Requirements of genetic interactions between Src42A, armadillo and shotgun, a gene encoding E-cadherin, for normal development in Drosophila

Toxicity Evaluation of Potassium Sorbate In Vivo with Drosophila Melanogaster

Potassium sorbate (PS) is a preservative widely used in the food, pharmaceutical, and cosmetics industries. Improper and careless use of PS can lead to various health issues and potential environmental problems. Drosophila is capable of making rapid and sensitive responses to stress or other stimuli. Here we utilized Drosophila as a model organism to evaluate the potential toxicity of PS. Our study revealed that PS ingestion reduced the lifespan and fecundity of Drosophila. In addition, excessive PS ingestion led to cell apoptosis and ROS accumulation in the midgut. Furthermore, PS intake also enhanced the mitophagy of midgut cells. Strikingly, PS affected the cell differentiation progression as well, leading to the production of more enteroendocrine (EE) cells. We further demonstrated that the expression of notch (N), a vital player in intestinal stem cell (ISC) differentiation, was down-regulated in the midgut. This indicates that the differentiation progression was affected potentially by repressing the N expression.

A cellular tilting mechanism important for dynamic tissue shape changes and cell differentiation in Drosophila

Dynamic changes in three-dimensional cell shape are important for tissue form and function. In the developing Drosophila eye, photoreceptor differentiation requires the progression across the tissue of an epithelial fold known as the morphogenetic furrow. Morphogenetic furrow progression involves apical cell constriction and movement of apical cell edges. Here, we show that cells progressing through the morphogenetic furrow move their basal edges in opposite direction to their apical edges, resulting in a cellular tilting movement. We further demonstrate that cells generate, at their basal side, oriented, force-generating protrusions. Knockdown of the protein kinase Src42A or photoactivation of a dominant-negative form of the small GTPase Rac1 reduces protrusion formation. Impaired protrusion formation stalls basal cell movement and slows down morphogenetic furrow progression and photoreceptor differentiation. This work identifies a cellular tilting mechanism important for the generation of dynamic tissue shape changes and cell differentiation.

Src42A is required for E-cadherin dynamics at cell junctions during Drosophila axis elongation

Src kinases are important regulators of cell adhesion. Here, we have explored the function of Src42A in junction remodelling during Drosophila gastrulation. Src42A is required for tyrosine phosphorylation at bicellular (bAJ) and tricellular (tAJ) junctions in germband cells, and localizes to hotspots of mechanical tension. The role of Src42A was investigated using maternal RNAi and CRISPR-Cas9-induced germline mosaics. We find that, during cell intercalations, Src42A is required for the contraction of junctions at anterior-posterior cell interfaces. The planar polarity of E-cadherin is compromised and E-cadherin accumulates at tricellular junctions after Src42A knockdown. Furthermore, we show that Src42A acts in concert with Abl kinase, which has also been implicated in cell intercalations. Our data suggest that Src42A is involved in two related processes: in addition to establishing tension generated by the planar polarity of MyoII, it may also act as a signalling factor at tAJs to control E-cadherin residence time.

Engineered kinases as a tool for phosphorylation of selected targets in vivo

Reversible protein phosphorylation by kinases controls a plethora of processes essential for the proper development and homeostasis of multicellular organisms. One main obstacle in studying the role of a defined kinase–substrate interaction is that kinases form complex signaling networks and most often phosphorylate multiple substrates involved in various cellular processes. In recent years, several new approaches have been developed to control the activity of a given kinase. However, most of them fail to regulate a single protein target, likely hiding the effect of a unique kinase–substrate interaction by pleiotropic effects. To overcome this limitation, we have created protein binder-based engineered kinases that permit a direct, robust, and tissue-specific phosphorylation of fluorescent fusion proteins in vivo. We show the detailed characterization of two engineered kinases based on Rho-associated protein kinase (ROCK) and Src. Expression of synthetic kinases in the developing fly embryo resulted in phosphorylation of their respective GFP-fusion targets, providing for the first time a means to direct the phosphorylation to a chosen and tagged target in vivo. We presume that after careful optimization, the novel approach we describe here can be adapted to other kinases and targets in various eukaryotic genetic systems to regulate specific downstream effectors.

Mitochondria Lead the Way: Mitochondrial Dynamics and Function in Cellular Movements in Development and Disease

The dynamics, distribution and activity of subcellular organelles are integral to regulating cell shape changes during various physiological processes such as epithelial cell formation, cell migration and morphogenesis. Mitochondria are famously known as the powerhouse of the cell and play an important role in buffering calcium, releasing reactive oxygen species and key metabolites for various activities in a eukaryotic cell. Mitochondrial dynamics and morphology changes regulate these functions and their regulation is, in turn, crucial for various morphogenetic processes. In this review, we evaluate recent literature which highlights the role of mitochondrial morphology and activity during cell shape changes in epithelial cell formation, cell division, cell migration and tissue morphogenesis during organism development and in disease. In general, we find that mitochondrial shape is regulated for their distribution or translocation to the sites of active cell shape dynamics or morphogenesis. Often, key metabolites released locally and molecules buffered by mitochondria play crucial roles in regulating signaling pathways that motivate changes in cell shape, mitochondrial shape and mitochondrial activity. We conclude that mechanistic analysis of interactions between mitochondrial morphology, activity, signaling pathways and cell shape changes across the various cell and animal-based model systems holds the key to deciphering the common principles for this interaction.

Hippo signaling suppresses tumor cell metastasis via a Yki-Src42A positive feedback loop

Metastasis is an important cause of death from malignant tumors. It is of great significance to explore the molecular mechanism of metastasis for the development of anti-cancer drugs. Here, we find that the Hippo pathway hampers tumor cell metastasis in vivo. Silence of hpo or its downstream wts promotes tumor cell migration in a Yki-dependent manner. Furthermore, we identify that inhibition of the Hippo pathway promotes tumor cell migration through transcriptional activating src42A, a Drosophila homolog of the SRC oncogene. Yki activates src42A transcription through direct binding its intron region. Intriguingly, Src42A further increases Yki transcriptional activity to form a positive feedback loop. Finally, we show that SRC is also a target of YAP and important for YAP to promote the migration of human hepatocellular carcinoma cells. Together, our findings uncover a conserved Yki/YAP-Src42A/SRC positive feedback loop promoting tumor cell migration and provide SRC as a potential therapeutic target for YAP-driven metastatic tumors.

