Drosophila Abelson kinase mediates cell invasion and proliferation through two distinct MAPK pathways

The Abelson (Abl) family of non-receptor tyrosine kinases plays important role in cell morphogenesis, motility and proliferation. Although the function of Abl has been extensively studied in leukemia, its role in epithelial cell invasion remains obscure. Using the Drosophila wing epithelium as an in-vivo model system, we demonstrate that overexpression (activation) of Drosophila Abl (dAbl) causes loss of epithelial apical/basal cell polarity and secretion of matrix metalloproteinases, resulting in a cellular invasion and apoptosis. Our in vivo data indicate that dAbl acts downstream of the Src kinases, which are known regulators of cell adhesion and invasion. Downstream of dAbl, Rac GTPases activate two distinct MAPK pathways: JNK signaling (required for cell invasion and apoptosis) and ERK signaling (inducing cell proliferation). Activated Abl also increases the activity of Src members through a positive feedback loop leading to signal amplification. Thus targeting Src-Abl, using available dual inhibitors, could be of therapeutic importance in tumor cell metastasis.

The ankyrin repeat protein Diego mediates Frizzled-dependent planar polarization

During planar polarization of the Drosophila wing epithelium, the homophilic adhesion molecule Flamingo localizes to proximal/distal cell boundaries in response to Frizzled signaling; perturbing Frizzled signaling alters Flamingo distribution, many cell diameters distant, by a mechanism that is not well understood. This work identifies a tissue polarity gene, diego, that comprises six ankyrin repeats and colocalizes with Flamingo at proximal/distal boundaries. Diego is specifically required for polarized accumulation of Flamingo and drives ectopic clustering of Flamingo when overexpressed. Our data suggest that Frizzled acts through Diego to promote local clustering of Flamingo, and that clustering of Diego and Flamingo in one cell nonautonomously propagates to others.

Drosophila Fos mediates ERK and JNK signals via distinct phosphorylation sites

During Drosophila development Fos acts downstream from the JNK pathway. Here we show that it can also mediate ERK signaling in wing vein formation and photoreceptor differentiation. Drosophila JNK and ERK phosphorylate D-Fos with overlapping, but distinct, patterns. Analysis of flies expressing phosphorylation site point mutants of D-Fos revealed that the transcription factor responds differentially to JNK and ERK signals. Mutations in the phosphorylation sites for JNK interfere specifically with the biological effects of JNK activation, whereas mutations in ERK phosphorylation sites affect responses to the EGF receptor-Ras-ERK pathway. These results indicate that the distinction between ERK and JNK signals can be made at the level of D-Fos, and that different pathway-specific phosphorylated forms of the protein can elicit different responses.

The role of the Drosophila TAK homologue dTAK during development

The TAK kinases belong to the MAPKKK group and have been implicated in a variety of signaling events. Originally described as a TGF-beta activated kinase (TAK) it has, however, subsequently been demonstrated to signal through p38, Jun N-terminal kinase (JNK) and Nemo types of MAP kinases, and the NFkappaB inducing kinase. Despite these multiple proposed functions, the in vivo role of TAK family kinases remains unclear. Here we report the isolation and genetic characterization of the Drosophila TAK homologue (dTAK). By employing overexpression and double-stranded RNA interference (RNAi) techniques we have analyzed its function during embryogenesis and larval development. Overexpression of dTAK in the embryonic epidermis is sufficient to induce the transcription of the JNK target genes decapentaplegic and puckered. Furthermore, overexpression of dominant negative (DN) or wild-type forms of dTAK in wing and eye imaginal discs, respectively, results in defects in thorax closure and ommatidial planar polarity, two well described phenotypes associated with JNK signaling activity. Surprisingly, RNAi and DN-dTAK expression studies in the embryo argue for a differential requirement of dTAK during developmental processes controlled by JNK signaling, and a redundant or minor role of dTAK in dorsal closure. In addition, dTAK-mediated activation of JNK in the Drosophila eye imaginal disc leads to an eye ablation phenotype due to ectopically induced apoptotic cell death. Genetic analyses in the eye indicate that dTAK can also act through the p38 and Nemo kinases in imaginal discs. Our results suggest that dTAK can act as a JNKKK upstream of JNK in multiple contexts and also other MAPKs in the eye. However, the loss-of-function RNAi studies indicate that it is not strictly required and thus either redundant or playing only a minor role in the context of embryonic dorsal closure.

Nuclear signaling by Rac and Rho GTPases is required in the establishment of epithelial planar polarity in the Drosophila eye

BACKGROUND

The small GTPases Rac and Rho act as cellular switches in many important biological processes. In the fruit fly Drosophila, RhoA participates in the establishment of planar polarity, a process mediated by the receptor Frizzled (Fz). Thus far, analysis of Rac in this process has not been possible because of the absence of mutant Rac alleles. Here, we have investigated the role of Rac and Rho in establishing the polarity of ommatidia in the Drosophila eye.

RESULTS

By expressing a dominant negative or a constitutively activated form of Rac1, we interfered specifically with Rac signaling and disrupted ommatidial polarity. The resulting defects were similar to the loss/gain-of-function phenotypes typical of tissue-polarity genes. Through genetic interaction and rescue experiments involving a polarity-specific, loss-of-function dishevelled (dsh) allele, we found that Rac1 acts downstream of Dsh in the Fz signaling pathway, but upstream of, or in parallel to, RhoA. Rac signaled to the nucleus through the Jun N-terminal kinase (JNK) cascade in this process. By generating point mutations in the effector loop of RhoA, we found that RhoA also signals to the nucleus during the establishment of ommatidial polarity. Nevertheless, Rac and RhoA activated transcription of distinct target genes.

CONCLUSIONS

Rac is specifically required downstream of Dsh in the Fz pathway. It functions upstream or in parallel to RhoA and both signal to the nucleus, through distinct effectors, to establish planar polarity in the Drosophila eye.

