cAMP regulation of early gene expression in signal transduction mutants of Dictyostelium

Regulation of aggregate size and pattern by adenosine and caffeine in cellular slime molds

BACKGROUND

Multicellularity in cellular slime molds is achieved by aggregation of several hundreds to thousands of cells. In the model slime mold Dictyostelium discoideum, adenosine is known to increase the aggregate size and its antagonist caffeine reduces the aggregate size. However, it is not clear if the actions of adenosine and caffeine are evolutionarily conserved among other slime molds known to use structurally unrelated chemoattractants. We have examined how the known factors affecting aggregate size are modulated by adenosine and caffeine.

RESULT

Adenosine and caffeine induced the formation of large and small aggregates respectively, in evolutionarily distinct slime molds known to use diverse chemoattractants for their aggregation. Due to its genetic tractability, we chose D. discoideum to further investigate the factors affecting aggregate size. The changes in aggregate size are caused by the effect of the compounds on several parameters such as cell number and size, cell-cell adhesion, cAMP signal relay and cell counting mechanisms. While some of the effects of these two compounds are opposite to each other, interestingly, both compounds increase the intracellular glucose level and strengthen cell-cell adhesion. These compounds also inhibit the synthesis of cAMP phosphodiesterase (PdsA), weakening the relay of extracellular cAMP signal. Adenosine as well as caffeine rescue mutants impaired in stream formation (pde4- and pdiA-) and colony size (smlA- and ctnA-) and restore their parental aggregate size.

CONCLUSION

Adenosine increased the cell division timings thereby making large number of cells available for aggregation and also it marginally increased the cell size contributing to large aggregate size. Reduced cell division rates and decreased cell size in the presence of caffeine makes the aggregates smaller than controls. Both the compounds altered the speed of the chemotactic amoebae causing a variation in aggregate size. Our data strongly suggests that cytosolic glucose and extracellular cAMP levels are the other major determinants regulating aggregate size and pattern. Importantly, the aggregation process is conserved among different lineages of cellular slime molds despite using unrelated signalling molecules for aggregation.

Regulated protein degradation controls PKA function and cell-type differentiation in Dictyostelium

Cullins function as scaffolds that, along with F-box/WD40-repeat-containing proteins, mediate the ubiquitination of proteins to target them for degradation by the proteasome. We have identified a cullin CulA that is required at several stages during Dictyostelium development. culA null cells are defective in inducing cell-type-specific gene expression and exhibit defects during aggregation, including reduced chemotaxis. PKA is an important regulator of Dictyostelium development. The levels of intracellular cAMP and PKA activity are controlled by the rate of synthesis of cAMP and its degradation by the cAMP-specific phosphodiesterase RegA. We show that overexpression of the PKA catalytic subunit (PKAcat) rescues many of the culA null defects and those of cells lacking FbxA/ChtA, a previously described F-box/WD40-repeat-containing protein, suggesting CulA and FbxA proteins are involved in regulating PKA function. Whereas RegA protein levels drop as the multicellular organism forms in the wild-type strain, they remain high in culA null and fbxA null cells. Although PKA can suppress the culA and fbxA null developmental phenotypes, it does not suppress the altered RegA degradation, suggesting that PKA lies downstream of RegA, CulA, and FbxA. Finally, we show that CulA, FbxA, and RegA are found in a complex in vivo, and formation of this complex is dependent on the MAP kinase ERK2, which is also required for PKA function. We propose that CulA and FbxA regulate multicellular development by targeting RegA for degradation via a pathway that requires ERK2 function, leading to an increase in cAMP and PKA activity.

Dictyostelium development-socializing through cAMP

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

Sequence-specific protein interaction with a transcriptional enhancer involved in the autoregulated expression of cAMP receptor 1 in Dictyostelium

Major stages of Dictyostelium development are regulated by secreted, extracellular cAMP through activation of a serpentine receptor family. During early development, oscillations of extracellular cAMP mobilize cells for aggregation; later, continuous exposure to higher extracellular cAMP concentrations downregulates early gene expression and promotes cytodifferentiation and cell-specific gene expression. The cAMP receptor 1 gene CAR1 has two promoters that are differentially responsive to these extracellular cAMP stimuli. The early CAR1 promoter is induced by nM pulses of cAMP, which in turn are generated by CAR1-dependent activation of adenylyl cyclase (AC). Higher, non-fluctuating concentrations of cAMP will adapt this AC stimulus-response, repress the activated early promoter and induce the dormant late promoter. We now identify a critical element of the pulse-induced CAR1 promoter and a nuclear factor with sequence-specific interaction. Mutation of four nucleotides within the element prevents both in vitro protein binding and in vivo expression of an otherwise fully active early CAR1 promoter and multimerization of the wild-type, but not mutant, sequence will confer cAMP regulation to a quiescent heterologous promoter. These cis and trans elements, thus, constitute a part of the molecular response to the cAMP transmembrane signal cascade that regulates early development of Dictyostelium.

