A chemoattractant receptor controls development in Dictyostelium discoideum

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

A novel Dictyostelium Cdk8 is required for aggregation, but is dispensable for growth

When Dictyostelium cells starve, they express genes necessary for aggregation. Using insertional mutagenesis, we have isolated a mutant that does not aggregate upon starvation and that forms small plaques on bacterial lawns, thus indicating slow growth. Sequencing of the mutated locus showed a strong similarity to the catalytic domain of cdc2-related kinase genes. Phylogenetic analysis further indicated that the amino acid sequence was more close to cyclin-dependent kinase 8 than to the sequence of other cyclin-dependent kinases. Thus, we designated this gene as Ddcdk8. The Ddcdk8-null cells do not aggregate and grow somewhat more slowly than parental cells when being shaken in axenic medium or laid on bacterial plates. To confirm whether these defective phenotypes were caused by disruption of this gene, the Ddcdk8-null cells were complemented with DdCdk8 protein expressed from an endogenous promoter, but not an actin promoter, and when the complemented cells were then allowed to grow on a bacterial lawn, they began to aggregate as the food supply was depleted and finally became fruiting bodies. The results suggest that properly regulated DdCdk8 activity is essential for aggregation. Because, when starved, Ddcdk8-null cells do not express the acaA transcripts required for aggregation, we deduce that Ddcdk8 is epistatic for acaA expression, indicating that the DdCdk8 products may regulate expression of acaA and/or other genes.

Protein kinase A-independent activation of ERK and H,K-ATPase by cAMP in native kidney cells: role of Epac I

This study aimed at determining the signaling pathways underlying calcitonin- and isoproterenol-induced stimulation of H,K-ATPase in rat renal collecting duct. H,K-ATPase activity was determined in microdissected collecting ducts preincubated with or without either specific inhibitors or antibodies directed against intracellular signaling proteins. Transient cell membrane permeabilization with streptolysin-O allowed intracellular access of antibodies. The stimulation of H,K-ATPase by calcitonin and isoproterenol was mimicked by cAMP analogues and was abolished by adenylyl cyclase inhibition. Protein kinase A inhibition abolished isoproterenol but not calcitonin effect on H,K-ATPase. Calcitonin increased the phosphorylation of extracellular signal-regulated kinase (ERK) in a protein kinase A-independent manner, and the inhibition of the ERK phosphorylation prevented the stimulation of H,K-ATPase by calcitonin. Antibodies directed against either the cAMP-activated guanine-nucleotide exchange factor Epac I, the monomeric G protein Rap-1 or the kinase Raf-B, curtailed the stimulation of H,K-ATPase by calcitonin, whereas antibodies against the related monomeric G protein Ras or kinase Raf-1 had no effect. In conclusion, calcitonin stimulates H,K-ATPase through a cAMP/Epac I/Rap-1/Raf-B/ERK cascade.

Extracellular cAMP inhibits proximal reabsorption: are plasma membrane cAMP receptors involved?

Glucagon binding to hepatocytes has been known for a long time to not only stimulate intracellular cAMP accumulation but also, intriguingly, induce a significant release of liver-borne cAMP in the blood. Recent experiments have shown that the well-documented but ill-understood natriuretic and phosphaturic actions of glucagon are actually mediated by this extracellular cAMP, which inhibits the reabsorption of sodium and phosphate in the renal proximal tubule. The existence of this "pancreato-hepatorenal cascade" indicates that proximal tubular reabsorption is permanently influenced by extracellular cAMP, the concentration of which is most probably largely dependent on the insulin-to-glucagon ratio. The possibility that renal cAMP receptors may be involved in this process is supported by the fact that cAMP has been shown to bind to brush-border membrane vesicles. In other cell types (i.e., adipocytes, erythrocytes, glial cells, cardiomyocytes), cAMP eggress and/or cAMP binding have also been shown to occur, suggesting additional paracrine effects of this nucleotide. Although not yet identified in mammals, cAMP receptors (cARs) are already well characterized in lower eukaryotes. The amoeba Dictyostelium discoideum expresses four different cARs during its development into a multicellular organism. cARs belong to the superfamily of seven transmembrane domain G protein-coupled receptors and exhibit a modest homology with the secretin receptor family (which includes PTH receptors). However, the existence of specific cAMP receptors in mammals remains to be demonstrated. Disturbances in the pancreato-hepatorenal cascade provide an adequate pathophysiological understanding of several unexplained observations, including the association of hyperinsulinemia and hypertension, the hepatorenal syndrome, and the hyperfiltration of diabetes mellitus. The observations reviewed in this paper show that cAMP should no longer be regarded only as an intracellular second messenger but also as a first messenger responsible for coordinated hepatorenal functions, and possibly for paracrine regulations in several other tissues.

A putative serpentine receptor gene tasA required for normal morphogenesis of primary stalk and branch structure in Polysphondylium pallidum

The fruiting body of Polysphondylium pallidum is composed of whorls of branches along the axis of a primary stalk. In the course of fruiting body formation, the interval between neighboring whorls and the number and the spacing of branches in a whorl are highly regulated. In this study, using restriction enzyme mediated integration mutagenesis, we have obtained a mutant (strain M6226) with thicker and aberrant primary stalk. The gene responsible for the mutant phenotype, confirmed by homologous recombination, encodes an open reading frame with 383 aa residues (46.3 kDa) and was named thick and aberrant stalk A (tasA). TasA is highly homologous to Dictyostelium discoideum cyclic adenosine 3',5'-monophosphate receptors. A tasA transcript is expressed strictly at the late aggregation stage. Cells expressing a tasA::gfp fusion DNA are localized at the posterior region of the primary sorogen where secondary sorogens and branches originate. This result indicates the existence of 'prebranch' and 'pretrunk' regions in P. pallidum instead of the prespore and prestalk regions in D. discoideum. The analyzes of the gene disruptant and chimeric fruiting bodies also suggests that TasA affects the normal morphogenesis of the primary stalk and the process of cell differentiation into prebranch cells, but not into spore or stalk cells directly.

