Control of cell fate determination by p21ras/Ras1, an essential component of torso signaling in Drosophila

Dynamics of an incoherent feedforward loop drive ERK-dependent pattern formation in the early Drosophila embryo

Positional information in development often manifests as stripes of gene expression, but how stripes form remains incompletely understood. Here, we use optogenetics and live-cell biosensors to investigate the posterior brachyenteron (byn) stripe in early Drosophila embryos. This stripe depends on interpretation of an upstream ERK activity gradient and the expression of two target genes, tailless (tll) and huckebein (hkb), that exert antagonistic control over byn. We find that high or low doses of ERK signaling produce transient or sustained byn expression, respectively. Although tll transcription is always rapidly induced, hkb converts graded ERK inputs into a variable time delay. Nuclei thus interpret ERK amplitude through the relative timing of tll and hkb transcription. Antagonistic regulatory paths acting on different timescales are hallmarks of an incoherent feedforward loop, which is sufficient to explain byn dynamics and adds temporal complexity to the steady-state model of byn stripe formation. We further show that 'blurring' of an all-or-none stimulus through intracellular diffusion non-locally produces a byn stripe. Overall, we provide a blueprint for using optogenetics to dissect developmental signal interpretation in space and time.

Dynamics of an incoherent feedforward loop drive ERK-dependent pattern formation in the early Drosophila embryo

Positional information in developing tissues often takes the form of stripes of gene expression that mark the boundaries of a particular cell type or morphogenetic process. How stripes form is still in many cases poorly understood. Here we use optogenetics and live-cell biosensors to investigate one such pattern: the posterior stripe of brachyenteron (byn) expression in the early Drosophila embryo. This byn stripe depends on interpretation of an upstream signal - a gradient of ERK kinase activity - and the expression of two target genes tailless (tll) and huckebein (hkb) that exert antagonistic control over byn . We find that high or low doses of ERK signaling produce either transient or sustained byn expression, respectively. These ERK stimuli also regulate tll and hkb expression with distinct dynamics: tll transcription is rapidly induced under both low and high stimuli, whereas hkb transcription converts graded ERK inputs into an output switch with a variable time delay. Antagonistic regulatory paths acting on different timescales are hallmarks of an incoherent feedforward loop architecture, which is sufficient to explain transient or sustained byn dynamics and adds temporal complexity to the steady-state model of byn stripe formation. We further show that an all-or-none stimulus can be 'blurred' through intracellular diffusion to non-locally produce a stripe of byn gene expression. Overall, our study provides a blueprint for using optogenetic inputs to dissect developmental signal interpretation in space and time.

Cross-species identification of PIP5K1-, splicing- and ubiquitin-related pathways as potential targets for RB1-deficient cells

The RB1 tumor suppressor is recurrently mutated in a variety of cancers including retinoblastomas, small cell lung cancers, triple-negative breast cancers, prostate cancers, and osteosarcomas. Finding new synthetic lethal (SL) interactions with RB1 could lead to new approaches to treating cancers with inactivated RB1. We identified 95 SL partners of RB1 based on a Drosophila screen for genetic modifiers of the eye phenotype caused by defects in the RB1 ortholog, Rbf1. We validated 38 mammalian orthologs of Rbf1 modifiers as RB1 SL partners in human cancer cell lines with defective RB1 alleles. We further show that for many of the RB1 SL genes validated in human cancer cell lines, low activity of the SL gene in human tumors, when concurrent with low levels of RB1 was associated with improved patient survival. We investigated higher order combinatorial gene interactions by creating a novel Drosophila cancer model with co-occurring Rbf1, Pten and Ras mutations, and found that targeting RB1 SL genes in this background suppressed the dramatic tumor growth and rescued fly survival whilst having minimal effects on wild-type cells. Finally, we found that drugs targeting the identified RB1 interacting genes/pathways, such as UNC3230, PYR-41, TAK-243, isoginkgetin, madrasin, and celastrol also elicit SL in human cancer cell lines. In summary, we identified several high confidence, evolutionarily conserved, novel targets for RB1-deficient cells that may be further adapted for the treatment of human cancer.

Optogenetic Rescue of a Patterning Mutant

Animal embryos are patterned by a handful of highly conserved inductive signals. Yet, in most cases, it is unknown which pattern features (i.e., spatial gradients or temporal dynamics) are required to support normal development. An ideal experiment to address this question would be to "paint" arbitrary synthetic signaling patterns on "blank canvas" embryos to dissect their requirements. Here, we demonstrate exactly this capability by combining optogenetic control of Ras/extracellular signal-related kinase (ERK) signaling with the genetic loss of the receptor tyrosine-kinase-driven terminal signaling patterning in early Drosophila embryos. Blue-light illumination at the embryonic termini for 90 min was sufficient to rescue normal development, generating viable larvae and fertile adults from an otherwise lethal terminal signaling mutant. Optogenetic rescue was possible even using a simple, all-or-none light input that reduced the gradient of Erk activity and eliminated spatiotemporal differences in terminal gap gene expression. Systematically varying illumination parameters further revealed that at least three distinct developmental programs are triggered at different signaling thresholds and that the morphogenetic movements of gastrulation are robust to a 3-fold variation in the posterior pattern width. These results open the door to controlling tissue organization with simple optical stimuli, providing new tools to probe natural developmental processes, create synthetic tissues with defined organization, or directly correct the patterning errors that underlie developmental defects.

Migrating cells control morphogenesis of substratum serving as track to promote directional movement of the collective

In Drosophila embryos, caudal visceral mesoderm (CVM) cells undergo bilateral migration along the trunk visceral mesoderm (TVM) in order to form midgut muscles. Mutation of FGF receptor Heartless (Htl) has been shown to cause CVM migration defects, particularly midline crossing of the bilateral groups. Here, we show that htl mutants also exhibit TVM defects including contralateral merging. Both CVM mismigration and TVM contralateral merging are attenuated by restoring FGF signaling specifically in the CVM, suggesting that migrating CVM cells influence TVM morphogenesis; however, the inverse, supplying FGF to the TVM, does not rescue CVM mismigration. In addition, we show that FGF regulates integrin expression in both tissues, but only providing a source of integrin specifically to the TVM attenuates the contralateral merging phenotype. Finally, we demonstrate that the CVM influences cell shape in the TVM, and a loss of CVM results in TVM morphological defects. In summary, this study provides insight into how a migrating collective of cells can influence their tissue substrate and supports the view that morphogenesis of tissues during development is interdependent.

