The phosphorylation of serine⁵⁵ in enamelin is essential for murine amelogenesis

Amelogenesis imperfecta (AI) is an inherited developmental enamel defect affecting tooth masticatory function, esthetic appearance, and the well-being of patients. As one of the major enamel matrix proteins (EMPs), enamelin (ENAM) has three serines located in Ser-x-Glu (S-x-E) motifs, which are potential phosphorylation sites for the Golgi casein kinase FAM20C. Defects in FAM20C have similarly been associated with AI. In our previous study of Enam^Rgsc514 mice, the Glu⁵⁷ in the S⁵⁵-X⁵⁶-E⁵⁷ motif was mutated into Gly, which was expected to cause a phosphorylation failure of Ser⁵⁵ because Ser⁵⁵ cannot be recognized by FAM20C. The severe enamel defects in ENAM^Rgsc514 mice reminiscent of Enam-knockout mouse enamel suggested a potentially important role of Ser⁵⁵ phosphorylation in ENAM function. However, the enamel defects and ENAM dysfunction may also be attributed to distinct physicochemical differences between Glu⁵⁷ and Gly⁵⁷. To clarify the significance of Ser⁵⁵ phosphorylation to ENAM function, we generated two lines of Enam knock-in mice using CRISPR-Cas9 method to eliminate or mimic the phosphorylation state of Ser⁵⁵ by substituting it with Ala⁵⁵ or Asp⁵⁵ (designated as S55A or S55D), respectively. The teeth of 6-day or 4-week-old mice were subjected to histology, micro-CT, SEM, TEM, immunohistochemistry, and mass spectrometry analyses to characterize the morphological, microstructural and proteomic changes in ameloblasts, enamel matrix and enamel rods. Our results showed that the enamel formation and EMP expression in S55D heterozygotes (Het) were less disturbed than those in S55A heterozygotes, while both homozygotes (Homo) had no mature enamel formation. Proteomic analysis revealed alterations of enamel matrix biosynthetic and mineralization processes in S55A Hets. Our present findings indicate that Asp⁵⁵ substitution partially mimics the phosphorylation state of Ser⁵⁵ in ENAM. Ser⁵⁵ phosphorylation is essential for ENAM function during amelogenesis.