Toll receptors remodel epithelia by directing planar-polarized Src and PI3K activity

Toll-like receptors are essential for animal development and survival, with conserved roles in innate immunity, tissue patterning, and cell behavior. The mechanisms by which Toll receptors signal to the nucleus are well characterized, but how Toll receptors generate rapid, localized signals at the cell membrane to produce acute changes in cell polarity and behavior is not known. We show that Drosophila Toll receptors direct epithelial convergent extension by inducing planar-polarized patterns of Src and PI3-kinase (PI3K) activity. Toll receptors target Src activity to specific sites at the membrane, and Src recruits PI3K to the Toll-2 complex through tyrosine phosphorylation of the Toll-2 cytoplasmic domain. Reducing Src or PI3K activity disrupts planar-polarized myosin assembly, cell intercalation, and convergent extension, whereas constitutive Src activity promotes ectopic PI3K and myosin cortical localization. These results demonstrate that Toll receptors direct cell polarity and behavior by locally mobilizing Src and PI3K activity.

A modifier screen identifies regulators of cytoskeletal architecture as mediators of Shroom-dependent changes in tissue morphology

Regulation of cell architecture is critical in the formation of tissues during animal development. The mechanisms that control cell shape must be both dynamic and stable in order to establish and maintain the correct cellular organization. Previous work has identified Shroom family proteins as essential regulators of cell morphology during vertebrate development. Shroom proteins regulate cell architecture by directing the subcellular distribution and activation of Rho-kinase, which results in the localized activation of non-muscle myosin II. Because the Shroom-Rock-myosin II module is conserved in most animal model systems, we have utilized Drosophila melanogaster to further investigate the pathways and components that are required for Shroom to define cell shape and tissue architecture. Using a phenotype-based heterozygous F1 genetic screen for modifiers of Shroom activity, we identified several cytoskeletal and signaling protein that may cooperate with Shroom. We show that two of these proteins, Enabled and Short stop, are required for ShroomA-induced changes in tissue morphology and are apically enriched in response to Shroom expression. While the recruitment of Ena is necessary, it is not sufficient to redefine cell morphology. Additionally, this requirement for Ena appears to be context dependent, as a variant of Shroom that is apically localized, binds to Rock, but lacks the Ena binding site, is still capable of inducing changes in tissue architecture. These data point to important cellular pathways that may regulate contractility or facilitate Shroom-mediated changes in cell and tissue morphology.

Summary: Using Drosophila as a model system, we identify F-actin and microtubules as important determinants of how cells and tissues respond to Shroom induced contractility.

Rab11 is essential for lgl mediated JNK-Dpp signaling in dorsal closure and epithelial morphogenesis in Drosophila

Dorsal closure during Drosophila embryogenesis provides a robust genetic platform to study the basic cellular mechanisms that govern epithelial wound healing and morphogenesis. As dorsal closure proceeds, the lateral epithelial tissue (LE) adjacent to the dorsal opening advance contra-laterally, with a simultaneous retraction of the amnioserosa. The process involves a fair degree of coordinated cell shape changes in the dorsal most epithelial (DME) cells as well as a few penultimate rows of lateral epithelial (LE) cells (collectively referred here as Dorsolateral Epithelial (DLE) cells), lining the periphery of the amnioserosa, which in due course of time extend contra-laterally and ultimately fuse over the dorsal hole, giving rise to a dorsal epithelial continuum. The JNK-Dpp signaling in the dorsolateral epidermis, plays an instrumental role in guiding their fate during this process. A large array of genes have been reported to be involved in the regulation of this core signaling pathway, yet the mechanisms by which they do so is hitherto unclear, which forms the objective of our present study. Here we show a probable mechanism via which lgl, a conserved tumour suppressor gene, regulates the JNK-Dpp pathway during dorsal closure and epithelial morphogenesis. A conditional/targeted knock-down of lgl in the dorsolateral epithelium of embryos results in failure of dorsal closure. Interestingly, we also observed a similar phenotype in a Rab11 knockdown condition. Our experiment suggests Rab11 to be interacting with lgl as they seem to synergize in order to regulate the core JNK-Dpp signaling pathway during dorsal closure and also during adult thorax closure process.

An RNAi screen for secreted factors and cell-surface players in coordinating neuron and glia development in Drosophila

The establishment of the functional nervous system requires coordinated development of neurons and glia in the embryo. Our understanding of underlying molecular and cellular mechanisms, however, remains limited. The developing Drosophila visual system is an excellent model for understanding the developmental control of the nervous system. By performing a systematic transgenic RNAi screen, we investigated the requirements of secreted proteins and cell-surface receptors for the development of photoreceptor neurons (R cells) and wrapping glia (WG) in the Drosophila visual system. From the screen, we identified seven genes whose knockdown disrupted the development of R cells and/or WG, including amalgam (ama), domeless (dome), epidermal growth factor receptor (EGFR), kuzbanian (kuz), N-Cadherin (CadN), neuroglian (nrg), and shotgun (shg). Cell-type-specific analysis revealed that ama is required in the developing eye disc for promoting cell proliferation and differentiation, which is essential for the migration of glia in the optic stalk. Our results also suggest that nrg functions in both eye disc and WG for coordinating R-cell and WG development.

Extracellular matrix stiffness cues junctional remodeling for 3D tissue elongation

Organs are sculpted by extracellular as well as cell-intrinsic forces, but how collective cell dynamics are orchestrated in response to environmental cues is poorly understood. Here we apply advanced image analysis to reveal extracellular matrix-responsive cell behaviors that drive elongation of the Drosophila follicle, a model system in which basement membrane stiffness instructs three-dimensional tissue morphogenesis. Through in toto morphometric analyses of wild type and round egg mutants, we find that neither changes in average cell shape nor oriented cell division are required for appropriate organ shape. Instead, a major element is the reorientation of elongated cells at the follicle anterior. Polarized reorientation is regulated by mechanical cues from the basement membrane, which are transduced by the Src tyrosine kinase to alter junctional E-cadherin trafficking. This mechanosensitive cellular behavior represents a conserved mechanism that can elongate edgeless tubular epithelia in a process distinct from those that elongate bounded, planar epithelia.

The extracellular matrix can shape developing organs, but how external forces direct intercellular morphogenesis is unclear. Here, the authors use 3D imaging to show that elongation of the Drosophila egg chamber involves polarized cell reorientation signalled by changes in stiffness of the surrounding extracellular matrix.

The membrane environment of cadherin adhesion receptors: a working hypothesis

Classical cadherin cell adhesion receptors are integral membrane proteins that mediate cell–cell interactions, tissue integrity and morphogenesis. Cadherins are best understood to function as membrane-spanning molecular composites that couple adhesion to the cytoskeleton. On the other hand, the membrane lipid environment of the cadherins is an under-investigated aspect of their cell biology. In this review, we discuss two lines of research that show how the membrane can directly or indirectly contribute to cadherin function. Firstly, we consider how modification of its local lipid environment can potentially influence cadherin signalling, adhesion and dynamics, focusing on a role for phosphoinositide-4,5-bisphosphate. Secondly, we discuss how caveolae may indirectly regulate cadherins by modifying either the lipid composition and/or mechanical tension of the plasma membrane. Thus, we suggest that the membrane is a frontier of cadherin biology that is ripe for re-exploration.

