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

The cytosolic lncRNA dutA affects STATa signaling and developmental commitment in Dictyostelium

STATa is a pivotal transcription factor for Dictyostelium development. dutA is the most abundant RNA transcribed by RNA polymerase II in Dictyostelium, and its functional interplay with STATa has been suggested. This study demonstrates that dutA RNA molecules are distributed as spot-like structures in the cytoplasm, and that its cell type-specific expression changes dramatically during development. dutA RNA was exclusively detectable in the prespore region of slugs and then predominantly localized in prestalk cells, including the organizer region, at the Mexican hat stage before most dutA transcripts, excluding those in prestalk O cells, disappeared as culmination proceeded. dutA RNA was not translated into small peptides from any potential open reading frame, which confirmed that it is a cytoplasmic lncRNA. Ectopic expression of dutA RNA in the organizer region of slugs caused a prolonged slug migration period. In addition, buffered suspension-cultured cells of the strain displayed reduced STATa nuclear translocation and phosphorylation on Tyr702. Analysis of gene expression in various dutA mutants revealed changes in the levels of several STATa-regulated genes, such as the transcription factors mybC and gtaG, which might affect the phenotype. dutA RNA may regulate several mRNA species, thereby playing an indirect role in STATa activation.

STAT proteins: a kaleidoscope of canonical and non-canonical functions in immunity and cancer

STAT proteins represent an important family of evolutionarily conserved transcription factors that play key roles in diverse biological processes, notably including blood and immune cell development and function. Classically, STAT proteins have been viewed as inducible activators of transcription that mediate cellular responses to extracellular signals, particularly cytokines. In this 'canonical' paradigm, latent STAT proteins become tyrosine phosphorylated following receptor activation, typically via downstream JAK proteins, facilitating their dimerization and translocation into the nucleus where they bind to specific sequences in the regulatory region of target genes to activate transcription. However, growing evidence has challenged this paradigm and identified alternate 'non-canonical' functions, such as transcriptional repression and roles outside the nucleus, with both phosphorylated and unphosphorylated STATs involved. This review provides a revised framework for understanding the diverse kaleidoscope of STAT protein functional modalities. It further discusses the implications of this framework for our understanding of STAT proteins in normal blood and immune cell biology and diseases such as cancer, and also provides an evolutionary context to place the origins of these alternative functional modalities.

Analysis of DrkA kinase's role in STATa activation

Dictyostelium STATa is a homologue of metazoan signal transducers and activators of transcription (STATs) and is important for morphogenesis. STATa is activated by phosphorylation on Tyr702 when cells are exposed to extracellular cAMP. Although two tyrosine kinase-like (TKL) proteins, Pyk2 and Pyk3, have been definitively identified as STATc kinases, no kinase is known for STATa activation. Based on homology to the previously identified tyrosine-selective TKLs, we identified DrkA, a member of the TKL family and the Dictyostelium receptor-like kinase (DRK) subfamily, as a candidate STATa kinase. The drkA gene is almost exclusively expressed in prestalk A (pstA) cells, where STATa is activated. Transient over-expression of DrkA increased STATa phosphorylation, although over-expression of the protein causes a severe growth defect and cell death. Furthermore, recombinant DrkA protein is auto-phosphorylated on tyrosine and threonine residues, and an in vitro kinase assay shows that DrkA can phosphorylate STATa on Tyr702 in a STATa-SH2 (phosphotyrosine binding) domain-dependent manner. These observations strongly suggest that DrkA is one of the key regulators of STATa tyrosine phosphorylation and is consistent with it being the kinase that directly activates STATa.

Two potential evolutionary origins of the fruiting bodies of the dictyostelid slime moulds

Dictyostelium discoideum and the other dictyostelid slime moulds ('social amoebae') are popular model organisms best known for their demonstration of sorocarpic development. In this process, many cells aggregate to form a multicellular unit that ultimately becomes a fruiting body bearing asexual spores. Several other unrelated microorganisms undergo comparable processes, and in some it is evident that their multicellular development evolved from the differentiation process of encystation. While it has been argued that the dictyostelid fruiting body had similar origins, it has also been proposed that dictyostelid sorocarpy evolved from the unicellular fruiting process found in other amoebozoan slime moulds. This paper reviews the developmental biology of the dictyostelids and other relevant organisms and reassesses the two hypotheses on the evolutionary origins of dictyostelid development. Recent advances in phylogeny, genetics, and genomics and transcriptomics indicate that further research is necessary to determine whether or not the fruiting bodies of the dictyostelids and their closest relatives, the myxomycetes and protosporangids, are homologous.

