Chen Lei, Liao Guanghong, Waclaw Ronald R, Burns Kevin A, Linquist Diana, Campbell Kenneth, Zheng Yi, Kuan Chia-Yi
Division of Experimental Hematology, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio 45229, USA.
J Neurosci. 2007 Apr 4;27(14):3884-93. doi: 10.1523/JNEUROSCI.3509-06.2007.
Previous studies using dominant-mutant constructs have implicated Rac1 GTPase in neuritogenesis and neuronal migration. However, overexpression of dominant mutants generally blocks Rho-GTPase activity; thus, it may not reveal the specific or physiological functions of Rac1. To address this issue, we have applied a conditional gene-targeting strategy, using Foxg1-Cre mice to delete Rac1 in the ventricular zone (VZ) of telencephalon and Dlx5/6-Cre-IRES (internal ribosomal entry site)-EGFP (enhanced green fluorescent protein) (Dlx5/6-CIE) in the subventricular zone (SVZ) of ventral telencephalon, respectively. Surprisingly, the deletion of Rac1 in VZ progenitors did not prevent axonal outgrowth of telencephalic neurons. However, the anterior commissure was absent, and the corpus callosal as well as hippocampal commissural axons failed to cross the midline in Rac1/Foxg1-Cre knock-out embryos. The thalamocortical and corticothalamic axons also showed defasciculation or projection defects. These results suggest that Rac1 controls axon guidance rather than neuritogenesis. In addition, although Rac1/Foxg1-Cre knock-out embryos showed delayed radial migration of cortical projection neurons and severe impairment of tangential migration by the ventral telencephalon-derived interneurons, deletion of Rac1 in the SVZ by Dlx5/6-CIE mice produced no discernible defects in tangential migration. These contrasting effects of Rac1 deletion on tangential migration suggest that Rac1 is dispensable for cellular motility per se during neuronal migration. Together, these results underscore the challenge of deciphering the biological functions of Rac1, and Rho-GTPases in general, during mammalian brain development. Moreover, they indicate that Rac1 has a critical role in axon guidance and in acquisition of migratory competency during differentiation of the progenitors for the ventral telencephalon-derived interneurons.
以往使用显性突变体构建体的研究表明，Rac1 GTP酶参与了神经突生成和神经元迁移。然而，显性突变体的过表达通常会阻断Rho-GTP酶的活性；因此，它可能无法揭示Rac1的特定或生理功能。为了解决这个问题，我们应用了一种条件性基因靶向策略，分别使用Foxg1-Cre小鼠在端脑的脑室区（VZ）中删除Rac1，以及使用Dlx5/6-Cre-IRES（内部核糖体进入位点）-EGFP（增强型绿色荧光蛋白）（Dlx5/6-CIE）在腹侧端脑的室下区（SVZ）中删除Rac1。令人惊讶的是，VZ祖细胞中Rac1的缺失并未阻止端脑神经元的轴突生长。然而，前连合缺失，并且在Rac1/Foxg1-Cre基因敲除胚胎中，胼胝体以及海马连合轴突未能穿过中线。丘脑皮质和皮质丘脑轴突也表现出脱束或投射缺陷。这些结果表明，Rac1控制轴突导向而非神经突生成。此外，尽管Rac1/Foxg1-Cre基因敲除胚胎显示皮质投射神经元的放射状迁移延迟，并且腹侧端脑来源的中间神经元的切向迁移严重受损，但Dlx5/6-CIE小鼠在SVZ中删除Rac1并未在切向迁移中产生明显缺陷。Rac1缺失对切向迁移的这些对比效应表明，Rac1在神经元迁移过程中对于细胞运动本身是可有可无的。总之，这些结果强调了在哺乳动物脑发育过程中解读Rac1以及一般Rho-GTP酶的生物学功能所面临的挑战。此外，它们表明Rac1在轴突导向以及腹侧端脑来源的中间神经元祖细胞分化过程中获得迁移能力方面具有关键作用。