Genome-Wide Screen for New Components of the Drosophila melanogaster Torso Receptor Tyrosine Kinase Pathway

Patterning of the Drosophila embryonic termini by the Torso (Tor) receptor pathway has long served as a valuable paradigm for understanding how receptor tyrosine kinase signaling is controlled. However, the mechanisms that underpin the control of Tor signaling remain to be fully understood. In particular, it is unclear how the Perforin-like protein Torso-like (Tsl) localizes Tor activity to the embryonic termini. To shed light on this, together with other aspects of Tor pathway function, we conducted a genome-wide screen to identify new pathway components that operate downstream of Tsl. Using a set of molecularly defined chromosomal deficiencies, we screened for suppressors of ligand-dependent Tor signaling induced by unrestricted Tsl expression. This approach yielded 59 genomic suppressor regions, 11 of which we mapped to the causative gene, and a further 29 that were mapped to <15 genes. Of the identified genes, six represent previously unknown regulators of embryonic Tor signaling. These include twins (tws), which encodes an integral subunit of the protein phosphatase 2A complex, and α-tubulin at 84B (αTub84B), a major constituent of the microtubule network, suggesting that these may play an important part in terminal patterning. Together, these data comprise a valuable resource for the discovery of new Tor pathway components. Many of these may also be required for other roles of Tor in development, such as in the larval prothoracic gland where Tor signaling controls the initiation of metamorphosis.

Illuminating developmental biology through photochemistry

Developmental biology has been continually shaped by technological advances, evolving from a descriptive science into one immersed in molecular and cellular mechanisms. Most recently, genome sequencing and 'omics' profiling have provided developmental biologists with a wealth of genetic and biochemical information; however, fully translating this knowledge into functional understanding will require new experimental capabilities. Photoactivatable probes have emerged as particularly valuable tools for investigating developmental mechanisms, as they can enable rapid, specific manipulations of DNA, RNA, proteins, and cells with spatiotemporal precision. In this Perspective, we describe optochemical and optogenetic systems that have been applied in multicellular organisms, insights gained through the use of these probes, and their current limitations. We also suggest how chemical biologists can expand the reach of photoactivatable technologies and bring new depth to our understanding of organismal development.

Reconstitution of Torso signaling in cultured cells suggests a role for both Trunk and Torso-like in receptor activation

Formation of the Drosophila embryonic termini is controlled by the localized activation of the receptor tyrosine kinase Torso. Both Torso and Torso's presumed ligand, Trunk, are expressed uniformly in the early embryo. Polar activation of Torso requires Torso-like, which is expressed by follicle cells adjacent to the ends of the developing oocyte. We find that Torso expressed at high levels in cultured Drosophila cells is activated by individual application of Trunk, Torso-like or another known Torso ligand, Prothoracicotropic Hormone. In addition to assays of downstream signaling activity, Torso dimerization was detected using bimolecular fluorescence complementation. Trunk and Torso-like were active when co-transfected with Torso and when presented to Torso-expressing cells in conditioned medium. Trunk and Torso-like were also taken up from conditioned medium specifically by cells expressing Torso. At low levels of Torso, similar to those present in the embryo, Trunk and Torso-like alone were ineffective but acted synergistically to stimulate Torso signaling. Our results suggest that Torso interacts with both Trunk and Torso-like, which cooperate to mediate dimerization and activation of Torso at the ends of the Drosophila embryo.

Receptor tyrosine kinases in Drosophila development

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

Essential role for Ptpn11 in survival of hematopoietic stem and progenitor cells

Src homology 2 domain-containing phosphatase 2 (Shp2), encoded by Ptpn11, is a member of the nonreceptor protein-tyrosine phosphatase family, and functions in cell survival, proliferation, migration, and differentiation in many tissues. Here we report that loss of Ptpn11 in murine hematopoietic cells leads to bone marrow aplasia and lethality. Mutant mice show rapid loss of hematopoietic stem cells (HSCs) and immature progenitors of all hematopoietic lineages in a gene dosage-dependent and cell-autonomous manner. Ptpn11-deficient HSCs and progenitors undergo apoptosis concomitant with increased Noxa expression. Mutant HSCs/progenitors also show defective Erk and Akt activation in response to stem cell factor and diminished thrombopoietin-evoked Erk activation. Activated Kras alleviates the Ptpn11 requirement for colony formation by progenitors and cytokine/growth factor responsiveness of HSCs, indicating that Ras is functionally downstream of Shp2 in these cells. Thus, Shp2 plays a critical role in controlling the survival and maintenance of HSCs and immature progenitors in vivo.

The role of receptor tyrosine kinases in primordial germ cell migration

During embryonic development in Drosophila, rodents, and other organisms, primordial germ cells (PGCs) migrate from their points of origin to the nascent gonads, where they give rise to germ line stem cells. Receptor tyrosine kinase (RTK) activity is required for normal migration of primordial germ cells in both Drosophila and rodents. In this chapter, we discuss in vivo as well as in vitro methods which have been used to elucidate the role of the RTK Torso in Drosophila germ cell migration. Included are protocols for embryo collection, fixation, and immunostaining; the dominant female sterile technique; in vitro culture and observation of PGCs; pole cell transplantation; and labeling of pole cells for in vivo observation.

Small G protein signaling in neuronal plasticity and memory formation: the specific role of ras family proteins

Small G proteins are an extensive family of proteins that bind and hydrolyze GTP. They are ubiquitous inside cells, regulating a wide range of cellular processes. Recently, many studies have examined the role of small G proteins, particularly the Ras family of G proteins, in memory formation. Once thought to be primarily involved in the transduction of a variety of extracellular signals during development, it is now clear that Ras family proteins also play critical roles in molecular processing underlying neuronal and behavioral plasticity. We here review a number of recent studies that explore how the signaling of Ras family proteins contributes to memory formation. Understanding these signaling processes is of fundamental importance both from a basic scientific perspective, with the goal of providing mechanistic insights into a critical aspect of cognitive behavior, and from a clinical perspective, with the goal of providing effective therapies for a range of disorders involving cognitive impairments.

Control of oligodendrocyte generation and proliferation by Shp2 protein tyrosine phosphatase

Extracellular signals play essential roles in controlling the proliferation and differentiation of oligodendrocyte progenitor cells in the developing central nervous system. However, the intracellular pathways that transduce these extrinsic signals remain to be elucidated. In this study, we showed that conditional ablation of the nonreceptor tyrosine phosphatase Shp2 in Olig1-expressing oligodendrocyte lineage resulted in dramatic reduction in the generation and proliferation of oligodendrocyte progenitor cells in the spinal cord. Maturation and myelination of oligodendrocytes were also compromised in the Shp2 mutants. The deficits in oligodendrocyte development in Shp2 mutants nearly phenocopied those observed in PDGF-A mutants, suggesting that Shp2 is a crucial component in transducing PDGF-A signals in the control of oligodendrocyte proliferation and maturation.

