以失活X染色体为模型研究晶状体纤维细胞核DNA清除的分子机制

The crystalline lens is a structure which can allow light to transmit. To ensure this, cells located around the central axis of the lens will lose their nuclei gradually during the process of their development. In female mammals, there are two X chromosomes in the nucleus, and one of the X chromosome will lost its activity and forms a highly condensed structure, thus difficult to be degraded. For the inactive X chromosome, we speculated that the inactive X chromosome should be reactive before to be degraded. Therefore, during the development of crystalline lens, the inactive X chromosome can be an important model to study the mechanism of losing nuclei. Our previous works have found that lack of heat shock factor 4 (HSF4), the nuclei of lens fiber cells cannot be eliminated completely. Therefore, HSF4 might be involved in the process of DNA epigenetic modification in nuclei, the conformation changes of chromosome, DNA degradation and so on. In this project, we’re going to use the inactive X chromosome as a model to study the mechanism of how the lens fiber cells lose their nuclei and how HSF4 participate in this process. This study will not only give us a deeper understanding about lens development and cataract formation, also make us learn more about the mechanism of the loss of nuclei in cells.

眼睛晶状体作为一个透光的结构，为了保证其对光线的透过能力，随着晶状体的发育，位于其中心轴附近的细胞会逐步丢失其细胞核。雌性哺乳动物两条X染色体中，有一条失活的X染色体形成高度浓缩致密的结构，难以被DNA酶降解。对于失活的X染色体而言，要实现降解完全，推测需要有一个解旋的过程，使其变为松散状态。因此，失活 X染色体是研究晶状体纤维细胞失去细胞核机制的重要模型。我们前期工作发现热休克转录因子HSF4敲除小鼠晶状体纤维细胞核DNA不能完全清除，推测HSF4可能涉及核DNA表观修饰、染色质构象变化、DNA酶解等等。本课题拟以失活X染色体为模型，利用HSF4基因敲除和野生型小鼠研究晶状体纤维细胞失去细胞核的机制。针对这一问题的研究，不仅可以加深人们对于晶状体发育、白内障形成的认识，还有助于对细胞脱核／去核这一重要生理现象发生机制的理解。

随着晶状体的发育，位于其中心轴附近的细胞会逐步丢失其细胞器以及细胞核来保证其对光线的透过能力，雌性哺乳动物两条X染色体中，有一条失活的X染色体形成高度浓缩致密的结构，难以被DNA酶降解。对于失活的X染色体而言，要实现降解完全，需要有一个解旋的过程，使其变为松散状态。因此，在晶状体发育的失核过程中，失活 X染色体晶状体纤维细胞失去细胞核机制是一个重要模型。我们发现，热休克转录因子HSF4在该过程中起重要作用，敲除HSF4的小鼠晶状体纤维细胞核DNA不能完全清除，从而导致白内障。此外，我们通过生物信息学分析发现，处于发育阶段14.5天和16.5天的小鼠晶状体细胞的X染色体上的基因表达模式具有更高的相似度；另外，从基因表达活性上看，10.5天和12.5天时期的晶状体中X染色体基因表达活性相对较低，而14.5天和16.5天的基因表达活性则较高。而对这些差异表达基因进行富集分析后发现，这些差异基因主要富集在了无义介导的mRNA降解，即NMD途径中。