A chemoattractant receptor controls development in Dictyostelium discoideum

Glycogen synthase kinase 3 regulates cell fate in Dictyostelium

Extracellular cyclic AMP (cAMP) induces the formation of prespore cells in Dictyostelium but inhibits stalk cell formation. We have cloned gskA, which encodes the Dictyostelium homolog of glycogen synthase kinase 3 (GSK-3), and discovered that it is required for both cAMP effects. Disruption of gskA creates a mutant that aggregates but forms few spores and an abnormally high number of stalk cells. These stalk cells probably arise from an expanded prestalk B (pstB) cell population, which normally produces the basal disc of the fruiting body. In cultured mutant cells, cAMP neither inhibits pstB cell differentiation nor induces efficient prespore cell differentiation. We propose that cAMP acts through a common pathway that requires GSK-3 and determines the proportion of prespore and pstB cells.

Attenuation of parasite cAMP levels in T. cruzi-host cell membrane interactions in vitro

Previous investigations have shown that the adhesion of T. cruzi plasma membrane vesicles (PMV) to monolayers of host cell myoblasts and to immobilized heart muscle sarcolemma membranes (PAM) on polyacrylamide beads is mediated by the interaction of T. cruzi attachment sites with the muscarinic cholinergic and beta-adrenergic receptors of the host cell membrane. It has also been shown that this interaction is blunted by the specific antagonists of the mammalian receptors atropine and propranol, respectively. In the studies reported here, PAM also rapidly attached to swimming T. cruzi trypomastigotes in a complex, concentration-dependent fashion and binding isotherms showed that the equilibrium between free and bound PAM is rapidly reached within 2 minutes of incubation in physiologically balanced salt solutions. In this time frame, trypomastigote cAMP levels are significantly reduced from steady state values within 30 seconds of the addition of PAM in a buffer system containing a diesterase inhibitor. Maximal attenuation of cAMP levels was measured between 1 and 2 minutes of the addition of PAM to T. cruzi trypomastigotes. The degree of cAMP level attenuation was reduced by blocking PAM attachment with either atropine or propranol. On the basis of these results we propose that a likely pathway for the negative parasite signal generated upon adhesion of host muscle cell membranes to the surface of the flagellates is from the parasite's surface attachment sites directly to a Pertussis toxin sensitive inhibitory protein Gi, thereby blunting adenyl cyclase activity and cAMP formation.

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

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

CRAC, a cytosolic protein containing a pleckstrin homology domain, is required for receptor and G protein-mediated activation of adenylyl cyclase in Dictyostelium

Adenylyl cyclase in Dictyostelium, as in higher eukaryotes, is activated through G protein-coupled receptors. Insertional mutagenesis into a gene designated dagA resulted in cells that cannot activate adenylyl cyclase, but have otherwise normal responses to exogenous cAMP. Neither cAMP treatment of intact cells nor GTP gamma S treatment of lysates stimulates adenylyl cyclase activity in dagA mutants. A cytosolic protein that activates adenylyl cyclase, CRAC, has been previously identified. We trace the signaling defect in dagA- cells to the absence of CRAC, and we demonstrate that dagA is the structural gene for CRAC. The 3.2-kb dagA mRNA encodes a predicted 78.5-kD product containing a pleckstrin homology domain, in agreement with the postulated interaction of CRAC with activated G proteins. Although dagA expression is tightly developmentally regulated, the cDNA restores normal development when constitutively expressed in transformed mutant cells. In addition, the megabase region surrounding the dagA locus was mapped. We hypothesize that CRAC acts to connect free G protein beta gamma subunits to adenylyl cyclase activation. If so, it may be the first member of an important class of coupling proteins.

The cAMP receptor CAR4 regulates axial patterning and cellular differentiation during late development of Dictyostelium

Pseudoplasmodia of developing Dictyostelium are organized with anteroposterior polarity. We have isolated CAR4, the gene for a new cell-surface, G protein-linked cAMP receptor. CAR4 mRNA is initially expressed during tip elongation and continues to accumulate into culmination. CAR4 is maximally expressed in pseudoplasmodia anteriors which are centers for extracellular cAMP signaling and for organization of cellular patterning. Although car4 null cells progress unperturbed through early development, they exhibit major patterning aberrations as the anteroposterior axis becomes established. Prestalk gene expression is significantly reduced in car4 nulls, whereas prespore-specific markers are overexpressed and detected in zones normally restricted to prestalk cells. Patterning defects are similarly apparent in terminally differentiated fruiting bodies. Our results show that cAMP signaling is required for pattern formation and cellular differentiation during late Dictyostelium development.

