PTEN-mediated apical segregation of phosphoinositides controls epithelial morphogenesis through Cdc42

Modeling morphogenesis and oncogenesis in three-dimensional breast epithelial cultures

Three-dimensional (3D) epithelial culture systems recreate the cardinal features of glandular epithelium in vivo and represent a valuable tool for modeling breast cancer initiation and progression in a structurally appropriate context. 3D models have emerged as a powerful method to interrogate the biological activities of cancer genes and oncogenic pathways, and recent studies have poignantly illustrated their utility in dissecting the emerging role of tensional force in regulating epithelial tissue homeostasis. We review how 3D models are being used to investigate fundamental cellular and biophysical mechanisms associated with breast cancer progression that have not been readily amenable to traditional genetic or biochemical analysis.

Chemical inhibition through conformational stabilization of Rho GTPase effectors

The Rho family of small GTP-binding proteins can activate a large number of downstream effectors and participate in a wide variety of biological processes, including cell motility, membrane trafficking, cell polarity, gene transcription, and mitosis. Specific small-molecule inhibitors of individual effector proteins downstream of Rho GTPases would be powerful tools to elucidate the contributions of particular effectors to these processes. In this chapter we describe the identification of a chemical inhibitor of a Rho effector and scaffolding protein neural-Wiskott-Aldrich syndrome protein (N-WASP), and the discovery of its novel mechanism of action, stabilization of N-WASP's native autoinhibited conformation. Inasmuch as several other Rho GTPase effectors are regulated by autoinhibition, we discuss how this regulatory mechanism could be exploited by small molecules to develop highly specific inhibitors of other Rho GTPase effectors. We illustrate this concept with the Rac/Cdc42 effector p21-activated kinase (Pak1) and the Rho effector mammalian diaphanous-related formin (mDia1).

Cell polarity in development and cancer

The development of cancer is a multistep process in which the DNA of a single cell accumulates mutations in genes that control essential cellular processes. Loss of cell-cell adhesion and cell polarity is commonly observed in advanced tumours and correlates well with their invasion into adjacent tissues and the formation of metastases. Growing evidence indicates that loss of cell-cell adhesion and cell polarity may also be important in early stages of cancer. The strongest hints in this direction come from studies on tumour suppressor genes in the fruitfly Drosophila melanogaster, which have revealed their importance in the control of apical-basal cell polarity.

Annexin A2 phosphorylation mediates cell scattering and branching morphogenesis via cofilin Activation

Dynamic remodeling of the actin cytoskeleton is required for cell spreading, motility, and migration and can be regulated by tyrosine kinase activity. Phosphotyrosine proteomic screening revealed phosphorylation of the lipid-, calcium-, and actin-binding protein annexin A2 (AnxA2) at Tyr23 as a major event preceding ts-v-Src kinase-induced cell scattering. Expression of the phospho-mimicking mutant Y23E-AnxA2 itself was sufficient to induce actin reorganization and cell scattering in MDCK cells. While Y23E-AnxA2, but not Y23A-AnxA2, enhanced Src- or hepatocyte growth factor (HGF)-induced cell scattering, short hairpin RNA-mediated knockdown of AnxA2 inhibited both v-Src- and HGF-induced cell scattering. Three-dimensional branching morphogenesis was induced in wild-type-AnxA2-expressing cells only in the presence of HGF, while Y23E-AnxA2 induced HGF-independent branching morphogenesis. Knockdown of AnxA2 prevented lumen formation during cystogenesis. The Y23E-AnxA2-induced scattering was associated with dephosphorylation/activation of the actin-severing protein cofilin. Likewise, inactive S3E-cofilin and constitutively active LIM kinase, a direct upstream kinase of cofilin, inhibited Y23E-AnxA2-induced scattering. Together, our studies indicate an essential role for AnxA2 phosphorylation in regulating cofilin-dependent actin cytoskeletal dynamics in the context of cell scattering and branching morphogenesis.

Analysis of detergent-resistant membranes of Helicobacter pylori infected gastric adenocarcinoma cells reveals a role for MARK2/Par1b in CagA-mediated disruption of cellular polarity

Detergent-resistant membranes of eukaryotic cells are enriched in many important cellular signalling molecules and frequently targeted by bacterial pathogens. To learn more about pathogenic mechanisms of Helicobacter pylori and to elucidate novel effects on host epithelial cells, we investigated how bacterial co-cultivation changes the protein composition of detergent-resistant membranes of gastric adenocarcinoma (AGS) tissue culture cells. Using iTRAQ (isobaric tags for relative and absolute quantification) analysis we identified several cellular proteins, which are potentially related to H. pylori virulence. One of the proteins, which showed a significant infection-dependent increase in detergent resistance, was the polarity-associated serine/threonine kinase MARK2 (EMK1/Par-1b). We demonstrate that H. pylori causes the recruitment of MARK2 from the cytosol to the plasma membrane, where it colocalizes with the bacteria and interacts with CagA. Using Mardin Darby Canine Kidney (MDCK) monolayers and a three-dimensional MDCK tissue culture model we showed that association of CagA with MARK2 not only causes disruption of apical junctions, but also inhibition of tubulogenesis and cell differentiation.

