Fig. 8

Schematic representation of the loss of vitamin A transport and its consequences on retinal homeostasis in Rbpr2−/− mice. Membrane receptor proteins mediate and recognize functions of RBP4 for ROL uptake. “Stimulated by retinoic acid 6” (STRA6) in the eye and “retinol binding protein receptor 2” (RBPR2, also known as STRA6Like) in liver and intestine are involved in retinol uptake and its coupling to RAR/RXR signaling in target cells. The specific RBP4 receptor in the liver (RBPR2) is thought to also mediate reverse transport of retinol (? Symbols), allowing a cycle of retinol between the circulation and liver. When compared to WT mice, Rbpr2−/− mice on either vitamin A-sufficient (VAS) or vitamin A-depleted diets (VAD) show reduced ocular retinoid concentrations and loss of visual function. Additionally, Rbpr2−/− mice show severe retinal phenotypes when challenged with a VAD diet. Thus, the retinal function impairment could be caused by a deficiency in systemic retinol transport to the eye, which is likely dependent on the function of RBPR2. The model was created with BioRender.com (https://help.biorender.com/en/articles/3619405-how-do-i-cite-biorender, accessed on 5 January 2022).