Non-cell autonomous control of apoptosis by ligand-independent Hedgehog signaling in Drosophila

Signaling plasticity in the integrated stress response

The Integrated Stress Response (ISR) is an essential homeostatic signaling network that controls the cell's biosynthetic capacity. Four ISR sensor kinases detect multiple stressors and relay this information to downstream effectors by phosphorylating a common node: the alpha subunit of the eukaryotic initiation factor eIF2. As a result, general protein synthesis is repressed while select transcripts are preferentially translated, thus remodeling the proteome and transcriptome. Mounting evidence supports a view of the ISR as a dynamic signaling network with multiple modulators and feedback regulatory features that vary across cell and tissue types. Here, we discuss updated views on ISR sensor kinase mechanisms, how the subcellular localization of ISR components impacts signaling, and highlight ISR signaling differences across cells and tissues. Finally, we consider crosstalk between the ISR and other signaling pathways as a determinant of cell health.

Patched and Costal-2 mutations lead to differences in tissue overgrowth autonomy

Genetic screens are used in Drosophila melanogaster to identify genes key in the regulation of organismal development and growth. These screens have defined signalling pathways necessary for tissue and organismal development, which are evolutionarily conserved across species, including Drosophila. Here, we have used an FLP/FRT mosaic system to screen for conditional regulators of cell growth and cell division in the Drosophila eye. The conditional nature of this screen utilizes a block in the apoptotic pathway to prohibit the mosaic mutant cells from dying via apoptosis. From this screen, we identified two different mutants that mapped to the Hedgehog signalling pathway. Previously, we described a novel Ptc mutation and here we add to the understanding of disrupting the Hh pathway with a novel allele of Cos2. Both of these Hh components are negative regulators of the pathway, yet they depict mutant differences in the type of overgrowth created. Ptc mutations lead to overgrowth consisting of almost entirely wild-type tissue (non-autonomous overgrowth), while the Cos2 mutation results in tissue that is overgrown in both the mutant and wild-type clones (both autonomous and non-autonomous). These differences in tissue overgrowth are consistent in the Drosophila eye and wing. The observed difference is correlated with different deregulation patterns of pMad, the downstream effector of DPP signalling. This finding provides insight into pathway-specific differences that help to better understand intricacies of developmental processes and human diseases that result from deregulated Hedgehog signalling, such as basal cell carcinoma.

Non-autonomous consequences of cell death and other perks of being metazoan

Drosophila melanogaster remains a foremost genetic model to study basic cell biological processes in the context of multi-cellular development. In such context, the behavior of one cell can influence another. Non-autonomous signaling among cells occurs throughout metazoan development and disease, and is too vast to be covered by a single review. I will focus here on non-autonomous signaling events that occur in response to cell death in the larval epithelia and affect the life-death decision of surviving cells. I will summarize the use of Drosophila to study cell death-induced proliferation, apoptosis-induced apoptosis, and apoptosis-induced survival signaling. Key insights from Drosophila will be discussed in the context of analogous processes in mammalian development and cancer biology.

Non-autonomous consequences of cell death and other perks of being metazoan

Drosophila melanogaster remains a foremost genetic model to study basic cell biological processes in the context of multi-cellular development. In such context, the behavior of one cell can influence another. Non-autonomous signaling among cells occurs throughout metazoan development and disease, and is too vast to be covered by a single review. I will focus here on non-autonomous signaling events that occur in response to cell death in the larval epithelia and affect the life-death decision of surviving cells. I will summarize the use of Drosophila to study cell death-induced proliferation, apoptosis-induced apoptosis, and apoptosis-induced survival signaling. Key insights from Drosophila will be discussed in the context of analogous processes in mammalian development and cancer biology.

