The membrane glycoprotein gp150 is encoded by the lagC gene and mediates cell-cell adhesion by heterophilic binding during Dictyostelium development

Cellular allorecognition and its roles in Dictyostelium development and social evolution

The social amoeba Dictyostelium discoideum is a tractable model organism to study cellular allorecognition, which is the ability of a cell to distinguish itself and its genetically similar relatives from more distantly related organisms. Cellular allorecognition is ubiquitous across the tree of life and affects many biological processes. Depending on the biological context, these versatile systems operate both within and between individual organisms, and both promote and constrain functional heterogeneity. Some of the most notable allorecognition systems mediate neural self-avoidance in flies and adaptive immunity in vertebrates. D. discoideum's allorecognition system shares several structures and functions with other allorecognition systems. Structurally, its key regulators reside at a single genomic locus that encodes two highly polymorphic proteins, a transmembrane ligand called TgrC1 and its receptor TgrB1. These proteins exhibit isoform-specific, heterophilic binding across cells. Functionally, this interaction determines the extent to which co-developing D. discoideum strains co-aggregate or segregate during the aggregation phase of multicellular development. The allorecognition system thus affects both development and social evolution, as available evidence suggests that the threat of developmental cheating represents a primary selective force acting on it. Other significant characteristics that may inform the study of allorecognition in general include that D. discoideum's allorecognition system is a continuous and inclusive trait, it is pleiotropic, and it is temporally regulated.

The polymorphic proteins TgrB1 and TgrC1 function as a ligand-receptor pair in Dictyostelium allorecognition

Allorecognition is a key factor in Dictyostelium development and sociality. It is mediated by two polymorphic transmembrane proteins, TgrB1 and TgrC1, which contain extracellular immunoglobulin domains. TgrB1 and TgrC1 are necessary and sufficient for allorecognition, and they carry out separate albeit overlapping functions in development, but their mechanism of action is unknown. Here, we show that TgrB1 acts as a receptor with TgrC1 as its ligand in cooperative aggregation and differentiation. The proteins bind each other in a sequence-specific manner; TgrB1 exhibits a cell-autonomous function and TgrC1 acts non-cell-autonomously. The TgrB1 cytoplasmic tail is essential for its function and it becomes phosphorylated upon association with TgrC1. Dominant mutations in TgrB1 activate the receptor function and confer partial ligand independence. These roles in development and sociality suggest that allorecognition is crucial in the integration of individual cells into a coherent organism.

Cell-alignment patterns in the collective migration of cells with polarized adhesion

Dictyostelium discoideum (Dd) utilizes inhomogeneities in the distribution of cell-cell adhesion molecules on cell membranes for collective cell migration. A simple example of an inhomogeneity is a front-side (leading-edge) polarization in the distribution at the early streaming stage. Experiments have shown that the polarized cell-cell adhesion induces side-by-side contact between cells [Beug et al., Nature (London) 274, 445 (1978)NATUAS0028-083610.1038/274445a0]. This result is counterintuitive, as one would expect cells to align front to front in contact with each other on the basis of front-side polarization. In this work, we theoretically examine whether front-side polarization induces side-by-side contact in collective cell migration. We construct a model for expressing cells with this polarization based on the two-dimensional cellular Potts model. By a numerical simulation with this model, we find cell-cell alignment wherein cells form lateral arrays with side-by-side contacts as observed in the experiments.

A polychromatic 'greenbeard' locus determines patterns of cooperation in a social amoeba

Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs. Cheater impact can be diminished if cooperators display a tag (‘greenbeard') and recognise and preferentially direct cooperation towards other tag carriers. Despite its popular appeal, the feasibility of such greenbeards has been questioned because the complex patterns of partner-specific cooperative behaviours seen in nature require greenbeards to come in different colours. Here we show that a locus (‘Tgr') of a social amoeba represents a polychromatic greenbeard. Patterns of natural Tgr locus sequence polymorphisms predict partner-specific patterns of cooperation by underlying variation in partner-specific protein–protein binding strength and recognition specificity. Finally, Tgr locus polymorphisms increase fitness because they help avoid potential costs of cooperating with incompatible partners. These results suggest that a polychromatic greenbeard can provide a key mechanism for the evolutionary maintenance of cooperation.

Cooperation can be stabilized against exploitation if cooperators can reliably recognize each other. Here, Gruenheit and colleagues show that different alleles of the Tgr locus of the social amoeba Dictyostelium discoideum underlie the ability of different strains to recognize and cooperate with socially compatible individuals.

Allorecognition, via TgrB1 and TgrC1, mediates the transition from unicellularity to multicellularity in the social amoeba Dictyostelium discoideum

The social amoeba Dictyostelium discoideum integrates into a multicellular organism when individual starving cells aggregate and form a mound. The cells then integrate into defined tissues and develop into a fruiting body that consists of a stalk and spores. Aggregation is initially orchestrated by waves of extracellular cyclic adenosine monophosphate (cAMP), and previous theory suggested that cAMP and other field-wide diffusible signals mediate tissue integration and terminal differentiation as well. Cooperation between cells depends on an allorecognition system comprising the polymorphic adhesion proteins TgrB1 and TgrC1. Binding between compatible TgrB1 and TgrC1 variants ensures that non-matching cells segregate into distinct aggregates prior to terminal development. Here, we have embedded a small number of cells with incompatible allotypes within fields of developing cells with compatible allotypes. We found that compatibility of the allotype encoded by the tgrB1 and tgrC1 genes is required for tissue integration, as manifested in cell polarization, coordinated movement and differentiation into prestalk and prespore cells. Our results show that the molecules that mediate allorecognition in D. discoideum also control the integration of individual cells into a unified developing organism, and this acts as a gating step for multicellularity.

