1 INSERM, U1229, RMeS, Nantes, France.
2 Université de Nantes, UMR_S1229, UFR Odontologie, Nantes, France.
J Dent Res. 2018 Feb;97(2):209-217. doi: 10.1177/0022034517729811. Epub 2017 Sep 11.
The importance of phosphate (Pi) as an essential component of hydroxyapatite crystals suggests a key role for membrane proteins controlling Pi uptake during mineralization in the tooth. To clarify the involvement of the currently known Pi transporters (Slc17a1, Slc34a1, Slc34a2, Slc34a3, Slc20a1, Slc20a2, and Xpr1) during tooth development and mineralization, we determined their spatiotemporal expression in murine tooth germs from embryonic day 14.5 to postnatal day 15 and in human dental samples from Nolla stages 6 to 9. Using real-time polymerase chain reaction, in situ hybridization, immunohistochemistry, and X-gal staining, we showed that the expression of Slc17a1, Slc34a1, and Slc34a3 in tooth germs from C57BL/6 mice were very low. In contrast, Slc34a2, Slc20a1, Slc20a2, and Xpr1 were highly expressed, mostly during the postnatal stages. The expression of Slc20a2 was 2- to 10-fold higher than the other transporters. Comparable results were obtained in human tooth germs. In mice, Slc34a2 and Slc20a1 were predominantly expressed in ameloblasts but not odontoblasts, while Slc20a2 was detected neither in ameloblasts nor in odontoblasts. Rather, Slc20a2 was highly expressed in the stratum intermedium and the subodontoblastic cell layer. Although Slc20a2 knockout mice did not show enamel defects, mutant mice showed a disrupted dentin mineralization, displaying unmerged calcospherites at the mineralization front. This latter phenotypical finding raises the possibility that Slc20a2 may play an indirect role in regulating the extracellular Pi availability for mineralizing cells rather than a direct role in mediating Pi transport through mineralizing plasma cell membranes. By documenting the spatiotemporal expression of Pi transporters in the tooth, our data support the possibility that the currently known Pi transporters may be dispensable for the initiation of dental mineralization and may rather be involved later during the tooth mineralization scheme.
磷酸盐（Pi）作为羟磷灰石晶体的重要组成部分，提示其在牙齿矿化过程中控制 Pi 摄取的膜蛋白可能具有关键作用。为了阐明目前已知的 Pi 转运体（Slc17a1、Slc34a1、Slc34a2、Slc34a3、Slc20a1、Slc20a2 和 Xpr1）在牙齿发育和矿化过程中的参与情况，我们从胚胎第 14.5 天到出生后第 15 天，以及从 Nolla 阶段 6 到 9 的人类牙胚中确定了它们的时空表达。通过实时聚合酶链反应、原位杂交、免疫组织化学和 X-gal 染色，我们发现 C57BL/6 小鼠牙胚中 Slc17a1、Slc34a1 和 Slc34a3 的表达水平非常低。相比之下，Slc34a2、Slc20a1、Slc20a2 和 Xpr1 的表达水平较高，主要在出生后阶段。Slc20a2 的表达水平是其他转运体的 2-10 倍。在人类牙胚中也得到了类似的结果。在小鼠中，Slc34a2 和 Slc20a1 主要在成釉细胞中表达，但不在成牙本质细胞中表达，而 Slc20a2 既不在成釉细胞中表达，也不在成牙本质细胞中表达。相反，Slc20a2 在中间层和亚成牙本质细胞层中高度表达。虽然 Slc20a2 敲除小鼠没有出现釉质缺陷，但突变小鼠的牙本质矿化出现了紊乱，在矿化前沿显示出未融合的钙化小球。这一表型发现提示 Slc20a2 可能在调节矿化细胞外 Pi 供应方面发挥间接作用，而不是在介导矿化质膜中的 Pi 转运方面发挥直接作用。通过记录 Pi 转运体在牙齿中的时空表达，我们的数据支持这样一种可能性，即目前已知的 Pi 转运体可能对牙矿化的启动不是必需的，而可能在牙齿矿化过程中较晚阶段参与。
