Cell-cell adhesion in Dictyostelium discoideum

Three separate mechanisms of cell-cell adhesion have been shown to appear at different stages of development in Dictyostelium discoideum. During the first few hours of development, the cells synthesize and accumulate a glycoprotein of 24,000 daltons (gp24) that is positioned in the membrane. The time of appearance of gp24 correlates exactly with the time of appearance of cell-cell adhesion in two strains in which temporal control varies by several hours. Antibodies specific to gp24 are able to block cell-cell adhesion during the first few hours of development but not during later development. By 8 hr of development, another glycoprotein, gp80, that is not recognized by antibodies to gp24 accumulates on the surface of cells. This membrane protein mediates an independent adhesion mechanism during the aggregation stage that is resistant to 10 mM EDTA. Antibodies specific to gp80 can block EDTA-resistant adhesion during this stage. During subsequent development, gp80 is removed from the cell surface and replaced by another adhesion mechanism that is insensitive to antibodies to either gp24 or gp80. A lambda gt11 expression vector carrying a Dictyostelium cDNA insert was isolated that directs the synthesis of a fusion protein recognized by antibodies specific to gp24. This cDNA was used to probe a genomic library. A clone carrying a 1.4-kb insert of genomic DNA was recognized by the cDNA and shown to hybridize to a 0.7-kb mRNA that accumulates early in development. This unusually small RNA could code for the small protein, gp24. Southern analysis of restriction fragments generated by various enzymes on Dictyostelium DNA with both the cDNA and genomic clones indicated the presence of two tandem copies of the gene. This may account for the failure to recover mutations resulting in the lack of gp24. Mutations have been recovered that result in the lack of accumulation of gp80, and cells carrying these mutations have been shown to be missing the second adhesion mechanism. These mutant strains are able to complete development because the other adhesion mechanisms are not impaired. Sequential addition of adhesion mechanisms provides a means for the formation of multicellular organisms from previously solitary cells.

Neutral mutations and repetitive DNA

We have previously shown that computer simulations of processes that generate selectively advantageous changes together with random duplications and deletions give rise to genomes with many different genes embedded in a large amount of dispensable DNA sequence. We now explore the consequences of neutral changes on the evolution of genomes. We follow the consequences of sequence divergences that are neutral when they occur in dispensable sequences or extra copies of genes present in multigene families. We find that when divergence occurs at about the same frequency as duplication/deletion events, genomes carry repetitive sequences in proportion to their size. Inspection of the genomes as they evolved showed that multigene families were generated by relatively recent duplications of single genes and so would be expected to be highly homogeneous.

Antisense RNA inactivation of myosin heavy chain gene expression in Dictyostelium discoideum

The role of myosin in the contraction of striated muscle cells is well known, but its importance in nonmuscle cells is not yet clear. The function of myosin in Dictyostelium discoideum has been investigated by isolating cells which specifically lack myosin heavy chain (MHC A) protein. Cells were transformed with a vector encoding RNA complementary to mhcA messenger RNA (antisense RNA). Stable transformants have a dramatic reduction in the amount of MHC A protein, grow slowly, and generate giant multinucleated progeny, indicating an impairment in cytokinesis. Surprisingly, the cells adhere to surfaces, extend pseudopods and are capable of ameboid locomotion. The developmental sequence that is initiated by starving cells is severely impaired by the lack of myosin. The cells are unable to form multicellular aggregates normally and do not undergo subsequent morphogenesis. By changing the food source from liquid medium to bacteria, expression of the endogenous mhcA messenger RNA can be increased relative to expression of antisense RNA. When grown in this way, the transformed cells accumulate MHC A protein, remain mononucleate, and proceed through development normally.

