Post-ovulatory ageing of mouse oocytes affects the distribution of specific spindle-associated proteins and Akt expression levels.

The aim of this study has been to determine the effects of in vivo post-ovulatory ageing (POA) on the distribution of spindle-associated proteins, histone H3/H4 post-translational modifications and on v-akt murine thymoma viral oncogene homolog 1 (Akt) expression levels. To this end, oocytes were retrieved 13, 29 and 33h after human chorionic gonadotrophin (hCG) treatment. The presence and distribution at the meiotic spindle of acetylated tubulin, γ-tubulin, polo kinase-1 and Ser473/Thr308 phosphorylated Akt (pAkt) as well as histone H3 and H4 acetylation and phosphorylation levels were assayed via immunofluorescence. Akt expression levels were determined via reverse transcription-polymerase chain reaction and western blotting analyses. Spindles from oocytes recovered 13h and 29h after hCG treatment showed similar levels of acetylated tubulin but ageing induced: (1) translocation of γ-tubulin from spindle poles to microtubules, (2) absence of Thr308- and Ser473-pAkt in 76% and 30% of oocytes, respectively, and (3) a significant reduction in phosphorylation levels of serine 10 on histone 3. At 29h, a significant decrease in Akt mRNA, but not in pAkt or Akt protein levels, was recorded. By contrast, protein content significantly decreased 33h after hCG. We conclude that POA impairs oocyte viability and fertilisability by altering the expression levels and spindle distribution of proteins that are implicated in cell survival and chromosome segregation. Together, these events could play a role in oocyte apoptosis.