DIF-1 induces the basal disc of the Dictyostelium fruiting body

NOVEL INVENTION OF SPORE INDUCTION IN A SISTER SPECIES TO GROUP 4 DICTYOSTELIA

Background

Dictyostelia are soil amoebas that aggregate to form fruiting bodies with spores and stalk cells in response to starvation. Where known, species across the dictyostelid phylogeny use secreted cAMP, detected by cAMP receptors (cARs) to induce the differentiation of spores and to organize fruiting body construction. However, recent deletion of the single cAR of Polyspondylium violaceum (Pvio) left both its fruiting bodies and spores intact.

Methods

To investigate whether Pvio sporulation can occur in the absence of secreted cAMP and to explore alternative inducers in a bioassay , three prespore genes were identified and gene fusions of their promoters with the LacZ reporter gene were transformed into Pvio cells. After assessing the spatial expression pattern of the genes and the stage at which prespore gene expression initiated, the effect of cAMP and other Dictyostelium discoideum ( Ddis) signal molecules were tested on prespore gene expression in vitro.

Results

Pvio genes g4562 (psp1), g2696 (psp2) and g2380 (psp3) were identified as homologs of Ddis spore coat genes. They were first expressed around 4 h of starvation in aggregation centres and later in the posterior 4/5 th of emerging sorogens and the spore head of early fruiting bodies. Cells from dissociated 4 h aggregates and shaken in suspension for 6 h increased prespore- LacZ reporter activity 4-fold for psp1 and 6-fold for psp2, but this increase was at least 5-fold higher when cells were plated on solid substratum for 6 h to develop normally. cAMP had no effect on prespore gene induction and neither had the Pvio chemoattractant glorin nor the Ddis chemoattractants and differentiation inducers folate, c-di-GMP, DIF-1, GABA, cGMP and 8Br-cAMP.

Conclusions

The Pvio lineage uniquely evolved a novel genetic network for synthesis, detection and processing of the signal that triggers its main survival strategy.

The chemical ecology and physiological functions of type I polyketide natural products: the emerging picture

Covering: up to 2024.For many years, the value of complex polyketides lay in their medical properties, including their antibiotic and antifungal activities, with little consideration paid to their native functions. However, more recent evidence gathered from the study of inter-organismal interactions has revealed the influence of these metabolites upon the ecological adaptation and distribution of their hosts, as well as their modes of communication. The increasing number of sequenced genomes and associated transcriptomes has also unveiled the widespread occurrence of the underlying biosynthetic enzymes across all kingdoms of life, and the important contributions they make to physiological events specific to each organism. This review depicts the diversity of roles fulfilled by type I polyketides, particularly in light of studies carried out during the last decade, providing an initial overall picture of their diverse functions.

Emerging roles for diguanylate cyclase during the evolution of soma in dictyostelia

BACKGROUND

Cyclic di-guanylate (c-di-GMP), synthesized by diguanylate cyclase, is a major second messenger in prokaryotes, where it triggers biofilm formation. The dictyostelid social amoebas acquired diguanylate cyclase (dgcA) by horizontal gene transfer. Dictyostelium discoideum (Ddis) in taxon group 4 uses c-di-GMP as a secreted signal to induce differentiation of stalk cells, the ancestral somatic cell type that supports the propagating spores. We here investigated how this role for c-di-GMP evolved in Dictyostelia by exploring dgcA function in the group 2 species Polysphondylium pallidum (Ppal) and in Polysphondylium violaceum (Pvio), which resides in a small sister clade to group 4.

RESULTS

Similar to Ddis, dgcA is upregulated after aggregation in Ppal and Pvio and predominantly expressed in the anterior region and stalks of emerging fruiting bodies. DgcA null mutants in Ppal and Pvio made fruiting bodies with very long and thin stalks and only few spores and showed delayed aggregation and larger aggregates, respectively. Ddis dgcA- cells cannot form stalks at all, but showed no aggregation defects. The long, thin stalks of Ppal and Pvio dgcA- mutants were also observed in acaA- mutants in these species. AcaA encodes adenylate cyclase A, which mediates the effects of c-di-GMP on stalk induction in Ddis. Other factors that promote stalk formation in Ddis are DIF-1, produced by the polyketide synthase StlB, low ammonia, facilitated by the ammonia transporter AmtC, and high oxygen, detected by the oxygen sensor PhyA (prolyl 4-hydroxylase). We deleted the single stlB, amtC and phyA genes in Pvio wild-type and dgcA- cells. Neither of these interventions affected stalk formation in Pvio wild-type and not or very mildly exacerbated the long thin stalk phenotype of Pvio dgcA- cells.

CONCLUSIONS

The study reveals a novel role for c-di-GMP in aggregation, while the reduced spore number in Pvio and Ppal dgcA- is likely an indirect effect, due to depletion of the cell pool by the extended stalk formation. The results indicate that in addition to c-di-GMP, Dictyostelia ancestrally used an as yet unknown factor for induction of stalk formation. The activation of AcaA by c-di-GMP is likely conserved throughout Dictyostelia.

Visualization of c-di-GMP in multicellular Dictyostelium stages

The bacterial signaling molecule cyclic diguanosine monophosphate (c-di-GMP) is only synthesized and utilized by the cellular slime mold Dictyostelium discoideum among eukaryotes. Dictyostelium cells undergo a transition from a unicellular to a multicellular state, ultimately forming a stalk and spores. While Dictyostelium is known to employ c-di-GMP to induce differentiation into stalk cells, there have been no reports of direct observation of c-di-GMP using fluorescent probes. In this study, we used a fluorescent probe used in bacteria to visualize its localization within Dictyostelium multicellular bodies. Cytosolic c-di-GMP concentrations were significantly higher at the tip of the multicellular body during stalk formation.

Yellow polyketide pigment suppresses premature hatching in social amoeba

Low-molecular-weight natural products from microbes are indispensable in the development of potent drugs. However, their biological roles within an ecological context often remain elusive. Here, we shed light on natural products from eukaryotic microorganisms that have the ability to transition from single cells to multicellular organisms: the social amoebae. These eukaryotes harbor a large number of polyketide biosynthetic genes in their genomes, yet virtually none of the corresponding products can be isolated or characterized. Using complementary molecular biology approaches, including CRISPR-Cas9, we generated polyketide synthase (pks5) inactivation and overproduction strains of the social amoeba Dictyostelium discoideum. Differential, untargeted metabolomics of wild-type versus mutant fruiting bodies allowed us to pinpoint candidate metabolites derived from the amoebal PKS5. Extrachromosomal expression of the respective gene led to the identification of a yellow polyunsaturated fatty acid. Analysis of the temporospatial production pattern of this compound in conjunction with detailed bioactivity studies revealed the polyketide to be a spore germination suppressor.

