Function Of Fibroblast Growth Factor Receptors Research Articles

Fibroblast growth factor receptor (FGFR) gene: pathogenesis and treatment implications in urothelial carcinoma of the bladder

Dysregulation of fibroblast growth factor receptors (FGFRs) has been implicated in several human malignancies, including urothelial carcinoma. In urothelial carcinoma, the oncogenic role of mutated FGFR is mediated by the...

FGFR Regulation of Dendrite Elaboration in Adult-born Granule Cells Depends on Intracellular Mediator and Proximity to the Soma

Fibroblast Growth Factor Receptors (FGFRs) play crucial roles in promoting dendrite growth and branching during development. In mice, three FGFR genes, Fgfr1, Fgfr2, and Fgfr3, remain expressed in the adult neurogenic niche of the hippocampal dentate gyrus. However, the function of FGFRs in the dendritic maturation of adult-born neurons remains largely unexplored. Here, using conditional alleles of Fgfr1, Fgfr2, and Fgfr3 as well as Fgfr1 alleles lacking binding sites for Phospholipase-Cγ (PLCγ) and FGF Receptor Substrate (FRS) proteins, we test the requirement for FGFRs in dendritogenesis of adult-born granule cells. We find that deleting all three receptors results in a small decrease in proximal dendrite elaboration. In contrast, specifically mutating Tyr766 in FGFR1 (a PLCγ binding site) in an Fgfr2;Fgfr3 deficient background results in a dramatic increase of overall dendrite elaboration, while blocking FGFR1-FRS signaling causes proximal dendrite deficits and, to a lesser extent than Tyr766 mutants, increases distal dendrite elaboration. These findings reveal unexpectedly complex roles for FGFRs and their intracellular mediators in regulating proximal and distal dendrite elaboration, with the most notable role in suppressing distal elaboration through the PLCγbinding site.

Fibroblast growth factor receptor as a potential candidate for phosphate sensing.

Phosphate plays essential roles in many biological processes. Serum phosphate level needs to be regulated because hypophosphatemia and hyperphosphatemia cause rickets/osteomalacia and ectopic calcification, respectively. Fibroblast growth factor (FGF) 23 is the principal hormone to regulate serum phosphate level. FGF23 is produced by the bone and works to reduce serum phosphate level by binding to FGF receptor (FGFR) 1c and α-Klotho complex in the kidney. It has been unclear how the bone senses the changes of serum phosphate level and how the bone regulates the production of FGF23. Our recent results indicate that high extracellular phosphate activates FGFR1c. Its downstream intracellular signalling pathway regulates the expression of GALNT3 encoding a protein involved in the regulation of the posttranslational modification of FGF23 protein. This FGFR1c-GALNT3 axis is considered to be the main regulatory mechanism of enhanced FGF23 production in response to high phosphate. We propose that FGFR1c works as a phosphate-sensing molecule in the regulation of FGF23 production and serum phosphate level. Feedback system is present in the regulation of serum phosphate involving FGFR1c and FGF23. These findings uncover so far unrecognized function of FGFR and molecular basis of phosphate sensing.

Fibroblast Growth Factor Receptor Functions in Glioblastoma.

Glioblastoma is the most lethal brain cancer in adults, with no known cure. This cancer is characterized by a pronounced genetic heterogeneity, but aberrant activation of receptor tyrosine kinase signaling is among the most frequent molecular alterations in glioblastoma. Somatic mutations of fibroblast growth factor receptors (FGFRs) are rare in these cancers, but many studies have documented that signaling through FGFRs impacts glioblastoma progression and patient survival. Small-molecule inhibitors of FGFR tyrosine kinases are currently being trialed, underlining the therapeutic potential of blocking this signaling pathway. Nevertheless, a comprehensive overview of the state of the art of the literature on FGFRs in glioblastoma is lacking. Here, we review the evidence for the biological functions of FGFRs in glioblastoma, as well as pharmacological approaches to targeting these receptors.

