Transcription profiling of first molar tooth germ in PPAR-alpha knock-out and wild type mice

Embryologically the tooth is derived from both the ectoderm and neural crest (ectomesenchyme). It is often used as a model to study how epithelial-mesenchymal interactions can control differentiation and morphogenesis. During early development organs of ectodermal origin share both a set of signalling molecules and exhibit common morphological features, subsequently proceeding along separate developmental programs.Tooth development is a continuous process that can be divided into the initiation -, bud -, cap -, and bell-stages. In mice, tooth development begins at embryonic day 11.5 (E11.5), by thickening of the dental epithelium, while mineralization of enamel and dentin in first molar starts at postnatal day 0 (P0) (5). A multistep and complex process of the gene expression are involved in the early stage of tooth development. So far expression of more than 1300 genes and/or proteins have been detected during tooth germ development by microarrays/immunocytochemistry/in situ hybridization. Studies with mutant mice have identified a number of genes that regulate tooth development and morphology. For example, deficiency of Lef-1 or P63 arrests tooth development at early stages. Deficiency of Msx1 or Pax9 results in arrest of tooth development at the bud stage , while deficiency of Runx2/Cbfa1 or Sp3 inhibits cyto-differentiation of ameloblasts and/or odontoblasts. Shh is required for normal growth and morphogenesis, but is not essential for cyto-differentiation of the ameloblast and odontoblast populations. Ameloblastin and amelogenin knock-out mice develop severe enamel hypoplasia with abnormal ameloblast differentiation. Recently, new connections between retinoid metabolism and PPAR responses have been identified. It has also been shown that endogenous retinoic acid is necessary for the initiation of odontogenesis , and that some of the genes that catalyze the oxidation of retinaldehyde into retinoic acid, exhibit distinct patterns of expression in developing murine teeth. Little is known about functions of PPAR-a as regards tooth germs or mature teeth. It is, however, likely that mitochondrial oxidative metabolism well as fatty acid metabolism is enhanced in late odontogenesis. These are metabolic activities which in other tissues are stimulated by PPAR-a agonists.For this reason it was of interest to carry out comparative gene expression profiling of the first molar tooth germs of PPAR-a knock-out mouse and of the corresponding wild-type mice. The results suggest marked differences in gene expression, parts of which may be associated with an observed hypomineralization of enamel in the mature PPAR-a knock-out murine tooth.从胚胎学角度而言，牙齿来源于外胚层与神经嵴（外胚间充质，ectomesenchyme），其常被用作研究上皮-间充质相互作用如何调控细胞分化与形态发生的经典模型。在发育早期，外胚层来源的器官共享一套信号分子，并具备共同的形态学特征，随后会沿各自独立的发育程序进行分化。牙齿发育是一个连续过程，可分为启动期、蕾状期、帽状期与钟状期。在小鼠中，牙齿发育始于胚胎第11.5天（E11.5），表现为牙上皮增厚；而第一磨牙的牙釉质与牙本质矿化则始于出生后第0天（P0）[5]。牙齿发育早期涉及多步骤且复杂的基因表达调控过程，截至目前，已有研究通过基因芯片、免疫细胞化学及原位杂交技术，在牙胚发育过程中检测到超过1300个基因和/或蛋白质的表达。通过突变小鼠开展的研究已鉴定出多个调控牙齿发育与形态建成的基因：例如，Lef-1或P63的基因缺陷会使牙齿发育停滞于早期阶段；Msx1或Pax9的基因缺陷会使牙齿发育停滞于蕾状期；而Runx2/Cbfa1或Sp3的基因缺陷则会抑制成釉细胞与/或成牙本质细胞的细胞分化。音猬因子（Shh）对于正常的生长与形态发生是必需的，但并非成釉细胞与成牙本质细胞群体的细胞分化所必需。成釉蛋白（Ameloblastin）与釉原蛋白（amelogenin）敲除小鼠会出现严重的釉质发育不全，并伴随成釉细胞分化异常。近期研究揭示了类视黄醇代谢与过氧化物酶体增殖物激活受体（PPAR）应答通路之间的新关联，另有研究证实，内源性视黄酸对于牙发生的启动是必需的；而部分催化视黄醛氧化为视黄酸的基因，在发育中小鼠的牙齿中呈现独特的表达模式。目前关于过氧化物酶体增殖物激活受体α（PPAR-α）在牙胚或成熟牙齿中的功能仍知之甚少，不过，在牙发生后期，线粒体氧化代谢与脂肪酸代谢可能会被增强——这类代谢活动在其他组织中可被PPAR-α激动剂所激活。基于此，本研究对PPAR-α敲除小鼠及其对应野生型小鼠的第一磨牙牙胚开展了基因表达谱比较分析，研究结果显示二者的基因表达存在显著差异，其中部分差异可能与成熟PPAR-α敲除小鼠牙齿中观察到的釉质矿化不全相关。