Kurokawa Daisuke, Sakurai Yusuke, Inoue Ai, Nakayama Rika, Takasaki Nobuyoshi, Suda Yoko, Miyake Tsutomu, Amemiya Chris T, Aizawa Shinichi
Laboratory for Vertebrate Body Plan, RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku, Kobe 650-0047, Japan.
Proc Natl Acad Sci U S A. 2006 Dec 19;103(51):19350-5. doi: 10.1073/pnas.0604686103. Epub 2006 Dec 11.
Otx2 is a paired type homeobox gene that plays essential roles in each step and site of head development in vertebrates. In the mouse, Otx2 expression in the anterior neuroectoderm is regulated primarily by two distinct enhancers: anterior neuroectoderm (AN) and forebrain/midbrain (FM) enhancers at 92 kb and 75 kb 5'of the Otx2 locus, respectively. The AN enhancer has activity in the entire anterior neuroectoderm at headfold and early somite stages, whereas the FM enhancer is subsequently active in the future caudal forebrain and midbrain ectoderm. In tetrapods, both AN and FM enhancers are conserved, whereas the AN region is missing in teleosts, despite overt Otx2 expression in the anterior neuroectoderm. Here, we show that zebrafish and fugu FM regions drive expression not only in the forebrain and midbrain but also in the anterior neuroectoderm at headfold stage. The analysis of coelacanth and skate genomic Otx2 orthologues suggests that the utilization of the two enhancers, AN and FM, is an ancestral condition. In contrast, the AN enhancer has been specifically lost in the teleost lineage with a compensatory establishment of AN activity within the FM enhancer. Furthermore, the AN activity in the fish FM enhancer was established by recruiting upstream factors different from those that direct the tetrapod AN enhancer, yet zebrafish FM enhancer is active in both mouse and zebrafish anterior neuroectoderm at the headfold stage.
Otx2是一种成对型同源框基因,在脊椎动物头部发育的每个步骤和部位都发挥着重要作用。在小鼠中,前神经外胚层中的Otx2表达主要由两个不同的增强子调控:分别位于Otx2基因座5'端92 kb处的前神经外胚层(AN)增强子和75 kb处的前脑/中脑(FM)增强子。AN增强子在头褶期和早期体节期的整个前神经外胚层中具有活性,而FM增强子随后在未来的尾侧前脑和中脑外胚层中活跃。在四足动物中,AN和FM增强子都是保守的,而在硬骨鱼中,尽管Otx2在前神经外胚层中有明显表达,但AN区域缺失。在这里,我们表明斑马鱼和河豚的FM区域不仅在前脑和中脑中驱动表达,而且在头褶期的前神经外胚层中也驱动表达。对腔棘鱼和鳐鱼基因组Otx2直系同源物的分析表明,AN和FM这两个增强子的利用是一种祖先状态。相比之下,AN增强子在硬骨鱼谱系中特异性丢失,同时在FM增强子内补偿性地建立了AN活性。此外,鱼类FM增强子中的AN活性是通过招募与指导四足动物AN增强子的上游因子不同的上游因子而建立的,然而斑马鱼FM增强子在头褶期的小鼠和斑马鱼前神经外胚层中均有活性。