Structural basis for FGF hormone signalling

Development of Zalfermin, a Long-Acting Proteolytically Stabilized FGF21 Analog

Here, we describe the development of the FGF21 analog zalfermin (NNC0194-0499, 15), intended for once-weekly sc dosing. Protein engineering was needed to address inherent druggability issues of the natural FGF21 hormone. Thus, deamidation of Asp121 was solved by mutation to glutamine, and oxidation of Met168 was solved by mutation to leucine. N-terminal region degradation by dipeptidyl peptidase IV was prevented by alanine residue elongation. To prevent inactivating metabolism by fibroblast activation protein and carboxypeptidase-like activity in the C-terminal region, and to achieve t1/2 extension (53 h in cynomolgus monkeys), we introduced a C18 fatty diacid at the penultimate position 180. The fatty diacid binds albumin in a reversible manner, such that the free fraction of zalfermin potently activates the FGF-receptor complex and retains receptor selectivity compared with FGF21, providing strong efficacy on body weight loss in diet-induced obese mice. Zalfermin is currently being clinically evaluated for the treatment of metabolic dysfunction-associated steatohepatitis.

A Tetravalent Bispecific Antibody Selectively Inhibits Diverse FGFR3 Oncogenic Variants

The receptor tyrosine kinase FGFR3 is frequently mutated in bladder cancer and is a validated therapeutic target. Although pan-FGFR tyrosine kinase inhibitors (TKI) have shown clinical efficacy, toxicity and acquired resistance limit the benefit of these agents. While antibody-based therapeutics can offer superior selectivity than TKIs, conventional ligand-blocking antibodies are usually ineffective inhibitors of constitutively active receptor tyrosine kinases. Furthermore, the existence of multiple oncogenic variants of FGFR3 presents an additional challenge for antibody-mediated blockade. Here, we developed a tetravalent FGFR3×FGFR3 bispecific antibody that inhibited FGFR3 point mutants and fusion proteins more effectively than any of the conventional FGFR3 antibodies that we produced. Each arm of the bispecific antibody contacted two distinct epitopes of FGFR3 through a cis mode of binding. The antibody blocked dimerization of the most common FGFR3 oncogenic variant (S249C extracellular domain mutation) and inhibited the function of FGFR3 variants that are resistant to pan-FGFR TKIs. The antibody was highly effective in suppressing growth of FGFR3-driven tumor models, providing efficacy comparable to that of the FDA-approved TKI erdafitinib. Thus, this bispecific antibody may provide an effective approach for broad and highly selective inhibition of oncogenic FGFR3 variants. Significance: Development of a bispecific antibody that broadly inhibits gain-of-function FGFR3 variants provides a therapeutic strategy to target tumors with oncogenic FGFR3 point mutations and fusions, a particularly difficult case for antibody blockade.

Fibroblast growth factor 23 is pumping iron: C-terminal-fibroblast growth factor 23 cleaved peptide and its function in iron metabolism

PURPOSE OF REVIEW

Iron deficiency regulates the production of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23) but also its cleavage, to generate both intact (iFGF23) and C-terminal (Cter)-FGF23 peptides. Novel studies demonstrate that independently of the phosphaturic effects of iFGF23, Cter-FGF23 peptides play an important role in the regulation of systemic iron homeostasis. This review describes the complex interplay between iron metabolism and FGF23 biology.

RECENT FINDINGS

C-terminal (Cter) FGF23 peptides antagonize inflammation-induced hypoferremia to maintain a pool of bioavailable iron in the circulation. A key mechanism proposed is the down-regulation of the iron-regulating hormone hepcidin by Cter-FGF23.

SUMMARY

In this manuscript, we discuss how FGF23 is produced and cleaved in response to iron deficiency, and the principal functions of cleaved C-terminal FGF23 peptides. We also review possible implications anemia of chronic kidney disease (CKD).

The endocrine FGFs axis: A systemic anti-fibrotic response that could prevent pulmonary fibrogenesis?

