Hairless-binding deficient Suppressor of Hairless alleles reveal Su(H) protein levels are dependent on complex formation with Hairless

Novel Genome-Engineered H Alleles Differentially Affect Lateral Inhibition and Cell Dichotomy Processes during Bristle Organ Development

Hairless (H) encodes the major antagonist in the Notch signaling pathway, which governs cellular differentiation of various tissues in Drosophila. By binding to the Notch signal transducer Suppressor of Hairless (Su(H)), H assembles repressor complexes onto Notch target genes. Using genome engineering, three new H alleles, HFA, HLLAA and HWA were generated and a phenotypic series was established by several parameters, reflecting the residual H-Su(H) binding capacity. Occasionally, homozygous HWA flies develop to adulthood. They were compared with the likewise semi-viable HNN allele affecting H-Su(H) nuclear entry. The H homozygotes were short-lived, sterile and flightless, yet showed largely normal expression of several mitochondrial genes. Typical for H mutants, both HWA and HNN homozygous alleles displayed strong defects in wing venation and mechano-sensory bristle development. Strikingly, however, HWA displayed only a loss of bristles, whereas bristle organs of HNN flies showed a complete shaft-to-socket transformation. Apparently, the impact of HWA is restricted to lateral inhibition, whereas that of HNN also affects the respective cell type specification. Notably, reduction in Su(H) gene dosage only suppressed the HNN bristle phenotype, but amplified that of HWA. We interpret these differences as to the role of H regarding Su(H) stability and availability.

OptIC-Notch reveals mechanism that regulates receptor interactions with CSL

Active Notch signalling is elicited through receptor-ligand interactions that result in release of the Notch intracellular domain (NICD), which translocates into the nucleus. NICD activates transcription at target genes, forming a complex with the DNA-binding transcription factor CSL [CBF1/Su(H)/LAG-1] and co-activator Mastermind. However, CSL lacks its own nuclear localisation sequence, and it remains unclear where the tripartite complex is formed. To probe the mechanisms involved, we designed an optogenetic approach to control NICD release (OptIC-Notch) and monitored the subsequent complex formation and target gene activation. Strikingly, we observed that, when uncleaved, OptIC-Notch sequestered CSL in the cytoplasm. Hypothesising that exposure of a juxta membrane ΦWΦP motif is key to sequestration, we masked this motif with a second light-sensitive domain (OptIC-Notch{ω}), which was sufficient to prevent CSL sequestration. Furthermore, NICD produced by light-induced cleavage of OptIC-Notch or OptIC-Notch{ω} chaperoned CSL into the nucleus and induced target gene expression, showing efficient light-controlled activation. Our results demonstrate that exposure of the ΦWΦP motif leads to CSL recruitment and suggest this can occur in the cytoplasm prior to nuclear entry.

The role of the Notch signalling pathway in regulating the balance between neuronal and nonneuronal cells in sympathetic ganglia and the adrenal gland

Sympathetic neurons and endocrine chromaffin cells of the adrenal medulla are catecholaminergic cells that derive from the neural crest. According to the classic model, they develop from a common sympathoadrenal (SA) progenitor that has the ability to differentiate into both sympathetic neurons and chromaffin cells depending on signals provided by their final environment. Our previous data revealed that a single premigratory neural crest cell can give rise to both sympathetic neurons and chromaffin cells, indicating that the fate decision between these cell types occurs after delamination. A more recent study demonstrated that at least half of chromaffin cells arise from a later contribution by Schwann cell precursors. Since Notch signalling is known to be implicated in the regulation of cell fate decisions, we investigated the early role of Notch signalling in regulating the development of neuronal and non-neuronal SA cells within sympathetic ganglia and the adrenal gland. To this end, we implemented both gain and loss of function approaches. Electroporation of premigratory neural crest cells with plasmids encoding Notch inhibitors revealed an elevation in the number of SA cells expressing the catecholaminergic enzyme tyrosine-hydroxylase, with a concomitant reduction in the number of cells expressing the glial marker P0 in both sympathetic ganglia and adrenal gland. As expected, gain of Notch function had the opposite effect. Numbers of neuronal and non-neuronal SA cells were affected differently by Notch inhibition depending on the time of its onset. Together our data show that Notch signalling can regulate the ratio of glial cells, neuronal SA cells and nonneuronal SA cells in both sympathetic ganglia and the adrenal gland.

