmA::lacZ construct to mark prestalk cells, we found that amdA(-) null slugs have twice the normal number of prestalk cells. The number of cells expressing an ecmO::lacZ construct was not affected by loss of AmdA, indicating that the mutation results in an increase in PST-A prestalk cells rather than PST-O cells. This alteration in cell-type proportioning is a cell-autonomous consequence of the loss of AMP deaminase since mutant cells developed together with wild-type cells still produced excess prestalk cells and wild-type cells carrying the ecmA::lacZ construct formed normal numbers of prestalk cells when developed together with an equal number of amdA(-) mutant cells.

Expression patterns of cell-type-specific genes in Dictyostelium

Cell-type specific genes were recognized by interrogating microarrays carrying Dictyostelium gene fragments with probes prepared from fractions enriched in prestalk and prespore cells. Cell-type specific accumulation of mRNA from 17 newly identified genes was confirmed by Northern analyses. DNA microarrays carrying 690 targets were used to determine expression profiles during development. The profiles were fit to a biologically based kinetic equation to extract the times of transcription onset and cessation. Although the majority of the genes that were cell-type enriched at the slug stage were first expressed as the prespore and prestalk cells sorted out in aggregates, some were found to be expressed earlier before the cells had even aggregated. These early genes may have been initially expressed in all cells and then preferentially turned over in one or the other cell type. Alternatively, cell type divergence may start soon after the initiation of development.

The MADS-box gene srfA is expressed in a complex pattern under the control of alternative promoters and is essential for different aspects of Dictyostelium development

srfA displays a complex temporal and cell type-specific pattern of expression in Dictyostelium and is expressed by most of its cell types at some stage of their development. This complexity is achieved by the use of alternative promoters. The promoter activity of the proximal region was found to be restricted to a subset of prestalk cells. Little or no associated expression was observed in the lower cup and basal disc during culmination. The middle promoter region was preferentially active in prestalk cells under usual conditions of filter development. Interestingly, during slug migration, the activity of this promoter in posterior prespore cells was strongly induced. The distal region displayed a dual pattern of expression. Thus, before culmination, this region drove lacZ expression in a few cells scattered along the entire structure. However, intense lacZ staining was found in the spores by the end of culmination. We have previously reported that srfA expression is essential for spore differentiation (R. Escalante and L. Sastre, Development 125, 3801-3808). Our novel finding of the expression of the gene in prestalk cells before culmination suggested that it might play additional roles in Dictyostelium development. The study of knockout strains revealed that srfA is also required for proper slug migration. Spore differentiation and slug migration defects were rescued by reexpression of srfA in the null mutant background, under the appropriate promoter control. The expression of srfA under the activity of the distal promoter region was able to rescue spore differentiation but not slug migration. Conversely, reexpression under the control of the middle promoter rescued slug morphogenesis and migration. Our results demonstrate that the correct spatial and temporal pattern of expression of srfA is essential for the different functions that this transcription factor plays in development.

The phosphorylated C-terminus of cAR1 plays a role in cell-type-specific gene expression and STATa tyrosine phosphorylation

cAMP receptors mediate some signaling pathways via coupled heterotrimeric G proteins, while others are G-protein-independent. This latter class includes the activation of the transcription factors GBF and STATa. Within the cellular mounds formed by aggregation of Dictyostelium, micromolar levels of cAMP activate GBF function, thereby inducing the transcription of postaggregative genes and initiating multicellular differentiation. Activation of STATa, a regulator of culmination and ecmB expression, results from cAMP receptor-dependent tyrosine phosphorylation and nuclear localization, also in mound-stage cells. During mound development, the cAMP receptor cAR1 is in a low-affinity state and is phosphorylated on multiple serine residues in its C-terminus. This paper addresses possible roles of cAMP receptor phosphorylation in the cAMP-mediated stimulation of GBF activity, STATa tyrosine phosphorylation, and cell-type-specific gene expression. To accomplish this, we have expressed cAR1 mutants in a strain in which the endogenous cAMP receptors that mediate postaggregative gene expression in vivo are deleted. We then examined the ability of these cells to undergo morphogenesis and induce postaggregative and cell-type-specific gene expression and STATa tyrosine phosphorylation. Analysis of cAR1 mutants in which the C-terminal tail is deleted or the ligand-mediated phosphorylation sites are mutated suggests that the cAR1 C-terminus is not essential for GBF-mediated postaggregative gene expression or STATa tyrosine phosphorylation, but may play a role in regulating cell-type-specific gene expression and morphogenesis. A mutant receptor, in which the C-terminal tail is constitutively phosphorylated, exhibits constitutive activation of STATa tyrosine phosphorylation in pulsed cells in suspension and a significantly impaired ability to induce cell-type-specific gene expression. The constitutively phosphorylated receptor also exerts a partial dominant negative effect on multicellular development when expressed in wild-type cells. These findings suggest that the phosphorylated C-terminus of cAR1 may be involved in regulating aspects of receptor-mediated processes, is not essential for GBF function, and may play a role in mediating subsequent development.

