cAMP-dependent protein kinase differentially regulates prestalk and prespore differentiation during Dictyostelium development

Multiple phosphorylation sites on the RegA phosphodiesterase regulate Dictyostelium development

In Dictyostelium, the intracellular cAMP-specific phosphodiesterase RegA is a negative regulator of cAMP-dependent protein kinase (PKA), a key determinant in the timing of developmental morphogenesis and spore formation. To assess the role of protein kinases in the regulation of RegA function, this study identified phosphorylation sites on RegA and characterized the role of these modifications through the analysis of phospho-mimetic and phospho-ablative mutations. Mutations affecting residue T676 of RegA, a presumed target of the atypical MAP kinase Erk2, altered the rate of development and impacted cell distribution in chimeric organisms suggesting that phosphorylation of this residue reduces RegA function and regulates cell localization during multicellular development. Mutations affecting the residue S142 of RegA also impacted the rate developmental morphogenesis but in a manner opposite of changes at T676 suggesting the phosphorylation of the S142 residue increases RegA function. Mutations affecting residue S413 residue altered aggregate sizes and delayed developmental progression suggesting that PKA operates in a negative feedback mechanism to increase RegA function. These results suggest that the phosphorylation of different residues on RegA can lead to increased or decreased RegA function and therefore in turn regulate developmental processes such as aggregate formation, cell distribution, and the kinetics of developmental morphogenesis.

The transcription factor Spores Absent A is a PKA dependent inducer of Dictyostelium sporulation

Sporulation in Dictyostelium fruiting bodies evolved from amoebozoan encystation with both being induced by cAMP acting on PKA, but with downstream components still being unknown. Using tagged mutagenesis to find missing pathway components, we identified a sporeless mutant defective in a nuclear protein, SpaA. Expression of prespore genes was strongly reduced in spaA- cells, while expression of many spore stage genes was absent. Chromatin immunoprecipitation (ChIP) of a SpaA-YFP gene fusion showed that (pre)spore gene promoters bind directly to SpaA, identifying SpaA as a transcriptional regulator. SpaA dependent spore gene expression required PKA in vivo and was stimulated in vitro by the membrane-permeant PKA agonist 8Br-cAMP. The PKA agonist also promoted SpaA binding to (pre)spore promoters, placing SpaA downstream of PKA. Sequencing of SpaA-YFP ChIPed DNA fragments revealed that SpaA binds at least 117 (pre)spore promoters, including those of other transcription factors that activate some spore genes. These factors are not in turn required for spaA expression, identifying SpaA as the major trancriptional inducer of sporulation.

Modulation of cyclic nucleotide-mediated cellular signaling and gene expression using photoactivated adenylyl cyclase as an optogenetic tool

Cyclic nucleotide signaling pathway plays a significant role in various biological processes such as cell growth, transcription, inflammation, in microbial pathogenesis, etc. Modulation of cyclic nucleotide levels by optogenetic tools has overcome certain limitations of studying transduction cascade by pharmacological agents and has allowed several ways to modulate biological processes in a spatiotemporal manner. Here, we have shown the optogenetic modulation of the cyclooxygenase 2 (Cox-2) gene expression and their downstream effector molecule (PGE₂) in HEK-293T cells and the development process of Dictyostelium discoideum via modulating the cyclic nucleotide (cAMP) signaling pathway utilizing photoactivated adenylyl cyclases (PACs) as an optogenetic tool. Light-induced activation of PACs in HEK-293T cells increases the cAMP level that leads to activation of cAMP response element-binding protein (CREB) transcription factor and further upregulates downstream Cox-2 gene expression and their downstream effector molecule prostaglandin E2. In D. discoideum, the light-regulated increase in cAMP level affects the starvation-induced developmental process. These PACs could modulate the cAMP levels in a light-dependent manner and have a potential to control gene expression and their downstream effector molecules with varying magnitude. It would enable one to utilize PAC as a tool to decipher cyclic nucleotide mediated signaling pathway regulations and their mechanism.

Adenylate cyclase A acting on PKA mediates induction of stalk formation by cyclic diguanylate at the Dictyostelium organizer

Coordination of cell movement with cell differentiation is a major feat of embryonic development. The Dictyostelium stalk always forms at the organizing tip, by a mechanism that is not understood. We previously reported that cyclic diguanylate (c-di-GMP), synthesized by diguanylate cyclase A (DgcA), induces stalk formation. Here we used transcriptional profiling of dgca- structures to identify target genes for c-di-GMP, and used these genes to investigate the c-di-GMP signal transduction pathway. We found that knockdown of cAMP-dependent protein kinase (PKA) activity in prestalk cells reduced stalk gene induction by c-di-GMP, whereas PKA activation bypassed the c-di-GMP requirement for stalk gene expression. c-di-GMP caused a persistent increase in cAMP, which still occurred in mutants lacking the adenylate cyclases ACG or ACR, or the cAMP phosphodiesterase RegA. However, both inhibition of adenylate cyclase A (ACA) with SQ22536 and incubation of a temperature-sensitive ACA mutant at the restrictive temperature prevented c-di-GMP-induced cAMP synthesis as well as c-di-GMP-induced stalk gene transcription. ACA produces the cAMP pulses that coordinate Dictyostelium morphogenetic cell movement and is highly expressed at the organizing tip. The stalk-less dgca- mutant regained its stalk by expression of a light-activated adenylate cyclase from the ACA promoter and exposure to light, indicating that cAMP is also the intermediate for c-di-GMP in vivo. Our data show that the more widely expressed DgcA activates tip-expressed ACA, which then acts on PKA to induce stalk genes. These results explain why stalk formation in Dictyostelia always initiates at the site of the morphogenetic organizer.