Genetic and Molecular Interactions between HΔCT, a Novel Allele of the Notch Antagonist Hairless, and the Histone Chaperone Asf1 in Drosophila melanogaster

Cellular differentiation relies on the highly conserved Notch signaling pathway. Notch activity induces gene expression changes that are highly sensitive to chromatin landscape. We address Notch gene regulation using Drosophila as a model, focusing on the genetic and molecular interactions between the Notch antagonist Hairless and the histone chaperone Asf1. Earlier work implied that Asf1 promotes the silencing of Notch target genes via Hairless (H). Here, we generate a novel HΔCT allele by genome engineering. Phenotypically, HΔCT behaves as a Hairless gain of function allele in several developmental contexts, indicating that the conserved CT domain of H has an attenuator role under native biological contexts. Using several independent methods to assay protein-protein interactions, we define the sequences of the CT domain that are involved in Hairless-Asf1 binding. Based on previous models, where Asf1 promotes Notch repression via Hairless, a loss of Asf1 binding should reduce Hairless repressive activity. However, tissue-specific Asf1 overexpression phenotypes are increased, not rescued, in the HΔCT background. Counterintuitively, Hairless protein binding mitigates the repressive activity of Asf1 in the context of eye development. These findings highlight the complex connections of Notch repressors and chromatin modulators during Notch target-gene regulation and open the avenue for further investigations.

The Binding of CSL Proteins to Either Co-Activators or Co-Repressors Protects from Proteasomal Degradation Induced by MAPK-Dependent Phosphorylation

The primary role of Notch is to specify cellular identities, whereby the cells respond to amazingly small changes in Notch signalling activity. Hence, dosage of Notch components is crucial to regulation. Central to Notch signal transduction are CSL proteins: together with respective cofactors, they mediate the activation or the silencing of Notch target genes. CSL proteins are extremely similar amongst species regarding sequence and structure. We noticed that the fly homologue suppressor of hairless (Su(H)) is stabilised in transcription complexes. Using specific transgenic fly lines and HeLa RBPJ^KO cells we provide evidence that Su(H) is subjected to proteasomal degradation with a half-life of about two hours if not protected by binding to co-repressor hairless or co-activator Notch. Moreover, Su(H) stability is controlled by MAPK-dependent phosphorylation, matching earlier data for RBPJ in human cells. The homologous murine and human RBPJ proteins, however, are largely resistant to degradation in our system. Mutating presumptive protein contact sites, however, sensitised RBPJ for proteolysis. Overall, our data highlight the similarities in the regulation of CSL protein stability across species and imply that turnover of CSL proteins may be a conserved means of regulating Notch signalling output directly at the level of transcription.

α-Phenylalanyl tRNA synthetase competes with Notch signaling through its N-terminal domain

The alpha subunit of the cytoplasmic Phenylalanyl tRNA synthetase (α-PheRS, FARSA in humans) displays cell growth and proliferation activities and its elevated levels can induce cell fate changes and tumor-like phenotypes that are neither dependent on the canonical function of charging tRNA^Phe with phenylalanine nor on stimulating general translation. In intestinal stem cells of Drosophila midguts, α-PheRS levels are naturally slightly elevated and human FARSA mRNA levels are elevated in multiple cancers. In the Drosophila midgut model, elevated α-PheRS levels caused the accumulation of many additional proliferating cells resembling intestinal stem cells (ISCs) and enteroblasts (EBs). This phenotype partially resembles the tumor-like phenotype described as Notch RNAi phenotype for the same cells. Genetic interactions between α-PheRS and Notch suggest that their activities neutralize each other and that elevated α-PheRS levels attenuate Notch signaling when Notch induces differentiation into enterocytes, type II neuroblast stem cell proliferation, or transcription of a Notch reporter. These non-canonical functions all map to the N-terminal part of α-PheRS which accumulates naturally in the intestine. This truncated version of α-PheRS (α-S) also localizes to nuclei and displays weak sequence similarity to the Notch intracellular domain (NICD), suggesting that α-S might compete with the NICD for binding to a common target. Supporting this hypothesis, the tryptophan (W) residue reported to be key for the interaction between the NICD and the Su(H) BTD domain is not only conserved in α-PheRS and α-S, but also essential for attenuating Notch signaling.

