Activation of the JNK pathway during dorsal closure in Drosophila requires the mixed lineage kinase, slipper

X-linked variation in immune response in Drosophila melanogaster

This study quantifies the effects of naturally occurring X-linked variation on immune response in Drosophila melanogaster to assess associations between immunity genotypes and innate immune response. We constructed a set of 168 X-chromosomal extraction lines, incorporating X chromosomes from a natural population into co-isogenic autosomal backgrounds, and genotyped the lines at 88 SNPs in 20 X-linked immune genes. We find that genetic variation in many of the genes is associated with immune response phenotypes, including bacterial load and immune gene expression. Many of the associations act in a sex-specific or sexually antagonistic manner, supporting the theory that with the selective pressures facing genes on the X chromosome, sexually antagonistic variation may be more easily maintained.

Correcting developmental errors by apoptosis: lessons from Drosophila JNK signaling

Spatio-temporal regulation of the cell death machinery is essential for normal development and homeostasis of multicellular organisms. While the molecular basis for the central cell death machinery driven by caspases is now well documented, its regulatory mechanisms, especially in the context of living animals, remain to be clarified. The c-Jun N-terminal kinase (JNK) pathway is an evolutionarily conserved kinase cascade that regulates the apoptotic machinery. In mammals, JNK signaling has been implicated in stress-induced apoptosis. Drosophila genetics has now provided evidence of a novel role for JNK-mediated cell death signaling in eliminating developmentally aberrant cells from a tissue. The JNK-dependent cell-elimination system is orchestrated by cell-cell communication between normal and aberrant cells and is essential for ensuring developmental robustness, as well as for protecting organisms against fatal abnormalities such as neoplastic development. These processes are mediated by cell competition, morphogenetic apoptosis, and intrinsic tumor suppression. A combinatorial approach using both genetic and live-imaging systems in Drosophila will be extremely powerful to decipher how JNK-mediated apoptosis works within multicellular communities.

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 PP2C Alphabet is a negative regulator of stress-activated protein kinase signaling in Drosophila

The Jun N-terminal kinase and p38 pathways, also known as stress-activated protein kinase (SAPK) pathways, are signaling conduits reiteratively used throughout the development and adult life of metazoans where they play central roles in the control of apoptosis, immune function, and environmental stress responses. We recently identified a Drosophila Ser/Thr phosphatase of the PP2C family, named Alphabet (Alph), which acts as a negative regulator of the Ras/ERK pathway. Here we show that Alph also plays an inhibitory role with respect to Drosophila SAPK signaling during development as well as under stress conditions such as oxidative or genotoxic stresses. Epistasis experiments suggest that Alph acts at a step upstream of the MAPKKs Hep and Lic. Consistent with this interpretation, biochemical experiments identify the upstream MAPKKKs Slpr, Tak1, and Wnd as putative substrates. Together with previous findings, this work identifies Alph as a general attenuator of MAPK signaling in Drosophila.

MLK3 limits activated Galphaq signaling to Rho by binding to p63RhoGEF

Mixed lineage kinase 3 (MLK3) is a MAP3K that activates the JNK-dependent MAPK pathways. Here, we show that MLK3 is required for cell migration in a manner independent of its role as a MAP3K or MLK3 kinase activity. Rather, MLK3 functions in a regulated way to limit levels of the activated GTPase Rho by binding to the Rho activator, p63RhoGEF/GEFT, which, in turn, prevents its activation by Galphaq. These findings demonstrate a scaffolding role for MLK3 in controlling the extent of Rho activation that modulates cell migration. Moreover, they suggest that MLK3 functions as a network hub that links a number of signaling pathways.

The Drosophila peptidoglycan recognition protein PGRP-LF blocks PGRP-LC and IMD/JNK pathway activation

Eukaryotic peptidoglycan recognition proteins (PGRPs) are related to bacterial amidases. In Drosophila, PGRPs bind peptidoglycan and function as central sensors and regulators of the innate immune response. PGRP-LC/PGRP-LE constitute the receptor complex in the immune deficiency (IMD) pathway, which is an innate immune cascade triggered upon Gram-negative bacterial infection. Here, we present the functional analysis of the nonamidase, membrane-associated PGRP-LF. We show that PGRP-LF acts as a specific negative regulator of the IMD pathway. Reduction of PGRP-LF levels, in the absence of infection, is sufficient to trigger IMD pathway activation. Furthermore, normal development is impaired in the absence of functional PGRP-LF, a phenotype mediated by the JNK pathway. Thus, PGRP-LF prevents constitutive activation of both the JNK and the IMD pathways. We propose a model in which PGRP-LF keeps the Drosophila IMD pathway silent by sequestering circulating peptidoglycan.

