The murine Fgfrl1 receptor is essential for the development of the metanephric kidney

Comprehending fibroblast growth factor receptor like 1: Oncogene or tumor suppressor?

Fibroblast Growth Factor Receptor Like 1 (FGFRL1) signaling has crucial role in a multitude of processes during genetic diseases, embryonic development and various types of cancer. Due to its partial structural similarity with its classical Fibroblast Growth Factor Receptor [FGFR] counterparts and lack of tyrosine kinase domain, FGFRL1 was thought to work as a decoy receptor in FGF/FGFR signaling. Later on, growing number evidences showed that expression of FGFRL1 affects major pathways like ERK1/2, Akt and others, which are dysfunctional in a wide range of human cancers. In this review, we provide an overview of the current understanding of FGFRL1 and its roles in cell differentiation, adhesion and proliferation pathways . Overexpression of FGFRL1 might lead to tumor progression and invasion. In this context, inhibitors for FGFRL1 might have therapeutic benefits in human cancer prognosis.

Fibroblast growth factor and kidney disease: Updates for emerging novel therapeutics

The discovery of fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) provided a profound new insight into physiological and metabolic functions. FGF has a large family by having divergent structural elements and enable functional divergence and specification. FGF and FGFRs are highly expressed during kidney development. Signals from the ureteric bud regulate morphogenesis, nephrogenesis, and nephron progenitor survival. Thus, FGF signaling plays an important role in kidney progenitor cell aggregation at the sites of new nephron formation. This review will summarize the current knowledge about functions of FGF signaling in kidney development and their ability to promote regeneration in injured kidneys and its use as a biomarker and therapeutic target in kidney diseases. Further studies are essential to determine the predictive significance of the various FGF/FGFR deviations and to integrate them into clinical algorithms.

Evidence that FGFRL1 contributes to congenital diaphragmatic hernia development in humans

Fibroblast growth factor receptor-like 1 (FGFRL1) encodes a transmembrane protein that is related to fibroblast growth factor receptors but lacks an intercellular tyrosine kinase domain. in vitro studies suggest that FGFRL1 inhibits cell proliferation and promotes cell differentiation and cell adhesion. Mice that lack FGFRL1 die shortly after birth from respiratory distress and have abnormally thin diaphragms whose muscular hypoplasia allows the liver to protrude into the thoracic cavity. Haploinsufficiency of FGFRL1 has been hypothesized to contribute to the development of congenital diaphragmatic hernia (CDH) associated with Wolf-Hirschhorn syndrome. However, data from both humans and mice suggest that disruption of one copy of FGFRL1 alone is insufficient to cause diaphragm defects. Here we report a female fetus with CDH whose 4p16.3 deletion allows us to refine the Wolf-Hirschhorn syndrome CDH critical region to an approximately 1.9 Mb region that contains FGFRL1. We also report a male infant with isolated left-sided diaphragm agenesis who carried compound heterozygous missense variants in FGFRL1. These cases provide additional evidence that deleterious FGFRL1 variants may contribute to the development of CDH in humans.

[Expression of fibroblast growth factor receptor like 1 protein in oral squamous cell carcinoma and its influence on tumor cell proliferation and migration]

OBJECTIVE

This study aims to investigate the expression of fibroblast growth factor receptor like 1 (FGFRL1) in oral squamous cell carcinoma (OSCC) and reveals its association with tumor cell proliferation and migration.

METHODS

Western blot was performed to detect the expression of FGFRL1 protein in OSCC tissues, adjacent normal tissues, OSCC cell lines and normal epithelial cells. After knocking down of FGFRL1 in HN4 cells, CCK-8 and Ki67 assays were performed to detect cell proliferation, wounding healing assay and transwell were performed to detect cell-migration. Western blot was used to detect the expression of protein related to epithelial-mesenchymal transition (EMT).

RESULTS

The expression of FGFRL1 in OSCC tissues was higher than that in adjacent nontumor tissues, respectively (t=2.820, P=0.047 8). Moreover, the expression of FGFRL1 in OSCC cells was higher than that in HOK cells. Quantitative real-time polymerase chain reaction (qRT-PCR) showed that FGFRL1 expression of FGFRL1 RNA in HOK cells was lower than that in OSCC cells. HN4 cells transfected with FGFRL1 siRNA were included in the experimental group, whereas HN4 cells treated with NC siRNA were included in the control group. CCK-8 experiment showed no significant difference between the experimental and control groups with regard to proliferation ability at 48 h (P=0.478 1) and 72 h (P=0.334 2). Migration experiment showed that the wound healing areas in the experimental group after 12 h (P=0.022 8), 24 h (P=0.005 1), and 36 h (P=0.009 5）were smaller than that in the control group. Transwell invasion assay showed that the number of invaded cells in the experimental group after 16 h (P=0.008 7) and 24 h (P=0.008 6) were lower than that in the control group. Knocking-down FGFRL1 up-regulated the expression of E-cadherin and down-regulated the expression of N-cadherin and Vimentin in HN4 cells.

CONCLUSIONS

FGFRL1 expression in the OSCC tissues was significantly higher than that in the adjacent nontumor tissues. FGFRL1 expression in the OSCC cells was significantly higher than that in the HOK cells, and FGFRL1 had no effect on cell proliferation but promoted tumor cell migration and EMT.

Dissecting the Interaction of FGF8 with Receptor FGFRL1

In mammals, the novel protein fibroblast growth factor receptor-like 1 (FGFRL1) is involved in the development of metanephric kidneys. It appears that this receptor controls a crucial transition of the induced metanephric mesenchyme to epithelial renal vesicles, which further develop into functional nephrons. FGFRL1 knockout mice lack metanephric kidneys and do not express any fibroblast growth factor (FGF) 8 in the metanephric mesenchyme, suggesting that FGFRL1 and FGF8 play a decisive role during kidney formation. FGFRL1 consists of three extracellular immunoglobulin (Ig) domains (Ig1-Ig2-Ig3), a transmembrane domain and a short intracellular domain. We have prepared the extracellular domain (Ig123), the three individual Ig domains (Ig1, Ig2, Ig3) as well as all combinations containing two Ig domains (Ig12, Ig23, Ig13) in recombinant form in human cells. All polypeptides that contain the Ig2 domain (Ig123, Ig12, Ig23, Ig2) were found to interact with FGF8 with very high affinity, whereas all constructs that lack the Ig2 domain (Ig1, Ig3, Ig13) poorly interacted with FGF8 as shown by ELISA and surface plasmon resonance. It is therefore likely that FGFRL1 represents a physiological receptor for FGF8 in the kidney and that the ligand primarily binds to the Ig2 domain of the receptor. With Biacore experiments, we also measured the affinity of FGF8 for the different constructs. All constructs containing the Ig2 domain showed a rapid association and a slow dissociation phase, from which a K_D of 2–3 × 10⁻⁹ M was calculated. Our data support the hypothesis that binding of FGF8 to FGFRL1 could play an important role in driving the formation of nephrons in the developing kidney.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Functional domains of the FgfrL1 receptor