Aminoacyl tRNA synthetases charge tRNAs with their cognate amino acid to ensure proper decoding of the genetic code during translation. Independent of its aminoacylation function, the alpha subunit of Drosophila cytoplasmic Phenylalanyl tRNA synthetase (α-PheRS, FARSA in humans) has an additional activity that promotes growth and proliferation. Here we describe that elevated α-PheRS levels also induce cell fate changes and tumorous phenotypes in Drosophila midguts. Excessive proliferating cells with stem and progenitor cell characteristics accumulate and the composition of the terminally differentiated cells changes, too. This phenotype together with observed genetic interactions between α-PheRS and Notch levels show that α-PheRS counteracts Notch signaling in many different tissues and developmental stages. This novel activity of α-PheRS maps to its N-terminal part, which is naturally produced. The fragment contains a DNA binding domain, translocates into nuclei, and displays essential similarities to a Notch domain that binds to the downstream transcription factor. This suggests that it might be competing with Notch for binding to a common target. Not only because Notch plays important roles in many tumors, but also because FARSA mRNA levels are considerably upregulated in many tumors, this novel activity deserves more attention for cancer research.

Enhancers with cooperative Notch binding sites are more resistant to regulation by the Hairless co-repressor

Notch signaling controls many developmental processes by regulating gene expression. Notch-dependent enhancers recruit activation complexes consisting of the Notch intracellular domain, the Cbf/Su(H)/Lag1 (CSL) transcription factor (TF), and the Mastermind co-factor via two types of DNA sites: monomeric CSL sites and cooperative dimer sites called Su(H) paired sites (SPS). Intriguingly, the CSL TF can also bind co-repressors to negatively regulate transcription via these same sites. Here, we tested how synthetic enhancers with monomeric CSL sites versus dimeric SPSs bind Drosophila Su(H) complexes in vitro and mediate transcriptional outcomes in vivo. Our findings reveal that while the Su(H)/Hairless co-repressor complex similarly binds SPS and CSL sites in an additive manner, the Notch activation complex binds SPSs, but not CSL sites, in a cooperative manner. Moreover, transgenic reporters with SPSs mediate stronger, more consistent transcription and are more resistant to increased Hairless co-repressor expression compared to reporters with the same number of CSL sites. These findings support a model in which SPS containing enhancers preferentially recruit cooperative Notch activation complexes over Hairless repression complexes to ensure consistent target gene activation.

Cell signaling provides a basic means of communication during development. Many signaling pathways, including the Notch pathway, convert extracellular signals into changes in gene expression via transcription factors that bind specific DNA sequences. Importantly, the Notch pathway transcription factor can either form activating complexes upon Notch activation to stimulate gene expression or repression complexes with co-repressors to inhibit gene expression. Prior studies showed that the Notch activation complex binds DNA as either an independent complex on monomer binding sites or as two cooperative complexes (dimer) on paired binding sites. In this study, we used synthetic biology to examine how these two types of DNA sites impact the binding of Notch activation versus repression complexes and the output of Notch target gene expression. Our studies reveal that unlike the Notch activation complex, the repression complex does not cooperatively bind dimer sites. Moreover, our findings support the model that the enhanced stability of the Notch activation complex on dimer sites makes target genes with dimer sites less sensitive to the repression complex than target genes with only monomer sites. Thus, our studies reveal how target genes with different binding sites differ in sensitivity to the ratio of Notch activation to repression complexes.

