Jun mediates Frizzled-induced R3/R4 cell fate distinction and planar polarity determination in the Drosophila eye

Wnt signaling modulates the response to DNA damage in the Drosophila wing imaginal disc by regulating the EGFR pathway

Despite the deep conservation of the DNA damage response (DDR) pathway, cells in different contexts vary widely in their susceptibility to DNA damage and their propensity to undergo apoptosis as a result of genomic lesions. One of the cell signaling pathways implicated in modulating the DDR is the highly conserved Wnt pathway, which is known to promote resistance to DNA damage caused by ionizing radiation in a variety of human cancers. However, the mechanisms linking Wnt signal transduction to the DDR remain unclear. Here, we use a genetically encoded system in Drosophila to reliably induce consistent levels of DNA damage in vivo, and demonstrate that canonical Wnt signaling in the wing imaginal disc buffers cells against apoptosis in the face of DNA double-strand breaks. We show that Wg, the primary Wnt ligand in Drosophila, activates epidermal growth factor receptor (EGFR) signaling via the ligand-processing protease Rhomboid, which, in turn, modulates the DDR in a Chk2-, p53-, and E2F1-dependent manner. These studies provide mechanistic insight into the modulation of the DDR by the Wnt and EGFR pathways in vivo in a highly proliferative tissue. Furthermore, they reveal how the growth and patterning functions of Wnt signaling are coupled with prosurvival, antiapoptotic activities, thereby facilitating developmental robustness in the face of genomic damage.

Opposing roles of Fos, Raw, and SARM1 in the regulation of axonal degeneration and synaptic structure

Introduction

The degeneration of injured axons is driven by conserved molecules, including the sterile armadillo TIR domain-containing protein SARM1, the cJun N-terminal kinase JNK, and regulators of these proteins. These molecules are also implicated in the regulation of synapse development though the mechanistic relationship of their functions in degeneration vs. development is poorly understood.

Results and discussion

Here, we uncover disparate functional relationships between SARM1 and the transmembrane protein Raw in the regulation of Wallerian degeneration and synaptic growth in motoneurons of Drosophila melanogaster. Our genetic data suggest that Raw antagonizes the downstream output MAP kinase signaling mediated by Drosophila SARM1 (dSarm). This relationship is revealed by dramatic synaptic overgrowth phenotypes at the larval neuromuscular junction when motoneurons are depleted for Raw or overexpress dSarm. While Raw antagonizes the downstream output of dSarm to regulate synaptic growth, it shows an opposite functional relationship with dSarm for axonal degeneration. Loss of Raw leads to decreased levels of dSarm in axons and delayed axonal degeneration that is rescued by overexpression of dSarm, supporting a model that Raw promotes the activation of dSarm in axons. However, inhibiting Fos also decreases dSarm levels in axons but has the opposite outcome of enabling Wallerian degeneration. The combined genetic data suggest that Raw, dSarm, and Fos influence each other's functions through multiple points of regulation to control the structure of synaptic terminals and the resilience of axons to degeneration.

Notch-dependent Abl signaling regulates cell motility during ommatidial rotation in Drosophila

A collective cell motility event that occurs during Drosophila eye development, ommatidial rotation (OR), serves as a paradigm for signaling-pathway-regulated directed movement of cell clusters. OR is instructed by the EGFR and Notch pathways and Frizzled/planar cell polarity (Fz/PCP) signaling, all of which are associated with photoreceptor R3 and R4 specification. Here, we show that Abl kinase negatively regulates OR through its activity in the R3/R4 pair. Abl is localized to apical junctional regions in R4, but not in R3, during OR, and this apical localization requires Notch signaling. We demonstrate that Abl and Notch interact genetically during OR, and Abl co-immunoprecipitates in complexes with Notch in eye discs. Perturbations of Abl interfere with adherens junctional organization of ommatidial preclusters, which mediate the OR process. Together, our data suggest that Abl kinase acts directly downstream of Notch in R4 to fine-tune OR via its effect on adherens junctions.

Temperature and Growth Selection Effects on Proliferation, Differentiation, and Adipogenic Potential of Turkey Myogenic Satellite Cells Through Frizzled-7-Mediated Wnt Planar Cell Polarity Pathway

Satellite cells (SCs) are a heterogeneous population of multipotential stem cells. During the first week after hatch, satellite cell function and fate are sensitive to temperature. Wingless-type mouse mammary tumor virus integration site family/planar cell polarity (Wnt/PCP) signaling pathway is significantly affected by thermal stress in turkey pectoralis major (p. major) muscle SCs. This pathway regulates the activity of SCs through a frizzled-7 (Fzd7) cell surface receptor and two intracellular effectors, rho-associated protein kinase (ROCK) and c-Jun. The objective of the present study was to determine the effects of thermal stress, growth selection, and the Fzd7-mediated Wnt/PCP pathway on proliferation, myogenic differentiation, lipid accumulation, and expression of myogenic and adipogenic regulatory genes. These effects were evaluated in SCs isolated from the p. major muscle of 1-week faster-growing modern commercial (NC) line of turkeys as compared to SCs of a slower-growing historic Randombred Control Line 2 (RBC2) turkey line. Heat stress (43°C) increased phosphorylation of both ROCK and c-Jun with greater increases observed in the RBC2 line. Cold stress (33°C) had an inhibitory effect on both ROCK and c-Jun phosphorylation with the NC line showing greater reductions. Knockdown of the expression of Fzd7 decreased proliferation, differentiation, and expression of myogenic regulatory genes: myoblast determination factor-1 and myogenin in both lines. Both lipid accumulation and expression of adipogenic regulatory genes: peroxisome proliferator-activated receptor-γ, CCAAT/enhancer-binding protein-β, and neuropeptide-Y were suppressed with the Fzd7 knockdown. The RBC2 line was more dependent on the Fzd7-mediated Wnt/PCP pathway for proliferation, differentiation, and lipid accumulation compared to the NC line. Thus, thermal stress may affect poultry breast muscle growth potential and protein to fat ratio by altering function and fate of SCs through the Fzd7-mediated Wnt/PCP pathway in a growth-dependent manner.

Usp8 promotes tumor cell migration through activating the JNK pathway

Tumor metastasis is the most cause of high mortality for cancer patients. Identification of novel factors that modulate tumor cell migration is of great significance for therapeutic strategies. Here, we find that the ubiquitin-specific protease 8 (Usp8) promotes tumor cell migration through activating the c-Jun N-terminal kinase (JNK) pathway. Genetic epistasis analyses uncover Usp8 acts upstream of Tak1 to control the JNK pathway. Consistently, biochemical results reveal that Usp8 binds Tak1 to remove ubiquitin modification from Tak1, leading to its stabilization. In addition, human USP8 also triggers tumor cell migration and activates the JNK pathway. Finally, we show that knockdown of USP8 in human breast cancer cells suppresses cell migration. Taken together, our findings demonstrate that a conserved Usp8-Tak1-JNK axis promotes tumor cell migration, and providing USP8 as a potential therapeutic target for cancer treatment.

Different strategies by distinct Wnt-signaling pathways in activating a nuclear transcriptional response

The Wnt family of secreted glycolipo-proteins signals through multiple signal transduction pathways and is essential for embryonic development and organ development and homeostasis. The Wnt-pathways are conserved and critical in all metazoans. Wnt signaling pathways comprise the canonical Wnt/β-catenin pathway and several non-canonical signaling branches, of which Wnt-Planar Cell Polarity (PCP) signaling and the Wnt/Calcium pathway have received the most attention and are best understood. nterestingly, all Wnt-pathways have a nuclear signaling branch and also can affect many cellular processes independent of its nuclear transcriptional regulation. Canonical Wnt/β-catenin signaling is the most critical for a nuclear transcriptional response, in both development and disease, yet the mechanism(s) on how the "business end" of the pathway, β-catenin, translocates to the nucleus to act as co-activator to the TCF/Lef transcription factor family still remains obscure. Here we discuss and compare the very different strategies on how the respective Wnt signaling pathways activate a nuclear transcriptional response. We also highlight some recent new insights into how β-catenin is translocated to the nucleus via an IFT-A, Kinesin-2, and microtubule dependent mechanism and how this aspect of canonical Wnt-signaling uses ciliary proteins in a cilium independent manner, conserved between Drosophila and mammalian cells.