FgfrL1 is a novel growth factor receptor that is primarily expressed in musculoskeletal tissues and the kidney. FgfrL1-deficient mice have a malformed diaphragm and no kidneys. Such animals die immediately after birth because they are not able to inflate their lungs. The FgfrL1 molecule is composed of three extracellular Ig domains, a transmembrane helix and a short intracellular domain. To investigate the contribution of each of these domains to the function of the novel receptor, we generated mice with deletions of the individual domains. Mice lacking the intracellular domain are viable and phenotypically normal. Mice lacking the first (N-terminal) Ig domain are also viable and normal, but have a reduced life span. Mice lacking the Ig2 or the Ig3 domain are born alive, but die within 24 ​h after birth. Ig2-deficient animals exhibit substantially smaller kidneys than wild-type littermates and contain a lower number of glomeruli. Ig3-deficient mice completely lack metanephric kidneys. Interestingly, both the Ig2 and the Ig3-deficient animals show only minor alterations in the diaphragm, which still enables them to inflate their lungs after birth. Our results demonstrate that the principal function of the FgfrL1 receptor is to control the growth of the metanephric kidneys by regulating nephrogenesis. It appears that this function is primarily accomplished by the Ig3 domain with some contribution of the Ig2 domain. It is conceivable that the two domains interact with an Fgf ligand and another molecule from the surface of neighboring cells to induce condensation of the metanephric mesenchyme to renal epithelia and glomeruli.

Fibroblast growth factor receptor-like-1: a new therapeutic target and unfavorable prognostic indicator for rectal adenocarcinoma

Fibroblast growth factor receptor-like-1 (FGFRL1) is important to cell motility and links with tumorigenic potential in various types of cancers. To investigate the biological function and underlying mechanism of FGFRL1 in rectal adenocarcinoma, we conducted this study. TCGA and Oncomine databases were used to analyze FGFRL1 expression and its association with clinical characteristics or overall survival (OS) in rectal adenocarcinoma patients. siRNA strategy was implemented to knockdown FGFRL1 expression in rectal adenocarcinoma cells. CCK8, colony formation, wound healing, and transwell assays were implemented to measure cell behaviors. qRT-PCR and western blot were utilized to identify mRNA and protein expression levels. FGFRL1 was significantly increased in rectal adenocarcinoma tissue samples, either colon or rectum. High-regulation of FGFRL1 expression induced poorer outcome of rectal adenocarcinoma patients. Downregulation of FGFRL1 inhibited the proliferation, colony formation, migration, and invasion of SW837 cells. The MAPK pathway-related proteins, phosphorylation of MEK and ERK, were also decreased after si-FGFRL1 transfection. These findings demonstrated that FGFRL1, acting as a potential inducator, may promote the progression of rectal adenocarcinoma via activating the MAPK signaling pathway.

The Genetic Basis of Scale-Loss Phenotype in the Rapid Radiation of Takifugu Fishes

Rapid radiation associated with phenotypic divergence and convergence provides an opportunity to study the genetic mechanisms of evolution. Here we investigate the genus Takifugu that has undergone explosive radiation relatively recently and contains a subset of closely-related species with a scale-loss phenotype. By using observations during development and genetic mapping approaches, we show that the scale-loss phenotype of two Takifugu species, T. pardalis Temminck & Schlegel and T. snyderi Abe, is largely controlled by an overlapping genomic segment (QTL). A search for candidate genes underlying the scale-loss phenotype revealed that the QTL region contains no known genes responsible for the evolution of scale-loss phenotype in other fishes. These results suggest that the genes used for the scale-loss phenotypes in the two Takifugu are likely the same, but the genes used for the similar phenotype in Takifugu and distantly related fishes are not the same. Meanwhile, Fgfrl1, a gene predicted to function in a pathway known to regulate bone/scale development was identified in the QTL region. Since Fgfr1a1, another memebr of the Fgf signaling pathway, has been implicated in scale loss/scale shape in fish distantly related to Takifugu, our results suggest that the convergence of the scale-loss phenotype may be constrained by signaling modules with conserved roles in scale development.

Fibroblast growth factor signaling mediates progenitor cell aggregation and nephron regeneration in the adult zebrafish kidney

The zebrafish kidney regenerates after injury by development of new nephrons from resident adult kidney stem cells. Although adult kidney progenitor cells have been characterized by transplantation and single cell RNA seq, signals that stimulate new nephron formation are not known. Here we demonstrate that fibroblast growth factors and FGF signaling is rapidly induced after kidney injury and that FGF signaling is required for recruitment of progenitor cells to sites of new nephron formation. Chemical or dominant negative blockade of Fgfr1 prevented formation of nephron progenitor cell aggregates after injury and during kidney development. Implantation of FGF soaked beads induced local aggregation of lhx1a:EGFP  ​+ ​kidney progenitor cells. Our results reveal a previously unexplored role for FGF signaling in recruitment of renal progenitors to sites of new nephron formation and suggest a role for FGF signaling in maintaining cell adhesion and cell polarity in newly forming kidney epithelia.

Spontaneous calcium activity in metanephric mesenchymal cells regulates branching morphogenesis in the embryonic kidney

The central role of calcium signaling during development of early vertebrates is well documented, but little is known about its role in mammalian embryogenesis. We have used immunofluorescence and time-lapse calcium imaging of cultured explanted embryonic rat kidneys to study the role of calcium signaling for branching morphogenesis. In mesenchymal cells, we recorded spontaneous calcium activity that was characterized by irregular calcium transients. The calcium signals were dependent on release of calcium from intracellular stores in the endoplasmic reticulum. Down-regulation of the calcium activity, both by blocking the sarco-endoplasmic reticulum Ca²⁺-ATPase and by chelating cytosolic calcium, resulted in retardation of branching morphogenesis and a reduced formation of primitive nephrons but had no effect on cell proliferation. We propose that spontaneous calcium activity contributes with a stochastic factor to the self-organizing process that controls branching morphogenesis, a major determinant of the ultimate number of nephrons in the kidney.-Fontana, J. M., Khodus, G. R., Unnersjö-Jess, D., Blom, H., Aperia, A., Brismar, H. Spontaneous calcium activity in metanephric mesenchymal cells regulates branching morphogenesis in the embryonic kidney.