Raw mediates antagonism of AP-1 activity in Drosophila

High baselines of transcription factor activities represent fundamental obstacles to regulated signaling. Here we show that in Drosophila, quenching of basal activator protein 1 (AP-1) transcription factor activity serves as a prerequisite to its tight spatial and temporal control by the JNK (Jun N-terminal kinase) signaling cascade. Our studies indicate that the novel raw gene product is required to limit AP-1 activity to leading edge epidermal cells during embryonic dorsal closure. In addition, we provide the first evidence that the epidermis has a Basket JNK-independent capacity to activate AP-1 targets and that raw function is required broadly throughout the epidermis to antagonize this activity. Finally, our mechanistic studies of the three dorsal-open group genes [raw, ribbon (rib), and puckered (puc)] indicate that these gene products provide at least two tiers of JNK/AP-1 regulation. In addition to Puckered phosphatase function in leading edge epidermal cells as a negative-feedback regulator of JNK signaling, the three dorsal-open group gene products (Raw, Ribbon, and Puckered) are required more broadly in the dorsolateral epidermis to quench a basal, signaling-independent activity of the AP-1 transcription factor.

A specific primed immune response in Drosophila is dependent on phagocytes

Drosophila melanogaster, like other invertebrates, relies solely on its innate immune response to fight invading microbes; by definition, innate immunity lacks adaptive characteristics. However, we show here that priming Drosophila with a sublethal dose of Streptococcus pneumoniae protects against an otherwise-lethal second challenge of S. pneumoniae. This protective effect exhibits coarse specificity for S. pneumoniae and persists for the life of the fly. Although not all microbial challenges induced this specific primed response, we find that a similar specific protection can be elicited by Beauveria bassiana, a natural fly pathogen. To characterize this primed response, we focused on S. pneumoniae–induced protection. The mechanism underlying this protective effect requires phagocytes and the Toll pathway. However, activation of the Toll pathway is not sufficient for priming-induced protection. This work contradicts the paradigm that insect immune responses cannot adapt and will promote the search for similar responses overlooked in organisms with an adaptive immune response.

Due to the common practice of vaccination and prominence of AIDS, people are already aware of the distinction between adaptive and innate immunity without realizing it. All organisms have an innate immune response, but only vertebrates possess T cells and the ability to produce antibodies. It has been a long-standing assumption that invertebrate immune systems are not adaptive and respond identically to multiple challenges. In this study, we demonstrate that the fly innate immune response adapts to repeated challenges; flies preinoculated with dead Streptococcus pneumoniae are protected against a second, otherwise-lethal dose. Although the underlying mechanisms are likely to be very different, this primed response is reminiscent to vaccine-induced protection in that it exhibits coarse specificity (dead S. pneumoniae only protects against itself), persists for the life of the fly and is dependent on phagocytic cells. This result prompts the obvious question of whether the innate immune system of vertebrates shares a similar biology. Such a finding is of particular interest since immunocompromised individuals only possess an innate immune system.

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.

Dynamic positive feedback phosphorylation of mixed lineage kinase 3 by JNK reversibly regulates its distribution to Triton-soluble domains

MLK3 (mixed lineage kinase 3) is a widely expressed, mammalian serine/threonine protein kinase that activates multiple MAPK pathways. Previously our laboratory used in vivo labeling/mass spectrometry to identify phosphorylation sites of activated MLK3. Seven of 11 identified sites correspond to the consensus motif for phosphorylation by proline-directed kinases. Based on these results, we hypothesized that JNK, or another proline-directed kinase, phosphorylates MLK3 as part of a feedback loop. Herein we provide evidence that MLK3 can be phosphorylated by JNK in vitro and in vivo. Blockade of JNK results in dephosphorylation of MLK3. The hypophosphorylated form of MLK3 is inactive and redistributes to a Triton-insoluble fraction. Recovery from JNK inhibition restores MLK3 solubility and activity, indicating that the redistribution process is reversible. This work describes a novel mode of regulation of MLK3, by which JNK-mediated feedback phosphorylation of MLK3 regulates its activation and deactivation states by cycling between Triton-soluble and Triton-insoluble forms.