Jun mediates Frizzled-induced R3/R4 cell fate distinction and planar polarity determination in the Drosophila eye

Jun acts as a signal-regulated transcription factor in many cellular decisions, ranging from stress response to proliferation control and cell fate induction. Genetic interaction studies have suggested that Jun and JNK signaling are involved in Frizzled (Fz)-mediated planar polarity generation in the Drosophila eye. However, simple loss-of-function analysis of JNK signaling components did not show comparable planar polarity defects. To address the role of Jun and JNK in Fz signaling, we have used a combination of loss- and gain-of-function studies. Like Fz, Jun affects the bias between the R3/R4 photoreceptor pair that is critical for ommatidial polarity establishment. Detailed analysis of jun(−) clones reveals defects in R3 induction and planar polarity determination, whereas gain of Jun function induces the R3 fate and associated polarity phenotypes. We find also that affecting the levels of JNK signaling by either reduction or overexpression leads to planar polarity defects. Similarly, hypomorphic allelic combinations and overexpression of the negative JNK regulator Puckered causes planar polarity eye phenotypes, establishing that JNK acts in planar polarity signaling. The observation that Dl transcription in the early R3/R4 precursor cells is deregulated by Jun or Hep/JNKK activation, reminiscent of the effects seen with Fz overexpression, suggests that Jun is one of the transcription factors that mediates the effects of fz in planar polarity generation.

Signaling specificity by Frizzled receptors in Drosophila

Wnt-Frizzled (Fz) signaling pathways play recurring important roles during the development and homeostasis of vertebrates and invertebrates. Fz receptors can signal through beta-catenin-dependent and -independent pathways. In Drosophila, Fz and Fz2 are redundant receptors for Wg. In addition, Fz conveys signals through a distinct pathway to organize planar polarization of epithelial structures. We demonstrate that the cytoplasmic sequences of Fz2 and Fz preferentially activate the beta-catenin and planar polarity cascade, respectively. Both receptors activate either pathway, but with different efficiencies. Intrinsic differences in signaling efficiency in closely related receptors might be a general mechanism for generating signaling specificity in vivo.

Rasputin, the Drosophila homologue of the RasGAP SH3 binding protein, functions in ras- and Rho-mediated signaling

The small GTPase Ras plays an important role in many cellular signaling processes. Ras activity is negatively regulated by GTPase activating proteins (GAPs). It has been proposed that RasGAP may also function as an effector of Ras activity. We have identified and characterized the Drosophila homologue of the RasGAP-binding protein G3BP encoded by rasputin (rin). rin mutants are viable and display defects in photoreceptor recruitment and ommatidial polarity in the eye. Mutations in rin/G3BP genetically interact with components of the Ras signaling pathway that function at the level of Ras and above, but not with Raf/MAPK pathway components. These interactions suggest that Rin is required as an effector in Ras signaling during eye development, supporting an effector role for RasGAP. The ommatidial polarity phenotypes of rin are similar to those of RhoA and the polarity genes, e.g. fz and dsh. Although rin/G3BP interacts genetically with RhoA, affecting both photoreceptor differentiation and polarity, it does not interact with the gain-of-function genotypes of fz and dsh. These data suggest that Rin is not a general component of polarity generation, but serves a function specific to Ras and RhoA signaling pathways.

The Drosophila tumor suppressor expanded regulates growth, apoptosis, and patterning during development

The Drosophila expanded (ex) gene encodes a protein thought to play a role in signaling at apical junctions of epithelial cells. Previous studies have characterized this gene as a tumor suppressor involved in regulating the growth of a subset of Drosophila imaginal discs (Boedigheimer, M., Laughon, A., 1993. expanded: a gene involved in the control of cell proliferation in imaginal discs, Development 118, 1291-1301); although ex negatively regulates cell proliferation in the developing wing, it appeared to have a conflicting role in the eye. In contrast, our analysis of the loss-of-function phenotype indicates that ex does, in fact, regulate growth in the eye. We also show that this gene plays a role in patterning of the eye, mainly at the level of planar polarity. Our studies further demonstrate that, contrary to what was expected based on loss-of-function data, the tissue reduction phenotypes resulting from Ex overexpression are attributable to the induction of apoptotic cell death. Taken together, our data suggest that Ex is a versatile molecule that plays a role in most of the processes that govern disc development.

Spiny legs and prickled bodies: new insights and complexities in planar polarity establishment

Epithelial cells can be polarized along two axes, namely in the apical-basolateral axis and in the horizontal plane of the epithelium. Vertebrate examples of planar polarization include aspects of skin development or features in internal organs, such as the inner ear epithelium. In insects like Drosophila, adult cuticular structures show planar polarization. Studies on planar polarity in Drosophila have identified several genes that regulate this process. Notably, the Frizzled receptor and its signaling cascade provide an entry point to the molecular aspects of planar polarization. A recent study by Gubb et al.((1)) of the prickle locus, which encodes a cytoplasmic protein with three LIM domains, provides new insights and raises several interesting questions that can now be addressed. Pk might serve a scaffolding function involved in assembling a protein complex required for planar polarity establishment.

Morphogenetic furrow initiation and progression during eye development in Drosophila: the roles of decapentaplegic, hedgehog and eyes absent

The Drosophila signaling factor decapentaplegic (dpp) mediates the effects of hedgehog (hh) in tissue patterning by regulating the expression of tissue-specific genes. In the eye disc, the transcription factors eyeless (ey), eyes absent (eya), sine oculis (so) and dachshund (dac) participate with these signaling molecules in a complex regulatory network that results in the initiation of eye development. Our analysis of functional relationships in the early eye disc indicates that hh and dpp play no role in regulating ey, but are required for eya, so and dac expression. We show that restoring expression of eya in loss-of-function dpp mutant backgrounds is sufficient to induce so and dac expression and to rescue eye development. Thus, once expressed, eya can carry out its functions in the absence of dpp. These experiments indicate that dpp functions downstream of or in parallel with ey, but upstream of eya, so and dac. Additional control is provided by a feedback loop that maintains expression of eya and so and includes dpp. The fact that exogenous overexpression of ey, eya, so and dac interferes with wild-type eye development demonstrates the importance of such a complicated mechanism for maintaining proper levels of these factors during early eye development. Whereas initiation of eye development fails in either Hh or Dpp signaling mutants, the subsequent progression of the morphogenetic furrow is only slowed down. However, we find that clones that are simultaneously mutant for Hh and Dpp signaling components completely block furrow progression and eye differentiation, suggesting that Hh and Dpp serve partially redundant functions in this process. Interestingly, furrow-associated expression of eya, so and dac is not affected by double mutant tissue, suggesting that some other factor(s) regulates their expression during furrow progression.

