The Drosophila homolog of the Exo84 exocyst subunit promotes apical epithelial identity

The Scribble-Dlg-Lgl polarity module in development and cancer: from flies to man

The Scribble, Par and Crumbs modules were originally identified in the vinegar (fruit) fly, Drosophila melanogaster, as being critical regulators of apico-basal cell polarity. In the present chapter we focus on the Scribble polarity module, composed of Scribble, discs large and lethal giant larvae. Since the discovery of the role of the Scribble polarity module in apico-basal cell polarity, these proteins have also been recognized as having important roles in other forms of polarity, as well as regulation of the actin cytoskeleton, cell signalling and vesicular trafficking. In addition to these physiological roles, an important role for polarity proteins in cancer progression has also been uncovered, with loss of polarity and tissue architecture being strongly correlated with metastatic disease.

The apical polarity protein network in Drosophila epithelial cells: regulation of polarity, junctions, morphogenesis, cell growth, and survival

Epithelial tissue formation and function requires the apical-basal polarization of individual epithelial cells. Apical polarity regulators (APRs) are an evolutionarily conserved group of key factors that govern polarity and several other aspects of epithelial differentiation. APRs compose a diverse set of molecules including a transmembrane protein (Crumbs), a serine/threonine kinase (aPKC), a lipid phosphatase (PTEN), a small GTPase (Cdc42), FERM domain proteins (Moesin, Yurt), and several adaptor or scaffolding proteins (Bazooka/Par3, Par6, Stardust, Patj). These proteins form a dynamic cooperative network that is engaged in negative-feedback regulation with basolateral polarity factors to set up the epithelial apical-basal axis. APRs support the formation of the apical junctional complex and the segregation of the junctional domain from the apical membrane. It is becoming increasingly clear that APRs interact with the cytoskeleton and vesicle trafficking machinery, regulate morphogenesis, and modulate epithelial cell growth and survival. Not surprisingly, APRs have multiple fundamental links to human diseases such as cancer and blindness.

Sec5 and Exo84 mediate distinct aspects of RalA-dependent cell polarization

Metastasis is a complex process during which several gross cellular changes occur. Cells must dissociate from the tumor mass and gain the ability to degrade extracellular matrix and migrate in order to ultimately attach and form a satellite tumor. Regulation of the actin cytoskeleton is an indispensible aspect of cell migration, and many different factors have been implicated in this process. We identified interactions between RalA and its effectors in the Exocyst complex as directly necessary for migration and invasion of prostate cancer tumor cells. Blocking RalA-Exocyst binding caused significant morphological changes and defects in single and coordinated cell migration.

Stem cells: keeping BMP signaling local

In stem cell niches, the spatial extent of growth factor signaling needs to be tightly controlled. A new study on the Drosophila testicular germline stem cell niche has revealed that BMP signaling is locally activated through linkage to adherens junctions.

Rab8a regulates the exocyst-mediated kiss-and-run discharge of the Dictyostelium contractile vacuole

A molecular dissection of contractile vacuole (CV) discharge shows that Rab8a is recruited to the CV a few seconds before the exocyst. Together they tether it to the plasma membrane and commit it to fusion. GTP hydrolysis is necessary for vacuole detethering, a process in which LvsA, a protein of the Chédiak–Higashi family, plays a crucial role.

Water expulsion by the contractile vacuole (CV) in Dictyostelium is carried out by a giant kiss-and-run focal exocytic event during which the two membranes are only transiently connected but do not completely merge. We present a molecular dissection of the GTPase Rab8a and the exocyst complex in tethering of the contractile vacuole to the plasma membrane, fusion, and final detachment. Right before discharge, the contractile vacuole bladder sequentially recruits Drainin, a Rab11a effector, Rab8a, the exocyst complex, and LvsA, a protein of the Chédiak–Higashi family. Rab8a recruitment precedes the nucleotide-dependent arrival of the exocyst to the bladder by a few seconds. A dominant-negative mutant of Rab8a strongly binds to the exocyst and prevents recruitment to the bladder, suggesting that a Rab8a guanine nucleotide exchange factor activity is associated with the complex. Absence of Drainin leads to overtethering and blocks fusion, whereas expression of constitutively active Rab8a allows fusion but blocks vacuole detachment from the plasma membrane, inducing complete fragmentation of tethered vacuoles. An indistinguishable phenotype is generated in cells lacking LvsA, implicating this protein in postfusion detethering. Of interest, overexpression of a constitutively active Rab8a mutant reverses the lvsA-null CV phenotype.

Mutational analysis supports a core role for Drosophila α-catenin in adherens junction function

α-catenin associates the cadherin-catenin complex with the actin cytoskeleton. α-catenin binds to β-catenin, which links it to the cadherin cytoplasmic tail, and F-actin, but also to a multitude of actin-associated proteins. These interactions suggest a highly complex cadherin-actin interface. Moreover, mammalian αE-catenin has been implicated in a cadherin-independent cytoplasmic function in Arp2/3-dependent actin regulation, and in cell signaling. The function and regulation of individual molecular interactions of α-catenin, in particular during development, are not well understood. We have generated mutations in Drosophila α-Catenin (α-Cat) to investigate α-Catenin function in this model, and to establish a setup for testing α-Catenin-related constructs in α-Cat-null mutant cells in vivo. Our analysis of α-Cat mutants in embryogenesis, imaginal discs and oogenesis reveals defects consistent with a loss of cadherin function. Compromising components of the Arp2/3 complex or its regulator SCAR ameliorate the α-Cat loss-of-function phenotype in embryos but not in ovaries, suggesting negative regulatory interactions between α-Catenin and the Arp2/3 complex in some tissues. We also show that the α-Cat mutant phenotype can be rescued by the expression of a DE-cadherin::α-Catenin fusion protein, which argues against an essential cytosolic, cadherin-independent role of Drosophila α-Catenin.

