Major changes in gene expression occur during at least four stages of development of Dictyostelium discoideum

Analysis of two-dimensional protein patterns from developmental stages of the potato cyst-nematode,Globodera rostochiensis

Two-dimensional polyacrylamide gel electrophoresis was used to examine the differences in total protein composition between two motile stages and two sedentary stages of the potato cyst-nematode,Globodera rostochiensis. Using a sensitive silver stain, 542 reproducible protein spots were distinguished. A list of these spots is presented, showing their apparent molecular weights, estimated isoelectric points, and occurrences in the different developmental stages. When the protein patterns were compared, 401 spots were found to change their presence or size in one or more of the four developmental stages. It is therefore estimated that during the post-embryonic development ofG. rostochiensis, 74% of the polypeptides undergo modulation of their synthesis, or are affected by protein degradation or modification. In the motile stages several abundant proteins were present, which disappeared or decreased in concentration in the sedentary stages. Some of these proteins are presumably muscle proteins, and their modulation may illustrate the degeneration of body-wall musculature in the sedentary stages. It is concluded that the potato cyst-nematode has a very dynamic protein metabolism.

Identification and characterization of DdRPB4, a subunit of Dictyostelium discoideum RNA polymerase II

Rpb4, the fourth largest subunit of the eukaryotic RNA polymerase II (RNAPII), is required for growth at extreme temperatures and for an appropriate response to nutrient starvation in yeast. Sequence homologs of Rpb4 are found in most sequenced genomes from yeast to humans. To elucidate the role of this subunit in nutrient starvation, we chose Dictyostelium discoideum, a soil amoeba, which responds to nutrient deprivation by undergoing a complex developmental program. Here we report the identification of homolog of Saccharomyces cerevisiae RPB4 in D. discoideum. Localization and complementation studies suggest that Rpb4 is functionally conserved. DdRPB4 transcript and protein levels are developmentally regulated. Although DdRPB4 could not be deleted, overexpression revealed that the Rpb4 protein is essential for cell survival and is regulated stringently at the post-transcriptional level in D. discoideum. Thus maintaining a critical level of Rpb4 is important for this organism.

FebA: a gene for eukaryotic translation initiation factor 4E-binding protein (4E-BP) in Dictyostelium discoideum

We have identified a gene encoding a eukaryotic initiation factor 4E-binding protein (4E-BP) in the EST database of the Dictyostelium cDNA project. The Dictyostelium 4E-BP, designated febA (four e-binding), showed significant similarity to mammalian 4E-BPs. Northern blot analysis revealed that febA was expressed at a high level in the vegetative growth phase but the level of expression decreased during late development. The gene was shown to be non-essential since disruption of the gene had no severe effect; the null mutant proliferated normally and formed normal fruiting bodies. However, strains overexpressing the gene could not be established, suggesting that an excess of FebA protein may have a lethal effect on the cells.

Developmental decisions in Dictyostelium discoideum

A few hours after the onset of starvation, amoebae of Dictyostelium discoideum start to form multicellular aggregates by chemotaxis to centers that emit periodic cyclic AMP signals. There are two major developmental decisions: first, the aggregates either construct fruiting bodies directly, in a process known as culmination, or they migrate for a period as "slugs." Second, the amoebae differentiate into either prestalk or prespore cells. These are at first randomly distributed within aggregates and then sort out from each other to form polarized structures with the prestalk cells at the apex, before eventually maturing into the stalk cells and spores of fruiting bodies. Developmental gene expression seems to be driven primarily by cyclic AMP signaling between cells, and this review summarizes what is known of the cyclic AMP-based signaling mechanism and of the signal transduction pathways leading from cell surface cyclic AMP receptors to gene expression. Current understanding of the factors controlling the two major developmental choices is emphasized. The weak base ammonia appears to play a key role in preventing culmination by inhibiting activation of cyclic AMP-dependent protein kinase, whereas the prestalk cell-inducing factor DIF-1 is central to the choice of cell differentiation pathway. The mode of action of DIF-1 and of ammonia in the developmental choices is discussed.

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 regulation of the alpha-mannosidase gene in Dictyostelium discoideum: control is at the level of transcription and is affected by cell density

In Dictyostelium discoideum, there is a group of genes that are expressed following starvation and when exponentially growing cells reach high densities. We have examined the expression of one of these genes, alpha-mannosidase. Using an alpha-mannosidase cDNA probe in Northern (RNA) blot analysis, we have shown that the previously observed increase in alpha-mannosidase enzyme-specific activity during development is due to an increase in the levels of alpha-mannosidase mRNA. mRNA levels reach a maximum by 8 h of development and then begin to decline by 14 to 22 h. Using nuclear run-on analysis, we have found that this gene is regulated at the level of transcription. We also examined the effects of cell-cell contacts, cyclic AMP levels, and protein synthesis on expression of this gene and found that they were not critical in regulating its expression. However, cell density did play a major role in the expression of alpha-mannosidase. High cell density or the presence of buffer conditioned by high-density cells was sufficient to induce expression of alpha-mannosidase, indicating that this is one of the prestarvation response genes. Finally, the alpha-mannosidase gene was not expressed in aggregation-negative mutant strain HMW 404.

