JNK and decapentaplegic signaling control adhesiveness and cytoskeleton dynamics during thorax closure in Drosophila

Essential roles of the Tap42-regulated protein phosphatase 2A (PP2A) family in wing imaginal disc development of Drosophila melanogaster

Protein ser/thr phosphatase 2A family members (PP2A, PP4, and PP6) are implicated in the control of numerous biological processes, but our understanding of the in vivo function and regulation of these enzymes is limited. In this study, we investigated the role of Tap42, a common regulatory subunit for all three PP2A family members, in the development of Drosophila melanogaster wing imaginal discs. RNAi-mediated silencing of Tap42 using the binary Gal4/UAS system and two disc drivers, pnr- and ap-Gal4, not only decreased survival rates but also hampered the development of wing discs, resulting in a remarkable thorax cleft and defective wings in adults. Silencing of Tap42 also altered multiple signaling pathways (HH, JNK and DPP) and triggered apoptosis in wing imaginal discs. The Tap42^RNAi-induced defects were the direct result of loss of regulation of Drosophila PP2A family members (MTS, PP4, and PPV), as enforced expression of wild type Tap42, but not a phosphatase binding defective Tap42 mutant, rescued fly survivorship and defects. The experimental platform described herein identifies crucial roles for Tap42•phosphatase complexes in governing imaginal disc and fly development.

The MYST-containing protein Chameau is required for proper sensory organ specification during Drosophila thorax morphogenesis

The adult thorax of Drosophila melanogaster is covered by a stereotyped pattern of mechanosensory bristles called macrochaetes. Here, we report that the MYST containing protein Chameau (Chm) contributes to the establishment of this pattern in the most dorsal part of the thorax. Chm mutant pupae present extra-dorsocentral (DC) and scutellar (SC) macrochaetes, but a normal number of the other macrochaetes. We provide evidences that chm restricts the singling out of sensory organ precursors from proneural clusters and genetically interacts with transcriptional regulators involved in the regulation of achaete and scute in the DC and SC proneural cluster. This function of chm likely relies on chromatin structure regulation since a protein with a mutation in the conserved catalytic site fails to rescue the formation of supernumerary DC and SC bristles in chm mutant flies. This is further supported by the finding that mutations in genes encoding chromatin modifiers and remodeling factors, including Polycomb group (PcG) and Trithorax group (TrxG) members, dominantly modulate the penetrance of chm extra bristle phenotype. These data support a critical role for chromatin structure modulation in the establishment of the stereotyped sensory bristle pattern in the fly thorax.

Drosophila DREF acting via the JNK pathway is required for thorax development

The Drosophila Jun N-terminal kinase (JNK) gene basket (bsk) promoter contains a DNA replication-related element (DRE)-like sequence, raising the possibility of regulation by the DNA replication-related element-binding factor (DREF). Chromatin immunoprecipitation assays with anti-DREF IgG showed the bsk gene promoter region to be effectively amplified. Luciferase transient expression assays revealed the DRE-like sequence to be important for bsk gene promoter activity, and knockdown of DREF decreased the bsk mRNA level and the bsk gene promoter activity. Furthermore, knockdown of DREF in the notum compartment of wing discs by pannier-GAL4 and UAS-DREFIR resulted in a split thorax phenotype. Monitoring of JNK activity in the wing disc by LacZ expression in a puckered (puc)-LacZ enhancer trap line revealed the reduction in DREF knockdown clones. These findings indicate that DREF is involved in regulation of Drosophila thorax development via actions on the JNK pathway.

The 5' → 3' exoribonuclease XRN1/Pacman and its functions in cellular processes and development

XRN1 is a 5' → 3' processive exoribonuclease that degrades mRNAs after they have been decapped. It is highly conserved in all eukaryotes, including homologs in Drosophila melanogaster (Pacman), Caenorhabditis elegans (XRN1), and Saccharomyces cerevisiae (Xrn1p). As well as being a key enzyme in RNA turnover, XRN1 is involved in nonsense-mediated mRNA decay and degradation of mRNAs after they have been targeted by small interfering RNAs or microRNAs. The crystal structure of XRN1 can explain its processivity and also the selectivity of the enzyme for 5' monophosphorylated RNA. In eukaryotic cells, XRN1 is often found in particles known as processing bodies (P bodies) together with other proteins involved in the 5' → 3' degradation pathway, such as DCP2 and the helicase DHH1 (Me31B). Although XRN1 shows little specificity to particular 5' monophosphorylated RNAs in vitro, mutations in XRN1 in vivo have specific phenotypes suggesting that it specifically degrades a subset of RNAs. In Drosophila, mutations in the gene encoding the XRN1 homolog pacman result in defects in wound healing, epithelial closure and stem cell renewal in testes. We propose a model where specific mRNAs are targeted to XRN1 via specific binding of miRNAs and/or RNA-binding proteins to instability elements within the RNA. These guide the RNA to the 5' core degradation apparatus for controlled degradation.

The cytoskeletal mechanisms of cell-cell junction formation in endothelial cells

Cell–cell contact is initiated by lamellipodia, followed by filopodia-like structure formation. Filopodia-like bridges maintain cell–cell contact through adherens junctions. Although bridges are structurally similar to filopodia, they are formed via a unique mechanism. Myosin II activity is important for bridge formation and cadherin accumulation.

The actin cytoskeleton and associated proteins play a vital role in cell–cell adhesion. However, the procedure by which cells establish adherens junctions remains unclear. We investigated the dynamics of cell–cell junction formation and the corresponding architecture of the underlying cytoskeleton in cultured human umbilical vein endothelial cells. We show that the initial interaction between cells is mediated by protruding lamellipodia. On their retraction, cells maintain contact through thin bridges formed by filopodia-like protrusions connected by VE-cadherin–rich junctions. Bridges share multiple features with conventional filopodia, such as an internal actin bundle associated with fascin along the length and vasodilator-stimulated phosphoprotein at the tip. It is striking that, unlike conventional filopodia, transformation of actin organization from the lamellipodial network to filopodial bundle during bridge formation occurs in a proximal-to-distal direction and is accompanied by recruitment of fascin in the same direction. Subsequently, bridge bundles recruit nonmuscle myosin II and mature into stress fibers. Myosin II activity is important for bridge formation and accumulation of VE-cadherin in nascent adherens junctions. Our data reveal a mechanism of cell–cell junction formation in endothelial cells using lamellipodia as the initial protrusive contact, subsequently transforming into filopodia-like bridges connected through adherens junctions. Moreover, a novel lamellipodia-to-filopodia transition is used in this context.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

Oscillatory behaviors and hierarchical assembly of contractile structures in intercalating cells

Fluctuations in the size of the apical cell surface have been associated with apical constriction and tissue invagination. However, it is currently not known if apical oscillatory behaviors are a unique property of constricting cells or if they constitute a universal feature of the force balance between cells in multicellular tissues. Here, we set out to determine whether oscillatory cell behaviors occur in parallel with cell intercalation during the morphogenetic process of axis elongation in the Drosophila embryo. We applied multi-color, time-lapse imaging of living embryos and SIESTA, an integrated tool for automated and semi-automated cell segmentation, tracking, and analysis of image sequences. Using SIESTA, we identified cycles of contraction and expansion of the apical surface in intercalating cells and characterized them at the molecular, cellular, and tissue scales. We demonstrate that apical oscillations are anisotropic, and this anisotropy depends on the presence of intact cell-cell junctions and spatial cues provided by the anterior-posterior patterning system. Oscillatory cell behaviors during axis elongation are associated with the hierarchical assembly and disassembly of contractile actomyosin structures at the medial cortex of the cell, with actin localization preceding myosin II and with the localization of both proteins preceding changes in cell shape. We discuss models to explain how the architecture of cytoskeletal networks regulates their contractile behavior and the mechanisms that give rise to oscillatory cell behaviors in intercalating cells.

