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

Glaucoma is characterized by progressive retinal ganglion cell degeneration. Nicotinamide supplementation has demonstrated neuroprotective potential in glaucoma by raising retinal and optic nerve nicotinamide adenine dinucleotide (NAD) via the salvage pathway, dependent on nicotinamide phosphoribosyltransferase (NAMPT). The NAMPT is essential for retinal function, and its extracellular form (eNAMPT) has been detected in blood. Reduced circulating eNAMPT could indicate impaired NAD biosynthetic capacity and glaucomatous neurodegeneration susceptibility.

We (i) developed a specific, semiquantitative assay to detect plasma eNAMPT, (ii) explored its potential as biomarker, and (iii) assessed the effect of 2-week nicotinamide supplementation on circulating levels. This was done in samples from a prospective clinical trial at the Eye Clinic, Umeå University Hospital (Sweden), including 30 controls and 90 patients with glaucoma.

A Western blotting assay was designed, detecting eNAMPT (52 kilodalton [kDa]) and transferrin (77 kDa) as housekeeping protein from 0.2 µL EDTA-plasma. Intra- and inter-assay variability were 14.9% and 37.9%, respectively. eNAMPT levels showed no difference between glaucoma and controls, nor changes after supplementation.

eNAMPT is readily and specifically detected by Western blotting in plasma from healthy controls and patients with glaucoma. Given the role of NAD/NAMPT in neurodegeneration, this study provides a platform for 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 nicotinamide.

This study established a method to sensitively detect plasma eNAMPT of patients with glaucoma and controls, providing a basis for future biomarker development.