Dissection of the Regulatory Elements of the Complex Expression Pattern of Puckered, a Dual-Specificity JNK Phosphatase

For developmental processes, we know most of the gene networks controlling specific cell responses. We still have to determine how these networks cooperate and how signals become integrated. The JNK pathway is one of the key elements modulating cellular responses during development. Yet, we still know little about how the core components of the pathway interact with additional regulators or how this network modulates cellular responses in the whole organism in homeostasis or during tissue morphogenesis. We have performed a promoter analysis, searching for potential regulatory sequences of puckered (puc) and identified different specific enhancers directing gene expression in different tissues and at different developmental times. Remarkably, some of these domains respond to the JNK activity, but not all. Altogether, these analyses show that puc expression regulation is very complex and that JNK activities participate in non-previously known processes during the development of Drosophila.

20-hydroxyecdysone (20E) signaling regulates amnioserosa morphogenesis during Drosophila dorsal closure: EcR modulates gene expression in a complex with the AP-1 subunit, Jun

Steroid hormones influence diverse biological processes throughout the animal life cycle, including metabolism, stress resistance, reproduction, and lifespan. In insects, the steroid hormone, 20-hydroxyecdysone (20E), is the central hormone regulator of molting and metamorphosis, and plays roles in tissue morphogenesis. For example, amnioserosa contraction, which is a major driving force in Drosophila dorsal closure (DC), is defective in embryos mutant for 20E biosynthesis. Here, we show that 20E signaling modulates the transcription of several DC participants in the amnioserosa and other dorsal tissues during late embryonic development, including zipper, which encodes for non-muscle myosin. Canonical ecdysone signaling typically involves the binding of Ecdysone receptor (EcR) and Ultraspiracle heterodimers to ecdysone-response elements (EcREs) within the promoters of responsive genes to drive expression. During DC, however, we provide evidence that 20E signaling instead acts in parallel to the JNK cascade via a direct interaction between EcR and the AP-1 transcription factor subunit, Jun, which together binds to genomic regions containing AP-1 binding sites but no EcREs to control gene expression. Our work demonstrates a novel mode of action for 20E signaling in Drosophila that likely functions beyond DC, and may provide further insights into mammalian steroid hormone receptor interactions with AP-1.

Summary: During Drosophila dorsal closure, 20E signaling acts non-canonically through an interaction between EcR and the AP-1 subunit, Jun, to control gene expression at regions containing AP-1 motifs but no EcREs.

Differential activation of eMI by distinct forms of cellular stress

As one of the major, highly conserved catabolic pathways, autophagy delivers cytosolic components to lysosomes for degradation. It is essential for development, cellular homeostasis, and coping with stress. Reduced autophagy increases susceptibility to protein aggregation diseases and leads to phenotypes associated with aging. Of the three major forms of autophagy, macroautophagy (MA) can degrade organelles or aggregated proteins, and chaperone-mediated autophagy is specific for soluble proteins containing KFERQ-related targeting motifs. During endosomal microautophagy (eMI), cytoplasmic proteins are engulfed into late endosomes in an ESCRT machinery-dependent manner. eMI can be KFERQ-specific or occur in bulk and be induced by prolonged starvation. Its physiological regulation and function, however, are not understood. Here, we show that eMI in the Drosophila fat body, akin to the mammalian liver, is induced upon oxidative or genotoxic stress in an ESCRT and partially Hsc70-4-dependent manner. Interestingly, eMI activation is selective, as ER stress fails to elicit a response. Intriguingly, we find that reducing MA leads to a compensatory enhancement of eMI, suggesting a tight interplay between these degradative processes. Furthermore, we show that mutations in DNA damage response genes are sufficient to trigger eMI and that the response to oxidative stress is under the control of MAPK/JNK signaling. Our data suggest that, controlled by various signaling pathways, eMI allows an organ to react and adapt to specific types of stress and is thus likely critical to prevent disease.Abbreviations:Atg: autophagy-related; CMA: chaperone-mediated autophagy; DDR: DNA damage repair; Df: deficiency (deletion); (E)GFP: (enhanced) green fluorescent protein; eMI: endosomal microautophagy; ER: endoplasmatic reticulum; ESCRT: endosomal sorting complexes required for transport; Eto: etoposide; FLP: flipase; Hsc: heat shock cognate protein; LAMP2A: lysosomal-associated membrane protein 2A; LE: late endosome; MA: macroautophagy; MI: microautophagy; MVB: multivesicular body; PA: photoactivatable; Para: paraquat; ROS: reactive oxygen species; SEM: standard error of means; Tor: target of rapamycin [serine/threonine kinase]; UPR: unfolded protein response; Vps: vacuolar protein sorting.

Vangl2 in the Dentate Network Modulates Pattern Separation and Pattern Completion

The organization of spatial information, including pattern completion and pattern separation processes, relies on the hippocampal circuits, yet the molecular and cellular mechanisms underlying these two processes are elusive. Here, we find that loss of Vangl2, a core PCP gene, results in opposite effects on pattern completion and pattern separation processes. Mechanistically, we show that Vangl2 loss maintains young postmitotic granule cells in an immature state, providing increased cellular input for pattern separation. The genetic ablation of Vangl2 disrupts granule cell morpho-functional maturation and further prevents CaMKII and GluA1 phosphorylation, disrupting the stabilization of AMPA receptors. As a functional consequence, LTP at lateral perforant path-GC synapses is impaired, leading to defects in pattern completion behavior. In conclusion, we show that Vangl2 exerts a bimodal regulation on young and mature GCs, and its disruption leads to an imbalance in hippocampus-dependent pattern completion and separation processes.

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Vangl2-dependent PCP signaling controls granule cell maturation and network integration

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Vangl2 stabilizes GluA1-containing receptors at the surface of dendritic spines

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Granule cells require Vangl2-dependent signaling to elicit LTP

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Vangl2 loss has opposite functional effects on pattern completion/separation processes

ZnT7 RNAi favors RafGOFscrib-/--induced tumor growth and invasion in Drosophila through JNK signaling pathway

The disruption of zinc homeostasis has been identified in patients suffering from various cancers, but a causative relationship has not yet been established. Drosophila melanogaster has become a powerful model to study cancer biology. Here using a Drosophila model of malignant tumor Raf^GOFscrib^-/-, we observed that the tumor growth, invasion and migration were enhanced by silencing dZnT7, a zinc transporter localized on the Golgi apparatus. Further study indicated that the zinc deficiency in Golgi of dZnT7 RNAi resulted in ER stress which could activate the c-Jun-N-terminal Kinase (JNK) signaling and this process is mediated by Atg9. Lastly, we demonstrated that the exacerbation of dZnT7 RNAi on tumor was promoted by JNK signaling-dependent cell autonomous and non-autonomous autophagy. These findings suggest that zinc homeostasis in secretory compartments may provide a new therapeutic target for tumor treatment.

Hippo signaling promotes Ets21c-dependent apical cell extrusion in the Drosophila wing disc

Cell extrusion is a crucial regulator of epithelial tissue development and homeostasis. Epithelial cells undergoing apoptosis, bearing pathological mutations or possessing developmental defects are actively extruded toward elimination. However, the molecular mechanisms of Drosophila epithelial cell extrusion are not fully understood. Here, we report that activation of the conserved Hippo (Hpo) signaling pathway induces both apical and basal cell extrusion in the Drosophila wing disc epithelia. We show that canonical Yorkie targets Diap1, Myc and Cyclin E are not required for either apical or basal cell extrusion induced by activation of this pathway. Another target gene, bantam, is only involved in basal cell extrusion, suggesting novel Hpo-regulated apical cell extrusion mechanisms. Using RNA-seq analysis, we found that JNK signaling is activated in the extruding cells. We provide genetic evidence that JNK signaling activation is both sufficient and necessary for Hpo-regulated cell extrusion. Furthermore, we demonstrate that the ETS-domain transcription factor Ets21c, an ortholog of proto-oncogenes FLI1 and ERG, acts downstream of JNK signaling to mediate apical cell extrusion. Our findings reveal a novel molecular link between Hpo signaling and cell extrusion.