Differential regulation of fibroblast growth factor receptor 1 trafficking and function by extracellular galectins

Fibroblast growth factor receptors (FGFRs) are integral membrane proteins that transmit signals through the plasma membrane. FGFRs signaling needs to be precisely adjusted as aberrant FGFRs function is associated with development of human cancers or severe metabolic diseases. The subcellular localization, trafficking and function of FGFRs rely on the formation of multiprotein complexes. In this study we revealed galectins, lectin family members implicated in cancer development and progression, as novel FGFR1 binding proteins. We demonstrated that galectin-1 and galectin-3 directly bind to the sugar chains of the glycosylated extracellular part of FGFR1. Although both galectins compete for the same binding sites on FGFR1, these proteins elicit different impact on FGFR1 function and cellular trafficking. Galectin-1 mimics fibroblast growth factor as it efficiently activates FGFR1 and receptor-downstream signaling pathways that result in cell proliferation and apoptotic evasion. In contrast, galectin-3 induces extensive clustering of FGFR1 on the cell surface that inhibits constitutive internalization of FGFR1. Our data point on the interplay between extracellular galectins and FGFRs in the regulation of cell fate.

Identification of transcripts potentially involved in neural tube closure using RNA sequencing

Anencephaly is a fatal human neural tube defect (NTD) in which the anterior neural tube remains open. Zebrafish embryos with reduced Nodal signaling display an open anterior neural tube phenotype that is analogous to anencephaly. Previous work from our laboratory suggests that Nodal signaling acts through induction of the head mesendoderm and mesoderm. Head mesendoderm/mesoderm then, through an unknown mechanism, promotes formation of the polarized neuroepithelium that is capable of undergoing the movements required for closure. We compared the transcriptome of embryos treated with a Nodal signaling inhibitor at sphere stage, which causes NTDs, to embryos treated at 30% epiboly, which does not cause NTDs. This screen identified over 3,000 transcripts with potential roles in anterior neurulation. Expression of several genes encoding components of tight and adherens junctions was significantly reduced, supporting the model that Nodal signaling regulates formation of the neuroepithelium. mRNAs involved in Wnt, FGF, and BMP signaling were also differentially expressed, suggesting these pathways might regulate anterior neurulation. In support of this, we found that pharmacological inhibition of FGF-receptor function causes an open anterior NTD as well as loss of mesodermal derivatives. This suggests that Nodal and FGF signaling both promote anterior neurulation through induction of head mesoderm.

Abstract A099: Safety, tolerability, pharmacokinetics, and efficacy of pan-fibroblast growth factor receptor inhibitor rogaratinib in Japanese patients with refractory, locally advanced metastatic solid tumors selected with a novel, mRNA-based patient selection strategy

Abstract Background: Fibroblast growth factor receptor (FGFR)-mediated pathways are considered to be promising anticancer therapy targets. There are several types of genetic evidence that support an oncogenic function of FGFRs: identification of gene amplifications, activating mutations, chromosomal translocations, and aberrant splicing at the post-transcriptional level. Since FGFR expression is regulated not only by genetic but also by epigenetic mechanisms, examination of tumor FGFR mRNA levels may identify cancer patients likely to benefit from rogaratinib (BAY 1163877), an oral, potent small-molecule pan-FGFR inhibitor. We conducted a phase I study of rogaratinib in Japanese subjects with refractory, locally advanced or metastatic solid tumors selected by elevated tumor FGFR1-3 mRNA expression to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy (NCT02592785). Methods: Japanese subjects were enrolled at 600 mg BID (Cohort1) or 800 mg BID as the recommended phase 2 dose (Cohort 2) on a continuous 21-day cycle based on high FGFR1-3 mRNA expression. Archival or newly obtained formalin-fixed, paraffin-embedded (FFPE) tumor tissue samples were analyzed for elevated FGFR1-3 mRNA levels by RNA in situ hybridization (RNAscope®; Advanced Cell Diagnostics, Inc., Newark, CA) and Nanostring® assay (NanoString Technologies, Inc., Seattle, WA). Tumor response was assessed according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Results: In the prescreening phase of this study, 62 FFPE samples from cancer patients with several tumor types were assessed for FGFR1-3 tumor mRNA expression levels and the valid prescreening results were obtained from 48 samples. Among them, 16 samples (16/48, 33%) were found to overexpress FGFR mRNA. Overall, 9 FGFR mRNA-positive patients were enrolled and treated with rogaratinib, 3 in Cohort 1 and 6 in Cohort 2. All subjects had experienced more than one adverse event (AE). Most of them were Grade 1 or 2. No DLT, no SAE, no Grade 4 or 5 AEs were reported. Two G3 drug-related AEs were hyponatremia and increased ALT. The most common treatment related AE was hyperphosphatemia with Grade 1 or 2 and all cases were controllable with anti-hyperphosphatemia treatment. PK exposure in Japanese subjects was comparable to Western subjects. Eight patients could be advanced to at least one tumor response assessment by CT imaging. As a result, 1 patient with epipharynx cancer achieved a partial response (PR) while 5 patients had stable disease (SD) as best response in target lesions, including three SD patients with tumor shrinkage compared to baseline CT imaging. Conclusions: Rogaratinib showed a good tolerability with a confirmed PR and a disease control rate (DCR) of 56% (5/9). We concluded that 800 mg BID is safe and well tolerable to Japanese patients confirming the global recommended dose. The high prevalence of FGFR mRNA overexpression in tumors as well as the clinical response of FGFR-positive tumors support an mRNA-based patient selection for FGFR inhibitor treatment of cancer. Citation Format: Makoto Tahara, Shunji Takahashi, Matthias Ocker, Stuart Ince, Hendrik Nogai, Changliang Zhang, Fumiyoshi Sei, Peter Ellinghaus. Safety, tolerability, pharmacokinetics, and efficacy of pan-fibroblast growth factor receptor inhibitor rogaratinib in Japanese patients with refractory, locally advanced metastatic solid tumors selected with a novel, mRNA-based patient selection strategy [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A099.

