Fibroblast growth factor receptor 1 (FGFR1) tyrosine phosphorylation regulates binding of FGFR substrate 2alpha (FRS2alpha) but not FRS2 to the receptor

FGFR1 SUMOylation coordinates endothelial angiogenic signaling in angiogenesis

Angiogenesis, the formation of new blood vessels, contributes fundamentally to embryonic development, tissue homeostasis, and wound healing. Basic fibroblast growth factor (FGF2) is recognized as the first proangiogenic molecule discovered and it facilitates angiogenesis by activating FGF receptor 1 (FGFR1) signaling in endothelial cells. However, the roles of FGFR and the FGF/FGFR signaling axis in angiogenesis remain unclear. Here, we report that, upon reversible, posttranslational, small ubiquitin-like modifier modification (SUMOylation), FGFR1 regulates angiogenesis by coordinating endothelial angiogenic signaling. Mechanistically, FGFR1 SUMOylation maintains the balance in the competitive recruitment of the adaptor protein FRS2α between FGFR1 and VEGFR2 receptor complexes. VEGFA/VEGFR2 signaling primarily operates under hypoxic conditions and FGF/FGFR1 signaling is more important under normoxic conditions.

Angiogenesis contributes fundamentally to embryonic development, tissue homeostasis, and wound healing. Basic fibroblast growth factor (FGF2) is recognized as the first proangiogenic molecule discovered, and it facilitates angiogenesis by activating FGF receptor 1 (FGFR1) signaling in endothelial cells. However, the precise roles of FGFR and the FGF/FGFR signaling axis in angiogenesis remain unclear, especially because of the contradictory phenotypes of in vivo FGF and FGFR gene deficiency models. Our previous study results suggested a potential role of posttranslational small ubiquitin-like modifier modification (SUMOylation), with highly dynamic regulatory features, in vascular development and disorder. Here, we identified SENP1-regulated endothelial FGFR1 SUMOylation at conserved lysines responding to proangiogenic stimuli, while SENP1 functioned as the deSUMOylase. Hypoxia-enhanced FGFR1 SUMOylation restricted the tyrosine kinase activation of FGFR1 by modulating the dimerization of FGFR1 and FGFR1 binding with its phosphatase PTPRG. Consequently, it facilitated the recruitment of FRS2α to VEGFR2 but limited additional recruitment of FRS2α to FGFR1, supporting the activation of VEGFA/VEGFR2 signaling in endothelial cells. Furthermore, SUMOylation-defective mutation of FGFR1 resulted in exaggerated FGF2/FGFR1 signaling but suppressed VEGFA/VEGFR2 signaling and the angiogenic capabilities of endothelial cells, which were rescued by FRS2α overexpression. Reduced angiogenesis and endothelial sprouting in mice bearing an endothelial-specific, FGFR1 SUMOylation-defective mutant confirmed the functional significance of endothelial FGFR1 SUMOylation in vivo. Our findings identify the reversible SUMOylation of FGFR1 as an intrinsic fine-tuned mechanism in coordinating endothelial angiogenic signaling during neovascularization; SENP1-regulated FGFR1 SUMOylation and deSUMOylation controls the competitive recruitment of FRS2α by FGFR1 and VEGFR2 to switch receptor-complex formation responding to hypoxia and normoxia angiogenic environments.

Functional Roles of FGF Signaling in Early Development of Vertebrate Embryos

Fibroblast growth factors (FGFs) comprise a large family of growth factors, regulating diverse biological processes including cell proliferation, migration, and differentiation. Each FGF binds to a set of FGF receptors to initiate certain intracellular signaling molecules. Accumulated evidence suggests that in early development and adult state of vertebrates, FGFs also play exclusive and context dependent roles. Although FGFs have been the focus of research for therapeutic approaches in cancer, cardiovascular disease, and metabolic syndrome, in this review, we mainly focused on their role in germ layer specification and axis patterning during early vertebrate embryogenesis. We discussed the functional roles of FGFs and their interacting partners as part of the gene regulatory network for germ layer specification, dorsal-ventral (DV), and anterior-posterior (AP) patterning. Finally, we briefly reviewed the regulatory molecules and pharmacological agents discovered that may allow modulation of FGF signaling in research.

