Drosophila Cip4 and WASp define a branch of the Cdc42-Par6-aPKC pathway regulating E-cadherin endocytosis

dCIP4 (Drosophila Cdc42-interacting protein 4) restrains synaptic growth by inhibiting the secretion of the retrograde Glass bottom boat signal

The bone morphogenetic protein (BMP) ligand Glass bottom boat (Gbb) acts as a retrograde growth signal at the Drosophila neuromuscular junction (NMJ). Endocytic regulation of presynaptic BMP receptors has been proposed to attenuate retrograde BMP signaling. However, it remains unknown whether the Gbb signal is also regulated by postsynaptic mechanisms. Here, we provide evidence that Drosophila Cdc42-interacting protein 4 (dCIP4) functions postsynaptically to inhibit synaptic growth. dCIP4 is localized postsynaptically at NMJs. dcip4 mutations lead to synaptic overgrowth and increased presynaptic phosphorylated mothers against decapentaplegic (Mad) levels, and these defects are rescued by muscle-specific expression of dCIP4. Biochemical and genetic analyses demonstrate that dCIP4 acts downstream of Cdc42 to activate the postsynaptic Wsp-Arp2/3 pathway. We also show that BMP signaling is necessary for synaptic overgrowth in larvae lacking postsynaptic dcip4 or wsp. Finally, dCIP4 and Wsp inhibit Gbb secretion. Thus, we propose that dCIP4 restrains synaptic growth by inhibiting postsynaptic Gbb secretion through the Wsp-Arp2/3 pathway.

The Cdc42 GEF Intersectin 2 controls mitotic spindle orientation to form the lumen during epithelial morphogenesis

Epithelial organs are made of tubes and cavities lined by a monolayer of polarized cells that enclose the central lumen. Lumen formation is a crucial step in the formation of epithelial organs. The Rho guanosine triphosphatase (GTPase) Cdc42, which is a master regulator of cell polarity, regulates the formation of the central lumen in epithelial morphogenesis. However, how Cdc42 is regulated during this process is still poorly understood. Guanine nucleotide exchange factors (GEFs) control the activation of small GTPases. Using the three-dimensional Madin-Darby canine kidney model, we have identified a Cdc42-specific GEF, Intersectin 2 (ITSN2), which localizes to the centrosomes and regulates Cdc42 activation during epithelial morphogenesis. Silencing of either Cdc42 or ITSN2 disrupts the correct orientation of the mitotic spindle and normal lumen formation, suggesting a direct relationship between these processes. Furthermore, we demonstrated this direct relationship using LGN, a component of the machinery for mitotic spindle positioning, whose disruption also results in lumen formation defects.

Crumbs/DaPKC-dependent apical exclusion of Bazooka promotes photoreceptor polarity remodeling

BACKGROUND

In Drosophila epithelial cells, specification and maintenance of the zonula adherens (za) is crucial to ensure epithelial tissue integrity. This depends on the intertwined function of Bazooka (Baz), Par6-DaPKC, and the Crumbs (Crb)-Stardust (Sdt)-PATJ complex. However, the detailed molecular basis for the interplay between these factors during this process is not fully understood.

RESULTS

We demonstrate that during photoreceptor apicobasal polarity remodeling, Crb is required to exclude Baz from the subapical domain. This is achieved by recruiting Par6 and DaPKC to this membrane domain. This molecular sorting depends on Baz phosphorylation by DaPKC at the conserved serine 980 and on the activity of the small GTPase Cdc42 associated with Par6. Our data indicate that although Cdc42 binding to Par6 is not required for Baz phosphorylation by DaPKC, it is required for optimum recruitment of Crb at the subapical membrane, a process necessary for delineating the nascent za from this membrane domain.

CONCLUSION

Binding of Cdc42 to the DaPKC regulatory subunit Par6 is required to promote Crb- and DaPKC-dependent apical exclusion of Baz. This molecular sorting mechanism results in setting up the boundary between the photoreceptor subapical membrane and the za.

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.

