The cyclic nucleotide phosphodiesterases of Dictyostelium discoideum: molecular genetics and biochemistry

Moving the Research Forward: The Best of British Biology Using the Tractable Model System Dictyostelium discoideum

The social amoeba Dictyostelium discoideum provides an excellent model for research across a broad range of disciplines within biology. The organism diverged from the plant, yeast, fungi and animal kingdoms around 1 billion years ago but retains common aspects found in these kingdoms. Dictyostelium has a low level of genetic complexity and provides a range of molecular, cellular, biochemical and developmental biology experimental techniques, enabling multidisciplinary studies to be carried out in a wide range of areas, leading to research breakthroughs. Numerous laboratories within the United Kingdom employ Dictyostelium as their core research model. This review introduces Dictyostelium and then highlights research from several leading British research laboratories, covering their distinct areas of research, the benefits of using the model, and the breakthroughs that have arisen due to the use of Dictyostelium as a tractable model system.

Extracellular signaling in Dictyostelium

In the last few decades, we have learned a considerable amount about how eukaryotic cells communicate with each other, and what it is the cells are telling each other. The simplicity of Dictyostelium discoideum, and the wide variety of available tools to study this organism, makes it the equivalent of a hydrogen atom for cell and developmental biology. Studies using Dictyostelium have pioneered a good deal of our understanding of eukaryotic cell communication. In this review, we will present a brief overview of how Dictyostelium cells use extracellular signals to attract each other, repel each other, sense their local cell density, sense whether the nearby cells are starving or stressed, count themselves to organize the formation of structures containing a regulated number of cells, sense the volume they are in, and organize their multicellular development. Although we are probably just beginning to learn what the cells are telling each other, the elucidation of Dictyostelium extracellular signals has already led to the development of possible therapeutics for human diseases.

Proteomic and Transcriptomic Profiling Identifies Early Developmentally Regulated Proteins in Dictyostelium Discoideum

Cyclic AMP acts as a secondary messenger involving different cellular functions in eukaryotes. Here, proteomic and transcriptomic profiling has been combined to identify novel early developmentally regulated proteins in eukaryote cells. These proteomic and transcriptomic experiments were performed in Dictyostelium discoideum given the unique advantages that this organism offers as a eukaryotic model for cell motility and as a nonmammalian model of human disease. By comparing whole-cell proteome analysis of developed (cAMP-pulsed) wild-type AX2 cells and an independent transcriptomic analysis of developed wild-type AX4 cells, our results show that up to 70% of the identified proteins overlap in the two independent studies. Among them, we have found 26 proteins previously related to cAMP signaling and identified 110 novel proteins involved in calcium signaling, adhesion, actin cytoskeleton, the ubiquitin-proteasome pathway, metabolism, and proteins that previously lacked any annotation. Our study validates previous findings, mostly for the canonical cAMP-pathway, and also generates further insight into the complexity of the transcriptomic changes during early development. This article also compares proteomic data between parental and cells lacking glkA, a GSK-3 kinase implicated in substrate adhesion and chemotaxis in Dictyostelium. This analysis reveals a set of proteins that show differences in expression in the two strains as well as overlapping protein level changes independent of GlkA.

Quantification of Live Bacterial Sensing for Chemotaxis and Phagocytosis and of Macropinocytosis

Initial immunological defense mechanisms to pathogen invasion rely on innate pathways of chemotaxis and phagocytosis, original to ancient phagocytes. Although chemotaxis has been well-studied in mammalian and model systems using purified chemoattractants in defined conditions, directed movement toward live bacteria has been more difficult to assess. Dictyostelium discoideum is a professional phagocyte that chemotaxes toward bacteria during growth-phase in a process to locate nutrient sources. Using Dictyostelium as a model, we have developed a system that is able to quantify chemotaxis to very high sensitivity. Here, Dictyostelium can detect various chemoattractants at concentrations <1 nM. Given this exceedingly sensitive signal response, Dictyostelium will migrate directionally toward live gram positive and gram negative bacteria, in a highly quantifiable manner, and dependent upon bacterially-secreted chemoattractants. Additionally, we have developed a real-time, quantitative assay for phagocytosis of live gram positive and gram negative bacteria. To extend the analyses of endocytic functions, we further modified the system to quantify cellular uptake via macropinocytosis of smaller (<100 kDa) molecules. These various approaches provide novel means to dissect potential for identification of novel chemoattractants and mechanistic factors that are essential for chemotaxis, phagocytosis, and/or macropinocytosis and for more detailed understanding in host-pathogen interactive defenses.

