Chemotaxis and cell motility in the cellular slime molds

A stochastic view of lymphocyte motility and trafficking within the lymph node

Two-photon microscopy is providing literal insight into the cellular dynamics of lymphoid organs and, guided by analysis of three-dimensional images, into mechanisms that underlie cell migration and antigen recognition in vivo. This review describes lymphocyte motility and antigen recognition in the native tissue environment and compares these results with a much more extensive literature on lymphocyte motility, signaling, and chemotaxis in vitro. We discuss the in vitro literature on dynamic aspects of lymphocyte motility, chemotaxis, and the response to antigen and present the view that random migration of lymphocytes may drive a stochastic mechanism of antigen recognition in lymphoid organs, rather than being guided by chemotaxis.

A discrete cell model with adaptive signalling for aggregation of Dictyostelium discoideum

Dictyostelium discoideum (Dd) is a widely studied model system from which fundamental insights into cell movement, chemotaxis, aggregation and pattern formation can be gained. In this system aggregation results from the chemotactic response by dispersed amoebae to a travelling wave of the chemoattractant cAMP. We have developed a model in which the cells are treated as discrete points in a continuum field of the chemoattractant, and transduction of the extracellular cAMP signal into the intracellular signal is based on the G protein model developed by Tang & Othmer. The model reproduces a number of experimental observations and gives further insight into the aggregation process. We investigate different rules for cell movement the factors that influence stream formation the effect on aggregation of noise in the choice of the direction of movement and when spiral waves of chemoattractant and cell density are likely to occur. Our results give new insight into the origin of spiral waves and suggest that streaming is due to a finite amplitude instability.

Tyrosine phosphorylation of actin in Dictyostelium associated with cell-shape changes

When Dictyostelium cells that have initiated their developmental program upon starvation are returned to growth medium, there is a rapid and transient de novo tyrosine phosphorylation of a 43-kilodalton protein. This protein was found to be actin. Most of the phosphorylation occurred in a single, minor acidic isoform of actin. Developing cells that had been returned to growth medium lost their pseudopod extensions, became round, and had reduced adhesion to the substratum. These effects occurred with kinetics that matched the increase in tyrosine phosphorylation of actin. In mutant cell lines in which the gene for the phosphotyrosine phosphatase PTP1 had been disrupted, tyrosine phosphorylation of actin was rapid and more prolonged. These cells responded with proportionally accelerated kinetics of cell rounding. Cell lines overexpressing PTP1 had diminished amplitude and duration of actin tyrosine phosphorylation and exhibited diminished cell-shape change and an accelerated return to the extended cell-shape morphology seen in starved cells.

Coactosin, a 17 kDa F-actin binding protein from Dictyostelium discoideum

A 17 kDa protein, designated as coactosin, has been purified from an actin-myosin complex reconstituted in vitro from a soluble fraction of Dictyostelium discoideum cells. The protein binds to F-actin in vitro without significantly altering its viscosity. Immunoblots labeled with monoclonal antibodies indicate that part of the protein is associated with the detergent-insoluble cytoskeleton. cDNA clones comprising the entire coding region of coactosin have been isolated from an expression library. The cDNA-derived amino-acid sequence reveals similarities of coactosin to the drebrins identified in neurons and to actin-binding proteins from other organisms, including yeast ABP1p, and yeast and vertebrate cofilins.

