Point mutation in FGF receptor eliminates phosphatidylinositol hydrolysis without affecting mitogenesis

Fibroblast growth factor receptor signaling in estrogen receptor-positive breast cancer: mechanisms and role in endocrine resistance

Fibroblast Growth Factor Receptors (FGFRs) play a significant role in Estrogen Receptor-positive (ER+) breast cancer by contributing to tumorigenesis and endocrine resistance. This review explores the structure, signaling pathways, and implications of FGFRs, particularly FGFR1, FGFR2, FGFR3, and FGFR4, in ER+ breast cancer. FGFR1 is frequently amplified, especially in aggressive Luminal B-like tumors, and its amplification is associated with poor prognosis and treatment resistance. The co-amplification of FGFR1 with oncogenes like EIF4EBP1 and NSD3 complicates its role as a standalone oncogenic driver. FGFR2 amplification, though less common, is critical in hormone receptor regulation, driving proliferation and treatment resistance. FGFR3 and FGFR4 also contribute to endocrine resistance through various mechanisms, including the activation of alternate signaling pathways like PI3K/AKT/mTOR and RAS/RAF/MEK/ERK. Endocrine resistance remains a major clinical challenge, with around 70% of breast cancers initially hormone receptor positive. Despite the success of CDK 4/6 inhibitors in combination with endocrine therapy (ET), resistance often develops, necessitating new treatment strategies. FGFR inhibitors have shown potential in preclinical studies, but clinical trials have yielded limited success due to off-target toxicities and lack of predictive biomarkers. Current clinical trials, including those evaluating FGFR inhibitors like erdafitinib, lucitanib, and dovitinib, have demonstrated mixed outcomes, underscoring the complexity of FGFR signaling in breast cancer. The interplay between FGFR and other signaling pathways highlights the need for comprehensive molecular profiling and personalized treatment approaches. Future research should focus on identifying robust biomarkers and developing combination therapies to enhance the efficacy of FGFR-targeted treatments. In conclusion, targeting FGFR signaling in ER+ breast cancer presents both challenges and opportunities. A deeper understanding of the molecular mechanisms and resistance pathways is crucial for the successful integration of FGFR inhibitors into clinical practice, aiming to improve outcomes for patients with endocrine-resistant breast cancer.

FGFR families: biological functions and therapeutic interventions in tumors

There are five fibroblast growth factor receptors (FGFRs), namely, FGFR1-FGFR5. When FGFR binds to its ligand, namely, fibroblast growth factor (FGF), it dimerizes and autophosphorylates, thereby activating several key downstream pathways that play an important role in normal physiology, such as the Ras/Raf/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase, phosphoinositide 3-kinase (PI3K)/AKT, phospholipase C gamma/diacylglycerol/protein kinase c, and signal transducer and activator of transcription pathways. Furthermore, as an oncogene, FGFR genetic alterations were found in 7.1% of tumors, and these alterations include gene amplification, gene mutations, gene fusions or rearrangements. Therefore, FGFR amplification, mutations, rearrangements, or fusions are considered as potential biomarkers of FGFR therapeutic response for tyrosine kinase inhibitors (TKIs). However, it is worth noting that with increased use, resistance to TKIs inevitably develops, such as the well-known gatekeeper mutations. Thus, overcoming the development of drug resistance becomes a serious problem. This review mainly outlines the FGFR family functions, related pathways, and therapeutic agents in tumors with the aim of obtaining better outcomes for cancer patients with FGFR changes. The information provided in this review may provide additional therapeutic ideas for tumor patients with FGFR abnormalities.

Metabolic Messengers: fibroblast growth factor 1

While fibroblast growth factor (FGF) 1 is expressed in multiple tissues, only adipose-derived and brain FGF1 have been implicated in the regulation of metabolism. Adipose FGF1 production is upregulated in response to dietary stress and is essential for adipose tissue plasticity in these conditions. Similarly, in the brain, FGF1 secretion into the ventricular space and the adjacent parenchyma is increased after a hypercaloric challenge induced by either feeding or glucose infusion. Potent anorexigenic properties have been ascribed to both peripheral and centrally injected FGF1. The ability of recombinant FGF1 and variants with reduced mitogenicity to lower glucose, suppress adipose lipolysis and promote insulin sensitization elevates their potential as candidates in the treatment of type 2 diabetes mellitus and associated comorbidities. Here, we provide an overview of the known metabolic functions of endogenous FGF1 and discuss its therapeutic potential, distinguishing between peripherally or centrally administered FGF1.

FGF20-FGFR1 signaling through MAPK and PI3K controls sensory progenitor differentiation in the organ of Corti

BACKGROUND

Fibroblast Growth Factor 20 (FGF20)-FGF receptor 1 (FGFR1) signaling is essential for cochlear hair cell (HC) and supporting cell (SC) differentiation. In other organ systems, FGFR1 signals through several intracellular pathways including MAPK (ERK), PI3K, phospholipase C ɣ (PLCɣ), and p38. Previous studies implicated MAPK and PI3K pathways in HC and SC development. We hypothesized that one or both would be important downstream mediators of FGF20-FGFR1 signaling for HC differentiation.

RESULTS

By inhibiting pathways downstream of FGFR1 in cochlea explant cultures, we established that both MAPK and PI3K pathways are required for HC differentiation while PLCɣ and p38 pathways are not. Examining the canonical PI3K pathway, we found that while AKT is necessary for HC differentiation, it is not sufficient to rescue the Fgf20-/- phenotype. To determine whether PI3K functions downstream of FGF20, we inhibited Phosphatase and Tensin Homolog (PTEN) in Fgf20-/- explants. Overactivation of PI3K resulted in a partial rescue of the Fgf20-/- phenotype, demonstrating a requirement for PI3K downstream of FGF20. Consistent with a requirement for the MAPK pathway for FGF20-regulated HC differentiation, we show that treating Fgf20-/- explants with FGF9 increased levels of dpERK.

CONCLUSIONS

Together, these data provide evidence that both MAPK and PI3K are important downstream mediators of FGF20-FGFR1 signaling during HC and SC differentiation.

Posttranslational Modification Defects in Fibroblast Growth Factor Receptor 1 as a Reason for Normosmic Isolated Hypogonadotropic Hypogonadism

Some mutations in FGFR1 affect the sense of smell while others do not, resulting in Kallmann syndrome (KS) and normosmic isolated hypogonadotropic hypogonadism (nIHH), respectively. The underlying mechanism is still unclear. FGFR1 variants are found in less than 10% of patients with KS and nIHH, and among them, only some have undergone functional analysis. Thus, the correlation between the phenotype and genotype cannot be clearly verified. This study reports a case of nIHH and explores the potential mechanism of the FGFR1 gene in the pathogenesis of nIHH. A preschooler with cryptorchidism, micropenis, strabismus, and hypopsia is described. As he had a normal sense of smell, he was diagnosed with nIHH. A de novo mutation in FGFR1 (c.2008G>A) was detected in the patient along with a novel variant in CEP290 (c.964G>A) inherited from his mother. We present compelling in vitro evidence that this FGFR1 mutation-induced posttranslational modification defect, including defective glycosylation and impaired trans-autophosphorylation, along with the final reduction in expression, could lead to impairment of the receptor and abnormal signaling and eventually result in developmental abnormalities and inhibition of GnRH neuron release. The identification of an additional variant suggests that CEP290 might play a potential role in GnRH development.