Genetic control of morphogenesis in Dictyostelium

Cells grow, move, expand, shrink and die in the process of generating the characteristic shapes of organisms. Although the structures generated during development of the social amoeba Dictyostelium discoideum look nothing like the structures seen in metazoan embryogenesis, some of the morphogenetic processes used in their making are surprisingly similar. Recent advances in understanding the molecular basis for directed cell migration, cell type specific sorting, differential adhesion, secretion of matrix components, pattern formation, regulation and terminal differentiation are reviewed. Genes involved in Dictyostelium aggregation, slug formation, and culmination of fruiting bodies are discussed.

TgrC1 Has Distinct Functions in Dictyostelium Development and Allorecognition

The cell adhesion glycoproteins, TgrB1 and TgrC1, are essential for Dictyostelium development and allorecognition, but it has been impossible to determine whether their pleiotropic roles are due to one common function or to distinct functions in separate pathways. Mutations in the respective genes, tgrB1 and tgrC1, abrogate both development and allorecognition and the defects cannot be suppressed by activation of the cyclic AMP dependent protein kinase PKA, a central regulator of Dictyostelium development. Here we report that mutations in genes outside the known PKA pathway partially suppress the tgrC1-null developmental defect. We separated the pleiotropic roles of tgrC1 by testing the effects of a suppression mutation, stc^insA under different conditions. stcA^ins modified only the developmental defect of tgrC1^– but not the allorecognition defect, suggesting that the two functions are separable. The suppressor mutant phenotype also revealed that tgrC1 regulates stalk differentiation in a cell-autonomous manner and spore differentiation in a non-cell-autonomous manner. Moreover, stcA^ins did not modify the developmental defect of tgrB1^–, but the less robust phenotype of tgrB1^– obscures the possible role of stcA relative to tgrB1.

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.

Assembly of the TgrB1-TgrC1 cell adhesion complex during Dictyostelium discoideum development

In Dictyostelium discoideum, TgrB1 and TgrC1 are partners of a heterophilic cell-adhesion system. To investigate its assembly process, the split GFP complementation assay was used to track the oligomeric status of both proteins. The ability of TgrC1 to form cis-homodimers spontaneously was demonstrated by fluorescence complementation studies and confirmed by chemical cross-linking. In contrast, TgrB1 failed to form cis-homodimers in the absence of TgrC1. Treatment of cell aggregates with antibodies against TgrB1 or TgrC1 did not affect TgrC1 dimerization, but inhibited TgrB1 dimer formation, suggesting that TgrB1 cis-homodimerization is dependent on trans-interaction with TgrC1. When TgrB1 and TgrC1 conjugated with the complementary halves of GFP were co-expressed in cells, cis-heterodimers were not detected. However, weak FRET signals were detected in cells expressing TgrB1-RFP and TgrC1-GFP, suggesting that TgrB1 dimers and TgrC1 dimers were arranged juxtapose to each other in the adhesion complex. The results of the present study suggest that the assembly process is initiated upon trans-interaction of monomeric TgrB1 with TgrC1 homodimers on adjacent cells, which triggers the formation of TgrB1 dimers. The homodimerization of TgrB1 in turn induces the clustering of TgrB1 and TgrC1, and the coalescence of TgrB1-TgrC1 clusters results in the formation of large adhesion complexes.

Loss of the histidine kinase DhkD results in mobile mounds during development of Dictyostelium discoideum

Background

Histidine kinases are receptors for sensing cellular and environmental signals, and in response to the appropriate cue they initiate phosphorelays that regulate the activity of response regulators. The Dictyostelium discoideum genome encodes 15 histidine kinases that function to regulate several processes during the multicellular developmental program, including the slug to culmination transition, osmoregulation, and spore differentiation. While there are many histidine kinases, there is only a single response regulator, RegA. Not surprisingly given the ubiquitous involvement of cAMP in numerous processes of development in Dictyostelium, RegA is a cAMP phosphodiesterase that is activated upon receiving phosphates through a phosphorelay. Hence, all of the histidine kinases characterized to date regulate developmental processes through modulating cAMP production. Here we investigate the function of the histidine kinase DhkD.

Principal Findings

The dhkD gene was disrupted, and the resulting cells when developed gave a novel phenotype. Upon aggregation, which occurred without streaming, the mounds were motile, a phenotype termed the pollywog stage. The pollywog phenotype was dependent on a functional RegA. After a period of random migration, the pollywogs attempted to form fingers but mostly generated aberrant structures with no tips. While prestalk and prespore cell differentiation occurred with normal timing, proper patterning did not occur. In contrast, wild type mounds are not motile, and the cAMP chemotactic movement of cells within the mound facilitates proper prestalk and prespore patterning, tip formation, and the vertical elongation of the mound into a finger.

Conclusions

We postulate that DhkD functions to ensure the proper cAMP distribution within mounds that in turn results in patterning, tip formation and the transition of mounds to fingers. In the absence of DhkD, aberrant cell movements in response to an altered cAMP distribution result in mound migration, a lack of proper patterning, and an inability to generate normal finger morphology.

