cAMP and cell sorting control the spatial expression of a developmentally essential cell-type-specific ras gene in Dictyostelium

Regulation by protein-tyrosine phosphatase PTP2 is distinct from that by PTP1 during Dictyostelium growth and development

We have cloned a gene encoding a second Dictyostelium discoideum protein-tyrosine phosphatase (PTP2) whose catalytic domain has approximately 30 to 39% amino acid identity with those of other PTPs and a 41% amino acid identity with D. discoideum PTP1. Like PTP1, PTP2 is a nonreceptor PTP with the catalytic domain located at the C terminus of the protein. PTP2 has a predicted molecular weight of 43,000 and possesses an acidic 58-amino-acid insertion 24 amino acids from the N terminus of the conserved catalytic domain. PTP2 transcripts are expressed at moderate levels in vegetative cells and are induced severalfold at the onset of development. Studies with a PTP2-lacZ reporter gene fusion indicate that PTP2, like PTP1, is preferentially expressed in prestalk and anterior-like cell types during the multicellular stages of development. PTP2 gene disruptants (ptp2 null cells) are not detectably altered in growth and show a temporal pattern of development similar to that of wild-type cells. ptp2 null slugs and fruiting bodies, however, are significantly larger than those of wild-type slugs, suggesting a role for PTP2 in regulating multicellular structures. D. discoideum strains overexpressing PTP2 from the PTP2 promoter exhibit growth rate and developmental abnormalities, the severity of which corresponds to the level of PTP2 overexpression. Strains with high overexpression of the PTP2 gene grow slowly on bacterial lawns and produce small cells in axenic medium. When development is initiated in these strains, cells are able to aggregate but then stop further morphogenesis for 6 to 8 h, after which time a variable fraction of these aggregates continue with normal timing, producing diminutive fruiting bodies. These disruption and overexpression phenotypes for PTP2 are distinct from the corresponding mutant PTP1 phenotypes. Immunoprobing PTP2 mutant strains during growth and development with antiphosphotyrosine antibodies reveals several changes in the tyrosine phosphorylation of proteins in PTP2 mutant strains compared with that in wild-type cells. These changes are different from those identified in the previously characterized corresponding PTP1 disruption and overexpression mutant strains. Thus, although PTP2 and PTP1 are nonreceptor PTPs with similar spatial patterns of expression, our findings suggest that they possess distinct regulatory functions in controlling D. discoideum growth and development.

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

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

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

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

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

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

Spatial and temporal expression of the Dictyostelium discoideum G alpha protein subunit G alpha 2: expression of a dominant negative protein inhibits proper prestalk to stalk differentiation

Previous results have shown that the G alpha protein subunit G alpha 2 is required for aggregation in Dictyostelium discoideum and is essential for coupling cell-surface cAMP receptors to downstream effectors in vivo during this stage of development. G alpha 2 expresses at least four distinct transcripts that are differentially regulated during development; two of the transcripts are expressed exclusively in the multicellular stages and their expression is restricted to prestalk cells. We partially dissected the G alpha 2 promoter and identified a component that is expressed exclusively during the multicellular stages using luciferase gene fusions. When this promoter region is coupled to lacZ, beta-gal expression is restricted to the multicellular stages and localized in prestalk cells with a pattern similar to that of the ecmA prestalk-specific promoter. We show that expression in wild-type cells of the G alpha 2 mutant protein [G alpha 2(G206T)] during the early stages of development blocks aggregation and cAMP-mediated activation of adenylyl cyclase and guanylyl cyclase, suggesting it functions as a dominant negatively active G alpha subunit. When this mutant G alpha protein is expressed from the ecmA prestalk-specific promoter, abnormal stalk differentiation during culmination is observed. Expression of the mutant G alpha 2 from the SP60 prespore promoter or wild-type G alpha 2 from either the ecmA or the SP60 promoter results in no detectable phenotype. The results suggest that G alpha 2 plays an essential role during the culmination stage in prestalk cells and may mediate cAMP receptor activation of these processes during multicellular development.