Fgf regulates dedifferentiation during skeletal muscle regeneration in adult zebrafish

Fibroblast growth factors (Fgfs) regulate critical biological processes such as embryonic development, tissue homeostasis, wound healing, and tissue regeneration. In zebrafish, Fgf signaling plays an important role in the regeneration of the spinal cord, liver, heart, fin, and photoreceptors, although its exact mechanism of action is not fully understood. Utilizing an adult zebrafish extraocular muscle (EOM) regeneration model, we demonstrate that blocking Fgf receptor function using either a chemical inhibitor (SU5402) or a dominant-negative transgenic construct (dnFGFR1a:EGFP) impairs muscle regeneration. Adult zebrafish EOMs regenerate through a myocyte dedifferentiation process, which involves a muscle-to-mesenchyme transition and cell cycle reentry by differentiated myocytes. Blocking Fgf signaling reduced cell proliferation and active caspase 3 levels in the regenerating muscle with no detectable levels of apoptosis, supporting the hypothesis that Fgf signaling is involved in the early steps of dedifferentiation. Fgf signaling in regenerating myocytes involves the MAPK/ERK pathway: inhibition of MEK activity with U0126 mimicked the phenotype of the Fgf receptor inhibition on both muscle regeneration and cell proliferation, and activated ERK (p-ERK) was detected in injured muscles by immunofluorescence and western blot. Interestingly, following injury, ERK2 expression is specifically induced and activated by phosphorylation, suggesting a key role in muscle regeneration. We conclude that the critical early steps of myocyte dedifferentiation in EOM regeneration are dependent on Fgf signaling.

Genetic inhibition of fibroblast growth factor receptor 1 in knee cartilage attenuates the degeneration of articular cartilage in adult mice

Fibroblast growth factor (FGF) family members are involved in the regulation of articular cartilage homeostasis. The aim of this study was to investigate the function of FGF receptor 1 (FGFR-1) in the development of osteoarthritis (OA) and its underlying mechanisms. FGFR-1 was deleted from the articular chondrocytes of adult mice in a cartilage-specific and tamoxifen-inducible manner. Two OA models (aging-associated spontaneous OA, and destabilization-induced OA), as well as an antigen-induced arthritis (AIA) model, were established and tested in Fgfr1-deficient and wild-type (WT) mice. Alterations in cartilage structure and the loss of proteoglycan were assessed in the knee joints of mice of either genotype, using these 3 arthritis models. Primary chondrocytes were isolated and the expression of key regulatory molecules was assessed quantitatively. In addition, the effect of an FGFR-1 inhibitor on human articular chondrocytes was examined. The gross morphologic features of Fgfr1-deficient mice were comparable with those of WT mice at both the postnatal and adult stages. The articular cartilage of 12-month-old Fgfr1-deficient mice displayed greater aggrecan staining compared to 12-month-old WT mice. Fgfr1 deficiency conferred resistance to the proteoglycan loss induced by AIA and attenuated the development of cartilage destruction after surgically induced destabilization of the knee joint. The chondroprotective effect of FGFR-1 inhibition was largely associated with decreased expression of matrix metalloproteinase 13 (MMP-13) and up-regulation of FGFR-3 in mouse and human articular chondrocytes. Disruption of FGFR-1 in adult mouse articular chondrocytes inhibits the progression of cartilage degeneration. Down-regulation of MMP-13 expression and up-regulation of FGFR-3 levels may contribute to the phenotypic changes observed in Fgfr1-deficient mice.

