F-actin patches nucleated on chromosomes coordinate capture by microtubules in oocyte meiosis

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-GTPdependent manner, and we propose that these structures sterically block kinetochore-microtubule attachments. Once F-actin-driven chromosome transport is complete, coordinated disassembly of these F-actin patches allows synchronous capture by microtubules. Our observations indicate that this coordination is necessary, as early capture of chromosomes by microtubules would interfere with F-actin-driven transport leading to chromosome loss and formation of aneuploid eggs.All rights reserved. No reuse allowed without permission.

Induction of a Spindle-Assembly-Competent M Phase in Xenopus Egg Extracts

“…In apparent contradiction to this observation, experiments in starfish oocytes show that injection of cyclin B-Cdk1 and activated Gwl results in subjectively normal-appearing meiotic spindles with properly aligned chromosomes [38]. This discrepancy might be explained, at least in part, by the fact that the studies in starfish oocytes addressed spindle assembly during meiosis I, during which F-actin plays a predominant role in chromosome alignment [60]. In contrast, the Xenopus egg extracts used in our experiments were treated with cytochalasin B to inhibit F-actin assembly and cycled into interphase by addition of calcium to mimic fertilization.…”