Cloning and targeted mutations of G alpha 7 and G alpha 8, two developmentally regulated G protein alpha-subunit genes in Dictyostelium

GTP-binding protein (G protein)-mediated signal transduction pathways play essential roles during the aggregation and differentiation process of Dictyostelium. In addition to the five known G protein alpha-subunit genes, we recently identified three novel alpha-subunit genes, G alpha 6, G alpha 7, and G alpha 8, using the polymerase chain reaction technique. We present here a more complete analysis of G alpha 7 and G alpha 8. The cDNAs of these two genes were cloned, and their complete nucleotide sequences were determined. Sequence analyses indicate that G alpha 8 possesses some unusual features. It lacks the "TCATDT" motif, a sequence of amino acids highly conserved among G alpha subunits, and has an additional 50 amino acids at its C-terminus consisting of long stretches of asparagine. Moreover, G alpha 8 is unusually resistant to protease digestion, which may indicate a slow GTP hydrolysis rate. The possible functions of these alpha-subunits were assessed by generating mutants lacking G alpha 7 or G alpha 8 by gene targeting through homologous recombination and by overexpressing G alpha 7 or G alpha 8 protein. Overexpression of G alpha 7 resulted in abnormal morphogenesis starting at the slug stage, whereas analysis of the other strains failed to reveal any obvious growth or developmental defects under either normal or stressful conditions. The implications of these results are discussed.

Two cAMP receptors activate common signaling pathways in Dictyostelium

Multiple signal transduction pathways within a single cell may share common components. In particular, seven different transmembrane helix receptors may activate identical pathways by interacting with the same G-proteins. Dictyostelium cells respond to cAMP using one such receptor, cAR1, coupled by a typical heterotrimeric G-protein to intracellular effectors. However, cells in which the gene for cAR1 has been deleted are unexpectedly still able to respond to cAMP. This implies either that certain responses are mediated by a different receptor than cAR1, or alternatively that a second, partially redundant receptor shares some of the functions of cAR1. We have examined the dose response and ligand specificity of one response, cAMP relay, and the dose response of another, cyclic GMP synthesis. In each case, the EC50 was approximately 100-fold higher and the maximal response was smaller in car1- than wild-type cells. These data indicate that cAR1 normally mediates responses to cAMP. The ligand specificity suggests that the responses seen in car1- mutants are mediated by a second receptor, cAR3. To test this hypothesis, we constructed a cell line containing deletions of both cAR1 and cAR3 genes. As predicted, these lines are totally insensitive to cAMP. We conclude that the functions of the cAR1 and cAR3 receptors are partially redundant and that both interact with the same heterotrimeric G-protein to mediate these and other responses.

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.

Constitutively active receptors as a disease-causing mechanism

Membrane receptors have appeared early in evolution as the means for the unicellular organism to sense its environment. With the emergence of social cellular life in multicellular organisms, membrane receptors have acquired the additional functions of sensing the presence of similar cells (as in the aggregation phenomenon of Dictyostelium discoideum) (Klein et al., 1988) or the presence of the mate (Saccharomyces cerevisiae) (Cross et al., 1988), and to detect endocrine signals emitted by cells in distant tissues. As the latter function is central to homeostasis and regulation of cell growth, the downstream regulatory cascades under receptor control are the subject of intense research with implications in virtually all fields of biomedical science. The impact of the analysis of tyrosine kinase-activated cascades on our understanding of carcinogenesis is but one example of such an advance.

G protein-linked signaling pathways control the developmental program of Dictyostelium

The similarity of the signal transduction systems controlling early development in Dictyostelium with those mediating the action of hormones and neurotransmitters in mammals suggests that these strategies were quickly refined as eukaryotic cells began to communicate. These simple, genetically tractable organisms thus offer a great opportunity to elucidate these pathways further. Combinations of the null mutants are being studied to address questions of redundancy, cross-talk, and networking. Since cAR1, cAR2, G alpha 2, G beta, ACA, CRAC, PKA, and PDE are essential to the program, the capacity to rescue these phenotypes also serves as a convenient screen for functional mutations in these proteins. Finally, random mutagenesis by the recently developed method of restriction enzyme-mediated insertion provides a means to isolate new genes (Kuspa et al., 1992). The clear phenotypes of the null mutants observed so far indicate that the Dictyostelium developmental program can be used as a guide to isolate novel components of G protein-linked pathways.

Evolution of the gastrointestinal endocrine system (with special reference to gastrin and CCK)

The evolution of gut endocrine cells can be seen to have depended in the first instance on the expression of genes encoding regulatory peptides in cells that had evolved the regulated pathway of secretion. It seems probable that the endocrine cells made use of molecules and mechanisms that first emerged in early nervous systems. However, by the start of the vertebrate line of evolution, most of the major families of gut hormones were already found in association with endocrine cells. A single common class of receptor with seven transmembrane domains and acting via association with G-proteins transduces many (perhaps all) gut peptide actions. The duplication and divergence of receptors and peptides can now be traced, in outline at least, for gastrin and CCK in vertebrates. Even in phylogenetically similar groups such as birds and mammals, quite different molecular approaches have been applied to solving the same physiological problem. Evolution of the modern gastrointestinal control system evidently depended in this case both on molecular evolution of peptides and receptors and on cells expressing the genes encoding them.

