Sonic Hedgehog Positively Regulates Odontoblast Differentiation by a BMP2/4-dependent Mechanism

Abstract

Sonic hedgehog (Shh) is a cytokine which has important roles in cell differentiation and is believed to interact with other signaling molecules including bone morphogenetic proteins (BMPs). Given that tooth development involves reciprocal epithelial-mesenchymal interactions and cytodifferentiation processes, we asked whether Shh regulates odontoblast differentiation along with BMPs. Shh mRNA was produced by inner dental epithelium; the expression was not constant but varied with development and cytodifferentiation of ameloblasts along the cusp-to-cervix axis. Shh protein (SHH), but not RNA, was present in the underlying differentiating preodontoblasts, probably resulting from gradual diffusion from the epithelial layers. Patched-1, one of hedgehog receptors, was expressed in dental mesenchyme, reflecting paracrine loops of action. Interestingly, BMPs were also expressed in both preameloblasts and preodontoblasts, indicating functional interactions between the two signaling molecules during their differentiation. A new culture system has been established and the process of odontoblast differentiation were analyzed. SHH upregulated the expression levels of odontoblast differentiation markers including Alp and Osteopontin. The stimulatory effect of SHH on ALPase activity was synergistically enhanced by BMP2 or BMP4. Taken together, our data provide clear evidence that SHH is synthesized by dental epithelial layers, reaches the underlying dental mesenchyme, and appears to act as a paracrine factor which regulates odontoblast differentiation in concerted fashion with BMPs.