The transcriptional factor CF2 is a mediator of EGF-R-activated dorsoventral patterning in Drosophila oogenesis

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.

CF2 transcription factor is involved in the regulation of Mef2 RNA levels, nuclei number and muscle fiber size

CF2 and Mef2 influence a variety of developmental muscle processes at distinct stages of development. Nevertheless, the exact nature of the CF2-Mef2 relationship and its effects on muscle building remain yet to be resolved. Here, we explored the regulatory role of CF2 in the Drosophila embryo muscle formation. To address this question and not having proper null CF2 mutants we exploited loss or gain of function strategies to study the contribution of CF2 to Mef2 transcription regulation and to muscle formation. Our data point to CF2 as a factor involved in the regulation of muscle final size and/or the number of nuclei present in each muscle. This function is independent of its role as a Mef2 collaborative factor in the transcriptional regulation of muscle-structural genes. Although Mef2 expression patterns do not change, reductions or increases in parallel in CF2 and Mef2 transcript abundance were observed in interfered and overexpressed CF2 embryos. Since CF2 expression variations yield altered Mef2 expression levels but with correct spatio-temporal Mef2 expression patterns, it can be concluded that only the mechanism controlling expression levels is de-regulated. Here, it is proposed that CF2 regulates Mef2 expression through a Feedforward Loop circuit.

The Drosophila retinoblastoma binding protein 6 family member has two isoforms and is potentially involved in embryonic patterning

The human retinoblastoma binding protein 6 (RBBP6) is implicated in esophageal, lung, hepatocellular and colon cancers. Furthermore, RBBP6 was identified as a strong marker for colon cancer prognosis and as a predisposing factor in familial myeloproliferative neoplasms. Functionally, the mammalian protein interacts with p53 and enhances the activity of Mdm2, the prototypical negative regulator of p53. However, since RBBP6 (known as PACT in mice) exists in multiple isoforms and pact-/- mice exhibit a more severe phenotype than mdm2-/- mutants, it must possess some Mdm2-independent functions. The function of the invertebrate homologue is poorly understood. This is complicated by the absence of the Mdm2 gene in both Drosophila and Caenorhabditis elegans. We have experimentally identified the promoter region of Snama, the Drosophila homologue, analyzed potential transcription factor binding sites and confirmed the existence of an additional isoform. Using band shift and co-immunoprecipitation assays combined with mass spectrometry, we found evidence that this gene may be regulated by, amongst others, DREF, which regulates hundreds of genes related to cell proliferation. The potential transcription factors for Snama fall into distinct functional groups, including anteroposterior embryonic patterning and nucleic acid metabolism. Significantly, previous work in mice shows that pact-/- induces an anteroposterior phenotype in embryos when rescued by simultaneous deletion of p53. Taken together, these observations indicate the significance of RBBP6 proteins in carcinogenesis and in developmental defects.

A discrete model of Drosophila eggshell patterning reveals cell-autonomous and juxtacrine effects

The Drosophila eggshell constitutes a remarkable system for the study of epithelial patterning, both experimentally and through computational modeling. Dorsal eggshell appendages arise from specific regions in the anterior follicular epithelium that covers the oocyte: two groups of cells expressing broad (roof cells) bordered by rhomboid expressing cells (floor cells). Despite the large number of genes known to participate in defining these domains and the important modeling efforts put into this developmental system, key patterning events still lack a proper mechanistic understanding and/or genetic basis, and the literature appears to conflict on some crucial points. We tackle these issues with an original, discrete framework that considers single-cell models that are integrated to construct epithelial models. We first build a phenomenological model that reproduces wild type follicular epithelial patterns, confirming EGF and BMP signaling input as sufficient to establish the major features of this patterning system within the anterior domain. Importantly, this simple model predicts an instructive juxtacrine signal linking the roof and floor domains. To explore this prediction, we define a mechanistic model that integrates the combined effects of cellular genetic networks, cell communication and network adjustment through developmental events. Moreover, we focus on the anterior competence region, and postulate that early BMP signaling participates with early EGF signaling in its specification. This model accurately simulates wild type pattern formation and is able to reproduce, with unprecedented level of precision and completeness, various published gain-of-function and loss-of-function experiments, including perturbations of the BMP pathway previously seen as conflicting results. The result is a coherent model built upon rules that may be generalized to other epithelia and developmental systems.

