Sorting of the initial cell types in Dictyostelium is dependent on the tipA gene

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.

Regulation of multiple tip formation by caffeine in cellular slime molds

BACKGROUND

The multicellular slug in Dictyostelium has a single tip that acts as an organising centre patterning the rest of the slug. High adenosine levels at the tip are believed to be responsible for this tip dominance and the adenosine antagonist, caffeine overrides this dominance promoting multiple tip formation.

RESULTS

Caffeine induced multiple tip effect is conserved in all the Dictyostelids tested. Two key components of cAMP relay namely, cAMP phosphodiesterase (Pde4) and adenyl cyclase-A (AcaA) levels get reduced during secondary tip formation in Dictyostelium discoideum. Pharmacological inhibition of cAMP phosphodiesterase also resulted in multiple tips. Caffeine reduces cAMP levels by 16.4, 2.34, 4.71 and 6.30 folds, respectively in D. discoideum, D. aureostipes, D. minutum and Polysphondylium pallidum. We propose that altered cAMP levels, perturbed cAMP gradient and impaired signalling may be the critical factors for the origin of multiple tips in other Dictyostelids as well. In the presence of caffeine, slug cell movement gets impaired and restricted. The cell type specific markers, ecmA (prestalk) and pspA (prespore) cells are not equally contributing during additional tip formation. During additional tip emergence, prespore cells transdifferentiate to compensate the loss of prestalk cells.

CONCLUSION

Caffeine decreases adenyl cyclase-A (AcaA) levels and as a consequence low cAMP is synthesised altering the gradient. Further if cAMP phosphodiesterase (Pde4) levels go down in the presence of caffeine, the cAMP gradient breaks down. When there is no cAMP gradient, directional movement is inhibited and might favour re-differentiation of prespore to prestalk cells.

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.

A user's guide to restriction enzyme-mediated integration in Dictyostelium

Restriction enzyme-mediated integration (REMI) has been used to study a number of cellular and developmental processes in Dictyostelium discoideum. In this paper we review the basics of this powerful method of introducing random mutations in Dictyostelium. Here we discuss several mutation screens that have been devised and some of the genes that have been discovered through this approach to mutagenesis. Included in this discussion is how one goes about isolating a gene that has been disrupted by REMI, and how one confirms that this disruption is actually responsible for the observed phenotype. Finally, we describe how REMI can be used as an effective teaching tool in undergraduate cell biology laboratory courses.

Changing patterns of gene expression in dictyostelium prestalk cell subtypes recognized by in situ hybridization with genes from microarray analyses

We used microarrays carrying most of the genes that are developmentally regulated in Dictyostelium to discover those that are preferentially expressed in prestalk cells. Prestalk cells are localized at the front of slugs and play crucial roles in morphogenesis and slug migration. Using whole-mount in situ hybridization, we were able to verify 104 prestalk genes. Three of these were found to be expressed only in cells at the very front of slugs, the PstA cell type. Another 10 genes were found to be expressed in the small number of cells that form a central core at the anterior, the PstAB cell type. The rest of the prestalk-specific genes are expressed in PstO cells, which are found immediately posterior to PstA cells but anterior to 80% of the slug that consists of prespore cells. Half of these are also expressed in PstA cells. At later stages of development, the patterns of expression of a considerable number of these prestalk genes changes significantly, allowing us to further subdivide them. Some are expressed at much higher levels during culmination, while others are repressed. These results demonstrate the extremely dynamic nature of cell-type-specific expression in Dictyostelium and further define the changing physiology of the cell types. One of the signals that affect gene expression in PstO cells is the hexaphenone DIF-1. We found that expression of about half of the PstO-specific genes were affected in a mutant that is unable to synthesize DIF-1, while the rest appeared to be DIF independent. These results indicate that differentiation of some aspects of PstO cells can occur in the absence of DIF-1.

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.

tip genes act in parallel pathways of early Dictyostelium development

Analysis of Dictyostelium strains carrying null mutations in tipA showed a primary defect in cell sorting and the formation of tips on the developing mound. To study the process affected in tipA- mutants further, other mutants with a similar phenotype were isolated and characterized. These studies showed three new Dictyostelium genes: tipB, tipC, and tipD. All the tip mutants aggregate into larger than average mounds, which split up and form many lips on their surfaces. Furthermore, each mutant exhibits reduced or aberrant cell-sorting behavior, never makes migrating slugs, and has severely reduced fruiting body and spore production. The mRNA of each tip gene is present in vegetative cells and does not vary significantly with development. Prespore and prestalk gene expression is reduced or delayed in the tip mutants indicating cell type differentiation is dependent on the function of these genes. Developing mutant cells in chimeric mixtures with wildtype cells demonstrated that the defects in each tip mutant behave cell autonomously. The overexpression of TipA in a tipB- background and the overexpression of TipB in a tipA- background significantly improved the morphogenesis of these mutants. These were the only situations in which the expression of one tip gene could compensate for the lack of a different tip gene. Except for the tipA-/tipB- strain, double mutations in the tip genes have additive effects, causing a more severe mutant phenotype with defects earlier in development than single mutants. The tipA-/tipB- double mutant does not show additive effects and is very similar to the tipA-single mutant. Analysis of the effects of double mutations and overexpression indicates that members of this class of genes appear to act through parallel pathways of differentiation and tip formation in early Dictyostelium development. Furthermore, TipA and TipB appear to have some overlapping functions or are involved in the same pathway. The multitipped phenotype observed in all the mutants may be a general result of perturbing early developmental events such as cell type differentiation and cell type proportioning.