Regulation of Renal Differentiation by Trophic Factors

Classically, trophic factors are considered as proteins which support neurons in their growth, survival, and differentiation. However, most neurotrophic factors also have important functions outside of the nervous system. Especially essential renal growth and differentiation regulators are glial cell line-derived neurotrophic factor (GDNF), bone morphogenetic proteins (BMPs), and fibroblast growth factors (FGFs). Here we discuss how trophic factor-induced signaling contributes to the control of ureteric bud (UB) branching morphogenesis and to maintenance and differentiation of nephrogenic mesenchyme in embryonic kidney. The review includes recent advances in trophic factor functions during the guidance of branching morphogenesis and self-renewal versus differentiation decisions, both of which dictate the control of kidney size and nephron number. Creative utilization of current information may help better recapitulate renal differentiation in vitro, but it is obvious that significantly more basic knowledge is needed for development of regeneration-based renal therapies.

Fibroblast growth factor receptor 5 (FGFR5) is a co-receptor for FGFR1 that is up-regulated in beta-cells by cytokine-induced inflammation

Fibroblast growth factor receptor-1 (FGFR1) activity at the plasma membrane is tightly controlled by the availability of co-receptors and competing receptor isoforms. We have previously shown that FGFR1 activity in pancreatic beta-cells modulates a wide range of processes, including lipid metabolism, insulin processing, and cell survival. More recently, we have revealed that co-expression of FGFR5, a receptor isoform that lacks a tyrosine-kinase domain, influences FGFR1 responses. We therefore hypothesized that FGFR5 is a co-receptor to FGFR1 that modulates responses to ligands by forming a receptor heterocomplex with FGFR1. We first show here increased FGFR5 expression in the pancreatic islets of nonobese diabetic (NOD) mice and also in mouse and human islets treated with proinflammatory cytokines. Using siRNA knockdown, we further report that FGFR5 and FGFR1 expression improves beta-cell survival. Co-immunoprecipitation and quantitative live-cell imaging to measure the molecular interaction between FGFR5 and FGFR1 revealed that FGFR5 forms a mixture of ligand-independent homodimers (∼25%) and homotrimers (∼75%) at the plasma membrane. Interestingly, co-expressed FGFR5 and FGFR1 formed heterocomplexes with a 2:1 ratio and subsequently responded to FGF2 by forming FGFR5/FGFR1 signaling complexes with a 4:2 ratio. Taken together, our findings identify FGFR5 as a co-receptor that is up-regulated by inflammation and promotes FGFR1-induced survival, insights that reveal a potential target for intervention during beta-cell pathogenesis.

FGFRL1 deficiency reduces motility and tumorigenic potential of cells derived from oesophageal squamous cell carcinomas

Oesophageal squamous cell carcinoma (ESCC) is an aggressive cancer that resulted in ~400,000 mortalities worldwide in 2012. It was reported previously that fibroblast growth factor receptor-like 1 (FGFRL1) is highly expressed in ESCC patients with lymph node metastasis and poor prognosis accordingly. FGFRL1 is an FGFR that lacks tyrosine kinase activity, whereas the activity is critical for other FGFRs to activate intracellular signalling. The mechanism by which FGFRL1 promotes the aggressiveness of ESCCs is unknown. In the present study, two independent FGFRL1-deficient cell lines were generated from human ESCC KYSE520 cells, in order to investigate the relationship of FGFRL1 with the aggressiveness of ESCCs. FGFRL1-deficiency did not affect proliferation of KYSE520 cells in vitro. However, a xenograft mouse model demonstrated that FGFRL1-deficiency decelerated tumour growth in vivo. The haematoxylin-eosin staining identified that FGFRL1-deficient cells formed well-differentiated squamous cell carcinomas, whereas wild-type cells formed moderately differentiated squamous cell carcinomas. Microarray analysis of mRNA expression revealed that FGFRL1-depletion resulted in decreased expression of proteins associated with motility and invasion of tumour cells, matrix metalloproteinase-1 and fibroblast growth factor binding protein 1. The wound-healing assay indicated that depleting FGFRL1 reduced cell motility. Furthermore, the invasiveness of FGFRL1-deficient cells was lesser than that of wild-type KYSE520 cells. In the FGFRL1-deficient KYSE520 cells, actin filaments around the nucleus were observed sparsely, whereas the filaments along the plasma membranes were observed as frequently as those in the parent KYSE520 cells. These results demonstrate that FGFRL1 may be involved in regulation of protein expression, actin filament assembly and tumorigenic potential of ESCC cells.

Receptor FGFRL1 acts as a tumor suppressor in nude mice when overexpressed in HEK 293 Tet-On cells

Fibroblast growth factor receptor-like 1 (FGFRL1) is a transmembrane receptor that interacts with heparin and FGF ligands. In contrast to the classical FGF receptors, FGFR1 to FGFR4, it does not appear to affect cell growth and proliferation. In the present study, an inducible gene expression system was utilized in combination with a xenograft tumor model to investigate the effects of FGFRL1 on cell adhesion and tumor formation. It was determined that recombinant FGFRL1 promotes the adhesion of HEK 293 Tet-On® cells in vitro. Moreover, when such cells are induced to express FGFRL1ΔC they aggregate into huge clusters. If injected into nude mice, the cells form large tumors. Notably, this tumor growth is completely inhibited when the expression of FGFRL1 is induced. The forced expression of FGFRL1 in the tumor tissue may restore contact inhibition, thereby preventing growth of the cells in nude mice. The results of the present study demonstrate that FGFRL1 acts as a tumor suppressor similar to numerous other cell adhesion proteins. It is therefore likely that FGFRL1 functions as a regular cell-cell adhesion protein.

