Two distinct populations of prestalk cells within the tip of the migratory Dictyostelium slug with differing fates at culmination

OSBPa, a predicted oxysterol binding protein of Dictyostelium, is required for regulated entry into culmination

The oxysterol binding proteins (OSBPs) are believed to control cholesterol homeostasis but their precise mechanism of action is not well understood. The Dictyostelium osbA gene encodes a predicted OSBP, OSBPa, which lacks the PH domain that in most other OSBPs directs targetting to the Golgi. OSBPa instead localises selectively to the cell periphery and also, in some cells, to the perinuclear region. OSBPa null strains form normal fruiting bodies but are defective in the regulation of the transition from slug migration to culmination. Thus a plasma membrane-enriched OSBP family member is essential for correct regulation of the slug-fruiting body switch.

Loss of the beta-catenin homologue aardvark causes ectopic stalk formation in Dictyostelium

Aardvark (Aar) is a Dictyostelium beta-catenin homologue with both cytoskeletal and signal transduction roles during development. Here, we show that loss of aar causes a novel phenotype where multiple stalks appear during late development. Ectopic stalks are preceded by misexpression of the stalk marker ST-lacZ in the surrounding tissue. This process does not involve the kinase GSK-3. Mixing experiments show that ectopic ST-lacZ expression and stalk formation are cell non-autonomous. The protein-cellulose matrix surrounding the stalk of aar mutant fruiting bodies is defective, and damage to the stalk of wild-type fruiting bodies leads to ectopic ST-lacZ expression. We postulate that poor synthesis of the stalk tube matrix allows diffusion of a stalk cell-inducing factor into the surrounding tissue.

A novel Dictyostelium gene encoding multiple repeats of adhesion inhibitor-like domains has effects on cell-cell and cell-substrate adhesion

The Dictyostelium protein AmpA (adhesion modulation protein A) is encoded by the gene originally identified by the D11 cDNA clone. AmpA contains repeated domains homologous to a variety of proteins that influence cell adhesion. The protein accumulates during development, reaching a maximal level at the finger stage. Much of the AmpA protein is found extracellularly during development, and in culminants, AmpA is found in association with anterior-like cells. Characterization of an ampA- strain generated by gene replacement reveals a significant increase in cell-cell clumping when cells are starved in nonnutrient buffer suspensions. Developing ampA- cells are also more adhesive to the underlying substrate and are delayed in developmental progression, with the severity of the delay increasing as cells are grown in the presence of bacteria or on tissue culture dishes rather than in suspension culture. Reintroduction of the ampA gene rescues the developmental defects of ampA- cells; however, expression of additional copies of the gene in wild-type cells results in more severe developmental delays and decreased clumping in suspension culture. We propose that the AmpA protein functions as an anti-adhesive to limit cell-cell and cell-substrate adhesion during development and thus facilitates cell migration during morphogenesis.

A gene encoding a novel anti-adhesive protein is expressed in growing cells and restricted to anterior-like cells during development of Dictyostelium

The Dictyostelium gene ampA, initially identified by the D11 cDNA, encodes a novel anti-adhesive-like protein. The ampA gene product inhibits premature cell agglutination during growth and modulates cell-cell and cell-substrate adhesion during development. Analysis of the promoter indicates that cap site-proximal sequence directs ampA expression during both growth and early development. Expression following tip formation is controlled by more distal sequence, which contains TTGA repeats known to regulate prestalk cell gene expression in other promoters. Comparison of reporter gene expression and endogenous mRNA accumulation indicates that during growth the ampA gene is expressed in an increasing number of cells as a function of density. The number of cells expressing the ampA gene drops as development initiates, but the cells that continue to express the gene do so at high levels. These cells are initially scattered throughout the entire aggregate. By the tip formation stage, however, the majority of ampA-expressing cells are localized to the mound periphery, with only a few cells remaining scattered in the upper portion of the mound. In the final culminant, ampA is expressed only in the upper cup, lower cup, and basal disc. Although reporter expression is observed in cells that migrate anteriorly to a banded region just posterior to the tip, expression is rarely observed in the extreme tip. AmpA protein however, is localized to the tip as well as to ALCs during late development. The results presented here suggest that ampA gene expression is shut off in ALCs that continue along the prestalk differentiation pathway before they are added to the primordial stalk.

