Induction of terminal differentiation of Dictyostelium by cAMP-dependent protein kinase and opposing effects of intracellulr and extracellular cAMP on stalk cell differentiation

Emerging roles for diguanylate cyclase during the evolution of soma in dictyostelia

BACKGROUND

Cyclic di-guanylate (c-di-GMP), synthesized by diguanylate cyclase, is a major second messenger in prokaryotes, where it triggers biofilm formation. The dictyostelid social amoebas acquired diguanylate cyclase (dgcA) by horizontal gene transfer. Dictyostelium discoideum (Ddis) in taxon group 4 uses c-di-GMP as a secreted signal to induce differentiation of stalk cells, the ancestral somatic cell type that supports the propagating spores. We here investigated how this role for c-di-GMP evolved in Dictyostelia by exploring dgcA function in the group 2 species Polysphondylium pallidum (Ppal) and in Polysphondylium violaceum (Pvio), which resides in a small sister clade to group 4.

RESULTS

Similar to Ddis, dgcA is upregulated after aggregation in Ppal and Pvio and predominantly expressed in the anterior region and stalks of emerging fruiting bodies. DgcA null mutants in Ppal and Pvio made fruiting bodies with very long and thin stalks and only few spores and showed delayed aggregation and larger aggregates, respectively. Ddis dgcA- cells cannot form stalks at all, but showed no aggregation defects. The long, thin stalks of Ppal and Pvio dgcA- mutants were also observed in acaA- mutants in these species. AcaA encodes adenylate cyclase A, which mediates the effects of c-di-GMP on stalk induction in Ddis. Other factors that promote stalk formation in Ddis are DIF-1, produced by the polyketide synthase StlB, low ammonia, facilitated by the ammonia transporter AmtC, and high oxygen, detected by the oxygen sensor PhyA (prolyl 4-hydroxylase). We deleted the single stlB, amtC and phyA genes in Pvio wild-type and dgcA- cells. Neither of these interventions affected stalk formation in Pvio wild-type and not or very mildly exacerbated the long thin stalk phenotype of Pvio dgcA- cells.

CONCLUSIONS

The study reveals a novel role for c-di-GMP in aggregation, while the reduced spore number in Pvio and Ppal dgcA- is likely an indirect effect, due to depletion of the cell pool by the extended stalk formation. The results indicate that in addition to c-di-GMP, Dictyostelia ancestrally used an as yet unknown factor for induction of stalk formation. The activation of AcaA by c-di-GMP is likely conserved throughout Dictyostelia.

Deletion of Htt cause alterations in cAMP signaling and spatial patterning in Dictyostelium discoideum

In the present study, we have analyzed in detail the functions of Htt during growth and development of the protist, Dictyostelium discoideum by creating mutants (both overexpressor and knockout). The mRNA was present at all stages of growth and development. Overexpression of htt did not show any major anomaly, while deletion resulted in delayed aggregation territory formation and showed asynchronous development especially after slug stage. The slugs formed by htt ^- cells showed aberration in anterior-posterior boundary, showing increased prestalk region. DdHtt regulates STAT transcription factors in the tip organizer region that help maintain patterning and culmination. In chimeras with the wild-type, htt ^- cells preferentially localized to the tip of the slug and basal disc regions of the fruiting body showing prestalk/stalk bias, while the overexpressing cells majorly populated the prespore/spore region showing spore bias. These differences could be attributed to protein kinase A (PKA)-regulated cyclic adenosine monophosphate (cAMP) signaling.

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.

A conserved signalling pathway for amoebozoan encystation that was co-opted for multicellular development

The evolution of multicellularity required novel mechanisms for intercellular communication, but their origin is unclear. Dictyostelium cells exchange signals to position specialized cell types in multicellular spore-bearing structures. These signals activate complex pathways that converge on activation of cAMP-dependent protein kinase (PKA). Genes controlling PKA were detected in the Dictyostelid unicellular ancestors, which like most protists form dormant cysts when experiencing environmental stress. We deleted PKA and the adenylate cyclases AcrA and AcgA, which synthesize cAMP for PKA activation, in the intermediate species Polysphondylium, which can develop into either cysts or into multicellular structures. Loss of PKA prevented multicellular development, but also completely blocked encystation. Loss of AcrA and AcgA, both essential for sporulation in Dictyostelium, did not affect Polysphondylium sporulation, but prevented encystation. We conclude that multicellular cAMP signalling was co-opted from PKA regulation of protist encystation with progressive refunctionalization of pathway components.

