Gene expression and chromatin organization during mouse oocyte growth

Area of Science:

• Developmental Biology
• Epigenetics
• Oocyte Biology

Background:

• Mouse oocytes are classified as surrounded nucleolus (SN) or nonsurrounded nucleolus (NSN) based on chromatin organization.
• Only SN oocytes support development beyond the two-cell stage after fertilization.
• Chromatin organization correlates with female gamete developmental competence, potentially via gene expression.

Purpose of the Study:

• To analyze gene expression differences between SN and NSN mouse oocytes using a reporter transgene.
• To investigate the role of epigenetic regulation in oocyte developmental competence.

Main Methods:

• Utilized the HSP70.1Luc transgene (murine HSP70.1 promoter + firefly luciferase reporter).
• Isolated oocytes from female mice aged 2 days to 13 weeks.
• Classified oocytes as SN or NSN and assayed luciferase activity for gene expression levels.

Main Results:

• SN oocytes consistently demonstrated higher luciferase activity, indicating elevated gene expression compared to NSN oocytes.
• Transgene expression levels became equal in metaphase II oocytes derived from both SN and NSN types after meiotic resumption.
• NSN oocytes showed limited transcript and protein availability in the cytoplasm until germinal vesicle breakdown (GVBD).

Conclusions:

• Higher gene expression in SN oocytes is linked to their superior developmental potential.
• The developmental block in NSN oocytes may stem from restricted transcript/protein availability.
• This study enhances understanding of epigenetic control over gene expression during oogenesis.