Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

Bioluminescence and Photoreception in Unicellular Organisms: Light-Signalling in a Bio-Communication Perspective

Bioluminescence, the emission of light catalysed by luciferases, has evolved in many taxa from bacteria to vertebrates and is predominant in the marine environment. It is now well established that in animals possessing a nervous system capable of integrating light stimuli, bioluminescence triggers various behavioural responses and plays a role in intra- or interspecific visual communication. The function of light emission in unicellular organisms is less clear and it is currently thought that it has evolved in an ecological framework, to be perceived by visual animals. For example, while it is thought that bioluminescence allows bacteria to be ingested by zooplankton or fish, providing them with favourable conditions for growth and dispersal, the luminous flashes emitted by dinoflagellates may have evolved as an anti-predation system against copepods. In this short review, we re-examine this paradigm in light of recent findings in microorganism photoreception, signal integration and complex behaviours. Numerous studies show that on the one hand, bacteria and protists, whether autotrophs or heterotrophs, possess a variety of photoreceptors capable of perceiving and integrating light stimuli of different wavelengths. Single-cell light-perception produces responses ranging from phototaxis to more complex behaviours. On the other hand, there is growing evidence that unicellular prokaryotes and eukaryotes can perform complex tasks ranging from habituation and decision-making to associative learning, despite lacking a nervous system. Here, we focus our analysis on two taxa, bacteria and dinoflagellates, whose bioluminescence is well studied. We propose the hypothesis that similar to visual animals, the interplay between light-emission and reception could play multiple roles in intra- and interspecific communication and participate in complex behaviour in the unicellular world.

Dictyostelium discoideum--a model for many reasons

The social amoeba or cellular slime mould Dictyostelium discoideum is a "professional" phagocyte that has long been recognized for its value as a biomedical model organism, particularly in studying the actomyosin cytoskeleton and chemotactic motility in non-muscle cells. The complete genome sequence of D. discoideum is known, it is genetically tractable, readily grown clonally as a eukaryotic microorganism and is highly accessible for biochemical, cell biological and physiological studies. These are the properties it shares with other microbial model organisms. However, Dictyostelium combines these with a unique life style, with motile unicellular and multicellular stages, and multiple cell types that offer for study an unparalleled variety of phenotypes and associated signalling pathways. These advantages have led to its recent emergence as a valuable model organism for studying the molecular pathogenesis and treatment of human disease, including a variety of infectious diseases caused by bacterial and fungal pathogens. Perhaps surprisingly, this organism, without neurons or brain, has begun to yield novel insights into the cytopathology of mitochondrial diseases as well as other genetic and idiopathic disorders affecting the central nervous system. Dictyostelium has also contributed significantly to our understanding of NDP kinase, as it was the Dictyostelium enzyme whose structure was first determined and related to enzymatic activity. The phenotypic richness and tractability of Dictyostelium should provide a fertile arena for future exploration of NDPK's cellular roles.

PldB, a putative phospholipase D homologue in Dictyostelium discoideum mediates quorum sensing during development

Quorum sensing, also known as cell-density sensing in the unicellular eukaryote Dictyostelium discoideum, is required for efficient entry into the differentiation and development segment of its life cycle. Quorum sensing is accomplished by simultaneously secreting and sensing the glycoprotein Conditioned Medium Factor, or CMF. When the density of starving cells is high, CMF levels are high, which leads to aggregation followed by development. Here, we describe the role of pldB, a gene coding for a putative phospholipase D (PLD) homologue, in quorum sensing. We find that in submerged culture, adding butanol, an inhibitor of PLD-catalyzed phosphatidic acid production, allows cells to bypass the requirement for CMF mediated quorum sensing and aggregate at low cell density. Deletion of pldB mimics the presence of butanol, allowing cells to aggregate at low cell density. pldB- cells also initiate and finish aggregation rapidly. Analysis of early developmental gene expression in pldB- cells reveals that the cyclic AMP receptor cAR1 is expressed at higher levels earlier than in wild-type cells, which could explain the rapid aggregation phenotype. As would be predicted, cells overexpressing pldB are unable to aggregate even at high cell density. Adding CMF to these pldB- overexpressing cells does not rescue aggregation. Both of these phenotypes are cell autonomous, as mixing a small number of pldB- cells with wild-type cells does not cause the wild-type cells to behave like pldB- cells.

Understanding the cortex of crawling cells: insights from Dictyostelium

All animal cells are believed to use the same basic molecular mechanisms for locomotion when crawling on a surface. Study of a wide range of crawling cells has tended to confirm this belief but has also led to a diversity of hypotheses for locomotion and a bewildering list of candidate effector proteins. The emergence of a powerful model system, Dictyostelium discoideum, for the study of crawling of cells makes definitive tests of hypotheses for locomotion a reality.

