Dynamin-related protein 1 (Drp1) is essential for mitochondrial fission and maintaining mitochondrial dynamics. Deletion of Drp1 from oocytes leads to defective follicular development and negatively affects embryo development and fertility. It is not understood how manipulating oocyte mitochondrial function leads to compromised development of ovarian follicles.
we have performed an extensive analysis of ovaries from Drp1 conditional knock-out mice (Drp1 cKO), in which Drp1 was deleted in primordial oocytes by crossing Drp1fl/fl female mice with Gdf9-Cre males. Follicle counts were performed on ovaries from 4-, 5-, and 7-week-old Drp1 cKO and control mice, while immunofluorescence on ovaries of 5-week-old mice was used to analyse properties of developing follicles. This analysis included granulosa cell proliferation (Ki67 antibody), apoptosis (cleaved Caspase 3 (CC3) antibody), and lysosomal activity (Lamp1 antibody).
Follicle counts revealed no difference in follicle numbers between the ovaries of 4-week-old DRP1 cKO and control mice. However, in 5- and 7-week-old Drp1 cKO mice, there was a significant increase in primary follicle numbers and a decrease in antral follicles compared to controls. These findings suggest that oocyte-specific deletion of Drp1 leads to an arrest of follicle development at the secondary follicle stage. Immunofluorescence analysis revealed a significant decrease in Ki67-positive granulosa cells/follicle, indicating that granulosa cell proliferation was inhibited. This was accompanied by a significant increase in CC3-positive cells, suggesting apoptosis was increased, while LAMP1 labelling was present in a greater proportion of follicles from Drp1 cKO mice compared to controls.
These findings show that manipulating mitochondrial function in the growing oocyte has dramatic effects on the function and viability of the accompanying granulosa cells, suggesting oocyte mitochondrial function may be a critical component of oocyte-granulosa cell metabolic cooperativity necessary for follicle development and fertility.