An association between type Iγ PI4P 5-kinase and Exo70 directs E-cadherin clustering and epithelial polarization

E-Cadherin-mediated formation of adherens junctions (AJs) is essential for the morphogenesis of epithelial cells. However, the mechanisms underlying E-cadherin clustering and AJ maturation are not fully understood. Here we report that type Iγ phosphatidylinositol-4-phosphate 5-kinase (PIPKIγ) associates with the exocyst via a direct interaction with Exo70, the exocyst subunit that guides the polarized targeting of exocyst to the plasma membrane. By means of this interaction, PIPKIγ mediates the association between E-cadherin and Exo70 and determines the targeting of Exo70 to AJs. Further investigation revealed that Exo70 is necessary for clustering of E-cadherin on the plasma membrane and extension of nascent E-cadherin adhesions, which are critical for the maturation of cohesive AJs. In addition, we observed phosphatidylinositol-4,5-bisphosphate (PI4,5P(2)) accumulation at E-cadherin clusters during the assembly of E-cadherin adhesions. PIPKIγ-generated PI4,5P(2) is required for recruiting Exo70 to newly formed E-cadherin junctions and facilitates the assembly and maturation of AJs. These results support a model in which PIPKIγ and PIPKIγ-generated PI4,5P(2) pools at nascent E-cadherin contacts cue Exo70 targeting and orient the tethering of exocyst-associated E-cadherin. This could be an important mechanism that regulates E-cadherin clustering and AJ maturation, which is essential for the establishment of solid, polarized epithelial structures.

RalA and RalB differentially regulate development of epithelial tight junctions

Tight junctions (TJs) are structures indispensable to epithelial cells and are responsible for regulation of paracellular diffusion and maintenance of cellular polarity. Although many interactions between TJ constituents have been identified, questions remain concerning how specific functions of TJs are established and regulated. Here we investigated the roles of Ral GTPases and their common effector exocyst complex in the formation of nascent TJs. Unexpectedly, RNA interference-mediated suppression of RalA or RalB caused opposing changes in TJ development. RalA reduction increased paracellular permeability and decreased incorporation of components into TJs, whereas RalB reduction decreased paracellular permeability and increased incorporation of components into TJs. Activities of both Ral GTPases were mediated through the exocyst. Finally, we show that TJ-mediated separation of apical-basal membrane domains is established prior to equilibration of barrier function and that it is unaffected by Ral knockdown or specific composition of TJs.

Deconstructing host-pathogen interactions in Drosophila

Many of the cellular mechanisms underlying host responses to pathogens have been well conserved during evolution. As a result, Drosophila can be used to deconstruct many of the key events in host-pathogen interactions by using a wealth of well-developed molecular and genetic tools. In this review, we aim to emphasize the great leverage provided by the suite of genomic and classical genetic approaches available in flies for decoding details of host-pathogen interactions; these findings can then be applied to studies in higher organisms. We first briefly summarize the general strategies by which Drosophila resists and responds to pathogens. We then focus on how recently developed genome-wide RNA interference (RNAi) screens conducted in cells and flies, combined with classical genetic methods, have provided molecular insight into host-pathogen interactions, covering examples of bacteria, fungi and viruses. Finally, we discuss novel strategies for how flies can be used as a tool to examine how specific isolated virulence factors act on an intact host.

Apical-basal polarity in Drosophila neuroblasts is independent of vesicular trafficking

Cell polarity in epithelia depends on the PAR proteins, which interact with the machinery for exocytic and endocytic vesicular trafficking. Polarity in Drosophila neural stem cells is independent of vesicular trafficking, although it depends on the PAR proteins, revealing different mechanisms of how polarity is controlled.

The possession of apical–basal polarity is a common feature of epithelia and neural stem cells, so-called neuroblasts (NBs). In Drosophila, an evolutionarily conserved protein complex consisting of atypical protein kinase C and the scaffolding proteins Bazooka/PAR-3 and PAR-6 controls the polarity of both cell types. The components of this complex localize to the apical junctional region of epithelial cells and form an apical crescent in NBs. In epithelia, the PAR proteins interact with the cellular machinery for polarized exocytosis and endocytosis, both of which are essential for the establishment of plasma membrane polarity. In NBs, many cortical proteins show a strongly polarized subcellular localization, but there is little evidence for the existence of distinct apical and basolateral plasma membrane domains, raising the question of whether vesicular trafficking is required for polarization of NBs. We analyzed the polarity of NBs mutant for essential regulators of the main exocytic and endocytic pathways. Surprisingly, we found that none of these mutations affected NB polarity, demonstrating that NB cortical polarity is independent of plasma membrane polarity and that the PAR proteins function in a cell type–specific manner.

