A non-redundant role for Drosophila Mkk4 and hemipterous/Mkk7 in TAK1-mediated activation of JNK

Pattern recognition receptors in Crustacea: immunological roles under environmental stress

Innate immunity is the first line of defense against infections and the only known available strategy for invertebrates. Crustaceans, being mostly aquatic invertebrates, are constantly exposed to potential pathogens in the surrounding water. Their immune system abolishes most microbes that enter and are recognized as a threat. However, the stress produced by high population densities and abiotic changes, in aquaculture, disrupts the host-pathogen balance, leading to severe economic losses in this industry. Consequently, crustacean immunology has become a prime area of research where significant progress has been made. This review provides our current understanding of the key pattern recognition receptors in crustaceans, with special focus on Decapoda, and their roles in triggering an immune response. We discuss recent developments in the field of signal transduction pathways such as Toll-like receptors (TLRs) and the immune deficiency (IMD) pathway, and examine the role of antimicrobial peptides (AMPs) in pathogen defense. Additionally, we analyze how environmental stressors-such as temperature fluctuations, ammonia levels, and pollution-impact immune responses and increase susceptibility to diseases. Finally, we highlight future research directions, emphasizing the need to explore the interactions between environmental stressors and immune signaling pathways and to develop strategies to enhance immune responses in crustaceans within aquaculture settings. Altogether, these advancements deepen our understanding of pathogen recognition in invertebrates and the specific defense mechanisms employed by crustaceans, particularly in response to infections triggered by pathogens under abiotic stressors.

Novel PI(3)K-p85α/p110δ-ITK-LAT-PLC-γ2 and Fyn-ADAP-Carma1-TAK1 Pathways Define Reverse Signaling via FasL

The role of FasL in initiating death signals through Fas is well characterized. However, the reverse signaling pathway downstream of FasL in effector lymphocytes is poorly understood. Here, we identify that FasL functions as an independent activation receptor in NK cells. Activation via FasL results in the production of LFN-γ, GM-CSF, RANTES, MIP-1α, and MIP1-β. Proximal signaling of FasL requires Lck and Fyn. Upon activation, FasL facilitates the phosphorylation of PI(3)K-p85α/p55α subunits. A catalytically inactive PI(3)K-p110δD910A mutation significantly impairs the cytokine and chemokine production by FasL. Activation of ITK and LAT downstream of FasL plays a central role in recruiting and phosphorylating PLC-γ2. Importantly, Fyn-mediated recruitment of ADAP links FasL to the Carmal/ Bcl10/Tak1 signalosome. Lack of Carma1, CARD domain of Carma1, or Tak1 significantly reduces FasL-mediated cytokine and chemokine production. These findings, for the first time, provide a detailed molecular blueprint that defines FasL-mediated reverse signaling.

Genome-wide identification and analysis of the MAPKK gene family in Chinese mitten crab (Eriocheir sinensis) and its response to bacterial challenge

Protein kinases of the MAPK cascade family (MAPKKK-MAPKK-MAPK) play an important role in the growth and development of organisms and their response to environmental stress. The MAPKK gene families in the Chinese mitten crab Eriocheir sinensis have never been systematically analyzed. We identified four MAPKK family genes, EsMEK, EsMAPKK4, EsMAPKK6, and EsMAPKK7, in E. sinensis and analyzed their molecular features and expression patterns. All four MAPKK genes are composed of multiple exons and introns, all have a conserved domain, and all have 10 conserved motifs (except EsMEK and EsMAPKK7 which are missing motif 10). The four MAPKK genes are on four different chromosomes and have no gene duplications, and the results of phylogenetic tree analysis indicate that the ESMAPKK gene family is highly conserved evolutionarily. The EsMAPKK genes were widely expressed in all the examined tissues with higher expression in hemocytes, hepatopancreas, and gills. Notably, EsMAPKK6 was also highly expressed in the ovary. Vibrio parahaemolyticus infection significantly increased the mRNA levels of the EsMAPKK genes in hemocytes. Further disruption of the EsMAPKK gene family expression affects the expression levels of multiple antimicrobial peptides in hemocytes. Our experimental results provide a starting point for a more in-depth study of the innate immunity functional roles of members of the MAPKK gene families in E. sinensis.

The tracheal immune system of insects - A blueprint for understanding epithelial immunity

The unique design of respiratory organs in multicellular organisms makes them prone to infection by pathogens. To cope with this vulnerability, highly effective local immune systems evolved that are also operative in the tracheal system of insects. Many pathogens and parasites (including viruses, bacteria, fungi, and metazoan parasites) colonize the trachea or invade the host via this route. Currently, only two modules of the tracheal immune system have been characterized in depth: 1) Immune deficiency pathway-mediated activation of antimicrobial peptide gene expression and 2) local melanization processes that protect the structure from wounding. There is an urgent need to increase our understanding of the architecture of tracheal immune systems, especially regarding those mechanisms that enable the maintenance of immune homeostasis. This need for new studies is particularly exigent for species other than Drosophila.

M1-linked ubiquitination facilitates NF-κB activation and survival during sterile inflammation

Methionine 1 (M1)-linked ubiquitination plays a key role in the regulation of inflammatory nuclear factor-κB (NF-κB) signalling and is important for clearance of pathogen infection in Drosophila melanogaster. M1-linked ubiquitin (M1-Ub) chains are assembled by the linear ubiquitin E3 ligase (LUBEL) in flies. Here, we have studied the role of LUBEL in sterile inflammation induced by different types of cellular stresses. We have found that the LUBEL catalyses formation of M1-Ub chains in response to hypoxic, oxidative and mechanical stress conditions. LUBEL is shown to be important for flies to survive low oxygen conditions and paraquat-induced oxidative stress. This protective action seems to be driven by stress-induced activation of the NF-κB transcription factor Relish via the immune deficiency (Imd) pathway. In addition to LUBEL, the intracellular mediators of Relish activation, including the transforming growth factor activating kinase 1 (Tak1), Drosophila inhibitor of apoptosis (IAP) Diap2, the IκB kinase γ (IKKγ) Kenny and the initiator caspase Death-related ced-3/Nedd2-like protein (Dredd), but not the membrane receptor peptidoglycan recognition protein (PGRP)-LC, are shown to be required for sterile inflammatory response and survival. Finally, we showed that the stress-induced upregulation of M1-Ub chains in response to hypoxia, oxidative and mechanical stress is also induced in mammalian cells and protects from stress-induced cell death. Taken together, our results suggest that M1-Ub chains are important for NF-κB signalling in inflammation induced by stress conditions often observed in chronic inflammatory diseases and cancer.

Genome-wide analysis of the hard clam mitogen-activated protein kinase kinase gene family and their transcriptional profiles under abiotic stress

Mitogen-activated protein kinase kinase (MAPKK) was the hub component of the Mitogen-activated protein kinase (MAPK) signaling pathway and played an important role in the cellular response to environmental stress. In this study, we identified five MmMAPKK genes in hard clam Mercenaria mercenaria and found that all MmMAPKK genes contain a conserved protein kinase domain. The MmMAPKK genes derived from dispersed duplication were unevenly distributed in three chromosomes. Although the genome size was highly variable among different bivalve mollusks, the number of MAPKK genes was relatively stable. Phylogenetic analysis showed that bivalve MAPKK was divided into five clades, and amino acid sequences of MAPKK from the same clade consisted of similar conserved motifs. The syntenic analysis demonstrated that MmMAPKKs had the highest number of homologous gene pairs with Cyclina sinensis. MmMAPKKs were ubiquitously expressed in all examined tissues, and all MmMAPKK genes were highly expressed in the ovary. MmMAPKK genes showed stress-specific expression under envirionmental stress. MmMAPKK7 showed an upregulated in heat and heat plus hypoxia stress while MmMAPKK1 showed an upregulated in hypoxic stress groups. Dynamic changes of MmMAPKK7, MmMAPKK6 and MmMAPKK1 in hemocytes were observed in response to air exposure. MmMAPKK4 significantly downregulated after air exposure for five days. MmMAPKK7 and MmMAPKK6 might participate in adaptation to low salinity stress. Our results provided useful information about MAPKK and laid a foundation for further studies on MAPKK evolution in the bivalve.