A second UDP-glucose pyrophosphorylase is required for differentiation and development in Dictyostelium discoideum

Uridine diphosphoglucose pyrophosphorylase (UDPGP) is a developmentally regulated enzyme in Dictyostelium discoideum, which is involved in trehalose, cellulose, and glycogen synthesis. Two independent UDPGP proteins are believed to be responsible for this activity. To determine the relative contributions of each protein, the genes encoding them were disrupted individually. Cells lacking the udpgp1 gene exhibit normal growth and development and make normal levels of cellulose. In agreement with these phenotypes, udpgp1(-) cells still have UDPGP activity, although at a reduced level. This supports the importance of the second UDPGP gene. This newly identified gene, ugpB, encodes an active UDPGP as determined by complementation in Escherichia coli. When this gene is disrupted, cells undergo aberrant differentiation and development ending with small, gnarled fruiting bodies. These cells also have decreased spore viability and decreased levels of glycogen, whose production requires UDPGP activity. These phenotypes suggest that UgpB constitutes the major UDPGP activity produced during development. Sequence analysis of the two UDPGP genes shows that UgpB has higher homology to other eukaryotic UDPGPs than does UDPGP1. This includes the presence of 5 conserved lysine residues. Udpgp1 only has 1 of these lysines.

Isolation and characterization of a symbiosis-regulated ras from the ectomycorrhizal fungus Laccaria bicolor

Ectomycorrhizae formed by the symbiotic interaction between ectomycorrhizal fungi and plant roots play a key role in maintaining and improving the health of a wide range of plants. Mycorrhizal initiation, development, and functional maintenance involve morphological changes that are mediated by activation and suppression of several fungal and plant genes. We identified a gene, Lbras, in the ectomycorrhizal fungus Laccaria bicolor that belongs to the ras family of genes, which has been shown in other systems to be associated with signaling pathways controlling cell growth and proliferation. The Lbras cDNA complemented ras2 function in Saccharomyces cerevisiae and had the ability to transform mammalian cells. Expression of Lbras, present as a single copy in the genome, was dependent upon interaction with host roots. Northern analysis showed that expression was detectable in L bicolor 48 h after interaction as well as in the established mycorrhizal tissue. Phylogenetic analysis with other Ras proteins showed that Lbras is related most closely to Aras of Aspergillus nidulans.

Soybean isoflavones trigger a calcium influx in Phytophthora sojae

Both the motile zoospores and the hyphal germ tubes of Phytophthora sojae respond chemotropically to the soybean isoflavones daidzein and genistein. The role of Ca(2+) in the cellular response to these host signals was investigated by using X-ray microanalysis of cells to monitor net changes in cellular levels of Ca(2+) and by quantifying the effects of exogenous Ca(2+) and daidzein on the developmental fate of encysted zoospores. Confirmation that isoflavones trigger a net influx of Ca(2+) into the cell was demonstrated by X-ray microanalysis of individual encysted zoospores. Zoospores exposed to 10 mM Ca(2+) and 1 microM daidzein at the time of encystment formed cysts that contained more Ca(2+) than zoospores exposed to Ca(2+) alone. The magnitude of internal Ca(2+) stores appears to be a determining factor affecting the developmental fate of P. sojae cysts.

A serpentine receptor-dependent, Gbeta- and Ca(2+) influx-independent pathway regulates mitogen-activated protein kinase ERK2 in Dictyostelium

Ca(2+) influx and mitogen-activated protein (MAP) kinase activation are important phenomena in signal transduction, which are often interconnected. We investigated whether serpentine receptor-dependent, Gbeta-independent activation of MAP kinase ERK2 by chemoattractant cyclic AMP (cAMP) is mediated by Ca(2+) influx in the social amoeba Dictyostelium discoideum. We generated a D. discoideum double mutant, which harbours a temperature-sensitive Gbeta subunit and expresses the apoaequorin protein. Utilizing this mutant, we demonstrate that cAMP induced Ca(2+) influx into intact D. discoideum cells can be blocked completely at both the permissive and the restrictive temperature, by using either gadolinium ions or Ruthenium Red. Under the same experimental conditions, these substances do not abolish cAMP stimulation of ERK2 at either temperature. We conclude that there is a Gbeta- and Ca(2+) influx-independent pathway for the receptor-dependent activation of MAP kinase ERK2 in D. discoideum.