Adenylyl cyclase functions downstream of the Galpha protein Gpa1 and controls mating and pathogenicity of Cryptococcus neoformans

The signaling molecule cyclic AMP (cAMP) is a ubiquitous second messenger that enables cells to detect and respond to extracellular signals. cAMP is generated by the enzyme adenylyl cyclase, which is activated or inhibited by the Galpha subunits of heterotrimeric G proteins in response to ligand-activated G-protein-coupled receptors. Here we identified the unique gene (CAC1) encoding adenylyl cyclase in the opportunistic fungal pathogen Cryptococcus neoformans. The CAC1 gene was disrupted by transformation and homologous recombination. In stark contrast to the situation for Saccharomyces cerevisiae, in which adenylyl cyclase is essential, C. neoformans cac1 mutant strains were viable and had no vegetative growth defect. Furthermore, cac1 mutants maintained the yeast-like morphology of wild-type cells, in contrast to the constitutively filamentous phenotype found upon the loss of adenylyl cyclase in another basidiomycete pathogen, Ustilago maydis. Like C. neoformans mutants lacking the Galpha protein Gpal, cac1 mutants were mating defective and failed to produce two inducible virulence factors: capsule and melanin. As a consequence, cac1 mutant strains were avirulent in animal models of cryptococcal meningitis. Reintroduction of the wild-type CAC1 gene or the addition of exogenous cAMP suppressed cac1 mutant phenotypes. Moreover, the overexpression of adenylyl cyclase restored mating and virulence factor production in gpal mutant strains. Physiological studies revealed that the Galpha protein Gpa1 and adenylyl cyclase controlled cAMP production in response to glucose, and no cAMP was detectable in extracts from cac1 or gpa1 mutant strains. These findings provide direct evidence that Gpal and adenylyl cyclase function in a conserved signal transduction pathway controlling cAMP production, hyphal differentiation, and virulence of this human fungal pathogen.

Functional segregation of the highly conserved basic motifs within the third endoloop of the human secretin receptor

In this study, a mutagenesis-based strategy was employed to assess the roles of two highly conserved motifs (KLR and RLAR) within the third endoloop of the human secretin receptor. Block deletion of KLRT and mutation of Lys323 (K(323)I) significantly reduced cAMP accumulation, and these mutations did not affect ligand interaction and receptor number expressed on the cell surface. Thus, the KLRT region at the N terminus of the third endoloop, particularly Lys323, is important for G protein coupling. For the RLAR motif, receptors with substitutions at positions 339 and 342 from Arg to Ala (R(339, 342)A), Glu (R(339, 342)E), or Ile (R(339, 342)I) as well as block deletion of the RLAR motif were all found to be defective in both secretin-binding and cAMP production. Interestingly, a single mutation at the corresponding positions of Arg339 or Arg342 responded as the wild-type human secretin receptor in all functional assays, indicating that the presence of one Arg at either position within the RLAR motif is sufficient for a normal receptor function. Immunofluorescent staining of these mutant receptors showed that these Arg residues are responsible for surface presentation and/or receptor stability.

Detection of calmodulin-binding proteins and calmodulin-dependent phosphorylation linked to calmodulin-dependent chemotaxis to folic and cAMP in Dictyostelium

Calmodulin (CaM) antagonists, trifluoperazine (TFP) or calmidazolium (R24571), dose-dependently inhibited cAMP and folic acid (FA) chemotaxis in Dictyostelium. Developing, starved, and refed cells were compared to determine if certain CaM-binding proteins (CaMBPs) and CaM-dependent phosphorylation events could be identified as potential downstream effectors. Recombinant CaM ([35S]VU-1-CaM) gel overlays coupled with cell fractionation revealed at least three dozen Ca(2+)-dependent and around 12 Ca(2+)-independent CaMBPs in Dictyostelium. The CaMBPs associated with early development were also found in experimentally starved cells (cAMP chemotaxis), but were different for the CaMBP population linked to growth-phase cells (FA chemotaxis). Probing Western blots with phosphoserine antibodies revealed several phosphoprotein bands that displayed increases when cAMP-responsive cells were treated with TFP. In FA-responsive cells, several but distinct phosphoproteins decreased when treated with TFP. These data show that unique CaMBPs are present in growing, FA-chemosensitive cells vs. starved cAMP-chemoresponsive cells that may be important for mediating CaM-dependent events during chemotaxis.

CRTF is a novel transcription factor that regulates multiple stages of Dictyostelium development

During aggregation, Dictyostelium establish nanomolar oscillation waves of extracellular cAMP, but as development progresses, cells become responsive to higher, non-fluctuating concentrations of cAMP. The regulation of the promoter responsible for expression of cAMP receptor subtype 1, CAR1, during aggregation reflects these signaling variations. Transcription of CAR1 from the early, aggregation promoter is activated by cAMP pulsing, but is repressed by continuous exposure to micromolar concentrations of cAMP. Deletion and mutation analyses of this promoter had defined an element essential for cAMP-regulated expression, and mobility shift assay, DNA crosslinking and DNase I footprinting experiments had identified a nuclear protein (CRTF) with zinc-dependent sequence binding specificity. In our study, CRTF was purified to homogeneity, peptides were sequenced and full-length cDNAs were obtained. The deduced CRTF protein is ∼100 kDa with a C-terminal, zinc finger-like motif required for DNA binding; CRTF purified from cells, however, represents only a 40 kDa C-terminal fragment that retains DNA-binding activity.