Retromer regulates apical-basal polarity through recycling Crumbs

Epithelial cells are characterized by an "apical-basal" polarization. The transmembrane protein Crumbs (Crb) is an essential apical determinant which confers apical membrane identity. Previous studies indicated that Crb did not constantly reside on the apical membrane, but was actively recycled. However, the cellular mechanism(s) underlying this process was unclear. Here we showed that in Drosophila, retromer, which was a retrograde complex recycling certain transmembrane proteins from endosomes to trans-Golgi network (TGN), regulated Crb in epithelial cells. In the absence of retromer, Crb was mis-targeted into lysosomes and degraded, causing a disruption of the apical-basal polarity. We further showed that Crb co-localized and interacted with retromer, suggesting that retromer regulated the retrograde recycling of Crb. Our data presented here uncover the role of retromer in regulating apical-basal polarity in epithelial cells and identify retromer as a novel regulator of Crb recycling.

Phosphorylation provides a negative mode of regulation for the yeast Rab GTPase Sec4p

The Rab family of Ras-related GTPases are part of a complex signaling circuitry in eukaryotic cells, yet we understand little about the mechanisms that underlie Rab protein participation in such signal transduction networks, or how these networks are integrated at the physiological level. Reversible protein phosphorylation is widely used by cells as a signaling mechanism. Several phospho-Rabs have been identified, however the functional consequences of the modification appear to be diverse and need to be evaluated on an individual basis. In this study we demonstrate a role for phosphorylation as a negative regulatory event for the action of the yeast Rab GTPase Sec4p in regulating polarized growth. Our data suggest that the phosphorylation of the Rab Sec4p prevents interactions with its effector, the exocyst component Sec15p, and that the inhibition may be relieved by a PP2A phosphatase complex containing the regulatory subunit Cdc55p.

Apical deficiency triggers JNK-dependent apoptosis in the embryonic epidermis of Drosophila

Epithelial homeostasis and the avoidance of diseases such as cancer require the elimination of defective cells by apoptosis. Here, we investigate how loss of apical determinants triggers apoptosis in the embryonic epidermis of Drosophila. Transcriptional profiling and in situ hybridisation show that JNK signalling is upregulated in mutants lacking Crumbs or other apical determinants. This leads to transcriptional activation of the pro-apoptotic gene reaper and to apoptosis. Suppression of JNK signalling by overexpression of Puckered, a feedback inhibitor of the pathway, prevents reaper upregulation and apoptosis. Moreover, removal of endogenous Puckered leads to ectopic reaper expression. Importantly, disruption of the basolateral domain in the embryonic epidermis does not trigger JNK signalling or apoptosis. We suggest that apical, not basolateral, integrity could be intrinsically required for the survival of epithelial cells. In apically deficient embryos, JNK signalling is activated throughout the epidermis. Yet, in the dorsal region, reaper expression is not activated and cells survive. One characteristic of these surviving cells is that they retain discernible adherens junctions despite the apical deficit. We suggest that junctional integrity could restrain the pro-apoptotic influence of JNK signalling.

Emerging role for epithelial polarity proteins of the Crumbs family as potential tumor suppressors

Defects in apical-basal polarity regulation are associated with tissue overgrowth and tumorogenesis, yet the molecular mechanisms linking epithelial polarity regulators to hyperplasia or neoplasia remain elusive. In addition, exploration of the expression and function of the full complement of proteins required for the polarized architecture of epithelial cells in the context of cancer is awaited. This paper provides an overview of recent studies performed on Drosophila and vertebrates showing that apical polarity proteins of the Crumbs family act to repress tissue growth and epithelial to mesenchymal transition. Thus, these proteins emerge as potential tumor suppressors. Interestingly, analysis of the molecular function of Crumbs proteins reveals a function for these polarity regulators in junctional complexes stability and control of signaling pathways regulating proliferation and apoptosis. Thereby, these studies provide a molecular basis explaining how regulation of epithelial polarity is coupled to tumorogenesis.

Local BMP receptor activation at adherens junctions in the Drosophila germline stem cell niche

According to the stem cell niche synapse hypothesis postulated for the mammalian haematopoietic system, spatial specificity of niche signals is maximized by subcellularly restricting signalling to cadherin-based adherens junctions between individual niche and stem cells. However, such a synapse has never been observed directly, in part, because tools to detect active growth factor receptors with subcellular resolution were not available. Here we describe a novel fluorescence-based reporter that directly visualizes bone morphogenetic protein (BMP) receptor activation and show that in the Drosophila testis a BMP niche signal is transmitted preferentially at adherens junctions between hub and germline stem cells, resembling the proposed synapse organization. Ligand secretion involves the exocyst complex and the Rap activator Gef26, both of which are also required for Cadherin trafficking towards adherens junctions. We, therefore, propose that local generation of the BMP signal is achieved through shared use of the Cadherin transport machinery.

Cadherin-dependent cell morphology in an epithelium: constructing a quantitative dynamical model

Cells in the Drosophila retina have well-defined morphologies that are attained during tissue morphogenesis. We present a computer simulation of the epithelial tissue in which the global interfacial energy between cells is minimized. Experimental data for both normal cells and mutant cells either lacking or misexpressing the adhesion protein N-cadherin can be explained by a simple model incorporating salient features of morphogenesis that include the timing of N-cadherin expression in cells and its temporal relationship to the remodeling of cell-cell contacts. The simulations reproduce the geometries of wild-type and mutant cells, distinguish features of cadherin dynamics, and emphasize the importance of adhesion protein biogenesis and its timing with respect to cell remodeling. The simulations also indicate that N-cadherin protein is recycled from inactive interfaces to active interfaces, thereby modulating adhesion strengths between cells.

