Patterning of cells in the Drosophila eye by Lozenge, which shares homologous domains with AML1

Synergistic activation by Glass and Pointed promotes neuronal identity in the Drosophila eye disc

The integration of extrinsic signaling with cell-intrinsic transcription factors can direct progenitor cells to differentiate into distinct cell fates. In the developing Drosophila eye, differentiation of photoreceptors R1-R7 requires EGFR signaling mediated by the transcription factor Pointed, and our single-cell RNA-Seq analysis shows that the same photoreceptors require the eye-specific transcription factor Glass. We find that ectopic expression of Glass and activation of EGFR signaling synergistically induce neuronal gene expression in the wing disc in a Pointed-dependent manner. Targeted DamID reveals that Glass and Pointed share many binding sites in the genome of developing photoreceptors. Comparison with transcriptomic data shows that Pointed and Glass induce photoreceptor differentiation through intermediate transcription factors, including the redundant homologs Scratch and Scrape, as well as directly activating neuronal effector genes. Our data reveal synergistic activation of a multi-layered transcriptional network as the mechanism by which EGFR signaling induces neuronal identity in Glass-expressing cells.

Drosophila immune cells transport oxygen through PPO2 protein phase transition

Insect respiration has long been thought to be solely dependent on an elaborate tracheal system without assistance from the circulatory system or immune cells1,2. Here we describe that Drosophila crystal cells-myeloid-like immune cells called haemocytes-control respiration by oxygenating Prophenoloxidase 2 (PPO2) proteins. Crystal cells direct the movement of haemocytes between the trachea of the larval body wall and the circulation to collect oxygen. Aided by copper and a neutral pH, oxygen is trapped in the crystalline structures of PPO2 in crystal cells. Conversely, PPO2 crystals can be dissolved when carbonic anhydrase lowers the intracellular pH and then reassembled into crystals in cellulo by adhering to the trachea. Physiologically, larvae lacking crystal cells or PPO2, or those expressing a copper-binding mutant of PPO2, display hypoxic responses under normoxic conditions and are susceptible to hypoxia. These hypoxic phenotypes can be rescued by hyperoxia, expression of arthropod haemocyanin or prevention of larval burrowing activity to expose their respiratory organs. Thus, we propose that insect immune cells collaborate with the tracheal system to reserve and transport oxygen through the phase transition of PPO2 crystals, facilitating internal oxygen homeostasis in a process that is comparable to vertebrate respiration.

A single cell genomics atlas of the Drosophila larval eye reveals distinct photoreceptor developmental timelines

The Drosophila eye is a powerful model system to study the dynamics of cell differentiation, cell state transitions, cell maturation, and pattern formation. However, a high-resolution single cell genomics resource that accurately profiles all major cell types of the larval eye disc and their spatiotemporal relationships is lacking. Here, we report transcriptomic and chromatin accessibility data for all known cell types in the developing eye. Photoreceptors appear as strands of cells that represent their dynamic developmental timelines. As photoreceptor subtypes mature, they appear to assume a common transcriptomic profile that is dominated by genes involved in axon function. We identify cell type maturation genes, enhancers, and potential regulators, as well as genes with distinct R3 or R4 photoreceptor specific expression. Finally, we observe that the chromatin accessibility between cones and photoreceptors is distinct. These single cell genomics atlases will greatly enhance the power of the Drosophila eye as a model system.

CgWnt-1 regulates haemocyte proliferation during immune response of oyster Crassostrea gigas

Recent findings regarding the immunomodulatory role of Wnt signaling suggest that it is significant in regulating the differentiation and proliferation of immune cells. In the present study, a Wnt-1 homolog (designated as CgWnt-1) with a conserved WNT1 domain was identified from oyster Crassostrea gigas. The transcripts of CgWnt-1 were barely expressed in egg to gastrula stage during early embryogenesis, and up-regulated significantly in the trochophore to juvenile stage. The mRNA transcripts of CgWnt-1 were detected in different tissues of adult oyster, with an extremely high expression level in the mantle, which was 77.38-fold (p < 0.05) of that in labial palp. After Vibrio splendidus stimulation, the mRNA expression levels of CgWnt-1 and Cgβ-catenin in haemocytes up-regulated significantly at 3, 12, 24, and 48 h (p < 0.05). After injection of recombinant protein (rCgWnt-1) into oyster in vivo, the expressions of Cgβ-catenin, cell proliferation related genes CgRunx-1 and CgCDK-2 in haemocytes significantly up-regulated, which were 4.86-fold (p < 0.05), 9.33-fold (p < 0.05), 6.09-fold (p < 0.05) of those in rTrx group, respectively. The percentage of EDU⁺ cells in haemocytes also significantly increased (2.88-fold of that in control group, p < 0.05) at 12 h after rCgWnt-1 treatment. When the Wnt signal inhibitor C59 was injected simultaneously with rCgWnt-1, the expressions of Cgβ-catenin, CgRunx-1, and CgCDK-2 were significantly reduced, which were 0.32-fold (p < 0.05), 0.16-fold (p < 0.05), and 0.25-fold (p < 0.05) of that in rCgWnt-1 group, respectively, and the percentage of EDU⁺ cells in haemocytes was also significantly inhibited (0.15-fold compared with that in rCgWnt-1 group, p < 0.05). These results suggested that the conserved CgWnt-1 could modulate haemocytes proliferation via regulating cell cycle related genes and involved in the immune response of oysters.

CgTNF-2 promotes the proliferation of haemocytes by regulating the expressions of CgRunx and cell cycle related genes in the Pacific oyster Crassostrea gigas

A TNF-α family member, CgTNF-2, was previously identified from the oyster Crassostrea gigas to involve in the antibacterial response. In the present study, the role of CgTNF-2 in mediating the proliferation of haemocytes was further explored. The mRNA expression of CgTNF-2 in granulocytes was significantly higher than that in semi-granulocytes and agranulocytes, and the percentages of CgTNF-2 antibody labeled cells in agranulocytes, semi-granulocytes and granulocytes were 19.15%, 40.25% and 94.07%, respectively. After the treatment with rCgTNF-2, the percentage of EdU⁺ cells in haemocytes increased significantly (1.77-fold, p < 0.05) at 6 h compared with that in rGST-treated group, and the mRNA expressions of CgRunx, CgCyclin A, CgCDK2 and CgCDC45 in haemocytes all increased significantly (p < 0.05), which were 1.94-fold, 2.13-fold, 1.97-fold, 1.76-fold of that in rGST-treated group, respectively. Meanwhile, the protein abundance of CgRunx and CgCyclin A in the haemocytes of oysters in the rCgTNF-2-treated group increased, and the percentage of PI⁺ haemocytes in S phase also increased significantly (2.19-fold, p < 0.05) compared with that in rGST-treated group. These results collectively confirmed that CgTNF-2 was highly expressed in granulocytes and involved in the proliferation of haemocytes by regulating the expressions of CgRunx and cell cycle related genes in C. gigas.