A deubiquitinating enzyme that disassembles free polyubiquitin chains is required for development but not growth in Dictyostelium

Although cell differentiation usually involves synthesis of new proteins, little is known about the role of protein degradation. In eukaryotes, conjugation to ubiquitin polymers often targets a protein for destruction. This process is regulated by deubiquitinating enzymes, which can disassemble ubiquitin polymers or ubiquitin-substrate conjugates. We find that a deubiquitinating enzyme, UbpA, is required for Dictyostelium development. ubpA cells have normal protein profiles on gels, grow normally, and show normal responses to starvation such as differentiation and secretion of conditioned medium factor. However, ubpA cells have defective aggregation, chemotaxis, cAMP relay, and cell adhesion. These defects result from low expression of cAMP pulse-induced genes such as those encoding the cAR1 cAMP receptor, phosphodiesterase, and the gp80 adhesion protein. Treatment of ubpA cells with pulses of exogenous cAMP allows them to aggregate and express these genes like wild-type cells, but they still fail to develop fruiting bodies. Unlike wild type, ubpA cells accumulate ubiquitin-containing species that comigrate with ubiquitin polymers, suggesting a defect in polyubiquitin metabolism. UbpA has sequence similarity with yeast Ubp14, which disassembles free ubiquitin chains. Yeast ubp14 cells have a defect in proteolysis, due to excess ubiquitin chains competing for substrate binding to proteasomes. Cross-species complementation and enzyme specificity assays indicate that UbpA and Ubp14 are functional homologs. We suggest that specific developmental transitions in Dictyostelium require the degradation of specific proteins and that this process in turn requires the disassembly of polyubiquitin chains by UbpA.

Cell density sensing mediated by a G protein-coupled receptor activating phospholipase C

When the unicellular eukaryote Dictyostelium discoideum starves, it senses the local density of other starving cells by simultaneously secreting and sensing a glycoprotein called conditioned medium factor (CMF). When the density of starving cells is high, the corresponding high density of CMF permits signal transduction through cAR1, the chemoattractant cAMP receptor. cAR1 activates a heterotrimeric G protein whose alpha-subunit is Galpha2. CMF regulates cAMP signal transduction in part by regulating the lifetime of the cAMP-stimulated Galpha2-GTP configuration. We find here that guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) inhibits the binding of CMF to membranes, suggesting that the putative CMF receptor is coupled to a G protein. Cells lacking Galpha1 (Galpha1 null) do not exhibit GTPgammaS inhibition of CMF binding and do not exhibit CMF regulation of cAMP signal transduction, suggesting that the putative CMF receptor interacts with Galpha1. Work by others has suggested that Galpha1 inhibits phospholipase C (PLC), yet when cells lacking either Galpha1 or PLC were starved at high cell densities (and thus in the presence of CMF), they developed normally and had normal cAMP signal transduction. We find that CMF activates PLC. Galpha1 null cells starved in the absence or presence of CMF behave in a manner similar to control cells starved in the presence of CMF in that they extend pseudopods, have an activated PLC, have a low cAMP-stimulated GTPase, permit cAMP signal transduction, and aggregate. Cells lacking Gbeta have a low PLC activity that cannot be stimulated by CMF. Cells lacking PLC exhibit IP3 levels and cAMP-stimulated GTP hydrolysis rates intermediate to what is observed in wild-type cells starved in the absence or in the presence of an optimal amount of CMF. We hypothesize that CMF binds to its receptor, releasing Gbetagamma from Galpha1. This activates PLC, which causes the Galpha2 GTPase to be inhibited, prolonging the lifetime of the cAMP-activated Galpha2-GTP configuration. This, in turn, allows cAR1-mediated cAMP signal transduction to take place.

Isolation of a Dictyostelium discoideum 14-3-3 homologue

A 1.0 kb cDNA clone (Dd14-3-3) encoding a 14-3-3 homologue was isolated from a Dictyostelium discoideum cDNA library. The putative Dd14-3-3 protein has highest sequence identity to a barley 14-3-3 isoform (74%). Southern blot analysis suggests that only one 14-3-3 gene is present in the Dictyostelium genome. Highest Dd14-3-3 expression is observed in vegetatively growing cells, and expression decreases during multicellular development. In contrast, Dd14-3-3 protein levels detected immunochemically remained constant during Dictyostelium development. Expression of the Dd14-3-3 cDNA in Saccharomyces cerevisiae complemented the lethal disruption of the two yeast genes encoding 14-3-3 proteins (BMH1 and BMH2). This shows that Dd14-3-3 can fulfil the same function(s) as the yeast 14-3-3 proteins.

Intracellular free calcium responses during chemotaxis of Dictyostelium cells

A calcium ion indicator, fura-2 bovine serum albumin, was introduced into Dictyostelium discoideum cells by electroporation. The concentration of intracellular calcium ions ([Ca2+]i) increased transiently in vegetative cells upon stimulation with submicromolar concentrations of folic acid, a chemoattractant for this organism at the vegetative stage. Similar [Ca2+]i responses were also observed in aggregation-competent cells upon stimulation with subnanomolar concentrations of cAMP, a chemoattractant at the aggregation stage. The [Ca2+]i response caused by cAMP was 2.1 times higher than that caused by folic acid. The magnitude of these responses depended on the concentration of Ca2+ in the external buffer. The presence of magnesium ions inhibited the [Ca2+]i responses in a dose-dependent manner. [Ca2+]i was higher in the rear region than in the anterior region of cells freely migrating on the surface, although such a gradient was not always maintained. When aggregation competent cells were locally stimulated by the application of a microcapillary containing cAMP, the cells extended pseudopods toward the microcapillary. In these cases, an increase in [Ca2+]i was transiently observed in the region opposite to the tip of the capillary. At the slug stage, [Ca2+]i was higher in prestalk cells than in prespore cells of slugs. The possibility that the [Ca2+]i is spatially regulated within a cell was discussed.

