Deactivation of gene expression upon the onset of development in dictyostelium discoideum

A Dictyostelium homologue of the metazoan Cbl proteins regulates STAT signalling

Cbl proteins downregulate metazoan signalling pathways by ubiquitylating receptor tyrosine kinases, thereby targeting them for degradation. They contain a phosphotyrosine-binding region, comprising an EF-hand and an SH2 domain, linked to an E3 ubiquitin-ligase domain. CblA, a Dictyostelium homologue of the Cbl proteins, contains all three conserved domains. In a cblA(-) strain early development occurs normally but migrating cblA(-) slugs frequently fragment and the basal disc of the culminants that are formed are absent or much reduced. These are characteristic features of mutants in signalling by DIF-1, the low-molecular-mass prestalk and stalk cell inducer. Tyrosine phosphorylation of STATc is induced by DIF-1 but in the cblA(-) strain this response is attenuated relative to parental cells. We present evidence that CblA fulfils this function, as a positive regulator of STATc tyrosine phosphorylation, by downregulating PTP3, the protein tyrosine phosphatase responsible for dephosphorylating STATc. Thus Cbl proteins have an ancient origin but, whereas metazoan Cbl proteins regulate tyrosine kinases, the Dictyostelium Cbl regulates via a tyrosine phosphatase.

Proteomic and microarray analyses of the Dictyostelium Zak1-GSK-3 signaling pathway reveal a role in early development

GskA, the Dictyostelium GSK-3 orthologue, is modified and activated by the dual-specificity tyrosine kinase Zak1, and the two kinases form part of a signaling pathway that responds to extracellular cyclic AMP. We identify potential cellular effectors for the two kinases by analyzing the corresponding null mutants. There are proteins and mRNAs that are altered in abundance in only one or the other of the two mutants, indicating that each kinase has some unique functions. However, proteomic and microarray analyses identified a number of proteins and genes, respectively, that are similarly misregulated in both mutant strains. The positive correlation between the array data and the proteomic data is consistent with the Zak1-GskA signaling pathway's functioning by directly or indirectly regulating gene expression. The discoidin 1 genes are positively regulated by the pathway, while the abundance of the H5 protein is negatively regulated. Two of the targets, H5 and discoidin 1, are well-characterized markers for early development, indicating that the Zak1-GskA pathway plays a role in development earlier than previously observed.

Dictyostelium ribosomal protein genes and the elongation factor 1B gene show coordinate developmental regulation which is under post-transcriptional control

Starvation for amino acids initiates the developmental program in the cellular slime mold, Dictyostelium discoideum [19, 20]. One of the earliest developmental events is the decline in ribosomal protein synthesis [2, 17, 29, 30]. The ribosomal protein mRNAs are excluded from polysomes with 20 min to 1 h following the removal of nutrients, and their mRNA levels decline sharply at about 9 h into the 24-h developmental cycle [28, 31, 35, 36]. It has been generally assumed that the decline in r-protein mRNA levels during late development reflected a decline in the transcription rate [12, 32, 43]. Here we demonstrate that this is not the case. The transcription rates of three ribosomal protein genes, rpL11, rpL23 and rpS9 as well as an elongation factor 1B gene have been determined during growth and development in Dictyostelium. Throughout growth and development the transcription rate of the ribosomal protein genes remains relatively constant at 0.2%-0.5% of the rate of rRNA transcription while the elongation factor 1B gene is transcribed at 0.4%-0.6% of the rRNA rate. This low but constant transcription rate is in contrast to a spore coat protein gene Psp D, which is transcribed at 6% of the rRNA rate in late developing cells. The elongation factor 1B gene appears to be co-regulated with the ribosomal protein genes both in terms of its transcription rate and mRNA accumulation. Dictyostelium has been a popular model for understanding signal transduction and the growth to differentiation transition, thus it is of significance that the regulation of ribosome biosynthesis in Dictyostelium resembles that of higher eukaryotes in being regulated largely at the post-transcriptional level in response to starvation as opposed to yeasts where the regulation is largely transcriptional [27].

Comparative analysis of chemotaxis in Dictyostelium using a radial bioassay method: protein tyrosine kinase activity is required for chemotaxis to folate but not to cAMP

The role of signal transduction during chemotaxis of Dictyostelium discoideum cells to cAMP and folic acid was investigated using a radial bioassay technique. The effects of signalling agonists were assessed by measuring the diameters of visible rings formed by the outward migration of amoebae up radial gradients of chemoattractant. This rapid and simple bioassay method yields chemotactic rates equivalent to more complex assay systems. In support of previous studies, chemotaxis toward both cAMP and folic acid was inhibited in a dose-dependent manner by LaCl3, EDTA, chlorotetracycline and A1F3, supporting the importance of calcium ions and G protein-mediated signalling in both chemotactic events. The work was extended by examining the effects of the protein tyrosine kinase inhibitor genistein. This agent inhibited chemotaxis to folate in a dose-dependent manner but had no observable effect on chemotaxis toward cAMP. The notion that phosphorylation of proteins on tyrosine residues is critical for chemotaxis to folic acid was supported by Western blotting experiments with monoclonal anti-phosphotyrosine antibodies which detected two candidate proteins of M(r) 52,000 and 38,000 in the membranes of folate-responsive amoebae. These two bands disappeared with starvation which leads to the loss of responsiveness of folic acid and the acquisition of responsiveness to cAMP. Time-lapse videomicrography also revealed some unique differences in chemotactic response. Starved cells responded to cAMP as individuals but feeding cells chemoattracted to folic acid on a populational basis. The ability to compare two different types of chemotaxis using a simple, rapid and accurate bioassay system should enhance future studies of chemotaxis in wild-type and mutant strains of D. discoideum.