The effectiveness of EGFR knockdown by RNAi lines varies depending on the tissue

In Drosophila , the Epidermal growth factor receptor (EGFR) signaling pathway is known to be critically involved in multiple stages of development. We induced a loss of function phenotype in the eyes, wings, and somatic follicle cells using four EGFR RNAi lines: HMS05003 and JF02283, which produce short hairpin RNAs, as well as JF01368 and KK100051, which produce long hairpin RNAs. Using these four lines, we completed a systematic comparison of the ability of short hairpin vs long hairpin RNAi lines to produce loss-of-function phenotypes in the above-mentioned tissues. Tissue specific knockdown was achieved by using Gal4 drivers specific to the three tissues being studied. In the eyes and wings, the KK100051 line induced the most severe phenotype, while the JF01368 line was the least severe, but in the somatic follicle cells, the KK100051 line was the least effective, while the JF01368 and JF02283 lines were comparable with respect to phenotypic severity. We conclude that there is significant tissue specific variability exhibited by the different RNAi lines.

Accessible chromatin reveals regulatory mechanisms underlying cell fate decisions during early embryogenesis

This study was conducted to investigate epigenetic landscape across multiple species and identify transcription factors (TFs) and their roles in controlling cell fate decision events during early embryogenesis. We made a comprehensively joint-research of chromatin accessibility of five species during embryogenesis by integration of ATAC-seq and RNA-seq datasets. Regulatory roles of candidate early embryonic TFs were investigated. Widespread accessible chromatin in early embryos overlapped with putative cis-regulatory sequences. Sets of cell-fate-determining TFs were identified. YOX1, a key cell cycle regulator, were found to homologous to clusters of TFs that are involved in neuron and epidermal cell-fate determination. Our research provides an intriguing insight into evolution of cell-fate decision during early embryogenesis among organisms.

A functional genomics screen identifying blood cell development genes in Drosophila by undergraduates participating in a course-based research experience

Undergraduate students participating in the UCLA Undergraduate Research Consortium for Functional Genomics (URCFG) have conducted a two-phased screen using RNA interference (RNAi) in combination with fluorescent reporter proteins to identify genes important for hematopoiesis in Drosophila. This screen disrupted the function of approximately 3500 genes and identified 137 candidate genes for which loss of function leads to observable changes in the hematopoietic development. Targeting RNAi to maturing, progenitor, and regulatory cell types identified key subsets that either limit or promote blood cell maturation. Bioinformatic analysis reveals gene enrichment in several previously uncharacterized areas, including RNA processing and export and vesicular trafficking. Lastly, the participation of students in this course-based undergraduate research experience (CURE) correlated with increased learning gains across several areas, as well as increased STEM retention, indicating that authentic, student-driven research in the form of a CURE represents an impactful and enriching pedagogical approach.

A novel site of haematopoiesis and appearance and dispersal of distinct haemocyte types in the Manduca sexta embryo (Insecta, Lepidoptera)

With a set of haemocyte specific markers novel findings on haematopoiesis in the Manduca sexta embryo are presented. We identify a hitherto unknown paired haematopoietic cluster, the abdominal haemocyte cluster in abdominal segment 7 (A7-HCC). These clusters are localised at distinct positions and are established at around katatrepsis. Later in embryogenesis, the A7-HCCs disintegrate, thereby releasing numerous embryonic plasmatocytes which disperse both anteriorly and posteriorly. These cells follow stereotypic migration routes projecting anteriorly. The thoracic larval haematopoietic organs are established at around midembryogenesis. We identify embryonic oenocytoids in the M. sexta embryo for the first time. They appear in the head region roughly at the same time as the A7-HCCs occur and successively disperse in the body cavity during development. Localisation of the prophenoloxidase (proPO) mRNA and of the proPO protein are identical. Morphological, cytometric and antigenic traits show three independently generated haemocyte types during embryogenesis.

The Leukemic Fly: Promises and Challenges

Leukemia involves different types of blood cancers, which lead to significant mortality and morbidity. Murine models of leukemia have been instrumental in understanding the biology of the disease and identifying therapeutics. However, such models are time consuming and expensive in high throughput genetic and drug screening. Drosophilamelanogaster has emerged as an invaluable in vivo model for studying different diseases, including cancer. Fruit flies possess several hematopoietic processes and compartments that are in close resemblance to their mammalian counterparts. A number of studies succeeded in characterizing the fly's response upon the expression of human leukemogenic proteins in hematopoietic and non-hematopoietic tissues. Moreover, some of these studies showed that these models are amenable to genetic screening. However, none were reported to be tested for drug screening. In this review, we describe the Drosophila hematopoietic system, briefly focusing on leukemic diseases in which fruit flies have been used. We discuss myeloid and lymphoid leukemia fruit fly models and we further highlight their roles for future therapeutic screening. In conclusion, fruit fly leukemia models constitute an interesting area which could speed up the process of integrating new therapeutics when complemented with mammalian models.

Identification of genomic enhancers through spatial integration of single-cell transcriptomics and epigenomics

Single-cell technologies allow measuring chromatin accessibility and gene expression in each cell, but jointly utilizing both layers to map bona fide gene regulatory networks and enhancers remains challenging. Here, we generate independent single-cell RNA-seq and single-cell ATAC-seq atlases of the Drosophila eye-antennal disc and spatially integrate the data into a virtual latent space that mimics the organization of the 2D tissue using ScoMAP (Single-Cell Omics Mapping into spatial Axes using Pseudotime ordering). To validate spatially predicted enhancers, we use a large collection of enhancer-reporter lines and identify ~ 85% of enhancers in which chromatin accessibility and enhancer activity are coupled. Next, we infer enhancer-to-gene relationships in the virtual space, finding that genes are mostly regulated by multiple, often redundant, enhancers. Exploiting cell type-specific enhancers, we deconvolute cell type-specific effects of bulk-derived chromatin accessibility QTLs. Finally, we discover that Prospero drives neuronal differentiation through the binding of a GGG motif. In summary, we provide a comprehensive spatial characterization of gene regulation in a 2D tissue.

The Role of Lozenge in Drosophila Hematopoiesis

Drosophila hematopoiesis is comparable to mammalian differentiation of myeloid lineages, and therefore, has been a useful model organism in illustrating the molecular and genetic basis for hematopoiesis. Multiple novel regulators and signals have been uncovered using the tools of Drosophila genetics. A Runt domain protein, lozenge, is one of the first players recognized and closely studied in the hematopoietic lineage specification. Here, we explore the role of lozenge in determination of prohemocytes into a special class of hemocyte, namely the crystal cell, and discuss molecules and signals controlling the lozenge function and its implication in immunity and stress response. Given the highly conserved nature of Runt domain in both invertebrates and vertebrates, studies in Drosophila will enlighten our perspectives on Runx-mediated development and pathologies.