A role of the Lowe syndrome protein OCRL in early steps of the endocytic pathway

Mutations in the inositol 5-phosphatase OCRL are responsible for Lowe syndrome, whose manifestations include mental retardation and renal Fanconi syndrome. OCRL has been implicated in membrane trafficking, but disease mechanisms remain unclear. We show that OCRL visits late-stage, endocytic clathrin-coated pits and binds the Rab5 effector APPL1 on peripheral early endosomes. The interaction with APPL1, which is mediated by the ASH-RhoGAP-like domains of OCRL and is abolished by disease mutations, provides a link to protein networks implicated in the reabsorptive function of the kidney and in the trafficking and signaling of growth factor receptors in the brain. Crystallographic studies reveal a role of the ASH-RhoGAP-like domains in positioning the phosphatase domain at the membrane interface and a clathrin box protruding from the RhoGAP-like domain. Our results support a role of OCRL in the early endocytic pathway, consistent with the predominant localization of its preferred substrates, PI(4,5)P(2) and PI(3,4,5)P(3), at the cell surface.

Modeling tissue morphogenesis and cancer in 3D

Three-dimensional (3D) in vitro models span the gap between two-dimensional cell cultures and whole-animal systems. By mimicking features of the in vivo environment and taking advantage of the same tools used to study cells in traditional cell culture, 3D models provide unique perspectives on the behavior of stem cells, developing tissues and organs, and tumors. These models may help to accelerate translational research in cancer biology and tissue engineering.

alpha6beta4 integrin activates Rac-dependent p21-activated kinase 1 to drive NF-kappaB-dependent resistance to apoptosis in 3D mammary acini

Malignant transformation and multidrug resistance are linked to resistance to apoptosis, yet the molecular mechanisms that mediate tumor survival remain poorly understood. Because the stroma can influence tumor behavior by regulating the tissue phenotype, we explored the role of extracellular matrix signaling and tissue organization in epithelial survival. We report that elevated (alpha6)beta4 integrin-dependent Rac-Pak1 signaling supports resistance to apoptosis in mammary acini by permitting stress-dependent activation of the p65 subunit of NF-kappaB through Pak1. We found that inhibiting Pak1 through expression of N17Rac or PID compromises NF-kappaB activation and renders mammary acini sensitive to death, but that resistance to apoptosis could be restored to these structures by overexpressing wild-type NF-kappaB p65. We also observed that acini expressing elevated levels of Pak1 can activate p65 and survive death treatments, even in the absence of activated Rac, yet will die if activation of NF-kappaB is simultaneously inhibited through expression of IkappaBalphaM. Thus, mammary tissues can resist apoptotic stimuli by activating NF-kappaB through alpha6beta4 integrin-dependent Rac-Pak1 signaling. Our data emphasize the importance of the extracellular matrix stroma in tissue survival and suggest that alpha6beta4 integrin-dependent Rac stimulation of Pak1 could be an important mechanism mediating apoptosis-resistance in some breast tumors.

AMER1 regulates the distribution of the tumor suppressor APC between microtubules and the plasma membrane

APC is a multifunctional tumor suppressor protein that negatively controls Wnt signaling, but also regulates cell adhesion and migration by interacting with the plasma membrane and the microtubule cytoskeleton. Although the molecular basis for the microtubule association of APC is well understood, molecular mechanisms that underlie its plasma membrane localization have remained elusive. We show here that APC is recruited to the plasma membrane by binding to APC membrane recruitment 1 (AMER1), a novel membrane-associated protein that interacts with the ARM repeat domain of APC. The N-terminus of AMER1 contains two distinct phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P(2)]-binding domains, which mediate its localization to the plasma membrane. Overexpression of AMER1 increases APC levels and redirects APC from microtubule ends to the plasma membrane of epithelial cells. Conversely, siRNA-mediated knockdown of AMER1 reduces the overall levels of APC, promotes its association with microtubule ends in cellular protrusions and disturbs intercellular junctions. These data indicate that AMER1 controls the subcellular distribution of APC between membrane- and microtubule-associated pools, and might thereby regulate APC-dependent cellular morphogenesis, cell migration and cell-cell adhesion.

