Seven helix chemoattractant receptors transiently stimulate mitogen-activated protein kinase in Dictyostelium. Role of heterotrimeric G proteins

An integrated, cross-regulation pathway model involving activating/adaptive and feed-forward/feed-back loops for directed oscillatory cAMP signal-relay/response during the development of Dictyostelium

Self-organized and excitable signaling activities play important roles in a wide range of cellular functions in eukaryotic and prokaryotic cells. Cells require signaling networks to communicate amongst themselves, but also for response to environmental cues. Such signals involve complex spatial and temporal loops that may propagate as oscillations or waves. When Dictyostelium become starved for nutrients, cells within a localized space begin to secrete cAMP. Starved cells also become chemotactic to cAMP. cAMP signals propagate as outwardly moving waves that oscillate at ∼6 min intervals, which creates a focused territorial region for centralized cell aggregation. Proximal cells move inwardly toward the cAMP source and relay cAMP outwardly to recruit additional cells. To ensure directed inward movement and outward cAMP relay, cells go through adapted and de-adapted states for both cAMP synthesis/degradation and for directional cell movement. Although many immediate components that regulate cAMP signaling (including receptors, G proteins, an adenylyl cyclase, phosphodiesterases, and protein kinases) are known, others are only inferred. Here, using biochemical experiments coupled with gene inactivation studies, we model an integrated large, multi-component kinetic pathway involving activation, inactivation (adaptation), re-activation (re-sensitization), feed-forward, and feed-back controls to generate developmental cAMP oscillations.

Atypical MAP kinases - new insights and directions from amoeba

Mitogen-activated protein kinases (MAPKs) have been the focus of many studies over the past several decades, but the understanding of one subgroup of MAPKs, orthologs of MAPK15, known as atypical MAPKs, has lagged behind others. In most organisms, specific activating signals or downstream responses of atypical MAPK signaling pathways have not yet been identified even though these MAPKs are associated with many eukaryotic processes, including cancer and embryonic development. In this Review, we discuss recent studies that are shedding new light on both the regulation and function of atypical MAPKs in different organisms. In particular, the analysis of the atypical MAPK in the amoeba Dictyostelium discoideum has revealed important roles in chemotactic responses and gene regulation. The rapid and transient phosphorylation of the atypical MAPK in these responses suggest a highly regulated activation mechanism in vivo despite the ability of atypical MAPKs to autophosphorylate in vitro. Atypical MAPK function can also impact the activation of other MAPKs in amoeba. These advances are providing new perspectives on possible MAPK roles in animals that have not been previously considered, and this might lead to the identification of potential targets for regulating cell movement in the treatment of diseases.

An atypical MAPK regulates translocation of a GATA transcription factor in response to chemoattractant stimulation

The Dictyostelium atypical mitogen-activated protein kinase (MAPK) Erk2 is required for chemotactic responses to cAMP as amoeba undergo multicellular development. In this study, Erk2 was found to be essential for the cAMP-stimulated translocation of the GATA transcription factor GtaC as indicated by the distribution of a GFP-GtaC reporter. Erk2 was also found to be essential for the translocation of GtaC in response to external folate, a foraging signal that directs the chemotaxis of amoeba to bacteria. Erk1, the only other Dictyostelium MAPK, was not required for the GtaC translocation to either chemoattractant, indicating that GFP-GtaC is a kinase translocation reporter specific for atypical MAPKs. The translocation of GFP-GtaC in response to folate was absent in mutants lacking the folate receptor Far1 or the coupled G-protein subunit Gα4. Loss of GtaC function resulted in enhanced chemotactic movement to folate, suggesting that GtaC suppresses responses to folate. The alteration of four Erk2-preferred phosphorylation sites in GtaC impacted the translocation of GFP-GtaC in response to folate and the GFP-GtaC-mediated rescue of aggregation and development of gtaC- cells. The ability of different chemoattractants to stimulate Erk2-regulated GtaC translocation suggests that atypical MAPK-mediated regulation of transcription factors can contribute to different cell fates.

MAPK docking motif in the Dictyostelium Gα2 subunit is required for aggregation and transcription factor translocation

Some G protein alpha subunits contain a mitogen-activated protein kinase (MAPK) docking motif (D-motif) near the amino terminus that can impact cellular responses to external signals. The Dictyostelium Gα2 G protein subunit is required for chemotaxis to cAMP during the onset of multicellular development and this subunit contains a putative D-motif near the amino terminus. The Gα2 subunit D-motif was altered to examine its potential role in chemotaxis and multicellular development. In gα2^- cells the expression of the D-motif mutant (Gα2^D-) or wild-type subunit from high copy number vectors rescued cell aggregation but blocked the transition of mounds into slugs. This phenotype was also observed in parental strains with a wild-type gα2 locus indicating that the heterologous Gα2 subunit expression interferes with multicellular morphogenesis. Expression of the Gα2^D- subunit from a low copy number vectors in gα2^- cells did not rescue aggregation whereas the wild-type Gα2 subunit rescued aggregation efficiently and allowed wild-type morphological development. The Gα2^D- and Gα2 subunit were both capable of restoring comparable levels of cAMP stimulated motility and the ability to co-aggregate with wild-type cells implying that the aggregation defect of Gα2^D- expressing cells is due to insufficient intercellular signaling. Expression of the Gα2 subunit but not the Gα2^D- subunit fully restored the ability of cAMP to stimulate the translocation of the GtaC transcription factor suggesting the D-motif is important for transcription factor regulation. These results suggest that the D-motif of Gα2 plays a role in aggregation and other developmental responses involved with cAMP signaling.

Cell-substrate adhesion drives Scar/WAVE activation and phosphorylation by a Ste20-family kinase, which controls pseudopod lifetime

The Scar/WAVE complex is the principal catalyst of pseudopod and lamellipod formation. Here we show that Scar/WAVE's proline-rich domain is polyphosphorylated after the complex is activated. Blocking Scar/WAVE activation stops phosphorylation in both Dictyostelium and mammalian cells, implying that phosphorylation modulates pseudopods after they have been formed, rather than controlling whether they are initiated. Unexpectedly, phosphorylation is not promoted by chemotactic signaling but is greatly stimulated by cell:substrate adhesion and diminished when cells deadhere. Phosphorylation-deficient or phosphomimetic Scar/WAVE mutants are both normally functional and rescue the phenotype of knockout cells, demonstrating that phosphorylation is dispensable for activation and actin regulation. However, pseudopods and patches of phosphorylation-deficient Scar/WAVE last substantially longer in mutants, altering the dynamics and size of pseudopods and lamellipods and thus changing migration speed. Scar/WAVE phosphorylation does not require ERK2 in Dictyostelium or mammalian cells. However, the MAPKKK homologue SepA contributes substantially-sepA mutants have less steady-state phosphorylation, which does not increase in response to adhesion. The mutants also behave similarly to cells expressing phosphorylation-deficient Scar, with longer-lived pseudopods and patches of Scar recruitment. We conclude that pseudopod engagement with substratum is more important than extracellular signals at regulating Scar/WAVE's activity and that phosphorylation acts as a pseudopod timer by promoting Scar/WAVE turnover.