JNK signalling regulates antioxidant responses in neurons

Reactive oxygen species (ROS) are generated during physiological bouts of synaptic activity and as a consequence of pathological conditions in the central nervous system. How neurons respond to and distinguish between ROS in these different contexts is currently unknown. In Drosophila mutants with enhanced JNK activity, lower levels of ROS are observed and these animals are resistant to both changes in ROS and changes in synapse morphology induced by oxidative stress. In wild type flies, disrupting JNK-AP-1 signalling perturbs redox homeostasis suggesting JNK activity positively regulates neuronal antioxidant defense. We validated this hypothesis in mammalian neurons, finding that JNK activity regulates the expression of the antioxidant gene Srxn-1, in a c-Jun dependent manner. We describe a conserved ‘adaptive’ role for neuronal JNK in the maintenance of redox homeostasis that is relevant to several neurodegenerative diseases.

The transcription factor Spalt and human homologue SALL4 induce cell invasion via the dMyc-JNK pathway in Drosophila

Cancer cell metastasis is a leading cause of mortality in cancer patients. Therefore, revealing the molecular mechanism of cancer cell invasion is of great significance for the treatment of cancer. In human patients, the hyperactivity of transcription factor Spalt-like 4 (SALL4) is sufficient to induce malignant tumorigenesis and metastasis. Here, we found that when ectopically expressing the Drosophila homologue spalt (sal) or human SALL4 in Drosophila, epithelial cells delaminated basally with penetration of the basal lamina and degradation of the extracellular matrix, which are essential properties of cell invasion. Further assay found that sal/SALL4 promoted cell invasion via dMyc-JNK signaling. Inhibition of the c-Jun N-terminal kinase (JNK) signaling pathway through suppressing matrix metalloprotease 1, or basket can achieve suppression of cell invasion. Moreover, expression of dMyc, a suppressor of JNK signaling, dramatically blocked cell invasion induced by sal/SALL4 in the wing disc. These findings reveal a conserved role of sal/SALL4 in invasive cell movement and link the crucial mediator of tumor invasion, the JNK pathway, to SALL4-mediated cancer progression.This article has an associated First Person interview with the first author of the paper.

Cell Competition Is Driven by Autophagy

Cell competition is a quality control process that selectively eliminates unfit cells from the growing tissue via cell-cell interaction. Despite extensive mechanistic studies, the mechanism by which cell elimination is triggered has been elusive. Here, through a genetic screen in Drosophila, we discover that V-ATPase, an essential factor for autophagy, is required for triggering cell competition. Strikingly, autophagy is specifically elevated in prospective "loser" cells nearby wild-type "winner" cells, and blocking autophagy in loser cells abolishes their elimination. Mechanistically, elevated autophagy upregulates a proapoptotic gene hid through NFκB, and the elevated hid cooperates with JNK signaling to effectively induce loser's death. Crucially, this mechanism generally applies to cell competition caused by differences in protein synthesis between cells. Our findings establish a common mechanism of cell competition whereby cells with higher protein synthesis induce autophagy in their neighboring cells, leading to elimination of unfit cells.

Reproduction disrupts stem cell homeostasis in testes of aged male Drosophila via an induced microenvironment

Stem cells rely on instructive cues from their environment. Alterations in microenvironments might contribute to tissue dysfunction and disease pathogenesis. Germline stem cells (GSCs) and cyst stem cells (CySC) in Drosophila testes are normally maintained in the apical area by the testicular hub. In this study, we found that reproduction leads to accumulation of early differentiating daughters of CySCs and GSCs in the testes of aged male flies, due to hyperactivation of Jun-N-terminal kinase (JNK) signaling to maintain self-renewal gene expression in the differentiating cyst cells. JNK activity is normally required to maintain CySCs in the apical niche. A muscle sheath surrounds the Drosophila testis to maintain its long coiled structure. Importantly, reproduction triggers accumulation of the tumor necrosis factor (TNF) Eiger in the testis muscle to activate JNK signaling via the TNF receptor Grindelwald in the cyst cells. Reducing Eiger activity in the testis muscle sheath suppressed reproduction-induced differentiation defects, but had little effect on testis homeostasis of unmated males. Our results reveal that reproduction in males provokes a dramatic shift in the testicular microenvironment, which impairs tissue homeostasis and spermatogenesis in the testes.

Proper differentiation of stem cell progeny is necessary for preservation of tissue homeostasis. In Drosophila testes, somatic cyst cells derived from the cyst stem cells (CySCs) control the differentiation of the neighboring germ cells. Disruption of CySC daughter cyst cell differentiation leads to failure in sperm production. Interestingly, we found that reproduction triggers hyperactivation of Jun-N-terminal kinase (JNK) signaling to sustain CySC self-renewal gene expression in differentiating cyst cells, leading to accumulation of immature cyst cell and germ cells at the expense of mature cells in the testes of aged males. Endogenous JNK signaling is also required for CySC maintenance. Moreover, we found that the JNK signaling is hyperactivated via reproduction-induced accumulation of tumor necrosis factor (TNF) in testicular smooth muscle that surrounds the testis to support its long coiled structure. The reproduction-induced phenotypes were only observed in the testes of aged and mated males, but not in testes form young mated males or aged unmated males, indicating that it is a combined effect of reproduction and aging. Our results reveal that reproduction impedes sperm production in aged males, and identify testicular muscle as an inducible signaling center for spermatogenesis in Drosophila.

A genome-wide Drosophila epithelial tumorigenesis screen identifies Tetraspanin 29Fb as an evolutionarily conserved suppressor of Ras-driven cancer

Oncogenic mutations in the small GTPase Ras contribute to ~30% of human cancers. However, Ras mutations alone are insufficient for tumorigenesis, therefore it is paramount to identify cooperating cancer-relevant signaling pathways. We devised an in vivo near genome-wide, functional screen in Drosophila and discovered multiple novel, evolutionarily-conserved pathways controlling Ras-driven epithelial tumorigenesis. Human gene orthologs of the fly hits were significantly downregulated in thousands of primary tumors, revealing novel prognostic markers for human epithelial tumors. Of the top 100 candidate tumor suppressor genes, 80 were validated in secondary Drosophila assays, identifying many known cancer genes and multiple novel candidate genes that cooperate with Ras-driven tumorigenesis. Low expression of the confirmed hits significantly correlated with the KRAS^G12 mutation status and poor prognosis in pancreatic cancer. Among the novel top 80 candidate cancer genes, we mechanistically characterized the function of the top hit, the Tetraspanin family member Tsp29Fb, revealing that Tsp29Fb regulates EGFR signaling, epithelial architecture and restrains tumor growth and invasion. Our functional Drosophila screen uncovers multiple novel and evolutionarily conserved epithelial cancer genes, and experimentally confirmed Tsp29Fb as a key regulator of EGFR/Ras induced epithelial tumor growth and invasion.

Cancer involves the cooperative interaction of many gene mutations. The Ras signaling pathway is upregulated in many human cancers, but upregulated Ras signaling alone is not sufficient to induce malignant tumors. We have undertaken a genome-wide genetic screen using a transgenic RNAi library in the vinegar fly, Drosophila melanogaster, to identify tumor suppressor genes that cooperate with the Ras oncogene (Ras^V12) in conferring overgrown invasive tumors. We stratified the hits by analyzing the expression of human orthologs of these genes in human epithelial cancers, revealing genes that were strongly downregulated in human cancer. By conducting secondary genetic interaction tests, we validated 80 of the top 100 genes. Pathway analysis of these genes revealed that 55 fell into known pathways involved in human cancer, whereas 25 were unique genes. We then confirmed the tumor suppressor properties of one of these genes, Tsp29Fb, encoding a Tetraspanin membrane protein, and showed that Tsp29Fb functions as a tumor suppressor by inhibiting Ras signaling and by maintaining epithelial cell polarity. Altogether, our study has revealed novel Ras-cooperating tumor suppressors in Drosophila and suggests that these genes may also be involved in human cancer.