Non-Catalytic Roles of Presenilin Throughout Evolution

Research into Alzheimer's disease pathology and treatment has often focused on presenilin proteins. These proteins provide the key catalytic activity of the γ-secretase complex in the cleavage of amyloid-β precursor protein and resultant amyloid tangle deposition. Over the last 25 years, screening novel drugs to control this aberrant proteolytic activity has yet to identify effective treatments for the disease. In the search for other mechanisms of presenilin pathology, several studies have demonstrated that mammalian presenilin proteins also act in a non-proteolytic role as a scaffold to co-localize key signaling proteins. This role is likely to represent an ancestral presenilin function, as it has been described in genetically distant species including non-mammalian animals, plants, and a simple eukaryotic amoeba Dictyostelium that diverged from the human lineage over a billion years ago. Here, we review the non-catalytic scaffold role of presenilin, from mammalian models to other biomedical models, and include recent insights using Dictyostelium, to suggest that this role may provide an early evolutionary function of presenilin proteins.

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

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

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

BACKGROUND

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

METHODOLOGY/PRINCIPAL FINDINGS

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

CONCLUSIONS/SIGNIFICANCE

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

G{alpha}5 subunit-mediated signalling requires a D-motif and the MAPK ERK1 in Dictyostelium

The Dictyostelium Galpha5 subunit has been shown to reduce cell viability, inhibit folate chemotaxis and accelerate tip morphogenesis and gene expression during multicellular development. Alteration of the D-motif (mitogen-activated protein kinase docking site) at the amino terminus of the Galpha 5 subunit or the loss of extracellular signal-regulated kinase (ERK)1 diminished the lethality associated with the overexpression or constitutive activation of the Galpha5 subunit. The amino-terminal D-motif of the Galpha5 subunit was also found to be necessary for the reduced cell size, small aggregate formation and precocious developmental gene expression associated with Galpha5 subunit overexpression. This D-motif also contributed to the aggregation delay in cells expressing a constitutively active Galpha5 subunit, but the D-motif was not necessary for the inhibition of folate chemotaxis. These results suggest that the amino-terminal D-motif is required for some but not all phenotypes associated with elevated Galpha5 subunit functions during growth and development and that ERK1 can function in Galpha5 subunit-mediated signal transduction.

Functional characterisation of the regulatory subunit of cyclic AMP-dependent protein kinase A homologue of Giardia lamblia: Differential expression of the regulatory and catalytic subunits during encystation

To understand the functional roles of protein kinase A (PKA) during vegetative and differentiating states of Giardia parasites, we studied the structural and functional characteristics of the regulatory subunit of PKA (gPKAr) and its involvement in the giardial encystment process. Molecular cloning and characterisation showed that gPKAr contains two tandem 3'5'-cyclic adenosine monphosphate (cyclic AMP) binding domains at the C-terminal end and the interaction domain for the catalytic subunit. A number of consensus residues including in vivo phosphorylation site for PKAc and dimerisation/docking domain are present in gPKAr. The regulatory subunit physically interacts with the catalytic subunit and inhibits its kinase activity in the absence of cyclic AMP, which could be partially restored upon addition of cyclic AMP. Western blot analysis showed a marked reduction in the endogenous gPKAr concentration during differentiation of Giardia into cysts. An increased activity of gPKAc was also detected during encystation without any significant change in the protein concentration. Distinct localisations of gPKAc to the anterior flagella, basal bodies and caudal flagella as noted in trophozoites were absent in encysting cells at later stages. Instead, PKAc staining was punctate and located mostly to the cell periphery. Our study indicates possible enrichment of the active gPKAc during late stages of encystation, which may have implications in completion of the encystment process or priming of cysts for efficient excystation.

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.

Hypertonic signal promotes stability of Dictyostelium spores via a PKA-independent pathway

Differentiation of Dictyostelium spores initiates with rapid encapsulation of prespore cells under the control of cAMP-dependent protein kinase (PKA), followed by further maturation processes involving cytoskeletal reorganization. Constitutive activation of PKA induces precocious formation of viable spores in development and confers the ability to encapsulate under specific submerged conditions. In this study, we show that the stability of these spores depends upon conditions of high osmotic strength during spore differentiation, indicating that a hypertonic signal is required in addition to PKA to induce maturation to stable spores. The formation of stable spores under hypertonic conditions requires high cell density, suggesting the involvement of additional cellular signaling.

Eme1 is involved in DNA damage processing and maintenance of genomic stability in mammalian cells

Yeast and human Eme1 protein, in complex with Mus81, constitute an endonuclease that cleaves branched DNA structures, especially those arising during stalled DNA replication. We identified mouse Eme1, and show that it interacts with Mus81 to form a complex that preferentially cleaves 3'-flap structures and replication forks rather than Holliday junctions in vitro. We demonstrate that Eme1-/- embryonic stem (ES) cells are hypersensitive to the DNA cross-linking agents mitomycin C and cisplatin, but only mildly sensitive to ionizing radiation, UV radiation and hydroxyurea treatment. Mammalian Eme1 is not required for the resolution of DNA intermediates that arise during homologous recombination processes such as gene targeting, gene conversion and sister chromatid exchange (SCE). Unlike Blm-deficient ES cells, increased SCE was seen only following induced DNA damage in Eme1-deficient cells. Most importantly, Eme1 deficiency led to spontaneous genomic instability. These results reveal that mammalian Eme1 plays a key role in DNA repair and the maintenance of genome integrity.