Planar polarity in the Drosophila eye: a multifaceted view of signaling specificity and cross-talk

Functional tissues not only polarize their epithelia in the apical-basolateral axis, but also often display a polarity within the plane of the epithelium. In Drosophila, all adult structures are derived from epithelia called imaginal discs and display planar polarization; the eye and the wing are particularly well suited for analysis. Studies of their polarization have identified several conserved genes that regulate both nuclear signaling and cytoskeletal architecture. In particular, the Frizzled (Fz) receptor has been identified as a key component of polarity establishment in all tissues. The Fz signaling pathway and associated events are beginning to be unraveled, shedding light on a novel Wnt/Fz signaling cascade.

Drosophila tumor suppressor PTEN controls cell size and number by antagonizing the Chico/PI3-kinase signaling pathway

The human tumor suppressor gene PTEN encodes a putative cytoskeleton-associated molecule with both protein phosphatase and phosphatidylinositol 3,4,5-trisphosphate (PIP3) 3-phosphatase activities. In cell culture, the lipid phosphatase activity of this protein is involved in regulating cell proliferation and survival, but the mechanism by which PTEN inhibits tumorigenesis in vivo is not fully established. Here we show that the highly evolutionarily conserved Drosophila PTEN homolog, DPTEN, suppresses hyperplastic growth in flies by reducing cell size and number. We demonstrate that DPTEN modulates tissue mass by acting antagonistically to the Drosophila Class I phosphatidylinositol 3-kinase, Dp110, and its upstream activator Chico, an insulin receptor substrate homolog. Surprisingly, although DPTEN does not generally affect cell fate determination, it does appear to regulate the subcellular organization of the actin cytoskeleton in multiple cell types. From these data, we propose that DPTEN has a complex role in regulating tissue and body size. It acts in opposition to Dp110 to control cell number and growth, while coordinately influencing events at the cell periphery via its effects on the actin cytoskeleton.

Sec61beta, a subunit of the protein translocation channel, is required during Drosophila development

We have identified and isolated mutations in the first Drosophila gene encoding a subunit of the Sec61 protein translocation channel, DSec61beta. While neither the Saccharomyces cerevisiae Sec61beta nor its functional Escherichia coli homologue are essential for viability or for protein translocation, we show that DSec61beta is essential for embryonic development. Homozygous mutant embryos die at the end of embryogenesis and are impaired in the secretion of cuticle proteins from the epidermis. DSec61beta germ line clones, result in defects in dorso-ventral patterning of the egg and are consistent with affected secretion of the protein Gurken from the oocyte to the follicle cells. Clonal analyses in the imaginal discs reveal defects in adult structures, including rhabdomere morphogenesis and a reduction of the size of tarsal segments in the leg. This is the first in vivo study of a component of the protein translocation machinery in higher eukaryotes, and illustrates how a protein that has an inessential, kinetic function in single-cell organisms can become critical for the complex development of a multicellular organism.

The Drosophila Jak kinase hopscotch is required for multiple developmental processes in the eye

Jak kinases are critical signaling components in hematopoiesis. While a large number of studies have been conducted on the roles of Jak kinases in the hematopoietic cells, much less is known about the requirements for these tyrosine kinases in other tissues. We have used loss of function mutations in the Drosophila Jak kinase Hopscotch (Hop) to determine the role of Hop in eye development. We find that Hop is required for cell proliferation/survival in the eye imaginal disc, for the differentiation of photoreceptor cells, and for the establishment of the equator and of ommatidial polarity. These results indicate that hop activity is required for multiple developmental processes in the eye, both cell-autonomously and nonautonomously.

The Drosophila STE20-like kinase misshapen is required downstream of the Frizzled receptor in planar polarity signaling

The Drosophila misshapen (msn) gene is a member of the STE20 kinase family. We show that msn acts in the Frizzled (Fz) mediated epithelial planar polarity (EPP) signaling pathway in eyes and wings. Both msn loss- and gain-of-function result in defective ommatidial polarity and wing hair formation. Genetic and biochemical analyses indicate that msn acts downstream of fz and dishevelled (dsh) in the planar polarity pathway, and thus implicates an STE20-like kinase in Fz/Dsh-mediated signaling. This demonstrates that seven-pass transmembrane receptors can signal via members of the STE20 kinase family in higher eukaryotes. We also show that Msn acts in EPP signaling through the JNK (Jun-N-terminal kinase) module as it does in dorsal closure. Although at the level of Fz/Dsh there is no apparent redundancy in this pathway, the downstream effector JNK/MAPK (mitogen-activated protein kinase) module is redundant in planar polarity generation. To address the nature of this redundancy, we provide evidence for an involvement of the related MAP kinases of the p38 subfamily in planar polarity signaling downstream of Msn.

Dishevelled: at the crossroads of divergent intracellular signaling pathways

During the development of multicellular organisms the formation of complex patterns relies on specific cell-cell signaling events. For tissues to become spatially organized and cells to become committed to specialized fates it is absolutely crucial for proper development that the underlying signaling systems receive and route information correctly. Recently, a wealth of genetic and biochemical experimental data has been collected about prevalent evolutionary conserved signaling families, such as the Wnts, Dpp/BMPs, and Hedgehogs, in flies, worms, and vertebrates. Paradoxically, members of a particular signaling family often have receptors with similar biochemical binding properties, though they activate different intracellular pathways in vivo and can be phenotypically distinguished. How are their specific biological responses then generated? With respect to signaling specificity in Wnt pathways, Dishevelled is an intriguing protein; in Drosophila melanogaster it is required in two distinct signaling pathways, that share Frizzled receptors of similar structure, but have distinct intracellular signaling routes. Recent results suggest that Dishevelled is a multifunctional protein at the crossroads of divergent Wnt/Fz pathways. Dishevelled appears to be a key factor in Wnt signaling to read' signals coming from the plasma membrane and route them into the correct intracellular pathways.

Six class homeobox genes in drosophila belong to three distinct families and are involved in head development

The vertebrate Six genes are homologues of the Drosophila homeobox gene sine oculis (so), which is essential for development of the entire visual system. Here we describe two new Six genes in Drosophila, D-Six3 and D-Six4, which encode proteins with strongest similarity to vertebrate Six3 and Six4, respectively. In addition, we report the partial sequences of 12 Six gene homologues from several lower vertebrates and show that the class of Six proteins can be subdivided into three major families, each including one Drosophila member. Similar to so, both D-Six3 and D-Six4 are initially expressed at the blastoderm stage in narrow regions of the prospective head and during later stages in specific groups of head midline neurectodermal cells. D-Six3 may also be essential for development of the clypeolabrum and several head sensory organs. Thus, the major function of the ancestral Six gene probably involved specification of neural structures in the cephalic region.

