The homeo domain protein rough is expressed in a subset of cells in the developing Drosophila eye where it can specify photoreceptor cell subtype

A single cell RNA sequence atlas of the early Drosophila larval eye

The Drosophila eye has been an important model to understand principles of differentiation, proliferation, apoptosis and tissue morphogenesis. However, a single cell RNA sequence resource that captures gene expression dynamics from the initiation of differentiation to the specification of different cell types in the larval eye disc is lacking. Here, we report transcriptomic data from 13,000 cells that cover six developmental stages of the larval eye. Our data show cell clusters that correspond to all major cell types present in the eye disc ranging from the initiation of the morphogenetic furrow to the differentiation of each photoreceptor cell type as well as early cone cells. We identify dozens of cell type-specific genes whose function in different aspects of eye development have not been reported. These single cell data will greatly aid research groups studying different aspects of early eye development and will facilitate a deeper understanding of the larval eye as a model system.

40 years of homeodomain transcription factors in the Drosophila nervous system

Drosophila nervous system development progresses through a series of well-characterized steps in which homeodomain transcription factors (HDTFs) play key roles during most, if not all, phases. Strikingly, although some HDTFs have only one role, many others are involved in multiple steps of the developmental process. Most Drosophila HDTFs engaged in nervous system development are conserved in vertebrates and often play similar roles during vertebrate development. In this Spotlight, we focus on the role of HDTFs during embryogenesis, where they were first characterized.

Ashwagandha- Withania somnifera (L.) Dunal as a multipotent neuroprotective remedy for genetically induced motor dysfunction and cellular toxicity in human neurodegenerative disease models of Drosophila

ETHNOPHARMACOLOGICAL RELEVANCE

Ashwagandha-Withania somnifera (L.) Dunal, well known for its multipotent therapeutic properties has been used in Ayurveda for 3000 years. The plant with more than 50 active phytoconstituents is recognised for its anti-cancerous, anti-diabetic, anti-inflammatory, anti-microbial, and neurotherapeutic properties demonstrated in in vitro studies and chemically induced rodent models. Genetically targeted Parkinson's, Alzheimer's and other neurodegenerative disease models have been created in Drosophila and have been used to get mechanistic insight into the in vivo cellular events, and genetic pathways that underlie respective neurodegenerative condition. But hitherto, there aren't enough attempts made to capitalize the genetic potential of these disease models to validate the therapeutic efficacy of different reagents used in traditional medicine, in the context of specific disease-causing genetic mutations.

AIM OF THE STUDY

Drugs discovered using in vitro platforms might fail in several instances of clinical trials because of the genetic heterogeneity and variability in the physiological context found among the patients. Drosophila by virtue of its genetically regulated experimental potential forms an ideal in vivo model to validate the candidate reagents discovered in in vitro screens for their efficacy under specific genetic situations. Here we have used genetically induced α-synucleinopathy and tauopathy transgenic fly models to study the efficacy of Ashwagandha treatment, assessing cellular and behavioural parameters.

METHODS

We have expressed the disease-causing human gene mutations in specific cell types of Drosophila using GAL4/UAS targeted expression system to create disease models. Human α-synuclein mutant (A30P) was expressed in dopaminergic neurons using Ddc-GAL4 driver strain to induce dopaminergic neurodegeneration and assayed for motor dysfunction. Human TauE14, mutant protein was expressed in photoreceptor neurons using GMR-GAL4 driver to induce photoreceptor degeneration. Microtubular destability and mitotic arrest in the dividing photoreceptor precursor cells were studied using αPH3 antibody. Lysosomal dysregulation caused necrotic black spots were induced by TauE14 with GMR-GAL4 driver, in a white mutant background. These flies mimicking neurodegenerative conditions were supplemented with different concentrations of Ashwagandha aqueous root extract mixed with regular fly food. The treated flies were analysed for cellular and behaviour parameters.

RESULTS

Lifespan assay shows that, Ashwagandha-root extract imparts an extended lifespan in male Drosophila flies which are intrinsically less stress resistant. Motor dysfunction caused due to human α-synuclein mutant protein expressed in dopaminergic neurons is greatly brought down. Further, Ashwagandha extract treatment significantly reduces TauE14 induced microtubular destability, mitotic arrest and neuronal death in photoreceptor neurons. Our experiment with tauopathy model in white mutant background exemplify that, Ashwagandha-root extract treatment can bring down lysosomal dysregulation induced necrosis of photoreceptor neurons.

CONCLUSION

We have carried out a multifaceted study which elucidates that Ashwagandha can serve as a comprehensive, phytotherapeutic formulation to combat neurodegeneration, targeting multiple causative genetically defective conditions.

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.

Patterning of the Drosophila retina by the morphogenetic furrow

Pattern formation is the process by which cells within a homogeneous epithelial sheet acquire distinctive fates depending upon their relative spatial position to each other. Several proposals, starting with Alan Turing's diffusion-reaction model, have been put forth over the last 70 years to describe how periodic patterns like those of vertebrate somites and skin hairs, mammalian molars, fish scales, and avian feather buds emerge during development. One of the best experimental systems for testing said models and identifying the gene regulatory networks that control pattern formation is the compound eye of the fruit fly, Drosophila melanogaster. Its cellular morphogenesis has been extensively studied for more than a century and hundreds of mutants that affect its development have been isolated. In this review we will focus on the morphogenetic furrow, a wave of differentiation that takes an initially homogeneous sheet of cells and converts it into an ordered array of unit eyes or ommatidia. Since the discovery of the furrow in 1976, positive and negative acting morphogens have been thought to be solely responsible for propagating the movement of the furrow across a motionless field of cells. However, a recent study has challenged this model and instead proposed that mechanical driven cell flow also contributes to retinal pattern formation. We will discuss both models and their impact on patterning.

Single cell RNA sequencing of the adult Drosophila eye reveals distinct clusters and novel marker genes for all major cell types

The adult Drosophila eye is a powerful model system for phototransduction and neurodegeneration research. However, single cell resolution transcriptomic data are lacking for this tissue. We present single cell RNA-seq data on 1-day male and female, 3-day and 7-day old male adult eyes, covering early to mature adult eyes. All major cell types, including photoreceptors, cone and pigment cells in the adult eye were captured and identified. Our data sets identified novel cell type specific marker genes, some of which were validated in vivo. R7 and R8 photoreceptors form clusters that reflect their specific Rhodopsin expression and the specific Rhodopsin expression by each R7 and R8 cluster is the major determinant to their clustering. The transcriptomic data presented in this report will facilitate a deeper mechanistic understanding of the adult fly eye as a model system.

