Biochemistry of Aggregation in Dictyostelium. A review

Migration of Dictyostelium discoideum to the Chemoattractant Folic Acid

Dictyostelium discoideum can be grown axenically in a cultured media or in the presence of a natural food source, such as the bacterium Klebsiella aerogenes (KA). Here we describe the advantages and methods for growing D. discoideum on a bacterial lawn for several processes studied using this model system. When grown on a bacterial lawn, D. discoideum show positive chemotaxis towards folic acid (FA). While these vegetative cells are highly unpolarized, it has been shown that the signaling and cytoskeletal molecules regulating the directed migration of these cells are homologous to those seen in the motility of polarized cells in response to the chemoattractant cyclic adenosine monophosphate (cAMP). Growing D. discoideum on KA stimulates chemotactic responsiveness to FA. A major advantage of performing FA-mediated chemotaxis is that it does not require expression of the cAMP developmental program and therefore has the potential to identify mutants that are purely unresponsive to chemoattractant gradients. The cAMP-mediated chemotaxis can appear to fail when cells are developmentally delayed or do not up-regulate genes needed for cAMP-mediated migration. In addition to providing robust chemotaxis to FA, cells grown on bacterial lawns are highly resistant to light damage during fluorescence microscopy. This resistance to light damage could be exploited to better understand other biological processes such as phagocytosis or cytokinesis. The cell cycle is also shortened when cells are grown in the presence of KA, so the chances of seeing a mitotic event increases.

Cell signaling during development of Dictyostelium

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

Identification of a farnesol analog as a Ras function inhibitor using both an in vivo Ras activation sensor and a phenotypic screening approach

Mutations in Ras isoforms such as K-Ras, N-Ras, and H-Ras contribute to roughly 85, 15, and 1% of human cancers, respectively. Proper membrane targeting of these Ras isoforms, a prerequisite for Ras activity, requires farnesylation or geranylgeranylation at the C-terminal CAAX box. We devised an in vivo screening strategy based on monitoring Ras activation and phenotypic physiological outputs for assaying synthetic Ras function inhibitors (RFI). Ras activity was visualized by the translocation of RBD Raf1 -GFP to activated Ras at the plasma membrane. By using this strategy, we screened one synthetic farnesyl substrate analog (AGOH) along with nine putative inhibitors and found that only m-CN-AGOH inhibited Ras activation. Phenotypic analysis of starving cells could be used to monitor polarization, motility, and the inability of these treated cells to aggregate properly during fruiting body formation. Incorporation of AGOH and m-CN-AGOH to cellular proteins was detected by western blot. These screening assays can be incorporated into a high throughput screening format using Dictyostelium discoideum and automated microscopy to determine effective RFIs. These RFI candidates can then be further tested in mammalian systems.

Delineating the core regulatory elements crucial for directed cell migration by examining folic-acid-mediated responses

Dictyostelium discoideum shows chemotaxis towards folic acid (FA) throughout vegetative growth, and towards cAMP during development. We determined the spatiotemporal localization of cytoskeletal and signaling molecules and investigated the FA-mediated responses in a number of signaling mutants to further our understanding of the core regulatory elements that are crucial for cell migration. Proteins enriched in the pseudopods during chemotaxis also relocalize transiently to the plasma membrane during uniform FA stimulation. In contrast, proteins that are absent from the pseudopods during migration redistribute transiently from the PM to the cytosol when cells are globally stimulated with FA. These chemotactic responses to FA were also examined in cells lacking the GTPases Ras C and G. Although Ras and phosphoinositide 3-kinase activity were significantly decreased in Ras G and Ras C/G nulls, these mutants still migrated towards FA, indicating that other pathways must support FA-mediated chemotaxis. We also examined the spatial movements of PTEN in response to uniform FA and cAMP stimulation in phospholipase C (PLC) null cells. The lack of PLC strongly influences the localization of PTEN in response to FA, but not cAMP. In addition, we compared the gradient-sensing behavior of polarized cells migrating towards cAMP to that of unpolarized cells migrating towards FA. The majority of polarized cells make U-turns when the cAMP gradient is switched from the front of the cell to the rear. Conversely, unpolarized cells immediately extend pseudopods towards the new FA source. We also observed that plasma membrane phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] levels oscillate in unpolarized cells treated with Latrunculin-A, whereas polarized cells had stable plasma membrane PtdIns(3,4,5)P3 responses toward the chemoattractant gradient source. Results were similar for cells that were starved for 4 hours, with a mixture of polarized and unpolarized cells responding to cAMP. Taken together, these findings suggest that similar components control gradient sensing during FA- and cAMP-mediated motility, but the response of polarized cells is more stable, which ultimately helps maintain their directionality.

