Oocytes Survival Requires Metabolic Activity

Abstract

Vertebrate females produce their full complement of oocytes during embryogenesis and, over time, these are either released (and fertilized or not) or undergo cell death. Oocyte death ultimately leads to sterility as the animals age and conditions that accelerate death of the eggs cause premature infertility. Nutt et al. used unfertilized Xenopus eggs and egg extracts that recapitulate many of the cell death events to investigate the molecular mechanisms that control oocyte survival. Caspase-2 was known to regulate mouse oocyte survival, and Nutt et al. show that caspase-2 activity is inhibited in Xenopus oocytes by NADPH, which is produced as a by-product of metabolic flux through the pentose-phosphate pathway. Addition of glucose-6-phosphate (G6P) or other intermediates in the pentose-phosphate pathway, but not the glycolytic pathway, inhibited activation of caspase-2 and caspase-3, cytochrome c release, and oocyte cell death. Inhibition of G6P dehydrogenase by dehydroisoandrosterone (DHEA), which inhibits the pentose-phosphate pathway, promoted oocyte cell death, increased the rate at which caspase activity rose in the egg extracts, and decreased the concentration of G6P and NADPH production. Addition of G6P or NADPH inhibited the binding of caspase-2 to its adaptor RAIDD [in a glutathione- S -transferase (GST) pull-down experiment]. Oligomerization of caspase-2 through interactions with adaptors, such as RAIDD, is essential for caspase-2 autocleavage and activation. G6P stimulated the phosphorylation of caspase-2, and pharmacological analysis, in vitro kinase assays, and extract depletion assays indicated that calcium/calmodulin-dependent protein kinase II (CaMKII) was the responsible enzyme. Mutation of one of the two putative CaMKII phosphorylation sites in caspase-2 prevented the G6P- and NADPH-stimulated phosphorylation of caspase-2, and this mutant caspase-2 also promoted cell death of oocytes when its mRNA was injected into the oocytes. Thus, these results define a path from metabolic status through the pentose-phosphate pathway that controls oocyte cell survival in a process that relies on the phosphorylation state of caspase-2. Lipinski and Yuan discuss the implications of this research for understanding aging-related cell death. L. K. Nutt, S. S. Margolis, M. Jensen, C. E. Herman, W. G. Dunphy, J. C. Rathmell, S. Kornbluth, Metabolic regulation of oocyte cell death through the CaMKII-mediated phosphorylation of caspase-2. Cell 123 , 89-103 (2005). [PubMed] M. M. Lipinski, J. Yuan, A cellular response to an internal energy crisis. Cell 123 , 3-5 (2005). [PubMed]