Phospho-Site Mutations in Transcription Factor Suppressor of Hairless Impact Notch Signaling Activity During Hematopoiesis in Drosophila

The highly conserved Notch signaling pathway controls a multitude of developmental processes including hematopoiesis. Here, we provide evidence for a novel mechanism of tissue-specific Notch regulation involving phosphorylation of CSL transcription factors within the DNA-binding domain. Earlier we found that a phospho-mimetic mutation of the Drosophila CSL ortholog Suppressor of Hairless [Su(H)] at Ser269 impedes DNA-binding. By genome-engineering, we now introduced phospho-specific Su(H) mutants at the endogenous Su(H) locus, encoding either a phospho-deficient [Su(H) S269A ] or a phospho-mimetic [Su(H) S269D ] isoform. Su(H) S269D mutants were defective of Notch activity in all analyzed tissues, consistent with impaired DNA-binding. In contrast, the phospho-deficient Su(H) S269A mutant did not generally augment Notch activity, but rather specifically in several aspects of blood cell development. Unexpectedly, this process was independent of the corepressor Hairless acting otherwise as a general Notch antagonist in Drosophila. This finding is in agreement with a novel mode of Notch regulation by posttranslational modification of Su(H) in the context of hematopoiesis. Importantly, our studies of the mammalian CSL ortholog (RBPJ/CBF1) emphasize a potential conservation of this regulatory mechanism: phospho-mimetic RBPJ S221D was dysfunctional in both the fly as well as two human cell culture models, whereas phospho-deficient RBPJ S221A rather gained activity during fly hematopoiesis. Thus, dynamic phosphorylation of CSL-proteins within the DNA-binding domain provides a novel means to fine-tune Notch signal transduction in a context-dependent manner.

Nucleo-cytoplasmic shuttling of murine RBPJ by Hairless protein matches that of Su(H) protein in the model system Drosophila melanogaster

Abstract

CSL transcription factors are central to signal transduction in the highly conserved Notch signaling pathway. CSL acts as a molecular switch: depending on the cofactors recruited, CSL induces either activation or repression of Notch target genes. Unexpectedly, CSL depends on its cofactors for nuclear entry, despite its role as gene regulator. In Drosophila, the CSL homologue Suppressor of Hairless (Su(H)), recruits Hairless (H) for repressor complex assembly, and eventually for nuclear import. We recently found that Su(H) is subjected to a dynamic nucleo-cytoplasmic shuttling, thereby strictly following H subcellular distribution. Hence, regulation of nuclear availability of Su(H) by H may represent a new layer of control of Notch signaling activity. Here we extended this work on the murine CSL homologue RBPJ. Using a ‘murinized’ fly model bearing RBPJ^wt in place of Su(H) at the endogenous locus we demonstrate that RBPJ protein likewise follows H subcellular distribution. For example, overexpression of a H^*NLS3 protein variant defective of nuclear import resulted in a cytosolic localization of RBPJ protein, whereas the overexpression of a H^*NES protein variant defective in the nuclear export signal caused the accumulation of RBPJ protein in the nucleus. Evidently, RBPJ is exported from the nucleus as well. Overall these data demonstrate that in our fly model, RBPJ is subjected to H-mediated nucleo-cytoplasmic shuttling as is Su(H). These data raise the possibility that nuclear availability of mammalian CSL proteins is likewise restricted by cofactors, and may hence present a more general mode of regulating Notch signaling activity.

Graphical abstract

Membrane-Anchored Hairless Protein Restrains Notch Signaling Activity

The Notch signaling pathway governs cell-to-cell communication in higher eukaryotes. In Drosophila, after cleavage of the transmembrane receptor Notch, the intracellular domain of Notch (ICN) binds to the transducer Suppressor of Hairless (Su(H)) and shuttles into the nucleus to activate Notch target genes. Similarly, the Notch antagonist Hairless transfers Su(H) into the nucleus to repress Notch target genes. With the aim to prevent Su(H) nuclear translocation, Hairless was fused to a transmembrane domain to anchor the protein at membranes. Indeed, endogenous Su(H) co-localized with membrane-anchored Hairless, demonstrating their binding in the cytoplasm. Moreover, adult phenotypes uncovered a loss of Notch activity, in support of membrane-anchored Hairless sequestering Su(H) in the cytosol. A combined overexpression of membrane-anchored Hairless with Su(H) lead to tissue proliferation, which is in contrast to the observed apoptosis after ectopic co-overexpression of the wild-type genes, indicating a shift to a gain of Notch activity. A mixed response, general de-repression of Notch signaling output, plus inhibition at places of highest Notch activity, perhaps reflects Su(H)’s role as activator and repressor, supported by results obtained with the Hairless-binding deficient Su(H)^LLL mutant, inducing activation only. Overall, the results strengthen the idea of Su(H) and Hairless complex formation within the cytosolic compartment.