Sestrin protects Drosophila midgut from mercury chloride-induced damage by inhibiting oxidative stress and stimulating intestinal regeneration

Overproduction of the deleterious reactive oxygen species (ROS) is one of the major causes of mercury, a heavy metal with diverse applications and environmental presence, induced neuronal and gastrointestinal adversities in exposed organism including Drosophila melanogaster. Sestrin, an oxidative stress responsive gene, emerges as a novel player in the management of oxidative stress response. Due to limited information regarding the role of sestrin in mercury-induced gastrointestinal adversities, it was hypothesized that modulation of sestrin may improve the mercury-induced gastrointestinal adversities in Drosophila. Here, we fed Drosophila with 400 μM HgCl₂ and found that sestrin transcriptional level was significantly increased in midguts. Sestrin knockdown in HgCl₂-exposed midguts decreased survival rates and climbing ability of flies, and inhibited superoxide dismutase and glutathione-S-transferase activities of midgut epithelieum. Meanwhile, sestrin knockdown in midgut aggravated the HgCl₂-induced disruption of intestinal organization by worsening ROS production and cell apoptosis. Immunohistochemical staining data revealed that sestrin knockdown inhibited intestinal stem cell division in HgCl₂-exposed midguts. Furthermore, JNK signaling was found to mediated sestrin expression in midgut. Taken together, the study demonstrated that sestrin protects Drosophila midgut from HgCl₂-induced oxidative damage by inhibiting ROS production and stimulating the tissue regeneration program under regulation of JNK signaling pathway. This work suggests therapeutic implications of sestrin against heavy metal-induced gastrointestinal adversities in mammals.

Identification of GOLPH3 Partners in Drosophila Unveils Potential Novel Roles in Tumorigenesis and Neural Disorders

Golgi phosphoprotein 3 (GOLPH3) is a highly conserved peripheral membrane protein localized to the Golgi apparatus and the cytosol. GOLPH3 binding to Golgi membranes depends on phosphatidylinositol 4-phosphate [PI(4)P] and regulates Golgi architecture and vesicle trafficking. GOLPH3 overexpression has been correlated with poor prognosis in several cancers, but the molecular mechanisms that link GOLPH3 to malignant transformation are poorly understood. We recently showed that PI(4)P-GOLPH3 couples membrane trafficking with contractile ring assembly during cytokinesis in dividing Drosophila spermatocytes. Here, we use affinity purification coupled with mass spectrometry (AP-MS) to identify the protein-protein interaction network (interactome) of Drosophila GOLPH3 in testes. Analysis of the GOLPH3 interactome revealed enrichment for proteins involved in vesicle-mediated trafficking, cell proliferation and cytoskeleton dynamics. In particular, we found that dGOLPH3 interacts with the Drosophila orthologs of Fragile X mental retardation protein and Ataxin-2, suggesting a potential role in the pathophysiology of disorders of the nervous system. Our findings suggest novel molecular targets associated with GOLPH3 that might be relevant for therapeutic intervention in cancers and other human diseases.

Identification of a mitogen-activated protein kinase kinase (AccMKK4) from Apis cerana cerana and its involvement in various stress responses

The mitogen-activated protein kinase (MAPK) cascade pathway is a ubiquitous signal transduction pathway in eukaryotes that regulates a variety of immune responses. This study accomplished the first isolation of an AccMKK4 gene from Apis cerana cerana and explored its function. Yeast two-hybrid experiments proved that AccMKK4 can interact with Accp38b, and the silencing of AccMKK4 in honeybees downregulated the expression level of Accp38b, which suggests that AccMKK4 might participate in the oxidative stress response through the p38 MAPK pathway. Tissue-specific expression levels of AccMKK4 analysis showed that AccMKK4 in the thorax, particularly muscle tissue, was higher than that in other tissues. The qRT-PCR results from different conditions demonstrated that AccMKK4 responds to various environmental stresses. After AccMKK4 silencing, the transcription level of some antioxidant genes and the activity of antioxidant-related enzymes are reduced, which indicated that AccMKK4 plays an important role in resistance against oxidative stress caused by external stimuli. In summary, our findings indicate that AccMKK4 probably plays an indispensable role in the response of honeybees to environmental stress and might aid for further research on the role of the MAPK cascade pathway in the antioxidant defence mechanisms of insects.

Genetic interactions regulate hypoxia tolerance conferred by activating Notch in excitatory amino acid transporter 1-positive glial cells in Drosophila melanogaster

Hypoxia is a critical pathological element in many human diseases, including ischemic stroke, myocardial infarction, and solid tumors. Of particular significance and interest of ours are the cellular and molecular mechanisms that underlie susceptibility or tolerance to low O2. Previous studies have demonstrated that Notch signaling pathway regulates hypoxia tolerance in both Drosophila melanogaster and humans. However, the mechanisms mediating Notch-conferred hypoxia tolerance are largely unknown. In this study, we delineate the evolutionarily conserved mechanisms underlying this hypoxia tolerant phenotype. We determined the role of a group of conserved genes that were obtained from a comparative genomic analysis of hypoxia-tolerant D.melanogaster populations and human highlanders living at the high-altitude regions of the world (Tibetans, Ethiopians, and Andeans). We developed a novel dual-UAS/Gal4 system that allows us to activate Notch signaling in the Eaat1-positive glial cells, which remarkably enhances hypoxia tolerance in D.melanogaster, and, simultaneously, knock down a candidate gene in the same set of glial cells. Using this system, we discovered that the interactions between Notch signaling and bnl (fibroblast growth factor), croc (forkhead transcription factor C), or Mkk4 (mitogen-activated protein kinase kinase 4) are important for hypoxia tolerance, at least in part, through regulating neuronal development and survival under hypoxic conditions. Becausethese genetic mechanisms are evolutionarily conserved, this group of genes may serve as novel targets for developing therapeutic strategies and have a strong potential to be translated to humans to treat/prevent hypoxia-related diseases.

Molecular mechanisms of FasL-mediated 'reverse-signaling'

Effector lymphocytes, including NK and T cells, express FasL. Expression of Fas, the receptor for FasL in tumor cells, renders them susceptible to NK and T cell-mediated killing. The functional relevance of FasL in initiating death signals in tumor cells is well-characterized. However, the cytoplasmic interacting partners and the potential signaling pathways downstream of FasL are far from fully defined. FasL possesses an 81 amino acid long cytoplasmic tail with multiple unique recruitment motifs. We predict multiple interdependent signaling complexes form the core of the 'reverse signaling' downstream of FasL. A direct interaction between the proline-rich domain of FasL and the SH3 domain of PI(3)K-p85α initiates the first pathway. This cascade helps FasL to link to PLC-γ2 via PIP₃ or the Akt-dependent activation of mTOR complexes. Independently, a GRB2/GADs-binding PXXP cytoplasmic motif of FasL can initiate a Ras-GTP-dependent PAK1→C-Raf→MEK1/2→ERK1/2 activation. FasL can recruit Fyn via the proline-rich domain leading to the recruitment of ADAP. Through its ability to directly interact with Carma1 and TAK1, ADAP initiates the formation of the Carma1/Bcl10/Malt1-based CBM signalosome that is primarily responsible for inflammatory cytokine production. Here, we explore the conserved cytoplasmic domains of FasL, the potential signaling molecules that interact, and the functional downstream consequences within the effector lymphocytes to define the FasL-mediated 'reverse signaling'.

JNK pathway plays a key role in the immune system of the pea aphid and is regulated by microRNA-184

Different from holometabolous insects, the hemipteran species such as pea aphid Acyrthosiphon pisum exhibit reduced immune responses with the absence of the genes coding for antimicrobial peptide (AMP), immune deficiency (IMD), peptidoglycan recognition proteins (PGRPs), and other immune-related molecules. Prior studies have proved that phenoloxidase (PO)-mediated melanization, hemocyte-mediated phagocytosis, and reactive oxygen species (ROS) participate in pea aphid defense against bacterial infection. Also, the conserved signaling, Jun N-terminal kinase (JNK) pathway, has been suggested to be involved in pea aphid immune defense. However, the precise role of the JNK signaling, its interplay with other immune responses and its regulation in pea aphid are largely unknown. In this study, using in vitro biochemical assays and in vivo bioassays, we demonstrated that the JNK pathway regulated hemolymph PO activity, hydrogen peroxide concentration and hemocyte phagocytosis in bacteria infected pea aphids, suggesting that the JNK pathway plays a central role in regulating immune responses in pea aphid. We further revealed the JNK pathway is regulated by microRNA-184 in response to bacterial infection. It is possible that in common the JNK pathway plays a key role in immune system of hemipteran insects and microRNA-184 regulates the JNK pathway in animals.

