Assembly of a persistent apical actin network by the formin Frl/Fmnl tunes epithelial cell deformability

Abstract

9Tissue remodeling during embryogenesis is driven by the apical contractility of the epithelial 10 cell cortex. This behavior arises notably from Rho1/Rok induced transient accumulation of non-11 muscle myosin II (MyoII pulses) pulling on actin filaments (F-Actin) of the medio-apical 12 cortex. While recent studies begin to highlight the mechanisms governing the emergence of 13 Rho1/Rok/MyoII pulsatility in different organisms, little is known about how the F-Actin 14 organization influences this process. Focusing on Drosophila ectodermal cells during germband 15 extension and amnioserosa cells during dorsal closure, we show that the medio-apical 16 actomyosin cortex consists of two entangled F-Actin subpopulations. One exhibits pulsatile 17 dynamics of actin polymerization in a Rho1 dependent manner. The other forms a persistent 18 and homogeneous network independent of Rho1. We identify the Frl/Fmnl formin as a critical 19 nucleator of the persistent network since modulating its level, in mutants or by overexpression, 20 decreases or increases the network density. Absence of this network yields sparse connectivity 21 affecting the homogeneous force transmission to the cell boundaries. This reduces the 22 propagation range of contractile forces and results in tissue scale morphogenetic defects. Our 23 work sheds new lights on how the F-Actin cortex offers multiple levels of regulation to affect 24 epithelial cells dynamics. 25The recent advances in live imaging have shown that cortical contractility can occur in a 31 pulsatile manner, by taking the form of local and transient accumulations of MyoII, known as 32 MyoII pulses. This phenomenon was first described in the C. elegans zygote and has since been 33 reported in many other species, in both embryonic and extra-embryonic tissues 3-11 . MyoII 34 pulses can underly a variety of morphogenetic processes, ranging from single cell polarization 35 to tissue scale remodeling. Although recent evidence suggests that MyoII pulses can emerge 36 spontaneously from stochastic fluctuations and local amplification 12-15 , the spatio-temporal 37 pattern of cortical contractility must be controlled in order to produce reproducible 38 morphogenetic outcomes. In most studied systems, this control is achieved through the 39 conserved RhoA GTPase signaling, which activate MyoII via Rho-associated kinase (ROCK) 40 dependent phosphorylations of its regulatory light chain (MyoII-RLC) 1,11,13,15,16 . 41 Besides MyoII regulation, another key parameter influencing cortical contractility resides in 42 actin filament network organization and dynamics. Typically, the cortex assembles as a thin 43 network of actin filaments bound to the plasma membrane. The cortical network is both highly 44 plastic and mechanically rigid and confer to the cells the ability to adapt and exert forces on 45 their surrounding environment 2,17-19 . These remarkable properties stem from the action of more 46 than a hundred actin binding proteins (ABPs) regulating the organization and the turnover of 47 the ...