QuBiT: a quantitative tool for analyzing epithelial tubes reveals unexpected patterns of organization in the Drosophila trachea

Biological tubes are essential for animal survival, and their functions are dependent on tube shape. Analyzing the contributions of cell shape and organization to the morphogenesis of small tubes has been hampered by the limitations of existing programs in quantifying cell geometry on highly curved tubular surfaces and calculating tube-specific parameters. We therefore developed QuBiT (Quantitative Tool for Biological Tubes) and used it to analyze morphogenesis of the embryonic Drosophila trachea (airway). In the main tube, we find previously unknown anterior-to-posterior (A-P) gradients of cell apical orientation and aspect ratio, and periodicity in the organization of apical cell surfaces. Inferred cell intercalation during development dampens an A-P gradient of the number of cells per cross-section of the tube, but does not change the patterns of cell connectivity. Computationally 'unrolling' the apical surface of wild-type trachea and the hindgut reveals previously unrecognized spatial patterns of the apical marker Uninflatable and a non-redundant role for the Na⁺/K⁺ ATPase in apical marker organization. These unexpected findings demonstrate the importance of a computational tool for analyzing small diameter biological tubes.

Mbt/PAK4 together with SRC modulates N-Cadherin adherens junctions in the developing Drosophila eye

Tissue morphogenesis is accompanied by changes of adherens junctions (AJ). During Drosophila eye development, AJ reorganization includes the formation of isolated N-Cadherin AJ between photoreceptors R3/R4. Little is known about how these N-Cadherin AJ are established and maintained. This study focuses on the kinases Mbt/PAK4 and SRC, both known to alter E-Cadherin AJ across phyla. Drosophila p21-activated kinase Mbt and the non-receptor tyrosine kinases Src64 and Src42 regulate proper N-Cadherin AJ. N-Cadherin AJ elongation depends on SRC kinase activity. Cell culture experiments demonstrate binding of both Drosophila SRC isoforms to N-Cadherin and its subsequent tyrosine phosphorylation. In contrast, Mbt stabilizes but does not bind N-Cadherin in vitro. Mbt is required in R3/R4 for zipping the N-Cadherin AJ between these cells, independent of its kinase activity and Cdc42-binding. The mbt phenotype can be reverted by mutations in Src64 and Src42. Because Mbt neither directly binds to SRC proteins nor has a reproducible influence on their kinase activity, the conclusion is that Mbt and SRC signaling converge on N-Cadherin. N-Cadherin AJ formation during eye development requires a proper balance between the promoting effects of Mbt and the inhibiting influences of SRC kinases.

Summary: N-Cadherin adherens junction formation in the Drosophila larval eye imaginal disc is controlled by the combined functions of the p21-activated kinase Mbt/PAK4 and the kinases Src64 and Src42.

The major β-catenin/E-cadherin junctional binding site is a primary molecular mechano-transductor of differentiation in vivo

In vivo, the primary molecular mechanotransductive events mechanically initiating cell differentiation remain unknown. Here we find the molecular stretching of the highly conserved Y654-β-catenin-D665-E-cadherin binding site as mechanically induced by tissue strain. It triggers the increase of accessibility of the Y654 site, target of the Src42A kinase phosphorylation leading to irreversible unbinding. Molecular dynamics simulations of the β-catenin/E-cadherin complex under a force mimicking a 6 pN physiological mechanical strain predict a local 45% stretching between the two α-helices linked by the site and a 15% increase in accessibility of the phosphorylation site. Both are quantitatively observed using FRET lifetime imaging and non-phospho Y654 specific antibody labelling, in response to the mechanical strains developed by endogenous and magnetically mimicked early mesoderm invagination of gastrulating Drosophila embryos. This is followed by the predicted release of 16% of β-catenin from junctions, observed in FRAP, which initiates the mechanical activation of the β-catenin pathway process.

Src Cooperates with Oncogenic Ras in Tumourigenesis via the JNK and PI3K Pathways in Drosophila epithelial Tissue

The Ras oncogene (Rat Sarcoma oncogene, a small GTPase) is a key driver of human cancer, however alone it is insufficient to produce malignancy, due to the induction of cell cycle arrest or senescence. In a Drosophila melanogaster genetic screen for genes that cooperate with oncogenic Ras (bearing the Ras^V12 mutation, or Ras^ACT), we identified the Drosophila Src (Sarcoma virus oncogene) family non-receptor tyrosine protein kinase genes, Src42A and Src64B, as promoting increased hyperplasia in a whole epithelial tissue context in the Drosophila eye. Moreover, overexpression of Src cooperated with Ras^ACT in epithelial cell clones to drive neoplastic tumourigenesis. We found that Src overexpression alone activated the Jun N-terminal Kinase (JNK) signalling pathway to promote actin cytoskeletal and cell polarity defects and drive apoptosis, whereas, in cooperation with Ras^ACT, JNK led to a loss of differentiation and an invasive phenotype. Src + Ras^ACT cooperative tumourigenesis was dependent on JNK as well as Phosphoinositide 3-Kinase (PI3K) signalling, suggesting that targeting these pathways might provide novel therapeutic opportunities in cancers dependent on Src and Ras signalling.

The Misshapen kinase regulates the size and stability of the germline ring canals in the Drosophila egg chamber

Intercellular bridges are conserved structures that allow neighboring cells to exchange cytoplasmic material; defects in intercellular bridges can lead to infertility in many organisms. Here, we use the Drosophila egg chamber to study the mechanisms that regulate intercellular bridges. Within the developing egg chamber, the germ cells (15 nurse cells and 1 oocyte) are connected to each other through intercellular bridges called ring canals, which expand over the course of oogenesis to support the transfer of materials from the nurse cells to the oocyte. The ring canals are enriched in actin and actin binding proteins, and many proteins have been identified that localize to the germline ring canals and control their expansion and stability. Here, we demonstrate a novel role for the Ste20 family kinase, Misshapen (Msn), in regulation of the size of the germline ring canals. Msn localizes to ring canals throughout most of oogenesis, and depletion of Msn led to the formation of larger ring canals. Over-expression of Msn decreased ring canal diameter, and expression of a membrane tethered form of Msn caused ring canal detachment and nurse cell fusion. Altering the levels or localization of Msn also led to changes in the actin cytoskeleton and altered the localization of E-cadherin, which suggests that Msn could be indirectly limiting ring canal size by altering the structure or dynamics of the actin cytoskeleton and/or adherens junctions.