Altered prestarvation response in a nystatin resistant Dictyostelium discoideum mutant

Wild-type Dictyostelium amoebae secrete an autocrine, prestarvation factor (PSF) that allows them to measure the amount of food bacteria compared to their cell density. When the ratio of PSF to bacteria reaches a threshold, the cells are signaled to prepare for eventual starvation. This prestarvation response (PSR) usually starts three to four generations before the end of exponential growth, leading to the accumulation of several aggregation specific genes during growth. We characterize a nystatin-resistant mutant, HK19, that expresses the PSR genes three generations earlier than wild type but has an otherwise wild-type PSR. Although HK19 has a full PSR during growth, HK19 continues to grow at the wild-type rate and reaches normal cell densities. Because HK19 temporally separates the PSR from starvation, it became possible to test whether starvation is required for development. Since HK19 growing at low density can be induced to clump with either cAMP or folate, it appears that the PSR and an external signal are sufficient for entry into development. These data suggest that the PSR is a complex genetic pathway that induces genes involved in the exit from growth and the entry into development.

Mutant Rac1B expression in Dictyostelium: effects on morphology, growth, endocytosis, development, and the actin cytoskeleton

Rac1 is a small G-protein in the Ras superfamily that has been implicated in the control of cell growth, adhesion, and the actin-based cytoskeleton. To investigate the role of Rac1 during motile processes, we have established Dictyostelium cell lines that conditionally overexpress epitope-tagged Dictyostelium discoideum wild-type Rac1B (DdRac1B) or a mutant DdRac1B protein. Expression of endogenous levels of myc- or GFP-tagged wild-type DdRac1B had minimal effect on cellular morphologies and behaviors. By contrast, expression of a constitutively active mutant (G12-->V or Q61-->L) or a dominant negative mutant (T17-->N) generated amoebae with characteristic cellular defects. The morphological appearance of actin-containing structures, intracellular levels of F-actin, and cellular responses to chemoattractant closely paralleled the amount of active DdRac1B, indicating a role in upregulating actin cytoskeletal activities. Expression of any of the three mutants inhibited cell growth and cytokinesis, and delayed multicellular development, suggesting that DdRac1B plays important regulatory role(s) during these processes. No significant effects were observed on binding or internalization of latex beads in suspension or on intracellular membrane trafficking. Cells expressing DdRac1B-G12V exhibited defects in fluid-phase endocytosis and the longest developmental delays; DdRac1B-Q61L produced the strongest cytokinesis defect; and DdRac1B-T17N generated intermediate phenotypes. These conditionally expressed DdRac1B proteins should facilitate the identification and characterization of the Rac1 signaling pathway in an organism that is amenable to both biochemical and molecular genetic manipulations.

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.

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.

