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

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.

Navigating the ERK1/2 MAPK Cascade

The RAS-ERK pathway is a fundamental signaling cascade crucial for many biological processes including proliferation, cell cycle control, growth, and survival; common across all cell types. Notably, ERK1/2 are implicated in specific processes in a context-dependent manner as in stem cells and pancreatic β-cells. Alterations in the different components of this cascade result in dysregulation of the effector kinases ERK1/2 which communicate with hundreds of substrates. Aberrant activation of the pathway contributes to a range of disorders, including cancer. This review provides an overview of the structure, activation, regulation, and mutational frequency of the different tiers of the cascade; with a particular focus on ERK1/2. We highlight the importance of scaffold proteins that contribute to kinase localization and coordinate interaction dynamics of the kinases with substrates, activators, and inhibitors. Additionally, we explore innovative therapeutic approaches emphasizing promising avenues in this field.

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.

A chemorepellent inhibits local Ras activation to inhibit pseudopod formation to bias cell movement away from the chemorepellent

The ability of cells to sense chemical gradients is essential during development, morphogenesis, and immune responses. Although much is known about chemoattraction, chemorepulsion remains poorly understood. Proliferating Dictyostelium cells secrete a chemorepellent protein called AprA. AprA prevents pseudopod formation at the region of the cell closest to the source of AprA, causing the random movement of cells to be biased away from the AprA. Activation of Ras proteins in a localized sector of a cell cortex helps to induce pseudopod formation, and Ras proteins are needed for AprA chemorepulsion. Here we show that AprA locally inhibits Ras cortical activation through the G protein–coupled receptor GrlH, the G protein subunits Gβ and Gα8, Ras protein RasG, protein kinase B, the p21-activated kinase PakD, and the extracellular signal–regulated kinase Erk1. Diffusion calculations and experiments indicate that in a colony of cells, high extracellular concentrations of AprA in the center can globally inhibit Ras activation, while a gradient of AprA that naturally forms at the edge of the colony allows cells to activate Ras at sectors of the cell other than the sector of the cell closest to the center of the colony, effectively inducing both repulsion from the colony and cell differentiation. Together, these results suggest that a pathway that inhibits local Ras activation can mediate chemorepulsion.

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.

An Autocrine Negative Feedback Loop Inhibits Dictyostelium discoideum Proliferation through Pathways Including IP3/Ca2

Little is known about how eukaryotic cells can sense their number or spatial density and stop proliferating when the local density reaches a set value. We previously found that Dictyostelium discoideum accumulates extracellular polyphosphate to inhibit its proliferation, and this requires the G protein-coupled receptor GrlD and the small GTPase RasC. Here, we show that cells lacking the G protein component Gβ, the Ras guanine nucleotide exchange factor GefA, phosphatase and tensin homolog (PTEN), phospholipase C (PLC), inositol 1,4,5-trisphosphate (IP3) receptor-like protein A (IplA), polyphosphate kinase 1 (Ppk1), or the TOR complex 2 component PiaA have significantly reduced sensitivity to polyphosphate-induced proliferation inhibition. Polyphosphate upregulates IP3, and this requires GrlD, GefA, PTEN, PLC, and PiaA. Polyphosphate also upregulates cytosolic Ca²⁺, and this requires GrlD, Gβ, GefA, RasC, PLC, IplA, Ppk1, and PiaA. Together, these data suggest that polyphosphate uses signal transduction pathways including IP3/Ca²⁺ to inhibit the proliferation of D. discoideum. IMPORTANCE Many mammalian tissues such as the liver have the remarkable ability to regulate their size and have their cells stop proliferating when the tissue reaches the correct size. One possible mechanism involves the cells secreting a signal that they all sense, and a high level of the signal tells the cells that there are enough of them and to stop proliferating. Although regulating such mechanisms could be useful to regulate tissue size to control cancer or birth defects, little is known about such systems. Here, we use a microbial system to study such a mechanism, and we find that key elements of the mechanism have similarities to human proteins. This then suggests the possibility that we may eventually be able to regulate the proliferation of selected cell types in humans and animals.

Mitogen-activated protein kinase regulation of the phosphodiesterase RegA in early Dictyostelium development

Mitogen-activated protein kinase (MAPK) regulation of cAMP-specific phosphodiesterase function has been demonstrated in mammalian cells and suspected to occur in other eukaryotes. Epistasis analysis in the soil amoeba Dictyostelium discoideum suggests the atypical MAPK Erk2 downregulates the function of the cAMP-specific phosphodiesterase RegA to regulate progression of the developmental life cycle. A putative MAPK docking motif located near a predicted MAPK phosphorylation site was characterized for contributions to RegA function and binding to Erk2 because a similar docking motif has been previously characterized in the mammalian PDE4D phosphodiesterase. The overexpression of RegA with alterations to this docking motif (RegA^D-) restored RegA function to regA- cells based on developmental phenotypes, but low-level expression of RegA^D- from the endogenous regA promoter failed to rescue wild-type morphogenesis. Co-immunoprecipitation analysis indicated that Erk2 associates with both RegA and RegA^D-, suggesting the docking motif is not required for this association. Epistasis analysis between regA and the only other Dictyostelium MAPK, erk1, suggests Erk1 and RegA can function in different pathways but that some erk1- phenotypes may require cAMP signalling. These results imply that MAPK downregulation of RegA in Dictyostelium is accomplished through a different mechanism than MAPK regulation of cAMP-specific phosphodiesterases in mammalian cells and that the regulation in Dictyostelium does not require a proximal MAPK docking motif.