FRS2α-dependent cell fate transition during endocardial cushion morphogenesis

Atrioventricular valve development requires endothelial-to-mesenchymal transition (EndMT) that induces cushion endocardial cells to give rise to mesenchymal cells crucial to valve formation. In the adult endothelium, deletion of the docking protein FRS2α induces EndMT by activating TGFβ signaling in a miRNA let-7-dependent manner. To study the role of endothelial FRS2α during embryonic development, we generated mice with an inducible endothelial-specific deletion of Frs2α (FRS2α^iECKO). Analysis of the FRS2α^iECKO embryos uncovered a combination of impaired EndMT in AV cushions and defective maturation of AV valves leading to development of thickened, abnormal valves when Frs2α was deleted early (E7.5) in development. At the same time, no AV valve developmental abnormalities were observed after late (E10.5) deletion. These observations identify FRS2α as a pivotal controller of cell fate transition during both EndMT and post-EndMT valvulogenesis.

Targeting the Oncogenic FGF-FGFR Axis in Gastric Carcinogenesis

Gastric cancer (GC) is one of the most wide-spread malignancies in the world. The oncogenic role of signaling of fibroblast growing factors (FGFs) and their receptors (FGFRs) in gastric tumorigenesis has been gradually elucidated by recent studies. The expression pattern and clinical correlations of FGF and FGFR family members have been comprehensively delineated. Among them, FGF18 and FGFR2 demonstrate the most prominent driving role in gastric tumorigenesis with gene amplification or somatic mutations and serve as prognostic biomarkers. FGF-FGFR promotes tumor progression by crosstalking with multiple oncogenic pathways and this provides a rational therapeutic strategy by co-targeting the crosstalks to achieve synergistic effects. In this review, we comprehensively summarize the pathogenic mechanisms of FGF-FGFR signaling in gastric adenocarcinoma together with the current targeted strategies in aberrant FGF-FGFR activated GC cases.

Fibroblast growth factor receptor-1 mediates internalization of pathogenic spotted fever rickettsiae into host endothelium

Rickettsial infections continue to cause serious morbidity and mortality in severe human cases around the world. Host cell adhesion and invasion is an essential requisite for intracellular growth, replication, and subsequent dissemination of pathogenic rickettsiae. Heparan sulfate proteoglycans [HSPGs] facilitate the interactions between fibroblast growth factor(s) and their tyrosine kinase receptors resulting in receptor dimerization/activation and have been implicated in bacterial adhesion to target host cells. In the present study, we have investigated the contributions of fibroblast growth factor receptors [FGFRs] in rickettsial entry into the host cells. Inhibition of HSPGs by heparinase and FGFRs by AZD4547 (a selective small-molecule inhibitor) results in significant reduction in rickettsial internalization into cultured human microvascular endothelial cells (ECs), which represent the primary targets of pathogenic rickettsiae during human infections. Administration of AZD4547 during R. conorii infection in a murine model of endothelial-target spotted fever rickettsiosis also diminishes pulmonary rickettsial burden in comparison to mock-treated controls. Silencing of FGFR1 expression using a small interfering RNA also leads to similar inhibition of R. rickettsii invasion into ECs. Consistent with these findings, R. rickettsii infection of ECs also results in phosphorylation of tyrosine 653/654, suggesting activation of FGFR1. Using isobaric tag for relative and absolute quantitation [iTRAQ]-based proteomics approach, we further demonstrate association of β-peptide of rickettsial outer membrane protein OmpA with FGFR1. Mechanistically, FGFR1 binds to caveolin-1 and mediates bacterial entry via caveolin-1 dependent endocytosis. Together, these results identify host cell FGFR1 and rickettsial OmpA as another novel receptor-ligand pair contributing to the internalization of pathogenic rickettsiae into host endothelial cells and the potential application of FGFR-inhibitor drugs as adjunct therapeutics against spotted fever rickettsioses.