Modulation of cyclic nucleotide-mediated cellular signaling and gene expression using photoactivated adenylyl cyclase as an optogenetic tool

Cyclic nucleotide signaling pathway plays a significant role in various biological processes such as cell growth, transcription, inflammation, in microbial pathogenesis, etc. Modulation of cyclic nucleotide levels by optogenetic tools has overcome certain limitations of studying transduction cascade by pharmacological agents and has allowed several ways to modulate biological processes in a spatiotemporal manner. Here, we have shown the optogenetic modulation of the cyclooxygenase 2 (Cox-2) gene expression and their downstream effector molecule (PGE₂) in HEK-293T cells and the development process of Dictyostelium discoideum via modulating the cyclic nucleotide (cAMP) signaling pathway utilizing photoactivated adenylyl cyclases (PACs) as an optogenetic tool. Light-induced activation of PACs in HEK-293T cells increases the cAMP level that leads to activation of cAMP response element-binding protein (CREB) transcription factor and further upregulates downstream Cox-2 gene expression and their downstream effector molecule prostaglandin E2. In D. discoideum, the light-regulated increase in cAMP level affects the starvation-induced developmental process. These PACs could modulate the cAMP levels in a light-dependent manner and have a potential to control gene expression and their downstream effector molecules with varying magnitude. It would enable one to utilize PAC as a tool to decipher cyclic nucleotide mediated signaling pathway regulations and their mechanism.

Adenylyl cyclase localization to the uropod of aggregating Dictyostelium cells requires RacC

The localization of adenylyl cyclase A (ACA) to uropod of cells is required for the stream formation during Dictyostelium development. RacC is a Dictyostelium orthologue of Cdc42. We identified a streaming defect of racC(-) cells as they are clearly less polarized and form smaller and fragmented streams. ACA-YFP is mainly associated with intracellular vesicular structures, but not with the plasma membrane in racC(-) cells. racC(-) cells have a slightly higher number of vesicles than Ax3 cells, suggesting that the defect of ACA trafficking is not simply due to the lack of vesicle formation. While the ACA-YFP vesicles traveled with an average velocity of 9.1 μm/min in Ax3 cells, a slow and diffusional movement without direction with an average velocity of 4 μm/min was maintained in racC(-) cells. Images acquired by using total internal reflection fluorescence (TIRF) microscopy and fluorescence recovery after photobleaching (FRAP) analysis revealed that a significantly decreased number of ACA-YFP vesicles appeared near the cell membrane, indicating a defect in ACA-YFP vesicle trafficking. These results suggest an important role of RacC in the rapid and directional movements of ACA vesicles on microtubules to the plasma membrane, especially to the back of polarized cell.

Cell signaling during development of Dictyostelium

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

Analysis of phenotypic evolution in Dictyostelia highlights developmental plasticity as a likely consequence of colonial multicellularity

Colony formation was the first step towards evolution of multicellularity in many macroscopic organisms. Dictyostelid social amoebas have used this strategy for over 600 Myr to form fruiting structures of increasing complexity. To understand in which order multicellular complexity evolved, we measured 24 phenotypic characters over 99 dictyostelid species. Using phylogenetic comparative methods, we show that the last common ancestor (LCA) of Dictyostelia probably erected small fruiting structures directly from aggregates. It secreted cAMP to coordinate fruiting body morphogenesis, and another compound to mediate aggregation. This phenotype persisted up to the LCAs of three of the four major groups of Dictyostelia. The group 4 LCA co-opted cAMP for aggregation and evolved much larger fruiting structures. However, it lost encystation, the survival strategy of solitary amoebas that is retained by many species in groups 1–3. Large structures, phototropism and a migrating intermediate ‘slug’ stage coevolved as evolutionary novelties within most groups. Overall, dictyostelids show considerable plasticity in the size and shape of multicellular structures, both within and between species. This probably reflects constraints placed by colonial life on developmental control mechanisms, which, depending on local cell density, need to direct from 10 to a million cells into forming a functional fructification.

