Seamless tube shape is constrained by endocytosis-dependent regulation of active Moesin

Crb3 is required to organize the apical domain of multiciliated cells

Cell shape changes mainly rely on the remodeling of the actin cytoskeleton. Multiciliated cells (MCCs) of the mucociliary epidermis of Xenopus laevis embryos, as they mature, dramatically reshape their apical domain to grow cilia, in coordination with the underlying actin cytoskeleton. Crumbs (Crb) proteins are multifaceted transmembrane apical polarity proteins known to recruit actin linkers and promote apical membrane growth. Here, we identify the homeolog Crb3.L as an important player for the migration of centrioles or basal bodies (collectively centrioles/BBs) and apical domain morphogenesis in MCCs. Crb3.L is present in cytoplasmic vesicles close to the ascending centrioles/BBs, where it partially colocalizes with Rab11a. Crb3.L morpholino-mediated depletion in MCCs caused abnormal migration of centrioles/BBs, a reduction of their apical surface, disorganization of their apical actin meshwork and defective ciliogenesis. Rab11a morpholino-mediated depletion phenocopied Crb3.L loss-of-function in MCCs. Thus, the control of centrioles/BBs migration by Crb3.L might be mediated by Rab11a-dependent apical trafficking. Furthermore, we show that both phospho-activated ERM (pERM; Ezrin-Radixin-Moesin) and Crb3.L are recruited to the growing apical domain of MCCs, where Crb3.L likely anchors pERM, allowing actin-dependent expansion of the apical membrane.

The biosynthetic-secretory pathway, supplemented by recycling routes, determines epithelial membrane polarity

In prevailing epithelial polarity models, membrane-based polarity cues (e.g., the partitioning-defective PARs) position apicobasal cellular membrane domains. Intracellular vesicular trafficking expands these domains by sorting polarized cargo toward them. How the polarity cues themselves are polarized in epithelia and how sorting confers long-range apicobasal directionality to vesicles is still unclear. Here, a systems-based approach using two-tiered C. elegans genomics-genetics screens identifies trafficking molecules that are not implicated in apical sorting yet polarize apical membrane and PAR complex components. Live tracking of polarized membrane biogenesis indicates that the biosynthetic-secretory pathway, linked to recycling routes, is asymmetrically oriented toward the apical domain during this domain's biosynthesis, and that this directionality is regulated upstream of PARs and independent of polarized target membrane domains. This alternative mode of membrane polarization could offer solutions to open questions in current models of epithelial polarity and polarized trafficking.

Rab11 is essential to pancreas morphogenesis, lumen formation and endocrine mass

The molecular links between tissue-level morphogenesis and the differentiation of cell lineages in the pancreas remain elusive despite a decade of studies. We previously showed that in pancreas both processes depend on proper lumenogenesis. The Rab GTPase Rab11 is essential for epithelial lumen formation in vitro, however few studies have addressed its functions in vivo and none have tested its requirement in pancreas. Here, we show that Rab11 is critical for proper pancreas development. Co-deletion of the Rab11 isoforms Rab11A and Rab11B in the developing pancreatic epithelium (Rab11pancDKO) results in ∼50% neonatal lethality and surviving adult Rab11^pancDKO mice exhibit defective endocrine function. Loss of both Rab11A and Rab11B in the embryonic pancreas results in morphogenetic defects of the epithelium, including defective lumen formation and lumen interconnection. In contrast to wildtype cells, Rab11pancDKO cells initiate the formation of multiple ectopic lumens, resulting in a failure to coordinate a single apical membrane initiation site (AMIS) between groups of cells. This results in a failure to form ducts with continuous lumens. Here, we show that these defects are due to failures in vesicle trafficking, as apical and junctional components remain trapped within Rab11^pancDKO cells. Together, these observations suggest that Rab11 directly regulates epithelial lumen formation and morphogenesis. Our report links intracellular trafficking to organ morphogenesis in vivo and presents a novel framework for decoding pancreatic development.

