Osteopontin deficiency disturbs dentin bridge formation after direct pulp capping with mineral trioxide aggregate

OBJECTIVES: After direct pulp capping with mineral trioxide aggregate (MTA), osteopontin (OPN) deposition occurs beneath the exposed pulp before the differentiation of odontoblast-like cells. This study aims to determine whether OPN is directly required for dentin bridge formation following direct pulp capping. METHODS: Pulp exposures were made on the occlusal surface of maxillary first molars of five to six week-old, wild-type (WT) and Opn knockout (KO) mice. The cavity was filled with MTA and then with glass ionomer cement. One to 28 days (PODs 1-28), specimens were subjected to immunohistochemistry for evaluating nestin, OPN, Ki67, dentin matrix protein (DMP)-1, F4/80, and CD206; and quantitative real-time polymerase-chain reaction was performed for evaluating nestin (Nes), dentin sialophosphoprotein (Dspp), Dmp-1, and Opn mRNA. RESULTS: In WT mice, OPN was deposited just beneath the exposed pulp after odontoblast degeneration on POD 3, followed by an increase in Nes mRNA expression and CD206 immunoreactivity on POD 5. Nestin-positive odontoblast-like cells aligned on POD 14, resulting in dentin bridge formation on POD 28. Conversely, the arrangement of nestin-positive odontoblast-like cells was disrupted in Opn KO mice, with decreased expression levels of Nes mRNA, Dspp mRNA, and CD206-immunoreactivity on PODs 3-7. Eventually, dentin bridge formation was suppressed, resulting in pulp necrosis on POD 28. CONCLUSIONS: Following direct pulp capping with MTA, Opn KO mice exhibited impaired arrangement of nestin-positive odontoblast-like cells and failed to form a dentin bridge, indicating that OPN contributes remarkably to these responses.