Sprouty, an intracellular inhibitor of Ras signaling

Graded Egfr activity patterns the Drosophila eggshell independently of autocrine feedback

The pattern of the Drosophila eggshell is determined by the establishment of a complex and stereotyped pattern of cell fates in the follicular epithelium of the ovary. Localized activation of the Epidermal growth factor receptor (Egfr) is essential for this patterning. Modulation of Egfr pathway activity in time and space determines distinct fates at their appropriate locations, but the details of how Egfr signaling is regulated and how the profile of Egfr activity corresponds to cell fate remain unclear. Here we analyze the effect of loss of various Egfr regulators and targets on follicle cell patterning, using a marker for follicle cell fate, and on the mature eggshell phenotype, using a novel eggshell marker. We show, contrary to current patterning models, that feedback regulation of Egfr activity by the autocrine ligand Spitz and the inhibitor Argos is not necessary for patterning. Given the cell-autonomous nature of the mutant phenotypes we observed, we propose instead that the pattern of cell fates is generated by spatial information derived directly from the germline ligand Gurken, without a requirement for subsequent patterning by diffusible Egfr regulators in the follicular epithelium.

Sprouty2 interacts with protein kinase C delta and disrupts phosphorylation of protein kinase D1

The Sprouty (Spry) proteins act as inhibitors of the Ras/ERK pathway downstream of receptor tyrosine kinases. In this study, we report a novel interaction between protein kinase C delta (PKCdelta) and Spry2. Endogenous PKCdelta and Spry2 interact in cells upon basic fibroblast growth factor stimulation, indicating a physiological relevance for the interaction. This interaction appeared to require the full-length Spry2 protein and was conformation-dependent. Conformational constraints were released upon FGFR1 activation, allowing the interaction to occur. Although this interaction did not affect the phosphorylation of PKCdelta by another kinase, it reduced the phosphorylation of a PKCdelta substrate, protein kinase D1 (PKD1). Spry2 was found to interact more strongly with PKCdelta with increasing amounts of PKD1, which indicated that instead of competing with PKD1 for binding with PKCdelta, it was more likely to form a trimeric complex with both PKCdelta and PKD1. Formation of the complex was found to be dependent on an existing PKCdelta-PKD1 interaction. By disrupting the interaction between PKCdelta and PKD1, Spry2 was unable to associate with PKCdelta to form the trimeric complex. As a consequence of this trimeric complex, the existing interaction between PKCdelta and PKD1 was increased, and the transfer of phosphate groups from PKCdelta to PKD1 was at least partly blocked by Spry2. The action of Spry2 on PKCdelta resulted in the inhibition of both ERK phosphorylation and invasion of PC-3 cells via PKCdelta signaling. By disrupting the capacity of PKCdelta to phosphorylate its cognate substrates, Spry2 may serve to modulate PKCdelta signaling downstream of receptor tyrosine kinases and to regulate the physiological outcome.

Fasciclin 2, the Drosophila orthologue of neural cell-adhesion molecule, inhibits EGF receptor signalling

Adhesion proteins not only control the degree to which cells adhere to each other but are increasingly recognised as regulators of intercellular signalling. Using genetic screening in Drosophila, we have identified Fasciclin 2 (Fas2), the Drosophila orthologue of neural cell adhesion molecule (NCAM), as a physiologically significant and specific inhibitor of epidermal growth factor receptor (EGFR) signalling in development. We find that loss of fas2 genetically interacts with multiple genetic conditions that perturb EGFR signalling. Fas2 is expressed in dynamic patterns during imaginal disc development, and in the eye we have shown that this depends on EGFR activity, implying participation in a negative-feedback loop. Loss of fas2 causes characteristic EGFR hyperactivity phenotypes in the eye, notum and wing, and also leads to downregulation of Yan, a transcriptional repressor targeted for degradation by EGFR activity. No significant genetic interactions were detected with the Notch, Wingless, Hedgehog or Dpp pathways, nor did Fas2 inhibit the FGF receptor or Torso, indicating specificity in the inhibitory role of Fas2 in EGFR signalling. Our results introduce a new regulatory interaction between an adhesion protein and a Drosophila signalling pathway and highlight the extent to which the EGFR pathway must be regulated at multiple levels.

Suppression of Sproutys has a therapeutic effect for a mouse model of ischemia by enhancing angiogenesis

Sprouty proteins (Sproutys) inhibit receptor tyrosine kinase signaling and control various aspects of branching morphogenesis. In this study, we examined the physiological function of Sproutys in angiogenesis, using gene targeting and short-hairpin RNA (shRNA) knockdown strategies. Sprouty2 and Sprouty4 double knockout (KO) (DKO) mice were embryonic-lethal around E12.5 due to cardiovascular defects. The number of peripheral blood vessels, but not that of lymphatic vessels, was increased in Sprouty4 KO mice compared with wild-type (WT) mice. Sprouty4 KO mice were more resistant to hind limb ischemia and soft tissue ischemia than WT mice were, because Sprouty4 deficiency causes accelerated neovascularization. Moreover, suppression of Sprouty2 and Sprouty4 expression in vivo by shRNA targeting accelerated angiogenesis and has a therapeutic effect in a mouse model of hind limb ischemia. These data suggest that Sproutys are physiologically important negative regulators of angiogenesis in vivo and novel therapeutic targets for treating peripheral ischemic diseases.

Spred1 is required for synaptic plasticity and hippocampus-dependent learning

Germline mutations in SPRED1, a negative regulator of Ras, have been described in a neurofibromatosis type 1 (NF1)-like syndrome (NFLS) that included learning difficulties in some affected individuals. NFLS belongs to the group of phenotypically overlapping neuro-cardio-facial-cutaneous syndromes that are all caused by germ line mutations in genes of the Ras/mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) pathway and that present with some degree of learning difficulties or mental retardation. We investigated hippocampus-dependent learning and memory as well as synaptic plasticity in Spred1(-/-) mice, an animal model of this newly discovered human syndrome. Spred1(-/-) mice show decreased learning and memory performance in the Morris water maze and visual-discrimination T-maze, but normal basic neuromotor and sensory abilities. Electrophysiological recordings on brain slices from these animals identified defects in short- and long-term synaptic hippocampal plasticity, including a disequilibrium between long-term potentiation (LTP) and long-term depression in CA1 region. Biochemical analysis, 4 h after LTP induction, demonstrated increased ERK-phosphorylation in Spred1(-/-) slices compared with those of wild-type littermates. This indicates that deficits in hippocampus-dependent learning and synaptic plasticity induced by SPRED1 deficiency are related to hyperactivation of the Ras/ERK pathway.

Osa, a subunit of the BAP chromatin-remodelling complex, participates in the regulation of gene expression in response to EGFR signalling in the Drosophila wing

Gene expression is regulated in part by protein complexes containing ATP-dependent chromatin-remodelling factors of the SWI/SNF family. In Drosophila there is only one SWI/SNF protein, named Brahma, which forms the catalytic subunit of two complexes composed of different proteins. The protein Osa defines the BAP complex, and the proteins Polybromo and Bap170 are only present in the complex named PBAP. In this work we have analysed the functional requirements of Osa during Drosophila wing development, and found that osa is needed for cell growth and survival in the wing imaginal disc, and for the correct patterning of sensory organs, veins and the wing margin. Other members of the BAP complex, such as Snr1, Bap55, Mor and Brm, also share these functions of Osa. We focused on the requirement of Osa during the formation of the wing veins. Genetic interactions between osa alleles and mutations affecting the activity of the EGFR pathway suggest that one aspect of Osa is intimately related to the response to EGFR activity. Thus, loss of osa and EGFR signalling results in similar wing vein phenotypes, and osa alleles enhance the loss of veins caused by reduced EGFR activity. In addition, Osa is required for the expression of several targets of EGFR signalling, such as Delta, rhomboid and argos. We suggest that one role of Osa and Brm in the wing is to establish a chromatin environment in the regulatory regions of EGFR target genes, making them available for both activators and repressors and facilitating transcription in response to EGFR signalling.