NDR kinase tricornered genetically interacts with Ccm3 and metabolic enzymes in Drosophila melanogaster tracheal development

The Germinal Center Kinase III (GckIII) pathway is a Hippo-like kinase module defined by sequential activation of Ste20 kinases Thousand and One (Tao) and GckIII, followed by nuclear dbf2-related (NDR) kinase Tricornered (Trc). We previously uncovered a role for the GckIII pathway in Drosophila melanogaster tracheal (respiratory) tube morphology. The trachea form a network of branched epithelial tubes essential for oxygen transport, and are structurally analogous to branched tubular organs in vertebrates, such as the vascular system. In the absence of GckIII pathway function, aberrant dilations form in tracheal tubes characterized by mislocalized junctional and apical proteins, suggesting that the pathway is important in maintaining tube integrity in development. Here, we observed a genetic interaction between trc and Cerebral cavernous malformations 3 (Ccm3), the Drosophila ortholog of a human vascular disease gene, supporting our hypothesis that the GckIII pathway functions downstream of Ccm3 in trachea, and potentially in the vertebrate cerebral vasculature. However, how GckIII pathway signaling is regulated and the mechanisms that underpin its function in tracheal development are unknown. We undertook biochemical and genetic approaches to identify proteins that interact with Trc, the most downstream GckIII pathway kinase. We found that known GckIII and NDR scaffold proteins are likely to control GckIII pathway signaling in tracheal development, consistent with their conserved roles in Hippo-like modules. Furthermore, we show genetic interactions between trc and multiple enzymes in glycolysis and oxidative phosphorylation, suggesting a potential function of the GckIII pathway in integrating cellular energy requirements with maintenance of tube integrity.

Shaping subcellular tubes through vesicle trafficking: Common and distinct pathways

Cells with subcellular lumens form some of the most miniature tubes in the tubular organs of animals. These are often crucial components of the system, executing functions at remote body locations. Unlike tubes formed by intercellular or autocellular junctions, the cells with junctionless subcellular lumens face unique challenges in modifying the cell shape and plasma membrane organization to incorporate a membrane-bound tube within, often associated with dramatic cellular growth and extensions. Results in the recent years have shown that membrane dynamics, including both the primary delivery and recycling, is crucial in providing the cell with the flexibility to face these challenges. A significant portion of this information has come from two in vivo invertebrate models; the Drosophila tracheal terminal cells and the C. elegans excretory cell. This review focuses on the data obtained from these systems in the recent past about how trafficking pathways influence subcellular tube and branching morphogenesis. Given that such tubes occur in vertebrate vasculature, these insights are relevant to human health, and we contrast our conclusions with the less understood subcellular tubes of angiogenesis.

Regulators of the secretory pathway have distinct inputs into single-celled branching morphogenesis and seamless tube formation in the Drosophila trachea

Biological tubes serve as conduits through which gas, nutrients and other important fluids are delivered to tissues. Most biological tubes consist of multiple cells connected by epithelial junctions. Unlike these multicellular tubes, seamless tubes are unicellular and lack junctions. Seamless tubes are present in various organ systems, including the vertebrate vasculature, C.elegans excretory system, and Drosophila tracheal system. The Drosophila tracheal system is a network of air-filled tubes that delivers oxygen to all tissues. Specialized cells within the tracheal system, called terminal cells, branch extensively and form seamless tubes. Terminal tracheal tubes are polarized; the lumenal membrane has apical identity whereas the outer membrane exhibits basal characteristics. Although various aspects of membrane trafficking have been implicated in terminal cell morphogenesis, the precise secretory pathway requirements for basal and apical membrane growth have yet to be elucidated. In the present study, we demonstrate that anterograde trafficking, retrograde trafficking and Golgi-to-plasma membrane vesicle fusion are each required for the complex branched architecture of the terminal cell, but their inputs during seamless lumen formation are more varied. The COPII subunit, Sec31, and ER exit site protein, Sec16, are critical for subcellular tube architecture, whereas the SNARE proteins Syntaxin 5, Syntaxin 1 and Syntaxin 18 are more generally required for seamless tube growth and maintenance. These data suggest that distinct components of the secretory pathway have differential contributions to basal and apical membrane growth and maintenance during terminal cell morphogenesis.