Developmental regulation of the alpha-mannosidase gene in Dictyostelium discoideum: control is at the level of transcription and is affected by cell density

In Dictyostelium discoideum, there is a group of genes that are expressed following starvation and when exponentially growing cells reach high densities. We have examined the expression of one of these genes, alpha-mannosidase. Using an alpha-mannosidase cDNA probe in Northern (RNA) blot analysis, we have shown that the previously observed increase in alpha-mannosidase enzyme-specific activity during development is due to an increase in the levels of alpha-mannosidase mRNA. mRNA levels reach a maximum by 8 h of development and then begin to decline by 14 to 22 h. Using nuclear run-on analysis, we have found that this gene is regulated at the level of transcription. We also examined the effects of cell-cell contacts, cyclic AMP levels, and protein synthesis on expression of this gene and found that they were not critical in regulating its expression. However, cell density did play a major role in the expression of alpha-mannosidase. High cell density or the presence of buffer conditioned by high-density cells was sufficient to induce expression of alpha-mannosidase, indicating that this is one of the prestarvation response genes. Finally, the alpha-mannosidase gene was not expressed in aggregation-negative mutant strain HMW 404.

Structure, expression, and regulation of members of the developmentally controlled V and H gene classes from Dictyostelium

We have examined the expression and structure of vegetative specific genes belonging to the V and H gene classes. Both classes of genes are deactivated at the onset of development by a reduction in the rate of transcription. Thus, the genes must be reactivated when the terminally differentiated spores germinate and the resulting amebae return to the vegetative state. During germination, activation of expression of most members of the V gene class was found to parallel the emergence of amoebae from the spore coats. The activation of the V genes did not occur when protein synthesis was inhibited. The timing of activation of the H genes was more heterogeneous and did not parallel emergence. H gene activation occurred even when protein synthesis was inhibited. V4 was found to be the only vegetative specific gene that was responsive to the presence of bacteria. V4 expression was induced by 25-100 fold via transcriptional activation when bacteria were added to amebae growing axenically. Isolation and sequence analysis of the corresponding genomic clones revealed that two V genes, V18 and V1, encode ribosomal proteins. Promoter analysis has delineated the sequences necessary for expression and regulation for several of the V and H genes. In all cases, expression was determined by sequences within the first several hundred base pairs of the transcription start site. For V18 and V14, a positive constitutive element was identified in addition to the sequences involved in regulation. Finally, all of the characterizations and findings are discussed in terms of postulated models for V and H gene expression and regulation.

Control of cAMP-induced gene expression by divergent signal transduction pathways

A compilation of literature data and recent experiments led to the following conclusions regarding cyclic adenosine 3':5' monophosphate (cAMP) regulation of gene expression. Several classes of cAMP-induced gene expression can be discriminated by sensitivity to stimulation kinetics. The aggregation-related genes respond only to nanomolar cAMP pulses. The prestalk-related genes respond both to nanomolar pulses and persistent micromolar stimulation. The prespore specific genes respond only to persistent micromolar stimulation. The induction of the aggregation- and prestalk-related genes by nanomolar cAMP pulses may share a common transduction pathway, which does not involve cAMP, while involvement of the inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway is unlikely. Induction of the expression of prespore and prestalk-related genes by micromolar cAMP stimuli utilizes divergent signal processing mechanisms. cAMP-induced prespore gene expression does not involve cAMP and probably also not cyclic guanosine 3'.5' monophosphate (cGMP) as intracellular intermediate. Involvement of cAMP-induced phospholipase C (PLC) activation in this pathway is suggested by the observation that IP3 and 1,2-diacylglycerol (DAG) can induce prespore gene expression, albeit in a somewhat indirect manner and by the observation that Li+ and Ca2+ antagonists inhibit prespore gene expression. Cyclic AMP induction of prestalk-related gene expression is inhibited by IP3 and DAG and promoted by Li+, and is relatively insensitive to Ca2+ antagonists, which indicates that PLC activation does not mediate prestalk-related gene expression. Neither prespore nor prestalk-related gene expression utilizes the sustained cAMP-induced pHi increase as intracellular intermediate.