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

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

Multiple phosphorylation sites on the RegA phosphodiesterase regulate Dictyostelium development

In Dictyostelium, the intracellular cAMP-specific phosphodiesterase RegA is a negative regulator of cAMP-dependent protein kinase (PKA), a key determinant in the timing of developmental morphogenesis and spore formation. To assess the role of protein kinases in the regulation of RegA function, this study identified phosphorylation sites on RegA and characterized the role of these modifications through the analysis of phospho-mimetic and phospho-ablative mutations. Mutations affecting residue T676 of RegA, a presumed target of the atypical MAP kinase Erk2, altered the rate of development and impacted cell distribution in chimeric organisms suggesting that phosphorylation of this residue reduces RegA function and regulates cell localization during multicellular development. Mutations affecting the residue S142 of RegA also impacted the rate developmental morphogenesis but in a manner opposite of changes at T676 suggesting the phosphorylation of the S142 residue increases RegA function. Mutations affecting residue S413 residue altered aggregate sizes and delayed developmental progression suggesting that PKA operates in a negative feedback mechanism to increase RegA function. These results suggest that the phosphorylation of different residues on RegA can lead to increased or decreased RegA function and therefore in turn regulate developmental processes such as aggregate formation, cell distribution, and the kinetics of developmental morphogenesis.

Dictyostelium Erk2 is an atypical MAPK required for chemotaxis

The Dictyostelium genome encodes only two MAPKs, Erk1 and Erk2, and both are expressed during growth and development. Reduced levels of Erk2 expression have been shown previously to restrict cAMP production during development but still allow for chemotactic movement. In this study the erk2 gene was disrupted to eliminate Erk2 function. The absence of Erk2 resulted in a complete loss of folate and cAMP chemotaxis suggesting that this MAPK plays an integral role in the signaling mechanisms involved with this cellular response. However, folate stimulation of early chemotactic responses, such as Ras and PI3K activation and rapid actin filament formation, were not affected by the loss of Erk2 function. The erk2^- cells had a severe defect in growth on bacterial lawns but assays of bacterial cell engulfment displayed only subtle changes in the rate of bacterial engulfment. Only cells with no MAPK function, erk1^-erk2^- double mutants, displayed a severe proliferation defect in axenic medium. Loss of Erk2 impaired the phosphorylation of Erk1 in secondary responses to folate stimulation indicating that Erk2 has a role in the regulation of Erk1 activation during chemotaxis. Loss of the only known Dictyostelium MAPK kinase, MekA, prevented the phosphorylation of Erk1 but not Erk2 in response to folate and cAMP confirming that Erk2 is not regulated by a conventional MAP2K. This lack of MAP2K phosphorylation of Erk2 and the sequence similarity of Erk2 to mammalian MAPK15 (Erk8) suggest that the Dictyostelium Erk2 belongs to a group of atypical MAPKs. MAPK activation has been observed in chemotactic responses in a wide range of organisms but this study demonstrates an essential role for MAPK function in chemotactic movement. This study also confirms that MAPKs provide critical contributions to cell proliferation.

The Dictyostelium GSK3 kinase GlkA coordinates signal relay and chemotaxis in response to growth conditions

GSK3 plays a central role in orchestrating key biological signaling pathways, including cell migration. Here, we identify GlkA as a GSK3 family kinase with functions that overlap with and are distinct from those of GskA. We show that GlkA, as previously shown for GskA, regulates the cell's cytoskeleton through MyoII assembly and control of Ras and Rap1 function, leading to aberrant cell migration. However, there are both qualitative and quantitative differences in the regulation of Ras and Rap1 and their downstream effectors, including PKB, PKBR1, and PI3K, with glkA^- cells exhibiting a more severe chemotaxis phenotype than gskA^- cells. Unexpectedly, the severe glkA^- phenotypes, but not those of gskA^-, are only exhibited when cells are grown attached to a substratum but not in suspension, suggesting that GlkA functions as a key kinase of cell attachment signaling. Using proteomic iTRAQ analysis we show that there are quantitative differences in the pattern of protein expression depending on the growth conditions in wild-type cells. We find that GlkA expression affects the cell's proteome during vegetative growth and development, with many of these changes depending on whether the cells are grown attached to a substratum or in suspension. These changes include key cytoskeletal and signaling proteins known to be essential for proper chemotaxis and signal relay during the aggregation stage of Dictyostelium development.

Acanthamoeba and Dictyostelium Use Different Foraging Strategies

Amoeba often use cell movement as a mechanism to find food, such as bacteria, in their environment. The chemotactic movement of the soil amoeba Dictyostelium to folate or other pterin compounds released by bacteria is a well-documented foraging mechanism. Acanthamoeba can also feed on bacteria but relatively little is known about the mechanism(s) by which this amoeba locates bacteria. Acanthamoeba movement in the presence of folate or bacteria was analyzed in above agar assays and compared to that observed for Dictyostelium. The overall mobility of Acanthamoeba was robust like that of Dictyostelium but Acanthamoeba did not display a chemotactic response to folate. In the presence of bacteria, Acanthamoeba only showed a marginal bias in directed movement whereas Dictyostelium displayed a strong chemotactic response. A comparison of genomes revealed that Acanthamoeba and Dictyostelium share some similarities in G protein signaling components but that specific G proteins used in Dictyostelium chemotactic responses were not present in current Acanthamoeba genome sequence data. The results of this study suggest that Acanthamoeba does not use chemotaxis as the primary mechanism to find bacterial food sources and that the chemotactic responses of Dictyostelium to bacteria may have co-evolved with chemotactic responses that facilitate multicellular development.

The Dictyostelium prestalk inducer differentiation-inducing factor-1 (DIF-1) triggers unexpectedly complex global phosphorylation changes

Differentiation-inducing factor-1 (DIF-1) is a polyketide that induces Dictyostelium amoebae to differentiate as prestalk cells. We performed a global quantitative screen for phosphorylation changes that occur within the first minutes after addition of DIF-1, using a triple-label SILAC approach. This revealed a new world of DIF-1-controlled signaling, with changes in components of the MAPK and protein kinase B signaling pathways, components of the actinomyosin cytoskeletal signaling networks, and a broad range of small GTPases and their regulators. The results also provide evidence that the Ca(2+)/calmodulin-dependent phosphatase calcineurin plays a role in DIF-1 signaling to the DimB prestalk transcription factor. At the global level, DIF-1 causes a major shift in the phosphorylation/dephosphorylation equilibrium toward net dephosphorylation. Of interest, many of the sites that are dephosphorylated in response to DIF-1 are phosphorylated in response to extracellular cAMP signaling. This accords with studies that suggest an antagonism between the two inducers and also with the rapid dephosphorylation of the cAMP receptor that we observe in response to DIF-1 and with the known inhibitory effect of DIF-1 on chemotaxis to cAMP. All MS data are available via ProteomeXchange with identifier PXD001555.

