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Objectives The alteration of bioenergetics by oocytes in response to the demands of various biological processes plays a critical role in maintaining normal cellular physiology. However, little is known about the association between energy sensing and energy production with energy‐dependent cellular processes like meiosis. Materials and methods We demonstrated that cell cycle‐dependent mitochondrial Ca2+ connects energy sensing to mitochondrial activity in meiosis progression within mouse oocytes. Further, we established a model in mouse oocytes using siRNA knockdowns that target mitochondrial calcium uniporters (MCUs) in order to inhibit mitochondrial Ca2+ concentrations. Results Decreased numbers of oocytes successfully progressed to the germinal vesicle stage and extruded the first polar body during in vitro culture after inhibition, while spindle checkpoint‐dependent meiosis was also delayed. Mitochondrial Ca2+ levels changed, and this was followed by altered mitochondrial masses and ATP levels within oocytes during the entirety of meiosis progression. Abnormal mitochondrial Ca2+ concentrations in oocytes then hindered meiotic progress and activated AMP‐activated protein kinase (AMPK) signalling that is associated with gene expression. Conclusions These data provide new insight into the protective role that MCU‐dependent mitochondrial Ca2+ signalling plays in meiotic progress, in addition to demonstrating a new mechanism of mitochondrial energy regulation by AMPK signalling that influences meiotic maturation.

Vitamin C is not only an antioxidant but also a regulator of epigenetic modifications that can enhance the activity of the ten‐eleven translocation (TET) family dioxygenases and promote the oxidation of 5‐methylcytosine (5mC) to 5‐hydroxymethylcytosine (5hmC). Here, we investigated the effects of vitamin C in regulating DNA methylation in sheep somatic cells or embryos in an effort to improve the cloned embryo development. Vitamin C treatment of sheep foetal fibroblast cells significantly increased the 5hmC levels but did not affect the 5mC levels in cells. After nuclear transfer, vitamin C‐treated donor cells could not support a higher blastocyst development rate than non‐treated cells. Although combination of serum starvation and vitamin C treatment could induce significant 5mC decrease in donor cells, it failed to promote the development of resultant cloned embryos. When cloned embryos were directly treated with vitamin C, the pre‐implantation development of embryos and the 5hmC levels in blastocysts were significantly improved. This beneficial role of vitamin C on embryo development was also observed in fertilized embryos. Our results suggest that vitamin C treatment of the embryos, but not the donor cells, can improve the development of cloned sheep embryos.

The homeostasis of mitochondrial calcium ([Ca2+]mt) in oocytes plays a critical role in maintaining normal reproductive cellular progress such as meiosis. However, little is known about the association between [Ca2+]mt homeostasis and early embryonic development. Two in vitro mouse MII oocyte models were established by using a specific agonist or inhibitor targeting mitochondrial calcium uniporters (MCU) to upregulate or downregulate [Ca2+]mt concentrations. The imbalance of [Ca2+]mt in MII oocytes causes mitochondrial dysfunction and morphological abnormity, leading to an abnormal spindle/chromosome structure. Oocytes in drug-treated groups are less likely to develop into blastocyst during in vitro culture. Abnormal [Ca2+]mt concentrations in oocytes hindered epigenetic modification and regulated mitogen-activated protein kinase (MAPK) signaling that is associated with gene expression. We also found that MAPK/ERK signaling is regulating DNA methylation in MII oocytes to modulate epigenetic modification. These data provide a new insight into the protective role of [Ca2+]mt homeostasis in early embryonic development and also demonstrate a new mechanism of MAPK signaling regulated by [Ca2+]mt that influences epigenetic modification.