Molecular cloning of a cDNA coding for GTP cyclohydrolase I from Dictyostelium discoideum

The GTP cyclohydrolase I (GTP-CH) gene of the cellular slime mould Dictyostelium discoideum has been cloned and sequenced. The 855 bp cDNA of this gene contains the open reading frame (ORF) encoding 232 amino acids with a predicted molecular mass of approx. 26 kDa. Southern blot analysis indicated the presence of a single gene for GTP-CH in Dictyostelium. PCR amplification of the ORF from chromosomal DNA and sequencing showed the existence of a 101 bp intron in the GTP-CH gene of Dictyostelium discoideum. The amino acid sequence has 47% and 49% positional identity to those of the human and yeast enzymes respectively. Most of the sequence variation between species is located in the N-terminal part of the protein. The overall identity with the E. coli protein is markedly lower. The enzyme was expressed in E. coli and purified as a 68 kDa fusion protein with the maltose-binding protein of E. coli. GTP-CH of Dictyostelium is heat-stable and showed maximal activity at 60 degrees C. The Km value for GTP is 50 microM.

Control of 6-(D-threo-1',2'-dihydroxypropyl) pterin (dictyopterin) synthesis during aggregation of Dictyostelium discoideum. Involvement of the G-protein-linked signalling pathway in the regulation of GTP cyclohydrolase I activity

6-(D-threo-1',2'-Dihydroxypropylpterin (dictyopterin) has been identified in extracts of growing Dictyostelium dicoideum cells [Klein, Thiery and Tatischeff (1990) Eur. J. Biochem. 187, 665-669]. We demonstrate that it originates from GTP by de novo biosynthesis and that the first committed step is catalysed by GTP cyclohydrolase I, yielding dihydroneopterin triphosphate [neopterin is 6-(D-erythro-1',2',3'-trihydroxypropyl) pterin]. The GTP cyclohydrolase I activity is found in the cytosolic fraction and in a membrane-associated form. The level of a 0.9 kb mRNA coding for GTP cyclohydrolase I decreases to about 10% of its initial value within 2 h after Dictyostelium cells start development induced by starvation. In the cytosolic fraction, the specific activities of GTP cyclohydrolase I, as well as the concentrations of (6R/S)-5,6,7,8-tetrahydrodictyopterin (H4dictyopterin), follow this decline of the mRNA level. In the particulate fraction, however, the specific activities of GTP cyclohydrolase I and, in consequence, H4dictyopterin synthesis, transiently increase and reach a maximum after 4-5 h of development. The time-course of H4dictyopterin concentrations in the starvation medium closely correlates with its production in the membrane fraction. The activity of membrane-associated GTP cyclohydrolase I can be increased by pre-incubation of the cell lysate with guanosine 5'-[gamma-thio]triphosphate and Mg2+. This GTP analogue does not serve as a substrate and has no direct effect on the enzyme activity, indicating that a G-protein-linked signalling pathway is involved in the regulation of GTP cyclohydrolase I activity and thus in H4dictyopterin production during early development of D. discoideum.

The RNase P of Dictyostelium discoideum

Ribonuclease P (RNase P) is a key enzyme involved in tRNA biosynthesis. It catalyses the endonucleolytic cleavage of nearly all tRNA precursors to produce 5'-end matured tRNA. RNase P activity has been found in all organisms examined, from bacteria to mammals. Eubacterial RNase RNA is the only known RNA enzyme which functions in trans in nature. Similar behaviour has not been demonstrated in RNase P enzymes examined from archaebacteria or eukaryotes. Characterisation of RNase P enzymes from more diverse eukaryotic species, including the slime mold Dictyostelium discoideum, is useful for comparative analysis of the structure and function of eukaryotic RNase P.

The Dictyostelium cytoskeleton

New avenues of cytoskeleton research in Dictyostelium discoideum have opened up with the cloning of the alpha- and beta-tubulin genes and the characterization of kinesins and cytoplasmic dynein. Much research, however, continues to focus on the actin cytoskeleton and its dynamics during chemotaxis, morphogenesis, and other motile processes. New actin-associated proteins are being identified and characterized by biochemical means and through isolation of mutants lacking individual components. This work is shedding light on the roles of specific actin assemblies in various biological processes.

