Selective Roles of Normal and Mutant Huntingtin in Neural Induction and Early Neurogenesis

Huntington's disease (HD) is a neurodegenerative disorder caused by abnormal polyglutamine expansion in the amino-terminal end of the huntingtin protein (Htt) and characterized by progressive striatal and cortical pathology. Previous reports have shown that Htt is essential for embryogenesis, and a recent study by our group revealed that the pathogenic form of Htt (mHtt) causes impairments in multiple stages of striatal development. In this study, we have examined whether HD-associated striatal developmental deficits are reflective of earlier maturational alterations occurring at the time of neurulation by assessing differential roles of Htt and mHtt during neural induction and early neurogenesis using an in vitro mouse embryonic stem cell (ESC) clonal assay system. We demonstrated that the loss of Htt in ESCs (KO ESCs) severely disrupts the specification of primitive and definitive neural stem cells (pNSCs, dNSCs, respectively) during the process of neural induction. In addition, clonally derived KO pNSCs and dNSCs displayed impaired proliferative potential, enhanced cell death and altered multi-lineage potential. Conversely, as observed in HD knock-in ESCs (Q111 ESCs), mHtt enhanced the number and size of pNSC clones, which exhibited enhanced proliferative potential and precocious neuronal differentiation. The transition from Q111 pNSCs to fibroblast growth factor 2 (FGF2)-responsive dNSCs was marked by potentiation in the number of dNSCs and altered proliferative potential. The multi-lineage potential of Q111 dNSCs was also enhanced with precocious neurogenesis and oligodendrocyte progenitor elaboration. The generation of Q111 epidermal growth factor (EGF)-responsive dNSCs was also compromised, whereas their multi-lineage potential was unaltered. These abnormalities in neural induction were associated with differential alterations in the expression profiles of Notch, Hes1 and Hes5. These cumulative observations indicate that Htt is required for multiple stages of neural induction, whereas mHtt enhances this process and promotes precocious neurogenesis and oligodendrocyte progenitor cell elaboration.

亨廷顿病（Huntington's disease, HD）是一种神经退行性疾病，由亨廷顿蛋白（huntingtin protein, Htt）氨基末端区域异常多谷氨酰胺扩增引发，以进行性纹状体及皮层病理损伤为特征。既往研究表明，Htt对胚胎发生至关重要；本团队近期的一项研究揭示，致病型Htt（mutant Htt, mHtt）会对纹状体发育的多个阶段造成损伤。本研究通过体外小鼠胚胎干细胞（mouse embryonic stem cell, ESC）克隆分析系统，评估Htt与mHtt在神经诱导及早期神经发生过程中的差异化作用，旨在探究HD相关的纹状体发育缺陷是否反映了神经胚形成时期发生的早期成熟改变。我们证实，胚胎干细胞中Htt的缺失（KO ESCs）会严重干扰神经诱导过程中原始神经干细胞（primitive neural stem cells, pNSCs）与终末神经干细胞（definitive neural stem cells, dNSCs）的特化。此外，克隆来源的KO型pNSCs与dNSCs表现出增殖潜能受损、细胞死亡增强及多谱系分化潜能改变的特征。与之相反，如在HD敲入ESC（Q111 ESCs）中观察到的那样，mHtt可增加pNSC克隆的数量与体积，并使其增殖潜能增强且出现过早的神经元分化。从Q111 pNSCs向成纤维细胞生长因子2（fibroblast growth factor 2, FGF2）响应型dNSCs的转化过程，以dNSCs数量增多及增殖潜能改变为标志。Q111 dNSCs的多谱系分化潜能同样增强，且伴随过早神经发生及少突胶质前体细胞生成增加。Q111表皮生长因子（epidermal growth factor, EGF）响应型dNSCs的生成也受到损害，但其多谱系分化潜能未发生改变。上述神经诱导异常与Notch、Hes1及Hes5的表达谱差异改变相关。综上，Htt是神经诱导多个阶段所必需的因子，而mHtt则会增强这一过程，并促进过早神经发生与少突胶质前体细胞生成。