Glucose 6-phosphate dehydrogenase deficiency enhances germ cell apoptosis and causes defective embryogenesis in Caenorhabditis elegans

Yang, H-C; Chen, T-L; Wu, Y-H; Cheng, K-P; Lin, Y-H; Cheng, M-L; Ho, H-Y; Lo, S J; Chiu, D T-Y

One possible mechanism responsible for the defective oogenesis induced by G6PD knockdown in C. elegans is increased oxidative stress and oxidative damage. Indeed, G6PD deficiency has been correlated with increased oxidative stress in cells. [10] [11] [12] [13] Presumably, elevated oxidative stress and oxidative damage should be present in G6PD-deficient animals. Toward this end, we have determined ROS and oxidative damage in G6PD-knockdown C. elegans. As expected, G6PD-knockdown C. elegans displays increased oxidative stress compared with mock in basal condition ( Figure 3a ) and such observation is comparable with G6PDknockdown cells. 12, 13 Previous study has shown that G6PD-knockdown cells do not display enhanced MDA under physiological condition. 12 Similarly, the MDA level in G6PDknockdown C. elegans is indistinguishable to mock control under physiological condition (data not shown). On the other hand, G6PD knockdown enhances DNA oxidative damages in C. elegans (Figure 3b ). The fact that G6PD deficiency enhances DNA oxidative damage but not lipid peroxidation suggests that G6PD-deficient C. elegans is particularly sensitive to genotoxic insult, which is in accord with previous finding in G6PD-deficient fibroblasts. 11 Previous studies showed that increased oxidative stress is associated with defective oogenesis 33 ( ex ROS, low oxidative stress) Figure 8 Proposed mechanism of defective oogenesis and embryogenesis induced by G6PD knockdown in C. elegans. G6PD deficiency reduces NADPH level, which may modulate signaling pathways of egg production and hatching. (a) The mechanism of G6PD knockdown-induced defective egg production is proposed. NADPH depletion impairs redox homeostasis as indicated by increased oxidative stress and DNA oxidative damage. As a result, the accumulated oxidative insult enhances germ cell death and leads to decreased egg production. (b) The mechanism of G6PD knockdown-induced defective hatching is proposed. Upon G6PD knockdown, the diminished NADPH reduces the activity of a yet to be identified sensor, for example dual oxidase. The decreased production of sensor-derived ROS, which serves as signaling molecules, activates mek-1 or sek-1 through negative regulation. In turn, activation of mek-1 or sek-1 impairs normal hatching possibly through multiple signaling pathways. Hence, the loss of function of mek-1 or sek-1 negates the deleterious effects on hatching due to G6PD deficiency in C. elegans G6PD-deficient C. elegans cannot be excluded. In the current study, short-term H 2 O 2 enhances germ cell apoptosis in mock C. elegans in a dose-dependent manner (Figure 5a) . However, the fact that H 2 O 2 cannot further enhance germ cell apoptosis in G6PD-deficient C. elegans suggests that it may be due to the buildup of endogenous oxidative stress in the oocytes that enhances germ cell apoptosis. Consequently, the enhanced germ cell death may lead to reduced egg production in G6PDknockdown C. elegans as possible mechanism illustrated in Figure 8a . Nevertheless, the fact that vitamin E feeding fails to rescue the defective egg production and hatching in G6PDknockdown C. elegans under our experimental condition (data not shown), suggesting the modulation of the redox homeostasis is a very complicated matter and these findings are consistent with previous report that antioxidant supplementation often displays mixed or negative outcomes in clinical trials. 34 Hence, the effect of increased oxidative stress or oxidativ