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

We and others have previously shown that cAMP-dependent protein kinase (PKA) activity is essential for aggregation, induction of prespore gene expression and multicellular development in Dictyostelium. In this manuscript, we further examine this regulatory role. We have overexpressed the Dictyostelium PKA catalytic subunit (PKAcat) in specific cell types during the multicellular stages, using prestalk and prespore cell-type-specific promoters to make PKA activity constitutive in these cells (independent of cAMP concentration). To examine the effects on cell-type differentiation, we cotransformed the PKAcat-expressing vectors with reporter constructs expressing lacZ from four cell-type-specific promoters: ecmA (specific for prestalk A cells); ecmB (specific for prestalk B and anterior-like cells in the slug); ecmB delta 89 (specific for stalk cells); and SP60 (prespore-cell-specific). By staining for beta-galactosidase expression histologically at various stages of development in individual strains, we were able to dissect the morphological changes in these strains, examine the spatial localization of the individual cell types, and understand the possible roles of PKA during multicellular development. Expression of PKAcat from either the ecmA or ecmB prestalk promoters resulted in abnormal development that arrested shortly after the mound stage, producing a mound with a round apical protrusion at the time of tip formation. Prestalk A and prestalk B cells were localized in the central region and the apical mound in the terminal differentiated aggregate, while prespore cells showed an aberrant spatial localization. Consistent with a developmental arrest, these mounds did not form either mature spores or stalk cells and very few cells expressed a stalk-cell-specific marker. Expression of PKAcat from the prespore promoter resulted in abnormal morphogenesis and accelerated spore cell differentiation. When cells were plated on agar, a fruiting body was formed with a very large basal region, containing predominantly spores, and a small, abnormal sorocarp. Mature spore cells were first detected by 14 hours, with maximal levels reached by 18–20 hours, in contrast to 24–26 hours in wild-type strains. When cells were plated on filters, they produced an elongated tip from a large basal region, which continued to elongate as a tubular structure and produce a ‘slug-like’ structure at the end. The slug was composed predominantly of prestalk cells with a few prespore cells restricted to the junction between the ‘slug’ and tube. As the slug migrated, these prespore cells were found in the tube, while new prespore cells appeared at the slug/tube junction, suggesting a continual differentiation of new prespore cells at the slug's posterior.(ABSTRACT TRUNCATED AT 400 WORDS)

Patterns in Dictyostelium discoideum: the role of myosin II in the transition from the unicellular to the multicellular phase

Dictyostelium discoideum amoebae which lack the myosin II gene are motile and aggregate to form rudimentary mounds, but do not undergo further morphological development (Manstein et al., 1989). Here we use scanning electron microscopy, light microscopy, immunofluorescence and computer analysis of time-lapse video films to study how D. discoideum myosin null cells of strains HS2205 and HS2206 aggregate. Myosin null cells are sufficiently coordinated in their movements to form two-dimensional aggregation streams, although mutant cells within streams lack the elongated shape and parallel orientation of wild-type strains. In the wild-type, cell movements are coordinated, cells usually joining streams that spiral inwards and upwards as the mound extends into the standing papilla. In the aggregates of mutant strains, cell movements are chaotic, only occasionally forming short-term spirals that rotate at less than half the speed of wild-type spirals and frequently change direction. Unlike the situation in the wild-type where spirals continue with mound elongation, cells within the mutant mound eventually cease translocation altogether as the terminal shape of the mound is reached and only intracellular particle movement is observed. Scanning electron micrographs show that the surface of the wild-type mound consists of flattened cells which fit neatly together. The myosin null cell mound has an uneven surface, the orientation of the cells is chaotic and no tip is formed. This is consistent with the results of synergy experiments in which myosin null cells were absent from the tips of chimeric HS2205/AX2 slugs and pre-culminates. Immunofluorescence microscopy using prespore and spore cell markers reveals that a prestalk/prespore pattern forms within the mutant mound but that terminal spore differentiation is incomplete. These results are discussed in relation to the role of myosin II in aggregation and morphogenesis.

CAR2, a prestalk cAMP receptor required for normal tip formation and late development of Dictyostelium discoideum