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

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

Signalling crosstalk during early tumorigenesis in the absence of Polycomb silencing

In response to stress and injury a coordinated activation of conserved signalling modules, such as JNK and JAK/STAT, is critical to trigger regenerative tissue restoration. While these pathways rebuild homeostasis and promote faithful organ recovery, it is intriguing that they also become activated in various tumour conditions. Therefore, it is crucial to understand how similar pathways can achieve context-dependent functional outputs, likely depending on cellular states. Compromised chromatin regulation, upon removal of the Polycomb group member polyhomeotic, leads to tumour formation with ectopic activation of JNK signalling, mediated by egr/grnd, in addition to JAK/STAT and Notch. Employing quantitative analyses, we show that blocking ectopic signalling impairs ph tumour growth. Furthermore, JAK/STAT functions in parallel to JNK, while Notch relies on JNK. Here, we reveal a signalling hierarchy in ph tumours that is distinct from the regenerative processes regulated by these pathways. Absence of ph renders a permissive state for expression of target genes, but our results suggest that both loss of repression and the presence of activators may collectively regulate gene expression during tumorigenesis. Further dissecting the effect of signalling, developmental or stress-induced factors will thus elucidate the regulation of physiological responses and the contribution of context-specific cellular states.

Planar cell polarity in moving cells: think globally, act locally

The planar cell polarity (PCP) pathway is best known for its role in polarizing epithelial cells within the plane of a tissue but it also plays a role in a range of cell migration events during development. The mechanism by which the PCP pathway polarizes stationary epithelial cells is well characterized, but how PCP signaling functions to regulate more dynamic cell behaviors during directed cell migration is much less understood. Here, we review recent discoveries regarding the localization of PCP proteins in migrating cells and their impact on the cell biology of collective and individual cell migratory behaviors.

Restraint of presynaptic protein levels by Wnd/DLK signaling mediates synaptic defects associated with the kinesin-3 motor Unc-104

The kinesin-3 family member Unc-104/KIF1A is required for axonal transport of many presynaptic components to synapses, and mutation of this gene results in synaptic dysfunction in mice, flies and worms. Our studies at the Drosophila neuromuscular junction indicate that many synaptic defects in unc-104-null mutants are mediated independently of Unc-104's transport function, via the Wallenda (Wnd)/DLK MAP kinase axonal damage signaling pathway. Wnd signaling becomes activated when Unc-104's function is disrupted, and leads to impairment of synaptic structure and function by restraining the expression level of active zone (AZ) and synaptic vesicle (SV) components. This action concomitantly suppresses the buildup of synaptic proteins in neuronal cell bodies, hence may play an adaptive role to stresses that impair axonal transport. Wnd signaling also becomes activated when pre-synaptic proteins are over-expressed, suggesting the existence of a feedback circuit to match synaptic protein levels to the transport capacity of the axon.

Feedback amplification loop drives malignant growth in epithelial tissues

Interactions between cells bearing oncogenic mutations and the surrounding microenvironment, and cooperation between clonally distinct cell populations, can contribute to the growth and malignancy of epithelial tumors. The genetic techniques available in Drosophila have contributed to identify important roles of the TNF-α ligand Eiger and mitogenic molecules in mediating these interactions during the early steps of tumor formation. Here we unravel the existence of a tumor-intrinsic-and microenvironment-independent-self-reinforcement mechanism that drives tumor initiation and growth in an Eiger-independent manner. This mechanism relies on cell interactions between two functionally distinct cell populations, and we present evidence that these cell populations are not necessarily genetically different. Tumor-specific and cell-autonomous activation of the tumorigenic JNK stress-activated pathway drives the expression of secreted signaling molecules and growth factors to delaminating cells, which nonautonomously promote proliferative growth of the partially transformed epithelial tissue. We present evidence that cross-feeding interactions between delaminating and nondelaminating cells increase each other's sizes and that these interactions can explain the unlimited growth potential of these tumors. Our results will open avenues toward our molecular understanding of those social cell interactions with a relevant function in tumor initiation in humans.

The Drosophila Duox maturation factor is a key component of a positive feedback loop that sustains regeneration signaling

Regenerating tissue must initiate the signaling that drives regenerative growth, and sustain that signaling long enough for regeneration to complete. How these key signals are sustained is unclear. To gain a comprehensive view of the changes in gene expression that occur during regeneration, we performed whole-genome mRNAseq of actively regenerating tissue from damaged Drosophila wing imaginal discs. We used genetic tools to ablate the wing primordium to induce regeneration, and carried out transcriptional profiling of the regeneration blastema by fluorescently labeling and sorting the blastema cells, thus identifying differentially expressed genes. Importantly, by using genetic mutants of several of these differentially expressed genes we have confirmed that they have roles in regeneration. Using this approach, we show that high expression of the gene moladietz (mol), which encodes the Duox-maturation factor NIP, is required during regeneration to produce reactive oxygen species (ROS), which in turn sustain JNK signaling during regeneration. We also show that JNK signaling upregulates mol expression, thereby activating a positive feedback signal that ensures the prolonged JNK activation required for regenerative growth. Thus, by whole-genome transcriptional profiling of regenerating tissue we have identified a positive feedback loop that regulates the extent of regenerative growth.

Regenerating tissue must initiate the signaling that drives regenerative growth, and then sustain that signaling long enough for regeneration to complete. Drosophila imaginal discs, the epithelial structures in the larva that will form the adult animal during metamorphosis, have been an important model system for tissue repair and regeneration for over 60 years. Here we show that damage-induced JNK signaling leads to the upregulation of a gene called moladietz, which encodes a co-factor for an enzyme, NADPH dual oxidase (Duox), that generates reactive oxygen species (ROS), a key tissue-damage signal. High expression of moladietz induces continuous production of ROS in the regenerating tissue. The sustained production of ROS then continues to activate JNK signaling throughout the course of regeneration, ensuring maximal tissue regrowth.

Axon Termination, Pruning, and Synaptogenesis in the Giant Fiber System of Drosophila melanogaster Is Promoted by Highwire

The ubiquitin ligase Highwire has a conserved role in synapse formation. Here, we show that Highwire coordinates several facets of central synapse formation in the Drosophila melanogaster giant fiber system, including axon termination, axon pruning, and synaptic function. Despite the similarities to the fly neuromuscular junction, the role of Highwire and the underlying signaling pathways are distinct in the fly's giant fiber system. During development, branching of the giant fiber presynaptic terminal occurs and, normally, the transient branches are pruned away. However, in highwire mutants these ectopic branches persist, indicating that Highwire promotes axon pruning. highwire mutants also exhibit defects in synaptic function. Highwire promotes axon pruning and synaptic function cell-autonomously by attenuating a mitogen-activated protein kinase pathway including Wallenda, c-Jun N-terminal kinase/Basket, and the transcription factor Jun. We also show a novel role for Highwire in non-cell autonomous promotion of synaptic function from the midline glia. Highwire also regulates axon termination in the giant fibers, as highwire mutant axons exhibit severe overgrowth beyond the pruning defect. This excessive axon growth is increased by manipulating Fos expression in the cells surrounding the giant fiber terminal, suggesting that Fos regulates a trans-synaptic signal that promotes giant fiber axon growth.

MAPK signaling promotes axonal degeneration by speeding the turnover of the axonal maintenance factor NMNAT2

Injury-induced (Wallerian) axonal degeneration is regulated via the opposing actions of pro-degenerative factors such as SARM1 and a MAPK signal and pro-survival factors, the most important of which is the NAD⁺ biosynthetic enzyme NMNAT2 that inhibits activation of the SARM1 pathway. Here we investigate the mechanism by which MAPK signaling facilitates axonal degeneration. We show that MAPK signaling promotes the turnover of the axonal survival factor NMNAT2 in cultured mammalian neurons as well as the Drosophila ortholog dNMNAT in motoneurons. The increased levels of NMNAT2 are required for the axonal protection caused by loss of MAPK signaling. Regulation of NMNAT2 by MAPK signaling does not require SARM1, and so cannot be downstream of SARM1. Hence, pro-degenerative MAPK signaling functions upstream of SARM1 by limiting the levels of the essential axonal survival factor NMNAT2 to promote injury-dependent SARM1 activation. These findings are consistent with a linear molecular pathway for the axonal degeneration program.

DOI:http://dx.doi.org/10.7554/eLife.22540.001

Fold formation at the compartment boundary of Drosophila wing requires Yki signaling to suppress JNK dependent apoptosis

Compartment boundaries prevent cell populations of different lineage from intermingling. In many cases, compartment boundaries are associated with morphological folds. However, in the Drosophila wing imaginal disc, fold formation at the anterior/posterior (A/P) compartment boundary is suppressed, probably as a prerequisite for the formation of a flat wing surface. Fold suppression depends on optomotor-blind (omb). Omb mutant animals develop a deep apical fold at the A/P boundary of the larval wing disc and an A/P cleft in the adult wing. A/P fold formation is controlled by different signaling pathways. Jun N-terminal kinase (JNK) and Yorkie (Yki) signaling are activated in cells along the fold and are necessary for the A/P fold to develop. While JNK promotes cell shape changes and cell death, Yki target genes are required to antagonize apoptosis, explaining why both pathways need to be active for the formation of a stable fold.

