Biochemistry and cell biology of silica formation in sponges

The main inorganic material forming the skeletal elements in Demospongiae as well as in Hexactinellida, the spicules, is amorphous silica. The spicules occur in the cytoplasm and the extracellular space and also in the nucleus (as silicate crystals) of some sponge cells; the function in the latter compartment is unknown. Recent evidence shows that the formation of spicules is mediated by the enzyme silicatein. The cDNA as well as the gene encoding this enzyme was cloned from Suberites domuncula. The recombinant silicatein catalyzes the synthesis of amorphous silicate using tetraethoxysilane as substrate. The enzyme is dependent on ferric iron. Silicatein also has proteolytic (cathepsin-like) activity. Incubation of primmorphs, a special form of 3D-cell aggregates, with silicon resulted in a strong increase of their size from 1-7 mm to approximately 10 mm. The morphogenetic activity of silicate is underscored by the finding that this ion increases gene expression of silicatein and collagen. Based on these findings, it is concluded that both iron and silicate stimulate the activity of silicatein. Furthermore, it is proposed that the growing spicules are surrounded by the scavenger receptor which might be considered as a docking molecule for the collagen matrix into which the spicules are embedded.