An IκB homologue (FcCactus) in Chinese shrimp Fenneropenaeus chinensis

This study firstly reports the characterization of a functional IκB homologue, FcCactus in Chinese shrimp Fenneropenaeus chinensis. The full length cDNA of FcCactus consists of a 1359 bp open reading frame (ORF) encoding a 453 amino acid protein with a predicted molecular weight (MW) of 48.46 kDa and theoretical pI of 5.23. Phylogenetic analysis and multiple alignments revealed that the deduced amino acid sequence of FcCactus cDNA had high similarities to Cactus or IκB reported in seven other arthropods. Genomic DNA sequence of FcCactus was also obtained with a length of more than 17698 bp and constituted of seven exons and six introns. Analysis on 5'-upstream regulatory region of its DNA sequence revealed that it contained the core promoter sequence with the TATA-box and transcription start site existing in it; furthermore, various transcription factor binding sites (HSF, Hb, BR-C Z, Dfd, CF2-II, Croc, Ttk, Dorsal, and c-Rel) were predicted. Spatial expression profiles showed that FcCactus mRNA had the highest expression level in muscle, hemocytes, heart and lymphoid organ. Gram-positive bacteria (Micrococcus lysodeikticus) and Gram-negative bacteria (Vibrio anguillarium) injection to shrimp caused the modulation of FcCactus at the transcription level. DsRNAi (double-strand RNA interference) approach was used to study the function of FcCactus and the data showed that FcCactus could regulate the expression of different antimicrobial peptides (AMPs) and antiviral factor (AV). The present data showed that FcCactus might play important roles in regulating the immune response of shrimp.

Molecular mechanisms of EGF signaling-dependent regulation of pipe, a gene crucial for dorsoventral axis formation in Drosophila

During Drosophila oogenesis the expression of the sulfotransferase Pipe in ventral follicle cells is crucial for dorsoventral axis formation. Pipe modifies proteins that are incorporated in the ventral eggshell and activate Toll signaling which in turn initiates embryonic dorsoventral patterning. Ventral pipe expression is the result of an oocyte-derived EGF signal which down-regulates pipe in dorsal follicle cells. The analysis of mutant follicle cell clones reveals that none of the transcription factors known to act downstream of EGF signaling in Drosophila is required or sufficient for pipe regulation. However, the pipe cis-regulatory region harbors a 31-bp element which is essential for pipe repression, and ovarian extracts contain a protein that binds this element. Thus, EGF signaling does not act by down-regulating an activator of pipe as previously suggested but rather by activating a repressor. Surprisingly, this repressor acts independent of the common co-repressors Groucho or CtBP.

Structural analysis of a 4414-bp element in Drosophila melanogaster

We cloned a 4414-bp element from a mutant of Drosophila melanogaster. Its insertion site was 18,929,626 bp. Analysis of the nucleotide and amino acid sequences demonstrated that the element is homologous to Pifo_I, first obtained from D. yabuka, which belongs to the gypsy/Ty3 subfamily. We also obtained a 3754-bp length element from a wild-type fly by PCR, with a pair of primers designed from the conserved region of the 4414-bp length element. The two elements included a pair of long terminal repeats and part of the GAG and ENV proteins, but the POL protein was completely lost. This element is found in the subgenus of D. melanogaster, but it is a degenerate type of Pifo_I and is not infective. Also, a 714-bp region structured in 5.0 tandem repeats of 143 bp each was found in the 5'UTR of the degenerate element; these could interact with transcription factor CF2. Phylogenetic analysis and alignment of amino acids indicated that the Pifo_I element was closer to the ZAM retrotransposon, which gave us some clues to their functional similarity. Based on these data, we propose that there is a relationship between the degenerate element and the mutant phenotype, which would provide a foundation for further research.

CF2 represses Actin 88F gene expression and maintains filament balance during indirect flight muscle development in Drosophila

The zinc finger protein CF2 is a characterized activator of muscle structural genes in the body wall muscles of the Drosophila larva. To investigate the function of CF2 in the indirect flight muscle (IFM), we examined the phenotypes of flies bearing five homozygous viable mutations. The gross structure of the IFM was not affected, but the stronger hypomorphic alleles caused an increase of up to 1.5X in the diameter of the myofibrils. This size increase did not cause any disruption of the hexameric arrangement of thick and thin filaments. RT-PCR analysis revealed an increase in the transcription of several structural genes. Ectopic overexpression of CF2 in the developing IFM disrupts muscle formation. While our results indicate a role for CF2 as a direct negative regulator of the thin filament protein gene Actin 88F (Act88F), effects on levels of transcripts of myosin heavy chain (mhc) appear to be indirect. This role is in direct contrast to that described in the larval muscles, where CF2 activates structural gene expression. The variation in myofibril phenotypes of CF2 mutants suggest the CF2 may have separate functions in fine-tuning expression of structural genes to insure proper filament stoichiometry, and monitoring and/or controlling the final myofibril size.

Haplotype structure and expression divergence at the Drosophila cellular immune gene eater

The protein Eater plays an important role in microbial recognition and defensive phagocytosis in Drosophila melanogaster. We sequenced multiple alleles of the eater gene from an African and a North American population of D. melanogaster and found signatures of a partial selective sweep in North America that is localized around the second intron. This pattern is consistent with local adaptation to novel selective pressures during range expansion out of Africa. The North American sample is divided into two predominant haplotype groups, and the putatively selected haplotype is associated with a significantly higher gene expression level, suggesting that gene regulation is a possible target of selection. The eater alleles contain from 22 to 40 repeat units that are characterized by the presence of a cysteine-rich NIM motif. NIM repeats in the structural stalk of the protein exhibit concerted evolution as a function of physical location in the repeat array. Several NIM repeats within eater have previously been implicated in binding to microbial ligands, a function which in principle might subject them to special evolutionary pressures. However, we find no evidence of elevated positive selection on these pathogen-interacting units. Our study presents an instance where gene expression rather than protein structure is thought to drive the adaptive evolution of a pathogen recognition molecule in the immune system.