Generating polyketide diversity in Dictyostelium: a Steely hybrid polyketide synthase produces alternate products at different developmental stages

The soil is a rich ecosystem where many ecological interactions are mediated by small molecules, and in which amoebae are low-level predators and also prey. The social amoeba Dictyostelium discoideum has a high genomic potential for producing polyketides to mediate its ecological interactions, including the unique 'Steely' enzymes, consisting of a fusion between a fatty acid synthase and a chalcone synthase. We report here that D. discoideum further increases its polyketide potential by using the StlB Steely enzyme, and a downstream chlorinating enzyme, to make both a chlorinated signal molecule, DIF-1, during its multi-cellular development, and a set of abundant polyketides in terminally differentiated stalk cells. We identify one of these as a chlorinated dibenzofuran with potent anti-bacterial activity. To do this, StlB switches expression from prespore to stalk cells in late development and is cleaved to release the chalcone synthase domain. Expression of this domain alone in StlB null cells allows synthesis of the stalk-associated, chlorinated polyketides. Thus, by altered expression and processing of StlB, cells make first a signal molecule, and then abundant secondary metabolites, which we speculate help to protect the mature spores from bacterial infection.

The polyketide synthase StlA is involved in inducing aggregation in Polysphondylium violaceum

In the social amoeba Dictyostelium discoideum, the polyketide MPBD (4-methyl-5-pentylbenzene-1,3-diol) regulates the gene expressions of cAMP signaling to make cells aggregation-competent and also induces spore maturation. The polyketide synthase StlA is responsible for MPBD biosynthesis in D. discoideum and appears to be conserved throughout the major groups of the social amoeba (Dictyostelia). In this study, we analyzed the function of StlA in Polysphondylium violaceum by identifying the gene sequence and creating the knockout mutants. We found that Pv-stlA- mutants had defects only in cell aggregation but not in spore maturation, indicating that the function of StlA in inducing spore maturation is species-specific. We also found that MPBD could rescue the aggregation defect in Pv-stlA- mutants whereas the mutants normally exhibited chemotaxis to their chemoattractant, glorin. Our data suggest that StlA is involved in inducing aggregation in P. violaceum by acting on signaling pathways other than chemotaxis in P. violaceum.

Novel RNAseq-Informed Cell-type Markers and Their Regulation Alter Paradigms of Dictyostelium Developmental Control

Cell differentiation is traditionally monitored with a few marker genes, which may bias results. To understand the evolution and regulation of the spore, stalk, cup and basal disc cells in Dictyostelia, we previously performed RNAseq on purified cell-types of taxon-group representative dictyostelids. Using promoter-lacZ constructs in D. discoideum, we here investigate the spatio-temporal expression pattern of 29 cell-type specific genes. Genes selected for spore- or cup-specificity in RNAseq were validated as such by lacZ expression, but genes selected for stalk-specificity showed variable additional expression in basal disc, early cup or prestalk populations. We measured responses of 25 genes to 15 single or combined regimes of induction by stimuli known to regulate cell differentiation. The outcomes of these experiments were subjected to hierarchical clustering to identify whether common modes of regulation were correlated with specific expression patterns. The analysis identified a cluster combining the spore and cup genes, which shared upregulation by 8-bromo cyclic AMP and down-regulation by Differentiation Inducing Factor 1 (DIF-1). Most stalk-expressed genes combined into a single cluster and shared strong upregulation by cyclic di-guanylate (c-di-GMP), and synergistic upregulation by combined DIF-1 and c-di-GMP. There was no clustering of genes expressed in other soma besides the stalk, but two genes that were only expressed in the stalk did not respond to any stimuli. In contrast to current models, the study indicates the existence of a stem-cell like soma population in slugs, whose members only acquire ultimate cell fate after progressing to their terminal location during fruiting body morphogenesis.

Possible Involvement of the Nutrient and Energy Sensors mTORC1 and AMPK in Cell Fate Diversification in a Non-Metazoan Organism

mTORC1 and AMPK are mutually antagonistic sensors of nutrient and energy status that have been implicated in many human diseases including cancer, Alzheimer’s disease, obesity and type 2 diabetes. Starved cells of the social amoeba Dictyostelium discoideum aggregate and eventually form fruiting bodies consisting of stalk cells and spores. We focus on how this bifurcation of cell fate is achieved. During growth mTORC1 is highly active and AMPK relatively inactive. Upon starvation, AMPK is activated and mTORC1 inhibited; cell division is arrested and autophagy induced. After aggregation, a minority of the cells (prestalk cells) continue to express much the same set of developmental genes as during aggregation, but the majority (prespore cells) switch to the prespore program. We describe evidence suggesting that overexpressing AMPK increases the proportion of prestalk cells, as does inhibiting mTORC1. Furthermore, stimulating the acidification of intracellular acidic compartments likewise increases the proportion of prestalk cells, while inhibiting acidification favors the spore pathway. We conclude that the choice between the prestalk and the prespore pathways of cell differentiation may depend on the relative strength of the activities of AMPK and mTORC1, and that these may be controlled by the acidity of intracellular acidic compartments/lysosomes (pHv), cells with low pHv compartments having high AMPK activity/low mTORC1 activity, and those with high pHv compartments having high mTORC1/low AMPK activity. Increased insight into the regulation and downstream consequences of this switch should increase our understanding of its potential role in human diseases, and indicate possible therapeutic interventions.

Loss of PIKfyve Causes Transdifferentiation of Dictyostelium Spores Into Basal Disc Cells

The 1-phosphatidylinositol-3-phosphate 5-kinase PIKfyve generates PtdIns3,5P2 on late phagolysosomes, which by recruiting the scission protein Atg18, results in their fragmentation in the normal course of endosome processing. Loss of PIKfyve function causes cellular hypervacuolization in eukaryotes and organ failure in humans. We identified pikfyve as the defective gene in a Dictyostelium mutant that failed to form spores. The amoebas normally differentiated into prespore cells and initiated spore coat protein synthesis in Golgi-derived prespore vesicles. However, instead of exocytosing, the prespore vesicles fused into the single vacuole that typifies the stalk and basal disc cells that support the spores. This process was accompanied by stalk wall biosynthesis, loss of spore gene expression and overexpression of ecmB, a basal disc and stalk-specific gene, but not of the stalk-specific genes DDB_G0278745 and DDB_G0277757. Transdifferentiation of prespore into stalk-like cells was previously observed in mutants that lack early autophagy genes, like atg5, atg7, and atg9. However, while autophagy mutants specifically lacked cAMP induction of prespore gene expression, pikfyve ^- showed normal early autophagy and prespore induction, but increased in vitro induction of ecmB. Combined, the data suggest that the Dictyostelium endosomal system influences cell fate by acting on cell type specific gene expression.

