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

Decoding FGF/FGFR Signaling: Insights into Biological Functions and Disease Relevance

Fibroblast Growth Factors (FGFs) and their cognate receptors, FGFRs, play pivotal roles in a plethora of biological processes, including cell proliferation, differentiation, tissue repair, and metabolic homeostasis. This review provides a comprehensive overview of FGF-FGFR signaling pathways while highlighting their complex regulatory mechanisms and interconnections with other signaling networks. Further, we briefly discuss the FGFs involvement in developmental, metabolic, and housekeeping functions. By complementing current knowledge and emerging research, this review aims to enhance the understanding of FGF-FGFR-mediated signaling and its implications for health and disease, which will be crucial for therapeutic development against FGF-related pathological conditions.

FGF receptor kinase inhibitors exhibit broad antiviral activity by targeting Src family kinases

The development of antiviral strategies is a key task of biomedical research, but broad-spectrum virus inhibitors are scarce. Here we show that fibroblast growth factor receptor (FGFR) tyrosine kinase inhibitors reduce infection of several cell types with DNA and RNA viruses by blocking early stages of infection, but not viral cell association. Unexpectedly, their antiviral activity was largely independent of FGFR kinase inhibition. RNA profiling showed upregulation of interferon response genes by FGFR inhibitors, but their expression did not correlate with the antiviral activity in infected cells. Using bioinformatics analysis of kinome data, targeted kinase assays, siRNA-mediated knock-down and pharmacological inhibition experiments, we show that blockade of Src family kinases, in particular Lyn, is mainly responsible for the antiviral activity of FGFR inhibitors. These results identify FGFR inhibitors as broad-spectrum antiviral agents and suggest the poorly studied Lyn kinase as a promising target for the treatment of viral infections.

Deciphering the etiology of undiagnosed ocular anomalies along with systemic alterations in pediatric patients through whole exome sequencing

Inherited and developmental eye diseases are quite diverse and numerous, and determining their genetic cause is challenging due to their high allelic and locus heterogeneity. New molecular approaches, such as whole exome sequencing (WES), have proven to be powerful molecular tools for addressing these cases. The present study used WES to identify the genetic etiology in ten unrelated Mexican pediatric patients with complex ocular anomalies and other systemic alterations of unknown etiology. The WES approach allowed us to identify five clinically relevant variants in the GZF1, NFIX, TRRAP, FGFR2 and PAX2 genes associated with Larsen, Malan, developmental delay with or without dysmorphic facies and autism, LADD1 and papillorenal syndromes. Mutations located in GZF1 and NFIX were classified as pathogenic, those in TRRAP and FGFR2 were classified as likely pathogenic variants, and those in PAX2 were classified as variants of unknown significance. Protein modeling of the two missense FGFR2 p.(Arg210Gln) and PAX2 p.(Met3Thr) variants showed that these changes could induce potential structural alterations in important functional regions of the proteins. Notably, four out of the five variants were not previously reported, except for the TRRAP gene. Consequently, WES enabled the identification of the genetic cause in 40% of the cases reported. All the syndromes reported herein are very rare, with phenotypes that may overlap with other genetic entities.

A homozygous missense mutation in the fibroblast growth factor 5 gene is associated with the long-hair trait in Angora rabbits

BACKGROUND

Rabbits are well-domesticated animals. As a crucial economic animal, rabbit has been successfully bred into wool-use, meat-use and fur-use breeds. Hair length is one of the most economically important traits affecting profitability in wool rabbits. In this study, to identify selection signatures with the long-hair trait, whole-genomic resequencing of long-haired rabbits (Angora rabbits) and short-haired rabbits (Rex and New Zealand rabbits) was performed.

RESULTS

By genome-wide selective sweeping analysis based on population comparison, we identified a total of 5.85 Mb regions (containing 174 candidate genes) with strong selection signals. Six of these genes (Dusp1, Ihh, Fam134a, Map3k1, Spata16, and Fgf5) were enriched in the MAPK signalling and Hedgehog signalling pathways, both of which are closely associated with hair growth regulation. Among these genes, Fgf5 encodes the FGF5 protein, which is a well-established regulator of hair growth. There was a nonsynonymous nucleotide substitution (T19234C) in the Fgf5 gene. At this locus, the C allele was present in all of the tested Angora rabbits, while the T allele was dominant in New Zealand and Rex rabbits. We further confirmed that the C allele was conserved in Angora rabbits by screening an additional 135 rabbits. Moreover, the results of functional predictions and co-immunoprecipitation revealed that the T19234C mutation impaired the binding capacity of FGF5 to its receptor FGFR1.

CONCLUSIONS

We discovered that the homozygous missense mutation T19234C within Fgf5 might contribute to the long-hair trait of Angora rabbits by reducing its receptor binding capacity. This finding will provide new insights into the genetic basis underlying the genetic improvement of Angora rabbits and benefit the improvement of rabbit breeding in the future.

