An oncogenic form of human raf can specify terminal body pattern in Drosophila

A timer gene network is spatially regulated by the terminal system in the Drosophila embryo

In insect embryos, anteroposterior patterning is coordinated by the sequential expression of the 'timer' genes caudal, Dichaete, and odd-paired, whose expression dynamics correlate with the mode of segmentation. In Drosophila, the timer genes are expressed broadly across much of the blastoderm, which segments simultaneously, but their expression is delayed in a small 'tail' region, just anterior to the hindgut, which segments during germband extension. Specification of the tail and the hindgut depends on the terminal gap gene tailless, but beyond this the regulation of the timer genes is poorly understood. We used a combination of multiplexed imaging, mutant analysis, and gene network modelling to resolve the regulation of the timer genes, identifying 11 new regulatory interactions and clarifying the mechanism of posterior terminal patterning. We propose that a dynamic Tailless expression gradient modulates the intrinsic dynamics of a timer gene cross-regulatory module, delineating the tail region and delaying its developmental maturation.

Actors of the tyrosine kinase receptor downstream signaling pathways in amphioxus

One of the major goals of evo-developmentalists is to understand how the genetic mechanisms controlling embryonic development have evolved to create the current diversity of bodyplans that we encounter in the animal kingdom. Tyrosine kinase receptors (RTKs) are transmembrane receptors present in all metazoans known to control several developmental processes. They act via the activation of various cytoplasmic signaling cascades, including the mitogen-activated protein kinase (MAPK), the PI3K/Akt, and the phospholipase C-gamma (PLCgamma)/protein kinase C (PKC) pathways. In order to address the evolution of these three pathways and their involvement during embryogenesis in chordates, we took advantage of the complete genome sequencing of a key evolutionarily positioned species, the cephalochordate amphioxus, and searched for the complete gene set of the three signaling pathways. We found that the amphioxus genome contains all of the most important modules of the RTK-activated cascades, and looked at the embryonic expression of two genes selected from each cascade. Our data suggest that although the PI3K/Akt pathway may have ubiquitous functions, the MAPK and the PLCgamma/PKC cascades may play specific roles in amphioxus development. Together with data known in vertebrates, the expression pattern of PKC in amphioxus suggests that the PLCgamma/PKC cascade was implicated in neural development in the ancestor of all chordates.

Raf activation is regulated by tyrosine 510 phosphorylation in Drosophila

The proto-oncoprotein Raf is pivotal for mitogen-activated protein kinase (MAPK) signaling, and its aberrant activation has been implicated in multiple human cancers. However, the precise molecular mechanism of Raf activation, especially for B-Raf, remains unresolved. By genetic and biochemical studies, we demonstrate that phosphorylation of tyrosine 510 is essential for activation of Drosophila Raf (Draf), which is an ortholog of mammalian B-Raf. Y510 of Draf is phosphorylated by the c-src homolog Src64B. Acidic substitution of Y510 promotes and phenylalanine substitution impairs Draf activation without affecting its enzymatic activity, suggesting that Y510 plays a purely regulatory role. We further show that Y510 regulates Draf activation by affecting the autoinhibitory interaction between the N- and C-terminal fragments of the protein. Finally, we show that Src64B is required for Draf activation in several developmental processes. Together, these results suggest a novel mechanism of Raf activation via Src-mediated tyrosine phosphorylation. Since Y510 is a conserved residue in the kinase domain of all Raf proteins, this mechanism is likely evolutionarily conserved.

Receptor tyrosine kinase (RTK)/Ras signaling pathways control many different biological processes during metazoan development. Mutations that disrupt this signaling pathway cause many human diseases, including cancer. The proto-oncoprotein Raf functions downstream of Ras in transducing signals from RTK. Activating mutations in both Ras and Raf have been linked to many types of human cancers. Despite the importance of these oncoproteins in tumorigenesis, the molecular mechanisms of Raf activation remains unresolved. Here, using a genetic screen in Drosophila, we show that the Src homolog Src64B is an activator of Drosophila Raf (Draf) .Src64B phosphorylates tyrosine Y510, in the Draf kinase domain and will activate a full-length Draf, but not a truncated Draf that contains only its kinase domain, suggesting that Y510 phosphorylation may relieve the autoinhibition of full-length Draf. Since Y510 is conserved among all the members of the Raf protein family, its phosphorylation may serve as a mechanism of Raf regulation in general.