Functions and mechanisms of receptor tyrosine kinase Torso signaling: lessons from Drosophila embryonic terminal development

The Torso receptor tyrosine kinase (RTK) is required for cell fate specification in the terminal regions (head and tail) of the early Drosophila embryo. Torso contains a split tyrosine kinase domain and belongs to the type III subgroup of the RTK superfamily that also includes the platelet-derived growth factor receptors, stem cell or steel factor receptor c-Kit proto-oncoprotein, colony-stimulating factor-1 receptor, and vascular endothelial growth factor receptor. The Torso pathway has been a model system for studying RTK signal transduction. Genetic and biochemical studies of Torso signaling have provided valuable insights into the biological functions and mechanisms of RTK signaling during early Drosophila embryogenesis.

Shp2 regulates SRC family kinase activity and Ras/Erk activation by controlling Csk recruitment

The protein-tyrosine phosphatase Shp2 plays an essential role in growth factor and integrin signaling, and Shp2 mutations cause developmental defects and/or malignancy. Previous work has placed Shp2 upstream of Ras. However, the mechanism of Shp2 action and its substrate(s) are poorly defined. Additional Shp2 functions downstream of, or parallel to, Ras/Erk activation also are proposed. Here, we show that Shp2 promotes Src family kinase (SFK) activation by regulating the phosphorylation of the Csk regulator PAG/Cbp, thereby controlling Csk access to SFKs. In Shp2-deficient cells, SFK inhibitory C-terminal tyrosines are hyperphosphorylated, and the tyrosyl phosphorylation of multiple SFK substrates, including Plcgamma1, is decreased. Decreased Plcgamma1 phosphorylation leads to defective Ras activation on endomembranes, and may help account for impaired Erk activation in Shp2-deficient cells. Decreased phosphorylation/activation of other SFK substrates may explain additional consequences of Shp2 deficiency, including altered cell spreading, stress fibers, focal adhesions, and motility.

Reciprocal interactions between neurons and glia are required for Drosophila peripheral nervous system development

A major developmental role of peripheral glia is to mediate sensory axon guidance; however, it is not known whether sensory neurons influence peripheral glial development. To determine whether glia and neurons reciprocally interact during embryonic development, we ablated each cell type by overexpressing the apoptosis gene, grim, and observed the effects on peripheral nervous system (PNS) development. When neurons are ablated, glial defects occur as a secondary effect, and vice versa. Therefore glia and neurons are codependent during embryogenesis. To further explore glial-neuronal interactions, we genetically disrupted glial migration or differentiation and observed the secondary effects on sensory neuron development. Glial migration and ensheathment of PNS axons was blocked by overexpression of activated Rho GTPase, a regulator of actin dynamics. Here, sensory axons extended to the CNS without exhibiting gross pathfinding errors. In contrast, disrupting differentiation by expression of dominant-negative Ras GTPase in glia resulted in major sensory axon pathfinding errors, similar to those seen in glial ablations. Glial overexpression of transgenic components of the epidermal growth factor receptor (EGFR) signaling pathway yielded similar sensory neuron defects and also downregulated the expression of the glial marker Neuroglian. Mutant analysis also suggested that the EGFR ligands Spitz and Vein play roles in peripheral glial development. The observations support a model in which glia express genes necessary for sensory neuron development, and these genes are potentially under the control of the EGFR/Ras signaling pathway.

Drosophila gain-of-function mutant RTK torso triggers ectopic Dpp and STAT signaling

Overactivation of receptor tyrosine kinases (RTKs) has been linked to tumorigenesis. To understand how a hyperactivated RTK functions differently from wild-type RTK, we conducted a genome-wide systematic survey for genes that are required for signaling by a gain-of-function mutant Drosophila RTK Torso (Tor). We screened chromosomal deficiencies for suppression of a gain-of-function mutation tor (tor(GOF)), which led to the identification of 26 genomic regions that, when in half dosage, suppressed the defects caused by tor(GOF). Testing of candidate genes in these regions revealed many genes known to be involved in Tor signaling (such as those encoding the Ras-MAPK cassette, adaptor and structural molecules of RTK signaling, and downstream target genes of Tor), confirming the specificity of this genetic screen. Importantly, this screen also identified components of the TGFbeta (Dpp) and JAK/STAT pathways as being required for Tor(GOF) signaling. Specifically, we found that reducing the dosage of thickveins (tkv), Mothers against dpp (Mad), or STAT92E (aka marelle), respectively, suppressed tor(GOF) phenotypes. Furthermore, we demonstrate that in tor(GOF) embryos, dpp is ectopically expressed and thus may contribute to the patterning defects. These results demonstrate an essential requirement of noncanonical signaling pathways for a persistently activated RTK to cause pathological defects in an organism.

A Ral guanine exchange factor-Ral pathway is conserved in Drosophila melanogaster and sheds new light on the connectivity of the Ral, Ras, and Rap pathways

Ras GTPases are central to many physiological and pathological signaling pathways and act via a combination of effectors. In mammals, at least three Ral exchange factors (RalGEFs) contain a Ras association domain and constitute a discrete subgroup of Ras effectors. Despite their ability to bind activated Rap as well as activated Ras, they seem to act downstream of Ras but not downstream of Rap. We have revisited the Ras/Rap-Ral connections in Drosophila melanogaster by using iterative two-hybrid screens with these three GTPases as primary baits and a subsequent genetic approach. We show that (i) the Ral-centered protein network appears to be extremely conserved in human and flies, (ii) in this network, RGL is a functional Drosophila orthologue of RalGEFs, and (iii) the RGL-Ral pathway functionally interacts with both the Ras and Rap pathways. Our data do not support the paradigmatic model where Ral is in the effector pathway of Ras. They reveal a signaling circuitry where Ral is functionally downstream of the Rap GTPase, at odds with the pathways described for mammalian cell lines. Thus, in vivo data show variations in the connectivity of pathways described for cell lines which might display only a subset of the biological possibilities.

Nuclear import of activated D-ERK by DIM-7, an importin family member encoded by the gene moleskin

The initiation of gene expression in response to Drosophila receptor tyrosine kinase signaling requires the nuclear import of the MAP kinase, D-ERK. However, the molecular details of D-ERK translocation are largely unknown. In this regard, we have identified D-Importin-7 (DIM-7), the Drosophila homolog of vertebrate importin 7, and its gene moleskin. DIM-7 exhibits a dynamic nuclear localization pattern that overlaps the spatial and temporal profile of nuclear, activated D-ERK. Co-immunoprecipitation experiments show that DIM-7 associates with phosphorylated D-ERK in Drosophila S2 cells. Furthermore, moleskin mutations enhance hypomorphic and suppress hypermorphic D-ERK mutant phenotypes. Deletion or mutation of moleskin dramatically reduces the nuclear localization of activated D-ERK. Directly linking DIM-7 to its nuclear import, this defect can be rescued by the expression of wild-type DIM-7. Mutations in the Drosophila Importin (β) homolog Ketel, also reduce the nuclear localization of activated D-ERK. Together, these data indicate that DIM-7 and Ketel are components of the nuclear import machinery for activated D-ERK.