Sprouty4 negatively regulates protein kinase C activation by inhibiting phosphatidylinositol 4,5-biphosphate hydrolysis

Sproutys have been shown to negatively regulate growth factor-induced extracellular signal-regulated kinase (ERK) activation, and suggested to be an anti-oncogene. However, molecular mechanism of the suppression has not yet been clarified completely. Sprouty4 inhibits vascular endothelial growth factor (VEGF)-A-induced ERK activation, but not VEGF-C-induced ERK activation. It has been shown that VEGF-A-mediated ERK activation is strongly dependent on protein kinase C (PKC), whereas that by VEGF-C is dependent on Ras. This suggests that Sprouty4 inhibits the PKC pathway more specifically than the Ras pathway. In this study, we confirmed that Sprouty4 suppressed various signals downstream of PKC, such as phosphorylation of MARCKS and protein kinase D (PKD), as well as PKC-dependent nuclear factor (NF)-kappaB activation. Furthermore, Sprouty4 suppressed upstream signals of PKC, such as Ca(2+) mobilization, phosphatidylinositol 4,5-biphosphate (PIP(2)) breakdown and inositol 1,4,5-triphosphate (IP(3)) production in response to VEGF-A. Those effects were dependent on the C-terminal cysteine-rich region, but not on the N-terminal region of Sprouty4, which is critical for the suppression of fibroblast growth factor (FGF)-mediated ERK activation. Sprouty4 overexpression or deletion of the Sprouty4 gene did not affect phospholipase C (PLC) gamma-1 activation, which is an enzyme that catalyzes PIP(2) hydrolysis. Moreover, Sprouty4 inhibited not only VEGF-A-mediated PIP(2) hydrolysis but also inhibited the lysophosphatidic acid (LPA)-induced PIP(2) breakdown that is catalyzed by PLC beta/epsilon activated by G-protein coupled receptor (GPCR). Taken together, Sprouty4 has broader suppression activity for various stimuli than previously thought; it may function as an inhibitor for various types of PLC-dependent signaling as well as for ERK activation.

Regulation of EphB2 activation and cell repulsion by feedback control of the MAPK pathway

In this study, we investigated whether the ability of Eph receptor signaling to mediate cell repulsion is antagonized by fibroblast growth factor receptor (FGFR) activation that can promote cell invasion. We find that activation of FGFR1 in EphB2-expressing cells prevents segregation, repulsion, and collapse responses to ephrinB1 ligand. FGFR1 activation leads to increased phosphorylation of unstimulated EphB2, which we show is caused by down-regulation of the leukocyte common antigen–related tyrosine phosphatase receptor that dephosphorylates EphB2. In addition, FGFR1 signaling inhibits further phosphorylation of EphB2 upon stimulation with ephrinB1, and we show that this involves a requirement for the mitogen-activated protein kinase (MAPK) pathway. In the absence of activated FGFR1, EphB2 activates the MAPK pathway, which in turn promotes EphB2 activation in a positive feedback loop. However, after FGFR1 activation, the induction of Sprouty genes inhibits the MAPK pathway downstream of EphB2 and decreases cell repulsion and segregation. These findings reveal a novel feedback loop that promotes EphB2 activation and cell repulsion that is blocked by transcriptional targets of FGFR1.

MicroRNA-21 contributes to myocardial disease by stimulating MAP kinase signalling in fibroblasts

MicroRNAs comprise a broad class of small non-coding RNAs that control expression of complementary target messenger RNAs. Dysregulation of microRNAs by several mechanisms has been described in various disease states including cardiac disease. Whereas previous studies of cardiac disease have focused on microRNAs that are primarily expressed in cardiomyocytes, the role of microRNAs expressed in other cell types of the heart is unclear. Here we show that microRNA-21 (miR-21, also known as Mirn21) regulates the ERK-MAP kinase signalling pathway in cardiac fibroblasts, which has impacts on global cardiac structure and function. miR-21 levels are increased selectively in fibroblasts of the failing heart, augmenting ERK-MAP kinase activity through inhibition of sprouty homologue 1 (Spry1). This mechanism regulates fibroblast survival and growth factor secretion, apparently controlling the extent of interstitial fibrosis and cardiac hypertrophy. In vivo silencing of miR-21 by a specific antagomir in a mouse pressure-overload-induced disease model reduces cardiac ERK-MAP kinase activity, inhibits interstitial fibrosis and attenuates cardiac dysfunction. These findings reveal that microRNAs can contribute to myocardial disease by an effect in cardiac fibroblasts. Our results validate miR-21 as a disease target in heart failure and establish the therapeutic efficacy of microRNA therapeutic intervention in a cardiovascular disease setting.

Overexpression screen in Drosophila identifies neuronal roles of GSK-3 beta/shaggy as a regulator of AP-1-dependent developmental plasticity

AP-1, an immediate-early transcription factor comprising heterodimers of the Fos and Jun proteins, has been shown in several animal models, including Drosophila, to control neuronal development and plasticity. In spite of this important role, very little is known about additional proteins that regulate, cooperate with, or are downstream targets of AP-1 in neurons. Here, we outline results from an overexpression/misexpression screen in Drosophila to identify potential regulators of AP-1 function at third instar larval neuromuscular junction (NMJ) synapses. First, we utilize >4000 enhancer and promoter (EP) and EPgy2 lines to screen a large subset of Drosophila genes for their ability to modify an AP-1-dependent eye-growth phenotype. Of 303 initially identified genes, we use a set of selection criteria to arrive at 25 prioritized genes from the resulting collection of putative interactors. Of these, perturbations in 13 genes result in synaptic phenotypes. Finally, we show that one candidate, the GSK-3beta-kinase homolog, shaggy, negatively influences AP-1-dependent synaptic growth, by modulating the Jun-N-terminal kinase pathway, and also regulates presynaptic neurotransmitter release at the larval neuromuscular junction. Other candidates identified in this screen provide a useful starting point to investigate genes that interact with AP-1 in vivo to regulate neuronal development and plasticity.

Sprouty2-mediated inhibition of fibroblast growth factor signaling is modulated by the protein kinase DYRK1A

Raf-MEK-extracellular signal-regulated kinase (Erk) signaling initiated by growth factor-engaged receptor tyrosine kinases (RTKs) is modulated by an intricate network of positive and negative feedback loops which determine the specificity and spatiotemporal characteristics of the intracellular signal. Well-known antagonists of RTK signaling are the Sprouty proteins. The activity of Sprouty proteins is modulated by phosphorylation. However, little is known about the kinases responsible for these posttranslational modifications. We identify DYRK1A as one of the protein kinases of Sprouty2. We show that DYRK1A interacts with and regulates the phosphorylation status of Sprouty2. Moreover, we identify Thr75 on Sprouty2 as a DYRK1A phosphorylation site in vitro and in vivo. This site is functional, since its mutation enhanced the repressive function of Sprouty2 on fibroblast growth factor (FGF)-induced Erk signaling. Further supporting the idea of a functional interaction, DYRK1A and Sprouty2 are present in protein complexes in mouse brain, where their expression overlaps in several structures. Moreover, both proteins copurify with the synaptic plasma membrane fraction of a crude synaptosomal preparation and colocalize in growth cones, pointing to a role in nerve terminals. Our results suggest, therefore, that DYRK1A positively regulates FGF-mitogen-activated protein kinase signaling by phosphorylation-dependent impairment of the inhibitory activity of Sprouty2.