Phosphorylation of a conserved tyrosine residue located in the kinase domain of Raf family proteins can serve as a mechanism of Raf activation.

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.

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.

Identification of autosomal regions involved in Drosophila Raf function

Raf is an essential downstream effector of activated p21(Ras) (Ras) in transducing proliferation or differentiation signals. Following binding to Ras, Raf is translocated to the plasma membrane, where it is activated by a yet unidentified "Raf activator." In an attempt to identify the Raf activator or additional molecules involved in the Raf signaling pathway, we conducted a genetic screen to identify genomic regions that are required for the biological function of Drosophila Raf (Draf). We tested a collection of chromosomal deficiencies representing approximately 70% of the autosomal euchromatic genomic regions for their abilities to enhance the lethality associated with a hypomorphic viable allele of Draf, Draf(Su2). Of the 148 autosomal deficiencies tested, 23 behaved as dominant enhancers of Draf(Su2), causing lethality in Draf(Su2) hemizygous males. Four of these deficiencies identified genes known to be involved in the Drosophila Ras/Raf (Ras1/Draf) pathway: Ras1, rolled (rl, encoding a MAPK), 14-3-3epsilon, and bowel (bowl). Two additional deficiencies removed the Drosophila Tec and Src homologs, Tec29A and Src64B. We demonstrate that Src64B interacts genetically with Draf and that an activated form of Src64B, when overexpressed in early embryos, causes ectopic expression of the Torso (Tor) receptor tyrosine kinase-target gene tailless. In addition, we show that a mutation in Tec29A partially suppresses a gain-of-function mutation in tor. These results suggest that Tec29A and Src64B are involved in Tor signaling, raising the possibility that they function to activate Draf. Finally, we discovered a genetic interaction between Draf(Su2) and Df(3L)vin5 that revealed a novel role of Draf in limb development. We find that loss of Draf activity causes limb defects, including pattern duplications, consistent with a role for Draf in regulation of engrailed (en) expression in imaginal discs.

Progression of the morphogenetic furrow in the Drosophila eye: the roles of Hedgehog, Decapentaplegic and the Raf pathway

During Drosophila eye development, Hedgehog (Hh) protein secreted by maturing photoreceptors directs a wave of differentiation that sweeps anteriorly across the retinal primordium. The crest of this wave is marked by the morphogenetic furrow, a visible indentation that demarcates the boundary between developing photoreceptors located posteriorly and undifferentiated cells located anteriorly. Here, we present evidence that Hh controls progression of the furrow by inducing the expression of two downstream signals. The first signal, Decapentaplegic (Dpp), acts at long range on undifferentiated cells anterior to the furrow, causing them to enter a ‘pre-proneural’ state marked by upregulated expression of the transcription factor Hairy. Acquisition of the pre-proneural state appears essential for all prospective retinal cells to enter the proneural pathway and differentiate as photoreceptors. The second signal, presently unknown, acts at short range and is transduced via activation of the Serine-Threonine kinase Raf. Activation of Raf is both necessary and sufficient to cause pre-proneural cells to become proneural, a transition marked by downregulation of Hairy and upregulation of the proneural activator, Atonal (Ato), which initiates differentiation of the R8 photoreceptor. The R8 photoreceptor then organizes the recruitment of the remaining photoreceptors (R1-R7) through additional rounds of Raf activation in neighboring pre-proneural cells. Finally, we show that Dpp signaling is not essential for establishing either the pre-proneural or proneural states, or for progression of the furrow. Instead, Dpp signaling appears to increase the rate of furrow progression by accelerating the transition to the pre-proneural state. In the abnormal situation in which Dpp signaling is blocked, Hh signaling can induce undifferentiated cells to become pre-proneural but does so less efficiently than Dpp, resulting in a retarded rate of furrow progression and the formation of a rudimentary eye.

