Developmentally and spatially regulated activation of a Dictyostelium STAT protein by a serpentine receptor

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.

LrrkA, a kinase with leucine-rich repeats, links folate sensing with Kil2 activity and intracellular killing

Phagocytic cells ingest bacteria by phagocytosis and kill them efficiently inside phagolysosomes. The molecular mechanisms involved in intracellular killing and their regulation are complex and still incompletely understood. Dictyostelium discoideum has been used as a model to discover and to study new gene products involved in intracellular killing of ingested bacteria. In this study, we performed random mutagenesis of Dictyostelium cells and isolated a mutant defective for growth on bacteria. This mutant is characterized by the genetic inactivation of the lrrkA gene, which encodes a protein with a kinase domain and leucine-rich repeats. LrrkA knockout (KO) cells kill ingested Klebsiella pneumoniae bacteria inefficiently. This defect is not additive to the killing defect observed in kil2 KO cells, suggesting that the function of Kil2 is partially controlled by LrrkA. Indeed, lrrkA KO cells exhibit a phenotype similar to that of kil2 KO cells: Intraphagosomal proteolysis is inefficient, and both intraphagosomal killing and proteolysis are restored upon exogenous supplementation with magnesium ions. Bacterially secreted folate stimulates intracellular killing in Dictyostelium cells, but this stimulation is lost in cells with genetic inactivation of kil2, lrrkA, or far1. Together, these results indicate that the stimulation of intracellular killing by folate involves Far1 (the cell surface receptor for folate), LrrkA, and Kil2. This study is the first identification of a signalling pathway regulating intraphagosomal bacterial killing in Dictyostelium cells.

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.

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

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

Eat Prey, Live: Dictyostelium discoideum As a Model for Cell-Autonomous Defenses

The soil-dwelling social amoeba Dictyostelium discoideum feeds on bacteria. Each meal is a potential infection because some bacteria have evolved mechanisms to resist predation. To survive such a hostile environment, D. discoideum has in turn evolved efficient antimicrobial responses that are intertwined with phagocytosis and autophagy, its nutrient acquisition pathways. The core machinery and antimicrobial functions of these pathways are conserved in the mononuclear phagocytes of mammals, which mediate the initial, innate-immune response to infection. In this review, we discuss the advantages and relevance of D. discoideum as a model phagocyte to study cell-autonomous defenses. We cover the antimicrobial functions of phagocytosis and autophagy and describe the processes that create a microbicidal phagosome: acidification and delivery of lytic enzymes, generation of reactive oxygen species, and the regulation of Zn²⁺, Cu²⁺, and Fe²⁺ availability. High concentrations of metals poison microbes while metal sequestration inhibits their metabolic activity. We also describe microbial interference with these defenses and highlight observations made first in D. discoideum. Finally, we discuss galectins, TNF receptor-associated factors, tripartite motif-containing proteins, and signal transducers and activators of transcription, microbial restriction factors initially characterized in mammalian phagocytes that have either homologs or functional analogs in D. discoideum.

A novel, lineage-primed prestalk cell subtype involved in the morphogenesis of D. discoideum

Dictyostelium morphogenesis requires the tip, which acts as an organizer and conducts orchestrated cell movement and cell differentiation. At the slug stage the tip region contains prestalk A (pstA) cells, which are usually recognized by their expression of reporter constructs that utilize a fragment of the promoter of the ecmA gene. Here, using the promoter region of the o-methyl transferase 12 gene (omt12) to drive reporter expression, we demonstrate the presence, also within the pstA region, of a novel prestalk cell subtype: the pstV(A) cells. Surprisingly, a sub-population of the vegetative cells express a pstV(A): GFP marker and, sort out to the tip, both when developing alone and when co-developed with an excess of unmarked cells. The development of such a purified GFP-marked population is greatly accelerated: by precocious cell aggregation and tip formation with accompanying precocious elevation of developmental gene transcription. We therefore suggest that the tip contains at least two prestalk cell subtypes: the developmentally-specified pstA cells and the lineage-primed pstV(A) cells. It is presumably the pstV(A) cells that play the dominant role in morphogenesis during the earlier stages of development. The basis for the lineage priming is, however, unclear because we can find no correlation between pstV(A) differentiation and nutrient status during growth or cell cycle position at the time of starvation, the two known determinants of probable cell fate.