As might have been predicted if CRTF were essential for the regulation of CAR1, crtf-null strains fail to develop under standard conditions or to exhibit induced expression of CAR1 or other cAMP-regulated genes. Furthermore, crtf-nulls also fail to sporulate, even under conditions that bypass the dependence on early cAMP signaling pathways. In addition, early developmental events of crtf-null strains could be rescued with exogenous cAMP treatment, constitutive expression of CAR1 or co-development with wild-type cells; however, these treatments were insufficient to promote sporulation. This suggests a cell-autonomous role for CRTF during late development that is separate from its capacity to control CAR1 expression. Finally, ablation of CRTF promotes a precocious induction of certain cAMP-dependent gene expression pathways. We suggest that CRTF may function to help insulate distinct pathways from simultaneous and universal activation by cAMP. CRTF, thus, exhibits multiple complex and independent regulatory functions during Dictyostelium development.

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

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

A cell number-counting factor regulates group size in Dictyostelium by differentially modulating cAMP-induced cAMP and cGMP pulse sizes

A secreted counting factor (CF), regulates the size of Dictyostelium discoideum fruiting bodies in part by regulating cell-cell adhesion. Aggregation and the expression of adhesion molecules are mediated by relayed pulses of cAMP. Cells also respond to cAMP with a short cGMP pulse. We find that CF slowly down-regulates the cAMP-induced cGMP pulse by inhibiting guanylyl cyclase activity. A 1-min exposure of cells to purified CF increases the cAMP-induced cAMP pulse. CF does not affect the cAMP receptor or its interaction with its associated G proteins or the translocation of the cytosolic regulator of adenylyl cyclase to the membrane in response to cAMP. Pulsing streaming wild-type cells with a high concentration of cAMP results in the formation of small groups, whereas reducing cAMP pulse size with exogenous cAMP phosphodiesterase during stream formation causes cells to form large groups. Altering the extracellular cAMP pulse size does not phenocopy the effects of CF on the cAMP-induced cGMP pulse size or cell-cell adhesion, indicating that CF does not regulate cGMP pulses and adhesion via CF's effects on cAMP pulses. The results suggest that regulating cell-cell adhesion, the cGMP pulse size, or the cAMP pulse size can control group size and that CF regulates all three of these independently.

Evolutionary relationships among G protein-coupled receptors using a clustered database approach

Guanine nucleotide-binding protein-coupled receptors (GPCRs) comprise large and diverse gene families in fungi, plants, and the animal kingdom. GPCRs appear to share a common structure with 7 transmembrane segments, but sequence similarity is minimal among the most distant GPCRs. To reevaluate the question of evolutionary relationships among the disparate GPCR families, this study takes advantage of the dramatically increased number of cloned GPCRs. Sequences were selected from the National Center for Biotechnology Information (NCBI) nonredundant peptide database using iterative BLAST (Basic Local Alignment Search Tool) searches to yield a database of approximately 1700 GPCRs and unrelated membrane proteins as controls, divided into 34 distinct clusters. For each cluster, separate position-specific matrices were established to optimize sequence comparisons among GPCRs. This approach resulted in significant alignments between distant GPCR families, including receptors for the biogenic amine/peptide, VIP/secretin, cAMP, STE3/MAP3 fungal pheromones, latrophilin, developmental receptors frizzled and smoothened, as well as the more distant metabotrobic glutamate receptors, the STE2/MAM2 fungal pheromone receptors, and GPR1, a fungal glucose receptor. On the other hand, alignment scores between these recognized GPCR clades with p40 (putative GPCR) and pm1 (putative GPCR), as well as bacteriorhodopsins, failed to support a finding of homology. This study provides a refined view of GPCR ancestry and serves as a reference database with hyperlinks to other sources. Moreover, it may facilitate database annotation and the assignment of orphan receptors to GPCR families.

Folate reception by vegetative Dictyostelium discoideum amoebae: distribution of receptors and trafficking of ligand

We report the first explicit demonstration of post-reception processing of a Dictyostelium chemoattractant. Folic acid stimulates reorganization of the cytoskeleton of vegetative amoebae of D. discoideum. In particular, folic acid is a potent chemoattractant and it causes enlargement of the filopodial array. The distribution of folic acid receptors and the fate of bound folate were investigated by presenting an agonist consisting of the conjugate, folic acid-lactalbumin-FITC (Folate*), to these amoebae. This novel probe was specifically bound to folic acid receptors of these amoebae and it stimulated chemotaxis and enlargement of their filopodial array. Hence, Folate* is a physiologically competent probe. The probe sans-folate moiety was not bound anywhere to living or fixed amoebae. Since Folate* did not bind to amoebae after incubation with equimolar folic acid, this probe is a receptor-specific agonist. We report here the first description, by confocal visualization of a competent agonist, of the distribution of folate receptors of D. discoideum vegetative amoebae and of the fate of this ligand. Examination of fixed amoebae revealed that bound Folate* was distributed generally over their entire surface including their filopodia. However, in living amoebae, Folate* was bound only at the cell body and this bound Folate* was almost completely internalized as concentrated packets into vacuoles. This endocytosis of the probe and the clustering of endocytosed Folate* is consistent with receptor-mediated internalization of a ligand. Possible routes for internalization of the folate probe and the implications of this endocytosis for signal molecule processing and temporal sensing are discussed.