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

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

Phospholipase C in Dictyostelium discoideum. Identification of stimulatory and inhibitory surface receptors and G-proteins

A combined biochemical and genetic approach was used to show that phospholipase C in the cellular slime mould Dictyostelium is under dual regulation by the chemoattractant cyclic AMP (cAMP). This dual regulation involves stimulatory and inhibitory surface receptors and G-proteins. In wild-type cells both cAMP and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulated phospholipase C. In contrast, mutant fgd A, lacking the G-protein alpha-subunit G alpha 2, showed no stimulation by either cAMP or GTP[S], indicating that G alpha 2 is the stimulatory G-protein. In mutant fgd C cAMP did not stimulate phospholipase C, but stimulation by GTP[S] was normal, suggesting that the defect in this mutant is upstream of the stimulatory G alpha 2. Inhibition of phospholipase C was achieved in wild-type cells by the partial antagonist 3'-deoxy-3'-aminoadenosine 3',5'-phosphate (3'NH-cAMP). This inhibition was no longer observed in transformed cell lines lacking either the surface cAMP receptor cAR1 or the G-protein alpha-subunit G alpha 1; in these cells the agonist cAMP still activated phospholipase C. These results indicate that Dictyostelium phospholipase C is regulated via a stimulatory and an inhibitory pathway. The inhibitory pathway is composed of the surface receptor cAR1 and the G-protein G1. The stimulatory pathway consists of an unknown cAMP receptor (possibly the fgd C gene product) and the G-protein G2.

Isolation of cDNAs for perilipins A and B: sequence and expression of lipid droplet-associated proteins of adipocytes

The major cAMP-dependent protein kinase (A-kinase) substrate in adipocytes is perilipin, a protein found exclusively at the surface of the lipid storage droplets. Using anti-perilipin serum, we have isolated two related classes of full-length coding cDNAs, designated perilipin A and B, from a rat adipocyte cDNA expression library. The two cDNAs derive from two mRNA species that arise by differential splicing. The mRNAs are predicted to encode perilipins A and B, proteins of 517 aa (56,870 Da) and 422 aa (46,420 Da), respectively, which share a common 406-aa N-terminal sequence. The predicted perilipin A contains peptides present in proteolytic digests of the purified 62-kDa form of perilipin from rat adipocytes, as well as the requisite consensus A-kinase phosphorylation sites. Like perilipin A, the B form is expressed in adipocytes and is associated with lipid storage droplets. Modeling of predicted secondary structures fails to reveal an underlying basis for the tenacious association of perilipins with lipid droplets. These proteins exhibit a significant sequence relationship (approximately 65% similarity through 105 aa) with only one other known protein, the adipocyte differentiation-related protein (ADRP). Like the perilipins, ADRP appears to be adipocyte-specific, which suggests that they interact in a related intracellular pathway. The molecular probes for perilipins A and B described here will permit detailed analyses of their functional role(s) in lipid metabolism.

The regulatory subunit of cAMP-dependent protein kinase as a target for chemotherapy of cancer and other cellular dysfunctional-related diseases

Three separate experimental approaches, using site-selective cAMP analogs, antisense strategy and retroviral vector-mediated gene transfer, have provided evidence that two isoforms, the RI- and RII-regulatory subunits of cAMP-dependent protein kinase, have opposite roles in cell growth and differentiation; RI being growth stimulatory while RII is a growth-inhibitory and differentiation-inducing protein. As RI expression is enhanced during chemical or viral carcinogenesis, in human cancer cell lines and in primary human tumors, it is a target for cancer diagnosis and therapy. 8-Cl-cAMP and RI antisense oligodeoxynucleotide, those that effectively down-regulate RI alpha and up-regulate RII beta, provide new approaches toward the treatment of cancer. This approach to modulation of RI vs RII cAMP transducers may also be beneficial toward therapy of endocrine or cellular dysfunction-related diseases where abnormal signal transduction of cAMP is critically involved.