Reconstruction and in silico analysis of the MAPK signaling pathways in the human blood fluke,Schistosoma japonicum

At present, little is known about signal transduction mechanisms in schistosomes, which cause the disease of schistosomiasis. The mitogen-activated protein kinase (MAPK) signaling pathways, which are evolutionarily conserved from yeast to Homo sapiens, play key roles in multiple cellular processes. Here, we reconstructed the hypothetical MAPK signaling pathways in Schistosoma japonicum and compared the schistosome pathways with those of model eukaryote species. We identified 60 homologous components in the S. japonciumMAPK signaling pathways. Among these, 27 were predicted to be full-length sequences. Phylogenetic analysis of these proteins confirmed the evolutionary conservation of the MAPK signaling pathways. Remarkably, we identified S. japonicum homologues of GTP-binding protein beta and alpha-I subunits in the yeast mating pathway, which might be involved in the regulation of different life stages and female sexual maturation processes as well in schistosomes. In addition, several pathway member genes, including ERK, JNK, Sja-DSP, MRAS and RAS, were determined through quantitative PCR analysis to be expressed in a stage-specific manner, with ERK, JNK and their inhibitor Sja-DSP markedly upregulated in adult female schistosomes.

JNK signaling coordinates integrin and actin functions during Drosophila embryogenesis

Epithelial movements are key morphogenetic events in animal development. They are driven by multiple mechanisms, including signal-dependent changes in cytoskeletal organization and in cell adhesion. Such processes must be controlled precisely and coordinated to accurately sculpt the three-dimensional form of the developing organism. By observing the Drosophila epidermis during embryonic development using confocal time-lapse microscopy, we have investigated how signaling through the Jun-N-terminal kinase (JNK) pathway governs the tissue sheet movements that result in dorsal closure (DC). We find that JNK controls the polymerization of actin into a cable at the epidermal leading edge as previously suggested, as well as the joining (zipping) of the contralateral epithelial cell sheets. Here, we show that zipping is mediated by regulation of the integrins myospheroid and scab. Our data demonstrate that JNK signaling regulates a set of target genes that cooperate to facilitate epithelial movement and closure.

Drosophila Dok is required for embryonic dorsal closure

Embryonic dorsal closure (DC) in Drosophila is a series of morphogenetic movements involving the bilateral dorsal movement of the epidermis (cell stretching) and dorsal suturing of the leading edge (LE) cells to enclose the viscera. The Syk family tyrosine kinase Shark plays a crucial role in this Jun amino-terminal kinase (JNK)-dependent process, where it acts upstream of JNK in LE cells. Using a yeast two-hybrid screen, the unique Drosophila homolog of the downstream of kinase (Dok) family, Ddok,was identified by its ability to bind Shark SH2 domains in a tyrosine phosphorylation-dependent fashion. In cultured S2 embryonic cells, Ddok tyrosine phosphorylation is Src dependent; Shark associates with Ddok and Ddok localizes at the cell cortex, together with a portion of the Shark protein. The embryonic expression pattern of Ddok resembles the expression pattern of Shark. Ddok loss-of-function mutant (DdokPG155)germ-line clones possess DC defects, including the loss of JNK-dependent expression of dpp mRNA in LE cells, and decreased epidermal F-actin staining and LE actin cable formation. Epistatic analysis indicates that Ddok functions upstream of shark to activate JNK signaling during DC. Consistent with these observations, Ddok mutant embryos exhibit decreased levels of tyrosine phosphorylated Shark at the cell periphery of LE and epidermal cells. As there are six mammalian Dok family members that exhibit some functional redundancy, analysis of the regulation of DC by Ddok is expected to provide novel insights into the function of the Dok adapter proteins.

Cdc42 induces activation loop phosphorylation and membrane targeting of mixed lineage kinase 3

Mixed lineage kinase 3 (MLK3) functions as a mitogen-activated protein kinase kinase kinase to activate multiple mitogen-activated protein kinase pathways. Our current studies demonstrate that lack of MLK3 blocks signaling of activated Cdc42 to c-Jun N-terminal kinase, giving strong support for the idea that Cdc42 is a physiological activator of MLK3. We show herein that Cdc42, in a prenylation-dependent manner, targets MLK3 from a perinuclear region to membranes, including the plasma membrane. Cdc42-induced membrane targeting of MLK3 is independent of MLK3 catalytic activity but depends upon an intact Cdc42/Rac-interactive binding motif, consistent with MLK3 membrane translocation being mediated through direct binding of Cdc42. Phosphorylation of the activation loop of MLK3 requires MLK3 catalytic activity and is induced by Cdc42 in a prenylation-independent manner, arguing that Cdc42 binding is sufficient for activation loop autophosphorylation of MLK3. However, membrane targeting is necessary for full activation of MLK3 and maximal signaling to JNK. We previously reported that MLK3 is autoinhibited through an interaction between its N-terminal SH3 domain and a proline-containing sequence found between the leucine zipper and the CRIB motif of MLK3. Thus we propose a model in which GTP-bound Cdc42/Rac binds MLK3 and disrupts SH3-mediated autoinhibition leading to dimerization and activation loop autophosphorylation. Targeting of this partially active MLK3 to membranes likely results in additional phosphorylation events that fully activate MLK3 and its ability to maximally signal through the JNK pathway.

