[WWP1 plays a positive role in ameloblast differentiation and enamel formation in mice]

Objective: To investigate the role of WW domain containing E3 ubiquitin protein ligase 1 (WWP1) in enamel development of mice. Methods: Single-cell RNA sequencing data of incisor tissues of postnatal day 7 (P7) mice and mandibular first molar tooth germs of P3.5 mice were used to analyze the expression of Wwp1 in dental epithelial cells. Immunohistochemistry was performed to observe the distribution and expression levels of WWP1 in the epithelium of mouse incisors and mandibular first molar tooth germs. Wwp1 knockout (Wwp1 KO) mice were generated and collected with their control littermates at P1, P7, three mice per group, as well as at P14, P28, 2 months (2M), and 3M, six mice per group. The enamel volumes of molars and incisors were analyzed using micro-CT. Scanning electron microscopy was employed to examine the enamel cross-sections of Wwp1 KO and control mice. Energy dispersive spectroscopy (EDS) was used to analyze the calcium and phosphorus content of the enamel rod of incisors. Immunofluorescence was performed to detect the expression of amelogenin (AMELX) in the ameloblasts of Wwp1 KO and control mice. Additionally, LS-8 ameloblast-like epithelial cells were cultured, and Wwp1 siRNA or overexpression plasmids were transfected to knock down or overexpress WWP1. The protein levels of AMELX were then assessed by Western blotting. Results: Single-cell sequencing result showed a high Wwp1 mRNA expression level in the epithelial cells of mouse incisors and mandibular molar tooth germs. Immunohistochemistry revealed the expression of WWP1 in presecretory, secretory, transitional, and mature ameloblasts. Wwp1 KO mice exhibited enamel developmental defects. The enamel volumes of molars and incisors in Wwp1 KO mice [(0.155±0.016), (0.300±0.017) μm³] were reduced by 23.95% (P<0.001) and 28.31% (P<0.001) compared with the control group [(0.203±0.062), (0.418±0.023) μm³] respectively. Scanning electron microscopy showed disorganized enamel structures in Wwp1 KO incisors and molars. EDS results showed the weight percent of calcium in the enamel rod of incisors decreased in Wwp1 KO mice [(20.74±0.91)%] compared with the control group [(30.30±3.83)%] (P<0.001), and the calcium-to-phosphorus ratio decreased in Wwp1 KO mice (1.93±0.01) compared with the control group (2.02±0.01) (P<0.001). Immunofluorescence showed weaker AMELX expression in ameloblasts of mandibular first molar tooth germs from P1 and P7 Wwp1 KO mice compared with the control group (P<0.001, P<0.001). In LS-8 cells, Wwp1 knocked-down led to a decrease of AMELX protein expression, while WWP1 overexpression resulted in an increased AMELX protein level. Conclusions: WWP1 promotes ameloblast differentiation and enamel matrix mineralization, playing a critical role in enamel formation.