TgrC1 mediates cell–cell adhesion by interacting with TgrB1 via mutual IPT/TIG domains during development of Dictyostelium discoideum

Cell–cell adhesion plays crucial roles in cell differentiation and morphogenesis during development of Dictyostelium discoideum. The heterophilic adhesion protein TgrC1 (Tgr is transmembrane, IPT, IG, E-set, repeat protein) is expressed during cell aggregation, and disruption of the tgrC1 gene results in the arrest of development at the loose aggregate stage. We have used far-Western blotting coupled with MS to identify TgrB1 as the heterophilic binding partner of TgrC1. Co-immunoprecipitation and pull-down studies showed that TgrB1 and TgrC1 are capable of binding with each other in solution. TgrB1 and TgrC1 are encoded by a pair of adjacent genes which share a common promoter. Both TgrB1 and TgrC1 are type I transmembrane proteins, which contain three extracellular IPT/TIG (immunoglobulin, plexin, transcription factor-like/transcription factor immunoglobulin) domains. Antibodies raised against TgrB1 inhibit cell reassociation at the post-aggregation stage of development and block fruiting body formation. Ectopic expression of TgrB1 and TgrC1 driven by the actin15 promoter leads to heterotypic cell aggregation of vegetative cells. Using recombinant proteins that cover different portions of TgrB1 and TgrC1 in binding assays, we have mapped the cell-binding regions in these two proteins to Lys537–Ala783 in TgrB1 and Ile336–Val360 in TgrC1, corresponding to their respective TIG3 and TIG2 domain.

The G alpha subunit Gα8 inhibits proliferation, promotes adhesion and regulates cell differentiation

Heterotrimeric G protein-mediated signal transduction plays a pivotal role in both vegetative and developmental stages in the eukaryote Dictyostelium discoideum. Here we describe novel functions of the G protein alpha subunit Gα8 during vegetative and development stages. Gα8 is expressed at low levels during vegetative growth. Loss of Gα8 promotes cell proliferation, whereas excess Gα8 expression dramatically inhibits growth and induces aberrant cytokinesis on substrates in a Gβ-dependent manner. Overexpression of Gα8 also leads to increased cell-cell cohesion and cell-substrate adhesion. We demonstrate that the increased cell-cell cohesion is mainly caused by induced CadA expression, and the induced cell-substrate adhesion is responsible for the cytokinesis defects. However, the expression of several putative constitutively active mutants of Gα8 does not augment the phenotypes caused by intact Gα8. Gα8 is strongly induced after starvation, and loss of Gα8 results in decreased expression of certain adhesion molecules including CsA and tgrC1. Interestingly, Gα8 is preferentially distributed in the upper and lower cup of the fruiting body. Lack of Gα8 decreases the expression of the specific marker of the anterior-like cells, suggesting that Gα8 is required for anterior-like cell differentiation.

Ectopic expression of cyclase associated protein CAP restores the streaming and aggregation defects of adenylyl cyclase a deficient Dictyostelium discoideum cells

Background

Cell adhesion, an integral part of D. discoideum development, is important for morphogenesis and regulated gene expression in the multicellular context and is required to trigger cell-differentiation. G-protein linked adenylyl cyclase pathways are crucially involved and a mutant lacking the aggregation specific adenylyl cyclase ACA does not undergo multicellular development.

Results

Here, we have investigated the role of cyclase-associated protein (CAP), an important regulator of cell polarity and F-actin/G-actin ratio in the aca^- mutant. We show that ectopic expression of GFP-CAP improves cell polarization, streaming and aggregation in aca^- cells, but it fails to completely restore development. Our studies indicate a requirement of CAP in the ACA dependent signal transduction for progression of the development of unicellular amoebae into multicellular structures. The reduced expression of the cell adhesion molecule DdCAD1 together with csA is responsible for the defects in aca^- cells to initiate multicellular development. Early development was restored by the expression of GFP-CAP that enhanced the DdCAD1 transcript levels and to a lesser extent the csA mRNA levels.

Conclusions

Collectively, our data shows a novel role of CAP in regulating cell adhesion mechanisms during development that might be envisioned to unravel the functions of mammalian CAP during animal embryogenesis.

Self-recognition in social amoebae is mediated by allelic pairs of tiger genes

Free-living cells of the social amoebae Dictyostelium discoideum can aggregate and develop into multicellular fruiting bodies in which many die altruistically as they become stalk cells that support the surviving spores. Dictyostelium cells exhibit kin discrimination--a potential defense against cheaters, which sporulate without contributing to the stalk. Kin discrimination depends on strain relatedness, and the polymorphic genes tgrB1 and tgrC1 are potential components of that mechanism. Here, we demonstrate a direct role for these genes in kin discrimination. We show that a matching pair of tgrB1 and tgrC1 alleles is necessary and sufficient for attractive self-recognition, which is mediated by differential cell-cell adhesion. We propose that TgrB1 and TgrC1 proteins mediate this adhesion through direct binding. This system is a genetically tractable ancient model of eukaryotic self-recognition.