Altered cell-type proportioning in Dictyostelium lacking adenosine monophosphate deaminase

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

Expression patterns of cell-type-specific genes in Dictyostelium

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

Identification of novel elements which regulate the cell-type specificity of Dictyostelium 7E gene expression

Previously, we have identified the Dictyostelium 7E gene promoter and shown that it is capable of driving expression in the same temporal and cAMP responsive manner as the endogenous gene during development. Furthermore, we have mapped the corresponding transcriptional regulatory sequences within the promoter. In the present study we used the lacZ reporter gene system to examine the role of 7E promoter elements in regulating cell-type specific expression during Dictyostelium morphogenesis. In situ detection of beta-galactosidase activity revealed that expression was induced within anterior prestalk cells at approximately 18 h of development. Subsequently, we found that promoter activity was independently regulated in subpopulations of prestalk cells. Element(s) upstream of position - 532 were necessary for expression in pstA cells while more proximal elements (located downstream of position - 426) were capable of directing expression in pstO cells. Deletion of a G-rich element ('GGT' box; 5'-GGT GAT GA-3') located between positions - 159 and - 152 resulted both in a loss of expression in pstA cells and aberrant expression in the prespore zone. Furthermore, the spatial organisation of reporter gene expression directed by this construct during culmination delineated a population of cells that have not been previously defined. These data suggest that the 7E gene is independently regulated in subpopulations of prestalk cells during development.

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

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

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

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

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

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

Tyrosine phosphorylation-independent nuclear translocation of a dictyostelium STAT in response to DIF signaling

We describe a Dictyostelium STAT, Dd-STATc, which regulates the speed of early development and the timing of terminal differentiation. Dd-STATc also functions as a repressor, which directs graded expression of the ecmA gene in different prestalk cell populations. Developing Dictyostelium cells produce a chlorinated hexaphenone, DIF, which directs prestalk cell differentiation. Dd-STATc is tyrosine phosphorylated, dimerizes, and translocates to the nucleus when cells are exposed to DIF. Surprisingly, however, SH2 domain-phosphotyrosine interaction is not necessary for the DIF-induced nuclear translocation of Dd-STATc. In this respect, Dd-STATc activation resembles several recently described, noncanonical mammalian STAT signaling processes. We show instead that DIF mediates nuclear translocation via sequences located in the divergent, N-terminal half of the Dd-STATc molecule.

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

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

Protein tyrosine phosphatase PTP1 negatively regulates Dictyostelium STATa and is required for proper cell-type proportioning

The protein tyrosine phosphatase PTP1, which mediates reversible phosphorylation on tyrosine, has been shown to play an important regulatory role during Dictyostelium development. Mutants lacking PTP1 develop more rapidly than normal, while strains that overexpress PTP1 display aberrant morphology. However, the signalling pathways involved have not been characterised. In reexamining these strains, we have found that there is an inverse correlation between levels of PTP1 activity, the extent of tyrosine phosphorylation on Dictyostelium STATa after treatment with cAMP, and the proportion of the slug population exhibiting STATa nuclear enrichment in vivo. This suggests that PTP1 acts to attenuate the tyrosine phosphorylation of STATa and downstream STATa-mediated pathways. Consistent with this, we show that when PTP1 is overexpressed, there is increased expression of a prestalk cell marker at the slug posterior, a phenocopy of STATa null slugs. In ptp1 null strains, STATa tyrosine phosphorylation and nuclear enrichment in the slug anterior is increased. There is also a change in the prestalk to prespore cell ratio. Synergy experiments suggest that this is due to a cell-autonomous defect in forming the subset of prespore cells that are located in the anterior prespore region.

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.