Temperature-sensitive Gbeta mutants discriminate between G protein-dependent and -independent signaling mediated by serpentine receptors

Deletion of the single gene for the Dictyostelium G protein beta-subunit blocks development at an early stage. We have now isolated temperature-sensitive alleles of Gbeta to investigate its role in later development. We show that Gbeta is directly required for adenylyl cyclase A activation and for morphogenetic signaling during the entire developmental program. Gbeta was also essential for induction of aggregative gene expression by cAMP pulses, a process that is mediated by serpentine cAMP receptors (cARs). However, Gbeta was not required for cAR-mediated induction of prespore genes and repression of stalk genes, and neither was Gbeta needed for induction of prestalk genes by the differentiation inducing factor (DIF). cAMP induction of prespore genes and repression of stalk genes is mediated by the protein kinase GSK-3. GSK-3 also determines cell-type specification in insects and vertebrates and is regulated by the wingless/wnt morphogens that are detected by serpentine fz receptors. The G protein-dependent and -independent modes of cAR-mediated signaling reported here may also exist for the wingless/wnt signaling pathways in higher organisms.

Temporal regulation of the Dictyostelium glycogen phosphorylase 2 gene

The product of the glycogen phosphorylase-2 gene in Dictyostelium functions to provide the glucose units that are used to construct the structural components of the terminal stage of development. In this report, we link a 1233 bp upstream gp2 fragment to a luciferase reporter gene in order to study the sequences that are involved in the temporal expression of the gene. Various deletions of the promoter-luciferase fusion were then transformed into Dictyostelium cells. All deletion constructs, from -1216 to -486 nucleotides from the translational start codon, showed the same temporal pattern of expression as the authentic gp2 gene, as well as similar luciferase activities. Removal of an additional 37 nucleotides resulted in nearly 100-fold decrease in activity, yet retained the normal temporal expression of luciferase. Analysis of DNA binding proteins with the gel shift assay revealed a stage-dependent pattern of proteins that bound to the gp2 promoter. A similar pattern of temporal expression of the binding proteins was observed with either the full-length probe or with oligonucleotide probes that contained sequences that were identified as putative regulatory sites. Likewise, the full-length and oligonucleotide probes demonstrated identical binding patterns during several steps of purification of the DNA binding proteins. SDS-PAGE and Southwestern blot analysis of a DNA-affinity purified fraction, identified a 23 kDa peptide as the binding protein.

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.

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.

Spatial and temporal expression of the Dictyostelium discoideum G alpha protein subunit G alpha 2: expression of a dominant negative protein inhibits proper prestalk to stalk differentiation

Previous results have shown that the G alpha protein subunit G alpha 2 is required for aggregation in Dictyostelium discoideum and is essential for coupling cell-surface cAMP receptors to downstream effectors in vivo during this stage of development. G alpha 2 expresses at least four distinct transcripts that are differentially regulated during development; two of the transcripts are expressed exclusively in the multicellular stages and their expression is restricted to prestalk cells. We partially dissected the G alpha 2 promoter and identified a component that is expressed exclusively during the multicellular stages using luciferase gene fusions. When this promoter region is coupled to lacZ, beta-gal expression is restricted to the multicellular stages and localized in prestalk cells with a pattern similar to that of the ecmA prestalk-specific promoter. We show that expression in wild-type cells of the G alpha 2 mutant protein [G alpha 2(G206T)] during the early stages of development blocks aggregation and cAMP-mediated activation of adenylyl cyclase and guanylyl cyclase, suggesting it functions as a dominant negatively active G alpha subunit. When this mutant G alpha protein is expressed from the ecmA prestalk-specific promoter, abnormal stalk differentiation during culmination is observed. Expression of the mutant G alpha 2 from the SP60 prespore promoter or wild-type G alpha 2 from either the ecmA or the SP60 promoter results in no detectable phenotype. The results suggest that G alpha 2 plays an essential role during the culmination stage in prestalk cells and may mediate cAMP receptor activation of these processes during multicellular development.

DdPK3, which plays essential roles during Dictyostelium development, encodes the catalytic subunit of cAMP-dependent protein kinase

We have previously reported the analysis of DdPK3, a developmentally regulated putative serine/threonine kinase that shares approximately 50% amino acid sequence identity with metazoan cAMP-dependent protein kinase A (PKA) and protein kinase C, within their catalytic domains. Cells in which the DdPK3 gene has been disrupted do not aggregate but they are able to induce aggregation-stage genes in response to cAMP pulses and the prestalk-specific ras gene DdrasD in response to high continuous levels of cAMP but will not induce prespore gene expression. In this report, we present conclusive evidence that DdPK3 encodes the catalytic subunit of the Dictyostelium PKA. DdPK3 null cells lack kinase activity that phosphorylates a PKA-specific substrate and is specifically inhibitable by recombinant cAMP-dependent protein kinase inhibitor. DdPK3 expressed in Escherichia coli has PKA activity that is inhibitable by protein kinase inhibitor. When Ddpk3 null cells are complemented with DdPK3 expressed from an actin promoter on an extrachromosomal vector (low copy number), PKA activity is restored and the cells proceed to the slug stage but will not culminate, suggesting that properly regulated PKA activity is essential for culmination. Moreover, overexpressing DdPK3 in wild-type cells on integrating vectors (high copy number) from either an actin or prespore-specific promoter results in accelerated development and the ability to form mature spores in monolayer culture in the presence of high cAMP, a developmental potential lacking in wild-type cells.

