F-Actin nucleated on chromosomes coordinates their capture by microtubules in oocyte meiosis.

Mariia Burdyniuk,François Nédélec,Péter Lénárt,Andrea Callegari,Masashi Mori

doi:10.1083/jcb.201802080

Mariia Burdyniuk, François Nédélec + Show 3 more

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https://doi.org/10.1083/jcb.201802080

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Abstract

Capture of each and every chromosome by spindle microtubules is essential to prevent chromosome loss and aneuploidy. In somatic cells, astral microtubules search and capture chromosomes forming lateral attachments to kinetochores. However, this mechanism alone is insufficient in large oocytes. We have previously shown that a contractile F-actin network is additionally required to collect chromosomes scattered in the 70-µm starfish oocyte nucleus. How this F-actin-driven mechanism is coordinated with microtubule capture remained unknown. Here, we show that after nuclear envelope breakdown Arp2/3-nucleated F-actin "patches" form around chromosomes in a Ran-GTP-dependent manner, and we propose that these structures sterically block kinetochore-microtubule attachments. Once F-actin-driven chromosome transport is complete, coordinated disassembly of F-actin patches allows synchronous capture by microtubules. Our observations indicate that this coordination is necessary because early capture of chromosomes by microtubules would interfere with F-actin-driven transport leading to chromosome loss and formation of aneuploid eggs.

Highlights

Capture of chromosomes by spindle microtubules is an early step of cell division essential for subsequent alignment and segregation of chromosomes by the spindle apparatus
Microtubule-capture events can be identified on highresolution chromosome trajectories Upon entry into meiosis, after the onset of nuclear envelope breakdown (NEBD), a contractile F-actin network forms in the 70-μm nucleus of starfish oocytes and transports chromosomes to the animal pole (AP) (Mori et al, 2011)
We have shown earlier by tubulin immunofluorescence that these microtubules extend to 30–40 μm from the AP (Lénárt et al, 2005)

Summary

Introduction

Capture of chromosomes by spindle microtubules is an early step of cell division essential for subsequent alignment and segregation of chromosomes by the spindle apparatus. The proposed microtubule “search-and-capture” has since been validated in live cells (Hayden et al, 1990; Rieder and Alexander, 1990), and the molecular details of the initial attachments have been understood (Tanaka, 2012) These so-called lateral attachments form between the kinetochore and the microtubule lattice and involve molecular motors, dynein in particular, which transport captured chromosomes poleward (Rieder and Alexander, 1990; Yang et al, 2007). These lateral attachments are replaced by end-on attachments to allow biorientation of chromosomes on the spindle (Shrestha and Draviam, 2013). Computer simulations recapitulated key features of search-and-capture, confirming that this mechanism is sufficient to reliably capture chromosomes in a typical, 30-μm, rounded somatic cell (Holy and Leibler, 1994; Wollman et al, 2005; Heald and Khodjakov, 2015)

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