A role for fibroblast growth factor receptor 1 in the pathogenesis of Neisseria meningitidis

Neisseria meningitidis (the meningococcus) remains an important cause of human disease, including meningitis and sepsis. Adaptation to the host environment includes many interactions with specific cell surface receptors, resulting in intracellular signalling and cytoskeletal rearrangements that contribute to pathogenesis. Here, we assessed the interactions between meningococci and Fibroblast Growth Factor Receptor 1-IIIc (FGFR1-IIIc): a receptor specific to endothelial cells of the microvasculature, including that of the blood-brain barrier. We show that the meningococcus recruits FGFR1-IIIc onto the surface of human blood microvascular endothelial cells (HBMECs). Furthermore, we demonstrate that expression of FGFR1-IIIc is required for optimal invasion of HBMECs by meningococci. We show that the ability of N. meningitidis to interact with the ligand-binding domain of FGFR1-IIIc is shared with the other pathogenic Neisseria species, N. gonorrhoeae, but not with commensal bacteria including non-pathogenic Neisseria species.

FGFR1 Is Critical for RBL2 Loss-Driven Tumor Development and Requires PLCG1 Activation for Continued Growth of Small Cell Lung Cancer

Small cell lung cancer (SCLC) remains a recalcitrant disease where limited therapeutic options have not improved overall survival, and approved targeted therapies are lacking. Amplification of the tyrosine kinase receptor FGFR1 (fibroblast growth factor receptor 1) is one of the few actionable alterations found in the SCLC genome. However, efforts to develop targeted therapies for FGFR1-amplified SCLC are hindered by critical gaps in knowledge around the molecular origins and mediators of FGFR1-driven signaling as well as the physiologic impact of targeting FGFR1. Here we show that increased FGFR1 promotes tumorigenic progression in precancerous neuroendocrine cells and is required for SCLC development in vivo. Notably, Fgfr1 knockout suppressed tumor development in a mouse model lacking the retinoblastoma-like protein 2 (Rbl2) tumor suppressor gene but did not affect a model with wild-type Rbl2. In support of a functional interaction between these two genes, loss of RBL2 induced FGFR1 expression and restoration of RBL2 repressed it, suggesting a novel role for RBL2 as a regulator of FGFR1 in SCLC. Additionally, FGFR1 activated phospholipase C gamma 1 (PLCG1), whereas chemical inhibition of PLCG1 suppressed SCLC growth, implicating PLCG1 as an effector of FGFR1 signaling in SCLC. Collectively, this study uncovers mechanisms underlying FGFR1-driven SCLC that involve RBL2 upstream and PLCG1 downstream, thus providing potential biomarkers for anti-FGFR1 therapy. SIGNIFICANCE: This study identifies RBL2 and PLCG1 as critical components of amplified FGFR1 signaling in SCLC, thus representing potential targets for biomarker analysis and therapeutic development in this disease.

Role of Signal Transduction Pathways and Transcription Factors in Cartilage and Joint Diseases

Osteoarthritis and rheumatoid arthritis are common cartilage and joint diseases that globally affect more than 200 million and 20 million people, respectively. Several transcription factors have been implicated in the onset and progression of osteoarthritis, including Runx2, C/EBPβ, HIF2α, Sox4, and Sox11. Interleukin-1 β (IL-1β) leads to osteoarthritis through NF-ĸB, IκBζ, and the Zn²⁺-ZIP8-MTF1 axis. IL-1, IL-6, and tumor necrosis factor α (TNFα) play a major pathological role in rheumatoid arthritis through NF-ĸB and JAK/STAT pathways. Indeed, inhibitory reagents for IL-1, IL-6, and TNFα provide clinical benefits for rheumatoid arthritis patients. Several growth factors, such as bone morphogenetic protein (BMP), fibroblast growth factor (FGF), parathyroid hormone-related protein (PTHrP), and Indian hedgehog, play roles in regulating chondrocyte proliferation and differentiation. Disruption and excess of these signaling pathways cause genetic disorders in cartilage and skeletal tissues. Fibrodysplasia ossificans progressive, an autosomal genetic disorder characterized by ectopic ossification, is induced by mutant ACVR1. Mechanistic target of rapamycin kinase (mTOR) inhibitors can prevent ectopic ossification induced by ACVR1 mutations. C-type natriuretic peptide is currently the most promising therapy for achondroplasia and related autosomal genetic diseases that manifest severe dwarfism. In these ways, investigation of cartilage and chondrocyte diseases at molecular and cellular levels has enlightened the development of effective therapies. Thus, identification of signaling pathways and transcription factors implicated in these diseases is important.

Targeting Cellular Trafficking of Fibroblast Growth Factor Receptors as a Strategy for Selective Cancer Treatment

Fibroblast growth factor receptors (FGFRs) in response to fibroblast growth factors (FGFs) transmit signals across the cell membrane, regulating important cellular processes, like differentiation, division, motility, and death. The aberrant activity of FGFRs is often observed in various diseases, especially in cancer. The uncontrolled FGFRs' function may result from their overproduction, activating mutations, or generation of FGFRs' fusion proteins. Besides their typical subcellular localization on the cell surface, FGFRs are often found inside the cells, in the nucleus and mitochondria. The intracellular pool of FGFRs utilizes different mechanisms to facilitate cancer cell survival and expansion. In this review, we summarize the current stage of knowledge about the role of FGFRs in oncogenic processes. We focused on the mechanisms of FGFRs' cellular trafficking-internalization, nuclear translocation, and mitochondrial targeting, as well as their role in carcinogenesis. The subcellular sorting of FGFRs constitutes an attractive target for anti-cancer therapies. The blocking of FGFRs' nuclear and mitochondrial translocation can lead to the inhibition of cancer invasion. Moreover, the endocytosis of FGFRs can serve as a tool for the efficient and highly selective delivery of drugs into cancer cells overproducing these receptors. Here, we provide up to date examples how the cellular sorting of FGFRs can be hijacked for selective cancer treatment.

Structure, activation and dysregulation of fibroblast growth factor receptor kinases: perspectives for clinical targeting

The receptor tyrosine kinase family of fibroblast growth factor receptors (FGFRs) play crucial roles in embryonic development, metabolism, tissue homeostasis and wound repair via stimulation of intracellular signalling cascades. As a consequence of FGFRs' influence on cell growth, proliferation and differentiation, FGFR signalling is frequently dysregulated in a host of human cancers, variously by means of overexpression, somatic point mutations and gene fusion events. Dysregulation of FGFRs is also the underlying cause of many developmental dysplasias such as hypochondroplasia and achondroplasia. Accordingly, FGFRs are attractive pharmaceutical targets, and multiple clinical trials are in progress for the treatment of various FGFR aberrations. To effectively target dysregulated receptors, a structural and mechanistic understanding of FGFR activation and regulation is required. Here, we review some of the key research findings from the last couple of decades and summarise the strategies being explored for therapeutic intervention.