A genetic screen targeting the tumor necrosis factor/Eiger signaling pathway: identification of Drosophila TAB2 as a functionally conserved component

Signaling by tumor necrosis factors (TNFs) plays a prominent role in mammalian development and disease. To fully understand this complex signaling pathway it is important to identify all regulators and transduction components. A single TNF family member, Eiger, is encoded in the Drosophila genome, offering the possibility of applying genetic approaches for pursuing this goal. Here we present a screen for the isolation of novel genes involved in the TNF/Eiger pathway. On the basis of Eiger's ability to potently activate Jun-N-terminal kinase (JNK) and trigger apoptosis, we used the Drosophila eye to establish an assay for dominant suppressors of this activity. In a large-scale screen the Drosophila homolog of TAB2/3 (dTAB2) was identified as an essential component of the Eiger-JNK pathway. Genetic epistasis and biochemical protein-protein interaction assays assign an adaptor role to dTAB2, linking dTRAF1 to the JNKKK dTAK1, demonstrating a conserved mechanism of TNF signal transduction in mammals and Drosophila. Thus, in contrast to morphogenetic processes, such as dorsal closure of the embryo, in which the JNK pathway is activated by the JNKKK Slipper, Eiger uses the dTAB2-dTAK1 module to induce JNK signaling activity.

The small GTPase Rac plays multiple roles in epithelial sheet fusion—dynamic studies of Drosophila dorsal closure

The coordinated migration and fusion of epithelial sheets is a crucial morphogenetic tool used on numerous occasions during the normal development of an embryo and re-activated as part of the wound healing response. Drosophila dorsal closure, whereby a hole in the embryonic epithelium is zipped closed late in embryogenesis, serves as an excellent, genetically tractable model for epithelial migration. Using live confocal imaging, we have dissected multiple roles for the small GTPase Rac in this process. We show that constitutive activation of Rac1 leads to excessive assembly of lamellipodia and precocious halting of epithelial sweeping, possibly through premature activation of contact-inhibition machinery. Conversely, blocking Rac activity, either by loss-of-function mutations or expression of dominant negative Rac1, disables the assembly of both actin cable and protrusions by epithelial cells. Movies of mutant embryos show that continued contraction of the amnioserosa is sufficient to draw the epithelial edges towards one another, allowing the zipper machinery to bypass non-functioning regions of leading edge. In addition to illustrating the key role of Rac in organization of leading edge actin, loss-of-function mutants also provide substantive proof that Rac acts upstream in the Jun N-terminal kinase (JNK) cascade to direct epithelial cell shape changes during dorsal closure.

Role of MLK3 in the regulation of mitogen-activated protein kinase signaling cascades

Mixed-lineage protein kinase 3 (MLK3) is a member of the mitogen-activated protein (MAP) kinase kinase kinase group that has been implicated in multiple signaling cascades, including the NF-kappaB pathway and the extracellular signal-regulated kinase, c-Jun NH(2)-terminal kinase (JNK), and p38 MAP kinase pathways. Here, we examined the effect of targeted disruption of the murine Mlk3 gene. Mlk3(-/-) mice were found to be viable and healthy. Primary embryonic fibroblasts prepared from these mice exhibited no major signaling defects. However, we did find that MLK3 deficiency caused a selective reduction in tumor necrosis factor (TNF)-stimulated JNK activation. Together, these data demonstrate that MLK3 contributes to the TNF signaling pathway that activates JNK.

PAK and other Rho-associated kinases--effectors with surprisingly diverse mechanisms of regulation

The Rho GTPases are a family of molecular switches that are critical regulators of signal transduction pathways in eukaryotic cells. They are known principally for their role in regulating the cytoskeleton, and do so by recruiting a variety of downstream effector proteins. Kinases form an important class of Rho effector, and part of the biological complexity brought about by switching on a single GTPase results from downstream phosphorylation cascades. Here we focus on our current understanding of the way in which different Rho-associated serine/threonine kinases, denoted PAK (p21-activated kinase), MLK (mixed-lineage kinase), ROK (Rho-kinase), MRCK (myotonin-related Cdc42-binding kinase), CRIK (citron kinase) and PKN (protein kinase novel), interact with and are regulated by their partner GTPases. All of these kinases have in common an ability to dimerize, and in most cases interact with a variety of other proteins that are important for their function. A diversity of known structures underpin the Rho GTPase-kinase interaction, but only in the case of PAK do we have a good molecular understanding of kinase regulation. The ability of Rho GTPases to co-ordinate spatial and temporal phosphorylation events explains in part their prominent role in eukaryotic cell biology.