Extracellular cAMP is sufficient to restore developmental gene expression and morphogenesis in Dictyostelium cells lacking the aggregation adenylyl cyclase (ACA)

Cell movement and cell-type-specific gene expression during Dictyostelium development are regulated by cAMP, which functions both as an extracellular hormone-like signal and an intracellular second messenger. Previous data indicated that aca- mutants, which lack adenylyl cyclase activity, fail to aggregate and do not express cell-type-specific genes. We show here that overexpression of ACG, a constitutively active adenylyl cyclase, which in wild-type cells is only expressed during spore germination, partially restores the coordination of cell movement and completely restores developmental gene expression. The aca- cells can also be induced to develop into viable spores by synergy with wild-type cells and, furthermore, form small but normal fruiting bodies, after a developmentally relevant regimen of stimulation with nanomolar cAMP pulses followed by micromolar cAMP concentrations. 2'-Deoxy cAMP, a cAMP analog that activates the cell-surface cAMP receptors but not cAMP-dependent protein kinase (PKA), also induces fruiting body formation as well as expression of prespore-specific and prestalk-enriched genes in aca- cells. Intracellular cAMP levels were not altered in aca- cells after stimulation with 2'-deoxy cAMP. Our data indicate that ACA is not required to provide intracellular cAMP for PKA activation but is essential to produce extracellular cAMP for coordination of cell movement during all stages of development and for induction of developmental gene expression.

Antisense RNA inactivation of gp138 gene expression results in repression of sexual cell fusion in Dictyostelium discoideum

A glycoprotein, gp138, is implicated in the sexual cell fusion of Dictyostelium discoideum. We previously cloned and sequenced the two genes encoding the gp138 protein, GP138A and GP138B (Fang et al. (1993) Dev. Biol. 156, 201–208). Here, we have constructed a vector producing antisense RNA for the gp138 genes and have transformed the vector into Dictyostelium cells. The transformed cells showed a reduction in the amounts of gp138 mRNA and protein and their sexual cell fusion activity was considerably repressed.

Identification of additional members of human G-protein-coupled receptor kinase multigene family

Human neutrophils express several distinct guanine nucleotide binding (G)-protein-coupled receptors that mediate their responsiveness to chemoattractants. Phosphorylation by receptor-specific and second messenger-activated protein kinases is a common mechanism for regulation of G-protein-coupled receptors. To explore the possibility that chemoattractant receptors are regulated by unique receptor kinases, we utilized PCR to identify receptor kinases in human neutrophils. Here, we report the isolation of three G-protein-coupled-receptor-kinase (GPRK)-like sequences termed GPRK5, GPRK6, and GPRK7 in addition to the beta-adrenergic receptor kinase (beta ARK) 1 and 2 isoforms (beta ARK1 and beta ARK2). Two, GPRK5 and GPRK6, showed high homology at the amino acid level to the recently identified receptor-kinase-like sequence localized close to the Huntington disease locus. GPRK7 is of interest in that it contains a DLG (Asp-Leu-Gly) amino acid motif of receptor kinases preceded by a DFD (Asp-Phe-Asp) motif. We isolated cDNAs corresponding to GPRK6; the complete sequence shows > 66% identity and 81% similarity at the amino acid level to the GPRK from the Huntington disease locus. The GPRK6 cDNA probe hybridizes to two mRNAs of 2.9 and 2.1 kb that were expressed in all the tested human tissues including HL-60 cells and neutrophils. Genomic Southern blot analysis and chromosome mapping showed that GPRK6 hybridizes to two closely related genes located on chromosomes 5 and 13 and are, therefore, distinct from the GPRK located near the Huntington disease locus on chromosome 4. The identification herein of three putative receptor kinases indicates that in addition to beta ARK and rhodopsin kinase subfamilies, there are other receptor-kinase subfamilies that regulate the broad spectrum of G-protein-coupled receptors.

Salmeterol, a long-acting beta 2-adrenoceptor agonist mediating cyclic AMP accumulation in a neuronal cell line

1. The accumulation of cyclic AMP stimulated by salmeterol, a long-acting beta 2-adrenoceptor agonist and by isoprenaline, a non-selective beta-adrenoceptor agonist have been compared in the B50 neuroblastoma cell line. 2. Salmeterol produced a concentration-dependent increase in the accumulation of total [3H]-cyclic AMP in B50 cells yielding an EC50 value of 37 nM which was lower than that obtained with isoprenaline (294 nM). The maximum response to salmeterol was only 46% of that obtained with isoprenaline. 3. The beta 2-adrenoceptor antagonist, ICI 118551, inhibited the responses to both salmeterol (apparent KD 2.2 nM) and isoprenaline (apparent KD 1.6 nM). However, the beta 1-adrenoceptor antagonist, atenolol, produced no significant effect at concentrations up to 100 microM. 4. Salmeterol (1 microM) changed the concentration-response curve of isoprenaline in the manner of a partial agonist interacting with a full agonist. The KD of salmeterol obtained from the interaction was 55.6 nM. 5. Whereas salmeterol has a slow onset of action in airway smooth muscle compared to other beta 2-adrenoceptor agonists, in B50 monolayers both salmeterol and isoprenaline produced a rapid increase in cyclic AMP accumulation (t1/2 1.1 min and 0.4 min respectively). 6. Despite the existence of cyclic AMP efflux mechanisms that exist in this cell line it was possible to investigate the duration of agonist action by measuring intracellular levels of the second messenger. Replacement of drug-containing medium with fresh buffer led to a rapid reduction in intracellular levels of cyclic AMP in isoprenaline-stimulated cells whereas cyclic AMP accumulation was sustained for much longer periods in salmeterol-stimulated cells. However, the persistent action of salmeterol could be reversed by the addition of a beta2-selective antagonist.7. These results confirm that salmeterol has a high affinity, but low efficacy (relative to isoprenaline) for beta2-adrenoceptors coupled to cyclic AMP accumulation and that the drug persists at its site of action for long periods in the B50 neuronal cell line.

