Positive and negative tissue-specific signaling by a nematode epidermal growth factor receptor

The Signaling Network Controlling C. elegans Vulval Cell Fate Patterning

EGF, emitted by the Anchor Cell, patterns six equipotent C. elegans vulval precursor cells to assume a precise array of three cell fates with high fidelity. A group of core and modulatory signaling cascades forms a signaling network that demonstrates plasticity during the transition from naïve to terminally differentiated cells. In this review, we summarize the history of classical developmental manipulations and molecular genetics experiments that led to our understanding of the signals governing this process, and discuss principles of signal transduction and developmental biology that have emerged from these studies.

The SWI/SNF chromatin remodeling complex exerts both negative and positive control over LET-23/EGFR-dependent vulval induction in Caenorhabditis elegans

Signaling by the epidermal growth factor receptor (EGFR) generates diverse developmental patterns. This requires precise control over the location and intensity of signaling. Elucidation of these regulatory mechanisms is important for understanding development and disease pathogenesis. In Caenorhabditis elegans, LIN-3/EGF induces vulval formation in the mid-body, which requires LET-23/EGFR activation only in P6.p, the vulval progenitor nearest the LIN-3 source. To identify mechanisms regulating this signaling pattern, we screened for mutations that cooperate with a let-23 gain-of-function allele to cause ectopic vulval induction. Here, we describe a dominant gain-of-function mutation in swsn-4, a component of SWI/SNF chromatin remodeling complexes. Loss-of-function mutations in multiple SWI/SNF components reveal that weak reduction in SWI/SNF activity causes ectopic vulval induction, while stronger reduction prevents adoption of vulval fates, a phenomenon also observed with increasing loss of LET-23 activity. High levels of LET-23 expression in P6.p are thought to locally sequester LIN-3, thereby preventing ectopic vulval induction, with slight reductions in its expression interfering with LIN-3 sequestration, but not vulval fate signaling. We find that SWI/SNF positively regulates LET-23 expression in P6.p descendants, providing an explanation for the similarities between let-23 and SWI/SNF mutant phenotypes. However, SWI/SNF regulation of LET-23 expression is cell-specific, with SWI/SNF repressing its expression in the ALA neuron. The swsn-4 gain-of-function mutation affects the PTH domain, and provides the first evidence that its auto-inhibitory function in yeast Sth1p is conserved in metazoan chromatin remodelers. Finally, our work supports broad use of SWI/SNF in regulating EGFR signaling during development, and suggests that dominant SWI/SNF mutations in certain human congenital anomaly syndromes may be gain-of-functions.

A lover and a fighter: the genome sequence of an entomopathogenic nematode Heterorhabditis bacteriophora

Heterorhabditis bacteriophora are entomopathogenic nematodes that have evolved a mutualism with Photorhabdus luminescens bacteria to function as highly virulent insect pathogens. The nematode provides a safe harbor for intestinal symbionts in soil and delivers the symbiotic bacteria into the insect blood. The symbiont provides virulence and toxins, metabolites essential for nematode reproduction, and antibiotic preservation of the insect cadaver. Approximately half of the 21,250 putative protein coding genes identified in the 77 Mbp high quality draft H. bacteriophora genome sequence were novel proteins of unknown function lacking homologs in Caenorhabditis elegans or any other sequenced organisms. Similarly, 317 of the 603 predicted secreted proteins are novel with unknown function in addition to 19 putative peptidases, 9 peptidase inhibitors and 7 C-type lectins that may function in interactions with insect hosts or bacterial symbionts. The 134 proteins contained mariner transposase domains, of which there are none in C. elegans, suggesting an invasion and expansion of mariner transposons in H. bacteriophora. Fewer Kyoto Encyclopedia of Genes and Genomes Orthologies in almost all metabolic categories were detected in the genome compared with 9 other sequenced nematode genomes, which may reflect dependence on the symbiont or insect host for these functions. The H. bacteriophora genome sequence will greatly facilitate genetics, genomics and evolutionary studies to gain fundamental knowledge of nematode parasitism and mutualism. It also elevates the utility of H. bacteriophora as a bridge species between vertebrate parasitic nematodes and the C. elegans model.