Sustained remission of type 2 diabetes in rodents by centrally administered fibroblast growth factor 4

Type 2 diabetes (T2D) is a major health and economic burden worldwide. Despite the availability of multiple drugs for short-term management, sustained remission of T2D is currently not achievable pharmacologically. Intracerebroventricular administration of fibroblast growth factor 1 (icvFGF1) induces sustained remission in T2D rodents, propelling intense research efforts to understand its mechanism of action. Whether other FGFs possess similar therapeutic benefits is currently unknown. Here, we show that icvFGF4 also elicits a sustained antidiabetic effect in both male db/db mice and diet-induced obese mice by activating FGF receptor 1 (FGFR1) expressed in glucose-sensing neurons within the mediobasal hypothalamus. Specifically, FGF4 excites glucose-excited (GE) neurons while inhibiting glucose-inhibited (GI) neurons. Moreover, icvFGF4 restores the percentage of GI neurons in db/db mice. Importantly, intranasal delivery of FGF4 alleviates hyperglycemia in db/db mice, paving the way for non-invasive therapy. We conclude that icvFGF4 holds significant therapeutic potential for achieving sustained remission of T2D.

Isoform-specific inhibition of FGFR signaling achieved by a de-novo-designed mini-protein

Cellular signaling by fibroblast growth factor receptors (FGFRs) is a highly regulated process mediated by specific interactions between distinct subsets of fibroblast growth factor (FGF) ligands and two FGFR isoforms generated by alternative splicing: an epithelial b- and mesenchymal c-isoforms. Here, we investigate the properties of a mini-protein, mb7, developed by an in silico design strategy to bind to the ligand-binding region of FGFR2. We describe structural, biophysical, and cellular analyses demonstrating that mb7 binds with high affinity to the c-isoforms of FGFR, resulting in inhibition of cellular signaling induced by a subset of FGFs that preferentially activate c-isoforms of FGFR. Notably, as mb7 blocks interaction between FGFR with Klotho proteins, it functions as an antagonist of the metabolic hormones FGF19 and FGF21, providing mechanistic insights and strategies for the development of therapeutics for diseases driven by aberrantly activated FGFRs.

Park et al. show that a de-novo-designed mini-protein, mb7, can specifically recognize c-isoforms of FGFRs. By masking the regions of FGFR that are critical for the FGFR activation, mb7 can potently inhibit cellular signaling by a subset of FGFs that preferentially activate FGFR c-isoform signaling.

Advances in FGFs for diabetes care applications

BACKGROUND

Diabetes mellitus (DM) is an endocrine and metabolic disease caused by a variety of pathogenic factors, including genetic factors, environmental factors and behavior. In recent decades, the number of cases and the prevalence of diabetes have steadily increased, and it has become one of the most threatening diseases to human health in the world. Currently, insulin is the most effective and direct way to control hyperglycemia for diabetes treatment at a low cost. However, hypoglycemia is often a common complication of insulin treatment. Moreover, with the extension of treatment time, insulin resistance, considered the typical adverse symptom, can appear. Therefore, it is urgent to develop new targets and more effective and safer drugs for diabetes treatment to avoid adverse reactions and the insulin tolerance of traditional hypoglycemic drugs.

SCOPE OF REVIEW

In recent years, it has been found that some fibroblast growth factors (FGFs), including FGF1, FGF19 and FGF21, can safely and effectively reduce hyperglycemia and have the potential to be developed as new drugs for the treatment of diabetes. FGF23 is also closely related to diabetes and its complications, which provides a new approach for regulating blood glucose and solving the problem of insulin tolerance.

MAJOR CONCLUSIONS

This article reviews the research progress on the physiology and pharmacology of fibroblast growth factor in the treatment of diabetes. We focus on the application of FGFs in diabetes care and prevention.

Gremlin1 is a therapeutically targetable FGFR1 ligand that regulates lineage plasticity and castration resistance in prostate cancer

Among the greatest hurdles in clinical management of prostate cancer (PCa) are the progression to lethal castration-resistant prostate cancer (CRPC) and the lack of suitable targeted therapies for advanced disease. Here we identify Gremlin1 as a ligand for fibroblast growth factor receptor 1 (FGFR1), which promotes lineage plasticity and drives castration resistance. Importantly, we generate a specific anti-Gremlin1 therapeutic antibody and demonstrate synergistic effect with androgen deprivation therapy (ADT) in CRPC. GREM1 transcription is suppressed by androgen receptor (AR) and released following ADT. We show that Gremlin1 binds to FGFR1 and activates downstream MAPK signaling. Gremlin1 interacts with FGFR1 differently to its canonical ligand FGF1, as revealed through protein structure docking and mutagenesis experiments. Altogether, our data indicate Gremlin1 as a promising candidate therapeutic target for CRPC.

Site-specific labeling and functional efficiencies of human fibroblast growth Factor-1 with a range of fluorescent Dyes in the flexible N-Terminal region and a rigid β-turn region

Human fibroblast growth factor-1 (hFGF1) binding to its receptor and heparin play critical roles in cell proliferation, angiogenesis and wound healing but is also implicated in cancer. Fluorescence imaging is a powerful approach to study such protein interactions, but it is not always obvious if the site chosen will be efficiently labeled, often relying on trial-and-error. To provide a more systematic approach towards an efficient site-specific labeling strategy, we labeled two structurally distinct regions of the protein - the flexible N-terminus and a rigid loop. Several dyes were chosen to cover the visible region and to investigate how the structure of the dye affects the labeling efficiency. Flexibility in either the protein labeling site or the dye structure was found to result in high labeling efficiency, but flexibility in both resulted in a significant decrease in labeling efficiency. Conversely, too much rigidity in both can result in dye-protein interactions that can aggregate the protein. Importantly, site-specifically labeling hFGF1 in these regions maintained biological activity. These results could be applicable to other proteins by considering the flexibility of both the protein labeling site and the dye structure.