A threshold model for receptor tyrosine kinase signaling specificity and cell fate determination

Upon ligand engagement, the single-pass transmembrane receptor tyrosine kinases (RTKs) dimerize to transmit qualitatively and quantitatively different intracellular signals that alter the transcriptional landscape and thereby determine the cellular response. The molecular mechanisms underlying these fundamental events are not well understood. Considering recent insights into the structural biology of fibroblast growth factor signaling, we propose a threshold model for RTK signaling specificity in which quantitative differences in the strength/longevity of ligand-induced receptor dimers on the cell surface lead to quantitative differences in the phosphorylation of activation loop (A-loop) tyrosines as well as qualitative differences in the phosphorylation of tyrosines mediating substrate recruitment. In this model, quantitative differences on A-loop tyrosine phosphorylation result in gradations in kinase activation, leading to the generation of intracellular signals of varying amplitude/duration. In contrast, qualitative differences in the pattern of tyrosine phosphorylation on the receptor result in the recruitment/activation of distinct substrates/intracellular pathways. Commensurate with both the dynamics of the intracellular signal and the types of intracellular pathways activated, unique transcriptional signatures are established. Our model provides a framework for engineering clinically useful ligands that can tune receptor dimerization stability so as to bias the cellular transcriptome to achieve a desired cellular output.

PLCγ1: Potential arbitrator of cancer progression

Phospholipase C (PLC) is an essential mediator of cellular signaling. PLC regulates multiple cellular processes by generating bioactive molecules such as inositol-1,4,5-triphosphate (IP₃) and diacylglycerol (DAG). These products propagate and regulate cellular signaling via calcium (Ca²⁺) mobilization and activation of protein kinase C (PKC), other kinases, and ion channels. PLCγ1, one of the primary subtypes of PLC, is directly activated by membrane receptors, including receptor tyrosine kinases (RTKs), and adhesion receptors such as integrin. PLCγ1 mediates signaling through direct interactions with other signaling molecules via SH domains, as well as its lipase activity. PLCγ1 is frequently enriched and mutated in various cancers, and is involved in the processes of tumorigenesis, including proliferation, migration, and invasion. Although many studies have suggested that PLCγ functions in cell mobility rather than proliferation in cancer, questions remain as to whether PLCγ regulates mitogenesis and whether PLCγ promotes or inhibits proliferation. Moreover, how PLCγ regulates cancer-associated cellular processes and the interplay among other proteins involved in cancer progression have yet to be fully elucidated. In this review, we discuss the current understanding of the role of PLCγ1 in cancer mobility and proliferation.

Cellular lifespan and senescence: a complex balance between multiple cellular pathways

The study of cellular senescence and proliferative lifespan is becoming increasingly important because of the promises of autologous cell therapy, the need for model systems for tissue disease and the implication of senescent cell phenotypes in organismal disease states such as sarcopenia, diabetes and various cancers, among others. Here, we explain the concepts of proliferative cellular lifespan and cellular senescence, and we present factors that have been shown to mediate cellular lifespan positively or negatively. We review much recent literature and present potential molecular mechanisms by which lifespan mediation occurs, drawing from the fields of telomere biology, metabolism, NAD(+) and sirtuin biology, growth factor signaling and oxygen and antioxidants. We conclude that cellular lifespan and senescence are complex concepts that are governed by multiple independent and interdependent pathways, and that greater understanding of these pathways, their interactions and their convergence upon specific cellular phenotypes may lead to viable therapies for tissue regeneration and treatment of age-related pathologies, which are caused by or exacerbated by senescent cells in vivo.

Neural stem cells secrete factors facilitating brain regeneration upon constitutive Raf-Erk activation

The intracellular Raf-Erk signaling pathway is activated during neural stem cell (NSC) proliferation, and neuronal and astrocytic differentiation. A key question is how this signal can evoke multiple and even opposing NSC behaviors. We show here, using a constitutively active Raf (ca-Raf), that Raf-Erk activation in NSCs induces neuronal differentiation in a cell-autonomous manner. By contrast, it causes NSC proliferation and the formation of astrocytes in an extrinsic autocrine/paracrine manner. Thus, treatment of NSCs with medium (CM) conditioned in ca-Raf-transduced NSCs (Raf-CM; RCM) became activated to form proliferating astrocytes resembling radial glial cells (RGCs) or adult-type NSCs. Infusion of Raf-CM into injured mouse brains caused expansion of the NSC population in the subventricular zone, followed by the formation of new neurons that migrated to the damaged site. Our study shows an example how molecular mechanisms dissecting NSC behaviors can be utilized to develop regenerative therapies in brain disorders.

Genetic insights into the mechanisms of Fgf signaling

The fibroblast growth factor (Fgf) family of ligands and receptor tyrosine kinases is required throughout embryonic and postnatal development and also regulates multiple homeostatic functions in the adult. Aberrant Fgf signaling causes many congenital disorders and underlies multiple forms of cancer. Understanding the mechanisms that govern Fgf signaling is therefore important to appreciate many aspects of Fgf biology and disease. Here we review the mechanisms of Fgf signaling by focusing on genetic strategies that enable in vivo analysis. These studies support an important role for Erk1/2 as a mediator of Fgf signaling in many biological processes but have also provided strong evidence for additional signaling pathways in transmitting Fgf signaling in vivo.

Overexpression of the Fibroblast Growth Factor Receptor 1 (FGFR1) in a Model of Spinal Cord Injury in Rats

Spinal cord injury (SCI) is a severe condition that affects many people and results in high health care costs. Therefore, it is essential to find new targets for treatment. The fibroblast growth factor receptor 1 (FGFR1) signalling pathway has a history of being explored for SCI treatment. Several groups have examined the effect of high availability of different FGFR1 ligands at the injury site and reported corticospinal tract (CST) regeneration as well as improved motor functions. In this study, we investigated overexpression of the FGFR1 in rat corticospinal neurons in vivo after injury (unilateral pyramidotomy) and in cerebellar granule neurons (CGNs) in vitro. We show that overexpression of FGFR1 using AAV1 intracortical injections did not increase sprouting of the treated corticospinal tract and did not improve dexterity or walking in a rat model of SCI. Furthermore, we show that overexpression of FGFR1 in vitro resulted in decreased neurite outgrowth compared to control. Thus, our results suggest that the FGFR1 is not a suitable therapeutic target after SCI.

Fibroblast growth factors, old kids on the new block

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

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.

Activation of Cardiac Fibroblast Growth Factor Receptor 4 Causes Left Ventricular Hypertrophy

Chronic kidney disease (CKD) is a worldwide public health threat that increases risk of death due to cardiovascular complications, including left ventricular hypertrophy (LVH). Novel therapeutic targets are needed to design treatments to alleviate the cardiovascular burden of CKD. Previously, we demonstrated that circulating concentrations of fibroblast growth factor (FGF) 23 rise progressively in CKD and induce LVH through an unknown FGF receptor (FGFR)-dependent mechanism. Here, we report that FGF23 exclusively activates FGFR4 on cardiac myocytes to stimulate phospholipase Cγ/calcineurin/nuclear factor of activated T cell signaling. A specific FGFR4-blocking antibody inhibits FGF23-induced hypertrophy of isolated cardiac myocytes and attenuates LVH in rats with CKD. Mice lacking FGFR4 do not develop LVH in response to elevated FGF23, whereas knockin mice carrying an FGFR4 gain-of-function mutation spontaneously develop LVH. Thus, FGF23 promotes LVH by activating FGFR4, thereby establishing FGFR4 as a pharmacological target for reducing cardiovascular risk in CKD.

