Mitochondrial regulation of [Ca2+]i oscillations during cell cycle resumption of the second meiosis of oocyte

Oocyte is arrested at metaphase of the second meiosis until fertilization switching on [Ca2+]i oscillations. Oocyte activation inefficiency is the most challenging problem for failed fertilization and embryonic development. Mitochondrial function and intracellular [Ca2+]i oscillations are two critical factors for the oocyte’s developmental potential. We aimed to understand the possible correlation between mitochondrial function and [Ca2+]i oscillations in oocytes. To this end, mitochondrial uncoupler CCCP which damages mitochondrial function and two small molecule mitochondrial agonists, L-carnitine (LC) and BGP-15, were used to examine the regulation of [Ca2+]i by mitochondrial functions. With increasing CCCP concentrations, [Ca2+]i oscillations were gradually diminished and high concentrations of CCCP led to oocyte death. LC enhanced mitochondrial membrane potential and [Ca2+]i oscillations and even improved the damage induced by CCCP, however, BGP-15 had no beneficial effect on oocyte activation. We have found that mitochondrial function plays a vital role in the generation of [Ca2+]i oscillations in oocytes, and thus mitochondria may interact with the ER to generate [Ca2+]i oscillations during oocyte activation. Improvement of mitochondrial functions with small molecules can be expected to improve oocyte activation and embryonic development in infertile patients without invasive micromanipulation.

卵母细胞（Oocyte）会停滞于第二次减数分裂中期，直至受精触发细胞内钙离子([Ca²⁺]i)振荡。卵母细胞激活效率低下是导致受精失败与胚胎发育障碍的最棘手问题。线粒体功能与细胞内钙离子振荡是决定卵母细胞发育潜能的两大关键因素。本研究旨在阐明卵母细胞内线粒体功能与[Ca²⁺]i振荡之间的潜在关联。为此，我们采用损伤线粒体功能的线粒体解偶联剂CCCP，以及两种小分子线粒体激动剂——左旋肉碱（L-carnitine, LC）与BGP-15，来探究线粒体功能对[Ca²⁺]i振荡的调控作用。随着CCCP浓度升高，[Ca²⁺]i振荡逐渐减弱，高浓度CCCP甚至会引发卵母细胞死亡。左旋肉碱可提升线粒体膜电位与[Ca²⁺]i振荡水平，还能改善CCCP诱导的细胞损伤；但BGP-15对卵母细胞激活并无有益效果。本研究发现，线粒体功能在卵母细胞[Ca²⁺]i振荡的产生过程中发挥关键作用，因此在卵母细胞激活阶段，线粒体可能与内质网（Endoplasmic Reticulum, ER）协同调控[Ca²⁺]i振荡的生成。通过小分子药物改善线粒体功能，有望在无需侵入性显微操作（micromanipulation）的前提下，提升不孕患者的卵母细胞激活效率与胚胎发育质量。