Cell division fidelity is crucial for stem cell propagation and the maintenance of pluripotency. Centrosomes, which organize the mitotic spindle microtubules to ensure proper bipolar cell division, have a core of a pair of centrioles that duplicate once per cell cycle. At the onset of centriole biogenesis, SAS-6 forms a cartwheel structure, which is the precursor for the forming procentrioles. SAS-6 is essential for centriole formation in human cell lines and other organisms. However, the functions of SAS-6 in mouse stem cells remain to be elucidated. Here, we report that Sas-6-null mouse embryos lack centrioles, activate the mitotic surveillance cell death pathway and arrest at mid-gestation. In contrast, SAS-6 is not strictly required for centriole formation in mouse embryonic stem cells (mESCs) in vitro, but is still important to regulate centriole length, symmetry and ability to template cilia. Remarkably, centrioles appeared after just one day of culture of Sas-6-null blastocysts, from which mESCs are normally derived. Finally, the number of cells with centrosomes is drastically decreased upon the exit from a pluripotent state. Collectively, our data suggest a differential requirement for mouse SAS-6 in centriole formation or integrity depending on the cellular context, and highlight the robustness of mESCs in using SAS-6-independent centriole-duplication pathways.
Centrosomes, composed of two centrioles and pericentriolar material, organize mitotic spindles during cell division and template cilia during interphase. The first few divisions during mouse development occur without centrioles, which form around embryonic day (E) 3. However, disruption of centriole biogenesis in Sas-4 null mice leads to embryonic arrest around E9. Centriole loss in Sas-4−/− embryos causes prolonged mitosis and p53-dependent cell death. Studies in vitro discovered a similar USP28-, 53BP1-, and p53-dependent mitotic surveillance pathway that leads to cell cycle arrest. In this study, we show that an analogous pathway is conserved in vivo where 53BP1 and USP28 are upstream of p53 in Sas-4−/− embryos. The data indicates that the pathway is established around E7 of development, four days after the centrioles appear. Our data suggest that the newly formed centrioles gradually mature to participate in mitosis and cilia formation around the beginning of gastrulation, coinciding with the activation of mitotic surveillance pathway upon centriole loss.
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