Fgfr1 regulates development through the combinatorial use of signaling proteins

Brewer et al. engineered an allelic series of knock-in point mutations designed to disrupt Fgfr1 signaling functions individually and in combination. They found that, in addition to Frs2, Crk proteins and Plcγ also contribute to Erk1/2 activation. Disruption of all known signaling functions diminished Erk1/2 and Plcγ activation but did not recapitulate the peri-implantation Fgfr1-null phenotype.

Fibroblast growth factor (Fgf) signaling governs multiple processes important in development and disease. Many lines of evidence have implicated Erk1/2 signaling induced through Frs2 as the predominant effector pathway downstream from Fgf receptors (Fgfrs), but these receptors can also signal through other mechanisms. To explore the functional significance of the full range of signaling downstream from Fgfrs in mice, we engineered an allelic series of knock-in point mutations designed to disrupt Fgfr1 signaling functions individually and in combination. Analysis of each mutant indicates that Frs2 binding to Fgfr1 has the most pleiotropic functions in development but also that the receptor uses multiple proteins additively in vivo. In addition to Frs2, Crk proteins and Plcγ also contribute to Erk1/2 activation, affecting axis elongation and craniofacial and limb development and providing a biochemical mechanism for additive signaling requirements. Disruption of all known signaling functions diminished Erk1/2 and Plcγ activation but did not recapitulate the peri-implantation Fgfr1-null phenotype. This suggests that Erk1/2-independent signaling pathways are functionally important for Fgf signaling in vivo.

Distinct sets of FGF receptors sculpt excitatory and inhibitory synaptogenesis

Neurons in the brain must establish a balanced network of excitatory and inhibitory synapses during development for the brain to function properly. An imbalance between these synapses underlies various neurological and psychiatric disorders. The formation of excitatory and inhibitory synapses requires precise molecular control. In the hippocampus, the structure crucial for learning and memory, fibroblast growth factor 22 (FGF22) and FGF7 specifically promote excitatory or inhibitory synapse formation, respectively. Knockout of either Fgf gene leads to excitatory-inhibitory imbalance in the mouse hippocampus and manifests in an altered susceptibility to epileptic seizures, underscoring the importance of FGF-dependent synapse formation. However, the receptors and signaling mechanisms by which FGF22 and FGF7 induce excitatory and inhibitory synapse differentiation are unknown. Here, we show that distinct sets of overlapping FGF receptors (FGFRs), FGFR2b and FGFR1b, mediate excitatory or inhibitory presynaptic differentiation in response to FGF22 and FGF7. Excitatory presynaptic differentiation is impaired in Fgfr2b and Fgfr1b mutant mice; however, inhibitory presynaptic defects are only found in Fgfr2b mutants. FGFR2b and FGFR1b are required for an excitatory presynaptic response to FGF22, whereas only FGFR2b is required for an inhibitory presynaptic response to FGF7. We further find that FGFRs are required in the presynaptic neuron to respond to FGF22, and that FRS2 and PI3K, but not PLCγ, mediate FGF22-dependent presynaptic differentiation. Our results reveal the specific receptors and signaling pathways that mediate FGF-dependent presynaptic differentiation, and thereby provide a mechanistic understanding of precise excitatory and inhibitory synapse formation in the mammalian brain.

Extended Synaptotagmin Interaction with the Fibroblast Growth Factor Receptor Depends on Receptor Conformation, Not Catalytic Activity

We previously demonstrated that ESyt2 interacts specifically with the activated FGF receptor and is required for a rapid phase of receptor internalization and for functional signaling via the ERK pathway in early Xenopus embryos. ESyt2 is one of the three-member family of Extended Synaptotagmins that were recently shown to be implicated in the formation of endoplasmic reticulum (ER)-plasma membrane (PM) junctions and in the Ca(2+) dependent regulation of these junctions. Here we show that ESyt2 is directed to the ER by its putative transmembrane domain, that the ESyts hetero- and homodimerize, and that ESyt2 homodimerization in vivo requires a TM adjacent sequence but not the SMP domain. ESyt2 and ESyt3, but not ESyt1, selectively interact in vivo with activated FGFR1. In the case of ESyt2, this interaction requires a short TM adjacent sequence and is independent of receptor autophosphorylation, but dependent on receptor conformation. The data show that ESyt2 recognizes a site in the upper kinase lobe of FGFR1 that is revealed by displacement of the kinase domain activation loop during receptor activation.

The ins and outs of fibroblast growth factor receptor signalling

FGFR (fibroblast growth factor receptor) signalling plays critical roles in embryogensis, adult physiology, tissue repair and many pathologies. Of particular interest over recent years, it has been implicated in a wide range of cancers, and concerted efforts are underway to target different aspects of FGFR signalling networks. A major focus has been identifying the canonical downstream signalling pathways in cancer cells, and these are now relatively well understood. In the present review, we focus on two distinct but emerging hot topics in FGF biology: its role in stromal cross-talk during cancer progression and the potential roles of FGFR signalling in the nucleus. These neglected areas are proving to be of great interest clinically and are intimately linked, at least in pancreatic cancer. The importance of the stroma in cancer is well accepted, both as a conduit/barrier for treatment and as a target in its own right. Nuclear receptors are less acknowledged as targets, largely due to historical scepticism as to their existence or importance. However, increasing evidence from across the receptor tyrosine kinase field is now strong enough to make the study of nuclear growth factor receptors a major area of interest.

FGFR1-Frs2/3 signalling maintains sensory progenitors during inner ear hair cell formation

Inner ear mechanosensory hair cells transduce sound and balance information. Auditory hair cells emerge from a Sox2-positive sensory patch in the inner ear epithelium, which is progressively restricted during development. This restriction depends on the action of signaling molecules. Fibroblast growth factor (FGF) signalling is important during sensory specification: attenuation of Fgfr1 disrupts cochlear hair cell formation; however, the underlying mechanisms remain unknown. Here we report that in the absence of FGFR1 signaling, the expression of Sox2 within the sensory patch is not maintained. Despite the down-regulation of the prosensory domain markers, p27^Kip1, Hey2, and Hes5, progenitors can still exit the cell cycle to form the zone of non-proliferating cells (ZNPC), however the number of cells that form sensory cells is reduced. Analysis of a mutant Fgfr1 allele, unable to bind to the adaptor protein, Frs2/3, indicates that Sox2 maintenance can be regulated by MAP kinase. We suggest that FGF signaling, through the activation of MAP kinase, is necessary for the maintenance of sensory progenitors and commits precursors to sensory cell differentiation in the mammalian cochlea.

