Endocytosis and recognition mechanisms in Dictyostelium discoideum

LrrA, a novel leucine-rich repeat protein involved in cytoskeleton remodeling, is required for multicellular morphogenesis in Dictyostelium discoideum

Cell sorting by differential cell adhesion and movement is a fundamental process in multicellular morphogenesis. We have identified a Dictyostelium discoideum gene encoding a novel protein, LrrA, which composes almost entirely leucine-rich repeats (LRRs) including a putative leucine zipper motif. Transcription of lrrA appeared to be developmentally regulated with robust expression during vegetative growth and early development. lrrA null cells generated by homologous recombination aggregated to form loose mounds, but subsequent morphogenesis was blocked without formation of the apical tip. The cells adhered poorly to a substratum and did not form tight cell-cell agglomerates in suspension; in addition, they were unable to polarize and exhibit chemotactic movement in the submerged aggregation and Dunn chamber chemotaxis assays. Fluorescence-conjugated phalloidin staining revealed that both vegetative and aggregation competent lrrA(-) cells contained numerous F-actin-enriched microspikes around the periphery of cells. Quantitative analysis of the fluorescence-stained F-actin showed that lrrA(-) cells exhibited a dramatically increase in F-actin as compared to the wild-type cells. When developed together with wild-type cells, lrrA(-) cells were unable to move to the apical tip and sorted preferentially to the rear and lower cup regions. These results indicate that LrrA involves in cytoskeleton remodeling, which is needed for normal chemotactic aggregation and efficient cell sorting during multicellular morphogenesis, particularly in the formation of apical tip.

Calcium requirement for efficient phagocytosis by Dictyostelium discoideum

Extracellular EDTA suppressed in a dose-dependent manner the phagocytosis of yeast particles by Dictyostelium discoideum cells. Activity was restored fully by the addition of Ca(2+), and partially by the addition of Mn(2+)or Zn(2+), but Mg(2+)was ineffective. The pH-sensitive, Ca(2+)-specific chelator EGTA also inhibited phagocytosis at pH 7.5, but not at pH 5, and Ca(2+)restored the inhibited phagocytosis. In contrast, pinocytosis was unaffected by EDTA. Consistent with the idea that Ca(2+)was required for phagocytosis, D. discoideum growth on bacteria was inhibited by EDTA, which was then restored by the addition of Ca(2+). It is concluded that Ca(2+)was needed for efficient phagocytosis by D. discoideum amoebae. A search for Ca(2+)-dependent membrane proteins enriched in phagosomes revealed the presence of p24, a Ca(2+)-dependent cell-cell adhesion molecule-1 (DdCAD-1) that could be the target of the observed EDTA and EGTA inhibition. DdCAD-1-minus cells, however, had normal phagocytic activity. Furthermore, phagocytosis was inhibited by EDTA and rescued by Ca(2+)in the mutant just as in wild type. Thus, DdCAD-1 was not responsible for the observed Ca(2+)-dependence of phagocytosis, indicating that one or more different Ca(2+)-dependent molecule(s) was involved in the process.

Role of esterase gp70 and its influence on growth and development of Dictyostelium discoideum

Gp70 is an esterase originally called crystal protein because of its presence in crystalline structures in aggregation-competent Dictyostelium discoideum cells. Although postulated to break down spore coats, the function of gp70 in vivo was incompletely investigated. Our immunolocalization and biochemical studies of vegetative D. discoideum amoebae show that gp70 was recruited to phagosomes and found in lysosomes. Purified gp70 was effective at hydrolyzing naphthyl substrates with acyl chains typical of lipids and lipopolysaccharides, indicating that the gp70 was involved in digesting endocytosed molecules. The activity of purified gp70 was inhibited by reductants that retarded its electrophoretic mobility and verified the presence of intramolecular disulfide bonds predicted by its amino acid sequence. Compared to wild-type cells, cells overexpressing gp70 were more phagocytically active, had shorter generation times, and produced more fruiting bodies per unit area, while cells lacking gp70 were phagocytically less active with longer doubling times, developed more slowly, and had significantly fewer fruiting bodies per unit area. Consistent with the phenotype of a disrupted metabolism, one-third of the gp70-minus cells were large and multinucleated. Together, these results indicated that despite its crystalline appearance, gp70 was an active esterase involved in both the growth and the development of D. discoideum.