The role of DIF-1 signaling in Dictyostelium development

We have constructed a mutant blocked in the biosynthesis of DIF-1, a chlorinated signal molecule proposed to induce differentiation of both major prestalk cell types formed during Dictyostelium development. Surprisingly, the mutant still forms slugs retaining one prestalk cell type, the pstA cells, and can form mature stalk cells. However, the other major prestalk cell type, the pstO cells, is missing. Normal pstO cell differentiation and their patterning in the slug are restored by development on a uniform concentration of DIF-1. We conclude that pstO and pstA cells are in fact induced by separate signals and that DIF-1 is the pstO inducer. Positional information, in the form of DIF-1 gradients, is evidently not required for pstO cell induction.

Cell-fate choice in Dictyostelium: intrinsic biases modulate sensitivity to DIF signaling

Cell fate in Dictyostelium development depends on intrinsic differences between cells, dating from their growth period, and on cell interactions occurring during development. We have sought for a mechanism linking these two influences on cell fate. First, we confirmed earlier work showing that the vegetative differences are biases, not commitments, since cells that are stalky-biased when developed with one partner are sporey with another. Then we tested the idea that these biases operate by modulating the sensitivity of cells to the signals controlling cell fate during development. Cells grown without glucose are stalky-biased when developed with cells grown with glucose. We find, using monolayer culture conditions, that they are more sensitive to each of the stalk-inducing signals, DIFs 1-3. Mixing experiments show that this bias is a cell-intrinsic property. Cells initiating development early in the cell cycle are stalky compared to those initiating development later in the cycle. Likewise, they are more sensitive to DIF-1. Assays of standard markers for prestalk and prespore cell differentiation reveal similar differences in DIF-1 sensitivity between biased cells; DIF-1 dechlorinase (an early prestalk cell marker enzyme) behaves in a consistent manner. We propose that cell-fate biases are manifest as differences in sensitivity to DIF.

An F-Box/WD40 repeat-containing protein important for Dictyostelium cell-type proportioning, slug behaviour, and culmination

FbxA is a novel member of a family of proteins that contain an F-box and WD40 repeats and that target specific proteins for degradation via proteasomes. In fruiting bodies formed from cells where the fbxA gene is disrupted (fbxA(-) cells), the spore mass fails to fully ascend the stalk. In addition, fbxA(-) slugs continue to migrate under environmental conditions where the parental strain immediately forms fruiting bodies. Consistent with this latter behaviour, the development of fbxA(-) cells is hypersensitive to ammonia, the signaling molecule that regulates the transition from the slug stage to terminal differentiation. The slug comprises an anterior prestalk region and a posterior prespore region and the fbxA mRNA is highly enriched in the prestalk cells. The prestalk zone of the slug is further subdivided into an anterior pstA region and a posterior pstO region. In fbxA(-) slugs the pstO region is reduced in size and the prespore region is proportionately expanded. Our results indicate that FbxA is part of a regulatory pathway that controls cell fate decisions and spatial patterning via regulated protein degradation.

Developmental changes in the spatial expression of genes involved in myosin function in Dictyostelium

We analyzed the spatial expression patterns of the genes involved in myosin function by in situ hybridization at the tipped aggregate and early culmination stages of Dictyostelium. Myosin heavy chain II mRNA was enriched in the anterior prestalk region of the tipped aggregates, whereas it disappeared from there and began to appear in both upper and lower cups of the early culminants. Similarly, mRNAs for essential light chain, regulatory light chain, myosin light chain kinase A, and myosin heavy chain kinase C were enriched in the prestalk region of the tipped aggregates. However, expression of these genes was distinctively regulated in the early culminants. These findings suggest the existence of mechanisms responsible for the expression of particular genes.

Analysis of the promoter of the cudA gene reveals novel mechanisms of Dictyostelium cell type differentiation