Asymmetric Notch activation specifies photoreceptors R3 and R4 and planar polarity in the Drosophila eye

Planar polarity is seen in epidermally derived structures throughout the animal kingdom. In the Drosophila eye, planar polarity is reflected in the mirror-symmetric arrangement of ommatidia (eye units) across the dorsoventral midline or equator; ommatidia on the dorsal and ventral sides of the equator exhibit opposite chirality. Photoreceptors R3 and R4 are essential in the establishment of the polarity of ommatidia. The R3 cell is thought to receive the polarizing signal, through the receptor Frizzled (Fz), before or at higher levels then the R4 cell, generating a difference between neighbouring R3 and R4 cells. Both loss-of-function and overexpression of Fz in the R3/R4 pair result in polarity defects and loss of mirror-image symmetry. Here we identify Notch and Delta (Dl) as dominant enhancers of the phenotypes produced by overexpression of fz and dishevelled (dsh), which encodes a signalling component downstream of Fz, and we show that D1-mediated activation of Notch is required for establishment of ommatidial polarity. Whereas fz signalling is required to specify R3, Notch signalling induces the R4 fate. Our data indicate that Dl is a transcriptional target of Fz/Dsh signalling in R3, and activates Notch in the neighbouring R4 precursor. This two-tiered mechanism explains how small differences in the level and/or timing of Fz activation reliably generate a binary cell-fate decision, leading to specification of R3 and R4 and ommatidial chirality.

Functional analysis of the fibrinogen-related scabrous gene from Drosophila melanogaster identifies potential effector and stimulatory protein domains

The scabrous (sca) gene encodes a secreted dimeric glycoprotein with putative coiled-coil domains N-terminally and a C-terminal region related to the blood clot protein fibrinogen. Homozygous sca mutants have extra bristle organs and rough eyes. We describe a GAL4-based expression system for testing rescue of the sca mutant phenotype by altered SCA proteins and for misexpression. We find that deletion of the fibrinogen-related domain (FReD) greatly decreases SCA function, confirming the importance of this conserved region. SCA function could not be restored by FReDs from human fibrinogen chain genes. However, proteins lacking any FReD still showed some function in both rescue and misexpression experiments, suggesting that putative effector-binding regions lie outside this domain. Consistent with this, proteins expressing only the FReD had no rescuing activity but were recessive negative; i.e., they enhanced the phenotype of sca mutations but had no phenotype in the presence of a wild-type sca allele. This suggests that the FReD contributes to SCA function by binding to other components of the bristle determination pathway, increasing the activity of the linked N-terminal region.

Dishevelled activates JNK and discriminates between JNK pathways in planar polarity and wingless signaling

Frizzled family proteins have been described as receptors of Wnt signaling molecules. In Drosophila, the two known Frizzled proteins are associated with distinct developmental processes. Genesis of epithelial planar polarity requires Frizzled, whereas Dfz2 affects morphogenesis by wingless-mediated signaling. Dishevelled is required in both signaling pathways. Here, we use genetic and overexpression assays to show that Dishevelled activates JNK cascades. Rescue analysis reveals different protein domain requirements in Dishevelled for the two pathways; the C-terminal DEP domain is essential to rescue planar polarity defects and induce JNK signaling. Furthermore, the planar polarity-specific dsh1 allele is mutated in the DEP domain. Our results indicate that different Wnt/Fz signals activate distinct intracellular pathways, and Dishevelled discriminates among them by distinct domain interactions.

Linking cell-fate specification to planar polarity: determination of the R3/R4 photoreceptors is a prerequisite for the interpretation of the Frizzled mediated polarity signal

The adult eye of Drosophila is a highly ordered structure composed of about 800 ommatidia, each displaying precise polarity. The planar polarity is reflected in the mirror-symmetric arrangement of ommatidia relative to the dorso-ventral midline, the equator. This arrangement is generated when ommatidia rotate towards the equator and the photoreceptor R3 displaces R4 creating different chiral forms in each half. Analysis of ommatidia mosaic for the tissue polarity gene frizzled (fz) has shown that the presence of a single Fz+ photoreceptor cell within the R3/ R4 pair is critical for the direction of rotation and chirality. By analysing clones mutant for seven-up (svp), in which R3/R4 precursors reside in their normal positions and become photoreceptor neurones but fail to adopt the normal R3/R4 fate, we find that the R3/R4 photoreceptor subtype specification is a prerequisite for planar polarisation in the eye. Moreover, in mosaic R3/R4 pairs we find that the svp- cell always adopts the R4 position. This bias is reminiscent of what happens in fz mosaic R3/R4 pairs, where the fz- cell also almost always adopts the R4 position. In addition, we find that in genotypes where too many cells adopt the R3/R4 fate, ommatidial polarity is also disturbed. Taken together, these data imply that correct specification of a single R3 cell per ommatidium is essential for the normal interpretation of the Fz-mediated polarity signal.

The muscleblind gene participates in the organization of Z-bands and epidermal attachments of Drosophila muscles and is regulated by Dmef2

We report the embryonic phenotype of muscleblind (mbl), a recently described Drosophila gene involved in terminal differentiation of adult ommatidia. mbl is a nuclear protein expressed late in the embryo in pharyngeal, visceral, and somatic muscles, the ventral nerve cord, and the larval photoreceptor system. All three mbl alleles studied exhibit a lethal phenotype and die as stage 17 embryos or first instar larvae. These larvae are partially paralyzed, show a characteristically contracted abdomen, and lack striation of muscles. Our analysis of the somatic musculature shows that the pattern of muscles is established correctly, and they form morphologically normal synapses. Ultrastructural analysis, however, reveals two defects in the terminal differentiation of the muscles: inability to differentiate Z-bands in the sarcomeric apparatus and reduction of extracellular tendon matrix at attachment sites to the epidermis. Failure to differentiate both structures could explain the partial paralysis and contracted abdomen phenotype. Analysis of mbl expression in embryos that are either mutant for Dmef2 or ectopically express Dmef2 places mbl downstream of Dmef2 function in the myogenic differentiation program. mbl, therefore, may act as a critical element in the execution of two Dmef2-dependent processes in the terminal differentiation of muscles.