Cloning and gene expression of signal transducers and activators of transcription (STAT) homologue provide new insights into the immune response and nucleus graft of the pearl oyster Pinctada fucata

The signal transducers and activators of the transcription (STAT) family play an important role in regulatory and cellular functions by regulating the expression of a variety of genes, including cytokines and growth factors. In the present study, a Pinctada fucata STAT protein, termed PfSTAT, was described. The deduced amino acid sequence of PfSTAT contains the conserved STAT_bind domain and the SH2 domain, and the additional Bin/Amphiphysin/Rvs (BAR) domain, but does not have STAT_alpha and STAT_int domains. Multiple sequence alignments revealed that PfSTAT showed relatively low identity with vertebrate and other invertebrate STATs, and phylogenetic analysis indicated that the evolution of STAT may have been more complex and ancient. Gene expression analysis revealed that PfSTAT is involved in the immune response to polyinosinic-polycytidylic acid (poly I:C) stimulation and in the nucleus insertion operation. This study contributes to a better understanding of PfSTAT in protecting the pearl oyster from disease or injury caused by grafting.

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.

Copine A is expressed in prestalk cells and regulates slug phototaxis and thermotaxis in developing Dictyostelium

Copines are calcium-dependent membrane-binding proteins found in many eukaryotic organisms. We are studying the function of copines using the model organism, Dictyostelium discoideum. When under starvation conditions, Dictyostelium cells aggregate into mounds that become migrating slugs, which can move toward light and heat before culminating into a fruiting body. Previously, we showed that Dictyostelium cells lacking the copine A (cpnA) gene are not able to form fruiting bodies and instead arrest at the slug stage. In this study, we compared the slug behavior of cells lacking the cpnA gene to the slug behavior of wild-type cells. The slugs formed by cpnA- cells were much larger than wild-type slugs and exhibited no phototaxis and negative thermotaxis in the same conditions that wild-type slugs exhibited positive phototaxis and thermotaxis. Mixing as little as 5% wild-type cells with cpnA- cells rescued the phototaxis and thermotaxis defects, suggesting that CpnA plays a specific role in the regulation of the production and/or release of a signaling molecule. Reducing extracellular levels of ammonia also partially rescued the phototaxis and thermotaxis defects of cpnA- slugs, suggesting that CpnA may have a specific role in regulating ammonia signaling. Expressing the lacZ gene under the cpnA promoter in wild-type cells indicated cpnA is preferentially expressed in the prestalk cells found in the anterior part of the slug, which include the cells at the tip of the slug that regulate phototaxis, thermotaxis, and the initiation of culmination into fruiting bodies. Our results suggest that CpnA plays a role in the regulation of the signaling pathways, including ammonia signaling, necessary for sensing and/or orienting toward light and heat in the prestalk cells of the Dictyostelium slug.

Control of prestalk-cell differentiation by transcription factors

Transcriptional control of developmental genes is important for cell differentiation and pattern formation. Developing Dictyostelium discoideum cells form a multicellular structure in which individual cells differentiate into either stalk cells or spores. This simplicity makes the organism an attractive model for studying fundamental problems in developmental biology. However, the morphogenetic process of forming a stalked fruiting body conceals a certain degree of complexity. This is reflected in the presence of multiple prestalk subtypes that have individual roles to generate the fruiting body. This review describes recent advances in understanding the molecular mechanisms, mediated by transcription factors that generate prestalk-cell heterogeneity.

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.

Tyrosine phosphorylation-mediated signaling pathways in dictyostelium

While studies on metazoan cell proliferation, cell differentiation, and cytokine signaling laid the foundation of the current paradigms of tyrosine kinase signaling, similar studies using lower eukaryotes have provided invaluable insight for the understanding of mammalian pathways, such as Wnt and STAT pathways. Dictyostelium is one of the leading lower eukaryotic model systems where stress-induced cellular responses, Wnt-like pathways, and STAT-mediated pathways are well investigated. These Dictyostelium pathways will be reviewed together with their mammalian counterparts to facilitate the comparative understanding of these variant and noncanonical pathways.

cAMP-dependent signal pathways in unicellular eukaryotes

The review summarizes current data about mechanisms of signal transduction with participation of cAMP (cyclic adenosine monophosphate) and elements of the complex cAMP-protein kinase A (PKA) signal pathway in unicellular eukaryotes. Conceptions of evolutionary origin of eukaryotic signal transduction systems are developed.

