silicatein may have a diameter of 50–70 nm (Tahir et al. 2004). Particles of that size
are readily taken up by cells through endocytosis (Jin et al. 2009). In sponges,
biosilica can be hydrolyzed to orthosilicate by the enzyme silicase which is related
to the metazoan carbonic anhydrases (Schr€ oder et al. 2003). Carbonic anhydrases
which have a little silica-hydrolyzing activity are also present in the extracellular
space (Wetzel et al. 2001; Gao et al. 2007) and hence might allow polymeric silica
to be taken up after hydrolysis via an orthosilicate-specific transporter.
Biosilica in sponges has been attributed to exhibit both spiculoinductive and
spiculoconductive activity (Wiens et al. 2010b). This assumption is based on
the fact that silica triggers differentiation of progenitor cells to spicule-forming
sclerocytes (spiculoinductivity) (M€ uller et al. 2006; Kaandorp et al. 2008; Le
Pennec et al. 2003). On the other hand, immature spicules which are released
into the extracellular space determine spicular morphogenesis through attracting
sclerocytes and triggering collagen synthesis (spiculoconductivity) (Schr€ oder et al.
2006). It should be noted that the development of biomimetic materials that show
osteoinductive and osteoconductive potential is a challenging task for bone tissue
engineering (Albrektsson and Johansson 2001). Osteoinduction describes processes
which involve the differentiation of progenitor cells from hematopoietic stem cells
to osteoblasts and osteoclasts, while osteoconduction concerns the growth of bone
on a surface directed by the surface structure and mediated by (incorporated)
osteogenic agents (Albrektsson and Johansson 2001; Glantz 1987).
10.5 Silicatein
The principle enzyme catalyzing formation of biosilica from soluble precursors is
silicatein (Fig. 10.3), an enzyme exclusively found in sponges (Shimizu et al. 1998;
Cha et al. 1999; Krasko et al. 2000; M€ uller et al. 2008b). Silicatein is the major
protein component of the axial filament which is present in the axial canal of
the sponge spicules (Fig. 10.1b, d). This protein is the first enzyme that has been
discovered to be capable of forming an inorganic polymer (silica) from a monomeric precursor; orthosilicic acid or tetraethoxysilane (TEOS; as an orthosilicic
acid precursor) are commonly used as a substrate. Harnessing the biocatalytic
potential of this unique enzyme is expected to open a variety of new applications
of silica in nanotechnology, nanomedicine, and material sciences (Schr€ oder et al.
2007a; M€ uller et al. 2009b).
Silicatein is related to the cathepsins, a group of proteases, but is characterized
by replacement of the Cys residue by a Ser residue in the catalytic center of the
molecule (Shimizu et al. 1998; Krasko et al. 2000). In addition, the silicatein sequences
comprise a Ser stretch not found in cathepsins. Several genes/cDNAs encoding
different isoforms of silicatein have been isolated both from demosponges and
hexactinellid sponges, e.g. the marine demosponge S. domuncula (two isoforms:
silicatein-a and silicatein-b; Shimizu et al. 1998; Cha et al. 1999; Krasko et al.
2000; Schr€ oder et al. 2005b), the freshwater demosponge Lubomirskia baicalensis
10 Biosilica-Based Strategies for Treatment of Osteoporosis and Other Bone Diseases
289
are readily taken up by cells through endocytosis (Jin et al. 2009). In sponges,
biosilica can be hydrolyzed to orthosilicate by the enzyme silicase which is related
to the metazoan carbonic anhydrases (Schr€ oder et al. 2003). Carbonic anhydrases
which have a little silica-hydrolyzing activity are also present in the extracellular
space (Wetzel et al. 2001; Gao et al. 2007) and hence might allow polymeric silica
to be taken up after hydrolysis via an orthosilicate-specific transporter.
Biosilica in sponges has been attributed to exhibit both spiculoinductive and
spiculoconductive activity (Wiens et al. 2010b). This assumption is based on
the fact that silica triggers differentiation of progenitor cells to spicule-forming
sclerocytes (spiculoinductivity) (M€ uller et al. 2006; Kaandorp et al. 2008; Le
Pennec et al. 2003). On the other hand, immature spicules which are released
into the extracellular space determine spicular morphogenesis through attracting
sclerocytes and triggering collagen synthesis (spiculoconductivity) (Schr€ oder et al.
2006). It should be noted that the development of biomimetic materials that show
osteoinductive and osteoconductive potential is a challenging task for bone tissue
engineering (Albrektsson and Johansson 2001). Osteoinduction describes processes
which involve the differentiation of progenitor cells from hematopoietic stem cells
to osteoblasts and osteoclasts, while osteoconduction concerns the growth of bone
on a surface directed by the surface structure and mediated by (incorporated)
osteogenic agents (Albrektsson and Johansson 2001; Glantz 1987).
10.5 Silicatein
The principle enzyme catalyzing formation of biosilica from soluble precursors is
silicatein (Fig. 10.3), an enzyme exclusively found in sponges (Shimizu et al. 1998;
Cha et al. 1999; Krasko et al. 2000; M€ uller et al. 2008b). Silicatein is the major
protein component of the axial filament which is present in the axial canal of
the sponge spicules (Fig. 10.1b, d). This protein is the first enzyme that has been
discovered to be capable of forming an inorganic polymer (silica) from a monomeric precursor; orthosilicic acid or tetraethoxysilane (TEOS; as an orthosilicic
acid precursor) are commonly used as a substrate. Harnessing the biocatalytic
potential of this unique enzyme is expected to open a variety of new applications
of silica in nanotechnology, nanomedicine, and material sciences (Schr€ oder et al.
2007a; M€ uller et al. 2009b).
Silicatein is related to the cathepsins, a group of proteases, but is characterized
by replacement of the Cys residue by a Ser residue in the catalytic center of the
molecule (Shimizu et al. 1998; Krasko et al. 2000). In addition, the silicatein sequences
comprise a Ser stretch not found in cathepsins. Several genes/cDNAs encoding
different isoforms of silicatein have been isolated both from demosponges and
hexactinellid sponges, e.g. the marine demosponge S. domuncula (two isoforms:
silicatein-a and silicatein-b; Shimizu et al. 1998; Cha et al. 1999; Krasko et al.
2000; Schr€ oder et al. 2005b), the freshwater demosponge Lubomirskia baicalensis
10 Biosilica-Based Strategies for Treatment of Osteoporosis and Other Bone Diseases
289