Cell speed, persistence and information transmission during signal relay and collective migration

Collective migration is a key feature of the social amoebae Dictyostelium discoideum, where the binding of chemoattractants leads to the production and secretion of additional chemoattractant and the relay of the signal to neighboring cells. This then guides cells to migrate collectively in a head-to-tail fashion. We used mutants that were defective in signal relay to elucidate which quantitative metrics of cell migration are most strongly affected by signal relay and collective motion. We show that neither signal relay nor collective motion markedly impact the speed of cell migration. Cells maintained a preferred overall direction of motion for several minutes with similar persistence, regardless of whether or not they were attracted to moving neighbors, moving collectively in contact with their neighbors, or simply following a fixed exogenous signal. We quantitatively establish that signal relay not only increases the number of cells that respond to a chemotactic signal, but most remarkably, also transmits information about the location of the source accurately over large distances, independently of the strength of the exogenous signal. We envision that signal relay has a similar key role in the migration of a variety of chemotaxing mammalian cells that can relay chemoattractant signals.

The group migration of Dictyostelium cells is regulated by extracellular chemoattractant degradation

Starvation of Dictyostelium induces a developmental program in which cells form an aggregate that eventually differentiates into a multicellular structure. The aggregate formation is mediated by directional migration of individual cells that quickly transition to group migration in which cells align in a head-to-tail manner to form streams. Cyclic AMP acts as a chemoattractant and its production, secretion, and degradation are highly regulated. A key protein is the extracellular phosphodiesterase PdsA. In this study we examine the role and localization of PdsA during chemotaxis and streaming. We find that pdsA(-) cells respond chemotactically to a narrower range of chemoattractant concentrations compared with wild-type (WT) cells. Moreover, unlike WT cells, pdsA(-) cells do not form streams at low cell densities and form unusual thick and transient streams at high cell densities. We find that the intracellular pool of PdsA is localized to the endoplasmic reticulum, which may provide a compartment for storage and secretion of PdsA. Because we find that cAMP synthesis is normal in cells lacking PdsA, we conclude that signal degradation regulates the external cAMP gradient field generation and that the group migration behavior of these cells is compromised even though their signaling machinery is intact.

Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge

Chemoattractant signaling induces the polarization and directed movement of cells secondary to the activation of multiple effector pathways. In addition, chemotactic signals can be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms underlying such remarkable features remain ill defined. We show that the asymmetrical distribution of adenylyl cyclase (ACA) at the back of Dictyostelium discoideum cells, an essential determinant of their ability to migrate in a head-to-tail fashion, requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. We also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. We propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis.

cGMP-phosphodiesterase antagonists inhibit Ca2+-influx in Dictyostelium discoideum and bovine cyclic-nucleotide-gated-channel