Elimination of oncogenic neighbors by JNK-mediated engulfment in Drosophila

A newly emerged oncogenic cell in the epithelial population has to confront antitumor selective pressures in the host tissue. However, the mechanisms by which surrounding normal tissue exerts antitumor effects against oncogenically transformed cells are poorly understood. In Drosophila imaginal epithelia, clones of cells mutant for evolutionarily conserved tumor suppressor genes such as scrib or dlg lose their epithelial integrity and are eliminated from epithelia when surrounded by wild-type tissue. Here, we show that surrounding normal cells activate nonapoptotic JNK signaling in response to the emergence of oncogenic mutant cells. This JNK activation leads to upregulation of PVR, the Drosophila PDGF/VEGF receptor. Genetic and time-lapse imaging analyses reveal that PVR expression in surrounding cells activates the ELMO/Mbc-mediated phagocytic pathway, thereby eliminating oncogenic neighbors by engulfment. Our data indicate that JNK-mediated cell engulfment could be an evolutionarily conserved intrinsic tumor-suppression mechanism that eliminates premalignant cells from epithelia.

Roles of stress-activated protein kinases in the replication of Singapore grouper iridovirus and regulation of the inflammatory responses in grouper cells

Stress-activated protein kinases (SAPKs), including p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK), are usually activated in response to different environmental stimuli, including virus infection. In the present study, the roles of SAPKs during Singapore grouper iridovirus (SGIV) infection were investigated in fish cells. The results showed that increased phosphorylation of JNK1/2 and p38 MAPK occurred during active replication of SGIV in grouper cell cultures. Moreover, downstream effectors (c-Jun, MAPK-activated protein kinase 2, p53, activator protein 1, Myc and nuclear factor of activated T cells) were activated after SGIV infection, suggesting that SGIV replication activated the JNK and p38 MAPK signalling pathways. Notably, using specific inhibitors, it was found that viral gene transcripts, protein expression and viral titres were not affected by inhibition of p38 MAPK but were suppressed significantly by inhibiting JNK1/2 activation. In addition, transcription of grouper immune genes including interferon regulatory factor 1, interleukin-8 and tumour necrosis factor alpha (TNF-α) were regulated by JNK, whilst only TNF-α was regulated by p38 MAPK. It is proposed that the JNK pathway is important for SGIV replication and modulates the inflammatory responses during virus infection.

Control of Gliotactin localization and levels by tyrosine phosphorylation and endocytosis is necessary for survival of polarized epithelia

The tricellular junction (TCJ) forms at the convergence of bicellular junctions from three adjacent cells in polarized epithelia and is necessary for maintaining the transepithelial barrier. In the fruitfly Drosophila, the TCJ is generated at the meeting point of bicellular septate junctions. Gliotactin was the first identified component of the TCJ and is necessary for TCJ and septate junction development. Gliotactin is a member of the neuroligin family and associates with the PDZ protein discs large. Beyond this interaction, little is known about the mechanisms underlying Gliotactin localization and function at the TCJ. In this study, we show that Gliotactin is phosphorylated at conserved tyrosine residues, a process necessary for endocytosis and targeting to late endosomes and lysosomes for degradation. Regulation of Gliotactin levels through phosphorylation and endocytosis is necessary as overexpression results in displacement of Gliotactin away from the TCJ throughout the septate junction domain. Excessive Gliotactin in polarized epithelia leads to delamination, paired with subsequent migration, and apoptosis. The apoptosis and the resulting compensatory proliferation resulting from high levels of Gliotactin are mediated by the Drosophila JNK pathway. Therefore, Gliotactin levels within the cell membrane are regulated to ensure correct protein localization and cell survival.

Dpp signaling directs cell motility and invasiveness during epithelial morphogenesis

Tissue remodeling in development and disease involves the coordinated invasion of neighboring territories and/or the replacement of entire cell populations. Cell guidance, cell matching, transitions from passive to migratory epithelia, cell growth and death, and extracellular matrix remodeling all impinge on epithelial spreading. Significantly, the extracellular signals that direct these activities and the specific cellular elements and mechanisms regulated by these signals remain in most cases to be identified. To address these issues, we performed an analysis of histoblasts (Drosophila abdominal epithelial founder cells) on their transition from a dormant state to active migration replacing obsolete larval epidermal cells (LECs). We found that during expansion, Decapentaplegic (Dpp) secreted from surrounding LECs leads to graded pathway activation in cells at the periphery of histoblast nests. Across nests, Dpp activity confers differential cellular behavior and motility by modulating cell-cell contacts, the organization and activity of the cytoskeleton, and histoblast attachment to the substrate. Furthermore, Dpp also prevents the premature death of LECs, allowing the coordination of histoblast expansion to LEC delamination. Dpp signaling activity directing histoblast spreading and invasiveness mimics transforming growth factor-beta and bone morphogenetic proteins' role in enhancing the motility and invasiveness of cancer cells, resulting in the promotion of metastasis.

Cdc42- and IRSp53-dependent contractile filopodia tether presumptive lens and retina to coordinate epithelial invagination

The vertebrate lens provides an excellent model with which to study the mechanisms required for epithelial invagination. In the mouse, the lens forms from the head surface ectoderm. A domain of ectoderm first thickens to form the lens placode and then invaginates to form the lens pit. The epithelium of the lens placode remains in close apposition to the epithelium of the presumptive retina as these structures undergo a coordinated invagination. Here, we show that F-actin-rich basal filopodia that link adjacent presumptive lens and retinal epithelia function as physical tethers that coordinate invagination. The filopodia, most of which originate in the presumptive lens, form at E9.5 when presumptive lens and retinal epithelia first come into close contact, and have retracted by E11.5 when invagination is complete. At E10.5--the lens pit stage--there is approximately one filopodium per epithelial cell. Formation of filopodia is dependent on the Rho family GTPase Cdc42 and the Cdc42 effector IRSp53 (Baiap2). Loss of filopodia results in reduced lens pit invagination. Pharmacological manipulation of the actin-myosin contraction pathway showed that the filopodia can respond rapidly in length to change inter-epithelial distance. These data suggest that the lens-retina inter-epithelial filopodia are a fine-tuning mechanism to assist in lens pit invagination by transmitting the forces between presumptive lens and retina. Although invagination of the archenteron in sea urchins and dorsal closure in Drosophila are known to be partly dependent on filopodia, this mechanism of morphogenesis has not previously been identified in vertebrates.

JNK signalling influences intracellular trafficking during Drosophila morphogenesis through regulation of the novel target gene Rab30

JNK-mediated closure of the Drosophila dorsal epidermis during embryogenesis is a well-characterised model for morphogenesis. However, little is known about how JNK signalling modifies particular cellular behaviours such as intracellular transport. Here we demonstrate that the gene encoding the small GTPase Rab30 is a new JNK transcriptional target whose function is required during embryonic and adult morphogenesis including JNK-dependent dorsal closure, embryonic head involution and thorax closure. Using immuno-fluorescence and live imaging, we show that EGFP-Rab30 localises to trans-Golgi in addition to small unidentified vesicles, and moves in a microtubule-dependent, polarised dorso-ventral manner in the leading edge during dorsal closure. We propose that JNK activity upregulates genes involved in intracellular transport in order to provide an increased level of trafficking activity in cells undergoing complex morphogenetic arrangements such as dorsal closure.