An efficient and multiple target transgenic RNAi technique with low toxicity in Drosophila

Being relatively simple and practical, Drosophila transgenic RNAi is the technique of top priority choice to quickly study genes with pleiotropic functions. However, drawbacks have emerged over time, such as high level of false positive and negative results. To overcome these shortcomings and increase efficiency, specificity and versatility, we develop a next generation transgenic RNAi system. With this system, the leaky expression of the basal promoter is significantly reduced, as well as the heterozygous ratio of transgenic RNAi flies. In addition, it has been first achieved to precisely and efficiently modulate highly expressed genes. Furthermore, we increase versatility which can simultaneously knock down multiple genes in one step. A case illustration is provided of how this system can be used to study the synthetic developmental effect of histone acetyltransferases. Finally, we have generated a collection of transgenic RNAi lines for those genes that are highly homologous to human disease genes.

Lgl reduces endosomal vesicle acidification and Notch signaling by promoting the interaction between Vap33 and the V-ATPase complex

Epithelial cell polarity is linked to the control of tissue growth and tumorigenesis. The tumor suppressor and cell polarity protein lethal-2-giant larvae (Lgl) promotes Hippo signaling and inhibits Notch signaling to restrict tissue growth in Drosophila melanogaster Notch signaling is greater in lgl mutant tissue than in wild-type tissue because of increased acidification of endosomal vesicles, which promotes the proteolytic processing and activation of Notch by γ-secretase. We showed that the increased Notch signaling and tissue growth defects of lgl mutant tissue depended on endosomal vesicle acidification mediated by the vacuolar adenosine triphosphatase (V-ATPase). Lgl promoted the activity of the V-ATPase by interacting with Vap33 (VAMP-associated protein of 33 kDa). Vap33 physically and genetically interacted with Lgl and V-ATPase subunits and repressed V-ATPase-mediated endosomal vesicle acidification and Notch signaling. Vap33 overexpression reduced the abundance of the V-ATPase component Vha44, whereas Lgl knockdown reduced the binding of Vap33 to the V-ATPase component Vha68-3. Our data indicate that Lgl promotes the binding of Vap33 to the V-ATPase, thus inhibiting V-ATPase-mediated endosomal vesicle acidification and thereby reducing γ-secretase activity, Notch signaling, and tissue growth. Our findings implicate the deregulation of Vap33 and V-ATPase activity in polarity-impaired epithelial cancers.

Modelling Cooperative Tumorigenesis in Drosophila

The development of human metastatic cancer is a multistep process, involving the acquisition of several genetic mutations, tumour heterogeneity, and interactions with the surrounding microenvironment. Due to the complexity of cancer development in mammals, simpler model organisms, such as the vinegar fly, Drosophila melanogaster, are being utilized to provide novel insights into the molecular mechanisms involved. In this review, we highlight recent advances in modelling tumorigenesis using the Drosophila model, focusing on the cooperation of oncogenes or tumour suppressors, and the interaction of mutant cells with the surrounding tissue in epithelial tumour initiation and progression.

Hedgehog signalling is required for cell survival in Drosophila wing pouch cells

An appropriate balance between cell survival and cell death is essential for correct pattern formation in the animal tissues and organs. Previous studies have shown that the short-range signalling molecule Hedgehog (Hh) is required for cell proliferation and pattern formation in the Drosophila central wing discs. Signal transduction by one of the Hh targets, the morphogen Decapentaplegic (Dpp), is required for not only cell proliferation, but also cell survival in the pouch cells. However, Hh function in cell survival and cell death has not been revealed. Here, we found that loss of Hh signal activity induces considerable Caspase-dependent cell death in the wing pouch cells, and this process was independent of both Dpp signalling and Jun-N-terminal kinase (JNK) signalling. Loss of Hh induced activation of the pro-apoptotic gene hid and inhibition of diap1. Therefore, we identified an important role of Hh signalling in cell survival during Drosophila wing development.