An endosome-associated actin network involved in directed apical plasma membrane growth

Membrane trafficking plays many roles in morphogenesis, from bulk membrane provision to targeted delivery of proteins and other cargos. In tracheal terminal cells of the Drosophila respiratory system, transport through late endosomes balances membrane delivery between the basal plasma membrane and the apical membrane, which forms a subcellular tube, but it has been unclear how the direction of growth of the subcellular tube with the overall cell growth is coordinated. We show here that endosomes also organize F-actin. Actin assembles around late endocytic vesicles in the growth cone of the cell, reaching from the tip of the subcellular tube to the leading filopodia of the basal membrane. Preventing nucleation of endosomal actin disturbs the directionality of tube growth, uncoupling it from the direction of cell elongation. Severing actin in this area affects tube integrity. Our findings show a new role for late endosomes in directing morphogenesis by organizing actin, in addition to their known role in membrane and protein trafficking.

Crosstalk between basal extracellular matrix adhesion and building of apical architecture during morphogenesis

Tissues build complex structures like lumens and microvilli to carry out their functions. Most of the mechanisms used to build these structures rely on cells remodelling their apical plasma membranes, which ultimately constitute the specialised compartments. In addition to apical remodelling, these shape changes also depend on the proper attachment of the basal plasma membrane to the extracellular matrix (ECM). The ECM provides cues to establish apicobasal polarity, and it also transduces forces that allow apical remodelling. However, physical crosstalk mechanisms between basal ECM attachment and the apical plasma membrane remain understudied, and the ones described so far are very diverse, which highlights the importance of identifying the general principles. Here, we review apicobasal crosstalk of two well-established models of membrane remodelling taking place during Drosophila melanogaster embryogenesis: amnioserosa cell shape oscillations during dorsal closure and subcellular tube formation in tracheal cells. We discuss how anchoring to the basal ECM affects apical architecture and the mechanisms that mediate these interactions. We analyse this knowledge under the scope of other morphogenetic processes and discuss what aspects of apicobasal crosstalk may represent widespread phenomena and which ones are used to build subsets of specialised compartments.

Cytoskeletal players in single-cell branching morphogenesis

Branching networks are a very common feature of multicellular animals and underlie the formation and function of numerous organs including the nervous system, the respiratory system, the vasculature and many internal glands. These networks range from subcellular structures such as dendritic trees to large multicellular tissues such as the lungs. The production of branched structures by single cells, so called subcellular branching, which has been better described in neurons and in cells of the respiratory and vascular systems, involves complex cytoskeletal remodelling events. In Drosophila, tracheal system terminal cells (TCs) and nervous system dendritic arborisation (da) neurons are good model systems for these subcellular branching processes. During development, the generation of subcellular branches by single-cells is characterized by extensive remodelling of the microtubule (MT) network and actin cytoskeleton, followed by vesicular transport and membrane dynamics. In this review, we describe the current knowledge on cytoskeletal regulation of subcellular branching, based on the terminal cells of the Drosophila tracheal system, but drawing parallels with dendritic branching and vertebrate vascular subcellular branching.

Transcytosis in the development and morphogenesis of epithelial tissues

Transcytosis is a form of specialized transport through which an extracellular cargo is endocytosed, shuttled across the cytoplasm in membrane-bound vesicles, and secreted at a different plasma membrane surface. This important process allows membrane-impermeable macromolecules to pass through a cell and become accessible to adjacent cells and tissue compartments. Transcytosis also promotes redistribution of plasma membrane proteins and lipids to different regions of the cell surface. Here we review transcytosis and highlight in vivo studies showing how developing epithelial cells use it to change shape, to migrate, and to relocalize signaling molecules.