Intracellular growth of Legionella pneumophila in Dictyostelium discoideum, a system for genetic analysis of host-pathogen interactions

Conditions were established in which Legionella pneumophila, an intracellular bacterial pathogen, could replicate within the unicellular organism Dictyostelium discoideum. By several criteria, L. pneumophila grew by the same mechanism within D. discoideum as it does in amoebae and macrophages. Bacteria grew within membrane-bound vesicles associated with rough endoplasmic reticulum, and L. pneumophila dot/icm mutants, blocked for growth in macrophages and amoebae, also did not grow in D. discoideum. Internalized L. pneumophila avoided degradation by D. discoideum and showed evidence of reduced fusion with endocytic compartments. The ability of L. pneumophila to grow within D. discoideum depended on the growth state of the cells. D. discoideum grown as adherent monolayers was susceptible to L. pneumophila infection and to contact-dependent cytotoxicity during high-multiplicity infections, whereas D. discoideum grown in suspension was relatively resistant to cytotoxicity and did not support intracellular growth. Some known D. discoideum mutants were examined for their effect on growth of L. pneumophila. The coronin mutant and the myoA/B double myosin I mutant were more permissive than wild-type strains for intracellular growth. Growth of L. pneumophila in a G(beta) mutant was slightly reduced compared to the parent strain. This work demonstrates the usefulness of the L. pneumophila-D. discoideum system for genetic analysis of host-pathogen interactions.

Cytoskeletal association of an esterase in Dictyostelium discoideum

A 70-kDa glycoprotein, gp70, was found enriched in the detergent-insoluble cytoskeletal fraction of axenically grown Dictyostelium discoideum cells. Its N-terminal amino acid sequence identified it as 'crystal protein' (L. Bomblies et al., 1990, J. Cell Biol. 110, 669-679). This finding was corroborated when antibody to crystal protein cross-reacted with gp70 and its deglycosylated form. The postulated esterase activity of gp70/crystal protein was verified through comparative enzyme assays of extracts derived from cells that either overexpressed or lacked gp70. Gp70 cosedimented with cytoskeletons on sucrose gradients, suggesting an interaction with the cytoskeleton. Coisolation of gp70 with detergent-extracted cells, observed by immunofluorescence microscopy, also implied a gp70-cytoskeletal association. These data supported the idea that the localization or secretion of gp70, or both, was cytoskeletally mediated. Although axenically grown cells contained high levels of gp70, the same cell lines had reduced levels of gp70 when grown in bacterial suspension or in nutrient media containing bacteria. Bacterially grown cells, compared to axenically grown cells, had lower fluid-phase uptake rates even when nutrient media was present, indicating that phagocytosis was a preferred mode of feeding. Thus, bacteria inhibited gp70 expression, which suggested a role for prestarvation factor, in regulating its synthesis.

G protein beta subunit-null mutants are impaired in phagocytosis and chemotaxis due to inappropriate regulation of the actin cytoskeleton

Chemotaxis and phagocytosis are basically similar in cells of the immune system and in Dictyostelium amebae. Deletion of the unique G protein beta subunit in D. discoideum impaired phagocytosis but had little effect on fluid-phase endocytosis, cytokinesis, or random motility. Constitutive expression of wild-type beta subunit restored phagocytosis and normal development. Chemoattractants released by cells or bacteria trigger typical transient actin polymerization responses in wild-type cells. In beta subunit-null cells, and in a series of beta subunit point mutants, these responses were impaired to a degree that correlated with the defect in phagocytosis. Image analysis of green fluorescent protein-actin transfected cells showed that beta subunit- null cells were defective in reshaping the actin network into a phagocytic cup, and eventually a phagosome, in response to particle attachment. Our results indicate that signaling through heterotrimeric G proteins is required for regulating the actin cytoskeleton during phagocytic uptake, as previously shown for chemotaxis. Inhibitors of phospholipase C and intracellular Ca2+ mobilization inhibited phagocytosis, suggesting the possible involvement of these effectors in the process.