Downregulation of lipopolysaccharide response in drosophila by negative crosstalk between the AP1 and NF-κB signaling modules

IkappaB kinase (IKK) and Jun N-terminal kinase (Jnk) signaling modules are important in the synthesis of immune effector molecules during innate immune responses against lipopolysaccharide and peptidoglycan. However, the regulatory mechanisms required for specificity and termination of these immune responses are unclear. We show here that crosstalk occurred between the drosophila Jnk and IKK pathways, which led to downregulation of each other's activity. The inhibitory action of Jnk was mediated by binding of drosophila activator protein 1 (AP1) to promoters activated by the transcription factor NF-kappaB. This binding led to recruitment of the histone deacetylase dHDAC1 to the promoter of the gene encoding the antibacterial protein Attacin-A and to local modification of histone acetylation content. Thus, AP1 acts as a repressor by recruiting the deacetylase complex to terminate activation of a group of NF-kappaB target genes.

Dissecting JNK signaling, one KKKinase at a time

Specificity in signal transduction is essential to ensure distinct and appropriate cellular responses to extracellular cues. Determining the mechanisms that mediate specificity is key to understanding complex cell behaviors in development, when multiple pathways fire simultaneously and individual pathways are used recurrently. Jun kinase (JNK) signal transduction exemplifies a pathway that is used multiple times in animal development and homeostasis. Indeed, molecular genetic analysis of JNK signaling in Drosophila has shown that a core signaling module consisting of Hep (JNKK), Bsk (JNK), and Jun regulates various processes, including tissue morphogenesis, wound repair, stress response, innate immune response, and others. Six putative JNKK kinase (JNKKK) family members are present in the fly genome, which could activate the core module in response to distinct stimuli. The diversity of kinases at this level of the signaling hierarchy could substantially increase the number of possible signals that feed into activation of the core module. Recent studies have described the distinct phenotypic consequences of mutations in three of the genes, Slpr (dMLK), Tak1, and Mekk1. These data, together with Drosophila cell culture and genomic array analyses support the contention that the choice of JNKKK may contribute to signaling specificity in vivo. Whether this is achieved by individual JNKKKs or by means of a combinatorial mechanism will require a systematic characterization of compound mutants and a toolbox of transcriptional reporters specific for distinct JNK-dependent processes.

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.

The Caenorhabditis elegans MAPK phosphatase VHP-1 mediates a novel JNK-like signaling pathway in stress response

Mitogen-activated protein kinases (MAPKs) are integral to the mechanisms by which cells respond to physiological stimuli and to a wide variety of environmental stresses. MAPK cascades can be inactivated at the MAPK activation step by members of the MAPK phosphatase (MKP) family. However, the components that act in MKP-regulated pathways have not been well characterized in the context of whole organisms. Here we characterize the Caenorhabditis elegans vhp-1 gene, encoding an MKP that acts preferentially on the c-Jun N-terminal kinase (JNK) and p38 MAPKs. We found that animals defective in vhp-1 are arrested during larval development. This vhp-1 defect is suppressed by loss-of-function mutations in the kgb-1, mek-1, and mlk-1 genes encoding a JNK-like MAPK, an MKK7-type MAPKK, and an MLK-type MAPKKK, respectively. The genetic and biochemical data presented here demonstrate a critical role for VHP-1 in the KGB-1 pathway. Loss-of-function mutations in each component in the KGB-1 pathway result in hypersensitivity to heavy metals. These results suggest that VHP-1 plays a pivotal role in the integration and fine-tuning of the stress response regulated by the KGB-1 MAPK pathway.

