The Spindle Assembly Checkpoint Functions during Early Development in Non-Chordate Embryos.

Janet Chenevert,Marianne Roca,Stefania Castagnetti,Richard R Copley,Elisabeth Christians,Antonella Ruggiero,Lydia Besnardeau,Dalileh Nabi,Alex Mcdougall

doi:10.3390/cells9051087

Abstract

In eukaryotic cells, a spindle assembly checkpoint (SAC) ensures accurate chromosome segregation, by monitoring proper attachment of chromosomes to spindle microtubules and delaying mitotic progression if connections are erroneous or absent. The SAC is thought to be relaxed during early embryonic development. Here, we evaluate the checkpoint response to lack of kinetochore-spindle microtubule interactions in early embryos of diverse animal species. Our analysis shows that there are two classes of embryos, either proficient or deficient for SAC activation during cleavage. Sea urchins, mussels, and jellyfish embryos show a prolonged delay in mitotic progression in the absence of spindle microtubules from the first cleavage division, while ascidian and amphioxus embryos, like those of Xenopus and zebrafish, continue mitotic cycling without delay. SAC competence during early development shows no correlation with cell size, chromosome number, or kinetochore to cell volume ratio. We show that SAC proteins Mad1, Mad2, and Mps1 lack the ability to recognize unattached kinetochores in ascidian embryos, indicating that SAC signaling is not diluted but rather actively silenced during early chordate development.

Highlights

The mitotic checkpoint, known as the spindle assembly checkpoint (SAC), operates during mitosis and monitors bipolar attachment of spindle microtubules to kinetochores, specialized multiprotein complexes assembled on duplicated sister chromatids
Instead we show that recognition of unattached kinetochores by the SAC machinery is lost in SAC-deficient ascidian embryos, suggesting that lack of SAC activity during early development is not due to passive dilution of checkpoint signal in large cells, but instead the mitotic checkpoint is actively silenced in early embryos of many chordate species
As histone H3 is phosphorylated during mitosis [24], mitotic progression was assessed by following the phosphorylation status of histone H3 (Phospho Histone H3, pH3), as well as chromosome condensation, over the equivalent of at least one cell cycle time in both control (+ DMSO) and nocodazole-treated (+ noco) embryos (Figure 1b)

Summary

Introduction

The mitotic checkpoint, known as the spindle assembly checkpoint (SAC), operates during mitosis and monitors bipolar attachment of spindle microtubules to kinetochores, specialized multiprotein complexes assembled on duplicated sister chromatids. Degradation of securin activates separase, resulting in cohesin cleavage and physical separation of sister chromatids, while cyclin B degradation inactivates Cyclin Dependent Kinase (CDK), resulting in mitotic exit [1,2]. This mechanism increases the fidelity of mitosis by preventing premature initiation of anaphase and subsequent generation of daughter cells with unequal chromosomal complements, a condition, known as aneuploidy, which is linked to cell and organismal lethality [3].

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