FGF-Dependent, Context-Driven Role for FRS Adapters in the Early Telencephalon

FGF signaling, an important component of intercellular communication, is required in many tissues throughout development to promote diverse cellular processes. Whether FGF receptors (FGFRs) accomplish such varied tasks in part by activating different intracellular transducers in different contexts remains unclear. Here, we used the developing mouse telencephalon as an example to study the role of the FRS adapters FRS2 and FRS3 in mediating the functions of FGFRs. Using tissue-specific and germline mutants, we examined the requirement of Frs genes in two FGFR-dependent processes. We found that Frs2 and Frs3 are together required for the differentiation of a subset of medial ganglionic eminence (MGE)-derived neurons, but are dispensable for the survival of early telencephalic precursor cells, in which any one of three FGFRs (FGFR1, FGFR2, or FGFR3) is sufficient for survival. Although FRS adapters are dispensable for ERK-1/2 activation, they are required for AKT activation within the subventricular zone of the developing MGE. Using an FRS2,3-binding site mutant of Fgfr1, we established that FRS adapters are necessary for mediating most or all FGFR1 signaling, not only in MGE differentiation, but also in cell survival, implying that other adapters mediate at least in part the signaling from FGFR2 and FGFR3. Our study provides an example of a contextual role for an intracellular transducer and contributes to our understanding of how FGF signaling plays diverse developmental roles.SIGNIFICANCE STATEMENT FGFs promote a range of developmental processes in many developing tissues and at multiple developmental stages. The mechanisms underlying this multifunctionality remain poorly defined in vivo Using telencephalon development as an example, we show here that FRS adapters exhibit some selectivity in their requirement for mediating FGF receptor (FGFR) signaling and activating downstream mediators that depend on the developmental process, with a requirement in neuronal differentiation but not cell survival. Differential engagement of FRS and non-FRS intracellular adapters downstream of FGFRs could therefore in principle explain how FGFs play several distinct roles in other developing tissues and developmental stages.

Fibroblast growth factors, old kids on the new block

The fibroblast growth factors (FGFs) are a family of cell intrinsic regulatory peptides that control a broad spectrum of cellular activities. The family includes canonic FGFs that elicit their activities by activating the FGF receptor (FGFR) tyrosine kinase and non-canonic members that elicit their activities intracellularly and via FGFR-independent mechanisms. The FGF signaling axis is highly complex due to the existence of multiple isoforms of both ligands and receptors, as well as cofactors that include the chemically heterogeneous heparan sulfate (HS) cofactors, and in the case of endocrine FGFs, the Klotho coreceptors. Resident FGF signaling controls embryonic development, maintains tissue homeostasis, promotes wound healing and tissue regeneration, and regulates functions of multiple organs. However, ectopic or aberrant FGF signaling is a culprit for various diseases, including congenital birth defects, metabolic disorder, and cancer. The molecular mechanisms by which the specificity of FGF signaling is achieved remain incompletely understood. Since its application as a druggable target has been gradually recognized by pharmaceutical companies and translational researchers, understanding the determinants of FGF signaling specificity has become even more important in order to get into the position to selectively suppress a particular pathway without affecting others to minimize side effects.

Hepatocyte FRS2α is essential for the endocrine fibroblast growth factor to limit the amplitude of bile acid production induced by prandial activity

In addition to being positively regulated by prandial activity, bile acid production is also negatively controlled by the endocrine fibroblast growth factor 19 (FGF19) or the mouse ortholog FGF15 from the ileum that represses hepatic cholesterol 7 α-hydroxylase (Cyp7a1) expression through activating FGF receptor four (FGFR4). However, how these two regulatory mechanisms interplay to control bile acid homeostasis in the body and the downstream pathways by which FGFR4 regulates Cyp7a1 expression are not fully understood. Here we report that hepatocyte FGFR substrate 2α (FRS2α), a scaffold protein essential for canonical FGFRs to activate the ERK and AKT pathways, was required for the regulation of bile acid production by the FGF15/19-FGFR4 signaling axis. This occurred through limiting the extent of increases in Cyp7a1 expression induced by prandial activity. Excess FGFR4 kinase activity reduced the amplitude of the increase whereas a lack of FGFR4 augmented the increase of Cyp7a1 expression in the liver. Ablation of Frs2α alleles in hepatocytes abrogated the regulation of Cyp7a1 expression by FGFR4. Together, the results demonstrate that FRS2α-mediated pathways are essential for the FGF15/FGF19-FGFR4 signaling axis to control bile acid homeostasis.