Targeting the JNK MAPK cascade for inhibition: basic science and therapeutic potential

The c-Jun N-terminal protein kinases (JNKs) form one subfamily of the mitogen-activated protein kinase (MAPK) group of serine/threonine protein kinases. The JNKs were first identified by their activation in response to a variety of extracellular stresses and their ability to phosphorylate the N-terminal transactivation domain of the transcription factor c-Jun. One approach to study the function of the JNKs has included in vivo gene knockouts of each of the three JNK genes. Whilst loss of either JNK1 or JNK2 alone appears to have no serious consequences, their combined knockout is embryonic lethal. In contrast, the loss of JNK3 is not embryonic lethal, but rather protects the adult brain from glutamate-induced excitotoxicity. This latter example has generated considerable enthusiasm with JNK3, considered an appropriate target for the treatment of diseases in which neuronal death should be prevented (e.g. stroke, Alzheimer's and Parkinson's diseases). More recently, these gene knockout animals have been used to demonstrate that JNK could provide a suitable target for the protection against obesity and diabetes and that JNKs may act as tumour suppressors. Considerable effort is being directed to the development of chemical inhibitors of the activators of JNKs (e.g. CEP-1347, an inhibitor of the MLK family of JNK pathway activators) or of the JNKs themselves (e.g. SP600125, a direct inhibitor of JNK activity). These most commonly used inhibitors have demonstrated efficacy for use in vivo, with the successful intervention to decrease brain damage in animal models (CEP-1347) or to ameliorate some of the symptoms of arthritis in other animal models (SP600125). Alternative peptide-based inhibitors of JNKs are now also in development. The possible identification of allosteric modifiers rather than direct ATP competitors could lead to inhibitors of unprecedented specificity and efficacy.

Immune Activation of NF-κB and JNK Requires Drosophila TAK1

Stimulation of the Drosophila immune response activates NF-kappaB and JNK signaling pathways. For example, infection by Gram-negative bacteria induces the Imd signaling pathway, leading to the activation of the NF-kappaB-like transcription factor Relish and the expression of a battery of genes encoding antimicrobial peptides. Bacterial infection also activates the JNK pathway, but the role of this pathway in the immune response has not yet been established. Genetic experiments suggest that the Drosophila homolog of the mammalian MAPK kinase kinase, TAK1 (transforming growth factor beta-activated kinase 1), activates both the JNK and NF-kappaB pathways following immune stimulation. In this report, we demonstrate that Drosophila TAK1 functions as both the Drosophila IkappaB kinase-activating kinase and the JNK kinase-activating kinase. However, we found that JNK signaling is not required for antimicrobial peptide gene expression but is required for the activation of other immune inducible genes, including Punch, sulfated, and malvolio. Thus, JNK signaling appears to play an important role in the cellular immune response and the stress response.

Axin utilizes distinct regions for competitive MEKK1 and MEKK4 binding and JNK activation

Axin is a multidomain protein that plays a critical role in Wnt signaling, serving as a scaffold for down-regulation of beta-catenin. It also activates the JNK mitogen-activated protein kinase by binding to MEKK1. However, it is intriguing that Axin requires several additional elements for JNK activation, including a requirement for homodimerization, sumoylation at the extreme C-terminal sites, and a region in the protein phosphatase 2A-binding domain. In our present study, we have shown that another MEKK family member, MEKK4, also binds to Axin in vivo and mediates Axin-induced JNK activation. Surprisingly MEKK4 binds to a region distinct from the MEKK1-binding site. Dominant negative mutant of MEKK4 attenuates the JNK activation by Axin. Activation of JNK by Axin in MEKK1-/- mouse embryonic fibroblast cells supports the idea that another MEKK can mediate Axin-induced JNK activation. Expression of specific small interfering RNA against MEKK4 effectively attenuates JNK activation by the MEKK1 binding-defective Axin mutant in 293T cells and inhibits JNK activation by wild-type Axin in MEKK1-/- cells, confirming that MEKK4 is indeed another mitogen-activated protein kinase kinase kinase that is specifically involved in Axin-mediated JNK activation independently of MEKK1. We have also identified an additional domain between MEKK1- and MEKK4-binding sites as being required for JNK activation by Axin. MEKK1 and MEKK4 compete for Axin binding even though they bind to sites far apart, suggesting that Axin may selectively bind to MEKK1 or MEKK4 depending on distinct signals or cellular context. Our findings will provide new insights into how scaffold proteins mediate ultimate activation of different mitogen-activated protein kinase kinase kinases.

