Sars International Centre for Marine Molecular Biology, University of Bergen, Thormøhlensgate 55, 5008, Bergen, Norway.
Department of Biology, University of Kentucky, Thomas Hunt Morgan Building, 675 Rose Street, Lexington, KY, 40508, USA.
BMC Ecol Evol. 2021 Nov 29;21(1):215. doi: 10.1186/s12862-021-01939-x.
The process of photoreception in most animals depends on the light induced isomerization of the chromophore retinal, bound to rhodopsin. To re-use retinal, the all-trans-retinal form needs to be re-isomerized to 11-cis-retinal, which can be achieved in different ways. In vertebrates, this mostly includes a stepwise enzymatic process called the visual cycle. The best studied re-isomerization system in protostomes is the rhodopsin-retinochrome system of cephalopods, which consists of rhodopsin, the photoisomerase retinochrome and the protein RALBP functioning as shuttle for retinal. In this study we investigate the expression of the rhodopsin-retinochrome system and functional components of the vertebrate visual cycle in a polyplacophoran mollusk, Leptochiton asellus, and examine the phylogenetic distribution of the individual components in other protostome animals.
Tree-based orthology assignments revealed that orthologs of the cephalopod retinochrome and RALBP are present in mollusks outside of cephalopods. By mining our dataset for vertebrate visual cycle components, we also found orthologs of the retinoid binding protein RLBP1, in polyplacophoran mollusks, cephalopods and a phoronid. In situ hybridization and antibody staining revealed that L. asellus retinochrome is co-expressed in the larval chiton photoreceptor cells (PRCs) with the visual rhodopsin, RALBP and RLBP1. In addition, multiple retinal dehydrogenases are expressed in the PRCs, which might also contribute to the rhodopsin-retinochrome system.
We conclude that the rhodopsin-retinochrome system is a common feature of mollusk PRCs and predates the origin of cephalopod eyes. Our results show that this system has to be extended by adding further components, which surprisingly, are shared with vertebrates.
大多数动物的光感受过程依赖于视蛋白结合的发色团视黄醛的光诱导异构化。为了再利用视黄醛，需要将全反式视黄醛重新异构化为 11-顺式视黄醛，这可以通过不同的方式实现。在脊椎动物中，这主要包括一个称为视觉循环的逐步酶促过程。在原口动物中，研究最多的再异构化系统是头足类动物的视蛋白-视黄质系统，它由视蛋白、光异构酶视黄质和作为视黄醇穿梭蛋白的 RALBP 组成。在这项研究中，我们研究了多板纲软体动物 Leptochiton asellus 中视蛋白-视黄质系统的表达以及脊椎动物视觉循环的功能成分，并检查了单个成分在其他原口动物中的系统发育分布。
基于树的同源性分配表明，头足类动物的视黄质和 RALBP 的同源物存在于头足类动物以外的软体动物中。通过在我们的数据集中文挖掘脊椎动物视觉循环成分，我们还在多板纲软体动物、头足类动物和磷虾目动物中发现了视黄醇结合蛋白 RLBP1 的同源物。原位杂交和抗体染色显示，L. asellus 视黄质与视觉视蛋白、RALBP 和 RLBP1 在幼虫石鳖光感受器细胞（PRC）中共表达。此外，多个视网膜脱氢酶在 PRC 中表达，这也可能对视蛋白-视黄质系统有贡献。
我们得出结论，视蛋白-视黄质系统是软体动物 PRC 的共同特征，早于头足类动物眼睛的起源。我们的结果表明，这个系统必须通过添加其他与脊椎动物共享的成分来扩展。
