A spatial gradient of expression of a cAMP-regulated prespore cell-type-specific gene in Dictyostelium

Loss of cAMP-specific phosphodiesterase rescues spore development in G protein mutant in dictyostelium

Cyclic AMP (cAMP) is an important intracellular signaling molecule for many G protein-mediated signaling pathways but the specificity of cAMP signaling in cells with multiple signaling pathways is not well-understood. In Dictyostelium, at least two different G protein signaling pathways, mediated by the Gα2 and Gα4 subunits, are involved with cAMP accumulation, spore production, and chemotaxis and the stimulation of these pathways results in the activation of ERK2, a mitogen-activated protein kinase that can down regulate the cAMP-specific phosphodiesterase RegA. The regA gene was disrupted in gα2(−) and gα4(−) cells to determine if the absence of this phosphodiesterase rescues the development of these G protein mutants as it does for erk2(−) mutants. There gA(−) mutation had no major effects on developmental morphology but enriched the distribution of the Gα mutant cells to the prespore/prestalk border in chimeric aggregates. The loss of RegA function had no effect on Gα4- mediated folate chemotaxis. However, the regA gene disruption in gα4(−) cells, but not in gα2(−) cells, resulted in a substantial rescue and acceleration of spore production. This rescue in sporulation required cell autonomous signaling because the precocious sporulation could not be induced through intercellular signaling in chimeric aggregates. However, intercellular signals from regA(−) strains increased the expression of the prestalk gene ecmB and accelerated the vacuolization of stalk cells. Intercellular signaling from the gα4(−)regA(−) strain did not induce ecmA gene expression indicating cell-type specificity in the promotion of prestalk cell development. regA gene disruption in a Gα4(HC) (Gα4 overexpression) strain did not result in precocious sporulation or stalk cell development indicating that elevated Gα4 subunit expression can mask regA(−) associated phenotypes even when provided with wild-type intercellular signaling. These findings indicate that the Gα2 and Gα4-mediated pathways provide different contributions to the development of spores and stalk cells and that the absence of RegA function can bypass some but not all defects in G protein regulated spore development.

Transcriptional regulation of Dictyostelium pattern formation

On starvation, Dictyostelium cells form a terminally differentiated structure, known as the fruiting body, which comprises stalk and spore cells. Their precursors--prestalk and prespore cells--are spatially separated and accessible in a migratory structure known as the slug. This simplicity and manipulability has made Dictyostelium attractive to both experimental and theoretical developmental biologists. However, this outward simplicity conceals a surprising degree of developmental sophistication. Multiple prestalk subtypes are formed and undertake a co-ordinated series of morphogenetic cell movements to generate the fruiting body. This review describes recent advances in understanding the signalling pathways that generate prestalk-cell heterogeneity, focusing on the roles of the prestalk-cell inducer differentiation-inducing factor-1 (DIF-1), the tip inducer cAMP and the transcription factors that mediate their actions; these include signal transducer and activator of transcription (STAT) proteins, basic leucine zipper (bZIP) proteins and a Myb protein of a class previously described only in plants.

Reduced glutathione levels affect the culmination and cell fate decision in Dictyostelium discoideum

Glutaredoxins have been known to be glutathione-dependent oxidoreductases that participate in the redox regulation of various cellular processes. To understand the role of glutaredoxins in the development, we examined glutaredoxin 1 (Grx1) of Dictyostelium discoideum. Its mRNA was highly accumulated at the mound and the culmination stages. When Grx1-overexpressing cells were developed, their culmination was delayed, and the expression of marker genes for prespore and spore decreased. Interestingly, they had about 1.5-fold higher amount of reduced glutathione (GSH) compared with parental cells and their prolonged migration was repressed by the oxidant such as hydrogen peroxide. To confirm the effect of GSH on the culmination, glutathione reductase (Gsr) was overexpressed or underexpressed. Similar to Grx1-overexpressing cells, Gsr-overexpressing cells contained about 1.5-fold higher amount of GSH and exhibited the delayed culmination. In contrast, the knockdown mutant of Gsr had nearly 50% lower amount of GSH and showed accelerated culmination. Taken together, these data suggest that the culmination of Dictyostelium is controlled by GSH. In addition, the cells having higher GSH levels showed a prestalk tendency in the chimeric slugs with parental cells, indicating that the difference in the amount of GSH may affect the determination of cell fate.