The GATA transcription factor gene gtaG is required for terminal differentiation in Dictyostelium

The GATA transcription factor GtaG is conserved in Dictyostelids and essential for terminal differentiation in Dictyostelium discoideum, but its function is not well understood. Here we show that gtaG is expressed in prestalk cells at the anterior region of fingers and in the extending stalk during culmination. The gtaG^- phenotype is cell-autonomous in prestalk cells and non-cell-autonomous in prespore cells. Transcriptome analyses reveal that GtaG regulates prestalk gene expression during cell differentiation before culmination and is required for progression into culmination. GtaG-dependent genes include genetic suppressors of the Dd-STATa-defective phenotype as well as Dd-STATa target-genes, including extra cellular matrix genes. We show that GtaG may be involved in the production of two culmination-signaling molecules, cyclic di-GMP and the spore differentiation factor SDF-1 and that addition of c-di-GMP rescues the gtaG^- culmination and spore formation deficiencies. We propose that GtaG is a regulator of terminal differentiation that functions in concert with Dd-STATa and controls culmination through regulating c-di-GMP and SDF-1 production in prestalk cells.

Signaling via the CytoR/JAK/STAT/SOCS pathway: Emergence during evolution

Cell-cell signaling represents an essential hallmark of multicellular organisms, which necessarily require a means of communicating between different cell populations, particularly immune cells. Cytokine receptor signaling through the Janus kinase/Signal Transducer and Activator of Transcription/Suppressor of Cytokine Signaling (CytoR/JAK/STAT/SOCS) pathway embodies one important paradigm by which this is achieved. This pathway has been extensively studied in vertebrates and protostomes and shown to play fundamental roles in development and function of immune and other cells. However, our understanding of the origins of the individual pathway components and their assembly into a functional pathway has remained limited. This study examined the origins of each component of this pathway through bioinformatics analysis of key extant species. This has revealed step-wise accretion of individual components over a large evolutionary time-frame, but only in bilateria did a series of innovations allow their final coalescence to form a complete pathway. Assembly of the CytoR/JAK/STAT pathway has followed the retrograde model of pathway evolution, whereas addition of the SOCS component has adhered to the patchwork model.

Implications of expansin-like 3 gene in Dictyostelium morphogenesis

Dictyostelium harbors multiple expansin-like genes with generally unknown functions. Thus, we analyzed the expansin-like 3 (expL3) gene and found that its expression was reduced in a null mutant for a STATa gene encoding a transcription factor. The expression of expL3 was developmentally regulated and its transcript was spliced only in the multicellular stages. The expL3 promoter was activated in the anterior prestalk region of the parental strain and downregulated in the STATa null slug, although the expL3 promoter was still expressed in the prestalk region. The expL3 overexpressing strain exhibited delayed development and occasionally formed an aberrant structure, i.e., a fruiting body-like structure with a short stalk. The ExpL3-myc protein bound cellulose.

Evidence for tissue-specific Jak/STAT target genes in Drosophila optic lobe development

The evolutionarily conserved JAK/STAT pathway plays important roles in development and disease processes in humans. Although the signaling process has been well established, we know relatively little about what the relevant target genes are that mediate JAK/STAT activation during development. Here, we have used genome-wide microarrays to identify JAK/STAT targets in the optic lobes of the Drosophila brain and identified 47 genes that are positively regulated by JAK/STAT. About two-thirds of the genes encode proteins that have orthologs in humans. The STAT targets in the optic lobe appear to be different from the targets identified in other tissues, suggesting that JAK/STAT signaling may regulate different target genes in a tissue-specific manner. Functional analysis of Nop56, a cell-autonomous STAT target, revealed an essential role for this gene in the growth and proliferation of neuroepithelial stem cells in the optic lobe and an inhibitory role in lamina neurogenesis.

Evidence for a functional link between Dd-STATa and Dd-PIAS, a Dictyostelium PIAS homologue

Several mammalian protein families inhibit the activity of signal transducer and activator of transcription (STAT) proteins. The protein inhibitor of activated STAT (PIAS) was initially identified through its ability to interact with human STAT proteins. We isolated a gene (pisA) encoding a Dictyostelium orthologue of PIAS, Dd-PIAS, which possesses almost all the representative motifs and domains of mammalian PIAS proteins. A Dd-PIAS null mutant strain displays a normal terminal morphology but with accelerated development once cells are aggregated. In contrast, Dd-PIAS overexpressor strains demonstrate delayed aggregation, almost no slug phototaxis, impaired slug motility, and a prolonged slug migration period. This strain is a near phenocopy of the Dd-STATa null mutant, although it eventually forms a fruiting body, albeit inefficiently. The expression of several Dd-STATa-activated genes is upregulated in the Dd-PIAS null mutant and there is ectopic expression of pstAB makers. The concentration of a PIAS-green fluorescent protein (GFP) fusion protein, expressed under the PIAS promoter, is greatest in the pstO cells and gradually decreases with proximity to the tip of the slug and culminant: a pattern diametrically opposite to that of Dd-STATa. Our results suggest a functional interrelationship between Dd-PIAS and Dd-STATa that influences gene expression and development.