Coordinated crosstalk between microtubules and actin by a spectraplakin regulates lumen formation and branching

Subcellular lumen formation by single-cells involves complex cytoskeletal remodelling. We have previously shown that centrosomes are key players in the initiation of subcellular lumen formation in Drosophila melanogaster, but not much is known on the what leads to the growth of these subcellular luminal branches or makes them progress through a particular trajectory within the cytoplasm. Here, we have identified that the spectraplakin Short-stop (Shot) promotes the crosstalk between MTs and actin, which leads to the extension and guidance of the subcellular lumen within the tracheal terminal cell (TC) cytoplasm. Shot is enriched in cells undergoing the initial steps of subcellular branching as a direct response to FGF signalling. An excess of Shot induces ectopic acentrosomal luminal branching points in the embryonic and larval tracheal TC leading to cells with extra-subcellular lumina. These data provide the first evidence for a role for spectraplakins in single-cell lumen formation and branching.

Cells into tubes: Molecular and physical principles underlying lumen formation in tubular organs

Tubular networks, such as the vascular and respiratory systems, transport liquids and gases in multicellular organisms. The basic units of these organs are tubes formed by single or multiple cells enclosing a luminal cavity. The formation and maintenance of correctly sized and shaped lumina are fundamental steps in organogenesis and are essential for organismal homeostasis. Therefore, understanding how cells generate, shape and maintain lumina is crucial for understanding normal organogenesis as well as the basis of pathological conditions. Lumen formation involves polarized membrane trafficking, cytoskeletal dynamics, and the influence of intracellular as well as extracellular mechanical forces, such as cortical tension, luminal pressure or blood flow. Various tissue culture and in vivo model systems, ranging from MDCK cell spheroids to tubular organs in worms, flies, fish, and mice, have provided many insights into the molecular and cellular mechanisms underlying lumenogenesis and revealed key factors that regulate the size and shape of cellular tubes. Moreover, the development of new experimental and imaging approaches enabled quantitative analyses of intracellular dynamics and allowed to assess the roles of cellular and tissue mechanics during tubulogenesis. However, how intracellular processes are coordinated and regulated across scales of biological organization to generate properly sized and shaped tubes is only beginning to be understood. Here, we review recent insights into the molecular, cellular and physical mechanisms underlying lumen formation during organogenesis. We discuss how these mechanisms control lumen formation in various model systems, with a special focus on the morphogenesis of tubular organs in Drosophila.

Transcytosis via the late endocytic pathway as a cell morphogenetic mechanism

Plasma membranes fulfil many physiological functions. In polarized cells, different membrane compartments take on specialized roles, each being allocated correct amounts of membrane. The Drosophila tracheal system, an established tubulogenesis model, contains branched terminal cells with subcellular tubes formed by apical plasma membrane invagination. We show that apical endocytosis and late endosome‐mediated trafficking are required for membrane allocation to the apical and basal membrane domains. Basal plasma membrane growth stops if endocytosis is blocked, whereas the apical membrane grows excessively. Plasma membrane is initially delivered apically and then continuously endocytosed, together with apical and basal cargo. We describe an organelle carrying markers of late endosomes and multivesicular bodies (MVBs) that is abolished by inhibiting endocytosis and which we suggest acts as transit station for membrane destined to be redistributed both apically and basally. This is based on the observation that disrupting MVB formation prevents growth of both compartments.