Tissues are intricate, heterogeneous systems, consisting of individual cells whose shapes and relative positions are of great importance to the tissue's function, as well as to its formation during morphogenesis. To make progress in our understanding of the formation of organs, their malfunction, and their therapeutic replacement in regenerative medicine, it is crucial to elucidate the connection between shape and function. We have developed a quantitative mechanical model of an epithelial tissue, the retina of Drosophila, and compare the modeling results with experimental data. The model successfully predicts shape changes induced by different expression levels of cell-cell adhesion molecules. Furthermore, the model gives new insight into the changes a tissue undergoes during morphogenesis. Comparing simulations and experiments, we are able to accept or reject different hypotheses about morphogenetic dynamics. In this way, we can identify the time course of adhesion molecule synthesis and of cell-cell contact, as well as gain new insight into the regulation of adhesion strength. Given the prominent role of adhesion in wound healing, cancer research, and many other fields, our fundamental work introduces a novel modeling tool of universal applicability and importance.

Retromer controls epithelial cell polarity by trafficking the apical determinant Crumbs

The evolutionarily conserved apical determinant Crumbs (Crb) is essential for maintaining apicobasal polarity and integrity of many epithelial tissues [1]. Crb levels are crucial for cell polarity and homeostasis, yet strikingly little is known about its trafficking or the mechanism of its apical localization. Using a newly established, liposome-based system described here, we determined Crb to be an interaction partner and cargo of the retromer complex. Retromer is essential for the retrograde transport of numerous transmembrane proteins from endosomes to the trans-Golgi network (TGN) and is conserved between plants, fungi, and animals [2]. We show that loss of retromer function results in a substantial reduction of Crb in Drosophila larvae, wing discs, and the follicle epithelium. Moreover, loss of retromer phenocopies loss of crb by preventing apical localization of key polarity molecules, such as atypical protein kinase C (aPKC) and Par6 in the follicular epithelium, an effect that can be rescued by overexpression of Crb. Additionally, loss of retromer results in multilayering of the follicular epithelium, indicating that epithelial integrity is severely compromised. Our data reveal a mechanism for Crb trafficking by retromer that is vital for maintaining Crb levels and localization. We also show a novel function for retromer in maintaining epithelial cell polarity.

Dynamics of adherens junctions in epithelial establishment, maintenance, and remodeling

The epithelial cadherin (E-cadherin)–catenin complex binds to cytoskeletal components and regulatory and signaling molecules to form a mature adherens junction (AJ). This dynamic structure physically connects neighboring epithelial cells, couples intercellular adhesive contacts to the cytoskeleton, and helps define each cell’s apical–basal axis. Together these activities coordinate the form, polarity, and function of all cells in an epithelium. Several molecules regulate AJ formation and integrity, including Rho family GTPases and Par polarity proteins. However, only recently, with the development of live-cell imaging, has the extent to which E-cadherin is actively turned over at junctions begun to be appreciated. This turnover contributes to junction formation and to the maintenance of epithelial integrity during tissue homeostasis and remodeling.

Elaborating polarity: PAR proteins and the cytoskeleton

Cell polarity is essential for cells to divide asymmetrically, form spatially restricted subcellular structures and participate in three-dimensional multicellular organization. PAR proteins are conserved polarity regulators that function by generating cortical landmarks that establish dynamic asymmetries in the distribution of effector proteins. Here, we review recent findings on the role of PAR proteins in cell polarity in C. elegans and Drosophila, and emphasize the links that exist between PAR networks and cytoskeletal proteins that both regulate PAR protein localization and act as downstream effectors to elaborate polarity within the cell.

Complex interactions between GSK3 and aPKC in Drosophila embryonic epithelial morphogenesis

Generally, epithelial cells must organize in three dimensions to form functional tissue sheets. Here we investigate one such sheet, the Drosophila embryonic epidermis, and the morphogenetic processes organizing cells within it. We report that epidermal morphogenesis requires the proper distribution of the apical polarity determinant aPKC. Specifically, we find roles for the kinases GSK3 and aPKC in cellular alignment, asymmetric protein distribution, and adhesion during the development of this polarized tissue. Finally, we propose a model explaining how regulation of aPKC protein levels can reorganize both adhesion and the cytoskeleton.

Protein complexes that control renal epithelial polarity

Establishment of epithelial apicobasal polarity is crucial for proper kidney development and function. In recent years, there have been important advances in our understanding of the factors that mediate the initiation of apicobasal polarization. Key among these are the polarity complexes that are evolutionarily conserved from simple organisms to humans. Three of these complexes are discussed in this review: the Crumbs complex, the Par complex, and the Scribble complex. The apical Crumbs complex consists of three proteins, Crumbs, PALS1, and PATJ, whereas the apical Par complex consists of Par-3, Par-6, and atypical protein kinase C. The lateral Scribble complex consists of Scribble, discs large, and lethal giant larvae. These complexes modulate kinase and small G protein activity such that the apical and basolateral complexes signal antagonistically, leading to the segregation of the apical and basolateral membranes. The polarity complexes also serve as scaffolds to direct and retain proteins at the apical membrane, the basolateral membrane, or the intervening tight junction. There is plasticity in apicobasal polarity, and this is best seen in the processes of epithelial-to-mesenchymal transition and the converse mesenchymal-to-epithelial transition. These transitions are important in kidney disease as well as kidney development, and modulation of the polarity complexes are critical for these transitions.