A Dictyostelium protein binds to distinct oligo(dA) x oligo(dT) DNA sequences in the C-module of the retrotransposable element DRE

The genome of the eukaryotic microbe Dictyostelium discoideum contains some 200 copies of the nonlong-terminal repeat retrotransposon DRE. Among several unique features of this retroelement, DRE is transcribed in both directions leading to the formation of partially overlapping plus strand and minus strand RNAs. The synthesis of minus strand RNAs is controlled by the C-module, a 134-bp DNA sequence located at the 3'-end of DRE. A nuclear protein (CMBF) binds to the C-module via interaction with two almost homopolymeric 24 bp oligo(dA) x oligo(dT) sequences. The DNA-binding drugs distamycin and netropsin, which bind to A x T-rich DNA sequences in the minor groove, competed efficiently for the binding of CMBF to the C-module. The CMBF-encoding gene, cbfA, was isolated and a DNA-binding domain was mapped to a 25-kDa C-terminal region of the protein. A peptide motif involved in the binding of A x T-rich DNA by high mobility group-I proteins ('GRP' box) was identified in the deduced CMBF protein sequence, and exchange of a consensus arginine residue for alanine within the CMBF GRP box abolished the interaction of CMBF with the C-module. The current data support the theory that CMBF binds to the C-module by detecting its long-range DNA conformation and interacting with A x T base pairs in the minor groove of oligo(dA) x oligo(dT) stretches.

SDF-2 induction of terminal differentiation in Dictyostelium discoideum is mediated by the membrane-spanning sensor kinase DhkA

SDF-2 is a peptide released by prestalk cells during culmination that stimulates prespore cells to encapsulate. Genetic evidence indicates that the response is dependent on the dhkA gene. This gene encodes a member of the histidine kinase family of genes that functions in two-component signal transduction pathways. The sequence of the N-terminal half of DhkA predicts two hydrophobic domains separated by a 310-amino-acid loop that could bind a ligand. By inserting MYC6 epitopes into DhkA, we were able to show that the loop is extracellular while the catalytic domain is cytoplasmic. Cells expressing the MYC epitope in the extracellular domain of DhkA were found to respond only if induced with 100-fold-higher levels of SDF-2 than required to induce dhkA+ cells; however, they could be induced to sporulate by addition of antibodies specific to the MYC epitope. To examine the enzymatic activity of DhkA, we purified the catalytic domain following expression in bacteria and observed incorporation of labelled phosphate from ATP consistent with histidine autophosphorylation. Site-directed mutagenesis of histidine1395 to glutamine in the catalytic domain blocked autophosphorylation. Furthermore, genetic analyses showed that histidine1395 and the relay aspartate2075 of DhkA are both critical to its function but that another histidine kinase, DhkB, can partially compensate for the lack of DhkA activity. Sporulation is drastically reduced in double mutants lacking both DhkA and DhkB. Suppressor studies indicate that the cyclic AMP (cAMP) phosphodiesterase RegA and the cAMP-dependent protein kinase PKA act downstream of DhkA.

A molecular network that produces spontaneous oscillations in excitable cells of Dictyostelium

A network of interacting proteins has been found that can account for the spontaneous oscillations in adenylyl cyclase activity that are observed in homogenous populations of Dictyostelium cells 4 h after the initiation of development. Previous biochemical assays have shown that when extracellular adenosine 3',5'-cyclic monophosphate (cAMP) binds to the surface receptor CAR1, adenylyl cyclase and the MAP kinase ERK2 are transiently activated. A rise in the internal concentration of cAMP activates protein kinase A such that it inhibits ERK2 and leads to a loss-of-ligand binding by CAR1. ERK2 phosphorylates the cAMP phosphodiesterase REG A that reduces the internal concentration of cAMP. A secreted phosphodiesterase reduces external cAMP concentrations between pulses. Numerical solutions to a series of nonlinear differential equations describing these activities faithfully account for the observed periodic changes in cAMP. The activity of each of the components is necessary for the network to generate oscillatory behavior; however, the model is robust in that 25-fold changes in the kinetic constants linking the activities have only minor effects on the predicted frequency. Moreover, constant high levels of external cAMP lead to attenuation, whereas a brief pulse of cAMP can advance or delay the phase such that interacting cells become entrained.

