Dept. of Physics, University of Wisconsin Madison, WI, USA.
Department of Oral and Craniofacial Sciences, Center for Craniofacial Regeneration, School of Dental Medicine, University of Pittsburgh, PA, USA.
J Struct Biol. 2022 Jun;214(2):107844. doi: 10.1016/j.jsb.2022.107844. Epub 2022 Feb 25.
Amelogenin, the most abundant enamel matrix protein, plays several critical roles in enamel formation. Importantly, we previously found that the singular phosphorylation site at Ser16 in amelogenin plays an essential role in amelogenesis. Studies of genetically knock-in (KI) modified mice in which Ser16 in amelogenin is substituted with Ala that prevents amelogenin phosphorylation, and in vitro mineralization experiments, have shown that phosphorylated amelogenin transiently stabilizes amorphous calcium phosphate (ACP), the initial mineral phase in forming enamel. Furthermore, KI mice exhibit dramatic differences in the enamel structure compared with wild type (WT) mice, including thinner enamel lacking enamel rods and ectopic surface calcifications. Here, we now demonstrate that amelogenin phosphorylation also affects the organization and composition of mature enamel mineral. We compared WT, KI, and heterozygous (HET) enamel and found that in the WT elongated crystals are co-oriented within each rod, however, their c-axes are not aligned with the rods' axes. In contrast, in rod-less KI enamel, crystalline c-axes are less co-oriented, with misorientation progressively increasing toward the enamel surface, which contains spherulites, with a morphology consistent with abiotic formation. Furthermore, we found significant differences in enamel hardness and carbonate content between the genotypes. ACP was also observed in the interrod of WT and HET enamel, and throughout aprismatic KI enamel. In conclusion, amelogenin phosphorylation plays crucial roles in controlling structural, crystallographic, mechanical, and compositional characteristics of dental enamel. Thus, loss of amelogenin phosphorylation leads to a reduction in the biological control over the enamel mineralization process.
釉原蛋白是最丰富的釉基质蛋白，在釉质形成中发挥着几个关键作用。重要的是，我们之前发现釉原蛋白丝氨酸 16 位的单一磷酸化位点在釉质发生中起着至关重要的作用。对釉原蛋白丝氨酸 16 被丙氨酸取代的基因敲入（KI）修饰小鼠的研究，以及体外矿化实验表明，磷酸化釉原蛋白暂时稳定无定形磷酸钙（ACP），ACP 是形成釉质的初始矿物相。此外，与野生型（WT）小鼠相比，KI 小鼠的釉质结构存在明显差异，包括缺乏釉柱的更薄的釉质和异位表面钙化。在这里，我们现在证明釉原蛋白磷酸化也会影响成熟釉质矿化的组织和组成。我们比较了 WT、KI 和杂合（HET）的釉质，发现 WT 中拉长的晶体在每个晶体中都有共同的取向，然而，它们的 c 轴与晶体的轴并不一致。相比之下，在无晶体的 KI 釉质中，晶体的 c 轴的共同取向性较低，随着向釉质表面的取向逐渐增加，其晶轴的取向逐渐增加，釉质表面包含球晶，其形态与非生物形成一致。此外，我们还发现基因型之间在釉质硬度和碳酸盐含量上存在显著差异。ACP 也存在于 WT 和 HET 釉质的晶间和无晶体的 KI 釉质中。总之，釉原蛋白磷酸化在控制牙釉质的结构、结晶学、力学和组成特性方面起着至关重要的作用。因此，釉原蛋白磷酸化的丧失导致对釉质矿化过程的生物学控制减少。
