Hyperglycemia-reduced NAD+ biosynthesis impairs corneal epithelial wound healing in diabetic mice

Abstract

ObjectiveNicotinamide adenine dinucleotide (NAD) is an essential molecule participating in multiple physiological and pathophysiological processes. In diabetic cornea, the NAD+-consuming enzyme SIRT1 was down-regulated and contributed to the delayed wound healing. However, the impact of hyperglycemia on corneal NAD+ biosynthesis remained elusive. This study was to investigate the relationship of NAD+ biosynthesis and the delayed corneal wound healing in diabetic mice. MethodsType 1 diabetes mellitus (DM) mice were induced by streptozotocin and corneal epithelial wound healing models were constructed by epithelial scraping. The NAD+ contents of corneal epithelium were measured using the NAD/NADH quantification kit. Expression of key enzymes involved in the NAD+ biosynthesis in type 1 DM mice and type 2 DM patients were analyzed. The nicotinamide phosphoribosyltransferase (NAMPT)-specific siRNA and the selective inhibitor FK866 were used to achieve the blockade of NAMPT, whereas exogenous NAD+ and its precursors were replenished to the corneal epithelial cells and DM mice. ResultsHyperglycemia attenuated NAD+ content and NAMPT expression in the corneal epithelium of both type 1 DM mice and type 2 DM patients. Local knockdown of NAMPT by siRNA or FK866 consistently recapitulated the delayed corneal epithelial wound healing in normal mice. Moreover, NAD+ replenishment recovered the impaired proliferation and migration capacity by either FK866 or high glucose treatment in cultured corneal epithelial cells. Furthermore, in DM mice, NAD+ and its precursors nicotinamide mononucleotide and nicotinamide riboside also facilitated corneal epithelial and nerve regeneration, accompanied with the recovered expression of SIRT1 and phosphorylated EGFR, AKT, and ERK1/2 in epithelium and corneal sensitivity. ConclusionHyperglycemia-reduced NAD+ biosynthesis and contributed to the impaired epithelial wound healing in DM mice. The replenishment of NAD+ and its precursors facilitated diabetic corneal wound healing and nerve regeneration, which may provide a novel therapeutic strategy for the treatment of diabetic corneal complications.