Inhibition of cell differentiation and cell cohesion by tunicamycin in Dictyostelium discoideum

Unfolding the Endoplasmic Reticulum of a Social Amoeba: Dictyostelium discoideum as a New Model for the Study of Endoplasmic Reticulum Stress

The endoplasmic reticulum (ER) is a membranous network with an intricate dynamic architecture necessary for various essential cellular processes. Nearly one third of the proteins trafficking through the secretory pathway are folded and matured in the ER. Additionally, it acts as calcium storage, and it is a main source for lipid biosynthesis. The ER is highly connected with other organelles through regions of membrane apposition that allow organelle remodeling, as well as lipid and calcium traffic. Cells are under constant changes due to metabolic requirements and environmental conditions that challenge the ER network’s maintenance. The unfolded protein response (UPR) is a signaling pathway that restores homeostasis of this intracellular compartment upon ER stress conditions by reducing the load of proteins, and by increasing the processes of protein folding and degradation. Significant progress on the study of the mechanisms that restore ER homeostasis was achieved using model organisms such as yeast, Arabidopsis, and mammalian cells. In this review, we address the current knowledge on ER architecture and ER stress response in Dictyostelium discoideum. This social amoeba alternates between unicellular and multicellular phases and is recognized as a valuable biomedical model organism and an alternative to yeast, particularly for the presence of traits conserved in animal cells that were lost in fungi.

Cloning of cDNA for the contact site A glycoprotein of Dictyostelium discoideum

A cell surface glycoprotein of Dictyostelium discoideum with M(r) 80,000 (gp80) has been shown to mediate the formation of the developmentally acquired EDTA-resistant cell-cell binding sites termed contact sites A. We have isolated cDNA clones encoding gp80 by immunological screening of an expression library prepared in Escherichia coli. Double-stranded cDNA was prepared from poly(A)(+) RNA isolated from cells at 8 hr of development and cloned into the bacteriophage expression vector lambdagt11. Two recombinant phages containing cDNA inserts of 1.2 and 0.8 kilobases were isolated and shown to contain sequences coding for gp80 by the immunoselect assay. Partial DNA sequence analysis also confirmed that one of these cDNA clones, lambdaDdgp80c-19, contained the coding sequence for the amino terminus of gp80. DNA-RNA hybridization showed that the insert of lambdaDdgp80c-19 hybridized to a single mRNA transcript of approximately 2.0 kilobases. gp80 mRNA became detectable after 6 hr of development, reached its maximum level at 9 hr, and dropped to a negligible level by 15 hr. This pattern of mRNA accumulation corresponded closely to that of gp80 synthesis in D. discoideum cells, suggesting that gp80 expression is regulated at the transcriptional level.

Characterization and distribution of O-glycosylated carbohydrates in the cell adhesion molecule, contact site A, from Dictyostelium discoideum

This paper presents further investigation of the properties of carbohydrate II in the cell adhesion molecule, contact site A, from Dictyostelium discoideum. A purified contact site A was digested with Achromobacter protease I to produce a 31-kDa fragment to which carbohydrate II was mainly bound and a 21-kDa fragment containing the NH2 terminus of contact site A, which was identified as Ala-Pro-Thr-Ile-Thr-Ala. The NH2 terminus of the 31-kDa fragment was Thr-Glu-Ala-Thr-Thr-Ser. It was estimated from the cDNA sequence data of contact site A that more than 20 Ser/Thr residues exist as target sites for the O-linked oligosaccharides in the 31-kDa fragment, but not for the N-linked oligosaccharides. These results suggest that carbohydrate II exists as clustered O-linked oligosaccharides in the COOH terminus of contact site A. The results of two-dimensional electrophoresis confirm that oligosaccharides of contact site A contain sialic acids. Immunoelectron microscopy was carried out to define the organelle in which O-glycosylation by carbohydrate II occurs and how carbohydrate II antigens are distributed on the cell surface. The results show that O-glycosylation can occur in the Golgi apparatus in D. discoideum as observed in other cells, although this O-glycosylation was inhibited by tunicamycin. Furthermore, gold particles were densely concentrated in cell-cell contact regions but sparsely distributed in noncontact regions.