Cyclic nucleotide phosphodiesterase of Dictyostelium discoideum and its glycoprotein inhibitor: structure and expression of their genes

The genes coding for the cyclic nucleotide phosphodiesterase (PD) and the PD inhibitory glycoprotein (PDI) have been cloned and characterized. The PDI gene was isolated as a 1.6 kb genomic fragment, which included the coding sequence containing two small introns and 510 nucleotides of non-translated 5' sequence. From the deduced amino acid sequence we predict a protein with a molecular weight (MW) of 26,000 that, in agreement with previous data, contains 15% cysteine residues. Genomic Southern blot analysis indicates that only one gene encodes the inhibitor. Northern blot analysis shows a single transcript of 0.95 kb. The PDI gene is expressed early in development with little transcript remaining following aggregation. The appearance of PDI mRNA is prevented by the presence of cAMP, but when cAMP is removed the transcript appears within 30 minutes. When cAMP is applied to cells expressing PDI the transcript disappears with a half-life of less than 30 minutes. The PD gene of D. discoideum is transcribed into three mRNAs: a 1.9 kb mRNA specific for growth, a 2.4 kb mRNA specific for aggregation, and a 2.2 kb mRNA specific for late development. The 2.2 kb mRNA is also specific for prestalk cells, and is induced by differentiation-inducing factor. All three mRNAs contain the same coding sequence, and differ only in their 5' non-coding sequences. Each mRNA is transcribed from a different promoter, and by using the chloramphenicol acyltransferase gene as a reporter, we have shown that each promoter displays the same regulation as its cognate mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Dictyostelium discoideum plasma membranes contain an actin-nucleating activity that requires ponticulin, an integral membrane glycoprotein

In previous equilibrium binding studies, Dictyostelium discoideum plasma membranes have been shown to bind actin and to recruit actin into filaments at the membrane surface. However, little is known about the kinetic pathway(s) through which actin assembles at these, or other, membranes. We have used actin fluorescently labeled with N-(1-pyrenyl)iodoacetamide to examine the kinetics of actin assembly in the presence of D. discoideum plasma membranes. We find that these membranes increase the rate of actin polymerization. The rate of membrane-mediated actin polymerization is linearly dependent on membrane protein concentrations up to 20 micrograms/ml. Nucleation (the association of activated actin monomers into oligomers) appears to be the primary step of polymerization that is accelerated. A sole effect on the initial salt-induced actin conformational change (activation) is ruled out because membranes accelerate the polymerization of pre-activated actin as well as actin activated in the presence of membranes. Elongation of preexisting filaments also is not the major step of polymerization facilitated by membranes since membranes stripped of all peripheral components, including actin, increase the rate of actin assembly to about the same extent as do membranes containing small amounts of endogenous actin. Acceleration of the nucleation step by membranes also is supported by an analysis of the dependence of polymerization lag time on actin concentration. The barbed ends of membrane-induced actin nuclei are not obstructed by the membranes because the barbed end blocking agent, cytochalasin D, reduces the rate of membrane-mediated actin nucleation. Similarly, the pointed ends of the nuclei are not blocked by membranes since the depolymerization rate of gelsolin-capped actin is unchanged in the presence of membranes. These results are consistent with previous observations of lateral interactions between membranes and actin filaments. These results also are consistent with two predictions from a model based on equilibrium binding studies; i.e., that plasma membranes should nucleate actin assembly and that membrane-bound actin nuclei should have both ends free (Schwartz, M. A., and E. J. Luna. 1988. J. Cell Biol. 107:201-209). Integral membrane proteins mediate the actin nucleation activity because activity is eliminated by heat denaturation, treatment with reducing agents, or proteolysis of membranes. Activity also is abolished by solubilization with octylglucoside but is reconstituted upon removal or dilution of the detergent. Ponticulin, the major actin-binding protein in plasma membranes, appears to be necessary for nucleation activity since activity is not reconstituted from detergent extracts depleted of ponticulin.

Density dependent diffusional model of an infective population

In this work, we present a density-dependent diffusional model which, coupled to three different types of growth, permitted us to study the infective potential of a bacteria species. The results show that those species with strong internal competency have the higher colonizing capacity in terms of invasion speed. Here, we also advanced a model for the static spatial inhomogeneous distribution that some species establish after migration. It is proposed that the origin of these patterns is the result of a balance between the dispersal tendency and the attractive behavior. The results obtained were compared with the observed behavior of Rhizobium spp. during infection of leguminous roots. A possible explanation of the observed morphologies of nodule development in different legumes is suggested.

