Epigenetic regulation of neuronal morphogenesis in development and regeneration

描述（由申请人提供）：发育期间的神经元形态发生，包括神经元迁移、极化、轴突生长/引导和树突发育，是形成功能性神经回路的关键步骤。这些过程的中断是许多神经发育障碍的基础。在成人神经系统中，损伤后需要类似的神经元形态发生步骤来再生和恢复受损的神经回路。不幸的是，哺乳动物中枢神经系统（CNS）中的神经元失去了支持神经再生的内在能力。基因表达的协调调节不仅控制发育过程中神经元的形态发生，而且也是成熟CNS神经元内在再生能力丧失的基础。表观遗传调控，包括DNA甲基化和组蛋白修饰，正在成为基因表达调控的主要细胞机制。然而，到目前为止，很少有研究探讨神经发育和再生过程中的神经元形态发生是如何在表观遗传水平上调节的。最近的两项研究已经确定了组蛋白甲基转移酶EZH2，它使组蛋白3在赖氨酸27（H3K27）处甲基化，作为导致韦弗综合征的突变基因，韦弗综合征是一种表现出智力残疾和大脑体积增大（大头畸形）的发育障碍。我们的初步研究表明，EZH2在有丝分裂后神经元发育过程中高度表达。在功能上，EZH2是发育中的皮层神经元轴突生长所必需的，并调节几个神经元形态发生相关基因，如Slit2和Pten。此外，在分化的神经元中EZH2的条件性缺失似乎导致更大的大脑。在成人神经系统中，EZH2在CNS中的水平非常低。然而，在周围神经损伤后，它在成人感觉神经元中急剧上调，同时下调H3K27脱甲基酶JMJD3/UTX。在功能上，敲低EZH2可能通过上调Pten和KLF4（两种众所周知的神经再生抑制因子），在体外和体内均损害感觉轴突再生。相反，异位下调去甲基化酶JMJD 3足以促进体内轴突再生。基于这些结果，我们假设在神经发育过程中，EZH2和H3K27甲基化是神经元形态发生的关键表观遗传调节因子，并且在成人神经系统中，它们通过抑制神经再生抑制因子来支持内在神经再生能力。在这项提出的研究中，我们将1）通过在有丝分裂后神经元中有条件地敲除或过表达EZH2来研究EZH2在发育期间调节神经元形态发生中的作用，2）研究EZH2和JMJD3/UTX对H3K27甲基化的调节对于哺乳动物轴突再生是否是必要的和足够的，阐明EZH2和H3K27甲基化调控发育和再生过程中基因表达的分子机制。

DESCRIPTION (provided by applicant): Neuronal morphogenesis during development, including neuronal migration, polarization, axon growth/guidance, and dendrite development, are crucial steps for forming the functional neural circuits. Disruption in these processes underlies many neurodevelopmental disorders. Similar neuronal morphogenesis steps are required in adult nervous system after injuries to regenerate and restore damaged neural circuits. Unfortunately, neurons in the mammalian central nervous system (CNS) lose their intrinsic ability to support neural regeneration. Coordinated regulation of gene expression not only controls neuronal morphogenesis during development, but also underlies the loss of intrinsic regeneration ability of mature CNS neurons. Epigenetic regulation, including DNA methylation and histone modification, is becoming a major cellular mechanism for regulation of gene expression. To date, however, very few studies have investigated how neuronal morphogenesis during neural development and regeneration is regulated at the epigenetic level. Two recent studies have identified the histone methyltransferase EZH2, which methylates histone 3 at lysine 27 (H3K27), as the mutant gene that causes the Weaver Syndrome, a developmental disorder showing intellectual disability and enlarged brain size (macrocephaly). Our preliminary studies have revealed that EZH2 is highly expressed in post-mitotic neurons during development. Functionally, EZH2 is necessary for axon growth of developing cortical neurons, and regulates several neuronal morphogenesis-associated genes, such as Slit2 and Pten. Moreover, conditional deletion of EZH2 in differentiated neurons seems to result in bigger brain. In the adult nervous system, the level of EZH2 is very low in the CNS. However, it is drastically up regulated in adult sensory neurons upon peripheral nerve injury, together with down regulation of H3K27 demethylases JMJD3/UTX. Functionally, knocking down EZH2 impairs sensory axon regeneration both in vitro and in vivo, probably through up regulation of Pten and KLF4, two well-known repressors of neural regeneration. Conversely, ectopically down regulation of the demethylase JMJD3 is sufficient to promote axon regeneration in vivo. Based on these results, we hypothesize that during neural development EZH2 and H3K27 methylation are key epigenetic regulators of neuronal morphogenesis, and in the adult nervous system they function to support the intrinsic neural regeneration ability through repressing neural regeneration repressors. In this proposed study we will 1) investigate the roles of EZH2 in regulation of neuronal morphogenesis during development by conditionally knock out or overexpress EZH2 in post- mitotic neurons, 2) investigate if regulation of H3K27 methylation by EZH2 and JMJD3/UTX is necessary and sufficient for mammalian axon regeneration, and 3) elucidate the molecular mechanisms by which EZH2 and H3K27 methylation regulate gene expression during development and regeneration.