Neurons derive from the more apical daughter in asymmetric divisions in the zebrafish neural tube

In the developing CNS, asymmetric cell division is critical for maintaining the balanced production of differentiating neurons while renewing the population of neural progenitors. In invertebrates, this process depends on asymmetric inheritance of fate determinants during progenitor divisions. A similar mechanism is widely believed to underlie asymmetrically fated divisions in vertebrates, but compelling evidence for this is missing. We used live imaging of individual progenitors in the intact zebrafish embryo CNS to test this hypothesis. We found that asymmetric inheritance of a subcellular domain is strongly correlated with asymmetric daughter fates and our results reveal an unexpected feature of this process. The daughter cell destined to become a neuron was derived from the more apical of the two daughters, whereas the more basal daughter inherited the basal process and replenished the apical progenitor pool.

Sertoli-germ cell junctions in the testis: a review of recent data

Spermatogenesis is a process that involves an array of cellular and biochemical events, collectively culminating in the formation of haploid spermatids from diploid precursor cells known as spermatogonia. As germ cells differentiate from spermatogonia into elongated spermatids, they also progressively migrate across the entire length of the seminiferous epithelium until they reach the luminal edge in anticipation of spermiation at late stage VIII of spermatogenesis. At the same time, these germ cells must maintain stable attachment with Sertoli cells via testis-unique intermediate filament- (i.e. desmosome-like junctions) and actin- (i.e. ectoplasmic specializations, ESs) based cell junctions to prevent sloughing of immature germ cells from the seminiferous epithelium, which may result in infertility. In essence, both desmosome-like junctions and basal ESs are known to coexist between Sertoli cells at the level of the blood-testis barrier where they cofunction with the well-studied tight junction in maintaining the immunological barrier. However, the type of anchoring device that is present between Sertoli and germ cells depends on the developmental stage of the germ cell, i.e. desmosome-like junctions are present between Sertoli and germ cells up to, but not including, step 8 spermatids after which this junction type is replaced by the apical ES. While little is known about the biology of the desmosome-like junction in the testis, we have a relatively good understanding of the molecular architecture and the regulation of the ES. Here, we discuss recent findings relating to these two junction types in the testis, highlighting prospective areas that should be investigated in future studies.

PAR-3 mediates the initial clustering and apical localization of junction and polarity proteins during C. elegans intestinal epithelial cell polarization

The apicobasal polarity of epithelial cells is critical for organ morphogenesis and function, and loss of polarity can promote tumorigenesis. Most epithelial cells form when precursor cells receive a polarization cue, develop distinct apical and basolateral domains and assemble junctions near their apical surface. The scaffolding protein PAR-3 regulates epithelial cell polarity, but its cellular role in the transition from precursor cell to polarized epithelial cell has not been determined in vivo. Here, we use a targeted protein-degradation strategy to remove PAR-3 from C. elegans embryos and examine its cellular role as intestinal precursor cells become polarized epithelial cells. At initial stages of polarization, PAR-3 accumulates in cortical foci that contain E-cadherin, other adherens junction proteins, and the polarity proteins PAR-6 and PKC-3. Using live imaging, we show that PAR-3 foci move apically and cluster, and that PAR-3 is required to assemble E-cadherin into foci and for foci to accumulate at the apical surface. We propose that PAR-3 facilitates polarization by promoting the initial clustering of junction and polarity proteins that then travel and accumulate apically. Unexpectedly, superficial epidermal cells form apical junctions in the absence of PAR-3, and we show that PAR-6 has a PAR-3-independent role in these cells to promote apical junction maturation. These findings indicate that PAR-3 and PAR-6 function sequentially to position and mature apical junctions, and that the requirement for PAR-3 can vary in different types of epithelial cells.

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.

A vertebrate-specific Chp-PAK-PIX pathway maintains E-cadherin at adherens junctions during zebrafish epiboly

Background

In early vertebrate development, embryonic tissues modulate cell adhesiveness and acto-myosin contractility to correctly orchestrate the complex processes of gastrulation. E-cadherin (E-cadh) is the earliest expressed cadherin and is needed in the mesendodermal progenitors for efficient migration [1], [2]. Regulatory mechanisms involving directed E-cadh trafficking have been invoked downstream of Wnt11/5 signaling [3]. This non-canonical Wnt pathway regulates RhoA-ROK/DAAM1 to control the acto-myosin network. However, in this context nothing is known of the intracellular signals that participate in the correct localization of E-cadh, other than a need for Rab5c signaling [3].

