Bone and dentin are closely related tissues, formed when a type I collagen-rich extracellular matrix (ECM) is secreted from the osteoblasts or odontoblasts and subsequently mineralized. This process is tightly regulated by type I collagen plus a number of non-collagenous proteins, including members of the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family. Antibodies associated with this group include the Dentin Matrix Protein 1 (DMP1), Dentin Sialophosphoprotein (DSPP), Bone Sialoprotein (BSP) and Osteopontin (OPN).
The newest member of the family is Matrix Extracellular Phosphoglycoprotein (MEPE), of which presently little is known. However BSP, DSPP, DMP1 and OPN antibody studies suggest the proteins regulate crystal growth and mineralization of collagen fibers within the ECM during its conversion to bone or dentin.
Osteopontin antibody assays suggest little similarity between the amino acid sequence of OPN and those of its relatives. Nonetheless, the SIBLING proteins do share a number of common features, including an Arg-Gly-Asp (RGD) motif which binds to cell-surface integrins, allowing cell attachment and signalling. The RGD tri-peptide is always located on the last one or two exons (which are the largest) and all the SIBLING genes occupy a common chromosome location: 4q21-23. Finally, recent studies with SIBLING family antibodies have shown the proteins undergo a number of similar post-translational modifications, including phosphorylation, sulfuration, glycosylation, transglutaminase cross-linking and proteolytic processing.
Many of the mechanisms controlling bone and dentin synthesis remain elusive, but BSP and osteopontin antibody assays clearly show a link between SIBLING protein activity and PTMs, which must alter the structure and therefore function of the proteins in the same way histone modifications do.
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