The aerotaxis of Dictyostelium discoideum is independent of mitochondria, nitric oxide and oxidative stress

Spatial and temporal variations of oxygen environments affect the behaviors of various cells and are involved in physiological and pathological events. Our previous studies with Dictyostelium discoideum as a model of cell motility have demonstrated that aerotaxis toward an oxygen-rich region occurs below 2% O₂. However, while the aerotaxis of Dictyostelium seems to be an effective strategy to search for what is essential for survival, the mechanism underlying this phenomenon is still largely unclear. One hypothesis is that an oxygen concentration gradient generates a secondary oxidative stress gradient that would direct cell migration towards higher oxygen concentration. Such mechanism was inferred but not fully demonstrated to explain the aerotaxis of human tumor cells. Here, we investigated the role on aerotaxis of flavohemoglobins, proteins that can both act as potential oxygen sensors and modulators of nitric oxide and oxidative stress. The migratory behaviors of Dictyostelium cells were observed under both self-generated and imposed oxygen gradients. Furthermore, their changes by chemicals generating or preventing oxidative stress were tested. The trajectories of the cells were then analyzed through time-lapse phase-contrast microscopic images. The results indicate that both oxidative and nitrosative stresses are not involved in the aerotaxis of Dictyostelium but cause cytotoxic effects that are enhanced upon hypoxia.

Microphase separation of living cells

Self-organization of cells is central to a variety of biological systems and physical concepts of condensed matter have proven instrumental in deciphering some of their properties. Here we show that microphase separation, long studied in polymeric materials and other inert systems, has a natural counterpart in living cells. When placed below a millimetric film of liquid nutritive medium, a quasi two-dimensional, high-density population of Dictyostelium discoideum cells spontaneously assembles into compact domains. Their typical size of 100 μm is governed by a balance between competing interactions: an adhesion acting as a short-range attraction and promoting aggregation, and an effective long-range repulsion stemming from aerotaxis in near anoxic condition. Experimental data, a simple model and cell-based simulations all support this scenario. Our findings establish a generic mechanism for self-organization of living cells and highlight oxygen regulation as an emergent organizing principle for biological matter.

Aerotaxis and aerokinesis of Dictyostelium discoideum under hypoxic microenvironments

Although spatiotemporal changes of oxygen in a microenvironment are known to affect the cellular dynamics of various eukaryotes, the details are not fully understood. Here, we describe the aerotaxis and aerokinesis of Dictyostelium discoideum (Dd), which has long been employed as a model organism for eukaryotic cells. We developed a microfluidic device capable of time-lapse observation of cultured cells while controlling oxygen concentrations in microchannels. Migratory behaviors of Dd were observed and quantitatively evaluated under an oxygen concentration gradient from 0% to 21% O₂, as well as in various uniform oxygen conditions. In a hypoxic region within the oxygen concentration gradient, Dd migrated toward regions of higher oxygen concentration at increased velocity, which was independent of cell density. Observed under uniform oxygen concentrations of 1%, 2%, 3%, and 21%, the migration velocity of Dd increased significantly in hypoxic environments of 2% O₂ or less. Thus, Dd shows aerotaxis, directed by the oxygen concentration gradient, and simultaneously shows aerokinesis, changing the migration velocity according to the oxygen concentration itself.

Hypoxia triggers collective aerotactic migration in Dictyostelium discoideum

Using a self-generated hypoxic assay, we show that the amoeba Dictyostelium discoideum displays a remarkable collective aerotactic behavior. When a cell colony is covered, cells quickly consume the available oxygen (O₂) and form a dense ring moving outwards at constant speed and density. To decipher this collective process, we combined two technological developments: porphyrin-based O₂ -sensing films and microfluidic O₂ gradient generators. We showed that Dictyostelium cells exhibit aerotactic and aerokinetic response in a low range of O₂ concentration indicative of a very efficient detection mechanism. Cell behaviors under self-generated or imposed O₂ gradients were modeled using an in silico cellular Potts model built on experimental observations. This computational model was complemented with a parsimonious ‘Go or Grow’ partial differential equation (PDE) model. In both models, we found that the collective migration of a dense ring can be explained by the interplay between cell division and the modulation of aerotaxis.

Cytokinins in Dictyostelia - A Unique Model for Studying the Functions of Signaling Agents From Species to Kingdoms

Cytokinins (CKs) are a diverse group of evolutionarily significant growth-regulating molecules. While the CK biosynthesis and signal transduction pathways are the most well-understood in plant systems, these molecules have been identified in all kingdoms of life. This review follows the recent discovery of an expanded CK profile in the social amoeba, Dictyostelium discoideum. A comprehensive review on the present knowledge of CK biosynthesis, signal transduction, and CK-small molecule interactions within members of Dictyostelia will be summarized. In doing so, the utility of social amoebae will be highlighted as a model system for studying the evolution of these hormone-like signaling agents, which will set the stage for future research in this area.

Collective regulation of cell motility using an accurate density-sensing system

The capacity of living cells to sense their population density and to migrate accordingly is essential for the regulation of many physiological processes. However, the mechanisms used to achieve such functions are poorly known. Here, based on the analysis of multiple trajectories of vegetative Dictyostelium discoideum cells, we investigate such a system extensively. We show that the cells secrete a high-molecular-weight quorum-sensing factor (QSF) in their medium. This extracellular signal induces, in turn, a reduction of the cell movements, in particular, through the downregulation of a mode of motility with high persistence time. This response appears independent of cAMP and involves a G-protein-dependent pathway. Using a mathematical analysis of the cells' response function, we evidence a negative feedback on the QSF secretion, which unveils a powerful generic mechanism for the cells to detect when they exceed a density threshold. Altogether, our results provide a comprehensive and dynamical view of this system enabling cells in a scattered population to adapt their motion to their neighbours without physical contact.

Large negatively charged organic host molecules as inhibitors of endonuclease enzymes

Endonuclease enzymes can be inhibited in the micromolar range by sulphonated calix-arenes, sulphated cyclodextrin and sulphated cyclodextrin nanoparticles.