Targeted genomic disruption of H-ras and N-ras, individually or in combination, reveals the dispensability of both loci for mouse growth and development

Mammalian cells harbor three highly homologous and widely expressed members of the ras family (H-ras, N-ras, and K-ras), but it remains unclear whether they play specific or overlapping cellular roles. To gain insight into such functional roles, here we generated and analyzed H-ras null mutant mice, which were then also bred with N-ras knockout animals to ascertain the viability and properties of potential double null mutations in both loci. Mating among heterozygous H-ras(+/-) mice produced H-ras(-/-) offspring with a normal Mendelian pattern of inheritance, indicating that the loss of H-ras did not interfere with embryonic and fetal viability in the uterus. Homozygous mutant H-ras(-/-) mice reached sexual maturity at the same age as their littermates, and both males and females were fertile. Characterization of lymphocyte subsets in the spleen and thymus showed no significant differences between wild-type and H-ras(-/-) mice. Analysis of neuronal markers in the brains of knockout and wild-type H-ras mice showed that disruption of this locus did not impair or alter neuronal development. Breeding between our H-ras mutant animals and previously available N-ras null mutants gave rise to viable double knockout (H-ras(-/-)/N-ras(-/-)) offspring expressing only K-ras genes which grew normally, were fertile, and did not show any obvious phenotype. Interestingly, however, lower-than-expected numbers of adult, double knockout animals were consistently obtained in Mendelian crosses between heterozygous N-ras/H-ras mice. Our results indicate that, as for N-ras, H-ras gene function is dispensable for normal mouse development, growth, fertility, and neuronal development. Additionally, of the three ras genes, K-ras appears to be not only essential but also sufficient for normal mouse development.

Identification of genomic regions that interact with a viable allele of the Drosophila protein tyrosine phosphatase corkscrew

Signaling by receptor tyrosine kinases (RTKs) is critical for a multitude of developmental decisions and processes. Among the molecules known to transduce the RTK-generated signal is the nonreceptor protein tyrosine phosphatase Corkscrew (Csw). Previously, Csw has been demonstrated to function throughout the Drosophila life cycle and, among the RTKs tested, Csw is essential in the Torso, Sevenless, EGF, and Breathless/FGF RTK pathways. While the biochemical function of Csw remains to be unambiguously elucidated, current evidence suggests that Csw plays more than one role during transduction of the RTK signal and, further, the molecular mechanism of Csw function differs depending upon the RTK in question. The isolation and characterization of a new, spontaneously arising, viable allele of csw, csw(lf), has allowed us to undertake a genetic approach to identify loci required for Csw function. The rough eye and wing vein gap phenotypes exhibited by adult flies homo- or hemizygous for csw(lf) has provided a sensitized background from which we have screened a collection of second and third chromosome deficiencies to identify 33 intervals that enhance and 21 intervals that suppress these phenotypes. We have identified intervals encoding known positive mediators of RTK signaling, e.g., drk, dos, Egfr, E(Egfr)B56, pnt, Ras1, rolled/MAPK, sina, spen, Src64B, Star, Su(Raf)3C, and vein, as well as known negative mediators of RTK signaling, e.g., aos, ed, net, Src42A, sty, and su(ve). Of particular interest are the 5 lethal enhancing intervals and 14 suppressing intervals for which no candidate genes have been identified.

Calmodulin and son of sevenless dependent signaling pathways regulate midline crossing of axons in the Drosophila CNS

The establishment of axon trajectories is ultimately determined by the integration of intracellular signaling pathways. Here, a genetic approach in Drosophila has demonstrated that both Calmodulin and Son of sevenless signaling pathways are used to regulate which axons cross the midline. A loss in either signaling pathway leads to abnormal projection of axons across the midline and these increase with roundabout or slit mutations. When both Calmodulin and Son of sevenless are disrupted, the midline crossing of axons mimics that seen in roundabout mutants, although Roundabout remains expressed on crossing axons. Calmodulin and Son of sevenless also regulate axon crossing in a commissureless mutant. These data suggest that Calmodulin and Son of sevenless signaling pathways function to interpret midline repulsive cues which prevent axons crossing the midline.

Ras1 promotes cellular growth in the Drosophila wing

The Ras GTPase links extracellular mitogens to intracellular mechanisms that control cell proliferation. To understand how Ras regulates proliferation in vivo, we activated or inactivated Ras in cell clones in the developing Drosophila wing. Cells lacking Ras were smaller, had reduced growth rates, accumulated in G1, and underwent apoptosis due to cell competition. Conversely, activation of Ras increased cell size and growth rates and promoted G1/S transitions. Ras upregulated the growth driver dMyc, and both Ras and dMyc increased levels of cyclin E posttranscriptionally. We propose that Ras primarily promotes growth and that growth is coupled to G1/S progression via cyclin E. Interestingly, upregulation of growth by Ras did not deregulate G2/M progression or a developmentally regulated cell cycle exit.

Genetic analysis of rolled, which encodes a Drosophila mitogen-activated protein kinase

Genetic and molecular characterization of the dominant suppressors of D-raf(C110) on the second chromosome identified two gain-of-function alleles of rolled (rl), which encodes a mitogen-activated protein (MAP) kinase in Drosophila. One of the alleles, rl(Su23), was found to bear the same molecular lesion as rl(Sem), which has been reported to be dominant female sterile. However, rl(Su23) and the current stock of rl(Sem) showed only a weak dominant female sterility. Detailed analyses of the rl mutations demonstrated moderate dominant activities of these alleles in the Torso (Tor) signaling pathway, which explains the weak dominant female sterility observed in this study. The dominant rl mutations failed to suppress the terminal class maternal-effect mutations, suggesting that activation of Rl is essential, but not sufficient, for Tor signaling. Involvement of rl in cell proliferation was also demonstrated by clonal analysis. Branching and integration of signals in the MAP kinase cascade is discussed.

Signal transduction pathway for anterior-posterior development in Drosophila

In Drosophila, the establishment of embryonic polarity along the anterior-posterior axis of the egg is determined by the activity of maternal gene products that accumulate during oogenesis. Amongst these are the Bicoid, the Nanos, and the terminal class gene products, some of which are oncoproteins involved in signal transduction for the formation of terminal structures in the embryo. Several signal transduction pathways have been described in Drosophila, and this review explores the potential of oncogene studies using one of those pathways - the terminal class signal transduction pathway - to better understand the cellular mechanisms of proto-oncogenes that mediate cellular responses in vertebrates including humans.

