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

Abstract

Objective

Nicotinamide 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.

Methods

Type 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.

Results

Hyperglycemia 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.

Conclusion

Hyperglycemia-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.