Different modes of Notch activation and strength regulation in the spermathecal secretory lineage

The strength of Notch signaling contributes to pleiotropic actions of Notch; however, we do not yet have a full understanding of the molecular regulation of Notch-signaling strength. We have investigated the mode of Notch activation in binary fate specification in the Drosophila spermathecal linage, where Notch is asymmetrically activated across three divisions to specify different cell fates. Using clonal analysis, we show that Delta (Dl) serves as the ligand for Notch in the first and second divisions. Dl and Serrate (Ser) function redundantly in the third division. Compared with the third division, cell-fate decision in the second division requires a lower level of Suppressor of Hairless protein, and, consequently, a lower level of Notch signaling. Several Notch endosomal trafficking regulators differentially regulate Notch signaling between the second and third divisions. Here, we demonstrate that cell differentiation in spermathecae involves different Notch-activation modes, Notch-signaling strengths and Notch-trafficking regulations. Thus, the Drosophila spermathecal lineage is an exciting model for probing the molecular mechanisms that modulate the Notch signaling pathway.

An RBPJ-Drosophila Model Reveals Dependence of RBPJ Protein Stability on the Formation of Transcription-Regulator Complexes

Notch signaling activity governs widespread cellular differentiation in higher animals, including humans, and is involved in several congenital diseases and different forms of cancer. Notch signals are mediated by the transcriptional regulator RBPJ in a complex with activated Notch (NICD). Analysis of Notch pathway regulation in humans is hampered by a partial redundancy of the four Notch receptor copies, yet RBPJ is solitary, allowing its study in model systems. In Drosophila melanogaster, the RBPJ orthologue is encoded by Suppressor of Hairless [Su(H)]. Using genome engineering, we replaced Su(H) by murine RBPJ in order to study its function in the fly. In fact, RBPJ largely substitutes for Su(H)’s function, yet subtle phenotypes reflect increased Notch signaling activity. Accordingly, the binding of RBPJ to Hairless (H) protein, the general Notch antagonist in Drosophila, was considerably reduced compared to that of Su(H). An H-binding defective RBPJ^LLL mutant matched the respective Su(H)^LLL allele: homozygotes were lethal due to extensive Notch hyperactivity. Moreover, RBPJ^LLL protein accumulated at lower levels than wild type RBPJ, except in the presence of NICD. Apparently, RBPJ protein stability depends on protein complex formation with either H or NICD, similar to Su(H), demonstrating that the murine homologue underlies the same regulatory mechanisms as Su(H) in Drosophila. These results underscore the importance of regulating the availability of RBPJ protein to correctly mediate Notch signaling activity in the fly.

Protein Kinase D Is Dispensable for Development and Survival of Drosophila melanogaster

Members of the Protein Kinase D (PKD) family are involved in numerous cellular processes in mammals, including cell survival after oxidative stress, polarized transport of Golgi vesicles, as well as cell migration and invasion. PKD proteins belong to the PKC/CAMK class of serine/threonine kinases, and transmit diacylglycerol-regulated signals. Whereas three PKD isoforms are known in mammals, Drosophila melanogaster contains a single PKD homolog. Previous analyses using overexpression and RNAi studies indicated likewise multi-facetted roles for Drosophila PKD, including the regulation of secretory transport and actin-cytoskeletal dynamics. Recently, involvement in growth regulation has been proposed based on the hypomorphic dPKD^H allele. We have generated PKD null alleles that are homozygous viable without apparent phenotype. They largely match control flies regarding fertility, developmental timing and weight. Males, but not females, are slightly shorter lived and starvation sensitive. Furthermore, migration of pole cells in embryos and border cells in oocytes appears normal. PKD mutants tolerate heat, cold and osmotic stress like the control but are sensitive to oxidative stress, conforming to the described role for mammalian PKDs. A candidate screen to identify functionally redundant kinases uncovered genetic interactions of PKD with Pkcδ, sqa and Drak mutants, further supporting the role of PKD in oxidative stress response, and suggesting its involvement in starvation induced autophagy and regulation of cytoskeletal dynamics. Overall, PKD appears dispensable for fly development and survival presumably due to redundancy, but influences environmental responses.

