The maternal ventralizing locus torpedo is allelic to faint little ball, an embryonic lethal, and encodes the Drosophila EGF receptor homolog

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Epidermal growth factor receptor signaling protects epithelia from morphogenetic instability and tissue damage in Drosophila

Dying cells in the epithelia communicate with neighboring cells to initiate coordinated cell removal to maintain epithelial integrity. Naturally occurring apoptotic cells are mostly extruded basally and engulfed by macrophages. Here, we have investigated the role of Epidermal growth factor (EGF) receptor (EGFR) signaling in the maintenance of epithelial homeostasis. In Drosophila embryos, epithelial tissues undergoing groove formation preferentially enhanced extracellular signal-regulated kinase (ERK) signaling. In EGFR mutant embryos at stage 11, sporadic apical cell extrusion in the head initiates a cascade of apical extrusions of apoptotic and non-apoptotic cells that sweeps the entire ventral body wall. Here, we show that this process is apoptosis dependent, and clustered apoptosis, groove formation, and wounding sensitize EGFR mutant epithelia to initiate massive tissue disintegration. We further show that tissue detachment from the vitelline membrane, which frequently occurs during morphogenetic processes, is a key trigger for the EGFR mutant phenotype. These findings indicate that, in addition to cell survival, EGFR plays a role in maintaining epithelial integrity, which is essential for protecting tissues from transient instability caused by morphogenetic movement and damage.

The Epidermal Growth Factor Ligand Spitz Modulates Macrophage Efferocytosis, Wound Responses and Migration Dynamics During Drosophila Embryogenesis

How multifunctional cells such as macrophages interpret the different cues within their environment and undertake an appropriate response is a key question in developmental biology. Understanding how cues are prioritized is critical to answering this - both the clearance of apoptotic cells (efferocytosis) and the migration toward damaged tissue is dependent on macrophages being able to interpret and prioritize multiple chemoattractants, polarize, and then undertake an appropriate migratory response. Here, we investigate the role of Spitz, the cardinal Drosophila epidermal growth factor (EGF) ligand, in regulation of macrophage behavior in the developing fly embryo, using activated variants with differential diffusion properties. Our results show that misexpression of activated Spitz can impact macrophage polarity and lead to clustering of cells in a variant-specific manner, when expressed either in macrophages or the developing fly heart. Spitz can also alter macrophage distribution and perturb apoptotic cell clearance undertaken by these phagocytic cells without affecting the overall levels of apoptosis within the embryo. Expression of active Spitz, but not a membrane-bound variant, can also increase macrophage migration speeds and impair their inflammatory responses to injury. The fact that the presence of Spitz specifically undermines the recruitment of more distal cells to wound sites suggests that Spitz desensitizes macrophages to wounds or is able to compete for their attention where wound signals are weaker. Taken together these results suggest this molecule regulates macrophage migration and their ability to dispose of apoptotic cells. This work identifies a novel regulator of Drosophila macrophage function and provides insights into signal prioritization and integration in vivo. Given the importance of apoptotic cell clearance and inflammation in human disease, this work may help us to understand the role EGF ligands play in immune cell recruitment during development and at sites of disease pathology.

Formation, interpretation, and regulation of the Drosophila Dorsal/NF-κB gradient

The morphogen gradient of the transcription factor Dorsal in the early Drosophila embryo has become one of the most widely studied tissue patterning systems. Dorsal is a Drosophila homolog of mammalian NF-κB and patterns the dorsal-ventral axis of the blastoderm embryo into several tissue types by spatially regulating upwards of 100 zygotic genes. Recent studies using fluorescence microscopy and live imaging have quantified the Dorsal gradient and its target genes, which has paved the way for mechanistic modeling of the gradient. In this review, we describe the mechanisms behind the initiation of the Dorsal gradient and its regulation of target genes. The main focus of the review is a discussion of quantitative and computational studies of the Dl gradient system, including regulation of the Dl gradient. We conclude with a discussion of potential future directions.

Genetic Screens to Analyze Pattern Formation of Egg and Embryo in Drosophila: A Personal History

In Drosophila development, the axes of the egg and future embryo are established during oogenesis. To learn about the underlying genetic and molecular pathways that lead to axis formation, I conducted a large-scale genetic screen at the beginning of my independent career. This led to the eventual understanding that both anterior-posterior and dorsal-ventral pattern information is transmitted from the oocyte to the surrounding follicle cells and in turn from the follicle cells back to the oocyte. How I came to conduct this screen and what further insights were gained by studying the mutants isolated in the screen are the topics of this autobiographical article.

A Gene Expression Screen in Drosophila melanogaster Identifies Novel JAK/STAT and EGFR Targets During Oogenesis

The Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) and epidermal growth factor receptor (EGFR) signaling pathways are conserved regulators of tissue patterning, morphogenesis, and other cell biological processes. During Drosophila oogenesis, these pathways determine the fates of epithelial follicle cells (FCs). JAK/STAT and EGFR together specify a population of cells called the posterior follicle cells (PFCs), which signal to the oocyte to establish the embryonic axes. In this study, whole genome expression analysis was performed to identify genes activated by JAK/STAT and/or EGFR. We observed that 317 genes were transcriptionally upregulated in egg chambers with ectopic JAK/STAT and EGFR activity in the FCs. The list was enriched for genes encoding extracellular matrix (ECM) components and ECM-associated proteins. We tested 69 candidates for a role in axis establishment using RNAi knockdown in the FCs. We report that the signaling protein Semaphorin 1b becomes enriched in the PFCs in response to JAK/STAT and EGFR. We also identified ADAM metallopeptidase with thrombospondin type 1 motif A (AdamTS-A) as a novel target of JAK/STAT in the FCs that regulates egg chamber shape. AdamTS-A mRNA becomes enriched at the anterior and posterior poles of the egg chamber at stages 6 to 7 and is regulated by JAK/STAT. Altering AdamTS-A expression in the poles or middle of the egg chamber produces rounder egg chambers. We propose that AdamTS-A regulates egg shape by remodeling the basement membrane.

