A novel Myb homolog initiates Dictyostelium development by induction of adenylyl cyclase expression

Genome-Wide Classification of Myb Domain-Containing Protein Families in Entamoeba invadens

Entamoeba histolytica, the causative agent of amebiasis, is the third leading cause of death among parasitic diseases globally. Its life cycle includes encystation, which has been mostly studied in Entamoeba invadens, responsible for reptilian amebiasis. However, the molecular mechanisms underlying this process are not fully understood. Therefore, we focused on the identification and characterization of Myb proteins, which regulate the expression of encystation-related genes in various protozoan parasites. Through bioinformatic analysis, we identified 48 genes in E. invadens encoding MYB-domain-containing proteins. These were classified into single-repeat 1R (20), 2R-MYB proteins (27), and one 4R-MYB protein. The in-silico analysis suggests that these proteins are multifunctional, participating in transcriptional regulation, chromatin remodeling, telomere maintenance, and splicing. Transcriptomic data analysis revealed expression signatures of eimyb genes, suggesting a potential orchestration in the regulation of early and late encystation-excystation genes. Furthermore, we identified probable target genes associated with reproduction, the meiotic cell cycle, ubiquitin-dependent protein catabolism, and endosomal transport. In conclusion, our findings suggest that E. invadens Myb proteins regulate stage-specific proteins and a wide array of cellular processes. This study provides a foundation for further exploration of the molecular mechanisms governing encystation and unveils potential targets for therapeutic intervention in amebiasis.

Independent Evolution of the MYB Family in Brown Algae

Myeloblastosis (MYB) proteins represent one of the largest families of eukaryotic transcription factors and regulate important processes in growth and development. Studies on MYBs have mainly focused on animals and plants; however, comprehensive analysis across other supergroups such as SAR (stramenopiles, alveolates, and rhizarians) is lacking. This study characterized the structure, evolution, and expression of MYBs in four brown algae, which comprise the biggest multicellular lineage of SAR. Subfamily 1R-MYB comprised heterogeneous proteins, with fewer conserved motifs found outside the MYB domain. Unlike the SHAQKY subgroup of plant 1R-MYB, THAQKY comprised the largest subgroup of brown algal 1R-MYBs. Unlike the expansion of 2R-MYBs in plants, brown algae harbored more 3R-MYBs than 2R-MYBs. At least ten 2R-MYBs, fifteen 3R-MYBs, and one 6R-MYB orthologs existed in the common ancestor of brown algae. Phylogenetic analysis showed that brown algal MYBs had ancient origins and a diverged evolution. They showed strong affinity with stramenopile species, while not with red algae, green algae, or animals, suggesting that brown algal MYBs did not come from the secondary endosymbiosis of red and green plastids. Sequence comparison among all repeats of the three types of MYB subfamilies revealed that the repeat of 1R-MYBs showed higher sequence identity with the R3 of 2R-MYBs and 3R-MYBs, which supports the idea that 1R-MYB was derived from loss of the first and second repeats of the ancestor MYB. Compared with other species of SAR, brown algal MYB proteins exhibited a higher proportion of intrinsic disordered regions, which might contribute to multicellular evolution. Expression analysis showed that many MYB genes are responsive to different stress conditions and developmental stages. The evolution and expression analyses provided a comprehensive analysis of the phylogeny and functions of MYBs in brown algae.

Sir2D, a Sirtuin family protein, regulates adenylate cyclase A expression through interaction with the MybB transcription factor early in Dictyostelium development upon starvation

Sirtuin interacts with many regulatory proteins involved in energy homeostasis, DNA repair, cell survival, and lifespan extension. We investigated the functional roles of Sir2D during early Dictyostelium development upon starvation. We found that ectopic expression of Sir2D accelerated development among three Sirtuins containing highly homologous catalytic domain sequences to mouse Sirt1. Sir2D expression upregulated adenylate cyclase A (aca) mRNA expression 2, 4 and 6 h after starvation. We have previously reported that nicotinamide, a Sirt1 inhibitor, treatment delayed the development and decreased the expression of aca at 4 h after starvation. Sir2D expressing cells showed resistance against the nicotinamide effect. RNAi-mediated Sir2D knockdown cells were generated, and their development was also delayed. Aca expression was decreased 4 h after starvation. Sir2D expression restored the developmental impairment of Sir2D knockdown cells. The induction of aca upon starvation starts with transcriptional activation of MybB. The ectopic expression of MybB accelerated the development and increased the expression of aca 2 and 4 h after starvation but did not restore the phenotype of Sir2D knockdown cells. Sir2D expression had no effects on MybB-null mutant cells during early development. Thus, MybB is necessary for the upregulation of aca by Sir2D, and Sir2D is necessary for the full induction of aca after 4 h by MybB. MybB was coimmunoprecipitated with Sir2D, suggesting an interaction between MybB and Sir2D. These results suggest that Sir2D regulates aca expression through interaction with the MybB transcription factor early in Dictyostelium development upon starvation.