Maternal behavior in transgenic mice with reduced fibroblast growth factor receptor function in gonadotropin-releasing hormone neurons

BackgroundFibroblast growth factors (FGFs) and their receptors (FGFRs) are necessary for the proper development of gonadotropin-releasing hormone (GnRH) neurons, which are key activators of the hypothalamo-pituitary-gonadal axis. Transgenic mice that have the targeted expression of a dominant negative FGFR (dnFGFR) in GnRH neurons (dnFGFR mice) have a 30% decrease of GnRH neurons. Additionally, only 30–40% of the pups born to the transgenic dams survive to weaning age. These data raised the possibility that FGFR defects in GnRH neurons could adversely affect maternal behavior via novel mechanisms.MethodsWe first determined if defective maternal behavior in dnFGFR mothers may contribute to poor pup survival by measuring pup retrieval and a battery of maternal behaviors in primiparous control (n = 10–12) and dnFGFR (n = 13–14) mothers. Other endocrine correlates of maternal behaviors, including plasma estradiol levels and hypothalamic pro-oxyphysin and GnRH transcript levels were also determined using enzyme-linked immunoassay and quantitative reverse transcription polymerase chain reaction, respectively.ResultsMaternal behaviors (% time crouching with pups, time off pups but not feeding, time feeding, and total number of nesting bouts) were not significantly different in dnFGFR mice. However, dnFGFR dams were more likely to leave their pups scattered and took significantly longer to retrieve each pup compared to control dams. Further, dnFGFR mothers had significantly lower GnRH transcripts and circulating E2, but normal pro-oxyphysin transcript levels.ConclusionsOverall, this study suggests a complex scenario in which a GnRH system compromised by reduced FGF signaling leads to not only suboptimal reproductive physiology, but also suboptimal maternal behavior.

Abstract 17023: PAI-1 Deficiency Protects Against Accelerated Aging in Klotho-Deficient Mice

The klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends life span when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. Klotho protein and FGF receptor function as co-receptors for FGF23 and regulate phosphate and vitamin D metabolism. Klotho also plays a role in the regulation of calcium channel TRPV5, insulin, IGF-1, and Wnt. Klotho deficient (kl -/- ) mice display a phenotype consistent with accelerated aging, including a markedly truncated lifespan (approx. 10 weeks), infertility, osteoporosis, renal sclerosis and obstructive pulmonary disease. The biochemical hallmarks of kl -/- include elevated plasma levels of FGF23, phosphate, calcium, vitamin D, and plasma PAI-1. Given the relationship between elevated PAI-1 and nephrosclerosis and tissue fibrosis, we hypothesized that PAI-1 deficiency would attenuate the phenotype of kl -/- mice. Partial or complete PAI-1 deficiency had no significant effects on serum levels of calcium, phosphate, and creatinine levels, while plasma levels of FGF23 were reduced more than 60-fold in kl -/- -pai-1 -/- (P=0.01) and kl -/- -pai-1 +/- (P=0.004) mice. Furthermore, both kl -/- -pai-1 -/- and kl -/- -pai-1 +/- mice exhibit normal fertility. Most importantly, partial and complete PAI-1 deficiency more than doubled the lifespan of kl -/- mice, with kl -/- -pai-1 -/- living 148±79 days, (n=9, p=0.009)) and kl -/- -pai-1 +/- surviving 208±155 days (n=17, p=0.002). These findings indicate that PAI-1 contributes to some of the critical components of the kl -/- phenotype. These results suggest that drugs that either directly or indirectly reduce PAI-1 might have beneficial effects in delaying age-related pathologies.

Compound deletion of Fgfr3 and Fgfr4 partially rescues the Hyp mouse phenotype.