Nucleo-cytoplasmic shuttling of Drosophila Hairless/Su(H) heterodimer as a means of regulating Notch dependent transcription

Activation and repression of Notch target genes is mediated by transcription factor CSL, known as Suppressor of Hairless (Su(H)) in Drosophila and CBF1 or RBPJ in human. CSL associates either with co-activator Notch or with co-repressors such as Drosophila Hairless. The nuclear translocation of transcription factor CSL relies on co-factor association, both in mammals and in Drosophila. The Drosophila CSL orthologue Su(H) requires Hairless for repressor complex formation. Based on its role in transcriptional silencing, H protein would be expected to be strictly nuclear. However, H protein is also cytosolic, which may relate to its role in the stabilization and nuclear translocation of Su(H) protein. Here, we investigate the function of the predicted nuclear localization signals (NLS 1-3) and single nuclear export signal (NES) of co-repressor Hairless using GFP-fusion proteins, reporter assays and in vivo analyses using Hairless wild type and shuttling-defective Hairless mutants. We identify NLS3 and NES to be critical for Hairless function. In fact, H^⁎NLS3 mutant flies match H null mutants, whereas H^{⁎NLS3⁎NES} double mutants display weaker phenotypes in agreement with a crucial role for NES in H export. As expected for a transcriptional repressor, Notch target genes are deregulated in H^⁎NLS3 mutant cells, demonstrating nuclear requirement for its activity. Importantly, we reveal that Su(H) protein strictly follows Hairless protein localization. Together, we propose that shuttling between the nucleo-cytoplasmic compartments provides the possibility to fine tune the regulation of Notch target gene expression by balancing of Su(H) protein availability for Notch activation.

The evolution of transcriptional repressors in the Notch signaling pathway: a computational analysis

Background

The Notch signaling pathway governs the specification of different cell types in flies, nematodes and vertebrates alike. Principal components of the pathway that activate Notch target genes are highly conserved throughout the animal kingdom. Despite the impact on development and disease, repression mechanisms are less well studied. Repressors are known from arthropods and vertebrates that differ strikingly by mode of action: whereas Drosophila Hairless assembles repressor complexes with CSL transcription factors, competition between activator and repressors occurs in vertebrates (for example SHARP/MINT and KyoT2). This divergence raises questions on the evolution: Are there common ancestors throughout the animal kingdom?

Results

Available genome databases representing all animal clades were searched for homologues of Hairless, SHARP and KyoT2. The most distant species with convincing Hairless orthologs belong to Myriapoda, indicating its emergence after the Mandibulata-Chelicarata radiation about 500 million years ago. SHARP shares motifs with SPEN and SPENITO proteins, present throughout the animal kingdom. The CSL interacting domain of SHARP, however, is specific to vertebrates separated by roughly 600 million years of evolution. KyoT2 bears a C-terminal CSL interaction domain (CID), present only in placental mammals but highly diverged already in marsupials, suggesting introduction roughly 100 million years ago. Based on the LIM-domains that characterize KyoT2, homologues can be found in Drosophila melanogaster (Limpet) and Hydra vulgaris (Prickle 3 like). These lack the CID of KyoT2, however, contain a PET and additional LIM domains. Conservation of intron/exon boundaries underscores the phylogenetic relationship between KyoT2, Limpet and Prickle. Most strikingly, Limpet and Prickle proteins carry a tetra-peptide motif resembling that of several CSL interactors. Overall, KyoT2 may have evolved from prickle and Limpet to a Notch repressor in mammals.