Down-regulation of Sprouty2 via p38 MAPK plays a key role in the induction of cellular apoptosis by tumor necrosis factor-alpha

Mammalian Sprouty2 (Spry2) is a key regulator of the receptor tyrosine kinase/ERK signaling pathway and involved in many biological processes, including cell growth, migration, and tumor suppression. Here, we demonstrated that the intracellular protein level of Spry2 was significantly down-regulated by tumor necrosis factor-alpha (TNF-alpha) in both murine Swiss 3T3 fibroblasts and MLE15 lung epithelial cells. Although TNF-alpha activates multiple signaling cascades, only the inhibitor of p38 MAPK pathway blocked TNF-alpha-induced Spry2 down-regulation. Moreover, since both the mRNA level and protein half-life of Spry2 were unaltered by TNF-alpha treatment, this indicated the possible involvement of a translational mechanism in mediating the inhibitory effect of TNF-alpha. Importantly, rescue of the TNF-alpha-induced down-regulation of Spry2 by gene overexpression led to reverse of the apoptotic effect of TNF-alpha in Swiss 3T3 cells. To our knowledge, this study is the first that reported the association of Spry2 with TNF-alpha signaling pathway.

Overexpression of Sprouty 2 in mouse lung epithelium inhibits urethane-induced tumorigenesis

Members of the Sprouty family encode novel proteins that are thought to function primarily as intracellular antagonists of the Ras-signaling pathway. Increased Ras signaling is a critical characteristic of human lung adenocarcinoma, the most common type of non-small cell lung cancer. Sprouty 2 is expressed in the lung epithelium, the tissue layer from which lung cancers arise. We hypothesized that overexpression of Sprouty 2 in the distal lung epithelium would inhibit lung tumorigenesis. To test the hypothesis, the consequences of overexpressing Sprouty 2 in the distal lung epithelium on urethane-induced mouse lung tumorigenesis were determined. Urethane is a chemical carcinogen found in tobacco smoke that causes activating mutations in Kras and induces lung tumors in mice. Sprouty 2-overexpressor mice developed significantly fewer lung tumors compared with their littermate controls (13.2 +/- 1.1 versus 18.1 +/- 1.3, P = 0.006). Tumor diameter was also significantly smaller in Sprouty 2 overexpressors (0.85 mm +/- 0.03 versus 0.95 mm +/- 0.02, P = 0.005). Sprouty 2 overexpression did not alter Kras mutational frequencies in urethane-induced tumors, suggesting that the tumor-suppressing effect of Sprouty 2 overexpression acts at a stage after Kras mutation, perhaps by interfering with receptor tyrosine kinase-induced signaling. These results demonstrate that Sprouty 2 overexpression inhibited both tumor initiation and subsequent tumor growth.

Modulation of endocrine pancreas development but not beta-cell carcinogenesis by Sprouty4

Sprouty (Spry) proteins modulate signal transduction pathways elicited by receptor tyrosine kinases (RTK). Depending on cell type and the particular RTK, Spry proteins exert dual functions: They can either repress RTK-mediated signaling pathways, mainly by interfering with the Ras/Raf/mitogen-activated protein kinase pathway or sustaining RTK signal transduction, for example by sequestering the E3 ubiquitin-ligase c-Cbl and thus preventing ubiquitylation, internalization, and degradation of RTKs. Here, by the inducible expression of murine Spry4 in pancreatic beta cells, we have assessed the functional role of Spry proteins in the development of pancreatic islets of Langerhans in normal mice and in the Rip1Tag2 transgenic mouse model of beta-cell carcinogenesis. beta cell-specific expression of mSpry4 provokes a significant reduction in islet size, an increased number of alpha cells per islet area, and impaired islet cell type segregation. Functional analysis of islet cell differentiation in cultured PANC-1 cells shows that mSpry4 represses adhesion and migration of differentiating pancreatic endocrine cells, most likely by affecting the subcellular localization of the protein tyrosine phosphatase PTP1B. In contrast, transgenic expression of mSpry4 during beta-cell carcinogenesis does not significantly affect tumor outgrowth and progression to tumor malignancy. Rather, tumor cells seem to escape mSpry4 transgene expression.

Helmsman is expressed in both trachea and photoreceptor development: partial inactivation alters tracheal morphology and visually guided behavior

We have identified helmsman (hlm), which is expressed in the fruit fly photoreceptor cells during neural network development. Hlm is also expressed in the elongating cells of the embryonic trachea. Both photoreceptor neurons and embryonic trachea cells elongate in precise, targeted growth for cell-to-cell specific recognition. Expression of antisense hlm-interfering RNA during embryogenesis arrests elongation of the developing tracheal cells and blocks maturation. Expression of hlm-interfering RNA during visual system formation results in reduced visual acuity and poor performance in optomotor response, indicative of abnormal neural network development. Hlm is a unique cell surface protein with complement-like protein interaction motifs. We have also cloned hlm from Lucilia cuprina (Australian blowfly), which is approximately 100 million years divergent from Drosophila, and find a remarkable 90% protein identity over the entire 558 amino acid protein. Analysis of the hlm sequence found in other species indicates a significant evolutionary pressure to maintain the hlm protein sequence. Our interpretation is that hlm is involved in cell maturation in both the elongating trachea and elongating photoreceptor cells. Cell adhesion and cell signaling, which are known to use immunoglobulin-like cell adhesion molecules, may use molecular systems analogous to complement to create protein complexes to regulate growth.

Trabid, a new positive regulator of Wnt-induced transcription with preference for binding and cleaving K63-linked ubiquitin chains

A key effector of the canonical Wnt pathway is beta-catenin, which binds to TCF/LEF factors to promote the transcription of Wnt target genes. In the absence of Wnt stimulation, beta-catenin is phosphorylated constitutively, and modified with K48-linked ubiquitin for subsequent proteasomal degradation. Here, we identify Trabid as a new positive regulator of Wnt signaling in mammalian and Drosophila cells. Trabid show a remarkable preference for binding to K63-linked ubiquitin chains with its three tandem NZF fingers (Npl4 zinc finger), and it cleaves these chains with its OTU (ovarian tumor) domain. These activities of Trabid are required for efficient TCF-mediated transcription in cells with high Wnt pathway activity, including colorectal cancer cell lines. We further show that Trabid can bind to and deubiquitylate the APC tumor suppressor protein, a negative regulator of Wnt-mediated transcription. Epistasis experiments indicate that Trabid acts below the stabilization of beta-catenin, and that it may affect the association or activity of the TCF-beta-catenin transcription complex. Our results indicate a role of K63-linked ubiquitin chains during Wnt-induced transcription.