Amino acid substitutions in the Dictyostelium G alpha subunit G alpha 2 produce dominant negative phenotypes and inhibit the activation of adenylyl cyclase, guanylyl cyclase, and phospholipase C

Previous studies have demonstrated that the Dictyostelium G alpha subunit G alpha 2 is essential for the cAMP-activation of adenylyl cyclase and guanylyl cyclase and that g alpha 2 null mutants do not aggregate. In this manuscript, we extend the analysis of the function of G alpha 2 in regulating downstream effectors by examining the in vivo developmental and physiological phenotypes of both wild-type and g alpha 2 null cells carrying a series of mutant G alpha 2 subunits expressed from the cloned G alpha 2 promoter. Our results show that wild-type cells expressing G alpha 2 subunits carrying mutations G40V and Q208L in the highly conserved GAGESG (residues 38-43) and GGQRS (residues 206-210) domains, which are expected to reduce the intrinsic GTPase activity, are blocked in multicellular development. Analysis of down-stream effector pathways essential for mediating aggregation indicates that cAMP-mediated activation of guanylyl cyclase and phosphatidylinositol-phospholipase C (PI-PLC) is almost completely inhibited and that there is a substantial reduction of cAMP-mediated activation of adenylyl cyclase. Moreover, neither mutant G alpha 2 subunit can complement g alpha 2 null mutants. Expression of G alpha 2(G43V) and G alpha 2(G207V) have little or no effect on the effector pathways and can partially complement g alpha 2 null cells. Our results suggest a model in which the dominant negative phenotypes resulting from the expression of G alpha 2(G40V) and G alpha 2(Q208L) are due to a constitutive adaptation of the effectors through a G alpha 2-mediated pathway. Analysis of PI-PLC in g alpha 2 null mutants and in cell lines expressing mutant G alpha 2 proteins also strongly suggests that G alpha 2 is the G alpha subunit that directly activates PI-PLC during aggregation. Moreover, overexpression of wild-type G alpha 2 results in the ability to precociously activate guanylyl cyclase by cAMP in vegetative cells, suggesting that G alpha 2 may be rate limiting in the developmental regulation of guanylyl cyclase activation. In agreement with previous results, the activation of adenylyl cyclase, while requiring G alpha 2 function in vivo, does not appear to be directly carried out by the G alpha 2 subunit. Our data are consistent with adenylyl cyclase being directly activated by either another G alpha subunit or by beta gamma subunits released on activation of the G protein containing G alpha 2.

A density-sensing factor controls development in Dictyostelium

For an unknown reason, several genes expressed during Dictyostelium development are regulated by cell density. This is mediated by an 80-kD glycoprotein, conditioned medium factor (CMF), which is slowly secreted and simultaneously sensed by starved cells. To examine further this eukaryotic cell density-sensing mechanism, we have isolated a cDNA encoding CMF. The derived amino acid sequence of CMF shows no obvious similarity to any known protein and thus may represent a new class of eukaryotic intercellular signal. CMF antisense transformants do not aggregate, whereas normal development is restored by the addition of purified CMF protein. This suggests that CMF might synchronize the onset of development in Dictyostelium by triggering aggregation when a majority of the cells in a given area have starved, as signaled by CMF secretion.

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.

cAMP signal transduction pathways regulating development of Dictyostelium discoideum

Dictyostelium discoideum development is regulated through receptor/G protein signal transduction using cAMP as a primary extracellular signal. Signaling pathways will be discussed as well as the regulation and function of individual cAMP receptors and G alpha subunits. Finally potential downstream targets including protein kinases and nuclear events will be explored.

A developmentally regulated, putative serine/threonine protein kinase is essential for development in Dictyostelium

Using PCR technology, we have cloned parts of three developmentally regulated putative serine/threonine kinases from Dictyostelium. All show significant homology to members of the cAMP-dependent protein kinase A/protein kinase C subfamilies. A genomic clone encoding one of these, DdPK3, has been isolated and sequenced. The open reading frame encodes a protein of 648 amino acids with the conserved kinase domain in the C-terminal half. The protein encoded by this gene is unusual in that it contains long homopolymer runs in the N-terminal half of the protein, including a long run of 88 amino acids in which 73 are glutamine residues. To examine the function of DdPK3, a gene disruption was created via homologous recombination. Ddpk3- cells do not aggregate by themselves but will co-aggregate with wild-type cells. However, after aggregation these cells are 'sloughed off' and do not proceed further through development, but are found as a discrete mass alongside the fruiting body formed by the wild-type cells. Analysis of signal transduction pathways indicates that cAMP pulse-induced expression of aggregation stage-specific genes is normal in Ddpk3- cells, as is induction of the prestalk gene Ddras in single cell assays. However, cAMP induction of the late promoters of cAMP receptor cAR1 and of two prespore-specific genes is absent under similar conditions. These cells show normal activation of adenylate cyclase and normal phosphorylation of the G alpha protein G alpha 2 in response to cAMP. The possible role of DdPK3 in Dictyostelium development is discussed.

DC6, a novel type of Dictyostelium discoideum gene regulated by secreted factors but not by cAMP

We have isolated a gene, DC6, which is induced in the early aggregative stages of development in Dictyostelium discoideum. The increase in DC6 expression is dependent on high cell density, indicating that cellular interactions are required for DC6 induction. In low-cell-density cultures, the induction of DC6 occurs if supplied with conditioned medium of developing cells, suggesting that secreted factors are involved in DC6 induction. The expression of DC6 is not affected (1) in the presence of caffeine or adenosine, which block the production or the action of cAMP pulses, (2) in the presence of high concentrations of cAMP, or (3) in mutant strains (Synag7 and FrigidA), which are defective in transduction pathways of cAMP pulse signals. These results indicate that the induction of DC6 does not require extracellular cAMP pulse signals, which are known to regulate the expression of many genes in the early development. Independence of cAMP signals and dependence on other unknown cellular interactions are prominent characteristics of DC6.