The Amoebozoa

The model organism Dictyostelium discoideum is a member of the Amoebozoa, one of the six major -divisions of eukaryotes. Amoebozoa comprise a wide variety of amoeboid and flagellate organisms with single cells measuring from 5 μm to several meters across. They have adopted many different life styles and sexual behaviors and can live in all but the most extreme environments. This chapter provides an overview of Amoebozoan diversity and compares roads towards multicellularity within the Amoebozoa with inventions of multicellularity in other protist divisions. The chapter closes with a scenario for the evolution of Dictyostelid multicellularity from an Amoebozoan stress response.

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

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

Protein kinase B gene homologue pkbR1 performs one of its roles at first finger stage of Dictyostelium

Dictyostelium discoideum has protein kinases AKT/PKBA and PKBR1 that belong to the AGC family of kinases. The protein kinase B-related kinase (PKBR1) has been studied with emphasis on its role in chemotaxis, but its roles in late development remained obscure. The pkbR1 null mutant stays in the first finger stage for about 16 h or longer. Only a few aggregates continue to the migrating slug stage; however, the slugs immediately go back probably to the previous first finger stage and stay there for approximately 37 h. Finally, the mutant fingers diversify into various multicellular bodies. The expression of the pkbR1 finger protein probably is required for development to the slug stage and to express ecmB, which is first observed in migrating slugs. The mutant also showed no ST-lacZ expression, which is of the earliest step in differentiation to one of the stalk cell subtypes. The pkbR1 null mutant forms a small number of aberrant fruiting bodies, but in the presence of 10% of wild-type amoebae the mutant preferentially forms viable spores, driving the wild type to form nonviable stalk cells. These results suggest that the mutant has defects in a system that changes the physiological dynamics in the prestalk cell region of a finger. We suggest that the arrest of its development is due to the loss of the second wave of expression of a protein kinase A catalytic subunit gene (pkaC) only in the prestalk region of the pkbR1 null mutant.

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.

Transcriptional regulation of post-aggregation genes in Dictyostelium by a feed-forward loop involving GBF and LagC

Expression profiles of developmental genes in Dictyostelium were determined on microarrays during development of wild type cells and mutant cells lacking either the DNA binding protein GBF or the signaling protein LagC. We found that the mutant strains developed in suspension with added cAMP expressed the pulse-induced and early adenylyl cyclase (ACA)-dependent genes, but not the later ACA-dependent, post-aggregation genes. Since expression of lagC itself is dependent on GBF, expression of the post-aggregation genes might be controlled only by signaling from LagC. However, expression of lagC in a GBF-independent manner in a gbfA- null strain did not result in expression of the post-aggregation genes. Since GBF is necessary for accumulation of LagC and both the DNA binding protein and the LagC signal transduction pathway are necessary for expression of post-aggregation genes, GBF and LagC form a feed-forward loop. Such network architecture is a common motif in diverse organisms and can act as a filter for noisy inputs. Breaking the feed-forward loop by expressing lagC in a GBF-independent manner in a gbfA+ strain does not significantly affect the patterns of gene expression for cells developed in suspension with added cAMP, but results in a significant delay at the mound stage and asynchronous development on solid supports. This feed-forward loop can integrate temporal information with morphological signals to ensure that post-aggregation genes are only expressed after cell contacts have been made.

Developmental decisions in Dictyostelium discoideum

Dictyostelium discoideum is an excellent system in which to study developmental decisions. Synchronous development is triggered by starvation and rapidly generates a limited number of cell types. Genetic and image analyses have revealed the elegant intricacies associated with this simple development system. Key signaling pathways identified as regulating cell fate decisions are likely to be conserved with metazoa and are providing insight into differentiation decisions under circumstances where considerable cell movement takes place during development.

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.

Mutation of the Dictyostelium fbxA Gene Affects Cell-Fate Decisions and Spatial Patterning

Cell-fate decisions and spatial patterning in Dictyostelium are regulated by a number of genes. Our studies have implicated a gene called fbxA, which codes for an F-box protein, in these pathways. The FbxA protein is one of the controls on a cAMP phosphodiesterase called RegA, mediating its degradation via ubiquitin-linked proteolysis. Using marked strains, we showed that the fbxA- mutant has defective cell-type proportioning, with a dearth of prestalk cells compared to prespore cells. In this work, we show that this effect occurs earlier during the 24 hour developmental cycle than previously thought. The normal sorting of the prestalk and prespore cells in aggregates and mounds is not affected by the mutation. The mutant cells sort abnormally at the tipped mound stage, when prespore and prestalk cells normally distribute into their proper compartments. The fbxA- mutant forms pre-stalk cells in low numbers when not in chimeras, but in the presence of wild-type amoebae the mutant preferentially forms viable spores, driving the wild type to form non-viable stalk cells. In an attempt to identify the signal transduction pathway that mediates proportionality in prestalk and prespore cells, we asked whether certain signal transduction mutants were immune to the effects of the fbxA- cells and formed spores in chimeras.