Sequence homology between bacteriorhodopsin and G-protein coupled receptors: exon shuffling or evolution by duplication?

Bacteriorhodopsin (BR) is a membrane protein of known structure, widely used for the homology modeling of G-protein-coupled receptors (GPCR). The observation of apparently transposed sequence similarities between some of the helical domains of BR and GPCR has led to the suggestion that exon shuffling may have occurred in the later evolution of GPCR, which would necessitate a different folding pattern for the seven transmembrane helices of GPCR. An alternate hypothesis is that duplication occurred in the evolution of an ancestral gene, such that helices 5-7 originated as duplicates of helices 1-3, leading to intragenic as well as intergenic similarities between helices 1-3 and 5-7 of BR and various GPCR. Analyses of GPCR and BR sequences suggest that such a duplication may have occurred; symmetry within the BR structure is also consistent with homology between these two regions. The hypothesis of evolution by duplication is consistent with the conventional, unshuffled homology model, which is also supported by the obvious conservation of the retinal binding Lys moiety on helix 7 in both BR and the mammalian opsins.

Two transmembrane signaling mechanisms control expression of the cAMP receptor gene CAR1 during Dictyostelium development

Dictyostelium discoideum is among the best characterized organisms for the study of receptor/guanine nucleotide binding protein-mediated control of differentiation. Dictyostelium grow unicellularly but form fully differentiated multicellular organisms through a developmental program regulated by secreted cAMP activating specific cell-surface receptors. Dictyostelium respond differentially to cAMP at different developmental stages. During early development, expression of certain genes is induced by low-level oscillations of extracellular cAMP. Later, continuous, high cAMP concentrations will promote expression of specific genes in multicellular structures. Here, we show that the cAMP receptor gene CAR1, which is essential for development, utilizes two promoters that are activated at distinct stages of development and respond to different extracellular cAMP conditions. One promoter is active with low-level oscillations of cAMP; exposure to high cAMP concentrations will repress this promoter and induce a second promoter. The CAR1 mRNAs are alternatively spliced but encode identical proteins. Thus, through differential sensitivity to its own ligand, cAMP, two promoters and alternative splicing regulate CAR1 expression during Dictyostelium development.

Regulation of Dictyostelium morphogenesis by cAMP-dependent protein kinase

During formation of the Dictyostelium slug extracellular cAMP signals direct the differentiation of prespore cells and DIF, a chlorinated hexaphenone, induces the differentiation of prestalk cells. At culmination the slug transforms into a fruiting body, composed of a stalk supporting a ball of spores. A dominant inhibitor of cAMP-dependent protein kinase (PKA) expressed under the control of a prestalk-specific promoter blocks the differentiation of prestalk cells into stalk cells. Analysis of a gene specifically expressed in stalk cells suggests that PKA acts to remove a repressor that prevents the premature induction of stalk cell differentiation by DIF during slug migration. PKA is also necessary for the morphogenetic movement of prestalk cells at culmination. Expression of the PKA inhibitor under control of a prespore-specific promoter blocks the accumulation of prespore mRNA sequences and prevents terminal spore cell differentiation. Thus PKA is essential for progression along both pathways of terminal differentiation but with different mechanisms of action. On the stalk cell pathway it acts to regulate the action of DIF while on the spore cell pathway PKA itself seems to act as the inducer of spore cell maturation. Ammonia, the extracellular signal which regulates the entry into culmination, acts by controlling the intracellular concentration of cAMP and thus exerts its effects via PKA. The fact that PKA is necessary for both prespore and spore gene expression leads us to postulate the existence of a signalling mechanism which converts the progressive rise in cAMP concentration during development into discrete, PKA-regulated gene activation events.

Cloning and functional expression of a brain G-protein-coupled ATP receptor

A cDNA encoding a novel member of the G-protein-coupled receptor (GCR) superfamily, an ATP receptor, has been isolated from an embryonic chick whole brain cDNA library by hybridization screening. The encoded protein has a sequence of 362 amino acids (41 kDa) and shares no more than 27% amino acid identity with any known GCR. When expressed as a complementary RNA (cRNA) in Xenopus oocytes a slowly-developing inward current was observed in response to application of ATP. The pharmacology of this expressed protein defines it as a P2Y purinoceptor.