Specific cell-cell contacts are essential for induction of gene expression during differentiation of Dictyostelium discoideum

Postaggregation Dictyostelium discoideum cells contain 2000-3000 mRNA species that are absent from pre-aggregation cells. These aggregation-dependent sequences compose 30% of the mass of the late mRNA and represent the transcription products of an additional 11% of the single-copy genome. By analysis of mutants that are blocked at different stages of differentiation, we show that induction of expression of these genes is correlated with the formation of tight cell-cell contacts that resist EDTA. In particular, mutants that exhibit chemotaxis and aggregate to form loose mounds but do not form cell-cell contacts that resist EDTA fail to induce these late mRNA and protein species. By contrast, mutants that form normal contacts but progress no further through development do express the late mRNA species. Thus, interactions at the cell surface are involved in developmental induction of a large group of coregulated mRNAs. We have employed two independent assays for these developmentally regulated mRNAs: hybridization of gel-separated RNAs to cloned nuclear DNAs and hybridization of mRNA to a cDNA probe specific for the population of 2000-3000 regulated sequences.

Thermotaxis of Dictyostelium discoideum amoebae and its possible role in pseudoplasmodial thermotaxis

Thermotaxis by individual amoebae of Dictyostelium discoideum on a temperature gradient is described. These amoebae show positive thermotaxis at temperatures between 14 degrees C and 28 degrees C shortly (3 hr) after food depletion. Increasing time on the gradient reduces the positive thermotactic response at the lower temperature gradients (midpoint temperatures of 14, 16, and 18 degrees C), and amoebae show an apparent negative thermotactic response after 12 hr on the gradient. The thermotaxis response curve for "wild-type" amoebae after 16 hr on the gradient is similar to that shown for the pseudoplasmodia. Growth of the amoebae at a different temperature causes a shift in the thermotaxis response curve for the amoebae. This adaptation is similar to that shown for the pseudoplasmodia. Two mutants in thermotaxis, HO428 and HO813, show changes in amoebal thermotaxis similar to the observed changes in pseudoplasmodial thermotaxis. On the basis of the similarities between these responses, thermotaxis by the amoebae is proposed to be the basis for thermotaxis by the multicellular pseudoplasmodium.

A Model Based on Receptor Desensitization for Cyclic AMP Signaling in Dictyostelium Cells

We analyze a model based on receptor modification for the cAMP signaling system that controls aggregation of the slime mold Dictyostelium discoideum after starvation. The model takes into account both the desensitization of the cAMP receptor by reversible phosphorylation and the activation of adenylate cyclase that follows binding of extracellular cAMP to the unmodified receptor. The dynamics of the signaling system is studied in terms of three variables, namely, intracellular and extracellular cAMP, and the fraction of receptor in active state. Using parameter values collected from experimental studies on cAMP signaling and receptor phosphorylation, we show that the model accounts qualitatively and, in a large measure, quantitatively for the various modes of dynamic behavior observed in the experiments: (a) autonomous oscillations of cAMP, (b) relay of suprathreshold cAMP pulses, i.e., excitability, characterized by both an absolute and a relative refractory period, and (c) adaptation to constant cAMP stimuli. A two-variable version of the model is used to demonstrate the link between excitability and oscillations by phase plane analysis. The response of the model to repetitive stimulation allows comprehension, in terms of receptor desensitization, of the role of periodic signaling in Dictyostelium and, more generally, the function of pulsatile patterns of hormone secretion.