RUNX3 suppresses metastasis and stemness by inhibiting Hedgehog signaling in colorectal cancer

Disabled tumor suppressor genes and hyperactive oncogenes greatly contribute to cell fates during cancer development because of their genetic alterations such as somatic mutations. However, little is known about how tumor suppressor genes react to diverse oncogenes during tumor progression. Our previous study showed that RUNX3 inhibits invasiveness by preventing vascular endothelial growth factor secretion and suppressed endothelial cell growth and tube formation in colorectal cancer (CRC). Hedgehog signaling is crucial for the physiological maintenance and self-renewal of stem cells, and its deregulation is responsible for their tumor development. The mechanisms that inhibit this pathway during proliferation remain poorly understood. Here, we found that the tumor suppressor RUNX3 modulates tumorigenesis in response to cancer cells induced by inhibiting oncogene GLI1 ubiquitination. Moreover, we demonstrated that RUNX3 and GLI1 expression were inversely correlated in CRC cells and tissues. We observed a direct interaction between RUNX3 and GLI1, promoting ubiquitination of GLI1 at the intracellular level. Increased ubiquitination of GLI1 was induced by the E3 ligase β-TrCP. This novel RUNX3-dependent regulatory loop may limit the extent and duration of Hedgehog signaling during extension of the tumor initiation capacity. On the basis of our results, identification of agents that induce RUNX3 may be useful for developing new and effective therapies for CRC.

RUNX3 enhances TRAIL-induced apoptosis by upregulating DR5 in colorectal cancer

RUNX3 is frequently inactivated by DNA hypermethylation in numerous cancers. Here, we show that RUNX3 has an important role in modulating apoptosis in immediate response to tumor necrosis factor-related apoptosis-including ligand (TRAIL). Importantly, no combined effect of TRAIL and RUNX3 was observed in non-cancerous cells. We investigated the expression of the death receptors (DRs) DR4 and DR5, which are related to TRAIL resistance. Overexpression of RUNX3 increased DR5 expression via induction of the reactive oxygen species (ROS)-endoplasmic reticulum (ER) stress-effector CHOP. Reduction of DR5 markedly decreased apoptosis enhanced by the combined therapy of TRAIL and RUNX3. Interestingly, RUNX3 induced reactive oxygen species production by inhibiting SOD3 transcription via binding to the Superoxide dismutase 3 (SOD3) promoter. Additionally, the combined effect of TRAIL and RUNX3 decreased tumor growth in xenograft models. Our results demonstrate a direct role for RUNX3 in TRAIL-induced apoptosis via activation of DR5 and provide further support for RUNX3 as an anti-tumor.

Novel roles of HP1a and Mcm10 in DNA replication, genome maintenance and photoreceptor cell differentiation

Both Mcm10 and HP1a are known to be required for DNA replication. However, underlying mechanism is not clarified yet especially for HP1. Knockdown of both HP1a and Mcm10 genes inhibited the progression of S phase in Drosophila eye imaginal discs. Proximity Ligation Assay (PLA) demonstrated that HP1a is in close proximity to DNA replication proteins including Mcm10, RFC140 and DNA polymerase ε 255 kDa subunit in S-phase. This was further confirmed by co-immunoprecipitation assay. The PLA signals between Mcm10 and HP1a are specifically observed in the mitotic cycling cells, but not in the endocycling cells. Interestingly, many cells in the posterior regions of eye imaginal discs carrying a double knockdown of Mcm10 and HP1a induced ectopic DNA synthesis and DNA damage without much of ectopic apoptosis. Therefore, the G1-S checkpoint may be affected by knockdown of both proteins. This event was also the case with other HP family proteins such as HP4 and HP6. In addition, both Mcm10 and HP1a are required for differentiation of photoreceptor cells R1, R6 and R7. Further analyses on several developmental genes involved in the photoreceptor cell differentiation suggest that a role of both proteins is mediated by regulation of the lozenge gene.

Parsimony and complexity: Cell fate assignment in the developing Drosophila eye

The specification of the R7 photoreceptor in the Drosophila eye has become a classic model for understanding how cell fates are assigned in developing systems. R7 is derived from a group of cells that also gives rise to the R1/6 photoreceptor class and the non-photoreceptor cone cells. Our studies examine the signals and cellular information that direct each of these cell types. The cell fates are directed by the combined actions of the Receptor Tyrosine Kinase (RTK) and Notch (N) signaling pathways. The RTK pathway acts to remove the transcription factor Tramtrack (Ttk) which represses the photoreceptor fate. If a cell receives an RTK signal sufficient to remove Ttk then the photoreceptor fate is specified; if not, the cone cell fate results. If Ttk is removed from a cell and its N activity is high then it is specified as an R7, but if its N activity is low then it becomes an R1/6 class photoreceptor. Thus, a remarkably simple molecular code underlies the specification of the fates: 1. Ttk degraded or not: 2. N activity high or low. In the R1/6 and cone cell precursors the molecular codes are achieved with relative simplicity but in the R7 precursor, manifold interactions occur between the RTK and N pathways, and to-date we have identified 4 distinct roles played by N in R7 fate specification. In this review we detail this molecular complexity, and describe how the RTK/N pathway crosstalk eventually leads to the simple molecular code of Tramtrack removed and N activity high. Furthermore, we describe the role played by the transcription factor Lozenge (Lz) in directing retinal precursor fates, and how the RTK/N signals specify different retinal cell types depending on the presence or absence of Lz.

RUNX in Invertebrates

Runx genes have been identified in all metazoans and considerable conservation of function observed across a wide range of phyla. Thus, insight gained from studying simple model organisms is invaluable in understanding RUNX biology in higher animals. Consequently, this chapter will focus on the Runx genes in the diploblasts, which includes sea anemones and sponges, as well as the lower triploblasts, including the sea urchin, nematode, planaria and insect. Due to the high degree of functional redundancy amongst vertebrate Runx genes, simpler model organisms with a solo Runx gene, like C. elegans, are invaluable systems in which to probe the molecular basis of RUNX function within a whole organism. Additionally, comparative analyses of Runx sequence and function allows for the development of novel evolutionary insights. Strikingly, recent data has emerged that reveals the presence of a Runx gene in a protist, demonstrating even more widespread occurrence of Runx genes than was previously thought. This review will summarize recent progress in using invertebrate organisms to investigate RUNX function during development and regeneration, highlighting emerging unifying themes.

An RNAi Screen for Genes Involved in Nanoscale Protrusion Formation on Corneal Lens in Drosophila melanogaster

The "moth-eye" structure, which is observed on the surface of corneal lens in several insects, supports anti-reflective and self-cleaning functions due to nanoscale protrusions known as corneal nipples. Although the morphology and function of the "moth-eye" structure, are relatively well studied, the mechanism of protrusion formation from cell-secreted substances is unknown. In Drosophila melanogaster, a compound eye consists of approximately 800 facets, the surface of which is formed by the corneal lens with nanoscale protrusions. In the present study, we sought to identify genes involved in "moth-eye" structure, formation in order to elucidate the developmental mechanism of the protrusions in Drosophila. We re-examined the aberrant patterns in classical glossy-eye mutants by scanning electron microscope and classified the aberrant patterns into groups. Next, we screened genes encoding putative structural cuticular proteins and genes involved in cuticular formation using eye specific RNAi silencing methods combined with the Gal4/UAS expression system. We identified 12 of 100 candidate genes, such as cuticular proteins family genes (Cuticular protein 23B and Cuticular protein 49Ah), cuticle secretion-related genes (Syntaxin 1A and Sec61 ββ subunit), ecdysone signaling and biosynthesis-related genes (Ecdysone receptor, Blimp-1, and shroud), and genes involved in cell polarity/cell architecture (Actin 5C, shotgun, armadillo, discs large1, and coracle). Although some of the genes we identified may affect corneal protrusion formation indirectly through general patterning defects in eye formation, these initial findings have encouraged us to more systematically explore the precise mechanisms underlying the formation of nanoscale protrusions in Drosophila.