Wnt-Frizzled/planar cell polarity signaling: cellular orientation by facing the wind (Wnt)

The establishment of planar cell polarity (PCP) in epithelial and mesenchymal cells is a critical, evolutionarily conserved process during development and organogenesis. Analyses in Drosophila and several vertebrate model organisms have contributed a wealth of information on the regulation of PCP. A key conserved pathway regulating PCP, the so-called core Wnt-Frizzled PCP (Fz/PCP) signaling pathway, was initially identified through genetic studies of Drosophila. PCP studies in vertebrates, most notably mouse and zebrafish, have identified novel factors in PCP signaling and have also defined cellular features requiring PCP signaling input. These studies have shifted focus to the role of Van Gogh (Vang)/Vangl genes in this molecular system. This review focuses on new insights into the core Fz/Vangl/PCP pathway and recent advances in Drosophila and vertebrate PCP studies. We attempt to integrate these within the existing core Fz/Vangl/PCP signaling framework.

Wallenda/DLK protein levels are temporally downregulated by Tramtrack69 to allow R7 growth cones to become stationary boutons

Dual leucine zipper kinase (DLK) promotes growth cone motility and must be restrained to ensure normal development. PHR (Pam/Highwire/RPM-1) ubiquitin ligases therefore target DLK for degradation unless axon injury occurs. Overall DLK levels decrease during development, but how DLK levels are regulated within a developing growth cone has not been examined. We analyzed the expression of the fly DLK Wallenda (Wnd) in R7 photoreceptor growth cones as they halt at their targets and become presynaptic boutons. We found that Wnd protein levels are repressed by the PHR protein Highwire (Hiw) during R7 growth cone halting, as has been observed in other systems. However, as R7 growth cones become boutons, Wnd levels are further repressed by a temporally expressed transcription factor, Tramtrack69 (Ttk69). Previously unobserved negative feedback from JNK also contributes to Wnd repression at both time points. We conclude that neurons deploy additional mechanisms to downregulate DLK as they form stable, synaptic connections. We use live imaging to probe the effects of Wnd and Ttk69 on R7 bouton development and conclude that Ttk69 coordinates multiple regulators of this process.

Remote Control of Intestinal Stem Cell Activity by Haemocytes in Drosophila

The JAK/STAT pathway is a key signaling pathway in the regulation of development and immunity in metazoans. In contrast to the multiple combinatorial JAK/STAT pathways in mammals, only one canonical JAK/STAT pathway exists in Drosophila. It is activated by three secreted proteins of the Unpaired family (Upd): Upd1, Upd2 and Upd3. Although many studies have established a link between JAK/STAT activation and tissue damage, the mode of activation and the precise function of this pathway in the Drosophila systemic immune response remain unclear. In this study, we used mutations in upd2 and upd3 to investigate the role of the JAK/STAT pathway in the systemic immune response. Our study shows that haemocytes express the three upd genes and that injury markedly induces the expression of upd3 by the JNK pathway in haemocytes, which in turn activates the JAK/STAT pathway in the fat body and the gut. Surprisingly, release of Upd3 from haemocytes upon injury can remotely stimulate stem cell proliferation and the expression of Drosomycin-like genes in the intestine. Our results also suggest that a certain level of intestinal epithelium renewal is required for optimal survival to septic injury. While haemocyte-derived Upd promotes intestinal stem cell activation and survival upon septic injury, haemocytes are dispensable for epithelium renewal upon oral bacterial infection. Our study also indicates that intestinal epithelium renewal is sensitive to insults from both the lumen and the haemocoel. It also reveals that release of Upds by haemocytes coordinates the wound-healing program in multiple tissues, including the gut, an organ whose integrity is critical to fly survival.

Localized JNK signaling regulates organ size during development

A fundamental question of biology is what determines organ size. Despite demonstrations that factors within organs determine their sizes, intrinsic size control mechanisms remain elusive. Here we show that Drosophila wing size is regulated by JNK signaling during development. JNK is active in a stripe along the center of developing wings, and modulating JNK signaling within this stripe changes organ size. This JNK stripe influences proliferation in a non-canonical, Jun-independent manner by inhibiting the Hippo pathway. Localized JNK activity is established by Hedgehog signaling, where Ci elevates dTRAF1 expression. As the dTRAF1 homolog, TRAF4, is amplified in numerous cancers, these findings provide a new mechanism for how the Hedgehog pathway could contribute to tumorigenesis, and, more importantly, provides a new strategy for cancer therapies. Finally, modulation of JNK signaling centers in developing antennae and legs changes their sizes, suggesting a more generalizable role for JNK signaling in developmental organ size control.

DOI:http://dx.doi.org/10.7554/eLife.11491.001

A key challenge in biology is to understand what determines size. As an animal grows, signals are produced that control the size of its organs. Many of the signaling pathways that regulate size during normal animal development also contribute to the formation of tumors. Therefore, it is important to find out exactly how the signaling molecules that regulate size are linked to those that regulate tumor growth.

A protein called JNK activates a signaling pathway that triggers tumor growth. JNK signaling also stimulates cells to multiply in tissues that need repair, but it is not known whether it also regulates the size of organs during animal development. Here, Willsey et al. investigate whether JNK is active in the developing wings of fruit flies, which are commonly used as models of animal development.

The experiments show that JNK is active in a stripe across the developing wing and is required for the wing to grow to its proper size. A master signal protein called Hedgehog is responsible for establishing this stripe of JNK activity. Unexpectedly, rather than acting through its usual signaling pathway, JNK activates the Hippo pathway in the wing to control organ size during development.

Willsey et al.’s findings highlight potential new targets for cancer therapies. A future challenge will be to find out whether small patches of JNK signaling are found in the developing organs of other animals, and whether they can help explain how size changes between species.

DOI:http://dx.doi.org/10.7554/eLife.11491.002

Unique and Overlapping Functions of Formins Frl and DAAM During Ommatidial Rotation and Neuronal Development in Drosophila

The noncanonical Frizzled/planar cell polarity (PCP) pathway regulates establishment of polarity within the plane of an epithelium to generate diversity of cell fates, asymmetric, but highly aligned structures, or to orchestrate the directional migration of cells during convergent extension during vertebrate gastrulation. In Drosophila, PCP signaling is essential to orient actin wing hairs and to align ommatidia in the eye, in part by coordinating the movement of groups of photoreceptor cells during ommatidial rotation. Importantly, the coordination of PCP signaling with changes in the cytoskeleton is essential for proper epithelial polarity. Formins polymerize linear actin filaments and are key regulators of the actin cytoskeleton. Here, we show that the diaphanous-related formin, Frl, the single fly member of the FMNL (formin related in leukocytes/formin-like) formin subfamily affects ommatidial rotation in the Drosophila eye and is controlled by the Rho family GTPase Cdc42. Interestingly, we also found that frl mutants exhibit an axon growth phenotype in the mushroom body, a center for olfactory learning in the Drosophila brain, which is also affected in a subset of PCP genes. Significantly, Frl cooperates with Cdc42 and another formin, DAAM, during mushroom body formation. This study thus suggests that different formins can cooperate or act independently in distinct tissues, likely integrating various signaling inputs with the regulation of the cytoskeleton. It furthermore highlights the importance and complexity of formin-dependent cytoskeletal regulation in multiple organs and developmental contexts.