Clonal cooperativity in heterogenous cancers

Tumor heterogeneity is a major obstacle to the development of effective therapies and is thus an important focus of cancer research. Genetic and epigenetic alterations, as well as altered tumor microenvironments, result in tumors made up of diverse subclones with different genetic and phenotypic characteristics. Intratumor heterogeneity enables competition, but also supports clonal cooperation via cell-cell contact or secretion of factors, resulting in enhanced tumor progression. Here, we summarize recent findings related to interclonal interactions within a tumor and the therapeutic implications of such interactions, with an emphasis on how different subclones collaborate with each other to promote proliferation, metastasis and therapy-resistance. Furthermore, we propose that disruption of clonal cooperation by targeting key factors (such as Wnt and Hedgehog, amongst others) can be an alternative approach to improving clinical outcomes.

Suppression of induced but not developmental apoptosis in Drosophila by Ayurvedic Amalaki Rasayana and Rasa-Sindoor

Earlier we showed formulation-specific beneficial effects of dietary supplement of Ayurvedic Amalaki Rasayana (AR, a herbal formulation) and Rasa-Sindoor (RS, a mercury-based organo-metallic formulation) on various biological parameters in Drosophila, parallel to traditional Ayurvedic literature. These formulations also suppressed cell death and pathology in fly models of neurodegeneration. To understand basis of inhibition of apoptosis, we examined effects of AR and RS on induced and developmental apoptosis in Drosophila. Dietary AR or RS significantly reduced apoptosis induced by GMR-GAL4-, sev-GAL4- or hs-GAL4-directed expression of Rpr, Hid or Grim (RHG) proapoptotic proteins or by GMR-GAL4-directed DIAP1-RNAi, resulting in significant restoration of organism's viability and eye morphology. AR or RS supplement enhanced levels of inhibitor of apoptosis proteins, DIAP1 and DIAP2, and of Bancal/Hrb57A, while the levels of RHG proteins and of initiator Dronc and effecter Drice caspases were reduced in non-apoptotic wild type as well as in RHG over-expressing tissues. Levels of Dronc or Drice remained unaffected in cells developmentally destined to die so that developmental apoptosis occurred normally. Elevated levels of DIAPs and reduced levels of RHG proteins and caspases reflect a more robust physiological state of AR or RS fed organisms allowing them to tolerate greater insults without triggering the cell-death response. Such homeostatic effects of these Rasayanas seem to contribute to 'healthy ageing', one of their effects suggested in traditional Ayurvedic practices.

Functional Interaction between HEXIM and Hedgehog Signaling during Drosophila Wing Development

Studying the dynamic of gene regulatory networks is essential in order to understand the specific signals and factors that govern cell proliferation and differentiation during development. This also has direct implication in human health and cancer biology. The general transcriptional elongation regulator P-TEFb regulates the transcriptional status of many developmental genes. Its biological activity is controlled by an inhibitory complex composed of HEXIM and the 7SK snRNA. Here, we examine the function of HEXIM during Drosophila development. Our key finding is that HEXIM affects the Hedgehog signaling pathway. HEXIM knockdown flies display strong phenotypes and organ failures. In the wing imaginal disc, HEXIM knockdown initially induces ectopic expression of Hedgehog (Hh) and its transcriptional effector Cubitus interuptus (Ci). In turn, deregulated Hedgehog signaling provokes apoptosis, which is continuously compensated by apoptosis-induced cell proliferation. Thus, the HEXIM knockdown mutant phenotype does not result from the apoptotic ablation of imaginal disc; but rather from the failure of dividing cells to commit to a proper developmental program due to Hedgehog signaling defects. Furthermore, we show that ci is a genetic suppressor of hexim. Thus, HEXIM ensures the integrity of Hedgehog signaling in wing imaginal disc, by a yet unknown mechanism. To our knowledge, this is the first time that the physiological function of HEXIM has been addressed in such details in vivo.

Your neighbours matter - non-autonomous control of apoptosis in development and disease

Traditionally, the regulation of apoptosis has been thought of as an autonomous process in which the dying cell dictates its own demise. However, emerging studies in genetically tractable multicellular organisms, such as Caenorhabditis elegans and Drosophila, have revealed that death is often a communal event. Here, we review the current literature on non-autonomous mechanisms governing apoptosis in multiple cellular contexts. The importance of the cellular community in dictating the funeral arrangements of apoptotic cells has profound implications in development and disease.