The polyketide MPBD initiates the SDF-1 signaling cascade that coordinates terminal differentiation in Dictyostelium

Dictyostelium uses a wide array of chemical signals to coordinate differentiation as it switches from a unicellular to a multicellular organism. MPBD, the product of the polyketide synthase encoded by stlA, regulates stalk and spore differentiation by rapidly stimulating the release of the phosphopeptide SDF-1. By analyzing specific mutants affected in MPBD or SDF-1 production, we delineated a signal transduction cascade through the membrane receptor CrlA coupled to Gα1, leading to the inhibition of GskA so that the precursor of SDF-1 is released. It is then processed by the extracellular protease of TagB on prestalk cells. SDF-1 apparently acts through the adenylyl cyclase ACG to activate the cyclic AMP (cAMP)-dependent protein kinase A (PKA) and trigger the production of more SDF-1. This signaling cascade shows similarities to the SDF-2 signaling pathway, which acts later to induce rapid spore encapsulation.

Comparative genomics of the social amoebae Dictyostelium discoideum and Dictyostelium purpureum

Background

The social amoebae (Dictyostelia) are a diverse group of Amoebozoa that achieve multicellularity by aggregation and undergo morphogenesis into fruiting bodies with terminally differentiated spores and stalk cells. There are four groups of dictyostelids, with the most derived being a group that contains the model species Dictyostelium discoideum.

Results

We have produced a draft genome sequence of another group dictyostelid, Dictyostelium purpureum, and compare it to the D. discoideum genome. The assembly (8.41 × coverage) comprises 799 scaffolds totaling 33.0 Mb, comparable to the D. discoideum genome size. Sequence comparisons suggest that these two dictyostelids shared a common ancestor approximately 400 million years ago. In spite of this divergence, most orthologs reside in small clusters of conserved synteny. Comparative analyses revealed a core set of orthologous genes that illuminate dictyostelid physiology, as well as differences in gene family content. Interesting patterns of gene conservation and divergence are also evident, suggesting function differences; some protein families, such as the histidine kinases, have undergone little functional change, whereas others, such as the polyketide synthases, have undergone extensive diversification. The abundant amino acid homopolymers encoded in both genomes are generally not found in homologous positions within proteins, so they are unlikely to derive from ancestral DNA triplet repeats. Genes involved in the social stage evolved more rapidly than others, consistent with either relaxed selection or accelerated evolution due to social conflict.

Conclusions

The findings from this new genome sequence and comparative analysis shed light on the biology and evolution of the Dictyostelia.

Pregnenolone sulfate and cortisol induce secretion of acyl-CoA-binding protein and its conversion into endozepines from astrocytes

Acyl-CoA-binding protein (ACBP) functions both intracellularly as part of fatty acid metabolism and extracellularly as diazepam binding inhibitor, the precursor of endozepine peptides. Two of these peptides, ODN and TTN, bind to the GABA(A) receptor and modulate its sensitivity to gamma-aminobutyric acid (GABA). We have found that depolarization of mouse primary astrocytes induces the rapid release and processing of ACBP to the active peptides. We previously showed that ODN can trigger the rapid sporulation of the social amoeba Dictyostelium. Using this bioassay, we now show that astrocytes release the endozepine peptides within 10 min of exposure to the steroids cortisol, pregnenolone, pregnenolone sulfate, or progesterone. ACBP lacks a signal sequence for secretion through the endoplasmic reticulum/Golgi pathway and its secretion is not affected by addition of brefeldin A, a well known inhibitor of the classical secretion pathway, suggesting that it follows an unconventional pathway for secretion. Moreover, induction of autophagy by addition of rapamycin also resulted in rapid release of ACBP indicating that this protein uses components of the autophagy pathway for secretion. Following secretion, ACBP is proteolytically cleaved to the active neuropeptides by protease activity on the surface of astrocytes. Neurosteroids, such as pregnenolone sulfate, were previously shown to modulate the excitatory/inhibitory balance in brain through increased release of glutamate and decreased release of GABA. These effects of steroids in neurons will be reinforced by the release of endozepines from astrocytes shown here, and suggest an orchestrated astrocyte-neuron cross-talk that can affect a broad spectrum of behavioral functions.

Unconventional secretion of Acb1 is mediated by autophagosomes

Starving Dictyostelium discoideum cells secrete AcbA, an acyl coenzyme A-binding protein (ACBP) that lacks a conventional signal sequence for entering the endoplasmic reticulum (ER). Secretion of AcbA in D. discoideum requires the Golgi-associated protein GRASP. In this study, we report that starvation-induced secretion of Acb1, the Saccharomyces cerevisiae ACBP orthologue, also requires GRASP (Grh1). This highlights the conserved function of GRASP in unconventional secretion. Although genes required for ER to Golgi or Golgi to cell surface transport are not required for Acb1 secretion in yeast, this process involves autophagy genes and the plasma membrane t-SNARE, Sso1. Inhibiting transport to vacuoles does not affect Acb1 secretion. In sum, our experiments reveal a unique secretory pathway where autophagosomes containing Acb1 evade fusion with the vacuole to prevent cargo degradation. We propose that these autophagosome intermediates fuse with recycling endosomes instead to form multivesicular body carriers that then fuse with the plasma membrane to release cargo.

Unconventional secretion of Pichia pastoris Acb1 is dependent on GRASP protein, peroxisomal functions, and autophagosome formation

Evidence is presented for an unconventional protein secretion pathway that is conserved from yeast to Dictyostelium discoideum in which Acb1 may be sequestered into autophagosomal vesicles, which then fuse (either directly or indirectly) with the plasma membrane (see also the companion paper from Duran et al. in this issue).