Disruption of Dictyostelium PI3K genes reduces [32P]phosphatidylinositol 3,4 bisphosphate and [32P]phosphatidylinositol trisphosphate levels, alters F-actin distribution and impairs pinocytosis

To understand how phosphatidylinositol 3-kinase (PI3K) modulates cell structure and function, we examined the molecular and cellular defects of a Dictyostelium mutant strain (pik1(Delta)2(Delta)) missing two (DdPIK1 and 2) of three PI3K genes, which are homologues of the mammalian p110 subunit. Levels of [32P]phosphatidylinositol 3, 4 bisphosphate (PI(3,4)P2) and [32P]phosphatidylinositol trisphosphate (PIP3) were reduced in pik1(Delta)2(Delta), which had major defects in morphological and functional correlates of macropinocytosis. This was accompanied by dramatic deficits in a subset of F-actin-enriched structures such as circular ruffles, actin crowns and pseudopodia. Although pik1(Delta)2(Delta) were mobile, they failed to aggregate into streams. Therefore we conclude that PIK1 and 2, possibly through modulation of the levels of PIP3 and PI(3,4)P2, regulate the organization of actin filaments necessary for circular ruffling during macropinocytosis, the extension of pseudopodia and the aggregation of cells into streams, but not the regulation of cell motility.

Cloning and characterization of a rhoGAP homolog from Dictyostelium discoideum

Small GTPases interact with a variety of proteins that affect nucleotide binding and cleavage. GTPase activating proteins (GAPs) are one class of these proteins that act by accelerating the intrinsic GTPase rate resulting in the formation of the biologically inactive GDP-bound form of the GTPase. For the Rho subfamily of GTPases, there is a growing number of proteins with rhoGAP activity that are identifiable by a homologous region of about 150 amino acids. We have exploited this homology using the polymerase chain reaction to clone the first rhoGAP homolog, called DdRacGAP, from the slime mold Dictyostelium discoideum. The GAP domain of DdRacGAP (amino acids 1-212), when expressed and purified from Escherichia coli, is active on both Dictyostelium and human Rho family GTPases but not human Ras. The full-length protein is 1356 amino acids in length and has several interesting homologies in addition to the GAP domain, including an SH3 domain, a dbl homology domain, and a pleckstrin homology domain.

The role of the cortical cytoskeleton: F-actin crosslinking proteins protect against osmotic stress, ensure cell size, cell shape and motility, and contribute to phagocytosis and development

We generated Dictyostelium double mutants lacking the two F-actin crosslinking proteins alpha-actinin and gelation factor by inactivating the corresponding genes via homologous recombination. Here we investigated the consequences of these deficiencies both at the single cell level and at the multicellular stage. We found that loss of both proteins severely affected growth of the mutant cells in shaking suspension, and led to a reduction of cell size from 12 microns in wild-type cells to 9 microns in mutant cells. Moreover the cells did not exhibit the typical polarized morphology of aggregating Dictyostelium cells but had a more rounded cell shape, and also exhibited an increased sensitivity towards osmotic shock and a reduced rate of phagocytosis. Development was heavily impaired and never resulted in the formation of fruiting bodies. Expression of developmentally regulated genes and the final developmental stages that were reached varied, however, with the substrata on which the cells were deposited. On phosphate buffered agar plates the cells were able to form tight aggregates and mounds and to express prespore and prestalk cell specific genes. Under these conditions the cells could perform chemotactic signalling and cell behavior was normal at the onset of multicellular development as revealed by time-lapse video microscopy. Double mutant cells were motile but speed was reduced by approximately 30% as compared to wild type. These changes were reversed by expressing the gelation factor in the mutant cells. We conclude that the actin assemblies that are formed and/or stabilized by both F-actin crosslinking proteins have a protective function during osmotic stress and are essential for proper cell shape and motility.