Caenorhabditis elegans fibroblast growth factor receptor signaling can occur independently of the multi-substrate adaptor FRS2

The components of receptor tyrosine kinase signaling complexes help to define the specificity of the effects of their activation. The Caenorhabditis elegans fibroblast growth factor receptor (FGFR), EGL-15, regulates a number of processes, including sex myoblast (SM) migration guidance and fluid homeostasis, both of which require a Grb2/Sos/Ras cassette of signaling components. Here we show that SEM-5/Grb2 can bind directly to EGL-15 to mediate SM chemoattraction. A yeast two-hybrid screen identified SEM-5 as able to interact with the carboxy-terminal domain (CTD) of EGL-15, a domain that is specifically required for SM chemoattraction. This interaction requires the SEM-5 SH2-binding motifs present in the CTD (Y(1009) and Y(1087)), and these sites are required for the CTD role of EGL-15 in SM chemoattraction. SEM-5, but not the SEM-5 binding sites located in the CTD, is required for the fluid homeostasis function of EGL-15, indicating that SEM-5 can link to EGL-15 through an alternative mechanism. The multi-substrate adaptor protein FRS2 serves to link vertebrate FGFRs to Grb2. In C. elegans, an FRS2-like gene, rog-1, functions upstream of a Ras/MAPK pathway for oocyte maturation but is not required for EGL-15 function. Thus, unlike the vertebrate FGFRs, which require the multi-substrate adaptor FRS2 to recruit Grb2, EGL-15 can recruit SEM-5/Grb2 directly.

Epidermal growth factor signaling induces behavioral quiescence in Caenorhabditis elegans

The epidermal growth factor receptor (EGFR)/ErbB receptor tyrosine kinases regulate several aspects of development, including the development of the mammalian nervous system. ErbB signaling also has physiological effects on neuronal function, with influences on synaptic plasticity and daily cycles of activity. However, little is known about the effectors of EGFR activation in neurons. Here we show that EGF signaling has a nondevelopmental effect on behavior in Caenorhabditis elegans. Ectopic expression of the EGF-like ligand LIN-3 at any stage induces a reversible cessation of feeding and locomotion. These effects are mediated by neuronal EGFR (also called LET-23) and phospholipase C-gamma (PLC-gamma), diacylglycerol-binding proteins, and regulators of synaptic vesicle release. Activation of EGFR within a single neuron, ALA, is sufficient to induce a quiescent state. This pathway modulates the cessation of pharyngeal pumping and locomotion that normally occurs during the lethargus period that precedes larval molting. Our results reveal an evolutionarily conserved role for EGF signaling in the regulation of behavioral quiescence.

Phosphorylation of ErbB4 on tyrosine 1056 is critical for ErbB4 coupling to inhibition of colony formation by human mammary cell lines

In many studies, ErbB4 expression in breast tumor samples correlates with a favorable patient prognosis. Similarly, ErbB4 signaling is coupled to cellular differentiation and growth arrest in a variety of model systems. However, in some studies, ErbB4 expression in breast tumor samples correlates with poor outcome. Likewise, studies using some human mammary tumor cell lines suggest that ErbB4 is coupled to malignant phenotypes. Thus, the roles that ErbB4 plays in human breast cancer are still poorly defined. Here we demonstrate that a constitutively active ErbB4 mutant (ErbB4-Q646C) inhibits colony formation on plastic by two human mammary tumor cell lines (SKBR3 and MCF7) and by the MCF10A immortalized human mammary cell line, but does not inhibit colony formation by the MDA-MB-453 and T47D human mammary tumor cell lines. ErbB4 kinase activity is necessary for ErbB4 function and phosphorylation of ErbB4 Tyr1056 is necessary and appears to be sufficient for ErbB4 function. The inhibition of colony formation by MCF10A cells is accompanied by growth arrest but not cell death. These data suggest that ErbB4 behaves as a mammary tumor suppressor and that loss of ErbB4 coupling to growth arrest may be an important event in mammary tumorigenesis.