Quantification of GPCR internalization by single-molecule microscopy in living cells

Receptor internalization upon ligand stimulation is a key component of a cell's response and allows a cell to correctly sense its environment. Novel fluorescent methods have enabled the direct visualization of the agonist-stimulated G-protein-coupled receptors (GPCR) trafficking in living cells. However, it is difficult to observe internalization of GPCRs in vivo due to intrinsic autofluorescence and cytosolic signals of fluorescently labeled GPCRs. This study uses the superior positional accuracy of single-molecule fluorescence microscopy to visualize in real time the internalization of Dictyostelium discoideum cAMP receptors, cAR1, genetically encoded with eYFP. This technique made it possible to follow the number of receptors in time revealing that the fraction of cytosolic receptors increases after persistent agonist stimulation and that the majority of the receptors were degraded after internalization. The observed internalization process was phosphorylation dependent, as shown with the use of a phosphorylation deficient cAR1 mutant, cm1234-eYFP, or stimulation with an antagonist, Rp-cAMPS that does not induce receptor phosphorylation. Furthermore, experiments done in mound-stage cells suggest that intrinsic, phosphorylation-induced internalization of cAR1 is necessary for Dictyostelium wild type cells to progress properly through multicellular development. To our knowledge, this observation illustrates for the first time phosphorylation-dependent internalization of single cAR1 molecules in living cells and its involvement in multicellular development. This very sensitive imaging of receptor internalization can be a useful and universal approach for pharmacological characterization of GPCRs in other cell types.

STAT signaling in Dictyostelium development

Signal transducers and activators of transcription (STAT) proteins are one of the important mediators of phosphotyrosine-regulated signaling in metazoan cells. These proteins are components of JAK/STAT signal transduction pathways, which regulate immune responses, cell fate, proliferation, cell migration, and programmed cell death in multicellular organisms. The cellular slime mould, Dictyostelium discoideum, is the simplest multicellular organism using molecules homologous to STATs, Dd-STATa-d. The Dd-STATa null mutant displays delayed aggregation, no phototaxis and fails culmination. Here, the functions of Dictyostelium STATs during development and their associated signaling molecules are discussed.

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

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

Characterization of the Dictyostelium homolog of chromatin binding protein DET1 suggests a conserved pathway regulating cell type specification and developmental plasticity

DET1 (De-etiolated 1) is a chromatin binding protein involved in developmental regulation in both plants and animals. DET1 is largely restricted to multicellular eukaryotes, and here we report the characterization of a DET1 homolog from the social amoeba Dictyostelium discoideum. As in other species, Dictyostelium DET1 is nuclear localized. In contrast to other species, where it is an essential protein, loss of DET1 is nonlethal in Dictyostelium, although viability is significantly reduced. The phenotype of the det1(-) mutant is highly pleiotropic and results in a large degree of heterogeneity in developmental parameters. Loss of DET1 results in delayed and abnormal development with enlarged aggregation territories. Mutant slugs displayed cell type patterning with a bias toward the prestalk pathway. A number of DET1-interacting proteins are conserved in Dictyostelium, and the apparently conserved role of DET1 in regulatory pathways involving the bZIP transcription factors DimB, c-Jun, and HY5 suggests a highly conserved mechanism regulating development in multicellular eukaryotes. While the mechanism by which DET1 functions is unclear, it appears that it has a key role in regulation of developmental plasticity and integration of information on environmental conditions into the developmental program of an organism.

Transcriptional targets of Drosophila JAK/STAT pathway signalling as effectors of haematopoietic tumour formation

Although many signal transduction pathways have been implicated in the development of human disease, the identification of pathway targets and the biological processes that mediate disease progression remains challenging. One such disease-related pathway is the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) cascade whose constitutive misactivation by the JAK2 V617F mutation underlies most human myeloproliferative disorders. Here, we use transcript profiling of Drosophila haemocyte-like cells to identify JAK/STAT target genes, combined with an in vivo model for JAK-induced blood cell overproliferation, to identify the main effectors required for haematopoietic tumour development. The identified human homologues of the Drosophila effectors were tested for potential V617F-mediated transcriptional regulation in human HeLa cells and compared with small interfering RNA-derived data, quantify their role in regulating the proliferation of cancer-derived cell lines. Such an inter-species approach is an effective way to identify factors with conserved functions that might be central to human disease.