Activated Ras/JNK driven Dilp8 in imaginal discs adversely affects organismal homeostasis during early pupal stage in Drosophila, a new checkpoint for development

BACKGROUND

Dilp8-mediated inhibition of ecdysone synthesis and pupation in holometabolous insects maintains developmental homeostasis through stringent control of timing and strength of molting signals. We examined reasons for normal pupation but early pupal death observed in certain cases.

RESULTS

Overexpression of activated Ras in developing eye/wing discs inhibited Ptth expression in brain via upregulated JNK signaling mediated Dilp8 secretion from imaginal discs, which inhibited ecdysone synthesis in prothoracic gland after pupariation, leading to death of ~25- to 30-hour-old pupae. Inhibition of elevated Ras signaling completely rescued early pupal death while post-pupation administration of ecdysone to organisms with elevated Ras signaling in eye discs partially rescued their early pupal death. Unlike the earlier known Dilp8 action in delaying pupation, hyperactivated Ras mediated elevation of pJNK signaling in imaginal discs caused Dilp8 secretion after pupariation. Ectopic expression of certain other transgene causing pupal lethality similarly enhanced pJNK and early pupal Dilp8 levels. Suboptimal ecdysone levels after 8 hours of pupation prevented the early pupal metamorphic changes and caused organismal death.

CONCLUSIONS

Our results reveal early pupal stage as a novel Dilp8 mediated post-pupariation checkpoint and provide further evidence for interorgan signaling during development, wherein a peripheral tissue influences the CNS driven endocrine function.

Characterization of TNF-induced cell death in Drosophila reveals caspase- and JNK-dependent necrosis and its role in tumor suppression

Tumor-necrosis factor (TNF) and its superfamily members are pleiotropic cytokines. Activation of TNF can lead to distinct cellular outcomes including inflammation, cell survival, and different forms of cell death, such as apoptosis and necrosis in a context-dependent manner. However, our understanding of what determines the versatile functions of TNF is far from complete. Here, we examined the molecular mechanisms that distinguish the forms of cell death induced by Eiger (Egr), the sole homolog of TNF in Drosophila. We show that expression of Egr in the developing Drosophila eye simultaneously induces apoptosis and apoptosis-independent developmental defects indicated by cellular disorganization, both of which rely on the c-Jun N-terminal kinase (JNK) signaling activity. Intriguingly, when effector caspases DrICE and Dcp-1 are defective or inhibited, expression of Egr triggers necrosis which is characterized by loss of cell membrane integrity, translucent cytoplasm, and aggregation of cellular organelles. Moreover, such Egr-induced necrosis depends on the catalytic activity of the initiator caspase Dronc and the input from JNK signaling but is independent of their roles in apoptosis. Further mosaic analysis with mutants of scribble (scrib), an evolutionarily conserved tumor suppressor gene regulating cell polarity, suggests that Egr/JNK-mediated apoptosis and necrosis establish a two-layered defense system to inhibit the oncogenic growth of scrib mutant cells. Together, we have identified caspase- and JNK-dependent mechanisms underlying Egr-induced apoptosis versus necrosis and their fail-safe roles in tumor suppression in an intact organism in vivo.

The Two NF-κB Pathways Regulating Bacterial and WSSV Infection of Shrimp

The outbreak of diseases ordinarily results from the disruption of the balance and harmony between hosts and pathogens. Devoid of adaptive immunity, shrimp rely largely on the innate immune system to protect themselves from pathogenic infection. Two nuclear factor-κB (NF-κB) pathways, the Toll and immune deficiency (IMD) pathways, are generally regarded as the major regulators of the immune response in shrimp, which have been extensively studied over the years. Bacterial infection can be recognized by Toll and IMD pathways, which activate two NF-κB transcription factors, Dorsal and Relish, respectively, to eventually lead to boosting the expression of various antimicrobial peptides (AMPs). In response to white-spot-syndrome-virus (WSSV) infection, these two pathways appear to be subverted and hijacked to favor viral survival. In this review, the recent progress in elucidating microbial recognition, signal transduction, and effector regulation within both shrimp Toll and IMD pathways will be discussed. We will also highlight and discuss the similarities and differences between shrimps and their Drosophila or mammalian counterparts. Understanding the interplay between pathogens and shrimp NF-κB pathways may provide new opportunities for disease-prevention strategies in the future.

Lic regulates JNK-mediated cell death in Drosophila

Objectives

The evolutionary conserved JNK pathway plays crucial role in cell death, yet factors that modulate this signalling have not been fully disclosed. In this study, we aim to identify additional factors that regulate JNK signalling in cell death, and characterize the underlying mechanisms.

Materials and Methods

Drosophila were raised on standard media, and cross was carried out at 25°C. The Gal4/UAS system was used to express proteins or RNAi in a specific temporal and spatial pattern. Gene expression was revealed by GFP fluorescence, X‐gal staining or immunostaining of 3rd instar larval eye and wing discs. Cell death was visualized by acridine orange (AO) staining. Images of fly eyes and wings were taken by OLYMPUS microscopes.

Results

We found that licorne (lic) encoding the Drosophila MKK3 is an essential regulator of JNK‐mediated cell death. Firstly, loss of lic suppressed ectopic Egr‐triggered JNK activation and cell death in eye and wing development. Secondary, lic is necessary for loss‐of‐cell polarity‐induced, physiological JNK‐dependent cell death in wing development. Thirdly, Lic overexpression is sufficient to initiate JNK‐mediated cell death in developing eyes and wings. Furthermore, ectopic Lic activates JNK signalling by promoting JNK phosphorylation. Finally, genetic epistatic analysis confirmed that Lic acts in parallel with Hep in the Egr‐JNK pathway.

Conclusions

This study not only identified Lic as a novel component of the JNK signalling, but also disclosed the crucial roles and mechanism of Lic in cell death.

[Expressions of TAK1 and TAB1 in esophageal cancer and their correlation with prognosis]

OBJECTIVE

To detect the expressions of transforming growth factor-β (TGF-β)-activated kinase (TAK1) and TGF-β- activated protein kinase 1 (TAB1) in esophageal cancer tissues and explore their correlations with the clinicopathological features and prognosis of the patients.

METHODS

The expressions of TAK1 and TAB1 in 84 esophageal cancer tissues and paired adjacent tissues was detected using immunohistochemical staining. The correlations of different patterns of TAK1 and TAB1 expressions (TAK1 alone, TAB1 alone, and both) with the clinicopathological features of the patients were analyzed. The correlation between TAK1 and TAB1 was assessed based on GEPIA datasets. Kaplan-Meier survival analysis was used to analyze the recurrence-free survival of the patients in relation with TAK1 and TAB1 expressions.

RESULTS

TAK1 and TAB1 were highly expressed in 65.5% (55/84) and 52.4% (44/84) of the esophageal cancer tissues, respectively. The expression of TAK1, TAB1 and their co-expression were all correlated with tumor invasion depth, lymph node metastasis, and TNM staging (P < 0.05). A strong correlation was found between TAK1 and TAB1 expressions. A high expression of TAK1 and TAK1/TAB1 co-expression both predicted a poor recurrence-freed survival of the patients (P < 0.05).

CONCLUSIONS

TAK1 and TAB1 are associated with the progression and prognosis of esophageal cancer and can serve as new prognostic biomarkers for esophageal cancer and as potential molecular targets for therapies.

Myc inhibits JNK-mediated cell death in vivo

The proto-oncogene Myc is well known for its roles in promoting cell growth, proliferation and apoptosis. However, in this study, we found from a genetic screen that Myc inhibits, rather than promotes, cell death triggered by c-Jun N-terminal kinase (JNK) signaling in Drosophila. Firstly, expression of Drosophila Myc (dMyc) suppresses, whereas loss of dMyc enhances, ectopically activated JNK signaling-induced cell death. Secondly, dMyc impedes physiologically activated JNK pathway-mediated cell death. Thirdly, loss of dMyc triggers JNK pathway activation and JNK-dependent cell death. Finally, the mammalian cMyc gene, when expressed in Drosophila, impedes activated JNK signaling-induced cell death. Thus, besides its well-studied apoptosis promoting function, Myc also antagonizes JNK-mediated cell death in Drosophila, and this function is likely conserved from fly to human.