The cell adhesion molecule DdCAD-1 regulates morphogenesis through differential spatiotemporal expression in Dictyostelium discoideum

During development of Dictyostelium, multiple cell types are formed and undergo a coordinated series of morphogenetic movements guided by their adhesive properties and other cellular factors. DdCAD-1 is a unique homophilic cell adhesion molecule encoded by the cadA gene. It is synthesized in the cytoplasm and transported to the plasma membrane by contractile vacuoles. In chimeras developed on soil plates, DdCAD-1-expressing cells showed greater propensity to develop into spores than did cadA-null cells. When development was performed on non-nutrient agar, wild-type cells sorted from the cadA-null cells and moved to the anterior zone. They differentiated mostly into stalk cells and eventually died, whereas the cadA-null cells survived as spores. To assess the role of DdCAD-1 in this novel behavior of wild-type and mutant cells, cadA-null cells were rescued by the ectopic expression of DdCAD-1-GFP. Morphological studies have revealed major spatiotemporal changes in the subcellular distribution of DdCAD-1 during development. Whereas DdCAD-1 became internalized in most cells in the post-aggregation stages, it was prominent in the contact regions of anterior cells. Cell sorting was also restored in cadA(-) slugs by exogenous recombinant DdCAD-1. Remarkably, DdCAD-1 remained on the surface of anterior cells, whereas it was internalized in the posterior cells. Additionally, DdCAD-1-expressing cells migrated slower than cadA(-) cells and sorted to the anterior region of chimeric slugs. These results show that DdCAD-1 influences the sorting behavior of cells in slugs by its differential distribution on the prestalk and prespore cells.

Diverse evolutionary paths to cell adhesion

The morphological diversity of animals, fungi, plants, and other multicellular organisms stems from the fact that each lineage acquired multicellularity independently. A prerequisite for each origin of multicellularity was the evolution of mechanisms for stable cell-cell adhesion or attachment. Recent advances in comparative genomics and phylogenetics provide critical insights into the evolutionary foundations of cell adhesion. Reconstructing the evolution of cell junction proteins in animals and their unicellular relatives exemplifies the roles of co-option and innovation. Comparative studies of volvocine algae reveal specific molecular changes that accompanied the evolution of multicellularity in Volvox. Comparisons between animals and Dictyostelium show how commonalities and differences in the biology of unicellular ancestors influenced the evolution of adhesive mechanisms. Understanding the unicellular ancestry of cell adhesion helps illuminate the basic cell biology of multicellular development in modern organisms.

Polymorphic members of the lag gene family mediate kin discrimination in Dictyostelium

Self and kin discrimination are observed in most kingdoms of life and are mediated by highly polymorphic plasma membrane proteins. Sequence polymorphism, which is essential for effective recognition, is maintained by balancing selection. Dictyostelium discoideum are social amoebas that propagate as unicellular organisms but aggregate upon starvation and form fruiting bodies with viable spores and dead stalk cells. Aggregative development exposes Dictyostelium to the perils of chimerism, including cheating, which raises questions about how the victims survive in nature and how social cooperation persists. Dictyostelids can minimize the cost of chimerism by preferential cooperation with kin, but the mechanisms of kin discrimination are largely unknown. Dictyostelium lag genes encode transmembrane proteins with multiple immunoglobulin (Ig) repeats that participate in cell adhesion and signaling. Here, we describe their role in kin discrimination. We show that lagB1 and lagC1 are highly polymorphic in natural populations and that their sequence dissimilarity correlates well with wild-strain segregation. Deleting lagB1 and lagC1 results in strain segregation in chimeras with wild-type cells, whereas elimination of the nearly invariant homolog lagD1 has no such consequences. These findings reveal an early evolutionary origin of kin discrimination and provide insight into the mechanism of social recognition and immunity.

Regulation of Rap1 activity is required for differential adhesion, cell-type patterning and morphogenesis in Dictyostelium

Regulated cell adhesion and motility have important roles during growth, development and tissue homeostasis. Consequently, great efforts have been made to identify genes that control these processes. One candidate is Rap1, as it has been implicated in the regulation of adhesion and motility in cell culture. To further study the role of Rap1 during multicellular development, we generated a mutant in a potential Rap1 GTPase activating protein (RapGAPB) in Dictyostelium. rapGAPB(-) cells have increased levels of active Rap1 compared with wild-type cells, indicating that RapGAPB regulates Rap1 activity. Furthermore, rapGAPB(-) cells exhibit hallmark phenotypes of other known mutants with hyperactivated Rap1, including increased substrate adhesion and abnormal F-actin distribution. However, unlike these other mutants, rapGAPB(-) cells do not exhibit impaired motility or chemotaxis, indicating that RapGAPB might only regulate specific roles of Rap1. Importantly, we also found that RapGAPB regulates Rap1 activity during multicellular development and is required for normal morphogenesis. First, streams of aggregating rapGAPB(-) cells break up as a result of decreased cell-cell adhesion. Second, rapGAPB(-) cells exhibit cell-autonomous defects in prestalk cell patterning. Using cell-type-specific markers, we demonstrate that RapGAPB is required for the correct sorting behaviour of different cell types. Finally, we show that inactivation of RapGAPB affects prestalk and prespore cell adhesion. We therefore propose that a possible mechanism for RapGAPB-regulated cell sorting is through differential adhesion.

A 3-D model used to explore how cell adhesion and stiffness affect cell sorting and movement in multicellular systems

A three-dimensional mathematical model is used to determine the effects of adhesion and cell signalling on cell movements during the aggregation and slug stages of Dictyostelium discoideum (Dd) and to visualize cell sorting. The building blocks of the model are individual deformable ellipsoidal cells, where movement depends on internal parameter state (cell size and stiffness) and on external cues from the neighboring cells, extracellular matrix, and chemical signals. Cell movement and deformation are calculated from equations of motion using the total force acting on each cell, ensuring that forces are balanced. The simulations show that the sorting patterns of prestalk and prespore cells, emerging during the slug stage, depend critically on the type of cell adhesion and not just on chemotactic differences between cells. This occurs because cell size and stiffness can prevent the otherwise faster cells from passing the slower cells. The patterns are distinctively different when the prestalk cells are more or less adhesive than the prespore cells. These simulations suggest that sorting is not solely due to differential chemotaxis, and that differences in both adhesion strength and type between different cell types play a very significant role, both in Dictyostelium and other systems.

