The siliceous sponge Monorhaphis chuni (Hexactinellida) synthesizes the largest biosilica structures on earth (3 m). Scanning electron microscopy has shown that these spicules are regularly composed of concentrically arranged lamellae (width: 3-10 μm). Between 400 and 600 lamellae have been counted in one giant basal spicule. An axial canal (diameter:~2 μm) is located in the center of the spicules; it harbors the axial filament and is surrounded by an axial cylinder (100-150 μm) of electron-dense homogeneous silica. During dissolution of the spicules with hydrofluoric acid, the axial filament is first released followed by the release of a proteinaceous tubule. Two major proteins (150 kDa and 35 kDa) have been visualized, together with a 24-kDa protein that cross-reacts with antibodies against silicatein. The spicules are surrounded by a collagen net, and the existence of a hexactinellidan collagen gene has been demonstrated by cloning it from Aphrocallistes vastus. During the axial growth of the spicules, silicatein or the silicatein-related protein is proposed to become associated with the surface of the spicules and to be finally internalized through the apical opening to associate with the axial filament. Based on the data gathered here, we suggest that, in the Hexactinellida, the growth of the spicules is mediated by silicatein or by a silicatein-related protein, with the orientation of biosilica deposition being controlled by lectin and collagen.

No abstract

No abstract

It has been shown that inorganic monomeric and polymeric silica/silicate, in the presence of the biomineralization cocktail, increases the expression of osteoprotegerin (OPG) in osteogenic SaOS-2 sarcoma cells in vitro. In contrast, silicate does not affect the steady-state gene expression level of the osteoclastogenic ligand receptor activator of NF-κB ligand (RANKL). In turn it can be expected that the concentration ratio of the mediators OPG/RANKL increases in the presence of silicate. In addition, silicate enhances the growth potential of SaOS-2 cells in vitro, while it causes no effect on RAW 264.7 cells within a concentration range of 10-100 µM. Applying a co-cultivation assay system, using SaOS-2 cells and RAW 264.7 cells, it is shown that in the presence of 10 µM silicate the number of RAW 264.7 cells in general, and the number of TRAP(+) RAW 264.7 cells in particular markedly decreases. The SaOS-2 cells retain their capacity of differential gene expression of OPG and RANKL in favor of OPG after exposure to silicate. It is concluded that after exposure of the cells to silicate a factor(s) is released from SaOS-2 cells that causes a significant inhibition of osteoclastogenesis of RAW 264.7 cells. It is assumed that it is an increased secretion of the cytokine OPG that is primarily involved in the reduction of the osteoclastogenesis of the RAW 264.7 cells. It is proposed that silicate might have the potential to stimulate osteogenesis in vivo and perhaps to ameliorate osteoporotic disorders.

The siliceous spicules of sponges (Porifera) are synthesized by the enzyme silicatein. This protein and its gene have been identified so far in the Demospongiae, e.g., Tethya aurantium and Suberites domuncula. In the Hexactinellida, the second class of siliceous sponges, the mechanism of synthesis of the largest bio-silica structures on Earth remains obscure. Here, we describe the morphology of the spicules (diactines and stauractines) of the hexactinellid Crateromorpha meyeri. These spicules are composed of silica lamellae concentrically arranged around a central axial canal and contain proteinaceous sheaths (within the siliceous mantel) and proteinaceous axial filaments (within the axial canal). The major protein in the spicules is a 24-kDa protein that strongly reacts with anti-silicatein antibodies in Western blots. Its cDNA has been successfully cloned; the deduced hexactinellid silicatein comprises, in addition to the characteristic catalytic triad amino acids Ser-His-Asn and the "conventional" serine cluster, a "hexactinellid C. meyeri-specific" Ser cluster. We show that anti-silicatein antibodies react specifically with the proteinaceous matrix of the C. meyeri spicules. The characterization of silicatein at the genetic level should contribute to an understanding of the molecular/biochemical mechanism of spiculogenesis in Hexactinellida. These data also indicate that silicatein is an autapomorphic molecule common to both classes of siliceous sponges.