Expression of BSP-GFPtpz Transgene During Osteogenesis and Reparative Dentinogenesis

Objectives: Bone Sialoprotein (BSP) is a member of the SIBLING family with essential roles in skeletogenesis. In the developing teeth, although the expression and function of BSP in the formation of acellular cementum and periodontal attachment is well documented, there are uncertainties regarding the expression and function of BSP by odontoblasts and dentin. Reporter mice are valuable animal models for biological research providing a gene expression readout that can contribute to cellular characterization within the context of a developmental process.
Methods: In the present study, we examined the expression of a BSP-GFPtpz reporter mouse line during odontoblast and osteoblast differentiation in vivo (at various stages of embryonic and post-natal development) and during in vitro differentiation of primary pulp cultures derived from 5 days old pups. The expression of BSP-GFPtpz transgene was also examined following experimental pulp injury in the maxillary molars of 4-week old animals. Methods of analysis included image analysis, qPCR, immunohistochemical and FACS.
Results: In alveolar bones, calvaria, and long bones, the expression of BSP-GFPtpz was detected in osteoblasts and osteocytes. In the developing teeth, BSP-GFPtpz was expressed at high levels in cementoblasts. However, the BSP-GFPtpz transgene, similar to endogenous Bsp and BSP, was not expressed by odontoblasts. Interestingly, despite its lack of expression in odontoblasts during tooth development, the BSP-GFPtpz transgene was detected during in vitro mineralization of primary pulp cultures and during reparative dentinogenesis following pulp exposures. Importantly, under these experimental contexts, the expression of BSP-GFPtpz was still exclusive of DSPP-Cerulean, an odontoblast-specific reporter gene.
Conclusions: Our findings suggest that the combinatorial use of BSP-GFPtpz and DSPP-Cerulean can be a valuable experimental tool to distinguish osteogenic from dentinogenic cells; thereby providing an avenue to investigate the mechanisms that distinctly regulate the lineage progression of progenitors into odontoblasts vs. osteoblasts.