Glutathione initiates the development of Dictyostelium discoideum through the regulation of YakA

Reduced glutathione (GSH) is an essential metabolite that performs multiple indispensable roles during the development of Dictyostelium. We show here that disruption of the gene (gcsA-) encoding y-glutamylcysteine synthetase, an essential enzyme in GSH biosynthesis, inhibited aggregation, and that this developmental defect was rescued by exogenous GSH, but not by other thiols or antioxidants. In GSH-depleted gcsA- cells, the expression ofa growth-stage-specific gene (cprD) was not inhibited, and we did not detect the expression of genes that encode proteins required for early development (cAMP receptor, carA/cAR1; adenylyl cyclase, acaA/ACA; and the catalytic subunit of protein kinase A, pkaC/PKA-C). The defects in gcsA cells were not restored by cAMP stimulation or by cAR1 expression. Further, the expression of yakA, which initiates development and induces the expression of PKA-C, ACA, and cAR1, was regulated by the intracellular concentration of GSH. Constitutive expression of YakA in gcsA- cells (YakA(OE)/gcsA-) rescued the defects in developmental initiation and the expression of early developmental genes in the absence of GSH. Taken together, these findings suggest that GSH plays an essential role in the transition from growth to development by modulating the expression of the genes encoding YakA as well as components thatact downstream in the YakA signaling pathway.

Nucleocytoplasmic shuttling of a GATA transcription factor functions as a development timer

Biological oscillations are observed at many levels of cellular organization. In the social amoeba Dictyostelium discoideum, starvation-triggered multicellular development is organized by periodic cyclic adenosine 3',5'-monophosphate (cAMP) waves, which provide both chemoattractant gradients and developmental signals. We report that GtaC, a GATA transcription factor, exhibits rapid nucleocytoplasmic shuttling in response to cAMP waves. This behavior requires coordinated action of a nuclear localization signal and reversible G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor-mediated phosphorylation. Although both are required for developmental gene expression, receptor occupancy promotes nuclear exit of GtaC, which leads to a transient burst of transcription at each cAMP cycle. We demonstrate that this biological circuit filters out high-frequency signals and counts those admitted, thereby enabling cells to modulate gene expression according to the dynamic pattern of the external stimuli.

Developmental changes in transcriptional profiles

Recent advances in quantitation of mRNA by hybridization to microarrayed gene sequences or by deep sequencing of cDNA (RNA-seq) have provided global views of the abundance of each transcript. Analyses of RNA samples taken at 2 or 4 h intervals throughout development of Dictyostelium discoideum have defined the developmental changes in transcriptional profiles. Comparisons of the transcriptome of wild-type cells to that of mutant strains lacking a gene critical to progression through the developmental stages have defined key steps in the progression. The transcriptional response to cAMP pulses depends on the expression of pulse-independent genes that have been identified by transcriptional profiling with microarrays. Similar techniques were used to discover that the DNA binding protein GBF functions in a feed-forward loop to regulate post-aggregation genes and that expression of a set of late genes during culmination is dependent on the DNA binding protein SrfA. RNA-seq is able to reliably measure individual mRNAs present as a single copy per cell as well as mRNAs present at a thousand fold higher abundance. Using this technique it was found that 65% of the genes in Dictyostelium change twofold or more during development. Many decrease during the first 8 h of development, while the rest increase at specific stages and this pattern is evolutionarily conserved as found by comparing the transcriptomes of D. discoideum and Dictyostelium purpureum. The transcriptional profile of each gene is readily available at dictyBase and more sophisticated analyses are available on DictyExpress.