In contrast to the enormous advances made regarding mechanisms of conventional protein secretion, mechanistic insights into the unconventional secretion of proteins are lacking. Acyl coenzyme A (CoA)–binding protein (ACBP; AcbA in Dictyostelium discoideum), an unconventionally secreted protein, is dependent on Golgi reassembly and stacking protein (GRASP) for its secretion. We discovered, surprisingly, that the secretion, processing, and function of an AcbA-derived peptide, SDF-2, are conserved between the yeast Pichia pastoris and D. discoideum. We show that in yeast, the secretion of SDF-2–like activity is GRASP dependent, triggered by nitrogen starvation, and requires autophagy proteins as well as medium-chain fatty acyl CoA generated by peroxisomes. Additionally, a phospholipase D implicated in soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor–mediated vesicle fusion at the plasma membrane is necessary, but neither peroxisome turnover nor fusion between autophagosomes and the vacuole is essential. Moreover, yeast Acb1 and several proteins required for its secretion are necessary for sporulation in P. pastoris. Our findings implicate currently unknown, evolutionarily conserved pathways in unconventional secretion.

Steroids initiate a signaling cascade that triggers rapid sporulation in Dictyostelium

Encapsulation of prespore cells of Dictyostelium discoideum is controlled by several intercellular signals to ensure appropriate timing during fruiting body formation. Acyl-CoA-binding protein, AcbA, is secreted by prespore cells and processed by the prestalk protease TagC to form the 34 amino acid peptide SDF-2 that triggers rapid encapsulation. AcbA is secreted when gamma-aminobutyric acid (GABA) is released from prespore cells and binds to GrlE, a G protein-coupled receptor (GPCR). Analysis of SDF-2 production in mutant strains lacking Galpha subunits and GPCRs, either as pure populations or when mixed with other mutant strains, uncovered the non-cell-autonomous roles of GrlA, Galpha4 and Galpha7. We found that Galpha7 is essential for the response to GABA and is likely to be coupled to GrlE. GrlA-null and Galpha4-null cells respond normally to GABA but fail to secrete it. We found that they are necessary for the response to a small hydrophobic molecule, SDF-3, which is released late in culmination. Pharmacological inhibition of steroidogenesis during development blocked the production of SDF-3. Moreover, the response to SDF-3 could be blocked by the steroid antagonist mifepristone, whereas hydrocortisone and other steroids mimicked the effects of SDF-3 when added in the nanomolar range. It appears that SDF-3 is a steroid that elicits rapid release of GABA by acting through the GPCR GrlA, coupled to G protein containing the Galpha4 subunit. SDF-3 is at the head of the cascade that amplifies the signal for encapsulation to ensure the rapid, synchronous formation of spores.

Cytokinins induce sporulation in Dictyostelium

The social amoeba Dictyostelium discoideum diverged from the line leading to animals shortly after the separation of plants and animals but it retained characteristics of both kingdoms. A GABA(B)-like receptor and a peptide, SDF-2, with homologs found only in animals, control sporulation, while cytokinins, which act as hormones in plants, keep spores dormant. When SDF-2 binds its receptor DhkA, it reduces the activity of the cAMP phosphodiesterase RegA such that cAMP levels can increase. It has been proposed that the cytokinin discadenine also produces in an increase in cAMP but acts through a different histidine kinase, DhkB. We have found that discadenine and its precursor, isopentenyl adenine, not only maintain spore dormancy but also initiate rapid encapsulation independently of the SDF-2 signal transduction pathway. DhkB and the adenylyl cyclase of late development, AcrA, are members of two component signal transduction families and both are required to transduce the cytokinin signal. As expected, strains lacking the isopentenyl-transferase enzyme chiefly responsible for cytokinin synthesis are defective in sporulation. It appears that SDF-2 and cytokinins are secreted during late development to trigger signal transduction pathways that lead to an increase in the activity of the camp-dependent protein kinase, PKA, which triggers rapid encapsulation as well as ensuring spore dormancy.

The Golgi-Associated Protein GRASP Is Required for Unconventional Protein Secretion during Development

During Dictyostelium development, prespore cells secrete acyl-CoA binding protein (AcbA). Upon release, AcbA is processed to generate a peptide called spore differentiation factor-2 (SDF-2), which triggers terminal differentiation of spore cells. We have found that cells lacking Golgi reassembly stacking protein (GRASP), a protein attached peripherally to the cytoplasmic surface of Golgi membranes, fail to secrete AcbA and, thus, produce inviable spores. Surprisingly, AcbA lacks a signal sequence and is not secreted via the conventional secretory pathway (endoplasmic reticulum-Golgi-cell surface). GRASP is not required for conventional protein secretion, growth, and the viability of vegetative cells. Our findings reveal a physiological role of GRASP and provide a means to understand unconventional secretion and its role in development.

Genetic evidence that the acyl coenzyme A binding protein AcbA and the serine protease/ABC transporter TagA function together in Dictyostelium discoideum cell differentiation

The acyl coenzyme A (CoA) binding protein AcbA is cleaved to form a peptide (SDF-2) that coordinates spore encapsulation during the morphogenesis of Dictyostelium discoideum fruiting bodies. We present genetic evidence that the misspecification of cell types seen in mutants of the serine protease/ABC transporter TagA results from the loss of normal interactions with AcbA. Developmental phenotypes resulting from aberrant expression of the TagA protease domain, such as the formation of supernumerary tips on aggregates and the production of excess prestalk cells, are suppressed by null mutations in the acbA gene. Phenotypes resulting from the deletion of tagA, such as overexpression of the prestalk gene ecmB and the misexpression of the prespore gene cotB in stalk cells, are also observed in acbA mutants. Moreover, tagA- mutants fail to produce SDF-2 during fruiting body morphogenesis but are able to do so if they are stimulated with exogenous SDF-2. These results indicate that the developmental program depends on TagA and AcbA working in concert with each other during cell type differentiation and suggest that TagA is required for normal SDF-2 signaling during spore encapsulation.