The role of Dpp and Wg in compensatory proliferation and in the formation of hyperplastic overgrowths caused by apoptotic cells in the Drosophila wing disc

Non-lethal stress treatments (X-radiation or heat shock) administered to Drosophila imaginal discs induce massive apoptosis, which may eliminate more that 50% of the cells. Yet the discs are able to recover to form final structures of normal size and pattern. Thus, the surviving cells have to undergo additional proliferation to compensate for the cell loss. The finding that apoptotic cells ectopically express dpp and wg suggested that ectopic Dpp/Wg signalling might be responsible for compensatory proliferation. We have tested this hypothesis by analysing the response to irradiation-induced apoptosis of disc compartments that are mutant for dpp, for wg, or for both. We find that there is compensatory proliferation in these compartments, indicating that the ectopic Dpp/Wg signalling generated by apoptotic cells is not involved. However, we demonstrate that this ectopic Dpp/Wg signalling is responsible for the hyperplastic overgrowths that appear when apoptotic ('undead') cells are kept alive with the caspase inhibitor P35. We also show that the ectopic Dpp/Wg signalling and the overgrowths caused by undead cells are due to a non-apoptotic function of the JNK pathway. We propose that the compensatory growth is simply a homeostatic response of wing compartments, which resume growth after massive cellular loss until they reach the final correct size. The ectopic Dpp/Wg signalling associated with apoptosis is inconsequential in compartments with normal apoptotic cells, which die soon after the stress event. In compartments containing undead cells, the adventitious Dpp/Wg signalling results in hyperplastic overgrowths.

G protein Galphai functions immediately downstream of Smoothened in Hedgehog signalling

The hedgehog (Hh) signalling pathway has an evolutionarily conserved role in patterning fields of cells during metazoan development, and is inappropriately activated in cancer. Hh pathway activity is absolutely dependent on signalling by the seven-transmembrane protein smoothened (Smo), which is regulated by the Hh receptor patched (Ptc). Smo signals to an intracellular multi-protein complex containing the Kinesin related protein Costal2 (Cos2), the protein kinase Fused (Fu) and the transcription factor Cubitus interruptus (Ci). In the absence of Hh, this complex regulates the cleavage of full-length Ci to a truncated repressor protein, Ci75, in a process that is dependent on the proteasome and priming phosphorylations by Protein kinase A (PKA). Binding of Hh to Ptc blocks Ptc-mediated Smo inhibition, allowing Smo to signal to the intracellular components to attenuate Ci cleavage. Because of its homology with the Frizzled family of G-protein-coupled receptors (GPCR), a likely candidate for an immediate Smo effector would be a heterotrimeric G protein. However, the role that G proteins may have in Hh signal transduction is unclear and quite controversial, which has led to widespread speculation that Smo signals through a variety of novel G-protein-independent mechanisms. Here we present in vitro and in vivo evidence in Drosophila that Smo activates a G protein to modulate intracellular cyclic AMP levels in response to Hh. Our results demonstrate that Smo functions as a canonical GPCR, which signals through Galphai to regulate Hh pathway activation.

The 5'-3' exoribonuclease pacman is required for epithelial sheet sealing in Drosophila and genetically interacts with the phosphatase puckered

BACKGROUND INFORMATION

Ribonucleases have been well studied in yeast and bacteria, but their biological significance to developmental processes in multicellular organisms is not well understood. However, there is increasing evidence that specific timed transcript degradation is critical for regulation of many cellular processes, including translational repression, nonsense-mediated decay and RNA interference. The Drosophila gene pacman is highly homologous to the major yeast exoribonuclease XRN1 and is the only known cytoplasmic 5'-3' exoribonuclease in eukaryotes. To determine the effects of this exoribonuclease in development we have constructed a number of mutations in pacman by P-element excision and characterized the resulting phenotypes.

RESULTS

Mutations in pacman resulted in flies with a number of specific phenotypes, such as low viability, dull wings, crooked legs, failure of correct dorsal/thorax closure and defects in wound healing. The epithelial sheet movement involved in dorsal/thorax closure is a conserved morphogenetic process which is similar to that of hind-brain closure in vertebrates and wound healing in humans. As the JNK (c-Jun N-terminal kinase) signalling pathway is known to be involved in dorsal/thorax closure and wound healing, we tested whether pacman affects JNK signalling. Our experiments demonstrate that pacman genetically interacts with puckered, a phosphatase that negatively regulates the JNK signalling pathway.

CONCLUSIONS

These results reveal that the 5'-3' exoribonuclease pacman is required for a critical aspect of epithelial sheet sealing in Drosophila. Since these mutations result in specific phenotypes, our data suggest that the exoribonuclease Pacman targets a specific subset of mRNAs involved in this process. One of these targets could be a member of the JNK signalling pathway, although it is possible that a parallel pathway may instead be affected. The exoribonuclease pacman is highly conserved in all eukaryotes, therefore it is likely that it is involved in similar morphological processes, such as wound healing in human cells.

Interactions between the juvenile Batten disease gene, CLN3, and the Notch and JNK signalling pathways

Mutations in the gene CLN3 are responsible for the neurodegenerative disorder juvenile neuronal ceroid lipofuscinosis or Batten disease. CLN3 encodes a multi-spanning and hydrophobic transmembrane protein whose function is unclear. As a consequence, the cell biology that underlies the pathology of the disease is not well understood. We have developed a genetic gain-of-function system in Drosophila to identify functional pathways and interactions for CLN3. We have identified previously unknown interactions between CLN3 and the Notch and Jun N-terminal kinase signalling pathways and have uncovered a potential role for the RNA splicing and localization machinery in regulating CLN3 function.

Medea SUMOylation restricts the signaling range of the Dpp morphogen in the Drosophila embryo

Morphogens are secreted signaling molecules that form concentration gradients and control cell fate in developing tissues. During development, it is essential that morphogen range is strictly regulated in order for correct cell type specification to occur. One of the best characterized morphogens is Drosophila Decapentaplegic (Dpp), a BMP signaling molecule that patterns the dorsal ectoderm of the embryo by activating the Mad and Medea (Med) transcription factors. We demonstrate that there is a spatial and temporal expansion of the expression patterns of Dpp target genes in SUMO pathway mutant embryos. We identify Med as the primary SUMOylation target in the Dpp pathway, and show that failure to SUMOylate Med leads to the increased Dpp signaling range observed in the SUMO pathway mutant embryos. Med is SUMO modified in the nucleus, and we provide evidence that SUMOylation triggers Med nuclear export. Hence, Med SUMOylation provides a mechanism by which nuclei can continue to monitor the presence of extracellular Dpp signal to activate target gene expression for an appropriate duration. Overall, our results identify an unusual strategy for regulating morphogen range that, rather than impacting on the morphogen itself, targets an intracellular transducer.

