Title

Role of Dlx5 in odontoblast differentiation

Date of Completion

January 2009

Keywords

Biology, Genetics|Biology, Cell|Health Sciences, Dentistry

Degree

Ph.D.

Abstract

Previous studies suggest that Dlx5 promotes differentiation in several cell types, including neurons of the olfactory bulb, chondrocytes and osteoblasts. Dlx5 is expressed in all stages of tooth development, suggesting a possible role for this gene in tooth formation. In adult incisors, endogenous Dlx5 is expressed in pulp cells, preodontoblasts, at decreasing levels in polarizing odontoblasts, but not in mature odontoblasts. This study examined the role of Dlx5 in odontoblast differentiation using Col3.6-Dlx5 incisors as a model. Incisors of seven-week-old Col3.6-Dlx5 expressed transgenic Dlx5 in polarizing and mature odontoblasts and pulp. Thus, the transgene caused prolonged, high level expression in polarizing and mature odontoblasts, when endogenous expression is decreased. In contrast, Dlx5 expression is reduced in Col3.6-Dlx5 pulp when compared to wild type pulp. Adult Col3.6-Dlx5 mice have an overt phenotype of malocclusion, excessive incisor wear and tendency to fracture. Histological analysis of the incisors showed that Col3.6-Dlx5 odontoblasts polarize earlier than wild type odontoblasts. The early differentiation of Col3.6-Dlx5 odontoblasts resulted in accelerated dentin secretion and mineralization and, subsequently, enamel secretion and mineralization. Dspp expression further confirmed the odontoblastic phenotype of Col3.6-Dlx5 odontoblasts. ^ To determine whether the early differentiation observed in Col3.6-Dlx5 incisors is cell autonomous, pulp culture studies were performed. No dramatic differences were observed in the ability of Col3.6-Dlx5 pulp cultures to mineralize when compared to controls. Gene expression analysis also revealed decreased Dlx5 expression in undifferentiated transgenic cultures. Increased Dlx5 expression in Col3.6-Dlx5 cultures was observed as the cells began to differentiate. Dspp expression paralleled the increased Dlx5 expression in Col3.6-Dlx5 cultures. We also identified a putative Dlx-binding site at the promoter region of Dspp. In contrast, D1x3 expression was decreased in Col3.6-Dlx5 cultures, when compared to controls. This decrease was confirmed in vivo where we observed suppression of endogenous D1x3 expression in mature odontoblasts where Col3.6-Dlx5 is highly expressed. In summary, our findings suggest that Dlx5 promotes odontoblast differentiation possibly by binding to the promoter of odontoblast expressed genes such as Dspp, and that Dlx genes regulate expression of fellow Dlx family member within the same cell and in adjacent cell type. ^