Intron loss is associated with gain of function in the evolution of the gloverin family of antibacterial genes in Bombyx mori

Gene duplication is a characteristic feature of eukaryotic genomes. Here we investigated the role of gene duplication in the evolution of the gloverin family of antibacterial genes (Bmglv1, Bmglv2, Bmglv3, and Bmglv4) in Bombyx mori. We observed the following two significant changes during the first duplication event: (i) loss of intronV, located in the 3'-untranslated region (UTR) of the ancestral gene Bmglv1, and (ii) 12-bp deletion in exon3. We show that loss of intronV during Bmglv1 to Bmglv2 duplication was associated with embryonic expression of Bmglv2. Gel mobility shift, chromatin immunoprecipitation, and immunodepletion assays identified chorion factor 2, a zinc finger protein, as the repressor molecule that bound to a 10-bp regulatory motif in intronV of Bmglv1 and repressed its transcription. gloverin paralogs that lacked intronV were independent of chorion factor 2 regulation and expressed in embryo. These results suggest that change in cis-regulation because of intron loss resulted in embryonic expression of Bmglv2-4, a gain of function over Bmglv1. Studies on the significance of intron loss have focused on introns present within the coding sequences for their potential effect on the open reading frame, whereas introns present in the UTRs of the genes were not given due attention. This study emphasizes the regulatory function of the 3'-UTR intron. In addition, we also studied the genomic loss and show that "in-frame" deletion of 12 nucleotides led to loss of amino acids IHDF resulting in the generation of a prepro-processing site in BmGlv2. As a result, the N-terminal pro-part of BmGlv2, but not of BmGlv1, gets processed in an infection-dependent manner suggesting that prepro-processing is an evolved feature in Gloverins.

CF2 activity and enhancer integration are required for proper muscle gene expression in Drosophila

The creation of the contractile apparatus in muscle involves the co-activation of a group of genes encoding muscle-specific proteins and the production of high levels of protein in a short period of time. We have studied the transcriptional control of six Drosophila muscle genes that have similar expression profiles and we have compared these mechanisms with those employed to control the distinct expression profiles of other Drosophila genes. The regulatory elements controlling the transcription of co-expressed muscle genes share an Upstream Regulatory Element and an Intronic Regulatory Element. Moreover, similar clusters of MEF2 and CF2 binding sites are present in these elements. Here, we demonstrate that CF2 depletion alters the relative expression of thin and thick filament components. We propose that the appropriate rapid gene expression responses during muscle formation and the maintenance of each muscle type is guaranteed in Drosophila by equivalent duplicate enhancer-like elements. This mechanism may be exceptional and restricted to muscle genes, reflecting the specific requirement to mediate rapid muscle responses. However, it may also be a more general mechanism to control the correct levels of gene expression during development in each cell type.

Myocyte enhancer factor 2 and chorion factor 2 collaborate in activation of the myogenic program in Drosophila

The process of myogenesis requires the coordinated activation of many structural genes whose products are required for myofibril assembly, function, and regulation. Although numerous reports have documented the importance of the myogenic regulator myocyte enhancer factor 2 (MEF2) in muscle differentiation, the interaction of MEF2 with cofactors is critical to the realization of muscle fate. We identify here a genomic region required for full MEF2-mediated activation of actin gene expression in Drosophila, and we identify the zinc finger transcriptional regulator chorion factor 2 (CF2) as a factor functioning alongside MEF2 via this region. Furthermore, although both MEF2 and CF2 can individually activate actin gene expression, we demonstrate that these two factors collaborate in regulating the Actin57B target gene in vitro and in vivo. More globally, MEF2 and CF2 synergistically activate the enhancers of a number of muscle-specific genes, and loss of CF2 function in vivo results in reductions in the levels of several muscle structural gene transcripts. These findings validate a general importance of CF2 alongside MEF2 as a critical regulator of the myogenic program, identify a new regulator functioning with MEF2 to control cell fate, and provide insight into the network of regulatory events that shape the developing musculature.