Methodology/Principal Findings

By studying loss of Chp induced by morpholino-oligonucleotide injection in zebrafish, we find that the vertebrate atypical Rho-GTPase Chp is essential for the proper disposition of cells in the early embryo. The underlying defect is not leading edge F-actin assembly (prominent in the cells of the envelope layer), but rather the failure to localize E-cadh and β-catenin at the adherens junctions. Loss of Chp results in delayed epiboly that can be rescued by mRNA co-injection, and phenocopies zebrafish E-cadh mutants [4], [5]. This new signaling pathway involves activation of an effector kinase PAK, and involvement of the adaptor PAK-interacting exchange factor PIX. Loss of signaling by any of the three components results in similar underlying defects, which is most prominent in the epithelial-like envelope layer.

Conclusions/Significance

Our current study uncovers a developmental pathway involving Chp/PAK/PIX signaling, which helps co-ordinate E-cadh disposition to promote proper cell adhesiveness, and coordinate movements of the three major cell layers in epiboly. Our data shows that without Chp signaling, E-cadh shifts to intracellular vesicles rather than the adhesive contacts needed for directed cell movement. These events may mirror the requirement for PAK2 signaling essential for the proper formation of the blood-brain barrier [6], [7].

GSK3beta affects apical-basal polarity and cell-cell adhesion by regulating aPKC levels

The dynamic rearrangement of cell-cell contacts is required for the establishment of functional epithelial cell sheets. However, the signaling pathways and cellular mechanisms that initiate and maintain this polarity are not well understood. We show that loss of the Wnt signaling component GSK3 beta results in increased levels of aPKC and leads to defects in apical-basal polarity. We find that GSK3 beta directly phosphorylates aPKC, which likely promotes its ubiquitin-mediated proteosomal degradation. aPKC increases the levels of Armadillo and stabilizes adherens junctions. These results suggest that the Wnt pathway component GSK3 beta regulates the polarity determinant aPKC, which in turn affects cell-cell contacts during the development of polarized tissues.

A modifier screen for Bazooka/PAR-3 interacting genes in the Drosophila embryo epithelium

Background

The development and homeostasis of multicellular organisms depends on sheets of epithelial cells. Bazooka (Baz; PAR-3) localizes to the apical circumference of epithelial cells and is a key hub in the protein interaction network regulating epithelial structure. We sought to identify additional proteins that function with Baz to regulate epithelial structure in the Drosophila embryo.

Methodology/Principal Findings

The baz zygotic mutant cuticle phenotype could be dominantly enhanced by loss of known interaction partners. To identify additional enhancers, we screened molecularly defined chromosome 2 and 3 deficiencies. 37 deficiencies acted as strong dominant enhancers. Using deficiency mapping, bioinformatics, and available single gene mutations, we identified 17 interacting genes encoding known and predicted polarity, cytoskeletal, transmembrane, trafficking and signaling proteins. For each gene, their loss of function enhanced adherens junction defects in zygotic baz mutants during early embryogenesis. To further evaluate involvement in epithelial polarity, we generated GFP fusion proteins for 15 of the genes which had not been found to localize to the apical domain previously. We found that GFP fusion proteins for Drosophila ASAP, Arf79F, CG11210, Septin 5 and Sds22 could be recruited to the apical circumference of epithelial cells. Nine of the other proteins showed various intracellular distributions, and one was not detected.

Conclusions/Significance

Our enhancer screen identified 17 genes that function with Baz to regulate epithelial structure in the Drosophila embryo. Our secondary localization screen indicated that some of the proteins may affect epithelial cell polarity by acting at the apical cell cortex while others may act through intracellular processes. For 13 of the 17 genes, this is the first report of a link to baz or the regulation of epithelial structure.

Polarity proteins and Rho GTPases cooperate to spatially organise epithelial actin-based protrusions

Different actin-filament-based structures co-exist in many cells. Here, we characterise dynamic actin-based protrusions that form at distinct positions within columnar epithelial cells, focusing on basal filopodia and sheet-like intermediate-level protrusions that extend between surrounding epithelial cells. Using a genetic analysis, we found that the form and distribution of these actin-filament-based structures depends on the activities of apical polarity determinants, not on basal integrin signalling. Bazooka/Par3 acts upstream of the RacGEF Sif/TIAM1 to limit filopodia to the basal domain, whereas Cdc42, aPKC and Par6 are required for normal protrusion morphology and dynamics. Downstream of these polarity regulators, Sif/TIAM1, Rac, SCAR and Arp2/3 complexes catalyse actin nucleation to generate lamellipodia and filopodia, whose form depends on the level of Rac activation. Taken together, these data reveal a role for Baz/Par3 in the establishment of an intercellular gradient of Rac inhibition, from apical to basal, and an intimate association between different apically concentrated Par proteins and Rho-family GTPases in the regulation of the distribution and structure of the polarised epithelial actin cytoskeleton.