Dual Role of Jun N-Terminal Kinase Activity in Bone Morphogenetic Protein-Mediated Drosophila Ventral Head Development

The Drosophila bone morphogenetic protein encoded by decapentaplegic (dpp) controls ventral head morphogenesis by expression in the head primordia, eye-antennal imaginal discs. These are epithelial sacs made of two layers: columnar disc proper cells and squamous cells of the peripodial epithelium. dpp expression related to head formation occurs in the peripodial epithelium; cis-regulatory mutations disrupting this expression display defects in sensory vibrissae, rostral membrane, gena, and maxillary palps. Here we document that disruption of this dpp expression causes apoptosis in peripodial cells and underlying disc proper cells. We further show that peripodial Dpp acts directly on the disc proper, indicating that Dpp must cross the disc lumen to act. We demonstrate that palp defects are mechanistically separable from the other mutant phenotypes; both are affected by the c-Jun N-terminal kinase pathway but in opposite ways. Slight reduction of both Jun N-terminal kinase and Dpp activity in peripodial cells causes stronger vibrissae, rostral membrane, and gena defects than Dpp alone; additionally, strong reduction of Jun N-terminal kinase activity alone causes identical defects. A more severe reduction of dpp results in similar vibrissae, rostral membrane, and gena defects, but also causes mutant maxillary palps. This latter defect is correlated with increased peripodial Jun N-terminal kinase activity and can be caused solely by ectopic activation of Jun N-terminal kinase. We conclude that formation of sensory vibrissae, rostral membrane, and gena tissue in head morphogenesis requires the action of Jun N-terminal kinase in peripodial cells, while excessive Jun N-terminal kinase signaling in these same cells inhibits the formation of maxillary palps.

Scabrous overexpression in the eye affects R3/R4 cell fate specification and inhibits notch signaling

BACKGROUND

Planar cell polarity (PCP) in the Drosophila eye is generated when immature ommatidial preclusters acquire opposite chirality in the dorsal and ventral halves of the eye imaginal disc and rotate 90 ° toward the equator. The scabrous (sca) gene is involved in R8 differentiation and in the correct spacing of ommatidial clusters in eye imaginal discs, but it was also suggested to be required during ommatidial rotation. However, no clear relationships between sca and other genes involved in the process were established.

RESULTS

To explore the role of Sca in PCP establishment, we performed an RNAi-based modifier genetic screen using the rough eye phenotype of sca-overexpressing flies. We found that sca overexpression mainly affects R3/R4 cell specification as it was reported in Notch mutants. Of the 86 modifiers identified in the screen, genes encoding components of Notch signaling and proteins involved in intracellular transport were of particular interest.

CONCLUSIONS

These and other results obtained with a reporter line of Notch activity indicate that sca overexpression antagonizes Notch signaling in the Drosophila eye, and are inconsistent with Sca being an ommatidial rotation-specific factor. We also found that microtubule motors and other proteins involved in intracellular transport are related with Sca function.

The orthologous Tbx transcription factors Omb and TBX2 induce epithelial cell migration and extrusion in vivo without involvement of matrix metalloproteinases

The transcription factors TBX2 and TBX3 are overexpressed in various human cancers. Here, we investigated the effect of overexpressing the orthologous Tbx genes Drosophila optomotor-blind (omb) and human TBX2 in the epithelium of the Drosophila wing imaginal disc and observed two types of cell motility. Omb/TBX2 overexpressing cells could move within the plane of the epithelium. Invasive cells migrated long-distance as single cells retaining or regaining normal cell shape and apico-basal polarity in spite of attenuated apical DE-cadherin concentration. Inappropriate levels of DE-cadherin were sufficient to drive cell migration in the wing disc epithelium. Omb/TBX2 overexpression and reduced DE-cadherin-dependent adhesion caused the formation of actin-rich lateral cell protrusions. Omb/TBX2 overexpressing cells could also delaminate basally, penetratingthe basal lamina, however, without degradation of extracellular matrix. Expression of Timp, an inhibitor of matrix metalloproteases, blocked neither intraepithelial motility nor basal extrusion. Our results reveal an MMP-independent mechanism of cell invasion and suggest a conserved role of Tbx2-related proteins in cell invasion and metastasis-related processes.

JAK/STAT signaling in Drosophila muscles controls the cellular immune response against parasitoid infection

The role of JAK/STAT signaling in the cellular immune response of Drosophila is not well understood. Here, we show that parasitoid wasp infection activates JAK/STAT signaling in somatic muscles of the Drosophila larva, triggered by secretion of the cytokines Upd2 and Upd3 from circulating hemocytes. Deletion of upd2 or upd3, but not the related os (upd1) gene, reduced the cellular immune response, and suppression of the JAK/STAT pathway in muscle cells reduced the encapsulation of wasp eggs and the number of circulating lamellocyte effector cells. These results suggest that JAK/STAT signaling in muscles participates in a systemic immune defense against wasp infection.

Kinesin-1-powered microtubule sliding initiates axonal regeneration in Drosophila cultured neurons

Microtubule sliding drives initial axon regeneration in Drosophila neurons. Axotomy leads to fast calcium influx and subsequent microtubule reorganization. Kinesin-1 heavy chain drives the sliding of antiparallel microtubules to power axonal regrowth, and the JNK pathway promotes axonal regeneration by enhancing microtubule sliding.

Understanding the mechanism underlying axon regeneration is of great practical importance for developing therapeutic treatment for traumatic brain and spinal cord injuries. Dramatic cytoskeleton reorganization occurs at the injury site, and microtubules have been implicated in the regeneration process. Previously we demonstrated that microtubule sliding by conventional kinesin (kinesin-1) is required for initiation of neurite outgrowth in Drosophila embryonic neurons and that sliding is developmentally down-regulated when neurite outgrowth is completed. Here we report that mechanical axotomy of Drosophila neurons in culture triggers axonal regeneration and regrowth. Regenerating neurons contain actively sliding microtubules; this sliding, like sliding during initial neurite outgrowth, is driven by kinesin-1 and is required for axonal regeneration. The injury induces a fast spike of calcium, depolymerization of microtubules near the injury site, and subsequent formation of local new microtubule arrays with mixed polarity. These events are required for reactivation of microtubule sliding at the initial stages of regeneration. Furthermore, the c-Jun N-terminal kinase pathway promotes regeneration by enhancing microtubule sliding in injured mature neurons.

Syd/JIP3 and JNK signaling are required for myonuclear positioning and muscle function

Highlighting the importance of proper intracellular organization, many muscle diseases are characterized by mispositioned myonuclei. Proper positioning of myonuclei is dependent upon the microtubule motor proteins, Kinesin-1 and cytoplasmic Dynein, and there are at least two distinct mechanisms by which Kinesin and Dynein move myonuclei. The motors exert forces both directly on the nuclear surface and from the cell cortex via microtubules. How these activities are spatially segregated yet coordinated to position myonuclei is unknown. Using Drosophila melanogaster, we identified that Sunday Driver (Syd), a homolog of mammalian JNK-interacting protein 3 (JIP3), specifically regulates Kinesin- and Dynein-dependent cortical pulling of myonuclei without affecting motor activity near the nucleus. Specifically, Syd mediates Kinesin-dependent localization of Dynein to the muscle ends, where cortically anchored Dynein then pulls microtubules and the attached myonuclei into place. Proper localization of Dynein also requires activation of the JNK signaling cascade. Furthermore, Syd functions downstream of JNK signaling because without Syd, JNK signaling is insufficient to promote Kinesin-dependent localization of Dynein to the muscle ends. The significance of Syd-dependent myonuclear positioning is illustrated by muscle-specific depletion of Syd, which impairs muscle function. Moreover, both myonuclear spacing and locomotive defects in syd mutants can be rescued by expression of mammalian JIP3 in Drosophila muscle tissue, indicating an evolutionarily conserved role for JIP3 in myonuclear movement and highlighting the utility of Drosophila as a model for studying mammalian development. Collectively, we implicate Syd/JIP3 as a novel regulator of myogenesis that is required for proper intracellular organization and tissue function.

A common pathology found in numerous cases of muscle diseases, including congenital myopathies and muscular dystrophies, is aberrantly located nuclei within individual multinucleated muscle cells. However, whether or not mispositioned myonuclei are a cause or consequence of muscle disease states is currently debated. Here, we take advantage of the model organism, Drosophila melanogaster, which shares the conserved myofiber found in mammalian systems, to identify Syd as a novel regulator of myonuclear positioning. We show that Syd is responsible for mediating the activities of Kinesin and Dynein, two motor proteins that exert forces to pull myonuclei into place. Moreover, we demonstrate that Syd-dependent myonuclear positioning also requires intracellular signaling from the JNK MAPK cascade to direct when and how myonuclei are moved into proper position. This work thus identifies developmental cues that direct proper muscle morphogenesis, suggesting that cases of muscle disease may result from a failure to achieve initial spacing of myonuclei. Supporting this notion, we find that loss of Syd impairs muscle function, but resupplying Syd restores proper myonuclear spacing and muscle function. These findings are particularly important as mispositioned myonuclei gain traction as a potential contributing factor in cases of muscle disease.