Evolution of Multicellular Complexity in The Dictyostelid Social Amoebas

Multicellularity evolved repeatedly in the history of life, but how it unfolded varies greatly between different lineages. Dictyostelid social amoebas offer a good system to study the evolution of multicellular complexity, with a well-resolved phylogeny and molecular genetic tools being available. We compare the life cycles of the Dictyostelids with closely related amoebozoans to show that complex life cycles were already present in the unicellular common ancestor of Dictyostelids. We propose frost resistance as an early driver of multicellular evolution in Dictyostelids and show that the cell signalling pathways for differentiating spore and stalk cells evolved from that for encystation. The stalk cell differentiation program was further modified, possibly through gene duplication, to evolve a new cell type, cup cells, in Group 4 Dictyostelids. Studies in various multicellular organisms, including Dictyostelids, volvocine algae, and metazoans, suggest as a common principle in the evolution of multicellular complexity that unicellular regulatory programs for adapting to environmental change serve as "proto-cell types" for subsequent evolution of multicellular organisms. Later, new cell types could further evolve by duplicating and diversifying the "proto-cell type" gene regulatory networks.

Loss of the Polyketide Synthase StlB Results in Stalk Cell Overproduction in Polysphondylium violaceum

Major phenotypic innovations in social amoeba evolution occurred at the transition between the Polysphondylia and group 4 Dictyostelia, which comprise the model organism Dictyostelium discoideum, such as the formation of a new structure, the basal disk. Basal disk differentiation and robust stalk formation require the morphogen DIF-1, synthesized by the polyketide synthase StlB, the des-methyl-DIF-1 methyltransferase DmtA, and the chlorinase ChlA, which are conserved throughout Dictyostelia. To understand how the basal disk and other innovations evolved in group 4, we sequenced and annotated the Polysphondylium violaceum (Pvio) genome, performed cell type-specific transcriptomics to identify cell-type marker genes, and developed transformation and gene knock-out procedures for Pvio. We used the novel methods to delete the Pvio stlB gene. The Pvio stlB- mutants formed misshapen curly sorogens with thick and irregular stalks. As fruiting body formation continued, the upper stalks became more regular, but structures contained 40% less spores. The stlB- sorogens overexpressed a stalk gene and underexpressed a (pre)spore gene. Normal fruiting body formation and sporulation were restored in Pvio stlB- by including DIF-1 in the supporting agar. These data indicate that, although conserved, stlB and its product(s) acquired both a novel role in the group 4 Dictyostelia and a role opposite to that in its sister group.

Calmodulin-mediated events during the life cycle of the amoebozoan Dictyostelium discoideum

This review focusses on the functions of intracellular and extracellular calmodulin, its target proteins and their binding proteins during the asexual life cycle of Dictyostelium discoideum. Calmodulin is a primary regulatory protein of calcium signal transduction that functions throughout all stages. During growth, it mediates autophagy, the cell cycle, folic acid chemotaxis, phagocytosis, and other functions. During mitosis, specific calmodulin-binding proteins translocate to alternative locations. Translocation of at least one cell adhesion protein is calmodulin dependent. When starved, cells undergo calmodulin-dependent chemotaxis to cyclic AMP generating a multicellular pseudoplasmodium. Calmodulin-dependent signalling within the slug sets up a defined pattern and polarity that sets the stage for the final events of morphogenesis and cell differentiation. Transected slugs undergo calmodulin-dependent transdifferentiation to re-establish the disrupted pattern and polarity. Calmodulin function is critical for stalk cell differentiation but also functions in spore formation, events that begin in the pseudoplasmodium. The asexual life cycle restarts with the calmodulin-dependent germination of spores. Specific calmodulin-binding proteins as well as some of their binding partners have been linked to each of these events. The functions of extracellular calmodulin during growth and development are also discussed. This overview brings to the forefront the central role of calmodulin, working through its numerous binding proteins, as a primary downstream regulator of the critical calcium signalling pathways that have been well established in this model eukaryote. This is the first time the function of calmodulin and its target proteins have been documented through the complete life cycle of any eukaryote.

Combinatorial Enzymatic Synthesis of Unnatural Long-Chain β-Branch Pyrones by a Highly Promiscuous Enzyme

In this study, we described in detail a combinatorial enzymatic synthesis approach to produce a series of unnatural long-chain β-branch pyrones. We attempted to investigate the catalytic potential of a highly promiscuous enzyme type III PKS to catalyze the non-decarboxylative condensation reaction by two molecules of fatty acyl diketide-N-acetylcysteines (diketide-NACs) units. Two non-natural long-chain (C16, C18) fatty acyl diketide-NACs were prepared successfully for testing the ability of non-decarboxylative condensation. In vitro, 12 novel naturally unavailable long-chain β-branch pyrones were generated by one-pot formation and characterized by ultraviolet-visible spectroscopy and high-resolution liquid chromatography-mass spectrometry. Interestingly, enzymatic kinetics result displays that this enzyme exhibits the remarkable compatibility to various non-natural long-chain substrates. These results would be useful to deeply understand the catalytic mechanism of this enzyme and further extend the application of enzymatic synthesis of non-natural products.

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.

Glycogen synthase kinase 3 promotes multicellular development over unicellular encystation in encysting Dictyostelia

BACKGROUND

Glycogen synthase kinase 3 (GSK3) regulates many cell fate decisions in animal development. In multicellular structures of the group 4 dictyostelid Dictyostelium discoideum, GSK3 promotes spore over stalk-like differentiation. We investigated whether, similar to other sporulation-inducing genes such as cAMP-dependent protein kinase (PKA), this role of GSK3 is derived from an ancestral role in encystation of unicellular amoebas.

RESULTS

We deleted GSK3 in Polysphondylium pallidum, a group 2 dictyostelid which has retained encystation as an alternative survival strategy. Loss of GSK3 inhibited cytokinesis of cells in suspension, as also occurs in D. discoideum, but did not affect spore or stalk differentiation in P. pallidum. However, gsk3^- amoebas entered into encystation under conditions that in wild type favour aggregation and fruiting body formation. The gsk3^- cells were hypersensitive to osmolytes, which are known to promote encystation, and to cyst-inducing factors that are secreted during starvation. GSK3 was not itself regulated by these factors, but inhibited their effects.

CONCLUSIONS

Our data show that GSK3 has a deeply conserved role in controlling cytokinesis, but not spore differentiation in Dictyostelia. Instead, in P. pallidum, one of many Dictyostelia that like their solitary ancestors can still encyst to survive starvation, GSK3 promotes multicellular development into fruiting bodies over unicellular encystment.