Genome-wide expression profiling in the Drosophila eye reveals unexpected repression of notch signaling by the JAK/STAT pathway

Although the JAK/STAT pathway regulates numerous processes in vertebrates and invertebrates through modulating transcription, its functionally relevant transcriptional targets remain largely unknown. With one jak and one stat (stat92E), Drosophila provides a powerful system for finding new JAK/STAT target genes. Genome-wide expression profiling on eye discs in which Stat92E is hyperactivated, revealed 584 differentially regulated genes, including known targets domeless, socs36E, and wingless. Other differentially regulated genes (chinmo, lama, Mo25, Imp-L2, Serrate, Delta) were validated and may represent new Stat92E targets. Genetic experiments revealed that Stat92E cell-autonomously represses Serrate, which encodes a Notch ligand. Loss of Stat92E led to de-repression of Serrate in the dorsal eye, resulting in ectopic Notch signaling and aberrant eye growth there. Thus, our micro-array documents a new Stat92E target gene and a previously unidentified inhibitory action of Stat92E on Notch signaling. These data suggest that this study will be a useful resource for the identification of additional Stat92E targets.

Unphosphorylated STATs go nuclear

The JAK/STAT signal transduction pathway has traditionally been viewed as a cytokine-stimulated activator of gene expression consisting of a straightforward receptor/JAK kinase/STAT transcription factor cascade. Recent studies in Drosophila, have, however consistently identified a range of chromatin-remodelling factors as regulators of in vivo JAK/STAT signalling. Now, the detailed analysis of one of these, heterochromatin protein 1 (HP1), has provided an insight into an unexpected non-canonical in vivo role for STAT. In this model, unphosphorylated STATs associate with and maintain the stability of transcriptionally repressed heterochromatin--an effect countered by the recruitment of STAT to the canonical pathway. We examine the background of this new model and its implications for JAK/STAT pathway requirements in stem cell maintenance and cancer.

GBF-dependent family genes morphologically suppress the partially active Dictyostelium STATa strain

Transcription factor Dd-STATa, a functional Dictyostelium homologue of metazoan signal transducers and activators of transcription proteins, is necessary for culmination during development. We have isolated more than 18 putative multicopy suppressors of Dd-STATa using genetic screening. One was hssA gene, whose expression is known to be G-box-binding-factor-dependent and which was specific to prestalk A (pstA) cells, where Dd-STATa is activated. Also, hssA mRNA was expressed in pstA cells in the Dd-STATa-null mutant. At least 40 hssA-related genes are present in the genome and constitute a multigene family. The tagged HssA protein was translated; hssA encodes an unusually high-glycine-serine-rich small protein (8.37 kDa), which has strong homology to previously reported cyclic-adenosine-monophosphate-inducible 2C and 7E proteins. Overexpression of hssA mRNA as well as frame-shifted versions of hssA RNA suppressed the phenotype of the partially active Dd-STATa strain, suggesting that translation is not necessary for suppression. Although overexpression of prespore-specific genes among the family did not suppress the parental phenotype, prestalk-specific family members did. Although overexpression of the hssA did not revert the expression of Dd-STATa target genes, and although its suppression mechanism remains unknown, morphological reversion implies functional relationships between Dd-STATa and hssA.

Evidence that noncoding RNA dutA is a multicopy suppressor of Dictyostelium discoideum STAT protein Dd-STATa

Dd-STATa, a Dictyostelium discoideum homologue of metazoan STAT transcription factors, is necessary for culmination. We created a mutant strain with partial Dd-STATa activity and used it to screen for unlinked suppressor genes. We screened approximately 450,000 clones from a slug-stage cDNA library for their ability to rescue the culmination defect when overexpressed. There were 12 multicopy suppressors of Dd-STATa, of which 4 encoded segments of a known noncoding RNA, dutA. Expression of dutA is specific to the pstA zone, the region where Dd-STATa is activated. In suppressed strains the expression patterns of several putative Dd-STATa target genes become similar to the wild-type strain. In addition, the amount of the tyrosine-phosphorylated form of Dd-STATa is significantly increased in the suppressed strain. These results indicate that partial copies of dutA may act upstream of Dd-STATa to regulate tyrosine phosphorylation by an unknown mechanism.