Hexagonal patterning of the Drosophila eye

A complex network of transcription factor interactions propagates across the larval eye disc to establish columns of evenly-spaced R8 precursor cells, the founding cells of Drosophila ommatidia. After the recruitment of additional photoreceptors to each ommatidium, the surrounding cells are organized into their stereotypical pattern during pupal development. These support cells - comprised of pigment and cone cells - are patterned to encapsulate the photoreceptors and separate ommatidia with an hexagonal honeycomb lattice. Since the proteins and processes essential for correct eye patterning are conserved, elucidating how these function and change during Drosophila eye patterning can substantially advance our understanding of transcription factor and signaling networks, cytoskeletal structures, adhesion complexes, and the biophysical properties of complex tissues during their morphogenesis. Our understanding of many of these aspects of Drosophila eye patterning is largely descriptive. Many important questions, especially relating to the regulation and integration of cellular events, remain.

α-Synuclein E46K Mutation and Involvement of Oxidative Stress in a Drosophila Model of Parkinson's Disease

Parkinson's disease (PD) is an age-associated neurodegenerative condition in which some genetic variants are known to increase disease susceptibility on interaction with environmental factors inducing oxidative stress. Different mutations in the SNCA gene are reported as the major genetic contributors to PD. E46K mutation pathogenicity has not been investigated as intensive as other SNCA gene mutations including A30P and A53T. In this study, based on the GAL4-UAS binary genetic tool, transgenic Drosophila melanogaster flies expressing wild-type and E46K-mutated copies of the human SNCA gene were constructed. Western blotting, immunohistochemical analysis, and light and confocal microscopy of flies' brains were undertaken along with the survival rate measurement, locomotor function assay, and ethanol and paraquat (PQ) tolerance to study α-synuclein neurotoxicity. Biochemical bioassays were carried out to investigate the activity of antioxidant enzymes and alterations in levels of oxidative markers following damages induced by human α-synuclein to the neurons of the transgenic flies. Overexpression of human α-synuclein in the central nervous system of these transgenic flies led to disorganized ommatidia structures and loss of dopaminergic neurons. E46K α-synuclein caused remarkable climbing defects, reduced survivorship, higher ethanol sensitivity, and increased PQ-mediated mortality. A noticeable decline in activity of catalase and superoxide dismutase enzymes besides considerable increase in the levels of lipid peroxidation and reactive oxygen species was observed in head capsule homogenates of α-synuclein-expressing flies, which indicates obvious involvement of oxidative stress as a causal factor in SNCA^E46K neurotoxicity. In all the investigations, E46K copy of the SNCA gene was found to impose more severe defects when compared to wild-type SNCA. It can be concluded that the constructed Drosophila models developed PD-like symptoms that facilitate comparative studies of molecular and cellular pathways implicated in the pathogenicity of different α-synuclein mutations.

E3 ubiquitin ligase-mediated regulation of vertebrate ocular development; new insights into the function of SIAH enzymes

Developmental regulation of the vertebrate visual system has been a focus of investigation for generations as understanding this critical time period has direct implications on our understanding of congenital blinding disease. The majority of studies to date have focused on transcriptional regulation mediated by morphogen gradients and signaling pathways. However, recent studies of post translational regulation during ocular development have shed light on the role of the ubiquitin proteasome system (UPS). This rather ubiquitous yet highly diverse system is well known for regulating protein function and localization as well as stability via targeting for degradation by the 26S proteasome. Work from many model organisms has recently identified UPS activity during various milestones of ocular development including retinal morphogenesis, retinal ganglion cell function as well as photoreceptor homeostasis. In particular work from flies and zebrafish has highlighted the role of the E3 ligase enzyme family, Seven in Absentia Homologue (Siah) during these events. In this review, we summarize the current understanding of UPS activity during Drosophila and vertebrate ocular development, with a major focus on recent findings correlating Siah E3 ligase activity with two major developmental stages of vertebrate ocular development, retinal morphogenesis and photoreceptor specification and survival.

Gene regulatory networks during the development of the Drosophila visual system

The Drosophila visual system integrates input from 800 ommatidia and extracts different features in stereotypically connected optic ganglia. The development of the Drosophila visual system is controlled by gene regulatory networks that control the number of precursor cells, generate neuronal diversity by integrating spatial and temporal information, coordinate the timing of retinal and optic lobe cell differentiation, and determine distinct synaptic targets of each cell type. In this chapter, we describe the known gene regulatory networks involved in the development of the different parts of the visual system and explore general components in these gene networks. Finally, we discuss the advantages of the fly visual system as a model for gene regulatory network discovery in the era of single-cell transcriptomics.

Transgenic Drosophila model to study apolipoprotein E4-induced neurodegeneration

The ε4 isoform of apolipoprotein E (ApoE4) that is involved in neuron-glial lipid metabolism has been demonstrated as the main genetic risk factor in late-onset of Alzheimer's disease. However, the mechanism underlying ApoE4-mediated neurodegeneration remains unclear. We created a transgenic model of neurodegenerative disorder by expressing ε3 and ε4 isoforms of human ApoE in the Drosophila melanogaster. The genetic models exhibited progressive neurodegeneration, shortened lifespan and memory impairment. Genetic interaction studies between amyloid precursor protein and ApoE in axon pathology of the disease revealed that over expression of hApoE in Appl-expressing neurons of Drosophila brain causes neurodegeneration. Moreover, acute oxidative damage in the hApoE transgenic flies triggered a neuroprotective response of hApoE3 while chronic induction of oxidative damage accelerated the rate of neurodegeneration. This Drosophila model may facilitate analysis of the molecular and cellular events implicated in hApoE4 neurotoxicity.

