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

Human parainfluenza virus 3 field strains undergo extracellular fusion protein cleavage to activate entry

Human parainfluenza virus 3 (HPIV3) infection is driven by the coordinated action of viral surface glycoproteins hemagglutinin-neuraminidase (HN) and fusion protein (F). Receptor-engaged HN activates F to insert into the target cell membrane and drive virion-cell membrane fusion. For F to mediate entry, its precursor (F0) must first be cleaved by host proteases. F0 cleavage has been thought to be executed during viral glycoprotein transit through the trans-Golgi network by the ubiquitously expressed furin because F0 proteins of laboratory-adapted viruses contain a furin recognition dibasic cleavage motif RXKR around residue 108. Here, we show that the F proteins of field strains have a different cleavage motif from laboratory-adapted strains and are cleaved by unidentified proteases expressed in only a narrow subset of cell types. We demonstrate that extracellular serine protease inhibitors block HPIV3 F0 cleavage for field strains, suggesting F0 cleavage occurs at the cell surface facilitated by transmembrane proteases. Candidate proteases that may process HPIV3 F in vivo were identified by a genome-wide CRISPRa screen in HEK293/dCas9-VP64 + MPH cells. The lung-expressed extracellular serine proteases TMPRSS2 and TMPRSS13 are both sufficient to cleave HPIV3 F and enable infectious virus release by otherwise non-permissive cells. Our findings support an alternative mechanism of F activation in vivo, reliant on extracellular membrane-bound serine proteases expressed in a narrow subset of cells. The proportion of HPIV3 F proteins cleaved and infectious virus release is determined by host cell expression of requisite proteases, allowing just-in-time activation of F and positioning F cleavage as another key regulator of HPIV3 spread.

IMPORTANCE

Enveloped viruses cause a wide range of diseases in humans. At the first step of infection, these viruses must fuse their envelope with a cell membrane to initiate infection. This fusion is mediated by viral proteins that require a critical activating cleavage event. It was previously thought that for parainfluenza virus 3, an important cause of respiratory disease and a representative of a group of important pathogens, this cleavage event was mediated by furin in the cell secretory pathways prior to formation of the virions. We show that this is only true for laboratory strain viruses, and that clinical viruses that infect humans utilize extracellular proteases that are only made by a small subset of cells. These results highlight the importance of studying authentic clinical viruses that infect human tissues for understanding natural infection.

No evidence for a direct extracellular interaction between human Fc receptor-like 3 (MAIA) and the sperm ligand IZUMO1

Fertilization involves the recognition and fusion of sperm and egg to form a previously unidentified organism. In mammals, surface molecules on the sperm and egg have central roles, and while adhesion is mediated by the IZUMO1-JUNO sperm-egg ligand-receptor pair, the molecule/s responsible for membrane fusion remain mysterious. Recently, MAIA/FCRL3 was identified as a mammalian egg receptor, which bound IZUMO1 and JUNO and might therefore have a bridging role in gamete recognition and fusion. Here, we use sensitive assays designed to detect extracellular protein binding to investigate the interactions between MAIA and both IZUMO1 and JUNO. Despite using reagents with demonstrable biochemical activity, we did not identify any direct binding between MAIA/FCRL3 and either IZUMO1 or JUNO. We also observed no fusogenic activity of MAIA/FCRL3 in a cell-based membrane fusion assay. Our findings encourage caution in further investigations on the role played by MAIA/FCRL3 in fertilization.

Germline and Somatic Cell Syncytia in Insects

Syncytia are common in the animal and plant kingdoms both under normal and pathological conditions. They form through cell fusion or division of a founder cell without cytokinesis. A particular type of syncytia occurs in invertebrate and vertebrate gametogenesis when the founder cell divides several times with partial cytokinesis producing a cyst (nest) of germ line cells connected by cytoplasmic bridges. The ultimate destiny of the cyst's cells differs between animal groups. Either all cells of the cyst become the gametes or some cells endoreplicate or polyploidize to become the nurse cells (trophocytes). Although many types of syncytia are permanent, the germ cell syncytium is temporary, and eventually, it separates into individual gametes. In this chapter, we give an overview of syncytium types and focus on the germline and somatic cell syncytia in various groups of insects. We also describe the multinuclear giant cells, which form through repetitive nuclear divisions and cytoplasm hypertrophy, but without cell fusion, and the accessory nuclei, which bud off the oocyte nucleus, migrate to its cortex and become included in the early embryonic syncytium.