Disruption of the ifkA and ifkB genes results in altered cell adhesion, morphological defects and a propensity to form pre-stalk O cells during development of Dictyostelium

IfkA and ifkB are two GCN2-like genes present in Dictyostelium. Disruption of either gene alone results in subtle developmental defects. However, disruption of ifkA and ifkB within the same strain results in severe morphological and patterning defects in the developing double null cells. The mutant cells aggregate in streams that give tightly clumped mounds. Fingers form from the mounds but remain attached to one another, especially at their bases. The fingers culminate to give fused and entangled structures lacking proper stalk but containing some spores. The morphological defects are consistent with an enhanced cell-cell and cell-substrate adhesiveness of the developing double null cells, which may result in inappropriate cell contacts and altered cell motility and sorting properties. In ifkA/ifkB nulls, cell type proportioning and patterning is altered in favor of ALC/pstO cell types. The bias toward the ALC/pstO cell types may be due, in part, to the nuclear localization of the transcription factor STATc in growing ifkA/ifkB null cells. STATc normally becomes localized to the nucleus during finger formation and only within the pre-stalk O zone. The precocious nuclear localization seen in the mutant cells may predispose the cells to a ALC/pstO cell fate. The findings indicate that IfkA and IfkB have redundant functions in Dictyostelium morphogenesis that involve maintaining proper cell-cell and cell-substrate adhesion and the equilibrium between different cell types for proper spatial patterning.

Proteomic and microarray analyses of the Dictyostelium Zak1-GSK-3 signaling pathway reveal a role in early development

GskA, the Dictyostelium GSK-3 orthologue, is modified and activated by the dual-specificity tyrosine kinase Zak1, and the two kinases form part of a signaling pathway that responds to extracellular cyclic AMP. We identify potential cellular effectors for the two kinases by analyzing the corresponding null mutants. There are proteins and mRNAs that are altered in abundance in only one or the other of the two mutants, indicating that each kinase has some unique functions. However, proteomic and microarray analyses identified a number of proteins and genes, respectively, that are similarly misregulated in both mutant strains. The positive correlation between the array data and the proteomic data is consistent with the Zak1-GskA signaling pathway's functioning by directly or indirectly regulating gene expression. The discoidin 1 genes are positively regulated by the pathway, while the abundance of the H5 protein is negatively regulated. Two of the targets, H5 and discoidin 1, are well-characterized markers for early development, indicating that the Zak1-GskA pathway plays a role in development earlier than previously observed.

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.

STAT5 acts as a repressor to regulate early embryonic erythropoiesis

STAT5 regulates definitive (adult stage) erythropoiesis through its ability to transduce signals from the erythropoietin receptor. A function for STAT-dependent signaling during primitive (embryonic) erythropoiesis has not been analyzed. We tested this in the Xenopus system, because STAT5 is expressed at the right time and place to regulate development of the embryonic primitive ventral blood island. Depletion of STAT5 activity results in delayed accumulation of the first globin-expressing cells, indicating that the gene does regulate primitive erythropoiesis. Our results suggest that in this context STAT5 functions as a repressor, since forced expression of an activator isoform blocks erythropoiesis, while embryos expressing a repressor isoform develop normally. The erythroid phenotype caused by the activator isoform of STAT5 resembles that caused by overexpression of fibroblast growth factor (FGF). We show that STAT5 isoforms can function epistatic to FGF and can be phosphorylated in response to hyperactivated FGF signaling in Xenopus embryos. Therefore, our data indicate that STAT5 functions in both primitive and definitive erythropoiesis, but by different mechanisms.

JAK/STAT signalling in Drosophila controls cell motility during germ cell migration

The gonad is formed from two populations of cells originating at different locations: the primordial germ cells (PGCs), giving rise to either sperm or oocytes, and the somatic gonadal mesoderm precursors (SGPs), which support development of the gametes. Following the PGCs' migration during gastrulation, these two populations meet, forming the immature gonad. We present evidence that during embryonic development, the PGCs require the canonical JAK/STAT signalling cascade to migrate efficiently towards the SGPs. Loss of function for any element of the JAK/STAT pathway causes frequent germ cell mislocalisation. We have found that wild-type germ cells produce filopodia while they migrate through the mesoderm towards the gonad. Our observations suggest that PGCs use filopodia to migrate and to keep contact with each other. Interestingly, activation of the JAK/STAT pathway is required for these filopodia to form, and ectopic JAK/STAT activation enhances their formation.