The role of Gbetagamma subunits in the organization, assembly, and function of GPCR signaling complexes

The role of Gbetagamma subunits in cellular signaling has become well established in the past 20 years. Not only do they regulate effectors once thought to be the sole targets of Galpha subunits, but it has become clear that they also have a unique set of binding partners and regulate signaling pathways that are not always localized to the plasma membrane. However, this may be only the beginning of the story. Gbetagamma subunits interact with G protein-coupled receptors, Galpha subunits, and several different effector molecules during assembly and trafficking of receptor-based signaling complexes and not simply in response to ligand stimulation at sites of receptor cellular activity. Gbetagamma assembly itself seems to be tightly regulated via the action of molecular chaperones and in turn may serve a similar role in the assembly of specific signaling complexes. We propose that specific Gbetagamma subunits have a broader role in controlling the architecture, assembly, and activity of cellular signaling pathways.

MsSOCS expression indicates a potential role for JAK/STAT signalling in the early stages of Manduca sexta spermatogenesis

Our understanding of the Drosophila melanogaster testis stem cell niche has identified the signalling pathways required to maintain stem cells and promote self-renewal. Here we present the first detailed examination of the testes stem cell niche in the lepidopteran Manduca sexta . We show that larval testes contain hub-like structures surrounded by mitotically active cells likely to represent a self-renewing stem cell population. In addition, we have cloned DNA fragments encoding parts of M. sexta Signal Transducer and Activators of Transcription (STAT) and Suppressor of Cytokine Signalling (SOCS) homologues and show that expression of MsSOCS is upregulated in hub-adjacent cells. Given the conservation of socs-like genes and their regulation as Janus Kinases/Signal Transducer and Activators Transcription (JAK/STAT) pathway targets, we suggest that increased expression within the testis stem cells indicates increased levels of JAK/STAT signalling and a conserved role for pathway signalling in testis stem cell maintenance.

Evidence that DIF-1 and hyper-osmotic stress activate a Dictyostelium STAT by inhibiting a specific protein tyrosine phosphatase

STATc becomes tyrosine phosphorylated and accumulates in the nucleus when Dictyostelium cells are exposed to the prestalk cell inducer Differentiation inducing factor 1 (DIF-1), or are subjected to hyper-osmotic stress. We show that the protein tyrosine phosphatase PTP3 interacts directly with STATc and that STATc is refractory to activation in PTP3 overexpressing cells. Conversely, overexpression of a dominant inhibitor of PTP3 leads to constitutive tyrosine phosphorylation and ectopic nuclear localisation of STATc. Treatment of cells with DIF-1 or exposure to hyper-osmotic stress induces a decrease in biochemically assayable PTP3 activity and both agents also induce serine-threonine phosphorylation of PTP3. These observations suggest a novel mode of STAT activation, whereby serine-threonine phosphorylation of a cognate protein tyrosine phosphatase results in the inhibition of its activity, shifting the phosphorylation-dephosphorylation equilibrium in favour of phosphorylation.

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.

Dosage-dependent switch from G protein-coupled to G protein-independent signaling by a GPCR

G-protein-coupled receptors (GPCRs) mostly signal through heterotrimeric G proteins. Increasing evidence suggests that GPCRs could function in a G-protein-independent manner. Here, we show that at low concentrations of an agonist, beta(2)-adrenergic receptors (beta(2)-ARs) signal through Galpha(s) to activate the mitogen-activated protein kinase pathway in mouse embryonic fibroblast cells. At high agonist concentrations, signals are also transduced through beta(2)-ARs via an additional pathway that is G-protein-independent but tyrosine kinase Src-dependent. This new dosage-dependent switch of signaling modes of GPCRs has significant implications for GPCR intrinsic properties and desensitization.

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.