Discs large in the Drosophila testis: an old player on a new task

Gamete development requires a coordinated soma-germ line interaction that ensures renewal and differentiation of germline and somatic stem cells. The physical contact between the germline and somatic cell populations is crucial because it allows the exchange of diffusible signals among them. The tumor suppressor gene discs large (dlg) encodes a septate junction protein with functions in epithelial cell polarity, asymmetric neuroblast division and formation of neuromuscular junctions. Our recent work reveals a new role of dlg in the Drosophila testis, as mutations in dlg lead to testis defects and cell death. Dlg is required throughout spermatogenesis in the somatic lineage and its localization changes from a uniform distribution along the plasma membrane of somatic cells in the testis apex, to a restricted localization on the distally located somatic cell in growing cysts. The extensive defects in dlg testis underline the importance of the somatic cells in the establishment and maintenance of the male stem cell niche and somatic cell differentiation. Here, we discuss our latest findings on the role of dlg in the Drosophila testis, supporting the view that junction proteins are dynamic structures, which can provide guiding cues to recruit scaffold proteins or other signaling molecules.

Recycling endosomes in apical plasma membrane domain formation and epithelial cell polarity

Recycling endosomes have taken central stage in the intracellular sorting and polarized trafficking of apical and basolateral plasma membrane components. Molecular players in the underlying mechanisms are now emerging, including small GTPases, class V myosins and adaptor proteins. In particular, defects in the expression or function of these recycling endosome-associated and endosome-regulating proteins have been implicated in cell surface polarity defects and diseases, including microvillus inclusion disease, arthrogryposis-renal dysfunction-cholestasis syndrome, and virus susceptibility. Key findings are that recycling endosomes recruit and deliver core polarity proteins to lateral cell surfaces and initiate the biogenesis of apical plasma membrane domains and epithelial cell polarity. Here, we review recent data that implicate recycling endosomes in the establishment and maintenance of epithelial cell polarity.

Remodeling epithelial cell organization: transitions between front-rear and apical-basal polarity

Polarized epithelial cells have a distinctive apical-basal axis of polarity for vectorial transport of ions and solutes across the epithelium. In contrast, migratory mesenchymal cells have a front-rear axis of polarity. During development, mesenchymal cells convert to epithelia by coalescing into aggregates that undergo epithelial differentiation. Signaling networks and protein complexes comprising Rho family GTPases, polarity complexes (Crumbs, PAR, and Scribble), and their downstream effectors, including the cytoskeleton and the endocytic and exocytic vesicle trafficking pathways, together regulate the distributions of plasma membrane and cytoskeletal proteins between front-rear and apical-basal polarity. The challenge is to understand how these regulators and effectors are adapted to regulate symmetry breaking processes that generate cell polarities that are specialized for different cellular activities and functions.

Spatially defined Dsh-Lgl interaction contributes to directional tissue morphogenesis

The process of epithelial morphogenesis defines the structure of epidermal tissue sheets. One such sheet, the ventral epidermis of the Drosophila embryo, shows both intricate segmental patterning and complex cell organization. Within a segment, cells produce hair-like denticles in a stereotypical and highly organized pattern over the surface of the tissue. To understand the cell biological basis of this process, we examined cell shapes and alignments, and looked for molecules that showed an asymmetric distribution in this tissue. We found that apical polarity determinants and adherens junctions were enriched at the dorsal and ventral borders of cells, whereas basolateral determinants were enriched at the anterior and posterior borders. We report that the basolateral determinant Lgl has a novel function in the planar organization of the embryonic epidermis, and this function depends on Dsh and myosin. We conclude that apical-basal proteins, used to establish polarity within a cell, can be independently co-opted to function in epithelial morphogenesis.

Cell polarity in eggs and epithelia: parallels and diversity

Cell polarity, the generation of cellular asymmetries, is necessary for diverse processes in animal cells, such as cell migration, asymmetric cell division, epithelial barrier function, and morphogenesis. Common mechanisms generate and transduce cell polarity in different cells, but cell type-specific processes are equally important. In this review, we highlight the similarities and differences between the polarity mechanisms in eggs and epithelia. We also highlight the prospects for future studies on how cortical polarity interfaces with other cellular processes, such as morphogenesis, exocytosis, and lipid signaling, and how defects in polarity contribute to tumor formation.

Rab-mediated vesicular transport is required for neuronal positioning in the developing Drosophila visual system

BACKGROUND

The establishment of tissue architecture in the nervous system requires the proper migration and positioning of newly born neurons during embryonic development. Defects in nuclear translocation, a key process in neuronal positioning, are associated with brain diseases such as lissencephaly in humans. Accumulated evidence suggests that the molecular mechanisms controlling neuronal movement are conserved throughout evolution. While the initial events of neuronal migration have been extensively studied, less is known about the molecular details underlying the establishment of neuronal architecture after initial migration.

RESULTS

In a search for novel players in the control of photoreceptor (R cell) positioning in the developing fly visual system, we found that misexpression of the RabGAP RN-Tre disrupted the apical localization of R-cell nuclei. RN-Tre interacts with Rab5 and Rab11 in the fly eye. Genetic analysis shows that Rab5, Shi and Rab11 are required for maintaining apical localization of R-cell nuclei.

CONCLUSIONS

We propose that Rab5, Shi and Rab11 function together in a vesicular transport pathway for regulating R-cell positioning in the developing eye.

Trafficking through COPII stabilises cell polarity and drives secretion during Drosophila epidermal differentiation

Background

The differentiation of an extracellular matrix (ECM) at the apical side of epithelial cells implies massive polarised secretion and membrane trafficking. An epithelial cell is hence engaged in coordinating secretion and cell polarity for a correct and efficient ECM formation.

