Department of Medicine, Medical University of South Carolina, Charleston, SC 29425, USA.
Glaucoma Research Chair, Department of Ophthalmology, College of Medicine, King Saud University, Riyadh 11411, Saudi Arabia.
Cells. 2020 Apr 29;9(5):1099. doi: 10.3390/cells9051099.
Dietary vitamin A/all- retinol/ROL plays a critical role in human vision. ROL circulates bound to the plasma retinol-binding protein (RBP4) as RBP4-ROL. In the eye, the STRA6 membrane receptor binds to circulatory RBP4 and internalizes ROL. STRA6 is, however, not expressed in systemic tissues, where there is high affinity RBP4 binding and ROL uptake. We tested the hypothesis that the second retinol binding protein 4 receptor 2 (Rbpr2), which is highly expressed in systemic tissues of zebrafish and mouse, contains a functional RBP4 binding domain, critical for ROL transport. As for STRA6, modeling and docking studies confirmed three conserved RBP4 binding residues in zebrafish Rbpr2. In cell culture studies, disruption of the RBP4 binding residues on Rbpr2 almost completely abolished uptake of exogenous vitamin A. CRISPR-generated RBP4 domain zebrafish mutants showed microphthalmia, shorter photoreceptor outer segments, and decreased opsins, which were attributed to impaired ocular retinoid content. Injection of WT-Rbpr2 mRNA into mutant or all- retinoic acid treatment rescued the mutant eye phenotypes. In conclusion, zebrafish Rbpr2 contains a putative extracellular RBP4-ROL ligand-binding domain, critical for yolk vitamin A transport to the eye for ocular retinoid production and homeostasis, for photoreceptor cell survival.
饮食中的维生素 A/全视黄醇/视黄醇(ROL)在人类视觉中起着至关重要的作用。ROL 与视黄醇结合蛋白 4(RBP4)结合成 RBP4-ROL 循环存在于血浆中。在眼睛中,STRA6 膜受体与循环中的 RBP4 结合并内化 ROL。然而,STRA6 在系统组织中不表达,在这些组织中,RBP4 具有高亲和力的结合和 ROL 摄取。我们检验了这样一个假设,即第二个视黄醇结合蛋白 4 受体 2(Rbpr2)在斑马鱼和小鼠的系统组织中高度表达,它含有一个功能性的 RBP4 结合域,对于 ROL 转运至关重要。与 STRA6 一样,建模和对接研究证实了斑马鱼 Rbpr2 中有三个保守的 RBP4 结合残基。在细胞培养研究中,破坏 Rbpr2 上的 RBP4 结合残基几乎完全消除了外源性维生素 A 的摄取。CRISPR 生成的 RBP4 结构域斑马鱼突变体表现出小眼症,光感受器外节缩短,视蛋白减少,这归因于眼部类视黄醇含量受损。将 WT-Rbpr2 mRNA 注射到突变体或全反式视黄酸处理的突变体中,可挽救突变体的眼部表型。总之,斑马鱼 Rbpr2 含有一个假定的细胞外 RBP4-ROL 配体结合域,对于卵黄维生素 A 向眼部的转运以及眼内类视黄醇的产生和稳态、光感受器细胞的存活至关重要。