Diversity in fibroblast growth factor receptor 1 regulation: learning from the investigation of Kallmann syndrome

The unravelling of the genetic basis of the hypogonadotrophic hypogonadal disorders, including Kallmann syndrome (KS), has led to renewed interest into the developmental biology of gonadotrophin-releasing hormone (GnRH) neurones and, more generally, into the molecular mechanisms of reproduction. KS is characterised by the association of GnRH deficiency with diminished olfaction. Until recently, only two KS-associated genes were known: KAL1 and KAL2. KAL1 encodes the cell membrane and extracellular matrix-associated secreted protein anosmin-1 which is implicated in the X-linked form of KS. Anosmin-1 shows high affinity binding to heparan sulphate (HS) and its function remains the focus of ongoing investigation, although a role in axonal guidance and neuronal migration, which are processes essential for normal GnRH ontogeny and olfactory bulb histogenesis, has been suggested. KAL2, identified as the fibroblast growth factor receptor 1 (FGFR1) gene, has now been recognised to be the underlying genetic defect for an autosomal dominant form of KS. The diverse signalling pathways initiated upon FGFR activation can elicit pleiotropic cellular responses depending on the cellular context. Signalling through FGFR requires HS for receptor dimerisation and ligand binding. Current evidence supports a HS-dependent interaction between anosmin-1 and FGFR1, where anosmin-1 serves as a co-ligand activator enhancing the signal activity, the finer details of whose mechanism remain the subject of intense investigation. Recently, mutations in the genes encoding prokineticin 2 (PK2) and prokineticin receptor 2 (PKR2) were reported in a cohort of KS patients, further reinforcing the view of KS as a multigenic trait involving divergent pathways. Here, we review the historical and current understandings of KS and discuss the latest findings from the molecular and cellular studies of the KS-associated proteins, and describe the evidence that suggests convergence of several of these pathways during normal GnRH and olfactory neuronal ontogeny.

Sprouty proteins, masterminds of receptor tyrosine kinase signaling

Angiogenesis relies on endothelial cells properly processing signals from growth factors provided in both an autocrine and a paracrine manner. These mitogens bind to their cognate receptor tyrosine kinases (RTKs) on the cell surface, thereby activating a myriad of complex intracellular signaling pathways whose outputs include cell growth, migration, and morphogenesis. Understanding how these cascades are precisely controlled will provide insight into physiological and pathological angiogenesis. The Sprouty (Spry) family of proteins is a highly conserved group of negative feedback loop modulators of growth factor-mediated mitogen-activated protein kinase (MAPK) activation originally described in Drosophila. There are four mammalian orthologs (Spry1-4) whose modulation of RTK-induced signaling pathways is growth factor- and cell context-dependent. Endothelial cells are a group of highly differentiated cell types necessary for defining the mammalian vasculature. These cells respond to a plethora of growth factors and express all four Spry isoforms, thus highlighting the complexity that is required to form and maintain vessels in mammals. This review describes Spry functions in the context of endothelial biology and angiogenesis, and provides an update on Spry-interacting proteins and Spry mechanisms of action.

Specific regions within the embryonic midbrain and cerebellum require different levels of FGF signaling during development

Prospective midbrain and cerebellum formation are coordinated by FGF ligands produced by the isthmic organizer. Previous studies have suggested that midbrain and cerebellum development require different levels of FGF signaling. However, little is known about the extent to which specific regions within these two parts of the brain differ in their requirement for FGF signaling during embryogenesis. Here, we have explored the effects of inhibiting FGF signaling within the embryonic mouse midbrain (mesencephalon) and cerebellum (rhombomere 1) by misexpressing sprouty2 (Spry2) from an early stage. We show that such Spry2 misexpression moderately reduces FGF signaling, and that this reduction causes cell death in the anterior mesencephalon, the region furthest from the source of FGF ligands. Interestingly, the remaining mesencephalon cells develop into anterior midbrain, indicating that a low level of FGF signaling is sufficient to promote only anterior midbrain development. Spry2 misexpression also affects development of the vermis, the part of the cerebellum that spans the midline. We found that, whereas misexpression of Spry2 alone caused loss of the anterior vermis, reducing FGF signaling further, by decreasing Fgf8 gene dose, resulted in loss of the entire vermis. Our data suggest that cell death is not responsible for vermis loss, but rather that it fails to develop because reducing FGF signaling perturbs the balance between vermis and roof plate development in rhombomere 1. We suggest a molecular explanation for this phenomenon by providing evidence that FGF signaling functions to inhibit the BMP signaling that promotes roof plate development.

Drosophila cbl is essential for control of cell death and cell differentiation during eye development

Background

Activation of cell surface receptors transduces extracellular signals into cellular responses such as proliferation, differentiation and survival. However, as important as the activation of these receptors is their appropriate spatial and temporal down-regulation for normal development and tissue homeostasis. The Cbl family of E3-ubiquitin ligases plays a major role for the ligand-dependent inactivation of receptor tyrosine kinases (RTKs), most notably the Epidermal Growth Factor Receptor (EGFR) through ubiquitin-mediated endocytosis and lysosomal degradation.

Methodology/Principal Findings

Here, we report the mutant phenotypes of Drosophila cbl (D-cbl) during eye development. D-cbl mutants display overgrowth, inhibition of apoptosis, differentiation defects and increased ommatidial spacing. Using genetic interaction and molecular markers, we show that most of these phenotypes are caused by increased activity of the Drosophila EGFR. Our genetic data also indicate a critical role of ubiquitination for D-cbl function, consistent with biochemical models.

Conclusions/Significance

These data may provide a mechanistic model for the understanding of the oncogenic activity of mammalian cbl genes.

FGF signals guide migration of mesenchymal cells, control skeletal morphogenesis and regulate gastrulation during sea urchin development

The sea urchin embryo is emerging as an attractive model to study morphogenetic processes such as directed migration of mesenchyme cells and cell sheet invagination, but surprisingly, few of the genes regulating these processes have yet been characterized. We present evidence that FGFA, the first FGF family member characterized in the sea urchin, regulates directed migration of mesenchyme cells, morphogenesis of the skeleton and gastrulation during early development. We found that at blastula stages, FGFA and a novel putative FGF receptor are expressed in a pattern that prefigures morphogenesis of the skeletogenic mesoderm and that suggests that FGFA is one of the elusive signals that guide migration of primary mesenchyme cells (PMCs). We first show that fgfA expression is correlated with abnormal migration and patterning of the PMCs following treatments that perturb specification of the ectoderm along the oral-aboral and animal-vegetal axes. Specification of the ectoderm initiated by Nodal is required to restrict fgfA to the lateral ectoderm, and in the absence of Nodal, fgfA is expressed ectopically throughout most of the ectoderm. Inhibition of either FGFA, FGFR1 or FGFR2 function severely affects morphogenesis of the skeleton. Furthermore,inhibition of FGFA and FGFR1 signaling dramatically delays invagination of the archenteron, prevents regionalization of the gut and abrogates formation of the stomodeum. We identified several genes acting downstream of fgfAin these processes, including the transcription factors pea3 and pax2/5/8 and the signaling molecule sprouty in the lateral ectoderm and SM30 and SM50 in the primary mesenchyme cells. This study identifies the FGF signaling pathway as an essential regulator of gastrulation and directed cell migration in the sea urchin embryo and as a key player in the gene regulatory network directing morphogenesis of the skeleton.

