Genetic Dissection of Dual Roles for the Transcription Factor six7 in Photoreceptor Development and Patterning in Zebrafish.
Genetic Dissection of Dual Roles for the Transcription Factor six7 in Photoreceptor Development and Patterning in Zebrafish.
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DOI：
10.1371/journal.pgen.1005968
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发表时间：
2016-04
期刊：
影响因子：
4.5
通讯作者：
Fadool JM
中科院分区：
文献类型：
--
作者：
Sotolongo-Lopez M;Alvarez-Delfin K;Saade CJ;Vera DL;Fadool JM
特定物种的视觉系统高度适应于传达对生存至关重要的详细生态和行为信息。Opsins的结构突变对光谱敏感性和环境适应的影响已被详细研究，但缺乏关于基因调控网络变化对锥体亚型多样性的潜在影响的知识，以及在许多昼夜和夜间物种中观察到的视锥杆/视锥比率的变化。利用锥体占优势的斑马鱼中的光感受器模式，我们发现了两个独立的机制，即正弦眼孔同源盒同系物7(Six7)调节光感受器的发育。在基因筛查中，我们分离了大量杆状幼虫(Ljrp23aHub)突变，该突变导致正常幼虫中杆状突变体的数量增加和分布均匀。序列分析、使用TALENS的基因组编辑和基因敲除策略证实ljrp23aHub是Six3的硬骨同源物Six7的一个亚型等位基因，在前脑模式和Opsins的表达中具有已知的作用。基于缺乏预测的蛋白质编码变化和转录起始点上游保守元件的缺失，提出了顺式调节突变作为Ljrp23aHub中Si67表达降低的基础。亚型和敲除等位基因表型的比较为光感受器发育中的两个独立作用提供了证据。EDU和PH3标记表明，视杆细胞数量的增加与光感受器前体细胞的有丝分裂延长有关，TUNEL表明，缺乏绿色敏感的视锥细胞是视锥前体细胞死亡的结果。这些数据为非哺乳动物脊椎动物的光感受器的规范和模式的基本基因列表增加了6个，但这些基因的数量很少，但不断增加，并突出了转录调控的变化，它是光感受器跨物种差异的潜在来源。当图像聚焦在神经视网膜上时，视觉就开始了，在那里，视杆细胞和视锥细胞的光感受器将光转化为大脑的电信号。大多数鱼类、蜥蜴和鸟类视网膜中的4个视锥亚型提供丰富的色觉。相比之下，大多数哺乳动物的视网膜更能适应昏暗的光线条件，视杆细胞的数量远远超过稀疏和多样性较差的视锥细胞亚型。然而，我们对光感受器发育的理解主要基于哺乳动物模型的发现，无法解释大多数脊椎动物锥体亚型的巨大多样性以及杆状和锥体数量的差异。利用富含视锥的斑马鱼视网膜，我们确定了一种核因子，它可以抑制视杆的数量，对于哺乳动物中不存在的视锥亚型的发育是必不可少的。结合以前的研究，这些发现为维持视锥主导的视网膜的适应机制提供了洞察。
The visual system of a particular species is highly adapted to convey detailed ecological and behavioral information essential for survival. The consequences of structural mutations of opsins upon spectral sensitivity and environmental adaptation have been studied in great detail, but lacking is knowledge of the potential influence of alterations in gene regulatory networks upon the diversity of cone subtypes and the variation in the ratio of rods and cones observed in numerous diurnal and nocturnal species. Exploiting photoreceptor patterning in cone-dominated zebrafish, we uncovered two independent mechanisms by which the sine oculis homeobox homolog 7 (six7) regulates photoreceptor development. In a genetic screen, we isolated the lots-of-rods-junior (ljrp23ahub) mutation that resulted in an increased number and uniform distribution of rods in otherwise normal appearing larvae. Sequence analysis, genome editing using TALENs and knockdown strategies confirm ljrp23ahub as a hypomorphic allele of six7, a teleost orthologue of six3, with known roles in forebrain patterning and expression of opsins. Based on the lack of predicted protein-coding changes and a deletion of a conserved element upstream of the transcription start site, a cis-regulatory mutation is proposed as the basis of the reduced expression of six7 in ljrp23ahub. Comparison of the phenotypes of the hypomorphic and knock-out alleles provides evidence of two independent roles in photoreceptor development. EdU and PH3 labeling show that the increase in rod number is associated with extended mitosis of photoreceptor progenitors, and TUNEL suggests that the lack of green-sensitive cones is the result of cell death of the cone precursor. These data add six7 to the small but growing list of essential genes for specification and patterning of photoreceptors in non-mammalian vertebrates, and highlight alterations in transcriptional regulation as a potential source of photoreceptor variation across species. Vision begins when an image is focused on the neural retina where rod and cone photoreceptors convert light into the electrical signals of the brain. The 4 cone subtypes in retinas of the majority of fishes, lizards and birds, provide rich color vision. In contrast, retinas of most mammals are better adapted for dim light conditions with rods vastly outnumbering the sparse and less diverse cone subtypes. However, our understanding of photoreceptor development largely based on findings from mammalian models fails to explain the tremendous diversity of cone subtypes and variation of rod and cone numbers across the majority of vertebrate species. Taking advantage of the cone-rich zebrafish retina, we identified a nuclear factor that suppresses the number of rods and is essential for the development of a cone subtype not present in mammals. Combined with prior studies, the findings provide insight into adaptive mechanisms underlying maintenance of a cone-dominated retina.