Principal Findings

We are studying the molecular mechanisms that Drosophila tracheal and epidermal cells deploy to form their specific apical ECM during differentiation. In this work we demonstrate that the two genetically identified factors haunted and ghost are essential for polarity maintenance, membrane topology as well as for secretion of the tracheal luminal matrix and the cuticle. We show that they code for the Drosophila COPII vesicle-coating components Sec23 and Sec24, respectively, that organise vesicle transport from the ER to the Golgi apparatus.

Conclusion

Taken together, epithelial differentiation during Drosophila embryogenesis is a concerted action of ECM formation, plasma membrane remodelling and maintenance of cell polarity that all three rely mainly, if not absolutely, on the canonical secretory pathway from the ER over the Golgi apparatus to the plasma membrane. Our results indicate that COPII vesicles constitute a central hub for these processes.

Phosphatidylinositol 4,5-bisphosphate directs spermatid cell polarity and exocyst localization in Drosophila

During spermiogenesis, Drosophila melanogaster spermatids coordinate their elongation in interconnected cysts that become highly polarized, with nuclei localizing to one end and sperm tail growth occurring at the other. Remarkably little is known about the signals that drive spermatid polarity and elongation. Here we identify phosphoinositides as critical regulators of these processes. Reduction of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) by low-level expression of the PIP(2) phosphatase SigD or mutation of the PIP(2) biosynthetic enzyme Skittles (Sktl) results in dramatic defects in spermatid cysts, which become bipolar and fail to fully elongate. Defects in polarity are evident from the earliest stages of elongation, indicating that phosphoinositides are required for establishment of polarity. Sktl and PIP(2) localize to the growing end of the cysts together with the exocyst complex. Strikingly, the exocyst becomes completely delocalized when PIP(2) levels are reduced, and overexpression of Sktl restores exocyst localization and spermatid cyst polarity. Moreover, the exocyst is required for polarity, as partial loss of function of the exocyst subunit Sec8 results in bipolar cysts. Our data are consistent with a mechanism in which localized synthesis of PIP(2) recruits the exocyst to promote targeted membrane delivery and polarization of the elongating cysts.

Doing cell biology in embryos: regulated membrane traffic and its implications for cadherin biology

Regulated trafficking of cadherin adhesion molecules is often invoked as a mechanism to generate dynamic adhesive cell-cell contacts for tissue modeling and morphogenesis. The past 2-3 years have seen several important papers that tackle the cell biology of cadherin trafficking in organismal systems to provide new insights into both mechanism and morphogenetic impact.

Characterization of the Arabidopsis thaliana exocyst complex gene families by phylogenetic, expression profiling, and subcellular localization studies

*The exocyst is a complex of eight proteins (Sec3p, Sec5p, Sec6p, Sec8p, Sec10p, Sec15p, Exo70p and Exo84p) involved in tethering vesicles to the plasma membrane during regulated or polarized secretion. Here, the plant exocyst complex was explored in phylogenetic, expression, and subcellular localization studies. *Evolutionary relationships of predicted exocyst subunits were examined in the complete genomes of Arabidopsis thaliana, Oryza sativa, Populus trichocarpa and Physcomitrella patens. Furthermore, detailed expression profiling of the A. thaliana microarray databases was performed and subcellular localization patterns were studied. *Several plant exocyst subunit genes appear to have undergone gene expansion in a common ancestor and subsequent duplication events in independent plant lineages. Expression profiling revealed that the A. thaliana Exo70 gene family exhibits dynamic expression patterns, while the remaining exocyst subunit genes displayed more static profiles. Subcellular localization patterns for A. thaliana exocyst subunits ranged from cytosolic to endosomal compartments (with enrichment in the early endosomes and the trans-Golgi network). Interestingly, two endosomal-localized AtExo70 proteins also recruited other exocyst subunits to these compartments. *Overall subcellular localization patterns were observed that were also found in yeast and animal cells, and this, coupled with the evolutionary relationships, suggests that the exocyst may perform similar conserved functions in plants.

Sec3-containing exocyst complex is required for desmosome assembly in mammalian epithelial cells

In epithelial cells, Sec3 associates with Exocyst complexes enriched at desmosomes and centrosomes, distinct from Sec6/8 complexes at the apical junctional complex. RNAi-mediated suppression of Sec3 alters trafficking of desmosomal cadherins and impairs desmosome morphology and function, without noticeable effect on adherens junctions.

The Exocyst is a conserved multisubunit complex involved in the docking of post-Golgi transport vesicles to sites of membrane remodeling during cellular processes such as polarization, migration, and division. In mammalian epithelial cells, Exocyst complexes are recruited to nascent sites of cell–cell contact in response to E-cadherin–mediated adhesive interactions, and this event is an important early step in the assembly of intercellular junctions. Sec3 has been hypothesized to function as a spatial landmark for the development of polarity in budding yeast, but its role in epithelial cells has not been investigated. Here, we provide evidence in support of a function for a Sec3-containing Exocyst complex in the assembly or maintenance of desmosomes, adhesive junctions that link intermediate filament networks to sites of strong intercellular adhesion. We show that Sec3 associates with a subset of Exocyst complexes that are enriched at desmosomes. Moreover, we found that membrane recruitment of Sec3 is dependent on cadherin-mediated adhesion but occurs later than that of the known Exocyst components Sec6 and Sec8 that are recruited to adherens junctions. RNA interference-mediated suppression of Sec3 expression led to specific impairment of both the morphology and function of desmosomes, without noticeable effect on adherens junctions. These results suggest that two different exocyst complexes may function in basal–lateral membrane trafficking and will enable us to better understand how exocytosis is spatially organized during development of epithelial plasma membrane domains.