A Drosophila ABC transporter regulates lifespan

MRP4 (multidrug resistance-associated protein 4) is a member of the MRP/ABCC subfamily of ATP-binding cassette (ABC) transporters that are essential for many cellular processes requiring the transport of substrates across cell membranes. Although MRP4 has been implicated as a detoxification protein by transport of structurally diverse endogenous and xenobiotic compounds, including antivirus and anticancer drugs, that usually induce oxidative stress in cells, its in vivo biological function remains unknown. In this study, we investigate the biological functions of a Drosophila homolog of human MRP4, dMRP4. We show that dMRP4 expression is elevated in response to oxidative stress (paraquat, hydrogen peroxide and hyperoxia) in Drosophila. Flies lacking dMRP4 have a shortened lifespan under both oxidative and normal conditions. Overexpression of dMRP4, on the other hand, is sufficient to increase oxidative stress resistance and extend lifespan. By genetic manipulations, we demonstrate that dMRP4 is required for JNK (c-Jun NH₂-terminal kinase) activation during paraquat challenge and for basal transcription of some JNK target genes under normal condition. We show that impaired JNK signaling is an important cause for major defects associated with dMRP4 mutations, suggesting that dMRP4 regulates lifespan by modulating the expression of a set of genes related to both oxidative resistance and aging, at least in part, through JNK signaling.

The drug transporters are often known for their ability to transport different physiological-related compounds across cell membranes. Although the abnormal up-regulation of some these transporters is believed to be the common cause of the clinic problem called drug resistance, the biological functions of these transporters remain largely unknown. Here we show that a Drosophila homolog of the mammalian drug transporter plays a role in lifespan regulation. Mutations of this gene increase the sensitivity to oxidative stress and reduce lifespan, while overexpression of this gene increases resistance to oxidative stress and extends lifespan. By molecular and genetic analyses, we have linked functions of this gene to a key signaling transduction pathway that has been known to be important in lifespan regulation.

βν integrin inhibits chronic and high level activation of JNK to repress senescence phenotypes in Drosophila adult midgut

Proper control of adult stem cells including their proliferation and differentiation is crucial in maintaining homeostasis of well-organized tissues/organs throughout an organism's life. The Drosophila adult midgut has intestinal stem cells (ISCs), which have been exploited as a simple model system to investigate mechanisms controlling adult tissue homeostasis. Here, we found that a viable mutant of βν integrin (βint-ν), encoding one of two Drosophila integrin β subunits, showed a short midgut and abnormal multilayered epithelia accompanied by an increase in ISC proliferation and misdifferentiation defects. The increase in ISC proliferation and misdifferentiation was due to frequent ISC duplication expanding a pool of ISCs, which was caused by depression of the Notch signalling, and up-regulation of unpaired (upd), a gene encoding an extracellular ligand in the JAK/STAT signalling pathway. In addition, we observed that abnormally high accumulation of filamentous actin (F-actin) was caused in the βint-ν mutant enterocytes. Furthermore, the defects were rescued by suppressing c-Jun N-terminal kinase (JNK) signalling, which was up-regulated in a manner correlated with the defect levels in the above-mentioned βint-ν mutant phenotype. These symptoms observed in young βint-ν mutant midgut were very similar to those in the aged midgut in wild type. Our results suggested that βint-ν has a novel function for the Drosophila adult midgut homeostasis under normal conditions and provided a new insight into possible age-related diseases caused by latent abnormality of an integrin function.

Planar cell polarity signaling: coordination of cellular orientation across tissues

Establishment of Planar Cell Polarity (PCP) in epithelia, in the plane of an epithelium, is an important feature of the development and homeostasis of most organs. Studies in different model organisms have contributed a wealth of information regarding the mechanisms that govern PCP regulation. Genetic studies in Drosophila have identified two signaling systems, the Fz/PCP and Fat/Dachsous system, which are both required for PCP establishment in many different tissues in a largely non-redundant manner. Recent advances in vertebrate PCP studies have added novel factors of PCP regulation and also new cellular features requiring PCP-signaling input, including the positioning and orientation of the primary cilium of many epithelial cells. This review focuses mostly on several recent advances made in the Drosophila and vertebrate PCP field and integrates these within the existing PCP-signaling framework.

APC loss-induced intestinal tumorigenesis in Drosophila: Roles of Ras in Wnt signaling activation and tumor progression

Adenomatous polyposis coli (APC) and K-ras are the two most frequently mutated genes found in human colorectal cancers. In human colorectal cancers, Wnt signaling activation after the loss of APC is hypothesized to be the key event for adenoma initiation, whereas additional mutations such as Ras activation are required for the progression from adenoma to carcinoma. However, accumulating data have led to conflicting views regarding the precise role of Ras in APC loss-induced tumorigenesis. Here, using Drosophila midgut as a model system, we show that in the absence of Ras, APC mutant epithelial cells cannot initiate hyperplasia, suggesting that Ras plays an essential role in tumor initiation. Conversely, activating Ras by expressing oncogenic Ras or Raf in APC-deficient cells led to a blockage of cell differentiation and to preinvasive tumor outgrowth, characteristics that are shared by advanced colorectal carcinoma in humans. Mechanistically, we find that Ras is not required for Wnt signaling activation after APC loss, although Ras hyperactivation is able to potentiate Wnt signaling by increasing the cytoplasmic and nuclear accumulation of Armadillo/β-catenin via mechanisms independent of JNK/Rac1 or PI3K-Akt signaling, partly owing to the downregulation of DE-cadherin. Together with the data from gene expression analyses, our results indicate that both parallel and cooperative mechanisms of Wnt and Ras signaling are responsible for the initiation and progression of intestinal tumorigenesis after APC loss.

Gliotactin and Discs-large are co-regulated to maintain epithelial integrity

Establishment and maintenance of permeability barriers is one of the most important functions of epithelial cells. Tricellular junctions (TCJs) maintain the permeability barriers at the contact site of three epithelial cells. Gliotactin (Gli), a member of the Neuroligin family, is the only known Drosophila protein exclusively localized to the TCJ and is necessary for maintenance of the permeability barrier. Over-expression triggers the spread of Gliotactin away from the TCJ and causes epithelial cells to delaminate, migrate and die. Furthermore, excess Gli at the cell membrane results in an extensive down regulation of Dlg at the septate junctions. The intracellular domain of Gli contains two highly conserved tyrosine residues, and a PDZ binding motif. We previously found that phosphorylation of the tyrosine residues are necessary to control the level of Gliotactin at the TCJ. In this study we demonstrate that the phenotypes associated with excess Gliotactin is due to a functional interaction between Gliotactin and Dlg that is dependent on both tyrosine phosphorylation as well as the PDZ binding motif. We further show that elevated levels of Dlg strongly enhance Gliotactin over-expression phenotypes to the point where tissue over-growth is observed. The exhibition of these phenotypes require phosphorylation of Dlg on serine 797, a known Par1 phosphorylation target. Blocking this phosphorylation completely suppresses the cell invasiveness and apoptotic phenotypes associated with Gliotactin overexpression. Additionally, we show that Drosophila JNK acts downstream of Gliotactin and Dlg to mediate the overgrowth and apoptosis caused by the functional interaction of Gliotactin and Dlg.