Evidence that sprouty 2 is necessary for sarcoma formation by H-Ras oncogene-transformed human fibroblasts

Sprouty 2 (Spry2) acts as an inhibitor of receptor tyrosine kinase signaling in various cellular contexts. Interestingly, Spry2 also prevents the c-Cbl-induced degradation of epidermal growth factor receptor (EGFR). We compared human fibroblasts malignantly transformed by overexpression of H-Ras(V12) oncogene to their nontransformed parental cells and found that the malignant cells express a high level of Spry2. These cells also exhibited an increase in the level of EGFR compared with their precursor cells. We found that intact EGFR was required if H-Ras-transformed cells were to grow in the absence of exogenous growth factors or form large colonies in agarose. When we decreased expression of Spry2, using a Spry2-specific shRNA, the H-Ras(V12)-transformed fibroblasts could no longer form large colonies in agarose, grow in reduced levels of serum, or form tumors in athymic mice. The level of active H-Ras in these cells remained unaltered. A similar, but less pronounced, effect in tumor formation was observed when Spry2 was down-regulated in human patient-derived fibrosarcoma cell lines. In H-Ras-transformed cells Spry2 sustained the level and the downstream signaling activity of EGFR. In the parental, non-H-Ras-transformed fibroblasts, expression of Spry2 resulted in the inhibition of H-Ras and ERK activation, suggesting that the positive effect of Spry2 in tumor formation is specific to H-Ras transformation. Co-immunoprecipitation studies with H-Ras-transformed cells revealed that Spry2 and H-Ras interact and that H-Ras interacts with Spry2-binding partners, c-Cbl and CIN85, in a Spry2-dependent manner. These data show that Spry2 plays a critical role in the ability of H-Ras-transformed cells to form tumors in athymic mice.

Tesk1 interacts with Spry2 to abrogate its inhibition of ERK phosphorylation downstream of receptor tyrosine kinase signaling

The Sprouty (Spry) proteins function as inhibitors of the Ras-ERK pathway downstream of various receptor tyrosine kinases. In this study, we have identified Tesk1 (testicular protein kinase 1) as a novel regulator of Spry2 function. Endogenous Tesk1 and Spry2 exist in a complex in cell lines and mouse tissues. Tesk1 coexpression relocalizes Spry2 to vesicles including endosomes, inhibiting its translocation to membrane ruffles upon growth factor stimulation. Independent of its kinase activity, Tesk1 binding leads to a loss of Spry2 function as an inhibitor of ERK phosphorylation and reverses inhibition of basic fibroblast growth factor (bFGF)- and nerve growth factor-induced neurite outgrowth in PC12 cells by Spry2. Furthermore, depletion of endogenous Tesk1 in PC12 cells leads to a reduction in neurite outgrowth induced by bFGF. Tesk1 nullifies the inhibitory effect of Spry2 by abrogating its interaction with the adaptor protein Grb2 and interfering with its serine dephosphorylation upon bFGF and FGF receptor 1 stimulation by impeding its binding to the catalytic subunit of protein phosphatase 2A. A construct of Tesk1 that binds to Spry2 but does not localize to the vesicles does not interfere with its function, highlighting the importance of subcellular localization of Tesk1 in this context. Conversely, Tesk1 does not affect interaction of Spry2 with the E3 ubiquitin ligase, c-Cbl, and consequently, does not affect its inhibition of Cbl-mediated ubiquitination of the epidermal growth factor receptor. By selectively modulating the downstream effects of Spry2, Tesk1 may thus serve as a molecular determinant of the signaling outcome.

Sprouty2 downregulation plays a pivotal role in mediating crosstalk between TGF-beta1 signaling and EGF as well as FGF receptor tyrosine kinase-ERK pathways in mesenchymal cells

Mammalian Sprouty2 (Spry2) is a key regulator of the receptor tyrosine kinase/ERK signaling pathway and is involved in many biological processes, including cell growth, differentiation, migration, and embryonic lung branching morphogenesis. Previous studies have shown that Spry2 expression is upregulated by many mitogens, particularly epidermal growth factor (EGF) and fibroblast growth factors (FGFs). In contrast, we report that transforming growth factor-beta1 (TGF-beta1), which stimulates the growth of quiescent Swiss 3T3 cells, induced a dose dependent decrease of mouse Spry2 protein level within 24-h of treatment, and this effect was mediated by a MAP kinase-independent pathway. A concomitant reduction of the level of Spry2 mRNA indicates the involvement of a transcriptional mechanism, which requires histone deacetylase (HDAC) activity and de novo protein synthesis. On the other hand, the turnover rate of Spry2 protein was increased by TGF-beta1 treatment, suggesting enhanced Spry2 degradation. Treatment with lysosomal inhibitors, but not proteasome inhibitors, prevented the degradation of Spry2, thus, indicating that the degradation of Spry2 is mediated through the lysosomal pathway in Swiss 3T3 cells. Furthermore, we demonstrate that TGF-beta1 signaling can modulate EGF and FGF-induced ERK-MAP kinase activation by controlling Spry2 expression and function. Moreover, rescue of the TGF-beta1-induced downregulation of Spry2 by gene over-expression led to inhibition of the mitogenic effect of TGF-beta1 in Swiss 3T3 cells. Together, the combined operation of transcriptional and post-translational mechanisms suggests that regulation of Spry2 is a crucial event and emphasizes the important role that Spry2 plays in controlling cell behaviors.

Unraveling the molecular mechanisms that lead to supernumerary teeth in mice and men: current concepts and novel approaches

Supernumerary teeth are defined as those that are present in excess of the normal complement of human dentition and represent a unique developmental anomaly of patterning and morphogenesis. Despite the wealth of information generated from studies on normal tooth development, the genetic etiology and molecular mechanisms that lead to congenital deviations in tooth number are poorly understood. For developmental biologists, the phenomenon of supernumerary teeth raises interesting questions about the development and fate of the dental lamina. For cell and molecular biologists, the anomaly of supernumerary teeth inspires several questions about the actions and interactions of transcription factors and growth factors that coordinate morphogenesis, cell survival and programmed cell death. For human geneticists, the condition as it presents itself in either syndromic or non-syndromic forms offers an opportunity to discover mutations in known or novel genes. For clinicians faced with treating the dental complications that arise from the presence of supernumerary teeth, knowledge about the basic mechanisms involved is essential. The purpose of this manuscript is to review current knowledge about how supernumerary teeth form, the molecular insights gained through studies on mice that are deficient in certain tooth signaling molecules and the questions that require further research in the field.

Sprouty2 binds Grb2 at two different proline-rich regions, and the mechanism of ERK inhibition is independent of this interaction

Sprouty2 has been widely implicated in the negative regulation of the fibroblast growth factor receptor-extracellular regulated kinase (ERK) pathway. Sprouty2 directly interacts with the adapter protein Grb2, member of the receptor tyrosine kinase-induced signaling pathways. In considering the functional role of Grb2, we investigated whether the interaction with this protein was responsible for ERK pathway inhibition. We found that the binding between Sprouty2 and Grb2 is constitutive, independent of Sprouty2 tyrosine phosphorylation, although it is increased when fibroblast growth factor receptor is activated. This connection is mediated by the N-terminal SH3 domain of Grb2 and two Sprouty2 proline-rich stretches (residues 59-64 and 303-307). Most importantly, a double Sprouty2 mutant (hSpry2 P59AP304A), which is unable to bind Grb2, developed at a similar inhibition level of fibroblast growth factor receptor-ERK pathway than that which originated from Sprouty2 wt. These results are evidence that the Sprouty2 mechanism of ERK inhibition is independent of Grb2 binding.