Predicted structural mimicry of spike receptor-binding motifs from highly pathogenic human coronaviruses

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Potential coronavirus spike protein mimicry revealed by structural comparison.

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Human and non-human protein potential interactions with virus identified.

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Predicted structural mimicry corroborated by protein–protein docking.

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Epitope-based alignments may help guide vaccine efforts.

Viruses often encode proteins that mimic host proteins in order to facilitate infection. Little work has been done to understand the potential mimicry of the SARS-CoV-2, SARS-CoV, and MERS-CoV spike proteins, particularly the receptor-binding motifs, which could be important in determining tropism and druggability of the virus. Peptide and epitope motifs have been detected on coronavirus spike proteins using sequence homology approaches; however, comparing the three-dimensional shape of the protein has been shown as more informative in predicting mimicry than sequence-based comparisons. Here, we use structural bioinformatics software to characterize potential mimicry of the three coronavirus spike protein receptor-binding motifs. We utilize sequence-independent alignment tools to compare structurally known protein models with the receptor-binding motifs and verify potential mimicked interactions with protein docking simulations. Both human and non-human proteins were returned for all three receptor-binding motifs. For example, all three were similar to several proteins containing EGF-like domains: some of which are endogenous to humans, such as thrombomodulin, and others exogenous, such as Plasmodium falciparum MSP-1. Similarity to human proteins may reveal which pathways the spike protein is co-opting, while analogous non-human proteins may indicate shared host interaction partners and overlapping antibody cross-reactivity. These findings can help guide experimental efforts to further understand potential interactions between human and coronavirus proteins.

FGFR4: A promising therapeutic target for breast cancer and other solid tumors

The fibroblast growth factor receptor (FGFR) signaling pathway has long been known to cancer researchers because of its role in cell survival, proliferation, migration, and angiogenesis. Dysregulation of FGFR signaling is frequently reported in cancer studies, but most of these studies focus on FGFR1-3. However, there is growing evidence implicating an important and unique role of FGFR4 in oncogenesis, tumor progression, and resistance to anti-tumor therapy in multiple types of cancer. Importantly, there are several novel FGFR4-specific inhibitors in clinical trials, making FGFR4 an attractive target for further research. In this review, we focus on assessing the role of FGFR4 in cancer, with an emphasis on breast cancer. First, the structure, physiological functions and downstream signaling pathways of FGFR4 are introduced. Next, different mechanisms reported to cause aberrant FGFR4 activation and their functions in cancer are discussed, including FGFR4 overexpression, FGF ligand overexpression, FGFR4 somatic hotspot mutations, and the FGFR4 G388R single nucleotide polymorphism. Finally, ongoing and recently completed clinical trials targeting FGFRs in cancer are reviewed, highlighting the therapeutic potential of FGFR4 inhibition for the treatment of breast cancer.

Structure-guided design, generation, and biofunction of PEGylated fibroblast growth factor 2 variants for wound healing

Fibroblast growth factor 2 (FGF2) plays an important role in multiple physiological functions such as tissue repair. However, FGF2 has a short half-life in vivo due to protease degradation, thus limiting its clinical application. Traditional PEGylation has typically focused on the N-terminal α-amino group of FGF2. These modifications do not consider potential effects on protein function or structure, and sometimes lead to decreased bioactivity. In this study, we generated three PEGylated FGF2 variants based on the structure of the FGF2-FGFR-heparin ternary complex via gene mutation and PEGylation, and investigated the effects of these PEGylated sites on protein stability and bioactivity. Compared with native FGF2, all PEG-FGF2 conjugates exhibited significantly improved stability. Conjugates PEGylated at a site separated from both binding regions more effectively promoted proliferation, migration and angiogenesis than FGF2 in vitro, and exhibited excellent wound healing activity in vivo, making these conjugates potential therapeutic candidates for wound healing. Computer-assisted modification based on structure reveals the detailed structural characteristics of proteins, allowing efficient protein modification for improved stability and activity. This structure-guided PEGylation offers a more reliable modification strategy and should be applied for the rational design of protein-based therapeutics.

Paracrine Fibroblast Growth Factor 1 Functions as Potent Therapeutic Agent for Intrahepatic Cholestasis by Downregulating Synthesis of Bile Acid

Endocrine fibroblast growth factor (FGF) 19 has been shown to be capable of maintaining bile acid (BA) homeostasis and thus hold promise to be a potential therapeutic agent for cholestasis liver disease. However, whether paracrine FGFs possess this BA regulatory activity remains to be determined. In our study, we identified that paracrine fibroblast growth factor 1 (FGF1) was selectively downregulated in the liver of alpha naphthylisothiocyanate (ANIT)-induced intrahepatic cholestasis mice, suggesting a pathological relevance of this paracrine FGF with abnormal BA metabolism. Therefore, we evaluated the effects of engineered FGF1 mutant - FGF1^ΔHBS on the metabolism of hepatic BA and found that this protein showed a more potent inhibitory effect of BA biosynthesis than FGF19 without any hepatic mitogenic activity. Moreover, the chronic administration of FGF1^ΔHBS protected liver against ANIT-induced injury by reducing hepatic BA accumulation. Taken together, these data suggest that FGF1^ΔHBS may function as a potent therapeutic agent for intrahepatic cholestasis liver disease.