The histidine kinase homologue DhkK/Sombrero controls morphogenesis in Dictyostelium

A key event in Dictyostelium development is the formation of the Mexican hat. This corresponds to a commitment step in morphogenesis that irreversibly signals progression from the slug stage to the fruiting body. We describe the characterization of the dhkK gene that controls this morphogenetic step. Null mutants of dhkK repeatedly attempt, and fail, to undergo morphogenesis from the slug to the Mexican hat, causing them to exhibit a "slugger" phenotype, which cannot be corrected by co-development with wild-type cells. The dhkK gene encodes a putative receptor histidine kinase whose expression is enriched in prestalk cells in the slug. Uniquely for a histidine kinase, DhkK is located in the nuclear envelope. Entry into culmination requires the DhkK response regulator domain, which appears to directly regulate cyclic AMP signaling.

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.

Ammonium transporter C of Dictyostelium discoideum is required for correct prestalk gene expression and for regulating the choice between slug migration and culmination

Ammonium transporter C (AmtC) is one of three transporters in Dictyostelium that have been proposed to regulate entry and exit of ammonia in a cell type dependent manner and to mediate ammonia signaling. Previous work demonstrated that disruption of the amtC gene results in a slugger phenotype in which the cells remain as migrating slugs when they should form fruiting bodies. More detailed studies on the null strain revealed that differentiation of prestalk cell types was delayed and maintenance of prestalk cell gene expression was defective. There was little or no expression of ecmB, a marker for the initiation of culmination. Normal expression of CudA, a nuclear protein required for culmination, was absent in the anterior prestalk zone. The absence of CudA within the tip region was attributable to the lack of nuclear localization of the transcription factor STATa, despite expression of adenylyl cyclase A mRNA in the slug tips. Disruption of the histidine kinase gene dhkC in the amtC null strain restored STATa and CudA expression and the ability to culminate. The results suggest that the lack of nuclear translocation of STATa results from low cAMP due to a misregulated and overactive DhkC phosphorelay in the amtC null strain.

Breaking symmetries: regulation of Dictyostelium development through chemoattractant and morphogen signal-response

Dictyostelium discoideum grow unicellularly, but develop as multicellular organisms. At two stages of development, their underlying symmetrical pattern of cellular organization becomes disrupted. During the formation of the multicellular aggregate, individual non-polarized cells re-organize their cytoskeletal structures to sequester specific intracellular signaling elements for activation by and directed movement within chemoattractant gradients. Subsequently, response to secreted morphogens directs undifferentiated populations to adopt different cell fates. Using a combination of cellular, biochemical and molecular approaches, workers have now begun to understand the mechanisms that permit Dictyostelium (and other chemotactic cells) to move directionally in shallow chemoattractant gradients and the transcriptional regulatory pathways that polarize cell-fate choice and initiate pattern formation.

Evidence that the Dictyostelium STAT protein Dd-STATa plays a role in the differentiation of inner basal disc cells and identification of a promoter element essential for expression in these cells

Dd-STATa, a Dictyostelium homolog of the metazoan STAT (signal transducers and activators of transcription) proteins, is necessary in the slug for correct entry into culmination. Dd-STATa-null mutant fails to culminate and its phenotype correlates with the loss of a funnel-shaped core region, the pstAB core region, which expresses both the ecmA and ecmB genes. To understand how the differentiation of pstAB core cells is regulated, we identified an EST that is expressed in the core cells of normal slugs but down-regulated in the Dd-STATa-null mutant. This EST, SSK348, encodes a close homolog of the Dictyostelium acetyl-CoA synthetase (ACS). A promoter fragment of the cognate gene, aslA (acetyl-CoA synthetase-like A), was fused to a lacZ reporter and the expression pattern determined. As expected from the behavior of the endogenous aslA gene, the aslA::lacZ fusion gene is not expressed in Dd-STATa-null slugs. In parental cells, the aslA promoter is first activated in the funnel-shaped core cells located at the slug anterior, the "pstAB core." During culmination, the pstAB core cells move down, through the prespore cells, to form the inner part of the basal disc. As the spore mass climbs the stalk, the aslA gene comes to be expressed in cells of the upper and lower cups, structures that cradle the spore head. Deletion and point mutation analyses of the promoter identified an AT-rich sequence that is necessary for expression in the pstAB core. This acts in combination with repressor regions that prevent ectopic aslA expression in the pre-stalk regions of slugs and the stalks of culminants. Thus, this study confirms that Dd-STATa is necessary for the differentiation of pstAB core cells, by showing that it is needed for the activation of the aslA gene. It also identifies aslA promoter elements that are likely to be regulated, directly or indirectly, by Dd-STATa.

