The Neuroprotective Role of Retbindin, a Metabolic Regulator in the Neural Retina

Abstract

Dysregulation of retinal metabolism is emerging as one of the major reasons for many inherited retinal diseases (IRDs), a leading cause of blindness worldwide. Thus, the identification of a common regulator that can preserve or revert the metabolic ecosystem to homeostasis is a key step in developing a treatment for different forms of IRDs. Riboflavin (RF) and its derivatives (flavins), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), are essential cofactors for a wide range of cellular metabolic processes; hence, they are particularly critical in highly metabolically active tissues such as the retina. Patients with RF deficiency (ariboflavinosis) often display poor photosensitivity resulting in impaired low-light vision. We have identified a novel retina-specific RF binding protein called retbindin (Rtbdn), which plays a key role in retaining flavin levels in the neural retina. This role is mediated by its specific localization at the interface between the neural retina and retinal pigment epithelium (RPE), which is essential for metabolite and nutrient exchange. As a consequence of this vital function, Rtbdn's role in flavin utilization and metabolism in retinal degeneration is discussed. The principal findings are that Rtbdn helps maintain high levels of retinal flavins, and its ablation leads to an early-onset retinal metabolic dysregulation, followed by progressive degeneration of rod and cone photoreceptors. Lack of Rtbdn reduces flavin levels, forcing the neural retina to repurpose glucose to reduce the production of free radicals during ATP production. This leads to metabolic breakdown followed by retinal degeneration. Assessment of the role of Rtbdn in several preclinical retinal disease models revealed upregulation of its levels by several folds prior to and during the degenerative process. Ablation of Rtbdn in these models accelerated the rate of retinal degeneration. In agreement with these in vivo studies, we have also demonstrated that Rtbdn protects immortalized cone photoreceptor cells (661W cells) from light damage in vitro. This indicates that Rtbdn plays a neuroprotective role during retinal degeneration. Herein, we discussed the specific function of Rtbdn and its neuroprotective role in retinal metabolic homeostasis and its role in maintaining retinal health.

Keywords: flavins; neuroprotection; retbindin; retinal metabolism; retinal regeneration; riboflavin.

FIGURE 1

A schematic showing the conversion of riboflavin to flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Bio Render was used to generate the schematic diagram showing the conversion of riboflavin to FMN and FAD by riboflavin kinase and FAD synthase, respectively. Two molecules of ATP are utilized for the conversion of riboflavin to FAD. The reverse conversion of FAD to FMN by FAD diphosphatase and FMN to riboflavin by FMN phosphohydrolase is also shown. The forward conversions are more favorable than the reverse conversion. Re-drawn from Tolomeo et al. (2020). Created with BioRender.

FIGURE 2

Ablation of Rtbdn accelerates retinal phenotype in several models of degeneration. Histologic analysis at the light microscopic level of retinal sections from WT and Rtbdn^−/− (A), Rho^P23H/+ and Rtbdn^−/−/Rho^P23H/+ (B), Prph^Y141C/+ and Rtbdn^−/−/Prph2^Y141C/+ (C), and Prph^R172W and Rtbdn^−/−/Prph2^R172W (D) mice at P30. Arrowheads point to the degenerated ONL and outer segments shortening in the Rtbdn^−/−/Rho^P23H/+ model. Scale bar: 50 µm. RPE, retinal pigment epithelium; OS, outer segment; ONL, outer nuclear layer; INL, inner nuclear layer. Images were taken from paraffin-embedded eyes of the relevant genotypes used by Genc et al. (2020a) and Genc et al. (2020b).

FIGURE 3

Ablation of Rtbdn exacerbated the changes in outer segment ultrastructure in models of retinal degeneration. Effects of Rtbdn ablation at the ultrastructural level in the retinas of disease models. (A–D) Representative TEM images of RPE and OS at two magnifications (5,000× for the upper image and 25,000×for the lower image for each genotype) are presented for the indicated genotypes at P30. Arrowheads point to the shorter and misoriented OSs discs, disorganized and misaligned, or abnormal membrane accumulation when Rtbdn is ablated. Scale bar: 2 µm (A), 0.5 µm (B). RPE, retinal pigment epithelium; OS, outer segment. Images were taken from paraffin-embedded eyes of the relevant genotypes used by Genc et al. (2020a) and Genc et al. (2020b).