LAUSR

Abstract As the most abundant organelles in oocytes, mitochondria play an important role in maintaining oocyte quality. Here, we report that March5, encoding a mitochondrial ubiquitin ligase that promotes mitochondrial elongation, plays a critical role in mouse oocyte meiotic maturation via regulating mitochondrial function. The subcellular localization of MARCH5 was similar to the mitochondrial distribution during mouse oocyte meiotic progression. Knockdown of March5 caused decreased ratios of the first polar body extrusion. March5-siRNA injection resulted in oocyte mitochondrial dysfunctions, manifested by increased reactive oxygen species, decreased ATP content as well as decreased mitochondrial membrane potential, leading to reduced ability of spindle formation and an increased ratio of kinetochore–microtubule detachment. Further study showed that the continuous activation of the spindle assembly checkpoint and the failure of Cyclin B1 degradation caused MI arrest and first polar body (PB1) extrusion failure in March5 knockdown oocytes. Taken together, our results demonstrated that March5 plays an essential role in mouse oocyte meiotic maturation, possibly via regulation of mitochondrial function and/or ubiquitination of microtubule dynamics- or cell cycle-regulating proteins. Summary Sentence Knockdown of MARCH5 blocks mouse oocyte maturation at MI stage with the continuous activation of the spindle assembly checkpoint (SAC) and the failure of Cyclin B1 degradation, perhaps through inducing mitochondrial dysfunctions. Graphical Abstract Schematic diagram of MARCH5 functions in mouse oocyte maturation, as March5-siRNA injection resulted in oocyte mitochondrial dysfunctions, manifested by increased ROS, decreased ATP content, and decreased mitochondrial membrane potential, leading to reduced ability of spindle formation and an increased ratio of kinetochore–microtubule detachment. Further study showed that the continuous activation of the SAC and accumulation of MPF caused MI arrest and PB1 extrusion failure in March5 knockdown oocytes.