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.

New prestalk and prespore inducing signals in Dictyostelium

The differentiation-inducing signals (DIFs) currently known in Dictyostelium appear unable to account for the full diversity of cell types produced in development. To search for new signals, we analyzed the differentiation in monolayers of cells expressing prestalk (ecmAO, ecmA, ecmO, ecmB and cAR2) and prespore (psA) markers. Expression of each marker drops off as the cell density is reduced, suggesting that cell interaction is required. Expression of each marker is inhibited by cerulenin, an inhibitor of polyketide synthesis, and can be restored by conditioned medium. However, the known stalk-inducing polyketide, DIF-1, could not replace conditioned medium and induce the ecmA or cAR2 prestalk markers, suggesting that they require different polyketide inducers. Polyketide production by fungi is stimulated by cadmium ions, which also dramatically stimulates differentiation in Dictyostelium cell cultures and the accumulation of medium factors. Factors produced with cadmium present were extracted from conditioned medium and fractionated by HPLC. A new factor inducing prespore cell differentiation, called PSI-2, and two inducing stalk cell differentiation (DIFs 6 and 7) were resolved. All are distinct from currently identified factors. DIF-6, but not DIF-7 or PSI-2, appears to have an essential carbonyl group. Thus Dictyostelium may use extensive polyketide signaling in its development.

NADPH oxidase homologs are required for normal cell differentiation and morphogenesis in Dictyostelium discoideum

Membrane-associated NADPH oxidase complexes catalyse the production of the superoxide anion radical from oxygen and NADPH. In mammalian systems, NADPH oxidases form a family of at least seven isoforms that participate in host defence and signalling pathways. We report here the cloning and the characterisation of slime mould Dictyostelium discoideum homologs of the mammalian heme-containing subunit of flavocytochrome b (gp91(phox)) (NoxA, NoxB and NoxC), of the small subunit of flavocytochrome b (p22(phox)) and of the cytosolic factor p67(phox). Null-mutants of either noxA, noxB, noxC or p22(phox) show aberrant starvation-induced development and are unable to produce spores. The overexpression of NoxA(myc2) in noxA null strain restores spore formation. Remarkably, the gene alg-2B, coding for one of the two penta EF-hand proteins in Dictyostelium, acts as a suppressor in noxA, noxB, and p22(phox) null-mutant strains. Knockout of alg-2B allows noxA, noxB or p22(phox) null-mutants to return to normal development. However, the knockout of gene encoding NoxC, which contains two penta EF-hands, is not rescued by the invalidation of alg-2B. These data are consistent with a hypothesis connecting superoxide and calcium signalling during Dictyostelium development.

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

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

Dd-Alix, a conserved endosome-associated protein, controls Dictyostelium development

We have characterized the Dictyostelium homolog of the mammalian protein Alix. Dd-Alix is encoded by a single gene and is expressed during vegetative growth and multicellular development. We showed that the alx null strain fails to complete its developmental program. Past the tight aggregate stage, morphogenesis is impaired, leading to markedly aberrant structures containing vacuolated and undifferentiated cells but no mature spores. The developmental defect is cell-autonomous as most cells remain of the PstB type even when mixed with wild-type cells. Complementation analysis with different Alix constructs allowed the identification of a 101-residue stretch containing a coiled-coil domain essential for Alix function. In addition, we showed that the protein associates in part with vesicular structures and that its distribution on a Percoll gradient overlaps that of the endocytic marker Vamp7. Dd-Alix also co-localizes with Dd-Vps32. In view of our data, and given the role of Vps32 proteins in membrane protein sorting and multivesicular body formation in yeast and mammals, we hypothesize that the developmental defects of the alx null strain result from abnormal trafficking of cell-surface receptors.

Talin B is required for force transmission in morphogenesis of Dictyostelium

Talin plays a key role in the assembly and stabilisation of focal adhesions, but whether it is directly involved in force transmission during morphogenesis remains to be elucidated. We show that the traction force of Dictyostelium cells mutant for one of its two talin genes talB is considerably smaller than that of wild-type cells, both in isolation and within tissues undergoing morphogenetic movement. The motility of mutant cells in tightly packed tissues in vivo or under strong resistance conditions in vitro was lower than that of wild-type cells, but their motility under low external force conditions was not impaired, indicating inefficient transmission of force in mutant cells. Antibody staining revealed that the talB gene product (talin B) exists as small units subjacent to the cell membrane at adhesion sites without forming large focal adhesion-like assemblies. The total amount of talin B on the cell membrane was larger in prestalk cells, which exert larger force than prespore cells during morphogenesis. We conclude that talin B is involved in force transmission between the cytoskeleton and cell exterior.