The adherens junction-associated LIM domain protein Smallish regulates epithelial morphogenesis

Cell–cell adhesion and cell shape are regulated at adherens junctions during embryonic morphogenesis. Beati et al. show that the Drosophila LIM domain protein Smallish interacts with Bazooka, Canoe, and Src42A at adherens junctions. Loss-of-function and gain-of-function phenotypes reveal a function for Smallish in regulation of actomyosin contractility and cell shape.

In epithelia, cells adhere to each other in a dynamic fashion, allowing the cells to change their shape and move along each other during morphogenesis. The regulation of adhesion occurs at the belt-shaped adherens junction, the zonula adherens (ZA). Formation of the ZA depends on components of the Par–atypical PKC (Par-aPKC) complex of polarity regulators. We have identified the Lin11, Isl-1, Mec-3 (LIM) protein Smallish (Smash), the orthologue of vertebrate LMO7, as a binding partner of Bazooka/Par-3 (Baz), a core component of the Par-aPKC complex. Smash also binds to Canoe/Afadin and the tyrosine kinase Src42A and localizes to the ZA in a planar polarized fashion. Animals lacking Smash show loss of planar cell polarity (PCP) in the embryonic epidermis and reduced cell bond tension, leading to severe defects during embryonic morphogenesis of epithelial tissues and organs. Overexpression of Smash causes apical constriction of epithelial cells. We propose that Smash is a key regulator of morphogenesis coordinating PCP and actomyosin contractility at the ZA.

Mapping Heart Development in Flies: Src42A Acts Non-Autonomously to Promote Heart Tube Formation in Drosophila

Congenital heart defects, clinically identified in both small and large animals, are multifactorial and complex. Although heritable factors are known to have a role in cardiovascular disease, the full genetic aetiology remains unclear. Model organism research has proven valuable in providing a deeper understanding of the essential factors in heart development. For example, mouse knock-out studies reveal a role for the Integrin adhesion receptor in cardiac tissue. Recent research in Drosophila melanogaster (the fruit fly), a powerful experimental model, has demonstrated that the link between the extracellular matrix and the cell, mediated by Integrins, is required for multiple aspects of cardiogenesis. Here we test the hypothesis that Integrins signal to the heart cells through Src42A kinase. Using the powerful genetics and cell biology analysis possible in Drosophila, we demonstrate that Src42A acts in early events of heart tube development. Careful examination of mutant heart tissue and genetic interaction data suggests that Src42A’s role is independent of Integrin and the Integrin-related Focal Adhesion Kinase. Rather, Src42A acts non-autonomously by promoting programmed cell death of the amnioserosa, a transient tissue that neighbors the developing heart.

Adherens Junctions on the Move-Membrane Trafficking of E-Cadherin

Cadherin-based adherens junctions are conserved structures that mediate epithelial cell-cell adhesion in invertebrates and vertebrates. Despite their pivotal function in epithelial integrity, adherens junctions show a remarkable plasticity that is a prerequisite for tissue architecture and morphogenesis. Epithelial cadherin (E-cadherin) is continuously turned over and undergoes cycles of endocytosis, sorting and recycling back to the plasma membrane. Mammalian cell culture and genetically tractable model systems such as Drosophila have revealed conserved, but also distinct, mechanisms in the regulation of E-cadherin membrane trafficking. Here, we discuss our current knowledge about molecules and mechanisms controlling endocytosis, sorting and recycling of E-cadherin during junctional remodeling.

Adherens Junction and E-Cadherin complex regulation by epithelial polarity

E-Cadherin-based Adherens Junctions (AJs) are a defining feature of all epithelial sheets. Through the homophilic association of E-Cadherin molecules expressed on neighboring cells, they ensure intercellular adhesion amongst epithelial cells, and regulate many key aspects of epithelial biology. While their adhesive role requires these structures to remain stable, AJs are also extremely plastic. This plasticity allows for the adaptation of the cell to its changing environment: changes in neighbors after cell division, cell death, or cell movement, and changes in cell shape during differentiation. In this review we focus on the recent advances highlighting the critical role of the apico-basal polarity machinery, and in particular of the Par3/Bazooka scaffold, in the regulation and remodeling of AJs. We propose that by regulating key phosphorylation events on the core E-Cadherin complex components, Par3 and epithelial polarity promote meta-stable protein complexes governing the correct formation, localization, and functioning of AJ.

Analysis of the Molecular Mechanisms of Reepithelialization in Drosophila Embryos

Significance: The epidermis provides the main barrier function of skin, and therefore its repair following wounding is an essential component of wound healing. Repair of the epidermis, also known as reepithelialization, occurs by collective migration of epithelial cells from around the wound edge across the wound until the advancing edges meet and fuse. Therapeutic manipulation of this process could potentially be used to accelerate wound healing.

Recent Advances: It is difficult to analyze the cellular and molecular mechanisms of reepithelialization in human tissue, so a variety of model organisms have been used to improve our understanding of the process. One model system that has been especially useful is the embryo of the fruit fly Drosophila, which provides a simple, accessible model of the epidermis and can be manipulated genetically, allowing detailed analysis of reepithelialization at the molecular level. This review will highlight the key insights that have been gained from studying reepithelialization in Drosophila embryos.

Critical Issues: Slow reepithelialization increases the risk of wounds becoming infected and ulcerous; therefore, the development of therapies to accelerate or enhance the process would be a great clinical advance. Improving our understanding of the molecular mechanisms that underlie reepithelialization will help in the development of such therapies.

Future Directions: Research in Drosophila embryos has identified a variety of genes and proteins involved in triggering and driving reepithelialization, many of which are conserved in humans. These novel reepithelialization proteins are potential therapeutic targets and therefore findings obtained in Drosophila may ultimately lead to significant clinical advances.

Elevated Src family kinase activity stabilizes E-cadherin-based junctions and collective movement of head and neck squamous cell carcinomas

EGF receptor (EGFR) overexpression is thought to drive head and neck carcinogenesis however clinical responses to EGFR-targeting agents have been modest and alternate targets are actively sought to improve results. Src family kinases (SFKs), reported to act downstream of EGFR are among the alternative targets for which increased expression or activity in epithelial tumors is commonly associated to the dissolution of E-cadherin-based junctions and acquisition of a mesenchymal-like phenotype. Robust expression of total and activated Src was observed in advanced stage head and neck tumors (N=60) and in head and neck squamous cell carcinoma lines. In cultured cancer cells Src co-localized with E-cadherin in cell-cell junctions and its phosphorylation on Y419 was both constitutive and independent of EGFR activation. Selective inhibition of SFKs with SU6656 delocalized E-cadherin and disrupted cellular junctions without affecting E-cadherin expression and this effect was phenocopied by knockdown of Src or Yes. These findings reveal an EGFR-independent role for SFKs in the maintenance of intercellular junctions, which likely contributes to the cohesive invasion E-cadherin-positive cells in advanced tumors. Further, they highlight the need for a deeper comprehension of molecular pathways that drive collective cell invasion, in absence of mesenchymal transition, in order to combat tumor spread.