A novel role of differentiation-inducing factor-1 in Dictyostelium development, assessed by the restoration of a developmental defect in a mutant lacking mitogen-activated protein kinase ERK2

It has been previously reported that the differentiating wild-type cells of Dictyostelium discoideum secrete a diffusible factor or factors that are able to rescue the developmental defect in the mutant lacking extracellular signal-regulated kinase 2 (ERK2), encoded by the gene erkB. In the present study, it is demonstrated that differentiation-inducing factor-1 (DIF-1) for stalk cells can mimic the role of the factor(s) and the mechanism of the action of DIF-1 in the erkB null mutant is also discussed. The mutant usually never forms multicellular aggregates, because of its defect in cyclic adenosine monophosphate (cAMP) signaling. In the presence of 100 nM DIF-1, however, the mutant cells formed tiny slugs, which eventually developed into small fruiting bodies. In contrast, DIF-1 never rescued the developmental arrest of other Dictyostelium mutants lacking adenylyl cyclase A (ACA), cAMP receptors cAR1 and cAR3, heterotrimeric G-protein, the cytosolic regulator of ACA, or the catalytic subunit of cAMP-dependent protein kinase (PKA-C). Most importantly, it was found that DIF-1 did not affect the cellular cAMP level, but rather elevated the transcriptional level of pka during the development of erkB null cells. These results suggest that DIF-1 may rescue the developmental defect in erkB null cells via the increase in PKA activity, thus giving the first conclusive evidence that DIF-1 plays a crucial role in the early events of Dictyostelium development as well as in prestalk and stalk cell induction.

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

8-Substituted cAMP analogues reveal marked differences in adaptability, hydrogen bonding, and charge accommodation between homologous binding sites (AI/AII and BI/BII) in cAMP kinase I and II

cAMP analogues, systematically substituted at position 8 of the adenine moiety (C8), were tested quantitatively for binding to each cAMP interaction site (A and B) of the regulatory subunits of cAMP-dependent protein kinase type I (RI) and II (RII). Site AII did not accommodate cAMP analogues with any bulk at position 8, whereas site AI accepted even bulky 8-substituents. This implies that the narrow, buried pocket of site AI facing position C8 of cAMP in the RI-cAMP crystal [Su, Y., Dostmann, W. R., Herberg, F. W., Durick, K., Xuong, N. H., Ten Eyck, L., Taylor, S. S., and Varughese, K. I. (1995) Science 269, 807-813] must undergo considerable conformational change and still support high-affinity cAMP analogue binding. The B sites of RI and RII differed in three respects. First, site BI had a lower affinity than site BII for cAMP analogues with hydrophobic, bulky 8-substituents. Second, site BI had a preference for substituents with hydrogen bonding donor potential close to C8, whereas site BII had a preference for substituents with hydrogen bonding acceptor potential. This implies that Tyr(371) of RI and the homologous Tyr(379) of RII differ in their hydrogen bonding preference. Third, site BI preferred analogues with a positively charged amino group that was an extended distance from C8, whereas site BII discriminated against a positive charge. The combined results allow refinement of the cAMP binding site geometry of RI and RII in solution, and suggest design of improved isozyme-specific cAMP analogues.

Amoeboid movement anchored by eupodia, new actin-rich knobby feet in Dictyostelium

To date, protrusion of pseudopodia has been considered to be primarily responsible for translocation of free-living amoebae and leukocytes of higher organisms. Although there is little question that the pseudopodium plays an important role, little attention has been given to the cortical structures that are responsible for cell-substratum anchorage in amoeboid movement. Here, we report on a new knobby foot-like structure in amoebae of a cellullar slime mold, Dictyostelium discoideum. These feet, each about 1 micron in diameter, appear transiently in multiple units at the base of certain pseudopodia where the amoeba contacts a partially deformable substrate. The feet were discovered, and their spatial and temporal behavior relative to pseudopodial anchorage and invasive locomotion were observed, by examining Dictyostelium amoebae using a DIC video microscope providing an 0.3 micron depth of field. Key evidence for the anchoring role of the knobby feet was obtained by investigating amoebae, flattened in a specially devised observation chamber, and attracted by chemotaxis towards 3',5' cyclic-adenosine monophosphate (cAMP). The cAMP was released by highly localized, pulsed UV-microbeam irradiation of caged cAMP. We show by indirect immunofluorescence that the knobby feet contain a high concentration of filamentous (F-) actin, myoB (a member of Dictyostelium myosin-I family), and alpha-actinin (an actin-binding protein). Interestingly, myoB exhibits a circular disposition around each foot. Neither myosin-II (conventional myosin) nor the 269 kD protein, which has been recently identified as a talin homologue of Dictyostelium [Kreitmeier et al., 1995: J. Cell Biol. 129:179-188], are concentrated at the feet. We propose that the knobby feet provide anchorage to the substratum needed by lamellipodia to exert projectile forces for invading narrow spaces or otherwise for a flattened amoeba to secure itself to the deformable substratum. Some forms of adhesion plaques in higher organisms such as "podosomes" or "invadopodia" may perform functions similar to the knobby feet, but appear to differ in life time, cytoskeletal organization and composition. We have named the knobby foot "eupodium."

Integration of signaling networks that regulate Dictyostelium differentiation

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

Regulation of cell-fate determination in Dictyostelium

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

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.

Functional promiscuity of gene regulation by serpentine receptors in Dictyostelium discoideum