Dissection and immunostaining of imaginal discs from Drosophila melanogaster

A significant portion of post-embryonic development in the fruit fly, Drosophila melanogaster, takes place within a set of sac-like structures called imaginal discs. These discs give rise to a high percentage of adult structures that are found within the adult fly. Here we describe a protocol that has been optimized to recover these discs and prepare them for analysis with antibodies, transcriptional reporters and protein traps. This procedure is best suited for thin tissues like imaginal discs, but can be easily modified for use with thicker tissues such as the larval brain and adult ovary. The written protocol and accompanying video will guide the reader/viewer through the dissection of third instar larvae, fixation of tissue, and treatment of imaginal discs with antibodies. The protocol can be used to dissect imaginal discs from younger first and second instar larvae as well. The advantage of this protocol is that it is relatively short and it has been optimized for the high quality preservation of the dissected tissue. Another advantage is that the fixation procedure that is employed works well with the overwhelming number of antibodies that recognize Drosophila proteins. In our experience, there is a very small number of sensitive antibodies that do not work well with this procedure. In these situations, the remedy appears to be to use an alternate fixation cocktail while continuing to follow the guidelines that we have set forth for the dissection steps and antibody incubations.

Apical accumulation of the Sevenless receptor tyrosine kinase during Drosophila eye development is promoted by the small GTPase Rap1

The Ras/MAPK-signaling pathway plays pivotal roles during development of metazoans by controlling cell proliferation and cell differentiation elicited, in several instances, by receptor tyrosine kinases (RTKs). While the internal mechanism of RTK-driven Ras/MAPK signaling is well understood, far less is known regarding its interplay with other co-required signaling events involved in developmental decisions. In a genetic screen designed to identify new regulators of RTK/Ras/MAPK signaling during Drosophila eye development, we identified the small GTPase Rap1, PDZ-GEF, and Canoe as components contributing to Ras/MAPK-mediated R7 cell differentiation. Rap1 signaling has recently been found to participate in assembling cadherin-based adherens junctions in various fly epithelial tissues. Here, we show that Rap1 activity is required for the integrity of the apical domains of developing photoreceptor cells and that reduced Rap1 signaling hampers the apical accumulation of the Sevenless RTK in presumptive R7 cells. It thus appears that, in addition to its role in cell-cell adhesion, Rap1 signaling controls the partitioning of the epithelial cell membrane, which in turn influences signaling events that rely on apico-basal cell polarity.

Eye development

The eye has been one of the most intensively studied organs in Drosophila. The wealth of knowledge about its development, as well as the reagents that have been developed, and the fact that the eye is dispensable for survival, also make the eye suitable for genetic interaction studies and genetic screens. This article provides a brief overview of the methods developed to image and probe eye development at multiple developmental stages, including live imaging, immunostaining of fixed tissues, in situ hybridizations, and scanning electron microscopy and color photography of adult eyes. Also summarized are genetic approaches that can be performed in the eye, including mosaic analysis and conditional mutation, gene misexpression and knockdown, and forward genetic and modifier screens.

Drosophila EHBP1 regulates Scabrous secretion during Notch-mediated lateral inhibition

Notch signaling is an evolutionarily conserved pathway that plays a central role in numerous developmental and disease processes. The versatility of the Notch pathway relies on the activity of context-dependent regulators. These include rab11, sec15, arp3 and Drosophila EHBP1 (dEHBP1), which control Notch signaling and cell fate acquisition in asymmetrically dividing mechanosensory lineages by regulating the trafficking of the ligand Delta. Here, we show that dEHBP1 also controls the specification of R8 photoreceptors, as its loss results in the emergence of supernumerary R8 photoreceptors. Given the requirements for Notch signaling during lateral inhibition, we propose that dEHBP1 regulates distinct aspects of Notch signaling in different developmental contexts. We show that dEHBP1 regulates the exocytosis of Scabrous, a positive regulator of Notch signaling. In conclusion, dEHBP1 provides developmental versatility of intercellular signaling by regulating the trafficking of distinct Notch signaling components.

Cellular reprogramming processes in Drosophila and C. elegans

The identity of individual cell types in a multicellular organism appears to be continuously maintained through active processes but is not irreversible. Changes in the identity of individual cell types can be brought about through ectopic mis-expression of regulatory factors, but in a number of cases also occurs in normal development. I will review here these natural cellular reprogramming processes occurring in the invertebrate model organisms Caenorhabditis elegans and Drosophila melanogaster. Furthermore, I will discuss the issue of why only certain cell types can be converted during induced reprogramming processes evoked by ectopic expression of regulatory factors and how recent work in model systems have shown that this cellular context-dependency can be manipulated.

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.

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.

A network of broadly expressed HLH genes regulates tissue-specific cell fates

Spatial and temporal expression of specific basic-helix-loop-helix (bHLH) transcription factors defines many types of cellular differentiation. We find that a distinct mechanism regulates the much broader expression of the heterodimer partners of these specific factors and impinges on differentiation. In Drosophila, a cross-interacting regulatory network links expression of the E protein Daughterless (Da), which heterodimerizes with bHLH proteins to activate them, with expression of the Id protein Extramacrochaetae (Emc), which antagonizes bHLH proteins. Coupled transcriptional feedback loops maintain the widespread Emc expression that restrains Da expression, opposing bHLH-dependent differentiation while enhancing growth and cell survival. Where extracellular signals repress emc, Da expression can increase. This defines regions of proneural ectoderm independently from the proneural bHLH genes. Similar regulation is found in multiple Drosophila tissues and in mammalian cells and therefore is likely to be a conserved general feature of developmental regulation by HLH proteins.

Preparation of Drosophila eye specimens for scanning electron microscopy

Scanning electron microscopy (SEM) allows for high-magnification analysis of the external structures of the Drosophila eye. In this article, three methods are provided: one for eyes that are to be critical-point-dried (CPD) and two alternatives if a critical point drier is not available. The major difference between the first method and the other two is a preliminary fixation step. Drosophila eyes have a tendency to collapse when CPD, so fixing the animals before critical point drying improves the yield of good specimens.

