Notch Signaling is Required for Attachment of the Ameloblast-SI Interface

Introduction: Little is known about the roles of Notch signaling and cell-cell adhesion during tooth development.  Here we utilize the continuously growing rodent incisor to determine that Notch signaling is an important regulator of enamel formation through effects on the interaction between enamel-forming ameloblasts and its subjacent attachment layer, the stratum intermedium (SI).

Objective: To examine the role of Notch signaling during amelogenesis in vivo and in vitro.

Methods: Inhibitory antibodies specific for components of the Notch signaling pathway (Notch1, Notch2, Jag1, Jag2) were injected into the peritoneum of adult mice for two weeks.  The lower incisors were then excised, sectioned, and evaluated using H&E staining and immunohistochemistry.  Notch signaling was inhibited in the ameloblast-like LS8 cell line and the relative expression levels of genes downstream of Notch signaling were assayed using real-time quantitative PCR.

Results: The inhibition of the Notch signaling pathway resulted in detachment between the ameloblasts and the SI.  Furthermore, there were changes in the localization of PERP and desmoplakin but not amelogenin and ameloblastin.  PERP is a transmembrane protein that is required in the formation and homeostasis of desmosomes, which are specialized cell structures for cell-cell adhesion.  Desmoplakin is a specific component of desmosomes.  Previously, a similar detachment of the ameloblast-SI interface was observed in Perp-null mice.  Inhibition of Notch signaling in vitro led to decreases in Perp expression, demonstrating that Notch signaling is involved in the regulation of Perp. 

Conclusion: There is little prior knowledge regarding the role of Notch signaling during tooth development.  Here we show for the first time that Notch signaling is required for the integrity of the ameloblast-SI interface through its regulation of Perp, a gene important in cell-cell adhesion.