Extracellular cAMP serves as a primary signaling molecule to regulate the development of Dictyostelium discoideum. It is required for chemotaxis, aggregation, cytodifferentiation, and morphogenetic movement. The receptors for cAMP are members of the family of cell-surface receptors that are linked to G proteins and characterized by seven putative transmembrane domains. Previously, we have isolated the gene for the cAMP receptor subtype 1 (CAR1) from Dictyostelium and suggested that several genes related to CAR1 were present in the genome. Here, we describe a family of cAMP receptor genes of Dictyostelium and the isolation and function of the gene for the cAMP receptor subtype 2, CAR2. CAR2 is structurally similar to CAR1. Overall, their transmembrane and loop domains are approximately 75% identical in amino acid sequence; however, their carboxyl termini are quite dissimilar; CAR2 possesses homopolymeric runs of histidines and asparagines that are absent from the corresponding region in CAR1. Although CAR1 is maximally expressed during the early stages of development, CAR2 is expressed only after cells have aggregated and, then, preferentially in prestalk cells. Transgenic Dictyostelium that have had their wild-type CAR2 gene replaced by a defective copy using homologous recombination proceed through early development but are detained at the tight mound stage. CAR2 may be required for cAMP-directed sorting of prestalk cells during pattern formation within the aggregation mound. Furthermore, although prestalk genes are expressed normally in aggregates that lack CAR2, they exhibit an enhanced expression of prespore-specific mRNA. Previously, we had shown that there was a requirement for CAR1 during early development. The present results demonstrate that the multiple responses of Dictyostelium to cAMP are regulated by distinct cAMP receptors that are encoded by unique genes.

Analysis of a spatially regulated phosphotyrosine phosphatase identifies tyrosine phosphorylation as a key regulatory pathway in Dictyostelium

We have cloned a Dictyostelium phosphotyrosine phosphatase (PTP1) with a catalytic domain showing approximately 38%-50% amino acid identity to those of other PTPs. PTP1 contains an approximately 99 amino acid insert and bacterially produced PTP1 possesses PTP activity. PTP1 is expressed at a very low level in vegetative cells, induced by 4 hr, and maximally expressed at the tight aggregate stage. PTP1-lacZ studies indicate that PTP1 is spatially localized to prestalk and anterior-like cell types. PTP1 gene disruptants show accelerated development, whereas strains overexpressing PTP1 to a high level fail to aggregate. Strains overexpressing moderate levels exhibit severe morphological defects following aggregation, including multiply tipped aggregates and morphologically aberrant fruiting bodies. Western blot analysis using anti-phosphotyrosine antibodies shows specific changes in the mutant strains when compared with wild-type cells. The results indicate that reversible protein-tyrosine phosphorylation and PTP1 play important regulatory roles during Dictyostelium development.

Sporulation in prokaryotes and lower eukaryotes

During the past year, highlights in sporulation research include the demonstration that phosphorylation of SpoOA is a critical factor in Bacillus subtilis development; the identification of C alpha proteins, adenylyl cyclase and protein kinase A genes in Dictyostelium; proof that an endogenous antisense RNA regulates gene expression in Dictyostelium; and characterization of a second type of differentiated cell in Myxococcus.

Chemotactic sorting to cAMP in the multicellular stages of Dictyostelium development

Dictyostelium transformants that overproduce the extracellular form of cyclic nucleotide phosphodiesterase and so accumulate a reduced amount of cAMP are blocked in development after aggregation in the form of a tight mound, prior to formation of the apical tip. In such mounds, prespore cell differentiation is repressed, and the apical accumulation of prestalk cells is greatly retarded. When a source of cAMP is placed below the arrested mounds, prestalk cells that would normally migrate in an apical direction instead sort downwards to the substratum. Thus, by acting as the chemoattractant that draws prestalk cells to the apex, cAMP signaling directs the formation of a patterned structure.

Regulation of the Dictyostelium cAMP-induced, prestalk-specific DdrasD gene: identification of cis-acting elements

We have shown previously that expression of the Dictyostelium ras gene DdrasD (previously denoted Ddras) is induced during multicellular development and in single-cell shaking culture in response to cAMP (1). Analysis of transformants carrying DdrasD/lacZ reporter constructs showed DdrasD expression to be prestalk-specific (2). The gene is transcribed from three start sites with transcription from the distal site producing an approximately 1.2 kb transcript, which is expressed at low levels in growing cells and is subsequently induced late in aggregation. This promoter is also induced to high levels by cAMP. Transcription from the two more proximal sites is coregulated and is induced during development, resulting in approximately 1.0 kb transcripts. In this study, we examine cis-acting regions required for proper regulation of DdrasD expression using a DdrasD/beta-glucuronidase reporter gene construct. We have identified distinct sequence elements required for developmental and vegetative expression of DdrasD. A domain containing a CA repeat, similar to ones found in other late, cAMP-induced Dictyostelium genes, is required for cAMP-induced and developmental expression.