A density-sensing factor regulates signal transduction in Dictyostelium

Dictyostelium discoideum initiates development when cells overgrow their bacterial food source and starve. To coordinate development, the cells monitor the extracellular level of a protein, conditioned medium factor (CMF), secreted by starved cells. When a majority of the cells in a given area have starved, as signaled by CMF secretion, the extracellular level of CMF rises above a threshold value and permits aggregation of the starved cells. The cells aggregate using relayed pulses of cAMP as the chemoattractant. Cells in which CMF accumulation has been blocked by antisense do not aggregate except in the presence of exogenous CMF. We find that these cells are viable but do not chemotax towards cAMP. Videomicroscopy indicates that the inability of CMF antisense cells to chemotax is not due to a gross defect in motility, although both video and scanning electron microscopy indicate that CMF increases the frequency of pseudopod formation. The activations of Ca2+ influx, adenylyl cyclase, and guanylyl cyclase in response to a pulse of cAMP are strongly inhibited in cells lacking CMF, but are rescued by as little as 10 s exposure of cells to CMF. The activation of phospholipase C by cAMP is not affected by CMF. Northern blots indicate normal levels of the cAMP receptor mRNA in CMF antisense cells during development, while cAMP binding assays and Scatchard plots indicate that CMF antisense cells contain normal levels of the cAMP receptor. In Dictyostelium, both adenylyl and guanylyl cyclases are activated via G proteins. We find that the interaction of the cAMP receptor with G proteins in vitro is not measurably affected by CMF, whereas the activation of adenylyl cyclase by G proteins requires cells to have been exposed to CMF. CMF thus appears to regulate aggregation by regulating an early step of cAMP signal transduction.

Partial purification and characterization of RNase P from Dictyostelium discoideum

Ribonuclease P (RNase P) from Dictyostelium discoideum has been purified 470-fold. D. discoideum RNase P cleaves the precursor to Schizosaccharomyces pombe suppressor tRNA(Ser) at the same site as S. pombe RNase P, producing the mature 5' end of tRNA(Ser). pH and temperature optima for enzyme activity are 7.6 and 37 degrees C, respectively. The enzyme shows optimal activity in the presence of 5 mM MgCl2 and 10 mM NH4Cl or 5 mM KCl. The apparent Km for the S. pombe tRNA precursor derived from the supS1 tRNA(Ser) gene is 240 nM, and the apparent Vmax is 3.6 pmol/min. Inhibition of D. discoideum RNase P by proteinase K and micrococcal nuclease strongly indicates that the activity requires both protein and RNA components. In cesium sulfate density gradients, the enzyme has a buoyant density of 1.23 g/ml, indicating a low RNA/protein ratio for the holoenzyme.

Intracellular adenosine 3',5'-phosphate formation is essential for down-regulation of surface adenosine 3',5'-phosphate receptors in Dictyostelium

Dictyostelium discoideum cells contain cell surface cyclic AMP (cAMP) receptors that bind cAMP as a first messenger and intracellular cAMP receptors that bind cAMP as a second messenger. Prolonged incubation of Dictyostelium cells with cAMP induces a sequential process of phosphorylation, sequestration and down-regulation of the surface receptors. The role of intracellular cAMP in down-regulation of surface receptors was investigated. Down-regulation of receptors does not occur under conditions that specifically inhibit the formation of intracellular cAMP (the drug caffeine or mutant cells lacking adenylate cyclase) or conditions that inhibit the function of intracellular cAMP (mutants lacking protein kinase A activity). Cell-permeable non-hydrolysable cAMP derivatives were used to investigate further the requirement of intracellular cAMP for down-regulation. The Sp isomer of 6-thioethylpurineriboside 3',5'-phosphorothioate (6SEth-cPuMPS) does not bind to the surface receptor, enters the cell and has relative high affinity for protein kinase A. 6SEth-cPuMPS alone has no effect on down-regulation. However, together with an agonist of the surface receptor, the analogue induces down-regulation in caffeine-treated wild-type cells and in mutant cells lacking adenylate cyclase, but not in mutant cells lacking protein kinase A. These results indicate that intracellular cAMP formation and activation of protein kinase A are essential for down-regulation of the surface cAMP receptor.