The cudA gene encodes a nuclear protein that is essential for normal multicellular development. At the slug stage cudA is expressed in the prespore cells and in a sub-region of the prestalk zone. We show that cap site distal promoter sequences direct cudA expression in prespore cells, while proximal sequences direct expression in the prestalk sub-region. The promoter domain that directs prespore-specific transcription consists of a positively acting region, that has the potential to direct expression in all cells within the slug, and a negatively acting region that prevents expression in the prestalk cells. Dd-STATa is the STAT protein that regulates commitment to stalk cell gene expression, where it is known to function as a transcriptional repressor. We show that Dd-STATa binds in vitro to the positively acting part of the prespore domain of the cudA promoter. However, Dd-STATa cannot be utilised for this purpose in vivo, because analysis of a Dd-STATa null mutant strain shows that Dd-STATa is not necessary for cudA transcription in prespore cells. In contrast, the part of the cudA promoter that directs prestalk-specific expression contains a binding site for Dd-STATa that is essential for its biological activity. Dd-STATa appears therefore to serve as a direct activator of cudA transcription in prestalk cells, while a protein with a DNA binding specificity highly related to that of Dd-STATa is utilised to activate cudA transcription in prespore cells.

Dictyostelium RasD is required for normal phototaxis, but not differentiation

RasD, a Dictyostelium homolog of mammalian Ras, is maximally expressed during the multicellular stage of development. Normal Dictyostelium aggregates are phototactic and thermotactic, moving towards sources of light and heat with great sensitivity. We show that disruption of the gene for rasDcauses a near-total loss of phototaxis and thermotaxis in mutant aggregates, without obvious effects on undirected movement. Previous experiments had suggested important roles for RasD in development and cell-type determination. Surprisingly, rasD− cells show no obvious changes in these processes. These cells represent a novel class of phototaxis mutant, and indicate a role for a Ras pathway in the connections between stimuli and coordinated cell movement.

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

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

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.

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.

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.

Regulation of the ribonucleotide reductase small subunit gene by DNA-damaging agents in Dictyostelium discoideum

In Escherichia coli, yeast and mammalian cells, the genes encoding ribonucleotide reductase, an essential enzyme for de novo DNA synthesis, are up-regulated in response to DNA damaging agents. We have examined the response of the rnrB gene, encoding the small subunit of ribonucleotide reductase in Dictyostelium discoideum, to DNA damaging agents. We show here that the accumulation of rnrB transcript is increased in response to methyl methane sulfonate, 4-nitroquinoline-1-oxide and irradiation with UV-light, but not to the ribonucleotide reductase inhibitor hydroxyurea. This response is rapid, transient and independent of protein synthesis. Moreover, cells from different developmental stages are able to respond to the drug in a similar fashion, regardless of the basal level of expression of the rnrB gene. We have defined the cis -acting elements of the rnrB promoter required for the response to methyl methane sulfonate and 4-nitroquinoline-1-oxide by deletion analysis. Our results indicate that there is one element, named box C, that can confer response to both drugs. Two other boxes, box A and box D, specifically conferred response to methyl methane sulfonate and 4-nitroquinoline-1-oxide, respectively.

A novel, putative MEK kinase controls developmental timing and spatial patterning in Dictyostelium and is regulated by ubiquitin-mediated protein degradation

We have identified a developmentally regulated, putative MEK kinase (MEKKalpha) that contains an F-box and WD40 repeats and plays a complex role in regulating cell-type differentiation and spatial patterning. Cells deficient in MEKKalpha develop precociously and exhibit abnormal cell-type patterning with an increase in one of the prestalk compartments (pstO), a concomitant reduction in the prespore domain, and a loss of the sharp compartment boundaries, resulting in overlapping prestalk and prespore domains. Overexpression of MEKKalpha or MEKKalpha lacking the WD40 repeats results in very delayed development and a severe loss of compartment boundaries. Prespore and prestalk cells are interspersed throughout the slug. Analysis of chimeric organisms suggests that MEKKalpha function is required for the proper induction and maintenance of prespore cell differentiation. We show that the WD40 repeats target MEKKalpha to the cortical region of the cell, whereas the F-box/WD40 repeats direct ubiquitin-mediated MEKKalpha degradation. We identify a UBC and a UBP (ubiquitin hydrolase) that interact with the F-box/WD40 repeats. Our findings indicate that cells lacking the ubiquitin hydrolase have phenotypes similar to those of MEKKalpha null (mekkalpha-) cells, further supporting a direct genetic and biochemical interaction between MEKKalpha, the UBC, and the UBP. We demonstrate that UBC and UBP differentially control MEKKalpha ubiquitination/deubiquitination and degradation through the F-box/WD40 repeats in a cell-type-specific and temporally regulated manner. Our results represent a novel mechanism that includes targeted protein degradation by which MAP kinase cascade components can be controlled. More importantly, our findings suggest a new paradigm of spatial and temporal control of the kinase activity controlling spatial patterning during multicellular development, which parallels the temporally regulated degradation of proteins required for cell-cycle progression.