Non-autonomous regulation of a graded, PKA-mediated transcriptional activation signal for cell patterning

The pseudoplasmodium or migrating slug of Dictyostelium is composed of non-terminally differentiated cells, organized along an anteroposterior axis. Cells in the anterior region of the slug define the prestalk compartment, whereas most of the posterior zone consists of prespore cells. We now present evidence that the cAMP-dependent protein kinase (PKA) and the RING domain/leucine zipper protein rZIP interact genetically to mediate a transcriptional activation gradient that regulates the differentiation of prespore cells within the posterior compartment of the slug. PKA is absolutely required for prespore differentiation. In contrast, rZIP negatively regulates prespore patterning; rzpA- cells, which lack rZIP, have reduced prestalk differentiation and a corresponding increase in prespore-specific gene expression. Using cell-specific markers and chimaeras of wild-type and rzpA- cells, we show that rZIP functions non-autonomously to establish a graded, prespore gene activation signal but autonomously to localize prespore expression. Overexpression of either the catalytic subunit or a dominant-negative regulatory subunit of PKA further demonstrates that PKA lies within the intracellular pathway that mediates the extracellular signal and regulates prespore patterning. Finally, we show that a 5′-distal segment within a prespore promoter that is responsive to a graded signal is also sensitive to PKA and rZIP, indicating that it acts directly at the level of prespore-specific gene transcription for regulation.

Production and activity of spore differentiation factors (SDFs) in Dictyostelium

SDF-1 and SDF-2 are peptides that promote terminal spore differentiation under submerged conditions. The present study shows that they accumulate differentially and are released during the development of wild-type cells and can promote spore formation in cells disaggregated from wild-type culminants. SDF-1 accumulates during the slug stage and is released in a single burst at the onset of culmination while SDF-2 accumulates during early culmination and is released in a single burst from mid-culminants. The effects of SDF-1 and SDF-2 on stalk cell formation in cell monolayers were investigated. SDF-1 by itself induces stalk cell formation in some strains and also synergizes with the stalk-cell-inducing factor, DIF-1. cAMP has an inhibitory effect on stalk cell formation when either DIF-1 or SDF-1 are present on their own but is almost not inhibitory when both are present. SDF-2 alone does not induce stalk cell formation and appears to inhibit the response to DIF-1. At the same time, it increases the extent of vacuolization of the stalk cells that are produced. We propose that the release of SDF-1 and then of SDF-2 may mark irreversible steps in the developmental programme associated, respectively, with culmination and spore maturation.

Cell-cell signaling during Dictyostelium development

Specific proteins and peptides, as well as cAMP, are used as intercellular signals in Dictyostelium. Our understanding of the signal transduction pathways activated by these signals has been expanded by inclusion of newly characterized proteins. cAMP-dependent protein kinase (PKA) and its associated phosphodiesterase, RegA, play multiple roles in these pathways.

A Serum Response Factor homolog is required for spore differentiation in Dictyostelium

A homolog of the Serum Response Factor (SRF) has been isolated from Dictyostelium discoideum and its function studied by analyzing the consequences of its gene disruption. The MADS-box region of Dictyostelium SRF (DdSRF) is highly conserved with those of the human, Drosophila and yeast homologs. srfA is a developmentally regulated gene expressed in prespore and spore cells. This gene plays an essential role in sporulation as its disruption leads to abnormal spore morphology and loss of viability. The mutant spores were round and cellulose deposition seemed to be partially affected. Initial prestalk and prespore cell differentiation did not seem to be compromised in the mutant since the expression of several cell-type-specific markers were found to be unaffected. However, the mRNA level of the spore marker spiA was greatly reduced. Activation of the cAMP-dependent protein kinase (PKA) by 8-Br-cAMP was not able to fully bypass the morphological defects of srfA- mutant spores, although this treatment induced spiA mRNA expression. Our results suggest that DdSRF is required for full maturation of spores and participates in the regulation of the expression of the spore-coat marker spiA and probably other maturation genes necessary for proper spore cell differentiation.

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.