Cell adhesion in the life cycle of Dictyostelium

Three forms of cell adhesion determine the life cycle of Dictyostelium: i) adhesion of bacteria to the surface of the growing amoebae, as the prerequisite for phagocytosis; ii) cell-substrate adhesion, necessary for both locomotion of the amoebae and migration of the slug; iii) cell-cell adhesion, essential for transition from the unicellular to the multicellular stage. Intercellular adhesion has received the most attention, and fruitful approaches have been developed over the past 25 years to identify, purify and characterize cell adhesion molecules. The csA glycoprotein, in particular, which mediates adhesion during the aggregation stage, is one of the best defined cell adhesion molecules. The molecular components involved in phagocytosis and cell-substratum adhesion are less well understood, but the basis has been laid for a systematic investigation of both topics in the near future.

Carbohydrate structures of the cell adhesion molecule, contact site A, from Dictyostelium discoideum

We determined the carbohydrate structures of contact site A from Dictyostelium discoideum. The carbohydrate moieties of contact site A were released by hydrazinolysis. Fractionation of the deacidified oligosaccharide mixture by Bio-Gel P-4 column chromatography revealed that it was composed of four major oligosaccharides. Their respective structures were determined by sequential exoglycosidase digestion. It is known that contact site A consists of two kinds of carbohydrates, I and II. Taking together the previous and the present results, it was deduced that carbohydrate I comprises N-linked oligosaccharides and carbohydrate II O-linked ones. Furthermore, the relative molar contents of GalNAc and GlcNAc in reducing terminal suggested that contact site A contains 67% of N-linked and 33% of O-linked oligosaccharides.

Function of the carbohydrates in contact site A glycoprotein of Dictyostelium discoideum affected by tunicamycin

1. The relationship between glycosylation of contact site A (csA) of 80 kDa with two types of N-linked carbohydrates, I and II, and EDTA-resistant cell contact of Dictyostelium was investigated by tunicamycin treatment. 2. Carbohydrate I glycosylation, involved in a shift of csA from 66 to 80 kDa, was more sensitive to tunicamycin than carbohydrate II glycosylation in its shift from 53 to 66 kDa. 3. The appearance of csA of 80 kDa corresponded to that of EDTA-resistant cell contact. Carbohydrate I may be essential for EDTA-resistant cell contact. 4. In starved cells treated with tunicamycin, only 4-8% of moieties labeled with wheat germ agglutinin in carbohydrate II were modified.

Mapping of the monoclonal antibody 80L5C4 epitope on the cell adhesion molecule gp80 of Dictyostelium discoideum

EDTA-resistant cell-cell binding sites are expressed on Dictyostelium discoideum cells at the aggregation stage of development. A cell surface glycoprotein of Mr 80,000 (gp80) has been found to mediate these binding sites via homophilic interaction. We have previously raised a monospecific monoclonal antibody 80L5C4 against gp80, which blocks the cell binding site of gp80 (Siu, C.-H., Lam, T.Y. and Choi, A.H.C. (1985) J. Biol. Chem. 260, 16030-16036). To map the 80L5C4 epitope, gp80 was digested with protease V8, and the smallest proteolytic fragment that retained immunoreactivity with 80L5C4 was about 27,000 Da, corresponding to the amino-terminal fragment predicted from the cleavage sites. In addition, cDNA fragments containing different gp80 coding regions were used to construct trpE/gp80 gene fusions in the expression vector pATH10. An analysis of these fusion proteins led to the mapping of the 80L5C4 epitope to a 51 amino-acid segment between residues 123 and 173.

Mapping of a cell-binding domain in the cell adhesion molecule gp80 of Dictyostelium discoideum

At the aggregation stage of Dictyostelium discoideum development, a cell surface glycoprotein of Mr 80,000 (gp80) has been found to mediate the EDTA-resistant type of cell-cell adhesion via homophilic interaction (Siu, C.-H., A. Cho, and A. H. C. Choi. 1987. J. Cell Biol. 105:2523-2533). To investigate the structure-function relationships of gp80, we have isolated full length cDNA clones for gp80 and determined the DNA sequence. The deduced structure of gp80 showed three major domains. An amino-terminal globular domain composed of the bulk of the protein is supported by a short stalk region, which is followed by a membrane anchor at the carboxy terminus. Structural analysis suggested that the cell-binding domain of gp80 resides within the globular domain near the amino terminus. To investigate the relationship of the cell-binding activity to this region of the polypeptide, three protein A/gp80 (PA80) gene fusions were constructed using the expression vector pRIT2T. These PA80 fusion proteins were assayed for their ability to bind to aggregation stage cells. Binding of 125I-labeled fusion proteins PA80I (containing the Val123 to Ile514 fragment of gp80) and PA80II (Val123 to Ala258) was dosage dependent and could be inhibited by precoating cells with the cell cohesion-blocking mAb 80L5C4. On the other hand, there was no appreciable binding of PA80III (Ile174 to Ile514) to cells. Reassociation of cells was significantly inhibited in the presence of PA80I or PA80II. In addition, 125I-labeled PA80II exhibited homophilic interaction with immobilized PA80I, PA80II, or gp80. The results of these studies lead to the mapping of a cell-binding domain in the region between Val123 and Leu173 of gp80 and provide direct evidence that the cell-binding activity of gp80 resides in the protein moiety.