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal disease for which therapeutic options are limited, with an unmet need to identify new therapeutic targets. IPF is thought to be the consequence of repeated microlesions of the alveolar epithelium, leading to aberrant epithelial-mesenchymal communication and the accumulation of extracellular matrix proteins. The reactivation of developmental pathways, such as Fibroblast Growth Factors (FGFs), is a well-described mechanism during lung fibrogenesis. Secreted FGFs with local paracrine effects can either exert an anti-fibrotic or a pro-fibrotic action during this pathological process through their FGF receptors (FGFRs) and heparan sulfate residues as co-receptors. Among FGFs, endocrine FGFs (FGF29, FGF21, and FGF23) play a central role in the control of metabolism and tissue homeostasis. They are characterized by a low affinity for heparan sulfate, present in the cell vicinity, allowing them to have endocrine activity. Nevertheless, their interaction with FGFRs requires the presence of mandatory co-receptors, alpha and beta Klotho proteins (KLA and KLB). Endocrine FGFs are of growing interest for their anti-fibrotic action during liver, kidney, or myocardial fibrosis. Innovative therapies based on FGF19 or FGF21 analogs are currently being studied in humans during liver fibrosis. Recent data report a similar anti-fibrotic action of endocrine FGFs in the lung, suggesting a systemic regulation of the pulmonary fibrotic process. In this review, we summarize the current knowledge on the protective effect of endocrine FGFs during the fibrotic processes, with a focus on pulmonary fibrosis.

Tumor-induced osteomalacia

Tumor-induced osteomalacia is one of paraneoplastic syndromes characterized by hypophosphatemia caused by excessive actions of fibroblast growth factor 23 (FGF23). Since the cloning of FGF23 about 20 years ago, more widespread awareness of this disease has been achieved. However, there still remain several difficulties in the management of patients with this disease. In this review, these clinical problems are discussed together with the physiological and pathophysiological functions of FGF23. Personal proposals in the management of patients with suspected patients with tumor-induced osteomalacia are also presented.

Dynamic Single Cell Transcriptomics Defines Kidney FGF23/KL Bioactivity and Novel Segment-Specific Inflammatory Targets

FGF23 via its coreceptor αKlotho (KL) provides critical control of phosphate metabolism, which is altered in rare and very common syndromes, however the spatial-temporal mechanisms dictating renal FGF23 functions remain poorly understood. Thus, developing approaches to modify specific FGF23-dictated pathways has proven problematic. Herein, wild type mice were injected with rFGF23 for 1, 4 and 12h and renal FGF23 bioactivity was determined at single cell resolution. Computational analysis identified distinct epithelial, endothelial, stromal, and immune cell clusters, with differential expressional analysis uniquely tracking FGF23 bioactivity at each time point. FGF23 actions were sex independent but critically relied upon constitutive KL expression mapped within proximal tubule (S1-S3) and distal tubule (DCT/CNT) cell sub-populations. Temporal KL-dependent FGF23 responses drove unique and transient cellular identities, including genes in key MAPK- and vitamin D-metabolic pathways via early- (AP-1-related) and late-phase (EIF2 signaling) transcriptional regulons. Combining ATACseq/RNAseq data from a cell line stably expressing KL with the in vivo scRNAseq pinpointed genomic accessibility changes in MAPK-dependent genes, including the identification of FGF23-dependent EGR1 distal enhancers. Finally, we isolated unexpected crosstalk between FGF23-mediated MAPK signaling and pro-inflammatory TNF receptor activation via NF-κB, which blocked FGF23 bioactivity in vitro and in vivo . Collectively, our findings have uncovered novel pathways at the single cell level that likely influence FGF23-dependent disease mechanisms.

Translational statement

Inflammation and elevated FGF23 in chronic kidney disease (CKD) are both associated with poor patient outcomes and mortality. However, the links between these manifestations and the effects of inflammation on FGF23-mediated mineral metabolism within specific nephron segments remain unclear. Herein, we isolated an inflammatory pathway driven by TNF/NF-κB associated with regulating FGF23 bioactivity. The findings from this study could be important in designing future therapeutic approaches for chronic mineral diseases, including potential combination therapies or early intervention strategies. We also suggest that further studies could explore these pathways at the single cell level in CKD models, as well as test translation of our findings to interactions of chronic inflammation and elevated FGF23 in human CKD kidney datasets.

Decreased FGF23 inhibits placental angiogenesis via the ERK1/2-EGR-1 signaling pathway in preeclampsia

PURPOSE

This study aimed to investigate the expression of fibroblast growth factor 23 (FGF23) in pregnant women with preeclampsia and elucidate its role in promoting placental angiogenesis through the ERK1/2-EGR-1 signaling pathway.

METHODS

Serum FGF23 levels were measured by ELISA in healthy pregnant women and patients with preeclampsia during the first, second, and third trimesters of pregnancy. Wound healing, Transwell, and tube formation assays were performed to investigate the effects of FGF23 on cell migration, invasion and tube formation. The expression of vascular endothelial growth factor A (VEGF-A) and its upstream signaling molecules, p-ERK, and EGR-1, in placental tissues was detected by RT-qPCR and western blotting. Additionally, the effect of FGF23 on VEGF-A, p-ERK, and EGR-1 expression was further explored in vitro.