Fibroblast growth factor receptor signaling in kidney and lower urinary tract development

Fibroblast growth factor receptors (FGFRs) and FGF ligands are highly expressed in the developing kidney and lower urinary tract. Several classic studies showed many effects of exogenous FGF ligands on embryonic renal tissues in vitro and in vivo. Another older landmark publication showed that mice with a dominant negative Fgfr fragment had severe renal dysplasia. Together, these studies revealed the importance of FGFR signaling in kidney and lower urinary tract development. With the advent of modern gene targeting techniques, including conditional knockout approaches, several publications have revealed critical roles for FGFR signaling in many lineages of the kidney and lower urinary tract at different stages of development. FGFR signaling has been shown to be critical for early metanephric mesenchymal patterning, Wolffian duct patterning including induction of the ureteric bud, ureteric bud branching morphogenesis, nephron progenitor survival and nephrogenesis, and bladder mesenchyme patterning. FGFRs pattern these tissues by interacting with many other growth factor signaling pathways. Moreover, the many genetic Fgfr and Fgf animal models have structural defects mimicking numerous congenital anomalies of the kidney and urinary tract seen in humans. Finally, many studies have shown how FGFR signaling is critical for kidney and lower urinary tract patterning in humans.

Receptor FGFRL1 does not promote cell proliferation but induces cell adhesion

Fibroblast growth factor receptor (FGFR)-like protein 1 (FGFRL1) is the most recently discovered member of the FGFR family. Owing to the fact that it interacts with FGF ligands, but lacks the intracellular tyrosine kinase domain, several researchers have speculated that it may function as a decoy receptor and exert a negative effect on cell proliferation. In this study, we performed overexpression experiments with TetOn‑inducible cell clones and downregulation experiments with siRNA oligonucleotides, and found that FGFRL1 had absolutely no effect on cell growth and proliferation. Likewise, we did not observe any influence of FGFRL1 on ERK1/2 activation and on the phosphorylation of 250 other signaling proteins analyzed by the Kinexus antibody microarray. On the other hand, with bacterial petri dishes, we observed a clear effect of FGFRL1 on cell adhesion during the initial hours after cell seeding. Our results suggest that FGFRL1 is a cell adhesion protein similar to the nectins rather than a signaling receptor similar to FGFR1-FGFR4.

Wnt, Ptk7, and FGFRL expression gradients control trunk positional identity in planarian regeneration

Mechanisms enabling positional identity re-establishment are likely critical for tissue regeneration. Planarians use Wnt/beta-catenin signaling to polarize the termini of their anteroposterior axis, but little is known about how regeneration signaling restores regionalization along body or organ axes. We identify three genes expressed constitutively in overlapping body-wide transcriptional gradients that control trunk-tail positional identity in regeneration. ptk7 encodes a trunk-expressed kinase-dead Wnt co-receptor, wntP-2 encodes a posterior-expressed Wnt ligand, and ndl-3 encodes an anterior-expressed homolog of conserved FGFRL/nou-darake decoy receptors. ptk7 and wntP-2 maintain and allow appropriate regeneration of trunk tissue position independently of canonical Wnt signaling and with suppression of ndl-3 expression in the posterior. These results suggest that restoration of regional identity in regeneration involves the interpretation and re-establishment of axis-wide transcriptional gradients of signaling molecules.

Renal dysplasia in the neonate

PURPOSE OF REVIEW

Renal dysplasia is classically described as a developmental disorder whereby the kidneys fail to undergo appropriate differentiation, resulting in the presence of malformed renal tissue elements. It is the commonest cause of chronic kidney disease and renal failure in the neonate. Although several genes have been identified in association with renal dysplasia, the underlying molecular mechanisms are often complex and heterogeneous in nature, and remain poorly understood.

RECENT FINDINGS

In this review, we describe new insights into the fundamental process of normal kidney development, and how the renal cortex and medulla are patterned appropriately during gestation. We review the key genes that are indispensable for this process, and discuss how patterning of the kidney is perturbed in the absence of these signaling pathways. The recent use of whole exome sequencing has identified genetic mutations in patients with renal dysplasia, and the results of these studies have increased our understanding of the pathophysiology of renal dysplasia.

SUMMARY

At present, there are no specific treatments available for patients with renal dysplasia. Understanding the molecular mechanisms of normal kidney development and the pathogenesis of renal dysplasia may allow for improved therapeutic options for these patients.

Fibroblast growth factor signaling in the vasculature

Despite their discovery as angiogenic factors and mitogens for endothelial cells more than 30 years ago, much remains to be determined about the role of fibroblast growth factors (FGFs) and their receptors in vascular development, homeostasis, and disease. In vitro studies show that members of the FGF family stimulate growth, migration, and sprouting of endothelial cells, and growth, migration, and phenotypic plasticity of vascular smooth muscle cells. Recent studies have revealed important roles for FGFs and their receptors in the regulation of endothelial cell sprouting and vascular homeostasis in vivo. Furthermore, recent work has revealed roles for FGFs in atherosclerosis, vascular calcification, and vascular dysfunction. The large number of FGFs and their receptors expressed in endothelial and vascular smooth muscle cells complicates these studies. In this review, we summarize recent studies in which new and unanticipated roles for FGFs and their receptors in the vasculature have been revealed.

Phylogenetic analysis of receptor FgfrL1 shows divergence of the C-terminal end in rodents

FGFRL1 is a member of the fibroblast growth factor receptor (FGFR) family. Similar to the classical receptors FGFR1-FGFR4, it contains three extracellular Ig-like domains and a single transmembrane domain. However, it lacks the intracellular tyrosine kinase domain that would be required for signal transduction, but instead contains a short intracellular tail with a peculiar histidine-rich motif. This motif has been conserved during evolution from mollusks to echinoderms and vertebrates. Only the sequences of FgfrL1 from a few rodents diverge at the C-terminal region from the canonical sequence, as they appear to have suffered a frameshift mutation within the histidine-rich motif. This mutation is observed in mouse, rat and hamster, but not in the closely related rodents mole rat (Nannospalax) and jerboa (Jaculus), suggesting that it has occurred after branching of the Muridae and Cricetidae from the Dipodidae and Spalacidae. The consequence of the frameshift is a deletion of a few histidine residues and an extension of the C-terminus by about 40 unrelated amino acids. A similar frameshift mutation has also been observed in a human patient with a craniosynostosis syndrome as well as in several patients with colorectal cancer and bladder tumors, suggesting that the histidine-rich motif is prone to mutation. The reason why this motif was conserved during evolution in most species, but not in mice, is not clear.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