The Drosophila Epidermal Growth Factor Receptor does not act in the nucleus

Mammalian members of the ErbB family, including the epidermal growth factor receptor (EGFR), can regulate transcription, DNA replication and repair through nuclear entry of either the full-length proteins or their cleaved cytoplasmic domains. In cancer cells, these nuclear functions contribute to tumor progression and drug resistance. Here, we examined whether the single Drosophila EGFR can also localize to the nucleus. A chimeric EGFR protein fused at its cytoplasmic C-terminus to DNA-binding and transcriptional activation domains strongly activated transcriptional reporters when overexpressed in cultured cells or in vivo However, this activity was independent of cleavage and endocytosis. Without an exogenous activation domain, EGFR fused to a DNA-binding domain did not activate or repress transcription. Addition of the same DNA-binding and transcriptional activation domains to the endogenous Egfr locus through genome editing led to no detectable reporter expression in wild-type or oncogenic contexts. These results show that, when expressed at physiological levels, the cytoplasmic domain of the Drosophila EGFR does not have access to the nucleus. Therefore, nuclear EGFR functions are likely to have evolved after vertebrates and invertebrates diverged.

Mapping Second Chromosome Mutations to Defined Genomic Regions in Drosophila melanogaster

Hundreds of Drosophila melanogaster stocks are currently maintained at the Bloomington Drosophila Stock Center with mutations that have not been associated with sequence-defined genes. They have been preserved because they have interesting loss-of-function phenotypes. The experimental value of these mutations would be increased by tying them to specific genomic intervals so that geneticists can more easily associate them with annotated genes. Here, we report the mapping of 85 second chromosome complementation groups in the Bloomington collection to specific, small clusters of contiguous genes or individual genes in the sequenced genome. This information should prove valuable to Drosophila geneticists interested in processes associated with particular phenotypes and those searching for mutations affecting specific sequence-defined genes.

The Spatiotemporal Limits of Developmental Erk Signaling

Animal development is characterized by signaling events that occur at precise locations and times within the embryo, but determining when and where such precision is needed for proper embryogenesis has been a long-standing challenge. Here we address this question for extracellular signal regulated kinase (Erk) signaling, a key developmental patterning cue. We describe an optogenetic system for activating Erk with high spatiotemporal precision in vivo. Implementing this system in Drosophila, we find that embryogenesis is remarkably robust to ectopic Erk signaling, except from 1 to 4 hr post-fertilization, when perturbing the spatial extent of Erk pathway activation leads to dramatic disruptions of patterning and morphogenesis. Later in development, the effects of ectopic signaling are buffered, at least in part, by combinatorial mechanisms. Our approach can be used to systematically probe the differential contributions of the Ras/Erk pathway and concurrent signals, leading to a more quantitative understanding of developmental signaling.

Epidermal Growth Factor Pathway Signaling in Drosophila Embryogenesis: Tools for Understanding Cancer

EGF signaling is a well-known oncogenic pathway in animals. It is also a key developmental pathway regulating terminal and dorsal-ventral patterning along with many other aspects of embryogenesis. In this review, we focus on the diverse roles for the EGF pathway in Drosophila embryogenesis. We review the existing body of evidence concerning EGF signaling in Drosophila embryogenesis focusing on current uncertainties in the field and areas for future study. This review provides a foundation for utilizing the Drosophila model system for research into EGF effects on cancer.

Translational control of gurken mRNA in Drosophila development

Localized mRNA translation is a widespread mechanism for targeting protein synthesis, important for cell fate, motility and pathogenesis. In Drosophila, the spatiotemporal control of gurken/TGF-α mRNA translation is required for establishing the embryonic body axes. A number of recent studies have highlighted key aspects of the mechanism of gurken mRNA translational control at the dorsoanterior corner of the mid-stage oocyte. Orb/CPEB and Wispy/GLD-2 are required for polyadenylation of gurken mRNA, but unlocalized gurken mRNA in the oocyte is not fully polyadenylated. ¹ At the dorsoanterior corner, Orb and gurken mRNA have been shown to be enriched at the edge of Processing bodies, where translation occurs. ² Over-expression of Orb in the adjacent nurse cells, where gurken mRNA is transcribed, is sufficient to cause mis-expression of Gurken protein. ³ In orb mutant egg chambers, reducing the activity of CK2, a Serine/Threonine protein kinase, enhances the ventralized phenotype, consistent with perturbation of gurken translation. ⁴ Here we show that sites phosphorylated by CK2 overlap with active Orb and with Gurken protein expression. Together with our new findings we consolidate the literature into a working model for gurken mRNA translational control and review the role of kinases, cell cycle factors and polyadenylation machinery highlighting a multitude of conserved factors and mechanisms in the Drosophila egg chamber.

The Heidelberg Screen for Pattern Mutants of Drosophila: A Personal Account

In large-scale mutagenesis screens performed in 1979-1980 at the EMBL in Heidelberg, we isolated mutations affecting the pattern or structure of the larval cuticle in Drosophila. The 600 mutants we characterized could be assigned to 120 genes and represent the majority of such genes in the genome. These mutants subsequently provided a rich resource for understanding many fundamental developmental processes, such as the transcriptional hierarchies controlling segmentation, the establishment of cell states by signaling pathways, and the differentiation of epithelial cells. Most of the Heidelberg genes are now molecularly known, and many of them are conserved in other animals, including humans. Although the screens were initially driven entirely by curiosity, the mutants now serve as models for many human diseases. In this review, we describe the rationale of the screening procedures and provide a classification of the genes on the basis of their initial phenotypes and the subsequent molecular analyses.

Drosophila Condensin II subunit Chromosome-associated protein D3 regulates cell fate determination through non-cell-autonomous signaling

The pattern of the Drosophila melanogaster adult wing is heavily influenced by the expression of proteins that dictate cell fate decisions between intervein and vein during development. dSRF (Blistered) expression in specific regions of the larval wing disc promotes intervein cell fate, whereas EGFR activity promotes vein cell fate. Here, we report that the chromatin-organizing protein CAP-D3 acts to dampen dSRF levels at the anterior/posterior boundary in the larval wing disc, promoting differentiation of cells into the anterior crossvein. CAP-D3 represses KNOT expression in cells immediately adjacent to the anterior/posterior boundary, thus blocking KNOT-mediated repression of EGFR activity and preventing cell death. Maintenance of EGFR activity in these cells depresses dSRF levels in the neighboring anterior crossvein progenitor cells, allowing them to differentiate into vein cells. These findings uncover a novel transcriptional regulatory network influencing Drosophila wing vein development, and are the first to identify a Condensin II subunit as an important regulator of EGFR activity and cell fate determination in vivo.

