How do individual cells stick together to form a cohesive organism? This problem is the same one that embryonic cells face, and the solution that evolved in these protists is the same one used by animal embryos: developmentally regulated cell adhesion molecules.
While growing mitotically and feeding, Dictyostelium cells do not adhere to one another. However, once cell division stops, the cells become increasingly adhesive, reaching a plateau of maximum adhesiveness about 8 hours after starvation. The initial cell-cell adhesion is mediated by a 24-kilodalton glycoprotein (gp24, or DdCad1; Figure 1A) that is absent in solitary myxamoebae but which appears shortly after mitotic division ceases (Knecht et al. 1987; Wong et al. 1996). This protein is synthesized from newly transcribed mRNA and becomes localized in the cell membranes of the myxamoebae. If myxamoebae are treated with antibodies that bind to and mask this protein, the cells will not stick to one another, and all subsequent development ceases.
Once this initial aggregation has occurred, it becomes stabilized by a second cell adhesion molecule (Figure 1B). This 80-kDa glycoprotein (gp80; CsaA) is also synthesized during the aggregation phase. If it is defective or absent in the cells, small slugs will form, and their fruiting bodies will be only about one-third the normal size. Thus, the second cell adhesion system seems to be needed for retaining a large enough number of cells to form large fruiting bodies (Müller and Gerisch 1978; Loomis 1988). During late aggregation, the levels of gp80 decrease, and its role is taken over by a third cell adhesion protein, a 150-kDa protein (gp150; LagC; Figure 1C) whose synthesis becomes apparent just prior to aggregation and which stays on the cell surface during grex migration (Wang et al. 2000). If Dictyostelium cells lack functional genes for gp150, development is arrested at the loose aggregate stage, and the prespore and prestalk cells fail to sort out into their respective regions. Thus, Dictyostelium has evolved three developmentally regulated systems of cell-cell adhesion that are necessary for the morphogenesis of individual cells into a coherent organism. As we will see in subsequent chapters, metazoan cells also use cell adhesion molecules to form the tissues and organs of the embryo.
Dictyostelium is a “part-time multicellular organism” that does not form many cell types (Kay et al. 1989), and the more complex multicellular organisms do not form by the aggregation of formerly independent cells. Nevertheless, many of the principles of development demonstrated by this “simple” organism also appear in the embryos of more complex phyla (see Loomis and Insall 1999). The ability of individual cells to sense a chemical gradient (as in the myxamoeba’s response to cAMP) is crucial for cell migration and morphogenesis throughout development in all animal species. The role of cell surface proteins in cell cohesion is also seen throughout the animal kingdom, and differentiation-inducing molecules are being isolated in metazoan organisms.