Metastasis suppressors: functional pathways

Metastasis is a complex process and a major contributor of death in cancer patients. Metastasis suppressor genes are identified by their ability to inhibit metastasis at a secondary site without affecting the growth of primary tumor. In this review, we have conducted a survey of the metastasis suppressor literature to identify common downstream pathways. The metastasis suppressor genes mechanistically target MAPK, G-protein-coupled receptor, cell adhesion, cytoskeletal, transcriptional regulatory, and metastasis susceptibility pathways. The majority of the metastasis suppressor genes are functionally multifactorial, inhibiting metastasis at multiple points in the cascade, and many operate in a context-dependent fashion. A greater understanding of common pathways/molecules targeted by metastasis suppressor could improve metastasis treatment strategies.

Metastasis suppressors: functional pathways

Metastasis is a complex process and a major contributor of death in cancer patients. Metastasis suppressor genes are identified by their ability to inhibit metastasis at a secondary site without affecting the growth of primary tumor. In this review, we have conducted a survey of the metastasis suppressor literature to identify common downstream pathways. The metastasis suppressor genes mechanistically target MAPK, G-protein-coupled receptor, cell adhesion, cytoskeletal, transcriptional regulatory, and metastasis susceptibility pathways. The majority of the metastasis suppressor genes are functionally multifactorial, inhibiting metastasis at multiple points in the cascade, and many operate in a context-dependent fashion. A greater understanding of common pathways/molecules targeted by metastasis suppressor could improve metastasis treatment strategies.

The equilibrium between antagonistic signaling pathways determines the number of synapses in Drosophila

The number of synapses is a major determinant of behavior and many neural diseases exhibit deviations in that number. However, how signaling pathways control this number is still poorly understood. Using the Drosophila larval neuromuscular junction, we show here a PI3K-dependent pathway for synaptogenesis which is functionally connected with other previously known elements including the Wit receptor, its ligand Gbb, and the MAPkinases cascade. Based on epistasis assays, we determined the functional hierarchy within the pathway. Wit seems to trigger signaling through PI3K, and Ras85D also contributes to the initiation of synaptogenesis. However, contrary to other signaling pathways, PI3K does not require Ras85D binding in the context of synaptogenesis. In addition to the MAPK cascade, Bsk/JNK undergoes regulation by Puc and Ras85D which results in a narrow range of activity of this kinase to determine normalcy of synapse number. The transcriptional readout of the synaptogenesis pathway involves the Fos/Jun complex and the repressor Cic. In addition, we identified an antagonistic pathway that uses the transcription factors Mad and Medea and the microRNA bantam to down-regulate key elements of the pro-synaptogenesis pathway. Like its counterpart, the anti-synaptogenesis signaling uses small GTPases and MAPKs including Ras64B, Ras-like-a, p38a and Licorne. Bantam downregulates the pro-synaptogenesis factors PI3K, Hiw, Ras85D and Bsk, but not AKT. AKT, however, can suppress Mad which, in conjunction with the reported suppression of Mad by Hiw, closes the mutual regulation between both pathways. Thus, the number of synapses seems to result from the balanced output from these two pathways.

Annotation of the Asian Citrus Psyllid Genome Reveals a Reduced Innate Immune System

Citrus production worldwide is currently facing significant losses due to citrus greening disease, also known as Huanglongbing. The citrus greening bacteria, Candidatus Liberibacter asiaticus (CLas), is a persistent propagative pathogen transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). Hemipterans characterized to date lack a number of insect immune genes, including those associated with the Imd pathway targeting Gram-negative bacteria. The D. citri draft genome was used to characterize the immune defense genes present in D. citri. Predicted mRNAs identified by screening the published D. citri annotated draft genome were manually searched using a custom database of immune genes from previously annotated insect genomes. Toll and JAK/STAT pathways, general defense genes Dual oxidase, Nitric oxide synthase, prophenoloxidase, and cellular immune defense genes were present in D. citri. In contrast, D. citri lacked genes for the Imd pathway, most antimicrobial peptides, 1,3-β-glucan recognition proteins (GNBPs), and complete peptidoglycan recognition proteins. These data suggest that D. citri has a reduced immune capability similar to that observed in A. pisum, P. humanus, and R. prolixus. The absence of immune system genes from the D. citri genome may facilitate CLas infections, and is possibly compensated for by their relationship with their microbial endosymbionts.

Diminished MTORC1-Dependent JNK Activation Underlies the Neurodevelopmental Defects Associated with Lysosomal Dysfunction

Here, we evaluate the mechanisms underlying the neurodevelopmental deficits in Drosophila and mouse models of lysosomal storage diseases (LSDs). We find that lysosomes promote the growth of neuromuscular junctions (NMJs) via Rag GTPases and mechanistic target of rapamycin complex 1 (MTORC1). However, rather than employing S6K/4E-BP1, MTORC1 stimulates NMJ growth via JNK, a determinant of axonal growth in Drosophila and mammals. This role of lysosomal function in regulating JNK phosphorylation is conserved in mammals. Despite requiring the amino-acid-responsive kinase MTORC1, NMJ development is insensitive to dietary protein. We attribute this paradox to anaplastic lymphoma kinase (ALK), which restricts neuronal amino acid uptake, and the administration of an ALK inhibitor couples NMJ development to dietary protein. Our findings provide an explanation for the neurodevelopmental deficits in LSDs and suggest an actionable target for treatment.

The immune signaling pathways of Manduca sexta

Signal transduction pathways and their coordination are critically important for proper functioning of animal immune systems. Our knowledge of the constituents of the intracellular signaling network in insects mainly comes from genetic analyses in Drosophila melanogaster. To facilitate future studies of similar systems in the tobacco hornworm and other lepidopteran insects, we have identified and examined the homologous genes in the genome of Manduca sexta. Based on 1:1 orthologous relationships in most cases, we hypothesize that the Toll, Imd, MAPK-JNK-p38 and JAK-STAT pathways are intact and operative in this species, as are most of the regulatory mechanisms. Similarly, cellular processes such as autophagy, apoptosis and RNA interference probably function in similar ways, because their mediators and modulators are mostly conserved in this lepidopteran species. We have annotated a total of 186 genes encoding 199 proteins, studied their domain structures and evolution, and examined their mRNA levels in tissues at different life stages. Such information provides a genomic perspective of the intricate signaling system in a non-drosophiline insect.

Independent Proteolytic Activities Control the Stability and Size of Drosophila Inhibitor of Apoptosis 2 Protein

The Drosophila immune deficiency pathway defends many bacterial pathogens and bears striking molecular similarities to the mammalian tumor necrosis factor signal transduction pathway. Orthologous inhibitors of apoptosis ubiquitin ligases act at a proximal stage of both responses to coordinate the assembly of signal transduction platforms that shape host immune responses. Despite the importance of inhibitor of apoptosis proteins within evolutionarily conserved innate immune responses, we know relatively little about the cellular machinery that controls inhibitor of apoptosis activity. In this study, we examined the molecular basis for inhibitor of apoptosis 2 protein regulation in the immune deficiency pathway. Our studies identified two distinct proteolytic events that determine the stability and composition of cellular inhibitor of apoptosis 2 protein pools. We found that apoptotic caspase activity cleaves inhibitor of apoptosis 2 at an N-terminal aspartate to generate a truncated protein that retains the ability to interact with immune deficiency pathway members. We also showed that a C-terminal ubiquitin ligase activity within inhibitor of apoptosis 2 directs the proteasomal destruction of full-length and truncated inhibitor of apoptosis 2 isoforms. These studies add to our appreciation of the regulation of innate immunity and suggest potential links between apoptotic caspases and innate defenses.