Repression of Wasp by JAK/STAT signalling inhibits medial actomyosin network assembly and apical cell constriction in intercalating epithelial cells

Tissue morphogenesis requires stereotyped cell shape changes, such as apical cell constriction in the mesoderm and cell intercalation in the ventrolateral ectoderm of Drosophila. Both processes require force generation by an actomyosin network. The subcellular localization of Myosin-II (Myo-II) dictates these different morphogenetic processes. In the intercalating ectoderm Myo-II is mostly cortical, but in the mesoderm Myo-II is concentrated in a medial meshwork. We report that apical constriction is repressed by JAK/STAT signalling in the lateral ectoderm independently of Twist. Inactivation of the JAK/STAT pathway causes germband extension defects because of apical constriction ventrolaterally. This is associated with ectopic recruitment of Myo-II in a medial web, which causes apical cell constriction as shown by laser nanosurgery. Reducing Myo-II levels rescues the JAK/STAT mutant phenotype, whereas overexpression of the Myo-II heavy chain (also known as Zipper), or constitutive activation of its regulatory light chain, does not cause medial accumulation of Myo-II nor apical constriction. Thus, JAK/STAT controls Myo-II localization by additional mechanisms. We show that regulation of actin polymerization by Wasp, but not by Dia, is important in this process. Constitutive activation of Wasp, a branched actin regulator, causes apical cell constriction and promotes medial 'web' formation. Wasp is inactivated at the cell cortex in the germband by JAK/STAT signalling. Lastly, wasp mutants rescue the normal cortical enrichment of Myo-II and inhibit apical constriction in JAK/STAT mutants, indicating that Wasp is an effector of JAK/STAT signalling in the germband. We discuss possible models for the role of Wasp activity in the regulation of Myo-II distribution.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Polarity proteins and cell-cell interactions in the testis

In mammalian testes, extensive junction restructuring takes place in the seminiferous epithelium at the Sertoli-Sertoli and Sertoli-germ cell interface to facilitate the different cellular events of spermatogenesis, such as mitosis, meiosis, spermiogenesis, and spermiation. Recent studies in the field have shown that Rho GTPases and polarity proteins play significant roles in the events of cell-cell interactions. Furthermore, Rho GTPases, such as Cdc42, are working in concert with polarity proteins in regulating cell polarization and cell adhesion at both the blood-testis barrier (BTB) and apical ectoplasmic specialization (apical ES) in the testis of adult rats. In this chapter, we briefly summarize recent findings on the latest status of research and development regarding Cdc42 and polarity proteins and how they affect cell-cell interactions in the testis and other epithelia. More importantly, we provide a new model in which how Cdc42 and components of the polarity protein complexes work in concert with laminin fragments, cytokines, and testosterone to regulate the events of cell-cell interactions in the seminiferous epithelium via a local autocrine-based regulatory loop known as the apical ES-BTB-basement membrane axis. This new functional axis coordinates various cellular events during different stages of the seminiferous epithelium cycle of spermatogenesis.

Cdc42 antagonizes Rho1 activity at adherens junctions to limit epithelial cell apical tension

Rho promotes actomyosin contractility during epithelial cell remodeling, but Cdc42 keeps the epithelium in shape by limiting RhoA activity.

In epithelia, cells are arranged in an orderly pattern with a defined orientation and shape. Cadherin containing apical adherens junctions (AJs) and the associated actomyosin cytoskeleton likely contribute to epithelial cell shape by providing apical tension. The Rho guanosine triphosphatases are well known regulators of cell junction formation, maintenance, and function. Specifically, Rho promotes actomyosin activity and cell contractility; however, what controls and localizes this Rho activity as epithelia remodel is unresolved. Using mosaic clonal analysis in the Drosophila melanogaster pupal eye, we find that Cdc42 is critical for limiting apical cell tension by antagonizing Rho activity at AJs. Cdc42 localizes Par6–atypical protein kinase C (aPKC) to AJs, where this complex limits Rho1 activity and thus actomyosin contractility, independent of its effects on Wiskott-Aldrich syndrome protein and p21-activated kinase. Thus, in addition to its role in the establishment and maintenance of apical–basal polarity in forming epithelia, the Cdc42–Par6–aPKC polarity complex is required to limit Rho activity at AJs and thus modulate apical tension so as to shape the final epithelium.