Cell-type-specific rescue of myosin function during Dictyostelium development defines two distinct cell movements required for culmination

Mutant Dictyostelium cells lacking any of the component polypeptides of myosin II exhibit developmental defects. To define myosin's role in establishing Dictyostelium's developmental pattern, we have rescued myosin function in a myosin regulatory light chain null mutant (mlcR-) using cell-type-specific promoters. While mlcR- cells fail to progress beyond the mound stage, expression of RLC from the prestalk promoter, ecmA, produces culminants with normal stalks but with defects in spore cell localization. When GFP-marked prestalk and prespore cells expressing ecmA-RLC are mixed with wild-type cells, the mislocalization of prestalk cells, but not prespore cells, is rescued. Time-lapse video recording of ecmA-RLC cells showed that the posterior prespore zone failed to undergo a contraction important for the upward movement of prespore cells. Prespore cells marked with green fluorescent protein (GFP) failed to move toward the tip with the spiral motion typical of wild type. In contrast, expression of RLC in prespore cells using the psA promoter produced balloon-like structures reminiscent of sorocarps but lacking stalks. GFP-labeled prespore cells showed a spiral movement toward the top of the structures. Expression of RLC from the psA promoter restores the normal localization of psA-GFP cells, but not ecmA-GFP cells. These results define two distinct, myosin-dependent movements that are required for establishing a Dictyostelium fruiting body: stalk extension and active movement of the prespore zone that ensures proper placement of the spores atop the stalk. The approach used in these studies provides a direct means of testing the role of cell motility in distinct cell types during a morphogenetic program.

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

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

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.

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

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

A ubiquitin-conjugating enzyme is essential for developmental transitions in Dictyostelium

We have identified a developmentally essential gene, UbcB, by insertional mutagenesis. The encoded protein (UBC1) shows very high amino acid sequence identity to ubiquitin-conjugating enzymes from other organisms, suggesting that UBC1 is involved in protein ubiquitination and possibly degradation during Dictyostelium development. Consistent with the homology of the UBC1 protein to UBCs, the developmental pattern of protein ubiquitination is altered in ubcB-null cells. ubcB-null cells are blocked in the ability to properly execute the developmental transition that occurs between the induction of postaggregative gene expression during mound formation and the induction of cell-type differentiation and subsequent morphogenesis. ubcB-null cells plated on agar form mounds with normal kinetics; however, they remain at this stage for approximately 10 h before forming multiple tips and fingers that then arrest. Under other conditions, some of the fingers form migrating slugs, but no culmination is observed. In ubcB-null cells, postaggregative gene transcripts accumulate to very high levels and do not decrease significantly with time as they do in wild-type cells. Expression of cell-type-specific genes is very delayed, with the level of prespore-specific gene expression being significantly reduced compared with that in wild-type cells. lacZ reporter studies using developmentally regulated and cell-type-specific promoters suggest that ubcB-null cells show an unusually elevated level of staining of lacZ reporters expressed in anterior-like cells, a regulatory cell population found scattered throughout the aggregate, and reduced staining of a prespore reporter. ubcB-null cells in a chimeric organism containing predominantly wild-type cells are able to undergo terminal differentiation but show altered spatial localization. In contrast, in chimeras containing only a small fraction of wild-type cells, the mature fruiting body is very small and composed almost exclusively of wild-type cells, with the ubcB-null cells being present as a mass of cells located in extreme posterior of the developing organism. The amino acid sequence analysis of the UbcB open reading frame (ORF) and the analysis of the developmental phenotypes suggest that tip formation and subsequent development requires specific protein ubiquitination, and possibly degradation.

cudA: a Dictyostelium gene with pleiotropic effects on cellular differentiation and slug behaviour