Xenopus laevis FGF receptor substrate 3 (XFrs3) is important for eye development and mediates Pax6 expression in lens placode through its Shp2-binding sites

Members of the fibroblast growth factor (FGF) family play important roles during various developmental processes including eye development. FRS (FGF receptor substrate) proteins bind to FGFR and serve as adapters for coordinated assembly of multi-protein complexes involved in Ras/MAPK and PI3 kinase/Akt pathways. Here, we identified Xenopus laevis Frs3 (XFrs3), a homolog of vertebrate Frs3, and investigated its roles during embryogenesis. XFrs3 is expressed maternally and zygotically with specific expression patterns throughout the early development. Knockdown of XFrs3 using a specific antisense morpholino oligonucleotide (MO) caused reduction of Pax6 expression in the lens placode, and defects in the eye ranging from microphthalmia to anophthalmia. XFrs3 MO-induced defects were alleviated by wild type XFrs3 or a mutant XFrs3 (XFrs3-4YF), in which the putative tyrosine phosphorylation sites served as Grb2-binding sites are mutated. However, another XFrs3 mutant (XFrs3-2YF), in which the putative Shp2-binding sites are mutated, could not rescue the defects of XFrs3 morphants. In addition, we found that XFrs3 is important for FGF or IGF-induced ERK activation in ectodermal tissue. Taken together, our results suggest that signaling through Shp2-binding sites of XFrs3 is necessary for the eye development in Xenopus laevis.

The signaling adapter, FRS2, facilitates neuronal branching in primary cortical neurons via both Grb2- and Shp2-dependent mechanisms

The neurotrophins are a family of closely related growth factors that regulate proliferation and differentiation in the developing and mature nervous systems. Neurotrophins stimulate a family of receptor tyrosine kinases (Trk receptors) and utilize an intracellular docking protein termed fibroblast growth factor (FGF) receptor substrate 2 (FRS2) as a major downstream adapter to activate Ras, phosphatidylinositide 3-kinase (PI3K), and mitogen-activated protein kinase (MAPK) signaling cascades. The goals of this study were twofold: first, to investigate the complexity of neurotrophin-induced FRS2 interactions in primary cortical neurons and to determine which pathway(s) are important in regulating neuronal growth and, second, to determine whether the related signaling adapter, FRS3, stimulates neuron growth comparable to FRS2. We find that neurotrophin treatment of primary cortical neurons stimulates the tyrosine phosphorylation of FRS2 and the subsequent recruitment of Shp2, Grb2, and Gab2. With FRS2 mutants deficient in Grb2 or Shp2 binding, we demonstrate that FRS2 binds Gab1 and Gab2 through Grb2, providing an alternative route to activate PI3 kinase and Shp2. Using recombinant adenoviruses expressing FRS2, we demonstrate that FRS2 overexpression promotes neurite outgrowth and branching in cortical neurons relative to controls. In contrast, overexpression of FRS3 does not stimulate neuronal growth. Moreover, we find that while loss of Shp2, but not Grb2, reduces brain-derived neurotrophic factor (BDNF)-induced MAPK activation, the loss of either pathway impairs neuronal growth. Collectively, these experiments demonstrate that FRS2 functions as an adapter of a multiprotein complex that is activated by the Trk receptors and that the activation of both Grb2- and Shp2-dependent pathways facilitates cortical neuronal growth.

The Role of fibroblast growth factor receptor substrate 2 (FRS2) in the regulation of two activity levels of the components of the extracellular signal-regulated kinase (ERK) pathway in the mouse epididymis

The components of the extracellular signal-regulated kinase (ERK) pathway are involved in the regulation of epididymal cellular processes. Interestingly, our previous studies showed that there are two different activity levels of the ERK pathway components in the epididymal epithelium: a basal level in most regions and a higher level in the differentiated initial segment (IS). In this study we analyzed the role of fibroblast growth factor receptor substrate 2 (FRS2) in the regulation of these two levels. Two mouse models were generated. In the first model, Frs2 was deleted from epithelial cells of most epididymal regions except for the IS from the embryonic period onward. Loss of Frs2 dampened the basal activity level of the ERK pathway components, which resulted in an increase in apoptosis along the epididymal duct. This was observed during the period when FRS2 expression level was highest in wild-type epididymides. In the second model, Frs2 was deleted from the proximal epididymal epithelium from Postnatal Day 17 onward. Most of the epididymides in this model exhibited normal morphology. Loss of Frs2 in these epididymides did not affect the high activity level of the ERK pathway components in the IS. However, a subgroup of epididymides in the second model showed increased apoptosis which resulted in an abnormally shaped proximal region or development of granulomas. Therefore, data from these two models showed that FRS2 played different roles in the regulation of two activity levels of the ERK pathway components in the epididymis.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

Molecular networks in FGF signaling: flotillin-1 and cbl-associated protein compete for the binding to fibroblast growth factor receptor substrate 2