A cis-acting element responsible for early gene induction by extracellular cAMP in Dictyostelium discoideum

We have analysed the promoter of the Dictyostelium discoideum alpha-L-fucosidase (ALF) gene, and have identified a 58 bp fragment responsible for transcriptional activation mediated by extracellular cAMP. Replacement of regulatory sequences in the cAMP-independent actin 15 promoter by this fragment confers cAMP regulation to the hybrid promoter. A cAMP analog with high affinity to the cell surface cAMP receptor can induce transcription from the endogenous as well as from the hybrid promoter. Gel-shift experiments show that the 58 bp fragment is a target for nuclear DNA-binding proteins, and that a specific complex is formed in response to cAMP stimulation. The major cAMP-dependent DNA.protein complex is formed with a 22 bp subfragment which we have termed DCRE (Dictyostelium cAMP responsive element).

Control of cAMP-induced gene expression by divergent signal transduction pathways

A compilation of literature data and recent experiments led to the following conclusions regarding cyclic adenosine 3':5' monophosphate (cAMP) regulation of gene expression. Several classes of cAMP-induced gene expression can be discriminated by sensitivity to stimulation kinetics. The aggregation-related genes respond only to nanomolar cAMP pulses. The prestalk-related genes respond both to nanomolar pulses and persistent micromolar stimulation. The prespore specific genes respond only to persistent micromolar stimulation. The induction of the aggregation- and prestalk-related genes by nanomolar cAMP pulses may share a common transduction pathway, which does not involve cAMP, while involvement of the inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway is unlikely. Induction of the expression of prespore and prestalk-related genes by micromolar cAMP stimuli utilizes divergent signal processing mechanisms. cAMP-induced prespore gene expression does not involve cAMP and probably also not cyclic guanosine 3'.5' monophosphate (cGMP) as intracellular intermediate. Involvement of cAMP-induced phospholipase C (PLC) activation in this pathway is suggested by the observation that IP3 and 1,2-diacylglycerol (DAG) can induce prespore gene expression, albeit in a somewhat indirect manner and by the observation that Li+ and Ca2+ antagonists inhibit prespore gene expression. Cyclic AMP induction of prestalk-related gene expression is inhibited by IP3 and DAG and promoted by Li+, and is relatively insensitive to Ca2+ antagonists, which indicates that PLC activation does not mediate prestalk-related gene expression. Neither prespore nor prestalk-related gene expression utilizes the sustained cAMP-induced pHi increase as intracellular intermediate.

Coordinate regulation of the spore coat genes in Dictyostelium discoideum

Genomic clones of the genes coding for the three major spore coat proteins, SP60, SP70, and SP96, were used to measure the accumulation of their respective mRNAs in mutant and wild-type cells allowed to develop under a variety of conditions. These prespore-specific mRNAs were found to be both temporally and quantitatively coordinate under all conditions indicating that they may be subject to identical regulatory processes. Accumulation of the spore coat mRNAs is dependent upon the function of both cAMP receptors and G alpha 2 proteins during the aggregation stage as well as upon concomitant protein synthesis. When cells are dissociated from aggregates at 10 hr of development and rapidly shaken in 0.1 mM EDTA they form clumps but do not accumulate any of the prespore-specific RNAs assayed. However, if either 0.1 mM Ca++ or 20 microM cAMP is added to these cells, the spore coat mRNAs accumulate. Lower concentrations of either Ca++ or cAMP had no effect. These results suggest that expression of the spore coat genes normally involves a Ca+(+)-dependent process, but the Ca++ requirement can be overcome by adding high concentrations of exogenous cAMP. Addition of 50 nM DIF to dissociated cell blocks the accumulation of the spore coat mRNAs even when cAMP or Ca++ is present. The upstream regions of the spore coat genes were compared to those of another gene, D19, that codes for the prespore-specific protein SP29. Short sequences related to CACCCAC were found at about the same position relative to the transcriptional start sites of these coordinately regulated genes.

Signal transduction pathways involved in the expression of the uridine diphosphoglucose pyrophosphorylase gene of Dictyostelium discoideum

The uridine diphosphoglucose pyrophosphorylase (UDPGP1) gene of Dictyostelium discoideum is an excellent marker to study the pathways that control the expression of genes during development. We have previously shown that the UDPGP1 gene is regulated by exogenous cAMP acting on cell-surface cAMP receptors. Various steps in the signal transduction pathway between receptor stimulation and the induction of the gene can now be studied. Induction does not require the synthesis of intracellular cAMP, but does require new protein synthesis. By deletion and transformation with altered genes, two cis-acting sequences that are required for UDPGP1 expression have been identified. A GC-rich palindromic sequence located between -410 and -374 is essential for induction of the gene by extracellular cAMP, but not for its basal expression. A sequence element located between -374 and -337 is required for any basal expression of this gene. When the polarity of the palindromic sequence was reversed such that it resembled the H2K enhancer element, the gene could still be induced by exogenous cAMP. Two DNA binding activities were detected in gel mobility shift assays using a fragment containing both of the regulatory sequence elements of UDPGP1 gene. Transformation with a vector that resulted in the synthesis of anti-sense UDPGP1 RNA led to almost total elimination of the enzyme antigen and no detectable enzyme activity. However, these transformants developed normally, indicating that either UDPGP is not required for development or residual synthesis of UDPGP may be sufficient for normal development.