Analysis of G alpha 4, a G-protein subunit required for multicellular development in Dictyostelium

The Dictyostelium G alpha 4 gene encodes a G-protein alpha subunit that is primarily expressed during the multicellular stages of development. g alpha 4 null mutants, created by gene disruption, show aberrant morphological differentiation, reduced levels of prespore gene expression, and a loss of the ability to produce spores. These developmental phenotypes can be rescued by complementation with the wild-type gene. Cells that overexpress the G alpha 4 gene (G alpha 4HC) also show reduced spore production but display an aberrant morphological phenotype distinct from that of g alpha 4 cells. The g alpha 4 phenotype can be partially rescued by the presence of wild-type or G alpha 4HC cells in chimeric organisms, suggesting that G alpha 4-expressing cells produce an intercellular signal that is essential for multicellular development.

Two independent promoters as well as 5' untranslated regions regulate Dd ras expression in Dictyostelium

The Dd ras gene produces three transcripts during Dictyostelium development. The largest transcript (L-) can be induced by external addition of cAMP even in cells prevented from aggregating, whereas shorter transcripts (S1- and S2-) expression requires cell aggregate formation. We show the presence of two independent promoters for L- and S-transcripts by deletion analysis of Dd ras fragments fused to CAT reporter genes reintroduced in Dictyostelium. A direct repeat upstream of S-transcript start sites which seems involved in S-promoter function, modulates also L-RNA accumulation. Furthermore removal of sequences between this repeat and the AUG protein start codon reduces the level of L-transcripts in aggregates. This study allowed to uncover the intricate pattern of sequences participating in the regulation of Dd ras expression.

Regulation of cellular differentiation during Dictyostelium morphogenesis

In Dictyostelium there are multiple prestalk cell types that have a complex pattern of directed cell movement during slug formation and culmination. Three extracellular signals, cyclic AMP, DIF and ammonia, control cell type differentiation. Recently there has been considerable progress in understanding their modes of action and interaction.

Identification of Dictyostelium G alpha genes expressed during multicellular development

Guanine nucleotide-binding protein (G protein)-mediated signal transduction constitutes a common mechanism by which cells receive and respond to a diverse set of environmental signals. Many of the signals involved in the developmental life cycle of the slime mold Dictyostelium have been postulated to be transduced by such pathways and, in some cases, these pathways have been demonstrated to be dependent on specific G proteins. Using the polymerase chain reaction, we have identified two additional Dictyostelium G alpha genes, G alpha 4 and G alpha 5, that are developmentally regulated. Transcripts from both of these genes are primarily expressed during the multicellular stages of development, suggesting possible roles in cell differentiation or morphogenesis. The entire G alpha 4 gene was sequenced and found to encode a protein consisting of 345 amino acids. The G alpha 4 subunit is homologous to other previously identified G alpha subunits, including the Dictyostelium G alpha 1 (43% identity) and G alpha 2 (41% identity) subunits. However, the G alpha 4 subunit contains some unusual sequence divergences in residues highly conserved among most eukaryotic G alpha subunits, suggesting that G alpha 4 may be a member of another class of G alpha subunits.

A developmentally regulated, putative serine/threonine protein kinase is essential for development in Dictyostelium

Using PCR technology, we have cloned parts of three developmentally regulated putative serine/threonine kinases from Dictyostelium. All show significant homology to members of the cAMP-dependent protein kinase A/protein kinase C subfamilies. A genomic clone encoding one of these, DdPK3, has been isolated and sequenced. The open reading frame encodes a protein of 648 amino acids with the conserved kinase domain in the C-terminal half. The protein encoded by this gene is unusual in that it contains long homopolymer runs in the N-terminal half of the protein, including a long run of 88 amino acids in which 73 are glutamine residues. To examine the function of DdPK3, a gene disruption was created via homologous recombination. Ddpk3- cells do not aggregate by themselves but will co-aggregate with wild-type cells. However, after aggregation these cells are 'sloughed off' and do not proceed further through development, but are found as a discrete mass alongside the fruiting body formed by the wild-type cells. Analysis of signal transduction pathways indicates that cAMP pulse-induced expression of aggregation stage-specific genes is normal in Ddpk3- cells, as is induction of the prestalk gene Ddras in single cell assays. However, cAMP induction of the late promoters of cAMP receptor cAR1 and of two prespore-specific genes is absent under similar conditions. These cells show normal activation of adenylate cyclase and normal phosphorylation of the G alpha protein G alpha 2 in response to cAMP. The possible role of DdPK3 in Dictyostelium development is discussed.