Myb transcription factors in the oomycete Phytophthora with novel diversified DNA-binding domains and developmental stage-specific expression

Transcription factors containing two or three imperfect tandem repeats of the Myb DNA-binding domain (named R2R3 and R1R2R3, respectively) regulate important processes in growth and development. This study characterizes the structure, evolution, and expression of these proteins in the potato pathogen Phytophthora infestans and other oomycetes. P. infestans was found to encode five R2R3 and nine R1R2R3 transcription factor-like proteins, plus several with additional configurations of Myb domains. Sets of R2R3 and R1R2R3 orthologs are well-conserved in three Phytophthora species. Analyses of sites that bind DNA in canonical Myb transcription factors, such as mammalian c-Myb, revealed unusual diversification in the DNA recognition helices of the oomycete proteins. While oomycete R2R3 proteins contain c-Myb-like helices, R1R2R3 proteins exhibit either c-Myb-like or novel sequences. This suggests divergence in their DNA-binding specificities, which was confirmed by electrophoretic mobility shift assays. Eight of the P. infestans R2R3 and R1R2R3 genes are up-regulated during sporulation and three during zoospore release, which suggests their involvement in spore development. This is supported by the observation that an oomycete that does not form zoospores, Hyaloperonospora arabidopsidis, contains one-third fewer of these genes than Phytophthora.

The carboxy-terminal domain of Dictyostelium C-module-binding factor is an independent gene regulatory entity

The C-module-binding factor (CbfA) is a multidomain protein that belongs to the family of jumonji-type (JmjC) transcription regulators. In the social amoeba Dictyostelium discoideum, CbfA regulates gene expression during the unicellular growth phase and multicellular development. CbfA and a related D. discoideum CbfA-like protein, CbfB, share a paralogous domain arrangement that includes the JmjC domain, presumably a chromatin-remodeling activity, and two zinc finger-like (ZF) motifs. On the other hand, the CbfA and CbfB proteins have completely different carboxy-terminal domains, suggesting that the plasticity of such domains may have contributed to the adaptation of the CbfA-like transcription factors to the rapid genome evolution in the dictyostelid clade. To support this hypothesis we performed DNA microarray and real-time RT-PCR measurements and found that CbfA regulates at least 160 genes during the vegetative growth of D. discoideum cells. Functional annotation of these genes revealed that CbfA predominantly controls the expression of gene products involved in housekeeping functions, such as carbohydrate, purine nucleoside/nucleotide, and amino acid metabolism. The CbfA protein displays two different mechanisms of gene regulation. The expression of one set of CbfA-dependent genes requires at least the JmjC/ZF domain of the CbfA protein and thus may depend on chromatin modulation. Regulation of the larger group of genes, however, does not depend on the entire CbfA protein and requires only the carboxy-terminal domain of CbfA (CbfA-CTD). An AT-hook motif located in CbfA-CTD, which is known to mediate DNA binding to A+T-rich sequences in vitro, contributed to CbfA-CTD-dependent gene regulatory functions in vivo.

The C-module DNA-binding factor mediates expression of the dictyostelium aggregation-specific adenylyl cyclase ACA

Aggregation of Dictyostelium discoideum amoebae into multicellular structures is organized by cyclic AMP (cAMP), which acts as a chemoattractant, as a second messenger, and as a morphogen. Aggregation of D. discoideum cells depends on the expression of adenylyl cyclase ACA, which provides extracellular cAMP for signal relay and intracellular cAMP for the induction of genes required at multicellular stages. We have identified a DNA-binding activity specific for a highly A+T-enriched motif in the upstream region of the ACA-encoding gene, acaA. The factor shows DNA-binding characteristics very similar to those of C-module-binding factor (CbfA). Although CbfA was originally identified as a putative regulator of the activity of D. discoideum retrotransposon TRE5-A, it also was found to be essential for aggregation of D. discoideum cells. The identified DNA-binding activity was absent in mutant cells depleted of CbfA, and CbfA could be precipitated using an acaA promoter fragment. We propose that CbfA binds to the acaA promoter to provide a basal transcription activity that is required for induction of ACA expression after the onset of D. discoideum development.

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.

