Autophagy contributes to the clearance and recycling of macromolecules and organelles in response to stress. We previously reported that vitrified mouse oocytes show acute increases in autophagy during warming. Herein, we investigate the potential role of Atg7 in oocyte vitrification by using an oocyte-specific deletion model of the Atg7 gene, a crucial upstream gene in the autophagic pathway.
Oocyte-specific Atg7 deficient mice were generated by crossing Atg7 floxed mice and Zp3-Cre transgenic mice. The oocytes were vitrified-warmed and then subjected to in vitro fertilization and development. The rates of survival, fertilization, and development were assessed in the Atg7 deficient oocytes in comparison with the wildtype oocytes. Light chain 3 (LC3) immunofluorescence staining was performed to determine whether this method effectively evaluates the autophagy status of oocytes.
The survival rate of vitrified-warmed Atg7 f/f ;Zp3-Cre ( Atg7 d/d ) metaphase II (MII) oocytes was not significantly different from that of the wildtype ( Atg7 f/f ) oocytes. Fertilization and development in the Atg7 d/d oocytes were significantly lower than the Atg7 f/f oocytes, comparable to the Atg5 d/d oocytes previously described. Notably, the developmental rate improved slightly in vitrified-warmed Atg7 d/d MII oocytes when compared to fresh Atg7 d/d oocytes. LC3 immunofluorescence staining showed that this method can be reliably used to assess autophagic activation in oocytes.
We confirmed that the LC3-positive signal is nearly absent in Atg7 d/d oocytes. While autophagy is induced during the warming process after vitrification of MII oocytes, the Atg7 gene is not essential for survival of vitrified-warmed oocytes. Thus, induction of autophagy during warming of vitrified MII oocytes seems to be a natural response to manage cold or other cellular stresses.