GABA induces terminal differentiation of Dictyostelium through a GABAB receptor

When prespore cells approach the top of the stalk in a Dictyostelium fruiting body, they rapidly encapsulate in response to the signalling peptide SDF-2. Glutamate decarboxylase, the product of the gadA gene, generates GABA from glutamate. gadA is expressed exclusively in prespore cells late in development. We have found that GABA induces the release of the precursor of SDF-2, AcbA, from prespore cells. GABA also induces exposure of the protease domain of TagC on the surface of prestalk cells where it can convert AcbA to SDF-2. The receptor for GABA in Dictyostelium, GrlE, is a seven-transmembrane G-protein-coupled receptor that is most similar to GABA(B) receptors. The signal transduction pathway from GABA/GrlE appears to be mediated by PI3 kinase and the PKB-related protein kinase PkbR1. Glutamate acts as a competitive inhibitor of GABA functions in Dictyostelium and is also able to inhibit induction of sporulation by SDF-2. The signal transduction pathway from SDF-2 is independent of the GABA/glutamate signal transduction pathway, but the two appear to converge to control release of AcbA and exposure of TagC protease. These results indicate that GABA is not only a neurotransmitter but also an ancient intercellular signal.

Peptide signaling during terminal differentiation of Dictyostelium

A wide variety of mechanisms have evolved for intercellular communication in metazoans, but some of the signaling molecules were already used in their predecessors. The social amoeba, Dictyostelium discoideum, is known to use peptides to trigger sporulation within fruiting bodies, but their sequences have not been defined. We found that the peptide signal spore differentiation factor 2 (SDF-2) is processed from acyl-CoA binding protein, AcbA. The mammalian homolog of AcbA is processed to diazepam binding inhibitor that binds to the GABA(A) receptor in the brain and to peripheral 1,4 benzodiazepine receptors. Although Dictyostelium has neither GABA(A) nor peripheral-type benzodiazepine receptors, we find that both a diazepam binding inhibitor peptide and diazepam (Valium) can mimic SDF-2 in a Dictyostelium bioassay. Mutants lacking AcbA sporulate well only when developed in chimeras with WT cells. Using a yeast system we show that ligand binding to the SDF-2 receptor histidine kinase, DhkA, inhibits phosphorelay, which can account for its ability to induce rapid sporulation.

The genome of the social amoeba Dictyostelium discoideum

The social amoebae are exceptional in their ability to alternate between unicellular and multicellular forms. Here we describe the genome of the best-studied member of this group, Dictyostelium discoideum. The gene-dense chromosomes of this organism encode approximately 12,500 predicted proteins, a high proportion of which have long, repetitive amino acid tracts. There are many genes for polyketide synthases and ABC transporters, suggesting an extensive secondary metabolism for producing and exporting small molecules. The genome is rich in complex repeats, one class of which is clustered and may serve as centromeres. Partial copies of the extrachromosomal ribosomal DNA (rDNA) element are found at the ends of each chromosome, suggesting a novel telomere structure and the use of a common mechanism to maintain both the rDNA and chromosomal termini. A proteome-based phylogeny shows that the amoebozoa diverged from the animal-fungal lineage after the plant-animal split, but Dictyostelium seems to have retained more of the diversity of the ancestral genome than have plants, animals or fungi.

Gdt2 regulates the transition of Dictyostelium cells from growth to differentiation

Background

Dictyostelium life cycle consists of two distinct phases – growth and development. The control of growth-differentiation transition in Dictyostelium is not completely understood, and only few genes involved in this process are known.

Results

We have isolated a REMI (restriction enzyme-mediated integration) mutant, which prematurely initiates multicellular development. When grown on a bacterial lawn, these cells aggregate before the bacteria are completely cleared. In bacterial suspension, mutant cells express the developmental marker discoidin Iγ even at low cell densities and high concentrations of bacteria. In the absence of nutrients, mutant cells aggregate more rapidly than wild type, but the rest of development is unaffected and normal fruiting bodies are formed. The disrupted gene shows substantial homology to the recently described gdt1 gene, and therefore was named gdt2. While GDT1 and GDT2 are similar in many ways, there are intriguing differences. GDT2 contains a well conserved protein kinase domain, unlike GDT1, whose kinase domain is probably non-functional. The gdt2 and gdt1 mRNAs are regulated differently, with gdt2 but not gdt1 expressed throughout development. The phenotypes of gdt2^- and gdt1^- mutants are related but not identical. While both initiate development early, gdt2^- cells grow at a normal rate, unlike gdt1^- mutants. Protein kinase A levels and activity are essentially normal in growing gdt2^- mutants, implying that GDT2 regulates a pathway that acts separately from PKA. Gdt1 and gdt2 are the first identified members of a family containing at least eight closely related genes.

Conclusions

We have isolated and characterised a new gene, gdt2, which acts to restrain development until conditions are appropriate. We also described a family of related genes in the Dictyostelium genome. We hypothesise that different family members might control similar cellular processes, but respond to different environmental cues.

cAMP-dependent protein kinase regulates Polysphondylium pallidum development

In eukaryotic cells, the universal second messenger cAMP regulates various aspects of development and differentiation. The primary target for cAMP is the regulatory subunit of cAMP-dependent protein kinase A (PKA), which, upon cAMP binding, dissociates from the catalytic subunit and thus activates it. In the soil amoeba Dictyostelium discoideum, the function of PKA in growth, development and cell differentiation has been thoroughly investigated and substantial information is available. To obtain a more general view, we investigated the influence of PKA on development of the related species Polysphondylium pallidum. Cells were transformed to overexpress either a dominant negative mutant of the regulatory subunit (Rm) from Dictyostelium that cannot bind cAMP, or the catalytic subunit (PKA-C) from Dictyostelium. Cells overexpressing Rm rarely aggregated and the few multicellular structures developed slowly into very small fruiting bodies without branching of secondary sorogens, the prominent feature of Polysphondylium. Few round spores with reduced viability were formed. When mixed with wild-type cells and allowed to develop, the Rm cells were randomly distributed in aggregation streams, but were later found in the posterior region of the culminating slug or were left behind on the surface of the substratum. The PKA-C overexpressing cells exhibited precocious development and formed more aggregates of smaller size. Moreover, expression of PKA-C under the control of the prestalk-specific ecmB promoter of Dictyostelium leads to protrusions from aggregation streams. We conclude that Dictyostelium PKA subunits introduced into Polysphondylium cells are functional as signal components, indicating that a biochemically similar PKA mechanism works in Polysphondylium.