Defective dorsal closure and loss of epidermal decapentaplegic expression in Drosophila fos mutants

Drosophila kayak mutant embryos exhibit defects in dorsal closure, a morphogenetic cell sheet movement during embryogenesis. Here we show that kayak encodes D-Fos, the Drosophila homologue of the mammalian proto-oncogene product, c-Fos. D-Fos is shown to act in a similar manner to Drosophila Jun: in the cells of the leading edge it is required for the expression of the TGFbeta-like Decapentaplegic (Dpp) protein, which is believed to control the cell shape changes that take place during dorsal closure. Defects observed in mutant embryos, and adults with reduced Fos expression, are reminiscent of phenotypes caused by 'loss of function' mutations in the Drosophila JNKK homologue, hemipterous. These results indicate that D-Fos is required downstream of the Drosophila JNK signal transduction pathway, consistent with a role in heterodimerization with D-Jun, to activate downstream targets such as dpp.

The deubiquitination enzyme fat facets negatively regulates RTK/Ras/MAPK signalling during Drosophila eye development

The Drosophila fat facets (faf) gene encodes a deubiquitination enzyme with a putative function in proteasomal protein degradation. Mutants lacking zygotic faf function develop to adulthood, but have rough eyes caused by the presence of one to two ectopic outer photoreceptors per ommatidium. Here we show that faf interacts genetically with the receptor tyrosine kinase (RTK)/Ras pathway, which induces photoreceptor differentiation in the developing eye. The results indicate that RTK/Ras signalling is increased in faf mutants, causing normally non-neuronal cells to adopt photoreceptor fate. Consistently, the protein level of at least one component of the Ras signal transduction pathway, the transcription factor D-Jun, is elevated in faf eye discs at the time when the ectopic photoreceptors are induced. We propose that defective ubiquitin-dependent proteolysis leads to increased and prolonged D-Jun expression, which together with other factors contributes to the induction of ectopic photoreceptors in faf mutants. These studies demonstrate the relevance of ubiquitin-dependent protein degradation in the regulation of RTK/Ras signal transduction in an intact organism.

muscleblind, a gene required for photoreceptor differentiation in Drosophila, encodes novel nuclear Cys3His-type zinc-finger-containing proteins

We have isolated the embryonic lethal gene muscleblind (mbl) as a suppressor of the sev-svp2 eye phenotype. Analysis of clones mutant for mbl during eye development shows that it is autonomously required for photoreceptor differentiation. Mutant cells are recruited into developing ommatidia and initiate neural differentiation, but they fail to properly differentiate as photoreceptors. Molecular analysis reveals that the mbl locus is large and complex, giving rise to multiple different proteins with common 5′ sequences but different carboxy termini. Mbl proteins are nuclear and share a Cys3His zinc-finger motif which is also found in the TIS11/NUP475/TTP family of proteins and is highly conserved in vertebrates and invertebrates. Functional analysis of mbl, the observation that it also dominantly suppresses the sE-Jun(Asp) gain-of-function phenotype and the phenotypic similarity to mutants in the photoreceptor-specific glass gene suggest that mbl is a general factor required for photoreceptor differentiation.

six-banded, a novel Drosophila gene, is expressed in 6 segmental stripes during embryonic development and in the eye imaginal disc

We have characterised a Drosophila P-element enhancer detector insertion F125, which is expressed in the embryonic head and CNS as well as in various third instar imaginal discs. In an attempt to identify the gene with the equivalent expression pattern, we have characterised an adjacent gene. It encodes two novel conceptual proteins: Type I (1182 amino acids) and Type II, representing a shorter form of 774 amino acids truncated at both termini relative to Type I that is generated by alternative splicing. Based on its embryonic expression pattern, the gene was called six-banded (sba). Both splice forms are expressed in a unique embryonic pattern: initially as 6, then 12 stripes during early stages of embryonic development. Subsequently, expression is found in the developing trachae and during larval development is restricted to the eye imaginal disc where both transcripts are present immediately anterior to and behind the morphogenetic furrow. While sba expression in the eye antennal disc is mirrored by the expression of the adjacent F125 P-element, other patterns reported by this enhancer detector are not mimicked by the sba gene suggesting that the expression of the P-element represents a 'composite' of the effects of nearby enhancers.

Hedgehog is an indirect regulator of morphogenetic furrow progression in the Drosophila eye disc

Pattern formation in the eye imaginal disc of Drosophila occurs in a wave that moves from posterior to anterior. The anterior edge of this wave is marked by a contracted band of cells known as the morphogenetic furrow, behind which photoreceptors differentiate. The movement of the furrow is dependent upon the secretion of the signalling protein Hedgehog (Hh) by more posterior cells, and it has been suggested that Hh acts as an inductive signal to induce cells to enter a furrow fate and begin differentiation. To further define the role of Hh in this process, we have analysed clones of cells lacking the function of the smoothened (smo) gene, which is required for transduction of the Hh signal and allows the investigation of the autonomous requirement for hh signalling. These experiments demonstrate that the function of hh in furrow progression is indirect. Cells that cannot receive/transduce the Hh signal are still capable of entering a furrow fate and differentiating normally. However, hh is required to promote furrow progression and regulate its rate of movement across the disc, since the furrow is significantly delayed in smo clones.

Jun in Drosophila development: redundant and nonredundant functions and regulation by two MAPK signal transduction pathways

Drosophila Jun is shown to be involved in different signal transduction pathways and developmental decisions. Dorsal closure, a morphogenetic process occurring during Drosophila embryogenesis, is regulated by Hemipterous (Hep) and Basket (Bsk), homologs of JNKK and JNK, respectively. Embryos lacking Jun activity exhibit a dorsal closure phenotype, very similar to that of bsk and hep mutants, indicating that Jun is a target of Hep/Bsk signaling. In eye and wing development Jun participates in a separate signaling pathway that is comprised of Ras, Raf, and the ERK-type kinase Rolled. In contrast to the strict requirement for Jun in dorsal closure, its role in the eye is redundant but can be uncovered by mutations in other signaling components. The redundant function of Jun in eye development may contribute to the precision of photoreceptor differentiation and ommatidial assembly.