The Dictyostelium MAPK ERK1 is phosphorylated in a secondary response to early developmental signaling

Previous reports have suggested that the two mitogen-activated protein kinases (MAPKs) in Dictyostelium discoideum, ERK1 and ERK2, can be directly activated in response to external cAMP even though these MAPKs play different roles in the developmental life cycle. To better characterize MAPK regulation, the levels of phosphorylated MAPKs were analyzed in response to external signals. Only ERK2 was rapidly phosphorylated in response to the chemoattractants, cAMP and folate. In contrast, the phosphorylation of ERK1 occurred as a secondary or indirect response to these stimuli and this phosphorylation was enhanced by cell-cell interactions, suggesting that other external signals can activate ERK1. The phosphorylation of ERK1 or ERK2 did not require the function of the other MAPK in these responses. Folate stimulation of a chimeric population of erk1- and gα4- cells revealed that the phosphorylation of ERK1 could be mediated through an intercellular signal other than folate. Loss of ERK1 function suppressed the developmental delay and the deficiency in anterior cell localization associated with gα5- mutants suggesting that ERK1 function can be down regulated through Gα5 subunit-mediated signaling. However, no major changes in the phosphorylation of ERK1 were observed in gα5- cells suggesting that the Gα5 subunit signaling pathway does not regulate the phosphorylation of ERK1. These findings suggest that the activation of ERK1 occurs as a secondary response to chemoattractants and that other cell-cell signaling mechanisms contribute to this activation. Gα5 subunit signaling can down regulate ERK1 function to promote prestalk cell development but not through major changes to the level of phosphorylated ERK1.

Arrestins function in cAR1 GPCR-mediated signaling and cAR1 internalization in the development of Dictyostelium discoideum

Evolutionarily conserved arrestin-like proteins are key components of the cAR1-mediated ERK2 activation that controls cAMP cell–cell signaling during Dictyostelium aggregation. They are also involved in ligand-induced cAR1 internalization, which is required for the switch of cAMP receptors during multicellular development.

Oscillation of chemical signals is a common biological phenomenon, but its regulation is poorly understood. At the aggregation stage of Dictyostelium discoideum development, the chemoattractant cAMP is synthesized and released at 6-min intervals, directing cell migration. Although the G protein–coupled cAMP receptor cAR1 and ERK2 are both implicated in regulating the oscillation, the signaling circuit remains unknown. Here we report that D. discoideum arrestins regulate the frequency of cAMP oscillation and may link cAR1 signaling to oscillatory ERK2 activity. Cells lacking arrestins (adcB⁻C⁻) display cAMP oscillations during the aggregation stage that are twice as frequent as for wild- type cells. The adcB⁻C⁻ cells also have a shorter period of transient ERK2 activity and precociously reactivate ERK2 in response to cAMP stimulation. We show that arrestin domain–containing protein C (AdcC) associates with ERK2 and that activation of cAR1 promotes the transient membrane recruitment of AdcC and interaction with cAR1, indicating that arrestins function in cAR1-controlled periodic ERK2 activation and oscillatory cAMP signaling in the aggregation stage of D. discoideum development. In addition, ligand-induced cAR1 internalization is compromised in adcB⁻C⁻ cells, suggesting that arrestins are involved in elimination of high-affinity cAR1 receptors from cell surface after the aggregation stage of multicellular development.

Loss of cAMP-specific phosphodiesterase rescues spore development in G protein mutant in dictyostelium

Cyclic AMP (cAMP) is an important intracellular signaling molecule for many G protein-mediated signaling pathways but the specificity of cAMP signaling in cells with multiple signaling pathways is not well-understood. In Dictyostelium, at least two different G protein signaling pathways, mediated by the Gα2 and Gα4 subunits, are involved with cAMP accumulation, spore production, and chemotaxis and the stimulation of these pathways results in the activation of ERK2, a mitogen-activated protein kinase that can down regulate the cAMP-specific phosphodiesterase RegA. The regA gene was disrupted in gα2(−) and gα4(−) cells to determine if the absence of this phosphodiesterase rescues the development of these G protein mutants as it does for erk2(−) mutants. There gA(−) mutation had no major effects on developmental morphology but enriched the distribution of the Gα mutant cells to the prespore/prestalk border in chimeric aggregates. The loss of RegA function had no effect on Gα4- mediated folate chemotaxis. However, the regA gene disruption in gα4(−) cells, but not in gα2(−) cells, resulted in a substantial rescue and acceleration of spore production. This rescue in sporulation required cell autonomous signaling because the precocious sporulation could not be induced through intercellular signaling in chimeric aggregates. However, intercellular signals from regA(−) strains increased the expression of the prestalk gene ecmB and accelerated the vacuolization of stalk cells. Intercellular signaling from the gα4(−)regA(−) strain did not induce ecmA gene expression indicating cell-type specificity in the promotion of prestalk cell development. regA gene disruption in a Gα4(HC) (Gα4 overexpression) strain did not result in precocious sporulation or stalk cell development indicating that elevated Gα4 subunit expression can mask regA(−) associated phenotypes even when provided with wild-type intercellular signaling. These findings indicate that the Gα2 and Gα4-mediated pathways provide different contributions to the development of spores and stalk cells and that the absence of RegA function can bypass some but not all defects in G protein regulated spore development.

Cell signaling during development of Dictyostelium

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

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

The role of epidermal growth factor-like module containing mucin-like hormone receptor 2 in human cancers

G-protein coupled receptors (GPCRs) are among the most diverse and ubiquitous proteins in all of biology. The epidermal growth factor-seven span transmembrane (EGF-TM7) subfamily of adhesion GPCRs is a small subset whose members are mainly expressed on the surface of leukocytes. The EGF domains on the N-terminus add significant size to these receptors and they are considered to be among the largest members of the TM7 family. Although not all of their ligands or downstream targets have been identified, there is evidence implicating the EGF-TM7 family diverse processes such as cell adhesion, migration, inflammation, and autoimmune disease. Recent studies have identified expression of EGF-TM7 family members on human neoplasms including those of the thyroid, stomach, colon, and brain. Their presence on these tissues is not surprising given the ubiquity of GPCRs, but because their functional significance and pathways are not completely understood, they are of tremendous clinical and scientific interest. Current evidence suggests that expression of certain EGF-TM7 receptors is correlated with tumor grade, confers a more invasive phenotype, and increases the likelihood of metastatic disease. In this review, we will discuss the structure, function, and regulation of these receptors. We also describe the expression of these receptors in human cancers and explore their potential mechanistic significance.

Chemoattractant stimulation of TORC2 is regulated by receptor/G protein-targeted inhibitory mechanisms that function upstream and independently of an essential GEF/Ras activation pathway in Dictyostelium

Protein kinase TORC2 is regulated by Ras response to distinct stimulatory ligands. Cells insensitive to one chemoattractant for TORC2 activation remain fully responsive to other ligands. Receptor-specific inhibitory circuits in Dictyostelium are found upstream and independent of GEF/Ras and downstream, feedback, or feedforward responses.