Cytoplasmic pH of Dictyostelium discoideum amebae during early development: identification of two cell subpopulations before the aggregation stage

Development of the cellular slime mold Dictyostelium discoideum is initiated by the removal of nutrients, and results in formation of a mature fruiting body composed of two cell types, the stalk and spore cells. A considerable body of evidence supports the hypothesis that cytoplasmic pH may be an essential regulator of the choice to differentiate in either the prestalk or prespore pathway. We have devised methods for measurement and analysis of intracellular pH in developing Dictyostelium amebae in order to assess directly the potential role of cytoplasmic pH in regulating the pathway of differentiation. The intracellular pH of single D. discoideum amebae during development and in intact slugs has been measured using the pH-sensitive indicator pyranine in a low light level microspectrofluorometer. We have used the ATP-mediated loading method to introduce pyranine into these cells. Cells loaded by the ATP method appear healthy, have no detectable defects in development, and exhibit a similar population distribution of intracellular pH to those loaded by sonication. The intracellular pH of populations comprised of single amebae was found to undergo a transient acidification during development resulting in a bimodal distribution of intracellular pH. The subpopulations were characterized by fitting two gaussian distributions to the data. The number of cells in the acidic intracellular pH subpopulation reached a maximum 4 h after initiation of development, and had returned to a low level by 7 h of development. In addition, a random sample of single amebae within a slug had a median intracellular pH of 7.2, nearly identical to the median pH (7.19) of similarly treated vegetative cells. No gradient of intracellular pH along the anterior to posterior axis of the slug was detected. Our data demonstrate the existence of two distinct subpopulations of cells before the aggregation stage of development in Dictyostelium, and offers support for the hypothesis that changes in intracellular pH contribute to development in D. discoideum.

Expression and organization of BP74, a cyclic AMP-regulated gene expressed during Dictyostelium discoideum development

We have characterized a cDNA and the corresponding gene for a cyclic AMP-inducible gene expressed during Dictyostelium development. This gene, BP74, was found to be first expressed about the time of aggregate formation, approximately 6 h after starvation. Accumulation of BP74 mRNA did not occur in Dictyostelium cells that had been starved in fast-shaken suspension cultures but was induced in similar cultures to which cyclic AMP pulses had been added. The BP74 cDNA and gene were characterized by DNA sequence analysis and transcriptional mapping. When the BP74 promoter region was fused with a chloramphenicol acetyltransferase reporter gene and reintroduced into Dictyostelium cells, the transfected chloramphenicol acetyltransferase gene displayed the same developmentally regulated pattern of expression as did the endogenous BP74 gene, suggesting that all of the cis-acting elements required for regulated expression were carried by a 2-kilobase cloned genomic fragment. On the basis of sequence analysis, the gene appeared to encode a protein containing a 20-residue hydrophobic sequence at the amino-terminal end and 26 copies of a 20-amino-acid repeat.

Expression and organization of BP74, a cyclic AMP-regulated gene expressed during Dictyostelium discoideum development

We have characterized a cDNA and the corresponding gene for a cyclic AMP-inducible gene expressed during Dictyostelium development. This gene, BP74, was found to be first expressed about the time of aggregate formation, approximately 6 h after starvation. Accumulation of BP74 mRNA did not occur in Dictyostelium cells that had been starved in fast-shaken suspension cultures but was induced in similar cultures to which cyclic AMP pulses had been added. The BP74 cDNA and gene were characterized by DNA sequence analysis and transcriptional mapping. When the BP74 promoter region was fused with a chloramphenicol acetyltransferase reporter gene and reintroduced into Dictyostelium cells, the transfected chloramphenicol acetyltransferase gene displayed the same developmentally regulated pattern of expression as did the endogenous BP74 gene, suggesting that all of the cis-acting elements required for regulated expression were carried by a 2-kilobase cloned genomic fragment. On the basis of sequence analysis, the gene appeared to encode a protein containing a 20-residue hydrophobic sequence at the amino-terminal end and 26 copies of a 20-amino-acid repeat.

Dictyostelium discoideum: a model system for cell-cell interactions in development

The cellular slime mold Dictyostelium discoideum undergoes a transition from single-celled amoebae to a multicellular organism as a natural part of its life cycle. A method of cell-cell signaling that controls chemotaxis, morphogenesis, and gene expression has developed in this organism, and a detailed understanding of this signaling system provides clues to mechanisms of intercellular communication in the development of metazoans.