The SWI/SNF chromatin remodeling complex exerts both negative and positive control over LET-23/EGFR-dependent vulval induction in Caenorhabditis elegans

Signaling by the epidermal growth factor receptor (EGFR) generates diverse developmental patterns. This requires precise control over the location and intensity of signaling. Elucidation of these regulatory mechanisms is important for understanding development and disease pathogenesis. In Caenorhabditis elegans, LIN-3/EGF induces vulval formation in the mid-body, which requires LET-23/EGFR activation only in P6.p, the vulval progenitor nearest the LIN-3 source. To identify mechanisms regulating this signaling pattern, we screened for mutations that cooperate with a let-23 gain-of-function allele to cause ectopic vulval induction. Here, we describe a dominant gain-of-function mutation in swsn-4, a component of SWI/SNF chromatin remodeling complexes. Loss-of-function mutations in multiple SWI/SNF components reveal that weak reduction in SWI/SNF activity causes ectopic vulval induction, while stronger reduction prevents adoption of vulval fates, a phenomenon also observed with increasing loss of LET-23 activity. High levels of LET-23 expression in P6.p are thought to locally sequester LIN-3, thereby preventing ectopic vulval induction, with slight reductions in its expression interfering with LIN-3 sequestration, but not vulval fate signaling. We find that SWI/SNF positively regulates LET-23 expression in P6.p descendants, providing an explanation for the similarities between let-23 and SWI/SNF mutant phenotypes. However, SWI/SNF regulation of LET-23 expression is cell-specific, with SWI/SNF repressing its expression in the ALA neuron. The swsn-4 gain-of-function mutation affects the PTH domain, and provides the first evidence that its auto-inhibitory function in yeast Sth1p is conserved in metazoan chromatin remodelers. Finally, our work supports broad use of SWI/SNF in regulating EGFR signaling during development, and suggests that dominant SWI/SNF mutations in certain human congenital anomaly syndromes may be gain-of-functions.

The Complexities and Unexpected Insights of Developmental Genetic Analysis

The study of development involves many important techniques. Here I am trying to reflect on the strength of genetic analysis and its ability to uncover unexpected relationships and regulatory inputs from seemingly unrelated pathways.

Multipotent versus differentiated cell fate selection in the developing Drosophila airways

Developmental potentials of cells are tightly controlled at multiple levels. The embryonic Drosophila airway tree is roughly subdivided into two types of cells with distinct developmental potentials: a proximally located group of multipotent adult precursor cells (P-fate) and a distally located population of more differentiated cells (D-fate). We show that the GATA-family transcription factor (TF) Grain promotes the P-fate and the POU-homeobox TF Ventral veinless (Vvl/Drifter/U-turned) stimulates the D-fate. Hedgehog and receptor tyrosine kinase (RTK) signaling cooperate with Vvl to drive the D-fate at the expense of the P-fate while negative regulators of either of these signaling pathways ensure P-fate specification. Local concentrations of Decapentaplegic/BMP, Wingless/Wnt, and Hedgehog signals differentially regulate the expression of D-factors and P-factors to transform an equipotent primordial field into a concentric pattern of radially different morphogenetic potentials, which gradually gives rise to the distal-proximal organization of distinct cell types in the mature airway.

DOI:http://dx.doi.org/10.7554/eLife.09646.001

Many organs are composed of tubes of different sizes, shapes and patterns that transport vital substances from one site to another. In the fruit fly species Drosophila melanogaster, oxygen is transported by a tubular network, which divides into finer tubes that allow the oxygen to reach every part of the body.

Different parts of the fruit fly’s airways develop from different groups of tracheal precursor cells. P-fate cells form the most 'proximal' tubes (which are found next to the outer layer of the fly). These cells are 'multipotent' stem cells, and have the ability to specialize into many different types of cells during metamorphosis. The more 'distal' branches that emerge from the proximal tubes develop from D-fate cells. These are cells that generally acquire a narrower range of cell identities.

By performing a genetic analysis of fruit fly embryos, Matsuda et al. have now identified several proteins and signaling molecules that control whether tracheal precursor cells become D-fate or P-fate cells. For example, several signaling pathways work with a protein called Ventral veinless to cause D-fate cells to develop instead of P-fate cells. However, molecules that prevent signaling occurring via these pathways help P-fate cells to form. Different amounts of the molecules that either promote or hinder these signaling processes are present in different parts of the fly embryo; this helps the airways of the fly to develop in the correct pattern.

This work provides a comprehensive view of how cell types with different developmental potentials are positioned in a complex tubular network. This sets a basis for future studies addressing how the respiratory organs – and indeed the entire organism – are sustained.

DOI:http://dx.doi.org/10.7554/eLife.09646.002

The intersection of the extrinsic hedgehog and WNT/wingless signals with the intrinsic Hox code underpins branching pattern and tube shape diversity in the drosophila airways

The tubular networks of the Drosophila respiratory system and our vasculature show distinct branching patterns and tube shapes in different body regions. These local variations are crucial for organ function and organismal fitness. Organotypic patterns and tube geometries in branched networks are typically controlled by variations of extrinsic signaling but the impact of intrinsic factors on branch patterns and shapes is not well explored. Here, we show that the intersection of extrinsic hedgehog(hh) and WNT/wingless (wg) signaling with the tube-intrinsic Hox code of distinct segments specifies the tube pattern and shape of the Drosophila airways. In the cephalic part of the airways, hh signaling induces expression of the transcription factor (TF) knirps (kni) in the anterior dorsal trunk (DTa1). kni represses the expression of another TF spalt major (salm), making DTa1 a narrow and long tube. In DTa branches of more posterior metameres, Bithorax Complex (BX-C) Hox genes autonomously divert hh signaling from inducing kni, thereby allowing DTa branches to develop as salm-dependent thick and short tubes. Moreover, the differential expression of BX-C genes is partly responsible for the anterior-to-posterior gradual increase of the DT tube diameter through regulating the expression level of Salm, a transcriptional target of WNT/wg signaling. Thus, our results highlight how tube intrinsic differential competence can diversify tube morphology without changing availabilities of extrinsic factors.

