Objective: The aim of this work is to investigate the possible role of Toll-like receptor 4 (TLR4) during the development of mouse tooth germ. TLR4 is well known to inhibit mineralization and cause inflammation in mature odontoblasts and dental pulp cells. However, unlike these pathological functions of TLR4, little is known about the developmental function(s) of TLR4 during tooth development.
Materials and methods: TLR4 expression was studied via Western blot in developing lower mouse incisors from E13.5 to E18.5. To generate functional data about the effects of TLR4, a specific agonist (LPS) was applied to the medium of in vitro tooth germ cultures, followed by Western blot, histochemical staining, ELISA assay, in situ hybridization and RT-qPCR.
Results: Increased accumulation of biotin-labelled LPS was detected in the enamel organ and in preodontoblasts. LPS treatment induced degradation of the inhibitor molecule (IκB) of the NF-κB signalling pathway. However, no morphological alterations were detected in cultured tissue after LPS addition at the applied dosage. Activation of TLR4 inhibited the mineralization of enamel and dentin, as demonstrated by alizarin red staining and as decreased levels of collagen type X. mRNA expression of ameloblastin was elevated after LPS administration.
Conclusion: These results demonstrate that TLR4 may decrease the mineralization of hard tissues of the tooth germ and may trigger the maturation of ameloblasts; it can give valuable information to understand better congenital tooth abnormalities.
Keywords: LPS; development; enamel organ; incisor.