A MAPK docking site is critical for downregulation of Capicua by Torso and EGFR RTK signaling

Early Drosophila development requires two receptor tyrosine kinase (RTK) pathways: the Torso and the Epidermal growth factor receptor (EGFR) pathways, which regulate terminal and dorsal-ventral patterning, respectively. Previous studies have shown that these pathways, either directly or indirectly, lead to post-transcriptional downregulation of the Capicua repressor in the early embryo and in the ovary. Here, we show that both regulatory effects are direct and depend on a MAPK docking site in Capicua that physically interacts with the MAPK Rolled. Capicua derivatives lacking this docking site cause dominant phenotypes similar to those resulting from loss of Torso and EGFR activities. Such phenotypes arise from inappropriate repression of genes normally expressed in response to Torso and EGFR signaling. Our results are consistent with a model whereby Capicua is the main nuclear effector of the Torso pathway, but only one of different effectors responding to EGFR signaling. Finally, we describe differences in the modes of Capicua downregulation by Torso and EGFR signaling, raising the possibility that such differences contribute to the tissue specificity of both signals.

Calcineurin function is required for myofilament formation and troponin I isoform transition in Drosophila indirect flight muscle

Mutations in the Drosophila calcineurin B2 gene cause the collapse of indirect flight muscles during mid stages of pupal development. Examination of cell fate-specific markers indicates that unlike mutations in genes such as vestigial, calcineurin B2 does not cause a shift in cell fate from indirect flight muscle to direct flight muscle. Genetic and molecular analyses indicate a severe reduction of myosin heavy chain gene expression in calcineurin B2 mutants, which accounts at least in part for the muscle collapse. Myofibrils in calcineurin B2 mutants display a variety of phenotypes, ranging from normal to a lack of sarcomeric structure. Calcineurin B2 also plays a role in the transition to an adult-specific isoform of troponin I during the late pupal stages, although the incompleteness of this transition in calcineurin B2 mutants does not contribute to the phenotype of muscle collapse. Together, these findings suggest a molecular basis for the indirect flight muscle hypercontractility phenotype observed in flies mutant for Drosophila calcineurin B2.

EGF-dependent and independent activation of MAP kinase during Drosophila oogenesis

Receptor tyrosine kinase (RTK) signaling is involved in multiple cell fate determination during Drosophila oogenesis. To address the problem of signaling specificity, we sought to systematically document the expression pattern of activated MAP kinase, the downstream effector of RTK signaling. We show that MAP kinase is activated in some of the cell types in which Drosophila EGF receptor signaling is known to function. MAP kinase activation is also associated with many cell migration events. Finally, MAP kinase is activated by heat stress without altering follicle cell fates. The implications of these findings are discussed.

The Drosophila zinc finger transcription factor CF2 is a myogenic marker downstream of MEF2 during muscle development

The Drosophila C(2)-H(2)-type zinc-finger transcription factor CF2 has been shown to regulate follicular cell fate determination during oogenesis. Here we show that CF2 is also expressed in the developing muscles of the embryo where it first appears at stage 12 at the time of skeletal myoblast fusion. Later it is expressed in all muscle lineages including skeletal, visceral and cardiac. Epistatic analysis showed that CF2 expression is dependent on the myogenic factor MEF2.

EGFR signalling inhibits Capicua-dependent repression during specification of Drosophila wing veins

Localised activation of the Ras/Raf pathway by Epidermal Growth Factor Receptor (EGFR) signalling specifies the formation of veins in the Drosophila wing. However, little is known about how the EGFR signal regulates transcriptional responses during the vein/intervein cell fate decision. We provide evidence that EGFR signalling induces expression of vein-specific genes by inhibiting the Capicua (Cic) HMG-box repressor, a known regulator of embryonic body patterning. Lack of Cic function causes ectopic expression of EGFR targets such as argos, ventral veinless and decapentaplegic and leads to formation of extra vein tissue. In vein cells, EGFR signalling downregulates Cic protein levels in the nucleus and relieves repression of vein-specific genes, whereas intervein cells maintain high levels of Cic throughout larval and pupal development, repressing the expression of vein-specific genes and allowing intervein differentiation. However, regulation of some EGFR targets such as rhomboid appears not to be under direct control of Cic, suggesting that EGFR signalling branches out in the nucleus and controls different targets via distinct mediator factors. Our results support the idea that localised inactivation of transcriptional repressors such as Cic is a rather general mechanism for regulation of target gene expression by the Ras/Raf pathway.

Cloning and functional analysis of pyruvate kinase promoter region from Drosophila melanogaster