Multiple Requirements for Rab GTPases in the Development of Drosophila Tracheal Dorsal Branches and Terminal Cells

The tracheal epithelium in fruit fly larvae is a popular model for multi- and unicellular migration and morphogenesis. Like all epithelial cells, tracheal cells use Rab GTPases to organize their internal membrane transport, resulting in the specific localization or secretion of proteins on the apical or basal membrane compartments. Some contributions of Rabs to junctional remodelling and governance of tracheal lumen contents are known, but it is reasonable to assume that they play important further roles in morphogenesis. This pertains in particular to terminal tracheal cells, specialized branch-forming cells that drastically reshape both their apical and basal membrane during the larval stages. We performed a loss-of-function screen in the tracheal system, knocking down endogenously tagged alleles of 26 Rabs by targeting the tag via RNAi. This revealed that at least 14 Rabs are required to ensure proper cell fate specification and migration of the dorsal branches, as well as their epithelial fusion with the contralateral dorsal branch. The screen implicated four Rabs in the subcellular morphogenesis of terminal cells themselves. Further tests suggested residual gene function after knockdown, leading us to discuss the limitations of this approach. We conclude that more Rabs than identified here may be important for tracheal morphogenesis, and that the tracheal system offers great opportunities for studying several Rabs that have barely been characterized so far.

Proximity interactions of the ubiquitin ligase Mind bomb 1 reveal a role in regulation of epithelial polarity complex proteins

MIB1 belongs to the RING domain containing family of E3 ubiquitin ligases. In vertebrates, MIB1 plays an essential role in activation of Notch signaling during development, through the ubiquitination and endocytosis of Notch ligands. More recently, Notch independent functions for MIB1 have been described in centriole homeostasis, dendritic spine outgrowth and directional cell migration. Here we use proximity-dependent biotin identification (BioID) to define the MIB1 interactome that included 163 high confidence interactions with polypeptides linked to centrosomes and cilia, endosomal trafficking, RNA and DNA processing, the ubiquitin system, and cell adhesion. Biochemical analysis identified several proteins within these groups including CCDC14 and EPS15 that were ubiquitinated but not degraded when co-expressed with MIB1. The MIB1 interactome included the epithelial cell polarity protein, EPB41L5. MIB1 binds to and ubiquitinates EPB41L5 resulting in its degradation. Furthermore, MIB1 ubiquitinates the EPB41L5-associated polarity protein CRB1, an important determinant of the apical membrane. In polarized cells, MIB1 localized to the lateral membrane with EPB41L5 and to the tight junction with CRB1, CRB3 and ZO1. Furthermore, over expression of MIB1 resulted in altered epithelial cell morphology and apical membrane expansion. These results support a role for MIB1 in regulation of polarized epithelial cell morphology.

Centrosomes in Branching Morphogenesis

The centrosome, a major microtubule organizer, has important functions in regulating the cytoskeleton as well as the position of cellular structures and orientation of cells within tissues. The centrosome serves as the main cytoskeleton-organizing centre in the cell and is the classical site of microtubule nucleation and anchoring. For these reasons, centrosomes play a very important role in morphogenesis, not just in the early stages of cell divisions but also in the later stages of organogenesis. Many organs such as lung, kidney and blood vessels develop from epithelial tubes that branch into complex networks. Cells in the nervous system also form highly branched structures in order to build complex neuronal networks. During branching morphogenesis, cells have to rearrange within tissues though multicellular branching or through subcellular branching, also known as single-cell branching. For highly branched structures to be formed during embryonic development, the cytoskeleton needs to be extensively remodelled. The centrosome has been shown to play an important role during these events.

Yorkie controls tube length and apical barrier integrity during airway development

Skouloudaki et al. identify an alternative role of the transcriptional coactivator Yorkie (Yki) in controlling water impermeability and tube size of developing Drosophila airways. Tracheal impermeability is triggered by Yki-mediated transcriptional regulation of δ-aminolevulinate synthase (Alas), whereas tube elongation is controlled by binding of Yki to the actin-severing factor Twinstar.