Mediation of cell-cell adhesion by the altered contact site A glycoprotein expressed in modB mutants of Dictyostelium discoideum

In Dictyostelium discoideum, a surface glycoprotein with Mr 80,000 (gp80) has been found to mediate the EDTA-resistant contact sites A at the aggregation stage of development. To evaluate the role of the carbohydrate moiety in cell-cell adhesion, we have examined the accumulation and activity of an altered gp80 molecule in two glycosylation (modB) mutants. Both mutants synthesize an altered gp80 of lower molecular size. This modB-gp80 can be detected by the monoclonal antibody 80L5C4, which is capable of blocking cell-cell adhesion (C. -H. Siu, T. Y. Lam, and A. Choi, (1985) J. Biol. Chem. 260, 16,030-16,036). The mutant cells exhibit both EDTA-sensitive and EDTA-resistant types of cell-cell binding, though to a lesser extent than that of the parental strain, and the EDTA-resistant binding sites are blocked in the presence of 80L5C4 Fab. Mutant cells can also bind Covaspheres conjugated with gp80. These results suggest that the modB-gp80 protein still retains the domain essential for its cell binding activity and the carbohydrate moiety affected by the modB mutation is not directly involved in cell-cell adhesion.

Inhibition of N-linked glycosylation in Dictyostelium discoideum: effects of aggregate formation

The aggregation program of Dictyostelium discoideum is extremely sensitive to the effects of tunicamycin when the drug is added to cells during the first few hours of starvation. Inhibition of development is observed with concentrations as low as 0.5 micrograms/ml, which cause only a 25%-30% inhibition of general N-linked glycosylation. However, 0.5 micrograms/ml tunicamycin can result in the total inhibition of N-linked glycosylation of specific, developmentally regulated, proteins, as exemplified by the glycoprotein 117 antigen. If added after the first hours of starvation, tunicamycin cannot inhibit aggregation even when present at 10 micrograms/ml, which maximally inhibits N-linked glycosylation. cAMP pulses can override the inhibitory effects of tunicamycin on cell aggregation. The data support the hypothesis that there is an early developmental pathway that is dependent on the N-linked glycosylation of one, or a small set of developmentally regulated proteins and that this pathway may involve the biogenesis of the chemotactic signalling system. In addition, the data raise questions as to the role of N-linked oligosaccharides in cell cohesion.

Cell surface oligosaccharides participate in cohesion during aggregation of Dictyostelium discoideum

Plasma membrane glycoproteins from Dictyostelium discoideum amoebae at three stages of early development were digested with Pronase and endoglycosidase H and fractionated by gel filtration. This gave three classes of glycans (polysaccharides, endoglycosidase H-resistant glycopeptides, and endoglycosidase H-released oligosaccharides), which were tested for their ability to block agglutination of amoebae from vegetative, aggregation (8-hr), and late-aggregation (13-hr) stages of development. The endoglycosidase H-resistant glycopeptides from 8-hr cells inhibited agglutination of disaggregated 8-hr cells but not vegetative or 13-hr cells. The 8-hr polysaccharide and endo H-sensitive oligosaccharides did not inhibit. The glycopeptides from 8-hr cells were resolved into five species by electrophoresis in borate-containing buffer. Two of these had agglutination-inhibiting activity, and three did not. None of the glycan fractions from vegetative or 13-hr cells inhibited agglutination of vegetative, 8-, or 13-hr cells. These data implicate specific cell surface glycans in aggregation-stage intercellular cohesion and suggest that both these glycans and receptors for them are developmentally regulated.

