The authors proposed that nutrient stores in the oocyte might contribute to cell survival by modulating metabolic pathways. To test this theory, they added glucose-6-phosphate (G6P) — which is used for glycogen deposition, or is metabolized through the pentose-phosphate pathway — directly to Xenopus laevis egg extracts in vitro, and found that apoptosis is inhibited, and that the block occurs either at the level, or upstream, of the mitochondria. Because they found that stimulation of the pentose-phosphate pathway or NADPH (nicotinamide-adenine dinucleotide phosphate (reduced)) could substitute for G6P to inhibit apoptosis in the egg extracts, Nutt et al. reasoned that it is in fact the generation of NADPH — a key metabolic by-product of the pentose -phosphate pathway — that transmits the anti-apoptotic signal. The authors also showed that inhibition of the pentose-phosphate pathway rapidly promotes the demise of X. laevis oocytes, highlighting the importance of this pathway in oocyte viability.
Having identified intermediates of the oocyte metabolic pathway as apoptosis inhibitors, the authors next set out to find the target of this inhibition. They focused on caspase-2, which is known to be an important constituent of oocyte apoptosis pathways. As well as confirming that caspase-2 is indeed a target for NADPH-mediated blocking of programmed cell death, they showed that this inhibition can be effected directly at the level of caspase-2 activation, rather than at an upstream signalling component.
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