JNK signaling controls border cell cluster integrity and collective cell migration

Collective cell movement is a mechanism for invasion identified in many developmental events. Examples include the movement of lateral-line neurons in Zebrafish, cells in the inner blastocyst, and metastasis of epithelial tumors [1]. One key model to study collective migration is the movement of border cell clusters in Drosophila. Drosophila egg chambers contain 15 nurse cells and a single oocyte surrounded by somatic follicle cells. At their anterior end, polar cells recruit several neighboring follicle cells to form the border cell cluster [2]. By stage 9, and over 6 hr, border cells migrate as a cohort between nurse cells toward the oocyte. The specification and directionality of border cell movement are regulated by hormonal and signaling mechanisms [3]. However, how border cells are held together while they migrate is not known. Here, we show that a negative-feedback loop controlling JNK activity regulates border cell cluster integrity. JNK signaling modulates contacts between border cells and between border cells and substratum to sustain collective migration by regulating several effectors including the polarity factor Bazooka and the cytoskeletal adaptor D-Paxillin. We anticipate a role for the JNK pathway in controlling collective cell movements in other morphogenetic and clinical models.

Three genes control the timing, the site and the size of blastema formation in Drosophila

Regeneration is a vital process to maintain and repair tissues. Despite the importance of regeneration, the genes responsible for regenerative growth remain largely unknown. In Drosophila, imaginal disc regeneration can be induced either by fragmentation and in vivo culture or in situ by ubiquitous expression of wingless (wg/wnt1). Imaginal discs, like appendages in lower vertebrates, initiate regeneration by wound healing and proliferation at the wound site, forming a regeneration blastema. Most blastema cells maintain their disc-specific identity during regeneration; a few cells however, exhibit stem-cell like properties and switch to a different fate, in a phenomenon known as transdetermination. We identified three genes, regeneration (rgn), augmenter of liver regeneration (alr) and Matrix metalloproteinase-1 (Mmp1) expressed specifically in blastema cells during disc regeneration. Mutations in these genes affect both fragmentation- and wg-induced regeneration by either delaying, reducing or positioning the regeneration blastema. In addition to the modifications of blastema homeostasis, mutations in the three genes alter the rate of regeneration-induced transdetermination. We propose that these genes function in regenerative proliferation, growth and regulate cellular plasticity.

Transcription factor NF-Y is involved in regulation of the JNK pathway during Drosophila thorax development

The CCAAT motif-binding factor, nuclear factor Y (NF-Y) consists of three different subunits, NF-YA, NF-YB and NF-YC. Knockdown of Drosophila NF-YA (dNF-YA) in the notum compartment of wing discs by a pannir-GAL4 and UAS-dNF-YAIR mainly resulted in a thorax disclosed phenotype. Reduction of the Drosophila c-Jun N-terminal kinase (JNK) basket (bsk) gene dose enhanced the knockdown of dNF-YA-induced phenotype. Monitoring of JNK activity in the wing disc by LacZ expression in a puckered (puc)-LacZ enhancer trap line revealed reduction in the level of the JNK reporter, puc-LacZ signals, in dNF-YA RNAi clones. In addition, expression of wild-type Bsk effectively suppressed the phenotype induced by knockdown of dNF-YA. The bsk gene promoter contains a CCAAT motif and this motif plays a positive role in the promoter activity. We performed chromatin immunoprecipitation (ChIP) assays in S2 cells with anti-dNF-YA IgG and quantitative real-time PCR. The bsk gene promoter region containing the CCAAT boxes was effectively amplified in the immunoprecipitates by PCR. However, this region was not amplified in the immunoprecipitates from dNF-YA knockdown cells. Furthermore, the level of endogenous bsk mRNA is reduced in the dNF-YA knockdown larvae. These results suggest that dNF-Y is necessary for proper bsk expression and activity of JNK pathway during thorax development.

Apical and lateral cell protrusions interconnect epithelial cells in live Drosophila wing imaginal discs

Communication among cells by means of the exchange of signaling cues is important for tissue and organ development. Recent reports indicate that one way that signaling cues can be delivered is by movement along cellular protrusions interconnecting cells. Here, by using confocal laser scanning microscopy and three-dimensional rendering, we describe in Drosophila melanogaster wing imaginal discs lateral protrusions interconnecting cells of the columnar epithelium. Moreover, we identified protrusions of the apical surface of columnar cells that reached and apparently contacted cells of the overlying squamous epithelium. Both apical and lateral protrusions could be visualized by expression of Tkv-GFP, a green fluorescent protein (GFP) -tagged version of a receptor of the Dpp/BMP4 signaling molecule, and the endosome marker GFP-Rab5. Our results demonstrate a previously unexpected richness of cellular protrusions within wing imaginal discs and support the view that cellular protrusions may provide a means for exchanging signaling cues between cells.

Modulation of Kaposi's sarcoma-associated herpesvirus infection and replication by MEK/ERK, JNK, and p38 multiple mitogen-activated protein kinase pathways during primary infection

Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically associated with Kaposi's sarcoma, a dominant AIDS-related tumor of endothelial cells, and several other lymphoproliferative malignancies. While activation of the phosphatidylinositol 3-kinase-protein kinase C-MEK-ERK pathway is essential for KSHV infection, we have recently shown that KSHV also activates JNK and p38 mitogen-activated protein kinase (MAPK) pathways during primary infection (J. Xie, H. Y. Pan, S. Yoo, and S.-J. Gao, J. Virol. 79:15027-15037, 2005). Here, we found that activation of both JNK and p38 pathways was also essential for KSHV infection. Inhibitors of all three MAPK pathways reduced KSHV infectivity in both human umbilical vein endothelial cells (HUVEC) and 293 cells. These inhibitory effects were dose dependent and occurred at the virus entry stage of infection. Consistently, inhibition of all three MAPK pathways with dominant-negative constructs reduced KSHV infectivity whereas activation of the ERK pathway but not the JNK and p38 pathways enhanced KSHV infectivity. Importantly, inhibition of all three MAPK pathways also reduced the yield of infectious virions during KSHV productive infection of HUVEC. While the reduction of infectious virions was in part due to the reduced infectivity, it was also the result of direct modulation of KSHV lytic replication by the MAPK pathways. Accordingly, KSHV upregulated the expression of RTA (Orf50), a master transactivator of KSHV lytic replication, and activated its promoter during primary infection. Furthermore, KSHV activation of RTA promoter during primary infection was modulated by all three MAPK pathways, predominantly through their downstream target AP-1. Together, these results indicate that, by modulating multiple MAPK pathways, KSHV manipulates the host cells to facilitate its entry into the cells and postentry productive lytic replication during primary infection.

Csrp1 regulates dynamic cell movements of the mesendoderm and cardiac mesoderm through interactions with Dishevelled and Diversin

Zebrafish Csrp1 is a member of the cysteine- and glycine-rich protein (CSRP) family and is expressed in the mesendoderm and its derivatives. Csrp1 interacts with Dishevelled 2 (Dvl2) and Diversin (Div), which control cell morphology and other dynamic cell behaviors via the noncanonical Wnt and JNK pathways. When csrp1 message is knocked down, abnormal convergent extension cell movement is induced, resulting in severe deformities in midline structures. In addition, cardiac bifida is induced as a consequence of defects in cardiac mesoderm cell migration. Our data highlight Csrp1 as a key molecule of the noncanonical Wnt pathway, which orchestrates cell behaviors during dynamic morphogenetic movements of tissues and organs.