Global stimulation of Dictyostelium with different chemoattractants elicits multiple transient signaling responses, including synthesis of cAMP and cGMP, actin polymerization, activation of kinases ERK2, TORC2, and phosphatidylinositide 3-kinase, and Ras-GTP accumulation. Mechanisms that down-regulate these responses are poorly understood. Here we examine transient activation of TORC2 in response to chemically distinct chemoattractants, cAMP and folate, and suggest that TORC2 is regulated by adaptive, desensitizing responses to stimulatory ligands that are independent of downstream, feedback, or feedforward circuits. Cells with acquired insensitivity to either folate or cAMP remain fully responsive to TORC2 activation if stimulated with the other ligand. Thus TORC2 responses to cAMP or folate are not cross-inhibitory. Using a series of signaling mutants, we show that folate and cAMP activate TORC2 through an identical GEF/Ras pathway but separate receptors and G protein couplings. Because the common GEF/Ras pathway also remains fully responsive to one chemoattractant after desensitization to the other, GEF/Ras must act downstream and independent of adaptation to persistent ligand stimulation. When initial chemoattractant concentrations are immediately diluted, cells rapidly regain full responsiveness. We suggest that ligand adaptation functions in upstream inhibitory pathways that involve chemoattractant-specific receptor/G protein complexes and regulate multiple response pathways.

Direct biochemical measurements of signal relay during Dictyostelium development

Upon starvation, individual Dictyostelium discoideum cells enter a developmental program that leads to collective migration and the formation of a multicellular organism. The process is mediated by extracellular cAMP binding to the G protein-coupled cAMP receptor 1, which initiates a signaling cascade leading to the activation of adenylyl cyclase A (ACA), the synthesis and secretion of additional cAMP, and an autocrine and paracrine activation loop. The release of cAMP allows neighboring cells to polarize and migrate directionally and form characteristic chains of cells called streams. We now report that cAMP relay can be measured biochemically by assessing ACA, ERK2, and TORC2 activities at successive time points in development after stimulating cells with subsaturating concentrations of cAMP. We also find that the activation profiles of ACA, ERK2, and TORC2 change in the course of development, with later developed cells showing a loss of sensitivity to the relayed signal. We examined mutants in PKA activity that have been associated with precocious development and find that this loss in responsiveness occurs earlier in these mutants. Remarkably, we show that this loss in sensitivity correlates with a switch in migration patterns as cells transition from streams to aggregates. We propose that as cells proceed through development, the cAMP-induced desensitization and down-regulation of cAMP receptor 1 impacts the sensitivities of chemotactic signaling cascades leading to changes in migration patterns.

MAPKs in development: insights from Dictyostelium signaling pathways

Mitogen activated protein kinases (MAPKs) play important roles in the development of eukaryotic organisms through the regulation of signal transduction pathways stimulated by external signals. MAPK signaling pathways have been associated with the regulation of cell growth, differentiation, and chemotaxis, indicating MAPKs contribute to a diverse set of developmental processes. In most eukaryotes, the diversity of external signals is likely to far exceed the diversity of MAPKs, suggesting that multiple signaling pathways might share MAPKs. Do different signaling pathways converge before MAPK function or can MAPKs maintain signaling specificity through interactions with specific proteins? The genetic and biochemical analysis of MAPK pathways in simple eukaryotes such as Dictyostelium offers opportunities to investigate functional specificity of MAPKs in G protein-mediated signal transduction pathways. This review considers the regulation and specificity of MAPK function in pathways that control Dictyostelium growth and development.

Expression of Y53A-actin in Dictyostelium disrupts the cytoskeleton and inhibits intracellular and intercellular chemotactic signaling

We showed previously that phosphorylation of Tyr(53), or its mutation to Ala, inhibits actin polymerization in vitro with formation of aggregates of short filaments, and that expression of Y53A-actin in Dictyostelium blocks differentiation and development at the mound stage (Liu, X., Shu, S., Hong, M. S., Levine, R. L., and Korn, E. D. (2006) Proc. Natl. Acad. Sci. U.S.A. 103, 13694-13699; Liu, X., Shu, S., Hong, M. S., Yu, B., and Korn, E. D. (2010) J. Biol. Chem. 285, 9729-9739). We now show that expression of Y53A-actin, which does not affect cell growth, phagocytosis, or pinocytosis, inhibits the formation of head-to-tail cell streams during cAMP-induced aggregation, although individual amoebae chemotax normally. We show that expression of Y53A-actin causes a 50% reduction of cell surface cAMP receptors, and inhibits cAMP-induced increases in adenylyl cyclase A activity, phosphorylation of ERK2, and actin polymerization. Trafficking of vesicles containing adenylyl cyclase A to the rear of the cell and secretion of the ACA vesicles are also inhibited. The actin cytoskeleton of cells expressing Y53A-actin is characterized by numerous short filaments, and bundled and aggregated filaments similar to the structures formed by copolymerization of purified Y53A-actin and wild-type actin in vitro. This disorganized actin cytoskeleton may be responsible for the inhibition of intracellular and intercellular cAMP signaling in cells expressing F-Y53A-actin.

G{alpha}5 subunit-mediated signalling requires a D-motif and the MAPK ERK1 in Dictyostelium

The Dictyostelium Galpha5 subunit has been shown to reduce cell viability, inhibit folate chemotaxis and accelerate tip morphogenesis and gene expression during multicellular development. Alteration of the D-motif (mitogen-activated protein kinase docking site) at the amino terminus of the Galpha 5 subunit or the loss of extracellular signal-regulated kinase (ERK)1 diminished the lethality associated with the overexpression or constitutive activation of the Galpha5 subunit. The amino-terminal D-motif of the Galpha5 subunit was also found to be necessary for the reduced cell size, small aggregate formation and precocious developmental gene expression associated with Galpha5 subunit overexpression. This D-motif also contributed to the aggregation delay in cells expressing a constitutively active Galpha5 subunit, but the D-motif was not necessary for the inhibition of folate chemotaxis. These results suggest that the amino-terminal D-motif is required for some but not all phenotypes associated with elevated Galpha5 subunit functions during growth and development and that ERK1 can function in Galpha5 subunit-mediated signal transduction.

MAP kinases have different functions in Dictyostelium G protein-mediated signaling

Extracellular signal regulated kinases (ERKs) are a class of MAP kinases that function in many signaling pathways in eukaryotic cells and in some cases, a single stimulus can activate more than one ERK suggesting functional redundancy or divergence from a common pathway. Dictyostelium discoideum encodes only two MAP kinases, ERK1 and ERK2, that both function during the developmental life cycle. To determine if ERK1 and ERK2 have overlapping functions, chemotactic and developmental phenotypes of erk1(-) and erk2(-) mutants were assessed with respect to G protein-mediated signal transduction pathways. ERK1 was specifically required for Galpha5-mediated tip morphogenesis and inhibition of folate chemotaxis but not for cAMP-stimulated chemotaxis or cGMP accumulation. ERK2 was the primary MAPK phosphorylated in response to folate or cAMP stimulation. Cell growth was not altered in erk1(-), erk2(-) or erk1(-)erk2(-) mutants but each mutant displayed a different pattern of cell sorting in chimeric aggregates. The distribution of GFP-ERK1 or GFP-ERK2 fusion proteins in the cytoplasm and nucleus was not grossly altered in cells stimulated with cAMP or folate. These results suggest ERK1 and ERK2 have different roles in G protein-mediated signaling during growth and development.

