Analysis of cell movement during the culmination phase of Dictyostelium development

Mound-cell movement and morphogenesis in Dictyostelium

To examine the mechanisms of cell locomotion within a three-dimensional (3-D) cell mass, we have undertaken a systematic 3-D analysis of individual cell movements in the Dictyostelium mound, the first 3-D structure to form during development of the fruiting body. We used time-lapse deconvolution microscopy to examine two strains whose motion represents endpoints on the spectrum of motile behaviors that we have observed in mounds. In AX-2 mounds, cell motion is slow and trajectories are a combination of random and radial, compared to KAX-3, in which motion is fivefold faster and most trajectories are rotational. Although radial or rotational motion was correlated with the optical-density wave patterns present in each strain, we also found small but significant subpopulations of cells that moved differently from the majority, demonstrating that optical-density waves are at best insufficient to explain all motile behavior in mounds. In examining morphogenesis in these strains, we noted that AX-2 mounds tended to culminate directly to a fruiting body, whereas KAX-3 mounds first formed a migratory slug. By altering buffering conditions we could interchange these behaviors and then found that mound-cell motions also changed accordingly. This demonstrates a correlation between mound-cell motion and subsequent development, but it is not obligatory. Chimeric mounds composed of only 10% KAX-3 cells and 90% AX-2 cells exhibited rotational motion, suggesting that a diffusible molecule induces rotation, but many of these mounds still culminated directly, demonstrating that rotational motion does not always lead to slug migration. Our observations provide a detailed analysis of cell motion for two distinct modes of mound and slug formation in Dictyostelium.

Modeling chemotactic cell sorting during Dictyostelium discoideum mound formation

Coordinated cell movement is a major mechanism of the multicellular development of most organisms. The multicellular morphogenesis of the slime mould Dictyostelium discoideum, from single cells into a multicellular fruiting body, results from differential chemotactic cell movement. During aggregation cells differentiate into prestalk and prespore cells that will form the stalk and spores in the fruiting body. These cell types arise in a salt and pepper pattern after what the prestalk cells chemotactically sort out to form a tip. The tip functions as an organizer because it directs the further development. It has been difficult to get a satisfactory formal description of the movement behavior of cells in tissues. Based on our experiments, we consider the aggregate as a drop of a viscous fluid and show that this consideration is very well suited to mathematically describe the motion of cells in the tissue. We show that the transformation of a hemispherical mound into an elongated slug can result from the coordinated chemotactic cell movement in response to scroll waves of the chemoattractant cAMP. The model calculations furthermore show that cell sorting can result from differences in chemotactic cell movement and cAMP relay kinetics between the two cell types. During this process, the faster moving and stronger signaling cells collect on the top of the mound to form a tip. The mound then extends into an elongated slug just as observed in experiments. The model is able to describe cell movement patterns in the complex multicellular morphogenesis of Dictyostelium rather well and we expect that this approach may be useful in the modeling of tissue transformations in other systems.

Production and activity of spore differentiation factors (SDFs) in Dictyostelium

SDF-1 and SDF-2 are peptides that promote terminal spore differentiation under submerged conditions. The present study shows that they accumulate differentially and are released during the development of wild-type cells and can promote spore formation in cells disaggregated from wild-type culminants. SDF-1 accumulates during the slug stage and is released in a single burst at the onset of culmination while SDF-2 accumulates during early culmination and is released in a single burst from mid-culminants. The effects of SDF-1 and SDF-2 on stalk cell formation in cell monolayers were investigated. SDF-1 by itself induces stalk cell formation in some strains and also synergizes with the stalk-cell-inducing factor, DIF-1. cAMP has an inhibitory effect on stalk cell formation when either DIF-1 or SDF-1 are present on their own but is almost not inhibitory when both are present. SDF-2 alone does not induce stalk cell formation and appears to inhibit the response to DIF-1. At the same time, it increases the extent of vacuolization of the stalk cells that are produced. We propose that the release of SDF-1 and then of SDF-2 may mark irreversible steps in the developmental programme associated, respectively, with culmination and spore maturation.

Cell-type-specific rescue of myosin function during Dictyostelium development defines two distinct cell movements required for culmination

Mutant Dictyostelium cells lacking any of the component polypeptides of myosin II exhibit developmental defects. To define myosin's role in establishing Dictyostelium's developmental pattern, we have rescued myosin function in a myosin regulatory light chain null mutant (mlcR-) using cell-type-specific promoters. While mlcR- cells fail to progress beyond the mound stage, expression of RLC from the prestalk promoter, ecmA, produces culminants with normal stalks but with defects in spore cell localization. When GFP-marked prestalk and prespore cells expressing ecmA-RLC are mixed with wild-type cells, the mislocalization of prestalk cells, but not prespore cells, is rescued. Time-lapse video recording of ecmA-RLC cells showed that the posterior prespore zone failed to undergo a contraction important for the upward movement of prespore cells. Prespore cells marked with green fluorescent protein (GFP) failed to move toward the tip with the spiral motion typical of wild type. In contrast, expression of RLC in prespore cells using the psA promoter produced balloon-like structures reminiscent of sorocarps but lacking stalks. GFP-labeled prespore cells showed a spiral movement toward the top of the structures. Expression of RLC from the psA promoter restores the normal localization of psA-GFP cells, but not ecmA-GFP cells. These results define two distinct, myosin-dependent movements that are required for establishing a Dictyostelium fruiting body: stalk extension and active movement of the prespore zone that ensures proper placement of the spores atop the stalk. The approach used in these studies provides a direct means of testing the role of cell motility in distinct cell types during a morphogenetic program.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.

