Dental enamel comprises interwoven arrays of extremely long and narrow crystals of carbonated hydroxyapatite called enamel rods. Amelogenin (AMELX) is the predominant extracellular enamel matrix protein and plays an essential role in enamel formation (amelogenesis). Previously, we have demonstrated that full-length AMELX forms higher-order supramolecular assemblies that regulate ordered mineralization in vitro, as observed in enamel rods. Phosphorylation of the sole AMELX phosphorylation site (Ser-16) in vitro greatly enhances its capacity to stabilize amorphous calcium phosphate (ACP), the first mineral phase formed in developing enamel, and prevents apatitic crystal formation. To test our hypothesis that AMELX phosphorylation is critical for amelogenesis, we generated and characterized a hemizygous knockin (KI) mouse model with a phosphorylation-defective Ser-16 to Ala-16 substitution in AMELX. Using EM analysis, we demonstrate that in the absence of phosphorylated AMELX, KI enamel lacks enamel rods, the hallmark component of mammalian enamel, and, unlike WT enamel, appears to be composed of less organized arrays of shorter crystals oriented normal to the dentinoenamel junction. KI enamel also exhibited hypoplasia and numerous surface defects, whereas heterozygous enamel displayed highly variable mosaic structures with both KI and WT features. Importantly, ACP-to-apatitic crystal transformation occurred significantly faster in KI enamel. Secretory KI ameloblasts also lacked Tomes' processes, consistent with the absence of enamel rods, and underwent progressive cell pathology throughout enamel development. In conclusion, AMELX phosphorylation plays critical mechanistic roles in regulating ACP-phase transformation and enamel crystal growth, and in maintaining ameloblast integrity and function during amelogenesis.

中文翻译：

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Amelogenin磷酸化通过稳定瞬态无定形矿物前体来调节牙釉质的形成。

牙釉质包括交织的非常长和狭窄的碳酸羟基磷灰石晶体交织而成的阵列，称为搪瓷棒。Amelogenin（AMELX）是主要的细胞外釉质基质蛋白，在釉质形成（釉质生成）中起重要作用。以前，我们已经证明全长AMELX会形成更高阶的超分子组装体，如在搪瓷棒中观察到的那样，它们在体外调节有序矿化。唯一的AMELX磷酸化位点（Ser-16）在体外进行磷酸化可极大地增强其稳定无定形磷酸钙（ACP）的能力，非晶态磷酸钙是在搪瓷发育过程中形成的第一个矿物相，并能防止磷灰石晶体形成。为了验证我们的假设，即AMELX磷酸化对于牙釉质形成至关重要，我们生成并表征了一个半合子敲除（KI）小鼠模型，在AMELX中具有磷酸化缺陷的Ser-16至Ala-16取代。使用EM分析，我们证明了在没有磷酸化的AMELX的情况下，KI搪瓷缺少搪瓷棒，这是哺乳动物搪瓷的标志性成分，与WT搪瓷不同，它似乎是由组织较少的较短的晶体阵列组成的，这些晶体的取向垂直于牙本质-搪瓷连接。KI釉质还表现出发育不全和许多表面缺陷，而杂合性釉质则显示出具有KI和WT特征的高度可变的镶嵌结构。重要的是，KI瓷釉中ACP到脂族晶体的转变明显更快。分泌型KI成釉细胞也缺乏Tomes的加工过程，这与搪瓷棒的缺乏相符，并在整个瓷釉发展过程中进行了渐进性细胞病理学检查。总之，AMELX磷酸化在调节ACP相变和釉质晶体生长，并在成釉过程中保持成釉细胞的完整性和功能中起着关键的机械作用。