Kynurenine pathway metabolism is involved in the maintenance of the intracellular NAD ⁺ concentration in human primary astrocytes

Efficient synthesis of NAD ⁺ is critical to maintaining cell viability in all organs of the body. However, little is known of the pathway(s) by which cells of the central nervous system produce NAD ⁺. The aim of this study was to investigate the relationship, between tryptophan degradation via the kynurenine pathway (KP) and de novo NAD ⁺ synthesis in human astrocytes, a major cell type within the brain. In this study we observed that inhibition of single enzymes of the KP resulted in significant decreases in NAD ⁺ levels in astroglial cells after a 24 hr period. We also observed that astrocytes cultured in media deficient in tryptophan, nicotinic acid and nicotinamide resulted in a 50% decrease in NAD ⁺ levels after 24 hrs. This decrease in NAD ⁺ was partially restored by supplementation of the culture media with either tryptophan or kynurenine, or nicotinic acid or with supply of the salvage pathway precursor nicotinamide.