Regulation of mitochondrial DNA accumulation during oocyte growth and meiotic maturation in the mouse

Abstract

Oocytes accumulate an enormous quantity of mitochondrial (mt) DNA, and an insufficient amount of mtDNA may underlie some cases of poor oocyte quality leading to infertility. Little is known, however, about the mechanisms that govern the timing and regulation of mtDNA accumulation during oogenesis. We report, through analysis of the mtDNA content of individual oocytes of the mouse, that mtDNA accumulates steadily during oocyte growth to reach a value of w175 000 copies per cell. MtDNA content ceases to increase once oocytes reach full size and remains unchanged during meiotic maturation. To test whether mtDNA accumulation depends on oocyte growth, we inhibited growth in vitro in two ways -by exposing complexes comprising partially grown oocytes enclosed by granulosa cells to a chemical inhibitor of the phosphatidylinositol-3-kinase signaling pathway and by removing the surrounding granulosa cells from partially grown oocytes. Under both conditions, the oocytes fail to grow, but mtDNA accumulation is unaffected, indicating that the two processes can be mechanistically uncoupled. Quantitative analysis of the mRNAs encoding proteins required for mtDNA replication revealed that Polg (Polga) (polymerase-g, a-subunit), Polg2 (Polgb), and Tfam (transcription factor A, mitochondrial) increase during oocyte growth but then decrease after fully grown oocytes become transcriptionally silent as indicated by the non-surrounded nucleolusto-surrounded nucleolus transition. Thus, there is a correlation between the decline in the quantity of mRNAs encoding mtDNA replication factors in fully grown oocytes and the arrest of mtDNA accumulation in these cells, suggesting that the two events may be causally linked.

Maternal age and in vitro culture affect mitochondrial number and function in equine oocytes and embryos

“…A similar general trend was observed for all target genes examined and may explain, in part, why mtDNA copy numbers have been reported not to increase during IVM of equine oocytes (Rambags et al 2014). A decrease in relative gene expression for TFAM and mtPOLB after oocyte maturation has previously been reported for porcine and murine oocytes Mahrous et al 2012) and related to the arrest in mitochondrial replication that is observed during early embryonic development. However, the absolute reduction in mtDNA copy number in oocytes of mares $12 years after IVM observed by Rambags et al (2014) was thought to be a factor of mitochondrial deterioration, as indicated by electron microscopic evidence of mitochondrial swelling and disintegration.…”

Mitochondrial biogenesis and degradation are induced by CCCP treatment of porcine oocytes

“…Mahrous et al (2012) suggested that the Mt number in mouse oocytes remains constant once the oocytes reach full size. By contrast, the Mt number in porcine oocytes was found to increase during maturation period (Mao et al 2012, Pawlak et al 2012.…”

“…Early embryonic development may be supported by whole, already existing mitochondria without de novo synthesis (Wai et al 2010), although a few studies suggest that de novo mitochondrial synthesis occurs during early embryonic development (Chiaratti et al 2010). Furthermore, it has been suggested that once mitochondrial number reaches a certain threshold during oocyte maturation, this number is maintained (Mahrous et al 2012, Cotterill et al 2013), but Mao et al (2012) reported that mitochondrial synthesis is induced by supplementation of maturation medium with follicular fluid (FF) and neuregulin 1. Despite the presence of both de novo synthesis and degradation pathways in somatic cells, mitochondrial turnover during oocyte maturation has not been investigated.…”

Mitochondrial dynamics and the cell cycle

“…There is an increase in the amount of mitochondrial DNA during meiosis I (Mahrous et al, 2012). In these oocytes, mitochondrial numbers reach over 100,000 per cell before the completion of meiosis II.…”

“…During the progression from immature to mature embryo sac in P. zonale , over a 900 fold increase in the amount of mitochondrial DNA is observed (Kuroiwa et al, 1996). This increase is mirrored in mouse oocytes, but estimates place the increase to be only 100-fold (Mahrous et al, 2012). These studies of pollen and megaspore development show that the mitochondria change shape, size, and cellular location during meiosis; yet linking these observations to the nuclear genes identified in Arabidopsis with a role in mitochondrial dynamics has not been established.…”