EGFR-induced cell migration is mediated predominantly by the JAK-STAT pathway in primary esophageal keratinocytes

The epidermal growth factor receptor (EGFR) activates several signaling cascades in response to epidermal growth factor stimulation. One of these signaling events involves tyrosine phosphorylation of signal transducer and activator of transcription (STAT), whereas another involves activation of the phosphatidylinositol 3-OH kinase pathway. Two possibilities for STAT activation exist: a janus kinase (JAK)-dependent and a JAK-independent mechanism. Herein, we demonstrate that EGFR overexpression in primary esophageal keratinocytes activates STAT in a JAK-dependent fashion with the functional consequence of enhanced cell migration, which can be abolished by use of a JAK-specific inhibitor, AG-490. We determined the mechanisms underlying the signal transduction pathway responsible for increased cell migration. Stimulation of EGFR induces Tyr701 phosphorylation of STAT1 and initiates complex formation of STAT1 and STAT3 with JAK1 and JAK2. Thereafter, the STATs translocate to the nucleus within 15 min. In addition, we found that activation of this signaling pathway results in matrix metalloproteinase-1 (MMP-1) activity. By contrast, Akt activation does not impact the EGFR-STATs-JAKs complex formation and nuclear translocation of the STATs with subsequent MMP-1 activity, although Akt activation may contribute to cell migration through an independent mechanism. Taken together, we find that the recruitment of the STAT-JAK complex by EGFR is responsible for keratinocyte migration that, in turn, might be mediated by MMP-1 activation.

Extracellular matrix family proteins that are potential targets of Dd-STATa in Dictyostelium discoideum

Dd-STATa is a functional Dictyostelium homologue of metazoan STAT (signal transducers and activators of transcription) proteins, which is activated by cAMP and is thereby translocated into the nuclei of anterior tip cells of the prestalk region of the slug. By using in situ hybridization analyses, we found that the SLF308 cDNA clone, which contains the ecmF gene that encodes a putative extracellular matrix protein and is expressed in the anterior tip cells, was greatly down-regulated in the Dd-STATa-null mutant. Disruption of the ecmF gene, however, resulted in almost no phenotypic change. The absence of any obvious mutant phenotype in the ecmF-null mutant could be due to a redundancy of similar genes. In fact, a search of the Dictyostelium whole genome database demonstrates the existence of an additional 16 homologues, all of which contain a cellulose-binding module. Among these homologues, four genes show Dd-STATa-dependent expression, while the others are Dd-STATa-independent. We discuss the potential role of Dd-STATa in morphogenesis via its effect on the interaction between cellulose and these extracellular matrix family proteins.

A homologue of Cdk8 is required for spore cell differentiation in Dictyostelium

The Cdk8 proteins are kinases which phosphorylate the carboxy terminal domain (CTD) of RNA polymerase II (Pol II) as well as some transcription factors and, therefore, are involved in the regulation of transcription. Here, we report that a Cdk8 homologue from Dictyostelium discoideum is localized in the nucleus where it forms part of a high molecular weight complex that has CTD kinase activity. Insertional mutagenesis was used to abrogate gene function, and analysis of the null strain revealed that the DdCdk8 protein plays an important role in spore formation during late development. As previously reported [Dev. Growth Differ. 44 (2002) 213] Ddcdk8- cells also exhibit impaired aggregation, although we report that the severity of the defect depends upon experimental conditions. When aggregation occurs, Ddcdk8- cells form abnormal terminally differentiated structures within which the Ddcdk8- cells differentiate into stalk cells but fail to form spores, indicating a role for DdCdk8 in cell differentiation. When Ddcdk8 is expressed from its own promoter, the protein is able to rescue both the late developmental defect and the impaired aggregation. However, when expressed from an heterologous promoter, only the impaired aggregation is rescued. This result demonstrates that the defect during late development is not a consequence of impaired aggregation and indicates a direct role for DdCdk8 in spore formation.

Structure of an activated Dictyostelium STAT in its DNA-unbound form

Dd-STATa is a STAT protein which transcriptionally regulates cellular differentiation in Dictyostelium discoideum, the only non-metazoan known to employ SH2 domain signaling. The 2.7 A crystal structure of a tyrosine phosphorylated Dd-STATa homodimer reveals a four-domain architecture similar to that of mammalian STATs 1 and 3, but with an inverted orientation for the coiled-coil domain. Dimerization is mediated by SH2 domain:phosphopeptide interactions and by a direct interaction between SH2 domains. The unliganded Dd-STATa dimer adopts a fully extended conformation remarkably different from that of the DNA-bound mammalian STATs, implying a large conformational change upon target site recognition. Buried hydrophilic residues predicted to destabilize the coiled-coil domain suggest how hydrophobic residues may become exposed and mediate nuclear export. Functional and evolutionary implications for metazoan STAT proteins are discussed.