The ability of our brain to perceive sound depends on its conversion into electrical impulses within the cochlea of the inner ear. The cochlea has dedicated specialized cells, called inner ear hair cells, which register sound energy. Environmental effects, genetic disorders or just the passage of time can damage these cells, and the damage impairs our ability to hear. If we could understand how these cells develop, we might be able to exploit this knowledge to generate new hair cells. In this study we address an old problem: how do signals from the fibroblast growth factor (FGF) family control hair cell number? We used mice in which one of the receptors for FGF (Fgfr1) is mutated and found that the expression of a stem cell protein, Sox2 is not maintained. Sox2 generally acts to keep precursors in the cochlea in a pre-hair cell state. However, in mutant mice Sox2 expression is transient, diminishing the ability of precursors to commit to a hair cell fate. These findings suggest that it may be possible to amplify the number of hair cell progenitors in culture by tuning FGF activity, providing a route to replace damaged inner ear hair cells.

FGFR signalling in women's cancers

FGFs, in a complex with their receptors (FGFRs) and heparan sulfate (HS), are responsible for a range of cellular functions, from embryogenesis to metabolism. Both germ line and somatic FGFR mutations are known to play a role in a range of diseases, most notably craniosynestosis dysplasias, dwarfism and cancer. Because of the ability of FGFR signalling to induce cell proliferation, migration and survival, FGFRs are readily co-opted by cancer cells. Mutations in, and amplifications of, these receptors are found in a range of cancers with some of the most striking clinical findings relating to their contribution to pathogenesis and progression of female cancers. Here, we outline the molecular mechanisms of FGFR signalling and discuss the role of this pathway in women's cancers, focusing on breast, endometrial, ovarian and cervical carcinomas, and their associated preclinical and clinical data. We also address the rationale for therapeutic intervention and the need for FGFR-targeted therapy to selectively target cancer cells in view of the fundamental roles of FGF signalling in normal physiology.

Phosphorylation of serine 779 in fibroblast growth factor receptor 1 and 2 by protein kinase C(epsilon) regulates Ras/mitogen-activated protein kinase signaling and neuronal differentiation

The FGF receptors (FGFRs) control a multitude of cellular processes both during development and in the adult through the initiation of signaling cascades that regulate proliferation, survival, and differentiation. Although FGFR tyrosine phosphorylation and the recruitment of Src homology 2 domain proteins have been widely described, we have previously shown that FGFR is also phosphorylated on Ser(779) in response to ligand and binds the 14-3-3 family of phosphoserine/threonine-binding adaptor/scaffold proteins. However, whether this receptor phosphoserine mode of signaling is able to regulate specific signaling pathways and biological responses is unclear. Using PC12 pheochromocytoma cells and primary mouse bone marrow stromal cells as models for growth factor-regulated neuronal differentiation, we show that Ser(779) in the cytoplasmic domains of FGFR1 and FGFR2 is required for the sustained activation of Ras and ERK but not for other FGFR phosphotyrosine pathways. The regulation of Ras and ERK signaling by Ser(779) was critical not only for neuronal differentiation but also for cell survival under limiting growth factor concentrations. PKCε can phosphorylate Ser(779) in vitro, whereas overexpression of PKCε results in constitutive Ser(779) phosphorylation and enhanced PC12 cell differentiation. Furthermore, siRNA knockdown of PKCε reduces both growth factor-induced Ser(779) phosphorylation and neuronal differentiation. Our findings show that in addition to FGFR tyrosine phosphorylation, the phosphorylation of a conserved serine residue, Ser(779), can quantitatively control Ras/MAPK signaling to promote specific cellular responses.

Low molecular weight fibroblast growth factor-2 signals via protein kinase C and myofibrillar proteins to protect against postischemic cardiac dysfunction

Among its many biological roles, fibroblast growth factor-2 (FGF2) acutely protects the heart from dysfunction associated with ischemia/reperfusion (I/R) injury. Our laboratory has demonstrated that this is due to the activity of the low molecular weight (LMW) isoform of FGF2 and that FGF2-mediated cardioprotection relies on the activity of protein kinase C (PKC); however, which PKC isoforms are responsible for LMW FGF2-mediated cardioprotection, and their downstream targets, remain to be elucidated. To identify the PKC pathway(s) that contributes to postischemic cardiac recovery by LMW FGF2, mouse hearts expressing only LMW FGF2 (HMWKO) were bred to mouse hearts not expressing PKCα (PKCαKO) or subjected to a selective PKCε inhibitor (εV(1-2)) before and during I/R. Hearts only expressing LMW FGF2 showed significantly improved postischemic recovery of cardiac function following I/R (P < 0.05), which was significantly abrogated in the absence of PKCα (P < 0.05) or presence of PKCε inhibition (P < 0.05). Hearts only expressing LMW FGF2 demonstrated differences in actomyosin ATPase activity as well as increases in the phosphorylation of troponin I and T during I/R compared with wild-type hearts; several of these effects were dependent on PKCα activity. This evidence indicates that both PKCα and PKCε play a role in LMW FGF2-mediated protection from cardiac dysfunction and that PKCα signaling to the contractile apparatus is a key step in the mechanism of LMW FGF2-mediated protection against myocardial dysfunction.

FGFR1-induced epithelial to mesenchymal transition through MAPK/PLCγ/COX-2-mediated mechanisms

Tumour invasion and metastasis is the most common cause of death from cancer. For epithelial cells to invade surrounding tissues and metastasise, an epithelial-mesenchymal transition (EMT) is required. We have demonstrated that FGFR1 expression is increased in bladder cancer and that activation of FGFR1 induces an EMT in urothelial carcinoma (UC) cell lines. Here, we created an in vitro FGFR1-inducible model of EMT, and used this model to identify regulators of urothelial EMT. FGFR1 activation promoted EMT over a period of 72 hours. Initially a rapid increase in actin stress fibres occurred, followed by an increase in cell size, altered morphology and increased migration and invasion. By using site-directed mutagenesis and small molecule inhibitors we demonstrated that combined activation of the mitogen activated protein kinase (MAPK) and phospholipase C gamma (PLCγ) pathways regulated this EMT. Actin stress fibre formation was regulated by PLCγ activation, and was also important for the increase in cell size, migration and altered morphology. MAPK activation regulated migration and E-cadherin expression, indicating that combined activation of PLCγ and MAPK is required for a full EMT. We used expression microarrays to assess changes in gene expression downstream of these signalling cascades. COX-2 was transcriptionally upregulated by FGFR1 and caused increased intracellular prostaglandin E(2) levels, which promoted migration. In conclusion, we have demonstrated that FGFR1 activation in UC cells lines promotes EMT via coordinated activation of multiple signalling pathways and by promoting activation of prostaglandin synthesis.