Uncertainty exists regarding the physiologically relevant fibroblast growth factor (FGF) receptor (FGFR) for FGF23 in the kidney and the precise tubular segments that are targeted by FGF23. Current data suggest that FGF23 targets the FGFR1c-Klotho complex to coordinately regulate phosphate transport and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] production in the proximal tubule. In studies using the Hyp mouse model, which displays FGF23-mediated hypophosphatemia and aberrant vitamin D, deletion of Fgfr3 or Fgfr4 alone failed to correct the Hyp phenotype. To determine whether FGFR1 is sufficient to mediate the renal effects of FGF23, we deleted Fgfr3 and Fgfr4 in Hyp mice, leaving intact the FGFR1 pathway by transferring compound Fgfr3/Fgfr4-null mice on the Hyp background to create wild-type (WT), Hyp, Fgfr3(-/-)/Fgfr4(-/-), and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice. We found that deletion of Fgfr3 and Fgfr4 in Fgfr3(-/-)/Fgfr4(-/-) and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice induced an increase in 1,25(OH)(2)D. In Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice, it partially corrected the hypophosphatemia (P(i) = 9.4 ± 0.9, 6.1 ± 0.2, 9.1 ± 0.4, and 8.0 ± 0.5 mg/dl in WT, Hyp, Fgfr3(-/-)/Fgfr4(-/-), and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice, respectively), increased Na-phosphate cotransporter Napi2a and Napi2c and Klotho mRNA expression in the kidney, and markedly increased serum FGF23 levels (107 ± 20, 3,680 ± 284, 167 ± 22, and 18,492 ± 1,547 pg/ml in WT, Hyp, Fgfr3(-/-)/Fgfr4(-/-), and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice, respectively), consistent with a compensatory response to the induction of end-organ resistance. Fgfr1 expression was unchanged in Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice and was not sufficient to transduce the full effects of FGF23 in Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice. These studies suggest that FGFR1, FGFR3, and FGFR4 act in concert to mediate FGF23 effects on the kidney and that loss of FGFR function leads to feedback stimulation of Fgf23 expression in bone.

Impact of Fibroblast Growth Factor 23 on Lipids and Atherosclerosis in Hemodialysis Patients

Levels of fibroblast growth factor (FGF) 23, a phosphatonin, are frequently elevated in patients with end-stage renal disease (ESRD) who are on maintenance hemodialysis (MHD). However, the role of FGF23 remains unclear because renal FGF receptor function might be impaired. The present cross-sectional study examines a cohort of patients (n = 196) on MHD who were not undergoing therapy with lipid-lowering drugs including sevelamer. Non-fasting venous blood samples were withdrawn before the hemodialysis (HD) session on the third day after the previous HD session to measure serum levels of albumin, calcium (Ca), phosphate (P), alkaline phosphatase, intact parathyroid hormone (PTH), total cholesterol (C), high-density lipoprotein (HDL)-C, low-density lipoprotein(LDL)-C, oxidative LDL-C, high-sensitivity C-reactive protein (HsCRP), interleukin-6 (IL-6), and FGF23. Nutritional status was assessed using the geriatric nutritional risk index (GNRI). Carotid intima-medial thickness (CIMT) was assessed using a B-mode ultrasound scanner. FGF23 was positively correlated with P, Ca(alb)xP product, and intact PTH, and inversely correlated with C and non-HDL-C. In the higher FGF23 tertile, levels of both non-HDL-C and C were significantly decreased and CIMT was less elevated compared to the lower FGF23 tertile. Multivariate analysis showed that the higher FGF23 tertile was independently associated with decreases in C (adjusted r(2) = 0.14) and non-HDL-C (adjusted r(2) = 0.20) levels and with a less-pronounced increase in CIMT (adjusted r(2) = 0.14). High FGF23 appears to be an independent biomarker of a decrease in C and non-HDL-C that is negatively associated with atherosclerosis in patients on MHD.

MAP'ing CNS Development and Cognition: An ERKsome Process

The ERK MAP kinase signaling cascade plays critical roles in brain development, learning, memory, and cognition. It has recently been appreciated that mutation or deletion of elements within this signaling pathway leads to developmental syndromes in humans that are associated with impaired cognitive function and autism. Here, we review recent studies that provide insight into the biological roles of the ERKs in the brain that may underlie the cognitive deficits seen in these syndromes.

