Epithelial-mesenchymal transition (EMT) is a cellular process during which epithelial cells acquire mesen chymal phenotypes and behaviour following the down regulation of epithelial features. EMT is triggered in response to signals that cells receive from their micro environment. The epithelial state of the cells in which EMT is initiated is characterized by stable epithelial cell-cell junctions, apical-basal polarity and interac tions with basement membrane. During EMT, changes in gene expression and posttranslational regulation mechanisms lead to the repression of these epithelial characteristics and the acquisition of mesenchymal char acteristics. Cells then display fibroblastlike morphol ogy and cytoarchitecture, as well as increased migratory capacity. Furthermore, these now migratory cells often acquire invasive properties (Fig. 1). EMT was first described by researchers studying early embryogenesis as a programme with welldefined cellular features 1,2. It is now widely accepted that EMT occurs normally during early embryonic development, to enable a variety of morphogenetic events, as well as later in development and during wound healing in adults.

To form epithelial organs cells must polarize and generate de novo an apical domain and lumen. Epithelial polarization is masterminded by polarity complexes, which are thought to direct downstream events such as polarized membrane traffic, though this interconnection is not well understood. We report that Rab11a regulates apical traffic and lumen formation via the Rab GEF Rabin8, and its target Rab8a. Rab8a/11a act via the exocyst to target Par3 to the apical surface, and control apical Cdc42 activation via the Cdc42 GEF, Tuba. These components assemble at a transient apical membrane initiation site to form the lumen. This Rab11a-directed network directs Cdc42-dependent apical exocytosis during lumen formation, revealing a novel interplay of the machineries of vesicular transport and polarization.Most internal epithelial organs consist of a monolayer of polarized epithelial cells surrounding a central lumen. Polarization requires the interaction of the signaling complexes and scaffolds that define cortical domains with the polarized membrane sorting machinery 1 . In yeast, traffic from the trans-Golgi network to the surface is regulated by Ypt32p and Sec4p 2 , homologs of mammalian Rab11 and Rab8, respectively. These are linked by Sec2p (homolog of mammalian Rabin8), a guanine nucleotide exchange factor (GEF) for Sec4p, which is recruited by Ypt32p. Sec2p and Sec4p in turn interact with the exocyst, which docks vesicles to the surface 3 .Definition of cortical domains in metazoa involves a complex of Par3, Par6, atypical PKC (aPKC), and the GTPase Cdc42 4 . This complex is a master regulator of polarity, conventionally depicted upstream of membrane trafficking machinery. How this complex interfaces with membrane transport is poorly understood.Here we show a molecular mechanism for lumen and apical surface formation, linking Rab8a/ 11a, exocyst, annexin2, Cdc42 and its GEF Tuba, and the Par3/aPKC complex. This novel NIH Public Access Author ManuscriptNat Cell Biol. Author manuscript; available in PMC 2010 November 8. Published in final edited form as:Nat Cell Biol. 2010 November ; 12(11): 1035-1045. doi:10.1038/ncb2106. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript pathway shows how the membrane traffic and cortical polarity machineries cooperate to generate the apical surface and lumen. RESULTS Apical polarization during lumen formationUpon plating into 3D culture, individual MDCK cells proliferate and assemble into cyst structures -a polarized spherical monolayer surrounding a central lumen. Lumenogenesis requires the apical membrane determinant gp135/podocalyxin 5 (PCX in figures). Initially, MDCK aggregates have podocalyxin at the ECM-contacting surface (Fig. 1a, 12 h; Fig. S1a), before polarity inversion occurs, with β-catenin and Na/K-ATPase at cell-cell junctions and podocalyxin now at the lumen (Fig. 1a, 24-48 h, arrows; Fig. S1d) 6,7 . Early lumens occur at a site previously termed the "Pre-Apical Patch" (PAP), where opposing plasma membranes are separated, but the po...