Biosynthesis and trafficking of seven transmembrane receptor signalling complexes

Recent studies have shown that 7-transmembrane receptors (7TM-Rs), their associated signalling molecules and scaffolding proteins are often constitutively associated under basal conditions. These studies highlight that receptor ontogeny and trafficking are likely to play key roles in the determination of both signalling specificity and efficacy. This review highlights information about how 7TM-Rs and their associated signalling molecules are trafficked to the cell surface as well as other intracellular destinations.

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.

Regulation of Dictyostelium prestalk-specific gene expression by a SHAQKY family MYB transcription factor

PstA and pstO cells are the two major populations in the prestalk region of the Dictyostelium slug and DIF-1 is a low molecular weight signalling molecule that selectively induces pstO cell-specific gene expression. The two cell types are defined by their differential use of spatially separated regions of the ecmA promoter. Additionally, there are anterior-like cells (ALCs) scattered throughout the rear, prespore region of the slug. They, like the pstO cells, use a cap-site distal ecmA promoter segment termed the ecmO region. When multimerised, a 22-nucleotide subsegment of the ecmO region directs expression in pstA cells, pstO cells and ALCs. It also directs DIF-inducible gene expression. The 22-nucleotide region was used to purify MybE, a protein with a single MYB DNA-binding domain of a type previously found only in a large family of plant transcription factors. Slugs of a mybE-null (mybE-) strain express an ecmAO:lacZ fusion gene (i.e. a reporter construct containing the ecmA and ecmO promoter regions) in pstA cells but there is little or no expression in pstO cells and ALCs. The ecmA gene is not induced by DIF-1 in a mybE-strain. Thus, MybE is necessary for DIF-1 responsiveness and for the correct differentiation of pstO cells and ALCs.

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.

Characterisation of Upd2, a Drosophila JAK/STAT pathway ligand

The characterisation of ligands that activate the JAK/STAT pathway has the potential to throw light onto a comparatively poorly understood aspect of this important signal transduction cascade. Here, we describe our analysis of the only invertebrate JAK/STAT pathway ligands identified to date, the Drosophila unpaired-like family. We show that upd2 is expressed in a pattern essentially identical to that of upd and demonstrate that the proteins encoded by this region activate JAK/STAT pathway signalling. Mutational analysis demonstrates a mutual semi-redundancy that can be visualised in multiple tissues known to require JAK/STAT signalling. In order to better characterise the in vivo function of these ligands, we developed a reporter based on a natural JAK/STAT pathway responsive enhancer and show that ectopic upd2 expression can effectively activate the JAK/STAT pathway. While both Upd and Upd2 are secreted JAK/STAT pathway agonists, tissue culture assays show that the signal-sequences of Upd and Upd2 confer distinct properties, with Upd associated primarily with the extracellular matrix and Upd2 secreted into the media. The differing biophysical characteristics identified for Upd-like molecules have implications for their function in vivo and adds another aspect to our understanding of cytokine signalling in Drosophila.

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.

Manifestations of multicellularity: Dictyostelium reports in

The recent release of the Dictyostelium genome sequence is important because Dictyostelium has become a much-favoured model system for cell and developmental biologists. The sequence has revealed a remarkably high total number of approximately 12 500 genes, only a thousand fewer than are encoded by Drosophila. Previous protein-sequence comparisons suggested that Dictyostelium is evolutionarily closer to animals and fungi than to plants, and the global protein sequence comparison, now made possible by the genome sequence, confirms this. This review focuses on several classes of proteins that are shared by Dictyostelium and animals: a highly sophisticated array of microfilament components, a large family of G-protein-coupled receptors and a diverse set of SH2 domain-containing proteins. The presence of these proteins strengthens the case for a relatively close relationship with animals and extends the range of problems that can be addressed using Dictyostelium as a model organism.