The Galpha4 G protein subunit interacts with the MAP kinase ERK2 using a D-motif that regulates developmental morphogenesis in Dictyostelium

G protein Galpha subunits contribute to the specificity of different signal transduction pathways in Dictyostelium discoideum but Galpha subunit-effector interactions have not been previously identified. The requirement of the Dictyostelium Galpha4 subunit for MAP kinase (MAPK) activation and the identification of a putative MAPK docking site (D-motif) in this subunit suggested a possible interaction between the Galpha4 subunit and MAPKs. In vivo association of the Galpha4 subunit and ERK2 was demonstrated by pull-down and co-immunoprecipitation assays. Alteration of the D-motif reduced Galpha4 subunit-ERK2 interactions but only slightly altered MAPK activation in response to folate. Expression of the Galpha4 subunit with the altered D-motif in galpha4(-)cells allowed for slug formation but not the morphogenesis associated with culmination. Expression of this mutant Galpha4 subunit was sufficient to rescue chemotactic movement to folate. Alteration of the D-motif also reduced the aggregation defect associated with constitutively active Galpha4 subunits. These results suggest Galpha4 subunit-MAPK interactions are necessary for developmental morphogenesis but not for chemotaxis to folate.

cAMP-dependent signal pathways in unicellular eukaryotes

The review summarizes current data about mechanisms of signal transduction with participation of cAMP (cyclic adenosine monophosphate) and elements of the complex cAMP-protein kinase A (PKA) signal pathway in unicellular eukaryotes. Conceptions of evolutionary origin of eukaryotic signal transduction systems are developed.

Biochemical responses to chemoattractants in Dictyostelium: ligand-receptor interactions and downstream kinase activation

Dictyostelium discoideum is one of the most facile eukaryotic systems for the study of chemotactic response to secreted chemical ligands. Dictyostelium grow as individual cells, using bacteria and fungi as primary nutrient sources; during growth, Dictyostelium moves directionally toward folate, a bacterial byproduct. Upon nutrient depletion Dictyostelium initiates a multicellular development program characterized by the production and secretion of cAMP. Cell surface receptors specifically recognize extracellular cAMP, which serves as both a morphogen to promote development and a chemoattractant to organize multicellularity. We discuss several approaches for the study of ligand-receptor interaction, with focus on affinity class determination and quantification of ligand binding sites (i.e., receptors) per cell. We further present examples for the application of biochemical assays to characterize the ligand-induced kinase activation of PI3K, GSK3, and ERK2.

Highlighting the role of Ras and Rap during Dictyostelium chemotaxis

Chemotaxis, the directional movement towards a chemical compound, is an essential property of many cells and has been linked to the development and progression of many diseases. Eukaryotic chemotaxis is a complex process involving gradient sensing, cell polarity, remodelling of the cytoskeleton and signal relay. Recent studies in the model organism Dictyostelium discoideum have shown that chemotaxis does not depend on a single molecular mechanism, but rather depends on several interconnecting pathways. Surprisingly, small G-proteins appear to play essential roles in all these pathways. This review will summarize the role of small G-proteins in Dictyostelium, particularly highlighting the function of the Ras subfamily in chemotaxis.

Isoprenylcysteine carboxy methylation is essential for development in Dictyostelium discoideum

Members of the Ras superfamily of small GTPases and the heterotrimeric G protein gamma subunit are methylated on their carboxy-terminal cysteine residues by isoprenylcysteine methyltransferase. In Dictyostelium discoideum, small GTPase methylation occurs seconds after stimulation of starving cells by cAMP and returns quickly to basal levels, suggesting an important role in cAMP-dependent signaling. Deleting the isoprenylcysteine methyltransferase-encoding gene causes dramatic defects. Starving mutant cells do not propagate cAMP waves in a sustained manner, and they do not aggregate. Motility is rescued when cells are pulsed with exogenous cAMP, or coplated with wild-type cells, but the rescued cells exhibit altered polarity. cAMP-pulsed methyltransferase-deficient cells that have aggregated fail to differentiate, but mutant cells plated in a wild-type background are able to do so. Localization of and signaling by RasG is altered in the mutant. Localization of the heterotrimeric Ggamma protein subunit was normal, but signaling was altered in mutant cells. These data indicate that isoprenylcysteine methylation is required for intercellular signaling and development in Dictyostelium.

A stress response kinase, KrsA, controls cAMP relay during the early development of Dictyostelium discoideum

Solitary amoebae of Dictyostelium discoideum are frequently exposed to stressful conditions in nature, and their multicellular development is one response to environmental stress. Here we analyzed an aggregation stage abundant gene, krsA, homologous to human krs1 (kinase responsive to stress 1) to understand the mechanisms for the initiation of development and cell fate determination. The krsA- cells exhibited reduced viability under hyperosmotic conditions. They produced smaller aggregates on membrane filters and did not form aggregation streams on a plastic surface under submerged starvation conditions, but were normal in sexual development. During early asexual development, the expression of cAMP-related genes peaked earlier in the knockout mutants. Neither cAMP oscillation in starved cells nor an increase in the cAMP level following osmotic stress was observed in krsA-. The nuclear export signal, as well as the kinase domain, in KrsA was necessary for stream formation. These results strongly suggest that krsA is involved in cAMP relay, and that signaling pathways for multicellular development have evolved in unison with the stress response.

The C-module DNA-binding factor mediates expression of the dictyostelium aggregation-specific adenylyl cyclase ACA

Aggregation of Dictyostelium discoideum amoebae into multicellular structures is organized by cyclic AMP (cAMP), which acts as a chemoattractant, as a second messenger, and as a morphogen. Aggregation of D. discoideum cells depends on the expression of adenylyl cyclase ACA, which provides extracellular cAMP for signal relay and intracellular cAMP for the induction of genes required at multicellular stages. We have identified a DNA-binding activity specific for a highly A+T-enriched motif in the upstream region of the ACA-encoding gene, acaA. The factor shows DNA-binding characteristics very similar to those of C-module-binding factor (CbfA). Although CbfA was originally identified as a putative regulator of the activity of D. discoideum retrotransposon TRE5-A, it also was found to be essential for aggregation of D. discoideum cells. The identified DNA-binding activity was absent in mutant cells depleted of CbfA, and CbfA could be precipitated using an acaA promoter fragment. We propose that CbfA binds to the acaA promoter to provide a basal transcription activity that is required for induction of ACA expression after the onset of D. discoideum development.