The novel ankyrin-repeat containing kinase ARCK-1 acts as a suppressor of the Spalten signaling pathway during Dictyostelium development

Spalten (Spn), a member of the PP2C family of Ser/Thr protein phosphatases, is required for Dictyostelium cell-type differentiation and morphogenesis. We have identified a new protein kinase, ARCK-1, through a second site suppressor screen for mutants that allow spn null cells to proceed further through development. ARCK-1 has a C-terminal kinase domain most closely related to Ser/Thr protein kinases and an N-terminal putative regulatory domain with ankyrin repeats, a 14-3-3 binding domain, and a C1 domain, which is required for binding to RasBGTP in a two-hybrid assay. Disruption of the gene encoding ARCK-1 results in weak, late developmental defects. However, overexpression of ARCK-1 phenocopies the spn null phenotype, consistent with Spn and ARCK-1 being on the same developmental pathway. Our previous analyses of Spn and the present analysis of ARCK-1 suggest a model in which Spn and ARCK-1 differentially control the phosphorylation state of a protein that regulates cell-type differentiation. Dephosphorylation of the substrate by Spn is required for cell-type differentiation. Control of ARCK-1 and Spn activities by upstream signals is proposed to be part of the developmental regulatory program mediating cell-fate decisions in Dictyostelium.

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

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

cAMP-dependent protein kinase regulates Polysphondylium pallidum development

In eukaryotic cells, the universal second messenger cAMP regulates various aspects of development and differentiation. The primary target for cAMP is the regulatory subunit of cAMP-dependent protein kinase A (PKA), which, upon cAMP binding, dissociates from the catalytic subunit and thus activates it. In the soil amoeba Dictyostelium discoideum, the function of PKA in growth, development and cell differentiation has been thoroughly investigated and substantial information is available. To obtain a more general view, we investigated the influence of PKA on development of the related species Polysphondylium pallidum. Cells were transformed to overexpress either a dominant negative mutant of the regulatory subunit (Rm) from Dictyostelium that cannot bind cAMP, or the catalytic subunit (PKA-C) from Dictyostelium. Cells overexpressing Rm rarely aggregated and the few multicellular structures developed slowly into very small fruiting bodies without branching of secondary sorogens, the prominent feature of Polysphondylium. Few round spores with reduced viability were formed. When mixed with wild-type cells and allowed to develop, the Rm cells were randomly distributed in aggregation streams, but were later found in the posterior region of the culminating slug or were left behind on the surface of the substratum. The PKA-C overexpressing cells exhibited precocious development and formed more aggregates of smaller size. Moreover, expression of PKA-C under the control of the prestalk-specific ecmB promoter of Dictyostelium leads to protrusions from aggregation streams. We conclude that Dictyostelium PKA subunits introduced into Polysphondylium cells are functional as signal components, indicating that a biochemically similar PKA mechanism works in Polysphondylium.

PRKX, a phylogenetically and functionally distinct cAMP-dependent protein kinase, activates renal epithelial cell migration and morphogenesis

The human protein kinase X gene (PRKX) is a member of an ancient family of cAMP-dependent serine/threonine kinases here shown to be phylogenetically distinct from the classical PKA, PKB/Akt, PKC, SGK, and PKG gene families. Renal expression of the PRKX gene is developmentally regulated and restricted to the ureteric bud epithelium of the fetal metanephric kidney. Aberrant adult kidney expression of PRKX was found in autosomal dominant polycystic kidney disease. PRKX kinase expression markedly activated migration of cultured renal epithelial cells in the presence of cAMP; this effect was blocked by cell treatment with the PKA inhibitor H89 and was not observed in PKA-transfected cells. In addition, expression of PRKX kinase activated branching morphogenesis of Madin-Darby canine kidney cells in collagen gels even in the absence of cAMP and/or hepatocyte growth factor, an effect not seen with either PKA expression or expression of a mutant, kinase-inactivated PRKX. These results suggest that the PRKX kinase may regulate epithelial morphogenesis during mammalian kidney development. Because another member of the PRKX gene family (the Dictyostelium discoideum gene KAPC-DICDI) also plays a role in cellular migration, these studies suggest that regulation of morphogenesis may be a distinctive property of these genes that has been conserved in evolution that is not shared with PKA family genes.

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.

Regulated protein degradation controls PKA function and cell-type differentiation in Dictyostelium

Cullins function as scaffolds that, along with F-box/WD40-repeat-containing proteins, mediate the ubiquitination of proteins to target them for degradation by the proteasome. We have identified a cullin CulA that is required at several stages during Dictyostelium development. culA null cells are defective in inducing cell-type-specific gene expression and exhibit defects during aggregation, including reduced chemotaxis. PKA is an important regulator of Dictyostelium development. The levels of intracellular cAMP and PKA activity are controlled by the rate of synthesis of cAMP and its degradation by the cAMP-specific phosphodiesterase RegA. We show that overexpression of the PKA catalytic subunit (PKAcat) rescues many of the culA null defects and those of cells lacking FbxA/ChtA, a previously described F-box/WD40-repeat-containing protein, suggesting CulA and FbxA proteins are involved in regulating PKA function. Whereas RegA protein levels drop as the multicellular organism forms in the wild-type strain, they remain high in culA null and fbxA null cells. Although PKA can suppress the culA and fbxA null developmental phenotypes, it does not suppress the altered RegA degradation, suggesting that PKA lies downstream of RegA, CulA, and FbxA. Finally, we show that CulA, FbxA, and RegA are found in a complex in vivo, and formation of this complex is dependent on the MAP kinase ERK2, which is also required for PKA function. We propose that CulA and FbxA regulate multicellular development by targeting RegA for degradation via a pathway that requires ERK2 function, leading to an increase in cAMP and PKA activity.

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.