Surface glycoprotein, gp24, involved in early adhesion of Dictyostelium discoideum

A membrane glycoprotein of 24,000 Da (gp24) was purified from developed cells of Dictyostelium discoideum and shown to neutralize a crude antiserum (R695) that blocks EDTA-sensitive cell-cell adhesion during the early developmental stages of this organism. Purified gp24 was used to raise rabbit polyclonal antibodies and mouse monoclonal antibodies. Rabbit antiserum R851 was shown to be highly specific to gp24 by both Western analysis and immunoprecipitation. IgG of R851 is able to block adhesion of dissociated cells swirled in suspension. Adhesion of wild-type cells is blocked by R851 antibodies during the first 8 hr of development but not thereafter when other adhesion mechanisms come into play. The glycoprotein gp80 plays an essential role in the second adhesion system that appears during the aggregation stage of D. discoideum. By adding both anti-gp24 and anti-gp80 antibodies, adhesion of aggregation stage cells could be blocked. Late in development a third adhesion mechanism appears that is not blocked by either antibodies to gp24 or gp80 or both antibodies together. Western analysis and immunoprecipitation with monoclonal antibody mLJ11, specific for gp24, indicated that gp24 is absent in cells growing exponentially on bacteria but is rapidly synthesized and accumulated following the initiation of development. Synthesis of gp24 is maximal during the first 4 hr of development and then continues at a reduced rate throughout the remainder of development. The coordinate appearance of gp24 and EDTA-sensitive cell-cell adhesion as well as the ability of this glycoprotein to neutralize the adhesion blocking activity of R695 and R851 antibodies indicates that it plays a role in early cell-cell adhesion.

DNA sequences required for expression of a Dictyostelium actin gene

A 2.8-kb fragment of 5' non-coding DNA from the Dictyostelium actin 15 gene has previously been shown to contain all of the cis-acting DNA sequence elements required for normal developmentally-regulated transcription of actin gene fusion RNAs when reintroduced into the genome by DNA-mediated transformation. Deletion analysis of this promoter fragment indicates that all of the necessary information is contained within a 270-bp fragment of actin 15 5' non-coding DNA. This fragment contains four short G/C-rich repeated sequences that are also found in other co-regulated Dictyostelium actin genes. A 12-bp consensus sequence, AAAAATGGGG/ATT, is present in the regions essential for expression of two different Dictyostelium actin genes, actin 6 and actin 15, but is absent from an actin gene showing a different temporal pattern of developmental regulation. Deletion analysis and DNase I footprinting implicate this sequence as a functional cis-acting element required for transcription of the actin 15 gene.

Developmental regulation of Dictyostelium discoideum actin gene fusions carried on low-copy and high-copy transformation vectors

The Dictyostelium discoideum genome contains an estimated 17 to 20 actin genes. We report the identification of a new member of this multigene family, actin 15, and its complete nucleotide sequence and transcription initiation sites. We constructed transformation vectors carrying either the actin 15 promoter fused to the neomycin phosphotransferase gene from transposon Tn903 or the actin 6 promoter fused to the neomycin phosphotransferase gene from Tn5. Cells transformed with the actin 15 vector carried less than five copies of vector DNA, while cells transformed with the actin 6 vector carried more than 200 copies. In both cases, the vector appeared to be integrated into the chromosome as a tandem array. Gene fusion RNAs transcribed from the actin 15 and actin 6 vectors were regulated like endogenous actin genes during D. discoideum development. DNA sequences required for temporal and cell type-specific regulation of these genes were contained within 2.8 kilobases of 5' noncoding DNA for actin 15 and 0.7 kilobases of 5' noncoding DNA for actin 6.

Hexapeptide repeat structure in Dictyostelium spore coat protein

The sequences of the NH2-termini of two spore coat proteins of Dictyostelium discoideum have been determined. One of them (SP60) consists of perfect hexapeptide repeats of the sequence Gly-Asp-Trp-Asn-Asn-Asx-. The sequence has some homology to the parvovirus capsid protein which does not display periodicity. The NH2-terminal sequence of the second protein, SP70, contains a modified amino acid in two positions and like SP60 is highly hydrophilic and acidic.