We used antagonists of cGMP-phosphodiesterases to examine the role of cGMP for light-scattering oscillations and cAMP-induced Ca(2+)-influx in Dictyostelium discoideum, however, SCH 51866 (cis-5,6a,7,8,9,9a-hexahydro-2-[4-(trifluoromethyl)phenylmethyl]-5-methyl-cyclopent[4,5]imidazo[2,1-b]purin-4(3H)-one) and sildenafil citrate (1-[[3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1-H-pyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxyphenyl]sulfonyl]-4-methylpiperazine citrate) were poor inhibitors of cGMP-hydrolysis. Instead, SCH 51866 (IC(50) = 16 microM) and sildenafil, blocked chemoattractant (cAMP)-induced Ca(2+)-influx as determined with a Ca(2+)-specific electrode. SCH 51866 (150 microM) affected neither spontaneous cGMP transients during light-scattering-oscillations nor cAMP-mediated K(+)-efflux. SCH 51866 and sildenafil are competitive inhibitors of cGMP phosphodiesterases. However, the activity of cGMP-dependent protein kinase Ialpha (PKGIalpha) was not altered by SCH 51866 (150 microM). By contrast, patch-clamp measurements of bovine cone cGMP-gated-channels (cyclic-nucleotide-gated-channel, CNGA3), stably expressed in human embryonic kidney cells, HEK 293 cells, revealed reversible, competitive and dose-dependent inhibition of sodium currents by SCH 51866 (IC(50) = 25 microM) and sildenafil, but not by another inhibitor of cGMP-phosphodiesterases, UK 114,542. The possibility that D. discoideum cells also express a cGMP-regulated channel is supported by our finding that LY 83583 (6-(phenylamino)-5,8-quinolinedione) (35 microM), known to inhibit cyclic-nucleotide-gated-channels as well as guanylyl-cyclases, reduced cAMP-induced Ca(2+)-influx in D. discoideum, but did not affect cAMP-induced cGMP accumulation. Utilizing a PDED null strain that exhibits a prolonged and elevated cGMP transient following receptor activation, we found that the inhibition of Ca(2+)-influx by SCH 51866 in the wildtype was absent in the mutant. Our results show that SCH 51866 and sildenafil are antagonists of a Ca(2+)-permeable channel (CNGA3) and that both compete with cGMP for a regulatory site of Ca(2+)-influx in D. discoideum.

Reverse genetic analyses of gamete-enriched genes revealed a novel regulator of the cAMP signaling pathway in Dictyostelium discoideum

Sexual development in Dictyostelium discoideum is initiated by the fusion of opposite mating type cells to form zygote giant cells, which subsequently gather and phagocytose surrounding cells for nutrition to form macrocysts. Here we performed the targeting of 24 highly gamete-enriched genes we previously isolated, and successfully generated knockout mutants for 16 genes and RNAi mutants for 20 genes including 6 genes without disruptants. In the knockout mutants of two genes, cell aggregation toward the giant cells was much less extensive and many cells remained around poorly formed macrocysts. We named these genes tmcB and tmcC. Although macrocyst formation of wild type cells was suppressed by the addition of exogenous cAMP, that of knockout mutants of tmcB was much less sensitive. The mRNA level of phosphodiesterase (pde) was higher and that of its inhibitor (pdi) was lower in the latter cells compared to the parental strains during sexual development. Thus, tmcB appeared to be a novel regulator of the cAMP signaling pathway specific to sexual development. Knockout mutants of tmcC were indistinguishable from the wild type cells with respect to the cAMP response, suggesting that this gene is relevant to other processes.

Dispatch. Dictyostelium chemotaxis: fascism through the back door?

Aggregating Dictyostelium cells secrete cyclic AMP to attract their neighbours by chemotaxis. It has now been shown that adenylyl cyclase is enriched in the rear of cells, and this localisation is required for normal aggregation.

Adenylyl cyclase A expression is tip-specific in Dictyostelium slugs and directs StatA nuclear translocation and CudA gene expression

cAMP oscillations, generated by adenylyl cyclase A (ACA), coordinate cell aggregation in Dictyostelium and have also been implicated in organizer function during multicellular development. We used a gene fusion of the ACA promoter with a labile lacZ derivative to study the expression pattern of ACA. During aggregation, most cells expressed ACA, but thereafter expression was lost in all cells except those of the anterior tip. Before aggregation, ACA transcription was strongly upregulated by nanomolar cAMP pulses. Postaggregative transcription was sustained by nanomolar cAMP pulses, but downregulated by a continuous micromolar cAMP stimulus and by the stalk-cell-inducing factor DIF. Earlier work showed that the transcription factor StatA displays tip-specific nuclear translocation and directs tip-specific expression of the nuclear protein CudA, which is essential for culmination. Both StatA and CudA were present in nuclei throughout the entire slug in an aca null mutant that expresses ACA from the constitutive actin15 promoter. This suggests that the tip-specific expression of ACA directs tip-specific nuclear translocation of StatA and tip-specific expression of CudA.