Identification and analysis of a gene that is essential for morphogenesis and prespore cell differentiation in Dictyostelium

We have identified a gene (PslA) that is expressed throughout Dictyostelium development and encodes a novel protein that is required for proper aggregation and subsequent cell-type differentiation and morphogenesis. pslA null (pslA-) cells produce large aggregation streams under conditions in which wild-type cells form discrete aggregates. Tips form along the stream, elongate to produce a finger, and eventually form a terminal structure that lacks a true sorus (spore head). More than half of the cells remain as a mass at the base of the developing fingers. The primary defect in the pslA- strain is the inability to induce prespore cell differentiation. Analyses of gene expression show a complete lack of prespore-specific gene expression and no mature spores are produced. In chimeras with wild-type cells, pslA- cells form the prestalk domain and normal, properly proportioned fruiting bodies can be produced. This indicates that pslA- cells are able to interact with wild-type cells and regulate patterning, even though pslA- cells are unable to express prespore cell-type-specific genes, do not participate in prespore cell differentiation and do not produce pslA- spores in the chimeras. While pslA- cells produce mature, vacuolated stalk cells during multicellular development, pslA- cells are unable to do so in vitro in response to exogenous DIF (a morphogen required for prestalk and stalk cell differentiation). These results indicate that pslA- cells exhibit a defect in the prestalk/stalk cell pathways under these experimental conditions. Our results suggest that PslA's primary function is to regulate prespore cell determination very early in the prespore pathway via a cell-autonomous mechanism, possibly at the time of the initial prestalk/prespore cell-fate decision. Indirect immunofluorescence of myc-tagged PslA localizes the protein to the nucleus, suggesting that PslA may function to control the prespore pathway at the level of transcription.

The homeobox-containing gene Wariai regulates anterior-posterior patterning and cell-type homeostasis in Dictyostelium

We have identified a Dictyostelium gene, Wariai (Wri), that encodes a protein with a homeobox and seven ankyrin repeats; both domains are required for function. A null mutation results in a more than doubling of the size of the prestalk O (pstO) compartment, one of the anterior prestalk compartments lying along the anterior-posterior axis of the migrating slug. There is a concomitant decrease in the more posterior prespore domain and no change in the more anterior prestalk A (pstA) and prestalk AB (pstAB) domains. wri null cells also have a morphological defect consistent with an increase in the pstO cell population. Wri itself is preferentially expressed in the pstA but not the pstO compartment, raising the possibility that Wri regulation of pstO compartment size is nonautonomous. Analysis of chimeric organisms is consistent with this model. Development in Dictyostelium is highly regulative, with cells within the prestalk and prespore populations being able to transdifferentiate into other cells to maintain proper cell-type proportioning. Our results suggest that Wri controls cell-type proportioning, possibly by functioning as a negative regulator of a pathway mediating pstO cell differentiation and controlling the mechanism of homeostasis regulating the size of one or more of the cell-type compartments. Our results also suggest that homeobox gene regulation of anterior-posterior axis patterning may have evolved prior to the evolution of metazoans.

Dictyostelium development in the absence of cAMP

Adenosine 3',5'-monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) are regulators of development in many organisms. Dictyostelium uses cAMP as an extracellular chemoattractant and as an intracellular signal for differentiation. Cells that are mutant in adenylyl cyclase do not develop. Moderate expression of the catalytic subunit of PKA in adenylyl cyclase-null cells led to near-normal development without detectable accumulation of cAMP. These results suggest that all intracellular cAMP signaling is effected through PKA and that signals other than extracellular cAMP coordinate morphogenesis in Dictyostelium.

The initiation of basal disc formation in Dictyostelium discoideum is an early event in culmination

We have analysed expression of the ecmA and ecmB genes of Dictyostelium by enzymatic double staining using beta-galactosidase and beta-glucuronidase reporter gene constructs. Cells expressing the ecmA gene first appear as scattered cells at the mound stage of development and we show that this is also true for cells expressing the ecmB gene. During tip formation the ecmA-expressing cells move to the apex of the mound, while the ecmB-expressing cells accumulate in the base. The ecmB-expressing cells constitute part of the basal disc if the culminant is formed in situ but are discarded if a migratory slug is formed. During slug migration they are replaced by a band of ecmB-expressing cells, situated in the front half of the prespore zone and tightly apposed to the substratum. When culmination is triggered these cells rapidly move to the back half of the prestalk zone, possibly acting as a point of attachment to the substratum. Ultimately, they are joined by cells at the back of the slug, the rearguard cells, to form the basal disc. Thus, contrary to previous belief, basal disc formation is initiated very early during culmination and occurs by the forward movement of cells located in the anterior of the prespore zone.