Evolutionary analyses of ABC transporters of Dictyostelium discoideum

The ABC superfamily of genes is one of the largest in the genomes of both bacteria and eukaryotes. The proteins encoded by these genes all carry a characteristic 200- to 250-amino-acid ATP-binding cassette that gives them their family name. In bacteria they are mostly involved in nutrient import, while in eukaryotes many are involved in export. Seven different families have been defined in eukaryotes based on sequence homology, domain topology, and function. While only 6 ABC genes in Dictyostelium discoideum have been studied in detail previously, sequences from the well-advanced Dictyostelium genome project have allowed us to recognize 68 members of this superfamily. They have been classified and compared to animal, plant, and fungal orthologs in order to gain some insight into the evolution of this superfamily. It appears that many of the genes inferred to have been present in the ancestor of the crown organisms duplicated extensively in some but not all phyla, while others were lost in one lineage or the other.

Requirements for the adenylyl cyclases in the development of Dictyostelium

It has been suggested that all intracellular signaling by cAMP during development of Dictyostelium is mediated by the cAMP-dependent protein kinase, PKA, since cells carrying null mutations in the acaA gene that encodes adenylyl cyclase can develop so as to form fruiting bodies under some conditions if PKA is made constitutive by overexpressing the catalytic subunit. However, a second adenylyl cyclase encoded by acrA has recently been found that functions in a cell autonomous fashion during late development. We have found that expression of a modified acaA gene rescues acrA− mutant cells indicating that the only role played by ACR is to produce cAMP. To determine whether cells lacking both adenylyl cyclase genes can develop when PKA is constitutive we disrupted acrA in a acaA− PKA-Cover strain. When developed at high cell densities, acrA−acaA− PKA-Cover cells form mounds, express cell type-specific genes at reduced levels and secrete cellulose coats but do not form fruiting bodies or significant numbers of viable spores. Thus, it appears that synthesis of cAMP is required for spore differentiation in Dictyostelium even if PKA activity is high.

Interaction of gdt1 and protein kinase A (PKA) in the growth-differentiation-transition in Dictyostelium

The gdt1 gene is a negative regulator of the growth-differentiation-transition (GDT) in Dictyostelium. gdt1- cells express the GDT marker discoidin earlier and at higher levels and prematurely enter the differentiation pathway. Protein kinase A is a positive regulator of the GDT and is required for multicellular development. Disruption of the PKA catalytic subunit or overexpression of a constitutively active mutant of the regulatory subunit results in cells which do not form multicellular aggregates and which show strongly reduced levels of discoidin. We have created PKA-/gdt1- double mutants and show that these display high levels of discoidin expression but no aggregation, suggesting that gdt1 may be a downstream target of PKA in a branched signaling cascade initiating differentiation. Data obtained with the PKA inhibitor H89 support these result: in wild type cells H89 inhibits discoidin expression while in gdt1- mutants there is no obvious effect. However, since PKA-/gdt1- cells display less discoidin expression than the single gdt1 mutant, we propose that PKA and gdt1 are in two parallel interacting pathways. To get insight into the mechanism how PKA may block gdt1, we have tested two putative PKA phosphorylation sites in the protein and found that one of them is efficiently phosphorylated by PKA in vitro. A model for the interplay between PKA and gdt1 during the growth-differentiation-transition is discussed.

gdt1, a new signal transduction component for negative regulation of the growth-differentiation transition in Dictyostelium discoideum

Discoidin I expression was used as a marker to screen for mutants affected in the growth-differentiation transition (GDT) of Dictyostelium. By REMI mutagenesis we have isolated mutant 2-9, an overexpressor of discoidin I. It displays normal morphogenesis but shows premature entry into the developmental cycle. The disrupted gene was denominated gdt1. The mutant phenotype was reconstructed by disruptions in different parts of the gene, suggesting that all had a complete loss of function. gdt1 was expressed in growing cells; the levels of protein and mRNA appear to increase with cell density and rapidly decrease with the onset of development. gdt1 encodes a 175-kDa protein with four putative transmembrane domains. In the C terminus, the derived amino acid sequence displays some similarity to the catalytic domain of protein kinases. Mixing experiments demonstrate that the gdt1(-) phenotype is cell autonomous. Prestarvation factor is secreted at wild-type levels. The response to folate, a negative regulator of discoidin expression, was not impaired in gdt1 mutants. Cells that lack the G protein alpha2 display a loss of discoidin expression and do not aggregate. gdt1(-)/Galpha2(-) double mutants show no aggregation but strong discoidin expression. This suggests that gdt1 is a negative regulator of the GDT downstream of or in a parallel pathway to Galpha2.

An adenylyl cyclase that functions during late development of Dictyostelium

A variety of extracellular signals lead to the accumulation of cAMP which can act as a second message within cells by activating protein kinase A (PKA). Expression of many of the essential developmental genes in Dictyostelium discoideum are known to depend on PKA activity. Cells in which the receptor-coupled adenylyl cyclase gene, acaA, is genetically inactivated grow well but are unable to develop. Surprisingly, acaA(−) mutant cells can be rescued by developing them in mixtures with wild-type cells, suggesting that another adenylyl cyclase is present in developing cells that can provide the internal cAMP necessary to activate PKA. However, the only other known adenylyl cyclase gene in Dictyostelium, acgA, is only expressed during germination of spores and plays no role in the formation of fruiting bodies. By screening morphological mutants generated by Restriction Enzyme Mediated Integration (REMI) we discovered a novel adenylyl cyclase gene, acrA, that is expressed at low levels in growing cells and at more than 25-fold higher levels during development. Growth and development up to the slug stage are unaffected in acrA(−) mutant strains but the cells make almost no viable spores and produce unnaturally long stalks. Adenylyl cyclase activity increases during aggregation, plateaus during the slug stage and then increases considerably during terminal differentiation. The increase in activity following aggregation fails to occur in acrA(−) cells. As long as ACA is fully active, ACR is not required until culmination but then plays a critical role in sporulation and construction of the stalk.