Fibroblast growth factor receptor-Frs2α signaling is critical for nephron progenitors

Previous studies using transgenic Pax3cre mice have revealed roles for fibroblast growth factor receptors (Fgfrs) and Fgfr substrate 2α (Frs2α) signaling in early metanephric mesenchyme patterning and in ureteric morphogenesis. The role of Fgfr/Frs2α signaling in nephron progenitors is unknown. Thus, we generated mouse models using BAC transgenic Six2EGFPcre (Six2cre) mediated deletion of Fgfrs and/or Frs2α in nephron progenitors. Six2cre mediated deletion of Fgfr1 or Fgfr2 alone led to no obvious kidney defects. Six2creFgfr1(flox/flox)Fgfr2(flox/flox) (Fgfr1/2(NP-/-)) mice generate a discernable kidney; however, they develop nephron progenitor depletion starting at embryonic day 12.5 (E12.5) and later demonstrate severe cystic dysplasia. To determine the role of Frs2α signaling downstream of Fgfr2 in Fgfr1/2(NP-/-) mice, we generated Six2cre(,)Fgfr1(flox/flox)Fgfr2(LR/LR) (Fgfr1(NP-/-)Fgfr2(LR/LR)) mice that have point mutations in the Frs2α binding site of Fgfr2. Like Fgfr1/2(NP-/-) mice, Fgfr1(NP-/-)Fgfr2(LR/LR) develop nephron progenitor depletion, but it does not start until E14.5 and older mice have less severe cystic dysplasia than Fgfr1/2(NP-/-) To determine the role of Frs2α alone in nephron progenitors, we generated Six2creFrs2'A(flox/flox) (Frs2a(NP-/-)) mice. Frs2a(NP-/-)mice also develop nephron progenitor depletion and renal cysts, although these occurred later and were less severe than in the other Six2cre mutant mice. The nephron progenitor loss in all Six2cre mutant lines was associated with decreased Cited1 expression and increased apoptosis versus controls. FAC-sorted nephron progenitors in Six2cre Frs2'A(flox/flox) mice demonstrated evidence of increased Notch activity versus controls, which likely drives the progenitor defects. Thus, Fgfr1 and Fgfr2 have synergistic roles in maintaining nephron progenitors; furthermore, Fgfr signaling in nephron progenitors appears to be mediated predominantly by Frs2α.

Targeted disruption of the intracellular domain of receptor FgfrL1 in mice

FgfrL1 is the fifth member of the fibroblast growth factor receptor (Fgfr) family. Studies with FgfrL1 deficient mice have demonstrated that the gene plays an important role during embryonic development. FgfrL1 knock-out mice die at birth as they have a malformed diaphragm and lack metanephric kidneys. Similar to the classical Fgfrs, the FgfrL1 protein contains an extracellular part composed of three Ig-like domains that interact with Fgf ligands and heparin. However, the intracellular part of FgfrL1 is not related to the classical receptors and does not possess any tyrosine kinase activity. Curiously enough, the amino acid sequence of this domain is barely conserved among different species, with the exception of three motifs, namely a dileucine peptide, a tandem tyrosine-based motif YXXΦ and a histidine-rich sequence. To investigate the function of the intracellular domain of FgfrL1, we have prepared genetically modified mice that lack the three conserved sequence motifs, but instead contain a GFP cassette (FgfrL1ΔC-GFP). To our surprise, homozygous FgfrL1ΔC-GFP knock-in mice are viable, fertile and phenotypically normal. They do not exhibit any alterations in the diaphragm or the kidney, except for a slight reduction in the number of glomeruli that does not appear to affect life expectancy. In addition, the pancreas of both FgfrL1ΔC-GFP knock-in and FgfrL1 knock-out mice do not show any disturbances in the production of insulin, in contrast to what has been suggested by recent studies. Thus, the conserved motifs of the intracellular FgfrL1 domain are dispensable for organogenesis and normal life. We conclude that the extracellular domain of the protein must conduct the vital functions of FgfrL1.

Gene regulatory network of renal primordium development

Animal development progresses through the stepwise deployment of gene regulatory networks (GRN) encoded in the genome. Comparative analyses in different species and organ systems have revealed that GRN blueprints are composed of subcircuits with stereotypical architectures that are often reused as modular units. In this review, we report the evidence for the GRN underlying renal primordium development. In vertebrates, renal development is initiated by the induction of a field of intermediate mesoderm cells competent to undergo lineage specification and nephric (Wolffian) duct formation. Definition of the renal field leads to the activation of a core regulatory subcircuit composed of the transcription factors Pax2/8, Gata3 and Lim1. These transcription factors turn on a second layer of transcriptional regulators while also activating effectors of tissue morphogenesis and cellular specialization. Elongation and connection of the nephric duct to the cloaca (bladder/urethra primordium) is followed by metanephric kidney induction through signals emanating from the metanephric mesenchyme. Central to this process is the activation and positioning of the glial cell line-derived neurotrophic factor (Gdnf)-Ret signaling pathway by network subcircuits located in the mesenchyme and epithelial tissues of the caudal trunk. Evidence shows that each step of the renal primordium developmental program is regulated by structured GRN subunits organized in a hierarchical manner. Understanding the structure and dynamics of the renal GRN will help us understand the intrinsic phenotypical variability of congenital anomalies of the kidney and urinary tract and guide our approaches to regenerative medicine.

Evidence that the novel receptor FGFRL1 signals indirectly via FGFR1

Fibroblast growth factor (FGF) receptor-like protein 1 (FGFRL1) is a recently discovered member of the FGF receptor (FGFR) family. Similar to the classical FGFRs, it contains three extracellular immunoglobulin-like domains and interacts with FGF ligands. However, in contrast to the classical receptors, it does not contain any intracellular tyrosine kinase domain and consequently cannot signal by transphosphorylation. In mouse kidneys, FgfrL1 is expressed primarily at embryonic stages E14–E15 in regions where nascent nephrons develop. In this study, we used whole-mount in situ hybridization to show the spatial pattern of five different Fgfrs in the developing mouse kidney. We compared the expression pattern of FgfrL1 with that of other Fgfrs. The expression pattern of FgfrL1 closely resembled that of Fgfr1, but clearly differed from that of Fgfr2–Fgfr4. It is therefore conceivable that FgfrL1 signals indirectly via Fgfr1. The mechanisms by which FgfrL1 affects the activity of Fgfr1 remain to be elucidated.

