Mitochondrial biogenesis occurs throughout oocyte growth, when the number of mitochondria increases from ~1000 to ~500,000 through tightly controlled processes of mtDNA replication and ultimately fission of mitochondria via the GTPase DRP1 (dynamin-related protein 1). In addition to providing increased capacity for ATP production, mitochondrial fission in other cells regulates mitochondrial quality and key cellular functions, such as redox balance. However, in oocytes it is not known whether mitochondria perform additional functions and how these impinge upon embryo reprogramming and developmental potential.
To investigate this, we developed an oocyte-specific Drp1 Cre-loxP knockout (KO) system in mice to irreversibly remove DRP1 protein at the primordial follicle stage and comprehensively characterised oocyte quality including mitochondrial function, transcriptomics, proteomics and epigenetic profile. We find that Drp1 KO oocytes are able to grow and meiotically mature, have normal ATP concentrations but reduced mitochondrial membrane potential and moderately increased mitochondrial reactive oxygen species levels compared to controls. Analysis of Drp1 KO oocytes reveals dramatic changes to the transcriptome and proteome, as well as disrupted levels of epigenetic marks in oocyte nuclei. To determine the impact of these changes on developmental competence, Drp1 KO oocytes were fertilised with WT sperm such that embryos expressed DRP1 following zygotic genome activation but had inherited mitochondria from the DRP1-deficient oocyte. Remarkably, Drp1 KO oocytes fertilized normally but embryos showed high rates of developmental failure after day 13 of gestation. To determine whether developmental failure is due to disrupted mito-nuclear communication, pronuclear transplantation experiments were conducted. These showed that placing the nuclear material from Drp1 KO oocytes into wild type cytoplasm did not rescue the developmental failure. These studies are the first reveal a regulatory role of mitochondria, specifically via DRP1, in the establishment of the maternal epigenome during oocyte growth, which is critical for normal fetal development.