Nucleocytoplasmic oscillations of the yeast transcription factor Msn2: evidence for periodic PKA activation

At intermediate intensities, stress induces oscillations in the nucleocytoplasmic shuttling of the transcription factor Msn2 in budding yeast. Activation by stress results in a reversible translocation of Msn2 from the cytoplasm to the nucleus. This translocation is negatively controlled by the cAMP-PKA pathway through Msn2 phosphorylation. Here we show that the nuclear localization signal (NLS) of Msn2 is necessary and sufficient to promote the nucleocytoplasmic oscillations of the transcription factor. Because the NLS is controlled by protein kinase A (PKA) phosphorylation, we use a computational model to investigate the possibility that the cAMP-PKA pathway could function as an oscillator driving the periodic shuttling of Msn2. The model indicates that sustained oscillations of cAMP can indeed occur in a range bounded by two critical values of stress intensity, owing to the negative feedback exerted by PKA on cAMP accumulation. We verify the predictions of the model in mutants by showing that suppressing this negative-feedback loop prevents the oscillatory shuttling but still promotes the stress-induced nuclear localization of Msn2. The physiological significance of Msn2 oscillations is discussed in the light of the frequency encoding of cellular rhythms.

Oscillations and waves of cyclic AMP in Dictyostelium: a prototype for spatio-temporal organization and pulsatile intercellular communication

The amoebae Dictyostelium discoideum aggregate after starvation in a wavelike manner in response to periodic pulses of cyclic AMP (cAMP) secreted by cells which behave as aggregation centers. In addition to autonomous oscillations, the cAMP signaling system that controls aggregation is also capable of excitable behavior, which consists in the transient amplification of suprathreshold pulses of extracellular cAMP. Since the first theoretical model for slime mold aggregation proposed by Keller and Segel in 1970, many theoretical studies have addressed various aspects of the mechanism and function of cAMP signaling in Dictyostelium. This paper presents a brief overview of these developments as well as some reminiscences of the author's collaboration with Lee Segel in modeling the dynamics of cAMP relay and oscillations. Considered in turn are models for cAMP signaling in Dictyostelium, the developmental path followed by the cAMP signaling system after starvation, the frequency encoding of cAMP signals, and the origin of concentric or spiral waves of cAMP.

Impact of mitochondrial Ca2+ cycling on pattern formation and stability

Energization of mitochondria significantly alters the pattern of Ca2+ wave activity mediated by activation of the inositol (1,4,5) trisphosphate (IP3) receptor (IP3R) in Xenopus oocytes. The number of pulsatile foci is reduced and spiral Ca2+ waves are no longer observed. Rather, target patterns of Ca2+ release predominate, and when fragmented, fail to form spirals. Ca2+ wave velocity, amplitude, decay time, and periodicity are also increased. We have simulated these experimental findings by supplementing an existing mathematical model with a differential equation for mitochondrial Ca2+ uptake and release. Our calculations show that mitochondrial Ca2+ efflux plays a critical role in pattern formation by prolonging the recovery time of IP3Rs from a refractory state. We also show that under conditions of high energization of mitochondria, the Ca2+ dynamics can become bistable with a second stable stationary state of high resting Ca2+ concentration.

A molecular network that produces spontaneous oscillations in excitable cells of Dictyostelium

A network of interacting proteins has been found that can account for the spontaneous oscillations in adenylyl cyclase activity that are observed in homogenous populations of Dictyostelium cells 4 h after the initiation of development. Previous biochemical assays have shown that when extracellular adenosine 3',5'-cyclic monophosphate (cAMP) binds to the surface receptor CAR1, adenylyl cyclase and the MAP kinase ERK2 are transiently activated. A rise in the internal concentration of cAMP activates protein kinase A such that it inhibits ERK2 and leads to a loss-of-ligand binding by CAR1. ERK2 phosphorylates the cAMP phosphodiesterase REG A that reduces the internal concentration of cAMP. A secreted phosphodiesterase reduces external cAMP concentrations between pulses. Numerical solutions to a series of nonlinear differential equations describing these activities faithfully account for the observed periodic changes in cAMP. The activity of each of the components is necessary for the network to generate oscillatory behavior; however, the model is robust in that 25-fold changes in the kinetic constants linking the activities have only minor effects on the predicted frequency. Moreover, constant high levels of external cAMP lead to attenuation, whereas a brief pulse of cAMP can advance or delay the phase such that interacting cells become entrained.