Role of FGFRL1 and other FGF signaling proteins in early kidney development

The mammalian kidney develops from the ureteric bud and the metanephric mesenchyme. In mice, the ureteric bud invades the metanephric mesenchyme at day E10.5 and begins to branch. The tips of the ureteric bud induce the metanephric mesenchyme to condense and form the cap mesenchyme. Some cells of this cap mesenchyme undergo a mesenchymal-to-epithelial transition and differentiate into renal vesicles, which further develop into nephrons. The developing kidney expresses Fibroblast growth factor (Fgf)1, 7, 8, 9, 10, 12 and 20 and Fgf receptors Fgfr1 and Fgfr2. Fgf7 and Fgf10, mainly secreted by the metanephric mesenchyme, bind to Fgfr2b of the ureteric bud and induce branching. Fgfr1 and Fgfr2c are required for formation of the metanephric mesenchyme, however the two receptors can substitute for one another. Fgf8, secreted by renal vesicles, binds to Fgfr1 and supports survival of cells in the nascent nephrons. Fgf9 and Fgf20, expressed in the metanephric mesenchyme, are necessary to maintain survival of progenitor cells in the cortical region of the kidney. FgfrL1 is a novel member of the Fgfr family that lacks the intracellular tyrosine kinase domain. It is expressed in the ureteric bud and all nephrogenic structures. Targeted deletion of FgfrL1 leads to severe kidney dysgenesis due to the lack of renal vesicles. FgfrL1 is known to interact mainly with Fgf8. It is therefore conceivable that FgfrL1 restricts signaling of Fgf8 to the precise location of the nascent nephrons. It might also promote tight adhesion of cells in the condensed metanephric mesenchyme as required for the mesenchymal-to-epithelial transition.

Crossroads of Wnt and Hippo in epithelial tissues

Epithelial tissues undergo constant growth and differentiation during embryonic development and to replace damaged tissue in adult organs. These processes are governed by different signaling pathways that ultimately control the expression of genes associated with cell proliferation, patterning, and death. One essential pathway is Wnt, which controls tubulogenesis in several epithelial organs. Recently, Wnt has been closely linked to other signaling pathways, such as Hippo, that orchestrate proliferation and apoptosis to control organ size. There is evidence that epithelial cell junctions may sequester the transcription factors that act downstream of these signaling pathways, which would represent an important aspect of their functional regulation and their influence on cell behavior. Here, we review the transcriptional control exerted by the Wnt and Hippo signaling pathways during epithelial growth, patterning, and differentiation and recent advances in understanding of the regulation and crosstalk of these pathways in epithelial tissues.

Defining the signals that constitute the nephron progenitor niche

For decades we have known that reciprocal inductive interactions between the embryonic ureteric bud and the metanephric mesenchyme are the basis for kidney development. Signals from the mesenchyme promote the branching of the bud, whereas signals from the bud regulate the survival, proliferation, and differentiation of nephron progenitors. Due to the complex nature of the bud-derived signals, progress in identifying these factors has been slow. However, in the last several years, tremendous advances have been made in identifying specific roles for various secreted proteins in nephron progenitor cell development. Here, we briefly review the roles for Fgfs and Wnts in induction of the nephron progenitors.

Transgenic expression of nonclassically secreted FGF suppresses kidney repair

FGF1 is a signal peptide-less nonclassically released growth factor that is involved in angiogenesis, tissue repair, inflammation, and carcinogenesis. The effects of nonclassical FGF export in vivo are not sufficiently studied. We produced transgenic mice expressing FGF1 in endothelial cells (EC), which allowed the detection of FGF1 export to the vasculature, and studied the efficiency of postischemic kidney repair in these animals. Although FGF1 transgenic mice had a normal phenotype with unperturbed kidney structure, they showed a severely inhibited kidney repair after unilateral ischemia/reperfusion. This was manifested by a strong decrease of postischemic kidney size and weight, whereas the undamaged contralateral kidney exhibited an enhanced compensatory size increase. In addition, the postischemic kidneys of transgenic mice were characterized by hyperplasia of interstitial cells, paucity of epithelial tubular structures, increase of the areas occupied by connective tissue, and neutrophil and macrophage infiltration. The continuous treatment of transgenic mice with the cell membrane stabilizer, taurine, inhibited nonclassical FGF1 export and significantly rescued postischemic kidney repair. It was also found that similar to EC, the transgenic expression of FGF1 in monocytes and macrophages suppresses kidney repair. We suggest that nonclassical export may be used as a target for the treatment of pathologies involving signal peptide-less FGFs.

Comparison of the gene expression profiles from normal and Fgfrl1 deficient mouse kidneys reveals downstream targets of Fgfrl1 signaling

Fgfrl1 (fibroblast growth factor receptor-like 1) is a transmembrane receptor that is essential for the development of the metanephric kidney. It is expressed in all nascent nephrogenic structures and in the ureteric bud. Fgfrl1 null mice fail to develop the metanephric kidneys. Mutant kidney rudiments show a dramatic reduction of ureteric branching and a lack of mesenchymal-to-epithelial transition. Here, we compared the expression profiles of wildtype and Fgfrl1 mutant kidneys to identify genes that act downstream of Fgfrl1 signaling during the early steps of nephron formation. We detected 56 differentially expressed transcripts with 2-fold or greater reduction, among them many genes involved in Fgf, Wnt, Bmp, Notch, and Six/Eya/Dach signaling. We validated the microarray data by qPCR and whole-mount in situ hybridization and showed the expression pattern of candidate genes in normal kidneys. Some of these genes might play an important role during early nephron formation. Our study should help to define the minimal set of genes that is required to form a functional nephron.

