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Dev Cell. 2018 Jun 4;45(5):551-564.e4. doi: 10.1016/j.devcel.2018.04.024. Epub 2018 May 24.

Collision of Expanding Actin Caps with Actomyosin Borders for Cortical Bending and Mitotic Rounding in a Syncytium.

Author information

1
Department of Cell & Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada.
2
Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON M5S 3G9, Canada.
3
Department of Cell & Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada; Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON M5S 3G9, Canada; Ted Rogers Centre for Heart Research, University of Toronto, Toronto, ON M5G 1M1, Canada; Developmental and Stem Cell Biology Program, The Hospital for Sick Children, Toronto, ON M5G 1X8, Canada.
4
Department of Cell & Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada. Electronic address: tony.harris@utoronto.ca.

Abstract

The early Drosophila embryo is a large syncytial cell that compartmentalizes mitotic spindles with furrows. Before furrow ingression, an Arp2/3 actin cap forms above each nucleus and is encircled by actomyosin. We investigated how these networks transform a flat cortex into a honeycomb-like compartmental array. The growing caps circularize and ingress upon meeting their actomyosin borders, which become the furrow base. Genetic perturbations indicate that the caps physically displace their borders and, reciprocally, that the borders resist and circularize their caps. These interactions create an actomyosin cortex arrayed with circular caps. The Rac-GEF Sponge, Rac-GTP, Arp3, and actin coat the caps as a growing material that can drive cortical bending for initial furrow ingression. Additionally, laser ablations indicate that actomyosin contraction squeezes the cytoplasm, producing counterforces that swell the caps. Thus, Arp2/3 caps form clearances of the actomyosin cortex and control buckling and swelling of these clearances for metaphase compartmentalization.

KEYWORDS:

Arp2/3; Drosophila; actin; cortical domains; embryo cleavage; forces; mesoscale; mitotic rounding; myosin; syncytium

PMID:
29804877
DOI:
10.1016/j.devcel.2018.04.024
[Indexed for MEDLINE]

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