Plasticity in interactions of fibroblast growth factor 1 (FGF1) N terminus with FGF receptors underlies promiscuity of FGF1

Tissue-specific alternative splicing in the second half of Ig-like domain 3 (D3) of fibroblast growth factor receptors 1-3 (FGFR1 to -3) generates epithelial FGFR1b-FGFR3b and mesenchymal FGFR1c-FGFR3c splice isoforms. This splicing event establishes a selectivity filter to restrict the ligand binding specificity of FGFRb and FGFRc isoforms to mesenchymally and epithelially derived fibroblast growth factors (FGFs), respectively. FGF1 is termed the "universal FGFR ligand" because it overrides this specificity barrier. To elucidate the molecular basis for FGF1 cross-reactivity with the "b" and "c" splice isoforms of FGFRs, we determined the first crystal structure of FGF1 in complex with an FGFRb isoform, FGFR2b, at 2.1 Å resolution. Comparison of the FGF1-FGFR2b structure with the three previously published FGF1-FGFRc structures reveals that plasticity in the interactions of the N-terminal region of FGF1 with FGFR D3 is the main determinant of FGF1 cross-reactivity with both isoforms of FGFRs. In support of our structural data, we demonstrate that substitution of three N-terminal residues (Gly-19, His-25, and Phe-26) of FGF2 (a ligand that does not bind FGFR2b) for the corresponding residues of FGF1 (Phe-16, Asn-22, and Tyr-23) enables the FGF2 triple mutant to bind and activate FGFR2b. These findings taken together with our previous structural data on receptor binding specificity of FGF2, FGF8, and FGF10 conclusively show that sequence divergence at the N termini of FGFs is the primary regulator of the receptor binding specificity and promiscuity of FGFs.

Fibronectin induces endothelial cell migration through β1 integrin and Src-dependent phosphorylation of fibroblast growth factor receptor-1 at tyrosines 653/654 and 766

The extracellular matrix microenvironment regulates cell phenotype and function. One mechanism by which this is achieved is the transactivation of receptor tyrosine kinases by specific matrix molecules. Here, we demonstrate that the provisional matrix protein, fibronectin (FN), activates fibroblast growth factor (FGF) receptor-1 (FGFR1) independent of FGF ligand in liver endothelial cells. FN activation of FGFR1 requires β1 integrin, as evidenced by neutralizing antibody and siRNA-based studies. Complementary genetic and pharmacologic approaches identify that the non-receptor tyrosine kinase Src is required for FN transactivation of FGFR1. Whereas FGF ligand-induced phosphorylation of FGFR1 preferentially activates ERK, FN-induced phosphorylation of FGFR1 preferentially activates AKT, indicating differential downstream signaling of FGFR1 in response to alternate stimuli. Mutation analysis of known tyrosine residues of FGFR1 reveals that tyrosine 653/654 and 766 residues are required for FN-FGFR1 activation of AKT and chemotaxis. Thus, our study mechanistically dissects a new signaling pathway by which FN achieves endothelial cell chemotaxis, demonstrates how differential phosphorylation profiles of FGFR1 can achieve alternate downstream signals, and, more broadly, highlights the diversity of mechanisms by which the extracellular matrix microenvironment regulates cell behavior through transactivation of receptor tyrosine kinases.

The structural biology of the FGF19 subfamily

The ability of the Fibroblast Growth Factor (FGF) 19 subfamily to signal in an endocrine fashion sets this subfamily apart from the remaining five FGF subfamilies known for their paracrine functions during embryonic development. Compared to the members of paracrine FGF subfamiles, the three members of the FGF19 subfamily, namely FGF19, FGF21 and FGF23, have poor affinity for heparan sulfate (HS) and therefore can diffuse freely in the HS-rich extracellular matrix to enter into the bloodstream. In further contrast to paracrine FGFs, FGF19 subfamily members have unusually poor affinity for their cognate FGF receptors (FGFRs) and therefore cannot bind and activate them in a solely HS-dependent fashion. As a result, the FGF19 subfamily requires α/βklotho coreceptor proteins in order to bind, dimerize and activate their cognate FGFRs. This klotho-dependency also determines the tissue specificity of endocrine FGFs. Recent structural and biochemical studies have begun to shed light onto the molecular basis for the klotho-dependent endocrine mode of action of the FGF19 subfamily. Crystal structures of FGF19 and FGF23 show that the topology of the HS binding site (HBS) of FGF19 subfamily members deviates drastically from the common topology adopted by the paracrine FGFs. The distinct topologies of the HBS of FGF19 and FGF23 prevent HS from direct hydrogen bonding with the backbone atoms of the HBS of these ligands and accordingly decrease the HS binding affinity of this subfamily. Recent biochemical data reveal that the ?klotho ectodomain binds avidly to the ectodomain of FGFR1c, the main cognate FGFR of FGF23, creating a de novo high affinity binding site for the C-terminal tail of FGF23. The isolated FGF23 C-terminus can be used to effectively inhibit the formation of the FGF23-FGFR1c-αklotho complex and alleviate hypophosphatemia in renal phosphate disorders due to elevated levels of FGF23.

Mechanisms of FGFR3 actions in endocrine resistant breast cancer

Although endocrine therapy has dramatically improved the treatment of breast cancer therapeutic resistance and tumour recurrence occurs, even in estrogen receptor (ER) positive cases. Identifying and understanding the molecular mechanisms which underpin endocrine resistance is therefore important if future therapeutic strategies are to be developed. Members of the fibroblast growth factor (FGF) and fibroblast growth factor receptor (FGFR) families have been implicated in breast cancer development and progression. Our results demonstrate that culture of michigan cancer foundation - 1 (MCF)7 cells with FGF1 results in reduced sensitivity to tamoxifen in vitro. Furthermore, our tissue microarray expression data demonstrates that FGFR3 expression is increased in tamoxifen resistant breast tumours. To confirm that activation of FGFR3 reduced sensitivity to tamoxifen we used an inducible activation system and a constitutively active mutant of FGFR3 expressed in MCF7 cells. Activation of FGFR3 reduced sensitivity to tamoxifen and Fulvestrant but did not lead to phosphorylation of ER demonstrating that FGFR3 does not feedback to modulate ER activity. FGFR3 activation in MCF7 cells stimulated activation of the mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) signalling pathways, both of which have been implicated in tamoxifen resistance in breast cancer. Furthermore, our data indicates that activation of phospholipase C gamma is a key-signalling event regulating MAPK and PI3K activation and that its activation reduces sensitivity to tamoxifen. Therefore, we hypothesise that FGFRs could play an integral part, not only in breast cancer development but also in resistance to endocrine-therapy.

Similar expression to FGF (Sef) reduces endometrial adenocarcinoma cells proliferation via inhibiting fibroblast growth factor 2-mediated MAPK/ERK signaling pathway

OBJECTIVE

Fibroblast growth factors (FGF) axis, and in particular FGF2 axis, is an important mitogenic stimulus in endometrial carcinogenesis. hSef is a key inhibitory regulator of FGF signaling and aberrant hSef expression is reported to be present in various human carcinomas. The objective of this study was to investigate the role of hSef in the growth and proliferation of endometrial adenocarcinoma cells and to explore the mechanism that may be involved.

METHODS

Using western blot analysis, we determined the expression of hSef in Ishikawa cells under different conditions. Using luciferase reporter assays and western blot analysis, we detected the effect of hSef on MAPK/ERK-mediated FGF2 signaling. Using MTT, cell counting and colony formation assays, we analyzed the growth and proliferation of Ishikawa cells under different conditions.