SDF-2 induction of terminal differentiation in Dictyostelium discoideum is mediated by the membrane-spanning sensor kinase DhkA

SDF-2 is a peptide released by prestalk cells during culmination that stimulates prespore cells to encapsulate. Genetic evidence indicates that the response is dependent on the dhkA gene. This gene encodes a member of the histidine kinase family of genes that functions in two-component signal transduction pathways. The sequence of the N-terminal half of DhkA predicts two hydrophobic domains separated by a 310-amino-acid loop that could bind a ligand. By inserting MYC6 epitopes into DhkA, we were able to show that the loop is extracellular while the catalytic domain is cytoplasmic. Cells expressing the MYC epitope in the extracellular domain of DhkA were found to respond only if induced with 100-fold-higher levels of SDF-2 than required to induce dhkA+ cells; however, they could be induced to sporulate by addition of antibodies specific to the MYC epitope. To examine the enzymatic activity of DhkA, we purified the catalytic domain following expression in bacteria and observed incorporation of labelled phosphate from ATP consistent with histidine autophosphorylation. Site-directed mutagenesis of histidine1395 to glutamine in the catalytic domain blocked autophosphorylation. Furthermore, genetic analyses showed that histidine1395 and the relay aspartate2075 of DhkA are both critical to its function but that another histidine kinase, DhkB, can partially compensate for the lack of DhkA activity. Sporulation is drastically reduced in double mutants lacking both DhkA and DhkB. Suppressor studies indicate that the cyclic AMP (cAMP) phosphodiesterase RegA and the cAMP-dependent protein kinase PKA act downstream of DhkA.

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.

Mucor rouxii delta9-desaturase gene is transcriptionally regulated during cell growth and by low temperature

Unsaturated fatty acids are essential lipid components of Mucor rouxii. Gamma-linolenic acid (GLA) is synthesized via the desaturase enzymes: delta9-desaturase catalyzes mono-unsaturated fatty acids that are utilized as substrate for GLA biosynthesis. We cloned and characterized a M. rouxii gene highly homologous to delta9-desaturase genes. This sequence encodes for a protein of 452 amino acids and contains two introns of 60 and 61 nucleotides. Delta9-desaturase of M. rouxii is expressed during cell growth when cells are subjected to temperature shifts. At 30 degrees C, the mRNA level of late log phase is about 6.4-fold higher than that of early log phase. A shift from 30 to 15 degrees C induced transcription of delta9-desaturase gene in both early and late log phases. However, the pattern of increased transcription by cold induction varied depending on growth conditions: transcription of late log phase is higher than that of early log phase. These results indicate that cell growth and low temperature influence the expression of delta9-desaturase gene and fatty acid composition of M. rouxii.

Production and activity of spore differentiation factors (SDFs) in Dictyostelium

SDF-1 and SDF-2 are peptides that promote terminal spore differentiation under submerged conditions. The present study shows that they accumulate differentially and are released during the development of wild-type cells and can promote spore formation in cells disaggregated from wild-type culminants. SDF-1 accumulates during the slug stage and is released in a single burst at the onset of culmination while SDF-2 accumulates during early culmination and is released in a single burst from mid-culminants. The effects of SDF-1 and SDF-2 on stalk cell formation in cell monolayers were investigated. SDF-1 by itself induces stalk cell formation in some strains and also synergizes with the stalk-cell-inducing factor, DIF-1. cAMP has an inhibitory effect on stalk cell formation when either DIF-1 or SDF-1 are present on their own but is almost not inhibitory when both are present. SDF-2 alone does not induce stalk cell formation and appears to inhibit the response to DIF-1. At the same time, it increases the extent of vacuolization of the stalk cells that are produced. We propose that the release of SDF-1 and then of SDF-2 may mark irreversible steps in the developmental programme associated, respectively, with culmination and spore maturation.

Functional analysis of the catalytic subunit of Dictyostelium PKA in vivo

The catalytic subunit of the cAMP-dependent protein kinase (PKA) from Dictyostelium discoideum contains several domains, including an unusually long N-terminal extension preceding a highly conserved catalytic core. We transformed the aggregationless PkaC-null strain with several deletion constructs of both domains. Strains transformed with genes expressing catalytically-inactive polypeptides could not rescue development. Cotransformation with constructs encoding the N-terminal extension and the catalytic core, both unable to rescue development by themselves, yielded transformants able to proceed to late development. A 27-amino acid long hydrophobic region, immediately upstream of the catalytic core, was found indispensable for PKA function. A putative role of this sequence in the acquisition of the active conformation of the protein is discussed.

Signal transduction pathways leading to spore differentiation in Dictyostelium discoideum

Cells that overexpress PKA as a consequence of carrying multiple copies of the gene encoding the catalytic subunit can be induced to sporulate when developing as single cells. A peptide phosphorylated by PKA, termed SDF-1, has recently been shown to stimulate this process (Anjard et al., 1997). Several genes have been implicated in a signal transduction pathway by which prestalk cells induce encapsulation of prespore cells during terminal differentiation including a prestalk-specific putative membrane protease (TagC) and a two-component system consisting of a receptor-histidine kinase (DhkA) and a response regulator with cAMP phosphodiesterase activity (RegA). To determine whether SDF-1 uses this pathway, strains carrying null mutations in the pertinent genes were transformed with a pkaC plasmid such that they can overexpress PKA. Since these mutant strains all sporulated efficiently when SDF-1 was added, it appears that other gene products mediate the response. However, we found that regA- mutant cells release a distinct factor, SDF-2, that rapidly induces encapsulation of test cells overexpressing pkaC. Since cells in which tagC is disrupted do not form SDF-2 and cells in which dhkA is disrupted do not respond to SDF-2, this peptide appears to use the two-component system that regulates PKA activity. SDF-2 is a small peptide released by prestalk cells in a manner dependent on TagC. It appears to act on prespore cells through the DhkA receptor to inhibit the cAMP phosphodiesterase of RegA, thereby activating PKA via cAMP. The process of induction by SDF-2 can be shown to be distinct from that by SDF-1. SDF-2 appears to stimulate prestalk cells to release additional SDF-2 by acting through a signal transduction pathway that also involves DhkA, RegA, and PKA. Based on these results we present a model for the signal transduction cascade regulating spore differentiation.