Developmental changes in protein composition and the actin-binding protein ponticulin in Dictyostelium discoideum plasma membranes purified by an improved method

We have used a new combination of previously-described methods to obtain a 29-fold purification of plasma membranes from Dictyostelium discoideum. In this procedure, the pellet from a cell lysate is centrifuged through a high-pH sucrose gradient and then through a Renografin gradient. Electron microscopy shows that the resultant "Renografin membranes" are essentially homogeneous. As measured by enzymatic marker assays, contamination with mitochondria, lysosomes, and endoplasmic reticulum is minimal. As assayed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), the protein composition of Renografin membranes is similar to that of highly purified membranes isolated using concanavalin A stabilization and detergent extraction. Using Renografin membranes, we have examined developmental changes in the membrane protein composition. In agreement with previous investigations, we observe major changes in lectin-binding glycoproteins and cell-surface-labeled proteins during the first 18 h of D. discoideum development. In contrast to most previous work, which may have employed plasma membranes of lesser purity, we also observe major changes in silver-stained membrane proteins. We conclude that many developmentally regulated proteins, previously thought to be minor membrane constituents, are a larger proportion of the plasma membrane than originally believed. The observed changes in membrane protein composition may correlate with changes in plasma membrane functions during development. For instance, ponticulin, the major salt-sensitive F-actin-binding protein in plasma membranes from vegetative cells, increases at least twofold in plasma membranes during early development when the cells are chemotaxing into large aggregates. The amount of plasma membrane ponticulin then decreases during the pseudoplasmodial stage.

Characterization of genes which are transiently expressed during the preaggregative phase of development of Dictyostelium discoideum

We have identified and characterized three genes, the I genes (I for induced), which are induced during the preaggregative phase of the developmental program of Dictyostelium discoideum. None of these genes are expressed in cells growing vegetatively on bacteria or in axenic broth, and their induction during early development is due to transcriptional activation. Developmental expression of I6, I8, and I11 occurs even in the absence of protein synthesis. Their induction is very rapid and occurs essentially at the onset of development. The expression is transient, peaking between 2 and 4 hr followed by a rapid loss of expression. These characteristics suggest that the induction of I6, I8, and I11 is a primary result of the initiation of development, and thus they represent the first such genes isolated. Although their expression behavior shares these characteristics, examination of their expression under various conditions of development and in a variety of aggregation-deficient mutant strains reveals that the details of the regulation and developmental control of these three genes are distinct.

Codon preference in Dictyostelium discoideum

Dictyostelium discoideum is of increasing interest as a model eukaryotic cell because its many attributes have recently been expanded to include improved genetic and biochemical manipulability. The ability to transform Dictyostelium using drug resistance as a selectable marker (1) and to gene target by high frequency homologous integration (2) makes this organism particularly useful for molecular genetic approaches to cell structure and function. Given this background, it becomes important to analyze the codon preference used in this organism. Dictyostelium displays a strong and unique overall codon preference. This preference varies between different coding regions and even varies between coding regions from the same gene family. The degree of codon preference may be correlated with expression levels but not with the developmental time of expression of the gene product. The strong codon preference can be applied to identify coding regions in Dictyostelium DNA and aid in the design of oligonucleotide probes for cloning Dictyostelium genes.

Genetics of early Dictyostelium discoideum development

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Effect of protein synthesis inhibition on gene expression during early development of Dictyostelium discoideum

Several genes which are deactivated on the initiation of development of Dictyostelium discoideum were identified by differential screening of various cDNA libraries. These genes have in common a decrease in the steady-state levels of their corresponding mRNAs on the onset of development and as development proceeds. When development was carried out in the absence of protein synthesis by inhibition with cycloheximide, the decrease in mRNA levels for most genes (V genes) was normal or slightly accelerated. For about 5% of the genes (H genes), however, cycloheximide caused an apparent induction of expression, as revealed by a slight or dramatic increase in mRNA levels, instead of the normal decrease. This effect was due to inhibition of protein synthesis and not to cycloheximide per se. The induction was found to be due to an enhancement of the transcription rate; normal rates of transcription for the H genes were dependent on continued protein synthesis during vegetative growth and development. Thus, two general regulatory classes exist for deactivation of gene expression on initiation of development, one of which is dependent on and one of which is independent of protein synthesis. Analysis of expression of these genes in mutant strains which are aggregation deficient allowed the classes to be subdivided further. Taken together, these characterizations allow several distinct regulatory mechanisms to be identified that are involved in the deactivation of gene expression on the onset of development in D. discoideum.

Characterization of an unusual cAMP receptor and its related polypeptides in Dictyostelium discoideum

Several lines of evidence indicate that cAMP modulates developmental gene activity via cell-surface receptors. We describe here a novel cAMP receptor, CABP1, whose properties are consistent with the idea that this protein is involved in gene regulation. Firstly, immunological techniques using anti-CABP1 antibodies as probes showed that this cAMP receptor can be detected on the surface of developing cells. Secondly, there is a steady migration of CABP1 to the nucleus during development. Thirdly, some genetic variants exhibiting an altered pattern of development are found to possess modified CABP1. We also showed that CABP1 co-purifies with at least seven other polypeptides which share common epitopes with CABP1. Interestingly, four of the CABP1-related polypeptides can be detected on the cell surface as well as in the nucleus.