Inverse agonism: more than reverting constitutively active receptor signaling

Seven-transmembrane receptors constitute one of the major families of proteins encoded by the genome. This type of receptor is one of the most important targets of the pharmaceutical industry, and many of the drugs with significant therapeutic action have been shown to be inverse agonists. Concepts regarding the mechanisms by which ligands activate and inactivate receptors are thought to be far more complex that a simple on-off switch. For both drug design and pharmacology principles, it is important to understand the mechanisms by which these drugs achieve their effects. Recent studies have demonstrated intriguing actions of inverse agonists. They have been shown not only to block constitutive responses of receptors but also to activate and regulate seven-transmembrane receptor signaling and trafficking. The activation of pathways by inverse agonists was shown to occur mainly via G-protein-independent mechanisms. These findings emphasize the importance of inverse agonism as a principle of receptor regulation. In this paper, we will review the evidence supporting inverse agonist promoted signaling and trafficking.

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.

Activation of soluble guanylyl cyclase at the leading edge during Dictyostelium chemotaxis

Dictyostelium contains two guanylyl cyclases, GCA, a 12-transmembrane enzyme, and sGC, a homologue of mammalian soluble adenylyl cyclase. sGC provides nearly all chemoattractant-stimulated cGMP formation and is essential for efficient chemotaxis toward cAMP. We show that in resting cells the major fraction of the sGC-GFP fusion protein localizes to the cytosol, and a small fraction is associated to the cell cortex. With the artificial substrate Mn2+/GTP, sGC activity and protein exhibit a similar distribution between soluble and particulate fraction of cell lysates. However, with the physiological substrate Mg2+/GTP, sGC in the cytosol is nearly inactive, whereas the particulate enzyme shows high enzyme activity. Reconstitution experiments reveal that inactive cytosolic sGC acquires catalytic activity with Mg2+/GTP upon association to the membrane. Stimulation of cells with cAMP results in a twofold increase of membrane-localized sGC-GFP, which is accompanied by an increase of the membrane-associated guanylyl cyclase activity. In a cAMP gradient, sGC-GFP localizes to the anterior cell cortex, suggesting that in chemotacting cells, sGC is activated at the leading edge of the cell.

GSK3 is a multifunctional regulator of Dictyostelium development

Glycogen synthase kinase 3 (GSK3) is a central regulator of metazoan development and the Dictyostelium GSK3 homologue, GskA, also controls cellular differentiation. The originally derived gskA-null mutant exhibits a severe pattern formation defect. It forms very large numbers of pre-basal disc cells at the expense of the prespore population. This defect arises early during multicellular development, making it impossible to examine later functions of GskA. We report the analysis of a gskA-null mutant, generated in a different parental strain, that proceeds through development to form mature fruiting bodies. In this strain, Ax2/gskA-, early development is accelerated and slug migration greatly curtailed. In a monolayer assay of stalk cell formation, the Ax2/gskA- strain is hypersensitive to the stalk cell-inducing action of DIF-1 but largely refractory to the repressive effect exerted by extracellular cAMP. During normal development, apically situated prestalk cells express the ecmB gene just as they commit themselves to stalk cell differentiation. In the Ax2/gskA- mutant, ecmB is expressed throughout the prestalk region of the slug, suggesting that GskA forms part of the repressive signalling pathway that prevents premature commitment to stalk cell differentiation. GskA may also play an inductive developmental role, because microarray analysis identifies a large gene family, the 2C family, that require gskA for optimal expression. These observations show that GskA functions throughout Dictyostelium development, to regulate several key aspects of cellular patterning.

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.

Chemotaxis: signalling the way forward

During random locomotion, human neutrophils and Dictyostelium discoideum amoebae repeatedly extend and retract cytoplasmic processes. During directed cell migration--chemotaxis--these pseudopodia form predominantly at the leading edge in response to the local accumulation of certain signalling molecules. Concurrent changes in actin and myosin enable the cell to move towards the stimulus. Recent studies are beginning to identify an intricate network of signalling molecules that mediate these processes, and how these molecules become localized in the cell is now becoming clear.