The in vivo role of annexin VII (synexin): characterization of an annexin VII-deficient Dictyostelium mutant indicates an involvement in Ca(2+)-regulated processes

Dictyostelium discoideum cells harbor two annexin VII isoforms of 47 and 51 kDa which are present throughout development. In immunofluorescence and cell fractionation studies annexin VII was found in the cytoplasm and on the plasma membrane. In gene disruption mutants lacking both annexin VII isoforms growth, pinocytosis, phagocytosis, chemotaxis and motility were not significantly impaired under routine laboratory conditions, and the cells were able to complete the developmental cycle on bacterial plates. On non-nutrient agar plates development was delayed by three to four hours and a significant number of aggregates was no longer able to form fruiting bodies. Exocytosis as determined by measuring extracellular cAMP phosphodiesterase, alpha-fucosidase and alpha-mannosidase activity was unaltered, the total amounts of these enzymes were however lower in the mutant than in the wild type. The mutant cells were markedly impaired when they were exposed to low Ca2+ concentrations by adding EGTA to the nutrient medium. Under these conditions growth, motility and chemotaxis were severely affected. The Ca2+ concentrations were similar in mutant and wild-type cells both under normal and Ca2+ limiting conditions; however, the distribution was altered under low Ca2+ conditions in SYN-cells. The data suggest that annexin VII is not required for membrane fusion events but rather contributes to proper Ca2+ homeostasis in the cell.

Ponticulin is the major high affinity link between the plasma membrane and the cortical actin network in Dictyostelium

Interactions between the plasma membrane and underlying actin-based cortex have been implicated in membrane organization and stability, the control of cell shape, and various motile processes. To ascertain the function of high affinity actin-membrane associations, we have disrupted by homologous recombination the gene encoding ponticulin, the major high affinity actin-membrane link in Dictyostelium discoideum amoebae. Cells lacking detectable amounts of ponticulin message and protein also are deficient in high affinity actin-membrane binding by several criteria. First, only 10-13% as much endogenous actin cosediments through sucrose and crude plasma membranes from ponticulin-minus cells, as compared with membranes from the parental strain. Second, purified plasma membranes exhibit little or no binding or nucleation of exogenous actin in vitro. Finally, only 10-30% as much endogenous actin partitions with plasma membranes from ponticulin-minus cells after these cells are mechanically unroofed with polylysine-coated coverslips. The loss of the cell's major actin-binding membrane protein appears to be surprisingly benign under laboratory conditions. Ponticulin-minus cells grow normally in axenic culture and pinocytose FITC-dextran at the same rate as do parental cells. The rate of phagocytosis of particles by ponticulin-minus cells in growth media also is unaffected. By contrast, after initiation of development, cells lacking ponticulin aggregate faster than the parental cells. Subsequent morphogenesis proceeds asynchronously, but viable spores can form. These results indicate that ponticulin is not required for cellular translocation, but apparently plays a role in cell patterning during development.

Clathrin heavy chain is required for pinocytosis, the presence of large vacuoles, and development in Dictyostelium

To investigate the intracellular role of the clathrin heavy chain in living cells, we have used "antisense" RNA to engineer mutant Dictyostelium discoideum cells that are severely deficient in clathrin heavy chain expression. Immunoblots stained with an anti-clathrin heavy chain antiserum revealed that mutant cells contained undetectable amounts of clathrin heavy chain protein. Similarly, Northern blots showed an absence of clathrin heavy chain mRNA. Clathrin heavy chain-deficient Dictyostelium cells were viable, but exhibited growth rates twofold slower than parental cells. Whereas many morphological features of the mutant cells were normal, mutant cells lacked coated pits and coated vesicles. Clathrin-deficient cells were also missing large translucent vacuoles that serve as endosomes and contractile vacuoles. In the absence of clathrin heavy chain, mutant cells displayed three distinct functional defects: (a) impairment in endocytosis of fluid phase markers, but competence in another endocytic pathway, the phagocytosis of solid particles; (b) defects in osmoregulation; and (c) inability to complete the starvation-induced development cycle.