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.

Regulated expression of the MADS-box transcription factor SrfA mediates activation of gene expression by protein kinase A during Dictyostelium sporulation

Cell differentiation and morphogenesis are tightly regulated during sporulation in the lower eukaryote Dictyostelium discoideum. The control of the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is essential to coordinate these processes. Several signal transduction pathways are being recognized that lead to the regulation of intracellular cAMP levels. However, very little is known about the events lying downstream of PKA that are essential to activate late gene expression and terminal differentiation of the spores. We have studied the relationship between PKA and the MADS-box transcription factor SrfA, essential for spore differentiation. Constitutive activation of PKA was not able to rescue sporulation in a strain that lacks srfA suggesting the possibility that srfA functions downstream of PKA in a signal transduction pathway leading to spore maturation. A distal promoter region regulates the induction of srfA expression in the prespore region during culmination. We found that this promoter can be induced precociously by activating PKA with 8-Br-cAMP suggesting a transcriptional regulation by PKA. Moreover, precocious sporulation and expression of the spore marker spiA in a strain that overexpresses PKA, correlates with a precocious induction of srfA expression. The temporal and spatial pattern of expression was also studied in a mutant strain lacking the main adenylyl cyclase that functions during culmination, ACR. This strain is expected to have lower PKA activity and consistently, the level of srfA expression was reduced. Moreover, the temporal induction of srfA in the prespore region was also delayed during culmination. Our results strongly suggest that PKA activation during culmination leads to the induction of the expression of srfA. The correct temporal and spatial pattern of srfA expression appears to be part of a mechanism that ensures the adequate coordination of gene expression and morphogenesis.

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.

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.

Expression of activated Ras during Dictyostelium development alters cell localization and changes cell fate

There is now a body of evidence to indicate that Ras proteins play important roles in development. Dictyostelium expresses several ras genes and each appears to perform a distinct function. Previous data had indicated that the overexpression of an activated form of the major developmentally regulated gene, rasD, caused a major aberration in morphogenesis and cell type determination. We now show that the developmental expression of an activated rasG gene under the control of the rasD promoter causes a similar defect. Our results indicate that the expression of activated rasG in prespore cells results in their transdifferentiation into prestalk cells, whereas activated rasG expression in prestalk causes gross mislocalization of the prestalk cell populations.

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.

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.

An adenylyl cyclase that functions during late development of Dictyostelium

A variety of extracellular signals lead to the accumulation of cAMP which can act as a second message within cells by activating protein kinase A (PKA). Expression of many of the essential developmental genes in Dictyostelium discoideum are known to depend on PKA activity. Cells in which the receptor-coupled adenylyl cyclase gene, acaA, is genetically inactivated grow well but are unable to develop. Surprisingly, acaA(−) mutant cells can be rescued by developing them in mixtures with wild-type cells, suggesting that another adenylyl cyclase is present in developing cells that can provide the internal cAMP necessary to activate PKA. However, the only other known adenylyl cyclase gene in Dictyostelium, acgA, is only expressed during germination of spores and plays no role in the formation of fruiting bodies. By screening morphological mutants generated by Restriction Enzyme Mediated Integration (REMI) we discovered a novel adenylyl cyclase gene, acrA, that is expressed at low levels in growing cells and at more than 25-fold higher levels during development. Growth and development up to the slug stage are unaffected in acrA(−) mutant strains but the cells make almost no viable spores and produce unnaturally long stalks. Adenylyl cyclase activity increases during aggregation, plateaus during the slug stage and then increases considerably during terminal differentiation. The increase in activity following aggregation fails to occur in acrA(−) cells. As long as ACA is fully active, ACR is not required until culmination but then plays a critical role in sporulation and construction of the stalk.

Signalling pathways that direct prestalk and stalk cell differentiation in Dictyostelium

Prestalk cell differentiation in Dictyostelium is induced by DIF and two DIF-induced genes, ecmA and ecmB, have revealed the existence of multiple prestalk and stalk cell sub-types. These different sub-types are defined by the pattern of expression of subfragments derived from the ecmA and ecmB promoters. These markers have been utilised in three ways; for fate mapping in vivo, to investigate the molecular mechanisms underlying DIF signalling and to explore the relative requirement for DIF and other signalling molecules for prestalk and stalk cell differentiation in vitro. The heterogeneity of the prestalk and stalk populations seems to be reflected in differences in the cell signalling pathways that they utilise.