The contact site A glycoprotein mediates cell-cell adhesion by homophilic binding in Dictyostelium discoideum

Dictyostelium discoideum expresses a developmentally regulated cell surface glycoprotein of Mr 80,000 (gp80), which has been implicated in the formation of the EDTA-resistant contact sites A at the cell aggregation stage. To determine whether gp80 participates directly in cell binding and, if so, its mode of action, we conjugated purified gp80 to Covaspheres (Covalent Technology Corp., Ann Arbor, MI) and investigated their ability to bind to cells. The binding of gp80-Covaspheres was dependent on the developmental stage of the cells, with maximal interaction at the late aggregation stage. Scanning electron microscopic studies revealed the clustering of gp80-Covaspheres at the polar ends of these cells, similar to the pattern of gp80 distribution on the cell surface as reported earlier (Choi, A. H. C., and Siu, C.-H., 1987, J. Cell Biol., 104:1375-1387). Precoating cells with an adhesion-blocking anti-gp80 monoclonal antibody inhibited the binding of gp80-Covaspheres, suggesting that Covasphere-associated gp80 might undergo homophilic interaction with gp80 on the cell surface. Quantitative binding of 125I-labeled gp80 to intact cells gave an estimate of 1.5 X 10(5) binding sites per cell at the aggregation stage. Binding of soluble gp80 to cells was blocked by precoating cells with the anti-gp80 monoclonal antibody. The ability of gp80 to undergo homophilic interaction was further tested in a filter-binding assay, which showed that 125I-labeled gp80 was able to interact with gp80 bound on nitrocellulose in a dosage-dependent manner. In addition, reassociation of cells was significantly inhibited in the presence of soluble gp80, suggesting that gp80 has a single cell-binding site. These results are consistent with the notion that gp80 mediates cell-cell binding at the aggregation stage of development via homophilic interaction.

Sulfate suicide selection of Dictyostelium discoideum mutants defective in protein glycosylation

The assembly and processing of glycoprotein-linked oligosaccharides in Dictyostelium discoideum has been shown to generate a wide array of glycan structures which undergo dramatic developmental regulation. As late steps in processing of these oligosaccharides involve sulfation, a sulfate suicide selection procedure was developed to select for temperature-sensitive glycoprotein-processing mutants. Of 673 clones derived from the survivors of suicide selection, 99 were classified by replica-plating fluorography as temperature sensitive for sulfate transport or incorporation. Of these, 74 were unable to complete the developmental program to the fruiting body stage at the restrictive temperature, 29 being blocked in some aspect of aggregation and 45 being blocked at some postaggregation stage. Quantitative metabolic labeling experiments with representative clones showed that they incorporated wild-type levels of [35S]methionine but reduced levels of sulfate at the restrictive temperature. The specific incorporation patterns in the mutants suggest that distinct oligosaccharide-processing steps are involved in different developmental events.

Sulfate suicide selection of Dictyostelium discoideum mutants defective in protein glycosylation

The assembly and processing of glycoprotein-linked oligosaccharides in Dictyostelium discoideum has been shown to generate a wide array of glycan structures which undergo dramatic developmental regulation. As late steps in processing of these oligosaccharides involve sulfation, a sulfate suicide selection procedure was developed to select for temperature-sensitive glycoprotein-processing mutants. Of 673 clones derived from the survivors of suicide selection, 99 were classified by replica-plating fluorography as temperature sensitive for sulfate transport or incorporation. Of these, 74 were unable to complete the developmental program to the fruiting body stage at the restrictive temperature, 29 being blocked in some aspect of aggregation and 45 being blocked at some postaggregation stage. Quantitative metabolic labeling experiments with representative clones showed that they incorporated wild-type levels of [35S]methionine but reduced levels of sulfate at the restrictive temperature. The specific incorporation patterns in the mutants suggest that distinct oligosaccharide-processing steps are involved in different developmental events.

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.

Protein-linked oligosaccharide implicated in cell-cell adhesion in two Dictyostelium species

Monoclonal antibody d-41, previously shown to block in vitro cell-cell adhesion in aggregating Dictyostelium discoideum, also blocks adhesion in aggregating D. purpureum. In both species the antibody reacts with proteins with Mr approximately 80,000, 37,000, and 27,000, presumed to be glycoproteins since the d-41 epitope is destroyed by periodate oxidation but unaffected by extensive Pronase digestion. Polyclonal antibodies raised against the mixture of d-41 reactive glycoproteins that had been purified by immunoaffinity chromatography are potent inhibitors of D. discoideum adhesion, and adhesion-blocking activity is neutralized extensively and equivalently by each of the purified glycoproteins from D. discoideum with which d-41 reacts. In contrast, polyclonal antibodies raised against the same purified glycoproteins after they had been oxidized with periodate do not block cell-cell adhesion although they react with the glycoproteins with Mr approximately 80,000, 37,000, and 27,000 and bind as extensively to the surface of aggregating D. discoideum cells as do the adhesion-blocking polyclonal antibodies. When taken together, these results raise the possibility that some component of the d-41 binding oligosaccharide participates in cell-cell adhesion.