Rap1 promotes VEGFR2 activation and angiogenesis by a mechanism involving integrin αvβ₃

Vascular endothelial growth factor (VEGF) acting through VEGF receptor 2 (VEGFR2) on endothelial cells (ECs) is a key regulator of angiogenesis, a process essential for wound healing and tumor metastasis. Rap1a and Rap1b, 2 highly homologous small G proteins, are both required for angiogenesis in vivo and for normal EC responses to VEGF. Here we sought to determine the mechanism through which Rap1 promotes VEGF-mediated angiogenesis. Using lineage-restricted Rap1-knockout mice we show that Rap1-deficiency in endothelium leads to defective angiogenesis in vivo, in a dose-dependent manner. Using ECs obtained from Rap1-deficient mice we demonstrate that Rap1b promotes VEGF-VEGFR2 kinase activation and regulates integrin activation. Importantly, the Rap1b-dependent VEGF-VEGFR2 activation is in part mediated via integrin α(v)β(3). Furthermore, in an in vivo model of zebrafish angiogenesis, we demonstrate that Rap1b is essential for the sprouting of intersomitic vessels, a process known to be dependent on VEGF signaling. Using 2 distinct pharmacologic VEGFR2 inhibitors we show that Rap1b and VEGFR2 act additively to control angiogenesis in vivo. We conclude that Rap1b promotes VEGF-mediated angiogenesis by promoting VEGFR2 activation in ECs via integrin α(v)β(3). These results provide a novel insight into the role of Rap1 in VEGF signaling in ECs.

Drosophila FMRP regulates microtubule network formation and axonal transport of mitochondria

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the fragile X mental retardation protein FMRP. The RNA-binding FMRP represses translation of the microtubule (MT)-associated protein 1B (MAP1B) during synaptogenesis in the brain of the neonatal mouse. However, the effect of FMRP on MTs remains unclear. Mounting evidence shows that the structure and the function of FMRP are well conserved across species from Drosophila to human. From a genetic screen, we identified spastin as a dominant suppressor of rough eye caused by dfmr1 over-expression. spastin encodes an MT-severing protein, and its mutations cause neurodegenerative hereditary spastic paraplegia. Epistatic and biochemical analyses revealed that dfmr1 acts upstream of or in parallel with spastin in multiple processes, including synapse development, locomotive behaviour and MT network formation. Immunostaining showed that both loss- and gain-of-function mutations of dfmr1 result in an apparently altered MT network. Western analysis revealed that the levels of α-tubulin and acetylated MTs remained normal in dfmr1 mutants, but increased significantly when dfmr1 was over-expressed. To examine the consequence of the aberrant MTs in dfmr1 mutants, we analysed the MT-dependent mitochondrial transport and found that the number of mitochondria and the flux of mitochondrial transport are negatively regulated by dfmr1. These results demonstrate that dFMRP plays a crucial role in controlling MT formation and mitochondrial transport. Thus, defective MTs and abnormal mitochondrial transport might account for, at least partially, the pathogenesis of fragile X mental retardation.

Bap170, a subunit of the Drosophila PBAP chromatin remodeling complex, negatively regulates the EGFR signaling

BAP and PBAP constitute the two different forms of the Drosophila melanogaster Brahma chromatin remodelers. A common multisubunit core, containing the Brahma ATPase, can associate either with Osa to form the BAP complex or with Bap170, Bap180, and Sayp to constitute the PBAP complex. Although required for many biological processes, recent genetic analyses revealed that one role of the BAP complex during Drosophila wing development is the proper regulation of EGFR target genes. Here, we show that Bap170, a distinctive subunit of the PBAP complex, participates instead in the negative regulation of EGFR signaling. In adults, loss of Bap170 generates phenotypes similar to the defects induced by hyperactivation of the EGFR pathway, such as overrecruitment of cone and photoreceptor cells and formation extra veins. In genetic interactions, bap170 mutations suppress the loss of veins and photoreceptors caused by mutations affecting the activity of the EGFR pathway. Our results suggest a dual requirement of the PBAP complex: for transcriptional repression of rhomboid and for efficient expression of argos. Interestingly, genetic evidence also indicates that Bap170-mediated repression of rho is inhibited by EGFR signaling, suggesting a scenario of mutual antagonism between EGFR signaling and PBAP function.

Enforcing biphasic eye development in a directly developing insect by transient knockdown of single eye selector genes

The visual system of indirectly developing insects such as Drosophila passes through two phases of development. Larval eyes form in the embryo, whereas the adult compound eyes form during metamorphosis. Comparative evidence implies that this biphasic mode of visual system development evolved from the continuously developing eye of directly developing insects. We investigated the developmental basis of this evolutionary transformation in a directly developing insect taking advantage of the time-limited nature of systemic RNAi in the grasshopper Schistocerca americana. Transient knockdown of the homologs of the early retinal genes eyes absent (eya) or sine oculis (so) both induced long-term arrest of eye development in grasshopper nymphs. Eye development, however, resumed after knockdown expiry. This finding sheds first light on the molecular regulation of postembryonic eye development in directly developing insects and unravels an inherent capacity of the underlying gene regulatory network to accommodate for partitioning visual system development into discrete phases, as in indirectly developing insects.

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.

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.

LKB1 regulates polarity remodeling and adherens junction formation in the Drosophila eye

The serine-threonine kinase LKB1 regulates cell polarity from Caenorhabditis elegans to man. Loss of lkb1 leads to a cancer predisposition, known as Peutz-Jeghers Syndrome. Biochemical analysis indicates that LKB1 can phosphorylate and activate a family of AMPK- like kinases, however, the precise contribution of these kinases to the establishment and maintenance of cell polarity is still unclear. Recent studies propose that LKB1 acts primarily through the AMP kinase to establish and/or maintain cell polarity. To determine whether this simple model of how LKB1 regulates cell polarity has relevance to complex tissues, we examined lkb1 mutants in the Drosophila eye. We show that adherens junctions expand and apical, junctional, and basolateral domains mix in lkb1 mutants. Surprisingly, we find LKB1 does not act primarily through AMPK to regulate cell polarity in the retina. Unlike lkb1 mutants, ampk retinas do not show elongated rhabdomeres or expansion of apical and junctional markers into the basolateral domain. In addition, nutrient deprivation does not reveal a more dramatic polarity phenotype in lkb1 photoreceptors. These data suggest that AMPK is not the primary target of LKB1 during eye development. Instead, we find that a number of other AMPK-like kinase, such as SIK, NUAK, Par-1, KP78a, and KP78b show phenotypes similar to weak lkb1 loss of function in the eye. These data suggest that in complex tissues, LKB1 acts on an array of targets to regulate cell polarity.