Down-regulation of Sprouty2 in non-small cell lung cancer contributes to tumor malignancy via extracellular signal-regulated kinase pathway-dependent and -independent mechanisms

Sprouty (Spry) proteins function as inhibitors of receptor tyrosine kinase signaling mainly by interfering with the Ras/Raf/mitogen-activated protein kinase cascade, a pathway known to be frequently deregulated in human non-small cell lung cancer (NSCLC). In this study, we show a consistently lowered Spry2 expression in NSCLC when compared with the corresponding normal lung epithelium. Based on these findings, we investigated the influence of Spry2 expression on the malignant phenotype of NSCLC cells. Ectopic expression of Spry2 antagonized mitogen-activated protein kinase activity and inhibited cell migration in cell lines homozygous for K-Ras wild type, whereas in NSCLC cells expressing mutated K-Ras, Spry2 failed to diminish extracellular signal-regulated kinase (ERK) phosphorylation. Nonetheless, Spry2 significantly reduced cell proliferation in all investigated cell lines and blocked tumor formation in mice. Accordingly, a Spry2 mutant unable to inhibit ERK phosphorylation reduced cell proliferation significantly but less pronounced compared with the wild-type protein. Therefore, we conclude that Spry2 interferes with ERK phosphorylation and another yet unidentified pathway. Our results suggest that Spry2 plays a role as tumor suppressor in NSCLC by antagonizing receptor tyrosine kinase-induced signaling at different levels, indicating feasibility for the usage of Spry in targeted gene therapy of NSCLC.

Spreds are essential for embryonic lymphangiogenesis by regulating vascular endothelial growth factor receptor 3 signaling

Spred/Sprouty family proteins negatively regulate growth factor-induced ERK activation. Although the individual physiological roles of Spred-1 and Spred-2 have been investigated using gene-disrupted mice, the overlapping functions of Spred-1 and Spred-2 have not been clarified. Here, we demonstrate that the deletion of both Spred-1 and Spred-2 resulted in embryonic lethality at embryonic days 12.5 to 15.5 with marked subcutaneous hemorrhage, edema, and dilated lymphatic vessels filled with erythrocytes. This phenotype resembled that of Syk(-/-) and SLP-76(-/-) mice with defects in the separation of lymphatic vessels from blood vessels. The number of LYVE-1-positive lymphatic vessels and lymphatic endothelial cells increased markedly in Spred-1/2-deficient embryos compared with WT embryos, while the number of blood vessels was not different. Ex vivo colony assay revealed that Spred-1/2 suppressed lymphatic endothelial cell proliferation and/or differentiation. In cultured cells, the overexpression of Spred-1 or Spred-2 strongly suppressed vascular endothelial growth factor-C (VEGF-C)/VEGF receptor (VEGFR)-3-mediated ERK activation, while Spred-1/2-deficient cells were extremely sensitive to VEGFR-3 signaling. These data suggest that Spreds play an important role in lymphatic vessel development by negatively regulating VEGF-C/VEGFR-3 signaling.

Intersection of signal transduction pathways and development

One of the challenges of modern biology is to understand how cells within a developing organism generate, integrate, and respond to dynamic informational cues. Based on over two decades of intensive research, many parts and subroutines of the responsible signal transduction networks have been identified and functionally characterized. From this work, it has become evident that a complicated interplay between signaling pathways, involving extensive feedback regulation and multiple levels of cross-talk, underlies even the "simplest" developmental decision. Thus a signaling pathway can no longer be thought of as a rigid linear process, but rather must be considered a dynamic, self-interacting, and self-adjusting network. The Epidermal Growth Factor Receptor tyrosine kinase signaling pathway provides a prime vantage point from which to explore emerging principles in developmental signal transduction.

Regulation of Sprouty2 stability by mammalian Seven-in-Absentia homolog 2

Mammalian Sprouty (Spry) gene expression is rapidly induced upon activation of the FGF receptor signaling pathway in multiple cell types including cells of mesenchymal and epithelial origin. Spry2 inhibits FGF-dependent ERK activation and thus Spry acts as a feedback inhibitor of FGF-mediated proliferation. In addition, Spry2 interacts with the ring-finger-containing E3 ubiquitin ligase, c-Cbl, in a manner that is dependent upon phosphorylation of Tyr55 of Spry2. This interaction results in the poly-ubiquitination and subsequent degradation of Spry2 by the proteasome. Here, we describe the identification of another E3 ubiquitin ligase, human Seven-in-Absentia homolog-2 (SIAH2), as a Spry2 interacting protein. We show by yeast two-hybrid analysis that the N-terminal domain of Spry2 and the ring finger domain of SIAH2 mediated this interaction. Co-expression of SIAH2 resulted in proteasomal degradation of Spry1, 2, and to a lesser extent Spry4. The related E3 ubiquitin-ligase, SIAH1, had little effect on Spry2 protein stability when co-expressed. Unlike c-Cbl-mediated degradation of Spry2, SIAH2-mediated degradation was independent of phosphorylation of Spry2 on Tyr55. Spry2 was also phosphorylated on Tyr227, and phosphorylation of this residue was also dispensable for SIAH2-mediated degradation of Spry2. Finally, co-expression of SIAH2 with Spry2 resulted in a rescue of FGF2-mediated ERK phosphorylation. These data suggest a novel mechanism whereby Spry2 stability is regulated in a manner that is independent of tyrosine phosphorylation, and provides an addition level of control of Spry2 protein levels.

Spatial regulation of EGFR signaling by Sprouty2

Ligand-induced activation of the epidermal growth factor receptor (EGFR) initiates multiple signal-transduction pathways as well as trafficking events that relocalize the receptors from the cell surface to intracellular endocytic compartments. Although there is growing awareness that endocytic transport can play a direct role in signal specification, relatively little is known about the molecular mechanisms underlying this link. Here we show that human Sprouty 2 (hSpry2), a protein that has been implicated in the negative regulation of receptor tyrosine kinase (RTK) signaling [1], interferes with the trafficking of activated EGFR specifically at the step of progression from early to late endosomes. This effect is mediated by the binding of hSpry2 to the endocytic regulatory protein, hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), and leads to a block in intracellular signal propagation. These observations suggest that EGFR signaling is controlled by a novel mechanism involving trafficking-dependent alterations in receptor compartmentalization.

Distinct roles of fibroblast growth factor receptor 1 and 2 in regulating cell survival and epithelial-mesenchymal transition

Two related receptor tyrosine kinases (RTKs), fibroblast growth factor receptor 1 and 2 (FGFR1 and FGFR2), exert distinct effects during carcinogenesis. To examine FGFR1 and FGFR2 signaling in polarized epithelia, we have developed an in vitro three-dimensional HC11 mouse mammary epithelial cell culture model combined with a chemically inducible FGFR (iFGFR) dimerization system. Although activation of both RTKs led to reinitiation of cell proliferation and loss of cell polarity, only iFGFR1 activation induced cell survival and epithelial to mesenchymal transition. In contrast, iFGFR2 activation induced cell apoptosis even in the cells in direct contact with the extracellular matrix. Activation of iFGFR2, but not iFGFR1, led to rapid receptor down-regulation and transient activation of downstream signaling, which were partially rescued by Cbl small interfering RNA knockdown or the proteasome inhibitor lactacystin. Importantly, inhibition of proteasome activity in iFGFR2-activated structures led to epithelial to mesenchymal transition and invasive phenotypes resembling those observed after iFGFR1 activation. These studies demonstrate, for the first time, that the duration of downstream signaling determines the distinct phenotypes mediated by very homologous RTKs in three-dimensional cultures.