Ponticulin, a developmentally-regulated plasma membrane glycoprotein, mediates actin binding and nucleation

Ponticulin is a 17,000-dalton transmembrane glycoprotein that is involved in the binding and nucleation of actin filaments by Dictyostelium discoideum plasma membranes. The major actin-binding protein isolated from these membranes by F-actin affinity chromatography, ponticulin also binds F-actin on blot overlays. The actin-binding activity of ponticulin in vitro is identical to that observed for purified plasma membranes: it resists extraction with 0.1 N NaOH, is sensitive to high salt concentrations, and is destroyed by heat, proteolysis, and thiol reduction and alkylation. A cytoplasmic domain of ponticulin mediates binding to actin because univalent antibody fragments directed against the cytoplasmic surface of this protein inhibit 96% of the actin-membrane binding in sedimentation assays. Antibody specific for ponticulin removes both ponticulin and the ability to reconstitute actin nucleation activity from detergent extracts of solubilized plasma membranes. Levels of plasma membrane ponticulin increase 2- to 3-fold during aggregation streaming, when cells adhere to each other and are highly motile. Although present throughout the plasma membrane, ponticulin is preferentially localized to some actin-rich membrane structures, including sites of cell-cell adhesion and arched regions of the plasma membrane reminiscent of the early stages of pseudopod formation. Ponticulin also is present but not obviously enriched at phagocytic cups of log-phase amebae. These results indicate that ponticulin may function in vivo to attach and nucleate actin filaments at the cytoplasmic surface of the plasma membrane. A 17,000-dalton analogue of ponticulin has been identified in human polymorphonuclear leukocyte plasma membranes by immunoblotting and immunofluorescence microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

A model for aggregation-dispersion dynamics of a population

An important feature that distinguishes the movement of living systems from the random motion of inorganic material is a delicate balance between spreading and concentrating. This movement is based on the kind of interactions which a bacterial colony may establish during migration. Namely, the antagonistic effects of dispersal which take place preferentially down the population gradient and the tendency in grouping together. In this work a model is proposed which considers these effects. The phase plane analysis and the numerical calculations reveal the existence of stable sharp wave front solutions. The speed of the wave front is modulated by the compromise between the tendencies of spreading and aggregating. The results obtained were compared with experimental observations in cultures of Escherichia coli and Streptococcus faecalis. The agreement between both types of results supports the hypothesis on which the model was based.

Transduction of the chemotactic signal to the actin cytoskeleton of Dictyostelium discoideum

Dictyostelium discoideum amebae chemotax toward folate during vegetative growth and toward extracellular cAMP during the aggregation phase that follows starvation. Stimulation of starving amebae with extracellular cAMP leads to both actin polymerization and pseudopod extension (Hall et al., 1988, J. Cell. Biochem. 37, 285-299). We have identified an actin nucleation activity (NA) from starving amebae that is regulated by cAMP receptors and controls actin polymerization (Hall et al., 1989, J. Cell Biol., in press). We show here that NA from vegetative cells is also regulated by chemotactic receptors for folate. Our studies indicate that NA is an essential effector in control of the actin cytoskeleton by chemotactic receptors. Guided by a recently proposed model for signal transduction from the cAMP receptor (Snaar-Jagalska et al., 1988, Dev. Genet. 9, 215-225), we investigated which of three signaling pathways activates the NA effector. Treatment of whole cells with a commercial pertussis toxin preparation (PT) inhibited cAMP-stimulated NA. However, endotoxin contamination of the PT appears to account for this effect. The synag7 mutation and caffeine treatment do not inhibit activation of NA by cAMP. Thus, neither activation of adenylate cyclase nor a G protein sensitive to PT treatment of whole cells is necessary for the NA response. Actin nucleation activity stimulated with folate is normal in vegetative fgdA cells. However, cAMP suppresses rather than activates NA in starving fgdA cells. This indicates that the components of the actin nucleation effector are present and that a pathway regulating the inhibitor(s) of nucleation remains functional in starving fgdA cells. The locus of the fgdA defect, a G protein implicated in phospholipase C activation, is directly or indirectly responsible for transduction of the stimulatory chemotactic signal from cAMP receptors to the nucleation effector in Dictyostelium.