EppA, a putative substrate of DdERK2, regulates cyclic AMP relay and chemotaxis in Dictyostelium discoideum

The mitogen-activated protein kinase DdERK2 is critical for cyclic AMP (cAMP) relay and chemotaxis to cAMP and folate, but the details downstream of DdERK2 are unclear. To search for targets of DdERK2 in Dictyostelium discoideum, 32PO4(3-)-labeled protein samples from wild-type and Dderk2- cells were resolved by 2-dimensional electrophoresis. Mass spectrometry was used to identify a novel 45-kDa protein, named EppA (ERK2-dependent phosphoprotein A), as a substrate of DdERK2 in Dictyostelium. Mutation of potential DdERK2 phosphorylation sites demonstrated that phosphorylation on serine 250 of EppA is DdERK2 dependent. Changing serine 250 to alanine delayed development of Dictyostelium and reduced Dictyostelium chemotaxis to cAMP. Although overexpression of EppA had no significant effect on the development or chemotaxis of Dictyostelium, disruption of the eppA gene led to delayed development and reduced chemotactic responses to both cAMP and folate. Both eppA gene disruption and overexpression of EppA carrying the serine 250-to-alanine mutation led to inhibition of intracellular cAMP accumulation in response to chemoattractant cAMP, a pivotal process in Dictyostelium chemotaxis and development. Our studies indicate that EppA regulates extracellular cAMP-induced signal relay and chemotaxis of Dictyostelium.

Nonadaptive regulation of ERK2 in Dictyostelium: implications for mechanisms of cAMP relay

It is assumed that ERK2 in Dictyostelium is subject to adaptive regulation in response to constant extracellular ligand stimulation. We now show, to the contrary, that ERK2 remains active under continuous stimulation, differing from most ligand-activated pathways in chemotactically competent Dictyostelium and other cells. We show that the upstream phosphorylation pathway, responsible for ERK2 activation, transiently responds to receptor stimulation, whereas ERK2 dephosphorylation (deactivation) is inhibited by continuous stimulation. We argue that the net result of these two regulatory actions is a persistently active ERK2 pathway when the extracellular ligand (i.e., cAMP) concentration is held constant and that oscillatory production/destruction of secreted cAMP in chemotaxing cells accounts for the observed oscillatory activity of ERK2. We also show that pathways controlling seven-transmembrane receptor (7-TMR) ERK2 activation/deactivation function independently of G proteins and ligand-induced production of intracellular cAMP and the consequent activation of PKA. Finally, we propose that this regulation enables ERK2 to function both in an oscillatory manner, critical for chemotaxis, and in a persistent manner, necessary for gene expression, as secreted ligand concentration increases during later development. This work redefines mechanisms of ERK2 regulation by 7-TMR signaling in Dictyostelium and establishes new implications for control of signal relay during chemotaxis.

Signal relay during the life cycle of Dictyostelium

A fundamental property of multicellular organisms is signal relay, the process by which information is transmitted from one cell to another. The integration of external information, such as nutritional status or developmental cues, is critical to the function of organisms. In addition, the spatial organizations of multicellular organisms require intricate signal relay mechanisms. Signal relay is remarkably exhibited during the life cycle of the social amoebae Dictyostelium discoideum, a eukaryote that retains a simple way of life, yet it has greatly contributed to our knowledge of the mechanisms cells use to communicate and integrate information. This chapter focuses on the molecules and mechanisms that Dictyostelium employs during its life cycle to relay temporal and spatial cues that are required for survival.

Release of Ca2+ from the endoplasmic reticulum contributes to Ca2+ signaling in Dictyostelium discoideum

Ca2+ responses to two chemoattractants, folate and cyclic AMP (cAMP), were assayed in Dictyostelium D. discoideum mutants deficient in one or both of two abundant Ca2+-binding proteins of the endoplasmic reticulum (ER), calreticulin and calnexin. Mutants deficient in either or both proteins exhibited enhanced cytosolic Ca2+ responses to both attractants. Not only were the mutant responses greater in amplitude, but they also exhibited earlier onsets, faster rise rates, earlier peaks, and faster fall rates. Correlations among these kinetic parameters and the response amplitudes suggested that key events in the Ca2+ response are autoregulated by the magnitude of the response itself, i.e., by cytosolic Ca2+ levels. This autoregulation was sufficient to explain the altered kinetics of the mutant responses: larger responses are faster in both mutant and wild-type cells in response to both folate (vegetative cells) and cAMP (differentiated cells). Searches of the predicted D. discoideum proteome revealed three putative Ca2+ pumps and four putative Ca2+ channels. All but one contained sequence motifs for Ca2+- or calmodulin-binding sites, consistent with Ca2+ signals being autoregulatory. Although cytosolic Ca2+ responses in the calnexin and calreticulin mutants are enhanced, the influx of Ca2+ from the extracellular medium into the mutant cells was smaller. Compared to wild-type cells, Ca2+ release from the ER in the mutants thus contributes more to the total cytosolic Ca2+ response while influx from the extracellular medium contributes less. These results provide the first molecular genetic evidence that release of Ca2+ from the ER contributes to cytosolic Ca2+ responses in D. discoideum.

Defect in peroxisomal multifunctional enzyme MFE1 affects cAMP relay in Dictyostelium

We have previously reported that cells of Dictyostelium discoideum lacking the fatty acid oxidation enzyme MFE1 accumulate excess cyclopropane fatty acids from ingested bacteria. Cells in which mfeA(-) is disrupted fail to develop when grown in association with bacteria but form normal fruiting bodies when grown in axenic media. Bacterially grown mfeA(-) cells express the genes for the cyclic AMP (cAMP) receptor (carA) and adenylyl cyclase (acaA) but fail to respond to a cAMP pulse by synthesis of additional cAMP which normally relays the signal. Moreover, they do not accumulate the adhesion protein, gp80, which is encoded by the cAMP-induced gene, csaA. As a consequence, they do not acquire developmentally regulated EDTA-resistant cell-cell adhesion. When mutant cells are mixed with wild-type cells and allowed to develop together, they co-aggregate and differentiate into both spores and stalk cells. Thus, most of the developmental consequences of excess cyclopropane fatty acids appear to result from impaired cAMP relay.

Activation and nuclear translocation of ERK1/2 by the formyl peptide receptor is regulated by G protein and is not dependent on beta-arrestin translocation or receptor endocytosis

G protein-coupled receptors (GPCRs) transmit diverse cellular signals in response to a large number of stimuli such as chemoattractants, lipids, neurotransmitters, odorants and light. The classical signaling pathway is through heterotrimeric G proteins, but GPCRs can also transmit signals through mechanisms that are not dependent on G proteins. In mammalian cells, the key component for this type of signaling is the family of scaffolding molecules called beta-arrestins. They can function as scaffolds for activation of mitogen-activated protein kinases, including extracellular signal-regulated kinases 1 and 2 (ERK1/2). In this study we examined the role of G protein and beta-arrestin in formyl peptide receptor (FPR)-mediated activation of chemotaxis, receptor endocytosis and ERK1/2 activation using wild type and mutant receptors. Our findings suggest that, unlike certain other GPCRs that can activate ERK1/2 without the involvement of G protein, FPR requires signaling through a G protein-mediated pathway. Previous observations have shown that ERK1/2, activated through G protein, translocates to the nucleus where it stimulates transcription factors. In contrast, the scaffolding protein beta-arrestin retains the activated ERK1/2 in the cytoplasm to allow phosphorylation of cytoplasmic targets. Our experimental data show that both wild-type FPR and a mutant FPR, defective in beta-arrestin binding, induce nuclear translocation of activated ERK1/2 with similar ligand concentration dependence as seen for activation of cytosolic ERK1/2. We propose that FPR-mediated activation of ERK1/2 takes place primarily through G protein and is physiologically important to ensure transcriptional activation of myeloid immunomodulators, such as cytokines.

