Asymmetric recruitment and actin-dependent cortical flows drive the neuroblast polarity cycle.

Chet Huan Oon,Kenneth E Prehoda

doi:10.7554/elife.45815

Abstract

During the asymmetric divisions of Drosophila neuroblasts, the Par polarity complex cycles between the cytoplasm and an apical cortical domain that restricts differentiation factors to the basal cortex. We used rapid imaging of the full cell volume to uncover the dynamic steps that underlie transitions between neuroblast polarity states. Initially, the Par proteins aPKC and Bazooka form discrete foci at the apical cortex. Foci grow into patches that together comprise a discontinuous, unorganized structure. Coordinated cortical flows that begin near metaphase and are dependent on the actin cytoskeleton rapidly transform the patches into a highly organized apical cap. At anaphase onset, the cap disassembles as the cortical flow reverses direction toward the emerging cleavage furrow. Following division, cortical patches dissipate into the cytoplasm allowing the neuroblast polarity cycle to begin again. Our work demonstrates how neuroblasts use asymmetric recruitment and cortical flows to dynamically polarize during asymmetric division cycles.

Highlights

Drosophila neuroblasts dynamically polarize to segregate fate determinants while dividing asymmetrically (Homem and Knoblich, 2012; Knoblich, 2010; Prehoda, 2009; Venkei and Yamashita, 2018)
We investigated the divisions of neuroblasts from Drosophila larval brain lobes (Homem and Knoblich, 2012), first focusing on a GFP fusion of atypical Protein Kinase C (aPKC) (Besson et al, 2015), as its catalytic activity is the direct output of the Par complex (Atwood and Prehoda, 2009; Bailey and Prehoda, 2015)
We collected optical sections throughout the full volume of the cell to visualize sections in the center along with those at the cortical edge and to allow for full three-dimensional projections at each time point. These data reveal a highly dynamic process that begins with aPKC in the cytoplasm as cells entered mitosis (Figure 1B and C; Figure 1—video 1)

Summary

Introduction

Drosophila neuroblasts dynamically polarize to segregate fate determinants while dividing asymmetrically (Homem and Knoblich, 2012; Knoblich, 2010; Prehoda, 2009; Venkei and Yamashita, 2018). We examine the dynamic processes that underlie aPKC polarization and depolarization during neuroblast asymmetric division cycles. Directional transport from the posterior to anterior cortical domain (i.e. cortical flow), potentially through the activity of actomyosin (Munro et al, 2004), is thought to play a key role in the worm embryo It has been unknown whether cortical flows play any role in aPKC polarization in neuroblasts. Because neuroblasts repeatedly cycle between polarized (apical aPKC at metaphase) and unpolarized (interphase cytoplasmic aPKC) states, depolarization is a necessary step in the neuroblast cortical. Little is known about the events that follow metaphase that regenerate the unpolarized state These events may be especially important for asymmetric division because the localization of aPKC at metaphase is distant from the site of cleavage furrow formation in anaphase, the exclusion point for basal fate determinants. The dynamic steps in neuroblast polarization that we have discovered provide further insight into the mechanisms underlying animal cell polarity and a new framework for using the neuroblast as a polarity model system

Results

Discussion

Materials and methods

Funding Funder