Peritoneal expression of Matrilysin helps identify early post-operative recurrence of colorectal cancer

Recurrence of colorectal cancer (CRC) following a potentially curative resection is a challenging clinical problem. Matrix metalloproteinase-7 (MMP-7) is over-expressed by CRC cells and supposed to play a major role in CRC cell diffusion and metastasis. MMP-7 RNA expression was assessed by real-time PCR using specific primers in peritoneal washing fluid obtained during surgical procedure. After surgery, patients underwent a regular follow up for assessing recurrence. transcripts for MMP-7 were detected in 31/57 samples (54%). Patients were followed-up (range 20-48 months) for recurrence prevention. Recurrence was diagnosed in 6 out of 55 patients (11%) and two patients eventually died because of this. Notably, all the six patients who had relapsed were positive for MMP-7. Sensitivity and specificity of the test were 100% and 49% respectively. Data from patients have also been corroborated by computational approaches. Public available coloncarcinoma datasets have been employed to confirm MMP7 clinical impact on the disease. Interestingly, MMP-7 expression appeared correlated to Tgfb-1, and correlation of the two factors represented a poor prognostic factor. This study proposes positivity of MMP-7 in peritoneal cavity as a novel biomarker for predicting disease recurrence in patients with CRC.

The E3 ligase Itch knockout mice show hyperproliferation and wound healing alteration

The HECT-type E3 ubiquitin ligase Itch is absent in the non-agouti-lethal 18H or Itchy mice, which develop a severe immunological disease. Several of the known Itch substrates are relevant for epidermal development and homeostasis, such as p63, Notch, c-Jun and JunB. By analysing Itchy mice before the onset of immunological alterations, we investigated the contribution of Itch in skin development and wound healing. Itchy newborn mice manifested hyperplastic epidermis, which is not present in adulthood. Itch(-/-) cultured keratinocytes showed overexpression of proliferating markers and increased capability to proliferate, migrate and to repair a scratch injury in vitro. These data correlated with improved in vivo wound healing in Itchy mice, at late time points of the repair process when Itch is physiologically upregulated. Despite healing acceleration, epidermal remodelling was delayed in the scars of Itch(-/-) mice, as indicated by enhanced epidermal thickening, keratinocyte proliferation and keratin 6 expression, and retarded keratin 14 polarization to the basal layer. Itch(-/-) keratinocyte prolonged activation was not associated with increased immune cell persistence in the scars. Our in vitro and in vivo results indicate that Itch plays a role in epidermal homeostasis and remodelling and this feature does not seem to depend on immunological alterations.

Neural stem cell progeny regulate stem cell death in a Notch and Hox dependent manner

Cell death is a prevalent, well-controlled and fundamental aspect of development, particularly in the nervous system. In Drosophila, specific neural stem cells are eliminated by apoptosis during embryogenesis. In the absence of apoptosis, these stem cells continue to divide, resulting in a dramatically hyperplastic central nervous system and adult lethality. Although core cell death pathways have been well described, the spatial, temporal and cell identity cues that activate the cell death machinery in specific cells are largely unknown. We identified a cis-regulatory region that controls the transcription of the cell death activators reaper, grim and sickle exclusively in neural stem cells. Using a reporter generated from this regulatory region, we found that Notch activity is required for neural stem cell death. Notch regulates the expression of the abdominalA homeobox protein, which provides important spatial cues for death. Importantly, we show that pro-apoptotic Notch signaling is activated by the Delta ligand expressed on the neighboring progeny of the stem cell. Thus we identify a previously undescribed role for progeny in regulating the proper developmental death of their parental stem cells.