Effect of protein synthesis inhibition on gene expression during early development of Dictyostelium discoideum

Several genes which are deactivated on the initiation of development of Dictyostelium discoideum were identified by differential screening of various cDNA libraries. These genes have in common a decrease in the steady-state levels of their corresponding mRNAs on the onset of development and as development proceeds. When development was carried out in the absence of protein synthesis by inhibition with cycloheximide, the decrease in mRNA levels for most genes (V genes) was normal or slightly accelerated. For about 5% of the genes (H genes), however, cycloheximide caused an apparent induction of expression, as revealed by a slight or dramatic increase in mRNA levels, instead of the normal decrease. This effect was due to inhibition of protein synthesis and not to cycloheximide per se. The induction was found to be due to an enhancement of the transcription rate; normal rates of transcription for the H genes were dependent on continued protein synthesis during vegetative growth and development. Thus, two general regulatory classes exist for deactivation of gene expression on initiation of development, one of which is dependent on and one of which is independent of protein synthesis. Analysis of expression of these genes in mutant strains which are aggregation deficient allowed the classes to be subdivided further. Taken together, these characterizations allow several distinct regulatory mechanisms to be identified that are involved in the deactivation of gene expression on the onset of development in D. discoideum.

Developmentally regulated expression of temporally distinct beta-glucosidase isozymes in Dictyostelium discoideum

During development of Dictyostelium discoideum, the cellular specific activity of beta-glucosidase increases before aggregation, declines to low levels during pseudoplasmodium formation, and increases rapidly during culmination. In addition, two electrophoretically distinct isozymes of beta-glucosidase are present at different times of development. Using enzyme-specific monoclonal antibodies, we have shown that changes in the level of enzyme specific activity are closely paralleled by changes in the relative rate of beta-glucosidase synthesis in vivo and by corresponding changes in the relative cellular concentration of functional beta-glucosidase mRNA. Thus, the developmental synthesis of beta-glucosidase is controlled at a pretranslational level. Furthermore, our experiments have demonstrated that both beta-glucosidase isozymes consist of a single subunit of identical molecular weight. This result is consistent with the previous finding that both isozymes are encoded by the same gene and suggests that the isozymes differ solely with respect to post-translational modification.

A late developmental change in lysosomal enzyme sulfation specific to newly synthesized proteins in Dictyostelium discoideum

During development in Dictyostelium discoideum, several lysosomal glycosidases undergo changes in post-translational modification that are thought to involve differences in the extent of sulfation or phosphorylation, and appear to be required for the maintenance of cellular enzyme activity late in development. We have used monoclonal antibodies specific to the lysosomal enzyme alpha-mannosidase-1 to study the major late (12 hr) developmental change in the modification system. Pulse-chase experiments performed both early and late in development reveal that the substrate for the late form of modification is restricted to newly synthesized alpha-mannosidase-1 precursor protein. We have identified one modification difference between the two developmentally distinct isozymes of alpha-mannosidase-1: 35SO4 pulse-chase data show that the newly synthesized "late" enzyme precursor is significantly undersulfated in comparison with the enzyme synthesized early in development. This apparent lack of sulfation is associated with the lack of acquisition of endoglycosidase H resistance. By contrast, an aggregation-deficient mutant, which is defective with regard to the accumulation of alpha-mannosidase-1 activity late in development, synthesizes the "early" sulfated form of the enzyme throughout development. We conclude that the late developmental change in post-translational modification specifically involves one of the biochemical steps in which the N-linked oligosaccharide side chains of the newly synthesized alpha-mannosidase-1 precursor are modified by sulfation.

Density-dependent induction of discoidin-I synthesis in exponentially growing cells of Dictyostelium discoideum

The synthesis of the lectin, discoidin I, by vegetative cells of Dictyostelium discoideum (strain NC4) was monitored using immunoblot analysis and indirect immunofluorescence. Suspension cultures were used, so that the D. discoideum cell density and the concentration of bacteria could be controlled. Discoidin-I production was found to be a function of the relative densities of D. discoideum cells and food bacteria. Synthesis was initiated in exponentially growing D. discoideum cells approximately three generations before depletion of the food supply. In the growth medium of cells producing discoidin I, a soluble activity was detected that caused low-density cells to begin discoidin-I synthesis. This activity was not dialyzable and was destroyed by heat. A similar activity was produced by AX3 cells during axenic growth. Density-dependent induction of other 'early developmental' proteins was also detected in wild-type cells. These findings suggest that the expression of several 'early developmental' genes is regulated by a mechanism that measures cell density relative to food supply, not by starvation per se.