Molecular regulation of receptor tyrosine kinases in hematopoietic malignancies

Dysregulation of receptor tyrosine kinase (RTK) activity has been implicated in the progression of a variety of human leukemias. Most notably, mutations and chromosomal translocations affecting regulation of tyrosine kinase activity in the Kit receptor, the Flt3 receptor, and the PDGFbeta/FGF1 receptors have been demonstrated in mast cell leukemia, acute myeloid leukemia (AML), and chronic myelogenous leukemias (CML), respectively. In addition, critical but non-overlapping roles for the Ron and Kit receptor tyrosine kinases in the progression of animal models of erythroleukemia have been demonstrated [Persons, D., Paulson, R., Loyd, M., Herley, M., Bodner, S., Bernstein, A., Correll, P. and Ney, P., 1999. Fv2 encodes a truncated form of the Stk receptor tyrosine kinase. Nat. Gen. 23, 159-165.; Subramanian, A., Teal, H.E., Correll, P.H. and Paulson, R.F., 2005. Resistance to friend virus-induced erythroleukemia in W/Wv mice is caused by a spleen-specific defect which results in a severe reduction in target cells and a lack of Sf-Stk expression. J. Virol. 79 (23), 14586-14594.]. The various classes of RTKs implicated in the progression of leukemia have been recently reviewed [Reilly, J., 2003. Receptor tyrosine kinases in normal and malignant haematopoiesis. Blood Rev. 17 (4), 241-248.]. Here, we will discuss the mechanism by which alterations in these receptors result in transformation of hematopoietic cells, in the context of what is known about the molecular regulation of RTK activity, with a focus on our recent studies of the Ron receptor tyrosine kinase.

The adaptor protein soc-1/Gab1 modifies growth factor receptor output in Caenorhabditis elegans

Previous genetic analysis has shown that dos/soc-1/Gab1 functions positively in receptor tyrosine kinase (RTK)-stimulated Ras/Map kinase signaling through the recruitment of csw/ptp-2/Shp2. Using sensitized assays in Caenorhabditis elegans for let-23/Egfr and daf-2/InsR (insulin receptor-like) signaling, it is shown that soc-1/Gab1 inhibits phospholipase C-gamma (PLCgamma) and phosphatidylinositol 3'-kinase (PI3K)-mediated signaling. Furthermore, as well as stimulating Ras/Map kinase signaling, soc-1/Gab1 stimulates a poorly defined signaling pathway that represses class 2 daf-2 phenotypes. In addition, it is shown that SOC-1 binds the C-terminal SH3 domain of SEM-5. This binding is likely to be functional as the sem-5(n2195)G201R mutation, which disrupts SOC-1 binding, behaves in a qualitatively similar manner to a soc-1 null allele in all assays for let-23/Egfr and daf-2/InsR signaling that were examined. Further genetic analysis suggests that ptp-2/Shp2 mediates the negative function of soc-1/Gab1 in PI3K-mediated signaling, as well as the positive function in Ras/Map kinase signaling. Other effectors of soc-1/Gab1 are likely to inhibit PLCgamma-mediated signaling and stimulate the poorly defined signaling pathway that represses class 2 daf-2 phenotypes. Thus, the recruitment of soc-1/Gab1, and its effectors, into the RTK-signaling complex modifies the cellular response by enhancing Ras/Map kinase signaling while inhibiting PI3K and PLCgamma-mediated signaling.

PDGF signaling in cells and mice

Since its discovery over three decades ago, platelet-derived growth factor (PDGF) has been a model system for learning how growth factors regulate biological processes. For the first several decades investigators used cells grown in tissue culture. More recently, PDGF signaling has also been investigated in mice. This review outlines the advances in these two systems, and highlights some of the directions for future investigation.

Tyrosine kinase signal specificity: lessons from the HGF receptor

There are two major lines of thinking concerning the mechanisms responsible for specificity in receptor tyrosine kinase signalling. On one hand, receptors might provide instructive signals that dictate cell-fate decisions and, on the other, they might generate permissive signals unleashing responses that are inherently defined in the protein repertoire of target cells. Recent data indicate that the signalling activity of the Met receptor for hepatocyte growth factor is affected by association with cell-specific surface molecules, namely the alpha6beta4 integrin, Plexin B1 and CD44. This suggests that integration of cell-restricted expression of receptor partners that modulate kinase outputs with the intrinsic signalling features of receptors is required for specification of biological responses.

