Drosophila CtBP regulates proliferation and differentiation of eye precursors and complexes with Eyeless, Dachshund, Dan, and Danr during eye and antennal development

The JNK and Hippo pathways control epithelial integrity and prevent tumor initiation by regulating an overlapping transcriptome

Epithelial organs maintain their integrity and prevent tumor initiation by actively removing defective cells, such as those that have lost apicobasal polarity. Here, we identify how transcription factors of two key signaling pathways-Jun-N-terminal kinase (JNK) and Hippo-regulate epithelial integrity by controlling transcription of an overlapping set of target genes. Targeted DamID experiments reveal that, in proliferating cells of the Drosophila melanogaster eye, the AP-1 transcription factor Jun and the Hippo pathway transcription regulators Yorkie and Scalloped bind to a common suite of target genes that promote organ growth. In defective neoplastic cells, AP-1 transcription factors repress transcription of growth genes together with the C-terminal binding protein (CtBP) co-repressor. If gene repression by AP-1/CtBP fails, neoplastic tumor growth ensues, driven by Yorkie/Scalloped. Thus, AP-1/CtBP eliminates defective cells and prevents tumor initiation by acting in parallel to Yorkie/Scalloped to repress expression of a shared transcriptome. These findings shed new light on the maintenance of epithelial integrity and tumor suppression.

Dachshund and C-terminal Binding Protein bind directly during Drosophila eye development

The transcription factor Dachshund (Dac) and the transcriptional co-regulator C-terminal Binding Protein (CtBP) were identified as the retinal determination factors during Drosophila eye development . A previous study established that Dac and CtBP interact genetically during eye development. Co-immunoprecipitation assays suggested that both molecules interact in the Drosophila larval eye-antennal disc. Our present study shows that Dac and CtBP bind each other directly, as determined by GST pull-down assays. Thus, our results demonstrate the molecular mechanism of Dac and CtBP interaction and suggest the direct binding of these two transcription regulators in the cells of the eye disc promotes the Drosophila eye specification.

Molecular and cytological analysis of widely-used Gal4 driver lines for Drosophila neurobiology

Background

The Drosophila central nervous system (CNS) is a convenient model system for the study of the molecular mechanisms of conserved neurobiological processes. The manipulation of gene activity in specific cell types and subtypes of the Drosophila CNS is frequently achieved by employing the binary Gal4/UAS system. However, many Gal4 driver lines available from the Bloomington Drosophila Stock Center (BDSC) and commonly used in Drosophila neurobiology are still not well characterized. Among these are three lines with Gal4 driven by the elav promoter (BDSC #8760, #8765, and #458), one line with Gal4 driven by the repo promoter (BDSC #7415), and the 69B-Gal4 line (BDSC #1774). For most of these lines, the exact insertion sites of the transgenes and the detailed expression patterns of Gal4 are not known. This study is aimed at filling these gaps.

Results

We have mapped the genomic location of the Gal4-bearing P-elements carried by the BDSC lines #8760, #8765, #458, #7415, and #1774. In addition, for each of these lines, we have analyzed the Gal4-driven GFP expression pattern in the third instar larval CNS and eye-antennal imaginal discs. Localizations of the endogenous Elav and Repo proteins were used as markers of neuronal and glial cells, respectively.

Conclusions

We provide a mini-atlas of the spatial activity of Gal4 drivers that are widely used for the expression of UAS–target genes in the Drosophila CNS. The data will be helpful for planning experiments with these drivers and for the correct interpretation of the results.