A new spore differentiation factor (SDF) secreted by Dictyostelium cells is phosphorylated by the cAMP dependent protein kinase

Upon starvation, Dictyostelium discoideum unicellular amoebae form a multicellular organism leading to the development of a fruiting body containing spores. Single cells of sporogenous mutants, unlike wild type cells, are able to differentiate into spores under specific conditions. We show in this report that overexpression of the catalytic subunit of the cAMP dependent protein kinase (PKA), not only renders the cells sporogenous, but is also accompanied by the production/release of a diffusible spore differentiation factor (SDF). SDF is a small, thermostable phospho-polypeptide. In vitro dephosphorylation reduces SDF spore differentiation capacity, which can be regained in vitro by PKA phosphorylation. These results indicate that SDF is a PKA substrate and might be activated in vivo by this protein kinase. Since spore differentiation requires PKA catalytic subunit activation, we conclude that the response of prespore cells to SDF involves an intracellular pathway dependent on PKA.

The catalytic subunit of Dictyostelium cAMP-dependent protein kinase -- role of the N-terminal domain and of the C-terminal residues in catalytic activity and stability

The C subunit of Dictyostelium cAMP-dependent protein kinase (PKA) is unusually large (73 kDa) due to the presence of 330 amino acids N-terminal to the conserved catalytic core. The sequence following the core, including a C-terminal -Phe-Xaa-Xaa-Phe-COOH motif, is highly conserved. We have characterized the catalytic activity and stability of C subunits mutated in sequences outside the catalytic core and we have analyzed their ability to interact with the R subunit and with the heat-stable protein-kinase inhibitor PKI. Mutants carrying deletions in the N-terminal domain displayed little difference in their kinetic properties and retained their capacity to be inhibited by R subunit and by PKI. In contrast, the mutation of one or both of the phenylalanine residues in the C-terminal motif resulted in a decrease of catalytic activity and stability of the proteins. Inhibition by the R subunit or by PKI were however unaffected. Sequence-comparison analysis of other protein kinases revealed that a -Phe-Xaa-Xaa-Phe- motif is present in many Ser/Thr protein kinases, although its location at the very end of the polypeptide is a particular feature of the PKA family. We propose that the presence of this motif may serve to identify isoforms of protein kinases.

Subunits I and II of Dictyostelium cytochrome c oxidase are specified by a single open reading frame transcribed into a large polycistronic RNA

A single open reading frame (ORF) encoding cytochrome c oxidase subunit I and II (cox1/2) was identified in the mitochondrial genome of the slime mold Dictyostelium discoideum. The cox1 coding region shares intron positions with its counterparts in fungi and algae. Northern blot analysis, using exon and intron-specific probes, suggests that the cox1/2 gene is transcribed as part of a large, efficiently processed, polycistronic RNA.

An unusual catalytic subunit for the cAMP-dependent protein kinase of Dictyostelium discoideum

The cAMP-dependent protein kinase (cAPK) plays an essential role during differentiation and fruit morphogenesis in Dictyostelium discoideum. The presence of an open reading frame on the gene, pkaC (previously named either Dd PK2 or Dd PK3 by different groups), predicts a 73-kDa polypeptide with 54% similarity to the catalytic subunits of cAPKs from other organisms. Using anti-peptide antibodies, we show that the pkaC gene product, PkaC, is a 73-kDa polypeptide. Despite the fact that PkaC is about twice the size of its mammalian counterparts, it possesses all of the properties required of a catalytic subunit. It is physically associated with the regulatory subunit, and this association results in an inhibition of the catalytic activity which is reverted by cAMP. PkaC copurifies with cAPK activity, and an increased cAPK activity is observed in cells overexpressing PkaC. We conclude that PkaC is a catalytic subunit of the Dictyostelium discoideum cAPK and discuss the unusual features of this protein with the highest molecular weight of known cAPKs.

Induction of terminal differentiation of Dictyostelium by cAMP-dependent protein kinase and opposing effects of intracellulr and extracellular cAMP on stalk cell differentiation

Expression of the catalytic (C) subunit of the cAMP-dependent protein kinase (PKA) of Dictyostelium under the control of heterologous, cell-type-specific promoters causes ectopic terminal differentiation. When expressed under the control of a prespore-specific promoter, development is accelerated, to yield highly aberrant fruiting bodies that contain a basal mass of spore cells surrounding a central stalk-like structure. When expressed under the control of a prestalk-specific promoter, development arrests much earlier, at the tight mound stage. Prestalk cells move to the apices of these mounds, apparently normally, but no tip is formed. Most of the prestalk cells remain arrested in their development but there are a few isolated stalk cells scattered within such mounds. We show that extracellular cAMP represses stalk cell-specific gene expression in cells where the kinase is constitutively active, suggesting that inhibition of stalk cell differentiation by cAMP in normal cells (Berks and Kay, 1988) occurs because of an effect of extracellular cAMP on an intracellular signalling pathway independent of PKA. We propose a scheme whereby two separate events, a rise in intracellular cAMP levels and a fall in extracellular cAMP concentration, are required to induce stalk cell differentiation.

