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Regulation of meiotic metaphase by a cytoplasmic maturation-promoting factor during mouse oocyte maturation
1Department of Developmental Biology, National Institute for Basic Biology, Okazaki, Japan.
Abstract:
During mouse oocyte maturation the regulation of the activity of a cytoplasmic maturation-promoting factor (MPF) was examined. The mouse MPF activity was determined based on its ability to induce maturation in immature starfish oocytes after microinjection with the cytoplasm from mouse oocytes. MPF appeared initially at germinal vesicle breakdown (GVBD), and its activity fluctuated in exact correspondence with meiotic cycles, reaching a peak at each metaphase and almost disappearing at the time of emission of the first polar body. Cycloheximide affected neither the initial MPF appearance nor GVBD. Thereafter, however, in the presence of cycloheximide the meiotic spindle was not formed and MPF disappeared, although the chromosomes remained condensed. After removing cycloheximide, MPF reappeared and was followed by the first metaphase and subsequently by polar body emission. Finally the meiotic cycle progressed to the second metaphase. Thus, for the appearance of MPF, there is a critical period shortly before the first metaphase, after which protein synthesis is required. In the presence of either cytochalasin D or colcemid, MPF activity remained at elevated levels. Addition of cycloheximide to such cytochalasin-treated oocytes, in which the meiotic cycle was arrested at the first metaphase, caused the MPF levels to decrease and was followed by movement of chromosomes to both poles where they decondensed and two nucleus-like structures were formed. Thus, the disappearance of MPF may initiate the metaphase-anaphase transition. Furthermore, detailed cytological examination revealed that chromosomes in cytochalasin-treated oocytes were monovalent while those treated only with cycloheximide were divalent, suggesting that dissociation of the synapsis is a prerequisite for chromosome decondensation after the disappearance of MPF. In all these respects, MPF seems to be a metaphase-promoting factor rather than just a maturation-promoting factor.
Insights
Maturation-promoting factor (MPF) activity in mouse oocytes peaks at metaphase and requires protein synthesis for its reappearance. MPF
Area of Science:
- Cell Biology
- Developmental Biology
- Reproductive Biology
Background:
- Understanding the regulation of cytoplasmic maturation-promoting factor (MPF) activity is crucial for deciphering oocyte maturation processes.
- MPF is a key regulator of meiotic progression in various organisms, but its precise role and regulation during mammalian oocyte maturation require further elucidation.
Purpose of the Study:
- To investigate the regulation of cytoplasmic maturation-promoting factor (MPF) activity during mouse oocyte maturation.
- To determine the role of protein synthesis in MPF activity and meiotic progression.
- To elucidate the relationship between MPF activity, chromosome dynamics, and the metaphase-anaphase transition.
Main Methods:
- Assessed MPF activity by microinjecting cytoplasm from mouse oocytes into immature starfish oocytes.
- Utilized cycloheximide to inhibit protein synthesis and assess its impact on MPF activity and meiotic progression.
- Employed cytochalasin D and colcemid to disrupt cytoskeletal functions and observe their effects on MPF and chromosome behavior.
Main Results:
- MPF activity appeared at germinal vesicle breakdown (GVBD), peaked at metaphase, and decreased during polar body emission, correlating with meiotic cycles.
- Protein synthesis, inhibited by cycloheximide, was essential for MPF reappearance after a critical pre-metaphase period, meiotic spindle formation, and progression beyond the first metaphase.
- Cytochalasin D or colcemid treatment maintained high MPF levels, while subsequent cycloheximide addition led to MPF decrease, chromosome decondensation, and formation of nucleus-like structures.
Conclusions:
- MPF activity regulation is tightly linked to the meiotic cycle, with a critical requirement for protein synthesis preceding metaphase.
- The disappearance of MPF appears to trigger the metaphase-anaphase transition, facilitating chromosome decondensation.
- Dissociation of chromosome synapsis is a prerequisite for decondensation following MPF disappearance, suggesting MPF acts as a metaphase-promoting factor.