The Jak/STAT pathway in model organisms: emerging roles in cell movement

The JAK/STAT pathway was originally identified in mammals. Studies of this pathway in the mouse have revealed that JAK/STAT signaling plays a central role during hematopoeisis and other developmental processes. The role of JAK/STAT signaling in blood appears to be conserved throughout evolution, as it is also required during fly hematopoeisis. Studies in Dictyostelium, Drosophila, and zebrafish have shown that the JAK/STAT pathway is also required in an unusually broad set of developmental decisions, including cell proliferation, cell fate determination, cell migration, planar polarity, convergent extension, and immunity. There is increasing evidence that the versatility of this pathway relies on its cooperation with other signal transduction pathways. In this review, we discuss the components of the JAK/STAT pathway in model organisms and what is known about its requirement in cellular and developmental processes. In particular, we emphasize recent insights into the role that this pathway plays in the control of cell movement.

CulB, a putative ubiquitin ligase subunit, regulates prestalk cell differentiation and morphogenesis in Dictyostelium spp

Dictyostelium amoebae accomplish a starvation-induced developmental process by aggregating into a mound and forming a single fruiting body with terminally differentiated spores and stalk cells. culB was identified as the gene disrupted in a developmental mutant with an aberrant prestalk cell differentiation phenotype. The culB gene product appears to be a homolog of the cullin family of proteins that are known to be involved in ubiquitin-mediated protein degradation. The culB mutants form supernumerary prestalk tips atop each developing mound that result in the formation of multiple small fruiting bodies. The prestalk-specific gene ecmA is expressed precociously in culB mutants, suggesting that prestalk cell differentiation occurs earlier than normal. In addition, when culB mutant cells are mixed with wild-type cells, they display a cell-autonomous propensity to form stalk cells. Thus, CulB appears to ensure that the proper number of prestalk cells differentiate at the appropriate time in development. Activation of cyclic AMP-dependent protein kinase (PKA) by disruption of the regulatory subunit gene (pkaR) or by overexpression of the catalytic subunit gene (pkaC) enhances the prestalk/stalk cell differentiation phenotype of the culB mutant. For example, culB- pkaR- cells form stalk cells without obvious multicellular morphogenesis and are more sensitive to the prestalk O (pstO) cell inducer DIF-1. The sensitized condition of PKA activation reveals that CulB may govern prestalk cell differentiation in Dictyostelium, in part by controlling the sensitivity of cells to DIF-1, possibly by regulating the levels of one or more proteins that are rate limiting for prestalk differentiation.

Adenylyl cyclase A expression is tip-specific in Dictyostelium slugs and directs StatA nuclear translocation and CudA gene expression

cAMP oscillations, generated by adenylyl cyclase A (ACA), coordinate cell aggregation in Dictyostelium and have also been implicated in organizer function during multicellular development. We used a gene fusion of the ACA promoter with a labile lacZ derivative to study the expression pattern of ACA. During aggregation, most cells expressed ACA, but thereafter expression was lost in all cells except those of the anterior tip. Before aggregation, ACA transcription was strongly upregulated by nanomolar cAMP pulses. Postaggregative transcription was sustained by nanomolar cAMP pulses, but downregulated by a continuous micromolar cAMP stimulus and by the stalk-cell-inducing factor DIF. Earlier work showed that the transcription factor StatA displays tip-specific nuclear translocation and directs tip-specific expression of the nuclear protein CudA, which is essential for culmination. Both StatA and CudA were present in nuclei throughout the entire slug in an aca null mutant that expresses ACA from the constitutive actin15 promoter. This suggests that the tip-specific expression of ACA directs tip-specific nuclear translocation of StatA and tip-specific expression of CudA.

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.

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

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

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

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

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

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

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

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

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

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

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.

STAT signalling in cell proliferation and in development

Recent advances in STAT signalling research include a better understanding of the roles of mammalian STAT proteins in cell proliferation and apoptosis, and of non-mammalian STAT proteins in morphogenesis. Two different ways in which STAT signalling pathways can interface with Smad signalling pathways significantly increasing combinatorial signalling possibilities, have also been described.

Toward the functional analysis of the Dictyostelium discoideum genome

Dictyostelium discoideum is a useful model for molecular studies of cell biology and development. The 34-megabase Dictyostelium genome is currently being sequenced through the efforts of an international consortium. The genome is expected to encode 8-10,000 genes, including all those required for a free-living eukaryote capable of multicellular development. A complete description of the Dictyostelium genome will open the way toward the application of genome-based experimental approaches to studies of cell biology and development in this organism, and allow detailed physiological and evolutionary comparisons to other species.

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

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