The internal phosphodiesterase RegA is essential for the suppression of lateral pseudopods during Dictyostelium chemotaxis

Dictyostelium strains in which the gene encoding the cytoplasmic cAMP phosphodiesterase RegA is inactivated form small aggregates. This defect was corrected by introducing copies of the wild-type regA gene, indicating that the defect was solely the consequence of the loss of the phosphodiesterase. Using a computer-assisted motion analysis system, regA(-) mutant cells were found to show little sense of direction during aggregation. When labeled wild-type cells were followed in a field of aggregating regA(-) cells, they also failed to move in an orderly direction, indicating that signaling was impaired in mutant cell cultures. However, when labeled regA(-) cells were followed in a field of aggregating wild-type cells, they again failed to move in an orderly manner, primarily in the deduced fronts of waves, indicating that the chemotactic response was also impaired. Since wild-type cells must assess both the increasing spatial gradient and the increasing temporal gradient of cAMP in the front of a natural wave, the behavior of regA(-) cells was motion analyzed first in simulated temporal waves in the absence of spatial gradients and then was analyzed in spatial gradients in the absence of temporal waves. Our results demonstrate that RegA is involved neither in assessing the direction of a spatial gradient of cAMP nor in distinguishing between increasing and decreasing temporal gradients of cAMP. However, RegA is essential for specifically suppressing lateral pseudopod formation during the response to an increasing temporal gradient of cAMP, a necessary component of natural chemotaxis. We discuss the possibility that RegA functions in a network that regulates myosin phosphorylation by controlling internal cAMP levels, and, in support of that hypothesis, we demonstrate that myosin II does not localize in a normal manner to the cortex of regA(-) cells in an increasing temporal gradient of cAMP.

The control of chemotactic cell movement during Dictyostelium morphogenesis

Differential cell movement is an important mechanism in the development and morphogenesis of many organisms. In many cases there are indications that chemotaxis is a key mechanism controlling differential cell movement. This can be particularly well studied in the starvation-induced multicellular development of the social amoeba Dictyostelium discoideum. Upon starvation, up to 10(5) individual amoebae aggregate to form a fruiting body The cells aggregate by chemotaxis in response to propagating waves of cAMP, initiated by an aggregation centre. During their chemotactic aggregation the cells start to differentiate into prestalk and prespore cells, precursors to the stalk and spores that form the fruiting body. These cells enter the aggregate in a random order but then sort out to form a simple axial pattern in the slug. Our experiments strongly suggest that the multicellular aggregates (mounds) and slugs are also organized by propagating cAMP waves and, furthermore, that cell-type-specific differences in signalling and chemotaxis result in cell sorting, slug formation and movement.

A Xestospongin C-sensitive Ca(2+) store is required for cAMP-induced Ca(2+) influx and cAMP oscillations in Dictyostelium

Xestospongin C (XeC) is known to bind to the inositol 1,4, 5-trisphosphate (IP(3))-sensitive store in mammalian cells and to inhibit IP(3)- and thapsigargin-induced Ca(2+) release. In this study we show that this is also true for Dictyostelium. In addition, XeC inhibited Ca(2+) uptake into purified vesicle fractions and induced Ca(2+) release. This suggests that, in the case of Dictyostelium, XeC opens rather than plugs the IP(3) receptor channel as was proposed for mammalian cells (Gafni, J., Munsch, J. A. , Lam, T. H., Catlin, M. C., Costa, L. G., Molinski, T. F., and Pessah, I. N. (1997) Neuron 19, 723-733). In order to elucidate the function of the XeC-sensitive Ca(2+) store in Dictyostelium during differentiation, we applied XeC to the cells and found that it caused a time-dependent increase of basal [Ca(2+)](i) and inhibited cAMP-induced Ca(2+) influx in single cells as well as in cell suspensions. Moreover, XeC blocked light scattering spikes and pulsatile cAMP signaling.

Integration of signaling networks that regulate Dictyostelium differentiation

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

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.

