(85) Investigating RNA Solubility Phase Transition During Oocyte-to-Embryo Transition

Sijie Chen¹, Meng Ma¹, Jing Yang¹

1. Department of Comparative Bioscience, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States

Abstract Text:

The oocyte-to-embryo transition (OET) is one of the most important developmental transitions. It is regulated by maternal products stored in the oocyte, independent of transcription. How maternal products are precisely remodeled to dictate the OET remains largely unclear.

RNAs can exist relatively freely in the cell, or be associated with large protein complexes such as ribosomes and membraneless condensates. The interaction between RNAs and large protein complexes influences the function and stability of RNAs, representing important post-transcriptional regulatory mechanisms. To investigate the post-transcriptional regulatory mechanisms governing the OET, we recently developed a centrifugation-based fractionation protocol to separate free RNAs from RNAs associated with large protein complexes. Using this method, we analyzed the transition of maternal RNAs between the soluble (supernatant) and insoluble (pellet) phases during the OET. Our work reveals that the majority of RNAs essential for germline development in early embryos undergo an insoluble-to-soluble (I-S) phase transition. The I-S phase transition of germline RNAs is mediated by germ plasm remodeling. In contrast, RNAs encoding key cell cycle regulators and some RNA-binding proteins undergo a soluble-to-insoluble (S-I) phase transition. Currently, the significance and underlying mechanism of the S-I phase transition remain poorly understood.

By performing fractionation RNAseq, here we report that the S-I phase transition can be controlled by a ribosome-dependent and independent mechanism. Among RNAs that undergo ribosome-independent S-I phase transition, some of them are transcriptionally repressed during the OET. Artificially relieving this repression led to impaired embryonic development. These findings suggest that S-I phase transition of RNA plays a crucial role in controlling the proper expression of OET regulators. Our results thus have identified novel post-transcriptional regulatory mechanisms that are fundamentally important for reproduction.