Polarizing receptor activation dissociates fibroblast growth factor 2 mediated inhibition of myelination from its neuroprotective potential

Fibroblast growth factor (FGF) signaling contributes to failure of remyelination in multiple sclerosis, but targeting this therapeutically is complicated by its functional pleiotropy. We now identify FGF2 as a factor up-regulated by astrocytes in active inflammatory lesions that disrupts myelination via FGF receptor 2 (FGFR2) mediated activation of Wingless (Wnt) signaling; pharmacological inhibition of Wnt being sufficient to abrogate inhibition of myelination by FGF2 in tissue culture. Using a novel FGFR1-selective agonist (F2 V2) generated by deleting the N-terminal 26 amino acids of FGF2 we demonstrate polarizing signal transduction to favor FGFR1 abrogates FGF mediated inhibition of myelination but retains its ability to induce expression of pro-myelinating and immunomodulatory factors that include Cd93, Lif, Il11, Hbegf, Cxcl1 and Timp1. Our data provide new insights into the mechanistic basis of remyelination failure in MS and identify selective activation of FGFR1 as a novel strategy to induce a neuroprotective signaling environment in multiple sclerosis and other neurological diseases.

Fgf3 is crucial for the generation of monoaminergic cerebrospinal fluid contacting cells in zebrafish

In most vertebrates, including zebrafish, the hypothalamic serotonergic cerebrospinal fluid-contacting (CSF-c) cells constitute a prominent population. In contrast to the hindbrain serotonergic neurons, little is known about the development and function of these cells. Here, we identify fibroblast growth factor (Fgf)3 as the main Fgf ligand controlling the ontogeny of serotonergic CSF-c cells. We show that fgf3 positively regulates the number of serotonergic CSF-c cells, as well as a subset of dopaminergic and neuroendocrine cells in the posterior hypothalamus via control of proliferation and cell survival. Further, expression of the ETS-domain transcription factor etv5b is downregulated after fgf3 impairment. Previous findings identified etv5b as critical for the proliferation of serotonergic progenitors in the hypothalamus, and therefore we now suggest that Fgf3 acts via etv5b during early development to ultimately control the number of mature serotonergic CSF-c cells. Moreover, our analysis of the developing hypothalamic transcriptome shows that the expression of fgf3 is upregulated upon fgf3 loss-of-function, suggesting activation of a self-compensatory mechanism. Together, these results highlight Fgf3 in a novel context as part of a signalling pathway of critical importance for hypothalamic development.

Summary: This study highlights Fgf3 in a novel context where it is part of a signalling pathway of critical importance for development of hypothalamic monoaminergic cells in zebrafish.

Resistance to visceral obesity is associated with increased locomotion in mice expressing an endothelial cell-specific fibroblast growth factor 1 transgene

Overdevelopment of visceral adipose is positively correlated with the etiology of obesity-associated pathologies including cardiovascular disease and insulin resistance. However, identification of genetic, molecular, and physiological factors regulating adipose development and function in response to nutritional stress is incomplete. Fibroblast Growth Factor 1 (FGF1) is a cytokine expressed and released by both adipocytes and endothelial cells under hypoxia, thermal, and oxidative stress. Expression of Fibroblast Growth Factor 1 (FGF1) in adipose is required for normal depot development and remodeling. Loss of FGF1 leads to deleterious changes in adipose morphology, metabolism, and insulin resistance. Conversely, diabetic and obese mice injected with recombinant FGF1 display improvements in insulin sensitivity and a reduction in adiposity. We report in this novel, in vivo study that transgenic mice expressing an endothelial-specific FGF1 transgene (FGF1-Tek) are resistant to high-fat diet-induced abdominal adipose accretion and are more glucose-tolerant than wild-type control animals. Metabolic chamber analyses indicate that suppression of the development of visceral adiposity and insulin resistance was not associated with alterations in appetite or resting metabolic rate in the FGF1-Tek strain. Instead, FGF1-Tek mice display increased locomotor activity that likely promotes the utilization of dietary fatty acids before they can accumulate in adipose and liver. This study provides insight into the impact that genetic differences dictating the production of FGF1 has on the risk for developing obesity-related metabolic disease in response to nutritional stress.