Epithelial organ size and shape depend on cell shape changes, cell–matrix communication, and apical membrane growth. The Drosophila melanogaster embryonic tracheal network is an excellent model to study these processes. Here, we show that the transcriptional coactivator of the Hippo pathway, Yorkie (YAP/TAZ in vertebrates), plays distinct roles in the developing Drosophila airways. Yorkie exerts a cytoplasmic function by binding Drosophila Twinstar, the orthologue of the vertebrate actin-severing protein Cofilin, to regulate F-actin levels and apical cell membrane size, which are required for proper tracheal tube elongation. Second, Yorkie controls water tightness of tracheal tubes by transcriptional regulation of the δ-aminolevulinate synthase gene (Alas). We conclude that Yorkie has a dual role in tracheal development to ensure proper tracheal growth and functionality.

The regulation of cell size and branch complexity in the terminal cells of the Drosophila tracheal system

The terminal cells of the larval Drosophila tracheal system extend dozens of branched cellular processes, most of which become hollow intracellular tubes that support gas exchange with internal tissues. Previously, we undertook a forward genetic mosaic screen to uncover the pathways regulating terminal cell size, morphogenesis, and the generation and maintenance of new intracellular tubes. Our initial work identified several mutations affecting terminal cell size and branch number, and suggested that branch complexity and cell size are typically coupled but could be genetically separated. To deepen our understanding of these processes, we have further characterized and determined the molecular identities of mutations in the genes sprout, denuded and asthmatic, that had been implicated in our initial screen. Here we reveal the molecular identity of these genes and describe their function in the context of the TOR and Hippo pathways, which are widely appreciated to be key regulators of cell and organ size.

Single-cell branching morphogenesis in the Drosophila trachea

The terminal cells of the tracheal epithelium in Drosophila melanogaster are one of the few known cell types that undergo subcellular morphogenesis to achieve a stable, branched shape. During the animal's larval stages, the cells repeatedly sprout new cytoplasmic processes. These grow very long, wrapping around target tissues to which the terminal cells adhere, and are hollowed by a gas-filled subcellular tube for oxygen delivery. Our understanding of this ramification process remains rudimentary. This review aims to provide a comprehensive summary of studies on terminal cells to date, and attempts to extrapolate how terminal branches might be formed based on the known genetic and molecular components. Next to this cell-intrinsic branching mechanism, we examine the extrinsic regulation of terminal branching by the target tissue and the animal's environment. Finally, we assess the degree of similarity between the patterns established by the branching programs of terminal cells and other branched cells and tissues from a mathematical and conceptual point of view.

Development and Function of the Drosophila Tracheal System

The tracheal system of insects is a network of epithelial tubules that functions as a respiratory organ to supply oxygen to various target organs. Target-derived signaling inputs regulate stereotyped modes of cell specification, branching morphogenesis, and collective cell migration in the embryonic stage. In the postembryonic stages, the same set of signaling pathways controls highly plastic regulation of size increase and pattern elaboration during larval stages, and cell proliferation and reprograming during metamorphosis. Tracheal tube morphogenesis is also regulated by physicochemical interaction of the cell and apical extracellular matrix to regulate optimal geometry suitable for air flow. The trachea system senses both the external oxygen level and the metabolic activity of internal organs, and helps organismal adaptation to changes in environmental oxygen level. Cellular and molecular mechanisms underlying the high plasticity of tracheal development and physiology uncovered through research on Drosophila are discussed.

The AFF-1 exoplasmic fusogen is required for endocytic scission and seamless tube elongation

Many membranes must merge during cellular trafficking, but fusion and fission events initiating at exoplasmic (non-cytosolic) membrane surfaces are not well understood. Here we show that the C. elegans cell-cell fusogen anchor-cell fusion failure 1 (AFF-1) is required for membrane trafficking events during development of a seamless unicellular tube. EGF-Ras-ERK signaling upregulates AFF-1 expression in the excretory duct tube to promote tube auto-fusion and subsequent lumen elongation. AFF-1 is required for scission of basal endocytic compartments and for apically directed exocytosis to extend the apical membrane. Lumen elongation also requires the transcytosis factor Rab11, but occurs independently of dynamin and clathrin. These results support a transcytosis model of seamless tube lumen growth and show that cell-cell fusogens also can play roles in intracellular membrane trafficking events.

