Coordinating cell differentiation with cell growth and division is crucial for the successful development, homeostasis and regeneration of multicellular tissues. Here, we use bristle patterning in the fly notum as a model system to explore the regulatory and functional coupling of cell cycle progression and cell fate decision-making. The pattern of bristles and intervening epithelial cells (ECs) becomes established through Notch-mediated lateral inhibition during G2 phase of the cell cycle, as neighbouring cells physically interact with each other via lateral contacts and/or basal protrusions. Since Notch signalling controls cell division timing downstream of Cdc25, ECs in lateral contact with a Delta-expressing cell experience higher levels of Notch signalling and divide first, followed by more distant neighbours, and lastly Delta-expressing cells. Conversely, mitotic entry and cell division makes ECs refractory to lateral inhibition signalling, fixing their fate. Using a combination of experiments and computational modelling, we show that this reciprocal relationship between Notch signalling and cell cycle progression acts like a developmental clock, providing a delimited window of time during which cells decide their fate, ensuring efficient and orderly bristle patterning.
Using a series of immunoprecipitation (IP)-tandem mass spectrometry (LC-MS/MS) experiments and reciprocal BLAST, we conducted a fly-human cross-species comparison of the phosphoinositide-3-kinase (PI3K) interactome in a drosophila S2R+ cell line and several NSCLC and human multiple myeloma cell lines to identify conserved interacting proteins to PI3K, a critical signaling regulator of the AKT pathway. Using H929 human cancer cells and drosophila S2R+ cells, our data revealed an unexpected direct binding of Corkscrew, the drosophila ortholog of the non-receptor protein tyrosine phosphatase type II (SHP2) to the Pi3k21B (p60) regulatory subunit of PI3K (p50/p85 human ortholog) but no association with Pi3k92e, the human ortholog of the p110 catalytic subunit. The p85-SHP2 association was validated in human cell lines, and formed a ternary regulatory complex with GRB2-associated-binding protein 2 (GAB2). Validation experiments with knockdown of GAB2 and Far-Western blots proved the direct interaction of SHP2 with p85, independent of adaptor proteins and transfected FLAG-p85 provided evidence that SHP2 binding on p85 occurred on the SH2 domains. A disruption of the SHP2-p85 complex took place after insulin/IGF1 stimulation or imatinib treatment, suggesting that the direct SHP2-p85 interaction was both independent of AKT activation and positively regulates the ERK signaling pathway.
Cellular and systemic responses to low oxygen levels are principally mediated by Hypoxia Inducible Factors (HIFs), a family of evolutionary conserved heterodimeric transcription factors, whose alpha- and beta-subunits belong to the bHLH-PAS family. In normoxia, HIFα is hydroxylated by specific prolyl-4-hydroxylases, targeting it for proteasomal degradation, while in hypoxia the activity of these hydroxylases decreases due to low oxygen availability, leading to HIFα accumulation and expression of HIF target genes. To identify microRNAs required for maximal HIF activity, we conducted an overexpression screen in Drosophila melanogaster, evaluating the induction of a HIF transcriptional reporter. miR-190 overexpression enhanced HIF-dependent biological responses, including terminal sprouting of the tracheal system, while in miR-190 loss of function embryos the hypoxic response was impaired. In hypoxic conditions, miR-190 expression was upregulated and required for induction of HIF target genes by directly inhibiting the HIF prolyl-4-hydroxylase Fatiga. Thus, miR-190 is a novel regulator of the hypoxia response that represses the oxygen sensor Fatiga, leading to HIFα stabilization and enhancement of hypoxic responses.