Structural Biology of the FGF7 Subfamily

Mammalian fibroblast growth factor (FGF) signaling is intricately regulated via selective binding interactions between 18 FGF ligands and four FGF receptors (FGFR1–4), three of which (FGFR1–3) are expressed as either epithelial (“b”) or mesenchymal (“c”) splice isoforms. The FGF7 subfamily, consisting of FGF3, FGF7, FGF10, and FGF22, is unique among FGFs in that its members are secreted exclusively by the mesenchyme, and specifically activate the “b” isoforms of FGFR1 (FGFR1b) and FGFR2 (FGFR2b) present in the overlying epithelium. This unidirectional mesenchyme-to-epithelium signaling contributes to the development of essentially all organs, glands, and limbs. Structural analysis has shown that members of the FGF7 subfamily achieve their restricted specificity for FGFR1b/FGFR2b by engaging in specific contacts with two alternatively spliced loop regions in the immunoglobulin-like domain 3 (D3) of these receptors. Weak basal receptor-binding affinity further constrains the FGF7 subfamily’s specificity for FGFR1b/2b. In this review, we elaborate on the structural determinants of FGF7 subfamily receptor-binding specificity, and discuss how affinity differences among the four members for the heparin sulfate (HS) co-receptor contribute to their disparate biological activities.

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.

Probing the role of proline -135 on the structure, stability, and cell proliferation activity of human acidic fibroblast growth factor

Human acidic fibroblast growth factor 1 (hFGF1) is a protein intricately involved in cell growth and tissue repair. In this study, we investigate the effect(s) of understanding the role of a conserved proline (P135), located in the heparin binding pocket, on the structure, stability, heparin binding affinity, and cell proliferation activity of hFGF1. Substitution of proline-135 with a positively charged lysine (P135K) resulted in partial destabilization of the protein; however, the overall structural integrity of the protein was maintained upon substitution of proline-135 with either a negative charge (P135E) or a polar amino acid (P135Q). Interestingly, upon heparin binding, an increase in thermal stability equivalent to that of wt-hFGF1 was observed when P135 was replaced with a positive (P135K) or a negative charge (P135E), or with a polar amino acid (P135Q). Surprisingly, introduction of negative charge in the heparin-binding pocket at position 135 (P135E) increased hFGF1's affinity for heparin by 3-fold, while the P135K mutation, did not alter the heparin-binding affinity. However, the enhanced heparin-binding affinity of mutant P135E did not translate to an increase in cell proliferation activity. Interestingly, the P135K and P135E double mutations, P135K/R136E and P135/R136E, reduced the heparin binding affinity by ∼3-fold. Furthermore, the cell proliferation activity was increased when the charge reversal mutation R136E was paired with both P135E (P135E/R136E) and P135K (P135K/R136E). Overall, the results of this study suggest that while heparin is useful for stabilizing hFGF1 on the cell surface, this interaction is not mandatory for activation of the FGF receptor.

Investigating the dynamics and polyanion binding sites of fibroblast growth factor-1 using hydrogen-deuterium exchange mass spectrometry

In this study, we examined the local dynamics of acidic fibroblast growth factor (FGF-1) as well as the binding sites of various polyanions including poly-sulfates (heparin and low MW heparin) and poly-phosphates (phytic acid and ATP) using hydrogen-deuterium exchange mass spectrometry (HX-MS). For local dynamics, results are analyzed at the peptide level as well as in terms of buried amides employing crystallographic B-factors and compared with a residue level heat map generated from HX-MS results. Results show that strand 4 and 5 and the turn between them to be the most flexible regions as was previously seen by NMR. On the other hand, the C-terminal strands 8, 9, and 10 appear to be more rigid which is also consistent with crystallographic B-factors as well as local dynamics studies conducted by NMR. Crystal structures of FGF-1 in complex with heparin have shown that heparin binds to N-terminal Asn18 and to C-terminal Lys105, Tryp107, Lys112, Lys113, Arg119, Pro121, Arg122, Gln127, and Lys128 indicating electrostatic forces as dominant interactions. Heparin binding as determined by HX-MS is consistent with crystallography data. Previous studies have also shown that other polyanions including low MW heparin, phytic acid and ATP dramatically increase the thermal stability of FGF-1. Using HX-MS, we find other poly anions tested bind in a similar manner to heparin, primarily targeting the turns in the lysine rich C-terminal region of FGF-1 along with two distinct N-terminal regions that contains lysines and arginines/histidines. This confirms the interactions between FGF-1 and polyanions are primary directed by electrostatics.

Effect of extension of the heparin binding pocket on the structure, stability, and cell proliferation activity of the human acidic fibroblast growth factor

Acidic human fibroblast growth factor (hFGF1) plays a key role in cell growth and proliferation. Activation of the cell surface FGF receptor is believed to involve the glycosaminoglycan, heparin. However, the exact role of heparin is a subject of considerable debate. In this context, in this study, the correlation between heparin binding affinity and cell proliferation activity of hFGF1 is examined by extending the heparin binding pocket through selective engineering via charge reversal mutations (D82R, D84R and D82R/D84R). Results of biophysical experiments such as intrinsic tryptophan fluorescence and far UV circular dichroism spectroscopy suggest that the gross native structure of hFGF1 is not significantly perturbed by the engineered mutations. However, results of limited trypsin digestion and ANS binding experiments show that the backbone structure of the D82R variant is more flexible than that of the wild type hFGF1. Results of the temperature and urea-induced equilibrium unfolding experiments suggest that the stability of the charge-reversal mutations increases in the presence of heparin. Isothermal titration calorimetry (ITC) data reveal that the heparin binding affinity is significantly increased when the charge on D82 is reversed but not when the negative charge is reversed at both positions D82 and D84 (D82R/D84R). However, despite the increased affinity of D82R for heparin, the cell proliferation activity of the D82R variant is observed to be reduced compared to the wild type hFGF1. The results of this study clearly demonstrate that heparin binding affinity of hFGF1 is not strongly correlated to its cell proliferation activity.