Ponticulin is the major high affinity link between the plasma membrane and the cortical actin network in Dictyostelium

Interactions between the plasma membrane and underlying actin-based cortex have been implicated in membrane organization and stability, the control of cell shape, and various motile processes. To ascertain the function of high affinity actin-membrane associations, we have disrupted by homologous recombination the gene encoding ponticulin, the major high affinity actin-membrane link in Dictyostelium discoideum amoebae. Cells lacking detectable amounts of ponticulin message and protein also are deficient in high affinity actin-membrane binding by several criteria. First, only 10-13% as much endogenous actin cosediments through sucrose and crude plasma membranes from ponticulin-minus cells, as compared with membranes from the parental strain. Second, purified plasma membranes exhibit little or no binding or nucleation of exogenous actin in vitro. Finally, only 10-30% as much endogenous actin partitions with plasma membranes from ponticulin-minus cells after these cells are mechanically unroofed with polylysine-coated coverslips. The loss of the cell's major actin-binding membrane protein appears to be surprisingly benign under laboratory conditions. Ponticulin-minus cells grow normally in axenic culture and pinocytose FITC-dextran at the same rate as do parental cells. The rate of phagocytosis of particles by ponticulin-minus cells in growth media also is unaffected. By contrast, after initiation of development, cells lacking ponticulin aggregate faster than the parental cells. Subsequent morphogenesis proceeds asynchronously, but viable spores can form. These results indicate that ponticulin is not required for cellular translocation, but apparently plays a role in cell patterning during development.

Involvement of cell-cell adhesion in the expression of the cell cohesion molecule gp80 in Dictyostelium discoideum

Soon after the initiation of the developmental cycle of Dictyostelium discoideum, cells acquire EDTA-sensitive cell-cell binding sites mediated by the glycoprotein gp24. Cells at the aggregation stage display a second type of cell adhesion site, the EDTA-resistant cell-cell binding sites, mediated by the glycoprotein gp80. The gene encoding gp80 is first turned on to a low basal level of expression in the preaggregation stage. At the onset of the aggregation stage, cells produce pulses of low levels of cAMP, which greatly augment the expression of gp80. To investigate the role of cell-cell adhesion in the regulation of gp80 expression, cells were developed in the presence of EDTA or carnitine to block the EDTA-sensitive cell binding sites. Alternatively, cell cohesion was disrupted by shaking low-density cultures at high shearing forces. In all three instances, gp80 was expressed at a substantially reduced level. In addition, exogenous cAMP pulses, which normally were capable of stimulating a precocious and enhanced expression of gp80, failed to restore the high level of gp80 expression. However, if the formation of cell-cell contact was permitted, exogenous cAMP pulses were able to rescue the expression of gp80 even when the cAMP signal relay was blocked. These results indicate that previous cell-cell contact, provided by the EDTA-sensitive binding sites, is required for the activation of the cAMP-mediated signal transduction pathway producing high levels of gp80 expression.

Phospholipase C in Dictyostelium discoideum. Cyclic AMP surface receptor and G-protein-regulated activity in vitro

The cellular slime mould Dictyostelium discoideum shows several responses after stimulation with the chemoattractant cAMP, including a transient rise in cyclic AMP (cAMP), cGMP and Ins(1,4,5)P3. In this paper the regulation of phospholipase C in vitro is described. Under our experimental conditions commercial PtdIns(4,5)P2 cannot be used to analyse phospholipase C activity in Dictyostelium lysates, because it is hydrolysed mainly to glycerophosphoinositol instead of Ins(1,4,5)P3. Enzyme activity was determined with endogenous unlabelled PtdInsP2 as a substrate. The product was measured by isotope-dilution assay and identified as authentic Ins(1,4,5)P3. Since phospholipase C is strictly Ca(2+)-dependent, with an optimal concentration range of 1-100 microM, cell lysates were prepared in EGTA and the enzyme reaction was started by adding 10 microM free Ca2+. Phospholipase C activity increased 2-fold during Dictyostelium development up to 8 h of starvation, after which the activity declined to less than 10% of the vegetative level. Enzyme activity in vitro increased up to 2-fold after stimulation of cells with the agonist cAMP in vivo. Addition of 10 microM guanosine 5'-[gamma-thio]triphosphate during lysis activated the enzyme to the same extent, and this effect was antagonized by guanosine 5'-[beta-thio]diphosphate. These results strongly suggest that surface cAMP receptors and G-proteins regulate phospholipase C during Dictyostelium development.

Cyclic AMP binding to the amphibian oocyte plasma membrane: possible interrelationship between meiotic arrest and membrane fluidity