Tissue-specific G1-phase cell-cycle arrest prior to terminal differentiation in Dictyostelium

The cell cycle status of developing Dictyostelium cells remains unresolved because previous studies have led to conflicting interpretations. We propose a new model of cell cycle events during development. We observe mitosis of about 50% of the cells between 12 and 18 hours of development. Cellular DNA content profiles obtained by flow cytometry and quantification of extra-chromosomal and chromosomal DNA suggest that the daughter cells have half the chromosomal DNA of vegetative cells. Furthermore, little chromosomal DNA synthesis occurs during development, indicating that no S phase occurs. The DNA content in cells sorted by fluorescent tissue-specific reporters indicates that prespore cells divide before prestalk cells and later encapsulate as G1-arrested spores. Consistent with this, germinating spores have one copy of their chromosomes, as judged by fluorescence in situ hybridization and they replicate their chromosomes before mitosis of the emergent amoebae. The DNA content of mature stalk cells suggests that they also attain a G1 state prior to terminal differentiation. As prestalk cells appear to be in G2 up to 22 hours of development, our data suggest that they divide just prior to stalk formation. Our results suggest tissue-specific regulation of G1 phase cell cycle arrest prior to terminal differentiation in Dictyostelium.

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

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

A novel developmental mechanism in Dictyostelium revealed in a screen for communication mutants

We performed a screen for signaling genes by selecting mutant strains of Dictyostelium that fail to develop spores in a pure population but sporulate well in chimerae with wild type cells. We found 9 strains whose sporulation was induced up to 10 million-fold in chimerae. Most strains were also able to sporulate in chimerae with each other, but 2 pairs failed to do so, suggesting that the genes in each pair participate in the production of 1 signal. One of the pairs, comD and comB, is described in detail. Sequence analysis revealed that both genes encode putative membrane proteins. ComD is predicted to have 15 transmembrane domains, and ComB has a region of high similarity to the Rab family of small GTPases and 1 transmembrane domain. Similarities between the developmental regulation and cell-type specificity of the genes' expression, the terminal developmental morphology, and the expression pattern of cell-type specific markers in the mutants suggest that comD and comB participate in 1 signal production pathway. This idea is also supported by a high similarity between the global transcriptional profiles of the mutant strains. Differences between the mutant phenotypes late in development suggest that comD and comB participate in separate processes as well. comD has a cell-autonomous role in the specialization of a novel prespore cell type, whereas comB has a cell-autonomous role in prestalk A cell differentiation.

TagA, a putative serine protease/ABC transporter of Dictyostelium that is required for cell fate determination at the onset of development

The tag genes of Dictyostelium are predicted to encode multi-domain proteins consisting of serine protease and ATP-binding cassette transporter domains. We have identified a novel tag gene, tagA, which is involved in cell type differentiation. The tagA mRNA accumulates during the first four hours of development, whereas TagA protein accumulates between two and ten hours of development and decreases thereafter. Wild-type cells express tagA in prespore cells and mature spores, defining tagA expression as prespore specific. However, tagA mutant cells that activate the tagA promoter do not sporulate, but instead form part of the outer basal disc and lower cup of the fruiting body. tagA mutant aggregates elaborate multiple prestalk cell regions during development and produce spores asynchronously and with low viability. tagA mutants produce about twice as many prestalk cells as the wild type as judged by a prestalk cell reporter construct. When mixed with wild-type cells, tagA(-) cells become overrepresented in the prestalk cell population, suggesting that this phenotype is cell-autonomous. These results suggest that TagA is required for the specification of an initial population of prespore cells in which tagA is expressed. Expression profiling uncovered a delay in the transcriptional program between 2 and 6 hours, coincident with TagA expression, revealing an early function for TagA. TagA also appears to play a general role in cell fate determination since tagA mutants express a spore coat protein gene (cotB) within vacuolated cells that form part of the stalk and they express a prestalk/stalk-specific gene (ecmB) within cells that become spores. The expression of TagA at two hours of development, the observed coincident delay in the transcriptional program and the subsequent mis-expression of cell-type specific genes provide evidence for cell fate determination beginning in some cells much earlier than previously believed.