Distinct effects of SDC3 and FGFRL1 on selective neurodegeneration in AD and PD

Alzheimer's disease (AD) and Parkinson's disease (PD) are age-dependent neurodegenerative disorders. There is a profound neuronal loss in the basal forebrain cholinergic system in AD and severe dopaminergic deficiency within the nigrostriatal pathway in PD. Swedish APP (APP_SWE ) and SNCA_A53T mutations promote Aβ generation and α-synuclein aggregation, respectively, and have been linked to the pathogenesis of AD and PD. However, the mechanisms underlying selective cholinergic and dopaminergic neurodegeneration in AD and PD are still unknown. We demonstrated that APP_SWE mutation enhanced Aβ generation and increased cell susceptibility to Aβ oligomer in cholinergic SN56 cells, whereas SNCA_A53T mutations promoted aggregates formation and potentiated mutant α-synuclein oligomer-induced cytotoxicity in MN9D cells. Furthermore, syndecan-3 (SDC3) and fibroblast growth factor receptor-like 1 (FGFRL1) genes were differentially expressed in SN56 and MN9D cells carrying APP_SWE or SNCA_A53T mutation. SDC3 and FGFRL1 proteins were preferentially expressed in the cholinergic nucleus and dopaminergic neurons of APP_SWE and SNCA_A53T mouse models, respectively. Finally, the knockdown of SDC3 and FGFRL1 attenuated oxidative stress-induced cell death in SN56-APP_SWE and MN9D-SNCAA53T cells. The results demonstrate that SDC3 and FGFRL1 mediated the specific effects of APP_SWE and SNCA_A53T on cholinergic and dopaminergic neurodegeneration in AD and PD, respectively. Our study suggests that SDC3 and FGFRL1 could be potential targets to alleviate the selective neurodegeneration in AD and PD.

An easy method for developing fusion enabled SARS-CoV2 virus fusion mimic (SCFM), bypassing the need of Bio Safety Level (BSL) facility

Widespread infection due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) has led to a global pandemic. Currently, various approaches are being taken up to develop vaccines and therapeutics to treat SARS-CoV2 infection. Consequently, the S protein has become an important target protein for developing vaccines and therapeutics against SARS-CoV2. However, the highly infective nature of SARS-CoV2 restricts experimentation with the virus to highly secure BSL3 facilities. The availability of fusion-enabled, nonreplicating, and nonbiohazardous mimics of SARS-CoV2 virus fusion, containing the viral S or S and M protein in their native conformation on mammalian cells, can serve as a useful substitute for studying viral fusion for testing various inhibitors of viral fusion. This would avoid the use of the BSL3 facility for fusion studies required to develop therapeutics. In the present study, we have developed SARS-CoV2 virus fusion mimics (SCFMs) using mammalian cells transfected with constructs coding for S or S and M protein. The fusogenic property of the mimic(s) and their interaction with the functional human ACE2 receptors was confirmed experimentally. We have also shown that such mimics can easily be used in an inhibition assay. These mimic(s) can be easily prepared on a large scale, and such SCFMs can serve as an invaluable resource for viral fusion inhibition assays and in vitro screening of antiviral agents, which can be shared/handled between labs/facilities without worrying about any biohazard while working under routine laboratory conditions, avoiding the use of BSL3 laboratory.Abbreviations :SCFM: SARS-CoV2 Virus Fusion Mimic; ACE2: Angiotensin-Converting Enzyme 2; hACE2: Human Angiotensin-Converting enzyme 2; MEF: Mouse Embryonic Fibroblasts; HBSS: Hanks Balanced Salt Solution; FBS: Fetal Bovine Serum.

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.

[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.

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.

The hallmarks of cell-cell fusion

Cell-cell fusion is essential for fertilization and organ development. Dedicated proteins known as fusogens are responsible for mediating membrane fusion. However, until recently, these proteins either remained unidentified or were poorly understood at the mechanistic level. Here, we review how fusogens surmount multiple energy barriers to mediate cell-cell fusion. We describe how early preparatory steps bring membranes to a distance of ∼10 nm, while fusogens act in the final approach between membranes. The mechanical force exerted by cell fusogens and the accompanying lipidic rearrangements constitute the hallmarks of cell-cell fusion. Finally, we discuss the relationship between viral and eukaryotic fusogens, highlight a classification scheme regrouping a superfamily of fusogens called Fusexins, and propose new questions and avenues of enquiry.

Inhibition of c-Myc expression accounts for an increase in the number of multinucleated cells in human cervical epithelial cells

The present study aimed to explore the mechanisms by which c-Myc is involved in mitotic catastrophe. HeLa-630 is a cell line stably silenced for c-Myc expression that was established in the laboratory of the School of Radiation Medicine and Protection. Multinucleated cells were observed in this line, and gene expression analysis was utilized to examine differences in gene expression in these cells compared with in the control cells transfected with the control plasmid. Gene ontology analysis was performed for differentially expressed genes. Expression profile analyses revealed that cells with silenced c-Myc exhibited abnormal expression patterns of genes involved in various functions, including the regulation of microtubule nucleation, centrosome duplication, the formation of pericentriolar material, DNA synthesis and metabolism, protein metabolism and the regulation of ion concentrations. Pathway analyses of differentially expressed genes demonstrated that these genes were primarily involved in diverse signal transduction pathways, including not only the adherens junction pathway, the transforming growth factor-β signaling pathway and the Wnt signaling pathway, among others, but also signaling pathways with roles in cytokine and immune regulation. The proportion of multinucleated cells with multipolar spindles was significantly higher in silenced c-Myc cells as compared with the control cells, and this discrepancy became more pronounced following cell irradiation. The inhibition of c-Myc in tumors may account for the radiosensitization of certain tumor cell types.