The chemoattractants, IL-8 and formyl-methionyl-leucyl-phenylalanine, regulate granulocyte colony-stimulating factor signaling by inducing suppressor of cytokine signaling-1 expression

Suppressors of cytokine signaling (SOCS) are encoded by immediate early genes known to inhibit cytokine responses in a classical feedback loop. SOCS gene expression has been shown to be induced by many cytokines, growth factors, and innate immune stimuli, such as LPS. In this paper, we report that the chemoattractants, IL-8 and fMLP, up-regulate SOCS1 mRNA in human myeloid cells, primary human neutrophils, PBMCs, and dendritic cells. fMLP rapidly up-regulates SOCS1, whereas the induction of SOCS1 upon IL-8 treatment is delayed. IL-8 and fMLP did not signal via Jak/STATs in primary human macrophages, thus implicating the induction of SOCS by other intracellular pathways. As chemoattractant-induced SOCS1 expression in neutrophils may play an important role in regulating the subsequent response to growth promoting cytokines like G-CSF, we investigated the effect of chemoattractant-induced SOCS1 on cytokine signal transduction. We show that pretreatment of primary human neutrophils with fMLP or IL-8 blocks G-CSF-mediated STAT3 activation. This study provides evidence for cross-talk between chemoattractant and cytokine signal transduction pathways involving SOCS proteins, suggesting that these chemotactic factors may desensitize neutrophils to G-CSF via rapid induction of SOCS1 expression.

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.

Dd-STATb, a Dictyostelium STAT protein with a highly aberrant SH2 domain, functions as a regulator of gene expression during growth and early development

Dictyostelium, the only known non-metazoan organism to employ SH2 domain:phosphotyrosine signaling, possesses STATs (signal transducers and activators of transcription) and protein kinases with orthodox SH2 domains. Here, however, we describe a novel Dictyostelium STAT containing a remarkably divergent SH2 domain. Dd-STATb displays a 15 amino acid insertion in its SH2 domain and the conserved and essential arginine residue, which interacts with phosphotyrosine in all other known SH2 domains, is substituted by leucine. Despite these abnormalities, Dd-STATb is biologically functional. It has a subtle role in growth, so that Dd-STATb-null cells are gradually lost from the population when they are co-cultured with parental cells, and microarray analysis identified several genes that are either underexpressed or overexpressed in the Dd-STATb null strain. The best characterised of these,discoidin 1, is a marker of the growth-development transition and it is overexpressed during growth and early development of Dd-STATb null cells. Dimerisation of STAT proteins occurs by mutual SH2 domain:phosphotyrosine interactions and dimerisation triggers STAT nuclear accumulation. Despite its aberrant SH2 domain, the Dd-STATb protein sediments at the size expected for a homodimer and it is constitutively enriched in the nucleus. Moreover, these properties are retained when the predicted site of tyrosine phosphorylation is substituted by phenylalanine. These observations suggest a non-canonical mode of activation of Dd-STATb that does not rely on orthodox SH2 domain:phosphotyrosine interactions.

cAMP signaling in Dictyostelium. Complexity of cAMP synthesis, degradation and detection

cAMP plays a pivotal role in control of cell movement, differentiation and response to stress in all phases of the Dictyostelium life cycle. The multitudinous functions of cAMP require precise spatial and temporal control of its production, degradation and detection. Many novel proteins have recently been identified that critically modulate the cAMP signal. We focus in this review on the properties and functions of the three adenylyl cyclases and the three cAMP-phosphodiesterases that are present in Dictyostelium, and the network of proteins that regulate the activity of these enzymes. We also briefly discuss the two modes of detection of cAMP.

A STAT-regulated, stress-induced signalling pathway in Dictyostelium

The Dictyostelium stalk cell inducer differentiation-inducing factor (DIF) directs tyrosine phosphorylation and nuclear accumulation of the STAT (signal transducer and activator of transcription) protein Dd-STATc. We show that hyperosmotic stress, heat shock and oxidative stress also activate Dd-STATc. Hyperosmotic stress is known to elevate intracellular cGMP and cAMP levels, and the membrane-permeant analogue 8-bromo-cGMP rapidly activates Dd-STATc, whereas 8-bromo-cAMP is a much less effective inducer. Surprisingly, however, Dd-STATc remains stress activatable in null mutants for components of the known cGMP-mediated and cAMP-mediated stress-response pathways and in a double mutant affecting both pathways. Also, Dd-STATc null cells are not abnormally sensitive to hyperosmotic stress. Microarray analysis identified two genes, gapA and rtoA, that are induced by hyperosmotic stress. Osmotic stress induction of gapA and rtoA is entirely dependent on Dd-STATc. Neither gene is inducible by DIF but both are rapidly inducible with 8-bromo-cGMP. Again, 8-bromo-cAMP is a much less potent inducer than 8-bromo-cGMP. These data show that Dd-STATc functions as a transcriptional activator in a stress-response pathway and the pharmacological evidence, at least, is consistent with cGMP acting as a second messenger.