An individual-level selection model for the apparent altruism exhibited by cellular slime moulds

In Dictyostelium discoideum, cells that become part of the stalk or basal disc display behaviour that can be interpreted as altruistic. Atzmony et al. (Curr Sci 72:142-145, 1997) had hypothesised that this behaviour could be the outcome of an adaptive strategy based on differing intrinsic quality as reflected by phenotypes that indicate differences in potential for survival and reproduction, followed by intercellular competition among amoebae of differing qualities. Low-quality amoebae would have a poor chance of succeeding in the competition to form spores; they could enhance their chances of survival by adopting a presumptive stalk strategy. Here we extend the hypothesis by making use of recent findings. Our approach is based on the view that an evolutionary explanation for the apparent altruism of stalk cells in D. discoideum must apply broadly to other cellular slime moulds (CSMs) that exhibit stalk cell death. Further, it must be capable of being modified to cover social behaviour in CSMs with an extracellular stalk, as well as in sorocarpic amoebae whose stalk cells are viable. With regard to D. discoideum, we suggest that (a) differentiation-inducing factor, thought of as a signal that inhibits amoebae from forming spores and induces them to differentiate into basal disc cells, is better viewed as a mediator of competition among post-aggregation amoebae and (b) the products of the 'recognition genes', tgrB and tgrC, allow an amoeba to assess its quality relative to that of its neighbours and move to a position within the aggregate that optimises its reproductive fitness. From this perspective, all cells behave in a manner that is 'selfish' rather than 'altruistic', albeit with different expectations of success.

Natural Products from Social Amoebae

Natural products are invaluable sources of structural diversity and complexity ideally suited for the development of therapeutic agents. The search for novel bioactive molecules has prompted scientists to explore various ecological niches. Microorganisms have been shown to constitute such an important source. Despite their biosynthetic potential, social amoebae, that is, microorganisms with both a uni- and multicellular lifestyle, are underexplored regarding their secreted secondary metabolome. In this review, we present the structural diversity of amoebal natural products and discuss their biological functions as well as their total syntheses.

A polycystin-type transient receptor potential (Trp) channel that is activated by ATP

ATP and ADP are ancient extra-cellular signalling molecules that in Dictyostelium amoebae cause rapid, transient increases in cytosolic calcium due to an influx through the plasma membrane. This response is independent of hetero-trimeric G-proteins, the putative IP3 receptor IplA and all P2X channels. We show, unexpectedly, that it is abolished in mutants of the polycystin-type transient receptor potential channel, TrpP. Responses to the chemoattractants cyclic-AMP and folic acid are unaffected in TrpP mutants. We report that the DIF morphogens, cyclic-di-GMP, GABA, glutamate and adenosine all induce strong cytoplasmic calcium responses, likewise independently of TrpP. Thus, TrpP is dedicated to purinergic signalling. ATP treatment causes cell blebbing within seconds but this does not require TrpP, implicating a separate purinergic receptor. We could detect no effect of ATP on chemotaxis and TrpP mutants grow, chemotax and develop almost normally in standard conditions. No gating ligand is known for the human homologue of TrpP, polycystin-2, which causes polycystic kidney disease. Our results now show that TrpP mediates purinergic signalling in Dictyostelium and is directly or indirectly gated by ATP.

Summary: We show that a Trp channel related to the mammalian polycystin channel, rather than a P2X receptor, is responsible for the purinergic stimulation of cytosolic calcium levels in Dictyostelium cells.

Effects of deletion of the receptor CrlA on Dictyostelium aggregation and MPBD-mediated responses are strain dependent and not evident in strain Ax2

The polyketide MPBD (4-methyl-5-pentylbenzene-1, 3-diol) is produced by the polyketide synthase SteelyA (StlA) in Dictyostelium discoideum. MPBD is required for appropriate expression of cAMP signalling genes involved in cell aggregation and additionally induces the spore maturation at the fruiting body stage. The MPBD signalling pathway for regulation of cell aggregation is unknown, but MPBD effects on sporulation were reported to be mediated by the G-protein coupled receptor CrlA in D. discoideum KAx3. In this study, we deleted the crlA gene from the same parental strain (Ax2) that was used to generate the MPBD-less mutant. We found that unlike the MPBD-less mutant, Ax2-derived crlA- mutants exhibited normal cell aggregation, indicating that in Ax2 MPBD effects on early development do not require CrlA. We also found that the Ax2/crlA- mutant formed normal spores in fruiting bodies. When transformed with PkaC, both Ax2 and Ax2/crlA- similarly responded to MPBD in vitro with spore encapsulation. Our data make it doubtful that CrlA acts as the receptor for MPBD signalling during the development of D. discoideum Ax2.

Adenylate cyclase A acting on PKA mediates induction of stalk formation by cyclic diguanylate at the Dictyostelium organizer

Coordination of cell movement with cell differentiation is a major feat of embryonic development. The Dictyostelium stalk always forms at the organizing tip, by a mechanism that is not understood. We previously reported that cyclic diguanylate (c-di-GMP), synthesized by diguanylate cyclase A (DgcA), induces stalk formation. Here we used transcriptional profiling of dgca- structures to identify target genes for c-di-GMP, and used these genes to investigate the c-di-GMP signal transduction pathway. We found that knockdown of cAMP-dependent protein kinase (PKA) activity in prestalk cells reduced stalk gene induction by c-di-GMP, whereas PKA activation bypassed the c-di-GMP requirement for stalk gene expression. c-di-GMP caused a persistent increase in cAMP, which still occurred in mutants lacking the adenylate cyclases ACG or ACR, or the cAMP phosphodiesterase RegA. However, both inhibition of adenylate cyclase A (ACA) with SQ22536 and incubation of a temperature-sensitive ACA mutant at the restrictive temperature prevented c-di-GMP-induced cAMP synthesis as well as c-di-GMP-induced stalk gene transcription. ACA produces the cAMP pulses that coordinate Dictyostelium morphogenetic cell movement and is highly expressed at the organizing tip. The stalk-less dgca- mutant regained its stalk by expression of a light-activated adenylate cyclase from the ACA promoter and exposure to light, indicating that cAMP is also the intermediate for c-di-GMP in vivo. Our data show that the more widely expressed DgcA activates tip-expressed ACA, which then acts on PKA to induce stalk genes. These results explain why stalk formation in Dictyostelia always initiates at the site of the morphogenetic organizer.

Early nucleolar disorganization in Dictyostelium cell death

Cell death occurs in all eukaryotes, but it is still not known whether some core steps of the cell death process are conserved. We investigated this using the protist Dictyostelium. The dissection of events in Dictyostelium vacuolar developmental cell death was facilitated by the sequential requirement for two distinct exogenous signals. An initial exogenous signal (starvation and cAMP) recruited some cells into clumps. Only within these clumps did subsequent cell death events take place. Contrary to our expectations, already this initial signal provoked nucleolar disorganization and irreversible inhibition of rRNA and DNA synthesis, reflecting marked cell dysfunction. The initial signal also primed clumped cells to respond to a second exogenous signal (differentiation-inducing factor-1 or c-di-GMP), which led to vacuolization and synthesis of cellulose encasings. Thus, the latter prominent hallmarks of developmental cell death were induced separately from initial cell dysfunction. We propose that (1) in Dictyostelium vacuolization and cellulose encasings are late, organism-specific, hallmarks, and (2) on the basis of our observations in this protist and of similar previous observations in some cases of mammalian cell death, early inhibition of rRNA synthesis and nucleolar disorganization may be conserved in some eukaryotes to usher in developmental cell death.