Gene expression profiling of ErbB receptor and ligand-dependent transcription

Overexpression of ErbB2 and ErbB4 receptors in breast cancers may be accompanied by contrasting clinical outcomes. To investigate the molecular mechanisms contributing to these differences, we undertook a comparative study of gene expression regulated by the two receptors. Agonistic antibodies were employed to activate ErbB2 and ErbB4 in isolation from the other ErbBs in breast cancer cells. Gene expression profiling using a 16 755-gene oligonucleotide array was performed to identify transcriptional targets of receptor activation. Our results indicate that, in the same cell line, ErbB2 and ErbB4 activation influence gene transcription differentially. Although there are genes that are regulated by signaling from both receptors, there are also receptor-specific targets that are preferentially regulated by each receptor. We further show that two ligands acting via the same receptor homodimer may activate different subsets of genes. Many of the induced genes are hitherto unidentified targets of ErbB signaling. These include ErbB4 targets EPS15R, GATA4, and RAB2 and ErbB2-activated HRY/HES1 and PPAP2A. Targets of ErbB2 homodimer signaling may be especially important as markers in breast cancer, where ErbB2 homodimerization mediated by overexpression and ligand-independent activation is common.

Temporal and molecular separation of the kit receptor tyrosine kinase's roles in zebrafish melanocyte migration and survival

The Kit receptor tyrosine kinase is required by vertebrate melanocytes for their migration and survival. The relationship between these developmental roles of Kit, however, remains poorly understood. Here, we use two genetic approaches to demonstrate that Kit's roles in the migration and survival of embryonic melanocytes in the zebrafish (Danio rerio) are temporally and functionally independent. We use a temperature-sensitive kit mutation to show that kit promotes melanocyte migration and survival during distinct stages of development. These experiments additionally reveal that melanocyte migration is neither necessary nor sufficient for subsequent survival. We also identify kit alleles that molecularly separate kits roles in migration and survival. These results suggest that the melanocyte changes its response to Kit receptor signaling and function during development, first to promote migration, then to promote survival through distinct Kit-dependent mechanisms.

Extracellular domain determinants of LET-23 (EGF) receptor tyrosine kinase activity in Caenorhabditis elegans

Negative regulation of ErbB/EGFR signalling pathways is important for normal development and the prevention of cancer. In a genetic screen to uncover mechanisms that negatively regulate ErbB signalling in Caenorhabditis elegans, we isolated a second-site mutation (sy621) that promotes the activity of a gain-of-function allele (sa62gf) of the let-23 (EGF) receptor tyrosine kinase. We show that activation by the sa62 mutation (C359Y) likely results from a break in the conserved disulphide-bonded eighth module at the junction of CR1 and L2. The sy621 mutation causes a G270E change in the third disulphide-bonded module of CR1, and causes no phenotype on its own, but cooperates with the sa62 mutation to promote receptor activity. Although both sa62 single- and double-mutant receptors can function in the absence of ligand, they can be further activated by ligand. Our results support the current model for ligand-induced dimerization based on the recent crystal structures of HER3 and the EGFR, and provide more evidence for the generation of distinctly activated ErbB family members through mutation.

Genetic identification of effectors downstream of Neu (ErbB-2) autophosphorylation sites in a Drosophila model

The ErbB-2/Neu receptor tyrosine kinase plays a causal role in tumorigenesis in mammals. Neu's carboxyl terminus contains five phosphorylated tyrosines that mediate transformation through interaction with cytoplasmic SH2 or PTB containing adaptor proteins. We show that Drosophila adaptors signal from individual phosphotyrosine sites of rat Neu. Activated Neu expression in the midline glia suppressed apoptosis, similar to that seen with activated Drosophila EGF-R expression. Expression in eye and wing tissues generated graded phenotypes suitable for dosage-sensitive modifier genetics. Suppression of ErbB-2/Neu-induced phenotypes in tissues haplosufficient for genes encoding adaptor protein or second messengers suggests that pTyr 1227(YD) signals require Shc, and that pTyr 1253 (YE) signalling does not employ Ras, but does require Raf function. Signalling from pTyr (YB) was affected by a haplosufficiency in drk (Grb-2), and in genes thought to function downstream of Grb-2: dab, sos, csw (Shp-2), and dos (Gab-1). These data demonstrate the power of Drosophila genetics to unmask the molecules that signal from oncogenic ErbB-2/Neu.

