Intracellular signal transduction pathways in sponges.

Sponges are the lowest multicellular eukaryotic organisms. Due to the relatively low specialization, and concomitantly the high differentiation and dedifferentiation potency of their cells, the sponge cell system has proven to be a useful model to study the mechanism of cell-cell adhesion on molecular levels. Results of detailed biochemical and cell biological studies with the main cell adhesion molecules, the aggregation factor (AF) and the aggregation receptor, led to the formation of the modulation theory of cell adhesion. The events of cell adhesion are contigent on a multiplicity of precisely coordinated intracellular signal transduction pathways. Using the marine sponge Geodia cydonium we showed that during the initial phase of cell-cell contact the AF causes a rapid stimulation of the phosphatidylinositol pathway, resulting in an activation of protein kinase C and a subsequent phosphorylation of DNA topoisomerase II. As one consequence of these processes, the cells undergo a phase of high DNA synthesis. However, at later stages, the AF loses its mitogenic activity; this function is then taken over by the matrix lectin. During this switch, the lectin receptor associates in the plasma membrane with the ras oncogene product. The description of these processes is subject of this review article.