Modeling oscillations and waves of cAMP in Dictyostelium discoideum cells

We examine the theoretical aspects of temporal and spatiotemporal organization in the cAMP signaling system of Dictyostelium discoideum amoebae which aggregate in a wavelike manner after starvation, in response to pulses of cAMP emitted with a periodicity of several minutes by cells behaving as aggregation centers. We first extend the model based on receptor desensitization, previously proposed by Martiel and Goldbeter, by incorporating the role of G proteins in signal transduction. The extended model accounts for observations on the response of the signaling system to successive step increases in extracellular cAMP. In the presence of the positive feedback loop in cAMP synthesis, this model generates sustained oscillations in cAMP and in the fraction of active cAMP receptor, similar to those obtained in the simpler model where the role of the G proteins is not taken into account explicitly. We use the latter model to address the formation of concentric and spiral waves of cAMP in the course of D. discoideum aggregation. Previous analyses of the model showed that a progressive increase in the activity of adenylate cyclase and phosphodiesterase can account for the transitions no relay-relay-oscillations-relay observed in the experiments. We show that the degree of cellular synchronization on such a developmental path in parameter space markedly affects the nature of the spatial patterns generated by the model. These patterns range from concentric waves to a small number of large spirals, and finally to a large number of smaller spirals, as the degree of developmental desynchronization between cells increases.

Desynchronization of cells on the developmental path triggers the formation of spiral waves of cAMP during Dictyostelium aggregation

Whereas it is relatively easy to account for the formation of concentric (target) waves of cAMP in the course of Dictyostelium discoideum aggregation after starvation, the origin of spiral waves remains obscure. We investigate a physiologically plausible mechanism for the spontaneous formation of spiral waves of cAMP in D. discoideum. The scenario relies on the developmental path associated with the continuous changes in the activity of enzymes such as adenylate cyclase and phosphodiesterase observed during the hours that follow starvation. These changes bring the cells successively from a nonexcitable state to an excitable state in which they relay suprathreshold cAMP pulses, and then to autonomous oscillations of cAMP, before the system returns to an excitable state. By analyzing a model for cAMP signaling based on receptor desensitization, we show that the desynchronization of cells on this developmental path triggers the formation of fully developed spirals of cAMP. Developmental paths that do not correspond to the sequence of dynamic transitions no relay-relay-oscillations-relay are less able or fail to give rise to the formation of spirals.

Cyclic AMP oscillations in suspensions of Dictyostelium discoideum

A model developed previously for signal relay and adaptation in the cellular slime mould Dictyostelium discoideum is shown to account for the observed oscillations of calcium and cyclic AMP in cellular suspensions. A qualitative argument is given which explains how the oscillations arise, and numerical computations show how characteristics such as the period and amplitude of the periodic solutions depend on parameters in the model. Several extensions of the basic model are investigated, including the effect of cell aggregation and the effect of time delays in the activation and adaptation processes. The dynamics of mixed cell populations in which only a small fraction of the cells are capable of autonomous oscillation are also studied.

Genetics of early Dictyostelium discoideum development

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Partial purification of two myosin heavy chain kinases from Dictyostelium discoideum

Myosin heavy chain kinase activity was identified in the high speed supernate of lysed Dictyostelium amoebae and was precipitated by 30-50% ammonium sulphate. In low ionic strength buffer, the activity bound tightly to a Cibacron Blue Sepharose column and eluted as a single peak with 1.0 M NaCl. Gel filtration chromatography resolved the kinase into two activities, each of which phosphorylated the tail portion of purified Dictyostelium myosin. One of these activities phosphorylated both serine and threonine residues of the heavy chain, while the other activity only phosphorylated threonine residues. Peptide mapping studies indicated that in vivo and in vitro phosphorylation sites were identical. The heavy chain kinases required Mg2+ for activity but were unaffected by Ca2+ or calmodulin. The heavy chain kinases did not phosphorylate Dictyostelium light chain, and also did not phosphorylate myosins from striated, smooth, or other nonmuscle sources.

