Impairment of pentose phosphate pathway mediates 6-PPD quinone-induced neurotoxicity by inhibiting NADPH-dependent antioxidant defense.

6-PPD quinone (6-PPDQ) is the derivative from N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) after reaction with environmental ozone. Pentose phosphate pathway (PPP) serves as a principal source of nicotinamide adenine dinucleotide phosphate (NADPH). However, association between disruption in PPP and 6-PPDQ neurotoxicity remains largely unclear. In Caenorhabditis elegans, we first examined the role of PPP derived NADPH in controlling 6-PPDQ neurotoxicity. Exposure to 0.1-10 µg/L 6-PPDQ decreased NADPH level and inhibited expression of gspd-1 and T25B9.9 encoding PPP related enzymes. Meanwhile, NADPH-dependent antioxidant defense was inhibited by 6-PPDQ, which was reflected by reduced glutathione (GSH) content and elevated H₂O₂ level. Additionally, expressions of glutathione peroxidase genes (gpx-1, gpx-2, gpx-3, gpx-4, gpx-5, and gpx-6) were decreased by 6-PPDQ. RNAi of gspd-1, T25B9.9, and glutathione peroxidase genes exacerbated 6-PPDQ induced inhibition in locomotion and degeneration of D-type motor neurons. Moreover, RNAi of these genes strengthened activation of neurodegeneration related genes (deg-3, crt-1, itr-1, and tra-3) and suppressed expression of genes related to stress response (jnk-1, glb-10, and dbl-1) in 6-PPDQ exposed nematodes. Therefore, our results suggested the possibility of disruption in PPP by 6-PPDQ in intensifying damage on NADPH-dependent antioxidant defense and thereby causing the neurotoxicity in nematodes.