Autophagy in Dictyostelium: Mechanisms, regulation and disease in a simple biomedical model

Autophagy is a fast-moving field with an enormous impact on human health and disease. Understanding the complexity of the mechanism and regulation of this process often benefits from the use of simple experimental models such as the social amoeba Dictyostelium discoideum. Since the publication of the first review describing the potential of D. discoideum in autophagy, significant advances have been made that demonstrate both the experimental advantages and interest in using this model. Since our previous review, research in D. discoideum has shed light on the mechanisms that regulate autophagosome formation and contributed significantly to the study of autophagy-related pathologies. Here, we review these advances, as well as the current techniques to monitor autophagy in D. discoideum. The comprehensive bioinformatics search of autophagic proteins that was a substantial part of the previous review has not been revisited here except for those aspects that challenged previous predictions such as the composition of the Atg1 complex. In recent years our understanding of, and ability to investigate, autophagy in D. discoideum has evolved significantly and will surely enable and accelerate future research using this model.

Glutathione S-transferase 4 is a putative DIF-binding protein that regulates the size of fruiting bodies in Dictyostelium discoideum

In the development of the cellular slime mold Dictyostelium discoideum, two chlorinated compounds, the differentiation-inducing factors DIF-1 and DIF-2, play important roles in the regulation of both cell differentiation and chemotactic cell movement. However, the receptors of DIFs and the components of DIF signaling systems have not previously been elucidated. To identify the receptors for DIF-1 and DIF-2, we here performed DIF-conjugated affinity gel chromatography and liquid chromatography-tandem mass spectrometry and identified the glutathione S-transferase GST4 as a major DIF-binding protein. Knockout and overexpression mutants of gst4 (gst4^- and gst4^OE, respectively) formed fruiting bodies, but the fruiting bodies of gst4^- cells were smaller than those of wild-type Ax2 cells, and those of gst4^OE cells were larger than those of Ax2 cells. Both chemotaxis regulation and in vitro stalk cell formation by DIFs in the gst4 mutants were similar to those of Ax2 cells. These results suggest that GST4 is a DIF-binding protein that regulates the sizes of cell aggregates and fruiting bodies in D. discoideum.

The multicellularity genes of dictyostelid social amoebas

The evolution of multicellularity enabled specialization of cells, but required novel signalling mechanisms for regulating cell differentiation. Early multicellular organisms are mostly extinct and the origins of these mechanisms are unknown. Here using comparative genome and transcriptome analysis across eight uni- and multicellular amoebozoan genomes, we find that 80% of proteins essential for the development of multicellular Dictyostelia are already present in their unicellular relatives. This set is enriched in cytosolic and nuclear proteins, and protein kinases. The remaining 20%, unique to Dictyostelia, mostly consists of extracellularly exposed and secreted proteins, with roles in sensing and recognition, while several genes for synthesis of signals that induce cell-type specialization were acquired by lateral gene transfer. Across Dictyostelia, changes in gene expression correspond more strongly with phenotypic innovation than changes in protein functional domains. We conclude that the transition to multicellularity required novel signals and sensors rather than novel signal processing mechanisms.

Unicellular social amoebae aggregate to form a multicellular life stage, making them a model system for the evolution of multicellularity. Here, Glöckner et al. use a comparative genomic and transcriptomic approach to determine the origin of the genes essential for multicellularity in the social amoebae.

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.

Evolution of developmental signalling in Dictyostelid social amoebas

Dictyostelia represent a tractable system to resolve the evolution of cell-type specialization, with some taxa differentiating into spores only, and other taxa with additionally one or up to four somatic cell types. One of the latter forms, Dictyostelium discoideum, is a popular model system for cell biology and developmental biology with key signalling pathways controlling cell-specialization being resolved recently. For the most dominant pathways, evolutionary origins were retraced to a stress response in the unicellular ancestor, while modifications in the ancestral pathway were associated with acquisition of multicellular complexity. This review summarizes our current understanding of developmental signalling in D. discoideum and its evolution.

Secreted Cyclic Di-GMP Induces Stalk Cell Differentiation in the Eukaryote Dictyostelium discoideum

Cyclic di-GMP (c-di-GMP) is currently recognized as the most widely used intracellular signal molecule in prokaryotes, but roles in eukaryotes were only recently discovered. In the social amoeba Dictyostelium discoideum, c-di-GMP, produced by a prokaryote-type diguanylate cyclase, induces the differentiation of stalk cells, thereby enabling the formation of spore-bearing fruiting bodies. In this review, we summarize the currently known mechanisms that control the major life cycle transitions of Dictyostelium and focus particularly on the role of c-di-GMP in stalk formation. Stalk cell differentiation has characteristics of autophagic cell death, a process that also occurs in higher eukaryotes. We discuss the respective roles of c-di-GMP and of another signal molecule, differentiation-inducing factor 1, in autophagic cell death in vitro and in stalk formation in vivo.

Stalk cell differentiation without polyketides in the cellular slime mold

Polyketides induce prestalk cell differentiation in Dictyostelium. In the double-knockout mutant of the SteelyA and B polyketide synthases, most of the pstA cells-the major part of the prestalk cells-are lost, and we show by whole mount in situ hybridization that expression of prestalk genes is also reduced. Treatment of the double-knockout mutant with the PKS inhibitor cerulenin gave a further reduction, but some pstA cells still remained in the tip region, suggesting the existence of a polyketide-independent subtype of pstA cells. The double-knockout mutant and cerulenin-treated parental Ax2 cells form fruiting bodies with fragile, single-cell layered stalks after cerulenin treatment. Our results indicate that most pstA cells are induced by polyketides, but the pstA cells at the very tip of the slug are induced in some other way. In addition, a fruiting body with a single-cell layered, vacuolated stalk can form without polyketides.

Differentiation-inducing factor 2 modulates chemotaxis via the histidine kinase DhkC-dependent pathway in Dictyostelium discoideum

Differentiation-inducing factor 1(DIF-1) and DIF-2 are signaling molecules that control chemotaxis in Dictyostelium discoideum. Whereas DIF-1 suppresses chemotaxis in shallow cAMP gradients, DIF-2 enhances chemotaxis under the same conditions via a phosphodiesterase, response regulator A (RegA), which is a part of the DhkC-RdeA-RegA two-component signaling system. In this study, to investigate the mechanism of the chemotaxis regulation by DIF-2, we examined the effects of DIF-2 (and DIF-1) on chemotaxis in rdeA(-) and dhkC(-) mutant strains. In the parental wild-type strains, chemotactic cell movement was suppressed with DIF-1 and enhanced with DIF-2 in shallow cAMP gradients. In contrast, in both rdeA(-) and dhkC(-) strains, chemotaxis was suppressed with DIF-1 but unaffected by DIF-2. The results suggest that DIF-2 modulates chemotaxis via the DhkC-RdeA-RegA signaling system.