Opsins and cell fate in the Drosophila Bolwig organ: tricky lessons in homology inference

The Drosophila Bolwig organs are small photoreceptor bundles that facilitate the phototactic behavior of the larva. Comparative literature suggests that these highly reduced visual organs share evolutionary ancestry with the adult compound eye. A recent molecular genetic study produced the first detailed account of the mechanisms controlling differential opsin expression and photoreceptor subtype determination in these enigmatic eyes of the Drosophila larva. Here, the evolutionary implications are examined, taking into account the dynamic diversification of opsin genes and the spatial regulation of opsin homolog expression in other insects. It is concluded that, consistent with their common evolutionary roots, the Drosophila larval and adult eyes use the same mechanisms for the regulation of opsin expression and photoreceptor cell fate specification. Strikingly, the structurally highly derived Bolwig organs retained a more ancestral state of opsin expression and regulation. Inconspicuous in size, the Drosophila larval eyes deliver useful lessons in the reconstruction of homology between neuronal cell types with gene expression data, and on the conservative nature of gene regulatory network evolution during the emergence of novel organs from ancestral templates.

Regulation of neurogenesis and epidermal growth factor receptor signaling by the insulin receptor/target of rapamycin pathway in Drosophila

Determining how growth and differentiation are coordinated is key to understanding normal development, as well as disease states such as cancer, where that control is lost. We have previously shown that growth and neuronal differentiation are coordinated by the insulin receptor/target of rapamycin (TOR) kinase (InR/TOR) pathway. Here we show that the control of growth and differentiation diverge downstream of TOR. TOR regulates growth by controlling the activity of S6 kinase (S6K) and eIF4E. Loss of s6k delays differentiation, and is epistatic to the loss of tsc2, indicating that S6K acts downstream or in parallel to TOR in differentiation as in growth. However, loss of eIF4E inhibits growth but does not affect the timing of differentiation. We also show, for the first time in Drosophila, that there is crosstalk between the InR/TOR pathway and epidermal growth factor receptor (EGFR) signaling. InR/TOR signaling regulates the expression of several EGFR pathway components including pointedP2 (pntP2). In addition, reduction of EGFR signaling levels phenocopies inhibition of the InR/TOR pathway in the regulation of differentiation. Together these data suggest that InR/TOR signaling regulates the timing of differentiation through modulation of EGFR target genes in developing photoreceptors.

Proneural basic helix-loop-helix proteins and epidermal growth factor receptor signaling coordinately regulate cell type specification and cdk inhibitor expression during development

Cell differentiation and cell cycle exit are coordinately regulated during development; however, the molecular logic underlying this regulation is not known. The Drosophila cdk inhibitor Dacapo (Dap) is one of the key cell cycle regulators that exhibit dynamic expression during development and contribute to the developmental regulation of the cell cycle. In this study, regulation of Dap expression during cell type specification was investigated. The expression of Dap in the R2 and R5 precursors of the developing eye and in the newly recruited leg disc femoral sense organ precursors was found to be controlled by the epidermal growth factor receptor signaling-regulated transcription factor Pointed (Pnt) and the proneural basic helix-loop-helix proteins Atonal (Ato) and Daughterless (Da). We show that Pnt, Ato, and Da regulate Dap expression directly through their respective binding sites precisely at the time when these transcription factors function to specify neural fates. These results show that Dap expression is directly regulated by developmental mechanisms that simultaneously control cell type specification. This is potentially a general mechanism by which the expression of key cell cycle regulators is coordinated with differentiation during normal development. The direct regulation of key cell cycle regulators by the differentiation factors ensures coordinated regulation of cell cycle and differentiation.

Relationships of Drosophila melanogaster RECQ5/QE to cell-cycle progression and DNA damage

Members of the RecQ family of DNA helicases are involved in the cellular response to DNA damage and are regulated in the cell-cycle. However, little is known about RecQ5, one of these members. The level of RECQ5/QE, Drosophila melanogaster RecQ5, was increased after the exposure of cultured cells to methyl-methanesulfonate. Transgenic flies that overexpressed RECQ5/QE in their developing eye primordia showed mild roughening of the ommatidial lattice. DNA-damaging agents and the mei-41 mutation enhanced the phenotype caused by RECQ5/QE overexpression. Overexpression of RECQ5/QE perturbed the progression of the cell-cycle in response to DNA damage in the eye imaginal discs. These results suggest that RECQ5/QE interacts with components of the cell-cycle during its progression in response to DNA damage.

The Ral/exocyst effector complex counters c-Jun N-terminal kinase-dependent apoptosis in Drosophila melanogaster

Ral GTPase activity is a crucial cell-autonomous factor supporting tumor initiation and progression. To decipher pathways impacted by Ral, we have generated null and hypomorph alleles of the Drosophila melanogaster Ral gene. Ral null animals were not viable. Reduced Ral expression in cells of the sensory organ lineage had no effect on cell division but led to postmitotic cell-specific apoptosis. Genetic epistasis and immunofluorescence in differentiating sensory organs suggested that Ral activity suppresses c-Jun N-terminal kinase (JNK) activation and induces p38 mitogen-activated protein (MAP) kinase activation. HPK1/GCK-like kinase (HGK), a MAP kinase kinase kinase kinase that can drive JNK activation, was found as an exocyst-associated protein in vivo. The exocyst is a Ral effector, and the epistasis between mutants of Ral and of msn, the fly ortholog of HGK, suggest the functional relevance of an exocyst/HGK interaction. Genetic analysis also showed that the exocyst is required for the execution of Ral function in apoptosis. We conclude that in Drosophila Ral counters apoptotic programs to support cell fate determination by acting as a negative regulator of JNK activity and a positive activator of p38 MAP kinase. We propose that the exocyst complex is Ral executioner in the JNK pathway and that a cascade from Ral to the exocyst to HGK would be a molecular basis of Ral action on JNK.