The EGF receptor family--multiple roles in proliferation, differentiation, and neoplasia with an emphasis on HER4

The EGF Receptor (EGFR), the first transmembrane receptor tyrosine kinase cloned and sequenced, and its closely related family members HER2, HER3, and HER4, play myriad roles in mammalian growth and development. Receptor activation involves ligand binding to separate receptors followed by formation of active dimers. These receptors can signal as homodimers or they can subtly alter signaling output by heterodimerizing with other family members. Adding complexity, these receptors with varying specificity bind at least 10 ligands from two ligand families, the EGF and neuregulin/heregulin families. This signaling system's impact on human neoplasia is underscored by the following: i.) EGFR is overexpressed or activated by autocrine or paracrine growth factor loops in at least 50% of epithelial malignancies; ii.) HER2 is amplified and dramatically overexpressed in approximately 20%-25% or breast cancers; iii) HER3 and HER4 are variably expressed in breast and other cancers. Overexpression and/or activation of EGFR, HER2 and HER3 has been correlated with poor tumor prognosis; antibody and small molecule inhibitors of their activity are being tested as therapy in cancer patients. However, the signaling complexity engendered by four interacting receptors and ten ligands makes it difficult to definitively measure receptor signaling output in human tumors and even makes mechanistic studies of the family's role in normal physiology and neoplastic transformation a challenge. In spite of the literature's emphasis on growth control, activation by some EGF receptor family member ligands can produce tumor cell differentiation, characterized by growth cessation and differentiation gene product synthesis. The present work delineates a role for HER4 in breast cancer cell differentiation and demonstrates that HER4 is both necessary and sufficient to produce an anti-proliferative signal. These

Phosphotyrosine-binding domains in signal transduction

Protein phosphorylation provides molecular control of complex physiological events within cells. In many cases, phosphorylation on specific amino acids directly controls the assembly of multi-protein complexes by recruiting phospho-specific binding modules. Here, the function, structure, and cell biology of phosphotyrosine-binding domains is discussed.

Interaction domains: from simple binding events to complex cellular behavior

Many of the signaling pathways and regulatory systems in eukaryotic cells are controlled by proteins with multiple interaction domains that mediate specific protein-protein and protein-phospholipid interactions, and thereby determine the biological output of receptors for external and intrinsic signals. Here, we discuss the basic features of interaction domains, and suggest that rather simple binary interactions can be used in sophisticated ways to generate complex cellular responses.

Update on HER-2 as a target for cancer therapy: intracellular signaling pathways of ErbB2/HER-2 and family members

ErbB (also termed HER) receptors are expressed in various tissues of epithelial, mesenchymal and neuronal origin, in which they are involved in the control of diverse biological processes such as proliferation, differentiation, migration and apoptosis. Furthermore, their deregulated expression has been implicated in many types of human cancers and is associated with poor clinical prognosis. Owing to the importance of ErbB proteins in both development and cellular transformation, a lot of attention has been drawn to the intracellular signals initiated by the engagement of this family of receptor tyrosine kinases. This review will focus on the membrane proximal events triggered by the ErbB receptor network and will address questions of how receptor heterodimerization may contribute to signal specification and diversification.