The Dictyostelium cudA gene encodes a nucleoplasmic protein that is essential for normal culmination. There are no functionally characterised homologues in other organisms but there is a related gene of unknown function in Entamoeba histolytica. The cudA gene is expressed by the prestalk cells that constitute the slug tip (the pstA cells), it is not detectably expressed in the band of prestalk cells that lies behind the tip (the pstO cells) but it is expressed in the prespore cells. This unusual pattern of expression suggests a role on both the stalk and spore pathways of differentiation and cudA- mutant cells are indeed defective in both stalk and spore formation. Furthermore, the slugs formed by cudA- cells continue to migrate under environmental conditions where normal slugs culminate immediately. This aspect of their behaviour can be reversed when the cudA gene is selectively expressed in the pstA cells. This shows that processes occurring in the pstA cells regulate entry into culmination.

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

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

Characterization of a Dictyostelium factor that acts synergistically with DIF to induce terminal stalk cell differentiation

The differentiation of Dictyostelium prestalk cells is induced by the chlorinated hexaphenone DIF-1 and their maturation into stalk cells at culmination occurs by activation of the cAMP-dependent protein kinase (PKA). Medium harvested from developing Dictyostelium cells will act synergistically with DIF-1 to induce prestalk cell differentiation in a low-density monolayer assay (Yamada and Okamoto, 1994). Using HPLC, we have partially purified from such conditioned medium an activity we term STIF (stalk-inducing factor). It is hydrophilic and of low molecular weight. There are multiple classes of prestalk cells, which are defined by their patterns of expression of the ecmA and ecmB genes and that can be further subcategorized because they utilize different elements within the promoters of the two genes. We show that, in a monolayer assay, STIF acts synergistically with DIF-1 to induce ecmB gene expression via promoter elements that are normally activated strongly only in cells that have entered the stalk tube and which are therefore committed to differentiate into stalk cells. The combination of STIF and DIF-1 also induces morphological maturation of prestalk cells into stalk cells but does not efficiently induce expression of ecmA: a gene that is selectively expressed in cells within the anterior, prestalk region of the slug. Inactivation of PKA, by cell type-specific expression of a dominant inhibitor, represses the action of STIF. These data suggest that STIF is an extracellular signal that acts to induce the terminal differentiation of stalk cells.

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.

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.

RtoA links initial cell type choice to the cell cycle in Dictyostelium

In Dictyostelium, initial cell type choice is correlated with the cell-cycle phase of the cell at the time of starvation. We have isolated a mutant, ratioA (rtoA), with a defect in this mechanism that results in an abnormally high percentage of prestalk cells. The rtoA gene has been cloned and sequenced and codes for a novel protein. The cell cycle is normal in rtoA. In the wild type, prestalk cells differentiate from those cells in S or early G2 phase at starvation and prespore cells from cells in late G2 or M phase at starvation. In rtoA mutants, both prestalk and prespore cells originate randomly from cells in any phase of the cell cycle at starvation.

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.

Evolution of promoter sequences: elements of a canonical promoter for prespore genes of Dictyostelium

An attempt is made to define a minimal prespore promoter which contains all elements essential for correct regulation of expression of a prespore gene. The prespore genes of Dictyostelium are coregulated during development. Most begin transcription at the same early stage, and activity of all is restricted to prespore tissue during the later slug stage. Sequences 5' to the coding sequences of eight prespore genes were searched for all elements proposed to control transcription and for new elements. The meaningfulness of occurrences of elements and pairs of elements in prespore promoters was evaluated by comparison with frequencies of occurrences in promoters of other, nonprespore genes. These comparisons resulted in definition of a canonical prespore promoter, a stretch of about 200 nucleotides containing at least one of each of three elements. Certain limitations were found on the spacing of elements. Orientation of elements with respect to each other appeared unrestricted. All elements often occurred in multiple copies. This structure suggests that individual copies of each element are not conserved during evolution, but instead continually appear and disappear.

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.