Postsynaptic cells can induce synaptic plasticity through the release of activity-dependent retrograde signals. We previously described a Ca(2+)-dependent retrograde signaling pathway mediated by postsynaptic Synaptotagmin 4 (Syt4). To identify proteins involved in postsynaptic exocytosis, we conducted a screen for candidates that disrupted trafficking of a pHluorin-tagged Syt4 at Drosophila neuromuscular junctions (NMJs). Here we characterize one candidate, the postsynaptic t-SNARE Syntaxin 4 (Syx4). Analysis of Syx4 mutants reveals that Syx4 mediates retrograde signaling, modulating the membrane levels of Syt4 and the transsynaptic adhesion protein Neuroligin 1 (Nlg1). Syx4-dependent trafficking regulates synaptic development, including controlling synaptic bouton number and the ability to bud new varicosities in response to acute neuronal stimulation. Genetic interaction experiments demonstrate Syx4, Syt4, and Nlg1 regulate synaptic growth and plasticity through both shared and parallel signaling pathways. Our findings suggest a conserved postsynaptic SNARE machinery controls multiple aspects of retrograde signaling and cargo trafficking within the postsynaptic compartment.
Adult stem cells or residential progenitor cells are critical to maintain the structure and function of adult tissues (homeostasis) throughout the lifetime of an individual. Mis-regulation of stem cell proliferation and differentiation often leads to diseases including cancer, however, how wildtype adult stem cells and cancer cells respond to cellular damages remains unclear. We find that in the adult Drosophila midgut, intestinal stem cells (ISCs), unlike tumor intestinal cells, are resistant to various cellular damages. Tumor intestinal cells, unlike wildtype ISCs, are easily eliminated by apoptosis. Further, their proliferation is inhibited upon autophagy induction, and autophagy-mediated tumor inhibition is independent of caspase-dependent apoptosis. Interestingly, inhibition of tumorigenesis by autophagy is likely through the sequestration and degradation of mitochondria, as compromising mitochondria activity in these tumor models mimics the induction of autophagy and increasing the production of mitochondria alleviates the tumor-suppression capacity of autophagy. Together, these data demonstrate that wildtype adult stem cells and tumor cells show dramatic differences in sensitivity to cellular damages, thus providing potential therapeutic implications targeting tumorigenesis.
Yolk provides an important source of nutrients during the early development of oviparous organisms. It is composed mainly of vitellogenin proteins packed into membrane-bound compartments called yolk platelets. Catabolism of yolk is initiated by acidification of the yolk platelet, leading to the activation of Cathepsin-like proteinases, but it is unknown how this process is triggered. Yolk catabolism initiates at cellularization in Drosophila melanogaster embryos. Using maternal shRNA technology we found that yolk catabolism depends on the Tor pathway and on the autophagy-initiating kinase Atg1. Whereas Atg1 was required for a burst of spatially regulated autophagy during late cellularization, autophagy was not required for initiating yolk catabolism. We propose that the conserved Tor metabolic sensing pathway regulates yolk catabolism, similar to Tor-dependent metabolic regulation on the lysosome.
Activating mutations in the Kirsten rat sarcoma viral oncogene homolog (KRAS) underlie the pathogenesis and chemoresistance of ∼ 30% of all human tumors, yet the development of high-affinity inhibitors that target the broad range of KRAS mutants remains a formidable challenge. Here, we report the development and validation of stabilized alpha helices of son of sevenless 1 (SAH-SOS1) as prototype therapeutics that directly inhibit wild-type and mutant forms of KRAS. SAH-SOS1 peptides bound in a sequence-specific manner to KRAS and its mutants, and dose-responsively blocked nucleotide association. Importantly, this functional binding activity correlated with SAH-SOS1 cytotoxicity in cancer cells expressing wild-type or mutant forms of KRAS. The mechanism of action of SAH-SOS1 peptides was demonstrated by sequence-specific down-regulation of the ERK-MAP kinase phosphosignaling cascade in KRAS-driven cancer cells and in a Drosophila melanogaster model of Ras85D(V12) activation. These studies provide evidence for the potential utility of SAH-SOS1 peptides in neutralizing oncogenic KRAS in human cancer.
