Retbindin Is Capable of Protecting Photoreceptors from Flavin-Sensitized Light-Mediated Cell Death In Vitro

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

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.

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

Purpose

Riboflavin and its cofactors are essential for cellular energy generation, responses to oxidative stress, and overall homeostasis. Retbindin is a novel retina-specific riboflavin binding protein essential for the maintenance of retinal flavin levels, but its function remains poorly understood. To further elucidate the function of retbindin in retinal health and disease, we evaluated its role in retinal degeneration in a cone-rod dystrophy model associated with the R172W mutation in the photoreceptor tetraspanin Prph2.

Methods

We performed structural, functional, and biochemical characterization of R172W-Prph2 mice with and without retbindin (Rtbdn-/-/Prph2R172W).

Results

Retbindin is significantly upregulated during degeneration in the R172W model, suggesting that retbindin plays a protective role in retinal degenerative diseases. This hypothesis was supported by our findings that R172W mice lacking retbindin (Rtbdn-/-/Prph2R172W) exhibit functional and structural defects in rods and cones that are significantly worse than in controls. Retinal flavin levels were also altered in the Rtbdn-/-/Prph2R172W retina. However, in contrast to the Rtbdn-/- retina which has reduced flavin levels compared to wild-type, Rtbdn-/-/Prph2R172W retinas exhibited elevated levels of riboflavin and the flavin cofactor FMN.

Conclusions

These results indicate that retbindin plays a protective role during retinal degeneration, but that its function is more complex than previously thought, and suggest a possible role for retbindin in protecting the retina from phototoxicity associated with unbound flavins. This study highlights the essential role of precisely regulated homeostatic mechanisms in photoreceptors, and shows that disruption of this metabolic balance can contribute to the degenerative process associated with other cellular defects.

Retbindin: A riboflavin Binding Protein, Is Critical for Photoreceptor Homeostasis and Survival in Models of Retinal Degeneration

The large number of inherited retinal disease genes (IRD), including the photopigment rhodopsin and the photoreceptor outer segment (OS) structural component peripherin 2 (PRPH2), has prompted interest in identifying common cellular mechanisms involved in degeneration. Although metabolic dysregulation has been shown to play an important role in the progression of the disease etiology, identifying a common regulator that can preserve the metabolic ecosystem is needed for future development of neuroprotective treatments. Here, we investigated whether retbindin (RTBDN), a rod-specific protein with riboflavin binding capability, and a regulator of riboflavin-derived cofactors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), is protective to the retina in different IRD models; one carrying the P23H mutation in rhodopsin (which causes retinitis pigmentosa) and one carrying the Y141C mutation in Prph2 (which causes a blended cone-rod dystrophy). RTBDN levels are significantly upregulated in both the rhodopsin (Rho)^P23H/+ and Prph2^Y141C/+ retinas. Rod and cone structural and functional degeneration worsened in models lacking RTBDN. In addition, removing Rtbdn worsened other phenotypes, such as fundus flecking. Retinal flavin levels were reduced in Rho^P23H/+/Rtbdn^−/− and Prph2^Y141C/+/Rtbdn^−/− retinas. Overall, these findings suggest that RTBDN may play a protective role during retinal degenerations that occur at varying rates and due to varying disease mechanisms.

Flavin Imbalance as an Important Player in Diabetic Retinopathy

The retina and RPE together constitute the most metabolically active ecosystem in the body, harboring high levels of flavins. Although diabetic patients have been reported to suffer from riboflavin deficiency and use of flavins as nutritional interventions to combat diabetic insult on other tissues have been investigated, such attempts have never been tested for the retina to avoid diabetic retinopathy. Furthermore, the role of flavins in pathophysiology of the retina and RPE has mostly been overlooked. Herein, we review the impact of flavins on various clinical manifestations of diabetic retinopathy and discuss possible ways to address them.