A Ca2+ transport system associated with the plasma membrane of Dictyostelium discoideum is activated by different chemoattractant receptors

Amebae of Dictyostelium exhibit a transient uptake of extracellular Ca2+ approximately 5 s after activation of surface folate or cAMP receptors (Bumann, J., B. Wurster, and D. Malchow. 1984. J. Cell Biol. 98:173-178). To further characterize these Ca2+ entry systems, we analyzed 45Ca2+ uptake by resting and activated amebae. Like the surface chemoreceptors, folate- and cAMP-induced Ca2+ uptake responses were developmentally regulated; the former response was evident in vegetative but not aggregation-competent cells, whereas the latter response displayed the opposite pattern of expression. In contrast, other characteristics of these Ca2(+)-uptake pathways were remarkably similar. Both systems (a) exhibited comparable kinetic properties, (b) displayed a high specificity for Ca2+, and (c) were inhibited effectively by Ruthenium Red, sodium azide, and carbonylcyanide m-chlorophenyl-hydrazone. These results, together with the finding that vegetative cells transformed with a plasmid expressing the surface cAMP receptor exhibit a cAMP-induced Ca2+ uptake, suggest that different chemoreceptors activate a single Ca2+ entry pathway. Additional pharmacological and ion competition studies indicated that receptor-mediated Ca2+ entry probably does not involve a Na+/Ca2+ exchanger or voltage-activated channels. Chemoattractant binding appears to generate intracellular signals that induce activation and adaption of the Ca2(+)-uptake response. Analysis of putative signaling mutants suggests that Ca2+ entry is not regulated by the guanine nucleotide-binding (G) protein subunits G alpha 1 or G alpha 2, or by G protein-mediated changes in intracellular cAMP or guanosine 3,'5'-cyclic monophosphate (cGMP).

Effects of DNA and synthetic oligodeoxyribonucleotides on the binding properties of a cGMP-binding protein from Dictyostelium discoideum

Previously, we identified a cGMP-binding protein (cGBP) in Dictyostelium discoideum that can exist in two forms: a fast-dissociating (F-type) activity and a slow-dissociating (S-type) activity. Moreover, the F-type activity was converted effectively to S-type by the addition of nucleic acids, especially DNA (Parissenti, A.M. and Coukell, M. B. (1989) J. Cell Sci. 92, 291-301). In this study, we examined the effects of heterologous DNA and various synthetic homo-oligodeoxyribonucleotides on the cGMP-binding properties of partially purified F-type activity. Equilibrium and kinetic binding experiments revealed that DNA increased the affinity of the protein for cGMP without altering the number of binding sites. However, the presence of DNA decreased only slightly the apparent Kd of the protein for cGMP because the nucleic acid also reduced the rate of cGMP association. Addition of oligo(dGMP)8 or oligo(dCMP)8 to the protein increased both total cGMP binding and the conversion of F-type activity to S-type; in contrast, oligo(dAMP)8 or oligo(dTMP)8, at the same concentration, had no effect. Oligodeoxycytidylic acids with chain lengths less than about eight nucleotides were also ineffective or inhibitory. Analysis of cGMP binding to intact, filipin-permeabilized cells revealed a binding activity with association and dissociation rates comparable to isolated S-type activity. This observation suggests that in vivo the cGBP might exist in its S-form.

Regulation of the Discoidin I gamma gene in Dictyostelium discoideum: identification of individual promoter elements mediating induction of transcription and repression by cyclic AMP

We dissected the promoter of the developmentally induced and cyclic AMP-repressed discoidin I gamma gene and identified a sequence element essential for developmental induction. Transfer of the element to an inactive heterologous promoter demonstrated that this sequence is sufficient to confer expression in axenically growing cells and to induce gene activity in development after growth on bacteria. A 16-base-pair sequence within this element was shown to be sufficient for induction in the discoidin promoter context and was used to reactivate different truncated promoter constructs. This led to the localization of an element necessary for down regulation of gene expression by extracellular cyclic AMP.

Regulation of the Discoidin I gamma gene in Dictyostelium discoideum: identification of individual promoter elements mediating induction of transcription and repression by cyclic AMP

We dissected the promoter of the developmentally induced and cyclic AMP-repressed discoidin I gamma gene and identified a sequence element essential for developmental induction. Transfer of the element to an inactive heterologous promoter demonstrated that this sequence is sufficient to confer expression in axenically growing cells and to induce gene activity in development after growth on bacteria. A 16-base-pair sequence within this element was shown to be sufficient for induction in the discoidin promoter context and was used to reactivate different truncated promoter constructs. This led to the localization of an element necessary for down regulation of gene expression by extracellular cyclic AMP.