The molecular biology of cyclic nucleotide phosphodiesterases

Recent progress in the field of cyclic nucleotides has shown that a large array of closely related proteins is involved in each step of the signal transduction cascade. Nine families of adenylyl cyclases catalyze the synthesis of the second messenger cAMP, and protein kinases A, the intracellular effectors of cAMP, are composed of four regulatory and three catalytic subunits. A comparable heterogeneity has been discovered for the enzymes involved in the inactivation of cyclic nucleotide signaling. In mammals, 19 different genes encode the cyclic nucleotide phosphodiesterases (PDEs), the enzymes that hydrolyze and inactivate cAMP and cGMP. This is only an initial level of complexity, because each PDE gene contains several distinct transcriptional units that give rise to proteins with subtle structural differences, bringing the number of the PDE proteins close to 50. The molecular biology of PDEs in Drosophila and Dictyostelium has shed some light on the role of PDE diversity in signaling and development. However, much needs to be done to understand the exact function of these enzymes, particularly during mammalian development and cell differentiation. With the identification and mapping of regulatory and targeting domains of the PDEs, modularity of the PDE structure is becoming an established tenet in the PDE field. The use of different transcriptional units and exon splicing of a single PDE gene generates proteins with different regulatory domains joined to a common catalytic domain, therefore expanding the array of isoforms with subtle differences in properties and sensitivities to different signals. The physiological context in which these different isoforms function is still largely unknown and undoubtedly will be a major area of expansion in the years to come.

Signal transduction and motility of Dictyostelium

This review is concerned with the roles of cyclic GMP and Ca(2+) ions in signal transduction for chemotaxis of Dictyostelium. These molecules are involved in signalling between the cell surface cyclic AMP receptors and cytoskeletal myosin II involved in chemotactic cell movement. Evidence is presented for uptake and/or efflux of Ca(2+) being regulated by cyclic GMP. The link between Ca(2+), cyclic GMP and chemotactic cell movement has been explored using "streamer F" mutants whose primary defect is in the structural gene for the cyclic GMP-specific phosphodiesterase. This mutation causes the mutants to produce an abnormally prolonged peak of cyclic GMP accumulation in response to stimulation with the chemoattractant cyclic AMP. The production and relay of cyclic AMP signals is normal in these mutants, but certain events associated with movement are (like the cyclic GMP response) abnormally prolonged in the mutants. These events include Ca(2+) uptake, myosin II association with the cytoskeleton and regulation of both myosin heavy and light chain phosphorylation. These changes can be correlated with changes in the shape of the amoebae after chemotactic stimulation. Other mutants in which the accumulation of cyclic GMP in response to cyclic AMP stimulation was absent produced no myosin II responses. A model is described in which cyclic GMP (directly or indirectly via Ca(2+) regulates accumulation of myosin II on the cytoskeleton by regulating phosphorylation of the myosin heavy and light chain kinases.

Integration of signaling information in controlling cell-fate decisions in Dictyostelium

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Genetic and physiologic modulation of the prestarvation response in Dictyostelium discoideum

Throughout vegetative growth, Dictyostelium amoebae secrete an autocrine factor, prestarvation factor, PSF, which accumulates in proportion to cell density. During late exponential growth, PSF induces the expression of several genes whose products are needed for cAMP signaling and cell aggregation. Among these genes are discoidin-I and the 2.4-kb transcript of cyclic nucleotide phosphodiesterase (PDE). We have identified several parameters that modulate expression of one or both of these prestarvation response genes; all effects were monitored in cells growing exponentially on bacteria. Under these conditions, axenic mutants produce higher levels of PSF activity than wild-type cells. Consistent with the high PSF levels, the 2.4-kb PDE transcript is more abundant in axenic strains than wild-type cells at the same cell density. In contrast, the density-dependent induction of discoidin-I is greatly delayed in axenic strains, occurring only at the very end of exponential growth. Analysis of axenic strains of independent origin suggested that this negative effect on discoidin-I expression is attributable to the axenic mutations themselves. The effects of two environmental factors that inhibit the prestarvation response (the bacteria upon which the cells feed and a bacterial product, folic acid) were also analyzed. We found that folate does not account for the inhibitory effect of bacteria. Cells deficient in the G-protein beta subunit, which is thought to be common to all heterotrimeric G-proteins in Dictyostelium, respond to PSF in the same manner as G beta+ cells, and this response is inhibited by bacteria. However, folate has no inhibitory effect on g beta- cells, indicating that folate inhibition is mediated by a heterotrimeric G-protein. In cells lacking the catalytic subunit of protein kinase A, the prestarvation response is severely impaired, but about 3% of the pka- cells manifest an apparently normal density-dependent induction of discoidin-I. This behavior and the heterogeneity of the prestarvation response in wild-type cells lead us to speculate that protein kinase A may not be required for PSF signal transduction per se, but rather may render the cells responsive to PSF. Based on analysis of adenylyl cyclase mutants (aca-), the effect of protein kinase A is not cAMP-dependent.