α-Klotho is a non-enzymatic molecular scaffold for FGF23 hormone signalling

The aging suppressor αKlotho binds to the fibroblast growth factor receptor (FGFR). This commits FGFR to respond to FGF23, a key hormone in the regulation of mineral ion/vitamin D homeostasis. The role and mechanism of this co-receptor are unknown. Here we present the atomic structure of a 1:1:1 ternary complex consisting of the shed extracellular domain of αKlotho, the FGFR1c ligand-binding domain, and FGF23. In this complex, αKlotho simultaneously tethers FGFR1c by its D3 domain and FGF23 by its C-terminal tail, thus implementing FGF23-FGFR1c proximity and conferring stability. The endocrine character of FGF23 notwithstanding, dimerization of the stabilized ternary complexes and receptor activation remain dependent on the binding of heparan sulfate, a mandatory cofactor of paracrine FGF signaling. The structure of αKlotho is incompatible with its purported glycosidase activity. Thus, shed αKlotho functions as an on-demand non-enzymatic scaffold protein that promotes FGF23 signaling.

Identification of new genes associated to senescent and tumorigenic phenotypes in mesenchymal stem cells

Although human mesenchymal stem cells (hMSCs) are a powerful tool for cell therapy, prolonged culture times result in replicative senescence or acquisition of tumorigenic features. To identify a molecular signature for senescence, we compared the transcriptome of senescent and young hMSCs with normal karyotype (hMSCs/n) and with a constitutional inversion of chromosome 3 (hMSC/inv). Senescent and young cells from both lineages showed differentially expressed genes (DEGs), with higher levels in senescent hMSCs/inv. Among the 30 DEGs in senescent hMSC/inv, 11 are new candidates for biomarkers of cellular senescence. The functional categories most represented in senescent hMSCs were related to cellular development, cell growth/proliferation, cell death, cell signaling/interaction, and cell movement. Mapping of DEGs onto biological networks revealed matrix metalloproteinase-1, thrombospondin 1, and epidermal growth factor acting as topological bottlenecks. In the comparison between senescent hMSCs/n and senescent hMSCs/inv, other functional annotations such as segregation of chromosomes, mitotic spindle formation, and mitosis and proliferation of tumor lines were most represented. We found that many genes categorized into functional annotations related to tumors in both comparisons, with relation to tumors being highest in senescent hMSCs/inv. The data presented here improves our understanding of the molecular mechanisms underlying the onset of cellular senescence as well as tumorigenesis.

Regulation of Receptor Binding Specificity of FGF9 by an Autoinhibitory Homodimerization

The epithelial fibroblast growth factor 9 (FGF9) subfamily specifically binds and activates the mesenchymal "c" splice isoform of FGF receptors 1-3 (FGFR1-3) to regulate organogenesis and tissue homeostasis. The unique N and C termini of FGF9 subfamily ligands mediate a reversible homodimerization that occludes major receptor binding sites within the ligand core region. Here we provide compelling X-ray crystallographic, biophysical, and biochemical data showing that homodimerization controls receptor binding specificity of the FGF9 subfamily by keeping the concentration of active FGF9 monomers at a level, which is sufficient for a normal FGFR "c" isoform binding/signaling, but is insufficient for an illegitimate FGFR "b" isoform binding/signaling. We show that deletion of the N terminus or alanine substitutions in the C terminus of FGF9 skews the delicate ligand equilibrium toward active FGF9 monomers causing off-target binding and activation of FGFR b isoforms. Our study is the first to implicate ligand homodimerization in the regulation of ligand-receptor specificity.

Engineering a Cysteine-Free Form of Human Fibroblast Growth Factor-1 for "Second Generation" Therapeutic Application

Human fibroblast growth factor-1 (FGF-1) has broad therapeutic potential in regenerative medicine but has undesirable biophysical properties of low thermostability and 3 buried cysteine (Cys) residues (at positions 16, 83, and 117) that interact to promote irreversible protein unfolding under oxidizing conditions. Mutational substitution of such Cys residues eliminates reactive buried thiols but cannot be accomplished simultaneously at all 3 positions without also introducing further substantial instability. The mutational introduction of a novel Cys residue (Ala66Cys) that forms a stabilizing disulfide bond (i.e., cystine) with one of the extant Cys residues (Cys83) effectively eliminates one Cys while increasing overall stability. This increase in stability offsets the associated instability of remaining Cys substitution mutations and permits production of a Cys-free form of FGF-1 (Cys16Ser/Ala66Cys/Cys117Ala) with only minor overall instability. The addition of a further stabilizing mutation (Pro134Ala) creates a Cys-free FGF-1 mutant with essentially wild-type biophysical properties. The elimination of buried free thiols in FGF-1 can substantially increase the protein half-life in cell culture. Here, we show that the effective cell survival/mitogenic functional activity of a fully Cys-free form is also substantially increased and is equivalent to wild-type FGF-1 formulated in the presence of heparin sulfate as a stabilizing agent. The results identify this Cys-free FGF-1 mutant as an advantageous "second generation" form of FGF-1 for therapeutic application.

