Bosch Peter J, Fuller Leah C, Sleeth Carolyn M, Weiner Joshua A
Department of Biology, The University of Iowa, Iowa City, IA, USA.
Department of Biology and Department of Psychiatry, The University of Iowa, 143 Biology Building, Iowa City, IA, 52242, USA.
Neural Dev. 2016 Nov 21;11(1):21. doi: 10.1186/s13064-016-0076-8.
The proper spatial and temporal regulation of dorsal telencephalic progenitor behavior is a prerequisite for the formation of the highly-organized, six-layered cerebral cortex. Premature differentiation of cells, disruption of cell cycle timing, excessive apoptosis, and/or incorrect neuronal migration signals can have devastating effects, resulting in a number of neurodevelopmental disorders involving microcephaly and/or lissencephaly. Though genes encoding many key players in cortical development have been identified, our understanding remains incomplete. We show that the gene encoding Akirin2, a small nuclear protein, is expressed in the embryonic telencephalon. Converging evidence indicates that Akirin2 acts as a bridge between transcription factors (including Twist and NF-κB proteins) and the BAF (SWI/SNF) chromatin remodeling machinery to regulate patterns of gene expression. Constitutive knockout of Akirin2 is early embryonic lethal in mice, while restricted loss in B cells led to disrupted proliferation and cell survival.
We generated cortex-restricted Akirin2 knockouts by crossing mice harboring a floxed Akirin2 allele with the Emx1-Cre transgenic line and assessed the resulting embryos using in situ hybridization, EdU labeling, and immunohistochemistry.
The vast majority of Akirin2 mutants do not survive past birth, and exhibit extreme microcephaly, with little dorsal telencephalic tissue and no recognizable cortex. This is primarily due to massive cell death of early cortical progenitors, which begins at embryonic day (E)10, shortly after Emx1-Cre is active. Immunostaining and cell cycle analysis using EdU labeling indicate that Akirin2-null progenitors fail to proliferate normally, produce fewer neurons, and undergo extensive apoptosis. All of the neurons that are generated in Akirin2 mutants also undergo apoptosis by E12. In situ hybridization for Wnt3a and Wnt-responsive genes suggest defective formation and/or function of the cortical hem in Akirin2 null mice. Furthermore, the apical ventricular surface becomes disrupted, and Sox2-positive progenitors are found to "spill" into the lateral ventricle.
Our data demonstrate a previously-unsuspected role for Akirin2 in early cortical development and, given its known nuclear roles, suggest that it may act to regulate gene expression patterns critical for early progenitor cell behavior and cortical neuron production.
背侧端脑祖细胞行为在空间和时间上的正确调控是形成高度有序的六层大脑皮质的前提条件。细胞过早分化、细胞周期时间紊乱、过度凋亡和/或不正确的神经元迁移信号可能产生毁灭性影响，导致许多涉及小头畸形和/或无脑回畸形的神经发育障碍。尽管已经鉴定出许多在皮质发育中起关键作用的基因，但我们的了解仍然不完整。我们发现，编码小核蛋白Akirin2的基因在胚胎端脑中表达。越来越多的证据表明，Akirin2在转录因子（包括Twist和NF-κB蛋白）与BAF（SWI/SNF）染色质重塑机制之间起桥梁作用，以调节基因表达模式。Akirin2的组成型敲除在小鼠胚胎早期致死，而在B细胞中的局限性缺失导致增殖和细胞存活受到破坏。
我们通过将携带floxed Akirin2等位基因的小鼠与Emx1-Cre转基因品系杂交，产生了皮质限制性Akirin2敲除小鼠，并使用原位杂交、EdU标记和免疫组织化学对所得胚胎进行评估。
绝大多数Akirin2突变体在出生后无法存活，并表现出极度小头畸形，背侧端脑组织很少，且没有可识别的皮质。这主要是由于早期皮质祖细胞的大量细胞死亡，这种死亡在Emx1-Cre激活后不久的胚胎第10天（E10）开始。使用EdU标记的免疫染色和细胞周期分析表明，Akirin2基因缺失的祖细胞不能正常增殖，产生的神经元较少，并经历广泛的凋亡。Akirin2突变体中产生的所有神经元在E12时也会发生凋亡。对Wnt3a和Wnt反应基因的原位杂交表明，Akirin2基因敲除小鼠的皮质下托形成和/或功能存在缺陷。此外，顶端脑室表面受到破坏，发现Sox2阳性祖细胞“溢出”到侧脑室。
我们的数据证明了Akirin2在早期皮质发育中以前未被怀疑的作用，鉴于其已知的核作用，表明它可能起到调节对早期祖细胞行为和皮质神经元产生至关重要的基因表达模式的作用。