Control of spatial patterning and cell-type proportioning in Dictyostelium

The spatial patterning of prestalk and prespore cells in the slug arises from the differential sorting of newly differentiated cell types as the mound forms. This pattern is highly organized along an anterior-posterior axis and is constant irrespective of the size of the organism. Cell-type differentiation is plastic until late in development. A change in the ratio of cell types resulting from removal of part of the slug leads to a rapid restoration of the original ratio of the cell types through a pathway involving dedifferentiation, redifferentiation, and sorting of the existing cells. This review provides insight into various molecules, morphogens, and pathways regulating spatial patterning and cell-type proportioning.

Dictyostelium development-socializing through cAMP

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

The RdeA-RegA system, a eukaryotic phospho-relay controlling cAMP breakdown

The regA and rdeA gene products of Dictyostelium are involved in the regulation of cAMP signaling. The response regulator, RegA, is composed of an N-terminal receiver domain linked to a C-terminal cAMP-phosphodiesterase domain. RdeA may be a phospho-transfer protein that supplies phosphates to RegA. We show genetically that phospho-RegA is the activated form of the enzyme in vivo, in that the predicted site of aspartate phosphorylation is required for full activity. We show biochemically that RdeA and RegA communicate, as evidenced by phospho-transfer between the two proteins in vitro. Phospho-transfer is dependent on the presumed phospho-accepting amino acids, histidine 65 of RdeA and aspartate 212 of RegA, and occurs in both directions. Phosphorylation of RegA by a heterologous phospho-donor protein activates RegA phosphodiesterase activity at least 20-fold. Our results suggest that the histidine phosphotransfer protein, RdeA, and the response regulator, RegA, constitute two essential elements in a eukaryotic His-Asp phospho-relay network that regulates Dictyostelium development and fruiting body maturation.

Starvation promotes Dictyostelium development by relieving PufA inhibition of PKA translation through the YakA kinase pathway

When nutrients are depleted, Dictyostelium cells undergo cell cycle arrest and initiate a developmental program that ensures survival. The YakA protein kinase governs this transition by regulating the cell cycle, repressing growth-phase genes and inducing developmental genes. YakA mutants have a shortened cell cycle and do not initiate development. A suppressor of yakA that reverses most of the developmental defects of yakA- cells, but none of their growth defects was identified. The inactivated gene, pufA, encodes a member of the Puf protein family of translational regulators. Upon starvation, pufA- cells develop precociously and overexpress developmentally important proteins, including the catalytic subunit of cAMP-dependent protein kinase, PKA-C. Gel mobility-shift assays using a 200-base segment of PKA-C's mRNA as a probe reveals a complex with wild-type cell extracts, but not with pufA- cell extracts, suggesting the presence of a potential PufA recognition element in the PKA-C mRNA. PKA-C protein levels are low at the times of development when this complex is detectable, whereas when the complex is undetectable PKA-C levels are high. There is also an inverse relationship between PufA and PKA-C protein levels at all times of development in every mutant tested. Furthermore, expression of the putative PufA recognition elements in wild-type cells causes precocious aggregation and PKA-C overexpression, phenocopying a pufA mutation. Finally, YakA function is required for the decline of PufA protein and mRNA levels in the first 4 hours of development. We propose that PufA is a translational regulator that directly controls PKA-C synthesis and that YakA regulates the initiation of development by inhibiting the expression of PufA. Our work also suggests that Puf protein translational regulation evolved prior to the radiation of metazoan species.

Taking the plunge. Terminal differentiation in Dictyostelium

During the last stage of Dictyostelium development a motile, cylindrical slug transforms into an immotile, stalked fruiting body and the constituent cells change from amoebae to either refractile spores or vacuolated stalk cells. Analysis of this process using genetics and simple culture techniques is becoming a powerful way of investigating a number of conserved signal transduction processes. A common pathway activating cAMP-dependent protein kinase (PKA) triggers the maturation of spore cells and those stalk cells forming the stalk. It uses a eukaryotic version of the 'bacterial' two-component phospho-relay system to control cAMP breakdown. A second pathway, inhibiting the GSK3 protein kinase, might control the maturation of a distinct set of stalk cells at the base of the fruiting body.

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.

Identification and analysis of a gene that is essential for morphogenesis and prespore cell differentiation in Dictyostelium

We have identified a gene (PslA) that is expressed throughout Dictyostelium development and encodes a novel protein that is required for proper aggregation and subsequent cell-type differentiation and morphogenesis. pslA null (pslA-) cells produce large aggregation streams under conditions in which wild-type cells form discrete aggregates. Tips form along the stream, elongate to produce a finger, and eventually form a terminal structure that lacks a true sorus (spore head). More than half of the cells remain as a mass at the base of the developing fingers. The primary defect in the pslA- strain is the inability to induce prespore cell differentiation. Analyses of gene expression show a complete lack of prespore-specific gene expression and no mature spores are produced. In chimeras with wild-type cells, pslA- cells form the prestalk domain and normal, properly proportioned fruiting bodies can be produced. This indicates that pslA- cells are able to interact with wild-type cells and regulate patterning, even though pslA- cells are unable to express prespore cell-type-specific genes, do not participate in prespore cell differentiation and do not produce pslA- spores in the chimeras. While pslA- cells produce mature, vacuolated stalk cells during multicellular development, pslA- cells are unable to do so in vitro in response to exogenous DIF (a morphogen required for prestalk and stalk cell differentiation). These results indicate that pslA- cells exhibit a defect in the prestalk/stalk cell pathways under these experimental conditions. Our results suggest that PslA's primary function is to regulate prespore cell determination very early in the prespore pathway via a cell-autonomous mechanism, possibly at the time of the initial prestalk/prespore cell-fate decision. Indirect immunofluorescence of myc-tagged PslA localizes the protein to the nucleus, suggesting that PslA may function to control the prespore pathway at the level of transcription.