CbfA, the C-module DNA-binding factor, plays an essential role in the initiation of Dictyostelium discoideum development

We recently isolated from Dictyostelium discoideum cells a DNA-binding protein, CbfA, that interacts in vitro with a regulatory element in retrotransposon TRE5-A. We have generated a mutant strain that expresses CbfA at <5% of the wild-type level to characterize the consequences for D. discoideum cell physiology. We found that the multicellular development program leading to fruiting body formation is highly compromised in the mutant. The cells cannot aggregate and stay as a monolayer almost indefinitely. The cells respond properly to prestarvation conditions by expressing discoidin in a cell density-dependent manner. A genomewide microarray-assisted expression analysis combined with Northern blot analyses revealed a failure of CbfA-depleted cells to induce the gene encoding aggregation-specific adenylyl cyclase ACA and other genes required for cyclic AMP (cAMP) signal relay, which is necessary for aggregation and subsequent multicellular development. However, the cbfA mutant aggregated efficiently when mixed with as few as 5% wild-type cells. Moreover, pulsing cbfA mutant cells developing in suspension with nanomolar levels of cAMP resulted in induction of acaA and other early developmental genes. Although the response was less efficient and slower than in wild-type cells, it showed that cells depleted of CbfA are able to initiate development if given exogenous cAMP signals. Ectopic expression of the gene encoding the catalytic subunit of protein kinase A restored multicellular development of the mutant. We conclude that sensing of cell density and starvation are independent of CbfA, whereas CbfA is essential for the pattern of gene expression which establishes the genetic network leading to aggregation and multicellular development of D. discoideum.

Chemoattractant signaling in dictyostelium discoideum

Dictyostelium is an accessible organism for studies of signaling via chemoattractant receptors. Chemoattractant-mediated signaling events and components are reviewed and presented as a series of connected modules, including excitation, inhibition, G protein-independent responses, early gene expression, inositol lipids, PH domain-containing proteins, cyclic AMP signaling, polarization acquisition, actin polymerization, and cortical myosin. The network incorporates information from biochemical, genetic, and cell biological experiments carried out on living cells. The modules and connections represent current understanding, and future information is expected to modify and build upon this structure.

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 novel Myb-related protein involved in transcriptional activation of encystation genes in Giardia lamblia

Giardia lamblia is an important human intestinal parasite that survives outside of the host by differentiation of trophozoites into infectious cysts. Transcriptional regulation is key for encystation gene expression, but the mechanisms are unknown. Giardia genome database searches identified a myb-like gene (gmyb2) whose expression increased during encystation. Epitope-tagged gMyb2 localized to both nuclei. DNA binding and mutation analysis showed that gMyb2 binds specifically to C(T/A)ACAG, a c-Myb-like target sequence in the promoters of encystation-induced genes encoding gMyb2, three cyst wall proteins and G6PI-B, a key enzyme in cyst wall polysaccharide biosynthesis. gMyb2 binding sites were not found in the upstream regions of 31 other giardial genes. Deletion of the putative gMyb2 binding site greatly reduced encystation-specific promoter activity of g6pi-b. Fusion of gMyb2 binding sites to the constitutive ran promoter or g6pi-b promoter deletion lacking the gMyb2 binding site in-duced encystation-specific expression. gMyb2 may play an important role in transcriptional regulation of encystation genes, and may help co-ordinate synthesis of cyst wall proteins and polysaccharide. gMyb2 is the first giardial transcription factor to be functionally identified and the first that is associated with upregulation of encystation genes. This work provides a model for study of differential gene regulation in early diverging eukaryotic organisms.

Existence of xenobiotic response element binding in Dictyostelium

Xenobiotic response element (XRE) is a core nucleotide sequence at the upstream of inducible target genes for the transcription factor aryl hydrocarbon receptor (AhR) that is responsible for recognition of exogenous environmental pollutants in eukaryotic cells. Gel retardation electrophoresis revealed the presence of binding of a radiolabeled probe containing XRE in both cytosolic and nuclear preparations of the slime mold Dictyostelium. Unlabeled XRE probe was more potent in competing for XRE binding in both fractions than unlabeled XRE probe with a point mutation at the core element. Limited proteolysis by V8 protease did not markedly affect XRE binding in both fractions, while XRE binding decreased during in vitro incubation at 30 degrees C for up to 24 h at decline rates proportional to increasing pHs at a range of 6.5-8.5 in cytosolic fractions in a manner different from those in nuclear fractions. Deprivation of nutrients induced aggregation of cells within 4-8 h later, followed by formation of first finger tips around 12 h later and subsequent development to mobile slugs within 16 h and then to fruit bodies between 20 and 24 h later. The starvation led to a marked decrement of XRE binding in cytosolic fractions 4-36 h later, followed by a robust but transient increment of that in nuclear extracts 12-20 h afterward. However, XRE binding was not affected by antibodies against AhR-related proteins known to date in both fractions irrespective of nutritional conditions. These results suggest the abundance of as-yet unidentified proteins with high affinity for XRE in the slime mold Dictyostelium. The possibility that those proteins may be translocated from the cytoplasm to the nucleus in response to cellular development during starvation is feasible.

