The H3.3 chaperone Hira complex orchestrates oocyte developmental competence.

Area of Science:

• Reproductive biology
• Epigenetics
• Developmental biology

Background:

• Oocyte competence is vital for successful reproduction and early embryonic development.
• The mechanisms and significance of transcriptional silencing in mature oocytes are poorly understood.
• Histone H3.3, a variant histone, plays unique roles in chromatin structure and gene expression.

Purpose of the Study:

• To characterize the function of the H3.3 chaperone Hira/Cabin1/Ubn1 complex in mouse oogenesis and early embryogenesis.
• To investigate the impact of Hira complex loss of function on oocyte transcription and chromatin.
• To elucidate the role of Zscan4d in oocyte developmental competence and its regulation by the Hira complex.

Main Methods:

• Generation and analysis of mouse mutants lacking functional subunits of the Hira/Cabin1/Ubn1 complex.
• Transcriptome and nascent RNA sequencing to assess transcriptional activity in mutant oocytes.
• Histone mark analysis (H3K4me3, H3K9me3) and chromatin accessibility assays.
• Zygote genome activation assays and manipulation of Zscan4 expression.

Main Results:

• Loss of any Hira/Cabin1/Ubn1 subunit leads to early embryogenesis failure in mice.
• Mutant oocytes exhibit aberrant transcriptional silencing, reduced H3K4me3 and H3K9me3 marks, and impaired chromatin accessibility.
• Misregulation of Zscan4d, a key gene for zygote genome activation, was observed in mutant oocytes.
• Oocyte-specific Zscan4 overexpression mimicked Hira mutant phenotypes, while Zscan4 knockdown partially rescued mutant oocyte development.

Conclusions:

• The H3.3 chaperone Hira complex is essential for maternal effects in oocyte developmental competence.
• This complex regulates chromatin condensation and transcriptional quiescence, ensuring proper oocyte maturation.
• Fine-tuning of Zscan4 expression at the oocyte-to-embryo transition is critical for embryogenesis, mediated by the Hira complex.