Cyclic AMP, which maintains the vertebrate oocyte in prophase arrest under physiological conditions, exhibits specific and saturable binding to the cytoplasmic face of the prophase-arrested Rana pipiens oocyte plasma membrane. Scatchard type analyses of [3H]cAMP binding to isolated plasma membranes indicate a single class of binding sites with a Kd = 19.3 +/- 7.0 nM at cAMP concentrations below 10(-6) M and additional low affinity site(s) and/or non-specific binding at concentrations above 10(-6) M. Photoaffinity labeling of prophase oocyte plasma membranes with [32P]-8-N3cAMP demonstrates cAMP/cGMP-displacable binding of 8-N3[32P]cAMP to a 100-110 kDa peptide doublet. Plasma membrane fluidity was monitored by electron spin resonance in isolated plasma-vitelline membranes using a 5-doxyl stearic acid probe. Exogenous dibutyryl cAMP (dbcAMP) produces an increase in membrane fluidity within minutes and blocks and/or reverses the progesterone-induced decrease in plasma membrane fluidity. The dbcAMP concentration that produced half-maximal fluidity increase (10 microM) corresponds to the half-maximal inhibiting dose of dbcAMP for progesterone induction of meiosis. Cholera toxin, which elevates intracellular cAMP and blocks meiosis, also increases membrane fluidity and inhibits progesterone-induced decrease in membrane fluidity. Elevated levels of intracellular cAMP thus appear to maintain meiotic arrest by binding to specific plasma membrane site(s) and maintaining the plasma membrane in a relatively fluid state. The progesterone-induced fall in intracellular cAMP first reported in Rana thus appears to be responsible for the progesterone-induced increase in membrane fluidity and further suggests that the change in membrane order is essential for the resumption of the meiotic divisions.

ABP50: an actin-binding elongation factor 1 alpha from Dictyostelium discoideum

ABP50 is a polypeptide elongation factor 1 alpha from Dictyostelium that is associated with the actin cytoskeleton. Upon chemotactic stimulation, ABP50 undergoes a dramatic cytoplasmic redistribution into newly formed surface projections and in vitro binds to and bundles actin filaments. Many questions are raised by this interaction pertaining to the spatiotemporal regulation of protein synthesis and cytoskeletal organization by extracellular signals.

Cyclophilin-dependent stimulation of transcription by cyclosporin A

Exposure to cyclosporin A (CspA) increased laccase (lac-1) transcript accumulation in the chestnut blight fungus Cryphonectria parasitica. This response was suppressed by compounds that interfere with calcium-dependent signal transduction and by the presence of a virulence-attenuating mycovirus. CspA stimulated the accumulation of mRNA from a nonhomologous reporter fused to the lac-1 promoter, indicating that the increased transcript levels resulted from an increase in promoter activity. Based on the current model for the regulation of lac-1 transcription, these results suggest that CspA interferes with a negative regulatory pathway that normally constrains lac-1 promoter activity. Significantly, CspA did not stimulate lac-1 transcription in mutant strains deficient in CspA binding activity, directly demonstrating a requirement for the interaction of CspA and cyclophilin in the modulation of lac-1 transcription. Our results establish that CspA treatment can stimulate gene transcription and that cyclophilin is the cellular receptor that mediates this activity.

Inducible expression of calmodulin antisense RNA in Dictyostelium cells inhibits the completion of cytokinesis

The single gene encoding calmodulin in the eukaryotic microorganism Dictyostelium discoideum was cloned and sequenced. The gene was found to contain three introns, one lying immediately after the translation initiation codon. The deduced amino acid sequence indicated that Dictyostelium calmodulin contains 19 amino acid differences from vertebrate calmodulin, including extensions at both termini. Northern blot analysis showed that similar levels of calmodulin mRNA are present throughout growth and development of wild-type cells. A complete copy of the calmodulin cDNA was prepared, and an 87-base pair fragment complementary to the 5'-end of the calmodulin mRNA was subcloned into the Dictyostelium transformation vector pVEII, such that expression of the antisense transcript was driven by the discoidin I gamma promoter. Transformed cells were selected and maintained at low cell density, a condition resulting in minimal activity of the discoidin I promoter. High level expression was induced by allowing the transformants to reach high cell density or by growing them in the presence of medium conditioned by high density cells. Under these conditions, in which calmodulin mRNA and protein levels were reduced about twofold, the calmodulin antisense transformants lost the ability to complete cytokinesis. A contractile ring formed and constricted, but the midbody linking daughter cells failed to break. The resulting cell population contained multinucleated cells and networks of cells connected by cytoplasmic bridges. Normal cell division was restored when the cells were diluted to low density. These observations have identified a new point at which calmodulin may regulate cell cleavage.

Inhibition of receptor-stimulated guanylyl cyclase by intracellular calcium ions in Dictyostelium cells

In Dictyostelium discoideum extracellular cAMP stimulates guanylyl cyclase and phospholipase C; the latter enzyme produces Ins(1,4,5)P3 which releases Ca2+ from internal stores. The following data indicate that intracellular Ca2+ ions inhibit guanylyl cyclase activity. 1) In vitro, Ca2+ inhibits guanylyl cyclase with IC50 = 41 nM Ca2+ and Hill-coefficient of 2.1. 2) Extracellular Ca2+ does not affect basal cGMP levels of intact cells. In electro-permeabilized cells, however, cGMP levels are reduced by 85% within 45 s after addition of 10(-6) M Ca2+ to the medium; halfmaximal reduction occurs at 200 nM extracellular Ca2+. 3) Receptor-stimulated activation of guanylyl cyclase in electro-permeabilized cells is also inhibited by extracellular Ca2+ with half-maximal effect at 200 nM Ca2+. 4) In several mutants an inverse correlation exists between receptor-stimulated Ins(1,4,5)P3 production and cGMP formation. We conclude that receptor-stimulated cytosolic Ca2+ elevation is a negative regulator of receptor-stimulated guanylyl cyclase.