The role of RhoA in tissue polarity and Frizzled signalling

The tissue polarity genes of Drosophila are required for correct establishment of planar polarity in epidermal structures, which in the eye is shown in the mirror-image symmetric arrangement of ommatidia relative to the dorsoventral midline. Mutations in the genes frizzled (fz), dishevelled (dsh) and prickle-spiny-legs (pk-sple) result in the loss of this mirror-image symmetry. fz encodes a serpentine receptor-like transmembrane protein required for reception and transmission of a polarity signal. Little else is known of the signalling pathway(s) involved other than that Dsh acts downstream of Fz. We have identified mutations in the Drosophila homologue of RhoA p21 GTPase, and by analysis of their phenotype show that RhoA is required for the generation of tissue polarity. Genetic interactions indicate a role for RhoA in signalling mediated by Fz and Dsh, and furthermore suggest that JNK/SAPK-like kinases are involved. These data are consistent with a Fz/RhoA signalling cascade analogous to the yeast pheromone signalling pathway and that proposed for activation of the serum response factor (SRF) in vertebrate cells.

Requirement for Drosophila 14-3-3 zeta in Raf-dependent photoreceptor development

Based on biochemical and functional data obtained with tissue culture cells and yeast, 14-3-3 proteins have been implicated in a number of different signal transduction processes, in particular in the signal-dependent activation of protein kinases. We performed a functional analysis of 14-3-3 in a multicellular organism, initiated by the cloning of a 14-3-3 zeta homolog of Drosophila melanogaster, termed D14-3-3 zeta. D14-3-3 zeta transcripts are strongly enriched in the developing central nervous system. In addition, they are predominantly expressed in the region posterior to the morphogenetic furrow of the eye imaginal disc where cells differentiate as photoreceptors. In these cells D14-3-3 zeta is localized apically. Both the expression pattern and the subcellular localization are consistent with the proposed function of 14-3-3 proteins in Ras/Raf/MAPK signaling. D14-3-3 zeta mutant analysis combined with rescue experiments involving gain-of-function alleles of Raf and Ras indicate that D14-3-3 zeta is an essential component of the Raf/Ras signaling pathway and necessary for photoreceptor differentiation. It acts upstream of Raf and downstream of Ras.

The regulation of hedgehog and decapentaplegic during Drosophila eye imaginal disc development

The hedgehog signalling pathway is a conserved mechanism which acts in inductive processes in both vertebrate and invertebrate development to direct growth and patterning. In Drosophila, the secreted Hedgehog protein acts as a signal to induce non-autonomous activation in adjacent cells of either the decapentaplegic or wingless genes (both of which encode growth factor-like molecules), via inactivation of patched activity. In the eye disc, this pathway drives progression of the morphogenetic furrow, while in the wing (and leg and antennal) discs it is required to set up an organising centre along the anteroposterior compartment boundary. We have compared the regulation and function of hedgehog pathway activity in the eye and wing discs, and find that there are significant differences. Whereas in the wing disc, engrailed function is required for hedgehog expression, in the eye disc activation and maintenance of hedgehog expression is achieved independently of engrailed. Regulation of decapentaplegic expression also differs: in the wing disc it is repressed in the anterior compartment by patched and in the posterior compartment by engrailed. In the eye disc, however, it is repressed posterior to the morphogenetic furrow in the absence of either patched or engrailed activity. We conclude that in the eye disc there are novel aspects to hedgehog pathway function. Moreover, engrailed does not play an essential conserved role.

Phosphorylation of Drosophila Jun by the MAP kinase rolled regulates photoreceptor differentiation

Drosophila Jun (D-Jun) is a nuclear component of the receptor tyrosine kinase/Ras signal transduction pathway which triggers photoreceptor differentiation during eye development. Here we show that D-Jun is a substrate for the ERK-related Drosophila MAP kinase Rolled, which has previously been shown to be a part of this pathway. A D-Jun mutant that carries alanines in place of the Rolled phosphorylation sites acts as a dominant suppressor of photoreceptor cell fate if expressed in the eye imaginal disc. In contrast, a mutant in which the phosphorylation sites are replaced by phosphate-mimetic Asp residues, as well as a VP16-D-Jun fusion protein, can promote photoreceptor differentiation. These data implicate Jun phosphorylation in the choice between neuronal and non-neuronal fate during Drosophila eye development.

Mad acts downstream of Dpp receptors, revealing a differential requirement for dpp signaling in initiation and propagation of morphogenesis in the Drosophila eye

Decapentaplegic (Dpp), a member of the TGF-betta family of cytokines, has been implicated in many patterning processes in Drosophila, including the initial steps of pattern formation in the developing eye. We show that the Mothers against dpp (Mad) gene is required for dpp signaling during eye development. Clonal analysis demonstrates a cell-autonomous function for Mad and genetic interactions indicate that Mad is an essential component of the signal transduction pathway downstream of the Dpp receptors in responding cells. Mad-mediated dpp signaling is absolutely required for the initiation of the morphogenetic furrow in the eye, but has only a minor role in its subsequent propagation across the eye

Drosophila TFIIA-S is up-regulated and required during Ras-mediated photoreceptor determination

Photoreceptor induction in the developing Drosophila eye is triggered by the activation of the Ras pathway. Subsequently, the Ras-mediated activation of site-specific transcription factors leads to the expression of putative "effector" genes. The coactivator function of the basal transcription factor TFIIA has been shown previously to enhance the trans-activation potential of site-specific transcription factors in vitro. Here, we show that the expression of the small subunit of TFIIA (dTFIIA-S) is specifically up-regulated in a transient manner during Ras-mediated photoreceptor induction. Furthermore, although null mutations in dTFIIA-S are cell lethal, a hypomorphic dTFIIA-S allele demonstrates an increased requirement for this factor during photoreceptor development. In addition, the cone cell to R7 photoreceptor transformation caused by ectopic activation of the Ras pathway during eye development is suppressed by the removal of one functional copy of the dTFIIA-S locus revealing the sensitivity of this process to reductions in dTFIIA-S activity. These results are the first in vivo evidence for the coactivator function in transcriptional enhancement proposed for TFIIA.

pipsqueak encodes a novel nuclear protein required downstream of seven-up for the development of photoreceptors R3 and R4

Photoreceptor induction in the Drosophila eye is mediated by activation of the Ras signal transduction cascade. Although this process is well understood, little is known about how the diversity of photoreceptor subtypes is generated. The pipsqueak (psq) gene is expressed at high levels in the R3/R4 precursors during eye development and this expression depends on seven-up (svp) gene function. Moreover, strong psq alleles are dominant suppressors of a svp-induced cone cell transformation phenotype. Although the gene was previously identified and described as a member of the maternal posterior group of genes, the strong semilethal alleles isolated here demonstrate a specific requirement for psq function downstream of svp for the development of photoreceptors R3/R4. The gene has three independent 5' ends and codes for several nuclear protein isoforms, some containing the POZ domain which has been implicated in protein-protein interactions. Interestingly, all viable alleles with a maternal posterior group phenotype cluster around one specific 5' exon, while all semilethal alleles have lesions which map to a different alternative 5' exon.

