Role of TNFR1-TNF-Alpha Signaling Pathway in Dental Enamel Biomineralization

Objectives: Genetic and environmental players may determine an increased risk for the development of dental enamel defects, including single nucleotide polymorphisms for genes involved in immune and inflammatory response. Therefore, the objective of this study was to evaluate the effect of ablation of TNFR1 gene, a pro-inflammatory receptor for pleiotropic tumor necrosis factor-alpha (TNF-alpha), on the biomineralization of dental enamel and to investigate the microscopic changes and molecular signaling during amelogenesis.
Methods: After Committee on Animal Research and Ethics approval, male and female C57BL/6 (wild-type) and TNFR1 knockout mice were used (n= 40). The lower incisors were collected for enamel visual photographic analysis, microcomputed tomography (microCT), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and surface microhardness evaluation. Blocks containing teeth and surrounding tissues were collected for in situ zymography and immunohistochemistry to investigate the synthesis of matrix metalloproteinase 20 (MMP-20). Data were analyzed using one-way ANOVA followed by Tukey's post-test (alpha= 5%).
Results: Macroscopically no changes were found in dental enamel regarding to coloring, texture and thickness in knockout compared to wild-type mice. SEM qualitative analysis revealed a difference in the conformation of dental enamel rods in TNFR1 knockout animals, yet no impact in mineral density of dental enamel was found in microCT analysis (p>0.05). Interestingly, a reduced calcium content and lower enamel surface microhardness were detected in TNFR1 knockout mice compared to wild-type (p<0.05). This was accompanied by an increased MMP-20 synthesis and activity in organic enamel matrix and in ameloblastic layer during amelogenesis (p<0.05). Those findings were similar in both male and female mice.
Conclusions: Genetic ablation of TNFR1 impacts the signaling for the formation of dental enamel, resulting in reduced surface microhardness and mineral composition albeit no macroscopic change could be detected. These results shed light on the role of immune genes in dental enamel biomineralization indicating that host mediated response might be involved in the etiology of dental enamel defects.