Absence of a carbohydrate modification does not affect the level or subcellular localization of three membrane glycoproteins in modB mutants of Dictyostelium discoideum

The accumulation and localization of four developmentally regulated membrane glycoproteins were examined in a glycosylation mutant of the cellular slime mold Dictyostelium discoideum. As judged by immunoblot procedures using antipeptide antibodies, the levels of three of the glycoproteins, WGA80B, SP29A, and SP29B, were unaffected, but their apparent molecular masses were reduced by 14,000, 3,500 and 3,500 daltons, respectively. The level of the fourth glycoprotein, gp80, was reduced to below detectable limits. The reduced molecular sizes were apparently due to the absence of certain carbohydrate structures as judged by labeling Western blots with anti-carbohydrate antibodies and a lectin. Using immunofluorescence labeling of permeabilized and intact cells, the localization of WGA80B, SP29A, and SP29B, in intracellular vesicles and on the cell surface of prespore cells, was observed to be unaffected in the mutant cells. The developmentally regulated oligosaccharide structure(s) affected by the modB locus does not influence the subcellular localization and accumulation of these three glycoproteins in the prespore cells of this phylogenetically primitive organism.

Adhesion mutants of Dictyostelium discoideum lacking the saccharide determinant recognized by two adhesion-blocking monoclonal antibodies

A mutant of Dictyostelium discoideum, strain HL260, was isolated based on its failure to bind d-41, a monoclonal antibody that blocks developmentally regulated cell-cell adhesion. The mutant fails to normally acquire cell-cell adhesion as assayed with cells shaken in 10 mM EDTA, but aggregates and and constructs fruiting bodies. Other mutant strains, HL216 and HL220, previously shown to have impaired cell-cell adhesion, also lack the determinant that binds d-41. The three strains all carry mutations in a gene designated mod B, which directs a post-translational modification of several developmentally regulated D. discoideum glycoproteins. Diploids formed between independent mod B mutant haploid strains also lack this determinant and show marked impairment of cell-cell adhesion in EDTA, indicating that mutations in mod B, rather than other mutations not shared by the haploid strains, are related to the adhesion defect. The results are consistent with other evidence that an oligosaccharide carried on several developmentally regulated glycoproteins plays an essential role in EDTA-resistant cell-cell adhesion in D. discoideum. However, this type of adhesion is not essential for morphogenesis in that the only defect detected thus far in mod B mutant strains is that they construct relatively smaller fruiting bodies that contain fewer spores.

Molecular characterization of anterior-like cells in Dictyostelium discoideum

Prestalk cells are found in the anterior 15% of slugs of Dictyostelium discoideum and prespore cells are found in the remaining posterior portions. Neutral red staining as well as immunostaining with antisera prepared against spores or a spore-coat protein has indicated that there are cells in the posterior region which have properties similar to prestalk cells. This cell population, called anterior-like cells, has been isolated by density separation of posterior slug fractions. This anterior-like cell preparation contained approximately 80% neutral red-stained cells, none of which carried a surface antigen specific to prespore cells (MUD-1 antigen). These cells also lacked antigens recognized by antisera to the spore-coat proteins or by a prespore-specific monoclonal antibody isolated for this study (mLJ1). Anterior-like cells were found to modify the lysosomal enzymes acid phosphatase and alpha-mannosidase in a manner found specifically in prestalk cells. They also synthesized two prestalk-specific proteins and contain two prestalk-specific mRNA transcripts. Anterior-like cells also failed to express a large number of prespore-specific characteristics. Although very similar to prestalk cells, anterior-like cells differ from them in that they express a prespore-specific protein PSP59 and the levels of the prestalk-specific mRNA transcripts and one of the prestalk-specific proteins ST430 appear to be lower.

Developmental changes in the modification of lysosomal enzymes in Dictyostelium discoideum