Neural cell adhesion molecule regulates the cellular response to fibroblast growth factor

Neural cell adhesion molecule (NCAM) mediates cell-cell adhesion and signaling in the nervous system, yet NCAM is also expressed in non-neural tissues, in which its function has in most parts remained elusive. We have previously reported that NCAM stimulates cell-matrix adhesion and neurite outgrowth by activating fibroblast growth factor receptor (FGFR) signaling. Here, we investigated whether the interplay between NCAM and FGFR has any impact on the response of FGFR to its classical ligands, FGFs. To this end, we employed two fibroblast cell lines, NCAM-negative L cells and NCAM-positive NIH-3T3 cells, in which the expression of NCAM was manipulated by means of transfection or RNAi technologies, respectively. The results demonstrate that NCAM expression reduces FGF-stimulated ERK1/2 activation, cell proliferation and cell-matrix adhesion, in both L and NIH-3T3 cells. Furthermore, our data show that NCAM inhibits the binding of FGF to its high-affinity receptor in a competitive manner, providing the mechanisms for the NCAM-mediated suppression of FGF function. In this context, a small peptide that mimics the binding of NCAM to FGFR was sufficient to block FGF-dependent cell proliferation. These findings point to NCAM as being a major regulator of FGF-FGFR interaction, thus introducing a novel type of control mechanism for FGFR activity and opening new therapeutic perspectives for those diseases characterized by aberrant FGFR function.

Regulation of Ephexin1, a Guanine Nucleotide Exchange Factor of Rho Family GTPases, by Fibroblast Growth Factor Receptor-mediated Tyrosine Phosphorylation

Fibroblast growth factor (FGF) signal is implicated in not only cell proliferation, but cell migration and morphological changes. Several different Rho family GTPases downstream of the Ras/ERK pathway are postulated to mediate the latter functions. However, none have been recognized to be directly coupled to FGF receptors (FGFRs). We have previously reported that EphA4 and FGFRs hetero-oligomerize through their cytoplasmic domains, trans-activate each other, and transduce a signal for cell proliferation through a docking protein, FRS2alpha (Yokote, H., Fujita, K., Jing, X., Sawada, T., Liang, S., Yao, L., Yan, X., Zhang, Y., Schlessinger, J., and Sakaguchi, K. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 18866-18871). Here, we have found that ephexin1, a guanine nucleotide exchange factor for Rho family GTPases, constitutes another downstream component of the receptor complex. Ephexin1 directly binds to the kinase domain of FGFR mainly through its DH and PH domains. The binding appears to become weaker and limited to the DH domain when FGFRs become activated. FGFR-mediated phosphorylation of ephexin1 enhances the guanine nucleotide exchange activity toward RhoA without affecting the activity to Rac1 or Cdc42. The FGFR-mediated tyrosine phosphorylation includes, but is not limited to, the residue (Tyr-87) phosphorylated by Src family kinase, which is known to be activated following EphA4 activation. The Tyr-to-Asp mutations that mimic the tyrosine phosphorylation in some of the putative FGFR-mediated phosphorylation sites increase the nucleotide exchange activity for RhoA without changing the activity for Rac1 or Cdc42. From these results, we conclude that ephexin1 is located immediately downstream of the EphA4-FGFR complex and the function is altered by the FGFR-mediated tyrosine phosphorylation at multiple sites.

FGF signaling delineates the cardiac progenitor field in the simple chordate, Ciona intestinalis.

Comprehensive gene networks in Ciona intestinalis embryos provide a foundation for characterizing complex developmental processes, such as the initial phases of chordate heart development. The basic helix-loop-helix regulatory gene Ci-Mesp is required for activation of cardiac transcription factors. Evidence is presented that Ci-Ets1/2, a transcriptional effector of receptor tyrosine kinase (RTK) signaling, acts downstream from Mesp to establish the heart field. Asymmetric activation of Ets1/2, possibly through localized expression of FGF9, drives heart specification within this field. During gastrulation, Ets1/2 is expressed in a group of four cells descended from two Mesp-expressing founder cells (the B7.5 cells). After gastrulation, these cells divide asymmetrically; the smaller rostral daughters exhibit RTK activation (phosphorylation of ERK) and form the heart lineage while the larger caudal daughters form the anterior tail muscle lineage. Inhibition of RTK signaling prevents heart specification. Targeted inhibition of Ets1/2 activity or FGF receptor function also blocks heart specification. Conversely, application of FGF or targeted expression of constitutively active Ets1/2 (EtsVp16) cause both rostral and caudal B7.5 lineages to form heart cells. This expansion produces an unexpected phenotype: transformation of a single-compartment heart into a functional multicompartment organ. We discuss these results with regard to the development and evolution of the multichambered vertebrate heart.

Gene regulatory networks for the development and evolution of the chordate heart: Figure 1.