Atrophin contributes to the negative regulation of epidermal growth factor receptor signaling in Drosophila

Dentato-rubral and pallido-luysian atrophy (DRPLA) is a dominant, progressive neurodegenerative disease caused by the expansion of polyglutamine repeats within the human Atrophin-1 protein. Drosophila Atrophin and its human orthologue are thought to function as transcriptional co-repressors. Here, we report that Drosophila Atrophin participates in the negative regulation of Epidermal Growth Factor Receptor (EGFR) signaling both in the wing and the eye imaginal discs. In the wing pouch, Atrophin loss of function clones induces cell autonomous expression of the EGFR target gene Delta, and the formation of extra vein tissue, while overexpression of Atrophin inhibits EGFR-dependent vein formation. In the eye, Atrophin cooperates with other negative regulators of the EGFR signaling to prevent the differentiation of surplus photoreceptor cells and to repress Delta expression. Overexpression of Atrophin in the eye reduces the EGFR-dependent recruitment of cone cells. In both the eye and wing, epistasis tests show that Atrophin acts downstream or in parallel to the MAP kinase rolled to modulate EGFR signaling outputs. We show that Atrophin genetically cooperates with the nuclear repressor Yan to inhibit the EGFR signaling activity. Finally, we have found that expression of pathogenic or normal forms of human Atrophin-1 in the wing promotes wing vein differentiation and acts as dominant negative proteins inhibiting endogenous fly Atrophin activity.

The Drosophila tumor suppressor vps25 prevents nonautonomous overproliferation by regulating notch trafficking

Cell-cell signaling coordinates proliferation of metazoan tissues during development, and its alteration can induce malignant transformation. Endocytosis regulates signaling by controlling the levels and activity of transmembrane receptors, both prior to and following ligand engagement. Here, we identify Vps25, a component of the ESCRT machinery that regulates endocytic sorting of signaling receptors, as an unconventional type of Drosophila tumor suppressor. vps25 mutant cells undergo autonomous neoplastic-like transformation, but they also stimulate nonautonomous cell proliferation. Endocytic trafficking defects in vps25 cells cause endosomal accumulation of the signaling receptor Notch and enhanced Notch signaling. Increased Notch activity leads to ectopic production of the mitogenic JAK-STAT pathway ligand Unpaired, which is secreted from mutant cells to induce overproliferation of the surrounding epithelium. Our data show that defects in endocytic sorting can both transform cells and, through heterotypic signaling, alter the behavior of neighboring wild-type tissue.

Dual regulation and redundant function of two eye-specific enhancers of the Drosophila retinal determination gene dachshund

Drosophila eye development is controlled by a conserved network of retinal determination (RD) genes. The RD genes encode nuclear proteins that form complexes and function in concert with extracellular signal-regulated transcription factors. Identification of the genomic regulatory elements that govern the eye-specific expression of the RD genes will allow us to better understand how spatial and temporal control of gene expression occurs during early eye development. We compared conserved non-coding sequences (CNCSs) between five Drosophilids along the approximately 40 kb genomic locus of the RD gene dachshund (dac). Our analysis uncovers two separate eye enhancers in intron eight and the 3' non-coding regions of the dac locus defined by clusters of highly conserved sequences. Loss- and gain-of-function analyses suggest that the 3' eye enhancer is synergistically activated by a combination of eya, so and dpp signaling, and only indirectly activated by ey, whereas the 5' eye enhancer is primarily regulated by ey, acting in concert with eya and so. Disrupting conserved So-binding sites in the 3' eye enhancer prevents reporter expression in vivo. Our results suggest that the two eye enhancers act redundantly and in concert with each other to integrate distinct upstream inputs and direct the eye-specific expression of dac.

Photoreceptor differentiation in Drosophila: from immature neurons to functional photoreceptors

How a pool of equipotent cells acquires a multitude of distinct fates is a major question in developmental biology. The study of photoreceptor (PR) cell differentiation in Drosophila has been used to address this question. PR differentiation is a process that extends over a period of 5 days: It begins in the larval eye imaginal disc when PRs are recruited and commit to particular PR fates, and it culminates in the pupal eye disc with the morphogenesis of the rhabdomeres and the initiation of rhodopsin expression. Several models for PR specification agree that the Ras and Notch signaling pathways are important for the specification of different PR subtypes (Freeman [1997] Development 124:261-270; Cooper and Bray [2000] Curr. Biol. 10:1507-1510; Tomlinson and Struhl [2001] Mol. Cell. 7:487-495). In the first part of this review, we briefly describe the different signaling pathways and transcription factors required for the specification and differentiation of the different PR subtypes in the larval eye disc. In the second part, we review the roles of several transcription factors, which are required for the terminal photoreceptor differentiation and rhodopsin expression.

A genetic screen identifies putative targets and binding partners of CREB-binding protein in the developing Drosophila eye

Drosophila CREB-binding protein (dCBP) is a very large multidomain protein, which belongs to the CBP/p300 family of proteins that were first identified by their ability to bind the CREB transcription factor and the adenoviral protein E1. Since then CBP has been shown to bind to >100 additional proteins and functions in a multitude of different developmental contexts. Among other activities, CBP is known to influence development by remodeling chromatin, by serving as a transcriptional coactivator, and by interacting with terminal members of several signaling transduction cascades. Reductions in CBP activity are the underlying cause of Rubinstein-Taybi syndrome, which is, in part, characterized by several eye defects, including strabismus, cataracts, juvenile glaucoma, and coloboma of the eyelid, iris, and lens. Development of the Drosophila melanogaster compound eye is also inhibited in flies that are mutant for CBP. However, the vast array of putative protein interactions and the wide-ranging roles played by CBP within a single tissue such as the retina can often complicate the analysis of CBP loss-of-function mutants. Through a series of genetic screens we have identified several genes that could either serve as downstream transcriptional targets or encode for potential CBP-binding partners and whose association with eye development has hitherto been unknown. The identification of these new components may provide new insight into the roles that CBP plays in retinal development. Of particular interest is the identification that the CREB transcription factor appears to function with CBP at multiple stages of retinal development.