Complementation of defective colony-stimulating factor 1 receptor signaling and mitogenesis by Raf and v-Src

Ras-activated signal transduction pathways are implicated in the control of cell proliferation, differentiation, apoptosis, and tumorigenesis, but the molecular mechanisms mediating these diverse functions have yet to be fully elucidated. Conditionally active forms of Raf, v-Src, and MEK1 were used to identify changes in gene expression that participate in oncogenic transformation, as well as in normal growth control. Activation of Raf, v-Src, and MEK1 led to induced expression of c-Myc and cyclin D1. Induction of c-Myc mRNA by Raf was an immediate-early response, whereas the induction of cyclin D1 mRNA was delayed and inhibited by cycloheximide. Raf activation also resulted in the induction of an established c-Myc target gene, ornithine decarboxylase (ODC). ODC induction by Raf was mediated, in part, by tandem E-boxes contained in the first intron of the gene. Activation of the human colony-stimulating factor 1 (CSF-1) receptor in NIH 3T3 cells leads to activation of the mitogen-activated protein (MAP) kinase pathway and induced expression of c-Fos, c-Myc, and cyclin D1, leading to a potent mitogenic response. By contrast, a mutated form of this receptor fails to activate the MAP kinases or induce c-Myc and cyclin D1 expression and fails to elicit a mitogenic response. The biological significance of c-Myc and cyclin D1 induction by Raf and v-Src was confirmed by the demonstration that both of these protein kinases complemented the signaling and mitogenic defects of cells expressing this mutated form of the human CSF-1 receptor. Furthermore, the induction of c-Myc and cyclin D1 by oncogenes and growth factors was inhibited by PD098059, a specific MAP kinase kinase (MEK) inhibitor. These data suggest that the Raf/MEK/MAP kinase pathway plays an important role in the regulation of c-Myc and cyclin D1 expression in NIH 3T3 cells. The ability of oncogenes such as Raf and v-Src to regulate the expression of these proteins reveals new lines of communication between cytosolic signal transducers and the cell cycle machinery.

Participation of small GTPases in dorsal closure of the Drosophila embryo: distinct roles for Rho subfamily proteins in epithelial morphogenesis

The Rho subfamily of Ras-related small GTPases participates in a variety of cellular events including organization of the actin cytoskeleton and signalling by c-Jun N-terminal kinase and p38 kinase cascades. These functions of the Rho subfamily are likely to be required in many developmental events. We have been studying the participation of the RHO subfamily in dorsal closure of the Drosophila embryo, a process involving morphogenesis of the epidermis. We have previously shown that Drac1, a Rho subfamily protein, is required for the presence of an actomyosin contractile apparatus believed to be driving the cell shape changes essential to dorsal closure. Expression of a dominant negative Drac1 transgene causes a loss of this contractile apparatus from the leading edge of the advancing epidermis and dorsal closure fails. We now show that two other Rho subfamily proteins, Dcdc42 and RhoA, as well as Ras1 are also required for dorsal closure. Dcdc42 appears to have conflicting roles during dorsal closure: establishment and/or maintenance of the leading edge cytoskeleton versus its down regulation. Down regulation of the leading edge cytoskeleton may be controlled by the serine/threonine kinase DPAK, a potential Drac1/Dcdc42 effector. RhoA is required for the integrity of the leading edge cytoskeleton specifically in cells flanking the segment borders. We have begun to characterize the interactions of the various small GTPases in regulating dorsal closure and find no evidence for the hierarchy of Rho subfamily activity described in some mammalian cell types. Rather, our results suggest that while all Ρ subfamily p21s tested are required for dorsal closure, they act largely in parallel.

Synergistic activities of multiple phosphotyrosine residues mediate full signaling from the Drosophila Torso receptor tyrosine kinase

Here, we identify four tyrosine residues (Y644, Y698, Y767, and Y772) that become phosphorylated after activation of the Torso (Tor) receptor tyrosine kinase. Previously, we characterized phosphotyrosine sites (P-Y630 and P-Y918). Of the six P-Y sites identified, three (Y630, Y644, and Y698) are located in the kinase domain insert region, one (Y918) is located in the C-terminal tail region, and two (Y767 and Y772) are located in the activation loop of the kinase domain. To investigate the function of each P-Y residue in Tor signaling, we have generated transgenic Drosophila embryos expressing mutant Tor receptors containing either single or multiple tyrosine to phenylalanine substitutions. Single P-Y mutations were found to have either positive, negative, or no effect on the signaling activity of the receptor. Elimination of all P-Y sites within the kinase insert region resulted in the complete loss of receptor function, indicating that some combination of these sites is necessary for Tor signaling. Mutation of the C-terminal P-Y918 site revealed that this site is responsible for negative signaling or down-regulation of receptor activity. Mutation of the P-Y sites in the kinase domain activation loop demonstrated that these sites are essential for enzymatic activity. Our analysis provides a detailed in vivo example of the extent of cooperativity between P-Y residues in transducing the signal received by a receptor tyrosine kinase and in vivo data demonstrating the function of P-Y residues in the activation loop of the kinase domain.

Activation of the Drosophila C3G leads to cell fate changes and overproliferation during development, mediated by the RAS-MAPK pathway and RAP1

The cellular signal transduction pathways by which C3G, a RAS family guanine nucleotide exchange factor, mediates v-crk transformation are not well understood. Here we report the identification of Drosophila C3G, which, like its human cognate, specifically binds to CRK but not DRK/GRB2 adaptor molecules. During Drosophila development, constitutive membrane binding of C3G, which also occurs during v-crk transformation, results in cell fate changes and overproliferation, mimicking overactivity of the RAS-MAPK pathway. The effects of C3G overactivity can be suppressed by reducing the gene dose of components of the RAS-MAPK pathway and of RAP1. These findings provide the first in vivo evidence that membrane localization of C3G can trigger activation of RAP1 and RAS resulting in the activation of MAPK, one of the hallmarks of v-crk transformation previously thought to be mediated through activation of SOS.

Opposing actions of CSW and RasGAP modulate the strength of Torso RTK signaling in the Drosophila terminal pathway

In Drosophila, specification of embryonic terminal cells is controlled by the Torso receptor tyrosine kinase. Here, we analyze the molecular basis of positive (Y630) and negative (Y918) phosphotyrosine (pY) signaling sites on Torso. We find that the Drosophila homolog of RasGAP associates with pY918 and is a negative effector of Torso signaling. Further, we show that the tyrosine phosphatase Corkscrew (CSW), which associates with pY630, specifically dephosphorylates the negative pY918 Torso signaling site, thus identifying Torso to be a substrate of CSW in the terminal pathway. CSW also serves as an adaptor protein for DRK binding, physically linking Torso to Ras activation. The opposing actions of CSW and RasGAP modulate the strength of the Torso signal, contributing to the establishment of precise boundaries for terminal structure development.