Evidence has been found for a generalized change in the post-translational modification of lysosomal enzymes during development of Dictyostelium discoideum. The physical and antigenic properties of four developmentally regulated lysosomal enzymes, N-acetylglucosaminidase, beta-glucosidase, alpha-mannosidase, and acid phosphatase, have been examined throughout the life cycle. In vegetative cells, a single major isoelectric species is detected for each enzymatic activity on native nonequilibrium isoelectric focusing gels. Between 6 and 10 hr of development, all activities, including the preformed enzyme, become less negatively charged, resulting in a modest but reproducible shift in the isoelectric focusing pattern. This alteration is not detected by native gel electrophoresis at constant pH. As development continues, the specific activity of beta-glucosidase, alpha-mannosidase, and acid phosphatase continues to increase and coincidentally, new, less acidic isozymic bands of activity can be observed on both gel systems. Some of these new isozymes accumulate preferentially in anterior cells, while others accumulate preferentially in posterior cells of migrating slugs. N-Acetylglucosaminidase does not increase in specific activity late in development and no new isozymic species appear. Using a monoclonal antibody that reacts with sulfated N-linked oligosaccharides shared by vegetative lysosomal enzymes in D. discoideum, the antigenicity of the developmental isozymes has been characterized. All of the enzymatic activity present during vegetative growth and early development is immunoprecipitable. However, the less negatively charged isozymes that accumulate after aggregation are not recognized by the antibody. Nonantigenic acid phosphatase and alpha-mannosidase are found in both anterior and posterior cells from migrating pseudoplasmodia. Since each enzyme is coded by a single structural gene, these results suggest that the isozymes present late in development arise from the synthesis of the same polypeptides with altered post-translational modifications. The appearance of anterior and posterior specific isozymes is likely to be the result of cell type specific changes in the glycoprotein modification pathway for newly synthesized proteins.

Cloning and expression of a cDNA that comprises part of the gene coding for a spore coat protein of Dictyostelium discoideum

The three major spore coat proteins of Dictyostelium discoideum are developmentally regulated, cell-type-specific proteins. They are packaged in prespore vesicles and then secreted to form the outer layer of spore coats. We have isolated a cDNA clone from the gene coding for one of these proteins, SP96, a glycoprotein of 96,000 daltons. We screened the cDNA bank by the method of hybrid select translation followed by immunoprecipitation of the translation products with SP96-specific polyclonal antiserum. We found that the gene was first transcribed into stable mRNA a few hours before the time of detection of SP96 synthesis and that the mRNA, like the protein, accumulated specifically in prespore cells and spores. SP96 constituted the same proportion of newly synthesized protein as the proportion of its message in polyadenylated RNA. SP96 appeared to be encoded by a single gene as judged by Southern blot analysis of digested genomic DNA hybridized to the cDNA clone.

Regulation of cell-type-specific differentiation in Dictyostelium

During development of Dictyostelium, there are at least a dozen discrete stages of differentiation that can be distinguished by the expression of specific genes. The early stages are triggered by amino acid starvation and are dependent on a small heat-stable effector secreted by the cells to indicate a critical cell density. After development has proceeded for 12 hours, late genes are expressed that are dependent on the conditions found in multicellular aggregates. Cells monitor these conditions and appear to respond by raising their internal cAMP levels to act as a second messenger. Multicellularity can be bypassed as an essential condition if high levels of cAMP are added to the environment. The EDTA-resistant cell-adhesion mechanism that develops by 12 hours is not a required aspect of multicellularity. A final set genes can be induced in 18-hour-developed cells by lowering the pNH3. The spore coat proteins are well-characterized markers for prespore differentiation; their genes are first expressed at 12 hours. Prestalk cells do not express these genes. A small number of prestalk cells become redistributed in the posterior during slug migration and appear to undergo respecification when their position is changed. Prestalk genes become repressed in these "anterior-like" cells and prespore genes are activated. These results clearly indicate that a fieldwide system of positional information regulates cell-type differentiation in Dictyostelium.

Antigenic determinants shared by lysosomal proteins of Dictyostelium discoideum. Characterization using monoclonal antibodies and isolation of mutations affecting the determinant