Visualizing signals moving in cells

Cells display a highly complex spatiotemporal organization, required to exert a wide variety of different functions, for example, detection, processing, and propagation of nerve impulses by neurons; contraction and relaxation by muscle cells; movement by leukocytes; and adsorption and secretion of nutrients and metabolites by epithelial cells lining the gut. Successful execution of these complex processes requires highly dynamic information transfer between different regions and compartments within cells. Through the development of fluorescent sensors for intracellular signaling molecules coupled with improved microscopic imaging techniques, it has now become possible to investigate signal propagation in cells with high spatial and temporal resolution.

The Dictyostelium prestalk cell inducer DIF regulates nuclear accumulation of a STAT protein by controlling its rate of export from the nucleus

Dd-STATc becomes tyrosine phosphorylated, dimerises and accumulates in the nuclei of Dictyostelium cells exposed to DIF, the chlorinated hexaphenone that directs prestalk cell differentiation. By performing cytoplasmic photobleaching of living cells, we show that DIF inhibits the nuclear export of Dd-STATc. Within Dd-STATc there is a 50 amino acid region containing several consensus CRM1 (exportin 1)-dependent nuclear export signals (NESs). Deletion of this region causes Dd-STATc to accumulate in the nucleus constitutively and, when coupled to GFP, the same region directs nuclear export. We show that the N-terminal-proximal 46 amino acids are necessary for nuclear accumulation of Dd-STATc and sufficient to direct constitutive nuclear accumulation when fused to GFP. Combining the photobleaching and molecular analyses, we suggest that DIF-induced dimerisation of Dd-STATc functionally masks the NES-containing region and that this leads to nett nuclear accumulation, directed by the N-terminal-proximal import signals. These results show that the regulated nuclear accumulation of a STAT protein can be controlled at the level of nuclear export and they also provide a better understanding of the mechanism whereby DIF directs cell type divergence.

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.

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

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

AT1 receptor mutant lacking heterotrimeric G protein coupling activates the Src-Ras-ERK pathway without nuclear translocation of ERKs

Angiotensin II (Ang II) type 1 receptors (AT1Rs) activate tyrosine kinases, including Src. Whether or not tyrosine kinase activation by AT1R occurs independently of heterotrimeric G protein coupling and, if so, the cellular function of such a mechanism are unknown. To address these questions, we used an AT1aR intracellular second loop mutant, which lacks heterotrimeric G protein coupling (AT1a-i2m). Surprisingly, Ang II-induced Src activation was preserved in AT1a-i2m, which was not attenuated by inhibiting protein kinase C and Ca(2+) or by inhibiting Galpha(i) or Galpha(q) in CHO-K1 cells. By contrast, Ang II-induced Src activation was abolished in a C-terminally truncated AT1a-(1--309), where Ang II-induced inositol phosphate response was preserved. Ang II activates ERKs via a Src-Ras-dependent mechanism in AT1a-i2m. ERKs activated by AT1a-i2m phosphorylate their cytoplasmic targets, including p90(RSK), but fail to translocate into the nucleus or to cause cell proliferation. Ang II-induced nuclear translocation of ERKs by wild type AT1aR was inhibited by overexpression of nuclear exportin Crm-1, while that by AT1a-i2m was restored by leptomycin B, an inhibitor of Crm-1. In summary, while Src and ERKs are activated by Ang II even without heterotrimeric G protein coupling, the carboxyl terminus of the AT1 receptor is required for activation of Src. Interestingly, ERKs activated by heterotrimeric G protein-independent mechanisms fail to phosphorylate nuclear targets due to lack of inhibition of Crm-1-induced nuclear export of ERKs. These results suggest that heterotrimeric G protein-dependent and -independent signaling mechanisms play distinct roles in Ang II-mediated cellular responses.