Functional Characterization and Heterogeneity Analysis of Ribosomal Proteins in Mouse Preimplantation Embryos

Translational control is important during the mammalian preimplantation phase, when maternal RNAs and proteins are degraded and de novo synthesis of RNAs and proteins increases. Proteins are synthesized in ribosomes, which are assembled from ~82 ribosomal proteins (RPs). The function of ribosomes varies depending on the resident RPs, suggesting that ribosome heterogeneity can lead to functional specialization. Only a few studies have investigated the function of RPs during preimplantation embryonic development. Here, we performed functional analyses on six RP-encoding genes-Rpl4, Rps9, Rps11, Rpl13a, Rpl19, and Rpl39-in mouse preimplantation embryos. Knockdown (KD) of each of these RP genes, except Rpl39, affected morula-to-blastocyst transition, producing phenotypes that varied somewhat in their details. Rpl4-, Rpl13a-, and Rpl19-KD embryos showed fragmentation and strong arrest of cell proliferation, whereas Rps9- and Rps11-KD embryos showed severe fragmentation with relatively weak arrest of cell proliferation. In the case of Rpl39, single-KD embryos developed normally, but double-KD embryos with its paralog Rpl39-like (Rpl39l) inhibited normal blastocyst development. Protein misfolding signals were also activated in Rpl39-KD and Rpl39l + Rpl39 double-KD embryos, confirming a previous finding that RPL39 and RPL39L are associated with ribosome exit tunnels. Our results suggest the presence of different groups of proteins that require an RPL39-containing ribosome or RPL39/RPL39L-containing ribosome for correct folding in early embryos. Taken together, the results of the present study demonstrate that ribosomal proteins are fundamentally important for normal blastocyst formation and development, but not all ribosomal proteins contribute equally to embryonic development, providing a novel example of ribosome heterogeneity in preimplantation embryos.