Two temporal functions of Glass: Ommatidium patterning and photoreceptor differentiation

Much progress has been made in elucidating the molecular networks required for specifying retinal cells, including photoreceptors, but the downstream mechanisms that maintain identity and regulate differentiation remain poorly understood. Here, we report that the transcription factor Glass has a dual role in establishing a functional Drosophila eye. Utilizing conditional rescue approaches, we confirm that persistent defects in ommatidium patterning combined with cell death correlate with the overall disruption of eye morphology in glass mutants. In addition, we reveal that Glass exhibits a separable role in regulating photoreceptor differentiation. In particular, we demonstrate the apparent loss of glass mutant photoreceptors is not only due to cell death but also a failure of the surviving photoreceptors to complete differentiation. Moreover, the late reintroduction of Glass in these developmentally stalled photoreceptors is capable of restoring differentiation in the absence of correct ommatidium patterning. Mechanistically, transcription profiling at the time of differentiation reveals that Glass is necessary for the expression of many genes implicated in differentiation, i.e. rhabdomere morphogenesis, phototransduction, and synaptogenesis. Specifically, we show Glass directly regulates the expression of Pph13, which encodes a transcription factor necessary for opsin expression and rhabdomere morphogenesis. Finally, we demonstrate the ability of Glass to choreograph photoreceptor differentiation is conserved between Drosophila and Tribolium, two holometabolous insects. Altogether, our work identifies a fundamental regulatory mechanism to generate the full complement of cells required for a functional rhabdomeric visual system and provides a critical framework to investigate the basis of differentiation and maintenance of photoreceptor identity.

Cross-talk between PRMT1-mediated methylation and ubiquitylation on RBM15 controls RNA splicing

RBM15, an RNA binding protein, determines cell-fate specification of many tissues including blood. We demonstrate that RBM15 is methylated by protein arginine methyltransferase 1 (PRMT1) at residue R578, leading to its degradation via ubiquitylation by an E3 ligase (CNOT4). Overexpression of PRMT1 in acute megakaryocytic leukemia cell lines blocks megakaryocyte terminal differentiation by downregulation of RBM15 protein level. Restoring RBM15 protein level rescues megakaryocyte terminal differentiation blocked by PRMT1 overexpression. At the molecular level, RBM15 binds to pre-messenger RNA intronic regions of genes important for megakaryopoiesis such as GATA1, RUNX1, TAL1 and c-MPL. Furthermore, preferential binding of RBM15 to specific intronic regions recruits the splicing factor SF3B1 to the same sites for alternative splicing. Therefore, PRMT1 regulates alternative RNA splicing via reducing RBM15 protein concentration. Targeting PRMT1 may be a curative therapy to restore megakaryocyte differentiation for acute megakaryocytic leukemia.

DOI:http://dx.doi.org/10.7554/eLife.07938.001

The many different cell types in an adult animal all develop from a single fertilized egg. The development of cells into more specialized cell types is called ‘differentiation’. Proteins and other molecules from both inside and outside of the cells regulate the differentiation process.

RNA is a molecule that is similar to DNA, and performs several important roles inside cells. Perhaps most importantly, RNA molecules act as messengers and carry genetic instructions during gene expression. RBM15 is an RNA-binding protein that is found throughout nature, and is involved in a number of developmental processes. Previous research has linked the incorrect control of RBM15 with an increased risk of certain cancers, including megakaryocytic leukemia. However, it is not clear what role RNA-binding proteins such as RBM15 play during differentiation.

Now, Zhang, Tran, Su et al. have investigated the role of RBM15 during the development of large cells found in human bone marrow (called megakaryocytes). First, the experiments demonstrated that an enzyme called PRMT1 modifies RBM15. This enzyme adds a chemical mark called a methyl group at a specific site (an arginine amino acid) on the RNA-binding protein. Next, Zhang, Tran, Su et al. showed that the addition of this methyl group earmarks RBM15 for destruction. This means that an increase in PRMT1 levels reduces the amount of RBM15 in cells, while decreases in PRMT1 have the opposite effect.

Further experiments showed that RBM15 normally processes the RNA messengers that carry the genetic instructions needed for the differentiation of bone marrow cells. An excess of PRMT1 enzyme leads to a lack of this RNA-binding protein. This in turn interferes with the differentiation process, and can contribute to the development of cancers such as megakaryocytic leukemia. Future work will therefore explore whether targeting PRMT1 with drugs could represent an effective treatment for these kinds of cancers.

DOI:http://dx.doi.org/10.7554/eLife.07938.002

Spen is required for pigment cell survival during pupal development in Drosophila

Apoptosis is required during development to eliminate superfluous cells and sculpt tissues; spatial and timed control of apoptosis ensures that the necessary number of cells is eliminated at a precise time in a given tissue. The elimination of supernumerary pigment or inter-ommatidial cells (IOCs) depends on cell-cell communication and is necessary for the formation of the honeycomb-like structure of the Drosophila eye. However, the mechanisms occurring during pupal development and controlling apoptosis of superfluous IOC in space and time remain unclear. Here, we found that split-ends (spen) is required for IOC survival at the time of removal of superfluous IOCs. Loss of spen function leads to abnormal removal of IOCs by apoptosis. We show that spen is required non-autonomously in cone cells for the survival of IOCs by positively regulating the Spitz/EGFR pathway. We propose that Spen is an important survival factor that ensures spatial control of the apoptotic wave that is necessary for the correct patterning and formation of the Drosophila eye.

RUNX family members are covalently modified and regulated by PIAS1-mediated sumoylation

Transcription factors of the RUNX family (RUNXs), which play pivotal roles in normal development and neoplasia, are regulated by various post-translational modifications. To understand the molecular mechanisms underlying the regulation of RUNXs, we performed a large-scale functional genetic screen of a fly mutant library. The screen identified dPias (the fly ortholog of mammalian PIASs), an E3 ligase for the SUMO (small ubiquitin-like modifier) modification, as a novel genetic modifier of lz (the fly ortholog of mammalian RUNX3). Molecular biological analysis revealed that lz/RUNXs are sumoylated by dPias/PIAS1 at an evolutionarily conserved lysine residue (K372 of lz, K144 of RUNX1, K181 of RUNX2 and K148 of RUNX3). PIAS1-mediated sumoylation inhibited RUNX3 transactivation activity, and this modification was promoted by the AKT1 kinase. Importantly, PIAS1 failed to sumoylate some RUNX1 mutants associated with breast cancer. In nude mice, tumorigenicity was promoted by RUNX3 bearing a mutation in the sumoylation site, but suppressed by wild-type RUNX3. Our results suggest that RUNXs are sumoylated by PIAS1, and that this modification could play a critical role in the regulation of the tumor-suppressive activity of these proteins.