Changes during differentiation in requirements for cAMP for expression of cell-type-specific mRNAs in the cellular slime mold, Dictyostelium discoideum

A number of genes encoding developmentally regulated mRNAs in the cellular slime mold, Dictyostelium discoideum, have been described. Many of these are regulated by cAMP. Analysis of the earliest time at which elevated levels of cAMP can induce the expression of these mRNAs reveals a more complex pattern of regulation in which genes change in their ability to be induced in response to cAMP with developmental stage. A prestalk mRNA (C1/D11) previously thought not be regulated by elevated levels of cAMP is inducible by cAMP between aggregation and loose mound stage; later in development its expression becomes independent of elevated cAMP. The early prespore genes (prespore class I) also show two modes of regulation; early in development they are induced independently of continuous elevated levels of cAMP, while later in development their expression is dependent upon elevated cAMP. The period during development when the prestalk genes are cAMP inducible precedes by 2 hr the first time at which either the early prespore class I or late prespore class II mRNAs are inducible by continuous elevated levels of cAMP. Previous analysis of these mRNAs has been carried out using Dictyostelium cells grown axenically. In this report we have studied the developmental expression of these mRNAs in cells grown on bacteria. A substantial shutoff of the class I prestalk and early prespore (class I) mRNAs not seen in axenically grown cells is observed when bacterially grown cells are plated for development. Less than 10% of the maximal level of these mRNAs remains in the cells at the time of mature spore and stalk differentiation. Additionally, in the bacterially grown cells two distinct patterns of developmental regulation are observed for mRNAs which in axenically growing cells appear to be constitutively expressed throughout growth and development.

The developmental regulation of single-cell motility in Dictyostelium discoideum

The velocity of single amebae in the absence of a chemotactic signal has been analyzed during growth, development, rapid recapitulation, and dedifferentiation in the cellular slime mold Dictyostelium discoideum. It is demonstrated that (1) the velocity of axenically grown cells in half that of bacterially grown cells, (2) the velocity of bacterially grown cells decreased to roughly the same low level as axenically grown cells approximately 5 hr after the removal of exogeneous bacteria, (3) the velocity remains low for a 7-hr period preceding the onset of aggregation in both axenically and bacterially grown cells, (4) the velocity increases transiently at the onset of aggregation for both axenically and bacterially grown cells, (5) the velocity decreases to a very low level after the formation of loose aggregates and remains at that level at least through the early culminate I stage, (6) the velocity is not stimulated in 13-hr developing cells (finger stage) by inducing rapid recapitulation, (7) the velocity decreases after the erasure event in cultures of 7-hr developing cells (ripple stage) stimulated to undergo dedifferentiation, but the inhibition of the erasure event by the addition of 10(-4) M cAMP does not block this decrease. These results demonstrate that the basal level of single-cell motility in growing cultures is significantly influenced by the nutrient composition of the supporting medium, and that the transient increase in single-cell motility at the onset of aggregation is under the rigid control of the initial developmental program. Both rapid recapitulation and the program of dedifferentiation appear to have no influence on the basal level of single-cell motility.

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.

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

The spectrum of proteins synthesized at different stages of development of the cellular slime mold Dictyostelium discoideum was analyzed by two-dimensional (2D) gel electrophoresis. Of the approximately 400 proteins detected by this method 189 show changes in their relative rate of synthesis. Most of these changes occur during four distinct stages of development: commencement of development immediately following removal of nutrients (early interphase), early aggregation, late aggregation, and culmination. During commencement the synthesis of 19 proteins begins, the relative rate of synthesis of 21 other proteins increases, and 16 proteins show a rapid decrease in their synthetic rate. During early aggregation the largest change occurs in the spectrum of proteins being synthesized. Specifically, the synthesis of 29 new proteins begins and an increase occurs in the relative synthetic rate of 43 others. During late aggregation, when tight cell-cell contacts form, a reduction takes place in the synthetic rate of most of these induced proteins in addition to the synthesis of 12 new proteins. At least two of these induced proteins are synthesized exclusively in prespore and eventually spore cells. Finally, during culmination, 23 new proteins begin to be synthesized and the synthetic rate of 12 other proteins increases. Five of the 23 newly synthesized proteins appear to be stalk-cell specific. In general, synthesis of spore-cell specific proteins begins just following the formation of tight aggregates while stalk-cell specific proteins are induced during culmination. The relative amounts of mRNAs coding for most of the early developmentally regulated proteins have been estimated by their translation in rabbit reticulocyte lysates and subsequent analysis of protein products by 2D gel electrophoresis. For most of those proteins whose rate of synthesis increases in vivo following starvation there is a parallel increase in the cellular level of the functional mRNAs encoding them. This suggests that the genes coding for these mRNAs may be under transcriptional control. In contrast, the mRNAs coding for most of the proteins whose synthetic rate decreases early in development are under translational control and persist in the cell in an inactive state.