Pits and CtBP Control Tissue Growth in Drosophila melanogaster with the Hippo Pathway Transcription Repressor Tgi

The Hippo pathway is an evolutionarily conserved signaling network that regulates organ size, cell fate, and tumorigenesis. In the context of organ size control, the pathway incorporates a large variety of cellular cues, such as cell polarity and adhesion, into an integrated transcriptional response. The central Hippo signaling effector is the transcriptional coactivator Yorkie, which controls gene expression in partnership with different transcription factors, most notably Scalloped. When it is not activated by Yorkie, Scalloped can act as a repressor of transcription, at least in part due to its interaction with the corepressor protein Tgi. The mechanism by which Tgi represses transcription is incompletely understood, and therefore we sought to identify proteins that potentially operate together with Tgi. Using an affinity purification and mass-spectrometry approach we identified Pits and CtBP as Tgi-interacting proteins, both of which have been linked to transcriptional repression. Both Pits and CtBP were required for Tgi to suppress the growth of the Drosophila melanogaster eye and CtBP loss suppressed the undergrowth of yorkie mutant eye tissue. Furthermore, as reported previously for Tgi, overexpression of Pits repressed transcription of Hippo pathway target genes. These findings suggest that Tgi might operate together with Pits and CtBP to repress transcription of genes that normally promote tissue growth. The human orthologs of Tgi, CtBP, and Pits (VGLL4, CTBP2, and IRF2BP2) have previously been shown to physically and functionally interact to control transcription, implying that the mechanism by which these proteins control transcriptional repression is conserved throughout evolution.

CtBP represses Dpp-dependent Mad activation during Drosophila eye development

Complex networks of signaling pathways maintain the correct balance between positive and negative growth signals, ensuring that tissues achieve proper sizes and differentiation pattern during development. In Drosophila, Dpp, a member of the TGFβ family, plays two main roles during larval eye development. In the early eye primordium, Dpp promotes growth and cell survival, but later on, it switches its function to induce a developmentally-regulated cell cycle arrest in the G1 phase and neuronal photoreceptor differentiation. To advance in the identification and characterization of regulators and targets of Dpp signaling required for retinal development, we carried out an in vivo eye-targeted double-RNAi screen to identify punt (Type II TGFβ receptor) interactors. Using a set of 251 genes associated with eye development, we identified CtBP, Dad, Ago and Brk as punt genetic interactors. Here, we show that downregulation of Ago, or conditions causing increased tissue growth including overexpression of Myc or CyclinD-Cdk4 are sufficient to partially rescue punt-dependent growth and photoreceptor differentiation. Interestingly, we show a novel role for the transcriptional co-repressor CtBP in inhibiting Dpp-dependent Mad activation by phosphorylation, downstream or in parallel to Dad, the inhibitory Smad. Furthermore, CtBP downregulation activates JNK signaling pathway, implying a complex regulation of signaling pathways by CtBP during eye development.

The Role of CtBP1 in Oncogenic Processes and Its Potential as a Therapeutic Target

Transcriptional corepressor proteins have emerged as an important facet of cancer etiology. These corepressor proteins are often altered by loss- or gain-of-function mutations, leading to transcriptional imbalance. Thus, research directed at expanding our current understanding of transcriptional corepressors could impact the future development of new cancer diagnostics, prognostics, and therapies. In this review, our current understanding of the CtBP corepressors, and their role in both development and disease, is discussed in detail. Importantly, the role of CtBP1 overexpression in adult tissues in promoting the progression of multiple cancer types through their ability to modulate the transcription of developmental genes ectopically is explored. CtBP1 overexpression is known to be protumorigenic and affects the regulation of gene networks associated with "cancer hallmarks" and malignant behavior, including increased cell survival, proliferation, migration, invasion, and the epithelial-mesenchymal transition. As a transcriptional regulator of broad developmental processes capable of promoting malignant growth in adult tissues, therapeutically targeting the CtBP1 corepressor has the potential to be an effective method for the treatment of diverse tumor types. Although efforts to develop CtBP1 inhibitors are still in the early stages, the current progress and the future perspectives of therapeutically targeting this transcriptional corepressor are also discussed. Mol Cancer Ther; 16(6); 981-90. ©2017 AACR.

