Sakagami Kiyo, Gan Lin, Yang Xian-Jie
Jules Stein Eye Institute and Department of Ophthalmology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90095, USA.
J Neurosci. 2009 May 27;29(21):6932-44. doi: 10.1523/JNEUROSCI.0289-09.2009.
Cell-extrinsic signals can profoundly influence the production of various neurons from common progenitors. Yet mechanisms by which extrinsic signals coordinate progenitor cell proliferation, cell cycle exit, and cell fate choices are not well understood. Here, we address whether Hedgehog (Hh) signals independently regulate progenitor proliferation and neuronal fate decisions in the embryonic mouse retina. Conditional ablation of the essential Hh signaling component Smoothened (Smo) in proliferating progenitors, rather than in nascent postmitotic neurons, leads to a dramatic increase of retinal ganglion cells (RGCs) and a mild increase of cone photoreceptor precursors without significantly affecting other early-born neuronal cell types. In addition, Smo-deficient progenitors exhibit aberrant expression of cell cycle regulators and delayed G(1)/S transition, especially during the late embryonic stages, resulting in a reduced progenitor pool by birth. Deficiency in Smo function also causes reduced expression of the basic helix-loop-helix transcription repressor Hes1 and preferential elevation of the proneural gene Math5. In Smo and Math5 double knock-out mutants, the enhanced RGC production observed in Smo-deficient retinas is abolished, whereas defects in the G(1)/S transition persist, suggesting that Math5 mediates the Hh effect on neuronal fate specification but not on cell proliferation. These findings demonstrate that Hh signals regulate progenitor pool expansion primarily by promoting cell cycle progression and influence cell cycle exit and neuronal fates by controlling specific proneural genes. Together, these distinct cellular effects of Hh signaling in neural progenitor cells coordinate a balanced production of diverse neuronal cell types.
细胞外信号可深刻影响共同祖细胞产生各种神经元的过程。然而,外在信号协调祖细胞增殖、细胞周期退出和细胞命运选择的机制尚不清楚。在此,我们探讨刺猬索尼克(Hh)信号是否独立调节胚胎小鼠视网膜中祖细胞的增殖和神经元命运决定。在增殖的祖细胞而非新生的有丝分裂后神经元中条件性敲除关键的Hh信号成分平滑蛋白(Smo),会导致视网膜神经节细胞(RGC)显著增加,视锥光感受器前体细胞轻度增加,而不会显著影响其他早期产生的神经元细胞类型。此外,Smo缺陷型祖细胞表现出细胞周期调节因子的异常表达和G(1)/S期转换延迟,尤其是在胚胎后期,导致出生时祖细胞池减少。Smo功能缺陷还会导致碱性螺旋-环-螺旋转录抑制因子Hes1表达降低,神经源性基因Math5优先升高。在Smo和Math5双敲除突变体中,Smo缺陷型视网膜中观察到的RGC产生增加被消除,而G(1)/S期转换缺陷仍然存在,这表明Math5介导Hh对神经元命运特化的影响,但不影响细胞增殖。这些发现表明,Hh信号主要通过促进细胞周期进程来调节祖细胞池的扩张,并通过控制特定的神经源性基因来影响细胞周期退出和神经元命运。总之,Hh信号在神经祖细胞中的这些不同细胞效应协调了多种神经元细胞类型的平衡产生。