Spalten, a protein containing Galpha-protein-like and PP2C domains, is essential for cell-type differentiation in Dictyostelium

We have identified a novel gene, Spalten (Spn) that is essential for Dictyostelium multicellular development. Spn encodes a protein with an amino-terminal domain that shows very high homology to Galpha-protein subunits, a highly charged inter-region, and a carboxy-terminal domain that encodes a functional PP2C. Spn is essential for development past the mound stage, being required cell autonomously for prestalk gene expression and nonautonomously for prespore cell differentiation. Mutational analysis demonstrates that the PP2C domain is the Spn effector domain and is essential for Spn function, whereas the Galpha-like domain is required for membrane targeting and regulation of Spn function. Moreover, Spn carrying mutations in the Galpha-like domain that do not affect membrane targeting but affect specificity of guanine nucleotide binding in known GTP-binding proteins are unable to fully complement the spn- phenotype, suggesting that the Galpha-like domain regulates Spn function either directly or indirectly by mediating its interactions with other proteins. Our results suggest that Spn encodes a signaling molecule with a novel Galpha-like regulatory domain.

Activation of the mitogen-activated protein kinase ERK2 by the chemoattractant folic acid in Dictyostelium

The Dictyostelium MAP kinase ERK2 is activated by extracellular cAMP in aggregation-competent cells and is required for receptor activation of adenylyl cyclase (Maeda, M., Aubry, L., Insall, R., Gaskins, C., Devreotes, P. N., and Firtel, R. A. (1996) J. Biol. Chem. 271, 3351-3354; Segall, J., Kuspa, A., Shaulsky, G., Ecke, M., Maeda, M., Gaskins, C., Firtel, R., and Loomis, W. (1995) J. Cell Biol. 128, 405-413). This cAMP-dependent activation of ERK2 is mediated by the serpentine, G protein-coupled cAMP receptors. However, ERK2 activation by cAMP is at least partially heterotrimeric G protein-independent, with a level of activation in cells lacking the sole Gbeta subunit or the G protein-coupled cAMP receptors-coupled Galpha2 subunit that is approximately 50% that of wild-type cells (Maeda, M., Aubry, L., Insall, R., Gaskins, C., Devreotes, P. N., and Firtel, R. A. (1996) J. Biol. Chem. 271, 3351-3354; Segall, J., Kuspa, A., Shaulsky, G., Ecke, M., Maeda, M., Gaskins, C., Firtel, R., and Loomis, W. (1995) J. Cell Biol. 128, 405-413). Folic acid, a chemoattractant in the vegetative cells that enables amoebae to find bacteria in the wild, also triggers the activation of adenylyl cyclase, which is impaired in the vegetative cells lacking the Galpha protein subunit Galpha4 (Hadwiger, J., Lee, S., and Firtel, R. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 10566-10570). In this study, we show that folic acid activates ERK2 in developmentally regulated manner and is required for ERK2 stimulation of adenylyl cyclase activity. Maximum levels of folate-stimulated ERK2 activity occur in cells from very early in development, prior to aggregation, and again at the tipped aggregate stages, corresponding to the stages in which folate receptors and the coupled Galpha subunit Galpha4 are maximally expressed. During the activation by folic acid, ERK2 is phosphorylated on tyrosine residue(s) and contemporaneously shows a mobility shift on SDS-PAGE. Interestingly, this activation is not elicited in the absence of Gbeta subunits, in contrast to the response to cAMP. This response also requires the Galpha4 subunit known to be required for other folic acid-mediated responses (Hadwiger, J., Lee, S., and Firtel, R. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 10566-10570). Furthermore, we show that the activation of ERK2 by cAMP is independent of the Galpha4 subunit, while the activation of ERK2 by folate is independent of Galpha2. Taken together, these data indicate that there are at least two pathways of ERK2 activation, heterotrimeric G protein-dependent and -independent pathways.

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.

SH2 signaling in a lower eukaryote: a STAT protein that regulates stalk cell differentiation in dictyostelium

The TTGA-binding factor is a transcriptional regulator activated by DIF, the chlorinated hexaphenone that induces prestalk cell differentiation in Dictyostelium. The same activity also functions as a repressor, controlling stalk cell differentiation. We show that the TTGA-binding factor is a STAT protein. Like the metazoan STATs, it functions via the reciprocal interaction of a phosphotyrosine residue on one molecule with an SH2 domain on a dimerizing partner. Furthermore, it will bind specifically to a mammalian interferon-stimulated response element. In Saccharomyces cerevisiae, where the entire genomic sequence is known, SH2 domains have not been identified. It would seem, therefore, that SH2 signaling pathways arose very early in the evolution of multicellular organisms, perhaps to facilitate intercellular comunication.

