Sequential Roles of Cdc42, Par-6, aPKC, and Lgl in the Establishment of Epithelial Polarity during Drosophila Embryogenesis

Abstract

How epithelial cells subdivide their plasma membrane into an apical and a basolateral domain is largely unclear. In Drosophila embryos, epithelial cells are generated from a syncytium during cellularization. We show here that polarity is established shortly after cellularization when Par-6 and the atypical protein kinase C concentrate on the apical side of the newly formed cells. Apical localization of Par-6 requires its interaction with activated Cdc42 and dominant-active or dominant-negative Cdc42 disrupt epithelial polarity, suggesting that activation of this GTPase is crucial for the establishment of epithelial polarity. Maintenance of Par-6 localization requires the cytoskeletal protein Lgl. Genetic and biochemical experiments suggest that phosphorylation by aPKC inactivates Lgl on the apical side. On the basolateral side, Lgl is active and excludes Par-6 from the cell cortex, suggesting that complementary cortical domains are maintained by mutual inhibition of aPKC and Lgl on opposite sides of an epithelial cell.

Phosphorylation-dependent interaction between tumor suppressors Dlg and Lgl

“…When unphosphorylated, apical cell cortex-localized Lgl can interact with and inhibit the enzyme activity of aPKC [15,26]. 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].…”

“…In vivo imaging data have demonstrated that Dlg, Lgl and Scribble interdependently colocalize to the lateral membranes of epithelial cells and thereby specify the lateral membrane domain by excluding apical components from this region [2,6]. However, it is still not clear how Lgl is brought to the lateral membrane, although it is well established that apical atypical protein kinase C (aPKC) activity is required for excluding Lgl from apical membranes [15]. In dlg mutant embryos, Lgl basal membrane association is lost, indicating that Dlg may play a vital role in recruiting Lgl to the basal membranes [6].…”

Expression of Hugl-1 is strongly reduced in malignant melanoma

“…Recently, evidence was provided that mammalian Lgl forms complexes with Par6/atypical PKC regulating epithelial cell polarity (Plant et al, 2003;Yamanaka et al, 2003). Upon cell-cell contact-induced cell polarization, Lgl is phosphorylated by atypical protein kinase C (aPKC) resulting in a dissociation of Lgl from Par6/ aPKC followed by an accumulation of Lgl along the basolateral membrane, where it contributes to the formation of the basolateral membrane domain (Tanentzapf and Tepass, 2003;Hutterer et al, 2004) and to correct positioning of epithelial junctions (Borg, 2004). Additionally, recent studies revealed that Lgl has the ability to inhibit notch signaling (Justice et al, 2003).…”

Cdc42 deficiency causes Sonic hedgehog-independent holoprosencephaly

“…First, Cdc42 regulates PAR protein localization and controls the anteriorposterior polarity in Caenorhabditis elegans embryos (17,18). Secondly, Cdc42 sequentially directs Par-6, aPKC, and Lgl to establish the epithelial polarity in Drosophila (19). Thirdly, the mammalian Par-aPKC protein complex interacts with Cdc42 and Rac1 (20).…”