An S116R Phosphorylation Site Mutation in Human Fibroblast Growth Factor-1 Differentially Affects Mitogenic and Glucose-Lowering Activities

Fibroblast growth factor-1 (FGF-1), a potent human mitogen and insulin sensitizer, signals through both tyrosine kinase receptor-mediated autocrine/paracrine pathways as well as a nuclear intracrine pathway. Phosphorylation of FGF-1 at serine 116 (S116) has been proposed to regulate intracrine signaling. Position S116 is located within a ∼17 amino acid C-terminal loop that contains a rich set of functional determinants including heparin∖heparan sulfate affinity, thiol reactivity, nuclear localization, pharmacokinetics, functional half-life, nuclear ligand affinity, stability, and structural dynamics. Mutational targeting of specific functionality in this region without perturbing other functional determinants is a design challenge. S116R is a non-phosphorylatable variant present in bovine FGF-1 and other members of the human FGF family. We show that the S116R mutation in human FGF-1 is accommodated with no perturbation of biophysical or structural properties, and is therefore an attractive mutation with which to elucidate the functional role of phosphorylation. Characterization of S116R shows reduction in NIH 3T3 fibroblast mitogenic stimulation, increase in fibroblast growth factor receptor-1c activation, and prolonged duration of glucose lowering in ob/ob hyperglycemic mice. A novel FGF-1/fibroblast growth factor receptor-1c dimerization interaction combined with non-phosphorylatable intracrine signaling is hypothesized to be responsible for these observed functional effects.

The Fibroblast Growth Factor signaling pathway

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

Demystifying heparan sulfate-protein interactions

Numerous proteins, including cytokines and chemokines, enzymes and enzyme inhibitors, extracellular matrix proteins, and membrane receptors, bind heparin. Although they are traditionally classified as heparin-binding proteins, under normal physiological conditions these proteins actually interact with the heparan sulfate chains of one or more membrane or extracellular proteoglycans. Thus, they are more appropriately classified as heparan sulfate-binding proteins (HSBPs). This review provides an overview of the various modes of interaction between heparan sulfate and HSBPs, emphasizing biochemical and structural insights that improve our understanding of the many biological functions of heparan sulfate.

Inclusion of the orientational entropic effect and low-resolution experimental information for protein-protein docking in Critical Assessment of PRedicted Interactions (CAPRI)

Inclusion of entropy is important and challenging for protein-protein binding prediction. Here, we present a statistical mechanics-based approach to empirically consider the effect of orientational entropy. Specifically, we globally sample the possible binding orientations based on a simple shape-complementarity scoring function using an FFT-type docking method. Then, for each generated orientation, we calculate the probability through the partition function of the ensemble of accessible states, which are assumed to be represented by the set of nearby binding modes. For each mode, the interaction energy is calculated using our ITScorePP scoring function that was developed in our laboratory based on principles of statistical mechanics. Using the above protocol, we present the results of our participation in Rounds 22-27 of the Critical Assessment of PRedicted Interactions (CAPRI) experiment for 10 targets (T46-T58). Additional experimental information, such as low-resolution small-angle X-ray scattering data, was used when available. In the prediction (or docking) experiments of the 10 target complexes, we achieved correct binding modes for six targets: one with high accuracy (T47), two with medium accuracy (T48 and T57), and three with acceptable accuracy (T49, T50, and T58). In the scoring experiments of seven target complexes, we obtained correct binding modes for six targets: one with high accuracy (T47), two with medium accuracy (T49 and T50), and three with acceptable accuracy (T46, T51, and T53).

Cooperative heparin-mediated oligomerization of fibroblast growth factor-1 (FGF1) precedes recruitment of FGFR2 to ternary complexes

Fibroblast growth factors (FGFs) utilize cell surface heparan sulfate as a coreceptor in the assembly of signaling complexes with FGF-receptors on the plasma membrane. Here we undertake a complete thermodynamic characterization of the assembly of the FGF signaling complex using isothermal titration calorimetry. Heparin fragments of defined length are used as chemical analogs of the sulfated domains of heparan sulfate and examined for their ability to oligomerize FGF1. Binding is modeled using the McGhee-von Hippel formalism for the cooperative binding of ligands to a monodimensional lattice. Oligomerization of FGFs on heparin is shown to be mediated by positive cooperativity (α = 6). Heparin octasaccharide is the shortest length capable of dimerizing FGF1 and on longer heparin chains FGF1 binds with a minimal footprint of 4.2 saccharide units. The thermodynamics and stoichiometry of the ternary complex suggest that in solution FGF1 binds to heparin in a trans-dimeric manner before FGFR recruitment.

FGFR1 mutations cause Hartsfield syndrome, the unique association of holoprosencephaly and ectrodactyly

Background

Harstfield syndrome is the rare and unique association of holoprosencephaly (HPE) and ectrodactyly, with or without cleft lip and palate, and variable additional features. All the reported cases occurred sporadically. Although several causal genes of HPE and ectrodactyly have been identified, the genetic cause of Hartsfield syndrome remains unknown. We hypothesised that a single key developmental gene may underlie the co-occurrence of HPE and ectrodactyly.

Methods

We used whole exome sequencing in four isolated cases including one case-parents trio, and direct Sanger sequencing of three additional cases, to investigate the causative variants in Hartsfield syndrome.

