Monoclonal antibody against cytoplasmic lectins of Dictyostelium discoideum: cross-reactivity with a membrane glycoprotein, contact site A, and with E. coli beta-galactosidase and lac repressor

The lectin Discoidin I acts in the cytoplasm to help assemble the contractile machinery

Cellular functions, such as division and migration, require cells to undergo robust shape changes. Through their contractility machinery, cells also sense, respond, and adapt to their physical surroundings. In the cytoplasm, the contractility machinery organizes into higher order assemblies termed contractility kits (CKs). Using Dictyostelium discoideum, we previously identified Discoidin I (DscI), a classic secreted lectin, as a CK component through its physical interactions with the actin crosslinker Cortexillin I (CortI) and the scaffolding protein IQGAP2. Here, we find that DscI ensures robust cytokinesis through regulating intracellular components of the contractile machinery. Specifically, DscI is necessary for normal cytokinesis, cortical tension, membrane-cortex connections, and cortical distribution and mechanoresponsiveness of CortI. The dscI deletion mutants also have complex genetic epistatic relationships with CK components, acting as a genetic suppressor of cortI and iqgap1, but as an enhancer of iqgap2. This work underscores the fact that proteins like DiscI contribute in diverse ways to the activities necessary for optimal cell function.

Mfsd8 Modulates Growth and the Early Stages of Multicellular Development in Dictyostelium discoideum

MFSD8 is a transmembrane protein that has been reported to transport chloride ions across the lysosomal membrane. Mutations in MFSD8 are associated with a subtype of Batten disease called CLN7 disease. Batten disease encompasses a family of 13 inherited neurodegenerative lysosomal storage diseases collectively referred to as the neuronal ceroid lipofuscinoses (NCLs). Previous work identified an ortholog of human MFSD8 in the social amoeba D. discoideum (gene: mfsd8, protein: Mfsd8), reported its localization to endocytic compartments, and demonstrated its involvement in protein secretion. In this study, we further characterized the effects of mfsd8 loss during D. discoideum growth and early stages of multicellular development. During growth, mfsd8⁻ cells displayed increased rates of proliferation, pinocytosis, and expansion on bacterial lawns. Loss of mfsd8 also increased cell size, inhibited cytokinesis, affected the intracellular and extracellular levels of the quorum-sensing protein autocrine proliferation repressor A, and altered lysosomal enzyme activity. During the early stages of development, loss of mfsd8 delayed aggregation, which we determined was at least partly due to impaired cell-substrate adhesion, defects in protein secretion, and alterations in lysosomal enzyme activity. Overall, these results show that Mfsd8 plays an important role in modulating a variety of processes during the growth and early development of D. discoideum.

Eat Prey, Live: Dictyostelium discoideum As a Model for Cell-Autonomous Defenses

The soil-dwelling social amoeba Dictyostelium discoideum feeds on bacteria. Each meal is a potential infection because some bacteria have evolved mechanisms to resist predation. To survive such a hostile environment, D. discoideum has in turn evolved efficient antimicrobial responses that are intertwined with phagocytosis and autophagy, its nutrient acquisition pathways. The core machinery and antimicrobial functions of these pathways are conserved in the mononuclear phagocytes of mammals, which mediate the initial, innate-immune response to infection. In this review, we discuss the advantages and relevance of D. discoideum as a model phagocyte to study cell-autonomous defenses. We cover the antimicrobial functions of phagocytosis and autophagy and describe the processes that create a microbicidal phagosome: acidification and delivery of lytic enzymes, generation of reactive oxygen species, and the regulation of Zn²⁺, Cu²⁺, and Fe²⁺ availability. High concentrations of metals poison microbes while metal sequestration inhibits their metabolic activity. We also describe microbial interference with these defenses and highlight observations made first in D. discoideum. Finally, we discuss galectins, TNF receptor-associated factors, tripartite motif-containing proteins, and signal transducers and activators of transcription, microbial restriction factors initially characterized in mammalian phagocytes that have either homologs or functional analogs in D. discoideum.

A microcyst-overproducing mutant of Polysphondylium pallidum

A mutant, PN6017, of the cellular slime mold Polysphondylium pallidum was selected by cell-surface labeling with a monoclonal antibody, mAb 293, and fluorescence-activated cell sorting. The antibody was directed against an L-fucose-containing epitope on glycoproteins, designated ep 293, and the mutant showed reduced and delayed expression of this epitope. PN6017 was distinguished from other mutants of this kind by extensive microcyst formation on agar plates under conditions where the wild type formed only sparse microcysts. In suspension cultures transformation of cells into microcysts was negligible in the wild type, and close to 100% in the mutant. Under these conditions microcyst formation in the mutant began at 5-7 h of starvation. At the same time expression of ep 293 and also of a developmentally regulated cytoplasmic protein, pallidin, became detectable. This coincidence in time suggests that microcyst formation in PN6017 is coupled to the same control mechanism as the two other developmentally regulated processes.

Selection of Dictyostelium mutants defective in cytoskeletal proteins: use of an antibody that binds to the ends of alpha-actinin rods

A monoclonal antibody, mAb 47-19-2, was used to study the subunit topology of the rod-shaped alpha-actinin molecules of Dictyostelium discoideum and to screen for mutants defective in the production of alpha-actinin. Electron microscopy of rotary-shadowed alpha-actinin-antibody complexes showed binding of mAb 47-19-2 to both ends of the alpha-actinin rods and cleavage of the rods into its subunits, indicating that the two subunits of alpha-actinin extend in an anti-parallel mode through the whole length of the rod. The antibody binding sites were located in close proximity to the sites responsible for actin cross-linking, which is consistent with the blocking activity of the antibody. In a mutant, HG1130, no antibody label was detected in colony blots, and by immunoblotting of mutant proteins separated by SDS-PAGE, only trace amounts of alpha-actinin were found. The mutant showed normal binding of antibodies directed against the actin-binding proteins severin and capping protein. The mutation responsible for the alpha-actinin defect was recessive and located on linkage group I of the genetic map of D. discoideum. HG1130 cells grew on bacteria at a normal rate and also axenically like cells of the parent strain AX2. After starvation the mutant cells expressed the contact site A glycoprotein, a marker of the aggregation-competent stage, and reacted chemotactically to cyclic AMP. The aggregation patterns and fruiting bodies of the mutant appeared to be normal. Patching and capping on the surface of HG1130 cells was induced by antibodies against the contact site A glycoprotein.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell surface carbohydrates and cell recognition in Dictyostelium

Carbohydrate ligands and complementary receptors have been detected on the surface of Dictyostelium cells, using lectins, monoclonal antibodies, and immobilized sugar probes. They have been implicated in cell recognition processes, such as phagocytosis and intercellular adhesion, and could act as membrane signals for differentiation. Specific glycoproteins have been proposed to mediate intercellular adhesion in Dictyostelium discoideum and Polysphondylium pallidum and to account for the species-specificity of adhesion displayed by these species. Recent studies with the inhibitor of N-glycosylation, tunicamycin, and with glycosylation defective mutants suggest that some carbohydrate groups in these glycoproteins play a role in cell adhesion.