Lgl regulates Notch signaling via endocytosis, independently of the apical aPKC-Par6-Baz polarity complex

BACKGROUND

The Drosophila melanogaster junctional neoplastic tumor suppressor, Lethal-2-giant larvae (Lgl), is a regulator of apicobasal cell polarity and tissue growth. We have previously shown in the developing Drosophila eye epithelium that, without affecting cell polarity, depletion of Lgl results in ectopic cell proliferation and blockage of developmental cell death due to deregulation of the Hippo signaling pathway.

RESULTS

Here, we show that Notch signaling is increased in lgl-depleted eye tissue, independently of Lgl's function in apicobasal cell polarity. The upregulation of Notch signaling is ligand dependent and correlates with accumulation of cleaved Notch. Concomitant with higher cleaved Notch levels in lgl- tissue, early endosomes (Avalanche [Avl+]), recycling endosomes (Rab11+), early multivesicular bodies (Hrs+), and acidified vesicles, but not late endosomal markers (Car+ and Rab7+), accumulate. Colocalization studies revealed that Lgl associates with early to late endosomes and lysosomes. Upregulation of Notch signaling in lgl- tissue requires dynamin- and Rab5-mediated endocytosis and vesicle acidification but is independent of Hrs/Stam or Rab11 activity. Furthermore, Lgl regulates Notch signaling independently of the aPKC-Par6-Baz apical polarity complex.

CONCLUSIONS

Altogether, our data show that Lgl regulates endocytosis to restrict vesicle acidification and prevent ectopic ligand-dependent Notch signaling. This Lgl function is independent of the aPKC-Par6-Baz polarity complex and uncovers a novel attenuation mechanism of ligand-activated Notch signaling during Drosophila eye development.

Multiple mechanisms modulate distinct cellular susceptibilities toward apoptosis in the developing Drosophila eye

Although apoptosis is mechanistically well understood, a comprehensive understanding of how cells modulate their susceptibility toward apoptosis in a developing tissue is lacking. Here, we reveal striking dynamics in the apoptotic susceptibilities of different cell types in the Drosophila retina over a period of only 24 hr. Mitotic cells are extremely susceptible to apoptotic signals, while postmitotic cells have developed several strategies to promote survival. For example, photoreceptor neurons accumulate the inhibitor of apoptosis, Diap1. In unspecified cells, Cullin-3-mediated degradation keeps Diap1 levels low. These cells depend on EGFR signaling for survival. As development proceeds, developmentally older photoreceptors degrade Diap1, resulting in increased apoptosis susceptibility. Finally, R8 photoreceptors have very efficient survival mechanisms independent of EGFR or Diap1. These examples illustrate how complex cellular susceptibility toward apoptosis is regulated in a developing organ. Similar complexities may regulate apoptosis susceptibilities in mammalian development, and tumor cells may take advantage of it.

Hedgehog is a positive regulator of FGF signalling during embryonic tracheal cell migration

Cell migration is a widespread and complex process that is crucial for morphogenesis and for the underlying invasion and metastasis of human cancers. During migration, cells are steered toward target sites by guidance molecules that induce cell direction and movement through complex intracellular mechanisms. The spatio-temporal regulation of the expression of these guidance molecules is of extreme importance for both normal morphogenesis and human disease. One way to achieve this precise regulation is by combinatorial inputs of different transcription factors. Here we used Drosophila melanogaster mutants with migration defects in the ganglionic branches of the tracheal system to further clarify guidance regulation during cell migration. By studying the cellular consequences of overactivated Hh signalling, using ptc mutants, we found that Hh positively regulates Bnl/FGF levels during embryonic stages. Our results show that Hh modulates cell migration non-autonomously in the tissues surrounding the action of its activity. We further demonstrate that the Hh signalling pathway regulates bnl expression via Stripe (Sr), a zinc-finger transcription factor with homology to the Early Growth Response (EGR) family of vertebrate transcription factors. We propose that Hh modulates embryonic cell migration by participating in the spatio-temporal regulation of bnl expression in a permissive mode. By doing so, we provide a molecular link between the activation of Hh signalling and increased chemotactic responses during cell migration.