Cross-talk between JNK/SAPK and ERK/MAPK pathways: sustained activation of JNK blocks ERK activation by mitogenic factors

Mixed lineage kinases (MLKs) are a family of serine/threonine kinases that function in the SAPK signaling cascade. MLKs activate JNK/SAPK in vivo by directly phosphorylating and activating the JNK kinase SEK-1 (MKK4 and -7). Importantly, the MLK member MLK3/SPRK has been shown recently to be a direct target of ceramide and tumor necrosis factor-alpha (TNF-alpha) and to mediate the TNF-alpha and ceramide-induced JNK activation in Jurkat cells. Here we report that MLK3 can phosphorylate and activate MEK-1 directly in vitro and also can induce MEK phosphorylation on its activation sites in vivo in COS-7 cells. Surprisingly, this induction of MEK phosphorylation does not result in ERK activation in vivo. Rather, in cells expressing active MLK3, ERK becomes resistant to activation by growth factors and mitogens. This restriction in ERK activation requires MLK3 kinase activity, is independent of Raf activation, and is reversed by JNK pathway inhibition either at the level of SEK-1, JNK, or Jun. These results demonstrate that sustained JNK activation uncouples ERK activation from MEK in a manner requiring Jun-mediated gene transcription. This in turn points to the existence of a negative cross-talk relationship between the stress-activated JNK pathway and the mitogen-activated ERK pathway. Thus, our findings imply that some of the biological functions of JNK activators, such as TNF-alpha and ceramide, may be attributed to their ability to block cell responses to growth and survival factors acting through the ERK/MAPK pathway.

Drosophila mixed lineage kinase/slipper, a missing biochemical link in Drosophila JNK signaling

Mixed lineage kinases (MLKs) belong to the family of mitogen activated protein kinase kinase kinase (MAPKKK) and cause neuronal cell death mediated through c-Jun, N-terminal kinase (JNK) pathway. Recently, genetic studies in Drosophila revealed the presence of an MLK termed slipper (slpr). However, its biochemical features like physiological substrate, role in different MAPK pathways and developmental and tissue-specific expression pattern were not reported. Here, we report cDNA cloning, expression analysis and biochemical characterization of a Drosophila mixed lineage kinase (dMLK) that is also known as slipper. The protein structure analysis of dMLK/slipper revealed, in addition to the conserved domains, a stretch of glutamine in the amino terminus and an asparagine-threonine stretch at the carboxy-terminus. In situ hybridization and reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that dMLK is expressed in early embryonic stages, adult brain and thorax. Ectopic expression of dMLK either in Drosophila S2 or in mammalian HEK293 cells leads to activation of JNK, p38 and extracellular signal regulated kinase (ERK) pathways. Further, dMLK directly phosphorylates Hep, dMKK4 and also their mammalian counterparts, MKK7 and SEK1, in an in vitro kinase assay. Taken together, our results provide for the first time a comprehensive expression profile and new biochemical insight of dMLK/slipper.

Stimulus-specific requirements for MAP3 kinases in activating the JNK pathway

Mitogen-activated protein kinases (MAPKs) are activated by numerous ligands typically through a protein kinase cascade minimally composed of the MAPK in series with a MAP2 kinase (MAP2K) and a MAP3K. This arrangement is thought to confer specificity and appropriate kinetic properties on the activation of MAPKs in response to physiological stimuli. Surprisingly, more than a dozen MAP3Ks have been identified that activate the c-Jun N-terminal kinases (JNKs) when overexpressed, but there is no clear understanding of which kinases actually mediate JNK activation by ligands. Here, we use double-stranded RNA-mediated interference of gene expression to reveal the explicit participation of discrete MAP3Ks in controlling JNK activity by multiple stimuli. Maximal activation of JNK by lipopolysaccharide requires the MAP3K TAK1. On the other hand, sorbitol requires expression of four MAP3Ks to cause maximal JNK activation. Thus, we demonstrate that specific stimuli use different mechanisms to recruit distinct MAP3Ks to regulate the JNK pathway.

Planar polarity and actin dynamics in the epidermis of Drosophila

Dorsal closure is a morphogenetic process involving the coordinated convergence of two epithelial sheets to enclose the Drosophila melanogaster embryo. Specialized populations of cells at the edges of each epithelial sheet, the dorsal-most epidermal cells, emit actin-based processes that are essential for the proper enclosure of the embryo. Here we show that actin dynamics at the leading edge is preceded by a planar polarization of the dorsal-most epidermal cells associated with a reorganization of the cytoskeleton. An important consequence of this planar polarization is the formation of actin-nucleating centres at the leading edge, which are important in the dynamics of actin. We show that Wingless (Wg) signalling and Jun amino-terminal kinase (JNK) signalling have overlapping but different roles in these events.