Dynamic decapentaplegic signaling regulates patterning and adhesion in the Drosophila pupal retina

The correct organization of cells within an epithelium is essential for proper tissue and organ morphogenesis. The role of Decapentaplegic/Bone morphogenetic protein (Dpp/BMP) signaling in cellular morphogenesis during epithelial development is poorly understood. In this paper, we used the developing Drosophila pupal retina--looking specifically at the reorganization of glial-like support cells that lie between the retinal ommatidia--to better understand the role of Dpp signaling during epithelial patterning. Our results indicate that Dpp pathway activity is tightly regulated across time in the pupal retina and that epithelial cells in this tissue require Dpp signaling to achieve their correct shape and position within the ommatidial hexagon. These results point to the Dpp pathway as a third component and functional link between two adhesion systems, Hibris-Roughest and DE-cadherin. A balanced interplay between these three systems is essential for epithelial patterning during morphogenesis of the pupal retina. Importantly, we identify a similar functional connection between Dpp activity and DE-cadherin and Rho1 during cell fate determination in the wing, suggesting a broader link between Dpp function and junctional integrity during epithelial development.

Drosophila SnoN modulates growth and patterning by antagonizing TGF-beta signalling

Signalling by TGF-beta ligands through the Smad family of transcription factors is critical for developmental patterning and growth. Disruption of this pathway has been observed in various cancers. In vertebrates, members of the Ski/Sno protein family can act as negative regulators of TGF-beta signalling, interfering with the Smad machinery to inhibit the transcriptional output of this pathway. In some contexts ski/sno genes function as tumour suppressors, but they were originally identified as oncogenes, whose expression is up-regulated in many tumours. These growth regulatory effects and the normal physiological functions of Ski/Sno proteins have been proposed to result from changes in TGF-beta signalling. However, this model is controversial and may be over-simplified, because recent findings indicate that Ski/Sno proteins can affect other signalling pathways. To address this issue in an in vivo context, we have analyzed the function of the Drosophila Ski/Sno orthologue, SnoN. We found that SnoN inhibits growth when overexpressed, indicating a tumour suppressor role in flies. It can act in multiple tissues to selectively and cell autonomously antagonise signalling by TGF-beta ligands from both the BMP and Activin sub-families. By contrast, analysis of a snoN mutant indicates that the gene does not play a global role in TGF-beta-mediated functions, but specifically inhibits TGF-beta-induced wing vein formation. We propose that SnoN normally functions redundantly with other TGF-beta pathway antagonists to finely adjust signalling levels, but that it can behave as an extremely potent inhibitor of TGF-beta signalling when highly expressed, highlighting the significance of its deregulation in cancer cells.

Basement membrane remodeling is essential for Drosophila disc eversion and tumor invasion

Organ and tissue integrity is often maintained in animals by a specialized extracellular matrix structure called the basement membrane (BM). Accumulated evidence indicates that BM remodeling occurs during development and tumor invasion. Although the BM organizes and functions at the organ level, most past studies have explored its biochemical and in vitro properties. In this study, we monitor the BM in vivo during developmental tissue invasion for disc eversion and tumor invasion in Drosophila and modulate BM integrity with genetic alterations affecting either the whole organism or the targeted discs or tumors. We observe that the degradation of BM by the discs or the tumors is an early event during invasion processes and that preventing BM degradation completely blocks both tissue and tumor invasion, indicating that modulation of BM is essential for developmental and tumor invasion. Furthermore, elements of the invasion machinery, including JNK-induced matrix metalloproteinase (MMP) expression, are shared by both disc eversion and tumor invasion processes. Moreover, we show that although expression of MMP inhibitor, TIMP, is sufficient to halt developmental invasion, inhibition of proteases by both TIMP and RECK are required to block tumor invasion. These data suggest that tumor cells have a more robust invasion mechanism and could acquire metastatic behavior by co-opting developmental invasion programs. This type of co-option may be a general feature contributing to the progression of tumors. Finally, although past efforts using MMP inhibitors have not yielded much success, our results strongly argue that BM modulation could be a critical target for cancer therapy.

JNK- and Fos-regulated Mmp1 expression cooperates with Ras to induce invasive tumors in Drosophila

Loss of the epithelial polarity gene scribble in clones of Drosophila imaginal disc cells can cooperate with Ras signaling to induce malignant tumors. Such mutant tissue overproliferates, resists apoptosis, leaves its place of origin and invades other organs, ultimately causing lethality. We show that increased Jun N-terminal kinase (JNK) signaling resulting from the loss of scribble promotes the movement of transformed cells to secondary sites. This effect requires Fos-dependent transcriptional activation of a matrix metalloprotease gene mmp1 downstream of JNK. Expression of the Mmp inhibitor Timp or Mmp RNAi knockdown suppresses cell invasiveness. The proinvasive function of the JNK pathway is revealed in a tumor context when active Ras signaling prevents the apoptotic response to JNK activity as it occurs in nontransformed cells. Based on these results, we present a model that explains the oncogenic cooperation between JNK and Ras, and describes how aberrant regulation of cell survival, proliferation and mobilization cooperate to incite malignant tumor formation.

Control of G2/M transition by Drosophila Fos

The transcription factors of the Fos family have long been associated with the control of cell proliferation, although the molecular and cellular mechanisms that mediate this function are poorly understood. We investigated the contributions of Fos to the cell cycle and cell growth control using Drosophila imaginal discs as a genetically accessible system. The RNA interference-mediated inhibition of Fos in proliferating cells of the wing and eye discs resulted in a specific defect in the G2-to-M-phase transition, while cell growth remained unimpaired, resulting in a marked reduction in organ size. Consistent with the conclusion that Fos is required for mitosis, we identified cyclin B as a direct transcriptional target of Fos in Drosophila melanogaster, with Fos binding to a region upstream of the cyclin B gene in vivo and cyclin B mRNA being specifically reduced under Fos loss-of-function conditions.

Targeted mutation of serine 697 in the Ret tyrosine kinase causes migration defect of enteric neural crest cells

The RET receptor tyrosine kinase plays a critical role in the development of the enteric nervous system (ENS) and the kidney. Upon glial-cell-line-derived neurotrophic factor (GDNF) stimulation, RET can activate a variety of intracellular signals, including the Ras/mitogen-activated protein kinase, phosphatidylinositol 3-kinase (PI3K)/AKT, and RAC1/JUN NH(2)-terminal kinase (JNK) pathways. We recently demonstrated that the RAC1/JNK pathway is regulated by serine phosphorylation at the juxtamembrane region of RET in a cAMP-dependent manner. To determine the importance of cAMP-dependent modification of the RET signal in vivo, we generated mutant mice in which serine residue 697, a putative protein kinase A (PKA) phosphorylation site, was replaced with alanine (designated S697A mice). Homozygous S697A mutant mice lacked the ENS in the distal colon, resulting from a migration defect of enteric neural crest cells (ENCCs). In vitro organ culture showed an impaired chemoattractant response of the mutant ENCCs to GDNF. JNK activation by GDNF but not ERK, AKT and SRC activation was markedly reduced in neurons derived from the mutant mice. The JNK inhibitor SP600125 and the PKA inhibitor KT5720 suppressed migration of the ENCCs in cultured guts from wild-type mice to comparable degrees. Thus, these findings indicated that cAMP-dependent modification of RET function regulates the JNK signaling responsible for proper migration of the ENCCs in the developing gut.