Developmental regulation of the cyclic-nucleotide-phosphodiesterase mRNA of Dictyostelium discoideum. Analysis by cell-free translation and immunoprecipitation

Extracellular cyclic-nucleotide phosphodiesterase of Dictyostelium discoideum has previously been purified and characterized [Orlow et al. (1981) J. Biol. Chem. 256, 7620-7627]. Antisera have been raised against the purified enzyme. Following cell-free translation of RNA extracted from cells at various stages of development and immunoprecipitation with anti-phosphodiesterase serum, cAMP phosphodiesterase synthesized in vitro and labeled with L-[35S]methionine can be detected by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and fluorography. The cell-free translation product is an Mr-48 000 polypeptide and can be immunoprecipitated with antiserum raised against active Mr-50 000 cAMP phosphodiesterase or antiserum raised against heat-denatured cAMP phosphodiesterase. Purified native cAMP phosphodiesterase blocks immunoprecipitation of the cAMP-phosphodiesterase polypeptide synthesized in vitro. A detectable level of cAMP-phosphodiesterase mRNA is present in axenically grown cells. After starvation of the cells in phosphate buffer for 1 h an increase of translatable cAMP-phosphodiesterase mRNA occurs, followed by a decrease and another increase. When cells are starved in the presence of the slowly hydrolyzed cAMP analogue, adenosine 3',5'-thiophosphate, the level of translatable cAMP-phosphodiesterase mRNA increases about tenfold and does not show a temporary decline. A maximum of 0.015% of the total acid-insoluble radioactivity is incorporated into the Mr-48 000 cAMP-phosphodiesterase polypeptide.

Accumulation of alpha-mannosidase-1 in Dictyostelium discoideum requires many developmentally essential genes

alpha-Mannosidase-1, one of the earliest known developmentally controlled gene products in the cellular slime mold Dictyostelium discoideum, accumulates intracellularly during both axenic growth and development. The accumulation of alpha-mannosidase-1 activity prematurely ceases in all of 125 randomly isolated aggregation-deficient mutants at discrete times in development resulting in significantly reduced levels of cellular enzyme activity. This suggests that, unlike other developmentally controlled enzymes in this organism, the continued accumulation of alpha-mannosidase-1 activity is controlled by a large number of genes essential for early development. alpha-Mannosidase-1 misregulation and the aggregation-deficient phenotype are caused by the same mutation since (1) morphological revertants exhibit a coreversion to both fruiting ability and wild-type alpha-mannosidase-1 accumulation and (2) normal enzyme accumulation depends on the ability to aggregate and ultimately fruit in a conditional aggregation-deficient mutant. This type of regulation does not appear to be due to differences in enzyme secretion or changes in the overall rate of total protein synthesis. Aggregation-deficient mutants continue to synthesize protein beyond the time in development at which alpha-mannosidase-1 accumulation ceases. Our studies indicate that most of the 50-125 genes required for aggregation in Dictyostelium are also required for the normal accumulation of alpha-mannosidase-1 activity.

Involvement of a cell-surface glycoprotein in the cell-sorting process of Dictyostelium discoideum

Cell sorting among prespore cells and prestalk cells in Dictyostelium discoideum was studied by using fluorochrome-labeled cells in an in vitro assay. Labeled prestalk cells first formed randomly mixed aggregates with unlabeled prespore cells. Then cells began to sort out from each other. About 3-4 hr later, prestalk cells became clustered at one pole of the aggregate. Aggregates deposited on an agar surface underwent morphogenesis and formed migrating slugs within 3 hr. The addition of Fab fragments directed against a cell-surface glycoprotein of Mr 150,000 (gp 150) to the cell mixture completely inhibited the cell-sorting phenomenon. Morphogenesis of such aggregates on agar was also delayed by 5 hr. However, inclusion of Fab fragments directed against the endogenous lectins, the contact site A glycoprotein, or vegetative cells had no detectable effect on cell sorting or morphogenesis of these reconstituted aggregates.