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.

A second-site noncomplementation screen for modifiers of Rho1 signaling during imaginal disc morphogenesis in Drosophila

Background

Rho1 is a small GTPase of the Ras superfamily that serves as the central component in a highly conserved signaling pathway that regulates tissue morphogenesis during development in all animals. Since there is tremendous diversity in the upstream signals that can activate Rho1 as well as the effector molecules that carry out its functions, it is important to define relevant Rho1-interacting genes for each morphogenetic event regulated by this signaling pathway. Previous work from our lab and others has shown that Rho signaling is necessary for the morphogenesis of leg imaginal discs during metamorphosis in Drosophila, although a comprehensive identification of Rho1-interacting genes has not been attempted for this process.

Methodology/Principal Findings

We characterized an amorphic allele of Rho1 that displays a poorly penetrant dominant malformed leg phenotype and is capable of being strongly enhanced by Rho1-interacting heterozygous mutations. We then used this allele in a second-site noncomplementation screen with the Exelixis collection of molecularly defined deficiencies to identify Rho1-interacting genes necessary for leg morphogenesis. In a primary screen of 461 deficiencies collectively uncovering ∼50% of the Drosophila genome, we identified twelve intervals harboring Rho1-interacting genes. Through secondary screening we identified six Rho1-interacting genes including three that were previously identified (RhoGEF2, broad, and stubbloid), thereby validating the screen. In addition, we identified Cdc42, Rheb and Sc2 as novel Rho1-interacting genes involved in adult leg development.

Conclusions/Significance

This screen identified well-known and novel Rho1-interacting genes necessary for leg morphogenesis, thereby increasing our knowledge of this important signaling pathway. We additionally found that Rheb may have a unique function in leg morphogenesis that is independent of its regulation of Tor.

Requirements for F-BAR proteins TOCA-1 and TOCA-2 in actin dynamics and membrane trafficking during Caenorhabditis elegans oocyte growth and embryonic epidermal morphogenesis

The TOCA family of F-BAR–containing proteins bind to and remodel lipid bilayers via their conserved F-BAR domains, and regulate actin dynamics via their N-Wasp binding SH3 domains. Thus, these proteins are predicted to play a pivotal role in coordinating membrane traffic with actin dynamics during cell migration and tissue morphogenesis. By combining genetic analysis in Caenorhabditis elegans with cellular biochemical experiments in mammalian cells, we showed that: i) loss of CeTOCA proteins reduced the efficiency of Clathrin-mediated endocytosis (CME) in oocytes. Genetic interference with CeTOCAs interacting proteins WSP-1 and WVE-1, and other components of the WVE-1 complex, produced a similar effect. Oocyte endocytosis defects correlated well with reduced egg production in these mutants. ii) CeTOCA proteins localize to cell–cell junctions and are required for proper embryonic morphogenesis, to position hypodermal cells and to organize junctional actin and the junction-associated protein AJM-1. iii) Double mutant analysis indicated that the toca genes act in the same pathway as the nematode homologue of N-WASP/WASP, wsp-1. Furthermore, mammalian TOCA-1 and C. elegans CeTOCAs physically associated with N-WASP and WSP-1 directly, or WAVE2 indirectly via ABI-1. Thus, we propose that TOCA proteins control tissues morphogenesis by coordinating Clathrin-dependent membrane trafficking with WAVE and N-WASP–dependent actin-dynamics.

Cells continuously remodel their shape especially during cell migration, differentiation, and tissues morphogenesis. This occurs through the dynamic reorganization of their plasma membrane and actin cytoskeleton: two processes that must therefore be intimately linked and coordinated. Molecules that sit at the crossroads of membrane remodeling and actin dynamics are predicted to play a pivotal role in coordinating these processes. The TOCA family of proteins represents a case in point. These proteins bind to and deform membranes during processes such as membrane trafficking. They also control actin dynamics through their interactions with actin remodeling factors, such as WASP and WAVEs. Here, we characterize the functional role of TOCA proteins in a model organism, the nematode Caenorhabditis elegans. We established that toca genes regulate Clathrin-mediated membrane trafficking during oocyte growth. We further discovered that these proteins play an important role in epithelial morphogenesis in developing embryos, and in egg production in adult nematodes. Moreover, the TOCA interacting proteins WASP/WSP-1 and WAVE/WVE-1, as well as other components of the WVE-1 complex, appear to be involved in TOCA-dependent processes. Thus, we propose that TOCA proteins control tissue morphogenesis by coordinating Clathrin-dependent membrane trafficking with WAVE and N-WASP–dependent actin-dynamics.

