Factors controlling prespore cell differentiation in Dictyostelium discoideum: minute amounts of differentiation-inducing factor promote prespore cell differentiation

Basis of self-organized proportion regulation resulting from local contacts

One of the fundamental problems in biology concerns the method by which a cluster of organisms can regulate the proportion of individuals that perform various roles or modes as if each individual is aware of the overall situation without a leader. In various species, a specific ratio exists at multiple levels, from the process of cell differentiation in multicellular organisms to the situation of social dilemma in a group of human beings. This study determines a common basis for regulating collective behavior that is realized by a series of local contacts between individuals. In this theory, the most essential behavior of individuals is to change their internal mode by sharing information when in contact with others. Our numerical simulations regulate the proportion of population in two kinds of modes. Furthermore, using theoretical analysis and numerical calculations, we show that asymmetric properties in local contacts are essential for adaptive regulation in response to global information such as group size and overall density. Particle systems are crucial in allowing flexible regulation in no-leader groups, and the critical condition that eliminates overlap with other individuals (the excluded volume effect) also affects the resulting proportion at high densities. The foremost advantage of this strategy is that no global information is required for each individual, and minimal mode switching can regulate the overall proportion. This simple mechanism indicates that proportion regulation in well-organized groups in nature can be realized through and limited to local contacts, and has the potential to explain various phenomena in which microscopic individual behavior results in orderly macroscopic behavior.

AmpA protein functions by different mechanisms to influence early cell type specification and to modulate cell adhesion and actin polymerization in Dictyostelium discoideum

The Dictyostelium discoideum ampA gene encodes a multifunctional regulator protein that modulates cell-cell and cell-substrate adhesions and actin polymerization during growth and is necessary for correct cell type specification and patterning during development. Insertional inactivation of the ampA gene results in defects that define two distinct roles for the ampA gene during development. AmpA is necessary in a non-cell autonomous manner to prevent premature expression of a prespore gene marker. It is also necessary in a cell autonomous manner for the anterior like cells, which express the ampA gene, to migrate to the upper cup during culmination. It is also necessary to prevent excessive cell-cell agglutination when cells are developed in a submerged suspension culture. Here, we demonstrate that a supernatant source of AmpA protein, added extracellularly, can prevent the premature mis-expression of the prespore marker. Synthetic oligopeptides are used to identify the domain of the AmpA protein that is important for preventing cells from mis-expressing the prespore gene. We further demonstrate that a factor capable of inducing additional cells to express the prespore gene marker accumulates extracellularly in the absence of AmpA protein. While the secreted AmpA acts extracellularly to suppress prespore gene expression, the effects of AmpA on cell agglutination and on actin polymerization in growing cells are not due to an extracellular role of secreted AmpA protein. Rather, these effects appear to reflect a distinct cell autonomous role of the ampA gene. Finally, we show that secretion of AmpA protein is brought about by elevating the levels of expression of ampA so that the protein accumulates to an excessive level.

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.

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.

A new spore differentiation factor (SDF) secreted by Dictyostelium cells is phosphorylated by the cAMP dependent protein kinase

Upon starvation, Dictyostelium discoideum unicellular amoebae form a multicellular organism leading to the development of a fruiting body containing spores. Single cells of sporogenous mutants, unlike wild type cells, are able to differentiate into spores under specific conditions. We show in this report that overexpression of the catalytic subunit of the cAMP dependent protein kinase (PKA), not only renders the cells sporogenous, but is also accompanied by the production/release of a diffusible spore differentiation factor (SDF). SDF is a small, thermostable phospho-polypeptide. In vitro dephosphorylation reduces SDF spore differentiation capacity, which can be regained in vitro by PKA phosphorylation. These results indicate that SDF is a PKA substrate and might be activated in vivo by this protein kinase. Since spore differentiation requires PKA catalytic subunit activation, we conclude that the response of prespore cells to SDF involves an intracellular pathway dependent on PKA.

A novel prespore-cell-inducing factor in Dictyostelium discoideum induces cell division of prespore cells

In Dictyostelium discoideum strain V12M2, at a very low cell density (approximately 10(2) cells/cm2), most amoebae differentiate into prespore cells in a salt solution containing cAMP if an adequately diluted conditioned medium (CM) is provided (Oohata, A. A. (1995) Differentiation 59, 283–288). This finding suggests the presence of factor(s) released into the medium that are involved in inducing prespore cell differentiation. In the present study, we report the presence of two types of factors that function synergistically in prespore cell induction; one is a heat-stable and dialysable factor(s) and the other is a heat-labile and non-dialysable factor termed psi (psi) factor (prespore-inducing factor). We purified and characterized the psi factor. Its relative molecular mass was determined to be 106x10(3) Mr by SDS-PAGE and 180x10(3) Mr by gel filtration HPLC, respectively. These results indicate that psi factor exists as a dimer under native conditions. In addition to inducing prespore cell differentiation, psi factor induced cell division of prespore cells in submerged culture. Our results suggest that psi factor plays important roles not only in prespore cell differentiation but also in the progress of the cell cycle in the prespore pathway in normal development.