A spatial and temporal map of FGF/Erk1/2 activity and response repertoires in the early chick embryo

During early vertebrate development Fibroblast Growth Factor (FGF) signalling is required for multiple activities including specification of mesodermal, neural and heart tissue, as well as gastrulation movements and regulation of differentiation and pattern onset in the extending body axis. A current challenge is to understand how FGF signalling generates such diverse outcomes. A key FGF downstream pathway is the Ras-MAPK/Erk1/2 cascade, which culminates in the phosphorylation of target proteins, such as the Ets family of transcription factors. To begin to assess specificity downstream of FGF in the chick embryo we have characterised the patterns of Fgfr1-4 expression and Erk1/2 activation, as well as expression of the Erk1/2 specific phosphatase, Mkp3 and of three Ets factor genes (Erm, Pea3 and Er81) from early blastula to the 10 somite stage. We identify new sites of Fgfr expression and show that nearly all regions of Erk1/2 activity are within Fgfr expression domains and require FGF signalling. Differences in intensity, duration, distribution and sub-cellular localisation of activated Erk1/2 are observed in distinct cell populations within the embryo and during wound healing. With few exceptions, a tight correspondence between Erk1/2 activation and Mkp3 expression is found, while specific combinations of Ets factors are associated with distinct regions of Erk1/2 activation. These findings provide a comprehensive spatial and temporal map of FGF/Erk1/2 activity during early chick development and identify region and tissue specific differences in expression of Fgfrs as well as Erk1/2 phosphorylation and transcriptional targets which help to define response specificity.

Direct binding of PP2A to Sprouty2 and phosphorylation changes are a prerequisite for ERK inhibition downstream of fibroblast growth factor receptor stimulation

In the context of fibroblast growth factor (FGF) signaling, Sprouty2 (Spry2) is the most profound inhibitor of the Ras/ERK pathway as compared with other Spry isoforms. An exclusive, necessary, but cryptic PXXPXR motif in the C terminus of Spry2 is revealed upon stimulation. The activation of Spry2 appears to be linked to sequences in the N-terminal half of the protein and correlated with a bandshifting seen on SDS-PAGE. The band-shifting is likely caused by changes in the phosphorylation status of key Ser and Thr residues following receptor stimulation. Dephosphorylation of at least two conserved Ser residues (Ser-112 and Ser-115) within a conserved Ser/Thr sequence is accomplished upon stimulation by a phosphatase that binds to Spry2 around residues 50-60. We show that human Spry2 co-immunoprecipitates with both the catalytic and the regulatory subunits of protein phosphatase 2A (PP2A-C and PP2A-A, respectively) in cells upon FGF receptor (FGFR) activation. PP2A-A binds directly to Spry2, but not to Spry2Delta50-60 (Delta50-60), and the activity of PP2A increases with both FGF treatment and FGFR1 overexpression. c-Cbl and PP2A-A compete for binding centered around Tyr-55 on Spry2. We show that there are at least two distinct pools of Spry2, one that binds PP2A and another that binds c-Cbl. c-Cbl binding likely targets Spry2 for ubiquitin-linked destruction, whereas the phosphatase binding and activity are necessary to dephosphorylate specific Ser/Thr residues. The resulting change in tertiary structure enables the Pro-rich motif to be revealed with subsequent binding of Grb2, a necessary step for Spry2 to act as a Ras/ERK pathway inhibitor in FGF signaling.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

Orchestrating ontogenesis: variations on a theme by sonic hedgehog

Embryonic development is an emergent process in which increasing complexity is generated by sequential cellular interactions. Recently, it has become clear that such interactions are mediated by just a few families of signalling molecules; but how does this limited repertoire elicit the diversity of form that is characteristic of multicellular organisms? Here we review the various ways in which a member of one such family, the sonic hedgehog (SHH) protein, is deployed during embryonic development. These examples of SHH function provide paradigms for inductive interactions that should help to inform attempts to recapitulate cellular programming and organogenesis in vitro.

Epithelial cell adhesion in the developing Drosophila retina is regulated by Atonal and the EGF receptor pathway

In the Drosophila retina, photoreceptor differentiation is preceded by significant cell shape rearrangements within and immediately behind the morphogenetic furrow. Groups of cells become clustered into arcs and rosettes in the plane of the epithelium, from which the neurons subsequently emerge. These cell clusters also have differential adhesive properties: adherens junction components are upregulated relative to surrounding cells. Little is known about how these morphological changes are orchestrated and what their relevance is for subsequent neuronal differentiation. Here, we report that the transcription factor Atonal and the canonical EGF receptor signalling cascade are both required for this clustering and for the accompanying changes in cellular adhesion. In the absence of either component, no arcs are formed behind the furrow, and all cells show low Armadillo and DE-cadherin levels, although in the case of EGFR pathway mutants, single, presumptive R8 cells with high levels of adherens junction components can be seen. Atonal regulates DE-cadherin transcriptionally, whereas the EGFR pathway, acting through the transcription factor Pointed, exerts its effects on adherens junctions indirectly, at a post-transcriptional level. These observations define a new function for EGFR signalling in eye development and illustrate a mechanism for the control of epithelial morphology by developmental signals.

SOCS3 is required to temporally fine-tune photoreceptor cell differentiation

Suppressor of cytokine signaling 3 (SOCS3) is an intracellular, ligand-induced negative feedback modulator of STAT3 activation that acts during inflammation. Here, we demonstrate that SOCS3 expression is important for normal retinal development in the perinatal period. STAT3 is highly activated in the late-embryonic retina, then downregulated at postnatal day 0 (P0), presumably by the depletion of upstream ligands. We found that SOCS3 was required after P0 to shut down the residual STAT3 activation; this loss of activated STAT3 leads to Rhodopsin expression and rod photoreceptor cell differentiation. SOCS3 deficiency failed to terminate STAT3 activation, thereby delaying expression of Rhodopsin and its upstream transcription factor, crx. Development subsequently continued, but its course was temporally erratic, probably because of faulty compensation. Interestingly, SOCS3 protein expression was first detected postnatally, after STAT3 activation was mostly downregulated. It initially appeared in some of the presumptive photoreceptor cells and gradually spread. SOCS3 mRNA level was constant from the late-embryonic to early-postnatal period. Post-transcriptional inhibition of SOCS3 protein expression maintains a high STAT3 activation during late embryogenesis, and after P0, releasing from the inhibition promptly terminates STAT3 activation. Thus, SOCS3 can act as a temporal fine-tuner of STAT3 activation during photoreceptor cell differentiation.

The Cdi/TESK1 kinase is required for Sevenless signaling and epithelial organization in the Drosophila eye

How cellular behaviors such as cell-to-cell communication, epithelial organization and cell shape reorganization are coordinated during development is poorly understood. The developing Drosophila eye offers an ideal model system to study these processes. Localized actin polymerization is required to constrict the apical surface of epithelial cells of the eye imaginal disc to maintain the refined arrangement of the developing ommatidia. The identity of each photoreceptor cell within the epithelium is determined by cell-to-cell contacts involving signal transduction events. The R7 photoreceptor cell requires the activity of the Sevenless RTK to adopt a proper cell fate. We performed an EP screen for negative regulators of this inductive process, and we identified the serine/threonine kinase Center divider (cdi) as a suppressor of the phenotype caused by an activated Sevenless receptor. Cdi is homologous to the human testis-specific kinase 1 (TESK1), a member of the LIM kinases involved in cytoskeleton control through ADF/cofilin phosphorylation. We have analyzed the effects of gain- and loss-of-function of cdi and found alterations in actin organization and in the adherens junctions proteins DE-cadherin and beta-catenin, as well as in Sevenless apical localization. Interference with the function of the ADF/cofilin phosphatase Slingshot (ssh), which antagonizes Cdi, also results in a suppression of signaling triggered by the Sevenless RTK. Our results reveal a critical interplay between the localization of molecules involved in epithelial organization and signal transduction.

