Mitochondrial biogenesis occurs throughout oocyte growth by Mitochondrial DNA (mtDNA) replication and mitochondrial division, during which the number of mitochondria increases from about 1000 to up to 500,000. mtDNA replication is driven by upregulation of Transcription Factor A, Mitochondrial (TFAM). The aim of this study is to address the role of mtDNA proliferation on mitochondrial function in oocytes and embryos.
To investigate this, we have generated oocyte-specific TFAM KO mice and studied oogenesis and embryo development. We find that TFAM KO oocytes have greatly reduced levels of mtDNA (12369 ± 718, n = 11) than control (114398 ± 7602, n = 11) oocytes but surprisingly similar levels of mitochondrial mass (control 41.4 ± 2, n = 7 Vs mutant 46.5 ± 2.5, n = 12). Mutant oocyte morphology was apparently normal and no difference in the levels of ATP were found. Upon fertilization with WT sperm, embryo development to the blastocyst stage was not affected. However, breeding experiments reveal that no offspring were derived from the KO oocytes due to post-implantation loss around mid-gestation despite these embryos recovered normal TFAM protein expression from WT paternal allele. We found that nuclear gene expression in TFAM KO oocytes was disrupted suggesting effects of mitochondrial biogenesis on oocyte nuclear regulation.
In conclusion, these studies show that inhibition of mtDNA replication specifically during oocyte growth results in embryo loss after implantation. Mitochondrial function in oocytes is clearly extending beyond normal role of ‘power supply’ for the purposes of maintaining oocyte function.