The Drosophila Wilms׳ Tumor 1-Associating Protein (WTAP) homolog is required for eye development

Sine Oculis (So), the founding member of the SIX family of homeobox transcription factors, binds to sequence specific DNA elements and regulates transcription of downstream target genes. It does so, in part, through the formation of distinct biochemical complexes with Eyes Absent (Eya) and Groucho (Gro). While these complexes play significant roles during development, they do not account for all So-dependent activities in Drosophila. It is thought that additional So-containing complexes make important contributions as well. This contention is supported by the identification of nearly two-dozen additional proteins that complex with So. However, very little is known about the roles that these additional complexes play in development. In this report we have used yeast two-hybrid screens and co-immunoprecipitation assays from Kc167 cells to identify a biochemical complex consisting of So and Fl(2)d, the Drosophila homolog of human Wilms׳ Tumor 1-Associating Protein (WTAP). We show that Fl(2)d protein is distributed throughout the entire eye-antennal imaginal disc and that loss-of-function mutations lead to perturbations in retinal development. The eye defects are manifested behind the morphogenetic furrow and result in part from increased levels of the pan-neuronal RNA binding protein Embryonic Lethal Abnormal Vision (Elav) and the RUNX class transcription factor Lozenge (Lz). We also provide evidence that So and Fl(2)d interact genetically in the developing eye. Wilms׳ tumor-1 (WT1), a binding partner of WTAP, is required for normal eye formation in mammals and loss-of-function mutations are associated with some versions of retinoblastoma. In contrast, WTAP and its homologs have not been implicated in eye development. To our knowledge, the results presented in this report are the first description of a role for WTAP in the retina of any seeing animal.

Bar represses dPax2 and decapentaplegic to regulate cell fate and morphogenetic cell death in Drosophila eye

The coordinated regulation of cell fate and cell survival is crucial for normal pattern formation in developing organisms. In Drosophila compound eye development, crystalline arrays of hexagonal ommatidia are established by precise assembly of diverse cell types, including the photoreceptor cells, cone cells and interommatidial (IOM) pigment cells. The molecular basis for controlling the number of cone and IOM pigment cells during ommatidial pattern formation is not well understood. Here we present evidence that BarH1 and BarH2 homeobox genes are essential for eye patterning by inhibiting excess cone cell differentiation and promoting programmed death of IOM cells. Specifically, we show that loss of Bar from the undifferentiated retinal precursor cells leads to ectopic expression of Prospero and dPax2, two transcription factors essential for cone cell specification, resulting in excess cone cell differentiation. We also show that loss of Bar causes ectopic expression of the TGFβ homolog Decapentaplegic (Dpp) posterior to the morphogenetic furrow in the larval eye imaginal disc. The ectopic Dpp expression is not responsible for the formation of excess cone cells in Bar loss-of-function mutant eyes. Instead, it causes reduction in IOM cell death in the pupal stage by antagonizing the function of pro-apoptotic gene reaper. Taken together, this study suggests a novel regulatory mechanism in the control of developmental cell death in which the repression of Dpp by Bar in larval eye disc is essential for IOM cell death in pupal retina.

RUNX3 regulates vimentin expression via miR-30a during epithelial-mesenchymal transition in gastric cancer cells

Runt-related transcription factor 3 (RUNX3) is a putative tumour suppressor via regulating the expression of a series of target genes. Clinical studies demonstrated that loss of RUNX3 expression is associated with gastric cancer progression and poor prognosis, but the underlying mechanism is not entirely clear. Accumulating evidence shows that the epithelial–mesenchymal transition (EMT) plays an important role in cancer relapse and metastasis. Therefore, we addressed whether RUNX3 has a role in the EMT in gastric cancer. Knockdown of RUNX3 promoted cell invasion and increased the protein expression of the mesenchymal marker vimentin in human gastric cancer cells. Overexpression of RUNX3 suppressed cell invasion and decreased the protein expression of vimentin in the cells and inhibited gastric cancer cells colonization in nude mice. Furthermore, overexpression of RUNX3 increased the expression of microRNA-30a (miR-30a), and miR-30a directly targeted the 3′ untranslated region of vimentin and decreased its protein level. miR-30a inhibitor abrogated RUNX3-mediated inhibition of cell invasion and downregulation of vimentin. Thus, RUNX3 suppressed gastric cancer cell invasion and vimentin expression by activating miR-30a. In gastric cancer patients, levels of RUNX3 were positively correlated with miR-30a and negatively associated with the levels of vimentin. Collectively, our data suggest a novel molecular mechanism for the tumour suppressor activity of RUNX3. Effective therapy targeting the RUNX3 pathway may help control gastric cancer cell invasion and metastasis by inhibiting the EMT.

Kicking it up a Notch for the best in show: Scalloped leads Yorkie into the haematopoietic arena

Maintenance and differentiation of progenitor cells is essential for proper organ development and adaptation to environmental stress and injury. In Drosophila melanogaster, the haematopietic system serves as an ideal model for interrogating the function of signaling pathways required for progenitor maintenance and cell fate determination. Here we focus on the role of the Hippo pathway effectors Yorkie and Scalloped in mediating and facilitating Notch signaling-mediated lineage specification in the lymph gland, the primary center for haematopoiesis within the developing larva. We discuss the regulatory mechanisms which promote Notch activity during normal haematopoiesis and its modulation during immune challenge conditions. We provide additional evidence establishing the hierarchy of signaling events during crystal cell formation, highlighting the relationship between Yorkie, Scalloped and Lozenge, while expanding on the role of Yorkie in promoting hemocyte survival and the developmental regulation of Notch and its ligand, Serrate, within the lymph gland. Finally, we propose additional areas of exploration that may provide mechanistic insight into the environmental and non-cell autonomous regulation of cell fate in the blood system.

Switching cell fates in the developing Drosophila eye

The developing Drosophila ommatidium is characterized by two distinct waves of pattern formation. In the first wave, a precluster of five cells is formed by a complex cellular interaction mechanism. In the second wave, cells are systematically recruited to the cluster and directed to their fates by developmental cues presented by differentiating precluster cells. These developmental cues are mediated through the receptor tyrosine kinase (RTK) and Notch (N) signaling pathways and their combined activities are crucial in specifying cell type. The transcription factor Lozenge (Lz) is expressed exclusively in second wave cells. Here, we ectopically supply Lz to precluster cells and concomitantly supply the various RTK/N codes that specify each of three second wave cell fates. We thereby reproduce molecular markers of each of the second wave cell types in precluster cells and draw three inferences. First, we confirm that Lz provides key intrinsic information to second wave cells. We can now combine this with the RTK/N signaling to provide a cell fate specification code that entails both extrinsic and intrinsic information. Second, the reproduction of each second wave cell type in the precluster confirms the accuracy of the RTK/N signaling code. Third, RTK/N signaling and Lz need only be presented to the cells for a short period of time in order to specify their fate.