An Ichor-dependent apical extracellular matrix regulates seamless tube shape and integrity

During sprouting angiogenesis in the vertebrate vascular system, and primary branching in the Drosophila tracheal system, specialized tip cells direct branch outgrowth and network formation. When tip cells lumenize, they form subcellular (seamless) tubes. How these seamless tubes are made, shaped and maintained remains poorly understood. Here we characterize a Drosophila mutant called ichor (ich), and show that ich is essential for the integrity and shape of seamless tubes in tracheal terminal cells. We find that Ich regulates seamless tubulogenesis via its role in promoting the formation of a mature apical extracellular matrix (aECM) lining the lumen of the seamless tubes. We determined that ich encodes a zinc finger protein (CG11966) that acts, as a transcriptional activator required for the expression of multiple aECM factors, including a novel membrane-anchored trypsin protease (CG8213). Thus, the integrity and shape of seamless tubes are regulated by the aECM that lines their lumens.

Dual function for Tango1 in secretion of bulky cargo and in ER-Golgi morphology

Tango1 enables ER-to-Golgi trafficking of large proteins. We show here that loss of Tango1, in addition to disrupting protein secretion and ER/Golgi morphology, causes ER stress and defects in cell shape. We find that the previously observed dependence of smaller cargos on Tango1 is a secondary effect. If large cargos like Dumpy, which we identify as a Tango1 cargo, are removed from the cell, nonbulky proteins reenter the secretory pathway. Removal of blocking cargo also restores cell morphology and attenuates the ER-stress response. Thus, failures in the secretion of nonbulky proteins, ER stress, and defective cell morphology are secondary consequences of bulky cargo retention. By contrast, ER/Golgi defects in Tango1-depleted cells persist in the absence of bulky cargo, showing that they are due to a secretion-independent function of Tango1. Therefore, maintenance of ER/Golgi architecture and bulky cargo transport are the primary functions for Tango1.

Neuralized regulates Crumbs endocytosis and epithelium morphogenesis via specific Stardust isoforms

The E3 ubiquitin ligase Neuralized is shown to interact with a subset of the Stardust isoforms to regulate the endocytosis of the apical protein Crumbs and thereby promote epithelial remodeling during Drosophila development.

Crumbs (Crb) is a conserved determinant of apical membrane identity that regulates epithelial morphogenesis in many developmental contexts. In this study, we identify the Crb complex protein Stardust (Sdt) as a target of the E3 ubiquitin ligase Neuralized (Neur) in Drosophila melanogaster. Neur interacts with and down-regulates specific Sdt isoforms containing a Neur binding motif (NBM). Using a CRISPR (clustered regularly interspaced short palindromic repeats)-induced deletion of the NBM-encoding exon, we found that Sdt is a key Neur target and that Neur acts via Sdt to down-regulate Crb. We further show that Neur promotes the endocytosis of Crb via the NBM-containing isoforms of Sdt. Although the regulation of Crb by Neur is not strictly essential, it contributes to epithelium remodeling in the posterior midgut and thereby facilitates the trans-epithelial migration of the primordial germ cells in early embryos. Thus, our study uncovers a novel regulatory mechanism for the developmental control of Crb-mediated morphogenesis.

F-actin dismantling through a redox-driven synergy between Mical and cofilin

Numerous cellular functions depend on actin filament (F-actin) disassembly. The best-characterized disassembly proteins, the ADF/cofilins/twinstar, sever filaments and recycle monomers to promote actin assembly. Cofilin is also a relatively weak actin disassembler, posing questions about mechanisms of cellular F-actin destabilization. Here we uncover a key link to targeted F-actin disassembly by finding that F-actin is efficiently dismantled through a post-translational-mediated synergism between cofilin and the actin-oxidizing enzyme Mical. We find that Mical-mediated oxidation of actin improves cofilin binding to filaments, where their combined effect dramatically accelerates F-actin disassembly compared to either effector alone. This synergism is also necessary and sufficient for F-actin disassembly in vivo, magnifying the effects of both Mical and cofilin on cellular remodeling, axon guidance, and Semaphorin/Plexin repulsion. Mical and cofilin, therefore, form a Redox-dependent synergistic pair that promotes F-actin instability by rapidly dismantling F-actin and generating post-translationally modified actin that has altered assembly properties.