Conditional deletion of Pten causes bronchiolar hyperplasia

Tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a lipid phosphatase that regulates multiple cellular processes including cell polarity, migration, proliferation, and carcinogenesis. In this work, we demonstrate that conditional deletion of Pten (Pten(Delta/Delta)) in the respiratory epithelial cells of the developing mouse lung caused epithelial cell proliferation and hyperplasia as early as 4 to 6 weeks of age. While bronchiolar cell differentiation was normal, as indicated by beta-tubulin and FOXJ1 expression in ciliated cells and by CCSP expression in nonciliated cells, cell proliferation (detected by expression of Ki-67, phospho-histone-H3, and cyclin D1) was increased and associated with activation of the AKT/mTOR survival pathway. Deletion of Pten caused papillary epithelial hyperplasia characterized by a hypercellular epithelium lining papillae with fibrovascular cores that protruded into the airway lumens. Cell polarity, as assessed by subcellular localization of cadherin, beta-catenin, and zonula occludens-1, was unaltered. PTEN is required for regulation of epithelial cell proliferation in the lung and for the maintenance of the normal simple columnar epithelium characteristics of bronchi and bronchioles.

Apical junctional complexes and cell polarity

Recent studies have greatly expanded our knowledge of initial events that lead to epithelial cell polarity. Epithelial polarity is defined, in part, by apical cell-cell tight junctions that separate the plasma membrane into the apical domain and the basolateral domain, as well as the zonula adherens that mediate intercellular adhesion. The process of epithelial polarization is closely coupled to the biogenesis of these junctions. Studies in mammalian epithelial cells and lower organisms have identified two evolutionarily conserved junctional complexes as important epithelia polarity regulators: the Crumbs complex and the partitioning defective complex. Disruption of the components of the two complexes leads to a disorder of epithelial cell polarity and defects in junction formation or maintenance. Recent discoveries have revealed more details of how the two junctional polarity complexes function to establish epithelial polarity. They also raised the question about the relationship between polarity and adhesion. Although it is widely accepted that cell-cell adhesion provides a landmark from which polarity can proceed, there are results pointing to the possibility that polarity complexes can regulate cell-cell adhesion. It seems likely that proteins that control cell adhesion and cell polarity work intimately together to establish final epithelial polarity.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Sequential pulses of apical epithelial secretion and endocytosis drive airway maturation in Drosophila

The development of air-filled respiratory organs is crucial for survival at birth. We used a combination of live imaging and genetic analysis to dissect respiratory organ maturation in the embryonic Drosophila trachea. We found that tracheal tube maturation entails three precise epithelial transitions. Initially, a secretion burst deposits proteins into the lumen. Solid luminal material is then rapidly cleared from the tubes, and shortly thereafter liquid is removed. To elucidate the cellular mechanisms behind these transitions, we identified gas-filling-deficient mutants showing narrow or protein-clogged tubes. These mutations either disrupt endoplasmatic reticulum-to-Golgi vesicle transport or endocytosis. First, Sar1 is required for protein secretion, luminal matrix assembly, and diametric tube expansion. Subsequently, a sharp pulse of Rab5-dependent endocytic activity rapidly internalizes and clears luminal contents. The coordination of luminal matrix secretion and endocytosis may be a general mechanism in tubular organ morphogenesis and maturation.

Rac1 is required for reorientation of polarity and lumen formation through a PI 3-kinase-dependent pathway

Epithelial cells are characterized by the ability to form sheets of cells that surround fluid-filled lumens. Cells in these sheets exhibit a characteristic subcellular polarity, with an apical pole that faces the lumen and a basolateral pole that is in contact with other cells and the extracellular matrix (ECM). To investigate the signaling events required for polarization and lumen formation, we have taken advantage of the ability of Madin-Darby canine kidney (MDCK) cells to dynamically remodel their polarity in response to changes in ECM cues. When MDCK cells are grown in suspension culture, they form multicellular "inside-out" cysts with apical proteins found on the peripheral surface and basolateral markers on the interior surface. When these inside-out cysts are embedded in ECM, they rapidly reorient their polarity: apical proteins become localized to the inside surface, and basolateral proteins are found on the surface that contacts ECM. Here we have characterized the signaling requirements for these early molecular reorientation events. Specifically, expression of a dominant-negative form of Rac1 (DN-Rac1) blocks the reorientation of polarity. Phosphoinositide 3'-kinase is required for apical membrane protein remodeling from the initial apical membrane surface. Cells expressing DN-Rac1 fail to detectably activate the PI 3-kinase/protein kinase B pathway. Last, we found that atypical protein kinase C (aPKC) is also required for reorientation of polarity, since an inhibitor of atypical PKC blocks reorientation. This effect cannot be overcome by constitutively active Rac1, demonstrating that both Rac1 and atypical PKC are required for reorientation of cellular polarity.

