Osteopontin Is Essential for Type I Collagen Secretion in Reparative Dentin

Osteopontin (OPN) is a highly phosphorylated glycoprotein that is a prominent component of the mineralized extracellular matrix of bone. The secretion of OPN by immunocompetent cells plays a role in the differentiation of odontoblast-like cells during pulpal healing following tooth transplantation. This study aimed to clarify the role of OPN during reparative dentinogenesis. A groove-shaped cavity was prepared on the mesial surface of the upper first molars of wild-type (WT) and Opn knockout (KO) mice, and the samples were collected at intervals of 1 to 14 d. The demineralized sections were processed for immunohistochemistry for Ki67, nestin, OPN, dentin sialoprotein (DSP), integrin αvβ3, and type I collagen; in situ hybridization for Opn, col1a1, and dentin sialophosphoprotein (Dspp); and apoptosis assay. For the loss and gain of function experiments, an in vitro culture assay for evaluating dentin-pulp complex regeneration was performed. On day 1 in WT mice, odontoblasts beneath the affected dentin lost nestin immunoreactivity. On day 3, the expression of Opn was recognized at the mesial dental pulp, and OPN was deposited along the predentin-dentin border. Nestin-positive newly differentiated odontoblast-like cells expressed both Dspp and col1a1 and showed positive immunoreactivity for integrin αvβ3, DSP, and type I collagen. Until day 14, reparative dentin formation continued next to the preexisting dentin at the mesial coronal pulp. In contrast, there was no reparative dentin in the Opn KO mice where nestin- and DSP-positive newly differentiated odontoblast-like cells lacked immunoreaction for type I collagen. The in vitro organ culture demonstrated that the administration of recombinant OPN rescued the type I collagen secretion by odontoblast-like cells in the Opn KO mice. The results suggested that the deposition of OPN at the calcification front is essential for the type I collagen secretion by newly differentiated odontoblast-like cells to form reparative dentin during pulpal healing following cavity preparation.