A novel Dictyostelium Cdk8 is required for aggregation, but is dispensable for growth

When Dictyostelium cells starve, they express genes necessary for aggregation. Using insertional mutagenesis, we have isolated a mutant that does not aggregate upon starvation and that forms small plaques on bacterial lawns, thus indicating slow growth. Sequencing of the mutated locus showed a strong similarity to the catalytic domain of cdc2-related kinase genes. Phylogenetic analysis further indicated that the amino acid sequence was more close to cyclin-dependent kinase 8 than to the sequence of other cyclin-dependent kinases. Thus, we designated this gene as Ddcdk8. The Ddcdk8-null cells do not aggregate and grow somewhat more slowly than parental cells when being shaken in axenic medium or laid on bacterial plates. To confirm whether these defective phenotypes were caused by disruption of this gene, the Ddcdk8-null cells were complemented with DdCdk8 protein expressed from an endogenous promoter, but not an actin promoter, and when the complemented cells were then allowed to grow on a bacterial lawn, they began to aggregate as the food supply was depleted and finally became fruiting bodies. The results suggest that properly regulated DdCdk8 activity is essential for aggregation. Because, when starved, Ddcdk8-null cells do not express the acaA transcripts required for aggregation, we deduce that Ddcdk8 is epistatic for acaA expression, indicating that the DdCdk8 products may regulate expression of acaA and/or other genes.

Myb-binding site regulates the expression of glucosamine-6-phosphate isomerase in Dictyostelium discoideum

A homolog of the glucosamine-6-phosphate isomerase in the cellular slime mold Dictyostelium discoideum has been analyzed. The gene disruption mutant was arrested at the mound stage, demonstrating that the gene is important for development. The gene was expressed in vegetatively growing cells, silenced on starvation and expressed again in prestalk cells during the multicellular stages. The upstream region of the gene (1376 bp relative to ATG) was cloned and sequenced to study the transcription control mechanisms. Analysis of deletion mutants and a site-directed mutant indicated that the Myb-binding sequence (5'-AACTG-3') localized in the upstream region is important for gene expression. The results of gel-shift assays showed the presence of an Myb-related protein binding to the sequence at the growing phase and another protein binding to the sequence at developmental stages.

CRTF is a novel transcription factor that regulates multiple stages of Dictyostelium development

During aggregation, Dictyostelium establish nanomolar oscillation waves of extracellular cAMP, but as development progresses, cells become responsive to higher, non-fluctuating concentrations of cAMP. The regulation of the promoter responsible for expression of cAMP receptor subtype 1, CAR1, during aggregation reflects these signaling variations. Transcription of CAR1 from the early, aggregation promoter is activated by cAMP pulsing, but is repressed by continuous exposure to micromolar concentrations of cAMP. Deletion and mutation analyses of this promoter had defined an element essential for cAMP-regulated expression, and mobility shift assay, DNA crosslinking and DNase I footprinting experiments had identified a nuclear protein (CRTF) with zinc-dependent sequence binding specificity. In our study, CRTF was purified to homogeneity, peptides were sequenced and full-length cDNAs were obtained. The deduced CRTF protein is ∼100 kDa with a C-terminal, zinc finger-like motif required for DNA binding; CRTF purified from cells, however, represents only a 40 kDa C-terminal fragment that retains DNA-binding activity.

