Polarization of Drosophila Neuroblasts During Asymmetric Division

Abstract

During Drosophila development, neuroblasts divide to generate progeny with two different fates. One daughter cell self-renews to maintain the neuroblast pool, whereas the other differentiates to populate the central nervous system. The difference in fate arises from the asymmetric distribution of proteins that specify either self-renewal or differentiation, which is brought about by their polarization into separate apical and basal cortical domains during mitosis. Neuroblast symmetry breaking is regulated by numerous proteins, many of which have only recently been discovered. The atypical protein kinase C (aPKC) is a broad regulator of polarity that localizes to the neuroblast apical cortical region and directs the polarization of the basal domain. Recent work suggests that polarity can be explained in large part by the mechanisms that restrict aPKC activity to the apical domain and those that couple asymmetric aPKC activity to the polarization of downstream factors. Polarized aPKC activity is created by a network of regulatory molecules, including Bazooka/Par-3, Cdc42, and the tumor suppressor Lgl, which represses basal recruitment. Direct phosphorylation by aPKC leads to cortical release of basal domain factors, preventing them from occupying the apical domain. In this framework, neuroblast polarity arises from a complex system that orchestrates robust aPKC polarity, which in turn polarizes substrates by coupling phosphorylation to cortical release.

Phosphorylation-dependent interaction between tumor suppressors Dlg and Lgl

“…Upon phosphorylation, Lgl undergoes conformational changes causing its dissociation from the apical cortex and concomitant release of its inhibition on aPKC, and subsequent accumulation at the basal-lateral membranes [26,27]. Notably, expression of an unphosphorylatable Lgl (Lgl2 3A) led to severe defects in the segregation of cell fate determinants in the asymmetric cell division of Drosophila neuroblasts [28][29][30], probably due to constitutive inhibition of aPKC at the apical cortex. In MDCK cells, the Lgl2 3A mutant was shown to localize at the apical membrane and result in the severe cell polarity defects, whereas the wild-type Lgl mainly localizes at the basal-lateral region [31].…”

Widely Conserved Signaling Pathways in the Establishment of Cell Polarity

“…In addition to counteracting the inhibition of aPKC by Par-6, Cdc42 can also recruit the Par-6/aPKC complex to specific regions of the cell cortex where the GTPase is activated (see Prehoda 2009). For example, in Drosophila neuroblasts, Cdc42 mutants mislocalize Par-6/ aPKC to the cytoplasm (Atwood et al 2007), and in mammalian epithelial cells, knockdown of Cdc42 can partially mislocalize aPKC from the apical cortex (Martin-Belmonte et al 2007).…”

“…This spatial antagonism probably operates in the C. elegans zygote to maintain the distinct anterior-posterior distributions of Par-3/aPKC/Par-6 and Par-1, as well as in epithelial cells of Drosophila and mammals, and may be a conserved and widely applied method for excluding proteins from specific areas of the cell cortex. For example, the cell fate determinants Numb and Miranda, and the polarity protein Lgl, are also removed from the plasma membrane by aPKC-dependent phosphorylations (Betschinger et al 2003;Smith et al 2007;Atwood and Prehoda 2009;Prehoda 2009). An Arf-GAP that is involved in convergent extension during Xenopus gastrulation is phosphorylated by aPKC/Par-6, which may alter its subcellular distribution (Hyodo-Miura et al 2006).…”

“…This is a key step in ensuring asymmetric stem cell divisions in the Drosophila embryo. At the onset of mitosis, Aurora-A phosphorylates Par-6 within the amino-terminal PB1 domain that binds to aPKC, releasing Par-6 and relieving its inhibition of aPKC (see Prehoda 2009). The activated aPKC phosphorylates Lgl, which causes it to dissociate from the Par-6/aPKC complex and allows the interaction of the complex with Par-3.…”

“…Nonetheless, rapid progress is being made in our understanding of polarity signaling, which is outlined in this article, with an emphasis on the Par proteins, because these proteins play major roles integrating diverse signals that regulate cell polarity ( Fig. 1) (see Munro and Bowerman 2009;Prehoda 2009;Nelson 2009). …”