Role of MLL1 chromatin remodeling factor in neural stem cells

描述(由申请人提供)： 神经干细胞(NSCs)有望用于神经系统疾病的治疗，了解NSC神经发生和神经胶质形成的分子机制是释放其治疗潜力的关键。成人的大脑--包括人类的大脑--在脑室下区(SVZ)中有大量的神经干细胞。神经干细胞如何“忘记”其干细胞身份的表观遗传记忆，并“学习”神经发生和神经胶质发生的发育程序，目前还知之甚少。MLL1是一种与果蝇三胸相关的染色质重塑因子，可通过调节其在细胞中的作用来调节细胞的特性 或移除特定的染色质修饰。我们之前证明了MLL1是出生后SVZ神经干细胞形成神经元所必需的，但不是神经胶质细胞(自然，2009,458：529-33)。新出现的研究表明，DNA调控元件上称为增强子的不同染色质信号决定了特定基因启动子的转录活性，但对增强子上的染色质状态是如何调控的知之甚少。在初步研究中，我们发现MLL1通过调节基因间增强子元件来促进神经性基因Dlx2和Dlx5的表达(提交给自然细胞生物学的手稿)。我们的染色质分析支持一种模型，在该模型中，MLL1将增强子保持在一种“稳定”的状态，以促进神经元分化过程中快速的基因激活。此外，我们发现MLL1是JMJD3定位所必需的，JMJD3是一种组蛋白去甲基酶，能够在稳定的增强子元件(H3K27me3)上去除组蛋白3赖氨酸27(H3K27me3)的抑制三甲基化。为了研究JMJD3在成年神经发生中的作用，我们在体内和体外用条件性基因敲除和shRNA介导的基因敲除方法研究了JMJD3-功能丧失。事实上，JMJD3缺陷的SVZ神经干细胞在神经发生方面是有缺陷的，但在胶质形成方面没有缺陷。基于这些新的数据，我们假设MLL1通过靶向特定的增强子元件，使其能够通过JMJD3的作用快速激活，从而协调了多潜能神经干细胞的神经元分化。在这一应用中，我们提出了特定的目标来测试这一调控模型，这对一般的发育具有广泛的意义，并进一步确定MLL1在神经发育中的作用。

DESCRIPTION (provided by applicant): Neural stem cells (NSCs) hold promise for the treatment of neurological disorders, and understanding the molecular mechanisms of NSC neurogenesis and gliogenesis is key to unlocking their therapeutic potential. The adult brain - including that of humans - harbors a population of NSCs in the subventricular zone (SVZ). How NSCs 'forget' the epigenetic memory of their stem cell identity and 'learn' the developmental programs for neurogenesis and gliogenesis is poorly understood. Mll1 (Mixed lineage leukemia-1), a chromatin-remodeling factor related to Drosophila Trithorax, can regulate cellular identity by mediating the application or removal of specific chromatin modifications. We previously showed that Mll1 is required for the genesis of neurons - but not glial cells - from postnatal SVZ NSCs (Nature 2009, 458:529- 33). Emerging studies indicate that distinct chromatin signatures at DNA regulatory elements called enhancers determine the transcriptional activity of specific gene promoters, but little is known about how the chromatin-state at enhancers is regulated. In Preliminary Studies, we show that MLL1 promotes the expression of neurogenic genes Dlx2 and Dlx5 by regulating intergenic enhancer elements (manuscript submitted to Nature Cell Bio). Our chromatin analysis supports a model in which MLL1 maintains enhancers in a 'poised' state to facilitate rapid gene activation during neuronal differentiation. Furthermore, we found that MLL1 is required for the localization of JMJD3, a histone demethylase capable of removing repressive trimethylation of histone 3 at lysine 27 (H3K27me3) at poised enhancer elements. To investigate the role of JMJD3 in adult neurogenesis, we studied JMJD3-loss of function in vivo and in vitro with conditional knockout and shRNA-mediate knockdown. Indeed, JMJD3-deficient SVZ NSCs were defective for neurogenesis, but not gliogenesis. Based on these new data, we hypothesize that MLL1 coordinates neuronal differentiation from multipotent NSCs by targeting specific enhancer elements, enabling their rapid activation by the action of JMJD3. In this application, we propose Specific Aims that test this regulatory model, which has broad implications for development in general, and further determine the role of MLL1 in neural development.