Serpentine receptors such as smoothened and frizzled play important roles in cell fate determination during animal development. In Dictyostelium discoideum, four serpentine cyclic AMP (cAMP) receptors (cARs) regulate expression of multiple classes of developmental genes. To understand their function, it is essential to know whether each cAR is coupled to a specific gene regulatory pathway or whether specificity results from the different developmental regulation of individual cARs. To distinguish between these possibilities, we measured gene induction in car1 car3 double mutant cell lines that express equal levels of either cAR1, cAR2, or cAR3 under a constitutive promoter. We found that all cARs efficiently mediate both aggregative gene induction by cAMP pulses and induction of postaggregative and prespore genes by persistent cAMP stimulation. Two exceptions to this functional promiscuity were observed. (i) Only cAR1 can mediate adenosine inhibition of cAMP-induced prespore gene expression, a phenomenon that was found earlier in wild-type cells. cAR1's mediation of adenosine inhibition suggests that cAR1 normally mediates prespore gene induction. (ii) Only cAR2 allows entry into the prestalk pathway. Prestalk gene expression is induced by differentiation-inducing factor (DIF) but only after cells have been prestimulated with cAMP. We found that DIF-induced prestalk gene expression is 10 times higher in constitutive cAR2 expressors than in constitutive cAR1 or cAR3 expressors (which still have endogenous cAR2), suggesting that cAR2 mediates induction of DIF competence. Since in wild-type slugs cAR2 is expressed only in anterior cells, this could explain the so far puzzling observations that prestalk cells differentiate at the anterior region but that DIF levels are actually higher at the posterior region. After the initial induction of DIF competence, cAMP becomes a repressor of prestalk gene expression. This function can again be mediated by cAR1, cAR2, and cAR3.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Dictyostelium TRFA homologous to yeast Ssn6 is required for normal growth and early development

The TPR (tetratricopeptide repeat) family became widespread during evolution, having been found from bacteria to mammals. By means of restriction enzyme-mediated integration, we have identified a Dictyostelium gene (trfA) highly homologous to a Saccharomyces cerevisiae gene encoding a TPR protein, Ssn6 (Cyc8), which functions as a global transcriptional repressor for diverse genes. The deduced amino acid sequence of the Dictyostelium gene product, TRFA, contains 10 consecutive TPR units as well as Gln repeats, Asn repeats, and a region rich in Glu, Lys, Ser, and Thr. The sequences of some of the 10 TPR units in TRFA are more than 70% identical to the corresponding units in Ssn6. The trfA- cells produced smooth plaques on a bacterial lawn and failed to aggregate normally when starved on a plain agar plate. Individual trfA- cells also failed to correctly respond to cAMP, although the adenylyl cyclase of trfA- cells was expressed upon starvation and activated by stimulation with cAMP as in the wild-type cells. When cultured in a rich medium in suspension, they grew more slowly and stopped growing at a lower density than the wild-type cells. Furthermore, they divided into cells of various sizes and tended to be much smaller than the wild-type cells. These pleiotropic defects of the trfA- cells suggest the possibility that Dictyostelium TRFA may regulate the transcription of diverse genes required for normal growth and early development.

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.

Role of PKA in the timing of developmental events in Dictyostelium cells

The cyclic AMP (cAMP)-dependent protein kinase, PKA, is dispensable for growth of Dictyostelium cells but plays a variety of crucial roles in development. The catalytic subunit of PKA is inhibited when associated with its regulatory subunit but is activated when cAMP binds to the regulatory subunit. Deletion of pkaR or overexpression of the gene encoding the catalytic subunit, pkaC, results in constitutive activity. Development is independent of cAMP in strains carrying these genetic alterations and proceeds rapidly to the formation of both spores and stalk cells. However, morphogenesis is aberrant in these mutants. In the wild type, PKA activity functions in a circuit that can spontaneously generate pulses of cAMP necessary for long-range aggregation. It is also essential for transcriptional activation of both prespore and prestalk genes during the slug stage. During culmination, PKA functions in both prespore and prestalk cells to regulate the relative timing of terminal differentiation. A positive feedback loop results in the rapid release of a signal peptide, SDF-2, when prestalk cells are exposed to low levels of SDF-2. The signal transduction pathway that mediates the response to SDF-2 in both prestalk and prespore cells involves the two-component system of DhkA and RegA. When the cAMP phosphodiesterase RegA is inhibited, cAMP accumulates and activates PKA, leading to vacuolation of stalk cells and encapsulation of spores. These studies indicate that multiple inputs regulate PKA activity to control the relative timing of differentiations in 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.

[Use of a G-protein-coupled receptor to communicate. An evolutionary success]

Among membrane-bound receptors, the seven transmembrane receptors are the most abundant (several thousand, 1% of the genome). They were the most successful during evolution. They are capable of transducing messages as different as photons, organic odorants, nucleotides, nucleosides, peptides, lipids, proteins, etc. They are catalysts of the GDP/GTP nucleotide exchange on heterotrimeric G proteins. They are therefore also called 'G-protein-coupled receptors' (GPCR). G proteins are composed of three subunits, G alpha and two undissociable subunits, G beta gamma. There are at least three families of GPCR showing no sequence similarity. Among G proteins, some have been crystallized (including under the heterotrimeric form) and their structure as well as their activation mechanisms are well known. The structures of GPCR are less known owing to the difficulty in crystallizing membrane-bound proteins. Indirect studies (mutations, 2D crystallization of rhodopsine, molecular modelling, etc.) lead to a useful model of the 'central core' composed of the seven transmembrane domains and of its structural modifications during activation. The intimate contact zones between GPCR and G proteins include, on the GPCR side, domains of intracellular loops and C-terminal, which are specific for each family and on the G protein side, essentially the N- et C-terminal domains plus the alpha 4-beta 6 loop. GPCR can adopt several 'active' conformations some of them being found in mutated receptors responsible for pathologies.

YakA, a protein kinase required for the transition from growth to development in Dictyostelium

When Dictyostelium cells starve they arrest their growth and induce the expression of genes necessary for development. We have identified and characterized a protein kinase, YakA, that is essential for the proper regulation of both events. Amino acid sequence and functional similarities indicate that YakA is a homolog of Yak1p, a growth-regulating protein kinase in S. cerevisiae. Purified YakA expressed in E. coli is able to phosphorylate myelin basic protein. YakA-null cells are smaller and their cell cycle is accelerated relative to wild-type cells. When starved, YakA-null cells fail to decrease the expression of the growth-stage gene cprD, and do not induce the expression of genes required for the earliest stages of development. YakA mRNA levels increase during exponential growth and reach a maximum at the point of starvation, consistent with a role in mediating starvation responses. YakA mRNA also accumulates when cells are grown in medium conditioned by cells grown to high density, suggesting that yakA expression is under the control of an extracellular signal that accumulates during growth. Expression of yakA from a conditional promoter causes cell-cycle arrest in nutrient-rich medium and promotes developmental events, such as the expression of genes required for cAMP signaling. YakA appears to regulate the transition from growth to development in Dictyostelium.