The Evolution of Aggregative Multicellularity and Cell-Cell Communication in the Dictyostelia

Aggregative multicellularity, resulting in formation of a spore-bearing fruiting body, evolved at least six times independently amongst both eukaryotes and prokaryotes. Amongst eukaryotes, this form of multicellularity is mainly studied in the social amoeba Dictyostelium discoideum. In this review, we summarise trends in the evolution of cell-type specialisation and behavioural complexity in the four major groups of Dictyostelia. We describe the cell-cell communication systems that control the developmental programme of D. discoideum, highlighting the central role of cAMP in the regulation of cell movement and cell differentiation. Comparative genomic studies showed that the proteins involved in cAMP signalling are deeply conserved across Dictyostelia and their unicellular amoebozoan ancestors. Comparative functional analysis revealed that cAMP signalling in D. discoideum originated from a second messenger role in amoebozoan encystation. We highlight some molecular changes in cAMP signalling genes that were responsible for the novel roles of cAMP in multicellular development.

Genetic control of morphogenesis in Dictyostelium

Cells grow, move, expand, shrink and die in the process of generating the characteristic shapes of organisms. Although the structures generated during development of the social amoeba Dictyostelium discoideum look nothing like the structures seen in metazoan embryogenesis, some of the morphogenetic processes used in their making are surprisingly similar. Recent advances in understanding the molecular basis for directed cell migration, cell type specific sorting, differential adhesion, secretion of matrix components, pattern formation, regulation and terminal differentiation are reviewed. Genes involved in Dictyostelium aggregation, slug formation, and culmination of fruiting bodies are discussed.

Comparative genome and transcriptome analyses of the social amoeba Acytostelium subglobosum that accomplishes multicellular development without germ-soma differentiation

BACKGROUND

Social amoebae are lower eukaryotes that inhabit the soil. They are characterized by the construction of a starvation-induced multicellular fruiting body with a spore ball and supportive stalk. In most species, the stalk is filled with motile stalk cells, as represented by the model organism Dictyostelium discoideum, whose developmental mechanisms have been well characterized. However, in the genus Acytostelium, the stalk is acellular and all aggregated cells become spores. Phylogenetic analyses have shown that it is not an ancestral genus but has lost the ability to undergo cell differentiation.

RESULTS

We performed genome and transcriptome analyses of Acytostelium subglobosum and compared our findings to other available dictyostelid genome data. Although A. subglobosum adopts a qualitatively different developmental program from other dictyostelids, its gene repertoire was largely conserved. Yet, families of polyketide synthase and extracellular matrix proteins have not expanded and a serine protease and ABC transporter B family gene, tagA, and a few other developmental genes are missing in the A. subglobosum lineage. Temporal gene expression patterns are astonishingly dissimilar from those of D. discoideum, and only a limited fraction of the ortholog pairs shared the same expression patterns, so that some signaling cascades for development seem to be disabled in A. subglobosum.

CONCLUSIONS

The absence of the ability to undergo cell differentiation in Acytostelium is accompanied by a small change in coding potential and extensive alterations in gene expression patterns.

The Dictyostelium prestalk inducer differentiation-inducing factor-1 (DIF-1) triggers unexpectedly complex global phosphorylation changes

Differentiation-inducing factor-1 (DIF-1) is a polyketide that induces Dictyostelium amoebae to differentiate as prestalk cells. We performed a global quantitative screen for phosphorylation changes that occur within the first minutes after addition of DIF-1, using a triple-label SILAC approach. This revealed a new world of DIF-1-controlled signaling, with changes in components of the MAPK and protein kinase B signaling pathways, components of the actinomyosin cytoskeletal signaling networks, and a broad range of small GTPases and their regulators. The results also provide evidence that the Ca(2+)/calmodulin-dependent phosphatase calcineurin plays a role in DIF-1 signaling to the DimB prestalk transcription factor. At the global level, DIF-1 causes a major shift in the phosphorylation/dephosphorylation equilibrium toward net dephosphorylation. Of interest, many of the sites that are dephosphorylated in response to DIF-1 are phosphorylated in response to extracellular cAMP signaling. This accords with studies that suggest an antagonism between the two inducers and also with the rapid dephosphorylation of the cAMP receptor that we observe in response to DIF-1 and with the known inhibitory effect of DIF-1 on chemotaxis to cAMP. All MS data are available via ProteomeXchange with identifier PXD001555.

c-di-GMP induction of Dictyostelium cell death requires the polyketide DIF-1

Two inducers, DIF-1 and c-di-GMP, each separately shown to play a major role in Dictyostelium cell death induction in vitro, in fact cooperate. A similar cooperation with polyketides might occur for c-di-GMP effects in other situations and organisms, in particular in innate immunity and cell death in animal cells.

Cell death in the model organism Dictyostelium, as studied in monolayers in vitro, can be induced by the polyketide DIF-1 or by the cyclical dinucleotide c-di-GMP. c-di-GMP, a universal bacterial second messenger, can trigger innate immunity in bacterially infected animal cells and is involved in developmental cell death in Dictyostelium. We show here that c-di-GMP was not sufficient to induce cell death in Dictyostelium cell monolayers. Unexpectedly, it also required the DIF-1 polyketide. The latter could be exogenous, as revealed by a telling synergy between c-di-GMP and DIF-1. The required DIF-1 polyketide could also be endogenous, as shown by the inability of c-di-GMP to induce cell death in Dictyostelium HMX44A cells and DH1 cells upon pharmacological or genetic inhibition of DIF-1 biosynthesis. In these cases, c-di-GMP–induced cell death was rescued by complementation with exogenous DIF-1. Taken together, these results demonstrated that c-di-GMP could trigger cell death in Dictyostelium only in the presence of the DIF-1 polyketide or its metabolites. This identified another element of control to this cell death and perhaps also to c-di-GMP effects in other situations and organisms.

Evolutionary reconstruction of pattern formation in 98 Dictyostelium species reveals that cell-type specialization by lateral inhibition is a derived trait

Background

Multicellularity provides organisms with opportunities for cell-type specialization, but requires novel mechanisms to position correct proportions of different cell types throughout the organism. Dictyostelid social amoebas display an early form of multicellularity, where amoebas aggregate to form fruiting bodies, which contain only spores or up to four additional cell-types. These cell types will form the stalk and support structures for the stalk and spore head. Phylogenetic inference subdivides Dictyostelia into four major groups, with the model organism D. discoideum residing in group 4. In D. discoideum differentiation of its five cell types is dominated by lateral inhibition-type mechanisms that trigger scattered cell differentiation, with tissue patterns being formed by cell sorting.