Rho1-Wnd signaling regulates loss-of-cell polarity-induced cell invasion in Drosophila

Both cell polarity and c-Jun N-terminal kinase (JNK) activity are essential to the maintenance of tissue homeostasis, and disruption of either is commonly seen in cancer progression. Despite the established connection between loss-of-cell polarity and JNK activation, much less is known about the molecular mechanism by which aberrant cell polarity induces JNK-mediated cell migration and tumor invasion. Here we show results from a genetic screen using an in vivo invasion model via knocking down cell polarity gene in Drosophila wing discs, and identify Rho1-Wnd signaling as an important molecular link that mediates loss-of-cell polarity-triggered JNK activation and cell invasion. We show that Wallenda (Wnd), a protein kinase of the mitogen-activated protein kinase kinase kinase family, by forming a complex with the GTPase Rho1, is both necessary and sufficient for Rho1-induced JNK-dependent cell invasion, MMP1 activation and epithelial-mesenchymal transition. Furthermore, Wnd promotes cell proliferation and tissue growth through wingless production when apoptosis is inhibited by p35. Finally, Wnd shows oncogenic cooperation with Ras(V12) to trigger tumor growth in eye discs and causes invasion into the ventral nerve cord. Together, our data not only provides a novel mechanistic insight on how cell polarity loss contributes to cell invasion, but also highlights the value of the Drosophila model system to explore human cancer biology.

Wallenda regulates JNK-mediated cell death in Drosophila

The c-Jun N-terminal kinase (JNK) pathway plays essential roles in regulating a variety of cellular processes including proliferation, migration and survival. Previous genetic studies in Drosophila have identified numerous cell death regulating genes, providing new insights into the mechanisms for related diseases. Despite the known role of the small GTPase Rac1 in regulating cell death, the downstream components and underlying mechanism remain largely elusive. Here, we show that Rac1 promotes JNK-dependent cell death through Wallenda (Wnd). In addition, we find that Wnd triggers JNK activation and cell death via its kinase domain. Moreover, we show that both MKK4 and Hep are critical for Wnd-induced cell death. Furthermore, Wnd is essential for ectopic Egr- or Rho1-induced JNK activation and cell death. Finally, Wnd is physiologically required for loss of scribble-induced JNK-dependent cell death. Thus, our data suggest that wnd encodes a novel essential cell death regulator in Drosophila.

Shrinkage of genome size in a plant RNA virus upon transfer of an essential viral gene into the host genome

Nonretroviral integrated RNA viruses (NIRVs) are genes of nonretroviral RNA viruses found in the genomes of many eukaryotic organisms. NIRVs are thought to sometimes confer virus resistance, meaning that they could impact spread of the virus in the host population. However, a NIRV that is expressed may also impact the evolution of virus populations within host organisms. Here, we experimentally addressed the evolution of a virus in a host expressing a NIRV using Tobacco etch virus (TEV), a plant RNA virus, and transgenic tobacco plants expressing its replicase, NIb. We found that a virus missing the NIb gene, TEV-ΔNIb, which is incapable of autonomous replication in wild-type plants, had a higher fitness than the full-length TEV in the transgenic plants. Moreover, when the full-length TEV was evolved by serial passages in transgenic plants, we observed genomic deletions within NIb--and in some cases the adjacent cistrons--starting from the first passage. When we passaged TEV and TEV-ΔNIb in transgenic plants, we found mutations in proteolytic sites, but these only occurred in TEV-ΔNIb lineages, suggesting the adaptation of polyprotein processing to altered NIb expression. These results raise the possibility that NIRV expression can indeed induce the deletion of the corresponding genes in the viral genome, resulting in the formation of viruses that are replication defective in hosts that do not express the same NIRV. Moreover, virus genome evolution was contingent upon the deletion of the viral replicase, suggesting NIRV expression could also alter patterns of virus evolution.

Genetic models of apoptosis-induced proliferation decipher activation of JNK and identify a requirement of EGFR signaling for tissue regenerative responses in Drosophila

Recent work in several model organisms has revealed that apoptotic cells are able to stimulate neighboring surviving cells to undergo additional proliferation, a phenomenon termed apoptosis-induced proliferation. This process depends critically on apoptotic caspases such as Dronc, the Caspase-9 ortholog in Drosophila, and may have important implications for tumorigenesis. While it is known that Dronc can induce the activity of Jun N-terminal kinase (JNK) for apoptosis-induced proliferation, the mechanistic details of this activation are largely unknown. It is also controversial if JNK activity occurs in dying or in surviving cells. Signaling molecules of the Wnt and BMP families have been implicated in apoptosis-induced proliferation, but it is unclear if they are the only ones. To address these questions, we have developed an efficient assay for screening and identification of genes that regulate or mediate apoptosis-induced proliferation. We have identified a subset of genes acting upstream of JNK activity including Rho1. We also demonstrate that JNK activation occurs both in apoptotic cells as well as in neighboring surviving cells. In a genetic screen, we identified signaling by the EGFR pathway as important for apoptosis-induced proliferation acting downstream of JNK signaling. These data underscore the importance of genetic screening and promise an improved understanding of the mechanisms of apoptosis-induced proliferation.

Work in recent years has revealed that apoptotic caspases not only induce apoptosis, but also have non-apoptotic functions. One of these functions is apoptosis-induced proliferation, a relatively recently discovered phenomenon by which apoptotic cells induce proliferation of surviving neighboring cells. This phenomenon may have important implications for stem cell activity, tissue regeneration and tumorigenesis. Here, we describe the development of a genetic model of apoptosis-induced proliferation and the use of this model for convenient and unbiased genetic screening to identify genes involved in the process. We tested mutants of our RNAi transgenic lines targeting the core components of the apoptotic pathway and of JNK signaling, a known mediator of apoptosis-induced proliferation. These assays demonstrate the feasibility of the system for systematic genetic screening and identified several new genes upstream of JNK that are involved in apoptosis-induced proliferation. Finally, we tested the model in a pilot screen for chromosome arm 2L and identified spi, the EGF ligand in flies, as important for apoptosis-induced proliferation. We confirmed the involvement of EGF in a genuine apoptosis-induced regeneration system. These data underscore the importance of genetic screening and promise an improved understanding of the mechanisms of apoptosis-induced proliferation and regeneration.

Domain specificity of MAP3K family members, MLK and Tak1, for JNK signaling in Drosophila

A highly diverse set of protein kinases functions as early responders in the mitogen- and stress-activated protein kinase (MAPK/SAPK) signaling pathways. For instance, humans possess 14 MAPK kinase kinases (MAP3Ks) that activate Jun kinase (JNK) signaling downstream. A major challenge is to decipher the selective and redundant functions of these upstream MAP3Ks. Taking advantage of the relative simplicity of Drosophila melanogaster as a model system, we assessed MAP3K signaling specificity in several JNK-dependent processes during development and stress response. Our approach was to generate molecular chimeras between two MAP3K family members, the mixed lineage kinase, Slpr, and the TGF-β activated kinase, Tak1, which share 32% amino acid identity across the kinase domain but otherwise differ in sequence and domain structure, and then test the contributions of various domains for protein localization, complementation of mutants, and activation of signaling. We found that overexpression of the wild-type kinases stimulated JNK signaling in alternate contexts, so cells were capable of responding to both MAP3Ks, but with distinct outcomes. Relative to wild-type, the catalytic domain swaps compensated weakly or not at all, despite having a shared substrate, the JNK kinase Hep. Tak1 C-terminal domain-containing constructs were inhibitory in Tak1 signaling contexts, including tumor necrosis factor-dependent cell death and innate immune signaling; however, depressing antimicrobial gene expression did not necessarily cause phenotypic susceptibility to infection. These same constructs were neutral in the context of Slpr-dependent developmental signaling, reflecting differential subcellular protein localization and by inference, point of activation. Altogether, our findings suggest that the selective deployment of a particular MAP3K can be attributed in part to its inherent sequence differences, cellular localization, and binding partner availability.

JNK signaling is needed to tolerate chromosomal instability

Chromosomal instability (CIN), as a common feature of tumors, represents a potential therapeutic target if ways can be found to specifically cause apoptosis in unstably dividing cells. We have previously shown that if signaling through the JNK pathway is reduced, apoptosis is triggered in models of chromosomal instability induced by loss of the spindle checkpoint. Here we identify components upstream and downstream of JNK that are able to mediate this effect, and test the involvement of p53 and DNA damage in causing apoptosis when JNK signaling is reduced in CIN cells. We show that cell cycle progression timing has a strong effect on the apoptosis seen when JNK signaling is reduced in genetically unstable cells: a shortened G 2 phase enhances the apoptosis, while lengthening G 2 rescues the JNK-deficient CIN cell death phenotype. Our findings suggest that chromosomal instability represents a significant stress to dividing cells, and that without JNK signaling, cells undergo apoptosis because they lack a timely and effective response to DNA damage.