Ligand discrimination by ErbB receptors: differential signaling through differential phosphorylation site usage

The four members of the ErbB family of receptor tyrosine kinases (RTKs) mediate a variety of cellular responses to epidermal growth factor (EGF)-like growth factors, and serve as a model for the generation of both diversity and specificity in RTK signaling. Previous studies indicate that receptor-receptor interactions figure prominently in signaling through ErbB receptors. In addition to a role in receptor kinase activation, ligand-induced ErbB receptor homo- and heterodimerization is thought to account for the diversity of biological responses stimulated by EGF-like growth factors. Since each receptor has the potential to couple to different complements of signaling pathways, EGF-like ligands specify cellular response by dictating which pairs of receptors become activated. More recently evidence has been uncovered for ligand discrimination by individual ErbB receptor dimers; receptors appear to realize which ligand is binding and differentially respond through autophosphorylation site usage. These observations indicate that ligand stimulation of RTKs is not generic, and point to another layer in the ErbB signal diversification mechanism. The mechanistic implications of ligand discrimination are discussed.

Requirements of multiple domains of SLI-1, a Caenorhabditis elegans homologue of c-Cbl, and an inhibitory tyrosine in LET-23 in regulating vulval differentiation

SLI-1, a Caenorhabditis elegans homologue of the proto-oncogene product c-Cbl, is a negative regulator of LET-23-mediated vulval differentiation. Lack of SLI-1 activity can compensate for decreased function of the LET-23 epidermal growth factor receptor, the SEM-5 adaptor, but not the LET-60 RAS, suggesting that SLI-1 acts before RAS activation. SLI-1 and c-Cbl comprise an N-terminal region (termed SLI-1:N/Cbl-N, containing a four-helix bundle, an EF hand calcium-binding domain, and a divergent SH2 domain) followed by a RING finger domain and a proline-rich C-terminus. In a transgenic functional assay, the proline-rich C-terminal domain is not essential for sli-1(+) function. A protein lacking the SH2 and RING finger domains has no activity, but a chimeric protein with the SH2 and RING finger domains of SLI-1 replaced by the equivalent domains of c-Cbl has activity. The RING finger domain of c-Cbl has been shown recently to enhance ubiquitination of active RTKs by acting as an E3 ubiquitin-protein ligase. We find that the RING finger domain of SLI-1 is partially dispensable. Further, we identify an inhibitory tyrosine of LET-23 requiring sli-1(+) for its effects: removal of this tyrosine closely mimics the loss of sli-1 but not of another negative regulator, ark-1. Thus, we suggest that this inhibitory tyrosine mediates its effects through SLI-1, which in turn inhibits signaling upstream of LET-60 RAS in a manner not wholly dependent on the ubiquitin-ligase domain.

Structural requirements for the tissue-specific and tissue-general functions of the Caenorhabditis elegans epidermal growth factor LIN-3

Caenorhabditis elegans lin-3 encodes a homolog of the epidermal growth factor (EGF) family of growth factors. LIN-3 is the inductive signal for hermaphrodite vulval differentiation, and it is required for animal viability, hermaphrodite fertility, and the specification of anterior cell fates in the male B cell lineage. We describe the cloning of a lin-3 homolog from C. briggsae, sequence comparison of C. elegans lin-3 with C. briggsae lin-3, and the determination of molecular lesions in alleles of C. elegans lin-3, including three new alleles. We also analyzed the severity of phenotypes caused by the new and existing alleles of lin-3. Correlation of mutant phenotypes and their molecular lesions, as well as sequence comparison between two species, reveal that the EGF motif and the N-terminal portion of the cytoplasmic domain are important for the functions of LIN-3 in all tissues, while the C-terminal portion of the cytoplasmic domain is involved in the tissue-specific functions of lin-3. We discuss how the structure of lin-3 contributes to its functions in multiple developmental processes.

Signaling specificity: the RTK/RAS/MAP kinase pathway in metazoans

The molecular basis by which commonly used signaling pathways are able to elicit tissue-specific responses in multicellular organisms is an important yet poorly understood problem. In this review, we use the receptor tyrosine kinase (RTK)/RAS/MAP kinase signaling cascade as a model to discuss various hypotheses that have been proposed to explain signaling specificity. Specificity can arise at the level of the receptor, through the modulation of signaling kinetics, through the interaction of different signaling pathways, and at the level of downstream signaling components. Mechanisms of specificity used by the RTK/RAS/MAP kinase signaling pathway might apply to other signaling pathways as well, and might help explain how multicellular organisms are able to generate tissues of diverse forms and functions from a small set of common signaling pathways.

