RNA localisation has indispensable roles for establishing asymmetries and coordinating cell fate decisions during early embryogenesis across a plethora of non-mammalian species. To direct the spatiotemporal distribution of RNA within the cells of an embryo, the microtubule cytoskeleton provides highly sophisticated trafficking pathways. Yet, the mechanisms of RNA localisation during mammalian preimplantation development and how they contribute to pluripotency remains unknown.
Here, using advanced live imaging we visualise global RNA transcripts at high spatiotemporal resolution from fertilisation to the blastocyst stage in the living preimplantation mouse embryo. For the first time, we discover apicobasal RNA asymmetries specific to outer cells of the 16-cell stage embryo, which coincides with cell fate decisions and the emergence of the pluripotent inner cell mass. Highly clustered RNAs accumulated proximal to the basal membrane, while more dispersed RNA foci were localised apically as the embryo reached the late 16-cell stage. The targeted distribution of membrane-less RNA molecules is facilitated by the microtubule cytoskeleton, associated with lysosomes which serve as RNA transport vehicles. Furthermore, real-time tracking of RNA dynamics revealed distinct RNA subpopulations located in apical and basal regions of outer 16-cell stage blastomeres. Apically located RNA foci were more dynamic and were accompanied by an enrichment of associated translation components. Intriguingly, our discoveries of spatiotemporal RNA heterogeneities determining differential translation capacity are unevenly inherited by outer and inner daughter cells during subsequent cell divisions.
Here we provide novel insights into a subcellular mechanism driving asymmetric RNA localisation and compartmentalised translational regulation in outer cells of the 16-cell stage mouse embryo, which may contribute to cell fate decisions and pluripotent cell identity during mammalian embryogenesis, by serving as a fine-tuned mechanism for the post-transcriptional spatiotemporal control of gene expression.