In summary, the SIBLING family members that include DMP1, DSPP, MEPE, OPN, and BSP share common features such as their location on human chromosome 4q21; their multiple phosphorylation sites, making them highly acidic in nature; and the presence of an RGD integrin-binding domain. Each of these proteins plays an important role in either promoting tissue mineralization or inhibiting the calcification process. A general paradigm that emerges is that macromolecules such as DMP1, DPP in solution can act as an inhibitor of crystal nucleation and growth or can act as a template for crystal nucleation when it becomes adsorbed on a solid surface. Nucleation processes usually present an activation energy barrier before allowing the formation of a solid phase from a supersaturated metastable solution. In biological microenvironments, the activation energy for nucleation can be reduced by lowering of the interfacial energy. DPP immobilized on a collagen surface can lower the interfacial energy for HAP nucleation, indicating that DPP bound on collagen template had a high capacity to nucleate HAP. Similar lowering of interfacial energy can be anticipated with other phosphorylated proteins. This might be a common theme for proteins containing highly anionic polyanions. Both BSP and OPN possess high affinity for calcium due to the abundance of acidic amino acids such as glutamic and aspartic acid. Acidic amino acids in OPN phosphopeptides appear to contribute to the HAP-inhibiting activity by producing an electrostatic repulsion of inorganic phosphate ions once the protein is adsorbed to the crystal surface. Even during the inhibition process, it is necessary that these proteins bind to specific crystal faces. However, BSP promotes HAP nucleation when immobilized on a surface. Thus, the various acidic phosphoproteins of the extracellular matrix appear to have different crystal growth regulatory activities, enabling them to present specific distinct biological activities. Cooperation of several of these proteins would effectively promote or retard crystallization, as well as regulate the growth kinetics and size of crystals. Gene knockout models 97,189,262-265 of the proteins localized on 4q21 revealed that mineralized tissue formation and growth can be compensated by other macromolecules either partially or completely as well as by other members of the SIBLING family. In vivo studies of the effects of gene knockouts of individual SIBLINGs show that they each do more than simply alter the mineralization process. They also have profound effects on development and tissue repair not directly related to mineralization, as indicated below. Targeted deletion of MEPE gene resulted in increased bone mass. This was attributed to the increased osteoblast number and mineral apposition rate. Thus, in vivo MEPE could function as a negative regulator of osteoblast number and activity. Similarly, targeted disruption of OPN gene resulted in increased mineral content and maturity in long bones. Fourier transform infrared microspectroscopy (FT-IRM) based characterization of the mineral in bones revealed that the relative amount of mineral in the more mature areas of the bone obtained from 12- and 16-week-old OPN-null mice was significantly increased. Moreover, mineral maturity (mineral crystal size and perfection) was prominent in OPN-deficient bone. These findings are in good agreement with in vitro data, indicating that OPN is a potent inhibitor of mineral formation and mineral crystal growth and proliferation. DMP1-null newborn mice display no gross abnormalities in mineralization, indicating that there must be redundant genes that compensate for DMP1 function during early development. During postnatal development, DMP1-null pups develop skeletal abnormalities like enlarged growth plate, osteomalacia, and development of short limbs. These defects have been proposed to be related to defective maturation of osteoblasts into osteocytes. The dentin phenotype in the null-mice shows a partial failure of maturation of predentin into dentin, increased width of the predentin zone with a reduced dentin wall hypomineralization, and a 3-fold reduction in dentin appositional rate. As the DSPP gene is abundantly synthesized by the odontoblasts, the tooth phenotype has been well-characterized. Overall, the DSPP-null mice develop tooth defects similar to human dentinogenesis imperfecta III with enlarged pulp chambers, increased width of predentin zone, hypomineralization, and pulp exposure. Electron microscopy revealed an irregular mineralization front and a lack of calcospherites coalescence in the dentin. Interestingly, the levels of biglycan and decorin, small leucine-rich proteoglycans, were increased in the widened predentin zone and in void spaces among the calcospherites in the dentin of null teeth. Thus, the protein products of the DSPP gene are required for proper dentin mineralization. BSP null mice are viable and breed normally. Bone is undermineralized. in fetuses and young adults, but not in older (>12 months) BSP-/- mice. At 4 months, BSP-/- mice display thinner cortical bones than wild type but greater trabecular bone volume with very low bone formation rate. Overall, BSP deficiency impairs bone growth and mineralization. These examples of in vivo consequences of gene ablation demonstrate both the wide range of system redundancies and the multiplicity of properties altered, which is evidence of the vital importance of controlled mineralization and the other effects of the SIBLINGs in both mechanical and metabolic functions.
ASJC Scopus subject areas