Pathways and mechanisms of endocytic recycling

Endocytic recycling is coordinated with endocytic uptake to control the composition of the plasma membrane. Although much of our understanding of endocytic recycling has come from studies on the transferrin receptor, a protein internalized through clathrin-dependent endocytosis, increased interest in clathrin-independent endocytosis has led to the discovery of new endocytic recycling systems. Recent insights into the regulatory mechanisms that control endocytic recycling have focused on recycling through tubular carriers and the return to the cell surface of cargoes that enter cells through clathrin-independent mechanisms. Recent work emphasizes the importance of regulated recycling in processes as diverse as cytokinesis, cell adhesion, morphogenesis, cell fusion, learning and memory.

Drosophila Cip4/Toca-1 integrates membrane trafficking and actin dynamics through WASP and SCAR/WAVE

BACKGROUND

Developmental processes are intimately tied to signaling events that integrate the dynamic reorganization of the actin cytoskeleton and membrane dynamics. The F-BAR-domain-containing proteins are prime candidates to couple actin dynamics and membrane trafficking in different morphogenetic processes.

RESULTS

Here, we present the functional analysis of the Drosophila F-BAR protein Cip4/Toca1 (Cdc42-interacting protein 4/transducer of Cdc42-dependent actin assembly 1). Cip4 is able to form a complex with WASP and SCAR/WAVE and recruits both actin-nucleation-promoting factors to invaginating membranes and endocytic vesicles. Actin-comet-tail-based movement of these vesicles depends not only on WASP but largely on WAVE function. In vivo, loss of cip4 function causes multiple wing hairs. A similar phenotype is observed when vesicle scission is affected after Dynamin suppression. Gene dosage experiments show that Cip4 and WAVE functionally interact to restrict wing hair formation. Further rescue experiments confirm that Cip4 is able to act through WAVE and WASP in vivo. Biochemical and functional data support a model in which Cdc42 acts upstream of Cip4 and recruits not only WASP but also SCAR/WAVE via Abi to control Dynamin-dependent cell polarization in the wing.

CONCLUSION

Cip4 integrates membrane trafficking and actin dynamics through WASP and WAVE. First, Cip4 promotes membrane invaginations and triggers the vesicle scission by recruiting Dynamin to the neck of nascent vesicles. Second, Cip4 recruits WASP and WAVE proteins to induce actin polymerization, supporting vesicle scission and providing the force for vesicle movement.

Origin and function of fluctuations in cell behaviour and the emergence of patterns

Morphogenesis is the process whereby cells assemble into tissues and organs. Recent studies of this process have revealed heterogeneity of individual cell behaviours that contrasts with the deterministic activity of tissues as a whole. Here we review these observations and suggest that fluctuations and heterogeneities are a central substrate for morphogenesis and that there might exist mechanisms dedicated to the averaging of these fluctuations to ensure robust and reproducible behaviours at the tissue level.

Adherens junction: molecular architecture and regulation

The adherens junction (AJ) is an element of the cell-cell junction in which cadherin receptors bridge the neighboring plasma membranes via their homophilic interactions. Cadherins associate with cytoplasmic proteins, called catenins, which in turn bind to cytoskeletal components, such as actin filaments and microtubules. These molecular complexes further interact with other proteins, including signaling molecules, rendering the AJs into highly dynamic and regulatable structures. The AJs of such nature contribute to the physical linking of cells, as well as to the regulation of cell-cell contacts, which is essential for morphogenesis and remodeling of tissues and organs. Thus, elucidating the molecular architecture of the AJs and their regulatory mechanisms are crucial for understanding how the multicellular system is organized.

