FGF8介导的信号通路对哺乳动物牙齿发育速率的调控作用

The pressing demand for replacement teeth in regenerative dental medicine has brought up a matter of great urgency to begin to explore the molecular mechanisms that regulate tooth development in humans, a prerequisite for ultimate realization of human tooth regeneration. While significant amount of information regarding tooth development and its underlying molecular mechanisms has been obtained from studies using mouse as a model system, very little is known about gene expression and function in human odotogenesis. Despite that human odontogenesis shares considerable homology with that of the mouse, differences also exist. One major noticeable difference is their development phase. In mice, the tooth developmental phase, from the initiation to tooth eruption, is about 20 days. In humans, however, this process takes about 400 days. The underlying genetic controls of this difference are completely unknown. Our previous and preliminary studies have identified Fibroblast Growth Factor-8 (FGF8) as a potential key regulator of the tooth development rate. We hypothesize that in addition to its known function in tooth initiation, FGF8-mediated signaling plays a critical role in controlling tooth development rate in humans. To test this hypothesis, we will first conduct an expression survey of FGF signaling molecules and receptors in the developing human tooth, and to determine if FGF8-mediated signaling plays a critical role in regulating tooth development rate in humans by tissue recombination and siRNA-mediated loss-of-function approaches. We will subsequently validate this novel function of Fgf8 in vivo in mice by transgenic gain-of-function approach. The results obtained from the proposed studies will provide comprehensive information for FGF signaling in the developing human tooth and reveal a novel function of FGF8 in the human tooth development, which will have direct impact on the practice of human tooth regeneration in the near future.

理解人类牙齿发育分子机制是实现牙齿再生的前提。然而，对牙齿发育分子机制的理解主要来自对模式动物小鼠的研究，对人类牙齿发育的机制所知甚少。尽管人和小鼠的牙齿发育在形态上有许多相似，但也有明显的差异。重要差异之一是人类的牙齿发育周期明显长于小鼠，在小鼠只需20天，而在人类长达400天。这显然是由遗传因素所决定的信号分子调控机制上的差别而造成的。前期研究表明，FGF8可能是调控牙齿发育速率的关键因子。认为，除了已知其在牙齿发育起始中的作用外，在人类，FGF8介导的信号通路可能通过抑制牙齿的分化来调控其发育的速率，是调控牙齿发育速率的关键因子之一。为验证这一科学假说，将检测FGF信号分子及其受体在人牙胚中的表达模式，进而用组织重组和siRNA的方法检验FGF8在调控人牙齿发育速率中的功能；再利用在牙胚中过表达Fgf8的转基因小鼠，模拟人类牙齿发育中FGF8的表达模式，验证其抑制牙齿分化的机制。

哺乳动物齿列的结构和组成各有不同和特色，但调控其发育的遗传通路是相对保守的。种属的形态差异不是由于形成新的遗传通路，而是对保守的遗传通路的修饰和调节产生的。牙齿发育周期和牙齿尺寸大小是人类和小鼠牙齿形态的重要差异之一。前期研究已确定，哺乳动物牙齿发育速率主要由牙间充质调控。在小鼠牙间充质中过表达Fgf8可延缓牙齿发育进程。说明FGF8 可能是调控牙齿发育速率的关键因子之一。本项目进一步利用Wnt1-Cre;Rosa-Fgf8转基因小鼠，确认了在牙胚间充质中特异过表达Fgf8后可引起牙齿发育进程延缓。同时，利用FGF通路抑制剂（SU5402）处理人牙胚或FGF8-shRNA慢病毒感染人牙间充质细胞后，牙齿发育速率加快了。这些结果充分说明FGF8控制哺乳动物牙齿发育速率及牙齿尺寸大小。进一步研究发现，在转基因小鼠牙间充质中提高Fgf8活性，主要由P38 MAPK胞内传导途径将信号传入细胞核内，通过上调Ccnd1及下调P21活性，促进细胞周期由G1向S期转变，进而刺激细胞增殖，抑制细胞分化，最终控制牙齿发育速率和尺寸。