Cross-induction of cell types in Dictyostelium: evidence that DIF-1 is made by prespore cells

To investigate how cell type proportions are regulated during Dictyostelium development, we have attempted to find out which cell type produces DIF-1, a diffusible signal molecule inducing the differentiation of prestalk-O cells. DIF-1 is a chlorinated alkyl phenone that is synthesized from a C12 polyketide precursor by chlorination and methylation, with the final step catalysed by the dmtA methyltransferase. All our evidence points to the prespore cells as the major source of DIF-1. (1) dmtA mRNA and enzyme activity are greatly enriched in prespore compared with prestalk cells. The chlorinating activity is also somewhat prespore-enriched. (2) Expression of dmtA is induced by cyclic-AMP and this induction is inhibited by DIF-1. This regulatory behaviour is characteristic of prespore products. (3) Short-term labelling experiments, using the polyketide precursor, show that purified prespore cells produce DIF-1 at more than 20 times the rate of prestalk cells. (4) Although DIF-1 has little effect on its own synthesis in short-term labelling experiments, in long-term experiments, using 36Cl– as label, it is strongly inhibitory (IC50 about 5 nM), presumably because it represses expression of dmtA; this is again consistent with DIF-1 production by prespore cells. Inhibition takes about 1 hour to become effective.

We propose that prespore cells cross-induce the differentiation of prestalk-O cells by making DIF-1, and that this is one of the regulatory loops that sets the proportion of prespore-to-prestalk cells in the aggregate.

Functional and regulatory analysis of the dictyostelium G-box binding factor

The Dictyostelium discoidium G-box binding factor (GBF) is required for the induction of known postaggregative and cell-type-specific genes. gbf-null cells undergo developmental arrest at the loose-mound stage due to the absence of GBF-targeted gene transcription. GBF-mediated gene expression is activated by stimulation of cell-surface, seven-span cAMP receptors, but this activation is independent of heterotrimeric G-proteins. To further characterize GBF, we assayed a series of GBF mutants for their ability to bind a G-box in vitro and to complement the gbf-null phenotype. In vitro DNA-binding activity resides in the central portion of the protein, which contains two predicted zinc fingers. However, in vivo GBF function requires only one intact zinc finger. In addition, expression of some GBF mutants results in a partial complementation phenotype, suggesting that these mutants are hypomorphic alleles. We used a 2.4-kb GBF-promoter fragment to examine the regulation of GBF expression. GBF promoter-reporter studies confirmed the previous finding that GBF transcription is induced by continuous, micromolar extracellular cAMP. We also show that, like the activation of GBF-regulated transcription, the induction of GBF expression requires cell-surface cAMP receptors, but not heterotrimeric G-proteins. Finally, reporter studies demonstrated that induction of GBF-promoter-regulated expression does not require the presence of GBF protein, indicating that GBF expression is not regulated by a positive autoregulatory loop.

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.

Signalling molecules involved in cellular differentiation during Dictyostelium morphogenesis

GSK-3, Dd-STATa, PKA, rZIP and Ras all play important roles in cell type determination of Dictyostelium discoideum. The fact that homologs of these proteins also function in metazoan development emphasizes the importance of Dictyostelium as a model microbial organism for studying the molecular mechanisms that regulate development. The recent elaboration of the central role for GSK-3 in cell type determination has been of particular importance. The stimulatory effect of extracellular cAMP on GSK-3 activity has been shown to act through the cell surface receptor cAR3 and a tyrosine protein kinase ZAK1, which directly activates and phosphorylates GSK-3. Several proteins, including Dd-STATa, have been identified as substrates for GSK-3, and are therefore potential transducers of the signals involved in cell type determination.