A G-protein beta-subunit is essential for Dictyostelium development

Recent studies have demonstrated that G-protein-linked signal transduction pathways play a significant role in the developmental program of the simple eukaryotic organism Dictyostelium. We have reported previously the isolation of a G-protein beta-subunit and present here a more complete analysis of this gene. Low-stringency Southern blots and RFLP mapping studies suggest that the beta-subunit is a unique gene found on linkage group II. Its deduced amino acid sequence of 347 residues is approximately 60% identical to those of the human, Drosophila, and Caenorhabditis elegans beta-subunits. The carboxy-terminal 300 residues are about 70% identical; the amino-terminal 50 residues are quite divergent, containing only 10 identities. At all stages of growth and development, a single 1.9-kb beta-subunit mRNA is present at a high level, and a specific antibody detects a single 37-kD protein. We propose that G-protein heterotrimers are formed when this beta-subunit couples with each of the eight distinct G-protein alpha-subunits that are transiently expressed during development. Targeted disruption of the beta-subunit gene had no effect on the viability of haploid cells, but resulted in the inability of cells to aggregate.

The surface cyclic AMP receptors, cAR1, cAR2, and cAR3, promote Ca2+ influx in Dictyostelium discoideum by a G alpha 2-independent mechanism

Activation of surface folate receptors or cyclic AMP (cAMP) receptor (cAR) 1 in Dictyostelium triggers within 5-10 s an influx of extracellular Ca2+ that continues for 20 s. To further characterize the receptor-mediated Ca2+ entry, we analyzed 45Ca2+ uptake in amoebas overexpressing cAR2 or cAR3, cARs present during multicellular development. Both receptors induced a cAMP-dependent Ca2+ uptake that had comparable kinetics, ion selectivity, and inhibitor profiles as folate- and cAR1-mediated Ca2+ uptake. Analysis of mutants indicated that receptor-induced Ca2+ entry does not require G protein alpha subunits G alpha 1, G alpha 2, G alpha 3, G alpha 4, G alpha 7, or G alpha 8. Overexpression of cAR1 or cAR3 in g alpha 2- cells did not restore certain G alpha 2-dependent events, such as aggregation, or cAMP-mediated activation of adenylate and guanylate cyclases, but these strains displayed a cAMP-mediated Ca2+ influx with kinetics comparable to wild-type aggregation-competent cells. These results suggest that a plasma membrane-associated Ca(2+)-influx system may be activated by at least four distinct chemoreceptors during Dictyostelium development and that the response may be independent of G proteins.

CAR2, a prestalk cAMP receptor required for normal tip formation and late development of Dictyostelium discoideum

Extracellular cAMP serves as a primary signaling molecule to regulate the development of Dictyostelium discoideum. It is required for chemotaxis, aggregation, cytodifferentiation, and morphogenetic movement. The receptors for cAMP are members of the family of cell-surface receptors that are linked to G proteins and characterized by seven putative transmembrane domains. Previously, we have isolated the gene for the cAMP receptor subtype 1 (CAR1) from Dictyostelium and suggested that several genes related to CAR1 were present in the genome. Here, we describe a family of cAMP receptor genes of Dictyostelium and the isolation and function of the gene for the cAMP receptor subtype 2, CAR2. CAR2 is structurally similar to CAR1. Overall, their transmembrane and loop domains are approximately 75% identical in amino acid sequence; however, their carboxyl termini are quite dissimilar; CAR2 possesses homopolymeric runs of histidines and asparagines that are absent from the corresponding region in CAR1. Although CAR1 is maximally expressed during the early stages of development, CAR2 is expressed only after cells have aggregated and, then, preferentially in prestalk cells. Transgenic Dictyostelium that have had their wild-type CAR2 gene replaced by a defective copy using homologous recombination proceed through early development but are detained at the tight mound stage. CAR2 may be required for cAMP-directed sorting of prestalk cells during pattern formation within the aggregation mound. Furthermore, although prestalk genes are expressed normally in aggregates that lack CAR2, they exhibit an enhanced expression of prespore-specific mRNA. Previously, we had shown that there was a requirement for CAR1 during early development. The present results demonstrate that the multiple responses of Dictyostelium to cAMP are regulated by distinct cAMP receptors that are encoded by unique genes.