Redundant regulatory elements account for the developmental control of a ribosomal protein gene of Dictyostelium discoideum

In Dictyostelium discoideum, ribosomal protein genes along with other growth specific genes appear to be coordinately regulated, primarily in response to differences in the translational capacity of developing versus growing cells. In particular, expression of the members of this large class of genes is rapidly and dramatically deactivated when the developmental program is initiated and growth and division cease. In order to understand the mechanisms behind the deactivation event and how it is coupled to the transition from growth to development, we have analyzed the promoter of the V18 gene, a ribosomal protein gene characteristic of this class of growth specific genes. We have delineated three discrete regions involved in the transcription and regulation of the V18 gene. A initiator region which appears to function in a TATA-independent manner was required for transcription and for establishing start site utilization. Two regions upstream of this were defined, both of which were found to independently confer proper developmental regulation.

V4, a gene required for the transition from growth to development in Dictyostelium discoideum

The V4 gene of Dictyostelium discoideum is regulated in a nutrient-dependent manner and is deactivated immediately upon the onset of development. V4 is expressed only during growth, but its expression is not required for growth. We propose that the V4 gene product plays a role in the transition from growth to development. We have tested this hypothesis by antisense mutagenesis. Cells transformed with a V4 antisense construct contained no detectable endogenous V4 mRNA. These cells grew normally, but they failed to aggregate. Under conditions which normally promote development, V4 antisense transformants failed to deactivate vegetative-specific genes. These cells also were unable to induce the expression of the cAMP cell surface receptor, the cyclic nucleic phosphodiesterase, and contact sites A, all of which are normally induced under such conditions. Surprisingly, cells transformed with a V4 sense construct displayed a similar morphological and biochemical phenotype as the antisense cells, whereas cells transformed with the parental vector exhibited a normal biochemical and morphological phenotype. These results demonstrate that expression of the V4 gene during growth is required for the proper initiation of development.

Developmental mechanisms regulating the rapid decrease in a cohesion glycoprotein mRNA in Dictyostelium function primarily at the level of mRNA degradation

During the morphogenetic program in Dictyostelium discoideum, the transcript of the gene encoding the cohesion glycoprotein gp80 accumulates to a maximum level between 4 and 6 hr, (just prior to the onset of aggregation), remains high between 6 and 10 hr (the ripple to loose aggregate stages), and then decreases to less than 10% of the maximum level between 10 and 12 hr (the tight aggregate stage). The level of gp80 transcript also decreases precipitously at the time of the erasure event in the program of dedifferentiation, or when cAMP is added to a dedifferentiating cell population prior to the erasure event. In the dedifferentiation-defective mutant HI4, the cAMP-stimulated system for rapidly reducing the level of gp80 transcript is intact, but the mechanism functioning at the time of the erasure event is defective, demonstrating that the two reduction mechanisms are dissociable. By comparing the levels of gp80 transcript with the levels of in vitro transcription of the gene in isolated nuclei, it is demonstrated that the rapid reduction of gp80 transcript immediately after aggregation and immediately after addition of 10(-4) M cAMP are the result of increased transcript degradation. The rapid reduction of gp80 transcript at the erasure event may also be due to increased transcript degradation, but transcriptional regulation cannot be completely ruled out in this case.

Primary structure and regulation of vegetative specific genes of Dictyostelium discoideum

We have examined the expression and structure of several genes belonging to two classes of vegetative specific genes of the simple eukaryote, Dictyostelium discoideum. In amebae grown on bacteria, deactivation of all vegetative specific genes occurred at the onset of development and very little mRNA exists by 8 to 10 hours. In contrast, when cells were grown in axenic broth, the mRNA levels remained constant until a dramatic drop occurred around 10 to 12 hours. Thus, regulation of both classes of genes during the first several hours of development is dependent upon the prior growth conditions. Analysis of genomic clones has resulted in the identification of two V genes, V1 and V18, as ribosomal protein genes. Several other V genes were not found to be ribosomal protein genes, suggesting that in Dictyostelium non-ribosomal protein genes may be coordinately regulated with the ribosomal protein genes. Finally, using deletion analysis we show that the promoters of two of the V genes are composed of a constitutive positive element(s) located upstream of sequences involved in the regulated expression of these genes and within the first 545 upstream bp for V18 and 850 bp for V14. The regions involved in regulated expression were localized between -7 and -222 for V18 and -70 and -368 for V14. The sequences conferring protein synthesis sensitivity were shown to reside between -502 and -61 of the H4 promoter.