Genetic analysis of slipper/mixed lineage kinase reveals requirements in multiple Jun-N-terminal kinase-dependent morphogenetic events during Drosophila development

Mixed lineage kinases (MLKs) function as Jun-N-terminal kinase (JNK) kinase kinases to transduce extracellular signals during development and homeostasis in adults. slipper (slpr), which encodes the Drosophila homolog of mammalian MLKs, has previously been implicated in activation of the JNK pathway during embryonic dorsal epidermal closure. To further define the specific functions of SLPR, we analyzed the phenotypic consequences of slpr loss and gain of function throughout development, using a semiviable maternal-effect allele and wild-type or dominant-negative transgenes. From these analyses we confirm that failure of dorsal closure is the null phenotype in slpr germline clones. In addition, there is a functional maternal contribution, which can suffice for embryogenesis in the zygotic null mutant, but rarely suffices for pupal metamorphosis, revealing later functions for slpr as the maternal contribution is depleted. Zygotic null mutants that eclose as adults display an array of morphological defects, many of which are shared by hep mutant animals, deficient in the JNK kinase (JNKK/MKK7) substrate for SLPR, suggesting that the defects observed in slpr mutants primarily reflect loss of hep-dependent JNK activation. Consistent with this, the maternal slpr contribution is sensitive to the dosage of positive and negative JNK pathway regulators, which attenuate or potentiate SLPR-dependent signaling in development. Although SLPR and TAK1, another JNKKK family member, are differentially used in dorsal closure and TNF/Eiger-stimulated apoptosis, respectively, a Tak1 mutant shows dominant genetic interactions with slpr, suggesting potential redundant or combinatorial functions. Finally, we demonstrate that SLPR overexpression can induce ectopic JNK signaling and that the SLPR protein is enriched at the epithelial cell cortex.

Genetic interactions between Drosophila melanogaster menin and Jun/Fos

Menin is a tumor suppressor required to prevent multiple endocrine neoplasia in humans. Mammalian menin protein is associated with chromatin modifying complexes and has been shown to bind a number of nuclear proteins, including the transcription factor JunD. Menin shows bidirectional effects acting positively on c-Jun and negatively on JunD. We have produced protein null alleles of Drosophila menin (mnn1) and have over expressed the Mnn1 protein. Flies homozygous for protein-null mnn1 alleles are viable and fertile. Localized over-expression of Mnn1 causes defects in thoracic closure, a phenotype that sometimes results from insufficient Jun activity. We observed complex genetic interactions between mnn1 and jun in different developmental settings. Our data support the idea that one function of menin is to modulate Jun activity in a manner dependent on the cellular context.

Mutation of an upstream cleavage site in the BMP4 prodomain leads to tissue-specific loss of activity

ProBMP4 is initially cleaved at a site adjacent to the mature ligand (the S1 site) allowing for subsequent cleavage at an upstream (S2) site. Mature BMP4 synthesized from a precursor in which the S2 site cannot be cleaved remains in a complex with the prodomain that is targeted for lysosomal degradation, and is thus less active when overexpressed in Xenopus. Here we report that mice carrying a point mutation that prevents S2 processing show severe loss of BMP4 activity in some tissues, such as testes and germ cells, whereas other tissues that are sensitive to Bmp4 dosage, such as the limb, dorsal vertebrae and kidney, develop normally. In a haploinsufficient background, inability to cleave the S2 site leads to embryonic and postnatal lethality due to defects in multiple organ systems including the allantois, placental vasculature, ventral body wall, eye and heart. These data demonstrate that cleavage of the S2 site is essential for normal development and, more importantly, suggest that this site might be selectively cleaved in a tissue-specific fashion. In addition, these studies provide the first genetic evidence that BMP4 is required for dorsal vertebral fusion and closure of the ventral body wall.

Chameau HAT and DRpd3 HDAC function as antagonistic cofactors of JNK/AP-1-dependent transcription during Drosophila metamorphosis

Gene regulation by AP-1 transcription factors in response to Jun N-terminal kinase (JNK) signaling controls essential cellular processes during development and in pathological situations. Here, we report genetic and molecular evidence that the histone acetyltransferase (HAT) Chameau and the histone deacetylase DRpd3 act as antagonistic cofactors of DJun and DFos to modulate JNK-dependent transcription during thorax metamorphosis and JNK-induced apoptosis in Drosophila. We demonstrate in cultured cells that DFos phosphorylation mediated by JNK signaling plays a central role in coordinating the dynamics of Chameau and DRpd3 recruitment and function at AP-1-responsive promoters. Activating the pathway stimulates the HAT function of Chameau, promoting histone H4 acetylation and target gene transcription. Conversely, in response to JNK signaling inactivation, DRpd3 is recruited and suppresses histone acetylation and transcription. This study establishes a direct link among JNK signaling, DFos phosphorylation, chromatin modification, and AP-1-dependent transcription and its importance in a developing organism.

Anthrax lethal factor and edema factor act on conserved targets in Drosophila

Many bacterial toxins act on conserved components of essential host-signaling pathways. One consequence of this conservation is that genetic model organisms such as Drosophila melanogaster can be used for analyzing the mechanism of toxin action. In this study, we characterize the activities of two anthrax virulence factors, lethal factor (LF) and edema factor, in transgenic Drosophila. LF is a zinc metalloprotease that cleaves and inactivates most human mitogen-activated protein kinase (MAPK) kinases (MAPKKs). We found that LF similarly cleaves the Drosophila MAPK kinases Hemipterous (Hep) and Licorne in vitro. Consistent with these observations, expression of LF in Drosophila inhibited the Hep/c-Jun N-terminal kinase pathway during embryonic dorsal closure and the related process of adult thoracic closure. Epistasis experiments confirmed that LF acts at the level of Hep. We also found that LF inhibits Ras/MAPK signaling during wing development and that LF acts upstream of MAPK and downstream of Raf, consistent with LF acting at the level of Dsor. In addition, we found that edema factor, a potent adenylate cyclase, inhibits the hh pathway during wing development, consistent with the known role of cAMP-dependent PKA in suppressing the Hedgehog response. These results demonstrate that anthrax toxins function in Drosophila as they do in mammalian cells and open the way to using Drosophila as a multicellular host system for studying the in vivo function of diverse toxins and virulence factors.

how functions in leg development duringDrosophila metamorphosis

The Drosophila how gene encodes a KH RNA binding protein with strong similarity to GLD-1 from nematodes and QK1 from mice. Here, we investigate the function of how during metamorphosis. We show that how RNA and protein are present in a variety of tissues, and phenotypic analyses of how mutants reveal multiple lethal phases and defects during metamorphosis. In addition to previously reported abnormalities in muscle and wing development, how mutants exhibit defects in leg development. how mutant leg imaginal discs undergo cell shape changes associated with elongation, but are oriented improperly, do not evert normally, and often remain incased in peripodial epithelium longer than normal. Consequently, how mutants exhibit short, crooked legs. Our findings suggest that how functions in interactions between imaginal epithelium, peripodial epithelium, and larval epidermal cells during imaginal disc eversion.

MAP kinases and cell migration

Recent studies have demonstrated that mitogen-activated protein kinases (MAPKs), including Jun N-terminus kinase (JNK), p38 and Erk, play crucial roles in cell migration. JNK, for example, regulates cell migration by phosphorylating paxillin, DCX, Jun and microtubule-associated proteins. Studies of p38 show that this MAPK modulates migration by phosphorylating MAPK-activated protein kinase 2/3 (MAPKAP 2/3), which appears to be important for directionality of migration. Erk governs cell movement by phosphorylating myosin light chain kinase (MLCK), calpain or FAK. Thus, the different kinases in the MAPK family all seem able to regulate cell migration but by distinct mechanisms.