Activation of zebrafish Src family kinases by the prion protein is an amyloid-β-sensitive signal that prevents the endocytosis and degradation of E-cadherin/β-catenin complexes in vivo

Background

Prions and amyloid-β (Aβ) oligomers trigger neurodegeneration by hijacking a poorly understood cellular signal mediated by the prion protein (PrP) at the plasma membrane. In early zebrafish embryos, PrP-1-dependent signals control cell-cell adhesion via a tyrosine phosphorylation-dependent mechanism.

Results

Here we report that the Src family kinases (SFKs) Fyn and Yes act downstream of PrP-1 to prevent the endocytosis and degradation of E-cadherin/β-catenin adhesion complexes in vivo. Accordingly, knockdown of PrP-1 or Fyn/Yes cause similar zebrafish gastrulation phenotypes, whereas Fyn/Yes expression rescues the PrP-1 knockdown phenotype. We also show that zebrafish and mouse PrPs positively regulate the activity of Src kinases and that these have an unexpected positive effect on E-cadherin-mediated cell adhesion. Interestingly, while PrP knockdown impairs β-catenin adhesive function, PrP overexpression enhances it, thereby antagonizing its nuclear, wnt-related signaling activity and disturbing embryonic dorsoventral specification. The ability of mouse PrP to influence these events in zebrafish embryos requires its neuroprotective, polybasic N-terminus but not its neurotoxicity-associated central region. Remarkably, human Aβ oligomers up-regulate the PrP-1/SFK/E-cadherin/β-catenin pathway in zebrafish embryonic cells, mimicking a PrP gain-of-function scenario.

Conclusions

Our gain- and loss-of-function experiments in zebrafish suggest that PrP and SFKs enhance the cell surface stability of embryonic adherens junctions via the same complex mechanism through which they over-activate neuroreceptors that trigger synaptic damage. The profound impact of this pathway on early zebrafish development makes these embryos an ideal model to study the cellular and molecular events affected by neurotoxic PrP mutations and ligands in vivo. In particular, our finding that human Aβ oligomers activate the zebrafish PrP/SFK/E-cadherin pathway opens the possibility of using fish embryos to rapidly screen for novel therapeutic targets and compounds against prion- and Alzheimer's-related neurodegeneration. Altogether, our data illustrate PrP-dependent signals relevant to embryonic development, neuronal physiology and neurological disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0076-5) contains supplementary material, which is available to authorized users.

Regulation of Endothelial Adherens Junctions by Tyrosine Phosphorylation

Endothelial cells form a semipermeable, regulated barrier that limits the passage of fluid, small molecules, and leukocytes between the bloodstream and the surrounding tissues. The adherens junction, a major mechanism of intercellular adhesion, is comprised of transmembrane cadherins forming homotypic interactions between adjacent cells and associated cytoplasmic catenins linking the cadherins to the cytoskeleton. Inflammatory conditions promote the disassembly of the adherens junction and a loss of intercellular adhesion, creating openings or gaps in the endothelium through which small molecules diffuse and leukocytes transmigrate. Tyrosine kinase signaling has emerged as a central regulator of the inflammatory response, partly through direct phosphorylation and dephosphorylation of the adherens junction components. This review discusses the findings that support and those that argue against a direct effect of cadherin and catenin phosphorylation in the disassembly of the adherens junction. Recent findings indicate a complex interaction between kinases, phosphatases, and the adherens junction components that allow a fine regulation of the endothelial permeability to small molecules, leukocyte migration, and barrier resealing.

The Notch-mediated hyperplasia circuitry in Drosophila reveals a Src-JNK signaling axis

Notch signaling controls a wide range of cell fate decisions during development and disease via synergistic interactions with other signaling pathways. Here, through a genome-wide genetic screen in Drosophila, we uncover a highly complex Notch-dependent genetic circuitry that profoundly affects proliferation and consequently hyperplasia. We report a novel synergistic relationship between Notch and either of the non-receptor tyrosine kinases Src42A and Src64B to promote hyperplasia and tissue disorganization, which results in cell cycle perturbation, JAK/STAT signal activation, and differential regulation of Notch targets. Significantly, the JNK pathway is responsible for the majority of the phenotypes and transcriptional changes downstream of Notch-Src synergy. We previously reported that Notch-Mef2 also activates JNK, indicating that there are commonalities within the Notch-dependent proliferation circuitry; however, the current data indicate that Notch-Src accesses JNK in a significantly different fashion than Notch-Mef2.

DOI:http://dx.doi.org/10.7554/eLife.05996.001

The cells within animals are organized into tissues and organs that perform particular roles. To develop and maintain these structures, the ability of individual cells to divide and grow is strictly controlled by the activities of many proteins, including one called Notch. This protein is found in all multicellular organisms and allows cells to communicate with each other. Mutations in the gene that encodes Notch can cause cells to divide excessively and lead to cancer and other diseases.

Notch regulates the growth and division of cells by interacting with many other proteins. For example, Mef2 works with Notch to activate a communication system called the JNK pathway. This pathway is involved in controlling cell division, cell death, and cell movement. However, it is thought that Notch may also interact with other proteins that have not yet been identified.

Now, Ho et al. have conducted a genome-wide screen in fruit flies to find proteins that interact with Notch. The experiments used flies that develop abnormally large eyes because they have an over-active Notch protein. Ho et al. identified hundreds of fruit fly genes that could increase or decrease the size of the flies' eyes in the presence of Notch activity. Many of these genes are known to be involved in development, cell division, or in controlling the activity of other genes.

Ho et al. found that two of these genes encode similar proteins called Src42A and Src64B, which are similar to the Src proteins that are involved in many types of human cancers. The experiments show that both proteins interact with Notch to promote uncontrolled cell division and lead to tissues in the flies becoming more disorganized. The JNK pathway is also activated by Notch working with Src42A or Src64B, but in a different manner to how it is activated by Mef2 and Notch, and with different consequences for cells.

This study provides new insights into how genes work together in order to influence cell division and other events in development. Also, it suggests that Notch activity may regulate the growth of cancers linked with defects in the Src proteins.

