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

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 hybrid type polyketide synthase SteelyA is required for cAMP signalling in early Dictyostelium development

BACKGROUND

In our previous study we found that the expression of stlA showed peaks both in the early and last stages of development and that a product of SteelyA, 4-methyl-5-pentylbenzene-1,3-diol (MPBD), controlled Dictyostelium spore maturation during the latter. In this study we focused on the role of SteelyA in early stage development.

PRINCIPAL FINDINGS

Our stlA null mutant showed aggregation delay and abnormally small aggregation territories. Chemotaxis analysis revealed defective cAMP chemotaxis in the stlA null mutant. cAMP chemotaxis was restored by MPBD addition during early stage development. Assay for cAMP relay response revealed that the stlA null mutant had lower cAMP accumulation during aggregation, suggesting lower ACA activity than the wild type strain. Exogenous cAMP pulses rescued the aggregation defect of the stlA null strain in the absence of MPBD. Expression analysis of cAMP signalling genes revealed lower expression levels in the stlA null mutant during aggregation.

CONCLUSION

Our data indicate a regulatory function by SteelyA on cAMP signalling during aggregation and show that SteelyA is indispensable for full activation of ACA.

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.

Novel development rescuing factors (DRFs) secreted by the developing Dictyostelium cells, that are involved in the restoration of a mutant lacking MAP-kinase ERK2

We found novel development rescuing factors (DRFs) secreted from developing Dictyostelium cells, by using a mutant (erkB-) which is missing MAP-kinase ERK2 as a test strain for bioassay. The mutant erkB- fails to undergo multicellular morphogenesis due to impaired cAMP signaling. However, such developmental defect can be restored by the presence of low-molecular weight DRFs that are secreted from developing wild-type cells. We previously showed that DIF-1 (Differentiation-Inducing Factor 1 for stalk cells) possesses this activity, indicating a newly discovered role of DIF-1. Surprisingly, however, the mutant dmtA-, which is incapable of DIF-1 synthesis still exerts a strong inducing activity of the multicellular morphogenesis of erkB-. After analysis of HPLC fractions of conditioned media prepared from both wild type Ax2 and dmtA- strains revealed that both strains secrete at least two novel DRF activities with DIF-like mobility. However, these activities were not derived from other DIFs such as DIF-2 and DIF-3. Identification of these DRFs found in this study would provide insight into the mechanism by which the development of the erkB- mutant is restored and how these factors act in the normal development of Dictyostelium.

Identification and characterization of novel calcium-binding proteins of Dictyostelium and their spatial expression patterns during development

Five putative Ca2(+)-binding proteins, CBP5, 6, 7, 8 and 9, all having EF-hand motifs, were found by searching the Dictyostelium cDNA database (http://www.csm.biol.tsukuba.ac.jp/cDNAproject.html). 45Ca2(+)-overlay experiments revealed that four of these (excluding CBP9) are real Ca2(+)-binding proteins. Northern blot analysis revealed that the genes encoding CBP5, 6, 7 and 8 are all developmentally regulated. In situ hybridization analyses revealed that spatial expression of these genes was regulated in several different ways. CBP1, 2, 3, 5, 6 and 7 are expressed in prespore cells in the slug stage. Transcripts of the genes for CBP1 and 5 are enriched in prestalk subtype PstO cells. In contrast, CBP4 is expressed predominantly in PstO cells. CBP8 is evenly expressed at a very low level throughout the whole slug. Such distinct spatial expression patterns suggest that the CBP might be involved in morphogenesis and might have their own roles either in prespore or in prestalk cell differentiation of Dictyostelium.

MFE1, a member of the peroxisomal hydroxyacyl coenzyme A dehydrogenase family, affects fatty acid metabolism necessary for morphogenesis in Dictyostelium spp

Beta-oxidation of long-chain fatty acids and branched-chain fatty acids is carried out in mammalian peroxisomes by a multifunctional enzyme (MFE) or D-bifunctional protein, with separate domains for hydroxyacyl coenzyme A (CoA) dehydrogenase, enoyl-CoA hydratase, and steroid carrier protein SCP2. We have found that Dictyostelium has a gene, mfeA, encoding MFE1 with homology to the hydroxyacyl-CoA dehydrogenase and SCP2 domains. A separate gene, mfeB, encodes MFE2 with homology to the enoyl-CoA hydratase domain. When grown on a diet of bacteria, Dictyostelium cells in which mfeA is disrupted accumulate excess cyclopropane fatty acids and are unable to develop beyond early aggregation. Axenically grown mutant cells, however, developed into normal fruiting bodies composed of spores and stalk cells. Comparative analysis of whole-cell lipid compositions revealed that bacterially grown mutant cells accumulated cyclopropane fatty acids that remained throughout the developmental stages. Such a persistent accumulation was not detected in wild-type cells or axenically grown mutant cells. Bacterial phosphatidylethanolamine that contains abundant cyclopropane fatty acids inhibited the development of even axenically grown mutant cells, while dipalmitoyl phosphatidylethanolamine did not. These results suggest that MFE1 protects the cells from the increase of the harmful xenobiotic fatty acids incorporated from their diets and optimizes cellular lipid composition for proper development. Hence, we propose that this enzyme plays an irreplaceable role in the survival strategy of Dictyostelium cells to form spores for their efficient dispersal in nature.