Receptor tyrosine kinases in Drosophila development

Tyrosine phosphorylation plays a significant role in a wide range of cellular processes. The Drosophila genome encodes more than 20 receptor tyrosine kinases and extensive studies in the past 20 years have illustrated their diverse roles and complex signaling mechanisms. Although some receptor tyrosine kinases have highly specific functions, others strikingly are used in rather ubiquitous manners. Receptor tyrosine kinases regulate a broad expanse of processes, ranging from cell survival and proliferation to differentiation and patterning. Remarkably, different receptor tyrosine kinases share many of the same effectors and their hierarchical organization is retained in disparate biological contexts. In this comprehensive review, we summarize what is known regarding each receptor tyrosine kinase during Drosophila development. Astonishingly, very little is known for approximately half of all Drosophila receptor tyrosine kinases.

Retinal differentiation in Drosophila

Drosophila eye development has been extensively studied, due to the ease of genetic screens for mutations disrupting this process. The eye imaginal disc is specified during embryonic and larval development by the Pax6 homolog Eyeless and a network of downstream transcription factors. Expression of these factors is regulated by signaling molecules and also indirectly by growth of the eye disc. Differentiation of photoreceptor clusters initiates in the third larval instar at the posterior of the eye disc and progresses anteriorly, driven by the secreted protein Hedgehog. Within each cluster, the combined activities of Hedgehog signaling and Notch-mediated lateral inhibition induce and refine the expression of the transcription factor Atonal, which specifies the founding R8 photoreceptor of each ommatidium. Seven additional photoreceptors, followed by cone and pigment cells, are successively recruited by the signaling molecules Spitz, Delta, and Bride of sevenless. Combinations of these signals and of intrinsic transcription factors give each ommatidial cell its specific identity. During the pupal stages, rhodopsins are expressed, and the photoreceptors and accessory cells take on their final positions and morphologies to form the adult retina. Over the past few decades, the genetic analysis of this small number of cell types arranged in a repetitive structure has allowed a remarkably detailed understanding of the basic mechanisms controlling cell differentiation and morphological rearrangement.

PDCD2 controls hematopoietic stem cell differentiation during development

Programmed cell death 2 (Pdcd2) is a highly conserved protein of undefined function, and is widely expressed in embryonic and adult tissues. The observations that knockout of Pdcd2 in the mouse is embryonic lethal at preimplantation stages, and that in Drosophila, Zfrp8, the ortholog of Pdcd2, is required for normal lymph gland development suggest that Pdcd2 is important for regulating hematopoietic development. Through genetic and functional studies, we investigated pdcd2 function during the zebrafish ontogeny. Knockdown of pdcd2 expression in zebrafish embryos resulted in defects in embryonic hematopoietic development. Loss of pdcd2 function caused increased expression of progenitor markers, and accumulation of erythroid progenitors during primitive hematopoiesis. Additionally, hematopoietic stem cells (HSCs) failed to appear in the aorta-gonad mesonephros, and were not able to terminally differentiate or reconstitute hematopoiesis. Pdcd2 effects on HSC emergence were cell autonomous and P53-independent, and loss of pdcd2 function was associated with mitotic defects and apoptosis. Restoration of runx1 function(s) and modulation of apoptosis through the inhibition of Jak/Stat signaling rescued the hematopoietic and erythroid defects resulting from pdcd2 knockdown. Our studies suggest that pdcd2 plays a critical role in regulating the transcriptional hierarchy controlling hematopoietic lineage determination. Furthermore, the effects of pdcd2 in regulating mitotic cell death may contribute to its role(s) in directing hematopoietic differentiation during development.

Runx1 dose-dependently regulates endochondral ossification during skeletal development and fracture healing

Runx1 is expressed in skeletal elements, but its role in fracture repair has not been analyzed. We created mice with a hypomorphic Runx1 allele (Runx1(L148A) ) and generated Runx1(L148A/-) mice in which >50% of Runx1 activity was abrogated. Runx1(L148A/-) mice were viable but runted. Their growth plates had extended proliferating and hypertrophic zones, and the percentages of Sox9-, Runx2-, and Runx3-positive cells were decreased. Femoral fracture experiments revealed delayed cartilaginous callus formation, and the expression of chondrogenic markers was decreased. Conditional ablation of Runx1 in the mesenchymal progenitor cells of the limb with Prx1-Cre conferred no obvious limb phenotype; however, cartilaginous callus formation was delayed following fracture. Embryonic limb bud-derived mesenchymal cells showed delayed chondrogenesis when the Runx1 allele was deleted ex vivo with adenoviral-expressed Cre. Collectively, our data suggest that Runx1 is required for commitment and differentiation of chondroprogenitor cells into the chondrogenic lineage.

Building an ommatidium one cell at a time

Since the discovery of a single white-eyed male in a population of red eyed flies over 100 years ago (Morgan, 1910), the compound eye of the fruit fly, Drosophila melanogaster, has been a favorite experimental system for identifying genes that regulate various aspects of development. For example, a fair amount of what we know today about enzymatic pathways and vesicular transport is due to the discovery and subsequent characterization of eye color mutants such as white. Likewise, our present day understanding of organogenesis has been aided considerably by studies of mutations, such as eyeless, that either reduce or eliminate the compound eyes. But by far the phenotype that has provided levers into the greatest number of experimental fields has been the humble "rough" eye. The fly eye is composed of several hundred unit-eyes that are also called ommatidia. These unit eyes are packed into a hexagonal array of remarkable precision. The structure of the eye is so precise that it has been compared with that of a crystal (Ready et al., 1976). Even the slightest perturbations to the structure of the ommatidium can be visually detected by light or electron microscopy. The cause for this is two-fold: (1) any defect that affects the hexagonal geometry of a single ommatidium can and will disrupt the positioning of surrounding unit eyes thereby propagating structural flaws and (2) disruptions in genes that govern the development of even a single cell within an ommatidium will affect all unit eyes. In both cases, the effect is the visual magnification of even the smallest imperfection. Studies of rough eye mutants have provided key insights into the areas of cell fate specification, lateral inhibition, signal transduction, transcription factor networks, planar cell polarity, cell proliferation, and programmed cell death just to name a few. This review will attempt to summarize the key steps that are required to assemble each ommatidium.

Transcriptional control of cell fate specification: lessons from the fly retina

It is now widely recognized that as cells of developing tissues transition through successive states of decreasing pluripotency into a state of terminal differentiation, they undergo significant changes in their gene expression profiles. Interestingly, these successive states of increasing differentiation are marked by the spatially and temporally restricted expression of sets of transcription factors. Each wave of transcription factors not only signals the arrival of a given stage in cellular differentiation, but it is also necessary for the activation of the next set of transcription factors, creating the appearance of a smooth, directed, and deterministic genetic program of cellular differentiation. Until recently, however, it was largely unknown which genes, besides each other, these transcription factors were activating. Thus, the molecular definition of any given step of differentiation, and how it gave rise to the following step remained unclear. Recent advances in transcriptomics, bioinformatics, and molecular genetics resulted in the identification of numerous transcription factor target genes (TGs). These advances have opened the door to using similar approaches in developmental biology to understand what the transcriptional cascades of cellular differentiation might be. Using the development of the Drosophila eye as a model system, we discuss the role of transcription factors and their TGs in cell fate specification and terminal differentiation.

