Département de Biologie Cellulaire et de Morphologie, University of Lausanne, Lausanne, Switzerland.
PLoS Genet. 2012;8(3):e1002606. doi: 10.1371/journal.pgen.1002606. Epub 2012 Mar 29.
The corpus callosum (CC) is the major commissure that bridges the cerebral hemispheres. Agenesis of the CC is associated with human ciliopathies, but the origin of this default is unclear. Regulatory Factor X3 (RFX3) is a transcription factor involved in the control of ciliogenesis, and Rfx3-deficient mice show several hallmarks of ciliopathies including left-right asymmetry defects and hydrocephalus. Here we show that Rfx3-deficient mice suffer from CC agenesis associated with a marked disorganisation of guidepost neurons required for axon pathfinding across the midline. Using transplantation assays, we demonstrate that abnormalities of the mutant midline region are primarily responsible for the CC malformation. Conditional genetic inactivation shows that RFX3 is not required in guidepost cells for proper CC formation, but is required before E12.5 for proper patterning of the cortical septal boundary and hence accurate distribution of guidepost neurons at later stages. We observe focused but consistent ectopic expression of Fibroblast growth factor 8 (Fgf8) at the rostro commissural plate associated with a reduced ratio of GLIoma-associated oncogene family zinc finger 3 (GLI3) repressor to activator forms. We demonstrate on brain explant cultures that ectopic FGF8 reproduces the guidepost neuronal defects observed in Rfx3 mutants. This study unravels a crucial role of RFX3 during early brain development by indirectly regulating GLI3 activity, which leads to FGF8 upregulation and ultimately to disturbed distribution of guidepost neurons required for CC morphogenesis. Hence, the RFX3 mutant mouse model brings novel understandings of the mechanisms that underlie CC agenesis in ciliopathies.
胼胝体（CC）是连接大脑半球的主要连合。CC 发育不全与人类纤毛病有关，但这种缺陷的起源尚不清楚。调节因子 X3（RFX3）是一种参与纤毛发生调控的转录因子，Rfx3 缺陷小鼠表现出多种纤毛病的特征，包括左右不对称缺陷和脑积水。在这里，我们表明 Rfx3 缺陷小鼠患有 CC 发育不全，伴有指导神经元的明显组织紊乱，这些神经元是轴突穿越中线的必需。通过移植实验，我们证明了突变中线区域的异常主要导致 CC 畸形。条件性基因敲除表明 RFX3 不是指导细胞形成 CC 所必需的，但在 E12.5 之前对于皮质隔界的正确模式形成以及随后指导神经元在后期的准确分布是必需的。我们观察到纤维母细胞生长因子 8（Fgf8）在喙状连合板处集中但一致的异位表达，与 GLI 瘤相关癌基因家族锌指 3（GLI3）抑制物与激活物形式的比值降低有关。我们在脑外植体培养物中证明，异位 FGF8 再现了 Rfx3 突变体中观察到的指导神经元缺陷。这项研究揭示了 RFX3 在早期大脑发育中的关键作用，它通过间接调节 GLI3 活性，导致 FGF8 的上调，最终导致 CC 形态发生所需的指导神经元分布紊乱。因此，RFX3 突变小鼠模型为纤毛病中 CC 发育不全的机制提供了新的认识。