A mutational analysis of Dictyostelium discoideum multicellular development

We have collectedDictyosteliummutants that arrest in development after aggregation, but before first finger formation. A total of 118 mutant strains were isolated and are referred to as mound(mnd)mutants. Nine complementation groups(mndA-mndl), containing 46 of the mutant strains, were defined by parasexual methods. A statistical analysis suggested that there are about 118 genes which, when mutated, confer the mound phenotype. Of these genes, about 60 are predicted to be mutated in our collection: the 9 assigned to complementation groups and another 51 unassigned mutants.mndA, G, HandIwere assigned to linkage groups VII, IV, II and VI, respectively. Development of the mutant strains was characterized by terminal morphology, neutral red staining and expression of marker mRNAs for prespore and prestalk cells. Three broad classes were recognized. (1) Postaggregative mutants - those blocked early in multicellular development. They did not express any of the prestalk or prespore marker mRNAs and generally arrested as low mounds or ridges. (2) Pathway mutants - those blocked specifically in either prestalk or prespore differentiation. They expressed either prestalk or prespore marker mRNAs, but not both, and generally proceeded further morphologically than post-aggregative mutants. (3) Morphogenesis mutants - those apparently blocked in morphogenesis rather than cell differentiation. They expressed all the cell-type marker mRNAs tested. Most arrested as tight mounds lacking a tip and of defined upper size, but some mutants produced aberrant tips. The majority of mutants tested synergized with wild-type: 24/28 strains which cannot make spores when developed alone, were able do so when allowed to develop with an equal number of wild-type cells. We suggest that some of the morphogenesis mutants have a cytoskeletal defect which prevents first finger formation and that these mutants can be physically carried through development by the wildtype (synergy by ‘piggy-backing’).

The initiation of basal disc formation in Dictyostelium discoideum is an early event in culmination

We have analysed expression of the ecmA and ecmB genes of Dictyostelium by enzymatic double staining using beta-galactosidase and beta-glucuronidase reporter gene constructs. Cells expressing the ecmA gene first appear as scattered cells at the mound stage of development and we show that this is also true for cells expressing the ecmB gene. During tip formation the ecmA-expressing cells move to the apex of the mound, while the ecmB-expressing cells accumulate in the base. The ecmB-expressing cells constitute part of the basal disc if the culminant is formed in situ but are discarded if a migratory slug is formed. During slug migration they are replaced by a band of ecmB-expressing cells, situated in the front half of the prespore zone and tightly apposed to the substratum. When culmination is triggered these cells rapidly move to the back half of the prestalk zone, possibly acting as a point of attachment to the substratum. Ultimately, they are joined by cells at the back of the slug, the rearguard cells, to form the basal disc. Thus, contrary to previous belief, basal disc formation is initiated very early during culmination and occurs by the forward movement of cells located in the anterior of the prespore zone.

The Dictyostelium MAP kinase ERK2 regulates multiple, independent developmental pathways

We showed previously that the MAP kinase ERK2 is essential for aggregation. erk2 null cells lack cAMP stimulation of adenylyl cyclase and thus cannot relay the cAMP chemotactic signal, although the cells chemotax to cAMP (Segall et al. 1995). In this paper we have examined the role of ERK2 in controlling developmental gene expression and morphogenesis during the multicellular stages, making use of a temperature-sensitive ERK2 mutation. Using suspension assays, we show that ERK2 is not essential for aggregation-stage, cAMP pulse-induced gene expression, or for the expression of postaggregative genes, which are induced at the onset of mound formation in response to cAMP in wild-type cells. In contrast, the prespore-specific gene SP60 is not induced and the prestalk-specific gene ecmA is induced but at a significantly reduced level. Chimeric organisms, comprised of wild-type and erk2 null cells expressing the prestalk-specific ecmA/lacZ reporter, show an abnormal spatial patterning, in which Erk2ts/erk2 cells are excluded from the very anterior prestalk A region. To further examine the function of ERK2 during the multicellular stages, we bypassed the requirement of ERK2 for aggregation by creating an ERK2 temperature-sensitive mutant. erk2 null cells expressing the ERK2ts mutant develop normally at 20 degrees C and express cell-type-specific genes but do not aggregate at temperatures above 25 degrees C. Using temperature shift experiments, we showed that ERK2 is essential for proper morphogenesis and for the induction and maintenance of prespore but not prestalk gene expression. Our results indicate that ERK2 functions at independent stages during Dictyostelium development to control distinct developmental programs: during aggregation, ERK2 is required for the activation of adenylyl cyclase and during multicellular development, ERK2 is essential for morphogenesis and cell-type-specific gene expression. Analysis of these results and other supports the conclusion that the requirement of ERK2 for cell-type differentiation is independent of its role in the activation of adenylyl cyclase.