Transcriptional modulation of genes encoding structural characteristics of differentiating enterocytes during development of a polarized epithelium in vitro

Although there is considerable evidence implicating posttranslational mechanisms in the development of epithelial cell polarity, little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized the temporal program of gene expression during cell-cell adhesion-initiated polarization of human Caco-2 cells in tissue culture, which develop structural and functional polarity similar to that of enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts between neighboring cells. Expression of genes involved in cell proliferation was down-regulated concomitant with induction of genes necessary for functional specialization of polarized epithelial cells. Transcriptional up-regulation of these latter genes correlated with formation of important structural and functional features in enterocyte differentiation and establishment of structural and functional cell polarity; components of the apical microvilli were induced as the brush border formed during polarization; as barrier function was established, expression of tight junction transmembrane proteins peaked; transcripts encoding components of the apical, but not the basal-lateral trafficking machinery were increased during polarization. Coordinated expression of genes encoding components of functional cell structures were often observed indicating temporal control of expression and assembly of multiprotein complexes.

S100A10/p11: family, friends and functions

S100A10, also known as p11 or annexin 2 light chain, is a member of the S100 family of small, dimeric EF hand-type Ca(2+)-binding proteins that generally modulate cellular target proteins in response to intracellular Ca(2+) signals. In contrast to all other S100 proteins, S100A10 is Ca(2+) insensitive because of amino acid replacements in its Ca(2+)-binding loops that lock the protein in a permanently active state. Within cells, the majority of S100A10 resides in a tight heterotetrameric complex with the peripheral membrane-binding protein annexin A2 that directs the complex to specific target membranes, in particular the plasma membrane and the membrane of early endosomes. Several other Ca(2+)-independent interaction partners of S100A10 have been described in the recent past. Many of these interactions, which have been shown to be of functional significance for the respective partner, involve plasma membrane-resident proteins. In most of these cases, S100A10, probably residing in a complex with annexin A2, appears to regulate the intracellular trafficking of the respective target protein and thus its functional expression at the cell surface. In this paper, we review the current information on S100A10 protein interactions placing a particular emphasis on data that contribute to an understanding of the mechanistic basis of the S100A10 action. Based on these data, we propose that S100A10 functions as a linker tethering certain transmembrane proteins to annexin A2 thereby assisting their traffic to the plasma membrane and/or their firm anchorage at certain membrane sites.

Regulation of the actin cytoskeleton by phosphatidylinositol 4-phosphate 5 kinases

Phosphatidylinositol (4,5)-bisphosphate (PIP(2)) is an important lipid mediator that has multiple regulatory functions. There is now increasing evidence that the phosphatidylinositol 4-phosphate 5 kinases (PIP5Ks), which synthesize PIP(2), are regulated spatially and temporally and that they have isoform-specific functions and regulations. This review will summarize the highlights of recent developments in understanding how the three major PIP5K isoforms regulate the actin cytoskeleton and other important cellular processes.

Recycling endosomes of polarized epithelial cells actively sort apical and basolateral cargos into separate subdomains

The plasma membranes of epithelial cells plasma membranes contain distinct apical and basolateral domains that are critical for their polarized functions. However, both domains are continuously internalized, with proteins and lipids from each intermixing in supranuclear recycling endosomes (REs). To maintain polarity, REs must faithfully recycle membrane proteins back to the correct plasma membrane domains. We examined sorting within REs and found that apical and basolateral proteins were laterally segregated into subdomains of individual REs. Subdomains were absent in unpolarized cells and developed along with polarization. Subdomains were formed by an active sorting process within REs, which precedes the formation of AP-1B-dependent basolateral transport vesicles. Both the formation of subdomains and the fidelity of basolateral trafficking were dependent on PI3 kinase activity. This suggests that subdomain and transport vesicle formation occur as separate sorting steps and that both processes may contribute to sorting fidelity.

Formation of cysts by alveolar type II cells in three-dimensional culture reveals a novel mechanism for epithelial morphogenesis

Many organs consist of a hollow cavity surrounded by a monolayer of epithelial cells. Despite their common structure, such organs form by diverse morphogenetic processes. Three-dimensional culture systems have been useful in analyzing the events. Most processes require a combination of cell proliferation and cell death to produce a hollow cavity. Here, we describe a new three-dimensional culture system in which primary human lung alveolar type II cells formed hollow epithelial cysts by a novel process. Individual cells moved, collided, and formed alveolar-like cysts without appreciable proliferation or apoptosis. The alveolar-like cysts consisted of a polarized monolayer of differentiated alveolar type II cells, which secreted surfactant into the central lumen. Blockage of beta1 integrin did not alter cell movement or collision, but it greatly reduced adhesion of cells after collision and subsequent formation of alveolar-like cysts. Treatment of preformed alveolar-like cysts with forskolin increased their diameter, possibly due to stimulation of fluid secretion into the lumen. We conclude that epithelial differentiation and cyst formation can occur without appreciable proliferation or apoptosis.