Vital staining methods used in the analysis of cell sorting in Dictyostelium discoideum

Confluence of Cellular Degradation Pathways During Interdigital Tissue Remodeling in Embryonic Tetrapods

Digits develop in the distal part of the embryonic limb primordium as radial prechondrogenic condensations separated by undifferentiated mesoderm. In a short time interval the interdigital mesoderm undergoes massive degeneration to determine the formation of free digits. This fascinating process has often been considered as an altruistic cell suicide that is evolutionarily-regulated in species with different degrees of digit webbing. Initial descriptions of interdigit remodeling considered lysosomes as the primary cause of the degenerative process. However, the functional significance of lysosomes lost interest among researcher and was displaced to a secondary role because the introduction of the term apoptosis. Accumulating evidence in recent decades has revealed that, far from being a unique method of embryonic cell death, apoptosis is only one among several redundant dying mechanisms accounting for the elimination of tissues during embryonic development. Developmental cell senescence has emerged in the last decade as a primary factor implicated in interdigit remodeling. Our review proposes that cell senescence is the biological process identified by vital staining in embryonic models and implicates lysosomes in programmed cell death. We review major structural changes associated with interdigit remodeling that may be driven by cell senescence. Furthermore, the identification of cell senescence lacking tissue degeneration, associated with the maturation of the digit tendons at the same stages of interdigital remodeling, allowed us to distinguish between two functionally distinct types of embryonic cell senescence, “constructive” and “destructive.”

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.

Dictyostelium Myb transcription factors function at culmination as activators of ancillary stalk differentiation

ecmB and mrrA are expressed in the cups that cradle Dictyostelium spore heads, and MybE is necessary for their expression in lower but not upper cup cells. A Myb site within the mrrA promoter is necessary for expression in both cups. Thus, multiple Myb proteins are required for ancillary stalk differentiation.

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.

Light affects cAMP signaling and cell movement activity in Dictyostelium discoideum

The multicellular, slug stage of the slime mould Dictyostelium discoideum lacks specific sensory cells and organs but can nevertheless respond in a very sensitive manner to external stimuli such as temperature and light. Within the migrating slug, the behavior of up to 100,000 individual amoebae is coordinated by cAMP mediated cell-cell signaling and chemotaxis. We report here the striking result that light directly modulates the cAMP cell-cell signaling system. Light-induced secretion of cAMP from the slug tips decreased the period length of optical density waves and speeded up cell movement. A local effect of light on cAMP release within the slug tip could modulate cell movement within the slug and thus control its phototactic turning and orientation toward a light source.

Cells at the center of Dictyostelium aggregates become spores

The cellular slime mold Dictyostelium discoideum undergoes a developmental life cycle on starvation to generate a fruiting body consisting of a mass of spores supported on a stalk of dead, vacuolated cells. The choice between alternative cell fates is influenced by a variety of factors including cell cycle position at the onset of starvation. We present evidence to suggest that the cell cycle position influences cell fate by determining the position of cells in the early aggregate. The existence of a strain which cannot initiate development on its own but which can respond to signals generated by nonmutant cells has allowed us to investigate the eventual cell fate of the initiating cells which are, by definition, at the center of the early aggregate. Cells which have a propensity to become prespore cells show an increased efficiency in initiating development of this strain. Labeling the initiating cells by the expression of green fluorescent protein reveals that these cells become spores. The higher levels of expression of genes characteristic of early development in cells with a prespore tendency are consistent with the earlier expression of the components of relay in prespore cells.

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.

A study of PstB cells during Dictyostelium migration and culmination reveals a unidirectional cell type conversion process

The prestalk region of the Dictyostelium slug has recently been shown by Williams and his collaborators to consist of two distinct cell types, pstA and pstB cells. Here the movement of these cells in both the slug and culmination stages has been examined with the use of vital dyes. In the slug some of the pstB cells are continually lost from the prestalk region as small clusters of cells. These cells move through the prespore region and temporarily lie in the rearguard region at the posterior end of the slug. They are finally left in the slug's slime track as single cells or groups of a few cells. When culmination is initiated the pstB cells move as a whole from the prestalk region to the base where they join the rearguard cells to form the basal disc of the fruiting body. Transplantation experiments reveal that the rearguard cells form an outer ring portion of the basal disc and the pstB cells form an inner portion to which the stalk attaches. The continuous loss of one cell type during the slug stage without any change in cell type proportions suggests that cell types are redifferentiating. Grafting and transplantation experiments reveal that there is a unidirectional flow of cells through successive steps of cell type conversion. Prespore cells redifferentiate as anterior-like cells which migrate to the prestalk region and become pstA cells. The pstA cells then replace the pstB cells that are lost from the slug.