Three-dimensional in vivo analysis of Dictyostelium mounds reveals directional sorting of prestalk cells and defines a role for the myosin II regulatory light chain in prestalk cell sorting and tip protrusion

During cell sorting in Dictyostelium, we observed that GFP-tagged prestalk cells (ecmAO-expressing cells) moved independently and directionally to form a cluster. This is consistent with a chemotaxis model for cell sorting (and not differential adhesion) in which a long-range signal attracts many of the prestalk cells to the site of cluster formation. Surprisingly, the ecmAO prestalk cluster that we observed was initially found at a random location within the mound of this Ax3 strain, defining an intermediate sorting stage not widely reported in Dictyostelium. The cluster then moved en masse to the top of the mound to produce the classic, apical pattern of ecmAO prestalk cells. Migration of the cluster was also directional, suggesting the presence of another long-range guidance cue. Once at the mound apex, the cluster continued moving upward leading to protrusion of the mound's tip. To investigate the role of the cluster in tip protrusion, we examined ecmAO prestalk-cell sorting in a myosin II regulatory light chain (RLC) null in which tips fail to form. In RLC-null mounds, ecmAO prestalk cells formed an initial cluster that began to move to the mound apex, but then arrested as a vertical column that extended from the mound's apex to its base. Mixing experiments with wild-type cells demonstrated that the RLC-null ecmAO prestalk-cell defect is cell autonomous. These observations define a specific mechanism for myosin's function in tip formation, namely a mechanical role in the upward movement of the ecmAO prestalk cluster. The wild-type data demonstrate that cell sorting can occur in two steps, suggesting that, in this Ax3 strain, spatially and temporally distinct cues may guide prestalk cells first to an initial cluster and then later to the tip.

Evidence that a cell-type-specific efflux pump regulates cell differentiation in Dictyostelium

We have identified a cellular efflux pump, RhT, with the properties of an MDR transporter-a type of ATP-binding cassette transporter whose substrates include small hydrophobic molecules. RhT transports rhodamine 123 (Rh123) and is inhibited by low temperature, energy poisons, and several MDR transport inhibitors, such as verapamil. All vegetative cells have RhT activity, but during development prestalk cells lose RhT activity while prespore cells retain it. We also identified several RhT inhibitors. The most effective inhibitor is the stalk cell-inducing chlorinated alkyl phenone, DIF-1. The RhT inhibitors disrupted development, to varying degrees, and induced stalk cell formation in submerged culture. The inhibitors displayed the same rank order of pharmacological efficacy for stalk cell induction as they did for Rh123 transport inhibition. We also found that cerulenin, a specific inhibitor of DIF-1 biosynthesis (R. R. Kay, 1998, J. Biol. Chem. 273, 2669-2675), abolished the induction of stalk cells by each of the RhT inhibitors, and this effect could be reversed by DIF-1. Thus, DIF-1 synthesis appears to be required for the induction of stalk cells by the RhT inhibitors. Since DIF-1 is the most potent inhibitor of RhT activity, and thus a likely transport substrate itself, we propose that RhT inhibitors induce stalk cell differentiation by blocking DIF-1 export, causing DIF-1 to build up within cells. Our results provide evidence for a prespore-specific efflux pump that regulates cell fate determination, perhaps by regulating the cellular concentration of DIF-1.

SDF-2 induction of terminal differentiation in Dictyostelium discoideum is mediated by the membrane-spanning sensor kinase DhkA

SDF-2 is a peptide released by prestalk cells during culmination that stimulates prespore cells to encapsulate. Genetic evidence indicates that the response is dependent on the dhkA gene. This gene encodes a member of the histidine kinase family of genes that functions in two-component signal transduction pathways. The sequence of the N-terminal half of DhkA predicts two hydrophobic domains separated by a 310-amino-acid loop that could bind a ligand. By inserting MYC6 epitopes into DhkA, we were able to show that the loop is extracellular while the catalytic domain is cytoplasmic. Cells expressing the MYC epitope in the extracellular domain of DhkA were found to respond only if induced with 100-fold-higher levels of SDF-2 than required to induce dhkA+ cells; however, they could be induced to sporulate by addition of antibodies specific to the MYC epitope. To examine the enzymatic activity of DhkA, we purified the catalytic domain following expression in bacteria and observed incorporation of labelled phosphate from ATP consistent with histidine autophosphorylation. Site-directed mutagenesis of histidine1395 to glutamine in the catalytic domain blocked autophosphorylation. Furthermore, genetic analyses showed that histidine1395 and the relay aspartate2075 of DhkA are both critical to its function but that another histidine kinase, DhkB, can partially compensate for the lack of DhkA activity. Sporulation is drastically reduced in double mutants lacking both DhkA and DhkB. Suppressor studies indicate that the cyclic AMP (cAMP) phosphodiesterase RegA and the cAMP-dependent protein kinase PKA act downstream of DhkA.