Results

We identified a novel FGFR1 mutation in six out of seven patients. Affected residues are highly conserved and are located in the extracellular binding domain of the receptor (two homozygous mutations) or the intracellular tyrosine kinase domain (four heterozygous de novo variants). Strikingly, among the six novel mutations, three are located in close proximity to the ATP's phosphates or the coordinating magnesium, with one position required for kinase activity, and three are adjacent to known mutations involved in Kallmann syndrome plus other developmental anomalies.

Conclusions

Dominant or recessive FGFR1 mutations are responsible for Hartsfield syndrome, consistent with the known roles of FGFR1 in vertebrate ontogeny and conditional Fgfr1-deficient mice. Our study shows that, in humans, lack of accurate FGFR1 activation can disrupt both brain and hand/foot midline development, and that FGFR1 loss-of-function mutations are responsible for a wider spectrum of clinical anomalies than previously thought, ranging in severity from seemingly isolated hypogonadotropic hypogonadism, through Kallmann syndrome with or without additional features, to Hartsfield syndrome at its most severe end.

Molecular mechanisms of fibroblast growth factor signaling in physiology and pathology

Fibroblast growth factors (FGFs) signal in a paracrine or endocrine fashion to mediate a myriad of biological activities, ranging from issuing developmental cues, maintaining tissue homeostasis, and regulating metabolic processes. FGFs carry out their diverse functions by binding and dimerizing FGF receptors (FGFRs) in a heparan sulfate (HS) cofactor- or Klotho coreceptor-assisted manner. The accumulated wealth of structural and biophysical data in the past decade has transformed our understanding of the mechanism of FGF signaling in human health and development, and has provided novel concepts in receptor tyrosine kinase (RTK) signaling. Among these contributions are the elucidation of HS-assisted receptor dimerization, delineation of the molecular determinants of ligand-receptor specificity, tyrosine kinase regulation, receptor cis-autoinhibition, and tyrosine trans-autophosphorylation. These structural studies have also revealed how disease-associated mutations highjack the physiological mechanisms of FGFR regulation to contribute to human diseases. In this paper, we will discuss the structurally and biophysically derived mechanisms of FGF signaling, and how the insights gained may guide the development of therapies for treatment of a diverse array of human diseases.

Divergent requirements for fibroblast growth factor signaling in zebrafish maxillary barbel and caudal fin regeneration

The zebrafish maxillary barbel is an integumentary organ containing skin, glands, pigment cells, taste buds, nerves, and endothelial vessels. The maxillary barbel can regenerate (LeClair & Topczewski 2010); however, little is known about its molecular regulation. We have studied fibroblast growth factor (FGF) pathway molecules during barbel regeneration, comparing this system to a well-known regenerating appendage, the zebrafish caudal fin. Multiple FGF ligands (fgf20a, fgf24), receptors (fgfr1-4) and downstream targets (pea3, il17d) are expressed in normal and regenerating barbel tissue, confirming FGF activation. To test if specific FGF pathways were required for barbel regeneration, we performed simultaneous barbel and caudal fin amputations in two temperature-dependent zebrafish lines. Zebrafish homozygous for a point mutation in fgf20a, a factor essential for caudal fin blastema formation, regrew maxillary barbels normally, indicating that the requirement for this ligand is appendage-specific. Global overexpression of a dominant negative FGF receptor, Tg(hsp70l:dn-fgfr1:EGFP)(pd1) completely blocked fin outgrowth but only partially inhibited barbel outgrowth, suggesting reduced requirements for FGFs in barbel tissue. Maxillary barbels expressing dn-fgfr1 regenerated peripheral nerves, dermal connective tissue, endothelial tubes, and a glandular epithelium; in contrast to a recent report in which dn-fgfr1 overexpression blocks pharyngeal taste bud formation in zebrafish larvae (Kapsimali et al. 2011), we observed robust formation of calretinin-positive tastebuds. These are the first experiments to explore the molecular mechanisms of maxillary barbel regeneration. Our results suggest heterogeneous requirements for FGF signaling in the regeneration of different zebrafish appendages (caudal fin versus maxillary barbel) and taste buds of different embryonic origin (pharyngeal endoderm versus barbel ectoderm).

Fibroblast growth factor 10 in human ovaries

The expression of fibroblast growth factor 10 (FGF-10) has not been studied in human ovarian cortical follicles. The aim of the present study was to investigate the expression of FGF-10 in preantral follicles from fetuses, girls and women. Ovarian samples were obtained from 14 human fetuses at 21-33 gestational weeks and from 35 girls and women aged 5-39 years. The specimens were prepared for detection of the FGF-10 protein by immunohistochemistry. Reverse-transcription PCR was applied to ovarian extracts to identify FGF-10 mRNA transcripts. In fetal tissue, the FGF-10 protein was detected in oocytes in 50% of the samples and in granulosa cells in 30%. In ovarian tissue from girls and women, the FGF-10 protein was detected in oocytes and granulosa cells in all samples. FGF-10 mRNA transcripts were present in all adult and fetal samples tested. The identification of FGF-10 at both the protein and mRNA levels suggests that FGF-10 may contribute to human preantral follicle development.