A novel Myb homolog initiates Dictyostelium development by induction of adenylyl cyclase expression

Dictyostelium development is induced by starvation. The adenylyl cyclase gene ACA is one of the first genes expressed upon starvation. ACA produces extracellular cAMP that induces chemotaxis, aggregation, and differentiation in neighboring cells. Using insertional mutagenesis we have isolated a mutant that does not aggregate upon starvation but is rescued by adding extracellular cAMP. Sequencing of the mutated locus revealed a new gene, DdMYB2, whose product contains three Myb repeats, the DNA-binding motif of Myb-related transcription factors. Ddmyb2-null cells show undetectable levels of ACA transcript and no cAMP production. Ectopic expression of ACA from a constitutive promotor rescues differentiation and morphogenesis of Ddmyb2-null mutants. The results suggest that development in Dictyostelium starts by starvation-mediated DdMyb2 activation, which induces adenylyl cyclase activity producing the differentiation-inducing signal cAMP.

Switching of chemoattractant receptors programs development and morphogenesis in Dictyostelium: receptor subtypes activate common responses at different agonist concentrations

One of the common functional features among G-protein coupled receptors is the occurrence of multiple subtypes involved in similar signal transduction events. The cAMP chemoattractant receptor family of Dictyostelium discoideum is composed of four receptors (cAR1-cAR4), which are expressed sequentially throughout the developmental transition from a unicellular to a multicellular organism. The receptors differ in affinity for cAMP and in the sequences of their C-terminal domains. In this study, we constitutively expressed cAR1, cAR2, and cAR3 as well as a series of chimeric and mutant receptors and assessed the capacity of each to mediate chemotaxis, activation of adenylyl cyclase and actin polymerization, and rescue the developmental defect of car1-/car3- cells. We found that various receptors and mutants sense different concentration ranges of cAMP but all can mediate identical responses during the aggregation stage of development. The responses displayed very similar kinetics, suggesting no major differences in regulatory properties attributable to the C-terminal domains. We speculate that switching of receptor subtypes during development enables the organism to respond to the changing concentrations of the chemoattractant and thereby program morphogenesis appropriately.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

The R33 G protein-coupled receptor gene of rat cytomegalovirus plays an essential role in the pathogenesis of viral infection

We have identified a rat cytomegalovirus (RCMV) gene that encodes a G-protein-coupled receptor (GCR) homolog. This gene (R33) belongs to a family that includes the human cytomegalovirus UL33 gene. R33 was found to be transcribed during the late phase of RCMV infection in rat embryo fibroblasts. Unlike the mRNAs from all the other members of the UL33 family that have been studied to date, the R33 mRNA is not spliced. To study the function of the R33 gene, we constructed an RCMV strain in which the R33 open reading frame is disrupted. The mutant strain (RCMV deltaR33) did not show differences in replication from wild-type RCMV upon infection of several rat cell types in vitro. However, marked differences were seen between the mutant and wild-type strain in the pathogenesis of infection in immunocompromised rats. First, the mutant strain induced a significantly lower mortality than the wild-type virus did. Second, in contrast to wild-type RCMV, the mutant strain did not efficiently replicate in the salivary gland epithelial cells of immunocompromised rats. Although viral DNA was detected in salivary glands of RCMV deltaR33-infected rats up to 14 days postinfection, it could not be detected at later time points. This indicates that although the strain with R33 deleted is probably transported to the salivary glands in a similar fashion to that for wild-type virus, the mutant virus is not able to either enter or replicate in salivary gland epithelial cells. We conclude that the RCMV R33 gene plays a vital role in the pathogenesis of infection.

Immunochemical visualization and quantitation of cyclic AMP in single cells

Adenosine 3':5'-cyclic monophosphate (cAMP) is a key second messenger in signaling pathways governing many cellular processes. To define the subcellular localization and relative abundance of cAMP, we developed a novel immunochemical approach based on acrolein fixation to visualize cAMP within cells. We describe here the fixation and immobilization of cAMP within cells and the production of specific, high titer polyclonal antibodies that recognize cAMP. Relative levels of cAMP immunofluorescence were quantitated in glial cells (oligodendrocytes, astrocytes, Schwann cells, and glioma cells) that were either untreated or treated with activators of endogenous adenylyl cyclase to raise cAMP levels. In treated cells, cAMP immunofluorescence is strongly localized in the perinuclear cytoplasm.

A novel cytosolic regulator, Pianissimo, is required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase in Dictyostelium

Genetic analysis was applied to identify novel genes involved in G protein-linked pathways controlling development. Using restriction enzyme-mediated integration (REMI), we have identified a new gene, Pianissimo (PiaA), involved in cAMP signaling in Dictyostelium discoideum. PiaA encodes a 130-kD cytosolic protein required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase. In piaA- null mutants, neither chemoattractant stimulation of intact cells nor GTPgammaS treatment of lysates activates the enzyme; constitutive expression of PiaA reverses these defects. Cytosols of wild-type cells that contain Pia protein reconstitute the GTPgammaS stimulation of adenylyl cyclase activity in piaA- lysates, indicating that Pia is directly involved in the activation. Pia and CRAC, a previously identified cytosolic regulator, are both essential for activation of the enzyme as lysates of crac- piaA- double mutants require both proteins for reconstitution. Homologs of PiaA are found in Saccharomyces cerevisiae and Schizosaccaromyces pombe; disruption of the S. cerevisiae homolog results in lethality. We propose that homologs of Pia and similar modes of regulation of these ubiquitous G protein-linked pathways are likely to exist in higher eukaryotes.

