Implications of NAD⁺ Metabolism in the Aging Retina and Retinal Degeneration

Figure 1

Overview of NAD⁺ biosynthesis pathways. NAD⁺ is mainly synthesized via the Preiss-Handler, de novo, and salvage pathways using tryptophan, nicotinic acid, and nicotinamide, respectively. The red dotted arrow highlights key enzyme alterations that underlie decreased NAD⁺ availability during disease conditions. QA: quinolinic acid; QAPRT: quinolinate phosphoribosyltransferase; NAPRT: nicotinate phosphoribosyltransferase; NAAD: nicotinic acid adenine dinucleotide; NADS: NAD synthase; NAMPT: nicotinamide phosphoribosyltransferase; NMNAT: nicotinamide mononucleotide adenylyltransferase; NAMN: nicotinic acid mononucleotide; NMN: nicotinamide mononucleotide; NAM: nicotinamide; NR: nicotinamide ribose; NRK: nicotinamide riboside kinase; LCA: Leber congenital amaurosis; DR: diabetic retinopathy; AMD: age-related macular degeneration; RVO: retinal vein occlusion; TON: traumatic optic neuropathy; NK: neurotrophic keratopathy.