Quantitative variations in the level of MAPK activity control patterning of the embryonic termini in Drosophila

We have examined the role in patterning of quantitative variations of MAPK activity in signaling from the Drosophila Torso (Tor) receptor tyrosine kinase (RTK). Activation of Tor at the embryonic termini leads to differential expression of the genes tailless and huckebein. We demonstrate, using a series of mutations in the signal transducers Corkscrew/SHP-2 and D-Raf, that quantitative variations in the magnitude of MAPK activity trigger both qualitatively and quantitatively distinct transcriptional responses. We also demonstrate that two chimeric receptors, Torextracellular-Egfrcytoplasmic and Torextracellular-Sevcytoplasmic, cannot fully functionally replace the wild-type Tor receptor, revealing that the precise activation of MAPK involves not only the number of activated RTK molecules but also the magnitude of the signal generated by the RTK cytoplasmic domain. Altogether, our results illustrate how a gradient of MAPK activity controls differential gene expression and, thus, the establishment of various cell fates. We discuss the roles of quantitative mechanisms in defining RTK specificity.

Dual function of Ras in Raf activation

The small guanine nucleotide binding protein p21(Ras) plays an important role in the activation of the Raf kinase. However, the precise mechanism by which Raf is activated remains unclear. It has been proposed that the sole function of p21(Ras)in Raf activation is to recruit Raf to the plasma membrane. We have used Drosophila embryos to examine the mechanism of Raf (Draf) activation in the complete absence of p21(Ras) (Ras1). We demonstrate that the role of Ras1 in Draf activation is not limited to the translocation of Draf to the membrane through a Ras1-Draf association. In addition, Ras1 is essential for the activation of an additional factor which in turn activates Draf.

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.

Variation in the number of activated torso receptors correlates with differential gene expression

Activation of receptor tyrosine kinases triggers many developmental decisions, yet we do not understand how activation of a single receptor can be transduced into different cell responses. The torso pathway in Drosophila provides a model to address this issue since it generates more than one response in the embryo. The torso receptor tyrosine kinase is activated at the embryonic poles under the control of trunk, a protein with similarities to several types of extracellular growth factors. Activation of torso is responsible for the development of a variety of structures, whose appearance can be correlated with activation of at least two different genes along the terminal region. In this study we have analyzed mutations in torso and trunk that express low levels of the respective proteins. We show that different amounts of torso or trunk molecules correlate with the expression of different zygotic genes, implicating changes in the number of activated torso molecules as one of the mechanisms defining differential gene expression. We suggest that variation in the number of activated receptors at the cell surface is a general mechanism that leads to differential gene expression and thus the generation of different cell responses.

Recent advances in understanding signal transduction pathways in worms and flies

One major challenge in the fields of signal transduction and pattern formation is to understand how multiple signals are integrated to determine cell fates. Two developmental systems, vulval development in Caenorhabditis elegans and axis formation during Drosophila melanogaster oogenesis, require the epidermal growth factor receptor tyrosine kinase and the NOTCH signaling pathways to specify cell fates. Current work in both systems has provided new opportunities to investigate the potential for the cross-talk between these different signaling pathways.

The torso receptor tyrosine kinase can activate Raf in a Ras-independent pathway

Activation of the receptor tyrosine kinase (RTK) torso defines the spatial domains of expression of the transcription factors tailless and huckebein. Previous analyses have demonstrated that Ras1 (p21ras) operates upstream of the D-Raf (Raf1) serine/threonine kinase in this signaling pathway. By using a recently developed technique of germline mosaics, we find that D-Raf can be activated by torso in the complete absence of Ras1. This result is supported by analysis of D-Raf activation in the absence of either the exchange factor Son of sevenless (Sos) or the adaptor protein drk (Grb2), as well as by the phenotype of a D-Raf mutation that abolishes binding of Ras1 to D-Raf. Our study provides in vivo evidence that Raf can be activated by an RTK in a Ras-independent pathway.