Nemo phosphorylates Eyes absent and enhances output from the Eya-Sine oculis transcriptional complex during Drosophila retinal determination

The retinal determination gene network comprises a collection of transcription factors that respond to multiple signaling inputs to direct Drosophila eye development. Previous genetic studies have shown that nemo (nmo), a gene encoding a proline-directed serine/threonine kinase, can promote retinal specification through interactions with the retinal determination gene network, although the molecular point of cross-talk was not defined. Here, we report that the Nemo kinase positively and directly regulates Eyes absent (Eya). Genetic assays show that Nmo catalytic activity enhances Eya-mediated ectopic eye formation and potentiates induction of the Eya-Sine oculis (So) transcriptional targets dachshund and lozenge. Biochemical analyses demonstrate that Nmo forms a complex with and phosphorylates Eya at two consensus mitogen-activated protein kinase (MAPK) phosphorylation sites. These same sites appear crucial for Nmo-mediated activation of Eya function in vivo. Thus, we propose that Nmo phosphorylation of Eya potentiates its transactivation function to enhance transcription of Eya-So target genes during eye specification and development.

The lens in focus: a comparison of lens development in Drosophila and vertebrates

The evolution of the eye has been a major subject of study dating back centuries. The advent of molecular genetics offered the surprising finding that morphologically distinct eyes rely on conserved regulatory gene networks for their formation. While many of these advances often stemmed from studies of the compound eye of the fruit fly, Drosophila melanogaster, and later translated to discoveries in vertebrate systems, studies on vertebrate lens development far outnumber those in Drosophila. This may be largely historical, since Spemann and Mangold's paradigm of tissue induction was discovered in the amphibian lens. Recent studies on lens development in Drosophila have begun to define molecular commonalities with the vertebrate lens. Here, we provide an overview of Drosophila lens development, discussing intrinsic and extrinsic factors controlling lens cell specification and differentiation. We then summarize key morphological and molecular events in vertebrate lens development, emphasizing regulatory factors and networks strongly associated with both systems. Finally, we provide a comparative analysis that highlights areas of research that would help further clarify the degree of conservation between the formation of dioptric systems in invertebrates and vertebrates.

Retinal determination genes function along with cell-cell signals to regulate Drosophila eye development: examples of multi-layered regulation by master regulators

It is thought that retinal determination (RD) gene products define the response made to cell-cell signals in the field of eye development by binding to enhancers of genes that are also regulated by cell-cell signaling pathways. In Drosophila, RD genes, including eyeless, teashirt, eyes absent, dachsous, and sine oculis, are required for normal eye development and can induce ectopic eyes when mis-expressed. Characterization of the enhancers responsible for eye expression of the hedgehog, shaven, and atonal genes, as well as the dynamics of RD gene expression themselves, now suggest a multilayered network whereby transcriptional regulation by either RD genes or cell-cell signaling pathways can sometimes be indirect and mediated by other transcription factor intermediates. In this updated view of the interaction between extracellular information and cell intrinsic programs during development, regulation of individual genes might sometimes be several steps removed from either the RD genes or the cell-cell signaling pathways that nevertheless govern their expression.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Building a fly eye: terminal differentiation events of the retina, corneal lens, and pigmented epithelia

In the past, vast differences in ocular structure, development, and physiology throughout the animal kingdom led to the widely accepted notion that eyes are polyphyletic, that is, they have independently arisen multiple times during evolution. Despite the dissimilarity between vertebrate and invertebrate eyes, it is becoming increasingly evident that the development of the eye in both groups shares more similarity at the genetic level than was previously assumed, forcing a reexamination of eye evolution. Understanding the molecular underpinnings of cell type specification during Drosophila eye development has been a focus of research for many labs over the past 25 years, and many of these findings are nicely reviewed in Chapters 1 and 4. A somewhat less explored area of research, however, considers how these cells, once specified, develop into functional ocular structures. This review aims to summarize the current knowledge related to the terminal differentiation events of the retina, corneal lens, and pigmented epithelia in the fly eye. In addition, we discuss emerging evidence that the different functional components of the fly eye share developmental pathways and functions with the vertebrate eye.

Ttk69-dependent repression of lozenge prevents the ectopic development of R7 cells in the Drosophila larval eye disc

Background

During the development of the Drosophila eye, specific cell types differentiate from an initially equipotent group of uncommitted precursor cells. The lozenge (lz) gene, which is a member of the Runt family of transcriptional regulators, plays a pivotal role in mediating this process through regulating the expression of several fate-specifying transcription factors. However, the regulation of lz, and the control of lz expression levels in different cell types is not fully understood.

Results

Here, we show a genetic interaction between Tramtrack69 (Ttk69) a key transcriptional repressor and an inhibitor of neuronal fate specification, and lz, the master patterning gene of cells posterior to the morphogenetic furrow in the Drosophila eye disc. Loss of Ttk69 expression causes the development of ectopic R7 cells in the third instar eye disc, with these cells being dependent upon Lz for their development. Using the binary UAS Gal4 system, we show that overexpression of Ttk69 causes the loss of lz-dependent differentiating cells, and a down-regulation of Lz expression in the developing eye. The loss of lz-dependent cells can be rescued by overexpressing lz via a GMR-lz transgene. We provide additional data showing that factors functioning upstream of Ttk69 in eye development regulate lz in a Ttk69-dependent manner.

Conclusions

Our results lead us to conclude that Ttk69 can either directly or indirectly repress lz gene expression to prevent the premature development of R7 precursor cells in the developing eye of Drosophila. We therefore define a mechanism for the tight regulatory control of the master pre-patterning gene, lz, in early Drosophila eye development and provide insight into how differential levels of lz expression can be achieved to effect specific cell fate outcomes.

Cross-species RNAi rescue platform in Drosophila melanogaster

RNAi-mediated gene knockdown in Drosophila melanogaster is a powerful method to analyze loss-of-function phenotypes both in cell culture and in vivo. However, it has also become clear that false positives caused by off-target effects are prevalent, requiring careful validation of RNAi-induced phenotypes. The most rigorous proof that an RNAi-induced phenotype is due to loss of its intended target is to rescue the phenotype by a transgene impervious to RNAi. For large-scale validations in the mouse and Caenorhabditis elegans, this has been accomplished by using bacterial artificial chromosomes (BACs) of related species. However, in Drosophila, this approach is not feasible because transformation of large BACs is inefficient. We have therefore developed a general RNAi rescue approach for Drosophila that employs Cre/loxP-mediated recombination to rapidly retrofit existing fosmid clones into rescue constructs. Retrofitted fosmid clones carry a selection marker and a phiC31 attB site, which facilitates the production of transgenic animals. Here, we describe our approach and demonstrate proof-of-principle experiments showing that D. pseudoobscura fosmids can successfully rescue RNAi-induced phenotypes in D. melanogaster, both in cell culture and in vivo. Altogether, the tools and method that we have developed provide a gold standard for validation of Drosophila RNAi experiments.

Notch activity in the nervous system: to switch or not switch?