Analysis of gene function in Dictyostelium

Over the past ten years, powerful molecular genetic techniques have been developed to analyze gene function in Dictyostelium. DNA-mediated transformation using a variety of selections and vectors has allowed the introduction of wild-type or modified genes that are under various forms of transcriptional control. Homologous recombination is efficient and can be used to modify the genome in precise ways. In addition, it is now possible to clone genes based on their mutant phenotype alone, either by insertional mutagenesis, or by screening antisense expression cDNA libraries. Finally, a nearly complete physical map of the genome is available and so genes are easily mapped by physical techniques. We discuss many of these advances within the context of major research problems presently under study.

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.

Evidence for positional differentiation of prestalk cells and for a morphogenetic gradient in Dictyostelium

We present evidence that Dictyostelium slug tip cells, the pstA cells, may arise by positional differentiation, but at a site remote from that which they will eventually occupy. When first detectable, the pstA cells form a peripheral ring surrounding the other prestalk cell subtype, the pstO cells, but subsequently move above the pstO cells to form the tip. Because pstA cell differentiation requires a 10-fold higher concentration of differentiation-inducing factor, the stalk cell inducer, the initial patterning seems likely to reflect the existence of a morphogenetic gradient. The subsequent redistribution of the two cell types is explicable by their different rates of chemotaxis to cyclic AMP. These results help reconcile the two apparently opposing views of pattern formation in Dictyostelium, that there is positional differentiation and that pattern formation occurs by cell sorting.

Targeted disruption of the Dictyostelium myosin essential light chain gene produces cells defective in cytokinesis and morphogenesis

We have previously demonstrated that the myosin essential light chain (ELC) is required for myosin function in a Dictyostelium cell line, 7–11, in which the expression of ELC was inhibited by antisense RNA overexpression. We have now disrupted the gene encoding the ELC (mlcE) in Dictyostelium by gene targeting. The mlcE- mutants provide a clean genetic background for phenotypic analysis and biochemical characterization by removing complications arising from the residual ELC present in 7–11 cells, as well as the possibility of mutations due to insertion of the antisense construct at multiple sites in the genome. The mlcE- mutants, when grown in suspension, exhibited the typical multinucleate phenotype observed in both myosin heavy chain mutants and 7–11 cells. This phenotype was rescued by introducing a construct that expressed the wild-type Dictyostelium ELC cDNA. Myosin purified from the mlcE- cells exhibited significant calcium ATPase activity, but the actin-activated ATPase activity was greatly reduced. The results obtained from the mlcE- mutants strengthen our previous conclusion based on the antisense cell line 7–11 that ELC is critical for myosin function. The proper localization of myosin in mlcE- cells suggests that its phenotypic defects primarily arise from defective contractile function of myosin rather than its mislocalization. The enzymatic defect of myosin in mlcE- cells also suggests a possible mechanism for the observed chemotactic defect of mlcE- cells. We have shown that while mlcE- cells were able to respond to chemoattractant with proper directionality, their rate of movement was reduced. During chemotaxis, proper directionality toward chemoattractant may depend primarily on proper localization of myosin, while efficient motility requires contractile function. In addition, we have analyzed the morphogenetic events during the development of mlcE- cells using lacZ reporter constructs expressed from cell type specific promoters. By analyzing the morphogenetic patterns of the two major cell types arising during Dictyostelium development, prespore and prestalk cells, we have shown that the localization of prespore cells is more susceptible to the loss of ELC than prestalk cells, although localization of both cell types is abnormal when developed in chimeras formed by mixing equal numbers of wild-type and mutant cells. These results suggest that the morphogenetic events during Dictyostelium development have different requirements for myosin.