The use of time series profiling to identify groups of functionally related genes (synexpression groups) is a powerful approach for the discovery of gene function. Here we apply this strategy during Ras(V12) immortalization of Drosophila embryonic cells, a phenomenon not well characterized. Using high-resolution transcriptional time-series datasets, we generated a gene network based on temporal expression profile similarities. This analysis revealed that common immortalized cells are related to adult muscle precursors (AMPs), a stem cell-like population contributing to adult muscles and sharing properties with vertebrate satellite cells. Remarkably, the immortalized cells retained the capacity for myogenic differentiation when treated with the steroid hormone ecdysone. Further, we validated in vivo the transcription factor CG9650, the ortholog of mammalian Bcl11a/b, as a regulator of AMP proliferation predicted by our analysis. Our study demonstrates the power of time series synexpression analysis to characterize Drosophila embryonic progenitor lines and identify stem/progenitor cell regulators.
We present a resource of high quality lists of functionally related Drosophila genes, e.g. based on protein domains (kinases, transcription factors, etc.) or cellular function (e.g. autophagy, signal transduction). To establish these lists, we relied on different inputs, including curation from databases or the literature and mapping from other species. Moreover, as an added curation and quality control step, we asked experts in relevant fields to review many of the lists. The resource is available online for scientists to search and view, and is editable based on community input. Annotation of gene groups is an ongoing effort and scientific need will typically drive decisions regarding which gene lists to pursue. We anticipate that the number of lists will increase over time; that the composition of some lists will grow and/or change over time as new information becomes available; and that the lists will benefit the scientific community, e.g. at experimental design and data analysis stages. Based on this, we present an easily updatable online database, available at www.flyrnai.org/glad, at which gene group lists can be viewed, searched and downloaded.
The RNA-guided nuclease Cas9 can be reengineered as a programmable transcription factor. However, modest levels of gene activation have limited potential applications. We describe an improved transcriptional regulator obtained through the rational design of a tripartite activator, VP64-p65-Rta (VPR), fused to nuclease-null Cas9. We demonstrate its utility in activating endogenous coding and noncoding genes, targeting several genes simultaneously and stimulating neuronal differentiation of human induced pluripotent stem cells (iPSCs).
The tuberous sclerosis complex (TSC) family of tumor suppressors, TSC1 and TSC2, function together in an evolutionarily conserved protein complex that is a point of convergence for major cell signaling pathways that regulate mTOR complex 1 (mTORC1). Mutation or aberrant inhibition of the TSC complex is common in various human tumor syndromes and cancers. The discovery of novel therapeutic strategies to selectively target cells with functional loss of this complex is therefore of clinical relevance to patients with nonmalignant TSC and those with sporadic cancers. We developed a CRISPR-based method to generate homogeneous mutant Drosophila cell lines. By combining TSC1 or TSC2 mutant cell lines with RNAi screens against all kinases and phosphatases, we identified synthetic interactions with TSC1 and TSC2. Individual knockdown of three candidate genes (mRNA-cap, Pitslre, and CycT; orthologs of RNGTT, CDK11, and CCNT1 in humans) reduced the population growth rate of Drosophila cells lacking either TSC1 or TSC2 but not that of wild-type cells. Moreover, individual knockdown of these three genes had similar growth-inhibiting effects in mammalian TSC2-deficient cell lines, including human tumor-derived cells, illustrating the power of this cross-species screening strategy to identify potential drug targets.
A number of approaches for Cas9-mediated transcriptional activation have recently been developed, allowing target genes to be overexpressed from their endogenous genomic loci. However, these approaches have thus far been limited to cell culture, and this technique has not been demonstrated in vivo in any animal. The technique involving the fewest separate components, and therefore the most amenable to in vivo applications, is the dCas9-VPR system, where a nuclease-dead Cas9 is fused to a highly active chimeric activator domain. In this study, we characterize the dCas9-VPR system in Drosophila cells and in vivo. We show that this system can be used in cell culture to upregulate a range of target genes, singly and in multiplex, and that a single guide RNA upstream of the transcription start site can activate high levels of target transcription. We observe marked heterogeneity in guide RNA efficacy for any given gene, and we confirm that transcription is inhibited by guide RNAs binding downstream of the transcription start site. To demonstrate one application of this technique in cells, we used dCas9-VPR to identify target genes for Twist and Snail, two highly conserved transcription factors that cooperate during Drosophila mesoderm development. In addition, we simultaneously activated both Twist and Snail to identify synergistic responses to this physiologically relevant combination. Finally, we show that dCas9-VPR can activate target genes and cause dominant phenotypes in vivo, providing the first demonstration of dCas9 activation in a multicellular animal. Transcriptional activation using dCas9-VPR thus offers a simple and broadly applicable technique for a variety of overexpression studies.