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.

Evidence that the Dictyostelium Dd-STATa protein is a repressor that regulates commitment to stalk cell differentiation and is also required for efficient chemotaxis

Dd-STATa is a structural and functional homologue of the metazoan STAT (Signal Transducer and Activator of Transcription) proteins. We show that Dd-STATa null cells exhibit several distinct developmental phenotypes. The aggregation of Dd-STATa null cells is delayed and they chemotax slowly to a cyclic AMP source, suggesting a role for Dd-STATa in these early processes. In Dd-STATa null strains, slug-like structures are formed but they have an aberrant pattern of gene expression. In such slugs, ecmB/lacZ, a marker that is normally specific for cells on the stalk cell differentiation pathway, is expressed throughout the prestalk region. Stalk cell differentiation in Dictyostelium has been proposed to be under negative control, mediated by repressor elements present in the promoters of stalk cell-specific genes. Dd-STATa binds these repressor elements in vitro and the ectopic expression of ecmB/lacZ in the null strain provides in vivo evidence that Dd-STATa is the repressor protein that regulates commitment to stalk cell differentiation. Dd-STATa null cells display aberrant behavior in a monolayer assay wherein stalk cell differentiation is induced using the stalk cell morphogen DIF. The ecmB gene, a general marker for stalk cell differentiation, is greatly overinduced by DIF in Dd-STATa null cells. Also, Dd-STATa null cells are hypersensitive to DIF for expression of ST/lacZ, a marker for the earliest stages in the differentiation of one of the stalk cell sub-types. We suggest that both these manifestations of DIF hypersensitivity in the null strain result from the balance between activation and repression of the promoter elements being tipped in favor of activation when the repressor is absent. Paradoxically, although Dd-STATa null cells are hypersensitive to the inducing effects of DIF and readily form stalk cells in monolayer assay, the Dd-STATa null cells show little or no terminal stalk cell differentiation within the slug. Dd-STATa null slugs remain developmentally arrested for several days before forming very small spore masses supported by a column of apparently undifferentiated cells. Thus, complete stalk cell differentiation appears to require at least two events: a commitment step, whereby the repression exerted by Dd-STATa is lifted, and a second step that is blocked in a Dd-STATa null organism. This latter step may involve extracellular cAMP, a known repressor of stalk cell differentiation, because Dd-STATa null cells are abnormally sensitive to the inhibitory effects of extracellular cyclic AMP.

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.

Two distinct signaling pathways mediate DIF induction of prestalk gene expression in Dictyostelium

During Dictyostelium development, the differentiation inducing factor (DIF) triggers expression of the prestalk gene ecmB and induces stalk cell differentiation, a form of programmed cell death. The effects of DIF are mediated by a sustained increase in cytosolic Ca2+ levels. The Ca2+ ATPase inhibitor BHQ causes a similar rise in Ca2+ levels and also induces prestalk gene expression. We show here that Ca2+ is a specific intermediate for prestalk gene induction, since BHQ represses transcription of the cAMP-inducible aggregative gene PDE, the postaggregative gene CP2, and the prespore gene D19. The prestalk gene ecmA is also induced by DIF, but induction appears to occur in two steps, which occur within 1 h and after 2 h, respectively. The slow step shows the same kinetics as ecmB induction and similar to ecmB induction, this step is BHQ inducible and requires an initial round of protein synthesis. The fast step does not require protein synthesis and cannot be induced by BHQ. This indicates that in addition to the slow Ca2+-mediated pathway, there is probably a second fast Ca2+-independent signal transduction pathway for DIF.

The phosphorylation of protein S6 modulates the interaction of the 40 S ribosomal subunit with the 5'-untranslated region of a dictyostelium pre-spore-specific mRNA and controls its stability

AC914 mRNA, a pre-spore-specific mRNA that accumulates only in the post-aggregation stage of development, is transcribed constitutively as shown by nuclear run-off experiments and by fusing its promoter to the luciferase reporter gene. The same mRNA disappears quickly from disaggregated cells. If the 5'-untranslated region (5'UTR) of the constitutively expressed Actin 15 mRNA is substituted for the 5'UTR of AC914 mRNA, this can no longer be destabilized and accumulates both in growing and disaggregated cells. If the 5'UTR of AC914 mRNA is substituted for the 5'UTR of Actin 15 mRNA, the latter accumulates only in aggregated cells. Pactamycin, but not other inhibitors of protein synthesis, prevents AC914 mRNA from being destabilized in disaggregated cells, suggesting a role of 40 S subunits in the destabilization. This has been confirmed by using an in vitro system in which the in vivo stability of different mRNAs is reproduced. A protein kinase A-dependent phosphorylation of ribosomal protein S6 determines whether 40 S subunits are capable or not of destabilizing AC914 mRNA in the in vitro system.