Translational control of ribosomal protein synthesis during early Dictyostelium discoideum development

Throughout the developmental program of Dictyostelium discoideum there are substantial changes in the rates of both ribosome utilization and rRNA transcription and processing. We examined the regulation of ribosomal protein (r-protein) gene expression and found that, at the start of development, expression of these genes was drastically and specifically reduced by a block to translational initiation. An apparently separate event signals a sudden decrease in the relative amount of r-protein mRNA at about 10 h of development, a time when aggregated amoebae are forming tight cell-cell contacts. For the first 9 h of development, the relative amount of r-protein mRNA remained essentially unchanged and comparable to levels detected in growing cells. While the r-protein mRNAs were almost fully loaded on polysomes during vegetative growth, they were specifically excluded from polysomes at the start of development. The translational block was not the result of irreversible structural changes which inactivate the r-protein mRNAs since they remained translatable both in vitro, in wheat germ extracts, and in vivo, where they were recruited onto polysomes in the presence of the elongation inhibitor cycloheximide. In addition, precise measurements of poly(A) tail lengths on individual hybrid-selected mRNA species showed that there is no difference in the poly(A) tail length of r-protein mRNA isolated from growing cells and 1-h developing cells. Therefore, changes in translational efficiency cannot be attributed to cleavage of poly(A) tails.

Gene regulation during dedifferentiation in Dictyostelium discoideum

During development of Dictyostelium discoideum, cells acquire the capacity to rapidly recapitulate morphogenesis. Therefore, when cells at the loose aggregate stage are disaggregated and challenged to reaggregate, they do so in a tenth of the original time. If loose aggregate cells are disaggregated and resuspended in buffered dextrose solution (erasure medium), they retain the capacity of rapid recapitulation for 80 min, then completely lose this capacity in a single, synchronous step referred to as the "erasure event." The erasure event sets in motion a program of dedifferentiation during which cells lose developmentally acquired characteristics at different times. The erasure event is inhibited by the addition of 10(-4) M cAMP to erasure medium. The synthesis of 33 growth-associated polypeptides, the synthesis of 53 development-associated polypeptides, and the level of 2 development-associated RNAs have been monitored during the erasure program and in cultures inhibited from erasing by the addition of 10(-4) M cAMP. Growth-associated polypeptides begin to be resynthesized and development-associated polypeptides exhibit dramatic decreases in rate of synthesis at different times throughout the first 240 min in erasure medium. Inhibiting the erasure event with cAMP has no major effect in the resynthesis of the majority of growth-associated polypeptides. Only one growth-associated polypeptide, V28, is completely inhibited by cAMP, suggesting that it may play a role in the erasure process. In contrast, inhibiting the erasure event with cAMP has a marked effect on the synthesis of development-associated polypeptides, causing a dramatic reduction in the rate at which synthesis decreases for 6 polypeptides, and completely inhibits the decrease in the synthetic rate of 8 polypeptides. The two development-associated RNAs, 16G1 and 10C3, exhibit two distinctly different patterns of loss during erasure, but in both cases cAMP added at time zero of the erasure process dramatically retards or inhibits loss. In addition, when cAMP is added just prior to the erasure event, it inhibits the erasure event and stimulates a rapid increase in the level of 16G1 RNA back to the developmental level. The level of 16G1 RNA then remains at this level for at least 400 min. When cAMP is added after the erasure event, it causes a low, transient increase in the level of 16G1 RNA. These results are considered both in relation to the program of erasure, and in relation to the role of cAMP in the expression of developmental genes during the forward program of development.

Translocation of an unusual cAMP receptor to the nucleus during development of Dictyostelium discoideum

cAMP has been implicated in the control of the expression of developmental genes in Dictyostelium discoideum. To determine the potential role of cAMP receptors as regulators of gene expression, we have used immunocytochemical and immunoblotting techniques to reveal the subcellular localization of a cAMP binding protein CABP1. Most of the CABP1 antigen in early developing cells is localized near the cell periphery, with a small amount found in the nucleus. The level of CABP1 in the nucleus increases approximately 30-fold during development. Moreover, immunofluorescence studies showed that CABP1 can also be detected on the cell surface. Binding of anti-CABP1 to intact cells followed by reaction with 125I-labeled secondary antibody revealed that the cell-surface CABP1 activity peaks during aggregation and culmination. In addition, several proteins related to CABP1 are found mainly in the nuclear fraction of developing cells. The possible role of these proteins in the regulation of developmental gene activity is discussed.

