The apicobasal polarity kinase aPKC functions as a nuclear determinant and regulates cell proliferation and fate duringXenopusprimary neurogenesis

Abstract

During neurogenesis in Xenopus, apicobasally polarised superficial and non-polar deep cells take up different fates: deep cells become primary neurons while superficial cells stay as progenitors. It is not known whether the proteins that affect cell polarity also affect cell fate and how membrane polarity information may be transmitted to the nucleus. Here, we examine the role of the polarity components, apically enriched aPKC and basolateral Lgl2, in primary neurogenesis. We report that a membrane-tethered form of aPKC (aPKC-CAAX) suppresses primary neurogenesis and promotes cell proliferation. Unexpectedly, both endogenous aPKC and aPKC-CAAX show some nuclear localisation. A constitutively active aPKC fused to a nuclear localisation signal has the same phenotypic effect as aPKC-CAAX in that it suppresses neurogenesis and enhances proliferation. Conversely, inhibiting endogenous aPKC with a dominant-negative form that is restricted to the nucleus enhances primary neurogenesis. These observations suggest that aPKC has a function in the nucleus that is important for cell fate specification during primary neurogenesis. In a complementary experiment, overexpressing basolateral Lgl2 causes depolarisation and internalisation of superficial cells, which form ectopic neurons when supplemented with a proneural factor. These findings suggest that both aPKC and Lgl2 affect cell fate, but that aPKC is a nuclear determinant itself that might shuttle from the membrane to the nucleus to control cell proliferation and fate; loss of epithelial cell polarity by Lgl2 overexpression changes the position of the cells and is permissive for a change in cell fate.

Myosin Vb and rab11a regulate ezrin phosphorylation in enterocytes

“…S3, 2dox, yellow arrows indicate positive PDM puncta). aPKCi was also found in the nucleus, which has been reported previously in other cell types (Sabherwal et al, 2009). Some T567-phosphorylated ezrin was randomly localized to the cell cortex (supplementary material Fig.…”

PAR-1 promotes primary neurogenesis and asymmetric cell divisions via control of spindle orientation

“…1, could occur by two types of mechanism: ingression (delamination) of epithelial cells to deeper layers, or an increase in cell divisions with a horizontal cleavage plane in which one of the two daughters lies below, rather than within, the epithelium. Cell ingression from the superficial epithelial layer has previously been seen upon overexpression of the basolateral polarity protein Lgl2 (Sabherwal et al, 2009). PAR-1 perturbation produced by expression of a dominant-negative PAR-1 causes cells to be expelled from cultured mammalian epithelia (Bohm et al, 1997), suggesting that this could be occurring in the superficial ectodermal epithelium.…”

“…First, promotion of vertical spindle orientation increases the number of cells that lack apical membrane. Since apical membrane contains aPKC, which is an inhibitor of neurogenesis (Sabherwal et al, 2009), partitioning a daughter cell away from apical membrane is a mechanism for relieving this inhibition and is thus a novel form of aPKC antagonism. It is important to note that physical displacement of superficial cells into deep positions, either by dissection or by overexpression of Lgl2, is not sufficient for neurogenesis (Chalmers et al, 2002;Sabherwal et al, 2009;Strauss et al, 2006), and so it is this partitioning rather than the deep non-epithelial microenvironment that is important.…”

“…Since apical membrane contains aPKC, which is an inhibitor of neurogenesis (Sabherwal et al, 2009), partitioning a daughter cell away from apical membrane is a mechanism for relieving this inhibition and is thus a novel form of aPKC antagonism. It is important to note that physical displacement of superficial cells into deep positions, either by dissection or by overexpression of Lgl2, is not sufficient for neurogenesis (Chalmers et al, 2002;Sabherwal et al, 2009;Strauss et al, 2006), and so it is this partitioning rather than the deep non-epithelial microenvironment that is important. Second, PAR-1 loss-offunction shows that its effect on the cleavage plane is normally required to maintain the total number of deep cells that are generated, and it is only deep cells that are competent to become neurons (Chalmers et al, 2002).…”

When cell cycle meets development

“…Although there seems to be a fundamental link between apicobasal polarisation and the proliferative properties of neural progenitors, from Drosophila to mammals, the mechanistic links between these two processes are not well defined. When an activated version of the apicobasal polarity regulator atypical protein kinase C (aPKC) is expressed, neurogenesis is blocked and proliferation enhanced (Sabherwal et al, 2009). Nancy Papalopulu reported that aPKC mediates these effects by binding to, and phosphorylating, a key component of the cell cycle.…”