GrlJ, a Dictyostelium GABAB-like receptor with roles in post-aggregation development

BACKGROUND

The G-protein-coupled receptor (GPCR) family represents the largest and most important group of targets for chemotherapeutics. They are extremely versatile receptors that transduce signals as diverse as biogenic amines, purins, odorants, ions and pheromones from the extracellular compartment to the interior via biochemical processes involving GTP-binding proteins. Until recently, the cyclic AMP receptors (cARs) were the only known G protein coupled receptors in Dictyostelium discoideum. The completed genome sequence revealed the presence of several families of GPCRs in Dictyostelium, among them members of the family 3 of GPCRs, the GABAB/glutamate like receptor family, which in higher eukaryotes is involved in neuronal signaling.

RESULTS

D. discoideum has seventeen Family 3 members of GPCRs, denoted GrlA through GrlR. Their transcripts are detected throughout development with increased levels during early and late development. We have examined here GrlJ. GFP-tagged GrlJ localises to the plasma-membrane and to internal membranes. Inactivation of the grlJ gene leads to precocious development, and the mutant completes development ~6 hours earlier. Alterations were also noted at the slug stage and in spore formation. grlJ- slugs were longer and broke apart several times on their way to culmination forming smaller but proportionate fruiting bodies. Spores from grlJ- fruiting bodies were malformed and less viable, although the spore differentiation factors were synthesized and sensed normally. Expression of a GFP-tagged full length GrlJ rescued the phenotype.

CONCLUSION

Our data suggest that GrlJ acts at several stages of Dictyostelium development and that it is a negative regulator in Dictyostelium development.

Solution structures of the adhesion molecule DdCAD-1 reveal new insights into Ca2+-dependent cell-cell adhesion

DdCAD-1 is a novel Ca(2+)-dependent cell adhesion molecule that lacks a hydrophobic signal peptide and a transmembrane domain. DdCAD-1 is expressed by the social amoeba Dictyostelium discoideum at the onset of development. It is synthesized as a soluble protein and then transported to the plasma membrane by contractile vacuoles. Here we describe the novel features of the solution structures of Ca(2+)-free and Ca(2+)-bound monomeric DdCAD-1. DdCAD-1 contains two beta-sandwich domains, belonging to the betagamma-crystallin and immunoglobulin fold classes, respectively. Whereas the N-terminal domain has a major role in homophilic binding, the C-terminal domain tethers the protein to the cell membrane. From structural and mutational analyses, we propose a model for the Ca(2+)-bound DdCAD-1 dimer as a basis for understanding DdCAD-1-mediated cell-cell adhesion at the molecular level. Our results provide new insights into Ca(2+)-dependent mechanisms for cell-cell adhesion.

Transcriptional regulation of post-aggregation genes in Dictyostelium by a feed-forward loop involving GBF and LagC

Expression profiles of developmental genes in Dictyostelium were determined on microarrays during development of wild type cells and mutant cells lacking either the DNA binding protein GBF or the signaling protein LagC. We found that the mutant strains developed in suspension with added cAMP expressed the pulse-induced and early adenylyl cyclase (ACA)-dependent genes, but not the later ACA-dependent, post-aggregation genes. Since expression of lagC itself is dependent on GBF, expression of the post-aggregation genes might be controlled only by signaling from LagC. However, expression of lagC in a GBF-independent manner in a gbfA- null strain did not result in expression of the post-aggregation genes. Since GBF is necessary for accumulation of LagC and both the DNA binding protein and the LagC signal transduction pathway are necessary for expression of post-aggregation genes, GBF and LagC form a feed-forward loop. Such network architecture is a common motif in diverse organisms and can act as a filter for noisy inputs. Breaking the feed-forward loop by expressing lagC in a GBF-independent manner in a gbfA+ strain does not significantly affect the patterns of gene expression for cells developed in suspension with added cAMP, but results in a significant delay at the mound stage and asynchronous development on solid supports. This feed-forward loop can integrate temporal information with morphological signals to ensure that post-aggregation genes are only expressed after cell contacts have been made.

Signaling to cytoskeletal dynamics during chemotaxis

We review insights in signaling pathways controlling cell polarization and cytoskeletal organization during chemotactic movement in Dictyostelium amoebae and neutrophils. We compare and contrast these insights with our current understanding of pathways controlling chemotactic movements in more-complex multicellular developmental contexts.

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.

Glycoprotein gp130 of dictyostelium discoideum influences macropinocytosis and adhesion

Glycoprotein gp130, found on the plasma membrane of Dictyostelium discoideum amoebae, was postulated previously to play a role in phagocytosis. The gene for gp130 was cloned and when translated, yielded a 768 amino acid preproprotein of 85.3 kDa. It had nearly 40% similarity to the 138 kDa family of glycoproteins implicated in sexual cell fusion during macrocyst formation in D. discoideum. The difference between the calculated size and observed M(r) of 130 kDa on protein gels likely was due to N-glycosylation that was confirmed by lectin blots. Consistent with its surface-exposure, an antibody raised against recombinant protein stained the plasma membrane of D. discoideum amoebae. Gp130 and its transcripts were high during axenic growth of cells, but relatively low during growth on bacteria. The gene for gp130 was disrupted and cell lines lacking the glycoprotein were efficient phagocytes, indicating that gp130 was dispensable for phagocytosis. Gp130-null cells were similar in size to parent DH1 cells, had enhanced macropinocytosis and grew faster to higher densities. They also exhibited weaker cell-substrate adhesion but displayed greater cell-cell cohesion. Collectively, the data indicated that gp130 influenced macropinocytosis and played a role in adhesion during vegetative growth.