Translational control of ribosomal protein synthesis during early Dictyostelium discoideum development

Throughout the developmental program of Dictyostelium discoideum there are substantial changes in the rates of both ribosome utilization and rRNA transcription and processing. We examined the regulation of ribosomal protein (r-protein) gene expression and found that, at the start of development, expression of these genes was drastically and specifically reduced by a block to translational initiation. An apparently separate event signals a sudden decrease in the relative amount of r-protein mRNA at about 10 h of development, a time when aggregated amoebae are forming tight cell-cell contacts. For the first 9 h of development, the relative amount of r-protein mRNA remained essentially unchanged and comparable to levels detected in growing cells. While the r-protein mRNAs were almost fully loaded on polysomes during vegetative growth, they were specifically excluded from polysomes at the start of development. The translational block was not the result of irreversible structural changes which inactivate the r-protein mRNAs since they remained translatable both in vitro, in wheat germ extracts, and in vivo, where they were recruited onto polysomes in the presence of the elongation inhibitor cycloheximide. In addition, precise measurements of poly(A) tail lengths on individual hybrid-selected mRNA species showed that there is no difference in the poly(A) tail length of r-protein mRNA isolated from growing cells and 1-h developing cells. Therefore, changes in translational efficiency cannot be attributed to cleavage of poly(A) tails.

Characterization of genes which are deactivated upon the onset of development in Dictyostelium discoideum

We have identified and begun characterizations of the differential expression of 15 genes whose corresponding mRNA levels decrease during the preaggregative period of the developmental program of Dictyostelium discoideum. Upon the onset of development, the mRNAs decrease from 5- to 1000-fold over the first 8 hr. The rates of loss of each mRNA were similar to one another but distinct, and the decreases were dependent on progress through the developmental program. One exception to this dependency was observed, and the decrease in this mRNA was dependent on the absolute time after initiation of development instead of progress through development. With two exceptions, the decreases in mRNA levels were dependent on developmental conditions and were not seen when cells were shaken in starvation buffer. When the polysomal distributions of each species were examined, three classes were found: most showed no significant shifts off of polysomes upon initiation of development, two were characterized by a 20% shift to nonpolysomal RNA fractions upon development, and two gave a 40-50% shift. Collectively, these characterizations reveal differences in behavior which suggest that deactivation of genes upon initiation of development in Dictyostelium involves more than one regulatory pathway.

Distinct developmental regulation and properties of the responsiveness of different genes to cyclic AMP in Dictyostelium discoideum

Cyclic AMP (cAMP) is known to be an important mediator of gene expression in eukaryotic cells. At present, little is known about the developmental events which render specific genes responsive to cAMP in distinct cell types, or about the biochemical mechanisms by which cAMP exerts these regulatory effects. By examining the effects of cAMP treatment on specific mRNA levels in Dictyostelium discoideum cells with different 'developmental histories', we defined the developmental states in which specific genes display responsiveness to cAMP. We focused on two specific rapid responses: the ability of cAMP to inhibit the expression of an 'early' developmentally regulated mRNA (discoidin-I) and to stimulate the expression of a 'late', prespore-specific mRNA (PL3). Using this approach, we showed that, for both mRNAs, the ability to respond rapidly to cAMP is absent from vegetative cells grown on bacteria, and is acquired during development on filters. Furthermore, we identified several developmental states in which the discoidin-I response to cAMP is present, but in which the PL3 response is not. In experiments designed to examine the effects of cAMP analogues on the levels of these two mRNAs, we demonstrated that the analogue specificities of the discoidin-I and PL3 responses are different, and that the specificity for the PL3 response depends on the developmental state. The developmental kinetics and analogue specificity of the PL3 response suggest a two-step mode of action of cAMP in activating the expression of this gene. We discuss possible implications of these findings for the mechanisms of action of exogenous cAMP as well as for the role of cAMP in controlling the changes in gene expression that accompany normal development.

Postaggregative differentiation induction by cyclic AMP in Dictyostelium: intracellular transduction pathway and requirement for additional stimuli

Cyclic AMP induces postaggregative differentiation in aggregation competent cells of Dictyostelium by interacting with cell surface cAMP receptors. We investigated the transduction pathway of this response and additional requirements for the induction of postaggregative differentiation. Optimal induction of postaggregative gene expression requires that vegetative cells are first exposed to 2-4 hr of nanomolar cAMP pulses, and subsequently for 4-6 hr to steady-state cAMP concentrations in the micromolar range. Cyclic AMP pulses, which are endogenously produced before and during aggregation, induce full responsiveness to cAMP as a morphogen. The transduction pathway from the cell surface cAMP receptor to postaggregative gene expression may involve Ca2+ ions as intracellular messengers. A cAMP-induced increase in intracellular cAMP or cGMP levels is not involved in the transduction pathway.