Tubes are common structural elements of many internal organs, facilitating fluid flow and material exchange. To meet the local needs of diverse tissues, the branching patterns and tube shapes vary regionally. Diametric tapering and specialized branch targeting to the brain represent two common examples of variations with organismal benefits in the Drosophila airways and our vascular system. Several extrinsic signals instruct tube diversifications but the impact of intrinsic factors remains underexplored. Here, we show that the local, tube-intrinsic Hox code instructs the pattern and shape of the dorsal trunk (DT), the main Drosophila airway. In the cephalic part (DT1), where Bithorax Complex (BX-C) Hox genes are not expressed, the extrinsic Hedgehog signal is epistatic to WNT/Wingless signals. Hedgehog instructs anterior DT1 cells to take a long and narrow tube fate targeting the brain. In more posterior metameres, BX-C genes make the extrinsic WNT/Wingless signals epistatic over Hedgehog. There, WNT/Wingless instruct all DT cells to take the thick and short tube fate. Moreover, BX-C genes modulate the outputs of WNT/wingless signaling, making the DT tubes thicker in more posterior metameres. We provide a model for how intrinsic factors modify extrinsic signaling to control regional tube morphologies in a network.

Maternal control of the Drosophila dorsal-ventral body axis

The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

EGFR-dependent network interactions that pattern Drosophila eggshell appendages

Similar to other organisms, Drosophila uses its Epidermal Growth Factor Receptor (EGFR) multiple times throughout development. One crucial EGFR-dependent event is patterning of the follicular epithelium during oogenesis. In addition to providing inductive cues necessary for body axes specification, patterning of the follicle cells initiates the formation of two respiratory eggshell appendages. Each appendage is derived from a primordium comprising a patch of cells expressing broad (br) and an adjacent stripe of cells expressing rhomboid (rho). Several mechanisms of eggshell patterning have been proposed in the past, but none of them can explain the highly coordinated expression of br and rho. To address some of the outstanding issues in this system, we synthesized the existing information into a revised mathematical model of follicle cell patterning. Based on the computational model analysis, we propose that dorsal appendage primordia are established by sequential action of feed-forward loops and juxtacrine signals activated by the gradient of EGFR signaling. The model describes pattern formation in a large number of mutants and points to several unanswered questions related to the dynamic interaction of the EGFR and Notch pathways.

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.

Golgi during development

The Golgi is essential for processing proteins and sorting them, as well as plasma membrane components, to their final destinations. Not surprisingly, this organelle, a major compartment of the secretory pathway, is an important venue for regulating many aspects of development in both invertebrates and vertebrates. Through its role as a site for protein cleavage and glycosylation as well as through changes in its spatial organization and secretory trafficking, the Golgi exerts highly specific effects on cellular differentiation and morphogenesis by spatially and temporally constraining developmental pathways.

EGFR Intron Recombination in Human Gliomas: Inappropriate Diversion of V(D)J Recombination?

The epidermal growth factor receptor (EGFR) is a membrane-bound, 170 kDa, protein tyrosine kinase that plays an important role in tumorigenesis. The EGFR gene, which is composed of over 168 kb of sequence, including a 123-kb first intron, is frequently amplified and rearranged in malignant gliomas leading to the expression of oncogenic deletion (DM) and tandem duplication (TDM) mutants. The most common DM in gliomas is EGFRvIII, which arises from recombination between introns 1 and 7 with deletion of exons 2 through 7 and intervening introns. In addition, some human gliomas express 180- to 190-kDa TDM, which are constitutively active and highly oncogenic. Both DM and TDM arise by recombination of introns that contain sequences with homology to the recombination signal sequence (RSS) heptamers and nonamers present in the V(D)J region of the immunoglobin and T lymphocyte antigen receptor genes. V(D)J RSS have also been identified in certain proto-oncogenes like bcl-2 that are involved in translocations associated with the development of human lymphomas and in other genes such as hypoxanthine-guainine phosphoribosyl transferase (HPRT) in which deletion mutations and intron rearrangements are a common phenomenon. Together with the expression of recombination associated gene (RAG) and nonhomologous end-joining (NHEJ) proteins in gliomas, these observation suggest that aberrant activity of the V(D)J recombinase may be involved in the activation of proto-oncogenes in both liquid and solid tumors.

Regulation of Hh signal transduction as Drosophila eye differentiation progresses

Differentiation of the Drosophila retina occurs as a morphogenetic furrow sweeps anteriorly across the eye imaginal disc, driven by Hedgehog secretion from photoreceptor precursors differentiating behind the furrow. A BTB protein, Roadkill, is expressed posterior to the furrow and targets the Hedgehog signal transduction component Cubitus interruptus for degradation by Cullin-3 and the proteosome. Clonal analysis and conditional mutant studies establish that roadkill transcription is activated by the EGF receptor and Ras pathway in most differentiating retinal cells, and by both EGF receptor/Ras and by Hedgehog signaling in cells that remain unspecified. These findings outline a circuit by which Hedgehog signal transduction is modified as Hedgehog signaling initiates retinal differentiation. A model is presented for regulation of the Cullin-3 and Cullin-1 pathways that modifies Hedgehog signaling as the morphogenetic furrow moves and the responses of retinal cells change.

Distinct activation patterns of EGF receptor signaling in the homoplastic evolution of eggshell morphology in genus Drosophila

Homoplasy is a phenomenon in which organisms in different phylogenetic groups independently acquire similar traits. However, it is largely unknown how developmental mechanisms are altered to give rise to homoplasy. In the genus Drosophila, all species of the subgenus Sophophora, including Drosophila (D.) melanogaster, have eggshells with two dorsal appendages (DAs); most species in the subgenus Drosophila, including D. virilis, and in the subgenus Dorsilopha, have four-DAs. D. melanica belongs to the Drosophila subgenus, but has two-DAs, and phylogenetic analyses suggest that it acquired this characteristic independently. The patterning of the DAs is tightly regulated by epidermal growth factor receptor (EGFR) signaling in D. melanogaster. Previous studies suggested that a change in the EGFR signal activation pattern could have led to the divergence in DA number between D. melanogaster and D. virilis. Here, we compared the patterns of EGFR signal activation across the Drosophila subgenera by immunostaining for anti-activated MAP kinase (MAPK). Our analysis revealed distinct patterns of EGFR signal activation in each subgenus that was consistent with their phylogenetic relationship. In addition, the number of DAs always corresponded to the number of EGFR signaling activation domains in two, three, and four-DA species. Despite their common two-DA characteristic, the EGFR signaling activation pattern in D. melanica diverged significantly from that of species in the subgenus Sophophora. Our results suggest that acquisition of the homoplastic two-DA characteristic could be explained by modifications of the EGFR signaling system in the genus Drosophila that occurred independently and at least twice during evolution.