The genome of the social amoeba Dictyostelium discoideum

The social amoebae are exceptional in their ability to alternate between unicellular and multicellular forms. Here we describe the genome of the best-studied member of this group, Dictyostelium discoideum. The gene-dense chromosomes of this organism encode approximately 12,500 predicted proteins, a high proportion of which have long, repetitive amino acid tracts. There are many genes for polyketide synthases and ABC transporters, suggesting an extensive secondary metabolism for producing and exporting small molecules. The genome is rich in complex repeats, one class of which is clustered and may serve as centromeres. Partial copies of the extrachromosomal ribosomal DNA (rDNA) element are found at the ends of each chromosome, suggesting a novel telomere structure and the use of a common mechanism to maintain both the rDNA and chromosomal termini. A proteome-based phylogeny shows that the amoebozoa diverged from the animal-fungal lineage after the plant-animal split, but Dictyostelium seems to have retained more of the diversity of the ancestral genome than have plants, animals or fungi.

Involvement of a Rab8-like protein of Dictyostelium discoideum, Sas1, in the formation of membrane extensions, secretion and adhesion during development

Establishment of cell-cell adhesions, regulation of actin, and secretion are critical during development. Rab8-like GTPases have been shown to modulate these cellular events, suggesting an involvement in developmental processes. To further elucidate the function of Rab8-like GTPases in a developmental context, a Rab8-related protein (Sas1) of Dictyostelium discoideum was examined, the expression of which increases at the onset of development. Dictyostelium cell lines expressing inactive (N128I mutant) and constitutively active (Q74L mutant) Sas1 as green fluorescent protein (GFP)-Sas1 chimeras were generated. Cells expressing Sas1Q74L displayed numerous actin-rich membrane protrusions, increased secretion, and were unable to complete development. In particular, these cells demonstrated a reduction in adhesion as well as in the levels of a cell adhesion molecule, gp24 (DdCAD-1). In contrast, cells expressing Sas1N128I exhibited increased cell-cell adhesion and increased levels of gp24. Counting factor is a multisubunit signalling complex that is secreted in early development and controls aggregate size by negatively regulating the levels of cell adhesion molecules, including gp24. Interestingly, the Sas1Q74L mutant demonstrated increased levels of extracellular countin, a subunit of counting factor, suggesting that Sas1 may regulate trafficking of counting factor components. Together, the data suggest that Sas1 may be a key regulator of actin, adhesion and secretion during development.

A cell-adhesion pathway regulates intercellular communication during Dictyostelium development

Cell adhesion molecules play an important physical role in shaping the structure of multicellular organisms. Recent studies show that they also play a role in intracellular and intercellular signaling. We describe a cell adhesion pathway that is mediated by the intercellular communication genes comC, lagC, and lagD during Dictyostelium development. Disruptions of these genes result in strains that are unable to generate spores when developed in a pure population but are capable of sporulation when developed in chimerae with wild-type cells. In contrast, any pair-wise chimera of the three mutants fails to form spores. We postulate that the wild-type cells supply the mutant cells with a signal that partially rescues their sporulation. We also propose that the three mutants are deficient in the production of that signal, suggesting that the three genes function in one signaling pathway. In support of that notion, the mutant cells share common non-cell-autonomous prespore and prestalk-specific defects and a common pattern of developmental progression and regression. We provide transcriptional and functional evidence for a network in which comC inhibits lagC and activates lagD expression, lagC and lagD are mutually inductive, and the cell adhesion gene lagC is the terminal node in this signaling network.

AmpA, a modular protein containing disintegrin and ornatin domains, has multiple effects on cell adhesion and cell fate specification

Proteins containing disintegrin domains play a variety of roles in regulating processes involving adhesion, migration and cell type specification during development of many metazoan organisms. Most disintegrin domain containing proteins belong to the ADAM (a disintegrin and a metalloprotease) family of proteins that also contain a metalloprotease domain. Here we describe a small secreted protein from Dictyostelium that contains multiple repeated domains sharing homology with both the disintegrin motif and with a second class of fibrinogen receptor antagonists, the ornatins. This protein, called AmpA for its role in modulating adhesion, differs from the ADAM family proteins in that it lacks a metalloprotease domain. Nonetheless, it appears to be involved in the same complex spectrum of developmental functions as the metazoan ADAM family proteins. Here we review the structure and evolution of this protein and its function in cell adhesion and cell type specification. We discuss possible mechanisms by which it might function and review the emerging evidence for a close coupling between cell adhesion and cell type specification.

Genome-wide expression analyses of gene regulation during early development of Dictyostelium discoideum

Using genome-wide microarrays, we recognized 172 genes that are highly expressed at one stage or another during multicellular development of Dictyostelium discoideum. When developed in shaken suspension, 125 of these genes were expressed if the cells were treated with cyclic AMP (cAMP) pulses at 6-min intervals between 2 and 6 h of development followed by high levels of exogenous cAMP. In the absence of cAMP treatment, only three genes, carA, gbaB, and pdsA, were consistently expressed. Surprisingly, 14 other genes were induced by cAMP treatment of mutant cells lacking the activatable adenylyl cyclase, ACA. However, these genes were not cAMP induced if both of the developmental adenylyl cyclases, ACA and ACR, were disrupted, showing that they depend on an internal source of cAMP. Constitutive activity of the cAMP-dependent protein kinase PKA was found to bypass the requirement of these genes for adenylyl cyclase and cAMP pulses, demonstrating the critical role of PKA in transducing the cAMP signal to early gene expression. In the absence of constitutive PKA activity, expression of later genes was strictly dependent on ACA in pulsed cells.