Ligands stimulate Notch receptors by inducing regulated intramembrane proteolysis (RIP) to produce a transcriptional effector. Notch activation requires unmasking of a metalloprotease cleavage site remote from the site of ligand binding, raising the question of how proteolytic sensitivity is achieved. Here, we show that application of physiologically relevant forces to the Notch1 regulatory switch results in sensitivity to metalloprotease cleavage, and bound ligands induce Notch signal transduction in cells only in the presence of applied mechanical force. Synthetic receptor-ligand systems that remove the native ligand-receptor interaction also activate Notch by inducing proteolysis of the regulatory switch. Together, these studies show that mechanical force exerted by signal-sending cells is required for ligand-induced Notch activation and establish that force-induced proteolysis can act as a mechanism of cellular mechanotransduction.
Characterization of the proteome of organelles and subcellular domains is essential for understanding cellular organization and identifying protein complexes as well as networks of protein interactions. We established a proteomic mapping platform in live Drosophila tissues using an engineered ascorbate peroxidase (APEX). Upon activation, the APEX enzyme catalyzes the biotinylation of neighboring endogenous proteins that can then be isolated and identified by mass spectrometry. We demonstrate that APEX labeling functions effectively in multiple fly tissues for different subcellular compartments and maps the mitochondrial matrix proteome of Drosophila muscle to demonstrate the power of APEX for characterizing subcellular proteomes in live cells. Further, we generate "MitoMax," a database that provides an inventory of Drosophila mitochondrial proteins with subcompartmental annotation. Altogether, APEX labeling in live Drosophila tissues provides an opportunity to characterize the organelle proteome of specific cell types in different physiological conditions.
RNA binding proteins (RBPs) are involved in many cellular functions. To facilitate functional characterization of RBPs, we generated an RNA interference (RNAi) library for Drosophila cell-based screens comprising reagents targeting known or putative RBPs. To test the quality of the library and provide a baseline analysis of the effects of the RNAi reagents on viability, we screened the library using a total ATP assay and high-throughput imaging in Drosophila S2R+ cultured cells. The results are consistent with production of a high-quality library that will be useful for functional genomics studies using other assays. Altogether, we provide resources in the form of an initial curated list of Drosophila RBPs; an RNAi screening library we expect to be used with additional assays that address more specific biological questions; and total ATP and image data useful for comparison of those additional assay results with fundamental information such as effects of a given reagent in the library on cell viability. Importantly, we make the baseline data, including more than 200,000 images, easily accessible online.
Given the diversity of autophagy targets and regulation, it is important to characterize autophagy in various cell types and conditions. We used a primary myocyte cell culture system to assay the role of putative autophagy regulators in the specific context of skeletal muscle. By treating the cultures with rapamycin (Rap) and chloroquine (CQ) we induced an autophagic response, fully suppressible by knockdown of core ATG genes. We screened D. melanogaster orthologs of a previously reported mammalian autophagy protein-protein interaction network, identifying several proteins required for autophagosome formation in muscle cells, including orthologs of the Rab regulators RabGap1 and Rab3Gap1. The screen also highlighted the critical roles of the proteasome and glycogen metabolism in regulating autophagy. Specifically, sustained proteasome inhibition inhibited autophagosome formation both in primary culture and larval skeletal muscle, even though autophagy normally acts to suppress ubiquitin aggregate formation in these tissues. In addition, analyses of glycogen metabolic genes in both primary cultured and larval muscles indicated that glycogen storage enhances the autophagic response to starvation, an important insight given the link between glycogen storage disorders, autophagy, and muscle function.