The multifaceted roles of the receptor tyrosine kinase ROS in development and cancer

The proto-oncogene receptor tyrosine kinase ROS was originally discovered through the identification of oncogenic variants isolated from tumors. These discoveries spearheaded a body of work aimed at elucidating the function of this evolutionarily conserved receptor in development and cancer. Through genetic and biochemical approaches, progress in the characterization of ROS points to distinctive roles in the program of epithelial cell differentiation during the development of a variety of organs. Although substantial, these advances remain hampered by the absence of an identified ligand, making ROS one of the last two remaining orphan receptor tyrosine kinases. Recent studies on the oncogenic activation of ROS as a result of different chromosomal rearrangements found in brain and lung cancers have shed light on the molecular mechanisms underlying ROS transforming activities. ROS and its oncogenic variants therefore constitute clinically relevant targets for cancer therapeutic intervention. This review highlights the various roles that this receptor plays in multiple system networks in normalcy and disease and points to future directions towards the elucidation of ROS function in the context of ligand identification, signaling pathways and clinical applications.

Squid, Cup, and PABP55B function together to regulate gurken translation in Drosophila

During Drosophila melanogaster oogenesis, the proper localization of gurken (grk) mRNA and protein is required for the establishment of the dorsal-ventral axis of the egg and future embryo. Squid (Sqd) is an RNA-binding protein that is required for the correct localization and translational regulation of the grk message. We show that Cup and polyA-binding protein (PABP) interact physically with Sqd and with each other in ovaries. We show that cup mutants lay dorsalized eggs, enhance dorsalization of weak sqd alleles, and display defects in grk mRNA localization and Grk protein accumulation. In contrast, pAbp mutants lay ventralized eggs and enhance grk haploinsufficiency. PABP also interacts genetically and biochemically with Encore. These data predict a model in which Cup and Sqd mediate translational repression of unlocalized grk mRNA, and PABP and Enc facilitate translational activation of the message once it is fully localized to the dorsal-anterior region of the oocyte. These data also provide the first evidence of a link between the complex of commonly used trans-acting factors and Enc, a factor that is required for grk translation.

Role of receptor tyrosine kinases during Drosophila development

In vertebrates, a tyrosine kinase activity has been identified as an integral component of growth factor receptors and the products of proto-oncogenes. Many of these receptor tyrosine kinases (RTKs) appear to play a key role in the regulation of cell growth. Recent analyses of several Drosophila genes encoding putative RTKs indicate that this class of proteins also plays an important role in decisions about cell fate that depend on cellular interactions during development. The sevenless RTK mediates the position-dependent specification of a particular photoreceptor cell type (R7) in the eye. The local specification of R7 cells requires a functional tyrosine kinase domain of the sevenless protein but does not depend on the spatially restricted expression of the sevenless gene. The Drosophila EGF receptor homologue serves multiple functions during development, some of which are clearly unrelated to regulation of cell growth. Finally, the torso gene encodes an RTK required for the specification of the terminal regions of the Drosophila larva. A number of other genes have been genetically identified that appear to function in the same developmental processes upstream or downstream of these three RTKs. These loci are excellent candidates for genes encoding other components of the signalling pathways, such as ligands or substrates of the RTKs.

Egfr signaling modulates VM32E gene expression during Drosophila oogenesis

Drosophila vitelline membrane gene VM32E is expressed in the follicle cells of the stage 10 egg chamber and shows a peculiar temporal and spatial expression pattern compared to the other members of the same gene family. Previous work has led us to demonstrate that Decapentaplegic (Dpp) signaling represses the expression of the VM32E gene in the centripetal follicle cells. In this paper, we describe another level of complexity of the VM32E gene expression regulation. Through clonal analyses, we show that the expression of the VM32E gene in the main body follicle cells is modulated by the epidermal growth factor receptor (Egfr) activity. In follicle cell clones expressing a constitutively active form of the Egfr, the VM32E gene is downregulated, while the loss of the Egfr activity upregulates VM32E expression. In addition, we show that the ectopic expression of the Egfr-induced ETS transcription factor PointedP2 (PntP2) affects the expression of the VM32E gene. From these results and our previously published data, it appears that the proper patterning of follicle cells, defined by Dpp and Egfr signaling pathways, controls the VM32E gene expression pattern. This may suggest that a fine tuning of the expression of specific eggshell structural genes could be part of the complex process that leads to a proper eggshell assembly.

Drosophila TRAP230/240 are essential coactivators for Atonal in retinal neurogenesis

The TRAP (thyroid hormone receptor associated proteins)/Mediator complex serves as a transcriptional coactivator. In Drosophila, Kohtalo (Kto) and Skuld (Skd), homologs of TRAP subunits, TRAP230 and TRAP240, respectively, are necessary for eye development. However, the transcriptional activators that require Kto and Skd have not been identified. Here we provide evidence that Kto and Skd are essential for the function of transcription factor Atonal (Ato) in spatial patterning of proneural clusters in the morphogenetic furrow. In the absence of Kto/Skd, Ato fails to induce its inhibitory target events such as EGFR signaling and Scabrous expression that result in ectopic Ato expression in the space between proneural groups. Kto/Skd are also required for positive Ato functions to induce Ato targets such as Ato itself and Senseless within the proneural clusters. We also show that Skd forms a protein complex with Ato in vivo. These data suggest that Kto/Skd act as essential coactivators for Ato expression during early retinal neurogenesis.

Emerging therapies in gastrointestinal cancers

Members of the receptor tyrosine kinase family, that include EGFR, ErbB-2/HER-2, ErbB-3/HER-3 and ErbB-4/HER-4, are frequently implicated in experimental models of epithelial cell neoplasia as well as in human cancers. Therefore, interference with the activation of these growth factor receptors represents a promising strategy for development of novel and selective anticancer therapies. Indeed, a number of inhibitors that target either EGFR or HER-2, with the exception of a few that target both; have been developed for treatment of epithelial cancers. Since most solid tumors express different ErbB receptors and/or their ligands, identification of inhibitor(s), targeting multiple EGFR family members may provide a therapeutic benefit to a broader patient population. Here we describe the significance of an ErbB family of receptors in epithelial cancers, and summarize different available therapeutics targeting these receptors. It also emphasizes the need to develop pan-ErbB inhibitors and discusses EGF-Receptor Related Protein, a recently isolated negative regulator of EGFR as a potential pan-ErbB therapeutic for a wide variety of epithelial cancers.