Fibroblast growth factor receptor substrate 2 (FRS2α) is a signaling adaptor protein that regulates downstream signaling of many receptor tyrosine kinases. During signal transduction, FRS2 can be both tyrosine and threonine phosphorylated and forms signaling complexes with other adaptor proteins and tyrosine phosphatases. We have here identified flotillin-1 and the cbl-associated protein/ponsin (CAP) as novel interaction partners of FRS2. Flotillin-1 binds to the phosphotyrosine binding domain (PTB) of FRS2 and competes for the binding with the fibroblast growth factor receptor. Flotillin-1 knockdown results in increased Tyr phosphorylation of FRS2, in line with the inhibition of ERK activity in the absence of flotillin-1. CAP directly interacts with FRS2 by means of its sorbin homology (SoHo) domain, which has previously been shown to interact with flotillin-1. In addition, the third SH3 domain in CAP binds to FRS2. Due to the overlapping binding domains, CAP and flotillin-1 appear to compete for the binding to FRS2. Thus, our results reveal a novel signaling network containing FRS2, CAP and flotillin-1, whose successive interactions are most likely required to regulate receptor tyrosine kinase signaling, especially the mitogen activated protein kinase pathway.

Chlamydia trachomatis co-opts the FGF2 signaling pathway to enhance infection

The molecular details of Chlamydia trachomatis binding, entry, and spread are incompletely understood, but heparan sulfate proteoglycans (HSPGs) play a role in the initial binding steps. As cell surface HSPGs facilitate the interactions of many growth factors with their receptors, we investigated the role of HSPG-dependent growth factors in C. trachomatis infection. Here, we report a novel finding that Fibroblast Growth Factor 2 (FGF2) is necessary and sufficient to enhance C. trachomatis binding to host cells in an HSPG-dependent manner. FGF2 binds directly to elementary bodies (EBs) where it may function as a bridging molecule to facilitate interactions of EBs with the FGF receptor (FGFR) on the cell surface. Upon EB binding, FGFR is activated locally and contributes to bacterial uptake into non-phagocytic cells. We further show that C. trachomatis infection stimulates fgf2 transcription and enhances production and release of FGF2 through a pathway that requires bacterial protein synthesis and activation of the Erk1/2 signaling pathway but that is independent of FGFR activation. Intracellular replication of the bacteria results in host proteosome-mediated degradation of the high molecular weight (HMW) isoforms of FGF2 and increased amounts of the low molecular weight (LMW) isoforms, which are released upon host cell death. Finally, we demonstrate the in vivo relevance of these findings by showing that conditioned medium from C. trachomatis infected cells is enriched for LMW FGF2, accounting for its ability to enhance C. trachomatis infectivity in additional rounds of infection. Together, these results demonstrate that C. trachomatis utilizes multiple mechanisms to co-opt the host cell FGF2 pathway to enhance bacterial infection and spread.

Chlamydia trachomatis is an obligate intracellular bacterium that is an important cause of human disease, including sexually transmitted diseases and acquired blindness in developing countries. The inability to carry out conventional genetic manipulations limits our understanding of the mechanisms of C. trachomatis binding, entry, and spread. Previous studies have shown that heparan sulfate proteoglycans (HSPGs) play a role in early binding events. As cell surface HSPGs facilitate the interactions of many growth factors with their receptors, we investigated whether HSPG-associated growth factors affect C. trachomatis binding or entry. Here, we report the novel finding that Fibroblast Growth Factor 2 (FGF2), a ubiquitously expressed growth factor, enhances C. trachomatis binding to host cells in an HSPG-dependent manner. Furthermore, C. trachomatis infection stimulates production and release of FGF2 through distinct signaling pathways. Released FGF2 is sufficient to enhance the subsequent rounds of infection. Together, these results demonstrate that C. trachomatis utilizes multiple mechanisms to co-opt the host cell FGF2 pathway that sets up a positive feedback loop to enhance bacterial infection and spread.

FRS2α is essential for the fibroblast growth factor to regulate the mTOR pathway and autophagy in mouse embryonic fibroblasts

Although the fibroblast growth factor (FGF) signaling axis plays important roles in cell survival, proliferation, and differentiation, the molecular mechanism underlying how the FGF elicits these diverse regulatory signals is not well understood. By using the Frs2α null mouse embryonic fibroblast (MEF) in conjunction with inhibitors to multiple signaling pathways, here we report that the FGF signaling axis activates mTOR via the FGF receptor substrate 2α (FRS2α)-mediated PI3K/Akt pathway, and suppresses autophagy activity in MEFs. In addition, the PI3K/Akt pathway regulated mTOR is crucial for the FGF signaling axis to suppress autophagy in MEFs. Since autophagy has been proposed to play important roles in cell survival, proliferation, and differentiation, the findings suggest a novel mechanism for the FGF signaling axis to transmit regulatory signals to downstream effectors.