Polyclonal antisera raised against a single lysosomal enzyme from Dictyostelium discoideum will cross-react with other lysosomal proteins due to the presence of common post-translational modification antigens. We have now isolated hybridoma cell lines secreting monoclonal antibodies that also recognize a shared lysosomal determinant. Although the same proteins are recognized by the monoclonal and polyclonal antibodies, the actual determinants differ. Also, the polyclonal serum contains antibodies which recognize at least one determinant not recognized by the monoclonal antibodies. All of the antigenic proteins are minor cell proteins but represent a majority of the proteins secreted from the cells during axenic growth. Anode-directed nonequilibrium two-dimensional gel electrophoresis of cellular and secreted protein indicates that the antigenic proteins have extremely acidic pI values; moreover, most are both sulfated and phosphorylated. Several sulfated polysaccharides, including dextran sulfate, chondroitin sulfate, and heparin inhibit binding of the monoclonal but not the polyclonal antibodies, indicating that the shared determinant recognized by the monoclonal antibodies may involve sulfated oligosaccharides. As a step in analyzing the function of the post-translational modification of lysosomal enzymes, we have screened a mutagenized population of amoeba for those lacking the antigenic determinant recognized by one of the monoclonal antibodies. Five cell lines have been isolated that completely lack the determinant. They appear to fall into at least three different classes based upon physical properties of the enzymes. One strain lacks any detectable sulfation of glycoproteins. Several of the strains are capable of forming fruiting bodies indicating that the antigenic determinant plays no crucial role in morphogenesis under laboratory conditions.

The timing of cell-type-specific differentiation in Dictyostelium discoideum

We have used two-dimensional gel electrophoresis to identify over 30 proteins which are specific to one or other of the two cell types of Dictyostelium discoideum, either at the slug stage or in mature fruiting bodies. Our results support the idea that there is a continuous developmental program that begins in prespore cells at the hemispherical mound stage (10-12 hr) and results in spore differentiation (24 hr). Prestalk differentiation, on the other hand, appeared largely unrelated to stalk differentiation, which was first detectable at the onset of culmination (18 hr). We have also used this approach to study the differentiation of stalk-only mutants and have found that the cells can switch from spore to stalk differentiation as late as 2 hr before the end of the wild-type developmental program.

Biochemical and genetic analysis of pre-stalk specific acid phosphatase in Dictyostelium

The only known enzymatic marker of pre-stalk cells at the slug stage of development in Dictyostelium discoideum is an isozyme of acid phosphatase, AP2. There is another isozyme of acid phosphatase, AP1, which is present in vegetative cells and is not cell-type specific. We have purified these isozymes and find they differ in Km and thermostability. Both isozymes are affected by mutations in a single locus, acpA. Two mutations in the acpA locus abolished all activity of both AP1 and AP2 while a third mutation reduced the activity and altered the thermostability of both isozymes. It is likely that acpA is the structural gene responsible for both AP1 and AP2. The cell-type specificity of AP2 appears to result from differences in the modification of the acpA gene product between pre-spore and pre-stalk cells. The resulting difference in AP2 provides a useful marker for pre-stalk cells.

Mutations affecting a surface glycoprotein, gp80, of Dictyostelium discoideum

We isolated two independent mutations in Dictyostelium discoideum that result in the absence of the antigenic determinant recognized by monoclonal antibody E28D8. This antibody reacts with a post-translational modification on the surface glycoprotein gp80 and several other proteins. Both of the mutations occur in the same locus, modB, which was mapped to linkage group VI. The modB mutations result in sufficient alteration of gp80 that it is absent or unrecognizable by two-dimensional gel electrophoresis. Strains carrying modB mutations exhibit "contact sites A"-mediated cell-cell adhesion although more weakly than do wild-type strains and develop to fruiting bodies carrying viable spores. Although gp80 has been implicated in the mechanism of cell-cell adhesion in D. discoideum, it is clear from the behavior of these mutant strains that the determinant on gp80 recognized by E28D8 is not necessary for either morphogenesis or reduced EDTA-resistant adhesion.

Spore coat proteins of Dictyostelium discoideum are packaged in prespore vesicles

Previous studies have shown that Dictyostelium discoideum spore coat proteins are found in prespore cells, which are localized to the posterior region of migrating slugs, and in the coats of mature spores. Prespore vesicles, identified by morphology and by staining with anti-D. mucoroides spore serum, are also localized in the posterior region of migrating slugs. Using antisera specific to the spore coat proteins, we show that the spore coat proteins are packaged in prespore vesicles. They are present in the vesicles as a complex which can be dissociated by denaturation. The anti-D. mucoroides spore serum reacts with at least five proteins in whole spore extracts including the spore coat proteins SP96 and SP70.