Cell-type specific utilization of multiple negative feedback loops generates developmental constancy

Signaling pathways generally contain multiple negative regulators that are induced by the signal they repress, constructing negative feedback loops. Although such negative regulators are often expressed in a tissue- or cell-type specific manner during development, little is known about the significance of their differential expression patterns and possible interactions. We show the role and interplay of two cell-type specific negative feedback loops during specification of photoreceptor neurons in the Drosophila compound eye, a process that occurs via epidermal growth factor (EGF)-mediated sequential induction through the activation of the Ras/MAPK signaling pathway. Inducing cells secreting EGF express a negative regulator Sprouty (SPRY) that lowers Ras/MAPK signaling activity, and as a consequence reduces the signal-dependent expression of a secreted EGF inhibitor, Argos (AOS). Induced cells in turn express an orphan nuclear receptor Seven-up (SVP), which represses SPRY expression thereby allowing expression and secretion of AOS, preventing further induction. When this intricate system fails, as in spry mutants, sequential induction is no longer constant and the number of photoreceptor neurons becomes variable. Thus, cell-type specific utilization of multiple negative feedback loops not only confers developmental robustness through functional redundancy, but is a key component in generating consistent patterning.

Regulation of Rho and Rac signaling to the actin cytoskeleton by paxillin during Drosophila development

Paxillin is a prominent focal adhesion docking protein that regulates cell adhesion and migration. Although numerous paxillin-binding proteins have been identified and paxillin is required for normal embryogenesis, the precise mechanism by which paxillin functions in vivo has not yet been determined. We identified an ortholog of mammalian paxillin in Drosophila (Dpax) and have undertaken a genetic analysis of paxillin function during development. Overexpression of Dpax disrupted leg and wing development, suggesting a role for paxillin in imaginal disc morphogenesis. These defects may reflect a function for paxillin in regulation of Rho family GTPase signaling as paxillin interacts genetically with Rac and Rho in the developing eye. Moreover, a gain-of-function suppressor screen identified a genetic interaction between Dpax and cdi in wing development. cdi belongs to the cofilin kinase family, which includes the downstream Rho target, LIM kinase (LIMK). Significantly, strong genetic interactions were detected between Dpax and Dlimk, as well as downstream effectors of Dlimk. Supporting these genetic data, biochemical studies indicate that paxillin regulates Rac and Rho activity, positively regulating Rac and negatively regulating Rho. Taken together, these data indicate the importance of paxillin modulation of Rho family GTPases during development and identify the LIMK pathway as a critical target of paxillin-mediated Rho regulation.

Bipartite inhibition of Drosophila epidermal growth factor receptor by the extracellular and transmembrane domains of Kekkon1

In Drosophila, signaling by the epidermal growth factor receptor (EGFR) is required for a diverse array of developmental decisions. Essential to these decisions is the precise regulation of the receptor's activity by both stimulatory and inhibitory molecules. To better understand the regulation of EGFR activity we investigated inhibition of EGFR by the transmembrane protein Kekkon1 (Kek1). Kek1 encodes a molecule containing leucine-rich repeats (LRR) and an immunoglobulin (Ig) domain and is the founding member of the Drosophila Kekkon family. Here we demonstrate with a series of Kek1-Kek2 chimeras that while the LRRs suffice for EGFR binding, inhibition in vivo requires the Kek1 juxta/transmembrane region. We demonstrate directly, and using a series of Kek1-EGFR chimeras, that Kek1 is not a phosphorylation substrate for the receptor in vivo. In addition, we show that EGFR inhibition is unique to Kek1 among Kek family members and that this function is not ligand or tissue specific. Finally, we have identified a unique class of EGFR alleles that specifically disrupt Kek1 binding and inhibition, but preserve receptor activation. Interestingly, these alleles map to domain V of the Drosophila EGFR, a region absent from the vertebrate receptors. Together, our results support a model in which the LRRs of Kek1 in conjunction with its juxta/transmembrane region direct association and inhibition of the Drosophila EGFR through interactions with receptor domain V.

Spalt transcription factors are required for R3/R4 specification and establishment of planar cell polarity in the Drosophila eye

The establishment of planar cell polarity in the Drosophila eye requires correct specification of the R3/R4 pair of photoreceptor cells. In response to a polarizing factor, Frizzled signaling specifies R3 and induces Delta, which activates Notch in the neighboring cell, specifying it as R4. Here, we show that the spalt zinc-finger transcription factors (spalt major and spalt-related) are part of the molecular mechanisms regulating R3/R4 specification and planar cell polarity establishment. In mosaic analysis, we find that the spalt genes are specifically required in R3 for the establishment of correct ommatidial polarity. In addition, we show that spalt genes are required for proper localization of Flamingo in the equatorial side of R3 and R4, and for the upregulation of Delta in R3. These requirements are very similar to those of frizzled during R3/R4 specification. We show that spalt genes are required cell-autonomously for the expression of seven-up in R3 and R4, and that seven-up is downstream of spalt genes in the genetic hierarchy of R3/R4 specification. Thus, spalt and seven-up are necessary for the correct interpretation of the Frizzled-mediated polarity signal in R3. Finally, we show that, posterior to row seven, seven-up represses spalt in R3/R4 in order to maintain the R3/R4 identity and to inhibit the transformation of these cells to the R7 cell fate.

Regulation of R7 and R8 differentiation by the spalt genes

Photoreceptor development begins in the larval eye imaginal disc, where eight distinct photoreceptor cells (R1-R8) are sequentially recruited into each of the developing ommatidial clusters. Final photoreceptor differentiation, including rhabdomere formation and rhodopsin expression, is completed during pupal life. During pupation, spalt was previously proposed to promote R7 and R8 terminal differentiation. Here we show that spalt is required for proper R7 differentiation during the third instar larval stage since the expression of several R7 larval markers (prospero, enhancer of split mdelta0.5, and runt) is lost in spalt mutant clones. In R8, spalt is not required for cell specification or differentiation in the larval disc but promotes terminal differentiation during pupation. We show that spalt is necessary for senseless expression in R8 and sufficient to induce ectopic senseless in R1-R6 during pupation. Moreover, misexpression of spalt or senseless is sufficient to induce ectopic rhodopsin 6 expression and partial suppression of rhodopsin 1. We demonstrate that spalt and senseless are part of a genetic network, which regulates rhodopsin 6 and rhodopsin 1. Taken together, our results suggest that while spalt is required for R7 differentiation during larval stages, spalt and senseless promote terminal R8 differentiation during pupal stages, including the regulation of rhodopsin expression.