Characterization and overexpression of the Aspergillus niger gene encoding the cAMP-dependent protein kinase catalytic subunit

The gene pkaC encoding the catalytic subunit of cAMP-dependent protein kinase has been isolated from the industrially important filamentous fungus Aspergillus niger. A probe for screening A. niger phage libraries was generated by a polymerase chain reaction using degenerate primers. cDNA and genomic DNA clones were isolated and sequenced. An open reading frame of 1440 bp, interrupted by three short introns, encodes a polypeptide of 480 amino acids with a calculated molecular mass of 53813 Da. The cAMP-dependent protein kinase catalytic subunit (PKA-C) from A. niger has a 126 amino acid extension at the N-terminus compared to the PKA-C of higher eukaryotes that-except for the first 15 amino acids, which are homologous to the Magnaporthe grisea PKA-C-shows no significant similarity to the N-terminal extension of PKA-C of other lower eukaryotes. The catalytic core of PKA-C of A. niger shows extensive homology with the PKA-C isolated from all other eukaryotes. Low-stringency hybridization did not reveal any other pkaC homologue in A. niger. The cloned pkaC was used for transformation of A. niger, leading to increased levels of pkaC mRNA and PKA-C activity. Transformants overexpressing pkaC were phenotypically different with respect to growth, showing a more compact colony morphology, accompanied by a more dense sporulation, especially on media containing trehalose and glycerol. A number of transformants also showed a strongly reduced or complete absence of sporulation. This phenotype was quickly lost upon propagation of the strains.

Role of cAMP-dependent protein kinase in controlling aggregation and postaggregative development in Dictyostelium

We have examined the role of cAMP-dependent protein kinase (PKA) in controlling aggregation and postaggregative development in Dictyostelium. We previously showed that cells in which the gene encoding the PKA catalytic subunit has been disrupted (pkacat- cells) are unable to aggregate [S. K. O. Mann and R. A. Firtel (1991). A developmentally regulated, putative serine/threonine protein kinase is essential for development in Dictyostelium. Mech. Dev. 35, 89-102]. We show that pkacat- cells are unable to activate adenylyl cyclase in response to cAMP stimulation due to the inability to express the aggregation-stage, G-protein-stimulated adenylyl cyclase (ACA). Constitutive expression of ACA from an actin promoter results in a high level of Mn(2+)-stimulated adenylyl cyclase activity and restores chemoattractant- and GTP gamma S-stimulated adenylyl cyclase activity but not the ability to aggregate. Similarly, expression of the constitutively active, non-G protein-coupled adenylyl cyclase ACG in pkacat- cells also does not restore the ability to aggregate, although ACG can complement cells in which the ACA gene has been disrupted. These results indicate that pkacat- cells lack multiple, essential aggregation-stage functions. As the mound forms, high, continuous levels of extracellular cAMP functioning through the cAMP serpentine receptors activate a transcriptional cascade that leads to cell-type differentiation and morphogenesis. The first step is the induction and activation of the transcription factor GBF and downstream postaggregative genes, followed by the induction of prestalk- and prespore-specific genes. We show that pkacat- cells induce postaggregative gene expression in response to exogenous cAMP, but the level of induction of some of these genes, including GBF, is reduced. SP60 (a prespore-specific gene) is not induced and ecmA (a prestalk-specific gene) is induced to very low levels. Expressing GBF constitutively in pkacat- cells restores ecmA expression to a moderate level, but SP60 is not detectably induced. Overexpression of PKAcat from the Actin 15 (Act15), ecmA prestalk, and the PKAcat promoters in pkacat- cells result in significant aberrant spatial patterning of prestalk and prespore cells, as determined by lacZ reporter studies. Our studies identify new, essential regulatory roles for PKA in mediating multicellular development.

rZIP, a RING-leucine zipper protein that regulates cell fate determination during Dictyostelium development

rZIP is an approx. 32 kDa, multi-domain protein of Dictyostelium discoideum whose structural motifs include a RING (zinc-binding) domain, a leucine zipper, a glutamine repeat, an SH3-binding region and a consensus phosphorylation site for MAP kinase. In vitro, rZIP forms homodimers and interacts specifically with the SH3 domain(s) of the Nck adaptor protein. rZIP is expressed maximally during cell differentiation at approximately equivalent levels in all cells. Disruption of the rZIP gene rzpA results in altered cellular aggregation, impaired slug migration, and aberrant patterning of prespore and prestalk cells, the major progenitor classes. In rzpA- strains, prespore-specific genes are overexpressed and prestalk expression zones are reduced. Conversely, constitutive overexpression of rzpA markedly decreases prespore-specific gene expression and significantly increases the expression of prestalk-specific genes. Further, induced transdifferentiation of prespore cells into prestalk cells is inhibited in rzpA-slugs. In light of these patterning defects, we suggest that the RING/zipper protein rZIP plays an important role in early cell fate decisions in Dictyostelium, acting as a positive regulator of prestalk differentiation and an inhibitor of prespore differentiation.