Pyruvate kinase (PK; EC 2.7.1.40) is a key glycolytic enzyme of Drosophila melanogaster. It catalyzes the conversion of phosphoenolpyruvate into pyruvate with the transfer of a phosphate group to ADP to form ATP. The ATP provides energy for cell growth and metabolism, and pyruvate participates in many metabolic reactions. Therefore, PK plays an important role in cell metabolism. Southern blot analysis, PCR, and sequencing were used to determine the content of a Drosophila pyruvate kinase (Pyk) genomic clone, lambdaPK61. The results indicated that the insert of lambdaPK61 comprised 8330 bp upstream of and 7186 bp downstream of the transcription start point of the Pyk gene. The size of the insert was 15,516 bp in total, which contained six genes including Pyk. Deletion mapping was applied to identify the promoter region and cis-acting elements 5' of PyK. Ten serial deletions produced by PCR were inserted upstream of the reporter gene (LacZ) to form recombinant plasmids, which were then transfected into Drosophila S2 cells. The results revealed that the regions -1475 approximately -1033 and -1033 approximately -534 of the 5' end of PyK possessed positive regulatory function for Pyk expression; i.e., increased gene expression. There were redundant putative cis-acting elements, including ecdysone response element (EcRE), E74A, and broad complex zinc finger (BRCZ) binding sites. Both E74A and BRCZ belong to the early genes regulated by ecdysone. This result suggested that Pyk might be regulated by ecdysone, directly or indirectly. However, the results of the developmental profile of Pyk expression by Northern blot analysis suggested that the effects of ecdysone on Pyk were repressive, not inductive. In addition, it was found that in these regions, there were many cis-acting elements related to egg and embryo development. Both -258 approximately -254 and -167 approximately -163 contained a CAAT box, and deletion of these regions decreased reporter gene expression. Therefore, it is suggested that both CAAT boxes are functional and that the promoter of Pyk might be located in the region of -258 approximately +109. No TATA box or downstream promoter element were identified around the transcription start site of Pyk. Additionally, PyK might share a regulatory region with an unknown neighboring gene. It was concluded that Pyk has the characteristics of a housekeeping gene.

Drosophila Pin1 prolyl isomerase Dodo is a MAP kinase signal responder during oogenesis

The mammalian cis-trans prolyl isomerase Pin1 and its yeast orthologue Ess1/Ptf1 have been implicated in cell cycle control but a correlation between biochemical and physiological functions has not been established conclusively. Pin1 targets the proline residue carboxy-terminal to the phosphorylated threonine or serine residue, which constitutes part of the phosphorylated mitogen-activated protein kinase (MAPK) site PXpT/SP. Here we show that the Drosophila Pin1 homologue, the Dodo protein, is involved in dorsoventral patterning of the follicular epithelium in the egg chamber. Its function is to facilitate the degradation of transcription factor CF2, which requires, a priori, activated epidermal growth factor receptor-MAPK signalling.

Sequence and functional properties of Ets genes in the model organism Drosophila

Detailed molecular and genetic studies, coupled with the recent sequencing of the fly genome, have identified eight Ets-related genes in the model organism Drosophila. All show homology to genes in vertebrate species. Functional analyses of some of the Drosophila ets genes have revealed their essential roles in developmental processes such as metamorphosis, oogenesis, neurogenesis, myogenesis, and eye development. Such studies have yielded important insights into our understanding of the genetic control of hormonally-regulated gene expression, programmed cell death, and signal transduction during cell fate determination and differentiation. The developmental roles of E74 (ELF1), pointed (Ets 1), yan (TEL), and D-elg (GABPalpha) will be reviewed in this article. The context of their participation in signal transduction and gene regulation will also be discussed. The information should be of significant value to the study of related processes in higher organisms due to the growing evidence for the cross species conservation of developmental mechanisms.

Expression of fringe is down regulated by Gurken/Epidermal Growth Factor Receptor signalling and is required for the morphogenesis of ovarian follicle cells

Signalling by the Gurken/Epidermal Growth Factor Receptor (Grk/EGFR) pathway is involved in epithelial cell fate decision, morphogenesis and axis establishment in Drosophila oogenesis. In the search for genes downstream of the Grk/EGFR signal transduction pathway (STP), we isolated a number of genes that are components of other STPs. One of them is a known gene, called fringe (fng). Drosophila fng encodes a putative secreted protein that is required at other development stages for mediating interactions between dorsal and ventral cells via Notch signalling. Here we show that fng has a dynamic expression pattern in oogenesis and that its expression in specific groups of follicle cells along the anterior-posterior and dorsal-ventral axes is defined by the repression of fng by Grk. Interfering with fng expression using antisense RNA experiments resulted in a typical fng mutant phenotype in the wing, and malformed egg chambers and abnormal organisation of the follicle cells in the ovaries, revealing that fng is essential in oogenesis for the proper formation of the egg chamber and for epithelial morphogenesis. This has been confirmed by re-examination of fng mutants and analysis of fng mutant clones in oogenesis.

D-cbl, a negative regulator of the Egfr pathway, is required for dorsoventral patterning in Drosophila oogenesis

During Drosophila oogenesis, asymmetrically localized Gurken activates the EGF receptor (Egfr) and determines dorsal follicle cell fates. Using a mosaic follicle cell system we have identified a mutation in the D-cbl gene which causes hyperactivation of the Egfr pathway. Cbl proteins are known to downregulate activated receptors. We find that the abnormal Egfr activation is ligand dependent. Our results show that the precise regulation of Egfr activity necessary to establish different follicle cell fates requires two levels of control. The localized ligand Gurken activates Egfr to different levels in different follicle cells. In addition, Egfr activity has to be repressed through the activity of D-cbl to ensure the absence of signaling in the ventral most follicle cells.