Results

To reconstruct the evolution of pattern formation in Dictyostelia, we used cell-type specific antibodies and promoter-reporter fusion constructs to investigate pattern formation in 98 species that represent all groupings. Our results indicate that in all early diverging Dictyostelia and most members of groups 1–3, cells differentiate into maximally two cell types, prestalk and prespore cells, with pattern formation being dominated by position-dependent transdifferentiation of prespore cells into prestalk cells. In clade 2A, prestalk and stalk cell differentiation are lost and the prespore cells construct an acellular stalk. Group 4 species set aside correct proportions of prestalk and prespore cells early in development, and differentiate into up to three more supporting cell types.

Conclusions

Our experiments show that positional transdifferentiation is the ancestral mode of pattern formation in Dictyostelia. The early specification of a prestalk population equal to the number of stalk cells is a derived trait that emerged in group 4 and a few late diverging species in the other groups. Group 4 spore masses are larger than those of other groups and the differentiation of supporting cell types by lateral inhibition may have facilitated this increase in size. The signal DIF-1, which is secreted by prespore cells, triggers differentiation of supporting cell types. The synthesis and degradation of DIF-1 were shown to be restricted to group 4. This suggests that the emergence of DIF-1 signalling caused increased cell-type specialization in this group.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-5-34) contains supplementary material, which is available to authorized users.

The hybrid type polyketide synthase SteelyA is required for cAMP signalling in early Dictyostelium development

BACKGROUND

In our previous study we found that the expression of stlA showed peaks both in the early and last stages of development and that a product of SteelyA, 4-methyl-5-pentylbenzene-1,3-diol (MPBD), controlled Dictyostelium spore maturation during the latter. In this study we focused on the role of SteelyA in early stage development.

PRINCIPAL FINDINGS

Our stlA null mutant showed aggregation delay and abnormally small aggregation territories. Chemotaxis analysis revealed defective cAMP chemotaxis in the stlA null mutant. cAMP chemotaxis was restored by MPBD addition during early stage development. Assay for cAMP relay response revealed that the stlA null mutant had lower cAMP accumulation during aggregation, suggesting lower ACA activity than the wild type strain. Exogenous cAMP pulses rescued the aggregation defect of the stlA null strain in the absence of MPBD. Expression analysis of cAMP signalling genes revealed lower expression levels in the stlA null mutant during aggregation.

CONCLUSION

Our data indicate a regulatory function by SteelyA on cAMP signalling during aggregation and show that SteelyA is indispensable for full activation of ACA.

Alkylbenzoquinone involved in development of cellular slime molds

The structure of the prespore-cell-promoting factor from Dictyostelium discoideum was determined to be 2-hydroxy-5-methyl-6-pentylbenzoquinone. The synthetic compound has prespore-cell-promoting activity similar to the natural one, with half-maximal induction at a concentration as low as 40 pM. It was also found that the factor induces aggregation in an aggregation-deficient mutant of a related species, Polysphodilium violaceum. Both these activities are sensitive to positional isomerism with the 6-methyl-5-pentyl isomer showing no detectable activity.

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.

Defects in the synthetic pathway prevent DIF-1 mediated stalk lineage specification cascade in the non-differentiating social amoeba, Acytostelium subglobosum

Separation of somatic cells from germ-line cells is a crucial event for multicellular organisms, but how this step was achieved during evolution remains elusive. In Dictyostelium discoideum and many other dictyostelid species, solitary amoebae gather and form a multicellular fruiting body in which germ-line spores and somatic stalk cells differentiate, whereas in Acytostelium subglobosum, acellular stalks form and all aggregated amoebae become spores. In this study, because most D. discoideum genes known to be required for stalk cell differentiation have homologs in A. subglobosum, we inferred functional variations in these genes and examined conservation of the stalk cell specification cascade of D. discoideum mediated by the polyketide differentiation-inducing factor-1 (DIF-1) in A. subglobosum. Through heterologous expression of A. subglobosum orthologs of DIF-1 biosynthesis genes in D. discoideum, we confirmed that two of the three genes were functional equivalents, while DIF-methyltransferase (As-dmtA) involved at the final step of DIF-1 synthesis was not. In fact, DIF-1 activity was undetectable in A. subglobosum lysates and amoebae of this species were not responsive to DIF-1, suggesting a lack of DIF-1 production in this species. On the other hand, the molecular function of an A. subglobosum ortholog of DIF-1 responsive transcription factor was equivalent with that of D. discoideum and inhibition of polyketide synthesis caused developmental arrest in A. subglobosum, which could not be rescued by DIF-1 addition. These results suggest that non-DIF-1 polyketide cascades involving downstream transcription factors are required for fruiting body development of A. subglobosum.

Developmental lineage priming in Dictyostelium by heterogeneous Ras activation

In cell culture, genetically identical cells often exhibit heterogeneous behavior, with only 'lineage primed' cells responding to differentiation inducing signals. It has recently been proposed that such heterogeneity exists during normal embryonic development to allow position independent patterning based on 'salt and pepper' differentiation and sorting out. However, the molecular basis of lineage priming and how it leads to reproducible cell type proportioning are poorly understood. To address this, we employed a novel forward genetic approach in the model organism Dictyostelium discoideum. These studies reveal that the Ras-GTPase regulator gefE is required for normal lineage priming and salt and pepper differentiation. This is because Ras-GTPase activity sets the intrinsic response threshold to lineage specific differentiation signals. Importantly, we show that although gefE expression is uniform, transcription of its target, rasD, is both heterogeneous and dynamic, thus providing a novel mechanism for heterogeneity generation and position-independent differentiation. DOI: http://dx.doi.org/10.7554/eLife.01067.001.

Novel chlorinated dibenzofurans isolated from the cellular slime mold, Polysphondylium filamentosum, and their biological activities

Cellular slime molds are expected to have the huge potential for producing secondary metabolites including polyketides, and we have studied the diversity of secondary metabolites of cellular slime molds for their potential utilization as new biological resources for natural product chemistry. From the methanol extract of fruiting bodies of Polysphondylium filamentosum, we obtained new chlorinated benzofurans Pf-1 (4) and Pf-2 (5) which display multiple biological activities; these include stalk cell differentiation-inducing activity in the well-studied cellular slime mold, Dictyostelium discoideum, and inhibitory activities on cell proliferation in mammalian cells and gene expression in Drosophila melanogaster.

