Circulating eNAMPT in Glaucoma: A Semi-Quantitative Plasma Analysis Before and After Nicotinamide Supplementation

Glaucoma is a neurodegenerative disease characterized by progressive retinal ganglion cell degeneration. Nicotinamide supplementation has demonstrated neuroprotective potential in glaucoma. Oral nicotinamide supplementation raises retinal and optic nerve levels of nicotinamide adenine dinucleotide (NAD) through the NAD salvage pathway, a process dependent on the rate-limiting enzyme nicotinamide phosphoribosyltransferase (NAMPT). Current evidence supports that NAMPT is essential for vision and retinal function, and its extracellular form (eNAMPT) has been detected in blood. Reduced levels of eNAMPT in blood could indicate impaired NAD biosynthetic capacity, and therefore, glaucomatous neurodegeneration susceptibility. This study aimed to (i) develop a specific, semi-quantitative assay to detect eNAMPT in plasma from glaucoma patients and controls, (ii) explore its potential as biomarker, and (iii) assess the effect of 2 weeks accelerated dosing nicotinamide supplementation on its circulating levels. This was done in samples from participants of a prospective clinical trial at the Eye Clinic, Umeå University Hospital (Sweden), which included 30 controls and 90 glaucoma patients that received oral 1.5 g/day nicotinamide for one week, followed by 3.0 g/day in the second week. A Western blotting assay was designed to detect eNAMPT (52 kDa) and transferrin (77 kDa) as housekeeping protein from 0.2 μL of EDTA plasma. Intra- and inter-assay variability of the assay were 14.9% and 37.9%, respectively. Normalized eNAMPT levels did not differ between glaucoma and controls, nor did they change following 2 weeks nicotinamide supplementation. In conclusion, eNAMPT is readily and specifically detected by Western blotting in EDTA plasma from controls and glaucoma patients. Given the role of NAD / NAMPT in neurodegenerative diseases, this study provides a platform for the specific detection of eNAMPT in liquid biopsies. Further studies specifically designed to study eNAMPT are needed to clarify its role in retinal ganglion cell degeneration and the therapeutic response to NAM.