Effects of various calcium transporters on mitochondrial Ca²⁺ changes and oocyte maturation

Ca²⁺ participates in many important cellular processes, but the underlying mechanisms are still poorly understood, especially during oocyte maturation. First, we confirmed that calcium in the culture medium was essential for oocyte maturation. Next, various inhibitors of Ca²⁺ channels were applied to investigate their roles in mitochondrial Ca²⁺ changes and oocyte maturation. Our results showed that Trmp7, Orai, T-type Ca²⁺ channels and Na⁺ /Ca²⁺ exchanger complex (NCLX) were important for oocyte maturation. Trmp7 inhibition delayed germinal vesicle breakdown. Orai and NCLX inhibition significantly weakened the distribution of mitochondrial Ca²⁺ around the nucleus compared to the Ctrl group. Interestingly, even T-type Ca²⁺ channels-specific inhibitor Mibefradil blocked germinal vesicle breakdown; mitochondrial Ca²⁺ surrounding the nucleus still was maintained at a high level without spindle formation. Two calcium transporter inhibitors, Thapsigargin and Ruthenium Red, which have been confirmed to inhibit oocyte activation, did not significantly affect oocyte maturation. Increasing the knowledge of calcium transport may provide a basis to build on for improving oocyte in vitro maturation in human assisted reproduction clinics.