Retbindin Dynamically Redistributes to Preserve Retinal Flavin-Dependent Homeostasis in Response to Riboflavin Availability and Circadian Demand.

Flavin cofactors (FMN and FAD) are essential mediators of mitochondrial respiration, antioxidant defense, and redox signaling in metabolically active tissues. The neural retina exhibits one of the highest energetic and redox demands in the body, yet mechanisms governing local flavin homeostasis remain poorly defined. Retbindin (RTBDN), a retina-specific riboflavin-binding protein, has previously been shown to regulate total retinal flavin levels and photoreceptor survival. Here, we demonstrate that RTBDN abundance and subcellular localization are dynamically modulated by developmental stage, riboflavin availability, and the light-dark cycle. RTBDN expression emerges during outer segment maturation, peaks in early adulthood, and declines with aging in parallel with retinal flavin content. Fasting reduces retinal flavin levels without altering total RTBDN protein abundance but induces redistribution of RTBDN from the outer segment-RPE interface toward cone-associated domains. Riboflavin repletion restores its localization. Similarly, RTBDN redistributes around cones during the dark phase and returns to outer segment tips at light onset, independent of changes in total protein levels. These findings identify RTBDN as a spatially dynamic regulator of flavin-dependent homeostasis that adapts to nutrient flux and circadian metabolic demand. This adaptive redistribution may represent a protective mechanism to preserve cone function under fluctuating energetic conditions and suggests that disruption of flavin homeostasis may contribute to cone degeneration.