JSAP1/JIP3 cooperates with focal adhesion kinase to regulate c-Jun N-terminal kinase and cell migration

c-Jun N-terminal kinase (JNK)/stress-activated protein kinase-associated protein 1 (JSAP1) (also termed JNK-interacting protein 3; JIP3) is a member of a family of scaffold factors for the mitogen-activated protein kinase (MAPK) cascades, and it also forms a complex with focal adhesion kinase (FAK). Here we demonstrate that JSAP1 serves as a cooperative scaffold for activation of JNK and regulation of cell migration in response to fibronectin (FN) stimulation. JSAP1 mediated an association between FAK and JNK, which was induced by either co-expression of Src or attachment of cells to FN. Complex formation of FAK with JSAP1 and p130 Crk-associated substrate (p130(Cas)) resulted in augmentation of FAK activity and phosphorylation of both JSAP1 and p130(Cas), which required p130(Cas) hyperphosphorylation and was abolished by inhibition of Src. JNK activation by FN was enhanced by JSAP1, which was suppressed by disrupting the FAK/p130(Cas) pathway by expression of a dominant-negative form of p130(Cas) or by inhibiting Src. We also documented the co-localization of JSAP1 with JNK and phosphorylated FAK at the leading edge and stimulation of cell migration by JSAP1 expression, which depended on its JNK binding domain and was suppressed by inhibition of JNK. The level of JSAP1 mRNA correlated with advanced malignancy in brain tumors, unlike other JIPs. We propose that the JSAP1.FAK complex functions cooperatively as a scaffold for the JNK signaling pathway and regulator of cell migration on FN, and we suggest that JSAP1 is also associated with malignancy in brain tumors.

Puckered, a Drosophila MAPK phosphatase, ensures cell viability by antagonizing JNK-induced apoptosis

MAPK phosphatases (MKPs) are important negative regulators of MAPKs in vivo, but ascertaining the role of specific MKPs is hindered by functional redundancy in vertebrates. Thus, we characterized MKP function by examining the function of Puckered (Puc), the sole Drosophila Jun N-terminal kinase (JNK)-specific MKP, during embryonic and imaginal disc development. We demonstrate that Puc is a key anti-apoptotic factor that prevents apoptosis in epithelial cells by restraining basal JNK signaling. Furthermore, we demonstrate that JNK signaling plays an important role in gamma-irradiation-induced apoptosis, and examine how JNK signaling fits into the circuitry regulating this process. Radiation upregulates both JNK activity and puc expression in a p53-dependent manner, and apoptosis induced by loss of Puc can be suppressed by p53 inactivation. JNK signaling acts upstream of both Reaper and effector caspases. Finally, we demonstrate that JNK signaling directs normal developmentally regulated apoptotic events. However, if cell death is prevented, JNK activation can trigger tissue overgrowth. Thus, MKPs are key regulators of the delicate balance between proliferation, differentiation and apoptosis during development.

Extrusion and Death of DPP/BMP-Compromised Epithelial Cells in the Developing Drosophila Wing

During animal development, epithelial cell fates are specified according to spatial position by extracellular signaling pathways. Among these, the transforming growth factor beta/bone morphogenetic protein (TGF-beta/BMP) pathways are evolutionarily conserved and play crucial roles in the development and homeostasis of a wide range of multicellular tissues. Here we show that in the developing Drosophila wing imaginal epithelium, cell clones deprived of the BMP-like ligand Decapentaplegic (DPP) do not die as previously thought but rather extrude from the cell layer as viable cysts exhibiting marked abnormalities in cell shape and cytoskeletal organization. We propose that in addition to assigning cell fates, a crucial developmental function of DPP/BMP signaling is the position-specific control of epithelial architecture.

Extrusion of Cells with Inappropriate Dpp Signaling from Drosophila Wing Disc Epithelia

Decapentaplegic (Dpp) is a signaling molecule that controls growth and patterning of the developing Drosophila wing. Mutant cells lacking Dpp signal transduction have been shown to activate c-Jun amino-terminal kinase (JNK)-dependent apoptosis and to be lost from the wing disc epithelium. These observations have led to the hypothesis that Dpp promotes cell survival by preventing apoptosis. Here, we show that in the absence of JNK-dependent apoptosis, mutant cells lacking Dpp signal transduction can survive; however, they are still lost from the wing disc epithelium. This loss correlates with extensive cytoskeletal changes followed by basal epithelial extrusion. We propose that Dpp promotes cell survival within disc epithelia by affecting cytoskeletal organization.

FGF signalling and the mechanism of mesoderm spreading inDrosophilaembryos

FGF signalling is needed for the proper establishment of the mesodermal cell layer in Drosophila embryos. The activation of the FGF receptor Heartless triggers the di-phosphorylation of MAPK in the mesoderm, which accumulates in a graded fashion with the highest levels seen at the dorsal edge of the mesoderm. We have examined the specific requirement for FGF signalling in the spreading process. We show that only the initial step of spreading, specifically the establishment of contact between the ectoderm and the mesoderm, depends upon FGF signalling, and that unlike the role of FGF signalling in the differentiation of heart precursors this function cannot be replaced by other receptor tyrosine kinases. The initiation of mesoderm spreading requires the FGF receptor to possess a functional kinase domain, but does not depend upon the activation of MAPK. Thus, the dispersal of the mesoderm at early stages is regulated by pathways downstream of the FGF receptor that are independent of the MAPK cascade. Furthermore, we demonstrate that the activation of MAPK by Heartless needs additional cues from the ectoderm. We propose that FGF signalling is required during the initial stages of mesoderm spreading to promote the efficient interaction of the mesoderm with the ectoderm rather than having a long range chemotactic function, and we discuss this in relation to the cellular mechanism of mesoderm spreading.

Genetic characterization of the Drosophila homologue of coronin

We report cloning and characterization of coro, which codes for the Drosophila homologue of the F-actin binding protein coronin. Viable alleles of coro produce a variety of phenotypes in leg, wing and eye development, which are similar to the phenotypes observed as a result of mutations in genes associated with the actin cytoskeleton and/or membrane trafficking. Homozygous lethal mutations in coro results in the disruption of the actin cytoskeleton in wing imaginal discs. Formation of both basolateral septate junctions and apical adherens junctions are also adversely affected in epithelial cells. Both viable and lethal alleles of coro show genetic interactions with syntaxin1A, a gene required for membrane trafficking. They also show enhanced response to over-expression of Decapentaplegic (Dpp) and its receptor Thick vein. Tracing of Dpp morphogen using a Dpp::GFP fusion construct suggested defects in the endocytic pathway, which resulted in uniform distribution of Dpp along the AP axis rather than a gradient from the AP boundary. Our results provide a genetic link between endocytosis/exocytosis events involving F actin-coated vesicles and the establishment of morphogen gradient.

The integrin effector PINCH regulates JNK activity and epithelial migration in concert with Ras suppressor 1

Cell adhesion and migration are dynamic processes requiring the coordinated action of multiple signaling pathways, but the mechanisms underlying signal integration have remained elusive. Drosophila embryonic dorsal closure (DC) requires both integrin function and c-Jun amino-terminal kinase (JNK) signaling for opposed epithelial sheets to migrate, meet, and suture. Here, we show that PINCH, a protein required for integrin-dependent cell adhesion and actin-membrane anchorage, is present at the leading edge of these migrating epithelia and is required for DC. By analysis of native protein complexes, we identify RSU-1, a regulator of Ras signaling in mammalian cells, as a novel PINCH binding partner that contributes to PINCH stability. Mutation of the gene encoding RSU-1 results in wing blistering in Drosophila, demonstrating its role in integrin-dependent cell adhesion. Genetic interaction analyses reveal that both PINCH and RSU-1 antagonize JNK signaling during DC. Our results suggest that PINCH and RSU-1 contribute to the integration of JNK and integrin functions during Drosophila development.