Role of PKA in the timing of developmental events in Dictyostelium cells

The cyclic AMP (cAMP)-dependent protein kinase, PKA, is dispensable for growth of Dictyostelium cells but plays a variety of crucial roles in development. The catalytic subunit of PKA is inhibited when associated with its regulatory subunit but is activated when cAMP binds to the regulatory subunit. Deletion of pkaR or overexpression of the gene encoding the catalytic subunit, pkaC, results in constitutive activity. Development is independent of cAMP in strains carrying these genetic alterations and proceeds rapidly to the formation of both spores and stalk cells. However, morphogenesis is aberrant in these mutants. In the wild type, PKA activity functions in a circuit that can spontaneously generate pulses of cAMP necessary for long-range aggregation. It is also essential for transcriptional activation of both prespore and prestalk genes during the slug stage. During culmination, PKA functions in both prespore and prestalk cells to regulate the relative timing of terminal differentiation. A positive feedback loop results in the rapid release of a signal peptide, SDF-2, when prestalk cells are exposed to low levels of SDF-2. The signal transduction pathway that mediates the response to SDF-2 in both prestalk and prespore cells involves the two-component system of DhkA and RegA. When the cAMP phosphodiesterase RegA is inhibited, cAMP accumulates and activates PKA, leading to vacuolation of stalk cells and encapsulation of spores. These studies indicate that multiple inputs regulate PKA activity to control the relative timing of differentiations in Dictyostelium.

Detection of subtle phenotypes: the case of the cell adhesion molecule csA in Dictyostelium

Dictyostelium amoebae aggregate into a multicellular organism by cAMP-driven chemotaxis and cell-cell adhesion. Cell adhesion is mediated by an EDTA-sensitive and an EDTA-resistant adhesion system. The latter is developmentally regulated and triggered by homophilic interactions of the membrane glycoprotein csA; on disruption of the encoding gene, EDTA-resistant contacts fail to form. Nevertheless, csA-null cells under usual laboratory conditions aggregate normally and complete development. By using experimental conditions that reproduce more closely the habitat of Dictyostelium amoebae, evidence is provided that csA is required for development and that its expression confers a selective advantage to populations of wild-type cells over csA-null mutants. The latter display reduced cell-cell adhesion, increased adhesiveness to the substratum, and slower motility, which lead to their sorting out from aggregating wild-type cells. It is proposed that the experimental conditions commonly used in the laboratory are not stringent enough to assess the developmental role of csA and other proteins. The assay described can be used to detect subtle phenotypes, to reexamine the developmental role of apparently nonessential genes, and to test the validity of recent models on emergence and maintenance of apparent genetic redundancy.

A developmentally regulated kinesin-related motor protein from Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is an attractive system for studying the roles of microtubule-based motility in cell development and differentiation. In this work, we report the first molecular characterization of kinesin-related proteins (KRPs) in Dictyostelium. A PCR-based strategy was used to isolate DNA fragments encoding six KRPs, several of which are induced during the developmental program that is initiated by starvation. The complete sequence of one such developmentally regulated KRP (designated K7) was determined and found to be a novel member of the kinesin superfamily. The motor domain of K7 is most similar to that of conventional kinesin, but unlike conventional kinesin, K7 is not predicted to have an extensive alpha-helical coiled-coil domain. The nonmotor domain is unusual and is rich in Asn, Gln, and Thr residues; similar sequences are found in other developmentally regulated genes in Dictyostelium. K7, expressed in Escherichia coli, supports plus end-directed microtubule motility in vitro at a speed of 0.14 micron/s, indicating that it is a bona fide motor protein. The K7 motor is found only in developing cells and reaches a peak level of expression between 12 and 16 h after starvation. By immunofluorescence microscopy, K7 localizes to a membranous perinuclear structure. To examine K7 function, we prepared a null cell line but found that these cells show no gross developmental abnormalities. However, when cultivated in the presence of wild-type cells, the K7-null cells are mostly absent from the prestalk zone of the slug. This result suggests that in a population composed largely of wild-type cells, the absence of the K7 motor protein interferes either with the ability of the cells to localize to the prestalk zone or to differentiate into prestalk cells.