Misexpression of argos, an inhibitor of EGFR signaling in oogenesis, leads to the production of bicephalic, ventralized, and lateralized Drosophila melanogaster eggs

Epidermal growth factor receptor (EGFR) signaling pathways are frequently involved in generating cell fate diversity in a number of organisms. During anterior-posterior and dorso-ventral polarity in the Drosophila egg chamber and eggshell, EGFR signaling leads to a number of determinative events in the follicle cell layer. A high level of Gurken signal leads to the expression of argos in dorsal midline cells. Lateral follicle cells, receiving a lower level of Gurken signal, can continue to express the Broad-Complex (BR-C) and differentiate into cells which produce chorionic appendages. Misexpression of argos in mid-oogenesis causes the midline cells to retain expression of BR-C, resulting in a single fused large appendage. Evidence that argos can directly repress Gurken-induced EGFR signaling is seen when premature expression of argos is induced earlier in oogenesis. It represses the Gurken signal at stage 5-6 of oogenesis which determines posterior follicle cells and occasionally leads to eggs with anteriors at both ends. We propose that the Gurken signal at stage 9 of oogenesis induces follicle cells to take on two fates, dorsal midline and lateral, each producing different parts of the eggshell and that argos is one of the key downstream genes required to select between these two fates.

A targeted gene silencing technique shows that Drosophila myosin VI is required for egg chamber and imaginal disc morphogenesis

We report that Drosophila unconventional myosin VI, encoded by Myosin heavy chain at 95F (Mhc95F), is required for both imaginal disc and egg chamber morphogenesis. During oogenesis, Mhc95F is expressed in migrating follicle cells, including the border cells, which migrate between the nurse cells to lie at the anterior of the oocyte; the columnar cells that migrate over the oocyte; the centripetal cells that migrate between the oocyte and nurse cells; and the dorsal-anterior follicle cells, which migrate to secrete the chorionic appendages. Its function during development has been studied using a targeted gene silencing technique, combining the Gal4-UAS targeted expression system and the antisense RNA technique. Antibody staining shows that the expression of myosin 95F is greatly decreased in follicle cells when antisense Mhc95F RNA is expressed. Interfering with expression of Drosophila myosin VI at various developmental stages frequently results in lethality. During metamorphosis it results in adult flies with malformed legs and wings, indicating that myosin VI is essential for imaginal disc morphogenesis. During oogenesis, abnormal follicle cell shapes and aberrant follicle cell migrations are observed when antisense Mhc95F is expressed in follicle cells during stages 9 to 10, suggesting that the Drosophila myosin VI is required for follicle cell epithelial morphogenesis.

Oogenic function of the myogenic factor D-MEF2: negative regulation of the decapentaplegic receptor gene thick veins

The myogenic factor D-MEF2 is required for the proper differentiation of muscle cells during Drosophila embryogenesis and the correct patterning of indirect flight muscles assembled during later metamorphosis. In addition to these essential myogenic functions, mutant D-mef2 adult females are weakly fertile and produce defective eggs. D-MEF2 is expressed in nurse and follicle cells of the wild-type egg chamber. We have analyzed the D-mef2 oogenic phenotype and show that the gene is required for the normal patterning and differentiation of the centripetally migrating follicle cells that are crucial for development of the anterior chorionic structures. D-mef2 alleles exhibit a genetic interaction with a dominant-negative allele of thick veins (tkv), which encodes a type I receptor of the Decapentaplegic-signaling pathway. tkv RNA is overexpressed in D-mef2 mutant egg chambers, and, conversely, forced expression of D-mef2 represses tkv expression. These results indicate a role for D-MEF2 in the regulation of tkv gene expression and Decapentaplegic signal transduction that are essential for proper determination and/or differentiation of the anterior follicle cells. Additionally, they demonstrate a vital function for the D-MEF2 transcription factor in multiple genetic pathways during Drosophila development.

EGF receptor signaling in Drosophila oogenesis

The spatial regulation of Egfr activity in the follicular epithelium of the ovary is achieved by the localization of its ligand, Gurken, within the germline. The final distribution of Gurken within the oocyte appears to be specified both by the localization of the gurken RNA and by regulation of Gurken protein accumulation, possibly at the level of translation. Localized activation of the Egfr distinguishes certain subpopulations of follicle cells, thereby generating asymmetry within the follicular epithelium. In early oogenesis, Egfr activation in posterior follicle cells defines the AP polarity of the egg chamber, and in midoogenesis restriction of Egfr activity to dorsal follicle cells determines DV polarity. A number of factors required downstream of the Egfr have been identified, but the mechanism by which the observed patterning of the follicular epithelium is achieved remains unclear. The dynamic expression patterns of some of these targets suggest that the initial Gurken-Egfr signal at the dorsal side of the follicular epithelium mediates an initial distinction between dorsal and ventral follicle cells and also initiates subsequent refinement processes that determine the final pattern of cell fates. In the dorsal follicle cells, this refinement appears to involve interactions between Egfr targets and may also involve feedback regulation of Egfr activity such that the profile of Egfr activity is modulated over time. In addition, the initial Gurken-Egfr signal negatively regulates the functional domain of another patterning process that governs the establishment of the DV axis of the developing embryo.