FGF/EGF signaling regulates the renewal of early nephron progenitors during embryonic development

Recent studies indicate that nephron progenitor cells of the embryonic kidney are arranged in a series of compartments of an increasing state of differentiation. The earliest progenitor compartment, distinguished by expression of CITED1, possesses greater capacity for renewal and differentiation than later compartments. Signaling events governing progression of nephron progenitor cells through stages of increasing differentiation are poorly understood, and their elucidation will provide key insights into normal and dysregulated nephrogenesis, as well as into regenerative processes that follow kidney injury. In this study, we found that the mouse CITED1(+) progenitor compartment is maintained in response to receptor tyrosine kinase (RTK) ligands that activate both FGF and EGF receptors. This RTK signaling function is dependent on RAS and PI3K signaling but not ERK. In vivo, RAS inactivation by expression of sprouty 1 (Spry1) in CITED1(+) nephron progenitors results in loss of characteristic molecular marker expression and in increased death of progenitor cells. Lineage tracing shows that surviving Spry1-expressing progenitor cells are impaired in their subsequent epithelial differentiation, infrequently contributing to epithelial structures. These findings demonstrate that the survival and developmental potential of cells in the earliest embryonic nephron progenitor cell compartment are dependent on FGF/EGF signaling through RAS.

Role of fibroblast growth factor receptor signaling in kidney development

Fibroblast growth factor receptors (Fgfrs) are expressed throughout the developing kidney. Several early studies have shown that exogenous fibroblast growth factors (Fgfs) affect growth and maturation of the metanephric mesenchyme (MM) and ureteric bud (UB). Transgenic mice that over-express a dominant negative receptor isoform develop renal aplasia/severe dysplasia, confirming the importance of Fgfrs in renal development. Furthermore, global deletion of Fgf7, Fgf10, and Fgfr2IIIb (isoform that binds Fgf7 and Fgf10) in mice leads to small kidneys with fewer collecting ducts and nephrons. Deletion of Fgfrl1, a receptor lacking intracellular signaling domains, causes severe renal dysgenesis. Conditional targeting of Fgf8 from the MM interrupts nephron formation. Deletion of Fgfr2 from the UB results in severe ureteric branching and stromal mesenchymal defects, although loss of Frs2α (major signaling adapter for Fgfrs) in the UB causes only mild renal hypoplasia. Deletion of both Fgfr1 and Fgfr2 in the MM results in renal aplasia with defects in MM formation and initial UB elongation and branching. Loss of Fgfr2 in the MM leads to many renal and urinary tract anomalies as well as vesicoureteral reflux. Thus, Fgfr signaling is critical for patterning of virtually all renal lineages at early and later stages of development.

Defining and redefining the nephron progenitor population

It has long been appreciated that the mammalian kidney arises via reciprocal interactions between an epithelial ureteric epithelium and the surrounding metanephric mesenchyme. More recently, lineage tracing has confirmed that the portion of the metanephric mesenchyme closest to the advancing ureteric tips, the cap mesenchyme, represents the progenitor population for the nephron epithelia. This Six2(+)Cited1(+) population undergoes self-renewal throughout nephrogenesis while retaining the potential to epithelialize. In contrast, the Foxd1(+) portion of the metanephric mesenchyme shows no epithelial potential, developing instead into the interstitial, perivascular, and possibly endothelial elements of the kidney. The cap mesenchyme rests within a nephrogenic niche, surrounded by the stroma and the ureteric tip. While the role of Wnt signaling in nephron induction is known, there remains a lack of clarity over the intrinsic and extrinsic regulation of cap mesenchyme specification, self-renewal, and nephron potential. It is also not known what regulates cessation of nephrogenesis, but there is no nephron generation in response to injury during the postnatal period. In this review, we will examine what is and is not known about this nephron progenitor population and discuss how an increased understanding of the regulation of this population may better explain the observed variation in final nephron number and potentially facilitate the reinitiation or prolongation of nephron formation.

The GUDMAP database--an online resource for genitourinary research

The GenitoUrinary Development Molecular Anatomy Project (GUDMAP) is an international consortium working to generate gene expression data and transgenic mice. GUDMAP includes data from large-scale in situ hybridisation screens (wholemount and section) and microarray gene expression data of microdissected, laser-captured and FACS-sorted components of the developing mouse genitourinary (GU) system. These expression data are annotated using a high-resolution anatomy ontology specific to the developing murine GU system. GUDMAP data are freely accessible at www.gudmap.org via easy-to-use interfaces. This curated, high-resolution dataset serves as a powerful resource for biologists, clinicians and bioinformaticians interested in the developing urogenital system. This paper gives examples of how the data have been used to address problems in developmental biology and provides a primer for those wishing to use the database in their own research.

Frequent loss of genome gap region in 4p16.3 subtelomere in early-onset type 2 diabetes mellitus

A small portion of Type 2 diabetes mellitus (T2DM) is familial, but the majority occurs as sporadic disease. Although causative genes are found in some rare forms, the genetic basis for sporadic T2DM is largely unknown. We searched for a copy number abnormality in 100 early-onset Japanese T2DM patients (onset age <35 years) by whole-genome screening with a copy number variation BeadChip. Within the 1.3-Mb subtelomeric region on chromosome 4p16.3, we found copy number losses in early-onset T2DM (13 of 100 T2DM versus one of 100 controls). This region surrounds a genome gap, which is rich in multiple low copy repeats. Subsequent region-targeted high-density custom-made oligonucleotide microarray experiments verified the copy number losses and delineated structural changes in the 1.3-Mb region. The results suggested that copy number losses of the genes in the deleted region around the genome gap in 4p16.3 may play significant roles in the etiology of T2DM.

Role of fibroblast growth factor receptor signaling in kidney development

Fibroblast growth factor receptors (Fgfrs) consist of four signaling family members and one nonsignaling "decoy" receptor, Fgfr-like 1 (Fgfrl1), all of which are expressed in the developing kidney. Several studies have shown that exogenous fibroblast growth factors (Fgfs) affect growth and maturation of the metanephric mesenchyme (MM) and ureteric bud (UB) in cultured tissues. Transgenic and conditional knockout approaches in whole animals have shown that Fgfr1 and Fgfr2 (predominantly the IIIc isoform) in kidney mesenchyme are critical for early MM and UB formation. Conditional deletion of the ligand, Fgf8, in nephron precursors or global deletion of Fgfrl1 interrupts nephron formation. Fgfr2 (likely the IIIb isoform signaling downstream of Fgf7 and Fgf10) is critical for ureteric morphogenesis. Moreover, Fgfr2 appears to act independently of Frs2α (the major signaling adapter for Fgfrs) in regulating UB branching. Loss of Fgfr2 in the MM leads to many kidney and urinary tract anomalies, including vesicoureteral reflux. Thus Fgfr signaling is critical for patterning of virtually all renal lineages at early and later stages of development.

