(49) Spatiotemporal dynamics and selectivity of mRNA translation during mouse pre-implantation development

Hao Ming ¹, Rajan Iyyappan ¹, Kianoush Kakavand ², Michal Dvoran ², Andrej Susor ², Zongliang Jiang ¹

1. Department of Animal Sciences, Genetics Institute, University of Florida, Gainesville, FL 32610, USA

2.  Laboratory of Biochemistry and Molecular Biology of Germ Cells, Institute of Animal Physiology and Genetics of the Czech Academy of Sciences, Rumburska 89, 277 21 Libechov, Czech Republic

Abstract Text:

Translational regulation is pivotal during preimplantation development. However, how mRNAs are selected for temporal regulation and their dynamic utilization and fate during this period are still unknown. Using a high-resolution ribosome profiling approach, we analyzed transcriptome, as well as translational profiles of mRNAs that are bounded on different number of translating ribosomes (ten equal volumes of fractions (F1-F10) were collected separately) in mouse oocytes at the germinal vesicle (GV) and metaphase II (MII) stages, as well as pre-implantation embryos at the zygote, 2-Cell, 4-Cell, 8-Cell, morula, and blastocyst stages.

To explore the dynamics of specific mRNA that bound to different number of ribosomes (termed mRNA translational fate) across different developmental stages, we refined our analysis by applying the fuzzy k-means algorithm to categorize profiles of free RNA (fractions F1-F2), monosome-occupied RNA (Fractions F3-F5), and polysome-occupied RNA (Fractions F6-F10) separately. While most genes showed stage-specific expression, many monosome-bound genes either remained as such for later use or transitioned to polysome-bound to support immediate protein synthesis for specific stages. Given the EGA triggers extensive gene activation and translation, we specifically focused on the mRNA translational fate in the 2-Cell embryos. We noticed three distinct patterns: 1) mRNAs are initially not translationally active, becoming highly active during EGA and subsequently returning to a quiescent state; 2) mRNAs remain low in abundance and are rarely ribosome-bound until a spike in transcription and translation in the 2-Cells; and 3) mRNAs are actively translated in the 2-Cells, then transition to monosome-bound status afterwards, either being degraded or reserved for later stages. These patterns demonstrate the linear progression of early embryonic development and emphasize the strategic regulation of mRNA storage and translation, which is critical for the subsequent cellular process in the pre-implantation development. We extended our analysis across all stages to investigate the translational fate of mRNAs based on the patterns identified, and found that these patterns occur extensively. Collectively, during pre-implantation development, discrepancies arise not only between the transcriptome and the translatome, but also within the translatome itself, as mRNAs selectively bind to monosomes or polysomes.

Furthermore, we showed that the eukaryotic initiation factor Eif1ad3, which is exclusively translated in the 2-Cell embryo, is required for post embryonic genome activation. Our study thus provides genome-wide datasets and analyses of spatiotemporal translational dynamics accompanying mammalian germ cell and embryonic development and reveals the contribution of a novel translation initiation factor to mammalian pre-implantation development.