Conclusions

Notch repressors appear to be specific to either chordates or arthropods. Orthologues of experimentally validated repressors were not found outside the phylogenetic group they have been originally identified. However, the data provide a hypothesis on the evolution of mammalian KyoT2 from Prickle like ancestors. The finding of a potential CSL interacting domain in Prickle homologues points to a novel, very ancestral CSL interactor present in the entire animal kingdom.

Electronic supplementary material

The online version of this article (10.1186/s41065-019-0081-0) contains supplementary material, which is available to authorized users.

The Notch repressor complex in Drosophila: in vivo analysis of Hairless mutants using overexpression experiments

During development of higher animals, the Notch signalling pathway governs cell type specification by mediating appropriate gene expression responses. In the absence of signalling, Notch target genes are silenced by repressor complexes. In the model organism Drosophila melanogaster, the repressor complex includes the transcription factor Suppressor of Hairless [Su(H)] and Hairless (H) plus general co-repressors. Recent crystal structure analysis of the Drosophila Notch repressor revealed details of the Su(H)-H complex. They were confirmed by mutational analyses of either protein; however, only Su(H) mutants have been further studied in vivo. Here, we analyse three H variants predicted to affect Su(H) binding. To this end, amino acid replacements Phenylalanine 237, Leucines 245 and 247, as well as Tryptophan 258 to Alanine were introduced into the H protein. A cell-based reporter assay indicates substantial loss of Su(H) binding to the respective mutant proteins H^FA, H^LLAA and H^WA. For in vivo analysis, UAS-lines H^FA, H^LLAA and H^WA were generated to allow spatially restricted overexpression. In these assays, all three mutants resembled the H^LD control, shown before to lack Su(H) binding, indicating a strong reduction of H activity. For example, the H variants were impaired in wing margin formation, but unexpectedly induced ectopic wing venation. Concurrent overexpression with Su(H), however, suggests that all mutant H protein isoforms are still able to bind Su(H) in vivo. We conclude that a weakening of the cohesion in the H-Su(H) repressor complex is sufficient for disrupting its in vivo functionality.

Overexpression of the Drosophila ATR homologous checkpoint kinase Mei-41 induces a G2/M checkpoint in Drosophila imaginal tissue

Background

DNA damage generally results in the activation of ATM/ATR kinases and the downstream checkpoint kinases Chk1/Chk2. In Drosophila melanogaster, the ATR homologue meiotic 41 (mei-41) is pivotal to DNA damage repair and cell cycle checkpoint signalling. Although various mei-41 mutant alleles have been analyzed in the past, no gain-of-function allele is yet available. To fill this gap, we have generated transgenic flies allowing temporal and tissue-specific induction of mei-41.

Results

Overexpression of mei-41 in wing and eye anlagen affects proliferation and a G2/M checkpoint even in the absence of genomic stress. Similar consequences were observed following the overexpression of the downstream kinase Grapes (Grp) but not of Loki (Lok), encoding the respective Drosophila Chk1 and Chk2 homologues, in agreement with their previously reported activities. Moreover, we show that irradiation induced cell cycle arrest was prolonged in the presence of ectopic mei-41 expression. Similar to irradiation stress, mei-41 triggered the occurrence of a slower migrating form of Grp, implying specific phosphorylation of Grp in response to either signal. Using a p53R-GFP biosensor, we further show that overexpression of mei-41 was sufficient to elicit a robust p53 activation in vivo.

Conclusion

We conclude that overexpression of the Drosophila ATR homologue mei-41 elicits an effectual DNA damage response irrespective of irradiation.

Electronic supplementary material

The online version of this article (10.1186/s41065-018-0066-4) contains supplementary material, which is available to authorized users.