A pharmacologically distinct cyclic AMP receptor is responsible for the regulation of gp80 expression in Dictyostelium discoideum

The EDTA-resistant cell-cell adhesion expressed at the aggregation stage of Dictyostelium discoideum is mediated by a cell surface glycoprotein of Mr 80,000 (gp80). The expression of gp80 is developmentally regulated by cyclic AMP (cAMP). In vitro nuclear run-on experiments show that transcription of the gp80 gene is initiated soon after the onset of development. The basal level of gp80 transcription is significantly augmented by exogenous cAMP pulses. Interestingly, in analog studies, 2'-deoxy-cAMP, 8-bromo-cAMP, and N6-monobutyryl-cAMP are all capable of inducing a rapid accumulation of gp80 mRNA, suggesting the presence of a unique cAMP receptor that responds equally well to these analogs. To determine whether intracellular cAMP plays a role in the regulation of gp80 expression, caffeine was used to block cAMP-induced receptor-mediated adenylate cyclase activation. Expression of gp80 mRNA was blocked in caffeine-treated cells but could be substantially restored by treatment with exogenous cAMP pulses, suggesting that adenylate cyclase activation is not required. gp80 expression was also examined in the signal transduction mutants synag 7 and frigid A. In both mutants, gp80 was expressed at the basal level. Pulses of cAMP as well as 2'-deoxy-cAMP and N6-monobutyryl-cAMP were capable of restoring the normal level of gp80 expression in synag 7 cells. These results, taken together, indicate bimodal regulation of gp80 expression during development and the involvement of a novel cAMP receptor in the transmembrane signalling pathway that regulates gp80 gene expression.

A pleiotropic defect in cAMP-regulated gene expression in the Dictyostelium agg- mutant synag 7

During differentiation of Dictyostelium discoideum, cAMP functions as a diffusible, extracellular signal to direct chemotaxis and regulate developmental gene expression. The availability of signal-transduction mutants of Dictyostelium now makes it feasible to pursue a genetic analysis of cAMP signaling. The synag 7 mutant is defective in receptor-mediated adenylate cyclase stimulation and cannot relay a cAMP signal. To further characterize this mutant, mRNA levels of several cAMP-regulated genes were measured during development. cAMP-regulated gene expression was found to be dramatically altered in synag 7:several different genes which require cAMP for expression in wild-type cells were induced in synag 7 in the absence of cAMP. In addition, the gene-encoding discoidin I, which is normally expressed in starved cells and repressed by cAMP, is expressed at very low levels in starved synag 7 cells, possibly due to precocious repression. These results suggest that a pleiotropic regulator of cAMP-regulated gene expression is uncoupled from its normal controls during development in synag 7.

A pharmacologically distinct cyclic AMP receptor is responsible for the regulation of gp80 expression in Dictyostelium discoideum

The EDTA-resistant cell-cell adhesion expressed at the aggregation stage of Dictyostelium discoideum is mediated by a cell surface glycoprotein of Mr 80,000 (gp80). The expression of gp80 is developmentally regulated by cyclic AMP (cAMP). In vitro nuclear run-on experiments show that transcription of the gp80 gene is initiated soon after the onset of development. The basal level of gp80 transcription is significantly augmented by exogenous cAMP pulses. Interestingly, in analog studies, 2'-deoxy-cAMP, 8-bromo-cAMP, and N6-monobutyryl-cAMP are all capable of inducing a rapid accumulation of gp80 mRNA, suggesting the presence of a unique cAMP receptor that responds equally well to these analogs. To determine whether intracellular cAMP plays a role in the regulation of gp80 expression, caffeine was used to block cAMP-induced receptor-mediated adenylate cyclase activation. Expression of gp80 mRNA was blocked in caffeine-treated cells but could be substantially restored by treatment with exogenous cAMP pulses, suggesting that adenylate cyclase activation is not required. gp80 expression was also examined in the signal transduction mutants synag 7 and frigid A. In both mutants, gp80 was expressed at the basal level. Pulses of cAMP as well as 2'-deoxy-cAMP and N6-monobutyryl-cAMP were capable of restoring the normal level of gp80 expression in synag 7 cells. These results, taken together, indicate bimodal regulation of gp80 expression during development and the involvement of a novel cAMP receptor in the transmembrane signalling pathway that regulates gp80 gene expression.

Cytoplasmic pH of Dictyostelium discoideum amebae during early development: identification of two cell subpopulations before the aggregation stage

Development of the cellular slime mold Dictyostelium discoideum is initiated by the removal of nutrients, and results in formation of a mature fruiting body composed of two cell types, the stalk and spore cells. A considerable body of evidence supports the hypothesis that cytoplasmic pH may be an essential regulator of the choice to differentiate in either the prestalk or prespore pathway. We have devised methods for measurement and analysis of intracellular pH in developing Dictyostelium amebae in order to assess directly the potential role of cytoplasmic pH in regulating the pathway of differentiation. The intracellular pH of single D. discoideum amebae during development and in intact slugs has been measured using the pH-sensitive indicator pyranine in a low light level microspectrofluorometer. We have used the ATP-mediated loading method to introduce pyranine into these cells. Cells loaded by the ATP method appear healthy, have no detectable defects in development, and exhibit a similar population distribution of intracellular pH to those loaded by sonication. The intracellular pH of populations comprised of single amebae was found to undergo a transient acidification during development resulting in a bimodal distribution of intracellular pH. The subpopulations were characterized by fitting two gaussian distributions to the data. The number of cells in the acidic intracellular pH subpopulation reached a maximum 4 h after initiation of development, and had returned to a low level by 7 h of development. In addition, a random sample of single amebae within a slug had a median intracellular pH of 7.2, nearly identical to the median pH (7.19) of similarly treated vegetative cells. No gradient of intracellular pH along the anterior to posterior axis of the slug was detected. Our data demonstrate the existence of two distinct subpopulations of cells before the aggregation stage of development in Dictyostelium, and offers support for the hypothesis that changes in intracellular pH contribute to development in D. discoideum.