Spatial and temporal expression of a Polysphondylium spore-specific gene

In the cellular slime mold Polysphondylium spherical masses of cells are periodically released from the base of the culminating sorogen. These whorls undergo a morphogenetic transformation from spherical to radial symmetry, marked by the early emergence of a radially symmetric prepattern on the whorl surface. In previous experiments, morphogenesis was followed by observing prestalk cell markers. Here we describe the isolation and characterization of a spore coat gene whose expression pattern is the negative image of the prestalk pattern. To study the molecular mechanism of sp-45 gene regulation, we have cloned and analyzed the sp-45 promoter. Deletion analysis localized a single positive regulatory element (PRE) to a 106-bp fragment between positions -246 and -352 of the upstream coding sequence. This fragment can be further divided into a promoter-proximal and promoter-distal PRE and a 29-bp sequence between them. The distal PRE can regulate prespore expression when fused to a nonfunctioning basal promoter. The distal PRE contains two adjacent essential elements, a Gr box (GTGATATAGTGG) and a TA box (TAATATATT). Each element can drive prespore cell-specific reporter gene expression independently when incorporated into a nonfunctional promoter. Our results also show that prespore cell-specific gene expression is solely under positive regulation, with no evidence for spore-specific enhancers or cis-acting negative regulatory elements. By fusing GFP to the C-terminus of sp-45, we have demonstrated that the graded gene expression of SP45 in the sorogen is regulated by a sequence lying within the sp-45 coding sequence. The temporal and spatial expression pattern of this protein, taken together with the prestalk expression pattern, demonstrates unambiguously that the radial symmetries that emerge in the whorl are established by a system of positional coordinates and that cell sorting plays little if any role in this process.

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

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

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.

DIF signalling and cell fate

The DIFs are a family of secreted chlorinated molecules that control cell fate during development of Dictyostelium cells in culture and probably during normal development too. They induce stalk cell differentiation and suppress spore cell formation. The biosynthetic and inactivation pathways of DIF-1 (the major bioactivity) have been worked out. DIF-1 is probably synthesised in prespore cells and inactivated in prestalk cells, by dechlorination. Thus, each cell type tends to alter DIF-1 level so as to favour differentiation of the other cell type. This relationship leads to a model for cell-type proportioning during normal development.

Identification of cis-regulating elements and trans-acting factors regulating the expression of the gene encoding the small subunit of ribonucleotide reductase in Dictyostelium discoideum

We have examined the promoter of rnrB, the gene encoding the small subunit of ribonucleotide reductase of Dictyostelium discoideum, using lacZ as a reporter gene. Deletion analysis showed that expression of this gene in vegetative cells involves an A/T-rich element, whereas its expression in prespore cells during development requires a region encompassing two G/C-rich elements, designated box A and box B. Removal of boxes A and B results in very low level of activity. When either box A or box B is deleted, prestalk cells adjacent to the prespore zone also express beta-galactosidase. The behavior of these cis-regulatory elements implies that the mechanism regulating the prespore-specific expression of rnrB is different from that regulating other known prespore genes. We have used electrophoretic mobility shift assays to identify factors that interact with box A and box B. Box A interacts with a factor that is found in the nuclear fraction. While box B interacts with a factor that is present in the cytosolic fraction throughout growth and development, its presence in the nuclear fraction is developmentally regulated. Results from competition assays suggest that both box A and box B interact with transcriptional activators that have not been characterized previously.

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

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

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.

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.

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.

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.

Control of cell-type specific gene expression in Dictyostelium by the general transcription factor GBF

To understand how positional information within an organism specifies patterning during development, we are analyzing spatially regulated gene expression in Dictyostelium. CAR3 is a member of the cAMP, 7-span receptor family which directs the transition from unicellular to multicellular organism and regulates cellular differentiation and pattern formation. CAR3 mRNA is expressed maximally at 8–10 hours of development, as individual cells aggregate and differentiate, and is accumulated to equivalent levels in all cells. CAR3 is also induced in shaking cultures by response to extracellular cAMP. We now show, by extensive mutagenesis, that the maximum length of contiguous sequences required for accurate spatiotemporal regulation of CAR3 is approx. 350 bp. These sequences include three significant elements located in upstream and transcribed regions. Arrays of G-boxes (GBF regulatory sites) are centered near positions −165 and +50 and, although either is sufficient for induction by cAMP and expression in prespore cells, both are required for expression in prestalk cells. Another GC-rich element near position −80 is required for maximal expression of prespore-specific constructs, although full-length promoters carrying clustered mutations through the −80 region are still expressed in all cells, but with slightly reduced expression. Spatiotemporal expression of CAR3 during development, thus, requires cell-specific combinatorial interactions of multiple but redundant regulatory components. These essential elements are located in upstream and transcribed regions. However, most surprisingly, a primary control for spatial patterning of CAR3 expression appears to be mediated by GBF, a general transcription factor expressed ubiquitously during Dictyostelium development following early aggregation.