The prokaryote messenger c-di-GMP triggers stalk cell differentiation in Dictyostelium

Cyclic di-(3′:5′)-guanosine monophosphate (c-di-GMP) is a major prokaryote signalling intermediate that is synthesized by diguanylate cyclases and triggers sessility and biofilm formation. We detected the first eukaryote diguanylate cyclases in all major groups of Dictyostelia. On food depletion, Dictyostelium discoideum amoebas collect into aggregates, which first transform into migrating slugs and then into sessile fruiting structures. These structures consist of a spherical spore mass that is supported by a column of stalk cells and a basal disk. A polyketide, DIF-1, which induces stalk-like cells in vitro, was isolated earlier. However, its role in vivo proved recently to be restricted to basal disk formation. Here we show that the Dictyostelium diguanylate cyclase, DgcA, produces c-di-GMP as the morphogen responsible for stalk cell differentiation. Dictyostelium discoideum DgcA synthesized c-di-GMP in a GTP-dependent manner and was expressed at the slug tip, which is the site of stalk cell differentiation. Disruption of the DgcA gene blocked the transition from slug migration to fructification and the expression of stalk genes. Fructification and stalk formation were restored by exposing DgcA-null slugs to wild-type secretion products or to c-di-GMP. Moreover, c-di-GMP, but not cyclic di-(3′:5′)-adenosine monophosphate, induced stalk gene expression in dilute cell monolayers. Apart from identifying the long-elusive stalk-inducing morphogen, our work also identifies a role for c-di-GMP in eukaryotes.

Ca2+ signaling regulates ecmB expression, cell differentiation and slug regeneration in Dictyostelium

Ca(2+) regulates cell differentiation and morphogenesis in a diversity of organisms and dysregulation of Ca(2+) signal transduction pathways leads to many cellular pathologies. In Dictyostelium Ca(2+) induces ecmB expression and stalk cell differentiation in vitro. Here we have analyzed the pattern of ecmB expression in intact and bisected slugs and the effect of agents that affect Ca(2+) levels or antagonize calmodulin (CaM) on this expression pattern. We have shown that Ca(2+) and CaM regulate ecmB expression and pstAB/pstB cell differentiation in vivo. Agents that increase intracellular Ca(2+) levels increased ecmB expression and/or pstAB and pstB cell differentiation, while agents that decrease intracellular Ca(2+) or antagonize CaM decreased it. In isolated slug tips agents that affect Ca(2+) levels and antagonize CaM had differential effect on ecmB expression and cell differentiation in the anterior versus posterior zones. Agents that increase intracellular Ca(2+) levels increased the number of ecmB expressing cells in the anterior region of slugs, while agents that decrease intracellular Ca(2+) levels or antagonize CaM activity increased the number of ecmB expressing cells in the posterior. We have also demonstrated that agents that affect Ca(2+) levels or antagonize CaM affect cells motility and regeneration of shape in isolated slug tips and backs and regeneration of tips in isolated slug backs. To our knowledge, this is the first study detailing the pattern of ecmB expression in regenerating slugs as well as the role of Ca(2+) and CaM in the regeneration process and ecmB expression.

Colchicine affects cell motility, pattern formation and stalk cell differentiation in Dictyostelium by altering calcium signaling

Previous work, verified here, showed that colchicine affects Dictyostelium pattern formation, disrupts morphogenesis, inhibits spore differentiation and induces terminal stalk cell differentiation. Here we show that colchicine specifically induces ecmB expression and enhances accumulation of ecmB-expressing cells at the posterior end of multicellular structures. Colchicine did not induce a nuclear translocation of DimB, a DIF-1 responsive transcription factor in vitro. It also induced terminal stalk cell differentiation in a mutant strain that does not produce DIF-1 (dmtA-) and after the treatment of cells with DIF-1 synthesis inhibitor cerulenin (100 μM). This suggests that colchicine induces the differentiation of ecmB-expressing cells independent of DIF-1 production and likely through a signaling pathway that is distinct from the one that is utilized by DIF-1. Depending on concentration, colchicine enhanced random cell motility, but not chemotaxis, by 3-5 fold (10-50 mM colchicine, respectively) through a Ca(2+)-mediated signaling pathway involving phospholipase C, calmodulin and heterotrimeric G proteins. Colchicine's effects were not due to microtubule depolymerization as other microtubule-depolymerizing agents did not have these effects. Finally normal morphogenesis and stalk and spore cell differentiation of cells treated with 10 mM colchicine were rescued through chelation of Ca2+ by BAPTA-AM and EDTA and calmodulin antagonism by W-7 but not PLC inhibition by U-73122. Morphogenesis or spore cell differentiation of cells treated with 50 mM colchicine could not be rescued by the above treatments but terminal stalk cell differentiation was inhibited by BAPTA-AM, EDTA and W-7, but not U-73122. Thus colchicine disrupts morphogenesis and induces stalk cell differentiation through a Ca(2+)-mediated signaling pathway involving specific changes in gene expression and cell motility.

Transcriptional repression by a bZIP protein regulates Dictyostelium prespore differentiation

In response to the signaling polyketide DIF-1 DimB directly activates transcription of the ecmB gene in pstB cells; a subset of the prestalk cells that are the precursors of the basal disc. We show that the promoter of pspA, a prespore-specific gene, also contains a DimB binding site. Mutation of this site causes ectopic expression in the prestalk region and ChIP analysis shows that DIF-1 induces binding of DimB to the pspA promoter. DIF-1 represses pspA gene expression in a suspension cell assay but this repression is abrogated in a dimB null strain. These results suggest a coupled control mechanism, whereby the same DIF-DimB signaling pathway that directly activates ecmB gene expression directly represses pspA gene expression.

An orthologue of the Myelin-gene Regulatory Transcription Factor regulates Dictyostelium prestalk differentiation

The prestalk region of the Dictyostelium slug is comprised of an anterior population of pstA cells and a posterior population of pstO cells. They are distinguished by their ability to utilize different parts of the promoter of the ecmA gene. We identify, by mutational analysis and DNA transformation, CA-rich sequence elements within the ecmA promoter that are essential for pstA-specific expression and sufficient to direct pstA-specific expression when multimerised. The CA-rich region was used in affinity chromatography with nuclear extracts and bound proteins were identified by mass spectrometry. The CA-rich elements purify MrfA, a protein with extensive sequence similarity to animal Myelin-gene Regulatory Factor (MRF)-like proteins. The MRF-like proteins and MrfA also display more spatially limited but significant sequence similarity with the DNA binding domain of the yeast Ndt80 sporulation-specific transcription factor. Furthermore, the ecmA CA-rich elements show sequence similarity to the core consensus Ndt80 binding site (the MSE) and point mutation of highly conserved arginine residues in MrfA, that in Ndt80 make critical contacts with the MSE, ablate binding of MrfA to its sites within the ecmA promoter. MrfA null strains are delayed in multicellular development and highly defective in pstA-specific gene expression. These results provide a first insight into the intracellular signaling pathway that directs pstA differentiation and identify a non-metazoan orthologue of a family of molecularly uncharacterised transcription factors.