A protein kinase C-like activity involved in the chemotactic response of Dictyostelium discoideum

During the developmental life cycle of the cellular slime mould Dictyostelium discoideum cells aggregate in response to pulses of extracellular cAMP. This chemotactic agent stimulates a number of signalling pathways in the cell including the activation of a phospholipase C activity leading to the transient generation of inositol 3,4,5-trisphosphate and diacylglycerol. The role of diacylglycerol in chemotactic response and development of Dictyostelium is not known. We have evidence to suggest that two protein kinase C-like enzymes exist in Dictyostelium due to the different cellular responses to two inhibitors specific for protein kinase C. One enzyme is preferentially sensitive to D-erythro-sphingosine, a diacylglycerol analogue, and is required for growth. A second is preferentially inhibited by bisindolylmaleimide GF109203X and is required for chemotaxis. We have identified protein kinase C-like kinase activity in Dictyostelium cell extracts which appears as the cells aggregate. This activity is stimulated by diacylglycerol, especially biologically relevant diacylglycerol species, and phosphorylates a peptide substrate which is an efficient substrate for mammalian protein kinase Cs. This activity is a candidate for the effector of diacylglycerol generated during the aggregative phase of Dictyostelium development and defines a role for diacylglycerol in the chemotactic response.

Control of cell-type specific gene expression in Dictyostelium by the general transcription factor GBF

To understand how positional information within an organism specifies patterning during development, we are analyzing spatially regulated gene expression in Dictyostelium. CAR3 is a member of the cAMP, 7-span receptor family which directs the transition from unicellular to multicellular organism and regulates cellular differentiation and pattern formation. CAR3 mRNA is expressed maximally at 8–10 hours of development, as individual cells aggregate and differentiate, and is accumulated to equivalent levels in all cells. CAR3 is also induced in shaking cultures by response to extracellular cAMP. We now show, by extensive mutagenesis, that the maximum length of contiguous sequences required for accurate spatiotemporal regulation of CAR3 is approx. 350 bp. These sequences include three significant elements located in upstream and transcribed regions. Arrays of G-boxes (GBF regulatory sites) are centered near positions −165 and +50 and, although either is sufficient for induction by cAMP and expression in prespore cells, both are required for expression in prestalk cells. Another GC-rich element near position −80 is required for maximal expression of prespore-specific constructs, although full-length promoters carrying clustered mutations through the −80 region are still expressed in all cells, but with slightly reduced expression. Spatiotemporal expression of CAR3 during development, thus, requires cell-specific combinatorial interactions of multiple but redundant regulatory components. These essential elements are located in upstream and transcribed regions. However, most surprisingly, a primary control for spatial patterning of CAR3 expression appears to be mediated by GBF, a general transcription factor expressed ubiquitously during Dictyostelium development following early aggregation.

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

Synthesis of the Ca(2+)-dependent cell adhesion molecule DdCAD-1 is regulated by multiple factors during Dictyostelium development

In Dictyostelium discoideum, the cadA gene encodes the cell adhesion molecule DdCAD-1, a protein of M(r) 24,000, which mediates Ca(2+)-dependent cell-cell adhesion during development. We have examined the effects of cAMP, cell-cell contact, and growth conditions on cadA expression. cadA has a unique pattern of expression, which appears to be a combination of the expression patterns of early genes and aggregation-stage genes. Expression of the cadA gene in bacterially grown cells is activated at the beginning of the developmental cycle, followed by a period of rapid DdCAD-1 accumulation. The mRNA level reaches its maximum at 9 h of development and then declines to the basal level at approximately 18 h, while the protein level remains constant after reaching its maximum at 12 h. Pulse-chase experiments have demonstrated that DdCAD-1 has a significantly longer half-life than the average cellular protein. Transcription of the cadA gene is stimulated by exogenous cAMP pulses, leading to a 3- to 5-fold increase in the transcription rate. In the fgdA mutant, which lacks a functional G alpha 2, cAMP fails to enhance cadA expression, suggesting that cAMP stimulates cadA transcription via a G protein-dependent pathway. However, inhibition of cell-cell contact has no effect on the synthesis of DdCAD-1. Growth conditions also have a major influence on cadA expression. Axenically grown cells produce a high level of cadA transcripts during vegetative growth. The mRNA level shows a steady decrease during development and is reduced to the basal level by 12 h. In contrast, the level of DdCAD-1 remains relatively high throughout development, suggesting that axenic growth affects the accumulation of cadA mRNA but not the stability of the protein. These results indicate that multiple mechanisms are involved to maintain a high level of DdCAD-1 during development.

Dictyostelium development in the absence of cAMP

Adenosine 3',5'-monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) are regulators of development in many organisms. Dictyostelium uses cAMP as an extracellular chemoattractant and as an intracellular signal for differentiation. Cells that are mutant in adenylyl cyclase do not develop. Moderate expression of the catalytic subunit of PKA in adenylyl cyclase-null cells led to near-normal development without detectable accumulation of cAMP. These results suggest that all intracellular cAMP signaling is effected through PKA and that signals other than extracellular cAMP coordinate morphogenesis in Dictyostelium.

