Investigation of osteocalcin, osteonectin, and dentin sialophosphoprotein in developing human teeth

Biochemical investigations in rodents have shown that numerous mineralized matrix proteins share expression in bone, dentin, and cementum. Little information is available regarding the expression pattern of these proteins in human tissues, particularly during tooth formation. The aim of this study was to identify the expression pattern of the two major noncollagenous proteins of bone and dentin, osteocalcin (OC) and osteonectin (ON), in comparison to the dentin-specific protein, dentin sialophosphoprotein (DSPP). Mandibles from fetuses (5-26 weeks), neonate autopsies, forming teeth from 10-12-year-old patients, third molars extracted for orthodontic reasons, and bone tumors were collected with approval from the National Ethics Committee. Human OC, ON, and DSPP mRNAs were detected by reverse transcription-polymerase chain reaction (RT-PCR) in fetal mandibles (5-11 weeks) and in primary cell cultures of dental pulp. In addition, OC, ON, and DSPP proteins were localized in forming human mineralized tissues using immunohistochemistry. In vivo, DSPP expression was associated with tooth terminal epithelial-mesenchymal interaction events, amelogenesis and dentinogenesis. Transient DSPP expression was seen in the presecretory ameloblasts with continuous expression in the odontoblasts. In contrast, both osteoblasts and odontoblasts showed a temporal gap between OC and ON expression in early development. ON was expressed in the initial stages of cytodifferentiation, whereas OC was expressed only during the later stages, especially in the teeth. At the maturation stage of enamel formation, both proteins were detected in odontoblasts and their processes within the extracellular matrix. In contrast to bone, OC was not localized extracellularly within the collagen-rich dentin matrix (predentin or intertubular dentin), but was found in the mature enamel. ON was present mostly in the nonmineralized predentin. These results demonstrate for the first time that both OC and ON are produced by human odontoblasts and determine the expression pattern of DSPP in human teeth, and suggest that OC and ON move inside the canalicule via odontoblast cell processes becoming localized to specific extracellular compartments during dentin and enamel formation. These distinct extracellular patterns may be related to the nature of DSPP, OC, and ON interactions with other matrix-specific macromolecules (i.e., amelogenin, dentin matrix protein-1) and/or to the polarized organization of odontoblast secretion as compared with osteoblasts.