A gene expressed in undifferentiated vegetative Dictyostelium is repressed by developmental pulses of cAMP and reinduced during dedifferentiation

We describe the gene M4-1, whose unique pattern of developmental expression will allow us to study the molecular mechanisms controlling expression in undifferentiated cells in addition to repression in response to cAMP during development and reinduction during dedifferentiation. M4-1 is a Dictyostelium gene expressed in the undifferentiated cell. We have shown that M4-1 continues to be expressed very early during the developmental cycle but is repressed at a later stage of development, at a time coincident with the establishment of oscillations in the cAMP pool. Studies on the expression of the M4-1 gene in shaking culture, under conditions that mimic early development, have established that pulsatile stimulation of cells with cAMP is sufficient to repress M4-1 expression. Consistent with this, cells that are exposed to high levels of cAMP are unable to respond normally to cAMP oscillations and continue to express M4-1 at vegetative levels. These data indicate that low-level oscillations of cAMP are required for the repression of M4-1 expression rather than the continuous high levels of cAMP responsible for the regulation of a different class of Dictyostelium genes. We suggest that cAMP may mediate developmental expression of the Dictyostelium genome by different mechanisms. We also show that cell-cell interaction, a developmental event that occurs subsequent to the cAMP pulse, does not normally influence the regulation of M4-1. Finally, we have shown that when cAMP-pulsed cells are induced to dedifferentiate, M4-1 RNA sequences rapidly reappear in nuclei and cytoplasm, suggesting that regulation of M4-1 expression is primarily mediated at the level of transcription.

Lysosomal enzyme inactivation associated with defects in post-translational modification during development in Dictyostelium discoideum

The developmental accumulation of lysosomal alpha-mannosidase-1 activity in Dictyostelium discoideum is controlled at the level of de novo enzyme precursor biosynthesis. Aggregation-deficient mutants are defective with regard to the accumulation of alpha-mannosidase-1 activity beyond 8-16 h of development. We used enzyme-specific monoclonal antibodies to show that the activity defect in aggregation-deficient strains is not due to a lack of alpha-mannosidase-1-precursor synthesis or processing, or to preferential degradation of the mature enzyme protein. Instead, the defect is a result of enzyme inactivation: cells of aggregation-deficient strains contain significant amounts of inactive alpha-mannosidase-1 protein late in development. The alpha-mannosidase-1 inactivation phenotype is associated with a more general defect in lysosomal enzyme modification. A change in the post-translational modification system occurs during normal slime-mold development, as shown by differences in enzyme isoelectric point, antigenicity, and thermolability. We found that this change in modification does not occur in mutant strains blocked early in development. We propose a model in which pleiotropic mutations in early aggregation-essential genes can indirectly affect the accumulation of alpha-mannosidase-1 activity by preventing the expression of a developmentally controlled change in the post-translational modification system, a change which is required for the stability of several lysosomal enzymes late in development.

alpha-Mannosidase-1 mutants of Dictyostelium discoideum: early aggregation-essential genes regulate enzyme precursor synthesis, modification, and processing

The lysosomal enzyme alpha-mannosidase-1 is one of the earliest developmentally controlled gene products in Dictyostelium discoideum. Although this enzyme is synthesized throughout the first 20 h of development, it is not required for complete morphogenesis, since structural gene (manA) mutants lacking activity develop normally. We isolated six strains deficient in alpha-mannosidase-1 activity which, unlike structural gene mutants, fail to aggregate. Fruiting revertants of these strains accumulate wild-type levels of alpha-mannosidase-1 activity, suggesting that both the enzymatic and morphological defects are caused by single mutations in nonstructural genes essential for early development. Direct genetic evidence for mutations outside of the structural gene was obtained by complementation analysis. We used alpha-mannosidase-1-specific monoclonal antibodies to analyze the biochemical defects in these mad (alpha-mannosidase-1-deficient) mutants. All mad mutants show a significantly reduced relative rate of enzyme precursor biosynthesis. The mad-404 mutation results in a complete lack of precursor biosynthesis, as well as a lack of functional alpha-mannosidase-1 mRNA. In some cases, however, the enzymatic defect results from improper post-translational modification which affects precursor processing. We conclude that a small number of aggregation-essential genes are involved in regulating the synthesis, modification, and processing of alpha-mannosidase-1 during development.

Regulation of lysosomal alpha-mannosidase-1 synthesis during development in Dictyostelium discoideum

The cellular specific activity of lysosomal alpha-mannosidase-1 increases dramatically during development in Dictyostelium discoideum. alpha-Mannosidase-1 is composed of two subunits (Mr = 58,000 and 60,000) which are derived from a common precursor polypeptide (Mr = 140,000). Using enzyme-specific monoclonal antibodies we have determined that throughout development (a) the relative rate of precursor biosynthesis closely parallels the rate of accumulation of cellular enzyme activity and (b) the newly synthesized precursor is efficiently processed to mature enzyme (t1/2 less than 10 min). This indicates that the developmental accumulation of alpha-mannosidase-1 activity is primarily controlled by de novo enzyme synthesis. Furthermore, the change in the relative rate of enzyme precursor synthesis can be accounted for by an increase in the cellular level of functional alpha-mannosidase-1 mRNA during development.