The G alpha subunit G alpha 4 couples to pterin receptors and identifies a signaling pathway that is essential for multicellular development in Dictyostelium

In this paper, we show that the G alpha subunit G alpha 4 couples to pterin receptors and identifies a signalling pathway that is essential for multicellular development in Dictyostelium. G alpha 4 is developmentally regulated, is essential for proper morphogenesis and spore production, and functions cell nonautonomously. We show that G alpha 4 is coupled to receptors (alpha FAR) that activate chemotaxis and adenylyl and guanylyl cyclases in response to folate during the early stages of development and to a late class of folate receptors (beta FAR) that have different specificities for pterins. G alpha 4 is preferentially expressed in cells randomly distributed within the aggregate that are a component of the anterior-like cell population, and it is not detectably expressed in prespore cells. Our results suggest that an endogenous factor, possibly a pterin, produced during multicellular development is a requisite signal for multicellular development, acting through G alpha 4. We propose that the G alpha 4-expressing cells function as a regulatory cell type controlling prespore cell fate, possibly in response to an endogenous pterin. Our results indicate that G alpha 4 and G alpha 2 have parallel functions in mediating cellular responses to folate (pterins) and cAMP, respectively.

LagC is required for cell-cell interactions that are essential for cell-type differentiation in Dictyostelium

Strain AK127 is a developmental mutant of Dictyostelium discoideum that was isolated by restriction enzyme-mediated integration (REMI). Mutant cells aggregate normally but are unable to proceed past the loose aggregate stage. The cloned gene, lagC (loose aggregate C), encodes a novel protein of 98 kD that contains an amino-terminal signal sequence and a putative carboxy-terminal transmembrane domain. The mutant strain AK127 shows no detectable lagC transcript upon Northern analysis, indicating that the observed phenotype is that of a null allele. Expression of the lagC cDNA in AK127 cells complements the arrest at the loose aggregate stage, indicating that the mutant phenotype results from disruption of the lagC gene. In wild-type cells, lagC mRNA is induced at the loose aggregate stage and is expressed through the remainder of development. lagC- null cells aggregate but then disaggregate and reaggregate to form small granular mounds. Mature spores are produced at an extremely low efficiency (< 0.1% of wild type), appearing only after approximately 72 hr, whereas wild-type strains produce mature spores by 26 hr. lagC- null cells accumulate reduced levels of transcripts for the prestalk-enriched genes rasD and CP2 and do not express the DIF-induced prestalk-specific gene ecmA or the cAMP-induced prespore-specific gene SP60 to significant levels. In chimeric organisms resulting from the coaggregation of lagC- null and wild-type cells, cell-type-specific gene expression is rescued in the lagC- null cells; however, lagC- prespore cells are localized to the posterior of the prespore region and do not form mature spores, suggesting that LagC protein has both no cell-autonomous and cell-autonomous functions. Overexpression of lagC from an actin promoter in both wild-type and lagC- cells causes a delay at the tight aggregate stage, the first stage requiring LagC activity. These results suggest that the LagC protein functions as a nondiffusible cell-cell signaling molecule that is required for multicellular development.