Abstract
Purpose :
The eyes acquire vitamin A derived from either the extrinsic or intrinsic pathway, distributing vitamin A from intestinal enterocytes or from hepatic stores. Vitamin A deficiency or excess are commonly associated with blinding and inflammatory diseases. However, we have a limited understanding of mechanisms controlling the distribution pathways. We used a genetic dissection approach in transgenic mice to investigate how ocular homeostasis is perturbed under different dietary supply conditions.
Methods :
We utilized mice with mutations in intestine-specific transcription factor (ISX) or stimulated by retinoic acid gene 6 (STRA6) which have been identified as gatekeepers of the extrinsic and intrinsic pathways, respectively. Mice were supplemented with preformed vitamin A (4,000 IU/kg) or β-carotene (25 mg/kg) for 7 wks. Scotopic and photopic responses were then recorded from mice that were dark adapted overnight using ERG. Retinoid concentrations were measured by HPLC, and retinoid-dependent processes were evaluated by gene and protein expression analyses. Rhodopsin and M-opsin levels were evaluated by immunohistochemistry in retinal cross sections and whole mounts.
Results :
We observed Isx-/- mice possessed normal ocular vitamin A concentrations but acquired high levels of retinoids in peripheral tissues in a STRA6-dependent manner. Ocular retinoid levels were significantly lower in Stra6-/- compared to WT and Isx-/- mice. Stra6-/- mice displayed significant reduced photopic responses and M-opsin staining. Interestingly, genetic deletion of ISX partially rescued ocular vitamin A deficiency in Isx-/-/Stra6-/- mice. Remarkably, photopic ERG responses were significantly improved in Isx-/-/Stra6-/- double-knockout mice compared to that of the Stra6-/- counterparts. The double-knockout mice displayed a significant recovery of cone photoreceptors. However, the rescue of vision came at the expense of a massive accumulation of vitamin A in other tissues, demonstrating that vitamin A is randomly distributed when present in excessive amounts.
Conclusions :
Our analysis in different genotypes revealed STRA6 is critical for maintaining ocular vitamin A homeostasis and cone photoreceptor function. The partial rescue of vision in Isx-/-/Stra6-/- double-knockout mice was associated with hyper-vitaminosis A in most peripheral tissues, demonstrating the requirement of an eye-specific delivery pathway of the essential nutrient.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.