Identification and targeted gene disruption of cAR3, a cAMP receptor subtype expressed during multicellular stages of Dictyostelium development

Extracellular cAMP acts through cell-surface receptors to coordinate the developmental program of Dictyostelium. A cAMP receptor (cAR1), which is expressed during early aggregation, has been cloned and sequenced previously. We have identified a new receptor subtype, cAR3, that has approximately 56% and 69% amino acid identity with cAR1 and cAR2, respectively. cAR1, cAR2, or cAR3 expressed from plasmid in growing Dictyostelium cells can be photoaffinity labeled with 8-N3[32P]cAMP and phosphorylated when stimulated with cAMP. cAR3 RNA was not present during growth but appeared during late aggregation. Its expression peaked at 9 hr and then fell to a reduced level that was maintained until culmination. The expression of cAR3 protein followed a similar pattern, but with a 3-hr lag, and reached a maximum at the mound stage. In contrast, cAR1 protein was expressed predominantly during early aggregation and at low levels during later stages. At their respective peaks of expression, there were approximately 5 x 10(3) cAR3 sites per cell compared with approximately 7 x 10(4) cAR2 sites per cell. The cAR3 gene was disrupted by homologous recombination in several different parental cell lines. Surprisingly, the car3- cell lines display no obvious phenotype.

Systematic analysis of antisense RNA inhibition of myosin II heavy-chain gene expression in Dictyostelium discoideum

The powerful potential of antisense nucleotide inhibition of gene expression is presently being exploited in many biological systems. Although use of the technique is widespread, little is known about the variables that contribute to experimental success. We sought to define those variables that affect inhibition of myosin II heavy-chain (MIIHC) gene expression by stable nuclear-derived antisense RNAs in Dictyostelium discoideum. Different fragments of the MIIHC gene cloned in the antisense orientation into several transformation vectors were introduced into cells, and the accumulation of MIIHC protein and mRNA was examined. Inhibition of expression ranged from slight to virtually complete, and depended only on the specific gene fragment used to generate the antisense RNA. Fragments of the 3' end of the gene were the most effective and resulted in almost complete inhibition, whereas 5' fragments gave very little reduction. The severity of the morphological phenotypes associated with MIIHC depletion reflected the different levels of MIIHC in the transformants. Important implications for the design of antisense vectors suggested by these results are discussed.

Chemotactic antagonists of cAMP inhibit Dictyostelium phospholipase C

In Dictyostelium discoideum extracellular cAMP induces chemotaxis via a transmembrane signal transduction cascade consisting of surface cAMP receptors, G-proteins and effector enzymes including adenylyl cyclase, guanylyl cyclase and phospholipase C. Previously it was demonstrated that some cAMP derivatives such as 3′-deoxy-3′-aminoadenosine 3′:5′-monophosphate (3′NH-cAMP) bind to the receptor and induce normal activation of adenylyl cyclase and guanylyl cyclase. However these analogues do not induce chemotaxis, probably because the signal is transduced in an inappropriate manner. We have now studied the regulation of phospholipase C by cAMP and these chemotactic antagonists. cAMP induced the two-fold activation of phospholipase C leading to a transient increase of Ins(1,4,5)P3 levels. In contrast, the analogues induced a rapid decrease of intracellular Ins(1,4,5)P3 levels, due to the inhibition of phospholipase C activity. In a transformed cell-line lacking the G-protein that mediates phospholipase C inhibition, 3′NH-cAMP did not decrease phospholipase C activity and was no longer an antagonist of chemotaxis. These results suggest that inhibition of phospholipase C leads to aberrant chemotaxis.

Platelet activating factor modulates signal transduction in Dictyostelium

During development, Dictyostelium discoideum cells produce platelet activating factor (PAF). When cells are stimulated with external cAMP pulses, PAF is transiently synthesized. To determine whether PAF is involved in signal transduction, we have tested the effect of PAF on some cellular responses which are regulated by cAMP, such as spontaneous light-scattering oscillations of suspended cells, cAMP relay, transient increases of cGMP level, and extracellular calcium uptake. Our results show that PAF specifically interferes with spontaneous spike-shaped oscillations, without affecting sinusoidal ones. PAF increases the amplitude of a spike, but has no effects on its phase or frequency. When cells fail to oscillate spontaneously, PAF does not induce spikes; however, if administered together with cAMP, it amplifies the light-scattering response to cAMP. Amplification of light-scattering changes is accompanied by a threefold increase in the concentration levels of both cellular cAMP and cGMP. Extracellular Ca2+ uptake is also stimulated by PAF. This latter response is independent of endogenous or exogenously added cAMP. All these effects are specific for the naturally occurring R-enantiomer of PAF, the S-enantiomer and lyso-PAF being inactive. These results suggest that PAF modulates signal transduction in Dictyostelium, probably by interacting with an intracellular acceptor, which is involved in the pathways regulating membrane Ca2+ channels, adenylate and guanylate cyclase.