Membrane organization and dynamics in cell polarity

The establishment and maintenance of cell polarity is important to a wide range of biological processes ranging from chemotaxis to embryogenesis. An essential feature of cell polarity is the asymmetric organization of proteins and lipids in the plasma membrane. In this article, we discuss how polarity regulators such as small GTP-binding proteins and phospholipids spatially and kinetically control vesicular trafficking and membrane organization. Conversely, we discuss how membrane trafficking contributes to cell polarization through delivery of polarity determinants and regulators to the plasma membrane.

Rab11 helps maintain apical crumbs and adherens junctions in the Drosophila embryonic ectoderm

BACKGROUND

Tissue morphogenesis and organogenesis require that cells retain stable cell-cell adhesion while changing shape and moving. One mechanism to accommodate this plasticity in cell adhesion involves regulated trafficking of junctional proteins.

METHODOLOGY/PRINCIPAL FINDINGS

Here we explored trafficking of junctional proteins in two well-characterized model epithelia, the Drosophila embryonic ectoderm and amnioserosa. We find that DE-cadherin, the transmembrane protein of adherens junctions, is actively trafficked through putative vesicles, and appears to travel through both Rab5-positive and Rab11-positive structures. We manipulated the functions of Rab11 and Rab5 to examine the effects on junctional stability and morphogenesis. Reducing Rab11 function, either using a dominant negative construct or loss of function alleles, disrupts integrity of the ectoderm and leads to loss of adherens junctions. Strikingly, the apical junctional regulator Crumbs is lost before AJs are destabilized, while the basolateral protein Dlg remains cortical. Altering Rab5 function had less dramatic effects, not disrupting adherens junction integrity but affecting dorsal closure.

CONCLUSIONS/SIGNIFICANCE

We contrast our results with what others saw when disrupting other trafficking regulators, and when disrupting Rab function in other tissues; together these data suggest distinct mechanisms regulate junctional stability and plasticity in different tissues.

Epithelial polarity: interactions between junctions and apical-basal machinery

Epithelial polarity is established and maintained by competition between determinants that define the apical and basolateral domains. Cell-cell adhesion complexes, or adherens junctions, form at the interface of these regions. Mutations in adhesion components as well as apical determinants normally lead to an expansion of the basolateral domain. Here we investigate the genetic relationship between the polarity determinants and adhesion and show that the levels of the adhesion protein Armadillo affect competition. We find that in arm mutants, even a modest reduction in the basolateral component lgl leads to a full apical domain expansion or lgl phenotype. By using an allelic series of Armadillo mutations, we show that there is a threshold at which basolateral expansion can be reversed. Further, in embryos lacking the Wingless signaling component zw3, the same full apical expansion occurs again with only a reduction in lgl. We propose a model where zw3 regulates protein levels of apical and adhesion components and suggest that a reciprocal interaction between junctions and polarity modules functions to maintain stable apical and basolateral domains.

Crumbs stabilises epithelial polarity during tissue remodelling

The apicobasal polarity of epithelia depends on the integrated activity of apical and basolateral proteins, and is essential for tissue integrity and body homeostasis. Yet these tissues are frequently on the move as they are sculpted by active morphogenetic cell rearrangements. How does cell polarity survive these stresses? We analyse this question in the renal tubules of Drosophila, a tissue that undergoes dramatic morphogenetic change as it develops. Here we show that, whereas the Bazooka and Scribble protein groups are required for the establishment of tubule cell polarity, the key apical determinant, Crumbs, is required for cell polarity in the tubules only from the time when morphogenetic movements start. Strikingly, if these movements are stalled, polarity persists in the absence of Crumbs. Similar rescue of the ectodermal phenotype of the crumbs mutant when germ-band extension is reduced suggests that Crumbs has a specific, conserved function in stabilising cell polarity during tissue remodelling rather than in its initial stabilisation. We also identify a requirement for the exocyst component Exo84 during tissue morphogenesis, which suggests that Crumbs-dependent stability of epithelial polarity is correlated with a requirement for membrane recycling and targeted vesicle delivery.

Junctional trafficking and epithelial morphogenesis

Epithelial monolayers are major determinants of three-dimensional tissue organization and provide the structural foundation for the body plan and all of its component organs. Epithelial cells are connected by junctional complexes containing the cell adhesion molecule E-cadherin. Adherens junctions mediate stable cohesion between cells but must be actively reorganized to allow tissue remodeling during development. Recent studies demonstrate that junctional proteins are dynamically turned over at the cell surface, even in cells that do not appear to be moving. The redistribution of E-cadherin through spatially regulated endocytosis and exocytosis contributes to cell adhesion, cell polarity, and cell rearrangement. Here we describe recent progress in understanding the roles of the vesicle transport machinery in regulating cell adhesion and junctional dynamics during epithelial morphogenesis in vivo.

Endocytosis and spatial restriction of cell signaling

Endocytosis and recycling are essential components of the wiring enabling cells to perceive extracellular signals and transduce them in a temporally and spatially controlled fashion, directly influencing not only the duration and intensity of the signaling output, but also their correct location. Here, we will discuss key experimental evidence that support how different internalization routes, the generation of diverse endomembrane platforms, and cycles of internalization and recycling ensure polarized compartmentalization of signals, regulating a number of physiological and pathologically-relevant processes in which the resolution of spatial information is vital for their execution.