The role of yan in mediating the choice between cell division and differentiation

An allele of the yan locus was isolated as an enhancer of the Ellipse mutation of the Drosophila epidermal growth factor receptor (Egfr) gene. This yan allele is an embryonic lethal and also fails to complement the lethality of anterior open (aop) mutations. Phenotypic and complementation analysis revealed that aop is allelic to yan and genetically the lethal alleles act as null mutations for the yan gene. Analysis of the lethal alleles in the embryo and in mitotic clones showed that loss of yan function causes cells to overproliferate in the dorsal neuroectoderm of the embryo and in the developing eye disc. Our studies suggest that the role of yan is defined by the developmental context of the cells in which it functions. An important role of this gene is in allowing a cell to choose between cell division and differentiation. The relationship of the Egfr and Notch pathways to this developmental role of yan is discussed.

JUN cooperates with the ETS domain protein pointed to induce photoreceptor R7 fate in the Drosophila eye

R7 photoreceptor fate in the Drosophila eye induced by the activation of the Sevenless receptor tyrosine kinase and the RAS/MAP kinase signal transduction pathway. We show that expression of a constitutively activated JUN isoform in ommatidial precursor cells is sufficient to induce R7 fate independent of upstream signals normally required for photoreceptor determination. We present evidence that JUN interacts with the ETS domain protein Pointed to promote R7 formation. This interaction is cooperative when both proteins are targeted to the same promoter and is antagonized by another ETS domain protein, YAN, a negative regulator of R7 development. Furthermore, phyllopod, a putative transcriptional target of RAS pathway activation during R7 induction, behaves as a suppressor of activated JUN. Taken together, these data suggest that JUN and Pointed act on common target genes to promote neuronal differentiation in the Drosophila eye, and that phyllopod might be such a common target.

Ommatidial polarity in the Drosophila eye is determined by the direction of furrow progression and local interactions

The adult eye of Drosophila is a highly ordered structure. It is composed of about 800 ommatidia, each displaying precise polarity. The ommatidia are arranged about an axis of mirror image symmetry, the equator, which lies along the dorsoventral midline of the eye. We use hedgehog pathway mutants to induce ectopic morphogenetic furrows and use these as a tool to investigate the establishment of ommatidial polarity. Our results show that ommatidial clusters are self-organising units whose polarity in one axis is determined by the direction of furrow progression, and which can independently define the position of an equator without reference to the global coordinates of the eye disc.

A novel vertebrate svp-related nuclear receptor is expressed as a step gradient in developing rhombomeres and is affected by retinoic acid

The protein encoded by the zebrafish gene svp[40] belongs to a distinct group within the steroid hormone receptor superfamily that includes Drosophila seven-up and several vertebrate orphan receptors. Svp[40] shares a particularly high degree of amino acid sequence identity (approximately 86%) with the mammalian transcription factors ARP-1 and COUP. The gene is expressed in specific regional and segmental domains within the developing brain. Correspondence between this expression pattern and early sites of neuronal differentiation and axonogenesis in the rostral brain may reflect an involvement in neural patterning. During the early embryonic stages when hindbrain rhombomeres are formed, a segmental expression pattern is established as a step gradient. The single steps of this gradient coincide directly with the four anteriormost segments suggesting a role in controlling rhombomere-specific expression of genes contributing to cell differentiation in the hindbrain. Since COUP/ARP-1 and retinoic acid receptors (RARs/RXRs) are known to have similar DNA-binding specificities, different levels of Svp[40] might modulate retinoid signaling through competition for binding to specific RAREs in the promoters of target genes. Treatment of zebrafish embryos with retinoic acid affects the svp[40] step gradient and causes an elimination of a regional expression domain in the retina. These observations are consistent with svp[40] being an integral part of the retinoid signaling network during hindbrain and eye development.

Regulation of furrow progression in the Drosophila eye by cAMP-dependent protein kinase A

The earliest physical sign of differentiation in the Drosophila retina is the passage of the morphogenetic furrow across the epithelium of the eye disc. Secreted factors encoded by hedgehog (hh) and decapentaplegic (dpp) have been implicated in propagation of the furrow and the subsequent initiation of photoreceptor differentiation. The morphogenetic furrow initiates at the posterior edge of the third larval instar eye imaginal disc. Its continued progression towards the anterior is believed to depend upon secretion of Hh protein by the differentiating clusters of photoreceptors that emerge posterior to the moving furrow. This progression is marked by the initiation of expression of the transforming growth factor-beta homologue Dpp in cells entering the furrow anteriorly, and loss of dpp expression in cells emerging posteriorly. Although the transmembrane protein encoded by the patched gene has been genetically implicated as the Hh receptor, the intercellular signalling pathways involved in these inductive processes remain uncharacterized. Here we show that the catalytic subunit of cyclic AMP-dependent protein kinase A (Pka-C1) is required for the correct spatial regulation of dpp expression during eye development. Loss of Pka-C1 function is sufficient to produce an ectopic morphogenetic wave marked by premature ectopic photoreceptor differentiation and non-autonomous propagation of dpp expression. Our results indicate that Pka-C1 lies in a signalling pathway that controls the orderly temporal progression of differentiation across the eye imaginal disc.