PTEN controls three-dimensional (3D) glandular morphogenesis by coupling juxtamembrane signaling to mitotic spindle machinery. While molecular mechanisms remain unclear, PTEN interacts through its C2 membrane-binding domain with the scaffold protein b-Arrestin1. Because b-Arrestin1 binds and suppresses the Cdc42 GTPase-activating protein ARHGAP21, we hypothesize that PTEN controls Cdc42-dependent morphogenic processes through a b-Arrestin1-ARHGAP21 complex. Here, we show that PTEN knockdown (KD) impairs b-Arrestin1 membrane localization, b-Arrestin1-ARHGAP21 interactions, Cdc42 activation, mitotic spindle orientation and 3D glandular morphogenesis. Effects of PTEN deficiency were phenocopied by b-Arrestin1 KD or inhibition of b-Arrestin1-ARHGAP21 interactions. Conversely, silencing of ARHGAP21 enhanced Cdc42 activation and rescued aberrant morphogenic processes of PTEN-deficient cultures. Expression of the PTEN C2 domain mimicked effects of full-length PTEN but a membrane-binding defective mutant of the C2 domain abrogated these properties. Our results show that PTEN controls multicellular assembly through a membrane-associated regulatory protein complex composed of b-Arrestin1, ARHGAP21 and Cdc42.

How do animal cells assemble into tissues and organs? A diverse array of tissue structures and shapes can be formed by organizing groups of cells into different polarized arrangements and by coordinating their polarity in space and time. Conserved design principles underlying this diversity are emerging from studies of model organisms and tissues. We discuss how conserved polarity complexes, signalling networks, transcription factors, membrane-trafficking pathways, mechanisms for forming lumens in tubes and other hollow structures, and transitions between different types of polarity, such as between epithelial and mesenchymal cells, are used in similar and iterative manners to build all tissues.The defining feature of metazoa is that their cells are organized into multicellular tissues and organs. Although almost every eukaryotic cell is spatially asymmetric or polarized, polarity must be coordinated in space and time for individual cells to form a tissue 1 . Cell polarity involves the asymmetric organization of most of the physical aspects of the cell, including the cell surface, intracellular organelles and the cytoskeleton 2,3 . Analysis of the polarization of unicellular eukaryotes, such as yeast, has yielded enormous insights into the mechanisms that underlie the polarity of individual cells 3 . Formation of a tissue, however, requires an ensemble cast; the emergent properties of the tissue result from the combined roles of the individual cells that are involved. Accordingly, several biological processes, including cell division, cell death, shape changes, cell migration and differentiation, must be coordinated with the polarity requirements of a tissue to form an organ 4 .Evolutionarily, epithelia are the most archetypal polarized tissues in metazoa, with ~60% of mammalian cell types being of epithelial or epithelial-derived origin 5 . Accordingly, the best studied polarized tissue is the simple epithelium of the mammalian intestine and kidney, the cells of which are columnar in shape (that is, they are taller than they are wide). The apical surfaces of these cells provide the luminal interface and are specialized to regulate the exchange of materials, such as nutrients from the intestine. The lateral surfaces of these cells contact adjacent cells and have specialized junctions and cell-cell adhesion structures 3,6 (FIG. 1a). The basal surfaces of these cells contact the underlying basement membrane, extracellular matrix (ECM) and, ultimately, underlying blood vessels. The basal and lateral surfaces are fairly similar in composition and organization and are often referred to together as the basolateral surface. The apical and basolateral surfaces, however, have very different NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript compositions. In vertebrates, tight junctions (TJs) are found at the apical-most portion of the lateral surfaces, where the TJs form barriers both between the apical and basolateral surfaces and between adjacent cells, limiting paracellular permeability...

How do individual cells organize into multicellular tissues? Here, we propose that the morphogenetic behaviour of epithelial cells is guided by two distinct elements: an intrinsic differentiation programme that drives formation of a lumen-enclosing monolayer, and a growth factor-induced, transient de-differentiation that allows this monolayer to be remodelled.