A Ras GAP is essential for cytokinesis and spatial patterning in Dictyostelium

Using the yeast two-hybrid system, we have identified developmentally regulated Dictyostelium genes whose encoded proteins interact with Ras-GTP but not Ras-GDP. By sequence homology and biochemical function, one of these genes encodes a Ras GAP (DdRasGAP1). Cells carrying a DdRasGAP1 gene disruption (ddrasgap1 null cells) have multiple, very distinct growth and developmental defects as elucidated by examining the phenotypes of ddrasgap1 null strains. First, vegetative ddrasgap1 null cells are very large and highly multinucleate cells when grown in suspension, indicating a severe defect in cytokinesis. When suspension-grown cells are plated in growth medium on plastic where they attach and can move, the cells rapidly become mono- and dinucleate by traction-mediated cell fission and continue to grow vegetatively with a number of nuclei (1–2) per cell, similar to wild-type cells. The multinucleate phenotype, combined with results indicating that constitutive expression of activated Ras does not yield highly multinucleate cells and data on Ras null mutants, suggest that Ras may need to cycle between GTP- and GDP-bound states for proper cytokinesis. After starvation, the large null cells undergo rapid fission when they start to move at the onset of aggregation, producing mononucleate cells that form a normal aggregate. Second, ddrasgap1 null cells also have multiple developmental phenotypes that indicate an essential role of DdRasGAP1 in controlling cell patterning. Multicellular development is normal through the mid-slug stage, after which morphological differentiation is very abnormal and no culminant is formed: no stalk cells and very few spores are detected. lacZ reporter studies show that by the mid-finger stage, much of the normal cell-type patterning is lost, indicating that proper DdRasGAP1 function and possibly normal Ras activity are necessary to maintain spatial organization and for induction of prestalk to stalk and prespore to spore cell differentiation. The inability of ddrasgap1 null cells to initiate terminal differentiation and form stalk cells is consistent with a model in which Ras functions as a mediator of inhibitory signals in cell-type differentiation at this stage. Third, DdRasGAP1 and cAMP dependent protein kinase (PKA) interact to control spatial organization within the organism. Overexpression of the PKA catalytic subunit in ddrasgap1 cells yields terminal structures that are multiply branched but lack spores. This suggests that RasGAP and PKA may mediate common pathways that regulate apical tip differentiation and organizer function, which in turn control spatial organization during multicellular development. It also suggests that DdRasGAP1 either lies downstream from PKA in the prespore to spore pathway or in a parallel pathway that is also essential for spore differentiation. Our results indicate that DdRasGAP1 plays an essential role in controlling multiple, potentially novel pathways regulating growth and differentiation in Dictyostelium and suggest a role for Ras in these processes.

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.

Adenylyl cyclase G, an osmosensor controlling germination of Dictyostelium spores

Dictyostelium cells express a G-protein-coupled adenylyl cyclase, ACA, during aggregation and an atypical adenylyl cyclase, ACG, in mature spores. The ACG gene was disrupted by homologous recombination. acg- cells developed into normal fruiting bodies with viable spores, but spore germination was no longer inhibited by high osmolarity, a fairly universal constraint for spore and seed germination. ACG activity, measured in aca-/ACG cells, was strongly stimulated by high osmolarity with optimal stimulation occurring at 200 milliosmolar. RdeC mutants, which display unrestrained protein kinase A (PKA) activity and a cell line, which overexpresses PKA under a prespore specific promoter, germinate very poorly, both at high and low osmolarity. These data indicate that ACG is an osmosensor controlling spore germination through activation of protein kinase A.

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.

Integration of signaling information in controlling cell-fate decisions in Dictyostelium

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Regulatory role of the G alpha 1 subunit in controlling cellular morphogenesis in Dictyostelium

To determine the function of the Dictyostelium G alpha 1 subunit during aggregation and multicellular development, we analyzed the phenotypes of g alpha 1 null cells and strains overexpressing either wild-type G alpha 1 or two putative constitutively active mutations of G alpha 1. Strains overexpressing the wild-type or mutant G alpha 1 proteins showed very abnormal culmination with an aberrant stalk differentiation. The similarity of the phenotypes between G alpha 1 overexpression and expression of a putative constitutively active G alpha 1 subunit suggests that these phenotypes are due to increased G alpha 1 activity rather than resulting from a non-specific interference of other pathways. In contrast, g alpha 1 null strains showed normal morphogenesis except that the stalks were thinner and longer than those of wild-type culminants. Analysis of cell-type-specific gene expression using lacZ reporter constructs indicated that strains overexpressing G alpha 1 show a loss of ecmB expression in the central core of anterior prestalk AB cells. However, expression of ecmB in anterior-like cells and the expression of prestalk A-specific gene ecmA and the prespore-specific gene SP60/cotC appeared normal. Using a G alpha 1/lacZ reporter construct, we show that G alpha 1 expression is cell-type-specific during the multicellular stages, with a pattern of expression similar to ecmB, being preferentially expressed in the anterior prestalk AB cells and anterior-like cells. The developmental and molecular phenotypes of G alpha 1 overexpression and the cell-type-specific expression of G alpha 1 suggest that G alpha 1-mediated signaling pathways play an essential role in regulating multicellular development by controlling prestalk morphogenesis, possibly by acting as a negative regulator of prestalk AB cell differentiation. During the aggregation phase of development, g alpha 1 null cells display a delayed peak in cAMP-stimulated accumulation of cGMP compared to wild-type cells, while G alpha 1 overexpressors and dominant activating mutants show parallel kinetics of activation but decreased levels of cGMP accumulation compared to that seen in wild-type cells. These data suggest that G alpha 1 plays a role in the regulation of the activation and/or adaptation of the guanylyl cyclase pathway. In contrast, the activation of adenylyl cyclase, another pathway activated by cAMP stimulation, was unaffected in g alpha 1 null cells and cell lines overexpressing wild-type G alpha 1 or the G alpha 1 (Q206L) putative dominant activating mutation.(ABSTRACT TRUNCATED AT 400 WORDS)

Progression of an inductive signal activates sporulation in Dictyostelium discoideum

spiA, a marker for sporulation, is expressed during the culmination stage of Dictyostelium development, when the mass of prespore cells has moved partly up the newly formed stalk. Strains containing a full-length spiA promoter/lacZ fusion were stained for beta-galactosidase activity at intervals during development. The results indicate that expression of spiA initiates in prespore cells at the prestalk/prespore boundary (near the apex) and extends downward into the prespore mass as culmination continues. A spatial gradient of staining expands from the top of the prespore mass and intensifies until the front of activation reaches the bottom, whereupon the entire region stains darkly. The spiA promoter can be deleted to within 301 bp of the transcriptional start site with no effect on the relative strength, timing or spatial localization of expression. Further 5′ deletions from −301 to −175 reduce promoter strength incrementally, although timing and spatial expression are not affected. Deletions to −159 and beyond result in inactive promoters. Treatment of early developmental structures with 8-Br-cAMP in situ activates the intracellular cAMP-dependent protein kinase (PKA) and precociously induces spiA expression and sporulation. The absence of an apparent gradient of staining in these structures suggest that PKA is equivalently activatable throughout the prespore region and that all prespore cells are competent to express spiA. Thus, we postulate that the pattern of expression of spiA reveals the progression of an inductive signal for sporulation and suggest that this signal may originate from the prestalk cells at the apex.