A repetitive Dictyostelium gene family that is induced during differentiation and by heat shock

Clone pB41-6 (2.5 kb) contains sequences that are repeated 200-300 times in the Dictyostelium genome; about 40 of these sequences are part of a 4.5 kb repeated and apparently transposable genomic element. Clone pB41-6 hybridizes to a large number of cytoplasmic polyadenylated RNAs whose accumulation begins in the first hour of differentiation. In order to understand the regulation of these repeated sequences, we have sequenced pB41-6. It contains three long open reading frames in the "sense" strand. Remarkably, about 70 bases upstream of the transcription initiation site is a sequence identical to that responsible for induction of the Drosophila heat shock genes. A search of published sequences also generated a similar sequence upstream of one of the Dictyostelium actin genes. Indeed, we found that both pB41-6-related RNAs and actin mRNAs are increased as a result of heat shocking growing cells, and that transcription of pB41-6 sequences is induced by heat shock. Thus Dictyostelium contains a set of genes that are induced as a response to heat shock or to the stresses that trigger the initiation of development. We show here that the principal component of this "stress" is not amino acid starvation but the high density of the cells.

Detection and regulation of the mRNA for the inhibitor of extracellular cAMP phosphodiesterase of Dictyostelium discoideum

The inhibitor of the cAMP phosphodiesterase of Dictyostelium discoideum is a cysteine-rich glycoprotein, which binds to the enzyme and inactivates it. When the inhibitor is removed, enzymatic activity is restored. Following translation in vitro of RNA from developing cells and immunoprecipitation with anti-inhibitor serum, newly synthesized inhibitor can be detected by sodium dodecylsulfate/polyacrylamide gel electrophoresis and fluorography. The inhibitor can be labeled using [35S]cysteine but not [35S]methionine, in agreement with the previously determined amino acid composition, and can be detected after cell-free translation only if it has been previously acetylated. Purified native inhibitor blocks immunoprecipitation of the inhibitor polypeptide synthesized in vitro. No inhibitor mRNA was detected in growing cells. Translatable mRNA was present 2 h after the beginning of starvation, reached a maximal level after 3 h, and decreased thereafter. Addition of 1 mM cAMP at the beginning of starvation delayed the appearance of translatable inhibitor mRNA. In the presence of 5 microM adenosine cyclic-3',5'-phosphorothioate, a slowly hydrolyzed cAMP analogue, no translatable mRNA could be detected. Following removal of the analogue, the mRNA appeared within one hour and inhibitor was secreted after another hour.

Identification of the acrasin of Dictyostelium minutum as a derivative of folic acid

The acrasin of the slime mold Dictyostelium minutum was isolated from aggregating cells and purified. The compound was species specific and more active in the aggregative than in the vegetative stage. Three observations strongly suggest a structural relationship between the acrasin and folic acid. (1) Folic acid inhibited acrasin degradation by D. minutum. (2) Methotrexate, an antagonist of chemotaxis towards folic acid, also inhibited the response to the acrasin. (3) The chemotactic response to an excess of folic acid was delayed. The response was also delayed to simultaneously tested low amounts of a related compound, but not to unrelated compounds (Van Haastert, 1982). The response to the acrasin was observed to be delayed by excess of folic acid. The acrasinase was identified as a folic acid C9-N10 splitting enzyme. Based on chromatographic properties and biological activity of the acrasin and folate derivatives, the chemical structure of the acrasin is discussed.

Identification of a pterin as the acrasin of the cellular slime mold Dictyostelium lacteum

Cell aggregation in Dictyostelium discoideum is mediated by chemotaxis to cyclic AMP. Aggregative cells of the simpler species D. lacteum are not attracted by this cyclic nucleotide. We describe how the cell aggregation-inducing factor, or acrasin, of D. lacteum was purified from aggregating amoebae and characterized. The acrasin, which is mainly secreted in the aggregative phase, is identified as a derivative of pterin. This identification is based on (i) its UV spectrum, (ii) the inhibition of the enzymatic degradation of acrasin by 6-methylpterin, (iii) the antagonistic effect of 6-aminopterin on chemotaxis towards both pterin and acrasin and not on the response towards folic acid or cyclic AMP, and (iv) the degradation of the acrasin to pterin. Its chromatographic properties show that the acrasin is an as yet unidentified pterin derivative. The acrasin is species specific and attracts cells at very low concentrations (0.1-0.01 microM). Also, several naturally occurring stereoisomers of 6-polyhydroxyalkylpterins attract aggregative cells at these low concentrations. Additionally, we identified a pterin deaminase, which converts pterin into 2-deamino-2-hydroxypterin (lumazin), as the acrasinase in D. lacteum.

