Polarized deposition of basement membrane proteins depends on Phosphatidylinositol synthase and the levels of Phosphatidylinositol 4,5-bisphosphate
ABSTRACT The basement membrane (BM), a specialized sheet of the extracellular matrix contacting the basal side of epithelial tissues, plays an important role in the control of the polarized structure of epithelial cells. However, little is known about how BM proteins themselves achieve a polarized distribution. Here, we identify phosphatidylinositol 4,5-bisphosphate (PIP2) as a critical regulator of the polarized secretion of BM proteins. A decrease of PIP2 levels, in particular through mutations in Phosphatidylinositol synthase (Pis) and other members of the phosphoinositide pathway, leads to the aberrant accumulation of BM components at the apical side of the cell without primarily affecting the distribution of apical and basolateral polarity proteins. In addition, PIP2 controls the apical and lateral localization of Crag (Calmodulin-binding protein related to a Rab3 GDP/GTP exchange protein), a factor specifically required to prevent aberrant apical secretion of BM. We propose that PIP2, through the control of Crag's subcellular localization, restricts the secretion of BM proteins to the basal side.
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ABSTRACT: During embryonic development, polarized epithelial cells are either formed during cleavage or formed from mesenchymal cells. Because the formation of epithelia during embryogenesis has to occur with high fidelity to ensure proper development, embryos allow a functional approach to study epithelial cell polarization in vivo. In particular, genetic model organisms have greatly advanced our understanding of the generation and maintenance of epithelial cell polarity. Many novel and important polarity genes have been identified and characterized in invertebrate systems, like Drosophila melanogaster and Caenorhabditis elegans. With the rapid identification of mammalian homologues of these invertebrate polarity genes, it has become clear that many important protein domains, single proteins and even entire protein complexes are evolutionarily conserved. It is to be expected that the field of epithelial cell polarity is just experiencing the 'top of the iceberg' of a large protein network that is fundamental for the specific adhesive, cell signalling and transport functions of epithelial cells.Mechanisms of Development 12/2003; 120(11):1231-56. DOI:10.1016/j.mod.2003.06.001 · 2.24 Impact Factor
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ABSTRACT: In a genetic screen we isolated mutations in CG10260, which encodes a phosphatidylinositol 4-kinase (PI4KIIIalpha), and found that PI4KIIIalpha is required for Hippo signaling in Drosophila ovarian follicle cells. PI4KIIIalpha mutations in the posterior follicle cells lead to oocyte polarization defects similar to those caused by mutations in the Hippo signaling pathway. PI4KIIIalpha mutations also cause misexpression of well-established Hippo signaling targets. The Merlin-Expanded-Kibra complex is required at the apical membrane for Hippo activity. In PI4KIIIalpha mutant follicle cells, Merlin fails to localize to the apical domain. Our analysis of PI4KIIIalpha mutants provides a new link in Hippo signal transduction from the cell membrane to its core kinase cascade.Development 03/2011; 138(9):1697-703. DOI:10.1242/dev.059279 · 6.27 Impact Factor
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ABSTRACT: Drosophila Crumbs (Crb) and its mammalian ortholog CRB3 control epithelial polarity through poorly understood molecular mechanisms. Elucidating these mechanisms is crucial, because the physiology of epithelia largely depends on the polarized architecture of individual epithelial cells. In addition, loss of CRB3 favors tumor cell growth, metastasis and epithelial to mesenchymal transition (EMT). Using Drosophila embryos, we report that Rac1 sustains PI3K signaling, which is required for Rac1 activation. Crb represses this positive-feedback loop. Notably, this property confers to Crb its ability to promote epithelial integrity in vivo, because attenuation of either Rac1 or PI3K activity rescues the crb mutant phenotype. Moreover, inhibition of Rac1 or PI3K results in Crb-dependent apical membrane growth, whereas Rac1 activation restricts membrane localization of Crb and interferes with apical domain formation. This illustrates that Crb and the Rac1-PI3K module are antagonists, and that the fine balance between the activities of these proteins is crucial to maintain epithelial organization and an appropriate apical to basolateral ratio. Together, our results elucidate a mechanism that mediates Crb function and further define the role of PI3K and Rac1 in epithelial morphogenesis, allowing for a better understanding of how distinct membrane domains are regulated in polarized epithelial cells.Journal of Cell Science 10/2011; 124(Pt 20):3393-8. DOI:10.1242/jcs.092601 · 5.33 Impact Factor