Cellularization in Drosophila melanogaster is disrupted by the inhibition of rho activity and the activation of Cdc42 function

Regulation of cytoskeletal dynamics is essential for cell shape change and morphogenesis. Drosophila melanogaster embryos offer a well-defined system for observing alterations in the cytoskeleton during the process of cellularization, a specialized form of cytokinesis. During cellularization, the actomyosin cytoskeleton forms a hexagonal array and drives invagination of the plasma membrane between the nuclei located at the cortex of the syncytial blastoderm. Rho, Rac, and Cdc42 proteins are members of the Rho subfamily of Ras-related G proteins that are involved in the formation and maintenance of the actin cytoskeleton throughout phylogeny and in D. melanogaster. To investigate how Rho subfamily activity affects the cytoskeleton during cellularization stages, embryos were microinjected with C3 exoenzyme from Clostridium botulinum or with wild-type, constitutively active, or dominant negative versions of Rho, Rac, and Cdc42 proteins. C3 exoenzyme ADP-ribosylates and inactivates Rho with high specificity, whereas constitutively active dominant mutations remain in the activated GTP-bound state to activate downstream effectors. Dominant negative mutations likely inhibit endogenous small G protein activity by sequestering exchange factors. Of the 10 agents microinjected, C3 exoenzyme, constitutively active Cdc42, and dominant negative Rho have a specific and indistinguishable effect: the actomyosin cytoskeleton is disrupted, cellularization halts, and embryogenesis arrests. Time-lapse video records of DIC imaged embryos show that nuclei in injected regions move away from the cortex of the embryo, thereby phenocopying injections of cytochalasin or antimyosin. Rhodamine phalloidin staining reveals that the actin-based hexagonal array normally seen during cellularization is disrupted in a dose-dependent fashion. Additionally, DNA stain reveals that nuclei in the microinjected embryos aggregate in regions that correspond to actin disruption. These embryos halt in cellularization and do not proceed to gastrulation. We conclude that Rho activity and Cdc42 regulation are required for cytoskeletal function in actomyosin-driven furrow canal formation and nuclear positioning.

Ras promotes cell survival in Drosophila by downregulating hid expression

Activation of Ras inhibits apoptosis during Drosophila development. Genetic evidence indicates that Ras antiapoptotic activity in the developing eye is regulated by the Drosophila EGF receptor and operates through the Raf/MAPK pathway. Decreased activity of this pathway enhances, and increased activity suppresses, apoptosis induced by ectopic expression of the cell death regulators reaper (rpr) and head involution defective (hid). In addition, ectopic activation of the Ras/MAPK pathway in the developing embryo and in the developing eye suppresses naturally occurring apoptosis and regulates the transcription of the proapoptotic gene hid. Null alleles of hid recapitulate the antiapoptotic activities of Ras/MAPK, providing genetic evidence that downregulation of hid is an important mechanism by which Ras promotes survival.

The Drosophila protein Dof is specifically required for FGF signaling

Receptor tyrosine kinases (RTKs) transmit signals to the cell nucleus via the MAP kinase (MAPK) cascade, using specific molecules to link the activated receptors to the MAPK cascade activator, Ras. We have identified a component of the FGF receptor (FGFR) signal transduction pathway, Downstream of FGFR (Dof). Dof is an intracellular protein that is essential for signal transmission by the FGFR and acts downstream of the receptor and upstream of Ras. Unlike other signaling molecules that act downstream of RTKs, Dof is not expressed ubiquitously but is present exclusively in cells that express FGFRs. Dof is needed in these cells for activation of the MAPK cascade via FGF signaling, but not for activation via other RTK ligands. Dof therefore appears to be committed exclusively to FGFR-mediated signal transduction.

Promoter sequence and expression of the leucine-rich repeat gene LRR47: evidence for cytoplasmic and nuclear localization in Drosophila embryos and cells

In Drosophila, proteins containing leucine-rich repeats (LRR) play diverse roles during embryonic development. In particular, they function in cell adhesion and cellular signalling and have in common the ability to mediate reversible protein-protein interactions. The sequence and chromosomal location of Drosophila LRR47, which encodes a protein with eight LRR repeats, were reported previously. In this paper the 5' flanking region of the LRR47 gene is described and the initiation point for the maternal transcription unit is defined. LRR47 belongs to a subfamily of LRR proteins that have in common the ability to interact with ras GTPase. Whole-mount in situ hybridization studies show that the LRR47 transcript is uniformly distributed in early cleavage embryos but becomes depleted at the termini by the blastoderm stage. There is a specific requirement for ras function in the embryonic termini at this developmental stage. The distribution of LRR47 protein in embryos and tissue culture cells was also studied. The protein is present in both the cytoplasm and nuclei of embryos until gastrulation and is seen to persist in the nuclei of the amnioserosa until later stages of development. The protein is also constitutively present in growing SL2 culture cells and again is present in both cytoplasm and nuclei. These results suggest that LRR47 function may be modulated in the cell or nuclear division cycle.

MAP kinase-dependent pathways in cell cycle control

Mitogen-activated protein kinases such as Erk1 and Erk2 serve as a paradigm for a growing family of proline-directed protein kinases that mediate entry, progression and exit from the cell cycle in diverse eukaryotic cells. These enzymes function within highly conserved modules of sequentially activating protein kinases that transduce signals from diverse extracellular stimuli. In vertebrates, at least three distinct kinases modules have been characterized. Mitogens induce the sequential activation of the kinases Raf1-->Mek1-->Erk2-->Rsk via the G-protein Ras. Stress factors stimulate c-Jun activation through a related kinase pathway involving Mekk-->Sek-->SAPK c-Jun, and hsp27 phosphorylation via the MKK3-->Hog-->MAPKAPK-2 hsp27 route. Genetic and biochemical studies, for example from budding yeast, imply the existence of several related protein kinase modules that can operate in parallel or within integrated systems.

The Caenorhabditis elegans SH2 domain-containing protein tyrosine phosphatase PTP-2 participates in signal transduction during oogenesis and vulval development

Src homology-2 (SH2) domain-containing protein tyrosine phosphatases (SHPs) have been identified as either positive or negative regulators of signaling events downstream of receptor protein tyrosine kinases (R-PTKs). We describe here our characterization of ptp-2, a Caenorhabditis elegans gene that encodes a 668-amino-acid SHP. We isolated a recessive ptp-2 loss-of-function allele, op194, that lacks the conserved protein tyrosine phosphatase catalytic domain by screening for transposon-mediated deletion mutations. Homozygous ptp-2(op194) hermaphrodites exhibit a completely penetrant zygotic semisterile/maternal effect lethal phenotype, characterized by the presence of abnormally large oocytes in the zygotic semisterile animals. These phenotypes indicate that PTP-2 activity is essential for proper oogenesis. Gain-of-function let-60 ras alleles rescued the defects associated with ptp-2(op194), suggesting that LET-60 Ras acts downstream of, or in parallel to, PTP-2 during oogenesis. Although ptp-2 function is not required for normal vulval development, ptp-2(op194) altered significantly the vulval phenotypes caused by mutations in several genes of the inductive signaling pathway. The penetrance of the multivulva phenotype caused by loss-of-function mutations in lin-15, and gain-of-function mutations in let-23 or let-60 ras, was reduced by ptp-2(op194). Moreover, ptp-2(op194) increased the penetrance of the vulvaless phenotype conferred by a weak loss-of-function sem-5 allele. Taken together, our genetic data positions PTP-2 activity downstream of LET-23 in the vulval induction signaling pathway. Although PTP-2 functions to transmit a requisite signal during oogenesis, PTP-2 function during C. elegans vulval cell differentiation appears to be directed at regulating the overall strength of the inductive signal, which may contribute to the quantitative differences in signaling required for the proper specification of the 1 degrees , 2 degrees , and 3 degrees vulval cell fates.