Interaction of the receptor FGFRL1 with the negative regulator Spred1

FGFRL1 is a member of the fibroblast growth factor receptor family. It plays an essential role during branching morphogenesis of the metanephric kidneys, as mice with a targeted deletion of the Fgfrl1 gene show severe kidney dysplasia. Here we used the yeast two-hybrid system to demonstrate that FGFRL1 binds with its C-terminal, histidine-rich domain to Spred1 and to other proteins of the Sprouty/Spred family. Members of this family are known to act as negative regulators of the Ras/Raf/Erk signaling pathway. Truncation experiments further showed that FGFRL1 interacts with the SPR domain of Spred1, a domain that is shared by all members of the Sprouty/Spred family. The interaction could be verified by coprecipitation of the interaction partners from solution and by codistribution at the cell membrane of COS1 and HEK293 cells. Interestingly, Spred1 increased the retention time of FGFRL1 at the plasma membrane where the receptor might interact with ligands. FGFRL1 and members of the Sprouty/Spred family belong to the FGF synexpression group, which also includes FGF3, FGF8, Sef and Isthmin. It is conceivable that FGFRL1, Sef and some Sprouty/Spred proteins work in concert to control growth factor signaling during branching morphogenesis of the kidneys and other organs.

Biology of FGFRL1, the fifth fibroblast growth factor receptor

FGFRL1 (fibroblast growth factor receptor like 1) is the most recently discovered member of the FGFR family. It contains three extracellular Ig-like domains similar to the classical FGFRs, but it lacks the protein tyrosine kinase domain and instead contains a short intracellular tail with a peculiar histidine-rich motif. The gene for FGFRL1 is found in all metazoans from sea anemone to mammals. FGFRL1 binds to FGF ligands and heparin with high affinity. It exerts a negative effect on cell proliferation, but a positive effect on cell differentiation. Mice with a targeted deletion of the Fgfrl1 gene die perinatally due to alterations in their diaphragm. These mice also show bilateral kidney agenesis, suggesting an essential role for Fgfrl1 in kidney development. A human patient with a frameshift mutation exhibits craniosynostosis, arguing for an additional role of FGFRL1 during bone formation. FGFRL1 contributes to the complexity of the FGF signaling system.

Extending the family table: Insights from beyond vertebrates into the regulation of embryonic development by FGFs

Since the discovery of fibroblast growth factors (FGFs) much focus has been placed on elucidating the roles for each vertebrate FGF ligand, receptor, and regulating molecules in the context of vertebrate development, human disorders and cancer. Studies in human, mouse, frog, chick, and zebrafish have made great contributions to our understanding of the role of FGFs in specific processes. However, in recent years, as more genomes are sequenced, information is becoming available from many non-vertebrate models and a more complete picture of the FGF superfamily as a whole is emerging. In some cases, less redundancy in these FGF signaling systems may allow for more mechanistic insights. Studies in sea anemones have highlighted how ancient FGF signaling is and helped provide insight into the evolution of the FGF gene family. Work in nematodes has shown that different splice forms can be used for functional specificity in invertebrate FGF signaling. Comparing FGFs between urochordates and vertebrates as well as between different insect species reveals important clues into the process of gene loss, duplication and subfunctionalization of FGFs throughout evolution. Finally, comparing all members of the FGF ligand superfamily reveals variability in many properties, which may point to a feature of FGFs as being highly adaptable with regards to protein structure and signaling mechanism. Further studies on FGF signaling outside of vertebrates is likely to continue to complement work in vertebrates by contributing additional insights to the FGF field and providing unexpected information that could be used for medical applications.

Rapid fusion and syncytium formation of heterologous cells upon expression of the FGFRL1 receptor

The fusion of mammalian cells into syncytia is a developmental process that is tightly restricted to a limited subset of cells. Besides gamete and placental trophoblast fusion, only macrophages and myogenic stem cells fuse into multinucleated syncytia. In contrast to viral cell fusion, which is mediated by fusogenic glycoproteins that actively merge membranes, mammalian cell fusion is poorly understood at the molecular level. A variety of mammalian transmembrane proteins, among them many of the immunoglobulin superfamily, have been implicated in cell-cell fusion, but none has been shown to actively fuse cells in vitro. Here we report that the FGFRL1 receptor, which is up-regulated during the differentiation of myoblasts into myotubes, fuses cultured cells into large, multinucleated syncytia. We used luciferase and GFP-based reporter assays to confirm cytoplasmic mixing and to identify the fusion inducing domain of FGFRL1. These assays revealed that Ig-like domain III and the transmembrane domain are both necessary and sufficient to rapidly fuse CHO cells into multinucleated syncytia comprising several hundred nuclei. Moreover, FGFRL1 also fused HEK293 and HeLa cells with untransfected CHO cells. Our data show that FGFRL1 is the first mammalian protein that is capable of inducing syncytium formation of heterologous cells in vitro.

The FGFRL1 receptor is shed from cell membranes, binds fibroblast growth factors (FGFs), and antagonizes FGF signaling in Xenopus embryos

FGFRL1 (fibroblast growth factor receptor like 1) is the fifth and most recently discovered member of the fibroblast growth factor receptor (FGFR) family. With up to 50% amino acid similarity, its extracellular domain closely resembles that of the four conventional FGFRs. Its intracellular domain, however, lacks the split tyrosine kinase domain needed for FGF-mediated signal transduction. During embryogenesis of the mouse, FGFRL1 is essential for the development of parts of the skeleton, the diaphragm muscle, the heart, and the metanephric kidney. Since its discovery, it has been hypothesized that FGFRL1 might act as a decoy receptor for FGF ligands. Here we present several lines of evidence that support this notion. We demonstrate that the FGFRL1 ectodomain is shed from the cell membrane of differentiating C2C12 myoblasts and from HEK293 cells by an as yet unidentified protease, which cuts the receptor in the membrane-proximal region. As determined by ligand dot blot analysis, cell-based binding assays, and surface plasmon resonance analysis, the soluble FGFRL1 ectodomain as well as the membrane-bound receptor are capable of binding to some FGF ligands with high affinity, including FGF2, FGF3, FGF4, FGF8, FGF10, and FGF22. We furthermore show that ectopic expression of FGFRL1 in Xenopus embryos antagonizes FGFR signaling during early development. Taken together, our data provide strong evidence that FGFRL1 is indeed a decoy receptor for FGFs.