Regulation of EGF receptor signaling establishes pattern across the developing Drosophila retina

Developing epithelia use a variety of patterning mechanisms to place individual cells into their correct positions. However, the means by which pattern elements are established are poorly understood. Here, we report evidence that regulation of Drosophila EGF receptor (DER) activity plays a central role in propagating the evenly spaced array of ommatidia across the developing Drosophila retina. DER activity is essential for establishing the first ommatidial cell fate, the R8 photoreceptor neuron. R8s in turn appear to signal through Rhomboid and Vein to create a patterned array of ‘proneural clusters’ which contain high levels of phosphorylated ERKA and the bHLH protein Atonal. Finally, secretion by the proneural clusters of Argos represses DER activity in less mature regions to create a new pattern of R8s. Propagation of this process anteriorly results in a retina with a precise array of maturing ommatidia.

An autoregulatory cascade of EGF receptor signaling patterns the Drosophila egg

Intercellular signaling through the EGF receptor (EGFR) patterns the Drosophila egg. The TGF alpha-like ligand Gurken signals from the oocyte to the receptor in the overlying somatic follicle cells. We show that in the dorsal follicle cells this initial paracrine signaling event triggers an autocrine amplification by two other EGFR ligands, Spitz and Vein. Spitz only becomes an effective ligand in the presence of the multitransmembrane domain protein Rhomboid. Consequent high-level EGFR activation leads to localized expression of the diffusible inhibitor Argos, which alters the profile of signaling. This sequential activation, amplification, and local inhibition of the EGFR forms an autoregulatory cascade that leads to the splitting of an initial single peak of signaling into two, thereby patterning the egg.

Down-regulation of transcription factor CF2 by Drosophila Ras/MAP kinase signaling in oogenesis: cytoplasmic retention and degradation

Dorsoventral (D/V) patterning in Drosophila oogenesis is initiated by the transmission of a TGF-alpha-like ligand, Gurken (Grk), from the oocyte to the anterodorsal follicle cells, activating the EGF receptor (Egfr) signaling pathway. The zinc-finger transcription factor CF2 is a negative regulator of the rhomboid (rho) gene that encodes an essential membrane-bound component of the dorsalizing pathway. Expression of CF2 itself is negatively regulated by the activated Egfr. In this report, we demonstrate that CF2 is the target of down-regulation by the MAPK kinase cascade, and that this down-regulation is independent of the Rho function. These results suggest that D/V patterning involves a two-step signaling process: the initial Egfr signal, which represses CF2 and induces rho expression; and the subsequent Egfr + Rho signal, which determines the dorsal cell fates. Furthermore, we show that CF2 down-regulation occurs at the post-translational level through a mechanism involving coupled cytoplasmic retention and degradation.

Sequential activation of the EGF receptor pathway during Drosophila oogenesis establishes the dorsoventral axis

Previous work has demonstrated a role for the Drosophila EGF receptor (Torpedo/DER) and its ligand, Gurken, in the determination of anterioposterior and dorsoventral axes of the follicle cells and oocyte. The roles of DER in establishing the polarity of the follicle cells were examined further, by following the expression of DER-target genes. One class of genes (e.g. kekon) is induced by the DER pathway at all stages. Broad expression of kekon at the stage in which the follicle cells migrate posteriorly over the oocyte, demonstrates the capacity of the pathway to pattern all follicle cells except the ventral-most rows. This may provide the spatial coordinates for the ventral-most follicle cell fates. A second group of target genes (e.g. rhomboid (rho)) is induced only at later stages of oogenesis, and may require additional inputs by signals emanating from the anterior, stretch follicle cells. The function of Rho was analyzed by ectopic expression in the stretch follicle cells, and shown to induce a non-autonomous dorsalizing activity that is independent of Gurken. Rho thus appears to be involved in processing a DER ligand in the follicle cells, to pattern the egg chamber and allow persistent activation of the DER pathway during formation of the dorsal appendages.

Two signalling pathways specify localised expression of the Broad-Complex in Drosophila eggshell patterning and morphogenesis

The Drosophila eggshell, which has a pair of chorionic appendages (dorsal appendages) located asymmetrically along both the anterior/posterior and dorsal/ventral axes, provides a good model to study signal instructed morphogenesis. We show that the Broad-Complex, a gene encoding zinc-finger transcription factors, is essential for the morphogenesis of dorsal appendages and is expressed in a bilaterally symmetrical pattern in the lateral-dorsal-anterior follicle cells during late oogenesis. This is induced and specified along the dorsoventral axis by an epidermal growth factor receptor signalling pathway, which includes a localised transforming growth factor-alpha like molecule, Gurken, in the oocyte and the Drosophila EGF receptor homologue, Torpedo, in the surrounding somatic follicle cells. Furthermore, the precisely localised expression of BR-C along the AP axis requires a separate signalling pathway, initiated by a transforming growth factor-beta homologue, Decapentaplegic, in nearby follicle cells. These two signalling pathways, one from the oocyte and the other from the follicle cells, co-ordinately specify patches of follicle cells to express the Broad-Complex in a unique position in respect to both major axes, which in turn directs the differentiation of the dorsal appendages in the correct position on the eggshell.

