Table4.DOC

Multiple signals control the balance between proliferation and differentiation of neural progenitor cells during corticogenesis. A key point of this regulation is the control of G1 phase length, which is regulated by the Cyclin/Cdks complexes. Using genome-wide chromatin immunoprecipitation assay and mouse genetics, we have explored the transcriptional regulation of Cyclin D1 (Ccnd1) during the early developmental stages of the mouse cerebral cortex. We found evidence that SP8 binds to the Ccnd1 locus on exon regions. In vitro experiments show SP8 binding activity on Ccnd1 gene 3′-end, and point to a putative role for SP8 in modulating PAX6-mediated repression of Ccnd1 along the dorso-ventral axis of the developing pallium, creating a medialLow-lateralHigh gradient of neuronal differentiation. Activation of Ccnd1 through the promoter/5′-end of the gene does not depend on SP8, but on βcatenin (CTNNB1). Importantly, alteration of the Sp8 level of expression in vivo affects Ccnd1 expression during early corticogenesis. Our results indicate that Ccnd1 regulation is the result of multiple signals and that SP8 is a player in this regulation, revealing an unexpected and potentially novel mechanism of transcriptional activation.

在大脑皮质发生过程中，多种信号共同调控神经祖细胞（neural progenitor cells）的增殖与分化平衡。该调控过程的核心节点之一是对G1期（G1 phase）时长的控制，而这一过程由细胞周期蛋白/细胞周期蛋白依赖性激酶复合体（Cyclin/Cdks complexes）介导。本研究借助全基因组染色质免疫共沉淀实验（genome-wide chromatin immunoprecipitation assay）与小鼠遗传学手段，探究了小鼠大脑皮质早期发育阶段中细胞周期蛋白D1（Cyclin D1, Ccnd1）的转录调控机制。研究证实，SP8可结合Ccnd1基因座的外显子区域。体外实验显示，SP8对Ccnd1基因的3'端（3′-end）具有结合活性，并提示SP8可能通过调控配对盒基因6（PAX6）介导的抑制作用，沿发育中大脑皮层的背腹轴（dorso-ventral axis）调控Ccnd1的表达，进而形成内侧低-外侧高的神经元分化梯度。通过基因启动子/5'端激活Ccnd1的过程并不依赖SP8，而是依赖β连环蛋白（βcatenin, CTNNB1）。值得注意的是，体内SP8表达水平的改变会影响早期皮质发生过程中Ccnd1的表达。本研究结果表明，Ccnd1的调控是多种信号共同作用的结果，且SP8是该调控网络中的重要参与者，这一发现揭示了一种此前未被报道的、具有潜在创新性的转录激活机制。