Apoptotic cells can induce non-autonomous apoptosis through the TNF pathway

Apoptotic cells can produce signals to instruct cells in their local environment, including ones that stimulate engulfment and proliferation. We identified a novel mode of communication by which apoptotic cells induce additional apoptosis in the same tissue. Strong induction of apoptosis in one compartment of the Drosophila wing disc causes apoptosis of cells in the other compartment, indicating that dying cells can release long-range death factors. We identified Eiger, the Drosophila tumor necrosis factor (TNF) homolog, as the signal responsible for apoptosis-induced apoptosis (AiA). Eiger is produced in apoptotic cells and, through activation of the c-Jun N-terminal kinase (JNK) pathway, is able to propagate the initial apoptotic stimulus. We also show that during coordinated cell death of hair follicle cells in mice, TNF-α is expressed in apoptotic cells and is required for normal cell death. AiA provides a mechanism to explain cohort behavior of dying cells that is seen both in normal development and under pathological conditions.

DOI:http://dx.doi.org/10.7554/eLife.01004.001

The tissues of developing organisms can be shaped by apoptosis, a form of regulated cell killing. Although this process can occur in individual cells, apoptotic signals may also dictate the ‘communal death’ of many cells simultaneously. This occurs frequently in animal development: in human fetuses, for example, cells in the hand are directed to die to remove webbing between the fingers.

Apoptosis has been thought to resemble a form of silent suicide by cells, but more recent work suggests that apoptotic cells can also transmit signals. Now, Pérez-Garijo et al. find that these cells can stimulate other cells to die in both fruit flies and mice.

In fruit flies, apoptosis is activated by proteins known as Grim, Hid and Reaper. To explore whether apoptotic cells could communicate with other cells, Pérez-Garijo et al. created ‘undead’ cells in which one of these proteins was turned on, but other downstream proteins (that are responsible for the cellular execution phase of apoptosis) had been turned off: these cells were undergoing apoptosis, but could not complete the process and die.

Strikingly, undead cells in the posterior (back) region of the wing imaginal disc—the tissue in the larva that gives rise to the wing in the adult fruit fly—could trigger apoptosis in cells in the anterior (front) half. Pérez-Garijo et al. found that the JNK pathway activated apoptosis in anterior cells. In fruit flies, the Eiger protein turns on this pathway; when Eiger was absent from posterior cells in the wing imaginal disc, apoptosis in anterior cells ceased, indicating that Eiger might signal at long range.

Eiger is related to a protein called TNF that has been implicated in cycles of destruction and renewal of hair follicles in mice. Pérez-Garijo et al. found that TNF is produced by apoptotic cells in hair follicles, and that blocking TNF inhibits the death of other cells in the same cohort: this suggests that a common mechanism could regulate the communal death of cells in flies and mammals. These studies therefore shed light on a conserved pathway in the modulation of tissue development.

DOI:http://dx.doi.org/10.7554/eLife.01004.002

The JNK pathway is a key mediator of Anopheles gambiae antiplasmodial immunity

The innate immune system of Anopheles gambiae mosquitoes limits Plasmodium infection through multiple molecular mechanisms. For example, midgut invasion by the parasite triggers an epithelial nitration response that promotes activation of the complement-like system. We found that suppression of the JNK pathway, by silencing either Hep, JNK, Jun or Fos expression, greatly enhanced Plasmodium infection; while overactivating this cascade, by silencing the suppressor Puckered, had the opposite effect. The JNK pathway limits infection via two coordinated responses. It induces the expression of two enzymes (HPx2 and NOX5) that potentiate midgut epithelial nitration in response to Plasmodium infection and regulates expression of two key hemocyte-derived immune effectors (TEP1 and FBN9). Furthermore, the An. gambiae L3–5 strain that has been genetically selected to be refractory (R) to Plasmodium infection exhibits constitutive overexpression of genes from the JNK pathway, as well as midgut and hemocyte effector genes. Silencing experiments confirmed that this cascade mediates, to a large extent, the drastic parasite elimination phenotype characteristic of this mosquito strain. In sum, these studies revealed the JNK pathway as a key regulator of the ability of An. gambiae mosquitoes to limit Plasmodium infection and identified several effector genes mediating these responses.

The mosquito Anopheles gambiae is a major vector of human malaria, a disease caused by Plasmodium falciparum parasites that results in more than half a million deaths each year. Several signaling pathways in the mosquito have been shown to mediate the mosquito immune responses to Plasmodium infection. In this manuscript we investigated the participation of the Jun-N-terminal kinase (JNK) pathway in mosquito defense responses. We found that JNK signaling is required for mosquito midgut cells to induce expression of two enzymes, HPx2 and NOX5, that mediate epithelial nitration in response to parasite invasion. These reactions modify the parasites and promote activation of the mosquito complement-like system that results in parasite lysis. The JNK pathway also regulates the basal level of expression of TEP1 and FBN9, two key components of the complement-like system that are produced by hemocytes and secreted into the mosquito hemolymph. Our studies revealed that JNK signaling plays a key role for mosquitoes to limit Plasmodium infection, making it an important determinant of malaria transmission to humans.

The role of the IAP E3 ubiquitin ligases in regulating pattern-recognition receptor signalling

An inflammatory response is initiated when innate immune pattern-recognition receptors (PRRs) expressed by different cell types detect constituents of invading microorganisms and endogenous intracellular molecules released by dying cells. The intracellular cascades activated by PRRs induce the expression and maturation of inflammatory molecules that coordinate the removal of the infectious agents and of the infected or damaged cells. In this Review, we discuss the findings implicating members of the inhibitor of apoptosis protein (IAP) family in the ubiquitylation-dependent regulation of PRR signalling. Understanding the role of IAPs in innate immunity may open new therapeutic perspectives for the treatment of PRR-dependent inflammatory diseases.

Sin3a acts through a multi-gene module to regulate invasion in Drosophila and human tumors

Chromatin remodeling proteins regulate multiple aspects of cell homeostasis, making them ideal candidates for misregulation in transformed cells. Here, we explore Sin3A, a member of the Sin3 family of proteins linked to tumorigenesis that are thought to regulate gene expression through their role as histone deacetylases (HDACs). We identified Drosophila Sin3a as an important mediator of oncogenic Ret receptor in a fly model of Multiple Endocrine Neoplasia Type 2. Reducing Drosophila Sin3a activity led to metastasis-like behavior and, in the presence of Diap1, secondary tumors distant from the site of origin. Genetic and Chip-Seq analyses identified previously undescribed Sin3a targets including genes involved in cell motility and actin dynamics, as well as signaling pathways including Src, Jnk and Rho. A key Sin3a oncogenic target, PP1B, regulates stability of β-Catenin/Armadillo: the outcome is to oppose T-cell factor (TCF) function and Wg/Wnt pathway signaling in both fly and mammalian cancer cells. Reducing Sin3A strongly increased the invasive behavior of A549 human lung adenocarcinoma cells. We show that Sin3A is downregulated in a variety of human tumors and that Src, JNK, RhoA and PP1B/β-Catenin are regulated in a manner analogous to our Drosophila models. Our data suggest that Sin3A influences a specific step of tumorigenesis by regulating a module of genes involved in cell invasion. Tumor progression may commonly rely on such 'modules of invasion' under the control of broad transcriptional regulators.

Drosophila heat shock response requires the JNK pathway and phosphorylation of mixed lineage kinase at a conserved serine-proline motif

Defining context specific requirements for proteins and pathways is a major challenge in the study of signal transduction. For example, the stress-activated protein kinase (SAPK) pathways are comprised of families of closely related transducers that are activated in a variety of tissues and contexts during development and organismal homeostasis. Consequently, redundant and pleiotropic effects have hampered a complete understanding of the individual contributions of transducers in distinct contexts. Here, we report on the function of a context-specific regulatory phosphorylation site, PXSP, in the Drosophila mixed lineage kinase protein, Slpr, a mitogen-activated protein kinase kinase kinase (MAP3K) in the Jun Kinase (JNK) pathway. Genetic analysis of the function of non-phosphorylatable (PXAP) and phosphomimetic mutant (PXEP) Slpr transgenes in several distinct contexts revealed minimal effects in JNK-dependent tissue closure processes but differential requirements in heat stress response. In particular, PXAP expression resulted in sensitivity of adults to sustained heat shock, like p38 and JNK pathway mutants. In contrast, PXEP overexpression conferred some resistance. Indeed, phosphorylation of the PXSP motif is enriched under heat shock conditions and requires in part, the p38 kinases for the enrichment. These data suggest that coordination of signaling between p38 and Slpr serves to maintain JNK signaling during heat stress. In sum, we demonstrate a novel role for JNK signaling in the heat shock response in flies and identify a posttranslational modification on Slpr, at a conserved site among MAP3K mixed lineage kinase family members, which bolsters stress resistance with negligible effects on JNK-dependent developmental processes.