Temporal regulation of the Dictyostelium glycogen phosphorylase 2 gene

The product of the glycogen phosphorylase-2 gene in Dictyostelium functions to provide the glucose units that are used to construct the structural components of the terminal stage of development. In this report, we link a 1233 bp upstream gp2 fragment to a luciferase reporter gene in order to study the sequences that are involved in the temporal expression of the gene. Various deletions of the promoter-luciferase fusion were then transformed into Dictyostelium cells. All deletion constructs, from -1216 to -486 nucleotides from the translational start codon, showed the same temporal pattern of expression as the authentic gp2 gene, as well as similar luciferase activities. Removal of an additional 37 nucleotides resulted in nearly 100-fold decrease in activity, yet retained the normal temporal expression of luciferase. Analysis of DNA binding proteins with the gel shift assay revealed a stage-dependent pattern of proteins that bound to the gp2 promoter. A similar pattern of temporal expression of the binding proteins was observed with either the full-length probe or with oligonucleotide probes that contained sequences that were identified as putative regulatory sites. Likewise, the full-length and oligonucleotide probes demonstrated identical binding patterns during several steps of purification of the DNA binding proteins. SDS-PAGE and Southwestern blot analysis of a DNA-affinity purified fraction, identified a 23 kDa peptide as the binding protein.

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.

Clathrin heavy chain is required for spore cell but not stalk cell differentiation in Dictyostelium discoideum

Previous studies of a clathrin-minus Dictyostelium cell line revealed important roles for clathrin heavy chain (clathrin) in endocytosis, secretion of lysosomal hydrolases and osmoregulation. In this paper, we examine the contribution of clathrin-mediated membrane traffic to development in Dictyostelium discoideum. Clathrin-minus cells were delayed in early development. When exposed to starvation conditions, clathrin-minus cells streamed and aggregated more slowly than wild-type cells. Although clathrin-minus cells displayed only 40% the level of extracellular cyclic AMP binding normally found in wild-type cells, they responded chemotactically to extracellular cyclic AMP. Clathrin-minus cells down-regulated cyclic AMP receptors, but only to half the extent of wild-type cells. We found that the extent of development of clathrin-minus cells was variable and influenced by environmental conditions. Although the mutant cells always progressed beyond the tipped mound stage, the final structure varied from a finger-like projection to a short, irregular fruiting body. Microscopic examination of these terminal structures revealed the presence of intact stalks but a complete absence of spores. Clathrin-minus cells expressed prestalk (ecmA and ecmB) and prespore (psA and cotB) genes normally, but were blocked in expression of the sporulation gene spiA. Using clathrin-minus cells that had been transformed with various promoter-lacZ reporter constructs, we saw only partial sorting of clathrin-minus prestalk and prespore cells. Even when mixed with wild-type cells, clathrin-minus cells failed to sort correctly and never constructed functional spores. These results suggest three roles for clathrin during Dictyostelium development. First, clathrin increases the efficiency of early development. Second, clathrin enables proper and efficient patterning of prestalk and prespore cells during culmination. Third, clathrin is essential for differentiation of mature spore cells.

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.

Multiple roles of the novel protein tyrosine phosphatase PTP3 during Dictyostelium growth and development

