Distribution of Mammalian-Like Melanopsin in Cyclostome Retinas Exhibiting a Different Extent of Visual Functions

Mammals contain 1 melanopsin (Opn4) gene that is expressed in a subset of retinal ganglion cells to serve as a photopigment involved in non-image-forming vision such as photoentrainment of circadian rhythms. In contrast, most nonmammalian vertebrates possess multiple melanopsins that are distributed in various types of retinal cells; however, their functions remain unclear. We previously found that the lamprey has only 1 type of mammalian-like melanopsin gene, which is similar to that observed in mammals. Here we investigated the molecular properties and localization of melanopsin in the lamprey and other cyclostome hagfish retinas, which contribute to visual functions including image-forming vision and mainly to non-image-forming vision, respectively. We isolated 1 type of mammalian-like melanopsin cDNA from the eyes of each species. We showed that the recombinant lamprey melanopsin was a blue light-sensitive pigment and that both the lamprey and hagfish melanopsins caused light-dependent increases in calcium ion concentration in cultured cells in a manner that was similar to that observed for mammalian melanopsins. We observed that melanopsin was distributed in several types of retinal cells, including horizontal cells and ganglion cells, in the lamprey retina, despite the existence of only 1 melanopsin gene in the lamprey. In contrast, melanopsin was almost specifically distributed to retinal ganglion cells in the hagfish retina. Furthermore, we found that the melanopsin-expressing horizontal cells connected to the rhodopsin-containing short photoreceptor cells in the lamprey. Taken together, our findings suggest that in cyclostomes, the global distribution of melanopsin in retinal cells might not be related to the melanopsin gene number but to the extent of retinal contribution to visual function.

哺乳动物仅携带1个黑视蛋白（melanopsin，Opn4）基因，该基因在部分视网膜神经节细胞中表达，作为光色素参与非成像视觉功能，例如介导昼夜节律的光同步过程。与之相反，多数非哺乳类脊椎动物拥有多个黑视蛋白基因，这些蛋白分布于多种视网膜细胞类型中，但其具体功能目前尚不明确。我们此前的研究发现，七鳃鳗仅存在1类类似哺乳动物的黑视蛋白基因，这一点与哺乳动物的情况一致。本研究针对七鳃鳗与另一类圆口纲动物盲鳗的视网膜展开研究，分别探究其中黑视蛋白的分子特性与表达定位——二者的视觉功能分别涵盖成像视觉与以非成像视觉为主的视觉通路。我们从两个物种的眼部组织中均分离得到了1类类似哺乳动物的黑视蛋白互补DNA（cDNA）。实验结果表明，重组七鳃鳗黑视蛋白是一种蓝光敏感性色素；且七鳃鳗与盲鳗的黑视蛋白均可在培养细胞中引发依赖光照的钙离子浓度升高，该现象与哺乳动物黑视蛋白的作用模式一致。尽管七鳃鳗仅携带1个黑视蛋白基因，但我们观察到其视网膜内的黑视蛋白分布于多种视网膜细胞类型中，包括水平细胞与神经节细胞。与之相反，盲鳗视网膜内的黑视蛋白几乎仅特异性分布于视网膜神经节细胞中。此外，我们还发现七鳃鳗体内表达黑视蛋白的水平细胞可与含有视紫红质的短型感光细胞形成连接。综合以上研究结果，我们认为在圆口纲动物中，黑视蛋白在视网膜细胞中的整体分布模式可能与黑视蛋白基因数量无关，而是取决于视网膜对视觉功能的贡献程度。