RESULTS

We found that the hSef expression was positively regulated by FGF2-induced MAPK/ERK signaling and inversely, hSef expression efficiently inhibited the activity of FGF2-induced MAPK/ERK signaling, indicating the presence of hSef-mediated negative feedback mechanism for FGF signaling in endometrial cancer cells. In addition, we found that MAPK/ERK signaling was essential for the growth and proliferation of endometrial cancer cells in vitro, and hSef expression significantly reduced the cell proliferation.

CONCLUSIONS

hSef expression can inhibit the growth and proliferation of endometrial cancer cells via acting on the FGF2/MAPK/ERK signaling.

Differential effects of growth factors on oligodendrocyte progenitor migration

Oligodendrocytes are myelinating cells of the CNS that originate as progenitor cells (OP) in discrete areas of the developing brain. During brain development, OP migrate significant distances prior to proliferating and myelinating the axons of the putative white matter tracts. Growth factors play a major regulatory role in the behavior of OP. Specifically, platelet-derived growth factor A (PDGF-A) and fibroblast growth factor 2 (FGF2) are two of the most well characterized regulators of OP development. Both growth factors interact with tyrosine kinase receptors, activating various intracellular signaling pathways. The current study advances our earlier research by comparing the effects of both PDGF-A and FGF2 on OP migration. Our results show that activation of ERK is required for OP migration. These findings correlate well with our previous demonstration of the ERK pathway mediating PDGF-A induced OP migration. We also demonstrate the significance of threshold levels of growth factors and temporal regulation for OP migration. In addition, ERK activation alone is not sufficient to induce OP migration. The current research supports the involvement of the non-ERK mediated signaling pathway in OP migration.

Extended-synaptotagmin-2 mediates FGF receptor endocytosis and ERK activation in vivo

Targeting of activated plasma membrane receptors to endocytic pathways is important in determining the outcome of growth factor signaling. However, the molecular mechanisms are still poorly understood. Here, we show that the synaptotagmin-related membrane protein E-Syt2 is essential for rapid endocytosis of the activated FGF receptor and for functional signal transduction during Xenopus development. E-Syt2 depletion prevents an early phase of activated FGF receptor endocytosis that we show is required for ERK activation and the induction of the mesoderm. E-Syt2 interacts selectively with the activated FGF receptor and with Adaptin-2, and is required upstream of Ras activation and of receptor autophosphorylation for ERK activation and the induction of the mesodermal marker Xbra. The data identify E-Syt2 as an endocytic adaptor for the clathrin-mediated pathway whose function is conserved in human and suggest a broader role for the E-Syt subfamily in growth factor signaling.

FGF signalling in prostate development, tissue homoeostasis and tumorigenesis

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

FGFR3 down-regulates PTH/PTHrP receptor gene expression by mediating JAK/STAT signaling in chondrocytic cell line

The signaling axis comprising the parathyroid hormone (PTH)-related peptide (PTHrP), the PTH/PTHrP receptor and the fibroblast growth factor receptor 3 (FGFR3) plays a central role in chondrocyte proliferation. The Indian hedgehog (IHH) gene is normally expressed in early hypertrophic chondrocytes, and its negative feedback loop was shown to regulate PTH/PTHrP receptor signaling. In this study, we examined the regulation of PTH/PTHrP receptor gene expression in a FGFR3-transfected chondrocytic cell line, CFK2. Expression of IHH could not be verified on these cells, with consequent absence of hypertrophic differentiation. Also, expression of the PTH/PTHrP receptor (75% reduction of total mRNA) and the PTHrP (50% reduction) genes was reduced in CFK2 cells transfected with FGFR3 cDNA. Interestingly, we verified significant reduction in cell growth and increased apoptosis in the transfected cells. STAT1 was detected in the nuclei of the CFK2 cells transfected with FGFR3 cDNA, indicating predominance of the JAK/STAT signaling pathway. The reduction in PTH/PTHrP receptor gene in CFK2 cells overexpressing FGFR3 was partially blocked by treatment with an inhibitor of JAK3 (WHI-P131), but not with an inhibitor of MAPK (SB203580) or JAK2 (AG490). Altogether, these findings suggest that FGFR3 down-regulates PTH/PTHrP receptor gene expression via the JAK/STAT signaling in chondrocytic cells.

Impaired fibroblast growth factor receptor 1 signaling as a cause of normosmic idiopathic hypogonadotropic hypogonadism

CONTEXT

FGFR1 mutations have been identified in about 10% of patients with Kallmann syndrome. Recently cases of idiopathic hypogonadotropic hypogonadism (IHH) with a normal sense of smell (nIHH) have been reported.

AIMS

The objective of the study was to define the frequency of FGFR1 mutations in a large cohort of nIHH, delineate the spectrum of reproductive phenotypes, assess functionality of the FGFR1 mutant alleles in vitro, and investigate genotype-phenotype relationships.

DESIGN

FGFR1 sequencing of 134 well-characterized nIHH patients (112 men and 22 women) and 270 healthy controls was performed. The impact of the identified mutations on FGFR1 function was assessed using structural prediction and in vitro studies.

RESULTS

Nine nIHH subjects (five males and four females; 7%) harbor a heterozygous mutation in FGFR1 and exhibit a wide spectrum of pubertal development, ranging from absent puberty to reversal of IHH in both sexes. All mutations impair receptor function. The Y99C, Y228D, and I239T mutants impair the tertiary folding, resulting in incomplete glycosylation and reduced cell surface expression. The R250Q mutant reduces receptor affinity for FGF. The K618N, A671P, and Q680X mutants impair tyrosine kinase activity. However, the degree of functional impairment of the mutant receptors did not always correlate with the reproductive phenotype, and variable expressivity of the disease was noted within family members carrying the same FGFR1 mutation. These discrepancies were partially explained by additional mutations in known IHH loci.

CONCLUSIONS

Loss-of-function mutations in FGFR1 underlie 7% of nIHH with different degrees of impairment in vitro. These mutations act in concert with other gene defects in several cases, consistent with oligogenicity.

Fibroblast growth factor receptor-1 phosphorylation requirement for cardiomyocyte differentiation in murine embryonic stem cells

Fibroblast growth factor receptor-1 (Fgfr1) gene knockout impairs cardiac and haematopoietic development in murine embryonic stem cells (mESC). In FGFR1, tyrosine residues Y653 and Y654 are responsible for its tyrosine kinase (TK) activity whereas phosphorylated Y463 and Y766 represent docking sites for intracellular substrates. Aim of this study was the characterization of FGFR1 signalling requirements necessary for cardiomyocyte differentiation in mESC. To this purpose, fgfr1(-/-) mESC were infected with lentiviral vectors harbouring human wild-type hFGFR1 or the Y653/654F, Y463F and Y766F hFGFR1 mutants. The resulting embryonic stem (ES) cell lines were differentiated as embryoid bodies (EBs) and beating foci formation was evaluated. In order to appraise the presence of cells belonging to cardiovascular and haematopoietic lineages, specific markers were analysed by quantitative PCR, whole mount in situ hybridization and immunofluorescence. Transduction with TK(+) hFGFR1 or the TK(+) Y766F-hFGFR1 mutant rescued cardiomyocyte beating foci formation in fgfr1(-/-) EBs whereas the TK(-) Y653/654F-hFGFR1 mutant and the TK(+) Y463F-hFGFR1 mutant were both ineffective. Analysis of the expression of early and late cardiac markers in differentiating EBs confirmed these observations. At variance with cardiomyocyte differentiation, all the transduced TK(+) FGFR1 forms were able to rescue haematopoietic differentiation in EBs originated by infected fgfr1(-/-) mESC, only the TK(-) Y653/654F-hFGFR1 mutant being ineffective. In keeping with these observations, treatment with different signalling pathway inhibitors indicates that protein kinase C and ERK activation are essential for cardiomyocyte but not for haematopoietic differentiation in EBs generated by fgfr1(+/-) approximately mESC. In conclusion, our results suggest that, although FGFR1 kinase activity is necessary for both cardiac and haematopoietic lineage maturation in mESC, phosphorylation of Y463 in the intracellular domain of the receptor is a specific requirement for cardiomyocyte differentiation.