RESULTS

Serum FGF23 levels increased with gestational age. During the third trimester, the control group exhibited a more pronounced increase in FGF23 levels than the preeclampsia group. Administering exogenous FGF23 promoted trophoblast cell migration, invasion and enhanced tube formation in vascular endothelial cells. The expression levels of VEGF-A, p-ERK, and EGR-1 in the placental tissues were significantly lower in the preeclampsia group than in the control group. In vitro experiments confirmed that FGF23 up-regulated VEGF-A expression through the p-ERK/EGR-1 signaling pathway.

CONCLUSION

The serum level of FGF23 decreased in pregnant women with preeclampsia, inhibiting the ERK1/2-EGR-1 pathway and resulting in decreased expression of VEGF-A, thereby inhibiting placental angiogenesis. This could be a potential mechanism involved in the progression of preeclampsia.

N-glycan on N262 of FGFR3 regulates the intracellular localization and phosphorylation of the receptor

N-glycosylation and proper processing of N-glycans are required for the function of membrane proteins including cell surface receptors. Fibroblast growth factor receptor (FGFR) is involved in a wide variety of biological processes including embryonic development, osteogenesis, angiogenesis, and cell proliferation. Human FGFR3 contains six potential N-glycosylation sites, however, the roles of glycosylation have not been elucidated. The site-specific profiles of N-glycans of the FGFR3 extracellular domain expressed and secreted by CHO-K1 cells were examined, and glycan occupancies and structures of four sites were determined. The results indicated that most sites were fully occupied by glycans, and the dominant populations were the complex type. By examining single N-glycan deletion mutants of FGFR3, it was found that N262Q mutation significantly increased the population with oligomannose-type N-glycans, which was localized in the endoplasmic reticulum. Protein stability assay suggested that fraction with oligomannose-type N-glycans in the N262Q mutant is more stable than those in the wild type and other mutants. Furthermore, it was found that ligand-independent phosphorylation was significantly upregulated in N262Q mutants with complex type N-glycans. The findings suggest that N-glycans on N262 of FGFR3 affect the intracellular localization and phosphorylation status of the receptor.

FGFR-targeted therapeutics: clinical activity, mechanisms of resistance and new directions

Fibroblast growth factor (FGF) signalling via FGF receptors (FGFR1-4) orchestrates fetal development and contributes to tissue and whole-body homeostasis, but can also promote tumorigenesis. Various agents, including pan-FGFR inhibitors (erdafitinib and futibatinib), FGFR1/2/3 inhibitors (infigratinib and pemigatinib), as well as a range of more-specific agents, have been developed and several have entered clinical use. Erdafitinib is approved for patients with urothelial carcinoma harbouring FGFR2/3 alterations, and futibatinib and pemigatinib are approved for patients with cholangiocarcinoma harbouring FGFR2 fusions and/or rearrangements. Clinical benefit from these agents is in part limited by hyperphosphataemia owing to off-target inhibition of FGFR1 as well as the emergence of resistance mutations in FGFR genes, activation of bypass signalling pathways, concurrent TP53 alterations and possibly epithelial-mesenchymal transition-related isoform switching. The next generation of small-molecule inhibitors, such as lirafugratinib and LOXO-435, and the FGFR2-specific antibody bemarituzumab are expected to have a reduced risk of hyperphosphataemia and the ability to overcome certain resistance mutations. In this Review, we describe the development and current clinical role of FGFR inhibitors and provide perspective on future research directions including expansion of the therapeutic indications for use of FGFR inhibitors, combination of these agents with immune-checkpoint inhibitors and the application of novel technologies, such as artificial intelligence.

Do Fibroblast Growth Factor Receptor (FGFR) 2 and 3 Proteins Play a Role in Prognosis of Invasive Urothelial Bladder Carcinoma?

Background & Objective

Bladder carcinoma ranks second in prevalence among males in Egypt. As a family of tyrosine kinases, fibroblast growth factor receptor (FGFR) dysregulation has been linked to some malignancies in humans. The aim of this study is to analyze the clinicopathological data of patients while investigating FGFR2 and FGFR3 immunohistochemical expression in invasive urothelial bladder carcinoma.