Ubiquitylation of the initiator caspase DREDD is required for innate immune signalling

Caspases have been extensively studied as critical initiators and executioners of cell death pathways. However, caspases also take part in non-apoptotic signalling events such as the regulation of innate immunity and activation of nuclear factor-κB (NF-κB). How caspases are activated under these conditions and process a selective set of substrates to allow NF-κB signalling without killing the cell remains largely unknown. Here, we show that stimulation of the Drosophila pattern recognition protein PGRP-LCx induces DIAP2-dependent polyubiquitylation of the initiator caspase DREDD. Signal-dependent ubiquitylation of DREDD is required for full processing of IMD, NF-κB/Relish and expression of antimicrobial peptide genes in response to infection with Gram-negative bacteria. Our results identify a mechanism that positively controls NF-κB signalling via ubiquitin-mediated activation of DREDD. The direct involvement of ubiquitylation in caspase activation represents a novel mechanism for non-apoptotic caspase-mediated signalling.

The Drosophila midgut: a model for stem cell driven tissue regeneration

The Drosophila and mammalian digestive systems bear striking similarities in genetic control and cellular composition, and the Drosophila midgut has emerged as an amenable model for dissecting the mechanisms of tissue homeostasis. The Drosophila midgut is maintained by multipotent intestinal stem cells (ISCs) that give rise to all cell types in the intestinal epithelium and are required for long-term tissue homeostasis. ISC proliferation rate increases in response to a myriad of chemical and bacterial insults through the release of JAK-STAT and EGFR ligands from dying enterocytes that activate the JAK-STAT and EGFR pathways in ISCs. The Hippo and JNK pathways converge upon JAK-STAT and EGFR signaling, presumably in response to specific stresses, and JNK and insulin signaling have been shown to be critical in response to age-related stresses. This review details these emerging mechanisms of tissue homeostasis and the proliferative response of ISCs to epithelial damage, environmental stresses, and aging.

The HIV-1 Vpu protein induces apoptosis in Drosophila via activation of JNK signaling

The genome of the human immunodeficiency virus type 1 (HIV-1) encodes the canonical retroviral proteins, as well as additional accessory proteins that enhance the expression of viral genes, the infectivity of the virus and the production of virions. The accessory Viral Protein U (Vpu), in particular, enhances viral particle production, while also promoting apoptosis of HIV-infected human T lymphocytes. Some Vpu effects rely on its interaction with the ubiquitin-proteasome protein degradation system, but the mechanisms responsible for its pro-apoptotic effects in vivo are complex and remain largely to be elucidated.We took advantage of the Drosophila model to study the effects of Vpu activity in vivo. Expression of Vpu in the developing Drosophila wing provoked tissue loss due to caspase-dependent apoptosis. Moreover, Vpu induced expression of the pro-apoptotic gene reaper, known to down-regulate Inhibitor of Apoptosis Proteins (IAPs) which are caspase-antagonizing E3 ubiquitin ligases. Indeed, Vpu also reduced accumulation of Drosophila IAP1 (DIAP1). Though our results demonstrate a physical interaction between Vpu and the proteasome-addressing SLIMB/β-TrCP protein, as in mammals, both SLIMB/βTrCP-dependent and -independent Vpu effects were observed in the Drosophila wing. Lastly, the pro-apoptotic effect of Vpu in this tissue was abrogated upon inactivation of the c-Jun N-terminal Kinase (JNK) pathway. Our results in the fly thus provide the first functional evidence linking Vpu pro-apoptotic effects to activation of the conserved JNK pathway.

The Drosophila melanogaster host model

The deleterious and sometimes fatal outcomes of bacterial infectious diseases are the net result of the interactions between the pathogen and the host, and the genetically tractable fruit fly, Drosophila melanogaster, has emerged as a valuable tool for modeling the pathogen-host interactions of a wide variety of bacteria. These studies have revealed that there is a remarkable conservation of bacterial pathogenesis and host defence mechanisms between higher host organisms and Drosophila. This review presents an in-depth discussion of the Drosophila immune response, the Drosophila killing model, and the use of the model to examine bacterial-host interactions. The recent introduction of the Drosophila model into the oral microbiology field is discussed, specifically the use of the model to examine Porphyromonas gingivalis-host interactions, and finally the potential uses of this powerful model system to further elucidate oral bacterial-host interactions are addressed.

Conserved metabolic energy production pathways govern Eiger/TNF-induced nonapoptotic cell death

Caspase-independent cell death is known to be important in physiological and pathological conditions, but its molecular regulation is not well-understood. Eiger is the sole fly ortholog of TNF. The ectopic expression of Eiger in the developing eye primordium caused JNK-dependent but caspase-independent cell death. To understand the molecular basis of this Eiger-induced nonapoptotic cell death, we performed a large-scale genetic screen in Drosophila for suppressors of the Eiger-induced cell death phenotype. We found that molecules that regulate metabolic energy production are central to this form of cell death: it was dramatically suppressed by decreased levels of molecules that regulate cytosolic glycolysis, mitochondrial β-oxidation of fatty acids, the tricarboxylic acid cycle, and the electron transport chain. Importantly, reducing the expression of energy production-related genes did not affect the cell death triggered by proapoptotic genes, such as reaper, hid, or debcl, indicating that the energy production-related genes have a specific role in Eiger-induced nonapoptotic cell death. We also found that energy production-related genes regulate the Eiger-induced cell death downstream of JNK. In addition, Eiger induced the production of reactive oxygen species in a manner dependent on energy production-related genes. Furthermore, we showed that this cell death machinery is involved in Eiger's physiological function, because decreasing the energy production-related genes suppressed Eiger-dependent tumor suppression, an intrinsic mechanism for removing tumorigenic mutant clones from epithelia by inducing cell death. This result suggests a link between sensitivity to cell death and metabolic activity in cancer.

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.

MK2-dependent p38b signalling protects Drosophila hindgut enterocytes against JNK-induced apoptosis under chronic stress

The integrity of the intestinal epithelium is crucial for the barrier function of the gut. Replenishment of the gut epithelium by intestinal stem cells contributes to gut homeostasis, but how the differentiated enterocytes are protected against stressors is less well understood. Here we use the Drosophila larval hindgut as a model system in which damaged enterocytes are not replaced by stem cell descendants. By performing a thorough genetic analysis, we demonstrate that a signalling complex consisting of p38b and MK2 forms a branch of SAPK signalling that is required in the larval hindgut to prevent stress-dependent damage to the enterocytes. Impaired p38b/MK2 signalling leads to apoptosis of the enterocytes and a subsequent loss of hindgut epithelial integrity, as manifested by the deterioration of the overlaying muscle layer. Damaged hindguts show increased JNK activity, and removing upstream activators of JNK suppresses the loss of hindgut homeostasis. Thus, the p38/MK2 complex ensures homeostasis of the hindgut epithelium by counteracting JNK-mediated apoptosis of the enterocytes upon chronic stress.

The gut epithelium forms the first barrier against pathogens and stressors in the gut lumen, and a loss of this defence function can result in intestinal diseases. Damage in the gut epithelium triggers the proliferation of intestinal stem cells to replenish the epithelium. However, little is known about how the enterocytes are protecting themselves against stressors. We addressed the function of stress-activated protein kinase (SAPK) signal transduction pathways in the larval gut of Drosophila. Our study revealed that a particular module of the p38 SAPK signal cascade is required to protect the larval hindgut epithelium against chronic salt stress. We identified the two kinases, p38b and MK2, as key components of this protective signal. In the absence of p38b or MK2, the stress-inducible JNK cascade is locally upregulated and eventually induces apoptosis. Although the function of the p38b/MK2 module is only required in the enterocytes, the elimination of the affected cells results in atrophy of the overlaying muscle layer and subsequent systemic defects in the larvae (e.g., induction of antimicrobial peptides). We hope that our findings will contribute to a better understanding of early (i.e., pre-inflammatory) events in the development of human intestinal diseases.