Activation of the Drosophila MLK by ceramide reveals TNF-alpha and ceramide as agonists of mammalian MLK3

Mixed lineage kinases (MLKs) are MAPKKK members that activate JNK and reportedly lead to cell death. However, the agonist(s) that regulate MLK activity remain unknown. Here, we demonstrate ceramide as the activator of Drosophila MLK (dMLK) and identify ceramide and TNF-alpha as agonists of mammalian MLK3. dMLK and MLK3 are activated by a ceramide analog and bacterial sphingomyelinase in vivo, whereas a low nanomolar concentration of natural ceramide activates them in vitro. Specific inhibition of dMLK and MLK3 significantly attenuates activation of JNK by ceramide in vivo without affecting ceramide-induced p38 or ERK activation. In addition, TNF-alpha also activates MLK3 and evidently leads to JNK activation in vivo. Thus, the ceramide serves as a common agonist of dMLK and MLK3, and MLK3 contributes to JNK activation induced by TNF-alpha.

Epithelial fusions in the embryo

Morphogenesis in the embryo involves the bending, folding and fusing of epithelial tissues to create the final complex shapes of the various organs and structures in the body. One essential process that occurs frequently during development is the drawing together and fusion of epithelial edges. Drosophila dorsal closure is perhaps the most genetically tractable of this type of movement, and several recent advances have revealed much about the signals regulating the dynamic actin cytoskeletal machineries that underlie the zippering-closed of this hole in the embryonic fly. It is now clear that there are intriguing parallels with more complex morphogenetic tissue movements in vertebrates.

Mixed-lineage kinase control of JNK and p38 MAPK pathways

Mixed-lineage kinases (MLKs) are serine/threonine protein kinases that regulate signalling by the c-Jun amino-terminal kinase (JNK) and p38 mitogen-activated-protein kinase (MAPK) pathways. MLKs are represented in the genomes of both Caenorhabditis elegans and Drosophila melanogaster. The Drosophila MLK Slipper regulates JNK to control dorsal closure during embryonic morphogenesis. In mammalian cells, MLKs are implicated in the control of apoptosis and are potential drug targets for many neurodegenerative diseases.

Dynamic analysis of dorsal closure in Drosophila: from genetics to cell biology

Throughout development a series of epithelial bendings, sweepings, and fusions occur that collectively give shape to the embryo. These morphogenetic movements are driven by coordinated assembly and contraction of the actomyosin cytoskeleton in restricted populations of epithelial cells. One well-studied example of such a morphogenetic episode is dorsal closure in Drosophila embryogenesis. This process is tractable at a genetic level and has recently become the focus of live cell biology analysis because of the availability of flies expressing GFP-fusion proteins. This marriage of genetics and cell biology is very powerful and is allowing the dissection of fundamental signaling mechanisms that regulate the cytoskeletal reorganizations and contractions underlying coordinated tissue movements in the embryo.

Eiger, a TNF superfamily ligand that triggers the Drosophila JNK pathway

Drosophila provides a powerful genetic model for studying the in vivo regulation of cell death. In our large-scale gain-of-function screen, we identified Eiger, the first invertebrate tumor necrosis factor (TNF) superfamily ligand that can induce cell death. Eiger is a type II transmembrane protein with a C-terminal TNF homology domain. It is predominantly expressed in the nervous system. Genetic evidence shows that Eiger induces cell death by activating the Drosophila JNK pathway. Although this cell death process is blocked by Drosophila inhibitor-of-apoptosis protein 1 (DIAP1), it does not require caspase activity. We also show genetically that Eiger is a physiological ligand for the Drosophila JNK pathway. Our findings demonstrate that Eiger can initiate cell death through an IAP-sensitive cell death pathway via JNK signaling.

Signaling pathways directing the movement and fusion of epithelial sheets: lessons from dorsal closure in Drosophila

Wound healing in embryos and various developmental events in metazoans require the spreading and fusion of epithelial sheets. The complex signaling pathways regulating these processes are being pieced together through genetic, cell biological, and biochemical approaches. At present, dorsal closure of the Drosophila embryo is the best-characterized example of epithelial sheet movement. Dorsal closure involves migration of the lateral epidermal flanks to close a hole in the dorsal epidermis occupied by an epithelium called the amnioserosa. Detailed genetic studies have revealed a network of interacting signaling molecules regulating this process. At the center of this network is a Jun N-terminal kinase cascade acting at the leading edge of the migrating epidermis that triggers signaling by the TGF-beta superfamily member Decapentaplegic and which interacts with the Wingless pathway. These signaling modules regulate the cytoskeletal reorganization and cell shape change necessary to drive dorsal closure. Activation of this network requires signals from the amnioserosa and input from a variety of proteins at cell-cell junctions. The Rho family of small GTPases is also instrumental, both in activation of signaling and regulation of the cytoskeleton. Many of the proteins regulating dorsal closure have been implicated in epithelial movement in other organisms, and dorsal closure has emerged as an ideal model system for the study of the migration and fusion of epithelial sheets.