Comparative analysis of spore coat formation, structure, and function in Dictyostelium

Dictyostelium produces spores at the end of its developmental cycle to propagate the lineage. The spore coat is an essential feature of spore biology contributing a semipermeable chemical and physical barrier to protect the enclosed amoeba. The coat is assembled from secreted proteins and a polysaccharide, and from cellulose produced at the cell surface. They are organized into a polarized molecular sandwich with proteins forming layers surrounding the microfibrillar cellulose core. Genetic and biochemical studies are beginning to provide insight into how the deliveries of protein and cellulose to the cell surface are coordinated and how cysteine-rich domains of the proteins interact to form the layers. A multidomain inner layer protein, SP85/PsB, seems to have a central role in regulating coat assembly and contributing to a core structural module that bridges proteins to cellulose. Coat formation and structure have many parallels in walls from plant, algal, yeast, protist, and animal cells.

Cytoskeleton interactions involved in the assembly and function of glycoprotein-80 adhesion complexes in dictyostelium

Adhesion complexes typically assemble from clustered receptors that link to the cytoskeleton via cytoplasmic adapter proteins. However, it is unclear how phospholipid-anchored adhesion molecules, such as the Dictyostelium receptor gp80, interact with the cytoskeleton. gp80 has been found to form adhesion complexes from raftlike membrane domains, which can be isolated as a Triton X-100-insoluble floating fraction (TIFF). We report here that the actin-binding protein ponticulin mediates TIFF-cytoskeleton interactions. Analysis of gp80-null cells revealed that these interactions were minimal in the absence of gp80. During development, gp80 was required to enhance these interactions as its adhesion complexes assembled. Whereas ponticulin and gp80 could partition independently into TIFF, gp80 was shown to recruit ponticulin to cell-cell contacts and to increase its partitioning into TIFF. However, these proteins did not co-immunoprecipitate. Furthermore, sterol sequestration abrogated the association of ponticulin with TIFF without affecting gp80, suggesting that sterols may mediate the interactions between ponticulin and gp80. In ponticulin-null cells, large gp80 adhesion complexes assembled in the absence of ponticulin despite the lack of cytoskeleton association. We propose that such nascent gp80 adhesion complexes produce expanded raftlike domains that recruit ponticulin and thereby establish stable cytoskeleton interactions to complete the assembly process.

SadA, a novel adhesion receptor in Dictyostelium

Little is known about cell-substrate adhesion and how motile and adhesive forces work together in moving cells. The ability to rapidly screen a large number of insertional mutants prompted us to perform a genetic screen in Dictyostelium to isolate adhesion-deficient mutants. The resulting substrate adhesion-deficient (sad) mutants grew in plastic dishes without attaching to the substrate. The cells were often larger than their wild-type parents and displayed a rough surface with many apparent blebs. One of these mutants, sadA-, completely lacked substrate adhesion in growth medium. The sadA- mutant also showed slightly impaired cytokinesis, an aberrant F-actin organization, and a phagocytosis defect. Deletion of the sadA gene by homologous recombination recreated the original mutant phenotype. Expression of sadA-GFP in sadA-null cells restored the wild-type phenotype. In sadA-GFP-rescued mutant cells, sadA-GFP localized to the cell surface, appropriate for an adhesion molecule. SadA contains nine putative transmembrane domains and three conserved EGF-like repeats in a predicted extracellular domain. The EGF repeats are similar to corresponding regions in proteins known to be involved in adhesion, such as tenascins and integrins. Our data combined suggest that sadA is the first substrate adhesion receptor to be identified in Dictyostelium.

Disruption of the gene encoding the cell adhesion molecule DdCAD-1 leads to aberrant cell sorting and cell-type proportioning during Dictyostelium development

The cadA gene in Dictyostelium encodes the Ca2+-dependent cell adhesion molecule DdCAD-1, which is expressed soon after the initiation of development. To investigate the biological role of DdCAD-1, the cadA gene was disrupted by homologous recombination. The cadA-null cells showed a 50% reduction in EDTA-sensitive cell adhesion. The remaining EDTA-sensitive adhesion sites were resistant to dissociation by anti-DdCAD-1 antibody, suggesting that they were distinct adhesion sites. Cells that lacked DdCAD-1 were able to complete development and form fruiting bodies. However, they displayed abnormal slug morphology and culmination was delayed by ∼6 hours. The yield of spores was reduced by ∼50%. The proportion of prestalk cells in cadA– slugs showed a 2.5-fold increase over the parental strain. When cadA– cells were transfected with pcotB::GFP to label prespore cells, aberrant cell-sorting patterns in slugs became apparent. When mutant prestalk cells were mixed with wild-type prespore cells, mutant prestalk cells were unable to return to the anterior position of chimeric slugs, suggesting defects in the sorting mechanism. The wild-type phenotype was restored when cadA– cells were transfected with a cadA-expression vector. These results indicate that, in addition to cell-cell adhesion, DdCAD-1 plays a role in cell type proportioning and pattern formation.

