Antiapoptotic role for ornithine decarboxylase during oocyte maturation

Ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, is a nonredundant and essential gene in all eukaryotes. During the mitotic cell cycle, ODC exhibits two activity peaks: one at the G₁/S transition and one during the G₂/M transition. The physiological role of this cell cycle-dependent ODC activity dynamic is not clear. Previous studies have reported a significant elevation of ODC activity during Xenopus oocyte maturation, which resembles mitotic G₂/M transition. In order to study the roles of ODC activity in the oocytes, we utilized antisense morpholino (xODC mo) oligonucleotides to inhibit ODC translation. We report here that xODC mo abolished ODC activity increase during oocyte maturation. xODC mo-injected oocytes underwent germinal vesicle breakdown, emitted the first polar body, and reached metaphase II, thus completing nuclear maturation. However, the metaphase II oocytes exhibited high levels of reactive oxygen species and became apoptotic. When transferred to host frogs and subsequently ovulated, these eggs were fertilized but exhibited embryo fragmentation. Translation of ODC is therefore integral to cytoplasmic maturation, protecting metaphase II oocytes from reactive oxygen species-induced apoptosis.