Chemoattractant signaling in dictyostelium discoideum

Dictyostelium is an accessible organism for studies of signaling via chemoattractant receptors. Chemoattractant-mediated signaling events and components are reviewed and presented as a series of connected modules, including excitation, inhibition, G protein-independent responses, early gene expression, inositol lipids, PH domain-containing proteins, cyclic AMP signaling, polarization acquisition, actin polymerization, and cortical myosin. The network incorporates information from biochemical, genetic, and cell biological experiments carried out on living cells. The modules and connections represent current understanding, and future information is expected to modify and build upon this structure.

Constitutively active G protein-coupled receptor mutants block dictyostelium development

cAR1, a G protein-coupled receptor (GPCR) for cAMP, is required for the multicellular development of Dictyostelium. The activation of multiple pathways by cAR1 is transient because of poorly defined adaptation mechanisms. To investigate this, we used a genetic screen for impaired development to isolate four dominant-negative cAR1 mutants, designated DN1-4. The mutant receptors inhibit multiple cAR1-mediated responses known to undergo adaptation. Reduced in vitro adenylyl cyclase activation by GTPgammaS suggests that they cause constitutive adaptation of this and perhaps other pathways. In addition, the DN mutants are constitutively phosphorylated, which normally requires cAMP binding and possess cAMP affinities that are approximately 100-fold higher than that of wild-type cAR1. Two independent activating mutations, L100H and I104N, were identified. These residues occupy adjacent positions near the cytoplasmic end of the receptor's third transmembrane helix and correspond to the (E/D)RY motif of numerous mammalian GPCRs, which is believed to regulate their activation. Taken together, these findings suggest that the DN mutants are constitutively activated and block development by turning on natural adaptation mechanisms.

Periodic signaling controlled by an oscillatory circuit that includes protein kinases ERK2 and PKA

Self-regulating systems often use robust oscillatory circuits. One such system controls the chemotactic signaling mechanism of Dictyostelium, where pulses of adenosine 3',5'-monophosphate (cAMP) are generated with a periodicity of 7 minutes. We have observed spontaneous oscillations in activation of the mitogen-activated protein (MAP) kinase ERK2 that occur in phase with peaks of cAMP, and we show that ERK2 modulates cAMP levels through the phosphodiesterase RegA. Computer modeling and simulations of the underlying circuit faithfully account for the ability of the cells to spontaneously generate periodic pulses during specific stages of development. Similar oscillatory processes may occur in cells of many different species.

cAMP signaling in Dictyostelium. Complexity of cAMP synthesis, degradation and detection

cAMP plays a pivotal role in control of cell movement, differentiation and response to stress in all phases of the Dictyostelium life cycle. The multitudinous functions of cAMP require precise spatial and temporal control of its production, degradation and detection. Many novel proteins have recently been identified that critically modulate the cAMP signal. We focus in this review on the properties and functions of the three adenylyl cyclases and the three cAMP-phosphodiesterases that are present in Dictyostelium, and the network of proteins that regulate the activity of these enzymes. We also briefly discuss the two modes of detection of cAMP.

Multiple signalling pathways connect chemoattractant receptors and calcium channels in Dictyostelium

Dictyostelium mutants expressing aequorin were used to study and compare the roles of heterotrimeric G-proteins and the second messengers IP3 and cGMP in regulating folate- and cAMP receptor-activated [Ca2+]i signals. The calcium responses of vegetative cells to folate were dramatically impaired in Gbeta and Galpha4 null mutants but were restored with altered kinetics and temperature-sensitivity in Gbeta null mutants overexpressing wild type and temperature-sensitive Gbeta isoforms. Folic acid receptors thus mediate changes in [Ca2+]i via a Galpha4betagamma-dependent pathway. Neither folate nor cAMP-induced [Ca2+]i signals were significantly altered in PLC null transformants, but [Ca2+]i changes elicited by both attractants were significantly prolonged in two stmF mutants lacking cGMP-specific phosphodiesterase activity. This confirms an important role of cGMP in regulating receptor-activated Ca2+ uptake and/or extrusion systems. This cGMP-dependent part of the Ca2+ response to cAMP stimuli was developmentally down-regulated and all but disappeared by the time the cells reached full aggregation competence after 8 h of starvation. The results suggest that folate and cAMP receptor-activated [Ca2+]i signals are regulated in a complex manner via multiple signalling pathways, one that is G-protein- and cGMP-dependent (present at the vegetative and early poststarvation stage) and another that is G-protein-independent (dominant in fully aggregation-competent cells at approximately 8 h poststarvation).

Two components of a secreted cell number-counting factor bind to cells and have opposing effects on cAMP signal transduction in Dictyostelium

A secreted 450-kDa complex of proteins called counting factor (CF) is part of a negative feedback loop that regulates the size of the groups formed by developing Dictyostelium cells. Two components of CF are countin and CF50. Both recombinant countin and recombinant CF50 decrease group size in Dictyostelium. countin- cells have a decreased cAMP-stimulated cAMP pulse, whereas recombinant countin potentiates the cAMP pulse. We find that CF50 cells have an increased cAMP pulse, whereas recombinant CF50 decreases the cAMP pulse, suggesting that countin and CF50 have opposite effects on cAMP signal transduction. In addition, countin and CF50 have opposite effects on cAMP-stimulated Erk2 activation. However, like recombinant countin, recombinant CF50 increases cell motility. We previously found that cells bind recombinant countin with a Hill coefficient of approximately 2, a KH of 60 pm, and approximately 53 sites/cell. We find here that cells also bind 125I-recombinant CF50, with a Hill coefficient of approximately 2, a KH of approximately 15 ng/ml (490 pm), and approximately 56 sites/cell. Countin and CF50 require each other's presence to affect group size, but the presence of countin is not necessary for CF50 to bind to cells, and CF50 is not necessary for countin to bind to cells. Our working hypothesis is that a signal transduction pathway activated by countin binding to cells modulates a signal transduction pathway activated by CF50 binding to cells and vice versa and that these two pathways can be distinguished by their effects on cAMP signal transduction.

Genome-wide expression analyses of gene regulation during early development of Dictyostelium discoideum

Using genome-wide microarrays, we recognized 172 genes that are highly expressed at one stage or another during multicellular development of Dictyostelium discoideum. When developed in shaken suspension, 125 of these genes were expressed if the cells were treated with cyclic AMP (cAMP) pulses at 6-min intervals between 2 and 6 h of development followed by high levels of exogenous cAMP. In the absence of cAMP treatment, only three genes, carA, gbaB, and pdsA, were consistently expressed. Surprisingly, 14 other genes were induced by cAMP treatment of mutant cells lacking the activatable adenylyl cyclase, ACA. However, these genes were not cAMP induced if both of the developmental adenylyl cyclases, ACA and ACR, were disrupted, showing that they depend on an internal source of cAMP. Constitutive activity of the cAMP-dependent protein kinase PKA was found to bypass the requirement of these genes for adenylyl cyclase and cAMP pulses, demonstrating the critical role of PKA in transducing the cAMP signal to early gene expression. In the absence of constitutive PKA activity, expression of later genes was strictly dependent on ACA in pulsed cells.