Outside-in signaling of cellulose synthesis by a spore coat protein in Dictyostelium

The spore coat of Dictyostelium is formed de novo from proteins secreted from vesicles and cellulose synthesized across the plasma membrane as differentiating spores rise up the stalk. The mechanism by which these events are coordinated is not understood. In the course of experiments designed to test the function of the inner layer coat protein SP85 (PsB), expression of a specific partial length fragment was found to interrupt coat assembly after protein secretion and prior to cellulose synthesis in 85% of the cells. This fragment consisted of SP85's N-terminal domain, containing prespore vesicle targeting information, and its Cys-rich C1 domain. The effect of the NC1 fusion was not cell autonomous in interstrain chimeras, suggesting that it acted at the cell surface. SP85-null spores presented an opposite phenotype in which spores differentiated prematurely before reaching the top of the stalk, and cellulose was slightly overproduced in a disorganized fashion. A similar though less severe phenotype occurred when a fusion of the N and C2 domains was expressed. In a double mutant, absence of SP85 was epistatic to NC1 expression, suggesting that NC1 inhibited SP85 function. Together, these results suggest the existence of an outside-in signaling pathway that constitutes a checkpoint to ensure that cellulose synthesis does not occur until coat proteins are properly organized at the cell surface and stalk formation is complete. Checkpoint execution is proposed to be regulated by SP85, which is in turn under the influence of other coat proteins that interact with SP85 via its C1 and C2 domains.

Assembly of glycoprotein-80 adhesion complexes in Dictyostelium. Receptor compartmentalization and oligomerization in membrane rafts

The phospholipid-anchored membrane glycoprotein (gp)-80 mediates cell-cell adhesion through a homophilic trans-interaction mechanism during Dictyostelium development and is enriched in a Triton X-100-insoluble floating fraction. To elucidate how gp80 adhesion complexes assemble in the plasma membrane, gp80-gp80 and gp80-raft interactions were investigated. A low density raft-like membrane fraction was isolated using a detergent-free method. It was enriched in sterols, the phospholipid-anchored proteins gp80, gp138, and ponticulin, as well as DdCD36 and actin, corresponding to components found in the Triton X-100-insoluble floating fraction. Chemical cross-linking revealed that gp80 oligomers were enriched in the raft-like membrane fraction, implicating stable oligomer-raft interactions. However, gp80 oligomers resisted sterol sequestration and were partially dissociated with Triton X-100, suggesting that compartmentalization in rafts was not solely responsible for their formation. The trans-dimer known to mediate adhesion was identified, but cis-oligomerization predominated and displayed greater accumulation during development. In fact, oligomerization was dependent on the level of gp80 expression and occurred among isolated gp80 extracellular domains, indicating that it was mediated by direct gp80-gp80 interactions. Rafts existed in gp80-null cells and such pre-existent membrane domains may provide optimal microenvironments for gp80 cis-oligomerization and the assembly of adhesion complexes.

Cell-cell adhesion and signal transduction duringDictyosteliumdevelopment

The development of the non-metazoan eukaryote Dictyostelium discoideum displays many of the features of animal embryogenesis, including regulated cell-cell adhesion. During early development, two proteins, DdCAD-1 and csA, mediate cell-cell adhesion between amoebae as they form a loosely packed multicellular mass. The mechanism governing this process is similar to epithelial sheet sealing in animals. Although cell differentiation can occur in the absence of cell contact, regulated cell-cell adhesion is an important component of Dictyostelium morphogenesis, and a third adhesion molecule, gp150, is required for multicellular development past the aggregation stage.

Cell-cell junctions that appear to be adherens junctions form during the late stages of Dictyostelium development. Although they are not essential to establish the basic multicellular body plan, these junctions are required to maintain the structural integrity of the fruiting body. The Dictyostelium β-catenin homologue Aardvark (Aar) is present in adherens junctions, which are lost in its absence. As in the case of its metazoan counterparts, Aar also has a function in cell signalling and regulates expression of the pre-spore gene psA.

It is becoming clear that cell-cell adhesion is an integral part of Dictyostelium development. As in animals, cell adhesion molecules have a mechanical function and may also interact with the signal-transduction processes governing morphogenesis.

Involvement of a triton-insoluble floating fraction in Dictyostelium cell-cell adhesion

We have isolated and characterized a Triton-insoluble floating fraction (TIFF) from Dictyostelium. Ten major proteins were consistently detected in TIFF, and six species were identified by mass spectrometry as actin, porin, comitin, regulatory myosin light chain, a novel member of the CD36 family, and the phospholipid-anchored cell adhesion molecule gp80. TIFF was enriched with many acylated proteins. Also, the sterol/phospholipid ratio of TIFF was 10-fold higher than that of the bulk plasma membrane. Immunoelectron microscopy showed that TIFF has vesicular morphology and confirmed the association of gp80 and comitin with TIFF membranes. Several TIFF properties were similar to those of Dictyostelium contact regions, which were isolated as a cytoskeleton-associated membrane fraction. Mass spectrometry demonstrated that TIFF and contact regions shared the same major proteins. During development, gp80 colocalized with F-actin, porin, and comitin at cell-cell contacts. These proteins were also recruited to gp80 caps induced by antibody cross-linking. Filipin staining revealed high sterol levels in both gp80-enriched cell-cell contacts and gp80 caps. Moreover, sterol sequestration by filipin and digitonin inhibited gp80-mediated cell-cell adhesion. This study reveals that Dictyostelium TIFF has structural properties previously attributed to vertebrate TIFF and establishes a role for Dictyostelium TIFF in cell-cell adhesion during development.