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.

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.

Divergent functions of fibroblast growth factor receptor-like 1 genes in grass carp (Ctenopharyngodon idella)

Fibroblast growth factor receptor-like 1 (FGFRL1) is a novel FGF receptor (FGFR) lacking an intracellular tyrosine kinase domain. FGFRs control the proliferation, differentiation and migration of cells in various tissues. However the functions of FGFRL1 in teleost fish are currently unknown. In this study, we report the identification of two fgfrl1 genes in grass carp (Ctenopharyngodon idella) that share 56% amino acid sequence identity. Both fgfrl1a and 1b were transcribed throughout embryogenesis, and mRNA levels were particularly high during somitogenesis. Using in situ hybridization, fgfrl1a transcripts were detected in notochord, somites, brain and eye at 14, 24 and 36 h post fertilization (hpf). In contrast, fgfrl1b was transcribed mainly in the endoderm at 14 hpf, in the gut and proctodeum at 24 hpf, and in the lens, pharyngeal arch and proctodeum at 36 hpf. In adult fish, fgfrl1a was abundantly expressed in heart, brain and muscle, while fgfrl1b was expressed strongly in eye, muscle and gill. Furthermore, both genes were significantly (p<0.05) up-regulated in muscle and brain during starvation and returned to normal levels rapidly after re-feeding. Exogenous treatment with different doses of human growth hormone down-regulated the expression of both genes in brain and muscle (p<0.05). These results suggest that Fgfrl1a and 1b play divergent roles in regulating growth and development in grass carp.

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.

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.

An integrated genomic, transcriptional and protein investigation of FGFRL1 as a putative 4p16.3 deletion target in bladder cancer

Loss of heterozygosity (LOH) of chromosome arm 4p is a common event in bladder and other malignancies. At least three distinct regions of deletion have been identified, but the deletion targets have so far remained elusive. In this study, we have identified a novel region of deletion mapping to 4p16.3 spanning 0-2.1 Mb, in 15% of bladder tumors and 24% of bladder cancer cell lines. FGFRL1, which maps within this region, was investigated as putative deletion target. The retained FGFRL1 allele was not mutated in cell lines and tumors with LOH, although in patients heterozygous for the rs4647930 functional polymorphism, the common allele was preferentially lost in tumor tissue. Epigenetic silencing of the retained allele was also excluded as levels of FGFRL1 mRNA and protein were similar in cell lines and tumors with and without 4p16.3 loss. However, while FGFRL1 protein was moderately expressed in all layers of the normal bladder epithelium, the majority of tumors showed areas of downregulation. Overall, average FGFRL1 protein expression was significantly lower in bladder tumors compared to normal tissue, but downregulation was independent from 4p16.3 LOH status, FGFR3 mutation, and tumor grade and stage. In conclusion, although we found no evidence supporting a "two-hit" inactivation of FGFRL1 in bladder carcinogenesis, the effect of heterozygous deletion coupled with functional polymorphisms, and the role of post-transcriptional downregulation deserves further investigation.

A net-like structure with pores is observed during cell fusion induced by the receptor FGFRL1

FGFRL1 is the fifth member of the fibroblast growth factor receptor (FGFR) family. Similar to the other members, it harbors three Ig loops in its extracellular domain, but in contrast to the other receptors, it lacks the intracellular protein tyrosine kinase domain that would be required for signaling by transphosphorylation. FGFRL1 is mainly found in the musculoskeletal system, where it appears to inhibit cell proliferation but to induce cell adhesion and differentiation. Mice with a targeted disruption of the FGFRL1 gene die during birth due to a malformed diaphragm muscle, which is not strong enough to inflate the lungs after birth. Expression of FGFRL1 is highly upregulated during the differentiation of myoblasts to multinucleated myotubes, suggesting an important role for FGFRL1 in cell-cell fusion. Recently we showed that FGFRL1 does indeed induce fusion of cultured cells into large syncytia. A reporter gene assay demonstrated that the third Ig domain and the transmembrane domain of FGFRL1 are both necessary and sufficient to fuse CHO cells into syncytia comprising several hundred nuclei. At the contact site, the fusing cells reveal a peculiar net-like structure with pores of about 1 µm diameter. It is possible that these structures represent membrane areas with fusion pores that set in motion the cell-cell fusion process. FGFRL1 is the first mammalian protein that is capable of triggering cell-cell fusion in vitro.

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.