Sex-lethal (Sxl) encodes the master regulator of the sex determination pathway in Drosophila and acts by controlling sex identity in both soma and germ line. In females Sxl maintains its own expression by controlling the alternative splicing of its own mRNA. Here, we identify a novel sex determination gene, spenito (nito) that encodes a SPEN family protein. Loss of nito activity results in stem cell tumors in the female germ line as well as female-to-male somatic transformations. We show that Nito is a ubiquitous nuclear protein that controls the alternative splicing of the Sxl mRNA by interacting with Sxl protein and pre-mRNA, suggesting that it is directly involved in Sxl auto-regulation. Given that SPEN family proteins are frequently mutated in cancers, our results suggest that these factors might be implicated in tumorigenesis through splicing regulation.
The propagation of force in epithelial tissues requires that the contractile cytoskeletal machinery be stably connected between cells through E-cadherin-containing adherens junctions. In many epithelial tissues, the cells' contractile network is positioned at a distance from the junction. However, the mechanism or mechanisms that connect the contractile networks to the adherens junctions, and thus mechanically connect neighboring cells, are poorly understood. Here, we identified the role for F-actin turnover in regulating the contractile cytoskeletal network's attachment to adherens junctions. Perturbing F-actin turnover via gene depletion or acute drug treatments that slow F-actin turnover destabilized the attachment between the contractile actomyosin network and adherens junctions. Our work identifies a critical role for F-actin turnover in connecting actomyosin to intercellular junctions, defining a dynamic process required for the stability of force balance across intercellular contacts in tissues.
Organ wasting, related to changes in nutrition and metabolic activity of cells and tissues, is observed under conditions of starvation and in the context of diseases, including cancers. We have developed a model for organ wasting in adult Drosophila, whereby overproliferation induced by activation of Yorkie, the Yap1 oncogene ortholog, in intestinal stem cells leads to wasting of the ovary, fat body, and muscle. These organ-wasting phenotypes are associated with a reduction in systemic insulin/IGF signaling due to increased expression of the secreted insulin/IGF antagonist ImpL2 from the overproliferating gut. Strikingly, expression of rate-limiting glycolytic enzymes and central components of the insulin/IGF pathway is upregulated with activation of Yorkie in the gut, which may provide a mechanism for this overproliferating tissue to evade the effect of ImpL2. Altogether, our study provides insights into the mechanisms underlying organ-wasting phenotypes in Drosophila and how overproliferating tissues adapt to global changes in metabolism.
In multicellular organisms, cell number is typically determined by a balance of intracellular signals that positively and negatively regulate cell survival and proliferation. Dissecting these signaling networks facilitates the understanding of normal development and tumorigenesis. Here, we study signaling by the Drosophila PDGF/VEGF Receptor (Pvr) in embryonic blood cells (hemocytes) and in the related cell line Kc as a model for the requirement of PDGF/VEGF receptors in vertebrate cell survival and proliferation. The system allows the investigation of downstream and parallel signaling networks, based on the ability of Pvr to activate Ras/Erk, Akt/TOR, and yet-uncharacterized signaling pathway/s, which redundantly mediate cell survival and contribute to proliferation. Using Kc cells, we performed a genome wide RNAi screen for regulators of cell number in a sensitized, Pvr deficient background. We identified the receptor tyrosine kinase (RTK) Insulin-like receptor (InR) as a major Pvr Enhancer, and the nuclear hormone receptors Ecdysone receptor (EcR) and ultraspiracle (usp), corresponding to mammalian Retinoid X Receptor (RXR), as Pvr Suppressors. In vivo analysis in the Drosophila embryo revealed a previously unrecognized role for EcR to promote apoptotic death of embryonic blood cells, which is balanced with pro-survival signaling by Pvr and InR. Phosphoproteomic analysis demonstrates distinct modes of cell number regulation by EcR and RTK signaling. We define common phosphorylation targets of Pvr and InR that include regulators of cell survival, and unique targets responsible for specialized receptor functions. Interestingly, our analysis reveals that the selection of phosphorylation targets by signaling receptors shows qualitative changes depending on the signaling status of the cell, which may have wide-reaching implications for other cell regulatory systems.