Molecular genetic analysis of FGFR1 signalling reveals distinct roles of MAPK and PLCgamma1 activation for self-renewal of adult neural stem cells

BACKGROUND

Neural stem cells (NSCs) are present in the adult mammalian brain and sustain life-long adult neurogenesis in the dentate gyrus of the hippocampus. In culture, fibroblast growth factor-2 (FGF-2) is sufficient to maintain the self-renewal of adult NSCs derived from the adult rat hippocampus. The underlying signalling mechanism is not fully understood.

RESULTS

In the established adult rat NSC culture, FGF-2 promotes self-renewal by increasing proliferation and inhibiting spontaneous differentiation of adult NSCs, accompanied with activation of MAPK and PLC pathways. Using a molecular genetic approach, we demonstrate that activation of FGF receptor 1 (FGFR1), largely through two key cytoplasmic amino acid residues that are linked to MAPK and PLC activation, suffices to promote adult NSC self-renewal. The canonical MAPK, Erk1/2 activation, is both required and sufficient for the NSC expansion and anti-differentiation effects of FGF-2. In contrast, PLC activation is integral to the maintenance of adult NSC characteristics, including the full capacity for neuronal and oligodendroglial differentiation.

CONCLUSION

These studies reveal two amino acid residues in FGFR1 with linked downstream intracellular signal transduction pathways that are essential for maintaining adult NSC self-renewal. The findings provide novel insights into the molecular mechanism regulating adult NSC self-renewal, and pose implications for using these cells in potential therapeutic applications.

FRS2 via fibroblast growth factor receptor 1 is required for platelet-derived growth factor receptor beta-mediated regulation of vascular smooth muscle marker gene expression

Vascular smooth muscle cells (VSMC) exhibit phenotypic plasticity and change from a quiescent contractile phenotype to a proliferative synthetic phenotype during physiological arteriogenesis and pathological conditions such as atherosclerosis and restenosis. Platelet-derived growth factor (PDGF)-BB is a potent inducer of the VSMC synthetic phenotype; however, much less is known about the role of fibroblast growth factor-2 (FGF2) in this process. Here, we show using signal transduction mutants of FGF receptor 1 (FGFR1) expressed in rat VSMC that the adaptor protein FRS2 is essential for FGFR1-mediated phenotypic modulation and down-regulation of VSMC smooth muscle alpha-actin (SMA) gene expression. In addition, we show that PDGF-BB and FGF2 act synergistically to induce cell proliferation and down-regulate SMA and SM22alpha in VSMC. Furthermore, we show that PDGF-BB induces tyrosine phosphorylation of FGFR1 and that this phosphorylation is mediated by PDGF receptor-beta (PDGFRbeta), but not c-Src. We demonstrate that FRS2 co-immunoprecipitates with PDGFRbeta in a complex that requires FGFR1 and that both the extracellular and the intracellular domains of FGFR1 are required for association with PDGFRbeta, whereas the cytoplasmic domain of FGFR1 is required for FRS2 association with the FGFR1-PDGFRbeta complex. Knockdown of FRS2 in VSMC by RNA interference inhibited PDGF-BB-mediated down-regulation of SMA and SM22alpha without affecting PDGF-BB mediated cell proliferation or ERK activation. Together, these data support the notion that PDGFRbeta down-regulates SMA and SM22alpha through formation of a complex that requires FGFR1 and FRS2 and prove novel insight into VSMC phenotypic plasticity.

Aberrant receptor internalization and enhanced FRS2-dependent signaling contribute to the transforming activity of the fibroblast growth factor receptor 2 IIIb C3 isoform

Alternative splice variants of fibroblast growth factor receptor 2 (FGFR2) IIIb, designated C1, C2, and C3, possess progressive reduction in their cytoplasmic carboxyl termini (822, 788, and 769 residues, respectively), with preferential expression of the C2 and C3 isoforms in human cancers. We determined that the progressive deletion of carboxyl-terminal sequences correlated with increasing transforming potency. The highly transforming C3 variant lacks five tyrosine residues present in C1, and we determined that the loss of Tyr-770 alone enhanced FGFR2 IIIb C1 transforming activity. Because Tyr-770 may compose a putative YXXL sorting motif, we hypothesized that loss of Tyr-770 in the 770YXXL motif may cause disruption of FGFR2 IIIb C1 internalization and enhance transforming activity. Surprisingly, we found that mutation of Leu-773 but not Tyr-770 impaired receptor internalization and increased receptor stability and activation. Interestingly, concurrent mutations of Tyr-770 and Leu-773 caused 2-fold higher transforming activity than caused by the Y770F or L773A single mutations, suggesting loss of Tyr and Leu residues of the 770YXXL773 motif enhances FGFR2 IIIb transforming activity by distinct mechanisms. We also determined that loss of Tyr-770 caused persistent activation of FRS2 by enhancing FRS2 binding to FGFR2 IIIb. Furthermore, we found that FRS2 binding to FGFR2 IIIb is required for increased FRS2 tyrosine phosphorylation and enhanced transforming activity by Y770F mutation. Our data support a dual mechanism where deletion of the 770YXXL773 motif promotes FGFR2 IIIb C3 transforming activity by causing aberrant receptor recycling and stability and persistent FRS2-dependent signaling.

A crystallographic snapshot of tyrosine trans-phosphorylation in action

Tyrosine trans-phosphorylation is a key event in receptor tyrosine kinase signaling, yet, the structural basis for this process has eluded definition. Here, we present the crystal structure of the FGF receptor 2 kinases caught in the act of trans-phosphorylation of Y769, the major C-terminal phosphorylation site. The structure reveals that enzyme- and substrate-acting kinases engage each other through elaborate and specific interactions not only in the immediate vicinity of Y769 and the enzyme active site, but also in regions that are as much of 18 A away from D626, the catalytic base in the enzyme active site. These interactions lead to an unprecedented level of specificity and precision during the trans-phosphorylation on Y769. Time-resolved mass spectrometry analysis supports the observed mechanism of trans-phosphorylation. Our data provide a molecular framework for understanding the mechanism of action of Kallmann syndrome mutations and the order of trans-phosphorylation reactions in FGFRs. We propose that the salient mechanistic features of Y769 trans-phosphorylation are applicable to trans-phosphorylation of the equivalent major phosphorylation sites in many other RTKs.

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

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