A negative regulatory function for the protein tyrosine phosphatase PTP2C revealed by reconstruction of platelet-derived growth factor receptor signalling in Schizosaccharomyces pombe

We have exploited reconstitution in the fission yeast Schizosaccharomyces pombe to investigate how activation of phospholipase Cgamma (PLCgamma) by the platelet-derived growth factor-beta receptor (PDGFbetaR) is regulated by the SH2 domain-containing protein tyrosine phosphatase PTP2C (also known as SHP-2). When co-expressed in S. pombe, PTP2C abolished PDGFbetaR autophosphorylation as well as its ability to phosphorylate and activate PLCgamma. Inhibition of PDGFbetaR signalling by PTP2C appears specific insofar that PTPIC, a close homologue of PTP2C, does not suppress activation of either PDGFbetaR or PLCgamma. Surprisingly, an inactive PTP2C mutant (C459S), which dephosphorylates neither PDGFbetaR nor PLCgamma, remains fully effective as an inhibitor of [3H]inositol phosphate generation indicating that negative regulation is at least in part independent of catalytic activity. This contrasts with PLCgamma activation by c-Src which, although blocked by active PTP2C, is not inhibited by the mutant PTP2C C459S. These observations indicate that in addition to a reported positive role relaying trophic signals, PTP2C can also exert a negative effect on the PDGFbetaR and its signalling to PLCgamma.

Dissecting the mechanism of torso receptor activation

Regulated activation of receptor tyrosine kinases depends both on the presence of the receptors at the cell surface and on the availability of their ligands. In Drosophila the torso (tor) tyrosine kinase receptor is distributed along the surface of the embryo but it is only activated at the poles by a diffusible extracellular ligand generated at each pole which is trapped by the receptor, thereby impeding further diffusion. However, it is not well understood how this signal is generated, although it is known to depend on the activity of many genes such as torso-like (tsl) and trunk (trk). To further investigate the mechanism involved in the local activation of the tor receptor we have altered the normal expression of the tsl protein by generating females in which the tsl gene is expressed in the oocyte under the control of the tor promoter rather than in the ovarian follicle cells. Analysis of the phenotypes generated by this hybrid gene and its interactions with mutations in other genes in the pathway has enabled us to further dissect the mechanism of tor receptor activation and to define more precisely the role of the different genes acting in this process.

Different levels of Ras activity can specify distinct transcriptional and morphological consequences in early Drosophila embryos

The terminal portions of the Drosophila body pattern are specified by the localized activity of the receptor tyrosine kinase Torso (Tor) at each pole of the early embryo. Tor activity elicits the transcription of two ‘gap’ genes, tailless (tll) and huckebein (hkb), in overlapping but distinct domains by stimulating the Ras signal transduction pathway. Here, we show that quantitative variations in the level of Ras activity can specify qualitatively distinct transcriptional and morphological responses. Low levels of Ras activity at the posterior pole direct tll but not hkb transcription; higher levels drive transcription of both genes. Correspondingly, low levels of Ras activity specify a limited subset of posterior terminal structures, whereas higher levels specify a larger subset. However, we also show that the response to Ras activity is not uniform along the body. Instead, levels of Ras activity which suffice to drive tll and hkb transcription at the posterior pole fail to drive their expression in more central portions of the body, apparently due to repression by other gap gene products. We conclude that tll and hkb transcription, as well as the terminal structures, are specified by two inputs: a gradient of Ras activity which emanates from the pole, and the opposing influence of more centrally deployed gap genes which repress the response to Ras.

The Drosophila 14–3-3 protein Leonardo enhances Torso signaling through D-Raf in a Ras 1-dependent manner

14-3-3 proteins have been shown to interact with Raf-1 and cause its activation when overexpressed. However, their precise role in Raf-1 activation is still enigmatic, as they are ubiquitously present in cells and found to associate with Raf-1 in vivo regardless of its activation state. We have analyzed the function of the Drosophila 14–3-3 gene leonardo (leo) in the Torso (Tor) receptor tyrosine kinase (RTK) pathway. In the syncytial blastoderm embryo, activation of Tor triggers the Ras/Raf/MEK pathway that controls the transcription of tailless (tll). We find that, in the absence of Tor, overexpression of leo is sufficient to activate tll expression. The effect of leo requires D-Raf and Ras1 activities but not KSR or DOS, two recently identified essential components of Drosophila RTK signaling pathways. Tor signaling is impaired in embryos derived from females lacking maternal expression of leo. We propose that binding to 14–3-3 by Raf is necessary but not sufficient for the activation of Raf and that overexpressed Drosophila 14–3-3 requires Ras1 to activate D-Raf.

Multiple Ras signals pattern the Drosophila ovarian follicle cells

During Drosophila oogenesis, spatially restricted activity of the TORPEDO receptor tyrosine kinase first recruits follicle cells adjacent to the oocyte to a posterior cell fate and then specifies dorsal follicle cells. Another receptor tyrosine kinase, BREATHLESS, stimulates migration of the anterior follicle cells known as border cells. Since Ras is known to mediate many receptor tyrosine kinase effects, we have investigated the role of Ras in follicle cell fate determination, differentiation, and migration throughout oogenesis. Early ectopic Ras activity induced transient expression of posterior follicle cell markers in anterior follicle cells, but did not inhibit anterior differentiation. Later ectopic Ras activity inhibited anterior follicle cell differentiation but did not induce posterior marker expression. Complete transformation of anterior follicle cells to posterior follicle cells required early ectopic Ras activity in egg chambers where terminal differentiation of anterior cells was inhibited. These results suggest that, in vivo as in vitro, Ras can have diverse effects on different cells, but, in addition, Ras activity can have different effects on the same cells at different stages in their development.

heartless encodes a fibroblast growth factor receptor (DFR1/DFGF-R2) involved in the directional migration of early mesodermal cells in the Drosophila embryo

