P-171 Mitochondrial complex I, a key component of the mitochondrial respiratory chain, is necessary for maintaining chromosomal stability during the final stages of oocyte maturation
Authors: Barragan M , Rodríguez-Nuevo A , Malioufa G , Zambelli F , Boke E , Popovic M 
Published: July 2024

Abstract

Study question
What is the role of mitochondrial complex I during the final stages of human oocyte maturation?

Summary answer
Mitochondrial complex I is necessary for the cytoplasmic maturation of germinal vesicle (GV) oocytes and the subsequent organization of the meiotic spindle.

What is known already
Mitochondria play a pivotal role in oogenesis. Primordial human oocytes are characterized by an abundance of immature mitochondria, which exhibit low activity and a diminished capacity to produce adenosine triphosphate (ATP). During oocyte recruitment, mitochondrial fusion and fission play an essential role, contributing to oocyte maturation and oocyte developmental competence. Furthermore, an increase in ATP levels during polar body extrusion has been associated with higher fertilization rates, suggesting a reliance on mitochondria as ATP suppliers during oocyte maturation. However, the precise role of mitochondria during the final stages of human oocyte maturation is yet to be understood.

Study design, size, duration
At total of 99 immature oocytes, including 56 GVs and 40 metaphase I (MI) oocytes, were included in the study. These were obtained from 40 oocyte donation cycles, performed at a single IVF centre between August 2023 and September 2023.

Participants/materials, setting, methods
Immature denuded oocytes were cultured in G2TM PLUS medium to reach nuclear maturation (rescue in vitro maturation; rIVM). rIVM was performed in the absence (0.1% DMSO, control) or presence of 0.5 µM Rotenone (an inhibitor of mitochondrial complex I) or 10 µM FCCP (an uncoupler of mitochondrial oxidative phosphorylation). To assess mitochondrial and nuclear status, we performed immunofluorescence analyses for TOMM20 and the meiotic spindle (DNA and TUBA).

Main results and the role of chance
After 40 hours of culture, 64.3% of control GV oocytes progressed to the MII stage, 7.1% reached MI, while 21.4% arrested, and 7.1% degenerated. Control MI oocytes showed a 90% rIVM rate after 24 hours, with none arresting and 10% degenerating. Different mitochondrial inhibitors had varied effects on GV and MI oocytes. FCCP resulted in the degeneration of all treated oocytes. When treated with rotenone, 87.5% of GVs advanced to the MI stage, after which they arrested, but no signs of degeneration were observed. Notably, 100% of MIs and MIIs derived from GVs subjected to rotenone treatment presented with misaligned chromosomes, compared to 33.3% in the control group. When starting from MIs, the rIVM rate decreased to 41.7% following rotenone treatment, with no signs of degeneration. Here, misalignments were observed in 40% of the rIVM-MII oocytes and 43% of those that arrested, compared to 33.3% in controls. These results suggest that both GV and MI oocytes have adequate complex I activity, essential for proper nuclear maturation at the MII stage. We further highlight the importance of complex I activity, especially in GVs, for processes such as polar body extrusion during oocyte maturation.

Limitations, reasons for caution
Rescue in vitro maturation poses a challenge due to its limited effectiveness in achieving sufficient MII developmental competence. Consequently, mitochondrial inhibitors may have unforeseen effects on mitochondria and spindle formation. The specific role of complex I should be confirmed by employing other complex I inhibitors not related to rotenone.

Wider implications of the findings
Our results suggest that complex I activity, a key component of the mitochondrial respiratory chain, plays a crucial role in spindle organization during oocyte maturation. This relationship between mitochondrial function and chromosomal stability in oocytes suggests that interventions targeting the mitochondrial respiratory chain may be promising for enhancing oocyte quality.

Trial registration number
Not applicable