Regulation of DdrasG gene expression during Dictyostelium development

DdrasG gene expression during the early development of Dictyostelium discoideum has been examined in detail. The amount of DdrasG-specific mRNA increased approximately twofold during the first 2 to 3 h of development and then declined rapidly, reaching negligible levels by the aggregation stage. The increase in mRNA levels that occurred during the first 2 to 3 h of development also occurred during differentiation in cell suspensions and was enhanced when cells were shaken rapidly. This initial increase was unaffected by cell density. When cells were set up to differentiate on filters, the addition of a glucose-amino acid mixture slightly delayed differentiation and had a similar effect on the expression of the gene. The decline in DdrasG expression during development did not occur when cells were treated with cycloheximide, suggesting that the expression of a developmentally regulated gene product is essential for the reduction of DdrasG gene mRNA. There was no decrease in DdrasG mRNA level during differentiation in shake suspension, but the decrease did occur upon application of pulses of cyclic AMP to shaking cultures. The application of a continuously high level of cyclic AMP delayed the increase in expression of the gene and did not result in the subsequent decline. These results suggest that the induction of a functional cyclic AMP relay system is important in reducing DdrasG gene mRNA levels.

Regulation of DdrasG gene expression during Dictyostelium development

DdrasG gene expression during the early development of Dictyostelium discoideum has been examined in detail. The amount of DdrasG-specific mRNA increased approximately twofold during the first 2 to 3 h of development and then declined rapidly, reaching negligible levels by the aggregation stage. The increase in mRNA levels that occurred during the first 2 to 3 h of development also occurred during differentiation in cell suspensions and was enhanced when cells were shaken rapidly. This initial increase was unaffected by cell density. When cells were set up to differentiate on filters, the addition of a glucose-amino acid mixture slightly delayed differentiation and had a similar effect on the expression of the gene. The decline in DdrasG expression during development did not occur when cells were treated with cycloheximide, suggesting that the expression of a developmentally regulated gene product is essential for the reduction of DdrasG gene mRNA. There was no decrease in DdrasG mRNA level during differentiation in shake suspension, but the decrease did occur upon application of pulses of cyclic AMP to shaking cultures. The application of a continuously high level of cyclic AMP delayed the increase in expression of the gene and did not result in the subsequent decline. These results suggest that the induction of a functional cyclic AMP relay system is important in reducing DdrasG gene mRNA levels.

A role for cAMP-dependent protein kinase A in early Dictyostelium development

In Dictyostelium, cAMP functions as an extracellular regulatory molecule that controls aggregation, expression of a number of classes of genes, and cellular differentiation by binding to cell-surface receptors that activate intracellular signal transduction pathways. To investigate possible roles for intracellular cAMP, we have overexpressed the wild-type mouse type-I regulatory subunit (RI) of cAMP-dependent protein C (PKA) in Dictyostelium cells, as well as mutant forms of the subunit that are altered in their ability to bind cAMP. We show that overexpression of a mutated RI, which lacks both cAMP-binding sites and presumably forms a complex with the endogenous Dictyostelium catalytic subunit that cannot be activated by cAMP, results in cells that do not aggregate or express sets of genes that are normally induced in the multicellular stages. Transformations that express the mutant subunit at low levels show no observable phenotype. We show that these cells can respond to pulses of cAMP and activate cAMP receptor/G protein-mediated processes, including the activation of adenylate and guanylate cyclases and the induction of a class of genes known to be regulated through the receptor-mediated pathways; however, the cells do show an altered pattern of expression of other genes normally active during the preaggregation/interphase and aggregation stages. Of interest is a substantial overexpression of the developmentally regulated PDE mRNA. Cell lines carrying constructs encoding the wild-type subunit or mutant subunits lacking one of the two binding sites show no visual phenotype. The results suggest that PKA-mediated functions, presumably controlled by increases in intracellular cAMP, are essential for Dictyostelium aggregation.

Regulation of sugar and ethanol metabolism in Saccharomyces cerevisiae

This review briefly surveys the literature on the nature, regulation, genetics, and molecular biology of the major energy-yielding pathways in yeasts, with emphasis on Saccharomyces cerevisiae. While sugar metabolism has received the lion's share of attention from workers in this field because of its bearing on the production of ethanol and other metabolites, more attention is now being paid to ethanol metabolism and the regulation of aerobic metabolism by fermentable and nonfermentable substrates. The utility of yeast as a highly manipulable organism and the discovery that yeast metabolic pathways are subject to the same types of control as those of higher cells open up many opportunities in such diverse areas as molecular evolution and cancer research.

Molecular complementation of a genetic marker in Dictyostelium using a genomic DNA library

We constructed a partial Sau3A Dictyostelium genomic DNA library in a shuttle vector that replicates extrachromosomally in Dictyostelium cells. This library was used to complement Dictyostelium strain HPS400, which lacks thymidylate synthase activity and requires exogenous thymidine for growth. We have used a modified high-frequency transformation protocol that allows the introduction of the library into a sufficient number of Dictyostelium cells to select complementing plasmids. Using this approach, we have isolated a gene (Thy1) that complements the thymidine growth requirement of HPS400. The gene encodes a 1.2-kilobase RNA and the derived amino acid sequence shows no homology to thymidylate synthase, a protein highly conserved throughout evolution, or any other protein sequence in the data base examined. Thy1 provides an important selectable marker for transforming Dictyostelium cells. In addition, this work suggests that it will be possible to isolate genes that are essential for developmental processes in Dictyostelium by complementation.

