Reconstitution of chromatin reorganization during mammalian oocyte development

SignificanceThe generation of developmentally competent eggs requires oocyte growth and maturation. When compared to meiotic maturation, relatively little is known about the key processes that happen in growing oocytes. Since the first visualization of oocyte chromatin under the electron microscope, we have known that growing oocytes undergo a fundamental transition from the non-surrounded nucleolus (NSN) to surrounded nucleolus (SN) chromatin configuration. NSN-to-SN transition is crucial for subsequent embryonic development after fertilization, but what drives NSN-to-SN transition during mammalian oocyte development is still unknown. Here we show that natural degradation of RNA polymerase II (RNAPII) by segregase and proteasome drives NSN-to-SN transition, challenging the conventional view that transcriptional silencing and NSN-to-SN transition are driven by independent epigenetic mechanisms in mammalian oocytes. The final step of oocyte growth, which reorganizes chromatin from the uncondensed, non-surrounded nucleolus (NSN) configuration to the condensed, surrounded nucleolus (SN) configuration, is essential for embryonic development after fertilization. However, the mechanism of NSN-to-SN transition remains unknown. Here, we identify RNA polymerase II (RNAPII) degradation as the key driver of this process. RNAPII inhibitors, but not nucleoside-based transcription inhibitors, swiftly induced RNAPII degradation and NSN-to-SN transition in oocytes. Strikingly, induced SN-like nuclei recapitulated epigenetic features, chromatin interactions and developmental potential in SN oocytes. Using segregase and proteasome inhibitors and the newly established scFv miniTrim-Away for nuclear proteins, we further demonstrate that RNAPII degradation is necessary and sufficient for NSN-to-SN transition in mouse and human. RNAPII degradation globally cleared RNAPII from transcriptionally active chromatin and increased chromatin dynamics, concomitantly leading to transcriptional silencing and chromatin rearrangement in oocytes. Our study elucidates the framework of chromatin reorganization in growing mammalian oocytes and provides an alternative source of fully grown oocyte nuclei, representing a milestone for the potential use of growing oocytes in reproductive medicine.