A novel Myb homolog initiates Dictyostelium development by induction of adenylyl cyclase expression

Dictyostelium development is induced by starvation. The adenylyl cyclase gene ACA is one of the first genes expressed upon starvation. ACA produces extracellular cAMP that induces chemotaxis, aggregation, and differentiation in neighboring cells. Using insertional mutagenesis we have isolated a mutant that does not aggregate upon starvation but is rescued by adding extracellular cAMP. Sequencing of the mutated locus revealed a new gene, DdMYB2, whose product contains three Myb repeats, the DNA-binding motif of Myb-related transcription factors. Ddmyb2-null cells show undetectable levels of ACA transcript and no cAMP production. Ectopic expression of ACA from a constitutive promotor rescues differentiation and morphogenesis of Ddmyb2-null mutants. The results suggest that development in Dictyostelium starts by starvation-mediated DdMyb2 activation, which induces adenylyl cyclase activity producing the differentiation-inducing signal cAMP.

The Dictyostelium MAP kinase DdERK2 functions as a cytosolic protein in complexes with its potential substrates in chemotactic signal transduction

A polyclonal antibody against a MAP kinase (DdERK2) in Dictyostelium has been made and used to study DdERK2 activation and localization. The activation of DdERK2 by the chemoattractants cAMP and folate is rapid and transient. Its activity peaks between 15 and 60 seconds after cAMP stimulation and declines to basal levels after 5 minutes. In parallel with the DdERK2 activation is the appearance of a higher mobility band on Western blots. An antibody specific for activated MAP kinase shows that only the shifted band is tyrosine phosphorylated, suggesting that it is the active form. Both unstimulated and stimulated DdERK2 are soluble. In vitro phosphorylation with cell lysate supernatants or immunoprecipitates demonstrates the presence of several potential substrates, as identified by SDS-PAGE with mobility corresponding to molecular weights of 150, 25, and 19 kDa. Furthermore, immunoprecipitation studies suggest that these substrates are in a complex with DdERK2. These data suggest that DdERK2 works via cytoplasmic proteins to mediate signaling responses in Dictyostelium.

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.

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

Dictyostelium development in the absence of cAMP

Adenosine 3',5'-monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) are regulators of development in many organisms. Dictyostelium uses cAMP as an extracellular chemoattractant and as an intracellular signal for differentiation. Cells that are mutant in adenylyl cyclase do not develop. Moderate expression of the catalytic subunit of PKA in adenylyl cyclase-null cells led to near-normal development without detectable accumulation of cAMP. These results suggest that all intracellular cAMP signaling is effected through PKA and that signals other than extracellular cAMP coordinate morphogenesis in Dictyostelium.

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.

Expression of an Arabidopsis plasma membrane aquaporin in Dictyostelium results in hypoosmotic sensitivity and developmental abnormalities

The rd28 gene of Arabidopsis thaliana encodes a water channel protein, or aquaporin, of the plasma membrane. A construct in which transcription of the rd28 cDNA is controlled by the Dictyostelium actin15 promoter was transformed into Dictyostelium discoideum cells. Transformants contained RD28 protein in their plasma membranes. When shifted to a low-osmotic-strength buffer, cells expressing rd28 swelled rapidly and burst, indicating that the plant aquaporin allowed rapid water entry in the amoebae. The rate of osmotic lysis was a function of the osmotic pressure of the buffer. We also selected transformants in which the expression of the rd28 cDNA is driven by the promoter of the prespore cotB gene. These transformants accumulated rd28 mRNA uniquely in prespore cells. In low-osmotic-strength buffer, the cotB::rd28 cells aggregated and formed normally proportioned slugs but failed to form normal fruiting bodies. The number of spores was reduced 20-fold, and the stalks of the fruiting bodies were abnormally short. The consequences of expressing RD28 in prespore cells could be partially overcome by increasing the osmolarity of the medium. Under these conditions, the cotB::rd28 cells formed fruiting bodies of more normal appearance, and the number of viable spores increased slightly. Because prespore cells have to shrink and dehydrate to form spores, it was not unexpected that expression of an aquaporin would disrupt this process, but it was surprising to find that stalk differentiation was also affected by expression of rd28 in prespore cells. It appears that osmotic stress on prespore cells alters their ability to signal terminal differentiation in prestalk cells. The results provide independent confirmation that plant aquaporins can function in the cells of other organisms, and that D. discoideum can be used to study the properties of these water channels.