Chemoresponsiveness to cAMP and folic acid during growth, development, and dedifferentiation in Dictyostelium discoideum

Chemoresponsiveness to cAMP and to folic acid are monitored in growing, developing, and dedifferentiating amebae of the cellular slime mold Dictyostelium discoideum. Two semiquantitative assays are employed, one measuring the directed movement of cells up a gradient of chemoattractant ('chemotaxis' assay) and the other measuring the outward spreading of cells in response to a chemical stimulant distributed equally throughout the substratum ('spreading' assay). Vegetative amebae possess relatively insignificant levels of chemotactic responsiveness to cAMP. Six h after the initiation of development, at approximately the same time as the onset of aggregation, cells rapidly acquire chemotactic responsiveness to cAMP. During 'erasure', a dedifferentiation induced by resuspending aggregating cells in fresh nutrient medium, chemotactic responsiveness to cAMP is lost just after the erasure event. By the same chemotactic assay, it is demonstrated that vegetative amebae possess a significant level of chemotactic responsiveness to folic acid. Two h after the initiation of development, cells completely lose chemotactic responsiveness to folic acid. During erasure, cells reacquire chemotactic responsiveness to folic acid at approximately the same time that they lose responsiveness to cAMP. Dramatically different results are obtained by the spreading assay. When cells lose chemotactic responsiveness to folic acid early in development and when erasing cells lose chemotactic responsiveness to cAMP, they retain the spreading response to the two stimulants, respectively. The different results obtained for chemoreception employing the two assays are discussed in terms of molecular mechanisms, and a testable hypothesis is proposed for the possible roles of chemoresponsiveness and erasure in late morphogenesis.

Regulation of dimorphism in Histoplasma capsulatum by cyclic adenosine 3',5'-monophosphate

During temperature-induced transition of the dimorphic pathogenic fungus Histoplasma capsulatum from the single yeast cell form to the multicellular mycelial form, there was an increase in intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels as well as a striking accumulation of cAMP in the medium. cAMP levels also changed during the reverse mycelium-to-yeast transition.

Developmental regulation of glycoprotein biosynthesis in Dictyostelium

We have examined the glycoprotein-linked oligosaccharides assembled during the life cycle of Dictyostelium discoideum, and found their expression to be dramatically dependent upon the stage of development. During early development mature glycans have a high mannose character, and a substantial proportion acquire a fucose residue that correlates with endo-H resistance. One-third of the glycans also acquire sulfate residues. These glycans diminish in importance during aggregation. The mature glycans expressed during aggregation. The mature glycans expressed during late development contain fewer mannose residues, from five to ten mannose residues, and are characterized by the absence of sulfate residues and by the presence of fucose residues on endo-H-sensitive glycans. These glycans make their appearance coincident with the construction of tips on tight cell mounds. At this stage glycans characteristic of both early and late stages occur simultaneously. Developmental regulation of the wide array of protein-linked glycans expressed during the life cycle of Dictyostelium discoideum may be as simple as the controlled transition from a group of structures that are assembled by the vegetative cells to a group of structures that are assembled by the terminally differentiating cells. The potential biological significance of this transition is discussed.

Control of developmental transitions in the cyclic AMP signalling system of Dictyostelium discoideum

In the first few hours after starvation, the developing cAMP secretory system in Dictyostelium discoideum has been observed to be successively in one of four states: (a) quiescent, (b) excitable (capable of relay), (c) autonomously oscillating, and (d) secreting at a high steady level. A theoretical model is presented which demonstrates that the proximal cause of the transitions between different types of behavior may be slow changes in the activities of the enzymes adenylate cyclase and phosphodiesterase. These changes affect the stability properties of the steady state admitted by the cAMP signalling system. Sustained oscillations develop when the steady state is unstable, whereas relay of cAMP signals occurs upon perturbation of a stable steady state for parameter values close to those which produce oscillations. The developmental path suggested in the adenylate cyclase-phosphodiesterase space for the sequential transitions compares with the time course observed for the synthesis of these enzymes after starvation. It is suggested that there is general significance for the understanding of differentiation in the example given of a state-point following a developmental path in parameter space, moving from one behavioral domain to another, and thereby bringing about shifts in qualitative behavior.