At the crossroads of polarity, proliferation and apoptosis: the use of Drosophila to unravel the multifaceted role of endocytosis in tumor suppression

Endocytosis is an important regulator of cell-cell signaling and endocytic trafficking has been increasingly implicated in control of tumor suppression. Recent insights from Drosophila indicate that impairment of multiple trafficking steps which lead to receptor degradation can cause tumor formation in epithelial organs. These tumors are characterized by sustained activation of a number of mitogenic signaling pathways, and by subversion of epithelial polarity and the apoptotic response. Cooperation between such alterations, as well as tumor-host interactions, is also observed. The recapitulation of several hallmarks of human cancers in fly tumors provides a framework to understand the role of defective endocytosis in cancer.

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.

Regulation of cell death by recycling endosomes and golgi membrane dynamics via a pathway involving Src-family kinases, Cdc42 and Rab11a

Actin dynamics and membrane trafficking influence cell commitment to programmed cell death through largely undefined mechanisms. To investigate how actin and recycling endosome (RE) trafficking can engage death signaling, we studied the death program induced by the adenovirus early region 4 open reading frame 4 (E4orf4) protein as a model. We found that in the early stages of E4orf4 expression, Src-family kinases (SFKs), Cdc42, and actin perturbed the organization of the endocytic recycling compartment and promoted the transport of REs to the Golgi apparatus, while inhibiting recycling of protein cargos to the plasma membrane. The resulting changes in Golgi membrane dynamics that relied on actin-regulated Rab11a membrane trafficking triggered scattering of Golgi membranes and contributed to the progression of cell death. A similar mobilization of RE traffic mediated by SFKs, Cdc42 and Rab11a also contributed to Golgi fragmentation and to cell death progression in response to staurosporine, in a caspase-independent manner. Collectively, these novel findings suggest that diversion of RE trafficking to the Golgi complex through a pathway involving SFKs, Cdc42, and Rab11a plays a general role in death signaling by mediating regulated changes in Golgi dynamics.

F-BAR-containing adaptor CIP4 localizes to early endosomes and regulates Epidermal Growth Factor Receptor trafficking and downregulation

Cdc42-Interacting Protein-4 (CIP4) family adaptors have been implicated in promoting Epidermal Growth Factor Receptor (EGFR) internalization, however, their unique or overlapping functions remain unclear. Here, we show that although CIP4 was not required for early events in clathrin-mediated endocytosis of EGFR, CIP4 localizes to vesicles containing EGFR and Rab5. Furthermore, expression of constitutively active Rab5 led to accumulation of CIP4 and the related adaptor Toca-1 in giant endosomes. Using a mutagenesis approach, we show that localization of CIP4 to endosomes is mediated in part via the curved phosphoinositide-binding face of the CIP4 F-BAR domain. Downregulation of CIP4 in A431 epidermoid carcinoma cells by RNA interference led to elevated EGFR levels, compared to control cells. Although surface expression of EGFR was not affected by CIP4 silencing, EGF-induced transit of EGFR from EEA1-positive endosomes to lysosomes was reduced compared to control cells. This correlated with more robust activation of ERK kinase and entry to S phase in CIP4-depleted A431 cells, compared to control cells. The combined silencing of CIP4 and Toca-1 was more effective in driving cells into S phase, suggesting a partial redundancy in their functions. Overall, our results implicate CIP4 and Toca-1 in regulating late events in EGFR trafficking from endosomes that serves to limit sustained ERK activation within the endosomal compartment.

EphrinB reverse signaling in cell-cell adhesion: is it just par for the course?

Cell-cell adhesion is a critical process for the formation and maintenance of tissue patterns during development, as well as invasion and metastasis of cancer cells. Although great strides have been made regarding our understanding of the processes that play a role in cell-cell adhesion, the precise mechanisms by which diverse signaling events regulate cell and tissue architecture is poorly understood. In this commentary we will focus on the Eph/ephrin signaling system, and specifically how the ephrinB1 transmembrane ligand for Eph receptor tyrosine kinases sends signals affecting cell-cell junctions. In a recent study using the epithelial cells of early stage Xenopus embryos, we have shown that loss- or gain-of function of ephrinB1 can disrupt cell-cell contacts and tight junctions. This study reveals a mechanism where ephrinB1 competes with active Cdc42 for binding to Par-6, a scaffold protein central to the Par polarity complex (Par-3/Par-6/Cdc42/aPKC) and disrupts the localization of tight junction-associated proteins (ZO-1, Cingulin) at tight junctions. This competition reduces aPKC activity critical to maintaining and/or forming tight junctions. Finally, phosphorylation of ephrinB1 on specific tyrosine residues can block the interaction between ephrinB1 and Par-6 at tight junctions, and restore tight junction formation. Recent evidence indicates that de-regulation of forward signaling through EphB receptors may play a role in metastatic progression in colon cancer. In light of the new data showing an effect of ephrinB reverse signaling on tight junctions, an additional mechanism can be hypothesized where de-regulation of ephrinB1 expression or phosphorylation may also impact metastatic progression.