Rhomboid proteins: conserved membrane proteases with divergent biological functions

The rhomboid gene was discovered in Drosophila, where it encodes a seven transmembrane protein that is the signal-generating component of epidermal growth factor (EGF) receptor signaling during development. Although metazoan developmental regulators are rarely conserved outside the animal kingdom, rhomboid proteins are conserved in all kingdoms of life, but the significance of this remains unclear. Recent biochemical reconstitution and high-resolution crystal structures have provided proof that rhomboid proteins function as novel intramembrane proteases, with a serine protease-like catalytic apparatus embedded within the membrane bilayer, buried in a hydrophilic cavity formed by a protein ring. A thorough consideration of all known examples of rhomboid function suggests that, despite biochemical similarity in mechanism and specificity, rhomboid proteins function in diverse processes including quorum sensing in bacteria, mitochondrial membrane fusion, apoptosis, and stem cell differentiation in eukaryotes; rhomboid proteins are also now starting to be linked to human disease, including early-onset blindness, diabetes, and parasitic diseases. Regulating cell signaling is at the heart of rhomboid protein function in many, but not all, of these processes. Further study of these novel enzymes promises to reveal the evolutionary path of rhomboid protein function, which could provide insights into the forces that drive the molecular evolution of regulatory mechanisms.

Drosophila alpha-actinin in ovarian follicle cells is regulated by EGFR and Dpp signalling and required for cytoskeletal remodelling

alpha-Actinin is an evolutionarily conserved actin filament crosslinking protein with functions in both muscle and non-muscle cells. In non-muscle cells, interactions between alpha-actinin and its many binding partners regulate cell adhesion and motility. In Drosophila, one non-muscle and two muscle-specific alpha-actinin isoforms are produced by alternative splicing of a single gene. In wild-type ovaries, alpha-actinin is ubiquitously expressed. The non-muscle alpha-actinin mutant Actn(Delta233), which is viable and fertile, lacks alpha-actinin expression in ovarian germline cells, while somatic follicle cells express alpha-actinin at late oogenesis. Here we show that this latter population of alpha-actinin, termed FC-alpha-actinin, is absent from the dorsoanterior follicle cells, and we present evidence that this is the result of a negative regulation by combined Epidermal growth factor receptor (EGFR) and Decapentaplegic signalling. Furthermore, EGFR signalling increased the F-actin bundling activity of ectopically expressed muscle-specific alpha-actinin. We also describe a novel morphogenetic event in the follicle cells that occurs during egg elongation. This event involves a transient repolarisation of the basal actin fibres and the assembly of a posterior beta-integrin-dependent adhesion site accumulating alpha-actinin and Enabled. Clonal analysis using Actn null alleles demonstrated that although alpha-actinin was not necessary for actin fibre formation or maintenance, the cytoskeletal remodelling was perturbed, and Enabled did not localise in the posterior adhesion site. Nevertheless, epithelial morphogenesis proceeded normally. This work provides the first evidence that alpha-actinin is involved in the organisation of the cytoskeleton in a non-muscle tissue in Drosophila.

Quantifying the Gurken morphogen gradient in Drosophila oogenesis

Quantitative information about the distribution of morphogens is crucial for understanding their effects on cell-fate determination, yet it is difficult to obtain through direct measurements. We have developed a parameter estimation approach for quantifying the spatial distribution of Gurken, a TGFalpha-like EGFR ligand that acts as a morphogen in Drosophila oogenesis. Modeling of Gurken/EGFR system shows that the shape of the Gurken gradient is controlled by a single dimensionless parameter, the Thiele modulus, which reflects the relative importance of ligand diffusion and degradation. By combining the model with genetic alterations of EGFR levels, we have estimated the value of the Thiele modulus in the wild-type egg chamber. This provides a direct characterization of the shape of the Gurken gradient and demonstrates how parameter estimation techniques can be used to quantify morphogen gradients in development.

The role of brinker in eggshell patterning

Drosophila oogenesis provides a useful system to study signal transduction pathways and their interactions. Through clonal analysis, we found that brinker (brk), a repressor of Dpp signaling, plays an important role in the Drosophila ovary, where its function is essential for dorsal appendage formation. In the absence of brk, operculum fates are specified at the expense of dorsal appendage fates. Brk is expressed by most of the oocyte associated follicle cells, starting from stage 8 of oogenesis. Transforming Growth Factor beta (TGFbeta) signaling represses brk expression in both the early stage egg chambers and in the anterior follicle cells. In brk mutant follicle cell clones at the dorsal anterior region, Broad Complex (BR-C) expression is down-regulated in a larger domain than in wild type. We show that BR-C is required for dorsal appendage development. In large anterior BR-C mutant clones, dorsal appendages are absent, and instead, the eggshell has an enlarged operculum like region at the anterior. In addition, we show that the Epidermal Growth Factor (EGF) receptor signaling represses the TGFbeta signaling in oogenesis by up-regulating brk expression. From our results and previously published data, it appears that anterior follicle cells integrate the levels of EGF receptor activation and TGFbeta receptor activation. Operculum fate results when the sum of the level of activation of both pathways reaches a threshold level, and reduction of activity of one pathway can be compensated to some extent by increase in the other pathway.

Maintenance of segment and appendage primordia by the Tribolium gene knödel

For homeotic and segment-polarity genes in Drosophila, a switch in gene regulation has been described that distinguishes patterning and maintenance phases. Maintenance of segment and organ primordia involves secondary patterning and differentiation steps, as well as survival factors regulating proliferation and organ size. In a screen for embryonic lethal mutations in the flour beetle Tribolium castaneum, we have recovered two alleles of the knödel gene, which result in short, bag-like embryos. These embryos have severely reduced appendages and differentiate a cuticle that lacks most overt signs of segmentation. In addition, they lack bristles and display defects in the nervous system. Early patterning in knödel mutant embryos is normal up to the extended germ band stage, as indicated by the formation of regular even-skipped (Tc'eve) and wingless (Tc'wg) stripes. Afterwards, however, these patterns degenerate. Similarly, proximo-distal growth and patterning of limbs are nearly normal initially, but limb primordia shrink, and proximo-distal patterns degenerate, during subsequent stages. knödel could be a segment polarity gene required for segment border maintenance in both trunk and appendages. Alternatively, it may have a more general role in tissue or organ maintenance.