Ectopic activation of torpedo/Egfr, a Drosophila receptor tyrosine kinase, dorsalizes both the eggshell and the embryo

The Drosophila gene torpedo/Egfr (top/Egfr) encodes a homolog of the vertebrate Epidermal Growth Factor receptor. This receptor is required several times during the life cycle of the fly for the transmisson of developmental cues. During oogenesis, Top/Egfr activation is required for the establishment of the dorsal/ventral axis of the egg and the embryo. To examine how ectopic Top/Egfr activation affects cell fate determination, we constructed an activated version of the protein. Expression of this activated form (lambda top) in the follicle cells of the ovary induces dorsal cell fates in both the follicular epithelium and the embryo. Different levels of expression resulted in different dorsal follicle cell fates. These dorsal cell fates were expanded in the anterior, but not the posterior, of the egg, even in cases where all the follicle cells covering the oocyte expressed lambda top. The expression of genes known to respond to top/Egfr activation, argos (aos), kekkon1 (kek 1) and rhomboid (rho), was also expanded in the presence of the lambda top construct. When lambda top was expressed in all the follicle cells covering the oocyte, kek 1 and argos expression was induced in follicle cells all along the anterior/posterior axis of the egg chamber. In contrast, rho RNA expression was only activated in the anterior of the egg chamber. These data indicate that the response to Top/Egfr signaling is regulated by an anterior/posterior prepattern in the follicle cells. Expression of lambda top in the entire follicular epithelium resulted in an embryo dorsalized along the entire anterior/posterior axis. Expression of lambda top in anterior or posterior subpopulations of follicle cells resulted in regionally autonomous dorsalization of the embryos. This result indicates that subpopulations of follicle cells along the anterior/posterior axis can respond to Top/Egfr activation independently of one another.

The Drosophila bunched gene is a homologue of the growth factor stimulated mammalian TSC-22 sequence and is required during oogenesis

A Drosophila melanogaster sequence homologous to the mammalian growth factor-stimulated TSC-22 gene was isolated in an enhancer trap screen for genes expressed in anterodorsal follicle cells during oogenesis. This sequence includes a 225 aa residue open reading frame that encompasses a leucine zipper motif immediately preceded by a highly conserved region (TSC box), similarly located but distinct from the basic domain of bZIP proteins. The gene encoding this sequence, bunched (bun), has been independently isolated and characterized with respect to its role in peripheral nervous system development and eye development (Treisman, J.E., Lai, Z.-C. and Rubin, G.M. (1995) Shortsighted acts in the decapentaplegic pathway in the Drosophila eye development and has homology to a mouse TGF-beta-responsive gene. Development 121, 2835-2845). In agreement with the expression of the enhancer detector insertion, in situ hybridization reveals that bun transcripts localize to the anterior dorsal follicle cells at stages 10-12 of oogenesis. Changes in bun enhancer trap expression in genetic backgrounds that disrupt the grk/Egfr signaling pathway suggest that bun is regulated by growth factor patterning of dorsal anterior follicle cell fates. Clonal analysis shows that bun is required for the proper elaboration of dorsal cell fates leading to the formation of the dorsal appendages.

Pointed, an ETS domain transcription factor, negatively regulates the EGF receptor pathway in Drosophila oogenesis

Spatially regulated activation of the Drosophila epidermal growth factor (EGF) receptor by its ligand, Gurken, is required for establishment of the dorsal/ventral axis of the oocyte and embryo. During mid-oogenesis, Gurken is concentrated at the dorsal-anterior of the oocyte and is thought to activate the EGF receptor pathway in adjacent follicle cells. In response to this signal, dorsal follicle cell fate is determined. These cells further differentiate into either appendage-producing or midline cells, resulting in patterning in the dorsal follicle cell layer. We show here that Pointed, an ETS transcription factor, is required in dorsal follicle cells for this patterning. Loss of pointed results in the loss of midline cells and an excess of appendage-forming cells, a phenotype associated with overactivation of the EGF receptor pathway in the dorsal region. Overexpression of pointed leads to a phenotype similar to that generated by loss of the EGF receptor pathway. This suggests that Pointed normally down-regulates EGF receptor signaling in the midline to generate patterning in the dorsal region. Interestingly, pointed expression is induced by the EGF receptor pathway. These data indicate a novel antagonistic function for Pointed in oogenesis; in response to activation of the EGF receptor, pointed is expressed and negatively regulates the EGF receptor pathway, possibly by integrating information from a second pathway.