Characterization of the clathrin heavy chain from Dictyostelium discoideum

We report the cloning and analysis of a clathrin heavy-chain cDNA from the eukaryotic microorganism, Dictyostelium discoideum. A single gene, designated chcA, for the clathrin heavy chain encoded a protein of 1,694 amino acids with a molecular mass of 193,618 daltons. Comparison of the amino acid sequence with the rat and with the yeast sequence showed that the highly conserved protein was more similar to the mammalian clathrin heavy chain (57% identity) than to the yeast heavy chain (45% identity). The mRNA for the clathrin heavy chain was regulated during development. mRNA levels were highest during vegetative growth and declined as the cells progressed through the 24-hr developmental cycle. The concentration of clathrin heavy-chain protein was the same in cells grown in liquid media (high rates of pinocytosis) as in cells grown with bacteria (low rates of pinocytosis), which suggests that regulation of pinocytosis in these cells is not achieved by altering the concentration of clathrin.

Redundancy in the microfilament system: abnormal development of Dictyostelium cells lacking two F-actin cross-linking proteins

We generated by gene disruption Dictyostelium cells that lacked both the F-actin cross-linking proteins, alpha-actinin and gelation factor. Several major cell functions, such as growth, chemotaxis, phagocytosis, and pinocytosis, were apparently unaltered. However, in all double mutants, development was greatly impaired. After formation of aggregates, cells were very rarely able to form fruiting bodies. This ability was rescued when mutant and wild-type strains were mixed in a ratio of 70 to 30. The developmental program in the mutant was not arrested, since the expression pattern of early and late genes remained unchanged. Development of the mutant was rendered normal when a functional alpha-actinin gene was introduced and expressed, showing the morphogenetic defect to be due to the absence of the two F-actin cross-linking proteins. These findings suggest the existence of a functional network allowing mutual complementation of certain actin-binding proteins.

A natural-abundance 13C-NMR study of Dictyostelium discoideum metabolism. Monitoring of the spore germination process

Amoebae and spores of the cellular slime mold Dictyostelium discoideum have been investigated by natural-abundance proton-decoupled 13C-NMR spectroscopy. Axenically grown vegetative amoebae have been found to contain, as prominent metabolites, the polyamines 1,3-diaminopropane (3.2 mM), putrescine (9.4 mM) and spermidine (1.7 mM). We also detected lactic acid (4.4 mM) and the following amino acids as free metabolites in concentrations ranging over 1-3 mM: glycine, alanine, glutamine and glutamate. The glycogen level is highly dependent upon growth state, being below the level of NMR detection in early-exponential cells and reaching about 110 mM glucose equivalents in plateau-phase cells. Dormant spores contained high amounts of trehalose (50 mM), glutamine (73 mM) and glutamate (20 mM). The latter two compounds were not reported previously to be present in such high concentrations in Dictyostelium spores. Germination induced by heat-shock activation was monitored by 13C NMR. No change in the major components occurred during the activation step. The progressive disappearance of trehalose during germination correlated with the decrease of glutamine and glutamate. In general, the data suggest that germinated spores contain a composition of free metabolites very similar to that of starved vegetative amoebae.

Cell size in Dictyostelium

Cellular slime mold amoebae have become a model system for the study of cell motility and the cytoskeleton. A basic problem which all cells face that involves the cytoskeleton is how to control their size. The varied ways in which cellular slime mold amoebae change their cell size--by changing the size at which division occurs, by cell fusion, and by control over cytokinesis--are reviewed. A model is presented which attempts to explain how the mechanisms affected in certain cytokinesis mutants in Dictyostelium discoideum known as phg mutants could be involved in control of cell size in the predatory slime mold Dictyostelium caveatum.