DOI:http://dx.doi.org/10.7554/eLife.05996.002

In vivo RNAi screen identifies candidate signaling genes required for collective cell migration in Drosophila ovary

Collective migration of loosely or closely associated cell groups is prevalent in animal development, physiological events, and cancer metastasis. However, our understanding of the mechanisms of collective cell migration is incomplete. Drosophila border cells provide a powerful in vivo genetic model to study collective migration and identify essential genes for this process. Using border cell-specific RNAi-silencing in Drosophila, we knocked down 360 conserved signaling transduction genes in adult flies to identify essential pathways and genes for border cell migration. We uncovered a plethora of signaling genes, a large proportion of which had not been reported for border cells, including Rack1 (Receptor of activated C kinase) and brk (brinker), mad (mother against dpp), and sax (saxophone), which encode three components of TGF-β signaling. The RNAi knock down phenotype was validated by clonal analysis of Rack1 mutants. Our data suggest that inhibition of Src activity by Rack1 may be important for border cell migration and cluster cohesion maintenance. Lastly, results from our screen not only would shed light on signaling pathways involved in collective migration during embryogenesis and organogenesis in general, but also could help our understanding for the functions of conserved human genes involved in cancer metastasis.

Balance between apical membrane growth and luminal matrix resistance determines epithelial tubule shape

The morphological stability of biological tubes is crucial for the efficient circulation of fluids and gases. Failure of this stability causes irregularly shaped tubes found in multiple pathological conditions. Here, we report that Drosophila mutants of the ESCRT III component Shrub/Vps32 exhibit a strikingly elongated sinusoidal tube phenotype. This is caused by excessive apical membrane synthesis accompanied by the ectopic accumulation and overactivation of Crumbs in swollen endosomes. Furthermore, we demonstrate that the apical extracellular matrix (aECM) of the tracheal tube is a viscoelastic material coupled with the apical membrane. We present a simple mechanical model in which aECM elasticity, apical membrane growth, and their interaction are three vital parameters determining the stability of biological tubes. Our findings demonstrate a mechanical role for the extracellular matrix and suggest that the interaction of the apical membrane and an elastic aECM determines the final morphology of biological tubes independent of cell shape.

RPTPα controls epithelial adherens junctions, linking E-cadherin engagement to c-Src-mediated phosphorylation of cortactin

Epithelial junctions are fundamental determinants of tissue organization, subject to regulation by tyrosine phosphorylation. Homophilic binding of E-cadherin activates tyrosine kinases, such as Src, that control junctional integrity. Protein tyrosine phosphatases (PTPs) also contribute to cadherin-based adhesion and signaling, but little is known about their specific identity or functions at epithelial junctions. Here, we report that the receptor PTP RPTPα (human gene name PTPRA) is recruited to epithelial adherens junctions at the time of cell-cell contact, where it is in molecular proximity to E-cadherin. RPTPα is required for appropriate cadherin-dependent adhesion and for cyst architecture in three-dimensional culture. Loss of RPTPα impairs adherens junction integrity, as manifested by defective E-cadherin accumulation and peri-junctional F-actin density. These effects correlate with a role for RPTPα in cellular (c)-Src activation at sites of E-cadherin engagement. Mechanistically, RPTPα is required for appropriate tyrosine phosphorylation of cortactin, a major Src substrate and a cytoskeletal actin organizer. Expression of a phosphomimetic cortactin mutant in RPTPα-depleted cells partially rescues F-actin and E-cadherin accumulation at intercellular contacts. These findings indicate that RPTPα controls cadherin-mediated signaling by linking homophilic E-cadherin engagement to cortactin tyrosine phosphorylation through c-Src.

Innexin 3, a new gene required for dorsal closure in Drosophila embryo

BACKGROUND

Dorsal closure is a morphogenetic event that occurs during mid-embryogenesis in many insects including Drosophila, during which the ectoderm migrates on the extraembryonic amnioserosa to seal the embryo dorsally. The contribution of the ectoderm in this event has been known for a long time. However, amnioserosa tension and contractibility have recently been shown also to be instrumental to the closure. A critical pre-requisite for dorsal closure is integrity of these tissues that in part is mediated by cell-cell junctions and cell adhesion. In this regard, mutations impairing junction formation and/or adhesion lead to dorsal closure. However, no role for the gap junction proteins Innexins has so far been described.

RESULTS AND DISCUSSION

Here, we show that Innexin 1, 2 and 3, are present in the ectoderm but also in the amnioserosa in plaques consistent with gap junctions. However, only the loss of Inx3 leads to dorsal closure defects that are completely rescued by overexpression of inx3::GFP in the whole embryo. Loss of Inx3 leads to the destabilisation of Inx1, Inx2 and DE-cadherin at the plasma membrane, suggesting that these four proteins form a complex. Accordingly, in addition to the known interaction of Inx2 with DE-cadherin, we show that Inx3 can bind to DE-cadherin. Furthermore, Inx3-GFP overexpression recruits DE-cadherin from its wildtype plasma membrane domain to typical Innexin plaques, strengthening the notion that they form a complex. Finally, we show that Inx3 stability is directly dependent on tissue tension. Taken together, we propose that Inx3 is a critical factor for dorsal closure and that it mediates the stability of Inx1, 2 and DE-cadherin by forming a complex.

Src kinases mediate the interaction of the apical determinant Bazooka/PAR3 with STAT92E and increase signalling efficiency in Drosophila ectodermal cells

Intercellular communication depends on the correct organization of the signal transduction complexes. In many signalling pathways, the mechanisms controlling the overall cell polarity also localize components of these pathways to different domains of the plasma membrane. In the Drosophila ectoderm, the JAK/STAT pathway components are highly polarized with apical localization of the receptor, the associated kinase and the STAT92E protein itself. The apical localization of STAT92E is independent of the receptor complex and is due to its direct association with the apical determining protein Bazooka (Baz). Here, we find that Baz-STAT92E interaction depends on the presence of the Drosophila Src kinases. In the absence of Src, STAT92E cannot bind to Baz in cells or in whole embryos, and this correlates with an impairment of JAK/STAT signalling function. We believe that the requirement of Src proteins for STAT92E apical localization is mediated through Baz, as we can co-precipitate Src with Baz but not with STAT92E. This is the first time that a functional link between cell polarity, the JAK/STAT signalling pathway and the Src kinases has been established in a whole organism.

