Genetic Dissection of Photoreceptor Subtype Specification by the Drosophila melanogaster Zinc Finger Proteins Elbow and No ocelli

The elbow/no ocelli (elb/noc) complex of Drosophila melanogaster encodes two paralogs of the evolutionarily conserved NET family of zinc finger proteins. These transcriptional repressors share a conserved domain structure, including a single atypical C2H2 zinc finger. In flies, Elb and Noc are important for the development of legs, eyes and tracheae. Vertebrate NET proteins play an important role in the developing nervous system, and mutations in the homolog ZNF703 human promote luminal breast cancer. However, their interaction with transcriptional regulators is incompletely understood. Here we show that loss of both Elb and Noc causes mis-specification of polarization-sensitive photoreceptors in the ‘dorsal rim area’ (DRA) of the fly retina. This phenotype is identical to the loss of the homeodomain transcription factor Homothorax (Hth)/dMeis. Development of DRA ommatidia and expression of Hth are induced by the Wingless/Wnt pathway. Our data suggest that Elb/Noc genetically interact with Hth, and we identify two conserved domains crucial for this function. Furthermore, we show that Elb/Noc specifically interact with the transcription factor Orthodenticle (Otd)/Otx, a crucial regulator of rhodopsin gene transcription. Interestingly, different Elb/Noc domains are required to antagonize Otd functions in transcriptional activation, versus transcriptional repression. We propose that similar interactions between vertebrate NET proteins and Meis and Otx factors might play a role in development and disease.

黑腹果蝇（Drosophila melanogaster）的肘突/无单眼（elbow/no ocelli，elb/noc）复合体编码两类进化保守的锌指蛋白NET家族的旁系同源物。这类转录阻遏蛋白共享保守的结构域组成，包含一个非典型C2H2锌指结构。在果蝇中，Elb与Noc对腿部、眼部以及气管的发育具有关键调控作用。脊椎动物的NET蛋白在发育中的神经系统中发挥关键调控作用，而人类同源基因ZNF703发生突变可促进管腔型乳腺癌的发生发展。然而，目前学界对其与转录调控因子的相互作用机制仍未完全阐明。本研究证实，同时缺失Elb与Noc会导致果蝇视网膜"背缘区（dorsal rim area，DRA）"内偏振敏感感光细胞的分化异常，该表型与同源域转录因子Homothorax（Hth）/dMeis缺失所引发的表型完全一致。背缘区小眼的发育以及Hth的表达均由Wingless/Wnt信号通路诱导调控。本研究数据表明，Elb/Noc与Hth存在遗传互作，并鉴定出两个对该功能至关重要的保守结构域。此外，本研究发现Elb/Noc可与转录因子Orthodenticle（Otd）/Otx特异性结合，而后者是视紫红质基因转录的关键调控因子。值得注意的是，Elb/Noc通过不同的结构域分别拮抗Otd在转录激活与转录阻遏过程中的功能。我们推测，脊椎动物NET蛋白与Meis、Otx因子之间的类似互作，可能在发育进程与疾病发生中发挥重要作用。