The Notch pathway is instrumental for cell fate diversification during development. Pioneer studies conducted in Drosophila and more recent work performed in vertebrates have shown that in the nervous system, Notch is reiteratively employed when cells choose between two alternative fates, a process referred to as a binary fate decision. While the early (neural versus epidermal) fate decisions mainly involve an inhibitory effect of Notch on the neural fate, late fate decisions (choice between different subtypes of neural cells) have been proposed to involve a binary switch activity whereby Notch would be instructive for one fate and inhibitory for the other. We re-examine this binary switch model in light of two recent findings made in the vertebrate nervous system. First, in the zebrafish epiphysis, Notch is required to resolve a mixed identity through the inhibition of one specific fate. Second, in the murine telencephalon, Notch regulates the competence of neural progenitors to respond to the JAK/STAT pathway, thereby allowing for the induction of an astrocyte fate. In neither case is Notch instructive for the alternative fate, but rather cooperates with another signalling pathway to coordinate binary fate choices. We also review current knowledge on the molecular cascades acting downstream of Notch in the context of neural subtype diversification, a crucial issue if one is to determine Notch function as an instructive, permissive or inhibitory signal in the various cellular contexts where it is implicated. Finally, we speculate as to how such a 'non-switch' activity could contribute to the expansion of neuronal subtype diversity.

Pattern formation in the Drosophila eye disc

Differentiation of the Drosophila compound eye from the eye imaginal disc is a progressive process: columns of cells successively differentiate in a posterior to anterior sequence, clusters of cells form at regularly spaced intervals within each column, and individual photoreceptors differentiate in a defined order within each cluster. The progression of differentiation across the eye disc is driven by a positive autoregulatory loop of expression of the secreted molecule Hedgehog, which is temporally delayed by the intercalation of a second signal, Spitz. Hedgehog refines the spatial position at which each column initiates its differentiation by inducing secondary signals that act over different ranges to control the expression of positive and negative regulators. The position of clusters within each column is controlled by secreted inhibitory signals from clusters in the preceding column, and a single founder neuron, R8, is singled out within each cluster by Notch-mediated lateral inhibition. R8 then sequentially recruits surrounding cells to differentiate by producing a short-range signal, Spitz, which induces a secondary short-range signal, Delta. Intrinsic transcription factors act in combination with these two signals to produce cell-type diversity within the ommatidium. The Hedgehog and Spitz signals are transported along the photoreceptor axons and reused within the brain as long-range and local cues to trigger the differentiation and assembly of target neurons.

Is Runx a linchpin for developmental signaling in metazoans?

The Runt domain (Runx) is a 128 amino acid sequence motif that defines a metazoan family of sequence-specific DNA binding proteins, which appears to have originated in concert with the intercellular signaling systems that coordinate multicellular development in animals. In the model organisms where they have been studied (fruit fly, mouse, sea urchin, and nematode) Runx genes are essential for normal development, and in humans they are causally associated with a variety of cancers, manifesting both oncogenic and tumor suppressive attributes. During development Runx proteins support both cell proliferation and differentiation, and function in both transcriptional activation and repression. Runx function is thus context-dependent, with the context provided genetically by cis-regulatory sequence architecture and epigenetically by development. This context dependency makes it difficult to formulate reductionistic generalizations concerning Runx function in normal and carcinogenic development. However, a growing body of literature links Runx function to each of the major intercellular signaling systems in animals, suggesting that the general function of Runx transcription factors may be to potentiate and govern genomic responsiveness to developmental signaling.

Hindsight modulates Delta expression during Drosophila cone cell induction

The induction of cone cells in the Drosophila larval eye disc by the determined R1/R6 photoreceptor precursor cells requires integration of the Delta-Notch and EGF receptor signaling pathways with the activity of the Lozenge transcription factor. Here, we demonstrate that the zinc-finger transcription factor Hindsight (HNT) is required for normal cone-cell induction. R-cells in which hindsight levels are knocked down using RNAi show normal subtype specification, but these cells have lower levels of the Notch ligand Delta. We show that HNT functions in the determined R1/R6 precursor cells to allow Delta transcription to reach high enough levels at the right time to induce the cone-cell determinants Prospero and D-Pax2 in neighboring cells. The Delta signal emanating from the R1/R6 precursor cells is also required to specify the R7 precursor cell by repressing seven-up. As hindsight mutants have normal R7 cell-fate determination, we infer that there is a lower threshold of Delta required for R7 specification than for cone-cell induction.

Determination of cell fates in the R7 equivalence group of the Drosophila eye by the concerted regulation of D-Pax2 and TTK88

In the developing Drosophila eye, the precursors of the neuronal photoreceptor cells R1/R6/R7 and non-neuronal cone cells share the same developmental potential and constitute the R7 equivalence group. It is not clear how cells of this group elaborate their distinct fates. Here we show that both TTK88 and D-Pax2 play decisive roles in cone cell development and act in concert to transform developing R1/R6/R7 into cone cells: while TTK88 blocks neuronal development, D-Pax2 promotes cone cell specification. In addition, ectopic TTK88 in R cells induces apoptosis, which is suppressed by ectopic D-Pax2. We further demonstrate that Phyllopod (Phyl), previously shown to promote the neuronal fate in R1/R6/R7 by targeting TTK for degradation, also inhibits D-Pax2 transcription to prevent cone cell specification. Thus, the fates of R1/R6/R7 and cone cells are determined by a dual mechanism that coordinately activates one fate while inhibiting the other.

SOCS36E specifically interferes with Sevenless signaling during Drosophila eye development

During the development of multicellular organisms the fate of individual cells is specified with great precision and reproducibility. Although classical genetic approaches led to the identification of many of the signaling pathways contributing to cell fate specification, they have provided little insight into the mechanisms that ensure robustness and reproducibility. We have used the specification of the R7 photoreceptor cells controlled by the Sevenless receptor tyrosine kinase (Sev) pathway to screen for modulators of pathway activity and to uncover the mechanisms underlying the robustness of cell fate decisions. Here we provide genetic evidence that the Drosophila SOCS36E adaptor protein containing an SH2 domain and a SOCS box acts as an attenuator of Sev signaling. Overexpression of Socs36E strongly suppresses the specification of extra R7 photoreceptor cells in response to constitutive activation of Sev, and loss of Socs36E function suppresses the loss of R7 cells when Sev activity is impaired. In a wild-type background, however, loss and gain of Socs36E function exhibits little effect on R7 specification. We also show that SH2 domain of SOCS36E is essential for this function in inhibiting Sev action and that Socs36E expression is suppressed by high Sev pathway activity. In our model, only the cell able to activate high levels of receptor tyrosine kinase signaling will repress SOCS36E expression, reduce the negative effect on Sev signaling and allow this cell to differentiate into R7. In contrast, the remaining cells fail to receive high signaling, and thus maintain high levels of SOCS36E. This represses residual Sev activity and blocks R7 development. Therefore, Socs36E constitutes a novel partially redundant feedback mechanism that contributes to the robustness of R7 specification. The SOCS family of adaptor proteins may have evolved as modulators of specific signaling pathways that contribute to the robustness and precision of cell fate specification.