Complex Polarity: Building Multicellular Tissues Through Apical Membrane Traffic

The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical-basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level.

Time to make the doughnuts: Building and shaping seamless tubes

A seamless tube is a very narrow-bore tube that is composed of a single cell with an intracellular lumen and no adherens or tight junctions along its length. Many capillaries in the vertebrate vascular system are seamless tubes. Seamless tubes also are found in invertebrate organs, including the Drosophila trachea and the Caenorhabditis elegans excretory system. Seamless tube cells can be less than a micron in diameter, and they can adopt very simple "doughnut-like" shapes or very complex, branched shapes comparable to those of neurons. The unusual topology and varied shapes of seamless tubes raise many basic cell biological questions about how cells form and maintain such structures. The prevalence of seamless tubes in the vascular system means that answering such questions has significant relevance to human health. In this review, we describe selected examples of seamless tubes in animals and discuss current models for how seamless tubes develop and are shaped, focusing particularly on insights that have come from recent studies in Drosophila and C. elegans.

Intracellular lumen formation in Drosophila proceeds via a novel subcellular compartment

Cellular tubes have diverse morphologies, including multicellular, unicellular and subcellular architectures. Subcellular tubes are found prominently within the vertebrate vasculature, the insect breathing system and the nematode excretory apparatus, but how such tubes form is poorly understood. To characterize the cellular mechanisms of subcellular tube formation, we have refined methods of high pressure freezing/freeze substitution to prepare Drosophila larvae for transmission electron microscopic (TEM) analysis. Using our methods, we have found that subcellular tube formation may proceed through a previously undescribed multimembrane intermediate composed of vesicles bound within a novel subcellular compartment. We have also developed correlative light/TEM procedures to identify labeled cells in TEM-fixed larval samples. Using this technique, we have found that Vacuolar ATPase (V-ATPase) and the V-ATPase regulator Rabconnectin-3 are required for subcellular tube formation, probably in a step resolving the intermediate compartment into a mature lumen. In general, our ultrastructural analysis methods could be useful for a wide range of cellular investigations in Drosophila larvae.

Compensatory branching morphogenesis of stalk cells in the Drosophila trachea

Tubes are essential for nutrient transport and gas exchange in multicellular eukaryotes, but how connections between different tube types are maintained over time is unknown. In the Drosophila tracheal system, mutations in oak gall (okg) and conjoined (cnj) confer identical defects, including late onset blockage near the terminal cell-stalk cell junction and the ectopic extension of autocellular, seamed tubes into the terminal cell. We determined that okg and cnj encode the E and G subunits of the vacuolar ATPase (vATPase) and showed that both the V0 and V1 domains are required for terminal cell morphogenesis. Remarkably, the ectopic seamed tubes running along vATPase-deficient terminal cells belonged to the neighboring stalk cells. All vATPase-deficient tracheal cells had reduced apical domains and terminal cells displayed mislocalized apical proteins. Consistent with recent reports that the mTOR and vATPase pathways intersect, we found that mTOR pathway mutants phenocopied okg and cnj. Furthermore, terminal cells depleted for the apical determinants Par6 or aPKC had identical ectopic seamed tube defects. We thus identify a novel mechanism of compensatory branching in which stalk cells extend autocellular tubes into neighboring terminal cells with undersized apical domains. This compensatory branching also occurs in response to injury, with damaged terminal cells being rapidly invaded by their stalk cell neighbor.