The highly ordered assembly of retinal axons and their synaptic partners is regulated by Hedgehog/Single-minded in the Drosophila visual system

During development of the Drosophila visual center, photoreceptor cells extend their axons (R axons) to the lamina ganglion layer, and trigger proliferation and differentiation of synaptic partners (lamina neurons) by delivering the inductive signal Hedgehog (Hh). This inductive mechanism helps to establish an orderly arrangement of connections between the R axons and lamina neurons, termed a retinotopic map because it results in positioning the lamina neurons in close vicinity to the corresponding R axons. We found that the bHLH-PAS transcription factor Single-minded (Sim) is induced by Hh in the lamina neurons and is required for the association of lamina neurons with R axons. In sim mutant brains, lamina neurons undergo the first step of differentiation but fail to associate with R axons. As a result, lamina neurons are set aside from R axons. The data reveal a novel mechanism for regulation of the interaction between axons and neuronal cell bodies that establishes precise neuronal networks.

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.

spoonbill, a new Drosophila female-sterile mutation, interferes with chromosome organization and dorsal-ventral patterning of the egg

We have identified a new mutation, spoonbill (spoon), which interferes with two developmental processes during Drosophila oogenesis, nurse cell-nuclei chromatin organization and anterior-dorsal patterning of the eggshell. Here, we describe the localization patterns of key regulators of axis determination and the expression of follicle cell-specific markers involved in eggshell patterning in egg chambers from spoonbill females. Our molecular characterization of the patterning defects associated with the mutation reveals abnormalities in two major signaling pathways, the grk/Egfr and the Dpp/TGF-beta, that together control the elaborate patterning of the anterior follicular epithelium. The function of spoonbill appears to be required for dpp transcription in a specialized population of follicle cells and for the selective transport of grk mRNA from the nurse cells into the oocyte, as well as for its proper localization and translation. This finding places the spoonbill gene upstream of both pathways.

The jing and ras1 pathways are functionally related during CNS midline and tracheal development

The Drosophila jing gene encodes a zinc finger protein required for the differentiation and survival of embryonic CNS midline and tracheal cells. We show that there is a functional relationship between jing and the Egfr pathway in the developing CNS midline and trachea. jing function is required for Egfr pathway gene expression and MAPK activity in both the CNS midline and trachea. jing over-expression effects phenocopy those of the Egfr pathway and require Egfr pathway function. Activation of the Egfr pathway in loss-of-function jing mutants partially rescues midline cell loss. Egfr pathway genes and jing show dominant genetic interactions in the trachea and CNS midline. Together, these results show that jing regulates signal transduction in developing midline and tracheal cells.

Activation of EGF receptor kinase by L1-mediated homophilic cell interactions

Neural cell adhesion molecules (CAMs) are important players during neurogenesis and neurite outgrowth as well as axonal fasciculation and pathfinding. Some of these developmental processes entail the activation of cellular signaling cascades. Pharmacological and genetic evidence indicates that the neurite outgrowth-promoting activity of L1-type CAMs is at least in part mediated by the stimulation of neuronal receptor tyrosine kinases (RTKs), especially FGF and EGF receptors. It has long been suspected that neural CAMs might physically interact with RTKs, but their activation by specific cell adhesion events has not been directly demonstrated. Here we report that gain-of-function conditions of the Drosophila L1-type CAM Neuroglian result in profound sensory axon pathfinding defects in the developing Drosophila wing. This phenotype can be suppressed by decreasing the normal gene dosage of the Drosophila EGF receptor gene. Furthermore, in Drosophila S2 cells, cell adhesion mediated by human L1-CAM results in the specific activation of human EGF tyrosine kinase at cell contact sites and EGF receptors engage in a physical interaction with L1-CAM molecules. Thus L1-type CAMs are able to promote the adhesion-dependent activation of EGF receptor signaling in vitro and in vivo.

EGF receptor signalling protects smooth-cuticle cells from apoptosis during Drosophila ventral epidermis development

Patterning of the Drosophila ventral epidermis is a tractable model for understanding the role of signalling pathways in development. Interplay between Wingless and EGFR signalling determines the segmentally repeated pattern of alternating denticle belts and smooth cuticle: spitz group genes, which encode factors that stimulate EGFR signalling, induce the denticle fate, while Wingless signalling antagonizes the effect of EGFR signalling, allowing cells to adopt the smooth-cuticle fate. Medial fusion of denticle belts is also a hallmark of spitz group genes, yet its underlying cause is unknown. We have studied this phenotype and discovered a new function for EGFR signalling in epidermal patterning. Smooth-cuticle cells, which are receiving Wingless signalling, are nevertheless dependent on EGFR signalling for survival. Reducing EGFR signalling results in apoptosis of smooth-cuticle cells between stages 12 and 14, bringing adjacent denticle regions together to result in denticle belt fusions by stage 15. Multiple factors stimulate EGFR signalling to promote smooth-cuticle cell survival: in addition to the spitz group genes, Rhomboid-3/roughoid, but not Rhomboid-2 or -4, and the neuregulin-like ligand Vein also function in survival signalling. Pointed mutants display the lowest frequency of fusions, suggesting that EGFR signalling may inhibit apoptosis primarily at the post-translational level. All ventral epidermal cells therefore require some level of EGFR signalling; high levels specify the denticle fate, while lower levels maintain smooth-cuticle cell survival. This strategy might guard against developmental errors, and may be conserved in mammalian epidermal patterning.

Species-specific activation of EGF receptor signaling underlies evolutionary diversity in the dorsal appendage number of the genus Drosophila eggshells

In Drosophila melanogaster, the patterning of dorsal appendages on the eggshell is strictly controlled by EGFR signaling. However, the number of dorsal appendages is remarkably diverse among Drosophila species. For example, D. melanogaster and D. virilis have two and four dorsal appendages, respectively. Here we show that during oogenesis the expression patterns of rhomboid (rho) and argos (aos), positive and negative regulators of EGFR signaling, respectively, were substantially different between D. melanogaster and D. virilis. Importantly, the number and position of both the rho expression and MAPK activation were consistent with those of the dorsal appendages in each species. Despite the differences in the spatial expression, these results suggest that the function of EGFR signaling in dorsal appendage formation is largely conserved between these two species. Thus, our results link the species-specific activation of EGFR signaling and the evolution of eggshell morphology in Drosophila.