Differentiation and patterning in Dictyostelium

Dictyostelium development is orchestrated by diffusible signals. Progress has been made in understanding how cAMP signaling triggers post-aggregative development and in defining the number of cell types that eventually differentiate. Ammonia is an unusual signal that may act by alkalinizing acidic vesicles. A chlorinated signal, differentiation-inducing factor (DIF), may be universal amongst the slime moulds. The first genes have been cloned using restriction enzyme mediated integration (REMI) insertional mutagenesis; one encodes a novel cytosolic protein essential for activation of adenylyl cyclase.

Characterization of a novel Dictyostelium discoideum prespore-specific gene, PspB, reveals conserved regulatory sequences

While Dictyostelium discoideum has been studied as a developmental system for decades, and many regulatory proteins have been cloned, the molecular mechanisms of cell-type-specific gene expression are poorly understood. In this paper we characterize a novel prespore gene, PspB, and undertake a comparative analysis of the regulatory regions in prespore-specific D. discoideum promoters. Sequence alignment of the PSPB gene product with other prespore-specific proteins identifies a conserved, repeated 12 amino acid cysteine-containing motif that may be involved in spore coat function or assembly. Analysis of the PspB promoter identifies two domains essential for developmentally induced promoter activity. The first region includes two CA-rich elements (CAEs) that we show to be functionally homologous to the cAMP-inducible elements previously identified in the SP60 (cotC) promoter. The PspB CAEs compete with the SP60 (cotC) CAEs for binding in vitro to a developmentally regulated nuclear activity. We identify this activity as G-box Binding Factor, a developmentally induced transcription factor. The PspB CAEs and adjacent nucleotides direct a very low level of prespore-enriched expression, but high levels of cell-type-specific expression requires a second promoter region: a 46-bp AT-rich sequence that does not resemble the CAEs or any other previously described late gene promoter elements. Comparison of the PspB AT element with regulatory regions of the SP60 (cotC), SP70 (cotB), and D19 (pspA) promoters reveals an extensive consensus sequence. We suggest that these AT-rich sequences may represent a common regulatory element (or elements) required for prespore gene activation.

LagC is required for cell-cell interactions that are essential for cell-type differentiation in Dictyostelium

Strain AK127 is a developmental mutant of Dictyostelium discoideum that was isolated by restriction enzyme-mediated integration (REMI). Mutant cells aggregate normally but are unable to proceed past the loose aggregate stage. The cloned gene, lagC (loose aggregate C), encodes a novel protein of 98 kD that contains an amino-terminal signal sequence and a putative carboxy-terminal transmembrane domain. The mutant strain AK127 shows no detectable lagC transcript upon Northern analysis, indicating that the observed phenotype is that of a null allele. Expression of the lagC cDNA in AK127 cells complements the arrest at the loose aggregate stage, indicating that the mutant phenotype results from disruption of the lagC gene. In wild-type cells, lagC mRNA is induced at the loose aggregate stage and is expressed through the remainder of development. lagC- null cells aggregate but then disaggregate and reaggregate to form small granular mounds. Mature spores are produced at an extremely low efficiency (< 0.1% of wild type), appearing only after approximately 72 hr, whereas wild-type strains produce mature spores by 26 hr. lagC- null cells accumulate reduced levels of transcripts for the prestalk-enriched genes rasD and CP2 and do not express the DIF-induced prestalk-specific gene ecmA or the cAMP-induced prespore-specific gene SP60 to significant levels. In chimeric organisms resulting from the coaggregation of lagC- null and wild-type cells, cell-type-specific gene expression is rescued in the lagC- null cells; however, lagC- prespore cells are localized to the posterior of the prespore region and do not form mature spores, suggesting that LagC protein has both no cell-autonomous and cell-autonomous functions. Overexpression of lagC from an actin promoter in both wild-type and lagC- cells causes a delay at the tight aggregate stage, the first stage requiring LagC activity. These results suggest that the LagC protein functions as a nondiffusible cell-cell signaling molecule that is required for multicellular development.

Cloning and characterization of the G-box binding factor, an essential component of the developmental switch between early and late development in Dictyostelium

During Dictyostelium development, the cAMP-regulated induction of cell-type-specific late genes marks a developmental switch from the initial formation of the multicellular organism to the differentiation of the various cell types that mediate morphogenesis and eventually give rise to the mature fruting body. The G-box binding factor (GBF) is a developmentally regulated Dictyostelium transcription factor whose affinity for a DNA sequence correlates with the ability of that sequence to confer inducibility to late gene promoters in response to high, continuous levels of extracellular cAMP. We report the purification of GBF and cloning of the gene that encodes it, as confirmed by in vitro production of GBF activity. The predicted protein is highly basic and contains two putative zinc fingers. Disruption of the GBF gene by homologous recombination results in the loss of all GBF DNA-binding activity, developmental arrest at the loose aggregate stage, and the loss of late gene induction during development or in response to extracellular cAMP. Constitutive expression of GBF complements the null phenotype and allows for the rapid activation of a class of late genes in response to cAMP. Our results indicate that GBF acts as an extracellular cAMP-responsive transcriptional activator regulating late gene expression and is an essential component of a developmental switch between aggregation and cellular morphogenesis.