FGF signalling in prostate development, tissue homoeostasis and tumorigenesis

The FGFs (fibroblast growth factors) regulate a broad spectrum of biological activities by activating transmembrane FGFR (FGF receptor) tyrosine kinases and their coupled intracellular signalling pathways. In the prostate, the mesenchymal-epithelial interactions mediated by androgen signalling and paracrine factors are essential for gland organogenesis, homoeostasis and tumorigenesis. FGFs mediate these mesenchymal-epithelial interactions in the prostate by paracrinal crosstalk through a diverse set of ligands and receptors. Gain- and loss-of-function studies in mouse models have demonstrated the requirement for the FGF signalling axis in prostate development and homoeostasis. The aberrant induction of this axis in either compartment of the prostate results in developmental disorders, disrupts the homoeostatic balance and leads to prostate carcinogenesis. FGFs are also implicated in mediating androgen signalling in the prostate between mesenchymal and epithelial compartments. Therefore studying FGF signalling in the prostate will help us to better understand the underlying molecular mechanisms by which the gland develops, maintains homoeostasis and undergoes carcinogenesis; as well as yield clues on how androgens mediate these processes and how advanced-tumour prostate cells escape strict androgen regulations.

Novel phosphotyrosine targets of FGFR2IIIb signaling

In partnership exclusively with the epithelial FGFR2IIIb isotype and a structurally-specific heparan sulfate motif, stromal-derived FGF7 delivers both growth-promoting and growth-limiting differentiation signals to epithelial cells that promote cellular homeostasis between stromal and epithelial compartments. Intercompartmental homeostasis supported by FGF7/FGFR2IIIb is subverted in many solid epithelial tumors. The normally mesenchymal-derived homologue FGFR1 drives proliferation and a progressive tumor-associated phenotype when it appears ectopically in epithelial cells. In order to understand the mechanism underlying the unique biological effects of FGFR2IIIb, we developed an inducible FGFR2IIIb expression system that is specifically dependent on FGF7 for activation in an initially unresponsive cell line to avoid selection for only the growth-promoting aspects of FGFR2IIIb signaling. We then determined FGF7/FGFR2IIIb signaling-specific tyrosine phosphorylated proteins within 5 min after FGF7 stimulation by phosphopeptide immunoaffinity purification and nano-LC-MS/MS. The FGF7/FGFR2 pair caused tyrosine phosphorylation of multiple proteins that have been implicated in the growth stimulating activities of FGFR1 that included multi-substrate organizers FRS2alpha and IRS4, ERK2 and phosphatases SHP2 and SHIP2. It uniquely phosphorylated CDK2 and phosphatase PTPN18 on sites involved in the attenuation of cell proliferation, and several factors that maintain nuclear-cytosolic relationships (emerin and LAP2), protein structure and other cellular fine structures as well as some proteins of unknown functions. Several of the FGF7/FGFR2IIIb-specific targets have been associated with maintenance of function and tumor suppression and disruption in tumors. In contrast, a number of pTyr substrates associated with FGF2/FGFR1 that are generally associated with intracellular Ca(2+)-phospholipid signaling, membrane and cytoskeletal plasticity, cell adhesion, migration and the tumorigenic phenotype were not observed with FGF7/FGFR2IIIb. Our findings provide specific downstream targets for dissection of causal relationships underlying the distinct role of FGF7/FGFR2IIIb signaling in epithelial cell homeostasis.

The insulin-like growth factor (IGF) receptor type 1 (IGF1R) as an essential component of the signalling network regulating neurogenesis

The insulin-like growth factor receptor type 1 (IGF1R) signalling pathway is activated in the mammalian nervous system from early developmental stages. Its major effect on developing neural cells is to promote their growth and survival. This pathway can integrate its action with signalling pathways of growth and morphogenetic factors that induce cell fate specification and selective expansion of specified neural cell subsets. This suggests that during developmental and adult neurogenesis cellular responses to many signalling factors, including ligands of Notch, sonic hedgehog, fibroblast growth factor family members, ligands of the epidermal growth factor receptor, bone morphogenetic proteins and Wingless and Int-1, may be modified by co-activation of the IGF1R. Modulation of cell migration is another possible role that IGF1R activation may play in neurogenesis. Here, I briefly overview neurogenesis and discuss a role for IGF1R-mediated signalling in the developing and mature nervous system with emphasis on crosstalk between the signalling pathways of the IGF1R and other factors regulating neural cell development and migration. Studies on neural as well as on non-neural cells are highlighted because it may be interesting to test in neurogenic paradigms some of the models based on the information obtained in studies on non-neural cell types.