Sprouty genes control diastema tooth development via bidirectional antagonism of epithelial-mesenchymal FGF signaling

Unlike humans, who have a continuous row of teeth, mice have only molars and incisors separated by a toothless region called a diastema. Although tooth buds form in the embryonic diastema, they regress and do not develop into teeth. Here, we identify members of the Sprouty (Spry) family, which encode negative feedback regulators of fibroblast growth factor (FGF) and other receptor tyrosine kinase signaling, as genes that repress diastema tooth development. We show that different Sprouty genes are deployed in different tissue compartments--Spry2 in epithelium and Spry4 in mesenchyme--to prevent diastema tooth formation. We provide genetic evidence that they function to ensure that diastema tooth buds are refractory to signaling via FGF ligands that are present in the region and thus prevent these buds from engaging in the FGF-mediated bidirectional signaling between epithelium and mesenchyme that normally sustains tooth development.

An FGF response pathway that mediates hepatic gene induction in embryonic endoderm cells

While particular combinations of mesodermal signals are known to induce distinct tissue-specific programs in the endoderm, there is little information about the response pathways within endoderm cells that control their specification. We have used signaling inhibitors on embryo tissue explants and whole-embryo cultures as well as genetic approaches to reveal part of an intracellular network by which FGF signaling helps induce hepatic genes and stabilize nascent hepatic cells within the endodermal epithelium. Specifically, we found that hepatic gene induction is elicited by an FGF/MAPK pathway. Although the PI3K pathway is activated in foregut endoderm cells, its inhibition does not block hepatic gene induction in explants; however, it does block tissue growth. We also found that at the onset of hepatogenesis, the FGF/MAPK and PI3K pathways do not crossregulate in the endoderm. The finding of separate pathways for endoderm tissue specification and growth provides insights for guiding cellular regeneration and stem cell differentiation.

Sprouty and cancer: The first terms report

The Ras/Erk signaling pathway has a central role in development of multi-cellular organisms as well as in signal transmission in the mature individual. Recently, a family of genes, designated Sprouty, induced by the Ras/Erk pathway was found to specify proteins that inhibited the upstream pathway. Being in a position that is likely to control well-characterized oncogene products suggested that the expression levels of the Sprouty genes may be relevant in human carcinogenesis. Early data on the deregulation of Sprouty expression in breast, prostate and liver cancers is discussed along with the notion that some of them might have potential as tumour markers or that the derived proteins may act as tumour suppressors.

Comparative mechanisms of branching morphogenesis in diverse systems

Much progress has been made in recent years toward understanding mechanisms controlling branching morphogenesis, a fundamental aspect of development in a variety of invertebrate and vertebrate organs. To gain a deeper understanding of how branching morphogenesis occurs in the mammary gland, we compare and contrast the cellular and molecular events underlying this process in both invertebrate and vertebrate organs. Thus, in this review, we focus on the common themes that have emerged from such comparative analyses and discuss how they are implemented via a battery of signaling pathways to ensure proper branching morphogenesis in diverse systems.

Argos Mutants Define an Affinity Threshold for Spitz Inhibition in Vivo

Argos, a secreted antagonist of Drosophila epidermal growth factor receptor (dEGFR) signaling, acts by sequestering the activating ligand Spitz. To understand how different domains in Argos contribute to efficient Spitz sequestration, we performed a genetic screen aimed at uncovering modifiers of an Argos misexpression phenotype in the developing eye. We identified a series of suppressors mapping to the Argos transgene that affect its activity in multiple developmental contexts. These point mutations map to both the N- and C-terminal cysteine-rich regions, implicating both domains in Argos function. We show by surface plasmon resonance that these Argos mutants are deficient in their ability to bind Spitz in vitro. Our data indicate that a mere approximately 2-fold decrease in K(D) is sufficient to compromise Argos activity in vivo. This effect could be recapitulated in a cell-based assay, where a higher molar concentration of mutant Argos was needed to inhibit Spitz-dependent dEGFR phosphorylation. In contrast, a approximately 37-fold decrease in the binding constant nearly abolishes Argos activity in vivo and in cellular assays. In agreement with previously reported computational studies, our results define an affinity threshold for optimal Argos inhibition of dEGFR signaling during development.

A Src homology 3-binding sequence on the C terminus of Sprouty2 is necessary for inhibition of the Ras/ERK pathway downstream of fibroblast growth factor receptor stimulation

Because the Sprouty (Spry) proteins were shown to be inhibitors of the mainstream Ras/ERK pathway, there has been considerable interest in ascertaining their mechanism of action especially since a possible role as tumor suppressors for these inhibitory proteins has been suggested. We compared the ability of the mammalian Spry isoforms to inhibit the Ras/ERK pathway in the context of fibroblast growth factor receptor (FGFR) signaling. Spry2 is considerably more inhibitory than Spry1 or Spry4, and this correlates with the binding to Grb2 via a C-terminal proline-rich sequence that is found exclusively on Spry2. This PXXPXR motif binds directly to the N-terminal Src homology domain 3 of Grb2, and when added onto the C terminus of Spry4 the resultant chimera inhibits the Ras/ERK pathway. The ability to inhibit neurite outgrowth in PC-12 cells correlates with the propensity of Spry isoforms and engineered constructs to inhibit the phosphorylation of ERK1/2. The PXXPXR motif is cryptic in unstimulated cells, and it is postulated that Spry2 undergoes a conformational change following FGFR stimulation, enabling the subsequent interaction with Grb2. We present evidence that Spry2 can compete with the RasGEF (guanine nucleotide exchange factor) SOS1 for binding to Grb2, resulting in the inhibition of phosphorylation of ERK1/2.

A functional interaction between sprouty proteins and caveolin-1

Growth factor-mediated signal transduction cascades can be regulated spatio-temporally by signaling modulators, such as Sprouty proteins. The four mammalian Sprouty family members are palmitoylated phosphoproteins that can attenuate or potentiate numerous growth factor-induced signaling pathways. Previously, we have shown that Sprouty-1 and Sprouty-2 associate with Caveolin-1, the major structural protein of caveolae. Like Sprouty, Caveolin-1 inhibits growth factor-induced mitogen-activated protein kinase activation. Here, we demonstrate that all four mammalian Sprouty family members physically interact with Caveolin-1. The C terminus of Caveolin-1 is the major Sprouty-binding site, whereas Sprouty binds Caveolin-1 via its conserved C-terminal domain. A single point mutation in this domain results in loss of Caveolin-1 interaction. Moreover, we demonstrate that the various Sprouty isoforms differ dramatically in their cooperation with Caveolin-1-mediated inhibition of mitogen-activated protein kinase activation and that such cooperation is also highly dependent on the type of growth factor investigated and on cell density. Together, the data suggest that the Sprouty/Caveolin-1 interaction modulates signaling in a growth factor- and Sprouty isoform-specific manner.