Nicotinamide Mononucleotide Improves Lens Elasticity By Preventing Oxidative Stress

Loss of crystalline lens elasticity with age is a major cause of presbyopia. Oxidative damage can increase lens stiffness, although the underlying mechanisms have not yet been studied. Here, we investigated the potential association between oxidative stress and age-related lens stiffening, and the efficacy of nicotinamide mononucleotide (NMN) to reverse oxidative damage of lens proteins and improve lens elasticity. 

We performed ex vivo and in vivo experiments on middle-aged (10-month-old) C57BL/6J mice and young (2-month-old) C57BL/6J mice, including lens ex vivo culture with H₂O₂ or NMN, and in vivo anterior chamber injection with NMN.

We used coverslips compressive test to determine lens biomechanical compressive properties. To identify the association of oxidative stress and lens elasticiy, gene expression pattern was profiled by RNA-Sequencing and the oxidative stress-related genes were selected by overlapping with genes intersected from GO consortium website. Redox states within lenses were assessed with determination of ROS  production with dye DHR and expression and activities of antioxidative enzymes with qRT-PCR and commercial kits. Contents of glutathione and NAD/NADH were also detected. We established an ex vivo oxidative stress model by incubating lenses with H₂O₂. SDS-PAGE and Coomassie brilliant blue staining were used for water-soluble lens protein analysis.

 The axial and equatorial strains were significantly lower in lens of middle-aged C57BL/6J mice than young C57BL/6J mice, accompanied by a higher content of ROS and increased proportion of high molecular weight (HMW) aggregates in lens water-soluble protein. Incubation of young mouse lens with H₂O₂ decreased the contents and activities of antioxidants and caused accumulation of ROS and HMW aggregates, resulted in remarkable reduction of strains. However, these impaired antioxidative capacity and aggregation of HMW were improved by the NMN administration. Lens incubation ex vivo and anterior chamber injection in vivo with NMN both showed amelioration to lens strains. 

Our study provides strong evidence that, oxidative stress is involved in the process of age-related lens stiffening, and the increased proportion of HMW proteins is a key player. NMN acts as a critical antioxidant that can suppress accumulation of HMW aggregates and improve lens stiffness. These findings provide important insights into the mechanisms of lens stiffening and suggest that the antioxidant supplementation of NMN has the prospect to sever as an agent in the treatment of lens sclerosis-related deficits including presbyopia.