Segregation of eye and antenna fates maintained by mutual antagonism in Drosophila

A general question in development is how do adjacent primordia adopt different developmental fates and stably maintain their distinct fates? In Drosophila melanogaster, the adult eye and antenna originate from the embryonic eye-antenna primordium. These cells proliferate in the larval stage to form the eye-antenna disc. The eye or antenna differs at mid second instar with the restricted expression of Cut (Ct), a homeodomain transcriptional repressor, in the antenna disc and Eyeless (Ey), a Pax6 transcriptional activator, in the eye disc. In this study, we show that ey transcription in the antenna disc is repressed by two homeodomain proteins, Ct and Homothorax (Hth). Loss of Ct and Hth in the antenna disc resulted in ectopic eye development in the antenna. Conversely, the Ct and Hth expression in the eye disc was suppressed by the homeodomain transcription factor Sine oculis (So), a direct target of Ey. Loss of So in the eye disc caused ectopic antenna development in the eye. Therefore, the segregation of eye and antenna fates is stably maintained by mutual repression of the other pathway.

Analysis of the transcriptomes downstream of Eyeless and the Hedgehog, Decapentaplegic and Notch signaling pathways in Drosophila melanogaster

Tissue-specific transcription factors are thought to cooperate with signaling pathways to promote patterned tissue specification, in part by co-regulating transcription. The Drosophila melanogaster Pax6 homolog Eyeless forms a complex, incompletely understood regulatory network with the Hedgehog, Decapentaplegic and Notch signaling pathways to control eye-specific gene expression. We report a combinatorial approach, including mRNAseq and microarray analyses, to identify targets co-regulated by Eyeless and Hedgehog, Decapentaplegic or Notch. Multiple analyses suggest that the transcriptomes resulting from co-misexpression of Eyeless+signaling factors provide a more complete picture of eye development compared to previous efforts involving Eyeless alone: (1) Principal components analysis and two-way hierarchical clustering revealed that the Eyeless+signaling factor transcriptomes are closer to the eye control transcriptome than when Eyeless is misexpressed alone; (2) more genes are upregulated at least three-fold in response to Eyeless+signaling factors compared to Eyeless alone; (3) based on gene ontology analysis, the genes upregulated in response to Eyeless+signaling factors had a greater diversity of functions compared to Eyeless alone. Through a secondary screen that utilized RNA interference, we show that the predicted gene CG4721 has a role in eye development. CG4721 encodes a neprilysin family metalloprotease that is highly up-regulated in response to Eyeless+Notch, confirming the validity of our approach. Given the similarity between D. melanogaster and vertebrate eye development, the large number of novel genes identified as potential targets of Ey+signaling factors will provide novel insights to our understanding of eye development in D. melanogaster and humans.

The pipsqueak-domain proteins Distal antenna and Distal antenna-related restrict Hunchback neuroblast expression and early-born neuronal identity

A fundamental question in brain development is how precursor cells generate a diverse group of neural progeny in an ordered manner. Drosophila neuroblasts sequentially express the transcription factors Hunchback (Hb), Krüppel (Kr), Pdm1/Pdm2 (Pdm) and Castor (Cas). Hb is necessary and sufficient to specify early-born temporal identity and, thus, Hb downregulation is essential for specification of later-born progeny. Here, we show that distal antenna (dan) and distal antenna-related (danr), encoding pipsqueak motif DNA-binding domain protein family members, are detected in all neuroblasts during the Hb-to-Cas expression window. Dan and Danr are required for timely downregulation of Hb in neuroblasts and for limiting the number of early-born neurons. Dan and Danr function independently of Seven-up (Svp), an orphan nuclear receptor known to repress Hb expression in neuroblasts, because Dan, Danr and Svp do not regulate each other and dan danr svp triple mutants have increased early-born neurons compared with either dan danr or svp mutants. Interestingly, misexpression of Hb can induce Dan and Svp expression in neuroblasts, suggesting that Hb might activate a negative feedback loop to limit its own expression. We conclude that Dan/Danr and Svp act in parallel pathways to limit Hb expression and allow neuroblasts to transition from making early-born neurons to late-born neurons at the proper time.