As might have been predicted if CRTF were essential for the regulation of CAR1, crtf-null strains fail to develop under standard conditions or to exhibit induced expression of CAR1 or other cAMP-regulated genes. Furthermore, crtf-nulls also fail to sporulate, even under conditions that bypass the dependence on early cAMP signaling pathways. In addition, early developmental events of crtf-null strains could be rescued with exogenous cAMP treatment, constitutive expression of CAR1 or co-development with wild-type cells; however, these treatments were insufficient to promote sporulation. This suggests a cell-autonomous role for CRTF during late development that is separate from its capacity to control CAR1 expression. Finally, ablation of CRTF promotes a precocious induction of certain cAMP-dependent gene expression pathways. We suggest that CRTF may function to help insulate distinct pathways from simultaneous and universal activation by cAMP. CRTF, thus, exhibits multiple complex and independent regulatory functions during Dictyostelium development.

A novel role of differentiation-inducing factor-1 in Dictyostelium development, assessed by the restoration of a developmental defect in a mutant lacking mitogen-activated protein kinase ERK2

It has been previously reported that the differentiating wild-type cells of Dictyostelium discoideum secrete a diffusible factor or factors that are able to rescue the developmental defect in the mutant lacking extracellular signal-regulated kinase 2 (ERK2), encoded by the gene erkB. In the present study, it is demonstrated that differentiation-inducing factor-1 (DIF-1) for stalk cells can mimic the role of the factor(s) and the mechanism of the action of DIF-1 in the erkB null mutant is also discussed. The mutant usually never forms multicellular aggregates, because of its defect in cyclic adenosine monophosphate (cAMP) signaling. In the presence of 100 nM DIF-1, however, the mutant cells formed tiny slugs, which eventually developed into small fruiting bodies. In contrast, DIF-1 never rescued the developmental arrest of other Dictyostelium mutants lacking adenylyl cyclase A (ACA), cAMP receptors cAR1 and cAR3, heterotrimeric G-protein, the cytosolic regulator of ACA, or the catalytic subunit of cAMP-dependent protein kinase (PKA-C). Most importantly, it was found that DIF-1 did not affect the cellular cAMP level, but rather elevated the transcriptional level of pka during the development of erkB null cells. These results suggest that DIF-1 may rescue the developmental defect in erkB null cells via the increase in PKA activity, thus giving the first conclusive evidence that DIF-1 plays a crucial role in the early events of Dictyostelium development as well as in prestalk and stalk cell induction.

Suppression of the growth/differentiation transition in Dictyostelium development by transient expression of a novel gene, dia1

In Dictyostelium discoideum Ax-2 cells, a specific checkpoint (PS point) from which cells enter the differentiation phase in response to starvation has been specified in the cell cycle. Using the differential display method, we isolated a novel gene, dia1 (differentiation-associated gene 1), that is specifically expressed in cells differentiating from the PS point. The dia1 mRNA has an open reading frame of 1,368 bp and is deduced to code for a 48.6 kDa protein (DIA1). The DIA1 protein is highly serine-rich and the serine residues are predominantly located in the C-terminal region. After the PSORT II search, the protein is predicted to be GPI-anchored at the plasma membrane. Unexpectedly, dia1 overexpression rather impaired the progression of differentiation, possibly coupled with the reduced expression of early genes such as cAMP receptor1 (car1). The inhibitory effect of dia1 expression on early differentiation was almost completely nullified by externally applied cAMP pulses. In contrast to dia1 overexpression, antisense RNA-mediated dia1 inactivation was found to enhance the initial step of cell differentiation, as exemplified by precocious expression of car1 and other early genes. We discuss the unique structure and function of DIA1 in relation to the cooperative development of cells during the establishment of multicellular organization.

A diffusible factor involved in MAP-kinase ERK2-regulated development of Dictyostelium

Mitogen-activated protein (MAP)-kinase extracellular signal regulated kinase (ERK2) is essential for regulation of the intracellular cyclic adenosine monophosphate (cAMP) level in Dictyostelium. The mutant lacking ERK2, erk2-null, is arrested at the pre-aggregation stage, but develops into a fruiting body in a mixed population of wild-type and mutant cells. This fact implies that wild-type cells provide a certain factor that is missing in erk2-null. It was clarified that both wild-type strains KAx3 and Ax2 secreted a diffusible factor that enables erk2-null to develop. The fruiting body formed from erk2-null cells was smaller than that formed by the wild-type cells and consisted of a small sorus supported by a slender stalk with a single row of vacuolated stalk cells. The resulting spores were able to germinate and multiply on a bacterial lawn, but they were unable to develop unless the factor was provided. After 8 h of starvation, wild-type cells started to secrete the factor, which had a molecular mass of less than 3 kDa and was heat stable. The effect of this factor could not be mimicked by either cAMP or folate. Adenylyl cyclase A and cell surface cAMP receptors cAR1 and cAR3 were all indispensable components for the factor to function. Considering the molecular mass and the mode of action, this factor could be a novel one. Possible targets of this factor are discussed in terms of cAMP-dependent protein kinase activation.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Cell cycle-dependent regulation of early developmental genes