Loss and resynthesis of a developmentally regulated membrane protein (gp80) during dedifferentiation and redifferentiation in Dictyostelium

When developing cultures of Dictyostelium discoideum are disaggregated and resuspended in nutrient medium, they lose the capacity to rapidly reaggregate after 90 min, in a rapid and synchronous step referred to as the "erasure event." They then proceed to lose remaining developmentally acquired functions in a program of dedifferentiation culuminating with the loss of EDTA-resistant cohesion roughly 5 hr later. Immediately following the erasure event, cells can be stimulated to reenter the developmental program even though they still possess a number of developmentally acquired functions. These cells therefore appear to undergo dedifferentiation and redifferentiation simultaneously (D. R. Soll and L. H. Mitchell, 1982, Dev. Biol. 91, 183-190). In this report, we have employed an antiserum made against a developmentally acquired membrane glycoprotein, gp80, to examine whether gp80 is lost during dedifferentiation and whether it is either reutilized or resynthesized during redifferentiation. Results are presented which demonstrate that (1) when 9-hr developing cells are disaggregated and resuspended in nutrient medium, gp80 continues to accumulate for several hours after the erasure event, then is lost at roughly the same time as EDTA-resistant cohesion; (2) when cells are stimulated to reenter the developmental program immediately after the erasure event, both gp80 and EDTA-resistant cohesion are still lost according to the program of dedifferentiation, but are then reacquired soon afterwards according to the program of redifferentiation; (3) during redifferentiation, cells do not reutilize gp80 which had been synthesized during initial development; rather they synthesize gp80 de novo; and (4) developing cells of a dedifferentiation-defective variant, HI4, when disaggregated and resuspended in nutrient medium, retain gp80, EDTA-resistant cohesion, and the capacity to rapidly reinitiate aggregation for at least 12 hr. This last result indicates that the loss of gp80 is regulated by the dedifferentiation process and is not an independent response to disaggregation or the reintroduction of nutrients. Together, these results reinforce the conclusion that dedifferentiation and redifferentiation can function independently and simultaneously in the same cells.

Adhesion-blocking antibodies prepared against gp150 react with gp80 of Dictyostelium

A membrane glycoprotein of 150 000 D, gp150, has been implicated in the mechanism of cell-cell adhesion which arises during development of Dictyostelium discoideum. This conclusion was founded on the observation that monovalent Fab' fragments prepared from an antiserum raised against partially purified gp150 are able to block cell-cell adhesion. We show that this serum contains antibodies to a distinct membrane glycoprotein, gp80, previously implicated in cell-cell adhesion. Reaction of Fab' to this surface molecule can account for the adhesion-blocking activity in the antiserum to gp 150. Moreover, binding of gp80 neutralized Fab' to gp150 does not block adhesion. If gp150 carries other determinants which bind adhesion-blocking Fab', these determinants must also be present on gp80. Thus, it is not clear that gp150 is directly involved in cell-cell adhesion of Dictyostelium.

Monoclonal antibody recognizing gp80, a membrane glycoprotein implicated in intercellular adhesion of Dictyostelium discoideum

WE have raised a monoclonal antibody, designated E28D8, which reacts with an 80,000-dalton membrane glycoprotein (gp80) of Dictyostelium discoideum. gp80 has been implicated in the formation of the EDTA-resistant adhesions ("contact sites A") which appear during development. The monoclonal antibody reacted with other developmentally regulated proteins of D. discoideum, confirming previous results indicating the presence of common antigenic determinants recognized by polyclonal rabbit antibodies directed to gp80. Periodate sensitivity of the determinants suggests that carbohydrate may be necessary for reactivity. Thus, the determinant recognized by E28D8 may result from a posttranslational modification common to a number of proteins. Some of the proteins that carry the determinant were preferentially localized to posterior cells in slugs. Monoclonal antibody E28D8 did not inhibit contact-sites-A-mediated intercellular adhesion. However, gp80 affinity purified on immobilized monoclonal antibody was able to neutralize the adhesion-blocking effect of rabbit antiserum to gp80. Although gp80 itself may not be essential for cell-cell adhesion, it appears to carry the determinants associated with adhesion.