Methods

This retrospective cross-sectional study included 60 invasive urothelial carcinoma (UC) cases in the Pathology department, Faculty of Medicine, Menoufia University, from 2009 to 2020. All biopsies were stained for FGFR2 and FGFR3 antibodies. Complete clinical data were available for 44 patients treated and followed in clinical oncology and nuclear medicine departments.

Results

Advanced stage and high grade are significantly correlated with FGFR2 positivity (P=0.048 and 0.044, respectively). Cases presented with Perineural invasion showed a higher percentage of FGFR2 (P=0.023). There is a significant indirect linear correlation between FGFR3 expression and lymph node positivity (r= -0.265, P=0.041).

Conclusion

A high FGFR2 expression could be associated with poor prognostic parameters, while high FGFR3 expression would be associated with good prognostic parameters. These findings might highlight the importance of FGFR-targeted therapy as a FGFR2 antagonist and FGFR3 agonist for the treatment of urothelial carcinoma patients.

Erythropoietin alleviates lung ischemia-reperfusion injury by activating the FGF23/FGFR4/ERK signaling pathway

Background

The purpose of the present study was to investigate the effect of erythropoietin (EPO) on lung ischemia-reperfusion injury (LIRI).

Methods

Sprague Dawley rats and BEAS-2B cells were employed to construct an ischemia-reperfusion (I/R)-induced model in vivo and in vitro, respectively. Afterward, I/R rats and tert-butyl hydroperoxide (TBHP)-induced cells were treated with different concentrations of EPO. Furthermore, 40 patients with LIRI and healthy controls were enrolled in the study.

Results

It was observed that lung tissue damage, cell apoptosis and the expression of BAX and caspase-3 were higher in the LIRI model in vivo and in vitro than in the control group, nevertheless, the Bcl-2, FGF23 and FGFR4 expression level was lower than in the control group. EPO administration significantly reduced lung tissue damage and cell apoptosis while also up-regulating the expression of FGF23 and FGFR4. Rescue experiments indicated that EPO exerted a protective role associated with the FGF23/FGFR4/p-ERK1/2 signal pathway. Notably, the expression of serum EPO, FGF23, FGFR4 and Bcl-2 was decreased in patients with LIRI, while the expression of caspase-3 and BAX was higher.

Conclusion

EPO could effectively improve LIRI, which might be related to the activation of the FGF23/FGFR4/p-ERK1/2 signaling pathway.

Conformational landscape of soluble α-klotho revealed by cryogenic electron microscopy

α-Klotho (KLA) is a type-1 membranous protein that can associate with fibroblast growth factor receptor (FGFR) to form co-receptor for FGF23. The ectodomain of unassociated KLA is shed as soluble KLA (sKLA) to exert FGFR/FGF23-independent pleiotropic functions. The previously determined X-ray crystal structure of the extracellular region of sKLA in complex with FGF23 and FGFR1c suggests that sKLA functions solely as an on-demand coreceptor for FGF23. To understand the FGFR/FGF23-independent pleiotropic functions of sKLA, we investigated biophysical properties and structure of apo-sKLA. Mass photometry revealed that sKLA can form a stable structure with FGFR and/or FGF23 as well as sKLA dimer in solution. Single particle cryogenic electron microscopy (cryo-EM) supported the dimeric structure of sKLA. Cryo-EM further revealed a 3.3Å resolution structure of apo-sKLA that overlays well with its counterpart in the ternary complex with several distinct features. Compared to the ternary complex, the KL2 domain of apo-sKLA is more flexible. 3D variability analysis revealed that apo-sKLA adopts conformations with different KL1-KL2 interdomain bending and rotational angles. The potential multiple forms and shapes of sKLA support its role as FGFR-independent hormone with pleiotropic functions. A comprehensive understanding of the sKLA conformational landscape will provide the foundation for developing klotho-related therapies for diseases.

Functional selectivity of Receptor Tyrosine Kinases regulates distinct cellular outputs

Functional selectivity refers to the activation of differential signalling and cellular outputs downstream of the same membrane-bound receptor when activated by two or more different ligands. Functional selectivity has been described and extensively studied for G-protein Coupled Receptors (GPCRs), leading to specific therapeutic options for dysregulated GPCRs functions. However, studies regarding the functional selectivity of Receptor Tyrosine Kinases (RTKs) remain sparse. Here, we will summarize recent data about RTK functional selectivity focusing on how the nature and the amount of RTK ligands and the crosstalk of RTKs with other membrane proteins regulate the specificity of RTK signalling. In addition, we will discuss how structural changes in RTKs upon ligand binding affects selective signalling pathways. Much remains to be known about the integration of different signals affecting RTK signalling specificity to orchestrate long-term cellular outcomes. Recent advancements in omics, specifically quantitative phosphoproteomics, and in systems biology methods to study, model and integrate different types of large-scale omics data have increased our ability to compare several signals affecting RTK functional selectivity in a global, system-wide fashion. We will discuss how such methods facilitate the exploration of important signalling hubs and enable data-driven predictions aiming at improving the efficacy of therapeutics for diseases like cancer, where redundant RTK signalling pathways often compromise treatment efficacy.

