Modulating gene expression by nucleus positioning in mouse oocytes – NuPos

In order to comprehensively study the phenomena associated with the centering of the huge organelle that constitutes the nucleus of a mouse oocyte, we used several complementary approaches. Different biological objects (cytoskeleton, nuclear membrane, chromatin, membrane-less nuclear bodies) were monitored by cell biology approaches in fixed cells and in dynamics in control and in genetically or drug-induced perturbed conditions. These studies were combined with quantitative computational biology analyses to describe the organization and dynamics of these objects in correlation with the perturbations. We quantified the fluctuations of the nuclear envelope, the movements of chromatin and of certain nuclear condensates in response to mechanical forces from the cytoskeleton. From these measurements, we modelled and compared the bio-physical properties of the nuclear envelope in different contexts. We were also able to model the impact of cytoplasmic forces on the movements within the nucleoplasm, thus testing the hypothesis of a direct transfer of mechanical energy from the cytoplasm to the nucleoplasm. 3D simulations were used to validate the potential of a bio-physical model of the transformation of mechanical energy into kinetic energy synthetizing our observations. Finally, via transcriptomic studies combined with bioinformatics, we showed that the expression of a hundred genes and the splicing of several hundred were altered, for a majority of genes regulating cell division, when the oocyte nucleus was not centred.

Using an interdisciplinary approach, combining cellular, molecular and computational biology, as well as bioinformatics and theoretical biophysics, we discovered that mouse oocytes develop mechanical forces from the cytoskeleton during their growth in the ovaries, in order to agitate the chromatin and reorganize nuclear organelles, called condensates. We have shown that this ability of cytoplasmic forces to reorganize condensates within the oocyte nucleus has been conserved throughout evolution. Without this remodeling, maternal messenger RNA stores and oocyte division fail, impacting gamete fertility. Beyond Reproduction, this new mechanism could shed new light on numerous anomalies linked to defects in cytoplasmic forces correlating with defects in condensate architecture, observed in cancers and neurodegenerative diseases.

Importantly, this project initiated a solid collaboration between the two partners, which extended beyond the initial objectives of the proposal, which is precisely what we are presenting in this scientific memoir.

We have published 6 original articles since the beginning of the project (Almonacid Dev Cell 2019; Colin J Cell Biol 2020; Bennabi Nat Commun 2020; Crozet Dev 2021; Letort J Cell Sci 2022; Al Jord Nat Commun 2022), in which the ANR is thanked. Two papers are the result of collaboration between the two project partners (Almonacid Dev Cell 2019; Bennabi Nat Commun 2020)