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.

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.

Propagating waves control Dictyostelium discoideum morphogenesis

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

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.

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.

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.

Identification of the cell fate gene stalky in Dictyostelium

Using insertional mutagenesis, we have isolated a "stalky" mutant in which cells destined to become spores end up as stalk cells. Similar mutants were previously observed after chemical mutagenesis, but the affected gene could not be isolated. Our mutant, like the previous ones, is in stkA. Its defect is cell-autonomous and not overcome by overexpressing cAMP-dependent protein kinase. stkA is strongly expressed in the prespore region of aggregates but not in the anterior prestalk zone. The mutant expresses normal levels of prespore-cell transcripts but fails to produce the spore transcript spiA. stkA encodes a predicted 99 kDa protein (STKA) with two putative C4 zinc fingers, one of which is a GATA-type finger, indicating that it may be a transcription factor. This conclusion is supported by localization of STKA in the nucleus.

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.

Dual role of cAMP during Dictyostelium development

cAMP plays an essential role during Dictyostelium development both outside and inside the cell. Membrane-bound receptors and adenylyl cyclase are responsible for sensing and producing extracellular cAMP, whereas a phosphodiesterase is responsible for maintaining a low basal level. The molecular events underlying this type of hormone like signalling, which are now beginning to be deciphered, will be presented, in the light of cAMP analogue studies. The importance of intracellular cAMP for cell differentiation has been demonstrated by the central role of the cAMP dependent protein kinase. Mutants as well as strains obtained by reverse genetics will be reviewed which lead to our current understanding of the role of intracelluar cAMP in the differentiation of both stalk and spore cells.

cAMP-dependent protein kinase activity is essential for preaggregative gene expression in Dictyostelium

Constitutive inhibition of cAMP-dependent protein kinase (PKA) in Dictyostelium cells blocks cell aggregation and development. We investigated the cause of the aggregation defect in transformants overexpressing dominant-negative PKA regulatory subunits (PKA-RM) under an actin 15 promoter. These mutants could not relay pulses of the chemoattractant cAMP, due to a defect in expression of the aggregative adenylyl cyclase (ACA) gene. Unstimulated and cAMP pulse-induced expression of other aggregative genes encoding the cAMP receptor cAR1, adhesive contact sites A and cAMP-phosphodiesterase were also strongly reduced in the mutants. Additionally, the expression of the discoidin I gene, that is expressed early in development in response to cell density sensing factors, was almost completely absent. These data are in interesting contrast with observations that cAMP relay and aggregative gene expression are normal in null mutants for the PKA catalytic (C) subunit and suggest the presence of multiple C subunit genes in Dictyostelium and an almost universal requirement for PKA activity in developmental gene expression.

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.

A Dictyostelium anterior-like cell mutant reveals sequential steps in the prespore prestalk differentiation pathway

We have isolated Dictyostelium cell type proportioning mutants in a screen for cAMP signal transduction mutants. All mutants showed reduced numbers of prespore cells as analyzed with prespore-specific antibodies and prespore- and prestalk-specific reporter gene constructs. Formation of prestalk and prespore cells in the mutant HP1 is initially almost normal; however, many prespore cells redifferentiate as anterior-like cells leading to altered cell type proportions. Synergy experiments of HP1 transformant cells with wild-type Ax2 slugs showed the existence of stable subpopulations of cells in the prestalk and prespore regions of the slug. HP1 cells sort exclusively to the back of the prestalk and prespore zone in slugs. These cells will form the upper and lower cup regions of the culminate. Ax2 transformant cells sort to the tip of the anterior prestalk and prespore zone in HP1 slugs and will form the stalk and spores in the culminate. These experiments show that upper and lower cup derive from cells in different differentiation pathways. The lower cup cell derive from prespore cells, which redifferentiate as anterior-like cells in the back of the prespore zone. The upper cup cells derive from cells from the back of the prestalk zone. Synergy experiments showed that the mutation in HP1 results in an altered sensitivity to proportioning signals. The mutation in HP1 may result in a reduced sensitivity to cAMP, which facilitates the redifferentiation of prespore cells via anterior-like to lower cup cells and inhibits the transition from upper cup to stalk cells.

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.