Src inhibits midline axon crossing independent of Frazzled/Deleted in Colorectal Carcinoma (DCC) receptor tyrosine phosphorylation

The phylogenetically conserved Netrin family of chemoattractants signal outgrowth and attractive turning of commissural axons through the Deleted in Colorectal Carcinoma (DCC) family of receptors. Src family kinases are thought to be major signaling effectors of Netrin/DCC. In vertebrates, Src and the closely related Fyn kinases phosphorylate DCC and form a receptor-bound signaling complex leading to activation of downstream effectors. Here we show that, in the Drosophila embryonic CNS, Src kinases are dispensable for midline attraction of commissural axons. Consistent with this observation, tyrosine phosphorylation of the Netrin receptor DCC or its Drosophila ortholog, Frazzled, is not necessary for attraction to Netrin. Moreover, we uncover an unexpected function of Src kinases: inhibition of midline axon crossing through a novel mechanism. We propose that distinct signaling outputs must exist for midline axon crossing independent of Src kinases in commissural neurons.

The Frizzled-dependent planar polarity pathway locally promotes E-cadherin turnover via recruitment of RhoGEF2

Polarised tissue elongation during morphogenesis involves cells within epithelial sheets or tubes making and breaking intercellular contacts in an oriented manner. Growing evidence suggests that cell adhesion can be modulated by endocytic trafficking of E-cadherin (E-cad), but how this process can be polarised within individual cells is poorly understood. The Frizzled (Fz)-dependent core planar polarity pathway is a major regulator of polarised cell rearrangements in processes such as gastrulation, and has also been implicated in regulation of cell adhesion through trafficking of E-cad; however, it is not known how these functions are integrated. We report a novel role for the core planar polarity pathway in promoting cell intercalation during tracheal tube morphogenesis in Drosophila embryogenesis, and present evidence that this is due to regulation of turnover and levels of junctional E-cad by the guanine exchange factor RhoGEF2. Furthermore, we show that core pathway activity leads to planar-polarised recruitment of RhoGEF2 and E-cad turnover in the epidermis of both the embryonic germband and the pupal wing. We thus reveal a general mechanism by which the core planar polarity pathway can promote polarised cell rearrangements.

Tubulogenesis: Src42A goes to great lengths in tube elongation

New work shows the instructive role of Src42A kinase in tube size regulation. By inducing polarized cell-shape changes, Src42A promotes tube elongation in the Drosophila tracheal system.

Cell adhesion in Drosophila: versatility of cadherin and integrin complexes during development

We highlight recent progress in understanding cadherin and integrin function in the model organism Drosophila. New functions for these adhesion receptors continue to be discovered in this system, emphasising the importance of cell adhesion within the developing organism and showing that the requirement for cell adhesion changes between cell types. New ways to control adhesion have been discovered, including controlling the expression and recruitment of adhesion components, their posttranslational modification, recycling and turnover. Importantly, even ubiquitous adhesion components can function differently in distinct cellular contexts.

Src42A-dependent polarized cell shape changes mediate epithelial tube elongation in Drosophila

Although many organ functions rely on epithelial tubes with correct dimensions, mechanisms underlying tube size control are poorly understood. We analyse the cellular mechanism of tracheal tube elongation in Drosophila, and describe an essential role of the conserved tyrosine kinase Src42A in this process. We show that Src42A is required for polarized cell shape changes and cell rearrangements that mediate tube elongation. In contrast, diametric expansion is controlled by apical secretion independently of Src42A. Constitutive activation of Src42A induces axial cell stretching and tracheal overelongation, indicating that Src42A acts instructively in this process. We propose that Src42A-dependent recycling of E-Cadherin at adherens junctions is limiting for cell shape changes and rearrangements in the axial dimension of the tube. Thus, we define distinct cellular processes that independently control axial and diametric expansion of a cylindrical epithelium in a developing organ. Whereas exocytosis-dependent membrane growth drives circumferential tube expansion, Src42A is required to orient membrane growth in the axial dimension of the tube.

Drosophila Src regulates anisotropic apical surface growth to control epithelial tube size

Networks of epithelial and endothelial tubes are essential for the function of organs such as the lung, kidney and vascular system. The sizes and shapes of these tubes are highly regulated to match their individual functions. Defects in tube size can cause debilitating diseases such as polycystic kidney disease and ischaemia. It is therefore critical to understand how tube dimensions are regulated. Here we identify the tyrosine kinase Src as an instructive regulator of epithelial-tube length in the Drosophila tracheal system. Loss-of-function Src42 mutations shorten tracheal tubes, whereas Src42 overexpression elongates them. Surprisingly, Src42 acts distinctly from known tube-size pathways and regulates both the amount of apical surface growth and, with the conserved formin dDaam, the direction of growth. Quantitative three-dimensional image analysis reveals that Src42- and dDaam-mutant tracheal cells expand more in the circumferential than the axial dimension, resulting in tubes that are shorter in length-but larger in diameter-than wild-type tubes. Thus, Src42 and dDaam control tube dimensions by regulating the direction of anisotropic growth, a mechanism that has not previously been described.

Adherens junctions and cadherins in Drosophila development

Drosophila represents a paradigm for the analysis of the cellular, molecular and genetic mechanisms of development and is an ideal model system to study the contribution of Adherens Junctions (AJs) and their major components, cadherins, to morphogenesis. The combination of different techniques and approaches has allowed researchers to identify the requirements of these epithelial junctions in vivo in the context of a whole organism. The functional analysis of mutants for AJ core components, particularly for Drosophila DE-cadherin, has shown that AJs play critical roles in virtually all stages of development. For instance, AJs maintain tissue integrity while allowing the remodelling and homeostasis of many tissues. They control cell shape, contribute to cell polarity, facilitate cell-cell recognition during cell sorting, orient cell divisions, or regulate cell rearrangements, among other activities. Remarkably, these activities require a very fine control of the organisation and turnover of AJs during development. In addition, AJs engage in diverse and complex interactions with the cytoskeleton, signalling networks, intracellular trafficking machinery or polarity cues to perform these functions. Here, by summarising the requirements of AJs and cadherins during Drosophila morphogenesis, we illustrate the capital contribution of this model system to our knowledge of the mechanisms and biology of AJs.

Mutations in the catalytic loop HRD motif alter the activity and function of Drosophila Src64

The catalytic loop HRD motif is found in most protein kinases and these amino acids are predicted to perform functions in catalysis, transition to, and stabilization of the active conformation of the kinase domain. We have identified mutations in a Drosophila src gene, src64, that alter the three HRD amino acids. We have analyzed the mutants for both biochemical activity and biological function during development. Mutation of the aspartate to asparagine eliminates biological function in cytoskeletal processes and severely reduces fertility, supporting the amino acid's critical role in enzymatic activity. The arginine to cysteine mutation has little to no effect on kinase activity or cytoskeletal reorganization, suggesting that the HRD arginine may not be critical for coordinating phosphotyrosine in the active conformation. The histidine to leucine mutant retains some kinase activity and biological function, suggesting that this amino acid may have a biochemical function in the active kinase that is independent of its side chain hydrogen bonding interactions in the active site. We also describe the phenotypic effects of other mutations in the SH2 and tyrosine kinase domains of src64, and we compare them to the phenotypic effects of the src64 null allele.