Identification and engineering of potent cyclic peptides with selective or promiscuous binding through biochemical profiling and bioinformatic data analysis

As our understanding of biological systems grows, so does the need to selectively target individual or multiple members of specific protein families in order to probe their function. Many targets of current biological and pharmaceutical interest are part of a large family of closely related proteins and achieving ligand selectivity often remains either an elusive or time-consuming endeavour. Cyclic peptides (CPs) occupy a key niche in ligand space, able to achieve high affinity and selectivity while retaining synthetic accessibility. De novo cyclic peptide ligands can be rapidly generated against a given target using mRNA display. In this study we harness mRNA display technology and the wealth of next generation sequencing (NGS) data generated to explore both experimental approaches and bioinformatic, statistical data analysis of peptide enrichment in cross-screen selections to rapidly generate high affinity CPs with differing intra-family protein selectivity profiles against fibroblast growth factor receptor (FGF-R) family proteins. Using these methods, CPs with distinct selectivity profiles can be generated which can serve as valuable tool compounds to decipher biological questions.

Comparing Western and traditional Chinese medicine for male sexual dysfunction: can Klotho represent a silk road?

Traditional Chinese medicine (TCM) and Western Medicine both have shown efficacy in treating male sexual dysfunction (MSD). The aim of this perspective paper is to discuss a possible link between Western medicine and TCM in the MSD field as represented by the entity of Klotho. Klotho is a recently discovered protein, mainly expressed in the kidney, encoded by the anti-aging gene klotho. Not only is Klotho significantly correlated with the development and progression of kidney diseases and their complications, but increasing evidence indicates that it is also closely related to MSD. A comprehensive search within PubMed database was performed to retrieve available evidence on Klotho's roles, particularly in kidney and in MSD. Indeed, in the TCM theory, the concept of the "kidney" is entirely different from the Western medicine: it is closely related to metabolism and to the reproductive, nervous, endocrine systems, being more than just a urinary organ. According to the "Kidney storing essence (jīng) and governing reproduction" (KSEGR) theory, a cornerstone in TCM, the treatment of MSD mainly consists of restoring the kidney's function. Signs of decreasing kidney essence show a consistent similarity to deficiencies of Klotho, also for what regards the male sexual function. Based on the current evidence, Klotho may represent a potential biological indicator for sexual desire and sexual function and a kind of new scientific Silk Road between TCM and Western medicine for MSD; nevertheless, there is a need to conduct further high-quality research to prove this hypothesis.

Structural asymmetry in FGF23 signaling

Chen et al. have derived cryogenic electron microscopy (cryo-EM) structures of signaling complexes of the endocrine hormone fibroblast growth factor 23 (FGF23) with fibroblast growth factor receptor (FGFR), α-Klotho, and heparin sulfate. These structures are asymmetric, leading to questions concerning in vivo function, and will facilitate structure-based drug design to modulate FGF23 signaling.

Fibroblast Growth Factor 21: A Fascinating Perspective on the Regulation of Muscle Metabolism

Fibroblast growth factor 21 (FGF21) plays a vital role in normal eukaryotic organism development and homeostatic metabolism under the influence of internal and external factors such as endogenous hormone changes and exogenous stimuli. Over the last few decades, comprehensive studies have revealed the key role of FGF21 in regulating many fundamental metabolic pathways, including the muscle stress response, insulin signaling transmission, and muscle development. By coordinating these metabolic pathways, FGF21 is thought to contribute to acclimating to a stressful environment and the subsequent recovery of cell and tissue homeostasis. With the emphasis on FGF21, we extensively reviewed the research findings on the production and regulation of FGF21 and its role in muscle metabolism. We also emphasize how the FGF21 metabolic networks mediate mitochondrial dysfunction, glycogen consumption, and myogenic development and investigate prospective directions for the functional exploitation of FGF21 and its downstream effectors, such as the mammalian target of rapamycin (mTOR).