After invagination of the mesodermal primordium in the gastrulating Drosophila embryo, the internalized cells migrate in a dorsolateral direction along the overlying ectoderm. This movement generates a stereotyped arrangement of mesodermal cells that is essential for their correct patterning by later position-specific inductive signals. We now report that proper mesodermal cell migration is dependent on the function of a fibroblast growth factor (FGF) receptor encoded by heartless (htl). In htl mutant embryos, the mesoderm forms normally but fails to undergo its usual dorsolateral migration. As a result, cardiac, visceral, and dorsal somatic muscle fates are not induced by Decapentaplegic (Dpp), a transforming growth factor beta family member that is derived from the dorsal ectoderm. Visceral mesoderm can nevertheless be induced by Dpp in the absence of htl function. Ras1 is an important downstream effector of Htl signaling because an activated form of Ras1 partially rescues the htl mutant phenotype. The evolutionary conservation of htl function is suggested by the strikingly similar mesodermal migration and patterning phenotypes associated with FGF receptor mutations in species as diverse as nematode and mouse. These studies establish that Htl signaling provides a vital connection between initial formation of the embryonic mesoderm in Drosophila and subsequent cell-fate specification within this germ layer.

The solution structure of the Raf-1 cysteine-rich domain: a novel ras and phospholipid binding site

The Raf-1 protein kinase is the best-characterized downstream effector of activated Ras. Interaction with Ras leads to Raf-1 activation and results in transduction of cell growth and differentiation signals. The details of Raf-1 activation are unclear, but our characterization of a second Ras-binding site in the cysteine-rich domain (CRD) and the involvement of both Ras-binding sites in effective Raf-1-mediated transformation provides insight into the molecular aspects and consequences of Ras-Raf interactions. The Raf-1 CRD is a member of an emerging family of domains, many of which are found within signal transducing proteins. Several contain binding sites for diacylglycerol (or phorbol esters) and phosphatidylserine and are believed to play a role in membrane translocation and enzyme activation. The CRD from Raf-1 does not bind diacylglycerol but interacts with Ras and phosphatidylserine. To investigate the ligand-binding specificities associated with CRDs, we have determined the solution structure of the Raf-1 CRD using heteronuclear multidimensional NMR. We show that there are differences between this structure and the structures of two related domains from protein kinase C (PKC). The differences are confined to regions of the CRDs involved in binding phorbol ester in the PKC domains. Since phosphatidylserine is a common ligand, we expect its binding site to be located in regions where the structures of the Raf-1 and PKC domains are similar. The structure of the Raf-1 CRD represents an example of this family of domains that does not bind diacylglycerol and provides a framework for investigating its interactions with other molecules.

Daughter of sevenless is a substrate of the phosphotyrosine phosphatase Corkscrew and functions during sevenless signaling

The SH2 domain-containing phosphotyrosine phosphatase Corkscrew (CSW) is an essential component of the signaling pathway initiated by the activation of the sevenless receptor tyrosine kinase (SEV) during Drosophila eye development. We have used genetic and biochemical approaches to identify a substrate for CSW. Expression of a catalytically inactive CSW was used to trap CSW in a complex with a 115 kDa tyrosine-phosphorylated substrate. This substrate was purified and identified as the product of the daughter of sevenless (dos) gene. Mutations of dos were identified in a screen for dominant mutations which enhance the phenotype caused by overexpression of inactive CSW during photoreceptor development. Analysis of dos mutations indicates that DOS is a positive component of the SEV signaling pathway and suggests that DOS dephosphorylation by CSW may be a key event during signaling by SEV.

Salivary duct determination in Drosophila: roles of the EGF receptor signalling pathway and the transcription factors fork head and trachealess

Organogenesis in Drosophila embryos begins at 4–5 hours of development as the expression of organ-specific genes is initiated. The salivary primordium, which occupies the ventral epidermis of parasegment 2, is among the earliest to be defined. It is soon divided into two distinct regions: the more dorsal pregland cells and the more ventral preduct cells. We show that it is the opposing activities of the Drosophila EGF receptor (DER) signaling pathway and the Fork head transcription factor that distinguish these cell types and set up the boundary between them. DER signaling acts ventrally to block fork head expression in the preduct cells, thereby restricting gland identity to the more dorsal cells. Fork head in turn blocks expression of duct-specific genes in the pregland cells, thereby restricting duct identity to the more ventral cells. A third regulatory activity, the Trachealess transcription factor, is also required to establish the identity of the preduct cells, but we show that it acts independently or downstream from the DER:fork head confrontation. In trachealess mutants, subdivision of the salivary primordium occurs normally and the dorsal cells form glands, but the ventral cells are undetermined. We present a model proposing that trachealess is the crucial duct-specific gene that Fork head represses to distinguish pregland from preduct cells.

Activated Ras signals differentiation and expansion of CD4+8+ thymocytes

We describe a novel approach to assay the ability of particular gene products to signal transitions in lymphocyte differentiation in vivo. The method involves transfection of test expression constructs into RAG-1-deficient embryonic stem cells, which are subsequently assayed by the RAG-2-deficient blastocyst complementation approach. We have used this method to demonstrate that expression of activated Ras in CD4-8- (double negative, DN) prothymocytes in vivo induces their differentiation into small CD4+8+ (double positive, DP) cortical thymocytes with accompanying expansion to normal thymocyte numbers. However, activated Ras expression in DP cells does not cause proliferation or maturation to CD4+8- or CD4-8+ (single positive) thymocytes. Therefore, signaling through Ras is sufficient for promoting differentiation of DN to DP cells, but further differentiation requires the activity of additional signaling pathways.

The SH2-containing tyrosine phosphatase corkscrew is required during signaling by sevenless, Ras1 and Raf

The sevenless gene encodes a receptor tyrosine kinase which is required for the development of the R7 photoreceptor cell in each ommatidium of the Drosophila eye. We have previously used a sensitized genetic screen to identify mutations, designated Enhancers of sevenless (E(sev)), which affect genes that encode components of the sevenless signaling pathway. Here, we report that one of these mutations, E(sev)1Ae0P is a dominantly inhibiting allele of corkscrew, which encodes an SH2 domain-containing protein tyrosine phosphatase (Perkins et al., 1992). We show that corkscrew function is essential for sevenless signaling and that expression of a membrane-targeted form of corkscrew can drive R7 photoreceptor development in the absence of sevenless function. Furthermore, we have used the dominantly inhibiting corkscrew allele to examine the role of corkscrew during signaling by activated forms of Ras1 and Raf. Our analysis indicates that corkscrew function is still required during signaling by activated forms Ras1 and Raf proteins. These results define a function for corkscrew that is either downstream of Ras1 activation or in a parallel pathway that acts with activated Ras1/Raf to specify R7 photoreceptor development.