Drosophila spoonbill encodes a dual-specificity A-kinase anchor protein essential for oogenesis

spoonbill is a Drosophila female-sterile mutation, which interferes with normal egg patterning during oogenesis. Previous analyzes linked the mutation to a number of seemingly unrelated pathways, including GRK/EGFR and DPP, two major pathways essential for Drosophila and vertebrate development. Further work suggested that spoonbill may also function in actin polymerization and border-cell migration. Here we describe the molecular cloning of the spoonbill gene and characterize new mutant alleles, further demonstrating that spoonbill's function is essential during oogenesis. We found spoonbill to be allelic to CG3249 (also known as yu), which encodes the only known dual-specificity A-kinase anchor protein in Drosophila. Our data indicate that similar to mammalian AKAPs, Spoonbill protein contains a number of potential kinase and phosphatase binding motifs, and is targeted, in the ovary, to mitochondria and Golgi. Finally, we address some of spoonbill's mutant phenotypes from the perspective of the published data on the AKAP protein family.

Moleskin is essential for the formation of the myotendinous junction in Drosophila

It is the precise connectivity between skeletal muscles and their corresponding tendon cells to form a functional myotendinous junction (MTJ) that allows for the force generation required for muscle contraction and organismal movement. The Drosophila MTJ is composed of secreted extracellular matrix (ECM) proteins deposited between integrin-mediated hemi-adherens junctions on the surface of muscle and tendon cells. In this paper, we have identified a novel, cytoplasmic role for the canonical nuclear import protein Moleskin (Msk) in Drosophila embryonic somatic muscle attachment. Msk protein is enriched at muscle attachment sites in late embryogenesis and msk mutant embryos exhibit a failure in muscle-tendon cell attachment. Although the muscle-tendon attachment sites are reduced in size, components of the integrin complexes and ECM proteins are properly localized in msk mutant embryos. However, msk mutants fail to localize phosphorylated focal adhesion kinase (pFAK) to the sites of muscle-tendon cell junctions. In addition, the tendon cell specific proteins Stripe (Sr) and activated mitogen-activated protein kinase (MAPK) are reduced in msk mutant embryos. Our rescue experiments demonstrate that Msk is required in the muscle cell, but not in the tendon cells. Moreover, muscle attachment defects due to loss of Msk are rescued by an activated form of MAPK or the secreted epidermal growth factor receptor (Egfr) ligand Vein. Taken together, these findings provide strong evidence that Msk signals non-autonomously through the Vein-Egfr signaling pathway for late tendon cell late differentiation and/or maintenance.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Endocytic and recycling endosomes modulate cell shape changes and tissue behaviour during morphogenesis in Drosophila

During development tissue deformations are essential for the generation of organs and to provide the final form of an organism. These deformations rely on the coordination of individual cell behaviours which have their origin in the modulation of subcellular activities. Here we explore the role endocytosis and recycling on tissue deformations that occur during dorsal closure of the Drosophila embryo. During this process the AS contracts and the epidermis elongates in a coordinated fashion, leading to the closure of a discontinuity in the dorsal epidermis of the Drosophila embryo. We used dominant negative forms of Rab5 and Rab11 to monitor the impact on tissue morphogenesis of altering endocytosis and recycling at the level of single cells. We found different requirements for endocytosis (Rab5) and recycling (Rab11) in dorsal closure, furthermore we found that the two processes are differentially used in the two tissues. Endocytosis is required in the AS to remove membrane during apical constriction, but is not essential in the epidermis. Recycling is required in the AS at early stages and in the epidermis for cell elongation, suggesting a role in membrane addition during these processes. We propose that the modulation of the balance between endocytosis and recycling can regulate cellular morphology and tissue deformations during morphogenesis.

A novel strategy to isolate ubiquitin conjugates reveals wide role for ubiquitination during neural development

Ubiquitination has essential roles in neuronal development and function. Ubiquitin proteomics studies on yeast and HeLa cells have proven very informative, but there still is a gap regarding neuronal tissue-specific ubiquitination. In an organism context, direct evidence for the ubiquitination of neuronal proteins is even scarcer. Here, we report a novel proteomics strategy based on the in vivo biotinylation of ubiquitin to isolate ubiquitin conjugates from the neurons of Drosophila melanogaster embryos. We confidently identified 48 neuronal ubiquitin substrates, none of which was yet known to be ubiquitinated. Earlier proteomics and biochemical studies in non-neuronal cell types had identified orthologs to some of those but not to others. The identification here of novel ubiquitin substrates, those with no known ubiquitinated ortholog, suggests that proteomics studies must be performed on neuronal cells to identify ubiquitination pathways not shared by other cell types. Importantly, several of those newly found neuronal ubiquitin substrates are key players in synaptogenesis. Mass spectrometry results were validated by Western blotting to confirm that those proteins are indeed ubiquitinated in the Drosophila embryonic nervous system and to elucidate whether they are mono- or polyubiquitinated. In addition to the ubiquitin substrates, we also identified the ubiquitin carriers that are active during synaptogenesis. Identifying endogenously ubiquitinated proteins in specific cell types, at specific developmental stages, and within the context of a living organism will allow understanding how the tissue-specific function of those proteins is regulated by the ubiquitin system.

Dcas supports cell polarization and cell-cell adhesion complexes in development

Mammalian Cas proteins regulate cell migration, division and survival, and are often deregulated in cancer. However, the presence of four paralogous Cas family members in mammals (BCAR1/p130Cas, EFS/Sin1, NEDD9/HEF1/Cas-L, and CASS4/HEPL) has limited their analysis in development. We deleted the single Drosophila Cas gene, Dcas, to probe the developmental function of Dcas. Loss of Dcas had limited effect on embryonal development. However, we found that Dcas is an important modulator of the severity of the developmental phenotypes of mutations affecting integrins (If and mew) and their downstream effectors Fak56D or Src42A. Strikingly, embryonic lethal Fak56D-Dcas double mutant embryos had extensive cell polarity defects, including mislocalization and reduced expression of E-cadherin. Further genetic analysis established that loss of Dcas modified the embryonal lethal phenotypes of embryos with mutations in E-cadherin (Shg) or its signaling partners p120- and beta-catenin (Arm). These results support an important role for Cas proteins in cell-cell adhesion signaling in development.