The Drosophila orphan nuclear receptor seven-up requires the Ras pathway for its function in photoreceptor determination

The Drosophila seven-up (svp) gene specifies outer photoreceptor cell fate in eye development and encodes an orphan nuclear receptor with two isoforms. Transient expression under the sevenless enhancer of either svp isoform leads to a dosage-dependent transformation of cone cells into R7 photoreceptors, and at a lower frequency, R7 cells into outer photoreceptors. To investigate the cellular pathways involved, we have taken advantage of the dosage sensitivity and screened for genes that modify this svp-induced phenotype. We show that an active Ras pathway is essential for the function of both Svp isoforms. Loss-of-function mutations in components of the Ras signal transduction cascade act as dominant suppressors of the cone cell transformation, whilst loss-of-function mutations in negative regulators of Ras-activity act as dominant enhancers. Furthermore, Svp-mediated transformation of cone cells to outer photoreceptors, reminiscent of its wild-type function in specifying R3/4 and R1/6 identity, requires an activated Ras pathway in the same cells, or alternatively dramatic increase in ectopic Svp protein levels. Our results indicate that svp is only fully functional in conjunction with activated Ras. Since we find that mutations in the Egf-receptor are also among the strongest suppressors of svp-mediated cone cell transformation, we propose that the Ras activity in cone cells is due to low level Egfr signaling. Several models that could account for the observed svp regulation by the Ras pathway are discussed.

Drosophila TFIIA directs cooperative DNA binding with TBP and mediates transcriptional activation

Drosophila transcription factor IIA (TFIIA) is composed of three subunits (30, 20, and 14 kD) that function during initiation of transcription. We reported previously the characterization of cDNAs that encode a precursor (dTFIIA-L) of the Drosophila TFIIA 30- and 20-kD subunits. In the absence of the smallest subunit, dTFIIA-S (14 kD), the unprocessed large subunit failed to exhibit any detectable promoter binding or transcriptional activity. Here, we report the molecular cloning and expression of dTFIIA-S, which has allowed the assembly of holo-dTFIIA (dTFIIA-L/S). Subunit interaction studies indicate that dTFIIA-S binds to an amino-terminal domain of dTFIIA-L, which likely corresponds to the endogenous 30-kD processed species. In addition, both dTFIIA-S and the carboxy-terminal domain of dTFIIA-L, which corresponds to the 20-kD species, independently interact weakly with the TATA-binding protein (TBP). In contrast, the holo-dTFIIA (L/S) binds TBP with high affinity. The dTFIIA-L/S complex also binds cooperatively with TBP to TATA box DNA sequences, generating an extended DNase footprint pattern. The reconstituted holo-dTFIIA is able to stimulate basal transcription of several core promoter templates. Interestingly, dTFIIA-L/S is also able to significantly enhance transcriptional activation by upstream transcription factors including Sp1, VP16, and NTF-1. These results suggest that dTFIIA is a multifunctional transcription factor capable of influencing DNA binding as well as interactions with the basal machinery, thereby enhancing activator-dependent transcription.

Drosophila Jun mediates Ras-dependent photoreceptor determination

The expression of the D. melanogaster transcription factor Jun in the eye imaginal disc correlates temporally and spatially with the determination of neuronal photoreceptor fate. Expression of dominant negative forms of Jun in photoreceptor precursor cells results in dose-dependent loss of photoreceptors in the adult fly. Conversely, localized overexpression of Jun in the eye imaginal disc can induce the differentiation of additional photoreceptor cells. Furthermore, the transformation of nonneuronal cone cells into R7 neurons elicited by constitutively active forms of sevenless, Ras1, Raf, and MAP kinase is relieved in the presence of Jun mutants. These results demonstrate a requirement of Jun downstream of the sevenless/ras signaling pathway for neuronal development in the Drosophila eye.

Confrontation of scabrous expressing and non-expressing cells is essential for normal ommatidial spacing in the Drosophila eye

The establishment of neural precursor cells in Drosophila depends on cell-cell interactions and lateral inhibition. Scabrous (sca) is involved in this process by preventing an excess of cells from adopting a neural precursor fate. Specifically in eye development, Sca protein function has been implicated in the spacing pattern that is essential for the ordered appearance of the ommatidial array. During this process sca expression is restricted to neurogenic groups of cells and later to the neural precursors. We report that ectopic sca expression in the morphogenetic furrow results in a rough eye phenotype with oversized and fused ommatidia. These defects in adult eyes are due to the generation of too many ommatidial preclusters in the morphogenetic furrow. Strikingly, sca loss-of-function mutants have an almost identical phenotype. Our results suggest that Sca plays a positive role in establishing the spacing pattern within the furrow and that the quantitative difference in sca expression between neighboring groups of cells is a determining factor in this process. Ectopic expression of Sca also represses endogenous sca expression in the furrow, suggesting that Sca is involved in a feedback loop affecting its own transcription. Interestingly, sca shares homology to a group of extracellular matrix proteins that have been implicated in neuronal differentiation. We present a model for sca function based on its phenotypic and molecular features.

Ectopic expression of seven-up causes cell fate changes during ommatidial assembly

During Drosophila ommatidial development, a single cell is selected within the ommatidial cluster to become the R7 photoreceptor neuron. The seven-up gene has been shown to play a role in this process by preventing four other photoreceptor precursors, R3/R4/R1/R6, from adopting the R7 cell fate. The seven-up gene encodes a steroid receptor-like molecule that is expressed only in those four cells that require seven-up function in the developing Drosophila ommatidium. We have examined the functional significance of the spatially restricted expression of seven-up by misexpressing seven-up isoforms. As expected from the function that seven-up performs in R3/R4/R1/R6, ubiquitous expression of seven-up causes transformation of the R7 cell to an R1-R6 cell fate. In addition, depending on the timing and spatial pattern of expression, various other phenotypes are produced including the loss of the R7 cell and the formation of extra R7 cells. Ubiquitous expression of seven-up close to the morphogenetic furrow interferes with R8 differentiation resulting in failure to express the boss protein, the ligand for the sevenless receptor tyrosine kinase, and the R7 cell is lost consequently. Extra R7 cells are formed by recruiting non-neuronal cone cells as photoreceptor neurons in a sevenless and bride of sevenless independent way. Thus, the spatiotemporal pattern of seven-up expression plays an essential role in controlling the number and cellular origin of the R7 neuron in the ommatidium. Our results also suggest that seven-up controls decisions not only between photoreceptor subtypes, but also between neuronal and non-neuronal fates.