Multiple positive and negative regulators of signaling by the EGF-receptor

Signaling via the epidermal growth factor (EGF)-receptor family is subject to regulation and modulation by multiple ligands, effectors and negative regulators, as well as regulation by heterodimerization between family members and crosstalk between heterologous signaling pathways. Besides serving as a paradigm for receptor tyrosine kinases in general, this family is crucial for development and is often mutated or amplified in human tumors.

Reciprocal EGF signaling back to the uterus from the induced C. elegans vulva coordinates morphogenesis of epithelia

BACKGROUND

Reciprocal signaling between distinct tissues is a general feature of organogenesis. Despite the identification of developmental processes in which coordination requires reciprocal signaling, little is known regarding the underlying molecular details. Here, we use the development of the uterine-vulval connection in the nematode Caenorhabditis elegans as a model system to study reciprocal signaling.

RESULTS

In C. elegans, development of the uterine-vulval connection requires the specification of uterine uv1 cells and morphogenesis of 1 degrees -derived vulval cells. LIN-3, an epidermal growth factor (EGF) family protein, is first produced by the gonadal anchor cell to induce vulval precursor cells to generate vulval tissue. We have shown that lin-3 is also expressed in the 1 degrees vulval lineage after vulval induction and that the 1 degrees vulva is necessary to induce the uv1 uterine cell fate. Using genetic and cell biological analyses, we found that the specification of uterine uv1 cells is dependent on EGF signaling from cells of the 1 degrees vulval lineages to a subset of ventral uterine cells of the gonad. RAS and RAF are necessary for this signaling. We also found that EGL-38, a member of the PAX family of proteins, is necessary for transcription of lin-3 in the vulva but not in the anchor cell. A let-23 mutation that confers ligand-independent activity bypasses the requirement for EGL-38 in specification of the uv1 cell fate.

CONCLUSIONS

We have shown how relatively simple EGF signals can be used reciprocally to specify the uterine-vulval connection during C. elegans development.

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.

Genetics of RAS signaling in C. elegans

Genetic analysis of the RAS function in Caenorhabditis elegans has not only clarified the functional relationship of signal transduction proteins, but also led to the discovery of new proteins involved positively or negatively in RAS signaling. The stereotyped development of C. elegans has allowed many of the functions of RAS to be elucidated at the level of fates of individual cells.

Interactions of EGF, Wnt and HOM-C genes specify the P12 neuroectoblast fate in C. elegans

We investigate how temporal and spatial interactions between multiple intercellular and intracellular factors specify the fate of a single cell in Caenorhabditis elegans. P12, which is a ventral cord neuroectoblast, divides postembryonically to generate neurons and a unique epidermal cell. Three classes of proteins are involved in the specification of P12 fate: the LIN-3/LET-23 epidermal growth factor signaling pathway, a Wnt protein LIN-44 and its candidate receptor LIN-17, and a homeotic gene product EGL-5. We show that LIN-3 is an inductive signal sufficient to promote the P12 fate, and the conserved EGF signaling pathway is utilized for P12 fate specification; egl-5 is a downstream target of the lin-3/let-23 pathway in specifying P12 fate; and LIN-44 and LIN-17 act synergistically with lin-3 in the specification of the P12 fate. The Wnt pathway may function early in development to regulate the competence of the cells to respond to the LIN-3 inductive signal.

Inositol trisphosphate mediates a RAS-independent response to LET-23 receptor tyrosine kinase activation in C. elegans

Activity of LET-23, the C. elegans homolog of the epidermal growth factor receptor, is required in multiple tissues. RAS activation is necessary and sufficient for certain LET-23 functions. We show that an inositol trisphosphate receptor can act as a RAS-independent, tissue-specific positive effector of LET-23. Moreover, an inositol trisphosphate kinase negatively regulates this transduction pathway. Signals transduced by LET-23 control ovulation through changes in spermathecal dilation, possibly dependent upon calcium release regulated by both IP3 and IP4. Our results demonstrate that one mechanism by which receptor tyrosine kinases can evoke tissue-specific responses is through activation of distinct signal transduction cascades in different tissues.

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.