Complex genetic interactions of novel Suppressor of Hairless alleles deficient in co-repressor binding

Throughout the animal kingdom, the Notch signalling pathway allows cells to acquire diversified cell fates. Notch signals are translated into activation of Notch target genes by CSL transcription factors. In the absence of Notch signals, CSL together with co-repressors functions as a transcriptional repressor. In Drosophila, repression of Notch target genes involves the CSL homologue Suppressor of Hairless (Su(H)) and the Notch (N) antagonist Hairless (H) that together form a repressor complex. Guided by crystal structure, three mutations Su(H)LL, Su(H)LLF and Su(H)LLL were generated that specifically affect interactions with the repressor H, and were introduced into the endogenous Su(H) locus by gene engineering. In contrast to the wild type isoform, these Su(H) mutants are incapable of repressor complex formation. Accordingly, Notch signalling activity is dramatically elevated in the homozygotes, resembling complete absence of H activity. It was noted, however, that heterozygotes do not display a dominant H loss of function phenotype. In this work we addressed genetic interactions the three H-binding deficient Su(H) mutants display in combination with H and N null alleles. We included a null mutant of Delta (Dl), encoding the ligand of the Notch receptor, as well as of Su(H) itself in our genetic analyses. H, N or Dl mutations cause dominant wing phenotypes that are sensitive to gene dose of the others. Moreover, H heterozygotes lack bristle organs and develop bristle sockets instead of shafts. The latter phenotype is suppressed by Su(H) null alleles but not by H-binding deficient Su(H) alleles which we attribute to the socket cell specific activity of Su(H). Modification of the dominant wing phenotypes of either H, N or Dl, however, suggested some lack of repressor activity in the Su(H) null allele and likewise in the H-binding deficient Su(H) alleles. Overall, Su(H) mutants are recessive perhaps reflecting self-adjusting availability of Su(H) protein.

p53 and cyclin G cooperate in mediating genome stability in somatic cells of Drosophila

One of the key players in genome surveillance is the tumour suppressor p53 mediating the adaptive response to a multitude of stress signals. Here we identify Cyclin G (CycG) as co-factor of p53-mediated genome stability. CycG has been shown before to be involved in double-strand break repair during meiosis. Moreover, it is also important for mediating DNA damage response in somatic tissue. Here we find it in protein complexes together with p53, and show that the two proteins interact physically in vitro and in vivo in response to ionizing irradiation. In contrast to mammals, Drosophila Cyclin G is no transcriptional target of p53. Genetic interaction data reveal that p53 activity during DNA damage response requires the presence of CycG. Morphological defects caused by overexpression of p53 are ameliorated in cycG null mutants. Moreover, using a p53 biosensor we show that p53 activity is impeded in cycG mutants. As both p53 and CycG are likewise required for DNA damage repair and longevity we propose that CycG plays a positive role in mediating p53 function in genome surveillance of Drosophila.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

Phosphorylation of Suppressor of Hairless impedes its DNA-binding activity

Notch signalling activity governs cellular differentiation in higher metazoa, where Notch signals are transduced by the transcription factor CSL, called Suppressor of Hairless [Su(H)] in Drosophila. Su(H) operates as molecular switch on Notch target genes: within activator complexes, including intracellular Notch, or within repressor complexes, including the antagonist Hairless. Mass spectrometry identified phosphorylation on Serine 269 in Su(H), potentially serving as a point of cross-regulation by other signalling pathways. To address the biological significance, we generated phospho-deficient [Su(H)S269A] and phospho-mimetic [Su(H)S269D] variants: the latter displayed reduced transcriptional activity despite unaltered protein interactions with co-activators and -repressors. Based on the Su(H) structure, Ser269 phosphorylation may interfere with DNA-binding, which we confirmed by electro-mobility shift assay and isothermal titration calorimetry. Overexpression of Su(H)S269D during fly development demonstrated reduced transcriptional regulatory activity, similar to the previously reported DNA-binding defective mutant Su(H)R266H. As both are able to bind Hairless and Notch proteins, Su(H)S269D and Su(H)R266H provoked dominant negative effects upon overexpression. Our data imply that Ser269 phosphorylation impacts Notch signalling activity by inhibiting DNA-binding of Su(H), potentially affecting both activation and repression. Ser269 is highly conserved in vertebrate CSL homologues, opening the possibility of a general and novel mechanism of modulating Notch signalling activity.