Cell cycle phase at the onset of development in Dictyostelium influences cell fate. Cells in the G2 phase, which tend to become spores, show a more rapid induction of expression of the cell surface receptor involved in the chemotaxis. We show that differential induction of developmental expression is restricted to some transcripts, including those encoding proteins required for chemotaxis, and thus is not due to general transcriptional repression during mitosis. We also show that cells showing rapid induction of one such gene are preferentially located at the centre of early aggregates. These results are consistent with cells derived from G2 phase being at the centre of early aggregates because selective differences in gene regulation render them more efficient at aggregation.

Dictyostelium development-socializing through cAMP

Starving Dictyostelium amoebae use cAMP as a chemoattractant to gather into aggregates, as a hormone-like signal to induce cell differentiation, and as an intracellular messenger to control stalk- and spore cell maturation and germination of spores. In this chapter we describe the respective roles of the three adenylyl cyclases ACA, ACB and ACG in controlling cAMP signaling during development and we discuss how cAMP signals are processed by the cells to trigger the large repertoire of gene regulatory events that is under control of this signal molecule.

Starvation promotes Dictyostelium development by relieving PufA inhibition of PKA translation through the YakA kinase pathway

When nutrients are depleted, Dictyostelium cells undergo cell cycle arrest and initiate a developmental program that ensures survival. The YakA protein kinase governs this transition by regulating the cell cycle, repressing growth-phase genes and inducing developmental genes. YakA mutants have a shortened cell cycle and do not initiate development. A suppressor of yakA that reverses most of the developmental defects of yakA- cells, but none of their growth defects was identified. The inactivated gene, pufA, encodes a member of the Puf protein family of translational regulators. Upon starvation, pufA- cells develop precociously and overexpress developmentally important proteins, including the catalytic subunit of cAMP-dependent protein kinase, PKA-C. Gel mobility-shift assays using a 200-base segment of PKA-C's mRNA as a probe reveals a complex with wild-type cell extracts, but not with pufA- cell extracts, suggesting the presence of a potential PufA recognition element in the PKA-C mRNA. PKA-C protein levels are low at the times of development when this complex is detectable, whereas when the complex is undetectable PKA-C levels are high. There is also an inverse relationship between PufA and PKA-C protein levels at all times of development in every mutant tested. Furthermore, expression of the putative PufA recognition elements in wild-type cells causes precocious aggregation and PKA-C overexpression, phenocopying a pufA mutation. Finally, YakA function is required for the decline of PufA protein and mRNA levels in the first 4 hours of development. We propose that PufA is a translational regulator that directly controls PKA-C synthesis and that YakA regulates the initiation of development by inhibiting the expression of PufA. Our work also suggests that Puf protein translational regulation evolved prior to the radiation of metazoan species.

A myb-related protein required for culmination in Dictyostelium

The avian retroviral v-myb gene and its cellular homologues throughout the animal and plant kingdoms contain a conserved DNA binding domain. We have isolated an insertional mutant of Dictyostelium unable to switch from slug migration to fruiting body formation i.e. unable to culminate. The gene that is disrupted, mybC, codes for a protein with a myb-like domain that is recognized by an antibody against the v-myb repeat domain. During development of myb+ cells, mybC is expressed only in prestalk cells. When developed together with wild-type cells mybC- cells are able to form both spores and stalk cells very efficiently. Their developmental defect is also bypassed by overexpressing cAMP-dependent protein kinase. However even when their defect is bypassed, mybC null slugs and culminates produce little if any of the intercellular signalling peptides SDF-1 and SDF-2 that are believed to be released by prestalk cells at culmination. We propose that the mybC gene product is required for an intercellular signaling process controlling maturation of stalk cells and spores and that SDF-1 and/or SDF-2 may be implicated in this process.