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.

An endogenous chemorepellent directs cell movement by inhibiting pseudopods at one side of cells

Eukaryotic chemoattraction signal transduction pathways, such as those used by Dictyostelium discoideum to move toward cAMP, use a G protein-coupled receptor to activate multiple conserved pathways such as PI3 kinase/Akt/PKB to induce actin polymerization and pseudopod formation at the front of a cell, and PTEN to localize myosin II to the rear of a cell. Relatively little is known about chemorepulsion. We previously found that AprA is a chemorepellent protein secreted by Dictyostelium cells. Here we used 29 cell lines with disruptions of cAMP and/or AprA signal transduction pathway components, and delineated the AprA chemorepulsion pathway. We find that AprA uses a subset of chemoattraction signal transduction pathways including Ras, protein kinase A, target of rapamycin (TOR), phospholipase A, and ERK1, but does not require the PI3 kinase/Akt/PKB and guanylyl cyclase pathways to induce chemorepulsion. Possibly as a result of not using the PI3 kinase/Akt/PKB pathway and guanylyl cyclases, AprA does not induce actin polymerization or increase the pseudopod formation rate, but rather appears to inhibit pseudopod formation at the side of cells closest to the source of AprA.

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.

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 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.

Mef2A, a homologue of animal Mef2 transcription factors, regulates cell differentiation in Dictyostelium discoideum

Background

Transcription factors from the MADS-box family play a relevant role in cell differentiation and development and include the animal SRF (serum response factor) and MEF2 (myocyte enhancer factor 2) proteins. The social amoeba Dictyostelium discoideum contains four genes coding for MADS-box transcription factors, two of these genes code for proteins that are more similar to SRF, and the other two code for proteins that are more similar to MEF2 animal factors.

Results

The biological function of one of the two genes that codes for MEF2-related proteins, a gene known as mef2A, is described in this article. This gene is expressed under the transcriptional control of two alternative promoters in growing cells, and its expression is induced during development in prespore cells. Mutant strains where the mef2A gene has been partially deleted were generated to study its biological function. The mutant strains showed reduced growth when feeding on bacteria and were able to develop and form fruiting bodies, but spore production was significantly reduced. A study of developmental markers showed that prespore cells differentiation was impaired in the mutant strains. When mutant and wild-type cells were set to develop in chimeras, mutant spores were underrepresented in the fruiting bodies. The mutant cells were also unable to form spores in vitro. In addition, mutant cells also showed a poor contribution to the formation of the tip-organizer and the upper region of slugs and culminant structures. In agreement with these observations, a comparison of the genes transcribed by mutant and wild-type strains during development indicated that prestalk gene expression was enhanced, while prespore gene expression decreased in the mef2A^- strain.

Conclusions

Our data shows that mef2A plays a role in cell differentiation in D. discoideum and modulates the expression of prespore and prestalk genes.

The dual-specificity protein phosphatase MkpB, homologous to mammalian MKP phosphatases, is required for D. discoideum post-aggregative development and cisplatin response

Dual-specificity protein phosphatases participate in signal transduction pathways inactivating mitogen-activated protein kinases (MAP kinases). These signaling pathways are of critical importance in the regulation of numerous biological processes, including cell proliferation, differentiation and development. The social ameba Dictyostelium discoideum harbors 14 genes coding for proteins containing regions very similar to the dual-specificity protein phosphatase domain. One of these genes, mkpB, additionally codes for a region similar to the Rhodanase domain, characteristic of animal MAP kinase-phosphatases, in its N-terminal region. Cells that over-express this gene show increased protein phosphatase activity. mkpB is expressed in D. discoideum ameba at growth but it is greatly induced at 12h of multicellular development. Although it is expressed in all the cells of developmental structures, mkpB mRNA is enriched in cells with a distribution typical of anterior-like cells. Cells that express a catalytically inactive mutant of MkpB grow and aggregate like wild-type cells but show a greatly impaired post-aggregative development. In addition, the expression of cell-type specific genes is very delayed, indicating that this protein plays an important role in cell differentiation and development. Cells expressing the MkpB catalytically inactive mutant show increased sensitivity to cisplatin, while cells over-expressing wild type MkpB, or MkpA, proteins or mutated in the MAP kinase erkB gene are more resistant to this chemotherapeutic drug, as also shown in human tumor cells.

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.

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.