Overexpression of Dd PK2 protein kinase causes rapid development and affects the intracellular cAMP pathway of Dictyostelium discoideum

The Dd PK2 gene codes for a putative protein of 648 amino acids with a C-terminal half sharing high homology with protein kinase A catalytic subunits from other organisms. In order to find out more about the physiological role of the Dd PK2 kinase, its gene, and a version having a frame shift mutation in the middle of the catalytic region, were overexpressed in developing Dictyostelium cells. Both the intact gene (K-) and the frame shift mutant (Kdel-) caused rapid development with spores formed in 16–18 hours compared to the 24 hours required by their parent. This result was confirmed by the pattern of expression of some developmentally regulated genes. Other rapid developing strains (rde) are activated in the cAMP second messenger system. Both K- and Kdel-containing strains have lower cAMP levels than the parental strain during late development, thus resembling rdeC mutants. K-cells (but not Kdel-cells) produced bizarre fruiting bodies with many prostrate forms. The parallel with rde mutants was confirmed by demonstrating that K-cells are able to form spores in submerged monolayer culture. Furthermore, K-cells have about four times more protein kinase A (cAPK) activity than wild-type cells. These results indicate that the N-terminal domain of Dd PK2 is sufficient to influence cAMP levels and to provoke rapid development, whereas kinase activity seems to be required for the sporogenous phenotype. The association between elevated cAPK and Dd PK2 overexpression phenotype further indicates a role for cAPK in the formation of spores.

Enhanced resistance to HIV-1 replication in U937 cells stably transfected with the human IFN-beta gene behind an MHC promoter fragment

Cells of the monocyte lineage act as a major reservoir for HIV, and ways of enhancing the resistance of mononuclear phagocytes to HIV replication would be useful for delaying the onset of AIDS in infected individuals. Seif et al. (J. Virol. 65:664, 1991) have recently shown the possibility of obtaining stable antiviral expression (SAVE), directed against three nonretroviral RNA viruses, and normal cell viability in a significant percentage of murine BALB/c 3T3 cells transformed with an IFN-beta expression plasmid under the control of the 0.6-kb XhoII-NruI promoter region of the murine H-2Kb MHC gene. In the present paper, we show that it is possible to establish SAVE in human promonocytic cells. Cells of the human promonocytic U937 line were stably transfected with a human IFN-beta expression plasmid carrying the neo- and human IFN-beta-coding sequences under the control of the H-2Kb promoter fragment previously used in murine cells. After selection with G418, two transformed clones were isolated that released small amounts of human IFN-beta into the culture medium, without affecting the expression of CD4 and leucocyte function-associated Ag-1 differentiation Ag. The presence of construct-derived IFN-beta mRNA was demonstrated by polymerase chain reaction amplification of cDNA, and the level of 2-5A synthetase, one of the major IFN-induced antiviral proteins, was shown to be constitutively increased. These clones were less permissive for HIV-1 than control clones transformed with the neo gene only. The antiviral state could be modulated by anti-IFN-beta antibodies, in that the continuous presence of antibodies in the culture medium abolished the enhanced resistance to HIV-1 replication, whereas the withdrawal of the antiserum restored the antiviral state, indicating that it did indeed result from the constitutive synthesis of human IFN-beta. These results demonstrate the possibility of restricting HIV-1 replication in human promonocytic cells by establishing SAVE. Further exploration of this method as a possible approach to somatic cell gene therapy of HIV infection appears worthwhile.

Enhanced resistance to HIV-1 replication in U937 cells stably transfected with the human IFN-beta gene behind an MHC promoter fragment

Cells of the monocyte lineage act as a major reservoir for HIV, and ways of enhancing the resistance of mononuclear phagocytes to HIV replication would be useful for delaying the onset of AIDS in infected individuals. Seif et al. (J. Virol. 65:664, 1991) have recently shown the possibility of obtaining stable antiviral expression (SAVE), directed against three nonretroviral RNA viruses, and normal cell viability in a significant percentage of murine BALB/c 3T3 cells transformed with an IFN-beta expression plasmid under the control of the 0.6-kb XhoII-NruI promoter region of the murine H-2Kb MHC gene. In the present paper, we show that it is possible to establish SAVE in human promonocytic cells. Cells of the human promonocytic U937 line were stably transfected with a human IFN-beta expression plasmid carrying the neo- and human IFN-beta-coding sequences under the control of the H-2Kb promoter fragment previously used in murine cells. After selection with G418, two transformed clones were isolated that released small amounts of human IFN-beta into the culture medium, without affecting the expression of CD4 and leucocyte function-associated Ag-1 differentiation Ag. The presence of construct-derived IFN-beta mRNA was demonstrated by polymerase chain reaction amplification of cDNA, and the level of 2-5A synthetase, one of the major IFN-induced antiviral proteins, was shown to be constitutively increased. These clones were less permissive for HIV-1 than control clones transformed with the neo gene only. The antiviral state could be modulated by anti-IFN-beta antibodies, in that the continuous presence of antibodies in the culture medium abolished the enhanced resistance to HIV-1 replication, whereas the withdrawal of the antiserum restored the antiviral state, indicating that it did indeed result from the constitutive synthesis of human IFN-beta. These results demonstrate the possibility of restricting HIV-1 replication in human promonocytic cells by establishing SAVE. Further exploration of this method as a possible approach to somatic cell gene therapy of HIV infection appears worthwhile.

Isolation of two genes encoding putative protein kinases regulated during Dictyostelium discoideum development

Two Dictyostelium discoideum protein kinase(PK)-encoding cDNAs (Dd PK1 and Dd PK2) have been isolated by hybridization with an oligodeoxyribonucleotide derived from a highly conserved region of eukaryotic PKs. The two nucleotide (nt) sequences encode new putative serine/threonine-specific PKs. Dd PK1 is a partial cDNA covering the entire catalytic domain. The derived amino acid (aa) sequence is about 30% identical to both cAMP-dependent protein kinase (cAPK) and protein kinase C. The Dd PK2 sequence was extended through the isolation of a genomic fragment encoding a complete putative protein. A single intron is present, as deduced from sequence comparison with the cDNA. The catalytic domain appears more closely related to the catalytic subunit of cAPK (54% sequence identity). However, our nt sequence potentially codes for a much larger protein (648 vs. about 350 aa for most cAPKs) with a N-terminal half containing long homopolymers of threonines, glutamines and asparagines. Similar repeats occur at the C terminus of Dd PK1, Dd PK1 is expressed in vegetatively growing cells and during development. Dd PK1 RNA decreases after 6 h of starvation to re-accumulate once the cells have aggregated. Dd PK2 transcripts, present at a low amount in growing cells, rise upon starvation. A switch to a shorter form of transcripts occurs between 3 and 6 h into development.