A trans-acting factor required for cAMP-induced gene expression in Dictyostelium is regulated developmentally and induced by cAMP

We have identified a nuclear activity that binds specifically to a GT-rich sequence or G-box shown previously by use of deletion analysis to be required for cAMP and for developmentally induced expression of the prestalk gene pst-cathepsin (CP2). We show that the insertion of an oligonucleotide that contains the CP2 G-box restores regulated expression whereas the insertion of oligonucleotides that contain mutations in some of the G residues does not. Moreover, the mutant oligonucleotides do not compete for binding of the factor to the wild-type sequence. The activity of the G-box binding factor (GBF) is regulated developmentally with induction of activity occurring at the time of induction of pst-cathepsin expression. In a single-cell culture, GBF activity is inducible by cAMP, and its appearance is inhibited by cycloheximide, which suggests that the factor, or a protein component required for binding of the factor, is directly induced by cAMP and may be the rate-limiting factor required for cAMP induction of pst-cathepsin expression. Models for cAMP induction of prestalk genes are described.

Regulation and function of G alpha protein subunits in Dictyostelium

We have examined the developmental regulation and function of two G alpha protein subunits, G alpha 1 and G alpha 2, from Dictyostelium. G alpha 1 is expressed in vegetative cells through aggregate stages while G alpha 2 is inducible by cAMP pulses and preferentially expressed in aggregation. Our results suggest that G alpha 2 encodes the G alpha protein subunit associated with the cAMP receptor and mediates all known receptor-activated intracellular signal transduction processes, including chemotaxis and gene regulation. G alpha 1 appears to function in both the cell cycle and development. Overexpression of G alpha 1 results in large, multinucleated cells that develop abnormally. The central role that these G alpha proteins play in signal transduction processes and in controlling Dictyostelium development is discussed.

Two-phase regulatory pathway controls cAMP receptor-mediated expression of early genes in Dictyostelium

Two classes of early genes in Dictyostelium are differentially regulated by extracellular pulses of cAMP interacting with its cell-surface receptor, conditions that also regulate chemotaxis and aggregation. The pulse-repressed genes, such as K5, are induced shortly after the onset of starvation and are repressed a few hr later during aggregation by cAMP pulses. The pulse-induced genes (including D2, M3, and those encoding contact sites A, the G alpha protein subunit G alpha 2, and the cell-surface cAMP receptor) are maximally induced just prior to aggregation by pulses of cAMP and are subsequently repressed by sustained moderate levels of cAMP--conditions that exist sequentially in development. In this manuscript, we further analyze the requirement for cAMP pulses and characterize a requirement for protein synthesis for the expression of these two classes of genes. Our results indicate that the control of expression of both the pulse-induced and pulse-repressed genes requires other developmentally regulated factors in addition to starvation and cAMP pulses. We also identified another early gene, F9, whose expression is stimulated upon starvation, is not responsive to cAMP, and is hyperstimulated by cycloheximide, in a manner similar to the cycloheximide stimulation of c-fos and other serum-induced genes in mammalian cells. Examination of the kinetics of expression of the pulse-induced genes in a mutant blocked in the cAMP relay pathway indicates that their expression is controlled by a two-phase process. The first phase requires starvation and CMF, an extracellular conditioned medium factor, and results in a low level of expression. The second phase requires establishment of the cAMP signal-relay system and induces the genes to a high level. Both phases require prior and concomitant protein synthesis. Some of the members of the pulse-induced class encode elements of the cAMP signal-relay system that controls aggregation, indicating a feedback autoregulation. The two-phase process might allow the "finetuning" of the level of expression of genes involved in aggregation.

Molecular analysis of a developmentally regulated gene required for Dictyostelium aggregation

We have previously shown that the developmentally regulated gene D2 is induced during aggregation by pulses of cAMP, which act via the cell surface receptor and consequent signal transduction pathways (W. Rowekamp and R.A. Firtel, 1980, Dev. Biol. 79, 409-418; S.K.O. Mann and R.A. Firtel, 1987, Mol. Cell. Biol. 7, 458-469; S.K.O. Mann, C. Pinko, and R.A. Firtel, 1988, Dev. Biol., in press). In this manuscript, we compare the complete derived amino acid sequence for D2 to two cloned and sequenced eukaryotic esterases and examine the requirement of the D2 gene product for development. Amino acid sequence data comparisons suggest that D2 encodes a serine esterase with strong sequence identity to Torpedo acetylcholine esterase and a Drosophila esterase. The protein has a putative leader sequence, suggesting that it is shunted into vesicles. Using an antisense gene construct driven by a Discoidin I promoter, whose transcriptional activity depends on the growth conditions of the cells, we show that inhibition of D2 mRNA accumulation results in an abnormal developmental program that includes the absence of normal streaming and incomplete aggregate formation and subsequent development. We suggest that D2 encodes an esterase function required for proper aggregation and subsequent development.