JNK pathway activation is controlled by Tao/TAOK3 to modulate ethanol sensitivity

Neuronal signal transduction by the JNK MAP kinase pathway is altered by a broad array of stimuli including exposure to the widely abused drug ethanol, but the behavioral relevance and the regulation of JNK signaling is unclear. Here we demonstrate that JNK signaling functions downstream of the Sterile20 kinase family gene tao/Taok3 to regulate the behavioral effects of acute ethanol exposure in both the fruit fly Drosophila and mice. In flies tao is required in neurons to promote sensitivity to the locomotor stimulant effects of acute ethanol exposure and to establish specific brain structures. Reduced expression of key JNK pathway genes substantially rescued the structural and behavioral phenotypes of tao mutants. Decreasing and increasing JNK pathway activity resulted in increased and decreased sensitivity to the locomotor stimulant properties of acute ethanol exposure, respectively. Further, JNK expression in a limited pattern of neurons that included brain regions implicated in ethanol responses was sufficient to restore normal behavior. Mice heterozygous for a disrupted allele of the homologous Taok3 gene (Taok3Gt) were resistant to the acute sedative effects of ethanol. JNK activity was constitutively increased in brains of Taok3Gt/+ mice, and acute induction of phospho-JNK in brain tissue by ethanol was occluded in Taok3Gt/+ mice. Finally, acute administration of a JNK inhibitor conferred resistance to the sedative effects of ethanol in wild-type but not Taok3Gt/+ mice. Taken together, these data support a role of a TAO/TAOK3-JNK neuronal signaling pathway in regulating sensitivity to acute ethanol exposure in flies and in mice.

Src controls tumorigenesis via JNK-dependent regulation of the Hippo pathway in Drosophila

Cell-cell interactions within the tumour microenvironment have crucial roles in epithelial tumorigenesis. Using Drosophila genetics, we show that the oncoprotein Src controls tumour microenvironment by Jun N-terminal kinase (JNK)-dependent regulation of the Hippo pathway. Clones of cells with elevated Src expression activate the Rac-Diaphanous and Ras-mitogen-activated protein kinase (MAPK) pathways, which cooperatively induce F-actin accumulation, thereby leading to activation of the Hippo pathway effector Yorkie (Yki). Simultaneously, Src activates the JNK pathway, which antagonizes the autonomous Yki activity and causes propagation of Yki activity to neighbouring cells, resulting in the overgrowth of surrounding tissue. Our data provide a mechanism to explain how oncogenic mutations regulate tumour microenvironment through cell-cell communication.

Planar cell polarity controls pancreatic beta cell differentiation and glucose homeostasis

Planar cell polarity (PCP) refers to the collective orientation of cells within the epithelial plane. We show that progenitor cells forming the ducts of the embryonic pancreas express PCP proteins and exhibit an active PCP pathway. Planar polarity proteins are acquired at embryonic day 11.5 synchronously to apicobasal polarization of pancreas progenitors. Loss of function of the two PCP core components Celsr2 and Celsr3 shows that they control the differentiation of endocrine cells from polarized progenitors, with a prevalent effect on insulin-producing beta cells. This results in a decreased glucose clearance. Loss of Celsr2 and 3 leads to a reduction of Jun phosphorylation in progenitors, which, in turn, reduces beta cell differentiation from endocrine progenitors. These results highlight the importance of the PCP pathway in cell differentiation in vertebrates. In addition, they reveal that tridimensional organization and collective communication of cells are needed in the pancreatic epithelium in order to generate appropriate numbers of endocrine cells.

Niche-associated activation of rac promotes the asymmetric division of Drosophila female germline stem cells

An intracellular polarity in activation of the Rac GTPase cooperates with an extracellular signal to ensure a robust pattern of stem cell division in Drosophila female germline stem cells.

Background

Drosophila female germline stem cells (GSCs) reside adjacent to a cellular niche that secretes Bone Morphogenetic Protein (BMP) ligands and anchors the GSCs through adherens junctions. The GSCs divide asymmetrically such that one daughter remains in the niche as a GSC, while the other is born away from the niche and differentiates. However, given that the BMP signal can be diffusible, it remains unclear how a local extracellular asymmetry is sufficient to result in a robust pattern of asymmetric division.

Methods and Findings

Here we show that GSCs are polarized with respect to the cellular niche. We first use a modified biosensor to demonstrate that the small GTPase Rac is asymmetrically activated within the GSC at the niche-GSC interface. Experiments using loss-of-function and gain-of-function mutations in Rac indicate that asymmetric Rac activity both localizes the microtubule binding protein Apc2 to orient one GSC centrosome at the niche-GSC interface during interphase and activates the Jun N-terminal kinase pathway to increase the ability of the GSC to respond to BMP ligands. Other processes act in concert with each function of Rac. Specifically, we demonstrate that the GSC cell cycle arrests at prometaphase if centrosomes are misoriented.

Conclusions

Thus, the GSCs, an adult stem cell present in a cellular niche, have a niche-associated polarity that couples control of the division plane with increased response to an extracellular maintenance signal. Other processes work in parallel with the Rac-mediated polarity to ensure a robust pattern of asymmetric division. We suggest that all adult stem cells likely employ multiple, independently acting mechanisms to ensure asymmetric division to maintain tissue homeostasis.

Adult stem cells are undifferentiated cells within an organism that undergo continual asymmetric division to produce two daughter cells that have different cell fates: one daughter remains a stem cell like its parent, while the other daughter differentiates. Thus the stock of stem cells is renewed while new cells are provided to ensure tissue maintenance. Often, an adult stem cell is present within a niche, a specific microenvironment that contains signals necessary to maintain the stem cell in an undifferentiated state. The plane of division of a niche-resident stem cell can be controlled so that one of its daughters arises outside of the niche. This daughter does not receive the maintenance signals and differentiates. However, because biological systems are inherently noisy, we postulated that there might be mechanisms that couple the control of the plane of stem cell division to the response to the extracellular maintenance signals to ensure a continued pattern of asymmetric division. In this study, we show that a well-characterized niche-resident stem cell, the Drosophila female germline stem cell (GSC), also has an intracellular polarity that contributes to the determination of daughter cell fate. We find that the small GTPase Rac is activated at the interface between the GSC and its niche and that this localized Rac activity has two functions: first, it orients the plane of GSC division to ensure that one GSC daughter is born outside the niche; and second, it increases the ability of the niche-resident GSC to respond to the maintenance signal. We propose that most stem cells integrate multiple mechanisms to ensure a robust pattern of asymmetric division.

Autocrine platelet-derived growth factor-vascular endothelial growth factor receptor-related (Pvr) pathway activity controls intestinal stem cell proliferation in the adult Drosophila midgut

A dynamic pool of undifferentiated somatic stem cells proliferate and differentiate to replace dead or dying mature cell types and maintain the integrity and function of adult tissues. Intestinal stem cells (ISCs) in the Drosophila posterior midgut are a well established model to study the complex genetic circuitry that governs stem cell homeostasis. Exposure of the intestinal epithelium to environmental toxins results in the expression of cytokines and growth factors that drive the rapid proliferation and differentiation of ISCs. In the absence of stress signals, ISC homeostasis is maintained through intrinsic pathways. In this study, we uncovered the PDGF- and VEGF-receptor related (Pvr) pathway as an essential regulator of ISC homeostasis under unstressed conditions in the posterior midgut. We found that Pvr is coexpressed with its ligand Pvf2 in ISCs and that hyperactivation of the Pvr pathway distorts the ISC developmental program and drives intestinal dysplasia. In contrast, we show that mutant ISCs in the Pvf/Pvr pathway are defective in homeostatic proliferation and differentiation, resulting in a failure to generate mature cell types. Additionally, we determined that extrinsic stress signals generated by enteropathogenic infection are epistatic to the hypoplasia generated in Pvf/Pvr mutants, making the Pvr pathway unique among all previously studied intrinsic pathways. Our findings illuminate an evolutionarily conserved signal transduction pathway with essential roles in metazoan embryonic development and direct involvement in numerous disease states.

A versatile ΦC31 based reporter system for measuring AP-1 and Nrf2 signaling in Drosophila and in tissue culture

This paper describes the construction and characterization of a system of transcriptional reporter genes for monitoring the activity of signaling pathways and gene regulation mechanisms in intact Drosophila, dissected tissues or cultured cells. Transgenic integration of the reporters into the Drosophila germline was performed in a site-directed manner, using ΦC31 integrase. This strategy avoids variable position effects and assures low base level activity and high signal responsiveness. Defined integration sites furthermore enable the experimenter to compare the activity of different reporters in one organism. The reporter constructs have a modular design to facilitate the combination of promoter elements (synthetic transcription factor binding sites or natural regulatory sequences), reporter genes (eGFP, or DsRed.T4), and genomic integration sites. The system was used to analyze and compare the activity and signal response profiles of two stress inducible transcription factors, AP-1 and Nrf2. To complement the transgenic reporter fly lines, tissue culture assays were developed in which the same synthetic ARE and TRE elements control the expression of firefly luciferase.