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F-actin mechanics control spindle centring in the mouse zygote

A. Chaigne,C. Campillo,R. Voituriez,N. Gov,C. Sykes,M. Verlhac,M. Terret

Published 2016 in Nature Communications

ABSTRACT

Mitotic spindle position relies on interactions between astral microtubules nucleated by centrosomes and a rigid cortex. Some cells, such as mouse oocytes, do not possess centrosomes and astral microtubules. These cells rely only on actin and on a soft cortex to position their spindle off-centre and undergo asymmetric divisions. While the first mouse embryonic division also occurs in the absence of centrosomes, it is symmetric and not much is known on how the spindle is positioned at the exact cell centre. Using interdisciplinary approaches, we demonstrate that zygotic spindle positioning follows a three-step process: (1) coarse centring of pronuclei relying on the dynamics of an F-actin/Myosin-Vb meshwork; (2) fine centring of the metaphase plate depending on a high cortical tension; (3) passive maintenance at the cell centre. Altogether, we show that F-actin-dependent mechanics operate the switch between asymmetric to symmetric division required at the oocyte to embryo transition. How the mitotic spindle is positioned in the centre of the cell during the first mitotic division is not clear. Here Chaigne et al.show that the pronucleus coarsely centres using F-actin/Myosin-Vb dynamics, and the metaphase plate is finely centred by an F-actin cage influenced by high cortical tension.

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