Eukaryotic-like protein serine/threonine kinases in Myxococcus xanthus, a developmental bacterium exhibiting social behavior

Myxococcus xanthus, a gram-negative bacterium exhibits a spectacular life cycle and social behavior. Its developmental cycle and multicellular morphogenesis resemble those of eukaryotic slime molds such as Dictyostelium discoideum. On the basis of this resemblance, we explored the existence of eukaryotic-like protein serine/threonine kinases which are known to play important roles in signal transduction during development of D. discoideum. It was indeed found that M. xanthus contains a large family of protein serine/threonine kinases related to the eukaryotic enzymes. This is the first unambiguous demonstration of eukaryotic-like serine/threonine kinases in the prokaryotes.

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

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

Cloning, characterization, and expression of a human calcitonin receptor from an ovarian carcinoma cell line

A human ovarian small cell carcinoma line (BIN-67) expresses abundant calcitonin (CT) receptors (CTR) (143,000 per cell) that are coupled, to adenylate cyclase. The dissociation constants (Kd) for the CTRs on these BIN-67 cells is approximately 0.42 nM for salmon CT and approximately 4.6 nM for human CT. To clone a human CTR (hCTR), a BIN-67 cDNA library was screened using a cDNA probe from a porcine renal CTR (pCTR) that we recently cloned. One positive clone of 3,588 bp was identified. Transfection of this cDNA into COS cells resulted in expression of receptors with high affinity for salmon CT (Kd = approximately 0.44 nM) and for human CT (Kd = approximately 5.4 nM). The expressed hCTR was coupled to adenylate cyclase. Northern analysis with the hCTR cDNA probe indicated a single transcript of approximately 4.2 kb. The cloned cDNA encodes a putative peptide of 490 amino acids with seven potential transmembrane domains. The amino acid sequence of the hCTR is 73% identical to the pCTR, although the hCTR contains an insert of 16 amino acids between transmembrane domain I and II. The structural differences may account for observed differences in binding affinity between the porcine renal and human ovarian CTRs. The CTRs are closely related to the receptors for parathyroid hormone-parathyroid hormone-related peptide and secretin; these receptors comprise a distinct family of G protein-coupled seven transmembrane domain receptors. Interestingly, the hCTR sequence is remotely related to the cAMP receptor of Dictyostelium discoideum (21% identical), but is not significantly related to other G protein-coupled receptor sequences now in the data bases.

Multiple cyclic AMP receptors are linked to adenylyl cyclase in Dictyostelium

cAMP receptor 1 and G-protein alpha-subunit 2 null cell lines (car1- and g alpha 2-) were examined to assess the roles that these two proteins play in cAMP stimulated adenylyl cyclase activation in Dictyostelium. In intact wild-type cells, cAMP stimulation elicited a rapid activation of adenylyl cyclase that peaked in 1-2 min and subsided within 5 min; in g alpha 2- cells, this activation did not occur; in car1- cells an activation occurred but it rose and subsided more slowly. cAMP also induced a persistent activation of adenylyl cyclase in growth stage cells that contain only low levels of cAMP receptor 1 (cAR1). In lysates of untreated wild-type, car1-, or g alpha 2- cells, guanosine 5'-O-'(3-thiotriphosphate) (GTP gamma S) produced a similar 20-fold increase in adenylyl cyclase activity. Brief treatment of intact cells with cAMP reduced this activity by 75% in control and g alpha 2- cells but by only 8% in the car1- cells. These observations suggest several conclusions regarding the cAMP signal transduction system. 1) cAR1 and another cAMP receptor are linked to activation of adenylyl cyclase in intact cells. Both excitation signals require G alpha 2. 2) cAR1 is required for normal adaptation of adenylyl cyclase. The adaptation reaction caused by cAR1 is not mediated via G alpha 2. 3) Neither cAR1 nor G alpha 2 is required for GTP gamma S-stimulation of adenylyl cyclase in cell lysates. The adenylyl cyclase is directly coupled to an as yet unidentified G-protein.

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.

Clathrin heavy chain is required for pinocytosis, the presence of large vacuoles, and development in Dictyostelium

To investigate the intracellular role of the clathrin heavy chain in living cells, we have used "antisense" RNA to engineer mutant Dictyostelium discoideum cells that are severely deficient in clathrin heavy chain expression. Immunoblots stained with an anti-clathrin heavy chain antiserum revealed that mutant cells contained undetectable amounts of clathrin heavy chain protein. Similarly, Northern blots showed an absence of clathrin heavy chain mRNA. Clathrin heavy chain-deficient Dictyostelium cells were viable, but exhibited growth rates twofold slower than parental cells. Whereas many morphological features of the mutant cells were normal, mutant cells lacked coated pits and coated vesicles. Clathrin-deficient cells were also missing large translucent vacuoles that serve as endosomes and contractile vacuoles. In the absence of clathrin heavy chain, mutant cells displayed three distinct functional defects: (a) impairment in endocytosis of fluid phase markers, but competence in another endocytic pathway, the phagocytosis of solid particles; (b) defects in osmoregulation; and (c) inability to complete the starvation-induced development cycle.