Comparison of molecular mechanisms mediating cell contact phenomena in model developmental systems: an exploration of universality

Are there universal molecular mechanisms associated with cell contact phenomena during metazoan ontogenesis? Comparison of adhesion systems in disparate model systems indicates the existence of unifying principles. Requirements for multicellularity are (a) the construction of three-dimensional structures involving a crucial balance between adhesiveness and motility; and (b) the establishment of integration at molecular, cellular, tissue, and organismal levels of organization. Mechanisms for (i) cell-cell and cell-substrate adhesion, (ii) cell movement, (iii) cell-cell communication, (iv) cellular responses, (v) regulation of these processes, and (vi) their integration with patterning, growth, and other developmental processes are all crucial to metazoan development, and must have been present for the emergence and radiation of Metazoa. The principal unifying themes of this review are the dynamics and regulation of cell contact phenomena. Our knowledge of the dynamic molecular mechanisms underlying cell contact phenomena remains fragmentary. Here we examine the molecular bases of cell contact phenomena using extant model developmental systems (representing a wide range of phyla) including the simplest i.e. sponges, and the eukaryotic protist Dictyostelium discoideum, the more complex Drosophila melanogaster, and vertebrate systems. We discuss cell contact phenomena in a broad developmental context. The molecular language of cell contact phenomena is complex; it involves a plethora of structurally and functionally diverse molecules, and diverse modes of intermolecular interactions mediated by protein and/or carbohydrate moieties. Reasons for this are presumably the necessity for a high degree of specificity of intermolecular interactions, the requirement for a multitude of different signals, and the apparent requirement for an increasingly large repertoire of cell contact molecules in more complex developmental systems, such as the developing vertebrate nervous system. However, comparison of molecular models for dynamic adhesion in sponges and in vertebrates indicates that, in spite of significant differences in the details of the way specific cell-cell adhesion is mediated, similar principles are involved in the mechanisms employed by members of disparate phyla. Universal requirements are likely to include (a) rapidly reversible intermolecular interactions; (b) low-affinity intermolecular interactions with fast on-off rates; (c) the compounding of multiple intermolecular interactions; (d) associated regulatory signalling systems. The apparent widespread employment of molecular mechanisms involving cadherin-like cell adhesion molecules suggests the fundamental importance of cadherin function during development, particularly in epithelial morphogenesis, cell sorting, and segregation of cells.

Invasive cell behavior during Drosophila imaginal disc eversion is mediated by the JNK signaling cascade

Drosophila imaginal discs are monolayered epithelial invaginations that grow during larval stages and evert at metamorphosis to assemble the adult exoskeleton. They consist of columnar cells, forming the imaginal epithelium, as well as squamous cells, which constitute the peripodial epithelium and stalk (PS). Here, we uncover a new morphogenetic/cellular mechanism for disc eversion. We show that imaginal discs evert by apposing their peripodial side to the larval epidermis and through the invasion of the larval epidermis by PS cells, which undergo a pseudo-epithelial-mesenchymal transition (PEMT). As a consequence, the PS/larval bilayer is perforated and the imaginal epithelia protrude, a process reminiscent of other developmental events, such as epithelial perforation in chordates. When eversion is completed, PS cells localize to the leading front, heading disc expansion. We found that the JNK pathway is necessary for PS/larval cells apposition, the PEMT, and the motile activity of leading front cells.

The control of cell motility and epithelial morphogenesis by Jun kinases

Originally identified as stress-activated protein kinases that control cell survival and proliferation through transcription factor c-Jun, the Jun N-terminal kinase (JNK) subgroup of MAP kinases (MAPKs) have recently emerged as crucial regulators of cell migration and the morphogenetic movement of epithelial sheets. In Drosophila, a well-orchestrated JNK signaling pathway controls formation of actin stress fibers and cell shape changes, which are required for the sealing of embryonic epidermis in a process known as dorsal closure. The JNK pathway is also involved in morphogenetic processes in mice including closure of the eyelid, neural tube and optic fissure. This article focuses on recent advances in understanding the role of JNK pathway in the regulation of cell migration, cytoskeleton rearrangement and the morphogenesis of epithelial sheets.

PVR plays a critical role via JNK activation in thorax closure during Drosophila metamorphosis

PVR, the Drosophila homolog of the PDGF/VEGF receptor, has been implicated in border cell migration during oogenesis and hemocyte migration during embryogenesis. It was earlier shown that Mbc, a CDM family protein, and its effector, Rac, transduced the guidance signal from PVR during border cell migration. Here we demonstrate that PVR is also required for the morphogenetic process, thorax closure, during metamorphosis. The results of genetic and biochemical experiments indicate that PVR activates the JNK pathway. We present evidence showing Crk (an adaptor molecule), Mbc, ELMO (a homolog of Caenorhabditis elegans CED-12 and mammalian ELMO), and Rac to be mediators of JNK activation by PVR. In addition, we suppose that not only Rac but also Cdc42 is activated and involved in JNK activation downstream of PVR.

Coactivator MBF1 preserves the redox-dependent AP-1 activity during oxidative stress in Drosophila

Basic leucine zipper proteins Jun and Fos form the dimeric transcription factor AP-1, essential for cell differentiation and immune and antioxidant defenses. AP-1 activity is controlled, in part, by the redox state of critical cysteine residues within the basic regions of Jun and Fos. Mutation of these cysteines contributes to oncogenic potential of Jun and Fos. How cells maintain the redox-dependent AP-1 activity at favorable levels is not known. We show that the conserved coactivator MBF1 is a positive modulator of AP-1. Via a direct interaction with the basic region of Drosophila Jun (D-Jun), MBF1 prevents an oxidative modification (S-cystenyl cystenylation) of the critical cysteine and stimulates AP-1 binding to DNA. Cytoplasmic MBF1 translocates to the nucleus together with a transfected D-Jun protein, suggesting that MBF1 protects nascent D-Jun also in Drosophila cells. mbf1-null mutants live shorter than mbf1+ controls in the presence of hydrogen peroxide (H2O2). An AP-1-dependent epithelial closure becomes sensitive to H2O2 in flies lacking MBF1. We conclude that by preserving the redox-sensitive AP-1 activity, MBF1 provides an advantage during oxidative stress.

Wg and Egfr signalling antagonise the development of the peripodial epithelium in Drosophila wing discs

Imaginal discs contain a population of cells, known as peripodial epithelium, that differ morphologically and genetically from the rest of imaginal cells. The peripodial epithelium has a small contribution to the adult epidermis, though it is essential for the eversion of the discs during metamorphosis. The genetic mechanisms that control the identity and cellular morphology of the peripodial epithelia are poorly understood. In this report, we investigate the mechanisms that pattern the peripodial side of the wing imaginal disc during early larval development. At this time, the activities of the Wingless (Wg) and Epidermal growth factor receptor (Egfr) signalling pathways specify the prospective wing and notum fields, respectively. We show that peripodial epithelium specification occurs in the absence of Wingless and Egfr signalling. The ectopic activity in the peripodial epithelium of any of these signalling pathways transforms the shape of peripodial cells from squamous to columnar and resets their gene expression profile. Furthermore, peripodial cells where Wingless signalling is ectopically active acquire hinge identity, while ectopic Egfr activation results in notum specification. These findings suggest that suppression of Wg and Egfr activities is an early step in the development of the peripodial epithelium of the wing discs.