# Modulating One-Carbon Metabolism with B-Vitamins to Protect the Retinal Barrier and Prevent Retinal Degeneration

## TL;DR

This study shows that B-vitamin deficiency disrupts retinal blood barriers and causes retinal damage, but these effects can be reversed with B-vitamin supplementation.

## Contribution

The study demonstrates that B-vitamin supplementation can restore retinal vascular integrity disrupted by B-vitamin deficiency.

## Key findings

- B-vitamin deficiency caused elevated homocysteine and retinal vascular leakage in mice.
- B-vitamin supplementation restored retinal barrier integrity and reduced albumin leakage.
- Deficiency led to decreased tight junction proteins, indicating blood-retinal barrier disruption.

## Abstract

Background/Objectives: Vitamin B12 deficiency is increasingly recognized as a contributor in both vascular and neurodegenerative aging-related disorders. Its deficiency disrupts one-carbon metabolism, leading to impaired homocysteine (Hcy) cycling. Elevated Hcy is a well-established risk factor for vascular dysfunction. Previously, we established that elevated Hcy contributes to aging retinal diseases and plays a central role in blood retinal barrier (BRB) dysfunction. Building on this foundation, the present study examines how B-vitamin deficiency disrupts one-carbon metabolism and whether restoring these vitamins can serve as a preventive or therapeutic strategy. Since B-vitamins (B6, B9, and B12) are crucial cofactors in the metabolism of Hcy, we investigated how dietary changes in these vitamins affect serum Hcy levels and retinal vascular integrity in mice. Methods: C57BL/6- Wild-type (WT) and cbs+/− mice (Cystathionine Beta-Synthase heterozygotes, common mouse model for elevated Hcy) were fed specially formulated diets, which contained different levels of B-vitamins (normal, deficient (B-Vit (−)) or enriched (B-Vit (+)). Initially, two groups of mice were placed on either a normal or a deficient diet. After 12–16 weeks, the success of the diet regimes was confirmed by observing serum B12 deficiency in the B-Vit (−) group, along with elevated Hcy levels. Subsequently, a subgroup of the B-Vit (−) mice was switched to an enriched diet. The BRB integrity was evaluated in living mice using fluorescein angiography (FA), optical coherence tomography (OCT), and in the perfused mice retinas with Western blot analysis of leaked retinal albumin and tight junction proteins (occludin and ZO-1) levels. Results: The B-vitamin deficiency caused significant drop in serum vitamin B12 and an increase in plasma Hcy, leading to vascular leakage, altered retinal thickness, choroidal neovascular changes, increased retinal albumin leak, and decreased tight junction protein expression, indicating BRB disruption, which was restored with B-vitamin supplementation. Conclusions: a long-term deficiency of vitamins B6, B9, and B12 can lead to disruptions in the BRB. However, supplementation with these B-vitamins has the potential to reverse these effects and help maintain the integrity of BRB. This under-score the significance of one-carbon metabolism for retinal health and suggests that ensuring adequate levels of B-vitamins may aid in preventing aging retinal diseases with BRB disruption such as diabetic retinopathy and age-related macular degeneration.