Constitutively active protein kinase A disrupts motility and chemotaxis in Dictyostelium discoideum

The deletion of the gene for the regulatory subunit of protein kinase A (PKA) results in constitutively active PKA in the pkaR mutant. To investigate the role of PKA in the basic motile behavior and chemotaxis of Dictyostelium discoideum, pkaR mutant cells were subjected to computer-assisted two- and three-dimensional motion analysis. pkaR mutant cells crawled at only half the speed of wild-type cells in buffer, chemotaxed in spatial gradients of cyclic AMP (cAMP) but with reduced efficiency, were incapable of suppressing lateral pseudopods in the front of temporal waves of cAMP, a requirement for natural chemotaxis, did not exhibit the normal velocity surge in response to the front of a wave, and were incapable of chemotaxing toward an aggregation center in natural waves generated by wild-type cells that made up the majority of cells in mixed cultures. Many of the behavioral defects appeared to be the result of the constitutively ovoid shape of the pkaR mutant cells, which forced the dominant pseudopod off the substratum and to the top of the cell body. The behavioral abnormalities that pkaR mutant cells shared with regA mutant cells are discussed by considering the pathway ERK2 perpendicular RegA perpendicular [cAMP] --> PKA, which emanates from the front of a wave. The results demonstrate that cells must suppress PKA activity in order to elongate along a substratum, suppress lateral-pseudopod formation, and crawl and chemotax efficiently. The results also implicate PKA activation in dismantling cell polarity at the peak and in the back of a natural cAMP wave.

Ggamma in dictyostelium: its role in localization of gbetagamma to the membrane is required for chemotaxis in shallow gradients

G-protein-mediated signal transduction pathways play an essential role in the developmental program of the simple eukaryotic organism Dictyostelium discoideum. Database searches have yielded 11 Galpha-subunits, a single Gbeta-subunit, but no Ggamma-subunits. We report here the purification, cDNA isolation, and functional analysis of a Ggamma-subunit. Like Gbeta, the Ggamma appears to be unique and hybridization studies show that Ggamma and Gbeta are expressed in parallel during development. Species-wide sequence comparisons of Ggamma-subunits and gamma-like domains of RGS proteins reveal short stretches of highly conserved residues as well as the common CXXL motif at the COOH-terminal of Ggammas that target Gbetagammas to plasma membrane. Overexpression of a CSVL-deleted Ggamma (GgammaDelta) in wild-type cells shifts Gbetagamma to the cytosol and selectively impairs certain G-protein-mediated signal transduction pathways. These cells are able to respond to increments in the stimulus, but are unable to sense chemoattractant gradients. They neither move directionally nor recruit PH-domains to their leading edge. Thus, a full complement of membrane-tethered Gbetagamma is required for sensing shallow gradients, but is not essential for responses to increments in extracellular stimuli.

RasC is required for optimal activation of adenylyl cyclase and Akt/PKB during aggregation

Disruption of Dictyostelium rasC, encoding a Ras subfamily protein, generated cells incapable of aggregation. While rasC expression is enriched in a cell type-specific manner during post-aggregative development, the defect in rasC(-) cells is restricted to aggregation and fully corrected by application of exogenous cAMP pulses. cAMP is not produced in rasC(-) cells stimulated by 2'-deoxy-cAMP, but is produced in response to GTPgammaS in cell lysates, indicating that G-protein-coupled cAMP receptor activation of adenylyl cyclase is regulated by RasC. However, cAMP-induced ERK2 phosphorylation is unaffected in rasC(-) cells, indicating that RasC is not an upstream activator of the mitogen-activated protein kinase required for cAMP relay. rasC(-) cells also exhibit reduced chemotaxis to cAMP during early development and delayed response to periodic cAMP stimuli produced by wild-type cells in chimeric mixtures. Furthermore, cAMP-induced Akt/PKB phosphorylation through a phosphatidylinositide 3-kinase (PI3K)-dependent pathway is dramatically reduced in rasC(-) cells, suggesting that G-protein-coupled serpentine receptor activation of PI3K is regulated by RasC. Cells lacking the RasGEF, AleA, exhibit similar defects as rasC(-) cells, suggesting that AleA may activate RasC.

Functional and regulatory analysis of the dictyostelium G-box binding factor

The Dictyostelium discoidium G-box binding factor (GBF) is required for the induction of known postaggregative and cell-type-specific genes. gbf-null cells undergo developmental arrest at the loose-mound stage due to the absence of GBF-targeted gene transcription. GBF-mediated gene expression is activated by stimulation of cell-surface, seven-span cAMP receptors, but this activation is independent of heterotrimeric G-proteins. To further characterize GBF, we assayed a series of GBF mutants for their ability to bind a G-box in vitro and to complement the gbf-null phenotype. In vitro DNA-binding activity resides in the central portion of the protein, which contains two predicted zinc fingers. However, in vivo GBF function requires only one intact zinc finger. In addition, expression of some GBF mutants results in a partial complementation phenotype, suggesting that these mutants are hypomorphic alleles. We used a 2.4-kb GBF-promoter fragment to examine the regulation of GBF expression. GBF promoter-reporter studies confirmed the previous finding that GBF transcription is induced by continuous, micromolar extracellular cAMP. We also show that, like the activation of GBF-regulated transcription, the induction of GBF expression requires cell-surface cAMP receptors, but not heterotrimeric G-proteins. Finally, reporter studies demonstrated that induction of GBF-promoter-regulated expression does not require the presence of GBF protein, indicating that GBF expression is not regulated by a positive autoregulatory loop.

Spatial expression patterns of genes involved in cyclic AMP responses in Dictyostelium discoideum development

The spatial expression patterns of genes involved in cyclic adenosine monophosphate (cAMP) responses during morphogenesis in Dictyostelium discoideum were analyzed by in situ hybridization. Genes encoding adenylyl cyclase A (ACA), cAMP receptor 1, G-protein alpha2 and beta subunits, cytosolic activator of ACA (CRAC and Aimless), catalytic subunit of protein kinase A (PKA-C) and cAMP phosphodiesterases (PDE and REG-A) were preferentially expressed in the anterior prestalk (tip) region of slugs, which acts as an organizing center. MAP kinase ERK2 (extracellular signal-regulated kinase-2) mRNA, however, was enriched in the posterior prespore region. At the culmination stage, the expression of ACA, CRAC and PKA-C mRNA increased in prespore cells in contrast with the previous stage. However, no alteration in the site of expression was observed for the other mRNA analyzed. Based on these findings, two and four classes of expression patterns were catalogued for these genes during the slug and culmination stages, respectively. Promoter analyses of genes in particular classes should enhance understanding of the regulation of dynamic and coordinated gene expression during morphogenesis.