Epidermal growth factor receptor: association of extracellular domain negatively regulates intracellular kinase activation in the absence of ligand

The epidermal growth factor receptor (EGFR) plays an important role in many types of human cancers. Receptor amplification, autocrine activation and/or deletion of exons 2-7 of EGFR gene have all been associated with tumor development. The traditional model of EGFR activation via ligand induced dimerization and consequential kinase activation does not provide full understanding of its tumorigenicity. The main function of the receptor extracellular domain (ECD) has been thought to be ligand recognition and binding. We report that the EGFR ECD, through its association also negatively regulates the activity of the intracellular kinase in the absence of ligand. Even in the absence of its ligands, the EGF receptor forms homodimers, however, the ECD prevents constitutive receptor kinase activation through its intrinsic ligand-independent interaction. The removal of this domain, either partial or total, results in constitutive activation of the receptor kinase as observed by its phosphorylation in intact cells. Furthermore, EGF receptors truncated in the ECD induce phosphorylation of the wild-type full-length receptor, indicating an inter-molecular inhibitory mechanism by the receptor ECD. The tumor associated delta2-7EGFR mutant also dimerizes with and phosphorylates the wild type EGFR in the absence of ligand. Thus, in addition to its role in ligand recognition, EGFR ECD interacts with each other, imposing an inhibitory effect on the activation of the intracellular kinase.

Cell cycle withdrawal, progression, and cell survival regulation by EGFR and its effectors in the differentiating Drosophila eye

Receptor tyrosine kinases such as the EGF receptor transduce extracellular signals into multiple cellular responses. In the developing Drosophila eye, EGFR activity triggers cell differentiation. Here we focus on three additional cell autonomous aspects of EGFR function and their coordination with differentiation, namely, withdrawal from the cell cycle, mitosis, and cell survival. We find that, whereas differentiation requires intense signaling, dependent on multiple reinforcing ligands, lesser EGFR activity maintains cell cycle arrest, promotes mitosis, and protects against cell death. Each response requires the same Ras, Raf, MAPK, and Pnt signal transduction pathway. Mitotic and survival responses also involve Pnt-independent branches, perhaps explaining how survival and mitosis can occur independently. Our results suggest that, rather than triggering all or none responses, EGFR coordinates partially independent processes as the eye differentiates.

Transcriptional activation in Drosophila spermatogenesis involves the mutually dependent function of aly and a novel meiotic arrest gene cookie monster

In Drosophila spermatogenesis, meiotic cell cycle progression and cellular differentiation are linked by the function of the meiotic arrest genes. The meiotic arrest genes control differentiation by regulating the transcriptional activation of many differentiation-specific genes. The meiotic arrest genes have been subdivided into aly and can classes, based on the mechanism by which they control cell cycle progression. aly has previously been shown to encode a chromatin-associated protein. We present the identification, cloning and characterisation of a novel Drosophila meiotic arrest gene, cookie monster (comr), that has a mutant phenotype indistinguishable from that of aly. A null mutant allele of comr is viable but male sterile. Mutant primary spermatocytes fail to initiate transcription of a large number of genes, and arrest before entry into the meiotic divisions. In adult males, expression of comr is testis specific, low levels of transcripts are detected at other stages of development. comr encodes a novel acidic protein, which is nuclear and primarily localised to regions of chromatin in primary spermatocytes. The nuclear localisation of Aly and Comr proteins are mutually dependent. Finally, we show that active RNA polymerase II is found in distinct domains in the nucleus that constitute a subset of the total Comr stained chromatin.

Regulation of Easter activity is required for shaping the Dorsal gradient in the Drosophila embryo

Dorsoventral polarity of the Drosophila embryo requires maternal spätzle-Toll signaling to establish a nuclear gradient of Dorsal protein. The shape of this gradient is altered in embryos produced by females carrying dominant alleles of easter (ea(D)). The easter gene encodes a serine protease that generates processed Spätzle, which is proposed to act as the Toll ligand. By examining the expression domains of the zygotic genes zen, sog, rho and twist, which are targets of nuclear Dorsal, we show that the slope of the Dorsal gradient is progressively flattened in stronger ea(D) alleles. In the wild-type embryo, activated Easter is found in a high M(r) complex called Ea-X, which is hypothesized to contain a protease inhibitor. In ea(D) embryo extracts, we detect an Easter form corresponding to the free catalytic domain, which is never observed in wild type. These mutant ea(D) proteins retain protease activity, as determined by the production of processed Spätzle both in the embryo and in cultured Drosophila cells. These experiments suggest that the ea(D) mutations interfere with inactivation of catalytic Easter, and imply that this negative regulation is essential for generating the wild-type shape of the Dorsal gradient.

Connecting Hh, Dpp and EGF signalling in patterning of theDrosophilawing; the pivotal role ofcollier/knotin the AP organiser

Hedgehog (Hh) signalling from posterior (P) to anterior (A) cells is the primary determinant of AP polarity in the limb field in insects and vertebrates. Hh acts in part by inducing expression of Decapentaplegic (Dpp), but how Hh and Dpp together pattern the central region of the Drosophila wing remains largely unknown. We have re-examined the role played by Collier (Col), a dose-dependent Hh target activated in cells along the AP boundary, the AP organiser in the imaginal wing disc. We found that col mutant wings are smaller than wild type and lack L4 vein, in addition to missing the L3-L4 intervein and mis-positioning of the anterior L3 vein. We link these phenotypes to col requirement for the local upregulation of both emc and N, two genes involved in the control of cell proliferation, the EGFR ligand Vein and the intervein determination gene blistered. We further show that attenuation of Dpp signalling in the AP organiser is also col dependent and, in conjunction with Vein upregulation, required for formation of L4 vein. A model recapitulating the molecular interplay between the Hh, Dpp and EGF signalling pathways in the wing AP organiser is presented.

rugose (rg), a Drosophila A kinase anchor protein, is required for retinal pattern formation and interacts genetically with multiple signaling pathways

In the developing Drosophila eye, cell fate determination and pattern formation are directed by cell-cell interactions mediated by signal transduction cascades. Mutations at the rugose locus (rg) result in a rough eye phenotype due to a disorganized retina and aberrant cone cell differentiation, which leads to reduction or complete loss of cone cells. The cone cell phenotype is sensitive to the level of rugose gene function. Molecular analyses show that rugose encodes a Drosophila A kinase anchor protein (DAKAP 550). Genetic interaction studies show that rugose interacts with the components of the EGFR- and Notch-mediated signaling pathways. Our results suggest that rg is required for correct retinal pattern formation and may function in cell fate determination through its interactions with the EGFR and Notch signaling pathways.