RalA and the exocyst complex influence neuronal polarity through PAR-3 and aPKC

Neuronal polarization requires localized cytoskeletal changes and polarized membrane traffic. Here, I report that the small GTPase RalA, previously shown to control neurite branching, also regulates neuronal polarity. RalA depletion, or ectopic expression of constitutively active RalA in cultured neurons inhibit axon formation. However, expression of a constitutively active RalA mutant that is unable to interact with the exocyst complex has no effect on neuronal polarization. Furthermore, depletion of the Sec6, Sec8 or Exo84 subunits of the exocyst complex also leads to unpolarized neurons. Early stages of neuronal polarization are accompanied by increasing levels of interaction of the exocyst complex with PAR-3 and atypical protein kinase C (aPKC), and by the RalA-dependent association of the exocyst complex with PAR-3. Thus, neuronal polarization involves a RalA-regulated association between mediators of vesicle trafficking (exocyst complex) and cell polarity (PAR-3).

Control of tumourigenesis by the Scribble/Dlg/Lgl polarity module

The neoplastic tumour suppressors, Scribble, Dlg and Lgl, originally discovered in the vinegar fly Drosophila melanogaster, are currently being actively studied for their potential role in mammalian tumourigenesis. In Drosophila, these tumour suppressors function in a common genetic pathway to regulate apicobasal cell polarity and also play important roles in the control of cell proliferation, survival, differentiation and in cell migration/invasion. The precise mechanism by which Scribble, Dlg and Lgl function is not clear; however, they have been implicated in the regulation of signalling pathways, vesicle trafficking and in the Myosin II-actin cytoskeleton. We review the evidence for the involvement of Scribble, Dlg, and Lgl in cancer, and how the various functions ascribed to these tumour suppressors in Drosophila and mammalian systems may impact on the process of tumourigenesis.

Cdc42 and Par proteins stabilize dynamic adherens junctions in the Drosophila neuroectoderm through regulation of apical endocytosis

Cell rearrangements require dynamic changes in cell-cell contacts to maintain tissue integrity. We investigated the function of Cdc42 in maintaining adherens junctions (AJs) and apical polarity in the Drosophila melanogaster neuroectodermal epithelium. About one third of cells exit the epithelium through ingression and become neuroblasts. Cdc42-compromised embryos lost AJs in the neuroectoderm during neuroblast ingression. In contrast, when neuroblast formation was suppressed, AJs were maintained despite the loss of Cdc42 function. Loss of Cdc42 function caused an increase in the endocytotic uptake of apical proteins, including apical polarity factors such as Crumbs, which are required for AJ stability. In addition, Cdc42 has a second function in regulating endocytotic trafficking, as it is required for the progression of apical cargo from the early to the late endosome. The Par complex acts as an effector for Cdc42 in controlling the endocytosis of apical proteins. This study reveals functional interactions between apical polarity proteins and endocytosis that are critical for stabilizing dynamic basolateral AJs.

Regulating polarity by directing traffic: Cdc42 prevents adherens junctions from crumblin' aPart

The GTPase Cdc42 was among the original genes identified with roles in cell polarity, and interest in its cellular roles from yeast to humans remains high. Cdc42 is a well-known regulator of the actin cytoskeleton, but also plays important roles in vesicular trafficking. In this issue, Harris and Tepass (Harris, K.P, and U. Tepass. 2008. J. Cell. Biol. 183:1129–1143) provide new insights into how Cdc42 and Par proteins work together to modulate cell adhesion and polarity during embryonic morphogenesis by regulating the traffic of key cell junction proteins.

Coordinated protein sorting, targeting and distribution in polarized cells

The polarized distribution of functions in polarized cells requires the coordinated interaction of three machineries that modify the basic mechanisms of intracellular protein trafficking and distribution. First, intrinsic protein-sorting signals and cellular decoding machineries regulate protein trafficking to plasma membrane domains; second, intracellular signalling complexes define the plasma membrane domains to which proteins are delivered; and third, proteins that are involved in cell-cell and cell-substrate adhesion orientate the three-dimensional distribution of intracellular signalling complexes and, accordingly, the direction of membrane traffic. The integration of these mechanisms into a complex and dynamic network is crucial for normal tissue function and is often defective in disease states.

Ribbon modulates apical membrane during tube elongation through Crumbs and Moesin

Although the formation and maintenance of epithelial tubes are essential for the viability of multicellular organisms, our understanding of the molecular and cellular events coordinating tubulogenesis is relatively limited. Here, we focus on the activities of Ribbon, a novel BTB-domain containing nuclear protein, in the elongation of two epithelial tubes: the Drosophila salivary gland and trachea. We show that Ribbon interacts with Lola Like, another BTB-domain containing protein required for robust nuclear localization of Ribbon, to upregulate crumbs expression and downregulate Moesin activity. Our ultrastructural analysis of ribbon null salivary glands by TEM reveals a diminished pool of subapical vesicles and an increase in microvillar structure, cellular changes consistent with the known role of Crumbs in apical membrane generation and of Moesin in the cross-linking of the apical membrane to the subapical cytoskeleton. Furthermore, the subapical localization of Rab11, a small GTPase associated with apical membrane delivery and rearrangement, is significantly diminished in ribbon mutant salivary glands and tracheae. These findings suggest that Ribbon and Lola Like function as a novel transcriptional cassette coordinating molecular changes at the apical membrane of epithelial cells to facilitate tube elongation.