PTP3, the third nonreceptor protein tyrosine phosphatase identified in Dictyostelium discoideum, has a single catalytic protein tyrosine phosphatase domain. Recombinant PTP3 exhibited phosphatase activity that was inhibited by vanadate. PTP3 is expressed at a moderate level during growth. The level of transcripts increased between growth and 8 h of development and declined thereafter. Expression of lacZ under the control of the PTP3 promoter indicated a spatial localization of PTP3 in the anterior-like and prestalk cell types. There are two copies of the PTP3 gene in this haploid organism. Disruption of one copy led to a slow-growth phenotype. We were unable to obtain a strain with disruptions in both PTP3 genes. Overexpression of wild-type PTP3 led to slower growth rates and the formation of large aggregation streams. These streams split into smaller aggregates, many of which then arrested in development. Overexpression of a catalytically inactive mutation (Cys to Ser) had no effect on growth rate; however, this strain also formed large aggregation streams that later split up into large and small mound structures and became fruiting bodies of various sizes. Antiphosphotyrosine Western blot (immunoblot) analysis of total cell proteins showed that the pattern of protein tyrosine phosphorylation was specifically altered in PTP3 mutants. Addition of growth medium to starving cells and a subsequent replacement with nonnutrient buffer led to reciprocal changes in the pattern of several phosphotyrosine proteins, including a protein of approximately 130 kDa. Analysis of strains overexpressing active or inactive PTP3 suggested that p130 is a potential substrate of PTP3. A transient posttranslational phosphorylation of PTP3 further supported the role of PTP3 in these processes. The data obtained strongly suggest new regulatory functions for PTP3 that are distinct from those described earlier for D. discoideum PTP1 and PTP2.

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.

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.

PkpA, a novel Phycomyces blakesleeanus serine/threonine protein kinase

This work reports the cloning and sequencing of pkpA, a gene of the filamentous fungus Phycomyces blakesleeanus, whose expression seems to be coupled to vegetative growth. This gene encodes a putative serine/threonine-specific protein kinase, whose sequence is related to that of the yeast protein STE20, involved in pheromone-response pathways, and to a number of MAPK kinase proteins. However, detailed analysis of the kinase sequence suggests that PkpA is a novel serine/threonine protein kinase that probably participates as an intermediate in an intracellular system controlling nuclear proliferation in P. blakesleeanus.

Morphogenesis in Dictyostelium: new twists to a not-so-old tale

Prespore differentiation requires both cAMP-dependent protein kinase and the transcription factor GBF, and for one class of prespore genes the two form part of a single pathway. It seems that differentiation-inducing factor, the inducer of prestalk cell differentiation, may operate via a calcium signalling pathway, and terminal stalk cell differentiation is in part regulated by glycogen synthase kinase 3.

Patterns of free calcium in multicellular stages of Dictyostelium expressing jellyfish apoaequorin

To examine the patterns of high free cytosolic calcium or [Ca2+]i during Dictyostelium's development, we expressed apoaequorin in D. discoideum, reconstituted aequorin and observed the resultant patterns of calcium-dependent luminescence. Specific, high calcium zones are seen throughout normal multicellular development and are roughly coincident with those regions that later differentiate into stalk or stalk-like cells. A slug, for example, shows a primary high calcium zone within its front quarter and a secondary one around its tail; while a mound shows such a zone around the periphery of its base. Combined with previous evidence, our findings support the hypothesis that high [Ca2+]i feeds back to favor the stalk pathway. We also discovered several high calcium zones within the mound's base that do not coincide with any known prepatterns in D. discoideum. These include two, relatively persistent, antipodal strips along the mound's periphery. These various persistent zones of high calcium are largely made up of frequent, 10 to 30 second long, semiperiodic calcium spikes. Each of these spikes generates a correspondingly short-lived, 200 to 500 microns long, high calcium band which extends along the nearby surface. Similar, but relatively large and infrequent, spikes generate cross bands which extend across migrating slugs and just behind their advancing tips as well as across the peripheries of rotating mounds and midway between their antipodal strips. Moreover, calcium has a doubling time of about a second as various spikes rise. This last observation suggests that the calcium bands seen in Dictyostelium may be generated by so-called fast calcium waves.

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

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Serpentine cAMP receptors may act through a G protein-independent pathway to induce postaggregative development in Dictyostelium

The transcription factor G box-binding factor (GBF) is required for the developmental switch between aggregative and postaggregative gene expression, cell-type differentiation, and morphogenesis. We show that constitutive expression of GBF allows ectopic expression of postaggregative genes, but only in response to exogenous cAMP. GBF activation requires the serpentine cAMP receptors required for aggregation, but not the coupled G alpha 2 or the G beta subunit, suggesting a novel signaling pathway. In response to high cAMP, g alpha 2-null cells can bypass the aggregation stage, expressing cell type-specific genes and forming fruiting bodies. Our results demonstrate that the same receptors regulate aggregation and cell-type differentiation, but via distinct pathways depending upon whether the receptor perceives a pulsatile or sustained signal.