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.

Prelysosomal acidic vacuoles in Dictyostelium discoideum

We have examined the ameba Dictyostelium discoideum for evidence of a discrete, prelysosomal, acidic receiving compartment in endocytosis. We observed in the cytoplasm abundant round vacuoles with diameters up to 2 microns that concentrated acridine orange by a process inhibited by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl). They were therefore taken to be acidic. The vacuoles were observed to fuse nearly quantitatively with primary phagosomes over 30 min and thereby to confer upon them the ability to accumulate acridine orange. The entry into lysosomes of phagocytic cargo occurred later. In the absence of phagocytosis, almost all of the acidic vacuoles rapidly accumulated fluorescent markers that had either been covalently coupled to the cell surface or fed as the soluble dextran conjugate. Therefore, these vacuoles also lie on the pathway of pinocytosis. A prominent subcellular ATPase activity inhibited by 25 microM NBD-Cl co-distributed on sucrose equilibrium density gradients with vacuoles capable of concentrating acridine orange in vitro. The peak was broad and more buoyant than that bearing lysosomal acid hydrolases, which contained only a minor amount of this ATPase. Also migrating in the buoyant peak were internalized plasma membrane markers; e.g., 3H-galactose had been covalently coupled to the surface of intact cells and allowed to enter pinosomes. We conclude that in D. discoideum an extensive prelysosomal vacuolar compartment provides the proton pumps that acidify both phagosomes and pinosomes.

Quantification of motility and area changes of Dictyostelium discoideum amoebae in response to chemoattractants

This report presents quantitative measurements of cell area and motility on the time scale of seconds. The response of Dictyostelium discoideum amoebae to step changes in chemoattractant concentration were followed using an image-processing system. Parameters reflecting total area and motility of several hundred to thousand cells were measured with a time resolution of 2.5 s. Responses of growth phase cells to folate and of starved cells to cAMP were similar. An increase in chemoattractant concentration produced a brief increase in motility followed by a longer-lasting decrease that returned to initial values in 90 s. At high cAMP concentrations the motility remained depressed. Area also increased transiently. Half-maximal responses were produced by 2 nM folate or 2 nM cAMP. Removal of chemoattractant produced a temporary increase in motility and decrease in area. These responses support a model in which antagonistic signals are used to orient cell movement.

Genetics of early Dictyostelium discoideum development

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Serum-activated T51B rat liver cells transiently accumulate cyclic AMP-dependent protein kinases on their surfaces during the G1 phase

Confluent T51B rat liver epithelial cells promptly began accumulating cyclic AMP-binding sites on their surfaces when they were stimulated from quiescence by serum growth factors in medium containing 1.8 mM Ca2+, but they began losing the accumulated binding sites shortly before initiating DNA replication. When the medium contained only 0.02 mM Ca2+, the cells still accumulated surface cyclic AMP-binding sites, but they did not initiate DNA replication and tended to continue accumulating the binding sites. The cyclic AMP-binding sites were eliminated completely by treating intact cells for 5 minutes with 0.005% trypsin (which did not damage the cells), and cyclic AMP caused them to be released from intact, undamaged cells into the medium. The binding sites also comigrated electrophoretically with purified regulatory subunits of type I cyclic AMP-dependent protein kinase, and to a lesser extent the regulatory subunit of type II cyclic AMP-dependent protein kinase. Therefore, it is likely that a transient accumulation of cyclic AMP-dependent protein kinases on the outer surface of the plasma membrane is part of the T51B rat liver cell's prereplicate program.