Gene regulatory networks consisting of subcircuits of transcription factors and intercellular signaling molecules are key to an understanding of the complex mechanisms of animal development and evolution (Davidson and Erwin 2006). One of the most intensively studied genetic networks is that for the formation of the animal heart. Since the discovery of the “tinman” gene (Bodmer 1993) and its vertebrate homolog, Nkx2.5 (Lyons et al. 1995), many genes have been found to be involved in heart development in a wide range of animals, from insects to mammals. Actually, it is well established that a gene circuit consisting of GATA, Nkx, and Hand is evolutionarily highly conserved. This gene circuit constitutes a “kernel,” which is evolutionarily inflexible and performs essential regulatory functions in building a body part (Davidson and Erwin 2006). Analyses of vertebrate heart development revealed that the differentiation of cardiac muscle cells and morphogenesis of the heart are governed by this heart kernel gene regulatory network (Cripps and Olson 2002; Harvey 2002; Buckingham et al. 2005). An important question to be answered about the formation of the heart in vertebrates and other chordates is how this kernel is turned on at the earliest stages of the heart cell specification process to establish the heart field. Another intriguing question in chordate heart formation is how the dualor multichambered heart of vertebrates evolved. It is generally believed that the ancestral chordate resembled the present-day ascidian tadpole. The morphogenetic movement of heart precursor cells during ascidian larval development and metamorphosis is reminiscent of those in vertebrates (Davidson and Levine 2003). However, the ascidian tube-like heart lacks chambers. The innovation of the chambered heart was a key event in vertebrate evolution, because the chambered heart generates one-way blood flow with high pressure, a critical requirement for the efficient blood supply of large-body vertebrates. In this issue of Genes & Development, Davidson et al. (2006) addressed these questions by examining the function of an Ets-containing transcription factor in the tunicate, Ciona intestinalis. They found that Ci-Ets1/2 (because this is one of two Ciona orthologs for vertebrate Ets1 and Ets2 and Drosophila pointed, it was originally called Ci-ets/pointed2) (Yagi et al. 2003) establishes the heart field, probably through an FGF signal acting downstream from Ci-Mesp, a basic helix–loop–helix transcription factor gene required for the initial specification of heart precursor cells (Satou et al. 2004). Targeted inhibition of Ci-Ets1/2 or FGF receptor function blocks heart formation. Moreover, targeted expression of a constitutively active Ci-Ets1/2 causes the expansion of the heart field by forced recruitment of larval tail muscle cells that express Ci-Mesp. Interestingly, this heart field expansion, evoked by the subtle alteration of the heart genetic program, caused a morphological change—that is, to a heart with two compartments.

Bad bones, absent smell, selfish testes: The pleiotropic consequences of human FGF receptor mutations

The discovery in 1994 that highly specific mutations of fibroblast growth factor (FGF) receptor 3 caused the most common form of human short-limbed dwarfism, achondroplasia, heralded a new era in FGF receptor (FGFR) biology. A decade later, the purpose of this review is to survey how the study of humans with FGFR mutations continues to provide insights into FGFR function in health and disease, and the clinical applications of these findings. Amongst the most interesting recent discoveries have been the description of novel phenotypes associated with FGFR1 and FGFR3 mutations; identification of fundamental differences in the cellular mechanisms of mutant FGFR2 and FGFR3 action; and the direct identification of FGFR2 and FGFR3 mutations in sperm. These clinical observations illustrate the pleiotropism of FGFR action and fuel ongoing efforts to understand the rich biology and pathophysiology of the FGF signalling system.

Bad bones, absent smell, selfish testes: The pleiotropic consequences of human FGF receptor mutations

The discovery in 1994 that highly specific mutations of fibroblast growth factor (FGF) receptor 3 caused the most common form of human short-limbed dwarfism, achondroplasia, heralded a new era in FGF receptor (FGFR) biology. A decade later, the purpose of this review is to survey how the study of humans with FGFR mutations continues to provide insights into FGFR function in health and disease, and the clinical applications of these findings. Amongst the most interesting recent discoveries have been the description of novel phenotypes associated with FGFR1 and FGFR3 mutations; identification of fundamental differences in the cellular mechanisms of mutant FGFR2 and FGFR3 action; and the direct identification of FGFR2 and FGFR3 mutations in sperm. These clinical observations illustrate the pleiotropism of FGFR action and fuel ongoing efforts to understand the rich biology and pathophysiology of the FGF signalling system.