Carbohydrate and other epitopes of the contact site A glycoprotein of Dictyostelium discoideum as characterized by monoclonal antibodies

A series of monoclonal antibodies against a developmentally regulated protein of Dictyostelium discoideum, the contact site A glycoprotein, were used in immunoblots to label proteins of cells harvested at three stages of development: during the growth phase, at the aggregation competent stage, and at the slug stage. The antibodies fell into two groups according to their reactivity with partially or fully deglycosylated forms of the 80 kDa glycoprotein. Group A antibodies reacted not only with a 66 kDa, but also with a 53 kDa product of tunicamycin-treated wild-type cells, and they reacted with a 68 kDa component produced by HL220, a mutant that carries a specific defect in glycosylation. The 68 kDa product of the mutant was not completely unglycosylated. Like the 80 kDa glycoprotein of the wild type, which carried sulfate at carbohydrate residues, the mutant product was sulfated. In the presence of tunicamycin, the mutant produced a 53 kDa component indistinguishable from that of the wild type, which represents, most likely, the non-N-glycosylated protein portion of the contact site A glycoprotein. The group A antibodies showed almost no cross-reactivity with other proteins of the developmental stages tested, in accord with their postulated specificity for the protein moiety of the contact site A molecule. Group B antibodies did not react with the 53 kDa product of tunicamycin-treated cells, nor with the 68 kDa component of mutant HL220. These antibodies were of varying specificity. Some of them were almost as specific as group A antibodies, others cross-reacted with many proteins, particularly of the slug stage. Competition or non-competition between various group B antibodies for binding to the contact site A glycoprotein allowed sub-classification of these antibodies. According to two criteria, group B antibodies were characterized as anti-carbohydrate antibodies: (1) some of these antibodies were blocked by N-acetylglucosamine; (2) none of them reacted with the 68 kDa product or any other protein of mutant HL220. These results indicate that the 80 kDa glycoprotein carries two types of carbohydrate: type 1 carbohydrate that is sulfated and present on the 68 kDa product of mutant HL220, and type 2 carbohydrate that reacts with group B antibodies and is present on the 66 kDa product of tunicamycin-treated wild-type cells. Type 2 carbohydrate moieties are also present on many glycoproteins that are enriched in the prespore area of the slugs.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of tunicamycin on cell-surface changes associated with development and differentiation of Dictyostelium discoideum

Developmental changes in surface properties of Dictyostelium discoideum cells have been detected by partitioning. Changes occurring between 0 and 6 h development were not affected by tunicamycin but subsequent changes associated with formation of presumptive spore cells were inhibited. It would seem that only the latter changes are dependent on glycoprotein synthesis.

Epithelial sheet movement: effects of tunicamycin on migration and glycoprotein synthesis

Corneas with central epithelial wounds, 3 mm in diameter, were organ cultured in the presence of tunicamycin (TM) (1 microgram/ml), an antibiotic that inhibits glycosylation of asparagine-linked glycoproteins. Compared with control corneas, which healed in 22 hr, corneas cultured in the presence of TM for the entire culture time or for only the first 6 hr displayed a progressively slower epithelial healing rate that essentially dropped to zero by 24 hr of culture time. At 24 hr, approximately 75% of the wound was covered. After repeated washings with TM-free culture media (6X, 10 min each), this effect could consistently be reversed in corneas exposed to TM for 6 hr. Incorporation of [3H]glucosamine into trichloroacetic acid-precipitable proteins of migrating epithelial sheets was reduced to 14% that of controls after 12 hr of culture with TM, whereas [14C]leucine incorporation was not significantly affected. The decreased glycosylation was reflected on the cell surface after 12 and 20 hr culture in the presence of TM: apical cell membranes of the first six cells of the leading edge of the migrating sheet bound significantly fewer ferritin-concanavalin A particles per micrometer of membrane than did controls. These results indicate that synthesis of asparagine-linked glycoproteins is required for continued migration of corneal epithelial sheets. The asparagine-linked glycoproteins that are required for migration probably include cell-surface glycoproteins.