Cortical ultrastructure and chemoreception in ciliated protists (Ciliophora)

The ciliated protists (ciliates) offer a unique opportunity to explore the relationship between chemoreception and cell structure. Ciliates resemble chemosensory neurons in their responses to stimuli and presence of cilia. Ciliates have highly patterned surfaces that should permit precise localization of chemoreceptors in relation to effector organelles. Furthermore, ciliates are easy to grow and to manipulate genetically; they can also be readily studied biochemically and by electrophysiological techniques. This review contains a comparative description of the ultrastructural features of the ciliate cell surface relevant to chemoreception, examines the structural features of putative chemoreceptive cilia, and provides a summary of the electron microscopic information available so far bearing on chemoreceptive aspects of swimming, feeding, excretion, endocytosis, and sexual responses of ciliates. The electron microscopic identification and localization of specific chemoreceptive macromolecules and organelles at the molecular level have not yet been achieved in ciliates. These await the development of specific probes for chemoreceptor and transduction macromolecules. Nevertheless, the electron microscope has provided a wealth of information about the surface features of ciliates where chemoreception is believed to take place. Such morphological information will prove essential to a complete understanding of reception and transduction at the molecular level. In the ciliates, major questions to be answered relate to the apportionment of chemoreceptive functions between the cilia and cell soma, the global distribution of receptors in relation to the anterior-posterior, dorsal-ventral, and left-right axes of the cell, and the relationship of receptors to ultrastructural components of the cell coat, cell membrane, and cytoskeleton.

Regulation of adenylate cyclase in electropermeabilized Dictyostelium discoideum cells

In Dictyostelium discoideum cells the enzyme adenylate cyclase is functionally coupled to cell surface receptors for cAMP. Coupling is known to involve one or more G-proteins. Receptor-mediated activation of adenylate cyclase is subject to adaptation. In this study we employ an electropermeabilized cell system to investigate regulation of D. discoideum adenylate cyclase. Conditions for selective permeabilization of the plasma membrane have been described by C.D. Schoen, J. C. Arents, T. Bruin, and R. Van Driel (1989, Exp. Cell Res. 181, 51-62). Only small pores are created in the membrane, allowing exchange of exclusively low molecular weight substances like nucleotides, and preventing the loss of macromolecules. Under these conditions functional protein-protein interactions are likely to remain intact. Adenylate cyclase in permeabilized cells was activated by the cAMP receptor agonist 2'-deoxy cAMP and by the nonhydrolyzable GTP-analogue GTP gamma S, which activates G-proteins. The time course of the adenylate cyclase reaction in permeabilized cells was similar to that of intact cells. Maximal adenylate cyclase activity was observed if cAMP receptor agonist or GTP-analogue was added just before cell permeabilization. If these activators were added after permeabilization adenylate cyclase was stimulated in a suboptimal way. The sensitivity of adenylate cyclase activity for receptor occupation was found to decay more rapidly than that for G-protein activation. Importantly, the adenylate cyclase reaction in permeabilized cells was subject to an adaptation-like process that was characterized by a time course similar to adaptation in vivo. In vitro adaptation was not affected by cAMP receptor agonists or by G-protein activation. Evidently electropermeabilized cells constitute an excellent system for investigating the positive and negative regulation of D. discoideum adenylate cyclase.

Selective induction of gene expression and second-messenger accumulation in Dictyostelium discoideum by the partial chemotactic antagonist 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate

During development of the cellular slime mold Dictyostelium discoideum, cAMP induces chemotaxis and expression of different classes of genes by means of interaction with surface cAMP receptors. We describe a cAMP derivative, 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate (8-CPT-cAMP), which inhibits cAMP-induced chemotaxis at low concentrations but induces chemotaxis at supersaturating concentrations. This compound, moreover, selectively activates expression of aggregative genes but not of postaggregative genes. 8-CPT-cAMP induces normal cGMP and cAMP accumulation but in contrast to cAMP, which increases inositol 1,4,5-trisphosphate levels, 8-CPT-cAMP decreases inositol 1,4,5-trisphosphate levels. The derivative induces reduced activation of guanine nucleotide regulatory proteins, which may cause its defective activation of inositol 1,4,5-trisphosphate production. Our data suggest that disruption of inositolphospholipid signaling impairs chemotaxis and expression of a subclass of cAMP-regulated genes.