Ablation of the riboflavin-binding protein retbindin reduces flavin levels and leads to progressive and dose-dependent degeneration of rods and cones

Abstract

The interface between the neural retina and the retinal pigment epithelium (RPE) is critical for several processes, including visual pigment regeneration and retinal attachment to the RPE. One of its most important functions is the exchange of metabolites between the photoreceptors and RPE because photoreceptor cells have very high energy demands, largely satisfied by oxidative metabolism. The riboflavin (RF) cofactors, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), are two key cofactors involved in oxidative metabolism. We have previously shown that retbindin is a photoreceptor-specific RF-binding protein exclusively expressed in the rods and present in the interphotoreceptor matrix at the interface between the RPE and photoreceptor outer segments. Here, we show that retbindin ablation in mice causes a retinal phenotype characterized by time- and dose-dependent declines in rod and cone photoreceptor functions as early as 120 days of age. Whereas minor retinal ultrastructural defects were observed at all ages examined, a significant decline occurred in photoreceptor nuclei at 240 days of age (∼36.8% rods and ∼19.9% cones). Interestingly, significant reductions in FAD and FMN levels were observed before the onset of degeneration (∼46.1% FAD and ∼45% FMN). These findings suggest that the reduced levels of these flavins result in the disruption of intracellular mechanisms, leading to photoreceptor cell death. Altogether, our results suggest that retbindin is a key player in the acquisition and retention of flavins in the neural retina, warranting future investigation into retbindin's role in photoreceptor cell death in models of retinal degenerative disorders.

Pluripotent Stem Cells for the Treatment of Retinal Degeneration: Current Strategies and Future Directions

“…Though many mutations are rod-specific, death of cones generally ensues (Wert et al, 2014). Many suggest that rods provide critical trophic components into the retinal environment that ensure cone survival (Kelley et al, 2017) and because rods are the major cellular component of the photoreceptor layer, they contribute essential infrastructure for the cones (Steinberg, 1994;Ripps, 2002). Nevertheless, RP has demonstrated that cone survival is contingent upon rod photoreceptor health.…”

Elimination of a Retinal Riboflavin Binding Protein Exacerbates Degeneration in a Model of Cone-Rod Dystrophy

“…3 Retbindin is secreted into the IPM and peripherally anchored to the plasma membrane via electrostatic forces, and we showed that mice lacking retbindin (Rtbdn −/− ) exhibit retinal degeneration as well as defects in rod and cone function. 2,4 Our previous studies using both degenerative models that lack rods but retain some cones and models in which GFP is knocked into cells that express retbindin have shown that retbindin expression is limited to rods. 2,4 Retbindin shares significant homology with the riboflavin binding domain of chicken oviduct riboflavin binding protein (RBP).…”

“…2,4 Our previous studies using both degenerative models that lack rods but retain some cones and models in which GFP is knocked into cells that express retbindin have shown that retbindin expression is limited to rods. 2,4 Retbindin shares significant homology with the riboflavin binding domain of chicken oviduct riboflavin binding protein (RBP). 1,2 Subsequently, we determined that retbindin binds riboflavin in vitro 2 and that flavin levels were reduced in Rtbdn −/− mice.…”

“…1,2 Subsequently, we determined that retbindin binds riboflavin in vitro 2 and that flavin levels were reduced in Rtbdn −/− mice. 4 Riboflavin is not synthesized by the body and is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential enzymatic cofactors. [5][6][7] FMN and FAD are involved in many processes, including antioxidant systems (where they are cofactors for glutathione reductase) and energy generation.…”

A Plasma Metabolomic Profiling of Exudative Age-Related Macular Degeneration Showing Carnosine and Mitochondrial Deficiencies

“…It is therefore surprising that three of the metabolites that accumulate in AMD are electrons suppliers to the respiratory chain through such electron transfer flavoproteins. Interestingly, reduced levels of riboflavin cofactors are known to induce photoreceptor cell death in mice [20] and alteration of retinal autofluorescence of mitochondrial flavoproteins has been evidenced in patients with AMD [21]. Our results suggest that this mitochondrial flavoproteins dysfunctionality in AMD may be indirectly perceptible in the blood of patients through the accumulation of their substrates.…”