Transforming growth factor-beta-activated kinase 1 regulates natural killer cell-mediated cytotoxicity and cytokine production

Carma1, a caspase recruitment domain-containing membrane-associated guanylate kinase, initiates a unique signaling cascade via Bcl10 and Malt1 in NK cells. Carma1 deficiency results in reduced phosphorylation of JNK1/2 and activation of NF-κB that lead to impaired NK cell-mediated cytotoxicity and cytokine production. However, the precise identities of the downstream signaling molecules that link Carma1 to these effector functions were not defined. Here we show that transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) is abundantly present in NK cells, and activation via NKG2D results in its phosphorylation. Lack of Carma1 considerably reduced TAK1 phosphorylation, demonstrating the dependence of TAK1 on Carma1 in NKG2D-mediated NK cell activations. Pharmacological inhibitor to TAK1 significantly reduced NK-mediated cytotoxicity and its potential to generate IFN-γ, GM-CSF, MIP-1α, MIP-1β, and RANTES. Conditional in vivo knockdown of TAK1 in NK cells from Mx1Cre(+)TAK1(fx/fx) mice resulted in impaired NKG2D-mediated cytotoxicity and cytokine/chemokine production. Inhibition or conditional knockdown of TAK1 severely impaired the NKG2D-mediated phosphorylation of ERK1/2 and JNK1/2 and activation of NF-κB and AP1. Our results show that TAK1 links Carma1 to NK cell-mediated effector functions.

The protein Dredd is an essential component of the c-Jun N-terminal kinase pathway in the Drosophila immune response

The Drosophila immune deficiency (IMD) pathway mobilizes c-Jun N-terminal kinase (JNK), caspase, and nuclear factor-κB (NF-κB) modules to counter infection with gram-negative bacteria. Dredd is an essential caspase in the IMD pathway, and it is widely established that NF-κB activation depends on Dredd. More recent cell culture studies suggested a role for Dredd in the activation of dJNK (Drosophila JNK). However, there are no epistatic or mechanistic data on the involvement of Dredd in dJNK activation. More importantly, there is no in vivo evidence to demonstrate a physiological requirement for Dredd in the IMD/dJNK pathway. We performed a comprehensive analysis of the role of Dredd in the IMD/dJNK pathway, and we demonstrated that Dredd is essential for the activation of IMD/dJNK in cell culture. We positioned Dredd activity at an early point of the IMD/dJNK pathway and uncovered a series of interactions between Dredd and additional proximal IMD pathway molecules. Mechanistically, we showed that the caspase activity inhibitor p35 blocked dJNK activation and the induction of dJNK-dependent genes in cell culture and in vivo. Most importantly, we demonstrated that dredd mutant flies are completely inhibited in their ability to activate dJNK or express dJNK-responsive target genes after bacterial infection in vivo. In conclusion, we established Dredd as an essential component of the IMD pathway required for the full activation of IMD/dJNK in cell culture and in vivo. Our data enhance our appreciation of Dredd-dependent IMD signal transduction events.

Drosophila Smt3 negatively regulates JNK signaling through sequestering Hipk in the nucleus

Post-translational modification by the small ubiquitin-related modifier (SUMO) is important for a variety of cellular and developmental processes. However, the precise mechanism(s) that connects sumoylation to specific developmental signaling pathways remains relatively less clear. Here, we show that Smt3 knockdown in Drosophila wing discs causes phenotypes resembling JNK gain of function, including ectopic apoptosis and apoptosis-induced compensatory growth. Smt3 depletion leads to an increased expression of JNK target genes Mmp1 and puckered. We show that, although knockdown of the homeodomain-interacting protein kinase (Hipk) suppresses Smt3 depletion-induced activation of JNK, Hipk overexpression synergistically enhances this type of JNK activation. We further demonstrate that Hipk is sumolylated in vivo, and its nuclear localization is dependent on the sumoylation pathway. Our results thus establish a mechanistic connection between the sumoylation pathway and the JNK pathway through the action of Hipk. We propose that the sumoylation-controlled balance between cytoplasmic and nuclear Hipk plays a crucial role in regulating JNK signaling.

Regulation of Drosophila lifespan by JNK signaling

Cellular responses to extrinsic and intrinsic insults have to be carefully regulated to properly coordinate cytoprotection, repair processes, cell proliferation and apoptosis. Stress signaling pathways, most prominently the Jun-N-terminal Kinase (JNK) pathway, are critical regulators of such cellular responses and have accordingly been implicated in the regulation of lifespan in various organisms. JNK signaling promotes cytoprotective gene expression, but also interacts with the insulin signaling pathway to influence growth, metabolism, stress tolerance and regeneration. Here, we review recent studies in Drosophila that elucidate the tissue-specific and systemic consequences of JNK activation that ultimately impact lifespan of the organism.

Response to Dengue virus infections altered by cytokine-like substances from mosquito cell cultures

Background

With both shrimp and commercial insects such as honey bees, it is known that stable, persistent viral infections characterized by absence of disease can sometimes shift to overt disease states as a result of various stress triggers and that this can result in serious economic losses. The main research interest of our group is to understand the dynamics of stable viral infections in shrimp and how they can be destabilized by stress. Since there are no continuous cell lines for crustaceans, we have used a C6/36 mosquito cell line infected with Dengue virus to test hypotheses regarding these interactions. As a result, we accidentally discovered two new cytokine-like substances in 5 kDa extracts from supernatant solutions of acutely and persistently infected mosquito cells.

Results

Naïve C6/36 cells were exposed for 48 h to 5 kDa membrane filtrates prepared from the supernatant medium of stable C6/36 mosquito cell cultures persistently-infected with Dengue virus. Subsequent challenge of naïve cells with a virulent stock of Dengue virus 2 (DEN-2) and analysis by confocal immunofluorescence microscopy using anti-DEN-2 antibody revealed a dramatic reduction in the percentage of DEN-2 infected cells when compared to control cells. Similar filtrates prepared from C6/36 cells with acute DEN-2 infections were used to treat stable C6/36 mosquito cell cultures persistently-infected with Dengue virus. Confocal immunofluorescence microscopy revealed destabilization in the form of an apoptosis-like response. Proteinase K treatment removed the cell-altering activities indicating that they were caused by small polypeptides similar to those previously reported from insects.

Conclusions

This is the first report of cytokine-like substances that can alter the responses of mosquito cells to Dengue virus. This simple model system allows detailed molecular studies on insect cytokine production and on cytokine activity in a standard insect cell line.

A targeted UAS-RNAi screen in Drosophila larvae identifies wound closure genes regulating distinct cellular processes

Robust mechanisms for tissue repair are critical for survival of multicellular organisms. Efficient cutaneous wound repair requires the migration of cells at the wound edge and farther back within the epidermal sheet, but the genes that control and coordinate these migrations remain obscure. This is in part because a systematic screening approach for in vivo identification and classification of postembryonic wound closure genes has yet to be developed. Here, we performed a proof-of-principle reporter-based in vivo RNAi screen in the Drosophila melanogaster larval epidermis to identify genes required for normal wound closure. Among the candidate genes tested were kinases and transcriptional mediators of the Jun N-terminal kinase (JNK) signaling pathway shown to be required for epithelial sheet migration during development. Also targeted were genes involved in actin cytoskeletal remodeling. Importantly, RNAi knockdown of both canonical and noncanonical members of the JNK pathway caused open wounds, as did several genes involved in actin cytoskeletal remodeling. Our analysis of JNK pathway components reveals redundancy among the upstream activating kinases and distinct roles for the downstream transcription factors DJun and DFos. Quantitative and qualitative morphological classification of the open wound phenotypes and evaluation of JNK activation suggest that multiple cellular processes are required in the migrating epidermal cells, including functions specific to cells at the wound edge and others specific to cells farther back within the epidermal sheet. Together, our results identify a new set of conserved wound closure genes, determine putative functional roles for these genes within the migrating epidermal sheet, and provide a template for a broader in vivo RNAi screen to discover the full complement of genes required for wound closure during larval epidermal wound healing.