Antisense-mediated regulation of Annexin VII gene expression during the transition from growth to differentiation in Dictyostelium discoideum

Annexin VII is believed to be required for proper Ca(2+)-homeostasis in Dictyostelium discoideum cells. As was previously reported, the expression of Annexin VII gene increased during the transition of D. discoideum Ax-2 cells from growth to differentiation. We have casually cloned an interesting gene, Quit3, by the differential plaque hybridization. Quit3 had no coding region, and was expressed more predominantly in the growth phase than in the differentiation phase. Unexpectedly, this gene was found to encode the complementary sequence of Annexin VII. Therefore, it is most likely that the Quit3 mRNA may regulate the Annexin VII synthesis by the natural antisense transcript via an antisense RNA-RNA interaction, thus resulting in striking increase of Annexin VII production in the phase-shift of cells from growth to differentiation. Since Annexin VII is known to be coded for by a single gene in Dictyostelium, the antisense RNA seemed to be encoded in the same genetic locus as the Annexin VII mRNA.

Random mutagenesis of the cAMP chemoattractant receptor, cAR1, of Dictyostelium. Evidence for multiple states of activation

cAMP receptor 1 (cAR1) of Dictyostelium couples to the G protein G2 to mediate activation of adenylyl and guanylyl cyclases, chemotaxis, and cell aggregation. Other cAR1-dependent events, including receptor phosphorylation and influx of extracellular Ca2+, do not require G proteins. To further characterize signal transduction through cAR1, we performed random mutagenesis of the third intracellular loop (24 amino acids), since the corresponding region of other seven helix receptors has been implicated in the coupling to G proteins. Mutant receptors were expressed in car1(-) cells and were characterized for G protein-dependent and -independent signal transduction. Our results demonstrate that cAR1 is remarkably tolerant to amino acid substitutions in the third intracellular loop. Of the 21 positions where amino acid substitutions were observed, one or more replacements were found that retained full biological function. However, certain alterations resulted in receptors with reduced ability to bind cAMP and/or transduce signals. There were specific signal transduction mutants that could undergo cAMP-dependent cAR1 phosphorylation but were impaired either in coupling to G proteins, in G protein-independent Ca2+ influx, or in both pathways. In addition, there were general activation mutants that failed to restore aggregation to car1(-) cells and displayed severe defects in all signal transduction events, including the most robust response, cAMP-dependent cAR1 phosphorylation. Certain of these mutant phenotypes were obtained in a complementary study, where the entire region of cAR1 from the third to the seventh transmembrane helices was randomly mutagenized. Considered together, these studies indicate that the activation cycle of cAR1 may involve a number of distinct receptor intermediates. A model of G protein-dependent and -independent signal transduction through cAR1 is discussed.

Random mutagenesis of the cAMP chemoattractant receptor, cAR1, of Dictyostelium. Mutant classes that cause discrete shifts in agonist affinity and lock the receptor in a novel activational intermediate

The cAMP chemoattractant receptor, cAR1, of Dictyostelium transduces extracellular cAMP signals via G protein-dependent and G protein-independent mechanisms. While site-directed mutagenesis studies of G protein-coupled receptors have provided a host of information regarding the domains essential for various functions, many mechanistic and structural questions remain to be resolved. We therefore carried out polymerase chain reaction-mediated random mutagenesis over a large part of the cAR1 sequence (from TMIII through the proximal part of the cytoplasmic tail). We devised a rapid screen for loss-of-function mutations based on the essential role of cAR1 in the developmental program of Dictyostelium. Although there were an average of two amino acid substitutions per receptor, approximately 90% of the mutants were able to substitute for wild-type cAR1 when expressed in receptor null cells. About 2% were loss-of-function mutants that expressed wild-type levels of receptor protein. We used biochemical screens to select about 100 of these mutants and chose eight representative mutants for extensive characterization. These fell into distinct classes. One class had a conditional defect in cAMP binding that was reversed by high salt. Another large class had decreased affinity under all conditions. Curiously, the decreases were clustered into three discrete intervals. One of the most interesting class of mutants lost all capacity for signal transduction but was phosphorylated in response to agonist binding. This latter finding suggests that there are at least two activated states of cAR1 that can be recognized by different downstream effectors.

Dictyostelium discoideum cells lacking the 34,000-dalton actin-binding protein can grow, locomote, and develop, but exhibit defects in regulation of cell structure and movement: a case of partial redundancy

Cells lacking the Dictyostelium 34,000-D actin-bundling protein, a calcium-regulated actin cross-linking protein, were created to probe the function of this polypeptide in living cells. Gene replacement vectors were constructed by inserting either the UMP synthase or hygromycin resistance cassette into cloned 4-kb genomic DNA containing sequences encoding the 34-kD protein. After transformation and growth under appropriate selection, cells lacking the protein were analyzed by PCR analyses on genomic DNA, Northern blotting, and Western blotting. Cells lacking the 34-kD protein were obtained in strains derived from AX2 and AX3. Growth, pinocytosis, morphogenesis, and expression of developmentally regulated genes is normal in cells lacking the 34-kD protein. In chemotaxis studies, 34-kD- cells were able to locomote and orient normally, but showed an increased persistence of motility. The 34-kD- cells also lost bits of cytoplasm during locomotion. The 34-kD- cells exhibited either an excessive number of long and branched filopodia, or a decrease in filopodial length and an increase in the total number of filopodia per cell depending on the strain. Reexpression of the 34-kD protein in the AX2-derived strain led to a "rescue" of the defect in the persistence of motility and of the excess numbers of long and branched filopodia, demonstrating that these defects result from the absence of the 34-kD protein. We explain the results through a model of partial functional redundancy. Numerous other actin cross-linking proteins in Dictyostelium may be able to substitute for some functions of the 34-kD protein in the 34-kD cells. The observed phenotype is presumed to result from functions that cannot be adequately supplanted by a substitution of another actin cross-linking protein. We conclude that the 34-kD actin-bundling protein is not essential for growth, but plays an important role in dynamic control of cell shape and cytoplasmic structure.