Frs2alpha-deficiency in cardiac progenitors disrupts a subset of FGF signals required for outflow tract morphogenesis

The cardiac outflow tract (OFT) is a developmentally complex structure derived from multiple lineages and is often defective in human congenital anomalies. Although emerging evidence shows that fibroblast growth factor (FGF) is essential for OFT development, the downstream pathways mediating FGF signaling in cardiac progenitors remain poorly understood. Here, we report that FRS2alpha (FRS2), an adaptor protein that links FGF receptor kinases to multiple signaling pathways, mediates crucial aspects of FGF-dependent OFT development in mouse. Ablation of Frs2alpha in mesodermal OFT progenitor cells that originate in the second heart field (SHF) affects their expansion into the OFT myocardium, resulting in OFT misalignment and hypoplasia. Moreover, Frs2alpha mutants have defective endothelial-to-mesenchymal transition and neural crest cell recruitment into the OFT cushions, resulting in OFT septation defects. These results provide new insight into the signaling molecules downstream of FGF receptor tyrosine kinases in cardiac progenitors.

A quantitative study of the recruitment potential of all intracellular tyrosine residues on EGFR, FGFR1 and IGF1R

Receptor tyrosine kinases transmit and process extracellular cues by recruiting intracellular signaling proteins to sites of tyrosine phosphorylation. Using protein microarrays comprising virtually every human SH2 and PTB domain, we generated quantitative protein interaction maps for three well-studied receptors--EGFR, FGFR1 and IGF1R--using phosphopeptides derived from every intracellular tyrosine residue on each receptor, regardless of whether or not they are phosphorylated in vivo. We found that, in general, peptides derived from physiological sites of tyrosine phosphorylation bind to substantially more SH2 or PTB domains than do peptides derived from nonphysiological sites, supporting the idea that kinases and interaction domains co-evolve and suggesting that new sites arise predominantly through selection favoring advantageous interactions, rather than through selection disfavoring unwanted interactions. We also found substantial qualitative overlap in the recruitment profiles of these three receptors, suggesting that their different biological effects arise, at least in part, from quantitative differences in their affinities for the proteins they recruit.

Role of epithelial cell fibroblast growth factor receptor substrate 2alpha in prostate development, regeneration and tumorigenesis

The fibroblast growth factor (FGF) regulates a broad spectrum of biological activities by activation of transmembrane FGF receptor (FGFR) tyrosine kinases and their coupled intracellular signaling pathways. FGF receptor substrate 2alpha (FRS2alpha) is an FGFR interactive adaptor protein that links multiple signaling pathways to the activated FGFR kinase. We previously showed that FGFR2 in the prostate epithelium is important for branching morphogenesis and for the acquisition of the androgen responsiveness. Here we show in mice that FRS2alpha is uniformly expressed in the epithelial cells of developing prostates, whereas it is expressed only in basal cells of the mature prostate epithelium. However, expression of FRS2alpha was apparent in luminal epithelial cells of regenerating prostates and prostate tumors. To investigate FRS2alpha function in the prostate, the Frs2alpha alleles were ablated specifically in the prostatic epithelial precursor cells during prostate development. Similar to the ablation of Fgfr2, ablation of Frs2alpha disrupted MAP kinase activation, impaired prostatic ductal branching morphogenesis and compromised cell proliferation. Unlike the Fgfr2 ablation, disrupting Frs2alpha had no effect on the response of the prostate to androgens. More importantly, ablation of Frs2alpha inhibited prostatic tumorigenesis induced by oncogenic viral proteins. The results suggest that FRS2alpha-mediated signals in prostate epithelial cells promote branching morphogenesis and proliferation, and that aberrant activation of FRS2-linked pathways might promote tumorigenesis. Thus, the prostate-specific Frs2alpha(cn) mice provide a useful animal model for scrutinizing the molecular mechanisms underlying prostatic development and tumorigenesis.