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.

Distinct functions for Rho1 in maintaining adherens junctions and apical tension in remodeling epithelia

Maintenance and remodeling of adherens junctions (AJs) and cell shape in epithelia are necessary for the development of functional epithelia and are commonly altered during cancer progression/metastasis. Although formation of nascent AJs has received much attention, whether shared mechanisms are responsible for the maintenance and remodeling of AJs in dynamic epithelia, particularly in vivo, is not clear. Using clonal analysis in the postmitotic Drosophila melanogaster pupal eye epithelium, we demonstrate that Rho1 is required to maintain AJ integrity independent of its role in sustaining apical cell tension. Rho1 depletion in a remodeling postmitotic epithelium disrupts AJs but only when depleted in adjacent cells. Surprisingly, neither of the Rho effectors, Rok or Dia, is necessary downstream of Rho1 to maintain AJs; instead, Rho1 maintains AJs by inhibiting Drosophila epithelial cadherin endocytosis in a Cdc42/Par6-dependent manner. In contrast, depletion of Rho1 in single cells decreases apical tension, and Rok and myosin are necessary, while Dia function also contributes, downstream of Rho1 to sustain apical cell tension.

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.

Dynamin participates in the maintenance of anterior polarity in the Caenorhabditis elegans embryo

Cell polarity is crucial for the generation of cell diversity. Recent evidence suggests that the actin cytoskeleton plays a key role in establishment of embryonic polarity, yet the mechanisms that maintain polarity cues in particular membrane domains during development remain unclear. Dynamin, a large GTPase, functions in both endocytosis and actin dynamics. Here, the Caenorhabditis elegans dynamin ortholog, DYN-1, maintains anterior polarity cues. DYN-1-GFP foci are enriched in the anterior cortex in a manner dependent on the anterior polarity proteins, PAR-6 and PKC-3. Membrane internalization and actin comet formation are enriched in the anterior, and are dependent on DYN-1. PAR-6-labeled puncta are also internalized from cortical accumulations of DYN-1-GFP. Our results demonstrate a mechanism for the spatial and temporal regulation of endocytosis in the anterior of the embryo, contributing to the precise localization and maintenance of polarity factors within a dynamic plasma membrane.

Rho GTPase function in tumorigenesis

Malignant tumor cells display uncontrolled proliferation, loss of epithelial cell polarity, altered interactions with neighboring cells and the surrounding extracellular matrix, and enhanced migratory properties. Proteins of the Rho GTPase family regulate all these processes in cell culture and, for that reason, Rho GTPases, their regulators, and their effectors have been suggested to control tumor formation and progression in humans. However, while the tumor-relevant functions of Rho GTPases are very well documented in vitro, we are only now beginning to assess their contribution to cancer in human patients and in animal models. This review will give a very brief overview of Rho GTPase function in general and then focus on in vivo evidence for a role of Rho GTPases in malignant tumors, both in human patients and in genetically modified mice.

Regulation of cadherin trafficking

Cadherins are a large family of cell-cell adhesion molecules that tether cytoskeletal networks of actin and intermediate filaments to the plasma membrane. This function of cadherins promotes tissue organization and integrity, as demonstrated by numerous disease states that are characterized by the loss of cadherin-based adhesion. However, plasticity in cell adhesion is often required in cellular processes such as tissue patterning during development and epithelial migration during wound healing. Recent work has revealed a pivotal role for various membrane trafficking pathways in regulating cellular transitions between quiescent adhesive states and more dynamic phenotypes. The regulation of cadherins by membrane trafficking is emerging as a key player in this balancing act, and studies are beginning to reveal how this process goes awry in the context of disease. This review summarizes the current understanding of how cadherins are routed and how the interface between cadherins and membrane trafficking pathways regulates cell surface adhesive potential. Particular emphasis is placed on the regulation of cadherin trafficking by catenins and the interplay between growth factor signaling pathways and cadherin endocytosis.

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.