Control of central synaptic specificity in insect sensory neurons

Synaptic specificity is the culmination of several processes, beginning with the establishment of neuronal subtype identity, followed by navigation of the axon to the correct subdivision of neuropil, and finally, the cell-cell recognition of appropriate synaptic partners. In this review we summarize the work on sensory neurons in crickets, cockroaches, moths, and fruit flies that establishes some of the principles and molecular mechanisms involved in the control of synaptic specificity. The identity of a sensory neuron is controlled by combinatorial expression of transcription factors, the products of patterning and proneural genes. In the nervous system, sensory axon projections are anatomically segregated according to modality, stimulus quality, and cell-body position. A variety of cell-surface and intracellular signaling molecules are used to achieve this. Synaptic target recognition is also controlled by transcription factors such as Engrailed and may be, in part, mediated by cadherin-like molecules.

Two functionally redundant isoforms of Drosophila melanogaster eukaryotic initiation factor 4B are involved in cap-dependent translation, cell survival, and proliferation

Eukaryotic initiation factor (eIF) 4B is part of the protein complex involved in the recognition and binding of mRNA to the ribosome. DrosophilaeIF4B is a single-copy gene that encodes two isoforms, termed eIF4B-L (52.2 kDa) and eIF4B-S (44.2 kDa), generated as a result of the alternative recognition of two polyadeynlation signals during transcription termination and subsequent alternative splicing of the two pre-mRNAs. Both eIF4B mRNAs and proteins are expressed during the entire embryogenesis and life cycle. The proteins are cytoplasmic with polarized distribution. The two isoforms bind RNA with the same affinity. eIF4B-L and eIF4B-S preferentially enhance cap-dependent over IRES-dependent translation initiation in a Drosophila cell-free translation system. RNA interference experiments suggest that eIF4B is required for cell survival, although only a modest reduction in rate of protein synthesis is observed. Overexpression of eIF4B in Drosophila cells in culture and in developing eye imaginal discs promotes cell proliferation.

Structure–function analysis of the Drosophila retinal determination protein Dachshund

Dachshund (Dac) is a highly conserved nuclear protein that is distantly related to the Ski/Sno family of corepressor proteins. In Drosophila, Dac is necessary and sufficient for eye development and, along with Eyeless (Ey), Sine oculis (So), and Eyes absent (Eya), forms the core of the retinal determination (RD) network. In vivo and in vitro experiments suggest that members of the RD network function together in one or more complexes to regulate the expression of downstream targets. For example, Dac and Eya synergize in vivo to induce ectopic eye formation and they physically interact through conserved domains. Dac contains two highly conserved domains, named DD1 and DD2, but no function has been assigned to either of them in an in vivo context. We performed structure-function studies to understand the relationship between the conserved domains of Dac and the rest of the protein and to determine the function of each domain during development. We show that only DD1 is essential for Dac function and while DD2 facilitates DD1, it is not absolutely essential in spite of more than 500 million years of conservation. Moreover, the physical interaction between Eya and DD2 is not required for the genetic synergy between the two proteins. Finally, we show that DD1 also plays a central role for nuclear localization of Dac.

Regulation of multimers via truncated isoforms: a novel mechanism to control nitric-oxide signaling

Nitric oxide (NO) is an essential regulator of Drosophila development and physiology. We describe a novel mode of regulation of NO synthase (NOS) function that uses endogenously produced truncated protein isoforms of Drosophila NOS (DNOS). These isoforms inhibit NOS enzymatic activity in vitro and in vivo, reflecting their ability to form complexes with the full-length DNOS protein (DNOS1). Truncated isoforms suppress the antiproliferative action of DNOS1 in the eye imaginal disc by impacting the retinoblastoma-dependent pathway, yielding hyperproliferative phenotypes in pupae and adult flies. Our results indicate that endogenous products of the dNOS locus act as dominant negative regulators of NOS activity during Drosophila development.

Drosophila Src-family kinases function with Csk to regulate cell proliferation and apoptosis

Elevated Src protein levels and activity are associated with the development and progression of a variety of cancers. The consequences of deregulated Src activity have been studied extensively in cell culture; however, the effects of this deregulation in vivo, as well as the mechanisms of Src-induced tumorigenesis, remain poorly understood. In this study, the effect of expressing wild-type and constitutively active Drosophila Src-family kinases (SFKs) in the developing eye was examined. Overexpression of either wild-type Drosophila SFK (Src64 and Src42) is sufficient to induce ectopic proliferation in G1/G0-arrested, uncommitted cells in eye imaginal discs. In addition, both kinases trigger apoptosis in vivo, in a dosage-dependent manner. Constitutively active mutants are hypermorphic as they trigger proliferation and death more potently than their wild-type counterparts. Moreover, SFK-induced proliferation and apoptosis are largely independent events, as blocking ectopic proliferation does not block cell death. Further, DCsk (the Drosophila homolog of the C-terminal Src kinase) phosphorylates and interacts genetically with the wild-type SFKs, but not with the constitutively active mutants in which a conserved C-terminal tyrosine was mutated to phenylalanine, providing the first in vivo evidence that Csk regulates SFKs during development through phosphorylation of their C-terminal tyrosine.

Signal integration during development: insights from the Drosophila eye

The Drosophila eye is a highly ordered epithelial tissue composed of approximately 750 subunits called ommatidia arranged in a reiterated hexagonal pattern. At higher resolution, observation of the constituent photoreceptors, cone cells, and pigment cells of the eye reveals a highly ordered mosaic of amazing regularity. This relatively simple organization belies the repeated requirement for spatially and temporally coordinated inputs from the Hedgehog (Hh), Wingless (Wg), Decapentaplegic (Dpp), JAK-STAT, Notch, and receptor tyrosine kinase (RTK) signaling pathways. This review will discuss how signaling inputs from the Notch and RTK pathways, superimposed on the developmental history of a cell, facilitate context-specific and appropriate cell fate specification decisions in the developing fly eye. Lessons learned from investigating the combinatorial signal integration strategies underlying Drosophila eye development will likely reveal cell-cell communication paradigms relevant to many aspects of invertebrate and mammalian development. Developmental Dynamics 229:162-175, 2004.